ELECTRIC RESISTANCE WELDED STAINLESS CLAD STEEL PIPE AND METHOD OF MANUFACTURING THE SAME

Abstract

Provided are an electric resistance welded stainless clad steel manufactured by forming a hot-rolled steel strip of clad steel including low-carbon low-alloy steel and stainless steel into a cylindrical shape, and electric resistance welding the edges of the hot-rolled steel strip, characterized in that the flattening characteristic of an electric resistance weld, as-welded, satisfies the formula h/D<0.3, wherein h is the flattened height at fracture (mm) and D is the outer diameter of the pipe (mm), and a method of manufacturing the same.

Claims

1. An electric resistance welded stainless clad steel pipe made of clad steel comprising low-carbon low-alloy steel and stainless steel, wherein the flattening characteristic of an electric resistance weld, as-welded, satisfies formula (1)-be-low:
h/D<0.3 (1) wherein h is the flattened height at fracture (mm) and D is the outer diameter of the pipe (mm).

2. The electric resistance welded stainless clad steel pipe according to claim 1, wherein the thickness tw of the electric resistance weld satisfies the formula (2):
0.7?tb<tw<1.6?tb (2) wherein tb is the thickness of base metal (mm) and tw is the thickness of the weld (mm).

3. A method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 1, wherein a shielding process for portions of an open pipe to be welded for an electric resistance welded steel pipe is used to shield portions to be welded with a shielding gas composed of an inert gas during the electric resistance welding, the method comprising blowing the shielding gas to the portions to be welded through a gas outlet of a shielding-gas blowing nozzle which is divided into three layers in the open pipe circumferential direction, the gas outlet being disposed at a position 5 to 300 mm above the upper ends of the portions to be welded, wherein the gas flow rate B at the gas outlet of a center layer among the three layers is set to be 0.5 to 50 m/s, and the gas flow rate A (m/s) at the gas outlet of the remaining two side layers is set so as to satisfy formula (3):
0.01?B/A?10 (3)

4. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 3, wherein the gas outlet is rectangular and has a length of 30 mm or more in a pipe-length direction and a width of 5 mm or more in a direction in which open pipe edges face each other.

5. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 3, wherein the relationship R/W>1.0 is satisfied, wherein R is the total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other, and W is the maximum distance between end surfaces of the portions to be welded directly below the gas outlet.

6. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 3, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

7. A method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 2, wherein a shielding process for portions of an open pipe to be welded for an electric-resistance-welded steel pipe is used to shield portions to be welded with a shielding gas composed of an inert gas during the electric resistance welding, the method comprising blowing the shielding gas to the portions to be welded through a gas outlet of a shielding-gas blowing nozzle which is divided into three layers in the open pipe circumferential direction, the gas outlet being disposed at a position 5 to 300 mm above the upper ends of the portions to be welded, wherein the gas flow rate B at the gas outlet of a center layer among the three layers is set to be 0.5 to 50 m/s, and the gas flow rate A (m/s) at the gas outlet of the remaining two side layers is set so as to satisfy formula (3):
0.01?B/A?10 (3)

8. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 7, wherein the gas outlet is rectangular and has a length of 30 mm or more in a pipe-length direction and a width of 5 mm or more in a direction in which open pipe edges face each other.

9. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 7, wherein the relationship R/W>1.0 is satisfied, wherein R is the total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other, and W is the maximum distance between end surfaces of the portions to be welded directly below the gas outlet.

10. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 4, wherein the relationship R/W>1.0 is satisfied, wherein R is the total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other, and W is the maximum distance between end surfaces of the portions to be welded directly below the gas outlet.

11. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 8, wherein the relationship R/W>1.0 is satisfied, wherein R is the total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other, and W is the maximum distance between end surfaces of the portions to be welded directly below the gas outlet.

12. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 7, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

13. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 4, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

14. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 8, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

15. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 5, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

16. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 9, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

17. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 10, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

18. The method of manufacturing the electric resistance welded stainless clad steel pipe according to claim 11, wherein the inert gas is replaced by a gas containing 0.1% by mass or more of a reducing gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 includes schematic views showing an embodiment of the present invention.

[0029] FIG. 2 includes schematic views showing examples of a nozzle structure which is divided into a plurality of layers.

[0030] FIG. 3 includes views illustrating the gas flow rate B of a shielding gas and the proper range of the gas flow rate ratio B/A.

[0031] FIG. 4 is a graph showing the relationship between the gas flow rate ratio B/A of a shielding gas and the oxygen concentration around portions to be welded (butting edges of an open pipe).

[0032] FIG. 5 is a graph showing the relationship between the flattening ratio h/D in the 90? flattening test of electric resistance welded stainless clad steel pipes and the oxygen concentration around portions to be welded (butting edges of an open pipe).

[0033] FIG. 6 includes schematic views showing cross sections of welds of electric resistance welded stainless clad steel pipes, in which the cladding material on the pipe inner-surface side is stainless steel and the base metal on the pipe outer-surface side is low-alloy steel, produced by electric resistance welding with different amounts of upset.

DETAILED DESCRIPTION OF THE INVENTION

[0034] FIG. 1 includes schematic views showing an embodiment of the present invention. A steel strip is continuously fed from an uncoiler (not shown) and is corrected with a leveler (not shown). While the steel strip is being conveyed in the pipe-length direction 20, the strip is rounded in the width direction into an open pipe 10 by a roll former (not shown). Portions to be welded (11), i.e., butting edges of the open pipe, in which the two end surfaces of the strip rounded in the width direction face each other, are electric resistance welded together by an electric resistance welding machine, which includes a power supply device for heating edges (not shown) and squeeze rolls for pressure welding (not shown), to obtain an electric resistance welded steel pipe 15. Reference sign 12 denotes a heating starting point at the edge of the open pipe, and reference sign 13 denotes a welding point corresponding to a position in the pipe-length direction at which the portions to be welded (11) are welded together by the pressure welding. In some cases, an impeder (not shown) may be disposed on the pipe inner-surface side of the open pipe 10 or the electric resistance welded steel pipe 15. After the electric resistance welded steel pipe 15 has left the electric resistance welding machine, the outer diameter thereof is adjusted by a sizer (not shown). In FIG. 1, reference sign 2 denotes a gas tube, and reference sign 3 denotes a gas controller.

[0035] In embodiments of the present invention, a shielding range is defined in the entire region extending in the pipe-length direction from the heating starting points 12 at the edges of the open pipe to the welding point 13, or in a zone within the region where oxides are likely to be formed on the portions to be welded (this zone can be specified by preliminary study). In the shielding range, a shielding-gas blowing nozzle (abbreviated as nozzle) 1 is disposed at a position directly above the portions to be welded (11).

[0036] The nozzle 1 is disposed such that a gas outlet 1A directly faces the upper ends of the portions to be welded (11).

[0037] In embodiments of the present invention, the nozzle 1 is divided into three layers in the open pipe circumferential direction, as shown in FIG. 1(b) and FIGS. 2(a) and 2(d). These layers constitute gas flow channels which are independent from each other. Furthermore, a center layer 1C among the three layers may be divided into two or more layers in the open pipe circumferential direction 30, as shown in FIGS. 2(b) and 2(c). Side layers 1E are each a single layer.

[0038] In embodiments of the present invention, a shield box that covers the circumference of the open pipe 10 in the shielding range, to which reference is made in the background art, may not be provided. It is preferable not to include a shield box from the viewpoint of the pipe-making efficiency and production cost for electric resistance welded steel pipes, and therefore, the shield box is not provided in this embodiment.

[0039] The flow of the shielding gas has been observed in detail. The influence of various shielding gas blowing conditions, such as the position and size of the gas outlet 1A and the flow rate of the shielding gas at the gas outlet 1A of each of the center layer 1C and the side layers 1E, on the oxygen concentration around the portions to be welded (11) during electric resistance welding and the oxide area fraction in the weld formed by electric resistance welding between the portions to be welded has also been examined in detail.

[0040] As a result, it has been determined, when the shielding gas blowing conditions are optimized, the oxygen concentration around the portions to be welded is 0.01% by mass or less, and the oxide area fraction in the weld is less than 0.1%. The oxide area fraction in the weld is determined as follows: A fracture surface formed by subjecting an electric resistance weld to a Charpy impact test is observed in at least ten fields of view with an electron microscope at a magnification of 500 times or more. Oxide-containing dimple fracture surface areas observed in the fracture surface are selected, and the total area thereof is measured. The ratio of the total area of the oxide-containing dimple fracture surface areas to the total area of the fields of view is defined as the oxide area fraction.

[0041] The optimum conditions which have been determined are as follows: The nozzle height, i.e., the height from the upper ends of the portions to be welded (11) to the gas outlet 1A, is 5 to 300 mm (refer to FIG. 1(c)). The flow rate B of the shielding gas 5 at the gas outlet 1A of the center layer 1C is 0.5 to 50 m/s, and the flow rate A of the shielding gas 5 at the gas outlet 1A of the side layers 15 satisfies the formula 0.01?B/A?10 (refer to FIG. 3).

[0042] When the nozzle height exceeds 300 mm, the shielding gas does not sufficiently reach the portions to be welded (11), and the oxygen concentration around the portions to be welded (11) does not become 100 ppm or less. A smaller nozzle height is desirable. However, when the nozzle height is less than 5 mm, the gas outlet 1A is likely to be damaged by radiation heat from the heated portions to be welded (11), and spatters produced in the portions to be welded (11) strike the nozzle 1, thus deteriorating the durability of the nozzle 1.

[0043] In order to control the flow rate within the range of the above-mentioned optimum conditions, in embodiments of the present invention, a gas controller 3 (refer to FIGS. 1(a) and 1(b)) is used, in which regarding the flow rate of the shielding gas blown through the gas outlet, the gas flow rate B at the gas outlet of the center layer 1C among the three layers is controlled to be 0.5 to 50 m/s, and the gas flow rate A at the gas outlet of the remaining two side layers 1E is controlled so as to satisfy the formula 0.01?B/A?10.

[0044] When the flow rate B is excessively low, the shielding gas spreads out and the portions to be welded (11) are insufficiently shielded with the gas. When the flow rate B is excessively high, the force of the shielding gas increases excessively, resulting in occurrence of air entrainment between the end surfaces of the portions to be welded (11). Therefore, the proper range of the flow rate B is 0.5 to 50 m/s. In the case where the center layer C is further divided into a plurality of layers (e.g., FIGS. 2(b) and 2(c)), the plurality of layers do not necessarily have the same flow rate B, and the flow rate may vary depending on the layer as long as it is within the proper range described above.

[0045] However, even when the flow rate B is maintained within the proper range, if the gas flow rate ratio B/A, i.e., the ratio of the flow rate B to the flow rate A, is inappropriate, it is difficult to prevent air entrainment 6 as shown in FIG. 3.

[0046] That is, in the case where B/A<0.01, the gas flow (flow of the shielding gas 5) from the side layers 15 is excessively strong, and the gas flow from the center layer 1C is excessively weak. Consequently, the gas flow from the side layers 15 is reflected by the outer surface of the open pipe 10 and deflected upward, and the gas flow rate in the reflection region becomes close to zero. Therefore, it is not possible to prevent air entrainment 6 along the outer surface of the open pipe 10 (refer to FIG. 3(a)), and the oxygen concentration around the portions to be welded (11) cannot be decreased sufficiently.

[0047] On the other hand, in the case where B/A>10, the gas flow from the center layer 1C is excessively strong, and the gas flow from the side layers 15 is excessively weak. Consequently, the air is drawn between the end surfaces of the portions to be welded (11) by the gas flow from the center layer 1C, which is likely to cause air entrainment 6 (refer to FIG. 3(c)). Therefore, the oxygen concentration around the portions to be welded (11) cannot be decreased sufficiently.

[0048] In contrast, by setting the ratio B/A to be 0.01 to 10, the shielding gas 5, neither excessively nor insufficiently, fills the space between the end surfaces of the portions to be welded (11) without causing air entrainment, and thus sufficient gas shielding can be achieved (refer to FIG. 3(b)). Regarding the flow rate B in the gas flow rate ratio B/A, in the case where the center layer 1C is divided into a plurality of layers, and at least one layer of the plurality of layers has a gas flow rate different from that of the other layers, the maximum flow rate among different gas flow rates is used.

[0049] FIG. 4 is a graph showing, as an example, the results obtained by measuring the oxygen concentration at the middle position between the end surfaces of the portions to be welded (11) in the case where the shielding gas 5 is blown over the portions to be welded (11) by setting the nozzle height to be 50 mm and varying the gas flow rate ratio B/A in the proper range of the flow rate B of 0.5 to 50 m/s.

[0050] As is evident from FIG. 4, by setting the gas flow rate ratio B/A to be 0.01 to 10 in the proper range of the flow rate B of 0.5 to 50 m/s, it is possible to achieve an oxygen concentration of 0.01% by mass or less with a large margin (i.e., reliably).

[0051] Furthermore, as is evident from FIG. 4, by setting the gas flow rate ratio B/A to be 0.03 to 5, it is possible to achieve a lower oxygen concentration level, i.e., 0.001% to 0.0001% by mass, which is preferable. It has been confirmed that this result can be obtained even when other conditions, such as the nozzle height, are changed. That is, in embodiments of the present invention, it is preferable to set the ratio B/A to be 0.03 to 5 from the viewpoint of achieving an oxygen concentration level of 0.001% to 0.0001% by mass.

[0052] By configuring the gas outlet 1A in which all the layers are combined to be rectangular in shape and to have a length of 30 mm or more in a pipe-length direction 20 and a width of 5 mm or more in a direction in which open pipe edges face each other, the gas can be more uniformly blown over the portions to be welded (11), which is preferable.

[0053] Furthermore, as shown in FIG. 1(c), when the relationship R/W>1.0 is satisfied, where R is the total width of all the layers combined at the gas outlet 1A in the direction in which open pipe edges face each other, and W is the maximum distance between end surfaces of the portions to be welded (11) directly below the gas outlet 1A, the oxygen concentration around the portions to be welded (11) can be more rapidly decreased, which is preferable.

[0054] As the shielding gas, an inert gas is used. As used herein, the term inert gas refers to a gas, such as nitrogen gas, helium gas, argon gas, neon gas, or xenon gas, or a mixture of two or more of these gases.

[0055] Furthermore, as the shielding gas, the inert gas may be replaced by a gas containing 0.1% by mass or more of a reducing gas. The gas containing 0.1% by mass or more of a reducing gas is preferred because it is more effective in suppressing the formation of oxides, which are the cause of penetrators, and it can more greatly improve the toughness or strength of the weld. As used herein, the term reducing gas refers to a gas, such as hydrogen gas, carbon monoxide gas, methane gas, or propane gas, or a mixture of two or more of these gases. Preferably, the gas containing 0.1% by mass or more of a reducing gas is a gas composed of only a reducing gas, or a gas containing 0.1% by mass or more of a reducing gas and the balance being an inert gas.

[0056] From the viewpoint of availability and low cost, the following gases are preferable as the shielding gas to be used:

[0057] (a) In the case where an inert gas is used alone: (G1) any one of nitrogen gas, helium gas, and argon gas, or a mixture of two or more of these gases.

[0058] (b) In the case where a reducing gas is used alone: (G2) one of hydrogen gas and carbon monoxide gas or a mixture of these two gases.

[0059] (c) In the case where a mixture of an inert gas and a reducing gas is used: a mixture of the gases (G1) and (G2).

[0060] In particular, in the case where a gas containing hydrogen gas and/or carbon monoxide gas is used, sufficient safety measures should be taken without omission.

[0061] The amount of upset is determined by measuring the circumference of the pipe immediately before welding, then measuring the circumference of the welded pipe after excess weld metal is removed, and calculating the difference between the two measured values.

[0062] The electric resistance welded steel pipe (electric resistance welded stainless clad steel pipe) 15 according to embodiments of the present invention obtained as described above is made of clad steel including low-carbon low-alloy steel and stainless steel. The flattening characteristic of an electric resistance weld, as-welded, satisfies the formula (1) below, and the electric resistance welded steel pipe has the electric resistance weld having excellent fracture characteristics:

h/D<0.3 (1)

[0063] where h is the flattened height at fracture (mm) and D is the outer diameter of the pipe (mm).

[0064] The flattening characteristics can be measured by a 90? flattening test. A pipe is cut to a length of 300 mm, and then placed such that the weld is located at the position 90? relative to the vertical at the top of the pipe (0?). The flattening test is conducted, and the flattened height at the time when fracture occurs (flattened height at fracture h) is divided by the outer diameter of pipe D to obtain the flattening ratio.

[0065] Furthermore, when the thickness tw (mm) of the electric resistance weld is 0.7?tb (mm) or less, the strength of the electric resistance weld may be deteriorated in some cases. Therefore, the thickness tw is preferably more than 0.7?tb. Furthermore, when the thickness tw (mm) of the electric resistance weld is 1.6?tb or more, a gap may be formed between welds during circumferential welding between pipes to deteriorate corrosion resistance in some cases. Therefore, the thickness tw is preferably less than 1.6?tb. Consequently, the thickness tw preferably satisfies the formula (2) below:

0.7?tb<tw<1.6?tb (2)

[0066] where tb is the thickness of base metal (mm) and tw is the thickness of the weld (mm).

EXAMPLE 1

[0067] Electric resistance welded stainless clad steel pipes with an outer diameter of 300 mm were manufactured by a method in which stainless clad steel sheets including a cladding material made of stainless steel (SUS316, SUS304, SUS310, or SUS429) with a thickness of 2 mm on the pipe inner-surface side and a base metal made of low-carbon low-alloy steel (0.05 mass % C-0.3 mass % Si-1.2 mass % MnFe) with a thickness of 5 mm on the pipe outer-surface side were used as a material, and the stainless clad steel sheets were passed through a pipe-making system including an uncoiler, a leveler, a roll former, an electric resistance welding machine, and a sizer arranged in this order. In the manufacturing process, during electric resistance welding, gas shielding for portions to be welded was performed by varying level of the gas blowing conditions and the amount of upset within or outside the ranges of embodiments of the present invention described in the embodiment as shown in Tables 1 and 2. The thickness tb (mm) of the base metal and the thickness tw (mm) of the electric resistance weld were measured. The oxygen concentration around portions to be welded was measured. A 90? flattening test was conducted on the weld, and a corrosion test by oxalic acid etching was conducted on the inner-surface side of the pipe. As the reducing gas, 3% by mass hydrogen gas was used. The amount of upset due to squeeze rolls was determined by measuring the circumference of the pipe before being subjected to squeeze rolls, then measuring the circumference of the pipe after welding was performed with squeeze rolls and molten beads on the outer surface were cut away, and calculating the difference between the two measured values. The thickness of the base metal was determined by measuring the thickness of the pipe at a pitch of 60? in the circumferential direction of the pipe, relative to the weld (0?), i.e., at positions of 60?, 120?, 180?, 240?, and 300?, and calculating the average value thereof. The thickness of the weld was determined by measuring the thickness of the weld seam portion.

[0068] In the corrosion test, the pipe in which intergranular corrosion was not observed was evaluated as pass, and the pipe in which intergranular corrosion was observed was evaluated as rejection.

[0069] As shown in Tables 1 and 2, it has been confirmed that, in the inventive examples, the flattening ratio h/D (h: flattened height at fracture (mm), D: outer diameter of pipe (mm)) of the weld is significantly low compared with the comparative examples, the fracture characteristics are excellent, the weld has a good shape, and the weld maintains corrosion resistance of the stainless steel.

TABLE-US-00001 TABLE 1 Oxygen concentration Gas around Results Gas flow portions of Thick- Thick- Results of flow rate Reducing to be flattening ness ness corrosion Nozzle rate B ratio gas Amount welded test on of base of test on Cladding height (m/s) B/A R/W content of upset (mass weld metal tb weld tw pipe inner Level material (mm) *1 *1 *2 (mass %) (mm) % ? 10.sup.2) h/D (mm) (mm) tw/tb surface Remarks 1 SUS316 100 0.5 0.5 5.0 None 4 0.03 0.2 7.0 7.5 1.1 Pass Inventive example 2 SUS316 100 1.0 0.5 5.0 None 4 0.04 0.1 7.0 7.5 1.1 Pass Inventive example 3 SUS316 100 5.0 0.5 5.0 None 4 0.02 0.1 7.0 7.5 1.1 Pass Inventive example 4 SUS316 100 10.0 0.1 5.0 None 4 0.04 0.1 7.0 7.5 1.1 Pass Inventive example 5 SUS316 100 50.0 0.03 5.0 None 4 0.06 0.1 7.0 7.5 1.1 Pass Inventive example 6 SUS316 200 0.5 3 5.0 None 4 0.08 0.1 7.0 7.5 1.1 Pass Inventive example 7 SUS316 200 1.0 2 5.0 None 4 0.09 0.1 7.0 7.5 1.1 Pass Inventive example 8 SUS316 200 5.0 10 5.0 None 4 0.3 0.2 7.0 7.5 1.1 Pass Inventive example 9 SUS316 200 10.0 0.01 5.0 None 4 0.2 0.2 7.0 7.5 1.1 Pass Inventive example 10 SUS316 200 50.0 0.05 5.0 None 4 0.1 0.2 7.0 7.5 1.1 Pass Inventive example 11 SUS316 50 0.5 0.5 5.0 None 4 0.1 0.2 7.0 7.5 1.1 Pass Inventive example 12 SUS316 50 1.0 3 5.0 None 4 0.07 0.2 7.0 7.5 1.1 Pass Inventive example 13 SUS316 50 5.0 0.2 5.0 None 4 0.04 0.1 7.0 7.5 1.1 Pass Inventive example 14 SUS316 50 10.0 0.3 5.0 None 4 0.05 0.1 7.0 7.5 1.1 Pass Inventive example 15 SUS316 50 50.0 0.5 5.0 None 4 0.1 0.2 7.0 7.5 1.1 Pass Inventive example 16 SUS316 50 10.0 0.5 10.0 None 2 <0.01 0.1 7.0 6.4 0.9 Pass Inventive example 17 SUS316 50 10.0 2 2.0 None 2 0.03 0.1 7.0 6.2 0.9 Pass Inventive example 18 SUS316 50 10.0 2 1.0 None 2 0.05 0.1 7.0 6.0 0.9 Pass Inventive example 19 SUS316 50 10.0 0.5 20.0 None 7 <0.01 0.1 6.5 7.5 1.2 Pass Inventive example 20 SUS316 50 10.0 0.5 5.0 None 7 <0.01 0.1 6.5 9.7 1.5 Pass Inventive example 21 SUS316 300 10.0 0.2 5.0 None 1 0.2 0.2 7.0 5.5 0.8 Pass Inventive example 22 SUS316 50 10.0 0.03 5.0 0.1 1 <0.01 0.1 7.0 5.5 0.8 Pass Inventive example 23 SUS316 50 10.0 0.5 5.0 3.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 24 SUS316 50 10.0 1 5.0 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 25 SUS316 50 5.0 3 1.0 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 26 SUS316 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 27 SUS304 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 28 SUS304 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 29 SUS310 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 30 SUS429 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example 31 SUS429 50 5.0 5 0.8 5.0 3 <0.01 0.1 7.0 7.1 1.0 Pass Inventive example *1: Gas flow rate B; Gas flow rate B at the gas outlet of the center layer among three layers. Gas flow rate A; Gas flow rate A at the gas outlet of the remaining two side layers among three layers. *2: R; Total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other. W; Maximum distance between end surfaces of portions to be welded directly below the gas outlet.

TABLE-US-00002 TABLE 2 Oxygen concen- tration Gas around Results Thick- Thick- Gas flow portions of ness ness Results of flow rate Reducing to be flattening of base of corrosion Nozzle rate B ratio gas Amount welded test on metal weld test on Cladding height (m/s) B/A R/W content of upset (mass weld tb tw pipe inner Level material (mm) *1 *1 *2 (mass %) (mm) % ? 10.sup.2) h/D (mm) (mm) tw/tb surface Remarks 32 SUS316 400 1.0 1 5.0 None 4 15 0.5 7.0 7.5 1.1 Pass Comparative example 33 SUS316 100 0.4 1 5.0 None 4 160 0.8 7.0 7.5 1.1 Pass Comparative example 34 SUS316 100 60.0 1 5.0 None 4 3 0.4 7.0 7.5 1.1 Pass Comparative example 35 SUS316 400 60.0 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example 36 SUS316 100 60.0 1 5.0 5.0 4 2 0.4 7.0 4.5 0.6 Pass Comparative example 37 SUS316 400 1.0 1 5.0 None 10 12 0.4 7.0 7.5 1.1 Rejection Comparative example 38 SUS304 400 60 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example 39 SUS304 400 60 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example 40 SUS310 400 60 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example 41 SUS429 400 60 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example 42 SUS429 400 60 1 5.0 None 4 40 0.6 7.0 7.5 1.1 Pass Comparative example *1: Gas flow rate B; Gas flow rate B at the gas outlet of the center layer among three layers. Gas flow rate A; Gas flow rate A at the gas outlet of the remaining two side layers among three layers. *2: R; Total width of all the layers combined at the gas outlet in the direction in which open pipe edges face each other. W; Maximum distance between end surfaces of portions to be welded directly below the gas outlet.

REFERENCE SIGNS LIST

[0070] 1 nozzle (shielding-gas blowing nozzle)

[0071] 1A gas outlet

[0072] 1C center layer

[0073] 1E side layer

[0074] 2 gas tube

[0075] 3 gas controller

[0076] 5 shielding gas

[0077] 6 air entrainment

[0078] 10 open pipe

[0079] 11 portion to be welded (butting edge of open pipe)

[0080] 12 heating starting point at edge of open pipe

[0081] 13 welding paint

[0082] 15 electric resistance welded steel pipe

[0083] 20 pipe-length direction

[0084] 30 open pipe circumferential direction