Manufacturing method of hot stamping parts

Abstract

The present invention provides a method for manufacturing a hot stamped part, which includes a heating step of heating a blank; a transfer step of transferring the heated blank to a press die including a punch; and a forming and punching step of hot forming the transferred blank into a shape of the hot stamped part and hot punching the transferred blank to form a punched portion in the hot stamped part.

Description

[Technical Field]

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

[0002] As environmental and fuel economy regulations become stricter worldwide, the need for lighter vehicle materials is increasing, which has led to active research and development into ultra-high strength steels and hot stamping steels.

[0003] Hot stamping is a process of manufacturing high-strength parts by heating steel sheets to high temperatures in a heating furnace and then quenching them in a press. A drilling process may also be performed to cut / machine holes in the high-strength parts.

[0004] The drilling process uses a laser device or a press die, but when using a laser device, the processing time increases, and when using a press die, the quality of the sheared surface may decrease.

[0005] Related technologies include Korean Patent Publication No. 10-2020-0080721 (Title of the Invention: Hot stamping part manufacturing apparatus and hot stamping part manufacturing method using the same). Summary of the Invention [Problem to be solved by the invention]

[0006] The problem to be solved by the present invention is to derive the maximum delay time for hot piercing taking into consideration the thickness of the blank, the forming start temperature, and the like. [Means for solving the problem]

[0007] One embodiment of the present invention includes a heating step of heating a blank; a transferring step of transferring the heated blank to a press die including a punch; and a forming and punching step of hot-forming the transferred blank into a shape of a hot stamped part and hot-punching the transferred blank to form a punched portion in the hot stamped part; wherein a hot-punching delay time in the forming and punching step is λ 0 seconds or more from the time when the press die reaches the bottom dead center. max is less than or equal to λ max is derived by the following formula:

number

[0008] In this embodiment, in the above formula, α p has a value greater than 0 and less than or equal to 0.001, and b p has a value of about 0 to 0.65, and c p may have a value of about 1 or more and about 1.2 or less.

[0009] In this embodiment, in the step of transferring the blank, the heated blank may be air-cooled at room temperature.

[0010] In this embodiment, the step of heating the blank may include a multi-stage heating step of heating the blank stepwise; and a soaking heating step of heating the blank at a temperature of Ac3 to 1000°C.

[0011] In this embodiment, the step of heating the blank is performed in a heating furnace, and the heating furnace may have a plurality of sections having different temperature ranges.

[0012] In this embodiment, during the forming and punching step the blank is allowed to cool in the press die.

[0013] In this embodiment, at least two or more perforations may be formed.

[0014] Other aspects, features, and advantages beyond those described above will become apparent from the following detailed description of the invention, the claims, and the drawings. [Effects of the Invention]

[0015] According to one embodiment of the present invention, a maximum hot piercing delay time can be calculated taking into consideration the thickness of the blank, the forming start temperature, etc., thereby enabling flexible process design and facilitating quality control of the manufactured hot stamped parts. It goes without saying that the scope of the present invention is not limited by such effects. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a perspective view showing a schematic diagram of a hot stamped part according to an embodiment of the present invention; FIG. [Figure 2] 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 3] 1 is a flow chart that schematically illustrates the preparatory steps of a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Figure 4] FIG. 1 is a plan view schematically illustrating a blank according to an embodiment of the present invention. [Figure 5] 1 is a flow chart that schematically illustrates a heating stage of a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Figure 6] 1 is a view illustrating a heating furnace having multiple sections in a heating step of a method for manufacturing a hot stamped part according to an embodiment of the present invention; [Figure 7]2 is a cross-sectional view illustrating the forming / punching stage of the manufacturing process of a hot stamped part according to one embodiment of the present invention. FIG. [Figure 8] 2 is a cross-sectional view illustrating the forming / punching stage of the manufacturing process of a hot stamped part according to one embodiment of the present invention. FIG. [Figure 9] 1 is a diagram showing a process window derived from blank thickness, forming start temperature, and hot piercing delay time. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention can be embodied in various forms and in various modifications, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. The advantages and features of the present invention, as well as methods for achieving them, will become more apparent from the following detailed description of the embodiments, taken in conjunction with the drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms.

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

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

[0020] In the following examples, terms such as "comprise" or "have" mean the presence of a feature or component described in the specification, and do not preclude the possibility of adding one or more other features or components.

[0021] In the drawings, the size of components may be exaggerated or reduced for the sake of convenience of explanation. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the sake of convenience of explanation, and the present invention is not necessarily limited to what is shown in the drawings.

[0022] If an embodiment can be implemented differently, the order of certain steps may be performed differently than that described. For example, two steps described in succession may be performed substantially simultaneously or may be performed in the reverse order of that described.

[0023] In this specification, "A and / or B" refers to A, B, or A and B. And "at least one of A and B" refers to A, B, or A and B.

[0024] Hereinafter, embodiments 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 denoted by the same reference numerals, and duplicate descriptions thereof will be omitted.

[0025] FIG. 1 is a perspective view that schematically illustrates a hot stamped part according to one embodiment of the present invention.

[0026] 1 , a hot stamped part 10 according to an embodiment of the present invention may include a punching portion 110. In one embodiment, the hot stamped part 10 may include two punching portions 110. The punching portion 110 may include a first punching portion 110a and a second punching portion 110b. However, the present invention is not limited thereto. The hot stamped part 10 may include one punching portion 110 or three or more punching portions 110.

[0027] Although not shown, the hot stamped part 10 may include additional drilling portions in addition to the drilling portion 110. In this case, the drilling portion 110 is formed by hot drilling, and the additional drilling portions may be formed by cold drilling or laser drilling. The hot stamped part 10 may be provided with one or more additional drilling portions.

[0028] In one embodiment, perforation 110 may be used to form additional perforations, as will be described in more detail below.

[0029] Although not shown, the hot stamped part 10 may also include an end portion. In this case, the end portion of the hot stamped part 10 may refer to a surface extending along the long side of the hot stamped part 10.

[0030] FIG. 2 is a flow chart that schematically illustrates a method for manufacturing a hot stamped part according to one embodiment of the present invention.

[0031] Referring to FIG. 2, a method for manufacturing a hot stamped part according to an embodiment may include a preparation step (S100), a heating step (S200), a transfer step (S300), and a forming / punching step (S400).

[0032] FIG. 3 is a flow chart that schematically illustrates the preparatory steps of a method for manufacturing a hot stamped part according to an embodiment of the present invention, and FIG. 4 is a plan view that schematically illustrates a blank according to an embodiment of the present invention.

[0033] 3 and 4, the preparation step (S100) is also a step of preparing a blank 100 for hot stamping. In one embodiment, the preparation step (S100) may include a hot rolling step (S110), a cooling / coiling step (S120), a cold rolling step (S130), an annealing step (S140), a plating step (S150), and a cutting step (S160).

[0034] First, a steel slab reheating step may be performed. In this step, the steel slab obtained through the continuous casting process is reheated to a predetermined temperature to redissolve elements that segregated during casting. In one embodiment, the slab reheating temperature (SRT) is approximately 1,200°C to 1,400°C. If the SRT is lower than approximately 1,200°C, elements that segregated during casting are not sufficiently redissolved, resulting in a less effective homogenization of alloy elements and a less effective dissolution of titanium (Ti). While a higher SRT is beneficial for homogenization, if the SRT exceeds approximately 1,400°C, the austenite grain size increases, making it difficult to ensure strength, and the excessive heating process may increase the cost of manufacturing the steel sheet.

[0035] In the hot rolling step (S110), the reheated sheet material may be hot-rolled at a predetermined finish rolling temperature. A hot-rolled steel sheet may be manufactured through the hot rolling step (S110). In one embodiment, the finish delivery temperature (FDT) is about 880°C to 950°C. If the FDT is lower than about 880°C, it may be difficult to ensure workability of the steel sheet due to the generation of a duplex structure caused by rolling in an abnormal region, and workability may be reduced due to a non-uniform microstructure. Furthermore, a rapid phase change may cause problems with sheet threadability during hot rolling. If the FDT exceeds about 950°C, austenite grains may become coarse and TiC precipitates may become coarse, resulting in reduced performance of the hot stamped part.

[0036] In the cooling / coiling step (S120), the hot-rolled steel sheet may be cooled to a predetermined coiling temperature (CT) and then coiled. In one embodiment, the coiling temperature in the cooling / coiling step (S120) is about 550°C to 800°C. The coiling temperature affects carbon redistribution. If the coiling temperature is less than about 550°C, the low-temperature fraction increases due to overcooling, increasing strength, and the rolling load during cold rolling may become severe, resulting in a rapid decrease in softness. Conversely, if the coiling temperature exceeds about 800°C, abnormal or excessive grain growth may occur, resulting in deterioration of formability and strength.

[0037] In the cold rolling step (S130), the coiled hot-rolled steel sheet may be uncoiled, pickled, and then cold-rolled. Here, the pickling may be performed to remove scale from the coiled hot-rolled steel sheet, i.e., the hot-rolled coil manufactured through the hot rolling process. A cold-rolled steel sheet may be manufactured through the cold rolling step (S130).

[0038] In the annealing step (S140), the cold-rolled steel sheet is annealed at a temperature of about 700°C or higher. For example, the annealing step (S140) may include heating the cold-rolled steel sheet and cooling the heated cold-rolled steel sheet at a predetermined cooling rate. In the annealing step (S140), the cold-rolled steel sheet may be annealed. The annealing step (S140) may be performed in an annealing furnace.

[0039] In one embodiment, the annealing temperature of the cold-rolled steel sheet is about 750°C to 900°C. If the annealing temperature of the cold-rolled steel sheet is less than about 750°C, the desired structure may not be obtained and recrystallization may not be fully completed. On the other hand, if the annealing temperature of the cold-rolled steel sheet exceeds about 900°C, the annealing temperature is too high and the efficiency of the manufacturing process may decrease. Therefore, if the annealing temperature of the cold-rolled steel sheet satisfies the range of about 750°C to 900°C, the desired structure may be obtained, recrystallization may be fully completed, and the efficiency of the manufacturing process may be improved.

[0040] The plating step (S150) is also a step of forming a plating layer on the annealed cold-rolled steel sheet. In one embodiment, a plating layer may be formed on the annealed cold-rolled steel sheet through the plating step (S150). In this case, the plating layer may include a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer.

[0041] Specifically, in the plating step (S150), the annealed cold-rolled steel sheet may be immersed in a plating bath. At this time, the plating bath may be maintained at a temperature of about 400°C to 700°C. The coating weight of the cold-rolled steel sheet may be about 40 g / m2 on both sides of the base material. 2 ~200g / m 2 After the plating step (S150), the cold-rolled steel sheet on which the plating layer is formed may be wound into a coil.

[0042] 3 illustrates that the cold rolling step (S130), the annealing step (S140), and the coating step (S150) are performed after the cooling / coiling step (S120), but the present invention is not limited thereto. At least one of the cold rolling step (S130), the annealing step (S140), and the coating step (S150) may be omitted. For example, the cold rolling step (S130) and the annealing step (S140) may be omitted. In this case, after the coating step (S150), the hot-rolled steel sheet on which the coating layer is formed may be wound into a coil.

[0043] In the subsequent cutting step (S160), a coiled steel sheet (e.g., a cold-rolled steel sheet or a hot-rolled steel sheet) may be uncoiled, and then the steel sheet may be cut into blanks 100 using a laser or a cold press die. In this case, the blank 100 may include an outer periphery (or edge) of the coil. For example, the blank 100 may include an outer periphery (or edge) of the steel sheet.

[0044] 2, after the preparation step (S100) of preparing the blank 100, a heating step (S200) of heating the blank 100 may be performed. The heat source method in the heating step (S200) may be direct heating or indirect heating. The heat source method in the heating step (S200) may be one of direct heating and indirect heating, or a combination of direct heating and indirect heating.

[0045] In one embodiment, in the heating step (S200), the blank 100 may be heated in a heating furnace. The heating furnace may have one section with one temperature range (or a single temperature), or may have multiple sections with different temperature ranges. When the heating furnace has one section with one temperature range, the blank 100 may be heated to a target temperature in the heating furnace. In this case, the target temperature is Ac3 to 1,000°C. That is, the blank 100 may be heated in a heating furnace having a temperature range of Ac3 to 1,000°C until the temperature of the blank 100 reaches Ac3 to 1,000°C.

[0046] On the other hand, if the heating furnace is provided with a plurality of sections having different temperature ranges, the blank 100 can be heated to a target temperature in the heating furnace in different temperature ranges.

[0047] FIG. 5 is a flowchart illustrating a heating step of a method for manufacturing a hot stamped part according to an embodiment of the present invention, and FIG. 6 is a diagram illustrating a heating furnace having multiple sections in the heating step of a method for manufacturing a hot stamped part according to an embodiment of the present invention.

[0048] 5 and 6, in the heating step (S200), the blank 100 (see FIG. 4) may be heated in a heating furnace having multiple sections with different temperature ranges. As shown in FIG. 5, the heating step (S200) may include a multi-stage heating step (S210) and a soaking heating step (S220). The multi-stage heating step (S210) and the soaking heating step (S220) are also steps in which the blank 100 is heated by passing through multiple sections provided in the heating furnace.

[0049] In one embodiment, the temperature of the entire furnace is about 680°C to 1,000°C. Specifically, the temperature of the entire furnace in which the multi-stage heating step (S210) and the soaking heating step (S220) are performed is about 680°C to 1,000°C. In this case, the temperature of the furnace in which the multi-stage heating step (S210) is performed is about 680°C to Ac3, and the temperature of the furnace in which the soaking heating step (S220) is performed is about Ac3 to 1,000°C.

[0050] In the multi-stage heating step (S210), the blank 100 may be heated (or heated) in stages as it passes through multiple sections provided in the heating furnace. There may be multiple sections in which the multi-stage heating step (S210) is performed among the multiple sections provided in the heating furnace, and the temperature may be set for each section so that the temperature increases from the entrance of the heating furnace where the blank 100 is inserted toward the exit of the heating furnace where the blank 100 is removed, thereby heating (or raising the temperature) the blank 100 in stages.

[0051] After the multi-stage heating step (S210), a soaking heating step (S220) is performed. In the soaking heating step (S220), the multi-stage heated blank is heated (or soaked) by passing through a section of the heating furnace set at a temperature of about Ac3 to 1,000°C. The soaking heating step (S220) is performed in at least one section among the multiple sections provided in the heating furnace.

[0052] According to one embodiment, the heating furnace includes a plurality of zones having different temperature ranges, specifically, a first zone P1 having a first temperature range T1, a second zone P2 having a second temperature range T2, a third zone P3 having a third temperature range T3, a fourth zone P4 having a fourth temperature range T4, a fifth zone P5 having a fifth temperature range T5, a sixth zone P6 having a sixth temperature range T6, and a seventh zone P7 having a seventh temperature range T7.

[0053] In one embodiment, in the multi-stage heating step (S210), the blank may be heated in stages by passing through first to fourth sections P1 to P4 defined in a heating furnace. In addition, in the soaking heating step (S220), the blank that has been multi-stage heated in the first to fourth sections P1 to P4 may be soaked by passing through fifth to seventh sections P5 to P7.

[0054] The first through seventh sections P1 through P7 may be arranged sequentially within the heating furnace. The first section P1 having the first temperature range T1 may be adjacent to the entrance of the heating furnace where the blanks are inserted, and the seventh section P7 having the seventh temperature range T7 may be adjacent to the exit of the heating furnace where the blanks are removed. Therefore, the first section P1 having the first temperature range T1 may be the first section of the heating furnace, and the seventh section P7 having the seventh temperature range T7 may be the last section of the heating furnace.

[0055] The temperatures of the multiple sections within the heating furnace, for example, the temperatures of the first section P1 through the seventh section P7, may increase from the entrance where the blanks are inserted to the exit where the blanks are removed. However, the temperatures of the fifth section P5, the sixth section P6, and the seventh section P7 are the same. In addition, the temperature difference between two adjacent sections within the heating furnace is greater than 0°C and less than 100°C. For example, the temperature difference between the first section P1 and the second section P2 is greater than 0°C and less than 100°C.

[0056] The furnace temperature in the soaking step (S220) is Ac3 to 1,000°C. If the furnace temperature in the soaking step (S220) is less than Ac3, the hot stamped part may not have the desired properties. On the other hand, if the furnace temperature in the soaking step (S220) is more than about 1,000°C, carbide-forming elements and nitride-forming elements in the blank 100, such as Ti, V, Nb, and Mo, dissolve in the base material, making it difficult to suppress grain coarsening.

[0057] 6, the furnace according to one embodiment is shown to have seven zones with different temperature ranges, but the present invention is not limited thereto. The furnace may have five, six, or eight zones with different temperature ranges.

[0058] In one embodiment, the heating step (S200) is comprised of a multi-stage heating step (S210) and a soaking heating step (S220), so that the temperature of the heating furnace can be set in stages, thereby improving the energy efficiency of the heating furnace.

[0059] In one embodiment, the heating furnace may have a length of approximately 20 to 40 meters along the transfer path of the blank 100. The heating furnace may include multiple sections having different temperature ranges, and the ratio of the length of the section in which the blank is heated in multiple stages to the length of the section in which the blank is soaked satisfies approximately 1:1 to 4:1. If the length of the section in which the blank is soaked in the heating furnace increases so that the ratio of the length of the section in which the blank is soaked to the length of the section in which the blank is soaked exceeds approximately 1:1, the amount of hydrogen permeating into the blank in the soaking section may increase, resulting in an increased incidence of delayed fracture. On the other hand, if the length of the section in which the blank is soaked decreases so that the ratio of the length of the section in which the blank is soaked to the length of the section in which the blank is soaked is less than approximately 4:1, the soaking section (or time) may not be sufficiently secured, resulting in uneven strength of the manufactured hot stamped part. For example, the length of the soaking section among the multiple sections provided in the heating furnace may be approximately 20 to 50% of the overall length of the heating furnace.

[0060] In one embodiment, the total heating time for the heating step (S200) is about 2 to 20 minutes. That is, the total time the blank remains in the heating furnace is about 2 to 20 minutes. If the total heating time for the heating step (S200) is about 2 minutes or less, the heating time is insufficient, and the resulting hot stamped part may not have the desired material properties. On the other hand, if the total heating time for the heating step (S200) is about 20 minutes or more, the heating time is too long, which may reduce the production rate and reduce economic efficiency. Therefore, if the total heating time for the heating step (S200) is about 2 to 20 minutes, the resulting hot stamped part may have the desired material properties, and at the same time, a reduction in the economic efficiency of the manufacturing process may be prevented or minimized.

[0061] 2, after the heating step (S200), a transfer step (S300) may be performed. In the transfer step (S300), the heated blank 100 may be transferred to a press die 400 (see FIG. 7). For example, the heated blank 100 may be removed from a heating furnace and then transferred to the press die 400.

[0062] In the transfer step (S300), the heated blank 100 may be cooled to ambient temperature (or room temperature). That is, the heated blank 100 may be air-cooled at ambient temperature during transfer. If the heated blank 100 is not air-cooled, the mold entry temperature (e.g., the molding start temperature) may become high, and wrinkles (or bends) may occur on the surface of the manufactured hot stamped part. In addition, since the use of a refrigerant may affect the subsequent process (hot stamping), it is preferable that the heated blank 100 be air-cooled during transfer.

[0063] 7 and 8 are cross-sectional views illustrating the forming / punching step of the manufacturing process of the hot stamped part according to an embodiment of the present invention. Specifically, Fig. 7 is a cross-sectional view of the blank and the press die before the forming / punching step is performed, and Fig. 8 is a cross-sectional view of the blank and the press die during the forming / punching step.

[0064] 2, 7, and 8, after the transfer step S300, a forming / punching step S400 may be performed. The forming / punching step S400 is a step of hot-forming the transferred blank 100 into the shape of a hot stamping part and forming a punched portion 110 in the transferred blank 100 through hot punching.

[0065] In one embodiment, the forming / punching step (S400) may be performed using a press die 400. The press die 400 may include a lower die 410, an upper die 420, and a punch 430. The lower die 410 may have a shape of a bottom surface of the hot stamping part. The upper die 420 faces the lower die 410 and may have a shape of an top surface of the hot stamping part. The upper die 420 may move up and down between a top dead center and a bottom dead center. In this regard, the top dead center may refer to the position where the upper die 420 is moved to the uppermost position, and the bottom dead center may refer to the lowest point where the upper die 420 moves toward the lower die 410 to form the blank 100. Although not shown, the upper die 420 may move up and down between the top dead center and the bottom dead center via a separately provided hydraulic cylinder. However, the present invention is not limited thereto.

[0066] The press die 400 includes at least one punch 430, preferably at least two punches 430. In one embodiment, the punches 430 may include a first punch and a second punch. However, the present invention is not limited thereto. For example, the press die 400 may be variously modified, such as including only one punch 430 or three or more punches 430.

[0067] Although not shown, if the press die 400 is equipped with only one punch 430, one foam bead may be formed on the edge of the blank 100 when the blank 100 is formed.

[0068] For example, as will be described later, a laser process or a cold drilling process may be performed using the perforation portion 110 formed through hot drilling. Specifically, the blank 100 is placed on a jig using the perforation portion 110 formed through hot drilling as a guide pattern (or reference point), and then a subsequent process is carried out. Therefore, at least two perforation portions 110 are formed in the blank 100 through hot drilling.

[0069] In one embodiment, the clearance between the punch 430 and the die (e.g., the lower die 410 and / or the upper die 420) is approximately 2% to 30%. If the clearance is less than approximately 2%, the punch 430 may be pinched between the dies due to thermal expansion of the dies. On the other hand, if the clearance is more than approximately 30%, the punch 430 may vibrate violently, resulting in uneven sheared surface quality. For example, grain flows may be formed only in a portion of the sheared surface, resulting in a decrease in uniformity of the sheared surface quality. Therefore, if the clearance between the punch 430 and the die (e.g., the lower die 410 and / or the upper die 420) is approximately 2% to 30%, the punch 430 may be prevented from being pinched between the dies due to thermal expansion, and grain flows may be formed uniformly around the sheared surface.

[0070] In one embodiment, the blank 100 may be hot-formed (or hot-pressed) in the forming / punching step (S400). The transferred (or heated) blank 100 may be hot-formed into the shape of the hot stamped part using a press die 400 including a lower die 410 and an upper die 420. Specifically, the blank 100 may be formed into the shape of the hot stamped part by pressing (e.g., hot-pressing) the blank 100 with the upper die 420 having the shape of the top surface of the hot stamped part against the lower die 410 having the shape of the bottom surface of the hot stamped part. For example, the press die may reach its bottom dead center, and the blank 100 may be formed into the shape of the hot stamped part 10 at the bottom dead center. Specifically, the upper die 420 may descend to the bottom dead center, and after reaching the bottom dead center, the blank 100 may be formed into the shape of the hot stamped part 10 at the bottom dead center.

[0071] In one embodiment, the blank 100 has a forming start temperature equal to or higher than the Ms temperature during hot forming in the forming / punching step (S400). Specifically, the forming start temperature is equal to or higher than the Ms temperature and equal to or lower than the furnace removal temperature of the blank 100 during hot forming in the forming / punching step (S400). If the blank 100 is hot formed below the Ms temperature, a large load is generated during hot forming, damaging the press die 400, reducing the formability of the blank 100, and preventing the hot stamped part from having the desired structure and physical properties. Meanwhile, because the heated blank 100 is air-cooled during removal from the furnace and transfer to the press die 400, the temperature of the blank 100 during hot forming must be lower than the temperature at which the blank 100 is removed from the furnace. Therefore, when the forming start temperature is equal to or higher than about the Ms temperature and equal to or lower than the furnace removal temperature of the blank 100, the formability of the blank is improved, and the produced hot stamped part can have the desired structure and physical properties.

[0072] In one embodiment, in the forming / punching step (S400), hot punching may be performed on the blank 100 after hot forming of the blank 100 is completed. Specifically, after hot forming of the blank 100 is completed, a punch 430 included in the press die 400 may descend to perform hot punching on the blank 100, thereby forming the punched portion 110 in the hot stamped part 10 (or blank 100). For example, after the upper die 420 reaches bottom dead center to complete hot forming of the blank 100, the punch 430 may descend to perform hot punching on the blank 100, thereby forming the punched portion 110 in the hot stamped part 10 (or blank 100). In other words, hot punching may be performed after the press die (or upper die) reaches bottom dead center. Although not shown, the punch 430 may be raised and lowered by a hydraulic cylinder. However, the present invention is not limited to this.

[0073] In one embodiment, after hot forming of the blank 100 using the press die 400 is completed, a first punch and a second punch included in the press die 400 may be lowered to form at least two perforations 110a and 110b in the blank 100. Specifically, after the upper die 420 presses the lower die 410 and the blank 100 to complete hot forming of the blank 100, the first punch and the second punch may be lowered to form the first perforation 110a and the second perforation 110b in the hot stamped part 10.

[0074] In one embodiment, hot punching may be performed after the hot forming has occurred.

[0075] 7 and 8, the punch 430 included in the press die 400 is shown descending to form the hole 110 in the blank 100, but the present invention is not limited thereto. Although not shown, the hole 110 may be formed in the blank 100 by a separately provided external device.

[0076] In one embodiment, the temperature of the blank 100 when hot punching is performed in the forming / punching step (S400) is equal to or higher than the Mf temperature. If the temperature of the blank 100 when hot punching is performed is lower than the Mf temperature, hot punching may be performed at a very low temperature, completing a phase transformation in the blank (or hot stamped part) and increasing shear stress. This may result in poor shear quality and hydrogen embrittlement. Furthermore, a large load may be generated during hot punching, causing damage to the press die 400 and / or punch 430.

[0077] In one embodiment, the blank 100 may be cooled while being formed into a final part shape (or during hot forming) in the press die 400. The press die 400 may be provided with cooling channels 440 through which a refrigerant circulates. For example, the lower die 410 and the upper die 420 may each have a cooling channel 440. Specifically, the cooling channels 440 may be disposed within the lower die 410 and the upper die 420 along the surfaces of the lower die 410 and the upper die 420. The heated blank 100 may be quenched by circulating the refrigerant supplied through the cooling channels 440 provided in the press die 400. In this case, to prevent springback of the sheet material and maintain the desired shape, quenching may be performed while the press die 400 is closed and pressurized. When performing the forming and cooling operations on the heated blank 100, the blank 100 may be cooled to the martensite finish temperature at an average cooling rate of at least 10°C / s. Desirably, the cooling rate is at least 20°C / s or more.

[0078] The blank 100 may be held in the press die 400 for 3 to 20 seconds. For example, the engagement state of the press die 400 may be held for 3 to 20 seconds. If the holding time in the press die 400 is less than 3 seconds, the blank 100 may not be sufficiently cooled, and residual heat and temperature variations between regions may cause thermal deformation, resulting in reduced dimensional quality. On the other hand, if the holding time in the press die 400 exceeds 20 seconds, the holding time in the press die 400 may be too long, resulting in reduced productivity.

[0079] In one embodiment, the hot forming, hot punching, and cooling processes of the heated blank 100 are performed within the press die 400. For example, the hot forming and cooling of the blank 100 may be performed from the start of forming the blank 100, and the hot punching of the blank 100 may be performed thereafter. That is, the hot forming and cooling of the blank 100 may be performed with the press die 400 in a closed state (or engaged state), and after a certain time has passed, the hot punching of the blank 100 may be performed. However, the present invention is not limited thereto.

[0080] As described above, hot punching is a process of punching the blank 100 during the cooling process of the heated, high-temperature blank 100. Hot punching can be applied between the time when the press die (or upper die) reaches the bottom dead center and the time when cooling (or quenching) of the blank 100 is completed. In other words, hot punching can be applied between the time when hot forming is completed and the time when cooling (or quenching) of the blank 100 is completed.

[0081] If hot drilling is performed on the blank 100 while the blank 100 is cooling (or quenching), the temperature of the blank 100 can affect the quality of the resulting hot stamped part, so the timing of hot drilling must be controlled. For example, if the temperature of the blank 100 is too low during hot drilling, the shear surface quality and shear load will be poor, which can lead to hydrogen embrittlement. Furthermore, because the cooling behavior of the blank 100 varies depending on the thickness of the blank 100, the shear surface quality and dimensions of each hot stamped part 10 may vary even if hot drilling is performed the same amount of time after the press die 400 reaches bottom dead center. This makes it difficult to achieve the desired shear surface quality and dimensions.

[0082] In addition, since the temperature behavior (or cooling behavior) of the blank 100 differs depending on the thickness of the blank (or material) and the forming start temperature, it is necessary to take this into consideration and differentially control the hot drilling timing for each blank.

[0083] Therefore, it is necessary to adjust the time point of hot piercing in the forming / piercing step (S400). However, in order to adjust the time point of hot piercing in the forming / piercing step (S400), various variables must be considered, including not only the forming start temperature and the thickness of the material, but also the hot piercing environment, the pressure of the press die, and the thickness sensitivity of the material. In this case, the material may refer to a blank.

[0084] Therefore, the inventors have conducted extensive experiments to derive a formula that can easily control the hot piercing timing in the forming / piercing step (S400). In one embodiment, the maximum hot piercing delay time in the forming / piercing step (S400) may satisfy the following formula:

number

[0085] In the formula, λ max is the maximum hot drilling delay time (s), α p is the correction coefficient considering the forming start temperature and the hot drilling environment, T is the forming start temperature (℃), b p is the correction coefficient that takes into account the pressure of the press die, c p is the correction coefficient considering the thickness sensitivity of the material, and t is the thickness of the material (mm). In this case, the material refers to the blank, and the unit of the maximum delay time at the time of hot piercing, s, can mean seconds.

[0086] First, the cooling behavior of a material (e.g., a blank) may vary depending on the blank's starting forming temperature and the hot piercing environment. Specifically, the cooling behavior of the material may vary depending on the starting forming temperature, the atmospheric temperature where hot piercing is performed, the design structure in which the material is placed in the press die, the layout of the press die, etc. p is a correction coefficient that takes into account the forming start temperature and the hot drilling environment, and may have a value of about more than 0 and 0.001 or less. p may have units of s / (°C x mm).

[0087] The heat transfer (or the amount of heat transfer) from the material to the press die can vary depending on the pressure applied by the press die to the material. For example, if the press die presses the material with a large pressure, the heat transfer from the material to the press die can increase. Therefore, the cooling behavior of the material can vary depending on the pressure applied by the press die to the material. p is a correction coefficient that takes into account the pressure of the press die, and b p may have a value of about greater than 0 and less than or equal to 0.65. pmay have units of s / mm.

[0088] Also, the thermal conductivity (or amount of heat transferred) from inside the material may vary depending on the thickness of the material. p is a correction coefficient that takes into account the difference in thermal conductivity (or heat transfer rate) due to the thickness of the material, and c p may have a value of about 1 or more and about 1.2 or less.

[0089] The forming start temperature (T) is the temperature at which forming of the material begins, and is equal to or higher than the Ms temperature. Specifically, the forming start temperature (T) is the temperature at which forming of the material begins, and is equal to or higher than the Ms temperature and equal to or lower than the temperature at which the material is removed from the heating furnace. The material thickness (t) is equal to or higher than about 1 mm and equal to or lower than about 2.3 mm.

[0090] In one embodiment, the hot piercing maximum delay time in the forming / piercing step (S400) can be calculated using the above-mentioned formula. Therefore, the hot piercing delay time in the forming / piercing step (S400) is set to λ 0 seconds or more from the time when the press die (or upper die) reaches the bottom dead center. max Here, the hot piercing delay time is the time from when the press die (or upper die) reaches the bottom dead center to when hot piercing is performed on the blank 100.

[0091] In other words, the hot piercing delay time in the forming / piercing step (S400) is 0 seconds or more from the time when the upper die reaches the bottom dead center and forming is completed. max If the hot piercing time in the forming / piercing step (S400) is 0 seconds, it may mean that hot piercing is performed immediately after hot forming is completed. In other words, if the hot piercing delay time in the forming / piercing step (S400) is 0 seconds, it may mean that hot piercing is performed as soon as hot forming is completed. If hot piercing is performed before hot forming, the dimensional quality of the pierced portion 110 formed by hot piercing may be inferior. In addition, if the hot piercing delay time in the forming / piercing step (S400) is λ 0 seconds from the time the press die reaches the bottom dead center, maxIf it is exceeded, hot drilling will be performed at too low a temperature, which may complete the phase transformation in the blank (or hot stamped part) and increase the shear stress, which may result in poor shear quality and hydrogen embrittlement.

[0092] Therefore, the hot piercing delay time in the forming / piercing stage (S400) is 0 seconds or more from the time the press die reaches the bottom dead center. max When the following is satisfied, the quality of the shear surface of the punched portion 110 formed through hot punching can be uniformly controlled. For example, when the hot punching delay time in the forming / punching step (S400) is 0 seconds or more λ from the time when the press die reaches the bottom dead center, max If the following is satisfied, the hydrogen embrittlement resistance of the produced hot stamped parts can be improved and the dimensional quality of the produced hot stamped parts can be maintained uniformly.

[0093] In one embodiment, the hot punching delay time in the forming / punching step (S400) is 0 seconds or more λ from the time when the press die reaches the bottom dead center. max If the following conditions are met, the amount of activated hydrogen in the manufactured hot stamped part is 0.5 ppm or less.

[0094] 9 is a diagram showing the process window derived from the thickness of the blank, the forming start temperature, and the maximum delay time of hot piercing, where the blank 100 is referred to as the blank, and Ta is the temperature at which the blank 100 is removed from the heating furnace.

[0095] 2 and 9, the method for manufacturing a hot stamped part may include a preparation step (S100), a heating step (S200), a transfer step (S300), and a forming / punching step (S400). In addition, as described above, the maximum hot punching delay time (λ) is calculated using the formula, taking into account the thickness of the material and the forming start temperature. max ) is derived, and when hot drilling the material, the hot drilling delay time is 0 seconds or more λ max The following is the result.

[0096] Therefore, the process window can be derived using the forming start temperature, the blank thickness, and the hot piercing delay time as parameters. In this case, the forming temperature in the process window is between the Ms temperature and the Ta temperature (the temperature at which the blank is removed from the heating furnace), the blank thickness is between about 1.0 mm and about 2.3 mm, and the hot piercing delay time is between 0 seconds and λ max The following is the result.

[0097] In one embodiment, a parameter window (or process window) for hot piercing in the forming / piercing step (S400) is derived using the blank thickness, the forming start temperature, and the hot piercing delay time, which allows for flexible process design, improves the quality of the produced hot stamped parts, and makes it easier to control the quality of the produced hot stamped parts.

[0098] Although not shown, a trimming step may be performed after the forming / punching step (S400). The trimming step is also a step of cutting off the outer periphery of the hot stamped part 10 formed using at least two perforations 110a, 110b formed in the hot stamped part 10. That is, the trimming step is also a step of cutting off the outer periphery of the hot stamped part 10 using the two perforations 110a, 110b formed in the hot stamped part 10 through hot punching.

[0099] In one embodiment, in the trimming step, the edge of the hot stamped component 10 may be cut using the at least two perforations 110a, 110b formed through hot punching as reference points (or guide patterns). That is, the hot stamped component 10 may be placed on a jig using the at least two perforations 110a, 110b formed in the molded hot stamped component 10, and the edge of the hot stamped component 10 may be cut. For example, after inserting fixing pins into the at least two perforations 110a, 110b formed in the hot stamped component 10, the outer periphery of the hot stamped component 10 may be cut.

[0100] In one embodiment, the trimming step may be performed using a laser or a press die (eg, a cold press die).

[0101] In a method for manufacturing a hot stamped part according to an embodiment of the present invention, after forming a hole 110 in a blank 100 through hot punching, an outer portion of the hot stamped part 10 may be cut off through cold trimming or laser trimming. That is, in a method for manufacturing a hot stamped part according to an embodiment of the present invention, a hot punching process may be performed on the blank 100, and after the blank 100 is cooled, cold trimming or laser trimming may be performed.

[0102] In one embodiment, a second drilling step may be performed after the forming / punching step (S400). The second drilling step is also a step of forming an additional drilling portion in the hot stamped component 10 using at least two drilling portions 110a, 110b formed in the hot stamped component 10. That is, the second drilling step is also a step of forming an additional drilling portion (e.g., an additional drilling portion) in the hot stamped component 10 using the two drilling portions 110a, 110b formed in the hot stamped component 10 through hot drilling.

[0103] In one embodiment, the second drilling step may be performed during the trimming step. For example, the second drilling step and the trimming step may be performed simultaneously. Alternatively, the second drilling step may be performed before or after the trimming step. For example, the second drilling step and the trimming step may be performed sequentially.

[0104] In a method for manufacturing a hot stamped part according to an embodiment of the present invention, after forming a hole 110 in the hot stamped part 10 through hot punching, a further hole (e.g., an additional hole) may be formed in the hot stamped part 10 through cold punching or laser punching. That is, in a method for manufacturing a hot stamped part according to an embodiment of the present invention, a hot punching process may be performed on a blank 100, and after the blank 100 has been subjected to the hot punching process, a cold punching process or a laser punching process may be performed.

[0105] In one embodiment, a portion of the outer periphery of the produced hot stamped part is also an edge of the coil blank, i.e., a portion of the outer periphery of the produced hot stamped part is also an edge of the coil blank in an uncut state.

[0106] In one embodiment, after the trimming step and / or the second punching step, a step of removing burrs formed on the blank may be performed, thereby removing burrs formed during processes such as hot punching, cold punching, and cold trimming.

[0107] Experimental Example The present invention will be described in more detail with reference to the following experimental examples. However, the following experimental examples are intended to more specifically explain the present invention, and the scope of the present invention is not limited to the following experimental examples. The following experimental examples may be appropriately modified or changed by those skilled in the art within the scope of the present invention.

[0108] [Table 1]

[0109] [Table 2]

[0110] Examples 1 to 3 and Comparative Examples 1 to 3 are specimens obtained by heating blanks having the compositions shown in Table 1 for 360 seconds in a heating furnace with a maximum temperature of 950°C, and then hot forming, cooling, and hot drilling were performed according to the conditions shown in Table 2. In this case, a multi-stage heating furnace was used as the heating furnace, which was configured to include multiple heating zones in which the temperature gradually increased from the entrance of the heating furnace where the blank (e.g., a plated steel sheet) was inserted to the exit of the heating furnace where the blank was removed.

[0111] In addition, in Table 2, the maximum hot piercing delay time λ max In the formula to derive a p , b p , c p are 0.001, 0.65, and 1.2, respectively.

[0112] Hydrogen embrittlement was evaluated using a thermal desorption spectroscopy (TDS) device for the specimens of Examples 1 to 3 and Comparative Examples 1 to 3 manufactured according to the conditions in Tables 1 and 2. Specifically, the specimens of Examples 1 to 3 and Comparative Examples 1 to 3 manufactured according to the conditions in Tables 1 and 2 were heated from room temperature to 500°C at a heating rate of 20°C / min, and the amount of hydrogen released from each specimen of Examples 1 to 3 and Comparative Examples 1 to 3 at temperatures below 350°C was measured.

[0113] Refer to Table 2, if the hot piercing delay time is 0 seconds or more, the hot piercing maximum delay time λ max If the following conditions are met, it can be confirmed that the amount of activated hydrogen in the manufactured hot stamped part is 0.5 ppm or less.

[0114] On the other hand, in the case of Comparative Examples 1 to 3, the material thickness and forming start temperature satisfy the range of the present invention, but the hot piercing delay time is less than the hot piercing maximum delay time λ max If the amount of activated hydrogen exceeds 0.5 ppm, it can be confirmed that the amount of activated hydrogen in the manufactured hot stamped part exceeds 0.5 ppm.

[0115] Therefore, the hot piercing delay time in the forming / piercing stage (S400) is 0 seconds or more from the time the press die reaches the bottom dead center. max If the following conditions are met, the amount of activated hydrogen in the manufactured hot stamped part is 0.5 ppm or less.

[0116] Although the present invention has been described based on the embodiments shown in the drawings, these are merely illustrative, and those skilled in the art will recognize that various modifications and equivalent embodiments are possible. Therefore, the true technical scope of the present invention should be determined by the technical spirit of the claims.

Claims

1. a heating step of heating the blank; a transfer step of transferring the heated blank to a press die including a punch; a forming and punching step of hot forming the transferred blank into a shape of a hot stamped part and hot punching the transferred blank to form a punched portion in the hot stamped part; The hot piercing delay time in the forming and piercing step is 0 seconds or more λ from the time when the press die reaches the bottom dead center. ｍａｘ is as follows: Said λ ｍａｘ is a manufacturing method of a hot stamped part, which is derived by the following formula: [Equation 1] (In the above formula, λ ｍａｘ is the maximum hot drilling delay time (s), α ｐ is the correction coefficient considering the forming start temperature and the hot drilling environment, T is the forming start temperature (℃), b ｐ is a correction coefficient that takes into account the pressure of the press die, c ｐ is a correction coefficient that takes into account the thickness sensitivity of the material, and t is the thickness of the material (mm).

2. In the above formula, α ｐ has a value greater than 0 and less than or equal to 0.001, and b ｐ has a value of about greater than 0 and less than or equal to 0.65, and c ｐ The method for producing a hot stamped part according to claim 1 , wherein R has a value of about 1 or more and about 1.2 or less.

3. In the step of transporting the blank, The method for producing a hot stamped part according to claim 1 , wherein the heated blank is air-cooled at room temperature.

4. The step of heating the blank comprises: A multi-stage heating step in which the blank is heated in stages; The method for producing a hot stamped part according to claim 1, further comprising a soaking step of heating the blank at a temperature of Ac3 to 1000°C.

5. The step of heating the blank is performed in a heating furnace; The method for manufacturing a hot stamped part according to claim 1 , wherein the heating furnace comprises a plurality of sections having different temperature ranges.

6. The method for producing a hot stamped part according to claim 1 , wherein the blank cools in the press die during the forming and punching steps.

7. The method for manufacturing a hot stamped part according to claim 6, wherein at least two or more holes are formed.

Images