Steel sheet for hot pressing and method for manufacturing the same

Abstract

To provide a hot press steel sheet capable of preventing or minimizing cracks from occurring in a plated layer during hot press molding, and a method for manufacturing the same.SOLUTION: A hot pressing steel sheet 10 includes a base steel plate 100 and a plated layer 200 positioned on the base steel plate and including a diffusion layer 210 and surface layer 220 successively laminated. The diffusion layer successively positioned on the base steel plate includes an Fe-Al alloyed layer 212 and Fe-Al intermetallic compound layer 214 including silicon therein, and the area fraction of the Fe-Al intermetallic compound layer to the diffusion layer is 84.5% or 98.0%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to steel sheets for hot pressing and a method for manufacturing the same. [Background technology]

[0002] Recently, with the strengthening of environmental regulations and safety standards in the automotive industry, the application of high-strength steel for vehicle weight reduction and stability has increased. On the other hand, while high-strength steel can ensure high strength relative to its weight, it is prone to material fracture and springback during processing, making it difficult to form complex and precise shapes. Therefore, hot press forming is being increasingly applied as a method to solve these problems.

[0003] Hot press forming is a method that facilitates the shaping of steel materials by heating and pressing steel sheets at high temperatures, and ensures the strength of the formed product by rapid cooling through a die. However, a problem arises in that the surface of the steel sheet oxidizes because it is heated to high temperatures for hot press forming. To solve this problem, U.S. Patent No. 6,296,805 proposes a method for hot press forming aluminum-plated steel sheets. According to U.S. Patent No. 6,296,805, since the aluminum plating layer is present on the surface of the steel sheet, oxidation of the steel sheet surface due to heating can be prevented.

[0004] However, when the steel sheet is heated, Fe diffuses from the steel sheet into the aluminum plating layer, causing the aluminum plating layer to alloy. If such an aluminum-plated steel sheet is hot-pressed, cracks may occur in the plating layer, which becomes brittle due to the alloying. Furthermore, since the aluminum plating layer does not provide sacrificial corrosion protection, if cracks occur in the plating layer and the surface of the steel sheet is exposed, the corrosion resistance of the hot-pressed product will rapidly decrease.

[0005] On the other hand, invention No. 10-2019-0077928 discloses an iron-aluminum alloy plated steel sheet that includes an Fe-Al alloy plating layer formed on the surface of a base steel sheet, but when the Fe-Al alloy plating layer is divided into four equal parts in the thickness direction, the hardness of the remaining layers excluding the outermost layer is lower than that of the outermost layer, thereby suppressing crack formation on the surface. However, since the hardness of the Fe-Al alloy plating layer decreases towards the outer edge, there is a concern that the Fe-Al alloy plating layer may adhere to the mold during the hot pressing process and peel off. [Overview of the project] [Problems that the invention aims to solve]

[0006] Embodiments of the present invention provide a steel sheet for hot pressing and a method for manufacturing the same that can prevent or minimize crack formation in the plating layer during hot press forming. [Means for solving the problem]

[0007] One embodiment of the present invention discloses a steel sheet for hot pressing, comprising a base steel sheet and a plating layer located on the base steel sheet and comprising sequentially laminated diffusion layers and a surface layer, wherein the diffusion layers are sequentially located on the base steel sheet and each comprises a silicon-containing Fe-Al alloy layer and an Fe-Al intermetallic compound layer, and the area fraction of the Fe-Al intermetallic compound layer to the diffusion layer is 84.5% to 98.0%.

[0008] In this embodiment, the Fe-Al intermetallic compound layer includes a first layer and a second layer that are sequentially stacked, and the hardness of the Fe-Al alloy layer is higher than the first hardness of the first layer and the second hardness of the second layer, with the second hardness being higher than the first hardness.

[0009] In this embodiment, the area fraction of the diffusion layer relative to the plating layer is 10% to 35%.

[0010] In this embodiment, in the Fe-Al alloy layer, the first layer, and the second layer, the aluminum content is lowest in the first layer, and the silicon content is highest in the first layer.

[0011] In this embodiment, the average thickness of the first layer is 50 nm to 500 nm, and the average thickness of the second layer is 1 μm to 16 μm. In this embodiment, the average thickness of the Fe-Al alloy layer is 50 nm to 500 nm.

[0012] In this embodiment, the area fraction of the Fe-Al alloy layer relative to the diffusion layer is 2.0% to 15.5%.

[0013] In this embodiment, the base steel sheet also contains 0.01 wt% to 0.5 wt% of carbon (C), 0.01 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 3.0 wt% of manganese (Mn), phosphorus (P) greater than 0 but less than 0.05 wt%, sulfur (S) greater than 0 but less than 0.01 wt%, aluminum (Al) greater than 0 but less than 0.1 wt%, nitrogen (N) greater than 0 but less than 0.001 wt%, the remainder being iron (Fe), and other unavoidable impurities.

[0014] In this embodiment, the base steel sheet further contains one or more components selected from niobium (Nb), titanium (Ti), chromium (Cr), molybdenum (Mo), and boron (B).

[0015] Another embodiment of the present invention discloses a method for manufacturing a steel sheet for hot pressing, comprising the steps of: immersing a base steel sheet that has been cold-rolled or hot-rolled in a plating bath having a temperature of 650°C to 700°C to form a molten plating layer on the surface of the base steel sheet; and cooling the base steel sheet on which the molten plating layer has been formed to form the plating layer, wherein the plating bath comprises 4 wt% to 12 wt% silicon, 1.0 wt% to 4.0 wt% iron, and the remainder being aluminum, and the cooling step comprises a first cooling step of cooling the base steel sheet to 550°C at a first average cooling rate, and a second cooling step of cooling the base steel sheet to room temperature at a second average cooling rate, wherein the first average cooling rate is faster than the second average cooling rate.

[0016] In this embodiment, the first average cooling rate is also 20°C / s or higher.

[0017] In this embodiment, the base steel plate passes through the plating bath and is immersed in the plating bath, and the speed at which the base steel plate passes through the plating bath is 1 mpm to 250 mpm.

[0018] In this embodiment, the process further includes a step of injecting air or gas onto the base substrate before the cooling step to adjust the thickness of the molten plating layer. [Effects of the Invention]

[0019] According to embodiments of the present invention, by including an Fe-Al intermetallic compound layer in the plating layer, it is possible to more effectively prevent or minimize cracks occurring in the hot-pressing steel sheet during the hot-pressing process.

[0020] Furthermore, by including a first layer in which Fe-Al intermetallic compound layers are sequentially laminated, and a second layer having a higher hardness than the first layer, the adhesion strength of the plating layer can be improved. [Brief explanation of the drawing]

[0021] [Figure 1] It is a cross-sectional view illustrating a cross-section of a steel plate for hot pressing according to an embodiment of the present invention. [Figure 2] It is a flowchart schematically illustrating the method for manufacturing the steel plate for hot pressing of FIG. 1. MODE FOR CARRYING OUT THE INVENTION

[0022] The present invention can be subjected to various modifications and can have various embodiments. Specific embodiments will be exemplified in the drawings and described in detail in the detailed description. The effects, features of the present invention, and methods for achieving them will become clear when the embodiments described in detail later with reference to the drawings are referred to. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various forms.

[0023] In the following embodiments, terms such as first and second are used for the purpose of distinguishing one constituent element from other constituent elements, and do not have a limiting meaning. In the following embodiments, singular expressions include plural expressions unless the context clearly dictates otherwise.

[0024] In the following embodiments, terms such as "comprising" or "having" mean that the features or constituent elements described in the specification exist, and do not preclude in advance the possibility that one or more other features or constituent elements may be added.

[0025] In the following embodiments, when a portion such as a film, a region, or a constituent element is referred to as being "on" or "above" another portion, this includes not only the case where it is directly above the other portion, but also the case where another film, region, constituent element or the like is interposed therebetween.

[0026] In the drawings, for the sake of explanation, the size of the components may be exaggerated or reduced. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the sake of explanation, and therefore the present invention is not necessarily limited to what is shown in the drawings.

[0027] If a particular embodiment can be manifested in a different way, a specific sequence of steps may also be performed in a different order than that described. For example, two steps described consecutively may be performed substantially simultaneously, or they may proceed in the reverse order of the description.

[0028] The embodiments of the present invention will be described in detail below with reference to the attached drawings. When describing with reference to the drawings, identical or corresponding components will be denoted by the same reference numerals.

[0029] Figure 1 is a cross-sectional view illustrating a cross-section of a steel sheet for hot pressing according to one embodiment of the present invention.

[0030] Referring to Figure 1, one embodiment of the present invention also includes a hot-press steel sheet 100 and a plating layer 200 located on the base steel sheet 100.

[0031] The base steel sheet 100 is also a steel sheet manufactured by hot rolling and cold rolling processes on a steel slab that has been cast to contain predetermined alloying elements in predetermined amounts. For example, the base steel sheet 100 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N), the remainder being iron (Fe), and other unavoidable impurities. Furthermore, the base steel sheet 100 may also contain one or more of the following components: niobium (Nb), titanium (Ti), chromium (Cr), molybdenum (Mo), and boron (B).

[0032] Carbon (C) is the main element that determines the strength and hardness of the base steel sheet 100. It is added after the hot pressing process to ensure the tensile strength and hardenability of the base steel sheet 100. Such carbon is present in amounts of 0.01 wt% to 0.5 wt% of the total weight of the base steel sheet 100. If the carbon content is less than 0.01 wt%, it is difficult to ensure the mechanical strength of the base steel sheet 100. On the other hand, if the carbon content exceeds 0.5 wt%, it can lead to a decrease in toughness or problems with brittleness control of the base steel sheet 100.

[0033] Silicon (Si) interacts with ferrite-stabilizing elements in the base steel sheet 100. Silicon (Si) is a solid solution strengthening element that improves the flexibility of the base steel sheet 100 and suppresses the formation of carbides in the low-temperature range, thereby improving the carbon concentration in the ostenate. Furthermore, silicon (Si) is a core element for homogenization of the structure during hot rolling, cold rolling, and hot pressing (control of pearlite and manganese segregation zones) and fine dispersion of ferrite. Such silicon is present in amounts of 0.01 wt% to 1.0 wt% of the total weight of the base steel sheet 100. If the silicon content is less than 0.01 wt%, the aforementioned effects are difficult to obtain, and conversely, if the silicon content exceeds 1.0 wt%, the hot rolling load and cold rolling load increase, leading to excessive hot-rolled red scale and a decrease in the plating properties of the base steel sheet 100.

[0034] Manganese (Mn) is added during heat treatment to increase hardenability and strength. Manganese is present in amounts ranging from 0.5 wt% to 3.0 wt% of the total weight of the base steel sheet. If the manganese content is less than 0.5 wt%, the grain refinement effect is insufficient, and the hard phase fraction in the molded product after hot pressing is insufficient. On the other hand, if the manganese content exceeds 3.0 wt%, manganese segregation or pearlite banding reduces softness and toughness, leading to decreased bending performance and the formation of a heterogeneous microstructure.

[0035] Phosphorus (P) is also included in the base steel plate 100 in an amount greater than 0% but less than 0.05 wt% of the total weight of the base steel plate 100, in order to prevent a decrease in the toughness of the base steel plate 100. If the amount of phosphorus exceeds 0.05 wt% in the base steel plate 100, iron phosphide compounds will be formed, reducing the toughness and inducing cracks in the base steel plate 100 during the manufacturing process.

[0036] Sulfur (S) is present in amounts exceeding 0% but less than 0.01 wt% of the total weight of the base steel plate. If the sulfur content exceeds 0.01 wt%, the hot workability decreases, and surface defects such as cracks occur due to the formation of large inclusions.

[0037] Aluminum (Al) acts as a deoxidizer to remove oxygen from the base steel plate 100. Aluminum is also present in the base steel plate 100 in amounts greater than 0% but less than 0.1 wt% of the total weight. If the aluminum content exceeds 0.1 wt%, it can cause nozzle clogging during steelmaking, and during casting, hot brittleness can occur due to aluminum oxides, leading to cracks in the base steel plate 100 or a decrease in its softness.

[0038] On the other hand, if the base steel plate 100 contains a large amount of nitrogen (N), the amount of dissolved nitrogen will increase, reducing the impact properties and elongation rate of the base steel plate 100, and decreasing the toughness of the joint. Therefore, it is desirable that the nitrogen content of the base steel plate 100 be greater than 0 and less than 0.001 wt% of the total weight.

[0039] Niobium (Nb) is added to increase strength and toughness by reducing the martensite packet size. Niobium is also present in amounts of 0.005 wt% to 0.1 wt% of the total weight of the base steel sheet 100. When niobium is included within the above range, it has an excellent grain refinement effect on the steel material during the hot rolling and cold rolling processes, preventing crack formation in the slab and brittle fracture of the product during steelmaking / continuous casting, and minimizing the formation of coarse precipitates in the steelmaking process.

[0040] Titanium (Ti) can be added after hot press heat treatment to enhance hardenability and improve material quality through precipitate formation. Furthermore, at high temperatures, it forms precipitate phases such as Ti(C,N), effectively contributing to the refinement of ostenitite grains. Titanium is present in amounts of 0.005 wt% to 0.1 wt% of the total weight of the base steel sheet. When titanium is included within this content range, continuous casting defects and precipitate coarsening can be prevented, ensuring the physical properties of the steel and preventing defects such as crack formation on the steel surface.

[0041] Chromium (Cr) is added to improve the hardenability and strength of the base steel plate 100. The amount of chromium is also 0.01 wt% to 0.5 wt% of the total weight of the base steel plate 100. When chromium is included within the above range, the hardenability and strength of the base steel plate 100 can be improved, preventing an increase in production costs and a decrease in the toughness of the steel.

[0042] Molybdenum (Mo) can contribute to improving the strength of the base steel sheet 100 by suppressing the coarsening of precipitates and increasing hardenability during hot rolling and hot pressing. Such molybdenum (Mo) may be present in amounts of 0.001 wt% to 0.008 wt% of the total weight of the base steel sheet 100.

[0043] Boron (B) is added to ensure the hardenability and strength of the base steel sheet 100 by securing a martensitic structure, and has a grain refinement effect by raising the osteostenite grain growth temperature. Boron is also included in amounts of 0.001 wt% to 0.008 wt% of the total weight of the base steel sheet 100. When boron is included within the above range, the occurrence of hard phase grain boundary brittleness can be prevented, and high toughness and bendability can be ensured.

[0044] The plating layer 200 is formed on at least one surface of the base steel sheet 100 to a thickness of 10 μm to 50 μm and contains aluminum (Al). Here, the thickness of the plating layer 200 refers to the average thickness of the plating layer 200 over the entire surface area of ​​the plating layer 200. If the thickness of the plating layer 200 is less than 10 μm, the corrosion resistance is reduced, and if the thickness of the plating layer 200 exceeds 50 μm, the productivity of the hot-press steel sheet 10 is reduced, the plating layer 200 adheres to the roller or die during the hot-press process, and the plating layer 200 peels off from the base steel sheet 100.

[0045] The plating layer 200 also includes a diffusion layer 210 and a surface layer 220 that are sequentially laminated on the base steel plate 100.

[0046] The surface layer 220 is a layer containing 80 wt% or more of aluminum (Al) to prevent oxidation of the base steel plate 100. The diffusion layer 210 is formed by the mutual diffusion of Fe from the base steel plate 100 and Al from the plating layer 200, and also contains aluminum-iron (Al-Fe) compounds and aluminum-iron-silicon (Al-Fe-Si) compounds. The diffusion layer 210 also contains 20 wt% to 60 wt% of iron (Fe), 30 wt% to 80 wt% of aluminum (Al), and 0.1 wt% to 40 wt% of silicon (Si).

[0047] Since such a diffusion layer 210 has a higher melting point than the surface layer 220, it is possible to prevent the surface layer 220 from melting during the hot pressing process and the occurrence of liquid metal embrittlement, in which Al penetrates into the structure of the base steel sheet 100.

[0048] Therefore, the area fraction of the diffusion layer 210 (cross-sectional area of ​​diffusion layer 210 ÷ cross-sectional area of ​​plating layer 200), which is the ratio of the cross-sectional area of ​​the diffusion layer 210 to the cross-sectional area of ​​the plating layer 200, is between 10% and 35%. Here, the cross-sectional area of ​​the plating layer 200 and the cross-sectional area of ​​the diffusion layer 210 refer to the cross-sectional area at the same arbitrary location. This can be applied similarly to the area fractions related to other layers below.

[0049] The diffusion layer 210 is sequentially located on the base steel plate 100 and also includes a silicon-containing Fe-Al alloy layer 212 and an Fe-Al intermetallic compound layer 214, respectively.

[0050] The Fe-Al alloy layer 212 contains 50 wt% to 75 wt% Al, 10 wt% to 50 wt% Fe, and 0.1 wt% to 15 wt% Si, with a density of 4.0 g / cm³. 3 or 4.8 g / cm³ 3 It can have a density of . For example, the Fe-Al alloy layer 212 contains Al5Fe2 and can have a higher hardness than the diffusion layer 210.

[0051] Such an Fe-Al alloy layer 212 plays a role in preventing liquid metal embrittlement. However, since the Fe-Al alloy layer 212 consists of a hard phase, its hardness is maintained at a high level even during the hot pressing process, which can induce crack formation during the hot pressing process and cause a decrease in the formability of the hot pressing steel sheet 10. Therefore, in order to prevent liquid metal embrittlement and prevent a decrease in the formability of the hot pressing steel sheet 10, the average thickness of the Fe-Al alloy layer 212 is formed to be 50 nm to 500 nm, preferably 50 nm to 300 nm. Furthermore, the area fraction of the Fe-Al alloy layer 212 relative to the diffusion layer 210 is between 2.0% and 15.5%.

[0052] The Fe-Al intermetallic compound layer 214 contains 35 wt% to 85 wt% Al, 25 wt% to 45 wt% Fe, and 8 wt% to 30 wt% Si, with a density of 2.9 g / cm³. 3 or 5.6 g / cm³ 3 It can have a density of . The Fe-Al intermetallic compound layer 214 has lower hardness than the Fe-Al alloy layer 212, and by providing a buffering effect against the pressing force during the hot pressing process of the hot pressing steel sheet 10, it can prevent cracks from occurring in the plating layer 200.

[0053] More specifically, during heating for hot pressing, further interdiffusion occurs between the plating layer 200 and the base steel plate 100. At this time, the Fe-Al alloy layer 212 maintains relatively high hardness, while the Fe-Al intermetallic compound layer 214 forms a tau phase and / or AlFe, resulting in lower hardness. Therefore, by including the Fe-Al intermetallic compound layer 214 in the diffusion layer 210, which can provide a buffering effect against the pressing force during the hot pressing process, crack resistance can be improved.

[0054] Such an Fe-Al intermetallic compound layer 214 has an area fraction of 84.5% to 98.0% relative to the diffusion layer 210. If the cross-sectional area of ​​the Fe-Al intermetallic compound layer 214 is formed to be 84.5% or more of the cross-sectional area of ​​the diffusion layer 210, the Fe-Al intermetallic compound layer 214 can effectively absorb external forces that induce cracks in the plating layer 200 during the hot pressing process. However, if the area fraction of the Fe-Al intermetallic compound layer 214 relative to the diffusion layer 210 exceeds 98.0%, the average thickness of the Fe-Al alloying layer 212 becomes relatively thinner, making it difficult to prevent the liquid metal embrittlement phenomenon, and it becomes extremely difficult to secure an Fe-Al intermetallic compound layer 214 with an area fraction exceeding 98.0% within the temperature range of the plating bath for melting Al, as described later.

[0055] Furthermore, the Fe-Al intermetallic compound layer 210 also includes a first layer 215 and a second layer 217 that are sequentially laminated. Although the first layer 215 and the second layer 217 are each formed of an Fe-Al intermetallic compound containing Si, the first hardness of the first layer 215 may be lower than the second hardness of the second layer 217. That is, the Fe-Al alloy layer 212, the second layer 217, and the first layer 215 can have progressively higher hardness values. Therefore, even if phase changes occur in each layer or the positions of each layer are changed during the hot pressing process, the layer structure can be made capable of absorbing external forces that would induce crack formation or a decrease in formability.

[0056] Since the Fe-Al intermetallic compound layer 214 is formed on the Fe-Al alloy layer 212, which has a low solid solubility of Si, the first layer 215 faces the surface of the plating layer 200, and the Si content gradually increases, while the second layer has a relatively higher Al content and lower Si content compared to the first layer 215.

[0057] For example, the first layer 215 contains 35 wt% to 51 wt% Al, 25 wt% to 45 wt% Fe, and 15 wt% to 30 wt% Si, with a density of 4.6 g / cm³. 3 or 5.6 g / cm³ 3 It can have a density of 2.9 g / cm³. The second layer 217 contains Al 55 wt% to 85 wt%, Fe 10 wt% to 30 wt%, and Si 8 wt% to 25 wt%, and has a density of 2.9 g / cm³. 3 or 3.9 g / cm³ 3 It can have a density of . Furthermore, the Fe-Al alloy layer 212, the second layer 217, and the first layer 215 can have high Al content (wt%) values ​​in that order, and the second layer 217, the first layer 215, and the Fe-Al alloy layer 212 can have high Si content (wt%) values ​​in that order. Therefore, even if phase changes occur in each layer or the position of each layer is changed during the hot pressing process, it is possible to have a layer structure that can absorb external forces that induce crack formation or a decrease in formability.

[0058] In other words, in the Fe-Al alloy layer 212, the first layer 215, and the second layer 217, the Al content is lowest in the first layer 215, and the Si content is highest in the first layer 215, which can result in the first layer 215 having the lowest hardness.

[0059] The first layer 215 can absorb external forces that would induce cracks in the plating layer 200 during the hot pressing process, thereby preventing cracks from occurring in the plating layer 200. Furthermore, even if cracks occur in the second layer 217 or the Fe-Al alloy layer 212, which have relatively higher hardness than the first layer 215, the soft first layer 215 not only provides a buffering effect, but also blocks crack propagation at the interface formed during the hot pressing process, effectively preventing cracks generated in the second layer 217 or the Fe-Al alloy layer 212 from being transmitted to the base steel sheet 100 or the plating layer 200. Therefore, if the Fe-Al intermetallic compound layer 214 has a laminated structure of the first layer 215 and the second layer 217, it can more effectively prevent or minimize cracks from occurring in the hot pressing steel sheet 10 during the hot pressing process.

[0060] The second layer 217 plays a role in absorbing external forces during the hot pressing process and can also improve the adhesion of the plating layer 200. Compared to the first layer 215, the second layer 217 has a higher Al content and a lower Si content, so it has a composition even more similar to the surface layer 220 than the Fe-Al alloy layer 212 and the first layer 215, but the second layer 217 can improve the adhesion of the plating layer 200.

[0061] On the other hand, if the average thickness of the first layer 215 is less than 50 nm, the effect of absorbing external forces that induce cracks in the plating layer 200 during the hot pressing process is sharply reduced. On the other hand, if the average thickness of the first layer 215 is greater than 500 nm, Kirkendall voids may occur due to the difference in diffusion rates between Al and Fe, which may reduce performance such as weldability. Therefore, the average thickness of the first layer 215 can be formed to 50 nm to 500 nm, preferably to 50 nm to 300 nm.

[0062] Furthermore, if the average thickness of the second layer 217 is less than 1 μm, a highly brittle Fe2Al5 layer may be formed during the hot pressing process due to the diffusion of Fe, potentially causing cracks or delamination of the plating layer 200. On the other hand, if the average thickness of the second layer 217 is greater than 16 μm, the residual stress within the plating layer 200 increases after the hot pressing process, potentially causing cracks or delamination of the plating layer 200. Therefore, the average thickness of the second layer 217 can be formed to be between 1 μm and 16 μm.

[0063] Thus, if the Fe-Al intermetallic compound layer 214 has a laminated structure of a first layer 215 and a second layer 217, it not only more effectively prevents cracks from occurring in the plating layer 200, but also improves the bonding strength of the surface layer 220, resulting in superior stability of the plating layer 200.

[0064] Figure 2 is a flowchart illustrating the manufacturing method of the hot-press steel sheet shown in Figure 1. The following explanation of the manufacturing method of the hot-press steel sheet will refer to both Figure 1 and Figure 2.

[0065] A method for manufacturing steel sheets for hot pressing according to one embodiment of the present invention also includes a hot rolling step (S310), a cooling / winding step (S320), a cold rolling step (S330), an annealing heat treatment step (S340), and a hot-dip galvanizing step (S350) of a steel slab.

[0066] First, a semi-finished steel slab, which will be the subject of the process for forming plated steel sheets, is prepared. The steel slab contains 0.01 wt% to 0.5 wt% of carbon (C), 0.01 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 3.0 wt% of manganese (Mn), 0% or less of phosphorus (P) exceeding 0.05 wt%, 0% or less of sulfur (S) exceeding 0.01 wt%, 0% or less of aluminum (Al) exceeding 0.1 wt%, 0% or less of nitrogen (N) exceeding 0.001 wt%, the remainder being iron (Fe), and other unavoidable impurities. Furthermore, the steel slab also contains one or more of the following components: 0.005 wt% to 0.1 wt% of niobium (Nb), 0.005 wt% to 0.1 wt% of titanium (Ti), 0.01 wt% to 0.5 wt% of chromium (Cr), 0.001 wt% to 0.008 wt% of molybdenum (Mo), and 0.001 wt% to 0.008 wt% of boron (B).

[0067] For hot rolling, the steel slab is reheated. In this steel slab reheating step, the steel slab, which was secured through the continuous casting process, is reheated to a predetermined temperature, thereby resolving the components that were segregated during casting. In one embodiment, the slab reheating temperature (SRT) is 1,200°C to 1,400°C. If the slab reheating temperature (SRT) is lower than 1,200°C, the components that were segregated during casting are not sufficiently resolving, making it difficult to increase the homogenization effect of the alloying elements and difficult to increase the solid solution effect of titanium (Ti). While a higher slab reheating temperature (SRT) is advantageous for homogenization, if it exceeds 1,400°C, the ostenitite grain size increases, making it difficult to secure strength, and the excessive heating process also increases the manufacturing cost of the steel sheet.

[0068] In the hot rolling stage (S310), the reheated steel slab is hot-rolled at a predetermined finishing delivery temperature (FDT). In one embodiment, the finishing delivery temperature (FDT) is 880°C to 950°C. If the finishing delivery temperature (FDT) is lower than 880°C, a mixed grain structure occurs due to abnormal rolling, making it difficult to ensure the workability of the steel sheet. This not only reduces workability due to non-uniform microstructure, but also causes problems with sheet passage during hot rolling due to rapid phase changes. If the finishing delivery temperature (FDT) exceeds 950°C, the ostenitite grains become coarser. In addition, TiC precipitates become coarser, which may reduce the performance of the final component.

[0069] In the cooling / cooling stage (S320), the hot-rolled steel sheet is cooled to a predetermined coiling temperature (CT) and coiled. In one embodiment, the coiling temperature is 550°C to 800°C. The coiling temperature affects the redistribution of carbon (C). When the coiling temperature is 550°C, the low-temperature phase fraction increases due to overcooling, which increases strength and may lead to a greater rolling load during cold rolling, as well as a rapid decrease in softness. Conversely, when the coiling temperature exceeds 800°C, abnormal or excessive crystal grain growth can occur, leading to a deterioration in formability and strength.

[0070] In the cold rolling stage (S330), the wound steel sheet is uncoiled, pickled, and then cold-rolled. At this time, the pickling is carried out for the purpose of removing scale from the wound steel sheet, i.e., the hot-rolled coil produced through the aforementioned hot-rolling process.

[0071] The annealing heat treatment step (S340) is a step in which the cold-rolled steel sheet is subjected to annealing heat treatment at a temperature of 700°C or higher. In one specific example, the annealing heat treatment includes the steps of heating the cold-rolled sheet material and cooling the heated cold-rolled sheet material at a predetermined cooling rate.

[0072] The hot-dip plating step (S350) is a step in which a plating layer is formed on the annealed steel sheet. In one embodiment, an Al-Si plating layer 200 can be formed on the annealed steel sheet, i.e., the base steel sheet 100, during the hot-dip plating step (S350).

[0073] Specifically, the hot-dip plating step (S350) includes the steps of immersing the base steel plate 100 in a plating bath having a temperature of 650°C to 700°C to form a hot-dip plating layer on the surface of the base steel plate 100, and cooling the base steel plate 100 on which the hot-dip plating layer has been formed to form a plating layer 200.

[0074] The plating bath also contains 4 wt% to 12 wt% Si, 1.0 wt% to 4.0 wt% Fe, and the remainder being Al. In particular, the Si contained in the plating bath can suppress the growth of the Fe-Al alloy layer 212 when the plating layer 200 is formed. Therefore, if the Si content is less than 4 wt%, the Fe-Al alloy layer 212 will be formed excessively thick, reducing the formability of the hot-press steel sheet 10 and making it easy for cracks to occur in the hot-press steel sheet 10. On the other hand, if the Si content is more than 12 wt%, the growth of the Fe-Al intermetallic compound layer 214, especially the second layer 217, will become dominant. Therefore, by adjusting the Si content in the plating bath, the area fraction of the Fe-Al alloy layer 212 relative to the diffusion layer 210 can be limited to 2.0% to 15.5%, thereby forming the Fe-Al intermetallic compound layer 214 at an area fraction of 84.5% to 98.0% relative to the diffusion layer 210, effectively preventing or minimizing crack formation in the plating layer 200 during the hot pressing process.

[0075] In addition, plating baths may also contain additive elements such as Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, and Bi.

[0076] In the cooling step of cooling the base steel sheet 100 on which the hot-dip plating layer is formed, the base steel sheet 100 may be subjected to a first cooling step of cooling from the temperature of the plating bath to 550°C at a first average cooling rate, and a second cooling step of cooling the base steel sheet 100 from 550°C to room temperature at a second average cooling rate, wherein the first average cooling rate is higher than the second average cooling rate. For example, the first average cooling rate is 20°C / s or more, and the overall average cooling rate for cooling from the temperature of the plating bath to room temperature may be 1°C / s to 50°C / s.

[0077] In addition, in order to form the hot-dip plating layer on the base steel sheet 100, the base steel sheet 100 can pass through a plating bath, wherein the passing speed of the base steel sheet 100 passing through the plating bath may be 1 mpm to 250 mpm.

[0078] In this way, after the base steel sheet 100 passes through the plating bath at a speed of 1 mpm to 250 mpm, by performing the first cooling step and the second cooling step, the Fe-Al intermetallic compound layer 214 can also be formed to include the first layer 215 and the second layer 217 that are sequentially laminated.

[0079] The formed plating layer 200 may be an Al-Si plating layer, and based on both sides of the base steel sheet 100, it is 40 g / m 2 to 200 g / m 2 It may be formed by plating, or may be formed to have a thickness of 10 µm to 50 µm. For this purpose, before cooling the base substrate 100 on which the hot-dip plating layer is formed, air or gas is injected onto the base substrate 100 to wipe the hot-dip plating layer, whereby the thickness of the hot-dip plating layer can be adjusted.

[0080] Hereinafter, the present invention will be described in further detail with reference to examples. However, the following examples are intended to further specifically illustrate the present invention, and the scope of the present invention is not limited by the following examples. The following examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

[0081] <Manufacturing of steel sheets for hot pressing> Steel slabs with the following components were subjected to hot rolling, cooling / winding, cold rolling, and annealing to form a base steel plate (1.2 mm thick). Then, hot-dip galvanizing was applied to the surface of the base steel plate to form a plating layer, thereby manufacturing steel plates for hot pressing.

[0082] [Table 1]

[0083] For hot-dip aluminum plating, we use a non-oxidizing furnace / reducing furnace type line. After plating, we use gas wiping to ensure the amount of hot-dip plating layer is 50g / m² per side. 2 or 90g / m 2 After adjusting the temperature, the process was carried out by cooling. At this time, the plating bath was set to contain 7 wt% Si, 2.5 wt% Fe, and the remainder Al at a temperature range of 600°C to 700°C. The base steel sheet was then passed through the plating bath at a speed of 100 mpm to 200 mpm, and then cooled to room temperature at an average cooling rate of 25°C / s to produce steel sheets for hot pressing.

[0084] <Crack inspection of the coating layer after the hot pressing process> By adjusting the wiping of the molten plating layer, the plating bath temperature, and the passage speed (immersion time) of the base steel plate through the plating bath, specimens with different average plating layer thicknesses, diffusion layer area fractions relative to the plating layer, Fe-Al alloy layer area fractions relative to the diffusion layer, and Fe-Al intermetallic compound layer area fractions relative to the diffusion layer were prepared as shown in Table 2 below. These specimens were then heated to a temperature of Ac3 or higher, subjected to external force with a press, and rapidly cooled, and the number of cracks formed in the plating layer was measured. Specifically, samples were taken from the specimen, and the area fraction of the diffusion layer relative to the plating layer, the area fraction of the Fe-Al alloy layer relative to the diffusion layer, and the area fraction of the Fe-Al intermetallic compound layer relative to the diffusion layer were measured. Then, the specimen was heated to a temperature of Ac3 or higher at an average heating rate of 3°C / s or higher, and then rapidly cooled to below 300°C at an average rate of 30°C / s or higher while applying external force with a press. Finally, the number of cracks in the plating layer per unit length (mm) at three arbitrary points on the specimen was measured.

[0085] [Table 2]

[0086] As can be seen from Table 2 above, in Examples 1 to 7, where the area fraction of the Fe-Al intermetallic compound layer relative to the diffusion layer was formed within the range of 84.5% to 98.0%, the number of cracks in the plating layer was far less than in Comparative Examples 1 to 6, where the area fraction of the Fe-Al intermetallic compound layer relative to the diffusion layer was formed at a lower rate than 84.5%. This is because the area fraction of the Fe-Al intermetallic compound layer relative to the diffusion layer is formed at 84.5% or higher, effectively absorbing the external forces that induce cracks in the plating layer during the hot pressing process, and as a result, cracks in the plating layer can be prevented or minimized. On the other hand, as mentioned above, if the area fraction of the Fe-Al intermetallic compound layer exceeds 98.0%, the average thickness of the Fe-Al alloy layer becomes relatively thinner, making it difficult to prevent the liquid metal embrittlement phenomenon, and it is extremely difficult to secure an Fe-Al intermetallic compound layer with an area fraction exceeding 98.0% within the temperature range of the plating bath as described above.

[0087] Table 3 below shows the results of measuring the number of cracks that formed in the plating layer after passing a base steel sheet through the plating bath under the same conditions as in Examples 1 to 7 in Table 2. The base steel sheet was cooled to an average cooling rate of 15°C / s up to 550°C, and then cooled from 550°C to room temperature at an average cooling rate of 30°C / s to produce a steel sheet for hot pressing. After that, a test piece was produced under the same conditions as in Table 2, and then heated to a temperature of Ac3 or higher. After applying external force with a press and rapidly cooling, the number of cracks that formed in the plating layer was measured.

[0088] [Table 3]

[0089] As can be seen from Table 3 above, in Examples 1 to 7, where the plating bath was cooled from the temperature to room temperature at an average cooling rate of 25°C / s, the Fe-Al intermetallic compound layer structure was formed as a single layer. In contrast, in Examples 8 to 17, where the base steel plate was cooled up to 550°C at an average cooling rate of 15°C / s and then cooled from 550°C to room temperature at an average cooling rate of 30°C / s, the Fe-Al intermetallic compound layer structure was found to have a two-layer structure with a first layer and a second layer stacked on top of each other. This shows that when the Fe-Al intermetallic compound layer has a two-layer structure, the number of cracks that occur in the plating layer is further reduced.

[0090] As mentioned above, this is because, in addition to the first and second layers acting as buffers to absorb external forces that induce cracks, even if cracks occur in the hard Fe-Al alloy layer, crack propagation is blocked at the interface formed during the hot pressing process, preventing the cracks originating in the Fe-Al alloy layer from being transmitted to the plating layer. Furthermore, because the Fe-Al intermetallic compound layer has a two-layer structure, the plating layer can be formed with excellent bonding strength.

[0091] As described above, the present invention has been explained with reference to an embodiment illustrated in the drawings, but these are merely illustrative, and a person skilled in the art will understand that a variety of modifications and variations of the embodiment are possible. Accordingly, the true scope of technical protection of the present invention is determined by the technical idea of ​​the claims.

Claims

1. Steel sheet for hot pressing, Base steel plate and The plating layer is located on the base steel plate and comprises sequentially laminated diffusion layers and surface layers, The diffusion layer is sequentially located on the base steel plate and includes, respectively, an Fe-Al alloy layer containing silicon and an Fe-Al intermetallic compound layer. The aforementioned surface layer contains 80 wt% or more of aluminum (Al), The Fe-Al alloy layer comprises aluminum (Al): 50 wt% to 75 wt%, iron (Fe): 10 wt% to 50 wt%, and silicon (Si): 0.1 wt% to 15 wt%. The Fe-Al intermetallic compound layer comprises aluminum (Al): 35 wt% to 85 wt%, iron (Fe): 25 wt% to 45 wt%, and silicon (Si): 8 wt% to 30 wt%. The Fe-Al alloy layer has a higher hardness than the Fe-Al intermetallic compound layer. The average thickness of the aforementioned plating layer is 10 μm to 50 μm. The average thickness of the Fe-Al alloy layer is 50 nm to 300 nm. A steel sheet for hot pressing, wherein the area fraction of the diffusion layer relative to the plating layer is 10% to 35%.

2. The steel sheet for hot pressing according to claim 1, wherein the area fraction of the Fe-Al intermetallic compound layer relative to the diffusion layer is 84.5% to 98.0%.

3. The Fe-Al intermetallic compound layer includes a first layer and a second layer that are sequentially laminated from the base steel sheet side. The first layer has a first hardness, and the second layer has a second hardness that is higher than the first hardness. The steel sheet for hot pressing according to claim 1, wherein the hardness of the Fe-Al alloy layer is higher than the first hardness and the second hardness.

4. The steel sheet for hot pressing according to claim 3, wherein the aluminum content in the Fe-Al alloy layer, the first layer, and the second layer is lowest in the first layer, and the silicon content is highest in the first layer.

5. The steel sheet for hot pressing according to claim 3, wherein the average thickness of the first layer is 50 nm to 500 nm, and the average thickness of the second layer is 1 μm to 16 μm.

6. The steel sheet for hot pressing according to claim 1, wherein the area fraction of the Fe-Al alloy layer relative to the diffusion layer is 2.0% to 15.5%.

7. The base steel sheet comprises, by wt%, carbon (C): 0.01 wt% to 0.5 wt%, silicon (Si): 0.01 wt% to 1.0 wt%, manganese (Mn): 0.5 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): greater than 0 and less than or equal to 0.1 wt%, with the remainder being iron (Fe) and other unavoidable impurities, as described in claim 1.

8. The hot press steel sheet according to claim 1, wherein the base steel sheet further comprises one or more components selected from niobium (Nb), titanium (Ti), chromium (Cr), molybdenum (Mo), and boron (B).

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