WO2024095533A1

WO2024095533A1 - Hot-rolled steel sheet

Info

Publication number: WO2024095533A1
Application number: PCT/JP2023/024346
Authority: WO
Inventors: 菜月大住; 顕吾畑; 武豊田
Original assignee: 日本製鉄株式会社
Priority date: 2022-11-02
Filing date: 2023-06-30
Publication date: 2024-05-10

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

Hot rolled steel plate

The present invention relates to hot-rolled steel sheets.

In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. In order to achieve both a lighter vehicle body and crashworthiness, increasing the strength of the steel plates used is one effective method, and against this background, the development of high-strength steel plates is underway. However, as strength increases, the workability of the steel plate generally decreases. For this reason, when developing high-strength steel plates, it is important to increase strength while maintaining a certain level of workability.

In this regard, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a Tief represented by [Ti]-48/14 x [N]-48/32 x [S] of 0.01 to 0.30%, crystal grains in which the misorientation of the grain boundaries between adjacent crystal grains is 15° or more, and the average misorientation within the crystal grains is 0 to 0.5° in terms of area fraction, the total of martensite, tempered martensite, and retained austenite is 2% to 10% in terms of area fraction, and Ti is present as Ti carbides at a mass percentage of 40% or more of Tief, and the mass of the Ti carbides having a circle-equivalent grain size of 7 nm to 20 nm or less accounts for 50% or more of the mass of the total Ti carbides. Patent Document 1 also teaches that grains with an average intragrain orientation difference of 0 to 0.5° have high ductility and are further strengthened by precipitation with Ti carbides, so by ensuring that such 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 more.

Patent Document 2 describes a high-strength hot-rolled steel sheet having a predetermined composition, a total volume fraction of the ferrite phase and the bainite phase in the entire structure of 95% or more, a volume fraction of the ferrite phase in the entire structure of 50-90%, precipitates of less than 20 nm in size containing 650-1100 ppm Ti precipitated in the ferrite phase, and a microstructure in which the ΔHv of the bainite phase (the difference between the maximum and minimum Vickers hardness values of the bainite phase measured at 1/4 of the sheet thickness position in the sheet thickness cross section along the rolling direction) is 150 or less. Patent Document 2 also teaches that if a microstructure is formed mainly of ferrite and bainite phases, precipitates of less than 20 nm in size containing 650-1100 ppm Ti precipitated in the ferrite phase, and the ΔHv of the bainite phase is set to 150 or less, a TS of 780 MPa or more can be secured, and excellent stretch flangeability (hole expandability) and impact resistance can be achieved at the same time.

JP 2016-204690 A JP 2011-068945 A

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 component shape in automobile chassis components, etc. Furthermore, for components that require impact resistance, plastic deformation occurs when they receive an impact that exceeds the yield strength, so from the perspective of ensuring the collision safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength, and therefore there is a need to increase the yield ratio, which is the ratio of yield strength to tensile strength.

The present invention aims to provide a hot-rolled steel sheet that has high strength, high hole expansion property and high yield ratio.

In order to achieve the above object, the inventors conducted research, focusing particularly on the metal structure of hot-rolled steel sheet. As a result, the inventors discovered that the strength of hot-rolled steel sheet can be significantly increased by configuring the metal structure of hot-rolled steel sheet having a specified chemical composition to mainly contain ferrite, bainite, and martensite, controlling the bainite fraction to a relatively high range, and utilizing dislocation strengthening, and further discovered that ferrite can be precipitation strengthened by causing TiC precipitates having a suitable diameter to exist in ferrite at a specified number density, thereby further increasing the strength of the hot-rolled steel sheet while reducing the difference in hardness between ferrite and bainite and increasing the hole expandability and yield ratio, thus completing the present invention.

The present invention, which 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 1.000%,
B: 0 to 0.0100%,
Sn: 0 to 1.000%,
Sb: 0 to 1.000%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.010%,
REM: 0 to 0.0100%,
As: 0 to 0.010%,
Zr: 0 to 0.010%,
Co: 0 to 2.000%,
Zn: 0 to 0.010%,
W: 0 to 1.000%, and the balance: Fe and impurities;
In terms of area percentage,
Ferrite: 30-60%,
Bainite: 30-60%; Martensite: 5-20%;
TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a density of 1.0 x ¹⁰¹⁶ particles/ ^cm3 or more,
A hot-rolled steel sheet, characterized in that it has a metal structure in which prior austenite grains in a region containing bainite and martensite have an average aspect ratio of 3.0 or more.
(2) The chemical composition is, in mass%,
Nb: 0.001 to 0.050%,
V: 0.001 to 1.000%,
Cr: 0.001 to 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 to 0.0100%,
Mg: 0.0001 to 0.0100%,
Hf: 0.0001 to 0.0100%,
Bi: 0.001 to 0.010%,
REM: 0.0001 to 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 to 1.000%
The hot-rolled steel sheet according to the above (1), characterized in that it contains at least one of the following:
(3) The hot-rolled steel sheet according to (1) or (2) above, characterized in that the ratio of the average nano-hardness of ferrite to the average nano-hardness of bainite is 0.75 to 1.20.

The present invention provides hot-rolled steel sheets that have high strength, high hole expansion properties, and high yield ratios.

<Hot-rolled steel sheets>
The hot-rolled steel sheet according to the embodiment of the present invention has, 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 1.000%,
B: 0 to 0.0100%,
Sn: 0 to 1.000%,
Sb: 0 to 1.000%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.010%,
REM: 0 to 0.0100%,
As: 0 to 0.010%,
Zr: 0 to 0.010%,
Co: 0 to 2.000%,
Zn: 0 to 0.010%,
W: 0 to 1.000%, and the balance: Fe and impurities;
In area %,
Ferrite: 30-60%,
Bainite: 30-60%; Martensite: 5-20%;
TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a density of 1.0 x ¹⁰¹⁶ particles/ ^cm3 or more,
The steel is characterized by having a metal structure in which the average aspect ratio of prior austenite grains in a region containing bainite and martensite is 3.0 or more.

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

On the other hand, when the fraction of bainite is controlled to a relatively high range in a three-phase structure mainly composed of ferrite, bainite, and martensite as described above, it is very effective in terms of increasing strength, but the hardness difference between the hard phase bainite and the soft phase ferrite becomes relatively large. If the hardness difference between each phase in the metal structure becomes large, the hole expandability and the yield ratio may decrease. For this reason, the present inventors have studied the improvement of hole expandability and the realization of a high yield ratio from the viewpoint of reducing the hardness difference between each phase in a metal structure including such a three-phase structure. As a result, the inventors found that precipitation strengthening of the softest ferrite in the three-phase structure, more specifically, by having TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite exist at a number density of 1.0 x 10 ¹⁶ pieces/cm ³ or more, not only contributes to improving the strength of the hot-rolled steel sheet as a whole, but also sufficiently reduces the hardness difference between the bainite, which is a hard phase relatively abundant in the metal structure, and the softest ferrite in the three-phase structure. In the hot-rolled steel sheet according to the embodiment of the present invention, in order to have TiC precipitates with a diameter of 2.0 to 8.0 nm exist in ferrite at a number density of 1.0 x 10 ¹⁶ pieces/cm ³ or more, in addition to the manufacturing method described 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 containing these elements in the steel in a predetermined amount or more, more specifically, by containing Si and Al in an amount of 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, and thereby to promote the formation of TiC precipitates during cooling after hot rolling. As a result, the inventors have found that, although the metal structure is constituted by a three-phase structure in which the hardness difference is relatively likely to become large by increasing the bainite fraction and utilizing dislocation strengthening to achieve high strength, it is possible to obtain a hot-rolled steel sheet having high strength, hole expansion property 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 that require both the contradictory properties of high strength and excellent workability, and further require impact resistance properties, and is therefore particularly useful in the automotive field.

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

[C: 0.03 to 0.10%]
C is an element effective in increasing the strength of steel plate. In addition, C forms carbides and/or carbonitrides with Ti and Nb in steel, and contributes to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates. In order to fully obtain these effects, the C content is set to 0.03% or more. The C content may be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, if C is excessively contained, the hole expandability and yield ratio may decrease due to the formation of cementite. Therefore, the C content is set to 0.10% or less. The C content may be 0.09% or less, 0.08% or less, or 0.07% or less.

[Si: 0.010 to 0.100%]
Si is an element effective in increasing strength as a solid solution strengthening element. In addition, Si also has the effect of suppressing the precipitation of cementite. Therefore, by containing Si, it is possible to suppress the consumption of C in the steel to form cementite, and thereby it is possible to promote the formation of TiC precipitates during cooling after hot rolling. In order to fully obtain these effects, the Si content is set to 0.010% or more. The Si content may be 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, if Si is contained excessively, a surface quality defect called Si scale may occur. Therefore, the Si content is set to 0.100% or less. The Si content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[Mn: 0.50 to 3.00%]
Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 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 Mn is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content is set to 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.

[Ti: 0.05 to 0.20%]
Ti is an element that finely precipitates in steel as carbide (TiC), improves the strength of steel by precipitation strengthening, and increases the hardness of ferrite. Ti also forms carbides to fix C, and is an element that suppresses the formation of cementite, which is harmful to hole expansion. In order to fully obtain these effects, the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.12% or more, or 0.14% or more. On the other hand, if Ti is contained excessively, the carbides become coarse, and the desired precipitation strengthening in ferrite may not be obtained. In addition, as the TiC precipitates become coarse, the number density of the TiC precipitates also decreases, so in this case, the hardness of ferrite cannot be sufficiently increased by precipitation strengthening. Therefore, the Ti content is set to 0.20% or less. The Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.

[Al: 0.20 to 0.40%]
Al is an element that acts as a deoxidizer for molten steel. In addition, Al also has the effect of suppressing the precipitation of cementite. Therefore, by containing Al, it is possible to suppress the consumption of C in the steel to form cementite, and thereby it is possible to promote the formation of TiC precipitates during cooling after hot rolling. In order to fully obtain these effects, the Al content is set to 0.20% or more. The Al content may be 0.22% or more, 0.25% or more, or 0.28% or more. On the other hand, if Al is contained excessively, coarse oxides may be formed, and toughness and ductility may be reduced. Therefore, the Al content is set to 0.40% or less. The Al content may be 0.38% or less, 0.35% or less, or 0.32% or less.

[P: 0.100% or less]
If P is contained in an excessive amount, it may adversely affect weldability, etc. Therefore, the P content is set to 0.100% or less. The P content may 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 an increase in costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[S: 0.0100% or less]
If S is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Si content is set to 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 an increase in cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[N: 0.010% or less]
If N is contained excessively, it may form coarse nitrides and reduce toughness. Therefore, the N content is set to 0.010% or less. The N content may 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 an increase in costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[O: 0.010% or less]
O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.010% or less. The O content may 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 the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

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 contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.

[Nb: 0 to 0.050%]
Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure and thus increasing the strength of the steel sheet by the pinning effect. The Nb content may be 0%, but in order to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content may 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 Nb is contained excessively, coarse carbides and the like may be generated in the steel, and the ductility of the steel sheet may decrease. Therefore, the Nb content is preferably 0.050% or less. The Nb content may be 0.040% or less, 0.030% or less, or 0.025% or less.

[V: 0 to 1.000%]
V is an element that contributes to improving strength by precipitation strengthening, etc. The V content may be 0%, but in order to obtain such an effect, the V content is preferably 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, even if V is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. Therefore, the V content is preferably 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.

[Cr: 0 to 2.00%]
Cr is an element that enhances the hardenability of steel and contributes to improving strength. The Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if Cr is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. Therefore, the Cr content is preferably 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.

[Ni: 0 to 2.00%]
[Cu: 0 to 2.00%]
Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening. The Ni and Cu contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if these elements are contained excessively, the effects are saturated and there is a risk of increasing manufacturing costs. Therefore, the Ni and Cu contents are preferably 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.

[Mo: 0 to 1.000%]
Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if Mo is excessively contained, the deformation resistance during hot working may increase, and the equipment load may become large. Therefore, the Mo content is preferably 1.000% or less. The Mo content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[B: 0 to 0.0100%]
B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but in order to obtain such an effect, the B content is preferably 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, even if B is contained excessively, the effect becomes saturated and there is a risk of causing an increase in manufacturing costs. Therefore, the B content is preferably 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.

[Sn: 0 to 1.000%]
[Sb: 0 to 1.000%]
Sn and Sb are elements effective for improving corrosion resistance. The Sn and Sb contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 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 inclusion of these elements may cause a decrease in toughness. Therefore, the Sn and Sb contents are preferably 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.

[Ca: 0 to 0.0100%]
[Mg: 0 to 0.0100%]
[Hf: 0 to 0.0100%]
Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions. The Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more or 0.0010% or more. On the other hand, even if these elements are contained in excess, the effect is saturated, and containing more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Bi: 0 to 0.010%]
Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.001% or more or 0.002% or more. On the other hand, even if Bi is contained excessively, the effect is saturated, and containing more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Bi content is preferably 0.010% or less. The Bi content may be 0.005% or less or 0.003% or less.

[REM: 0 to 0.0100%]
REM is an element capable of controlling the form of nonmetallic inclusions. The REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more or 0.0010% or more. On the other hand, even if REM is contained excessively, the effect is saturated, and the inclusion of more REM than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the REM content is preferably 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, namely, scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[As: 0 to 0.010%]
As is an element effective in improving corrosion resistance. The As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if As is excessively contained, the effect is saturated, and containing more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[Zr: 0 to 0.010%]
Zr is an element capable of controlling the form of nonmetallic inclusions. The Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content may be 0.002% or more or 0.003% or more. On the other hand, even if Zr is contained excessively, the effect is saturated, and the inclusion of more Zr than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Zr content is preferably 0.010% or less. The Zr content may be 0.008% or less or 0.005% or less.

[Co: 0 to 2.000%]
Co is an element that contributes to improving hardenability and/or heat resistance. The Co content may be 0%, but in order to obtain these effects, the Co content is preferably 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, excessive Co content may deteriorate hot workability and lead to an increase in raw material costs. Therefore, the Co content is preferably 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.

[Zn: 0 to 0.010%]
Zn is an element that may be contained in a steel sheet when scrap or the like is used as a steel raw material. 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 the Zn content to less than 0.001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[W: 0 to 1.000%]
W is an element that enhances the hardenability of steel and contributes to improving strength. The W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.001% or more. The W content may 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, the W content is preferably 1.000% or less. The W content may be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

In the hot-rolled steel sheet according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.

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

[Metal structure]
The metal structure of the hot rolled steel sheet according to the embodiment of the present invention includes, in terms of area percentage, ferrite: 30 to 60%, bainite: 30 to 60%, and martensite: 5 to 20%. By configuring the metal structure of the hot rolled steel sheet to mainly include these three structures and to include a relatively large amount of bainite, it is possible to increase the hole expandability and yield ratio while increasing the strength of the hot rolled steel sheet. In addition to including these three structures at the specific area percentage as described above, the strength of the hot rolled steel sheet can be further increased by utilizing dislocation strengthening, which will be described in detail later. In addition, by including these three structures at the specific area percentage as described above, it is possible to appropriately reduce the hardness difference in the metal structure by precipitation strengthening of ferrite, which will be described 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 the hardness difference in the metal structure, more specifically the hardness difference between ferrite and bainite, may not be appropriately reduced even by precipitation strengthening of ferrite. In such a case, the desired hole expandability and/or yield ratio cannot be achieved. Therefore, the area ratio of ferrite needs to 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 ratio of bainite and martensite, which are hard phases, decreases, and as a result, the desired strength, for example, a tensile strength of 780 MPa or more, may not be achieved. Therefore, the area ratio of ferrite is 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.

From the viewpoint of improving tensile strength, it is preferable that the area ratios of the hard phases bainite and martensite are high. 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 the hole expandability and yield ratio, it is preferable that the area ratios of bainite and martensite are low. 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.

[Remainder structure]
As described above, the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes ferrite, bainite, and martensite, and may include other remaining structures, but the area ratio of the remaining structure is preferably small and may be 0%. The area ratio of the remaining structure is not particularly limited, and may be, for example, 0 to 5%, 0 to 4%, or 0 to 3%. In other words, the total area ratio of ferrite, bainite, and martensite may be, for example, 95 to 100%, 96 to 100%, or 97 to 100%. The lower limit of the remaining structure may be 1% or 2%. When a remaining structure is present, the remaining structure may include at least one of pearlite and retained austenite or may be at least one of them.

[Identification of metal structure and calculation of area ratio]
The structure observation is performed with a scanning electron microscope. Prior to the observation, the sample for structure observation is polished by wet polishing with emery paper and diamond abrasive grains having an average particle size of 1 μm, and the observation surface is mirror-finished, and then the structure is etched with a 3% nitric acid alcohol solution. The magnification of the observation is 2000 times, and 10 random images of a 30 μm x 40 μm field of view at a position of 1/4 of the plate thickness from the surface are taken. The ratio of the structure is obtained by the point count method. A total of 225 lattice points are set at intervals of 3 μm vertically and 4 μm horizontally for the obtained structure image, and the structure present under the lattice points is identified, and the structure ratio contained in the steel material is obtained from the average value of the 10 sheets. Ferrite is a blocky crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more inside. Bainite is a collection of lath-shaped crystal grains, and does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, the group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less. Bainite is counted as one bainite grain when it is surrounded by grain boundaries with an orientation difference of 15° or more. In addition, martensite, which contains a large amount of dissolved carbon, has a higher brightness and appears whiter than other structures, so it can 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%. Although it is not necessary to specifically identify the remaining structure, when the remaining structure includes pearlite and retained austenite, etc., pearlite has a unique structure in which cementite is precipitated in a lamellar form, and therefore can be identified by a scanning electron microscope. In addition, the volume fraction of the retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of the retained austenite is equivalent to the area fraction, this can be regarded as the area fraction of the retained austenite.

[Number density of TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite: 1.0 x 10 ¹⁶ /cm ³ or more]
In the hot-rolled steel sheet according to the embodiment of the present invention, TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a number density of 1.0 x 10 ¹⁶ pieces/cm ³ or more. Here, TiC precipitates include not only TiC but also composite carbides containing Ti and other elements other than Ti, such as V and Nb. By allowing TiC precipitates having a diameter of 2.0 to 8.0 nm to exist in the ferrite at such a number density, the hardness of the ferrite can be increased by precipitation strengthening. More specifically, by increasing the hardness of ferrite and reducing the hardness difference with bainite, a hard phase that is relatively abundant in the metal structure, the hardness difference in the metal structure mainly composed of ferrite, bainite and martensite can be reduced. As a result, it is possible to significantly increase the hole expandability and yield ratio of the hot-rolled steel sheet, and it is possible to achieve, for example, a hole expansion ratio (λ) of 60.0% or more and a yield ratio (YR) of 0.70 or more. Naturally, the precipitation strengthening by TiC precipitates also contributes to improving the strength of the entire hot-rolled steel sheet. If the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot act sufficiently as obstacles to dislocation motion, and therefore the effect of improving the hardness of ferrite by precipitation strengthening cannot be fully obtained. In addition, the effect of improving the strength of the hot-rolled steel sheet may not be fully exhibited. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in ferrite may not be obtained.

Although it is not intended to be bound by any particular theory, this is believed to be because the strengthening mechanism changes in relation to dislocation motion as the TiC precipitates become coarse, and for example, dislocation lines do not pass across the TiC precipitates, but pass around the coarse TiC precipitates while leaving a loop of dislocation lines, resulting in a small amount of precipitation strengthening. In addition, as the TiC precipitates become coarse, the number density of the TiC precipitates also decreases significantly, making it impossible to sufficiently increase the hardness of ferrite by precipitation strengthening. Therefore, in order to effectively increase the hardness of ferrite by precipitation strengthening, it is effective to control the diameter of the TiC precipitates to a range of 2.0 to 8.0 nm, and in order to increase the hardness of ferrite to a desired level by precipitation strengthening, it is important to control the number density of TiC precipitates having such a diameter within a predetermined range. From this viewpoint, TiC precipitates having a diameter of 2.0 to 8.0 nm need to be present in the ferrite at a number density of 1.0×10 ¹⁶ /cm ³ or more as described above. In order to further enhance the effect of improving the hardness of ferrite, the higher the number density, the more preferable it is, and it may be, for example, 1.2×10 ¹⁶ /cm ³ or more, 1.5×10 ¹⁶ /cm ³ or more, 2.0×10 ¹⁶ /cm ³ or more, 5.0×10 ¹⁶ /cm ³ or more, or 10.0×10 ¹⁶ /cm ³ or more. On the other hand, since there is a limit to the contents of C and Ti, which are the supply sources of TiC precipitates, if the number density becomes too high, it may be difficult to control the diameter of the TiC precipitates within the desired range. Therefore, the number density is not particularly limited as long as the diameter of 2.0 to 8.0 nm is satisfied, but may be, for example, 75.0 x 10 ¹⁶ pieces/cm ³ or less, 50.0 x 10 ¹⁶ pieces/cm ³ or less, 30.0 x 10 ¹⁶ pieces/cm ³ or less, or 20.0 x 10 ¹⁶ pieces/cm ³ or less. In the hot-rolled steel sheet according to the embodiment of the present invention, when measured by a three-dimensional atom probe measurement method described in detail later, TiC precipitates having a diameter of 2.0 to 8.0 nm may be present in the ferrite at a number density of 1.0 x 10 ¹⁶ pieces/cm ³ or more, and therefore, as long as the above diameter and number density requirements are satisfied, for example, coarse TiC precipitates may be present in the ferrite.

[Calculation of diameter and number density of TiC precipitates]
The diameter and number density of TiC precipitates are calculated by 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 a focused ion beam processing method in combination with the electrolytic polishing method. In the three-dimensional atom probe measurement, the accumulated data can be reconstructed to obtain an actual atomic distribution image in real space. In the case of a fine TiC precipitate with a Na-Cl structure, the unit cell is 4.33 Å, so the interatomic distance between Ti and Ti is 4.33×√2=6.1 Å. Therefore, when a plurality of Ti atoms are present at approximately the same coordinate position (7 Å or less), it is determined that these Ti atoms are in the same precipitate, and the number of Ti atoms determined to be in the same precipitate is counted, and when the number is 50 or more, the precipitate is defined as a fine TiC precipitate. The diameter of the fine TiC precipitate is the circle equivalent diameter calculated from the number of Ti atoms constituting the observed fine Ti precipitate and the lattice constant of the fine Ti precipitate, assuming that the fine Ti precipitate is spherical. The method of calculating the diameter (circle equivalent diameter) 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 number N of all atoms of the target sample is measured by the three-dimensional atom probe measurement method, but in reality, the number N of all atoms of the target sample cannot be detected by the three-dimensional atom probe measurement method. Since each device has its own atomic detection rate α (= number of detected atoms / total number of atoms), the number N of atoms that would have existed 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, the TiC precipitate of the Na-Cl structure has 8 Ti atoms in the unit lattice, and the lattice constant a of the Na-Cl structure is 4.33 Å, and the diameter (circle equivalent diameter) R of the TiC precipitate is calculated by the following formula.
Diameter of TiC precipitate R = {(6/8) x (1/π) x N x a ³ } ^(1/3)
Finally, the number density of TiC precipitates is calculated using the measurement field of view as the denominator and the number of fine TiC precipitates as the numerator.

[Ratio of average nanohardness of ferrite to average nanohardness of bainite: 0.75 to 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 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 within such a range, the hardness difference between ferrite and bainite in the metal structure can be further reduced. As a result, it is possible to further increase the hole expandability and yield ratio of the hot-rolled steel sheet, and for example, it is possible to achieve a hole expansion ratio (λ) of 65.0% or more and a yield ratio (YR) of 0.75 or more. From the viewpoint of further increasing the hole expansion ratio and the 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 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 to 5.0 GPa, 3.2 to 4.8 GPa, or 3.5 to 4.5 GPa. Similarly, the average nanohardness of ferrite is not particularly limited, but may be, for example, 2.5 to 5.0 GPa, 2.8 to 4.8 GPa, or 3.0 to 4.5 GPa.

[Method for determining the ratio of the average nanohardness of ferrite to that 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 out from a hot-rolled steel sheet so that a cross section perpendicular to the surface can be observed. The cross section of the sample is polished to a mirror finish by wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 μm. The mirror-finished cross section is indented with a test load of 3 gf at a depth position of 1/4 of the plate thickness from the surface using a microhardness tester, and the nanohardness is measured, obtaining a total of 100 or more measured values. Next, the same sample is measured using a scanning electron microscope, and only measurement points with indentations inside the ferrite grains and inside the bainite are extracted with reference to the obtained structure analysis results. Finally, the arithmetic average of the nanohardnesses for the 10 or more extracted ferrite crystal grains is determined as the average nanohardness of ferrite, and the arithmetic average of the nanohardnesses for the 10 or more extracted bainite grains is determined as the average nanohardness of bainite, and the ratio thereof (average nanohardness of ferrite)/(average nanohardness of bainite) is determined as the ratio of the average nanohardness of ferrite to the average nanohardness of bainite.

[Average aspect ratio of prior austenite grains in region containing bainite and martensite: 3.0 or more]
In the hot-rolled steel sheet according to the embodiment of the present invention, the average aspect ratio of the prior austenite grains in the region containing bainite and martensite must be 3.0 or more. 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 bainite, 30 to 60 area %, of the hard phase. However, there are cases where the desired high strength cannot be reliably achieved simply by increasing the area ratio of bainite. 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, dislocation strengthening is utilized to further increase the strength. To explain in more detail, dislocations can be introduced into the steel sheet while suppressing recrystallization of the metal structure by applying an appropriate reduction during hot rolling. The metal structure in which dislocations are appropriately introduced in this way has a relatively large aspect ratio because recrystallization is suppressed. That is, by appropriately controlling the average aspect ratio of the 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 increased due to dislocation strengthening. In the hot-rolled steel sheet according to the embodiment of the present invention, the metal structure is configured to contain 30 to 60 area % of bainite, and the average aspect ratio of the prior austenite grains in the region containing the bainite and martensite is controlled to 3.0 or more, so that the strength of the hot-rolled steel sheet can be significantly increased due to a combination of the strength improving effect of bainite and dislocation strengthening. From the viewpoint of further increasing the strength of the hot-rolled steel sheet, the larger the average aspect ratio, the more preferable it is, and it may be, for example, 3.2 or more, 3.5 or more, 3.8 or more, or 4.0 or more. The upper limit of the average aspect ratio is not particularly limited, but for example, the average aspect ratio may be 6.0 or less, 5.5 or less, or 5.0 or less.

[Measurement of the average aspect ratio of prior austenite grains in the region containing bainite and martensite]
The average aspect ratio of the prior austenite grains in the region containing bainite and martensite is measured by a scanning electron microscope. Prior to the measurement, the sample for structure observation is first polished by wet polishing with emery paper and diamond abrasives with an average particle size of 1 μm, and the L-direction cross section (cross section parallel to the rolling direction and the plate thickness direction) is mirror-finished as the observation surface, and then the structure is etched with a picric acid solution. The magnification of the observation is 1000 times, and 10 random photographs are taken of a 60 μm x 80 μm field of view at a position of 1/4 of the plate thickness from the surface. The grain size in each of the plate thickness direction and the rolling direction is obtained by a cutting method, with the grain boundaries revealed by corrosion with picric acid as the target. In the cutting method, five straight lines parallel to the plate thickness direction and the rolling direction of the photographed image are drawn at equal intervals, and the intersections with the grain boundaries are counted. The total length of the five straight lines divided by the number of intersections is taken as the grain size. The grain size in the rolling direction is divided by the grain size in the sheet thickness direction to determine the average aspect ratio of the prior austenite grains in the region containing bainite and martensite.

[Thickness]
The hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto. 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.

[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 more can be achieved. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. According to the hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of the chemical composition and metal structure described above can realize an improvement in hole expandability and a high yield ratio. 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 a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.

[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 be increased, and more specifically, a yield ratio of 0.70 or more can be achieved. The yield ratio is preferably 0.75 or more, more preferably 0.80 or more. The upper limit is not particularly limited, but for example, the yield ratio may be 0.90 or less or 0.85 or less. 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 piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet and performing a tensile test in accordance with JIS Z 2241:2011.
Yield ratio YR = 0.2% proof stress / tensile strength TS

[Hole expansion ratio: λ]
According to the hot-rolled steel sheet having the above chemical composition and metal structure, high hole expansion property, specifically, a hole expansion ratio of 60.0% or more can be achieved. The hole expansion ratio may be preferably 65.0% or more, more preferably 70.0% or more or 80% or more. The upper limit of the hole expansion ratio is not particularly limited, 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 having a width of 100 mm and a length of 100 mm is taken from the hot-rolled steel sheet, and a punch 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, the initial hole is expanded with a conical punch having an apex angle of 60° until a crack penetrating the plate thickness occurs, with the burr facing the die side, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated using the following formula. This hole expansion test is carried out three times, and the average value is determined as the hole expansion ratio λ.
λ = 100 × {(d1 - d0) / d0}

<Method of manufacturing hot-rolled steel sheet>
Next, a preferred method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention, and is not intended to limit the hot-rolled steel sheet to one manufactured by the manufacturing method described below.

The method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes:
A hot rolling process comprising heating a slab having the chemical composition described above in relation to the hot rolled steel sheet and then finish rolling the slab, the hot rolling process satisfying the following conditions (a) to (e):
(a) The heating temperature of the slab is 1200 to 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 in the rolling passes of the front stages other than the rear three stages is 60 to 90%;
(d) the rolling reduction in each rolling pass of the latter three stages is 10% or more, and the total rolling reduction in the rolling passes of the latter three stages is 30 to 50%, and (e) the end temperature of finish rolling is 900 to 1000°C. The method is characterized by including an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 670 to 750°C at an average cooling rate of 50 to 200°C/s, and then intermediate air-cooled for 3 to 10 seconds, and a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/s, and then coiled at a coiling temperature of 20 to 200°C. Each step will be described in detail below.

[Hot rolling process]
[(a) Slab heating temperature: 1200 to 1300° C.]
[(b) Holding time in the temperature range of 1200 to 1300° C.: 1000 to 4000 seconds]
First, a slab having the chemical composition described above in relation to the hot-rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting 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 dissolve the alloying elements in the slab. If the heating temperature is low, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity and productivity of the heating equipment. In addition, by setting the holding time in the temperature range of 1200 to 1300°C to 1000 seconds or more, the alloying elements can be reliably dissolved in the slab. The upper limit of the holding time is not particularly limited, but is preferably 4000 seconds or less from the viewpoint of productivity, etc. When rough rolling is performed, holding in the temperature range of 1200 to 1300° C. may be performed after the rough rolling.

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

[(c) Total reduction in rolling passes prior to finish rolling: 60 to 90%]
The heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. In this manufacturing method, the 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 in the rolling passes of the front stages other than the rear three stages (final three stages) is controlled to 60 to 90%. By performing rolling at such a high reduction rate in the rolling passes of the front stages, recrystallization can be promoted to refine the metal structure. However, the recrystallization rate in each rolling pass of the front stages does not need to be 100%. Refining the metal structure by such recrystallization is very advantageous in forming a desired metal structure and improving properties such as hole expandability. If the total reduction rate in the rolling passes of the front stages is less than 60%, the desired metal structure containing ferrite, bainite, and martensite in a specific ratio cannot be obtained, and properties such as hole expandability may be reduced. Therefore, the total reduction in the front rolling passes is set to 60% or more, and preferably 70% or more. On the other hand, if the total reduction in the front rolling passes is too high, the rolling load becomes excessive, and the load on the rolling mill increases. For this reason, the total reduction in the front rolling passes is set to 90% or less.

[(d) Reduction in each rolling pass of the last three stages: 10% or more, and total reduction in the last three rolling passes of the finish rolling: 30 to 50%]
In the finish rolling of the present manufacturing method, the reduction in each rolling pass of the latter three stages is controlled to 10% or more, and the total reduction in the rolling passes of the latter three stages is controlled to within a range of 30 to 50%. In the latter rolling passes of the finish rolling, unlike the rolling passes of the former stages, it is necessary to suppress recrystallization. By controlling the reduction in each rolling pass of the latter three stages to 10% or more and the total reduction in the rolling passes of the latter three stages to within a range of 30 to 50%, the reduction in each rolling pass is substantially limited to within a range of 10 to 30%. By performing the rolling passes of the latter three stages under such relatively light reduction conditions, it is possible to suppress recrystallization in each rolling pass and reliably introduce dislocations, and it is possible to control the average aspect ratio of the prior austenite grains in the region containing bainite and martensite in the metal structure of the finally obtained hot-rolled steel sheet to 3.0 or more.

If the reduction rate in each rolling pass of the last three stages is less than 10%, recrystallization is suppressed in each rolling pass, but dislocations cannot be sufficiently introduced in each rolling pass, and the desired aspect ratio cannot be achieved in the final metal structure. Similarly, if the total reduction rate in the rolling passes of the last three stages is less than 30%, dislocations cannot be sufficiently introduced in at least one rolling pass of the last three stages, and the average aspect ratio of the old austenite grains in the region containing bainite and martensite cannot be achieved in the final metal structure. On the other hand, if the reduction rate in each rolling pass of the last three stages is more than 30% or the total reduction rate in the last three stages is more than 50%, recrystallization is promoted and dislocations cannot be sufficiently introduced, and similarly, the average aspect ratio of the old austenite grains in the region containing bainite and martensite cannot be achieved in the final metal structure. The total reduction rate in the last three rolling passes of the finish rolling is preferably controlled within the range of 35 to 50%.

[(e) End temperature of finish rolling: 900 to 1000° C.]
In this manufacturing method, in addition to controlling the reduction ratios in the front and rear stages of the finish rolling, the finish rolling end temperature is also important in controlling the metal structure of the steel sheet. If the finish rolling end temperature is low, the metal structure may become non-uniform, and the strength and/or hole expandability may decrease. For this reason, the finish rolling end temperature is set to 900°C or higher. On the other hand, if the finish rolling end temperature is high, recrystallization is promoted in the rolling passes in the rear three stages of the finish rolling, and dislocations cannot be sufficiently introduced. As a result, in the finally obtained metal structure, it is not possible to achieve an average aspect ratio of 3.0 or more of the prior austenite grains in the region including bainite and martensite. Therefore, the finish rolling end temperature is set to 1000°C or lower.

[Intermediate air cooling process]
In the next intermediate cooling step, the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) to an intermediate cooling temperature of 670 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, and then intermediate cooling is performed for 3 to 10 seconds. By primarily cooling to an intermediate cooling temperature of 670 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, it is possible to suppress excessive generation of ferrite while promoting the precipitation of TiC precipitates by the next intermediate cooling at high temperatures. As a result, ferrite is sufficiently precipitation strengthened, thereby reducing the hardness difference between ferrite and bainite, and improving the hole expandability and yield ratio. To explain in more detail, in order to sufficiently precipitation strengthen ferrite by promoting the generation and grain growth of TiC precipitates during intermediate cooling, it is necessary to set the intermediate cooling temperature to a relatively high temperature range, i.e., a temperature range of 670 to 750 ° C. However, in this case, ferrite is generated excessively, and the area ratio of ferrite in the final metal structure exceeds 60%, making it impossible to obtain the desired characteristics. Therefore, in this manufacturing method, the average cooling rate in the primary cooling from the finish rolling to the intermediate air cooling temperature is set to 50 ° C / sec or more, thereby suppressing excessive generation of ferrite and allowing TiC precipitates to be sufficiently precipitated by the next intermediate air cooling at high temperature. On the other hand, if the average cooling rate of the primary cooling exceeds 200 ° C, the generation of ferrite is excessively suppressed, and the area ratio of ferrite in the final metal structure is less than 30%, and properties such as hole expansion property are deteriorated. Therefore, the average cooling rate of the primary cooling is set to 200 ° C / sec or less, preferably 160 ° C / sec or less.

If the intermediate cooling temperature is above 750°C or the intermediate cooling time is more than 10 seconds, excessive ferrite is generated or the TiC precipitates become coarse. If excessive ferrite is generated, the desired metal structure containing ferrite, bainite and martensite in a specific ratio cannot be formed in the final hot-rolled steel sheet. In addition, if the TiC precipitates become coarse, the number density of the TiC precipitates also decreases significantly, and the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. On the other hand, if the intermediate cooling temperature is below 670°C or the intermediate cooling time is less than 3 seconds, the generation and grain growth of TiC precipitates are suppressed, and the desired diameter and/or number density cannot be obtained. In this case, too, the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. In addition, the formation of ferrite may be suppressed too much, which makes it impossible to form the desired metal structure containing ferrite, bainite, and martensite in specific proportions in the final hot-rolled steel sheet.

In contrast, in the intermediate cooling step, the intermediate cooling temperature is primarily cooled to 670 to 750 ° C., preferably 690 to 750 ° C. at an average cooling rate of 50 to 200 ° C./s, preferably 50 to 160 ° C./s, and then intermediate cooling is performed for 3 to 10 seconds, preferably 4 to 9 seconds, to precipitate ferrite in a desired ratio, generate TiC precipitates in the ferrite, and allow them to grow appropriately, so that TiC precipitates having a diameter of 2.0 to 8.0 nm are finally present at a number density of 1.0 × 10 ¹⁶ / cm ³ or more. As a result, by fully exerting the effect of improving the hardness of ferrite by precipitation strengthening, it is possible to reduce the hardness difference between ferrite and bainite in the metal structure and significantly increase the hole expandability and yield ratio.

[Cooling process]
The steel sheet after the intermediate air cooling is secondarily cooled at an average cooling rate of 50 to 200 ° C./s in the next cooling step, and then wound at a winding temperature of 20 to 200 ° C. By subjecting the steel sheet after the intermediate air cooling to such a relatively fast average cooling rate for the second cooling, bainite and martensite can be appropriately precipitated, so that it is possible to form a metal structure containing ferrite, bainite and martensite in a specific ratio in the finally obtained hot rolled steel sheet. On the other hand, if the average cooling rate of the second cooling is less than 50 ° C./s, bainite and/or martensite cannot be appropriately precipitated, and therefore the desired metal structure cannot be obtained in the finally obtained hot rolled steel sheet. In such a case, it becomes impossible to achieve a tensile strength of 780 MPa or more. On the other hand, if the average cooling rate of the second cooling is more than 200 ° C., bainite is not sufficiently generated and/or martensite is excessively generated, and similarly, the desired metal structure cannot be obtained in the finally obtained hot rolled steel sheet. Therefore, the average cooling rate of the secondary cooling is set to 200° C./sec or less, and preferably 180° C./sec or less.

On the other hand, if the coiling temperature is above 200°C, a relatively large amount of cementite may precipitate. In such a case, the C in the steel is consumed in the formation of cementite. As a result, the formation of TiC precipitates is suppressed, and the effect of improving the hardness of ferrite by precipitation strengthening using the TiC precipitates cannot be fully obtained. In addition, the effect of improving the strength of the hot-rolled steel sheet may not be fully achieved. On the other hand, if the coiling temperature is too low, excessive water cooling, etc. will be required, reducing productivity. It may also cause embrittlement of the hot-rolled steel sheet. Therefore, the coiling temperature is set to 20°C or higher.

According to the hot-rolled steel sheet manufactured by the above manufacturing method, the area ratio of bainite, which is a hard layer in the metal structure, is controlled within a relatively high range of 30 to 60%, and the average aspect ratio of the prior austenite grains in the region containing the bainite and martensite is 3.0 or more, and the dislocation strengthening associated therewith can be utilized, and as a result, the strength of the hot-rolled steel sheet can be significantly increased. Furthermore, since TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a number density of 1.0 x ¹⁰¹⁶ precipitates/ ^cm3 or more, the precipitation strengthening naturally contributes to improving the strength of the entire hot-rolled steel sheet, and the hardness difference between the bainite, which is a hard phase relatively abundant in the metal structure, and the ferrite, which is the softest of the three-phase structure, can be sufficiently reduced, and as a result, the hole expandability and yield ratio of the hot-rolled steel sheet can be significantly increased. Therefore, the hot-rolled steel sheet manufactured by the above-mentioned manufacturing method is particularly useful in the automotive field because it can be effectively used in components that require both the contradictory properties of high strength and excellent workability, and furthermore, impact resistance.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

In the following examples, hot-rolled steel sheets according to the embodiments 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.

First, ingots having various chemical compositions shown in Table 1 were produced in a vacuum melting furnace, then reheated to the heating temperature shown in Table 2, and rough rolled to produce rough bars with a thickness of 30 mm. The rough bars were held at a temperature range of 1200 to 1300°C for 3600 seconds, and then finish rolling was performed using a rolling mill consisting of multiple rolling stands, with two or more rolling passes in the front stage and three rolling passes in the rear stage, under the conditions shown in Table 2. The end temperature of the finish rolling was as shown in Table 2. Next, the finish-rolled steel plate was primarily cooled to the intermediate air-cooling temperature under the conditions shown in Table 2, and then intermediate air-cooled. Finally, the intermediate air-cooled steel plate was secondarily cooled to the coiling temperature under the conditions shown in Table 2, and then coiled at the coiling temperature, to obtain a hot-rolled steel plate with a thickness of 2.5 mm.

The properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.

[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 technique detailed herein, with an instrument specific atomic detection rate α of 0.35.

[Tensile strength (TS) and yield ratio (YR)]
The tensile strength (TS) was measured by taking a JIS No. 5 test piece having a length of 200 mm and a thickness of 2.5 mm from a direction (C direction) in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the hot-rolled steel plate, 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 piece, and strain was applied until it broke. The yield ratio (YR) was determined by the following formula 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 piece in the same manner.
Yield ratio YR = 0.2% proof stress / tensile strength TS

[Hole expansion ratio: λ]
The hole expansion ratio was determined as follows. First, a test piece having a thickness of 2.5 mm x width of 100 mm x length of 100 mm was taken from the hot-rolled steel sheet, and a punching tool having a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to make a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was set to be on the die side, and the initial hole was pushed out with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the crack occurrence was measured, and the hole expansion ratio λ (%) of 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}

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

With reference to Tables 1 to 3, in Comparative Example 18, the average cooling rate in the primary cooling to the intermediate air-cooling temperature was high, so that the generation of ferrite was excessively suppressed, and the area ratio of ferrite in the final metal structure was less than 30%. As a result, λ was reduced. In Comparative Example 19, it is considered that the total reduction rate in the rolling passes in the front stages of the finish rolling was low, so that recrystallization was suppressed in the front rolling passes, and the metal structure could not be refined. As a result, the desired metal structure was not obtained, and λ was reduced. In Comparative Example 20, it is considered that the total reduction rate in the rolling passes in the rear three stages of the finish rolling was high, so that recrystallization was promoted in the rear rolling passes, and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in the region including bainite and martensite was less than 3.0, and TS was reduced. In Comparative Example 21, it is considered that the coiling temperature was high, so that a relatively large amount of cementite was precipitated, and 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 the TiC precipitates was less than ^1.0 x ¹⁰¹⁶ /cm3, and the effect of improving the hardness of ferrite by precipitation strengthening and the effect of improving the strength of the hot-rolled steel sheet could not be sufficiently obtained, and TS, λ, and YR were reduced. In Comparative Example 22, the intermediate air-cooling temperature was high, so that ferrite was excessively generated, and further the TiC precipitates were coarsened, and in connection with this, the number density of the TiC precipitates was also reduced. As a result, TS, λ, and YR were reduced. In Comparative Example 23, it is considered that the end temperature of the finish rolling was high, so that recrystallization was promoted in the rolling passes in the latter three stages of the finish rolling, and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in the region including bainite and martensite was less than 3.0, and TS was reduced. In Comparative Example 24, the average cooling rate in the primary cooling to the intermediate air-cooling temperature was slow, so that ferrite was generated excessively, and the area ratio of ferrite in the final metal structure was more than 60%, resulting in a decrease in TS.

In Comparative Example 25, the average cooling rate in the secondary cooling after the intermediate air cooling was slow, so the area ratio of martensite was less than 5%, and TS was reduced. In Comparative Example 26, the intermediate air cooling temperature was low, so the generation and grain growth of TiC precipitates were suppressed, and the desired diameter of the TiC precipitates could not be obtained. As a result, the hardness improvement effect of ferrite due to precipitation strengthening could not be fully obtained, and λ and YR were reduced. In Comparative Example 27, the intermediate air cooling time was long, so ferrite was generated excessively, and further the TiC precipitates became coarse, and in connection with this, the number density of the TiC precipitates also decreased. As a result, TS, λ, and YR were reduced. In Comparative Example 28, the C content was high, so λ and YR were reduced due to the formation of cementite. In Comparative Example 29, the Ti content was low, so the TiC precipitates could not be precipitated at a sufficient number density. As a result, TS, λ, and YR were reduced. In Comparative Example 30, since the Si content was low, the precipitation of cementite could not be sufficiently suppressed, and it is considered 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 the TiC precipitates was less than 1.0 × 10 ¹⁶ / cm ³ , and the effect of improving the hardness of ferrite by precipitation strengthening could not be sufficiently obtained, and the YR was reduced. In Comparative Example 31, since the Al content was low, it is considered that the precipitation of cementite could not be sufficiently suppressed, and 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 the TiC precipitates was less than 1.0 × 10 ¹⁶ / cm ³ , and the effect of improving the hardness of ferrite by precipitation strengthening and the effect of improving the strength of the hot-rolled steel sheet could not be sufficiently obtained, and the TS and YR were reduced.

In contrast, in all the hot-rolled steel sheets according to the invention, a hot-rolled steel sheet having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, which contains ferrite: 30-60%, bainite: 30-60%, and martensite: 5-20% in terms of area ratio, TiC precipitates having a diameter of 2.0-8.0 nm are present in the ferrite at a number density of 1.0 x ¹⁰¹⁶ /cm3 ^or more, and the average aspect ratio of the prior austenite grains in the region containing bainite and martensite is 3.0 or more, was obtained. As a result, due to the relatively high bainite fraction, dislocation strengthening, and precipitation strengthening by TiC precipitates, it was possible to achieve a high strength of tensile strength of 780 MPa or more, while reducing the hardness difference between ferrite and bainite, and increasing the hole expandability and yield ratio. In addition, in the hot-rolled steel sheets according to 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, a hole expansion ratio of 65.0% or more and a yield ratio of 0.75 or more could be achieved, and therefore the hole expandability and yield ratio of the hot-rolled steel sheets could be further significantly improved. In addition, when a residual structure was present in the examples, the residual structure was at least one of pearlite and retained austenite.

Claims

In mass percent,
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 1.000%,
B: 0 to 0.0100%,
Sn: 0 to 1.000%,
Sb: 0 to 1.000%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.010%,
REM: 0 to 0.0100%,
As: 0 to 0.010%,
Zr: 0 to 0.010%,
Co: 0 to 2.000%,
Zn: 0 to 0.010%,
W: 0 to 1.000%, and the balance: Fe and impurities;
In terms of area percentage,
Ferrite: 30-60%,
Bainite: 30-60%; Martensite: 5-20%;
TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a density of 1.0 x 1016 particles/ cm3 or more,
A hot-rolled steel sheet, characterized in that it has a metal structure in which prior austenite grains in a region containing bainite and martensite have an average aspect ratio of 3.0 or more.
The chemical composition, in mass%,
Nb: 0.001 to 0.050%,
V: 0.001 to 1.000%,
Cr: 0.001 to 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 to 0.0100%,
Mg: 0.0001 to 0.0100%,
Hf: 0.0001 to 0.0100%,
Bi: 0.001 to 0.010%,
REM: 0.0001 to 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 to 1.000%
The hot-rolled steel sheet according to claim 1, characterized in that it contains at least one of the following:
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.