Hot Stamping Parts

Abstract

TECHNICAL FIELD The present application relates to a hot-stamped part and a manufacturing method thereof. According to the hot-stamped part and the manufacturing method thereof of the present application, it is possible to obtain a hot-stamped part that is suppressed in hydrogen embrittlement and has ultra-high strength.

Description

[Technical Field]

[0001] This application relates to hot stamped parts and methods of making same. [Background technology]

[0002] In recent years, environmental and fuel efficiency regulations and safety standards have been strengthened in the automotive industry. As a result, the application rate of ultra-high strength steel and hot stamping steel has been steadily increasing. In particular, research and development is being conducted on hot stamping steel to improve its toughness and strength, including the existing 1.5G hot stamping steel.

[0003] The hot stamping process is a technology that applies ultra-high strength to the microstructure based on the phase transformation of the material during the process, and is generally performed in the following steps: heating, forming, cooling, and trimming.

[0004] Unlike conventional press forming, which is performed in a cold state, the hot stamping process is usually performed at high temperatures of over 900°C, so aluminum (Al) alloy-plated steel sheets are generally used to ensure the high-temperature stability of the body components and for surface treatment. Among Al alloys, Al-Si, Al-Cu, and Al-Zn alloys have good mechanical properties such as fluidity, shrinkage during solidification, and corrosion resistance, and are widely used as structural castings and coating materials for aircraft and automobiles, and are also widely used as materials for hot stamping.

[0005] Meanwhile, during the hot stamping process, high-temperature heat treatment using a heating furnace involves the decomposition of moisture in the air, resulting in the formation of hydrogen. In this case, as in Patent Document 1 (Korean Patent Publication No. 2012-0134709), hot-stamped steel sheets coated with aluminum-silicon (Al-Si) transform into a liquid state due to their low melting point during high-temperature heat treatment, resulting in the incorporation of a large amount of hydrogen from the surface. After cooling, the coating transforms into an Al-Fe intermetallic compound layer, trapping the hydrogen that had infiltrated into the material. Over time, the infiltrated hydrogen accumulates in one location, resulting in hydrogen embrittlement. Therefore, to address this issue, there is a need for hot-stamped parts with ultra-high strength that suppress hydrogen embrittlement. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a hot stamped part that is suppressed in hydrogen embrittlement and has ultra-high strength, and a method for manufacturing the same. [Means for solving the problem]

[0007] In order to solve the above problems, the hot stamped part of the present application comprises a steel sheet and a zinc-containing plating layer formed on at least one surface of the steel sheet, which satisfies the following general formula 1:

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[0008] In addition, the plating layer can satisfy the following general formula 3.

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[0009] In addition, the plating layer can satisfy the following general formula 4.

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[0010] The hot stamped part may be manufactured by hot stamping a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet.

[0011] Furthermore, the hot stamping forming can include a heating step of preparing a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet and heating the prepared plated steel sheet, a forming step of stamping the heated plated steel sheet with a press die to form a formed body, and a cooling step of cooling the formed body.

[0012] Furthermore, when the prepared plated steel sheet is heated in the heating step, the plated steel sheet may be heated in a temperature range of 600°C to 700°C at an average temperature increase rate of 3°C / s to 8°C / s.

[0013] Furthermore, when the prepared plated steel sheet is heated in the heating step, the plated steel sheet may be heated at an average temperature increase rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to X°C, where X°C is the maximum temperature to which the plated steel sheet is heated in the heating step minus 10°C.

[0014] In addition, the total heating time during the hot stamping molding may be 150 seconds to 450 seconds.

[0015] Furthermore, the plating layer may have a thickness of 10 μm to 30 μm.

[0016] The present application also provides a method for producing a hot stamped part, which includes a steel sheet and a zinc-containing plating layer formed on at least one surface of the steel sheet, the zinc-containing plating layer satisfying General Formula 1 below. The method includes the steps of: preparing a plated steel sheet having the zinc-containing plating layer formed on at least one surface of the steel sheet; heating the prepared plated steel sheet; stamping the heated plated steel sheet with a press die to form a formed body; and cooling the formed body.

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[0017] Furthermore, when the prepared plated steel sheet is heated in the heating step, the plated steel sheet may be heated in a temperature range of 600°C to 700°C at an average temperature increase rate of 3°C / s to 8°C / s.

[0018] Furthermore, when the prepared plated steel sheet is heated in the heating step, the plated steel sheet may be heated at an average temperature increase rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to X°C, where X°C is the maximum temperature to which the plated steel sheet is heated in the heating step minus 10°C.

[0019] In addition, the plating layer formed on the plated steel sheet prepared in the heating step may include a zeta layer, a delta layer, and a gamma layer. [Effects of the Invention]

[0020] According to the hot-pressed part and the manufacturing method thereof of the present application, it is possible to obtain a hot-pressed part that is suppressed in hydrogen embrittlement and has ultra-high strength. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is an exemplary diagram of a hot stamped part according to one embodiment of the present application.

[0022] [Figure 2] FIG. 2 is a scanning electron microscope image of the hot stamped part produced in Example 2 at the maximum heating temperature.

[0023] [Figure 3] FIG. 3 shows images taken with a camera of the fracture test evaluation results of the hot stamped parts manufactured in Example 2, Comparative Example 1, and Comparative Example 2, respectively.

[0024] [Figure 4] FIG. 4 shows images taken with a camera of the fracture test evaluation results of the hot stamped parts manufactured in Example 2, Comparative Example 1, and Comparative Example 2, respectively.

[0025] [Figure 5] FIG. 5 shows images taken with a camera of the fracture test evaluation results of the hot stamped parts manufactured in Example 2, Comparative Example 1, and Comparative Example 2, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, the hot stamping part of the present application will be described with reference to the accompanying drawings, which are for illustrative purposes only and are not intended to limit the scope of the hot stamping part of the present application.

[0027] 1 is a diagram illustrating an example of a hot stamped part according to one embodiment of the present application. As shown in FIG. 1, the hot stamped part 10 of the present application includes a steel sheet 11 and a plating layer 12. The hot stamped part 10 of the present application can suppress hydrogen embrittlement and have ultra-high strength.

[0028] The steel plate 11 refers to a steel plate used for automobile bodies. For example, the steel plate 11 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), boron (B), the balance being iron (Fe), and other unavoidable impurities. Specifically, the steel plate 11 may contain 0.1 to 0.5 weight percent carbon (C), 0.1 to 0.8 weight percent silicon (Si), 0.3 to 3.0 weight percent manganese (Mn), more than 0 to 0.05 weight percent phosphorus (P), more than 0 to 0.03 weight percent sulfur (S), 0.0005 to 0.005 weight percent boron (B), the balance being iron (Fe), and other unavoidable impurities.

[0029] The steel sheet 11 may further contain one or more selected from titanium (Ti) and / or niobium (Nb), chromium (Cr), molybdenum (Mo), and nickel (Ni). Specifically, the steel sheet 11 may further contain 0.01 to 0.1 wt% of titanium (Ti) and / or niobium (Nb), 0.01 to 1.0 wt% of chromium (Cr), 0.01 to 1.0 wt% of molybdenum (Mo), and 0.001 to 1.0 wt% of nickel (Ni).

[0030] Carbon (C) is a major element that determines the strength and hardness of steel and may be added to ensure the tensile strength of the steel after hot stamping or hot pressing. Carbon (C) may also be added to ensure the hardenability characteristics of the steel. If the carbon (C) content in the steel sheet 11 is less than the above-mentioned amount, it may be difficult to achieve the desired mechanical strength. On the other hand, if the carbon (C) content in the steel sheet 11 is greater than the above-mentioned amount, it may cause problems such as reduced toughness or brittleness control.

[0031] Silicon (Si) can act as a ferrite stabilizer in the steel sheet 11. Silicon (Si) improves softness by purifying ferrite and enhances carbon concentration in austenite by suppressing the formation of carbides in the low-temperature region. Silicon (Si) can be a key element for homogenizing the structure during hot rolling, cold rolling, and hot stamping (controlling pearlite and manganese segregation zones) and for finely dispersing ferrite. If the silicon (Si) content in the steel sheet 11 is less than the above-mentioned amount, the above-mentioned functions may not be fully achieved. On the other hand, if the silicon (Si) content in the steel sheet 11 is greater than the above-mentioned amount, the hot rolling and cold rolling loads may increase, excessive hot-rolling red scale may occur, and bondability may be impaired.

[0032] Manganese (Mn) may be added to increase hardenability and strength during heat treatment. If the manganese (Mn) content in the steel sheet 11 is less than the above-mentioned amount, the hardenability may be insufficient, and the material properties after hot stamping, for example, the hard phase fraction, may be insufficient. On the other hand, if the manganese (Mn) content in the steel sheet 11 is more than the above-mentioned amount, the softness and toughness may be reduced due to manganese segregation or pearlite bands, which may cause a decrease in bending performance and the occurrence of a heterogeneous microstructure.

[0033] Phosphorus (P) is an element that is prone to segregation and may impair the toughness of steel. When the phosphorus (P) is contained in the steel sheet 11 in the amount described above, it is possible to prevent a decrease in the toughness of the steel. On the other hand, when the phosphorus (P) is contained in the steel sheet 11 in an amount exceeding the amount described above, cracks may occur during processing, and iron phosphide compounds may be formed, which may reduce the toughness of the steel.

[0034] Sulfur (S) can be an element that impairs workability and physical properties. If the sulfur (S) content in the steel sheet 11 exceeds the above-mentioned amount, hot workability may be reduced, and large inclusions may be formed, resulting in surface defects such as cracks.

[0035] Boron (B) is added to ensure the hardenability and strength of the steel by ensuring a martensite structure, and also has the effect of increasing the growth temperature of austenite grains, thereby reducing the grain size. When boron (B) is contained in the steel sheet 11 in the above-mentioned amount, it is possible to prevent the occurrence of brittleness in the hard phase grains and ensure high toughness and bendability.

[0036] Titanium (Ti) can be added to strengthen hardenability and improve material properties by forming precipitates after heat treatment in hot stamping. Titanium (Ti) also forms precipitate phases such as Ti(C,N) at high temperatures, effectively contributing to the refinement of austenite grains.

[0037] Niobium (Nb) can be added for the purpose of increasing strength and toughness by reducing the packet size of martensite.

[0038] Chromium (Cr) can be added to improve the hardenability and strength of steel. When chromium (Cr) is contained in the steel sheet 11 in the above-mentioned amount, the hardenability and strength of the steel can be improved, an increase in production costs can be prevented, and a decrease in the toughness of the steel can be prevented.

[0039] Molybdenum (Mo) can contribute to improving strength by suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot stamping. When contained in the steel sheet 11 in the above-mentioned amount, molybdenum (Mo) is excellent in suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot stamping.

[0040] Nickel (Ni) may be added to ensure hardenability and strength. Nickel (Ni) is an austenite-stabilizing element, and can contribute to improving elongation by controlling austenite transformation. If nickel (Ni) is contained in the steel sheet 11 in an amount less than the above-mentioned amount, the above-mentioned effects may not be fully realized. If nickel (Ni) is contained in the steel sheet 11 in an amount exceeding the above-mentioned amount, the toughness may be reduced, the cold workability may be reduced, and the manufacturing cost of the product may be increased.

[0041] As an example, the steel sheet 11 may have a microstructure containing 90% or more of martensite. Specifically, the steel sheet 11 may have a microstructure containing 90% or more of martensite, with the remainder being other unavoidable structures and other precipitates being less than 10%. Preferably, the steel sheet 11 may be full martensite. By having the above-mentioned microstructure, the steel sheet 11 can ensure ultra-high strength.

[0042] The steel plate 11 may have a tensile strength (TS) of 1,350 MPa or more, for example, 1,680 MPa to 2,300 MPa, a yield stress (YP) of 900 MPa to 1,300 MPa, for example, 1,150 MPa to 1,300 MPa, and an elongation (EL) of 4% to 15%.

[0043] The coating layer 12 is a layer to be coated on the surface of the steel sheet 11, is formed on at least one surface of the steel sheet 11, satisfies the following general formula 1, and contains zinc. In this specification, "surface" refers to a surface located on one side, both sides, or all sides of the steel sheet. For example, one surface of the steel sheet 11 may be the top surface of the steel sheet 11.

[0044]

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[0045] <Electrochemical hydrogen permeation experiment method> - A specimen made of the kth layer is placed in the center of two cells, and hydrogen is electrically generated in one cell, allowing the hydrogen to permeate to the other cell. - Measure the time it takes for hydrogen to permeate to the other cell. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D k Let's say.

[0046]

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[0047] Furthermore, by satisfying the above general formula 1, the coating layer 12 can suppress the amount of diffusible hydrogen to less than 0.5 ppm, thereby suppressing hydrogen embrittlement of the hot stamped part and enabling it to have ultra-high strength. On the other hand, if the coating layer 12 does not satisfy the above general formula 1, the amount of hydrogen that flows into the steel sheet of the hot stamped part, i.e., the amount of diffusible hydrogen, will increase, and hydrogen embrittlement may increase.

[0048] In this specification, the "kth layer in the coating layer" refers to the kth layer in the thickness direction from the surface of the coating layer toward the steel sheet. For example, when n = 3, the kth layer in the coating layer can be the first layer, the second layer, or the third layer. In addition, in this specification, the "shortening direction" refers to the stacking direction in which the steel sheet and the coating layer are stacked, based on the hot stamped part.

[0049] In another example, the plating layer 12 may satisfy the following general formula 3:

[0050]

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[0051] <Electrochemical hydrogen permeation experiment method> - A specimen made of the kth layer is placed in the center of two cells, and hydrogen is electrically generated in one cell, allowing the hydrogen to permeate to the other cell. - Measure the time it takes for hydrogen to permeate to the other cell. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D k Let's say.

[0052]

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[0053] Furthermore, by satisfying the above general formula 3, the coating layer 12 can suppress the amount of diffusible hydrogen to less than 0.1 ppm, thereby further suppressing hydrogen embrittlement of the hot stamped part and enabling it to have ultra-high strength. On the other hand, if the coating layer 12 exceeds the upper limit of the above general formula 3, the amount of hydrogen that flows into the steel sheet of the hot stamped part increases, and hydrogen embrittlement may increase.

[0054] In yet another example, the plating layer 12 may satisfy the following general formula 4:

[0055]

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[0056] <Electrochemical hydrogen permeation experiment method> - A specimen made of the kth layer is placed in the center of two cells, and hydrogen is electrically generated in one cell, allowing the hydrogen to permeate to the other cell. - Measure the time it takes for hydrogen to permeate to the other cell. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D k Let's say.

[0057]

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[0058] When the coating layer 12 satisfies the above general formula 4, hydrogen embrittlement of the hot stamped part can be further suppressed and ultra-high strength can be achieved. On the other hand, if the coating layer 12 exceeds the upper limit of the above general formula 4, the amount of hydrogen that flows into the steel sheet of the hot stamped part increases, resulting in increased hydrogen embrittlement. Also, if the coating layer 12 is below the lower limit of the above general formula 4, the total heating time becomes too short or the thickness of the coating layer 12 becomes too thin, making it impossible to ensure the desired material properties or reducing corrosion resistance.

[0059] As described above, the coating layer 12 may be a zinc-based coating layer containing zinc. For example, the zinc-based coating layer formed on the steel sheet may be at least one of a galvanized iron (GI) steel sheet, an electrogalvanized iron (EGI) steel sheet, and a galvannealed iron (GA) steel sheet. Specifically, the coating layer 12 may contain iron (Fe), aluminum (Al), manganese (Mn), the balance being zinc (Zn), and other unavoidable impurities. More specifically, the coating layer 12 may contain 10 wt% to 70 wt% iron (Fe), 0 wt% to 5 wt% aluminum (Al), 0 wt% to 1 wt% manganese (Mn), the balance being zinc (Zn), and other unavoidable impurities. Because the coating layer 12 is a zinc-based coating layer containing the above-described composition, the hot stamped part may have ultra-high strength.

[0060] The plating layer 12 may have a thickness of 10 μm to 30 μm. By having such a thickness, the plating layer 12 can protect the surface of the steel sheet 11 and simultaneously prevent or minimize a decrease in the toughness of the hot stamped component 10. The plating layer 12 satisfies the above-described general formula 1, general formula 3, or general formula 4, thereby suppressing hydrogen embrittlement of the hot stamped component 10 and enabling the hot stamped component 10 to have ultrahigh strength. If the thickness of the plating layer 12 is less than the above-described lower limit, the sacrificial corrosion protection effect specific to zinc may be reduced, and the hot stamped component 10 may not satisfy the above-described general formula 1, general formula 3, or general formula 4. Furthermore, if the thickness of the plating layer 12 exceeds the above-described upper limit, the toughness of the hot stamped component 10 including the plating layer 12 may be reduced.

[0061] In one example, the hot stamped part 10 may be a part manufactured by hot stamping a plated steel sheet (not shown) having a plating layer containing zinc formed on at least one surface thereof.

[0062] Specifically, the hot stamping molding may include a heating step, a molding step, and a cooling step.

[0063] The heating step is a step of preparing a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet 11, and heating the prepared plated steel sheet, and can be performed in a heating furnace having a specific temperature range. For example, the temperature range of the heating furnace may be Ac1 to 950°C, specifically Ac3 to 950°C, or 860°C to 920°C.

[0064] In one example, when the plated steel sheet prepared in the heating step is heated, the plated steel sheet may be heated at an average heating rate of 3°C / s to 8°C / s in a temperature range of 600°C to 700°C. Specifically, the plated steel sheet may be heated at an average heating rate of 4°C / s to 7°C / s or 5°C / s to 6°C / s in the aforementioned temperature range. By heating the plated steel sheet at the aforementioned average heating rate in the aforementioned temperature range, it is possible to satisfy the aforementioned general formula 1, general formula 3, or general formula 4, thereby suppressing hydrogen embrittlement and obtaining a hot stamped part with ultra-high strength.

[0065] In another example, when the plated steel sheet prepared in the heating step is heated, the plated steel sheet may be heated at an average heating rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to X°C. In this case, X°C is the maximum temperature to which the plated steel sheet is heated in the heating step minus 10°C. For example, if the maximum temperature to which the plated steel sheet is heated is 860°C to 920°C, X°C may be 850°C to 910°C. By heating the plated steel sheet in the aforementioned temperature range at the aforementioned average heating rate, the plated steel sheet can satisfy the aforementioned general formula 1, general formula 3, or general formula 4, thereby suppressing hydrogen embrittlement and providing a hot stamped part with ultra-high strength.

[0066] During the hot stamping, the total heating time, i.e., the total heating time for the heating step, may be 150 seconds to 450 seconds. Specifically, the total heating time during the hot stamping may be 165 seconds to 410 seconds, 180 seconds to 370 seconds, 195 seconds to 330 seconds, 210 seconds to 290 seconds, or 225 seconds to 240 seconds. By heating the plated steel sheet for the heating time described above during hot stamping, the above-described general formula 1, general formula 3, or general formula 4 can be satisfied, thereby suppressing hydrogen embrittlement and obtaining a hot stamped part with ultrahigh strength.

[0067] The forming step is a step of forming the plated steel sheet heated in the heating step into a formed body, and can be performed by stamping the plated steel sheet with a press die. For example, the forming start temperature in the forming step can be 500°C to 700°C. When the forming start temperature in the forming step satisfies the above-mentioned range, the formability of the plated steel sheet can be improved, the resulting hot stamped part 10 can have the desired structure and physical properties, and the occurrence of wrinkles or bends on the surface of the resulting hot stamped part 10 can be prevented or minimized. On the other hand, when the forming start temperature in the forming step is below the above-mentioned range, the formability of the plated steel sheet can be reduced, and the resulting hot stamped part 10 can fail to have the desired structure and physical properties. Furthermore, when the forming start temperature in the forming step exceeds the above-mentioned range, the resulting hot stamped part 10 can have wrinkles or bends on the surface, and the plating layer 12 can be seized onto the die.

[0068] The cooling step is a step of cooling the formed body formed in the forming step. The cooling step may be performed in a press mold that presses the plated steel sheet.

[0069] Specifically, the press die may be used to form the final part shape, and the formed body may be cooled to form the final product. The die may have cooling channels through which a refrigerant circulates. The refrigerant supplied through the cooling channels may be circulated to rapidly cool the formed body. Rapid cooling may be performed while the die is closed and pressurized to prevent spring back of the sheet material and maintain the desired shape. During the forming and cooling of the formed body, an average cooling rate of 10°C / s or more may be maintained up to the martensitic transformation finish temperature (Mf). If the cooling end temperature in the cooling step satisfies the above-described range, productivity of the manufacturing process may be improved and distortion of the manufactured hot stamped part 10 may be prevented or minimized. On the other hand, the cooling end temperature at which the cooling step is completed may be 25°C to 200°C. If the cooling end temperature in the cooling step is below the above-described range, productivity of the manufacturing process may be reduced. Furthermore, if the cooling end temperature in the cooling step exceeds the above-mentioned range, the hot stamped part 10 may become twisted, and it may be difficult to obtain the desired material quality.

[0070] The present application also relates to a method for manufacturing a hot stamped part, which relates to a method for manufacturing the hot stamped part described above. Details of the hot stamped part described below are omitted because the same applies to the hot stamped part described above.

[0071] The method for producing a hot stamped part of the present application relates to a method for producing a hot stamped part including a steel sheet and a zinc-containing coating layer formed on at least one surface of the steel sheet, the coating layer satisfying General Formula 1 below, and the method includes a heating step, a forming step, and a cooling step.

[0072]

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[0073] <Electrochemical hydrogen permeation experiment method> - A specimen made of the kth layer is placed in the center of two cells, and hydrogen is electrically generated in one cell, allowing the hydrogen to permeate to the other cell. - Measure the time it takes for hydrogen to permeate to the other cell. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D k Let's say.

[0074]

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[0075] The specific description of the configuration and manufacturing method of the hot stamped part is omitted here because they are the same as those described in the hot stamping section. The hot stamped part manufactured by the manufacturing method of the hot stamped part satisfies General Formula 1 as described above, thereby suppressing hydrogen embrittlement and achieving ultra-high strength.

[0076] The entire plating layer formed on the plated steel sheet prepared in the heating step may contain 10% to 70% by weight of iron (Fe), with the remainder being zinc (Zn) and other unavoidable impurities.

[0077] The thickness of the coating layer formed on the plated steel sheet prepared in the heating step may be 5 μm to 20 μm, specifically 8 μm to 18 μm or 10 μm to 15 μm. The coating layer formed on the plated steel sheet prepared in the heating step has the aforementioned thickness, which can protect the surface of the steel sheet and simultaneously prevent or minimize a decrease in toughness of the hot stamped part manufactured by the aforementioned manufacturing method. If the thickness of the coating layer formed on the plated steel sheet prepared in the heating step is less than the aforementioned range, the sacrificial corrosion protection effect unique to zinc may be reduced. Furthermore, if the thickness of the coating layer formed on the plated steel sheet prepared in the heating step exceeds the aforementioned range, the toughness of the hot stamped part including the coating layer manufactured by the aforementioned manufacturing method may be reduced.

[0078] In one example, the plating layer formed on the plated steel sheet prepared in the heating step may include a zeta phase, a delta phase, and a gamma phase after alloying by including the composition as described above.

[0079] Because the plated steel sheet prepared in the heating step includes the layers described above, the hot stamped part manufactured through the method for manufacturing a hot stamped part can satisfy General Formula 1, General Formula 3, or General Formula 4, thereby suppressing hydrogen embrittlement and achieving ultrahigh strength. The order in which the zeta layer, delta layer, and gamma layer are laminated in the coating layer formed on the plated steel sheet prepared in the heating step is not particularly limited. For example, the coating layer formed on the plated steel sheet prepared in the heating step can include a zeta layer, delta layer, and gamma layer, in that order, along the thickness direction from the surface toward the steel sheet.

[0080] The zeta layer is a layer that exhibits a zeta phase, and may contain 3% to 8% by weight of iron (Fe), with the remainder being zinc (Zn) and other unavoidable impurities.

[0081] The thickness of the zeta layer may be greater than 0 to 4 μm, specifically greater than 0 to 3 μm, or greater than 0 to 2 μm. When the zeta layer has the above-mentioned thickness, it may be more advantageous that the hot stamped part satisfies the above general formula 1, general formula 3, or general formula 4.

[0082] The delta layer is a layer representing the delta phase, and may contain 8% to 12% by weight of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities.

[0083] The thickness of the delta layer may be greater than 3 μm to 10 μm, specifically greater than 6 μm to 10 μm, or greater than 8 μm to 9 μm. It may be more advantageous for the delta layer to have such a thickness so that the hot stamped part satisfies the above-mentioned general formula 1, general formula 3, or general formula 4.

[0084] The gamma layer is a layer that exhibits a gamma phase and may contain 19% to 32% by weight of iron (Fe), with the remainder being zinc (Zn) and other unavoidable impurities.

[0085] The gamma layer may have a thickness of 1 μm to 6 μm, specifically 1 μm to 5 μm or 1 μm to 4 μm. When the gamma layer has the above thickness, it may be more advantageous that the hot stamped part satisfies the above general formula 1, general formula 3, or general formula 4.

[0086] In one example, a method for producing a galvanized steel sheet having a coating layer including a layer having the above-described structure on the steel sheet includes immersing a steel sheet in a coating bath to produce a hot-dip galvanized steel sheet having a coating layer including a layer having the above-described structure formed on the steel sheet, and alloying the hot-dip galvanized steel sheet to produce a galvannealed steel sheet. The coating bath may contain less than 0.3 wt. % aluminum, less than 0.1 wt. % iron, the balance being zinc and other unavoidable impurities. The coating layer may be deposited on both sides of the steel sheet to a thickness of 10 μm to 15 μm. The difference between the temperature of the steel sheet immediately before alloying and the alloying temperature may be 80°C or more. When the difference between the temperature of the steel sheet immediately before alloying and the alloying temperature satisfies the above-mentioned range, it is possible to ensure that the coating layer formed on the steel sheet contains zeta, delta, and gamma layers, and to satisfy the above-mentioned coating layer thickness, thereby satisfying General Formula 1, General Formula 3, or General Formula 4, thereby suppressing hydrogen embrittlement and providing a hot stamped part with ultra-high strength.

[0087] In one example, when the plated steel sheet prepared in the heating step is heated, the steel sheet may be heated at an average temperature rise rate of 3°C / s to 8°C / s in a temperature range of 600°C to 700°C. A detailed description of the average temperature rise rate of the steel sheet in the above temperature range when the plated steel sheet prepared in the heating step is the same as that described above in connection with hot stamping, and therefore will be omitted here.

[0088] Furthermore, when heating the plated steel sheet prepared in the heating step, the plated steel sheet may be heated at an average temperature rise rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to 900°C. When heating the plated steel sheet prepared in the heating step, a detailed description of the average temperature rise rate of the plated steel sheet in the above temperature range is the same as that described above in connection with hot stamping, and therefore will not be repeated here.

[0089] The present application will be described in more detail below through examples according to the present application and comparative examples not according to the present application, but the scope of the present application is not limited to the examples presented below.

[0090] Example 1 Hot stamping parts manufacturing A 1.2 mm thick steel sheet containing 0.29 wt% carbon, 0.2 wt% silicon, 1.5 wt% manganese, up to 0.02 wt% phosphorus, up to 0.015 wt% sulfur, 0.2 wt% chromium, 0.0025 wt% boron, 0.035 wt% titanium, with the remainder being iron and other unavoidable impurities, was immersed in a coating bath to form a coating layer consisting of a zeta layer, a delta layer, and a gamma layer on both the top and bottom surfaces of the steel sheet in the thickness direction from the surface toward the steel sheet. The galvannealed hot-dip coating was carried out by immersing the steel sheet in the coating bath, adjusting the coating weight with an air knife, and then alloying the steel sheet to an average coating thickness of 12 μm. Specifically, the coating bath contained 0.13 wt. % aluminum, 0.06 wt. % iron, and the remainder zinc and other unavoidable impurities. The temperature of the coating bath was set to 450°C. The steel sheet was passed through the coating bath at a speed of 120 mpm until molten zinc adhered to both sides of the uppermost portion of the steel sheet. The steel sheet was then charged into a galvannealing furnace at 530°C, where iron atoms constituting the steel sheet diffused into the coating layer, alloying the steel sheet with the molten zinc to produce a galvannealed steel sheet. The position of the galvannealing furnace was adjusted so that the temperature of the steel sheet immediately before charging was 420°C during the galvannealing heating, and the temperature difference between the steel sheet and the galvannealing furnace was 80°C or more. The time for maintaining the alloying temperature, i.e., the alloying time, was 30 seconds.

[0091] Thereafter, the manufactured plated steel sheet was prepared and heated for 450 seconds in a heating furnace with a maximum temperature of 900°C. The heating furnace was a multi-stage heating furnace configured to include multiple heating zones in which the temperature gradually increased from the inlet where the plated steel sheet was charged to the outlet where the heated plated steel sheet was discharged, and the furnace temperature in each heating zone was controlled so that the plated steel sheet was heated at an average heating rate of 7.20°C / s in the temperature zone from 600°C to 700°C, and at an average heating rate of 2.22°C / s in the temperature zone from 800°C to 890°C.

[0092] The heated plated steel sheet was then stamped using a press to form a compact, which was then rapidly cooled to below 300°C at an average cooling rate of 10°C / s to produce a hot stamped part. The physical properties of the produced hot stamped part are shown in Table 1 below.

[0093] Example 2 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 5.20°C / s and the average heating rate in the temperature section from 800°C to 890°C was 1.21°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0094] Example 3 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 4.10°C / s and the average heating rate in the temperature section from 800°C to 890°C was 0.86°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0095] Example 4 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 3.00°C / s and the average heating rate in the temperature section from 800°C to 890°C was 0.53°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0096] Comparative Example 1 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 3.50°C / s and the average heating rate in the temperature section from 800°C to 890°C was 0.30°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0097] Comparative Example 2 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 2.91°C / s and the average heating rate in the temperature section from 800°C to 890°C was 0.52°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0098] Comparative Example 3 Hot stamping parts manufacturing A hot stamped part was manufactured in the same manner as in Example 1, except that the average heating rate in the temperature section from 600°C to 700°C was 2.98°C / s and the average heating rate in the temperature section from 800°C to 890°C was 0.45°C / s. The physical properties of the manufactured hot stamped part are shown in Table 1 below.

[0099] Evaluation example 1. Evaluation of plating layer shape analysis The coating layers of the hot stamped parts manufactured in the examples and comparative examples were photographed using a scanning electron microscope (SEM), and the results are shown in Table 1. In particular, an image of the coating layer of the hot stamped part manufactured in Example 2 photographed using a scanning electron microscope is shown in Figure 2. The structure formed in the coating layer was described in the thickness direction from the surface toward the steel sheet.

[0100] Evaluation example 2: Evaluation of electrochemical hydrogen permeation experiments For the hot stamped parts manufactured in the examples and comparative examples, the structure of each layer (n layers) formed on the plating layer was confirmed, and a specimen consisting of each structure was manufactured. Each manufactured specimen was placed in the center of two cells, and hydrogen was electrically generated in one cell so that the hydrogen permeated to the other cell. The time it took for the hydrogen to permeate to the other cell was then measured. Using the measured time and the thickness of the specimen, D was calculated using the following general formula 2: k The value was calculated.

[0101] The specimens of the examples and comparative examples were measured using the following general formula 2: k The values ​​are 8 x 10 -14 m 2 / s~8×10 -11 m 2 / s.

[0102]

number

[0103] Evaluation example 3: Evaluation of whether general formula 3 is satisfied Using the results of Evaluation Examples 1 and 2 above, it was evaluated whether the hot stamped parts manufactured in the Examples and Comparative Examples satisfied the following General Formula 3, and the results are shown in Table 1 below.

[0104]

number

[0105] [Table 1]

[0106] As shown in Table 1 above, it was confirmed that the hot stamped parts manufactured in Examples 1 to 4 satisfied General Formula 3. In contrast, it was confirmed that the hot stamped parts manufactured in Comparative Examples 1 to 3 did not satisfy General Formula 3.

[0107] Evaluation example 4: Evaluation of hydrogen embrittlement The amount of diffusible hydrogen for the hot stamped parts manufactured in each of the Examples and Comparative Examples was measured using thermal desorption spectroscopy. Specifically, the hot stamped parts manufactured in each of the Examples and Comparative Examples were heated from room temperature (±20°C) to 500°C at a heating rate of 20°C / min, and the amount of hydrogen released from the hot stamped parts manufactured in each of the Examples and Comparative Examples was measured at temperatures below 350°C. The results are shown in Table 2 below.

[0108] [Table 2]

[0109] As shown in Table 2 above, the hot stamped parts manufactured in Examples 1 to 4 had a diffusible hydrogen content of less than 0.1 ppm, which was significantly lower than that of the hot stamped parts manufactured in Comparative Examples 1 to 5, and it was confirmed that hydrogen embrittlement could be suppressed as a result.

[0110] Evaluation example 5. Evaluation of fracture experiment A four-point bending test was performed to evaluate fracture of the hot stamped parts manufactured in Example 2 and Comparative Examples 1 and 2. Specifically, the four-point bending test involved applying a stress below the elastic limit, specifically 1,000 MPa in air, to a specific point on each specimen, which was then exposed to a corrosive environment. The test then examined whether stress corrosion cracking, i.e., fracture, occurred. The results are shown in Figures 3 to 5. Stress corrosion cracking refers to cracking that occurs when corrosion and sustained tensile stress act simultaneously.

[0111] As shown in Figure 3, it was confirmed that the hot stamped part manufactured in Example 2 did not fracture. In contrast, it was confirmed that the hot stamped parts manufactured in Comparative Examples 1 and 2, shown in Figures 4 and 5, respectively, fractured. In other words, it was confirmed that the hot stamped part manufactured in Example 2, by satisfying General Formula 3 above, exhibited less hydrogen embrittlement than the hot stamped parts manufactured in Comparative Examples 1 and 2, which did not satisfy General Formula 3 above. [Explanation of symbols]

[0112] 1: Alpha iron layer 2: Gamma layer 10: Hot stamping parts 11: Steel plate 12: Plating layer

Claims

1. Steel plate, and The hot stamped part further comprises a zinc-containing plating layer formed on at least one surface of the steel sheet, the zinc-containing plating layer satisfying the following general formula 1: [Equation 1] (In the above general formula 1, when the plating layer is composed of n layers with distinct structures, L ｋ is the average thickness (μm) measured along the shortening direction at 10 arbitrary positions in the k-th layer of the plating layer observed in the SEM image, and D ｋ is the value (m ２ / s), t is the total heating time (sec) during hot stamping, and n is an integer of 2 or more. <Electrochemical hydrogen permeation experiment method> A specimen made from the kth layer is placed in the center of two cells, and hydrogen is electrochemically generated in one cell, allowing it to permeate to the other cell. - The time it takes for hydrogen to permeate through to the other cell is measured. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D ｋ Let's say. [Equation 2] (In the above general formula 2, L ｓａｍｐｌｅ is the thickness of the specimen (m), and t ｓａｍｐｌｅ is the time (sec) it took to penetrate the specimen.

2. The hot stamped part according to claim 1 , wherein the plating layer satisfies the following general formula 3: [Equation 3] (In the above general formula 3, when the plating layer is composed of n layers with distinct structures, L ｋ is the average thickness (μm) measured along the shortening direction at 10 arbitrary positions in the k-th layer of the plating layer observed in the SEM image, and D ｋ is the value (m ２ / s), t is the total heating time (sec) during hot stamping, and n is an integer of 2 or more. <Electrochemical hydrogen permeation experiment method> A specimen made from the kth layer is placed in the center of two cells, and hydrogen is electrochemically generated in one cell, allowing it to permeate to the other cell. - The time it takes for hydrogen to permeate through to the other cell is measured. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D ｋ Let's say. [Equation 4] (In the above general formula 2, L ｓａｍｐｌｅ is the thickness of the specimen (m), and t ｓａｍｐｌｅ is the time (sec) it took to penetrate the specimen.

3. The hot stamped component according to claim 1 , wherein the plating layer satisfies the following general formula 4: [Equation 5] (In the above general formula 4, when the plating layer is composed of n layers with distinct structures, L ｋ is the average thickness (μm) measured along the shortening direction at 10 arbitrary positions in the k-th layer of the plating layer observed in the SEM image, and D ｋ is the value (m ２ / s), t is the total heating time (sec) during hot stamping, and n is an integer of 2 or more. <Electrochemical hydrogen permeation experiment method> A specimen made from the kth layer is placed in the center of two cells, and hydrogen is electrochemically generated in one cell, allowing it to permeate to the other cell. - The time it takes for hydrogen to permeate through to the other cell is measured. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D ｋ Let's say. [Equation 6] (In the above general formula 2, L ｓａｍｐｌｅ is the thickness of the specimen (m), and t ｓａｍｐｌｅ is the time (sec) it took to penetrate the specimen.

4. 2. The hot stamped part according to claim 1, which is a part manufactured by hot stamping a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet.

5. The hot stamping includes a heating step of preparing a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet, and heating the prepared plated steel sheet; a forming step of stamping the heated plated steel sheet with a press die to form a formed body; and The hot stamped part of claim 4 , further comprising a cooling step of cooling the compact.

6. 6. The hot stamped part according to claim 5, wherein when the prepared plated steel sheet is heated in the heating step, the plated steel sheet is heated in a temperature range of 600°C to 700°C at an average heating rate of 3°C / s to 8°C / s.

7. In the heating step, when the prepared plated steel sheet is heated, the plated steel sheet is heated at an average temperature rising rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to X°C, The hot stamped part according to claim 5, wherein X°C is the maximum temperature to which the plated steel sheet is heated in the heating step minus 10°C.

8. The hot stamped part according to claim 4, wherein the total heating time during the hot stamping is 150 seconds to 450 seconds.

9. 2. The hot stamped part according to claim 1, wherein the plating layer has a thickness of 10 μm to 30 μm.

10. The present invention relates to a method for producing a hot stamped part including a steel sheet and a zinc-containing plating layer formed on at least one surface of the steel sheet, the zinc plating layer satisfying the following general formula 1: a heating step of preparing a plated steel sheet having a zinc-containing plating layer formed on at least one surface of the steel sheet, and heating the prepared plated steel sheet; a forming step of stamping the heated plated steel sheet with a press die to form a formed body; and A method for producing a hot stamped part, comprising a cooling step of cooling the compact. [Equation 7] (In the above general formula 1, when the plating layer is composed of n layers with distinct structures, L ｋ is the average thickness (μm) measured along the shortening direction at 10 arbitrary positions in the k-th layer of the plating layer observed in the SEM image, and D ｋ is the value (m ２ / s), t is the total heating time (sec) during hot stamping, and n is an integer of 2 or more. <Electrochemical hydrogen permeation experiment method> A specimen made from the kth layer is placed in the center of two cells, and hydrogen is electrochemically generated in one cell, allowing it to permeate to the other cell. - The time it takes for hydrogen to permeate through to the other cell is measured. - Using the measured time and thickness of the specimen, the value calculated by the following general formula 2 is D ｋ Let's say. [Equation 8] (In the above general formula 2, L ｓａｍｐｌｅ is the thickness of the specimen (m), and t ｓａｍｐｌｅ is the time (sec) it took to penetrate the specimen.

11. 11. The method for producing a hot stamped part according to claim 10, wherein when the prepared plated steel sheet is heated in the heating step, the plated steel sheet is heated in a temperature range of 600°C to 700°C at an average temperature increase rate of 3°C / s to 8°C / s.

12. In the heating step, when the prepared plated steel sheet is heated, the plated steel sheet is heated at an average temperature rising rate of 0.5°C / s to 3°C / s in a temperature range of 800°C to X°C, The method for manufacturing a hot stamped part according to claim 10, wherein the X°C is a maximum temperature to which the plated steel sheet is heated in the heating step minus 10°C.

13. The method for manufacturing a hot stamped part according to claim 10, wherein the plating layer formed on the plated steel sheet prepared in the heating step includes a zeta layer, a delta layer, and a gamma layer.

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