Hot stamping part and its manufacturing method

Abstract

The present invention discloses a method for manufacturing a hot stamped part, which includes the steps of: placing a blank in a heating furnace; heating the blank; and transferring the heated blank from the heating furnace to a mold; wherein the air-cooling time of the blank during the step of transferring the blank satisfies Equation 1.

Description

[Technical field]

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

[0002] As environmental and fuel efficiency regulations become more stringent around the world, the need for lighter vehicle materials is increasing, which has led to active research and development of ultra-high strength steels and hot stamping steels.

[0003] The hot stamping process generally includes heating / forming / cooling / trim, and may utilize phase transformation and microstructural changes of the material during the process. The heating process of the hot stamping process is a process of heating a blank in a heating furnace, and the cooling process of the hot stamping process is a process of cooling the hot stamped compact in a die. In addition, the blank heated through the heating process may be exposed to room temperature and air-cooled while being fed from the heating furnace into the die.

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

[0005] The present invention can improve the quality of the hot stamped parts produced by controlling the heating time, air cooling time, and die cooling time in consideration of various parameters such as the blank material, blank thickness, and heating temperature. [Means for solving the problem]

[0006] According to one embodiment of the present invention, there is provided a method for manufacturing a hot stamped part, the method including the steps of: inserting a blank into a heating furnace; heating the blank; and transferring the heated blank from the heating furnace to a die, wherein an air-cooling time of the blank during the step of transferring the blank satisfies Equation 1 below.

number

[0007] In this embodiment, in the formula 1, t is between 0.0160 and 0.0165, and T t is Ac3 or more and 1000°C or less, b t is -10 or more and 0.5 or less, t is 1 mm or more and 2.6 mm or less, and c t is also greater than or equal to 0.7 and less than or equal to 0.9.

[0008] In this embodiment, in the above formula 1, λ t is between 5s and 20s.

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

[0010] In this embodiment, the step of heating the blank includes 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, in the step of heating the blank, the heating time of the blank satisfies the following Equation 2.

number

[0012] In this embodiment, in the formula 2, n is between -0.60 and -0.55, and T n is Ac3 or more and 1000°C or less, b n is 700 or more and 900 or less, t is 1 mm or more and 2.6 mm or less, and c n is also greater than or equal to 0.7 and less than or equal to 0.9.

[0013] In this embodiment, in the above formula 2, λ n is between 100s and 900s.

[0014] In this embodiment, the furnace may have a number of sections each having a different temperature range.

[0015] In this embodiment, in the plurality of sections, the ratio of the length of the section in which the blank is multistage heated to the length of the section in which the blank is soaked heated satisfies 1:1 to 4:1.

[0016] In this embodiment, after the step of transferring the blank, the method further includes the steps of pressing the transferred blank with the die to form a green body, and cooling the green body.

[0017] In this embodiment, in the step of forming the compact, the forming start temperature of the blank is 500°C or more and 700°C or less.

[0018] In the present embodiment, in the compact cooling step, a mold cooling time during which the compact is cooled in the mold satisfies the following Equation 3.

number

[0019] In this embodiment, in the formula 3, q is -1.0 or more and -0.2 or less, P is 0.1 MPa or more and 5 MPa or less, b q is 11 or more and 15 or less, t is 1 mm or more and 2.6 mm or less, and c q is greater than or equal to 1.00 and less than or equal to 1.05.

[0020] In this embodiment, in the above formula 3, λ q is between 6s and 40s.

[0021] In this embodiment, the mold cooling end temperature at which the molding cooling step ends is equal to or higher than room temperature and equal to or lower than about 200°C.

[0022] In another embodiment of the present invention, a hot stamped part is provided having a tensile strength of 1350 MPa or greater and less than 2300 MPa.

[0023] Other aspects, features, and advantages will become apparent from the following detailed description, claims, and drawings. Effect of the Invention

[0024] According to one embodiment of the present invention as described above, the quality of a manufactured hot stamped part can be improved by controlling the heating time, air cooling time, and die cooling time in consideration of various parameters such as the blank material, the blank thickness, and the heating temperature.

[0025] In addition, according to an embodiment of the present invention, a process window can be derived using material thickness, heating time, air cooling time, and die cooling time as parameters, which allows for flexible process design and facilitates quality control of the manufactured hot stamped parts. It goes without saying that the scope of the present invention is not limited by these effects. [Brief description of the drawings]

[0026] [Figure 1] 1 is a flow chart that illustrates a schematic diagram of a method for manufacturing a hot stamped part according to an embodiment of the present invention. [Diagram 2] 2 is a flow chart illustrating a heating step of a method for manufacturing a hot stamping part according to an embodiment of the present invention. [Diagram 3] 2 is a view illustrating a heating furnace having multiple sections in a heating step of a method for manufacturing a hot stamped part according to an embodiment of the present invention; [Figure 4] 1 is a diagram showing the cooling behavior of a heated blank over time. [Diagram 5] 1 is a diagram showing heating time depending on material thickness and heating temperature. [Figure 6] 1 is a diagram showing air-cooling time depending on material thickness and heating temperature. [Figure 7] 1 is a diagram showing a die cooling time according to a material thickness and a die cooling time according to a pressure. [Figure 8] 1 is a diagram showing a process window derived using material thickness, heating time, air cooling time, and mold cooling time as parameters. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention can be modified in various ways and can have various embodiments, and specific embodiments are illustrated in the drawings and will be described in detail in the detailed description. The advantages and features of the present invention, and the methods for achieving the same, will become apparent from the following detailed description of the embodiments in conjunction with the drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms.

[0028] In the following examples, the terms "first", "second" and the like are not used in a limiting sense but are used for the purpose of distinguishing one component from another component.

[0029] In the following examples, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0030] 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.

[0031] In the following examples, when a part such as a film, region, or component is said to be on or above another part, this does not only include the case where it is directly on top of the other part, but also the case where another film, region, component, etc. is interposed between them.

[0032] In the drawings, the size of components may be exaggerated or reduced for the convenience of explanation. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the convenience of explanation, and the present invention is not necessarily limited to what is shown in the drawings.

[0033] In this specification, "A and / or B" refers to the case of A, B, or A and B. In addition, in this specification, "at least one of A and B" refers to the case of A, B, or A and B.

[0034] In the following embodiments, the term "extending in a first or second direction" means not only that the wiring extends in a straight line, but also that the wiring extends in a zigzag or curved manner along the first or second direction.

[0035] In the following examples, when we say "on a plane", this means a top view of the target part, and when we say "on a cross section", this means a side view of a cross section cut vertically through the target part. In the following examples, when we say "overlap", this includes "on a plane" and "on a cross section" overlap.

[0036] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description with reference to the drawings, the same or corresponding elements are designated by the same reference numerals.

[0037] Fig. 1 is a flow chart showing a schematic diagram of a method for manufacturing a hot stamped part according to an embodiment of the present invention, and Fig. 2 is a flow chart showing a detailed heating step of the method for manufacturing a hot stamped part according to an embodiment of the present invention. Hereinafter, the method for manufacturing a hot stamped part will be described with reference to Figs. 1 and 2.

[0038] Referring to FIG. 1, a method for manufacturing a hot stamping part according to an embodiment includes a blank inserting step (S100), a heating step (S200), a transferring step (S300), a forming step (S400), and a cooling step (S500).

[0039] First, the blank inserting 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 is prepared in the form of a base material having a coating layer formed on at least one surface thereof. The base material may be a steel sheet manufactured by performing a hot rolling process and / or a cold rolling process on a steel slab that is cast to contain a predetermined content of a predetermined alloy element as a base steel sheet.

[0040] In one embodiment, the base steel sheet contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), the balance being iron (Fe), and other inevitable impurities. For example, the base steel sheet contains carbon (C) of 0.01% or more and 0.5% or less by weight, silicon (Si) of 0.01% or more and 1.00% or less by weight, manganese (Mn) of 0.3% or more and 2.0% or less by weight, phosphorus (P) of more than 0 and 0.1% or less by weight, sulfur (S) of more than 0 and 0.1% or less by weight, the balance being iron (Fe) and other inevitable impurities.

[0041] The base steel sheet further contains one or more of boron (B), titanium (Ti), niobium (Nb), chromium (Cr), molybdenum (Mo), and nickel (Ni). For example, the base steel sheet further contains one or more of boron (B) 0.0001% to 0.005% by weight, titanium (Ti) 0.01% to 0.1% by weight, niobium (Nb) 0.01% to 0.1% by weight, chromium (Cr) 0.01% to 0.5% by weight, molybdenum (Mo) 0.01% to 0.5% by weight, and nickel (Ni) 0.01% to 1.0% by weight.

[0042] In one embodiment, the blank is used to manufacture a hot stamped part, so that the manufactured hot stamped part also includes the aforementioned components.

[0043] Carbon (C) is a main element that determines the strength and hardness of steel, and may be added to ensure the tensile strength of the steel after the hot stamping (or hot pressing) process. Carbon may also be added to ensure the hardenability characteristics of the steel. In one embodiment, carbon is included in an amount of 0.01 to 0.5 wt% based on the total weight of the base steel sheet. If carbon is included in an amount less than 0.01 wt% based on the total weight of the base steel sheet, it is difficult to achieve the mechanical strength of the present invention. On the other hand, if carbon is included in an amount more than 0.5 wt% based on the total weight of the base steel sheet, a problem of reduced toughness of the steel or a problem of controlling the brittleness of the steel may occur.

[0044] Silicon (Si) can act as a ferrite stabilizing element in the base steel sheet. Silicon improves ductility by purifying ferrite, and improves carbon concentration in austenite by suppressing the formation of carbides in low temperature regions. In addition, silicon is a key element for homogenizing hot rolling, cold rolling, and hot stamping structure (control of pearlite and manganese segregation zones) and finely dispersing ferrite. In one embodiment, silicon is included in an amount of 0.01% to 1.0% by weight based on the total weight of the base steel sheet. If silicon is included in an amount of less than 0.01% by weight based on the total weight of the base steel sheet, the above-mentioned functions may not be fully exerted. On the other hand, if silicon is included in an amount of more than 1.0% by weight based on the total weight of the base steel sheet, the hot rolling and cold rolling loads may increase, and hot rolling red scale may become excessive, resulting in reduced bondability.

[0045] Manganese (Mn) may be added to increase hardenability and strength during heat treatment. In one embodiment, manganese is included in the range of 0.3 wt% to 2.0 wt% based on the total weight of the base steel sheet. If manganese is included in the range of less than 0.3 wt% based on the total weight of the base steel sheet, there is a high possibility that the material quality after hot stamping will be insufficient (insufficient hard phase fraction) due to insufficient hardenability. On the other hand, if manganese is included in the range of more than 2.0 wt% based on the total weight of the base steel sheet, ductility and toughness will decrease due to manganese segregation or pearlite bands, causing a decrease in bending performance and the generation of a non-uniform microstructure.

[0046] Phosphorus (P) is an element that easily segregates and also inhibits the toughness of steel. In one embodiment, phosphorus (P) is contained in an amount of more than 0 to 0.1 wt% based on the total weight of the base steel sheet. When phosphorus is contained within the above range based on the total weight of the base steel sheet, a decrease in the toughness of the steel can be avoided. On the other hand, when phosphorus is contained in an amount of more than 0.1 wt% based on the total weight of the base steel sheet, cracks can be induced during processing and iron phosphide compounds can be formed, which can decrease the toughness of the steel.

[0047] Sulfur (S) is an element that impairs workability and physical properties. In one embodiment, sulfur is contained in an amount of more than 0 to 0.1 wt% based on the total weight of the base steel sheet. If sulfur is contained in an amount of more than 0.1 wt% based on the total weight of the base steel sheet, hot workability is reduced and large inclusions are formed, causing surface defects such as cracks.

[0048] Boron (B) is added to ensure the hardenability and strength of steel by ensuring a martensite structure, and has a grain refinement effect by increasing the austenite grain growth temperature. In one embodiment, boron is included in the base steel sheet in a range of 0.0001 to 0.005% by weight based on the total weight of the base steel sheet. When boron is included in the above range based on the total weight of the base steel sheet, it is possible to prevent the occurrence of hard phase grain boundary embrittlement and ensure high toughness and bendability.

[0049] Titanium (Ti) may be added to enhance hardenability and improve material properties by forming precipitates after hot stamping heat treatment. Titanium also forms precipitate phases such as Ti(C,N) at high temperatures, effectively contributing to refinement of austenite grains. In one embodiment, titanium is included in an amount of 0.01 to 0.1 wt% based on the total weight of the base steel sheet. When titanium is included in the above range based on the total weight of the base steel sheet, performance degradation is prevented, coarsening of precipitates is prevented, the physical properties of the steel are easily secured, and the occurrence of cracks on the steel surface can be prevented or minimized.

[0050] Niobium (Nb) may be added to increase strength and toughness by reducing martensite packet size. In one embodiment, niobium is included in an amount of 0.01 to 0.1 wt% based on the total weight of the base steel sheet. When niobium is included in the above range based on the total weight of the base steel sheet, it has an excellent effect of refining the crystal grains of the steel material during hot rolling and cold rolling processes, prevents cracks in slabs and brittle fractures in products during steelmaking / continuous casting, and minimizes the generation of coarse precipitates in steelmaking.

[0051] Chromium (Cr) may be added to improve the hardenability and strength of steel. In one embodiment, chromium is included in an amount of 0.01 to 0.5 wt% based on the total weight of the base steel sheet. When chromium is included in the above range based on the total weight of the base steel sheet, the hardenability and strength of the steel can be improved, and an increase in production costs and a decrease in the toughness of the steel can be prevented.

[0052] Molybdenum (Mo) can contribute to improving strength by suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot stamping. Molybdenum (Mo) is included in an amount of 0.01 to 0.5 wt% based on the total weight of the base steel sheet. When molybdenum is included in the above range based on the total weight of the base steel sheet, it has excellent effects of suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot stamping.

[0053] Nickel (Ni) may be added to improve hardenability and strength. Nickel is also an austenite stabilizing element, and contributes to improving elongation by controlling austenite transformation. In one embodiment, nickel is included in an amount of 0.01 to 1.0 wt% based on the total weight of the base steel sheet. If nickel is included in an amount less than 0.01 wt% based on the total weight of the base steel sheet, the above-mentioned effects are not fully realized. If nickel is included in an amount more than 1.0 wt% based on the total weight of the base steel sheet, toughness is reduced, cold workability is reduced, and the manufacturing cost of the product is increased.

[0054] In one embodiment, in the blank inserting step (S100), the blank inserted into the heating furnace may be mounted on rollers and then transported along the transport direction.

[0055] 1 and 2, a heating step (S200) may be performed after the blank insertion step (S100). In one embodiment, the heating step (S200) includes a multi-stage heating step (S210) and a soaking heating step (S220). Therefore, after the blank insertion step (S100), the multi-stage heating step (S210) and the soaking heating step (S220) may be performed. The multi-stage heating step (S210) and the soaking heating step (S220) are also steps in which the blank is heated by passing through multiple sections provided in a heating furnace.

[0056] In one embodiment, the entire furnace temperature is 680° C. to 1000° C. In particular, the entire furnace temperature in which the multi-stage heating step (S210) and the soaking heating step (S220) are performed is 680° C. to 1000° C. In this case, the furnace temperature in which the multi-stage heating step (S210) is performed is 680° C. to Ac3, and the furnace temperature in which the soaking heating step (S220) is performed is Ac3 to 1000° C.

[0057] In the multi-stage heating step (S210), the blank may be heated stepwise as it passes through a plurality of sections provided in the heating furnace. Among the plurality of sections provided in the heating furnace, there are a plurality of sections in which the multi-stage heating step (S210) is performed, and the temperature of the blank may be set for each section so that the temperature increases from the entrance of the heating furnace where the blank is inserted to the exit of the heating furnace where the blank is removed, thereby heating the blank stepwise.

[0058] After the multi-stage heating step (S210), a soaking heating step (S220) may be performed. In the soaking heating step (S220), the multi-stage heated blank may be heat-treated while passing through a section of a heating furnace set at a temperature of Ac3 to 1000°C. Preferably, in the soaking heating step (S220), the multi-stage heated blank may be soaked at a temperature of 830°C to 1000°C. In addition, the soaking heating step (S220) is performed in at least one section among a plurality of sections provided in the heating furnace.

[0059] FIG. 3 is a view for explaining 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.

[0060] 3, the furnace according to an embodiment includes a plurality of zones having different temperature ranges. More specifically, the furnace includes 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.

[0061] In one embodiment, in the multi-stage heating step (S210), the blank may be heated in multiple stages by passing through a first section P1 to a fourth section P4 defined in a heating furnace. Also, in the soaking heating step (S220), the blank heated in multiple stages in the first section P1 to the fourth section P4 may be soaked by passing through a fifth section P5 to a seventh section P7.

[0062] The first section P1 to the seventh section P7 may be arranged in sequence in 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 discharged. Therefore, the first section P1 having the first temperature range T1 is also the first section of the heating furnace, and the seventh section P7 having the seventh temperature range T7 is also the last section of the heating furnace. Among the multiple sections of the heating furnace, the fifth section P5, the sixth section P6, and the seventh section P7 are also sections where soaking heating is performed, not sections where multi-stage heating is performed.

[0063] The temperatures of the multiple sections provided in the heating furnace, for example, the temperatures of the first section P1 to the seventh section P7, may increase from the entrance of the heating furnace where the blanks are inserted to the exit of the heating furnace where the blanks are removed. However, the temperatures of the fifth section P5, the sixth section P6, and the seventh section P7 may be the same. Also, the temperature difference between two adjacent sections among the multiple sections provided in the heating furnace may be greater than 0°C and less than 100°C. For example, the temperature difference between the first section P1 and the second section P2 may be greater than 0°C and less than 100°C.

[0064] In one embodiment, the first temperature range T1 of the first section P1 is 680°C to 870°C. The second temperature range T2 of the second section P2 is 700°C to 900°C. The third temperature range T3 of the third section P3 is 750°C to 930°C. The fourth temperature range T4 of the fourth section P4 is 800°C to 950°C. The fifth temperature range T5 of the fifth section P5 is Ac3 to 1000°C. Preferably, the fifth temperature range T5 of the fifth section P5 is 830°C or more and 1000°C or less. The sixth temperature range T6 of the sixth section P6 and the seventh temperature range T7 of the seventh section P7 are the same as the fifth temperature range T5 of the fifth section P5.

[0065] 3, the furnace according to the embodiment is illustrated as having seven sections with different temperature ranges, but the present invention is not limited thereto, and the furnace may have five, six, eight, etc. sections with different temperature ranges.

[0066] In one embodiment, in the multi-stage heating step (S210), the blank may be heated stepwise through a plurality of sections (eg, a first section P1 to a fourth section P4) defined in a heating furnace.

[0067] After the multi-stage heating step (S210), a soaking heating step (S220) is performed. The soaking heating step (S220) is performed in the last part of the multiple sections of the heating furnace. In one embodiment, the soaking heating step (S220) is performed in the fifth section P5, the sixth section P6, and the seventh section P7 of the heating furnace. When multiple sections are provided in the heating furnace, if one section is long, there may be a problem that temperature changes occur within the section. Therefore, the sections in which the soaking heating step (S220) is performed are divided into the fifth section P5, the sixth section P6, and the seventh section P7, and the fifth section P5, the sixth section P6, and the seventh section P7 may have the same temperature range within the heating furnace.

[0068] In the soaking heating step (S220), the multi-stage heated blank may be soaked at a temperature of Ac3 to 1,000° C. Preferably, in the soaking heating step (S220), the multi-stage heated blank may be soaked at a temperature of 830° C. to 1,000° C.

[0069] In one embodiment, the heating step (S200) is divided into a multi-stage heating step (S210) and a soaking heating step (S220), so that the temperature of the furnace can be set in stages, thereby improving the energy efficiency of the furnace.

[0070] In one embodiment, the heating furnace has a length of 20m to 40m along the blank transfer path. The heating furnace has a plurality of sections having different temperature ranges, and the ratio of the length D1 of the section where the blank is heated in multiple stages among the plurality of sections to the length D2 of the section where the blank is heated soakingly among the plurality of sections satisfies 1:1 to 4:1. When the length of the section where the blank is heated soakingly in the heating furnace increases and the ratio of the length D1 of the section where the blank is heated in multiple stages to the length D2 of the section where the blank is heated soakingly exceeds 1:1, the amount of hydrogen permeation into the blank increases in the soaking heating section, and delayed fracture increases. On the other hand, when the length of the section where the blank is heated soakingly decreases and the ratio of the length D1 of the section where the blank is heated in multiple stages to the length D2 of the section where the blank is heated soakingly is less than 4:1, the soaking heating section (time) is not sufficiently secured, and the strength of the hot stamped part manufactured by the hot stamping part manufacturing process is also non-uniform.

[0071] In one embodiment, the length of the uniform heating section among the multiple sections provided in the heating furnace is 20% to 50% of the total length of the heating furnace.

[0072] Referring to FIG. 1, after the heating step (S200), a transferring step (S300), a molding step (S400), and a cooling step (S500) may be further performed.

[0073] In one embodiment, the transferring step (S300) is also a step of transferring the heated blank from the heating furnace to the die. In this case, in the transferring step (S300), the heated blank may be cooled to atmospheric temperature (or room temperature) while being transferred to the die. The heated blank may be air-cooled during the transfer. If the heated blank is not air-cooled, the die entry temperature (e.g., molding start temperature) becomes high, and wrinkles (or bends) may occur on the surface of the manufactured hot stamped part. In addition, when a coolant is used, it may affect the subsequent process (hot stamping), so it is preferable that the heated blank be air-cooled during the transfer.

[0074] In one embodiment, the forming step (S400) is a step of hot stamping the transferred blank to form a green body. Specifically, in the forming step (S400), the blank may be pressed in a mold to form a green body.

[0075] In one embodiment, the cooling step (S500) is also a step of cooling the molded compact. The cooling step (S500) is performed in the mold in which the blank is pressed.

[0076] Fig. 4 is a diagram showing the cooling behavior of a heated blank over time. Specifically, Fig. 4 is a graph showing the cooling behavior of a blank heated through a heating step (S200) while it is being removed from a heating furnace and then undergoing a transfer step (S300), a forming step (S400), and a cooling step (S500).

[0077] Referring to FIG. 4, the blank heated in the heating step (S200) is removed from the heating furnace and then cooled while passing through a conveying step (S300), a forming step (S400), and a cooling step (S500).

[0078] In one embodiment, the heated blank may be cooled at atmospheric temperature (or room temperature) in the transfer step (S300). Specifically, in the transfer step (S300), the blank heated through the heating step (S200) may be cooled at atmospheric temperature (or room temperature) while being transferred to a mold after being removed from the heating furnace.

[0079] Next, in the forming step (S400), forming of the blank cooled to the atmospheric temperature (or room temperature) is started. At this time, the forming start temperature of the blank is set to a forming start temperature T A That is, in the transfer step (S300), the blank heated in the heating step (S200) is taken out of the heating furnace and heated to a forming start temperature T A The device can be cooled (or air-cooled) to room temperature.

[0080] In one embodiment, the forming start temperature T AThe forming start temperature T is 500°C or more and 700°C or less. A If the forming start temperature T A If the forming start temperature T is too low, the blank formability is reduced, and the hot stamped part produced cannot have the desired structure and physical properties. A If the forming start temperature T exceeds 700°C, wrinkles (or bends) will occur on the surface of the hot stamped parts produced. In addition, the plating layer of the blank may be burned onto the die. Therefore, A When the temperature is 500°C or higher and 700°C or lower, the formability of the blank is improved, the produced hot stamped part has the desired structure and physical properties, and the occurrence of wrinkles (or bends) on the surface of the produced hot stamped part can be prevented or minimized.

[0081] Then, in one embodiment, in the molding step (S400), the blank transferred to the die through the transfer step (S300) is molded to form a green body, and the green body is cooled in the cooling step (S500). At this time, the cooling step (S500) of cooling the green body is performed in the die.

[0082] Specifically, the final product may be formed by forming the final part shape in a mold and cooling the molded body. The mold may have a cooling channel inside through which a refrigerant circulates. The molded body may be quenched by circulating the refrigerant supplied through the cooling channel provided in the mold. In this case, in order to prevent the spring back phenomenon of the plate material and to maintain the desired shape, quenching may be performed while pressurizing the mold in a closed state. In forming and cooling the molded body, the average cooling rate may be at least 10° C. / s to the martensite finish temperature.

[0083] In one embodiment, the die cooling end temperature at which the cooling step (S500) is completed is between room temperature and 200°C. If the die cooling end temperature is below room temperature, the productivity of the manufacturing process is reduced. Meanwhile, if the die cooling end temperature exceeds 200°C, the manufactured hot stamped part is air-cooled at room temperature, at which time, the hot stamped part may be twisted and it may be difficult to secure the desired material. Therefore, if the die cooling end temperature at which the cooling step (S500) is completed satisfies the range between room temperature and about 200°C, the productivity of the manufacturing process may be improved, and the manufactured hot stamped part may be air-cooled at room temperature, preventing or minimizing the twisting of the hot stamped part.

[0084] Fig. 5 is a diagram showing the heating time according to the material thickness and the heating time according to the heating temperature. Specifically, Fig. 5 is a graph for explaining the minimum heating time according to the material thickness and the heating temperature. In Fig. 5, the heating temperature means the soaking heating temperature in the soaking heating step (S220), and the heating time means the total heating time in the heating step (S200).

[0085] 1, 2 and 5, it can be seen that, when the material thickness is the same, the minimum heating time increases as the heating temperature decreases, and also, when the heating temperature is the same, the minimum heating time increases as the material thickness increases.

[0086] If the heating time (e.g., total heating time) for which the blank is heated in the heating step (S200) is short, the blank will not undergo sufficient phase transformation. On the other hand, if the heating time for the blank in the heating step (S200) is too long, not only will austenite grains become coarse and hydrogen embrittlement resistance decrease, but the coating layer will become thicker and weldability may decrease. Therefore, it is necessary to adjust the heating time in the heating step (S200). However, in order to adjust the heating time in the heating step (S200), various variables must be considered, such as the heating temperature and the thickness of the blank (e.g., the thickness of the material), as well as heat loss in the heating furnace caused by the airtightness, atmosphere, heat source, etc. of the heating furnace, and the components of the blank.

[0087] Therefore, the present inventors have derived Equation 1, which allows easy control of the heating time, through extensive repeated experiments. In one embodiment, the heating time of the blank in the heating step (S200) satisfies the following Equation 1.

number

[0088] Since different heat sources are used for different types of furnaces, the heat loss generated by each type of furnace is also different. n is a correction coefficient taking into account the heat loss of the heating furnace, and has a value of about -0.60 to about -0.55. n has units of s / (℃xmm).

[0089] When the components of each material are different, the phase transformation temperatures are different. n is a correction coefficient taking into account the Ac3 temperature difference due to the material composition, and has a value of about 700 to about 900. n has units of s / mm.

[0090] The thermal conductivity (or amount of heat transferred) from inside the material varies depending on the thickness of the material. n is a correction coefficient that takes into account the difference in thermal conductivity (or heat transfer rate) due to the thickness of the material at high temperatures, and may have a value of about 0.7 to about 0.9. In this case, high temperature may mean 600°C or higher. However, high temperature may mean 500°C or higher, or 700°C or higher.

[0091] Heating temperature T nmeans the soaking temperature in the soaking heating step (S220), and the heating temperature T n has a value of about Ac3 or more and about 1000° C. or less. Also, the material thickness t may have a value of about 1 mm or more and about 2.6 mm or less.

[0092] In one embodiment, the heating time λ according to Equation 1 n The heating time λ is about 100 s or more and about 900 s or less. n If the heating time λ is less than 100 s, the blank will not undergo sufficient phase transformation. n If the heating time λ exceeds 900s, not only will the austenite grains become coarse and hydrogen resistance deteriorate, but the coating layer will become thicker and weldability will deteriorate. n When the time satisfies the range of about 100 s or more and about 900 s or less, sufficient phase transformation is performed in the blank, the occurrence of austenite grain coarsening is prevented or minimized, and the deterioration of hydrogen embrittlement resistance and / or weldability can be prevented or minimized.

[0093] Fig. 6 is a diagram showing the air cooling time according to the material thickness and the heating temperature. Specifically, Fig. 6 is a graph shown to explain the maximum allowable air cooling time according to the material thickness and the heating temperature. In Fig. 6, a high heating temperature means a high heating furnace removal temperature.

[0094] 1, 2 and 6, it can be seen that the maximum allowable cooling time increases as the heating temperature decreases for the same material thickness. It can also be seen that the maximum allowable cooling time increases as the material thickness increases for the same heating temperature.

[0095] If the heated blank is exposed to room temperature too much, not only will the productivity decrease, but the blank will undergo a phase transformation during air cooling, which will decrease formability and make it difficult to obtain the desired material. Meanwhile, if the heated blank is exposed to room temperature for too short a time, the hot stamping part manufactured will begin to be formed at an excessively high temperature, causing wrinkles (or bending). Also, the plating layer of the blank may be burned onto the die. Therefore, it is necessary to adjust the air cooling time in the transfer step (S300). However, in order to adjust the air cooling time in the transfer step (S300), various variables must be considered, including not only the heating temperature and the thickness of the blank (e.g., the thickness of the material), but also the blank's composition, blank thickness, plating amount, thermal conductivity, thermal conductivity, and heat transfer rate due to surface emissivity, and the blank's removal temperature from the heating furnace and the air temperature.

[0096] Therefore, the present inventors have derived Equation 2, which allows easy control of the air-cooling time through extensive repeated experiments. In one embodiment, the air-cooling time of the blank in the transfer step (S300) satisfies the following Equation 2.

number

[0097] a t is a correction coefficient taking into account the temperature at which the heated blank is removed from the heating furnace and the atmospheric temperature, and has a value of about 0.0160 to about 0.0165. t has units of s / (℃xmm).

[0098] b tis a correction coefficient that takes into account the fact that the components of each material are different from each other, and has a value of about -10.0 to about 0.5. t has units of s / mm.

[0099] Also, the amount of heat transferred from inside the material varies depending on the thickness of the material. t is a correction coefficient that takes into account the difference in heat transfer rate due to the thickness of the material at high temperatures, and has a value of about 0.7 to about 0.9. In this case, high temperature means 600°C or higher. However, high temperature means 500°C or higher or 700°C or higher.

[0100] Heating temperature T t means the soaking temperature in the soaking heating step (S220), and the heating temperature T t has a value of about Ac3 or more and about 1000° C. or less. In this case, the heating temperature T t can also mean the temperature at which the material is removed from the heating furnace. Also, the material thickness t has a value of about 1 mm or more and about 2.6 mm or less.

[0101] In one embodiment, the air-cooling time λ according to Equation 2 t The cooling time λ is about 5 s or more and about 20 s or less. t If the air cooling time λ is less than 5s, the forming start temperature at which the blank starts to be formed is excessively high, and the hot stamped parts manufactured by forming the blank at a high temperature will have wrinkles (or bends). t On the other hand, the air cooling time λ t If the cooling time λ exceeds 20 s, not only will the productivity decrease, but the blank will undergo phase transformation during the blank transfer process, reducing the blank's formability, and the hot stamped parts produced will not have the desired properties. t When the time t is in the range of about 5 s to about 20 s, the formability of the blank and the productivity of the process are improved, and the produced hot stamped part has a desired material.

[0102] Fig. 7 is a diagram showing the die cooling time according to the material thickness and the pressurizing force. Specifically, Fig. 7 is a graph for explaining the minimum allowable die cooling time according to the material thickness and the pressurizing force.

[0103] 1, 2 and 7, it can be seen that, when the material thickness is the same, the minimum allowable mold cooling time decreases as the pressurizing force increases, and also, when the pressurizing force is the same, it can be seen that the minimum allowable mold cooling time increases as the material thickness increases.

[0104] If the die cooling time during which the molded body formed in the cooling step (S500) is cooled is short, the die cooling is completed at an excessively high temperature, and the hot stamped part manufactured is air-cooled at room temperature for a long time, and the hot stamped part manufactured during air-cooling is distorted, making it difficult to secure the desired dimensions. Meanwhile, if the die cooling time during which the molded body formed in the cooling step (S500) is cooled is long, productivity decreases. Therefore, it is necessary to adjust the die cooling time in the cooling step (S500). However, in order to adjust the cooling time in the cooling step (S500), various variables such as the thermal conductivity of the die, the cooling behavior due to the blank components, and the hardening ability due to the blank components, as well as the pressure force of the die and the thickness of the blank (e.g., the thickness of the material), must be considered.

[0105] Therefore, the present inventors have derived Equation 3, which can easily control the mold cooling time through extensive repeated experiments. In one embodiment, the mold cooling time of the compact in the cooling step (S500) satisfies the following Equation 3.

number

[0106] Thermal conductivity differs depending on the material of the mold. In addition, the local thermal conductivity that occurs within the same part differs depending on the deformation amount at each molding position (flat part, edge part, side wall part, etc.). q is a correction coefficient that takes into account the thermal conductivity of the mold and the local thermal conductivity generated within the part, and has a value of about -1.0 to about -0.2. q has units of s / (MPaxmm).

[0107] When the components of each material are different, the Continuous Cooling Transformation (CCT) curves of the compacts containing them are different, and the martensitic transformation start temperatures are different. q is a correction coefficient taking into account the continuous cooling transformation (CCT) curve and / or martensite transformation start temperature of the formed body depending on the material composition, and has a value of about 11 to about 15. q has units of s / mm.

[0108] The amount of heat transferred from inside the material also varies depending on the thickness of the material. q is a correction coefficient that takes into account the difference in heat transfer rate due to the thickness of the material at low temperatures, and has a value of about 1.00 to about 1.05. In this case, low temperature means 600°C or lower. However, low temperature means 500°C or lower or 700°C or lower.

[0109] The pressure force P is also the minimum pressure force in the die cooling process. Specifically, the pressure force P is the minimum value of the pressure forces applied to each part of the blank (e.g., flat part, edge part, side wall part, etc.), and is also the pressure force in the part where the force of the die does not act vertically (e.g., side wall part). For example, the pressure force P has a value of about 0.1 MPa or more. In an actual process, the pressure force P is about 5 MPa or more. However, even when the pressure force P is 5 MPa or more, the die cooling time λ qIn order to easily derive the above, the applied pressure P in Equation 3 has a value of 5 MPa.

[0110] The material thickness t has a value of about 1 mm or more and about 2.6 mm or less.

[0111] In one embodiment, the mold cooling time λ according to Equation 3 q is about 6 seconds or more and about 40 seconds or less. q If the die cooling time λ is less than 6s, the die cooling ends at a high temperature and requires a long air cooling period, which causes the hot stamped part to be distorted and not to have the desired dimensions. q If the mold cooling time λ exceeds 40 s, productivity may decrease. q In the case where the time period is in the range of about 6 seconds or more and about 40 seconds or less, when the temperature of the blank is from room temperature to 200°C or less, the distortion of the hot stamped part manufactured after the die cooling is completed is prevented or minimized, and the productivity of the manufacturing process can be improved.

[0112] Fig. 8 is a diagram showing a process window derived using material thickness, heating time, air-cooling time, and mold cooling time as parameters. The process window in Fig. 8 is a graph derived using material thickness, heating time, air-cooling time, and mold cooling time as parameters.

[0113] 1 and 8, a method for manufacturing a hot stamping part includes a blank inserting step (S100), a heating step (S200), a transferring step (S300), a forming step (S400), and a cooling step (S500).

[0114] In one embodiment, the heating time in the heating step (S200) can be easily derived using the above-mentioned Equation 1, the air-cooling time in the transfer step (S300) can be easily derived using the above-mentioned Equation 2, and the mold cooling time in the cooling step (S500) can be easily derived using the above-mentioned Equation 3. In addition, the process window can be derived using the heating time, air-cooling time, and mold cooling time derived through Equation 1, Equation 2, and Equation 3, respectively, and the material thickness as parameters. That is, the process window can be derived using the heating time in the heating step (S200), the air-cooling time in the transfer step (S300), the mold cooling time in the cooling step (S500), and the material thickness as parameters. In this case, in the process window, the material thickness is about 1.0 mm to about 2.6 mm, the heating time is about 100 s to about 900 s, the air-cooling time is about 5 s to about 20 s, and the mold cooling time is about 6 s to about 40 s.

[0115] In one embodiment, an integrated parameter window for the heating step (S200), the transfer step (S300) and the cooling step (S500) is derived using material thickness, heating time, air cooling time and die cooling time as parameters, which can induce flexible process design before the hot stamping process, improve the quality of the manufactured hot stamped parts and make it easier to control the quality of the manufactured hot stamped parts.

[0116] In one embodiment, the hot stamped parts produced according to one embodiment of the present invention have a tensile strength of about 1350 MPa or more and less than about 2300 MPa. Desirably, the hot stamped parts produced have a tensile strength of about 1350 MPa or more and less than about 1680 MPa. Alternatively, the hot stamped parts produced can have a tensile strength of about 1680 MPa or more and less than about 2300 MPa.

[0117] 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 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. placing a blank into a heating furnace; heating the blank; transferring the heated blank from the furnace to a die; The air-cooling time λ t of the blank during the blank transfer step is controlled to satisfy the following Equation 1 in consideration of the furnace removal temperature and atmospheric temperature correction coefficient a t , the heating temperature T t , the material component correction coefficient b t , the material thickness t, and the high temperature material thickness sensitivity correction coefficient c t : In the above formula 1, a t is 0.0160 or more and 0.0165 or less, T t is Ac3 or more and 1000° C. or less, b t is −10 or more and 0.5 or less, t is 1 mm or more and 2.6 mm or less, and c t is 0.7 or more and 0.9 or less; In the above formula 1, The method for producing a hot stamped part, wherein λ t is 5 s or more and 20 s or less. [0010] (At this time, λ ｔ is the air cooling time (s), a ｔ is the blank removal temperature from the heating furnace and the atmospheric temperature correction coefficient, T ｔ is the heating temperature (℃) at which the steel plate is heated, b is the ｔ is the material component correction coefficient, t is the material thickness (mm), c ｔ is the high temperature material thickness sensitivity correction factor).

2. 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.

3. The step of heating the blank comprises: A multi-stage heating step for heating the blank 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.

4. In the step of heating the blank, The heating time λ n of the blank is controlled to satisfy the following formula 2 in consideration of the furnace heat loss correction coefficient a n , the heating temperature T n , the Ac3 temperature correction coefficient b n , the material thickness t, and the high temperature material thickness sensitivity coefficient c n : In the above formula 2, a n is -0.60 or more and -0.55 or less, T n is Ac3 or more and 1000° C. or less, b n is 700 or more and 900 or less, t is 1 mm or more and 2.6 mm or less, and c n is 0.7 or more and 0.9 or less; In the above formula 2, The method of claim 3, wherein λ n is ≧100 s and ≦900 s. [0025] (At this time, λ ｎ is the heating time (s), a ｎ is the furnace heat loss correction coefficient, T ｎ is the heating temperature (°C) at which the steel plate is heated, b ｎ is the Ac3 temperature correction coefficient, t is the material thickness (mm), c ｎ is the high temperature material thickness sensitivity coefficient).

5. The method for producing a hot stamped part according to claim 3 , wherein the heating furnace has a plurality of zones having different temperature ranges.

6. 6. The method for manufacturing a hot stamped part according to claim 5, wherein a ratio of a length of a section in which the blank is multi-stage heated to a length of a section in which the blank is soaked heated in the plurality of sections satisfies 1:1 to 4:

1.

7. After the step of transporting the blank, pressing the transferred blank with the die to form a compact; The method of claim 1 , further comprising the step of: cooling the formed body.

8. In the step of molding the molded body, The method for producing a hot stamped part according to claim 7, wherein the blank has a forming start temperature of 500°C or more and 700°C or less.

9. The step of cooling the molded body includes: The method of claim 7, wherein the method is performed in the mold.

10. In the compact cooling step, The mold cooling time λ q during which the molded body is cooled in the mold is controlled to satisfy the following Equation 3 in consideration of the mold heat conduction correction coefficient a q , the pressurizing force P, the workpiece hardening ability correction coefficient b q , the workpiece thickness t, and the low-temperature workpiece thickness sensitivity coefficient c q : In the above formula 3, a q is −1.0 or more and −0.2 or less, P is 0.1 MPa or more and 5 MPa or less, b q is 11 or more and 15 or less, t is 1 mm or more and 2.6 mm or less, and c q is 1.00 or more and 1.05 or less, In the above formula 3, The method for producing a hot stamped part according to claim 9, wherein λ q is ≧6s and ≦40s. [0030] (At this time, λ ｑ is the mold cooling time (s), a ｑ is the die thermal conductivity correction coefficient, pressure P is the minimum pressure (MPa) applied by the die to the blank when the compact is cooled in the die, b ｑ is the material hardening correction coefficient, t is the material thickness (mm), c ｑ is the low temperature material thickness sensitivity coefficient).

11. In the compact cooling step, 8. The method for producing a hot stamped part according to claim 7, wherein the die cooling end temperature at which the cooling step ends is equal to or higher than room temperature and equal to or lower than about 200°C.

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