JP7713480B2 - Manufacturing method of hot stamping parts - Google Patents

Description

The present invention relates to a method for manufacturing hot stamped parts.

As environmental and fuel economy regulations become stricter worldwide, the need for lighter vehicle materials is increasing. This has led to active research and development of ultra-high strength steels and hot stamping steels. The hot stamping process generally consists of heating/forming/cooling/trim, and utilizes phase transformation and microstructural changes in the material during the process.

Recently, research has been actively conducted to improve delayed fracture, corrosion resistance, and weldability of hot stamped parts manufactured by the hot stamping process. Related technology is Korean Patent Publication No. 10-2018-0095757 (Invention title: Manufacturing method for hot stamped parts).

Republic of Korea Patent Publication No. 10-2018-0095757

The problem that the present invention aims to solve is to provide a hot stamped part with improved hydrogen embrittlement resistance, corrosion resistance, and weldability, and a manufacturing method thereof.

One embodiment of the present invention discloses a method for manufacturing a hot stamped part, comprising the steps of: manufacturing a plated steel sheet by immersing a base steel sheet in a plating bath containing aluminum and silicon; applying an oil containing an ester-based compound onto the plated steel sheet; cutting the plated steel sheet to which the oil has been applied to form a blank; heating the blank in a heating furnace; and transferring the heated blank from the heating furnace to a press die; wherein, in the step of transferring the heated blank to the press die, an air-cooling time of the blank satisfies the following mathematical formula:

(In this case, _λt is the air cooling time (s), _at is a correction coefficient considering the temperature at the time of removal from the heating furnace and the atmospheric temperature, _Tt is the heating temperature (°C), _bt is a correction coefficient considering the material composition, t is the material thickness (mm), and _ct is a correction coefficient considering the material thickness sensitivity to high temperatures.)
In this embodiment, in the formula, the a _t is 0.0160 or more and 0.0165 or less, the T _t is Ac3 or more and 1000° C. or less, the b _t is 10 or more and 0.5 or less, the t is 1 mm or more and 2.6 mm or less, and the c _t is 0.7 or more and 0.9 or less.

In this embodiment, in the above formula, λ _t is equal to or greater than 5 s and equal to or less than 20 s.

In this embodiment, the oil may be applied to the plated steel sheet in an amount of 0.1 g/m ² to 10 g/m ² .

In this embodiment, the oil may contain 10 wt% to 30 wt% hydrotreated heavy paraffin refined oil, 30 wt% to 50 wt% solvent-dewaxed heavy paraffin refined oil, 1 wt% to 5 wt% solvent-refined heavy paraffin refined oil, and 10 wt% to 40 wt% ester compounds.

In this embodiment, in the step of heating the blank in a heating furnace, the heating furnace has a plurality of sections having different temperature ranges, and the blank can be heated in stages in the heating furnace.

In this embodiment, the blank is heated in the furnace at a heating rate of 4.5°C/s to 10°C/s.

In this embodiment, in the step of heating the blank in the heating furnace, at least two blanks having different thicknesses may be simultaneously transported into the heating furnace.

According to an embodiment of the present invention, by hot stamping a blank coated with oil containing an ester-based compound, the hydrogen embrittlement resistance, corrosion resistance, and weldability of the parts produced by hot stamping can be improved.

FIG. 1 is a cross-sectional view showing a cross section of a hot stamped part according to one embodiment. 1 is a table showing peel resistance evaluation results of a second layer included in a hot stamped part according to one embodiment. 1 is a flow chart that illustrates a schematic diagram of a blank preparation process used in a method for manufacturing a hot stamped part according to an embodiment. 1 is a flow chart that generally illustrates a method for manufacturing a hot stamped part according to one embodiment. 4 is a diagram for explaining the blank heating step of FIG. 3; 1 is a graph showing temperature changes over time of a blank coated with oil containing an ester compound and a blank coated with general oil. 1 is a diagram showing air-cooling time depending on material thickness and heating temperature. 1 is a graph showing the results of thermal degassing analysis of Example 1 and Comparative Example 1. 1 is a graph showing the results of thermal degassing analysis of Example 2 and Comparative Example 2. 1 is a graph showing the results of thermal degassing analysis of Example 3 and Comparative Example 3. 1 is a photograph showing a surface layer of Example 1. 1 is a photograph showing a surface layer of Comparative Example 1. 1 is a graph showing resistance measurement results during spot welding of hot stamping parts according to Example 1 and Comparative Example 1. 1 is a graph showing the results of an experiment evaluating corrosion resistance for Example 1 and Comparative Example 1.

The present invention can be modified in various ways and can have various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. The advantages and features of the present invention, as well as the methods for achieving them, will become clearer with reference to the embodiments described in detail below in conjunction with the drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms.

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

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

In the following examples, terms such as "comprise" or "have" mean that the features or components described in the specification are present, but do not preclude the possibility that one or more other features or components may also be added.

In the following examples, when a film, region, component, or other part is said to be "on" or "above" another part, this does not only mean that it is directly on top of the other part, but also includes cases where there is another film, region, component, etc. in between.

In the drawings, the size of components is 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.

When an embodiment can be implemented differently, the order of certain steps may be performed out of the order described. For example, two steps described as successive may be performed substantially simultaneously or may be performed in the reverse order of that described.

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

Figure 1 is a cross-sectional view showing a cross section of a hot stamped part according to one embodiment of the present invention.

Referring to FIG. 1, a hot stamping part 10 according to one embodiment of the present invention includes a base steel sheet 100 and a plating layer 200 located on the base steel sheet 100.

The base steel sheet 100 is also a steel sheet manufactured by carrying out a hot rolling process and a cold rolling process on a steel slab that has been cast to contain a predetermined content of a predetermined alloy element. As an example, the base steel sheet 100 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), titanium (Ti), boron (B), the balance iron (Fe), and other unavoidable impurities. The base steel sheet 100 may also further contain one or more of niobium (Nb), molybdenum (Mo), and aluminum (Al).

Carbon (C) is a major element that determines the strength and hardness of the base steel sheet 100, and is added to ensure the tensile strength and hardenability characteristics of the base steel sheet 100 after the hot stamping process. As an example, carbon may be included in an amount of 0.19 wt% to 0.38 wt% of the total weight of the base steel sheet 100. If the carbon content is less than 0.19 wt%, it is difficult to ensure the mechanical strength of the base steel sheet 100. On the other hand, if the carbon content exceeds 0.38 wt%, the toughness of the base steel sheet 100 may decrease or a brittleness control problem may occur.

Silicon (Si) is a solid solution strengthening element and can improve the strength and softness of the base steel sheet 100. Silicon can also play a role in suppressing the formation of cementite, which is the starting point for cracks due to hydrogen embrittlement. Such silicon can be included in an amount of 0.1 wt% to 1 wt% of the total weight of the base steel sheet 100. If the silicon content is less than 0.1 wt%, it is difficult to obtain the above-mentioned effects, and conversely, if the silicon content exceeds 1 wt%, the plating characteristics of the base steel sheet 100 may be deteriorated.

Manganese (Mn) is added during heat treatment to improve hardenability and increase strength. Manganese can be included in an amount of 1 wt% to 2 wt% based on the total weight of the base steel sheet 100. If the manganese content is less than 1 wt%, the grain refinement effect may be insufficient, and the hard phase fraction of the hot stamped part may be insufficient. On the other hand, if the manganese content exceeds 2 wt%, softness and toughness may decrease due to manganese segregation or pearlite bands, causing a decrease in bending performance and the occurrence of inhomogeneous fine structures.

Phosphorus (P) is added to prevent a decrease in the toughness of the base steel sheet 100. Phosphorus may be contained in an amount of more than 0 and not more than 0.03 wt% based on the total weight of the base steel sheet 100. If the phosphorus content exceeds 0.03 wt%, iron phosphide compounds are formed, which reduces the toughness and may induce cracks in the base steel sheet 100 during the manufacturing process.

Sulfur (S) may be contained in an amount of more than 0 and not more than 0.01 wt% based on the total weight of the base steel sheet 100. If the sulfur content exceeds 0.01 wt%, hot workability may decrease and surface defects such as cracks may occur due to the formation of large inclusions.

Chromium (Cr) is added to improve the hardenability and strength of the base steel sheet 100. Chromium may be included in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the base steel sheet 100. If the chromium content is less than 0.1 wt%, the effect of improving hardenability and strength may be insufficient. On the other hand, if the chromium content exceeds 0.6 wt%, the manufacturing cost may increase and the toughness of the base steel sheet 100 may decrease.

Titanium (Ti) is added to strengthen 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, which can effectively contribute to austenite grain refinement. Titanium can be included in an amount of 0.01wt% to 0.05wt% based on the total weight of the base steel sheet. If the titanium content is less than 0.01wt%, precipitate formation is negligible and the grain refinement effect is insufficient. On the other hand, if the titanium content exceeds 0.05wt%, a decrease in elongation rate and toughness can occur.

Boron (B) is added to ensure the hardenability and strength of the base steel sheet 100 by ensuring a martensite structure, and has a grain refinement effect by increasing the austenite grain growth temperature. Boron may be included in an amount of 0.001 wt% to 0.005 wt% based on the total weight of the base steel sheet 100. If the boron content is less than 0.001 wt%, the effect of improving hardenability may be insufficient. On the other hand, if the boron content exceeds 0.005 wt%, the risk of brittleness and elongation deterioration may increase.

As an example, if the tensile strength of the manufactured hot stamped part is targeted to be 1680 MPa or more, the base steel sheet 100 should contain carbon (C): 0.20 wt% to 0.50 wt%, silicon (Si): 0.15 wt% to 0.70 wt%, manganese (Mn): 0.5 wt% to 2.0 wt%, phosphorus (P): more than 0 and not more than 0.05 wt%, sulfur (S): more than 0 and not more than 0.01 wt%, and the remainder being iron (Fe) and other unavoidable elements. It contains impurities, and optionally contains one or more of the following: boron (B): 0.001 wt% to 0.005 wt%, chromium (Cr): 0.05 wt% to 0.5 wt%, molybdenum (Mo): 0.05 wt% to 0.3 wt%, nickel (Ni): 0.05 wt% to 0.6 wt%, and also contains one or more of titanium (Ti), niobium (Nb), and vanadium (V) in a total amount exceeding 0 and not exceeding 0.1 wt%.

The plating layer 200 is formed on at least one surface of the base steel sheet 100 to a thickness of 10 μm to 50 μm and contains aluminum (Al). Here, the thickness of the plating layer 200 refers to the average thickness of the plating layer 200 over the entire area of the plating layer 200. If the thickness of the plating layer 200 is less than 10 μm, the corrosion resistance decreases, and if the thickness of the plating layer 200 exceeds 50 μm, the productivity of the hot stamped part 10 decreases, and the plating layer 200 may peel off from the base steel sheet 100 because the plating layer 200 is attached to a roller or a die during the hot stamping process.

The plating layer 200 includes a first layer 210 and a second layer 220 that are sequentially laminated on the base steel sheet 100. The plating layer 200 also includes a surface layer 240 that is laminated on the second layer 220. The surface layer 240 is a layer that contains 80 wt% or more of aluminum (Al), and the surface layer 240 disposed on the base steel sheet 100 can prevent the base steel sheet 100 from being oxidized. As an example, the average thickness of the surface layer 240 disposed on the base steel sheet 100 is 100 nm to 500 nm.

The first layer 210 and the second layer 220 may be made of polycrystals. For example, the second layer 220 may be thicker than the first layer 210. For example, the second layer 220 may be 1.6 to 3.6 times thicker than the first layer 210. The plating layer 200 may be finally formed into an iron-aluminum (Fe-Al)-based plating layer having silicon (Si) dissolved therein by a thermal reaction during the blank heating process of the hot stamping process in the manufacturing method of the hot stamping part described below.

The coating layer 200 includes a first layer 210 located on the base steel sheet 100. The first layer 210 may include iron (Fe), aluminum (Al), and silicon (Si) that are mixed together by thermal diffusion during a hot stamping manufacturing process. As an example, the first layer 210 has an α-Fe phase or an Fe ₃ Al ₂ phase. The first layer 210 may further include voids.

The plating layer 200 includes a second layer 220 located on the first layer 210. The plating layer 200 further includes an intermetallic compound portion 230 arranged in an island shape within the second layer 220. The second layer 220 includes at least one of an FeAl ₃ phase and an Fe ₂ Al ₅ phase, and the average crystal grain size of the FeAl ₃ phase and the Fe ₂ Al ₅ phase is 3 μm to 15 μm.

Figure 2 is a diagram showing the peel resistance evaluation results of the second layer included in a hot stamping part according to one embodiment. Specifically, Figure 2 is a diagram showing the results of measuring the strength at which the second layer 220 peels off from the base steel sheet 100 through a dolly test.

2, when the average crystal grain size of the second layer 220 is 3 μm (case a), the adhesive strength is 6.52 MPa, when the average crystal grain size of the second layer 220 is 7 μm (case b), the adhesive strength is 6.09 MPa, when the average crystal grain size of the second layer 220 is 10 μm (case c), the adhesive strength is 5.37 MPa, and when the average crystal grain size of the second layer 220 is 15 μm (case d), the adhesive strength is 5.21 MPa. In addition, when the average crystal grain size of the second layer 220 is 17 μm (case e), the adhesive strength is 3.94 MPa, and when the average crystal grain size of the second layer 220 is 20 μm (case f), the adhesive strength is 3.85 MPa. It can be seen that when the average crystal grain size of the second layer 220 exceeds 15 μm, the adhesive strength of the second layer 220 is reduced.

Therefore, if the average grain size of the second layer 220 exceeds 15 μm, the plating layer 200 may easily peel off from the base steel sheet 100. More specifically, if the average grain size of the second layer 220 exceeds 15 μm, the adhesive strength of the second layer 220 may be reduced, and the second layer 220 may easily peel off from the base steel sheet 100 and/or the first layer 210.

In addition, when using the hot stamping part manufacturing method described below, it is difficult to achieve an average grain size of less than 3 μm.

Therefore, when the average crystal grain size of the second layer 220 satisfies 3 μm to 15 μm, the adhesive strength of the plating layer 200 including the second layer 220 can be improved, and the peel resistance of the plating layer 200 can be improved.

The intermetallic compound portion 230 may be located within the second layer 220. The intermetallic compound portion 230 may be distributed in an island-like manner within the second layer 220. The intermetallic compound portion 230 may be arranged discontinuously within the second layer 220. The island-like arrangement of the intermetallic compound portion 230 within the second layer 220 may improve the weldability and peel resistance of the hot stamping part.

Each intermetallic compound portion 230 discontinuously disposed in the second layer 220 may have a size of 1 μm to 5 μm. The intermetallic compound portion 230 may be distributed at a ratio of 20% to 60% of the total cross-sectional area of the second layer 220. That is, the area ratio of the intermetallic compound portion 230 in the second layer 220 is also 20% to 60%. When using the manufacturing method of the hot stamping part described later, it is difficult to form the area ratio of the intermetallic compound portion 230 in the second layer 220 to be less than 20%, and if the area ratio of the intermetallic compound portion 230 in the second layer 220 exceeds 60%, the weldability of the hot stamping part may be reduced.

The intermetallic compound portion 230 contains an iron-aluminum (Fe-Al) compound, and the aluminum content contained in the intermetallic compound portion 230 is higher than the aluminum content contained in the second layer 220.

As an example, the first layer 210 includes iron (Fe): 82 wt% to 90 wt%, silicon (Si): 0 to 5 wt%, and aluminum (Al): 9 wt% to 15 wt%, the second layer 220 includes iron (Fe): 39 wt% to 47 wt%, silicon (Si): 0 to 2 wt%, and aluminum (Al): 53 wt% to 61 wt%, and the intermetallic compound portion 230 includes iron (Fe): 62 wt% to 67 wt%, silicon (Si): 2 wt% to 6 wt%, and aluminum (Al): 30 wt% to 34 wt%.

Figure 3 is a flow chart that shows an outline of a method for manufacturing a hot stamped part according to one embodiment of the present invention, and Figure 4 is a flow chart that shows an outline of a process for manufacturing the plated steel sheet of Figure 3. Hereinafter, the method for manufacturing a hot stamped part will be described with reference to Figures 3 and 4.

Referring to FIG. 3, the method for manufacturing a hot stamping part according to one embodiment may include a plated steel sheet manufacturing step (S110), an oil application step (S120), a blank forming step (S130), a blank heating step (S140), a blank transfer step (S150), a compact forming step (S160), and a compact cooling step (S170).

As an example, the method for manufacturing a hot stamping part includes an oil application step (S120), a blank formation step (S130), a blank heating step (S140), a blank transfer step (S150), a compact formation step (S160), and a compact cooling step (S170).

As an example, in the method for manufacturing a hot stamping part, the blank forming step (S130) may be performed, followed by the oil application step (S120). That is, the method for manufacturing a hot stamping part may be performed in the order of the blank forming step (S130), the oil application step (S120), the blank heating step (S140), the blank transferring step (S150), the compact forming step (S160), and the compact cooling step (S170).

The plated steel sheet manufacturing step (S110) includes a hot rolling step (S210) of the steel slab, a cooling/coil step (S220), a cold rolling step (S230), an annealing heat treatment step (S240), and a hot-dip plating step (S250), as shown in FIG. 3.

First, a semi-finished steel slab is prepared, which will be the subject of the process of forming the plated steel sheet. 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 not more than 0.03 wt%, sulfur (S): more than 0 and not more than 0.01 wt%, 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 is iron (Fe) and unavoidable impurities.

In order to perform hot rolling, the steel slab is reheated. In the steel slab reheating step, the steel slab obtained through the continuous casting process is reheated to a predetermined temperature to redissolve the elements that were segregated during casting. For example, the slab reheating temperature (SRT) is 1,200°C to 1,400°C. If the SRT is lower than 1,200°C, the elements that were segregated during casting are not sufficiently redissolved, and the homogenization effect of the alloy elements is not large, and the dissolution effect of titanium (Ti) is not large. The higher the SRT, the more favorable it is for homogenization, but if the SRT exceeds 1,400°C, the austenite grain size increases, making it difficult to ensure strength, and the manufacturing cost of the steel plate increases due to the excessive heating process.

In the hot rolling step (S210), the reheated steel slab is hot rolled at a predetermined finishing rolling temperature. For example, the finishing delivery temperature (FDT) is 880°C to 950°C. If the finishing delivery temperature (FDT) is lower than 880°C, it is difficult to ensure the workability of the steel sheet due to the generation of a mixed grain structure caused by abnormal region rolling, and there is a problem that the workability is reduced due to the non-uniformity of the microstructure. In addition, there may be a problem of sheet threadability during hot rolling due to a sudden phase change. If the finishing delivery temperature (FDT) exceeds 950°C, the austenite grains may become coarse. In addition, the TiC precipitates may become coarse, and the performance of the hot stamped parts may be reduced.

In the cooling/coiling step (S220), the hot-rolled steel sheet is cooled to a predetermined coiling temperature (CT) and coiled. As an example, the coiling temperature is 550°C to 800°C. The coiling temperature affects the redistribution of carbon (C), and if the coiling temperature is less than 550°C, the low-temperature phase fraction increases due to overcooling, increasing strength, and the rolling load during cold rolling may become intense, causing a rapid decrease in ductility. Conversely, if the coiling temperature exceeds 800°C, abnormal or excessive crystal grain growth may cause deterioration in formability and strength.

In the cold rolling step (S230), the coiled steel sheet is uncoiled, pickled, and then cold rolled. At this time, the pickling is performed for the purpose of removing scale from the coiled steel sheet, i.e., the hot-rolled coil produced through the above-mentioned hot rolling process.

The annealing heat treatment step (S240) is a step of annealing the cold-rolled steel sheet at a temperature of 700°C or more. As an example, the annealing heat treatment includes a step of heating the cold-rolled sheet material and cooling the heated cold-rolled sheet material at a predetermined cooling rate.

The hot-dip plating step (S250) is a step of forming a plating layer on the annealed heat-treated steel sheet. As an example, in the hot-dip plating step (S250), an aluminum-silicon (Al-Si) plating layer 200 can be formed on the annealed heat-treated steel sheet, i.e., the base steel sheet 100.

Specifically, in the hot-dip plating step (S250), the base steel sheet 100 may be immersed in a hot-dip plating bath containing 8 wt % to 12 wt % silicon (Si), the remainder aluminum (Al), and unavoidably added impurities. At this time, the hot-dip plating bath may be maintained at a temperature of 400° C. to 700° C. The plating layer 200 may be formed by plating both sides of the base steel sheet 100 at 40 to 180 g/ ^m2 .

The oil application step (S120) is a step of applying oil containing an ester-based compound onto a plated steel sheet manufactured by immersing a base steel sheet 100 in a plating bath containing aluminum and silicon. The oil includes 10 wt% to 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% to 50 wt% of solvent-dewaxed heavy paraffin refined oil, 1 wt% to 5 wt% of solvent-refined heavy paraffin refined oil, and 10 wt% to 40 wt% of ester-based compound. As an example, the oil includes 10 wt% to 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% to 50 wt% of solvent-dewaxed heavy paraffin refined oil, 1 wt% to 5 wt% of solvent-refined heavy paraffin refined oil, and 3 wt% to 40 wt% of polyester. The ester component may be included in the solvent-dewaxed heavy paraffin refined oil and the polyester. In another embodiment, the oil includes 10 wt% to 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% to 50 wt% of solvent-dewaxed heavy paraffin refined oil, 1 wt% to 5 wt% of solvent-refined heavy paraffin refined oil, and 3 wt% to 40 wt% of methyl esters. The ester component may be included in the solvent-dewaxed heavy paraffin refined oil and the methyl esters. In one embodiment, the oil may be "FERROCOTE (registered trademark) 6130", a commercial product of Quaker.

In the oil application step (S120), the oil may be applied on the plated steel sheet in an amount of 0.1 g/m ² to 10 g/m ^2. When the oil is applied on the plated steel sheet in an amount of 0.1 g/m ² to 10 g/m ² , an oil film may be formed on the surface of the aluminum-silicon (Al-Si) plating layer. The oil film formed on the surface of the aluminum-silicon (Al-Si) plating layer may affect the reaction between the base steel sheet and the aluminum-silicon (Al-Si) plating layer when the blank is heated, as described below, and may reduce the amount of hydrogen that flows into the base steel sheet from the outside when the blank is heated. If the amount of oil applied on the plated steel sheet is less than 0.1 g/m ² , the surface of the part manufactured by the method for manufacturing a hot stamped part may be corroded. On the other hand, if the amount of oil applied on the plated steel sheet exceeds 10 g/m ² , the coil may be deformed (buckled) in the radial direction.

As an example, a method for manufacturing a plated steel sheet for hot stamping includes a plated steel sheet manufacturing step (S110) and an oil application step (S120). Through the method for manufacturing a plated steel sheet for hot stamping including a plated steel sheet manufacturing step (S110) and an oil application step (S120), a plated steel sheet for hot stamping having oil applied to its surface can be manufactured.

The blank formation step (S130) is a step in which the plated steel sheet with oil applied thereon is cut to form a blank. The blank formation step (S130) is a step in which the plated steel sheet with oil applied thereon is cut into a desired shape depending on the purpose to form a blank. A blank for hot stamping can be provided by cutting the plated steel sheet with oil applied thereon.

For example, the oil application step (S120) may be performed after the blank formation step (S130). In this case, in the blank formation step (S130), a plated steel sheet may be cut into a desired shape depending on the purpose to form a blank. Also, in the oil application step (S120), oil containing an ester-based compound may be applied onto the blank formed through the blank formation step (S130). In the oil application step (S120), the oil may be applied onto the blank at 0.1 g/m ² to 10 g/m ² .

The blank heating step (S140) is also a step of heating the cut blanks in a heating furnace. Specifically, the blank heating step (S140) is also a step of heating the cut blanks in a heating furnace maintained at 800°C to 1,000°C.

As an example, if the oil application step (S120) is performed after the blank formation step (S130), the blank heating step (S140) is also a step of heating the blank coated with oil in a heating furnace. Specifically, the blank heating step (S140) is also a step of heating the blank on which the blank formation step (S130) and the oil application step (S120) have been sequentially performed in a heating furnace maintained at 800°C to 1,000°C.

As an example, the blank heating step (S140) includes a multi-stage heating step and a soaking heating step. In the multi-stage heating step, the blank may be heated in stages, and in the soaking heating step, the blank may be heated at a uniform temperature. Specifically, in the multi-stage heating step, the blank may be heated in stages 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 is performed, and the temperature of the blank may be set for each section so that it 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 in stages. After the multi-stage heating step, a soaking heating step may be performed. In the soaking heating step, the multi-stage heated blank may be heat-treated by passing through a section of the heating furnace set at a temperature of Ac3 to 1,000°C. Preferably, in the soaking heating step, the multi-stage heated blank may be soaked at a temperature of 930°C to 1,000°C. More preferably, in the soaking heating step, the multi-stage heated blank can be soaked at a temperature of 950°C to 1,000°C. In addition, among the multiple sections provided in the heating furnace, at least one section is where the soaking heating step is performed.

Figure 5 is a diagram illustrating a heating furnace having multiple sections in the blank heating step of a method for manufacturing a hot stamped part according to one embodiment of the present invention.

5, the furnace according to an embodiment may include a plurality of sections having different temperature ranges. More specifically, the furnace may 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 .}

For example, in the multi-stage heating step of the blank heating step (S140), the blank may be heated in multiple stages in a heating furnace through a plurality of sections (e.g., a first section _P1 to a fourth section _P4 ). In addition, in the soaking heating step of the blank heating step (S140), the blank heated in multiple stages in the first section _P1 to the fourth section _P4 may be soaked in the fifth section _P5 to the seventh section ^P7 .

The first section _P1 to the seventh section _P7 may be arranged in the furnace in the specified order. The first section _P1 having the first temperature range _T1 may be adjacent to the entrance of the furnace where the blank is inserted, and the seventh section _P7 having the seventh temperature range _T7 may be adjacent to the exit of the furnace where the blank is discharged. Therefore, the first section _P1 having the first temperature range _T1 may be the first section of the furnace, and the seventh section _P7 having the seventh temperature range _T7 may be the last section of the furnace. Among the sections of the furnace, the fifth section _P5 , the sixth section _P6 , and the seventh section _P7 may be sections where soaking heating is performed, not sections where multi-stage heating is performed.

The temperatures of the 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 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.

As an example, the first temperature range _T1 of the first section _P1 may be 840°C to 860°C or 835°C to 865°C. The second temperature range _T2 of the second section _P2 may be 870°C to 890°C or 865°C to 895°C. The third temperature range _T3 of the third section _P3 may be 900°C to 920°C or 895°C to 925°C. The fourth temperature range _T4 of the _fourth section P4 may be 920°C to 940°C or 915°C to 945°C. The fifth temperature range _T5 of the fifth section _P5 may be Ac3 to 1,000°C. Preferably, the fifth temperature range _T5 of the fifth section _P5 may be 930°C or more and 1,000°C or less. More preferably, the fifth temperature range _T5 of the fifth section _P5 is 950° C. to 1,000° C. 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 .

In FIG. 5, the furnace according to one embodiment is illustrated as having seven zones with different temperature ranges, but the present invention is not limited thereto. The furnace may have five, six, eight, etc. zones with different temperature ranges.

As an example, the blank may be heated in multiple stages and then soaked. The soaking of the blank may be performed in the last of the multiple sections in the heating furnace at a temperature of Ac3 to 1,000°C.

The soaking step may be performed in the last part of the multiple sections of the furnace. For example, the soaking step may be performed in the fifth section _P5 , the sixth section _P6 , and the seventh section _P7 of the furnace. When multiple sections are provided in the furnace, if one section is long, there may be a problem that the temperature changes within the section. Therefore, the sections in which the soaking step 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 furnace.

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

As an example, 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 soaked may be 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 are the sections where the blank is heated in multiple stages, to the sum of the lengths of the fifth section _P5 to the seventh section _P7 , which are the sections where the blank is soaked, may satisfy 1:1 to 4:1. If the length of the section where the blank is soaked is increased so that 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 soaked exceeds 1:1, an austenite (FCC) structure may be generated in the soaked heating section, increasing the amount of hydrogen permeation into the blank and increasing delayed fracture. In addition, if the length of the section in which the blank is soaked is reduced and the ratio of the length _D1 of the section in which the blank is heated in multiple stages to the length _D2 of the section in which the blank is soaked is less than 4:1, the soaked heating section (time) is not sufficiently secured, and the strength of the part manufactured by the manufacturing process of the hot stamped part becomes non-uniform.

As an example, among the multiple sections provided in the heating furnace, the length of the uniform heating section may be 20% to 50% of the total length of the heating furnace.

In addition, in the blank heating step (S140), at least two blanks having different thicknesses may be simultaneously transported into the heating furnace.

As an example, the blanks are retained in the heating furnace for 180 to 500 seconds. More specifically, the time for which the blanks are multi-stage heated and soaked in the heating furnace is 180 to 500 seconds. If the blanks are retained in the heating furnace for less than 180 seconds, it is difficult to sufficiently soak them at the desired soaking temperature. Also, if the blanks are retained in the heating furnace for more than 500 seconds, the amount of hydrogen that penetrates into the blanks increases, increasing the risk of delayed fracture and reducing corrosion resistance after hot stamping.

Figure 6 is a graph showing the temperature change over time for a blank coated with oil containing an ester compound and a blank coated with general oil.

Referring to FIG. 6, blank 310 coated with oil containing an ester-based compound and blank 320 coated with general oil show substantially the same temperature rise rate up to 600°C at room temperature, but it can be seen that from 600°C to 900°C, the temperature rise rate of blank 310 coated with oil containing an ester-based compound is faster than the temperature rise rate of blank 320 coated with general oil. In other words, it can be seen that the temperature rise rate of blank 310 coated with oil containing 10wt% to 40wt% of ester-based compound is faster than the temperature rise rate of blank 320 coated with general oil.

The hydrolysis reaction of the ester-based compound occurs in the blank temperature range of 600°C to 900°C. The hydrolysis reaction of the ester-based compound is an endothermic reaction, and by increasing the furnace temperature in this range to promote the hydrolysis reaction, hydrogen is prevented from flowing into the blank. As described above in FIG. 1, a plating layer 200 including a first layer 210, a second layer 220, and an intermetallic compound portion 230 provided in an island shape in the second layer 220 can be formed on the base steel sheet 100. In this case, the average heating rate of the blank 310 coated with oil containing 10 wt% to 40 wt% of an ester compound in the range of 600°C to 900°C is also 4.5°C/s to 10°C/s.

As an example, the oil may contain 10 wt% to 40 wt% of an ester-based compound. When the oil contains 10 wt% to 40 wt% of an ester-based compound, the amount of diffusible hydrogen is reduced, improving hydrogen delayed breakdown performance, and a dense oxide film may be formed by the hydrolysis reaction of the ester-based compound. When the oil contains less than 10 wt% of an ester-based compound, the amount of diffusible hydrogen increases, and hydrogen delayed breakdown may occur. On the other hand, when the oil contains more than 40 wt% of an ester-based compound, stains may be present on the surface of parts manufactured by the hot stamping part manufacturing method.

The blank transfer step (S150) is also a step of transferring the heated blank from the heating furnace to the press die. In this case, in the blank transfer step (S150), the heated blank may be cooled to ambient temperature (or room temperature) while being transferred to the press die. The heated blank may be air-cooled during transfer. If the heated blank is not air-cooled, the die entry temperature (e.g., molding start temperature) may become high, and wrinkles (or bends) may occur on the surface of the manufactured hot stamped part. In addition, when a refrigerant is used, it may affect the subsequent process (hot stamping), so it is preferable that the heated blank is air-cooled during transfer.

The compact formation step (S160) is a step in which the transferred blank is hot stamped to form a compact. The compact cooling step (S170) is a step in which the formed compact is cooled.

The blank may be formed into a final part shape in a press die and cooled to form a final product. The press die may have a cooling channel through which a refrigerant circulates. The heated blank may be quenched by circulating the refrigerant supplied through the cooling channel provided in the press die. In this case, in order to prevent springback of the plate material and to maintain the desired shape, quenching may be performed while the press die is closed and pressurized. When forming and cooling the heated blank, the blank may be cooled to the martensite finish temperature at an average cooling rate of at least 10°C/s. The blank may be held in the press die for 3 to 20 seconds. If the holding time in the press die is less than 3 seconds, the material may not be sufficiently cooled, and the temperature deviation in each part due to the residual heat may affect the embrittlement quality. In addition, a sufficient amount of martensite may not be generated, and mechanical properties may not be secured. On the other hand, if the holding time in the press die exceeds 20 seconds, the holding time in the press die may be extended, and productivity may decrease.

Figure 7 is a diagram showing the air cooling time according to material thickness and heating temperature. Specifically, Figure 7 is a graph shown to explain the maximum allowable air cooling time according to material thickness and heating temperature. In Figure 7, a high heating temperature can be understood to mean that the heating furnace removal temperature is high.

Referring to Figures 3 and 7, it can be seen that for the same material thickness, the maximum allowable air cooling time increases as the heating temperature decreases. It can also be seen that for the same heating temperature, the maximum allowable air cooling time increases as the material thickness increases.

If the heated blank is exposed to room temperature too much, not only will productivity decrease, but the blank will undergo phase transformation during air cooling, reducing formability and making it difficult to obtain the desired material. On the other hand, if the heated blank is exposed to room temperature for a short period of time, wrinkles (or bending) may occur in the hot stamping part manufactured by starting forming at an excessively high temperature. In addition, the plating layer of the blank may be burned onto the press die. Therefore, it is necessary to adjust the air cooling time in the blank transfer step (S150). However, in order to adjust the air cooling time in the blank transfer step (S150), various variables must be considered, including not only the heating temperature and the blank thickness (e.g., the material thickness), but also the blank components, blank thickness, plating amount, and thermal conductivity, thermal conductivity, and heat transfer amount due to surface emissivity, as well as the blank removal temperature from the heating furnace and the air temperature.

Therefore, the present inventors have derived a formula for easily controlling the air-cooling time through extensive repeated experiments. In one embodiment, the air-cooling time of the blank in the blank transferring step (S150) may satisfy the following formula:

In the formula, _λt is the air-cooling time (s), _at is a correction coefficient considering the temperature at the time of removal from the heating furnace and the atmospheric temperature, _Tt is the heating temperature (°C), _bt is a correction coefficient considering the material composition, _ct is a correction coefficient considering the material thickness sensitivity to high temperatures, and t is the material thickness (mm). In this case, the material means a blank, and the unit of air-cooling time, s, means seconds.

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, inclusive. In this case, a _t may have a unit of s/(° C.×mm).

_bt is a correction coefficient that takes into account the case where each material has a different composition, _{and bt} has a value of about 10.0 to about _0.5 inclusive, and has a unit of s/mm.

Also, the amount of heat transfer from inside the material varies depending on the thickness of the material. _ct is a correction coefficient that takes into account the difference in heat transfer 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.

The heating temperature _Tt refers to the soaking temperature in the blank heating step (S140), _and may have a value of about Ac3 or more and about 1000° C. or less. In this case, the heating temperature _Tt refers to the temperature at which the blank is removed from the heating furnace. In addition, the blank thickness t may have a value of about 1 mm or more and about 2.6 mm or less.

In one embodiment, the air cooling time λ _t according to the formula is about 5 s to about 20 s. If the air cooling time λ _t is less than 5 s, the forming start temperature at which the blank starts to be formed is excessively high, and the hot stamping part manufactured by forming the blank at a high temperature may have wrinkles (or bends), making it difficult to realize an air cooling time λ _t of less than 5 s due to equipment. On the other hand, if the air cooling time λ _t exceeds 20 s, not only the productivity decreases, but also the blank undergoes phase transformation during the transfer of the blank, reducing the formability of the blank, and the manufactured hot stamping part does not have the desired material. Therefore, if the air cooling time λ _t satisfies the range of about 5 s to about 20 s, the blank formability and the process productivity can be improved, and the manufactured hot stamping part can have the desired material.

By applying the above-mentioned composition and process conditions, a hot stamped part (component) having delayed fracture resistance can be realized. The base layer microstructure of the hot stamped part has a full martensite structure and can satisfy a yield strength of 900 MPa or more, a tensile strength of 1,350 MPa or more, and an elongation rate of 5% or more.

The present invention will be described in more detail with reference to the following examples. However, the following examples are provided to more specifically explain the present invention, and the scope of the present invention is not limited to the following examples. The following examples may be appropriately modified or altered by those skilled in the art within the scope of the present invention.

Table 1 shows the composition of the base steel sheet according to the experimental example of the present invention, and Table 2 shows the steel composition, oil conditions, whether or not multi-stage heating was performed, soaking temperature, and residence time in the heating furnace for manufacturing hot stamped parts according to the experimental example of the present invention.

Component system 1 in Table 1 was set with a target tensile strength of 1,350 MPa or more, and component system 2 was set with a target tensile strength of 1,680 MPa or more.

Referring to Tables 1 and 2, the steel materials of Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 4 contain the composition of Component System 1 in Table 1 with the balance being iron, and the steel materials of Example 3, Comparative Example 3, and Comparative Example 5 contain the composition of Component System 2 in Table 1 with the balance being iron. Hot rolling, cooling/coiling, cold rolling, annealing heat treatment, and hot-dip galvanizing treatment were performed under the same conditions for the steel materials having the compositions of Examples 1 to 3 and Comparative Examples 1 to 5.

Meanwhile, in the case of Examples 1 to 3 and Comparative Examples 4 and 5, 0.1 g/ ^m2 of Oil 1 was applied to the surface of the aluminum-silicon (Al-Si) plating layer to form an oil film, and in the case of Comparative Examples 1 to 3, 0.1 g/ ^m2 of Oil 2 was applied to the surface of the aluminum-silicon (Al-Si) plating layer to form an oil film. In this case, Quaker's "Ferrocote 6130" product was used as Oil 1, and Buhmwoo's "BW-80HG" commercial product was used as Oil 2. Oil 1 contains 10 wt% to 40 wt% of polyester, while Oil 2 contains less than 10 wt% of ester-based compounds.

Then, the blanks of Examples 1 to 3, in which Oil 1 was applied to the surface, and Comparative Examples 1 to 3, in which Oil 2 was applied to the surface, were heated according to the conditions in Table 2, and the heated blanks were transferred to a press die to perform press forming, and were cooled at a cooling rate of 10°C/s or more to manufacture hot stamped parts. The blanks of Comparative Examples 4 and 5, in which Oil 1 was applied to the surface, were heated at a uniform temperature (single heating) according to the conditions in Table 2, and the heated blanks were transferred to a press die to perform press forming, and were cooled at a cooling rate of 10°C/s or more to manufacture hot stamped parts.

<Tensile strength test>
After the hot stamping parts of Examples 1 to 3 and Comparative Examples 1 to 5 in Table 3 were manufactured, the yield strength, tensile strength, and elongation rate at room temperature were measured.

Table 3 shows the yield strength, tensile strength, and elongation ratio at room temperature for Examples 1 to 3 and Comparative Examples 1 to 5. Referring to Table 3, it can be seen that the yield strength of Examples 1 and 2 is 900 MPa or more, the tensile strength is 1,350 MPa or more, and the elongation ratio is 5% or more. It can also be seen that the tensile strength of Example 3 is 1,680 MPa or more, and the yield strength is 1,000 MPa or more.

Therefore, even if a hot stamping part is manufactured using a blank in which oil (oil 1) containing 10 wt% to 40 wt% of an ester-based compound is applied onto the plating layer, it can have a tensile strength greater than the preset target (e.g., 1,350 MPa or 1,680 MPa).

<Evaluation of diffusible hydrogen content and hydrogen delayed fracture characteristics>
Thermal desorption spectroscopy was performed on Examples 1 to 3 and Comparative Examples 1 to 5. More specifically, the amount of diffusible hydrogen released from the hot stamped parts at 350°C or less was measured while increasing the temperature from room temperature to 500°C at a heating rate of 20°C/min. In addition, hydrogen delayed fracture evaluation was performed on Examples 1 to 3 and Comparative Examples 1 to 5. The hydrogen delayed fracture evaluation was performed by a 4-point bending test. The 4-point bending test is a test method in which a stress level below the elastic limit is applied to a specific point of a specimen manufactured by reproducing a state exposed to a corrosive environment, and the occurrence of stress corrosion cracks is confirmed. In this case, the stress corrosion crack refers to a crack that occurs when corrosion and continuous tensile stress act simultaneously.

Specifically, for Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 4, which have a tensile strength of 1,350 MPa or more, a stress of 1,000 MPa was applied to each sample in air for 100 hours to check whether or not breakage occurred. Also, for Example 3, Comparative Example 3, and Comparative Example 5, which have a tensile strength of 1,680 MPa or more, a stress of 1,200 MPa was applied to each sample in air for 100 hours to check whether or not breakage occurred.

Figures 8A to 8C are graphs showing the results of the thermal degassing analysis of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3, respectively, and Table 4 is a table showing the amount of diffusible hydrogen released and the hydrogen delayed fracture evaluation results from Examples 1 to 3 and Comparative Examples 1 to 5.

Referring to FIG. 8A and Table 4, it can be seen that the amount of diffusible hydrogen released from Example 1 at temperatures below 300°C is less than the amount of diffusible hydrogen released from Comparative Example 1. In addition, the results of the hydrogen delayed fracture evaluation showed that no fracture occurred in Example 1, but fracture occurred in Comparative Example 1.

Referring to FIG. 8B and Table 4, it can be seen that the amount of diffusible hydrogen released from Example 2 at temperatures below 300°C is less than the amount of diffusible hydrogen released from Comparative Example 2. In addition, the hydrogen delayed fracture evaluation showed that no fracture occurred in Example 2, but fracture occurred in Comparative Example 2.

Referring to FIG. 8C and Table 4, it can be seen that the amount of diffusible hydrogen released from Example 3 at temperatures below 300°C is less than the amount of diffusible hydrogen released from Comparative Example 3. In addition, the hydrogen delayed fracture evaluation showed that no fracture occurred in Example 3, but fracture occurred in Comparative Example 3.

Therefore, when oil containing an ester compound (Oil 1) is applied to the surface of the plating layer, the amount of hydrogen inflow from the outside is reduced, which indicates that the resistance to hydrogen delayed breakdown is excellent.

In addition, the amount of diffusible hydrogen released from Examples 1 and 3, which were subjected to multi-stage heating, was smaller than the amount of diffusible hydrogen released from Comparative Examples 4 and 5, which were subjected to single heating, and the hydrogen delayed fracture evaluation showed that no fracture occurred in Examples 1 and 3, but fracture occurred in Comparative Examples 4 and 5.

Therefore, when multi-stage heating is performed, the amount of hydrogen inflow from the outside is reduced compared to when single heating is performed, which indicates that resistance to hydrogen delayed fracture is superior.

<Observation of surface layer thickness>
9A and 9B are photographs showing the surface layers of Example 1 and Comparative Example 1, respectively, and are drawings showing the results of measuring the cross sections of Example 1 and Comparative Example 1 by TEM.

9A and 9B, the thickness of the surface layer 240 of Example 1 is about 165 nm, and the thickness of the surface layer 450 of Comparative Example 1 is about 92 nm. Therefore, Example 1 has a thicker surface layer than Comparative Example 1, which is believed to be caused by the difference in the heating rate of the blank of Example 1 and the blank of Comparative Example 1, as shown in FIG. 6. Since the surface layer 240 of Example 1 is thicker than the surface layer 450 of Comparative Example 1, when oil (oil 1) containing an ester-based compound is applied onto the plating layer, the base steel sheet 100 can be more effectively prevented from being oxidized.

<Weldability evaluation>
A weldability evaluation was carried out for Example 1 and Comparative Example 1. In the weldability evaluation, a 6 mm welding tip was applied to the welded portion at 930° C. for 4 minutes with a pressure of 350 kgf, and contact resistance was measured with a current being applied.

Figure 10 is a graph showing the resistance measurement results during spot welding for Example 1 and Comparative Example 1.

Referring to FIG. 10, it can be seen that the contact resistance of Example 1 is lower than that of Comparative Example 1. In particular, it can be seen that the contact resistance of Example 1 at around 5 ms after the initial stage of welding is lower than that of Comparative Example 1. Therefore, since the contact resistance of Example 1 is lower than that of Comparative Example 1, it can be seen that the hot stamped part of Example 1 has superior weldability compared to the hot stamped part of Comparative Example 1.

<Corrosion resistance evaluation>
A corrosion resistance evaluation experiment was carried out on the hot stamped parts of Example 1 and Comparative Example 1. In the corrosion resistance evaluation, a potentiodynamic polarization test was carried out by forming a three-electrode electrochemical cell using a specimen as a working electrode, a high-purity carbon rod as a counter electrode, and a saturated calomel electrode as a reference electrode. The potentiodynamic polarization test was carried out after confirming electrochemical stabilization by measuring the open-circuit potential (OCP) in a 3.5% NaCl solution for 10 hours, and applying a potential from -250 mV to 0 mVSCE at a scanning rate of 0.166 mV/s.

Figure 11 is a graph showing the results of a corrosion resistance evaluation experiment for Example 1 and Comparative Example 1. The graph in Figure 11 shows the results of measuring the corrosion current for Example 1 and Comparative Example 1, and the corrosion current is a value corresponding to the current density at the point where a branch occurs in the stably held potential.

11, the corrosion current of Example 1 was measured as 5×10 ⁻⁴ A, and the corrosion current of Comparative Example 1 was measured as 5.5×10 ⁻⁴ A. Thus, it can be seen that the current density related to corrosion of Example 1 is lower than that of Comparative Example 1. Thus, it can be seen that the corrosion resistance of Example 1 is superior to that of Comparative Example 1. As described above, the present invention has been described based on one embodiment shown in the drawings, but this is merely illustrative, and a person skilled in the art will understand that various modifications and variations of the embodiments are possible therefrom. Therefore, the true technical scope of the present invention should be determined by the technical ideas of the claims.

Reference Signs List 100 Base steel sheet 200 Plating layer 210 First layer 220 Second layer 230 Intermetallic compound portion 240 Surface layer

Claims (5)

A method for producing a plated steel sheet by immersing a base steel sheet in a plating bath containing aluminum and silicon;
applying oil containing an ester-based compound onto the plated steel sheet;
cutting the oil-coated plated steel sheet to form a blank;
heating the blank in a heating furnace;
and transferring the heated blank from the furnace to a press die;
In the step of transferring the heated blank to the press die, the air-cooling time of the blank satisfies the following formula:
(wherein, _λt is the air cooling time (s), at _is a correction coefficient taking into consideration the temperature at which the blank is removed from the heating furnace and the atmospheric temperature, _Tt is the heating temperature at which the steel plate is heated (°C), _bt is a correction coefficient taking into consideration the material composition, t is the material thickness (mm), and _ct is a correction coefficient taking into consideration the material thickness sensitivity to high temperatures).
In the formula, 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, and λ _t is 5 s or more and 20 s or less. The method for producing a hot stamped part according to claim 1, wherein the oil is applied to the plated steel sheet in an amount of 0.1 g/m ² to 10 g/m ² . The method for producing a hot stamping part according to claim 2, wherein the oil contains 10 wt% to 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% to 50 wt% of solvent-dewaxed heavy paraffin refined oil, 1 wt% to 5 wt% of solvent-refined heavy paraffin refined oil, and 10 wt% to 40 wt% of an ester-based compound. In the step of heating the blank in a heating furnace,
The method for producing a hot stamped part according to claim 1 , wherein the heating furnace has a plurality of sections having different temperature ranges, and the blank is heated in the heating furnace in stages. In the step of heating the blank in a heating furnace,
The method for producing a hot stamped part according to claim 4, wherein at least two blanks having different thicknesses are transferred into the heating furnace at the same time.