Hot-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to steel suitable for automobile chassis components and, more specifically, to a high yield ratio type hot-rolled steel sheet having ultra-high strength and excellent hole expandability, and a manufacturing method thereof.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to steel suitable for automobile chassis parts, and more specifically, to a high yield ratio hot-rolled steel sheet having ultra-high strength and excellent hole expandability, and a method for manufacturing the same.

[0002] Hot-rolled steel sheets have been used as materials for automotive components such as chassis and frames. To reduce vehicle weight, hot-rolled steel sheets are required to be manufactured with high strength and thinner profiles. Furthermore, to manufacture these sheets into parts of a specific shape, excellent formability is required. Furthermore, there is a growing demand for a certain yield strength to maximize component durability.

[0003] In recent years, as automotive development has shifted from internal combustion engines to electric vehicle platforms, there has been a pressing need to offset the inevitable weight gain caused by batteries. Consequently, the importance of steel materials has become even more crucial than in the past.

[0004] Meanwhile, as steel strength increases, local formability (typically, hole expandability) tends to deteriorate. Therefore, the development of technologies that can enhance both high strength (ultra-high strength) and formability (hole expandability) is necessary. To this end, various studies have been conducted in the past on the microstructural aspect.

[0005] As a related technology, Patent Document 1 discloses a hot-rolled steel sheet with excellent workability, forming a microstructure composed of hard phases of bainite and martensite in addition to ferrite. However, this technology does not mention a guaranteed tensile strength of 950 MPa or higher. Therefore, it does not consider aspects such as impact resistance for higher-strength hot-rolled steel sheets.

[0006] Patent Document 2 relates to a technology for manufacturing a composite phase steel having a yield strength of 750 MPa or more, a tensile strength of 950 MPa or more, and a certain hole expandability (HER) by forming a certain amount of MA phase (a mixed structure of martensite and austenite) in addition to bainitic ferrite and controlling the fraction according to the intragranular orientation difference of bainitic ferrite and the grain size of the MA phase. However, the technology does not include a technical idea for simultaneously securing a steel plate having an even higher tensile strength and excellent hole expandability, for example, a tensile strength of 1.2 GPa and a hole expandability of 30% or more.

[0007] Therefore, in order to provide hot-rolled steel sheets as a material applicable to automobile chassis, frame, and other parts, there is a need to develop steel sheets with improved formability while having a tensile strength of 1.2 GPa.

[0008] (Patent Document 1) Japanese Patent Publication No. 1994-049591

[0009] (Patent Document 2) Korean Patent Publication No. 2022-0039946

[0010] One aspect of the present invention provides a hot-rolled steel sheet having ultra-high strength and excellent hole expandability, making it suitable for use in automotive chassis components and other parts. Furthermore, a method for manufacturing the hot-rolled steel sheet is also provided.

[0011] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0012] According to one aspect of the present invention, a composite material comprising, in weight %, carbon (C): 0.05 to 0.25%, silicon (Si): 0.01 to 1.20%, manganese (Mn): 1.0 to 2.5%, aluminum (Al): 0.010 to 0.100%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, boron (B): 0.0010 to 0.0040%, niobium (Nb): 0.005 to 0.050%, titanium (Ti): 0.005 to 0.100%, and at least one of chromium (Cr), molybdenum (Mo), vanadium (V), and nickel (Ni) of 1.50% or less based on the total content, the remainder being Fe and other unavoidable impurities. We provide hot rolled steel sheets.

[0013] In one embodiment of the present invention, a hot-rolled steel sheet includes a surface portion and a central portion, and the surface portion may include ferrite as a microstructure with an area fraction of 10% or more, and the central portion may include one or a combination of tempered martensite and lower bainite as a microstructure with an area fraction of 85% or more.

[0014] In one embodiment of the present invention, the surface layer may include ferrite having an area fraction of 45% or less, and the remaining structure may be at least one of tempered martensite, lower bainite phase, granular bainite, upper bainite, fresh martensite, MA (martensite and austenite composite phase), and pearlite.

[0015] In one embodiment of the present invention, the tempered martensite phase of the surface layer and the central portion may include carbides so as to satisfy the following relationship 2.

[0016] [Relationship 2]

[0017] A ≥ 70

[0018] (Here, A represents the proportion (%) of carbides having an aspect ratio of 1.2 or more among all carbides inside the tempered martensite block, and the size of the carbides is limited to those with a long axis length of 200 to 1000 nm.)

[0019] In one embodiment of the present invention, the central portion may have a residual structure of at least one of granular bainite, upper bainite, fresh martensite, MA (martensite and austenite composite phase), and pearlite.

[0020] In this way, according to one embodiment of the present invention, by controlling the microstructure in the thickness direction along with the alloy composition, it is possible to secure not only ultra-high strength of the hot-rolled steel sheet but also excellent formability (hole expandability).

[0021] In one embodiment of the present invention, the H value defined in the following [Relationship 1] can satisfy 0.90 to 1.90.

[0022] [Relationship 1]

[0023] H = C + (0.5×Mn) + (0.4×Cr) + (0.2×Mo)

[0024] (In equation 1, each element is a weight content, and if not added, 0 is substituted.)

[0025] According to another aspect of the present invention, a method for manufacturing a hot-rolled steel sheet is provided, comprising the steps of: preparing a steel slab; heating the steel slab in a temperature range of 1150 to 1350°C; finishing hot-rolling the steel slab after the heating in a temperature range of 850 to 1150°C to obtain a hot-rolled steel sheet; first cooling the hot-rolled steel sheet and then coiling it in a temperature range of Ms (martensite transformation initiation temperature) -30°C to Ms -120°C; and second cooling the coiled hot-rolled steel sheet.

[0026] In one embodiment of the present invention, the hot rolling may be performed by the steps of: rough rolling the heated steel slab to obtain a bar; descaling the bar at a pressure of 150 bar or more; and finishing hot rolling after the descaling.

[0027] In one embodiment of the present invention, the descaling process can be performed 3 to 5 times.

[0028] In this way, by simultaneously removing the scale formed on the surface during the hot rolling process and lowering the surface temperature, a surface layer containing a certain percentage of ferrite can be ultimately secured. Consequently, both the ultra-high strength targeted by the present invention and improved formability can be achieved.

[0029] According to the present invention, a hot-rolled steel sheet having both ultra-high strength and excellent formability (hole expandability) can be provided. The hot-rolled steel sheet of the present invention is advantageously applicable as a material for automobile chassis components, etc.

[0030] The various advantageous and beneficial advantages and 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.

[0031] Figure 1 shows a microstructure observation photograph of a hot-rolled steel sheet (invention example and comparative example) according to one embodiment of the present invention.

[0032] Figure 2 shows a cross-sectional structure observation photograph of the surface layer of a hot-rolled steel sheet (invention example) according to one embodiment of the present invention.

[0033] The inventors of the present invention have conducted in-depth research to provide a hot-rolled steel sheet having ultra-high strength and further improved hole expandability in response to the demand for continuous weight reduction, while providing a hot-rolled steel sheet that has been applied as a material for conventional automobile chassis parts, etc.

[0034] As a result, it was confirmed that a hot-rolled steel sheet having a tensile strength of 1.2 GPa, which exceeds the existing tensile strength of 980 MPa, while also having improved hole expandability could be provided, and the present invention was completed.

[0035] Hereinafter, the present invention will be described in detail.

[0036] A hot-rolled steel sheet according to one aspect of the present invention may contain, in wt%, carbon (C): 0.05 to 0.25%, silicon (Si): 0.01 to 1.20%, manganese (Mn): 1.0 to 2.5%, aluminum (Al): 0.010 to 0.100%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, boron (B): 0.0010 to 0.0040%, niobium (Nb): 0.005 to 0.050%, and titanium (Ti): 0.005 to 0.100%.

[0037] Below, the reasons for limiting the alloy composition of a hot-rolled steel sheet according to one embodiment of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0038] Carbon (C): 0.05~0.25%

[0039] Carbon (C) is the most economical and effective element for strengthening steel. As C content increases, the precipitation strengthening effect increases, contributing to increased strength. Furthermore, higher C content enhances the hardenability of steel, promoting the formation of low-temperature phases such as bainite and martensite, thereby contributing to increased strength.

[0040] In one embodiment of the present invention, if the C content is less than 0.05%, the aforementioned effects cannot be sufficiently achieved, making it difficult to secure the target strength. On the other hand, if the C content exceeds 0.25%, excessive carbide formation within the steel may result in excessively high strength, resulting in poor formability. Furthermore, the steel's carbon equivalent increases, which has the disadvantage of poor weldability.

[0041] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.05 to 0.25%. In another embodiment of the present invention, C may be included in an amount of 0.07% or more, or 0.10% or more. In yet another embodiment, C may be included in an amount of 0.23% or less, or 0.22% or less.

[0042] Silicon (Si): 0.01~1.20%

[0043] Silicon (Si) is an element that deoxidizes molten steel, enhances solid solution strengthening, and delays the formation of coarse carbides at grain boundaries, thereby improving the formability of steel. By adding Si appropriately, a small amount of retained austenite can be secured, thereby increasing the elongation of the steel, thereby improving its formability.

[0044] Meanwhile, according to one embodiment of the present invention, the Si has an effect of suppressing the formation of carbides during heat treatment in a temperature range of 300 to 600°C.

[0045] In one embodiment of the present invention, in order to sufficiently obtain the above-mentioned effects, Si may be included in an amount of 0.01% or more. If the Si content is less than 0.01%, the effect of delaying the formation of carbides is insufficient, making it difficult to improve formability. On the other hand, if the Si content exceeds 1.20%, there is a concern that red scale due to Si may be formed on the steel surface during hot rolling, resulting in significantly poor surface quality, and there is also a problem of reduced weldability.

[0046] Therefore, in one embodiment of the present invention, Si may be included in an amount of 0.01 to 1.20%. In another embodiment of the present invention, Si may be included in an amount of 0.02% or more, or 0.03% or more. In yet another embodiment, Si may be included in an amount of 1.10% or less, or 1.00% or less.

[0047] Manganese (Mn): 1.0~2.5%

[0048] Manganese (Mn) is an effective element for solid-solution strengthening of steel. Furthermore, Mn increases the hardenability of steel, delaying the transformation of ferrite during cooling after hot rolling while facilitating the formation of hard phases (low-temperature phases) such as bainite and martensite.

[0049] In one embodiment of the present invention, if the content of Mn is less than 1.0%, the effect of adding Mn cannot be obtained. That is, the effects of strengthening and improving hardenability are insufficient, so that the intended structure and corresponding strength cannot be secured. On the other hand, if the content exceeds 2.5%, when the slab is cast in the casting process, a segregation zone develops significantly in the center of the thickness of the slab, which not only results in poor formability but also causes deterioration of the welding quality.

[0050] Accordingly, in one embodiment of the present invention, Mn may be included in an amount of 1.0 to 2.5%. In another embodiment of the present invention, Mn may be included in an amount of 1.2% or more, or 1.3% or more. In yet another embodiment, Mn may be included in an amount of 2.4% or less, or 2.3% or less.

[0051] Aluminum (Al): 0.010~0.100%

[0052] Aluminum (Al) is an element mainly added for deoxidation. In one embodiment of the present invention, Al refers to available aluminum (Sol.Al).

[0053] In one embodiment of the present invention, if the content of Al is less than 0.010%, the aforementioned effect cannot be sufficiently obtained. On the other hand, if the content exceeds 0.100%, AlN is formed by combining with nitrogen (N) in the steel, which makes it easy for corner cracks to occur in the slab during continuous casting, and there is a concern that defects due to the formation of inclusions may easily occur.

[0054] Therefore, in one embodiment of the present invention, Al may be included in an amount of 0.010 to 0.100%. In another embodiment of the present invention, Al may be included in an amount of 0.015% or more, or 0.020% or more. In yet another embodiment, Al may be included in an amount of 0.90% or less, or 0.80% or less.

[0055] Phosphorus (P): 0.001~0.050%

[0056] Phosphorus (P) is an element that has a solid-solution strengthening effect similar to Si and also has a ferrite transformation promoting effect.

[0057] In one embodiment of the present invention, manufacturing with a P content of less than 0.001% incurs excessive manufacturing costs, which is economically disadvantageous, and is also disadvantageous in obtaining the target level of strength. On the other hand, if the P content exceeds 0.050%, there is a risk of brittleness due to grain boundary segregation, and microcracks are likely to occur during steel forming, which significantly deteriorates ductility, impact resistance, etc.

[0058] Therefore, in one embodiment of the present invention, P may be included in an amount of 0.001 to 0.050%. According to another embodiment of the present invention, P may be included in an amount of 0.003% or more, or 0.005% or more. According to yet another embodiment, P may be included in an amount of 0.045% or less, or 0.040% or less.

[0059] Sulfur (S): 0.001~0.010%

[0060] Sulfur (S) is an impurity that inevitably exists in steel. In one embodiment of the present invention, if the S content exceeds 0.010%, it combines with manganese (Mn) and other elements in the steel to form non-metallic inclusions, which makes it easy for microcracks to occur during cutting of the steel and significantly reduces impact resistance. On the other hand, in order to manufacture steel with an S content of less than 0.001%, excessive time is required during steelmaking, which reduces productivity.

[0061] Therefore, in one embodiment of the present invention, S can be limited to 0.001 to 0.010%.

[0062] Nitrogen (N): 0.001~0.010%

[0063] Nitrogen (N), along with carbon (C), is a representative solid-solution strengthening element in steel. N can combine with titanium (Ti), aluminum (Al), etc. in steel to form coarse precipitates. In general, the solid-solution strengthening effect of N is similar to that of C, but as the content of N in steel increases, the toughness of the steel decreases significantly, so the content can be limited to 0.010% or less. In one embodiment of the present invention, in order to manufacture the steel with an N content of less than 0.001%, a lot of time is required during the steelmaking process, which causes a problem of reduced productivity.

[0064] Therefore, in one embodiment of the present invention, N can be limited to 0.001 to 0.010%.

[0065] Boron (B): 0.0010~0.0040%

[0066] Boron (B) can enhance the hardenability of steel even with small additions, making it the most economical and efficient element for achieving this. Therefore, for those seeking to fully enhance the hardenability of steel, boron (B) is the most preferred element among the elements beneficial for enhancing hardenability.

[0067] In one embodiment of the present invention, if the B content is less than 0.0010%, the hardenability of the steel cannot be sufficiently increased, making it impossible to form the intended structure. Consequently, securing the target strength also becomes difficult. On the other hand, if the B content exceeds 0.0040%, there is a risk of secondary embrittlement occurring due to segregation of boron at grain boundaries due to excessive addition of B.

[0068] Therefore, in one embodiment of the present invention, B may be included in an amount of 0.0010 to 0.0040%. According to another embodiment of the present invention, B may be included in an amount of 0.0013% or more, or 0.0015% or more. According to yet another embodiment, B may be included in an amount of 0.0038% or less, or 0.0035% or less.

[0069] Niobium (Nb): 0.005~0.050%

[0070] Niobium (Nb) is a representative precipitation-strengthening element, along with titanium (Ti) and vanadium (V), which will be described later. Nb precipitates during hot rolling and is effective in improving the strength and impact toughness of steel by refining grains through delayed recrystallization.

[0071] In one embodiment of the present invention, if the content of Nb is less than 0.005% when added, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the content exceeds 0.050%, there is a problem in that excessive delay in recrystallization occurs during hot rolling, resulting in the formation of elongated grains and the formation of coarse composite precipitates, which deteriorates the formability of the steel.

[0072] Therefore, in one embodiment of the present invention, Nb may be included in an amount of 0.005 to 0.050%. In another embodiment of the present invention, Nb may be 0.010% or more, and in another embodiment, Nb may be 0.045% or less.

[0073] Titanium (Ti): 0.005~0.100%

[0074] As previously mentioned, titanium (Ti), along with niobium (N) and vanadium (V), is a representative precipitation-strengthening element. Its strong affinity for nitrogen (N) in steel allows it to form coarse TiN. TiN inhibits grain growth during the heating process of steel slabs for hot rolling. Furthermore, the remaining Ti, after reacting with nitrogen, forms a solid solution in the steel, combining with carbon to form TiC precipitates, making it a useful element for enhancing the strength of steel.

[0075] In one embodiment of the present invention, in order to sufficiently obtain the aforementioned effects, Ti may be included in an amount of 0.005% or more. However, if the content exceeds 0.100%, there is a problem in that the formability of the steel is deteriorated due to the formation of coarse precipitates (TiN, TiC, etc.).

[0076] Therefore, in one embodiment of the present invention, Ti may be included in an amount of 0.005 to 0.100%. In another embodiment of the present invention, Ti may be 0.010% or more, and in another embodiment, Ti may be 0.095% or less.

[0077] A hot-rolled steel sheet according to one embodiment of the present invention may further include the following elements in addition to the above-described alloy composition.

[0078] As an example, one or more of chromium (Cr), molybdenum (Mo), vanadium (V), and nickel (Ni) may be further included in an amount of 1.50% or less based on the total content.

[0079] Among these, chromium (Cr) strengthens steel and can therefore be added to enhance strength. Furthermore, Cr delays the phase transformation of ferrite during cooling and induces the formation of the bainite phase.

[0080] Molybdenum (Mo) increases the hardenability of steel and promotes the formation of fine precipitates. This Mo facilitates the formation of the bainite phase in the steel structure, thereby contributing to the enhancement of steel strength.

[0081] Vanadium (V), along with the aforementioned Nb and Ti, is a representative precipitation-strengthening element. It rarely precipitates during hot rolling, but forms precipitates during the cooling and coiling processes, thereby enhancing the strength of steel. Therefore, it is advantageous for further enhancing strength without increasing deformation resistance or rolling load due to the delay in recrystallization during hot rolling.

[0082] Nickel (Ni) is also an element that is advantageous in improving the hardenability of steel, and especially when adding Ni during the production of ultra-high strength steel, the target strength can be advantageously secured.

[0083] In one embodiment of the present invention, if the sum of the contents of the above-mentioned elements exceeds 1.50%, a martensite phase is excessively formed as the final microstructure, which is advantageous in securing strength, but there is a concern that formability may be greatly inferior.

[0084] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0085] A hot-rolled steel sheet according to one embodiment of the present invention may be composed of the above-described alloy elements, and the relationship between some of the alloy elements may be limited to specific conditions.

[0086] In one embodiment of the present invention, the content relationship between C, Mn, Cr, and Mo in the alloy composition may be defined by the following relational expression 1. According to one embodiment of the present invention, the H value defined by the following relational expression 1 may be 0.90 to 1.90.

[0087] [Relationship 1]

[0088] H = C + (0.5×Mn) + (0.4×Cr) + (0.2×Mo)

[0089] (In equation 1, each element is a weight content, and if not added, 0 is substituted.)

[0090] Among the alloy compositions, C, Mn, Cr, and Mo are elements that contribute to improving the hardenability of steel. In one embodiment of the present invention, in order to secure ultra-high strength, not only the formation of hard phases (low-temperature phases) such as bainite and martensite phases in the microstructure, but also the formation of specific carbides is required, and to achieve this, it is necessary to add a certain amount of hardenable elements. However, if the content of these elements is excessive, it becomes difficult to secure the target formability, and therefore the content needs to be appropriately controlled. Accordingly, the relationship between the contents of the hardenable elements is expressed as Relationship 1. Relationship 1 at this time also affects the securing of a microstructure that is advantageous for securing the intended physical properties.

[0091] According to one embodiment of the present invention, as the H value according to relational expression 1 increases, the hardenability of the steel increases. When the value exceeds 1.90%, it is advantageous for securing strength, and the tensile strength can be significantly improved. However, as the strength increases excessively, the formability becomes significantly inferior. In other words, it becomes difficult to provide a steel sheet having both the ultra-high strength and excellent formability targeted by the present invention.

[0092] Meanwhile, if the value of relational expression 1 according to one embodiment of the present invention is less than 0.90%, the hardenable elements are not sufficiently contained, so that sufficient strength improvement cannot be obtained, and thus ultra-high strength cannot be secured.

[0093] Therefore, in one embodiment of the present invention, the H value defined by relational expression 1 may be 0.90 to 1.90. In another embodiment of the present invention, the H value may be 1.00 or greater, and in another embodiment, may be 1.80 or less.

[0094] Hereinafter, the microstructural characteristics of a hot-rolled steel sheet according to one embodiment of the present invention will be described in detail.

[0095] The inventors of the present invention conducted in-depth research to obtain a hot-rolled steel sheet suitable for automotive chassis components, while maintaining an ultra-high tensile strength of 1.2 GPa and enhanced formability. As a result, they discovered that the phase composition of hot-rolled steel sheets is important in terms of microstructure, along with the alloy composition.

[0096] In particular, a hot-rolled steel sheet according to one embodiment of the present invention controls the surface layer structure differently from the center structure in terms of improving formability, forms a certain fraction of ferrite in the surface layer, and forms a composite structure of a tempered martensite phase and a lower bainite phase, which is advantageous for securing strength, as the main phase in the center structure. As a result, it is possible to provide a hot-rolled steel sheet that achieves both ultra-high strength and high formability.

[0097] Accordingly, a hot-rolled steel sheet according to one embodiment of the present invention can be divided into a surface portion including a certain fraction of a ferrite phase in the thickness direction and a central portion, which is the remaining region excluding the surface portion, and the microstructure can be controlled for each region.

[0098] In one embodiment of the present invention, the surface layer of the hot-rolled steel sheet may be set to a region corresponding to 10% of the total thickness in the thickness direction from the surface. That is, in one embodiment of the present invention, the thickness (depth) of the surface layer may vary depending on the thickness of the hot-rolled steel sheet being manufactured, and thus may be proposed as a ratio depending on the thickness of the corresponding hot-rolled steel sheet.

[0099] In one embodiment of the present invention, the microstructure of the surface layer may include ferrite at an area fraction of 10% or more. If the ferrite fraction within the surface layer is less than 10%, the effect of improving the formability of the hot-rolled steel sheet cannot be obtained. On the other hand, the surface layer may include a ferrite phase of 45% or less, and if the fraction exceeds 45%, the fraction of the hard phase is relatively low, making it impossible to secure the target strength.

[0100] In one embodiment of the present invention, the microstructure of the surface layer, excluding the ferrite phase, is not particularly limited in type. As a non-limiting example, the remaining structure excluding the ferrite phase may be composed of one or more phases selected from the group consisting of tempered martensite, lower bainite, granular bainite, upper bainite, fresh martensite, MA (martensite and austenite composite phase), and pearlite. However, in the case of a series of processes according to one embodiment of the present invention, the tempered martensite phase and the lower bainite phase may exist in a relatively high fraction among the remaining structure excluding the ferrite phase.

[0101] Meanwhile, a hot-rolled steel sheet according to one embodiment of the present invention may have a central portion distinct from a surface portion, and the central portion may correspond to the entire area excluding the surface portion. In one embodiment of the present invention, the central portion may include at least one of a tempered martensite phase and a lower bainite phase as a main phase, and may include one or a combination of these at an area fraction of 85% or more. The tempered martensite phase is a structure in which the fresh martensite phase is tempered, and has relatively lower strength and softness compared to the fresh martensite phase. Therefore, in terms of improving the tensile strength and yield strength of the steel sheet, the introduction of the tempered martensite phase is advantageous compared to the fresh martensite phase. In addition, the lower bainite phase is a structure formed at a lower temperature than the upper bainite phase, and has a characteristic of higher strength than the upper bainite phase.

[0102] In this way, by including a ferrite phase of 10% or more in the surface layer and at least one of a tempered martensite phase and a lower bainite phase of 85% or more in the center, the strength and formability of the hot-rolled steel sheet can be improved simultaneously. As a non-limiting example, it is possible to provide a hot-rolled steel sheet having an ultra-high strength of 1.2 GPa and a hole expandability (HER) of 30% or more.

[0103] In one embodiment of the present invention, when configuring the tempered martensite phase and the lower bainite phase as the main phase as the microstructure of the central portion, the fraction of each phase is not limited, and if the combined fraction of these structures is 85% or more, there is no problem in securing the target properties.

[0104] In one embodiment of the present invention, the residual structure excluding the main phase within the central portion is not particularly limited. As a non-limiting example, the residual structure may be composed of one or more phases selected from the group consisting of granular bainite, upper bainite, fresh martensite, MA (a composite phase of martensite and austenite), and pearlite. It should also be noted that these residual structures are not necessarily included.

[0105] As described above, the inventors of the present invention have discovered that the behavior of carbides within a specific phase in hot-rolled steel sheets with controlled microstructures across the thickness direction is a critical factor in securing desired properties. Therefore, after extensive research, we propose carbide properties defined by Equation 2.

[0106] Specifically, the hot-rolled steel sheet according to one embodiment of the present invention may mainly include carbides having a value of (major axis / minor axis), which represents the length ratio of the minor axis to the major axis (Aspect ratio), of 1.2 or more among the carbides when carbides exist in the tempered martensite, as represented by the following Relationship Formula 2. Here, “mainly included” means that the proportion of carbides having an aspect ratio of 1.2 or more among the entire carbides is 70% or more, as represented by Relationship Formula 2.

[0107] [Relationship 2]

[0108] A ≥ 70

[0109] (Here, A represents the proportion of carbides having an aspect ratio of 1.2 or more among the carbides inside the tempered martensite block, and the size of the carbides is limited to those with a major axis length of 200 to 1000 nm.)

[0110] In this way, since the shape of the carbide existing in the tempered martensite has a columnar shape rather than a spherical shape, a tempered martensite phase having a desired target material can be secured. In one embodiment of the present invention, if the A value defined by relational expression 2 is less than 70, it may mean that the carbide existing in the tempered martensite is mainly formed in a spherical shape or the formation of the carbide is insufficient, and in these cases, it is difficult to secure the target strength.

[0111] In one embodiment of the present invention, carbide means an iron (Fe)-based carbide, and its type is not particularly limited, but may mainly be a transition carbide or cementite (Fe3C).

[0112] In one embodiment of the present invention, it is to be noted that the tempered martensite phase in which carbides exist corresponds to the entire thickness of the hot-rolled steel sheet, that is, it refers to both the tempered martensite phase formed in the surface portion and the tempered martensite phase formed in the center portion.

[0113] A hot-rolled steel sheet according to one embodiment of the present invention may have both ultra-high strength and excellent hole expandability. As an example, the hot-rolled steel sheet may have a yield strength of 950 MPa or greater, a tensile strength of 1180 MPa or greater, and an elongation of 6% or greater.

[0114] In this way, by securing a certain level of ductility along with ultra-high strength and further adjusting the structure of the surface layer of the steel plate, the hot-rolled steel plate can have excellent formability with a hole expansion ratio (HER) of 30% or more.

[0115] Hereinafter, a method for manufacturing a hot-rolled steel sheet 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 hot-rolled steel sheet according to one embodiment of the present invention.

[0116] According to one embodiment of the present invention, a hot-rolled steel plate can be manufactured by performing certain processes such as rolling, cooling, and coiling on a prepared steel slab, and each specific process step is described in detail below.

[0117] [Heating of steel slabs]

[0118] 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. As one example, the steel slab may have the same alloy composition and alloy composition relationship as the hot-rolled steel sheet according to one embodiment of the present invention, and the description of each alloy element and each composition relationship are replaced with the above-mentioned matters.

[0119] In one embodiment of the present invention, the heating of the steel slab may be performed at a temperature range of 1150 to 1350°C. If the heating temperature is lower than 1150°C, precipitates (e.g., precipitates of Ti, Nb, Mo, V, etc.) are not sufficiently re-dissolved, thereby reducing the formation of precipitates in subsequent processes following the hot rolling process. On the other hand, if the temperature exceeds 1350°C, the austenite grains become coarser, thereby coarsening the grain size of the final microstructure, resulting in a decrease in strength.

[0120] [Hot rolling]

[0121] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0122] In one embodiment of the present invention, the hot rolling may be performed at a temperature range of 850 to 1150°C. Specifically, the hot rolling may proceed to a step of performing rough rolling at a certain temperature to obtain a bar, then descaling the bar, and then performing final finishing rolling.

[0123] First, in one embodiment of the present invention, if hot rolling is initiated at a temperature exceeding 1150°C, the temperature of the hot-rolled steel sheet may increase, resulting in a coarser grain size and excessive oxide formation on the surface of the hot-rolled steel sheet, potentially degrading surface quality. On the other hand, if hot rolling is terminated at a temperature lower than 850°C, excessive recrystallization delay may occur, resulting in the development of elongated grains, which may exacerbate anisotropy and deteriorate formability.

[0124] Meanwhile, in one embodiment of the present invention, the rough rolling process is a process for obtaining a bar of a certain thickness before obtaining a hot-rolled steel sheet of the final thickness through a series of hot rolling processes. This rough rolling process can be performed under conventional conditions, and as an example, can be performed at a temperature range of 900 to 1100°C.

[0125] According to one embodiment of the present invention, a descaling process may be performed prior to finish rolling on a bar obtained by the rough rolling process. The descaling process is a process for removing oxides from the surface side of a hot-rolled steel sheet manufactured through a series of processes and locally lowering the temperature of the surface layer to below the ferrite transformation initiation temperature, thereby allowing a soft ferrite phase to exist. The inventors of the present invention, in an attempt to improve the formability of a hot-rolled steel sheet having ultra-high strength, have confirmed that when a soft phase exists at a certain fraction on the surface, different from the center in the thickness direction, the resistance to stress applied to the surface during processes such as steel sheet processing can be increased. Accordingly, a descaling process may be performed as one method for forming a ferrite phase at a certain fraction on the surface of a hot-rolled steel sheet, specifically, on the surface layer according to one embodiment of the present invention.

[0126] In one embodiment of the present invention, the descaling process may be a process of applying a certain pressure to the entire surface of a bar to remove oxides present on the surface of the bar and simultaneously providing cooling to the surface. To this end, the descaling process may be a process of spraying water at a water pressure of 150 bar or more, and for example, a water spraying device may be used. This lowers the temperature of the surface of the bar compared to the center, and in the process, a transformation from austenite to a ferrite phase occurs. If the pressure of the sprayed water during the descaling process is less than 150 bar, scale removal may take an excessive amount of time, which may lead to a temperature drop in areas other than the surface. In this case, the intended microstructure may not be formed in areas other than the surface. The upper limit of the pressure is not particularly limited, and it is to be understood that a person skilled in the art may appropriately select the pressure as long as the pressure does not cause the surface to become uneven due to the sprayed water.

[0127] In one embodiment of the present invention, the descaling process may be performed 3 to 5 times. That is, the number of times water is sprayed at a pressure of 150 bar or more may be performed from a minimum of 3 times to a maximum of 5 times. If the descaling process is performed less than 3 times, the ferrite phase may not be sufficiently secured in the surface layer as the final microstructure of the hot-rolled steel sheet. On the other hand, if the descaling process is performed more than 5 times, the cooling of the bar may proceed excessively in the depth direction, and the ferrite fraction in the surface layer may become excessive as the final microstructure.

[0128] In this way, by performing a descaling process during the process of manufacturing a hot-rolled steel sheet so that the temperature of the surface is lower than that of the center in the thickness direction, a certain fraction of a ferrite phase can be formed in the surface. At this time, the ferrite phase formed in the surface is maintained during the subsequent process, and thus exists as is in the final microstructure. However, the austenite structure remaining after the ferrite phase is formed during the descaling process forms a residual structure through a subsequent process, and this residual structure can be replaced with the above-described content.

[0129] In one embodiment of the present invention, a bar subjected to a descaling process may be subjected to finish rolling to obtain a hot-rolled steel sheet. The finish rolling may be performed at a lower temperature than the previously performed rough rolling process, and preferably at 850°C or higher.

[0130] [First cooling and winding]

[0131] The hot rolled steel sheet obtained through the above series of hot rolling processes can be cooled and then wound into a coil shape.

[0132] In one embodiment of the present invention, the cooling performed after the hot rolling process is referred to as the first cooling, and the first cooling process can be performed up to the coiling temperature range. As an example, it can be performed up to the temperature range of Ms (martensite transformation initiation temperature) -30°C to Ms -120°C at a cooling rate of 50°C / s or more.

[0133] If the cooling rate during the first cooling is less than 50°C / s, ferrite and upper bainite phases are excessively formed as microstructures during the cooling process, making it impossible to secure the target strength and formability.

[0134] In addition, if the cooling end temperature during the first cooling, i.e., the temperature at which the coiling process is performed, is lower than Ms-120°C, there is a concern that a large amount of martensite phase will be formed during this process, resulting in the final microstructure, especially the main phase in the center, being composed only of the tempered martensite phase. On the other hand, if the temperature exceeds Ms-30°C, the bainite phase will be formed excessively, thereby reducing the fraction of the tempered martensite phase in the final microstructure, and relatively low-strength granular bainite or upper bainite phase will be introduced by reheating during the coiling process after cooling, which is detrimental to securing ultra-high strength. In addition, formability, i.e., hole expandability, will be inferior. According to another embodiment of the present invention, the first cooling may be performed at Ms-100°C or higher, and according to another embodiment, at Ms-50°C or lower. Here, Ms represents the martensite transformation initiation temperature, and an equation for calculating it is shown in the examples below.

[0135] [Second Cooling]

[0136] The above first cooled and coiled hot-rolled steel sheet can be cooled again, and can be cooled in the coil form.

[0137] In one embodiment of the present invention, the cooling process at this time is referred to as second cooling, and as an example, it can be performed at a cooling rate of 0.1 to 25.0°C / hour to a temperature range of room temperature to 150°C. In this way, by slowly cooling the coiled hot-rolled steel sheet, the martensite phase generated during the first cooling and coiling process can be tempered.

[0138] If the cooling rate during the second cooling exceeds 25.0°C / hour, tempering of the martensite phase will not occur at the intended level, and thus an excessive amount of fresh martensite phase may exist as the final microstructure. In this case, there is no problem in securing ultra-high strength, but formability will be poor. On the other hand, in order to control the cooling rate to less than 0.1°C / hour, separate equipment is required, which is economically disadvantageous. According to another embodiment of the present invention, the second cooling may be performed at 1.0°C / hour or more, and in another embodiment, at 10.0°C / hour or less.

[0139] According to one embodiment of the present invention, a hot-rolled steel sheet can be manufactured through the aforementioned series of processes, and this hot-rolled steel sheet can exhibit both ultra-high strength and excellent hole expandability. This ensures excellent crack resistance during processing of the steel sheet, ultimately enabling more effective weight reduction of the vehicle body.

[0140] According to one embodiment of the present invention, the step of pickling and oiling the hot-rolled steel sheet manufactured through a series of processes may be further included.

[0141] In one embodiment of the present invention, a step of hot-dip galvanizing the hot-rolled steel sheet that has undergone the acid washing and oiling steps may be further included.

[0142] As a non-limiting example, the hot-dip galvanizing step may be performed by heating the hot-rolled steel sheet to a temperature range of 450 to 740°C, and a zinc-based plating bath may be used. As an example, the zinc-based plating bath may be a plating bath containing, but is not limited to, 0.01 to 30.00 wt% magnesium (Mg), 0.01 to 50.00 wt% aluminum (Al), and the remainder zinc and unavoidable impurities.

[0143] 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.

[0144] (Example)

[0145] A steel slab having the alloy composition and component relationship shown in Table 1 below was prepared, heat-treated at 1150 to 1350°C, and then rough-rolled at 900 to 1000°C, and then a hot-rolled steel sheet was manufactured through a series of processes under the conditions shown in Table 2 below.

[0146] For each hot-rolled steel sheet, the phase types and fractions of the microstructure were measured, and the mechanical properties were also measured.

[0147] The microstructure of each hot-rolled steel sheet was analyzed using SEM at 4000x and 8000x magnifications for specimens taken from the surface and center (thickness cross-section at 1 / 4t point), and the fraction of each phase was measured. However, in order to interpret the tempered martensite phase and fresh martensite phase, the specimens were etched with Nital and Lepera, and then analyzed at 1000x magnification using an optical microscope (OM) and an image analyzer. The phase fractions of the measured microstructures are shown in Table 3 below. Considering that the lower bainite (LB) and tempered martensite (TM) phases among the microstructures have very similar properties in terms of structure, they were calculated as a combined fraction and the results are presented. In addition, the combined fractions were also expressed for the granular bainite phase and the upper bainite phase, and for the fresh martensite phase and the MA phase.

[0148] The size of carbides present in the tempered martensite phase was measured using an image analyzer, and the ratio of the long and short sides of the carbides was measured in an image at 15,000x magnification of the tempered martensite phase obtained previously through SEM.

[0149] In addition, the mechanical properties of each hot-rolled steel sheet were evaluated by tensile and hole expandability tests. For the tensile test, JIS standard test specimens were collected in a direction perpendicular to the rolling direction and measured at room temperature using a universal tensile testing machine. In addition, the hole expandability (HER) test was conducted with a punching clearance (cl) of 12% and was performed based on the JFST 1001-1996 standard. The HER evaluation value was expressed as the average value after three tests.

[0150] Steel alloy composition (weight%) relationship equation 1 (H)Ms(℃)CSiMnAlPSNTiNbCrMoB10.060.601.40.0310.0070.0010.0030.0800.0300.10.10.00120.8246920.030.702.20.0250.0110.0020.0030.1100.0400.60.20.00111.4145130.140.600.70.0230.0130.00 30.0040.0700.0200.20.10.00160.5945540.280.402.30.0250.0120.0010.0030.0600.0200.20.30.00221.5734650.160.803.60.0350.0120.0020.0030.0900.0400.30.20.00282.1235760.210.901.90.0280 .0120.0020.0030.1200.0502.20.50.00212.1436270.080.702.10.0280.0090.0010.0040.0900.0300.20.10.00251.2343880.150.902.20.0250.0120.0020.0040.0800.0400.40.20.00181.4540290.170.802 .10.0310.0110.0020.0030.0500.0200.30.20.00221.38398100.210.401.80.0270.0090.0010.0040.0 900.0200.10.10.00171.17393110.220.501.50.0280.0090.0020.0030.0700.0300.400.00311.13396Ms = 539-(423×C)-(30.4×Mn)-(12.1×Cr)-(17.7×Ni)-(7.5×Mo) (Enter the weight content of each element, and if not added, enter 0)

[0151] Steel grade descaling finish rolling temperature (℃) 1st cooling (℃ / s) Coiling (℃ / s) 2nd cooling division hydraulic pressure (MPa) Number of times speed (℃ / hour) End temperature (℃ / s) 11603912654121936 Comparative example 121603907723961624 Comparative example 231603916593881529 Comparative example 341603889623051342 Comparative example 45160392282297851 Comparative example 56160487667311750 Comparative example 67160589672365640 Invention example 18160491276334236 Invention example 291603886813151022 Invention example 3101603902633512029 Invention example 4111604899702891130 Invention example 571604901765311237 Comparative example 78160386626342627 Comparative example 89160379871360945 Comparative example 91016049246151126 Comparative example 101116039136305625 Comparative example 11111601916653211335 Comparative example 12111607907773151428 Comparative example 13111603910773205732Comparative Example 14 When the descaling process is performed more than once, the same water pressure is applied for each cycle (number of times).

[0152] ClassificationMicrostructural characteristicsMechanical propertiesSurface areaCentral areaRelationship 2YS(MPa)TS(MPa)El(%)HER(%)FTM+LBGB+UBFM+MAComparative example 127672675279811421121Comparative example 22559329487649841419Comparative example 3144842105581210981226Comparative example 42197128113221672528Comparative example 51598118612981635425Comparative example 62995237212851563522Invention example 13994338010351232736Invention example 23598118311651384841Invention example 32699019112121422739 Invention example 42594429311211320742 Invention example 53194519411651298835 Comparative example 740117217246258311632 Comparative example 82888111196328711528 Comparative example 926751213738421295726 Comparative example 103699013212851492416 Comparative example 11303952768421135620 Comparative example 1249433719641252711 Comparative example 137584124748761106839 Comparative example 143363334507251215624 The fraction of each structure means the area fraction (%). F: ferrite, TM: tempered martensite, LB: lower bainite, GB: granular bainite, UB: upper bainite, MA: martensite-austenite composite structure (MA). Although not specified, the remaining structures excluding the F fraction of the surface layer were confirmed to be at least one of TM, LB, GB, UB, and MA phases, and in the case of the invention examples, the TM+LB phase was formed in a high fraction.

[0153] As shown in Tables 1 to 3, Invention Examples 1 to 5 are examples that satisfy both the alloy composition and manufacturing conditions according to an embodiment of the present invention. These invention examples formed the intended microstructures in the surface and center portions of the hot-rolled steel sheet. As a result, the target ultra-high strength of 1.2 GPa was achieved along with a hole expandability (HER) of 30% or more. As such, it can be seen that the hot-rolled steel sheet according to an embodiment of the present invention has both ultra-high strength and excellent formability.

[0154] Meanwhile, Comparative Examples 1 to 14 are cases in which at least one of the alloy composition, alloy component relationship, and manufacturing conditions according to one embodiment of the present invention is not satisfied.

[0155] Comparative examples 1, 3 and 5, which did not satisfy the alloy composition system (Relationship 1) according to one embodiment of the present invention, had poor hole expandability, and among these, Comparative examples 1 and 3 could not secure the target level of strength because they exceeded the lower limit of Relationship 1.

[0156] Comparative Example 2, in which the content of C was insufficient according to one embodiment of the present invention, could not sufficiently secure strength and hole expandability, and Comparative Example 4, in which the content of C was excessive, was able to secure ultra-high strength, but had low ductility and hole expandability.

[0157] Comparative Example 7 was a case where coiling was performed at an excessively high temperature, and Comparative Examples 8 and 11 were cases where a relatively slow cooling rate was applied during cooling after finish rolling. In all of these cases, relatively low-strength granular bainite and upper bainite phases were excessively formed as the final microstructure, rather than tempered martensite and lower bainite phases. As a result, ultra-high strength could not be secured.

[0158] Comparative Example 9 is a case where finishing rolling was performed at too low a temperature, and hole expandability was poor due to the excessive formation of fresh martensite phase and MA phase in the center as the final microstructure.

[0159] Comparative Example 10 is a case where coiling was performed at an excessively low temperature, and as a result of tempering at an excessively low temperature, the intended carbide was not formed within the tempered martensite phase. In this way, the martensite structure tempered at a low temperature exhibits properties similar to the fresh martensite phase, and while it is possible to secure ultra-high strength, it is difficult to secure the desired formability due to poor ductility and hole expandability.

[0160] Meanwhile, Comparative Examples 12 and 13 are examples in which the conditions of the descaling process do not satisfy the present invention. In Comparative Example 12, the number of descaling processes was too small to sufficiently lower the surface temperature of the rough-rolled bar, and as a result, the ferrite fraction in the surface layer was insufficient, resulting in poor hole expandability. On the contrary, in Comparative Example 13, the number of descaling processes was too large, and the surface of the rough-rolled bar was excessively cooled, resulting in the formation of a ferrite phase exceeding the target fraction, and as a result, the ultimate ultra-high strength could not be secured.

[0161] In addition, in Comparative Example 14, the cooling rate was applied too quickly during the second cooling during coiling, so that tempering of martensite was not properly performed, and the intended carbide was not formed within the tempered martensite phase, and the yield strength was low and the hole expandability was poor.

[0162] Figure 1 shows a tissue photograph of the center (1 / 4t point) of Invention Example 1 and Comparative Example 7.

[0163] As shown in Fig. 1, it can be confirmed that in Inventive Example 1, a tempered martensite phase and a lower bainite phase were mainly formed, whereas in Comparative Example 7, a relatively coarse granular bainite phase and an upper bainite phase were formed in a high fraction.

[0164] Figure 2 shows a tissue photograph of a cross-section observed from the surface to the thickness direction of Invention Example 1.

[0165] As shown in Fig. 2, it can be confirmed that a ferrite phase is formed to a certain thickness based on the surface excluding the oxide layer. Here, the oxide layer corresponds to the hot-rolled scale, and a process to remove it can be performed later.

Claims

1. Contains, in weight%, carbon (C): 0.05 to 0.25%, silicon (Si): 0.01 to 1.20%, manganese (Mn): 1.0 to 2.5%, aluminum (Al): 0.010 to 0.100%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, boron (B): 0.0010 to 0.0040%, niobium (Nb): 0.005 to 0.050%, titanium (Ti): 0.005 to 0.100%, and one or more of chromium (Cr), molybdenum (Mo), vanadium (V), and nickel (Ni) of 1.50% or less based on the total content, the remainder being iron and other unavoidable impurities. Includes the surface and the center, The above surface layer contains ferrite as a microstructure with an area fraction of 10% or more, The above hot-rolled steel sheet having a microstructure of tempered martensite and lower bainite, or a combination of the two, with an area fraction of 85% or more.

2. In paragraph 1, The above surface layer contains ferrite having an area fraction of 45% or less, Hot-rolled steel sheet having a residual structure of at least one of tempered martensite, lower bainite, granular bainite, upper bainite, fresh martensite, MA (martensite and austenite composite phase), and pearlite.

3. In paragraph 1 or 2, A hot-rolled steel sheet in which the tempered martensite phase in the surface layer and the center contains carbides so as to satisfy the following relationship 2. [Relationship 2] A ≥ 70 (Here, A represents the ratio (%) of carbides having an aspect ratio of 1.2 or more among all carbides inside the tempered martensite block, and the size of the carbides is limited to those whose major axis length falls within the range of 200 to 1000 nm.) 4. In paragraph 1, The above hot-rolled steel sheet having a residual structure of at least one of granular bainite, upper bainite, fresh martensite, MA (martensite and austenite composite phase), and pearlite.

5. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet that satisfies the H value defined in [Relational Expression 1] below, which is 0.90 to 1.

90. [Relationship 1] H = C + (0.5×Mn) + (0.4×Cr) + (0.2×Mo) (In equation 1, each element is a weight content, and 0 is substituted if not added.) 6. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a yield strength of 950 MPa or more, a tensile strength of 1180 MPa or more, and an elongation of 6% or more.

7. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a hole expandability (HER) of 30% or more.

8. Steel containing carbon (C): 0.05 to 0.25%, silicon (Si): 0.01 to 1.20%, manganese (Mn): 1.0 to 2.5%, aluminum (Al): 0.010 to 0.100%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, boron (B): 0.0010 to 0.0040%, niobium (Nb): 0.005 to 0.050%, titanium (Ti): 0.005 to 0.100%, and one or more of chromium (Cr), molybdenum (Mo), vanadium (V), and nickel (Ni) with a total content of 1.5% or less, the remainder being iron and other unavoidable impurities. Steps to prepare the Slavs; A step of heating the above steel slab in a temperature range of 1150 to 1350℃; A step of obtaining a hot-rolled steel sheet by final hot-rolling the steel slab after heating in a temperature range of 850 to 1150℃; A step of coiling the hot-rolled steel sheet in a temperature range of Ms (martensite transformation initiation temperature) -30℃ to Ms-120℃ after the first cooling; and A step of second cooling the above-mentioned hot-rolled steel sheet is included, A method for manufacturing a hot-rolled steel sheet, wherein the hot rolling is performed by the steps of: rough rolling the heated steel slab to obtain a bar; descaling the bar at a pressure of 150 bar or more; and finishing hot rolling after the descaling.

9. In paragraph 8, A method for manufacturing a hot-rolled steel plate, wherein the above-mentioned steel slab satisfies the following relational expression 1. [Relationship 1] H = C + (0.5×Mn) + (0.4×Cr) + (0.2×Mo) (In equation 1, each element is a weight content, and 0 is substituted if not added.) 10. In paragraph 8, A method for manufacturing a hot rolled steel sheet, wherein the above rough rolling is performed in a temperature range of 900 to 1100°C.

11. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, wherein the above descaling step is performed 3 to 5 times.

12. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, wherein the first cooling is performed at a cooling rate of 50°C / s or higher up to a coiling temperature range.

13. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, wherein the second cooling is performed at a cooling rate of 0.1 to 25.0°C / hour to a temperature range of room temperature to 150°C.

14. In paragraph 8, A method for manufacturing a hot-rolled steel sheet, further comprising the steps of pickling and oiling the hot-rolled steel sheet after the second cooling.

15. In paragraph 14, A method for manufacturing a hot-rolled steel sheet further comprising the step of heating the hot-rolled steel sheet to a temperature range of 450 to 740°C after the above-mentioned acid washing and galvanizing, and then performing hot-dip zinc plating.

16. In paragraph 15, A method for manufacturing a hot-rolled steel sheet, wherein the above-mentioned hot-dip galvanizing is performed in a plating bath containing 0.01 to 30.00 wt% of magnesium (Mg), 0.01 to 50.00 wt% of aluminum (Al), and the remainder of zinc and unavoidable impurities.

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