Hot-rolled steel sheet and manufacturing method therfor

Abstract

The present invention relates to steel suitable for automobile chassis components and, more specifically, to a hot-rolled steel sheet having excellent shear formability and crack propagation resistance, and a manufacturing method therefor.

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 hot-rolled steel sheet having excellent shear formability and crack propagation resistance and a method for manufacturing the same.

[0002] Among conventional chassis components, the lower arm and upper arm, which have the largest forming volumes, have primarily been made of high-strength hot-rolled steel sheets with a tensile strength of 590 to 780 MPa, which are highly burr-resistant. These hot-rolled steel sheets can have a microstructure consisting of a single-phase ferrite structure, a two-phase ferrite-bainite structure, or a composite structure consisting of ferrite-bainite-martensite. Hot-rolled steel sheets with such microstructures exhibit excellent burr-resistant formability due to a small difference in hardness between phases.

[0003] As an example, patent documents 1 to 3 propose a method of simultaneously securing both elongation flangeability and strength by forming a composite structure of polygonal ferrite and bainite by applying a three-stage cooling process to a hot-rolled steel sheet, which involves cooling it, air-cooling it for a certain period of time, and then cooling it again and coiling it.

[0004] Hot-rolled steel sheets with the above composite structure mainly utilize alloying elements such as Si, Mn, Al, Cr, and Mo to improve the elongation flangeability and strength of the hot-rolled steel sheets. However, if the alloying elements are excessively added to obtain higher tensile strength, there is a problem of causing segregation of the alloying elements and non-uniformity of the microstructure. In addition, since the hardenability of the steel increases, different microstructures are formed depending on the cooling conditions, and there is a problem of inconsistency in the strength, ductility, and elongation flangeability and severe deviation.

[0005] Moreover, the edge part of the hot-rolled steel sheet has a lower temperature after hot rolling than the center part of the hot-rolled steel sheet, and the cooling rate is faster when the hot-rolled steel sheet is cooled immediately after hot rolling. Therefore, the phase fraction of martensite or bainite increases significantly in the edge part of the hot-rolled steel sheet, so that not only the ductility but also the bending workability and the extension flangeability are all inferior. In this way, when the formability of the edge part of the hot-rolled steel sheet is deteriorated, the edge part must be removed, which has the disadvantage of lowering productivity due to unnecessary processes and reducing the actual use of the hot-rolled steel sheet, which is economically disadvantageous.

[0006] Meanwhile, Patent Document 4 discloses a technology for ferrite single-phase structure steel that utilizes fine precipitate-forming elements such as Ti, Nb, Mo, and V while lowering the content of the aforementioned alloying components. Patent Document 4 discloses a method for securing strength by maximizing fine intragranular precipitation, and limits the formation of a second phase or prevents the formation of a hard phase such as a martensite phase in advance. However, Patent Document 4 requires that the heating furnace conditions be constant, and if the high temperature is not maintained uniformly over the entire length and width of the rolled sheet during the rolling and cooling processes, the precipitation behavior becomes non-uniform, preventing the intended strength and elongation flangeability from being secured. In addition, there is a problem that fatigue cracks easily occur in holes or edges when using parts obtained by forming such steel.

[0007] Meanwhile, the use of electric furnaces utilizing scrap iron has been expanding as a means of minimizing carbon emissions in the steelmaking process. However, steel made from scrap iron is likely to contain a large amount of impurities. These impurities tend to form oxides or segregate in the steel's surface layer and grain boundaries, potentially promoting crack initiation and propagation. This could negatively impact the steel's porosity, but technical review of this issue remains insufficient.

[0008] (Patent Document 1) Japanese Patent Publication No. 1994-293910

[0009] (Patent Document 2) Korean Registration No. 10-1114672

[0010] (Patent Document 3) Korean Registration No. 10-1528084

[0011] (Patent Document 4) Japanese Patent Publication No. 2003-321739

[0012] One aspect of the present invention provides a hot-rolled steel sheet having excellent shear formability and crack propagation resistance, and thus is suitable for use in automotive chassis components requiring high formability. Furthermore, a method for manufacturing the hot-rolled steel sheet is also provided.

[0013] 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.

[0014] According to one aspect of the present invention, a hot-rolled steel sheet contains, in wt%, carbon (C): 0.030 to 0.080%, silicon (Si): 0.01 to 1.00%, manganese (Mn): 1.0 to 2.5%, chromium (Cr): 0.005 to 0.500%, aluminum (Al): 0.01 to 0.80%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, and at least one of copper (Cu): 0.01 to 0.30%, nickel (Ni): 0.010 to 0.200%, antimony (Sb): 0.100% or less, tin (Sn): 0.100% or less, and arsenic (As): 0.100% or less. It may contain one or more of niobium (Nb): 0.050% or less, titanium (Ti): 0.120% or less, molybdenum (Mo): 0.300% or less, and vanadium (V): 0.200% or less, with the remainder being Fe and other unavoidable impurities.

[0015] In one embodiment of the present invention, the hot-rolled steel sheet can satisfy a T value defined in [Relationship 1] of 30.0 to 60.0, an X value defined in [Relationship 2] of 0.300 to 0.800, and an A value defined in [Relationship 3] of 10.000 or less.

[0016] [Relationship 1]

[0017] T = 33.02 × (1.07([C] / 10)^0.5) × (0.7[Si]+1) × (5[Mn]+1) × (2.16[Cr]+1) × (3[Mo]+1) × (0.36[Ni]+1) × (0.35[Cu]+1)

[0018] (In equation 1, each element is a weight content, and if not added, 0 is substituted.)

[0019] [Relationship 2]

[0020] X = ((Nb / 93) + (Ti* / 48) + (V / 51) + (Mo / 96)) / ((C / 12) + (N / 14))

[0021] (In equation 2, Ti* is calculated as (Ti-3.42N-1.5S), and each element is a weight content, and 0 is substituted if not added.)

[0022] [Relationship 3]

[0023] A = 85([Cu] + 3[Sb] + 2[As] + 3[Sn] - [Ni]) 1.2

[0024] (In equation 3, each element is a weight content, and if not added, 0 is substituted.)

[0025] According to one embodiment of the present invention, by controlling the alloy composition and alloy component relationship, not only can high strength of the hot-rolled steel sheet be secured, but also excellent hole expandability can be secured.

[0026] In one embodiment of the present invention, the microstructure of the hot-rolled steel sheet has an average dislocation density (Geometrical Necessary Dislocation) value of 2.5×10 14m -2 ~4.0×10 14 m -2 can satisfy the range of

[0027] According to one embodiment of the present invention, the hole expandability of a hot-rolled steel sheet can be improved by controlling the average dislocation density value of the microstructure, thereby ensuring excellent formability, particularly shear formability, during processing of the steel sheet.

[0028] In one embodiment of the present invention, the microstructure of the hot-rolled steel sheet may include ferrite and bainite phases at a combined area fraction of 95% or more, and the remaining structure may include at least one of a martensite phase and a pearlite phase.

[0029] In one embodiment of the present invention, the hot-rolled steel sheet can satisfy a segregation index (η) defined by relational expression (4) of 4.000 or less.

[0030] [Relationship 4]

[0031] η = ηgrain boundary / η grain

[0032] (η in relation 4 grain η grain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grains, and η grain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grain boundaries.

[0033] In one embodiment of the present invention, the hot-rolled steel sheet may have a yield strength of 650 MPa or more and a hole expandability (HER) of 60% or more.

[0034] According to another aspect of the present invention, a method for manufacturing a hot-rolled steel sheet may include the steps of preparing a steel slab; heating the steel slab at a temperature range of 1150 to 1350°C; hot-rolling the heated steel slab at a temperature range of 850 to 1150°C to obtain a hot-rolled steel sheet; and cooling the hot-rolled steel sheet and then coiling it at a temperature range of 300 to 500°C.

[0035] In one embodiment of the present invention, the cooling may include a step of first cooling to a temperature range of 550 to 700°C at a cooling rate of 10 to 100°C / s; a step of air cooling after the first cooling; and a step of second cooling to a temperature range of 300 to 500°C at a cooling rate of 10 to 100°C / s after the air cooling.

[0036] In one embodiment of the present invention, the coiled hot-rolled steel sheet can be pickled and oiled.

[0037] In one embodiment of the present invention, hot-dip galvanizing can be performed on a hot-rolled steel sheet that has been pickled and oiled, and at this time, the hot-rolled steel sheet can be heated to a temperature range of 450 to 740°C and then the plating process can be performed.

[0038] In one embodiment of the present invention, hot-dip galvanizing can be 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.

[0039] According to the present invention, a hot-rolled steel sheet having excellent shear formability and crack propagation resistance can be provided. In addition, the hot-rolled steel sheet according to the present invention can have high strength and excellent hole expandability.

[0040] The hot-rolled steel sheet of the present invention has the effect of being suitably applicable as a material for automobile chassis parts, especially lower arms, upper arms, and link parts that require the largest amount of forming.

[0041] FIG. 1 is a graph showing the relationship between the average dislocation density (GDN) of a hot-rolled steel sheet microstructure and the segregation index (η) defined by Equation 4 according to one embodiment of the present invention.

[0042] The inventors of the present invention have conducted in-depth research to solve the problems of hot-rolled steel sheets that have been applied as materials for conventional automobile chassis parts, etc., and to provide hot-rolled steel sheets having properties suitable as materials for lower arms and upper arms, which have the largest molding volume among automobile chassis parts.

[0043] Through repeated research, the inventors of the present invention have recognized that when applying a hot-rolled steel sheet having a yield strength of 650 MPa or more as the aforementioned material, cracks should not occur in the holes and edges of a shear- and punch-formed molded part, and that it is necessary to suppress the propagation of microcracks so that fractures do not occur during molding or fatigue failure occurs during use of the part.

[0044] In particular, the inventors of the present invention investigated the physical properties (strength, ductility, and hole expandability) according to the characteristics of the alloy composition and microstructure of high-strength hot-rolled steel sheets having various alloy compositions and different microstructures, and noted that the formation and propagation of cracks mainly occurred at grain boundaries and their surroundings during the entire process of forming into parts and during the use of the parts.

[0045] Accordingly, the present invention is technically significant in that it proposes a method for controlling the microstructure of hot-rolled steel sheets, thereby eliminating factors that weaken grain boundaries and their surrounding structures. Furthermore, the present invention was completed after examining the dependence of alloy composition and microstructure on securing the strength and formability of hot-rolled steel sheets.

[0046] Hereinafter, the present invention will be described in detail.

[0047] A hot-rolled steel sheet according to one aspect of the present invention contains, in wt%, carbon (C): 0.030 to 0.080%, silicon (Si): 0.01 to 1.00%, manganese (Mn): 1.0 to 2.5%, chromium (Cr): 0.005 to 0.500%, aluminum (Al): 0.01 to 0.80%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, and at least one of copper (Cu): 0.01 to 0.30%, nickel (Ni): 0.010 to 0.200%, antimony (Sb): 0.100% or less, tin (Sn): 0.100% or less, and arsenic (As): 0.100% or less. It may contain one or more of niobium (Nb): 0.050% or less, titanium (Ti): 0.120% or less, molybdenum (Mo): 0.300% or less, and vanadium (V): 0.200% or less.

[0048] 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.

[0049] Carbon (C): 0.030~0.080%

[0050] Carbon (C) is the most economical and effective element for strengthening steel, and it has a significant impact on the hardness and dislocation density of each constituent phase, that is, the microstructure. The higher the C content, the higher the hardenability of the steel, which increases the fraction of hard phases such as bainite and martensite in the microstructure, thereby increasing the tensile strength and dislocation density of the steel. In addition, by combining with Ti and Nb, which have a high affinity for C, it forms fine precipitates, inducing grain refinement and enhancing the precipitation strengthening effect, which increases both the yield strength and tensile strength of the steel.

[0051] In one embodiment of the present invention, if the C content is less than 0.030%, the strengthening effect of the steel cannot be sufficiently obtained, and when manufacturing the steel, the ferrite phase transformation occurs quickly during cooling after hot rolling, making it difficult to obtain a structure having a high dislocation density. On the other hand, if the C content exceeds 0.080%, there is a problem in that carbides and fine martensite phases and MA (martensite and austenite mixed structure) phases are formed locally at the grain boundaries, which weakens the grain boundaries. In addition, the fraction of hard phases such as bainite and martensite phases increases, and the hardness value and dislocation density of the phases also increase, so that the strength excessively increases, and there is a problem in that the ductility and formability are reduced.

[0052] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.030 to 0.080%. In another embodiment of the present invention, C may be included in an amount of 0.035% or more, or 0.040% or more. In yet another embodiment, C may be included in an amount of 0.075% or less, or 0.070% or less.

[0053] Silicon (Si): 0.01~1.00%

[0054] Silicon (Si) is an element that is advantageous in improving the formability of steel by deoxidizing molten steel, strengthening it through solid solution, and delaying the formation of coarse carbides at grain boundaries.

[0055] In one embodiment of the present invention, in order to sufficiently obtain the aforementioned effects, Si may be included in an amount of 0.01% or more. On the other hand, if the content exceeds 1.00%, red scale due to Si may be formed on the steel surface during hot rolling, which may significantly deteriorate the surface quality, and there is also a problem of deterioration in the ductility and weldability of the steel.

[0056] Therefore, in one embodiment of the present invention, Si may be included in an amount of 0.01 to 1.00%. In another embodiment of the present invention, Si may be included in an amount of 0.20% or more, or 0.30% or more. In yet another embodiment, Si may be included in an amount of 0.70% or less, or 0.60% or less.

[0057] Manganese (Mn): 1.0~2.5%

[0058] Manganese (Mn) is an effective element for solid-solution strengthening of steel. Furthermore, Mn increases the hardenability of steel, facilitating the formation of hard phases such as bainite and martensite during cooling after hot rolling. Furthermore, Mn inhibits the formation of fine precipitates within grains, thereby preventing grain boundaries and their surroundings from becoming brittle.

[0059] 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. On the other hand, if the content exceeds 2.5%, the hardenability of the steel increases significantly, so that the fraction of hard phases, including the bainite phase and the martensite phase, increases, and the hardness value and dislocation density of the phase also increase, so that the strength excessively increases, and there is a problem of reduced formability. In addition, when the slab is cast in the casting process, a segregation zone is greatly developed in the center of the thickness of the slab, so that the microstructure in the thickness direction is formed unevenly when cooling after hot rolling, resulting in inferior stretch flangeability. In particular, it becomes difficult to uniformly control the dislocation density when cooling in the full length and full width of the hot-rolled steel sheet.

[0060] 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 1.3% or more, or 1.5% or more. In yet another embodiment, Mn may be included in an amount of 2.3% or less, or 2.2% or less.

[0061] Chromium (Cr): 0.005~0.500%

[0062] Chromium (Cr) strengthens steel and delays the phase transformation of ferrite during the cooling process, thereby suppressing the formation of low-density equiaxed ferrite and inducing the formation of bainite.

[0063] In one embodiment of the present invention, if the content of Cr is less than 0.005%, the effect of addition cannot be sufficiently obtained. On the other hand, if the content of Cr exceeds 0.500%, the ferrite transformation is excessively delayed, and the martensite phase is excessively formed around the grain boundaries, resulting in poor formability. In addition, similar to the Mn, the segregation zone is greatly developed in the center of the thickness direction of the steel, and the microstructure becomes non-uniform in the thickness direction, which may result in poor elongation flangeability.

[0064] Therefore, in one embodiment of the present invention, Cr may be included in an amount of 0.005 to 0.500%. In another embodiment of the present invention, Cr may be included in an amount of 0.010% or more, or 0.015% or more. In yet another embodiment, Cr may be included in an amount of 0.450% or less, or 0.400% or less.

[0065] Aluminum (Al): 0.01~0.80%

[0066] Aluminum (Al) is an element added primarily for deoxidation and has the effect of promoting ferrite transformation. The above Al refers to available aluminum (Sol.Al).

[0067] In one embodiment of the present invention, if the content of Al is less than 0.01%, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the content exceeds 0.80%, 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.

[0068] Therefore, in one embodiment of the present invention, Al may be included in an amount of 0.01 to 0.08%. In another embodiment of the present invention, Al may be included in an amount of 0.02% or more, or 0.03% or more. In yet another embodiment, Al may be included in an amount of 0.70% or less, or 0.60% or less.

[0069] Phosphorus (P): 0.001~0.050%

[0070] Phosphorus (P), similar to Si, is an element that simultaneously has the effects of strengthening solid solution and promoting ferrite transformation.

[0071] 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 deteriorates ductility, elongation flangeability, impact resistance, etc.

[0072] 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 0.005% or more, or 0.008% or more. According to yet another embodiment, P may be included in an amount of 0.030% or less, or 0.015% or less.

[0073] Sulfur (S): 0.001~0.010%

[0074] 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 elongation flangeability and impact resistance.

[0075] In one embodiment of the present invention, there is no particular limitation on the lower limit of the content of S, but in order to manufacture the S content to be less than 0.001%, a lot of time is required during steelmaking, which causes a problem of reduced productivity.

[0076] Therefore, in one embodiment of the present invention, S may be included in an amount of 0.001 to 0.010%. In another embodiment of the present invention, S may be included in an amount of 0.002% or more, and in another embodiment, S may be included in an amount of 0.005% or less.

[0077] Nitrogen (N): 0.001~0.010%

[0078] Nitrogen (N), along with carbon (C), is a representative solid-solution strengthening element in steel. N can combine with elements such as titanium (Ti) and aluminum (Al) in steel to form coarse precipitates. While the solid-solution strengthening effect of N is generally similar to that of C, increasing the N content in steel significantly reduces its toughness. Therefore, its content can be limited to 0.010% or less.

[0079] In one embodiment of the present invention, in order to manufacture the N content to be less than 0.001%, there is a problem that productivity is reduced because a lot of time is required during steelmaking.

[0080] Therefore, in one embodiment of the present invention, N may be included in an amount of 0.001 to 0.010%. In another embodiment of the present invention, N may be included in an amount of 0.002% or more, and in another embodiment, N may be included in an amount of 0.008% or less.

[0081] 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.

[0082] As one example, it may further include one or more of copper (Cu): 0.01 to 0.30%, nickel (Ni): 0.010 to 0.200%, antimony (Sb): 0.100% or less, tin (Sn): 0.100% or less, and arsenic (As): 0.100% or less, and as another example, it may further include one or more of niobium (Nb): 0.050% or less, titanium (Ti): 0.120% or less, molybdenum (Mo): 0.300% or less, and vanadium (V): 0.200% or less.

[0083] Copper (Cu): 0.01~0.30%

[0084] Copper (Cu) has the effect of improving the hardenability of steel. To achieve this effect, when adding Cu, according to one embodiment of the present invention, the content may be 0.01% or more. However, if the Cu content is excessive, intergranular cracking may occur in the slab. Therefore, in one embodiment of the present invention, the Cu content may be 0.30% or less.

[0085] In another embodiment of the present invention, when adding Cu, it may be included in an amount of 0.03% or more, or 0.05% or more, and in another embodiment, it may be included in an amount of 0.20% or less.

[0086] Nickel (Ni): 0.010~0.200%

[0087] Nickel (Ni) is an element that is advantageous in improving the hardenability of steel, and when added together with Cu, it has the effect of suppressing slab grain boundary cracking caused by Cu.

[0088] In one embodiment of the present invention, in order to obtain the effect of adding Ni, it may be included at 0.010% or more. Since Ni is an expensive element, an excessive content may be economically disadvantageous, and therefore, taking this into consideration, it may be included at 0.200% or less.

[0089] Therefore, in one embodiment of the present invention, Ni may be included in an amount of 0.010 to 0.200% when added. According to another embodiment of the present invention, Ni may be included in an amount of 0.050% or more, and according to yet another embodiment, Ni may be included in an amount of 0.150% or less.

[0090] Antimony (Sb): 0.100% or less

[0091] According to one embodiment of the present invention, antimony (Sb) may inevitably be contained in steel due to the use of iron scrap. When contained at an appropriate level, Sb exists in a solid solution state in the steel, thereby promoting solid solution strengthening and thereby enhancing the strength of the steel.

[0092] In one embodiment of the present invention, when Sb is added, if the content is added so as to satisfy the relational expression 3 described below, the effect of the addition by Sb can be obtained, and there is no particular limitation on the lower limit content, and as an example, it may be more than 0%. However, when the content exceeds 0.100%, there is an effect of suppressing the formation of red-scale (Fayalite, Fe2SiO4) by Si, but since this effect acts locally, defects by red-scale rather become more uneven, and there is a concern that Sb may be locally concentrated on the steel surface, causing surface defects.

[0093] Therefore, in one embodiment of the present invention, the addition of Sb may be comprised in an amount of more than 0% and less than or equal to 0.100%. According to another embodiment of the present invention, the Sb may be comprised in an amount of 0.003% or more, or 0.005% or more, and in yet another embodiment, it may be comprised in an amount of 0.080% or less.

[0094] Tin (Sn): 0.100% or less

[0095] According to one embodiment of the present invention, tin (Sn) may inevitably be contained in steel due to the use of iron scrap. This Sn has a solid solution strengthening effect similar to the Sb.

[0096] In one embodiment of the present invention, when Sn is added, if the content is added so as to satisfy the relational expression 3 described below, the effect of the addition by Sn can be obtained, and there is no particular limitation on the lower limit content, and as an example, it may be more than 0%. However, if Sn is present in excess, there is a concern that surface defects of the steel may increase due to its characteristic of concentrating on the surface, and therefore the content may be limited to 0.100% or less.

[0097] Therefore, in one embodiment of the present invention, the Sn content may be from 0% to 0.100%. According to another embodiment of the present invention, the Sn content may be 0.001% or more, or 0.003% or more, and according to another embodiment, it may be 0.080% or less.

[0098] Arsenic (As): 0.100% or less

[0099] According to one embodiment of the present invention, arsenic (As) may inevitably be contained in steel due to the use of iron scrap. Such As has a solid solution strengthening effect similar to that of Sb and Sn.

[0100] In one embodiment of the present invention, when As is added, if the content is added so as to satisfy the relational expression 3 described below, the effect of the addition by As can be obtained, and the lower limit content is not particularly limited, and as an example, it may be more than 0%. Meanwhile, As is an element that is easily segregated on the steel surface and grain boundaries, and if the content is excessive, the ductility of the steel may be poor, and there is a concern that surface defects may occur. Considering this, the content of As can be limited to 0.100% or less.

[0101] Therefore, in one embodiment of the present invention, As may be included in an amount of more than 0% and less than or equal to 0.100% when added. According to another embodiment of the present invention, As may be 0.005% or more, or 0.010% or more, and according to another embodiment, it may be 0.080% or less.

[0102] According to one embodiment of the present invention, the hot-rolled steel sheet may further contain elements such as tungsten (W), tantalum (Ta), zirconium (Zr), yttrium (Y), lanthanum (La), and cerium (Ce), which are rare-earth metals (REMs), in addition to the Sb, Sn, and As. If these elements are present in excess in the hot-rolled steel sheet, inclusions may be formed, which may adversely affect the intended physical properties. Therefore, in one embodiment of the present invention, the elements may be limited to 0.020% or less based on the sum of the contents of each element.

[0103] Niobium (Nb): 0.050% or less

[0104] Niobium (Nb), along with titanium (Ti) and vanadium (V), which will be described later, is a representative precipitation-strengthening element. Nb precipitates during hot rolling and effectively improves the strength and impact toughness of steel by refining grains through delayed recrystallization.

[0105] In one embodiment of the present invention, when Nb is added, if the content is added so as to satisfy the relational expression 2 described below, the effect of the addition by Nb can be obtained, and there is no particular limitation on the lower limit content, and as an example, it may exceed 0%. However, if the content exceeds 0.050%, there is a problem that excessive delay in recrystallization occurs during rolling, resulting in the formation of elongated grains and the formation of coarse composite precipitates, which deteriorates the elongation flangeability of the steel.

[0106] Therefore, in one embodiment of the present invention, the addition of Nb may be comprised in an amount of more than 0% and less than or equal to 0.050%. According to another embodiment of the present invention, the Nb may be greater than or equal to 0.005%, or greater than or equal to 0.010%, and in yet another embodiment, less than or equal to 0.040%.

[0107] Titanium (Ti): 0.120% or less

[0108] 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.

[0109] In one embodiment of the present invention, when adding Ti, if the content is added so as to satisfy the relational expression 2 described below, the effect of addition by Ti can be obtained, and there is no particular limitation on the lower limit content, and as an example, it may exceed 0%. However, if the content exceeds 0.120%, there is a problem of deteriorating the elongation flangeability of the steel due to the formation of coarse precipitates.

[0110] Therefore, in one embodiment of the present invention, when adding Ti, it may be included in an amount of more than 0% and less than or equal to 0.120%. According to another embodiment of the present invention, the Ti may be 0.005% or more, or 0.010% or more, and according to another embodiment, it may be 0.115% or less.

[0111] Molybdenum (Mo): 0.300% or less

[0112] 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.

[0113] In one embodiment of the present invention, when Mo is added, if the content is added so as to satisfy the following relational expression 2, the effect of the addition by Mo can be obtained, and the lower limit content is not particularly limited, and as an example, it may be more than 0%. However, if the content exceeds 0.300%, the hardenability becomes excessive and the martensite phase is excessively formed, so the formability of the steel is rapidly deteriorated. In addition, as an expensive element, a higher content is economically disadvantageous and also has a negative effect on weldability.

[0114] Therefore, in one embodiment of the present invention, when Mo is added, it may be included in an amount of more than 0% and less than or equal to 0.300%. According to another embodiment of the present invention, the Mo may be 0.005% or more, or 0.010% or more, and in yet another embodiment, it may be 0.250% or less, or 0.200% or less.

[0115] Vanadium (V): 0.200% or less

[0116] 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 process after coiling, thereby enhancing the strength of the 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.

[0117] In one embodiment of the present invention, when V is added, if the content is added so as to satisfy the relational expression 2 described below, the effect of addition by V can be obtained, and there is no particular limitation on the lower limit content, and as an example, it may exceed 0%. However, if the content exceeds 0.200%, coarse precipitates are formed, which deteriorates the elongation flangeability of the steel, and since it is an expensive element, it is also economically disadvantageous.

[0118] Therefore, in one embodiment of the present invention, when V is added, it may be included in an amount of more than 0% and less than or equal to 0.200%. According to another embodiment of the present invention, the V may be 0.003% or more, or 0.005% or more, and according to another embodiment, it may be 0.150% or less.

[0119] 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.

[0120] 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.

[0121] In one embodiment of the present invention, the content relationship between C, Si, Mn, Cr, Mo, Ni, and Cu in the alloy composition may be defined by the following relational expression 1. According to one embodiment of the present invention, the T value defined by the following relational expression 1 may be 30.0 to 60.0.

[0122] [Relationship 1]

[0123] T = 33.02 × (1.07([C] / 10)^0.5) × (0.7[Si]+1) × (5[Mn]+1) × (2.16[Cr]+1) × (3[Mo]+1) × (0.36[Ni]+1) × (0.35[Cu]+1)

[0124] (In equation 1, each element is a weight content, and if not added, 0 is substituted.)

[0125] The inventors of the present invention have repeatedly studied and have factored the combination of specific alloy elements into equation 1 so that the microstructure of the hot-rolled steel sheet maintains an appropriate level of dislocation density while preventing local deviations.

[0126] According to one embodiment of the present invention, as the T value of relational expression 1 increases, the hardenability of the steel increases, and the local deviation of the microstructure becomes larger. Specifically, when the T value exceeds 60.0, a martensite phase is formed in the microstructure, and the hardness difference between the phases becomes large, making it difficult to secure burring resistance of the steel, and the elongation may also become insufficient. In addition, the microstructure becomes non-uniform in the width direction of the hot-rolled steel sheet, which increases material deviation. On the other hand, when the T value is less than 30.0, the hardenability of the steel is insufficient, and the dislocation density of the formed microstructure is not sufficiently high, so the resistance to crack propagation is low, and the strength may also be reduced.

[0127] Therefore, in one embodiment of the present invention, the T value defined by relational expression 1 may be 30.0 to 60.0. In another embodiment of the present invention, the T value may be 35.0 or more, and in another embodiment, it may be 55.0 or less.

[0128] In one embodiment of the present invention, the content relationship between C, N, S, Nb, Ti, V and Mo in the alloy composition can be defined by the following relational expression 2. According to one embodiment of the present invention, the X value defined by the following relational expression 2 can be 0.300 to 0.800.

[0129] [Relationship 2]

[0130] X = ((Nb / 93) + (Ti* / 48) + (V / 51) + (Mo / 96)) / ((C / 12) + (N / 14))

[0131] (In equation 2, Ti* is calculated as (Ti-3.42N-1.5S), and each element is a weight content, and 0 is substituted if not added.)

[0132] Relationship equation 2 according to one embodiment of the present invention factors out the relationship between elements that affect the formation of precipitates.

[0133] In one embodiment of the present invention, when the X value according to relational expression 2 is less than 0.300, grain growth occurs rapidly during slab heating, and recrystallization becomes non-uniform during hot rolling, resulting in the formation of locally coarse grains, which also makes the final microstructure non-uniform. In addition, the precipitation strengthening effect is insufficient, resulting in inferior strength and formability of the steel. On the other hand, when the X value exceeds 0.800, the formation of precipitates in the austenite region during hot rolling increases, which leads to excessive recrystallization delay during hot rolling, making it easy for a microstructure elongated in the rolling direction to be formed. This reduces the elongation in the vertical direction of rolling, and although fine precipitates within ferrite grains increase during the phase transformation process during cooling, there is a region where precipitates are not formed relatively at the boundary between grain boundaries and within grains, making the grain boundaries and their surroundings vulnerable. In addition, when hot-rolled steel sheets are cooled, the atoms of dissolved C and dissolved N in the untransformed phase become insufficient, making it difficult for a hard phase to be stably formed. As a result, the quality of the shear surface is deteriorated during shear processing of the steel, and the burring formability is also deteriorated.

[0134] Therefore, in one embodiment of the present invention, the X value defined by relational expression 2 may be 0.300 to 0.800. According to another embodiment of the present invention, the X value may be 0.350 or more, and according to another embodiment, it may be 0.750 or less.

[0135] In one embodiment of the present invention, the content relationship between Cu, Sb, As, Sn, and Ni in the alloy composition may be defined by the following relational expression 3. According to one embodiment of the present invention, the A value defined by the following relational expression 3 may be 10.000 or less.

[0136] [Relationship 3]

[0137] A = 85([Cu] + 3[Sb] + 2[As] + 3[Sn] - [Ni]) 1.2

[0138] (In equation 3, each element is a weight content, and if not added, 0 is substituted.)

[0139] Equation 3 according to one embodiment of the present invention factors in the relationship between elements that, while enhancing the hardenability of steel and providing a solid solution strengthening effect, have the property of segregating in the surface layer or grain boundaries of steel and have a low melting point, which may cause defects during the steel manufacturing process. That is, if the content of these elements is excessive, there is a problem in that the grain boundaries are weakened, making it easy for cracks to occur and propagate in the shear forming section of the steel during shear processing.

[0140] In one embodiment of the present invention, when the value of A defined by relational expression 3 exceeds 10.000, segregation of Cu, Sb, As, and Sn at grain boundaries becomes excessive. As a result, grain boundaries in the microstructure become weaker than the grain interior, making it easier for microcracks to occur during shear forming of the steel, and the crack propagation resistance also becomes inferior.

[0141] Therefore, in one embodiment of the present invention, the A value according to relational expression 3 may be 10.000 or less. In one embodiment of the present invention, each element of relational expression 3 may inevitably exist or may be optionally added, and therefore, there is no particular limitation on the lower limit of the A value.

[0142] According to one embodiment of the present invention, a hot-rolled steel sheet can be provided in which the relationship between grain boundaries of a microstructure and segregated elements existing within grains is controlled.

[0143] In one embodiment of the present invention, the relationship between segregation elements in a microstructure can be defined by the segregation index (η) of the following relational expression (4), which can be expressed as the concentration ratio of elements present at grain boundaries and elements present within grains.

[0144] [Relationship 4]

[0145] η = ηgrain boundary / η grain

[0146] (η in relation 4 grain η grain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grains, and η grain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grain boundaries.

[0147] As one embodiment of the present invention, the segregation index (η) value defined by relational expression 4 may be 4.000 or less. The value of η exceeding 4.000 means that the segregation of Cu, Sb, As, and Sn at the grain boundaries is excessive, which ultimately causes a problem of facilitating the occurrence and propagation of cracks in the shear forming section during steel processing.

[0148] In one embodiment of the present invention, the average concentration of segregating elements Cu, Sb, As, and Sn present within grains and at grain boundaries can be measured using a 3-dimensional atomic force microscope (3-Dimensional Atom Probe). As a non-limiting example, a specimen in which both grains and grain boundaries exist can be collected at a point 1 / 4t in the thickness direction of a hot-rolled steel sheet (where t represents the thickness (mm) of the hot-rolled steel sheet), and then the concentration of each component can be measured.

[0149] Hereinafter, the microstructural characteristics of a hot-rolled steel sheet according to one embodiment of the present invention will be described in detail.

[0150] The inventors of the present invention, in an attempt to improve the shear formability of a hot-rolled steel sheet according to an embodiment of the present invention, have recognized that, when approached from the perspective of the microstructural characteristics of the hot-rolled steel sheet, there is a limit to clearly distinguishing the excellence of shear formability, stability of punching clearance fluctuations, etc., based solely on the ratio of microstructural phases. Accordingly, through repeated research, the inventors of the present invention have discovered that not only the microstructural phase composition of a hot-rolled steel sheet, but also the geometrically necessary dislocation density of the microstructure is an important influencing factor in the occurrence of microcracks in the shear plane of the hot-rolled steel sheet.

[0151] Accordingly, the hot-rolled steel sheet according to one embodiment of the present invention has an average dislocation density (Geometrical Necessary Dislocation) of microstructure of 2.5×10 14 m -2 ~4.0×10 14 m -2 can satisfy the range of . The average dislocation density value of the above microstructure is 2.5×10 14 m -2 If the value is less than 4.0×10, the strength of the hot-rolled steel sheet is insufficient, making it difficult to secure the target material. On the other hand, if the value is less than 4.0×10 14 m -2 If it exceeds , the strength of the steel increases significantly, and a martensite phase is formed locally, which increases the difference in hardness between the phases, which increases the occurrence of microcracks during steel processing, such as punching and shear forming, and there is a problem that the crack propagation resistance decreases.

[0152] In one embodiment of the present invention, the dislocation density of the microstructure can be measured using Electron Back Scattered Diffraction (EBSD, (JEOL JSM-7001F)) based on a cross-section parallel to the rolling direction at a point 1 / 4t in the thickness direction of the hot-rolled steel sheet (where t represents the thickness (mm) of the hot-rolled steel sheet). As an example, the dislocation density (Geometrical Necessary Dislocation) can be measured using Kernel Average Misorientation (KAM) data for the results analyzed at a magnification of 1000 to 3000. As a non-limiting example, it can be calculated through the [Formula] below, and for convenience, such calculations can be performed using software such as OIM analysis™ (EDAX) that can analyze EBSD measurement results.

[0153] [ceremony]

[0154] Average potential density value (m -2 ) = 2θ / ub

[0155] (In the equation, θ represents the KAM value, u represents the unit length (step zise in the EBED measurement), and b represents the burgers vector.)

[0156] A hot-rolled steel sheet according to one embodiment of the present invention may include ferrite and bainite phases as the main phase in the microstructure, and the main phase may have an area fraction of 95% or more.

[0157] In one embodiment of the present invention, if the main body is formed with an area fraction of less than 95%, the shear formability and crack propagation resistance of the hot-rolled steel sheet cannot be secured.

[0158] In one embodiment of the present invention, the area fractions of each of the ferrite and bainite phases constituting the main phase are not particularly limited, and one embodiment of the present invention can be achieved by satisfying the limitations of the combined fraction of these phases.

[0159] In addition, the above-mentioned main phase may mean one or more phases of ferrite and bainite, and as an example, the above-mentioned main phase may be composed of only the bainite phase.

[0160] In one embodiment of the present invention, the remaining structure excluding the main phase may be at least one of a martensite phase and a pearlite phase, wherein the pearlite may include coarse carbides. The martensite phase is a harder phase than the bainite phase, and if formed locally, it may have a negative effect on burring formability. Therefore, the martensite phase may be limited to an area fraction of 5% or less.

[0161] Meanwhile, the hot-rolled steel sheet according to one embodiment of the present invention has a microstructure that contains little MA (Martensite and Austenite composite phase) phase and retained austenite phase. However, since the MA phase is a harder phase than the bainite phase, if the MA phase exists in the hot-rolled steel sheet, the MA phase can be regarded as a martensite phase.

[0162] A hot-rolled steel sheet according to one embodiment of the present invention can have high strength and excellent hole expandability.

[0163] As an example, hot-rolled steel sheets may have a yield strength of 650 MPa or more and a hole expansion ratio (HER) of 60% or more. Here, the hole expansion ratio refers to the average value of the results obtained by applying different punching clearances (cl) of 5%, 10%, and 20%, respectively.

[0164] 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.

[0165] According to one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing a [heating - hot rolling - cooling - coiling] process on a prepared steel slab, and each process step is described in detail below.

[0166] [Heating of steel slabs]

[0167] 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.

[0168] 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, the precipitates are not sufficiently re-dissolved, thereby reducing the formation of precipitates in the subsequent hot rolling process. In addition, the remaining coarse TiN causes insufficient maturation of the slab, making it difficult to control the temperature of the steel sheet at a constant level during the subsequent hot rolling. On the other hand, if the temperature exceeds 1350°C, the strength is reduced due to anomalous grain growth of austenite crystal grains.

[0169] [Hot rolling]

[0170] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0171] In one embodiment of the present invention, the hot rolling may be performed at a temperature range of 850 to 1150°C. If the hot rolling is started at a temperature exceeding 1150°C, the temperature of the hot-rolled steel sheet may increase, resulting in a coarser grain size and a deterioration in the surface quality of the hot-rolled steel sheet. On the other hand, if the hot rolling is terminated at a temperature lower than 850°C, excessive delay in recrystallization may occur, resulting in the development of elongated grains, which may result in aggravated anisotropy and poor formability. In addition, the shape quality of the rolled sheet may also deteriorate.

[0172] [cooling]

[0173] The hot-rolled steel sheet obtained by the above hot rolling can be cooled.

[0174] In one embodiment of the present invention, the cooling may be performed by step-by-step cooling. As an example, the step-by-step cooling may be a process of cooling the hot-rolled steel sheet at a constant cooling rate, then air-cooling it in a specific temperature range, and then cooling it again at a constant cooling rate.

[0175] In one embodiment of the present invention, the multi-stage cooling may include a step of first cooling the hot-rolled steel sheet to a temperature range of 550 to 700°C at a cooling rate of 10 to 100°C / s; a step of air cooling after the first cooling; and a step of second cooling after the air cooling to a temperature range of 300 to 500°C at a cooling rate of 10 to 100°C / s.

[0176] If the cooling rate during the first and second cooling is less than 10℃ / s, the fraction of the polygonal ferrite phase becomes excessive, and the average dislocation density value of the microstructure becomes 2.5×10 14 m -2 If the cooling rate exceeds 100°C / s, the average dislocation density of the microstructure becomes 4.0×10 14 m -2When the strength is exceeded, the burring formability becomes poor due to excessively high strength.

[0177] If the cooling end temperature during the first cooling is less than 550°C, there is a risk that a hard phase will be excessively formed in the subsequent process, resulting in excessive increase in strength. On the other hand, if the temperature exceeds 700°C, there is a risk that an equiaxed ferrite phase will be excessively formed.

[0178] After the above primary cooling is completed, an air cooling process may be performed, and according to one embodiment of the present invention, the air cooling may be performed for 2 to 8 seconds. Through the air cooling process, fine precipitates and ferrite may be formed within the hot-rolled steel sheet, thereby securing a balance between strength, ductility, and hole expandability. If the air cooling time exceeds 8 seconds, the final microstructure is mainly composed of a ferrite or bainitic ferrite phase, which makes it impossible to improve shear formability and crack propagation resistance, and there is a problem that the strength becomes insufficient.

[0179] After completing the above air cooling process, secondary cooling can be performed up to the temperature range of the coiling process described below. The end temperature of the secondary cooling is replaced by the description of the coiling process below.

[0180] [Winding]

[0181] The above cooled hot-rolled steel sheet can be coiled.

[0182] In one embodiment of the present invention, the coiling may be performed at a temperature range of 300 to 500°C. If the temperature during the coiling is below 300°C, there is a problem in that the martensite phase is excessively formed in the microstructure, resulting in an excessive increase in the strength of the steel. On the other hand, if the temperature exceeds 500°C, the bainite phase is not sufficiently formed in the microstructure, and carbides are formed at the grain boundaries, making it impossible to secure the intended physical properties and also resulting in poor shear formability.

[0183] 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 have both high strength and excellent hole expandability. Consequently, the hot-rolled steel sheet exhibits excellent shear formability during processing, and also exhibits excellent crack propagation resistance.

[0184] 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.

[0185] 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.

[0186] 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 (Zn) and unavoidable impurities.

[0187] 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.

[0188] (Example)

[0189] A steel slab having the alloy composition of Table 1 below and the component relationship of Table 2 was prepared, heat-treated at 1150 to 1350°C, and then final hot-rolled at the temperature shown in Table 3 below, and then each hot-rolled steel sheet was manufactured through a series of cooling processes and coiling processes.

[0190] For each hot-rolled steel sheet, the phase types and fractions of the microstructure and the average dislocation density values ​​were measured, and the mechanical properties were also measured and shown in Tables 4 and 5.

[0191] At this time, the microstructure was analyzed by collecting specimens at positions corresponding to 1 / 4t to 1 / 2t in the thickness direction for the cross-section in the rolling direction of each hot-rolled steel plate. In order to distinguish each phase and measure the area fraction, a backscatter electron diffraction (EBSD, JEOL JSM-7001F) device was used for each specimen, and analysis was performed at 3000 to 5000 times magnification.

[0192] The average dislocation density (Geometrical Necessary Dislocation, GND) for the observed microstructure was measured using OIM analysis™ (EDAX) after EBSD measurement based on the cross-section parallel to the rolling direction at the 1 / 4t position in the thickness direction of each hot-rolled steel sheet.

[0193] In addition, the average concentration of segregated elements Cu, Sb, As, and Sn present within grains and grain boundaries was measured using a 3-Dimensional Atom Probe (LA-WATAP, Spatial Resolution: 0.2 nm). In this case, a specimen was collected at the 1 / 4t point in the thickness direction of the hot-rolled steel sheet, and the concentration of each element was measured.

[0194] Meanwhile, the mechanical properties of each hot-rolled steel plate were evaluated through tensile tests and hole expandability tests.

[0195] At this time, the yield strength (off-set yield strength, YS), tensile strength (TS), and elongation at break (T-El) were measured as tensile tests. A JIS 5 standard test piece was taken in a direction perpendicular to the rolling direction, and the test piece was measured using a universal tensile tester (UTM-600kN, Zwick).

[0196] In addition, the hole expandability (HER) test was conducted by applying different punching clearances (cl) of 5%, 10%, and 20%, and was performed based on the JFST 1001-1996 standard. Each HER evaluation value was expressed as the average value after taking the average value after performing the test three times.

[0197] Steel alloy composition (weight %) CSiMnCrMoNbTiVCuNiSbAsSnAlPSN10.0550.301.20.010.050.0200.100.0030.010.0050.0060.0200.0050.030.0090.0040.00620.0600.302.00.030.100.0200.100.0030.080.1100.0050.0070.0500.040.0080.0020.00430.0550.301.90.010.030.0030.070.0030.100.1000.0100.02 00.0100.040.0080.0020.00840.0550.301.90.010.150.0200.130.0500.100.1000.0100.0200.0100.040.0080.0020.00850.0600.101.50.010.010 .0150.070.0020.060.0800.0100.0200.0100.030.0090.0020.00660.0450.302.00.030.200.0200.100.0030.090.0900.0100.0080.0090.040.0100 .0040.00670.0600.301.90.010.010.0200.120.0030.120.0900.0500.0500.0090.040.0100.0040.00680.0550.302.00.400.150.0200.120.0030.1 00.0800.0300.0100.0600.040.0100.0040.00590.0550.051.90.010.100.0200.120.0030.100.0800.0100.0100.0100.030.0100.0030.006100.055 0.301.90.010.010.0200.110.0500.030.0110.0120.0060.0080.030.0090.0020.006110.0500.302.00.010.050.0200.100.0030.100.1200.0100.0 050.0200.030.0100.0040.005120.0600.301.70.200.010.0200.100.0030.030.0900.0050.0100.0320.030.0110.0060.006130.0500.301.70.300.010.0200.100.0030.090.1000.0040.0200.0300.030.0120.0080.005140.0500.301.90.010.100.0200.110.0030.020.0500.0050.0220.0410.030.0130.0100.006150.0600.301.70.010.200.0200.100.0030.100.1000.0300.0040.0050.030.0140.0040.006.

[0198] Steel alloy composition relationship T (Relationship 1) Ti*X (Relationship 2) A (Relationship 3) 126.2 0.07 30.46 4 3.98 0 2 8 3.40 08 30.57 7 8.65 5 339.70.04 00.239 5.36 34 5 2.90.100 0.938 5.36 35 27.50.04 60.235 4.10 36 8 8.60.07 30.930 3.67 67 39.40.09 30.42 8 20.60 68 9 5.70.09 70. 85220.847938.40.0950.7175.3631035.50.0860.6194.7891142.20.0770.5304.1031248.40.0700.3403.5561352.10.0710.4107.4841443.20.0740.6248.8641555.10.0730.7166.210

[0199] Steel grade Finish rolling temperature (℃) Primary cooling air cooling time (sec) Secondary division speed (℃ / s) End temperature (℃) Speed ​​(℃ / s) End temperature (℃) 188566634641443 Comparative example 1289065615644461 Comparative example 2388263624643438 Comparative example 3487055630638440 Comparative example 4587570635645443 Comparative example 5687066627647445 Comparative example 6788072628648439 Comparative example 7887668615646452 Comparative example 8987069630643460 Invention example 11087372625641455 Invention example 21188860633644440 Invention example 31289058617647447 Invention example 41389556620650439 Invention example 51488745622654446 Invention example 61589255609657452 Invention example 72 The end temperature at the time of cooling is the same as the coiling temperature (℃).

[0200] Classification microstructure relationship 4FBF+BPMGND(×10 14 )ηGBηGηComparative example 1187997302.340.0430.0261.654Comparative example 229395054.490.2550.1242.057Comparative example 3158398202.410.1670.1281.305Comparative example 489098024.700.1440.1251.152Comparative example 5227395412.260.1600.0861.861Comparative example 658994065.130.1330.1051.267Comparative example 7188199103.011.3560.2106.457Comparative example Invention example 849195054.551.4200.1887.553 Invention example 139598113.120.1350.1161.164 Invention example 229698022.920.0920.0521.769 Invention example 339598023.070.1450.1151.261 Invention example 449498203.220.1050.0671.567 Invention example 559398113.500.2510.1341.873 Invention example 649599103.120.2200.0723.056 Invention example 709898023.370.1940.1351.437F: FerriteB: BainiteP: Pearlite (also includes coarse carbides)M: MartensiteThe fraction of each phase means the area fraction (%), and the unit of the average dislocation density (GND) value is m -2 am.

[0201] Classification Tensile Test Hole Expandability (HER) YS (MPa) TS (MPa) T-El (%) Comparative Example 16026852168.7 Comparative Example 27068161756.0 Comparative Example 35806702167.7 Comparative Example 47408621649.0 Comparative Example 55656482259.7 Comparative Example 68139051347.7 Comparative Example 77108221746.3 Comparative Example 88028921745.0 Invention Example 16767672073.3 Invention Example 27158201765.3 Invention Example 37218501565.3 Invention Example 46757782070.7 Invention Example 56887861965.7 Invention Example 67048151764.7 Invention Example 77188451664.0

[0202] As shown in Tables 1 to 5, Inventive Examples 1 to 7 are examples that satisfy all of the alloy composition, alloy component relationship, and manufacturing conditions according to one embodiment of the present invention, in which the main phase of the microstructure was formed with a combined fraction of ferrite and bainite of 95% or more, and the average dislocation density value of the microstructure was also secured within the intended range. As a result, it can be seen that the workability is excellent, as it exhibits a high strength of 650 MPa or more in addition to a hole expandability of 60% or more.

[0203] Meanwhile, Comparative Examples 1 to 8 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.

[0204] Among these, Comparative Examples 1 and 2 are cases where the alloy composition relation equation 1 is not satisfied. Comparative Example 1, which has a relatively low value of relation equation 1, could not secure high strength because the yield strength was less than 650 MPa. Comparative Example 2, which has an excessive value of relation equation 1, showed a result of poor hole expandability, which is because the fraction of the martensite phase formed around the grain boundary in the microstructure exceeds 5%.

[0205] Comparative Examples 3 and 4 are cases where equation 2 among the alloy component equations is not satisfied. Comparative Example 3, which has a relatively low value of equation 2, had insufficient precipitation strengthening and grain refinement effects due to a lack of precipitate-forming elements, and thus could not secure high strength with a yield strength of less than 650 MPa. On the other hand, Comparative Example 4, which has an excessive value of equation 2, had an excessively high average dislocation density of the microstructure due to the excessive addition of alloy components with high hardenability, resulting in poor pore expandability.

[0206] Comparative Examples 5 and 6 are cases where neither Relationships 1 and 2 among the alloy composition relationships were satisfied. Among these, Comparative Example 5 not only did the precipitation strengthening effect not fully develop, but also had low strength due to insufficient hardenability, and also had poor pore expandability due to the formation of coarse carbides at grain boundaries in the microstructure. In the case of Comparative Example 6, the average dislocation density of the microstructure was high due to excessive intragranular precipitation and excessive formation of martensite phase. As a result, the strength increased significantly, showing poor elongation, and poor pore expandability.

[0207] Comparative Examples 7 and 8 are cases where equation 3 among the alloy composition equations was not satisfied. In the case of Comparative Example 7, segregation occurred at the grain boundaries, resulting in poor pore expandability. On the other hand, Comparative Example 8 is a case where not only equation 3 but also equations 1 and 2 were not satisfied. The average dislocation density value of the microstructure was high, and the grain boundaries became weak, resulting in poor pore expandability.

Claims

1. In weight%, carbon (C): 0.030 to 0.080%, silicon (Si): 0.01 to 1.00%, manganese (Mn): 1.0 to 2.5%, chromium (Cr): 0.005 to 0.500%, aluminum (Al): 0.01 to 0.80%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, and at least one of copper (Cu): 0.01 to 0.30%, nickel (Ni): 0.010 to 0.200%, antimony (Sb): 0.100% or less, tin (Sn): 0.100% or less, and arsenic (As): 0.100% or less, niobium (Nb): Contains at least one of the following: 0.050% or less, titanium (Ti): 0.120% or less, molybdenum (Mo): 0.300% or less, and vanadium (V): 0.200% or less, the remainder being Fe and other unavoidable impurities. The T value defined in [Relationship 1] is 30.0 to 60.0, The X value defined in [Relationship 2] is 0.300 to 0.800, Hot rolled steel sheet that satisfies the A value defined in [Relationship 3] as 10,000 or less. [Relationship 1] T = 33.02 × (1.07([C] / 10)^0.5) × (0.7[Si]+1) × (5[Mn]+1) × (2.16[Cr]+1) × (3[Mo]+1) × (0.36[Ni]+1) × (0.35[Cu]+1) (In equation 1, each element is a weight content, and 0 is substituted if not added.) [Relationship 2] X = ((Nb / 93) + (Ti* / 48) + (V / 51) + (Mo / 96)) / ((C / 12) + (N / 14)) (In equation 2, Ti* is calculated as (Ti-3.42N-1.5S), and each element is a weight content, and 0 is substituted if not added.) [Relationship 3] A = 85([Cu] + 3[Sb] + 2[As] + 3[Sn] - [Ni]) 1.2 (In equation 3, each element is a weight content, and 0 is substituted if not added.) 2. In paragraph 1, The microstructure of the above hot-rolled steel sheet has an average dislocation density (Geometrical Necessary Dislocation) value of 2.5×10 14 m -2 ~4.0×10 14 m -2 Hot rolled steel plate satisfying the range.

3. In paragraph 1, A hot-rolled steel sheet having a microstructure comprising ferrite and bainite phases with an area fraction of 95% or more, and a residual structure comprising at least one of a martensite phase and a pearlite phase.

4. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet that satisfies the segregation index (η) defined by equation (4) of 4.000 or less. [Relationship 4] η = ηgrain boundary / η grain (η in relation 4 grain ηgrain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grains, and ηgrain boundary means the sum of the average concentrations (at%) of the segregation elements Cu, Sb, As, and Sn present in the grain boundaries.) 5. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a yield strength of 650 MPa or more.

6. In paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a hole expandability (HER) of 60% or more.

7. In weight%, carbon (C): 0.030 to 0.080%, silicon (Si): 0.01 to 1.00%, manganese (Mn): 1.0 to 2.5%, chromium (Cr): 0.005 to 0.500%, aluminum (Al): 0.01 to 0.80%, phosphorus (P): 0.001 to 0.050%, sulfur (S): 0.001 to 0.010%, nitrogen (N): 0.001 to 0.010%, and at least one of copper (Cu): 0.01 to 0.30%, nickel (Ni): 0.010 to 0.200%, antimony (Sb): 0.100% or less, tin (Sn): 0.100% or less, and arsenic (As): 0.100% or less, niobium (Nb): Contains at least one of the following: 0.050% or less, titanium (Ti): 0.120% or less, molybdenum (Mo): 0.300% or less, and vanadium (V): 0.200% or less, the remainder being Fe and other unavoidable impurities. A step for preparing a steel slab that satisfies the T value defined in [Relationship 1] of 30 to 60, the X value defined in [Relationship 2] of 0.3 to 0.8, and the A value defined in [Relationship 3] of 10 or less; A step of heating the above steel slab in a temperature range of 1150 to 1350℃; A step of hot rolling the above heated steel slab at a temperature range of 850 to 1150℃ to obtain a hot rolled steel sheet; and A method for manufacturing a hot-rolled steel sheet, comprising the step of cooling the hot-rolled steel sheet and then coiling it at a temperature range of 300 to 500°C. [Relationship 1] T = 33.02 × (1.07([C] / 10)^0.5) × (0.7[Si]+1) × (5[Mn]+1) × (2.16[Cr]+1) × (3[Mo]+1) × (0.36[Ni]+1) × (0.35[Cu]+1) (In equation 1, each element is a weight content, and 0 is substituted if not added.) [Relationship 2] X = ((Nb / 93) + (Ti* / 48) + (V / 51) + (Mo / 96)) / ((C / 12) + (N / 14)) (In equation 2, Ti* is calculated as (Ti-3.42N-1.5S), and each element is a weight content, and 0 is substituted if not added.) [Relationship 3] A = 85([Cu] + 3[Sb] + 2[As] + 3[Sn] - [Ni]) 1.2 (In equation 3, each element is a weight content, and 0 is substituted if not added.) 8. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, wherein the cooling is performed by: first cooling to a temperature range of 550 to 700°C at a cooling rate of 10 to 100°C / s; air cooling after the first cooling; and second cooling to a temperature range of 300 to 500°C at a cooling rate of 10 to 100°C / s after the air cooling.

9. In paragraph 7, A method for manufacturing a hot-rolled steel sheet, further comprising the steps of pickling and oiling the hot-rolled steel sheet after the coiling.

10. In paragraph 9, 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.

11. In paragraph 10, 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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