Butt welding method

Abstract

Provided is a butt welding method capable of increasing the strength of a weld metal and preventing breakage in a weld portion when passing through a continuous treatment line, even if the weld metal is soft like a weld portion formed when a high Ni-containing austenite type or a low-carbon ferrite type filler wire is used. When the rear end of a preceding steel plate and the front end of a following steel plate are butted on the inlet side of the treatment line and the solidified weld portion is pressurized at the butted location, an excess weld height h [mm] of the weld portion satisfies h (mm) > 0, and the draft at the time of pressurization is controlled so as to satisfy the relationship of 0≤Hw-I1<100, where Hw is the hardness of the weld portion after pressurization and H1 is the base material hardness of the steel plate having the lower hardness among 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 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, discovered that by applying pressure to the weld bead at an appropriate reduction rate depending on the gap at the butt joint between the steel sheets (hereinafter referred to as the "butt gap") and the thickness of the weld bead after welding (hereinafter referred to as the "reinforcement height"), it is possible to increase the strength of the weld metal and prevent fracture of the weld in a continuous processing line.

[0018] The inventors consider the mechanism by which the weld metal is strengthened 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 difference in strength 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 close to the composition 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 close to the composition of the filler wire, so the strength difference is thought to be large. Therefore, in order to eliminate the strength difference, the larger the butt gap, the higher the reduction rate required for pressurization.

[0019] Furthermore, the greater the reinforcement height, the more gently convex the cross-sectional shape of the weld bead becomes, and the greater the volume of pressure applied at the same reduction rate compared to when the reinforcement height is small. As a result, the processing strain introduced into the pressurized area becomes larger. On the other hand, when the reinforcement height is small, a higher reduction rate is required to achieve the desired 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 plates are continuously passed through a processing line, the rear end of a preceding steel plate and the front end of a following steel plate 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 reinforcement height h [mm] of the weld formed by the laser irradiation satisfies h > 0, and the reduction rate during the pressurization is set to H, and the hardness of the weld after pressurization 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 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 reduction rate R [%] during the application of pressure satisfies any one of the following formulas (1) to (6) depending on the butt gap G [mm] at the butted portion and the weld metal height h [mm]: For weld fill height 0<h≦0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(1.0G+0.5) / (0.1h) ...(1) For 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(0.7G+1.3) / (0.2h) ...(2) For 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(0.5G+3.0) / (0.4h) ...(3) For weld fill height h>0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(40G+220) / (10+h) ...(4) If 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(20G+330) / (15+h) ... (5) If 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(10G+450) / (20+h) ... (6)

[0022] 3. The butt welding method according to 1 or 2 above, wherein either or both of ferritic and austenitic filler wires are used as the filler wire.

[0023] 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 plate is passed through a continuous processing line.

[0024] The present invention relates to a method for welding a welded portion, a method for welding a welded portion, and a method for welding a welded portion.

[0025] 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.

[0026] 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 weld is formed by pressing a solidified weld bead reinforcement 6 formed after welding.

[0027] 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 reinforcement 6, the composition is preferably a ferritic type with a low content of C and Si or an austenitic type 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.

[0028] In addition, the means for applying pressure to the welded portion formed by the weld bead 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 Figure 1 so as to face the front and back surfaces of the welded portion, and to apply pressure to the weld bead reinforcement 6 from the top and bottom surfaces of the steel sheet while rotating the flattening rolls in the welding direction.

[0029] 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.

[0030] 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 Si or C may be contained, or both may be contained.

[0031] 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.

[0032] 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).

[0033] 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.

[0034] The welding method of the present invention includes the steps of performing butt welding by laser welding, monitoring the height of the weld bead formed by the laser welding, and applying pressure to the weld formed after the laser welding in accordance with the height of the weld bead.

[0035] 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.

[0036] The strength of such weld metal is significantly affected by the dilution rate of the base metal. The larger the butt gap, the smaller the base metal dilution rate, and the closer the weld zone to the filler wire's composition, which is thought to result in a lower weld metal strength. Furthermore, the greater the reinforcement height, the more gently convex the cross-sectional shape of the weld bead becomes. This means that, compared to when the reinforcement height is small, the volume of the weld bead that is pressurized is larger even with the same reduction, and the processing strain introduced into the pressurized zone is also thought to be greater. 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 reduction rate according to the butt gap and reinforcement height.

[0037] That is, in order to improve the strength of the weld metal, the reduction ratio 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.

[0038] In the butt welding method of the present invention, when the butt gap is G [mm], the weld height is h [mm], and the reduction ratio of the weld is R [%], the reduction ratio R should satisfy any of the following formulas (1) to (6) depending on the butt gap G.

[0039] For weld fill height 0<h≦0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(1.0G+0.5) / (0.1h) ...(1) For 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(0.7G+1.3) / (0.2h) ...(2) For 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(0.5G+3.0) / (0.4h) ...(3) For weld fill height h>0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(40G+220) / (10+h) ...(4) If 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(20G+330) / (15+h) ... (5) If 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(10G+450) / (20+h) ... (6)

[0040] The reduction ratio R is a value calculated by (w1-w2) / w1 x 100, where w1 [mm] is the thickness of the weld bead before rolling and w2 [mm] is the thickness of the weld bead after rolling, in a cross section of the weld perpendicular to the welding direction.

[0041] When the reduction ratio R is below the lower limit of the formulas (1) to (6), the hardening of the welded portion due to the application of pressure is insufficient.On the other hand, when the reduction ratio R is above the upper limit of the formulas (1) to (6), the hardening of the welded portion is excessive.

[0042] In the above formulas (1) to (6), for example, when the base metal is ferritic steel and the wire is a nickel alloy, the structure of the weld metal tends to be a single-phase α-phase structure when 0≦G<0.20, a structure of either α or γ phase or a mixture of both phases depending on the welding conditions when 0.20≦G<0.50, and a single-phase γ-phase structure when 0.50≦G.

[0043] 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 sheet. In other words, as the butt gap G increases, it is necessary to increase the reduction ratio R and increase the processing strain introduced into the weld metal. Note that, since burn-through is more likely to occur if the butt gap G [mm] exceeds 2.00, it is preferable that the butt gap G [mm] be between 0 and 2.00.

[0044] As shown in Figure 2, the above-mentioned reinforcement height h [mm] is the sum (h1 + h2) of the reinforcement height h1 [mm] protruding upward from the thin plate surface and the reinforcement height h2 [mm] protruding downward from the thin plate surface in a cross section of the weld perpendicular to the welding direction, and is measured using a shape measurement sensor such as a laser displacement meter, a micrometer, etc. In Figure 2, h1 = (h1s + h1e) / 2 and h2 = (h2s + h2e) / 2. (h1s, h1e, ​​h2s, h2e: units [mm])

[0045] As the weld bead height increases, the cross-sectional shape of the weld bead becomes gently convex, and the volume of pressure applied increases with the same reduction compared to when the weld bead height is small. Therefore, appropriate processing strain can be introduced with a small reduction. Here, as the weld bead height increases, the reduction required to press the weld bead weld to the base metal thickness increases. Therefore, the range of appropriate reduction R expands, as shown in equations (1) to (6) above.

[0046] In the present invention, it is necessary that the height h of the weld formed by the laser irradiation before pressurization satisfies h > 0 (i.e., there is a so-called reinforcement). This is because, if this relationship is not satisfied when pressurizing the weld, it is physically impossible to pressurize the weld metal within the range of the present invention. Note that such a condition (the presence of a reinforcement) is usually adopted in butt welding. Furthermore, because excessive pressurization reduces the thickness of the base metal, it is preferable that the average thickness of the weld after pressurization be equal to or greater than the maximum thickness of the butted steel sheets.

[0047] In the present invention, the measurement method using a laser displacement meter is a reflective light receiving method. The reinforcement height h is preferably 0.1 to 2.0 mm, and more preferably 0.1 to 1.0 mm.

[0048] 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.

[0049] In the butt welding method according to the present invention, any items not described in this specification can be performed by conventional methods.

[0050] 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.

[0051] 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 flat rolls arranged opposite the front and back surfaces of the weld, and the weld formed by the laser welding was pressurized while the flat rolls were rotated in the welding direction.

[0052] Before welding, the butt gap G was measured by measuring the gap between the leading steel sheet and the trailing steel sheet using a laser displacement meter. That is, the butt gap G was measured using the laser displacement meter in the same manner as above, and was found by averaging the gap in a range excluding regions 20 mm from each end of the steel sheet in the width direction toward the center. Specifically, the gap G was calculated as the average of three values: a position 20 mm from each end of the steel sheet in the width direction toward the center and a position in the center of the weld length direction. The reason for this measurement being in a range excluding a region 20 mm from each end of the steel sheet in the width direction toward the center 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 non-steady-state portions that may be removed in the production line.

[0053] The reinforcement height h was measured using a laser displacement meter and calculated by adding together the reinforcement height h1 protruding upward from the thin plate surface and the reinforcement height h2 protruding downward from the thin plate surface, as shown in Figure 2. Note that both h1 and h2 are values ​​obtained by calculating the average of the distances from the maximum value of the measurement point to the surfaces of the preceding steel plate and the succeeding steel plate. That is, in Figure 2, as mentioned above, h1 = (h1s + h1e) / 2 and h2 = (h2s + h2e) / 2.

[0054] Here, the reinforcement height was measured using a laser displacement meter as described above, and was found by averaging the height over a range excluding a region 20 mm from each of the start and end of the weld toward the center. Specifically, the height was calculated as the average of three values: a position 20 mm from each end of the steel plate in the width direction toward the center, and a position 20 mm from the center in the weld length direction. The reason for excluding 20 mm from each of the start and end is that the start and end of the weld are unsteady portions and may be removed in the production line.

[0055] The reduction ratio in the present invention was determined by calculating the change rate of the reinforcement heights before and after pressing, as described above, using the reinforcement heights calculated by the above method. ｗThe thickness was determined by using the laser displacement meter to calculate the average thickness of the weld in an area excluding regions 20 mm from the start and end of the weld toward the center. Specifically, the average was calculated from three points: a position 20 mm from each end of the steel plate in the width direction toward the center and a position in the center of the weld length direction. The welds obtained as described above were evaluated as follows.

[0056] <Evaluation of hardness of welded portion> As shown in Figure 3, the specimen was cut in a direction perpendicular to the welding direction, and the maximum welded portion 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 hardness of the base material, at a position 10 mm away from the boundary between the base material and the molten 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 value 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.

[0057] <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 (×).

[0058] The above evaluation results are shown in Table 1, which also lists the welding conditions.

[0059]

[0060] From these results, the butt-welded components (invention examples) in which the reduction ratio was set to satisfy the requirements of the present invention were evaluated as passing (◯) in all of the above evaluations.

[0061] 1 preceding steel sheet 2 following steel sheet 3 butt joint (butt gap) 4 filler wire 5 laser 6 weld bead excess 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 laser is irradiated while a filler wire is supplied to the butted part, the filler wire is melted and solidified to weld, and the solidified weld is pressurized, wherein the excess height h [mm] of the weld formed by the irradiation of the laser satisfies h>0, and the reduction rate during the pressurization is H, and the hardness of the weld after pressurization is 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. A butt welding method as set forth in claim 1, wherein the reduction rate R [%] during the application of pressure satisfies any one of the following formulas (1) to (6) depending on the butt gap G [mm] at the butted portion and the weld height h [mm]. For weld height 0<h≦0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(1.0G+0.5) / (0.1h) ...(1) When 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(0.7G+1.3) / (0.2h) ...(2) When 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(0.5G+3.0) / (0.4h) ...(3) For weld height h>0.4mm, when 0≦G<0.20, (0.05G+0.06) / (0.1h)≦R<(40G+220) / (10+h) ...(4) In the case of 0.20≦G<0.50, (0.1G+0.1) / (0.2h)≦R<(20G+330) / (15+h) …(5) In the case of 0.50≦G, (0.2G+0.2) / (0.4h)≦R<(10G+450) / (20+h) …(6) 3. A butt welding method according to claim 1 or 2, wherein the filler wire is either or both of a ferritic and austenitic filler wire.

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