Resistance spot welding process of 2000MPa grade hot-formed steel with aluminum-silicon coating

Abstract

The present invention discloses a resistance spot welding process for 2000MPa-grade hot-formed steel with an aluminum-silicon coating. The process steps are as follows: 1) performing a first weld on the steel plate to be welded; the first weld has a welding current I1=I2-(0.8-1.2)KA, a welding time of 500ms-1000ms, and a cooling time of 20ms-40ms; 2) performing a second weld on the steel plate to be welded; the second weld has a welding current I2=I-0.2KA, a welding time of 200ms-320ms, and a cooling time of 20ms-40ms, where I is the spatter current; and 3) performing a temper weld on the steel plate to be welded; the temper weld has a welding current I3=0.5I1-0.7I1, and a temper pulse time of 120ms-500ms. This process significantly improves cross-tensile properties, prevents the generation of macroscopic defects in spot weld joints, and solves the problem of low cross-tensile properties in aluminum-silicon coated weld joints, thereby improving the overall performance of the weld.

Description

Technical Field

[0001] The invention relates to a welding process, in particular to a resistance spot welding process for 2000MPa grade hot-formed steel with an aluminum-silicon coating. Background Art

[0002] Hot-formed ultra-high-strength steel, due to its high strength, high energy absorption rate, strong forming ability, low springback, and excellent dent resistance, has been widely used in the manufacture of impact- and collision-resistant automotive safety structures. To prevent surface oxidation and decarburization during the hot stamping process, an aluminum-silicon coating is often applied to the steel sheet, which has been accepted and widely used by most automotive manufacturers. Low-cost, easy-to-use, and highly efficient resistance spot welding is a key welding method in the automotive industry, with approximately 4,000 to 6,000 resistance spot welds on each car body. Resistance spot welding, which applies pressure to the assembled welded parts using electrodes and generates resistance heat through the contact surface and adjacent areas of the joint, has been widely used in aerospace, electronics, automotive, and household appliances. Due to the special physical and chemical properties of ultra-high-strength hot stamping steel, its large thermal expansion coefficient and high high-temperature plasticity easily lead to defects such as shrinkage holes in the weld core center. To ensure the quality of automotive plate welds, the comprehensive mechanical properties of the welds are extremely important, especially the cross-tension performance index, which is the main performance indicator. Currently, the cross-tension performance of 2000MPa-grade hot-formed steel with aluminum-silicon coating after resistance spot welding in the automotive manufacturing process is low, which seriously affects the use of 2000MPa-grade hot-formed steel. Therefore, it is necessary to improve the cross-tension performance of 2000MPa-grade hot-formed steel with aluminum-silicon coating after resistance spot welding.

[0003] Chinese patent application No. 202110897788.0 provides "a welding process for improving the tensile shear force and plasticity of 2GPa steel welded joints"; this process solves the problem of poor mechanical properties of 2GPa steel welded joints due to the higher carbon equivalent, but only the tensile shear performance was tested, and the cross-tensile performance of the important plasticity index of the weld was not tested. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a resistance spot welding process for 2000MPa grade hot-formed steel with aluminum silicon coating, so as to effectively improve the comprehensive performance of the welding spot.

[0005] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows: (1) the steel plate to be welded is welded for the first time; the welding current of the first welding is I1=I2-(0.8~1.2)KA, the welding time is 500ms~1000ms, and the cooling time is 20ms~40ms;

[0006] (2) Perform a second welding on the steel plate to be welded; the welding current of the second welding is I2=I-0.2KA, the welding time is 200ms~320ms, and the cooling time is 20ms~40ms, where I is the spatter current;

[0007] (3) Temper welding is performed on the steel plate to be welded; the welding current of temper welding is I3=0.5I1~0.7I1, and the tempering pulse time is 120ms~500ms.

[0008] Furthermore, the electrode pressure during the welding process is 3.5 kN to 4.5 kN. Furthermore, the electrode pressure is maintained for 200 ms to 300 ms after the welding current stops.

[0009] Furthermore, the hot-formed steel is in a quenched state and has a thickness of 1.2 mm to 1.8 mm, and the number of welds is two layers of overlapping steel plates.

[0010] The beneficial effects of the above technical solution are as follows: the present invention adopts a first welding-second welding-tempering welding process, and during the first welding, a spatter-free welding is applied to the aluminum-silicon coated 2000MPa grade hot-formed steel for a long time and a high welding current, thereby widening the width of the softening zone; the second welding further melts the melted weld spot to obtain a larger weld core diameter and a higher tensile shear strength; the tempering welding slowly cools the weld core metal to improve the plasticity of the weld spot; thereby greatly improving the cross-tensile performance and preventing the occurrence of macro defects of the spot welding joint, such as shrinkage, shrinkage cavities, and cracks, thereby solving the problem of low cross-tensile performance of the aluminum-silicon coated 2000MPa grade hot-formed steel weld joint, thereby improving the comprehensive performance of the weld spot. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0012] Figure 1 This is the resistance spot welding process diagram of the present invention;

[0013] Figure 2 is a diagram of the softening zone width under the process conditions described in Examples 1, 2, and 3 of the present invention;

[0014] Figure 3 This is a photograph of the fracture of the cross tensile specimen of Example 3 of the present invention;

[0015] Figure 4 This is a photograph of the fracture of the cross tensile specimen of Example 6 of the present invention;

[0016] Figure 5 This is a photograph of the fracture of the cross tensile specimen of Example 7 of the present invention. DETAILED DESCRIPTION

[0017] The resistance spot welding process of the aluminum-silicon coated 2000MPa grade hot-formed steel is as follows: the grade of the 2000MPa grade aluminum-silicon coated hot stamping steel plate is CR1300 / 2000HS+AS, the thickness is 1.2mm-1.8mm, and the steel plate is in the quenched state; the chemical composition of the steel plate is calculated by mass percentage as follows: C 0.30%-0.40%, Si 0.20%-0.30%, Mn 1.10%-1.50%, P≤0.02%, S≤0.02%, Als 0.030%-0.050%, B 0.0015%-0.0025%, Ti 0.020%-0.050%, Cr 0.10%-0.50%, V 0.10%~0.50%, the balance is Fe and unavoidable impurity elements; the thickness of the Al-Si coating on one side is 25±5um, and its composition and mass percentage are: Al≥85%, Si 5%~11%, Fe 2%~4%, and the balance is other unavoidable impurity elements.

[0018] Figure 1 As shown, the process steps of the resistance spot welding process of the aluminum-silicon coated 2000MPa grade hot-formed steel are as follows:

[0019] (1) After cleaning the surface of the steel plates to be welded, overlap the two layers of steel plates; use a medium frequency inverter resistance welding machine and use a larger electrode pressure during welding, which is 3.5KN to 4.5KN;

[0020] (2) First welding: The first welding is performed on the steel plate to be welded; the welding current for the first welding is I1=I2-(0.8~1.2)KA, and the welding time is t w1 500ms~1000ms, cooling time t c1 20ms~40ms;

[0021] (3) Second welding: perform the second welding on the steel plate to be welded; the welding current of the second welding is I2=I-0.2KA, and the welding time is t w2 200ms~320ms, cooling time t c2 The time duration is 20ms to 40ms; I is the spatter current, which is the minimum current that produces welding spatter when single-pulse spot welding is used;

[0022] (5) Tempering welding: Temper welding is performed on the steel plate to be welded; the welding current for tempering welding is I3 = 0.5I1 ~ 0.7I1, and the tempering pulse time is t w3 The cooling time is 120ms~500ms, and the cooling time t c3 20ms~40ms; holding time t after welding current stops H It is 200ms to 300ms.

[0023] Example 1-7: The resistance spot welding process of the aluminum-silicon coated 2000MPa grade hot-formed steel adopts the following specific process.

[0024] (1) The steel plate is CR1300 / 2000HS+AS with a thickness of 1.2 mm to 1.8 mm. The above-mentioned first welding-second welding-tempering welding process is adopted. The specific process parameters of each embodiment are shown in Table 1;

[0025] Table 1: Process parameters of various examples

[0026]

[0027] In Table 1, the splashing current I of each embodiment and comparative example is 7.8 kA; the comparative example is a comparative case using a conventional process.

[0028] (2) The performance of the welded joints obtained in each embodiment was tested. The tensile shear test specimen of the comparative example showed interface fracture, and the cross tensile force showed a button fracture mode. The tensile shear force and cross tensile force of the weld point of the embodiment both showed a button fracture mode. The test results are shown in Table 2.

[0029] Table 2: Welding joint performance results of various examples

[0030]

[0031] Figure 2 The softening zone width diagram of different process conditions described in Examples 1, 2 and 3; Figure 2 It can be seen that since this process adopts a method of first welding with a small current for a long time and then welding with a large current for a short time, and then tempering, spatter-free welding is guaranteed during the welding process of 2000MPa-grade hot-formed steel with aluminum silicon coating; first welding with a small current for a long time and then welding with a large current for a short time, a larger weld core diameter and heat-affected softening zone are obtained, and increasing the weld core diameter can improve the weld strength; the tempering current is slow cooling, which improves the plasticity of the weld; thereby greatly improving the cross tensile properties and slightly improving the tensile shear strength. Figure 3 、 Figure 4 and Figure 5 The fracture photos of the cross tensile specimens described in Examples 3, 6 and 7 are respectively Figure 3 、 Figure 4 and Figure 5 It can be seen that under the same process conditions, the cross tensile macro fracture diagram shows that with the increase of tempering time, the cross tensile specimen button becomes larger, which improves the comprehensive performance of the solder joint.

Claims

1. A resistance spot welding process for 2000MPa grade hot-formed steel with aluminum silicon coating, characterized in that: The process steps are as follows: (1) After cleaning the surface of the steel plates to be welded, overlap the two layers of steel plates; use a medium frequency inverter resistance welding machine and use a large electrode pressure of 3.5KN to 4.5KN during the welding process; (2) First welding: The first welding is performed on the steel plate to be welded; the welding current for the first welding is I1=I2-(0.8~1.2)KA, and the welding time is t w1 500ms~1000ms, cooling time t c1 20ms~40ms; (3) Second welding: perform the second welding on the steel plate to be welded; the welding current of the second welding is I2=I-0.2KA, and the welding time is t w2 200ms~320ms, cooling time t c2 The time duration is 20ms to 40ms; I is the spatter current, which is the minimum current that produces welding spatter when single-pulse spot welding is used; (4) Tempering welding: Temper welding is performed on the steel plate to be welded; the welding current for tempering welding is I3 = 0.5I1 ~ 0.7I1, and the tempering pulse time is t w3 The cooling time is 120ms~500ms, and the cooling time t c3 20ms~40ms; holding time t after welding current stops H It is 200ms to 300ms.

2. The resistance spot welding process for 2000MPa grade hot-formed steel with aluminum-silicon coating according to claim 1, characterized in that: The hot-formed steel is in a quenched state and has a thickness of 1.2 mm to 1.8 mm.

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