Butt welding method

Abstract

Provided is a butt welding method capable of increasing the strength of a weld metal, and preventing a fracture in a weld when the plate is passed through a processing line, even if the weld metal is soft like the weld formed when a high Ni-containing austenitic or a low-carbon ferritic filler wire is used. The rear end of the preceding steel plate and the tip end of the succeeding steel plate are butted on the inlet side of the processing line. When the solidified weld is pressurized at the butted part, a pressurizing force at the time of the pressurization is controlled so as to satisfy the relationship of 0 ≤ Hw-H1 < 100, where Hw is the hardness of the weld after the pressurization, and H1 is the base material hardness of the lower in hardness of the butted steel plates.

Description

Butt welding method

[0001] The present invention relates to a butt welding method for welding steel sheets together by laser welding the rear end of a preceding steel sheet and the front end of a succeeding steel sheet at the entry side of the processing line when the steel sheets are passed through various processing lines.

[0002] In recent years, in the steel industry, in hot rolling facilities, pickling facilities, cold rolling facilities, annealing facilities and the like that manufacture steel sheets, a form in which a coiled steel sheet is continuously processed while being uncoiled has become mainstream.

[0003] In this way, in a manufacturing line that continuously processes steel plates, in order to pass the steel plates to be processed through the line without interruption, the rear end of the preceding steel plate and the front end of the following steel plate are butt-welded together at the entry side of the line.

[0004] Flash butt welding and laser welding are generally used to weld such steel plates together, but laser welding is often used particularly for welding hard steel plates with poor toughness, such as electrical steel plates and high-strength steel plates.

[0005] However, in such laser welding, the welded portion is rapidly cooled after welding, and therefore hardens and does not ensure sufficient toughness. In particular, in steel plates containing large amounts of C and Si, the embrittlement of the welded portion becomes more pronounced as the content of these elements increases.

[0006] Therefore, laser welds of steel sheets containing large amounts of C and Si are prone to fracture during passage through a continuous processing line. If a steel sheet fractures during passage through a processing line, it takes a long time to recover, resulting in a significant drop in operating rate and productivity.

[0007] Furthermore, in rolling mills and the like, rolls are damaged and must be replaced, which not only reduces productivity but also increases maintenance costs.

[0008] Therefore, in the past, filler wires with low alloy contents have been used for welding steel plates with high alloy contents, such as low-carbon ferritic or high-Ni austenitic filler wires, such as YGW12Y (C: approximately 0.08 mass%, Si: approximately 0.5 mass%) specified in JIS Z 3312 (2009) or SNI6625 (Si: approximately 0.29 mass%, Ni: approximately 61.3 mass%) specified in JIS Z 3334 (2017).

[0009] That is, by diluting the C and Si concentrations in the weld metal with the filler wire, it is possible to prevent the formation of a coarse ferrite structure or a brittle martensite structure, thereby preventing the weld metal from becoming embrittled.

[0010] However, when the above-mentioned high-Ni austenitic or low-carbon ferritic filler wire is used, the weld metal tends to be softer than the steel sheet, and deformation concentrates on the weld metal when the sheet is passed through a continuous processing line, causing fracture at the weld.

[0011] Therefore, Patent Document 1 proposes a technique for obtaining a welded joint with excellent toughness of the weld metal by applying pressure to the excess weld bead after welding when butt welding is performed while supplying a filler wire, and controlling the structure of the weld metal that is formed.

[0012] Furthermore, Patent Document 2 proposes a technique in which a pressure device such as a press is used to apply pressure to a weld bead after welding.

[0013] Patent No. 4486639 Patent No. 2948843

[0014] However, the technology described in Patent Document 1 does not involve laser welding. １ -Ac ３This technology refines the austenite grain size, promotes the transformation into ferrite and pearlite, and reduces brittle martensite by applying pressure within this temperature range. Therefore, it cannot be applied to strengthening weld metals that do not undergo phase transformation, such as single-phase ferrite or single-phase austenite. In addition, even in weld metals that undergo phase transformation, if there is a concentration distribution of the chemical composition within the weld metal, the above-mentioned transformations have the problem that the strength of the entire weld metal cannot necessarily be increased.

[0015] Furthermore, the technology described in Patent Document 2 is a technology for removing irregularities in the weld bead, alleviating stress concentration at the weld, and preventing fracture at the weld when the sheet is passed through a continuous processing line. However, fracture may occur at the weld when there is a difference in strength between the weld and the base metal, such as in welds formed when using austenitic or low-carbon ferritic filler wire with a high Ni content.

[0016] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a butt welding method that can increase the strength of the weld metal and prevent fracture at the weld when the sheet is passed through a continuous processing line, even when the weld metal is soft, such as a weld formed when a high-Ni austenitic or low-carbon ferritic filler wire is used.

[0017] The inventors conducted extensive research to achieve the above object, and as a result, found that by controlling the pressure applied to the welded portion with rolls after the welded portion has solidified so that a predetermined relationship is satisfied between the hardness of the welded portion after pressing and the hardness of the lower base metal of the butted steel sheets, the strength of the weld metal can be appropriately increased and fracture of the welded portion in a continuous processing line can be prevented.

[0018] In addition, it was also discovered that by appropriately pressurizing the weld bead reinforcement in accordance with the wire feed rate relative to the gap at the butt joint between the steel sheets, the strength of the weld metal can be increased and fracture of the weld can be more effectively prevented in a continuous processing line. Here, the wire feed rate is the ratio b / a of the volume b of the filler wire supplied per unit weld length to the volume a per unit weld length in the gap at the butt joint between the steel sheets. Hereinafter, the gap at the butt joint between the steel sheets will be referred to as the "butt gap." Furthermore, the butt gap G refers to the butt gap length, i.e., the distance between the rear end of the preceding steel sheet and the front end of the following steel sheet.

[0019] The inventors believe the mechanism by which the weld metal is strengthened as described above is as follows. Specifically, by applying pressure to the weld metal formed by laser welding under appropriate conditions, processing strain is introduced into the pressurized portion, thereby increasing the strength of the weld metal. Regarding the strength difference between the weld metal and the steel plate, when the butt gap is small, the dilution rate of the weld metal with the base steel plate (the proportion of base metal components in the weld metal) is high, and the composition of the weld metal is similar to that of the steel plate, so the strength difference is thought to be small. On the other hand, when the butt gap is large, the dilution rate of the weld metal with the base steel plate is low, and the composition of the weld metal is similar to that of the filler wire, so the strength difference is thought to be large. Therefore, to eliminate the strength difference, the larger the butt gap, the more pressing force is required to apply. Furthermore, to eliminate the strength difference, the higher the wire feed rate, the larger the volume of the pressurized portion, so it is necessary to apply a high pressing force to apply a large processing strain.

[0020] The present invention is based on the above findings and has the following gist: 1. A butt welding method in which, when a plurality of steel sheets are continuously passed through a processing line, the rear end of a preceding steel sheet and the front end of a succeeding steel sheet are butted together at the entry side of the processing line, a filler wire is supplied to the butted location while irradiating with a laser, the filler wire is melted and solidified to form a weld, and the solidified weld is pressurized, wherein the pressure during the pressurization is set to a value that makes the hardness of the weld after pressurization H ｗThe hardness of the base material of the butted steel plates with the lower hardness is H １ When 0≦H ｗ -H １ A butt welding method in which the welding is controlled to satisfy the relationship of <100.

[0021] 2. The butt welding method according to item 1, wherein the applied pressure P [kN] satisfies any one of the following formulas (1) to (3) depending on the butt gap G [mm] between the butted parts and the wire feed rate X for the gap between the butted parts: if 0≦G<0.20, 1.1X(1.5G+2.1)≦P<10X(2G+0.5) ... (1); if 0.20≦G<0.50, 1.2X(2G+1.8)≦P<5X(3G+1.2) ... (2); if 0.50≦G, 1.3X(3G+1.1)≦P<1.2X(3.5G+9.5) ... (3).

[0022] 3. The butt welding method according to paragraph 1 or 2, wherein the applied pressure P [kN] satisfies any one of the following formulas (4) to (6) depending on the butt gap G [mm] between the butted parts and the wire feed rate X for the gap between the butted parts: if 0≦G<0.20, (1+X / 30)×(1.5G+2.5)≦P<(1+X / 30)×(15G+18)...(4); if 0.20≦G<0.50, (1+X / 20)×(2G+2.3)≦P<(1+X / 20)×(10G+18)...(5); if 0.50≦G, (1+X / 10)×(2.5G+1.8)≦P<(1+X / 10)×(9G+17)...(6).

[0023] 4. The butt welding method according to any one of 1, 2 and 3 above, wherein the filler wire is a ferritic or austenitic filler wire, or both.

[0024] According to the present invention, it is possible to provide a butt welding method that can increase the strength of the weld metal even when the weld metal is soft, and can prevent fracture at the welded portion when the sheet is passed through a continuous processing line.

[0025] 1 is a schematic diagram showing a laser welding method and a pressurizing method, and FIG. 2 is a schematic diagram showing hardness measurement points in hardness evaluation of a welded portion.

[0026] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0027] The present invention relates to a technology for producing a butt-welded component, in which, when steel sheets are passed through a continuous processing line, as shown in FIG. 1 , the rear end of a preceding steel sheet 1 and the front end of a following steel sheet 2 are butted together at the line entry side, a filler wire 4 is supplied to the butt joint 3, and a laser 5 is irradiated to melt and weld the steel sheets 1, 2 and filler wire 4 at the butt joint 3, and a solidified weld bead 6 formed after welding is pressed.

[0028] First, a description will be given of a welding apparatus and the like used in carrying out the present invention. The type of laser used in the welding of the present invention is not particularly limited, and may be a fiber laser, a disk laser, a semiconductor laser, or a CO ２ A laser or the like may be used. The composition and diameter of the filler wire are not particularly limited, but from the viewpoint of preventing embrittlement of the weld metal constituting the weld bead 6, for example, the composition is preferably ferritic with a low content of C and Si or austenitic with a high content of Ni. Furthermore, the type and installation method of the shielding gas are not particularly limited, but CO ２ In the case of a laser, it is preferable to use He gas because He gas has a higher ionization voltage than Ar gas and is less likely to generate plasma.

[0029] In addition, the means for applying pressure to the welded portion made of the weld metal is not particularly limited as long as it satisfies the conditions described below. As an example, it is preferable to arrange rolls for flattening the welded portion (hereinafter also referred to as flattening rolls) such as pressure rolls 7 shown in Fig. 1 so as to face the front and back surfaces of the welded portion, and to apply pressure to the weld bead from the top and bottom surfaces of the steel sheet while rotating the flattening rolls in the welding direction.

[0030] The steel sheet used in the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, but preferably contains 0.5 mass% or more of Si. In order to more advantageously obtain the effects of the present invention, the steel sheet should contain 2.5 mass% or more of Si, at which point fracture at the weld becomes significant. On the other hand, the upper limit of the Si content is not particularly limited, but from the viewpoint of ensuring cold rolling properties, it is preferably 4.5 mass% or less.

[0031] The steel sheet used in the present invention may contain 0.010 mass% or more of C. As the C content increases, fracture at the weld becomes more pronounced, so the C content is more preferably 0.100 mass% or more. There is no particular upper limit for the C content, but it is preferably 0.500 mass% or less. Here, either one of Si and C may be contained, or both may be contained.

[0032] The other components (including unavoidable impurities) of the steel sheet used in the present invention are not particularly limited, and any of the components or amounts thereof of the steel sheet passed through the continuous processing line described below can be suitably used.

[0033] Furthermore, the steel plate used in the present invention preferably has a thickness in the range of 0.8 to 6.0 mm. If the plate thickness is less than 0.8 mm, the amount of steel plate to be melted is small, which may cause the weld to melt through and prevent the formation of a weld bead. On the other hand, if the plate thickness exceeds 6.0 mm, in order to melt the steel plate to the back surface, it is necessary to increase the laser output or reduce the welding speed to increase the heat input, which may reduce the toughness of the heat-affected zone (HAZ).

[0034] The processing line in the present invention refers to a line that continuously processes the above-mentioned steel sheets. That is, the present invention is preferably applied to a continuous processing line in which the rear end of a preceding steel sheet is laser welded to the front end of a succeeding steel sheet at the line entry side of a hot rolling facility, a pickling facility, a cold rolling facility, a cleaning facility, a continuous annealing facility, a temper rolling facility, a fine-tuning facility, or a continuous processing line in which such welded steel sheets are passed through. Note that the continuous processing line in the present invention includes a processing line in which the rear end of a preceding steel sheet is laser welded to the front end of a succeeding steel sheet at the line entry side and such welded steel sheets are passed through, and also includes a case in which an operator manually installs steel sheets one by one.

[0035] The welding method of the present invention includes a step of performing butt welding by laser welding and a step of applying pressure to the welded portion formed by the laser welding.

[0036] Here, as described above, when a low-alloy ferritic or austenitic filler wire with a high Ni content is supplied, the weld metal becomes soft, and it is thought that the strength of the weld metal becomes excessively low compared to the steel plate.

[0037] The strength of such weld metal is thought to be significantly affected by the dilution rate of the base metal, and the strength of the weld metal decreases as the butt gap increases and the wire feed rate increases. Therefore, in the present invention, it is necessary to improve the strength of the weld metal to be at least as strong as that of the steel plate by adjusting the pressing force in accordance with the butt gap and the wire feed rate.

[0038] That is, in order to improve the strength of the weld metal, the pressure is set to H ｗ The hardness of the base material of the butted steel plates with the lower hardness is H １ When 0≦H ｗ -H １ It is necessary to control (adjust) it so that the relationship of <100 is satisfied.

[0039] More specifically, in the butt welding method of the present invention, when the butt gap is G [mm], the wire feed rate is X, and the pressing force when pressing the weld is P [kN], the pressing force P may satisfy any one of the following formulas (1) to (3) depending on the butt gap G.

[0040] When 0≦G<0.20, 1.1X(1.5G+2.1)≦P<10X(2G+0.5) ...(1) When 0.20≦G<0.50, 1.2X(2G+1.8)≦P<5X(3G+1.2) ...(2) When 0.50≦G, 1.3X(3G+1.1)≦P<1.2X(3.5G+9.5) ...(3)

[0041] Alternatively, any of the following formulas (4) to (6) may be satisfied: If 0≦G<0.20, (1+X / 30)×(1.5G+2.5)≦P<(1+X / 30)×(15G+18)...(4) If 0.20≦G<0.50, (1+X / 20)×(2G+2.3)≦P<(1+X / 20)×(10G+18)...(5) If 0.50≦G, (1+X / 10)×(2.5G+1.8)≦P<(1+X / 10)×(9G+17)...(6)

[0042] Here, for example, when G = 0.10, either one of formula (1) and formula (4) may be satisfied, or both formulas (1) and (4) may be satisfied. Similarly, when 0.20 ≦ G < 0.50, either one of formula (2) and formula (5) may be satisfied, or both formulas (2) and (5) may be satisfied. Furthermore, when 0.50 ≦ G, either one of formula (3) and formula (6) may be satisfied, or both formulas (3) and (6) may be satisfied.

[0043] Here, the pressure P in the present invention is the force applied to the excess reinforcement of the weld bead by, for example, the roll 7 shown in FIG. 1, and is a value measured by a load cell or the like attached to the roll 7 of the pressure device.

[0044] When the pressure P is below the lower limit of the formulas (1) to (3), the welded portion is insufficiently hardened by the pressure applied, whereas when the pressure P is above the upper limit of the formulas (1) to (3), the welded portion is excessively hardened.

[0045] Furthermore, when the pressure P is below the lower limit of the formulas (4) to (6), the hardening of the welded portion due to the application of pressure is insufficient, whereas when the pressure P is above the upper limit of the formulas (4) to (6), the welded portion is excessively hardened.

[0046] In the formulas (1) to (3) and the formulas (4) to (6), for example, if the base metal is ferritic steel and the wire is a nickel alloy, the structure of the weld metal tends to be a single α-phase structure when 0≦G<0.20, a structure of α or γ phase or a mixture of both phases depending on the welding conditions when 0.20≦G<0.50, and a single γ-phase structure when 0.50≦G.

[0047] Here, the aforementioned butt gap G is the size of the gap that occurs when the rear end of the preceding steel sheet and the front end of the following steel sheet are butted together, and is a value measured by a sensor or the like before welding. A laser displacement meter, for example, can be used as the sensor. As the butt gap G increases, the base metal dilution in the weld metal decreases, and the filler wire components become the main component. Therefore, the strength of the weld metal becomes lower than that of the steel sheets.

[0048] That is, as the butt gap G increases, the pressing force P must be increased, and the processing strain introduced into the weld metal must be increased. Furthermore, as the wire feed rate X increases, both the width and height of the weld bead increase, which increases the volume of the pressurized portion and requires a larger pressing force. Note that, since burn-through is more likely to occur if the butt gap G [mm] exceeds 2.00, it is preferable that the pressing force P [kN] be between 0 and 2.00. Furthermore, it is preferable that the pressing force P [kN] be between 2 and 20.

[0049] As described above, the wire feed rate X is the ratio b / a of the volume b of the filler wire fed per unit weld length to the volume a per unit weld length in the gap between the butted steel sheets. Therefore, X can be calculated from the geometric relationship of the amount of weld metal required to fill the butt gap, where F is the filler wire feed rate [mm / min], A is the cross-sectional area of ​​the filler wire [mm ２], W is the welding speed [mm / min], G is the butt gap length [mm], and T is the average thickness of the pair of steel plates [mm], then X is calculated using the following formula (7): X = (F × A) / (W × G × T) (7)

[0050] In order to apply pressure to the weld bead, it is desirable that the thickness of the weld bead is at least greater than the thicker of the steel plate thicknesses (this is also referred to as having excess weld in the present invention). In butt welding, such a condition (having excess weld) is usually adopted. Specifically, the average thickness of the weld bead before pressure is t ｗ [mm], the thickness of the thicker of the butted plates is t １ When [mm], t ｗ >t １ In order to effectively carry out the present invention, it is desirable that the above conditions be satisfied. Moreover, since excessive pressure reduces the thickness of the base metal, it is preferable that the average thickness of the weld after pressure is equal to or greater than the thickness of the thicker of the butted plates.

[0051] Furthermore, in actual welding, the molten metal is scattered from the weld pool due to spattering, plumes, etc., so the wire feed rate X becomes less than the value required by the above-mentioned geometric relationship. Taking these factors into consideration, X is preferably 1.1 or more. On the other hand, although there is no particular upper limit for X, it is preferably less than 5.0 because the cross-sectional shape of the weld becomes asymmetrical above and below the center of the plate thickness.

[0052] Furthermore, in the present invention, the application of pressure after laser welding must be performed while the weld is solidified. If the weld is at or above the recrystallization temperature before solidification, dislocations are less likely to accumulate due to processing, and work hardening ability decreases. Therefore, the time from the end of laser irradiation to the start of pressure application is not particularly limited, but is preferably 0.1 seconds or more.

[0053] In the butt welding method according to the present invention, any items not described in this specification can be performed by conventional methods.

[0054] Examples of the present invention are described below. For each of the steel plates shown in Table 1, two of each steel plate were butt-welded to each other using a laser welder and a pressure device installed at the entrance of a line for continuously processing the steel plates. That is, butt welding was performed under the welding and pressure conditions shown in Table 1, and each steel plate was made into a butt-welded member.

[0055] The laser welding used a disk laser with a wavelength of 1030 nm, and the filler wire used was WEL MIG625 (WEL: registered trademark) with a wire diameter of 0.9 mm manufactured by Nippon Welding Rod Co., Ltd. The pressure device had flattening rolls arranged opposite the front and back surfaces of the weld, and the weld formed by the laser welding was pressurized while the flattening rolls were rotated in the welding direction.

[0056] The butt gap G was determined by measuring the gap between the rear end of the preceding steel sheet and the front end of the following steel sheet using a laser displacement meter before welding, and averaging the measurements within a range excluding a region 20 mm from each end of the steel sheet in the width direction toward the center. Specifically, the average value was determined from three points: a position 20 mm from each end of the steel sheet in the width direction toward the center and a position 20 mm from the center in the weld length direction. Note that the reason for this measurement being within a range excluding 20 mm from each end of the steel sheet in the width direction is that the regions 20 mm from each end of the steel sheet in the width direction correspond to the start and end of the weld after welding, and these start and end portions are unsteady portions that may be removed in the production line.

[0057] The average thickness of the weld before pressure is t ｗ The thickness of the weld was measured using the laser displacement meter and averaged over a range excluding a region 20 mm from the start and end of the weld toward the center. The average was taken as the average of three values: 20 mm from each end of the steel plate in the width direction and the center of the weld length.

[0058] The wire feed rate X was calculated according to the above-mentioned formula (7). That is, the filler wire feed rate F and the welding speed W were set values, the cross-sectional area A of the filler wire was calculated from the wire diameter measured with a vernier caliper or the like, and the butt gap G and the average thickness T of the pair of steel sheets were actual measured values, and the calculation was performed according to formula (7).

[0059] The pressure in the present invention is a value measured by a load cell or the like attached to the roll 7 shown in Figure 1, and specifically, it is the average of values ​​measured at three points: positions 20 mm from each end of the steel sheet in the width direction and a center position in the weld length direction. The welds obtained as described above were evaluated as follows.

[0060] <Evaluation of hardness of welded portion> As shown in Figure 2, the specimen was cut in a direction perpendicular to the welding direction, and the maximum welded thickness t ｗ１ Based on this, 1 / 8t ｗ１ , 1 / 2 t ｗ１ and 7 / 8t ｗ１ The Vickers hardness in the weld metal at each position is HV0.2 (10), and the average of all the measured values ​​is the hardness H of the weld after pressure application. ｗ Regarding the base material hardness, at a position 10 mm away from the boundary between the base material and the weld metal, when the steel plate thickness is T, Vickers hardness was measured at each position of 1 / 8T, 1 / 2T, and 7 / 8T in the plate thickness direction, HV0.2 (10) as one point each, and the average of the three points was taken, and the lower value of the butted steel plates was taken as H １ And then, H ｗ -H １ The hardness Hs of the welded portion before pressurization was determined by measuring HV0.2(10) at each position in the same manner as above for welded portions produced under the same welding conditions.

[0061] <Plate passing test> The weld beads were passed through a cold rolling mill at a rolling tension of 150 MPa and a reduction rate relative to the steel plate thickness of 50%, and those that did not fracture at the weld metal were rated as pass (◯), and those that fractured at the weld metal were rated as fail (×).

[0062] The above evaluation results are shown in Table 1, which also lists the welding conditions.

[0063]

[0064] From these results, the butt-welded member (invention example) in which the pressure was set to satisfy the requirements of the present invention was passed (◯) in all of the above evaluations.

[0065] 1 preceding steel sheet 2 following steel sheet 3 butt joint (butt gap) 4 filler wire 5 laser 6 weld bead 7 pressure roll

Claims

1. A butt welding method in which, when multiple steel sheets are continuously passed through a processing line, the rear end of a preceding steel sheet and the front end of a following steel sheet are butted together at the entry side of the processing line, a filler wire is supplied to the butted part while irradiating it with a laser, the filler wire is melted and solidified to weld, and the solidified welded part is pressurized. The pressing force during the pressing is set to a value that indicates the hardness of the welded part after pressing, and is expressed as H. ｗ The hardness of the base material of the butted steel plates with the lower hardness is H １ When 0≦H ｗ -H １ A butt welding method controlled to satisfy the relationship:

2. The butt welding method according to claim 1, wherein the applied pressure P [kN] satisfies any one of the following formulas (1) to (3) depending on the butt gap G [mm] at the butted parts and the wire feed rate X for the gap at the butted parts: if 0≦G<0.20, 1.1X(1.5G+2.1)≦P<10X(2G+0.5) ... (1) if 0.20≦G<0.50, 1.2X(2G+1.8)≦P<5X(3G+1.2) ... (2) if 0.50≦G, 1.3X(3G+1.1)≦P<1.2X(3.5G+9.5) ... (3) 3. The butt welding method according to claim 1 or 2, wherein the applied pressure P [kN] satisfies any one of the following formulas (4) to (6) depending on the butt gap G [mm] at the butted parts and the wire feed rate X for the gap at the butted parts: if 0≦G<0.20, (1+X / 30)×(1.5G+2.5)≦P<(1+X / 30)×(15G+18) ... (4) if 0.20≦G<0.50, (1+X / 20)×(2G+2.3)≦P<(1+X / 20)×(10G+18) ... (5) if 0.50≦G, (1+X / 10)×(2.5G+1.8)≦P<(1+X / 10)×(9G+17) ... (6) 4. A butt welding method according to claim 1, 2 or 3, wherein either or both of ferritic and austenitic filler wires are used as the filler wire.

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