A resistance spot welding method for 800mpa grade hot-rolled pickled multiphase steel plate

Abstract

The present application relates to the technical field of resistance spot welding, and particularly relates to a resistance spot welding method for 800MPa-grade hot-rolled pickled multiphase steel plate, which adopts a pre-pressing, pre-welding, cooling, welding + heat treatment and post-welding air cooling process, and can solve problems such as easy splashing, easy interface failure, easy molten core deviation and molten core asymmetry of the resistance spot welded joint of the hot-rolled pickled multiphase steel plate, and most of the failure modes of the spot welded joint are drawing failure, the load bearing capacity and energy absorption capacity of the spot welded joint are high, and the performance of the spot welded joint is stable and the quality is good.

Description

Technical Field

[0001] This invention relates to the field of resistance spot welding technology, and in particular to a resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates. Background Technology

[0002] With the continuous advancement of steel rolling technology, the performance of hot-rolled thin plates is increasingly approaching that of cold-rolled plates, making technological breakthroughs in "replacing cold with hot rolling" possible. Enterprises are placing increasingly higher demands on the high performance and low cost of steel, especially in the automotive industry. With the rapid development of the automotive industry, the proportion of high-strength steel in vehicle bodies is increasing. Pickled steel sheet is a cost-effective product positioned between cold-rolled and hot-rolled steel sheets. Among them, 800MPa grade hot-rolled pickled multiphase steel sheet possesses high strength and good elongation, meeting the strength and plasticity requirements of automotive parts. Simultaneously, it also exhibits excellent hole-expanding performance, i.e., extended flange performance, making it particularly suitable for manufacturing safety components such as door anti-collision bars, bumpers, and B-pillars, and has a promising market prospect. Resistance spot welding is a primary process for connecting metal parts in the automotive industry, widely used in welding workshops. The failure mode of the weld joint is a qualitative measure of the quality of the spot weld, categorized into interface failure and pull-out failure. Compared to pull-out failure, weld joints that fail under interface failure have lower load-bearing and energy absorption capacities. Therefore, ensuring pull-out failure during spot welding is of great significance for actual industrial production. Compared to traditional low-carbon steel resistance spot welded joints, 800MPa-grade hot-rolled pickled multiphase steel plate resistance spot welded joints have complex microstructures, are prone to spatter, interface failure, weld nugget displacement, and weld nugget asymmetry. How to obtain stable and high-performance 800MPa-grade hot-rolled pickled multiphase steel plate resistance spot welded joints is one of the urgent problems to be solved in the automotive industry. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, the technical problem solved by this invention is to provide a resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates. The method employs a pre-pressing, pre-welding, cooling, welding + heat treatment and post-weld air cooling process, which can solve the problems of easy spattering, interface failure, weld nugget displacement and weld nugget asymmetry in resistance spot welded joints of 800MPa grade hot-rolled pickled multiphase steel plates. The failure mode of the spot welded joint is mostly pull-out failure. The spot welded joint has high load-bearing capacity and energy absorption capacity, and the spot welded joint has stable performance and good quality.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] A resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates, wherein the metallographic structure of the hot-rolled pickled multiphase steel plates to be welded is ferrite + bainite + Mao islands, and the plates are 800MPa grade hot-rolled pickled multiphase steel plates of the same thickness, characterized by comprising the following steps:

[0006] S1. Before welding, clean the surface of the hot-rolled pickled multiphase steel plate sample to remove foreign objects that may affect the welding quality. Then clean the sample with alcohol and let it air dry or air dry.

[0007] S2. Overlap the samples that have been cleaned and dried in step S1.

[0008] S3. Pre-compress the lapped sample from step S2 for 100ms to 600ms.

[0009] S4. Pre-weld the sample under the pre-compression state in step S3. The pre-welding current is I. P The pre-welding time is T. p The number of pre-welding times is N. p ,

[0010] Pre-welding welding current I p =a+t p Where a is the welding current, a = 8.0~10.4kA; t p The additional current, which is related to the sample thickness t (in mm), is determined as follows:

[0011] When 2.0 ≤ t < 3.0, t p =2.0kA;

[0012] When 3.0 ≤ t ≤ 4.0, t p =3.0kA;

[0013] Pre-welding welding time T p The values ​​related to the sample thickness t are as follows:

[0014] When 2.0 ≤ t < 3.0, T p =40~60ms;

[0015] When 3.0 ≤ t ≤ 4.0, T p =60~80ms;

[0016] Number of pre-welded welding operations N p The values ​​related to the sample thickness t are as follows:

[0017] When 2.0 ≤ t < 3.0, N p =1;

[0018] When 3.0 ≤ t ≤ 4.0, Np =2;

[0019] S5. Allow the pre-welded solder joints from step S4 to cool naturally for 20-40ms.

[0020] S6. Weld the initial weld point formed in step S5 under constant electrode pressure, wherein the welding current is a, the welding time is 200-400ms, and the weld point does not need to be cooled after welding and directly enters heat treatment, that is, the cooling time between welding and heat treatment is 0ms, wherein the heat treatment current is (0.5-0.9)×a, and the heat treatment time is 200-600ms.

[0021] After the heat treatment in step S7 and S6 is completed, immediately lift the electrode to allow the solder joint to air cool to room temperature.

[0022] Preferably, in S4, when N p When the value is 2, the cooling time between two adjacent pre-welding operations is 20-40 ms.

[0023] Preferably, the electrode head used in S3 to S6 has an end face diameter of 8 mm and an end face that is hemispherical.

[0024] Preferably, the pre-welding, welding + heat treatment pressure in S3 to S6 is F = (5.0 + 0.5 × t) kN, where t is the thickness of the sample in mm.

[0025] Preferably, a medium-frequency DC resistance spot welding machine is used for welding, and the angle between the central axis of the electrode cap and the welding plane of the sample is within 90°±3°.

[0026] Preferably, in step S4, the first pre-welding adopts a current ramp-up mode with a ramp-up time of 20-40ms.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1) The 800MPa grade hot-rolled pickled multiphase steel plate to be welded is relatively thick, and the multiphase steel has a high yield strength, so the springback tendency is more obvious. By pre-welding to form a plastic ring and initial weld nugget, the large gap, high contact resistance and easy spatter caused by springback can be eliminated.

[0029] 2) The first pre-welding adopts a current slow-rise mode. The smooth change of current can avoid spatter caused by rapid heating during the pre-welding process;

[0030] 3) Due to the presence of the plastic ring and the initial weld nugget, the weld nugget grows together with the weld nugget throughout the subsequent welding process, which can prevent the surrounding gas from entering and avoid the liquid weld nugget from losing its constraint and breaking the plastic ring, thus preventing spattering.

[0031] 4) After welding, the weld joint does not need to be cooled and can directly enter the heat treatment, which reduces the cooling rate after welding and tempers the weld nugget. By generating a more tough microstructure in the welding area, the failure mode of the spot weld joint changes from interface failure to pull-out failure, thereby improving the load-bearing capacity and energy absorption capacity of the spot weld joint.

[0032] 5) After heat treatment, the solder joints are air-cooled to room temperature to avoid the quenching effect of water-cooled copper electrodes, which would increase the brittleness of the solder joint structure.

[0033] 6) Good alignment of the upper and lower electrode caps can improve the sealing effect of the plastic ring on the liquid metal in the weld nugget area, avoid the impact of splashing on the size of the weld nugget and thus ensure the strength of the weld joint. At the same time, it can effectively solve the problems of weld nugget offset and weld nugget asymmetry. Detailed Implementation

[0034] The specific embodiments of the present invention will be further described below with reference to examples:

[0035] Taking 800MPa grade hot-rolled pickled multiphase steel plates with thicknesses t=2.0mm, 2.6mm and 3.5mm, all with a metallographic structure of ferrite + bainite + Mao Island as an example, welding was performed using a medium-frequency DC resistance spot welding machine. The electrode cap used had an end face diameter of 8mm and a hemispherical end face.

[0036]

Example 1

[0037] A resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates involves preparing 20 welding samples of multiphase pickled steel with a thickness of t=2.0mm, and spot welding 10 times according to the following steps:

[0038] S1. Before welding, clean the surface of the sample to remove foreign objects that may affect the welding quality, then clean the sample with alcohol and let it air dry.

[0039] S2. Overlap the samples that have been cleaned and dried in step S1.

[0040] S3. Pre-compress the sample after overlapping in step S2. The electrode pressure F = (5.0 + 0.5 × t) = (5.0 + 0.5 × 2) = 6.00 kN, and the pre-compression time is 200 ms.

[0041] S4. Perform a pre-welding once on the sample under the pre-pressure state in step S3, using a current ramp-up mode with a ramp-up time of 20ms, wherein:

[0042] When t = 2.0 mm, t p =2.0kA; N p =1;

[0043] Take T p=40ms;

[0044] Given a welding current a = 8.2 kA, the pre-welding current I is... P =a+t p =8.2 + 2.0 = 10.2 kA;

[0045] S5. Cool the pre-welded weld joints from step S4 under welding pressure for 20ms.

[0046] S6. Weld the initial weld joint formed in step S5 under constant electrode pressure F = 6.00kN. The welding current a is the same as in S4, i.e., a = 8.2kA. The welding time is 220ms. After welding, the weld joint does not need to be cooled and directly enters the heat treatment. That is, the cooling time between welding and heat treatment is 0ms. The heat treatment current is (0.5~0.9)×a=(0.5~0.9)×8.2=4.1~7.38, take 4.1kA, and the heat treatment time is 200ms.

[0047] After the heat treatment of the weld joints in step S7 and S6 is completed, the electrodes are immediately lifted to allow the weld joints to be air-cooled to room temperature, thus obtaining 10 welded samples with a thickness of t = 2.0 mm.

[0048]

Example 2

[0049] A resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates involves preparing 20 welding samples with a thickness of t = 2.6mm and spot welding them 10 times according to the following steps:

[0050] S1. Before welding, clean the surface of the sample to remove foreign objects that may affect the welding quality, then clean the sample with alcohol and dry it with air.

[0051] S2. Overlap the samples that have been cleaned and dried in step S1.

[0052] S3. Pre-compress the sample after overlapping in step S2. The electrode pressure F = (5.0 + 0.5 × t) = (5.0 + 0.5 × 2.6) = 6.30 kN, and the pre-compression time is 300 ms.

[0053] S4. Perform a pre-welding once on the sample under the pre-pressure state in step S3, using a current ramp-up mode with a ramp-up time of 20ms, wherein:

[0054] When t = 2.6 mm, t p =2.0kA; N p =1;

[0055] Take T p =60ms;

[0056] Given a welding current a = 8.2 kA, the pre-welding current I is... P =a+t p =8.2 + 2.0 = 10.2 kA;

[0057] S5. Cool the pre-welded weld joints from step S4 under welding pressure for 20ms.

[0058] S6. Weld the initial weld joint formed in step S5 under constant electrode pressure F = 6.30kN. The welding current a is the same as in S4, i.e., a = 8.2kA. The welding time is 240ms. After welding, the weld joint does not need to be cooled and directly enters the heat treatment. That is, the cooling time between welding and heat treatment is 0ms. The heat treatment current is (0.5~0.9)×a=(0.5~0.9)×8.2=4.1~7.38, which is taken as 5.7kA. The heat treatment time is 300ms.

[0059] After the heat treatment of the weld joints in step S7 and S6 is completed, the electrodes are immediately lifted to allow the weld joints to be air-cooled to room temperature, thus obtaining 10 welded samples with a thickness of t = 2.6 mm.

[0060]

Example 3

[0061] A resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates involves preparing 20 welding samples with a thickness of t=3.5mm and spot welding them 10 times according to the following steps:

[0062] S1. Before welding, clean the surface of the sample to remove foreign objects that may affect the welding quality, then clean the sample with alcohol and dry it with air.

[0063] S2. Overlap the samples that have been cleaned and dried in step S1.

[0064] S3. Pre-compress the sample after overlapping in step S2. The electrode pressure F = (5.0 + 0.5 × t) = (5.0 + 0.5 × 3.5) = 6.75 kN, and the pre-compression time is 500 ms.

[0065] S4. Perform two pre-welding operations on the sample under pre-compression conditions from step S3. The first pre-welding uses a current ramp-up mode with a ramp-up time of 40ms. The cooling time between the first and second pre-welding operations is 20ms.

[0066] When t = 3.5 mm, t p =3.0kA; N p =2;

[0067] Take T p =80ms;

[0068] Taking the welding current a = 10.0 kA, then the pre-welding current I P =a+t p =10.0 + 3.0 = 13.0 kA;

[0069] S5. Cool the pre-welded weld joints from step S4 under welding pressure for 20ms.

[0070] S6. Weld the initial weld joint formed in step S5 under constant electrode pressure F = 6.75kN. The welding current a is the same as in S4, i.e., a = 10.0kA. The welding time is 300ms. After welding, the weld joint does not need to be cooled and directly enters the heat treatment. That is, the cooling time between welding and heat treatment is 0ms. The heat treatment current is (0.5~0.9)×a=(0.5~0.9)×10.0=5.0~9.0, which is taken as 8.0kA. The heat treatment time is 500ms.

[0071] After the heat treatment of the weld joints in step S7 and S6 is completed, the electrodes are immediately lifted to allow the weld joints to be air-cooled to room temperature, thus obtaining 10 welded samples with a thickness of t = 3.5 mm.

[0072] The welding process parameters for Examples 1 to 3 are summarized in Table 1.

[0073] Table 1

[0074]

[0075]

[0076] The weld samples after welding in Examples 1 to 3 were subjected to tensile shear tests according to the method specified in ISO-18278. The tensile speed was 10 mm / min. The tensile shear load and weld nugget diameter of 10 spot weld samples in each example were averaged and the failure modes were statistically analyzed. The results are shown in Table 2.

[0077] Table 2

[0078]

[0079] As can be seen from Table 2, the technical solution provided by the present invention enables the resistance spot welded joint of 800MPa grade hot-rolled pickled multiphase steel plate to fail primarily through pull-out failure of the weld nugget. Compared with the weld joint in the interface failure mode, its load-bearing capacity and energy absorption capacity are significantly improved. Therefore, ensuring pull-out failure is of great significance for actual industrial production.

[0080] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plates, wherein the metallographic structure of the hot-rolled pickled multiphase steel plates to be welded is ferrite + bainite + Mao islands, and the plates are 800MPa grade hot-rolled pickled multiphase steel plates of the same thickness, characterized in that... Includes the following steps: S1. Before welding, clean the surface of the hot-rolled pickled multiphase steel plate sample to remove foreign objects that may affect the welding quality. Then clean the sample with alcohol and let it air dry or air dry. S2. Overlap the samples that have been cleaned and dried in step S1. S3. Pre-compress the lapped sample from step S2 for 100ms to 600ms. S4. Pre-weld the sample under the pre-compression state in step S3. The pre-welding current is I. P The pre-welding time is T. p The number of pre-welding times is N. p , Pre-welding current I p =a+t p Where a is the welding current, a = 8.0~10.4kA; t p The additional current, which is related to the sample thickness t, is determined as follows: When 2.0 ≤ t < 3.0, t p =2.0kA; When 3.0 ≤ t ≤ 4.0, t p =3.0kA; Pre-welding time T p The values ​​related to the sample thickness t are as follows: When 2.0 ≤ t < 3.0, T p =40~60ms; When 3.0 ≤ t ≤ 4.0, T p =60~80ms; Number of pre-welding times N p The values ​​related to the sample thickness t are as follows: When 2.0 ≤ t < 3.0, N p =1; When 3.0 ≤ t ≤ 4.0, N p =2; The unit of sample thickness t is mm; S5. Allow the pre-welded solder joints from step S4 to cool naturally for 20~40ms; S6. Weld the initial weld point formed in step S5 under constant electrode pressure, wherein the welding current is a, the welding time is 200~400ms, and the weld point does not need to be cooled after welding and directly enters heat treatment, that is, the cooling time between welding and heat treatment is 0ms, wherein the heat treatment current is (0.5~0.9)×a, and the heat treatment time is 200~600ms. S7. After the heat treatment in step S6 is completed, immediately lift the electrode to allow the solder joint to air cool to room temperature. In S4, when N p When the ratio is 2, the cooling time between two adjacent pre-welding operations is 20~40ms.

2. The resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plate according to claim 1, characterized in that, The electrode head used in S3~S6 has an end face diameter of 8mm and a hemispherical shape.

3. The resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plate according to claim 1, characterized in that, The pre-welding, welding, and heat treatment pressures in S3~S6 are F = (5.0 + 0.5 × t) kN, where t is the thickness of the sample in mm.

4. The resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plate according to claim 1, characterized in that, Welding was performed using a medium-frequency DC resistance spot welder, with the angle between the central axis of the electrode cap and the welding plane of the sample within 90°±3°.

5. The resistance spot welding method for 800MPa grade hot-rolled pickled multiphase steel plate according to claim 1, characterized in that, In step S4, the first pre-welding adopts a current ramp-up mode with a ramp-up time of 20~40ms.

Images