Low-melting-point coating part with low microcrack depth and hot stamping forming process thereof

Abstract

The invention relates to the technical field of hot stamping forming of parts, in particular to a low-melting-point coating part with the low microcrack depth and a hot stamping forming technology thereof.The part comprises a base body and a low-melting-point coating located on the surface of the base body, the part is formed through hot stamping, and in the forming process, the base body and the coating or the coating generate microcracks; most of the microcracks are located at the indent fillet tangency point of the part. According to the method, under the direct forming technology, the micro-crack growth process of a part coating and a steel plate base body is effectively restrained, the micro-crack depth is controlled within 10 microns, the service life of a part is guaranteed, and meanwhile the part shows excellent corrosion resistance.

Description

[0001] This application claims priority to a Chinese patent application filed with the Patent Office of China on March 15, 2024, with application number CN 202410300810.2, entitled “Low-melting-point coated components with low microcrack depth and hot stamping forming process thereof,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the technical field of hot stamping forming of parts, and specifically to a hot stamping forming process for low-melting-point coated parts with low microcrack depth and a hot stamping forming process thereof. 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. Hot stamping of 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 cold stamping 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. During hot stamping, the coating is prone to cracking, but the cracks do not extend into the substrate, so a direct hot stamping process can be used. Low-melting-point coated hot stamping steels (such as GI, GA, ZnAlMg, etc.) are prone to LME cracking during high-temperature forming due to the low melting point of Zn, which transforms into liquid zinc during high-temperature heating. However, the high Fe content in the GA coating (~10%) allows for rapid element diffusion reactions during hot stamping, effectively reducing the occurrence of LME cracking. Therefore, GA-coated steel sheets are suitable for direct hot stamping. However, currently, pure zinc-coated hot stamping steel sheets can only be used in the indirect hot stamping process for the manufacture of automotive parts. This 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] CN114032475A discloses a high-strength cold-rolled hot-dip galvanized medium-manganese steel, its preparation method, and hot-formed components. This method utilizes pre-nickel plating, adjusts the Mn elemental composition in the matrix, and combines it with appropriate hot-forming processes to mitigate the technical problem of liquid metal embrittlement (LME) cracking in galvanized steel products caused by post-hot-forming processing. However, this method does not address a direct forming process for component manufacturing, nor does it investigate cracking in formed components.

[0008] CN115369317A discloses a nickel-plated and zinc-plated hot-formed steel sheet, a preparation method, and a method for preparing hot-formed parts. This method forms a Zn-Ni alloy by coating the zinc coating with a metallic nickel layer, thereby raising the melting point of the coating and suppressing the liquid crack embrittlement caused by cracks extending into the substrate. However, this method increases the difficulty and cost of producing the coated hot-formed steel sheet, making it unsuitable for conventional low-melting-point coated hot-formed steel.

[0009] CN115029632A discloses a highly corrosion-resistant, galvanized, hot-formed hardened steel, its components, and a method for producing the same. 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 for liquid metal brittleness during hot forming, and enabling the production of components with complex cross-sectional shapes. However, this method still fails to address the issues of liquid metal brittleness or coating crack propagation during direct forming.

[0010] Therefore, it is extremely important to develop a coating hot stamping forming component and its manufacturing process with controllable microcracks in the coating and substrate, low sensitivity to liquid metal embrittlement, low cost, wide process window, and suitable for low-melting-point coating hot-formed steel and direct hot forming process. Summary of the Invention

[0011] In response to the technical problem of the risk of substrate cracking caused by liquid metal embrittlement (LME) during the direct hot stamping process of low-melting-point coated steel plates, the present application provides a low-melting-point coated component with low microcrack depth and a hot stamping forming method thereof. The present application can effectively inhibit the microcrack propagation process of the low-melting-point coating of the component and the steel plate substrate under the direct hot stamping process, control the microcrack depth within 10μm, ensure the service life of the component, and at the same time the component exhibits excellent corrosion resistance.

[0012] In a first aspect, the present application 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 does not have microcracks or has microcracks with a depth of less than 10 μm, and most of the microcracks with a depth of less than 10 μm are located at the intersection of the concave fillet of the low-melting-point coating component; the microcracks include coating microcracks; or the microcracks include coating microcracks and substrate microcracks, and the number of substrate microcracks is less than the number of coating microcracks.

[0013] In the present invention, the microcracks include coating microcracks and substrate microcracks. The coating microcracks refer to microcracks that do not extend to the substrate, and the substrate microcracks refer to microcracks that extend from the coating to the substrate.

[0014] In the second aspect, the present application 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 hot stamping process, wherein the microcracks include a first microcrack and a second microcrack extending from the coating to the substrate, wherein the first microcrack is located at the intersection of the concave fillet of the component, and the second microcrack is located outside the intersection of the concave fillet of the component, the number of the first microcracks is greater than the number of the second microcracks, the depth of the first microcrack is less than 8 μm, and the depth of the second microcrack is less than 4 μm.

[0015] Furthermore, 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 heat preservation stage of the austenitizing heating process of the coated steel plate, and the late heating stage is heating to ≥750°C.

[0016] Furthermore, the low melting point coating is GI, GA, ZnAlMg or 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; preferably, the substrate 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%, Al0.02% to 0.06%, N≤0.006%, and the remaining elements are Fe.

[0017] In a third aspect, the present application also provides a hot stamping method for low-melting-point coated parts with low microcrack depth, comprising the first method or the second method,

[0018] The first method comprises the following steps: after heating the low-melting-point coated steel plate until it is completely austenitized, transferring it to a hot bath mold, wherein the hot bath mold comprises an upper mold and a lower mold, wherein the lower mold is fully or partially immersed in the hot bath medium, the upper mold and the lower mold are closed, and the low-melting-point coated steel plate is stamped and formed in the hot bath medium;

[0019] The second method comprises the following steps: heating the low-melting-point coated steel plate until it is completely austenitized, transferring it to a hot bath medium for cooling, transferring it to a mold for hot stamping after cooling, and pressure-holding quenching.

[0020] Furthermore, in the first method and the second method, the heat bath medium is water or a water-based solution; when the heat bath medium is water, the heat bath temperature is 60-100°C.

[0021] Furthermore, in the first method and the second method, the low-melting-point coated steel plate is heated to a temperature of 850-920° C., the holding time is 3-8 minutes, and the closing time of the upper mold and the lower mold is 2-10 seconds.

[0022] Furthermore, in the second method, the mold includes an upper mold and a lower mold, and the inner cavity surfaces of the upper mold and the lower mold are coated with a lubricant;

[0023] Preferably, the lubricant is a boron nitride lubricant or a graphite lubricant.

[0024] Furthermore, in the first method, the heat bath medium also includes a lubricant.

[0025] Furthermore, after the low melting point coated steel plate is heated, it is transferred to a hot bath mold, which includes an upper mold and a lower mold. The lower mold is fully or partially immersed in a hot bath medium, which includes a lubricant. 下 The mold is closed, and the low-melting-point coated steel plate is stamped and formed in the hot bath medium. The lubricant or other water-based solute in the hot bath medium adheres to the surface of the high-temperature sheet and works together with the vapor film on the high-temperature surface to improve the friction state, stress state and cooling rate between the sheet and the mold during the forming process, thereby further reducing the hot bath temperature.

[0026] This application explores the formation rules of microcracks in coated hot-formed steel during the forming process, and considers the surface friction characteristics, lubrication status and different stress state factors during the hot stamping forming process of the material. It is found that the areas where coating microcracks are prone to expand in components are not the areas where the maximum strain occurs during the forming process (such as the deep drawing side walls), but often appear at the tangent points of the concave fillets in the components.

[0027] The beneficial effects of this application are:

[0028] (1) For the hot-stamped parts with low-melting-point coatings produced in this application, cracks in the coating at most locations on the product surface do not extend to the substrate (the depth of the substrate cracks is less than 4 μm); a small number of microcracks extend from the low-melting-point coating to the substrate. The substrate microcracks appear at the intersection of the concave fillet of the parts, and the depth is less than 8 μm, which does not affect the fatigue performance and service life of the substrate.

[0029] (2) The hot stamping forming method of the present application is applicable to hot-formed steel with low-melting-point coating, and does not require subsequent treatment of the surface low-melting-point coating. Within a wide process window, it can avoid cracking of parts caused by liquid metal embrittlement (LME), and achieve one-time direct forming of parts with complex shapes or large drawing depths, with qualified performance and dimensional accuracy of the parts, thereby improving the production efficiency of parts and reducing production costs.

[0030] (3) The low-melting-point coating of the components of the present application contains a zinc-rich phase, which has a lower electrode potential, can provide good cathodic protection for the components, and the incision also exhibits excellent corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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 or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0032] Figure 1 This is a cross-sectional view of a component after hot stamping in a specific embodiment of the present application;

[0033] Figure 2 This is a microscopic picture of the coating and microcracks of the parts after hot stamping in the specific embodiment of this application;

[0034] Figure 3 This is a microstructure diagram of the coating phase of a component after hot stamping in a specific embodiment of the present application;

[0035] Figure 4 This is a microscopic morphology of the coating and microcracks of the component (A-pillar lower reinforcement plate) obtained in Example 8 of the specific implementation method of this application;

[0036] Figure 5 This is a schematic diagram of the unilateral corrosion expansion of the surface of the component (A-pillar lower reinforcement plate) after the cyclic corrosion test of the coating obtained in Comparative Example 3 of the specific embodiment of the present application;

[0037] Figure 6 is the surface friction coefficient of galvanized sheet under different forming methods;

[0038] Figure 7 This is a schematic diagram of the appearance of the components of Example 8 and the location where the maximum depth of microcracks occurs;

[0039] Figure 8 3 is a correlation diagram between (strain rate×equivalent plastic strain) and microcrack depth in Example 6. DETAILED DESCRIPTION

[0040] In order to enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0041] The substrate of the coated steel plate of Examples 1-7 and Comparative Examples 1-2 is a steel plate with a thickness of 1.8 mm and a coating thickness of 190 g / m 2 (Double-sided) hot-dip pure zinc-coated hot-formed sheet, using a U-shaped die for hot stamping forming test, such as Figure 1 This is a cross-sectional view of the component after hot stamping.

[0042] The main components of the steel plate material in the following embodiment are (in percentage by weight): 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.

[0043] The following examples use water as the heating bath medium.

[0044] Examples 1 to 3 all adopt the following hot stamping process:

[0045] The coated steel plate is heated in a heating furnace until it is completely austenitized and kept warm. The heated coated steel plate is transferred to a hot bath medium for cooling. After cooling, it is transferred to a mold for hot stamping and pressure quenching.

[0046] Example 4

[0047] The coated steel plate is heated in a heating furnace until it is completely austenitized and kept warm. The heated coated steel plate is transferred to a hot bath medium for cooling. The inner cavity surfaces of the upper and lower molds of the mold are coated with boron nitride lubricant for surface lubrication. After cooling, it is transferred to a mold coated with lubricant for hot stamping and pressure quenching.

[0048] Example 5

[0049] The difference between Example 5 and Example 4 is that the boron nitride lubricant is replaced by a graphite lubricant.

[0050] Both Example 6 and Example 7 adopt the following hot stamping process:

[0051] The coated steel plate is heated in a heating furnace until it is completely austenitized and kept warm. The heated coated steel plate is transferred to a mold located in a hot bath medium. The molding surface of the lower mold is below the liquid level of the hot bath medium. The hot bath medium contains lubricant, and a vapor film forms on the surface of the high-temperature coated steel plate in the hot bath medium. The upper mold moves downward to close the mold, and the coated steel plate and the mold are hot stamped in the hot bath medium. The vapor film on the surface of the coated steel plate and the liquid medium can greatly improve the friction and stress state between the coated steel plate and the mold during the forming process. Pressure quenching is performed, and the parts continue to be quenched in the hot bath medium during the upward movement of the upper mold.

[0052] Comparative Example 1

[0053] The difference between Comparative Example 1 and Example 1 is that the step of transferring the heated coated steel plate to a hot bath medium for cooling is omitted in Comparative Example 1.

[0054] Comparative Example 2

[0055] Comparative Example 2 differs from Example 1 in that the heated coated steel sheet is transferred to a hot bath for cooling, and then cooled at room temperature. The hot forming process parameters for each example and comparative example are shown in Table 1:

[0056] Table 1 Thermoforming process parameters of various embodiments and comparative examples

[0057]

[0058] A metallographic microscope was used to characterize the coating thickness and microcracks at various locations on the parts. The microscopic images of the coating and microcracks on the parts after hot forming are shown in the figure below. Figure 2 The cracking conditions of the parts of each embodiment and comparative example are shown in Table 2.

[0059] Table 2 Cracking of parts in various embodiments and comparative examples

[0060]

[0061]

[0062] Depend on Figure 2As can be seen from Table 2, for the same U-shaped parts, for different hot forming processes, the extension of the coating microcracks to the substrate occurs above the R corner tangent point of the part, which is repeatable. In most parts of the product, the coating cracks do not extend to the substrate (the substrate crack depth is less than 4μm); a small number of microcracks extend from the coating to the substrate (the substrate microcracks are located at the tangent point of the concave fillet of the part, and the depth is less than 8μm). It was found that the area where the coating microcracks are prone to extension is not the area where the maximum strain occurs during the forming process (such as the deep drawing side wall), but often appears at the tangent point of the concave fillet (R corner) of the part (such as Figure 1 , the crack position P is shown at the intersection of the R angle).

[0063] In the hot stamping process, when the high-temperature sheet metal is cooled in a hot bath medium and then transferred to the mold for forming, the depth of microcracks can be controlled within 10μm by controlling the temperature of the sheet metal during forming; after lubricating the inner cavity surfaces of the upper and lower molds with solid lubricants, the depth and number of microcracks in the matrix are reduced to varying degrees; when the hot stamping process is that the high-temperature sheet metal and the lower mold surface of the mold are simultaneously in the hot bath medium, the sheet metal 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 metal in the hot bath medium, evenly covering the surface of the sheet metal. The simultaneous existence of the vaporized film and the liquid medium can simultaneously improve the friction and stress state between the sheet metal and the mold during the forming process. Only microcracks in the coating appear in the parts, and no microcracks extending to the matrix are found. When the hot bath medium contains lubricant, no cracking occurs.

[0064] Figure 3 The coating of a hot-formed component was observed using a scanning electron microscope in backscattered mode. The coating structure after hot-forming reveals that the off-white portion is the zinc-rich phase (Γ phase), while the dark gray portion is the iron-rich phase (α-Fe(Zn) phase). By volume percentage, the zinc-rich phase accounts for ≥4%, and the coating thickness is 16-35 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%.

[0065] Example 8

[0066] 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) are formed under the liquid level of a hot bath medium using high-temperature sheets and molds, and subjected to pressure-holding quenching process to verify the batch stability of hot stamping. The mechanical properties of hot stamping parts after forming (test standard: GB / T228.1-2010 "Tensile Test of Metallic Materials Part 1: Room Temperature Test Method") are as follows: tensile strength > 1600 MPa, elongation after fracture > 5%; the micromorphology of the coating and microcracks of the parts is as follows Figure 4 As shown in the figure, no matrix microcracks larger than 8μm were found in any part of the component.

[0067] Comparative Example 3

[0068] Comparative Example 3: The mass-produced parts of the same model as Example 8 (aluminum-silicon coated hot-formed steel) were subjected to cyclic salt spray corrosion test (test standard: GWTAH01-03:2022-03 "Cyclic Corrosion Test Method and Control Requirements for Covering Layer Parts in High Corrosion Areas"). The unilateral corrosion expansion of the incision after corrosion of the parts is as follows: Figure 5 After an accelerated salt spray cycle, the hot-dip galvanized thermoformed parts showed no obvious red rust or corrosion, while the aluminum-silicon coated thermoformed parts showed obvious red rust and cut corrosion.

[0069] The friction coefficient of the galvanized sheet surface obtained under different forming methods (direct hot forming - comparative example 1, forming after hot bath cooling - example 1, forming in hot bath medium - example 7) was measured using a friction tester. The experimental results are as follows: Figure 6 As shown by Figure 6 It can be seen that the surface of the galvanized hot-formed parts of the present application is in complete contact with water and is wrapped in water, and the friction coefficient between the mold and the sheet is less than 0.4.

[0070] Figure 7 This is a schematic diagram of the appearance of the parts of Example 8 and the location where the maximum depth of microcracks occurs. It can be seen that the law of the location of microcracks that diffuse to the substrate obtained on the U-shaped mold is applicable to the molding results of complex parts and has universal applicability.

[0071] Different forming methods were used to prepare a complex component (A-pillar lower reinforcement plate) of a certain vehicle model. Microcrack detection and verification of batch A-pillar lower reinforcement plate components of a certain vehicle model showed that part A was prone to microcracks that extended to the substrate. The microcracks appeared at the intersection of the deep-drawn side wall and the bottom flange edge. The maximum crack depth data is shown in Table 3.

[0072] Table 3 Evaluation of friction coefficient of galvanized sheet surface obtained by different forming methods and maximum depth of microcracks in part A

[0073] Forming method Average friction coefficient Maximum depth of microcracks at part A / μm Comparative Example 1 0.45 15 Example 1 0.39 8 Example 6 0.33 2

[0074] Using a U-shaped mold for verification, under the condition of simultaneous immersion of the mold and the sheet in the hot bath medium in Example 6, the strain rate during the component forming process was changed to obtain the equivalent plastic strain value during the forming process, and the maximum microcrack depth data of the component was measured. It was found that the maximum microcrack depth on the component surface is related to the product of the equivalent plastic strain and the strain rate (strain rate × equivalent plastic strain). Figure 8 This is a correlation diagram between (strain rate × equivalent plastic strain) and microcrack depth in Example 6. During the entire component forming process, the strain rate × equivalent plastic strain is less than 0.6.

[0075] Although the present application has been described in detail with reference to the accompanying drawings and in conjunction with preferred embodiments, the present application is not limited thereto. Without departing from the spirit and substance of the present application, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present application, and such modifications or substitutions shall be within the scope of the present application. Any changes or substitutions that can be easily conceived by persons skilled in the art within the technical scope disclosed in the present application shall be within the scope of protection of the present application.

Claims

1. A low-melting-point coating component, 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 does not have microcracks or has microcracks with a depth of less than 10 μm, and most of the microcracks with a depth of less than 10 μm are located at the intersection of the concave fillet of the low-melting-point coating component; the microcracks include coating microcracks; or the microcracks include coating microcracks and substrate microcracks, and the number of substrate microcracks is less than the number of coating microcracks.

2. 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 hot stamping process, microcracks are generated in the substrate and the low-melting-point coating. The microcracks include a first microcrack and a second microcrack extending from the coating to the substrate. The first microcrack is located at the tangent point of the concave fillet of the component, and the second microcrack is located outside the tangent point of the concave fillet of the component. The number of the first microcracks is greater than the number of the second microcracks. The depth of the first microcrack is less than 8 μm, and the depth of the second microcrack is less than 4 μm.

3. The low melting point coating component with low microcrack depth according to claim 2, 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 late heating stage is heating to ≥750°C.

4. The low melting point coating component with low microcrack depth according to claim 2 or 3, characterized in that: The low melting point coating is GI, GA, ZnAlMg or 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.

5. The low melting point coating component with low microcrack depth according to claim 2, characterized in that: The matrix includes the following element 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.

6. A hot stamping forming method for low-melting-point coated parts with low microcrack depth, characterized in that: Including the first method or the second method, The first method comprises the following steps: after heating the low-melting-point coated steel plate until it is completely austenitized, transferring it to a hot bath mold, wherein the hot bath mold comprises an upper mold and a lower mold, wherein the lower mold is fully or partially immersed in the hot bath medium, the upper mold and the lower mold are closed, and the low-melting-point coated steel plate is stamped and formed in the hot bath medium; The second method comprises the following steps: heating the low-melting-point coated steel plate until it is completely austenitized, transferring it to a hot bath medium for cooling, transferring it to a mold for hot stamping after cooling, and pressure-holding quenching.

7. The hot stamping method according to claim 6, wherein: In the second method, the mold includes an upper mold and a lower mold, and the inner cavity surfaces of the upper mold and the lower mold are coated with a lubricant. Preferably, the lubricant is a boron nitride lubricant or a graphite lubricant.

8. The hot stamping method according to claim 6, wherein: In the first method, the heat bath medium further includes a lubricant.

9. The hot stamping method according to claim 6, wherein: In the first method and the second method, the heat bath medium is water or a water-based solution; when the heat bath medium is water, the heat bath temperature is 60-100°C.

10. The hot stamping method according to claim 6, wherein: In the first and second methods, the low-melting-point coated steel plate is heated to a temperature of 850-920° C., the holding time is 3-8 minutes, and the closing time of the upper and lower molds is 2-10 seconds.

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