Hot stamping parts and their manufacturing method

Abstract

A hot stamped part according to an embodiment of the present invention includes a steel sheet; a plating layer located on the steel sheet and containing Zn; and a surface layer located on the plating layer; the surface layer includes a post-treatment layer containing a Si-based inorganic post-treatment agent; a Zn oxide layer located on the plating layer in the same layer as the post-treatment layer; and an interdiffusion layer located between at least one of the post-treatment layer and the Zn oxide layer and the plating layer, overlapping with at least one of the post-treatment layer and the Zn oxide layer, and containing at least one of Si, Mn, O, Fe, Zn, and SiO, and the steel sheet has a tensile strength of 1,680 MPa or more.

Description

Technical Field

[0001] The present invention relates to hot stamping parts and a manufacturing method thereof.

Background Art

[0002] For automotive parts, high-strength steel is applied for weight reduction and stability. On the other hand, high-strength steel can ensure high-strength characteristics with respect to weight, but as the strength increases, the press formability decreases, resulting in breakage of the material during processing or the occurrence of the springback phenomenon, making it difficult to form products with complex and precise shapes.

[0003] As a solution to such problems, there is a hot stamping method, and as the interest in it is increasing, research on materials for hot stamping is also being actively conducted. For example, as disclosed in the invention of Korean Patent Publication No. 10-2017-0076009, the hot stamping method is a forming technique for manufacturing high-strength parts by heating a hot stamping steel sheet at a high temperature and then rapidly cooling it during forming in a press die.

[0004] Related technologies include Korean Patent Publication No. 10-2018-0095757 (title of the invention: Method for manufacturing hot stamping parts), etc.

[0005] On the other hand, for hot stamping steel sheets, the surface of the steel sheet is oxidized during heating to generate scale, which reduces the surface characteristics and paintability of the product and also makes the corrosion resistance inferior to that of plating materials. Therefore, after product forming, a separate process such as shot blasting or shot peening to remove the scale is required. Alternatively, in order to prevent the above problems, a method of using an Al-based plating layer to suppress the oxidation reaction on the steel sheet surface and induce the formation of a passive film of Al to increase the corrosion resistance of the steel sheet has been used. However, in the case of the above-mentioned Al plating materials, although the heat resistance is excellent, there are problems in that the corrosion resistance is inferior to that of Zn plating materials and the manufacturing cost increases.

[0006] However, Zn-based hot-stamped steel sheets can suffer a significant decrease in paint adhesion and appearance due to the oxidation of Zn that occurs during the hot-stamping heat treatment. To address this, post-treatment processes such as shot peening have been introduced, but these have the drawback of incurring additional costs and processes, thus reducing productivity. [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide hot stamped parts with improved surface properties and paintability, and a method for manufacturing the same. [Means for solving the problem]

[0008] A hot stamped part according to one embodiment of the present invention comprises a steel sheet; a plating layer located on the steel sheet and containing Zn; and a surface layer located on the plating layer, wherein the surface layer comprises a post-treatment layer containing a Si-based inorganic post-treatment agent; a Zn oxide layer located on the plating layer in the same layer as the post-treatment layer; and an interdiffusion layer located between the plating layer and at least one of the post-treatment layer and the Zn oxide layer, superimposed on the post-treatment layer and at least one of the Zn oxide layer, and containing at least one of Si, Mn, O, Fe, Zn, and SiO, and the tensile strength of the steel sheet is 1680 MPa or more.

[0009] The area fraction of the interdiffusion layer is also between 10% and 80% of the total area fraction of the post-treatment layer.

[0010] The post-treatment layer may contain an oxide of a Si-based post-treatment agent and at least one of Si, Mn, O, Fe, Zn, and SiO as components diffused from the steel sheet and the plating layer.

[0011] The average thickness of the post-treatment layer is thinner than the average thickness of the Zn oxide layer, and the average thickness of the post-treatment layer is 5% or more but less than 100% of the average thickness of the Zn oxide layer.

[0012] The average thickness of the post-treatment layer is 0.5 μm to 3 μm, and the average thickness of the Zn oxide layer is 1 μm to 10 μm.

[0013] The average thickness of the aforementioned interdiffusion layer is 0.1 μm to 2 μm.

[0014] A method for manufacturing a hot-stamped part according to one embodiment of the present invention includes a heating step of placing a steel sheet coated with a Si-based inorganic post-treatment agent on a Zn-containing plating layer into a heating furnace and heating it; a transfer step of transferring the heated steel sheet from the heating furnace to a press die; a molding step of hot-stamping the transferred steel sheet to form a molded body; and a cooling step of the molded body. In the heating step, the inorganic post-treatment agent and the components of the plating layer are diffused to form an interdiffusion layer, a post-treatment layer, and a Zn oxide layer formed by the oxidation of the plating layer. The interdiffusion layer is located between the plating layer and at least one of the post-treatment layer and the Zn oxide layer, superimposed on at least one of the post-treatment layer and the Zn oxide layer, and contains at least one of Si, Mn, O, Fe, Zn, and SiO. The tensile strength of the steel sheet is 1680 MPa or more.

[0015] The area fraction of the interdiffusion layer may be formed to be between 10% and 80% of the total area fraction of the post-treatment layer.

[0016] The post-treatment layer may contain at least one of Si, Mn, O, Fe, Zn, and SiO as an oxide of the inorganic post-treatment agent and a component diffused from the steel sheet and the plating layer.

[0017] The average thickness of the post-treatment layer is thinner than the average thickness of the Zn oxide layer, and the average thickness of the post-treatment layer is 5% or more of the average thickness of the Zn oxide layer.

[0018] The average thickness of the post-treatment layer is 0.5 μm to 3 μm, and the average thickness of the Zn oxide layer is 1 μm to 10 μm.

[0019] The average thickness of the interdiffusion layer is also 0.1 μm to 2 μm.

[0020] In the heating stage, the steel sheet is heated to a target heating temperature having a temperature range of Ac3 to 910° C. in the heating furnace and can be heated while staying for 120 seconds to 600 seconds.

[0021] Before the heating stage, a post-treatment step may further be included in which the Si-based inorganic post-treatment agent is applied onto the steel sheet on which the plating layer is formed, and dried to form a pre-post-treatment layer.

[0022] In the post-treatment step, the inorganic post-treatment agent is applied to the steel sheet to a thickness of 0.5 μm to 3 μm to form the pre-post-treatment layer, and the application amount of the inorganic post-treatment agent is 0.5 g / m 2 ~3 g / m 2 as well.

[0023] In the post-treatment step, the steel sheet coated with the inorganic post-treatment agent is dried at a temperature of 70° C. to 150° C. for 1 second to 10 seconds.

Effect of the Invention

[0024] According to an embodiment of the present invention, it is possible to provide a hot stamping part having improved surface properties and paintability using an inorganic post-treatment, and a method for manufacturing the same.

Brief Description of the Drawings

[0025] [Figure 1] It is a flowchart for explaining a method for manufacturing a hot stamping part according to an embodiment of the present invention. [Figure 2] It is a TEM (transmission electron microscope) image showing a part of a material before a hot stamping process according to an embodiment of the present invention. [Figure 3] It is a TEM image showing a part of a part after a hot stamping process according to an embodiment of the present invention. [Figure 4] It is a cross-sectional view showing a part after a hot stamping process according to an embodiment of the present invention. [Figure 5] Figure 2 is a magnified TEM image of a portion of the part after the hot stamping process. [Figure 6] This drawing compares the adhesion evaluation of a hot stamped part according to a comparative example and a hot stamped part according to one embodiment of the present invention. [Modes for carrying out the invention]

[0026] The present invention can have various embodiments through diverse modifications, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. The effects and features of the present invention, and the methods for achieving them, will become clear when you refer to the embodiments described in detail below along with the drawings. However, such inventions are not limited to the embodiments disclosed below and can be embodied in a variety of forms.

[0027] In the following embodiments, terms such as "first," "second," etc., are used not in a restrictive sense, but to distinguish one component from another.

[0028] In the following examples, the singular expression includes plural expressions unless the context clearly indicates otherwise.

[0029] In the following embodiments, terms such as "includes" or "has" mean that the features or components described in the specification are present, and do not preclude the possibility of adding one or more other features or components.

[0030] In the following embodiments, when a part such as a film, region, or component is located on or above another part, this includes not only cases where it is directly above the other part, but also cases where another film, region, or component is interposed between them.

[0031] In drawings, the size of components may be exaggerated or reduced for illustrative purposes. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for illustrative purposes, and the present invention is not necessarily limited to what is shown.

[0032] If a particular embodiment can be manifested in a different way, a specific sequence of steps may be performed in a different order than that described. For example, two steps described consecutively may be performed substantially simultaneously, or they may be performed in the reverse order of the description.

[0033] Hereinafter, embodiments of the present invention will be described in detail 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.

[0034] Figure 1 is a flowchart illustrating a method for manufacturing hot-stamped parts according to one embodiment of the present invention.

[0035] Referring to Figure 1, a method for manufacturing a hot stamped part according to one embodiment may include a post-processing step (S100), a heating step (S200), a transfer step (S300), a molding step (S400), and a cooling step (S500).

[0036] In the post-treatment stage (S100), a Si-based inorganic post-treatment agent is applied to the steel sheet on which a Zn-containing plating layer has been formed to form a pre-post-treatment layer, and the steel sheet is dried to prepare the post-treated steel sheet. The step of preparing the steel sheet with the plating layer formed prior to the post-treatment stage (S100) can be carried out by preparing a steel slab having a known composition applicable for hot stamping, performing at least one of known hot rolling and cold rolling on the steel slab, and then performing an annealing heat treatment to manufacture the steel sheet material. After the annealing heat treatment, a Zn plating layer can be formed on the steel sheet material by a known method.

[0037] In the post-treatment stage (S100), the steel sheet coated with an inorganic post-treatment agent can be dried to form a pre-post-treatment layer. In this case, the drying stage may be carried out at a temperature of 70°C to 150°C for 1 to 10 seconds, for example.

[0038] All hot stamped parts generated in the post-processing stage (S100) will be explained in more detail in Figure 2, which will be described later.

[0039] Thereafter, in the heating stage (S200), the steel plate coated with the inorganic post-treatment agent is placed in a heating furnace and heated. In the heating stage, the steel plate is heated in the heating furnace to a target heating temperature in order to ensure the material quality of the steel plate and to prevent vaporization of the Zn plating layer, and the target heating temperature is approximately Ac3 to 910°C. The steel plate may be heated while remaining in the heating stage (S200) for approximately 120 seconds to 600 seconds.

[0040] In heating stage 200, the average heating rate of the steel plate in the range from approximately 700°C to the target heating temperature is approximately 1.5°C / sec to 7°C / sec. When the average heating rate is approximately 1.5°C / sec to 7°C / sec, the components of the plating layer and the post-treatment layer diffuse with each other, and the material properties of both the steel plate and the plating layer can be maintained. If the heating rate is less than approximately 1.5°C / sec, there are problems with insufficient material properties of the steel plate and reduced productivity. If the heating rate exceeds approximately 7°C / sec, it is difficult to maintain the target plating layer structure due to over-alloying of the plating layer.

[0041] Depending on the example, multi-stage heating or crack heating can be performed during the heating phase.

[0042] The multi-stage heating stage is a stage in which the steel plate is heated in stages, and the crack heating stage is a stage in which the multi-stage heated steel plate is heated at a uniform temperature. In the multi-stage heating stage, the steel plate can be heated in stages by passing through multiple sections provided in the heating furnace. Of the multiple sections provided in the heating furnace, there are multiple sections in which the multi-stage heating stage is performed, and the temperature of each section is set to increase from the entrance of the heating furnace where the steel plate is fed in to the exit of the heating furnace where the steel plate is removed, thereby gradually heating the steel plate. The crack heating stage is performed after the multi-stage heating stage. In the crack heating stage, the multi-stage heated steel plate can be heat-treated by passing through sections of the heating furnace set to a temperature of approximately Ac3 to 910°C. Also, of the multiple sections provided in the heating furnace, there is at least one section in which the crack heating stage is performed.

[0043] During the heating step (S200), the components of the pre-treatment layer containing the post-treatment agent and the plating layer may diffuse to each other, forming an interdiffusion layer, a second post-treatment layer, and a Zn oxide layer formed by the oxidation of the plating layer. The interdiffusion layer is located between the second post-treatment layer and the Zn oxide layer and the plating layer, superimposed on at least one of the second post-treatment layer and the Zn oxide layer, and may contain Si, Mn, O, Fe, Zn, and SiO. The above-mentioned layers of the component after the heating step (S200) will be described in more detail in Figures 3 to 5, which will be described later.

[0044] Thereafter, in the transfer stage (S300), the heated steel plate can be transferred from the heating furnace to the press die. In the transfer stage (S300), the heated blank can be air-cooled for 5 to 30 seconds.

[0045] The forming stage (S400) is the stage in which the transferred steel sheet is hot-stamped to form a molded body. The forming start temperature in the forming stage (S400) is approximately 550°C to 750°C. If the forming temperature is above approximately 750°C, there is a problem in that cracks of approximately 10 μm or more occur on the side walls of the part due to the liquid metal embrittlement (LME) phenomenon, reducing durability. Conversely, if the forming temperature is below approximately 550°C, there is a problem in that the material properties of the steel sheet are insufficient.

[0046] The cooling stage (S500) is the stage in which the formed molded body is cooled. The average cooling rate in the cooling stage (S500) is also approximately 25°C / sec or higher.

[0047] The final product can be formed by shaping the blank into the final part shape using a press die and then cooling the molded body. The press die may be equipped with cooling channels through which a coolant circulates. The blank, heated by circulation, can be rapidly cooled by the coolant supplied through the cooling channels in the press die. In this case, to prevent the springback phenomenon of the sheet metal and to maintain the desired shape, rapid cooling can be performed while the press die is closed and pressurized. In the shaping and cooling operation of the heated blank, the average cooling rate can be reduced to at least 10°C / s until the martensite completion temperature is reached. The blank can be held in the press die for 3 to 20 seconds. If the holding time in the press die is less than 3 seconds, the material will not be sufficiently cooled, and dimensional quality will be affected by temperature variations in different parts due to residual heat. Also, a sufficient amount of martensite will not be generated, and mechanical properties will not be ensured. On the other hand, if the holding time in the press die exceeds 20 seconds, the holding time in the press die will be long, which may reduce productivity.

[0048] Hot stamped parts manufactured using a manufacturing method according to one embodiment of the present invention may have improved surface properties and paint adhesion through an interdiffusion layer formed by the mutual diffusion of the inorganic post-treatment agent and the components of the plating layer. In this case, the area fraction of the interdiffusion layer is 10% to 80% of the total area fraction of the second post-treatment layer. Furthermore, the average thickness of the second post-treatment layer is thinner than the average thickness of the Zn oxide layer formed by the oxidation of the Zn plating layer. The specific relationship between these layers will be explained later in Figures 3 to 5.

[0049] Figure 2 is a TEM (transmission electron microscope) image showing a part of material 10 (hereinafter also referred to as "hot stamping material") before the hot stamping process (or heating) according to one embodiment of the present invention.

[0050] One embodiment of the present invention may include a steel plate 100, a plating layer 200p before hot stamping, and a pre-post-treatment layer 300p on the plating layer 200p. Hereinafter, the post-treatment layer 300p of the hot stamping material 10 will be referred to as the "pre-post-treatment layer 300p" in order to distinguish it from the post-treatment layer 310 of the part 20 after the hot stamping process, which will be described later.

[0051] The steel sheet 100 of the present invention is also a steel sheet manufactured by hot rolling and / or cold rolling processes applied to a slab cast to contain predetermined alloying elements in predetermined amounts. The steel sheet 100 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), molybdenum (Mo), nickel (Ni), additives, and the remainder being iron (Fe) and other unavoidable impurities. In one embodiment, the steel sheet 100 may further contain at least one of titanium (Ti), niobium (Nb), and vanadium (V) as additives. In other embodiments, the steel sheet 100 may further contain a predetermined amount of calcium (Ca).

[0052] Carbon (C) acts as an austenite-stabilizing element in the steel sheet 100. Carbon is the main element that determines the strength and hardness of the steel sheet 100, and is added after the hot stamping process to ensure the tensile strength of the steel sheet 100 (e.g., a tensile strength of 1,680 MPa or more) and to ensure hardenability. Such carbon may be present in amounts of 0.25 wt% to 0.50 wt% of the total weight of the steel sheet 100. If the carbon content is less than 0.25 wt%, it is difficult to secure a hard phase (such as martensite), and it is difficult to satisfy the mechanical strength of the steel sheet 100. Conversely, if the carbon content exceeds 0.50 wt%, problems such as brittleness of the steel sheet 100 or a reduction in bending performance may occur.

[0053] Silicon (Si) acts as a ferrite-stabilizing element in the steel sheet 100. Silicon (Si) is a solid solution strengthening element that improves the ductility of the steel sheet 100 and suppresses the formation of carbides in the low-temperature range, thereby improving the carbon concentration in the austenite. Silicon is also a core element for hot rolling, cold rolling, hot pressing, and structural homogenization (control of pearlite and manganese segregation zones), as well as fine ferrite dispersion. Silicon also acts as a martensite strength heterogeneity control element, improving impact performance. Such silicon may be present in the steel sheet 100 at a concentration of 0.10 wt% to 0.8 wt% of the total weight. If the silicon content is less than 0.10 wt%, the above-mentioned effects will not be obtained, cementite formation and coarsening will occur in the final hot-stamped martensite structure, the homogenization effect of the steel sheet 100 will be minimal, and it will become impossible to secure a V-bending angle. Conversely, if the silicon content exceeds 0.8 wt%, the load of hot rolling and cold rolling increases, and the hot-rolled red scale may become excessive, potentially degrading the plating properties of steel sheet 100.

[0054] Manganese (Mn) acts as an austenite-stabilizing element in the steel sheet 100. Manganese is added during heat treatment to increase hardenability and strength. Such manganese may be present in amounts of 0.30 wt% to 3.0 wt% of the total weight of the steel sheet 100. If the manganese content is less than 0.30 wt%, the grain refinement effect is insufficient, and the hard phase fraction in the hot-stamped molded product becomes insufficient due to inadequate hardenability. On the other hand, if the manganese content exceeds 3.0 wt%, ductility and toughness decrease due to manganese segregation or pearlite banding, which can lead to a decrease in bending performance and the formation of a heterogeneous microstructure.

[0055] To prevent a decrease in the toughness of the steel plate 100, phosphorus (P) may be included in the steel plate 100 in an amount greater than 0% but less than or equal to 0.05 wt% of the total weight of the steel plate 100. If the phosphorus content exceeds 0.05 wt%, iron phosphide compounds will be formed, reducing toughness and weldability, and cracks may be induced in the steel plate 100 during the manufacturing process.

[0056] Sulfur (S) may be present in amounts greater than 0 but less than 0.01 wt% of the total weight of the steel plate 100. If the sulfur content exceeds 0.01 wt%, hot workability, weldability, and impact properties will decrease, and surface defects such as cracks may occur due to the formation of large inclusions.

[0057] Boron (B) is added to ensure the hardenability and strength of the steel sheet 100 by suppressing ferrite, pearlite, and bainite transformations and securing a martensitic structure. In addition, boron segregates at grain boundaries, lowering grain boundary energy and increasing hardenability, and has a grain refinement effect by increasing the austenite grain growth temperature. Such boron may be present in amounts of 0.0005 wt% to 0.005 wt% of the total weight of the steel sheet 100. When boron is present within this range, the occurrence of hard phase grain boundary brittleness can be prevented, and high toughness and bendability can be ensured. If the boron content is less than 0.0005 wt%, the hardenability effect is insufficient. Conversely, if the boron content exceeds 0.005 wt%, the solid solubility is low, and depending on the heat treatment conditions, it can easily precipitate at the grain boundaries, degrading the hardenability or causing high-temperature embrittlement. This can lead to a decrease in toughness and bendability due to the occurrence of hard phase grain boundary brittleness.

[0058] Chromium (Cr) is added to steel sheet 100 to improve its hardenability and strength. Chromium enables grain refinement and strength assurance through precipitation hardening. Such chromium may be present in amounts of 0.01 wt% to 1.0 wt% of the total weight of steel sheet 100. If the chromium content is less than 0.01 wt%, the precipitation hardening effect is weak, while if the chromium content exceeds 1.0 wt%, the amount of Cr-based precipitates and matrix solid solution increases, reducing toughness and potentially increasing production costs due to increased costs.

[0059] Molybdenum (Mo) may be present in amounts of 0.01 wt% to 1.0 wt% or less relative to the total weight of the steel plate 100.

[0060] Nickel (Ni) may be present in 0.001 wt% to 1.0 wt% or less of the total weight of the steel plate 100.

[0061] The additives are also nitride or carbide-forming elements. Specifically, the additives may contain at least one of titanium (Ti), niobium (Nb), and vanadium (V). Titanium (Ti), niobium (Nb), and vanadium (V) can ensure the strength of hot-stamped and quenched components by forming fine precipitates in the form of nitrides or carbides. They are also contained in Fe-Mn composite oxides and function as hydrogen trapping sites that are effective in improving delayed fracture resistance, making them elements necessary for improving delayed fracture resistance. The total amount of such additives may be 0.01 wt% to 0.1 wt% or less of the total weight of 100g of steel sheet. If the content of the additives exceeds 0.1 wt%, the increase in yield strength will be excessively large.

[0062] Titanium (Ti) can be added after hot press heat treatment to enhance hardenability and improve material properties through precipitate formation. Furthermore, it forms precipitate phases such as Ti(C,N) at high temperatures, effectively contributing to austenite grain refinement. Titanium can prevent continuous casting defects and precipitate coarsening, easily ensuring the physical properties of steel materials and preventing defects such as crack formation on the steel surface.

[0063] Niobium (Nb) and vanadium (V) are added to increase strength and toughness by reducing the martensite packet size. Niobium and vanadium have an excellent grain refinement effect on steel materials during hot rolling and cold rolling processes, preventing slab crack formation and brittle fracture of products during steelmaking / continuous casting, and minimizing the generation of coarse precipitates in steelmaking.

[0064] Steel sheet 100 has, for example, the composition shown in [Table 1] below. Referring to [Table 1] below, C, Si, Mn, P, S, and B are essential components, while Cr, Mo, Ni, and additives are optional components. The additives include, for example, at least one of Ti, Nb, and V, and their total amount is about 0.01 to 0.1 wt%.

[0065] [Table 1]

[0066] The structure of the steel sheet 100 of the present invention may, for example, include ferrite, pearlite, the remainder being other unavoidable structures and other precipitates. The physical properties of the steel sheet 100 of the present invention may, for example, be a tensile strength (TS) of about 1,680 to 2,300 MPa, a yield temperature (YP) of about 1,150 to 1,500 MPa, and an elongation (EL) of about 4% to about 10% after hot stamping. The plating layer 200p of the present invention may also be a Zn-based plating layer. That is, the hot stamping material 10 of the present invention is a galvanized steel sheet, and for example, it may be a galva-annealed iron (GA) in which a plating layer 200p is formed by alloying the steel sheet 100 with zinc by thermal diffusion, or it may be a granular hot-dip galvanized iron (GI) in which the plating layer 200p is formed by fine zinc crystal grains formed while molten zinc solidifies on the surface of the steel sheet 100.

[0067] The plating layer 200p before hot stamping in the present invention may contain, but is not limited to, Zn and Fe. The plating layer 200p is applied to one surface of the steel sheet 100 at a density of approximately 15 g / m². 2 ~100g / m 2 (Based on both sides, approximately 30-200g / m²) 2 The plating is applied with a certain amount of adhesion, and the thickness h1 of the 200p plating layer is approximately 4 μm to 30 μm.

[0068] The pre-post-treatment layer 300p of the present invention is a layer formed by coating and drying in the post-treatment step (S100) described above in Figure 1. The pre-post-treatment layer 300p will be explained in relation to the post-treatment step (S100) described above. The pre-post-treatment layer 300p contains an inorganic post-treatment agent, and as an example, it is a Si-based inorganic post-treatment agent. The Si in the pre-post-treatment layer 300p forms an interdiffusion layer containing SiO on the surface of the steel plate 100 or the plating layer 200p during the hot stamping heat treatment process, thereby improving the part characteristics of the hot stamped part. This will be explained in more detail in the related drawings described later. In the present invention, by using an inorganic post-treatment agent, it is possible to improve the problem that occurs when using an organic post-treatment agent containing carbon (C) chains, where the chains are broken by heating during the hot stamping process, reducing durability.

[0069] In the post-treatment stage (S100), the inorganic post-treatment agent can be applied to the steel plate 100 to a thickness of approximately 0.5 μm to 3 μm to form a pre-post-treatment layer 300p. That is, the thickness h0 of the pre-post-treatment layer 300p is also approximately 0.5 μm to 3 μm. In this case, the amount of inorganic post-treatment agent applied is approximately 0.5 g / m². 2 ~3g / m 2 Furthermore, only when the inorganic post-treatment agent of the present invention is applied in such an amount can the hot-stamped parts and their surface properties and paint adhesion according to one embodiment of the present invention, described later, be ensured.

[0070] The final part after the hot stamping process will be described below with reference to both Figures 3 and 4. Figure 3 is a TEM image showing a part of part 20 after the hot stamping process according to one embodiment of the present invention, and Figure 4 is a cross-sectional view showing part 20 after the hot stamping process according to one embodiment of the present invention (hereinafter referred to as "hot stamped part"). Hereinafter, the hot stamped part 20 refers to the part after all of the S200 to S500 steps described in Figure 1 have been performed. Content that is substantially the same as what was described in Figure 2 will be omitted or simplified in explanation.

[0071] The hot stamping part 20 of the present invention may include a steel sheet 100, a Zn-containing plating layer 200, and a surface layer 300 formed through a hot stamping process. The surface layer 300 is located on the plating layer 200 and may include a post-treatment layer 310, a Zn oxide layer 320, and an interdiffusion layer 330. The composition of the steel sheet 100 after the hot stamping process is similar to that of the steel sheet 100 before the hot stamping process as described in Figure 2, but its structure is different. After the hot stamping process of the present invention, the structure of the steel sheet 100 undergoes a phase transformation, and as an example, it contains 90% or more martensite and less than 10% of the remainder being other unavoidable structures and other precipitates.

[0072] The plating layer 200 after the hot stamping process can be formed by the diffusion of components between the steel sheet 100 of the hot stamping material 10 and the plating layer 200p. The plating layer 200 may contain at least one of Zn, Fe, Al, Mn, and Si. The composition of the plating layer 200 may, for example, include 10-70 wt% Fe, 0-5 wt% Al, 0-5 wt% Mn, 0-5 wt% Si, the remainder being Zn and other impurities, but is not limited to these. The plating layer 200 can be formed to a thickness of approximately 5 μm to 50 μm. If the thickness of the plating layer 200 is less than 5 μm, corrosion resistance will decrease, and if the thickness exceeds 50 μm, productivity may decrease.

[0073] The surface layer 300 may be formed by the diffusion of components of the steel sheet 100 and the plating layer 200p of the hot stamping material 10 in the pre-post-treatment layer 300p. Alternatively, during the hot stamping process, the components of the plating layer 200 of the hot stamped part 20, which is formed by the diffusion of components of the steel sheet 100 and the plating layer 200p of the hot stamping material 10, may be formed by the diffusion of these components into the post-treatment layer 300p. The surface layer 300 is located on the plating layer 200 and may include a post-treatment layer 310, a Zn oxide layer 320, and an interdiffusion layer 330.

[0074] The post-treatment layer 310 is located on the plating layer 200 and may contain an inorganic post-treatment agent. For example, the post-treatment layer 310 may contain a Si-based inorganic post-treatment agent, and in relation to the pre-post-treatment layer 300p, the post-treatment layer 310 may further contain silicon oxide such as SiO, which is obtained by oxidizing Si, and components diffused from the plating layer 200p and the steel sheet 100. The post-treatment layer 310 may be formed by mixing the Si-based post-treatment agent, the oxide of the post-treatment agent, and the components of the plating layer 200 on the steel sheet 100. Alternatively, it may be formed by mixing the components of the steel sheet 100 and the plating layer 200p of the hot stamping material 10. For example, the components diffused from the steel sheet 100 and the plating layer 200, 200p may include at least one of Si, O, Mn, Zn, and Fe.

[0075] The post-treatment layer 310 can suppress and minimize the formation of a Zn oxide layer on the plating layer 200, where the Zn components of the plating layers 200 and 200p combine with oxygen, reducing paint adhesion. In this way, the post-treatment layer 310 can suppress the generation of Zn oxides that hinder binding with the paint layer and induce the generation of Si oxides that improve binding with the paint layer, ultimately improving the paint adhesion of the hot-stamped part 20. The average Zn content of the post-treatment layer 310 is lower than the average Zn content of the plating layer 200. The average Zn content of the post-treatment layer 310 is also higher than the average Si content of the post-treatment layer 310. As an example, a phosphate-containing phosphate treatment layer may be further interposed between the post-treatment layer 310 and the paint layer (not shown) formed thereon, and the post-treatment layer 310 may have a strong bonding force with such a phosphate treatment layer, thereby improving paint adhesion.

[0076] The average thickness h3 of the post-treatment layer 310 is thinner than the average thickness h4 of the Zn oxide layer 320, which will be described later. The average thickness h3 of the post-treatment layer 310 is also approximately 5% to less than approximately 100% of the average thickness h4 of the Zn oxide layer 200. For example, the average thickness h3 of the post-treatment layer 310 is also approximately 0.5 μm to 3 μm, and if it is less than 0.5 μm, the paintability may decrease. The area fraction of the post-treatment layer 310 relative to the overall surface layer 300 is also approximately 20% to 100%, and if it is less than 20%, the paintability may decrease. The post-treatment layer 310 may contain at least one of SiO, Zn, Fe, and Mn. For example, the composition of the post-treatment layer 310 is as follows: Si 20-90 wt%, Mn 0-15 wt%, O 10-80 wt%, Fe 0-15 wt%, and Zn 0-15 wt%.

[0077] [Table 2]

[0078] The Zn oxide layer 320 may be located on the plating layer 200 in the same layer as the post-treatment layer 310. The Zn oxide layer 320 may be formed when the zinc in the plating layer 200 is oxidized during the hot stamping heat treatment process. The Zn oxide layer 320 may cause problems such as a decrease in the surface quality and paint adhesion of the part. The average thickness h4 of the Zn oxide layer 320 is about 1 μm to 10 μm, and when it exceeds 10 μm, the paintability of the part may decrease. In this invention, the decrease in surface quality and paintability is addressed by reducing the thickness and area fraction of the Zn oxide layer 320 through the formation of a post-treatment layer 310 composed of the aforementioned inorganic post-treatment agent and an interdiffusion layer 330 formed therefrom, which will be described later. The interdiffusion layer 330 of this invention may be located between the plating layer 200 and at least one of the post-treatment layer 310 and the Zn oxide layer 320, overlapping with at least one of the post-treatment layer 310 and the Zn oxide layer 320. In this drawing, the interdiffusion layer 330 is illustrated in an example where at least a portion of the interdiffusion layer 330 overlaps with the post-treatment layer 310 and the plating layer 200. The interdiffusion layer 330 may contain at least one of Si, Mn, O, Fe, Zn, and SiO. As an example, the composition of the interdiffusion layer 330 is as follows: Si 15-35 wt%, Mn 0-15 wt%, O 35-80 wt%, Fe 0-15 wt%, and Zn 5-40 wt%. The average Zn content of the interdiffusion layer 330 is also greater than the average Zn content of the post-treatment layer 310 mentioned above.

[0079] [Table 3]

[0080] The area fraction of the interdiffusion layer 330 is approximately 10% to 80% of the total area fraction of the post-treatment layer 310. Only when the interdiffusion layer 330 accounts for 10% or more of the post-treatment layer 310 can the bonding force between the plating layer 200 and the coating layer (not shown) above it be ensured, thereby sufficiently reducing the Zn oxide layer 320 and ensuring the necessary coating properties. The average thickness of the interdiffusion layer 330 is, for example, approximately 0.1 μm to 2 μm. The average thickness of the interdiffusion layer 330 is also approximately 10% to 80% of the average thickness of the post-treatment layer 310.

[0081] Thus, according to one embodiment of the present invention, the surface layer 300, which includes a post-treatment layer 310 and an interdiffusion layer 330 formed from the (first) post-treatment layer 300p through hot stamping heat treatment, can improve the paint adhesion of the hot stamped part 20 by additional bonding force with the plating layer 200. Furthermore, by forming an interdiffusion layer 330 using an inorganic post-treatment agent, the Zn of the plating layer 200 is not diffused to its surface but rather consumed by the interdiffusion layer 330, that is, by forming an interdiffusion layer 330 with a higher average Zn content than the post-treatment layer 310, the amount of Zn oxide layer 320 can be reduced, ultimately improving the paint adhesion.

[0082] Figure 5 is a magnified TEM image 21 of a portion of the part after the hot stamping process shown in Figure 3. The content that is substantially the same as described above can be omitted or simplified.

[0083] Referring to Figure 5, the hot stamping part 21 is shown, consisting of a Zn plating layer 200, a surface layer 300 located thereon, and a sampling plate 400 for analytical sampling applied on the surface layer 300. The surface layer 300, including a second post-treatment layer 310 and an interdiffusion layer 330, plays a role in binding the underlying plating layer 200 and the coating layer (not shown) coated on top of it to each other. The interdiffusion layer 330 performs binding between the plating layer 200 and the post-treatment layer 310, ultimately improving the bonding strength between the plating layer 200 and the coating layer, thereby increasing the surface properties and paint adhesion of the hot stamping part 20.

[0084] Figure 6 is a diagram comparing the adhesion evaluation of hot stamped parts according to comparative examples and hot stamped parts according to one embodiment of the present invention. Comparative Example 1 is a case where the inorganic post-treatment agent of the present invention is not applied to alloyed hot-dip galvanized steel sheet (GA) and the shot peening process is not performed. Comparative Example 2 is a case where the shot peening post-treatment process is performed on the same GA steel sheet using shot peening powder (the inorganic post-treatment agent of the present invention is not applied). Example 1 shows an example of a hot stamped part manufactured according to one embodiment of the present invention (no post-treatment process such as shot peening is performed).

[0085] The adhesion strength evaluation in Figure 6 is an example of performing a Dolly test. In this test, the adhesion strength of the parts can be evaluated by how much material is removed from the surface of the test parts when the dolly is fixed to the hot stamping parts prepared by each comparative example and example using adhesive and then separated.

[0086] First, Comparative Example 1, an example in which the shot peening process was not performed, shows that a considerable portion of the painted or plated layer on the surface of the part adheres to the dolly, confirming a decrease in paintability. On the other hand, Comparative Example 2 is an example in which the shot peening process was performed to improve the paintability of a conventional hot-stamped part, and it can be seen that the surface of the hot-stamped part is almost completely retained even after the dolly is removed, indicating improved paintability. Referring to Example 1 of the present invention, even when post-treatment processes such as shot peening or shot blasting are not performed and the inorganic post-treatment agent of the present invention is used, the surface of the part is almost completely retained. In other words, it can be confirmed that by using the inorganic post-treatment agent of the present invention, the same surface quality, paintability, and adhesion as when complex and expensive processes such as shot peening are performed can be ensured without them.

[0087] Although the present invention has been described based on one embodiment illustrated in the drawings, this is merely an example, and a person with ordinary skill in the art will understand that various modifications and variations of the embodiment are possible therefrom. Therefore, the true scope of technical protection of the present invention must be determined by the technical idea of ​​the claims.

Claims

1. Steel plate and, Located on the aforementioned steel plate, a plating layer containing Zn, The plated layer includes a surface layer located on the aforementioned plating layer, The aforementioned surface layer is A post-treatment layer containing a Si-based inorganic post-treatment agent, A Zn oxide layer located on the aforementioned plating layer in the same layer as the post-treatment layer, The plated layer comprises at least one of the post-treatment layer and the Zn oxide layer, and an interdiffusion layer located between the post-treatment layer and the at least one of the Zn oxide layer, which is superimposed on the post-treatment layer and the Zn oxide layer and contains at least one of Si, Mn, O, Fe, Zn, and SiO. The tensile strength of the steel plate is 1,680 MPa or more. The average Zn content of the interdiffusion layer is greater than the average Zn content of the post-treatment layer, and the average Zn content of the Zn oxide layer is greater than the average Zn content of the interdiffusion layer. A hot stamping part wherein the area fraction of the interdiffusion layer is 10% or more and 80% or less of the total area fraction of the post-processing layer.

2. The hot stamped part according to claim 1, wherein the post-treatment layer comprises an oxide of the Si-based inorganic post-treatment agent and at least one of Si, Mn, O, Fe, Zn, and SiO as a component diffused from the steel sheet and the plating layer.

3. The average thickness of the post-treatment layer is thinner than the average thickness of the Zn oxide layer. The hot stamping part according to claim 1, wherein the average thickness of the post-treatment layer is 5% or more and less than 100% of the average thickness of the Zn oxide layer.

4. The average thickness of the post-treatment layer is 0.5 μm to 3 μm. The hot stamping part according to claim 3, wherein the average thickness of the Zn oxide layer is 1 μm to 10 μm.

5. The hot stamping component according to claim 4, wherein the average thickness of the interdiffusion layer is 0.1 μm to 2 μm.

6. A heating step in which a steel plate with a Si-based inorganic post-treatment agent applied to a Zn-containing plating layer is placed in a heating furnace and heated, A transfer step of transferring the heated steel plate from the heating furnace to a press die, The molding step involves hot stamping the transferred steel plate to form a molded body, The step includes cooling the molded body, In the aforementioned heating step, The inorganic post-treatment agent and the components of the plating layer are diffused to form an interdiffusion layer, a post-treatment layer, and a Zn oxide layer obtained by oxidizing the plating layer. The interdiffusion layer is located between the post-treatment layer and the Zn oxide layer and the plating layer, superimposed on the post-treatment layer and the Zn oxide layer, and contains at least one of Si, Mn, O, Fe, Zn, and SiO. The tensile strength of the steel plate is 1680 MPa or more. The average Zn content of the interdiffusion layer is greater than the average Zn content of the post-treatment layer, and the average Zn content of the Zn oxide layer is greater than the average Zn content of the interdiffusion layer. In the aforementioned heating step, The steel plate is heated in the heating furnace to a target heating temperature having a temperature range of Ac1 to 910°C. The steel plate is heated so that the average heating rate of the steel plate in the section from 700°C to the target heating temperature is 1.5°C / sec to 7°C / sec, thereby forming the interdiffusion layer. A method for manufacturing hot stamped parts, wherein the area fraction of the interdiffusion layer is formed to be 10% or more and 80% or less of the total area fraction of the post-treatment layer.

7. The method for manufacturing a hot stamped part according to claim 6, wherein the post-treatment layer contains at least one of Si, Mn, O, Fe, Zn, and SiO as an oxide of the inorganic post-treatment agent and a component diffused from the steel sheet and the plating layer.

8. The average thickness of the post-treatment layer is thinner than the average thickness of the Zn oxide layer. The method for manufacturing a hot stamped part according to claim 6, wherein the average thickness of the post-treatment layer is 5% or more of the average thickness of the Zn oxide layer.

9. A heating step of placing a steel plate on which a Si-based inorganic post-treatment agent has been applied to a plating layer containing Zn into a heating furnace and heating it, A transfer step of transferring the heated steel plate from the heating furnace to a press die, The molding step involves hot stamping the transferred steel plate to form a molded body, The step includes cooling the molded body, In the aforementioned heating step, The inorganic post-treatment agent and the components of the plating layer are diffused to form an interdiffusion layer, a post-treatment layer, and a Zn oxide layer obtained by oxidizing the plating layer. The interdiffusion layer is located between the post-treatment layer and the Zn oxide layer and the plating layer, superimposed on the post-treatment layer and the Zn oxide layer, and contains at least one of Si, Mn, O, Fe, Zn, and SiO. The tensile strength of the steel plate is 1680 MPa or more. The average Zn content of the interdiffusion layer is greater than the average Zn content of the post-treatment layer, and the average Zn content of the Zn oxide layer is greater than the average Zn content of the interdiffusion layer. In the aforementioned heating step, The steel plate is heated in the heating furnace to a target heating temperature having a temperature range of Ac1 to 910°C. The steel plate is heated so that the average heating rate of the steel plate in the section from 700°C to the target heating temperature is 1.5°C / sec to 7°C / sec, thereby forming the interdiffusion layer. The average thickness of the post-treatment layer is thinner than the average thickness of the Zn oxide layer. The average thickness of the post-treatment layer is 5% or more of the average thickness of the Zn oxide layer. The average thickness of the post-treatment layer is 0.5 μm to 3 μm. A method for manufacturing hot-stamped parts, wherein the average thickness of the Zn oxide layer is formed to be 1 μm to 10 μm.

10. The method for manufacturing a hot stamped part according to claim 9, wherein the average thickness of the interdiffusion layer is formed to be 0.1 μm to 2 μm.

11. In the aforementioned heating step, The method for manufacturing a hot stamped part according to claim 6, wherein the steel plate is heated while remaining in place for 120 to 600 seconds.

12. Prior to the aforementioned heating step, The method for manufacturing a hot stamped part according to claim 6, further comprising a post-treatment step of applying the Si-based inorganic post-treatment agent onto the steel sheet on which the plating layer is formed, and drying to form a pre-post-treatment layer.

13. In the aforementioned post-processing step, The inorganic post-treatment agent is applied to the steel plate to a thickness of 0.5 μm to 3 μm to form the pre-post-treatment layer, and the amount of the inorganic post-treatment agent applied is 0.5 g / m². ２ ~3g / m ２ The method for manufacturing hot stamped parts according to claim 12.

14. In the aforementioned post-processing step, The method for manufacturing hot stamped parts according to claim 12, wherein the steel plate to which the inorganic post-treatment agent has been applied is dried at a temperature of 70°C to 150°C for 1 to 10 seconds.

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