Resistance spot welding method of 1500mpa grade antioxidant hot formed steel

Abstract

The application belongs to the field of resistance spot welding, and particularly relates to a resistance spot welding method for 1500MPa-grade antioxidant hot forming steel. The antioxidant hot forming steel has cost and environmental protection advantages in the environment of cost reduction and efficiency increase in the automobile industry, but the existing hot forming steel spot welding process is prone to cracking in the fusion core part of the welding spot when the antioxidant hot forming steel is spot welded, and the cracking expands to the fusion core part, which seriously affects the strength and performance of the welding spot. Therefore, the application is directed to the spot welding process of the antioxidant hot forming steel with high alloy content, so that the antioxidant hot forming steel has great economic value and social benefits when applied to the automobile industry.

Description

Technical Field

[0001] This invention belongs to the field of resistance spot welding, specifically relating to a resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel. Background Technology

[0002] Ultra-high strength hot-formed steel represents the future direction of automotive steel development. While lightweight materials such as aluminum alloys and magnesium alloys are currently being developed, the automotive industry is undergoing a series of cost-reduction and efficiency-enhancing reforms, resulting in lower cost-effectiveness in production. Therefore, high-strength hot-formed steel will remain a necessary component for automobiles for a considerable period. Common welding processes for hot-formed steel include laser welding and resistance spot welding. Laser welding has high equipment requirements, leading to high investment costs, and also suffers from the drawback of poor heating and difficulty in welding high-reflectivity materials. Resistance spot welding has lower equipment requirements and production costs, and can weld a wider range of materials. It can meet the requirements of OEMs developing new models. A car has 3,000-6,000 spot welds, each affecting the vehicle's safety and quality; therefore, the quality of spot welding on the car body is closely related to vehicle safety.

[0003] Antioxidant hot-formed steel, based on ordinary hot-formed steel, incorporates silicon (Si) and chromium (Cr) elements. This improves the steel's hardenability, allowing it to achieve martensite formation even under air cooling. It also creates a 1-micron oxide layer on the steel substrate surface to protect it from oxidation. In actual production, compared to bare steel, antioxidant hot-formed steel allows for lower heating temperatures, shorter heating times, and faster stamping cycles. Furthermore, it eliminates the need for the environmentally unfriendly shot blasting step. Compared to aluminum-silicon coated steel, it offers similar advantages in lower heating temperatures, shorter heating times, and faster stamping cycles, and is also more affordable. However, due to its higher alloy content and carbon equivalent, antioxidant hot-formed steel exhibits significantly increased hardenability. When welded using conventional spot welding processes (low welding current with long welding time or high welding current with short welding time), spatter is severe, and cracks may appear at the weld nugget, seriously affecting the weld's strength and performance.

[0004] Based on the above research, oxidation-resistant hot-formed steel has great value in the automotive industry under the premise of cost reduction and efficiency improvement. However, under the rapid cooling conditions of resistance spot welding, the weld joints of oxidation-resistant hot-formed steel undergo hardening, which can lead to weld nugget cracking, spattering, and other problems. Therefore, the use of oxidation-resistant hot-formed steel requires the search for suitable welding processes, and systematic research on the resistance spot welding process of oxidation-resistant hot-formed steel has certain theoretical and practical value.

[0005] CN202211589297.0 discloses an anti-oxidation hot-formed steel and its production method. The microstructure of this steel after hot forming is martensitic, with a yield strength ≥1000MPa, tensile strength ≥1500MPa, elongation ≥6%, and no oxide scale peeling after hot forming. The oxide layer thickness is within 2µm. The welding current range is 8.5KA-11KA, the welding time range is 170ms-230ms, the spot welding process window is ≥1.0KA, and a hard specification is adopted, namely, high welding current and short welding time. However, the specific welding method is not specified, only the relevant parameter values ​​of the welding machine are simply described.

[0006] CN202110246842.5 discloses a resistance spot welding method for 2000MPa grade hot-stamped steel plates. When welding steel plates together, a resistance spot welding machine is used. The electrode end face diameter is 6-8mm, the welding current is 4.7-7.0KA, the spot welding time is 200-400ms, and the holding time is 250-350ms. Using this resistance spot welding method, the weld nugget diameter of each weld joint is 4.5-6.7mm, the shear resistance of each weld joint is greater than 13.23KN, and the shear and tensile properties are good, resulting in excellent welding performance.

[0007] CN108890110A discloses a resistance spot welding process to improve the quality of Q&P steel welded joints. The process employs a double-pulse welding technique, in which a single pulse current I1 is applied to the Q&P steel for one welding operation, followed by a second pulse current I2 applied to the melted weld spot to remelt the partially solidified weld nugget. This solves the problems of poor mechanical properties and severe shrinkage defects in Q&P steel welded joints. Summary of the Invention

[0008] 1. Using traditional hot-formed steel spot welding technology, single-pulse spot welding, compared to double-pulse spot welding, is prone to excessive energy and spatter, affecting mechanical properties. Double-pulse welding, on the other hand, can remelt the weld nugget from a single pulse, effectively suppressing the formation of shrinkage cavities in the center of the weld nugget and improving its overall strength, especially tensile and shear strength. Furthermore, using rigid specifications—high welding current and short welding time—increases energy consumption and electrode wear.

[0009] 2. Taking preheating measures before spot welding can reduce the cooling rate after spot welding, improve the tendency of the weld nugget to crack, and improve the strength and performance of the weld.

[0010] To address the aforementioned technical problems, this application provides the following technical solution:

[0011] This invention provides a resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel, characterized by including a preheating stage and a welding stage. The 1500MPa grade oxidation-resistant hot-formed steel, by weight percentage, comprises 0.15-0.30% C, 1.10-1.80% Mn, 0.001-0.010% B, 0.02-0.08% Ti, 0.05-0.10% Nb, Si+Cr≤5.5%, and the remainder being Fe.

[0012] During the preheating stage, the preheating current is 2-4KA and the preheating time is 100-300ms.

[0013] During the welding stage, the number of pulses is 2, the welding current I2 = I4 - I5, and the welding time is 100-600ms; where I4 is the spatter current value and I5 is 0.4-0.8KA.

[0014] Preferably, the preheating stage is followed by cooling, and the cooling time is 0-50ms.

[0015] Preferably, during the preheating stage, the preheating energy Q1 is 30-60J.

[0016] Preferably, during the welding stage, the input energy Q2 is 200-300J.

[0017] Preferably, the electrode pressure is 3-4 kN during both the preheating and welding stages.

[0018] Preferably, during the welding stage, the pulse interval time T3 is 20-40ms.

[0019] Preferably, the electrode pressure is maintained for 45-55 ms after the welding stage is completed.

[0020] Preferably, during the welding stage, the welding current I3 is [value missing] for different plate thicknesses. Where t is the thickness of the sheet metal.

[0021] Furthermore, the thickness t of the sheet metal is 1-1.8 mm.

[0022] Preferably, during the welding stage, I4 is the minimum current value that produces spatter.

[0023] Specifically, the empirical formula for carbon equivalent (CE) is as follows:

[0024]

[0025] 1. CE≤0.45%, preheating is usually not mandatory;

[0026] 2. 0.45% ≤ CE ≤ 0.6%, preheating temperature 100℃-200℃;

[0027] 3. CE > 0.6%, preheating temperature 200℃-350℃;

[0028] Antioxidant hot-formed steel has a high alloy content, resulting in a CE value greater than 0.45%, thus requiring an additional preheating step. The spot welding process consists of two stages: a preheating stage and a welding stage. The preheating stage reduces the cooling rate, decreases brittleness, improves weldability, reduces shrinkage stress to prevent weld cracking, reduces spatter, and eliminates structural rigidity of the material. The welding stage is used to form a weld nugget, employing multiple pulses to increase the microstructure and stress at the weld nugget location.

[0029] The spot welding process parameters are as follows:

[0030] The whole process:

[0031] The electrode pressure P is maintained at 3KN-4KN; the electrode pressure remains constant throughout the entire spot welding process cycle, and continues to be maintained for about 50ms after the welding current stops.

[0032] Preheating phase:

[0033] Preheating energy Q = I 2 RT = CmΔt, where: C is the specific heat capacity, m is the mass, and R is the resistance, which are all constants; Δt is the temperature obtained through the carbon equivalent formula, 30J≤Q≤60J; preheating current I1 = 2KA~4KA; preheating time T1 = 100ms~300ms; cooling time T1 = 0ms~50ms.

[0034] Welding stage:

[0035] Input energy Q = I 2 RT = CmΔt, where: C is the specific heat capacity, m is the mass, and R is the resistance, all of which are constants; Δt is the temperature obtained from the melting temperature of the steel; input energy 200J ≤ Q ≤ 300J; welding current I2 = spatter current - (0.4KA ~ 0.8KA); welding time T2 = 100ms ~ 600ms; number of pulses W = 2; pulse interval T3 = 20ms ~ 40ms; welding current for different plate thicknesses. t is the thickness of the sheet metal;

[0036] Among them, the splash current is the minimum current that produces splash.

[0037] Regarding CN202211589297.0, the steel plate involved in this invention is also an anti-oxidation hot-formed steel. A reasonable welding method is designed based on the carbon equivalent calculated according to the chemical composition. Moreover, the welding method adopts a soft specification, namely, small welding current, long welding time, and spot welding process window ≥1.0KA. Furthermore, the small welding current can reduce energy consumption and slow down the wear rate of the electrode tip.

[0038] Regarding CN202110246842.5, the material involved in this invention is oxidation-resistant hot-formed steel, which has a carbon equivalent different from that of ordinary hot-formed steel. Before the spot welding process, a preheating step is required to reduce the cooling rate and alleviate internal stress that could lead to cracking. Moreover, the spot welding process is a secondary pulse welding process. Therefore, this technical solution is not applicable to the spot welding of the oxidation-resistant hot-formed steel described in this invention.

[0039] Regarding CN108890110A, the material involved in this invention is anti-oxidation hot-formed steel. Although it is also a two-pulse spot welding process, the lack of a preheating step before the spot welding process can easily cause the weld to crack. Therefore, this technical solution is not applicable to the spot welding of the anti-oxidation hot-formed steel described in this invention.

[0040] This invention provides a spot welding process for oxidation-resistant hot-formed steel. Based on the calculation of carbon equivalent according to chemical composition, a reasonable welding method is designed, which can improve the microstructure and stress state of the weld nugget. It effectively solves the problem of easy cracking at the weld nugget when spot welding high-carbon-equivalent oxidation-resistant hot-formed steel, and improves the spot welding quality.

[0041] The technical solution of the present invention has the following advantages compared with the prior art:

[0042] Based on the calculation of carbon equivalent of the chemical composition of anti-oxidation hot-formed steel, a preheating + secondary pulse spot welding process was designed. This process can improve the microstructure and stress state of the weld nugget, solve the problem of easy cracking of the weld, improve the welding quality, and the new process design is simple and applicable to anti-oxidation hot-formed steel with different carbon equivalents. It helps to improve and solve the problem of poor weld quality in anti-oxidation hot forming, and helps the automotive industry reduce costs and increase efficiency. Attached Figure Description

[0043] Figure 1 The graph shows the comparison of resistance with and without splashing during the KSR secondary pulse.

[0044] Figure 2 A comparison curve of resistance for KSR preheating + secondary pulse without splashing.

[0045] Figure 3 This is an overall morphological image of the material after spot welding.

[0046] Figure 4 This is a morphological image of the weld nugget pulled out after spot welding, shearing, and stretching of the material of this invention.

[0047] Figure 5 This is a morphological image of the melt core falling off after spot welding, shearing, and stretching of the material of this invention. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0049] Example 1

[0050] In this embodiment, a spot welding test was conducted on 1.0 mm anti-oxidation hot-formed steel (Ceq = 1.1) according to the process method described in this invention. The welding electrode pressure was 3.5 KN. During the preheating stage, the preheating current was 3 KA and the preheating time was 200 ms. During the welding stage, the welding current was 8 KA and the welding time was 300 ms, with two pulses.

[0051] Comparative Example 1

[0052] In this comparative example, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. Spot welding tests were conducted on 1.0 mm anti-oxidation hot-formed steel (Ceq = 1.55) according to the process method described in this invention. No preheating was performed, the welding electrode pressure was 3.5 kN, the welding current was 8 kA, the welding time was 300 ms, and two pulses were used.

[0053] Example 2

[0054] In this embodiment, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, with the remainder being Fe. Spot welding tests were conducted on 1.2mm anti-oxidation hot-formed steel (Ceq = 1.55) according to the process method described in this invention. The welding electrode pressure was 3.5 kN. During the preheating stage, the preheating current was 3 kA and the preheating time was 200 ms. During the welding stage, the welding current was 8 kA and the welding time was 300 ms, with two pulses.

[0055] Comparative Example 2

[0056] In this comparative example, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. Spot welding tests were conducted on 1.2mm anti-oxidation hot-formed steel (Ceq=1.55) according to the process method described in this invention. No preheating was performed, the welding electrode pressure was 3.5KN, the welding current was 8KA, the welding time was 300ms, and two pulses were used.

[0057] Example 3

[0058] In this embodiment, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. A spot welding test was conducted on 1.4mm anti-oxidation hot-formed steel (Ceq = 1.55) according to the process method described in this invention. The welding electrode pressure was 3.5 kN. During the preheating stage, the preheating current was 3 kA and the preheating time was 200 ms. During the welding stage, the welding current was 8 kA and the welding time was 300 ms, with two pulses.

[0059] Comparative Example 3

[0060] In this comparative example, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. Spot welding tests were conducted on 1.4mm anti-oxidation hot-formed steel (Ceq=1.55) according to the process method described in this invention. No preheating was performed, the welding electrode pressure was 3.5KN, and during the welding stage, the welding current was 8KA, the welding current was 300ms, and two pulses were used.

[0061] Example 4

[0062] In this embodiment, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, with the remainder being Fe. Spot welding tests were conducted on 1.6mm anti-oxidation hot-formed steel (Ceq = 1.55) according to the process method described in this invention. The welding electrode pressure was 3.5 kN. During the preheating stage, the preheating current was 3 kA and the preheating time was 200 ms. During the welding stage, the welding current was 8 kA and the welding time was 300 ms, with two pulses.

[0063] Comparative Example 4

[0064] In this comparative example, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. Spot welding tests were conducted on 1.6mm anti-oxidation hot-formed steel (Ceq=1.55) according to the process method described in this invention. No preheating was performed, the welding electrode pressure was 3.5KN, the welding current was 8KA, the welding time was 300ms, and two pulses were used.

[0065] Example 5

[0066] In this embodiment, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, with the remainder being Fe. A spot welding test was conducted on 1.8mm anti-oxidation hot-formed steel (Ceq = 1.55) according to the process method described in this invention. The welding electrode pressure was 3.5 kN. During the preheating stage, the preheating current was 3 kA and the preheating time was 200 ms. During the welding stage, the welding current was 8 kA and the welding time was 300 ms, with two pulses.

[0067] Comparative Example 5

[0068] In this comparative example, the chemical composition is 0.25% C, 1.5% Mn, 0.005% B, 0.05% Ti, 0.08% Nb, 1.5% Si, 4% Cr, and the remainder is Fe. Spot welding tests were conducted on 1.8mm anti-oxidation hot-formed steel (Ceq=1.55) according to the process method described in this invention, without preheating, with a welding electrode pressure of 3.5KN and two pulses.

[0069] Table 1. Spot welding process parameters and results of various embodiments and comparative examples of the present invention.

[0070]

[0071] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel, characterized in that, Including the preheating and welding stages, the 1500MPa grade oxidation-resistant hot-formed steel, by weight percentage, comprises 0.15-0.30%C, 1.10-1.80%Mn, 0.001-0.010%B, 0.02-0.08%Ti, 0.05-0.10%Nb, Si+Cr=5.5%, and the remainder is Fe; During the preheating stage, the preheating current is 2-4 kA and the preheating time is 100-300 ms. During the welding stage, there are 2 pulses, the welding current I2 = I4 - I5, and the welding time is 100-600 ms; where I4 is the spatter current value and I5 is 0.4-0.8 KA; during the preheating stage, the preheating energy Q1 is 30-60 J; during the welding stage, the input energy Q2 is 200-300 J; during both the preheating and welding stages, the electrode pressure is 3-4 kN; during the welding stage, the pulse interval T3 is 20-40 ms.

2. The resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel as described in claim 1, characterized in that, The preheating stage is followed by cooling, with a cooling time of 0-50 ms.

3. The resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel as described in claim 1, characterized in that, After the welding stage is completed, the electrode pressure is maintained for 45-55 ms.

4. The resistance spot welding method for 1500MPa grade oxidation-resistant hot-formed steel as described in claim 1, characterized in that, During the welding stage, I4 is the minimum current value that will produce spatter.

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