Method for manufacturing hot stamping parts

Abstract

To provide a method for manufacturing a hot-stamping component.SOLUTION: A method for manufacturing a hot-stamping component includes: a step of heating a steel plate formed with a plated layer at a first temperature for a first time; cutting the heated steel plate and forming a blank; a multi-stage heating step of gradually heating the blank; a uniform heating step of heating the multi-stage heated blank at temperatures of Ac3 to 1,000°C; and a transfer step of transferring the uniformly heated blank to a press mold from the heating furnace.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing hot stamping parts.

Background Art

[0002] Worldwide, environmental regulations and fuel consumption regulations are being strengthened, and the need for lighter vehicle materials is increasing. As a result, research and development on ultra-high-strength steel and hot stamping steel have become active. The hot stamping process generally consists of heating / formning / cooling / trimming, and during the process, the phase transformation of the material and the change of the microstructure are utilized.

[0003] Recently, research has been actively carried out to improve the delayed fracture, corrosion resistance, and weldability of hot stamping parts produced by the hot stamping process. Related technologies include Korean Patent Publication No. 10-2018-0095757 (Title of the Invention: Method for Manufacturing Hot Stamping Parts).

Prior Art Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problems to be solved by the present invention are to provide a blank for hot stamping, a method for manufacturing the same, a hot stamping part, and a method for manufacturing the same, which can prevent or minimize the problems of hydrogen delayed fracture and die sticking during the hot stamping process.

Means for Solving the Problems

[0006] One embodiment of the present invention includes the steps of heating a steel sheet with a plating layer at a first temperature for a first hour; cutting the heated steel sheet to form a blank; a multi-stage heating step of heating the blank step by step; a soaking heating step of heating the multi-stage heated blank at a temperature of Ac3 to 1,000 °C; and a step of transferring the soaked heated blank from the heating furnace to a press die, wherein the first temperature is 540 °C to 600 °C, and the air cooling time of the blank in the transfer step satisfies the following formula, and a method for manufacturing hot stamping parts is provided.

Equation

[0007] In this embodiment, when the first temperature is 600 °C, the first hour is also 10 minutes or more.

[0008] In this embodiment, in the above formula, the 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 also 0.7 or more and 0.9 or less.

[0009] In this embodiment, in the above formula, λ t is also 5 s or more and 20 s or less.

[0010] In this embodiment, the multi-stage heating step and the soaking heating step can be performed in a heating furnace having a plurality of sections with different temperature ranges from each other.

[0011] In this embodiment, in the plurality of sections, the ratio of the length of the section for multi-stage heating the blank to the length of the section for soaking heating the blank satisfies 1:1 to 4:1.

[0012] In this embodiment, the temperatures of the plurality of sections can increase in the direction from the inlet of the heating furnace to the outlet of the heating furnace.

[0013] In this embodiment, among the sections for multi-stage heating the blank, the temperature difference between two adjacent sections is greater than 0 °C and not more than 100 °C.

[0014] In this embodiment, among the plurality of sections, the temperature of the section for soaking heating the blank is higher than the temperature of the section for multi-stage heating the blank.

[0015] In this embodiment, after the transfer step, the step of hot stamping the transferred blank to form a molded body; and the step of cooling the formed molded body; are further included.

Advantages of the Invention

[0016] According to an embodiment of the present invention, by heating a steel sheet having an aluminum-silicon (Al-Si) plating layer at a first temperature for a first time before the hot stamping process, an overall alloying layer provided as a single layer on the steel sheet can be formed.

[0017] Also, by multi-stage heating and soaking heating a blank having an alloying layer formed on a steel sheet to manufacture a hot stamping part, the hydrogen embrittlement and peel resistance of the manufactured part can be improved.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0019] The present invention can be subjected to various transformations and can have various embodiments. Specific embodiments are illustrated in the drawings and will be described in detail by detailed description. The effects and features of the present invention, and the methods for achieving them, will become clear by referring to the embodiments described in detail hereinafter together with the drawings. However, the present invention is not limited to the embodiments disclosed below and can also be embodied in various forms.

[0020] In the following embodiments, terms such as first and second are not used in a limiting sense and are used for the purpose of distinguishing one component from another.

[0021] In the following embodiments, singular expressions include plural expressions unless the context clearly dictates otherwise.

[0022] In the following embodiments, terms such as "comprising" or "having" mean that the features or components described in the specification exist, and do not preclude the possibility of adding one or more other features or components in advance.

[0023] In the following embodiments, when a part such as a film, region, component, etc. is "on" or "above" another part, it includes not only the case where it is directly above the other part, but also the case where other films, regions, components, etc. are interposed in between.

[0024] In the drawings, for convenience of explanation, the sizes of the components may be exaggerated or reduced. For example, the sizes and thicknesses of each configuration shown in the drawings are arbitrarily shown for convenience of explanation, and the present invention is not necessarily limited to the places shown in the drawings.

[0025] When a certain embodiment can be embodied differently, a specific process order can be carried out differently from the described order. For example, two processes described continuously may be carried out substantially simultaneously, or may be carried out in the reverse order to the described order.

[0026] 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 are given the same reference numerals.

[0027] FIG. 1 is a cross-sectional view showing a cross-section of a blank for hot stamping according to an embodiment.

[0028] Referring to FIG. 1, a blank for hot stamping according to an embodiment includes a steel sheet 100 and a first alloying layer 200 disposed on the steel sheet 100.

[0029] The steel sheet 100 is also a steel sheet manufactured by performing a hot rolling process and / or a cold rolling process on a steel slab cast to contain a predetermined alloy element in a predetermined content. As an example, the steel sheet 100 can contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), titanium (Ti), boron (B), the balance iron (Fe), and other inevitable impurities. Further, the steel sheet 100 can further contain one or more components among niobium (Nb), molybdenum (Mo), and aluminum (Al).

[0030] Carbon (C) is a main element that determines the strength and hardness of the steel sheet 100, and is added for the purpose of ensuring the tensile strength and hardenability characteristics of the steel sheet 100 after the hot stamping process. As an example, carbon can be contained in an amount of 0.19 wt% to 0.38 wt% based on the total weight of the steel sheet 100. When the content of carbon is less than 0.19 wt%, it is difficult to ensure the mechanical strength of the steel sheet 100. On the other hand, if the content of carbon exceeds 0.38 wt%, the toughness of the steel sheet 100 may decrease or a brittle control problem may be caused.

[0031] Silicon (Si) can improve the strength and softness of the steel sheet 100 as a solid solution strengthening element. Also, silicon can play a role in suppressing the formation of cementite that becomes the origin of cracks due to hydrogen embrittlement. Such silicon can be contained in an amount of 0.1 wt% to 1 wt% based on the total weight of the steel sheet 100. When the content of silicon is less than 0.1 wt%, it is difficult to obtain the above-described effects. On the contrary, if the content of silicon exceeds 1 wt%, the plating characteristics of the steel sheet 100 may deteriorate.

[0032] Manganese (Mn) is added for the purpose of hardenability and strength increase during heat treatment. Manganese can be contained in an amount of 1 wt% to 2 wt% based on the total weight of the steel sheet 100. When the content of manganese is less than 1 wt%, the effect of grain refinement is not sufficient, and the hard phase fraction of the hot stamping parts may be insufficient. On the other hand, if the content of manganese exceeds 2 wt%, the softness and toughness due to manganese segregation or pearlite banding decrease, which causes a decrease in bending performance and may generate a non-uniform fine structure.

[0033] Phosphorus (P) is added to prevent a decrease in the toughness of the steel sheet 100. Phosphorus can be contained in an amount exceeding 0 and 0.03 wt% or less based on the total weight of the steel sheet 100. If the content of phosphorus exceeds 0.03 wt%, an iron phosphide compound is formed and the toughness decreases, and cracks may be induced in the steel sheet 100 during the manufacturing process.

[0034] Sulfur (S) can be contained in an amount exceeding 0 and 0.01 wt% or less based on the total weight of the steel sheet 100. If the content of sulfur exceeds 0.01 wt%, the hot workability decreases, and surface defects such as cracks may occur due to the formation of large inclusions.

[0035] Chromium (Cr) is added for the purpose of improving the hardenability and strength of the steel sheet 100. Chromium can be contained in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the steel sheet 100. When the content of chromium is less than 0.1 wt%, the effects of hardenability and strength improvement are also insufficient. On the other hand, if the content of chromium exceeds 0.6 wt%, the manufacturing cost increases and the toughness of the steel sheet 100 may decrease.

[0036] Titanium (Ti) is added for the purpose of strengthening hardenability by precipitate formation after hot stamping heat treatment and improving material properties. Also, titanium can effectively contribute to refining austenite crystal grains by forming precipitate phases such as Ti(C,N) at high temperatures. Titanium may be contained in an amount of 0.01 wt% to 0.05 wt% based on the total weight of the steel sheet 100. When the content of titanium is less than 0.01 wt%, the formation of precipitates is negligible and the effect of grain refinement is insufficient. On the other hand, when titanium exceeds 0.05 wt%, a decrease in elongation and a decrease in toughness may occur.

[0037] Boron (B) is added for the purpose of ensuring the hardenability and strength of the steel sheet 100 by ensuring a martensite structure, and has an effect of grain refinement by increasing the austenite crystal grain growth temperature. Boron may be contained in an amount of 0.001 wt% to 0.005 wt% based on the total weight of the steel sheet 100. When the content of boron is less than 0.001 wt%, the effect of improving hardenability is insufficient. On the other hand, when the content of boron exceeds 0.005 wt%, the risk of brittleness and the risk of inferior elongation may increase.

[0038] As an example, the first alloying layer 200 may be formed on at least one surface of the steel sheet 100 and provided as a single layer. For example, the first alloying layer 200 may be provided by an FeAlSi alloy.

[0039] As described later, by heating the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer formed on the steel sheet 100 at a first temperature for a first time, the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer can be alloyed to form the first alloying layer 200. At this time, no aluminum-silicon (Al-Si) plating layer remains on the steel sheet 100, and at least a part of the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer can be alloyed as a whole.

[0040] FIG. 2 is a flowchart schematically showing a method for manufacturing a blank for hot stamping according to an embodiment, and FIG. 3 is a flowchart schematically showing a process for manufacturing the steel sheet of FIG. 2. Hereinafter, a method for manufacturing a blank for hot stamping will be described with reference to FIGS. 2 and 3.

[0041] Referring to FIG. 2, a method for manufacturing a blank for hot stamping according to an embodiment includes a plating layer forming step (S110) and a heating step (S120).

[0042] As shown in FIG. 3, the plating layer forming step (S110) includes a hot rolling step (S210), a cooling / winding step (S220), a cold rolling step (S230), an annealing heat treatment step (S240), and a molten plating step (S250) of a steel slab. First, a semi-finished steel slab that is the object of the process of forming a steel sheet is prepared. At this time, the steel slab contains carbon (C): 0.19 wt% to 0.38 wt%, silicon (Si): 0.1 wt% to 1 wt%, manganese (Mn): 1 wt% to 2 wt%, phosphorus (P): more than 0 and 0.03 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, chromium (Cr): 0.1 wt% to 0.6 wt%, titanium (Ti): 0.01 wt% to 0.05 wt%, boron (B): 0.001 wt% to 0.005 wt%, and the balance of iron (Fe) and inevitable impurities.

[0043] The reheating stage of the steel slab is carried out for hot rolling. In the steel slab reheating stage, by reheating the steel slab secured through the continuous casting process to a predetermined temperature, the segregated components during casting will be redissolved. As an example, the slab reheating temperature (SRT) is also 1,200°C to 1,400°C. When the slab reheating temperature (SRT) is lower than 1,200°C, the segregated components during casting cannot be sufficiently redissolved during casting, and it cannot be said that the homogenization effect of alloying elements is significant, nor can it be said that the solution effect of titanium (Ti) is significant. The higher the slab reheating temperature (SRT), the more favorable it is for homogenization. However, when the slab reheating temperature (SRT) exceeds 1,400°C, the austenite crystal grain size increases and it is difficult to ensure strength, and the manufacturing cost of the steel plate may increase due to excessive heating processes.

[0044] In the hot rolling stage (S210), the reheated steel slab is hot rolled at a predetermined finishing rolling temperature. As an example, the finishing delivery temperature (FDT) is also from 880°C to 950°C. At this time, if the finishing delivery temperature (FDT) is lower than 880°C, it is difficult to ensure the workability of the steel plate due to the generation of a mixed grain structure by abnormal area rolling. There are not only problems with the reduction of workability due to the non-uniformity of the fine structure, but also problems with the through-feedability during hot rolling due to sudden phase changes. When the finishing delivery temperature (FDT) exceeds 950°C, the austenite crystal grains may coarsen. In addition, the TiC precipitates may coarsen and the performance of hot stamping parts may deteriorate.

[0045] In the cooling / coiling stage (S220), the hot-rolled steel plate is cooled to a predetermined coiling temperature (CT) and then coiled. As an example, the coiling temperature is also 550°C to 800°C. The coiling temperature affects the redistribution of carbon (C). When the coiling temperature is less than 550°C, the low-temperature phase fraction due to supercooling increases, resulting in increased strength. During cold rolling, the rolling load may become intense, and the softness may suddenly decrease. Conversely, when the coiling temperature exceeds 800°C, formability and strength deterioration may occur due to abnormal grain growth or excessive grain growth.

[0046] In the cold rolling stage (S230), the coiled steel plate is uncoiled, pickled, and then cold rolled. At this time, pickling is carried out for the purpose of removing the scale of the coiled steel plate, that is, the hot-rolled coil manufactured through the hot rolling process.

[0047] The annealing heat treatment stage (S240) is a stage in which the cold-rolled steel plate is annealed heat-treated at a temperature of 700°C or higher. As an example, the annealing heat treatment may include heating the cold-rolled sheet and cooling the heated cold-rolled sheet at a predetermined cooling rate.

[0048] The molten plating stage (S250) is a stage in which a plating layer is formed on the steel plate annealed heat-treated. As an example, in the molten plating stage (S250), an aluminum-silicon (Al-Si) plating layer can be formed on the annealed heat-treated steel plate, that is, on the steel plate.

[0049] Specifically, in the molten plating stage (S250), the steel plate can be immersed in a molten plating bath containing 8 wt% to 12 wt% silicon (Si) and excess aluminum (Al). At this time, the molten plating bath can maintain a temperature of 400°C to 700°C. The plating layer can be formed by plating at 40 g / m 2 ~200 g / m 2 on both sides of the steel plate.

[0050] The heating step (S120) is a step of heating a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed. More specifically, the heating step (S120) is a step of heating a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed to form a first alloying layer.

[0051] In the heating step (S120), the heating time and heating temperature for heating the steel sheet on which the aluminum-silicon (Al-Si) plating layer is formed satisfy the following formula 1.

Equation

[0052] In the method for manufacturing a blank for hot stamping according to an embodiment, the plating amount of the aluminum-silicon (Al-Si) plating layer formed on the steel sheet is 40 g / m 2 ~200 g / m 2 Therefore, the correction coefficient α based on the plating amount can have a value of -31.09 to -10.36.

[0053] Also, β can have a value of 84752.2 J / mol to 254256.5 J / mol in consideration of the silicon (Si) content contained in the aluminum-silicon (Al-Si) plating layer and the activation energy of silicon (Si).

[0054] As an example, in the heating step (S120), a steel sheet with an aluminum-silicon (Al-Si) plating layer can be heated at a heating temperature T. For example, in the heating step (S120), a steel sheet with an aluminum-silicon (Al-Si) plating layer can be heated at a first temperature. At this time, the first temperature is also 540°C to 600°C. If the first temperature is less than 540°C, the time required for alloying at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer increases, and process losses may occur. On the other hand, if the first temperature exceeds 600°C, during the manufacturing process of the hot stamping parts described later, a liquid state may be formed on the surface of the blank, and die welding problems may occur.

[0055] As an example, in the heating step (S120), a steel sheet with an aluminum-silicon (Al-Si) plating layer is heated during a heating time (ta). For example, in the heating step (S120), a steel sheet with an aluminum-silicon (Al-Si) plating layer can be heated for a first time. At this time, the first time is also 10 minutes to 120 minutes. Specifically, when the first temperature is 540°C, the first time is also 60 minutes or more and less than 120 minutes, and when the first temperature is 600°C, the first time is also 10 minutes or more and less than 60 minutes. If the first time is less than 10 minutes, the steel sheet and the aluminum-silicon (Al-Si) plating layer are not completely alloyed. On the other hand, if the first time exceeds 120 minutes, the productivity of the hot stamping parts may decrease due to excessive heating time.

[0056] In the heating step (S120), when the first temperature is 540°C, the minimum time required for complete alloying of the aluminum-silicon (Al-Si) plating layer is also about 60 minutes. That is, in the heating step (S120), the steel sheet with the aluminum-silicon (Al-Si) plating layer can be heated at 540°C for about 60 minutes or more before the complete alloying layer can be formed. Therefore, when the first temperature is higher than 540°C, the minimum time required to form the complete alloying layer becomes less than 60 minutes, and the minimum time is also 10 minutes to 60 minutes.

[0057] Also, in the heating step (S120), when the first temperature is 600°C, the minimum time required for the overall alloying of the aluminum-silicon (Al-Si) plating layer is also about 10 minutes. That is, in the heating step (S120), the steel sheet with the aluminum-silicon (Al-Si) plating layer formed thereon can be heated at 600°C for about 10 minutes or more before the overall alloying layer can be formed. Therefore, when the first temperature is lower than 600°C, the minimum time for forming the overall alloying layer becomes greater than 10 minutes, and the minimum time is also in the range of 10 minutes to 60 minutes.

[0058] FIG. 4 is a graph showing the section where the steel sheet and the plating layer are alloyed in the method for manufacturing a blank for hot stamping according to an embodiment. More specifically, FIG. 4 is a graph showing the heating time at a heating temperature that satisfies Equation 1, and is a drawing showing the section where the plating layer is alloyed.

[0059] As described above, in the heating step (S120), by heating the steel sheet with the aluminum-silicon (Al-Si) plating layer formed thereon at the first temperature for the first time, at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer can be alloyed to form an overall alloying layer.

[0060] Therefore, when corresponding to the A section in FIG. 4, at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer can be interdiffused to form a first alloying layer. At this time, the formed first alloying layer can be provided as a single layer. For example, the first alloying layer can be provided by an FeAlSi alloy. As an example, on the steel sheet, no aluminum-silicon (Al-Si) plating layer remains, and at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer can be overall alloyed.

[0061] Thereafter, the blank forming step can be performed. The blank forming step is also a step of cutting the steel sheet with the first alloying layer formed thereon to form a blank. In the blank forming step, the steel sheet with the first alloying layer formed thereon is cut into a desired shape according to the purpose to form a blank.

[0062] As an example, the blank forming step can be carried out before the heating step (S120). For example, after cutting a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed to form a blank, a heating step (S120) of heating the formed blank can be carried out.

[0063] FIG. 5 is a cross-sectional view showing a cross-section of a hot stamping blank manufactured using a method for manufacturing a hot stamping blank in which the heating step (S120) is omitted.

[0064] Referring to FIGS. 1 and 5, a hot stamping blank manufactured using a method for manufacturing a hot stamping blank including the heating step (S120) can include a steel sheet 100 and a first alloying layer 200 disposed on the steel sheet 100. The first alloying layer 200 is also one in which the steel sheet 100 and an aluminum-silicon (Al-Si) plating layer formed on the steel sheet 100 are alloyed. Specifically, by heating the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer formed on the steel sheet 100 at a first temperature for a first time, the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer can be alloyed to form the first alloying layer 200. At this time, no aluminum-silicon (Al-Si) plating layer remains on the steel sheet 100, and at least a part of the steel sheet 100 and the aluminum-silicon (Al-Si) plating layer can be entirely alloyed.

[0065] A hot stamping blank manufactured using a method for manufacturing a hot stamping blank in which the heating step (S120) is omitted can include a steel sheet 100, a first alloying layer 200' disposed on the steel sheet, and an aluminum-silicon (Al-Si) plating layer 250' disposed on the first alloying layer 200'.

[0066] Through this, in the hot stamping blank in which the heating step (S120) has been performed, all the aluminum-silicon (Al-Si) plating layers formed on the steel sheet 100 are alloyed as the first alloying layer 200. However, in the hot stamping blank in which the heating step (S120) is omitted, it can be confirmed that a part of the aluminum-silicon (Al-Si) plating layer remains on the steel sheet 100.

[0067] Therefore, since all the aluminum-silicon (Al-Si) plating layers formed on the hot stamping blank in which the heating step (S120) has been performed are alloyed as the first alloying layer 200, it is possible to prevent or minimize hydrogen inflow due to the formation of aluminum-silicon (Al-Si) liquid on the surface and die sticking problems during the austenitizing process through high-temperature heat treatment.

[0068] FIG. 6 is a cross-sectional view showing a cross-section of a hot stamping part according to an embodiment.

[0069] Referring to FIG. 6, a hot stamping part according to an embodiment may include a steel sheet 100 and a second alloying layer 300 including a first layer 310, a second layer 320, and a third layer 330 that are sequentially stacked on the steel sheet 100.

[0070] The second alloying layer 300 is formed on at least one surface of the steel sheet 100 and contains aluminum (Al). The second alloying layer 300 includes a first layer 310, a second layer 320, and a third layer 330 that are sequentially stacked on the steel sheet 100. The third layer 330 contains an FeAl phase.

[0071] The first layer 310 contains alloyed iron (Fe), aluminum (Al), and silicon (Si). As an example, the first layer 310 may have an α-Fe phase.

[0072] The second layer 320 contains a Fe2Al5 phase. As an example, the area fraction of the second layer 320 with respect to the alloying layer 300 is also more than 0% and 33% or less. When the area fraction of the second layer 320 with respect to the alloying layer 300 exceeds 33%, the peel resistance and hydrogen embrittlement of the hot stamping part manufactured by the method for manufacturing a hot stamping part may decrease.

[0073] On the other hand, the first layer 310 has a hardness of 200 Hv to 800 Hv, the second layer 320 has a hardness of 700 Hv to 1,200 Hv, and the third layer 330 may have a hardness of 200 Hv to 800 Hv.

[0074] As an example, the hot stamping part may further include a surface layer disposed on the second alloying layer 300. The surface layer is a layer containing 80 wt% or more of aluminum (Al), and can prevent the steel sheet 100 from being oxidized. As an example, the average thickness of the surface layer disposed on the steel sheet is also 100 nm to 200 nm.

[0075] FIG. 7 is a flowchart schematically showing a method for manufacturing a hot stamping part according to an embodiment.

[0076] Referring to FIG. 7, a method for manufacturing a hot stamping part according to an embodiment includes a heating step (S310), a blank forming step (S320), a multi-stage heating step (S330), a soaking heating step (S340), a transfer step (S350), a forming step (S360), and a cooling step (S370).

[0077] The heating step (S310) is a step of heating a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed. More specifically, the heating step (S310) is a step of heating a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed to form a first alloying layer. At this time, the steel sheet on which the aluminum-silicon (Al-Si) plating layer is formed can be manufactured through the hot rolling step (S210), the cooling / winding step (S220), the cold rolling step (S230), the annealing heat treatment step (S240), and the electroplating step (S250) of the steel slab shown in FIG. 3.

[0078] In the heating stage (S310), the heating time and heating temperature for heating a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed satisfy the following Mathematical Formula 1.

Equation

[0079] In the method for manufacturing a hot stamping part according to an embodiment, the plating amount of the aluminum-silicon (Al-Si) plating layer formed on the steel sheet is 40 g / m 2 ~200 g / m 2 Therefore, the correction coefficient α based on the plating amount can have a value of -31.09 to -10.36.

[0080] Also, β can have a value of 84752.2 J / mol to 254256.5 J / mol in consideration of the silicon (Si) content contained in the aluminum-silicon (Al-Si) plating layer and the activation energy of silicon (Si).

[0081] As an example, in the heating stage (S310), the steel sheet on which the aluminum-silicon (Al-Si) plating layer is formed can be heated at the heating temperature T. For example, in the heating stage (S120), the steel sheet on which the aluminum-silicon (Al-Si) plating layer is formed can be heated at the first temperature. At this time, the first temperature is also 540°C to 600°C. When the first temperature is less than 540°C, the time required for alloying at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer increases, and losses may occur in the process. On the other hand, when the first temperature exceeds 600°C, liquid may be formed on the surface of the blank during the manufacturing process of the hot stamping part, and die sticking problems may occur.

[0082] As an example, in the heating step (S310), a steel sheet with an aluminum-silicon (Al-Si) plating layer can be heated for a heating time ta. For example, in the heating step (S120), a steel sheet with an aluminum-silicon (Al-Si) plating layer can be heated for the first hour. At this time, the first hour is also 10 minutes to 120 minutes. Specifically, when the first temperature is 540°C, the first hour is also 60 minutes or more and less than 120 minutes, and when the first temperature is 600°C, the first hour is also 10 minutes or more and less than 60 minutes. If the first hour is less than 10 minutes, complete alloying of the steel sheet and the aluminum-silicon (Al-Si) plating layer is difficult. On the other hand, if the first hour exceeds 120 minutes, the productivity of hot stamping parts may decrease due to excessive heating time.

[0083] In the heating step (S310), when the first temperature is 540°C, the minimum time required for complete alloying of the aluminum-silicon (Al-Si) plating layer is also about 60 minutes. That is, in the heating step (S310), only when the steel sheet with the aluminum-silicon (Al-Si) plating layer is heated at 540°C for about 60 minutes or more can a complete alloying layer be formed. Therefore, when the first temperature is higher than 540°C, the minimum time required to form a complete alloying layer becomes shorter than 60 minutes, and the minimum time is also 10 minutes to 60 minutes.

[0084] Also, in the heating step (S310), when the first temperature is 600°C, the minimum time required for complete alloying of the aluminum-silicon (Al-Si) plating layer is also about 10 minutes. That is, in the heating step (S120), only when the steel sheet with the aluminum-silicon (Al-Si) plating layer is heated at 600°C for about 10 minutes or more can a complete alloying layer be formed. Therefore, when the first temperature is lower than 600°C, the minimum time required to form a complete alloying layer becomes longer than 10 minutes, and the minimum time is also 10 minutes to 60 minutes.

[0085] The blank forming step (S320) is also a step of cutting the steel sheet on which the first alloying layer is formed to form a blank. In the blank forming step (S320), the steel sheet on which the first alloying layer is formed can be cut into a desired shape according to the purpose to form a blank.

[0086] As an example, the blank forming step (S320) may be performed before the heating step (S310). For example, after cutting a steel sheet on which an aluminum-silicon (Al-Si) plating layer is formed to form a blank, a heating step (S120) of heating the formed blank can be performed.

[0087] The multi-stage heating step (S330) is also a step of heating the blank step by step, and the soaking heating step (S340) is also a step of heating the blank heated in multiple stages at a uniform temperature. In the multi-stage heating step (S330), the blank can be heated step by step by passing 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 (S330) is performed, and the temperature is set for each section so as to increase in the direction from the inlet of the heating furnace where the blank is introduced to the outlet of the heating furnace where the blank is taken out, and the blank can be heated step by step. The soaking heating step (S340) can be performed after the multi-stage heating step (S330). In the soaking heating step (S340), the blank heated in multiple stages can be heat-treated by passing through a section of the heating furnace set at a temperature of Ac3 to 1,000 °C. Desirably, in the soaking heating step (S340), the blank heated in multiple stages can be soaked and heated at a temperature of 930 °C to 1,000 °C. More desirably, in the soaking heating step (S340), the blank heated in multiple stages can be soaked and heated at a temperature of 950 °C to 1,000 °C. Also, among the plurality of sections provided in the heating furnace, the section in which the soaking heating step (S340) is performed is at least one or more.

[0088] FIG. 8 is a drawing showing a heating furnace having a plurality of sections in the multi-stage heating step and the soaking heating step of the method for manufacturing a hot stamping part according to an embodiment for explanation.

[0089] Referring to FIG. 8, a heating furnace according to an embodiment can include a plurality of sections having different temperature ranges. More specifically, the heating furnace can include a first section P1 having a first temperature range T1, a second section P2 having a second temperature range T2, a third section P3 having a third temperature range T3, a fourth section P4 having a fourth temperature range T4, a fifth section P5 having a fifth temperature range T5, a sixth section P6 having a sixth temperature range T6, and a seventh section P7 having a seventh temperature range T7.

[0090] As an example, in the multi-stage heating step (S330), the blank can be gradually heated in multiple stages by passing through a plurality of sections (for example, the first section P1 to the fourth section P4) defined in the heating furnace. Also, in the soaking heating step (S340), the blank multi-stage heated in the first section P1 to the fourth section P4 can be soaked heated in the fifth section P5 to the seventh section P7.

[0091] The first section P1 to the seventh section P7 can be arranged in order in the heating furnace. The first section P1 having the first temperature range T1 can be adjacent to the inlet of the heating furnace where the blank is introduced, and the seventh section P7 having the seventh temperature range T7 can be adjacent to the outlet of the heating furnace where the blank is discharged. Thus, 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 plurality of 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 rather than sections where multi-stage heating is performed.

[0092] The temperatures of the plurality of sections provided in the heating furnace, for example, the temperatures of the first section P1 to the seventh section P7, increase in the direction from the inlet of the heating furnace where the blank is introduced to the outlet of the heating furnace where the blank is taken out. However, the temperatures of the fifth section P5, the sixth section P6, and the seventh section P7 are also the same. Also, among the plurality of sections provided in the heating furnace, the temperature difference between two adjacent sections is greater than 0°C and not more than 100°C. For example, the temperature difference between the first section P1 and the second section P2 is greater than 0°C and not more than 100°C.

[0093] As an example, the first temperature range T1 of the first section P1 is also 820°C to 860°C and also 835°C to 865°C. The second temperature range T2 of the second section P2 is also 850°C to 890°C and also 865°C to 895°C. The third temperature range T3 of the third section P3 is also 880°C to 920°C and also 895°C to 925°C. The fourth temperature range T4 of the fourth section P4 is also 910°C to 940°C and also 915°C to 945°C. The fifth temperature range T5 of the fifth section P5 is also Ac3 to 1,000°C. Desirably, the fifth temperature range T5 of the fifth section P5 is also 930°C or higher and 1,000°C or lower. More desirably, the fifth temperature range T5 of the fifth section P5 is also 950°C or higher and 1,000°C or lower. 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.

[0094] In FIG. 8, a heating furnace according to an embodiment is illustrated as having seven sections with different temperature ranges, but the present invention is not limited thereto. In the heating furnace, five, six, eight, or the like sections with different temperature ranges may be provided.

[0095] The soaking heating stage (S340) can be performed in the last part of a plurality of sections of the heating furnace. As an example, the soaking heating stage can be performed in the fifth section P5, the sixth section P6, and the seventh section P7 of the heating furnace. When a plurality of sections are provided in the heating furnace, if the length of one section is long, there may be problems such as temperature changes occurring within the section. Therefore, the sections in which the soaking heating stage is performed are divided into the fifth section P5, the sixth section P6, and the seventh section P7, but the fifth section P5, the sixth section P6, and the seventh section P7 may have the same temperature range in the heating furnace.

[0096] In the soaking heating stage (S340), the multi-stage heated blank can be soaked and heated at a temperature of Ac3 to 1,000 °C. Desirably, in the soaking heating stage (S340), the multi-stage heated blank can be soaked and heated at a temperature of 930 °C to 1,000 °C. More desirably, in the soaking heating stage (S340), the multi-stage heated blank can be soaked and heated at a temperature of 950 °C to 1,000 °C.

[0097] As an example, the ratio of the length D1 of the section where the blank is multi-stage heated to the length D2 of the section where the blank is soaked and heated is also 1:1 to 4:1. More specifically, the ratio of the sum of the lengths of the first section P1 to the fourth section P4, which is the section where the blank is multi-stage heated, to the sum of the lengths of the fifth section P5 to the seventh section P7, which is the section where the blank is soaked and heated, can satisfy 1:1 to 4:1. When the length of the section where the blank is soaked and heated increases and the ratio of the length D1 of the section where the blank is multi-stage heated to the length D2 of the section where the blank is soaked and heated exceeds 1:1, an austenite (FCC) structure is generated in the soaking heating section, the amount of hydrogen penetration into the blank increases, and delayed fracture may increase. Also, when the length of the section where the blank is soaked and heated decreases and the ratio of the length D1 of the section where the blank is multi-stage heated to the length D2 of the section where the blank is soaked and heated is less than 4:1, the soaking heating section (time) is not sufficiently ensured, and the strength of the parts manufactured by the manufacturing process of the hot stamping parts becomes non-uniform.

[0098] As an example, among the plurality of sections provided in the heating furnace, the length of the section where the soaking heating stage (S340) is performed is also 20% to 50% of the total length of the heating furnace. Also, in the multi-stage heating stage (S330) and the soaking heating stage (S340), at least two blanks having different thicknesses can be simultaneously transferred in the heating furnace.

[0099] As an example, the blank stays in the heating furnace for 180 to 360 seconds. That is, the time during which the blank is multi-stage heated and soaking heated is also 180 to 360 seconds. If the residence time of the blank in the heating furnace is less than 180 seconds, it is difficult to be sufficiently soaked at the desired soaking temperature. Also, if the residence time of the blank in the heating furnace exceeds 360 seconds, the amount of hydrogen penetrating into the blank increases, the risk of delayed fracture becomes high, and the corrosion resistance after hot stamping may decrease.

[0100] In one embodiment, the transfer step (S350) is also the step of transferring the heated blank from the heating furnace to the press die. At this time, in the transfer step (S350), the heated blank can be cooled at the atmospheric temperature (or normal temperature) while being transferred to the press die. The heated blank can be air-cooled during transfer. If the heated blank is not air-cooled, the die entry temperature (for example, the forming start temperature) will be high, and wrinkles (or buckling) may occur on the surface of the manufactured hot stamping part. Also, when using a refrigerant, it may affect the subsequent process (hot stamping), so it is desirable that the heated blank be air-cooled during transfer.

[0101] The forming step (S360) is the step of hot stamping the transferred blank to form a formed body. The cooling step (S370) is the step of cooling the formed formed body.

[0102] In a press mold, it is possible to form the final part shape and cool the formed body to form the final product. The press mold can be provided with a cooling channel through which a refrigerant circulates inside. By the circulation of the refrigerant supplied through the cooling channel provided in the press mold, the heated blank can be rapidly cooled. At this time, in order to prevent the spring back phenomenon of the plate material and maintain the desired shape, rapid cooling can be carried out while pressurizing the press mold in the closed state. When performing the forming and cooling operations on the heated blank, it is possible to cool the average cooling rate to at least 10 °C / s or more until the martensite finish temperature. The blank can be held in the press mold for 3 seconds to 20 seconds. If the holding time in the press mold is less than 3 seconds, the material cannot be sufficiently cooled, and the temperature deviation by part due to the residual heat can affect the embrittlement quality. Also, a sufficient amount of martensite cannot be generated, and the mechanical properties cannot be ensured. On the other hand, if the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes long and the productivity may decrease.

[0103] FIG. 9 is a drawing showing the air-cooling time according to the material thickness and the air-cooling time according to the heating temperature. Specifically, FIG. 9 is a graph shown to explain the maximum allowable air-cooling time according to the material thickness and the maximum allowable air-cooling time according to the heating temperature. In FIG. 9, it can be understood that a high heating temperature means a high heating furnace extraction temperature.

[0104] Referring to FIGS. 7 and 9, it can be confirmed that the maximum allowable air-cooling time increases as the heating temperature decreases at the same material thickness. Also, it can be confirmed that the maximum allowable air-cooling time increases as the material thickness increases at the same heating temperature.

[0105] When the heated blank is excessively exposed to room temperature, not only does productivity decrease, but a phase transformation occurs in the blank during air cooling, resulting in a decrease in formability and difficulty in ensuring the desired material properties. On the other hand, when the room temperature exposure time of the heated blank is short, wrinkles (or buckling) may occur in the hot stamping parts manufactured with forming starting at an excessively high temperature. Also, the plating layer of the blank may adhere to the mold. Therefore, it is necessary to adjust the air cooling time in the transfer stage (S350). However, in order to adjust the air cooling time in the transfer stage (S350), not only the heating temperature and the thickness of the blank (e.g., the thickness of the material), but also various variables such as the composition of the blank, the thickness of the blank, the plating amount and the thermal conductivity due to the surface emissivity, the thermal conductivity and the heat transfer amount, and the heating furnace extraction temperature of the blank and the atmospheric temperature must be considered.

[0106] Therefore, the inventor of the present invention derived Equation 2 that can easily control the air cooling time through excessive repeated experiments. In one embodiment, the air cooling time of the blank in the transfer stage (S350) can satisfy the following Equation 2.

Equation

[0107] a t is the correction coefficient considering the heating furnace extraction temperature and the atmospheric temperature of the heated blank, and can have a value of about 0.0160 or more and about 0.0165 or less. At this time, a t can have the unit of s / (°C xmm).

[0108] b tconsiders the case where the components of each material are different from each other, and b t is a correction coefficient considering the material components. At this time, b t can have a value of about 10.0 or more and about 0.5 or less. At this time, b t can have the unit of s / mm.

[0109] Also, the amount of heat transfer transmitted from inside the material varies depending on the thickness of the material. c t is a correction coefficient considering the difference in the amount of heat transfer due to the thickness of the material at high temperature, and can have a value of about 0.7 or more and about 0.9 or less. At this time, high temperature can mean 600 °C or more. However, high temperature means 500 °C or more or 700 °C or more.

[0110] Heating temperature T t means the soaking heating temperature in the soaking heating stage (S340), and the heating temperature T t can have a value of about Ac3 or more and about 1000 °C or less. At this time, the heating temperature T t can mean the heating furnace outlet temperature. Also, the material thickness t can have a value of about 1 mm or more and about 2.6 mm or less.

[0111] In one embodiment, the air cooling time λ according to Equation 2 t is also about 5 s or more and about 20 s or less. When the air cooling time λ t is less than 5 s, the forming start temperature at which the blank forming starts is excessively high, and wrinkles (or buckling) occur in the hot stamping parts manufactured with the blank forming proceeding at a high temperature. In terms of equipment, it is difficult to implement an air cooling time λ of less than 5 s t . On the other hand, when the air cooling time λ t exceeds 20 s, the productivity decreases, and during the process of transporting the blank, a phase transformation occurs in the blank and the formability of the blank decreases, and the manufactured hot stamping parts do not have the desired material. Therefore, when the air cooling time λ t satisfies the range of about 5 s or more and about 20 s or less, the formability of the blank and the productivity of the process can be improved, and the manufactured hot stamping parts can also have the desired material.

[0112] FIG. 10 is a drawing showing a cross section of a component manufactured using a method for manufacturing a hot stamping component in which a heating step (S310) is omitted.

[0113] Referring to FIGS. 6 and 10, a hot stamping component manufactured using a method for manufacturing a hot stamping component including a heating step (S310) includes a second alloying layer 300, and the second alloying layer 300 includes a first layer 310, a second layer 320, and a third layer 330 that are sequentially laminated. At this time, the first layer 310 may include α-Fe, the second layer 320 may include Fe2Al5, and the third layer 330 may include FeAl.

[0114] The hot stamping component manufactured using the manufacturing method in which the heating step (S310) is omitted also includes a second alloying layer 300'. The second alloying layer 300' of the hot stamping component manufactured using the manufacturing method in which the heating step (S310) is omitted includes a first layer 310', a second layer 320', a third layer 330', and a fourth layer 340' that are sequentially laminated. At this time, the first layer 310' includes α-Fe, the second layer 320' includes Fe2Al5, the third layer 330' includes FeAl, and the fourth layer 340' includes Fe2Al5.

[0115] It can be confirmed that the outermost layer of the second alloying layer 300 included in the hot stamping component manufactured using the manufacturing method including the heating step (S310) is a FeAl phase, while the outermost layer of the second alloying layer 300' included in the hot stamping component manufactured using the manufacturing method in which the heating step (S310) is omitted is a Fe2Al5 phase.

[0116] Therefore, when the blank is pre-heated (heating step (S310)) before the multi-stage heating step (S330) and the soaking heating step (S340), it can be confirmed that a Fe2Al5 phase is not formed in the second alloying layer 300 included in the final hot stamping component.

[0117] The Fe2Al5 phase is known to have a higher crack generation frequency and a higher crack propagation rate compared to the FeAl phase. When manufacturing parts through the manufacturing method of hot stamping parts according to an embodiment, since the Fe2Al5 phase existing on the outermost contour of the second alloying layer 300 disappears, the crack generation frequency decreases, crack propagation is suppressed, and the peel resistance of the manufactured parts can be improved.

[0118] When heating a blank with an aluminum-silicon (Al-Si) plating layer formed on a steel sheet at the hot stamping process temperature, there were cases where at least a part of the aluminum-silicon (Al-Si) plating layer could not form an alloying layer and remained, forming a liquid aluminum-silicon (Al-Si) layer. At this time, since the liquid aluminum-silicon (Al-Si) layer has a faster hydrogen diffusion rate and a stronger surface hydrogen adsorption force compared to the alloying layer, hydrogen flows into the steel sheet through the liquid aluminum-silicon (Al-Si) layer, and there was a problem of hydrogen-induced delayed fracture. In addition, there was a problem that the liquid aluminum-silicon (Al-Si) layer adhered to the mold, resulting in a decrease in productivity.

[0119] FIG. 11 is a drawing showing an iron (Fe)-aluminum (Al) phase diagram.

[0120] Referring to FIG. 11, in the iron (Fe)-aluminum (Al) phase diagram, it can be confirmed that the melting point of FeAl3 is about 1160°C and the melting point of Fe2Al5 is about 1169°C. Therefore, when forming an FeAl alloy, it can have a melting point higher than that of aluminum-silicon (Al-Si) (about 660°C).

[0121] As an example, before heating (multi-stage heating and soaking) a steel sheet with an aluminum-silicon (Al-Si) plating layer, by heating the steel sheet with the aluminum-silicon (Al-Si) plating layer at a first temperature for a first time, at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer can be alloyed to form a first alloyed layer. Thereafter, by heating (multi-stage heating and soaking) the steel sheet with the first alloyed layer formed thereon to manufacture hot stamping parts, hydrogen delayed fracture of the manufactured parts can be prevented or minimized, and at the same time, the occurrence of die sticking phenomenon during the process can be prevented or minimized.

[0122] Hereinafter, the present invention will be described in more detail through examples. However, the following examples are for further specifically explaining the present invention, and the scope of the present invention is not limited by the following examples. The following examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

[0123] Examples

Table 1

[0124]

Table 2

[0125]

Table 3

[0126]

Table 4

[0127] Table 1 is a table showing the composition of the steel sheet, Table 2 is a table showing the heating temperature and heating time of Example 1, Example 2, and Comparative Examples 1 to 9, Table 3 is a table showing the length by section of the heating furnace, and Table 4 is a table showing the set temperature of the sectional heating furnace and the residence time in the heating furnace.

[0128] A blank with an aluminum-silicon (Al-Si) plating layer formed on a steel sheet having the composition of Table 1 was heated under the conditions of Table 2, and then the heated blank was multi-stage heated and soaking heated in a heating furnace satisfying the conditions of Table 3 and Table 4 to manufacture hot stamping parts.

[0129] In Table 2, Example 1 and Example 2 correspond to the case where the heating temperature and heating time in the heating stages (S120, S310) are satisfied. Comparative Example 1 corresponds to the case where the heating stages (S120, S310) are not performed. Comparative Examples 2 and 3 correspond to the case where the heating time in the heating stages (S120, S310) is not satisfied. Comparative Examples 4 to 6 correspond to the case where the heating temperature in the heating stages (S120, S310) is not satisfied.

[0130] When the heating temperature and heating time in the heating stages (S120, S310) are satisfied as in Example 1 and Example 2, at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer formed on the steel sheet can be alloyed to form an overall alloyed layer.

[0131] However, when the heating stages (S120, S310) are omitted or the heating temperature and / or heating time in the heating stages (S120, S310) are not satisfied as in Comparative Examples 1 to 6, the aluminum-silicon (Al-Si) plating layer may remain on the steel sheet.

[0132] In addition, Comparative Examples 7 to 9 correspond to cases where the heating time in the heating steps (S120, S310) is exceeded. Even in the cases of Comparative Examples 7 to 9, at least a part of the steel sheet and the aluminum-silicon (Al-Si) plating layer formed on the steel sheet can be alloyed to form an overall alloyed layer. However, in the cases of Comparative Examples 7 to 9, due to the excessive heating time, the productivity of the hot stamping parts will decrease.

[0133] <Diffusible hydrogen amount and hydrogen stress corrosion cracking evaluation> Thermal Desorption Spectroscopy was performed on Example 1, Example 2, and Comparative Examples 1 to 9. More specifically, while raising the temperature from room temperature to 500 °C at a heating rate of 20 °C / min, the amount of diffusible hydrogen released from the hot stamping parts at 350 °C or lower was measured. The hydrogen stress corrosion cracking evaluation was carried out by the 4 point bending test method. The 4 point bending test is a test method in which a stress below the elastic limit is applied to a specific point of a specimen manufactured to reproduce the state of being exposed to a corrosive environment, and the presence or absence of stress corrosion cracks is confirmed. At this time, stress corrosion cracks mean cracks that occur when corrosion and a continuous tensile stress act simultaneously.

Table 5

[0134] Table 5 is a table showing the amount of diffusible hydrogen released from Example 1, Example 2, and Comparative Examples 1 to 9, and the results of the hydrogen stress corrosion cracking evaluation.

[0135] Referring to Table 5, it can be confirmed that the amount of diffusible hydrogen released from Comparative Example 1 is even more than that released from Example 1 and Example 2. Through this, it can be seen that when performing the heating steps (S120, S310) before performing the hot stamping process, the amount of hydrogen inflow from the outside decreases.

[0136] Moreover, it can be confirmed that the amount of diffusible hydrogen released from Comparative Example 2 and Comparative Example 3 (when the heating time is not satisfied) is larger than the amount of diffusible hydrogen released from Example 1 and Example 2.

[0137] Moreover, it can be confirmed that the amount of diffusible hydrogen released from Comparative Example 4, Comparative Example 5, and Comparative Example 6 (when the heating temperature is not satisfied) is larger than the amount of diffusible hydrogen released from Example 1 and Example 2.

[0138] However, it can be confirmed that the amount of diffusible hydrogen released from Comparative Example 7, Comparative Example 8, and Comparative Example 9 (when the heating temperature is satisfied but the heating time exceeds 60 minutes) is larger than the amount of diffusible hydrogen released from Example 1 and Example 2, but smaller than the amount of diffusible hydrogen released from Comparative Example 1 to Comparative Example 6.

[0139] Through this, when performing the heating stage (S120, S310) before performing the hot stamping process, it can be seen that the amount of diffusible hydrogen released from the manufactured hot stamping parts decreases, so the hydrogen inflow amount from the outside decreases.

[0140] However, even when heating a steel sheet formed with aluminum-silicon (Al-Si) at a first temperature for more than 120 minutes before performing the hot stamping process, the amount of diffusible hydrogen released from the manufactured hot stamping parts decreases. However, when the heating time exceeds 120 minutes, the productivity of the hot stamping parts may decrease due to the excessive heating time.

[0141] As a result of the hydrogen delayed fracture evaluation, fracture did not occur in the case of Example 1, Example 2, and Comparative Example 7 to Comparative Example 9, but fracture occurred in the case of Comparative Example 1 to Comparative Example 6.

[0142] Through this, when performing the heating steps (S120, S310) before performing the hot stamping process, the hydrogen inflow from the outside decreases, and thereby, it is shown to be excellent in resistance to hydrogen delayed fracture. Therefore, when heating a steel sheet on which aluminum-silicon (Al-Si) is formed at a first temperature for a first time before performing the hot stamping process, the hydrogen embrittlement of the manufactured hot stamping parts can be improved.

[0143] <Adhesion resistance evaluation> To perform the adhesion resistance evaluation on Example 1, Example 2, and Comparative Examples 1 to 6, the adhesion of the alloyed layer was measured through a dolly test. The adhesion was measured under the conditions of a speed of 0.6 MPa / s, a peeling area of 20π, a maximum load of 24 MPa, a curing temperature of 120°C, and a curing time of 20 minutes.

Table 6

[0144] Table 6 is a table showing the dolly test results of Example 1, Example 2, and Comparative Examples 1 to 6.

[0145] Referring to Table 6, it can be confirmed that the adhesive strengths of Example 1 and Example 2 are greater than those of Comparative Examples 1 to 6.

[0146] Through this, it can be seen that the adhesion resistance of the hot stamping parts in which the heating steps (S120, S310) are performed before performing the hot stamping process is greater than that of the hot stamping parts in which the heating steps (S120, S310) are omitted or the heating steps that do not satisfy the heating conditions are performed.

[0147] Therefore, when the heating steps (S120, S310) are performed before performing the hot stamping process, the adhesion resistance of the manufactured hot stamping parts can be improved.

[0148] As described above, the present invention has been described based on one embodiment illustrated in the drawings, but this is merely exemplary, and those having ordinary knowledge in the art will understand that various modifications and variations of the embodiments are possible from these. Therefore, the true technical protection scope of the present invention must be determined by the technical idea of the claims.

Explanation of Reference Numerals

[0149] 100 Steel plate 200 First alloying layer 300 Second alloying layer

Claims

1. heating a steel plate with a plating layer at a first temperature for a first time; cutting the heated steel plate to form a blank; a multi-stage heating step of heating the blank step by step; a soaking heating step of heating the multi-stage heated blank at a temperature of Ac3 to 1,000 °C; transferring the soaked heated blank from a heating furnace to a press die, including: the first temperature is 540 °C to 600 °C; the air cooling time of the blank in the transfer step satisfies the following formula; 【Number 1】 (At this time, λ ｔ is the air cooling time (s), a ｔ is the correction coefficient considering the heating furnace outlet temperature and the atmospheric temperature of the blank, T ｔ is the heating temperature (°C) for heating the steel plate, b ｔ is the correction coefficient considering the material components, t is the material thickness (mm), c ｔ is the correction coefficient considering the high-temperature material thickness sensitivity), In the above formula, the 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, c ｔ is 0.7 or more and 0.9 or less, λ ｔ is 5 s or more and 20 s or less, a method for manufacturing a hot stamping part.

2. The method for manufacturing a hot stamping part according to claim 1, wherein when the first temperature is 540 °C, the first time is 60 minutes or more.

3. The method for manufacturing a hot stamping part according to claim 1, wherein when the first temperature is 600 °C, the first time is 10 minutes or more.

4. The multi-stage heating step and the soaking heating step are carried out in a heating furnace having a plurality of sections with different temperature ranges, in the multi-stage heating step, the blank passes through the plurality of sections, the soaking heating step is carried out in at least one or more of the plurality of sections. The method for manufacturing a hot stamping part according to claim 1.

5. In the plurality of sections, the ratio of the length of the section for multi-stage heating the blank to the length of the section for soaking heating the blank satisfies 1:1 to 4:

1. The method for manufacturing a hot stamping part according to claim 4.

6. The temperatures of the plurality of sections increase in the direction from the inlet of the heating furnace to the outlet of the heating furnace. The method for manufacturing a hot stamping part according to claim 4.

7. Among the sections for multi-stage heating the blank, the temperature difference between two adjacent sections is greater than 0 °C and 100 °C or less. The method for manufacturing a hot stamping part according to claim 4.

8. Among the plurality of sections, the temperature of the section for soaking heating the blank is higher than the temperature of the section for multi-stage heating the blank. The method for manufacturing a hot stamping part according to claim 4.

9. After the transfer step, hot stamping the transferred blank to form a molded body; further including a step of cooling the formed molded body. The method for manufacturing a hot stamping part according to claim 1. ​

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