Precoated steel sheet for variable gauge hot forming and production method, part hot formed using same and method of manufacturing same

Abstract

The present application provides a variable thickness pre-coated steel sheet for hot forming and a production method, a part hot formed by using the same and a manufacturing method thereof; the present application controls the thickness of the pre-coated layer to be 13-20 microns, the continuous variable cross-section rolling reduction rate to be less than or equal to 30%, and further adds optional Ti and Sr in the pre-coated layer to effectively inhibit the occurrence of large and dense cavity defects in the coating layer of the aluminum-silicon coated steel sheet after the continuous variable cross-section rolling hot forming. The thickness of the pre-coated layer after the continuous variable cross-section rolling is less than or equal to 14 microns, and the prepared variable thickness aluminum-silicon pre-coated steel sheet can obtain a part with more excellent cold bending toughness after hot stamping quenching forming, has excellent coating quality, and has good mechanical properties, coating quality and cold bending toughness.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel and hot-formed parts manufacturing, specifically relating to pre-coated steel sheets for variable thickness hot forming and their production methods, as well as parts formed using these materials and their manufacturing methods, which can be used in automobile manufacturing. Background Technology

[0002] In recent years, with the development of the automobile industry, the accompanying energy crisis and environmental problems have become increasingly prominent. Energy conservation and emission reduction have become major problems that the automobile industry urgently needs to solve. Among them, automobile lightweighting is an effective means of energy conservation and emission reduction.

[0003] With the increasing demand for lightweight vehicles, high-strength hot-stamped steel parts are widely used in automotive body structural components due to their advantages such as high strength, no springback, high forming precision, significant reduction in vehicle weight, improved vehicle safety and collision performance, energy saving and emission reduction, etc., such as A and B pillar reinforcement plates, front and rear door anti-collision bars, and front and rear anti-collision beams.

[0004] With the increasing demands for lightweighting in automobiles, continuous variable cross-section rolling (TRB) hot forming technology and laser welding (TWB) hot forming technology have also developed. Utilizing these two technologies to optimize the design of automotive structural components can further reduce the weight of parts. TWB technology allows for the arbitrary splicing of sheets of different strengths and thicknesses, offering great flexibility. However, due to abrupt thickness changes and the influence of weld seams, it negatively impacts subsequent hot forming processes. In contrast, TRB technology offers better mechanical and forming properties at a lower cost.

[0005] Currently, the market mainly uses aluminum-silicon coated hot-formed steel. When using TWB hot forming technology for lightweight design, during the welding process of aluminum-silicon coated plates, Al in the coating enters the weld to form high-temperature ferrite, affecting the mechanical properties after hot forming. This problem does not exist when using TRB technology.

[0006] Patent EP3358037B1 discloses a method for producing coated steel strip for producing rolled blanks suitable for thermomechanical forming, the steel strip produced therefrom, and the uses of such coated steel strip. The method is characterized by hot rolling of coated (aluminum or aluminum alloy, zinc or zinc alloy) strip followed by cold rolling of the strip, such that the strip has a variable thickness in its length direction, having at least a thicker portion, a thinner portion, and a transition portion between the thicker and thinner portions, wherein the thinner portion is at least 15% thinner than the thicker portion, and the coating thickness is 1 to 50 μm. The product is then obtained by hot forming. However, for aluminum-silicon coated sheets, the intermetallic compound layer at the interface between the steel substrate and the coating is destroyed after rolling, affecting the coating quality after hot forming. Summary of the Invention

[0007] The purpose of this invention is to provide a pre-coated steel sheet for variable thickness hot forming and a production method thereof. In view of the problem that large and dense void defects appear in the coating when manufacturing aluminum-silicon coated hot-formed steel parts using existing TRB hot forming technology, the coating is easy to fall off, which affects the subsequent coating and welding performance, the provided pre-coated steel sheet for variable thickness hot forming is rolled by TRB continuous variable cross-section rolling, which effectively suppresses the void defects in the coating after hot forming.

[0008] Another objective of this invention is to provide a component hot-formed from a pre-coated steel sheet with variable thickness for hot forming, and a method for manufacturing the same. The hot-stamped component has good coating quality and cold bending toughness.

[0009] The specific technical solution of this invention is as follows:

[0010] A pre-coated steel sheet for variable thickness hot forming includes a base steel sheet and a pre-coating layer;

[0011] The pre-coating thickness is ≥13μm; the pre-coated steel sheet for variable thickness hot forming is subjected to continuous variable cross-section rolling with a reduction rate of ≤30%; the present invention controls the reduction rate of continuous variable cross-section rolling to ≤30% and the pre-coating thickness to ≥13μm, which can reduce the breakage of the Fe-Al-Si intermetallic compound layer in the coating and reduce large void defects in the coating after hot forming.

[0012] Preferably, the pre-coating thickness is controlled to be ≥13μm and the continuous variable cross-section rolling reduction rate is ≤15%; good coating quality can be obtained after the coating is thermoformed.

[0013] Preferably, the thickness of the pre-coating is 13μm≤20μm, and the thickness of the pre-coating after continuous variable cross-section rolling is ≤14μm. The components formed by hot stamping and quenching of the prepared coated plate have better cold bending toughness.

[0014] Furthermore, when the continuous variable cross-section rolling reduction rate is >15%, the pre-coating contains 0 to 0.5 wt% Ti, and the Ti content is not 0.

[0015] Furthermore, when the continuous variable cross-section rolling reduction rate is >15%, the pre-coating also contains 0-0.1 wt% Sr; Ti+Sr≤0.5 wt%.

[0016] In this invention, the addition of Ti can reduce the thickness of the intermetallic compound layer and form TiAl3 metal compound, increasing the nucleation density during solidification, refining the coating structure, and making the coating structure more uniform. The addition of Sr can reduce the size of primary silicon particles and improve the plastic processing performance of the coating.

[0017] The pre-coating is applied to at least one surface of the base steel plate.

[0018] The pre-coating contains Fe, Al, and Si components;

[0019] The pre-coating includes an Fe-Al-Si intermetallic compound layer close to the base steel plate and a surface aluminum layer away from the base steel plate. The thickness of the surface aluminum layer accounts for more than 60% of the total thickness of the pre-coating, which can more effectively suppress coating structure breakage during processing.

[0020] The pre-coating comprises the following components by weight percentage: 7-15 wt% Si, 0-5 wt% Fe and unavoidable impurity elements, with the remainder being Al.

[0021] Preferably, the pre-coating contains 8-13 wt% Si; more preferably, it contains 8.5-12 wt% Si.

[0022] The Fe-Al-Si intermetallic layer will inevitably form during the coating process. If elements such as Si are not added to inhibit the growth of the intermetallic compound layer, the intermetallic compound layer will be very thick, hard and brittle after coating. It is easy to cause a lot of breakage in subsequent uncoiling or TRB rolling processes. The broken areas lose their blocking effect. In addition, the coating becomes thinner after rolling. During the heating process, the Fe and Al elements diffuse violently, resulting in large voids. Therefore, the pre-coating layer of this invention includes 7 to 15 wt% Si.

[0023] Preferably, the pre-coating layer further contains 0 to 0.5 wt% Ti, and the Ti content is not zero;

[0024] More preferably, the pre-coating contains 0 to 0.5 wt% Ti and Ti is not 0, and 0 to 0.1 wt% Sr, wherein Ti + Sr ≤ 0.5 wt% in the pre-coating; most preferably 0.2% ≤ Ti + Sr ≤ 0.5 wt%.

[0025] Preferably, the pre-coating contains Ti+Sr ≤ 0.4 wt%.

[0026] The base steel plate is a hot-stamped steel plate;

[0027] The base steel plate may include the following components by weight percentage:

[0028] C: 0.2–0.3 wt%, Si: 0.1–0.4 wt%, Mn: 1.0–2.0 wt%, Al ≤0.06 wt%, Ti: 0.01–0.05 wt%, Cr: 0.1–0.4 wt%, B: ≤0.003 wt%, with the balance being Fe and unavoidable impurity elements.

[0029] Alternatively, C: 0.25–0.4 wt%, Si: 0.1–0.4 wt%, Mn: 1.0–2.5 wt%, Al ≤0.06 wt%, Ti: 0.01–0.05 wt%, Cr: 0.1–0.4 wt%, B: ≤0.003 wt%, Nb: 0.01–0.06 wt%, with the balance being Fe and unavoidable impurity elements.

[0030] The present invention provides a method for producing pre-coated steel plates for variable thickness hot forming, comprising the following process flow: steelmaking → continuous casting → hot rolling → pickling → cold rolling → annealing → hot-dip galvanizing → continuous variable cross-section rolling.

[0031] After the above steelmaking → continuous casting → hot rolling → pickling → cold rolling → annealing process, the basic steel plate is obtained.

[0032] The hot-dip plating solution contains the following components by mass percentage: 7-15% Si, 0-5% Fe and unavoidable impurity elements, with the remainder being Al.

[0033] Preferably, the plating solution further includes 0-0.5 wt% Ti, and the Ti content is not zero;

[0034] More preferably, the plating solution contains 0-0.5 wt% Ti (not 0), 0-0.1 wt% Sr, and Ti+Sr≤0.5 wt%; most preferably, 0.2%≤Ti+Sr≤0.5 wt%.

[0035] The hot-dip plating is performed at a temperature of 640℃~700℃, preferably 650℃~690℃, and for a time of 2s~8s, preferably 2s~5s.

[0036] After hot-dip galvanizing, the steel sheet is cooled to 350-380°C at a cooling rate of 10°C / s to 30°C / s, and then air-cooled to room temperature to obtain a pre-coated steel sheet.

[0037] The pre-coated steel sheet obtained after hot-dip galvanizing has a pre-coating thickness of preferably 13-20 μm, more preferably 14-20 μm, and even more preferably 14-18 μm;

[0038] The thickness of the surface aluminum layer in the pre-coating accounts for more than 60% of the entire pre-coating; if it is less than 60%, the effect of suppressing the breakage of the coating structure during processing is not obvious.

[0039] The pre-coated steel sheet obtained after hot-dip galvanizing is subjected to continuous variable cross-section rolling to obtain a pre-coated steel sheet for hot forming with variable thickness; the rolling reduction rate of the continuous variable cross-section rolling is ≤30%.

[0040] The thickness of the pre-coating after continuous variable cross-section rolling is preferably 9–14 μm, more preferably 9–12 μm.

[0041] The present invention provides a method for manufacturing a component using a pre-coated steel sheet for variable thickness hot forming, specifically comprising: heating and holding the pre-coated steel sheet for variable thickness hot forming at a certain temperature, then placing it in a mold for cooling, stamping, quenching and forming to produce the component.

[0042] The heating and heat preservation refers to a temperature of 860℃~970℃ for 1min~10min; it can be achieved by box-type heating furnace, roller hearth heating furnace or electromagnetic induction heating.

[0043] The cooling stamping quenching refers to stamping and quenching forming at a cooling rate of ≥30℃ / s.

[0044] The component obtained by the present invention is a component hot-formed using a pre-coated steel sheet for variable thickness hot forming. It is obtained by the above-described manufacturing method. The component has a microstructure of martensite, or martensite and a small amount of bainite, wherein the volume content of martensite is ≥95%.

[0045] For components manufactured using the above methods, the coating has a density of 2000 μm. 2 The average number of voids with an internal size greater than 3μm is 0.

[0046] The applicant's research revealed that while aluminum-silicon coated hot-formed steel parts manufactured using TRB hot forming technology exhibit good mechanical properties, large and dense voids appear in the coating after continuous variable cross-section rolling hot forming of the aluminum-silicon coated sheet under varying degrees of pressure. The coating easily peels off at these defects, failing to provide adequate protection and affecting subsequent coating and welding performance. To address these problems, this invention provides a pre-coated steel sheet for hot forming that effectively suppresses coating void defects after continuous variable cross-section rolling hot forming, and a method for manufacturing parts formed using this sheet. The applicant's research found that during the continuous variable cross-section rolling process of the aluminum-silicon coated sheet, the continuous intermetallic compound (i.e., the Fe-Al-Si intermetallic compound layer formed by the interdiffusion of aluminum, silicon in the plating solution, and iron in the matrix) brittle-hard layer is destroyed. After hot forming, the damaged areas lack the barrier effect of the intermetallic compound layer, leading to intense diffusion of Fe and Al elements, forming large and dense voids, causing the coating to easily peel off. To address this issue, the inventors further discovered that after continuous variable cross-section rolling of the aluminum-silicon coated sheet, the higher the reduction rate, the more severe the breakage of the Fe-Al-Si intermetallic compound layer in the coating, resulting in more severe void defects in the coating after hot forming. Furthermore, the thinner the pre-coating thickness, the easier it is for the intermetallic compound layer to break after rolling. Even under relatively low reduction rates, the breakage of the Fe-Al-Si intermetallic compound layer in the coating is still severe, leading to poor coating quality after hot forming. This is mainly because after the coating is thinned, the proportion of the softer aluminum layer on the coating surface is smaller, resulting in less buffering during rolling and making it easier to press down on the harder intermetallic compound layer, leading to more severe breakage and poor coating quality after hot forming. Further, the inventors found that adding a certain amount of Ti or Sr to the coating can further refine the coating structure, reduce the thickness of the intermetallic compound layer, increase the proportion of the surface aluminum layer, and improve the coating's plastic processing performance. By controlling the pre-coating thickness, rolling reduction rate, and adding a certain amount of Ti and optionally Sr, void defects appearing in the coating after continuous variable cross-section rolling hot forming can be effectively suppressed. Through research, the inventors also discovered that when the coating thickness after continuous variable cross-section rolling is controlled below 14μm, the carbon enrichment at the substrate-coating interface is significantly reduced after hot stamping due to the thinner coating, forming a martensitic structure with excellent toughness. This results in a significant improvement in the cold bending toughness of the parts after hot stamping and quenching. Furthermore, during the dip-coating process, the interdiffusion of Fe and Al elements forms an Fe-Al-Si intermetallic compound layer. This intermetallic compound layer is brittle and hard; if it is too thick, it is prone to breakage during subsequent processing. During hot forming, this intermetallic compound layer can inhibit the intense diffusion of Fe and Al elements at high temperatures, preventing the formation of large voids. If this intermetallic compound layer is severely broken before hot forming, it will affect the coating quality after hot forming. Therefore, it is undesirable for this intermetallic compound layer to be too thick to avoid significant breakage during subsequent processing.This invention increases the thickness of the outer aluminum layer of the intermetallic compound layer, with the surface aluminum layer accounting for more than 60% of the total thickness of the pre-coating layer, which can also reduce breakage during processing.

[0047] Compared with existing technologies, this invention controls the thickness of the pre-coating to be 13-20 μm, and the reduction rate of continuous variable cross-section rolling is ≤30%. Furthermore, when the reduction rate is >15%, the addition of Ti and optional Sr to the pre-coating can effectively suppress large and dense void defects in the coating after hot forming of aluminum-silicon coated steel sheets through continuous variable cross-section rolling. This results in a pre-coating thickness of ≤14 μm after continuous variable cross-section rolling. The prepared variable-thickness aluminum-silicon pre-coated steel sheet, after hot stamping and quenching, can produce parts with superior cold bending toughness, excellent coating quality, and good mechanical properties, coating quality, and cold bending toughness. Attached Figure Description

[0048] Figure 1 The morphology of the pre-coated section;

[0049] Figure 2 The figures show the cross-sectional morphology of pre-coated layers of different thicknesses after hot forming by variable cross-section rolling at a reduction rate of 15%. In the figures, a is the pre-coated layer of Comparative Example 1 with a thickness of 10 μm, b is the pre-coated layer of Example 4 with a thickness of 13 μm, and c is the pre-coated layer of Example 5 with a thickness of 20 μm.

[0050] Figure 3 The figures show the cross-sectional morphology of pre-coated layers of different thicknesses and compositions after hot forming by variable cross-section rolling at a reduction rate of 30%. In the figures, d represents the pre-coated layer of Comparative Example 2 with a thickness of 13 μm, e represents the pre-coated layer of Comparative Example 3 with a thickness of 20 μm, f represents the pre-coated layer of Example 2 containing Ti and Sr with a thickness of 13 μm, and g represents the pre-coated layer of Comparative Example 3 containing Ti and Sr with a thickness of 20 μm.

[0051] Figure 4 The figure shows the cross-sectional morphology of a 20 μm thick pre-coating at a 45% reduction rate after variable cross-section rolling thermoforming; in the figure, h represents the pre-coating of Comparative Example 5 containing Ti and Sr, and i represents the pre-coating of Comparative Example 6 without Ti and Sr. Detailed Implementation

[0052] The present invention will be further described below with reference to specific embodiments and accompanying drawings. The following embodiments or experimental data are intended to illustrate the present invention by way of example, and those skilled in the art should understand that the present invention is not limited to these embodiments or experimental data.

[0053] This invention provides a pre-coated steel sheet for variable thickness hot forming, comprising a base steel sheet and a pre-coating layer, wherein the cross-sectional morphology of the pre-coated steel sheet is as follows: Figure 1As shown; the following uses a 1.8mm thick steel plate as the base steel plate to illustrate the embodiments of the present invention. It should be noted that the base steel plate described in the present invention is not limited to a certain composition and is not limited by the manufacturing method.

[0054] The chemical composition of the base steel plate used in this invention is C: 0.23wt%, Si: 0.24wt%, Mn: 1.24wt%, Al: 0.049wt%, Ti: 0.031wt%, Cr: 0.17wt%, B: 0.003wt%, with the balance being Fe and unavoidable impurity elements.

[0055] The pre-coating is formed on both sides of the base steel plate to obtain the pre-coated steel plate.

[0056] The pre-coated steel sheet is manufactured through the following process: steelmaking → continuous casting → hot rolling → pickling → cold rolling → continuous annealing → hot-dip galvanizing.

[0057] The hot-dip galvanizing process specifically involves immersing the base steel plate in a bath at 640°C to 700°C for 2 to 8 seconds, preferably 2 to 5 seconds, removing it from the bath, and cooling it to 350°C to 380°C at a cooling rate of 20°C / s to 30°C / s. After cooling to room temperature, a pre-coated steel plate is obtained. The thickness of the pre-coating is preferably 13 to 20 μm, more preferably 14 to 20 μm, and even more preferably 14 to 18 μm. The thickness of the surface aluminum layer in the pre-coating accounts for more than 60% of the total coating thickness.

[0058] The plating solution contains the following components by mass percentage: 7-15 wt% Si, 0-5 wt% Fe and unavoidable impurity elements, with the remainder being Al;

[0059] Preferably, the plating solution further includes 0-0.5 wt% Ti, and the Ti content is not zero;

[0060] More preferably, the plating solution contains 0-0.5 wt% Ti (not 0), 0-0.1 wt% Sr, and Ti+Sr≤0.5 wt%.

[0061] The Si content in the plating solution is preferably 7-15 wt%, more preferably 8-13 wt%, and even more preferably 8.5-12 wt%. If the silicon content is too low, the effect of inhibiting the growth of the Fe-Al-Si intermetallic layer is not obvious, resulting in an excessively thick, brittle intermetallic layer that is prone to breakage and peeling during continuous variable cross-section rolling. If the silicon content is too high, coarse primary silicon crystals will form in the coating, affecting the machinability of the coating.

[0062] The plating solution contains optional Ti and Sr, with Ti+Sr ≤ 0.5 wt%, preferably Ti+Sr ≤ 0.4 wt%. Ti can refine the coating structure and reduce the thickness of the intermetallic layer, while Sr can transform the β-FeAlSi phase in the coating into the α-FeAlSi phase, reducing the size of the primary silicon crystals and improving plastic processing performance. The above-mentioned improvement effects no longer increase when the Ti+Sr content exceeds 0.5 wt%.

[0063] The 22MnB5 pre-coated steel sheet prepared according to the above method is subjected to continuous variable cross-section rolling to obtain a 22MnB5 pre-coated steel sheet with variable thickness. The reduction rate of the continuous variable cross-section rolling is preferably ≤30%, and more preferably ≤15%.

[0064] The manufacturing method of hot-formed parts using 22MnB5 pre-coated steel sheets with variable thickness is as follows: the 22MnB5 pre-coated steel sheet with variable thickness is heated to 860℃~970℃ in a box furnace, held for 3min~10min, and then quickly transferred to a mold and stamped and quenched to form the required parts at a cooling rate of ≥30℃ / s.

[0065] This invention investigates the effects of Ti and Sr content in aluminum-silicon based plating solutions, pre-coating thickness, and reduction rate of continuous variable cross-section rolling on the coating quality of aluminum-silicon based coatings after continuous variable cross-section rolling hot forming. Specific examples are shown in Table 1 (Examples 1-6 and Comparative Examples 1-6). The selected base steel plate thickness was 1.8 mm, and the results are shown in Table 1. The composition of each plating solution, pre-coating thickness, reduction rate of continuous variable cross-section rolling, and the corresponding coating quality after hot forming are shown in Table 1. The coating quality after hot forming is determined by the presence of Ti and Sr in the plating solution, pre-coating thickness, reduction rate of continuous variable cross-section rolling, and the corresponding coating quality after hot forming. 2 The average number of voids with an internal size greater than 3μm (the void size in a normal coating is generally less than 2μm) is 0, indicating good coating quality; 1 to 3 voids indicate average coating quality; and more than 3 voids indicate insufficient coating quality.

[0066] Table 1 Pre-coating parameters for each embodiment and comparative example

[0067]

[0068]

[0069] Comparative Examples 2, 3, and 4 can reduce large void defects in the coating after variable cross-section rolling thermoforming of aluminum-silicon coating, but they are not as effective as Examples 1-6 in suppressing large void defects in the coating after variable cross-section rolling thermoforming of aluminum-silicon coating.

[0070] The cross-sectional morphology of the coating after thermoforming in Comparative Example 1 is as follows: Figure 1As shown in Figure a, it can be seen that when the pre-coating thickness is 10 μm, after hot forming with variable cross-section rolling at a reduction rate of 15%, the coating has a large number of large voids, resulting in insufficient coating quality. The cross-sectional morphology of the coatings after hot forming in Examples 4 and 5 is shown below. Figure 2 As shown in Figures b and c, it can be seen that when the pre-coating thickness is 13 μm and 20 μm, after continuous variable cross-section rolling hot forming with a 15% reduction rate, there are no large void defects in the coating, and the coating quality is good. The cross-sectional morphology of the coatings after hot forming in Comparative Examples 2 and 3 is as follows. Figure 3 As shown in Figures d and e, when the pre-coating thickness is 13 μm and 20 μm, large void defects appear in the coating after continuous variable cross-section rolling hot forming with a 30% reduction rate, but they are few. In Example 1, Ti and Sr were added to the coating, and the coating quality was better after continuous variable cross-section rolling hot forming with a 15% reduction rate. Comparing Example 1, Example 4, and Example 5 with Example 1, it can be seen that when the pre-coating thickness is ≥13 μm and the reduction rate is not more than 15%, good coating quality can be obtained after hot forming with or without the addition of Ti and Sr.

[0071] The cross-sectional morphology of the coating after thermoforming in Examples 2 and 3 is as follows: Figure 3 As shown in f and g, after adding 0.4 wt% Ti+Sr to the coating, compared with Comparative Examples 2 and 3 without addition, the pre-coatings with thicknesses of 13 μm and 20 μm showed no large void defects after continuous variable cross-section rolling hot forming at a reduction rate of 30%, exhibiting better quality. Comparative Examples 2, 4, and 6 compared to Example 2 revealed that adding 0.2 wt% Ti+Sr to the coating reduced the number of large voids after continuous variable cross-section rolling hot forming at a reduction rate. Adding 0.4 wt% Ti+Sr further reduced the number of large voids; adding 0.5 wt% Ti+Sr did not increase the improvement in coating quality.

[0072] The cross-sectional morphology of the coatings after thermoforming in Comparative Examples 5 and 6 is as follows: Figure 4 As shown in h and i, when the rolling reduction rate is 45%, whether Ti and Sr are added to the coating or not, the large void defects in the coating after hot forming are more serious, the coating is easy to peel off, the quality is poor, and it is not conducive to subsequent coating and welding.

[0073] In summary, achieving a pre-coating thickness ≥13μm and a variable cross-section rolling reduction rate ≤30% can reduce large void defects in the aluminum-silicon coating after variable cross-section rolling hot forming. When the variable cross-section rolling reduction rate >15%, adding ≤0.5wt% Ti+Sr to the coating can further effectively suppress large void defects in the aluminum-silicon coating after variable cross-section rolling hot forming, resulting in parts with good coating quality. When the pre-coating thickness ≥13μm and the variable cross-section rolling reduction rate ≤15%, adding Ti+Sr or not adding Ti+Sr to the coating can also effectively suppress large void defects in the aluminum-silicon coating after variable cross-section rolling hot forming, resulting in parts with good coating quality.

[0074] In addition, the present invention also conducted experiments on the cold bending toughness of pre-coated plates of different thicknesses after variable cross-section rolling hot forming (test standard: VDA238-100), and obtained the following results, as shown in Table 2 below.

[0075] Table 2. Bending properties of each embodiment and comparative example.

[0076]

[0077] Based on the above-mentioned results, pre-coatings with thicknesses of 13μm, 20μm, and 25μm were rolled using a 30% reduction rate. The corresponding coating thicknesses after rolling were 8.96μm, 13.89μm, and 17.64μm, respectively, with average bending angles of 65°, 63°, and 58° after hot pressing. Comparison of these results shows that, while ensuring good coating quality after hot forming, the coatings rolled in Examples 7 and 8 are thinner, have larger bending angles, and exhibit better cold bending toughness. To obtain components with good coating quality and cold bending toughness, the pre-coating thickness should be ≤20μm. In general, a pre-coating thickness of ≤20μm and a coating thickness of ≤14μm after variable cross-section rolling can increase the cold bending angle by more than 5 degrees, which is beneficial to improving the cold bending toughness of aluminum-silicon coated hot-formed parts. This is mainly because the coating is thinned, and the C enrichment layer at the substrate / coating interface is significantly reduced after hot forming, forming a low-carbon martensite structure with better toughness, which improves the cold bending toughness of the parts after hot stamping and quenching.

[0078] By implementing the above-described inventive scheme, the thickness of the pre-coating is controlled to be between 13 and 20 μm, and the continuous variable cross-section rolling reduction rate is ≤30%. Furthermore, the optional addition of Ti and Sr to the pre-coating effectively suppresses large and dense void defects in the coating after hot forming of the aluminum-silicon coated steel sheet via continuous variable cross-section rolling, resulting in a hot-formed part with variable thickness and good coating quality. Further controlling the coating thickness after continuous variable cross-section rolling to ≤14 μm allows the prepared variable-thickness aluminum-silicon based coated sheet to achieve parts with superior cold bending toughness after hot stamping and quenching. In summary, this invention provides a variable-thickness hot-stamped part and a pre-coated steel sheet for hot stamping, which exhibits good coating quality, mechanical properties, and cold bending toughness after hot forming.

[0079] The above embodiments have described in detail the purpose and effects of the present invention. It should be understood that the above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. All modifications, equivalent substitutions, improvements, etc., made by those skilled in the art or by adopting the technical concept and technical solution of the present invention within the spirit and principles of the present invention and without departing from the design concept of the present invention are within the protection scope of the present invention.

Claims

1. A method for producing a pre-coated steel sheet for variable thickness hot forming, characterized in that, The process includes the following steps: Obtain the base steel plate; The base steel plate is hot-dip galvanized to form a pre-coated layer, resulting in a pre-coated steel plate. The thickness of the pre-coated layer is 13 μm ≤ 20 μm. The hot-dip galvanizing solution contains the following components: 7-15 wt% Si, 0-5 wt% Fe, 0-0.5 wt% Ti (not zero), 0-0.1 wt% Sr (Ti+Sr≤0.5 wt%), unavoidable impurity elements, and the remainder being Al. The pre-coated layer comprises an Fe-Al-Si intermetallic compound layer close to the base steel plate and a surface aluminum layer away from the base steel plate, with the surface aluminum layer accounting for more than 60% of the total thickness of the pre-coated layer. The pre-coated steel sheet is continuously rolled with variable cross-section to obtain the variable thickness pre-coated steel sheet for hot forming; wherein, the reduction rate is controlled to be ≤30%; and the thickness of the pre-coating after continuous variable cross-section rolling is ≤14μm.

2. The production method according to claim 1, characterized in that, The base steel plate is obtained after steelmaking, continuous casting, hot rolling, pickling, cold rolling, and annealing.

3. A pre-coated steel sheet for variable thickness hot forming, characterized in that, It is produced by the production method described in claim 1 or 2.

4. A method for manufacturing a thermoformed part, characterized in that, The manufacturing method is as follows: after heating and holding the pre-coated steel plate for variable thickness hot forming as described in claim 3, it is placed in a mold for cooling, stamping, quenching and forming to produce the hot-formed part.

5. A thermoformed part manufactured by the manufacturing method of claim 4.

6. The thermoformed part according to claim 5, characterized in that, After hot forming using the aforementioned pre-coated steel sheet for variable thickness hot forming, the coating thickness per 2000 μm 2 The average number of voids with an internal size greater than 3μm is 0.

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