Hot-stamping component and manufacturing method therefor

Abstract

To improve component performance of a hot-stamping blank such as strength characteristic, hydrogen brittleness depending on a mixed hydrogen quantity, and weldability depending on a plated layer structure by controlling a temperature rise rate for each section.SOLUTION: A manufacturing method for a hot-stamping component includes: a step at which a blank, in which a plated layer is formed on at least one face of a base material, is inputted into a heating furnace including a plurality of sections having temperature rise rate ranges different from one another; and a multistage heating step at which the blank passes the plurality of sections, and is gradually heated. The plurality of sections include: a first heating section that has a first average temperature rise speed change rate; a second heating section after the first heating section that has a second average temperature rise speed change rate which is different from the first average temperature rise speed change rate; and a third heating section after the second heating section that has a third average temperature rise speed change rate which is different from the first average temperature rise speed change rate and the second average temperature rise speed change rate. The third average temperature rise speed change rate includes a section where values change from positive to negative.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a hot stamped part and a method for manufacturing the same. [Background technology]

[0002] In the automotive industry, the need for lightweight yet high-strength automotive materials is increasing due to the tightening of environmental regulations and safety standards. Hot stamping technology has attracted attention as a manufacturing method for such high-strength and lightweight automotive materials, and research and development related to hot stamping materials is being actively conducted.

[0003] The hot stamping process generally involves heating, forming, cooling, and trimming, and can utilize microstructural changes such as phase transformation of the material and alloying of the coating during the process. During the hot stamping process, the amount of hydrogen incorporation increases during heating or heat treatment, which can lead to problems such as reduced weldability due to the coating layer structure, resulting in reduced component performance. In particular, the microstructural characteristics of the hot stamped material can vary depending on the heating or heat treatment temperature, which can affect hydrogen embrittlement and weldability.

[0004] Therefore, precise control of the part performance of hot stamped materials using the hot stamping heat treatment temperature conditions is required. Related technologies include Korean Patent Publication No. 10-2013-0136565 (Title of Invention: Steel Sheet for Hot Stamping Members and Manufacturing Method Thereof). Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a hot stamped part and a manufacturing method thereof, in which the part performance of a hot stamped material, such as strength characteristics, hydrogen embrittlement due to the amount of hydrogen entrainment, and weldability due to the coating layer structure, is improved by controlling the hot stamping heating temperature. [Means for solving the problem]

[0006] A method for manufacturing a hot stamped part according to an embodiment of the present invention includes: placing a blank having a coating layer formed on at least one surface of a base material into a heating furnace having a plurality of zones with different heating rate ranges; and heating the blank in a stepwise manner through the plurality of zones, wherein the plurality of zones include: a first heating zone having a first average heating rate change rate; a second heating zone subsequent to the first heating zone, having a second average heating rate change rate that is different from the first average heating rate change rate; and a third heating zone subsequent to the second heating zone, having a third average heating rate change rate that is different from the first average heating rate change rate and the second average heating rate change rate, wherein the third average heating rate change rate includes a zone in which the third average heating rate change rate changes from a positive value to a negative value.

[0007] The change from the first average rate of change of heating rate to the second average rate of change of heating rate is also discontinuous between the first heating section and the second heating section.

[0008] The third heating section includes a 3-1 heating section having a 3-1 average heating rate change rate and a 3-2 heating section having a 3-2 average heating rate change rate, but the 3-1 average heating rate change rate has a positive value, the 3-2 average heating rate change rate has a negative value, and the absolute value of the 3-1 average heating rate change rate is smaller than the absolute value of the 3-2 average heating rate change rate.

[0009] The first average heating rate change rate and the second average heating rate change rate each have a negative value, and the absolute value of the first average heating rate change rate is greater than the absolute value of the second average heating rate change rate.

[0010] The plurality of sections further includes, after the third heating section, a fourth heating section having a fourth average heating rate change rate different from the first average heating rate change rate, the second average heating rate change rate, and the third average heating rate change rate, and the absolute value of the fourth average heating rate change rate is smaller than the absolute values ​​of the first average heating rate change rate, the second average heating rate change rate, and the third average heating rate change rate.

[0011] The 3-1 average temperature rise rate change rate is 0 to 0.25°C / s 2 The 3-2 average temperature rise rate change rate is -0.3°C / s 2 It can have a value of greater than or equal to 0.

[0012] The first average temperature rise rate change rate is -0.5°C / s 2 the second average temperature rise rate change rate is -0.25°C / s 2 It can have a value of greater than or equal to 0.

[0013] In the second heating zone, the plating layer may be alloyed, and in the third heating zone, the base material may undergo a phase transformation.

[0014] The hot stamped part manufactured by the method according to one embodiment of the present invention has an amount of hydrogen contamination of 0 to less than 0.21 ppm and a dynamic resistance of more than 0 to 0.8 mΩ. [Effects of the Invention]

[0015] According to an embodiment of the present invention, by controlling the temperature rise rate in each section in the hot stamping heating process, it is possible to improve the part performance of the hot stamped material, such as strength characteristics, hydrogen embrittlement due to the amount of hydrogen mixed in, and weldability due to the coating layer structure, for the hot stamped steel. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a flow chart that schematically illustrates a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Figure 2] 1 is a graph showing the temperature change of a blank when the blank is heated singly by a conventional method. [Figure 3] 4 is a graph showing temperature changes when a blank is subjected to multi-stage heating and soak heating in a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Figure 4] 4 is a graph showing a rate of change in temperature rise rate in a plurality of sections according to heating time in a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Figure 5] 1 is a scanning electron microscope (SEM) image showing a cross section of a hot stamped part according to one embodiment of the present invention. [Figure 6] 1 is a scanning electron microscope (SEM) image showing a cross section of a hot stamped part according to a comparative embodiment of the present invention. [Figure 7] 1 is a plan view schematically illustrating a blank used in a method for manufacturing a hot stamped part according to an embodiment of the present invention; FIG. [Figure 8] 1 is a plan view schematically illustrating a blank placed in a heating furnace in a method for manufacturing a hot stamped part according to an embodiment of the present invention; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention can be modified in various ways and can have various embodiments, but specific embodiments are illustrated in the drawings and will be described in detail in the detailed description. The advantages, features, and methods of achieving the same of the present invention will become clearer with reference to the embodiments described in detail below together with the drawings. However, the present invention is not limited to the embodiments disclosed below and may be embodied in various forms.

[0018] In the following embodiments, terms such as "first" and "second" are not used in a limiting sense but are used to distinguish one component from another.

[0019] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0020] In the following embodiments, terms such as "comprise" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0021] In the following embodiments, when a part such as a film, region, or component is said to be "on" or "above" another part, this does not only mean that it is directly on top of the other part, but also means that there is another film, region, component, etc. interposed between them.

[0022] In the drawings, the size of components may be exaggerated or reduced for the sake of clarity. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the sake of clarity, but the present invention is not necessarily limited to those shown in the drawings.

[0023] If an embodiment can be implemented differently, the order of certain steps may be performed differently than described. For example, two steps described as successive steps may be performed substantially simultaneously or may be performed in the reverse order from that described.

[0024] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. When describing with reference to the drawings, identical or corresponding components will be designated by the same reference numerals.

[0025] 1 is a flow chart that schematically illustrates a method for manufacturing a hot stamped part according to an embodiment of the present invention. In the following, the method for manufacturing a hot stamped part will be described with reference to FIG.

[0026] A method for manufacturing a hot stamped part according to an embodiment of the present invention may include a blank insertion step (S100) and a multi-stage heating step (S200), or may further include a transfer step (S300), a forming step (S400), and a cooling step (S500) after the multi-stage heating step (S200).

[0027] First, the blank insertion step (S100) is a step of inserting a blank into a heating furnace having a plurality of sections each having a different heating rate range. The blank may be in the form of a base material having a plating layer formed on at least one surface thereof.

[0028] The blanks fed into the heating furnace are also formed by cutting a sheet material (or base material) for forming hot stamped parts. The sheet material can also be produced by hot-rolling or cold-rolling a steel slab and then annealing it. After the annealing, an Al-Si based plating layer or a Zn plating layer can be formed on at least one side of the annealed sheet material, but the type of plating layer formed on one side of the base material is not limited thereto.

[0029] After the blank insertion step (S100), a multi-stage heating step (S200) may be performed. The multi-stage heating step (S200) is a step in which the blank passes through multiple sections provided in the heating furnace and is heated in stages. The multi-stage heating step (S200) will be described in more detail with reference to the graphs of FIGS. 2 to 4, which will be described later.

[0030] After the multi-stage heating step (S200), a transferring step (S300), a forming step (S400), and a cooling step (S500) may be further performed.

[0031] The transfer step (S300) is also a step of transferring the soaked blank from the heating furnace to the press die in the multi-stage heating. In the step of transferring the soaked blank from the heating furnace to the press die, the soaked blank is air-cooled for 7 to 15 seconds, preferably 10 to 15 seconds.

[0032] The forming step (S400) is a step of hot stamping the transferred blank to form a compact, and the cooling step (S500) is a step of cooling the formed compact.

[0033] After being formed into the final part shape in a press die, the formed body can be cooled to form the final product. The press die can be equipped with cooling channels through which a refrigerant circulates. The refrigerant supplied through the cooling channels of the press die can circulate the heated blank, allowing it to be rapidly cooled simultaneously with forming. Rapid cooling can be performed while the press die is closed and pressurized to prevent springback of the sheet material and maintain the desired shape. During the forming and cooling of the heated blank, the blank can be cooled to the martensite finish temperature at an average cooling rate of at least 10°C / s. The blank can be maintained in the press die for 3 to 20 seconds. If the maintenance time in the press die is less than 3 seconds, a sufficient amount of martensite is not generated, resulting in insufficient mechanical properties. Furthermore, if the maintenance time in the press die exceeds 20 seconds, the maintenance time in the press die becomes too long, reducing productivity.

[0034] FIG. 2 is a graph showing the temperature change of a blank when the blank is heated by a single heating method according to the prior art. Specifically, FIG. 2 shows the temperature change of a blank when the internal temperature of the heating furnace is higher than the target temperature T t 3 is a graph showing the temperature change of a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm over time when the blanks are simultaneously heated (310, 320) after the temperature of the heating furnace is set to maintain the same value.

[0035] At this time, the target temperature of the blank T t is also equal to or greater than Ac3 (the temperature at which the transformation from ferrite to austenite is complete). tis also about 930°C. t The temperature at which the blanks are heated is about 950°C. However, the present invention is not limited to this. Furthermore, the single heating does not mean that a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm are placed in a heating furnace and heated separately, but means that the temperature of the heating furnace is set to a single temperature, and then a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm are placed in the heating furnace at the same time and heated.

[0036] Referring to Figure 2, the temperature inside the heating furnace is set to the target temperature T t After setting the temperature to the same value, when a blank with a thickness of 1.2 mm and a blank with a thickness of 1.6 mm are heated simultaneously, the blank with a thickness of 1.2 mm reaches the target temperature T t It is clear that this is the first step.

[0037] That is, a blank having a thickness of 1.2 mm is first heated to a target temperature T t The blanks having a thickness of 1.2 mm are soaked for a first time S1 (310), and the blanks having a thickness of 1.6 mm are soaked for a second time S2, which is shorter than the first time S1 (320). Because the soaking time is adjusted based on the blank that reaches the target temperature later, the 1.2 mm blank that reaches the target temperature first is overheated, which increases the hydrogen delayed fracture of the 1.2 mm blank and reduces weldability.

[0038] In addition, the target temperature T tSetting the control range based only on temperature and time has the problem of being unable to effectively control part performance. For example, a part including an Al-Si plating layer may undergo alloying of the plating layer and phase transformation of the base material during the hot stamping process. The part's performance, such as the structure of the plating layer, the thickness of the interdiffusion layer, plating layer peeling, formability, hydrogen embrittlement, and weldability, is determined differently depending on the temperature history applied to the part and the hot stamping process control. Existing hot stamping processes use the aforementioned final target temperature T t Alternatively, the temperature may be controlled based on the overall heating rate, but there is a limit to the precise control of part performance when the process is controlled solely based on the target temperature and time. Therefore, in the following embodiments of the present invention, the temperature rise rate change rate for the blank is controlled to easily and precisely control part performance.

[0039] Hereinafter, with reference to Figs. 3 and 4, a description will be given of the multiple sections through which the blank passes and is heated in stages in the multi-stage heating stage.

[0040] 3 is a graph showing temperature changes over time when a blank is subjected to multi-stage heating and soak heating in a method for producing a hot stamped part according to one embodiment of the present invention. More specifically, FIG. 3 is a graph showing temperature changes over time when a 1.2 mm thick blank is subjected to multi-stage heating (510) and a 1.6 mm thick blank is subjected to multi-stage heating (520) in one embodiment of the present invention.

[0041] Referring to FIG. 3, a heating furnace according to one embodiment may have multiple sections P1, P2, P3, and P4 having different temperature ranges. More specifically, the heating furnace may 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. In this case, the third heating section T3 may have two sections having different temperature ranges. The third heating section T3 may also include a 3-1 heating section P3-1 having a 3-1 temperature range T3-1 and a 3-2 heating section P3-2 having a 3-2 temperature range T3-2. According to one embodiment, the second heating section T2 may have multiple sections having different temperature ranges. The second heating section T2 may include a 2-1 heating section P2-1 having a 2-1 temperature range T2-1 through a 2-n heating section P2-n having a 2-n temperature range T2-n. The first heating section T1 may also include multiple sections having different temperature ranges. The first heating section T1 may include a 1-1 heating section P1-1 having a 1-1 temperature range T1-1 through a 1-n heating section P1-n having a 1-n temperature range T1-n.

[0042] The first through fourth heating zones P1 through P4 are arranged in order within the heating furnace. The first heating zone P1 may be adjacent to the entrance of the heating furnace where the blanks are inserted, and the fourth heating zone P4 may be adjacent to the exit of the heating furnace where the blanks are removed. Therefore, the first heating zone P1 having the first temperature range T1 is the first zone of the heating furnace, and the fourth heating zone P4 having the fourth temperature range T4 is the last zone of the heating furnace. As will be described later, the fourth heating zone P4, which is the last zone of the heating furnace, is not a zone where multi-stage heating is performed, but rather a zone where soaking heating is performed.

[0043] The temperatures of the multiple zones within the heating furnace, for example, the temperatures of the first heating zone P1 through the fourth heating zone P4, may increase from the entrance of the heating furnace where the blanks are inserted toward the exit of the heating furnace where the blanks are removed. In addition, the temperature difference between two adjacent zones within the multiple zones within the heating furnace may be greater than 0°C and less than 100°C. For example, the temperature difference between the first heating zone P1 and the second heating zone P2 may be greater than 0°C and less than 100°C.

[0044] In one embodiment, the first temperature range T1 of the first heating section P1 is 840°C to 860°C or 835°C to 865°C. The second temperature range T2 of the second heating section P2 is 870°C to 920°C or 865°C to 925°C. The third-first temperature range T3-1 of the third-first section P3-1 is 920°C to 940°C or 915°C to 945°C. The third-second temperature range T3-2 of the third-second section P3-2 is 940°C to 960°C or 935°C to 965°C. The fourth temperature range T4 of the fourth heating section P4 is Ac3 to 1,000°C. Preferably, the fourth temperature range T4 of the fourth heating section P4 is 930°C to 1,000°C. More preferably, the fourth temperature range T4 of the fourth heating section P4 is 950°C or higher and 1,000°C or lower.

[0045] According to one embodiment, when the second heating section P2 includes the 2-1 heating section P2-1 and the 2-2 heating section P2-2 having the above-mentioned different temperature ranges, the 2-1 temperature range T2-1 is either 870°C to 890°C or 865°C to 895°C, and the 2-2 temperature range T2-2 of the 2-2 heating section P2-2 is either 900°C to 920°C or 895°C to 925°C.

[0046] The boundary values ​​defining the aforementioned multiple sections will now be described. The boundary values ​​are represented by the horizontal axis of the graph, representing the heating time range (seconds). First, the first boundary value e1 located between the first heating section P1 and the second heating section P2 is about 30 to about 50, or about 40 seconds. The second boundary value e2 located between the second heating section P2 and the third heating section P3 is about 80 to about 130, or about 85 seconds. The third boundary value e3 located between the third-first heating section P3-1 and the third-second heating section P3-2 is about 110 to about 180, or about 120 seconds. The fourth boundary value e4 located between the third-second heating section P3-2 and the fourth heating section P4 is about 140 to about 230, or about 150 seconds.

[0047] According to one embodiment, when the second heating section P2 includes the 2-1 heating section P2-1 and the 2-2 heating section P2-2 having the above-mentioned different temperature ranges, the 2-1 boundary value e2' located between the 2-1 heating section P2-1 and the 2-2 heating section P2-2 is about 50 to about 110, or about 60 (seconds).

[0048] 3, the heating furnace according to one embodiment of the present invention is shown as having five sections P1, P2, P3-1, P3-2, and P4 having different temperature ranges, but the present invention is not limited thereto. The heating furnace may have five, six, or eight sections having different temperature ranges.

[0049] 4 is a graph showing the rate of change in temperature rise rate for multiple sections as a function of heating time in a method for manufacturing a hot stamped part according to an embodiment of the present invention, showing the temperature rise rate (°C / s) of a blank as a function of heating time (s). The multiple sections and boundary values ​​shown in FIG. 4 are the same as those described in FIG. 3, and their description will be simplified or omitted.

[0050] Referring to FIG. 4, the heating rate (°C / s) or the rate of change of the heating rate (°C / s) in the multiple sections where the blank is heated in multiple stages is shown. 2) will be described later. Hereinafter, the "temperature rise rate change rate" refers to the average slope of each section of the graph shown in FIG. 4, and can be described as the "average temperature rise rate change rate." Hereinafter, the "average temperature rise rate change rate" in a section can also be defined as the difference between the initial temperature rise rate and the final temperature rise rate in that section, divided by the time of that section. FIG. 4 shows a first temperature rise rate control curve 610 according to one embodiment of the present invention and a second temperature rise rate control curve 620 according to a comparative embodiment.

[0051] First, the first temperature rise rate control curve 610 according to one embodiment of the present invention will be described.

[0052] The first heating section P1 may have a first average heating rate change rate r1. The second heating section P2, located after the first heating section P1, may have a second average heating rate change rate r2 that is different from the first average heating rate change rate r1. The third heating section P3, located after the second heating section P2, may have a third average heating rate change rate r3 that is different from the first average heating rate change rate r1 and the second average heating rate change rate r2. In this case, the third average heating rate change rate r3 may include a section where it changes from a positive value to a negative value. The fourth heating section P4, located after the third heating section P3, may have a fourth average heating rate change rate r4 that is different from the first average heating rate change rate r1, the second average heating rate change rate r2, and the third average heating rate change rate r3.

[0053] The first heating section P1 is also a general heating section, and in the second heating section P2, the heating rate is gradually reduced compared to the first heating section P1 (|r1| > |r2|), allowing alloying of the coating layer to be performed. The third heating section P3 is a phase transformation section in which the blank's base metal undergoes phase transformation. The third-first heating section P3-1 may have a positive (+) heating rate change rate, and the third-second heating section P3-2 may have a negative (-) heating rate change rate. The fourth heating section P4 is also a stabilization section in which the blank is heated uniformly and soakingly.

[0054] Referring to the first control curve 610, the first average rate of change of heating rate r1 and the second average rate of change of heating rate r2 each have a negative value, and the absolute value of the first average rate of change of heating rate r1 is greater than the absolute value of the second average rate of change of heating rate r2 (|r1|>|r2|). The first average rate of change of heating rate r1 is approximately -0.5°C / s 2 It is also equal to or less than 0, for example, about -0.3℃ / s 2 The second average temperature rise rate change rate r2 is approximately -0.25°C / s 2 It is also equal to or less than 0, for example, about -0.07℃ / s 2 It is also.

[0055] Between the first heating section P1 and the second heating section P2, i.e., 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 is discontinuous. More specifically, in the first heating section P1, the heating rate v1 at the first boundary value e1 that defines the first average heating rate change rate r1 and the heating rate v2 at the first boundary value e1 that defines the second average heating rate change rate r2 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 are also different values. When the heating rate change rate changes discontinuously (r1 → r2) near the first boundary value e1 (610), the weldability of the hot stamped part can be improved compared to when it changes continuously (620).

[0056] The discontinuous change in the average rate of change of heating rate between the first heating section P1 and the second heating section P2 is due to the large amount of energy required for the coating layer to change. The necessary energy must be supplied for the Fe in the base metal to diffuse into the Al coating layer and for the initial formation and growth of an Al-Fe phase within the coating layer. The Fe diffused into the base metal forms an Al-Fe-Si alloy layer over time. The more discontinuous the change in the rate of change of heating rate near the first boundary value e1, the more uniform the diffusion to the surface, resulting in better weldability. On the other hand, if the change is continuous, the Al-Fe-Si diffusion to the surface will be rapid and uneven, resulting in the formation of a phase with high welding resistance on the surface, resulting in poor weldability.

[0057] The third heating section P3 includes a third heating section P3-1 having a third-first heating rate change rate r3-1 and a third-second heating section P3-2 having a third-second heating rate change rate r3-2. The third-first average heating rate change rate r3-1 has a positive value, and the third-second average heating rate change rate r3-2 has a negative value. The third average heating rate change rate r3 may have a section where it changes from a positive value to a negative value. In this case, the absolute value of the third-first average heating rate change rate r3-1 is smaller than the absolute value of the third-second average heating rate change rate r3-2 (|r3-1|<|r3-2|). The third-first average heating rate change rate r3-1 is greater than or equal to 0 and less than about 0.25°C / s. 2 For example, the 3-1 average temperature rise rate change rate r3-1 is about 0.07 ° C / s 2 The 3-2 average temperature rise rate change rate r3-2 is about -0.3℃ / s 2 It is also equal to or less than 0, for example, about -0.08℃ / s 2 It is also.

[0058] In the third-1 heating section P3-1, the smaller the third average heating rate change rate r3 and the gentler the slope of the first control curve 610, the less hydrogen is trapped, thereby improving hydrogen embrittlement. In contrast, the second control curve 620 has a shape in which the heating rate change rate increases abruptly or discontinuously in the third-1 heating section P3-1. In such cases, the amount of hydrogen trapped increases, which can lead to poor hydrogen embrittlement. Thus, unlike the section between the first heating section P1 and the second heating section P2, the third heating section P3 is a section in which the base metal undergoes phase transformation. Since abrupt temperature changes can cause problems such as hydrogen embrittlement and delayed fracture, a lower heating rate change rate is advantageous.

[0059] Between the second heating section P2 and the third-first heating section P3-1, i.e., near the second boundary value e2, the change in the second average heating rate change rate r2 to the third-first average heating rate change rate r3-1 changes from a negative value to a positive value. That is, the phase transformation of the base material occurs as the heating rate decreases and then increases. For example, during the phase transformation of the base material, an endothermic reaction occurs during the transformation to austenite in this section, which requires an energy supply. Therefore, the heating rate must be further increased in the third-first heating section P3-1 to induce a reasonable level of phase transformation to austenite.

[0060] Between the 3-1 heating section P3-1 and the 3-2 heating section P3-2, i.e., near the third boundary value e3, the change from the 3-1 average heating rate change rate r3-1 to the 3-2 average heating rate change rate r3-2 changes from a positive value to a negative value. That is, the heating rate increases, and then decreases, and a phase transformation of the base material can occur. As the phase transformation progresses, a larger amount of heat energy is required for the endothermic reaction, and the heating rate increases. However, as the phase transformation progresses, the amount of austenite increases, and in the 3-2 heating section P3-2, the heat energy required for the endothermic reaction gradually decreases, resulting in a slower heating rate.

[0061] The absolute value of the fourth average heating rate change rate r4 is smaller than the absolute values ​​of the first average heating rate change rate r1, the second average heating rate change rate r2, and the third average heating rate change rate r3. For example, the fourth average heating rate change rate r4 is close to 0, and the fourth heating section P4 is also a section where the blank is soaked at a uniform temperature. The time t4 during which the blank is heated in the fourth heating section P4 is approximately 50% or less of the total heating time t. This is because the longer the soaking time t4 in the fourth heating section P4 is compared to the time t1 during which the blank is multistage heated in the first heating section P1, the second heating section P2, and the third heating section P3, the more likely part properties such as weldability, hydrogen embrittlement, and bendability will be degraded. For example, the ratio (t1:t4) of the length of the multistage heating sections P1, P2, and P3 to the length of the soaking section P4 may satisfy a range of 1:1 to 4:1.

[0062] The characteristics of the second control curve 620 will be described below in comparison with the first control curve 610, focusing on the differences from the first control curve 610. Referring to the second control curve 620, the first' average heating rate change rate r1' changes continuously between the first heating section P1 and the second heating section P2. More specifically, the heating rate at the first boundary value e1 that defines the first' average heating rate change rate r1 in the first heating section P1 and the heating rate v1' at the first boundary value e1 that defines the first' average heating rate change rate r1' in the second heating section P2 may have the same value.

[0063] The first 'average temperature rise rate change rate r1' is approximately -0.26℃ / s 2 It is also equal to or less than 0, for example, about -0.2℃ / s 2The change characteristics of the heating rate change rates r3', r3-1', and r3-2' in the third heating section P3 of the second control curve 620 may be the same as those described for the first control curve 610. However, the 3-1' heating rate change rate r3-1' may have a discontinuous or unstable value compared to the 3-1 average heating rate change rate r3-1 of the first control curve 610. In this case, the 3-1' heating rate change rate r3-1' may refer to the rate of change at the front end where the heating rate shows an increasing trend in the 3-1 heating section P3-1. The 3-1' heating rate change rate r3-1' is approximately 0.04°C / s 2 Approximately 0.16℃ / s or more 2 For example, it is about 0.1℃ / s 2 The temperature rise rate change rate r3-2' is about -0.16℃ / s 2 More than about -0.04℃ / s 2 It is also less than, for example, about -0.1℃ / s 2 The fourth heating section P4 of the second control curve 620 is also a soaking heating section in which the fourth average temperature rise rate of change r4 has a value close to zero, similar to the first control curve 610.

[0064] In this manner, in a method for manufacturing a hot stamped part according to an embodiment of the present invention, by controlling the temperature rise rate change rate for each section according to the above-described multiple section characteristics, it is possible to precisely control and improve part characteristics of the hot stamped part, such as ultra-high strength characteristics, weldability, hydrogen embrittlement, and bending performance. The part characteristics of the hot stamped part according to an embodiment will be described in further detail below with reference to FIGS. 5 and 6.

[0065] The relationship between the heating time (s) and the boundary values ​​shown on the horizontal axis of Figure 4 is not limited to that shown in Figure 4, and various modifications may be applied as long as the performance of the hot stamped part of the present disclosure is improved. Although the above description has been given assuming that the multiple sections include five sections, the multiple sections may be divided differently depending on the distribution of the temperature rise rate change rate.

[0066] 5 is a scanning electron microscope (SEM) image showing a cross section of a hot stamped part according to an embodiment of the present invention. The hot stamped part according to FIG. 5 is also a part manufactured by the above-described method for manufacturing a hot stamped part according to an embodiment of the present invention (e.g., curve 610 in FIG. 4).

[0067] Referring to FIG. 5, the hot stamped part 1 includes a base material 10 and a coating layer 20 including multiple layers 21, 22, 23, and 24 disposed on the base material 10. The base material 10 is a steel sheet, which is manufactured by hot-rolling and / or cold-rolling a steel slab that has been cast to contain predetermined alloying elements in predetermined amounts. For example, the base steel sheet 100 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), titanium (Ti), boron (B), the remainder being iron (Fe), and other unavoidable impurities. The base steel sheet 10 may further contain one or more of niobium (Nb), molybdenum (Mo), and aluminum (Al).

[0068] The plating layer 20 is an alloyed layer formed on at least one surface of the base material 10 and may contain aluminum (Al), iron (Fe), or the like. The plating layer 20 may include multiple layers 21, 22, 23, and 24 sequentially stacked on the base material 10. In a hot stamped part according to an embodiment of the present invention, as shown in FIG. 5, the plating layer 20 may be clearly divided into four layers. As an example, the multiple layers 21, 22, 23, and 24 may have, in order, an α-Fe phase, an Fe2Al5 phase, an AlFe phase, and an Fe2Al5 phase, but the composition of the multiple layers is not limited thereto. The hot stamped part 1 shown in FIG. 5 has an amount of hydrogen inclusion of 0 to less than 0.21 ppm and a dynamic resistance of 0 to 0.8 mΩ.

[0069] Figure 6 is a scanning electron microscope (SEM) image showing a cross section of a hot stamped part according to a comparative embodiment of the present invention, and the differences from the cross section of Figure 5 will be mainly described below. The hot stamped part 1' shown in Figure 6 is also a part manufactured by the above-described hot stamped part manufacturing method (e.g., curve 620 in Figure 4).

[0070] The hot stamped part 1' shown in FIG. 6 also includes a base material 10 and a plating layer 30 disposed on the base material 10. Unlike the plating layer 20 of FIG. 5, the plating layer 30 is a single layer rather than a multi-layer structure, and is thinner than the plating layer 20. In one embodiment, the plating layer 30 may include multiple layers, but the boundaries between the layers may be unclear. The plating layer 30 may also include at least one element selected from the group consisting of Al, Fe, and Si. The hot stamped part 1' shown in FIG. 6 has a hydrogen content of 0 to less than 0.35 ppm and a dynamic resistance of 0.5 to 1.5 mΩ.

[0071] Hereinafter, the characteristics of the hot stamped parts shown in Figures 5 and 6 will be described together with Table 1.

[0072] The evaluation results shown in Table 1 below are also relative comparison results. [Table 1] 5 is composed of multiple layers, as compared with the plating layer 30 of FIG. 6, and the boundaries between the multiple layers are also clearly defined, resulting in a thicker plating layer 20 than the plating layer 30. Referring to FIG. 4, the thickness characteristics of the plating layer and the control characteristics of the first control curve 610 are derived from the mutual influence of each other. For example, when the plating layer is thick, during heating between the first heating section P1 and the second heating section P2, the rate of change of the temperature rise rate changes discontinuously between the two sections P1 and P2, and in the third heating section P3, the slope of the first control curve 610 is gentle, i.e., the rate of change of the temperature rise rate is small.

[0073] On the other hand, the thicker the plating layer, the less hydrogen is trapped, resulting in better hydrogen embrittlement of the blank. Because the plating layer 20 in Figure 5 is thicker, the amount of trapped hydrogen is smaller in part 1 in Figure 5 (less than about 0.21 ppm) than in part 1' in Figure 6 (less than about 0.35 ppm), as mentioned above. This results in better hydrogen embrittlement and a reduced risk of hydrogen-delayed fracture.

[0074] Furthermore, due to the thickness characteristics of the plating layer described above, the surface resistance of component 1 in Figure 5 is lower than that of component 1' in Figure 6. Furthermore, the lower the kinetic resistance, the better the weldability. As mentioned above, the kinetic resistance value of component 1 in Figure 5 (approximately 0.8 mΩ or less) is lower than that of component 1' in Figure 6 (approximately 0.8 mΩ or less), so it can be confirmed that component 1 in Figure 5 also has an even greater advantage in weldability.

[0075] According to the method for manufacturing a hot stamped part according to an embodiment of the present invention, precise control of the part is possible by controlling the rate of change of the temperature rise rate for each of the multiple sections, which has the advantage of improving the part performance such as weldability, hydrogen embrittlement resistance, and super strength characteristics of the hot stamped part.

[0076] FIG. 7 is a plan view that schematically illustrates a blank used in a method for manufacturing a hot stamped part according to an embodiment of the present invention. Referring to FIG. 7, a blank 200 according to one embodiment may include at least one of a blank 210 having a single thickness, a tailor welded blank (TWB) 220 made by cutting different types of plate material having different thicknesses into a required shape and welding them together, a tailor rolled blank (TRB) 230 made by rolling a single thickness plate material and having different thicknesses, and a patchwork 240 made by welding a small patch blank to a large blank.

[0077] The tailor-welded blank 220 can be manufactured by welding a first plate 221 and a second plate 223 having different thicknesses. The B pillar, a key component of a vehicle's collision equipment, is made by joining two plates with different strengths to form an upper crash support section and a lower impact absorption section. The two plates are welded and then shaped. The tailor-welded blank method, which is commonly used, refers to a series of processes in which different types of plate, each with different thicknesses, strengths, and materials, are cut to the required shape, welded, and then press-formed to manufacture a part. By welding plates with different thicknesses to manufacture blanks with different thicknesses, different portions of the blank can have different properties. For example, the upper crash support section of the B pillar can be made of ultra-high-strength plate material with a strength of 120 to 150 K, and the lower end of the B pillar, where stress is concentrated, can be connected to a plate material with excellent impact absorption performance, thereby improving impact absorption during a vehicle collision.

[0078] The tailor rolled blank 230 can be manufactured by rolling a cold-rolled steel material to have a specific thickness profile. When a hot stamped part is manufactured using the tailor rolled blank 230, the resulting thickness profile can be effectively reduced in weight. For example, the thickness profile can be achieved using a conventional method. For example, when the cold-rolled steel material is cold-rolled, the reduction ratio can be adjusted to form a tailor rolled blank 230 including a first region 231 having a first thickness, a second region 232 having a second thickness, a third region 233 having a third thickness, and a fourth region 234 having a fourth thickness. The first, second, third, and fourth thicknesses may be different from each other, and transition sections 235 may exist between the first region 231 and the second region 232, between the second region 232 and the third region 233, and between the third region 233 and the fourth region 234. 2, the tailor rolled blank 230 is illustrated as including the first region 231 to the fourth region 234, but the present invention is not limited to this. The tailor rolled blank 230 may also be formed to include the first region 231, the second region 232, ..., the nth region.

[0079] The patchwork 240 is a method of partially reinforcing a base material using at least two or more plates, and the base material and the patch may be formed simultaneously by joining the patch to the base material before the molding process. For example, a patch 243 having a second size smaller than the first size may be welded to a base material 241 having a first size, and then the two may be formed simultaneously.

[0080] 8 is a plan view schematically illustrating a blank placed in a heating furnace in a method for manufacturing a hot stamped part according to an embodiment of the present invention. Hereinafter, a description will be given with reference to FIG.

[0081] In the blank loading step (S100), at least two blanks 200 having different thicknesses and sizes may be loaded into a heating furnace at the same time.

[0082] As an example, FIG. 8 illustrates two first blanks 250 and two second blanks 260 being placed simultaneously in a heating furnace. The first blanks 250 and the second blanks 260 may have different sizes and thicknesses. For example, the first blank 250 may have a thickness of approximately 1.2 mm, and the second blank 260 may have a thickness of approximately 1.6 mm. However, the present invention is not limited thereto, and one first blank 250 and one second blank 260 may be placed simultaneously in the heating furnace. Furthermore, the first blanks 250 and the second blanks 260 may be variously modified, such as being the same size but different thicknesses, or being the same thickness but different sizes.

[0083] In another embodiment, at least two blanks 200 having a single thickness may be simultaneously loaded into the heating furnace in the blank loading step (S100). For example, at least two blanks 240 having a thickness of 1.2 mm may be simultaneously loaded, and at least two blanks 250 having a thickness of 1.6 mm may be simultaneously loaded. Also, in the blank loading step (S100), the tailor welded blank 220 (FIG. 7) or the tailor rolled blank 230 (FIG. 7) may be loaded into the heating furnace.

[0084] The blanks introduced into the heating furnace may be mounted on rollers and then transported along the transport direction (S300).

[0085] After the blank insertion step (S100), a multi-stage heating step (S200) may be performed. The multi-stage heating step (S200) is a step in which the blank passes through multiple sections provided in a heating furnace and is heated in stages, and may include at least one section where soaking is performed.

[0086] As described above, the present invention has been described with reference to the embodiments shown in the drawings, but these are merely illustrative, and those skilled in the art will understand that various modifications and variations of the embodiments are possible therefrom. Therefore, the true technical scope of protection of the present invention is defined by the technical ideas of the claims.

Claims

1. placing a blank having a coating layer formed on at least one surface of a base material into a heating furnace having a plurality of sections each having a different heating rate range; a multi-stage heating step in which the blank passes through the plurality of sections and is heated in stages; The plurality of sections are: a first heating section having a first average heating rate; a second heating section after the first heating section, the second heating section having a second average heating rate change rate different from the first average heating rate change rate; a third heating section after the second heating section, the third heating section having a third average heating rate change rate different from the first average heating rate change rate and the second average heating rate change rate; the third average temperature rise rate change rate includes a section in which it changes from a positive value to a negative value, The third heating section includes a third-1 heating section having a third-1 average heating rate change rate and a third-2 heating section having a third-2 average heating rate change rate, The third-first average rate of change of heating rate has a positive value, and the third-second average rate of change of heating rate has a negative value, a third boundary value located between the 3-1 heating section and the 3-2 heating section is 110 seconds or more and 180 seconds or less; The blanks include a first blank and a second blank having different sizes, The method for manufacturing a hot stamped part, wherein the step of introducing the blanks comprises simultaneously introducing the first blank and the second blank into the heating furnace.

2. 2. The method of claim 1, wherein the change from the first average rate of change of heating rate to the second average rate of change of heating rate is discontinuous between the first heating zone and the second heating zone.

3. The third heating section includes a third-1 heating section having a third-1 average temperature rise rate change rate and a third-2 heating section having a third-2 average temperature rise rate change rate, The third-first average rate of change of heating rate has a positive value, and the third-second average rate of change of heating rate has a negative value, The method for manufacturing a hot stamped part according to claim 1, wherein the absolute value of the third-first average temperature rise rate change rate is smaller than the absolute value of the third-second average temperature rise rate change rate.

4. the first average heating rate change rate and the second average heating rate change rate each have a negative value, The method for manufacturing a hot stamped part according to claim 1 , wherein an absolute value of the first average rate of change of heating rate is greater than an absolute value of the second average rate of change of heating rate.

5. The plurality of sections include, after the third heating section, a fourth heating section having a fourth average heating rate change rate different from the first average heating rate change rate, the second average heating rate change rate, and the third average heating rate change rate; 2. The method for manufacturing a hot stamped part according to claim 1, wherein an absolute value of the fourth average heating rate change rate is smaller than an absolute value of each of the first average heating rate change rate, the second average heating rate change rate, and the third average heating rate change rate.

6. The 3-1 average temperature rise rate change rate is 0 to 0.25°C / s ２ The third-second average temperature rise rate change rate is -0.3°C / s ２ The method of claim 3 , wherein the value of ρ is greater than or equal to 0 and less than or equal to 0.

7. The first average temperature rise rate change rate is −0.5° C. / s ２ the second average temperature rise rate change rate is -0.25°C / s ２ The method for producing a hot stamped part according to claim 4 or 6, wherein the value of t is greater than or equal to 0 and less than or equal to 0.

8. In the second heating section, the plating layer is alloyed, The method for producing a hot stamped part according to claim 1 , wherein the base material undergoes a phase transformation in the third heating zone.

9. 9. The method for manufacturing a hot stamped part according to claim 1, wherein the amount of hydrogen mixed in the hot stamped part is 0 or more and less than 0.21 ppm, and the dynamic resistance value is greater than 0 mΩ and less than 0.8 mΩ.

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