Patent Description:
Exemplary embodiments of the present invention relate to a steel sheet for hot press forming and a method of manufacturing the steel sheet.

As the environmental regulations and safety standards in the automobile industry have been recently strengthened, the application of high-strength steel has increased for weight reduction and stability of automobiles. High-strength steel may have high-strength characteristics compared to its weight. However, during processing, a material may break or a spring back phenomenon may occur, and it is difficult to form a high-steel product having a complex and precise shape. Therefore, as a method to solve this, use of hot press forming has been expanded.

In hot press forming, a steel sheet is pressed by being heated at a high temperature to easily form steel, and the strength of a formed product may be secured by performing rapid cooling through a mold. However, since the steel sheet is heated at a high temperature for hot press forming, the surface of the steel sheet is oxidized. In order to solve this issue, the invention of <CIT> proposes a method of hot press forming a steel sheet subjected to aluminum plating. According to the invention of <CIT>, because an aluminum plating layer exists on the surface of the steel sheet, the surface of the steel sheet may be prevented from oxidizing by heating the steel sheet.

However, when the steel sheet is heated, Fe is diffused from the steel sheet into the aluminum plating layer, and then the aluminum plating layer is alloyed. Also, when such an aluminum plated steel sheet is hot press formed, cracks may occur in a plating layer, which becomes brittle due to alloying. In the meantime, since the aluminum plating layer has no sacrificial corrosion resistance, cracks may occur in the plating layer, and when the surface of the steel sheet is exposed, the corrosion resistance of a hot press-formed product may rapidly deteriorate.

The <CIT> discloses an iron-aluminum-based alloy-plated steel sheet which includes an Fe-Al alloy plating layer formed on the surface of a holding steel sheet, and when the Fe-Al alloy plating layer is divided into four equal portions in the thickness direction to form four layers, the hardness of the other layers except the outermost layer is less than that of the outer layer thereof, thereby suppressing the occurrence of cracks in the surface thereof. However, because the hardness of the Fe-Al alloy plating layer decreases toward the outside, during a hot press process, the Fe-Al alloy plating layer may be attached to a mold and peeled.

<CIT> relates to a hot press formed article which has a melted aluminum plating layer formed on the surface of a base steel sheet, wherein the plating layer consists of a soft diffusion layer and a hard alloy layer, the hard alloy layer having a tau layer irregularly and non-continuously dispersed and distributed on the inside thereof at <NUM>% or more of the entire area fraction so that the difference in hardness between the alloy layer and the diffusion layer is <NUM> (Hv) or less, <CIT> relates to a Fe-Al-based plated hot-stamped member including a Fe-Al-based plated layer located on one surface or both surfaces of a base material, the Fe-Al-based plated layer having a thickness of <NUM> or more and <NUM> or less, formed by four layers, wherein each of the four layers is a Fe-Al-based intermetallic compound containing Al, Fe, Si, Mn and Cr with the balance made up of impurities, <CIT> relates to a hot press formed article having a hot dipped aluminized layer formed on the surface of a base steel sheet, the aluminized layer comprising a single soft diffusion layer, <CIT> relates to an aluminum plated steel sheet including an aluminum plating layer, wherein the aluminum plating layer comprises finely dispersed Mg<NUM>Si particles with a long axis of <NUM> or less and an aspect ratio of <NUM> to <NUM>, and <CIT> relates to an elongated steel wire comprising a steel filament and a metal coating for corrosion resistance upon the steel filament, the steel filament having a microstructure comprising more than <NUM>% tempered martensite.

The present invention provides a steel sheet for hot press forming according to claim <NUM>, the steel sheet preventing or reducing the occurrence of cracks in a plating layer during hot press forming, and a method of manufacturing the steel sheet according to claim <NUM>.

In the following embodiments, the terms, "first", "second", etc. are only used to distinguish one element from another rather than a limited meaning.

The singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It will be understood that the terms "comprises," "comprising," "includes," or "including," when used herein, specify the presence of stated features or elements, but do not preclude the presence or addition of one or more other features or elements.

When a layer, area, or element is referred to as being on another layer, area, or element, it may be directly or indirectly on the other layer, area, or element, and intervening layers, areas, or elements may be present.

In the drawings, the sizes of elements may be exaggerated or reduced for convenience of description. Since the size and thickness of each element shown in the drawings are shown for convenience of description.

When a certain embodiment is capable of being implemented differently, a particular process order may be performed differently from the described order. Two processes described in succession may be performed substantially simultaneously or may be performed in an order opposite to the described order.

Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings, and when describing with reference to the drawings, the same or corresponding elements will be given the same reference numerals.

<FIG> is a cross-sectional view of a steel sheet for hot press according to an exemplary embodiment.

Referring to <FIG>, a steel sheet <NUM> for hot press according to one embodiment may include a base steel sheet <NUM> and a plating layer <NUM> disposed on the base steel sheet <NUM>.

The base steel sheet <NUM> may be a steel sheet which is manufactured by performing a hot rolling process and a cold rolling process on a steel slab that is cast to include a certain alloy element in a certain content. For example, the base steel sheet <NUM> include carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N), balance iron (Fe), and other inevitable impurities. In addition, the base steel sheet <NUM> may further include one or more of niobium (Nb), titanium (Ti), chromium (Cr), molybdenum (Mo), and boron (B).

Carbon (C) is a major element that determines the strength and hardness of the base steel sheet <NUM> and, after a hot press process, is added to secure the tensile strength of the base steel sheet <NUM> and secure hardenability characteristics. Such carbon may be included in an amount of <NUM> wt% to <NUM> wt% with respect to a total weight of the base steel sheet <NUM>. When a content of carbon is less than <NUM> wt%, the mechanical strength of the base steel sheet <NUM> may not be secured. When the content of carbon exceeds <NUM> wt%, the toughness of the base steel sheet <NUM> may be reduced or the brittleness of the base steel sheet <NUM> may not be controlled.

Silicon (Si) serves as a ferrite stabilizing element in the base steel sheet <NUM>. Silicon (Si) is a solid solution strengthening element, improves the ductility of the base steel sheet <NUM>, and may improve carbon concentration in austenite by suppressing the formation of low-temperature region carbide. In addition, silicon (Si) is a key element in hot-rolled, cold-rolled, and hot-pressed structure homogenization (perlite, manganese segregation control) and ferrite fine dispersion. Such silicon may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When silicon is included less than <NUM> wt%, the above-described effects may not be acquired. When the content of silicon exceeds <NUM> wt%, hot rolling and cold rolling loads increase, hot-rolling red scale becomes excessive, and plating characteristics of the base steel sheet <NUM> may be deteriorated.

Manganese (Mn) is added to increase hardenability and strength during heat treatment. Manganese may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When a content of manganese is less than <NUM> wt%, a grain refinement effect is insufficient, and thus, a hard phase fraction in a formed product may be insufficient after hot press. When the content of manganese exceeds <NUM> wt%, ductility and toughness may be reduced due to manganese segregation or a pearlite band, thereby causing a decrease in a bending performance and generating an inhomogeneous microstructure.

Phosphorus (P) may be included in an amount greater than <NUM> wt% and less than or equal to <NUM> wt% with respect to the total weight of the base steel sheet <NUM> to prevent a decrease in the toughness of the base steel sheet <NUM>. When phosphorus exceeds <NUM> wt% and is included in the base steel sheet <NUM>, an iron phosphide compound may be formed to reduce the toughness, and cracks may be generated in the base steel sheet <NUM> during a manufacturing process.

Sulfur (S) may be included in an amount greater than <NUM> wt% and less than or equal to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When the content of sulfur exceeds <NUM> wt%, hot workability may be deteriorated, and a surface detect such as cracks may occur due to formation of a large inclusion.

Aluminum (Al) serves as a deoxidizing agent for removing oxygen in the base steel sheet <NUM>. Aluminum may be included in an amount greater than <NUM> wt% and less than or equal to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When a content of aluminum exceeds <NUM> wt%, a nozzle may be clogged during steel making, and, during casting, hot brittleness may occur due to aluminum oxide or the like, and thus, cracks may occur in the base steel sheet <NUM> or ductility may be reduced.

When a large amount of nitrogen is included in the base steel sheet <NUM>, an amount of solid solution nitrogen may increase, thereby decreasing impact characteristics and elongation of the base steel sheet <NUM> and decreasing the toughness of a joint. Therefore, nitrogen may be included in an amount greater than <NUM> wt% and less than or equal to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>.

Niobium (Nb) is added to increase strength and toughness according to a decrease in the size of a martensite packet. Niobium may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When niobium is included in the above range, a grain refinement effect of steel may be high in hot rolling and cold rolling processes, the occurrence of cracks in a slab and the occurrence of brittle fractures of a product may be prevented during steel making/performing, and the generation of steel-making coarse precipitates may be minimized.

Titanium (Ti) may be added to strengthen hardenability and increase a material by forming precipitates after hot press heat treatment. In addition, titanium effectively contributes to refinement of austenite grains by forming a precipitated phase such as Ti (C, N) at a high temperature. Titanium may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When titanium is included in the above content range, poor performance and coarsening of precipitates may be prevented, physical properties of steel may be easily secured, and defects such as the occurrence of cracks in the surface of the steel may be prevented.

Chromium (Cr) is added to improve the hardenability and strength of the base steel sheet <NUM>. Chromium may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When chromium is included in the above range, the hardenability and strength of the base steel sheet <NUM> may be improved, and an increase in production cost and a decrease in toughness of steel may be prevented.

Molybdenum (Mo) may contribute to improving the strength of the base steel sheet <NUM> by suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot press. Molybdenum (Mo) as described above may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>.

Boron (B) is added to secure the hardenability and strength of the base steel sheet <NUM> by securing a martensite structure and has a grain refinement effect by increasing an austenite grain growth temperature. Boron may be included in an amount of <NUM> wt% to <NUM> wt% with respect to the total weight of the base steel sheet <NUM>. When boron is included in the above range, the occurrence of hard grain boundary brittleness may be prevented, and high toughness and bendability may be secured.

The plating layer <NUM> is formed in a thickness of <NUM> to <NUM> on at least one surface of the base steel sheet <NUM> and includes aluminum (Al). Here, the thickness of the plating layer <NUM> refers to an average thickness of the plating layer <NUM> over the entire area of the plating layer <NUM>. When the thickness of the plating layer <NUM> is less than <NUM>, corrosion resistance is lowered. When the thickness of the plating layer <NUM> exceeds <NUM>, the productivity of the steel sheet <NUM> for hot press may be reduced, and the plating layer <NUM> may be attached to a roller or a mold and peeled from the base steel sheet <NUM> during the hot press process.

The plating layer <NUM> includes a diffusion layer <NUM> and a surface layer <NUM> sequentially laminated on the base steel sheet <NUM>.

The surface layer <NUM> includes aluminum (Al) greater than or equal to <NUM> wt% and prevents oxidation of the base steel sheet <NUM> or the like. The diffusion layer <NUM> may be formed by mutually diffusing Fe of the base steel sheet <NUM> and Al of the plating layer <NUM>, and may include an aluminum-iron (Al-Fe) and aluminum-iron-silicon (Al-Fe-Si) compound. The diffusion layer <NUM> may include iron (Fe) in an amount of <NUM> wt% to <NUM> wt%, aluminum (Al) in an amount of <NUM> wt% to <NUM> wt%, and silicon (Si) in an amount of <NUM> wt% to <NUM> wt%.

The diffusion layer <NUM> as described above may have a higher melting point than the surface layer <NUM> to prevent the occurrence of liquid metal embrittlement in which the surface layer <NUM> is melted during the hot press process, and thus, Al penetrates into a structure of the base steel sheet <NUM>.

For this, an area fraction of the diffusion layer <NUM> (a cross-sectional area of the diffusion layer <NUM> ÷ a cross-sectional area of the plating layer <NUM>), which is a ratio of the cross-sectional area of the diffusion layer <NUM> to the cross-sectional of the plating layer <NUM>, may be <NUM> % to <NUM> %. Here, the cross-sectional area of the plating layer <NUM> and the cross-sectional area of the diffusion layer <NUM> refer to cross-sectional areas at the same certain location. This may be applied equally to area fractions for other layers below.

The diffusion layer <NUM> includes an Fe-Al alloy layer <NUM> and an Fe-Al intermetallic compound layer <NUM> sequentially disposed on the base steel sheet <NUM> and each including silicon.

The Fe-Al alloy layer <NUM> includes Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt% and may have a density of <NUM>/cm<NUM> to <NUM>/cm<NUM>. As an example, the Fe-Al alloy layer <NUM> may include Al<NUM>Fe<NUM> and may have a greater hardness than the Fe-Al intermetallic compound layer <NUM>.

The Fe-Al alloy layer <NUM> as described above prevents the liquid metal embrittlement. However, the Fe-Al alloy layer <NUM> is made of a hard phase and maintains high hardness even during the hot press process, and thus may generate cracks and decrease the formability of the steel sheet <NUM> for hot press during the hot press process. Therefore, for preventing the liquid metal embrittlement and preventing the decrease in the formability of the steel sheet <NUM> for hot press, an average thickness of the Fe-Al alloy layer <NUM> may be <NUM> to <NUM>, alternatively, <NUM> to <NUM>. In addition, an area fraction of the Fe-Al alloy layer <NUM> with respect to the diffusion layer <NUM> may be <NUM> % to <NUM> %.

The Fe-Al intermetallic compound layer <NUM> includes Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%, and may have a density of <NUM>/cm<NUM> to <NUM>/cm<NUM>. The Fe-Al intermetallic compound layer <NUM> may have a lower hardness than the Fe-Al alloy layer <NUM> and operates as a buffer against a compressive force during the hot press process of the steel sheet <NUM> for hot press, thereby preventing cracks from occurring in the plating layer <NUM>.

In more detail, during heating for hot press, additional mutual diffusion occurs between the plating layer <NUM> and the base steel sheet <NUM>. Here, the Fe-Al alloy layer <NUM> may maintain a relatively high hardness, but the Fe-Al intermetallic compound layer <NUM> may form a tau phase or/and AlFe, and a hardness thereof may be lowered. Therefore, the diffusion layer <NUM> may include the Fe-Al intermetallic compound layer <NUM> capable of operating as a buffer against the compressive force during the hot press process to thereby improve crack resistance.

An area fraction of the Fe-Al intermetallic compound layer <NUM> as described above with respect to the diffusion layer <NUM> is <NUM> % to <NUM> %. When the cross-sectional area of the Fe-Al intermetallic compound layer <NUM> with respect to the cross-sectional area of the diffusion layer <NUM> is formed to be greater than or equal to <NUM> %, the Fe-Al intermetallic compound layer <NUM> may effectively absorb an external force that generates cracks in the plating layer <NUM> during the hot press process. However, when the area fraction of the Fe-Al intermetallic compound layer <NUM> with respect to the diffusion layer <NUM> exceeds <NUM> %, an average thickness of the Fe-Al alloy layer <NUM> may be relatively reduced, and thus the liquid metal embrittlement may not be prevented. Also, the Fe-Al intermetallic compound layer <NUM>, which has the area fraction exceeding <NUM> %, may not be secured in the temperature range of a plating bath for melting Al described later.

In addition, the Fe-Al intermetallic compound layer <NUM> includes a first layer <NUM> and a second layer <NUM> that are sequentially laminated. The first layer <NUM> and the second layer <NUM> are each formed of an Fe-Al intermetallic compound including Si, and a first hardness of the first layer <NUM> is less than a second hardness of the second layer <NUM>. In other words, the Fe-Al alloy layer <NUM>, the second layer <NUM>, and the first layer <NUM> have a high hardness value in that order. Accordingly, although a phase change of each layer occurs or the location of each layer is changed during the hot press process, a layer structure capable of absorbing an external force that causes the occurrence of cracks or reducing of formability may be obtained.

Because the Fe-Al intermetallic compound layer <NUM> is formed on the Fe-Al alloy layer <NUM> including Si having low solid solubility, a content of Si of the first layer <NUM> may gradually increase toward the surface of the plating layer <NUM>, and the second layer <NUM> may have relatively higher Al content and lower Si content than the first layer <NUM>.

The first layer <NUM> includes Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%, and may have a density of <NUM>/cm<NUM> to <NUM>/cm<NUM>. The second layer <NUM> includes Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%, and may have a density of <NUM>/cm<NUM> to <NUM>/cm<NUM>. In addition, the second layer <NUM>, the Fe-Al alloy layer <NUM>, and the first layer <NUM> may have a high Al content (wt%) value in that order, and the first layer <NUM>, the second layer <NUM>, and the Fe-Al alloy layer <NUM> may have a high Si content (wt%) value in that order. Therefore, although a phase change of each layer occurs or the location of each layer is changed during the hot pressing process, a layer structure capable of absorbing an external force that causes the occurrence of cracks or lowering of formability may be secured.

In other words, in the Fe-Al alloy layer <NUM>, the first layer <NUM>, and the second layer <NUM>, a content of Al in the first layer <NUM> is the least, and a content of Si in the first layer <NUM> is the greatest, and thus, the first layer <NUM> may have the lowest hardness.

The first layer <NUM> may prevent cracks from occurring in the plating layer <NUM> by absorbing an external force causing cracks in the plating layer <NUM> during the hot press process. In addition, although cracks occur in the second layer <NUM> or the Fe-Al alloy layer <NUM> having a relatively high hardness than the first layer <NUM> during the hot press process, the first layer <NUM> that is soft not only operates as a buffer but also prevents crack propagation at an interface formed during the hot press process, thereby effectively preventing cracks generated in the second layer <NUM> or the Fe-Al alloy layer <NUM> from being transmitted to the base steel sheet <NUM> or the plating layer <NUM>. Accordingly, when the Fe-Al intermetallic compound layer <NUM> has a laminated structure of the first layer <NUM> and the second layer <NUM>, the occurrence of cracks in the steel sheet <NUM> for hot press during the hot press process may be more effectively prevented or minimized.

The second layer <NUM> may absorb an external force and improve adhesion of the plating layer <NUM> during the hot press process. The second layer <NUM> has a greater Al content and a less Si content than the first layer <NUM>, and thus has a composition more similar to the surface layer <NUM> than the Fe-Al alloy layer <NUM> and the first layer <NUM>. Therefore, the second layer <NUM> may improve the adhesion of the plating layer <NUM>.

When an average thickness of the first layer <NUM> is less than <NUM>, an effect of absorbing an external force causing cracks in the plating layer <NUM> during the hot press process decreases sharply. When the average thickness of the first layer <NUM> is greater than <NUM>, Kirkendal voids may be generated due to a difference in diffusion rates of Al and Fe, thereby decreasing performance such as weldability. Therefore, the average thickness of the first layer <NUM> may be <NUM> to <NUM>, preferably, <NUM> to <NUM>.

In addition, when an average thickness of the second layer <NUM> is less than <NUM>, an Fe2Al5 layer having high brittleness may be formed due to diffusion of Fe during the hot press process, and thus, cracks may occur in the plating layer <NUM> or the plating layer <NUM> may be peeled. When the average thickness of the second layer <NUM> is greater than <NUM>, stress remaining in the plating layer <NUM> may increase after the hot press process, and thus, cracks may occur in the plating layer <NUM> or the plating layer <NUM> may be peeled. Accordingly, the average thickness of the second layer <NUM> may be <NUM> to <NUM>.

As described above, when the Fe-Al intermetallic compound layer <NUM> has the laminated structure of the first layer <NUM> and the second layer <NUM>, not only cracks may be more effectively prevented from occurring in the plating layer <NUM>, but also a bonding strength of the surface layer <NUM> may be improved, thereby increasing the stability of the plating layer <NUM>.

<FIG> is a flowchart schematically illustrating a method of manufacturing a steel sheet for hot press of <FIG>. Hereinafter, a method of manufacturing a steel sheet for hot press will be described with reference to <FIG> and <FIG>.

A method of manufacturing a steel sheet for hot press according to one embodiment may include hot rolling operation S310, cooling/coiling operation S320, a cold rolling operation S330, annealing heat treatment operation S340, and hot-dip plating operation S350 for a steel slab.

A semi-finished steel slab that is an object of a process of forming a plated steel sheet is provided. The steel slab may include carbon (C) in an amount of <NUM> wt% to <NUM> wt%, silicon (S) in an amount of <NUM> wt% to <NUM> wt%, manganese (Mn) in an amount of <NUM> wt% to <NUM> wt%, phosphorus in an amount greater than <NUM> wt% and less than or equal to <NUM> wt%, sulfur (S) in an amount greater than <NUM> wt% and less than or equal to <NUM> wt%, aluminum (Al) in an amount greater than <NUM> wt% and less than or equal to <NUM> wt%, nitrogen in an amount greater than <NUM> wt% and less than or equal to <NUM> wt%, balance iron (Fe), and other inevitable impurities. In addition, the steel slab may further include one or more of niobium (Nb) in an amount of <NUM> wt% to <NUM> wt%, titanium (Ti) in an amount of <NUM> wt% to <NUM> wt%, chromium (Cr) in an amount of <NUM> wt% to <NUM> wt%, molybdenum (Mo) in an amount of <NUM> wt% to <NUM> wt%, and boron (B) in an amount of <NUM> wt% to <NUM> wt%.

Reheating operation of the steel slab is performed for hot rolling. In the reheating operation of the steel slab, components segregated during casting are resolved by reheating, to a certain temperature, the steel slab secured through a continuous casting process. In one exemplary embodiment, a slab reheating temperature (SRT) may be <NUM> to <NUM>. When the slab reheating temperature (SRT) is lower than <NUM>, the components segregated during casting may not be sufficiently resolved, and thus, a homogenization effect of alloy elements may not be significantly shown, and a solid solution effect of titanium (Ti) may not be significantly shown. As the slab reheating temperature (SRT) is a high, the slab reheating temperature (SRT) is appropriate for homogenization. However, when the slab reheating temperature (SRT) exceeds <NUM>, an austenite grain size may increase, and thus, strength may not be secured, and only manufacturing cost of the steel sheet may increase due to an excessive heating process.

In hot rolling operation S310, the reheated steel slab is hot rolled at a certain finishing delivery temperature (FDT). In one embodiment, the finishing delivery temperature may be <NUM> to <NUM>. Here, when the finishing delivery temperature (FDT) is lower than <NUM>, workability of the steel sheet may not be secured due to the occurrence of a mixed structure due to rolling over an abnormal area. Also, the workability may be deteriorated due to an uneven microstructure, and a passing ability may be deteriorated during hot rolling due to a rapid phase change. When the finishing delivery temperature (FDT) exceeds <NUM>, austenite grains are coarsened. In addition, TiC precipitates may be coarsened to thereby deteriorate the performance of a final part.

In cooling/coiling operation S320, the hot-rolled steel sheet is cooled to a certain coiling temperature (CT) and coiled. In one embodiment, the coiling temperature (CT) may be <NUM> to <NUM>. The coiling temperature (CT) affects the redistribution of carbon (C). When the coiling temperature (CT) is less than <NUM>, a low-temperature phase fraction may increase due to subcooling, thereby increasing the strength, intensifying a rolling road during cold rolling, and rapidly deteriorating ductility. In contrast, when the coiling temperature (CT) exceeds <NUM>, formability and strength deterioration may occur due to abnormal grain growth or excessive grain growth.

In cold rolling operation S330, the coiled steel sheet is uncoiled, pickled, and then cold-rolled. Here, pickling is performed to remove scale of the coiled steel sheet, i.e., a hot-rolled coil manufactured through the hot rolling process described above.

Annealing heat treatment operation S340 is operation of performing, on the cold-rolled steel sheet, annealing heat treatment at a temperature higher than or equal to <NUM>. In one embodiment, annealing heat treatment includes operation of heating a cold-rolled sheet material and cooling the heated cold-rolled sheet material at a certain cooling rate.

Hot-dip plating operation S350 is operation of forming a plating layer on the annealed heat-treated steel sheet. In one embodiment, in hot-dip plating operation S350, the plating layer <NUM> of Al-Si may be formed on the annealed heat-treated steel sheet, i.e., on the base steel sheet <NUM>.

In detail, according to the present invention, the hot-dip plating operation S350 includes: operation of forming a hot-dip plating layer on the surface of the base steel sheet <NUM> by immersing the base steel sheet <NUM> in a plating bath having a temperature of <NUM> to <NUM>; and cooling operation of forming the plating layer <NUM> by cooling the base steel sheet <NUM> on which the hot-dip plating layer is formed.

The plating bath includes Si in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and balance Al. In particular, Si included in the plating bath may suppress the growth of the Fe-Al alloy layer <NUM> when the plating layer <NUM> is formed. Therefore, when a content of Si is less than <NUM> wt%, the Fe-Al alloy layer <NUM> may be formed too thick, thereby reducing the formability of the steel sheet <NUM> for hot press and easily generating cracks in the steel sheet <NUM> for hot press. In contrast, when the content of Si is greater than <NUM> wt%, the growth of the Fe-Al intermetallic compound layer <NUM>, in particular, the second layer <NUM> may become dominant. Accordingly, an area fraction of the Fe-Al alloy layer <NUM> with respect to the diffusion layer <NUM> may be limited to <NUM> % to <NUM> % by adjusting the content of Si in the plating bath. Therefore, the Fe-Al intermetallic compound layer <NUM> may be formed in an area fraction of <NUM> % to <NUM> % with respect to the diffusion layer <NUM>, and thus, the occurrence of cracks in the plating layer <NUM> during a hot press process may be effectively prevented or minimized.

In addition, the plating bath may include, as additional elements, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, and the like.

The cooling operation of cooling the base steel sheet <NUM> on which the hot-dip plating layer is formed includes: first cooling operation of cooling the base steel sheet <NUM> at a first average cooling rate from a temperature of the plating bath to <NUM>; and second cooling operation of cooling the base steel sheet <NUM> at a second average cooling rate from <NUM> to room temperature. Here, the first average cooling rate is greater than the second average cooling rate. The first average cooling rate is greater than or equal to <NUM>/s, and an overall average cooling rate for cooling from the temperature of the plating bath to the room temperature may be <NUM>/ to <NUM>/s.

In addition, the base steel sheet <NUM> passes through the plating bath to form the hot-dip plating layer on the base steel sheet <NUM>. Here, a passing rate of the base steel sheet <NUM> passing through the plating bath is <NUM> mpm (meter per minute) to <NUM> mpm.

As described above, after the base steel sheet <NUM> passes through the plating bath at the rate of <NUM> mpm to <NUM> mpm, the first cooling operation and the second cooling operation are performed to form the Fe-Al intermetallic compound layer <NUM> so that the Fe-Al intermetallic compound layer <NUM> includes the first layer <NUM> and the second layer <NUM> sequentially laminated.

The plating layer <NUM> that is formed may be an Al-Si plating layer and may be plated and formed at <NUM>/m<NUM> to <NUM>/m<NUM> with respect to both sides of the base steel sheet <NUM> or may be formed to have a thickness of <NUM> to <NUM>. For this, before the base steel sheet <NUM> on which the hot-dip plating layer is formed is cooled, air or gas may be sprayed onto the base steel sheet <NUM> to wipe the hot-dip plating layer to thereby adjust a thickness of the hot-dip plating layer.

Hereinafter, exemplary embodiments will be described in more detail. The following embodiments may be appropriately modified and changed by one of ordinary skill in the art within the scope of the invention as defined by the appended claims.

After performing hot rolling, cooling/coiling, cold rolling, and annealing heat treatment on a steel slab of the following components to form a base steel sheet (to a sheet thickness of <NUM>), a steel sheet for hot press was manufactured by performing hot-dip plating on a surface of the base steel sheet to form a plating layer.

Hot-dip Al plating was performed by using an oxidation-free furnace-reduction furnace type line and, after plating, adjusting an adhesion amount of a hot-dip plating layer from <NUM>/m<NUM> to <NUM>/m<NUM> on one side by gas wiping and then cooling the hot-dip plating layer. Here, a plating bath was set to include Si of <NUM> wt%, Fe of <NUM> wt%, and a component of balance Al in a temperature range of <NUM> to <NUM>. In addition, the base steel sheet passed through the plating bath at a rate of <NUM> mpm to <NUM> mpm and then was cooled at an average cooling rate of <NUM>/s to room temperature to manufacture a steel sheet for hot press.

Wiping of the hot-dip plating layer, the temperature of the plating bath, or the rate (an immersion time) of the base steel sheet passing through the plating path was adjusted to manufacture a specimen having different average thickness of a plating layer, area fraction of a diffusion layer with respect to the plating layer, area fraction of an Fe-Al alloy layer with respect to the diffusion layer, and area fraction of an Fe-Al intermetallic compound layer with respect to the diffusion layer as shown in Table <NUM> below. Thereafter, the specimen was heated up to a temperature higher than or equal to Ac3, an external force was applied to the specimen with a press, and at the same time, the specimen was quenched to measure the number of cracks generated in the plating layer. In detail, a sample was taken from the specimen to measure the area fraction of the diffusion layer with respect to the plating layer, the area fraction of the Fe-Al alloy layer with respect to the diffusion layer, and the area fraction of the Fe-Al intermetallic compound layer with respect to the diffusion layer. The specimen was heated at an average heating rate higher than or equal to <NUM>/s to the temperature higher than or equal to Ac3, an external force was applied to the specimen with the press, and at the same time, the specimen was quenched at an average rate higher than or equal to <NUM>/s to the temperature less than or equal to <NUM>. The number of cracks generated in the plating layer per unit length (mm) at certain three points of the specimen was measured.

As shown in Table <NUM> above, in the case of embodiments <NUM> to <NUM> in which the area fraction of the Fe-Al intermetallic compound layer with respect to the diffusion layer was within a range of <NUM> % to <NUM> %, the number of cracks generated in the plating layer was much less than in comparative examples <NUM> to <NUM> in which the area fraction of the Fe-Al intermetallic compound layer with respect to the diffusion layer was less than <NUM>%. This is because the area fraction of the Fe-Al intermetallic compound layer with respect to the diffusion layer was greater than or equal to <NUM>% to effectively absorb an external force causing cracks in the plating layer during a hot press process. As a result, the occurrence of cracks in the plating layer may be prevented or minimized. As described above, when the area fraction of the Fe-Al intermetallic compound layer exceeds <NUM>%, an average thickness of the Fe-Al alloy layer is relatively reduced. Therefore, liquid metal embrittlement may not be prevented, and the Fe-Al intermetallic compound layer, which has an area fraction exceeding <NUM>% within a temperature range of the plating bath as described above, may not be secured.

Table <NUM> below shows a result of measuring the number of cracks generated in the plating layer as follows. In the same conditions in addition to embodiments <NUM> to <NUM>, a base steel, which passed through a plating bath, was cooled at an average cooling rate of <NUM>/s up to <NUM> and cooled at an average cooling rate of <NUM>/s from <NUM> to room temperature to manufacture a steel sheet for hot press. In addition, a specimen was manufactured in the same conditions as in Table <NUM> above, the specimen was heated up to a temperature higher than or equal to Ac3, an external force was applied to the specimen with a press, and at the same time, the specimen was quenched to thereby the number of cracks generated in the plating layer.

As shown in Table <NUM> above, in embodiments <NUM> to <NUM> in which cooling was performed at an average cooling rate of <NUM>/ from the temperature of the plating bath to the room temperature, the Fe-Al intermetallic compound layer structure was formed in one layer. However, in the case of embodiments <NUM> to <NUM> in which the base steel was cooled at an average cooling rate of <NUM>/s up to <NUM> and cooled at an average cooling rate of <NUM>/s from <NUM> to the room temperature, the Fe-Al intermetallic compound layer structure had a two-layer structure in which a first layer and a second layer were laminated. In addition, when the Fe-Al intermetallic compound layer had a two-layer structure, the number of cracks generated in the plating layer was more reduced.

This is, as described above, because the first layer and the second layer operate as a buffer absorbing an external force causing cracks, and, although cracks occur in a hard Fe-Al alloy layer, crack propagation at an interface formed during the hot press process blocks transmission of cracks generated in the Fe-Al alloy layer to the plating layer. In addition, as the Fe-Al intermetallic compound layer has a two-layer structure, the plating layer may be formed to have a high bonding strength.

According to exemplary embodiments of the present invention, as a plating layer may include an Fe-Al intermetallic compound layer, the occurrence of cracks in a steel sheet for hot press during a hot press process may be more effectively prevented or minimized.

In addition, the Fe-Al intermetallic compound layer includes a first layer and a second layer, having a greater hardness than the first layer, which are sequentially laminated, thereby improving an adhesion of the plating layer.

Claim 1:
A steel sheet (<NUM>) for hot press forming, the steel sheet (<NUM>) comprising:
a base steel sheet (<NUM>); and
a plating layer (<NUM>) disposed on the base steel sheet (<NUM>) and having a diffusion layer (<NUM>) and a surface layer (<NUM>) that are sequentially laminated,
wherein the diffusion layer (<NUM>) comprises an Fe-Al alloy layer (<NUM>) and an Fe-Al intermetallic compound layer (<NUM>) that are sequentially disposed on the base steel sheet (<NUM>) and each of which includes silicon, and
an area fraction of the Fe-Al intermetallic compound layer (<NUM>) with respect to the diffusion layer (<NUM>) is <NUM> % to <NUM> %, wherein the Fe-Al intermetallic compound layer (<NUM>) comprises a first layer (<NUM>) and a second layer (<NUM>) that are sequentially laminated,
wherein the Fe-Al alloy layer (<NUM>) comprises Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%,
wherein the Fe-Al intermetallic compound layer (<NUM>) comprises Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%,
wherein the first layer (<NUM>) comprises Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%,
wherein the second layer (<NUM>) comprises Al in an amount of <NUM> wt% to <NUM> wt%, Fe in an amount of <NUM> wt% to <NUM> wt%, and Si in an amount of <NUM> wt% to <NUM> wt%, and
wherein a hardness of the Fe-Al alloy layer (<NUM>) is greater than a first hardness of the first layer (<NUM>) and a second hardness of the second layer (<NUM>), and the second hardness is greater than the first hardness.