Method for eliminating spot welding stress cracks of high-strength steel for cold stamping forming

Abstract

The invention relates to the field of metal material welding, in particular to a method for eliminating spot welding stress cracks of high-strength steel for cold stamping forming, which comprises the following steps: stamping a ring-shaped reinforcing rib with the inner diameter of 8-20mm around a welding spot of a steel plate to be welded, and enabling the ring-shaped reinforcing rib to penetrate through all welded steel plates; heat treatment is conducted on the inner area of the annular reinforcing rib, the heat treatment temperature ranges from 200 DEG C to 700 DEG C, and the heat treatment time ranges from 30 seconds to 300 seconds; and spot welding is carried out on the center position of the annular reinforcing rib, and a spot welding process is adopted. The method has the advantages that the annular reinforcing ribs are stamped around the welding spots in advance, the stress balance state of the welding area is achieved, the stress of the welding area is reduced or eliminated through the heat treatment technology, and the structure inducement caused by cracks is fundamentally reduced; in combination with a multi-pulse and current gradually-increasing welding process, through staged control of heat input, slow growth of nuggets and gradual release of stress in the welding process are promoted, and the crack sensitivity of a heat affected zone is remarkably reduced.

Description

Technical Field

[0001] This invention relates to the field of metal welding, and more particularly to a method for eliminating stress cracks in spot welding of high-strength steel used in cold stamping. Background Technology

[0002] Spot welding, as a highly efficient joining process, is widely used in the manufacturing of automobiles, home appliances, and steel structures, especially in the joining of cold-rolled steel sheets, galvanized steel sheets, and their composite steel sheets. However, during the spot welding of these materials, cracks often appear in the heat-affected zone of the weld joint, particularly noticeable in the spot welding of galvanized steel sheets. Cracks typically occur in the heat-affected zone at the weld interface of the two steel sheets, severely impacting the mechanical properties, sealing performance, and durability of the joint, thus reducing product quality and safety.

[0003] Long-term experiments and analysis have revealed that if there is significant tension or residual stress in the weld zone of steel plates before welding, the probability of welding cracks will increase significantly. This is mainly because the welding heat input reduces the interatomic bonding force of the material, leading to brittle cracking in the weld nugget and heat-affected zone under tensile stress.

[0004] To address the problem of spot welding cracks in galvanized steel sheets, existing technologies have proposed several solutions. For example, patent CN113909660A, entitled "Resistance Spot Welding Method and Resistance Spot Welding Device," controls the application of welding current by setting the slope of the current rise phase, thereby suppressing crack formation during welding of galvanized steel sheets. Patent CN108015401A, entitled "Resistance Spot Welding Method for Galvanized High-Strength Steel with Good Joint Performance," employs a three-stage welding process including preheating, formal welding, and post-heating, controlling heat input in stages to improve joint microstructure and reduce cracking tendency.

[0005] Although the above methods have improved upon these methods in terms of current control strategy and multi-stage welding process, certain limitations remain: stress in the weld area is not fully released, stress concentration still occurs during welding, and the risk of cracking is not fundamentally eliminated. Especially in situations with high welding tension or significant differences in plate thickness, existing methods still struggle to guarantee stable, crack-free welding quality. Summary of the Invention

[0006] The purpose of this invention is to provide a method for eliminating stress cracks in spot welding of high-strength steel used in cold stamping. This method involves pre-stamping annular reinforcing ribs around the weld point to create a uniform stress state in the welding zone. Heat treatment is then used to reduce or eliminate stress in the welding zone. Simultaneously, a multi-pulse spot welding process with gradually increasing current is employed to release thermal stress in stages, promoting slow formation of the weld nugget and homogenization of the microstructure. Without affecting the structural design, this method significantly reduces cracking tendency and improves the quality and reliability of the welded joint.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for eliminating stress cracks in spot welding of high-strength steel used in cold stamping includes: A circular reinforcing rib with an inner diameter of 8mm to 20mm is punched around the weld point of the steel plate to be welded, and the circular reinforcing rib runs through all the steel plates to be welded. Heat treatment is performed on the inner area of ​​the annular reinforcing rib at a temperature of 200℃~700℃ for 30 seconds~300 seconds. Spot welding is performed at the center of the annular reinforcing rib using a spot welding process. The spot welding process consists of five sequential energizing processes: The initial energizing current is I1, and the energizing time is t1. The second energizing current is I2 = A × I1, and the energizing time is t2; The third energizing current is I3 = A × I2, and the energizing time is t3; The current for the fourth energization is I4 = A × I3, and the energization time is t4; The fifth energizing current is I5 = A × I4, and the energizing time is t5; There is an interval time t0 between each power-on, and a voltage holding time t after the fifth power-on. in: The value range of I1 is 3~5KA; The values ​​of t1, t2, t3, and t4 range from 60 to 300 ms; t5 = t4 + 100ms; The value of t0 ranges from 10 to 60 ms; The value of t ranges from 100 to 300 ms; A is the current coefficient.

[0008] The inner diameter of the circular reinforcing rib is in the range of 10~18mm.

[0009] The spot welding process is a multi-pulse spot welding process, with an electrode pressure of 3.5~4.5kN and a current coefficient A of 1.2~1.4.

[0010] The value of the initial energizing current I1 ranges from 3.5 to 4.5 kA.

[0011] The energizing times t1, t2, t3, and t4 are the same, and their values ​​range from 80 to 200 ms.

[0012] The interval time t0 ranges from 20 to 40 ms.

[0013] The holding time t ranges from 150 to 250 ms.

[0014] The steel plate to be welded is cold-rolled plate, galvanized plate, or a combination of both.

[0015] The stamping process of the steel plate to be welded with circular reinforcing ribs is completed before welding, and the position of the circular reinforcing ribs is concentric with the weld point.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. By pre-pressing circular reinforcing ribs around the weld point, a stress balance in the welding area is achieved. Heat treatment reduces or eliminates stress in the welding area, fundamentally reducing the structural factors that induce cracking. Combined with a multi-pulse welding process with gradually increasing current, and further through staged control of heat input, the slow growth of the weld nugget and the gradual release of stress during welding are promoted, significantly reducing the crack sensitivity of the heat-affected zone. Experimental results show that no cracks appeared in the embodiments using this invention, while cracking problems were prevalent in the comparative examples. 2. The multi-pulse, gradually increasing current welding process proposed in this invention adapts to the welding requirements of different plate thicknesses and materials (such as cold-rolled plates, galvanized plates, and their combinations) through five gradual heat inputs via energization, and has a wide operating window. All process parameters (such as current coefficient A, energization time, interval time, and holding time) are set within the optimized range, which ensures sufficient formation of the weld nugget while avoiding welding defects such as spatter and overheating, thereby improving the controllability and repeatability of the process. 3. The design of the circular reinforcing ribs not only serves to disperse stress but also provides local reinforcement, which helps to improve the overall rigidity and load-bearing capacity of the welded joint. The inner diameter of the reinforcing ribs is set within the range of 8-20mm, which does not affect the original structural design of the sheet metal parts and ensures the feasibility of welding positioning and the operability of the stamping process. 4. The phased heat input method results in a smoother welding thermal cycle, which helps to refine the weld nugget structure, reduce intergranular segregation and embrittlement tendency, thereby improving the toughness, fatigue performance and sealing performance of the joint. It is especially suitable for manufacturing fields such as automobiles and home appliances where high welding quality is required. 5. This invention does not require complex equipment modifications or special electrode structures. It only requires adjusting the welding program on conventional spot welding equipment and adding a stamping process. The implementation cost is low and it is easy to promote and apply on existing production lines. Attached Figure Description

[0017] Figure 1 This is a schematic diagram showing the location of the reinforcing ribs and weld points.

[0018] Figure 2 This is a cross-sectional diagram.

[0019] In the diagram: 1. Welded steel plate; 2. Weld point; 3. Circular reinforcing rib; 4. Circular diameter. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0021] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0022] Example I. Step-by-step instructions; First, determine the spot welding location, and then stamp a circular reinforcing rib with an inner diameter of 8-20mm around the weld point 2. The circular reinforcing rib 3 runs through all the steel plates 1 being welded. The purpose of stamping the circular reinforcing rib 3 is to create a uniform stress area around the weld point 2, with the circular reinforcing rib 3 bearing the deformation tension of the workpiece, thereby reducing stress concentration in the welding area. If the inner diameter of the circular reinforcing rib is less than 8mm, subsequent welding positioning will be difficult, the sheet metal deformation area will be small, and the stamping difficulty will be high. If the inner diameter of the ring is greater than 20mm, it will affect the structural design of the sheet metal.

[0023] The steel plate is heat-treated inside the annular reinforcing ribs at a temperature of 200℃ to 700℃ for 30 to 300 seconds.

[0024] Spot welding is performed at the center of the annular reinforcing rib 3 using a multi-pulse spot welding process with the following parameters: The electrode pressure P is 3~5kN, and the current coefficient A is 1.1~1.5.

[0025] The welding process involves five sequential applications of electricity: The initial energizing current is I1, and the energizing time is t1. The second energizing current is I2 = A × I1, and the energizing time is t2; The third energizing current is I3 = A × I2, and the energizing time is t3. The fourth energizing current is I4 = A × I3, and the energizing time is t4. The fifth energizing current is I5 = A × I4, and the energizing time is t5.

[0026] There is an interval time t0 between each power-on, and a voltage holding time t after the fifth power-on.

[0027] II. Basis for parameter selection; The heat treatment temperature is set between 200℃ and 700℃, and the heat treatment time is between 30 and 300 seconds. This is to eliminate stress in the weld zone without altering the microstructure of the steel plate. For high-strength steel, the heat treatment temperature must not exceed the material's annealing temperature to prevent excessively high temperatures from altering the material's properties. Conversely, if the heat treatment temperature is too low, the stress-relieving effect decreases, requiring a longer heat treatment time, which directly impacts production efficiency.

[0028] The starting current I1 is selected based on the plate thickness and material of the welding material, with a value range of 3~5KA. The energizing times t1, t2, t3, and t4 can be the same or different within the range of 60~300ms; t5 is 100ms longer than t4, i.e., t5 = t4 + 100ms, to ensure the final weld nugget size. The interval time t0 is set to 10~60ms, and the holding time t is set to 100~300ms.

[0029] III. Explanation of the process principle; A multi-pulse energizing scheme is adopted to gradually input heat to weld point 2, causing the weld nugget to grow slowly. The weld nugget formed during the first energizing is released due to the welding thermal cycle. With subsequent energizing, the weld nugget grows further, and the stress in the welding area continues to be released. This process is repeated until the stress in the welding area is completely released before the fifth energizing. At this point, high current and long-term welding parameters are used to obtain the target weld nugget diameter.

[0030] Gradually increasing the current ensures that the resistance heat generated each time is higher than the previous time, which is beneficial for expanding the weld nugget diameter, further releasing the stress on the steel plate, and eliminating defects that may have occurred in the previous welding. The current coefficient A ranges from 1.1 to 1.5. When the current coefficient A is below 1.1, the increase in welding current is not significant, and the increase in welding resistance heat is small, which cannot achieve the desired effect of this invention. When the current coefficient A is above 1.5, the welding current increases too quickly, which can easily cause weld nugget spatter during welding, resulting in a decrease in the quality of the weld joint. The energizing current I1 is the starting current of the spot welding process of this invention. Due to the different resistances of different steel plates, the resistance heat generated during welding varies slightly. In order to achieve the gradually increasing welding process, this current design is such that when it is less than 3KA, a weld nugget cannot be formed; when it is greater than 5KA, the starting circuit is too large, the current increases too quickly, and welding spatter is likely to occur. The energizing times t1, t2, t3, t4, and t5 are important parameters for the formation of the weld nugget. A shorter energizing time results in less resistance heat and cannot form a weld nugget. A longer energizing time results in more resistance heat, which cannot meet the process requirements of the multiple pulse welding of this invention. Therefore, the welding time is between 60 and 300 ms. The fifth energizing time t5 is added to increase the welding heat input and obtain the target weld nugget size. Based on welding experience and actual welding results, it is more appropriate to increase the energizing time t5 by 100 ms compared to t4.

[0031] The interval time t0 is the time for each power-on and power-off cycle. To ensure the continuity of power supply to solder joint 2, this time is set to 10~60ms. A time less than 10ms results in poor stability during production control, while a time greater than 60ms severely affects the welding cycle and has no positive effect on the process effect of this invention.

[0032] The holding time t is to allow the solder joint 2 to obtain the cooling time provided by the electrode. When the holding time is less than 100ms, hot cracks are likely to occur at the joint. When the holding time exceeds 300ms, the weld has already cooled to a lower temperature, and increasing the holding time has no positive effect on the present invention.

[0033] To verify the effectiveness of this method, seven sets of exemplary examples (Examples 1-7) and three sets of comparative examples (Comparative Examples 1-3) were designed. The thickness of the steel plates used and the inner diameter of the circular reinforcing ribs are shown in Table 1. Table 1 shows the data for welded steel plates and circular reinforcing ribs.

[0034] Table 1 shows that all embodiments are provided with annular reinforcing ribs, while the comparative examples are not provided with reinforcing ribs.

[0035] Table 2 shows the welding process specifications.

[0036] Examples 1-7 employ a multi-pulse current incremental process, with the current coefficient A ranging from 1.1 to 1.6, and the energizing time, interval time, and holding time all within the specified range. Comparative Examples 1-3 utilize a traditional single-pulse welding process, involving only one large current energization over a long period, without a multi-pulse process.

[0037] After welding, crack detection was performed on the weld joints, and the results are shown in Table 3: Table 3 shows the results of the weld point inspection.

[0038] Experimental results show that no welding cracks appeared in all embodiments using the method of this invention (circular reinforcing ribs combined with multi-pulse incremental current process), and all were evaluated as qualified; while the comparative examples using the traditional process without reinforcing ribs all showed cracks and were evaluated as unqualified. This indicates that the present invention can effectively eliminate spot weld cracks and improve the quality of welded joints.

[0039] This invention achieves stress equilibrium in the welding area by pre-pressing annular reinforcing ribs around the weld point. Heat treatment further reduces or eliminates stress in the welding area, fundamentally minimizing structural factors that induce cracking. Combined with a multi-pulse, gradually increasing current welding process, and through staged control of heat input, it promotes slow nugget growth and gradual stress release during welding, significantly reducing the crack sensitivity of the heat-affected zone. Experimental results show that no cracks appeared in the embodiments using this invention, while cracking was prevalent in the comparative examples. The multi-pulse, gradually increasing current welding process proposed in this invention, through five stages of gradual heat input, adapts to the welding needs of different plate thicknesses and materials (such as cold-rolled steel, galvanized steel, and combinations thereof), and has a wide operating window. All process parameters (such as current coefficient A, energizing time, interval time, and holding time) are set within the optimized range. This design ensures sufficient weld nugget formation while avoiding welding defects such as spatter and overheating, thus improving process controllability and repeatability. The circular reinforcing ribs not only disperse stress but also provide local reinforcement, helping to enhance the overall rigidity and load-bearing capacity of the welded joint. The inner diameter of the reinforcing ribs is set within the range of 8-20mm, which does not affect the original structural design of the sheet metal parts and ensures the feasibility of welding positioning and the operability of the stamping process. The staged heat input method results in a smoother welding thermal cycle, which is conducive to refining the weld nugget structure, reducing intergranular segregation and embrittlement tendency, thereby improving the toughness, fatigue performance, and sealing performance of the joint. It is particularly suitable for manufacturing fields such as automobiles and home appliances where high welding quality is required. No complex equipment modification or special electrode structure is required. Only the welding program needs to be adjusted on conventional spot welding equipment and an additional stamping process needs to be added, resulting in low implementation costs and easy promotion and application on existing production lines.

Claims

1. A method for eliminating stress cracks in spot welding of high-strength steel used in cold stamping, characterized in that, include: A circular reinforcing rib with an inner diameter of 8mm to 20mm is punched around the weld point of the steel plate to be welded, and the circular reinforcing rib runs through all the steel plates to be welded. Heat treatment is performed on the inner area of ​​the annular reinforcing rib at a temperature of 200℃~700℃ for 30 seconds~300 seconds. Spot welding is performed at the center of the annular reinforcing rib using a spot welding process. The spot welding process consists of five sequential energizing processes: The initial energizing current is I1, and the energizing time is t1. The second energizing current is I2 = A × I1, and the energizing time is t2; The third energizing current is I3 = A × I2, and the energizing time is t3; The current for the fourth energization is I4 = A × I3, and the energization time is t4; The fifth energizing current is I5 = A × I4, and the energizing time is t5; There is an interval time t0 between each power-on, and a voltage holding time t after the fifth power-on. in: The value range of I1 is 3~5kA; The values ​​of t1, t2, t3, and t4 range from 60 to 300 ms; t5 = t4 + 100ms; The value of t0 ranges from 10 to 60 ms; The value of t ranges from 100 to 300 ms; A is the current coefficient.

2. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The inner diameter of the circular reinforcing rib is in the range of 10~18mm.

3. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The spot welding process is a multi-pulse spot welding process, with an electrode pressure of 3.5~4.5kN and a current coefficient A of 1.2~1.

4.

4. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The value range of the first energizing current I1 is 3.5~4.5KA.

5. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The energizing times t1, t2, t3, and t4 are the same, and their values ​​range from 80 to 200 ms.

6. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The interval time t0 ranges from 20 to 40 ms.

7. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The holding time t ranges from 150 to 250 ms.

8. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The steel plate to be welded is a cold-rolled plate, a galvanized plate, or a combination of both.

9. The method for eliminating stress cracks in spot welding of high-strength steel for cold stamping as described in claim 1, characterized in that, The stamping process of the circular reinforcing ribs on the steel plate to be welded is completed before welding, and the position of the circular reinforcing ribs is concentric with the weld point.

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