Hot-rolled steel sheet

Abstract

Provided is a hot-rolled steel plate characterized by having a predetermined chemical composition, by containing, in terms of area ratio, 60%-80% of ferrite, 15%-30% of bainite, and 3%-10% of martensite, by having a microstructure in which TiC precipitates with a diameter of 1.0-5.0 nm are present in the ferrite at a number density of 1.0 × 1016 to 100.0 × 1016 precipitates / cm3, and by having a tensile strength of 780 MPa or more.

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 plates used is one effective method, and for this reason, the development of high-strength steel plates is progressing.

[0003] On the other hand, many automotive components are manufactured by press forming. Generally, it is known that as strength increases, the formability of steel sheets decreases, and properties such as ductility decline.

[0004] In this regard, for example, Patent Document 1 describes a high-strength thin steel sheet characterized by a steel structure consisting of a ferrite phase and a martensite phase, wherein carbonitrides are precipitated in the ferrite phase by interphase interface precipitation, and the interplanar spacing of the precipitated surfaces of the interphase interface precipitation in a region of 40% or more of the ferrite phase is 20 nm to 60 nm. Furthermore, Patent Document 1 teaches that sufficient strength can be ensured by precipitation strengthening due to precipitates in the ferrite phase, and in addition, by mixed structure formation with the martensite phase, it is possible to obtain high ductility characteristic of mixed structures while ensuring high fatigue properties. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-225935 [Overview of the project] [Problems that the invention aims to solve]

[0006] As described above, with the increase in strength, the formability of the steel sheet decreases, and it is known that, in addition to ductility, properties such as hole expansion property also decrease. When the hole expansion property decreases, for example, it may not be possible to form the desired member shape in automotive underbody members or the like. Therefore, in the development of high-strength steel sheets, it is important to increase the strength while ensuring properties according to the intended use to a certain level or more. Further, high-strength steel sheets are manufactured by subjecting cast slabs to hot rolling, and it is known that there may be anisotropy in strength between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular thereto in relation to the hot rolling. When the anisotropy in strength increases, generally the workability of the steel sheet deteriorates, which becomes a problem.

[0007] The present invention has been made in view of the above, and an object thereof is to provide a hot-rolled steel sheet having a novel configuration, which has improved hole expansion property and reduced anisotropy in strength despite being high-strength.

Means for Solving the Problems

[0008] In order to achieve the above object, the present inventors focused particularly on the microstructure of the hot-rolled steel sheet and conducted studies. As a result, the present inventors have found that by configuring the microstructure of a hot-rolled steel sheet having a predetermined chemical composition from a three-phase structure containing ferrite, bainite, and martensite in specific ratios, it is possible to achieve a high strength with a tensile strength of 780 MPa or more while improving the anisotropy in strength. Furthermore, by causing TiC precipitates having an appropriate diameter to be present in ferrite at a predetermined number density, the ferrite is precipitation-strengthened, thereby reducing the hardness difference of the three-phase structure and improving the hole expansion property, and completed the present invention.

[0009] The present invention that has achieved the above object is as follows. (1) In mass%, C: 0.010 to 0.100%, Si: 0.01 to 0.10%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, Nb: 0.010 - 0.020%, Al: 0.100 - 1.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Ni: 0 - 2.000%, Mo: 0 - 1.000%, Cr: 0 - 2.000%, B: 0 - 0.0100%, Co: 0 - 2.000%, V: 0 - 1.000%, Cu: 0 - 2.000%, W: 0 - 1.0000%, Ta: 0 - 1.0000%, Sn: 0 - 1.0000%, Sb: 0 - 1.0000%, As: 0 - 0.0100%, Mg: 0 - 0.0100%, Ca: 0 - 0.0100%, Zr: 0 - 0.0100%, Hf: 0 - 0.0100%, Bi: 0 - 0.0100%, REM: 0 - 0.0100%, and the balance: having a chemical composition consisting of Fe and impurities, in terms of area ratio, ferrite: 60 - 80%, bainite: 15 - 30%, and martensite: containing 3 - 10%, in the ferrite, TiC precipitates with a diameter of 1.0 - 5.0 nm are present at a number density of 1.0×10 , , , , , 16 , , , , , , 3 , , , , 16 , ~100.0×10 16 per cm 3 and having a microstructure, a hot-rolled steel sheet characterized by having a tensile strength of 780 MPa or more. (2) The chemical composition is, in mass%, Ni: 0.001 - 2.000%, Mo: 0.001~1.000%, Cr: 0.001~2.000%, B: 0.0001~0.0100%, Co: 0.001~2.000%, V: 0.001~1.000%, Cu: 0.001~2.000%, W: 0.0001~1.0000%, Ta: 0.0001~1.0000%, Sn: 0.0001~1.0000%, Sb: 0.0001~1.0000%, As: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Ca: 0.0001~0.0100%, Zr: 0.0001~0.0100%, Hf: 0.0001~0.0100%, Bi: 0.0001~0.0100%, and REM: 0.0001~0.0100% The hot-rolled steel sheet according to (1) above, characterized in that it includes at least one of the above. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a hot-rolled steel sheet that has high strength, yet has improved hole-expandability and reduced strength anisotropy. [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: 0.010~0.100%, Si: 0.01~0.10%, Mn: 0.50~3.00%, Ti: 0.050~0.200%, Nb: 0.010~0.020%, Al: 0.100~1.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Ni: 0~2,000%, Mo: 0~1.000%, Cr: 0~2,000%, B: 0~0.0100%, Co: 0~2.000%, V: 0~1.000%, Cu: 0~2.000%, W: 0~1.0000%, Ta: 0~1.0000%, Sn: 0~1.0000%, Sb: 0~1.0000%, As: 0~0.0100%, Mg: 0~0.0100%, Ca: 0~0.0100%, Zr: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM: 0~0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. In terms of area ratio, Ferrite: 60-80%, Bainite: 15-30%, and Martensite: Contains 3-10%, TiC precipitates with a diameter of 1.0 to 5.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻¹⁶. 16 ~100.0 × 10 16 pieces / cm 3 It has a microstructure that exists at a number density of, It is characterized by having a tensile strength of 780 MPa or higher.

[0012] As described above, with the increase in the strength of the steel sheet, characteristics such as the hole expansion property deteriorate, and it is known that there may be anisotropy in strength between the strength in the rolling direction (L direction) related to hot rolling during steel sheet manufacturing and the strength in the width direction (C direction) perpendicular thereto. More specifically regarding the strength anisotropy, due to the non-isotropic microstructure obtained by hot rolling during steel sheet manufacturing, there is a tendency for the tensile strength to be different between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto. Generally, there is a tendency for the strength anisotropy to show that the tensile strength in the L direction of the hot rolled steel sheet is lower than the tensile strength in the C direction. Although the workability of high-strength steel sheets used in applications such as automobiles can be greatly improved by improving the hole expansion property and reducing such strength anisotropy, it is generally very difficult to achieve both the increase in the strength of the steel sheet and the improvement of these characteristics. Therefore, in addition to making the chemical composition of the hot rolled steel sheet appropriate, the present inventors focused particularly on the microstructure of the hot rolled steel sheet and conducted studies. First, the present inventors constituted the microstructure of a hot rolled steel sheet having a predetermined chemical composition by a three-phase structure containing ferrite, bainite, and martensite in specific ratios, more specifically, in terms of area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%. As a result, while achieving a high strength with a tensile strength of 780 MPa or more, it was found that the strength anisotropy could be significantly reduced compared to the case of a DP steel (dual-phase steel) mainly composed of soft ferrite and hard martensite. Furthermore, in the case of a three-phase structure composed of ferrite, bainite, and martensite, since the hardness difference between each phase is relatively large and there is a possibility that the hole expansion property may deteriorate due to such a hardness difference, the present inventors examined the improvement of the hole expansion property from the perspective of reducing the hardness difference in the three-phase structure. As a result, the present inventors strengthened the precipitation of the softest ferrite in the three-phase structure, more specifically, TiC precipitates with a diameter of 1.0 to 5.0 nm were precipitated in ferrite at 1.0×10 16 ~100.0×10 16 per cm 3We discovered that by precipitation strengthening of ferrite through the presence of TiC precipitates at a specific number density, the hardness difference in the three-phase structure can be reduced. As a result, we found that even though the microstructure was constructed using a three-phase structure, which tends to have a relatively large hardness difference, in order to reduce the anisotropy of strength, the hardness of the ferrite can be increased by precipitation strengthening using TiC precipitates of a specific diameter and number density, thereby simultaneously achieving a reduction in anisotropy of strength and an improvement in hole-expanding properties.

[0013] While not intended to be bound by any particular theory, in the case of the three-phase structure described above, the dispersion of bainite and martensite, which are different hard structures, within the soft ferrite is thought to result in a more isotropic structure compared to the case of a two-phase structure such as DP steel. Furthermore, since the hot-rolled steel sheet according to the embodiment of the present invention contains Nb, it is thought that this forms carbides, nitrides and / or carbonitrides in the steel, and that their pinning effect promotes ferrite transformation and refines the ferrite grains. The refinement of the ferrite grains that constitute the main phase of the three-phase structure is thought to contribute not only to improving the overall strength of the hot-rolled steel sheet but also to reducing the hardness difference of the three-phase structure consisting of ferrite, bainite and martensite. As a result, according to the embodiment of the present invention, it is possible to achieve improved hole-expandability and reduced strength anisotropy despite having a high strength of 780 MPa or more. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both high strength and excellent workability, which are conflicting properties, and is therefore particularly useful in the automotive sector where both of these properties are required.

[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:0.010~0.100%] Carbon (C) is an effective element for increasing the strength of steel sheets. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in ferrite, contributing to ferrite precipitation strengthening based on these precipitates, as well as grain refinement of ferrite due to the pinning effect of these precipitates. To fully obtain these effects, the C content should be 0.010% or higher. The C content may also be 0.012% or higher, 0.015% or higher, 0.018% or higher, 0.020% or higher, or 0.022% or higher. On the other hand, excessive C content may reduce elongation. Also, the desired three-phase structure may not be obtained, and the anisotropy of strength may not be sufficiently reduced. Therefore, the C content should be 0.100% or lower. The C 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.

[0016] [Si: 0.01~0.10%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect sufficiently, the Si content should be 0.01% or more. The Si content may be 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, excessive Si content may cause a surface quality defect called Si scale. In addition, Si scale may increase the surface roughness of the hot-rolled steel sheet, and excessive Si content may increase the ferrite content, making it impossible to obtain the desired three-phase structure. As a result, the anisotropy of strength in the tensile strength in the L direction and C direction may become significant. Therefore, the Si content should be 0.10% or less. The Si content may be 0.09% or less, 0.08% or less, 0.07% or less, or 0.06% or less.

[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 0.80% 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, the diffusion coefficient of C decreases, which reduces the diameter of TiC precipitates, and the hardness improvement effect of ferrite due to precipitation strengthening based on TiC precipitates may not be fully obtained. Therefore, the Mn content should be 3.00% or less. The Mn content may be 2.70% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0018] [Ti: 0.050~0.200%] Ti has the effect of increasing the hardness of ferrite by forming TiC precipitates, which are carbides, within the ferrite through precipitation strengthening. To obtain this effect sufficiently, the Ti content should be 0.050% or more. The Ti content may be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if the Ti content is excessive, the TiC precipitates will become coarse, and the desired precipitation strengthening in the ferrite may not be obtained. In addition, as the TiC precipitates become coarser, the number density of the TiC precipitates also decreases, and in this case, the hardness of the ferrite cannot be sufficiently increased through precipitation strengthening. Therefore, the Ti content should be 0.200% or less. The Ti content may be 0.190% or less, 0.180% or less, 0.160% or less, or 0.140% or less.

[0019] [Nb:0.010~0.020%] Nb is an element that contributes to microstructure refinement through a pinning effect by forming carbides, nitrides, and / or carbonitrides in steel. This pinning effect suppresses the coarsening of austenite grains, promotes ferrite transformation, and refines ferrite grains. Refinement of ferrite grains contributes not only to increasing the strength of the steel sheet but also to reducing the hardness difference in the three-phase structure. If the Nb content is low, these effects may not be fully achieved. Therefore, the Nb content should be 0.010% or higher. The Nb content may also be 0.012% or higher, 0.013% or higher, or 0.015% or higher. On the other hand, excessive Nb content may lead to the formation of coarse carbides in the steel, reducing the strength of the steel sheet. In addition, the formation of coarse carbides may prevent the pinning effect from being fully realized, making it impossible to obtain the desired three-phase structure. Therefore, the Nb content should be 0.020% or lower. The Nb content may also be 0.018% or lower, or 0.016% or lower.

[0020] [Al: 0.100~1.000%] Al is an element that acts as a deoxidizing agent. If the Al content is too low, this effect may not be sufficiently obtained, and / or the desired three-phase structure may not be achieved. Therefore, the Al content should be 0.100% or more. The Al content may also be 0.120% or more, 0.150% or more, or 0.020% or more. On the other hand, if the Al content is excessive, coarse oxides may be formed, reducing the strength and / or the desired three-phase structure may not be achieved. Therefore, the Al content should be 1.000% or less. The Al content may also be 0.800% or less, 0.600% or less, or 0.400% or less.

[0021] [P:0.1000% or less] Excessive phosphorus (P) content can negatively affect weldability and other properties. Therefore, the P content should be 0.1000% or less. The P content may also be 0.0800% or less, 0.0500% or less, 0.0300% or less, or 0.0250% 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, 0.0010% or more, or 0.0050% or more.

[0022] [S:0.0100% or less] Excessive sulfur content can lead to the formation of large amounts of manganese sulfur (MnS), which can reduce toughness. Therefore, the Si content should be 0.0100% or less. The S content may be 0.0080% or less, 0.0060% or less, or 0.0050% 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, or 0.0005% or more.

[0023] [N:0.0100% or less] Excessive nitrogen (N) content can form coarse nitrides, reducing toughness. Therefore, the N content should be 0.0100% or less. The N content may also be 0.0080% or less, 0.0060% or less, or 0.0050% 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, or 0.0005% or more.

[0024] [O:0.0100% 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 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% 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.

[0025] 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 optionally contain at least one of the following optional elements in place of a portion of the remaining Fe. For example, the hot-rolled steel sheet may contain at least one of the following: Ni: 0-2,000%, Mo: 0-1,000%, Cr: 0-2,000%, B: 0-0.0100%, Co: 0-2,000%, V: 0-1,000%, Cu: 0-2,000%, W: 0-1,0000%, and Ta: 0-1,0000%. Alternatively, the hot-rolled steel sheet may also contain at least one of the following: Sn: 0-1,0000%, Sb: 0-1,0000%, and As: 0-0.0100%. Furthermore, hot-rolled steel sheets may contain at least one of the following: Mg: 0-0.0100%, Ca: 0-0.0100%, Zr: 0-0.0100%, and Hf: 0-0.0100%. Hot-rolled steel sheets may also contain Bi: 0-0.0100%. Furthermore, hot-rolled steel sheets may contain REM: 0-0.0100%. These optional elements will be explained in detail below.

[0026] [Ni: 0~2.000%] Ni is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. The Ni content may be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more, and may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the Ni content is excessive, the effect will saturate, and there is a risk of increased manufacturing costs. Therefore, the Ni content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[0027] [Mo: 0~1.000%] Mo is an element that enhances the hardenability of steel, contributes to improved strength, and also contributes to improved corrosion resistance. 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 increase. 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.

[0028] [Cr: 0~2.000%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the Cr content is excessive, the effect will saturate, and there is a risk of increased manufacturing costs. Therefore, the Cr content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[0029] [B: 0~0.0100%] B is an element that enhances the hardenability of steel and contributes to improving its strength. The B content may be 0%, but to obtain such effects, it is preferable that the B content be 0.0001% or more. The B content may 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 toughness and / or weldability may decrease. Therefore, it is preferable that the B content be 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0030] [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, 0.020% or more, or 0.050% 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 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[0031] [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, a large amount of precipitate may be formed, which may reduce toughness. Therefore, it is preferable that the V content be 1.000% or less. The V content may be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.

[0032] [Cu: 0~2.000%] Cu is an element that contributes to improving strength and / or corrosion resistance. The Cu content may be 0%, but to obtain these effects, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Cu content may lead to deterioration of toughness and weldability. Therefore, it is preferable that the Cu content be 2.000% or less. The Cu content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[0033] [W: 0~1.0000%] 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 0.0001% or more to obtain such effects. The W content may also be 0.0010% or more, 0.0020% or more, or 0.0050% or more. On the other hand, excessive W content may reduce weldability. Therefore, it is preferable that the W content be 1.0000% or less. The W content may also be 0.8000% or less, 0.5000% or less, 0.2000% or less, 0.1000% or less, or 0.0500% or less.

[0034] [Ta:0~1.0000%] Ta (Ta) is an effective element for controlling the morphology of carbides and improving the strength of steel sheets. While the Ta content may be 0%, it is preferable that the Ta content be 0.0001% or higher to obtain these effects. The Ta content may also be 0.0010% or higher, 0.0020% or higher, or 0.0050% or lower. On the other hand, if the Ta content is excessive, the effect will saturate, and including more Ta than necessary in the steel sheet will lead to increased manufacturing costs. For this reason, it is preferable that the Ta content be 1.0000% or lower. The Ta content may also be 0.8000% or lower, 0.5000% or lower, 0.2000% or lower, 0.1000% or lower, or 0.0500% or lower.

[0035] [Sn: 0~1.0000%] [Sb: 0~1.0000%] 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 Sn and Sb content be 0.0001% or more, and may be 0.0010% or more, 0.0020% or more, or 0.0050% or more. On the other hand, excessive Sn and Sb content may lead to a decrease in toughness. Therefore, it is preferable that the Sn and Sb content be 1.0000% or less, and may be 0.8000% or less, 0.5000% or less, 0.3000% or less, 0.1000% or less, or 0.0500% or less.

[0036] [As:0~0.0100%] As is an element effective in improving corrosion resistance. While the As content may be 0%, to obtain such an effect, the As content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% 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, the As content is preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0037] [Mg: 0~0.0100%] [Ca: 0~0.0100%] [Zr:0~0.0100%] [Hf: 0~0.0100%] Mg, Ca, Zr, and Hf are elements that can control the form of sulfides. The content of Mg, Ca, Zr, and Hf may be 0%, but to obtain such an effect, it is preferable that the content of each of these elements be 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% 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 content of Mg, Ca, Zr, and Hf is 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.002 each is preferable.

[0038] [Bi:0~0.0100%] Bi is an element effective in improving corrosion resistance. While the Bi content may be 0%, to obtain such an effect, it is preferable that the Bi content be 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. 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 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0039] [REM:0~0.0100%] REM is an element that can control the form of sulfides. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% 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, the REM content is preferably 0.0100% or less, and 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 lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[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] [Microorganisms] The microstructure of the hot-rolled steel sheet according to the embodiment of the present invention consists of ferrite: 60-80%, bainite: 15-30%, and martensite: 3-10% by area ratio. By structuring the microstructure of the hot-rolled steel sheet with such a three-phase structure, it is possible to significantly reduce the anisotropy of strength in the tensile strength in the L and C directions of the hot-rolled steel sheet compared to, for example, DP steel which is mainly composed of soft ferrite and hard martensite. In addition, by including these three structures in the specific area ratios described above, it is possible to appropriately reduce the hardness difference of each phase, which will be explained in detail later in relation to hole expansion properties, while maintaining a high tensile strength of 780 MPa or more. For example, if the area ratio of ferrite is small, the proportion of the hard phases bainite and martensite will be high, and even with ferrite precipitation strengthening, which will be explained in detail later, it will not be possible to appropriately reduce the hardness difference of each phase, more specifically the hardness difference between ferrite and martensite, and it may not be possible to achieve the desired hole expansion properties. Therefore, the ferrite area ratio should be 60% or more, for example, 62% or more, 65% or more, or 68% or more. On the other hand, if the ferrite area ratio is high, the proportion of the hard phases, bainite and martensite, decreases, and as a result, it may not be possible to achieve a tensile strength of 780 MPa or more. Therefore, the ferrite area ratio should be 80% or less, for example, 78% or less, 75% or less, or 72% 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 18% or more, 20% or more, or 22% or more. Similarly, the area ratio of martensite may be more than 3%, 3.1% or more, 3.2% or more, 3.3% or more, 3.5% or more, 3.8% or more, 4% or more, 4.5% or more, 5% or more, or 6% or more. On the other hand, from the viewpoint of reducing the hardness difference between each phase and improving hole expansion properties, a lower area ratio of bainite and martensite is preferable. From this viewpoint, for example, the area ratio of bainite may be 28% or less, 26% or less, or 24% or less. Similarly, the area ratio of martensite may be 9% or less, 8% or less, or 7% or less.

[0044] [Remaining tissue] As described above, the microstructure of the hot-rolled steel sheet according to the embodiment of the present invention consists of ferrite, bainite, and martensite, and does not contain or substantially contains any other microstructure (remaining microstructure). "Substantially contained" means that the area ratio of the remaining microstructure other than ferrite, bainite, and martensite is 3% or less. Therefore, the area ratio of the remaining microstructure is 0 to 3%, and may be, for example, 0 to 1.5%, 0 to 1%, or 0 to 0.5%. In other words, the total area ratio of ferrite, bainite, and martensite is 97 to 100%, and may be, for example, 98.5 to 100%, 99 to 100%, or 99.5 to 100%. If the remaining microstructure is present, it may be, for example, pearlite.

[0045] [Identification of microstructure and calculation of area ratio] Microstructure observation is performed using a scanning electron microscope. Prior to observation, the sample for microstructure observation is polished using wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 μm to a mirror finish on the observation surface, and then the microstructure is etched with a 3% nitric acid alcohol solution. The observation magnification is set to 3000x, 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 proportion of the microstructure is determined by the point counting method. For the obtained microstructure images, a total of 100 grid points are determined at intervals of 3 μm vertically and 4 μm horizontally, the microstructure present beneath the grid points is identified, and the proportion of the microstructure contained in the steel is determined from the average value of the 10 images. Ferrite is defined as a massive crystalline grain that does not contain iron-based carbides with a major axis of 100 nm or more. Bainite is an aggregate of lath-like crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, and these carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction means that the difference in the elongation direction of the iron-based carbide group is within 5°. Bainite is counted as one bainite grain when surrounded by grain boundaries with an orientation difference of 15° or more. In addition, martensite, which contains a large amount of solid-solution carbon, has less corrosion loss during etching compared to other structures, and its height in the observation field after etching is relatively higher than that of other structures. For this reason, it appears relatively whiter than other structures, which allows martensite to be distinguished from other structures. When structures other than ferrite, bainite, and martensite are present, the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite, bainite, and martensite from 100%. While it is not necessary to specifically identify the remaining tissue, if the remaining tissue contains perlite, it can be identified by scanning electron microscopy because perlite has a distinctive structure in which cementite precipitates in a lamellar pattern.

[0046] [Number density of TiC precipitates with a diameter of 1.0-5.0 nm in ferrite: 1.0 × 10 16 ~100.0 × 10 16 pieces / cm 3 ] In the hot-rolled steel sheet according to an embodiment of the present invention, the ferrite contains TiC precipitates with a diameter of 1.0 to 5.0 nm, totaling 1.0 × 10⁻¹⁴ 16 ~100.0 × 10 16 pieces / cm 3 TiC precipitates exist at a number density of 1.0 to 5.0 nm in diameter. By introducing TiC precipitates with a diameter of 1.0 to 5.0 nm into 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 the hardest martensite, the hardness difference between each phase in the three-phase structure composed of ferrite, bainite, and martensite can be reduced, and as a result, the hole-expanding properties of hot-rolled steel sheets can be significantly improved. If the diameter of the TiC precipitates is smaller than 1.0 nm, the TiC precipitates cannot adequately act as obstacles to dislocation movement, and therefore the effect of increasing the hardness of the ferrite through precipitation strengthening cannot be fully obtained. 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 1.0 to 5.0 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 1.0 to 5.0 nm are placed in the ferrite as described above, at a density of 1.0 × 10⁻⁶ 16 pieces / cm 3It 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, 2.0 × 10⁻⁶. 16 pieces / cm 3 The above is 5.0 x 10 16 pieces / cm 3 The above is 10.0 × 10 16 pieces / cm 3 or more or 20.0 × 10 16 pieces / cm 3 The above is also acceptable. On the other hand, if the number density becomes too high, it becomes difficult to control the diameter of the TiC precipitates within the desired range. Therefore, the number density should be 100.0 × 10⁻⁶. 16 pieces / cm 3 The following applies, for example, 80.0 × 10 16 pieces / cm 3 The following or 50.0 × 10 16 pieces / cm 3 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 1.0 to 5.0 nm is 1.0 × 10⁻⁶. 16 ~100.0 × 10 16 pieces / cm 3 It is sufficient that they exist in the ferrite at a number density such that, and therefore, as long as the above-mentioned diameter and number density requirements 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] [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.

[0050] [Mechanical properties] [Tensile strength: TS] According to the hot-rolled steel sheet having the above chemical composition and microstructure, a high tensile strength, specifically a tensile strength of 780 MPa or higher, can be achieved. The tensile strength is preferably 850 MPa or higher, 900 MPa or higher, or 980 MPa or higher. According to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of chemical composition and microstructure described above makes it possible to achieve both improved hole-expandability and reduced strength anisotropy. 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 1470 MPa or lower, 1250 MPa or lower, 1180 MPa or lower, or 1080 MPa or lower. The tensile strength is 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.

[0051] [Total growth: EL] According to the hot-rolled steel sheet having the above chemical composition and microstructure, in addition to high tensile strength, it is also possible to improve total elongation, more specifically achieving a total elongation of 16.0% or more. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more. There is no particular upper limit, but for example, the total elongation may be 30.0% or less or 25.0% or less. The total elongation is 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.

[0052] (Hole expansion ratio: λ) According to the hot-rolled steel sheet having the above chemical composition and microstructure, high hole expansion properties, specifically a hole expansion ratio of 50% or more, can be achieved. The hole expansion ratio may preferably be 55% or more, 60% or more, or 65% 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 circular hole with a diameter of 10 mm (initial hole: hole diameter d0 = 10 mm) is punched into the test piece under conditions where the clearance is 12.5%, with the burr facing the die side, and the initial hole is expanded with a conical punch with a vertex angle of 60° until a crack that penetrates the thickness of the sheet occurs, and the hole diameter d1 mm 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 5 times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0053] <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.

[0054] 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 a hot-rolled steel sheet, comprising heating a slab having the chemical composition described above to a temperature of 1100 to 1300°C, and then performing finish rolling, wherein the finishing temperature of the finish rolling is 900 to 1000°C. An intermediate cooling process in which the finish-rolled steel sheet is cooled to an intermediate air-cooling temperature of 620-700°C at an average cooling rate of 10°C / second or more, and then air-cooled for 5-10 seconds at an average cooling rate of 2°C / second or more and less than 10°C / second, and The cooling process involves first cooling the intercooled steel sheet at an average cooling rate of 10-20°C / second for 1-3 seconds, followed by second cooling to below 200°C at an average cooling rate of 25°C / second or higher, and then winding it up. It is characterized by including [specific features]. The following describes each process in detail.

[0055] [Hot rolling process] [Slab heating] First, a slab having the chemical composition described above in relation to hot-rolled steel sheets is heated. From the viewpoint of productivity, 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 in order to obtain a high-strength steel sheet. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid-solve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements will not sufficiently solid-solve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, it is preferable that the heating temperature be 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferable that it be 1300°C or lower.

[0056] [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.

[0057] [Finishing Rolling] The heated slab, or slab that has been roughly rolled as needed, is then subjected to finish rolling. As the slabs used as described above contain a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature. In particular, the finishing temperature of the finish rolling is important in terms of controlling the microstructure of the steel sheet. If the finishing temperature of the finish rolling is low, recrystallization is suppressed, the microstructure becomes non-uniform, and the strength and / or hole-expanding properties may decrease. For this reason, the finishing temperature of the finish rolling should be 900°C or higher. On the other hand, if the finishing temperature of the finish rolling is high, the austenite coarsens and the proportion of ferrite decreases, and the desired three-phase microstructure cannot be obtained. For this reason, the finishing temperature of the finish rolling should be 1000°C or lower.

[0058] [Intermediate cooling process] In the next intermediate cooling process, the finish-rolled steel sheet is cooled to an intermediate air-cooling temperature of 620-700°C at an average cooling rate of 10°C / second or more, and then air-cooled for 5-10 seconds at an average cooling rate of 2°C / second or more and less than 10°C / second. By cooling to the intermediate air-cooling temperature of 620-700°C at an average cooling rate of 10°C / second or more, ferrite can be precipitated in the desired proportion, and TiC precipitates with the desired diameter can be formed within the ferrite. On the other hand, if the intermediate air-cooling temperature is above 700°C or the average cooling rate to the intermediate air-cooling temperature is less than 10°C / second, the ferrite transformation proceeds too much, and it becomes impossible to form a three-phase structure containing ferrite, bainite, and martensite in specific proportions in the final hot-rolled steel sheet. In addition, if the intermediate air-cooling temperature is above 700°C, a relatively large amount of coarse TiC precipitates may precipitate, and the desired precipitation strengthening in the ferrite may not be obtained. The average cooling rate up to the intermediate air cooling temperature of 620-700°C is preferably 20°C / second or higher. There is no particular upper limit, but for example, the average cooling rate may be 30°C / second or lower. On the other hand, if the intermediate air cooling temperature is below 600°C, ferrite cannot be sufficiently precipitated, and similarly, it becomes impossible to form a three-phase structure containing ferrite, bainite, and martensite in specific proportions in the final hot-rolled steel sheet. In addition, if the intermediate air cooling temperature is below 600°C, TiC precipitates with the desired diameter cannot be formed at a sufficient number density even with subsequent air cooling, and as a result, the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained.

[0059] Similarly, the average cooling rate and time during air cooling after cooling to an intermediate air cooling temperature of 620-700°C are important for precipitating ferrite in the desired proportion and forming TiC precipitates with the desired diameter within the ferrite. First, when the temperature range for ferrite transformation is in the relatively high temperature range of 620-700°C, it is possible to precipitate ferrite in the desired proportion and to promote grain growth of the TiC precipitates that precipitate within the ferrite. This is advantageous because it increases the amount of ferrite precipitation strengthening by allowing the TiC precipitates to grow to a certain extent. However, even at the intermediate air cooling temperature of 620-700°C, if the average cooling rate of air cooling is less than 2°C / second, a relatively large amount of coarse TiC precipitates will precipitate, making it impossible to obtain the desired precipitation strengthening in the ferrite. On the other hand, if forced cooling such as water cooling is performed instead of air cooling, it is not possible to form TiC precipitates with the desired diameter at a sufficient number density, and as a result, the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. Furthermore, if the air cooling time is less than 5 seconds, sufficient ferrite cannot be precipitated, and the resulting hot-rolled steel sheet will not be able to form a three-phase structure containing ferrite, bainite, and martensite in specific proportions. On the other hand, if the air cooling time exceeds 10 seconds, excessive ferrite precipitates, reducing the proportion of the hard phase, resulting in the inability to obtain the desired three-phase structure, leading to significant anisotropy in strength and the inability to achieve the desired tensile strength. In contrast, by cooling the steel sheet to an intermediate air cooling temperature of 620-700°C and then air-cooling it for 5-10 seconds at an average cooling rate of 2°C / second or more, ferrite can be precipitated in the desired proportion, TiC can be precipitated within the ferrite, and the resulting TiC precipitates can be appropriately grown to produce TiC precipitates with a diameter of 1.0-5.0 nm, resulting in a final product of 1.0 × 10⁻⁶ TiC precipitates. 16 ~100.0 × 10 16 pieces / cm 3This makes it possible to have them present at a number density. As a result, the hardness of the ferrite can be increased by precipitation strengthening, reducing the hardness difference in the three-phase structure and significantly improving hole-expanding properties. In addition, the above air-cooling control is an extremely important operation not only for precipitation strengthening of ferrite by TiC precipitates, but also for precipitating Nb carbides, nitrides and / or carbonitrides in the ferrite and promoting grain growth, and for fully utilizing the pinning effect of such precipitates to refine the ferrite grains and, consequently, achieve high strength in the hot-rolled steel sheet.

[0060] [Cooling process] After intermediate cooling, the steel sheet is first cooled for 1 to 3 seconds at an average cooling rate of 10 to 20°C / second in the next cooling process, and then secondarily cooled to below 200°C at an average cooling rate of 25°C / second or more before being rolled up. This two-stage cooling process allows for the appropriate precipitation of bainite during the first cooling at a relatively slow average cooling rate, and similarly, the appropriate precipitation of martensite during the second cooling at a relatively fast average cooling rate. As a result, the final hot-rolled steel sheet can form a three-phase structure containing ferrite, bainite, and martensite in specific proportions. In contrast, if the average cooling rate of the first cooling is less than 10°C / second, or if the average cooling rate is greater than 20°C / second, effectively eliminating the first cooling and resulting in a single-stage cooling process consisting only of second cooling, bainite cannot be appropriately precipitated. Therefore, the final hot-rolled steel sheet cannot form a three-phase structure consisting of ferrite, bainite, and martensite. In such cases, the tensile strength in the L direction of the hot-rolled steel sheet tends to be significantly lower than the tensile strength in the C direction, meaning that the anisotropy of strength between the L and C directions becomes pronounced. Preferably, the average cooling rate of the primary cooling is 12 to 18°C / second.

[0061] On the other hand, if the average cooling rate of secondary cooling is less than 25°C / second or the winding temperature is greater than 200°C, it may not be possible to precipitate martensite in the desired amount and / or excessive bainite precipitation may occur, making it impossible to obtain the desired three-phase structure. Preferably, the average cooling rate of secondary cooling is 27°C / second or higher. There is no particular upper limit to the average cooling rate of secondary cooling, but for example, it may be 50°C / second or less or 40°C / second or less. Similarly, there is no particular lower limit to the winding temperature, but if the winding temperature is too low, excessive water cooling or the like will be required, reducing productivity. Therefore, it is preferable that the winding temperature be, for example, 100°C or higher.

[0062] According to the hot-rolled steel sheet manufactured by the above manufacturing method, the microstructure is composed of a three-phase structure consisting of ferrite: 60-80%, bainite: 15-30%, and martensite: 3-10% by area ratio. Therefore, it is possible to achieve high strength of 780 MPa or more while significantly reducing the anisotropy of the strength in the L-direction and C-direction of the hot-rolled steel sheet. In addition, 1.0 × 10 TiC precipitates with a diameter of 1.0-5.0 nm are present in the ferrite. 16 ~100.0 × 10 16 pieces / cm 3 Because they exist at a high number density, precipitation strengthening can increase the hardness of ferrite and reduce the hardness difference between each phase in the three-phase structure, resulting in a significant improvement in the hole-expanding properties of hot-rolled steel sheets. Therefore, hot-rolled steel sheets manufactured by the above manufacturing method can reliably achieve both high strength and excellent workability, which are conflicting properties, making them particularly useful in the automotive sector where both properties are required.

[0063] 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]

[0064] 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), total elongation (EL), hole expansion ratio (λ), and strength anisotropy of the obtained hot-rolled steel sheets were investigated.

[0065] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs were then heated under the conditions shown in Table 2 and subsequently hot-rolled. Hot rolling was carried out by rough rolling and finish rolling, with the finishing temperatures shown in Table 2. Next, the finish-rolled steel sheets were intercooled under the conditions shown in Table 2. The intercooled steel sheets were then primary-cooled for 2 seconds under the conditions shown in Table 2, followed by secondary cooling and coiling to obtain hot-rolled steel sheets with a thickness of 3.2 mm.

[0066] [Table 1-1]

[0067] [Table 1-2]

[0068] [Table 2]

[0069] The properties of the obtained hot-rolled steel sheets were measured and evaluated by the following method.

[0070] [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.

[0071] [Tensile strength (TS) and total elongation (EL)] Tensile strength (TS) and total elongation (EL) were 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 (C direction), and performing a tensile test in accordance with JIS Z 2241:2011. The tensile strength obtained here is also called C-direction TS.

[0072] [Hole expansion ratio (λ)] The hole expansion ratio (λ) was determined as follows: First, a circular hole with a diameter of 10 mm (initial hole: hole diameter d0 = 10 mm) was punched into the test piece under conditions of a clearance of 12.5%, with the burr facing the die side. The initial hole was then expanded using a conical punch with a 60° apex angle until a crack penetrating the plate 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 five times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0073] [Intensity of anisotropy] To assess the anisotropy of strength, a JIS No. 5 test specimen was first taken from the direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the hot-rolled steel sheet (L direction), and the tensile strength in the L direction, i.e., L-direction TS, was measured by performing a tensile test in accordance with JIS Z 2241:2011. Next, if the obtained L-direction TS and the previously determined C-direction TS satisfied the following formula, the anisotropy of strength was reduced and the specimen was evaluated as passing (○), and if the formula was not satisfied, it was evaluated as failing (×). 0.95≦L direction TS / C direction TS≦1.00

[0074] Hot-rolled steel sheets with a tensile strength (TS) of 780 MPa or higher, a hole expansion ratio (λ) of 50% or higher, and passing the strength anisotropy evaluation were evaluated as hot-rolled steel sheets that, despite their high strength, have improved hole expansion properties and reduced strength anisotropy. The results are shown in Table 3.

[0075] [Table 3]

[0076] Referring to Tables 1-3, it is believed that in Comparative Example 42, the high finishing temperature of the end rolling caused the austenite to coarseen and the proportion of ferrite to decrease. As a result, the desired three-phase structure was not obtained, and the anisotropy of strength increased. In Comparative Example 43, the low finishing temperature of the end rolling resulted in a non-uniform metal structure, leading to a decrease in TS and λ. In Comparative Example 44, the high intermediate air cooling temperature caused the ferrite transformation to proceed too much, resulting in a failure to obtain the desired three-phase structure and a decrease in strength anisotropy. In Comparative Example 45, the low intermediate air cooling temperature prevented sufficient ferrite precipitation, similarly resulting in a failure to obtain the desired three-phase structure and a decrease in strength anisotropy. In addition, the low intermediate air cooling temperature prevented the formation of TiC precipitates with the desired diameter at a sufficient number density even after subsequent air cooling. As a result, the hardness improvement effect of ferrite due to precipitation strengthening was not fully obtained, and λ decreased. In Comparative Example 46, the intermediate air cooling time was too long, resulting in excessive ferrite precipitation and a low proportion of the hard phase. As a result, the desired three-phase structure was not obtained, strength anisotropy was significant, and the desired tensile strength could not be achieved. In Comparative Example 47, the intermediate air cooling time was too short, preventing sufficient ferrite precipitation. As a result, the desired three-phase structure was not obtained, and strength anisotropy was high. In Comparative Example 48, the average cooling rate of the primary cooling in the cooling process was too fast, preventing proper bainite precipitation. As a result, the desired three-phase structure was not obtained, and strength anisotropy was high. Furthermore, in Comparative Example 48, the high proportion of martensite likely prevented the hardness difference between the phases from being adequately reduced even with ferrite precipitation strengthening. Consequently, λ also decreased. In Comparative Example 49, the average cooling rate of the primary cooling in the cooling process was too slow, similarly preventing proper bainite precipitation. As a result, the desired three-phase structure was not obtained, and strength anisotropy was high. In Comparative Example 50, the average cooling rate of the secondary cooling in the cooling process was slow, resulting in a large amount of bainite precipitation. Consequently, the desired three-phase structure could not be obtained, and the anisotropy of the strength was increased.In Comparative Example 51, the high winding temperature prevented sufficient precipitation of martensite, resulting in a failure to obtain the desired three-phase structure and increased anisotropy of strength.

[0077] Comparative Examples 52 and 55 had low C and Mn content, respectively, resulting in a decrease in TS. Comparative Example 53 had a high C content, which prevented the acquisition of the desired three-phase structure and resulted in increased strength anisotropy. Comparative Example 54 had a high Si content, which increased the surface roughness of the hot-rolled steel sheet due to Si scale, and further increased strength anisotropy due to the inability to obtain the desired three-phase structure. Comparative Example 56 had a high Mn content, which is thought to have reduced the diffusion coefficient of C and decreased the diameter of the TiC precipitates. As a result, the hardness improvement effect of ferrite due to precipitation strengthening based on TiC precipitates could not be fully obtained, and λ decreased. Comparative Example 57 had a low Ti content, which prevented the formation of TiC precipitates at a sufficient number density, resulting in a decrease in λ. Comparative Example 58 had a high Ti content, which led to the coarsening of TiC precipitates and a decrease in the number density of said TiC precipitates, similarly resulting in a decrease in λ. Comparative Examples 59 and 61 failed to achieve the desired three-phase structure and exhibited high strength anisotropy due to their low Nb and Al content, respectively. In particular, Comparative Example 59's low Nb content likely resulted in insufficient pinning effect from carbides, and consequently, the ferrite transformation was not promoted. Comparative Example 60's high Nb content likely led to the formation of coarse carbides in the steel, and the desired three-phase structure was also not achieved. As a result, the TS decreased and strength anisotropy increased. Comparative Example 62's high Al content likely led to the formation of coarse oxides, and the desired three-phase structure was also not achieved. As a result, the TS decreased and strength anisotropy increased.

[0078] In contrast, in all the examples of the invention, the hot-rolled steel sheets had a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, the microstructure was composed of a three-phase structure consisting of ferrite: 60-80%, bainite: 15-30%, and martensite: 3-10% by area ratio, achieving high strength of 780 MPa or more while significantly reducing the anisotropy of the strength. In addition, 1.0 × 10 TiC precipitates with a diameter of 1.0-5.0 nm were found in the ferrite. 16 ~100.0 × 10 16 pieces / cm 3 By precipitation-strengthening ferrite at a specific number density, the hardness difference in the three-phase structure was reduced, significantly improving hole-expanding properties. Furthermore, while the area ratio of the residual structure was 0% in many of the invention examples, when residual structure was present, it was pearlite.

Claims

1. In mass percent, C: 0.010-0.100%, Si: 0.01 to 0.10%, Mn: 0.50-3.00%, Ti: 0.050-0.200%, Nb: 0.010-0.020%, Al: 0.100-1.000%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Ni: 0-2.000%, Mo: 0-1.000%, Cr: 0-2.000%, B: 0 to 0.0100%, Co: 0-2.000%, V: 0-1.000%, Cu: 0-2.000%, W: 0 to 1.0000%, Ta: 0 to 1.0000%, Sn: 0 to 1.0000%, Sb: 0 to 1.0000%, As: 0 to 0.0100%, Mg: 0 to 0.0100%, Ca: 0-0.0100%, Zr: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM: 0-0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. In terms of area ratio, Ferrite: 60-80%, Bainite: 15-30%, and Martensite: Contains 3-10%, TiC precipitates with a diameter of 1.0 to 5.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻⁶. 16 ~100.0 x 10 16 pieces / cm 3 It has a microstructure that exists at a number density of, A hot-rolled steel sheet characterized by having a tensile strength of 780 MPa or more.

2. The aforementioned chemical composition, in mass%, Ni: 0.001 to 2.000%, Mo: 0.001 to 1.000%, Cr: 0.001-2.000%, B: 0.0001 to 0.0100%, Co: 0.001 to 2.000%, V: 0.001-1.000%, Cu: 0.001-2.000%, W: 0.0001-1.0000%, Ta: 0.0001 to 1.0000%, Sn: 0.0001 to 1.0000%, Sb: 0.0001 to 1.0000%, As: 0.0001 to 0.0100%, Mg: 0.0001-0.0100%, Ca: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM: 0.0001~0.0100% The hot-rolled steel sheet according to claim 1, characterized in that it includes at least one of the following.

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