Steel sheet for hot press forming, hot press formed member and methods for producing same

Abstract

The present invention relates to a steel sheet suitable for the material of a vehicle body component and, more specifically, to a steel sheet for hot press forming, suitable for hot press forming, a hot press formed member manufactured therefrom, and methods for producing same.

Description

Steel plates for hot forming, hot forming members and methods for manufacturing them

[0001] The present invention relates to a steel sheet suitable as a material for automobile body parts, and more specifically, to a steel sheet suitable for hot forming, a hot forming member manufactured therefrom, and a method for manufacturing the same.

[0002] Recently, with the push for improved fuel efficiency through lightweight automobiles and increasingly stringent regulations on passenger protection, the application of hot-formed components as structural components in automobiles has been increasing. Hot-formed components are produced by applying a hot forming process to a specific steel plate, which allows them to exhibit high strength. Therefore, hot-formed components are ideally suited for applications requiring ultra-high strength or high energy absorption, such as bumpers, doors, and pillar reinforcements.

[0003] As a technology related to hot forming of steel plates, Patent Document 1 has been proposed. This document describes a method for producing a member by heating an Al-Si-based plated steel plate to 850°C or higher, hot forming using a press, and then rapidly cooling the member, thereby forming the member into a martensite structure, thereby ensuring ultra-high tensile strength. As such, hot-formed members are obtained by forming steel plates at high temperatures, and thus have the advantage of being easy to form when manufacturing parts with complex shapes. In addition, since rapid cooling within the forming process can promote an increase in strength, a weight reduction effect due to high strength can be expected.

[0004] Meanwhile, martensite is known to have low resistance to hydrogen embrittlement. However, hot-formed parts whose structure is mainly composed of martensite phase become more susceptible to hydrogen embrittlement when hydrogen dissociated from moisture in the furnace atmosphere flows in during high-temperature heating for hot-forming steel sheets, resulting in a high amount of diffusible hydrogen in the steel. In particular, when the strength required for hot-formed parts is 1.8 GPa or higher, hydrogen embrittlement occurs more easily as the strength increases, limiting its expanded application to various automotive parts.

[0005] Therefore, in order to expand the application of hot-formed parts of 1.8 GPa or higher to various automotive parts, the problem of hydrogen embrittlement of hot-formed parts must be fundamentally resolved, and technological development for this purpose is necessary.

[0006] (Patent Document 1) U.S. Patent Publication No. 6296805

[0007] One aspect of the present invention relates to a steel sheet suitable as a material for structural members of automobiles, and particularly to a hot-forming galvanized steel sheet suitable for a hot-forming process and a method for manufacturing the same.

[0008] In addition, another aspect of the present invention is to provide a hot-formed member obtained from a hot-formed plated steel sheet, and in particular, a hot-formed member having ultra-high strength and improved hydrogen embrittlement resistance.

[0009] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0010] According to one aspect of the present invention, a hot-forming plated steel sheet is provided, which includes a base steel sheet containing, by weight %, carbon (C): 0.30 to 0.50%, and an aluminum-based plated layer formed on at least one surface of the base steel sheet.

[0011] In one embodiment of the present invention, the minimum carbon content (C) is defined within the surface layer within 30 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer. min ) and the ratio of carbon content (C0) of the steel sheet (C min / C0) can include an area satisfying 0.5 to 0.8.

[0012] In one embodiment of the present invention, an internal oxide may be included within a depth of 0.5 to 5.0 μm in the thickness direction of the base steel sheet from the interface between the base steel sheet and the plating layer. As one example, the internal oxide may be formed at a grain boundary.

[0013] In one embodiment of the present invention, the steel sheet may include at least one carbide selected from V-type carbide, Mo-type carbide, and V-Mo composite carbide, and the carbides may be 10 5 dog / mm 2 It can be included as above. In addition, in one embodiment of the present invention, the carbide can have an average size of 1 to 100 nm.

[0014] In this way, the steel sheet for hot forming according to one aspect of the present invention includes a region with a low carbon content near the interface between the base steel sheet and the plating layer and an internal oxide at grain boundaries, thereby having the effect of preventing embrittlement due to hydrogen penetration during high-temperature heating for hot forming the steel sheet for hot forming. This effect can also be obtained by the carbide according to one embodiment of the present invention.

[0015] In one embodiment of the present invention, the steel sheet may further include, in wt%, silicon (Si): 0.05 to 1.00%, manganese (Mn): 0.4 to 3.0%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.0001 to 0.0200%, aluminum (Al): 0.01 to 0.10%, nitrogen (N): 0.001 to 0.020%, boron (B): 0.0001 to 0.0100%, chromium (Cr): 0.01 to 0.50%, titanium (Ti): 0.010 to 0.050%, and the sum of the contents of at least one of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%, the remainder being iron and unavoidable impurities.

[0016] In one embodiment of the present invention, the aluminum-based plating layer may include, in weight percent, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder Al, and other unavoidable impurities. In addition, in one embodiment of the present invention, the aluminum-based plating layer may have a thickness of 5 to 40 μm.

[0017] According to another aspect of the present invention, there is provided a method of manufacturing a steel sheet, comprising: preparing a steel slab containing, by weight %, carbon (C): 0.30 to 0.50%; heating the steel slab at a temperature range of 1050 to 1300°C; finishing hot-rolling the steel slab at a temperature range of 800 to 950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 500 to 700°C; heating the hot-rolled steel sheet after the coiling at a temperature range of 740 to 860°C and then annealing the steel sheet in an atmosphere having a dew point temperature of -10 to +20°C for 50 to 150 seconds; and a first carbon diffusion step of cooling the hot-rolled steel sheet at a cooling rate of 0.3 to 2.0°C / s in an atmosphere having a dew point temperature of -30 to -60°C after the annealing treatment. A method for manufacturing a plated steel sheet for hot forming is provided, comprising: a step of immersing a hot-rolled steel sheet in an aluminum-based plating bath after the first carbon diffusion step to obtain an aluminum-based plated steel sheet; and a second carbon diffusion step of cooling the aluminum-based plated steel sheet to 400°C at a cooling rate of 4 to 12°C / s.

[0018] In one embodiment of the present invention, the annealing treatment can be performed in an atmosphere gas controlled by 5 to 15 vol% of H2, the remainder N2, and unavoidable impurities.

[0019] According to one embodiment of the present invention, a steel sheet for hot forming can be provided that includes a region with a reduced carbon content and an internal oxide at the interface between the plating layer and the base steel sheet by performing a series of annealing treatments, carbon diffusion processes, etc.

[0020] In one embodiment of the present invention, a step of obtaining a cold-rolled steel sheet by cold rolling at a reduction ratio of 20 to 80% before annealing the hot-rolled steel sheet after coiling may be further included.

[0021] In one embodiment of the present invention, the aluminum-based plating bath may be composed of, in weight %, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder Al and other unavoidable impurities, and the step of obtaining the plated steel sheet is 30 to 130 g / m on one side.2 It can be done with the amount of plating.

[0022] A steel slab according to one embodiment of the present invention may have the above-described alloy composition.

[0023] According to another aspect of the present invention, a hot-formed member is provided, which includes a steel sheet containing carbon (C) in a weight percent amount of 0.30 to 0.50 and an aluminum-based plating layer formed on at least one surface of the steel sheet.

[0024] In one embodiment of the present invention, the maximum carbon content (C) is defined as a region within 30 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer. max ) and the ratio of carbon content (C0) of the steel sheet (C max / C0) can include an area satisfying 0.8 to 1.1.

[0025] In this way, a hot-formed member according to one embodiment of the present invention can have excellent resistance to hydrogen introduced during hot forming by including a region in which carbon exists at a certain ratio near the interface between the base steel sheet and the plating layer.

[0026] According to one embodiment of the present invention, the hot-formed member may include carbides present in the base steel sheet of the hot-formed steel sheet mentioned above.

[0027] In one embodiment of the present invention, the hot-formed member may have a tensile strength of 1800 MPa or more.

[0028] The steel sheet and the plating layer according to one embodiment of the present invention may have the above-described alloy composition.

[0029] According to the present invention, a hot-forming galvanized steel sheet suitable as a material for automobile body parts can be provided, and a hot-forming member obtained using such a hot-forming galvanized steel sheet can be provided.

[0030] In addition, according to the present invention, a hot-formed member having ultra-high strength and excellent hydrogen embrittlement resistance can be provided.

[0031] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0032] Figure 1 shows the results of component analysis using a glow discharge spectrometer (GDS) in the thickness direction from the surface of the plating layer of a steel sheet for hot forming (Invention Example 1) according to one embodiment of the present invention, and C measured therefrom. min This is a graph showing the calculated value of / C0.

[0033] FIG. 2 is a photograph of a cross-section of a steel sheet for hot forming (Invention Example 1) according to one embodiment of the present invention, taken using an SEM, from the interface between the base steel sheet and the plating layer in the thickness direction of the base steel sheet.

[0034] FIG. 3 is a result of observing carbides present in a steel sheet for hot forming (invention example 1) according to one embodiment of the present invention using TEM.

[0035] Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can also be usefully applied to other aspects of the present invention. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to achieve beneficial effects.

[0036] Hereinafter, preferred embodiments of the present invention will be described. However, the 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. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0037] The inventors of the present invention have conducted in-depth research on a method for improving the low hydrogen resistance of a hot-formed part obtained by hot-forming a steel sheet suitable for use as a material for automobile body parts, etc., and particularly a hot-formable steel sheet suitable for hot-forming.

[0038] In particular, the inventors of the present invention conducted in-depth research on a method for securing hydrogen embrittlement resistance along with the ultra-high strength characteristics of hot-formed parts, and thus completed the present invention.

[0039] Hereinafter, the present invention will be described in detail.

[0040] According to one aspect of the present invention, a steel sheet for hot forming is provided, which includes a base steel sheet and an aluminum-based plating layer formed on at least one surface of the base steel sheet.

[0041] The inventors of the present invention have conducted in-depth research into methods for improving the ultra-high strength and hydrogen embrittlement resistance of hot-formed parts when manufacturing hot-formed parts using hot-formed steel plates. As a result, they discovered that by using carbon steel containing a certain amount of carbon (C) as the base steel for hot-formed steel plates and partially decarburizing the surface of such carbon steel, a hot-formed part with excellent resistance to hydrogen embrittlement can be provided.

[0042] Accordingly, in one embodiment of the present invention, the base steel plate may be carbon steel containing 0.30 to 0.50% by weight of carbon (C). Meanwhile, when using carbon steel containing a certain amount of carbon as the base steel plate, the carbon steel may further contain other components other than C. Although the components are not specifically limited, as a non-limiting example, the composition may further include, in weight %, silicon (Si): 0.05 to 1.00%, manganese (Mn): 0.4 to 3.0%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.0001 to 0.0200%, aluminum (Al): 0.01 to 0.10%, nitrogen (N): 0.001 to 0.020%, boron (B): 0.0001 to 0.0100%, chromium (Cr): 0.01 to 0.50%, titanium (Ti): 0.010 to 0.050%, and the sum of the contents of at least one of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%.

[0043] Hereinafter, the alloy composition of a steel plate, for example, carbon steel, according to one embodiment of the present invention will be described, and unless otherwise specified, the content unit of each alloy composition means weight%.

[0044] Carbon (C): 0.30~0.50%

[0045] Carbon (C) is an element that significantly increases the strength of hot-formed parts and enhances hardenability, and it must be added in an appropriate amount to control strength.

[0046] In one embodiment of the present invention, if the C content is less than 0.30%, it is difficult to secure a target level of strength, particularly a tensile strength of 1.8 GPa or more for hot-formed parts. On the other hand, if the C content exceeds 0.50%, the strength of the steel may increase excessively, which may cause brittleness and may lead to poor weldability.

[0047] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.30 to 0.50%. In another embodiment of the present invention, C may be included in an amount of 0.32% or more, or 0.34% or more. In yet another embodiment of the present invention, C may be included in an amount of 0.48% or less, or 0.46% or less.

[0048] Silicon (Si): 0.05~1.00%

[0049] Silicon (Si) is an element added for deoxidation during the steelmaking process, has a solid solution strengthening effect, and suppresses the formation of carbides, effectively achieving uniformity of the internal structure of steel. In one embodiment of the present invention, it not only contributes to increasing the strength of hot-formed parts, but is also an element effective in uniformizing material properties.

[0050] In one embodiment of the present invention, if the Si content is less than 0.05%, the aforementioned effects cannot be sufficiently achieved, and there is a concern that excessive process costs will be incurred due to the attempt to control the Si content to a low level. On the other hand, if the content exceeds 1.00%, excessive Si oxide is formed on the surface of the steel sheet during the annealing process, resulting in significantly inferior plating properties during the subsequent plating process. This makes it difficult to ensure the surface quality of the plated steel sheet.

[0051] Therefore, in one embodiment of the present invention, Si may be included in an amount of 0.05 to 1.00%. According to another embodiment of the present invention, Si may be included in an amount of 0.07% or more, or 0.10% or more. According to yet another embodiment of the present invention, Si may be included in an amount of 0.95% or less, or 0.90% or less.

[0052] Manganese (Mn): 0.4~3.0%

[0053] Manganese (Mn) is an effective element for securing target strength due to its solid solution strengthening effect, and can be added to suppress the formation of ferrite during hot forming as it improves the hardenability of steel.

[0054] In one embodiment of the present invention, if the manganese content is less than 0.4%, the steel's hardenability cannot be sufficiently secured. Furthermore, the addition of other expensive alloying elements may be necessary to compensate for the insufficient hardenability. In this case, there is a problem that the manufacturing cost increases significantly. On the other hand, if the manganese content exceeds 3.0%, the band-like structure arranged in the rolling direction in the microstructure is aggravated, causing non-uniformity in the internal structure, which may result in deterioration of the steel's physical properties, such as bendability.

[0055] Accordingly, in one embodiment of the present invention, Mn may be included in an amount of 0.4 to 3.0%. In another embodiment of the present invention, the Mn may be included in an amount of 0.5% or more, or 0.6% or more. In yet another embodiment of the present invention, the Mn may be included in an amount of 2.9% or less, or 2.8% or less.

[0056] Phosphorus (P): 0.001~0.050%

[0057] Phosphorus (P) is an element that is inevitably added during the steel manufacturing process.

[0058] In one embodiment of the present invention, if the P content exceeds 0.050%, the weldability of the hot-formed part may be impaired, and the material properties may deteriorate due to segregation at grain boundaries at high temperatures. On the other hand, considering that excessive manufacturing costs occur in order to control the P content to the extreme and the level that must be added inevitably, in one embodiment of the present invention, the lower limit may be limited to 0.001%.

[0059] Therefore, in one embodiment of the present invention, P can be limited to 0.001 to 0.050%.

[0060] Sulfur (S): 0.0001~0.0200%

[0061] Sulfur (S) is an element that is inevitably introduced during the steel manufacturing process, and it is an element that impairs the ductility, impact toughness, and weldability of steel, so it is advantageous to control it as low as possible.

[0062] In one embodiment of the present invention, when the content of S exceeds 0.0200%, the aforementioned problems may occur, and therefore, the upper limit may be limited to 0.0200%. On the other hand, in order to control the content of S to the extreme, excessive manufacturing costs are incurred, and considering the level that must be added, in one embodiment of the present invention, the lower limit may be limited to 0.0001%.

[0063] Therefore, in one embodiment of the present invention, S can be limited to 0.0001 to 0.0200%.

[0064] Aluminum (Al): 0.01~0.10%

[0065] Aluminum (Al) can be added together with the above Si for deoxidation during the steelmaking process, thereby improving the cleanliness of the steel.

[0066] In one embodiment of the present invention, if the Al content is less than 0.01%, not only cannot the aforementioned effect be sufficiently obtained, but also manufacturing costs may increase excessively in order to control the Al content to a low level, so the content may be limited to 0.01% or more. On the other hand, if the Al content exceeds 0.10%, AlN precipitates are excessively formed during the continuous casting process, which reduces the high-temperature ductility of the steel and causes slab cracks.

[0067] Therefore, in one embodiment of the present invention, Al may be included in an amount of 0.01 to 0.10%. According to another embodiment of the present invention, Al may be included in an amount of 0.015% or more, or 0.020% or more. According to yet another embodiment of the present invention, Al may be included in an amount of 0.090% or less, or 0.085% or less.

[0068] Nitrogen (N): 0.001~0.020%

[0069] Nitrogen (N) may be added unavoidably during the steel manufacturing process.

[0070] In one embodiment of the present invention, when the content of N exceeds 0.020%, slab cracks may easily occur due to the formation of AlN precipitates by combining with Al added for deoxidation and other effects. On the other hand, considering that excessive manufacturing costs occur in order to control the content of N to the extreme and the level that must be added inevitably, in one embodiment of the present invention, the lower limit may be limited to 0.001%.

[0071] Therefore, in one embodiment of the present invention, N can be limited to 0.001 to 0.020%.

[0072] Boron (B): 0.0001~0.0100%

[0073] Boron (B) is an element that can improve the hardenability of steel even with a small amount of addition, and is effective in preventing embrittlement due to grain boundary segregation of P and / or S added as impurities by segregating at the grain boundaries of old austenite.

[0074] In one embodiment of the present invention, if the content of B is less than 0.0001%, the aforementioned effect cannot be sufficiently obtained. On the other hand, if the content exceeds 0.0100%, Fe 23 It can cause embrittlement in steel during hot rolling by forming complex compounds such as CB6.

[0075] Therefore, in one embodiment of the present invention, B may be included in an amount of 0.0001 to 0.0100%. According to another embodiment of the present invention, B may be included in an amount of 0.0003% or more, or 0.0005% or more. According to yet another embodiment of the present invention, B may be included in an amount of 0.0090% or less, or 0.0080% or less.

[0076] Chromium (Cr): 0.01~0.50%

[0077] Chromium (Cr), similar to Mn and B in steel, is an element that improves the hardenability of steel.

[0078] In one embodiment of the present invention, if the content of Cr is less than 0.01%, it is difficult to sufficiently improve the hardenability of the steel, whereas if the content exceeds 0.50%, the effect of improving the hardenability becomes saturated, and the cost of manufacturing the steel may rather increase.

[0079] Therefore, in one embodiment of the present invention, Cr may be included in an amount of 0.01 to 0.50%. In another embodiment of the present invention, Cr may be included in an amount of 0.02% or more, or 0.03% or more. In yet another embodiment of the present invention, Cr may be included in an amount of 0.48% or less, or 0.45% or less.

[0080] Titanium (Ti): 0.010~0.050%

[0081] Titanium (Ti) combines with N added as an impurity in steel to form TiN precipitates, thereby protecting B, which can be added to secure the hardenability of steel, from being precipitated as BN.

[0082] In one embodiment of the present invention, if the Ti content is less than 0.010%, it is difficult to expect the aforementioned effect. On the other hand, if the Ti content exceeds 0.050%, the aforementioned effect becomes saturated, and manufacturing costs may increase. In addition, coarse precipitates may be formed, which may reduce the strength of the steel.

[0083] Therefore, in one embodiment of the present invention, Ti may be included in an amount of 0.010 to 0.050%. According to another embodiment of the present invention, Ti may be included in an amount of 0.012% or more, or 0.015% or more. According to yet another embodiment of the present invention, Ti may be included in an amount of 0.048% or less, or 0.045% or less.

[0084] Sum of one or more of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%

[0085] Molybdenum (Mo), niobium (Nb), and vanadium (V) are effective elements in refining grains by forming precipitates such as carbides and / or carbonitrides in steel. Precipitates formed by these elements act as hydrogen trapping sites within the steel, effectively improving the hydrogen embrittlement resistance of the steel.

[0086] In one embodiment of the present invention, Mo, Nb, and V may be included in at least one kind, and the aforementioned effect can be expected by adding them in an amount of 0.10% or more based on the total content thereof. On the other hand, if the total content thereof exceeds 0.50%, it is difficult to expect the effect of improving hydrogen embrittlement resistance as well as the effect of refining crystal grains by forming coarse precipitates rather than securing the number (number density) of precipitates formed.

[0087] Meanwhile, in one embodiment of the present invention, the steel sheet may further include additional components in addition to the aforementioned alloy composition. As a non-limiting example, the steel sheet may further include at least one selected from the group consisting of calcium (Ca): 0.01% or less, tungsten (W): 1.0% or less, rare earth elements (REM): 0.3% or less, and at least one Group 1 element: 0.005 to 1.000%. All of these elements may be optionally added, and may include 0% in content.

[0088] Calcium (Ca): 0.01% or less

[0089] Calcium (Ca) plays a role in removing phosphorus and sulfur to a certain level during the steelmaking process, thereby improving the purity of steel. Therefore, it can be added for this purpose. However, if its content exceeds 0.01%, the cost of steel manufacturing increases significantly. Therefore, when Ca is added, it can be included at 0.01% or less.

[0090] Tungsten (W): 1.0% or less

[0091] Tungsten (W) forms carbides in steel and serves to refine grain boundaries. In one embodiment of the present invention, W may be included for the purpose of improving the hydrogen embrittlement resistance of steel by refining grain boundaries. However, if the content exceeds 1.0%, the cost of manufacturing steel increases significantly, so when W is added, it may be included at 1.0% or less.

[0092] Rare earth elements (REM): 0.3% or less

[0093] Rare earth metals (REM) are elements that can be contained in steel during the steelmaking process and tend to segregate at grain boundaries. If the content of these REMs exceeds 0.3%, excessive segregation of REMs at grain boundaries can deteriorate the hydrogen embrittlement resistance of the steel. Therefore, when adding REMs, they can be included at 0.3% or less. Typically, rare earth elements (REM) refer to a total of 17 metallic elements, including scandium (Sc), yttrium (Y), lanthanum (La), and cerium (Ce).

[0094] One or more Group 1 elements: 0.005–1.000%

[0095] According to one embodiment of the present invention, the base steel sheet may further include one or more elements selected from the group consisting of first group elements, specifically antimony (Sb), tin (Sn), arsenic (As), bismuth (Bi), copper (Cu), and nickel (Ni). Among these, Sb, Sn, and Bi may be added to improve plating performance, such as plating wettability and plating adhesion, and Cu and Ni may be added to improve the corrosion resistance of the steel. In particular, when Ni is added together with Cu, the effect of preventing red-hot embrittlement that may occur due to the added Cu can be obtained. When these elements are added, each may be included in an amount of 0.005 to 1.000%, and even when two or more types are added, they may be added within a range of 0.005 to 1.000%.

[0096] In one embodiment of the present invention, the remaining components, excluding the aforementioned components, are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art, their full details are not specifically discussed in this specification.

[0097] At least one side of the steel plate according to one embodiment of the present invention may include a plating layer, and the plating layer may be an aluminum-based plating layer.

[0098] In one embodiment of the present invention, an aluminum-based plating layer formed on at least one surface of a base steel sheet is a plating layer containing aluminum (Al) as a main element, and as an example, the aluminum-based plating layer may include, in weight %, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder aluminum (Al), and unavoidable impurities. As will be described in detail later, such an aluminum-based plating layer can be formed by immersing the base steel sheet in an aluminum-based molten plating bath, and it is well known that the composition of the aluminum-based plating layer is determined according to the composition of the aluminum-based molten plating bath.

[0099] The above silicon (Si) not only lowers the melting point of the plating bath, but also prevents excessive alloying during high-temperature heating for hot forming. Accordingly, the Si content may be 6.0% or more. However, if the content exceeds 12.0%, there is a concern that the melting point of the plating bath may rapidly increase, and in this case, there is a problem that the amount of ash generated from the plating bath increases rapidly. In addition, the fluidity of the plating bath is reduced, making it difficult to form a uniform plating layer.

[0100] The above iron (Fe) may exist as an impurity in the plating bath, and may partially flow into the plating layer during the plating process. In addition, as Fe present in the base steel sheet is eluted into the plating layer during the plating process, it may be included in the aluminum-based plating layer. Accordingly, the aluminum-based plating layer may contain Fe at 1.0% or more and 9.0% or less. Meanwhile, in order to control the Fe content in the plating layer to less than 1.0%, the Fe eluted from the base steel sheet must be diluted, and in this case, there is a problem in that the manufacturing cost excessively increases. In addition, if the Fe content in the aluminum-based plating layer exceeds 9.0%, there is a concern that an Al-Fe alloy phase may be excessively formed, which may deteriorate the plating quality.

[0101] According to one example of the present invention, the aluminum-based plating layer may further include, in addition to the above-described components, general elements included in the plating layer. Examples of such elements include one or two or more selected from Mg, Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, and Sr.

[0102] In one embodiment of the present invention, the aluminum-based plating layer may have a thickness of 5.0 to 40.0 μm. If the thickness of the aluminum-based plating layer is less than 5.0 μm, the corrosion resistance of the member after hot forming may be reduced. On the other hand, if the thickness of the plating layer exceeds 40.0 μm, although it is advantageous for improving corrosion resistance, it is difficult to completely alloy the plating layer within a given heat treatment time, which may reduce productivity, and there is a problem that the possibility of aluminum sticking to the roll in the heating furnace increases.

[0103] As mentioned above, in order to simultaneously secure ultra-high strength and hydrogen embrittlement resistance of a hot-formed part, according to one embodiment of the present invention, the surface of a steel plate for hot forming to obtain a hot-formed part can be partially decarburized.

[0104] Specifically, in one embodiment of the present invention, the surface layer defined as within 30 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer has a minimum carbon content (C min ) and the ratio of carbon content (C0) of the steel sheet (C min / C0) can satisfy 0.5 to 0.8.

[0105] That is, the base steel sheet constituting the steel sheet for hot forming may be, for example, carbon steel containing a certain amount of carbon, and a region with a relatively low C content, a so-called decarburized region, may exist at the interface between the base steel sheet and the plating layer in the thickness direction of the base steel sheet for a certain depth. In this way, the steel sheet for hot forming according to one embodiment of the present invention can prevent embrittlement due to hydrogen during a subsequent hot forming process by having a region with a relatively low C content in the surface layer portion existing in the thickness direction of the base steel sheet at the interface between the base steel sheet and the plating layer.

[0106] In one embodiment of the present invention, the ratio of the minimum C content in the surface layer to the C content in the steel sheet (C min / C0) is less than 0.5, there is a problem that the strength of the member after hot forming is reduced due to excessive decarburization of the surface layer of the base steel plate. In addition, there is a possibility that the fatigue properties may be inferior due to the decrease in the hardness of the surface layer. On the other hand, if the value exceeds 0.8, the hydrogen embrittlement resistance of the hot formed member cannot be secured because the surface layer does not decarburize to the intended level. At this time, the description was made based on the minimum carbon content of the surface layer of the base steel plate, but even when viewed based on the maximum carbon content, the value does not exceed 1.0.

[0107] Meanwhile, a region with a relatively low C content within the surface layer of the steel plate, i.e. a decarburized region, may exist mainly near the interface, which is the area in contact with the plating layer.

[0108] When hot forming is performed using a plated steel sheet, alloying occurs at the interface between the base steel sheet and the plating layer during the heating process. When this alloying occurs, hydrogen from the outside (for example, the atmosphere inside the furnace where hot forming is performed) penetrates into the steel, and the carbon present on the surface of the base steel sheet cannot diffuse into the plating layer due to the low carbon solubility of the plating layer, but only diffuses toward the base steel sheet directly under the plating layer. At this time, if the alloying speed is faster than the speed at which carbon diffuses into the center of the base steel sheet (base material), the carbon is concentrated on the surface of the base steel sheet, and as a result, a region where carbon is concentrated in the thickness direction of the base steel sheet is created based on the interface between the plating layer and the base steel sheet.

[0109] Typically, hot-forming steel sheets used to obtain hot-formed parts are coated steel sheets with non-decarburized surfaces, meaning that the average carbon content is the same or similar to that of the base steel sheet throughout its entire thickness. When these coated steel sheets are heated to high temperatures and then hot-formed (pressed) using a mold or similar device, alloying occurs. During this process, carbon concentrates, creating more brittle martensite in areas with locally high carbon content. These areas become more brittle due to hydrogen entering the steel during the hot-forming process. This adversely affects the physical properties and hydrogen embrittlement resistance of the resulting hot-formed parts, ultimately rendering them unusable as products.

[0110] The inventors of the present invention have conducted in-depth research into a solution that can fundamentally solve the aforementioned problems. As a result, they have come to provide a steel sheet for hot forming, which has a decarburized region with a relatively low carbon content for a certain thickness from the interface between the base steel sheet and the plating layer in the direction of the thickness of the base steel sheet, and thus has a relatively low strength, in other words, a softer property than the interior of the base steel sheet. In this way, when hot forming a steel sheet for hot forming, which has a region with a relatively low carbon content on the surface side of the base steel sheet, alloying occurs at the interface between the base steel sheet and the plating layer during high-temperature heating, and hydrogen simultaneously intrudes from the outside, and after subsequent forming and cooling, the hydrogen-induced fracture stress increases due to the low strength of the region with a low carbon content, thereby enhancing the hydrogen embrittlement resistance of the hot-formed member.

[0111] In one embodiment of the present invention, an internal oxide may be included within a depth of 0.5 to 5.0 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the aluminum-based plating layer.

[0112] That is, as described above, an internal oxide may exist at a certain depth on the surface side of the surface layer corresponding to a certain thickness in the thickness direction of the base steel sheet at the interface between the base steel sheet and the plating layer. At this time, the internal oxide may be formed at the grain boundary and may be referred to as a grain boundary internal oxide. Such grain boundary internal oxide can act as a strong trap site that traps hydrogen flowing into the steel, and thus can contribute to improving the hydrogen embrittlement resistance of the component after hot forming.

[0113] In one embodiment of the present invention, if the depth of the intergranular oxide existing in the thickness direction of the base steel sheet from the interface between the base steel sheet and the plating layer is less than 0.5 μm, hydrogen trap sites cannot be sufficiently provided. On the other hand, if the depth exceeds 5.0 μm, the internal oxide is formed at an excessive depth, which may act as a stress concentration site during bending deformation and may actually deteriorate hydrogen embrittlement resistance.

[0114] In one embodiment of the present invention, the type of internal oxide is not particularly limited. However, since the internal oxide is formed in the substrate steel sheet, its type may be determined depending on the elements within the substrate steel sheet. As an example, among the elements within the substrate steel sheet, elements with a high affinity for oxygen will preferentially form oxides, and the oxides in this case may be silicon oxide, manganese oxide, etc.

[0115] In one embodiment of the present invention, the substrate steel plate may include a certain amount of carbide. The carbide may exist not only within the substrate steel plate but also on the surface layer of the substrate steel plate. As an example, the results of the number of carbides measured at the t / 4 point in the thickness direction of the substrate steel plate may be representatively represented.

[0116] In one embodiment of the present invention, the steel sheet comprises at least one carbide selected from among V-type carbide, Mo-type carbide, and V-Mo composite carbide. 5 dog / mm 2It can be included as above. In general, it is known that V and Mo are elements that are advantageous in forming precipitates when added in a certain amount to steel, and that grain refinement can be promoted by these precipitates. In addition, in one embodiment of the present invention, it is significant to utilize the precipitates (carbides) by the V and Mo as sites for hydrogen absorption. That is, during the hot forming process of a steel sheet for hot forming, hydrogen can flow into the interior of the steel sheet (base steel sheet), and at this time, the carbides mainly present inside the steel sheet trap the introduced hydrogen, thereby improving the hydrogen embrittlement resistance of the member.

[0117] In one embodiment of the present invention, the number of the carbides is 10 5 dog / mm 2 If it is less than this, hydrogen absorption sites cannot be sufficiently provided, making it difficult to improve the hydrogen embrittlement resistance of the component after hot forming. Meanwhile, the greater the number of carbides, the more advantageous it is for hydrogen absorption, and there is no particular limitation regarding the upper limit. However, the number of carbides can be formed appropriately depending on the content of the elements forming carbides in the base steel sheet.

[0118] In one embodiment of the present invention, when including carbides in the base steel sheet as described above, the average size of the carbides may be 1 to 100 nm. In this way, by the presence of a large amount of fine carbides, the hydrogen embrittlement resistance of the member after hot forming can be further improved. If the average size of the carbides is less than 1 nm, the grain refinement and hydrogen embrittlement resistance improvement effects are saturated, while the cooling rate may have to be excessively increased during the manufacturing process in order to control the size to a relatively finer size. In this case, it may become difficult to manufacture the intended steel sheet. On the other hand, if the size exceeds 100 nm, the hydrogen absorption effect cannot be sufficiently obtained due to the presence of mainly coarse carbides.

[0119] Hereinafter, a method for manufacturing a steel sheet for hot forming according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a steel sheet for hot forming according to one embodiment of the present invention.

[0120] According to one embodiment of the present invention, a prepared steel slab can be manufactured through a process of [heating - hot rolling - coiling - annealing - cooling - plating - cooling], and each process step is specifically described below.

[0121] [Heating of steel slabs]

[0122] After preparing a steel slab according to one embodiment of the present invention, the steel slab may be heated. The heating process of the steel slab is a process for smoothly performing the hot rolling process described below and sufficiently obtaining the target physical properties of the steel plate. As one example, the steel slab may have the same alloy composition as the steel plate according to one embodiment of the present invention, and the description of each alloy element is replaced with the above-mentioned matters.

[0123] In one embodiment of the present invention, the process of heating the steel slab may be performed under normal conditions, and as an example, may be performed at a temperature range of 1050 to 1300°C. If the heating temperature is lower than 1050°C, the structure of the slab may not be sufficiently homogenized. On the other hand, if the temperature exceeds 1300°C, not only will an oxide layer be excessively formed on the surface, but decarburization will also excessively occur in the surface layer of the slab during heating, making it impossible to obtain the steel sheet for hot forming targeted in the present invention.

[0124] [Hot rolling]

[0125] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0126] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature range of 800 to 950°C during hot rolling. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature below 800°C, a mixed grain structure is generated in the surface layer due to the ferrite and austenite dual-phase rolling, making it difficult to control the plate shape. On the other hand, if the temperature exceeds 950°C, there is a problem of the grains becoming coarse.

[0127] [Winding]

[0128] The hot-rolled steel sheet manufactured above can be wound, and a hot-rolled coil can be obtained therefrom.

[0129] In one embodiment of the present invention, the coiling process may be performed at a temperature range of 500 to 700°C. If the coiling temperature is lower than 500°C, internal oxides may not be formed to serve as hydrogen trap sites. In addition, the microstructure of the hot-rolled steel sheet may be formed as a martensite phase in whole or in part, making it difficult to control the shape of the sheet. In addition, the cold-rollability may be reduced when cold rolling is performed as a subsequent process due to the increase in strength of the hot-rolled steel sheet. On the other hand, if the coiling temperature exceeds 700°C, excessive surface decarburization of the base steel sheet in the coated steel sheet may occur, and internal oxidation may occur in too deep a region, which may reduce the strength of the part after hot forming, and there is a problem that hydrogen embrittlement is worsened.

[0130] [Sodun]

[0131] The above-mentioned hot-rolled steel sheet can be annealed.

[0132] In one embodiment of the present invention, the annealing treatment of the steel plate can be performed by heating the steel plate in a temperature range of 740 to 860°C and then in an atmosphere having a dew point temperature of -10 to +20°C.

[0133] If the heating temperature for the above annealing treatment is below 740°C, sufficient recrystallization of the structure may not occur, resulting in a poor plate shape. Furthermore, the strength of the plated steel sheet may increase excessively after the subsequent plating process, potentially causing mold wear during the blanking process. Conversely, if the temperature exceeds 860°C, surface oxides such as Si and Mn may form during the annealing process, resulting in a poor plated surface.

[0134] During the annealing process, decarburization can be induced on the surface of the steel sheet, and in order to sufficiently cause the decarburization, it is important to control the atmosphere after heating, and in one embodiment of the present invention, this can be achieved by controlling the dew point temperature of the atmosphere. In one embodiment of the present invention, if the dew point temperature of the atmosphere is below -10°C, decarburization of the surface of the steel sheet cannot sufficiently occur, whereas if the temperature exceeds +20°C, the surface may be excessively decarburized, which may reduce the strength of the member after hot forming. In addition, fatigue properties may be inferior due to excessively low carbon in the surface layer of the base steel sheet.

[0135] According to one embodiment of the present invention, the heated steel plate can be maintained in a controlled dew point temperature environment, for example, for 50 to 150 seconds. If the maintenance time is less than 50 seconds, surface decarburization may not occur sufficiently, whereas if it exceeds 150 seconds, there is a risk of excessive surface decarburization.

[0136] In one embodiment of the present invention, the annealing treatment may be performed in a reducing atmosphere. As a non-limiting example, a mixed gas of hydrogen and nitrogen may be used, and the annealing treatment may be performed in 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 in a reducing atmosphere, whereas if it exceeds 15 vol%, there is a risk of explosion due to hydrogen.

[0137] Meanwhile, in one embodiment of the present invention, it has been described that the annealing process can be performed on a hot-rolled steel sheet, and a subsequent plating process can be performed on the annealed hot-rolled steel sheet. In addition, if necessary, annealing, plating, etc. can be performed on a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet. The cold rolling can be performed to control the thickness and correct the shape of the steel sheet, and there is no particular limitation on the cold reduction ratio. As an example, when applied to an automotive steel sheet, it can be performed at a reduction ratio of 20 to 80%. If the cold reduction ratio is less than 20%, it is difficult to secure a uniform structure in the subsequent annealing process due to the low recrystallization driving force. On the other hand, if the cold reduction ratio exceeds 80%, not only the shape of the cold-rolled steel sheet becomes poor, but the load applied to the roll can also become severe.

[0138] Cooling (first carbon diffusion)

[0139] The above-mentioned annealed hot-rolled steel sheet or cold-rolled steel sheet can be cooled. Hereinafter, hot-rolled steel sheet and cold-rolled steel sheet are collectively referred to as steel sheet.

[0140] In accordance with one embodiment of the present invention, a steel sheet annealed under the conditions has a portion of its surface decarburized, so that a region or portion within the surface layer of the steel sheet has a carbon (C) content close to 0. In order to provide a certain amount of carbon to the surface layer where the decarburized region or portion exists, in one embodiment of the present invention, the annealed steel sheet is cooled, and as an example, cooling may be performed at a cooling rate of 0.3 to 2.0°C / s in an atmosphere having a dew point temperature of -30 to -60°C. The cooling process at this time is referred to as the first carbon diffusion step.

[0141] In one embodiment of the present invention, there is a problem that the process cost increases excessively in order to control the dew point temperature of the atmosphere during cooling of the annealed steel sheet to below -60℃. On the other hand, if the dew point temperature exceeds -30℃, there is a concern that additional decarburization may occur on the surface of the annealed steel sheet. In addition, if the cooling rate is less than 0.3℃ / s during cooling, the amount of carbon diffused is excessive, so that the decarburized area or decarburized portion within the surface layer may have a C content similar to that before decarburization. On the other hand, if the cooling rate exceeds 2.0℃ / s, there is not enough time for carbon to diffuse, making it impossible to secure a decarburized area or decarburized portion with a target carbon content within the surface layer.

[0142] By this cooling process, a certain amount of carbon can be diffused from the inside of the annealed steel plate into the surface layer having the decarburized area or decarburized portion.

[0143] [Plating]

[0144] According to the above, a steel sheet that has been cooled (first carbon diffusion) can be plated, and a plated steel sheet can be obtained therefrom.

[0145] In one embodiment of the present invention, the plating process of the steel sheet may be a hot-dip plating process using a hot-dip plating bath. As an example, a hot-dip plating bath whose main component is aluminum (a hot-dip aluminum-based plating bath) may be used. Plating can be performed by immersing the steel sheet in such a hot-dip plating bath, thereby obtaining an aluminum-based plated steel sheet.

[0146] In one embodiment of the present invention, the hot dip plating can be performed at a temperature range of 600 to 700°C, and this temperature range refers to the temperature of the plating bath at the time when the steel sheet is immersed during hot dip plating. If the temperature during the plating is less than 600°C, it is difficult to secure fluidity of the plating bath, making it difficult to precisely control the amount of plating attached to the surface of the steel sheet. On the other hand, if the temperature exceeds 700°C, Fe is excessively eluted from the steel sheet immersed in the plating bath, resulting in a large amount of Al-Fe compounds attached to the surface, thereby deteriorating the plating quality.

[0147] In one embodiment of the present invention, the molten aluminum-based plating bath is a plating bath containing aluminum (Al) as a main component, and as an example, the plating bath may contain 6.0 to 12.0 wt% of silicon (Si). In addition, the plating bath may contain some iron (Fe), and the iron (Fe) may be mainly eluted from a steel sheet immersed in the plating bath and may have a content of 1.0 to 9.0%. In addition, the component composition of the plating bath may be replaced with the aforementioned content. Meanwhile, the molten aluminum-based plating bath according to one embodiment of the present invention may further contain other elements in addition to the aforementioned components, and as a non-limiting example, may contain one or two or more of Mg, Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Zn, etc.

[0148] In one embodiment of the present invention, when performing molten plating, 30 to 130 g / m on one side 2 It can be done with a plating amount of . The plating amount for the above plating is 30g / m on one side. 2 If it is less than 130 g / m2, the plating layer is not sufficiently formed, making it impossible to secure corrosion resistance of the part after hot forming. On the other hand, if the plating amount is 130 g / m2 on one side, 2If it exceeds , not only will the amount of plating adhesion become excessive, increasing the manufacturing cost, but it will also be difficult to form a uniform plating layer in the width and length directions of the steel plate.

[0149] Cooling (secondary carbon diffusion)

[0150] A process of cooling the plated steel sheet obtained by the above, for example, an aluminum-based plated steel sheet, can be performed.

[0151] In one embodiment of the present invention, a plated steel sheet manufactured through a plating process has a plating layer of a certain thickness formed on the surface of a base steel sheet, and a cooling process can be performed to allow carbon (C) to diffuse from the inside of the base steel sheet toward the surface thereof. That is, the base steel sheet undergoes a series of annealing and cooling processes prior to the plating process so that a surface portion having a decarburized region or decarburized portion having a certain C content exists on the surface thereof, and after forming a plating layer on this surface portion, carbon diffusion can be additionally induced from the inside of the base steel sheet to the decarburized portion. Accordingly, this cooling step can be referred to as a second carbon diffusion step.

[0152] In one embodiment of the present invention, the cooling for the additional carbon diffusion may be performed at a cooling rate of 4 to 12°C / s up to 400°C. If the cooling rate is less than 4°C / s, excessive time may be required to diffuse carbon to the target level, which may reduce economic feasibility, and furthermore, the target level of carbon content may not be secured in the surface layer of the base steel plate. On the other hand, if the cooling rate exceeds 12°C / s, the carbon may not be sufficiently diffused and cooling may be completed, which also makes it impossible to secure the target level of carbon content.

[0153] Meanwhile, below 400°C, the diffusion rate of carbon is relatively slow. Therefore, in the temperature range below 400°C, there is no need to specifically limit the cooling rate to facilitate further carbon diffusion. Therefore, in one embodiment of the present invention, to increase productivity in the temperature range below 400°C, rapid cooling at a rate exceeding 12°C / s may be performed, or natural cooling may be allowed.

[0154] By completing the cooling process described above, a steel plate for hot forming is obtained.

[0155] In this way, when a series of processes according to an embodiment of the present invention are performed, the aluminum-based coated steel sheet, i.e., the steel sheet for hot forming, may have a decarburization region or portion having a certain carbon content in the thickness direction of the base steel sheet based on the interface between the base steel sheet and the coating layer. At this time, the decarburization region or portion may exist within the surface layer, and furthermore, an internal oxide may exist at a certain depth on the surface side of this surface layer. The internal oxide at this time is mainly formed at the grain boundary, and this internal oxide acts as a site that traps hydrogen during the subsequent hot forming process, thereby improving the hydrogen embrittlement resistance of the part obtained by hot forming. In addition, a certain amount of carbide may exist within the base steel sheet of the aluminum-based coated steel sheet, and this carbide also acts as a hydrogen trap site, thereby further improving the hydrogen embrittlement resistance of the hot-formed part.

[0156] Hereinafter, a hot-formed member and a manufacturing method thereof according to another aspect of the present invention will be described in detail.

[0157] 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 at this time can be according to one embodiment of the present invention.

[0158] A steel sheet for hot forming according to one embodiment of the present invention may include a base steel sheet and an aluminum-based plating layer formed on at least one surface of the base steel sheet, and the alloy composition of the base steel sheet and the composition of the aluminum-based plating layer may be replaced with the above-described contents.

[0159] In one embodiment of the present invention, the hot-formed member has a maximum carbon content (C) in a region within 30 μm in the thickness direction of the base steel sheet from the interface between the base steel sheet and the plating layer. max ) and the ratio of carbon content (C0) of the above steel plate (C max / C0) can satisfy 0.8~1.1.

[0160] According to one embodiment of the present invention, a hot forming steel sheet has a decarburized region (decarburized region) with a relatively low carbon content compared to the carbon content of the base steel sheet from the interface between the base steel sheet and the plating layer to a certain region in the thickness direction of the base steel sheet, and it has been revealed that this region exists in the surface layer of the base steel sheet. When such a hot forming steel sheet is heated at a high temperature for hot forming, alloying occurs at the interface between the base steel sheet and the plating layer, and carbon existing on the surface side of the base steel sheet diffuses into the interface area, thereby relatively increasing the carbon concentration in the surface layer. As an example, a hot forming member may have a maximum carbon content (C) existing in the surface layer as mentioned above. max ) as a standard, C max / C0 value can be 0.8 to 1.1. In the hot-formed member, C of the surface layer of the steel plate max If the / C0 value is less than 0.8, it is impossible to provide hot-formed parts with ultra-high strength. In particular, as the surface hardness of the base steel plate decreases, not only does the strength of the part decrease, but the durability of the part affected by the surface hardness also deteriorates. On the other hand, if the value exceeds 1.1, the effect of decarburizing the surface of the base steel plate through a series of processes cannot be achieved, resulting in difficulty in ensuring hydrogen embrittlement resistance of the hot-formed part.

[0161] Meanwhile, when explaining the surface layer of the base steel plate of a hot-formed part, it is necessary to note that it is distinct from the surface layer of the base steel plate of the hot-formed steel plate mentioned above. The range of the distinguished area is similar, but in the hot-formed part, the surface layer of the base steel plate is alloyed at the interface between the base steel plate and the plating layer during the high-temperature heating process, and the interface between the plating layer and the base steel plate moves downward in the direction of the base steel plate (in the depth direction). Therefore, the interface point between the base steel plate and the plating layer, which refers to the surface layer of the base steel plate in the hot-formed part, is different from that interface point of the hot-formed steel plate.

[0162] In a hot-formed member obtained by hot-forming a steel sheet for hot forming according to one embodiment of the present invention, the base steel sheet contains at least one carbide among V-type carbide, Mo-type carbide, and V-Mo composite carbide. 5 dog / mm 2 It can be included as above. In this way, since the carbides present in the base steel sheet of the hot-forming steel sheet also exist in the hot-formed member, a trapping effect for hydrogen that has penetrated during the hot-forming process can be obtained, thereby improving the hydrogen embrittlement resistance of the hot-formed member. In addition, the carbides at this time can have an average size of 1 to 100 nm. The content regarding the number and size of the carbides can be replaced with the content described above.

[0163] In one embodiment of the present invention, the base steel plate of the hot-formed member may have a hard structure by performing forming after high-temperature heat treatment. As one example, the base steel plate may have a microstructure of a combined martensite and bainite phases with an area fraction of 90% or more, and may also include pearlite, ferrite, etc. as other structures. However, the present invention is not limited thereto. Accordingly, the hot-formed member according to one embodiment of the present invention may have ultra-high strength, and for example, may have a tensile strength of 1800 MPa or more.

[0164] In addition, the hot-formed member according to one embodiment of the present invention has excellent hydrogen embrittlement resistance, which can be confirmed from the hydrogen embrittlement evaluation results of the hot-formed member.

[0165] Meanwhile, a method for manufacturing a hot-formed member according to one embodiment of the present invention is not particularly limited, and can be manufactured through a process of heating a steel plate for hot forming to a temperature higher than the austenitizing temperature, maintaining the temperature, and then rapidly cooling and forming (pressing) the steel plate at the same time, as is widely known in the art.

[0166] However, as one example of the present invention, after obtaining a blank using a hot forming steel sheet according to one embodiment of the present invention, a step of heating the blank to a temperature range of 860 to 970°C and then maintaining it for 3 to 15 minutes may be performed. By performing a step of forming (pressing) the blank heated and maintained in a hot state and cooling it at a cooling rate higher than the critical cooling rate, an intended hot forming member can be manufactured. As a non-limiting example, the cooling may be performed at a cooling rate of 30°C / s or higher.

[0167] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0168] (Example)

[0169] Ingots were vacuum melted to produce slabs with a thickness of 40 mm having the alloy composition shown in Table 1 below. The produced slabs were heated to 1200°C, maintained for 1.5 hours, and then final hot-rolled at 900°C to obtain hot-rolled steel sheets. Thereafter, each hot-rolled steel sheet was coiled according to the conditions shown in Table 2, and then cold-rolled at a reduction ratio of 50% to obtain cold-rolled steel sheets. Next, each cold-rolled steel sheet was subjected to annealing, cooling, plating, and cooling processes according to the conditions shown in Table 2 to produce final hot-forming steel sheets. At this time, the annealing process was performed in an atmosphere of 5 vol% hydrogen (H2) and the balance nitrogen (N2), and the plating process was performed in a plating bath composed of 9.0% Si, 2.0% Fe, the balance Al, and unavoidable impurities. At this time, the cooling performed before the plating process was set as the first carbon diffusion step, and the cooling performed after the plating process was set as the second carbon diffusion step.

[0170] Steel alloy composition (weight %) (Mo+V+Nb)CSiMnPSAlCrTiNBMoVNbA0.350.301.30.0200.00300.030.200.030.0040.00300.100.2000.30B0.380.301.30.0200.00300.030.200.030.0040.00300000C0.420.200.80.0100.00200.040.300.040.0050.00300.030.0600.09D0.260.201.80.0150 .00300.030.200.030.0050.00300.100.1000.20E0.350.300.30.0200.00300.030.200.030.0040.00300.100.2000.30F0.420.700 .80.0100.00200.040.300.040.0050.00300.150.300.030.48G0.330.302.50.0100.00100.060.400.040.0060.00250.100.2000.30

[0171] Steel plate removal annealing agent 1 Carbon diffusion plating bath agent 2 Carbon diffusion Classification Temperature (℃) Temperature (℃) Dew point temperature (℃) Time (sec) Dew point temperature (℃) Cooling speed (℃ / s) Immersion temperature (℃) Immersion time (sec) Adhesion weight (g / m) 2 ) Cooling speed (℃ / s) A-155078010100-451.36708808 Invention example 1A-2650800-25120-451.36708808 Comparative example 1A-352082010140105.067088015 Comparative example 2A-4480780-45100-451.36708808 Comparative example 3A-575078025100-451.367088018 Comparative example 4B550780-20100-451.06708808 Comparative example 5C550780-15100-451.06708808 Comparative example 6D62078015100-501.767088010Comparative Example 7E5507800100-351.767088010Comparative Example 8F60076015120-501.26656807Invention Example 2G6408201570-501.866598010Invention Example 3

[0172] For each hot forming steel sheet manufactured as described above, a region of 30 ㎛ in depth in the thickness direction of the base steel sheet was set as the surface layer based on the interface between the base steel sheet and the plating layer, and the minimum carbon content (Cmin) of this surface layer and the depth of internal oxide existing at the grain boundary (depth of internal oxide layer) were measured.

[0173] Specifically, a Glow Discharge Spectrometer (GDS), which can quantitatively analyze various components in the depth direction of the steel plate, was used to analyze the concentration of carbon, etc. Typically, GDS analysis is performed in the depth direction for a circular area of ​​2 to 6 mm, so it is difficult to designate the exact interface of the plating layer / base material (base steel plate) based on the concentration profile in this depth direction. However, based on the results of various optical microscope and SEM analyses to date, the point where the Al content is 1.0% was set as the outermost layer of the base steel plate.

[0174] Additionally, the depth of internal oxides present within the surface layer of the steel plate was measured by observing three random points along the width of the steel plate using a scanning electron microscope (SEM) at a magnification of 3000x, and then measuring the depth of internal oxidation from each of the three observed images. The average value was then taken and presented.

[0175] And, in order to analyze the size and number of carbides existing in the base steel sheet, a cross-section was taken in the direction perpendicular to the rolling direction at the t / 4 point in the thickness direction of the base steel sheet, and then a replica specimen for transmission electron microscope (TEM) analysis was prepared. Then, 15 random locations in each specimen were observed by TEM, and the composition of the carbides was confirmed through EDS analysis. The size and number (number density) of the V carbides, Mo carbides, and V-Mo composite carbides confirmed therefrom were calculated, and the total number of carbides is shown in Table 3 below. At this time, the size of each precipitate was measured as the average of the major and minor axes for each observed carbide, and the average size was calculated based on this.

[0176] Meanwhile, a hot forming process was applied to each of the previously manufactured steel plates for hot forming to manufacture hot-formed parts. At this time, the heating temperature for hot forming was set to 900℃, and after reaching the target temperature, it was maintained for 6 minutes, and then transferred to a mold to perform rapid cooling (cooling rate of approximately 30℃ / s or more) along with forming, thereby manufacturing hot-formed parts. At this time, the transfer time to the mold was applied as 10 seconds, and the mold for forming was used as a flat mold.

[0177] For each hot-formed member, tensile specimens in accordance with ASTM standards were collected, and the yield strength and tensile strength were measured using a universal tensile testing machine.

[0178] In addition, the carbon concentration in the thickness direction at the interface between the base steel plate and the plating layer of the hot-formed member and the carbide present in the base steel plate were measured using the same method as that applied to the hot-formed steel plate.

[0179] In addition, to evaluate the hydrogen embrittlement resistance of hot-formed parts, three specimens with a bending stress were prepared by applying a stress at the yield strength level through a 4-pt bend. Each specimen was immersed in a 0.1 N HCl solution and then maintained for 120 hours. If fracture occurred in one or more specimens during the maintenance period, the hydrogen embrittlement resistance was considered poor and evaluated as defective.

[0180] The measurement values ​​of the steel plates and hot-formed parts for hot forming mentioned above are all shown in Table 3 below.

[0181] Hot-formed steel plate, hot-formed member C min / C0 carbide number (10 5 dog / mm 2 )Internal oxidation layer (depth, ㎛)Yield strength (MPa)Tensile strength (MPa)C max / C0 carbide number (10 5 dog / mm 2 ) Hydrogen embrittlement invention example 10.679.32.0124819800.9410.9 Good comparison example 10.9112.61.2131020341.2314.2 Bad comparison example 20.1811.43.7109817200.329.3 Good comparison example 30.9713.80.3128719921.3416.8 Bad comparison example 40.457.79.4117918390.729.7 Bad comparison example 50.8701.6134720921.190 Bad comparison example 60.820.42.2138722571.130.8 Bad comparison example 70.625.22.6107815980.858.1Good Comparison Example 80.686.11.994514030.909.7Good Invention Honor Example 20.5318.34.6136022030.8218.7Good Invention Honor Example 30.7414.63.1121219071.0414.2Good

[0182] As shown in Tables 1 to 3, a steel sheet having an alloy composition according to an embodiment of the present invention can be used to obtain a steel sheet for hot forming and a hot forming member as intended by performing a series of processes according to an embodiment of the present invention.

[0183] Specifically, in the case of steel sheets for hot forming, there exists a region with a relatively low carbon content (decarburization region) on the surface of the base steel sheet, and also, while internal oxides exist at a certain depth from the surface of the base steel sheet, carbides are formed as intended within the base steel sheet. It can be seen that a hot-formed part obtained by performing hot forming using such steel sheets for hot forming has an ultra-high tensile strength of 1800 MPa or more, while also ensuring hydrogen embrittlement resistance.

[0184] On the other hand, Comparative Examples 1 to 4, in which the alloy composition of the base steel plate satisfies one embodiment of the present invention but the manufacturing process does not satisfy one embodiment of the present invention, failed to secure ultra-high strength or had poor hydrogen embrittlement resistance. Among these, Comparative Examples 1 and 3, in which a decarburization region was not sufficiently formed on the surface of the base steel plate, had poor hydrogen embrittlement resistance of the hot-formed part.

[0185] Meanwhile, Comparative Example 5 using steel that does not contain carbide forming elements showed poor hydrogen embrittlement resistance due to the absence of any carbides inside the steel plate.

[0186] In addition, in Comparative Example 6, in which the content of precipitate-forming elements added to the steel was low and the dew point temperature was controlled low during annealing, not only was a decarburization area not sufficiently formed on the surface of the base steel plate, but carbides were formed in too small an amount within the base steel plate, so there was no problem in securing the strength of the hot-formed part, but the hydrogen embrittlement resistance was inferior.

[0187] Comparative Example 7, in which the carbon content of the steel was lower than the range according to one embodiment of the present invention, could not secure a strength of the hot-formed member of 1800 MPa or more.

[0188] In addition, Comparative Example 8 is an example in which the manufacturing process satisfies one embodiment of the present invention, but the alloy composition deviates from the present invention. Due to the low manganese content of the steel, the hardenability could not be sufficiently secured, and thus the strength of the hot-formed part was inferior.

[0189] Figure 1 is a drawing showing the GDS measurement results of a steel sheet for hot forming corresponding to Invention Example 1.

[0190] As shown in Fig. 1, it can be confirmed that there is a region where the carbon content is lower than the carbon content of the steel sheet at a certain depth in the thickness direction of the steel sheet at the interface between the steel sheet and the plating layer.

[0191] Figure 2 shows a photograph of the results of measuring the thickness cross-section of a steel plate for hot forming corresponding to Invention Example 1 using SEM.

[0192] As shown in Fig. 2, it can be confirmed that an internal oxidation layer exists at a certain depth in the thickness direction of the base steel sheet from the interface between the base steel sheet and the plating layer.

[0193] Figure 3 is a TEM result observed at the point t / 4 in the thickness direction of the steel sheet for hot forming corresponding to Invention Example 1.

[0194] As shown in Fig. 3, it can be confirmed that spherical micro-carbides are formed within the steel plate.

Claims

1. A steel sheet containing carbon (C) of 0.30 to 0.50% by weight and an aluminum-based plating layer formed on at least one surface of the steel sheet, The minimum carbon content (C) in the surface layer defined within 30 ㎛ in the thickness direction of the steel sheet from the interface between the steel sheet and the aluminum-based plating layer min ) and the ratio of carbon content (C0) of the steel sheet (C min / C0) is a hot-forming galvanized steel sheet having a range satisfying 0.5 to 0.

8.

2. In paragraph 1, It contains internal oxide within a depth of 0.5 to 5.0 ㎛ in the thickness direction of the steel sheet from the interface between the steel sheet and the aluminum-based plating layer, A hot-forming galvanized steel sheet, wherein the internal oxide is formed at the grain boundary.

3. In paragraph 1, The above steel plate contains at least one type of carbide among V-type carbide, Mo-type carbide, and V-Mo composite carbide. 5 dog / mm 2 A hot-forming galvanized steel sheet comprising the above.

4. In paragraph 3, A hot-forming galvanized steel sheet, wherein the above carbides have an average size of 1 to 100 nm.

5. In paragraph 1, The above-mentioned steel sheet is a hot-forming galvanized steel sheet further containing, in wt%, silicon (Si): 0.05 to 1.00%, manganese (Mn): 0.4 to 3.0%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.0001 to 0.0200%, aluminum (Al): 0.01 to 0.10%, nitrogen (N): 0.001 to 0.020%, boron (B): 0.0001 to 0.0100%, chromium (Cr): 0.01 to 0.50%, titanium (Ti): 0.010 to 0.050%, and the sum of the contents of at least one of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%, the remainder being iron and unavoidable impurities.

6. In paragraph 1, A hot-forming galvanized steel sheet, wherein the aluminum-based plating layer contains, in weight %, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder Al, and other unavoidable impurities.

7. In paragraph 1, A hot-forming galvanized steel sheet having an aluminum-based plating layer having a thickness of 5 to 40 μm.

8. A step of preparing a steel slab containing carbon (C) of 0.30 to 0.50% by weight and heating it at a temperature range of 1050 to 1300℃; A step of obtaining a hot-rolled steel sheet by final hot-rolling the steel slab after heating in a temperature range of 800 to 950°C; A step of coiling the above hot-rolled steel plate at a temperature range of 500 to 700°C; An annealing treatment step in which the hot-rolled steel sheet is heated to a temperature range of 740 to 860°C after the above-mentioned coiling, and then maintained in an atmosphere having a dew point temperature of -10 to +20°C for 50 to 150 seconds; A first carbon diffusion step of cooling the hot-rolled steel sheet after the above annealing treatment at a cooling rate of 0.3 to 2.0°C / s in an atmosphere having a dew point temperature of -30 to -60°C; A step of immersing the hot-rolled steel sheet after the first carbon diffusion into an aluminum-based plating bath to obtain an aluminum-based plated steel sheet; and A method for manufacturing a hot-forming coated steel sheet, comprising a second carbon diffusion step of cooling the aluminum-based coated steel sheet to 400°C at a cooling rate of 4 to 12°C / s.

9. In paragraph 8, A method for manufacturing a galvanized steel sheet for hot forming, wherein the above annealing treatment is performed in an atmosphere gas controlled by 5 to 15 vol% of H2, the remainder N2, and unavoidable impurities.

10. In paragraph 8, A method for manufacturing a plated steel sheet for hot forming, further comprising the step of obtaining a cold rolled steel sheet by cold rolling at a reduction ratio of 20 to 80% before annealing the hot rolled steel sheet after the above-mentioned coiling.

11. In paragraph 8, A method for manufacturing a galvanized steel sheet for hot forming, wherein the steel slab further contains, in wt%, silicon (Si): 0.05 to 1.00%, manganese (Mn): 0.4 to 3.0%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.0001 to 0.0200%, aluminum (Al): 0.01 to 0.10%, nitrogen (N): 0.001 to 0.020%, boron (B): 0.0001 to 0.0100%, chromium (Cr): 0.01 to 0.50%, titanium (Ti): 0.010 to 0.050%, and the sum of the contents of at least one of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%, the remainder being iron and unavoidable impurities.

12. In paragraph 8, A method for manufacturing a hot-forming galvanized steel sheet, wherein the aluminum-based plating bath is composed of, in weight %, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder Al, and other unavoidable impurities.

13. In paragraph 8, The step of obtaining the above-mentioned galvanized steel sheet is 30 to 130 g / m on one side. 2 A method for manufacturing a hot-forming galvanized steel sheet by using a galvanizing amount of .

14. A steel sheet comprising carbon (C): 0.30 to 0.50 by weight% and an aluminum-based plating layer formed on at least one surface of the steel sheet, The maximum carbon content (C) in the surface layer defined within 30 ㎛ in the thickness direction of the steel sheet from the interface between the steel sheet and the aluminum-based plating layer max ) and the ratio of carbon content (C0) of the steel sheet (C max A hot-formed member including a region where / C0) satisfies 0.8 to 1.

1.

15. In paragraph 14, The above steel plate contains at least one type of carbide among V-type carbide, Mo-type carbide, and V-Mo composite carbide. 5 dog / mm 2 A hot formed member including the above.

16. In paragraph 15, The above carbide is a hot-formed member having an average size of 1 to 100 nm.

17. In paragraph 14, The above-mentioned steel plate is a hot-formed member further including, in wt%, silicon (Si): 0.05 to 1.00%, manganese (Mn): 0.4 to 3.0%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.0001 to 0.0200%, aluminum (Al): 0.01 to 0.10%, nitrogen (N): 0.001 to 0.020%, boron (B): 0.0001 to 0.0100%, chromium (Cr): 0.01 to 0.50%, titanium (Ti): 0.010 to 0.050%, and the sum of the contents of at least one of molybdenum (Mo), niobium (Nb), and vanadium (V): 0.10 to 0.50%, the remainder being iron and unavoidable impurities.

18. In paragraph 14, A hot-formed member, wherein the aluminum-based plating layer contains, in weight %, silicon (Si): 6.0 to 12.0%, iron (Fe): 1.0 to 9.0%, the remainder Al, and other unavoidable impurities.

19. In paragraph 14, The above hot-formed member is a hot-formed member having a tensile strength of 1800 MPa or more.

Images