Hot press-formed member

Abstract

The present invention provides a hot press-formed member having excellent corrosion resistance and hydrogen embrittlement resistance as well as excellent wear resistance.

Description

Hot-formed member

[0001] The present invention relates to a hot-formed member.

[0002] Recently, due to the depletion of petroleum energy resources and heightened environmental concerns, regulations regarding the improvement of automobile fuel efficiency are becoming increasingly stringent. From a materials perspective, reducing the thickness of steel plates can be one method to improve fuel efficiency; however, since reducing thickness can compromise vehicle safety, it is essential that the strength of the steel plates be enhanced to support this.

[0003] For these reasons, there has been a continuous demand for high-strength steel sheets, and various types of steel sheets have been developed. However, these steel sheets have a problem of poor workability because they possess high strength in themselves. In other words, since the product of strength and elongation tends to remain constant for each grade of steel sheet, there was a problem in that as the strength of the steel sheet increases, the elongation, which serves as an indicator of workability, decreases.

[0004] To address these issues, the hot press forming method has been proposed. The hot press forming method involves processing a steel sheet to a high temperature suitable for processing and then rapidly cooling it to a low temperature to form a low-temperature structure, such as martensite, within the sheet, thereby increasing the strength of the final product. This approach has the advantage of minimizing processability issues when manufacturing high-strength components.

[0005] When undergoing hot press forming, steel sheets can have a strength of 1000 MPa or more, and in some cases, 1400 MPa or more. Recently, the required strength level has increased further, and there are cases where they have a strength of 1800 MPa or more. However, as the strength of the steel sheet increases, it becomes sensitive to hydrogen delayed fracture, and the steel sheet may fracture even when containing a small amount of hydrogen. In addition, when aluminum-plated steel sheets are hot press formed, diffusion of Fe occurs from the base steel of the steel sheet to the plating layer on the surface, causing alloying in the plating layer. Due to this alloying layer, hydrogen that has penetrated during hot press forming cannot easily escape, resulting in a problem where the hydrogen resistance of the hot press formed member deteriorates.

[0006] Meanwhile, although the hot-formed member using the above-mentioned aluminum-plated steel can secure corrosion resistance by aluminum plating, there is a problem that the Al plating layer may stick to the rolls, known as roll adhesion, during the process in which the aluminum-plated steel sheet heated to a high temperature for hot forming moves along the rolls.

[0007] Typically, since the temperature at which it is heated for hot forming is higher than the melting point of the aluminum plating layer, a portion of the plating layer of the aluminum-plated steel sheet heated to a high temperature may remain in a molten state. When the sheet undergoes a roll transfer process in this state, roll adhesion occurs, where the molten plating layer adheres to the contacted roll. As the aforementioned roll adhesion frequently occurs in aluminum-plated steel sheets, much research is being conducted to resolve this problem.

[0008] (Patent Document 1) Korean Published Patent Application No. 10-2024-0098870

[0009] One aspect of the present invention is to provide a hot-formed member having excellent corrosion resistance and hydrogen embrittlement resistance, as well as excellent wear resistance.

[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 hot-formed member is provided comprising a base steel plate and an Al-based plating layer formed on the surface of the base steel plate, wherein the Al-based plating layer comprises: a first alloy layer formed on the base steel plate and comprising Al, Fe, and Cu; a second alloy layer formed on the first alloy layer and comprising Al and Fe; and a first alloy layer formed on or inside the second alloy layer and comprising Al, Fe, and Cu.

[0012] In one embodiment of the present invention, the Cu content in the first alloy phase may be 5.0 to 50.0 weight%.

[0013] In one embodiment of the present invention, the first alloy layer may include a layer a formed on the base steel plate and a layer b formed on the layer a. In this case, the Fe content of the layer a may be greater than the Fe content of the layer b.

[0014] In one embodiment of the present invention, the average Cu content in the first alloy layer is 30.0 wt% or less (including 0%), and the Cu content of the a layer may be smaller than the Cu content of the b layer.

[0015] In one embodiment of the present invention, the second alloy layer may further contain 5.0 weight% or less of Cu, and the Cu content in the first alloy layer may be greater than the Cu content in the second alloy layer.

[0016] In one embodiment of the present invention, the exposure level of the first alloy phase may be 10.0% or more.

[0017] In one embodiment of the present invention, the first alloy phase exists as a layer on the second alloy layer, and the thickness of the layer may be 20 μm or less (excluding 0).

[0018] In one embodiment of the present invention, the Al-based plating layer may further include one or more of Si, Mn, Mg, and Fe.

[0019] 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): 0.10~4.00%, 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 have a composition containing 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.

[0020] According to the present invention, a hot-formed member with excellent corrosion resistance and hydrogen embrittlement resistance can be provided.

[0021] In addition, according to the present invention, the problem of the plating layer adhering to the mold during hot forming to obtain a hot formed member can be efficiently solved, and a hot formed member having excellent wear resistance can be provided.

[0022] The various and beneficial advantages and effects of the present invention are not limited to those described above and may be more easily understood in the process of explaining specific embodiments of the present invention.

[0023] FIG. 1 schematically illustrates one example of a cross-section of a plating layer of a hot-formed member according to one embodiment of the present invention.

[0024] Figure 2 shows a cross-section of a plating layer of a hot-formed member according to one embodiment of the present invention, taken with an SEM.

[0025] 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.

[0026] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0027] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0028] 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 conventions 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.

[0029] 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.

[0030] In addition, in the present invention, the term "steel plate" refers to a coil or sheet material that has not yet been processed into a specific shape, and the term "member" refers to a material that has been processed into a non-plate shape through a forming process.

[0031] It should be noted that in the present invention, when expressing the content of each element, the basis is weight (mass%) unless specifically otherwise specified. Additionally, the proportion of crystals or structures is based on area (area%) unless specifically otherwise expressed, and the gas content is based on volume unless specifically otherwise expressed.

[0032] 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.

[0033] Typically, hot-formed components are obtained by hot-forming steel sheets suitable for hot forming, such as galvanized steel sheets. When galvanized steel sheets are used as steel sheets for hot forming, effects such as corrosion resistance can be obtained; therefore, galvanized steel sheets having a specific plating layer on the surface of a base steel sheet are primarily used. As one example, active technological development is underway to obtain hot-formed components from galvanized steel sheets having an Al-Si-based plating layer on a base steel sheet.

[0034] Meanwhile, the inventors of the present invention confirmed that when a plated steel sheet having an Al-Si plating layer is used as a hot-forming steel sheet to obtain a hot-forming member, the problem of the plating layer sticking to the mold frequently occurs during forming using a mold after heating and holding the plated steel sheet. Consequently, it was confirmed that as the surface non-uniformity of the manufactured hot-forming member increases, problems such as deterioration of surface quality and deterioration of the corrosion resistance of the hot-forming member are also entailed. Through in-depth research to solve these problems, it was confirmed that when a hot-forming member is obtained from a steel sheet having an Al-Cu plating layer on a base steel sheet instead of an Al-Si plating layer as a hot-forming steel sheet, not only the corrosion resistance and hydrogen embrittlement resistance characteristics but also the anti-sticking resistance can be improved, leading to the completion of the present invention.

[0035] The present invention will be described in detail below.

[0036] According to one aspect of the present invention, a hot-formed member is provided, wherein the hot-formed member may include a base steel plate and an Al-based plating layer formed on the surface of the base steel plate.

[0037] First, the base steel sheet constituting the hot-formed member 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): 0.10~4.00%, 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.

[0038] 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.

[0039] In one embodiment of the present invention, the Al-based plating layer comprises aluminum (Al) as a main component and copper (Cu) as an other main component.

[0040] In one embodiment of the present invention, the Al-based plating layer may contain additional elements other than the aforementioned Al and Cu, and as an example, may further include one or more of Si, Mn, Mg, and Fe. The content of the elements that may be additionally added is not specifically limited, but as a non-limiting example, Si may be included in an amount of 12% or less, Mn in an amount of 10% or less, Mg in an amount of 5% or less, and Fe in an amount of 3% or less.

[0041] According to one embodiment of the present invention, the Al-based plating layer may include a first alloy layer formed on the base steel plate, a second alloy layer formed thereon, and a first alloy layer formed on the upper or internal portion of the second alloy layer.

[0042] In one embodiment of the present invention, the first alloy layer may be a layer that includes an Fe-Al alloy phase formed by combining Fe of the base steel plate and a portion of Al, which is the main component of the plating layer, and also includes an Fe-Al-Cu alloy phase by combining with Cu constituting the Al-based plating layer. Referring to the drawings, as shown in FIG. 1, the Al-based plating layer according to one embodiment of the present invention may include a first alloy layer in which an Fe-Al-Cu alloy phase, Fe3Al, α-Fe, etc., exist on the base steel plate.

[0043] In one embodiment of the present invention, the first alloy layer may include Al, Fe, and Cu. In this case, the first alloy layer may have a Fe content greater than the Al content and may include Cu at an average of 3.0 wt% or less. As another example, the Cu content in the first alloy layer may be an average of 25.0% or less, or 20.0% or less.

[0044] In one embodiment of the present invention, the first alloy layer may include a layer a formed on the base steel plate and a layer b formed on the layer a. In this case, the Fe content of the layer a may be greater than the Fe content of the layer b. Additionally, the Cu content of the layer a may be smaller than the Cu content of the layer b.

[0045] In one embodiment of the present invention, the upper portion of the first alloy layer may include a second alloy layer.

[0046] In one embodiment of the present invention, the second alloy layer is composed of an Fe-Al alloy phase in which Fe of the base steel plate and Al, which is the main component of the plating layer, are combined, and the second alloy layer may include Al and Fe. Here, the Fe-Al alloy phase mentioned may be an alloy phase of the Fe-Al type, and as a non-limiting example, it may be one or more of FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al.

[0047] In one embodiment of the present invention, the second alloy layer may further include Cu, and in one embodiment of the present invention, the second alloy layer may include 15.0 weight% or less (including 0%) of the Cu.

[0048] In one embodiment of the present invention, when both the aforementioned first alloy layer and the second alloy layer contain Cu, the Cu content of the first alloy layer may be greater than the Cu content of the second alloy layer. In other words, the proportion of the Fe-Al-Cu alloy phase within the first alloy layer may be higher than that of the second alloy layer. However, the region within the first alloy layer that has a higher Cu content than the second alloy layer may be the b layer.

[0049] As previously mentioned, an Al-based plating layer according to one embodiment of the present invention may include a first alloy phase on or inside the second alloy layer, and the first alloy phase may include Al, Fe, and Cu.

[0050] In one embodiment of the present invention, the first alloy phase is a region with a relatively high content of Cu, and the Cu content in the first alloy phase may be greater than the Cu content of the first alloy layer and the second alloy layer mentioned above. As one example, the first alloy phase may contain Cu in an amount of 5.0 to 50.0 weight%. As another example, the content of Cu may be 10.0% or more, or 20.0% or more. As yet another example, the content of Cu may be 48.0% or less, or 45.0% or less.

[0051] Thus, according to the present invention, by having an alloy phase derived from Cu on the uppermost part, i.e., the outermost surface, of the plating layer of a hot-formed member, the surface of the hot-formed member can have a relatively soft property. From this, the anti-sticking properties of the hot-formed member can be improved.

[0052] In one embodiment of the present invention, if the Cu content in the first alloy phase is less than 5.0%, the Fe-Al-Cu alloy phase is not sufficiently formed, and thus physical properties such as hydrogen embrittlement resistance and corrosion resistance of the hot-formed member may be inferior. On the other hand, if the Cu content in the first alloy phase exceeds 50.0%, it means that the Al content is relatively low, and in this case, not only is the intended Al-based plating layer not obtained, but there is also a risk that the corrosion resistance of the hot-formed member may be significantly reduced.

[0053] Meanwhile, in one embodiment of the present invention, the first alloy phase may be exposed on the upper surface of the plating layer. Although the fraction of the exposed area is not specifically limited, as one example, the exposure level may be 10.0% or more. In addition, in one embodiment of the present invention, the first alloy phase may exist as a layer. Although the thickness of the layer is not specifically limited, as one example, it may be 20 μm (excluding 0) or less. Thus, by exposing the first alloy phase on the upper surface of the plating layer or including the first alloy phase as the uppermost layer of the plating layer, it is possible to contribute to improving the corrosion resistance and anti-sintering properties of the hot-formed member. Here, the fact that the first alloy phase exists as a layer means that, in terms of exposure level, it is 10.0% or more to 100% or less.

[0054] Here, the exposure level can be measured using SEM. As an example, the exposure level can be expressed as the average value of the exposure levels measured at each cross-section after measuring five or more cross-sections of the Al-based plating layer using SEM. Additionally, the thickness of the first alloy phase can be measured in the thickness direction from the outermost layer where the first alloy phase is formed, based on the thickness cross-section of the Al-based plating layer. In one embodiment of the present invention, if the thickness of the first alloy phase exceeds 20 μm, the relatively soft first alloy phase is excessively present, while the relatively hard second alloy layer is not sufficiently formed, leading to a risk of surface defects occurring, such as the surface of the hot-formed member being easily pressed by a mold. The lower limit of the thickness of the first alloy phase is not specifically limited, and the thickness of the first alloy phase layer will be determined according to the content of Cu contained in the plating bath during the plating process. Furthermore, as mentioned above, since the intended effect can be obtained even if only the exposure level is satisfied, it is not problematic to limit the lower limit.

[0055] According to one embodiment of the present invention, the first alloy phase tends to have a higher exposed surface area or a thicker thickness as the Cu content increases, and this is determined by the Cu content present in the plating layer, so the lower limit of the exposure or thickness is not specifically limited.

[0056] Hereinafter, a method for manufacturing a hot-formed member according to another aspect of the present invention will be described. However, it should be noted that the method for manufacturing the hot-formed member described below is merely an example, and that the hot-formed member according to one embodiment of the present invention must not necessarily be manufactured by this manufacturing method. Furthermore, any manufacturing method that satisfies the claims of the present invention may be used without issue to implement each embodiment of the present invention.

[0057] According to one embodiment of the present invention, a hot-formed member can be manufactured by following the steps of: preparing a base steel plate; manufacturing a plated steel plate having an Al-based plating layer on the base steel plate; heating and maintaining the plated steel plate; and forming and cooling the heated and maintained plated steel plate to manufacture the member.

[0058] In one embodiment of the present invention, the base steel plate for obtaining a hot-formed member may be the base steel plate mentioned above, and it is noted that there are no particular restrictions on its composition and that it is replaced by the aforementioned details.

[0059] First, a method for manufacturing the above-mentioned base steel sheet will be described. However, it should be noted that the following manufacturing method is merely one example for manufacturing the above-mentioned base steel sheet and does not mean that it must be manufactured using this method.

[0060] Accordingly, as one embodiment, the above-mentioned steel sheet can be manufactured by a method comprising the steps of: preparing a steel slab and then heating the steel slab; hot-rolling the heated steel slab to obtain a hot-rolled steel sheet; cooling and coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet.

[0061] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0062] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0063] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in the present invention (e.g., Cr, Cu, Mo, Ni, Sn, etc.) may be included in the molten steel within permissible limits.

[0064] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0065] In one embodiment of the present invention, the steel slab may have the aforementioned alloy composition. The steel slab may be heated, and in order to facilitate subsequent hot rolling, the heating may be performed in a temperature range of, for example, 1050 to 1300°C. If the heating temperature is below 1050°C, the slab structure may not be sufficiently homogenized, whereas if the temperature exceeds 1300°C, an excessive oxide layer may be formed on the surface of the slab, leading to increased manufacturing costs for removing this oxide layer and potentially causing surface defects after hot rolling.

[0066] A hot-rolled steel sheet can be obtained by hot-rolling the above-mentioned heated steel slab. Since the above-mentioned hot rolling is a conventional hot-rolling process, the conditions thereof are not specifically limited. As a non-limiting example, finish hot rolling can be performed in a temperature range of 800 to 950°C. If the temperature during the finish hot rolling is below 800°C, two-phase rolling occurs, making it difficult to control the plate shape. On the other hand, if the temperature exceeds 950°C, there is a risk that the grains will coarsen.

[0067] The above hot-rolled steel sheet can be coiled after cooling. The cooling can be performed up to the coiling temperature, and the conditions are not specifically limited and can be performed under normal conditions. However, as an example, the coiling can be performed in a temperature range of 500 to 700°C. If the coiling temperature is below 500°C, the tension may become excessively high during the coiling process, and the width shape of the hot-rolled coil may become defective. On the other hand, if the temperature exceeds 700°C, coarse carbides may be excessively formed, and these carbides may cause cracks to occur when stress is generated in the hot-formed member.

[0068] A cold-rolled steel sheet can be obtained by cold-rolling the coiled hot-rolled steel sheet. At this time, the cold-rolling process can be performed while uncoiling the coiled hot-rolled steel sheet, and rolling can be performed to obtain a target thickness, and the conditions are not specifically limited. However, as a non-limiting example, cold-rolling can be performed with a reduction rate of 30 to 80%. If the reduction rate is less than 30%, a cold-rolled steel sheet of the target thickness cannot be obtained, whereas if it exceeds 80%, the rolling load becomes excessive, and there is a risk that the shape will become defective.

[0069] The above cold-rolled steel sheet can be subjected to continuous annealing. The continuous annealing can be performed using a continuous annealing furnace and can be carried out within a temperature range where the microstructure of the cold-rolled steel sheet can be completely recrystallized. As an example, the cold-rolled steel sheet can be heated to a temperature range of Ac1 to Ac3. If the heating temperature for the annealing of the cold-rolled steel sheet is below Ac1, complete recrystallization does not occur, and a uniform microstructure cannot be obtained during the subsequent cooling process. On the other hand, if the temperature exceeds Ac3, the recrystallization effect becomes saturated, and excessive cooling is required during subsequent cooling, which may lead to a risk of defective shape.

[0070] Meanwhile, decarburization can be induced on the surface of the cold-rolled steel sheet during the continuous annealing process described above. To achieve this, it is necessary to control the atmosphere during the process of heating to and maintaining the temperature range described above. In one embodiment of the present invention, a decarburization effect can be obtained by controlling the dew point temperature of the atmosphere. As an example, the dew point temperature of the atmosphere can be controlled to -10 to +20°C. If the dew point temperature is below -20°C, sufficient decarburization of the steel sheet surface does not occur; on the other hand, if the temperature exceeds +20°C, the surface becomes excessively decarburized, which may lead to a decrease in the strength of the member after hot forming. Furthermore, fatigue characteristics may be inferior due to the excessively low carbon content of the surface layer of the base steel sheet.

[0071] According to one embodiment of the present invention, a heated cold-rolled steel sheet can be maintained while the dew point temperature of the atmosphere is controlled, and as one example, this can be done for 50 to 150 seconds. If the maintenance time is less than 50 seconds, surface decarburization does not occur sufficiently, whereas if it exceeds 150 seconds, there is a risk that surface decarburization may occur excessively.

[0072] In one embodiment of the present invention, the continuous annealing treatment may be performed in a reducing atmosphere. As a non-limiting example, the atmosphere may be controlled using a mixed gas of hydrogen and nitrogen, and may be performed with an atmosphere gas controlled by 5 to 15 vol% H2, the remainder N2, and unavoidable impurities. If the content of H2 in the atmosphere gas is less than 5 vol%, it may be difficult to control it as a reducing atmosphere, whereas if it exceeds 15 vol%, there is a risk of explosion due to hydrogen.

[0073] According to the above, a cold-rolled steel sheet that has completed continuous annealing treatment can be cooled to obtain a base steel sheet for subsequent plating treatment. Although the conditions of the cooling process are not specifically limited, as an example, cooling to a temperature of 100°C or lower can be performed at a cooling rate of 10°C / s or more. If the cooling rate is less than 10°C / s, carbon within the steel diffuses to the decarburized area during the cooling process, and the decarburization effect during continuous annealing treatment can be eliminated.

[0074] A plating layer can be formed on one or both sides of a steel sheet obtained according to one embodiment of the present invention by loading the steel sheet into a molten plating facility and performing a molten plating treatment. As one example, when forming an Al-based plating layer as the plating layer, the molten plating treatment may be a molten aluminum plating method in which the steel sheet is immersed in a molten aluminum plating bath to perform plating. At this time, the temperature of the molten aluminum plating bath may be set to a temperature range for normal aluminum plating, and as a non-limiting example, it may be performed in a temperature range of 620 to 680°C.

[0075] In one embodiment of the present invention, the molten aluminum plating bath is a plating bath containing aluminum (Al) as a main component, specifically containing copper (Cu): 5.0 to 50.0 wt%, the remainder being Al and other unavoidable impurities. Thus, by adding a certain amount of Cu to the plating bath, the outermost surface layer of the final hot-formed member may be provided with a first alloy phase containing Cu, for example, an Fe-Al-Cu alloy phase. Accordingly, the hot-formed member of the present invention may have the effect of improved anti-sintering properties.

[0076] Meanwhile, the addition of components other than the above Cu is not excluded, and as a non-limiting example, the plating bath may further include one or more selected from Si, Fe, Mg, Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, and Sr in addition to the above Cu. Although not limited thereto, among the elements that can be optionally added, Si may be 12% or less, Mn may be 10% or less, Mg may be 5% or less, and Fe may be 3% or less.

[0077] In one embodiment of the present invention, the plating amount (also referred to as the adhesion amount) during plating is 20 to 100 g / m² on a single side. 2 It may be. The above plating amount is 20g / m² 2 If it is less than that, there is a concern that corrosion resistance may decrease. On the other hand, if the plating amount is 100g / m² 2 If it exceeds [amount], there is a risk that unalloyed Al may adhere to the rolls inside the furnace during high-temperature heating for hot forming.

[0078] Meanwhile, in one embodiment of the present invention, a step of alloying heat treatment may be further performed after performing the aluminum plating. The step of alloying heat treatment is not particularly limited, but may be performed by online heating while the plated steel sheet (plated steel sheet having an Al-based plating layer) obtained by the molten aluminum plating is in motion. When performing alloying heat treatment by the aforementioned online heating, as one example, the heating temperature range may be 670 to 900°C, and the holding time may be 1 to 20 seconds.

[0079] A hot-formed member can be obtained by hot-forming a plated steel sheet having an Al-based plating layer obtained as described above. At this time, the hot-forming to obtain the member can be manufactured through a process in which the plated steel sheet having an Al-based plating layer is heated to a temperature above the austenitizing temperature, maintained at that temperature, and then formed simultaneously with rapid cooling, as is widely known in the past.

[0080] However, as one example of the present invention, after obtaining a blank from a plated steel sheet having an Al-based plating layer according to one embodiment of the present invention, the blank may be heated to a temperature range of Ac3 to 950°C and then maintained for 20 to 1000 seconds. A hot-formed member may be manufactured by hot-press forming the blank heated and maintained in this manner 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 20°C / s or more.

[0081] 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.

[0082] (Example)

[0083] An annealed cold-rolled material having an alloy composition of 0.22%C-0.25%Si-1.2%Mn-0.03%Al-0.03%Ti-0.2%Cr-0.0025%B-0.006%N-balance of Fe and unavoidable impurities was prepared as a base steel sheet. The annealed cold-rolled material was manufactured by the following method. First, a steel slab having the aforementioned alloy composition was heated to 1250°C, then finished hot-rolled at 900°C, and coiled at 600°C to obtain a hot-rolled steel sheet. Then, this hot-rolled steel sheet was cold-rolled at a cold reduction rate of 50% to obtain a cold-rolled steel sheet. After that, this cold-rolled steel sheet was heated to 800°C and subjected to continuous annealing heat treatment (dew point temperature: +5°C) to manufacture an annealed cold-rolled material with a thickness of 1.5 mm. For the annealed cold-rolled material produced in this way, a plated steel sheet having an Al-based plating layer was obtained by immersing it in an Al-based plating bath (residue Al and unavoidable impurities) having the composition shown in Table 1 below and performing plating. Subsequently, a plated steel sheet was obtained by performing an alloying heat treatment under the conditions shown in Table 1 below in succession to the plating process.

[0084] Subsequently, a hot forming process was applied to each plated steel sheet to manufacture a hot-formed member. At this time, the heating temperature for hot forming was set to 930°C. After heating for 5 minutes in a furnace with this set temperature, the member was transferred to a mold and rapidly cooled (at a cooling rate of approximately 30°C / s or more) simultaneously with forming to manufacture the hot-formed member. At this time, the transfer time to the mold was set to 10 seconds, and a flat plate mold was used for forming.

[0085] Specimen (No.) Plating Process Alloying Heat Treatment Process Plating Bath Composition (Wet%) Coating Amount (g / m²) 2Temperature (°C) Holding Time (s) Cubel.1 3.0 Al and impurities 609303002 10.0 Al and impurities 609303003 15.0 Fe 2%, Al and impurities 209303004 20.0 Mn 5%, Al and impurities 609303005 25.0 Si 10%, Al and impurities 809303006 30.0 Mg 1%, Al and impurities 1009303007 50.0 Al and impurities 409303008 - Si 8%, Al and impurities 60930300

[0086] To analyze the plating layer of each component manufactured according to the above, the cross-section in the thickness direction of each plated steel sheet was observed. Specifically, each cross-section in the thickness direction was observed using SEM, and after confirming the formation of each alloy layer within the plating layer, the thickness of each alloy layer was measured. In addition, the thickness and exposure of the region corresponding to the first alloy phase were measured. Furthermore, the component content within each alloy layer was measured by point analysis at five random locations using the Energy Dispersive Spectroscopy (EDS) method, and the results were expressed as the average value. The results are shown in Table 2.

[0087] Specimen No. Plating Layer Structure Thickness Exposure of First Alloy Phase Plating Layer Composition (Wet%) Classification (㎛) (%) AlFeCu Other 1 First Alloy Layer B Layer 7-3466--Comparative Example 1 Layer a 10-1090--Second Alloy Layer 27-4852--First Alloy Phase 0.1834642-2 First Alloy Layer B Layer 6-32618-Inventive Example 1 Layer a 9-1288--Second Alloy Layer 24-5050--First Alloy Phase 112325711-3 First Alloy Layer B Layer 2-295912-Inventive Example 2 Layer a 3-1189--Second Alloy Layer 8-4753--First Alloy Phase 350315118-4 First Alloy Layer B Layer 5-314722-Inventive Example 3 Layer a Layer 7-1288--2nd alloy layer 20-4852--1st alloy phase 795304327-5 1st alloy layer b Layer 3-294123Si: 7 Invention Example 4 a Layer 4-1286-Si: 2 2nd alloy layer 30-4752-Si: 1 1st alloy phase 690304125Si: 46 1st alloy layer b Layer 4-304324Mg: 3 Invention Example 5 a Layer 7-1288--2nd alloy layer 29-4852--1st alloy phase 9100303335Mg: 27 1st alloy layer b Layer 4-304327- Invention Example 6 a Layer 5-10882-2nd alloy layer 17-474211-1st alloy phase 13100302347-8 1 Alloy layer b Layer 3-3257-Si: 11 Comparative Example 2 a Layer 6-1186-Si: 3 2nd alloy layer 21-5048-Si: 2 1st alloy phase 4-3157-Si: 12

[0088] As shown in Tables 1 and 2, in the inventive examples in which the Cu content in the plating bath satisfies the range according to one embodiment of the present invention and alloying heat treatment is performed after plating using this plating bath, it can be confirmed that the first alloy layer and the second alloy layer are clearly formed, and the content of each element and the thickness of the second alloy layer are formed as intended.

[0089] On the other hand, in Comparative Example 1, in which plating was performed using a plating bath with a Cu content of 3.0%, a Cu-based alloy phase was not sufficiently formed on the outermost layer of the plating layer, so the first alloy phase was not formed as intended. Furthermore, in Comparative Example 8, in which plating was performed without containing Cu in the plating bath, the first alloy phase was not formed at all. Among these, it should be noted that in Comparative Example 2, the first alloy phase is not the outermost layer, but only the composition of the first alloy layer, the second alloy layer, and the layer corresponding to the first alloy phase is described based on the inventive example.

[0090] Figure 2 shows a photograph of a cross-section of the plating layer of Invention Example 4 taken using a scanning electron microscope (SEM). As shown in Figure 2, it can be confirmed that a first alloy phase containing a Cu alloy phase (Fe-Al-Cu alloy phase) is formed on the surface (outermost layer) of the plating layer. In addition, it can be confirmed that a second alloy layer is formed below the first alloy phase, and a first alloy layer composed of a layer a and a layer b is formed below that.

Claims

1. A base steel plate and an Al-based plating layer formed on the surface of the base steel plate, and The above Al-based plating layer is, A first alloy layer formed on the above-mentioned steel plate and comprising Al, Fe, and Cu; A second alloy layer formed on the first alloy layer and comprising Al and Fe; Formed on or inside the second alloy layer and comprising a first alloy phase including Al, Fe, and Cu, A hot-formed member having a Cu content of 5.0 to 50.0 weight% in the first alloy phase.

2. In Paragraph 1, The first alloy layer comprises a layer a formed on the base steel plate and a layer b formed on the layer a. A hot-formed member in which the Fe content of the above-mentioned layer a is greater than the Fe content of the above-mentioned layer b.

3. In Paragraph 1 or 2, The average Cu content in the first alloy layer is 30.0 wt% or less (including 0%), and A hot-formed member in which the Cu content of the above-mentioned layer a is smaller than the Cu content of the above-mentioned layer b.

4. In any one of paragraphs 1 to 3, The second alloy layer may further include 5.0 weight% or less of Cu, and A hot-formed member in which the Cu content in the first alloy layer is greater than the Cu content in the second alloy layer.

5. In any one of paragraphs 1 through 4, A hot-formed member having an exposure degree of 10.0% or more of the first alloy phase.

6. In any one of paragraphs 1 through 5, The first alloy phase exists as a layer on the second alloy layer, and A hot-formed member having a thickness of 20㎛ or less (excluding 0) of the above layer.

7. In any one of paragraphs 1 through 6, A hot-formed member in which the above Al-based plating layer further comprises one or more of Si, Mn, Mg, and Fe.

8. In any one of paragraphs 1 through 7, 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): 0.10~4.00%, 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): A hot-formed member having a composition comprising 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.

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