Method for improving the resistance to hydrogen embrittlement of a welded joint of an aluminum-silicon coated hot-formed steel tailor-welded blank and applications thereof

Abstract

The application discloses a method for improving the hydrogen embrittlement resistance of a welded joint of an aluminum-silicon plated hot-formed steel tailor-welded blank and application thereof, and belongs to the technical field of laser heat treatment. The method for improving the hydrogen embrittlement resistance comprises the following steps: S1, providing a tailor-welded blank; and S2, performing laser heat treatment under a protective atmosphere. In the first step, a circular light spot is used, a laser power P=200-300 W is adopted, a welding speed v=2-5 mm / s is adopted, and cooling is performed at room temperature; in the second step, a rectangular light spot is used, a laser power P=100-150 W is adopted, a welding speed v=1-2 mm / s is adopted, and no cooling is needed; and in the third step, a rectangular light spot is used, a laser power P=600-800 W is adopted, and a welding speed v=8-10 mm / s is adopted. The method does not need post-welding stress relief annealing and other treatments for reducing production efficiency, is simple, and can greatly improve product quality and production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of laser heat treatment technology, and relates to a method and its application for preventing hydrogen embrittlement of welded joints of hot-formed steel welded plates with aluminum-silicon coating. In particular, it relates to a method for preventing hydrogen embrittlement of welded joints of hot-formed steel welded plates with aluminum-silicon coating, including strength levels of 2000MPa and above. Specifically, it relates to a laser heat treatment process for preventing hydrogen embrittlement of hot-formed steel with aluminum-silicon coating, including strength levels of 2000MPa and above. Background Technology

[0002] With the surge in demand for lightweight vehicles, the penetration rate of 2000MPa grade aluminum-silicon coated hot-formed steel in collision safety components such as A / B pillars, anti-collision beams, and door sills has increased from 18% in 2020 to 41% in 2025. However, with the increase in strength of hot-formed steel, hydrogen can easily accumulate in the steel matrix after laser welding, resulting in a serious hydrogen-induced delayed cracking problem.

[0003] Traditional methods for preventing hydrogen-induced delayed cracking in high-strength steel after welding include post-weld stress-relief annealing. However, this method cannot completely eliminate residual stress in the welded joint; it only eliminates a large portion. To completely eliminate residual stress, the welded plates must be heated to very high temperatures, which alters the microstructure of the welded joint and may render it unsuitable for actual production needs, thus reducing industrial competitiveness. Therefore, stress-relief annealing, to some extent, reduces production efficiency and fails to meet the urgent needs of efficient and environmentally friendly industrial production. Therefore, developing a new and efficient method for resisting hydrogen embrittlement in high-strength hot-formed steel is of great significance for improving industrial production efficiency, enhancing the performance of high-strength hot-formed steel welded joints, and reducing energy consumption and environmental pollution. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preventing hydrogen embrittlement in welded joints of hot-formed steel plates with aluminum-silicon coating. This method utilizes circular or rectangular laser spots to cover the welded joint with laser energy, thereby applying a localized heat treatment effect to the joint and improving the hydrogen embrittlement sensitivity of the high-strength hot-formed steel welded plate, preventing delayed cracking after welding. The method includes setting different laser spots, appropriate laser power, and scanning speed to prevent discoloration of the aluminum-silicon coating, thus meeting industrial production standards. This invention satisfies the hydrogen embrittlement resistance requirements of high-strength hot-formed steel in automobile manufacturing without requiring stress-relief annealing or post-weld tempering of the welded plate, achieving good hydrogen embrittlement resistance conditions, simplifying production processes, improving production efficiency, and reducing production costs.

[0005] Meanwhile, the purpose of this invention is to provide an application of aluminum-silicon coated hot-formed steel welded plate.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preventing hydrogen embrittlement of welded joints of aluminum-silicon coated hot-formed steel plates. The apparatus used in the method includes a laser, a robotic arm, a welding fixture, and a base. The welding fixture fixes two aluminum-silicon coated hot-formed steel plates onto a platform, and the robotic arm drives the laser to act on the welded joint of the plates according to a programmed path, applying circular and rectangular light spots.

[0008] In industrial production, this invention can weld 2000MPa aluminum-silicon galvanized steel together, or weld 2000MPa aluminum-silicon galvanized steel with other aluminum-silicon coating steels of different strength levels together, such as welding 2000MPa aluminum-silicon galvanized steel with 1500MPa, or welding 2000MPa aluminum-silicon galvanized steel with 2200MPa, thus achieving the same effect of resisting hydrogen embrittlement.

[0009] When a circular laser spot is used to perform a single laser heat treatment on the joint of a welded plate, it releases some tensile stress concentration, reducing stress concentration caused by rapid cooling after laser welding. A second laser heat treatment using a rectangular laser spot preheats the welded plate, preventing stress concentration and secondary embrittlement due to rapid temperature increases. A third laser heat treatment using a rectangular laser spot, reaching the austenitic temperature or a suitable tempering temperature, eliminates the hardened structure in the heat-affected zone (HAZ). The HAZ microstructure transforms from lath martensite to tempered martensite and carbides, reducing the HAZ microhardness to approximately 400 HV and significantly improving the welded joint's toughness. Furthermore, this localized laser heat treatment effectively reduces residual tensile stress in the welded joint, particularly at the fusion line and in the HAZ. After eliminating hardened structures and residual stress, the welded plate's resistance to hydrogen embrittlement is significantly improved.

[0010] In laser heat treatment, the precise control of the three laser heat treatments has a decisive influence on the welding quality. The mechanism of action is mainly reflected in the following three aspects: (1) The first laser heat treatment can release a certain amount of stress in the weld joint, and to a certain extent avoid the stress concentration caused by rapid cooling after laser welding; (2) The second laser heat treatment is mainly for preheating the third laser heat treatment, and avoids the tensile stress caused by the rapid rise in temperature during the third laser heat treatment; (3) The third laser heat treatment requires control of heat input. When the heat input is insufficient: the laser power is too low or the welding speed is too fast, when the line energy density is lower than the critical threshold, the temperature cannot reach Ac3 or the appropriate tempering temperature, the microstructure of the joint does not change from lath martensite to tempered martensite, and the microhardness of its heat-affected zone is still as high as 600 HV, which is about 150 HV higher than the center of the weld. When the heat input is too high: the welded joint remelts, and its microstructure is still lath martensite; when the heat input is too high, even if the temperature reaches Ac3 or the tempering temperature, the microhardness of the heat-affected zone drops to about 400 HV, but the temperature at this time has reached the melting temperature of the aluminum-silicon coating, which will cause the coating to discolor and not meet the industrial production standards.

[0011] Furthermore, the diameter of the circular light spot is 5-10 mm, and the area of ​​the rectangular light spot ranges from 2-100 mm. 2 The connection is made using a robotic arm and a laser.

[0012] The second aspect of this invention provides a method for laser heat treatment, hydrogen purging, and slow stretching to resist hydrogen embrittlement of welded joints in aluminum-silicon coated hot-formed steel welded plates, employing the laser generating device described in the first aspect, and comprising the following steps:

[0013] S1. Provide a welded plate of aluminum-silicon coated hot-formed steel, wherein the chemical composition of the steel plate includes C: 0.3-0.4 wt.%, Si: 0.2-0.3 wt.%, Mn: 1.1-1.5 wt.%, P: 0.001-0.01 wt.%, Cu: 0.1-0.2 wt.%, Cr: 0.2-0.3 wt.%, Ti: 0.02-0.04 wt.%, Mo: 0.004-0.005 wt.%, B: 0.002-0.004 wt.%, N: 0.001-0.004 wt.%, with the remainder being Fe.

[0014] The hot-formed steel welded plates all include steel plates and aluminum-silicon coatings disposed on the steel plates;

[0015] S2. Under a protective atmosphere, the welded plates of the hot-formed steel are laser-heat treated by laser welding using a laser.

[0016] The lasers used in this invention are not limited to fiber lasers, but also include semiconductor lasers and other types of lasers.

[0017] The laser heat treatment process is as follows: First, a circular laser spot is used with a laser power of 200-300W and a scanning speed of 2-5mm / s for a primary heat input, followed by cooling at room temperature (25°C); second, a rectangular laser spot is used with a laser power of 100-150W and a scanning speed of 1-2mm / s for a secondary heat input, without requiring further cooling; third, a rectangular laser spot is used with a laser power of 600-800W and a scanning speed of 8-10mm / s for a tertiary heat input, followed by cooling at room temperature (25°C).

[0018] S3. After laser heat treatment, hydrogen is added and then slowly stretched.

[0019] The electrolyte solution used for hydrogen charging is a 0.1 mol / L Na2SO4 aqueous solution, with 1.0 g / L thiourea added as a poisoning agent.

[0020] The hydrogen charging current density is 2 mA / cm². 2 The hydrogen charging time is 4 hours.

[0021] Slow stretching refers to stretching at a constant strain rate of 5 × 10⁻⁶ at room temperature. -5 The tensile specimen is subjected to slow strain rate tensile testing at a rate of / s.

[0022] The laser heat treatment technology of this invention is highly efficient and has good resistance to hydrogen embrittlement.

[0023] Furthermore, the thickness of the hot-formed steel welded plate is selected from 1-3 mm.

[0024] Furthermore, the thickness of the aluminum-silicon coating on the hot-formed steel welded plate is independently selected from 5-40 μm. The coating comprises two main layers: first, the outer aluminum-silicon layer has a chemical composition of Al: 85-90 wt.% and Si: 10-15 wt.%; second, the inner iron-aluminum intermetallic compound has a chemical composition of Al: 50-55 wt.% and Fe: 45-50 wt.%.

[0025] Furthermore, the welding of the hot-formed steel welded plate uses a self-developed welding wire. The composition of the welding wire has been disclosed in patent CN202310229599.5, "A Welding Wire for Hot-Formed Steel Welded Plates with Aluminum-Silicon Coating and Its Application." The preparation method of the aluminum-silicon coated hot-formed steel welded plate of this invention is also the same as the method in patent CN202310229599.5, "A Welding Wire for Hot-Formed Steel Welded Plates with Aluminum-Silicon Coating and Its Application" (the weld of this invention is prepared using a laser filler wire welding process, and the welding wire is a solid welding wire with a diameter of 1.0-1.2 mm. The solid welding wire preparation process follows the conventional welding wire production process, including raw material preparation and smelting, welding wire pretreatment, drawing and heat treatment, surface treatment and finished product processing, etc.).

[0026] Furthermore, in S1, the laser power of the hot-formed steel welded plate is 0.5-6kW, the welding rate is 1-10m / min, the wire feeding speed is 1-10m / min, and the defocusing amount is 0-5mm.

[0027] Furthermore, in S2, the protective atmosphere is pure argon gas with a flow rate of 10-25 L / min.

[0028] Furthermore, the weld of the welded component (i.e., the welded plate after laser local heat treatment) contains tempered martensite and austenite. Specifically, the weld microstructure does not include δ-ferrite, the volume percentage of tempered martensite is above 97%, and the volume percentage of austenite is below 3%.

[0029] The chemical composition of the weld includes C: 0.2-0.4 wt.%, Si: 0.1-0.2 wt.%, Mn: 1.0-5.0 wt.%, P: 0.001-0.003 wt.%, Cu: 0.01-2 wt.%, Cr: 1-5 wt.%, Ni: 1-10 wt.%, Ti: 0.01-0.02 wt.%, Mo: 0.001-0.004 wt.%, B: 0.001-0.003 wt.%, N: 0.001-0.002 wt.%, with the remainder being Fe; the purpose is to ensure that the tensile strength of the weld after hot stamping is not lower than that of the base metal.

[0030] The aluminum-silicon coated hot-formed steel welded plate obtained by the hydrogen embrittlement resistance method of the present invention has a weld joint tensile strength of 670-700 MPa, a fracture location in the base material under slow tension, an elongation after fracture of 6.5-7%, and a microhardness of <500HV.

[0031] The present invention relates to the application of aluminum-silicon coated hot-formed steel welded plates in automotive collision safety components, including A / B pillars, anti-collision beams, and door sills.

[0032] This invention improves the hydrogen embrittlement resistance of welded joints in hot-formed steel welded plates with aluminum-silicon coatings without resorting to traditional, complex, and inefficient post-weld stress-relief annealing or tempering. Localized laser heat treatment allows energy to be precisely applied to the entire welded joint through a rectangular spot, with minimal impact on the base material properties, avoiding discoloration and deformation of the coating caused by overall heating. Laser heat treatment is effective in eliminating hardened microstructure in the heat-affected zone, improving toughness, and reducing residual stress. It enables highly localized, rapid, and controllable microstructural regulation of the heat-affected zone of the welded joint, thereby reducing the hydrogen embrittlement resistance of high-strength steel welded plates and preventing hydrogen-induced delayed cracking. Laser heat treatment offers high scanning speed and production efficiency, making it extremely easy to integrate into automated production lines.

[0033] The heat treatment method of this invention comprises two parts: first, the laser head uses both rectangular and circular laser spots; second, the laser heat treatment employs three different process parameters to improve the welded plate. By using rectangular and circular laser spots and three heat inputs to perform laser heat treatment on the weld joint of the aluminum-silicon coated hot-formed steel welded plate, the hardened structure in the heat-affected zone of the welded plate can be eliminated, stress concentration in the weld joint can be avoided, and the resistance to hydrogen embrittlement of the welded plate can be significantly improved. Simultaneously, the microstructure of the weld joint is optimized, thereby preventing delayed cracking of the welded plate. The aluminum-silicon coated hot-formed steel welded plate prepared by this invention has a reduced microhardness in the heat-affected zone to approximately 400 HV, and the microstructure changes from hardened martensite to tempered martensite + carbides. After pre-charging with hydrogen, the slow tensile fracture location remains in the base material, significantly improving resistance to hydrogen embrittlement. This method eliminates the need for post-weld stress-relief annealing and other processes that reduce production efficiency. The method is simple and can greatly improve product quality and production efficiency. Attached Figure Description

[0034] The technical solution and experimental results of the present invention will be further described in detail below with reference to the accompanying drawings and microscopic tissue photographs.

[0035] Figure 1 This is a schematic diagram of the structure of a laser heat treatment device for hot-formed steel with aluminum-silicon coating according to the present invention.

[0036] Figure 2 The figures show the tensile test results of Comparative Examples 1-2 and Examples 1-3 of the present invention;

[0037] Figure 3 This is a microstructure diagram of the location of the heat-affected zone in Embodiment 1 of the present invention;

[0038] Figure 4 The image shows the microhardness distribution of the welded joints of Comparative Examples 1-3 and Examples 1-2 of the present invention.

[0039] Figure 5This is a diagram showing the tensile test results of Example 4 of the present invention;

[0040] Figure 6 This is a diagram showing the tensile test results of Comparative Example 5 of the present invention. Detailed Implementation

[0041] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0042] It should be noted that the aluminum-silicon coated hot-formed steel welded plate described in this invention is a welded plate that has not undergone hot stamping. Furthermore, the method for judging hydrogen embrittlement sensitivity is as follows: there is no hardened structure in the heat-affected zone of the weld joint, and the microhardness is below 500 HV; a tensile member taken from the weld joint of the welded plate, after slow tensile testing with hydrogen purging, fractures at the base material, not in the heat-affected zone. Figure 1 As shown, a laser heat treatment device for aluminum-silicon coated hot-formed steel includes a laser, a protective gas, a weld seam, an aluminum-silicon coated hot-formed steel welded plate, a clamping plate, etc. Example 1

[0043] This embodiment provides a laser heat treatment process for hydrogen embrittlement resistance of welded joints in aluminum-silicon coated hot-formed steel welded plates.

[0044] S1. Provide a 1mm thick 2000MPa grade aluminum-silicon coated hot-formed steel welded plate;

[0045] The chemical composition of the steel plate includes C: 0.3 wt.%, Si: 0.2 wt.%, Mn: 1.1 wt.%, P: 0.001 wt.%, Cu: 0.1 wt.%, Cr: 0.2 wt.%, Ti: 0.02 wt.%, Mo: 0.004 wt.%, B: 0.002 wt.%, N: 0.001 wt.%, with the remainder being Fe.

[0046] The welded plate of hot-formed steel with a strength of 2000MPa includes a steel plate and an aluminum-silicon coating disposed on the steel plate.

[0047] The aluminum-silicon coating has a thickness of 5 μm. The aluminum-silicon coating comprises two layers with a thickness ratio of 1:1. The outer aluminum-silicon layer has a chemical composition of Al: 85 wt.% and Si: 15 wt.%. The inner iron-aluminum intermetallic compound layer has a chemical composition of Al: 50 wt.% and Fe: 50 wt.%. The substrate of the welded plate has a microstructure of pearlite + ferrite.

[0048] S2. Under a protective atmosphere, a laser (preferably a fiber laser) is used to perform local heat treatment on the entire welded joint of the welded plate.

[0049] The laser heat treatment process is as follows: First, a circular laser spot is used with a laser power of 300W and a scanning speed of 5mm / s for a primary heat input, followed by cooling at room temperature (25℃). Then, a rectangular laser spot is used with a laser power of 150W and a scanning speed of 2mm / s for a secondary heat input, without cooling. Finally, a rectangular laser spot is used with a laser power of 600W and a scanning speed of 8mm / s for a tertiary heat input, followed by cooling at room temperature (25℃). The diameter of the circular laser spot is 10mm, the size of the rectangular laser spot is 1*10mm, and the defocusing amount is 5mm. The protective atmosphere is pure argon gas with a flow rate of 20L / min.

[0050] The weld of the welded component (i.e., the welded plate after laser local heat treatment) obtained in this embodiment contains tempered martensite and austenite structures. Specifically, the microstructure in the weld does not include δ-ferrite, the volume percentage of tempered martensite is 98%, and the volume percentage of austenite is 2%.

[0051] The chemical composition of the weld includes C: 0.3 wt.%, Si: 0.15 wt.%, Mn: 3.0 wt.%, P: 0.002 wt.%, Cu: 0.1 wt.%, Cr: 4 wt.%, Ni: 5 wt.%, Ti: 0.015 wt.%, Mo: 0.003 wt.%, B: 0.002 wt.%, N: 0.0015 wt.%, with the remainder being Fe; the purpose is to ensure that the tensile strength of the weld after hot stamping is not lower than that of the base metal.

[0052] S3. After the laser local heat treatment of the welded plate is completed, hydrogen is charged.

[0053] The electrolyte solution used for hydrogen charging is a 0.1 mol / L Na₂SO₄ aqueous solution with 1.0 g / L thiourea added as a poisoning agent. The hydrogen charging current density is 2 mA / cm². 2 Hydrogen charging time: 4 hours.

[0054] S4. After hydrogen charging is completed, a slow tensile test is performed.

[0055] The slow stretching is performed at a constant strain rate of 5 × 10⁻⁶ at room temperature. -5 Slow strain rate tensile test was performed on the tensile specimen at a strain rate of / s.

[0056] In Example 1, the first laser heat treatment serves to initially release stress and prevent stress concentration in the welded plates. The second laser heat treatment preheats the joint for the third laser heat treatment, preventing a rapid increase in joint stress due to a rapid temperature rise. The third laser heat treatment transforms the lath martensite in the joint into tempered martensite and carbides, improving toughness while maintaining high strength and reducing crack susceptibility. During tempering, carbide precipitation occurs, and its interface becomes a strong hydrogen trap. Laser tempering reduces dislocation density, enhancing overall resistance to hydrogen diffusion. Localized heat treatment significantly reduces residual tensile stress and eliminates the driving force for hydrogen-induced delayed cracking. Localized laser heat treatment causes thermal expansion of the material, resulting in compressive plastic deformation constrained by the surrounding cold metal. Upon cooling, contraction is hindered, adding compressive stress to the rectangular laser scanning area and significantly reducing residual tensile stress, directly inhibiting the initiation of hydrogen-induced cracks. Laser local heat treatment can reduce the microhardness of the entire welded joint to about 400 HV and make the microstructure of the entire joint more uniform, reducing the electrochemical potential difference caused by microstructure differences, thereby reducing the driving force for the directional migration of hydrogen.

[0057] Example 1 demonstrates how laser-guided local heat treatment significantly improves the hydrogen embrittlement resistance of welded joints in high-strength hot-formed steel plates. Aluminum-silicon coated hot-formed steel with a strength of 2000 MPa (i.e., in this example, 2000 MPa and 2000 MPa aluminum-silicon galvanized steel are welded together) undergoes hydrogen purging after laser-guided local heat treatment. The slow tensile fracture location shifts from the heat-affected zone to the base material, and it is a ductile fracture. The tensile strength, toughness, and other mechanical properties of the welded joint are significantly improved, meeting the performance requirements of high-strength steel plate welded joints.

[0058] This embodiment describes the application of 2000MPa-grade aluminum-silicon coated hot-formed steel welded plates in automotive collision safety components, including A / B pillars, anti-collision beams, and door sills. Example 2

[0059] This embodiment provides another laser heat treatment method for hydrogen embrittlement resistance of welded joints of aluminum-silicon coated hot-formed steel welded plates.

[0060] S1. Provide a 3mm thick aluminum-silicon coated hot-formed steel welded plate;

[0061] The chemical composition of the steel plate includes C: 0.4 wt.%, Si: 0.3 wt.%, Mn: 1.5 wt.%, P: 0.01 wt.%, Cu: 0.2 wt.%, Cr: 0.3 wt.%, Ti: 0.04 wt.%, Mo: 0.005 wt.%, B: 0.004 wt.%, N: 0.004 wt.%, with the remainder being Fe.

[0062] The hot-formed steel welded plate includes a steel plate and an aluminum-silicon coating disposed on the steel plate.

[0063] The aluminum-silicon coating has a thickness of 40 μm. The aluminum-silicon coating consists of two layers with a thickness ratio of 1:1. The outer aluminum-silicon layer has a chemical composition of Al: 90 wt.% and Si: 10 wt.%. The inner iron-aluminum intermetallic compound layer has a chemical composition of Al: 55 wt.% and Fe: 45 wt.%.

[0064] S2. Under a protective atmosphere, a laser (preferably a semiconductor laser) is used to perform local heat treatment on the entire weld joint of the welded plate through circular and rectangular light spots.

[0065] The parameters for the laser heat treatment are as follows: The laser heat treatment process is as follows: First, a circular laser spot is used with a laser power of P=200W and a scanning speed of v=2mm / s for a first heat input, followed by cooling at room temperature (25℃). Then, a rectangular laser spot is used with a laser power of P=100W and a scanning speed of v=1mm / s for a second heat input, without cooling. Finally, a rectangular laser spot is used with a laser power of P=800W and a scanning speed of v=10mm / s for a third heat input, followed by cooling at room temperature (25℃). The diameter of the circular laser spot is 5mm, the size of the rectangular laser spot is 5*20mm, and the defocusing amount is 0mm. The protective atmosphere is pure argon gas with a flow rate of 10L / min.

[0066] The weld seam of the welded component (i.e., the welded plate after laser local heat treatment) obtained in this embodiment contains tempered martensite and austenite. Specifically, the weld seam microstructure does not include δ-ferrite, the volume percentage of tempered martensite is 97%, and the volume percentage of austenite is 3%.

[0067] The chemical composition of the weld includes C: 0.4 wt.%, Si: 0.2 wt.%, Mn: 5.0 wt.%, P: 0.003 wt.%, Cu: 2 wt.%, Cr: 5 wt.%, Ni: 10 wt.%, Ti: 0.02 wt.%, Mo: 0.004 wt.%, B: 0.003 wt.%, N: 0.002 wt.%, with the remainder being Fe; the purpose is to ensure that the tensile strength of the weld after hot stamping is not lower than that of the base metal.

[0068] S3. After the laser local heat treatment of the welded plate is completed, hydrogen is charged.

[0069] The electrolyte solution used for hydrogen charging is a 0.1 mol / L Na₂SO₄ aqueous solution with 1.0 g / L thiourea added as a poisoning agent. The hydrogen charging current density is 2 mA / cm². 2 The hydrogen charging time is 4 hours.

[0070] S4. After hydrogen charging is completed, a slow tensile test is performed.

[0071] The slow stretching is performed at a constant strain rate of 5 × 10⁻⁶ at room temperature. -5 Slow strain rate tensile test was performed on the tensile specimen at a strain rate of / s.

[0072] As a specific example, in S1, the steel plate is made of hot-formed steel.

[0073] The weld seam of the welded plate contains tempered martensite and austenite, but does not contain δ-ferrite.

[0074] This embodiment uses laser heat treatment technology, which has low assembly requirements, high efficiency, and significant resistance to hydrogen embrittlement.

[0075] This embodiment describes the application of aluminum-silicon coated hot-formed steel welded plates in automotive collision safety components, including A / B pillars, anti-collision beams, and door sills. Example 3

[0076] The only difference between this embodiment and Embodiment 1 is that the diameter of the circular spot is 6mm, the size of the rectangular spot is 1*2mm, the defocusing amount is 3mm, and the protective atmosphere is pure argon with a flow rate of 25L / min.

[0077] The weld seam of the welded component (i.e., the welded plate after laser local heat treatment) obtained in this embodiment contains tempered martensite and austenite. Specifically, the weld seam microstructure does not include δ-ferrite, the volume percentage of tempered martensite is 97%, and the volume percentage of austenite is 3%.

[0078] The chemical composition of the weld includes C: 0.2 wt.%, Si: 0.1 wt.%, Mn: 1.0 wt.%, P: 0.001 wt.%, Cu: 0.01 wt.%, Cr: 1 wt.%, Ni: 1 wt.%, Ti: 0.01 wt.%, Mo: 0.001 wt.%, B: 0.001 wt.%, N: 0.001 wt.%, with the remainder being Fe; the purpose is to ensure that the tensile strength of the weld after hot stamping is not lower than that of the base metal. Example 4

[0079] The difference between this embodiment and Embodiment 1 lies only in that: hot stamping is performed after localized laser heat treatment. The purpose is to evaluate whether the tensile strength of the weld after hot stamping (hot stamping involves heating a steel plate to between 900 and 950°C for hot deformation and subsequent die quenching to obtain a desired shape) can reach the level of the base material. For example... Figure 5As shown, after hot stamping, the tensile strengths of the two welded plates in Example 1 are 2021 MPa and 2005 MPa, respectively. That is, after hot stamping, the tensile strength of the weld of the present invention reaches the level of the base material. It has excellent resistance to hydrogen embrittlement, and the fracture location is in the base material. Comparative Example 1

[0080] The laser heat treatment process in this comparative example is as follows: First, a primary heat treatment is performed using a circular laser spot with a laser power of P=200W and a scanning speed of v=3mm / s, followed by cooling at room temperature of 25℃. Second, a secondary heat treatment is performed using a rectangular laser spot with a laser power of P=200W and a scanning speed of v=8mm / s, followed by cooling at room temperature of 25℃.

[0081] The specific hydrogen charging method is as follows: Prepare a 1L electrolyte solution, including 0.1mol of Na2SO4 aqueous solution and adding 1.0g of thiourea as a poisoning agent. Immerse the tensile member in the electrolyte solution and then turn on the current. The hydrogen charging current density is 2mA / cm. 2 The hydrogen charging time is 4 hours.

[0082] The specific slow stretching method is the same as in Example 1.

[0083] That is, the only difference between this comparative example and Example 1 is that the three laser heat treatments are changed to two laser heat treatments. Comparative Example 2

[0084] The laser heat treatment process in this comparative example involves a single heat treatment using a rectangular laser spot with a laser power of P=800W and a scanning speed of v=8mm / s, followed by cooling at room temperature (25°C). The slow stretching process is the same as in Comparative Example 1.

[0085] That is, the only difference between this comparative example and Example 1 is that the three laser heat treatments are replaced with one laser heat treatment. Comparative Example 3

[0086] The laser heat treatment process of Comparative Example 3 is as follows: First, a primary heat treatment is performed using a circular laser spot with a laser power of P=200W and a scanning speed of v=3mm / s, without the need for cooling. Then, a secondary heat treatment is performed using a rectangular laser spot with a laser power of P=1000W and a scanning speed of v=10mm / s, followed by cooling at room temperature (25℃).

[0087] That is, the only difference between this comparative example and Example 1 is that the three laser heat treatments are changed to two laser heat treatments. Comparative Example 4

[0088] The laser heat treatment process in this comparative example is as follows: First, a circular laser spot is used for a first heat treatment with a laser power of P=200W and a scanning speed of v=3mm / s, followed by cooling at room temperature of 25℃. Then, a rectangular laser spot is used for a second heat treatment with a laser power of P=100W and a scanning speed of v=5mm / s, without cooling. Finally, a rectangular laser spot is used for a third heat treatment with a laser power of P=1000W and a scanning speed of v=10mm / s, followed by cooling at room temperature of 25℃.

[0089] That is, the only difference between this comparative example and Example 1 is that the process parameters for the three laser heat treatments are different. Comparative Example 5

[0090] The only difference between this comparative example and Comparative Example 3 is that hot stamping is performed after localized laser heat treatment. The purpose of this is to evaluate whether the tensile strength of the weld after hot stamping can reach the level of the base material. Figure 6 As shown, after hot stamping, the tensile strengths of the two welded plates in this comparative example are 1672 MPa and 1483 MPa, respectively. That is, after hot stamping, the tensile strength of the weld in this comparative example is lower than that of the base material, the resistance to hydrogen embrittlement is poor, and the fracture location is the weld.

[0091] Because the coating of Comparative Example 4 changed color after laser heat treatment, which did not meet the requirements of industrial production, the performance of the welded joints of the aluminum-silicon coated hot-formed steel welded plates of Comparative Examples 1-3 and Examples 1-2 after hydrogen purging was tested, and the results are shown in Table 1.

[0092] Table 1

[0093]

[0094] From Table 1 and Figure 2 It can be seen that the aluminum-silicon coated hot-formed steel welded plates in Examples 1 and 2 of the present invention have better resistance to hydrogen embrittlement, with the fracture location being the base material, and also exhibit good tensile strength and elongation after fracture. In contrast, the fracture locations in the comparative examples are all in the heat-affected zone.

[0095] Depend on Figure 4 As can be seen from Examples 1-2, after laser heat treatment, the hardened martensite of the welded joint transforms into tempered martensite, the microhardness decreases to around 400 HV, and the microstructure of the entire welded joint becomes more uniform (e.g., Figure 3 As shown in the figure, there is no potential difference caused by structural differences, which reduces the driving force for hydrogen migration and causes the fracture location to shift from the heat-affected zone to the base material. However, the laser heat treatment process in Comparative Examples 1-3 is inappropriate, and the heat-affected zone of the welded joint still contains hardened martensite with a microhardness as high as 650 HV.

[0096] As can be seen from the comparison between the examples and the comparative examples, the first laser heat treatment allows the welded joint to release a certain amount of stress. The second laser heat treatment serves as preheating for the third laser heat treatment, reducing the possibility of stress concentration in the heat-affected zone due to a rapid temperature rise. The heat input for the third laser heat treatment must reach 1.0 KJ / cm, or, as described in the comparative examples and examples, a laser power of 600-800W and a scanning speed of 8-10 mm / s, to reach the Ac3 temperature or a suitable tempering temperature. Otherwise, it will strengthen the welded joint, increase its microhardness, and make the welded plate more susceptible to hydrogen embrittlement, increasing the probability of delayed cracking after welding. When the heat input is too high, the temperature will reach the melting temperature of the aluminum-silicon coating, causing discoloration and deterioration of the aluminum-silicon coating.

[0097] In the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all features of the foregoingly disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0098] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0099] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preventing hydrogen embrittlement in welded joints of hot-formed steel plates with aluminum-silicon coating, characterized in that, Includes the following steps: S1. Provide a welded plate of aluminum-silicon coated hot-formed steel; S2. Under a protective atmosphere, a laser is used to perform laser heat treatment on the welded joints of the welded plates by laser welding. First, use a circular laser spot with a laser power of P=200-300W and a scanning speed of v=2-5mm / s for a single heat input, and then cool it at room temperature; Second, using a rectangular spot and a secondary heat input with laser power P=100-150W and scanning speed v=1-2mm / s, no cooling is required; Third, the rectangular spot is used with three heat inputs at a laser power of P=600-800W and a scanning speed of v=8-10mm / s, and then cooled at room temperature. The diameter of the circular light spot is 5-10 mm, and the area of ​​the rectangular light spot ranges from 2-100 mm. 2 .

2. The method for resisting hydrogen embrittlement according to claim 1, characterized in that, The welded plates of aluminum-silicon coated hot-formed steel all include steel plates and aluminum-silicon coatings disposed on the steel plates.

3. The method for resisting hydrogen embrittlement according to claim 2, characterized in that, The chemical composition of the steel plate includes C: 0.3-0.4 wt.%, Si: 0.2-0.3 wt.%, Mn: 1.1-1.5 wt.%, P: 0.001-0.01 wt.%, Cu: 0.1-0.2 wt.%, Cr: 0.2-0.3 wt.%, Ti: 0.02-0.04 wt.%, Mo: 0.004-0.005 wt.%, B: 0.002-0.004 wt.%, N: 0.001-0.004 wt.%, with the remainder being Fe.

4. The method for resisting hydrogen embrittlement according to claim 2, characterized in that, The aluminum-silicon coating consists of two layers. The outer aluminum-silicon layer has a chemical composition of Al: 85-90 wt.% and Si: 10-15 wt.%. The inner iron-aluminum intermetallic compound layer has a chemical composition of Al: 50-55 wt.% and Fe: 45-50 wt.%.

5. The method for resisting hydrogen embrittlement according to claim 2, characterized in that, The thickness of the welded plate of hot-formed steel with aluminum-silicon coating is 1-3mm; the thickness of the aluminum-silicon coating is 5-40μm.

6. The method for resisting hydrogen embrittlement according to claim 1, characterized in that, The protective gas consists of pure argon, with a flow rate of 10-25 L / min.

7. The aluminum-silicon coated hot-formed steel welded plate obtained by the method for resisting hydrogen embrittlement according to any one of claims 1 to 6, characterized in that, The tensile strength of its welded joint is 670-700 MPa, the fracture location under slow tension is the base material, the elongation after fracture is 6.5-7%, and the microhardness is <500HV.

8. The aluminum-silicon coated hot-formed steel welded plate according to claim 7, characterized in that, After laser heat treatment, the welded joint contains tempered martensite and austenite structures, with no δ-ferrite structure formed; the volume percentage of tempered martensite is above 97%, and the volume percentage of austenite is below 3%; the chemical composition of the weld includes C: 0.2-0.4 wt.%, Si: 0.1-0.2 wt.%, Mn: 1.0-5.0 wt.%, P: 0.001-0.003 wt.%, Cu: 0.01-2 wt.%, Cr: 1-5 wt.%, Ni: 1-10 wt.%, Ti: 0.01-0.02 wt.%, Mo: 0.001-0.004 wt.%, B: 0.001-0.003 wt.%, N: 0.001-0.002 wt.%, with the remainder being Fe.

9. The application of the aluminum-silicon coated hot-formed steel welded plate according to claim 8 in automotive collision safety components, characterized in that, Automotive crash safety components include A / B pillars, crash beams, and door sills.

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