Thermoforming method for high-strength steel part, and low-melting-point coating part with low micro-crack depth, hot stamping forming method therefor and hot bath edge cutting device therefor

Abstract

A thermoforming method for a high-strength steel part. A hot stamping die comprises an upper die (1) and a lower die (2); the lower die is located in a water tank (3); the water tank is used for containing hot water above 60°C; during use, the lower die is partially or completely immersed in hot water; the lower die is provided with at least two sheet supports; each sheet support comprises a fixed rod (7) vertically fixed on the bottom surface of the water tank; the upper end of the fixed rod is rotatably connected to a cross beam (8) by means of a rotating shaft (9); one ends of the cross beams of the sheet supports cooperate with each other to support sheets, and the other ends of the cross beams are rotatably connected to the upper ends of pull rods (10); the lower ends of the pull rods are connected to the fixed rods by means of springs (11); and in an initial state, the height of the cross beams is greater than the liquid level of hot water. Further provided is a low-melting-point coating part with a low micro-crack depth. The low-melting-point coating part comprises a substrate and a low-melting-point coating located on the surface of the substrate; the low-melting-point coating part is formed by hot stamping; micro-cracks are generated in the substrate and the low-melting-point coating in the forming process; and most of the micro-cracks extending from the low-melting-point coating to the substrate are located at concave fillet tangent points of the low-melting-point coating part, and a small part of the micro-cracks extending from the low-melting-point coating to the substrate are located at positions other than the concave fillet tangent points of the low-melting-point coating part. Further provided are a thermoforming method for the low-melting-point coating part, a hot bath edge cutting device for the high-strength steel part, and a method for using the hot bath edge cutting device. According to the thermoforming method, in the direct forming process, the micro-crack expansion process of the low-melting-point coating and the substrate is effectively inhibited, the service life of the low-melting-point coating part is ensured, and excellent corrosion resistance is shown.

Description

A hot forming method for high-strength steel parts, a low-melting-point coated part with low microcrack depth, a hot stamping forming method thereof, and a hot bath trimming device

[0001] This application claims the priority of the Chinese patent application filed with the Patent Office of China on March 15, 2024, with application number 202410300810.2, entitled “Low-melting-point coated parts with low microcrack depth and hot stamping forming process thereof”, and the priority of the Chinese patent application filed with the Patent Office of China on June 18, 2024, with application number 202410786969.X, entitled “A hot stamping forming device and hot forming method for high-strength steel parts”. The priority of the Chinese patent application filed with the Patent Office of China on February 2, 2024, with application number 202410151931.5 and invention name “A hot bath trimming device and use method for high-strength steel parts”, and the priority of the Chinese patent application filed with the Patent Office of China on May 31, 2024, with application number 202410700062.7 and invention name “A hot stamping forming device and hot forming method for high-strength steel parts”, all of which are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of hot stamping forming of parts, and specifically relates to a hot forming method of high-strength steel parts, low-melting-point coated parts with low microcrack depth and their hot stamping forming method and a hot bath trimming device. Background Art

[0003] With the increasing demands for lightweighting and safety standards in automobiles, the use of ultra-high-strength steel is gradually increasing. Ultra-high-strength steel sheets with a strength of 1000MPa or higher suffer from issues such as cracking, severe springback, difficulty forming complex parts, and significant mold loss during cold stamping. The hot stamping process for ultra-high-strength steel sheets (referred to as high-strength steel) is considered an effective solution to these challenges.

[0004] At present, the hot stamping process is divided into direct hot stamping process (one-step method) and indirect hot stamping process (two-step method). The direct hot stamping process is to heat the steel plate and completely austenitize it, and then quickly transfer it to a mold with a water cooling system to complete rapid stamping and rapid cooling. It is mainly used to produce parts with a small degree of deformation; the indirect hot stamping process is to first complete 90-95% of the cold stamping pre-forming of the parts, and then transfer the pre-formed parts to the cooling mold after austenitizing heat treatment to complete quenching and part size correction. It is used to produce parts with complex shapes or large drawing depths.

[0005] After heat treatment of the aluminum-silicon coating, the interdiffusion of the coating elements forms iron-aluminum and iron-aluminum-silicon phase compounds, which are brittle phases. The coating is prone to cracking during hot stamping, but the cracks will not extend into the substrate, so a direct hot stamping process can be used. Low-melting-point coated hot stamping steel (such as GI, GA, ZnAlMg, etc.) is prone to LME cracking due to the low melting point of Zn, which is converted into liquid zinc during high-temperature heating. However, the Fe content in the GA coating is relatively high (about 10%), which can quickly cause element diffusion reactions during hot stamping, effectively reducing the occurrence of LME cracks. Therefore, GA-coated steel sheets are suitable for direct hot stamping processes. However, at present, pure zinc-coated hot stamping steel sheets can only be used in the manufacture of automotive parts using an indirect hot stamping process, which increases processing steps, wastes energy, increases costs, and reduces production efficiency.

[0006] Furthermore, aluminum-silicon coatings are prone to cracking during hot stamping. Although these cracks do not spread to the substrate, the lack of cathodic protection results in low corrosion resistance at the cut. Zinc-based hot-formed steel, through sacrificial anode protection, offers superior corrosion resistance to aluminum-silicon coatings. Furthermore, zinc-based hot-formed steels require lower heating temperatures and shorter heating times than aluminum-silicon coatings, resulting in greater energy efficiency and lower production costs.

[0007] Chinese patent CN115029632A discloses a highly corrosion-resistant, galvanized, hot-formed hardened steel, its components, and a method for preparing them. This method distributes most of the deformation during component forming to the cold forming process, broadening the window for subsequent heating and forming, reducing the tendency of liquid metal brittleness during hot forming, and enabling the production of components with complex cross-sectional shapes. However, this method does not address the issue of liquid metal brittleness that occurs during direct forming. Summary of the Invention

[0008] The purpose of this application is to provide a hot forming method for high-strength steel parts, low-melting-point coated parts with low microcrack depth and their hot stamping forming method and hot bath trimming device. This application can effectively suppress the microcrack propagation process of the low-melting-point coating and the substrate of the low-melting-point coated parts under the direct forming process, control the microcrack depth within 10μm, ensure the service life of the low-melting-point coated parts, and at the same time the low-melting-point coated parts exhibit excellent corrosion resistance.

[0009] In order to achieve the above objectives, this application provides the following technical solutions:

[0010] In a first aspect, the present application provides a method for hot forming a high-strength steel component, comprising the following steps:

[0011] During the hot stamping process, as the upper die descends, the high-strength steel sheet is first cooled in the air medium. Then, driven by the upper die, it gradually moves from the air medium into the hot water medium until it is completely immersed in the hot water and formed. The die is then pressure-maintained to achieve component quenching, thereby obtaining the high-strength steel component. The temperature of the hot water is above 60°C.

[0012] The hot stamping forming device includes a hot stamping die, which includes an upper die and a lower die. The lower die is located in a water tank, which is used to hold hot water above 60°C. When in use, part or all of the lower die is immersed in hot water. The lower die is provided with at least two sheet metal supports, and the sheet metal support includes a fixed rod vertically fixed to the bottom surface of the water tank. The upper end of the fixed rod is rotatably connected to a crossbeam through a rotating shaft. One end of the crossbeam of the sheet metal support cooperates with each other to support the sheet metal, and the other end of the crossbeam is rotatably connected to the upper end of the pull rod. The lower end of the pull rod is connected to the fixed rod through a spring. In the initial state, the height of the crossbeam is higher than the hot water level.

[0013] Furthermore, the hot stamping forming device also includes a controller; a water level sensor and a thermometer are also provided in the water tank, a water inlet and a water outlet are provided on the side wall of the water tank, a water inlet valve is provided at the water inlet, and a water outlet valve is provided at the water outlet, the water level sensor and the thermometer are respectively connected to the controller signal, and the water inlet valve and the water outlet valve are respectively connected to the controller control.

[0014] Furthermore, the temperature of the hot water is 60-100°C.

[0015] Furthermore, the high-strength steel sheet is heated to above AC3 before use to obtain an austenitic structure.

[0016] Furthermore, the forming start temperature of the high-strength steel sheet is not less than 680° C., and the forming start temperature refers to the sheet temperature when the upper die moves downward to contact the sheet.

[0017] Furthermore, the mold pressure holding time is 1 to 10 seconds.

[0018] Furthermore, the chemical element composition of the high-strength steel sheet is: C 0.05wt%~0.35wt%, Si 0.05wt%~0.6wt%, Mn 0.5wt%~2.2wt%, Cr≤0.5wt%, Mo≤0.5wt%, Ni≤0.5wt%, Ti≤0.04wt%, Nb≤0.2wt%, V≤0.2wt%, B 0.002wt%~0.006wt%, P≤0.020wt%, S≤0.003wt%, Al≤0.8wt%, N≤0.006wt%, and the balance is Fe and unavoidable impurities.

[0019] Furthermore, the high-strength steel sheet is an uncoated sheet or a coated sheet, the coated sheet is a zinc-based coated sheet or an aluminum-silicon coated sheet, and the zinc-based coated sheet is a pure zinc-coated sheet (GI), a zinc-iron alloy-coated sheet (GA) or a zinc-aluminum-magnesium-coated sheet (Zn-5Al-3Mg).

[0020] Furthermore, the high-strength steel sheet is a zinc-based coated sheet, and the proportion of zinc-rich phase in the surface zinc-based coating of the high-strength steel component is not higher than 40%; the zinc-rich phase proportion is controlled by specifying appropriate heating temperature and time based on the thickness of the high-strength steel sheet and the initial thickness of the coating.

[0021] In the second aspect, the present application also provides a low-melting-point coating component with low microcrack depth, comprising a substrate and a low-melting-point coating located on the surface of the substrate, wherein the low-melting-point coating component is formed by hot stamping, and microcracks are generated in the substrate and the low-melting-point coating during the forming process, most of the microcracks extending from the low-melting-point coating to the substrate are located at the intersection of the concave fillet of the low-melting-point coating component, and a small number of microcracks extending from the low-melting-point coating to the substrate are located outside the intersection of the concave fillet of the low-melting-point coating component; wherein the microcrack depth of the substrate located at the intersection of the concave fillet of the low-melting-point coating component is less than 8 μm, and the microcrack depth of the substrate located outside the intersection of the concave fillet of the low-melting-point coating component is less than 4 μm; the substrate is a steel plate.

[0022] Furthermore, at locations other than the tangent points of the concave fillet of the low-melting-point coating component, most of the microcracks do not extend to the substrate (microcrack depth is less than 4 μm).

[0023] Furthermore, the low-melting-point coating is a coating in which a certain proportion of liquid phase remains on the coated steel plate during the later heating stage or the holding stage of the austenitizing heating process, and the final temperature of the heating stage is ≥750°C. Furthermore, the low-melting-point coating is GI, GA, or ZnAlMg, preferably a hot-dip pure zinc coating.

[0024] Furthermore, the low-melting-point coating is a zinc-iron dual-phase alloy layer, the zinc-iron dual-phase alloy layer includes an iron-rich phase and a zinc-rich phase, and the thickness of the low-melting-point coating is 16 to 35 μm.

[0025] Furthermore, the matrix includes the following elemental components in weight percentage: C 0.05% to 0.38%, Si 0.05% to 0.4%, Mn 0.5% to 2.2%, Cr≤0.4%, Mo≤0.3%, Ti 0.02% to 0.04%, Nb≤0.2%, V≤0.2%, B 0.002% to 0.006%, P≤0.020%, S≤0.003%, Al 0.02% to 0.06%, N≤0.006%, and the remaining elements are Fe.

[0026] In a third aspect, the present application also provides a hot stamping forming method for low-melting-point coated parts with low microcrack depth as described in the above-mentioned scheme, the method including method one or method two; the method one includes the following steps: heating the coated steel plate and transferring it to a hot stamping die, the hot stamping die including an upper die and a lower die, the lower die being fully or partially immersed in a hot bath medium, the upper die and the lower die being closed, and the coated steel plate being stamped in the hot bath medium to obtain the low-melting-point coated parts with low microcrack depth; a layer of vaporized film is formed on the sheet surface of the coated steel plate in a high-temperature state in the hot bath medium, and the vaporized film and the liquid medium greatly improve the friction state and stress state between the sheet and the hot stamping die during the forming process;

[0027] The second method comprises the following steps: heating the coated steel plate and then transferring it to a hot bath medium for cooling; transferring the cooled coated steel plate to a mold for hot stamping and pressure quenching to obtain the low-melting-point coated component with low microcrack depth.

[0028] Furthermore, the heat bath medium is water or a water-based solution, and when the heat bath medium is water, the temperature of the heat bath medium is 60-100° C. Furthermore, when the heat bath medium is a water-based solution, the temperature of the heat bath medium is reduced.

[0029] Furthermore, the coated steel plate is heated to a temperature of 850-920° C., the holding time is 3-8 minutes, and the clamping time of the upper mold and the lower mold is 2-10 seconds.

[0030] Furthermore, in the second method, the inner cavity surfaces of the upper die and the lower die of the mold are coated with lubricant before the hot stamping.

[0031] Furthermore, in the method one, the hot bath medium includes a lubricant, the upper mold and the lower mold are closed, and the coated steel plate is stamped in the hot bath medium to obtain the low-melting-point coated parts with low microcrack depth; the lubricant or other water-based solute in the hot bath medium adheres to the surface of the coated steel plate in a high-temperature state and acts together with the vaporized film on the surface of the coated steel plate in a high-temperature state to improve the friction state, stress state and cooling rate between the coated steel plate and the mold during the forming process, thereby further reducing the hot bath temperature.

[0032] This application explores the formation rules of microcracks in low-melting-point coated parts during the forming process, and considers the surface friction characteristics, lubrication state and different stress state factors during the hot stamping forming process of the material. It is found that the area where low-melting-point coated parts are prone to the expansion of coating microcracks is not the area with the maximum strain during the forming process (such as the deep drawing side wall), but often appears at the tangent point of the concave fillet of the low-melting-point coated parts.

[0033] Furthermore, the device used in method one or method two is the hot stamping forming device used in the hot forming method described in the above scheme.

[0034] Furthermore, the hot stamping forming method of the low-melting-point coated parts with low microcrack depth has the same preparation steps as the hot forming method of the high-strength steel parts described in the above scheme.

[0035] Fourthly, the present application also provides a hot bath trimming device for high-strength steel parts, comprising a water tank, a heating and temperature control component and a second lower mold installed inside the water tank, lower trimming knives are arranged on both sides of the second lower mold, the heating and temperature control component is located on the outside of the lower trimming knife, an upper mold is arranged above the second lower mold, upper trimming knives are arranged on both sides of the upper mold, the top of the upper mold and the bottom of the lower trimming knife are connected with elastic parts, when in use, the high-temperature plate is first placed on the second lower mold, and then the upper mold and the upper trimming knife are driven downward by the equipment, the upper mold first contacts the plate and presses it on the second lower mold to form the plate, and then starts to compress the elastic part, as the elastic part is compressed, the upper trimming knife starts to contact the plate, and cooperates with the lower trimming knife to cut the edge of the formed plate, and at the same time, the plate is quenched in the water tank to harden it, the overall use effect is good, and the practicality is strong.

[0036] Furthermore, the elastic member is a delayed nitrogen spring or an oil cylinder.

[0037] Furthermore, an elastic member installed at the bottom of the lower trimming knife passes through the bottom of the water tank and is fixed on the second lower mold base.

[0038] Furthermore, an elastic member installed on the top of the upper trimming knife is fixed on the upper die base.

[0039] Furthermore, the liquid in the water tank is higher than the highest position of the second lower mold before mold closing, and is lower than the edge of the water tank after mold closing.

[0040] Furthermore, the liquid in the water tank is water or an aqueous solution.

[0041] Furthermore, the temperature of the liquid in the water tank is not lower than 50°. The liquid at not lower than 50° can prevent the high-temperature plate from directly contacting cold water and causing fracture.

[0042] In a fifth aspect, the present application provides a method for using the hot bath trimming device for high-strength steel parts described in the above solution, comprising the following steps:

[0043] 1) After high-temperature austenitization, the hot-formed steel plate is moved above the second lower die and immersed in the liquid in the water tank;

[0044] 2) The upper die and the upper trimming knife move downward simultaneously under the action of the equipment. The upper die first contacts the sheet and presses it onto the second lower die, forming the sheet.

[0045] 3) Continue to move downward. As the elastic member is compressed, the upper trimming knife begins to contact the sheet and cooperates with the lower trimming knife to cut the edge of the formed sheet;

[0046] 4) Cut to obtain the formed sheet, and use the elastic member to maintain pressure. Quench the formed sheet in the water tank to harden it before opening the mold.

[0047] The present application provides a method for hot forming of high-strength steel parts using the hot stamping forming device for high-strength steel parts described in the above-mentioned scheme. The working principle of the hot forming method of high-strength steel parts of the present application is as follows: when the high-strength steel sheet is transferred to the mold, it does not come into contact with hot water, and air is used as a cooling medium to cool the sheet. At this time, the cooling rate is slow, ensuring that the sheet begins to form at a higher temperature. As the upper mold continues to descend and close the mold, the sheet begins to deform after contacting the upper mold. At this time, the sheet is still in the air medium, and the strength of the sheet will not increase due to excessive cooling speed, resulting in precision deviation of the parts. The sheet then gradually enters the hot water from the air medium, and a uniform vapor film is formed on the surface of the sheet in the hot water. The vapor film can act as an insulating layer to reduce the heat transfer rate between the sheet and the water, ensuring that the cooling rate of the sheet is slow and controllable. It can also improve the friction and stress state between the sheet and the mold during the forming process together with the hot water, and inhibit the microcracks of the low-melting-point coating from extending to the substrate during the forming process. After the parts are fully formed, the temperature of the parts decreases, and the mold is completely closed. The surface vapor film is destroyed, and the heat transfer rate between the low-temperature parts and the hot water is accelerated, which can achieve rapid quenching and shorten the mold holding time.

[0048] The beneficial effects of the present application are: (1) The method for hot forming high-strength steel parts provided in the present application effectively controls the cooling rate and forming state of different forming stages by controlling the high-temperature sheet to be formed in different media in succession, thereby improving the dimensional accuracy of the parts and components, and can effectively avoid the risk of LME cracking of low-melting-point coated sheets during direct hot forming, and is particularly suitable for the production of hot-formed parts with low-melting-point and high-corrosion-resistant coatings.

[0049] (2) The hot forming method of high-strength steel parts provided in this application utilizes the uniform vapor film generated on the surface of the high-temperature sheet material during the cooling process in hot water to slowly control the cooling rate of the sheet material. After the parts are fully formed, the surface vapor film is destroyed, and the heat transfer rate between the low-temperature parts and the hot water is accelerated, thereby achieving rapid quenching, reducing the holding time, shortening the production cycle, and improving production efficiency.

[0050] This application can realize the forming and cooling of high-temperature sheet materials in different media in sequence. First, the sheet material is slowly cooled by the air medium to ensure a high forming temperature. Then, the vapor film generated by the contact between hot water and the high-temperature sheet material on the surface of the steel plate is used to uniformly and slowly control the cooling rate. After the parts are formed, the cooling rate between the low-temperature parts and the hot water is faster, which can shorten the holding time and effectively solve the problem of LME cracking of the parts, and directly mass-produce them in the existing parts production line. The hot stamping forming device provided by this application is matched with the hot forming method. The lower mold is placed in hot water. After the mold is closed, the upper and lower mold surfaces are both in hot water, which effectively ensures that the temperature distribution on the mold surface is uniform and constant. There is no need to design and process cooling water channels, which greatly reduces the mold processing cost and manufacturing cycle.

[0051] The present application also provides a low-melting-point coating component. In the low-melting-point coating component provided by the present application, the cracks in the low-melting-point coating at most locations on the product surface do not extend to the substrate (the depth of the substrate cracks is less than 4μm); a small number of microcracks extend from the low-melting-point coating to the substrate, and the substrate microcracks appear at the intersection of the concave fillet of the low-melting-point coating component, and the depth is less than 8μm, which does not affect the fatigue performance and service life of the substrate. The low-melting-point coating in the low-melting-point coating component provided by the present application contains a zinc-rich phase, has a lower electrode potential, can provide good cathodic protection for the low-melting-point coating component, and also exhibits excellent corrosion resistance at the incision.

[0052] The present application also provides a hot stamping forming method for low-melting-point coated parts with low microcrack depth as described in the above scheme. The method provided in the present application is applicable to hot-formed steel with low-melting-point coating, does not require subsequent treatment of the surface low-melting-point coating, and can avoid cracking of low-melting-point coated parts caused by liquid metal embrittlement (LME) within a wide process window. It can achieve one-time direct forming of low-melting-point coated parts with complex shapes or large drawing depths, and the performance and dimensional accuracy of the low-melting-point coated parts are qualified, thereby improving the production efficiency of low-melting-point coated parts and reducing production costs.

[0053] This application also provides a hot-bath trimming device for high-strength steel parts. This device features a simple structure, eliminating complex water-cooling pipes and reducing mold costs. The lower trimming blade is submerged in water, providing direct contact with the liquid, ensuring effective blade cooling and a long lifespan. This application integrates the hot forming and trimming processes for high-strength steel, significantly improving trimming efficiency and productivity compared to laser trimming.

[0054] This application also provides a method for using the hot bath trimming device for high-strength steel parts described in the above solution. This application uses hot water as the contact cooling medium. By controlling the hot water temperature, the forming temperature can be precisely controlled. This also avoids the risk of high-temperature sheet metal directly contacting cold water and causing fracture, making it suitable for existing production lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0056] FIG1 is a cross-sectional view of a component after hot stamping in this application;

[0057] FIG2 is a microscopic picture of microcracks in the low-melting-point coating of a component after hot stamping in this application;

[0058] FIG3 is a microstructure diagram of the low-melting-point coating phase of the component after hot stamping in this application;

[0059] FIG4 is a microscopic morphology of a low-melting-point coating and microcracks of a component (A-pillar lower reinforcement plate) prepared in Example 8 of the present application;

[0060] FIG5 is a schematic diagram of the unilateral corrosion expansion of the surface of the component (A-pillar lower reinforcement plate) prepared in Comparative Example 3 of the present application after the low-melting-point coating cyclic corrosion test;

[0061] FIG6 is a schematic structural diagram of a hot stamping forming device according to Example 9;

[0062] FIG7 is a schematic structural diagram of one of the sheet metal supports in the hot stamping forming device of Example 9;

[0063] FIG8 is a diagram showing the working principle of one of the sheet metal supports in the hot stamping forming device of Example 9;

[0064] FIG9 is a cooling temperature drop curve of a high-strength steel sheet in hot water according to Example 10;

[0065] FIG10 is a microscopic image of the coating and microcracks on both sides of the high-strength steel component prepared in Example 10;

[0066] FIG11 is a two-phase distribution morphology of the coating on a high-strength steel component prepared in Example 10;

[0067] FIG12 is a microscopic image of the coating and microcracks on both sides of the high-strength steel component prepared in Example 11;

[0068] FIG13 is a two-phase distribution morphology of the coating on a high-strength steel component prepared in Example 11;

[0069] FIG14 is a microscopic image of the coating and microcracks on both sides of a high-strength steel component prepared in Comparative Example 4;

[0070] FIG15 is a two-phase distribution morphology of the coating on a high-strength steel component prepared in Comparative Example 4;

[0071] FIG16 is a microscopic image of the coating and microcracks on both sides of the high-strength steel component prepared in Comparative Example 5;

[0072] FIG17 is a two-phase distribution morphology of the coating on a high-strength steel component prepared in Comparative Example 5;

[0073] FIG18 is a schematic diagram of the structure of a hot bath trimming device for high-strength steel parts provided in this application;

[0074] Figure numerals: 1 is the upper mold, 2 is the lower mold, 3 is the water tank, 4 is the first sheet material support, 5 is the second sheet material support, 6 is the sheet material, 7 is the fixing rod, 8 is the beam, 9 is the rotating shaft, 10 is the pull rod, 11 is the spring, 201 is the water tank, 202 is the heating and temperature control component, 203 is the upper mold, 2041 is the first elastic member, 2042 is the second elastic member, 205 is the upper trimming knife, 206 is the lower mold, and 207 is the lower trimming knife. DETAILED DESCRIPTION

[0075] In order to further illustrate the present application, the scheme of the present application is described in detail below with reference to the accompanying drawings and embodiments, but they should not be understood as limiting the scope of protection of the present application.

[0076] The coated steel sheets of Examples 1 to 7 and Comparative Examples 1 to 2 are made of steel sheets with a thickness of 1.8 mm and a coating thickness of 190 g / m 2 The hot-dip pure zinc-coated (double-sided) hot-formed sheet was subjected to hot stamping forming tests using a U-shaped die. Figure 1 shows a cross-sectional view of the component after hot stamping.

[0077] The main components of the coated steel plates in Examples 1 to 7 and Comparative Examples 1 to 2 are (in weight percentage): C 0.20%, Si 0.3%, Mn 1.6%, Cr 0.2%, Nb≤0.2%, V≤0.2%, B 0.002%, P 0.020%, and the remaining elements are Fe.

[0078] The hot bath medium used in Examples 1 to 7 and Comparative Examples 1 to 2 was water.

[0079] Examples 1 to 3

[0080] Examples 1 to 3 all adopt the following hot stamping method:

[0081] The coated steel plate is heated in a heating furnace until it is fully austenitized and kept warm. The heated plate is transferred to a hot bath medium for cooling. After cooling, it is transferred to a mold for hot stamping and pressure quenching to obtain low-melting-point coated parts with low microcrack depth.

[0082] Example 4

[0083] The coated steel plate is heated in a heating furnace until it is completely austenitized and kept warm. The heated plate is transferred to a hot bath medium for cooling. The inner cavity surfaces of the upper and lower dies are coated with boron nitride lubricant for surface lubrication. The plate is then transferred to the die for hot stamping and pressure quenching to obtain low-melting-point coated parts with low microcrack depth.

[0084] Example 5

[0085] The preparation method of Example 5 is the same as that of Example 4, except that the boron nitride lubricant is replaced by a graphite lubricant.

[0086] Examples 6-7

[0087] Examples 6 to 7 all adopt the following hot stamping method:

[0088] The coated steel plate is heated in a heating furnace until it is completely austenitized and kept warm. The heated plate is transferred to a hot bath medium containing a lubricant. A vaporized film forms on the surface of the high-temperature plate in the hot bath medium, and the lower mold surface is below the liquid level of the hot bath medium. The upper mold descends to close the mold, and the plate and mold are hot stamped in the hot bath medium. The vaporized film on the surface of the plate and the liquid medium can greatly improve the friction and stress state between the plate and the mold during the forming process. Pressure-maintaining quenching is performed, and the parts continue to be quenched in the hot bath medium during the upward movement of the upper mold to obtain low-melting-point coated parts with low microcrack depth.

[0089] Comparative Example 1

[0090] The preparation method of Comparative Example 1 is the same as that of Example 1, except that the step of transferring the heated sheet material to a hot bath medium for cooling is omitted in Comparative Example 1.

[0091] Comparative Example 2

[0092] The preparation method of Comparative Example 2 is the same as that of Example 1, except that, in Comparative Example 2, the heated sheet material is transferred to a hot bath medium for cooling, and then cooled at room temperature.

[0093] The parameters of the thermoforming methods of Examples 1 to 7 and Comparative Examples 1 to 2 are shown in Table 1:

[0094] Table 1 Thermoforming process parameters of Examples 1 to 7 and Comparative Examples 1 to 2

[0095] The coating thickness and microcracks at various locations on the low-melting-point coating components with low microcrack depths in Examples 1-7 and Comparative Examples 1-2 were characterized using a metallographic microscope. Microscopic images of the low-melting-point coating and microcracks on the components after hot forming are shown in Figure 2. The cracking conditions of the low-melting-point coating components with low microcrack depths in Examples 1-7 and Comparative Examples 1-2 are shown in Table 2.

[0096] Table 2 Cracking of low-melting-point coated parts with low microcrack depth in Examples 1 to 7 and Comparative Examples 1 to 2

[0097] As can be seen from Figure 2 and Table 2, for the same U-shaped component and different hot forming processes, the extension of microcracks in the low-melting-point coating to the substrate always occurs above the tangent point of the component's rounded corner, showing repeatability. In most locations of the product, coating cracks do not extend into the substrate (substrate crack depth is less than 4 μm); a small number of microcracks extend from the low-melting-point coating to the substrate (substrate microcracks are located at the tangent point of the component's concave fillet, and are less than 8 μm deep). It was found that the areas where microcracks in the low-melting-point coating are most likely to extend are not the areas where the greatest strain occurs during the forming process (such as the deep-drawn sidewalls), but often occur at the tangent point of the component's concave fillet (R corner) (as shown in Figure 1, the crack position P is shown at the tangent point of the R corner).

[0098] The hot forming process is that the high-temperature sheet is cooled in the hot bath medium and then transferred to the mold for forming. By controlling the temperature of the sheet during forming, the depth of microcracks can be controlled within 10μm; after lubricating the upper and lower model surfaces of the mold with solid lubricant, the depth and number of microcracks in the matrix are reduced to varying degrees; when the hot forming process is that the high-temperature sheet and the lower model surface of the mold are simultaneously in the hot bath medium, the sheet and the mold are formed and pressure-maintained quenched in the hot bath medium, a layer of vaporized film will form on the surface of the high-temperature sheet in the hot bath medium, evenly covering the surface of the sheet. The simultaneous existence of the vaporized film and the liquid medium can simultaneously improve the friction and stress state between the sheet and the mold during the forming process. Only microcracks in the low-melting-point coating appear in the parts, and no microcracks extending to the matrix are found. When the hot bath medium contains lubricant, no cracking occurs.

[0099] Figure 3 shows the low-melting-point coating phase of a thermoformed component, observed using a scanning electron microscope in backscattered mode. The structure of the low-melting-point coating phase after thermoforming reveals that the off-white portion represents the zinc-rich phase (Γ phase), while the dark gray portion represents the iron-rich phase (α-Fe(Zn) phase). By volume percentage, the zinc-rich phase accounts for ≥4%, and the coating thickness ranges from 16 to 35 μm. By weight percentage, the zinc content of the zinc-rich phase is ≥65%, while the zinc content of the iron-rich phase is ≤45%.

[0100] Example 8

[0101] A complex part of a certain car model (A-pillar lower reinforcement plate) was selected, with a sheet thickness of 1.6mm and a coating thickness of 130g / m 2 and 160g / m 2Hot-dip pure zinc-coated hot-formed sheets with two coating thicknesses (on both sides) were formed using high-temperature sheets and molds under the liquid level of a hot bath medium, and a pressure-holding quenching process was used to verify the batch stability of hot stamping. The mechanical properties of the hot-stamped parts after forming (test standard: GB / T 228.1-2010 "Tensile testing of metallic materials Part 1: Room temperature test method") were as follows: tensile strength > 1600 MPa, elongation after fracture > 5%; the micromorphology of the low-melting-point coating and microcracks of the parts are shown in Figure 4, and no substrate microcracks > 8 μm were found in any part of the parts.

[0102] Comparative Example 3

[0103] Comparative Example 3: Cyclic salt spray corrosion testing was performed on mass-produced parts (hot-formed steel with aluminum-silicon coating) of the same vehicle model as in Example 8 (test standard: GWT A H01-03:2022-03, "Cyclic Corrosion Test Method and Control Requirements for Covering Layers in High-Corrosion Areas"). The unilateral corrosion expansion of the cutouts on the corroded parts is shown in Figure 5. After one accelerated salt spray cycle, the hot-dip galvanized hot-formed parts showed no obvious red rust or corrosion expansion, while the hot-formed parts with aluminum-silicon coating showed obvious red rust and corrosion expansion on the cutouts.

[0104] Example 9

[0105] A hot stamping forming device for high-strength steel parts, the specific structure of which is shown in Figures 6 and 7, includes a hot stamping die and a controller. The hot stamping die includes an upper die 1 and a lower die 2. The lower die 2 is located in a water tank 3. A water level sensor and a thermometer are provided in the water tank 3. A water inlet and a water outlet are opened on the side wall of the water tank 3. A water inlet valve is provided at the water inlet, and a water outlet valve is provided at the water outlet. The water level sensor and the thermometer are respectively connected to the controller signal, and the water inlet valve and the water outlet valve are respectively connected to the controller control.

[0106] Sheet material supports (a first sheet material support 4 and a second sheet material support 5) are symmetrically arranged on both sides of the lower mold 2. Each sheet material support includes a fixed rod 7 vertically fixed to the bottom surface of the water tank 3. The upper end of the fixed rod 7 is rotatably connected to a crossbeam 8 through a rotating shaft 9. One end of the crossbeam 8 of the two sheet material supports cooperates with each other to support the sheet material 6. The other end of the crossbeam 8 is rotatably connected to the upper end of the pull rod 10. The lower end of the pull rod 10 is connected to the fixed rod 7 through a spring 11. In the initial state, the height of the crossbeam 8 is higher than the hot water level.

[0107] During use, the water tank 3 is filled with hot water above 60°C, and part or all of the lower mold 2 is immersed in the hot water. The sheet 6 is placed on the beam 8. At this time, the sheet 6 does not contact the hot water and the molding surface of the lower mold 2. The upper mold 1 moves downward to contact the sheet 6. The end of the beam 8 that contacts the sheet 6 is forced to swing downward around the rotating shaft 9, driving the pull rod 10 connected to the other end to move upward, the spring 11 extends, and the sheet 6 falls and contacts the lower mold 2; after the sheet 6 completely leaves the beam, the spring 11 returns to its initial length and automatically moves downward, and the pull rod 10 drives the beam 8 to move upward and reset.

[0108] When the amount of hot water in the water tank 3 is small and the sheet material 6 cannot be completely immersed in the hot water medium, open the water inlet valve and add hot water into the water tank 3 through the water inlet until the hot water level rises to the point where the sheet material is completely immersed in the hot water; when the water temperature in the water tank 3 is lower than the required temperature, open the water outlet valve and discharge the low-temperature water outward through the water outlet, open the water inlet valve and add hot water into the water tank 3 through the water inlet until the water temperature in the water tank 3 reaches the required temperature. The working principle is shown in Figure 8.

[0109] In other embodiments, depending on the shape of the sheet material, the number of sheet material supports may be two or more, and the distribution of the sheet material supports may be symmetrical or asymmetrical, so as to support the sheet material and ensure that the sheet material does not slip during the thermoforming process.

[0110] Example 10

[0111] Select thickness of 1.8mm and coating thickness of 150g / m 2 (Double-sided) hot-dip pure zinc-coated high-strength steel sheet was used to trial-produce the A-pillar upper reinforcement plate components of a certain vehicle model on a mass production line using the apparatus of Example 9. The main components of the sheet were C 0.20wt%, Si 0.3wt%, Mn 1.6wt%, Cr 0.2wt%, Nb 0.02wt%, V 0.002wt%, B 0.002wt%, and P 0.020wt%, with the remaining elements being Fe and unavoidable impurities.

[0112] The specific steps are as follows:

[0113] (1) The sheet was kept warm in a 900°C box-type heating furnace for 380 seconds to obtain an austenitic structure. The heated sheet was transferred to the hot stamping forming device of Example 9. The two ends of the sheet were placed on the first sheet support and the second sheet support, respectively. The water tank of the hot stamping forming device was filled with 95°C hot water, and the hot water level was flush with the upper end surface of the lower die.

[0114] (2) The upper die of the hot stamping forming device moves downward and then contacts the sheet. At this time, the sheet is in the air medium and the cooling speed is slow. The sheet temperature is kept above 680°C. The upper die continues to move downward, and the first sheet support and the second sheet support move downward in coordination. The sheet begins to deform in the air medium and cools slowly, avoiding the increase in strength caused by excessive cooling and the resulting deviation in dimensional accuracy.

[0115] (3) The upper mold continues to move downward, and the sheet metal is gradually immersed in the hot water. The cooling temperature drop curve in the hot water is shown in Figure 9. The mold is closed and the sheet metal is completely formed into a component in the hot water. The mold closing time (i.e., the time spent in steps (2) and (3)) is 3 seconds in total.

[0116] (4) The mold is kept under pressure for 10 seconds. At this time, the vapor film on the surface of the sheet is destroyed, and the heat transfer rate between the low-temperature parts and the hot water is accelerated, so that the parts are quenched and high-strength steel parts are obtained.

[0117] The mechanical properties of the obtained high-strength steel components were tested according to the test method GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The results showed that the tensile strength of the high-strength steel components was greater than 1500MPa, and the elongation after fracture was greater than 5%. The dimensional accuracy test of the high-strength steel components was fully qualified.

[0118] The micromorphology of the coating and microcracks on a high-strength steel component is shown in Figure 10. The upper and lower images in Figure 10 correspond to the micromorphology of the coating on the front and back of the high-strength steel component. No matrix microcracks larger than 8μm were found in any part of the high-strength steel component. The two-phase distribution morphology of the coating on the high-strength steel component is shown in Figure 11. The bright white areas in Figure 11 represent the zinc-rich phase, while the darker gray areas represent the iron-rich phase. Analysis of Figure 11 indicates that the zinc-rich phase accounts for approximately 11%.

[0119] Example 11

[0120] The hot forming method of Example 11 is basically the same as that of Example 10, with the only difference being that a roller bottom heating furnace is used to heat the coated sheet in step (1), the sheet heating temperature is 890°C, the holding time is 320s, and the mold holding time in step (4) is shortened to 3s.

[0121] The mechanical properties of the obtained high-strength steel components were tested according to the test method GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The results showed that the tensile strength of the high-strength steel components was greater than 1500MPa, and the elongation after fracture was greater than 5%. The dimensional accuracy test of the high-strength steel components was fully qualified.

[0122] The coating and microcracks of the high-strength steel component were observed, as shown in Figure 7. Figure 7 shows that no matrix microcracks larger than 8μm were found in any part of the high-strength steel component. The two-phase distribution of the coating on the high-strength steel component was observed, as shown in Figure 8. Analysis in Figure 8 shows that the bright white zinc-rich phase accounts for approximately 20%.

[0123] Comparative Example 4

[0124] The thermoforming method of Comparative Example 4 is substantially the same as that of Example 10, with the only difference being that the water tank of the hot stamping forming device is not filled with hot water, and the sheet material is continuously thermoformed in an air medium.

[0125] The mechanical properties and dimensional accuracy of the obtained high-strength steel parts are qualified, and the proportion of zinc-rich phase is about 18% (as shown in Figure 15). However, the microcracks in the coating extend into the substrate, with a depth of more than 30μm (as shown in Figure 14), which cannot meet the fatigue service requirements of high-strength steel parts.

[0126] Comparative Example 5

[0127] The thermoforming methods of Comparative Example 5 and Example 10 are basically the same, with the only difference being that in step (1), the hot water level in the water tank is flush with the upper end surface of the sheet, and the sheet is continuously thermoformed in the hot water medium.

[0128] The mechanical properties, coating and microcracks of the obtained high-strength steel parts are all qualified, and the proportion of zinc-rich phase is about 19% (as shown in Figure 17). However, the dimensional accuracy deviation of the parts is large and cannot meet the fatigue service requirements of the parts.

[0129] Example 12

[0130] Select thickness of 1.8mm and coating thickness of 150g / m 2 An aluminum-silicon coated high-strength steel sheet was used to trial-produce an A-pillar upper reinforcement plate component for a certain vehicle model on a mass production line using the apparatus of Example 9. The main components of the sheet were C 0.2wt%, Si 0.2wt%, Mn 1.2wt%, Cr 0.2wt%, Ti 0.04wt%, B 0.003wt%, S 0.001wt%, and N 0.001wt%, with the remaining elements being Fe and unavoidable impurities.

[0131] The specific steps are as follows:

[0132] (1) The sheet was kept warm in a box-type heating furnace at 930°C for 300 seconds to obtain an austenitic structure. The heated sheet was transferred to the hot stamping forming apparatus of Example 9. The two ends of the sheet were placed on the first sheet support and the second sheet support, respectively. The water tank of the hot stamping forming apparatus was filled with 75°C hot water, and the hot water level was flush with the upper end surface of the lower die.

[0133] (2) The upper die of the hot stamping forming device moves downward and then contacts the sheet. At this time, the sheet is in the air medium and the cooling speed is slow. The sheet temperature is kept above 700°C. The upper die continues to move downward, and the first sheet support and the second sheet support move downward in coordination. The sheet begins to deform in the air medium and cools slowly, avoiding the increase in strength caused by excessive cooling and the resulting deviation in dimensional accuracy.

[0134] (3) The upper mold continues to move downward, the sheet metal is gradually immersed in the hot water, the mold is closed, and the sheet metal is completely formed into a component in the hot water. The mold closing time (i.e., the time spent in steps (2) and (3)) is 5 seconds in total;

[0135] (4) The mold maintains pressure for 8 seconds. At this time, the vapor film on the surface of the sheet is destroyed, and the heat transfer rate between the low-temperature parts and the hot water is accelerated, so the parts are quenched and high-strength steel parts are obtained.

[0136] The mechanical properties of the obtained high-strength steel components were tested according to the test method GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The results showed that the tensile strength of the high-strength steel components was greater than 1500MPa, and the elongation after fracture was greater than 5%. The dimensional accuracy test of the high-strength steel components was fully qualified, and no matrix microcracks greater than 8μm were found in any part of the high-strength steel components.

[0137] Example 13

[0138] A high-strength steel sheet with a thickness of 2.0 mm was selected, and a trial production of an A-pillar upper reinforcement plate component for a certain vehicle model was carried out on a mass production line using the apparatus of Example 9. The main components of the sheet were C 0.18 wt%, Si 0.2 wt%, Mn 1.5 wt%, Cr 0.3 wt%, B 0.002 wt%, and S 0.001 wt%, with the remaining elements being Fe and unavoidable impurities.

[0139] The specific steps are as follows:

[0140] (1) The sheet was kept warm in a box-type heating furnace at 930°C for 360 seconds to obtain an austenitic structure. The heated sheet was transferred to the hot stamping forming apparatus of Example 9. The two ends of the sheet were placed on the first sheet support and the second sheet support, respectively. The water tank of the hot stamping forming apparatus was filled with 85°C hot water, and the hot water level was flush with the upper end surface of the lower die.

[0141] (2) The upper die of the hot stamping forming device moves downward and then contacts the sheet. At this time, the sheet is in the air medium and the cooling speed is slow. The sheet temperature is kept above 695°C. The upper die continues to move downward, and the first sheet support and the second sheet support move downward in coordination. The sheet begins to deform in the air medium and cools slowly, avoiding the increase in strength caused by excessive cooling and the resulting deviation in dimensional accuracy.

[0142] (3) The upper mold continues to move downward, the sheet metal is gradually immersed in the hot water, the mold is closed, and the sheet metal is completely formed into a component in the hot water. The mold closing time (i.e., the time spent in steps (2) and (3)) is 8 seconds in total.

[0143] (4) The mold maintains pressure for 3 seconds. At this time, the vapor film on the surface of the sheet is destroyed, and the heat transfer rate between the low-temperature parts and the hot water is accelerated, so the parts are quenched and high-strength steel parts are obtained.

[0144] The mechanical properties of the obtained high-strength steel components were tested according to the test method GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The results showed that the tensile strength of the high-strength steel components was greater than 1500MPa and the elongation after fracture was greater than 5%. The dimensional accuracy test of the high-strength steel components was fully qualified, and no matrix microcracks greater than 8μm were found in any part of the high-strength steel components.

[0145] Example 14

[0146] As shown in Figure 18, in this embodiment, the present application provides a hot bath trimming device for high-strength steel parts, including a water tank 201. The liquid in the water tank 201 is water or an aqueous solution. A heating and temperature control component 202 and a lower mold 206 are installed inside the water tank 201. The temperature of the liquid in the water tank 201 is not lower than 50°C. The liquid not lower than 50°C can prevent the high-temperature plate from directly contacting cold water and causing fracture. Lower trimming knives 207 are provided on both sides of the lower mold 206. The liquid in the water tank 201 is higher than the highest position of the lower mold 206 before mold closing, and is lower than the edge of the water tank 201 after mold closing. The heating and temperature control component 202 is located on the outside of the lower trimming knife 207. An upper mold 203 is provided above the lower mold 206. Upper trimming knives 205 are provided on both sides of the upper mold 203. The top of the upper mold 203 and the bottom of the lower trimming knife 207 They are all connected with elastic parts 204, which are delayed nitrogen springs or oil cylinders. The first elastic part 2041 installed at the bottom of the lower trimming knife 207 passes through the bottom of the water tank 201 and is fixed on the lower die base. The second elastic part 2042 installed on the top of the upper trimming knife 205 is fixed on the upper die base. When in use, the high-temperature plate is first placed on the lower die 206, and then the upper die 203 and the upper trimming knife 205 are driven downward by the equipment. The upper die 203 first contacts the plate and presses it on the lower die 206 to form the plate, and then begins to compress the elastic part 204. As the elastic part 204 is compressed, the upper trimming knife 205 begins to contact the plate and cooperates with the lower trimming knife 207 to cut the edge of the formed plate. At the same time, the plate is quenched in the water tank 201 to harden it. The overall use effect is good and the practicality is strong.

[0147] Example 15

[0148] This embodiment provides a method for using a hot bath trimming device for high-strength steel parts. The hot bath trimming device for high-strength steel parts prepared in Example 14 above includes the following steps:

[0149] (1) After being austenitized at high temperature, the hot-formed steel plate is moved above the lower die 206 and immersed in the liquid in the water tank 201;

[0150] (2) The upper die 203 and the upper trimming knife 205 move downward simultaneously under the action of the equipment. The upper die 203 first contacts the plate and presses it onto the lower die 206, so that the plate is formed;

[0151] (3) Continuing to move downward, as the elastic member 204 is compressed, the upper trimming knife 205 begins to contact the plate and cooperates with the lower trimming knife 207 to cut the edge of the formed plate;

[0152] (4) After the formed sheet is cut, the elastic member 204 maintains pressure, and the formed sheet is quenched in the water tank 201 to harden it before the mold is opened.

[0153] Although the above embodiment provides a detailed description of the present application, it is only a part of the embodiments of the present application, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present application.

Claims

1. A hot forming method for high-strength steel parts, characterized in that: The steps include: During the mold closing process of the hot stamping forming device, as the upper mold (1) descends, the high-strength steel sheet is first cooled in the air medium, and then gradually enters the hot water medium from the air medium under the drive of the upper mold (1) until it is completely immersed in the hot water and formed, and then the mold is pressure-maintained to achieve component quenching to obtain a high-strength steel component; the temperature of the hot water is above 60°C; The hot stamping forming device includes a hot stamping die, and the hot stamping die includes an upper die (1) and a lower die (2). The lower die (2) is located in a water tank (3). The water tank (3) is used to hold hot water above 60°C. When in use, part or all of the lower die (2) is immersed in hot water. The lower die (2) is provided with at least two sheet material supports. The sheet material support includes a fixed rod (7) vertically fixed to the bottom surface of the water tank (3). The upper end of the fixed rod (7) is rotatably connected to a crossbeam (8) through a rotating shaft (9). One end of the crossbeam (8) of the sheet material support cooperates with each other to support the sheet material. The other end of the crossbeam (8) is rotatably connected to the upper end of the pull rod (10). The lower end of the pull rod (10) is connected to the fixed rod (7) through a spring (11). In the initial state, the height of the crossbeam (8) is higher than the hot water liquid level.

2. The hot forming method according to claim 1, characterized in that The hot stamping forming device further includes a controller; A water level sensor and a thermometer are also provided in the water tank (3). A water inlet and a water outlet are provided on the side wall of the water tank (3). A water inlet valve is provided at the water inlet, and a water outlet valve is provided at the water outlet. The water level sensor and the thermometer are respectively connected to controller signals, and the water inlet valve and the water outlet valve are respectively connected to the controller for control.

3. The hot forming method according to claim 1, characterized in that The high-strength steel sheet is further heated to above AC3 before use to obtain an austenitic structure.

4. The hot forming method according to claim 1, characterized in that The temperature at which the high-strength steel sheet begins to be formed is not less than 680°C.

5. The thermoforming method according to claim 1, characterized in that The mold pressure holding time is 1 to 10 seconds.

6. The thermoforming method according to claim 1, characterized in that The chemical element composition of the high-strength steel sheet is: C 0.05wt% to 0.35wt%, Si 0.05wt% to 0.6wt%, Mn 0.5wt% to 2.2wt%, Cr≤0.5wt%, Mo≤0.5wt%, Ni≤0.5wt%, Ti≤0.04wt%, Nb≤0.2wt%, V≤0.2wt%, B 0.002wt% to 0.006wt%, P≤0.020wt%, S≤0.003wt%, Al≤0.8wt%, N≤0.006wt%, and the balance is Fe and unavoidable impurities.

7. The thermoforming method according to claim 1, characterized in that: The high-strength steel sheet material is an uncoated sheet or a coated sheet, and the coated sheet is a zinc-based coated sheet or an aluminum-silicon coated sheet.

8. The thermoforming method according to claim 1, characterized in that: The high-strength steel sheet material is a zinc-based coating sheet, and the proportion of the zinc-rich phase in the zinc-based coating on the surface of the high-strength steel component is not higher than 40%.

9. A low-melting-point coating component with low microcrack depth, comprising a substrate and a low-melting-point coating located on the surface of the substrate, characterized in that: The low-melting-point coating component is formed by hot stamping. During the forming process, microcracks are generated in the substrate and the low-melting-point coating. Most of the microcracks extending from the low-melting-point coating to the substrate are located at the tangent point of the concave fillet of the low-melting-point coating component, and a small number of microcracks extending from the low-melting-point coating to the substrate are located at positions other than the tangent point of the concave fillet of the low-melting-point coating component; the depth of the microcracks in the substrate located at the tangent point of the concave fillet of the low-melting-point coating component is less than 8 μm, and the depth of the microcracks in the substrate located outside the tangent point of the concave fillet of the low-melting-point coating component is less than 4 μm; the substrate is a steel plate.

10. The low melting point coating component with low microcrack depth according to claim 9, characterized in that: At locations other than the tangent points of the concave fillet of the low-melting-point coating component, most of the microcracks have a depth of less than 4 μm.

11. The low melting point coating component with low microcrack depth according to claim 9, characterized in that: The low melting point coating is a coating in which a certain proportion of liquid phase still exists in the late heating stage or the insulation stage of the austenitizing heating process of the coated steel plate, and the final temperature of the heating stage is ≥750°C.

12. The low melting point coating component with low microcrack depth according to claim 9 or 11, characterized in that: The low-melting-point coating is a zinc-iron dual-phase alloy layer, which includes an iron-rich phase and a zinc-rich phase. The thickness of the low-melting-point coating is 16 to 35 μm.

13. The low melting point coating component with low microcrack depth according to claim 9, characterized in that: The matrix includes the following elemental components in weight percentage: C 0.05% to 0.38%, Si 0.05% to 0.4%, Mn 0.5% to 2.2%, Cr≤0.4%, Mo≤0.3%, Ti 0.02% to 0.04%, Nb≤0.2%, V≤0.2%, B 0.002% to 0.006%, P≤0.020%, S≤0.003%, Al 0.02% to 0.06%, N≤0.006%, and the remaining elements are Fe.

14. The hot stamping method for forming a low-melting-point plated component with a low microcrack depth according to any one of claims 9 to 13, characterized in that: The method includes method 1 or method 2; The first method comprises the following steps: heating the coated steel sheet and transferring it to a hot stamping die, wherein the hot stamping die comprises an upper die and a lower die, wherein the lower die is fully or partially immersed in a hot bath medium, the upper die and the lower die are closed, and the coated steel sheet is stamped in the hot bath medium to obtain the low-melting-point coated component with a low microcrack depth; The second method comprises the following steps: heating the coated steel plate and then transferring it to a hot bath medium for cooling; transferring the cooled coated steel plate to a mold for hot stamping and pressure quenching to obtain the low-melting-point coated parts with low microcrack depth.

15. The hot stamping method according to claim 14, wherein: The heat bath medium is water or a water-based solution. When the heat bath medium is water, the temperature of the heat bath medium is 60-100°C.

16. The hot stamping method according to claim 14, wherein: The coating steel plate is heated to a temperature of 850-920° C., the heat preservation time is 3-8 minutes, and the clamping time of the upper mold and the lower mold is 2-10 seconds.

17. The hot stamping method according to claim 14, wherein: In the second method, the inner cavity surfaces of the upper die and the lower die of the mold are coated with lubricant before the hot stamping.

18. The hot stamping method according to claim 14, wherein: In the first method, the heat bath medium includes a lubricant.

19. The hot stamping method according to claim 14, wherein: The device used in method one or method two is a hot stamping forming device used in the hot forming method according to any one of claims 1 to 8.

20. The hot stamping method for forming a low-melting-point plated component with a low microcrack depth according to any one of claims 9 to 13, characterized in that: The preparation steps are the same as the hot forming method of the high-strength steel component according to any one of claims 1 to 8.

21. A hot bath trimming device for high-strength steel parts, comprising a water tank (201), characterized in that: A heating and temperature control component (202) and a second lower mold (206) are installed inside the water tank (201), lower trimming knives (207) are provided on both sides of the second lower mold (206), the heating and temperature control component (202) is located on the outside of the lower trimming knife (207), an upper mold (203) is provided above the second lower mold (206), upper trimming knives (205) are provided on both sides of the upper mold (203), and elastic parts (204) are connected to the top of the upper mold (203) and the bottom of the lower trimming knife (207).

22. The hot bath trimming device for high-strength steel parts according to claim 21, characterized in that: The elastic member (204) is a time-delay nitrogen spring or an oil cylinder.

23. The hot bath trimming device for high-strength steel parts according to claim 21, characterized in that: The elastic member (204) installed at the bottom of the lower trimming knife (207) passes through the bottom of the water tank (201) and is fixed on the seat of the second lower mold (206).

24. The hot bath trimming device for high-strength steel parts according to claim 21, characterized in that: The elastic member (204) installed on the top of the upper trimming knife (205) is fixed on the upper die (203) seat.

25. The hot bath trimming device for high-strength steel parts according to claim 21, characterized in that: The liquid in the water tank (201) is higher than the highest position of the second lower mold (206) before mold closing, and is lower than the edge of the water tank (201) after mold closing.

26. The hot bath trimming device for high-strength steel parts according to claim 25, characterized in that: The liquid in the water tank (201) is water or an aqueous solution.

27. The hot bath trimming device for high-strength steel parts according to claim 26, characterized in that: The temperature of the liquid in the water tank (201) is not lower than 50°C.

28. A method for using the hot bath trimming device for high-strength steel parts according to any one of claims 21 to 27, characterized in that: The following steps are involved: (1) After being austenitized at high temperature, the hot-formed steel plate is moved to the top of the second lower die (206) and immersed in the liquid in the water tank (201); (2) The upper die (203) and the upper trimming knife (205) move downward simultaneously under the action of the equipment, and the upper die (203) first contacts the plate and presses it onto the second lower die (206), so that the plate is formed; (3) Continuing to move downward, as the elastic member (204) is compressed, the upper trimming knife (205) begins to contact the plate and cooperates with the lower trimming knife (207) to cut the edge of the formed plate; (4) After cutting, a formed plate is obtained, and the elastic member (204) maintains pressure, allowing the formed plate to be quenched in the water tank (201) to harden it before the mold is opened.

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