Hot-rolled steel sheet and method for manufacturing the same

Abstract

One embodiment of the present invention provides a hot-rolled steel sheet having a microstructure containing, by weight%, 0.06-0.12% C, 0.004-0.4% Si, 0.8-2.0% Mn, 0.01-0.05% Al, 0.05-1.0% Cr, 0.001-0.3%, 0.001-0.05%, S: 0.001-0.005%, N: 0.001-0.01%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, B: 0.001-0.005%, the balance being Fe and other unavoidable impurities, satisfying the following Relational Formula 1, and containing, by area%, 5-15% austenite, 80% or more autotempered martensite, and the balance being one or more of bainite and ferrite. [Relationship 1] (10[C] + [Si] + 2.5[Mn]) / (1.5[Cr] + 2.0[Mo] - 3.2[Nb]) ≦ 20

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.

Background Art

[0002] High-strength hot-rolled steel sheets are applied to various uses, such as boom arms of special vehicles such as cranes and concrete pump trucks, and frames of trucks and trailers. The thickness of the steel sheets used for such applications is generally at a level of about 3 to 10 mm. The high-strength hot-rolled steel sheet, which is thicker than general automotive steel sheets, requires not only a high yield strength to support the design load but also excellent shape quality for part processing and stability. In particular, when the high-strength hot-rolled steel sheet has excellent shape quality, there is an advantage that the quality remains sound even after processing and the stability of large structures can be enhanced.

[0003] Patent Document 1 is a technique for minimizing residual stress and ensuring quality shape by controlling alloy composition, annealing, and cooling conditions. Patent Document 2 is a technique for ensuring quality shape by controlling alloy composition, annealing, and cooling conditions and at the same time adding a heat treatment step.

[0004] However, the above Patent Documents 1 and 2 include an annealing process to which various cooling conditions can be applied as a method for manufacturing cold-rolled steel sheets. In the case of hot-rolled steel sheets, unlike cold-rolled steel sheets, they are manufactured without additional processes after hot rolling. Therefore, due to the rapid phase transformation during cooling and the high yield strength, even if shape correction is performed, the shape quality of the steel sheet deteriorates significantly. Furthermore, in the case of general high-strength hot-rolled steel sheets, although it is possible to manufacture them so as to actually obtain target physical properties, in the case of high-strength hot-rolled steel sheets with a yield strength of 900 MPa or more, it is practically difficult to improve the shape quality through the commonly used shape correction.

[0005] Thus, there is a demand for the development of a technology capable of increasing the shape correction property for high-strength hot-rolled steel sheets having a high yield strength.

Prior Art Documents

[0006] [Patent Document 1] Korean Registered Patent Publication No. 10-1228753 [Patent Document 2] Korean Registered Patent Publication No. 10-1568495 [Overview of the project] [Problems that the invention aims to solve]

[0007] One aspect of the present invention is to provide a high-strength hot-rolled steel sheet with excellent shape correction properties and a method for manufacturing the same. [Means for solving the problem]

[0008] One embodiment of the present invention provides a hot-rolled steel sheet having a microstructure that, by weight percent, contains C: 0.06~0.12%, Si: 0.004~0.4%, Mn: 0.8~2.0%, Al: 0.01~0.05%, Cr: 0.05~1.0%, Mo: 0.001~0.3%, P: 0.001~0.05%, S: 0.001~0.005%, N: 0.001~0.01%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, B: 0.001~0.005%, with the remainder being Fe and other unavoidable impurities, satisfies the following relational formula 1, and has a microstructure that, by area percent, contains austenite: 5~15%, auto-tempered martensite: 80% or more, with the remainder being one or more of bainite and ferrite. [Relationship 1] (10[C]+[Si]+2.5[Mn]) / (1.5[Cr]+2.0[Mo]-3.2[Nb])≦20

[0009] Another embodiment of the present invention provides a hot-rolled steel sheet having a microstructure in which, by weight%, C: 0.06~0.12%, Si: 0.004~0.4%, Mn: 0.8~2.0%, Al: 0.01~0.05%, Cr: 0.05~1.0%, Mo: 0.001~0.3%, P: 0.001~0.05%, S: 0.001~0.005%, N: 0.001~0.01%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, B: 0.001~0.005%, the remainder being Fe and other unavoidable impurities, satisfying the following relational formula 1, and having a microstructure in which, by area%, auto-tempered martensite: 80% or more, the remainder being one or more of fresh martensite, bainite, and ferrite, and having a wave height in the longitudinal direction of the steel sheet of 10 mm or less. [Relationship 1] (10[C]+[Si]+2.5[Mn]) / (1.5[Cr]+2.0[Mo]-3.2[Nb])≦20

[0010] Another embodiment of the present invention involves a composition of C: 0.06~0.12% by weight, Si: 0.004~0.4%, Mn: 0.8~2.0%, Al: 0.01~0.05%, Cr: 0.05~1.0%, Mo: 0.001~0.3%, P: 0.001~0.05%, S: 0.001~0.005%, N: 0.001~0.01%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, B: 0. The present invention provides a method for manufacturing a hot-rolled steel sheet, comprising the steps of: reheating a slab containing 0.001 to 0.005%, with the remainder being Fe and other unavoidable impurities, and satisfying the following relational formula 1, at 1200 to 1350°C; hot-rolling the reheated slab at 800 to 1200°C to satisfy the following relational formula 2 to obtain a hot-rolled steel sheet; and primary cooling, secondary cooling, and winding of the hot-rolled steel sheet to satisfy the following relational formulas 3 to 6. [Relationship 1] (10[C]+[Si]+2.5[Mn]) / (1.5[Cr]+2.0[Mo]-3.2[Nb])≦20 [Relationship 2] FDT ≥ 896 - 251[C] + 37.5[Si] - 31.6[Mn] - 7.16[Cr] + 29.5[Mo] + 129[Ti] - 107[Nb] [Relationship 3] MTL ≤ MT ≤ MTU [Relationship 4] CRL ≤ ICR [Relationship 5] TCR ≤ 80°C / sec [Relationship 6] MTL - 100 ≤ CT (However, in the above relational equations 2 to 6, FDT means the surface temperature of the hot-rolled steel sheet at the end of hot rolling, MT means the surface temperature of the hot-rolled steel sheet at the end of primary cooling and the start of secondary cooling, MTL means 430-380[C]-13.4[Si]-47.3[Mn]-16.0[Cr]-24.2[Mo], MTU means 481-358[C]-16.6[Si]-45.6[Mn]-15.2[Cr]-24.1[Mo], ICR means the primary cooling rate of the hot-rolled steel sheet surface from FDT to MT, CRL means 10[2.9-(0.1[C]+0.9[Mn]+0.5[Cr]+1.2[Mo])]+10, TCR means the average cooling rate of the hot-rolled steel sheet surface from FDT to CT, and CT means the coiling temperature.)

[0011] Another embodiment of the present invention is a composition in weight percent of C: 0.06~0.12%, Si: 0.004~0.4%, Mn: 0.8~2.0%, Al: 0.01~0.05%, Cr: 0.05~1.0%, Mo: 0.001~0.3%, P: 0.001~0.05%, S: 0.001~0.005%, N: 0.001~0.01%, Nb: 0.001~0.05%, Ti: 0.001~0.05%, B: 0.001~0.005%, remaining The present invention provides a method for manufacturing a hot-rolled steel sheet, comprising the steps of: reheating a slab containing partial Fe and other unavoidable impurities and satisfying the following relational formula 1 at 1200 to 1350°C; hot-rolling the reheated slab at 800 to 1200°C to satisfy the following relational formula 2 to obtain a hot-rolled steel sheet; primary cooling, secondary cooling and winding of the hot-rolled steel sheet to satisfy the following relational formulas 3 to 6; and leveling the wound hot-rolled steel sheet. [Relationship 1] (10[C]+[Si]+2.5[Mn]) / (1.5[Cr]+2.0[Mo]-3.2[Nb])≦20 [Relationship 2] FDT ≥ 896 - 251[C] + 37.5[Si] - 31.6[Mn] - 7.16[Cr] + 29.5[Mo] + 129[Ti] - 107[Nb] [Relationship 3] MTL ≤ MT ≤ MTU [Relationship 4] CRL ≤ ICR [Relationship 5] TCR ≤ 80°C / sec [Relationship 6] MTL - 100 ≤ CT (However, in the above relational equations 2 to 6, FDT means the surface temperature of the hot-rolled steel sheet at the end of hot rolling, MT means the surface temperature of the hot-rolled steel sheet at the end of primary cooling and the start of secondary cooling, MTL means 430-380[C]-13.4[Si]-47.3[Mn]-16.0[Cr]-24.2[Mo], MTU means 481-358[C]-16.6[Si]-45.6[Mn]-15.2[Cr]-24.1[Mo], ICR means the primary cooling rate of the hot-rolled steel sheet surface from FDT to MT, CRL means 10[2.9-(0.1[C]+0.9[Mn]+0.5[Cr]+1.2[Mo])]+10, TCR means the average cooling rate of the hot-rolled steel sheet surface from FDT to CT, and CT means the coiling temperature.) [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a high-strength hot-rolled steel sheet with excellent shape correction properties and a method for manufacturing the same. [Brief explanation of the drawing]

[0013] [Figure 1] This graph shows the relationship between yield strength and wave height after leveling for Invention Examples 1-6 and Comparative Examples 1-10. [Figure 2] These are photographs of the microstructure of Invention Example 16, observed before (left) and after (right) leveling, using EBSD and an electron microscope, respectively. [Modes for carrying out the invention]

[0014] In order to produce a hot-rolled steel sheet having high strength and excellent shape rectification property, the above physical properties must be ensured simultaneously during the hot-rolling process without further heat treatment steps. Generally, in the case of steel materials having high strength, a low-temperature transformation phase such as martensite or bainite must be included as essential for ensuring strength. In order to ensure such a low-temperature transformation phase, cooling must be performed at a high cooling rate and a low cooling stop temperature during the cooling process in the hot-rolling process. However, in this case, since the wave height of the hot-rolled steel sheet is extremely large and the yield strength also increases, shape rectification becomes difficult.

[0015] The inventors of the present invention have completed the present invention based on the finding that a hot-rolled steel sheet having high yield strength and excellent shape rectification property can be produced by precisely controlling the alloy composition and manufacturing conditions.

[0016] Hereinafter, the present invention will be described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below means weight%.

[0017] C: 0.06 to 0.12% C is the most economical and effective element for strengthening steel. As the addition amount increases, the martensite and bainite fractions increase, and the tensile strength and yield strength increase. In particular, the strength of tempered martensite or martensite is absolutely affected by the content of the above C. When the content of the above C is less than 0.06%, it is difficult to obtain a sufficient strengthening effect compared to the yield strength to be obtained in the present invention. When it exceeds 0.12%, the martensite becomes too hard, resulting in an increase in brittleness and a decrease in shape rectification property. In addition, there are disadvantages such as deterioration of weldability and material uniformity. Therefore, the content of the above C preferably has a range of 0.06 to 0.12%. The lower limit of the above C content is more preferably 0.065%, and even more preferably 0.07%. The upper limit of the above C content is more preferably 0.115%, and even more preferably 0.110%.

[0018] Si: 0.004 to 0.4% Si is an element that is advantageous for deoxidizing molten steel, exerting a solid solution strengthening effect in the matrix, delaying the formation of coarse carbides, allowing carbon to be concentrated, and retaining austenite after cooling when certain cooling conditions are met. If the Si content is less than 0.004%, the effect of delaying carbide formation is insufficient, making it difficult to retain austenite, and the process cost of controlling the Si content becomes excessive. If the Si content exceeds 0.4%, a red scale due to Si forms on the surface of the steel sheet during hot rolling, resulting in extremely poor surface quality of the steel sheet, as well as a decrease in bendability and material uniformity, ultimately leading to a deterioration in shape correction ability. Therefore, it is preferable that the Si content be in the range of 0.004 to 0.4%. The lower limit of the Si content is more preferably 0.01%, even more preferably 0.03%, and most preferably 0.05%. The upper limit of the Si content is more preferably 0.25%, even more preferably 0.18%, and most preferably 0.15%.

[0019] Mn: 0.8~2.0% Mn, like Si, is an effective element for solid solution strengthening of steel, increasing its hardening ability and facilitating the formation of low-temperature transformation structures such as martensite and bainite during cooling. However, if the Mn content is less than 0.8%, the above effect is excessively low, and the insufficient hardening ability must be compensated for with other elements, resulting in a significant increase in the cost of the alloy. On the other hand, if the Mn content exceeds 2.0%, segregation develops significantly in the center of the thickness during slab casting in the continuous casting process, and during cooling, the microstructure is formed unevenly in the thickness direction, resulting in poor material uniformity and deterioration of shape correction properties. It also weakens grain boundaries and excessively increases the brittleness of the steel. Therefore, the Mn content is preferably in the range of 0.8 to 2.0%. The lower limit of the Mn content is more preferably 0.9%. The upper limit of the Mn content is more preferably 1.8%, and even more preferably 1.7%.

[0020] Al: 0.01~0.05% Al is an ingredient added primarily for deoxidation, and if its content is less than 0.01%, the above effect becomes insufficient. On the other hand, if the Al content exceeds 0.05%, it combines with nitrogen to form AlN, which makes corner cracks more likely to occur in the slab during continuous casting and prone to defects due to the formation of inclusions. In addition, it may also adversely affect the shape correction properties. Therefore, it is preferable that the Al content be in the range of 0.01 to 0.05%. The lower limit of the Al content is more preferably 0.015%, and even more preferably 0.02%. The upper limit of the Al content is more preferably 0.045%, and even more preferably 0.04%.

[0021] Cr: 0.05~1.0% Cr strengthens steel through solid solution, increases its hardening ability during cooling, suppresses ferrite formation, and simultaneously aids in the formation of low-temperature transformation structures such as martensite and bainite. If the Cr content is less than 0.05%, the above effects cannot be obtained or become excessively small. On the other hand, if the Cr content exceeds 1.0%, segregation begins to develop significantly in the center of the thickness, similar to Mn, resulting in uneven microstructure in the thickness direction and deterioration of material uniformity and shape correctability. Furthermore, it promotes bainite formation more than the tempered martensite targeted by this invention, making it difficult to ensure strength. Therefore, the Cr content is preferably in the range of 0.05 to 1.0%. The lower limit of the Cr content is more preferably 0.07%, and even more preferably 0.1%. The upper limit of the Cr content is more preferably 0.9%.

[0022] Mo: 0.001~0.3% Mo increases the hardening ability of steel and facilitates the formation of low-temperature transformation structures such as martensite and bainite, and it is known that this effect is as strong as that of Mn. However, unlike Mn, it plays a role in increasing strength while suppressing brittleness by reinforcing grain boundaries. If the Mo content is less than 0.001%, the above effect cannot be fully obtained, and if it exceeds 0.3%, precipitates grow coarsely by bonding with C formed during coiling after hot rolling, which may result in areas where material uniformity and shape correctability are partially deteriorated. Furthermore, it is an expensive element, which is disadvantageous from the standpoint of manufacturing costs, and it is also detrimental to weldability. Therefore, it is preferable that the Mo content be in the range of 0.001 to 0.3%. The lower limit of the Mo content is more preferably 0.03%, even more preferably 0.05%, and most preferably 0.07%.

[0023] P: 0.001~0.05% P is an element that provides a high solid solution strengthening effect, but causes brittleness due to grain boundary segregation, which hinders material uniformity and shape correction. If the P content exceeds 0.05%, as mentioned above, sudden fracture may occur during shape correction due to brittleness caused by grain boundary segregation, potentially degrading shape correction. It is advantageous to control the P content to be as low as possible, but manufacturing it to less than 0.001% incurs high manufacturing costs and is economically disadvantageous. Therefore, it is preferable that the P content be in the range of 0.001 to 0.05%. The lower limit of the P content is more preferably 0.002%, even more preferably 0.003%, and most preferably 0.005%. The upper limit of the P content is more preferably 0.03%, even more preferably 0.02%, and most preferably 0.015%.

[0024] S: 0.001~0.005% S is an impurity present in steel. When its content exceeds 0.005%, it combines with Mn and other elements to form nonmetallic inclusions, which makes the steel prone to developing fine cracks when bent, significantly reducing its impact resistance and impairing material uniformity and shape correctability. On the other hand, it is advantageous to control the S content to be as low as possible, but producing it at less than 0.001% requires a lot of time and energy during steelmaking operations, reducing productivity. Therefore, it is preferable that the S content be in the range of 0.001 to 0.005%. It is more preferable that the lower limit of the S content be 0.002%. It is more preferable that the upper limit of the S content be 0.004%.

[0025] N: 0.001~0.01% The above-mentioned N, along with C, is a typical solid solution strengthening element, and may form coarse precipitates together with Ti, Al, etc. Generally, the solid solution strengthening effect of N is known to be superior to that of carbon. However, if the N content exceeds 0.01%, there is a problem in that the toughness decreases significantly. On the other hand, producing N with a content of less than 0.001% requires a lot of time during steelmaking operations, reducing productivity. Therefore, it is preferable that the N content be in the range of 0.001 to 0.01%. The lower limit of the N content is more preferably 0.002%, even more preferably 0.003%, and most preferably 0.004%. The upper limit of the N content is more preferably 0.009%, and even more preferably 0.008%.

[0026] Nb: 0.001~0.05% Nb, along with Ti and V, is a representative precipitation strengthening element. It precipitates during hot rolling and is effective in improving strength and impact toughness through grain refinement due to delayed recrystallization. It is also advantageous for retaining austenite under specific cooling conditions. This improvement in physical properties can improve shape correction. If the Nb content is less than 0.001%, the above effects cannot be obtained. On the other hand, if the Nb content exceeds 0.05%, it grows into coarse composite precipitates, which degrades the uniformity of the material. Therefore, it is preferable that the Nb content be in the range of 0.001 to 0.05%. The upper limit of the Nb content is more preferably 0.03%, even more preferably 0.02%, and most preferably 0.01%.

[0027] Ti: 0.001~0.05% Ti, along with Nb and V, is a representative precipitation strengthening element, and its strong affinity with N forms TiN in steel. TiN has the effect of suppressing grain growth during the heating process for hot rolling, and is advantageous for utilizing B, which is added to improve hardening ability through the stabilization of solid-solution N. In addition, the remaining Ti that reacts with nitrogen dissolves in the steel and combines with carbon to form TiC precipitates, which is a useful component for additionally improving the strength of the steel. If the above Ti content is less than 0.001%, the above effect cannot be obtained, on the other hand, if it exceeds 0.05%, there is a problem that the uniformity of the material deteriorates due to the generation of coarse TiN and the coarsening of precipitates during heat treatment. Therefore, it is preferable that the above Ti content be in the range of 0.001 to 0.05%. The lower limit of the above Ti content is more preferably 0.005%, even more preferably 0.01%, and most preferably 0.02%. The upper limit of the above Ti content is more preferably 0.04%, and even more preferably 0.03%.

[0028] B: 0.001~0.005% When present in a solid solution state in steel, B has the effect of improving hardening ability, stabilizing grain boundaries and improving the brittleness of steel in the low-temperature range, and strengthening grain boundaries even in trace amounts. If the content of B is less than 0.001%, it is difficult to obtain the above effects, on the other hand, if it exceeds 0.005%, it delays recrystallization behavior during hot rolling, greatly increases hardening ability, deteriorates formability, and may form precipitates such as coarse BN, thus actually increasing the brittleness of the steel. Therefore, it is preferable that the content of B be in the range of 0.001 to 0.005%. The upper limit of the B content is more preferably 0.004%, and even more preferably 0.003%.

[0029] On the other hand, it is preferable that the hot-rolled steel sheet of the present invention satisfies the alloy composition described above, as well as the following relational equation 1 (hereinafter, the left side of relational equation 1 is also referred to as "T"). In this case, the content of each alloying element in relational equation 1 is in weight percent.

[0030] [Relationship 1] (10[C]+[Si]+2.5[Mn]) / (1.5[Cr]+2.0[Mo]-3.2[Nb])≦20

[0031] The above relational expression 1 is a component relational expression for controlling the microstructure. When the value of T exceeds 20, while a sufficient low-temperature structure is obtained, the non-uniform distribution of Mn segregation zones and retained austenite increases, making it impossible to obtain uniform physical properties, and therefore, a sufficient shape correction effect cannot be obtained. For this reason, it is preferable that the value of T is 20 or less. It is more preferable that the value of T is 19 or less, even more preferable that it is 17 or less, and most preferable that it is 16 or less. On the other hand, since a smaller value of T is advantageous for ensuring a uniform microstructure and physical properties, the present invention does not particularly limit the lower limit of the value of T.

[0032] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from the raw materials or the surrounding environment, and therefore cannot be eliminated. Since these impurities are easily recognizable to any technician in the normal manufacturing process, their details are not specifically mentioned in this specification.

[0033] In this case, the above-mentioned unavoidable impurities may include Ni: 0.01% or less. Since Ni is an expensive element, the present invention has the advantage of being economical because excellent shape correction properties can be ensured without adding Ni. The Ni content is more preferably 0.008% or less, even more preferably 0.006% or less, and most preferably 0.005% or less.

[0034] The hot-rolled steel sheet of the present invention preferably has a microstructure before shape correction by leveling that, by area percent, contains 5-15% austenite, 80% or more auto-tempered martensite, and the remainder being one or more of bainite and ferrite. The austenite lowers the yield strength, allowing for effective shape correction, and at the same time, after leveling, it transforms into martensite, improving strength. If the austenite fraction is less than 5%, it is difficult to obtain the above effect sufficiently, and if it exceeds 15%, a sufficient low-temperature structure cannot be secured, resulting in the disadvantage of a decrease in the strength of the final steel sheet. The lower limit of the austenite fraction is more preferably 6%, and even more preferably 7%. The upper limit of the austenite fraction is more preferably 13%, even more preferably 11%, and most preferably 10%. The auto-tempered martensite has the effect of having ductility that is advantageous for localized and limited deformation such as shape correction, while also having high strength. If the proportion of auto-tempered martensite is less than 80%, the resulting steel sheet will have the disadvantage of being excessively weak. It is more preferable that the proportion of auto-tempered martensite be 82% or more. The more auto-tempered martensite that is formed, the more advantageous it is for ensuring strength, but it is unavoidable in the manufacturing process that one or more of bainite and ferrite may be formed. On the other hand, auto-tempered martensite has almost the same structure as tempered martensite formed by a low-temperature, short-duration tempering treatment, without the need for a separate tempering treatment, and is characterized by the formation of fine epsilon carbide within the lath.

[0035] The hot-rolled steel sheet of the present invention preferably has a microstructure after shape correction by leveling that, by area percentage, contains 80% or more auto-tempered martensite and the remainder being one or more of fresh martensite, bainite, and ferrite. By the transformation of austenite before leveling into martensite after leveling, not only shape correction properties but also superior strength can be ensured.

[0036] Furthermore, the hot-rolled steel sheet of the present invention, after leveling, has excellent shape correction properties, with a wave height of 10 mm or less in the longitudinal direction of the steel sheet. In this case, the wave height refers to the height from the trough to the peak when the steel sheet has a wave shape in the longitudinal direction.

[0037] As described above, the hot-rolled steel sheets provided before and after leveling preferably have a prior austenite average grain size of 10 to 30 μm. If the prior austenite average grain size is less than 10 μm, there is a disadvantage that hardenability decreases and a sufficient low-temperature structure cannot be secured. If it exceeds 30 μm, there is a disadvantage that retained austenite cannot be formed in the steel sheet due to the excessively large hardenability, resulting in a significant decrease in ductility. The lower limit of the prior austenite average grain size is more preferably 12 μm, even more preferably 15 μm, and most preferably 17 μm. The upper limit of the prior austenite average grain size is more preferably 28 μm, and even more preferably 26 μm.

[0038] Furthermore, the hot-rolled steel sheet after leveling can have excellent strength, with a yield strength of 900 MPa or more.

[0039] The following describes a method for manufacturing hot-rolled steel sheets according to one embodiment of the present invention. On the other hand, the method for manufacturing hot-rolled steel sheets according to one embodiment of the present invention can also use a process in which the continuous casting and hot-rolling processes are directly linked.

[0040] First, the slab satisfying the alloy composition and relational equation 1 described above is reheated to 1200-1350°C. If the reheating temperature is below 1200°C, the precipitates are not sufficiently redissolved, reducing precipitate formation in subsequent processes after hot rolling, resulting in the retention of coarse TiN, and making it difficult to resolve the segregation generated during continuous casting by diffusion. On the other hand, if the temperature exceeds 1350°C, abnormal grain growth of austenite crystal grains occurs, leading to a decrease in strength and unevenness in the microstructure. Therefore, it is preferable that the reheating temperature be in the range of 1200-1350°C. The lower limit of the reheating temperature is more preferably 1220°C, even more preferably 1230°C, and most preferably 1250°C. The upper limit of the reheating temperature is more preferably 1330°C, even more preferably 1310°C, and most preferably 1300°C.

[0041] Subsequently, the reheated slab is hot-rolled at 800-1200°C to satisfy the following relational equation 2 to obtain a hot-rolled steel sheet. If the hot-rolling temperature exceeds 1200°C, the temperature of the hot-rolled steel sheet becomes high, the size of the crystal grains becomes coarse, and the surface quality of the hot-rolled steel sheet deteriorates. On the other hand, if it is below 800°C, when the process is completed, the stretched crystal grains develop due to excessive delay in recrystallization, resulting in severe anisotropy, poor formability, and ultimately poor material uniformity and shape correctability. The lower limit of the hot-rolling temperature is more preferably 810°C, even more preferably 820°C, and most preferably 830°C. The upper limit of the hot-rolling temperature is more preferably 1180°C.

[0042] On the other hand, in the present invention, it is preferable that the following relational equation 2 (hereinafter, the right-hand side of relational equation 2 below is also referred to as "FDTL") is satisfied during hot rolling. In relational equation 2 below, FDT means the surface temperature of the hot-rolled steel sheet at the end of hot rolling.

[0043] [Relationship 2] FDT ≥ 896 - 251[C] + 37.5[Si] - 31.6[Mn] - 7.16[Cr] + 29.5[Mo] + 129[Ti] - 107[Nb]

[0044] When the above FDT is lower than FDTL, as ferrite partially forms on the surface of the steel plate, which is at a lower temperature than the center of the steel plate, it becomes impossible to form a sufficient fraction of martensite after cooling. This results in material variation between the center and the surface, and ultimately, a deterioration in shape correctability. In other words, if relational equation 2 is satisfied, high strength and excellent shape correctability can be obtained simultaneously.

[0045] Subsequently, the hot-rolled steel sheet is subjected to primary cooling, secondary cooling, and winding so as to satisfy the following relational equations 3 to 6. In relational equations 3 to 6, MT represents the surface temperature of the hot-rolled steel sheet at the end of primary cooling and the start of secondary cooling, MTL represents 430-380[C]-13.4[Si]-47.3[Mn]-16.0[Cr]-24.2[Mo], MTU represents 481-358[C]-16.6[Si]-45.6[Mn]-15.2[Cr]-24.1[Mo], ICR represents the primary cooling rate of the hot-rolled steel sheet surface from FDT to MT, CRL represents 10[2.9-(0.1[C]+0.9[Mn]+0.5[Cr]+1.2[Mo])]+10, TCR represents the average cooling rate of the hot-rolled steel sheet surface from FDT to CT, and CT represents the winding temperature.

[0046] If the MT exceeds the MTU, martensite cannot be formed, and if it is less than the MTL, fine and uniformly distributed austenite cannot be formed. If the ICR is lower than the CRL, sufficient martensite cannot be formed, and a large amount of ferrite and bainite are formed unintentionally, resulting in a failure to obtain high strength. This also suppresses austenite formation and deteriorates shape correction properties. On the other hand, in this invention, even if the ICR value is extremely high, there is no significant change in the strength of the martensite, so there is no particular upper limit. If the TCR exceeds 80°C / sec, a problem occurs in which the austenite is exposed to a low temperature and disappears before it can stabilize. If the CT is less than MTL-100, the coil temperature becomes excessively low, making winding difficult in the process. In addition, the fine retained austenite formed during the process of generating a large amount of excessively hard and brittle martensite phase disappears, resulting in uneven material properties and poor shape of the rolled sheet. On the other hand, the present invention does not particularly limit the upper limit of the winding temperature, but from the standpoint of ensuring strength, the upper limit may be 350°C. That is, the fine and uniformly distributed austenite formed by appropriately controlling the cooling and winding processes is characterized in that it makes shape correction easier during leveling, and at the same time disappears completely after correction. Hot-rolled steel sheets manufactured through the control of the aforementioned processes can have high yield strength while having excellent shape correctability.

[0047] [Relationship 3] MTL ≤ MT ≤ MTU [Relationship 4] CRL ≤ ICR [Relationship 5] TCR ≤ 80°C / sec [Relationship 6] MTL - 100 ≤ CT

[0048] The process may then include a step of leveling the wound hot-rolled steel sheet. This leveling is for shape correction, and the present invention does not particularly limit the leveling process; any conventional technique used in the art can be used. On the other hand, this leveling is a shape correction method that does not apply reduction to the steel sheet, and can be distinguished from temper rolling (skin pass rolling), which applies a reduction of 0.1 to 2.0%. [Examples]

[0049] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes and to illustrate the present invention, and are not intended to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0050] (Examples) Hot-rolled steel sheets were manufactured from slabs having the alloy composition described in Table 1 below, using the conditions described in Table 2 below. The slab reheating temperature was 1250°C, and the thickness of the hot-rolled sheet immediately after hot rolling was 4 mm. Leveling was then performed using a tension leveler. The microstructure, average prior austenite grain size, wave height, and mechanical properties of the hot-rolled steel sheets before and after leveling were measured, and the results are shown in Tables 3 and 4 below, respectively. On the other hand, trace amounts of impurities were detected in this example despite the absence of Ni addition.

[0051] The microstructure was measured using an electron microscope with an Electron Back-Scattered Diffraction (EBSD) testing device.

[0052] The average grain size of the prior austenite was measured by immersing test specimens taken from the hot-rolled steel sheets manufactured as described above in a solution prepared by mixing 200 ml of supersaturated picric acid aqueous solution with 10 ml of 10% sodium dodecylbenzenesulfonate aqueous solution, adding 10 ml of 10% ferric chloride aqueous solution, and etching the specimens for 10 minutes, then measuring the grain size using an optical microscope.

[0053] Wave height was determined after unraveling the hot-rolled coil, by the value representing the largest difference between the trough and crest of the wave for a 2m length of steel plate.

[0054] Yield strength (YS), tensile strength (TS), and elongation at fracture (El) were measured by taking test specimens according to JIS No. 5 standard from the hot-rolled coil in a direction parallel to the rolling direction.

[0055] [Table 1]

[0056] [Table 2]

[0057] [Table 3]

[0058] [Table 4]

[0059] As shown in Tables 1 to 4 above, in the case of Invention Examples 1 to 6, which satisfy all of the alloy composition, relational formulas, and manufacturing conditions proposed by the present invention, it can be seen that the mechanical properties and shape quality targeted by the present invention are ensured.

[0060] In the case of Comparative Example 1, although the manufacturing conditions of the present invention are met, the C content exceeds the range of the present invention and does not satisfy relational formula 1, indicating that the wave height after leveling is at a high level due to high strength and low shape correction properties.

[0061] In the case of Comparative Example 2, although the manufacturing conditions of the present invention are met, the yield strength is at a low level because it does not reach the C content range of the present invention.

[0062] In the case of Comparative Example 3, although the manufacturing conditions of the present invention are met, the Mn content exceeds the range of the present invention and does not satisfy relational formula 1. This leads to a deterioration in shape correction due to increased segregation and embrittlement, and it can be seen that the wave height after leveling is at a high level.

[0063] In the case of Comparative Example 4, although the manufacturing conditions of the present invention are met, the C and Cr content range of the present invention is not reached, and relational formula 1 is not satisfied. Therefore, it can be seen that the yield strength is low and the wave height after leveling is at a high level.

[0064] In the case of Comparative Example 5, although the alloy composition of the present invention is satisfied, it is clear that an appropriate fraction of austenite before leveling could not be secured, and the yield strength was low, because an existing hot rolling process using a single-stage cooling was employed, and the MT did not satisfy the conditions of the present invention.

[0065] In the case of Comparative Example 6, since the C content exceeds the range of the present invention and the FDT is lower than the FDTL, it can be seen that the anisotropy is large due to the ferrite formed during rolling, resulting in material non-uniformity, low yield strength, and a high wave height after leveling.

[0066] In the case of Comparative Example 7, although the alloy composition of the present invention is satisfied, the CT does not satisfy the conditions of the present invention, so it is not possible to secure an appropriate fraction of austenite before leveling, and the wave height after leveling is at a high level.

[0067] In the case of Comparative Example 8, although the alloy composition of the present invention is satisfied, the yield strength is at a low level because the ICR is lower than the CRL.

[0068] In the case of Comparative Example 9, although the alloy composition of the present invention is satisfied, the TCR exceeds the conditions of the present invention, so it is not possible to secure an appropriate fraction of austenite before leveling, and the wave height after leveling is at a high level.

[0069] In the case of Comparative Example 10, although the manufacturing conditions of the present invention are met, relational equation 1 is not satisfied, indicating that the wave height after leveling is at a high level.

[0070] Figure 1 is a graph showing the relationship between yield strength and wave height after leveling for Invention Examples 1-6 and Comparative Examples 1-10. As can be seen from Figure 1, Invention Examples 1-6 can be confirmed to have both a yield strength of 900 MPa or more and a wave height of 10 mm or less.

[0071] Figure 2 shows photographs of the microstructure of Invention Example 16 before (left) and after (right) leveling, observed using EBSD and an electron microscope, respectively. As can be seen from Figure 2, in Invention Example 16, the austenite formed before leveling disappears after leveling, and it can be confirmed that the microstructure to be obtained by the present invention is formed.

Claims

1. In weight percent, it contains C: 0.06-0.12%, Si: 0.004-0.4%, Mn: 0.8-2.0%, Al: 0.01-0.05%, Cr: 0.05-1.0%, Mo: 0.001-0.3%, P: 0.001-0.05%, S: 0.001-0.005%, N: 0.001-0.01%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, and B: 0.001-0.005%, with the remainder being Fe and other unavoidable impurities. The following relation 1 is satisfied, The microstructure has the following composition in area percent: austenite: 5-15%, auto-tempered martensite: 80% or more, with the remainder being one or more of bainite and ferrite. Hot-rolled steel sheet with a yield strength of 815 MPa or higher. [Relationship 1] (10[C] + [Si] + 2.5[Mn]) / (1.5[Cr] + 2.0[Mo] - 3.2[Nb]) ≤ 20

2. In weight percent, it contains C: 0.06-0.12%, Si: 0.004-0.4%, Mn: 0.8-2.0%, Al: 0.01-0.05%, Cr: 0.05-1.0%, Mo: 0.001-0.3%, P: 0.001-0.05%, S: 0.001-0.005%, N: 0.001-0.01%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, and B: 0.001-0.005%, with the remainder being Fe and other unavoidable impurities. The following relation 1 is satisfied, In terms of area percentage, the microstructure consists of 80% or more auto-tempered martensite, with the remainder being one or more of fresh martensite, bainite, and ferrite. The yield strength is 900 MPa or higher. Hot-rolled steel sheet with a wave height of 10 mm or less in the longitudinal direction of the steel sheet. [Relationship 1] (10[C] + [Si] + 2.5[Mn]) / (1.5[Cr] + 2.0[Mo] - 3.2[Nb]) ≤ 20

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the aforementioned unavoidable impurities include Ni: 0.01% or less.

4. The hot-rolled steel sheet according to claim 1 or 2, wherein the hot-rolled steel sheet has a prior austenite average grain size of 10 to 30 μm.

5. The first step is to reheat a slab at 1200-1350°C containing, by weight percent, C: 0.06-0.12%, Si: 0.004-0.4%, Mn: 0.8-2.0%, Al: 0.01-0.05%, Cr: 0.05-1.0%, Mo: 0.001-0.3%, P: 0.001-0.05%, S: 0.001-0.005%, N: 0.001-0.01%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, and B: 0.001-0.005%, with the remainder being Fe and other unavoidable impurities, satisfying the following relational formula 1; The steps include: and obtaining a hot-rolled steel sheet by hot-rolling the reheated slab at 800 to 1200°C to satisfy the following relational equation 2; and A method for manufacturing a hot-rolled steel sheet, comprising the steps of primary cooling, secondary cooling, and winding the hot-rolled steel sheet so as to satisfy the following relational equations 3 to 6. [Relationship 1] (10[C] + [Si] + 2.5[Mn]) / (1.5[Cr] + 2.0[Mo] - 3.2[Nb]) ≤ 20 [Relationship 2] FDT ≥ 896 - 251[C] + 37.5[Si] - 31.6[Mn] - 7.16[Cr] + 29.5[Mo] + 129[Ti] - 107[Nb] [Relationship 3] MTL ≤ MT ≤ MTU [Relationship 4] CRL ≤ ICR [Relationship 5] TCR ≤ 80°C / sec [Relationship 6] MTL - 100 ≤ CT (However, in the above relational equations 2 to 6, FDT means the surface temperature of the hot-rolled steel sheet at the end of hot rolling, MT means the surface temperature of the hot-rolled steel sheet at the end of primary cooling and the start of secondary cooling, MTL means 430-380[C]-13.4[Si]-47.3[Mn]-16.0[Cr]-24.2[Mo], and MTU means 481-358[C]-16.6[Si]-45 6[Mn] - 15.2[Cr] - 24.1[Mo] means ICR represents the primary cooling rate of the hot-rolled steel sheet surface from FDT to MT, CRL represents 10[2.9 - (0.1[C] + 0.9[Mn] + 0.5[Cr] + 1.2[Mo])] + 10, TCR represents the average cooling rate of the hot-rolled steel sheet surface from FDT to CT, and CT represents the coiling temperature.

6. The first step is to reheat a slab at 1200-1350°C containing, by weight percent, C: 0.06-0.12%, Si: 0.004-0.4%, Mn: 0.8-2.0%, Al: 0.01-0.05%, Cr: 0.05-1.0%, Mo: 0.001-0.3%, P: 0.001-0.05%, S: 0.001-0.005%, N: 0.001-0.01%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, and B: 0.001-0.005%, with the remainder being Fe and other unavoidable impurities, satisfying the following relational formula 1; The step of obtaining a hot-rolled steel sheet by hot-rolling the reheated slab at 800 to 1200°C to satisfy the following relational equation 2; The steps of primary cooling, secondary cooling, and winding the hot-rolled steel sheet so as to satisfy the following relational equations 3 to 6; and A method for manufacturing a hot-rolled steel sheet, comprising the step of leveling the wound hot-rolled steel sheet. [Relationship 1] (10[C] + [Si] + 2.5[Mn]) / (1.5[Cr] + 2.0[Mo] - 3.2[Nb]) ≤ 20 [Relationship 2] FDT ≥ 896 - 251[C] + 37.5[Si] - 31.6[Mn] - 7.16[Cr] + 29.5[Mo] + 129[Ti] - 107[Nb] [Relationship 3] MTL ≤ MT ≤ MTU [Relationship 4] CRL ≤ ICR [Relationship 5] TCR ≤ 80°C / sec [Relationship 6] MTL - 100 ≤ CT (However, in the above relational equations 2 to 6, FDT means the surface temperature of the hot-rolled steel sheet at the end of hot rolling, MT means the surface temperature of the hot-rolled steel sheet at the end of primary cooling and the start of secondary cooling, MTL means 430-380[C]-13.4[Si]-47.3[Mn]-16.0[Cr]-24.2[Mo], and MTU means 481-358[C]-16.6[Si]-45 6[Mn] - 15.2[Cr] - 24.1[Mo] means ICR represents the primary cooling rate of the hot-rolled steel sheet surface from FDT to MT, CRL represents 10[2.9 - (0.1[C] + 0.9[Mn] + 0.5[Cr] + 1.2[Mo])] + 10, TCR represents the average cooling rate of the hot-rolled steel sheet surface from FDT to CT, and CT represents the coiling temperature.

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