Hot stamped parts

Abstract

The present invention relates to a composition containing carbon (C): 0.15 to 0.27 wt%, silicon (Si): 0.15 to 1.0 wt%, manganese (Mn): 0.5 to 1.10 wt%, phosphorus (P): 0.018 wt% or less, sulfur (S): 0.005 wt% or less, chromium (Cr): 0.1 to 1.0 wt%, aluminum (Al): 0.1 to 1.0 wt%, titanium (Ti): 0.015 to 0.080 wt%, niobium (Nb): 0.015 to 0.080 wt%, molybdenum (Mo): 0.1 The present invention provides a hot-stamped part including a base steel sheet containing 0.01 wt% or less of Cr, 0.0 ...5 wt% or less of Cr, 0.005 wt% or less of Cr, and the remainder being iron (Fe) and other unavoidable impurities, wherein the base steel sheet includes a martensitic structure, and the nano-indentation hardness of the martensitic structure is 3.0 GPa or more and 5.0 GPa or less, and the standard deviation of the nano-indentation hardness is 0.8 GPa or less.

Description

[Technical Field]

[0001] The present invention relates to a hot stamped part. [Background technology]

[0002] High-strength steel is used in automotive parts to reduce weight and ensure stability. While high-strength steel can achieve high strength relative to its weight, as its strength increases, its press formability and bendability decrease, leading to breakage of the material during processing and springback, making it difficult to form products with complex and precise shapes.

[0003] One solution to address these issues is the hot stamping process, and as interest in this process grows, active research into materials for hot stamping is also being conducted. For example, as disclosed in Korean Patent Publication No. 10-2017-0076009, the hot stamping process is a forming technology in which a steel sheet for hot stamping is heated to a high temperature and then rapidly cooled while being formed in a press die to produce high-strength parts. Specifically, the hot stamping process generally consists of heating, forming, cooling, and trimming, and can utilize phase transformation and microstructural changes of the material during the process. The heating process of the hot stamping process involves heating a blank in a heating furnace, and the cooling process involves cooling the hot-stamped formed body in a die. Furthermore, the blank heated through the heating process can be exposed to room temperature and air-cooled while flowing from the heating furnace into the die.

[0004] Related technologies include Korean Patent Registration Publication No. 10-2070579 (title of invention: hot stamping method). Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide a hot stamped part having excellent mechanical properties such as high strength and high toughness by controlling the uniformity of the nano-indentation hardness of the martensitic structure, but this is merely an example and does not limit the scope of the present invention. [Means for solving the problem]

[0006] According to one aspect of the present invention, the composition of the present invention includes carbon (C): 0.15 to 0.27 wt%, silicon (Si): 0.15 to 1.0 wt%, manganese (Mn): 0.5 to 1.10 wt%, phosphorus (P): 0.018 wt% or less, sulfur (S): 0.005 wt% or less, chromium (Cr): 0.1 to 1.0 wt%, aluminum (Al): 0.1 to 1.0 wt%, titanium (Ti): 0.015 to 0.080 wt%, niobium (Nb): 0.015 to 0.080 wt%, molybdenum (Mo): The present invention provides a hot-stamped part including a base steel sheet containing 0.1 to 0.7 wt% of manganese (Mn): 0.001 to 0.008 wt% of boron (B): 0.001 to 0.008 wt% of manganese (Mn): 0.005 wt% or less of nitrogen (N): 0.005 wt% or less, and the remainder being iron (Fe) and other unavoidable impurities, wherein the base steel sheet includes a martensitic structure, and the nano-indentation hardness of the martensitic structure is 3.0 GPa or more and 5.0 GPa or less, and the standard deviation of the nano-indentation hardness is 0.8 GPa or less. [Effects of the Invention]

[0007] According to one embodiment of the present invention configured as described above, the uniformity of the nano-indentation hardness of the martensitic structure can be controlled, thereby ensuring that the hot stamped part has excellent mechanical properties such as high strength and high toughness. Of course, the scope of the present invention is not limited to these effects. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a flow chart that schematically illustrates a method for manufacturing a hot stamped component according to an embodiment of the present invention. [Figure 2]2 is a flowchart illustrating a heating step of a method for manufacturing a hot stamped component according to an embodiment of the present invention. [Figure 3] 1 is a view illustrating a heating furnace having multiple sections in a heating step of a method for manufacturing a hot stamped part according to an embodiment of the present invention; [Figure 4] 1 is a diagram showing a temperature rise rate change rate in a plurality of sections depending on heating time in a method for manufacturing a hot stamped part according to an embodiment of the present invention; [Figure 5] 1 is a diagram showing heating time depending on material thickness and heating temperature. [Figure 6] 1 is a flowchart illustrating a method for manufacturing a blank for producing a hot stamped component according to an embodiment of the present invention. [Figure 7] FIG. 1 is a cross-sectional view showing a schematic view of a portion of a hot stamped component according to an embodiment of the present invention. [Figure 8] 1 is a photomicrograph showing a cross section of a base steel sheet of a hot stamped component according to an embodiment of the present invention. [Figure 9] 1 is a photomicrograph showing a cross section of a base steel sheet of a hot stamped component according to an embodiment of the present invention. [Figure 10] 1 is a photomicrograph showing a cross section of a base steel sheet of a hot-stamped part according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0009] In this embodiment, the bend angle of the hot stamped part is 70° or more and 85° or less.

[0010] In this embodiment, when the value obtained by dividing the standard deviation of the nanoindentation hardness by the average of the nanoindentation hardness is called a coefficient of variation, the coefficient of variation is 0.2 or less.

[0011] In this embodiment, the standard deviation of the carbon (C) content of the martensite structure is less than 0.04 wt%.

[0012] In this embodiment, the hot stamped component further includes fine precipitates distributed in the base steel sheet, and the fine precipitates include carbides of at least one of titanium (Ti), niobium (Nb), and molybdenum (Mo).

[0013] In this embodiment, the unit area (100 μm 2 The number of the fine precipitates distributed per 1000 particles is 9,000 or more and 30,000 or less.

[0014] In this embodiment, the average diameter of the fine precipitates is 0.003 μm or more and 0.006 μm or less.

[0015] In this embodiment, the tensile strength of the hot stamped part is 1,350 MPa or more and 1,650 MPa or less.

[0016] In this embodiment, the hot stamped part has a yield strength of 950 MPa or more and 1,200 MPa or less.

[0017] In this embodiment, the elongation of the hot stamped part is 6% or more.

[0018] In this embodiment, the martensitic structure includes a plurality of lath structures.

[0019] In this embodiment, the hot stamped part further includes a plating layer disposed on the base steel sheet.

[0020] Other aspects, features, and advantages beyond those described above will become apparent from the following detailed description of the invention, the claims, and the drawings.

[0021] While the present invention can be modified in various ways and can have various embodiments, specific embodiments are illustrated in the drawings and will be described in detail in the detailed description. The advantages and features of the present invention, as well as methods for achieving them, will become more apparent with reference to the following detailed description of the embodiments, taken in conjunction with the drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms.

[0022] In the following embodiments, terms such as first and second are not used in a limiting sense but are used to distinguish one component from another component.

[0023] In the following embodiments, singular expressions include plural expressions unless the context clearly dictates otherwise.

[0024] In the following embodiments, the terms "comprise" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0025] In the following embodiments, when a part such as a film, region, or component is said to be on top of or above another part, this does not only mean that it is directly on top of the other part, but also means that there is another film, region, component, etc. interposed between them.

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

[0027] In this specification, "A and / or B" means A, B, or A and B. Also, in this specification, "at least one of A and B" means A, B, or A and B.

[0028] In the following embodiments, "on a plane" means when the target part is viewed from above, and "on a cross section" means when the target part is viewed from the side along a vertical cross section. In the following embodiments, "superimposition" means "on a plane" and "on a cross section" superimposition.

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. When referring to the drawings, the same or corresponding elements are designated by the same reference numerals.

[0030] Fig. 1 is a flowchart schematically illustrating a method for manufacturing a hot-stamped component according to an embodiment of the present invention, and Fig. 2 is a flowchart specifically illustrating a heating step of the method for manufacturing a hot-stamped component according to an embodiment of the present invention. The method for manufacturing a hot-stamped component will be described below with reference to Figs. 1 and 2.

[0031] Referring to FIG. 1 , in one embodiment, a method for manufacturing a hot-stamped part includes a blank-introducing step (S100), a heating step (S200), a transferring step (S300), a forming step (S400), and a cooling step (S500). According to an embodiment, the method for manufacturing a hot-stamped part of the present invention further includes a blank-preparing step (S1). The blank-preparing step (S1) refers to a step of preparing a hot-stamping blank (hereinafter simply referred to as a "blank") for manufacturing a hot-stamped part according to an embodiment of the present invention, which will be described in detail with reference to FIG. 6 below.

[0032] First, the blank introduction step (S100) involves introducing a blank into a heating furnace having a plurality of sections with different heating rate ranges. The blank is prepared in the form of a base steel sheet having a coating layer formed on at least one surface. The base steel sheet is a base steel sheet, and may be manufactured by hot rolling and / or cold rolling a steel slab that is cast to contain predetermined alloying elements in predetermined amounts.

[0033] In one embodiment, in the blank loading step (S100), the blank loaded into the heating furnace is mounted on rollers and then transported along the transport direction.

[0034] 1 and 2, a heating step (S200) is performed after a blank insertion step (S100). In one embodiment, the heating step (S200) includes a multi-stage heating step (S210) and a soaking heating step (S220). Therefore, after the blank insertion step (S100), the multi-stage heating step (S210) and the soaking heating step (S220) are performed. The multi-stage heating step (S210) and the soaking heating step (S220) are steps in which the blank is heated while passing through multiple sections provided in a heating furnace.

[0035] In one embodiment, the overall temperature of the heating furnace is 680° C. to 1000° C. Specifically, the overall temperature of the heating furnace in which the multi-stage heating step (S210) and the soaking heating step (S220) are performed is 680° C. to 1000° C. At this time, the temperature of the heating furnace in which the multi-stage heating step (S210) is performed is 680° C. to Ac3, and the temperature of the heating furnace in which the soaking heating step (S220) is performed is Ac3 to 1000° C.

[0036] Specifically, in the multi-stage heating step (S210), the blank is heated in stages as it passes through multiple sections provided in the heating furnace. Among the multiple sections provided in the heating furnace, there are multiple sections in which the multi-stage heating step (S210) is performed, and the temperature of the blank is set to increase from the entrance of the heating furnace where the blank is inserted toward the exit of the heating furnace where the blank is removed, so that the temperature of the blank is raised in stages.

[0037] After the multi-stage heating step (S210), a soaking heating step (S220) is performed. In the soaking heating step (S220), the multi-stage heated blank is heat-treated while passing through sections of a heating furnace set at a temperature of Ac3 to 1000°C. Preferably, in the soaking heating step (S220), the multi-stage heated blank may be soaked at a temperature of 830°C to 1000°C. In addition, the soaking heating step (S220) is performed in at least one section among the multiple sections provided in the heating furnace.

[0038] FIG. 3 is a view illustrating a heating furnace having multiple sections in a heating step of a method for manufacturing a hot stamped part according to an embodiment.

[0039] 3, a heating furnace according to one embodiment includes multiple sections P1, P2, P3, and P4 having different temperature ranges. More specifically, the heating furnace includes a first heating section P1 having a first temperature range T1, a second heating section P2 having a second temperature range T2, a third heating section P3 having a third temperature range T3, and a fourth heating section P4 having a fourth temperature range T4. Here, the third heating section P3 includes two sections having different temperature ranges. The third heating section P3 includes a 3-1 heating section P3-1 having a 3-1 temperature range T3-1 and a 3-2 heating section P3-2 having a 3-2 temperature range T3-2.

[0040] In one embodiment, the second heating section P2 includes multiple sections having different temperature ranges. For example, the second heating section P2 includes a 2-1 heating section P2-1 having a 2-1 temperature range T2-1 and a 2-2 heating section P2-2 having a 2-2 temperature range T2-2. However, the present invention is not limited thereto. The second heating section P2 includes a 2-1 heating section P2-1 having a 2-1 temperature range T2-1 through a 2-n heating section P2-n having a 2-n temperature range T2-n, where n is a natural number greater than or equal to 2.

[0041] In one embodiment, the first heating section P1 also includes multiple sections having different temperature ranges. For example, the first heating section P1 includes a first-1 heating section P1-1 having a first-1 temperature range T1-1 and a first-2 heating section P1-2 having a first-2 temperature range T1-2. However, the present invention is not limited thereto. The first heating section P1 includes a first-1 heating section P1-1 having a first-1 temperature range T1-1 through a first-n heating section P1-n having a first-n temperature range T1-n, where n is a natural number greater than or equal to 2.

[0042] 2 and 3, in one embodiment, in the multi-stage heating step (S210), the blank is heated in stages (or multi-stage heated) while passing through a first heating section P1, a second heating section P2, and a third-first heating section P3-1 defined in a heating furnace. In addition, in the soaking heating step (S220), the multi-stage heated blank is soaked while passing through a third-second heating section P3-2 and a fourth heating section P4. That is, the first heating section P1, the second heating section P2, and the third-first heating section P3-1 correspond to sections in which the blank is multi-stage heated, and the third-second heating section P3-2 and the fourth heating section P4 correspond to sections in which the blank is soaked.

[0043] In one embodiment, the fourth heating section P4 includes multiple sections, such as two sections or three sections, and the temperature ranges (or temperatures) of the multiple sections included in the fourth heating section P4 are the same.

[0044] The first heating zone P1 through the fourth heating zone P4 are arranged in order within the heating furnace. The first heating zone P1 is adjacent to the entrance of the heating furnace where the blanks are inserted, and the fourth heating zone P4 is adjacent to the exit of the heating furnace where the blanks are removed. Therefore, the first heating zone P1 having the first temperature range T1 is the first zone of the heating furnace, and the fourth heating zone P4 having the fourth temperature range T4 is the last zone of the heating furnace. As will be described later, among the multiple zones of the heating furnace, the third-second heating zone P3-2 and the fourth heating zone P4 are zones where soaking heating is performed rather than zones where multi-stage heating is performed.

[0045] The temperatures of the multiple zones within the heating furnace, for example, the temperatures of the first heating zone P1 to the fourth heating zone P4, increase from the entrance of the heating furnace where the blanks are inserted toward the exit of the heating furnace where the blanks are removed. The temperature difference between two adjacent zones within the heating furnace may be greater than 0°C and less than 100°C. For example, the temperature difference between the first heating zone P1 and the second heating zone P2 is greater than 0°C and less than 100°C.

[0046] In one embodiment, the first temperature range T1 of the first heating section P1 is 680°C to 870°C. The second temperature range T2 of the second heating section P2 is 700°C to 930°C. The third-first temperature range T3-1 of the third-first heating section P3-1 is 800°C to 950°C. The third-second temperature range T3-2 of the third-second heating section P3-2 is AC3 to 1000°C. The fourth temperature range T4 of the fourth heating section P4 is AC3 to 1000°C. Preferably, the fourth temperature range T4 of the fourth heating section P4 is 830°C to 1000°C. The third-second temperature range T3-2 of the third-second heating section P3-2 and the fourth temperature range T4 of the fourth heating section P4 are the same.

[0047] In one embodiment, when the second heating section P2 includes the 2-1 heating section P2-1 and the 2-2 heating section P2-2 having different temperature ranges as described above, the 2-1 temperature range T2-1 is 700°C to 900°C, and the 2-2 temperature range T2-2 of the 2-2 heating section P2-2 is 750°C to 930°C.

[0048] The boundary values ​​defining the aforementioned multiple sections will now be described. The boundary values ​​are represented by the heating time (s) on the horizontal axis of the graph. First, the first boundary value e1 located between the first heating section P1 and the second heating section P2 is about 30 to about 50 seconds, and may be about 40 seconds. The second boundary value e2 located between the second heating section P2 and the third heating section P3 is about 80 to about 130 seconds, and may be about 85 seconds. The third boundary value e3 located between the third-first heating section P3-1 and the third-second heating section P3-2 is about 110 to about 180 seconds, and may be about 120 seconds. The fourth boundary value e4 located between the third-second heating section P3-2 and the fourth heating section P4 is about 140 to about 230 seconds, and may be about 150 seconds.

[0049] In one embodiment, when the second heating section P2 includes the 2-1 heating section P2-1 and the 2-2 heating section P2-2 having different temperature ranges as described above, the 2-1 boundary value e2' located between the 2-1 heating section P2-1 and the 2-2 heating section P2-2 is about 50 s to about 110 s, and may be about 60 s.

[0050] 3, the heating furnace according to one embodiment of the present invention is shown to have five sections P1, P2, P3-1, P3-2, and P4, each having a different temperature range, but the present invention is not limited thereto. The heating furnace may have six, seven, eight, or more sections having different temperature ranges.

[0051] In one embodiment, the heating furnace has a length of 20 to 40 meters along the blank transfer path. The heating furnace has multiple sections with different temperature ranges, and the ratio of the length of the section in which the blank is heated in multiple stages to the length of the section in which the blank is soaked is 1:1 to 4:1. If the length of the section in which the blank is soaked in the heating furnace increases and the ratio of the length of the section in which the blank is soaked to the length of the section in which the blank is soaked exceeds 1:1, the amount of hydrogen permeation into the blank in the soaking section increases, increasing the risk of delayed fracture. On the other hand, if the length of the section in which the blank is soaked decreases and the ratio of the length of the section in which the blank is soaked to the length of the section in which the blank is soaked is less than 4:1, the soaking section (time) is insufficient, which may result in non-uniform strength of the hot stamped part manufactured by the hot stamped part manufacturing process.

[0052] In one embodiment, the length of the uniform heating section among the sections provided in the heating furnace is 20% to 50% of the total length of the heating furnace.

[0053] 4 is a graph showing the rate of change in temperature rise rate for multiple sections as a function of heating time in a method for manufacturing a hot stamped part according to an embodiment of the present invention. In this case, FIG. 4 shows a graph of the temperature rise rate (°C / s) of a blank as a function of heating time (s). The multiple sections and boundary values ​​shown in FIG. 4 are the same as those described above in FIG. 3, and their descriptions may be simplified or omitted.

[0054] Referring to FIG. 4, the heating rate (°C / s) or the rate of change of the heating rate (°C / s) in the multiple sections where the blank is heated in multiple stages is shown. 2 ) will be described later. Hereinafter, the "temperature rise rate change rate" refers to the average slope of each section of the graph shown in Figure 4, and will be referred to as the "average temperature rise rate change rate." Figure 4 shows a first temperature rise rate control curve 410 according to one embodiment of the present invention and a second temperature rise rate control curve 420 according to a comparative embodiment.

[0055] First, a first control curve 410 of the temperature rise rate according to one embodiment of the present invention will be described.

[0056] The first heating section P1 has a first average heating rate change rate r1. The second heating section P2, located after the first heating section P1, has a second average heating rate change rate r2 that is different from the first average heating rate change rate r1. The third heating section P3, located after the second heating section P2, has a third average heating rate change rate r3 that is different from the first average heating rate change rate r1 and the second average heating rate change rate r2. Here, the third average heating rate change rate r3 includes a section where it changes from a positive value to a negative value. The fourth heating section P4, located after the third heating section P3, has a fourth average heating rate change rate r4 that is different from the first average heating rate change rate r1, the second average heating rate change rate r2, and the third average heating rate change rate r3.

[0057] The first heating section P1 is a general heating section, and the second heating section P2 has a gradually decreasing heating rate (|r1| > |r2|) compared to the first heating section P1, allowing alloying of the coating layer. The third heating section P3 is a phase transformation section where the blank's base steel sheet undergoes phase transformation. The third-first heating section P3-1 has a positive (+) heating rate change rate, and the third-second heating section P3-2 has a negative (-) heating rate change rate. The fourth heating section P4 is a stabilization section where the blank is heated to a uniform temperature.

[0058] Referring to the first control curve 410, the first average rate of change of heating rate r1 and the second average rate of change of heating rate r2 each have a negative value, and the absolute value of the first average rate of change of heating rate r1 is greater than the absolute value of the second average rate of change of heating rate r2 (|r1|>|r2|). In one embodiment, the first average rate of change of heating rate r1 is approximately -0.5°C / s 2 The first average temperature rise rate change rate r1 is about -0.3°C / s 2 In one embodiment, the second average heating rate change rate r2 is about −0.25° C. / s 2 For example, the second average temperature rise rate change rate r2 is about -0.07°C / s 2In one embodiment, the change from the first average heating rate change rate r1 to the second average heating rate change rate r2 is discontinuous between the first heating section P1 and the second heating section P2, i.e., near the first boundary value e1. Specifically, the heating rate v1 at the first boundary value e1 that defines the first average heating rate change rate r1 in the first heating section P1 and the heating rate v2 at the first boundary value e1 that defines the second average heating rate change rate r2 in the second heating section P2 have different values. In other words, the final heating rate v1 of the first average heating rate change rate r1 and the initial heating rate v2 of the second average heating rate change rate r2 have different values. When the heating rate change rate changes discontinuously (r1 → r2) near the first boundary value e1 (410), the weldability of the hot stamped part can be improved compared to when it changes continuously (420).

[0059] Because the coating layer transformation requires a large amount of energy, the average heating rate change rate changes discontinuously between the first heating section P1 and the second heating section P2. The necessary energy must be supplied for the Fe from the base steel sheet to diffuse into the aluminum coating layer and for the initial formation and growth of the Al-Fe phase within the coating layer. Furthermore, the Fe diffused into the base steel sheet forms an Al-Fe-Si alloy layer over time. The more discontinuous the change in the heating rate change rate near the first boundary value e1, the more uniform the diffusion to the surface, resulting in better weldability. On the other hand, if the change is continuous, the Al-Fe-Si diffusion to the surface will be rapid and uneven, resulting in the formation of a phase with high welding resistance on the surface, resulting in reduced weldability.

[0060] In one embodiment, the third heating section P3 includes a third heating section P3-1 having a third-1 average heating rate change rate r3-1 and a third heating section P3-2 having a third-2 average heating rate change rate r3-2. The third-1 average heating rate change rate r3-1 has a positive value, the third-2 average heating rate change rate r3-2 has a negative value, and the third average heating rate change rate r3 has a section where it changes from positive to negative. Here, the absolute value of the third-1 average heating rate change rate r3-1 is smaller than the absolute value of the third-2 average heating rate change rate r3-2 (|r3-1|<|r3-2|). In one embodiment, the third-1 average heating rate change rate r3-1 is greater than or equal to 0 and less than about 0.25°C / s. 2 For example, the 3-1 average temperature rise rate change rate r3-1 is approximately 0.07°C / s 2 In one embodiment, the third-second average heating rate change rate r3-2 is about -0.3°C / s 2 For example, the 3-2 average temperature rise rate change rate r3-2 is approximately -0.08°C / s 2 is.

[0061] The smaller the average heating rate change rate r3-1 in the heating section P3-1, the more gradual the slope of the first control curve 410. The more gradual the slope of the first control curve 410, the less hydrogen is trapped, thereby improving hydrogen embrittlement. In contrast, the second control curve 420 exhibits a rapid or discontinuous increase in the heating rate change rate in the heating section P3-1. In such cases, the amount of hydrogen trapped increases, thereby worsening hydrogen embrittlement. Unlike the section between the first heating section P1 and the second heating section P2, the third heating section P3 is a section where the phase transformation of the base steel sheet occurs. Since sudden temperature changes can cause problems such as hydrogen embrittlement and delayed fracture, a lower heating rate change rate is advantageous.

[0062] Between the second heating section P2 and the -1 heating section P3-1, i.e., near the second boundary value e2, the change in the second average heating rate change rate r2 to the 3-1 average heating rate change rate r3-1 changes from a negative value to a positive value. That is, the heating rate decreases and then increases as the phase transformation of the base steel sheet occurs. For example, during the phase transformation of the base steel sheet, an endothermic reaction occurs during the transformation to austenite in this section, and this requires an energy supply. Therefore, the heating rate must increase again in the 3-1 heating section P3-1 to induce a reasonable level of phase transformation to austenite.

[0063] Between the 3-1 heating section P3-1 and the 3-2 heating section P3-2, i.e., near the third boundary value e3, the change in the 3-1 average heating rate change rate r3-1 to the 3-2 average heating rate change rate r3-2 changes from a positive value to a negative value. That is, the heating rate increases and then decreases as the phase transformation of the base steel sheet occurs.

[0064] In one embodiment, the absolute value of the fourth average heating rate change rate r4 is smaller than the absolute values ​​of the first average heating rate change rate r1, the second average heating rate change rate r2, and the third average heating rate change rate r3. For example, the fourth average heating rate change rate r4 is close to 0, and the fourth heating section P4 is a section where heating is performed at a uniform temperature.

[0065] The time t4 during which the blank is heated in the third-second heating section P3-2 and the fourth heating section P4 is approximately 50% or less of the total heating time t. This is because the longer the time t4 during which the blank is soaked in the third-second heating section P3-2 and the fourth heating section P4 compared to the time t1 during which the blank is heated in multiple stages in the first heating section P1, the second heating section P2, and the third-first heating section P3-1, the more the part properties such as weldability, hydrogen embrittlement, and bendability deteriorate.

[0066] The characteristics of the second control curve 420, compared to the first control curve 410 described above, will now be described, focusing on the differences from the first control curve 410. Referring to the second control curve 420, the first' average heating rate change rate r1' changes continuously between the first heating section P1 and the second heating section P2. Specifically, the heating rate at the first boundary value e1 that defines the first' average heating rate change rate r1' in the first heating section P1 and the heating rate v1' at the first boundary value e1 that defines the first' average heating rate change rate r1' in the second heating section P2 have equal values.

[0067] In one embodiment, the first average heating rate change rate r1' is about -0.26°C / s 2 The first average temperature rise rate change rate r1' is about -0.2°C / s 2 is.

[0068] The change characteristics of the temperature rise rate change rate (r3'; r3-1', r3-2') in the third heating section P3 of the second control curve 420 are the same as those described for the first control curve 410. However, the 3-1' temperature rise rate change rate r3-1' has a discontinuous and unstable value compared to the 3-1 average temperature rise rate change rate r3-1 of the first control curve 410. In this case, the 3-1' temperature rise rate change rate r3-1' refers to the rate of change at the front end of the 3-1 heating section P3-1 where the temperature rise rate shows an increasing tendency. The 3-1' temperature rise rate change rate r3-1' is approximately 0.04°C / s 2 Approximately 0.16℃ / s 2 For example, the temperature rise rate change rate r3-1' is about 0.1°C / s 2 The temperature rise rate change rate r3-2' is about -0.16℃ / s 2 Approximately -0.04℃ / s 2 For example, the temperature rise rate change rate r3-2' is about -0.1°C / s 2 The fourth heating section P4 of the second control curve 420 is a soaking heating section in which the fourth average heating rate change rate r4 has a value close to zero, similar to the first control curve 410.

[0069] As described above, in the method for manufacturing a hot stamped part according to an embodiment of the present invention, by controlling the temperature rise rate change rate for each section according to the characteristics of the plurality of sections as described above, it is possible to precisely control and improve part properties such as ultra-high strength properties, weldability, hydrogen embrittlement, and bendability of the hot stamped part.

[0070] The relationship between the heating time s and the boundary values ​​shown on the horizontal axis of Figure 4 is not limited to that shown in Figure 4, and may be modified in various ways as long as the part performance of the hot stamped part of the present invention is improved. Although the above description has been given of five sections, the sections may be divided differently depending on the distribution of the temperature rise rate change rate.

[0071] 5 is a graph showing the heating time depending on the thickness of the material and the heating temperature. Specifically, FIG. 5 is a graph showing the minimum heating time depending on the thickness of the material and the heating temperature. In FIG. 5, the heating temperature refers to the soaking temperature in the soaking heating step (S220), and the heating time refers to the total heating time in the heating step (S200).

[0072] 1, 2, and 5, it can be seen that when the thickness of the material is the same, the minimum heating time increases as the heating temperature decreases. Also, when the heating temperature is the same, it can be seen that when the thickness of the material increases, the minimum heating time increases.

[0073] If the heating time (e.g., total heating time) for the blank in the heating step (S200) is too short, the blank may not undergo sufficient phase transformation. On the other hand, if the heating time for the blank in the heating step (S200) is too long, the austenite grains may become coarse, reducing hydrogen embrittlement resistance, and the coating layer may become thicker, resulting in reduced weldability. Therefore, the heating time in the heating step (S200) must be adjusted. However, to adjust the heating time in the heating step (S200), various variables must be considered, including not only the heating temperature and blank thickness (e.g., material thickness), but also heat loss within the heating furnace due to factors such as the airtightness of the heating furnace, the atmosphere, and the heat source, as well as the blank's composition.

[0074] In one embodiment, the heating time of the blank in the heating step (S200) satisfies the following Equation 2.

[0075]

number

[0076] In the above formula 2, λ n is the heating time s, a n is the furnace heat loss correction factor, T n is the heating temperature (℃), b n is the Ac3 temperature correction coefficient, c n is the thickness sensitivity correction coefficient for the high-temperature material, t is the thickness of the material (mm), and the material refers to the blank, and the unit of heating time, s, is seconds.

[0077] Since different heat sources are used for different types of heating furnaces, the heat loss generated by each type of heating furnace is also different. n is a correction coefficient that takes into account the heat loss of the heating furnace, and has a value of about -0.60 or more and about -0.55 or less. n has units of s / (℃×mm).

[0078] If the components of each material are different, the temperatures at which the phase transformation occurs will be different. n is a correction coefficient that takes into account the Ac3 temperature difference due to the material components, and has a value of approximately 700 or more and approximately 900 or less. n has units of s / mm.

[0079] The thickness of the material affects the thermal conductivity of the material. n is a correction coefficient that takes into account the difference in thermal conductivity due to the thickness of the material at high temperatures, and has a value of approximately 0.7 to approximately 0.9. In this case, high temperature means 600°C or higher. However, high temperature can also mean 500°C or higher, or 700°C or higher.

[0080] Heating temperature T nmeans the soaking temperature in the soaking heating step (S220), and the heating temperature T n has a value of about Ac3 or more and about 1000° C. or less. The thickness of the material has a value of about 1 mm or more and about 2.6 mm or less.

[0081] In one embodiment, the heating time λ according to the formula n The heating time λ is between about 100 seconds and about 900 seconds. n If the heating time λ is less than 100 s, the blank may not undergo sufficient phase transformation. n If the heating time λ exceeds 900 seconds, not only will the austenite grains become coarse and hydrogen resistance deteriorate, but the plating layer will become thicker and weldability may deteriorate. n When the heating time satisfies the range of about 100 seconds or more and about 900 seconds or less, sufficient phase transformation occurs in the blank, coarsening of austenite grains is prevented or minimized, and deterioration of hydrogen embrittlement resistance and / or weldability is prevented or minimized.

[0082] Referring back to FIG. 1, after the heating step (S200), a transferring step (S300), a molding step (S400), and a cooling step (S500) are further performed.

[0083] In one embodiment, the transferring step (S300) is a step of transferring the heated blank from the heating furnace to the die. At this time, in the transferring step (S300), the heated blank is cooled to ambient temperature (or room temperature) while being transferred to the die. The heated blank may be air-cooled during transfer. If the heated blank is not air-cooled, the die entry temperature (e.g., the forming start temperature) may become high, which may cause wrinkles (or bends) on the surface of the manufactured hot-stamped part. In addition, since the use of a coolant may affect subsequent processes (hot stamping), it is preferable to air-cool the heated blank during transfer.

[0084] In one embodiment, the forming step (S400) is a step of hot stamping the transferred blank to form a compact. Specifically, in the forming step (S400), the blank is pressed in a mold to form a compact.

[0085] In one embodiment, the cooling step (S500) is a step of cooling the molded body. The cooling step (S500) is performed in a mold.

[0086] In one embodiment, the heated blank is cooled to ambient temperature (or room temperature) in the transfer step (S300). Specifically, in the transfer step (S300), the blank heated in the heating step (S200) is removed from the heating furnace and then cooled to ambient temperature (or room temperature) while being transferred to a mold. Then, in the forming step (S400), forming of the blank cooled to ambient temperature (or room temperature) begins. At this time, the temperature at which forming of the blank begins is referred to as the forming start temperature. That is, in the transfer step (S300), the blank heated in the heating step (S200) is removed from the heating furnace and then cooled from ambient temperature to the forming start temperature.

[0087] In one embodiment, the starting temperature for forming is 500°C or higher and 700°C or lower. If the starting temperature for forming is lower than 500°C, the blank's formability may be reduced, and the resulting hot-stamped part may not have the desired structure and physical properties. On the other hand, if the starting temperature for forming is higher than 700°C, wrinkles (or warping) may occur on the surface of the resulting hot-stamped part. In addition, the plating layer of the blank may be burned onto the die. Therefore, if the starting temperature for forming is higher than 500°C and lower than 700°C, the blank's formability is improved, the resulting hot-stamped part may have the desired structure and physical properties, and wrinkles (or warping) on ​​the surface of the resulting hot-stamped part may be prevented or minimized.

[0088] In one embodiment, in the forming step (S400), the blank transferred to the mold in the transferring step (S300) is formed to form a green body, and the formed green body is cooled in the cooling step (S500). At this time, the cooling step (S500) of cooling the formed green body is performed within the mold.

[0089] Specifically, the final product is formed by forming the final part shape in a mold and cooling the formed body. The mold is equipped with cooling channels through which a refrigerant circulates. The formed body can be rapidly cooled by circulating the refrigerant supplied through the cooling channels equipped in the mold. At this time, in order to prevent the springback phenomenon of the sheet material and maintain the desired shape, rapid cooling is performed while applying pressure with the mold closed. During the forming and cooling operations of the formed body, the average cooling rate may be set to a minimum of 10°C / s or more until the martensite finish temperature.

[0090] In one embodiment, the die cooling end temperature at which the cooling step (S500) is completed is between about room temperature and about 200°C. If the die cooling end temperature is below room temperature, the productivity of the manufacturing process decreases. On the other hand, if the die cooling end temperature exceeds 200°C, the manufactured hot stamped part is air-cooled at room temperature, which may cause distortion in the hot stamped part and make it difficult to obtain the desired material quality. Therefore, if the die cooling end temperature at which the cooling step (S500) is completed is between room temperature and about 200°C, the productivity of the manufacturing process can be increased, and the manufactured hot stamped part is air-cooled at room temperature, preventing or minimizing distortion in the hot stamped part.

[0091] In one embodiment, the air-cooling time for cooling the blank in the transfer step (S300) is about 5 seconds to about 20 seconds. If the air-cooling time is less than 5 seconds, the forming start temperature at which the blank begins to form is excessively high, and the blank proceeds at a high temperature, resulting in wrinkles (or bends) in the manufactured hot-stamped part. Therefore, it is difficult to achieve an air-cooling time of less than 5 seconds due to equipment limitations. On the other hand, if the air-cooling time exceeds 20 seconds, not only will productivity decrease, but phase transformation may occur in the blank during transfer, reducing the blank's formability, and the manufactured hot-stamped part may not have the desired material properties. Therefore, if the air-cooling time is within the range of about 5 seconds to about 20 seconds, the blank's formability and process productivity can be improved, and the manufactured hot-stamped part can have the desired material properties.

[0092] In one embodiment, the die cooling time in the cooling step (S500) is between about 6 seconds and about 40 seconds. If the die cooling time is less than 6 seconds, the die cooling at a high temperature requires a long air cooling period, which can cause distortion in the hot stamped part and prevent the desired dimensions from being achieved. On the other hand, if the die cooling time exceeds 40 seconds, productivity decreases. Therefore, if the die cooling time is within the range of between about 6 seconds and about 40 seconds, when the blank temperature is between room temperature and 200°C, distortion in the hot stamped part produced after die cooling is prevented or minimized, thereby improving productivity of the manufacturing process.

[0093] FIG. 6 is a flowchart illustrating a method for manufacturing a blank for producing a hot stamped part according to one embodiment of the present invention.

[0094] The size, density and area fraction of the pearlite domain in the microstructure of the blank can be controlled by adjusting the process conditions of the blank manufacturing process as described above with reference to FIG.

[0095] Specifically, the microstructure of the blank includes ferrite and pearlite. Meanwhile, carbon (C) and / or manganese (Mn) are segregated in the pearlite. That is, the microstructure of the blank includes pearlite with a relatively high carbon (C) content and / or manganese (Mn) content. Furthermore, pearlite with a relatively high carbon (C) content and / or manganese (Mn) content is locally accumulated within the blank. That is, the microstructure of the blank includes "pearlite regions" with a relatively high carbon (C) content and / or manganese (Mn) content. The pearlite regions refer to a structure (layered structure) in which different steel structures, ferrite and cementite (FeC), are alternately layered. In one embodiment, the pearlite regions are formed in a strip (or band) shape within the hot-rolled steel sheet. In this specification, the term "region where pearlite is locally accumulated" in relation to the "pearlite region" refers to a region where, when ferrite and cementite (FeC) are formed in the form of bands within the pearlite region, the division between the different bands is clear and a lamellar structure is clearly visible.

[0096] The extent to which the pearlite region affects the mechanical properties of a hot stamped part varies depending on the carbon (C) and manganese (Mn) contents of the pearlite accumulated in the pearlite region. Specifically, regions with locally concentrated pearlite containing 0.19 wt% or more of carbon and 0.8 wt% or more of manganese affect the mechanical properties of a hot stamped part. On the other hand, regions with locally concentrated pearlite containing less than 0.19 wt% of carbon or less than 0.8 wt% of manganese have little effect on the mechanical properties of a hot stamped part.

[0097] In one embodiment, the blank includes a first region in which pearlite containing 0.19 to 0.55 wt% carbon (C) and / or pearlite containing 0.8 to 6.0 wt% manganese (Mn) is locally accumulated, and the size, density, and area fraction of the first region are controlled to satisfy predetermined conditions.

[0098] Specifically, when the long side of the first region is defined as the length of the first region, the average length of the first region is controlled to be 0.01 μm or more and 300 μm or less. When the short side of the first region is defined as the thickness of the first region, the average thickness of the first region is controlled to be 0.01 μm or more and 5 μm or less. The line density in the short side direction of the first region is controlled to be 0.001 / μm or more and 0.1 / μm or less. The area fraction of the first region is controlled to be 0.01% or more and 15% or less.

[0099] Meanwhile, the blank further includes a second region in which pearlite containing more than 0.55 wt% carbon (C) and / or pearlite containing more than 6.0 wt% manganese (Mn) is locally accumulated. Because this second region can reduce the tensile strength and bending properties of the hot stamped part, it is controlled to be equal to or less than a predetermined area fraction. Specifically, the area fraction of the second region is controlled to be equal to or greater than 0% and equal to or less than 5%.

[0100] That is, the pearlite contained in the blank is controlled to include a first region having an area fraction of 0.01% to 15% and a second region having an area fraction of 0% to 5%. Here, the first region is a region where pearlite containing 0.19 to 0.55 wt% carbon (C) and / or pearlite containing 0.8 to 6.0 wt% manganese (Mn) is locally accumulated. The second region is a region where pearlite containing more than 0.55 wt% carbon (C) and / or pearlite containing more than 6.0 wt% manganese (Mn) is locally accumulated. Meanwhile, the region of pearlite contained in the blank excluding the first and second regions is understood to be pearlite containing less than 0.19 wt% carbon (C) and less than 0.8 wt% manganese (Mn).

[0101] Referring to FIG. 6, a blank manufacturing method (or blank preparation step; S1) according to one embodiment of the present invention includes a reheating step (S10), a hot rolling step (S20), a cooling / coiling step (S30), a cold rolling step (S40), an annealing heat treatment step (S50), and a plating step (S60).

[0102] For reference, in FIG. 6, steps S10 to S60 are shown as independent steps, but some of steps S10 to S60 may be performed in a single process, and some of steps S10 to S60 may be omitted as necessary.

[0103] First, a semi-finished slab to be processed into a hot stamping blank is prepared, which contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), aluminum (Al), titanium (Ti), niobium (Nb), molybdenum (Mo), boron (B), nitrogen (N), and the remainder iron (Fe) and other unavoidable impurities.

[0104] The reheating step (S10) is a step of reheating a slab having the above composition for hot rolling at a predetermined slab reheating temperature (SRT) range. In the reheating step (S10), the slab obtained through the continuous casting process is reheated at a predetermined temperature range to reuse elements that segregated during casting. The slab reheating temperature (SRT) is controlled within a preset temperature range to refine austenite and maximize the precipitation hardening effect.

[0105] In one embodiment, the slab reheating temperature (SRT) is controlled to 1,100°C to 1,300°C. If the slab reheating temperature (SRT) is less than 1,100°C, elements that segregated during casting (e.g., Ti, Nb, Mo, etc.) are not fully reused, resulting in a problem of making it difficult to achieve homogenization of alloy elements. While a higher slab reheating temperature (SRT) is more favorable for homogenization, if the slab reheating temperature (SRT) exceeds 1,300°C, the austenite grain size increases, making it difficult to ensure strength, and the excessive heating process can increase the manufacturing costs of the steel sheet.

[0106] The hot rolling step (S20) is a step of manufacturing a steel plate by hot rolling the slab reheated in the reheating step (S10) within a predetermined finishing delivery temperature (FDT) range.

[0107] In one embodiment, the finish rolling temperature FDT is controlled to a range of 800°C to 1000°C. If the finish rolling temperature FDT is less than 800°C, it is difficult to ensure the workability of the steel sheet due to the generation of a duplex structure caused by abnormal region rolling, and there are problems with reduced workability due to variations in the microstructure. In addition, a rapid phase change causes problems with sheet threadability during hot rolling. Conversely, if the finish rolling temperature FDT exceeds 1000°C, austenite grains become coarse, making it difficult to ensure strength.

[0108] In one embodiment, the reduction rate during hot rolling is controlled to satisfy 90% or more, thereby controlling the size, density, and area fraction of the pearlite region (pearlite region) in the manufactured blank, where pearlite has a relatively high carbon (C) content and / or manganese (Mn) content, to satisfy predetermined conditions.

[0109] Meanwhile, in the reheating step (S10) and the hot rolling step (S20), some fine precipitates are precipitated from the grain boundaries where energy is unstable. At this time, the fine precipitates precipitated from the grain boundaries act as an element that hinders the growth of austenite grains, thereby providing the effect of improving strength through austenite refinement.

[0110] The cooling / coiling step (S30) includes the steps of cooling the steel sheet hot-rolled in the hot-rolling step (S20) and coiling the cooled steel sheet.

[0111] The step of cooling the hot-rolled steel sheet is a step of ROT (Run Out Table) cooling the hot-rolled steel sheet to a predetermined cooling end temperature range for a predetermined cooling time.

[0112] In one embodiment, the cooling end temperature range is between the martensitic transformation start temperature (Ms) and the pearlite transformation start temperature (Ps) + 40°C, and the predetermined time is 30 seconds or less. The cooling end temperature range and cooling time in the step of cooling the hot-rolled steel sheet affect the size, density, and area fraction of the pearlite region (pearlite region) in the manufactured blank, where pearlite with a relatively high carbon (C) and / or manganese (Mn) content is locally accumulated. Specifically, when the cooling end temperature range and cooling time are satisfied, the size, density, and area fraction of the pearlite region are controlled to meet the predetermined conditions, and a uniform ferrite matrix hot-rolled structure is formed. On the other hand, when cooling is completed in a temperature range exceeding the cooling end temperature range or the cooling time is exceeded, the size, density, and / or area fraction of the pearlite region do not meet the predetermined conditions, resulting in reduced strength and bending properties.

[0113] The step of coiling the cooled steel sheet is a step of coiling the cooled steel sheet at a predetermined coiling temperature (CT) range.

[0114] In one embodiment, the coiling temperature CT is controlled to be equal to or higher than Ms+50°C and lower than 650°C. The coiling temperature CT affects the size, density, and area fraction of the pearlite region (pearlite region) where pearlite, which has a relatively high carbon (C) content and / or manganese (Mn) content, is locally accumulated in the manufactured blank. Specifically, when the coiling temperature CT is equal to or higher than Ms+50°C and lower than 650°C, the size, density, and area fraction of the pearlite region are controlled to satisfy predetermined conditions. On the other hand, when the coiling temperature CT is lower than Ms+50°C, the low-temperature phase fraction increases due to supercooling, which may increase strength and increase the rolling load during cold rolling, resulting in a rapid decrease in softness. In contrast, when the coiling temperature is higher than 650°C, the size, density, and / or area fraction of the pearlite region may not satisfy predetermined conditions, resulting in decreased strength and bending properties, and deterioration of formability and strength due to abnormal or excessive grain growth.

[0115] Specifically, when the coiling temperature is 650°C or higher, pearlite bands are formed due to the formation of excessively large areas of pearlite regions (pearlite regions) where pearlite with relatively high carbon (C) and / or manganese (Mn) contents accumulates locally. This results in uneven distribution of carbon (C) and / or manganese (Mn) within the blank. This also affects the uniformity of carbon (C) and / or manganese (Mn) within the formed part after hot stamping. This, in turn, affects the uniformity of nano-indentation hardness within the martensite structure within the formed part after hot stamping, resulting in deterioration of the bending properties of the formed part after hot stamping.

[0116] In contrast, when the coiling temperature CT is greater than or equal to Ms + 50°C and less than 650°C, pearlite regions with relatively high carbon (C) and / or manganese (Mn) contents locally accumulate and form over a relatively small area, resulting in the formation of pearlite bands or their formation over a relatively small area. This results in a uniform distribution of carbon (C) and / or manganese (Mn) within the blank. This also affects the uniformity of carbon (C) and / or manganese (Mn) within the formed part after hot stamping. This, in turn, affects the uniformity of nanoindentation hardness within the martensite structure within the formed part after hot stamping, improving the bending properties of the formed part after hot stamping.

[0117] On the other hand, when the coiling temperature CT is less than Ms+50°C, locally accumulated regions (pearlite regions) are formed in a relatively small area, but the formation of low-temperature phases causes problems with the sheet shape during the cold rolling process, increasing the possibility of sheet breakage.

[0118] The cold rolling step (S40) involves uncoiling the steel sheet coiled in the cooling / coiling step (S30), pickling the steel sheet, and then cold rolling the steel sheet. The pickling is carried out to remove scale from the coiled steel sheet, i.e., the hot-rolled coil produced through the hot rolling process.

[0119] In one embodiment, the reduction rate during cold rolling is controlled to 5% to 80%. This allows the size, density, and area fraction of the pearlite regions (pearlite regions) in the manufactured blank, where pearlite has a relatively high carbon (C) and / or manganese (Mn) content, to be controlled to meet predetermined conditions. For example, if the reduction rate is less than 5%, the spacing between the pearlite particles becomes narrower, increasing the number of regions where pearlite is locally concentrated, thereby reducing strength and bending properties.

[0120] The annealing step (S50) is a step of annealing the cold-rolled steel sheet at a temperature of 700°C or higher after the cold rolling step (S40). In one embodiment, the annealing step (S50) is a step of annealing the cold-rolled steel sheet at a temperature in the range of Ae3±200°C. Meanwhile, the annealing step includes the steps of heating the cold-rolled steel sheet and cooling the heated cold-rolled steel sheet at a predetermined cooling rate. In this case, the cooling rate may be 1 to 40°C / sec.

[0121] The plating step (S60) is a step of forming a plating layer on the annealed heat-treated steel sheet. In one embodiment, the plating step (S60) includes a step of forming an Al-Si plating layer on the annealed heat-treated steel sheet in the annealing heat-treatment step (S50).

[0122] Specifically, the plating step (S60) includes a step of immersing the steel sheet in a plating bath having a temperature in the range of Bs±150°C to form a hot-dip coating layer on the surface of the steel sheet, and a step of cooling the steel sheet with the hot-dip coating layer to form a coating layer. The plating bath may contain, but is not limited to, Si, Fe, Al, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, and / or Bi as additive elements. For example, the plating bath may contain 5 to 12% Si, 1 to 4% Fe, and Al. The coating weight on the front and back surfaces is 40 to 200 g / m. 2 is controlled to satisfy the following.

[0123] By performing the hot stamping process on the hot stamping blank manufactured through steps S10 to S60, a hot stamped formed part having desired mechanical properties (e.g., tensile strength, yield strength, bending properties, elongation, etc.) can be manufactured.

[0124] Fig. 7 is a cross-sectional view schematically illustrating a portion of a hot-stamped component 1 according to one embodiment of the present invention, and Fig. 8 is a photomicrograph illustrating a cross-section of a base steel plate 10 of the hot-stamped component 1 according to one embodiment of the present invention. The base steel plate 10 shown in Fig. 8 is the base steel plate 10 of the hot-stamped component 1 manufactured by the method for manufacturing a hot-stamped component described above with reference to Figs. 1 to 6 (e.g., curve 410 in Fig. 4).

[0125] 7, the hot stamped component 1 includes a base steel sheet 10 and a plating layer 20 disposed on the base steel sheet 10. The plating layer 20 is an alloying layer formed on at least one surface of the base steel sheet 10 and includes aluminum (Al), iron (Fe), etc. Although not shown, the plating layer 20 includes multiple layers (not shown) sequentially stacked on the base steel sheet 10. In one embodiment, the multiple layers sequentially include an α-Fe phase, an Fe2Al5 phase, an AlFe phase, and an Fe2Al5 phase, although the compositions of the multiple layers are not limited thereto.

[0126] The base steel plate 10 is a base steel plate, and may be a steel plate manufactured by hot rolling and / or cold rolling a steel slab that has been cast to contain predetermined alloy elements in predetermined amounts.

[0127] In one embodiment, the base steel sheet 10 comprises carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), aluminum (Al), titanium (Ti), niobium (Nb), molybdenum (Mo), boron (B), nitrogen (N), and the balance iron (Fe) along with other unavoidable impurities.

[0128] In one embodiment, the base steel plate 10 contains carbon (C): 0.15 to 0.27 wt%, silicon (Si): 0.15 to 1.0 wt%, manganese (Mn): 0.5 to 1.10 wt%, phosphorus (P): 0.018 wt% or less, sulfur (S): 0.005 wt% or less, chromium (Cr): 0.1 to 1.0 wt%, aluminum (Al): 0.1 to 1.0 wt%, titanium (Ti): 0.015 to 0.080 wt%, niobium (Nb): 0.015 to 0.080 wt%, molybdenum (Mo): 0.1 to 0.7 wt%, boron (B): 0.001 to 0.008 wt%, nitrogen (N): 0.005 wt% or less, and the remainder iron (Fe) and other unavoidable impurities.

[0129] Meanwhile, in one embodiment, when the respective contents of carbon (C), manganese (Mn), chromium (Cr), and molybdenum (Mo) contained in the base steel plate 10 are expressed as [C], [Mn], [Cr], and [Mo] in wt%, the following mathematical formula 1 is satisfied.

[0130]

number

[0131] Carbon (C) acts as an austenite stabilizer in the base steel sheet 10. Carbon is the main element that determines the strength and hardness of the base steel sheet 10 and is added to increase hardenability and strength during heat treatment. The carbon content of the base steel sheet 10 is 0.15 wt% to 0.27 wt% based on the total weight. If the carbon content is less than 0.15 wt%, it is difficult to secure a hard phase (e.g., martensite) and the mechanical strength of the formed part after hot stamping is difficult to meet. On the other hand, if the carbon content exceeds 0.27 wt%, it may cause a decrease in the workability of the base steel sheet 10 or a decrease in the bending performance of the formed part after hot stamping.

[0132] Silicon (Si) acts as a ferrite stabilizer in the base steel sheet 10. Silicon is a solid-solution strengthening element that increases the strength of the base steel sheet 10 and suppresses the formation of low-temperature carbides, thereby improving the carbon concentration in austenite. Silicon is also a key element for homogenizing the hot-rolled, cold-rolled, and hot-pressed structures and finely dispersing ferrite. Silicon also acts as an element for controlling martensite strength heterogeneity, improving crashworthiness. The base steel sheet 10 contains silicon in an amount of 0.15 wt% to 1.0 wt% based on its total weight. If the silicon content is less than 0.15 wt%, the aforementioned effects are difficult to achieve, and cementite formation and coarsening occur in the martensite structure of the formed part after hot stamping. On the other hand, if the silicon content exceeds 1.0 wt%, the hot-rolled and cold-rolled loads increase, degrading the coating properties of the base steel sheet 10.

[0133] Manganese (Mn) acts as an austenite stabilizer in the base steel sheet 10. Manganese is added during heat treatment to increase hardenability and strength. The manganese content of the base steel sheet 10 is 0.5 wt% to 1.1 wt% based on the total weight. If the manganese content is less than 0.5 wt%, the hardening effect is insufficient, and the hardenability may not be achieved, resulting in an insufficient hard phase fraction in the formed part after hot stamping. On the other hand, if the manganese content exceeds 1.1 wt%, concentrated areas of pearlite where manganese segregates may occur, reducing softness and toughness, causing a decrease in the bending performance of the formed part after hot stamping, and the formation of an inhomogeneous microstructure.

[0134] Phosphorus (P) is an element that contributes to improving strength. To prevent a decrease in the toughness of the base steel plate 10, the phosphorus content is more than 0 and not more than 0.018 wt% based on the total weight of the base steel plate 10. If the phosphorus content exceeds 0.018 wt%, iron phosphide compounds are formed, which reduces toughness and weldability and may cause cracks in the base steel plate 10 during the manufacturing process.

[0135] Sulfur (S) is an element that contributes to improving workability. The sulfur content is more than 0 and not more than 0.005 wt% of the total weight of the base steel sheet 10. If the sulfur content exceeds 0.005 wt%, hot workability, weldability, and impact properties may be reduced, and the formation of large inclusions may cause surface defects such as cracks.

[0136] Aluminum (Al) acts as a ferrite stabilizer in the base steel sheet 10. Aluminum is a solid-solution strengthening element that increases the strength of the base steel sheet 10 and suppresses the formation of low-temperature carbides, thereby increasing the carbon concentration in austenite. Aluminum also acts as a martensite strength heterogeneity control element, improving crashworthiness. Aluminum is included in the base steel sheet 10 at 0.1 wt% to 1.0 wt% of the total weight. If the aluminum content is less than 0.1 wt%, the aforementioned effects are difficult to achieve, and cementite formation and coarsening occur in the martensite structure of the formed part after hot stamping. On the other hand, if the aluminum content exceeds 1.0 wt%, the hot rolling and cold rolling loads increase, resulting in a deterioration in the coating properties of the steel sheet.

[0137] Meanwhile, in one embodiment, in order to improve plating properties, the sum of the contents of silicon (Si) and aluminum (Al) contained in the base steel sheet 10 is controlled to satisfy a predetermined range. For example, the sum of the contents of silicon (Si) and aluminum (Al) contained in the base steel sheet 10 satisfies 0.4 to 1.5 wt%.

[0138] Chromium (Cr) is added to enhance the hardenability and strength of the base steel sheet 10 during heat treatment. Chromium refines grains and ensures strength through precipitation hardening. Chromium is included in an amount of 0.1 wt% to 1.0 wt% of the total weight of the base steel sheet 10. If the chromium content is less than 0.1 wt%, the precipitation hardening effect is low. Conversely, if the chromium content exceeds 1.0 wt%, the amount of Cr-based precipitates and matrix solid solution increases, reducing toughness and increasing production costs.

[0139] Titanium (Ti) forms precipitates at high temperatures and effectively contributes to grain refinement. Titanium is included in the base steel sheet 10 in an amount of 0.015 wt% to 0.080 wt% based on the total weight. This titanium content range prevents continuous casting defects and coarsening of precipitates, ensuring the proper physical properties of the steel and preventing defects such as cracks on the steel surface. If the titanium content is less than 0.015 wt%, the above effect cannot be fully achieved. On the other hand, if the titanium content exceeds 0.080 wt%, the precipitates become coarse, resulting in a decrease in elongation and bendability.

[0140] Titanium (Ti), niobium (Nb), and molybdenum (Mo) form fine precipitates in the form of nitrides or carbides, thereby ensuring the strength of hot-stamped and hardened parts. Furthermore, these elements are contained in Fe-Mn composite oxides and function as hydrogen trapping sites, which are effective in improving delayed fracture resistance. Therefore, they are essential elements for improving delayed fracture resistance.

[0141] More specifically, niobium (Nb) can increase strength and toughness by reducing martensite packet size. Niobium is included in the base steel sheet 10 in an amount of 0.015 wt% to 0.080 wt% based on the total weight. When niobium is included in this range, it effectively refines the grains of the base steel sheet 10 during hot rolling and cold rolling, preventing slab cracking and brittle fracture during steelmaking / continuous casting, and minimizing the formation of coarse precipitates during steelmaking. If the niobium content is less than 0.015 wt%, the above effects cannot be fully achieved. On the other hand, if the niobium content exceeds 0.080 wt%, the strength and toughness do not improve further due to the increased niobium content, but rather, the niobium remains dissolved in ferrite, potentially reducing impact toughness.

[0142] Molybdenum (Mo) is a substitutional element that increases the strength of steel through its solid solution strengthening effect. Molybdenum is added to inhibit coarsening of precipitates and improve hardenability. Molybdenum also improves the hardenability of steel. Molybdenum is included in an amount of 0.1 wt% to 0.7 wt% of the total weight of the base steel sheet 10. If the molybdenum content is less than 0.1 wt%, the above effects cannot be fully achieved. On the other hand, if the molybdenum content exceeds 0.7 wt%, there is a risk of a decrease in rolling productivity and elongation, which increases manufacturing costs without any additional benefits.

[0143] Boron (B) is added to suppress ferrite, pearlite, and bainite transformations and maintain a martensite structure, thereby ensuring hardenability and strength during heat treatment. Boron also segregates at grain boundaries, lowering grain boundary energy and improving hardenability. It also increases the growth temperature of austenite grains, resulting in grain refinement. The boron content of the base steel sheet 10 is 0.001 wt% to 0.008 wt% based on the total weight. When boron is present within this range, it prevents hard phase grain boundary embrittlement and ensures high toughness and bendability. If the boron content is less than 0.001 wt%, the hardenability effect is insufficient. Conversely, if the boron content exceeds 0.008 wt%, the solid solubility is low and boron easily precipitates from grain boundaries depending on the heat treatment conditions, resulting in poor hardenability or high-temperature embrittlement, and the occurrence of hard phase grain boundary embrittlement may reduce toughness and bendability. Nitrogen (N) is contained in an amount of 0.005 wt% or less based on the total weight of the base steel plate 10. If the nitrogen content exceeds 0.005 wt%, coarse TiN inclusions are formed, reducing bendability.

[0144] Meanwhile, the base steel sheet 10 of the hot stamped component 1 according to one embodiment of the present invention includes fine precipitates. That is, the fine precipitates are distributed within the base steel sheet 10. Some of the elements contained in the base steel sheet 10 are nitride or carbide forming elements that contribute to the formation of the fine precipitates. Specifically, titanium (Ti), niobium (Nb), and molybdenum (Mo) can form fine precipitates in the form of nitride or carbide.

[0145] Accordingly, the hot stamped part 1 according to one embodiment of the present invention includes fine precipitates distributed within the base steel sheet 10, and these fine precipitates include nitrides or carbides of at least one of titanium (Ti), niobium (Nb), and molybdenum (Mo). These fine precipitates can suppress propagation of cracks during bending deformation of the hot stamped part 1. In other words, the movement of dislocations during bending deformation is restricted by the fine precipitates.

[0146] The number of fine precipitates formed in the base steel sheet 10 and the average diameter of the fine precipitates are controlled to satisfy a predetermined range. In one embodiment, fine precipitates having a diameter of about 0.02 μm or less are formed in the base steel sheet 10 per unit area (100 μm 2 ) 9,000 pieces / 100μm 2 More than 30,000 pieces / 100μm 2 In one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet 10 is 0.006 μm or less, and preferably 0.003 μm or more and 0.006 μm or less.

[0147] The number and average diameter of these fine precipitates affect the suppression of crack propagation. If the number and average diameter of the fine precipitates are formed within the above-mentioned ranges, the required tensile strength after hot stamping can be secured and bendability can be improved. On the other hand, 2 ) The number of fine precipitates is 9,000 / 100μm 2 If the thickness is less than 100μm, the strength of the hot stamped part will decrease. 2) The number of fine precipitates is 30,000 / 100μm 2 If the average diameter of the fine precipitates exceeds 0.003 μm, the bendability of the hot stamped part will be reduced. On the other hand, if the average diameter of the fine precipitates is less than about 0.003 μm, the fine precipitates are too small to restrict dislocation movement. If the average diameter of the fine precipitates exceeds about 0.006 μm, the number of fine precipitates will be relatively reduced, and dislocation movement will not be effectively restricted.

[0148] Meanwhile, the base steel sheet 10 includes a martensite microstructure with a distributed microstructure. The martensite microstructure is the result of a diffusionless transformation of austenite γ below the martensitic transformation start temperature Ms during cooling. The microstructure within the martensite microstructure is a diffusionless transformation microstructure formed during rapid cooling within grains called prior austenite grain boundaries (PAGBs), and includes multiple lath structures. The multiple lath structures further comprise units such as blocks and packets. More specifically, multiple lath structures form a block, multiple blocks form a packet, and multiple packets form a PAGB.

[0149] As shown in Fig. 8, the base steel plate 10 of the hot stamped component 1 according to one embodiment of the present invention includes a martensite structure having a plurality of blocks. These blocks form a packet, and each of these blocks is formed by laths in the form of elongated rods oriented in one direction. In other words, the martensite structure includes a plurality of lath structures.

[0150] These multiple blocks, packets, or laths form boundaries between unit structures within the martensite structure. Specifically, multiple laths form lath boundaries within the martensite structure. Similarly, multiple blocks form block boundaries within the martensite structure, and multiple packets form packet boundaries within the martensite structure. That is, the boundaries between unit structures herein are lath boundaries, block boundaries, or packet boundaries. These boundaries between unit structures have the property of resisting external deformation.

[0151] Specifically, cracks generated during bending deformation of the hot stamped part 1 are generated by the movement of one-dimensional defects called dislocations due to interaction within the martensitic structure. Therefore, in the process of dislocations moving within the martensitic structure during bending deformation, the movement of dislocations is restricted at the boundaries between unit structures.

[0152] On the other hand, if the hardness of regions within the martensite structure adjacent to the boundary between unit structures differs, the boundary between these unit structures will have a weaker ability to resist external deformation. In other words, the greater the difference in hardness, for example, nanoindentation hardness, within the martensite structure, the weaker the boundary between these unit structures will have a weaker ability to resist external deformation. That is, if the nanoindentation hardness within the martensite structure is uniform at a certain level or above, the bendability of the hot-stamped part can be ensured. Therefore, it is necessary to appropriately control the uniformity of the nanoindentation hardness within the martensite structure.

[0153] Table 1 shows the nanoindentation hardness, standard deviation of nanoindentation hardness, average nanoindentation hardness, coefficient of variation, and bending angle of the hot stamped parts according to the examples of the present invention and the comparative examples.

[0154] The nanoindentation hardness was measured using a nanoindenter. Specifically, the nanoindentation hardness was measured at more than 20 different points within one PAGB using a cube-corner tip (centerline-to-face angle = 35.3°, indentation strain rate = 0.22). The present invention is not limited to this. For example, a Berkovich tip (centerline-to-face angle = 65.3°, indentation strain rate = 0.072) may also be used as the nanoindenter.

[0155] Using these nanoindentation hardness values, the average and standard deviation of the nanoindentation hardness were calculated. The coefficient of variation is defined as the standard deviation of the nanoindentation hardness divided by the average nanoindentation hardness. Therefore, the coefficient of variation also indicates the uniformity of the nanoindentation hardness. Measuring nanoindentation hardness using a nanoindenter is a common method for measuring nanoindentation hardness, so a detailed explanation will be omitted.

[0156] Meanwhile, the nanoindentation hardness in Table 1 was measured using one PAGB, but the same or similar nanoindentation hardness was also measured using other PAGBs other than the PAGB related to Table 1. Therefore, the nanoindentation hardness, standard deviation of nanoindentation hardness, mean and coefficient of variation of nanoindentation hardness in Table 1 represent the nanoindentation hardness, standard deviation of nanoindentation hardness, mean and coefficient of variation of nanoindentation hardness in a martensitic structure. The uniformity of the nanoindentation hardness described above is evaluated using the standard deviation or coefficient of variation of the nanoindentation hardness.

[0157] Meanwhile, the bending performance of hot-stamped parts is evaluated by the bending angle. In this specification, "bending angle" refers to the V-bending angle in the rolling direction (RD). The V-bending angle is a parameter used to evaluate the bending deformation properties in the maximum load range among the deformations that appear in the bending performance of hot-stamped parts. That is, by examining the tensile deformation region during bending at macroscopic and microscopic scales through a load-displacement evaluation of the hot-stamped part 1, the bending performance is evaluated as the V-bending angle when microcracks are generated and propagated in the local tensile region.

[0158] [Table 1]

[0159] Referring to Table 1, in the embodiment of the present invention, the nanoindentation hardness is 3.0 GPa or more and 5.0 GPa or less. Preferably, the nanoindentation hardness is 3.05 GPa or more and 4.95 GPa or less. More preferably, the nanoindentation hardness is 3.18 GPa or more and 4.95 GPa or less. The standard deviation of the nanoindentation hardness is more than 0 GPa and 0.8 GPa or less. Preferably, the standard deviation of the nanoindentation hardness is 0.2 GPa or more and 0.6 GPa or less. More preferably, the standard deviation of the nanoindentation hardness is more than 0.28 GPa and less than 0.59 GPa. The coefficient of variation is more than 0 and 0.2 or less. Preferably, the coefficient of variation is 0.05 or more and 0.15 or less. More preferably, the coefficient of variation is 0.06 or more and 0.14 or less.

[0160] The comparative example had a nanoindentation hardness of 2.4 GPa to 5.2 GPa, a standard deviation of over 0.8 GPa, and a coefficient of variation of over 0.2. In other words, the comparative example had a relatively low uniformity of nanoindentation hardness within the martensite structure compared to the examples, resulting in poor resistance to external deformation at the boundaries between unit structures. This resulted in relatively poor bendability below 70° due to the ease of microcracks and / or bending.

[0161] That is, when the nanoindentation hardness within the martensite structure is uniformly formed above a certain level, the hot-stamped part 1 can ensure sufficient bendability. The bend angle of the hot-stamped part 1 according to the embodiment of the present invention is between 70° and 85°. On the other hand, when the standard deviation or coefficient of variation of the nanoindentation hardness exceeds the aforementioned range, the uniformity of the nanoindentation hardness within the martensite structure decreases, making it difficult for the hot-stamped part 1 to ensure sufficient bendability. When the standard deviation or coefficient of variation of the nanoindentation hardness is below the aforementioned range, the uniformity of the nanoindentation hardness within the martensite structure increases, but the manufacturing cost of the hot-stamped part 1 may increase excessively.

[0162] The uniformity of nanoindentation hardness within the martensite structure is affected by the uniformity of the carbon content and the manganese content within the martensite structure. That is, when the carbon content and manganese content within the martensite structure are uniform, the nanoindentation hardness within the martensite structure is also uniform.

[0163] Table 2 shows the carbon content and the standard deviation of the carbon content of the hot stamped parts according to the examples of the present invention and the comparative examples.

[0164] The carbon content was measured at more than 20 different points within one PAGB. The carbon content in Table 2 was measured at one PAGB, but the same or similar carbon content was measured at other PAGBs besides the PAGB associated with Table 2. Therefore, the carbon content and standard deviation of the carbon content in Table 1 represent the carbon content and standard deviation of the carbon content in the martensite structure. The uniformity of the carbon content described above is evaluated based on the standard deviation of the carbon content.

[0165] [Table 2]

[0166] Referring to Table 2, in the embodiment of the present invention, the carbon content is 0.14 wt% or more and 0.22 wt% or less. The standard deviation of the carbon content is more than 0 wt% and less than 0.04 wt%. Preferably, the standard deviation of the carbon content is 0.01 wt% or more and 0.03 wt% or less.

[0167] In the comparative example, the carbon content was 0.11 wt% or more and 0.23 wt% or less, and the standard deviation of the carbon content was 0.04 wt% or more. In other words, the comparative example had a lower uniformity of the carbon content within the martensite structure than the examples.

[0168] FIG. 9 is a photomicrograph showing a cross section of the base steel sheet 10 of the hot-stamped component 1 according to one embodiment of the present invention. FIG. 10 is a photomicrograph showing a cross section of the base steel sheet of the hot-stamped component according to a comparative example. Specifically, FIG. 9 is a photomicrograph illustrating the uniformity of the manganese content in the base steel sheet 10 of the hot-stamped component 1 according to one embodiment of the present invention, and FIG. 10 is a photomicrograph illustrating the uniformity of the manganese content in the base steel sheet of the hot-stamped component according to a comparative example. In FIGS. 9 and 10, the brightness or contrast of the corresponding regions varies depending on the manganese content. That is, the manganese content in the dark regions is relatively higher than that in the light regions.

[0169] The base steel sheet 10 of the hot stamped part 1 according to one embodiment of the present invention exhibited uniformly bright regions as shown in Fig. 9. However, the base steel sheet of the hot stamped part according to the comparative example exhibited darker regions in some areas than the remaining areas as shown in Fig. 10. Specifically, a line extending from side to side and darker than the remaining areas is shown in the center of Fig. 10. In other words, the base steel sheet of the hot stamped part according to the comparative example had a high manganese content in some areas, and therefore the uniformity of the manganese content within the martensite structure of the comparative example was relatively lower than that of the examples.

[0170] As described above, the Comparative Example had relatively lower uniformity of carbon content and manganese content within the martensite structure than the Examples. Therefore, the Comparative Example had relatively lower uniformity of nano-indentation hardness within the martensite structure than the Examples because carbon and manganese were not uniformly distributed within the martensite structure. Meanwhile, if the standard deviation of the carbon content exceeds the aforementioned range, the uniformity of nano-indentation hardness within the martensite structure decreases, making it difficult for the hot-stamped part 1 to ensure sufficient bendability. If the standard deviation of the carbon content is less than the aforementioned range, the uniformity of nano-indentation hardness within the martensite structure increases, but the manufacturing cost of the hot-stamped part 1 may increase excessively.

[0171] Meanwhile, the nanoindentation hardness and standard deviation of nanoindentation hardness of the base steel sheet 10 can be controlled by adjusting the process conditions of the manufacturing process of the hot stamped part described above. Of course, since the carbon content and standard deviation of the carbon content are related to the nanoindentation hardness and standard deviation of nanoindentation hardness, the carbon content and standard deviation of the carbon content can also be controlled by adjusting the process conditions of the manufacturing process of the hot stamped part.

[0172] Table 3 shows the contents of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), aluminum (Al), titanium (Ti), niobium (Nb), molybdenum (Mo), boron (B), and nitrogen (N) contained in each of the base steel sheets of the hot stamped parts according to the examples of the present invention and the comparative examples.

[0173] [Table 3]

[0174] The base steel sheets of the hot stamped parts according to the examples and comparative examples of the present invention satisfy the above-mentioned ranges of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), aluminum (Al), titanium (Ti), niobium (Nb), molybdenum (Mo), boron (B), and nitrogen (N) contents. That is, the base steel sheets of the hot stamped parts according to the examples and comparative examples of the present invention each contain carbon (C): 0.15 to 0.27 wt%, silicon (Si): 0.15 to 1.0 wt%, manganese (Mn): 0.5 to 1.10 wt%, phosphorus (P): 0.018 wt% or less, sulfur (S): 0.005 wt% or less, chromium (Cr): 0.1 to 1.0 wt%, aluminum (Al): 0.1 to 1.0 wt%, titanium (Ti): 0.015 to 0.080 wt%, niobium (Nb): 0.015 to 0.080 wt%, molybdenum (Mo): 0.1 to 0.7 wt%, boron (B): 0.001 to 0.008 wt%, nitrogen (N): 0.005 wt% or less, and the remainder being iron (Fe) and other unavoidable impurities.

[0175] Table 4 shows the process conditions for manufacturing hot stamped parts according to the examples and comparative examples of the present invention. Specifically, Table 4 shows the slab reheating temperature SRT in the reheating step (S10), the finish rolling temperature FDT in the hot rolling step (S20), the coiling temperature CT in the cooling / coiling step (S30), and the annealing temperature in the annealing heat treatment step (S50) in the manufacturing process of the hot stamped parts according to the examples and comparative examples of the present invention.

[0176] [Table 4]

[0177] The hot-stamped parts of the examples and comparative examples of the present invention were manufactured through a reheating step (S10) with a slab reheating temperature SRT of 1,100°C to 1,300°C, a hot-rolling step (S20) with a finish rolling temperature FDT of 800°C to 1,000°C, and an annealing heat treatment step (S50) with an annealing temperature of Ae3±200°C. However, the examples of the present invention were manufactured through a cooling / coiling step (S30) with a coiling temperature CT of Ms+50°C or higher and lower than 650°C, while the comparative examples were manufactured through a cooling / coiling step (S30) with a coiling temperature CT exceeding 650°C. That is, the manufacturing processes of the hot-stamped parts of the examples and comparative examples of the present invention were identical or similar in terms of process conditions other than the coiling temperature. Meanwhile, both the hot-stamped parts of the examples and comparative examples were manufactured through a plating step (S60), and therefore both included an Al-Si plating layer.

[0178] As described above, the size, density, and area fraction of pearlite regions (pearlite regions) in the blank, where pearlite has a relatively high carbon (C) and / or manganese (Mn) content, are affected by the coiling temperature (CT). That is, the uniformity of the carbon (C) and / or manganese (Mn) content in the blank is also affected by the coiling temperature (CT). These pearlite regions are minimized or absent in the hot stamped part due to heating after the blank preparation step (S1). However, the carbon (C) and / or manganese (Mn) content in the blank has a certain effect on the carbon (C) and / or manganese (Mn) content in the hot stamped part 1. Therefore, the carbon (C) and / or manganese (Mn) content in the blank also affect the nano-indentation hardness and bend angle of the hot stamped part 1. Therefore, by controlling the coiling temperature CT in the cooling / coiling step (S30) of the manufacturing process of the hot stamped part, it is possible to optimize the martensitic structure characteristics of the hot stamped part 1 and ensure excellent mechanical properties such as high strength and high toughness of the hot stamped part 1. However, the present invention is not limited to this. For example, by controlling the process conditions of the manufacturing process of the hot stamped part other than the coiling temperature CT, it is possible to manufacture a hot stamped part 1 having a nanoindentation hardness and standard deviation of nanoindentation hardness within the aforementioned range.

[0179] This allows for control of the mechanical properties of the hot-stamped part 1, such as tensile strength, yield strength, bending properties, and elongation. For example, the tensile strength of the hot-stamped part 1 is 1,350 MPa or more, and preferably 1,350 MPa to 1,650 MPa. The yield strength of the hot-stamped part 1 is 950 MPa or more, and preferably 950 MPa to 1,200 MPa. The hot-stamped part 1 also satisfies a bending angle of 70° to 85° and has an elongation of 6% or more. Preferably, the hot-stamped part 1 has an elongation of 6% to 9%.

[0180] Table 5 shows the tensile strength, yield strength, and elongation of the hot stamped parts according to the examples of the present invention and the comparative examples. As described above, each of the hot stamped parts according to the examples of the present invention has a tensile strength of 1,350 MPa or more and 1,650 MPa or less, a yield strength of 950 MPa or more and 1,200 MPa or less, and an elongation of 6% or more and 9% or less.

[0181] [Table 5]

[0182] Although the present invention has been described with reference to the embodiments shown in the drawings, these are merely illustrative, and those skilled in the art will recognize that various modifications and equivalent embodiments are possible. Therefore, the true technical scope of protection of the present invention should be determined by the technical spirit of the claims.

Claims

1. 1. A hot stamped part comprising a base steel plate containing carbon (C): 0.15 to 0.27 wt%, silicon (Si): 0.15 to 1.0 wt%, manganese (Mn): 0.5 to 1.10 wt%, phosphorus (P): 0.018 wt% or less, sulfur (S): 0.005 wt% or less, chromium (Cr): 0.1 to 1.0 wt%, aluminum (Al): 0.1 to 1.0 wt%, titanium (Ti): 0.015 to 0.080 wt%, niobium (Nb): 0.015 to 0.080 wt%, molybdenum (Mo): 0.1 to 0.7 wt%, boron (B): 0.001 to 0.008 wt%, nitrogen (N): 0.005 wt% or less, and the remainder being iron (Fe) and other unavoidable impurities, The base steel plate includes a martensite structure, The nanoindentation hardness of the martensitic structure is 3.0 GPa or more and 5.0 GPa or less, The standard deviation of the nanoindentation hardness is 0.8 GPa or less.

2. 2. The hot-stamped part according to claim 1, wherein the bend angle of the hot-stamped part is equal to or greater than 70° and equal to or less than 85°.

3. 2. The hot stamped part according to claim 1, wherein a coefficient of variation is a value obtained by dividing the standard deviation of the nanoindentation hardness by the average of the nanoindentation hardness, and the coefficient of variation is 0.2 or less.

4. 2. The hot stamped component according to claim 1, wherein the standard deviation of the carbon (C) content of the martensitic structure is less than 0.04 wt%.

5. Further comprising fine precipitates distributed within the base steel sheet, The hot stamped component according to claim 1 , wherein the fine precipitates include carbides of at least one of titanium (Ti), niobium (Nb), and molybdenum (Mo).

6. Unit area (100 μm ２ 6. The hot stamped component according to claim 5, wherein the number of the fine precipitates distributed per 1000 micrometers is 9,000 or more and 30,000 or less.

7. The hot stamped component according to claim 5, wherein the fine precipitates have an average diameter of 0.003 μm or more and 0.006 μm or less.

8. 2. The hot-stamped part according to claim 1, wherein the hot-stamped part has a tensile strength of 1,350 MPa or greater and 1,650 MPa or less.

9. 2. The hot stamped component according to claim 1, wherein the hot stamped component has a yield strength of 950 MPa or greater and 1,200 MPa or less.

10. 2. The hot stamped component according to claim 1, wherein the elongation of the hot stamped component is 6% or greater.

11. The hot stamped component according to claim 1 , wherein the martensitic structure includes a plurality of lath structures.

12. The hot stamped part of claim 1 , further comprising a plating layer disposed on the base steel sheet.

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