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 the steps of: immersing a base steel plate in a plating bath containing aluminum and silicon, and manufacturing a plated steel plate; coating oil containing an ester-based compound onto the plated steel plate; cutting the plated steel plate to which oil is coated, and forming a blank; heating the blank in a heating furnace; and transferring the 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, while environmental regulations and fuel efficiency regulations are being strengthened, the need for lighter vehicle materials is increasing. As a result, research and development on ultra-high strength steel and hot stamping steel are being actively carried out. 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

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problem to be solved by the present invention is to provide a hot stamping part with improved hydrogen embrittlement, corrosion resistance, and weldability, and a method for manufacturing the same.

Means for Solving the Problems

[0006] One embodiment of the present invention discloses a method for manufacturing hot stamping parts, including the steps of immersing a base steel plate in a plating bath containing aluminum and silicon to produce a plated steel plate; applying an oil containing an ester-based compound onto the plated steel plate; cutting the plated steel plate with the applied oil to form a blank; heating the blank in a heating furnace; and transferring the heated blank from the heating 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.

Equation

[0007] In this embodiment, in the above formula, λ t is also not less than 5 s and not more than 20 s.

[0008] In this embodiment, the oil can be applied onto the plated steel plate at 0.1 g / m 2 ~10 g / m 2 .

[0009] In this embodiment, the oil may contain 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.

[0010] In this embodiment, in the step of heating the blank in a heating furnace, the heating furnace includes a plurality of sections having different temperature ranges, and the blank can be heated step - by - step in the heating furnace.

[0011] In this embodiment, in the step of heating the blank in a heating furnace, the heating rate of the blank is also 4.5 °C / s to 10 °C / s.

[0012] In this embodiment, in the step of heating the blank in a heating furnace, at least two blanks having different thicknesses can be transferred simultaneously in the heating furnace.

Advantages of the Invention

[0013] According to the embodiment of the present invention, by hot - stamping a blank coated with an oil containing an ester - based compound, the hydrogen embrittlement, corrosion resistance, and weldability of parts manufactured by hot - stamping can be improved.

Brief Description of the Drawings

[0014]

Figure 1

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Embodiments for Carrying Out the Invention

[0015] The present invention can be subjected to various conversions and can have various embodiments. Specific embodiments are illustrated in the drawings and will be described in detail by detailed description. The effects, features, and methods for achieving them will become clear by referring to the embodiments described in detail below together with the drawings. However, the present invention is not limited to the embodiments disclosed below and can also be embodied in various forms.

[0016] 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 other components.

[0017] In the following examples, the singular forms include the plural forms unless the context clearly indicates otherwise.

[0018] In the following examples, terms such as "including" 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.

[0019] In the following examples, when a part such as a film, region, or component is "above" or "upper" of 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.

[0020] In the drawings, for the sake of convenience of explanation, the sizes of the components may be exaggerated or reduced. For example, the sizes and thicknesses of the respective configurations shown in the drawings are arbitrarily shown for the sake of convenience of explanation, and the present invention is not necessarily limited to the illustrated locations.

[0021] If a certain example can be implemented differently, the specific process order may be performed differently from the described order. For example, two processes described continuously may be performed substantially simultaneously or in the reverse order of the described order.

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. When referring to the drawings for explanation, the same or corresponding components are given the same reference numerals.

[0023] FIG. 1 is a cross-sectional view showing a cross-section of a hot stamping part according to an embodiment of the present invention.

[0024] Referring to FIG. 1, a hot stamping part 10 according to an embodiment of the present invention includes a base steel plate 100 and a plating layer 200 located on the base steel plate 100.

[0025] The base steel plate 100 is also a steel plate manufactured by subjecting a steel slab cast to contain a predetermined alloy element in a predetermined content to a hot rolling process and a cold rolling process. As an example, the base steel plate 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 base steel plate 100 can further contain one or more components among niobium (Nb), molybdenum (Mo), and aluminum (Al).

[0026] Carbon (C) is a main element that determines the strength and hardness of the base steel plate 100, and is added for the purpose of ensuring the tensile strength and hardenability characteristics of the base steel plate 100 after the hot stamping process. As an example, carbon can be contained at 0.19 wt% to 0.38 wt% based on the total weight of the base steel plate 100. When the content of carbon is less than 0.19 wt%, it is difficult to ensure the mechanical strength of the base steel plate 100. On the other hand, if the content of carbon exceeds 0.38 wt%, the toughness of the base steel plate 100 may decrease or a brittle control problem may be caused.

[0027] Silicon (Si) is a solid solution strengthening element and can improve the strength and softness of the base steel plate 100. Further, silicon can play a role in suppressing the formation of cementite that is the origin of cracks due to hydrogen embrittlement. Such silicon can be contained at 0.1 wt% to 1 wt% based on the total weight of the base steel plate 100. When the content of silicon is less than 0.1 wt%, it is difficult to obtain the above-described effects. Conversely, if the content of silicon exceeds 1 wt%, the plating characteristics of the base steel plate 100 may deteriorate.

[0028] 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 base steel sheet 100. When the content of manganese is less than 1 wt%, the grain refinement effect 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 are reduced due to manganese segregation or pearlite banding, which may cause a decrease in bending performance and the occurrence of a non-uniform fine structure.

[0029] Phosphorus (P) is added to prevent the reduction of toughness of the base steel sheet 100. Phosphorus can be contained in an amount exceeding 0 and not exceeding 0.03 wt% based on the total weight of the base steel sheet 100. If the content of phosphorus exceeds 0.03 wt%, an iron phosphide compound is formed and the toughness is reduced, and cracks may be induced in the base steel sheet 100 during the manufacturing process.

[0030] Sulfur (S) can be contained in an amount exceeding 0 and not exceeding 0.01 wt% based on the total weight of the base steel sheet 100. If the content of sulfur exceeds 0.01 wt%, the hot workability is reduced, and surface defects such as cracks may occur due to the formation of large inclusions.

[0031] Chromium (Cr) is added for the purpose of improving the hardenability and strength of the base 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 base 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 base steel sheet 100 may be reduced.

[0032] Titanium (Ti) is added for the purpose of strengthening hardenability and improving material properties by the formation of precipitates after hot stamping heat treatment. Also, titanium can effectively contribute to the refinement of austenite grain size by forming precipitate phases such as Ti(C,N) at high temperatures. Titanium can be contained in an amount of 0.01 wt% to 0.05 wt% based on the total weight of the base steel sheet. When the content of titanium is less than 0.01 wt%, the formation of precipitates is negligible and the grain refinement effect is insufficient. On the other hand, when titanium exceeds 0.05 wt%, a decrease in elongation and a decrease in toughness may occur.

[0033] Boron (B) is added for the purpose of ensuring the hardenability and strength of the base steel sheet 100 by ensuring a martensite structure, and has a grain refinement effect due to an increase in the austenite grain growth temperature. Boron can be contained in an amount of 0.001 wt% to 0.005 wt% based on the total weight of the base steel sheet 100. When the content of boron is less than 0.001 wt%, the effect of improving hardenability is also 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.

[0034] As an example, when the target tensile strength of the manufactured hot stamping part is 1680 MPa or more, the base steel sheet 100 contains 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 0.05 wt% or less, sulfur (S): more than 0 and 0.01 wt% or less, and the remaining iron (Fe) and other inevitable impurities, and selectively contains one or more of 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 of more than 0 and 0.1 wt% or less.

[0035] The plating layer 200 is formed on at least one surface of the base steel sheet 100 with a thickness of 10 μm to 50 μm and contains aluminum (Al). Here, the thickness of the plating layer 200 means the average thickness of the plating layer 200 over the entire area of the plating layer 200. When the thickness of the plating layer 200 is less than 10 μm, the corrosion resistance decreases. If the thickness of the plating layer 200 exceeds 50 μm, the productivity of the hot stamping part 10 decreases, and during the hot stamping process, the plating layer 200 may adhere to the roller or the mold and be peeled off from the base steel sheet 100.

[0036] The plating layer 200 includes a first layer 210 and a second layer 220 sequentially laminated on the base steel sheet 100. Further, the plating layer 200 may further include a surface layer 240 laminated on the second layer 220. The surface layer 240 is a layer containing 80 wt% or more of aluminum (Al). By disposing the surface layer 240 on the base steel sheet 100, oxidation of the base steel sheet 100 can be prevented. As an example, the average thickness of the surface layer 240 disposed on the base steel sheet 100 is also 100 nm to 500 nm.

[0037] The first layer 210 and the second layer 220 can be composed of polycrystals. As an example, the second layer 220 can be provided thicker than the first layer 210. For example, the thickness of the second layer 220 can be 1.6 times to 3.6 times thicker than the thickness of the first layer 210. In the manufacturing method of the hot stamping part described later, the plating layer 200 can be finally formed into an iron-aluminum (Fe-Al) based plating layer in which silicon (Si) is dissolved by a thermal reaction during the blank heating process of the hot stamping process.

[0038] The plating layer 200 includes a first layer 210 located on the base steel sheet 100. The first layer 210 can contain iron (Fe), aluminum (Al), and silicon (Si) mixed with each other by thermal diffusion during the hot stamping manufacturing process. As an example, the first layer 210 has an α-Fe phase or a Fe3Al2 phase. Further, the first layer 210 may further contain voids.

[0039] The plating layer 200 includes a second layer 220 located on the first layer 210. Further, the plating layer 200 may further include an intermetallic compound portion 230 disposed in an island shape within the second layer 220. The second layer 220 includes at least one of an FeAl3 phase and an Fe2Al5 phase, and the average grain size of the FeAl3 phase and the Fe2Al5 phase is also 3 μm to 15 μm.

[0040] FIG. 2 is a drawing showing the peel resistance evaluation result of the second layer included in the hot stamping part according to an embodiment. Specifically, FIG. 2 is a drawing showing the result of measuring the strength at which the second layer 220 is peeled from the base steel sheet 100 through a dolly test.

[0041] Referring to FIG. 2, when the average grain size of the second layer 220 is 3 μm (case a), the adhesive strength is 6.52 MPa. When the average grain size of the second layer 220 is 7 μm (case b), the adhesive strength is 6.09 MPa. When the average grain size of the second layer 220 is 10 μm (case c), the adhesive strength is 5.37 MPa. When the average grain size of the second layer 220 is 15 μm (case d), it can be confirmed that the adhesive strength is 5.21 MPa. Also, when the average grain size of the second layer 220 is 17 μm (case e), it can be confirmed that the adhesive strength is 3.94 MPa. When the average grain size of the second layer 220 is 20 μm (case f), it can be confirmed that the adhesive strength is 3.85 MPa. Through this, it can be seen that when the average grain size of the second layer 220 exceeds 15 μm, the adhesive strength of the second layer 220 decreases.

[0042] Therefore, when the average grain size of the second layer 220 exceeds 15 μm, the plating layer 200 can be easily peeled from the base steel sheet 100. More specifically, when the average grain size of the second layer 220 exceeds 15 μm, the adhesive strength of the second layer 220 decreases, and the second layer 220 can be easily peeled from the base steel sheet 100 and / or the first layer 210.

[0043] Also, when using the method for manufacturing hot stamping parts described later, it is difficult to form the average crystal grain size to less than 3 μm.

[0044] 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.

[0045] An intermetallic compound part 230 can be located within the second layer 220. The intermetallic compound part 230 can be distributed in an island shape within the second layer 220. The intermetallic compound part 230 can be arranged discontinuously within the second layer 220. By arranging the intermetallic compound part 230 in an island shape within the second layer 220, the weldability and peel resistance of the hot stamping parts can be improved.

[0046] Each intermetallic compound part 230 arranged discontinuously within the second layer 220 can have a size of 1 μm to 5 μm. The intermetallic compound part 230 can be distributed at a fraction of 20% to 60% with respect to the total cross-sectional area of the second layer 220. That is, the area fraction of the intermetallic compound part 230 with respect to the second layer 220 is also 20% to 60%. When using the method for manufacturing hot stamping parts described later, it is difficult to form the area fraction of the intermetallic compound part 230 with respect to the second layer 220 to less than 20%. When the area fraction of the intermetallic compound part 230 with respect to the second layer 220 exceeds 60%, the weldability of the hot stamping parts may decrease.

[0047] The intermetallic compound part 230 contains an iron-aluminum (Fe-Al) compound, and the content of aluminum contained in the intermetallic compound part 230 is higher than the content of aluminum contained in the second layer 220.

[0048] As an example, the first layer 210 contains iron (Fe): 82 wt% to 90 wt%, silicon (Si): more than 0 and 5 wt% or less, and aluminum (Al): 9 wt% to 15 wt%. The second layer 220 contains iron (Fe): 39 wt% to 47 wt%, silicon (Si): more than 0 and 2 wt% or less, and aluminum (Al): 53 wt% to 61 wt%. The intermetallic compound part 230 contains iron (Fe): 62 wt% to 67 wt%, silicon (Si): 2 wt% to 6 wt%, and aluminum (Al): 30 wt% to 34 wt%.

[0049] FIG. 3 is a flowchart schematically showing a method for manufacturing a hot stamping part according to an embodiment of the present invention, and FIG. 4 is a flowchart schematically showing a process for manufacturing the plated steel sheet of FIG. 3. Hereinafter, with reference to FIGS. 3 and 4, a method for manufacturing a hot stamping part will be described.

[0050] Referring to FIG. 3, a method for manufacturing a hot stamping part according to an embodiment may include a plated steel sheet manufacturing step (S110), an oil coating step (S120), a blank forming step (S130), a blank heating step (S140), a blank transfer step (S150), a formed body forming step (S160), and a formed body cooling step (S170).

[0051] As an example, a method for manufacturing a hot stamping part includes an oil coating step (S120), a blank forming step (S130), a blank heating step (S140), a blank transfer step (S150), a formed body forming step (S160), and a formed body cooling step (S170).

[0052] As an example, in a method for manufacturing a hot stamping part, after the blank forming step (S130) is performed, the oil coating step (S120) may be performed. That is, the method for manufacturing a hot stamping part may be performed in the order of the blank forming step (S130), the oil coating step (S120), the blank heating step (S140), the blank transfer step (S150), the formed body forming step (S160), and the formed body cooling step (S170).

[0053] The plating steel sheet manufacturing stage (S110) includes, as shown in FIG. 3, a hot rolling stage (S210), a cooling / coiling stage (S220), a cold rolling stage (S230), an annealing heat treatment stage (S240), and a molten plating stage (S250) of a steel slab.

[0054] First, a semi-finished steel slab that is the object of the process for forming a plating 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.

[0055] For hot rolling, the reheating stage of the steel slab is carried out. In the steel slab reheating stage, by reheating the steel slab secured through the continuous casting process to a predetermined temperature, the components segregated during casting are 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 components segregated during casting are not sufficiently redissolved, the homogenization effect of alloying elements is not considered significant, and the solid solution effect of titanium (Ti) is not considered significant. The higher the slab reheating temperature (SRT), the more advantageous it is for homogenization. However, when the slab reheating temperature (SRT) exceeds 1,400°C, the austenite crystal grain size increases, making it difficult to ensure strength, and the manufacturing cost of the steel sheet increases due to excessive heating processes.

[0056] In the hot rolling stage (S210), the reheated steel slab is hot rolled at a predetermined finishing rolling temperature. As an example, the finishing rolling temperature (Finishing Delivery Temperature: FDT) is also 880°C to 950°C. At this time, if the finishing rolling 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, and there is a problem that the workability decreases due to the non-uniformity of the fine structure, and during hot rolling due to a sudden phase change, a problem of sheet passing property may occur. When the finishing rolling temperature (FDT) exceeds 950°C, the austenite crystal grains may coarsen. Also, the TiC precipitates may coarsen and the performance of the hot stamping parts may deteriorate.

[0057] In the cooling / coiling stage (S220), the hot rolled steel plate is cooled to a predetermined coiling temperature (Coiling Temperature: CT) and 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 becomes high, the strength increases, and during cold rolling, the rolling load may become intense and the ductility may decrease rapidly. Conversely, when the coiling temperature exceeds 800°C, formability and strength deterioration may occur due to abnormal crystal grain growth or excessive crystal grain growth.

[0058] 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 above hot rolling process.

[0059] The annealing heat treatment stage (S240) is a stage of annealing heat treatment of the cold rolled steel plate at a temperature of 700°C or higher. As an example, the annealing heat treatment includes a stage of heating the cold rolled sheet material and cooling the heated cold rolled sheet material at a predetermined cooling rate.

[0060] The melting plating step (S250) is a step of forming a plating layer on the annealed steel sheet. As an example, in the melting plating step (S250), an aluminum-silicon (Al-Si) plating layer 200 can be formed on the annealed steel sheet, that is, the base steel sheet 100.

[0061] Specifically, in the melting plating step (S250), the base steel sheet 100 can be immersed in a melting plating bath containing 8 wt% to 12 wt% silicon (Si), the balance aluminum (Al), and unavoidably attached impurities. At this time, the melting plating bath can maintain a temperature of 400°C to 700°C. The plating layer 200 can be formed by plating at 40 to 180 g / m² based on both sides of the base steel sheet 100. 2

[0062] ​The oil coating step (S120) is a step of applying an oil containing an ester compound onto a plated steel sheet manufactured by immersing a base steel sheet 100 in a plating bath containing aluminum and silicon. The oil contains 10 wt% - 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% - 50 wt% of solvent - dewaxed heavy paraffin refined oil, 1 wt% - 5 wt% of solvent - refined heavy paraffin refined oil, and 10 wt% - 40 wt% of an ester compound. As an example, the oil contains 10 wt% - 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% - 50 wt% of solvent - dewaxed heavy paraffin refined oil, 1 wt% - 5 wt% of solvent - refined heavy paraffin refined oil, and 3 wt% - 40 wt% of polyester. The ester component may be included in the solvent - dewaxed heavy paraffin refined oil and the polyester. In other embodiments, the oil contains 10 wt% - 30 wt% of hydrotreated heavy paraffin refined oil, 30 wt% - 50 wt% of solvent - dewaxed heavy paraffin refined oil, 1 wt% - 5 wt% of solvent - refined heavy paraffin refined oil, and 3 wt% - 40 wt% of methyl ester. The ester component may be included in the solvent - dewaxed heavy paraffin refined oil and the methyl ester. In one embodiment, the oil may be "FERROCOTE (registered trademark) 6130", a commercial product of Quaker company.

[0063] In the oil coating step (S120), the oil can be applied onto the plated steel sheet at 0.1 g / m 2 ~10 g / m 2 The oil can be applied onto the plated steel sheet at 0.1 g / m 2 ~10 g / m 2By being applied, an oil film can 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 affects the reaction between the base steel sheet and the aluminum-silicon (Al-Si) plating layer during the heating of the blank described later, and can reduce the amount of hydrogen flowing into the base steel sheet from the outside during the heating of the blank. When the amount of oil applied on the plated steel sheet is less than 0.1 g / m 2 If it is less than, the surface of the parts manufactured by the method for manufacturing hot stamping parts can be corroded. On the other hand, when the amount of oil applied on the plated steel sheet exceeds 10 g / m 2 If it is excessive, the coil may be deformed (buckled) in the radial direction.

[0064] As an example, the 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 the plated steel sheet manufacturing step (S110) and the oil application step (S120), a plated steel sheet for hot stamping with oil applied on its surface can be manufactured.

[0065] The blank forming step (S130) is a step of cutting the plated steel sheet with oil applied to form a blank. In the blank forming step (S130), the plated steel sheet with oil applied on its surface is cut into a desired shape according to the purpose to form a blank. By cutting the plated steel sheet with oil applied on its surface, a blank for hot stamping can be provided.

[0066] As an example, after the blank forming step (S130) is performed, the oil application step (S120) can be performed. In that case, in the blank forming step (S130), the plated steel sheet can be cut into a desired shape according to the purpose to form a blank. Also, in the oil application step (S120), oil containing an ester-based compound can be applied on the blank formed through the blank forming step (S130). In the oil application step (S120), the oil is 0.1 g / m on the blank2 ~10 g / m 2 can be applied.

[0067] The blank heating step (S140) is also a step of heating the cut blank in a heating furnace. Specifically, the blank heating step (S140) is also a step of heating the cut blank in a heating furnace maintained at 800°C to 1,000°C.

[0068] As an example, when the oil application step (S120) is performed after the blank forming 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 in which the blank forming step (S130) and the oil application step (S120) are sequentially performed in a heating furnace maintained at 800°C to 1,000°C.

[0069] 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 is heated step by step, and in the soaking heating step, the blank can be heated at a uniform temperature. Specifically, in the multi-stage heating step, the blank can be gradually heated 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 where the multi-stage heating step is performed, and the temperature can be set for each section so that the blank is gradually heated to be higher 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. A soaking heating step can be performed after the multi-stage heating step. In the soaking heating step, 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, 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, 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 where the soaking heating step is performed is at least one or more.

[0070] FIG. 5 is a drawing showing a heating furnace having a plurality of sections in the blank heating stage of a method for manufacturing a hot stamping part according to an embodiment of the present invention.

[0071] Referring to FIG. 5, 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.

[0072] As an example, in the multi-stage heating stage of the blank heating stage (S140), 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 stage of the blank heating stage (S140), 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 P 7 and can be soaked heated.

[0073] 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. Therefore, the first section P1 having the first temperature range T1 is also the first section of the heating furnace, and the seventh section P7 having the seventh temperature range T7 is also the last section of the heating furnace. In 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, which is not a section where multi-stage heating is performed.

[0074] The temperatures of a plurality of sections provided in the heating furnace, for example, the temperatures of the first section P1 to the seventh section P7, can increase in the direction from the inlet of the heating furnace where the blank is charged 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 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.

[0075] As an example, the first temperature range T1 of the first section P1 is also 840°C to 860°C and also 835°C to 865°C. The second temperature range T2 of the second section P2 is also 870°C to 890°C and also 865°C to 895°C. The third temperature range T3 of the third section P3 is also 900°C to 920°C and also 895°C to 925°C. The fourth temperature range T4 of the fourth section P4 is also 920°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.

[0076] In FIG. 5, 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. The heating furnace may be provided with five, six, or eight sections, etc. having different temperature ranges.

[0077] As an example, after the blank is multi-stage heated, the blank can be soaking heated. The soaking heating of the blank is performed in the last part among the plurality of sections provided in the heating furnace and can be performed at a temperature of Ac3 to 1,000°C.

[0078] The soaking heating stage can be carried out in the last part of a plurality of sections of the heating furnace. As an example, the soaking heating stage can be carried out 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 section where the soaking heating stage is carried out is 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 within the heating furnace.

[0079] In the soaking heating stage, 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, 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, the multi-stage heated blank can be soaked and heated at a temperature of 950 °C to 1,000 °C.

[0080] 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.

[0081] As an example, among a plurality of 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.

[0082] Also, in the blank heating stage (S140), at least two blanks having different thicknesses can be simultaneously transferred in the heating furnace.

[0083] As an example, the blank stays in the heating furnace for 180 seconds to 500 seconds. More specifically, the time during which the blank is multi-stage heated and soaked in the heating furnace is also 180 seconds to 500 seconds. When the residence time of the blank in the heating furnace is less than 180 seconds, it is difficult to achieve sufficient soaking at the desired soaking temperature. Also, when the residence time of the blank in the heating furnace exceeds 500 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.

[0084] FIG. 6 is a graph showing the temperature change over time of a blank coated with an oil containing an ester-based compound and a blank coated with a general oil.

[0085] Referring to FIG. 6, in the case of the blank 310 coated with an oil containing an ester-based compound and the blank 320 coated with a general oil, the heating rate is substantially the same from room temperature to 600 °C, but it can be confirmed that the heating rate of the blank 310 coated with an oil containing an ester-based compound is faster than that of the blank 320 coated with a general oil at 600 °C to 900 °C. That is, it can be confirmed that the heating rate of the blank 310 coated with an oil containing 10 wt% to 40 wt% of an ester-based compound is faster than that of the blank 320 coated with a general oil.

[0086] The hydrolysis reaction of the ester compound occurs in the section where the blank temperature is 600°C to 900°C. However, the hydrolysis reaction of the ester compound is an endothermic reaction. By increasing the heating furnace temperature in this section to promote the hydrolysis reaction, the inflow of hydrogen into the blank is blocked. As described above with reference to FIG. 1, the plating layer 200 including the first layer 210, the second layer 220, and the intermetallic compound portion 230 provided in an island shape within the second layer 220 can be formed on the base steel plate 100. At this time, the average temperature increase rate of the blank in the section where the temperature of the blank 310 coated with the oil containing 10 wt% to 40 wt% of the ester compound is 600°C to 900°C is also 4.5°C / s to 10°C / s.

[0087] As an example, the oil may contain 10 wt% to 40 wt% of the ester compound. When the oil contains 10 wt% to 40 wt% of the ester compound, the amount of diffusible hydrogen is reduced, the hydrogen delayed fracture performance is improved, and a dense oxide film can be formed by the hydrolysis reaction of the ester compound. When the oil contains less than 10 wt% of the ester compound, the amount of diffusible hydrogen increases, and hydrogen delayed fracture may occur. On the other hand, when the oil contains more than 40 wt% of the ester compound, stains may exist on the surface of the parts manufactured by the method for manufacturing hot stamping parts.

[0088] The blank transfer step (S150) is also a step of transferring the heated blank from the heating furnace to the press die. At this time, in the blank transfer step (S150), 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 buckles) may occur on the surface of the manufactured hot stamping parts. Also, when using a refrigerant, it may affect the subsequent process (hot stamping). Therefore, it is desirable that the heated blank be air-cooled during transfer.

[0089] The formed body forming step (S160) is a step of hot stamping the transferred blank to form a formed body. The formed body cooling step (S170) is a step of cooling the formed body.

[0090] The blank can be formed into the final part shape with a press die and the formed body can be cooled to form the final product. The press die can be provided with cooling channels through which a refrigerant circulates inside. The heated blank can be rapidly cooled by the circulation of the refrigerant supplied through the cooling channels provided in the press die. At this time, in order to prevent the spring back phenomenon of the sheet material and maintain the desired shape, rapid cooling can be carried out while pressurizing the press die in the closed state. When performing the forming and cooling operations on the heated blank, the average cooling rate can be cooled to at least 10 °C / s or more up to the martensite finishing temperature. The blank can be held in the press die for 3 seconds to 20 seconds. If the holding time in the press die is less than 3 seconds, the material cannot be sufficiently cooled, and the embrittlement quality can be affected by the temperature deviation by part due to the residual heat. 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 die exceeds 20 seconds, the holding time in the press die becomes long and the productivity can decrease.

[0091] FIG. 7 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. 7 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. 7, it can be understood that a high heating temperature means a high heating furnace take-out temperature.

[0092] Referring to FIGS. 3 and 7, it can be confirmed that the maximum allowable air cooling time increases as the heating temperature decreases with 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.

[0093] When a heated blank is excessively exposed to room temperature, not only does productivity decrease, but also 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 by starting forming at an excessively high temperature. Also, the plating layer of the blank may adhere to the press die. Therefore, it is necessary to adjust the air cooling time in the blank transfer step (S150). However, to adjust the air cooling time in the blank transfer step (S150), various variables such as not only the heating temperature and the thickness of the blank (e.g., the thickness of the material), but also 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 blank heating furnace extraction temperature and the atmospheric temperature must be considered.

[0094] Therefore, the inventor of the present invention derived a mathematical formula that can easily control the air cooling time through excessive repeated experiments. In one embodiment, the air cooling time of the blank in the blank transfer step (S150) can satisfy the following mathematical formula.

Equation

[0095] a t is the correction coefficient considering the heating furnace extraction temperature and the atmospheric temperature of the heated blank, and has a value of about 0.0160 or more and about 0.0165 or less. At this time, a t may have a unit of s / (°C xmm).

[0096] b tIt takes into account 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 has a value of about 10.0 or more and about 0.5 or less. At this time, b t has the unit of s / mm.

[0097] Also, the heat transfer amount transmitted from inside the material varies depending on the thickness of the material. c t is a correction coefficient considering the heat transfer amount difference due to the material thickness at high temperature, and has a value of about 0.7 or more and about 0.9 or less. At this time, high temperature means 600 °C or more. However, high temperature means 500 °C or more or 700 °C or more.

[0098] Heating temperature T t means the soaking heating temperature in the blank heating stage (S140), and heating temperature T t can have a value of about Ac3 or more and about 1000 °C or less. At this time, heating temperature T t means 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.

[0099] In one embodiment, the air cooling time λ by a mathematical formula 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 the blank is formed at a high temperature, resulting in wrinkles (or buckling) in the manufactured hot stamping part, and it is difficult to implement an air cooling time λ t less than 5 s in terms of equipment. On the other hand, when the air cooling time λ t exceeds 20 s, not only does the productivity decrease, but a phase transformation occurs in the blank during the process of the blank being transferred, resulting in a decrease in the formability of the blank, and the manufactured hot stamping part does 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, it can improve the formability of the blank and the productivity of the process, and enable the manufactured hot stamping part to have the desired material.

[0100] By applying the above-described composition and process conditions, hot stamping parts (members) having delayed fracture resistance performance can be realized. The base layer microstructure of the hot stamping parts 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 of 5% or more.

[0101] 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.

Example

[0102]

Table 1

[0103]

Table 2

[0104] Table 1 is a table showing the composition of the base steel sheet according to the experimental example of the present invention, and Table 2 is a table showing the steel material components, oil conditions, implementation of multi-stage heating, soaking temperature, and heating furnace residence time for the production of hot stamping parts according to the experimental example of the present invention.

[0105] 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.

[0106] Referring to Table 1 and Table 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 and the balance 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 and the balance iron. Hot rolling, cooling / winding, cold rolling, annealing heat treatment, and hot dip plating treatment were performed on the steel materials having the compositions of Example 1 to Example 3 and Comparative Example 1 to Comparative Example 5 under the same conditions.

[0107] On the other hand, in the case of Example 1 to Example 3, Comparative Example 4, and Comparative Example 5, 0.1 g / m 2 of Oil 1 was applied to the surface of the aluminum-silicon (Al-Si) plating layer to form an oil film. In the case of Comparative Example 1 to Comparative Example 3, 0.1 g / m 2 of Oil 2 was applied to the surface of the aluminum-silicon (Al-Si) plating layer to form an oil film. At this time, for Oil 1, the product "Ferrocote 6130" of Quaker was used, and for Oil 2, the commercial product "BW-80HG" of Buhmwoo was used. Oil 1 contains 10 wt% to 40 wt% of polyester, while Oil 2 contains less than 10 wt% of an ester-based compound.

[0108] Next, the blanks of Example 1 to Example 3 with Oil 1 applied to the surface and Comparative Example 1 to Comparative Example 3 with Oil 2 applied to the surface were heated according to the conditions in Table 2, the heated blanks were transferred to a press die to perform press forming, and hot stamping parts were manufactured by cooling at a cooling rate of 10 °C / s or more. Also, the blanks of Comparative Example 4 and Comparative Example 5 with Oil 1 applied to the surface were heated at a uniform temperature (single heating) according to the conditions in Table 2, the heated blanks were transferred to a press die to perform press forming, and hot stamping parts were manufactured by cooling at a cooling rate of 10 °C / s or more.

[0109] <Tensile Strength Test> After manufacturing the hot stamping parts of Example 1 to Example 3 and Comparative Example 1 to Comparative Example 5 in Table 3, the yield strength, tensile strength, and elongation at room temperature were measured.

[0110]

Table 3

[0111] Table 3 is a table showing the yield strength, tensile strength, and elongation at normal temperature of Examples 1 to 3 and Comparative Examples 1 to 5. Referring to Table 3, it can be confirmed 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 is 5% or more. Also, it can be confirmed that the tensile strength of Example 3 is 1,680 MPa or more and the yield strength is 1,000 MPa or more.

[0112] Therefore, even when manufacturing hot stamping parts with a blank coated with an oil (Oil 1) containing 10 wt% to 40 wt% of an ester-based compound on the plating layer, it can have a tensile strength equal to or higher than a preset target (for example, 1,350 MPa or 1,680 MPa).

[0113] <Diffusible hydrogen amount and hydrogen delayed fracture property evaluation> Thermal Desorption Spectroscopy was performed on Examples 1 to 3 and Comparative Examples 1 to 5. 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. Also, a 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 method. The 4 point bending test is a test method for applying a stress at a level below the elastic limit to a specific point of a specimen manufactured to reproduce the state of being exposed to a corrosive environment and checking for the occurrence of stress corrosion cracks. At this time, stress corrosion cracks mean cracks that occur when corrosion and a continuous tensile stress act simultaneously.

[0114] Specifically, Examples 1 and 2 and Comparative Examples 1, 2, and 4, each having a tensile strength of 1,350 MPa or more, are the results of applying a stress of 1,000 MPa in air for 100 hours to each sample to check for the occurrence of fracture. Further, Examples 3, Comparative Example 3, and Comparative Example 5, each having a tensile strength of 1,680 MPa or more, are the results of applying a stress of 1,200 MPa in air for 100 hours to each sample to check for the occurrence of fracture.

[0115]

Table 4

[0116] 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 from Examples 1 to 3 and Comparative Examples 1 to 5 and the results of the hydrogen delayed fracture evaluation.

[0117] Referring to Figure 8A and Table 4, it can be confirmed that the amount of diffusible hydrogen released from Example 1 at a temperature of 300°C or lower is less than the amount of diffusible hydrogen released from Comparative Example 1. Further, as a result of the hydrogen delayed fracture evaluation, no fracture occurred in Example 1, but fracture occurred in Comparative Example 1.

[0118] Referring to Figure 8B and Table 4, it can be confirmed that the amount of diffusible hydrogen released from Example 2 at a temperature of 300°C or lower is less than the amount of diffusible hydrogen released from Comparative Example 2. Further, as a result of the hydrogen delayed fracture evaluation, no fracture occurred in Example 2, but fracture occurred in Comparative Example 2.

[0119] Referring to Figure 8C and Table 4, it can be confirmed that the amount of diffusible hydrogen released from Example 3 at a temperature of 300°C or lower is less than the amount of diffusible hydrogen released from Comparative Example 3. Further, as a result of the hydrogen delayed fracture evaluation, no fracture occurred in Example 3, but fracture occurred in Comparative Example 3.

[0120] Therefore, when applying oil (Oil 1) containing an ester compound to the surface of the plating layer, the amount of hydrogen inflow from the outside decreases, and as a result, it has been shown to be excellent in resistance to hydrogen delayed fracture.

[0121] Also, the amount of diffusible hydrogen released from Example 1 and Example 3 that performed multi-stage heating was less than the amount of diffusible hydrogen released from Comparative Example 4 and Comparative Example 5 that performed single-stage heating, respectively. As a result of the hydrogen delayed fracture evaluation, no fracture occurred in Example 1 and Example 3, but fracture occurred in Comparative Example 4 and Comparative Example 5.

[0122] Therefore, when performing multi-stage heating, it has been shown that the amount of hydrogen inflow from the outside decreases compared to the case of performing single-stage heating, and as a result, it is excellent in resistance to hydrogen delayed fracture.

[0123] <Observation of the thickness of the surface layer> FIGS. 9A and 9B are photographs showing the surface layers of Example 1 and Comparative Example 1, respectively. FIGS. 9A and 9B are drawings showing the results of measuring the cross-sections of Example 1 and Comparative Example 1 by TEM.

[0124] Referring to FIGS. 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 judged to be caused by the difference in the heating rate of the blank of Example 1 and the heating rate of the blank of Comparative Example 1 as shown in FIG. 6. Since the surface layer 240 of Example 1 is provided thicker than the surface layer 450 of Comparative Example 1, when oil (Oil 1) containing an ester compound is applied on the plating layer, the base steel plate 100 can be more effectively prevented from being oxidized.

[0125] <Weldability evaluation> Weldability evaluation was performed on Example 1 and Comparative Example 1. In the weldability evaluation, a welding tip of 6 mm was applied to the welded part with a pressure of 350 kgf at 930° C. for 4 minutes, and the contact resistance was measured while applying current.

[0126] Figure 10 is a graph showing the resistance measurement results during spot welding for Example 1 and Comparative Example 1.

[0127] Referring to Figure 10, it can be confirmed that the contact resistance of Example 1 is lower than that of Comparative Example 1. In particular, it can be confirmed that the contact resistance of Example 1 around 5 ms before the start 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 confirmed that the hot stamping parts of Example 1 are superior in weldability compared to the hot stamping parts of Comparative Example 1.

[0128] <Corrosion Resistance Evaluation> A corrosion resistance evaluation experiment was conducted on the hot stamping parts of Example 1 and Comparative Example 1. For the corrosion resistance evaluation, a specimen was used as the working electrode, a high-purity carbon rod was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode to construct a three-electrode electrochemical cell and conduct a potentiodynamic polarization test. The potentiodynamic polarization test was carried out after measuring the open-circuit potential (OCP) in a 3.5% NaCl solution for 10 hours to confirm electrochemical stabilization, and then a potential was applied at a scanning rate of 0.166 mV / s from -250 mV relative to the corrosion potential (Ecorr) to 0 mV SCE.

[0129] Figure 11 is a graph showing the results of the corrosion resistance evaluation experiment for Example 1 and Comparative Example 1. The graph in Figure 11 shows the corrosion current measurement results of Example 1 and Comparative Example 1, and the corrosion current is a numerical value corresponding to the current density at the time when a branch of the steadily maintained potential (Potential) occurs.

[0130] Referring to Figure 11, the corrosion current of Example 1 was measured to be 5×10 -4 A, and the corrosion current of Comparative Example 1 was 5.5×10 -4It was measured as A. Thereby, it can be confirmed that the current density related to the corrosion of Example 1 is lower than the current density related to the corrosion of Comparative Example 1. Therefore, it can be confirmed that the corrosion resistance of Example 1 is excellent compared to the corrosion resistance of Comparative Example 1. As described above, although the present invention has been described based on one embodiment illustrated in the drawings, 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 Signs

[0131] 100 Base steel plate 200 Plating layer 210 First layer 220 Second layer 230 Intermetallic compound part 240 Surface layer

Claims

1. immersing a base steel plate in a plating bath containing aluminum and silicon to produce a plated steel plate; applying an oil containing an ester-based compound onto the plated steel plate; cutting the plated steel plate with the applied oil to form a blank; heating the blank in a heating furnace; 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, the air cooling time of the blank 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 extraction 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 oil is applied onto the plated steel sheet at 0.1 g / m ２ to 10 g / m ２ The manufacturing method of the hot stamping part according to claim 1, wherein the oil is applied.

3. The method for manufacturing a hot stamping part according to claim 2, wherein the oil contains 10 wt% to 30 wt% of hydrogen-treated 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.

4. In the step of heating the blank in a heating furnace, the heating furnace includes a plurality of sections having different temperature ranges, and the blank is heated step by step in the heating furnace. The method for manufacturing a hot stamping part according to claim 1.

5. In the step of heating the blank in a heating furnace, at least two blanks having different thicknesses are simultaneously transferred in the heating furnace. The method for manufacturing a hot stamping part according to claim 4.

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