Electric-resistance-welded stainless clad steel pipe or tube

Abstract

An electric-resistance-welded stainless clad steel pipe or tube that is excellent in both the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface as electric resistance welded without additional welding treatment such as weld overlaying after electric resistance welding is provided. An electric-resistance-welded stainless clad steel pipe or tube comprises: an outer layer of carbon steel or low-alloy steel; and an inner layer of austenitic stainless steel having a predetermined chemical composition, wherein a flatness value h/D in a 90 flattening test in accordance with JIS G 3445 is less than 0.3, and a pipe or tube inner surface has no crack in a sulfuric acid-copper sulfate corrosion test in accordance with ASTM A262-10, Practice E, where h is a flattening crack height (mm), and D is a pipe or tube outer diameter (mm).

Claims

1. An electric-resistance-welded stainless clad steel pipe or tube, comprising: an outer layer of carbon steel or low-alloy steel; and an inner layer of austenitic stainless steel having a chemical composition containing, in mass %, C: 0.1% or less, Si: 1.5% or less, Mn: 2.5% or less, Ni: 7.0% to 35.0%, Cr: 16.0% to 35.0%, Mo: 0.1% to 10.0%, and the balance being Fe and inevitable impurities, wherein a welded seam part exits in the electric-resistance-welded stainless clad steel pipe or tube, and wherein a flatness value h/D in a 90 flattening test in accordance with JIS G 3445 is less than 0.3, and an inner surface of the electric-resistance-welded stainless clad steel pipe or tube has no crack in a sulfuric acid-copper sulfate corrosion test in accordance with ASTM A262-10, Practice E, where h is a flattening crack height in mm, and D is an outer diameter of the electric-resistance-welded stainless clad steel pipe or tube in mm.

2. The electric-resistance-welded stainless clad steel pipe or tube according to claim 1, wherein the chemical composition further contains at least one of (i) N: 2.0% or less, (ii) Cu: 3.0% or less, (iii) at least one of Ti, Nb, V, and Zr: 0.01% to 0.5% in total, (iv) at least one of Ca, Mg, B, and REM: 0.1% or less each, and (v) Al: 0.2% or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is a schematic diagram of a production line for an electric-resistance-welded stainless clad steel pipe or tube according to one of the disclosed embodiments;

(3) FIG. 2A is a schematic diagram for explaining gas shielding according to one of the disclosed embodiments, and is a perspective diagram of an open pipe or tube 16 and an electric-resistance-welded stainless clad steel pipe or tube 20 which are being transferred;

(4) FIG. 2B 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. 2A;

(5) FIG. 2C is a schematic diagram for explaining gas shielding, and is a sectional diagram of a Z2 portion in FIG. 2A;

(6) FIG. 3A is a schematic diagram illustrating an example of a nozzle usable in this embodiment;

(7) FIG. 3B is a schematic diagram illustrating an example of the nozzle usable in this embodiment;

(8) FIG. 3C is a schematic diagram illustrating an example of the nozzle usable in this embodiment;

(9) FIG. 3D is a schematic diagram illustrating an example of the nozzle usable in this embodiment;

(10) FIG. 4A 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;

(11) FIG. 4B 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;

(12) FIG. 4C 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;

(13) FIG. 5 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;

(14) FIG. 6 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;

(15) FIG. 7A is a schematic sectional diagram of an electric resistance weld and its vicinity in the case where the amount of upset is large; and

(16) FIG. 7B is a schematic sectional diagram of an electric resistance weld and its vicinity in the case where the amount of upset is small.

DETAILED DESCRIPTION

(17) (Method of Producing Electric-Resistance-Welded Stainless Clad Steel Pipe or Tube)

(18) A process of producing an electric-resistance-welded stainless clad steel pipe or tube according to one of the disclosed embodiments is described below, with reference to FIG. 1. The production of an electric-resistance-welded stainless clad steel pipe or tube according to one of the disclosed embodiments includes the following steps: First, a stainless clad steel strip 10 in the form of a hot rolled coil is continuously uncoiled by an uncoiler 30. The stainless clad steel strip 10 is then formed into a pipe or tube shape by a roll former 50. Following this, the transverse ends of the steel strip 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 an electric-resistance-welded stainless clad steel 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. The weld after the electric resistance welding is then heat-treated by a heating device 92, and further cooled by a cooling device 94. Subsequently, the pipe or tube 20 is cut to a predetermined length by a cutter 96.

(19) 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.

(20) In this embodiment, electric resistance welding is performed using the stainless clad steel strip 10 obtained by pressure-bonding a first layer 11 made of carbon steel or low-alloy steel as base metal and a second layer 12 made of austenitic stainless steel as cladding metal where the second layer 12 as cladding metal is the inner layer and the first layer 11 as base metal is the outer layer, as illustrated in FIG. 2C. As used herein, the base metal denotes the material of the thicker layer in a clad steel strip composed of two layers that differ in thickness and material from each other, and the cladding metal denotes the material of the thinner layer. In this embodiment, carbon steel or low-alloy steel as the base metal is a material for ensuring the strength of the steel pipe or tube, and austenitic stainless steel as the cladding metal is a material for ensuring the corrosion resistance of the pipe or tube inner surface.

(21) The carbon steel used as the base metal in this embodiment 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 steel pipe or tube, because the mechanical properties of the clad steel pipe or tube depend on the properties of the base metal occupying the major portion of the steel pipe or tube volume.

(22) The low-alloy steel used as the base metal in this embodiment 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 steel pipe or tube, as with the above-mentioned carbon steel.

(23) The chemical composition of the austenitic stainless steel used as the cladding metal in this embodiment is described below. In the following description, % regarding content denotes mass % unless otherwise noted.

(24) C: 0.1% or Less

(25) C combines with Cr in the steel and causes a decrease in corrosion resistance, and accordingly a lower C content is more desirable. If the C content is 0.1% or less, the corrosion resistance does not decrease significantly. The C content is therefore 0.1% or less. The C content is preferably 0.08% or less. No lower limit is placed on the C content, yet the lower limit is 0.001% from the industrial point of view.

(26) Si: 1.5% or Less

(27) Si is an element effective in deoxidation. However, if the Si content is excessive, an oxide tends to form in the electric resistance weld, which causes a decrease in weld properties. The Si content is therefore 1.5% or less. The Si content is preferably 1.0% or less. No lower limit is placed on the Si content, yet the lower limit is 0.01% from the industrial point of view.

(28) Mn: 2.5% or Less

(29) Mn is an element effective in strength improvement. However, if the Mn content is excessive, an oxide tends to form in the electric resistance weld, which causes a decrease in weld properties. The Mn content is therefore 2.5% or less. The Mn content is preferably 2.0% or less. No lower limit is placed on the Mn content, yet the lower limit is 0.001% from the industrial point of view.

(30) Ni: 7.0% to 35.0%

(31) Ni is an element that stabilizes austenite phase. If the Ni content is less than 7.0%, stable austenite phase cannot be obtained in the case where 16.0% or more Cr that stabilizes ferrite phase is contained. If the Ni content is more than 35.0%, the production cost increases, which is economically disadvantageous. Accordingly, the Ni content needs to be in a range of 7.0% to 35.0%.

(32) Cr: 16.0% to 35.0%

(33) Cr is an important element that forms a passive film on the surface of the steel pipe or tube to maintain corrosion resistance. This effect is achieved if the Cr content is 16.0% or more. If the Cr content is more than 35.0%, hot workability decreases, and austenite single phase microstructure is difficult to be obtained. The Cr content is therefore 16.0% to 35.0%. The Cr content is preferably 18.0% to 30.0%.

(34) Mo: 0.1% to 10.0%

(35) Mo is an element effective in inhibiting local corrosion such as crevice corrosion. To achieve this effect, the Mo content needs to be 0.1% or more. If the Mo content is more than 10.0%, the austenitic stainless steel is embrittled significantly. The Mo content is therefore 0.1% to 10.0%. The Mo content is preferably 0.5% to 7.0%.

(36) In addition to C, Si, Mn, Ni, Cr, and Mo described above, the following elements may be contained as appropriate.

(37) N: 2.0% or Less

(38) N has an effect of inhibiting local corrosion. It is, however, industrially difficult to have a N content of more than 2.0%, and so the upper limit of the N content is 2.0%. Moreover, in a typical steelmaking method, if the N content is more than 0.4%, adding N in the steelmaking stage requires a long time, which causes a decrease in productivity. Accordingly, the N content is more preferably 0.4% or less, in terms of cost. The N content is further preferably in a range of 0.01% to 0.3%.

(39) Cu: 3.0% or Less

(40) Cu is an element having an effect of improving corrosion resistance. To achieve this effect, the Cu content is preferably 0.01% or more. If the Cu content is more than 3.0%, hot workability decreases, causing a decrease in productivity. Accordingly, in the case of containing Cu, the Cu content is preferably 3.0% or less. The Cu content is more preferably in a range of 0.01% to 2.5%.

(41) At Least One of Ti, Nb, V, and Zr: 0.01% to 0.5% in Total

(42) Ti, Nb, V, and Zr are each an element effective in improving the intergranular corrosion resistance of the austenitic stainless steel, by reacting with C in the austenitic stainless steel to form a carbide and fix C. To achieve this effect, the total content of at least one of Ti, Nb, V, and Zr is preferably 0.01% or more. If the total content of at least one of Ti, Nb, V, and Zr, whether added singly or in combination, is more than 0.5%, the effect is saturated. Accordingly, in the case of containing at least one of Ti, Nb, V, and Zr, the total content is 0.5% or less.

(43) Besides the above-mentioned elements, at least one of Ca, Mg, B, and rare earth element (REM) may be contained in an amount of 0.1% or less each in order to improve the hot workability of the austenitic stainless steel, and Al may be contained in an amount of 0.2% or less for deoxidation in the molten steel stage.

(44) The balance of the chemical composition is Fe and inevitable impurities. Of the inevitable impurities, O is preferably contained in an amount of 0.02% or less.

(45) As illustrated in FIGS. 2A and 2C, in this embodiment, the stainless clad steel strip 10 is formed into a pipe or tube shape so that the first layer 11 is the outer layer and the second layer 12 is the inner layer, 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 electric-resistance-welded stainless clad steel pipe or tube 20.

(46) In FIG. 2A, 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 between 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.

(47) The nozzle 81 is split into three layers in the open pipe or tube butting direction Y, as illustrated in FIGS. 2B, 3A, and 3D. 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. 3B and 3C. 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.

(48) 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.

(49) 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.

(50) 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. 2C), and the shielding gas is blown under the conditions that B is 0.5 m/s to 50 m/s and 0.01B/A10, 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.

(51) 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. 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.

(52) 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. 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. 3B and 3C), 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.

(53) 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. 4A to 4C.

(54) With reference to FIG. 4A, in the case where B/A<0.01, 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.

(55) With reference to FIG. 4C, 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.

(56) With reference to FIG. 4B, in the case where 0.01B/A10, 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. 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.

(57) FIG. 5 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 0.5B50. A stainless clad steel strip having low-carbon low-alloy steel with a thickness of 5 mm as the base metal on the pipe or tube outer surface side and austenitic stainless steel (SUS316L) with a thickness of 2 mm as the cladding metal on the pipe or tube inner surface side was used.

(58) As illustrated in FIG. 5, 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 0.01B/A10 within the appropriate range of 0.5 B 50. Moreover, as illustrated in FIG. 5, 0.03B/A5 is preferable because a lower oxygen concentration level of 0.001 mass % to 0.0001 mass % can be achieved.

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

(60) FIG. 6 is a graph illustrating the relationship between the oxygen concentration around the parts to be welded and the flatness value h/D of each electric-resistance-welded stainless clad steel pipe or tube in a 90 flattening test. A stainless clad steel strip having low-carbon low-alloy steel with a thickness of 5 mm as the base metal on the pipe or tube outer surface side and austenitic stainless steel (SUS316L) with a thickness of 2 mm as the cladding metal on the pipe or tube inner surface side was used. As illustrated in FIG. 7B, electric-resistance-welded stainless clad steel pipes or tubes were produced with varying oxygen concentrations around the parts to be welded, while limiting the amount of upset to 1.0 mm which is not greater than the thickness of the stainless clad steel strip so as to prevent the base metal on the pipe or tube outer surface side from being exposed at the pipe or tube inner surface after the electric resistance welding. Subsequently, the weld was subjected to post-welding heat treatment with the heating temperature at the pipe or tube inner surface of the weld being 1000 C. and the cooling rate of the temperature at the pipe or tube inner surface being 10 C./s. A test piece of 50 mm in length was then collected, and a 90 flattening test in accordance with JIS G 3445 was performed to obtain the flatness value h/D.

(61) As illustrated in FIG. 6, 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.

(62) 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. 2C), 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.

(63) 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. 2C. This allows for a more rapid reduction in the oxygen concentration around the parts 17 to be welded.

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

(65) 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.

(66) 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.

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

(68) (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.

(69) (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.

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

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

(72) In this embodiment, the amount of upset is limited to not greater than the thickness of the stainless clad steel strip. This prevents the carbon steel or low-alloy steel of the outer layer in the weld from being exposed at the inner surface of the steel pipe or tube. The amount of upset is preferably not less than 20% of the thickness of the stainless clad steel strip, in terms of ensuring the effect of discharging penetrators from the weld during the electric resistance welding. 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.

(73) In this embodiment, the conditions of the heat treatment performed on the weld after the electric resistance welding are optimized. In detail, the weld after the electric resistance welding is subjected to heat treatment with the temperature at the pipe or tube inner surface of the weld being 800 C. to 1200 C., and then subjected to cooling with the cooling rate from 800 C. to 400 C. of the temperature at the pipe or tube inner surface of the weld being 4 C./s to 30 C./s. Both the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface can be improved in this way.

(74) If the heating temperature at the pipe or tube inner surface of the weld is less than 800 C., the homogenization and grain refinement of the microstructure of the weld of the base metal and the solutionizing of the cladding metal are insufficient, and so the effect of improving the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface cannot be achieved. If the heating temperature is more than 1200 C., the microstructure of the weld of the base metal coarsens, which causes a decrease in the toughness, i.e. fracture property, of the weld.

(75) If the cooling rate is less than 4 C./s, the austenitic stainless steel as the cladding metal is sensitized, and as a result the corrosion resistance of the weld at the pipe or tube inner surface decreases. If the cooling rate is more than 30 C./s, the carbon steel or low-alloy steel as the base metal forms a high-hardness quenched microstructure, and the fracture property of the weld decreases.

(76) The heating conditions and the cooling conditions in the post-welding heat treatment are controlled by the heating device 92 and the cooling device 94 illustrated in FIG. 1. The heating method and the cooling method are not limited, as long as the above-mentioned heating temperature and cooling rate are achieved. Examples of the heating method include induction heating, direct current heating, and heating by an atmosphere heating furnace. Examples of the cooling method include air cooling, air blast cooling, water cooling, and heat releasing by a pipe or tube formation tool (such as rolls or rollers).

(77) (Electric-Resistance-Welded Stainless Clad Steel Pipe or Tube)

(78) An electric-resistance-welded stainless clad steel pipe or tube according to this embodiment is obtained by the production method described above. The electric-resistance-welded stainless clad steel pipe or tube is composed of an outer layer of carbon steel or low-alloy steel and an inner layer of austenitic stainless steel having the chemical composition described above, and has an electric resistance weld that has undergone weld bead cutting and post-welding heat treatment as electric resistance welded. As used herein, the expression as electric resistance welded means that additional welding treatment such as weld overlaying is not performed after electric resistance welding.

(79) The electric-resistance-welded stainless clad steel pipe or tube according to this embodiment is excellent in both the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface as electric resistance welded, that is, the flatness value h/D in a 90 flattening test in accordance with JIS G 3445 is less than 0.3 and the pipe or tube inner surface has no crack in a sulfuric acid-copper sulfate corrosion test in accordance with ASTM A262-10, Practice E, where h is the flattening crack height (mm) and D is the pipe or tube outer diameter (mm). Cracks at the pipe or tube inner surface can be clearly determined based on the criteria defined in the standard.

EXAMPLES

Example 1

(80) Each stainless clad steel strip composed of: cladding metal of austenitic stainless steel having a thickness of 2 mm and a chemical composition containing, in mass %, C: 0.015%, Si: 0.76%, Mn: 1.06%, Ni: 12.2%, Cr: 17.4%, Cu: 0.24%, Mo: 2.32%, and the balance being Fe and inevitable impurities; and base metal of low-carbon low-alloy steel having a thickness of 5 mm and a chemical composition containing, in mass %, C: 0.04%, Si: 0.2%, Mn: 1.60%, V: 0.04%, Nb: 0.05%, Ti: 0.01%, and the balance being Fe and inevitable impurities was prepared.

(81) An electric-resistance-welded stainless clad steel pipe or tube of 300 mm in outer diameter was produced under various conditions by the electric-resistance-welded steel pipe or tube production line illustrated in FIG. 1, using the prepared stainless clad steel strip as raw material with the base metal forming the outer layer and the cladding metal forming the inner layer. During the electric resistance welding, the parts to be welded were shielded with shielding gas using the nozzle illustrated in FIGS. 2A to 2C under the conditions of nozzle height H, gas release flow rate B, flow rate ratio B/A, and R/W shown in Table 1. The shielding gas was nitrogen as inert gas. In some of the levels in Table 1, inert gas was mixed with propane gas as reducing gas. The amount of upset is shown in Table 1.

(82) The weld after the electric resistance welding was subjected to heat treatment with the temperature at the pipe or tube inner surface of the weld being the value shown in Table 1, and then subjected to cooling with the cooling rate from 800 C. to 400 C. of the temperature at the pipe or tube inner surface of the weld being the value shown in Table 1.

(83) In each level, the oxygen concentration around the parts to be welded was measured. Moreover, a test piece was collected from each produced steel pipe or tube, and a 90 flattening test in accordance with JIS G 3445 was performed to obtain the flatness value h/D. The results are shown in Table 1.

(84) In addition, the corrosion resistance of the pipe or tube inner surface was evaluated by a sulfuric acid-copper sulfate corrosion test in accordance with ASTM A262-10, Practice E, with reference to API specification 5LD, 4.sup.th Edition. To evaluate the corrosion resistance of the pipe or tube inner surface, the pipe or tube outer surface side (base metal side) was removed by grinding while leaving the pipe or tube inner surface side, to obtain a test piece made only of the stainless steel. 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 pass and each test piece observed to have any crack was determined as fail.

(85) As shown in Table 1, the Examples exhibited huge reductions in the flatness value h/D of the weld as compared with the Comparative Examples, i.e. had a weld having excellent fracture property and also maintaining the corrosion resistance of the austenitic stainless steel at the inner surface.

(86) TABLE-US-00001 TABLE 1 Post-welding heat treatment conditions Gas shielding conditions Heating Cooling rate Evaluation Gas Gas Oxygen temperature of pipe or Corrosion Nozzle release flow Reducing concentration of pipe or tube inner Flatness test on height flow rate gas Amount around parts tube inner surface value pipe or H rate B ratio content of upset to be welded surface temperature of weld tube inner Level (mm) (m/s) B/A R/W (mass %) (mm) (mass % 10.sup.2) ( C.) (C/s) h/D surface Category 1 100 0.5 0.5 5 0 4 0.03 1150 5 0.2 Pass Example 2 100 1.0 0.5 5 0 4 0.04 950 5 0.1 Pass Example 3 100 5 0.5 5 0 4 0.02 1100 6 0.1 Pass Example 4 100 10 0.1 5 0 4 0.04 1000 8 0.1 Pass Example 5 100 50 0.03 5 0 4 0.06 1100 8 0.1 Pass Example 6 200 0.5 3 5 0 4 0.08 900 4 0.1 Pass Example 7 200 1.0 2 5 0 4 0.09 1100 5 0.1 Pass Example 8 200 5 10 5 0 4 0.3 1000 6 0.2 Pass Example 9 200 10 0.01 5 0 4 0.2 1000 9 0.2 Pass Example 10 200 50 0.05 5 0 4 0.1 1000 10 0.2 Pass Example 11 50 0.5 0.5 5 0 4 0.1 950 5 0.2 Pass Example 12 50 1.0 3 5 0 4 0.07 950 8 0.2 Pass Example 13 50 5 0.2 5 0 4 0.04 950 5 0.1 Pass Example 14 50 10 0.3 5 0 4 0.05 950 4 0.1 Pass Example 15 50 50 0.5 5 0 4 0.1 950 5 0.2 Pass Example 16 50 10 0.5 10 0 2 <0.01 1000 6 0.1 Pass Example 17 50 10 2 2 0 2 0.03 1000 5 0.1 Pass Example 18 50 10 2 1.0 0 2 0.05 1000 8 0.1 Pass Example 19 20 10 0.5 20 0 7 <0.01 1000 5 0.1 Pass Example 20 20 10 0.5 5 0 7 <0.01 1000 5 0.1 Pass Example 21 300 10 0.2 5 0 1 0.2 1000 6 0.2 Pass Example 22 50 10 0.03 5 0.1 1 <0.01 1000 5 0.1 Pass Example 23 50 10 0.5 5 3 3 <0.01 1000 8 0.1 Pass Example 24 50 10 1 5 5 3 <0.01 1000 5 0.1 Pass Example 25 50 5 3 1.0 5 3 <0.01 1000 4 0.1 Pass Example 26 50 5 5 0.8 5 3 <0.01 1000 5 0.1 Pass Example 27 400 1.0 1 5 0 4 15 1000 5 0.5 Pass Comparative Example 28 100 0.4 1 5 0 4 160 1000 6 0.8 Pass Comparative Example 29 100 60 1 5 0 4 3 1000 5 0.4 Pass Comparative Example 30 400 60 1 5 0 4 40 1000 4 0.6 Pass Comparative Example 31 100 60 1 5 5 4 2 1000 8 0.4 Pass Comparative Example 32 50 10 0.5 10 0 8 <0.01 1000 4 0.1 Fail Comparative Example 33 50 10 2 2 0 8 0.02 1000 5 0.1 Fail Comparative Example 34 50 10 2 1.0 0 8 0.05 1000 6 0.1 Fail Comparative Example 35 20 10 0.5 20 0 8 <0.01 1000 5 0.1 Fail Comparative Example 36 20 10 0.5 5 0 8 <0.01 1000 8 0.1 Fail Comparative Example 37 400 1.0 1 5 0 10 12 1000 7 0.4 Fail Comparative Example 38 100 0.5 0.5 5 0 4 0.03 1000 2 0.2 Fail Comparative Example 39 100 0.5 0.5 5 0 4 0.03 1000 50 0.5 Pass Comparative Example 40 100 0.5 0.5 5 0 4 0.03 1180 5 0.2 Pass Example 41 100 0.5 0.5 5 0 4 0.03 810 5 0.2 Pass Example 42 100 0.5 0.5 5 0 4 0.03 1150 30 0.2 Pass Example 43 100 0.5 0.5 5 0 4 0.03 1150 35 0.4 Pass Comparative Example 44 100 0.5 0.5 5 0 4 0.03 750 5 0.4 Fail Comparative Example 45 100 0.5 0.5 5 0 4 0.03 1230 5 0.5 Pass Comparative Example

Example 2

(87) Each stainless clad steel strip composed of: cladding metal of austenitic stainless steel having a thickness of 2 mm and the chemical composition (balance being Fe and inevitable impurities) shown in Table 2; and base metal of low-carbon low-alloy steel having a thickness of 5 mm and a chemical composition containing, in mass %, C: 0.04%, Si: 0.2%, Mn: 1.60%, V: 0.04%, Nb: 0.05%, Ti: 0.01%, and the balance being Fe and inevitable impurities was prepared.

(88) An electric-resistance-welded stainless clad steel pipe or tube of 300 mm in outer diameter was produced by the electric-resistance-welded steel pipe or tube production line illustrated in FIG. 1, using the prepared stainless 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. During the electric resistance welding, the parts to be welded were shielded with shielding gas using the nozzle illustrated in FIGS. 2A to 2C under the conditions of nozzle height H=50 mm, gas release flow rate B=10 m/s, flow rate ratio B/A=0.5, and R/W=5. The shielding gas was nitrogen as inert gas. The amount of upset was 4 mm.

(89) The weld after the electric resistance welding was subjected to heat treatment with the temperature at the pipe or tube inner surface of the weld being 1000 C., and then subjected to cooling with the cooling rate from 800 C. to 400 C. of the temperature at the pipe or tube inner surface of the weld being 10 C./s.

(90) The measurement of the oxygen concentration around the parts to be welded and the evaluation of the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface were performed by the same methods as in Example 1. The results are shown in Table 3.

(91) As is clear from Table 3, the electric-resistance-welded stainless clad steel pipe or tube in each Example was excellent in both the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface, whereas each Comparative Example in which the chemical composition of the cladding metal was outside the presently disclosed range was inferior in the fracture property of the weld or the corrosion resistance of the pipe or tube inner surface.

(92) TABLE-US-00002 TABLE 2 (mass %) Steel C Si Mn Cr Ni Mo Others Category a 0.057 0.33 0.99 18.5 8.2 Comparative Example b 0.015 0.76 1.06 17.4 12.2 2.32 Cu: 0.24 Example c 0.012 0.72 0.97 22.2 25.1 6.08 Example d 0.150 0.51 1.21 18.1 8.5 1.02 Comparative Example e 0.061 1.8 0.85 18.3 8.1 0.59 Comparative Example f 0.062 0.68 3.2 18.7 8.6 2.51 Comparative Example g 0.071 0.71 1.15 15.1 8.3 0.23 Comparative Example

(93) TABLE-US-00003 TABLE 3 Oxygen concentration around parts Flatness value Corrosion test to be welded of weld on pipe or tube Steel (mass % 10.sup.2) h/D inner surface Category a 0.05 0.1 Fail Comparative Example b 0.05 0.1 Pass Example c 0.05 0.1 Pass Example d 0.05 0.3 Fail Comparative Example e 0.05 0.3 Pass Comparative Example f 0.05 0.3 Pass Comparative Example g 0.05 0.1 Fail Comparative Example

INDUSTRIAL APPLICABILITY

(94) The presently disclosed method of producing an electric-resistance-welded stainless clad steel pipe or tube can produce an electric-resistance-welded stainless clad steel pipe or tube that is excellent in both the fracture property of the weld and the corrosion resistance of the pipe or tube inner surface as electric resistance welded without conventionally required additional welding treatment such as weld overlaying after electric resistance welding.

REFERENCE SIGNS LIST

(95) 10 stainless clad steel strip 11 first layer (base metal) 12 second layer (cladding metal) 13 clad interface 14 welded seam part 16 open pipe or tube 17 part to be welded (butted part of open pipe or tube) 18 butted part heating starting point of open pipe or tube 19 welding point 20 electric-resistance-welded stainless clad steel pipe or tube 30 uncoiler 50 roll former 60 high-frequency heating device 70 squeeze roll 80 shielding-gas blowing device 81 shielding-gas blowing nozzle 82 gas pipe 83 gas flow adjusting device 84A first split nozzle (both ends) 84B second split nozzle (center) 85A first gas outlet (both ends) 85B second gas outlet (center) 86 shielding gas 87 air entrainment 90 bead cutter 92 heating device 94 cooling device 96 cutter X pipe or tube passage direction Y open pipe or tube butting direction