Resistance spot welding method for strip cast-rolling hot forming steel

Abstract

The invention belongs to the field of steel production, and particularly relates to a resistance spot welding method for thin-strip cast-rolled hot-formed steel. The invention relates to a resistance spot welding method for thin strip cast rolling hot forming steel. The hot forming steel comprises the following chemical components in percentage by mass: 0.19-0.30% of C, 0.15-0.80% of Cr, 0.2-0.6% of Si, 1.20-1.80% of Mn, less than or equal to 0.0012% of B, less than or equal to 0.01% of Ti, less than or equal to 0.06% of Nb and the balance of Fe and inevitable impurities. The hot forming steel is manufactured through a double-roller casting and rolling process, the carbon equivalent of the hot forming steel is larger than 0.5%, the thickness of the hot forming steel ranges from 0.9 mm to 1.8 mm, and a first hot forming part is manufactured based on the hot forming steel; the first hot forming part is connected with a second part manufactured through another technology through a resistance spot welding technology, and the resistance spot welding technology comprises the following technological parameter combination that the electrode pressure F is controlled to be 2 kN to 3 kN; the welding time T1 is controlled to be 200-400 ms; and the welding current I1 is controlled to be 7.5-13.5 kA according to the thickness of the plate.

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to a resistance spot welding method for thin strip cast and rolled hot-formed steel. Background Art

[0002] Ultra-high-strength hot-formed steel is the future of automotive steel. While lightweight materials like aluminum and magnesium alloys are currently being developed, the automotive industry is undergoing a series of cost-cutting and efficiency-enhancing changes, resulting in these materials being less cost-effective in practical applications and often limited to high-end models. Therefore, high-strength hot-formed steel will remain the mainstream steel for body-in-white (BIW) applications for a long time to come.

[0003] Shagang's thin strip casting and rolling process is a short-process twin-roll cast and rolling strip steel production technology that has significant cost advantages over continuous casting and rolling production technology. This is mainly because thin strip cast and rolled steel production omits the billet stage compared to conventional hot rolling production. The molten steel is directly solidified into a 1.4mm thick steel strip, which is then hot-rolled in one pass to produce hot-rolled thin strip steel with a thickness of 0.7 to 1.9mm, thus shortening the production line. At the same time, fewer production processes result in less energy consumption and CO2 emissions. Therefore, the future application of hot-formed thin strip cast and rolled steel in white bodies will not only help automakers reduce costs and increase efficiency, improve the market competitiveness of some models, but will also lead my country's automotive manufacturing industry towards a more low-carbon direction.

[0004] Currently, welding processes for steel car bodies include laser welding and resistance spot welding. Laser welding requires high equipment, resulting in high investment costs, and also has drawbacks such as difficulty welding in corners and confined spaces. Resistance spot welding offers lower equipment requirements and production costs, faster production cycles, greater flexibility, and the ability to weld a wider range of materials. Resistance spot welding can meet the requirements of OEMs developing new models. A car has 3,000 to 6,000 spot welds, each of which impacts the safety and quality of the car body. Therefore, the quality of spot welds on a car body is closely linked to the safety of the car.

[0005] Based on the good development trend of high-performance hot-formed thin-strip cast steel, it is very necessary to find a suitable resistance spot welding process for this material in the test phase before actual production, which has high practical value.

[0006] Currently, the commonly used processes for spot welding high-strength steel plates include low current and long welding times, welding followed by tempering, or preheating, welding, and tempering. These processes all increase the total welding time and cannot meet the actual needs of high-performance hot-formed thin-strip cast steel.

[0007] Patent document CN109079304A discloses a spot welding process method for high carbon equivalent cold-rolled dual-phase steel, and provides a spot welding process method for cold-rolled dual-phase steel with a carbon equivalent ≥0.5%. The method adopts a method of multiple pulses plus preheating, welding, and slow cooling in multiple stages, which is essentially different from the hot-rolled thin-strip cast steel targeted by the present invention. The process is not suitable for high-performance hot-formed thin-strip cast steel, and the process has many steps and takes a long time, which is not conducive to ensuring production efficiency.

[0008] Patent document CN115889958A discloses a resistance spot welding method for pickled high-strength automotive steel. This method replaces the copper alloy end face material where the electrode contacts the steel plate with pure tungsten and employs a three-stage pulse process. This process is also unsuitable for high-performance hot-formed thin-strip cast steel, increases electrode tip cost, reduces electrode heat dissipation efficiency, prolongs the welding cycle, and increases process costs.

[0009] It can be seen that in the existing technology, the resistance welding process for cold-rolled dual-phase steel or various other materials cannot be perfectly applied to high-performance hot-formed thin-strip cast steel. Many of these processes have the characteristics of multiple pulses and multiple stages, which significantly reduces production efficiency and is not conducive to manufacturers reducing costs and increasing efficiency. Summary of the Invention

[0010] This invention proposes a resistance welding process for high-performance hot-formed thin-strip cast steel. The welded parts produced by this process meet the performance requirements of automotive parts and resolve the resistance spot welding issues faced by high-performance hot-formed thin-strip cast steel in automotive parts production.

[0011] According to a first aspect of the present invention, a resistance spot welding method for thin strip cast hot-formed steel is provided:

[0012] (1) The chemical composition and mass fraction of the hot-formed steel are:

[0013] C: 0.19-0.30%, Cr: 0.15-0.80%, Si: 0.2-0.6%, Mn: 1.20-1.80%, B: ≤0.0012%, Ti: ≤0.01%, Nb: ≤0.06%, the rest is Fe and unavoidable impurities;

[0014] (2) The hot-formed steel is manufactured by a twin-roll casting process, the carbon equivalent of the hot-formed steel is greater than 0.5%, and the thickness is 0.9 to 1.8 mm, and a first hot-formed part is manufactured based on the hot-formed steel;

[0015] (3) connecting the first thermoformed part to a second part made by another process by resistance spot welding;

[0016] The resistance spot welding process includes the following process parameter combinations:

[0017] The electrode pressure F is controlled as: F = 2 ~ 3kN;

[0018] The welding time T1 is controlled to be T1=200~400ms;

[0019] The welding current I1 is controlled to be 7.5~13.5kA according to the thickness of the plate.

[0020] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the electrode pressure F remains unchanged during the entire spot welding process cycle.

[0021] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, after the welding current I1 stops, the electrode pressure F is continuously maintained, wherein the holding time is controlled to be T2 = 100-300 ms.

[0022] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, when the thickness of the first hot-formed part and / or the second part exceeds 1.6 mm, the welding time is increased to T1 = 300-400 ms.

[0023] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the welding in the resistance spot welding process adopts a single pulse.

[0024] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the welding current I1 is further controlled to be 9.5-10.5 kA.

[0025] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the welding current I1 is further controlled to be 10-10.5 kA.

[0026] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the second part made by the other process is a part made of thin strip cast steel, or the second part made by the other process is made of 22MnB5 hot-formed steel with a thickness range of 1.2 to 1.8 mm.

[0027] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the first hot-formed part and the second part have the same thickness.

[0028] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the hot-formed steel is subjected to a pickling process to remove surface iron oxide scale.

[0029] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the first hot-formed part is manufactured by a hot stamping process, and the mechanical properties of the first hot-formed part are: yield strength ≥950MPa, tensile strength ≥1300MPa, and elongation ≥5%.

[0030] According to the resistance spot welding method for thin strip cast hot-formed steel of the present invention, preferably, the hot-formed steel is shot peened to remove surface iron oxide scale generated during the hot forming heating process.

[0031] According to a second aspect of the present invention, a welded part is provided. The welded part is formed by welding the first thermoformed part and the second part using the aforementioned resistance spot welding method.

[0032] In the present invention, the carbon equivalent of high-performance hot-formed thin-strip cast steel is calculated to be greater than 0.5% according to the carbon equivalent calculation formula, and its weldability is worse than that of traditional materials. Therefore, the spot welding process suitable for this material is tested by adjusting the empirical parameters.

[0033] The high-performance hot-formed thin-strip cast steel involved in the present invention has a thickness of 1.2 mm, and the other plate overlapped with it is a thin-strip cast steel of the same thickness or a traditional 22MnB5 hot-formed steel with a thickness of 1.2 to 1.8 mm.

[0034] The parameter settings are as follows:

[0035] The electrode pressure F = 2 ~ 3kN, the electrode pressure remains unchanged during the entire spot welding process cycle, and when the welding current stops, the electrode pressure continues to be maintained for T2 = 100 ~ 300ms;

[0036] Welding time T1 = 200 ~ 400ms. When the thickness of a plate exceeds 1.6mm, the welding time should be appropriately increased to T1 = 300 ~ 400ms.

[0037] The welding current I1 is adjusted appropriately according to the thickness of the plate. In all cases, I1=I min ~I max .

[0038] Specifically for each set of overlapping material combinations, the process ranges are:

[0039] Table 1 1.2mm thin strip casting and 1.2mm thin strip casting process

[0040]

[0041] Table 2 1.2mm thin strip casting and 1.2mm ordinary bare plate process

[0042]

[0043] Table 3 1.2mm thin strip casting and 1.4mm ordinary bare plate process

[0044]

[0045] Table 4 1.2mm thin strip casting and 1.6mm ordinary bare plate process

[0046]

[0047] Table 5 1.2mm thin strip casting and 1.8mm ordinary bare plate process

[0048]

[0049] Beneficial technical effects

[0050] Compared with the prior art, the technical concept and corresponding technical solutions of the present invention can at least achieve the following beneficial technical effects:

[0051] This invention provides a resistance welding process for high-performance hot-formed thin-strip cast steel. The resulting welds meet the performance requirements of automotive parts, resolving the resistance spot welding issues faced by the application of high-performance hot-formed thin-strip cast steel in automotive parts production. The welds produced within this new process exhibit shear performance that meets the requirements for automotive parts. The new process is simple in design, requiring only a single pulse to produce high-quality welds. This reduces welding time and helps improve production efficiency, contributing to cost reduction and efficiency gains for the automotive industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings of the embodiments. Obviously, the drawings described below only relate to some embodiments of the present invention, and are not intended to limit the present invention.

[0053] Figure 1 This is the sample obtained in Example 1. The nugget was pulled out after testing.

[0054] Figure 2 This is the sample obtained in Comparative Example 1. The bonding surface was broken after the test.

[0055] Figure 3 This is the sample obtained in Example 2. The nugget was pulled out after testing.

[0056] Figure 4 This is the sample obtained in Comparative Example 2. The bonding surface was broken after the test.

[0057] Figure 5 This is the sample obtained in Example 3. The nugget was pulled out after testing.

[0058] Figure 6 This is the sample obtained in Comparative Example 3. The bonding surface was broken after the test.

[0059] Figures 7-11 It is a graphic representation of the process range for each set of overlapping material combinations. DETAILED DESCRIPTION

[0060] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0061] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0062] The following are embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments that can be obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention.

[0063] In the present invention, the specific description of the spot welding process parameters used in each embodiment and comparative example is as follows:

[0064] Example 1

[0065] In the welding test of 1.2 mm ordinary bare plate and 1.2 mm thin strip cast steel, the process parameters within the process range of the process of the present invention were adopted: electrode pressure 3 kN, welding time 200 ms, single pulse, and welding current 10 kA. Figure 1 This is the sample obtained in Example 1. The nugget was pulled out after testing.

[0066] Comparative Example 1

[0067] In the welding test of 1.2mm ordinary bare plate and 1.2mm thin strip cast steel, the welding parameters are: electrode pressure 3kN, welding time 200ms, single pulse, welding current 9kA. Figure 2 This is the sample obtained in Comparative Example 1. The bonding surface was broken after the test.

[0068] Example 2

[0069] In the welding test of 1.6 mm ordinary bare plate and 1.2 mm thin strip cast steel, the process parameters within the process range of the process of the present invention were adopted: electrode pressure 3 kN, welding time 400 ms, single pulse, and welding current 10 kA. Figure 3 This is the sample obtained in Example 2. The nugget was pulled out after testing.

[0070] Comparative Example 2

[0071] In the welding test of 1.6mm ordinary bare plate and 1.2mm thin strip cast steel, the welding parameters are: electrode pressure 3kN, welding time 300ms, single pulse, welding current 10kA. Figure 4 This is the sample obtained in Comparative Example 2. The bonding surface was broken after the test.

[0072] Example 3

[0073] In welding tests of 1.2 mm thin strip cast steel and 1.2 mm thin strip cast steel, the process parameters within the process range of the process of the present invention were adopted: electrode pressure 3 kN, welding time 400 ms, single pulse, and welding current 10.5 kA. Figure 5 This is the sample obtained in Example 3. The nugget was pulled out after testing.

[0074] Comparative Example 3

[0075] In the welding test of 1.2 mm thin strip cast steel and 1.2 mm thin strip cast steel, the welding parameters are: electrode pressure 2 kN, welding time 200 ms, one pulse, and welding current 7.5 kA. Figure 6 This is the sample obtained in Comparative Example 3. The bonding surface was broken after the test.

[0076] Table 6 Spot welding process parameters and results of various embodiments of the present invention and comparative examples

[0077]

[0078] The key technical points of the present invention are:

[0079] High-performance hot-formed thin-strip cast steel is an emerging high-performance green cost-reducing material. In its future application in the automotive manufacturing industry, it lacks a mature resistance welding process and a detailed welding process window. Therefore, the present invention provides the following key parameters for this high-performance hot-formed thin-strip cast steel in this process range: electrode pressure F = 2~3kN, the electrode pressure remains unchanged during the entire spot welding process cycle, and when the welding current stops, the electrode pressure continues to maintain T2 = 100~300ms; the welding time is controlled to T1 = 200~400ms, and when a plate thickness exceeds 1.6mm, the reliable process range of the welding application resistance welding process is appropriately increased. When the time is T1 = 300~400ms, the welding current I1 is appropriately adjusted according to the thickness of the plate, and the welding current I1 in all cases is controlled to I1 = I min ~I max .

[0080] The present invention specifically relates to each set of overlapping material combinations, and the process ranges are as follows: Figures 7-11 shown.

[0081] The above description is only a specific embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, they can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A resistance spot welding method for thin strip cast hot-formed steel, characterized in that: (1) The chemical composition and mass fraction of the hot-formed steel are: C: 0.19-0.30%, Cr: 0.15-0.80%, Si: 0.2-0.6%, Mn: 1.20-1.80%, B: ≤0.0012%, Ti: ≤0.01%, Nb: ≤0.06%, the rest is Fe and unavoidable impurities; (2) The hot-formed steel is manufactured by a twin-roll casting process, the carbon equivalent of the hot-formed steel is greater than 0.5%, and the thickness is 0.9 to 1.8 mm, and a first hot-formed part is manufactured based on the hot-formed steel; (3) connecting the first thermoformed part to a second part made by another process by resistance spot welding; The resistance spot welding process includes the following process parameter combinations: The electrode pressure F is controlled as: F = 2 ~ 3kN; The welding time T1 is controlled to be T1=200~400ms; The welding current I1 is controlled to be 7.5~13.5kA according to the thickness of the plate.

2. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: During the entire spot welding process cycle, the electrode pressure F remains unchanged.

3. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: When the welding current I1 stops, the electrode pressure F continues to be maintained. Among them, the holding time is controlled to be T2=100~300ms.

4. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: When the thickness of the first thermoformed part and / or the second part exceeds 1.6 mm, the welding time is increased to T1 = 300-400 ms.

5. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: The resistance spot welding process uses a single pulse for welding.

6. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: The welding current I1 is further controlled to be 9.5-10.5 kA.

7. The resistance spot welding method for thin strip cast hot-formed steel according to claim 6, characterized in that: The welding current I1 is further controlled to be 10-10.5 kA.

8. The resistance spot welding method for thin strip cast hot-formed steel according to any one of claims 1 to 7, characterized in that: The second part manufactured by the other process is a part made of thin strip cast steel, or The second part manufactured by the other process is made of 22MnB5 hot-formed steel with a thickness ranging from 1.2 to 1.8 mm.

9. The resistance spot welding method for thin strip cast hot-formed steel according to claim 8, characterized in that: The first thermoformed part and the second part have the same thickness.

10. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: The hot-formed steel is pickled to remove surface iron oxide scale.

11. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: The first thermoformed part is manufactured by a hot stamping process, and the mechanical properties of the first thermoformed part are: Yield strength ≥950MPa, tensile strength ≥1300MPa, elongation ≥5%.

12. The resistance spot welding method for thin strip cast hot-formed steel according to claim 1, characterized in that: The hot forming steel is shot blasted to remove surface iron oxide scale generated during the hot forming heating process.

13. A welded part, characterized in that: The welded part is formed by welding the first thermoformed part and the second part using the resistance spot welding method according to any one of claims 1 to 12.

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