Lap welding method and lap welding device

Abstract

To provide a lap-welding method and a lap-welding device that inhibit welding troubles by taking into account changes in hardness of steel sheets before the lap-welding.SOLUTION: A method for lap-welding a rear end of a preceding steel sheet and a tip part of a succeeding steel sheet includes: a hardness measurement step for measuring a hardness of the preceding steel sheet and a hardness of the succeeding steel sheet; and a welding pressure determination step for determining a welding pressure of a welding electrode with respect to an overlapped portion, according to a measured average hardness of a measured hardness of the preceding steel sheet and a measured hardness of the succeeding steel sheet that were measured in the hardness measurement step. Further, the lap-welding method includes a step for correcting an estimated welding pressure in accord with a hardness estimated in advance, by using the measured hardness, in order to determine the welding pressure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lap welding method and apparatus for lap welding a preceding steel plate and a following steel plate, and more particularly to a lap welding method and apparatus that can ensure excellent weldability even when the hardness of the steel plate serving as the base plate varies in the plate width direction. [Background technology]

[0002] In continuous steel plate processing lines, continuous processing is made possible by joining the preceding and succeeding steel plates by welding at the entry side of the line. There are various welding methods, such as butt resistance welding, lap resistance welding (also known as "mash seam welding," hereafter simply referred to as "lap welding"), and laser welding, but lap welding is generally used for thin steel plates with a thickness of 2 mm or less.

[0003] In order to improve the welding quality of welded joints formed by welding methods such as lap welding, various techniques for improving the accuracy of determining welding quality have been studied.

[0004] For example, it is known that in order to measure the brittleness of a weld, a hardness test is performed after welding to determine the strength of the weld. Patent Document 1 discloses a method in which, in butt welding, the hardness is measured on the weld surface after welding, and if the hardness is high, decorative welding is performed to reinforce the weld in order to avoid concentration of strain in the softened part.

[0005] Next, as a method for measuring steel plates and changing welding conditions before starting welding in lap welding, for example, Patent Document 2 discloses a method for measuring the overlap, which is the overlapping portion of the leading and trailing steel plates, in advance and determining whether the displacement of the overlap is within a preset tolerance range. Also, Patent Document 3 discloses a method for measuring the plate thicknesses of the leading and trailing steel plates and setting welding conditions in accordance with the actual plate thicknesses. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-210653 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-160070 [Patent Document 3] Japanese Patent Application Publication No. 5-111775 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the case of Patent Document 1, the pass / fail judgment is made after welding, which poses the problem of the need for rewelding in a line that operates continuously, resulting in reduced productivity. Also, in the cases of Patent Documents 2 and 3, the overlap and plate thickness are measured in advance, but no consideration is given to hardness, which affects the amount of heat input in lap welding, which poses the problem of weld fracture problems.

[0008] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a lap welding method and lap welding apparatus that take into account changes in hardness of steel sheets before lap welding and prevent welding problems. [Means for solving the problem]

[0009] In order to achieve the above object, the inventors of the present invention have thoroughly investigated the relationship between the hardness of steel sheets and weldability. Figure 4 shows examples of cross-sectional photographs of welds with different hardnesses. Figure 4(a) shows the case of lap welding performed under normal welding conditions. The hardness (Vickers hardness) of the steel sheets was 420 HV, and the welding conditions were a welding current of 12.0 kA, a welding electrode pressure of 2.2 ton, and a welding speed of 5 mpm. As a result, when the cross section of the lap weld (lapped portion) was observed, a large nugget (molten portion), indicated by a circle in the figure, was formed, and the molten state was sound, with no problems in weldability. However, as shown in Figure 4(b), when the hardness of the steel sheets was reduced to 330 HV, no nugget was formed when the cross section was observed, even when lap welding was performed under the same welding conditions as above. This is because the reduced hardness of the steel sheets reduced the contact resistance between the leading and trailing steel sheets, resulting in insufficient heat input and no nugget formation. If a nugget does not form, it can cause problems with fractures in the welded joint. From these investigations, it was discovered that the difference in hardness between the preceding and following steel plates has a significant impact on weldability.

[0010] The present invention was completed based on this finding and further investigation.

[0011] The gist of the present invention is as follows. [1] In the lap welding method for the rear end of the preceding steel plate and the front end of the following steel plate, a hardness measurement step of measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step of determining a pressure (P) of the welding electrode applied to the overlapping portion between the rear end portion and the front end portion based on the pressure (P); A lap welding method comprising: [2] In a method of lap welding the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation step of estimating the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA a pressure determination step of determining a pressure (P) of the welding electrode applied to the overlapping portion between the rear end portion and the front end portion based on the pressure (P); A lap welding method comprising: [3] In a method of lap welding the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation step of estimating the hardness of the preceding steel sheet and the hardness of the following steel sheet; a hardness measurement step of measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA ) is the estimated welding electrode pressure (P E ) is the measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step of determining the welding electrode pressure (P) by correcting the pressure (P) based on the calculated pressure (P); A lap welding method comprising: [4] The lap welding method according to [3], characterized in that the correction in the pressing force determination step determines the pressing force (P) based on the following formula (1): P=P E ×(H MA / H EA ) (1) [5] The lap welding method according to any one of [1] to [4], wherein the hardness (H) is a Vickers hardness (HV) specified in JIS Z 2244 and is in the range of 100 HV to 500 HV. [6] The lap welding method according to any one of the above [1] to [5], wherein the applied pressure (P) is in the range of 1.0 ton to 3.0 ton.

[0012] [7] In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness measuring means for measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a welding force determining means for determining a welding electrode pressure (P) applied to the overlapping portion between the rear end portion and the front end portion based on the welding force (P); A lap welding device comprising: [8] In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation means for estimating the hardness of the preceding steel plate and the hardness of the following steel plate; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA a welding force determining means for determining a welding electrode pressure (P) applied to the overlapping portion between the rear end portion and the front end portion based on the welding force (P); A lap welding device comprising: [9] In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation means for estimating the hardness of the preceding steel plate and the hardness of the following steel plate; a hardness measuring means for measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA ) is the estimated welding electrode pressure (P E ) is the measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MAa pressure determining means for determining the welding electrode pressure (P) by correcting the pressure (P) based on the calculated pressure (P); A lap welding device comprising:

[10] The lap welding device according to [9], wherein the pressing force determining means has a correction function for determining the pressing force (P) based on the following formula (1): P=P E ×(H MA / H EA ) (1)

[11] In any one of [7] to

[10] , the lap welding device is characterized in that the hardness (H) is a Vickers hardness (HV) specified in JIS Z 2244 and is in the range of 100HV to 500HV.

[12] The lap welding device according to any one of the above [7] to

[11] , wherein the applied pressure (P) is in the range of 1.0 ton to 3.0 ton. [Effects of the Invention]

[0013] According to the present invention, by changing the welding electrode pressure based on the hardness of the preceding and following steel sheets, it is possible to avoid insufficient strength of the welded portion due to insufficient heat input and reduce the risk of weld fracture, thereby achieving an industrially significant effect of contributing to improved productivity of coils (steel strips). [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic side view showing an example of a lap welding device provided with a hardness meter according to the present invention. FIG. [Figure 2] FIG. 2 is a schematic plan view showing an example of hardness measurement points in the lap welding device according to the present invention. [Figure 3] 1A is a schematic diagram showing the estimated hardness value and the measured hardness at each measurement point, and FIG. 1B is a schematic diagram showing the estimated applied pressure value and the applied pressure corrected based on the measured hardness at each measurement point. [Figure 4]These are cross-sectional photographs of welds resulting from a study of lap welding. (a) shows the result of welding normal steel plates, and (b) shows the result of welding steel plates with reduced hardness. [Figure 5] 1A and 1B are schematic diagrams showing an example of a conventional lap welding device, in which (a) is a front cross-sectional view of the entire device, and (b) is a cross-sectional view of a welding electrode wheel as seen from the side. DETAILED DESCRIPTION OF THE INVENTION

[0015] First, before describing the lap welding method and lap welding apparatus according to the present invention, an outline of a conventional lap welding apparatus will be described.

[0016] [Conventional lap welding equipment] Fig. 5 is a schematic diagram showing the general configuration of a conventional lap welding device, in which Fig. 5(a) is a front cross-sectional view of the device and Fig. 5(b) is a side cross-sectional view.

[0017] In a lap welding device, the rear end 1e of a leading steel sheet 1 and the front end 2f of a trailing steel sheet 2, which are held by a clamp (not shown), are simultaneously cut by a shearing machine (also called a "sear," not shown), and then overlapped in parallel. The overlapped portion (also called an "overlapped portion" 6) is clamped between a pair of upper and lower electrode wheels 3 (upper electrode wheel 3a and lower electrode wheel 3b), and welded by passing electricity from a power supply (not shown).

[0018] The electrode wheels 3 are placed on a traveling device 7 that moves in the width direction of the plate in order to weld the overlapping portion 6 over the entire width from one end to the other in the width direction. A pressure device 5 is provided above the upper electrode wheel 3a to press the electrode wheel 3a and apply pressure to the overlapping portion 6. The lower electrode wheel 3b is attached to the traveling device 7 and is structured to support pressure from above. The pair of upper and lower electrode wheels 3 can each be driven in the vertical direction by, for example, a hydraulic or pneumatic cylinder device (not shown), and an electrode wheel drive motor (not shown) is provided to rotate the electrode wheels 3.

[0019] Behind (downstream of) the electrode wheel 3, planish rolls 4 (upper planish roll 4a and lower planish roll 4b) are provided to further pressurize the welded portion to flatten the overlapping portion, and a pressure device 5 is also provided above them. Similarly, the pair of upper and lower planish rolls 4 can each be driven vertically by, for example, a hydraulic or pneumatic cylinder device (not shown), and a planish roll drive motor (not shown) is provided to rotate the planish rolls 4.

[0020] The electrode wheel 3, planish roll 4, pressure device 5, etc. are attached to a traveling device 7 for transporting them as a unit. When performing lap welding, the traveling device 7 moves in the direction of the arrow while the leading steel sheet 1 and the trailing steel sheet 2 are overlapped.

[0021] Here, the thickness t of the preceding steel sheet 1 and the following steel sheet 2 is 0.4 mm to 2.3 mm, and the width w thereof is 800 mm to 1800 mm. The width k of the overlapping portion 6 is 2 mm to 3 mm. The diameter R of the electrode wheel 3 is 160 mmφ to 180 mmφ, and the thickness m thereof is 10 mm to 12 mm. It is preferable to set the thickness m of the electrode wheel 3 larger than the width k of the overlapping portion.

[0022] [Lap welding method and lap welding device according to the present invention] The lap welding method and lap welding apparatus according to the present invention aim to improve the weldability of the weld, and are essentially a method and apparatus for controlling the welding electrode pressure based on the hardness of the preceding steel sheet and the following steel sheet. As specific lap welding methods and apparatus, the inventors have invented three methods and apparatus, which are described below. Note that, while the following description describes the invention of a welding method, the invention of a welding apparatus is an invention in which the "steps" of the corresponding welding method are replaced with the "means" of the welding apparatus, and is shown in parentheses. Specific examples of welding apparatus will be described as they arise.

[0023] The "first invention" is a method (apparatus) for measuring the hardness of a steel plate and determining the welding electrode pressure based on the measured hardness. Specifically, the method comprises the following two steps (means): [A1] Hardness measurement process (means) for measuring the hardness of the preceding steel plate and the hardness of the succeeding steel plate [B1] [A1] The measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step (means) for determining the welding electrode pressure (P) applied to the overlapping portion based on the Here, the hardness may be Vickers hardness (HV), Brinell hardness, Knove hardness, etc., and is not particularly limited, but in the present invention, it is preferable to use the widely used Vickers hardness. This Vickers hardness test method is specified in JIS Z 2244. The Vickers hardness (HV) range is preferably 100HV to 500HV.

[0024] The hardness measuring means in the device invention refers to a hardness tester. There are hardness testers that correspond to the various hardnesses mentioned above, but it is preferable to use a hardness tester for Vickers hardness.

[0025] FIG. 1 is a schematic diagram showing an overview of an lap welding apparatus according to the present invention. The welding apparatus shown in FIG. 5(b) described above is provided with hardness meters 8 (hardness meter 8a for measuring the hardness of the preceding steel sheet and hardness meter 8b for measuring the hardness of the following steel sheet) in front of and behind the overlapping portion 6. In FIG. 1, hardness meter 8a is placed on the back side (lower side) of the preceding steel sheet 1, but this position is not particularly limited. Both hardness meters 8a and 8b may be placed on either side of the steel sheet. The hardness measured by these hardness meters 8a and 8b, i.e., the measured hardness (H M1 ) and the measured hardness (H M2 ) is transmitted to the controller 9, where it calculates the average measured hardness (H MA ) is calculated. The average measured hardness (H MA) and transmits the data to the pressure device 5 to correct the pressure of the electrode wheel 3a. The pressure (P) is preferably in the range of 1.0 ton to 3.0 ton.

[0026] FIG. 2 is a schematic diagram showing measurement points for hardness measurement. The number of measurement points can be any number from one onward. However, taking into account the influence of variations in hardness in the sheet width direction, it is preferable to measure at multiple points. More preferably, as shown in FIG. 2, five measurement points are set at the center in the sheet width direction, at both ends, and at intervals between them. That is, five measurement points are set at equal intervals in the sheet width direction (five points 11a, 11b to 11e for the preceding sheet, and five points 12a, 12b to 12e for the following sheet). The measurement points are preferably positioned at a distance d of 50 mm or more from the edge of the steel sheet (rear edge 1e of the preceding sheet). This is because if the distance is less than 50 mm, the hardness measurement position may become a fusion zone in subsequent welding, affecting weld quality. Therefore, if hardness is measured at a distance less than 50 mm, a separate device is required to cut off the edge by 50 mm or more after hardness measurement.

[0027] Furthermore, the timing of the measurement may be before the start of welding or during welding (before pressure is applied by the electrode wheel). In order to minimize the influence of the lead time (the time required for measurement), it is preferable to measure the hardness of the succeeding steel sheet while the preceding steel sheet is being threaded. On the other hand, it is preferable to measure the hardness of the preceding steel sheet near the weld (overlapped portion), but this would result in downtime. In order to eliminate this downtime, it is also possible to measure the hardness of the leading edge of the preceding steel sheet and regard this value as the value of the trailing edge of the preceding steel sheet.

[0028] In the control device 9, an average measured hardness is calculated from the measured hardness at each of the respective positions. For example, the average measured hardness (H MA ) and determines the pressure (P) that corresponds to that hardness. When the electrode wheel welds that point (the frontmost side), it transmits a signal of the determined pressure (P) to the pressure device, which applies pressure to the overlapping part.

[0029] The method for determining the pressure corresponding to the calculated hardness is not particularly limited, but for example, the relationship with the optimum pressure is determined in advance from data such as the thickness and type of steel of the steel plate and welding conditions, and a chart or the like that summarizes this data is prepared. One example is a method for determining the pressure from the obtained measured hardness data using the chart or the like.

[0030] The "second invention" is a method (apparatus) for estimating the hardness of a steel plate and determining the welding electrode pressure based on the estimated hardness. Specifically, the method comprises the following two steps (means): [A2] Hardness estimation step (means) for estimating the hardness of the preceding steel plate and the hardness of the succeeding steel plate [B2] [A2] The estimated hardness (H E1 ) and the estimated hardness of the subsequent steel plate (H E2 ) and the average estimated hardness (H EA a pressure determination step (means) for determining the welding electrode pressure (P) applied to the overlapping portion based on the Here, the method (means) for estimating hardness is not particularly limited, but examples include a method in which the relationship between the hardness of a steel plate and data such as the plate thickness, steel type, and welding conditions of the steel plate is determined in advance, and a chart or the like that organizes the data is prepared, and the hardness of the steel plate is estimated from data related to the steel plate to be used using the chart or the like.

[0031] The pressing force determining step (means) of [B2] above is the same as the pressing force determining step (means) of [B1] described above, and is a step (means) of determining the pressing force using the estimated hardness of [A2] instead of the measured hardness of [A1] described above. In other words, the method of determining the pressing force from the estimated hardness is the same as the method of determining the pressing force from the measured hardness described above.

[0032] The "third invention" is a method (apparatus) for estimating the hardness of a steel sheet, estimating the welding electrode pressure based on the estimated hardness, measuring the actual hardness of the steel sheet, and correcting the welding electrode pressure based on the measured hardness to determine the actual welding pressure. Specifically, the method comprises the following three steps (means): [A2] Hardness estimation step (means) for estimating the hardness of the preceding steel plate and the hardness of the succeeding steel plate [A1] Hardness measurement process (means) for measuring the hardness of the preceding steel plate and the hardness of the succeeding steel plate [B3] [A2] The estimated hardness (H E1 ) and the estimated hardness of the subsequent steel plate (H E2 ) and the average estimated hardness (H EA ) based on the estimated welding electrode pressure (P E ) and estimate the applied pressure (P E ) is the measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step (means) for determining the actual welding electrode pressure (P) by correcting the pressure (P) by the Here, the [A2] hardness estimation step (means) is the same as the [A2] step (means) described in the second invention described above, and the [A1] hardness measurement step (means) is the same as the [A1] step (means) described in the first invention described above.

[0033] The pressure determining step (means) of [B3] is a combination of the above-mentioned [B2] and [B1], and is used to determine the average estimated hardness (H EA ) to estimate the applied pressure (P E ) is found. E ) is the average measured hardness (H MA ) to determine the applied pressure (P).

[0034] [How to correct pressure] Here, in the step (means) of [B3] of the third invention, the average estimated hardness (H EA ) to estimate the applied pressure (PE ) and calculate the average measured hardness (H MA ), but a preferable correction method for determining the applied pressure (P) by this correction will be described below.

[0035] The welding troubles that are the subject of the present invention occur partly due to insufficient heat input to the welding, and this insufficient heat input occurs due to fluctuations in the hardness of the steel plate.

[0036] This welding heat input is calculated using the following formula (2). Q=I 2 ×R×t (2) Here, Q is the amount of heat generated (kJ), I is the current value (A), R is the contact resistance (Ω), and t is the time (min).

[0037] In the formula (2), R: contact resistance, can be calculated by the following formula (3) when the contact surface undergoes plastic deformation. R=ρ×√(πH) / 2√P ··· (3) Here, ρ is the specific resistance (Ω) of the material, H is the hardness, and P is the applied pressure (unit).

[0038] From this equation (3), it can be seen that the contact resistance R is proportional to H / P. Therefore, since H(estimated) / P(estimated) is proportional to H(measured) / P(determined), P(determined) = P(estimated) × {H(measured) / H(estimated)}, or the following equation (1) can be derived. P=P E ×(H MA / H EA ) (1) where P: corrected determined pressure, P E : Estimated pressure, H EA : Average estimated hardness, H MA : Average measured hardness.

[0039] In other words, when welding by moving the estimated applied pressure in the plate width direction in accordance with the fluctuations in the measured average hardness, by fluctuating the applied pressure to the value determined by the above correction, it is possible to sufficiently resolve the shortage of welding heat input and prevent the occurrence of welding problems. [Example]

[0040] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0041] [Example 1] First, an example of implementing the third aspect of the present invention will be described with reference to Fig. 3. The hardness of steel sheets was measured, and welding was performed while correcting the electrode wheel pressure based on the hardness value. Fig. 3(a) is a schematic diagram showing the estimated hardness values ​​and the measured hardness at each measurement point. Fig. 3(b) is a schematic diagram showing the estimated pressure values ​​and the corrected pressure values ​​based on the measured hardness at each measurement point.

[0042] First, estimate the hardness of the steel plate (for example, Vickers hardness: HV) from data such as the type of steel and thickness of the steel plate to be used, and then estimate the electrode wheel pressure (ton) corresponding to that hardness. These are the values ​​shown by the dotted lines in Figure 3(a) and (b).

[0043] Next, hardness measurement points 11a-11e and 12a-12e (five points each in the sheet width direction at a distance d = 100 mm from the rear end 1e of the preceding steel sheet 1 and from the front end 2f of the following steel sheet 2) shown in Figure 2 were determined, and the Vickers hardness of each measurement point was measured. For this measurement, hardness meters 8a and 8b shown in Figure 1 were used. The obtained measured hardness data was sent to the control device 9 in Figure 1. The control device 9 performs a process to calculate the average value of the opposing measurement points on the preceding steel sheet and the following steel sheet. Specifically, the average value of 11a and 12a, the average value of 11b and 12b, and so on up to the average value of 11e and 12e are calculated. These average values ​​are the measured hardness values ​​of measurement points a-e shown in Figure 3(a). As in this example, in ordinary steel sheets (cold-rolled steel sheets), the ends of the steel sheet in the width direction (near points a and e) cool faster than the center of the steel sheet (near point c), and therefore generally have higher hardness.

[0044] Based on this average measured hardness data, calculation processing is performed to correct the estimated pressure using the aforementioned equation (1). The corrected pressure value obtained is shown by the solid line in Figure 3(b). This pressure data is sent to the pressure device 5 of the electrode wheel 3a, and the pressure of the electrode wheel is adjusted. By performing this type of control, the welding heat input is stabilized, and the occurrence of welding problems is significantly reduced.

[0045] [Example 2] An actual operation example will be described. Conventionally, the hardness of steel sheets was not measured, and therefore, if the hardness fluctuated due to changes in the cold reduction rate, the hot-rolling coiling temperature, or the annealing conditions before cold rolling, welding problems would occur due to insufficient welding heat input or spatter during lap welding. In conventional operation examples, welding problems occurred with a frequency of 1 / 100 coils, and rewelding was required, but by introducing the lap welding method and lap welding device according to the present invention, the frequency of rewelding has been reduced to 1 / 1000 coils. [Explanation of symbols]

[0046] 1 preceding steel plate, 1e rear end of preceding steel plate 2 Trailing steel plate, 2f Tip of trailing steel plate 3 electrode ring 3a Upper electrode ring, 3b Lower electrode ring 4 Planish Roll 4a Upper Planish Roll, 4b Lower Planish Roll 5. Pressure device 6 Overlapped section 7 Running gear 8 Hardness meter 8a Hardness tester for leading steel plate, 8b Hardness tester for trailing steel plate 9 Control Device 11a~11e Hardness measurement points of preceding steel plates 12a~12e Hardness measurement points of the following steel plate t plate thickness, w plate width R is the diameter of the electrode ring, m is the thickness of the electrode ring k Width of overlap d Distance from the edge of the steel plate to the hardness measurement point

Claims

1. In a method of lap welding the rear end of a preceding steel plate and the front end of a following steel plate, a hardness measurement step of measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step of determining a pressure (P) of the welding electrode applied to the overlapping portion between the rear end portion and the front end portion based on the pressure (P) of the welding electrode; A lap welding method comprising:

2. In a method of lap welding the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation step of estimating the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA a pressure determination step of determining a pressure (P) of the welding electrode applied to the overlapping portion between the rear end portion and the front end portion based on the pressure (P) of the welding electrode; A lap welding method comprising:

3. In a method of lap welding the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation step of estimating the hardness of the preceding steel sheet and the hardness of the following steel sheet; a hardness measurement step of measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA ) is estimated based on the welding electrode pressure (P E ) is the measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determination step of determining the welding electrode pressure (P) by correcting the pressure (P) based on the calculated pressure (P); A lap welding method comprising:

4. 4. The lap welding method according to claim 3, wherein the correction in the pressing force determination step determines the pressing force (P) based on the following formula (1): P=P E ×(H MA / H EA ) ・・・・・ (1)

5. The lap welding method according to any one of claims 1 to 4, characterized in that the hardness (H) is a Vickers hardness (HV) defined in JIS Z 2244 and is in the range of 100 HV to 500 HV.

6. 5. The lap welding method according to claim 1, wherein the applied pressure (P) is in the range of 1.0 tons to 3.0 tons.

7. 6. The lap welding method according to claim 5, wherein the applied pressure (P) is in the range of 1.0 tons to 3.0 tons.

8. In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness measuring means for measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a welding force determining means for determining a welding electrode pressure (P) applied to the overlapping portion between the rear end portion and the front end portion based on the welding force (P); A lap welding device comprising:

9. In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation means for estimating the hardness of the preceding steel plate and the hardness of the following steel plate; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA a welding force determining means for determining a welding electrode pressure (P) applied to the overlapping portion between the rear end portion and the front end portion based on the welding force (P); A lap welding device comprising:

10. In a lap welding device for the rear end of a preceding steel plate and the front end of a following steel plate, a hardness estimation means for estimating the hardness of the preceding steel plate and the hardness of the following steel plate; a hardness measuring means for measuring the hardness of the preceding steel sheet and the hardness of the following steel sheet; The estimated hardness (H E1 ) and the estimated hardness (H E2 ) and the average estimated hardness (H EA ) is estimated based on the welding electrode pressure (P E ) is the measured hardness (H M1 ) and the measured hardness (H M2 ) and the average measured hardness (H MA a pressure determining means for determining the welding electrode pressure (P) by correcting the pressure (P) based on the calculated pressure (P); A lap welding device comprising:

11. 11. The lap welding device according to claim 10, wherein the pressing force determining means has a correction function for determining the pressing force (P) based on the following formula (1): P=P E ×(H MA / H EA ) ・・・・・ (1)

12. 12. The lap welding device according to claim 8, wherein the hardness (H) is a Vickers hardness (HV) defined in JIS Z 2244 and is in the range of 100 HV to 500 HV.

13. 12. The lap welding device according to claim 8, wherein the pressure (P) is in the range of 1.0 tons to 3.0 tons.

14. 13. The lap welding device according to claim 12, wherein the pressure (P) is in the range of 1.0 tons to 3.0 tons.

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