Clad welded pipe or tube and method of producing same

Abstract

Provided is a clad welded pipe or tube that has improved pipe or tube mechanical properties by reducing the width of a weld without its function as a clad pipe or tube being impaired. A clad welded pipe or tube comprises: a first layer made of base metal; and a second layer placed on one surface of the first layer, and made of first cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length L1 of weld metal at a pipe or tube inner surface and a pipe or tube circumferential length L2 of the weld metal at a pipe or tube outer surface in a weld are each 0.0010 mm or more and 1.0 mm or less, and the base metal is not exposed at a first cladding metal-side surface of the clad welded pipe or tube in the weld.

Claims

1. A clad welded pipe or tube having a two-layer structure composed of a first layer made of base metal and a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at a first cladding metal-side surface of the clad welded pipe or tube in the weld, and wherein a pipe or tube circumferential length of the weld metal at a thickness center of the clad welded pipe or tube is 0.0010 mm or more and 0.3 mm or less.

2. A clad welded pipe or tube having a two-layer structure composed of a first layer made of base metal and a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at a first cladding metal-side surface of the clad welded pipe or tube in the weld, and wherein a thickness of the first cladding metal in the weld metal is 20% or more and 300% or less of a thickness of the first cladding metal in parts other than the weld metal.

3. A clad welded pipe or tube having a three-layer structure composed of a middle layer that is a first layer made of base metal, an inner layer that is a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal, and an outer layer that is a third layer placed on the other surface of the first layer and made of second cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at the inner surface and the outer surface of the clad welded pipe or tube in the weld, and wherein a pipe or tube circumferential length of the weld metal at a thickness center of the clad welded pipe or tube is 0.0010 mm or more and 0.3 mm or less.

4. A clad welded pipe or tube having a three-layer structure composed of a middle layer that is a first layer made of base metal, an inner layer that is a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal, and an outer layer that is a third layer placed on the other surface of the first layer and made of second cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at the inner surface and the outer surface of the clad welded pipe or tube in the weld, and wherein a thickness of the first cladding metal and a thickness of the second cladding metal in the weld metal are respectively 20% or more and 300% or less of a thickness of the first cladding metal and a thickness of the second cladding metal in parts other than the weld metal, and are each 35% or less of a thickness of the clad welded pipe or tube.

5. A clad welded pipe or tube having a two-layer structure composed of a first layer made of base metal and a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at a first cladding metal-side surface of the clad welded pipe or tube in the weld, wherein the base metal is carbon steel or low-alloy steel, and the first cladding metal is stainless steel or a nickel-containing alloy, and wherein a width of the weld metal in the weld is 1.0 μm or more and 1000 μm or less, over a whole thickness of the clad welded pipe or tube.

6. The clad welded pipe or tube according to claim 5, wherein the first cladding metal is stainless steel having a chemical composition containing, in mass %, C: 0.15% or less, Si: 5.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.1% or less, Ni: 1.0% or less, Cr: 11.0% or more, and N: 0.5% or less, with the balance being Fe and inevitable impurities.

7. The clad welded pipe or tube according to claim 5, wherein the first cladding metal is stainless steel or a nickel-containing alloy having a chemical composition containing, in mass %, C: 0.15% or less, Si: 5.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.1% or less, Ni: 6.0% or more, Cr: 15.0% or more, and N: 0.5% or less, with the balance being Fe and inevitable impurities.

8. The clad welded pipe or tube according to claim 6, wherein the chemical composition of the first cladding metal further contains, in mass %, one or more selected from the group consisting of Mo: 20.0% or less, Cu: 5.0% or less, Al: 2.0% or less, Co: 3.0% or less, W: 5.0% or less, Ta: 5.0% or less, Ti: 2.0% or less, Nb: 5.0% or less, V: 2.0% or less, Zr: 2.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less.

9. The clad welded pipe or tube according to claim 5, wherein the base metal is carbon steel or low-alloy steel having a chemical composition containing, in mass %, C: 0.02% to 0.20%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.05% or less, S: 0.01% or less, and Al: 0.1% or less, with the balance being Fe and inevitable impurities.

10. The clad welded pipe or tube according to claim 9, wherein the chemical composition of the base metal further contains, in mass %, one or more selected from the group consisting of Ti: 0.1% or less, Nb: 0.2% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, and Ca: 0.0005% to 0.0050%.

11. The clad welded pipe or tube according to claim 7, wherein the chemical composition of the first cladding metal further contains, in mass %, one or more selected from the group consisting of Mo: 20.0% or less, Cu: 5.0% or less, Al: 2.0% or less, Co: 3.0% or less, W: 5.0% or less, Ta: 5.0% or less, Ti: 2.0% or less, Nb: 5.0% or less, V: 2.0% or less, Zr: 2.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less.

12. A clad welded pipe or tube comprising: a first layer made of base metal; and a second layer placed on one surface of the first layer, and made of first cladding metal that is a material different from the base metal, wherein a pipe or tube circumferential length of weld metal in a weld at each of an inner surface and an outer surface of the clad welded pipe or tube is 0.0010 mm or more and 1.0 mm or less, wherein the base metal is not exposed at a first cladding metal-side surface of the clad welded pipe or tube in the weld, and wherein a flatness value h/D in a 90° flattening test in accordance with JIS G 3445 is less than 0.3, where h is a flattening crack height in mm, and D is a pipe or tube outer diameter in mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is a sectional diagram of the vicinity of a weld of a clad welded pipe or tube 20 according to Embodiment 1 of the present disclosure perpendicular to the pipe or tube longitudinal direction;

(3) FIG. 2 is a schematic diagram of a production line for a clad welded pipe or tube according to Embodiments 1 to 3 of the present disclosure;

(4) FIG. 3A is a sectional diagram illustrating a groove shape of both transverse ends (butted parts) of a clad steel strip in Embodiment 1 of the present disclosure;

(5) FIG. 3B is a sectional diagram illustrating the weld and its vicinity after electric resistance welding the clad steel strip;

(6) FIG. 4 is a schematic diagram illustrating a groove machine (beveling machine) usable in Embodiment 1 of the present disclosure;

(7) FIG. 5A is a schematic diagram for explaining gas shielding in Embodiment 1 of the present disclosure, and is a perspective diagram of an open pipe or tube 16 and the clad welded pipe or tube 20 which are being transferred;

(8) FIG. 5B is a schematic diagram for explaining gas shielding, and is an enlarged perspective diagram of a shielding-gas blowing nozzle 81 in a Z1 portion in FIG. 5A;

(9) FIG. 5C is a schematic diagram for explaining gas shielding, and is a sectional diagram of a Z2 portion in FIG. 5A;

(10) FIG. 6A is a schematic diagram illustrating an example of a nozzle usable in Embodiments 1 to 3 of the present disclosure;

(11) FIG. 6B is a schematic diagram illustrating an example of the nozzle usable in Embodiments 1 to 3 of the present disclosure;

(12) FIG. 6C is a schematic diagram illustrating an example of the nozzle usable in Embodiments 1 to 3 of the present disclosure;

(13) FIG. 6D is a schematic diagram illustrating an example of the nozzle usable in Embodiments 1 to 3 of the present disclosure;

(14) FIG. 7A is a diagram for explaining an appropriate range of a gas release flow rate B and a gas flow rate ratio B/A of shielding gas in Embodiment 1 of the present disclosure;

(15) FIG. 7B is a diagram for explaining the appropriate range of the gas release flow rate B and the gas flow rate ratio B/A of the shielding gas in Embodiment 1 of the present disclosure;

(16) FIG. 7C is a diagram for explaining the appropriate range of the gas release flow rate B and the gas flow rate ratio B/A of the shielding gas in Embodiment 1 of the present disclosure;

(17) FIG. 8 is a graph illustrating the relationship between the gas flow rate ratio B/A of the shielding gas and the oxygen concentration around the parts to be welded;

(18) FIG. 9 is a graph illustrating the relationship between the oxygen concentration around the parts to be welded and the flatness value h/D of the electric-resistance-welded stainless clad steel pipe or tube in a 90° flattening test;

(19) FIG. 10 is a sectional diagram of the vicinity of a weld of a clad welded pipe or tube 20 according to Embodiment 2 of the present disclosure perpendicular to the pipe or tube longitudinal direction;

(20) FIG. 11A is a sectional diagram illustrating a groove shape of both transverse ends (butted parts) of a clad steel strip in Embodiment 2 of the present disclosure;

(21) FIG. 11B is a sectional diagram illustrating the weld and its vicinity after electric resistance welding the clad steel strip;

(22) FIG. 12 is a schematic diagram illustrating a groove machine usable in Embodiment 2 of the present disclosure;

(23) FIG. 13A is a schematic diagram for explaining gas shielding in Embodiment 2 of the present disclosure, and is a perspective diagram of an open pipe or tube 16 and the clad welded pipe or tube 20 which are being transferred;

(24) FIG. 13B is a schematic diagram for explaining gas shielding, and is an enlarged perspective diagram of a shielding-gas blowing nozzle 81 in a Z1 portion in FIG. 13A;

(25) FIG. 13C is a schematic diagram for explaining gas shielding, and is a sectional diagram of a Z2 portion in FIG. 13A;

(26) FIG. 14A is a diagram for explaining an appropriate range of a gas release flow rate B and a gas flow rate ratio B/A of shielding gas in Embodiment 2 of the present disclosure;

(27) FIG. 14B is a diagram for explaining the appropriate range of the gas release flow rate B and the gas flow rate ratio B/A of the shielding gas in Embodiment 2 of the present disclosure;

(28) FIG. 14C is a diagram for explaining the appropriate range of the gas release flow rate B and the gas flow rate ratio B/A of the shielding gas in Embodiment 2 of the present disclosure;

(29) FIG. 15A is a schematic sectional diagram of an electric resistance weld and its vicinity in the case where a two-layer clad steel sheet is used as raw material and the amount of upset is large;

(30) FIG. 15B is a schematic sectional diagram of an electric resistance weld and its vicinity in the case where a three-layer clad steel sheet is used as raw material and the amount of upset is large; and

(31) FIG. 15C is a schematic sectional diagram of an electric resistance weld and its vicinity in the case where a two-layer clad steel sheet is used as raw material and the amount of upset is small.

DETAILED DESCRIPTION

(32) A clad welded pipe or tube according to the present disclosure includes at least a first layer made of base metal and a second layer placed on one surface of the first layer and made of first cladding metal that is a material different from the base metal. A clad welded pipe or tube according to one of the disclosed embodiments may be a clad welded pipe or tube having a two-layer structure composed of the first layer and the second layer. A clad welded pipe or tube according to another one of the disclosed embodiments may further include a third layer placed on the other surface of the first layer. In other words, the clad welded pipe or tube may be a clad welded pipe or tube having a three-layer structure composed of the first layer (middle layer), the second layer (inner layer) placed on one surface of the first layer, and the third layer (outer layer) placed on the other surface of the first layer.

(33) Herein, “base metal” denotes the material of the thickest layer in a clad steel strip made up of two or more layers that differ in thickness, and “cladding metal” denotes the material of the other layer(s). For example, for a clad steel strip used to produce a clad welded pipe or tube, a material for ensuring the strength of the pipe or tube may be selected as the base metal, and a material for ensuring properties (e.g. corrosion resistance) not obtainable with the base metal may be selected as the cladding metal.

(34) A method for carrying out the presently disclosed techniques will be described in detail below, using an example in which a clad welded pipe or tube has a two-layer structure (Embodiments 1 and 3) and an example in which a clad welded pipe or tube has a three-layer structure (Embodiment 2).

Embodiment 1

(35) An example in which a clad welded pipe or tube has a two-layer structure will be described first.

(36) [Clad Welded Pipe or Tube]

(37) A clad welded pipe or tube 20 according to Embodiment 1 of the present disclosure will be described below, with reference to FIG. 1. The clad welded pipe or tube 20 according to this embodiment is a two-layer clad welded pipe or tube composed of a first layer 11 made of base metal and a second layer 12 made of first cladding metal that is a material different from the base metal.

(38) (Base Metal)

(39) The base metal is not limited, and may be a metal containing a steel material such as carbon steel or low-alloy steel.

(40) The carbon steel used as the base metal is not limited. It is, however, preferable to select a carbon steel whose specifications and mechanical properties are appropriate to the application of the clad welded pipe or tube, because the mechanical properties of the clad welded pipe or tube depend on the properties of the base metal occupying the major portion of the pipe or tube volume.

(41) The low-alloy steel used as the base metal is not limited, as long as its total content of alloying elements is 5 mass % or less. A low-alloy steel may be selected according to the application of the clad welded pipe or tube, as with the carbon steel.

(42) (First Cladding Metal)

(43) The first cladding metal is not limited, and may be a corrosion resistant alloy of a metal containing a steel material such as stainless steel or a nickel-containing alloy. The stainless steel is preferably SUS316L, and the nickel-containing alloy is preferably Alloy625 or Alloy825, in terms of having particularly high corrosion resistance.

(44) Which of the first layer 11 made of the base metal and the second layer 12 made of the first cladding metal forms the inner layer of the pipe or tube and which of the first layer 11 and the second layer 12 forms the outer layer of the pipe or tube are not limited, and may be determined depending on the use of the clad welded pipe or tube. For example, in a line pipe used in a high corrosive environment, the pipe or tube inner surface where a product fluid flows is required to have high corrosion resistance. Accordingly, in the case of using the clad welded pipe or tube for a line pipe, the first cladding metal may be used as the inner layer and the base metal as the outer layer. In applications where the pipe or tube outer surface is required to have high corrosion resistance, on the other hand, the base metal may be used as the inner layer and the first cladding metal as the outer layer.

(45) (Width of Weld Metal and Exposure of Base Metal)

(46) With reference to FIG. 1, in the clad welded pipe or tube 20 according to this embodiment, the width (pipe or tube circumferential length) L1 at the pipe or tube inner surface and the width (pipe or tube circumferential length) L2 at the pipe or tube outer surface of weld metals 15B and 15A in the weld are each 0.0010 mm or more and 1.0 mm or less, and the base metal is not exposed at the first cladding metal-side surface of the clad welded pipe or tube in the weld. As a result of the weld metal being present from the inner surface through to the outer surface of the pipe or tube and also having a narrow width (L1 and L2) of 1.0 mm or less, the clad welded pipe or tube 20 has excellent mechanical properties. Moreover, since the base metal is not exposed at the cladding metal-side surface of the clad welded pipe or tube in the weld, the function as a clad pipe or tube is not impaired.

(47) If L1 is less than 0.0010 mm, the amount of weld metal on the pipe or tube inner surface side is excessively small, and the strength of the weld decreases. Likewise, if L2 is less than 0.0010 mm, the amount of weld metal on the pipe or tube outer surface side is excessively small, and the strength of the weld decreases. If at least one of L1 and L2 is more than 1.0 mm, the range of weld metal increases, so that the mechanical properties of the pipe or tube degrade. In view of this, L1 and L2 are preferably 0.0100 mm to 0.5 mm.

(48) The width (pipe or tube circumferential length) L3 of weld metal at the pipe or tube thickness center is preferably 0.0010 mm or more and 0.3 mm or less. If L3 is 0.0010 mm or more, a decrease in the strength of the weld caused by the amount of weld metal being excessively small is prevented. If L3 is 0.3 mm or less, degradation in the mechanical properties of the pipe or tube caused by an increase in the range of weld metal is prevented. In view of this, L3 is more preferably 0.0100 mm to 0.3 mm.

(49) The thickness t.sub.w1 of the first cladding metal in the weld metal (the thickness of the second layer in the welded seam part) is preferably 20% or more and 300% or less of the thickness t.sub.c1 of the first cladding metal in the parts other than the weld metal. If t.sub.w1 is 20% or more of t.sub.c1, the entry of the weld metal 15A of the base metal into the weld metal 15B of the first cladding metal can be suppressed sufficiently, and sufficient properties (e.g. corrosion resistance) of the first cladding metal can be obtained at the first cladding metal-side surface of the pipe or tube. If t.sub.w1 is 300% or less of t.sub.c1, the weld metal 15B of the first cladding metal is not dominant in the weld, and sufficient properties (e.g. strength) of the base metal can be obtained. In view of this, t.sub.w1 is more preferably 50% to 200% of t.sub.c1.

(50) Herein, “weld metal” denotes metal that has melted during welding and subsequently solidified, in a welded pipe or tube. A welded pipe or tube is composed of a weld made up of weld metal and a heat-affected zone surrounding the weld metal, and a non-weld not heat-affected by welding.

(51) In the present disclosure, the weld metal is identified by the following method. A region that reliably includes a weld in a section of the welded pipe or tube perpendicular to the pipe or tube longitudinal direction is etched by an appropriate method, and a region exhibiting morphology different from a non-weld is identified as the weld metal. Here, an appropriate etching solution may be selected depending on the type of the metal. For example, for weld metal of carbon steel or low-alloy steel, the section is etched with natal, and a region observed as white by an optical microscope is identified as the weld metal. For weld metal of stainless steel or a Ni-containing alloy, the section is etched with aqua regia, and a region observed as black by an optical microscope and containing solidification microstructure in, for example, dendritic or cellular form is identified as the weld metal. The weld metal 15A of the base metal and the weld metal 15B of the cladding metal in FIG. 1 can thus be identified. Both circumferential edges of the weld metal at each of the outer surface and the inner surface in the identified region of the weld metal in the section are connected by a straight line, and each of the length of the straight line is taken to be the outer surface width and the inner surface width of the weld metal, respectively. Regarding the thickness center width of the weld metal, too, both edges of the weld metal region at a thickness center of the clad welded pipe or tube are connected by a straight line, and the length of the straight line is taken to be the thickness center width of the weld metal.

(52) Alternatively, weld metal of carbon steel or low-alloy steel may be identified as a region observed to have no segregation line when metal flow is revealed by etching the section with picric acid. Likewise, weld metal of stainless steel or a Ni-containing alloy may be identified as a region observed to have no segregation line when metal flow is revealed.

(53) The weld in the clad welded pipe or tube 20 according to this embodiment preferably has high fracture property. Specifically, the flatness value h/D in a 90° flattening test in accordance with JIS G 3445 is preferably less than 0.3, where h is the flattening crack height (mm) and D is the pipe or tube outer diameter (mm).

(54) [Method of Producing Clad Welded Pipe or Tube]

(55) The above-described clad welded pipe or tube 20 according to Embodiment 1 of the present disclosure can be produced by electric resistance welding under specific conditions described below.

(56) A process of producing a clad welded pipe or tube according to Embodiment 1 of the present disclosure is described below, with reference to FIG. 2. The production of a clad welded pipe or tube according to Embodiment 1 of the present disclosure includes the following steps: First, a clad steel strip 10 in the form of a hot rolled coil is continuously uncoiled by an uncoiler 30. After this, both transverse ends of the uncoiled clad steel strip 10 are subjected to groove machining by a groove machine 40. The clad steel strip 10 is then formed into a pipe or tube shape by a roll former 50. Following this, the transverse ends as butted parts (parts to be welded) are, while being heated to a melting point or more by a high-frequency heating device 60, butt pressed by squeeze rolls 70 to be electric resistance welded, thus obtaining a clad welded pipe or tube 20. During this, the butted parts are subjected to gas shielding by a shielding-gas blowing device 80. After this, the weld bead on the outer surface and inner surface of the weld is cut by a bead cutter 90. Subsequently, the pipe or tube 20 is cut to a predetermined length by a cutter 96.

(57) The high-frequency heating device 60 may be any of a direct current heating device and an induction heating device. Electric resistance welding may be performed with an impeder (not illustrated) inserted on the pipe or tube inner surface side within a pipe or tube passage direction region including a current passage portion of high-frequency current.

(58) This embodiment describes an example in which electric resistance welding is performed using the clad steel strip 10 obtained by pressure-bonding the first layer 11 made of the base metal and the second layer 12 made of the first cladding metal where the second layer 12 is the inner layer and the first layer 11 is the outer layer, as illustrated in FIG. 3A.

(59) (Groove Machining)

(60) In this embodiment, both transverse ends of the clad steel strip are subjected to groove machining to form a groove. This groove machining will be described below, with reference to FIG. 3A.

(61) In the groove machining, both transverse ends of the clad steel strip 10 are indented from the second layer 12 side. The groove formed is a Y-groove as illustrated in FIG. 3A, and satisfies the following conditions (i) to (iv):

(62) (i) A clad interface 13 which is the interface between the second layer 12 and the first layer 11 turns from the second layer side toward the thickness center of the clad steel strip.

(63) (ii) The bevel angle θ1 on the second layer side is 10° or more and 50° or less.

(64) (iii) The groove depth d1 is 10.0% or more and 45.0% or less of the thickness t of the clad steel strip.

(65) (iv) The projection clad ratio R1 defined by the following Expression (1) is 25% or more and 50% or less:
R1=(t.sub.c1*+d1)/t×100(%)   (1)

(66) where R1 is the projection clad ratio, t.sub.c1* is the thickness (mm) of the second layer at the root surface, d1 is the groove depth (mm) on the second layer side, and t is the thickness (mm) of the clad steel strip.

(67) It is important that the Y-groove is formed by indenting the transverse ends of the clad steel strip 10 from the second layer 12 side, and not by cutting off the second layer-side corners of the transverse ends of the clad steel strip 10. As a result, the clad interface 13 is also indented from the second layer side toward the thickness center of the clad steel strip. In addition, the bevel angle θ1, the groove depth d1, and the projection clad ratio R1 are set to the foregoing ranges. Thus, the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal after electric resistance welding can be suppressed. Consequently, the base metal is kept from being exposed at the first cladding metal-side surface of the steel pipe or tube (the inner surface in this embodiment) in the weld. This makes it possible to obtain an electric-resistance-welded clad steel pipe or tube whose inner surface is entirely, including the weld, covered with the first cladding metal, after cutting the inner surface bead of the weld.

(68) Moreover, since the groove shape has no corners at which current concentrates, the temperature distribution of the whole parts to be welded is uniform. This facilitates discharging of penetrators from the weld, and thus prevents a decrease in the toughness and strength of the weld.

(69) If the bevel angle θ1 is less than 10°, the uniformity of the temperature distribution of the whole parts to be welded cannot be maintained, as a result of which discharging of penetrators tends to be insufficient. This causes insufficient properties such as toughness and strength of the weld. Besides, the width L1 or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(70) If the bevel angle θ1 is more than 50°, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal is insufficient. Consequently, the base metal is exposed at the first cladding metal-side surface of the steel pipe or tube in the weld, which impairs the function as a clad pipe or tube. Besides, the width L1 or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(71) If the groove depth d1 is less than 10.0% of the thickness t of the clad steel strip, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal is insufficient. Consequently, the base metal is more likely to be exposed at the first cladding metal-side surface of the steel pipe or tube in the weld. Besides, the width L1 or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(72) If the groove depth d1 is more than 45.0% of the thickness t of the clad steel strip, the composition of the weld is a high-alloy composition similar to the composition of the cladding metal. This causes insufficient properties such as toughness and strength of the weld. Besides, the width L1 or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(73) In terms of achieving both the prevention of the exposure of the base metal at the first cladding metal-side surface and the prevention of a decrease in the properties of the weld at higher level, the bevel angle θ1 is preferably 15° or more, the bevel angle θ1 is preferably 35° or less, the groove depth d1 is preferably 15% or more of the thickness t of the clad steel strip, and the groove depth d1 is preferably 35% or less of the thickness t of the clad steel strip.

(74) It is also very important in this embodiment that the projection clad ratio R1 defined by Expression (1) is 25% or more and 50% or less. If the projection clad ratio R1 is less than 25%, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal is insufficient. Consequently, the base metal is exposed at the first cladding metal-side surface of the steel pipe or tube in the weld. Moreover, with a low projection clad ratio of less than 25%, the groove shape is close to a rectangle. In such a case, due to the properties of high-frequency current, current tends to concentrate only at corners, facilitating heating. Besides, given that the metal of high-alloy composition such as stainless steel or a Ni alloy as the first cladding metal has a lower melting point than the base metal such as low-carbon low-alloy steel, with a low projection clad ratio of less than 25%, the first cladding metal of low melting point at the corners melts easily. This causes the width of the weld metal of the first cladding metal to be not uniform, as a result of which L1 or L2 exceeds 1.0 mm.

(75) If the projection clad ratio R1 is more than 50%, that is, if the position of the clad interface at the root surface of the Y-groove is on the base metal side of the wall thickness center of the clad steel strip 10, the most part of the welded seam 14 after electric resistance welding is a welded seam resulting from electric resistance welding the metal of high-alloy composition as the first cladding metal, so that the properties of the weld such as toughness and strength decrease. Besides, given that the metal of high-alloy composition such as stainless steel or a Ni alloy as the first cladding metal has a lower melting point than the base metal such as low-carbon low-alloy steel, with a high projection clad ratio of more than 50%, the first cladding metal of low melting point melts excessively. As a result, the width L1 or L2 of the weld metal exceeds 1.0 mm.

(76) In the case of using the electric-resistance-welded clad steel pipe or tube as a line pipe for transporting a corrosive substance, to maintain the corrosion resistance of the inner surface of the clad steel pipe or tube for the long term, the thickness t.sub.w1 of the first cladding metal in the weld metal is preferably 20% or more of the thickness t.sub.c1 of the first cladding metal in the parts other than the weld metal, as mentioned earlier. To achieve this, the projection clad ratio R1 of the Y-groove of the butted parts in the electric resistance welding is preferably set to 30% or more.

(77) The groove machining in this embodiment can be performed using, for example, a groove machine 40 illustrated in FIG. 4. The groove machine 40 is a rolling-type groove machine capable of continuously machining the running clad steel strip 10, and includes a pair of right and left upper side rolls 42 and a pair of right and left lower side rolls 44. As a result of the upper side rolls 42 having a rolling portion 42A that increases in diameter upward in reverse taper shape as illustrated in FIG. 4, the Y-groove can be formed at both transverse ends of the clad steel strip 10.

(78) By changing the reverse taper shape of the rolling portion 42A, the groove shape of the transverse ends of the clad steel strip can be adjusted as desired. As illustrated in FIG. 3A, the projection clad ratio R1 depends on the ratio of the thickness t.sub.m of the first layer (base metal) and the thickness t.sub.c1 of the second layer (first cladding metal) in the clad steel strip 10 and the groove shape by the indentation. Accordingly, the projection clad ratio R1 can be set to a desired value by selecting the ratio appropriately and also adjusting the reverse taper shape of the rolling portion 42A to form an appropriate groove shape.

(79) (Gas Shielding)

(80) Next, as illustrated in FIGS. 5A and 5C, the clad steel strip 10 is formed into a pipe or tube shape, to obtain an open pipe or tube 16 that is a cylindrical strip before welding. A pair of butted parts (parts to be welded) 17 of the open pipe or tube facing each other are, while being subjected to gas shielding, butt pressed and electric resistance welded, to obtain the clad welded pipe or tube 20.

(81) In FIG. 5A, reference sign 18 is the butted part heating starting point of the open pipe or tube, and reference sign 19 is the welding point representing the position in the pipe or tube passage direction at which the parts 17 to be welded are joined. In this embodiment, the entire region in the pipe or tube passage direction from the heating starting point 18 to the welding point 19 or a zone within that region where oxides tend to form in the parts to be welded (this zone can be located by preliminary investigation) is defined as the shielding range in the electric resistance welding, and a shielding-gas blowing nozzle 81 (hereafter also simply referred to as “nozzle”) is placed directly above the parts 17 to be welded in the shielding range.

(82) The nozzle 81 is split into three layers in the open pipe or tube butting direction Y, as illustrated in FIGS. 5B, 6A, and 6D. Alternatively, the nozzle 81 may be split into four or more layers in the open pipe or tube butting direction Y, as illustrated in FIGS. 6B and 6C. Thus, the nozzle 81 has three or more split nozzles arranged in parallel with and adjacent to each other in the open pipe or tube butting direction Y. The three or more split nozzles are made up of a pair of first split nozzles 84A located at both ends and a remaining second split nozzle (or nozzles) 84B. Each split nozzle is hollow inside, and forms a gas flow path independent of the other split nozzles. Each of the split nozzles 84A and 84B is supplied with shielding gas from a corresponding gas pipe 82, and the amount of the gas supplied is controlled by a gas flow adjusting device 83. The tip of each of the pair of first split nozzles 84A defines a slit-shaped first gas outlet 85A, and the tip of each second split nozzle 84B defines a slit-shaped second gas outlet 85B. The nozzle 81 is placed so that the gas outlets 85A and 85B face the upper ends of the parts 17 to be welded.

(83) We examined in detail the flow of the shielding gas. We also researched in detail the influence of various shielding gas blowing conditions, such as the position and size of each of the gas outlets 85A and 85B and the flow rate of the shielding gas through each of the gas outlets 85A and 85B, on the oxygen concentration around the parts 17 to be welded during electric resistance welding and the oxide area ratio in the weld formed by electric resistance welding the parts to be welded.

(84) We consequently discovered that, under the optimum shielding gas blowing conditions, the oxygen concentration around the parts to be welded is 0.01 mass % or less, and as a result the oxide area ratio in the weld is less than 0.1%, with it being possible to obtain a weld having excellent fracture property. Herein, the oxide area ratio in the weld is defined as follows: A fracture surface formed by subjecting an electric resistance weld to a Charpy impact test is observed for at least 10 observation fields at 500 or more magnifications using an electron microscope. The total area of oxide-containing dimple fracture surface areas found in the fracture surface is measured, and the ratio of this total area to the total observation field area is taken to be the oxide area ratio.

(85) The determined optimum conditions are as follows: The nozzle height H, i.e. the height from the upper ends of the parts 17 to be welded to the gas outlets 85A and 85B, is 5 mm or more and 300 mm or less (see FIG. 5C), and the shielding gas is blown under the conditions that B is 0.5 m/s to 50 m/s and 0.010≤B/A≤10, where A (m/s) is the gas release flow rate from the pair of first gas outlets 85A located at both ends, and B (m/s) is the gas release flow rate from the remaining second gas outlet (or outlets) 85B. We also discovered that the width of the weld can be sufficiently reduced by employing these gas shielding conditions in addition to the above-described groove machining conditions.

(86) If the nozzle height H is more than 300 mm, the amount of shielding gas reaching the parts 17 to be welded is insufficient, so that the oxygen concentration around the parts 17 to be welded is more than 0.01 mass %, and a weld having excellent fracture property cannot be obtained. Besides, penetrators tend to occur in the weld, and the flatness value h/D is likely to be 0.3 or more. Further, the width L1 or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade. If the nozzle height H is less than 5 mm, radiant heat from the parts 17 to be welded being heated tends to damage the gas outlets 85A and 85B, and also a spatter from the parts 17 to be welded collides with the nozzle 81 and decreases the durability of the nozzle 81. In addition, the weld metal is easily blown away by the shielding gas. In such a case, the width L1, L2, or L3 of the weld metal falls below 0.0010 mm, and the strength of the weld decreases.

(87) If the flow rate B is excessively low, the shielding gas spreads out and the gas shielding of the parts 17 to be welded is insufficient, so that the oxygen concentration around the parts 17 to be welded is more than 0.01 mass % and a weld having excellent fracture property cannot be obtained. If the flow rate B is excessively high, the shielding gas blows too intensely and causes air entrainment between the end surfaces of the parts 17 to be welded. Both if the flow rate B is excessively low and if the flow rate B is excessively high, the width L1, L2, or L3 of the weld metal falls below 0.0010 mm, and the strength of the weld decreases. The appropriate range of the flow rate B is therefore 0.5 m/s to 50 m/s. In the case where there are a plurality of second gas outlets 85B at the center (e.g. FIGS. 6B and 6C), the flow rates B at the respective second gas outlets need not necessarily be the same, and may be different from each other as long as the flow rates B are within the appropriate range.

(88) Even when the flow rate B is within the appropriate range, however, if the gas flow rate ratio B/A, i.e. the ratio between the flow rate B and the flow rate A, is inappropriate, air entrainment 87 occurs as illustrated in FIGS. 7A to 7C.

(89) As illustrated in FIG. 7A, in the case where B/A<0.010, the gas flows from the first gas outlets 85A at both ends are too intense, and the gas flow from the second gas outlet 85B at the center is too weak. Accordingly, the gas flows from the first gas outlets 85A at both ends reflect off the outer surface of the open pipe or tube 16, and deflect upward. This causes the gas flow rate in the reflection region to be close to 0, as a result of which air entrainment 87 occurs along the outer surface of the open pipe or tube 16. Consequently, the oxygen concentration around the parts 17 to be welded cannot be reduced sufficiently, and a weld having excellent fracture property cannot be obtained. In addition, the weld metal on the outer surface side is easily blown away by gas entrainment. In such a case, L2 falls below 0.0010 mm, and the strength of the weld decreases.

(90) As illustrated in FIG. 7C, in the case where B/A>10, the gas flow from the second gas outlet 85B at the center is too intense, and the gas flows from the first gas outlets 85A at both ends are too weak. Accordingly, the gas flow from the second gas outlet 85B at the center draws air into the gap between the end surfaces of the parts 17 to be welded, and facilitates air entrainment 87. Consequently, the oxygen concentration around the parts 17 to be welded cannot be reduced sufficiently, and a weld having excellent fracture property cannot be obtained. In addition, the weld metal is easily blown away by the shielding gas. In such a case, the width L1, L2, or L3 of the weld metal falls below 0.0010 mm, and the strength of the weld decreases.

(91) As illustrated in FIG. 7B, in the case where 0.010≤B/A≤10, shielding gas 86 sufficiently but not excessively fills the gap between the end surfaces of the parts 17 to be welded, without air entrainment. Consequently, the oxygen concentration around the parts 17 to be welded is 0.01 mass % or less, and a weld having excellent fracture property can be obtained. Moreover, the widths L1 and L2 of the weld metal can be limited to 0.0010 mm or more and 1.0 mm or less. In the case where there are a plurality of second gas outlets 85B at the center and the flow rates at the respective second gas outlets are different from each other, B/A calculated using the maximum one of the flow rates as the “flow rate B” is to satisfy the above-mentioned conditions.

(92) FIG. 8 is a graph illustrating, as an example, the results of measuring the oxygen concentration at an intermediate position between the end surfaces of the parts 17 to be welded. The shielding gas 86 was blown over the parts 17 to be welded, with a nozzle height H of 50 mm and varying gas flow rate ratios B/A within the appropriate range of 0.5≤B≤50. A stainless clad steel strip having low-carbon low-alloy steel with a thickness of 16 mm as the base metal on the pipe or tube outer surface side and austenitic stainless steel (SUS316L) with a thickness of 4 mm as the cladding metal on the pipe or tube inner surface side was used.

(93) As illustrated in FIG. 8, an oxygen concentration of 0.01 mass % or less around the parts to be welded can be well (i.e. reliably) achieved by controlling the gas flow rate ratio B/A to 0.010≤B/A≤10 within the appropriate range of 0.5≤B≤50. Moreover, as illustrated in FIG. 8, 0.030≤B/A≤5 is preferable because a lower oxygen concentration level of 0.001 mass % to 0.0001 mass % can be achieved.

(94) We confirmed that the same results were obtained even when other conditions such as the nozzle height H were changed.

(95) FIG. 9 is a graph illustrating the relationship between the oxygen concentration around the parts to be welded and the flatness value h/D of each clad welded pipe or tube in a 90° flattening test. The data in the graph was obtained by the following procedure. First, a stainless clad steel strip was electric resistance welded with each of various oxygen concentrations around the parts to be welded, to produce an electric-resistance-welded stainless clad steel pipe or tube. As the stainless clad steel strip, a clad steel strip composed of a first layer having a thickness of 5 mm and made of low-carbon low-alloy steel as base metal and a second layer having a thickness of 2 mm and made of austenitic stainless steel (SUS316L) as cladding metal was used. The electric-resistance-welded stainless clad steel pipe or tube was produced to have the first layer on the pipe or tube outer surface side and the second layer on the pipe or tube inner surface side. In the electric resistance welding, the amount of upset was limited to 1.0 mm which is not greater than the thickness of the stainless clad steel strip so as to prevent the base metal from being exposed at the pipe or tube inner surface, as illustrated in FIG. 15C. A test piece of 50 mm in length was then collected from the obtained electric-resistance-welded stainless clad steel pipe or tube, and a 90° flattening test in accordance with JIS G 3445 was performed to obtain the flatness value h/D.

(96) As illustrated in FIG. 9, each electric-resistance-welded stainless clad steel pipe or tube produced in an atmosphere of an oxygen concentration of 0.01 mass % or less around the parts to be welded showed a flatness value h/D (h: flattening crack height, D: pipe or tube outer diameter) of less than 0.3 in the 90° flattening test, i.e. had a weld with excellent fracture property.

(97) The combined shape of all of the gas outlets 85A and 85B is preferably a rectangular shape whose length, i.e. an X component of the size in the pipe or tube passage direction, is 30 mm or more and width (total width R in FIG. 5C), i.e. a Y component of the size in the open pipe or tube butting direction, is 5 mm or more. Such a shape contributes to more uniform gas blowing over the parts 17 to be welded.

(98) It is also preferable to satisfy R/W>1.0, where R is the total width of all of the gas outlets 85A and 85B, and W is the maximum distance between the butted parts of the open pipe or tube directly below the gas outlets, as illustrated in FIG. 5C. This allows for a more rapid reduction in the oxygen concentration around the parts 17 to be welded.

(99) In this embodiment, the shielding gas is composed of at least one of inert gas and reducing gas.

(100) As used herein, the term “inert gas” refers to gases such as nitrogen gas, helium gas, argon gas, neon gas, and xenon gas, mixtures of two or more of these gases, and the like.

(101) The shielding gas is preferably a gas containing 0.1 mass % or more reducing gas. Such a gas is more effective in suppressing the formation of oxides responsible for penetrators, thus further improving the toughness or strength of the weld. As used herein, the term “reducing gas” refers to gases such as hydrogen gas, carbon monoxide gas, methane gas, and propane gas, mixtures of two or more of these gases, and the like. The gas containing 0.1 mass % or more reducing gas is preferably reducing gas alone or a gas containing 0.1 mass % or more reducing gas and the balance being inert gas.

(102) The following shielding gases are preferred for their availability and low cost:

(103) (a) If inert gases are used alone, (G1) any one of nitrogen gas, helium gas, and argon gas or a mixture of two or more of these gases is preferred.

(104) (b) If reducing gases are used alone, (G2) any one of hydrogen gas and carbon monoxide gas or a mixture of these gases is preferred.

(105) (c) If mixtures of inert gases and reducing gases are used, a mixture of the gases (G1) and (G2) is preferred.

(106) Note that careful safety measures are to be taken if gases containing hydrogen gas and/or carbon monoxide gas are used.

(107) (Amount of Upset)

(108) In this embodiment, the amount of upset is limited to not greater than the thickness t of the clad steel strip. This prevents the base metal in the weld from being exposed at the first cladding metal-side surface of the steel pipe or tube. The amount of upset is preferably 20% or more of the thickness t.sub.c1 of the first cladding metal of the clad steel strip, in terms of ensuring the effect of discharging penetrators from the weld during the electric resistance welding. In addition, by limiting the amount of upset to 20% or more of the thickness t.sub.c1 of the first cladding metal of the clad steel strip, the pipe or tube circumferential length L3 of the weld metal at the pipe or tube thickness center can be kept at 0.0010 mm or more and 0.3 mm or less. The amount of upset by the squeeze rolls is determined by measuring the outer perimeter of the pipe or tube situated in front of the squeeze rolls, then measuring the outer perimeter of the pipe or tube after welding the parts to be welded by the squeeze rolls and cutting the weld bead portion on the outer surface, and calculating the difference between these outer perimeters.

(109) By controlling the combination of the conditions of the groove machining, the shielding gas, and the amount of upset as described above, the exposure of the base metal at the first cladding metal-side surface can be completely prevented. Consequently, an electric-resistance-welded clad steel pipe or tube excellent in performance such as corrosion resistance can be obtained without performing post-treatment such as weld overlaying or melting and solidification as in the conventional techniques.

(110) Although the foregoing embodiment describes an example of producing the clad welded pipe or tube so that the second layer 12 as the first cladding metal is the inner layer and the first layer 11 as the base metal is the outer layer, the clad welded pipe or tube is not limited to such, and may have a structure in which the second layer as the first cladding metal is the outer layer and the first layer as the base metal is the inner layer. In this case, too, the same advantageous effects can be achieved by forming a Y-groove through indentation from the second layer side, i.e. the cladding metal side.

Embodiment 2

(111) An example in which a clad welded pipe or tube has a three-layer structure will be described next. In Embodiment 2, the description of the same matters as those in Embodiment 1 is omitted as appropriate.

(112) [Clad Welded Pipe or Tube]

(113) A clad welded pipe or tube 20 according to Embodiment 2 of the present disclosure will be described below, with reference to FIG. 10. The clad welded pipe or tube 20 according to this embodiment is a three-layer clad welded pipe or tube composed of a middle layer 11 made of base metal, an inner layer 12A made of first cladding metal that is a material different from the base metal, and an outer layer 12B made of second cladding metal that is a material different from the base metal.

(114) In this embodiment, the same materials as those in Embodiment 1 can be used as the base metal and the first cladding metal. The second cladding metal may be the same material as the first cladding metal in Embodiment 1. The first cladding metal forming the inner layer and the second cladding metal forming the outer layer may be the same material or different materials.

(115) By forming such a clad welded pipe or tube of a three-layer structure, a welded pipe or tube having excellent properties by benefiting from the properties of both base metal and cladding metal can be obtained. For example, the middle layer (wall thickness center portion) is made of base metal such as carbon steel or low-alloy steel which is a low-cost material with excellent mechanical strength and the inner layer and the outer layer (the inner and outer surfaces of the pipe or tube) are made of cladding metal such as stainless steel or a nickel-containing alloy which is a material with excellent corrosion resistance. In this way, a steel pipe or tube having the same level of corrosion resistance as in the case where the whole steel pipe or tube is made of stainless steel or a nickel-containing alloy and also having excellent strength can be provided at low cost.

(116) (Width of Weld Metal and Exposure of Base Metal)

(117) With reference to FIG. 10, in the clad welded pipe or tube 20 according to this embodiment, the width (pipe or tube circumferential length) L1 at the pipe or tube inner surface and the width (pipe or tube circumferential length) L2 at the pipe or tube outer surface of weld metals 15B and 15C in the weld are each 0.0010 mm or more and 1.0 mm or less, and the base metal is not exposed at the inner surface and the outer surface of the clad welded pipe or tube in the weld. As a result of the weld metal being present from the inner surface through to the outer surface of the pipe or tube and also having a narrow width (L1 and L2) of 1.0 mm or less, the clad welded pipe or tube 20 has excellent mechanical properties. Moreover, since the base metal is not exposed at the inner surface and the outer surface of the clad welded pipe or tube in the weld, the function as a clad pipe or tube is not impaired.

(118) If L1 is less than 0.0010 mm, the amount of weld metal of the cladding metal is excessively small, and the strength of the weld decreases. Likewise, if L2 is less than 0.0010 mm, the amount of weld metal is excessively small, and the strength of the weld decreases. If at least one of L1 and L2 is more than 1.0 mm, the range of weld metal increases, so that the mechanical properties of the pipe or tube degrade. In view of this, L1 and L2 are preferably 0.0100 mm to 0.5 mm.

(119) The width (pipe or tube circumferential length) L3 of weld metal at the pipe or tube thickness center is preferably 0.0010 mm or more and 0.3 mm or less. If L3 is 0.0010 mm or more, a decrease in the strength of the weld caused by the amount of weld metal being excessively small is prevented. If L3 is 0.3 mm or less, degradation in the mechanical properties of the pipe or tube caused by an increase in the range of weld metal is prevented. In view of this, L3 is more preferably 0.0100 mm to 0.3 mm.

(120) The thickness t.sub.w1 of the first cladding metal in the weld metal (the thickness of the second layer in the welded seam part) is preferably 20% or more and 300% or less of the thickness t.sub.c1 of the first cladding metal in the parts other than the weld metal. If t.sub.w1 is 20% or more of t.sub.c1, the entry of the weld metal 15A of the base metal into the weld metal 15B of the first cladding metal can be suppressed sufficiently, and sufficient properties (e.g. corrosion resistance) of the first cladding metal can be obtained at the inner surface of the pipe or tube. If t.sub.w1 is 300% or less of t.sub.c1, the weld metal 15B of the first cladding metal is not dominant in the weld, and sufficient properties (e.g. strength) of the base metal can be obtained. In view of this, t.sub.w1 is more preferably 50% to 200% of t.sub.c1.

(121) The thickness t.sub.w2 of the second cladding metal in the weld metal (the thickness of the third layer in the welded seam part) is preferably 20% or more and 300% or less of the thickness t.sub.c2 of the second cladding metal in the parts other than the weld metal. If t.sub.w2 is 20% or more of t.sub.c2, the entry of the weld metal 15A of the base metal into the weld metal 15C of the second cladding metal can be suppressed sufficiently, and sufficient properties (e.g. corrosion resistance) of the second cladding metal can be obtained at the outer surface of the pipe or tube. If t.sub.w2 is 300% or less of t.sub.c2, the weld metal 15C of the second cladding metal is not dominant in the weld, and sufficient properties (e.g. strength) of the base metal can be obtained. In view of this, t.sub.w2 is more preferably 50% to 200% of t.sub.c2.

(122) The thickness of the first cladding metal and the thickness of the second cladding metal in the weld metal are each preferably 35% or less of the thickness of the pipe or tube. If the thickness of the first cladding metal and the thickness of the second cladding metal in the weld metal are each 35% or less of the thickness of the pipe or tube, the weld metal 15B of the first cladding metal and the weld metal 15C of the second cladding metal are not dominant in the weld, and sufficient properties (e.g. strength) of the base metal can be obtained.

(123) In this embodiment, the weld metal identification and the weld metal width measurement can be performed by the same methods as in Embodiment 1.

(124) The weld in the clad welded pipe or tube 20 according to this embodiment preferably has high fracture property. Specifically, the flatness value h/D in a 90° flattening test in accordance with JIS G 3445 is preferably less than 0.3, where h is the flattening crack height (mm) and D is the pipe or tube outer diameter (mm).

(125) [Method of Producing Clad Welded Pipe or Tube]

(126) The above-described clad welded pipe or tube 20 according to Embodiment 2 of the present disclosure can be produced by electric resistance welding under specific conditions described below.

(127) In Embodiment 2, the clad welded pipe or tube can be produced using the line illustrated in FIG. 2, as in Embodiment 1. The production process in this case may be the same as that in Embodiment 1.

(128) This embodiment describes an example in which electric resistance welding is performed using the clad steel strip 10 composed of the first layer 11 made of the base metal, the second layer 12A made of the first cladding metal pressure-bonded to the first layer, and the third layer 12B made of the second cladding metal pressure-bonded to the first layer where the second layer 12A is the inner layer and the third layer 12B is the outer layer, as illustrated in FIG. 11A.

(129) (Groove Machining)

(130) In this embodiment, both transverse ends of the clad steel strip are subjected to groove machining to form a groove. This groove machining will be described below, with reference to FIG. 11A.

(131) In the groove machining, both transverse ends of the clad steel strip 10 are indented from the second layer 12A side and the third layer 12B side. The groove formed is an X-groove as illustrated in FIG. 11A, and satisfies the same conditions (i) to (iv) as in Embodiment 1 and further satisfies the following conditions (v) to (viii):

(132) (i) A clad interface 13A between the second layer 12A and the first layer 11 turns from the second layer side toward the thickness center of the clad steel strip.

(133) (ii) The bevel angle θ1 on the second layer side is 10° or more and 50° or less.

(134) (iii) The groove depth d1 is 10.0% or more and 45.0% or less of the thickness t of the clad steel strip.

(135) (iv) The projection clad ratio R1 defined by the following Expression (1) is 25% or more and 50% or less:
R1=(t.sub.c1*+d1)/t×100(%)   (1)

(136) where R1 is the projection clad ratio (second layer side), t.sub.c1* is the thickness (mm) of the second layer at the root surface, d1 is the groove depth (mm) on the second layer side, and t is the thickness (mm) of the clad steel strip.

(137) (v) A clad interface 13B between the third layer 12B and the first layer 11 turns from the third layer side toward the thickness center of the clad steel strip.

(138) (vi) The bevel angle θ2 on the third layer side is 10° or more and 50° or less.

(139) (vii) The groove depth d2 is 10.0% or more and 45.0% or less of the thickness t of the clad steel strip.

(140) (viii) The projection clad ratio R2 defined by the following Expression (2) is 25% or more and 50% or less:
R2=(t.sub.c2* +d2)/t×100(%)   (2)

(141) where R2 is the projection clad ratio (third layer side), t.sub.c2* is the thickness (mm) of the third layer at the root surface, d2 is the groove depth (mm) on the third layer side, and t is the thickness (mm) of the clad steel strip.

(142) It is important that the X-groove is formed by indenting the transverse ends of the clad steel strip 10 from the second layer 12A side and the third layer 12B side, and not by cutting off the second layer-side corners and the third layer-side corners of the transverse ends of the clad steel strip 10. As a result, the clad interface 13A is indented from the second layer side toward the thickness center of the clad steel strip, and the clad interface 13B is indented from the third layer side toward the thickness center of the clad steel strip. In addition, the bevel angles θ1 and θ2, the groove depths d1 and d2, and the projection clad ratios R1 and R2 are set to the foregoing ranges. Thus, the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal or the second cladding metal after electric resistance welding can be suppressed. Consequently, the base metal is kept from being exposed at the inner or outer surface of the steel pipe or tube in the weld. This makes it possible to obtain an electric-resistance-welded clad steel pipe or tube whose inner and outer surfaces are entirely, including the weld, covered with the cladding metal, after cutting the inner surface bead of the weld.

(143) Moreover, since the groove shape has no corners at which current concentrates, the temperature distribution of the whole parts to be welded is uniform. This facilitates discharging of penetrators from the weld, and as a result prevents a decrease in the toughness and strength of the weld.

(144) If the bevel angle θ1 and/or θ2 is less than 10°, the uniformity of the temperature distribution of the whole parts to be welded cannot be maintained, as a result of which discharging of penetrators tends to be insufficient. This causes insufficient properties such as toughness and strength of the weld. Besides, the width L1 and/or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(145) If the bevel angle θ1 and/or θ2 is more than 50°, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal and/or the second cladding metal is insufficient. Consequently, the base metal is exposed at the inner surface and/or the outer surface of the steel pipe or tube in the weld, which impairs the function as a clad pipe or tube. Besides, the width L1 and/or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(146) If the groove depth d1 and/or d2 is less than 10.0% of the thickness t of the clad steel strip, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal and/or the second cladding metal is insufficient. Consequently, the base metal is more likely to be exposed at the inner surface and/or the outer surface of the steel pipe or tube in the weld. Besides, the width L1 and/or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(147) If the groove depth d1 and/or d2 is more than 45.0% of the thickness t of the clad steel strip, the composition of the weld is a high-alloy composition similar to the composition of the cladding metal. This causes insufficient properties such as toughness and strength of the weld. Besides, the width L1 and/or L2 of the molten metal exceeds 1.0 mm, so that the mechanical properties of the pipe or tube degrade.

(148) In terms of achieving both the prevention of the exposure of the base metal at the inner surface and the outer surface and the prevention of a decrease in the properties of the weld at higher level, the bevel angle θ1 is preferably 15° or more, the bevel angle θ1 is preferably 35° or less, the bevel angle θ2 is preferably 15° or more, the bevel angle θ2 is preferably 35° or less, the groove depth d1 is preferably 15% or more of the thickness t of the clad steel strip, the groove depth d1 is preferably 35% or less of the thickness t of the clad steel strip, the groove depth d2 is preferably 15% or more of the thickness t of the clad steel strip, and the groove depth d2 is preferably 35% or less of the thickness t of the clad steel strip.

(149) It is also very important in this embodiment that the projection clad ratio R1 defined by Expression (1) and the projection clad ratio R2 defined by Expression (2) are 25% or more and 50% or less. If the projection clad ratio R1 and/or R2 is less than 25%, the effect of suppressing the entry of the molten steel and the heat-affected zone of the base metal into the welded seam part of the first cladding metal and/or the second cladding metal is insufficient. Consequently, the base metal is exposed at the inner surface and/or the outer surface of the steel pipe or tube in the weld. Moreover, with a low projection clad ratio of less than 25%, the groove shape is close to a rectangle. In such a case, due to the properties of high-frequency current, current tends to concentrate only at corners, facilitating heating. Besides, given that the metal of high-alloy composition such as stainless steel or a Ni alloy as the cladding metal has a lower melting point than the base metal such as low-carbon low-alloy steel, with a low projection clad ratio of less than 25%, the cladding metal of low melting point at the corners melts easily. This causes the width of the weld metal at the inner surface or the outer surface to increase, as a result of which L1 and/or L2 exceeds 1.0 mm.

(150) If the projection clad ratio R1 and/or R2 is more than 50%, the most part or all of the welded seam 14 after electric resistance welding is a welded seam resulting from electric resistance welding the metal of high-alloy composition as the cladding metal, so that the properties of the weld such as toughness and strength decrease. Besides, given that the metal of high-alloy composition such as stainless steel or a Ni alloy as the cladding metal has a lower melting point than the base metal such as low-carbon low-alloy steel, with a high projection clad ratio of more than 50%, the cladding metal of low melting point melts excessively. As a result, the width L1 and/or L2 of the weld metal exceeds 1.0 mm.

(151) In the case of using the electric-resistance-welded clad steel pipe or tube as a line pipe for transporting a corrosive substance, to maintain the corrosion resistance of the inner surface of the clad steel pipe or tube for the long term, the thickness t.sub.w1 of the first cladding metal in the weld metal is preferably 20% or more of the thickness t.sub.c1 of the first cladding metal in the parts other than the weld metal, as mentioned earlier. To achieve this, the projection clad ratio R1 of the X-groove of the butted parts in the electric resistance welding is preferably set to 30% or more. Likewise, the projection clad ratio R2 on the third layer side is preferably set to 30% or more.

(152) The groove machining in this embodiment can be performed using, for example, a groove machine 40 illustrated in FIG. 12. The groove machine 40 is a rolling-type groove machine capable of continuously machining the running clad steel strip 10, and includes a pair of right and left upper side rolls 42 and a pair of right and left lower side rolls 44. As a result of the upper side rolls 42 having a rolling portion 42A that increases in diameter upward in reverse taper shape and the lower side rolls 44 having a rolling portion 44A that increases in diameter downward in reverse taper shape as illustrated in FIG. 12, the X-groove can be formed at both transverse ends of the clad steel strip 10.

(153) By changing the reverse taper shape of each of the rolling portions 42A and 44A, the groove shape of the transverse ends of the clad steel strip can be adjusted as desired. As illustrated in FIG. 11A, the projection clad ratio R1 depends on the ratio of the thickness t.sub.m of the first layer (base metal) and the thickness t.sub.c1 of the second layer (first cladding metal) in the clad steel strip 10 and the groove shape by the indentation, and the projection clad ratio R2 depends on the ratio of the thickness t.sub.m of the first layer (base metal) and the thickness t.sub.c2 of the third layer (second cladding metal) in the clad steel strip 10 and the groove shape by the indentation. Accordingly, each of the projection clad ratios R1 and R2 can be set to a desired value by selecting the ratio appropriately and also adjusting the reverse taper shape of the rolling portion 42A or 44A to form an appropriate groove shape.

(154) (Gas Shielding)

(155) Next, as illustrated in FIGS. 13A and 13C, the clad steel strip 10 is formed into a pipe or tube shape, to obtain an open pipe or tube 16 that is a cylindrical strip before welding. A pair of butted parts (parts to be welded) 17 of the open pipe or tube facing each other are, while being subjected to gas shielding, butt pressed and electric resistance welded, to obtain the clad welded pipe or tube 20.

(156) The gas shielding can be performed by the same method as in Embodiment 1 (see FIGS. 13A to 14). The shielding gas conditions may be the same conditions as in Embodiment 1. The same results as those illustrated in FIGS. 8 and 9 in Embodiment 1 were obtained in an experiment using a three-layer clad steel pipe or tube.

(157) (Amount of Upset)

(158) In this embodiment, the amount of upset is limited to not greater than the thickness t of the clad steel strip. This prevents the base metal in the weld from being exposed at the inner or outer surface of the steel pipe or tube. The amount of upset is preferably 20% or more of the thickness of the clad steel strip, in terms of ensuring the effect of discharging penetrators from the weld during the electric resistance welding. In addition, by limiting the amount of upset to 20% or more of each of the thicknesses t.sub.c1 and t.sub.c2 of the cladding metal of the clad steel strip, the pipe or tube circumferential length of the weld metal at the pipe or tube thickness center can be kept at 0.0010 mm or more and 0.3 mm or less.

Embodiment 3

(159) An example in which a clad welded pipe or tube has a two-layer structure will be described next, as a more specific embodiment of Embodiment 1. In Embodiment 3, the description of the same matters as those in Embodiment 1 is omitted as appropriate.

(160) [Clad Welded Pipe or Tube]

(161) A clad welded pipe or tube 20 according to Embodiment 3 of the present disclosure will be described below, with reference to FIG. 1. The clad welded steel pipe or tube 20 according to this embodiment is a two-layer clad welded pipe or tube composed of a first layer 11 made of base metal and a second layer 12 made of first cladding metal.

(162) In this embodiment, carbon steel or low-alloy steel which is a material with excellent mechanical strength is used as the base metal, and stainless steel or a nickel-containing alloy which is a material with excellent corrosion resistance is used as the cladding metal. Combining two types of materials in this way makes it possible to obtain a steel pipe or tube having both strength and corrosion resistance. Moreover, the production cost can be reduced as compared with the case where the whole steel pipe or tube is made of stainless steel or a nickel-containing alloy.

(163) (Base Metal)

(164) In this embodiment, carbon steel or low-alloy steel is used as the base metal. Herein, “low-carbon steel” denotes a carbon steel whose C content is 0.25 mass % or less.

(165) The chemical composition of low-carbon steel and low-alloy steel suitable as the base metal will be described in more detail below. Herein, “%” denotes “mass %” unless otherwise noted.

(166) In one of the disclosed embodiments, the base metal may be carbon steel or low-alloy steel having a chemical composition containing

(167) C: 0.02% to 0.20%,

(168) Si: 0.01% to 1.0%,

(169) Mn: 0.1% to 2.0%,

(170) P: 0.05% or less,

(171) S: 0.01% or less, and

(172) Al: 0.1% or less,

(173) with the balance being Fe and inevitable impurities.

(174) The reasons for limiting the content of each element will be explained below.

(175) C: 0.02% to 0.20%

(176) C is a necessary element to improve the strength of the steel. If the C content is less than 0.02%, it is difficult to achieve strength required of the base metal. The C content is therefore 0.02% or more. The C content is preferably 0.03% or more. If the C content is more than 0.20%, martensite tends to form during heat treatment such as welding, making the material brittle. The C content is therefore 0.20% or less. The C content is preferably 0.15% or less.

(177) Si: 0.01% to 1.0%

(178) Si is an element used for deoxidation and strength adjustment of the steel. If the Si content is less than 0.01%, the effect is insufficient. The Si content is therefore 0.01% or more. If the Si content is more than 1.0%, the material hardens, and the toughness decreases. The Si content is therefore 1.0% or less. The Si content is preferably 0.8% or less.

(179) Mn: 0.1% to 2.0%

(180) Mn is a useful element to adjust the strength of the steel. If the Mn content is less than 0.1%, the effect is insufficient. The Mn content is therefore 0.1% or more. If the Mn content is more than 2.0%, problems such as a decrease in weldability and an increase of inclusions arise. The Mn content is therefore 2.0% or less. The Mn content is preferably 1.8% or less.

(181) P: 0.05% or Less

(182) P is an element that exists in the steel as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The P content is desirably as low as possible, but 0.05% or less P is allowable. The P content is therefore 0.05% or less. The P content is preferably 0.03% or less. Excessively reducing the P content leads to excessively high refining cost while the effect lessens. Accordingly, the P content is preferably 0.001% or more.

(183) S: 0.01% or Less

(184) S is an element that exists in the steel as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The S content is desirably as low as possible, but 0.01% or less S is allowable. The S content is therefore 0.01% or less. The S content is preferably 0.005% or less. Excessively reducing the S content leads to excessively high refining cost while the effect lessens. Accordingly, the S content is preferably 0.001% or more.

(185) Al: 0.1% or Less

(186) Al is an element used for deoxidation of the steel. If the Al content is more than 0.1%, a large amount of Al oxide forms, and decreases the cleanliness of the steel. The Al content is therefore limited to 0.1% or less. Although no lower limit is placed on the Al content, if the Al content is less than 0.001%, the deoxidation effect is unlikely to be achieved, and thus the Al content is preferably 0.001% or more.

(187) In one of the disclosed embodiments, the chemical composition of the base metal may further contain one or more selected from the group consisting of

(188) Ti: 0.1% or less,

(189) Nb: 0.2% or less,

(190) Cu: 0.5% or less,

(191) Ni: 0.5% or less,

(192) Cr: 0.5% or less,

(193) Mo: 0.5% or less,

(194) V: 0.1% or less, and

(195) Ca: 0.0005% to 0.0050%.

(196) Ti: 0.1% or less, Nb: 0.2% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less

(197) Ti, Nb, Cr, Mo, and V each have an effect of improving the strength of the steel by forming carbide or dissolving in the steel. If the contents of the respective elements are more than 0.1%, 0.2%, 0.5%, 0.5%, and 0.1%, the toughness decreases. Accordingly, the contents of the respective elements are limited to 0.1% or less, 0.2% or less, 0.5% or less, 0.5% or less, and 0.1% or less. If the content of each element is less than 0.01%, the effect is unlikely to be achieved. Accordingly, in the case of containing any of these elements, the content of each element is preferably 0.01% or more.

(198) Cu: 0.5% or less, Ni: 0.5% or Less

(199) Cu and Ni each have an effect of improving the strength of the steel through quench hardenability improvement. If the content of each element is more than 0.5%, the effect is saturated, and an unnecessary cost increase ensues. The content of each element is therefore limited to 0.5% or less. Although no lower limit is placed on the content of each element, if the content of each element is less than 0.01%, the effect is unlikely to be achieved. Accordingly, in the case of containing any of these elements, the content of each element is preferably 0.01% or more.

(200) Ca: 0.0005% to 0.0050%

(201) Ca is an element that contributes to inclusion morphological control of transforming elongated coarse sulfide into spherical sulfide. If the Ca content is less than 0.0005%, the effect is insufficient. The Ca content is therefore 0.0005% or more. The Ca content is preferably 0.0010% or more. If the Ca content is more than 0.0050%, the cleanliness of the steel decreases. The Ca content is therefore 0.0050% or less.

(202) (Cladding Metal)

(203) In this embodiment, stainless steel or a nickel-containing alloy is used as the cladding metal.

(204) The stainless steel is not limited, and may be any stainless steel. In terms of corrosion resistance, austenitic stainless steel is preferable, and SUS316L is more preferable.

(205) The nickel-containing alloy may be any alloy containing Ni. Preferable examples of the nickel-containing alloy include Alloy625 and Alloy825.

(206) The chemical composition of stainless steel or a nickel-containing alloy suitable as the cladding metal will be described in more detail below.

(207) In one of the disclosed embodiments, the cladding metal may be stainless steel having a chemical composition containing

(208) C: 0.15% or less,

(209) Si: 5.0% or less,

(210) Mn: 2.0% or less,

(211) P: 0.1% or less,

(212) S: 0.1% or less,

(213) Ni: 1.0% or less,

(214) Cr: 11.0% or more, and

(215) N: 0.5% or less,

(216) with the balance being Fe and inevitable impurities.

(217) C: 0.15% or Less

(218) C has an effect of improving the strength of the stainless steel. If the C content is more than 0.15%, martensite tends to form during heat treatment such as welding, making the material brittle. The C content is therefore 0.15% or less. The C content is preferably 0.1% or less. Excessively reducing the C content leads to excessively high refining cost while the effect lessens. Accordingly, the C content is preferably 0.001% or more.

(219) Si: 5.0% or Less

(220) Si is an element that contributes to improved corrosion resistance of the stainless steel. If the Si content is more than 5.0%, however, the material hardens, and the toughness decreases. The Si content is therefore 5.0% or less. The Si content is preferably 3.0% or less. Si is an element which inevitably mixes in from the raw material and is hard to be removed. The Si content is therefore preferably 0.01% or more.

(221) Mn: 2.0% or Less

(222) Mn is a useful element to adjust the strength of the steel. If the Mn content is more than 2.0%, however, problems such as a decrease in weldability and an increase of inclusions arise. The Mn content is therefore 2.0% or less. The Mn content is preferably 1.8% or less. Although no lower limit is placed on the Mn content, the Mn content is preferably 0.1% or more because Mn has an effect of suppressing grain boundary segregation of S by combining with S inevitably mixed in.

(223) P: 0.1% or Less

(224) P is an element that exists in the stainless steel as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The P content is desirably as low as possible, but 0.1% or less P is allowable. The P content is therefore 0.1% or less. The P content is preferably 0.05% or less. Excessively reducing the P content leads to excessively high refining cost while the effect lessens. Accordingly, the P content is preferably 0.001% or more.

(225) S: 0.1% or Less

(226) S is an element that exists in the stainless steel as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The S content is desirably as low as possible, but 0.1% or less S is allowable. The S content is therefore 0.1% or less. The S content is preferably 0.05% or less. Excessively reducing the S content leads to excessively high refining cost while the effect lessens. Accordingly, the S content is preferably 0.0001% or more.

(227) Ni: 1.0% or Less

(228) Ni is an element that contributes to improved corrosion resistance of the stainless steel. However, because Ni is an austenite forming element, the Ni content is limited to 1.0% or less in ferritic stainless steel, in order to control the structure to ferrite single phase. Although no lower limit is placed on the Ni content, the Ni content is preferably 0.01% or more in terms of corrosion resistance improvement.

(229) Cr: 11.0% or More

(230) Cr is an important element that forms a passive film on the surface of the stainless steel to maintain corrosion resistance. If the Cr content is less than 11.0%, the effect is insufficient. The Cr content is therefore 11.0% or more. The Cr content is preferably 13.0% or more. Although no upper limit is placed on the Cr content, if the Cr content is more than 35.0%, hot workability tends to decrease, and thus the Cr content is preferably 35.0% or less.

(231) N: 0.5% or Less

(232) N is an element that contributes to improved corrosion resistance of the stainless steel. If the N content is more than 0.5%, however, the effect is saturated. The N content is therefore 0.5% or less. Although no lower limit is placed on the N content, the N content is preferably 0.001% or more to prevent an excessive increase in refining cost.

(233) In another one of the disclosed embodiments, the cladding metal may be stainless steel or a nickel-containing alloy having a chemical composition containing

(234) C: 0.15% or less,

(235) Si: 5.0% or less,

(236) Mn: 2.0% or less,

(237) P: 0.1% or less,

(238) S: 0.1% or less,

(239) Ni: 6.0% or more,

(240) Cr: 15.0% or more, and

(241) N: 0.5% or less,

(242) with the balance being Fe and inevitable impurities.

(243) C: 0.15% or Less

(244) C has an effect of improving the strength of the stainless steel or nickel-containing alloy. If the C content is more than 0.15%, martensite tends to form during heat treatment such as welding, making the material brittle. The C content is therefore 0.15% or less. The C content is preferably 0.1% or less. Although no lower limit is placed on the C content, the C content is preferably 0.001% or more to prevent an excessive increase in refining cost.

(245) Si: 5.0% or Less

(246) Si is an element that contributes to improved corrosion resistance of the stainless steel or nickel-containing alloy. If the Si content is more than 5.0%, the material hardens, and the toughness decreases. The Si content is therefore 5.0% or less. The Si content is preferably 3.0% or less. Si is an element which inevitably mixes in from the raw material and is hard to be removed. The Si content is therefore preferably 0.01% or more.

(247) Mn: 2.0% or Less

(248) Mn is an austenite forming element, and may be optionally added to control the phase stability of the stainless steel or nickel-containing alloy. If the Mn content is more than 2.0%, however, problems such as a decrease in weldability and an increase of inclusions arise. The Mn content is therefore 2.0% or less. Although no lower limit is placed on the Mn content, the Mn content is preferably 0.1% or more because Mn has an effect of suppressing grain boundary segregation of S by combining with S inevitably mixed in.

(249) P: 0.1% or Less

(250) P is an element that exists in the stainless steel or nickel-containing alloy as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The P content is desirably as low as possible, but 0.1% or less P is allowable. The P content is therefore 0.1% or less. The P content is preferably 0.05% or less. Excessively reducing the P content leads to excessively high refining cost while the effect lessens. Accordingly, the P content is preferably 0.001% or more.

(251) S: 0.1% or Less

(252) S is an element that exists in the stainless steel or nickel-containing alloy as an impurity and segregates to crystal grain boundaries or the like to cause a decrease in properties such as toughness. The S content is desirably as low as possible, but 0.1% or less S is allowable. The S content is therefore 0.1% or less. The S content is preferably 0.05% or less. Excessively reducing the S content leads to excessively high refining cost while the effect lessens. Accordingly, the S content is preferably 0.0001% or more.

(253) Ni: 6.0% or More

(254) Ni is an element that contributes to improved corrosion resistance of the stainless steel or nickel-containing alloy. Moreover, since Ni is an austenite forming element, Ni is used to control the phase stability of the stainless steel or nickel-containing alloy based on the balance with a ferrite forming element such as Cr. For this purpose, in the case where the Cr content is 15.0% or more, the Ni content is limited to 6.0% or more. Although no upper limit is placed on the Ni content, the Ni content is preferably 80% or less to prevent an increase in cost.

(255) Cr: 15.0% or More

(256) Cr is an important element that forms a passive film on the surface of the stainless steel or nickel-containing alloy to maintain corrosion resistance. Moreover, since Cr is a ferrite forming element, Cr is used to control the phase stability of the stainless steel or nickel-containing alloy based on the balance with an austenite forming element such as Ni. For this purpose, in the case where the Ni content is 6.0% or more, the Cr content is limited to 15.0% or more. Although no upper limit is placed on the Cr content, the Cr content is preferably 80% or less to prevent an increase in cost.

(257) N: 0.5% or Less

(258) N is an element that contributes to improved corrosion resistance of the stainless steel. If the N content is more than 0.5%, however, the effect is saturated. The N content is therefore limited to 0.5% or less. Although no lower limit is placed on the N content, the N content is preferably 0.001% or more to prevent an excessive increase in refining cost.

(259) In one of the disclosed embodiments, the chemical composition of the cladding metal may further contain one or more selected from the group consisting of

(260) Mo: 20.0% or less,

(261) Cu: 5.0% or less,

(262) Al: 2.0% or less,

(263) Co: 3.0% or less,

(264) W: 5.0% or less, and

(265) Ta: 5.0% or less.

(266) Mo: 20.0% or Less, Cu: 5.0% or Less, Al: 2.0% or Less, Co: 3.0% or Less, W: 5.0% or Less, Ta: 5.0% or Less

(267) Mo, Cu, Al, Co, W, and Ta may be added to improve the corrosion resistance or strength of the stainless steel or nickel alloy. If the contents of the respective elements are more than 20.0%, 5.0%, 2.0%, 3.0%, 5.0%, and 5.0%, however, the effect is saturated. The contents of the respective elements are therefore limited to 20.0% or less, 5.0% or less, 2.0% or less, 3.0% or less, 5.0% or less, and 5.0% or less. To achieve the effect, the content of each of these elements is preferably 0.005% or more.

(268) In one of the disclosed embodiments, the chemical composition of the cladding metal may further contain one or more selected from the group consisting of

(269) Ti: 2.0% or less,

(270) Nb: 5.0% or less,

(271) V: 2.0% or less, and

(272) Zr: 2.0% or less.

(273) Ti: 2.0% or Less, Nb: 5.0% or Less, V: 2.0% or Less, Zr: 2.0% or Less

(274) Ti, Nb, V, and Zr each have an effect of suppressing sensitization by fixing C, and thus may be optionally added. However, in a range of C: 0.15% or less, if the contents of the respective elements are more than 2.0%, 5.0%, 2.0%, and 2.0%, the effect is saturated. The contents of the respective elements are therefore limited to 2.0% or less, 5.0% or less, 2.0% or less, and 2.0% or less. To achieve the effect, the content of each of these elements is preferably 0.001% or more.

(275) In one of the disclosed embodiments, the chemical composition of the cladding metal may further contain one or more selected from the group consisting of

(276) B: 0.0050% or less,

(277) Ca: 0.0050% or less,

(278) Mg: 0.0030% or less, and

(279) REM: 0.10% or less.

(280) B is an element that contributes to improved properties such as toughness through grain boundary strengthening, and Ca, Mg, and REM (rare-earth metal) are each an element that contributes to improved properties such as toughness through inclusion morphological control. If the contents of the respective elements are more than 0.0050%, 0.0050%, 0.0030%, and 0.10%, however, the effect is saturated, or the cleanliness decreases. The contents of the respective elements are therefore limited to 0.0050% or less, 0.0050% or less, 0.0030% or less, and 0.10% or less. To achieve the effect, the content of each of these elements is preferably 0.0001% or more.

(281) (Exposure of Base Metal)

(282) In the clad welded pipe or tube according to this embodiment, the base metal is not exposed at the first cladding metal-side surface in the weld. If the base metal is exposed to the cladding metal side as illustrated in FIG. 15A, the properties such as corrosion resistance in the exposed part decrease. This makes it impossible to achieve performance originally expected as a clad steel pipe or tube. In this embodiment, the base metal is not exposed at the first cladding metal-side surface, so that the function as a clad welded pipe or tube is not impaired.

(283) (Width of Weld Metal)

(284) In the clad welded pipe or tube according to this embodiment, the width of the weld metal in the weld is 1.0 μm or more and 1000 μm or less over the whole thickness of the clad steel pipe or tube. As illustrated in FIG. 1, the weld metal 15A and the weld metal 15B formed by welding are present in the weld of the clad welded pipe or tube 20. The weld metal has degraded properties as compared with the steel sheet before the welding, due to melting and solidification during the welding. Hence, by limiting the width W of the weld metal to 1.0 μm or more and 1000 μm or less over the whole thickness, the property degradation caused by the welding is reduced, with it being possible to obtain a clad welded pipe or tube having excellent strength.

(285) Herein, the “width of the weld metal” denotes the width W of the weld metal in a direction perpendicular to the pipe or tube radial direction, in a section perpendicular to the longitudinal direction of the clad welded pipe or tube 20. Note that FIG. 1 is a schematic diagram for describing this embodiment, and is not based on the actual dimensional ratios.

(286) In this embodiment, the weld metal identification can be performed by the same method as in Embodiment 1. The length of the line segment perpendicular to the center line 14 in the pipe or tube circumferential direction of the identified weld metal in the section is taken to be the width of the weld metal.

(287) [Method of Producing Clad Welded Pipe or Tube]

(288) The above-described clad welded pipe or tube 20 according to Embodiment 3 of the present disclosure can be suitably produced by electric resistance welding under specific conditions described below.

(289) In Embodiment 3, the clad welded pipe or tube can be produced using the line illustrated in FIG. 2, as in Embodiment 1. The production process in this case may be the same as that in Embodiment 1.

(290) (Groove Machining, Gas Shielding, and Amount of Upset)

(291) The groove machining conditions, the gas shielding conditions, and the amount of upset are as described in Embodiment 1.

(292) (Post-Welding Heat Treatment)

(293) In this embodiment, after the electric resistance welding, post-welding heat treatment of heat-treating and then cooling the clad welded pipe or tube may be performed. The post-welding heat treatment is not limited, and may be performed by any method. It is, however, preferable to perform the post-welding heat treatment under the following conditions:

(294) heating temperature at cladding metal-side surface of clad welded pipe or tube: 750° C. to 1250° C.

(295) holding time at heating temperature at cladding metal-side surface of clad welded pipe or tube: 10 sec or more

(296) heating temperature at base metal-side surface of clad welded pipe or tube: 750° C. to 1200° C.

(297) holding time at heating temperature at base metal-side surface of clad welded pipe or tube: 10 sec or more

(298) average cooling rate from 750° C. to 400° C. at cladding metal-side surface of clad welded pipe or tube: 4° C./sec to 100° C./sec

(299) average cooling rate from 750° C. to 400° C. at base metal-side surface of clad welded pipe or tube: 8° C./sec to 70° C./sec.

(300) Conditions on Cladding Metal Side

(301) If the heating temperature of the cladding metal is less than 750° C., the corrosion resistance decreases due to sensitization. If the heating temperature of the cladding metal is more than 1250° C., crystal grains coarsen, which facilitates sensitization in the subsequent cooling process. If the holding time is less than 10 sec, sensitization in the heating process is not resolved. If the average cooling rate from 750° C. to 400° C. is less than 4° C./sec, sensitization occurs in the cooling process, and the corrosion resistance decreases. If the average cooling rate is more than 100° C./sec, strain caused by rapid cooling increases stress corrosion crack sensitivity.

(302) Conditions on Base Metal Side

(303) If the heating temperature of the base metal is less than 750° C. or if the holding time is less than 10 sec, good weld microstructure is not obtained, and the workability and the toughness are insufficient. If the heating temperature of the base metal is more than 1200° C., crystal grains coarsen, and the toughness decreases. If the average cooling rate from 750° C. to 400° C. is less than 8° C./sec, ferrite-pearlite formation causes insufficient strength. If the average cooling rate is more than 70° C./sec, martensite formation causes insufficient toughness.

(304) By performing heat treatment and cooling under the conditions described above, the base metal microstructure in the vicinity of the weld can be formed as microstructure mainly composed of ferrite and bainite (total area ratio of 90% or more), and the average grain size of ferrite and bainite can be limited to 15 μm or less. This further improves the toughness of the weld.

EXAMPLES

Example 1

(305) Electric-resistance-welded two-layer clad steel pipes or tubes were produced by the following procedure. Each two-layer clad hot-rolled steel strip composed of base metal of low-carbon low-alloy steel having a chemical composition containing, in mass %, C: 0.05%, Si: 0.2%, and Mn: 1.0% with the balance being Fe and inevitable impurities and having the thickness shown in Table 1 and cladding metal of SUS316L stainless steel or a nickel-containing alloy (Alloy625) having the thickness shown in Table 1 was prepared.

(306) A clad welded pipe or tube of 600 mm in outer diameter was produced under various conditions by the electric-resistance-welded steel pipe or tube production line illustrated in FIG. 2, using the prepared clad steel strip as raw material with the base metal forming the outer layer and the cladding metal forming the inner layer. Here, both transverse ends of the clad steel strip were subjected to groove machining by the rolling-type groove machine 40, to form a Y-groove of the shape shown in Table 1 at the transverse ends.

(307) The amount of upset in the electric resistance welding is shown in Table 1. During the electric resistance welding, the parts to be welded were shielded with shielding gas using the nozzle illustrated in FIGS. 5A to 5C under the conditions of nozzle height H, gas release flow rate B, and flow rate ratio B/A shown in Table 1. The shielding gas was argon gas.

(308) In each level, the oxygen concentration around the parts to be welded was measured. Moreover, a test piece was collected from each produced clad welded pipe or tube, and a 90° flattening test in accordance with JIS G 3445 was performed to determine the flatness value h/D. Further, the dimensions of the weld metal were measured by the above-mentioned method. The results are shown in Table 1.

(309) In addition, a V-notch test piece was collected so that the longitudinal direction of the test piece was the circumferential direction of the steel pipe or tube, the notch position was the weld, and the notch direction was the longitudinal direction of the steel pipe or tube, and a Charpy impact test in accordance with ASTM A370 was performed to determine the fracture appearance transition temperature vTrs (° C.). The toughness of the weld was evaluated using the obtained value of vTrs, based on the following criteria. The evaluation results are shown in Table 1.

(310) Good: vTrs: −30° C. or less

(311) Poor: vTrs: more than −30° C.

(312) TABLE-US-00001 TABLE 1 Groove machining conditions Cladding Cladding Base metal metal metal Projection thickness thickness Whole Groove depth thickness t.sub.c1* clad ratio Amount of t.sub.m t.sub.c1 thickness t Bevel angle d1 d1/t at root surface R1 upset Level (mm) (mm) (mm) θ1 (°) (mm) (%) (mm) (%) (mm) 1 16 4 20 20 5.0 25 3 40 10 2 16 4 20 40 5.0 25 3 40 10 3 16 4 20 30 3.0 15 3.4 32 10 4 16 4 20 20 5.0 25 3 40 20 5 16 4 20 20 5.0 25 3 40 10 6 16 4 20 20 5.0 25 3 40 10 7 16 4 20 20 5.0 25 3 40 10 8 16 4 20  8 5.0 25 3 40 10 9 16 4 20 60 5.0 25 3 40 10 10 16 4 20 20 5.0 25 3 40   0.5 11 16 4 20 20 8.0 40 2.4 52 10 12 19 1 20 20 4.0 20 0.8 24 10 13 19 1 20 20 9.2 46 0.54 49 10 14 11.3 8.7 20 20 1.8   9.0 7.9 49 10 15 16 4 20 20 5.0 25 3 40 22 16 16 4 20 20 5.0 25 3 40 10 17 16 4 20 20 7.0 35 2.6 48 10 18 16 4 20 20 7.0 35 2.6 48 20 19 16 4 20 20 7.0 35 2.6 48 10 20 16 4 20 20 7.0 35 2.6 48 20 21 16 4 20 20 5.0 25 3 40 10 22 16 4 20 20 5.0 25 3 40 10 23 16 4 20 45 5.0 25 3 40  8 24 16 4 20 20 6.4 32 2.72   45.6 10 25 16 4 20 20 2.4 12 3.52   29.6 10 26 16 4 20 20 5.0 25 3 40 10 27 16 4 20 20 5.0 25 3 40 10 28 16 4 20  8 5.0 25 3 40 10 29 16 4 20 60 5.0 25 3 40 20 30 16 4 20 20 8.0 40 2.4 52 10 31 19 1 20 20 9.2 46 0.54 49 10 32 16 4 20 20 7.0 35 2.6 48 10 33 16 4 20 20 7.0 35 2.6 48 10 34 16 4 20 20 5.0 25 3 40 10 Oxygen Weld metal dimensions Gas shielding conditions concentration Inner surface Outer surface Thickness Nozzle Gas release Gas flow around parts width width center width height flow rate B rate ratio to be welded L1 L2 L3 Level H (mm) (m/s) B/A (mass % × 10.sup.2) (mm) (mm) (mm) 1 100 10 0.5 0.03 0.17 0.25 0.07 2 100 10 0.5 0.04 0.08 0.19 0.06 3 100 10 0.5 0.11 0.16 0.20 0.12 4 100 10  0.05 0.13 0.07 0.16 0.04 5  8 10 0.5 0.01 0.02 0.09 0.009 6 280 10 5   0.86 0.15 0.15 0.11 7 100 45 0.5 0.37 0.27 0.28 0.09 8 100 10 0.5 0.02 1.22 0.33 1.11 9 100 10 0.5 0.06 1.23 0.23 1.04 10 100 10 0.5 0.06 0.45 0.51 0.74 11 100 10 0.5 0.05 1.34 0.15 0.51 12 100 10 0.5 0.05 1.18 0.16 0.11 13 100 10 0.5 0.05 1.52 0.44 0.69 14 100 10 0.5 0.04 1.33 0.43 0.61 15 100 10 0.5 0.03  0.0008  0.0008 0.0008 16  3 10 0.5 0.07  0.0009  0.001 0.0008 17 350 10 0.5 3 1.18 1.19 0.32 18 100   0.2 0.5 40  0.0008  0.0008 0.0007 19 100 60 0.5 6  0.0008  0.0008 0.0007 20 100 10  0.005 25 0.11  0.0008 0.05 21 100 40 12   10  0.0008  0.0009 0.0008 22 100 10 0.5 0.02 0.21 0.23 0.10 23 100 10 0.5 0.03 0.36 0.44 0.28 24 100 10 0.5 0.08 0.14 0.22 0.07 25 100 10 8   0.25 0.17 0.19 0.08 26  10  1 0.5 0.19 0.08 0.15 0.05 27 200 10 0.5 0.29 0.19 0.25 0.12 28 100 10 0.5 0.04 1.11 0.31 1.13 29 100 10 0.5 0.04 1.21 0.23 1.11 30 100 10 0.5 0.03 1.51 0.18 0.71 31 100 10 0.5 0.03 1.61 0.44 0.45 32  3 10 0.5 21  0.0009  0.0008 0.0007 33 100 60 0.5 18  0.0008  0.0008 0.0006 34 100 10 12   10  0.0009  0.0009 0.0008 Weld metal dimensions Cladding metal thickness ratio t.sub.w1 t.sub.w1/t.sub.c1 Flatness value Level (mm) (%) of weld h/D Toughness Cladding metal Category  1 2.9 72.5 0.22 Good SUS316 Example  2 5.5 137.5  0.26 Good SUS316 Example  3 2.1 52.5 0.20 Good SUS316 Example  4 1.5 37.5 0.19 Good SUS316 Example  5 3.4 85   0.27 Good SUS316 Example  6 4.2 105   0.25 Good SUS316 Example  7 6.6 165   0.28 Good SUS316 Example  8 12.1  302.5  0.44 Poor SUS316 Comparative Example  9 0   0  0.59 Poor SUS316 Comparative Example 10 1.1 27.5 0.75 Good SUS316 Example 11 13   325   0.52 Poor SUS316 Comparative Example 12 0   0  0.49 Poor SUS316 Comparative Example 13 3.7 370   0.49 Poor SUS316 Comparative Example 14 3.3 37.9 0.55 Poor SUS316 Comparative Example 15 0   0  0.33 Poor SUS316 Comparative Example 16 4.2 105   0.34 Poor SUS316 Comparative Example 17 1.6 40   0.77 Poor SUS316 Comparative Example 18 1.6 40   0.75 Poor SUS316 Comparative Example 19 1.7 42.5 0.64 Poor SUS316 Comparative Example 20 1.7 42.5 0.60 Poor SUS316 Comparative Example 21 3.1 77.5 0.58 Poor SUS316 Comparative Example 22 4.4 110   0.25 Good Alloy625 Example 23 2.8 70   0.29 Good Alloy625 Example 24 8.1 202.5  0.26 Good Alloy625 Example 25 1.5 37.5 0.28 Good Alloy625 Example 26 3.5 87.5 0.21 Good Alloy625 Example 27 4   100   0.23 Good Alloy625 Example 28 14   350   0.41 Poor Alloy625 Comparative Example 29 0   0  0.59 Poor Alloy625 Comparative Example 30 12.8  320   0.61 Poor Alloy625 Comparative Example 31 3.8 380   0.82 Poor Alloy625 Comparative Example 32 1.3 32.5 0.81 Poor Alloy625 Comparative Example 33 1.4 35   0.74 Poor Alloy625 Comparative Example 34 3.3 82.5 0.59 Poor Alloy625 Comparative Example Underline indicates outside the range according to present disclosure.

(313) Furthermore, whether or not the base metal was exposed in the weld was examined. Specifically, a section perpendicular to the pipe or tube longitudinal direction of the obtained clad welded pipe or tube was etched with a nital solution. In the case where the cladding metal-side surface was continuously covered by a region not etched at all (i.e. the cladding metal), it was determined that the base metal was not exposed. Otherwise, i.e. in the case where an etched region was observed at the cladding metal-side surface, it was determined that the base metal was exposed. As a result, in each clad welded pipe or tube obtained by a production method satisfying the conditions according to the present disclosure, no exposure of the base metal at the cladding metal-side surface in the weld was found. In level 12 with a low projection clad ratio, level 15 with a large amount of upset, and levels 9 and 29 with a large bevel angle, on the other hand, exposure of the base metal was found.

(314) These results indicate that, in each clad welded pipe or tube satisfying the conditions according to the present disclosure, mechanical properties such as toughness and flatness value were excellent, the base metal was not exposed, and the function as a clad pipe or tube was not impaired.

Example 2

(315) Electric-resistance-welded three-layer clad steel pipes or tubes were produced by the following procedure. Each three-layer clad hot-rolled steel strip composed of base metal of low-carbon low-alloy steel having a chemical composition containing, in mass %, C: 0.05%, Si: 0.2%, and Mn: 1.0% with the balance being Fe and inevitable impurities and having the thickness shown in Table 2, first cladding metal of SUS316L stainless steel or a nickel-containing alloy (Alloy625) having the thickness shown in Table 2, and second cladding metal of SUS316L stainless steel or a nickel-containing alloy (Alloy625) having the thickness shown in Table 2 was prepared.

(316) A clad welded pipe or tube of 600 mm in outer diameter was produced under various conditions by the electric-resistance-welded steel pipe or tube production line illustrated in FIG. 2, using the prepared clad steel strip as raw material with the base metal forming the middle layer, the first cladding metal forming the inner layer, and the second cladding metal forming the outer layer. Here, both transverse ends of the clad steel strip were subjected to groove machining by the rolling-type groove machine 40, to form an X-groove of the shape shown in Table 2 at the transverse ends.

(317) The amount of upset in the electric resistance welding is shown in Table 2. During the electric resistance welding, the parts to be welded were shielded with shielding gas using the nozzle illustrated in FIGS. 5A to 5C under the conditions of nozzle height H, gas release flow rate B, and flow rate ratio B/A shown in Table 2. The shielding gas was argon gas.

(318) In each level, the oxygen concentration around the parts to be welded was measured. Moreover, a test piece was collected from each produced clad welded pipe or tube, and a 90° flattening test in accordance with JIS G 3445 was performed to determine the flatness value h/D. Further, the dimensions of the weld metal were measured by the above-mentioned method. The results are shown in Table 3.

(319) In addition, a V-notch test piece was collected so that the longitudinal direction of the test piece was the circumferential direction of the steel pipe or tube, the notch position was the weld, and the notch direction was the longitudinal direction of the steel pipe or tube, and a Charpy impact test in accordance with ASTM A370 was performed to determine the fracture appearance transition temperature vTrs (° C.). The toughness of the weld was evaluated using the obtained value of vTrs, based on the following criteria. The evaluation results are shown in Table 3.

(320) Good: vTrs: −30° C. or less

(321) Poor: vTrs: more than −30° C.

(322) TABLE-US-00002 TABLE 2 Groove machining conditions (first cladding metal side) Cladding metal Projection Base metal First cladding Second cladding Whole Groove depth thickness t.sub.c1* clad ratio thickness metal thickness metal thickness thickness Bevel angle d1 d1/t at root surface R1 Level t.sub.m (mm) t.sub.c1 (mm) t.sub.c2 (mm) t (mm) θ1 (°) (mm) (%) (mm) (%) 1 16 4 4 24 20 5.0 20.8 3.2 34.0 2 16 4 4 24 40 5.0 20.8 3.2 34.0 3 16 4 4 24 30 3.0 12.5 3.5 27.1 4 16 4 4 24 20 5.0 20.8 3.2 34.0 5 16 4 4 24 20 5.0 20.8 3.2 34.0 6 16 4 4 24 20 5.0 20.8 3.2 34.0 7 16 4 4 24 20 5.0 20.8 3.2 34.0 8 18 3 3 24  8 5.0 20.8 3.2 34.0 9 16 4 4 24 60 5.0 20.8 3.2 34.0 10 16 4 4 24 20 5.0 20.8 3.2 34.0 11 18 3 3 24 20 10.0 41.7 2.3 51.4 12 20 2 2 24 20 4.2 17.5 1.7 24.4 13 22 1 1 24 20 10.9 45.4 0.5 47.7 14 3.8 10.1 10.1 24 20 2.3  9.6 9.1 47.6 15 16 4 4 24 20 5.0 20.8 3.2 34.0 16 16 4 4 24 20 5.0 20.8 3.2 34.0 17 16 4 4 24 20 7.0 29.2 2.8 41.0 18 16 4 4 24 20 7.0 29.2 2.8 41.0 19 16 4 4 24 20 7.0 29.2 2.8 41.0 20 16 4 4 24 20 7.0 29.2 2.8 41.0 21 16 4 4 24 20 7.0 29.2 2.8 41.0 22 16 4 4 24 20 5.0 20.8 3.2 34.0 23 16 4 4 24 45 5.0 20.8 3.2 34.0 24 16 4 4 24 20 6.4 26.7 2.9 38.9 25 16 4 4 24 20 2.4 10.0 3.6 25.0 26 16 4 4 24 20 5.0 20.8 3.2 34.0 27 16 4 4 24 20 5.0 20.8 3.2 34.0 28 18 3 3 24  8 5.0 20.8 3.2 34.0 29 16 4 4 24 60 5.0 20.8 3.2 34.0 30 18 3 3 24 20 10.0 41.7 2.3 51.4 31 22 1 1 24 20 10.9 45.4 0.5 47.7 32 16 4 4 24 20 7.0 29.2 2.8 41.0 33 16 4 4 24 20 7.0 29.2 2.8 41.0 34 16 4 4 24 20 5.0 20.8 3.2 34.0 Groove machining conditions (second cladding metal side) Cladding Groove metal Projection Gas shielding conditions depth thickness t.sub.c2* clad ratio Amount of Nozzle Gas release Gas flow Bevel angle d2 d2/t at root surface R2 upset height flow rate B rate ratio Level θ2 (°) (mm) (%) (mm) (%) (mm) H (mm) (m/s) B/A Category 1 20 5.0 20.8 3.2 34.0 10 100 10 0.5 Example 2 40 5.0 20.8 3.2 34.0 10 100 10 0.5 Example 3 30 3.0 12.5 3.5 27.1 10 100 10 0.5 Example 4 20 5.0 20.8 3.2 34.0 20 100 10  0.05 Example 5 20 5.0 20.8 3.2 34.0 10  8 10 0.5 Example 6 20 5.0 20.8 3.2 34.0 10 280 10 5   Example 7 20 5.0 20.8 3.2 34.0 10 100 45 0.5 Example 8  8 5.0 20.8 2.4 30.7 10 100 10 0.5 Comparative Example 9 60 5.0 20.8 3.2 34.0 10 100 10 0.5 Comparative Example 10 20 5.0 20.8 3.2 34.0 0.5 100 10 0.5 Example 11 20 10.0 41.7 1.8 49.0 10 100 10 0.5 Comparative Example 12 20 4.2 17.5 1.7 24.4 10 100 10 0.5 Comparative Example 13 20 10.9 45.4 0.5 47.7 10 100 10 0.5 Comparative Example 14 20 2.3  9.6 9.1 47.6 10 100 10 0.5 Comparative Example 15 20 5.0 20.8 3.2 34.0 26 100 10 0.5 Comparative Example 16 20 5.0 20.8 3.2 34.0 10  3 10 0.5 Comparative Example 17 20 7.0 29.2 2.8 41.0 10 350 10 0.5 Comparative Example 18 20 7.0 29.2 2.8 41.0 20 100   0.2 0.5 Comparative Example 19 20 7.0 29.2 2.8 41.0 10 100 60 0.5 Comparative Example 20 20 7.0 29.2 2.8 41.0 20 100 10  0.005 Comparative Example 21 20 7.0 29.2 2.8 41.0 10 100 40 12   Comparative Example 22 20 5.0 20.8 3.2 34.0 10 100 10 0.5 Example 23 45 5.0 20.8 3.2 34.0  8 100 10 0.5 Example 24 20 6.4 26.7 2.9 38.9 10 100 10 0.5 Example 25 20 2.4 10.0 3.6 25.0 10 100 10 8   Example 26 20 5.0 20.8 3.2 34.0 10  10  1 0.5 Example 27 20 5.0 20.8 3.2 34.0 10 200 10 0.5 Example 28  8 5.0 20.8 2.4 30.7 10 100 10 0.5 Example 29 60 5.0 20.8 3.2 34.0 20 100 10 0.5 Example 30 20 10.0 41.7 1.8 49.0 10 100 10 0.5 Comparative Example 31 20 10.9 45.4 0.5 47.7 10 100 10 0.5 Comparative Example 32 20 7.0 29.2 2.8 41.0 10  3 10 0.5 Comparative Example 33 20 7.0 29.2 2.8 41.0 10 100 60 0.5 Comparative Example 34 20 5.0 20.8 3.2 34.0 10 100 10 12   Comparative Example Underline indicates outside the range according to present disclosure.

(323) TABLE-US-00003 TABLE 3 Oxygen Weld metal dimensions concentration Inner surface Outer surface Thickness First cladding Second cladding around parts to width width center width metal thickness ratio metal thickness ratio be welded L1 L2 L3 t.sub.w1 t.sub.w1/t.sub.c1 t.sub.w1/t t.sub.w2 t.sub.w2/t.sub.c2 t.sub.w2/t Level (mass % × 10.sup.2) (mm) (mm) (mm) (mm) (%) (%) (mm) (%) (%) 1 0.03 0.17 0.25 0.07 2.9 72.5 12.1 2.9 72.5 12.1 2 0.04 0.08 0.19 0.06 5.5 137.5 22.9 5.5 137.5 22.9 3 0.11 0.16 0.20 0.12 2.1 52.5 8.8 2.1 52.5 8.8 4 0.13 0.07 0.16 0.04 1.5 37.5 6.3 1.5 37.5 6.3 5 0.01 0.02 0.09 0.009 3.4 85.0 14.2 3.4 85.0 14.2 6 0.86 0.15 0.15 0.11 4.2 105.0 17.5 4.2 105.0 17.5 7 0.37 0.27 0.28 0.09 6.6 165.0 27.5 6.6 165.0 27.5 8 0.02 1.23 1.31 0.35 9.2 306.7 38.3 9.2 306.7 38.3 9 0.05 1.42 1.52 0.69 0 0.0 0.0 0 0.0 0.0 10 0.06 0.45 0.47 0.74 0.9 22.5 3.8 0.9 22.5 3.8 11 0.05 1.34 1.41 0.51 9.2 306.7 38.3 9.3 310.0 38.8 12 0.05 1.18 1.22 0.11 0 0.0 0.0 0 0.0 0.0 13 0.05 1.52 1.48 0.69 3.8 380.0 15.8 3.8 380.0 15.8 14 0.04 1.35 1.41 0.61 4.7 46.5 19.6 4.7 46.5 19.6 15 0.03  0.0008  0.0008 0.0008 0 0.0 0.0 0 0.0 0.0 16 0.07  0.0009  0.001 0.0008 4 100.0 16.7 4 100.0 16.7 17 3 1.18 1.19 0.32 1.7 42.5 7.1 1.7 42.5 7.1 18 40  0.0008  0.0008 0.0007 1.8 45.0 7.5 1.8 45.0 7.5 19 6  0.0007  0.0006 0.0007 1.7 42.5 7.1 1.7 42.5 7.1 20 25  0.132  0.0006 0.062 1.5 37.5 6.3 1.5 37.5 6.3 21 10  0.0007  0.0006 0.0006 3.2 80.0 13.3 3.2 80.0 13.3 22 0.02 0.22 0.21 0.13 4.5 112.5 18.8 4.5 112.5 18.8 23 0.03 0.35 0.42 0.25 2.4 60.0 10.0 2.4 60.0 10.0 24 0.08 0.13 0.23 0.08 7.9 197.5 32.9 7.9 197.5 32.9 25 0.25 0.17 0.19 0.08 1.8 45.0 7.5 1.8 45.0 7.5 26 0.19 0.07 0.15 0.06 3.2 80.0 13.3 3.2 80.0 13.3 27 0.29 0.21 0.24 0.13 4.1 102.5 17.1 4.1 102.5 17.1 28 0.04 1.22 1.31 0.41 9.3 310.0 38.8 9.2 306.7 38.3 29 0.04 1.33 1.34 0.35 0 0.0 0.0 0 0.0 0.0 30 0.03 1.72 1.72 0.69 9.3 310.0 38.8 9.2 306.7 38.3 31 0.03 1.65 1.42 0.71 3.5 350.0 14.6 3.5 350.0 14.6 32 21  0.0008  0.0008 0.0006 1.2 30.0 5.0 1.2 30.0 5.0 33 18  0.0007  0.0008 0.0006 1.6 40.0 6.7 1.6 40.0 6.7 34 10  0.0009  0.0009 0.0007 3.2 80.0 13.3 3.2 80.0 13.3 Flatness value of Level weld h/D Toughness Cladding metal Category  1 0.24 Good SUS316 Example  2 0.28 Good SUS316 Example  3 0.23 Good SUS316 Example  4 0.22 Good SUS316 Example  5 0.28 Good SUS316 Example  6 0.27 Good SUS316 Example  7 0.28 Good SUS316 Example  8 0.44 Poor SUS316 Comparative Example  9 0.59 Poor SUS316 Comparative Example 10 0.82 Good SUS316 Example 11 0.66 Poor SUS316 Comparative Example 12 0.59 Poor SUS316 Comparative Example 13 0.49 Poor SUS316 Comparative Example 14 0.73 Poor SUS316 Comparative Example 15 0.33 Poor SUS316 Comparative Example 16 0.41 Poor SUS316 Comparative Example 17 0.81 Poor SUS316 Comparative Example 18 0.81 Poor SUS316 Comparative Example 19 0.66 Poor SUS316 Comparative Example 20 0.58 Poor SUS316 Comparative Example 21 0.66 Poor SUS316 Comparative Example 22 0.26 Good Alloy625 Example 23 0.28 Good Alloy625 Example 24 0.27 Good Alloy625 Example 25 0.28 Good Alloy625 Example 26 0.22 Good Alloy625 Example 27 0.24 Good Alloy625 Example 28 0.44 Poor Alloy625 Comparative Example 29 0.59 Poor Alloy625 Comparative Example 30 0.71 Poor Alloy625 Comparative Example 31 0.78 Poor Alloy625 Comparative Example 32 0.77 Poor Alloy625 Comparative Example 33 0.70 Poor Alloy625 Comparative Example 34 0.54 Poor Alloy625 Comparative Example Underline indicates outside the range according to present disclosure.

(324) Furthermore, whether or not the base metal was exposed in the weld was examined. Specifically, a section perpendicular to the pipe or tube longitudinal direction of the obtained clad welded pipe or tube was etched with a nital solution. In the case where both the inner surface and the outer surface of the clad welded pipe or tube were continuously covered by a region not etched at all (i.e. the cladding metal), it was determined that the base metal was not exposed. Otherwise, i.e. in the case where an etched region was observed at one or both of the inner surface and the outer surface, it was determined that the base metal was exposed. As a result, in each clad welded pipe or tube obtained by a production method satisfying the conditions according to the present disclosure, no exposure of the base metal at the cladding metal-side surface in the weld was found. In level 12 with a low projection clad ratio, level 15 with a large amount of upset, and levels 9 and 29 with a large bevel angle, on the other hand, exposure of the base metal was found.

(325) These results indicate that, in each clad welded pipe or tube satisfying the conditions according to the present disclosure, mechanical properties such as toughness and flatness value were excellent, the base metal was not exposed, and the function as a clad pipe or tube was not impaired.

Example 3

(326) Electric-resistance-welded two-layer clad steel pipes or tubes were produced by the following procedure. First, each two-layer clad hot-rolled steel strip composed of a first layer made of cladding metal having the chemical composition shown in Table 4 and a second layer made of base metal having the chemical composition shown in Table 5 was produced. The thickness of the first layer and the thickness of the second layer are shown in Table 6.

(327) Next, a clad steel pipe or tube of 400 mm in outer diameter was produced under various conditions by the electric-resistance-welded steel pipe or tube production line illustrated in FIG. 2, using the prepared clad steel strip as raw material with the base metal being on the pipe or tube outer surface side and the cladding metal being on the pipe or tube inner surface side. Here, both transverse ends of the clad steel strip were subjected to groove machining by the rolling-type groove machine 40, to form a Y-groove of the shape shown in Table 6 at the transverse ends.

(328) The amount of upset in the electric resistance welding is shown in Table 6. During the electric resistance welding, the parts to be welded were shielded with shielding gas using the nozzle illustrated in FIGS. 5A to 5C under the conditions of nozzle height H, gas release flow rate B, and flow rate ratio B/A shown in Table 6. The shielding gas was nitrogen.

(329) After this, post-welding heat treatment was performed under the conditions shown in Table 7.

(330) For each obtained electric-resistance-welded clad steel pipe or tube, the width of the weld metal, whether or not the base metal was exposed, the fracture property in the flattening test, the toughness of the weld, and the corrosion resistance of the cladding metal were evaluated by the following methods. The evaluation results are shown in Table 8.

(331) (Width of Weld Metal)

(332) The width of the weld metal was measured by the following procedure. First, a section perpendicular to the longitudinal direction of the clad steel pipe or tube was etched to reveal the microstructure, and the weld metal was identified. For example, for weld metal of carbon steel or low-alloy steel, the section is etched with natal, and a region observed as white by an optical microscope is identified as the weld metal. For weld metal of stainless steel or a Ni-containing alloy, the section is etched with aqua regia, and a region observed as black by an optical microscope and containing solidification microstructure in, for example, dendritic or cellular form is identified as the weld metal.

(333) The section was then photographed using the optical microscope, and analyzed to determine the width of the weld metal. Since the width of the weld metal differs in the thickness direction, the maximum value in the thickness direction of the width of the weld metal is shown in Table 8. Each case with a maximum width of the weld metal of less than 1.0 μm or 1000 μm or more in Table 8 is Comparative Example. All Examples in Table 8 were found to have a minimum value in the thickness direction of the width of the weld metal of 1.0 μm or more.

(334) (Exposure of Base Metal)

(335) A section perpendicular to the pipe or tube longitudinal direction of the obtained electric-resistance-welded clad steel pipe or tube was etched with a nital solution. In the case where the cladding metal-side surface was continuously covered by a region not etched at all (i.e. the cladding metal), it was determined that the base metal was not exposed. Otherwise, i.e. in the case where an etched region was observed at the cladding metal-side surface, it was determined that the base metal was exposed.

(336) (Fracture Property)

(337) To evaluate the fracture property of each obtained electric-resistance-welded clad steel pipe or tube in the weld, a test piece was collected from the produced clad steel pipe or tube, and a 90° flattening test in accordance with JIS G 3445 was performed to determine the flatness value h/D defined by the flattening crack height h (mm) and the pipe or tube outer diameter D (mm).

(338) (Toughness)

(339) A V-notch test piece was collected so that the longitudinal direction of the test piece was the circumferential direction of the steel pipe or tube, the notch position was the weld, and the notch direction was the longitudinal direction of the steel pipe or tube, and a Charpy impact test in accordance with ASTM A370 was performed to determine the fracture appearance transition temperature vTrs (° C.). The toughness of the weld was evaluated using the obtained value of vTrs, based on the following criteria.

(340) Excellent: vTrs: −45° C. or less

(341) Good: vTrs: more than −45° C. and −30° C. or less

(342) Poor: vTrs: more than −30° C.

(343) (Corrosion Resistance of Cladding Metal)

(344) In addition, the corrosion resistance of the cladding metal was evaluated by a sulfuric acid-copper sulfate corrosion test in accordance with ASTM A262-13, Practice E, with reference to API specification 5LD, 4.sup.th Edition. To evaluate the corrosion resistance of the cladding metal, the base metal side was removed by grinding while leaving the cladding metal side, to obtain a test piece made only of the cladding metal. In the evaluation of the corrosion resistance, the test piece after the test was observed visually or observed at 10 magnifications using a stereoscopic microscope or the like as appropriate, and each test piece observed to have no crack was determined as “good” and each test piece observed to have any crack was determined as “poor”. Further, the corrosion resistance of each test piece determined as “good” was also evaluated by a ferric chloride test described in ASTM G48-A. Each test piece whose corrosion loss in a 72 hr corrosion test was less than 0.5 g/m.sup.2 was determined as “excellent”.

(345) TABLE-US-00004 TABLE 4 Cladding metal Chemical composition (mass %)* ID C Si Mn P S Ni Cr Al N Mo Cu Ti Nb Category C1 0.04 0.20 0.60 0.025 0.001 0.20 16.1 0.01 0.04 — — — — Stainless steel C2 0.06 0.33 0.99 0.040 0.007 8.21 18.5 0.01 0.05 — — — — Stainless steel C3 0.02 0.13 0.03 0.004 0.001 61.60 21.3 0.12 0.05 8.6 — 0.25 3.31 Nickel-containing alloy C4 0.01 0.20 0.40 0.017 0.002 38.90 22.5 0.1 0.05 3.1 2.2  0.7  — Nickel-containing alloy C5 0.02 0.76 1.06 0.032 0.002 12.18 17.44 0.01 0.03 2.3 0.24 — — Stainless steel C6 0.01 0.16 0.11 0.024 0.004 0.21 29.71 0.15 0.01 2.0 0.02 — 0.15 Stainless steel *the balance being Fe and inevitable impurities

(346) TABLE-US-00005 TABLE 5 Base metal Chemical composition (mass %)* ID C Si Mn P S Al Ca Cr Mo Cu Ti Nb Category B1 0.04 0.19 1.6 0.006 0.003 0.04 0.0012 — — — 0.01 0.05 Low-carbon steel B2 0.05 0.25 1.5 0.013 0.003 0.03 — 0.3 0.4 — 0.01 — Low-alloy steel *the balance being Fe and inevitable impurities

(347) TABLE-US-00006 TABLE 6 Groove machining conditions Cladding metal Base metal Groove Cladding metal Cladding Thickness Base Thickness Whole Bevel depth thickness t.sub.cl* metal t.sub.cl metal t.sub.m thickness t angle d1 d1/t at root surface No. ID (mm) ID (mm) (mm) θ1 (°) (mm) (%) (mm) 1 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 2 C1 4.0 B1 16.0 20.0 40 5.0 25 3.7 3 C1 4.0 B1 16.0 20.0 30 3.0 15 4.2 4 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 5 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 6 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 7 C1 4.0 B1 16.0 20.0 8 5.0 25 4.3 8 C1 4.0 B1 16.0 20.0 60 5.0 25 3.5 9 C1 4.0 B1 16.0 20.0 20 10 50 3.5 10 C1 4.0 B1 16.0 20.0 20 1.0 5 3.2 11 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 12 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 13 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 14 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 15 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 16 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 17 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 18 C1 1.0 B1 19.0 20.0 20 4 20 0.8 19 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 20 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 21 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 22 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 23 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 24 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 25 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 26 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 27 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 28 C1 4.0 B2 16.0 20.0 20 5.0 25 4.1 29 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 30 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 31 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 32 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 33 C1 4.0 B1 16.0 20.0 20 5.0 25 4.1 34 C2 4.0 B1 16.0 20.0 20 5.0 25 4.1 35 C2 4.0 B2 16.0 20.0 20 5.0 25 4.1 36 C3 4.0 B1 16.0 20.0 20 5.0 25 4.1 37 C4 4.0 B1 16.0 20.0 20 5.0 25 4.1 38 C5 4.0 B1 16.0 20.0 20 5.0 25 4.1 39 C6 4.0 B1 16.0 20.0 20 5.0 25 4.1 40 C6 4.0 B2 16.0 20.0 20 5.0 25 4.1 Groove machining conditions Projection Gas shielding conditions clad ratio Amount of Nozzle Gas release Gas flow R1 upset height H flow rate B rate ratio No. (%) (mm) (mm) (m/s) B/A  1 45.5 10 100 10 0.5  2 43.5 10 100 10 0.5  3 36.0 10 100 10 0.5  4 45.5 18 100 10 0.05  5 45.5 10 8 10 0.5  6 45.5 10 280 10 5  7 46.5 10 100 10 0.5  8 42.5 10 100 10 0.5  9 67.5 10 100 10 0.5 10 21.0 10 100 10 0.5 11 45.5 25 100 10 0.5 12 45.5 10 3 10 0.5 13 45.5 10 350 10 0.5 14 45.5 10 100 0.2 0.5 15 45.5 10 100 60 0.5 16 45.5 10 100 10 12 17 51.0 10 100 10 0.5 18 24.0 10 100 10 0.5 19 45.5 10 100 10 0.5 20 45.5 10 100 10 0.5 21 45.5 10 100 10 0.5 22 45.5 10 100 10 0.5 23 45.5 10 100 10 0.5 24 45.5 10 100 10 0.5 25 45.5 10 100 10 0.5 26 45.5 10 100 10 0.5 27 45.5 10 100 10 0.5 28 45.5 10 100 10 0.5 29 45.5 10 100 10 0.5 30 45.5 10 100 10 0.5 31 45.5 10 100 10 0.5 32 45.5 10 100 10 0.5 33 45.5 10 100 10 0.5 34 45.5 10 100 10 0.5 35 45.5 10 100 10 0.5 36 45.5 10 100 10 0.5 37 45.5 10 100 10 0.5 38 45.5 10 100 10 0.5 39 45.5 10 100 10 0.5 40 45.5 10 100 10 0.5

(348) TABLE-US-00007 TABLE 7 Post-welding heat treatment conditions Cladding metal-side surface Base metal-side surface Heating temperature Holding time Cooling rate Heating temperature Holding time Cooling rate No. (° C.) (s) (° C./sec) (° C.) (s) (° C./sec) 1 1000 10 20 1000 10 20 2 1000 10 20 1000 10 20 3 1000 10 20 1000 10 20 4 1000 10 20 1000 10 20 5 1000 10 20 1000 10 20 6 1000 10 20 1000 10 20 7 1000 10 20 1000 10 20 8 1000 10 20 1000 10 20 9 1000 10 20 1000 10 20 10 1000 10 20 1000 10 20 11 1000 10 20 1000 10 20 12 1000 10 20 1000 10 20 13 1000 10 20 1000 10 20 14 1000 10 20 1000 10 20 15 1000 10 20 1000 10 20 16 1000 10 20 1000 10 20 17 1000 10 20 1000 10 20 18 1000 10 20 1000 10 20 19 1000 30 100 1000 30 50 20 1300 10 20 1000 10 20 21 1000 10 20 1000 10 20 22 1000 10 20 1000 10 20 23 1000 10 20 1000 10 20 24 1000 10 20 1000 10 20 25 1000 10 20 1000 10 20 26 1000 10 20 1000 10 20 27 1000 10 20 1000 10 20

(349) TABLE-US-00008 TABLE 8 Width of weld metal Flatness value Cladding metal (maximum value) Base metal of weld corrosion No. (μm) exposure h/D Toughness resistance Remarks 1 600 Not exposed 0.22 Excellent Excellent Example 2 800 Not exposed 0.26 Excellent Excellent Example 3 100 Not exposed 0.20 Excellent Excellent Example 4  3 Not exposed 0.19 Excellent Excellent Example 5  10 Not exposed 0.27 Excellent Excellent Example 6 780 Not exposed 0.25 Excellent Excellent Example 7 1020  Not exposed 0.44 Poor Excellent Comparative Example 8 1010  Exposed 0.23 Poor Poor Comparative Example 9 1200  Not exposed 0.44 Poor Excellent Comparative Example 10 1250  Exposed 0.49 Poor Poor Comparative Example 11    0.8 Exposed 0.33 Poor Poor Comparative Example 12    0.9 Not exposed 0.34 Poor Excellent Comparative Example 13 1050  Not exposed 0.77 Poor Excellent Comparative Example 14    0.8 Not exposed 0.75 Poor Excellent Comparative Example 15    0.8 Not exposed 0.64 Poor Excellent Comparative Example 16    0.9 Not exposed 0.58 Poor Excellent Comparative Example 17 1020  Not exposed 0.34 Poor Poor Comparative Example 18 1030  Exposed 0.41 Poor Poor Comparative Example 19 250 Not exposed 0.32 Good Good Example 20 250 Not exposed 0.22 Excellent Good Example 21 250 Not exposed 0.22 Excellent Excellent Example 22 250 Not exposed 0.22 Excellent Excellent Example 23 250 Not exposed 0.22 Excellent Excellent Example 24 250 Not exposed 0.22 Excellent Excellent Example 25 250 Not exposed 0.22 Excellent Excellent Example 26 250 Not exposed 0.22 Excellent Excellent Example 27 250 Not exposed 0.22 Excellent Excellent Example

(350) As can be understood from the results in Table 8, each Comparative Example with a width of the weld metal of less than 1.0 μm was inferior in the corrosion resistance of the cladding metal, and each Comparative Example with a width of the weld metal of more than 1000 μm was inferior in the toughness of the weld. On the other hand, each clad welded steel pipe or tube satisfying the conditions according to the present disclosure was superior in the toughness of the weld, and also had superior performance in the corrosion resistance required of clad steel pipes or tubes.

INDUSTRIAL APPLICABILITY

(351) With the method of producing a clad welded pipe or tube according to the present disclosure, it is possible to produce a clad welded pipe or tube that has improved pipe or tube mechanical properties by reducing the width of a weld without its function as a clad pipe or tube being impaired.

REFERENCE SIGNS LIST

(352) 10 clad steel strip

(353) 11 first layer (base metal)

(354) 12 second layer (first cladding metal)

(355) 12A second layer (first cladding metal, inner layer)

(356) 12B third layer (second cladding metal, outer layer)

(357) 13 clad interface

(358) 13A clad interface between first layer and second layer

(359) 13B clad interface between first layer and third layer

(360) 14 welded seam part

(361) 15A weld metal (base metal)

(362) 15B weld metal (first cladding metal)

(363) 15C weld metal (second cladding metal)

(364) 16 open pipe or tube

(365) 17 part to be welded (butted part of open pipe or tube)

(366) 18 butted part heating starting point of open pipe or tube

(367) 19 welding point

(368) 20 clad welded pipe or tube

(369) 30 uncoiler

(370) 40 groove machine

(371) 42 upper side roll

(372) 42A rolling portion

(373) 44 lower side roll

(374) 44A rolling portion

(375) 50 roll former

(376) 60 high-frequency heating device

(377) 70 squeeze roll

(378) 80 shielding-gas blowing device

(379) 81 shielding-gas blowing nozzle

(380) 82 gas pipe

(381) 83 gas flow adjusting device

(382) 84A first split nozzle (both ends)

(383) 84B second split nozzle (center)

(384) 85A first gas outlet (both ends)

(385) 85B second gas outlet (center)

(386) 86 shielding gas

(387) 87 air entrainment

(388) 90 bead cutter

(389) 96 cutter

(390) X pipe or tube passage direction

(391) Y open pipe or tube butting direction

(392) θ1 bevel angle on second layer side

(393) θ2 bevel angle on third layer side

(394) d1 groove depth on second layer side

(395) d2 groove depth on third layer side

(396) t thickness of clad steel strip (steel pipe or tube)

(397) t.sub.m thickness of first layer

(398) t.sub.c1 thickness of second layer

(399) t.sub.c1* thickness of second layer at root surface

(400) t.sub.w1 thickness of second layer in welded seam part

(401) t.sub.c2 thickness of third layer

(402) t.sub.c2* thickness of third layer at root surface

(403) t.sub.w2 thickness of third layer in welded seam part

(404) L1 pipe or tube circumferential length of weld metal at pipe or tube inner surface

(405) L2 pipe or tube circumferential length of weld metal at pipe or tube outer surface

(406) L3 pipe or tube circumferential length of weld metal at pipe or tube thickness center

(407) W width of weld metal