Steel sheet for hot forming, hot-formed member, and method for manufacturing same

Abstract

The present invention relates to a steel sheet suitable for hot forming. More specifically, the present invention relates to a steel sheet for hot forming, a hot-formed member, and a method for manufacturing same.

Description

Steel sheet for hot forming, hot formed member, and method for manufacturing the same

[0001] The present invention relates to a steel plate suitable for hot forming, and more specifically, to a steel plate for hot forming, a hot forming member, and a method for manufacturing the same.

[0002] Hot-formed steel sheets are widely used as materials for automobiles, and galvanized steel sheets having a plating layer on the surface of a base steel sheet are mainly used as said hot-formed steel sheets.

[0003] Recently, as the demand for improved fuel efficiency and passenger protection through vehicle lightweighting has increased, the use of hot-formed steel sheets is rising as a material for reinforcing components requiring high strength, such as automotive bumpers and pillars, with aluminum-plated steel sheets being primarily used in this context.

[0004] Hot forming is a technique in which a steel sheet for hot forming, such as an aluminum-based plated steel sheet, or in particular an Al-Si plated steel sheet having a plating layer containing components such as Si and Fe in addition to Al, is heated to a high temperature and then pressed using a mold or the like to perform forming and simultaneous rapid cooling. By doing so, the microstructure of the base steel sheet is formed into a low-temperature transformation phase (e.g., a martensite phase), thereby ensuring ultra-high strength in the manufactured hot-formed member.

[0005] This hot forming technology has the advantage of easily manufacturing complex shapes because forming is performed at high temperatures. In addition, by performing rapid cooling simultaneously with pressing, it is possible to expect not only increased material strength but also a lightweight effect.

[0006] Meanwhile, the martensitic phase is known to have low resistance to hydrogen embrittlement. However, hot-formed components produced after hot forming contain residual stress due to rapid cooling following high-temperature heating, and if the amount of diffusible hydrogen in the steel increases, delayed fracture caused by hydrogen embrittlement may occur. Consequently, there is a disadvantage in that the application of this material is limited, and there is a need for measures to resolve this problem.

[0007] Conventionally, Patent Document 1 discloses that hydrogen embrittlement can be improved by controlling the alloy composition and utilizing an Al-Fe plating layer. However, this document has problems such as increased costs due to additional processes, as there are significant limitations on the alloy composition and Ni pre-plating is required.

[0008] (Patent Document 1) U.S. Published Patent Application 11725255

[0009] One aspect of the present invention is to provide a steel sheet for hot forming capable of improving the resistance to hydrogen embrittlement of a hot-formed member, a hot-formed member produced therefrom, and a method for manufacturing the same.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] According to one aspect of the present invention, a steel sheet for hot forming is provided, comprising a base steel sheet and an alloy plating layer formed on at least one surface of the base steel sheet, wherein the alloy plating layer comprises, in weight percent, silicon (Si): 1.0~12.0%, zinc (Zn): 2.0~20.0%, iron (Fe): 15.0~50.0%, and the remainder being aluminum (Al) and other unavoidable impurities.

[0012] In one embodiment of the present invention, the ratio of Fe to Zn (Fe / Zn) in the alloy plating layer may satisfy 1.0 to 25.0.

[0013] In this way, when providing an aluminum-based plated steel sheet as a hot-forming steel sheet, by providing the plating layer of the plated steel sheet as an alloyed plating layer containing zinc, silicon, iron, etc., it is possible to improve the hydrogen embrittlement resistance and corrosion resistance of a hot-formed member obtained by hot-forming the hot-forming steel sheet.

[0014] In one embodiment of the present invention, the ratio of Fe to Al (Fe / Al) in the alloy plating layer may satisfy 0.3 to 1.4.

[0015] In one embodiment of the present invention, the alloy plating layer may include one or more substitutional intermetallic compounds among Fe2Al5(Zn,Si) and FeAl3(Zn,Si).

[0016] In one embodiment of the present invention, the alloy plating layer may have a thickness of 1.2 to 25.0 μm.

[0017] In one embodiment of the present invention, the base steel sheet comprises, in weight%, carbon (C): 0.02~0.60%, silicon (Si): 0.001~2.000%, aluminum (Al): 0.001~1.000%, manganese (Mn): 0.1~4.0%, phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, nitrogen (N): 0.0200% or less, titanium (Ti): 0~1.0000%, niobium (Nb): 0~1.0000%, vanadium (V): 0~1.0000%, boron (B): 0~0.0100%, chromium (Cr): 0~1.00%, molybdenum (Mo): 0~1.00%, tungsten (W): 0~1.00%, copper (Cu): It may contain 0~1.0%, Nickel (Ni): 0~1.0%, Tin (Sn): 0~1.00%, Antimony (Sb): 0~1.00%, Calcium (Ca): 0~0.10%, Magnesium (Mg): 0~0.10%, Cobalt (Co): 0~1.00%, Arsenic (As): 0~1.00%, Zirconium (Zr): 0~1.00%, Bismuth (Bi): 0~1.00%, Rare Earth elements (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

[0018] According to another aspect of the present invention, a method for manufacturing a steel sheet for hot forming is provided, comprising the steps of: preparing a base steel sheet; immersing the base steel sheet in an aluminum-based plating bath having a composition excluding Fe, wherein in weight percent, silicon (Si): 1.0~12.0%, zinc (Zn): 2.0~30.0%, the remainder being aluminum (Al) and other unavoidable impurities, to obtain a plated steel sheet having an aluminum-based plating layer on at least one surface; heat-treating the plated steel sheet; and cooling after the heat treatment.

[0019] In one embodiment of the present invention, the heat treatment can be performed in a heat treatment furnace connected to the plating bath at a temperature range of 650 to 750°C.

[0020] By the aforementioned heat treatment process, the plating layer of the steel sheet for hot forming can be formed into an alloyed plating layer containing Zn, Si, Fe, etc.

[0021] In one embodiment of the present invention, the step of obtaining the plated steel sheet comprises: the temperature of the aluminum-based plating bath is 550 to 650°C, and 5 g / m² on one side. 2 Up to 100g / m² 2 It can be done with the amount of plating.

[0022] In one embodiment of the present invention, cooling after the heat treatment can be performed at a cooling rate of 20℃ / s or higher.

[0023] According to another aspect of the present invention, a hot-formed member is provided comprising a base steel plate and an alloy plating layer formed on at least one surface of the base steel plate, wherein the alloy plating layer comprises, in weight percent, silicon (Si): 1.0~12.0%, zinc (Zn): 2.0~20.0%, iron (Fe): 15.0~50.0%, the remainder being aluminum (Al) and other unavoidable impurities, and the hydrogen content of the base steel plate is 0.09 ppm or less.

[0024] In one embodiment of the present invention, the ratio of Fe to Zn (Fe / Zn) in the alloy plating layer may satisfy 1.0 to 25.0, and the ratio of Fe to Al (Fe / Al) may satisfy 0.3 to 1.4.

[0025] According to another aspect of the present invention, a method for manufacturing a hot-formed member is provided, comprising the steps of: preparing a steel plate for hot forming; forming the steel plate for hot forming into a blank; heating and maintaining the blank to a temperature of Ac3 or higher; and forming and cooling simultaneously using a mold after heating and maintaining.

[0026] In one embodiment of the present invention, the hot forming steel plate may be a hot forming steel plate according to one embodiment of the present invention.

[0027] According to the present invention, a steel sheet for hot forming can be provided, which can obtain a hot forming member with improved resistance to hydrogen embrittlement. In addition, a hot forming member with excellent resistance to hydrogen embrittlement and corrosion resistance can be provided.

[0028] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0029] Figure 1 shows a cross-section in the thickness direction of a steel plate for hot forming according to one embodiment of the present invention, observed by SEM.

[0030] Figure 2 is a graph showing the measurement results of the hydrogen content of the steel sheet according to the Fe content in the plating layer of a hot-formed member according to one embodiment of the present invention.

[0031] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0032] In addition, embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the relevant technical field.

[0033] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0034] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or practices of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0035] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0036] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0037] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0038] The inventors of the present invention have conducted in-depth research to obtain a hot-formed member with improved resistance to hydrogen embrittlement when using an aluminum-based plated steel sheet as a hot-forming steel sheet and manufacturing a hot-formed member therefrom.

[0039] As a result, it was confirmed that a hot-formed member with excellent resistance to hydrogen embrittlement can be provided by preventing the liquefaction of the plating layer when heating a galvanized steel sheet, which is a steel sheet for hot forming, to a high temperature for hot forming, thereby suppressing the phenomenon of hydrogen penetrating into the base steel sheet, and thus the present invention was completed.

[0040] The present invention will be described in detail below.

[0041] In the following description, steel plates, members, etc. according to the present invention are described. Unless otherwise specifically stated, the content of each element of the alloy composition constituting the steel plates and members is based on weight. Additionally, it should be noted that when describing the steel structure in terms of ratios, the basis is area.

[0042] According to one aspect of the present invention, a steel sheet for hot forming can be provided, comprising a base steel sheet and an alloy plating layer formed on at least one surface of the base steel sheet, wherein the composition of the alloy plating layer is limited as follows.

[0043] An alloyed plating layer according to one embodiment of the present invention may comprise, in weight percent, silicon (Si): 1.0~12.0%, zinc (Zn): 2.0~20.0%, iron (Fe): 15.0~50.0%, and the remainder being aluminum (Al) and other unavoidable impurities.

[0044] That is, the alloyed plating layer according to one embodiment of the present invention is in a state in which an aluminum-based plating layer mainly comprising aluminum (Al) is alloyed, and the alloyed plating layer may include an Al-Fe alloy phase. At this time, the Al-Fe alloy phase may be one or more substitutional intermetallic compounds among Fe2Al5(Zn,Si) and FeAl3(Zn,Si). The Fe2Al5(Zn,Si) and FeAl3(Zn,Si) are compounds that exist in the form of a solid solution in which zinc (Zn) and silicon (Si) are substituted at the positions of aluminum (Al) atoms within the crystal structure of the Fe-Al intermetallic compound, and are specified as substitutional intermetallic compounds in the present invention.

[0045] Thus, the steel sheet for hot forming according to one embodiment of the present invention has a plating layer in an alloyed state prior to hot forming, thereby suppressing the melting of the plating layer during the high-temperature heating process for subsequent hot forming, and consequently suppressing the penetration of hydrogen into the base steel sheet. Accordingly, the steel sheet for hot forming according to one embodiment of the present invention may have excellent resistance to hydrogen embrittlement.

[0046] The composition of the alloyed plating layer described above will be explained in more detail below.

[0047] Silicon (Si): 1.0~12.0%

[0048] Silicon (Si) is a useful element for suppressing the formation of Al-Fe alloy phases during the plating process. When plating is performed using a plating bath primarily containing Al, Al has high reactivity with Fe in the substrate steel sheet; consequently, when the substrate steel sheet is immersed in the Al plating bath, an excessive Al-Fe alloy phase may be formed, which poses a problem of causing plating peeling during the processing of the material (e.g., steel sheet, component, etc.). Accordingly, the above-mentioned Si can be added to the Al plating bath.

[0049] If the Si content in the above alloy plating layer is less than 1.0%, the formation of the Al-Fe alloy phase may be excessive, which may result in poor powdering performance, whereas if the content exceeds 12.0%, the formation of the Al-Fe alloy phase may be excessively delayed, which may make continuous operation impossible.

[0050] Zinc (Zn): 2.0~20.0%

[0051] Zinc (Zn) reacts with Al during the plating process to form an Al-Zn alloy phase, which plays a crucial role in enhancing the sacrificial protection of galvanized steel sheets. In other words, Zn acts as the sacrificial protector in place of Al, thereby contributing to the improvement of the steel sheet's corrosion resistance.

[0052] If the content of Zn is less than 2.0%, the sacrificial protection may not be sufficiently exhibited, which can result in inferior corrosion resistance of the steel sheet. On the other hand, if the content exceeds 20.0%, Zn may segregate around the Al-Fe alloy phase, causing powdering problems during the processing of the material.

[0053] Accordingly, the above Zn may be 2.0 to 20.0%. In another embodiment of the present invention, the above Zn may be 5.0% or more or 7.0% or more, and in yet another embodiment, the above Zn may be 15.0% or less, or 12.0% or less.

[0054] Iron (Fe): 15.0~50.0%

[0055] Iron (Fe) is not an element artificially added to the plating bath; rather, it is an element present in the plating layer as the Fe in the base steel reacts with Al, Zn, etc., within the plating bath during the plating process to form an alloy phase at the interface between the base steel and the plating layer.

[0056] Meanwhile, in one embodiment of the present invention, an aluminum-based plating layer formed by a plating process is alloyed. During this alloying treatment, as an example, Fe reacts with Al or Zn during the heat treatment process after plating to form an Al-Fe alloy phase, i.e., an intermetallic compound.

[0057] If the Fe content is less than 15.0%, the reaction with Al and Zn is insufficient, so the aluminum-based plating layer is not sufficiently alloyed into an Al-Fe alloy phase, and there is a high possibility of hydrogen embrittlement occurring. On the other hand, if the content exceeds 50.0%, the thickness of the Al-Fe alloy phase becomes thicker, and there is a problem that the plating adhesion deteriorates during processing of the material.

[0058] The alloy plating layer according to one embodiment of the present invention may contain other unavoidable impurities in addition to the aforementioned alloy composition. Since such impurities may be unintentionally introduced from raw materials or the surrounding environment during the conventional steel manufacturing process, they cannot be completely eliminated. As any skilled technician in the conventional steel manufacturing process would be aware of these impurities, the present invention does not specifically mention all such details.

[0059] An alloyed plating layer according to one embodiment of the present invention may satisfy a ratio of Fe to Zn (Fe / Zn) of 1.0 to 25.0 and a ratio of Fe to Al (Fe / Al) of 0.3 to 1.4.

[0060] In one embodiment of the present invention, if the ratio of Fe to Zn content (Fe / Zn) in the alloyed plating layer is less than 1.0, the alloying of the plating layer becomes insufficient, and the intended hydrogen embrittlement resistance effect cannot be obtained. On the other hand, if the value exceeds 25.0, the relative Zn content becomes low, and the corrosion resistance of the material is reduced.

[0061] In addition, in one embodiment of the present invention, if the content ratio (Fe / Al) of Fe and Al in the alloy plating layer is less than 0.3, the alloying by Fe is insufficient, whereas even if the content exceeds 1.4, the effect of hydrogen embrittlement resistance cannot be obtained.

[0062] An alloy plating layer according to one embodiment of the present invention may have a thickness of 1.2 to 25.0 μm. If the thickness of the alloy plating layer is less than 1.2 μm, the corrosion resistance of the material may be reduced, whereas if the thickness exceeds 25.0 μm, plating peeling may occur during processing of the material. Generally, the thickness of the plating layer can be controlled according to the amount of adhesion during plating.

[0063] The base steel sheet constituting the hot-forming steel sheet according to one embodiment of the present invention may include elements that can typically be added to steel, and the types and contents thereof are not specifically limited. However, non-limiting examples of elements that may be added to a base steel sheet according to one embodiment of the present invention are, in weight%, carbon (C): 0.02~0.60%, silicon (Si): 0.001~2.000%, aluminum (Al): 0.001~1.000%, manganese (Mn): 0.1~4.0%, phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, nitrogen (N): 0.0200% or less, titanium (Ti): 0~1.0000%, niobium (Nb): 0~1.0000%, vanadium (V): 0~1.0000%, boron (B): 0~0.0100%, chromium (Cr): 0~1.00%, molybdenum (Mo): 0~1.00%, tungsten (W): It may contain 0~1.00%, copper (Cu): 0~1.0%, nickel (Ni): 0~1.0%, tin (Sn): 0~1.00%, antimony (Sb): 0~1.00%, calcium (Ca): 0~0.10%, magnesium (Mg): 0~0.10%, cobalt (Co): 0~1.00%, arsenic (As): 0~1.00%, zirconium (Zr): 0~1.00%, bismuth (Bi): 0~1.00%, rare earth elements (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

[0064] Among the alloy compositions described above, C, Mn, etc., may be added to ensure the strength of the steel; Si is effective not only for its deoxidation effect but also for reducing the segregation of Mn, etc., within the base steel sheet, and Al has a deoxidation effect. It should be noted that P, S, N, etc., may be elements inevitably introduced during the steel manufacturing process, but are not limited to these. Furthermore, it will be obvious to those skilled in the art that, in addition to the aforementioned composition, Ti, B, Cu, Mo, Cr, Ni, V, Ca, Nb, Sn, W, Sb, Mg, Co, As, Zr, Bi, REM, etc., may be additionally included in consideration of the target physical properties of the final product.

[0065] As described above, a steel sheet for hot forming according to one embodiment of the present invention comprises an alloy plating layer on at least one surface of a base steel sheet. The alloy plating layer may be formed by alloying an aluminum-based plating layer; however, it should be noted that while the entire aluminum plating layer may be alloyed, the state in which a partial Al plating layer remains is not excluded. For example, an Al plating layer in which Zn and Si are dissolved may exist on the upper surface of the alloy plating layer, that is, on the outermost surface of the alloy plating layer, and the thickness of such Al plating layer is not specifically limited. This is because the present invention can improve the intended effect, specifically hydrogen embrittlement resistance, if the alloy plating layer satisfies the aforementioned alloy composition and thickness.

[0066] Hereinafter, a method for manufacturing a steel sheet for hot forming according to another aspect of the present invention will be described in detail.

[0067] However, it should be noted that the following method is merely one example for manufacturing a steel sheet for hot forming, and that a steel sheet for hot forming according to one embodiment of the present invention must not necessarily be manufactured by this manufacturing method, and that any manufacturing method that satisfies the claims of the present invention may be used to implement each embodiment of the present invention without any problem.

[0068] According to one embodiment of the present invention, a steel sheet for hot forming can be manufactured by the steps of: preparing a base steel sheet; manufacturing an aluminum-plated steel sheet by plating the base steel sheet with an aluminum-based plate; and heat treating and cooling the aluminum-plated steel sheet.

[0069] In one embodiment of the present invention, the base steel sheet for obtaining a hot-forming steel sheet may be the base steel sheet mentioned above, and it is noted that there are no particular restrictions on its composition and that it is replaced by the aforementioned details.

[0070] In one embodiment of the present invention, the prepared steel substrate can be plated with an aluminum-based material to produce an aluminum-plated steel substrate having an aluminum-based plating layer on at least one surface of the steel substrate. As one example, the aluminum-based plating may be performed by a molten aluminum plating method in which the steel substrate is immersed in a molten aluminum plating bath. At this time, the temperature of the molten aluminum plating bath may be set to a temperature range for conventional aluminum plating, and as a non-limiting example, it may be performed in a temperature range of 550 to 650°C. Additionally, the plating may be performed by immersing the steel substrate in the plating bath for approximately 3 to 5 seconds, and the plating may be 5 g / m² on one side. 2 Up to 100g / m² 2 It can be done with the amount of plating.

[0071] In one embodiment of the present invention, the molten aluminum plating bath is a plating bath comprising aluminum (Al) as the main component, and may further comprise zinc (Zn) in an amount of 2.0 to 30.0% and silicon (Si) in an amount of 1.0 to 12.0% in addition to the Al.

[0072] In this way, by adding Zn to the plating bath, the corrosion resistance of the hot-formed steel sheet and the hot-formed member obtained using this steel sheet can be secured. In addition, by adding Si together to the plating bath, effects such as improving the fluidity of the molten metal and suppressing the diffusion of Al into the base steel sheet during plating can be obtained.

[0073] In one embodiment of the present invention, the Zn content in the molten aluminum plating bath is at most 30.0%, but the Zn content in the plating layer described above is lower than this. This is because, as Zn vaporizes and is lost during the process of manufacturing plated steel sheets, particularly during the subsequent heat treatment process after plating, the Zn content in the final plating layer becomes lower than the Zn content in the plating bath. At this time, the amount of Zn lost is not specifically limited, but as one example, approximately 30 to 50% of the Zn content in the plating bath may be lost.

[0074] Meanwhile, in another embodiment of the present invention, the plating bath may further include one or more elements selected from Mg, Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, and Sr in addition to Zn and Si. These elements may be added in trace amounts, and their content is not specifically limited.

[0075] In one embodiment of the present invention, an aluminum-based plated steel sheet produced by the aluminum plating can be heat-treated. The step of heat treatment is not particularly limited, but may be performed by on-line heating in which the plated steel sheet (aluminum-based plated steel sheet) obtained by the molten aluminum plating is heated while in motion. For example, a heat treatment furnace may exist above the plating bath, and the steel sheet removed after being introduced into the plating bath may be loaded into the heat treatment furnace existing above the plating bath to perform heat treatment.

[0076] As a non-limiting example, the heat treatment may be a process of heating to a temperature range of 650 to 750°C continuously after the aluminum-based plating and then maintaining it at that temperature. The holding time may be 1 to 20 seconds, although not specifically limited.

[0077] By the heat treatment described above, the plating layer of the aluminum-based plated steel sheet can be formed into an alloyed plating layer containing an Al-Fe alloy phase. As one example, the alloyed plating layer may be composed of an intermetallic compound of FexAly, and the description of the intermetallic compound is replaced with the previously explained content.

[0078] In one embodiment of the present invention, cooling may be performed after the heat treatment, and as one example, it may be performed at a cooling rate of 20°C / s or more. The cooling may be performed using air or nitrogen gas, etc.

[0079] If the above cooling rate is less than 20℃ / s, sufficient cooling may not be achieved, and there is a risk of powdering occurring in the top roll section, where the plating layer detaches. Although there is no specific upper limit for the above cooling rate, if the above cooling rate exceeds 50℃ / s, the cooling effect becomes saturated, so it may be limited to 50℃ / s or less in consideration of this.

[0080] Hereinafter, a hot-formed member and a method for manufacturing the same according to another aspect of the present invention will be described in detail.

[0081] In one embodiment of the present invention, a hot-formed member can be obtained by hot-forming a steel plate for hot forming, and the steel plate for hot forming may be according to one embodiment of the present invention.

[0082] A steel sheet for hot forming according to one embodiment of the present invention may include a base steel sheet and an alloy plating layer formed on at least one surface of the base steel sheet, and the alloy composition of the base steel sheet and the alloy composition, component content ratio, thickness, etc. of the alloy plating layer may be replaced with the above-described details.

[0083] When a steel sheet for hot forming is heated to a high temperature to obtain a hot forming member according to one embodiment of the present invention, the penetration of hydrogen into the base steel sheet is minimized by the alloy plating layer. Accordingly, the hot forming member according to one embodiment of the present invention has excellent resistance to hydrogen embrittlement, and as one example, the base steel sheet of the hot forming member may have a hydrogen content of 0.09 ppm or less.

[0084] In one embodiment of the present invention, the base steel sheet of the hot-formed member may have a hard structure by forming it after high-temperature heat treatment. As one example, the base steel sheet may have a microstructure with an area fraction of 90% or more of a combination of martensite and bainite phases, and may also include pearlite, ferrite, etc. as other structures. However, it is not limited thereto. Accordingly, the hot-formed member according to one embodiment of the present invention may have ultra-high strength, and as an example, may have a tensile strength of 1800 MPa or more.

[0085] Meanwhile, the method for manufacturing a hot-formed member according to one embodiment of the present invention is not specifically limited, and as is widely known in the past, it can be manufactured through a process of heating a steel plate for hot forming to a temperature above the austenitizing temperature (Ac3), maintaining it, and then forming (pressing) it simultaneously with rapid cooling.

[0086] However, as one example of the present invention, after obtaining a blank using a steel plate for hot forming according to one embodiment of the present invention, the blank may be heated to a temperature range of 860 to 970°C and maintained for 3 to 15 minutes. An intended hot-formed member can be manufactured by forming (pressing) the blank heated and maintained in a hot state and then cooling it at a cooling rate greater than or equal to the critical cooling rate. As a non-limiting example, the cooling may be performed at a cooling rate of 30°C / s or more.

[0087] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0088] (Example)

[0089] A cold-rolled steel sheet with a thickness of 1.6 t (mm) was prepared, containing, in weight percent, carbon (C): 0.20%, silicon (Si): 0.250%, manganese (Mn): 1.2%, boron (B): 30 ppm, and the remainder being Fe and unavoidable impurities. At this time, the cold-rolled steel sheet was an annealed steel sheet.

[0090] The above cold-rolled steel sheet was immersed for 3 seconds in an aluminum (Al)-based plating bath (composition of Table 1, remainder Al and other unavoidable impurities) maintained at 630°C, and then a steel sheet for hot forming was manufactured by applying the heat treatment and cooling conditions shown in Table 1 below. At this time, the plating amount was 70 g / m² per side. 2 The above process was carried out, and a plating layer with a thickness of 15.0 μm was obtained after the heat treatment and cooling.

[0091] SEM and EDS were used to evaluate the microstructure of the plating layer of each hot-formed steel sheet. The alloy composition of the plating layer was analyzed using the EDS, and 10 random points were measured in the cross-section in the thickness direction of the plating layer, and the average value of each component was calculated.

[0092] Each hot-forming steel sheet manufactured according to the above was made into a blank, and each blank was heat-treated by heating it to 900°C and holding it for 5 minutes. Afterward, the hydrogen content inside each steel sheet (base steel sheet) was measured, and corrosion resistance was evaluated using salt spray.

[0093] At this time, the hydrogen content was measured by using Thermal Desorption Spectroscopy (TDS) to determine the hydrogen content present inside the steel plate.

[0094] In addition, corrosion resistance was determined by spraying a 5% concentration brine (35℃, pH 6.8) onto the surface of each blank at a rate of 2 ml / 80 cm² per hour, and then measuring the maximum depth of corrosion after removing the corrosion products after 240 hours.

[0095] The component content in the plating layer, hydrogen content in the substrate steel sheet, and corrosion resistance evaluation results are shown together in Table 2 below.

[0096] Specimen Plating Bath Composition (Wt%) Heat Treatment Temperature (°C) Cooling Rate (°C / s) Classification ZnSi 1 23.0 10.0 730 20 Invention Example 1 230.0 12.0 750 20 Invention Example 2 325.08.0 710 30 Invention Example 3 426.06.0 70 30 Invention Example 4 520.04.0 69 0 35 Invention Example 5 613.03.0 68 0 40 Invention Example 6 711.02.0 66 0 45 Invention Example 7 85.01.0 65 0 50 Invention Example 8 921.0 13.0 76 0 25 Comparative Example 1 1023.0 -65 0 15 Comparative Example 2 113.0 13.0 78 0 20 Comparative Example 3 1236.0 12.0 76 0 60 Comparative Example 4133.01.066015 Comparative Example 5

[0097] Classification Plating Layer Composition (WJ%) Physical Properties FeZnSiOAlFe / ZnFe / Al Hydrogen Content (ppm) Corrosion Resistance (mm) Invention Example 1 15.0 13.0 10.0 8.0 54.0 1.2 0.3 0.0 90.2 Invention Example 2 20.0 20.0 12.0 4.0 44.0 1.0 0.5 0.0 90.1 Invention Example 3 27.0 14.0 8.0 6.0 45.0 1.9 0.6 0.0 80.3 Invention Example 4 32.0 13.0 6.0 9.0 40.0 2.5 0.8 0.0 60.2 Invention Example 5 42.0 10.0 4.0 4.0 40.0 4.2 1.10 0 60.3 644.07.03.05.041.06.31.10.030.4 Invention Example 745.05.02.09.039.09.01.20.030.5 Invention Example 850.02.01.011.036.025.01.40.010.5 Comparative Example 115.010.013.013.049.01.50.30.351.2 Comparative Example 251.012.0-10.027.04.31.90.371.0 Comparative Example 340.01.013.09.037.040.01.10.391.5 Comparative Example 414.021.012.017.036.00.70.40.520.7Comparative Example 552.02.01.015.030.026.01.70.151.4

[0098] As shown in Tables 1 and 2 above, it can be seen that there are differences in hydrogen content and corrosion resistance depending on the content of Fe, Zn, and Si in the composition of the plating layer.

[0099] Specifically, it can be confirmed that the inventive examples in which the Fe content in the plating layer is 15.0 to 50.0% have excellent resistance to hydrogen embrittlement with a hydrogen content of 0.09 or less (see FIG. 2). On the other hand, in the case of Comparative Example 4, in which the Fe content is 14.0%, it can be seen that the resistance to hydrogen embrittlement is inferior with a hydrogen content of 0.52, and it can also be seen that the hydrogen content increases when the Fe content exceeds 50.0% (Comparative Examples 2 and 5).

[0100] In addition, it can be confirmed that the invention examples, in which the Zn content in the plating layer is 2.0 to 20.0%, have excellent corrosion resistance with a maximum corrosion depth of 0.5 mm or less. On the other hand, in the case of Comparative Example 3, in which the Zn content is 1.0%, it can be confirmed that the corrosion resistance is very inferior with a corrosion depth of 1.5 mm.

[0101] The Si content in the plating layer is an element closely related to the Fe content, and when it is contained within 1.0 to 12.0%, the Fe content in the plating layer is found to be 15.0 to 50.0%. This means that the aluminum-based plating layer is sufficiently alloyed to form an alloyed plating layer containing an Al-Fe alloy phase, and as a result, resistance to hydrogen embrittlement can be secured.

[0102] Meanwhile, Comparative Example 1, in which the Si content in the plating bath was excessive and the heat treatment temperature exceeded 750°C, showed increased hydrogen content and inferior corrosion resistance. Additionally, Comparative Example 4, in which the Zn content in the plating bath was excessive and the heat treatment temperature exceeded 750°C, satisfied the Si content in the plating layer of 1.0–12.0%, but the Fe / Zn value was secured at less than 1.0, and it was confirmed that the hydrogen content increased due to the lower Fe alloying. Furthermore, it was confirmed that when the Fe / Zn value exceeded 25.0, the Zn content became insufficient, resulting in reduced corrosion resistance (Comparative Example 5).

Claims

1. A base steel plate and an alloy plating layer formed on at least one surface of the base steel plate, and The above alloyed plating layer comprises, in weight percent, silicon (Si): 1.0–12.0%, zinc (Zn): 2.0–20.0%, iron (Fe): 15.0–50.0%, the remainder being aluminum (Al) and other unavoidable impurities, and A steel sheet for hot forming that satisfies a ratio of Fe to Zn (Fe / Zn) in the alloyed plating layer of 1.0 to 25.

0.

2. In Paragraph 1, A steel sheet for hot forming that satisfies a ratio of Fe to Al (Fe / Al) in the alloyed plating layer of 0.3 to 1.

4.

3. In Paragraph 1, The above alloyed plating layer comprises one or more substitutional intermetallic compounds selected from Fe2Al5(Zn,Si) and FeAl3(Zn,Si), for a hot-forming steel sheet.

4. In Paragraph 1, The above alloyed plating layer is a steel sheet for hot forming having a thickness of 1.2 to 25.0 μm.

5. In Paragraph 1, The above base steel sheet comprises, in weight%, Carbon (C): 0.02~0.60%, Silicon (Si): 0.001~2.000%, Aluminum (Al): 0.001~1.000%, Manganese (Mn): 0.1~4.0%, Phosphorus (P): 0.050% or less, Sulfur (S): 0.0200% or less, Nitrogen (N): 0.0200% or less, Titanium (Ti): 0~1.0000%, Niobium (Nb): 0~1.0000%, Vanadium (V): 0~1.0000%, Boron (B): 0~0.0100%, Chromium (Cr): 0~1.00%, Molybdenum (Mo): 0~1.00%, Tungsten (W): 0~1.00%, Copper (Cu): 0~1.0%, A steel sheet for hot forming comprising nickel (Ni): 0~1.0%, tin (Sn): 0~1.00%, antimony (Sb): 0~1.00%, calcium (Ca): 0~0.10%, magnesium (Mg): 0~0.10%, cobalt (Co): 0~1.00%, arsenic (As): 0~1.00%, zirconium (Zr): 0~1.00%, bismuth (Bi): 0~1.00%, rare earth elements (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

6. Step of preparing the base steel plate; A step of obtaining a plated steel sheet having an aluminum-based plating layer on at least one surface by immersing the above-mentioned steel sheet in an aluminum-based plating bath having a composition excluding Fe, comprising, in weight percent, silicon (Si): 1.0~12.0%, zinc (Zn): 2.0~30.0%, and the remainder being aluminum (Al) and other unavoidable impurities; A step of heat-treating the above-mentioned plated steel sheet; and It includes a cooling step after the above heat treatment, A method for manufacturing a steel sheet for hot forming, wherein the heat treatment is performed in a heat treatment furnace connected to the plating bath at a temperature range of 650 to 750°C.

7. In Paragraph 6, The step of obtaining the above-mentioned plated steel sheet is, The temperature of the above aluminum-based plating bath is 550~650℃, and the plating thickness is 5g / m² per side. 2 Up to 100g / m² 2 A method for manufacturing a steel sheet for hot forming, performed with a plating amount.

8. In Paragraph 6, A method for manufacturing a steel sheet for hot forming, wherein cooling after the above heat treatment is performed at a cooling rate of 20℃ / s or higher.

9. A base steel plate and an alloy plating layer formed on at least one surface of the base steel plate, and The above alloyed plating layer comprises, in weight percent, silicon (Si): 1.0–12.0%, zinc (Zn): 2.0–20.0%, iron (Fe): 15.0–50.0%, the remainder being aluminum (Al) and other unavoidable impurities, and A hot-formed member having a hydrogen content of 0.09 ppm or less of the above-mentioned base steel plate.

10. In Paragraph 9, A hot-formed member satisfying a ratio of Fe to Zn (Fe / Zn) in the alloyed plating layer of 1.0 to 25.

0.

11. In Paragraph 9, A hot-formed member satisfying a ratio of Fe to Al (Fe / Al) in the alloyed plating layer of 0.3 to 1.

4.

12. A step of preparing a steel sheet for hot forming according to any one of claims 1 to 5; A step of producing a blank from the above-mentioned hot-forming steel plate; The step of heating and maintaining the above blank at a temperature of Ac3 or higher; and A method for manufacturing a hot-formed member comprising the step of forming and cooling simultaneously using a mold after heating and maintaining as described above.

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