An austenite alloyed 2000mpa grade hot-stamped zinc-aluminum-magnesium-silicon plated steel sheet and a method for manufacturing the same

Abstract

The application discloses a 2000MPa-grade hot stamping zinc-aluminum-magnesium-silicon coated steel plate based on austenite alloying and a preparation method thereof, and belongs to the technical field of coated steel for automobiles. The steel plate is obtained through hot-dip coating and austenitizing; the steel plate is composed of a base plate and a zinc-aluminum-magnesium-silicon coating, and the zinc-aluminum-magnesium-silicon coating is composed of an intermetallic compound layer, an alpha-Fe(Zn / Al / Si) layer and an Al-Si alloy layer, and the intermetallic compound layer is composed of Fe-Zn alloy phases and eutectic structures; the hot-dip coating solution is a Zn-Al-Mg-Si dipping solution, and the mass percentage of components of the Zn-Al-Mg-Si dipping solution is as follows: Al 3.0%-7.5%, Mg 1.0%-4.5%, Si 0.005%-0.8%, and the rest is Zn and inevitable impurities. The application solves the problem that molten zinc enters the base plate and causes liquid metal cracks in the zinc-coated hot-formed steel.

Description

Technical Field

[0001] This invention relates to the field of coated steel technology for automobiles, and more particularly to a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying and its preparation method. Background Technology

[0002] With the automotive industry's increasing demands for lightweighting and crash safety, hot-stamped high-strength steel coated sheets are widely used in vehicle body structural components. Currently, mainstream products include aluminum-silicon coated 22MnB5 steel sheets and zinc-magnesium coated high-strength steel. The traditional hot stamping process is as follows: sheet metal pretreatment → heating to austenitizing temperature → holding at that temperature for a period of time → rapid transfer to a die → stamping → pressure holding and quenching → part removal.

[0003] There are two main existing hot stamping methods: direct hot stamping and indirect hot stamping. Direct hot stamping involves heating the steel sheet to a temperature higher than the austenitizing temperature, typically 880-950℃. After holding at this temperature for 3-8 minutes, the heated steel sheet is transferred to a forming die and formed into a finished component in a one-step process. Simultaneously, hardening is achieved through die cooling (the die's cooling rate is greater than the steel sheet's critical cooling rate). However, for hot stamping steels with low-melting-point coatings (such as GI, GA, ZnAlMg, etc.), Zn, due to its low melting point, transforms into liquid zinc during the high-temperature heating process of direct hot stamping. Forming at high temperatures easily leads to liquid metal embrittlement (LME) cracking. Therefore, direct hot stamping methods are difficult to implement for mass production of hot-formed parts with low-melting-point, high-corrosion-resistant coatings.

[0004] Indirect hot forming involves a multi-step forming process to shape a component to almost complete completion (typically 90% pre-formed). The nearly formed component is then placed in a furnace and heated until fully austenitized and held at that temperature. Finally, the heated component is transferred to a die for final hot forming. Chinese invention patent application number 202310321793.6 discloses a method for preparing a 1500MPa grade hot-formed steel bent pipe with a zinc-based coating. This method employs indirect hot forming, first bending and then quenching, which allows for control over the pipe's processing accuracy and avoids the low-molecular-weight elongation (LME) problem associated with zinc-based hot-formed steel. However, indirect hot forming has limited adaptability to complex parts, and the cold stamping step during pre-forming can easily cause cracking in deep-drawn or high-curvature parts.

[0005] To address the aforementioned issues, Chinese Patent Application No. 202311693507.5 discloses a zinc-aluminum-magnesium coated steel sheet and its production process. Targeting the problem of embrittlement caused by liquid metal in hot-formed zinc-based coatings, it develops a low-aluminum, low-magnesium zinc-aluminum-magnesium coating, optimizes the coating structure, and forms a dense Fe2Al5 inhibition layer. Combined with rapid hot-dip galvanizing and low-temperature hot-forming processes, it produces a hot-formed steel sheet with a tensile strength ≥1900MPa, high corrosion resistance, and no cracking. Its performance surpasses that of pure zinc and aluminum-silicon coatings, achieving domestic substitution.

[0006] However, with the increasing demand for higher strength grades in automotive bodies, especially when strength reaches 2000 MPa, higher heating temperatures or longer holding times exacerbate element diffusion and interfacial reactions between the coating and the substrate. This makes liquid zinc more susceptible to LME cracking, and the stress state during the forming process becomes more demanding. Therefore, there is an urgent need to develop a hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet capable of achieving 2000 MPa strength while possessing excellent LME resistance, as well as its manufacturing method. Summary of the Invention

[0007] The purpose of this invention is to provide a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying and its preparation method, in order to solve the problem of liquid metal cracking caused by molten zinc entering the substrate in current galvanized hot-formed steel.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: The first aspect of this invention provides a method for preparing a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying, comprising the following steps: Step 1, Prepare the substrate; Step 2: The substrate obtained in Step 1 is subjected to hot-dip galvanizing to obtain a steel plate based on hot-dip galvanizing technology; In step 2, the immersion solution is a Zn-Al-Mg-Si immersion solution, and its composition by mass percentage is: Al 3.0%~7.5%, Mg 1.0%~4.5%, Si 0.005%~0.8%, with the remainder being Zn and unavoidable impurities; Step 3: Austenitize the steel sheet obtained in Step 2 based on hot-dip galvanizing technology to obtain a zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying. Step 4: The zinc-aluminum-magnesium-silicon coated steel sheet obtained in Step 3 is stamped and cooled to room temperature to obtain a 2000MPa hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloy.

[0009] Furthermore, in the above technical solution, in step 1, the mass percentage of the substrate composition is as follows: C 0.30%~0.35%, Si 0.05%~0.20%, Mn 1.30%~1.50%, P≤0.020%, S≤0.004%, Al 0.050%~0.090%, Ti 0.010%~0.025%, V 0.16%~0.20%, N≤0.0045%, B 0.0010%~0.0030%, with the remainder being Fe and unavoidable impurities.

[0010] Furthermore, in the above technical solution, in step 1, the substrate is prepared by: pre-treating molten iron, smelting in a converter, refining outside the ladle, and continuously casting to obtain a steel billet; then, heating, rolling, controlled cooling, coiling, pickling, and cold rolling to obtain a cold-rolled plate; finally, continuously annealing the cold-rolled plate at an annealing temperature of 755~805℃ and a dew point of -60~-20℃.

[0011] Furthermore, in the above technical solution, in step 2, the conditions for hot-dip plating are: plating temperature of 430~470℃, plating time of 2~10s, and cooling to room temperature at a cooling rate of 15~40℃ / s after plating.

[0012] Furthermore, in the above technical solution, in step 3, the austenitizing conditions are: austenitizing temperature of 875~930℃ and holding time of 1~8min.

[0013] A second aspect of the present invention provides a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying, wherein the steel sheet is prepared by the above-described preparation method; The steel plate is composed of a substrate and a zinc-aluminum-magnesium-silicon coating. The zinc-aluminum-magnesium-silicon coating is composed of an intermetallic compound layer, an α-Fe(Zn / Al / Si) layer, and an Al-Si alloy layer. The intermetallic compound layer is composed of an Fe-Zn alloy phase and a eutectic structure.

[0014] Furthermore, in the above technical solution, the total thickness of the zinc-aluminum-magnesium-silicon coating is 6-50 micrometers, wherein the thickness of the intermetallic compound layer is 5.8-49.8 micrometers, the thickness of the α-Fe(Zn / Al / Si) layer is 0.2-10 micrometers, and the thickness of the Al-Si alloy layer is 0.3-12 micrometers.

[0015] Furthermore, in the above technical solution, the Fe, Zn, Al, Mg, and Si elements are uniformly distributed in the α-Fe(Zn / Al / Si) layer, with the Zn element content being 3.5wt%~10.3wt%, the Al element content being 0.5wt%~12.2wt%, the Mg element content being 0~0.2wt%, the Si element content being 0.01wt%~5.0wt%, and the O element content being 0~8.2wt%.

[0016] The beneficial effects of this invention are as follows: 1. The hot-dip galvanizing solution used in this invention is a Zn-Al-Mg-Si composition system. Silicon has a dual function of reducing zinc dross formation and stabilizing composition. On the one hand, it can effectively inhibit the Fe-Al reaction and reduce the formation of iron, aluminum and zinc dross, thereby reducing zinc consumption and extending the service life of the zinc pot. On the other hand, silicon can also stabilize the composition of the zinc solution and reduce the burn-off of Al and Mg elements, which helps to improve the stability of long-term production.

[0017] 2. The steel plate obtained by this invention is based on the eutectic structure formed by hot-dip galvanizing. During the austenitization process of hot stamping, Zn, Al, Mg, and Si in the coating are uniformly exchanged with Fe, Mn and other elements in the steel matrix, which plays a binding role for zinc. At the same time, the Al-Si alloy layer pre-formed at the interface plays a blocking role for liquid zinc penetration, thereby reducing LME sensitivity. Attached Figure Description

[0018] Figure 1 The surface morphology and composition of samples 1-2 prepared in Example 1 were obtained using scanning electron microscopy. Figure 2 The cross-sectional scanning electron microscope morphology and energy dispersive spectroscopy (EDS) results of samples 1-2 prepared in Example 1 are shown. Figure 3 The cross-sectional scanning electron microscope morphology and energy dispersive spectroscopy (EDS) results of the finished steel plate prepared in Example 1 are shown. Figure 4 The scanning electron microscope morphology of the section at the bending deformation of the finished steel plate prepared in Example 1; Figure 5 A macroscopic photograph of the finished steel plate prepared in Example 2; Figure 6 A macroscopic photograph of the finished steel plate prepared in Example 3; Figure 7 Macroscopic photograph of the finished steel plate prepared for Comparative Example 1. Detailed Implementation

[0019] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0020] Example 1 Step 1: After hot metal pretreatment, converter smelting, ladle refining, and continuous casting, a steel billet is obtained. Then, the steel billet is heated, rolled, controlled cooled, coiled, pickled, and cold rolled to obtain a cold-rolled sheet with a thickness of 1.4 mm. Finally, the cold-rolled sheet is continuously annealed at an annealing temperature of 780±5℃ and a dew point of -40±10℃ to obtain a substrate. The mass percentage of the substrate composition determined by chemical analysis is as follows: C 0.33%, Si 0.10%, Mn 1.44%, P 0.010%, S 0.004%, Al 0.070%, Ti 0.020%, V 0.18%, N 0.0035%, B 0.0020%, with the remainder being Fe and unavoidable impurities. Step 2: Prepare the immersion plating solution. The composition of the immersion plating solution by mass percentage is: Al 4.5%, Mg 3.0%, Si 0.2%, with the remainder being Zn and unavoidable impurities. Under the conditions of immersion plating temperature of 460℃, immersion time of 3s, controlling the double-sided weight of the coating to 180g (i.e., coating thickness of 14~17 micrometers), and cooling to room temperature at a cooling rate of 30℃ / s after immersion plating, the substrate obtained in Step 1 is subjected to hot-dip plating to obtain a zinc-aluminum-magnesium-silicon coated steel sheet based on hot-dip plating technology, named Sample 1-2. Figure 1 and Figure 2 The surface and cross-sectional scanning electron microscope morphologies of samples 1-2 prepared in Example 1 are shown. It can be seen that samples 1-2 include a substrate, a zinc-aluminum-magnesium-silicon coating-1, and an intermediate compound layer-1. The zinc-aluminum-magnesium-silicon coating-1 is a eutectic structure-1 consisting of Zn-Al two-phase eutectic and Zn-Al-MgZn2 three-phase eutectic. The content of the eutectic structure-1 is 85%. Zn element exists in the form of Zn-rich phase and MgZn2 phase, and Mg element exists in the form of MgZn2 phase and Mg2Si phase. The interlamellar spacing of the eutectic structure-1 is less than 1 micrometer, and the thickness of the eutectic structure-1 is 22 micrometers. The intermediate compound layer-1 is an Al-Si intermetallic compound with a thickness of 2.8 micrometers. EDS tests were performed on the surfaces of samples 1-2, and the results are shown in Table 1. Figure 1 Middle spectrum Figures 1-6 Marker, spectrum Figure 1 Spectrum Figure 2 The main phases are MgZn2 and Mg2Si, and the spectrum is... Figure 3 Spectrum Figure 4 Predominantly zinc-rich phase, spectrum Figure 5 Spectrum Figure 6 It is predominantly composed of eutectic structures.

[0021] Table 1. Element content (wt%)

[0022] Step 3: The samples 1-2 obtained in Step 2 are austenitized at an austenitizing temperature of 880℃ and a holding time of 4min to obtain zinc-aluminum-magnesium-silicon coated steel plates based on austenitizing technology, which are named samples 1-3. Step 4: After austenitization, samples 1-3 obtained in step 3 are immediately stamped and rapidly cooled to room temperature to obtain 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheets based on austenitic alloying.

[0023] Example 1 yielded a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying (hereinafter referred to as the finished steel sheet), the microstructure of which is as follows: Figure 3 As shown, the substrate and zinc-aluminum-magnesium-silicon coating-2 are composed of an intermetallic compound layer (17.2 μm thick, consisting of a eutectic structure and Fe-Zn alloy phase), an Al-Si alloy layer (3.1 μm thick), an α-Fe(Zn / Al / Si) layer (10 μm thick), and the substrate. In the α-Fe(Zn / Al / Si) layer, Fe, Zn, Al, Mg, and Si elements are uniformly distributed, with Zn content of 5.3 wt%, Al content of 10 wt%, Mg content of 0.2 wt%, Si content of 0.12 wt%, and O content of 5.3 wt%. The scanning electron microscope morphology of the cross-section at the bending deformation point is shown in the figure. Figure 4 As shown, there are no LME cracks, and a small number of cracks caused by brittle metal alloy phases terminate within the coating. This indicates that the structure of the multilayer eutectic intermetallic compound effectively prevents the penetration of liquid zinc during the austenitization process.

[0024] Example 2 Step 1: After hot metal pretreatment, converter smelting, ladle refining, and continuous casting, a steel billet is obtained. Then, the steel billet is heated, rolled, cooled under controlled conditions, coiled, pickled, and cold rolled to obtain a cold-rolled sheet with a thickness of 1.2 mm. Finally, the cold-rolled sheet is continuously annealed at an annealing temperature of 800±5℃ and a dew point of -50±10℃ to obtain a substrate. The substrate composition by mass percentage is: C 0.33%, Si 0.09%, Mn 1.38%, P 0.015%, S 0.004%, Al 0.080%, Ti 0.015%, V 0.17%, N 0.0040%, B 0.0018%, with the remainder being Fe and unavoidable impurities. Step 2: Prepare the immersion plating solution. The composition of the immersion plating solution by mass percentage is: Al 5.0%, Mg 3.0%, Si 0.5%, with the remainder being Zn and unavoidable impurities. Under the conditions of immersion plating temperature of 465℃, immersion time of 3s, controlling the double-sided weight of the coating to 160g (i.e., coating thickness of 12~15 micrometers), and cooling to room temperature at a cooling rate of 25℃ / s after immersion plating, the substrate obtained in Step 1 is subjected to hot-dip plating to obtain a zinc-aluminum-magnesium-silicon coated steel plate based on hot-dip plating technology, named Sample 2-2. Step 3: The sample 2-2 obtained in Step 2 is austenitized at an austenitizing temperature of 880℃ and a holding time of 3min to obtain a zinc-aluminum-magnesium-silicon coated steel sheet based on austenitizing technology, named sample 2-3. Step 4: After austenitization, samples 2-3 obtained in Step 3 were immediately stamped and rapidly cooled to room temperature to obtain a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying. The macroscopic morphology is as follows: Figure 5 As shown.

[0025] Example 3 Step 1: After hot metal pretreatment, converter smelting, ladle refining, and continuous casting, a steel billet is obtained. Then, the steel billet is heated, rolled, cooled under controlled conditions, coiled, pickled, and cold rolled to obtain a cold-rolled sheet with a thickness of 1.2 mm. Finally, the cold-rolled sheet is continuously annealed at an annealing temperature of 790±5℃ and a dew point of -50±10℃ to obtain a substrate. The substrate composition by mass percentage is: C 0.34%, Si 0.08%, Mn 1.40%, P 0.015%, S 0.003%, Al 0.068%, Ti 0.018%, V 0.18%, N 0.0040%, B 0.0018%, with the remainder being Fe and unavoidable impurities. Step 2: Prepare the immersion plating solution. The composition of the immersion plating solution by mass percentage is: Al 4.5%, Mg 3.0%, Si 0.8%, with the remainder being Zn and unavoidable impurities. Under the conditions of immersion plating temperature of 460℃, immersion time of 3s, controlling the double-sided weight of the coating to 80g (i.e., coating thickness of 6~10 micrometers), and cooling to room temperature at a cooling rate of 20℃ / s after immersion plating, the substrate obtained in Step 1 is subjected to hot-dip plating to obtain a zinc-aluminum-magnesium-silicon coated steel sheet based on hot-dip plating technology, named Sample 3-2. Step 3: The sample 3-2 obtained in Step 2 is austenitized at an austenitizing temperature of 900℃ and a holding time of 3min to obtain a zinc-aluminum-magnesium-silicon coated steel plate based on austenitizing technology, named sample 3-3. Step 4: After austenitization, sample 3-3 obtained in step 3 was immediately stamped and rapidly cooled to room temperature to obtain a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying. The macroscopic morphology is as follows: Figure 6 As shown.

[0026] Comparative Example 1 The difference between Comparative Example 1 and Example 1 lies in the composition of the immersion plating solution, specifically: Al 2.8%, Mg 2.5%, Si 0.001%, with the remainder being Zn and unavoidable impurities. All other conditions are the same as in Example 1. The surface morphology after hot stamping is as follows: Figure 7 As shown, visible LME cracks can be seen on the surface.

[0027] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing a 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying, characterized in that, Includes the following steps: Step 1, Prepare the substrate; Step 2: The substrate obtained in Step 1 is subjected to hot-dip galvanizing to obtain a steel plate based on hot-dip galvanizing technology; In step 2, the immersion solution is a Zn-Al-Mg-Si immersion solution, and its composition by mass percentage is: Al 3.0%~7.5%, Mg 1.0%~4.5%, Si 0.005%~0.8%, with the remainder being Zn and unavoidable impurities; Step 3: Austenitize the steel sheet obtained in Step 2 based on hot-dip galvanizing technology to obtain a zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying. Step 4: The zinc-aluminum-magnesium-silicon coated steel sheet obtained in Step 3 is stamped and cooled to room temperature to obtain a 2000MPa hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloy.

2. The method for preparing 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 1, characterized in that, In step 1, the substrate composition by mass percentage is as follows: C 0.30%~0.35%, Si 0.05%~0.20%, Mn 1.30%~1.50%, P≤0.020%, S≤0.004%, Al 0.050%~0.090%, Ti 0.010%~0.025%, V 0.16%~0.20%, N≤0.0045%, B 0.0010%~0.0030%, with the remainder being Fe and unavoidable impurities.

3. The method for preparing 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 1, characterized in that, In step 1, the substrate is prepared by: pre-treating molten iron, smelting in a converter, refining in a ladle, and continuously casting to obtain a steel billet. Then, the steel billet is heated, rolled, cooled under controlled conditions, coiled, pickled, and cold-rolled to obtain a cold-rolled plate. Finally, the cold-rolled plate is continuously annealed at an annealing temperature of 755~805℃ and a dew point of -60~-20℃.

4. The method for preparing 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 1, characterized in that, In step 2, the hot-dip plating conditions are: plating temperature of 430~470℃, plating time of 2~10s, and cooling to room temperature at a cooling rate of 15~40℃ / s after plating.

5. The method for preparing 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 1, characterized in that, In step 3, the austenitizing conditions are: austenitizing temperature of 875~930℃ and holding time of 1~8min.

6. A 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying, characterized in that, The steel plate is prepared by the preparation method according to any one of claims 1-5; The steel plate is composed of a substrate and a zinc-aluminum-magnesium-silicon coating. The zinc-aluminum-magnesium-silicon coating is composed of an intermetallic compound layer, an α-Fe(Zn / Al / Si) layer, and an Al-Si alloy layer. The intermetallic compound layer is composed of an Fe-Zn alloy phase and a eutectic structure.

7. The 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 6, characterized in that, The total thickness of the zinc-aluminum-magnesium-silicon coating is 6-50 micrometers, of which the thickness of the intermetallic compound layer is 5.8-49.8 micrometers, the thickness of the α-Fe(Zn / Al / Si) layer is 0.2-10 micrometers, and the thickness of the Al-Si alloy layer is 0.3-12 micrometers.

8. The 2000MPa grade hot-stamped zinc-aluminum-magnesium-silicon coated steel sheet based on austenitic alloying according to claim 6, characterized in that, The α-Fe(Zn / Al / Si) layer contains uniformly distributed Fe, Zn, Al, Mg, and Si elements, with Zn content ranging from 3.5 wt% to 10.3 wt%, Al content ranging from 0.5 wt% to 12.2 wt%, Mg content ranging from 0 to 0.2 wt%, Si content ranging from 0.01 wt% to 5.0 wt%, and O content ranging from 0 to 8.2 wt%.

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