Hot-rolled steel sheet

Abstract

Provided is a hot-rolled steel sheet characterized by having a predetermined chemical composition, and having a metal structure which includes, in area%, 30-60 % of ferrite, 30-60 % of bainite, and 5-20 % of martensite and in which TiC deposits having a diameter of 2.0-8.0 nm exist at a number density of at least 1.0 × 1016 / cm3 in the ferrite, and the average aspect ratio of prior austenite grains in a region containing bainite and martensite is at least 3.0.

Description

[Technical Field]

[0001] This invention relates to hot-rolled steel sheets. [Background technology]

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel sheets used is one effective method, and for this reason, the development of high-strength steel sheets is progressing. On the other hand, increasing the strength of steel sheets generally reduces their workability. Therefore, in the development of high-strength steel sheets, it is important to achieve high strength while ensuring a certain level of workability.

[0003] In this regard, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, with a Tief of 0.01 to 0.30% represented by [Ti]-48 / 14×[N]-48 / 32×[S], grains having an orientation difference of 15° or more at the grain boundaries of adjacent grains, and containing 50% or more by area of ​​grains where the average orientation difference within the grain is 0 to 0.5°, further comprising a total of 2% to 10% by area fraction of martensite, tempered martensite, and retained austenite, and further comprising 40% or more by mass of Ti as Ti carbides, wherein the mass of Ti carbides with a circular equivalent grain size of 7 nm to 20 nm is 50% or more of the total mass of Ti carbides. Furthermore, Patent Document 1 teaches that crystal grains with an average orientation difference of 0 to 0.5° within the crystal grain have high ductility and are further strengthened by precipitation due to Ti carbide, and that by ensuring that such crystal grains account for 50% or more of the area, it is possible to improve ductility while maintaining a tensile strength (TS) of 540 MPa or higher.

[0004] Patent Document 2 describes a high-strength hot-rolled steel sheet characterized by having a microstructure having a predetermined component composition, in which the total volume fraction of the ferrite phase and bainite phase in the entire structure is 95% or more, the volume fraction of the ferrite phase in the entire structure is 50-90%, precipitates smaller than 20 nm containing 650-1100 ppm of Ti are precipitated in the ferrite phase, and the ΔHv of the bainite phase (the difference between the maximum and minimum values ​​of the Vickers hardness of the bainite phase measured at 1 / 4 of the thickness of the sheet cross-section along the rolling direction) is 150 or less. Furthermore, Patent Document 2 teaches that if the microstructure is mainly composed of ferrite and bainite phases, with precipitates smaller than 20 nm containing 650-1100 ppm of Ti precipitated in the ferrite phase, and the ΔHv of the bainite phase is 150 or less, a TS of 780 MPa or more can be secured, achieving both excellent ductility (hole-expanding ability) and impact resistance. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-204690 [Patent Document 2] Japanese Patent Publication No. 2011-068945 [Overview of the project] [Problems that the invention aims to solve]

[0006] In relation to the hole-expandability and impact resistance properties described in Patent Document 2, for example, if the hole-expandability decreases, it may not be possible to form the desired member shape in the case of automobile suspension components. Furthermore, for components that require impact resistance, plastic deformation occurs when subjected to an impact exceeding the yield strength. Therefore, from the viewpoint of ensuring the collision safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength, and thus it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

[0007] Therefore, an object of the present invention is to provide a hot-rolled steel sheet having high strength, high hole expansion property, and yield ratio.

Means for Solving the Problems

[0008] In order to achieve the above object, the inventors of the present invention focused on the metallographic structure of the hot-rolled steel sheet and conducted studies. As a result, the inventors of the present invention configured the metallographic structure of the hot-rolled steel sheet having a predetermined chemical composition to mainly contain ferrite, bainite, and martensite, and controlled the bainite fraction in a relatively high range and utilized dislocation strengthening to significantly increase the strength of the hot-rolled steel sheet. Furthermore, by allowing TiC precipitates having an appropriate diameter to exist in ferrite at a predetermined number density, the ferrite is precipitation-strengthened, thereby further increasing the strength of the hot-rolled steel sheet while reducing the hardness difference between ferrite and bainite to improve the hole expansion property and yield ratio. Based on these findings, the present invention was completed.

[0009] The present invention that has achieved the above object is as follows. (1) In mass%, C: 0.03 to 0.10%, Si: 0.010 to 0.100%, Mn: 0.50 to 3.00%, Ti: 0.05 to 0.20%, Al: 0.20 to 0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.ooo%, B: 0 to 0.0100%, Sn: 0 to 1.000%, Sb: 0 to 1.000%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.010%, REM: 0~0.0100%, As: 0~0.010%, Zr: 0~0.010%, Co: 0~2.000%, Zn: 0~0.010%, W: 0~1.000%, and The remainder has a chemical composition consisting of Fe and impurities. In area percentage, Ferrite: 30-60%, Bainite: 30-60%, and Martensite: Contains 5-20%, TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻⁶. 16 pieces / cm 3 They exist at the above number density, A hot-rolled steel sheet characterized by having a microstructure in which the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or greater. (2) The chemical composition is, in mass%, Nb: 0.001~0.050%, V: 0.001~1.000%, Cr: 0.001~2.00%, Ni: 0.001~2.00%, Cu: 0.001~2.00%, Mo: 0.001~1.000%, B: 0.0001~0.0100%, Sn: 0.001~1.000%, Sb: 0.001~1.000%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Hf: 0.0001~0.0100%, Bi: 0.001~0.010%, REM: 0.0001~0.0100%, As: 0.001~0.010%, Zr: 0.001~0.010%, Co: 0.001~2.000%, Zn: 0.001~0.010%, and W: 0.001~1.000% The hot-rolled steel sheet according to (1) above, characterized in that it includes at least one of the above. (3) The hot-rolled steel sheet according to (1) or (2) above, characterized in that the ratio of the average nanohardness of ferrite to the average nanohardness of bainite is 0.75 to 1.20. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a hot-rolled steel sheet that has high strength, high hole-expandability, and a high yield ratio. [Modes for carrying out the invention]

[0011] <Hot-rolled steel sheet> The hot-rolled steel sheet according to the embodiment of the present invention is, by mass%, C: 0.03~0.10%, Si: 0.010~0.100%, Mn: 0.50~3.00%, Ti: 0.05~0.20%, Al: 0.20-0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0~0.050%, V: 0~1.000%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.000%, B: 0~0.0100%, Sn: 0~1.000%, Sb: 0~1.000%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.010%, REM: 0~0.0100%, As: 0~0.010%, Zr: 0~0.010%, Co: 0~2.000%, Zn: 0~0.010%, W: 0~1.000%, and The remainder has a chemical composition consisting of Fe and impurities. In area percentage, Ferrite: 30-60%, Bainite: 30-60%, and Martensite: Contains 5-20%, TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻⁶. 16 pieces / cm 3 They exist at the above number density, It is characterized by having a metallic structure in which the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or higher.

[0012] As the strength of steel sheets increases, workability such as hole-expandability generally decreases. Therefore, it is generally very difficult to improve hole-expandability while ensuring sufficient strength of the steel sheet, and furthermore, to achieve a high yield ratio from the viewpoint of automobile crash safety, etc. So, the inventors investigated by focusing on the metal structure of the hot-rolled steel sheet, in addition to making the chemical composition of the hot-rolled steel sheet appropriate. To explain in more detail, first, the inventors found that by configuring the metal structure of a hot-rolled steel sheet having a predetermined chemical composition to mainly contain ferrite, bainite, and martensite, it is possible to increase hole-expandability and yield ratio while maintaining the strength of the hot-rolled steel sheet at a reasonably high level. Next, the inventors found that in order to further increase strength, the strength of the hot-rolled steel sheet can be significantly increased by controlling the fraction of bainite, which is a hard phase in the above metal structure, to a relatively high range and utilizing dislocation strengthening. More specifically, the inventors have found that by controlling the area ratio of bainite in the metal structure to within the range of 30-60%, and by introducing dislocations into the steel sheet during hot rolling such that the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or higher, as will be explained in more detail later in relation to the manufacturing method of hot-rolled steel sheets, the strength of the hot-rolled steel sheet can be significantly increased.

[0013] On the other hand, in a three-phase structure mainly composed of ferrite, bainite, and martensite, controlling the bainite fraction to a relatively high range as described above is very effective from the viewpoint of increasing strength, but the hardness difference between the hard phase, bainite, and the soft phase, ferrite, becomes relatively large. When the hardness difference between each phase in the metal structure becomes large, the hole-expanding properties and yield ratio may decrease as a result. For this reason, the inventors investigated improving hole-expanding properties and achieving a high yield ratio from the viewpoint of reducing the hardness difference between each phase in metal structures including such three-phase structures. As a result, the inventors found that precipitation strengthening of the softest ferrite in the three-phase structure, more specifically, precipitation of TiC precipitates with a diameter of 2.0 to 8.0 nm into the ferrite, resulting in a density of 1.0 × 10⁻⁶ 16 pieces / cm3 By precipitation-strengthening ferrite at the above number density, it is naturally possible to improve the overall strength of the hot-rolled steel sheet, and it has been found that the hardness difference between bainite, a relatively abundant hard phase in the metal structure, and ferrite, the softest of the three phase structures, can be sufficiently reduced. In the hot-rolled steel sheet according to the embodiment of the present invention, 1.0 × 10 TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite. 16 pieces / cm 3 In order to achieve the above number density, in addition to the manufacturing method which will be explained in detail later, it is necessary to make the chemical composition of the hot-rolled steel sheet appropriate. For example, Si and Al contained in the steel have the effect of suppressing the precipitation of cementite. Therefore, by including these elements in the steel in amounts above a certain level, more specifically, including Si and Al at 0.010 mass% or more and 0.20 mass% or more, respectively, it is possible to suppress the consumption of C in the steel to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. As a result, the inventors have found that even though the metal structure is composed of a three-phase structure which tends to have a relatively large hardness difference by increasing the bainite fraction and utilizing dislocation strengthening to achieve high strength, it is possible to obtain a hot-rolled steel sheet with high strength and high hole-expanding properties and yield ratio by increasing the hardness of ferrite through precipitation strengthening using TiC precipitates of a specific diameter and number density. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can be effectively used in components where both high strength and excellent workability, which are conflicting properties, are required, and furthermore, impact resistance is also required, making it particularly useful in the automotive sector.

[0014] The hot-rolled steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0015] [C:0.03~0.10%] Carbon (C) is an effective element for increasing the strength of steel plates. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in the steel, contributing to precipitation strengthening based on these precipitates and to microstructural refinement due to the pinning effect of these precipitates. To fully obtain these effects, the C content should be 0.03% or higher. The C content may also be 0.04% or higher, 0.05% or higher, or 0.06% or higher. On the other hand, excessive C content may reduce hole expansion properties and yield ratio due to cementite formation. Therefore, the C content should be 0.10% or lower. The C content may also be 0.09% or lower, 0.08% or lower, or 0.07% or lower.

[0016] [Si: 0.010~0.100%] Si is an effective element for increasing strength as a solid solution strengthening element. Si also has the effect of suppressing cementite precipitation. Therefore, including Si can suppress the consumption of carbon (C) in the steel for cementite formation, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully obtain these effects, the Si content should be 0.010% or higher. The Si content may also be 0.020% or higher, 0.030% or higher, or 0.040% or higher. On the other hand, excessive Si content can cause surface quality defects called Si scale. Therefore, the Si content should be 0.100% or lower. The Si content may also be 0.090% or lower, 0.080% or lower, 0.070% or lower, 0.060% or lower, or 0.050% or lower.

[0017] [Mn: 0.50~3.00%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To fully obtain these effects, the Mn content should be 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if the Mn content is excessive, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content should be 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0018] [Ti: 0.05~0.20%] Ti (Ti) precipitates finely in steel as carbides (TiC), improving the strength of the steel through precipitation strengthening and increasing the hardness of ferrite. Furthermore, Ti fixes carbon (C) by forming carbides, suppressing the formation of cementite, which is detrimental to hole expansion. To fully obtain these effects, the Ti content should be 0.05% or higher. The Ti content may also be 0.08% or higher, 0.10% or higher, 0.12% or higher, or 0.14% or higher. On the other hand, excessive Ti content can lead to coarser carbides, making it impossible to obtain the desired precipitation strengthening in ferrite. Additionally, the coarser TiC precipitates reduce the number density of TiC precipitates, preventing sufficient hardness increase of ferrite through precipitation strengthening. Therefore, the Ti content should be 0.20% or lower. The Ti content may also be 0.18% or lower, 0.17% or lower, 0.16% or lower, or 0.15% or lower.

[0019] [Al: 0.20~0.40%] Al is an element that acts as a deoxidizing agent for molten steel. Al also has the effect of suppressing cementite precipitation. Therefore, including Al can suppress the consumption of carbon (C) in the steel to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully obtain these effects, the Al content should be 0.20% or higher. The Al content may also be 0.22% or higher, 0.25% or higher, or 0.28% or higher. On the other hand, excessive Al content can lead to the formation of coarse oxides, which can reduce toughness and ductility. Therefore, the Al content should be 0.40% or lower. The Al content may also be 0.38% or lower, 0.35% or lower, or 0.32% or lower.

[0020] [P:0.100% or less] Excessive phosphorus (P) content can negatively affect weldability and other properties. Therefore, the P content should be 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0021] [S:0.0100% or less] Excessive S content can lead to the formation of large amounts of MnS, which can reduce toughness. Therefore, the Si content should be 0.0100% or less. The S content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0022] [N:0.010% or less] Excessive nitrogen (N) content can form coarse nitrides, reducing toughness. Therefore, the N content should be 0.010% or less. The N content may also be 0.008% or less, 0.005% or less, or 0.003% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0023] [O:0.010% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel sheet. Therefore, the O content should be 0.010% or less. The O content may also be 0.008% or less, 0.006% or less, or 0.004% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[0024] The basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0025] [Nb:0~0.050%] Nb is an element that contributes to the refinement of the microstructure and, consequently, the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. The Nb content may be 0%, but to obtain such an effect, it is preferable that the Nb content be 0.001% or more. The Nb content may also be 0.005% or more, 0.010% or more, 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if the Nb content is excessive, coarse carbides and the like may be formed in the steel, which may reduce the ductility of the steel sheet. Therefore, it is preferable that the Nb content be 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.025% or less.

[0026] [V: 0~1.000%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 1.000% or less. The V content may be 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0027] [Cr: 0~2.00%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. While the Cr content may be 0%, it is preferable that the Cr content be 0.001% or more to obtain such effects. The Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 2.00% or less. The Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0028] [Ni: 0~2.00%] [Cu: 0~2.00%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. The Ni and Cu content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni and Cu content be 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0029] [Mo: 0~1.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. While the Mo content may be 0%, it is preferable that the Mo content be 0.001% or more to obtain these effects. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may become larger. Therefore, it is preferable that the Mo content be 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0030] [B: 0~0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0031] [Sn: 0~1.000%] [Sb: 0~1.000%] Sn and Sb are elements that are effective in improving corrosion resistance. The Sn and Sb content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive content of these elements may lead to a decrease in toughness. Therefore, it is preferable that the Sn and Sb content be 1.000% or less, and may be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.

[0032] [Ca: 0~0.0100%] [Mg: 0~0.0100%] [Hf: 0~0.0100%] Ca, Mg, and Hf are elements that can control the morphology of nonmetallic inclusions. The Ca, Mg, and Hf content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the Ca, Mg, and Hf content be 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less, each.

[0033] [Bi: 0~0.010%] Bi is an effective element for improving corrosion resistance. While the Bi content may be 0%, it is preferable that the Bi content be 0.001% or higher to obtain this effect. The Bi content may be 0.001% or higher, or 0.002% or higher. On the other hand, if the Bi content is excessive, the effect will saturate, and including more Bi than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the Bi content be 0.010% or lower. The Bi content may be 0.005% or lower, or 0.003% or lower.

[0034] [REM:0~0.0100%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, it is preferable that the REM content be 0.0001% or more. The REM content may be 0.0005% or more, or 0.0010% or more. On the other hand, if the REM content is excessive, the effect will saturate, and including more REM in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the REM content be 0.0100% or less. The REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0035] [As: 0~0.010%] As is an effective element for improving corrosion resistance. While the As content may be 0%, it is preferable that the As content be 0.001% or more to obtain this effect. The As content may also be 0.002% or more, or 0.003% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.010% or less. The As content may also be 0.008% or less, or 0.005% or less.

[0036] [Zr:0~0.010%] Zr is an element that can control the morphology of nonmetallic inclusions. While the Zr content may be 0%, it is preferable that the Zr content be 0.001% or higher to obtain such an effect. The Zr content may also be 0.002% or higher, or 0.003% or higher. On the other hand, if the Zr content is excessive, the effect will saturate, and including more Zr than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the Zr content be 0.010% or lower. The Zr content may also be 0.008% or lower, or 0.005% or lower.

[0037] [Co: 0~2.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, it is preferable that the Co content be 0.001% or more. The Co content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Co content is excessive, the hot workability may decrease, and this can lead to an increase in raw material costs. Therefore, it is preferable that the Co content be 2.000% or less. The Co content may be 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[0038] [Zn: 0~0.010%] Zn is an element that can be contained in steel sheets when scrap or similar materials are used as steel raw materials. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less or 0.005% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires more time for refining, leading to a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0039] [W: 0~1.000%] W is an element that enhances the hardenability of steel and contributes to improving its strength. While the W content may be 0%, it is preferable that the W content be 0.001% or more to obtain such effects. The W content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive W content may reduce weldability. Therefore, it is preferable that the W content be 1.000% or less. The W content may also be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[0040] In the hot-rolled steel sheet according to an embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of hot-rolled steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0041] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the hot-rolled steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0042] [Metal structure] The microstructure of the hot-rolled steel sheet according to the embodiment of the present invention includes, by area percentage, ferrite: 30-60%, bainite: 30-60%, and martensite: 5-20%. By configuring the microstructure of the hot-rolled steel sheet to mainly consist of these three structures, and with a relatively high proportion of bainite, it is possible to increase the strength of the hot-rolled steel sheet while also improving its hole-expanding properties and yield ratio. In addition to including these three structures in the specific area percentages described above, the strength of the hot-rolled steel sheet can be further increased by utilizing dislocation strengthening, which will be explained in detail later. Furthermore, by including these three structures in the specific area percentages described above, it is possible to appropriately reduce the hardness difference in the microstructure through ferrite precipitation strengthening, which will be explained in detail later. For example, if the area percentage of ferrite is small, the proportion of bainite and martensite, which are hard phases, and especially the proportion of bainite, will be high, and even with ferrite precipitation strengthening, it may not be possible to appropriately reduce the hardness difference in the microstructure, more specifically the hardness difference between ferrite and bainite. In such cases, it becomes impossible to achieve the desired hole-expanding properties and / or yield ratio. Therefore, the area ratio of ferrite must be 30% or more, and may be, for example, 35% or more, 40% or more, or 45% or more. On the other hand, if the area ratio of ferrite is high, the proportion of the hard phases, bainite and martensite, decreases, and as a result, it may not be possible to achieve the desired strength, for example, a tensile strength of 780 MPa or more. Therefore, the area ratio of ferrite should be 60% or less, and may be, for example, less than 60%, 59% or less, 58% or less, 55% or less, 52% or less, or 50% or less.

[0043] From the viewpoint of improving tensile strength, a higher area ratio of the hard phases, bainite and martensite, is preferable. From this viewpoint, for example, the area ratio of bainite may be more than 30%, 31% or more, 32% or more, 35% or more, 40% or more, or 45% or more. Similarly, the area ratio of martensite may be 8% or more, 10% or more, or 12% or more. On the other hand, from the viewpoint of reducing the hardness difference in the metal structure and further improving hole-expanding properties and yield ratio, a lower area ratio of bainite and martensite is preferable. From this viewpoint, for example, the area ratio of bainite may be 58% or less, 55% or less, 52% or less, or 50% or less. Similarly, the area ratio of martensite may be 18% or less, 16% or less, or 14% or less.

[0044] [Remaining tissue] The microstructure of the hot-rolled steel sheet according to the embodiment of the present invention includes ferrite, bainite, and martensite as described above, and may also include other residual structures, but the area ratio of the residual structures is preferably small, and may be 0%. The area ratio of the residual structures is not particularly limited, but may be, for example, 0-5%, 0-4%, or 0-3%. In other words, the total area ratio of ferrite, bainite, and martensite may be, for example, 95-100%, 96-100%, or 97-100%. The lower limit of the residual structures may be 1% or 2%. If residual structures are present, they may include at least one of pearlite and retained austenite, or at least one of them.

[0045] [Identification of metallographic structure and calculation of area ratio] The tissue observation is performed using a scanning electron microscope. Prior to observation, the sample for tissue observation is polished by wet polishing with emery paper and then polished with diamond abrasive grains having an average particle size of 1 μm to finish the observation surface into a mirror surface, and then the tissue is etched with a 3% nitric acid alcohol solution. The magnification of the observation is set at 2000 times, and 10 random images of a 30 μm × 40 μm field of view at a position 1 / 4 of the plate thickness from the surface are taken. The ratio of the tissue is determined by the point count method. For the obtained tissue images, a total of 225 grid points arranged at intervals of 3 μm in the vertical direction and 4 μm in the horizontal direction are defined, the tissue existing under the grid points is discriminated, and the tissue ratio contained in the steel material is obtained from the average value of the 10 images. Ferrite is massive crystal grains that do not contain iron-based carbides with a major axis of 100 nm or more inside. Bainite is an aggregate of lath-shaped crystal grains that do not contain iron-based carbides with a major axis of 20 nm or more inside, or that contain iron-based carbides with a major axis of 20 nm or more inside, and the carbides belong to a single variant, that is, an iron-based carbide group extending in the same direction. Here, the iron-based carbide group extending in the same direction means that the difference in the extension direction of the iron-based carbide group is within 5°. Bainite is counted as one bainite grain when the bainite surrounded by grain boundaries with an azimuth difference of 15° or more. Also, martensite containing a large amount of dissolved carbon can be distinguished from other tissues because it has a higher brightness and appears white compared to other tissues. When there are tissues other than ferrite, bainite, and martensite, the area ratio of the remaining tissues is determined by subtracting the total area ratio of ferrite, bainite, and martensite from 100%. It is not necessary to specifically identify the remaining tissues, but when the remaining tissues contain pearlite, retained austenite, etc., pearlite has a unique structure in which cementite precipitates in a lamellar shape and can be identified by a scanning electron microscope. Also, the volume ratio of retained austenite can be calculated by X-ray diffraction measurement, and since the volume ratio of retained austenite is equivalent to the area ratio, this can be taken as the area ratio of retained austenite.

[0046] [Number density of TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite: 1.0×10 16 per cm 3 or more] In the hot-rolled steel sheet according to an embodiment of the present invention, the ferrite contains 1.0 × 10 TiC precipitates with a diameter of 2.0 to 8.0 nm. 16 pieces / cm 3 These precipitates exist at the number density described above. Here, TiC precipitates include not only TiC but also composite carbides containing Ti and other elements besides Ti, such as V and Nb. By introducing TiC precipitates with a diameter of 2.0 to 8.0 nm into the ferrite at this number density, the hardness of the ferrite can be increased through precipitation strengthening. More specifically, by increasing the hardness of the ferrite and reducing the hardness difference with bainite, a relatively abundant hard phase in the metal structure, the hardness difference in the metal structure, which is mainly composed of ferrite, bainite, and martensite, can be reduced. As a result, it becomes possible to significantly improve the hole expansion properties and yield ratio of hot-rolled steel sheets, for example, achieving a hole expansion ratio (λ) of 60.0% or more and a yield ratio (YR) of 0.70 or more. Naturally, precipitation strengthening by TiC precipitates also contributes to improving the overall strength of the hot-rolled steel sheet. If the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot adequately act as obstacles to dislocation movement, and therefore the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. In addition, the strength improvement effect of the hot-rolled steel sheet may not be fully realized. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in the ferrite may not be obtained.

[0047] While not intended to be bound by any particular theory, this is thought to be because the strengthening mechanism changes in relation to dislocation motion as the TiC precipitates become coarser. For example, dislocation lines no longer pass across the TiC precipitates but instead pass around the coarse TiC precipitates, leaving loops of dislocation lines, thus reducing the amount of precipitation strengthening. In addition, as the TiC precipitates become coarser, the number density of these TiC precipitates also decreases significantly, making it impossible to sufficiently increase the hardness of the ferrite through precipitation strengthening. Therefore, in order to effectively increase the hardness of the ferrite through precipitation strengthening, it is effective to control the diameter of the TiC precipitates in the range of 2.0 to 8.0 nm, and in order to improve the hardness of the ferrite to the desired level through precipitation strengthening, it is important to control the number density of TiC precipitates with such diameters within a predetermined range. From this perspective, TiC precipitates with a diameter of 2.0 to 8.0 nm are placed in the ferrite as described above, at a density of 1.0 × 10⁻⁶ 16 pieces / cm 3 It is necessary to have them present at the above number density. To further enhance the hardness-improving effect of ferrite, a higher number density is preferable, for example, 1.2 × 10⁻⁶. 16 pieces / cm 3 The above is 1.5 × 10 16 pieces / cm 3 The above is 2.0 × 10 16 pieces / cm 3 The above is 5.0 x 10 16 pieces / cm 3 or more or 10.0 × 10 16 pieces / cm 3 The above is also acceptable. On the other hand, because there are limitations on the content of C and Ti, which are the sources of TiC precipitates, if the number density becomes too high, it may become difficult to control the diameter of the TiC precipitates within the desired range. Therefore, the number density is not particularly limited as long as it satisfies a diameter of 2.0 to 8.0 nm, but for example, 75.0 × 10 16 pieces / cm 3 Below, 50.0 × 10 16 pieces / cm 3 Below, 30.0 × 10 16 pieces / cm 3 The following or 20.0 × 10 16 pieces / cm3 The following may also apply. In the hot-rolled steel sheet according to the embodiment of the present invention, when measured by the three-dimensional atom probe measurement method which will be described in detail later, the number of TiC precipitates with a diameter of 2.0 to 8.0 nm is 1.0 × 10⁻⁶. 16 pieces / cm 3 It is sufficient that the above number densities are present in the ferrite, and therefore, as long as the above requirements for diameter and number density are satisfied, for example, coarse TiC precipitates may be present in the ferrite.

[0048] [Calculation of diameter and number density of TiC precipitates] The diameter and number density of TiC precipitates are calculated using the three-dimensional atom probe measurement method as follows. First, a needle-shaped sample is prepared from the sample to be measured by cutting and electrolytic polishing, and if necessary, by using focused ion beam processing in combination with electrolytic polishing. In three-dimensional atom probe measurement, the accumulated data can be reconstructed to obtain an actual distribution image of atoms in real space. In the case of fine TiC precipitates with a Na-Cl structure, the unit cell is 4.33 Å, so the interatomic distance between Ti atoms is 4.33 × √2 = 6.1 Å. Therefore, if multiple Ti atoms exist at approximately the same coordinate position (less than 7 Å), these Ti atoms are judged to be in the same precipitate, and the number of Ti atoms judged to be in this same precipitate is counted. If this number is 50 or more, this precipitate is defined as a fine TiC precipitate. The diameter of the fine TiC precipitate described above is the equivalent diameter of a circle calculated by assuming the fine Ti precipitate is spherical, based on the number of Ti atoms constituting the observed fine Ti precipitate and the lattice constant of the fine Ti precipitate. The method for determining the diameter (equivalent diameter of a circle) R of the fine TiC precipitate using the number of Ti atoms of the fine TiC precipitate obtained by the three-dimensional atom probe measurement method is shown below. The three-dimensional atom probe measurement method measures the total number of atoms N in the target sample, but in reality, the three-dimensional atom probe measurement method cannot detect all of the number of atoms N in the target sample. Since there is an atom detection rate α (= number of atoms detected / total number of atoms) specific to each instrument, the number of atoms N that would have been present is calculated from the actual measured value n. That is, the total number of atoms N = n / α. Next, for this total number of atoms N, assuming that there are 8 Ti atoms in the unit cell of the Na-Cl structure TiC precipitate, and assuming the lattice constant a of the Na-Cl structure is 4.33 Å, the diameter (equivalent diameter of a circle) R of the TiC precipitate is calculated using the following formula. TiC precipitate diameter R = {(6 / 8)·(1 / π)·N·a} 3} (1 / 3) Finally, the number density of TiC precipitates is calculated by using the measurement field of view as the denominator and the number of fine TiC precipitates as the numerator.

[0049] [Ratio of average nanohardness of ferrite to average nanohardness of bainite: 0.75~1.20] In a preferred embodiment of the present invention, the ratio of the average nanohardness of ferrite to the average nanohardness of bainite, i.e., (average nanohardness of ferrite) / (average nanohardness of bainite), is controlled to be within the range of 0.75 to 1.20. By controlling the ratio of the average nanohardness of ferrite to the average nanohardness of bainite to be within this range, the hardness difference between ferrite and bainite in the metal structure can be further reduced. As a result, it becomes possible to further improve the hole expansion rate and yield ratio of the hot-rolled steel sheet, making it possible to achieve, for example, a hole expansion rate (λ) of 65.0% or more and a yield ratio (YR) of 0.75 or more. From the viewpoint of further improving the hole expansion rate and yield ratio, the ratio of the average nanohardness of ferrite to the average nanohardness of bainite may be 0.80 or more, 0.85 or more, or 0.90 or more, and similarly, it may be 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less. The average nanohardness of bainite is not particularly limited, but may be, for example, 3.0–5.0 GPa, 3.2–4.8 GPa, or 3.5–4.5 GPa. Similarly, the average nanohardness of ferrite is not particularly limited, but may be, for example, 2.5–5.0 GPa, 2.8–4.8 GPa, or 3.0–4.5 GPa.

[0050] [Method for determining the ratio of the average nanohardness of ferrite to the average nanohardness of bainite] The ratio of the average nanohardness of ferrite to the average nanohardness of bainite is determined as follows: First, a sample is cut from a hot-rolled steel sheet so that a cross-section perpendicular to the surface thickness can be observed. The cross-section of the sample is polished to a mirror finish using wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 μm. An indentation is made on the mirror-finished cross-section at a depth of 1 / 4 of the sheet thickness from the surface using a microhardness tester with a test load of 3 gf, and the nanohardness is measured, obtaining a total of more than 100 measurement points. Next, the same sample is measured using a scanning electron microscope, and by referring to the obtained microstructural analysis results, only the measurement points where indentations are found inside the ferrite crystal grains and inside the bainite are extracted. Finally, the arithmetic mean of the nanohardness of 10 or more extracted ferrite crystal grains is taken as the average nanohardness of ferrite, and similarly, the arithmetic mean of the nanohardness of 10 or more extracted bainite is taken as the average nanohardness of bainite. The ratio of these (average nanohardness of ferrite) / (average nanohardness of bainite) is then determined as the ratio of the average nanohardness of ferrite to the average nanohardness of bainite.

[0051] [Average aspect ratio of former austenite grains in regions containing bainite and martensite: 3.0 or higher] In the hot-rolled steel sheet according to the embodiment of the present invention, the average aspect ratio of prior austenite grains in the region containing bainite and martensite must be 3.0 or higher. As described above, the strength of the hot-rolled steel sheet can be increased by configuring the metal structure to contain a relatively large amount of hard phase bainite, amounting to 30-60 area percent. However, simply increasing the area percentage of bainite may not reliably achieve the desired high strength. Therefore, in the hot-rolled steel sheet according to the embodiment of the present invention, in addition to containing a relatively large amount of bainite, further strength improvement is made possible by utilizing dislocation strengthening. More specifically, by applying appropriate reduction during hot rolling, dislocations can be introduced into the steel sheet while suppressing recrystallization of the metal structure. In this way, the metal structure in which dislocations have been appropriately introduced has a relatively large aspect ratio because recrystallization is suppressed. That is, by appropriately controlling the average aspect ratio of prior austenite grains in the region containing bainite and martensite, the strength improvement effect obtained by containing a relatively large amount of bainite can be further enhanced by dislocation strengthening. In the hot-rolled steel sheet according to the embodiment of the present invention, the metal structure is configured to include 30 to 60 area percent of bainite, and the average aspect ratio of prior austenite grains in the region containing bainite and martensite is controlled to 3.0 or higher. This significantly increases the strength of the hot-rolled steel sheet due to the combination of the strength-enhancing effect of bainite and dislocation strengthening. From the viewpoint of further increasing the strength of the hot-rolled steel sheet, a larger average aspect ratio is preferable, for example, it may be 3.2 or higher, 3.5 or higher, 3.8 or higher, or 4.0 or higher. There is no particular upper limit to the average aspect ratio, but for example, the average aspect ratio may be 6.0 or lower, 5.5 or lower, or 5.0 or lower.

[0052] [Measurement of the average aspect ratio in prior austenite grains in regions containing bainite and martensite] The average aspect ratio of prior austenite grains in regions containing bainite and martensite is measured using a scanning electron microscope. Prior to the measurement, the sample for microstructure observation is first polished using wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 μm. The L-shaped cross section (a cross section parallel to the rolling direction and the thickness direction) is used as the observation surface and is finished to a mirror surface. Then, the microstructure is etched with a picric acid solution. The observation magnification is set to 1000x, and 10 random images of a 60 μm × 80 μm field of view at a position 1 / 4 of the thickness from the surface are taken. The grain size in the thickness direction and rolling direction is determined by the sectioning method, targeting the grain boundaries revealed by the picric acid etching. In the sectioning method, five straight lines are drawn at equal intervals parallel to the thickness direction and rolling direction of the captured image, and the intersections with the grain boundaries are counted. The grain size is obtained by dividing the total length of the five straight lines by the number of intersections. The average aspect ratio of prior austenite grains in the region containing bainite and martensite is determined by dividing the grain size in the rolling direction by the grain size in the thickness direction.

[0053] [plate thickness] The hot-rolled steel sheet according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 6.0 mm. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 5.0 mm or less, or 4.0 mm or less.

[0054] [Mechanical properties] [Tensile strength: TS] According to the hot-rolled steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 780 MPa or higher, can be achieved. The tensile strength is preferably 800 MPa or higher, 820 MPa or higher, or 840 MPa or higher. According to the hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, improved hole-expandability and a high yield ratio can be achieved by the specific combination of chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1180 MPa or less, 980 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from the direction in which the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[0055] [Yield ratio: YR] According to the hot-rolled steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can also be increased, and more specifically, a yield ratio of 0.70 or higher can be achieved. The yield ratio is preferably 0.75 or higher, more preferably 0.80 or higher. There is no particular upper limit, but for example, the yield ratio may be 0.90 or lower or 0.85 or lower. The yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet (direction C), and performing a tensile test in accordance with JIS Z 2241:2011. Yield ratio YR = 0.2% proof stress / Tensile strength TS

[0056] [Hole expansion ratio: λ] According to the hot-rolled steel sheet having the above chemical composition and metal structure, high hole expansion properties, specifically a hole expansion ratio of 60.0% or more, can be achieved. The hole expansion ratio is preferably 65.0% or more, more preferably 70.0% or more, or 80% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 120% or less, 110% or less, or 100% or less. The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length is taken from the hot-rolled steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is widened using a conical punch with a 60° apex angle until a crack penetrates the plate thickness. The hole diameter d1mm at the time of crack occurrence is measured, and the hole expansion ratio λ(%) for each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0057] <Manufacturing method for hot-rolled steel sheets> Next, preferred manufacturing methods for hot-rolled steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing hot-rolled steel sheets according to embodiments of the present invention, and is not intended to limit the hot-rolled steel sheets to those manufactured by the manufacturing methods described below.

[0058] A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention is as follows: A hot rolling process relating to hot-rolled steel sheets, comprising heating a slab having the chemical composition described above, and then finish rolling it, and satisfying the following conditions (a) to (e): (a) The heating temperature of the slab is 1200-1300°C. (b) The holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds. (c) The finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction ratio in the preceding rolling passes, excluding the last three stages, is 60-90%. (d) The reduction ratio in each of the three subsequent rolling passes is 10% or more, and the total reduction ratio in the three subsequent rolling passes is 30-50%, (e) The finishing rolling temperature is 900 to 1000°C. An intermediate air cooling process is performed in which the finish-rolled steel sheet is first cooled to an intermediate air cooling temperature of 670-750°C at an average cooling rate of 50-200°C / second, followed by intermediate air cooling for 3-10 seconds, and The intermediate-cooled steel sheet is secondarily cooled at an average cooling rate of 50-200°C / second, followed by a cooling process where it is wound at a winding temperature of 20-200°C. It is characterized by including [specific features]. The following describes each process in detail.

[0059] [Hot rolling process] [(a) Slab heating temperature: 1200~1300℃] [(b) Holding time in the temperature range of 1200~1300℃: 1000~4000 seconds] First, a slab having the chemical composition described above in relation to hot-rolled steel sheets is heated. From a productivity standpoint, the slab used is preferably cast by a continuous casting method, but it may also be manufactured by an ingot-making method or a thin-slab casting method. The slab used contains a relatively large amount of alloying elements to obtain high-strength steel sheets. Therefore, it is necessary to heat the slab before hot-rolling to solid-solve the alloying elements in it. If the heating temperature is too low, the alloying elements may not sufficiently solid-solve in the slab, leaving behind coarse alloy carbides, which may cause brittle cracking during hot-rolling. For this reason, a heating temperature of 1200°C or higher is preferable. While there is no particular upper limit to the heating temperature, it is preferable to be 1300°C or lower from the viewpoint of heating equipment capacity and productivity. Furthermore, by holding the slab in the 1200-1300°C temperature range for 1000 seconds or more, the alloying elements can be reliably solid-solved in the slab. While there is no particular upper limit to the holding time, it is preferable to be 4000 seconds or lower from the viewpoint of productivity, etc. When rough rolling is performed, holding at a temperature range of 1200-1300°C may be done after rough rolling.

[0060] [Rough rolling] In this method, for example, a heated slab may be subjected to rough rolling before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are ensured.

[0061] [(c) Total reduction ratio in the rolling pass preceding the finish rolling: 60-90%] The heated slab, or a slab that has been roughly rolled as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, more specifically, five to eight rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, the total reduction ratio in the preceding rolling passes, excluding the last three passes, is controlled to 60-90%. By performing rolling at such a high reduction ratio in the preceding rolling passes, recrystallization can be promoted and the metal structure can be refined. However, the recrystallization rate in each preceding rolling pass does not need to be 100%. Such refinement of the metal structure by recrystallization is very advantageous in forming the desired metal structure and improving properties such as hole-expanding ability. If the total reduction ratio in the preceding rolling passes is less than 60%, the desired metal structure containing ferrite, bainite, and martensite in specific proportions may not be obtained, and properties such as hole-expanding ability may be reduced. Therefore, the total reduction ratio in the preceding rolling pass should be 60% or more, preferably 70% or more. On the other hand, if the total reduction ratio in the preceding rolling pass is too high, the rolling load will be excessive, and the load on the rolling mill will increase. For this reason, the total reduction ratio in the preceding rolling pass should be 90% or less.

[0062] [(d) Reduction ratio in each of the three subsequent rolling passes: 10% or more, and total reduction ratio in the three subsequent rolling passes of finish rolling: 30-50%] In the finish rolling process of this manufacturing method, the reduction ratio in each of the three subsequent rolling passes is controlled to be 10% or more, and the total reduction ratio in the three subsequent rolling passes is controlled to be within the range of 30-50%. Unlike the preceding rolling passes, recrystallization must be suppressed in the subsequent rolling passes of the finish rolling process. By controlling the reduction ratio in each of the three subsequent rolling passes to be 10% or more, while keeping the total reduction ratio in the three subsequent rolling passes within the range of 30-50%, the reduction ratio in each rolling pass is effectively limited to within the range of 10-30%. By carrying out the three subsequent rolling passes under these relatively light reduction conditions, recrystallization can be suppressed in each rolling pass, ensuring the introduction of dislocations, and ultimately making it possible to control the average aspect ratio of prior austenite grains in the region containing bainite and martensite in the microstructure of the hot-rolled steel sheet to 3.0 or more.

[0063] If the reduction ratio in each of the three subsequent rolling passes is less than 10%, recrystallization is suppressed in each rolling pass, but sufficient dislocations cannot be introduced in each rolling pass, and the desired aspect ratio cannot be achieved in the final microstructure. Similarly, if the total reduction ratio in the three subsequent rolling passes is less than 30%, sufficient dislocations cannot be introduced in at least one of the three subsequent rolling passes, and the average aspect ratio of prior austenite grains in the region containing bainite and martensite cannot be achieved in the final microstructure. On the other hand, if the reduction ratio in each of the three subsequent rolling passes is greater than 30% or the total reduction ratio in the three subsequent rolling passes is greater than 50%, recrystallization is promoted and sufficient dislocations cannot be introduced, and similarly, the average aspect ratio of prior austenite grains in the region containing bainite and martensite cannot be achieved in the final microstructure. The total reduction ratio in the three subsequent rolling passes of the finish rolling process is preferably controlled within the range of 35 to 50%.

[0064] [(e) End temperature for finish rolling: 900~1000℃] In this manufacturing method, in addition to controlling the reduction ratio in the pre- and post-finish rolling stages, the finishing temperature of the finishing rolling is also important for controlling the microstructure of the steel sheet. If the finishing temperature of the finishing rolling is too low, the microstructure may become non-uniform, and the strength and / or hole-expanding properties may decrease. For this reason, the finishing temperature of the finishing rolling should be 900°C or higher. On the other hand, if the finishing temperature of the finishing rolling is too high, recrystallization will be promoted in the three rolling passes after the finishing rolling stage, making it impossible to sufficiently introduce dislocations. As a result, it will be impossible to achieve an average aspect ratio of 3.0 or higher for prior austenite grains in the region containing bainite and martensite in the final microstructure. For this reason, the finishing temperature of the finishing rolling should be 1000°C or lower.

[0065] [Intermediate air cooling process] In the subsequent intermediate air cooling process, the finish-rolled steel sheet is first cooled on a runout table (ROT) at an average cooling rate of 50-200°C / second to an intermediate air cooling temperature of 670-750°C, and then further air-cooled for 3-10 seconds. By first cooling to an intermediate air cooling temperature of 670-750°C at an average cooling rate of 50-200°C / second, excessive ferrite formation can be suppressed, while the subsequent high-temperature intermediate air cooling promotes the precipitation of TiC precipitates. As a result, the ferrite is sufficiently strengthened by precipitation, thereby reducing the hardness difference between ferrite and bainite and improving hole expansion properties and yield ratio. More specifically, in order to promote the formation of TiC precipitates and grain growth during intermediate air cooling and sufficiently strengthen the ferrite by precipitation, the intermediate air cooling temperature needs to be set in a relatively high temperature range, namely 670-750°C. However, in this case, excessive ferrite is formed, and the area ratio of ferrite in the final metal structure exceeds 60%, making it impossible to obtain the desired properties. Therefore, in this manufacturing method, the average cooling rate during primary cooling from finish rolling to the intermediate air cooling temperature is set to 50°C / second or higher to suppress excessive ferrite formation, and to ensure that TiC precipitates are sufficiently precipitated by the subsequent intermediate air cooling at a high temperature. On the other hand, if the average cooling rate of primary cooling exceeds 200°C, ferrite formation is excessively suppressed, and the area ratio of ferrite in the final metal structure becomes less than 30%, resulting in a decrease in properties such as hole expansionability. Therefore, the average cooling rate of primary cooling is set to 200°C / second or less, preferably 160°C / second or less.

[0066] If the intermediate air cooling temperature exceeds 750°C or the intermediate air cooling time exceeds 10 seconds, excessive ferrite formation or coarseness of TiC precipitates may occur. Excessive ferrite formation prevents the formation of the desired microstructure containing ferrite, bainite, and martensite in specific proportions in the final hot-rolled steel sheet. Furthermore, coarseness of TiC precipitates significantly reduces the number density of these precipitates, preventing the full hardness improvement effect of ferrite due to precipitation strengthening. On the other hand, if the intermediate air cooling temperature is below 670°C or the intermediate air cooling time is less than 3 seconds, TiC precipitate formation and grain growth are suppressed, preventing the acquisition of the desired diameter and / or number density. Similarly, in this case, the full hardness improvement effect of ferrite due to precipitation strengthening cannot be obtained. In addition, ferrite formation may be excessively suppressed, in which case the desired microstructure containing ferrite, bainite, and martensite in specific proportions in the final hot-rolled steel sheet cannot be formed.

[0067] In contrast, in the intermediate air cooling step, primary cooling is performed to an intermediate air cooling temperature of 670-750°C, preferably 690-750°C, at an average cooling rate of 50-200°C / second, preferably 50-160°C / second. Then, intermediate air cooling is performed for 3-10 seconds, preferably 4-9 seconds, to precipitate ferrite in the desired proportion and generate TiC precipitates within the ferrite. These precipitates are then appropriately grown to produce TiC precipitates with a diameter of 2.0-8.0 nm, resulting in a final product of 1.0 × 10⁻¹⁶ TiC precipitates. 16 pieces / cm 3 This makes it possible to create ferrite at the above number density. As a result, by fully utilizing the hardness-enhancing effect of ferrite through precipitation strengthening, it becomes possible to reduce the hardness difference between ferrite and bainite in the metal structure and significantly improve hole-expanding properties and yield ratio.

[0068] [Cooling process] After intermediate air cooling, the steel sheet is secondarily cooled in the next cooling step at an average cooling rate of 50-200°C / second, and then wound at a winding temperature of 20-200°C. By secondarily cooling the steel sheet after intermediate air cooling at such a relatively fast average cooling rate, bainite and martensite can be appropriately precipitated, making it possible to form a metal structure in the final hot-rolled steel sheet that contains ferrite, bainite, and martensite in specific proportions. In contrast, if the average cooling rate of the secondary cooling is less than 50°C / second, bainite and / or martensite cannot be appropriately precipitated, and therefore the desired metal structure cannot be obtained in the final hot-rolled steel sheet. In such cases, it becomes impossible to achieve a tensile strength of 780 MPa or more. On the other hand, if the average cooling rate of the secondary cooling exceeds 200°C, bainite will not be sufficiently formed and / or martensite will be excessively formed, similarly making it impossible to obtain the desired metal structure in the final hot-rolled steel sheet. Therefore, the average cooling rate of the secondary cooling should be 200°C / second or less, preferably 180°C / second or less.

[0069] On the other hand, if the winding temperature exceeds 200°C, a relatively large amount of cementite may precipitate. In such cases, the carbon in the steel is consumed in the formation of cementite. As a result, the formation of TiC precipitates is suppressed, and the hardness-enhancing effect of ferrite through precipitation strengthening using these TiC precipitates cannot be fully obtained. In addition, the strength-enhancing effect of the hot-rolled steel sheet may not be fully realized. On the other hand, if the winding temperature is too low, excessive water cooling or other measures will be required, reducing productivity. It may also cause embrittlement of the hot-rolled steel sheet. Therefore, the winding temperature should be 20°C or higher.

[0070] According to the hot-rolled steel sheet manufactured by the above manufacturing method, the area ratio of bainite, a hard layer in the metal structure, is controlled to a relatively high range of 30-60%. In addition, the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or higher, which allows for the utilization of dislocation strengthening, and as a result, the strength of the hot-rolled steel sheet can be significantly increased. Furthermore, 1.0 × 10⁻¹⁶ TiC precipitates with a diameter of 2.0-8.0 nm are present in the ferrite. 16 pieces / cm 3 Because of the number density of these particles, precipitation strengthening naturally contributes to improving the overall strength of the hot-rolled steel sheet. Furthermore, it is possible to sufficiently reduce the hardness difference between bainite, a relatively abundant hard phase in the metal structure, and ferrite, the softest of the three phases. As a result, it is possible to significantly improve the hole-expanding properties and yield ratio of the hot-rolled steel sheet. Therefore, hot-rolled steel sheets manufactured by the above manufacturing method can be effectively used in components where a balance between high strength and excellent workability, and where impact resistance is also required, are necessary, making them particularly useful in the automotive sector.

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples. [Examples]

[0072] In the following examples, hot-rolled steel sheets according to the embodiment of the present invention were manufactured under various conditions, and the tensile strength (TS), hole expansion ratio (λ), and yield ratio (YR) of the obtained hot-rolled steel sheets were investigated.

[0073] First, ingots with various chemical compositions shown in Table 1 were produced in a vacuum melting furnace. Then, after reheating to the heating temperature shown in Table 2, rough bars with a thickness of 30 mm were produced by rough rolling. These rough bars were held at a temperature range of 1200-1300°C for 3600 seconds. Then, using a rolling mill consisting of multiple rolling stands, finish rolling was performed under the conditions shown in Table 2, with at least two rolling passes in the first stage and three rolling passes in the second stage. The final temperatures of the finish rolling were as shown in Table 2. Next, the finish-rolled steel sheets were primary-cooled to the intermediate air-cooling temperature under the conditions shown in Table 2, followed by intermediate air cooling. Finally, the intermediate-air-cooled steel sheets were secondary-cooled to the winding temperature under the conditions shown in Table 2, and then wound at that winding temperature to obtain hot-rolled steel sheets with a thickness of 2.5 mm.

[0074] [Table 1]

[0075] [Table 2]

[0076] The properties of the obtained hot-rolled steel sheets were measured and evaluated by the following method.

[0077] [Calculation of diameter and number density of TiC precipitates] The diameter and number density of TiC precipitates were calculated using the three-dimensional atom probe measurement method detailed herein, with an instrument-specific atomic detection rate α of 0.35.

[0078] [Tensile strength (TS) and yield ratio (YR)] The tensile strength (TS) was measured by taking a JIS No. 5 test specimen, 200 mm in length and 2.5 mm in thickness, from the direction (C direction) where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test specimen, allowing strain to be introduced until fracture. The yield ratio (YR) was determined based on the tensile strength (TS) and 0.2% proof stress measured by performing a tensile test in accordance with JIS Z 2241:2011 using a JIS No. 5 test specimen, and was determined by the following formula. Yield ratio YR = 0.2% proof stress / Tensile strength TS

[0079] [Hole expansion ratio: λ] The hole expansion ratio was determined as follows. First, a test piece measuring 2.5 mm thick x 100 mm wide x 100 mm long was taken from a hot-rolled steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) was created using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole was expanded using a conical punch with a 60° apex angle until a crack penetrating the sheet thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0080] Hot-rolled steel sheets with a tensile strength (TS) of 780 MPa or higher, a hole expansion ratio (λ) of 60.0% or higher, and a yield ratio (YR) of 0.70 or higher were evaluated as having high strength, high hole expansion properties, and a high yield ratio. The results are shown in Table 3.

[0081] [Table 3]

[0082] Referring to Tables 1-3, in Comparative Example 18, the average cooling rate during the primary cooling up to the intermediate air cooling temperature was too fast, which excessively suppressed ferrite formation, resulting in a ferrite area ratio of less than 30% in the final microstructure. As a result, λ decreased. In Comparative Example 19, the total reduction ratio in the rolling pass preceding the finish rolling was low, which is thought to have suppressed recrystallization in the preceding rolling pass, preventing refinement of the microstructure. As a result, the desired microstructure was not obtained, and λ decreased. In Comparative Example 20, the total reduction ratio in the three rolling passes after the finish rolling was too high, which is thought to have promoted recrystallization in the subsequent rolling passes, preventing sufficient dislocation introduction. As a result, the average aspect ratio of prior austenite grains in the region containing bainite and martensite was less than 3.0, and TS decreased. In Comparative Example 21, the high winding temperature caused a relatively large amount of cementite to precipitate, and it is thought that the carbon in the steel was consumed in cementite formation. As a result, the formation of TiC precipitates is suppressed, and the number density of these TiC precipitates is reduced to 1.0 × 10⁻⁶. 16 pieces / cm 3 In Comparative Example 22, the hardness-enhancing effect of ferrite due to precipitation strengthening, and furthermore, the strength-enhancing effect of the hot-rolled steel sheet, were not sufficiently obtained, resulting in a decrease in TS, λ, and YR. In Comparative Example 22, the intermediate air cooling temperature was too high, resulting in excessive ferrite formation and further coarsening of TiC precipitates, which in turn reduced the number density of the TiC precipitates. As a result, TS, λ, and YR decreased. In Comparative Example 23, the high end temperature of the finish rolling likely promoted recrystallization in the last three rolling passes of the finish rolling, preventing sufficient dislocation introduction. As a result, the average aspect ratio of prior austenite grains in the region containing bainite and martensite was less than 3.0, resulting in a decrease in TS. In Comparative Example 24, the average cooling rate during the primary cooling up to the intermediate air cooling temperature was too slow, resulting in excessive ferrite formation, and the ferrite area ratio in the final metal structure exceeded 60%. As a result, TS decreased.

[0083] In Comparative Example 25, the average cooling rate during secondary cooling after intermediate air cooling was slow, resulting in a martensite area ratio of less than 5% and a decrease in TS. In Comparative Example 26, the low intermediate air cooling temperature suppressed the formation and grain growth of TiC precipitates, preventing the acquisition of the desired diameter of the TiC precipitates. As a result, the hardness improvement effect of ferrite due to precipitation strengthening was not fully obtained, and λ and YR decreased. In Comparative Example 27, the long intermediate air cooling time resulted in excessive ferrite formation and further coarsening of the TiC precipitates, which in turn reduced the number density of the TiC precipitates. As a result, TS, λ, and YR decreased. In Comparative Example 28, the high carbon content led to a decrease in λ and YR due to cementite formation. In Comparative Example 29, the low titanium content prevented the precipitation of TiC precipitates at a sufficient number density. As a result, TS, λ, and YR decreased. In Comparative Example 30, the low Si content prevented sufficient suppression of cementite precipitation, and it is believed that the carbon in the steel was consumed in cementite formation. As a result, the formation of TiC precipitates was suppressed, and the number density of these TiC precipitates was 1.0 × 10⁻⁶. 16 pieces / cm 3 The result was less than 10⁻¹⁰, and the hardness-improving effect of ferrite due to precipitation strengthening could not be fully obtained, resulting in a decrease in YR. In Comparative Example 31, because the Al content was low, the precipitation of cementite could not be sufficiently suppressed, and it is thought that the C in the steel was consumed in the formation of cementite. As a result, the formation of TiC precipitates was suppressed, and the number density of said TiC precipitates was 1.0 × 10⁻¹⁰. 16 pieces / cm 3 As a result, the hardness-enhancing effect of ferrite due to precipitation strengthening, and furthermore, the strength-enhancing effect of the hot-rolled steel sheet, could not be fully obtained, and TS and YR decreased.

[0084] In contrast, in all the examples of the invention, the hot-rolled steel sheets have a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, they contain, by area ratio, ferrite: 30-60%, bainite: 30-60%, and martensite: 5-20%, with 1.0 × 10 TiC precipitates with a diameter of 2.0-8.0 nm in the ferrite. 16 pieces / cm 3 We were able to obtain a hot-rolled steel sheet having a metallic structure in which the above number density is present and the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or higher. As a result, due to a relatively high bainite fraction and precipitation strengthening due to dislocation strengthening and TiC precipitates, we were able to achieve high strength of 780 MPa or higher while reducing the hardness difference between ferrite and bainite, thereby improving hole expansion properties and yield ratio. Furthermore, in the hot-rolled steel sheets according to Invention Examples 2 to 17, in which the ratio of the average nanohardness of ferrite to the average nanohardness of bainite was controlled within the range of 0.75 to 1.20, we were able to achieve a hole expansion rate of 65.0% or higher and a yield ratio of 0.75 or higher, thus further significantly improving the hole expansion properties and yield ratio of the hot-rolled steel sheet. In addition, when residual structures were present in the Invention Examples, these residual structures were at least one of pearlite and retained austenite.

Claims

1. In mass percent, C: 0.03 to 0.10%, Si: 0.010-0.100%, Mn: 0.50-3.00%, Ti: 0.05-0.20%, Al: 0.20-0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0-1.000%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.000%, B: 0 to 0.0100%, Sn: 0-1.000%, Sb: 0 to 1.000%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.010%, REM: 0-0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0-2.000%, Zn: 0 to 0.010%, W: 0-1,000%, and The remainder has a chemical composition consisting of Fe and impurities. In area percentage, Ferrite: 30-60%, Bainite: 30-60%, and Martensite: Contains 5-20%, TiC precipitates with a diameter of 2.0–8.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻⁶. 16 pieces / cm 3 They exist at the above number density, A hot-rolled steel sheet characterized by having a metallic structure in which the average aspect ratio of prior austenite grains in the region containing bainite and martensite is 3.0 or greater.

2. The aforementioned chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001-1.000%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, REM: 0.0001-0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.000%, Zn: 0.001 to 0.010%, and W: 0.001-1.000% The hot-rolled steel sheet according to claim 1, characterized in that it includes at least one of the following.

3. The hot-rolled steel sheet according to claim 1 or 2, characterized in that the ratio of the average nanohardness of ferrite to the average nanohardness of bainite is 0.75 to 1.20.

Images