Laser beam welding method and steel plate manufacturing method

Abstract

To provide a laser beam welding method capable of adjusting both the shape and the hardness of a weld metal part.SOLUTION: A method that forms a butted gap by mutually butting a plurality of steel strips, and performs laser beam welding while feeding a filler wire to the butted gap includes: measuring thicknesses of each of the butted gap and the steel strips in the width direction of the steel strips, prior to the laser beam welding; and controlling torch speed of the laser beam welding and feeding speed of the filler wire on the basis of the measured values, and performing the laser beam welding.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a laser welding method for butt-joining a plurality of steel strips to each other. In particular, the present invention relates to a method for producing a laser welded portion that contributes to realizing continuous cold rolling by connecting steel strips to each other and then subjecting them to a cold rolling process in a continuous steel sheet manufacturing process.

Background Art

[0002] For example, for the purpose of improving the productivity of steel sheets, in the cold rolling process, the trailing end of a preceding material to be rolled and the leading end of a succeeding material to be rolled are joined to continuously perform rolling. As welding methods applied to such continuous rolling lines, it is common to use flash butt welding and laser beam welding.

[0003] Laser welding has high energy density and low heat input, so excellent quality characteristics can be obtained compared to flash butt welding. However, in the welding of high-carbon steel sheets and electromagnetic steel sheets, cracks due to poor quality occur in the welded metal part. Therefore, a filler wire is used to reduce the carbon and alloy content in the welded metal part to stabilize the quality.

[0004] Regarding this laser welding using a filler wire, Patent Document 1 proposes a method of controlling the feeding speed of the filler wire based on the butting gap between steel sheets.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When continuously rolling a steel strip joined by laser welding using a rolling mill, the quality of the welded metal part after laser welding, which is the joint part of the steel strip, is important for realizing continuous rolling. Two particularly important points regarding the quality of this welded metal part are the shape and hardness of the welded metal part.

[0007] As described above, in the "laser welding method excellent in weld strength" described in Patent Document 1, the feeding speed of the filler wire is controlled based on the gap between the steel strips (workpieces) to be welded. However, with only this control based on the gap, it is difficult to adjust both the shape and hardness of the above-described welded metal part, leaving a problem in this regard.

[0008] Therefore, an object of the present invention is to propose a laser welding method capable of adjusting both the shape and hardness of the welded metal part. Another object of the present invention is to propose a method for manufacturing a steel sheet that realizes continuous cold rolling using the laser welding method.

Means for Solving the Problems

[0009] The inventors newly found that, as a factor affecting the quality of the welded metal part, in addition to the gap between the steel sheets to be welded, the thickness deviation of the steel strip (workpiece) cannot be ignored. That is, the thickness of the steel strip may vary by around 10% with respect to the set thickness. When the thickness changes with respect to the set value in this way, the shape of the welded metal part becomes underfill where there is not enough welded metal in the thickness direction, or conversely, overfill where the welded metal bulges and the excess amount becomes excessive. Also, regarding the hardness of the welded metal part, it may become a hardness different from the expected hardness. In any case, as a result, it will cause breakage at the welded metal part in the rolling mill.

[0010] In order to solve the above problems, the present inventors have found that it is important to measure the butting gap between butted steel strips and, for each of the leading and trailing steel strips, measure the plate thickness at at least one point in the width direction, preferably over the entire width. Based on the measurement results, it has been found that reflecting the plate thickness deviation of the steel strip in the welding conditions is effective in improving the quality of the welded metal part, and thus the present invention has been completed.

[0011] That is, the gist of the present invention is as follows. 1. A laser welding method in which a plurality of steel strips are butted against each other to form a butting gap, and laser welding is performed while feeding a filler wire into the butting gap, wherein prior to the laser welding, the butting gap and the plate thickness of the steel strip are measured in the width direction of the steel strip, respectively, and based on these measured values, the laser output of the laser welding and the feeding speed of the filler wire are controlled to perform laser welding.

[0012] 2. A laser welding method in which a plurality of steel strips are butted against each other to form a butting gap, and laser welding is performed while feeding a filler wire into the butting gap, wherein prior to the laser welding, the butting gap and the plate thickness of the steel strip are measured in the width direction of the steel strip, respectively, and based on these measured values, the torch speed of the laser welding and the feeding speed of the filler wire are controlled to perform laser welding.

[0013] 3. A method for manufacturing a steel plate, wherein a plurality of steel strips are butted and joined together by the laser welding method according to 1 or 2 above, and then the joined steel strips are continuously subjected to a cold rolling process to perform cold rolling.

Advantages of the Invention

[0014] According to the present invention, by taking into account not only the variation in the butt gap between the steel strips when butt-joining but also the variation in the thickness of the steel strip, and by varying the filler wire feed speed in the width direction of the steel strip as well as the laser output or the torch movement speed while welding, it is possible to achieve laser welding with uniform weld metal shape and weld hardness. [Brief description of the drawings]

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0016] The laser welding method of the present invention will be described in detail below. First, Figure 1 shows a schematic diagram of welding a leading steel strip 1 and a trailing steel strip 2 to be butt-jointed. When welding the leading steel strip 1 and the trailing steel strip 2, the two steel strips are butted together, but a small gap (space) 3 is created when butting, and the mixing ratio of the steel strip to be welded and the filler wire 4 is adjusted depending on the size of this gap 3.

[0017] When feeding the filler wire 4 into the gap 3 set as described above to weld the two steel strips 1 and 2 together, the filler wire 4 is supplied to the welding point in the gap 3 while a laser 6 from a torch 5 is irradiated onto the welding point, thereby melting the welding point of the steel strip together with the filler wire 4 to perform the welding.

[0018] [Hardness of the Weld Metal Part] Now, in order to melt the welded part of the steel strip and the filler wire 4 by laser irradiation, heat input by laser irradiation is required at the welding location of the gap 3. Actually, the melting of the steel strip and the filler wire 4 by laser irradiation is promoted when · The laser output (P) is high (proportional relationship) · The moving speed (Y speed) of the torch 5 is slow (inverse proportional relationship) · The plate thickness (t) is thin (inverse proportional relationship) under each of these conditions. And the relationship between these conditions and the above heat input amount can be expressed by the following formula (1). Since the above laser output P (kW), the moving speed Y speed (m / min) of the torch 5, and the plate thickness t (mm) are set values determined at the start of welding, the heat input amount Q (kW / mpm*mm) shown by the following formula (1) is a fixed value. Note that the plate thickness t is the average value of the actually measured plate thicknesses of the preceding steel strip and the succeeding steel strip to be joined. JPEG2025099433000002.jpg20170

[0019] Since the above heat input amount Q varies depending on the steel strip which is the material to be rolled, prior consideration is required for each steel strip to be welded. This is because the components and structures of the steel strips are different for each steel strip, and the hardness of the steel strips is also different for each steel strip, so the hardness required for the weld metal part also varies.

[0020] Here, regarding the results of measuring the Vickers hardness of the weld metal part and the periphery (base material) of the weld (excluding the weld metal part) when laser welding was performed on the same steel strip under different heat input amounts Q according to the various conditions shown in Table 1, it is shown in Figure 2. Note that the welding conditions not shown in Table 1 were made common for all conditions. As shown in Figure 2, as the heat input amount increases, the penetration of the metal part of the steel strip increases, so the base material position is different depending on the conditions.

[0021]

Table 1

[0022] As shown in Fig. 2, it can be seen that the hardness of the welded metal changes due to the variation in the heat input during laser welding. This is presumably because the penetration amount of the base material at the welding location has changed. Therefore, in order to suppress such hardness variations in the welded metal part, it is necessary to keep the penetration amount of the base material uniform. And to keep the penetration amount of the base material uniform, it is necessary to keep the heat input Q constant.

[0023] As a factor for the variation of the heat input Q, since the laser output P and the moving speed Y of the torch 5 on the right side of the above formula (1) follow the set values, it is due to the steel strip thickness t. As described above, this plate thickness t may deviate from the set thickness. Therefore, in order to suppress the variation of the heat input Q, it is necessary to correct the variation of the plate thickness t, specifically the plate thickness deviation, with the laser output P or the moving speed Y of the torch.

[0024] That is, in order to suppress the hardness variation of the welded metal part due to the influence of the plate thickness variation, when adjusting the laser output P, the laser output P may be determined according to the following formula (2) obtained by transforming the above formula (1). JPEG2025099433000004.jpg17170

[0025] Also, in order to suppress the hardness variation of the welded metal part due to the influence of the plate thickness variation, when adjusting the moving speed Y of the torch, the Y speed of the torch may be determined according to the following formula (3) obtained by transforming the above formula (1). JPEG2025099433000005.jpg22170

[0026] Regarding the measurement of the plate thickness, it is ideal to install a thickness gauge inside the welding machine and measure each of the preceding steel strip and the following steel strip. However, since it is necessary to add two thickness gauges, the method of measuring during the plate feeding in front of the welding machine is practical.

[0027] [Shape of the welded metal part] Next, when welding both steel strips 1 and 2 by feeding filler wire 4 into gap 3, the filler wire 4 is supplied to the welding location of gap 3 at a predetermined Fw input rate. Here, as shown in Fig. 3, the Fw input rate is the ratio of the filler wire 4 occupying the space R partitioned at the end faces where the gap 3 and the steel strips are joined. That is, it can be defined as the ratio of the filler wire input amount (to the space R) to the gap (space R) volume, and can be expressed by the following formula (4). In the following formula (4), Fw speed is the feeding speed of the filler wire into the space R (mm / min), and GAP is the average value in the steel strip width direction of the gap (mm). JPEG2025099433000006.jpg26170

[0028] Ideally, the above Fw input rate is 1.0, but in practice, it is often set to about 1.5 to 2.0 with a margin. It is necessary for this set value to be stable throughout the welding process. If the Fw input rate becomes unstable, it will cause the occurrence of defects shown in Fig. 4. That is, as shown in Fig. 4(a), when the Fw input rate is smaller than the set value, underfill occurs where there is not enough welding metal in the plate thickness direction. This underfill will result in insufficient thickness of the welded part and cause breakage. Conversely, when the Fw input rate is larger than the set value, overfill occurs where the welding metal bulges and the excess amount becomes excessive. This overfill will result in excessive thickness of the welded part and cause shear fracture.

[0029] Also, due to the installation accuracy and wear of the shear blade of the welding machine, fluctuations in the gap occur in the width direction of the steel strip. That is, as shown in Fig. 5, which shows the average value in the width direction of the gap when welding is repeated 5 times under the same conditions, it can be seen that the average value in the width direction of the gap fluctuates for each welding.

[0030] Furthermore, Fig. 6 shows the results of an investigation on the excess height of the welded metal part obtained by these 5 weldings. Fig. 6 plots all the measurement results of the 5 weldings. As shown in Fig. 6, it can be seen that there are also large fluctuations in the excess height.

[0031] As described above, the factor causing the fluctuation of the Fw input rate lies in the GAP (gap) on the right side of the above formula (4). Therefore, in order to suppress the fluctuation of the Fw input rate, it is necessary to correct the fluctuation of the gap, specifically the deviation of the gap, at the Fw speed.

[0032] That is, when adjusting the Fw speed to suppress the fluctuation of the shape (excess height) of the welded metal part due to the influence of the gap fluctuation, the Fw speed may be determined according to the following formula (5) obtained by transforming the above formula (4).

[0033] JPEG2025099433000007.jpg29170

[0034] [Adjustment of hardness and shape of welded metal part] Taking the above into consideration, in the present invention, the butting gap and the steel strip thickness are measured in the width direction of the steel strip respectively, and based on these measured values, the shape and hardness of the welded metal part by laser welding are adjusted according to the following two forms. (Embodiment 1) Based on the measured values of the butting gap and the steel strip thickness measured in the width direction of the steel strip respectively, the laser output of the laser welding (P) and the feeding speed (Fw speed) of the filler wire are controlled. Specifically, the adjustment is performed according to the following formulas (2) and (4). JPEG2025099433000008.jpg12170JPEG2025099433000009.jpg29170(Embodiment 2) Based on the measured values of the butting gap and the steel strip thickness measured in the width direction of the steel strip respectively, the torch moving speed (Y speed) and the feeding speed (Fw speed) of the filler wire are controlled. Specifically, the adjustment is performed according to the following formulas (3) and (5). Note that the following formula (5) is obtained by substituting the following formula (3) into the above formula (4), and the technical meaning of the formula is the same as that of the above formula (4). JPEG2025099433000010.jpg22170JPEG2025099433000011.jpg33170

Example

[0035] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the description of the following embodiments, and appropriate modifications can be made within the scope of the present invention. Using a laser welding machine equipped with a thickness gauge and a GAP gauge on the carriage, welding was performed while measuring the thickness of the steel strip and the gap (GAP). That is, after butting a steel strip with a set thickness of 2.0 mm at a set gap of 0.30 mm, a filler wire was fed into the gap according to the Fw speed and the Fw input rate shown as set values in Table 2, and laser irradiation was performed according to the laser output P and the torch moving speed Y speed shown as set values in Table 2 to give the heat input Q shown as the set value in Table 2 to the welding location to perform laser welding. Note that a YAG solid laser was used for the laser. Also, Inconel was used for the filler wire.

[0036] Before each welding of the butt laser welding according to Table 2 above, the butt gap and the thickness of the butted part were measured in the width direction respectively, the average value in the width direction was obtained, and the variations in the gap and the thickness were detected. And if any variation in the gap and the thickness was detected, control according to the present invention was executed in the present invention section. On the other hand, no control was performed in the comparative example section. That is, among the present invention sections, Invention 1 is a pattern in which the laser output (P) is controlled, and Invention 2 is a pattern in which the torch moving speed (Y speed) is controlled.

[0037] Note that in Table 2, set values are described on the left side and actual results are described on the right side, and the presence or absence of variations is described at the intermediate position between them. Incidentally, in the comparative example, the set value = the actual result.

[0038] In any case, for the welded metal part after welding, its hardness and shape were evaluated as follows. [Hardness of Welded Metal Part] Regarding the hardness of the welded metal part, the Vickers hardness of the base material (parent material) and the welded metal part was measured respectively, and the difference in hardness between the welded metal part and the parent material was used for evaluation. The evaluation results are shown in Table 2, where if the hardness of the welded metal part is within the range of ±20% of the hardness of the base plate, it is "good"; if it exceeds this range, it is "up"; if it is below this range, it is "down".

[0039] [Shape of the welded metal part] Regarding the shape of the welded metal part, the height of the bulge of the welded metal part from the surface of the base material was evaluated. That is, the evaluation results are shown in Table 2, where if the maximum distance in the thickness direction of the base material from the surface of the base material to the surface of the welded metal part is within the range of 0 to 10% of the plate thickness of the base material, it is good; if it exceeds 10%, it is overfill; if it is less than 0%, it is underfill.

[0040] From Table 2, in the first and second inventions, depending on the presence or absence of plate thickness variation and gap variation, by controlling the Fw speed calculated from the above formulas according to the laser output P or the Y speed, the hardness of the welded metal part and the shape of the welded metal part can be made good. Note that each of Test Nos. 1 to 8 in the first invention section corresponds to each of Test Nos. 9 to 16 in the second invention section, and each of Test Nos. 17 to 24 in the comparative example section. By comparing between these sections, it can be seen that when there are plate thickness variations or gap variations, the hardness and shape of the welded metal part can be improved by applying the present invention.

[0041] Regarding Test Nos. 1, 2 and Test Nos. 9, 10, since there are variations in the gap, if there is no control according to the present invention, underfill or overfill will occur as in Test Nos. 17, 18. On the other hand, for Test Nos. 1, 2, 9, 10, by controlling the Fw speed according to the present invention, a welded part with a good shape of the welded metal part can be obtained.

[0042] For Tests No. 3, 4, 11, 12, 19, and 20, there is no gap variation but there is plate thickness variation. In the case without the control of the present invention as in Tests No. 19 and 20, underfill and a decrease in welding hardness or overfill and an increase in welding hardness will occur. On the other hand, in Tests No. 3, 4, 11, and 12, by means of the Fw speed control and heat input control according to the present invention, a good shape of the welded metal part and a welded part with the targeted hardness can be obtained.

[0043] For Tests No. 5 to 8, 13 to 16, and 21 to 24, they are cases where gap variation and plate thickness variation are combined. In Tests No. 21 to 24 without control, it is impossible to avoid poor shape and poor hardness of the welded metal part. On the other hand, in Tests No. 5 to 8 and 13 to 16, by performing control, it is possible to avoid poor shape and poor hardness of the welded metal part.

[0044]

Table 2

Explanation of Signs

[0045] 1, 2 Steel strips 3 Gap 4 Filler wire 5 Torch 6 Laser

Claims

1. A laser welding method in which a plurality of steel strips are abutted against each other to form a butting gap, and laser welding is performed while feeding a filler wire into the butting gap. Prior to the laser welding, the butting gap and the thickness of the steel strip are measured in the width direction of the steel strip respectively, and based on these measured values, the laser output of the laser welding and the feeding speed of the filler wire are controlled to perform laser welding.

2. A laser welding method in which a plurality of steel strips are abutted against each other to form a butting gap, and laser welding is performed while feeding a filler wire into the butting gap. Prior to the laser welding, the butting gap and the thickness of the steel strip are measured in the width direction of the steel strip respectively, and based on these measured values, the torch speed of the laser welding and the feeding speed of the filler wire are controlled to perform laser welding.

3. A method for manufacturing a steel sheet, in which a plurality of steel strips are abutted and joined together by the laser welding method according to Claim 1 or 2, and then the joined steel strips are continuously subjected to a cold rolling process to perform cold rolling.

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