Hot stamping part

Abstract

The invention relates to a hot stamping part which comprises a base steel plate. The base steel sheet contains 0.15 wt% to 0.27 wt% of carbon (C), 0.15 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 1.10 wt% of manganese (Mn), 0.018 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), 0.1 wt% to 1.0 wt% of chromium (Cr), 0.1 wt% to 1.0 wt% of aluminum (Al), 0.015 wt% to 0.080 wt% of titanium (Ti), 0.015 wt% to 0.080 wt% of niobium (Nb), 0.1 wt% to 0.7 wt% of molybdenum (Mo), and the balance of steel. The present invention relates to a steel sheet which comprises a base steel sheet, 0.001 wt% to 0.008 wt% of boron (B), 0.005 wt% or less of nitrogen (N), and a remainder of iron (Fe) and other unavoidable impurities, in which the base steel sheet comprises a martensite structure having a nanoindentation hardness of 3.0 GPa to 5.0 GPa, and a standard deviation of the nanoindentation hardness of 0.8 GPa or less.

Description

Technical Field

[0001] The present invention relates to a hot stamping part. Background Art

[0002] High-strength steel is applied to vehicle parts to achieve lightweight and stability. On the other hand, although high-strength steel can ensure its high-strength characteristics in terms of weight, as the strength increases, the stamping formability or bendability decreases, which may cause material fracture or springback phenomenon during the processing, so it is difficult to form a product with a complex and precise shape.

[0003] As a method for improving these problems, a hot stamping method has been proposed. With the increasing interest in the hot stamping method, research on hot stamping materials is also actively underway. For example, as disclosed in Korean Patent Publication No. 10-2017-0076009, the hot stamping method is a forming technology for manufacturing high-strength parts, which heats a steel sheet for hot stamping to a high temperature and then rapidly cools the steel sheet in a die while forming. Specifically, the hot stamping process generally consists of heating / forming / cooling / trimming, and the phase transformation and microstructure change of the material during the hot stamping process can be utilized. The heating process during the hot stamping process is a process of heating a blank in a heating furnace, and the cooling process during the hot stamping process is a process of cooling a molded body that has undergone hot stamping in a die. In addition, the blank heated through the heating process can be exposed to room temperature and air-cooled when introduced from the heating furnace into the die.

[0004] Related technologies include Korean Patent Registration Publication No. 10-2070579 (invention title: Hot Stamping Method), etc. Summary of the Invention

[0005] Technical Problem

[0006] Embodiments of the present invention provide a hot stamping part having excellent mechanical properties such as high strength and high toughness by controlling the uniformity of the nanoindentation hardness of the martensite structure. However, these technical problems are only examples, and the scope of the inventive concept of the present invention is not limited thereto.

[0007] Solution

[0008] Embodiments of the present invention provide a hot stamping part, the hot stamping part including a base steel plate, the base steel plate containing 0.15 wt% to 0.27 wt% of carbon (C), 0.15 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 1.10 wt% of manganese (Mn), 0.018 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), 0.1 wt% to 1.0 wt% of chromium (Cr), 0.1 wt% to 1.0 wt% of aluminum (Al), 0.015 wt% to 0.080 wt% of titanium (Ti), 0.015 wt% to 0.080 wt% of niobium (Nb), 0.1 wt% to 0.7 wt% of molybdenum (Mo), 0.001 wt% to 0.008 wt% of boron (B), 0.005 wt% or less of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein the base steel plate includes a martensitic structure, the nanoindentation hardness of the martensitic structure being 3.0 GPa or greater and 5.0 GPa or less, and the standard deviation of the nanoindentation hardness being 0.8 GPa or less.

[0009] Advantages of the invention

[0010] According to the embodiments of the present invention as described above, by controlling the uniformity of the nanoindentation hardness of the martensitic structure, the hot stamping part can ensure excellent mechanical properties such as high strength and high toughness. Of course, the scope of the present invention is not limited by these effects. Description of the drawings

[0011] Figure 1 is a flowchart schematically showing a manufacturing method of a hot stamping part according to an embodiment of the present invention.

[0012] Figure 2 is a flowchart specifically showing a heating operation of a manufacturing method of a hot stamping part according to an embodiment of the present invention.

[0013] Figure 3 is a view showing a heating furnace having a plurality of zones in a heating operation of a manufacturing method of a hot stamping part according to an embodiment of the present invention.

[0014] Figure 4 is a graph showing the change rate of the heating rate of a plurality of zones with heating time in a manufacturing method of a hot stamping part according to an embodiment of the present invention.

[0015] Figure 5 is a graph showing the change of heating time with material thickness and the change of heating time with heating temperature.

[0016] Figure 6 is a flowchart specifically showing a manufacturing method of a blank for manufacturing a hot stamping part according to an embodiment of the present invention.

[0017] Figure 7 is a cross-sectional view schematically showing a part of a hot stamping component according to an embodiment of the present invention.

[0018] Figure 8 is a micrograph showing a cross-section of a base steel plate of a hot stamping component according to an embodiment of the present invention.

[0019] Figure 9 is a micrograph showing a cross-section of a base steel plate of a hot stamping component according to an embodiment of the present invention.

[0020] Figure 10 is a micrograph showing a cross-section of a base steel plate of a hot stamping component according to a comparative example. Detailed Description

[0021] In this embodiment, the bending angle of the hot stamping component can be 70° or greater and 85° or less.

[0022] In this embodiment, the value obtained by dividing the standard deviation of the nanoindentation hardness by the average value of the nanoindentation hardness is called the coefficient of variation, and the coefficient of variation can be 0.2 or less.

[0023] In this embodiment, the standard deviation of the carbon content of the martensite structure can be less than 0.04 wt%.

[0024] In this embodiment, the hot stamping component may further include fine precipitates distributed in the base steel plate, and the fine precipitates may include at least one carbide of Ti, Nb, and Mo.

[0025] In this embodiment, the number of fine precipitates distributed per unit area (100 square microns) can be 9,000 or more and 30,000 or less.

[0026] In this embodiment, the average diameter of the fine precipitates can be 0.003 microns or greater and 0.006 microns or less.

[0027] In this embodiment, the tensile strength of the hot stamping component can be 1,350 MPa or greater and 1,650 MPa or less.

[0028] In this embodiment, the yield strength of the hot stamping component can be 950 MPa or greater and 1,200 MPa or less.

[0029] In this embodiment, the elongation of the hot stamping component can be 6% or greater.

[0030] In this embodiment, the martensite structure may include a plurality of lath structures.

[0031] In this embodiment, the hot stamping part may further include a coating provided on the base steel plate.

[0032] Other aspects, features, and advantages in addition to the above will become apparent from the following detailed description, claims, and drawings for implementing the present invention.

[0033] Modes of Implementing the Present Invention

[0034] The present invention can be modified into various forms and can have various embodiments. In this regard, the embodiments will now be disclosed in detail, and examples thereof are shown in the drawings. Referring to the embodiments described below in conjunction with the drawings, the advantages, features, and methods for achieving the advantages will be clear. However, the present invention can have different forms and should not be construed as limited to the descriptions herein.

[0035] The terms "first", "second", etc. may be used herein to describe various elements, and these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

[0036] In the following embodiments, unless the context clearly indicates otherwise, the singular form includes the plural form.

[0037] When the terms "comprise" and / or "include" are used in this specification, they indicate the presence of the described features or constituent elements, but do not exclude the presence or addition of one or more other features or constituent elements.

[0038] It should also be understood that when a film, region, or constituent element is referred to as being "on" or "above" another element, the film, region, or constituent element may be in direct contact with the other element, or there may be other intermediate films, regions, or constituent elements.

[0039] In the drawings, for ease of explanation, the thicknesses of layers and regions may be enlarged or reduced. For example, for ease of explanation, the dimensions and thicknesses of the elements in the drawings are arbitrarily shown, and thus, the present inventive concept is not limited to the drawings.

[0040] In the present invention, an expression such as "A and / or B" may include A, B, or A and B. In addition, in the present invention, an expression such as "at least one of A and B" may include A, B, or A and B.

[0041] In the following embodiments, when referring to a "plane", this means when observing the target part from above, and when referring to a "cross-section", this means when observing a cross-section perpendicularly cut from the target part from the side. In the following embodiments, when referring to "overlap", this includes "plane" and "cross-section" overlap.

[0042] Hereafter, the present invention will be described more fully with reference to the accompanying drawings. When describing the present invention with reference to the accompanying drawings, the same reference numerals are used to denote substantially the same or corresponding elements to each other.

[0043] Figure 1 is a flowchart schematically showing a method for manufacturing a hot stamping part according to an embodiment of the present invention. Figure 2 is a flowchart specifically showing a heating operation of a method for manufacturing a hot stamping part according to an embodiment of the present invention. The following will refer to Figure 1 and Figure 2 to describe the method for manufacturing a hot stamping part.

[0044] Refer to Figure 1 , in one embodiment, the method for manufacturing a hot stamping part may include a blank input operation (S100), a heating operation (S200), a transfer operation (S300), a forming operation (S400), and a cooling operation (S500). The method for manufacturing a hot stamping part according to the present invention may further include a blank preparation operation (S1). The blank preparation operation (S1) may be an operation for manufacturing a blank for hot stamping (hereinafter simply referred to as "blank") used for manufacturing a hot stamping part according to an embodiment of the present invention, which will be described in detail with reference to the following Figure 6 .

[0045] First, the blank input operation (S100) may be an operation of inputting a blank into a heating furnace having a plurality of zones with different heating rate ranges from each other. The blank may be provided in a form in which a coating is formed on at least one side of a base steel plate. The base steel plate may be a steel plate made by performing a hot rolling process and / or a cold rolling process on a cast steel slab and containing a predetermined alloy element in a predetermined content as the base steel plate.

[0046] In one embodiment, in the blank input operation (S100), the blank input into the heating furnace may be loaded onto rollers and then moved in the transfer direction.

[0047] Refer to Figure 1 and Figure 2 , the heating operation (S200) may be performed after the blank input operation (S100). In one embodiment, the heating operation (S200) may include multiple heating operations (S210) and a soaking operation (S220). Therefore, multiple heating operations (S210) and a soaking operation (S220) may be performed after the blank input operation (S100). The multiple heating operations (S210) and the soaking operation (S220) may be operations in which the blank is heated when passing through a plurality of zones provided in the heating furnace.

[0048] In one embodiment, the overall temperature of the heating furnace can be in the range of about 680 °C to about 1000 °C. Specifically, the overall temperature of the heating furnace that performs multiple heating operations (S210) and soaking operations (S220) can be in the range of about 680 °C to about 1000 °C. In this regard, the temperature of the heating furnace that performs multiple heating operations (S210) can be in the range of about 680 °C to Ac3, and the temperature of the heating furnace that performs the soaking operation (S220) can be in the range of Ac3 to 1000 °C.

[0049] Specifically, in the multiple heating operations (S210), the blank can be heated multiple times when passing through multiple sections provided in the heating furnace. Among the multiple sections provided in the heating furnace, there can be multiple sections that perform the multiple heating operations (S210), and a temperature is set for each section such that the temperature increases from the inlet of the heating furnace where the blank is input to the outlet of the heating furnace where the blank is taken out, thereby gradually heating the blank.

[0050] The soaking operation (S220) can be performed after the multiple heating operations (S210). In the soaking operation (S220), the heat-treated blank that has been heated multiple times can be heat-treated when passing through the section in the heating furnace where the temperature is set from Ac3 to 1000 °C. Preferably, in the soaking operation (S220), the heat-treated blank that has been heated multiple times can be soaked at a temperature in the range of about 830 °C to about 1000 °C. In addition, among the multiple sections provided in the heating furnace, there can be at least one section that performs the soaking operation (S220).

[0051] Figure 3 FIG. is a diagram showing a heating furnace having multiple sections in the heating operation of a method for manufacturing a hot-stamped component according to an embodiment.

[0052] Reference Figure 3 , a heating furnace according to an embodiment can have multiple sections P1, P2, P3, and P4 with different temperature ranges. More specifically, the heating furnace can have 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). At this time, the third heating section P3 can have two sections with different temperature ranges. The third heating section P3 can include a third-first heating section P3-1 having a third-first temperature range T3-1 and a third-second heating section P3-2 having a third-second temperature range T3-2.

[0053] In one embodiment, the second heating section P2 may have multiple sections with different temperature ranges. For example, the second heating section P2 may include a second-first heating section P2-1 having a second-first temperature range T2-1 and a second-second heating section P2-2 having a second-second temperature range T2-2. However, the present invention is not limited thereto. The second heating section P2 may include a second-first heating section P2-1 having a second-first temperature range T2-1 to a second-nth heating section P2-n having a second-nth temperature range T2-n. At this time, n may be a natural number greater than or equal to 2.

[0054] In one embodiment, the first heating section P1 may also have multiple sections with different temperature ranges. For example, the first heating section P1 may include a first-first heating section P1-1 having a first-first temperature range T1-1 and a first-second heating section P1-2 having a first-second temperature range T1-2. However, the present invention is not limited thereto. The first heating section P1 may include a first-first heating section P1-1 having a first-first temperature range T1-1 to a first-nth heating section P1-n having a first-nth temperature range T1-n. At this time, n may be a natural number greater than or equal to 2.

[0055] In one embodiment, referring to Figure 2 and Figure 3 , in multiple heating operations (S210), the billet may be gradually heated (or heated multiple times) while passing through the first heating section P1, the second heating section P2, and the third-first heating section P3-1 defined in the heating furnace. In addition, in the soaking operation (S220), the billet heated multiple times may be soaked while passing through the third-second heating section P3-2 and the 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 the sections for heating the billet multiple times, and the third-second heating section P3-2 and the fourth heating section P4 correspond to the sections for soaking the billet.

[0056] In one embodiment, the fourth heating section P4 may have multiple sections. For example, the fourth heating section P4 may have two sections, three sections, etc. At this time, the temperature ranges (or temperatures) of the multiple sections provided in the fourth heating section P4 may be the same.

[0057] The first heating section P1 to the fourth heating section P4 can be arranged in sequence in the heating furnace. The first heating section P1 can be adjacent to the heating furnace inlet for inputting the blank, and the fourth heating section P4 can be adjacent to the heating furnace outlet for taking out the blank. Therefore, the first heating section P1 with the first temperature range T1 can be the first section of the heating furnace, and the fourth heating section P4 with the fourth temperature range T4 can be the last section of the heating furnace. As will be described later, among the multiple sections of the heating furnace, the third-second heating section P3-2 and the fourth heating section P4 can be the soaking sections rather than the multiple heating sections.

[0058] The temperatures of the multiple sections provided in the heating furnace, such as the temperatures of the first heating section P1 to the fourth heating section P4, can increase from the heating furnace inlet for inputting the blank towards the heating furnace outlet for taking out the blank. Additionally, the temperature difference between two adjacent sections among the multiple sections provided in the heating furnace can be greater than 0°C and less than or equal to 100°C. For example, the temperature difference between the first heating section P1 and the second heating section P2 can be greater than 0°C and less than or equal to 100°C.

[0059] In one embodiment, the first temperature range T1 of the first heating section P1 can be in the range of about 680°C to about 870°C. The second temperature range T2 of the second heating section P2 can be in the range of about 700°C to about 930°C. The third-first temperature range T3-1 of the third-first heating section P3-1 can be in the range of about 800°C to about 950°C. The third-second temperature range T3-2 of the third-second heating section P3-2 can be in the range of about AC3 to about 1000°C. The fourth temperature range T4 of the fourth heating section P4 can be in the range of about Ac3 to about 1,000°C. Preferably, the fourth temperature range T4 of the fourth heating section P4 can be in the range of about 830°C or higher and 1,000°C or lower. 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 can be the same.

[0060] In one embodiment, when the second heating section P2 includes the second-first heating section P2-1 and the second-second heating section P2-2 having different temperature ranges as described above, the second-first temperature range T2-1 can be in the range of about 700°C to about 900°C, and the second-second temperature range T2-2 of the second-second heating section P2-2 can be in the range of about 750°C to about 930°C.

[0061] The boundary values defining the above-described plurality of zones will be described. The boundary values represent the heating time s (as the horizontal axis of the graph). First, the first boundary value e1 between the first heating zone P1 and the second heating zone P2 can be in the range of about 30 seconds to about 50 seconds and can be about 40 seconds. The second boundary value e2 between the second heating zone P2 and the third heating zone P3 can be in the range of about 80 seconds to about 130 seconds and can be about 85 seconds. The third boundary value e3 between the third-first heating zone P3-1 and the third-second heating zone P3-2 can be in the range of about 110 seconds to about 180 seconds and can be about 120 seconds. The fourth boundary value e4 between the third-second heating zone P3-2 and the fourth heating zone P4 can be in the range of about 140 seconds to about 230 seconds and can be about 150 seconds.

[0062] In one embodiment, when the second heating zone P2 includes the second-first heating zone P2-1 and the second-second heating zone P2-2 having different temperature ranges as described above, the second-first boundary value e2' between the second-first heating zone P2-1 and the second-second heating zone P2-2 can be in the range of about 50 seconds to about 110 seconds and can be about 60 seconds.

[0063] In Figure 3 , the heating furnace according to an embodiment of the present invention is shown to have five zones P1, P2, P3-1, P3-2, and P4, which have different temperature ranges, but the present invention is not limited thereto. The heating furnace can be provided with six, seven, or eight zones having different temperature ranges.

[0064] In one embodiment, the length of the heating furnace along the transport path of the blank can be 20 m to 40 m. The heating furnace can have a plurality of zones having different temperature ranges, and the ratio of the length of the zone in which the blank is heated multiple times to the length of the zone in which the blank is soaked in the plurality of zones can satisfy 1:1 to 4:1. If the length of the zone in which the blank is soaked in the furnace increases and the ratio of the length of the zone in which the blank is heated multiple times to the length of the zone in which the blank is soaked exceeds 1:1, the amount of hydrogen infiltrated into the blank in the soaking zone will increase, thereby increasing delayed fracture. On the other hand, if the length of the zone in which the blank is soaked decreases and the ratio of the length of the zone in which the blank is heated multiple times to the length of the zone in which the blank is soaked is less than 4:1, it may not be possible to sufficiently ensure the soaking zone (time), resulting in uneven strength of the hot-stamped part manufactured by the hot stamping part manufacturing process.

[0065] In one embodiment, the length of the uniformly heated zone among the plurality of zones provided in the furnace can be 20% to 50% of the total length of the furnace.

[0066] Figure 4 is a graph showing the rate of change of the heating rate with heating time for multiple sections in a method for manufacturing a hot stamping part according to an embodiment of the present invention. At this time, Figure 4 is a graph showing the heating rate (°C / s) of the blank changing with heating time s. Figure 4 The multiple sections and boundary values shown in Figure 3 are the same as those described with reference to

[0067] Reference Figure 4 , the heating rate (°C / s) or the rate of change of the heating rate (°C / s) of multiple sections where the blank is heated multiple times is as described below. Hereinafter, the term "rate of change of heating rate" refers to 2 the average slope of each section of the graph shown in Figure 4 and can be described as "average rate of change of heating rate" hereinafter. Figure 4 shows a first control curve 410 of the heating rate according to an embodiment of the present invention and a second control curve 420 of the heating rate according to a comparative example.

[0068] First, the first control curve 410 of the heating rate according to the embodiment of the present invention will be described.

[0069] The first heating section P1 may have a first average rate of change of heating rate r1. The second heating section P2 located after the first heating section P1 may have a second average rate of change of heating rate r2 different from the first average rate of change of heating rate r1. The third heating section P3 located after the second heating section P2 may have a third average rate of change of heating rate r3 different from the first average rate of change of heating rate r1 and the second average rate of change of heating rate r2. At this time, the third average rate of change of heating rate r3 may include a section where the positive value becomes negative. The fourth heating section P4 located after the third heating section P3 may have a fourth average rate of change of heating rate r4 different from the first average rate of change of heating rate r1, the second average rate of change of heating rate r2, and the third average rate of change of heating rate r3.

[0070] The first heating section P1 may be a general heating section. In the second heating section P2, compared with the first heating section P1, the heating rate may gradually decrease (|r1| > |r2|), so that alloying of the coating can be carried out. The third heating section P3 is a phase change section where the base steel plate of the blank undergoes a phase change. The third-first heating section P3-1 may have a positive (+) rate of change of heating rate, and the third-second heating section P3-2 may have a negative (-) rate of change of heating rate. The fourth heating section P4 may be a stable section where the blank is heated to a uniform temperature.

[0071] Referring to the first control curve 410, the first average heating rate change rate r1 and the second average heating rate change rate r2 can each have negative values, and the absolute value of the first average heating rate change rate r1 can be greater than the absolute value of the second average heating rate change rate r2 (|r1| > |r2|). In one embodiment, the first average heating rate change rate r1 can be about -0.5 °C / s 2 or greater and 0 or less. For example, the first average heating rate change rate r1 can be about -0.3 °C / s 2 . In one embodiment, the second average heating rate change rate r2 can be about -0.25 °C / s 2 or greater and 0 or less. For example, the second average heating rate change rate r2 can be about -0.07 °C / s 2 .

[0072] In one embodiment, between the first heating section P1 and the second heating section P2, that is, near the first boundary value e1, the change from the first average heating rate change rate r1 to the second average heating rate change rate r2 can be discontinuous. 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 can 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 can be different values. When the heating rate change rate changes discontinuously near the first boundary value e1 (r1 → r2) (410), the weldability of the hot stamping part can be improved compared to the case where the heating rate change rate changes continuously (420).

[0073] Since a large amount of energy is required for the change of the coating, the average heating rate change rate can change discontinuously between the first heating section P1 and the second heating section P2. In order to diffuse Fe in the base steel plate into the Al coating and preliminarily form and grow the Al-Fe phase in the coating, necessary energy must be provided. In addition, the Fe diffused into the base steel plate forms an Al-Fe-Si alloy layer over time, and the more discontinuous the change of the heating rate change rate near the first boundary value e1 is, the more uniform the diffusion to the surface is, thereby obtaining good weldability. On the other hand, if the change is continuous, since the diffusion of Al-Fe-Si to the surface is rapid and non-uniform, there can be a phase with high welding resistance on the surface, resulting in a phenomenon of reduced weldability.

[0074] In one embodiment, the third heating section P3 may include a third-first heating section P3-1 having a third-first average heating rate change rate r3-1 and a third-second heating section P3-2 having a third-second average heating rate change rate r3-2. The third-first average heating rate change rate r3-1 may have a positive value, and the third-second average heating rate change rate r3-2 may have a negative value. Thus, the third average heating rate change rate r3 may have a section where the heating rate change rate r3 changes from a positive value to a negative value. At this time, the absolute value of the third-first average heating rate change rate r3-1 may be less than the absolute value of the third-second average heating rate change rate r3-2 (|r3-1| < |r3-2|). In one embodiment, the third-first average heating rate change rate r3-1 may be 0 or greater and about 0.25 °C / s 2 or less. For example, the third-first average heating rate change rate r3-1 may be about 0.07 °C / s 2 . In one embodiment, the third-second average heating rate change rate r3-2 may be about -0.3 °C / s 2 or greater and 0 or less. For example, the third-second average heating rate change rate r3-2 may be about -0.08 °C / s 2 .

[0075] In the third-first heating section P3-1, the smaller the third-first average heating rate change rate r3-1, the gentler the slope of the first control curve 410 can be. The gentler the slope of the first control curve 410, the smaller the amount of hydrogen mixed in. Thus, hydrogen embrittlement can be improved. Conversely, the second control curve 420 shows a form in which the heating rate change rate increases rapidly or discontinuously in the third-first heating section P3-1. In this case, the amount of hydrogen mixed in increases, and the hydrogen brittleness may deteriorate accordingly. Thus, different from between the first heating section P1 and the second heating section P2, the phase transformation of the base steel plate takes place in the third heating section P3. If the temperature changes rapidly, problems such as hydrogen embrittlement and delayed fracture may occur. Therefore, the lower the heating rate change rate, the more advantageous it is.

[0076] Between the second heating section P2 and the third-first heating section P3-1, that is, near the second boundary value e2, the change from the second average heating rate change rate r2 to the third-first average heating rate change rate r3-1 may change from a negative value to a positive value. In other words, as the heating rate decreases and then increases, a phase transformation of the base steel plate may occur. For example, during the phase transformation of the base steel plate, when transforming to austenite, an endothermic reaction occurs in the corresponding section, and the endothermic reaction requires energy supply. Therefore, the heating rate must be increased again in the third-first heating section P3-1 to induce a reasonable level of phase transformation to austenite.

[0077] Between the third-first heating section P3-1 and the third-second heating section P3-2, that is, near the third boundary value e3, the change in the third-first average heating rate change rate r3-1 and the third-second average heating rate change rate r3-2 may change from a positive value to a negative value. That is, as the heating rate increases and then decreases again, a phase change of the base steel plate may occur.

[0078] In one embodiment, the absolute value of the fourth average heating rate change rate r4 may be less than the absolute value of each 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 may be a value close to 0, and the fourth heating section P4 may be a soaking section at a uniform temperature.

[0079] The time t4 for heating the billet in the third-second heating section P3-2 and the fourth heating section P4 may be about 50% or less of the total heating time t. This is because the longer the soaking time t4 in the third-second heating section P3-2 and the fourth heating section P4 is compared with the multiple heating times t1 in the first heating section P1, the second heating section P2, and the third-first heating section P3-1, the worse the component performance (such as weldability, hydrogen embrittlement, and bending performance) may be.

[0080] The characteristics of the second control curve 420 compared with the above-mentioned first control curve 410 will be described below, with an emphasis on the differences from the first control curve 410. Referring to the second control curve 420, the first 'average heating rate change rate r1' may continuously change between the first heating section P1 and the second heating section P2. Specifically, the heating rate at the first boundary value e1 defining 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 defining the first 'average heating rate change rate r1' in the second heating section P2 may have the same value.

[0081] In one embodiment, the first 'average heating rate change rate r1' may be about -0.26 °C / s 2 or greater and 0 or less. For example, the first 'average heating rate change rate r1' may be about -0.2 °C / s 2 。

[0082] Variation characteristics of the heating rate change rate r3'; r3-1' and r3-2' in the third heating section P3 of the second control curve 420 may have the same characteristics as described in the first control curve 410. However, the third-first' heating rate change rate r3-1' may have a discontinuous and unstable value compared to the third-first average heating rate change rate r3-1 of the first control curve 410. At this time, the third-first' heating rate change rate r3-1' may represent the rate of change in the previous section in which the heating rate showed an increasing trend during the third-first heating section P3-1. The third-first' heating rate change rate r3-1' may be approximately 0.04°C / s 2 or greater and about 0.16°C / s 2 For example, the third-first 'temperature increase rate change rate r3-1' may be about 0.1°C / s 2 The third to second 'temperature rise rate change rate r3-2' may be about -0.16°C / s 2 or greater and about -0.04℃ / s 2 For example, the third-second 'temperature increase rate change rate r3-2' may be about -0.1°C / s 2 The fourth heating section P4 of the second control curve 420 may be a soaking section in which the fourth average temperature increase rate change rate r4 has a value close to 0, similar to the first control curve 410 .

[0083] In this way, in the manufacturing method of the hot stamping parts according to the embodiment of the present invention, by controlling the rate of change of the heating rate of each section according to multiple cross-sectional characteristics as described above, the ultra-high strength characteristics, weldability, hydrogen embrittlement resistance, bending performance, etc. of the hot stamping parts can be accurately controlled and improved.

[0084] Figure 4 The relationship between the heating time s shown on the horizontal axis and the boundary value is not limited to Figure 4 The relationship shown in , and various changes and applications can be made within the scope of improving the performance of the hot stamping part of the present invention. In the above, it has been described that the plurality of cross sections have five cross sections, but the plurality of cross sections can be distinguished differently according to the distribution of the temperature increase rate change rate.

[0085] Figure 5 is a graph showing the variation of heating time with material thickness and the variation of heating time with heating temperature. Specifically, Figure 5 It is a graph for explaining the minimum heating time according to material thickness and the minimum heating time according to heating temperature. Figure 5 In the figure, the heating temperature indicates the soaking temperature of the soaking operation (S220), and the heating time indicates the total heating time of the heating operation (S200).

[0086] refer to Figure 1, Figure 2 and Figure 5 When the material thickness is the same, it can be seen that as the heating temperature decreases, the minimum heating time increases. In addition, when the heating temperature is the same, it can be seen that when the material thickness increases, the minimum heating time increases.

[0087] If the heating time (e.g., total heating time) for heating the blank in the heating operation (S200) is short, sufficient phase transformation may not occur in the blank. On the other hand, if the heating time for heating the blank in the heating operation (S200) is too long, the austenite grains may become coarse, the hydrogen embrittlement resistance may decrease, and the coating thickness may increase, which may reduce the weldability. Therefore, it is necessary to control the heating time in the heating operation (S200). However, in order to control the heating time in the heating operation (S200), various variables must be considered, such as the heating temperature and the thickness of the blank (e.g., the thickness of the material), as well as the sealing, atmosphere, heat source, heat loss in the heating furnace, and the composition of the blank.

[0088] In one embodiment, the heating time of the blank in the heating operation (S200) may satisfy the following [Equation 2].

[0089] [Equation 2]

[0090]

[0091] In Equation 2, λ n is the heating time in s, a n is the heat loss correction factor of the heating furnace, T n is the heating temperature (°C), b n is the Ac3 temperature correction factor, c n is the high-temperature material thickness sensitivity correction factor, and t is the material thickness (mm). At this time, the material can represent the blank, and the unit s of the heating time can represent seconds.

[0092] Since the heat sources used for each type of heating furnace are different, the heat losses occurring in each type of heating furnace may also be different. a n is the correction factor considering the heat loss of the heating furnace, and its value can be about -0.60 or greater and about -0.55 or less. At this time, the unit of a n can be s / (°C×mm).

[0093] If the compositions of each material are different, the temperatures at which phase transformation occurs may be different. b n is the correction factor considering the Ac3 temperature difference varying with the material composition, and its value can be about 700 or greater and about 900 or less. At this time, the unit of b n can be s / mm.

[0094] The thermal conductivity that is transferred inside the material may vary according to the thickness of the material. c n is a correction factor that takes into account the difference in thermal conductivity due to the change in the thickness of the material at high temperatures, and its value can be about 0.7 or greater and about 0.9 or less. At this time, the high temperature can represent 600 °C or higher. However, the high temperature can refer to 500 °C or higher or 700 °C or higher.

[0095] Heating temperature T n refers to the soaking temperature of the soaking operation (S220), and the heating temperature T n can have a value of about Ac3 or greater and about 1000 °C or less. In addition, the material thickness can have a value of about 1 mm or greater and about 2.6 mm or less.

[0096] In one embodiment, the heating time λ according to the mathematical formula n can be about 100 seconds or longer and about 900 seconds or shorter. If the heating time λ n is less than 100 seconds, sufficient phase transformation may not occur in the blank. On the other hand, if the heating time λ n exceeds 900 seconds, the austenite grains may become coarser, the hydrogen embrittlement resistance may decrease, and the coating thickness may become thicker, which may reduce the weldability. Therefore, when the heating time λ n satisfies the range of about 100 seconds to about 900 seconds, sufficient phase transformation may occur in the blank, coarsening of the austenite grains can be prevented or minimized, and a decrease in hydrogen embrittlement resistance and / or weldability can be prevented or minimized.

[0097] Referring again to Figure 1 , after the heating operation (S200), a transfer operation (S300), a forming operation (S400), and a cooling operation (S500) can be further performed.

[0098] In one embodiment, the transfer operation (S300) can be an operation of transferring the heated blank from the heating furnace to the die. At this time, in the transfer operation (S300), the heated blank can be cooled at the ambient temperature (or room temperature) while being transferred to the die. During the transfer, the heated blank can be air-cooled. If the heated blank is not air-cooled, the die inlet temperature (for example, the forming start temperature) will increase, which will cause wrinkles (or bending) on the surface of the produced hot stamping part. In addition, if a refrigerant is used, the subsequent process (hot stamping) will be affected. Therefore, it is preferable to air-cool the heated blank during the transfer.

[0099] In one embodiment, the forming operation (S400) may be an operation of hot stamping the transferred blank to form a molded body. Specifically, in the forming operation (S400), the blank may be pressed by a mold to form a molded body.

[0100] In one embodiment, the cooling operation (S500) may be an operation of cooling the formed molded body. The cooling operation (S500) may be performed within the mold.

[0101] In one embodiment, in the transfer operation (S300), the heated blank may be cooled at ambient temperature (or room temperature). Specifically, in the transfer operation (S300), when the blank heated by the heating operation (S200) is taken out of the heating furnace and transferred to the mold, the blank may be cooled at ambient temperature (or room temperature). Thereafter, in the forming operation (S400), the forming of the blank cooled at ambient temperature (or room temperature) may be started. At this time, the temperature at which the blank starts to be formed may be referred to as the forming start temperature. That is, in the transfer operation (S300), the blank heated by the heating operation (S200) may be cooled from ambient temperature to the forming start temperature after being taken out of the heating furnace.

[0102] In one embodiment, the forming start temperature may be 500 °C or higher and 700 °C or lower. If the forming start temperature is lower than 500 °C, the forming start temperature may be too low, which may deteriorate the formability of the blank, and the manufactured hot stamping part may not have the target structure and performance. On the other hand, if the forming start temperature exceeds 700 °C, wrinkles (or bends) may occur on the surface of the manufactured hot stamping part. In addition, the plating of the blank may adhere to the mold. Therefore, if the forming start temperature is 500 °C or higher and 700 °C or lower, the formability of the blank can be improved, the manufactured hot stamping part can have the target structure and performance, and the appearance of wrinkles (or bends) on the surface of the manufactured hot stamping part can be prevented or minimized.

[0103] Subsequently, in one embodiment, in the forming operation (S400), the blank transferred to the mold through the transfer operation (S300) may be formed to form a molded body, and in the cooling operation (S500), the formed molded body may be cooled. At this time, the cooling operation (S500) for cooling the formed molded body may be performed within the mold.

[0104] Specifically, the molded body can be cooled while forming the final component shape in the mold, thereby forming the final product. The mold can be provided with cooling channels through which a coolant circulates internally. The coolant supplied through the cooling channels provided in the mold can be circulated to rapidly cool the molded body. At this time, in order to prevent the springback phenomenon of the sheet material and maintain the desired shape, rapid cooling can be performed while applying pressure in the closed state of the mold. When performing the forming and cooling operations of the molded body, the molded body can be cooled to the end temperature of martensite at an average cooling rate of at least 10 °C / s or more.

[0105] In one embodiment, the mold cooling end temperature at the end of the cooling operation (S500) can be about room temperature or higher and about 200 °C or lower. If the mold cooling end temperature is lower than room temperature, the productivity of the manufacturing process will decrease. On the other hand, if the mold cooling end temperature exceeds 200 °C, when the manufactured hot-stamped component is air-cooled at room temperature, at this time, the hot-stamped component may warp, and it may be difficult to fix the target material. Therefore, if the mold cooling end temperature at the end of the cooling operation (S500) satisfies the range of about room temperature or higher and about 200 °C or lower, the productivity of the manufacturing process can be improved, and the manufactured hot-stamped component can be air-cooled at room temperature, thereby preventing or minimizing warping in the hot-stamped component.

[0106] In one embodiment, the air cooling time for cooling the blank in the transfer operation (S300) can be about 5 seconds or longer and about 20 seconds or shorter. If the air cooling time is less than 5 seconds, the forming start temperature during blank forming is too high, so the blank is formed at a high temperature, which may cause wrinkles (or bending) in the manufactured hot-stamped component, and it may be difficult to achieve an air cooling time of less than 5 seconds in the equipment. On the other hand, if the air cooling time exceeds 20 seconds, the productivity may decrease, and a phase change may occur in the blank during the transfer of the blank, which may reduce the formability of the blank, and the manufactured hot-stamped component may not have the target material. Therefore, if the air cooling time satisfies the range of about 5 seconds or longer and about 20 seconds or shorter, the formability of the blank and the productivity of the process can be improved, and the manufactured hot-stamped component can have the target material.

[0107] In one embodiment, the die cooling time in the cooling operation (S500) can be about 6 seconds or longer and about 40 seconds or shorter. If the die cooling time is less than 6 seconds, the die cooling will end at a high temperature, which may cause long-term air cooling, and thus may cause warping of the manufactured hot-stamped part. Therefore, it may not be possible to ensure the target dimensions. On the other hand, if the die cooling time exceeds 40 seconds, the productivity will decrease. Therefore, if the die cooling time satisfies the range of about 6 seconds or greater and about 40 seconds or less, the die cooling ends when the temperature of the blank is above room temperature and below 200 °C, warping in the manufactured hot-stamped part can be prevented or minimized, and the productivity of the manufacturing process can be improved.

[0108] Figure 6 is a flowchart specifically showing a manufacturing method of a blank for manufacturing a hot-stamped part according to an embodiment of the present invention.

[0109] can be controlled by adjusting the process conditions of the blank manufacturing process according to the description of reference Figure 6 the size, density, and area fraction conditions of the pearlite region in the microstructure configuration of the blank.

[0110] Specifically, the microstructure of the blank can include ferrite and pearlite. Carbon (C) and / or manganese (Mn) can segregate in the pearlite. That is, the microstructure of the blank can include pearlite with a relatively high C and / or Mn content. In addition, pearlite with a relatively high C and / or Mn content can be locally concentrated within the blank. That is, the microstructure of the blank can include a "pearlite region" with a relatively high C and / or Mn content. The pearlite region can represent a structural shape (stacked structure) in which different steel structures of ferrite and cementite (Fe3C) alternately form layers. In one embodiment, the pearlite region can be formed as a strip (or band) within the hot-rolled steel sheet. In this specification, the "locally concentrated region of pearlite" in the term "pearlite region" can refer to a region where, when the ferrite and cementite (Fe3C) in the pearlite region each form a band, the difference between different bands is obvious, thus forming a clearly visible layered structure.

[0111] The pearlite region can have different degrees of influence on the mechanical properties of the hot-stamped part according to the content of C and Mn contained in the pearlite concentrated in the pearlite region. Specifically, a region where pearlite with 0.19 wt% or more of C and 0.8 wt% or more of Mn is locally concentrated affects the mechanical properties of the hot-stamped part. On the other hand, a region where pearlite with less than 0.19 wt% of C or less than 0.8 wt% of Mn is locally concentrated has a minimal influence on the mechanical properties of the hot-stamped part.

[0112] In one embodiment, the blank may include a first region where pearlite containing 0.19 wt% to 0.55 wt% C and / or pearlite containing 0.8 wt% to 6.0 wt% Mn is locally concentrated. The size, density, and area fraction of these first regions can be controlled to meet predetermined conditions.

[0113] 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 can be controlled to meet 0.01 μm or greater and 300 μm or less. Additionally, when the short side of the first region is defined as the thickness of the first region, the average thickness of the first region can be controlled to meet 0.01 μm or greater and 5 μm or less. The linear density in the short side direction of the first region can be controlled to meet 0.001 / μm or greater and 0.1 / μm or less. The area fraction of the first region can be controlled to meet 0.01% or greater and 15% or less.

[0114] The blank may further include a second region where pearlite containing more than 0.55 wt% C and / or pearlite containing more than 6.0 wt% Mn is locally concentrated. Since the second region may reduce the tensile strength and bending properties of the hot-stamped part, the second region can be controlled to be below a preset area fraction. Specifically, the area fraction of the second region can be controlled to meet 0% or greater and 5% or less.

[0115] That is, the pearlite contained in the blank can be controlled to include a first region with an area fraction of 0.01% or greater and 15% or less and a second region with an area fraction of 0% or greater and 5% or less. Here, the first region is a region where pearlite containing 0.19 wt% to 0.55 wt% C and / or pearlite containing 0.8 wt% to 6.0 wt% Mn is locally concentrated. In addition, the second region is a region where pearlite containing more than 0.55 wt% C and / or pearlite containing more than 6.0 wt% Mn is locally concentrated. Among the pearlite contained in the blank, the pearlite in the region other than the first region and the second region can be understood as pearlite containing less than 0.19 wt% C and less than 0.8 wt% Mn.

[0116] Refer to Figure 6 , the method for manufacturing a blank (or the blank preparation operation (S1)) according to an embodiment of the present invention may include a reheating operation (S10), a hot rolling operation (S20), a cooling / coiling operation (S30), a cold rolling operation (S40), an annealing heat treatment operation (S50), and an electroplating operation (S60).

[0117] For reference, although operations S10 to S60 are in Figure 6are depicted as independent operations in the figure, but some of operations S10 to S60 can be performed in one process, and some of operations S10 to S60 can be omitted as needed.

[0118] First, a semi-finished slab is prepared, which is the target of the process for forming a blank for hot stamping. The slab may contain 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 of iron (Fe) and other inevitable impurities.

[0119] The reheating operation (S10) is an operation of reheating the slab having the above composition for hot rolling within a predetermined slab reheating temperature (SRT) range. In the reheating operation (S10), the slab obtained by the continuous casting process is reheated within a predetermined temperature range, thereby reusing the components segregated during casting. SRT can be controlled within a preset temperature range to maximize the austenite refinement and precipitation hardening effects.

[0120] In one embodiment, SRT can be controlled in the range of about 1,100 °C to about 1,300 °C. If SRT is lower than 1,100 °C, there is a problem that the components segregated during casting (such as Ti, Nb, Mo, etc.) cannot be fully reused, so it may be difficult to significantly ensure the homogenization effect of alloy elements. On the other hand, the higher the SRT, the more favorable it is for homogenization, but if it exceeds 1300 °C, the grain size of austenite crystals will increase, so it may be difficult to ensure strength, and the manufacturing cost of the steel plate will increase due to the excessive heating process.

[0121] The hot rolling operation (S20) is an operation of manufacturing a steel plate by hot rolling the slab reheated in the reheating operation (S10) within a predetermined finish rolling temperature (FDT) range.

[0122] In one embodiment, the FDT range can be controlled in the range of about 800 °C to about 1000 °C. If FDT is less than 800 °C, it is difficult to ensure the workability of the steel plate due to the occurrence of a mixed grain structure caused by abnormal area rolling, and there may be problems of reduced workability due to non-uniform microstructure, and there may also be sheet-like problems due to rapid phase transformation during the hot rolling process. On the other hand, if FDT exceeds 1,000 °C, the austenite grains become coarser, and it is difficult to ensure strength.

[0123] In one embodiment, the reduction ratio during hot rolling can be controlled to meet 90% or higher. Through the above control, the size, density, and area fraction of the pearlite locally concentrated region (pearlite region) having a relatively high carbon content and / or manganese content in the obtained blank can be controlled to meet the preset conditions.

[0124] During the reheating operation (S10) and the hot rolling operation (S20), some fine precipitates may precipitate at the grain boundaries with unstable energy. At this time, the fine precipitates precipitated at the grain boundaries can act as a factor hindering the growth of austenite grains, thereby providing an effect of improving strength through austenite refinement.

[0125] The cooling / coiling operation (S30) may include an operation of cooling the hot-rolled steel sheet during the hot rolling operation (S20) and an operation of coiling the cooled steel sheet.

[0126] The operation of cooling the hot-rolled steel sheet may be an output roller table (ROT) operation of cooling the hot-rolled steel sheet to a predetermined end-cooling temperature range and maintaining a preset cooling time.

[0127] In one embodiment, the end-cooling temperature range is from the martensite start temperature (Ms) to the pearlite start temperature (Ps) + 40 °C, and the preset time may be 30 seconds or less. The end-cooling temperature range and the cooling time in the operation of cooling the hot-rolled steel sheet affect the size, density, and area fraction of the pearlite locally concentrated region (pearlite region) with a relatively high carbon content and / or manganese content in the obtained blank. Specifically, if the end-cooling temperature range and the cooling time are satisfied, the size, density, and area fraction of the pearlite region can be controlled to meet the preset conditions, and a uniform hot-rolled structure of a ferrite matrix can be formed. On the other hand, if the cooling is completed within a temperature range exceeding the end-cooling temperature range or the cooling time is exceeded, the size, density, and / or area fraction of the pearlite region may not meet the preset conditions, resulting in poor strength and bending properties.

[0128] The operation of coiling the cooled steel sheet may be an operation of coiling the cooled steel sheet within a predetermined coiling temperature (CT) range.

[0129] In one embodiment, the CT can be controlled to be Ms + 50°C or higher and less than 650°C. The CT affects the size, density, and area fraction of the regions where pearlite is locally concentrated (pearlite regions) in the as-produced billet that have a relatively high carbon content and / or manganese content. Specifically, when the CT satisfies Ms + 50°C or greater and less than 650°C, the size, density, and area fraction of the pearlite regions can be controlled to meet predetermined conditions. On the other hand, if the CT is less than Ms + 50°C, the fraction of the low-temperature phase due to supercooling increases, which may lead to an increase in strength and rolling load during cold rolling, and there is a problem of rapid deterioration of ductility. Conversely, if the coiling temperature is 650°C or greater, the size, density, and / or area fraction of the pearlite regions may not meet the predetermined conditions, which may lead to a decrease in strength and bending properties, and there is a problem of deterioration of formability and strength due to abnormal or excessive grain growth.

[0130] Specifically, when the coiling temperature is 650°C or higher, a pearlite band can be formed by forming a region (pearlite region) where pearlite with a relatively high carbon content and / or manganese content is locally concentrated in an overly large area. As a result, C and / or Mn are unevenly distributed within the billet. This also affects the uniformity of C and / or Mn within the molded part after hot stamping. Therefore, this may affect the uniformity of the nanoindentation hardness within the martensite structure of the molded part after hot stamping, which may deteriorate the bending characteristics of the molded part after hot stamping.

[0131] Conversely, when the CT satisfies the range of Ms + 50°C or higher and lower than 650°C, the regions where pearlite with a relatively high carbon content and / or manganese content is locally concentrated (pearlite regions) are formed in relatively small areas, so a pearlite band may not be formed, or a pearlite band may be formed in a relatively small area. Therefore, C and / or Mn can be evenly distributed in the billet. This also affects the uniformity of C and / or Mn in the formed part after hot stamping. Therefore, this also affects the uniformity of the nanoindentation hardness in the martensite structure of the molded part after hot stamping, thereby improving the bending characteristics of the molded part after hot stamping.

[0132] On the other hand, when the CT is less than Ms + 50°C, the locally concentrated regions (pearlite regions) are formed in relatively small areas, but the formation of the low-temperature phase may cause sheet shape problems during the cold rolling process, and the possibility of sheet breakage may increase.

[0133] The cold rolling operation (S40) is an operation of uncoiling, pickling, and then cold rolling the steel sheet coiled in the cooling / coiling operation (S30). At this time, the purpose of pickling is to remove the scale of the rolled steel sheet (i.e., the hot-rolled coil manufactured by the hot rolling process).

[0134] In one embodiment, the reduction ratio during cold rolling can be controlled within a range of about 5% to about 80%. By reduction, the size, density, and area fraction of the regions where pearlite is locally concentrated (pearlite regions) with a relatively high carbon content and / or manganese content in the produced blank can be controlled to meet preset conditions. For example, when the reduction ratio is less than 5%, the gap between pearlite may become narrow, so the regions where pearlite is locally concentrated may increase, which may reduce strength and bending properties.

[0135] The annealing operation (S50) is an operation of annealing the cold-rolled steel sheet in the cold rolling operation (S40) at a temperature of 700 °C or higher. In one embodiment, the annealing operation (S50) can be an operation of annealing the cold-rolled steel sheet in a temperature range of Ae3 ± 200 °C. Annealing can include an operation of heating the cold-rolled sheet and cooling the heated cold-rolled sheet at a predetermined cooling rate. At this time, the cooling rate can be within a range of about 1 °C / second to about 40 °C / second.

[0136] The electroplating operation (S60) is an operation of forming a coating on the annealed steel sheet. In one embodiment, the electroplating operation (S60) can include an operation of forming an Al-Si coating on the steel sheet annealed in the annealing operation (S50).

[0137] Specifically, the electroplating operation (S60) can include an operation of immersing the steel sheet in a plating bath with a temperature range of Bs ± 150 °C to form a molten coating on the steel sheet surface, and a cooling operation of cooling the steel sheet on which the molten coating has been formed to form a coating. At this time, the plating bath can include, 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 can contain 5% to 12% of Si, 1% to 4% of Fe, and the rest Al. In addition, the electroplating amounts on the front and back can be controlled to meet about 40 g / m 2 to about 200 g / m 2 range.

[0138] By performing a hot stamping process on the blank for hot stamping manufactured through steps S10 to S60 as described above, a hot-stamped molded part that meets the required mechanical properties (such as tensile strength, yield strength, bending properties, elongation, etc.) can be manufactured.

[0139] Figure 7 is a cross-sectional view schematically showing a part of the hot stamping part 1 according to an embodiment of the present invention, Figure 8 is a micrograph showing the cross-section of the base steel sheet 10 of the hot stamping part 1 according to an embodiment of the present invention. Figure 8 The base steel sheet 10 shown in can be obtained by referring to the aboveFigures 1 to 6 The base steel plate 10 of the hot-stamped component 1 obtained by the described method for manufacturing a hot-stamped component (for example, Figure 4 the first control curve 410).

[0140] See Figure 7 , the hot-stamped component 1 may include a base steel plate 10 and a coating 20 provided on the base steel plate 10. The coating 20 is formed as an alloy layer on at least one surface of the base steel plate 10, and the coating 20 may include Al, Fe, etc. Although not shown, the coating 20 may include a plurality of layers (not shown) stacked in sequence on the base steel plate 10. In one embodiment, the plurality of layers may sequentially have an α-Fe phase, an Fe2Al5 phase, an AlFe phase, and an Fe2Al5 phase, but the composition of the plurality of layers is not limited thereto.

[0141] The base steel plate 10 may be a steel plate made by performing a hot rolling process and / or a cold rolling process on a cast steel slab so as to contain a predetermined alloy element in a predetermined content as a base steel plate.

[0142] In one embodiment, the base steel plate 10 may contain 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 of iron (Fe) and other inevitable impurities.

[0143] In one embodiment, the base steel plate 10 may contain 0.15 wt% to 0.27 wt% of C, 0.15 wt% to 1.0 wt% of Si, 0.5 wt% to 1.10 wt% of Mn, 0.018 wt% or less of P, 0.005 wt% or less of S, 0.1 wt% to 1.0 wt% of Cr, 0.1 wt% to 1.0 wt% of Al, 0.015 wt% to 0.080 wt% of Ti, 0.015 wt% to 0.080 wt% of Nb, 0.1 wt% to 0.7 wt% of Mo, 0.001 wt% to 0.008 wt% of B, 0.005 wt% or less of N, and the balance of Fe and other inevitable impurities,

[0144] As one embodiment, when the contents of C, Mn, Cr, and Mo contained in the base steel plate 10 are expressed in wt% as [C], [Mn], [Cr], and [Mo], the following mathematical formula 1 may be satisfied.

[0145] [Formula 1]

[0146] 370 < 539 - 423*[C] - 30.4*[Mn] - 12.1[Cr] - 7.5[Mo] < 470

[0147] In this way, an increase in brittleness can be prevented, hardenability can be improved, and the welding strength of the molded part after hot stamping can be enhanced. For example, the cross-tensile strength (CTS) of the welded part can meet 10 kN / point or higher.

[0148] Carbon acts as an austenite stabilizing element in the base steel sheet 10. Carbon is the main element that determines the strength and hardness of the base steel sheet 10, and carbon is added during the heat treatment process to improve hardenability and strength. The content of carbon can be 0.15 wt% to 0.27 wt% of the total weight of the base steel sheet 10. When the carbon content is less than 0.15 wt%, it is difficult to ensure a hard phase (e.g., martensite, etc.), so it is difficult to meet the mechanical strength of the molded part after hot stamping. On the other hand, if the carbon content exceeds 0.27 wt%, the workability of the base steel sheet 10 may decrease or the bending performance of the molded part after hot stamping may deteriorate.

[0149] Silicon acts as a ferrite stabilizing element in the base steel sheet 10. As a solid-solution strengthening element, silicon increases the strength of the base steel sheet 10 and increases the carbon concentration in austenite by suppressing the formation of low-temperature carbides. In addition, silicon is a key element for homogenizing the hot-rolled, cold-rolled, and hot-pressed structures and for micro-dispersing ferrite. Silicon acts as an element for controlling the non-uniformity of martensite strength and plays a role in improving the collision performance. The content range of such silicon can be about 0.15 wt% to about 1.0 wt% of the total weight of the base steel sheet 10. When the silicon content is less than 0.15 wt%, it is difficult to obtain the above effects, and the formation and coarsening of cementite may occur in the martensite structure of the formed part after hot stamping. Conversely, if the silicon content exceeds 1.0 wt%, the hot-rolling and cold-rolling loads increase, and the plating characteristics of the base steel sheet 10 may deteriorate.

[0150] Manganese acts as an austenite stabilizing element in the base steel sheet 10. Manganese is added during the heat treatment process to improve hardenability and strength. The content of such manganese can be about 0.5 wt% to about 1.1 wt% of the total weight of the base steel sheet 10. When the manganese content is less than 0.5 wt%, the hardenability effect is insufficient, and due to insufficient hardenability, the fraction of the hard phase in the molded part after hot stamping may be insufficient. On the other hand, if the manganese content exceeds 1.1 wt%, a pearlite concentration region with manganese segregation may appear, which reduces ductility and toughness, causes the bending performance of the molded part after hot stamping to deteriorate, and leads to non-uniform microstructure.

[0151] Phosphorus is an element that helps to increase strength. To prevent the toughness of the base steel sheet 10 from decreasing, phosphorus can be included in an amount greater than 0 wt% and less than 0.018 wt% of the total weight of the base steel sheet 10. If the phosphorus content exceeds 0.018 wt%, iron-phosphorus compounds will be formed, which reduces toughness and weldability and may cause cracks in the base steel sheet 10 during the manufacturing process.

[0152] Sulfur is an element that helps improve workability. The content of this sulfur can be greater than 0% by weight and less than 0.005% by weight of the total weight of the base steel plate 10. If the sulfur content exceeds 0.005% by weight, hot workability, weldability, and impact properties will deteriorate, and surface defects such as cracks may occur due to the formation of large inclusions.

[0153] Aluminum acts as a ferrite stabilizing element in the base steel plate 10. Aluminum, as a strengthening element, increases the strength of the base steel plate 10 and increases the carbon concentration in austenite by suppressing the formation of carbide at low temperatures. Aluminum acts as a control element for the non-uniformity of martensite strength to improve collision performance. The content range of this aluminum can be about 0.1% by weight to about 1.0% by weight of the total weight of the base steel plate 10. If the content of aluminum is less than 0.1% by weight, it is difficult to obtain the above effects, and cementite formation and coarsening may occur in the martensite structure of the molded part after hot stamping. On the other hand, if the content of aluminum exceeds 1.0% by weight, the hot rolling and cold rolling loads increase, and the plating characteristics of the steel plate may deteriorate.

[0154] As an embodiment, in order to improve plating performance, the total content of Si and Al contained in the base steel plate 10 can be controlled to meet a preset range. For example, the total content of Si and Al contained in the base steel plate 10 can meet 0.4% by weight to 1.5% by weight.

[0155] The addition of chromium is to improve the hardenability and strength of the base steel plate 10 during the heat treatment process. Chromium can obtain grain refinement and strength through precipitation hardening. The content of chromium can be about 0.1% by weight to about 1.0% by weight of the total weight of the base steel plate 10. When the content of chromium is less than 0.1% by weight, the precipitation hardening effect is low. On the contrary, when the content of chromium exceeds 1.0% by weight, Cr-based precipitates and matrix solid solutions increase, which reduces toughness and increases production costs due to increased costs.

[0156] Titanium can effectively contribute to grain refinement by forming precipitates at high temperatures. The content range of this titanium can be about 0.015% by weight to about 0.080% by weight of the total weight of the base steel plate 10. When the content of titanium is within the above range, poor performance and coarsening of precipitates can be prevented, the physical properties of the steel can be easily ensured, and defects such as cracks on the steel surface can be prevented. When the titanium content is less than 0.015% by weight, the above effects may not be obtained properly. On the other hand, when the titanium content exceeds 0.080% by weight, the precipitates may coarsen, resulting in a decrease in elongation and bendability.

[0157] Ti, Nb, and Mo form fine precipitates in the form of nitrides or carbides, thereby ensuring the strength of hot-stamped and quenched components. Additionally, these are contained in the Fe-Mn-based composite oxide and act as hydrogen trapping sites effective in improving the delayed fracture property, and are elements necessary for improving the delayed fracture property.

[0158] More specifically, niobium can increase strength and toughness due to the reduction in martensite packet size. The content range of niobium can be from about 0.015 wt% to about 0.080 wt% of the total weight of the base steel plate 10. When the content of niobium is within the above range, the base steel plate 10 has an excellent grain refinement effect during hot rolling and cold rolling, and can prevent slab cracks and product brittle fracture during the steelmaking / rolling process, and can minimize the formation of coarse precipitates during the steelmaking process. If the content of niobium is less than 0.015 wt%, the above effects may not be obtained properly. On the other hand, when the niobium content exceeds 0.080 wt%, the strength and toughness no longer increase due to the increase in niobium content, and since niobium exists in a solid solution state in ferrite, there is a risk of strength reduction.

[0159] Molybdenum is a substitutional element that improves the strength of steel through its solid solution strengthening effect. The purpose of adding molybdenum is to inhibit the coarsening of precipitates and improve hardenability. In addition, molybdenum can play a role in improving the hardenability of steel. The content range of this molybdenum can be from about 0.1 wt% to about 0.7 wt% of the total weight of the base steel plate 10. If the content of molybdenum is less than 0.1 wt%, the above effects may not be obtained properly. On the other hand, if the molybdenum content exceeds 0.7 wt%, there is a risk of a decrease in rolling productivity and elongation, and there is a problem of an increase in manufacturing cost without an increased effect.

[0160] The purpose of adding boron is to ensure a martensite structure by inhibiting the transformation of ferrite, pearlite, and bainite, thereby ensuring hardenability and strength during the heat treatment process. In addition, boron segregates at the grain boundaries to reduce the grain boundary energy, thereby improving hardenability and having a grain refinement effect by increasing the austenite grain growth temperature. The content range of this boron can be from about 0.001 wt% to about 0.008 wt% of the total weight of the base steel plate 10. When the content of boron is within the above range, embrittlement of the hard phase grain boundaries can be prevented, and high toughness and bendability can be ensured. When the boron content is less than 0.001 wt%, the hardenability effect is insufficient. On the contrary, when the boron content exceeds 0.008 wt%, the solid solubility is low, and therefore, depending on the heat treatment conditions, it may easily precipitate at the grain boundaries, which may reduce the hardenability or cause high-temperature embrittlement, and the toughness and bendability may decrease due to the occurrence of hard grain boundary embrittlement. When the nitrogen content exceeds 0.005 wt%, large TiN inclusions may be generated, which may reduce the bendability.

[0161] According to an embodiment of the present invention, the base steel sheet 10 of the hot stamping part 1 may include fine precipitates. That is, the fine precipitates may be distributed within the base steel sheet 10. Some of the elements contained in the above base steel sheet 10 may be nitride or carbide forming elements that contribute to the formation of fine precipitates. Specifically, Ti, Nb, and Mo may form fine precipitates in the form of nitrides or carbides.

[0162] Therefore, the hot stamping part 1 according to the embodiment of the present invention may include fine precipitates distributed within the base steel sheet 10, and these fine precipitates may include nitrides or carbides of at least one of Ti, Nb, and Mo. These fine precipitates may inhibit the propagation of cracks when the hot stamping part 1 is bent. In other words, during the bending deformation process, the movement of dislocations may be restricted by the fine precipitates during the movement of the dislocations.

[0163] The number and average diameter of the fine precipitates formed in the base steel sheet 10 can be controlled to meet a preset range. In one embodiment, fine precipitates with a diameter of about 0.02 microns or less may be distributed in the base steel sheet 10 at a ratio of 9,000 / 100 square microns or more and 30,000 / 100 square microns or less per unit area (100 square microns). In addition, in one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet 10 may be 0.006 microns or less, preferably 0.003 microns or more and 0.006 microns or less.

[0164] The number and average diameter of such fine precipitates may affect the inhibition of crack propagation. When the number and average diameter of the fine precipitates are formed within the above ranges, the required tensile strength after hot stamping can be ensured and the bendability can be improved. If the number of fine precipitates per unit area (100 square microns) is less than 9,000 / 100 square microns, the strength of the hot stamping part 1 may be reduced. If the number of fine precipitates per unit area (100 square microns) exceeds 30,000 / 100 square microns, the bendability of the hot stamping part 1 may be reduced. On the other hand, if the average diameter of the fine precipitates is less than about 0.003 microns, the size of the fine precipitates may be small, so it may be difficult to restrict the movement of dislocations. If the average diameter of the fine precipitates exceeds about 0.006 microns, the number of fine precipitates may be relatively reduced, so the movement of dislocations may not be effectively restricted.

[0165] On the other hand, the base steel plate 10 may include a martensitic structure having a distributed microstructure. The martensitic structure is the result of a diffusionless transformation of austenite γ below the martensite transformation start temperature (Ms) during the cooling process. The microstructure within the martensitic structure is a diffusionless transformation structure formed during rapid cooling within grains called prior austenite grain boundaries (PAGBs), and may include a plurality of lath structures. The plurality of lath structures may also form units such as blocks or bundles. More specifically, the plurality of lath structures may form a block, a plurality of blocks may form a bundle, and a plurality of bundles may form a PAGB.

[0166] As Figure 8 shown, the base steel plate 10 of the hot stamping part 1 according to an embodiment of the present invention may include a martensitic structure having a plurality of blocks. The plurality of blocks may form a bundle, and each of the plurality of blocks may be formed by elongated rod-shaped laths oriented in one direction. In other words, the martensitic structure may include a plurality of lath structures.

[0167] The boundaries between the unit structures may be formed within the martensitic structure by a plurality of blocks, a plurality of bundles, or a plurality of laths. Specifically, lath boundaries (which are the boundaries between laths) may be formed within the martensitic structure by a plurality of laths. Similarly, block boundaries (which are the boundaries between blocks) may be formed within the martensitic structure by a plurality of blocks, and bundle boundaries (which are the boundaries between bundles) may be formed within the martensitic structure by a plurality of bundles. That is to say, in this specification, the boundaries between the unit structures may be lath boundaries, block boundaries, or bundle boundaries. These boundaries between the unit structures may have the property of resisting external deformation.

[0168] Specifically, the cracks generated during the bending deformation process of the hot stamping part 1 may be due to the movement of one-dimensional defects (called dislocations) within the martensitic structure through interaction. Therefore, during the movement of dislocations within the martensitic structure during the bending deformation process, the movement of dislocations at the boundaries between the unit structures can be restricted.

[0169] In addition, if the hardness of the regions adjacent to the boundaries between the unit structures within the martensitic structure is different, the performance of resisting external deformation at these boundaries between the unit structures may be weakened. In other words, the greater the difference in hardness (such as nanoindentation hardness) within the martensitic structure, the weaker the performance of resisting external deformation at the boundaries between the unit structures. In other words, if the nanoindentation hardness within the martensitic structure is uniformly formed above a certain level, the hot stamping part 1 can ensure bendability. Therefore, it is necessary to appropriately control the uniformity of the nanoindentation hardness within the martensite.

[0170] [Table 1] shows the nanoindentation hardness, standard deviation of nanoindentation hardness, average value of nanoindentation hardness, coefficient of variation, and bending angle of the hot stamping parts according to the embodiments and comparative examples of the present invention.

[0171] The nanoindentation hardness is measured using a nanoindenter. Specifically, a cubic - corner tip (angle between the centerline and the face = 35.3°, indentation strain rate = 0.22) is used as the nanoindenter, and the nanoindentation hardness is measured at more than 20 different points within a PAGB. However, the present invention is not limited thereto. For example, a Berkovich tip (angle between the centerline and the face = 65.3°, indentation strain rate = 0.072) can also be used as the nanoindenter.

[0172] Using these nanoindentation hardness values, the average value and standard deviation of the nanoindentation hardness are calculated. The coefficient of variation can be defined as the standard deviation of the nanoindentation hardness divided by the average value of the nanoindentation hardness. Therefore, the coefficient of variation can also represent the uniformity of the nanoindentation hardness. Since measuring the nanoindentation hardness using a nanoindenter is a common practice in nanoindentation hardness measurement, its detailed description is omitted.

[0173] The nanoindentation hardness in [Table 1] is measured on one PAGB, but the same or similar nanoindentation hardness is measured on another PAGB other than the PAGB related to [Table 1]. Therefore, the nanoindentation hardness, the standard deviation of the nanoindentation hardness, the average value of the nanoindentation hardness, and the coefficient of variation in [Table 1] represent the nanoindentation hardness, the standard deviation of the nanoindentation hardness, the average value of the nanoindentation hardness, and the coefficient of variation in the martensitic structure. The above - mentioned uniformity of the nanoindentation hardness can be evaluated by the standard deviation or the coefficient of variation of the nanoindentation hardness.

[0174] In addition, the bending performance of the hot - stamped part can be evaluated by the bending angle. In the specification, the "bending angle" can represent the V - shaped bending angle in the rolling direction (RD). The V - shaped bending angle is a parameter for evaluating the bending deformation performance of the maximum load section among the deformations that occur in the bending performance of the hot - stamped part. That is, according to the load - displacement evaluation of the hot - stamped part 1, when examining the tensile deformation region during bending at the macro and micro scales, if microcracks are generated and propagated in the local tensile region, the bending performance called the V - shaped bending angle can be evaluated.

[0175] [Table 1]

[0176]

[0177] Referring to [Table 1], in the embodiments of the present invention, the nanoindentation hardness can be 3.0 GPa or greater and 5.0 GPa or less. Preferably, the nanoindentation hardness can be 3.05 GPa or greater and 4.95 GPa or less. More preferably, the nanoindentation hardness can be 3.18 GPa or greater and 4.95 GPa or less. The standard deviation of the nanoindentation hardness can be greater than 0 GPa and less than 0.8 GPa. Preferably, the standard deviation of the nanoindentation hardness can be 0.2 GPa or greater and 0.6 GPa or less. More preferably, the standard deviation of the nanoindentation hardness can be greater than 0.28 GPa and less than 0.59 GPa. The coefficient of variation can be greater than 0 and less than 0.2. Preferably, the coefficient of variation can be 0.05 or greater and 0.15 or less. More preferably, the coefficient of variation can be 0.06 or greater and 0.14 or less.

[0178] In the case of the comparative example, it can be confirmed that the nanoindentation hardness is 2.4 GPa or greater and 5.2 GPa or less, the standard deviation of the nanoindentation hardness exceeds 0.8 GPa, and the coefficient of variation exceeds 0.2. In other words, compared with the embodiment, the nanoindentation hardness uniformity within the martensite structure of the comparative example is relatively low. Therefore, the anti-external deformation characteristics at the boundaries between the unit structures may be poor. Therefore, it can be confirmed that the comparative example has relatively low bending characteristics of less than 70° due to the easier occurrence and / or propagation of microcracks.

[0179] That is, when the nanoindentation hardness within the martensite structure is uniformly formed above a certain level, the hot stamping part 1 can ensure bendability. The bending angle of the hot stamping part 1 according to the embodiment of the present invention can be 70° or greater and 85° or less. On the other hand, when the standard deviation or the coefficient of variation of the nanoindentation hardness exceeds the above range, the uniformity of the nanoindentation hardness within the martensite structure deteriorates. Therefore, it may be difficult to ensure sufficient bendability of the hot stamping part 1. When the standard deviation or the coefficient of variation of the nanoindentation hardness is less than the above range, the uniformity of the nanoindentation hardness within the martensite structure increases, but the manufacturing cost of the hot stamping part 1 may increase excessively.

[0180] The uniformity of the nanoindentation hardness within the martensite structure may be affected by the uniformity of the carbon content and the manganese content within the martensite structure. That is, when the carbon content and the manganese content within the martensite structure are uniformly formed, the nanoindentation hardness within the martensite structure may also be uniformly formed.

[0181] [Table 2] shows the carbon content and the standard deviation of the carbon content of the hot stamping parts according to the embodiments and comparative examples of the present invention.

[0182] The carbon content was measured at over 20 different points within a PAGB. In addition, the carbon content in [Table 2] was measured in one PAGB, but the same or similar carbon content was measured in PAGBs other than the PAGB related to [Table 2]. Therefore, the carbon content and the standard deviation of the carbon content in [Table 1] represent the carbon content and the standard deviation of the carbon content in the martensitic structure. The uniformity of the above carbon content can be evaluated by the standard deviation of the carbon content.

[0183] [Table 2]

[0184]

[0185] Referring to [Table 2], in an embodiment of the present invention, the carbon content can be 0.14 wt% or more and 0.22 wt% or less. The standard deviation of the carbon content can be greater than 0 wt% and less than 0.04 wt%. Preferably, the standard deviation of the carbon content can be 0.01 wt% or more and 0.03 wt% or less.

[0186] In the case of the comparative example, it can be confirmed that the carbon content is 0.11 wt% or more and 0.23 wt% or less, and the standard deviation of the carbon content is 0.04 wt% or more. In other words, compared with the embodiment, the uniformity of the carbon content in the martensitic structure in the comparative example may be relatively low.

[0187] Figure 9 is a micrograph showing a cross-section of the base steel plate 10 of the hot stamping part 1 according to an embodiment of the present invention. Figure 10 is a micrograph showing a cross-section of the base steel plate of the hot stamping part according to the comparative example. Specifically, Figure 9 is a micrograph for explaining the manganese content uniformity of the base steel plate 10 of the hot stamping part 1 according to an embodiment of the present invention, Figure 10 is a micrograph for explaining the manganese content uniformity of the base steel plate of the hot stamping part according to the comparative example. Figure 9 and Figure 10 the brightness or contrast of the corresponding regions in may vary due to different manganese contents. That is to say, the manganese content in the dark regions may be relatively higher than that in the bright regions.

[0188] In the base steel plate 10 of the hot stamping part 1 according to an embodiment of the present invention, an overall uniform bright region as shown in Figure 9 is observed. However, as shown in Figure 10 , it is observed that the base steel plate of the hot stamping part according to the comparative example has darker regions in some areas than the remaining areas. Specifically, in Figure 10A line that extends left and right and is darker than the rest of the area appears at the center. That is, since the base steel plate of the hot stamping part of the comparative example has a high manganese content in some areas, the uniformity of the manganese content in the martensite structure of the comparative example may be relatively low compared to the embodiment.

[0189] As described above, compared with the embodiment, the uniformity of the carbon content and the manganese content in the martensite structure of the comparative example may be relatively low. Therefore, compared with the embodiment, since carbon and manganese are unevenly distributed in the martensite structure, the uniformity of the nanoindentation hardness in the martensite structure of the comparative example may be relatively low. When the standard deviation of the carbon content exceeds the above range, the uniformity of the nanoindentation hardness in the martensite structure decreases, and it may be difficult for the hot stamping part 1 to ensure sufficient bendability. When the standard deviation of the carbon content is less than the above range, the uniformity of the nanoindentation hardness in the martensite structure is improved, but the manufacturing cost of the hot stamping part 1 may increase excessively.

[0190] The nanoindentation hardness and the standard deviation of the nanoindentation hardness of the base steel plate 10 can be controlled by adjusting the process conditions of the manufacturing process of the above hot stamping part 1. Of course, since the carbon content and the standard deviation of the carbon content are related to the nanoindentation hardness and the standard deviation of the nanoindentation hardness, the carbon content and the standard deviation of the carbon content can also be controlled by adjusting the process conditions of the manufacturing process of the hot stamping part.

[0191] [Table 3] shows the contents of C, Si, Mn, P, S, Cr, Al, Ti, Nb, Mo, B, and N contained in each base steel plate of the hot stamping parts according to the embodiments and comparative examples of the present invention.

[0192] [Table 3]

[0193]

[0194] The base steel plates of the hot stamping parts according to the embodiments and comparative examples of the present invention satisfy the above content ranges of C, Si, Mn, P, S, Cr, Al, Ti, Nb, Mo, B, and N. That is, each base steel plate of the hot stamping parts according to the embodiments and comparative examples of the present invention contains: 0.15 wt% to 0.27 wt% of C, 0.15 wt% to 1.0 wt% of Si, 0.5 wt% to 1.10 wt% of Mn, 0.018 wt% or less of P, 0.005 wt% or less of S, 0.1 wt% to 1.0 wt% of Cr, 0.1 wt% to 1.0 wt% of Al, 0.015 wt% to 0.080 wt% of Ti, 0.015 wt% to 0.080 wt% of Nb, 0.1 wt% to 0.7 wt% of Mo, 0.001 wt% to 0.008 wt% of B, 0.005 wt% or less of N, the balance of Fe, and other inevitable impurities.

[0195] [Table 4] shows the process conditions of the manufacturing processes of the hot stamping parts according to the embodiments and comparative examples of the present invention. Specifically, [Table 4] shows the SRT in the reheating operation (S10), the FDT in the hot rolling operation (S20), the CT in the cooling / coiling operation (S30), and the annealing temperature in the annealing operation (S50) in the manufacturing processes of the hot stamping parts according to the embodiments and comparative examples of the present invention.

[0196] [Table 4]

[0197]

[0198] The hot stamping parts of the embodiments and comparative examples of the present invention are manufactured through a reheating operation (S10) with an SRT in the range of about 1,100 °C to about 1,300 °C, a hot rolling operation (S20) with an FDT in the range of about 800 °C to about 1,000 °C, and an annealing heat treatment operation (S50) with an annealing temperature of Ae3 ± 200 °C. However, the hot stamping parts according to the embodiments of the present invention are manufactured through a cooling / coiling operation (S30) with a CT of Ms + 50 °C or greater and less than 650 °C, while the hot stamping parts according to the comparative examples are manufactured through a cooling / coiling operation (S30) with a CT greater than 650 °C. That is, the manufacturing processes of the hot stamping parts according to the embodiments and comparative examples of the present invention are the same or similar in terms of process conditions except for the coiling temperature. Since the hot stamping parts of the embodiments and comparative examples of the present invention are both manufactured through an electroplating operation (S60), the hot stamping parts of the embodiments and comparative examples of the present invention can both include an Al-Si coating.

[0199] As described above, the size, density, and area fraction of the regions where pearlite locally aggregates (pearlite regions) with a relatively high content of C and / or Mn in the blank are affected by CT. That is, the uniformity of the content of C and / or Mn in the blank is affected by CT. After the blank preparation operation (S1), such pearlite regions in the hot-stamped part can be made to disappear or minimized by heating or the like. However, the content of C and / or Mn in the blank has a certain influence on the content of C and / or Mn in the stamped part 1. Therefore, the content of C and / or Mn in the blank also affects the nanoindentation hardness and bending angle in the stamped part 1. Therefore, by controlling CT in the cooling / coiling operation (S30) in the hot-stamped part manufacturing process, the martensite structure characteristics of the hot-stamped part 1 can be optimized, thereby ensuring that the hot-stamped part 1 has excellent mechanical properties of high strength and high toughness. However, the present invention is not limited thereto. For example, by controlling process conditions other than CT in the hot-stamped part manufacturing process, a hot-stamped part 1 having a nanoindentation hardness and a standard deviation of nanoindentation hardness within the above ranges can be manufactured.

[0200] Thereby, the mechanical properties of the hot-stamped part 1, such as tensile strength, yield strength, bending characteristics, and elongation, can be controlled. For example, the tensile strength of the hot-stamped part 1 can satisfy 1,350 MPa or more, preferably 1,350 MPa or more and 1,650 MPa or less. In addition, the yield strength of the hot-stamped part 1 can satisfy 950 MPa or more, preferably 950 MPa or more and 1,200 MPa or less. In addition, the hot-stamped part 1 can satisfy a bending angle of 70° or more and 85° or less and have an elongation of 6% or more. Preferably, the elongation of the hot-stamped part 1 is 6% or more and 9% or less.

[0201] [Table 5] shows the tensile strength, yield strength, and elongation of the hot-stamped parts according to the embodiments and comparative examples of the present invention. As described above, each hot-stamped part according to the embodiments of the present invention can have 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.

[0202] [Table 5]

[0203] Tensile strength (MPa) Yield strength (MPa) Elongation at break (%) Embodiment 1 1460 1050 7 Embodiment 2 1448 1041 8 Embodiment 3 1445 1056 8 Embodiment 4 1447 1046 8 Embodiment 5 1447 1045 8 Embodiment 6 1447 1059 9 Comparative Example 1 1442 1060 8 Comparative Example 2 1467 1064 7 Comparative Example 3 1466 1060 8

[0204] Although the present invention has been described with reference to the embodiments shown in the drawings, those skilled in the art will understand that various changes can be made to its form and details without departing from the spirit and scope of the inventive concept. Therefore, the scope of the present invention is not defined by the detailed description of the present invention, but by the appended claims.

Claims

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

2. The hot stamping part according to claim 1, wherein The bending angle of the hot stamping part is 70° or greater and 85° or less.

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

4. The hot stamping part according to claim 1, wherein The standard deviation of the carbon content of the martensite structure is less than 0.04 wt%.

5. The hot stamping part according to claim 1, which further includes: Fine precipitates distributed in the base steel plate, and The fine precipitates include at least one carbide of Ti, Nb, and Mo.

6. The hot stamping part according to claim 5, wherein The number of fine precipitates distributed per unit area (100 square microns) is 9,000 or more and 30,000 or less.

7. The hot stamping part according to claim 5, wherein The average diameter of the fine precipitates is 0.003 microns or greater and 0.006 microns or less.

8. The hot stamping part according to claim 1, wherein The tensile strength of the hot stamping part is 1350 MPa or greater and 1650 MPa or less.

9. The hot stamping part according to claim 1, wherein The yield strength of the hot stamping part is 950 MPa or greater and 1200 MPa or less.

10. The hot stamping part according to claim 1, wherein The elongation of the hot stamping part is 6% or greater.

11. The hot stamping part according to claim 1, wherein The martensite structure includes a plurality of lath structures.

12. The hot stamping part according to claim 1, which further includes: A coating provided on the base steel plate.

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