EP4056723A1

EP4056723A1 - Hot rolled steel sheet and production method thereof

Info

Publication number: EP4056723A1
Application number: EP20884487.8A
Authority: EP
Inventors: Shohei Yabu; Kunio Hayashi; Koutarou Hayashi; Kazumasa TSUTSUI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-11-06
Filing date: 2020-10-12
Publication date: 2022-09-14
Also published as: EP4056723A4; US20220259692A1; CN114630917A; JP7243854B2; WO2021090642A1; MX2022004885A; JPWO2021090642A1; KR20220068250A; CN114630917B

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area%, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L7 of a grain boundary having a crystal orientation difference of 7° and a length L68 of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 µm/µm2, and a tensile strength is 780 MPa or more.

Description

[Technical Field of the Invention]

The present invention relates to a hot-rolled steel sheet and a method of manufacturing the same. Specifically, the present invention relates to a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2019-201427, filed on November 6, 2019 , the content of which is incorporated herein by reference.

[Background Art]

In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision properties to secure the safety of the occupants.
In order to achieve both vehicle body weight reduction and collision properties, an investigation has been conducted to make a member thin by using a high strength steel sheet. Therefore, a steel sheet having both high strength and excellent formability is strongly desired. A steel sheet having excellent ductility and hole expansibility particularly among the formability is desired. In addition, a steel sheet applied to a vehicle body of a vehicle is also required to have excellent toughness in order to sufficiently absorb impact at the time of a collision.
For example, Patent Document 1 discloses a high strength hot-rolled steel sheet which has excellent fatigue properties and stretch flangeability and in which when a bainite fraction is 80% or more, an average grain size r (nm) of a precipitation satisfies an expression of (r ≥207/(27.4 × (V) + 23.5 × (Nb) + 31.4 × (Ti) + 17.6 × (Mo) + 25.5 × (Zr) + 23.5 × (W)), and the average grain size r and a precipitation fraction f satisfies an expression of (r/f ≤ 12000).
Patent Document 2 discloses a hot-rolled steel sheet in which a steel structure at a position at a depth of 1/4 of a sheet thickness from a surface of the steel sheet contains, by area%, 60% or more of bainite, 5% or more and less than 30% of polygonal ferrite, less than 3% of residual austenite, and 10% or less of a remainder excluding the bainite, the residual austenite, and the polygonal ferrite, and a polygonal ferrite area ratio at a position at a depth of 100 µm from the surface of the steel sheet and a polygonal ferrite area ratio at a position at a depth of 1/4 of the sheet thickness satisfy an expression of (Vas > 1.5 Vaq, where Vas is an area ratio (%) of the polygonal ferrite at a position at a depth of 100 µm from the surface of the steel sheet, and Vaq is an area ratio of the polygonal ferrite at a position of a depth of 1/4 of the sheet thickness from the surface of the steel sheet).

[Prior Art Document]

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2009-84637
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2016-50335

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

However, in Patent Documents 1 and 2, toughness is not considered. The present inventors have found that it is necessary not only to improve ductility and hole expansibility but also to secure toughness, in order to achieve both weight reduction of a vehicle body and collision properties.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same.
In addition, a steel sheet applied to a vehicle body of a vehicle may be required to have excellent punching properties in addition to the above-mentioned properties in some cases. Therefore, an object of the present invention is, preferably, to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.

[Means for Solving the Problem]

In view of the above-mentioned problems, as a result of intensive investigations on the chemical composition of a hot-rolled steel sheet and the relationship between a microstructure and mechanical properties, the present inventors have obtained the following findings (a) to (e) and thus completed the present invention.

(a) In order to obtain excellent ductility and hole expansibility, it is necessary to make a total area ratio of bainite 80.0% or more.
(b) By controlling a grain boundary density with a specific orientation in bainite, the ductility, the hole expansibility, and the toughness can be further improved.
(c) In order to make the grain boundary density with the specific orientation in bainite within a desired range, it is necessary to control a winding temperature and a retention temperature and retention time after winding.
(d) In order to improve the punching properties, it is necessary to control an average grain size and an aspect ratio of prior austenite grains.
(e) In order to obtain the desired average grain size and the aspect ratio of the prior austenite grains, it is necessary to control a hot rolling condition more strictly. Specifically, in a hot rolling step, it is necessary to control a total rolling reduction of rough rolling and rolling reductions of final three stages of finish rolling.

The gist of the present invention made based on the above findings is as follows.

(1) A hot-rolled steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass%,
- C: 0.030% to 0.200%;
- Si: 0.05% to 2.50%;
- Mn: 1.00% to 4.00%;
- sol. Al: 0.001% to 2.000%;
- Ti: 0.030% to 0.200%,
- P: 0.020% or less;
- S: 0.020% or less;
- N: 0.010% or less;
- Nb: 0% to 0.200%;
- B: 0% to 0.010%;
- V: 0% to 1.00%;
- Mo: 0% to 1.00%;
- Cu: 0% to 1.00%;
- W: 0% to 1.00%;
- Cr: 0% to 1.00%;
- Ni: 0% to 1.00%;
- Co: 0% to 1.00%;
- Ca: 0% to 0.010%;
- Mg: 0% to 0.010%;
- REM: 0% to 0.010%;
- Zr: 0% to 0.010%; and
- a remainder consisting of iron and impurities,
- in which a microstructure contains, by area%,
- bainite: 80.0% or more,
- ferrite: 10.0% or less, and
- a remainder in the microstructure: 10.0% or less,
- a total density of a length L₇ of a grain boundary having a crystal orientation difference of 7° and a length L₆₈ of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 µm/µm², and
- a tensile strength is 780 MPa or more.
(2) The hot-rolled steel sheet according to (1) may include, as a chemical composition, by mass%, one or two or more selected from the group consisting of:
- Nb: 0.005% to 0.200%;
- B: 0.001% to 0.010%;
- V: 0.005% to 1.00%;
- Mo: 0.005% to 1.00%;
- Cu: 0.005% to 1.00%;
- W: 0.005% to 1.00%;
- Cr: 0.005% to 1.00%;
- Ni: 0.005% to 1.00%;
- Co: 0.005% to 1.00%;
- Ca: 0.0005% to 0.010%;
- Mg: 0.0005% to 0.010%;
- REM: 0.0005% to 0.010%; and
- Zr: 0.0005% to 0.010%.
(3) The hot-rolled steel sheet according to (1) or (2), in which in the microstructure, an average grain size of prior austenite grains is 10 to 30 µm, and a ratio ld/Sd between a long axis l_d and a short axis S_d of the prior austenite grains may be 2.0 or less.
(4) A method of manufacturing a hot-rolled steel sheet according to another aspect of the present invention includes:
- a heating step of retaining a slab having the chemical composition according to (1) above at a heating temperature of 1200°C or higher for 1.0 hour or longer;
- a hot rolling step of performing rough rolling so that a rough rolling completion temperature is 1000°C or higher and a total rolling reduction is more than 65%, and performing finish rolling so that a finish rolling completion temperature is 860°C to 980°C; and
- a cooling step of performing cooling to a temperature range of 570°C to 620°C at an average cooling rate of 20 °C/s or higher and performing winding, then, performing retaining at a temperature range of 500°C to 580°C for 2.0 to 12.0 hours, and then performing cooling to a room temperature.
(5) The method of manufacturing a hot-rolled steel sheet according to (4) above, in which in the hot rolling step, the total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling may be performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.

[Effects of the Invention]

According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same. According to the above preferred aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.

[Embodiments of the Invention]

The chemical composition and the microstructure of a hot-rolled steel sheet (hereinafter, sometimes simply referred to as a steel sheet) according to the present embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention.
The numerical limit range described with "to" in between includes the lower limit and the upper limit. Regarding the numerical value indicated by "less than" or "more than", the value does not fall within the numerical range. In the following description, % regarding the chemical composition is mass% unless otherwise specified.

Chemical Composition

The hot-rolled steel sheet according to the present embodiment includes, by mass%, C: 0.030% to 0.200%, Si: 0.05% to 2.50%, Mn: 1.00% to 4.00%, sol. Al: 0.001% to 2.000%, Ti: 0.030% to 0.200, P: 0.020% or less, S: 0.020% or less, N: 0.010% or less, and a remainder: Fe and impurities. Each element will be described in detail below.

C: 0.030% to 0.200%

C is an element that promotes formation of bainite by improving a strength of a hot-rolled steel sheet and also improving hardenability. In order to obtain this effect, a C content is set to 0.030% or more. The C content is preferably 0.040% or more.
On the other hand, when the C content is more than 0.200%, it becomes difficult to control the formation of bainite, a large amount of martensite is formed, and one or both of ductility and hole expansibility of the hot-rolled steel sheet is decreased. Therefore, the C content is set to 0.200% or less. The C content is preferably 0.180% or less.

Si: 0.05% to 2.50%

Si is an element that contributes to solid solution strengthening and is an element that contributes to improving the strength of the hot-rolled steel sheet. In addition, Si has an action of making steel soundness by deoxidation (suppressing an occurrence of a defect such as blow holes in the steel). When a Si content is less than 0.05%, an effect by the action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more and more preferably 1.00% or more.
On the other hand, Si is an element that promotes formation of a mixture (MA) of full hard martensite (hereinafter, when simply referred to as martensite, this martensite means fresh martensite) and residual austenite. When the Si content is more than 2.50%, MA is formed and the hole expansibility of the hot-rolled steel sheet is decreased. Therefore, the Si content is set to 2.50% or less. The Si content is preferably 2.30% or less and more preferably 2.00% or less.

Mn: 1.00% to 4.00%

Mn dissolves in steel to contribute to an increase in the strength of the hot-rolled steel sheet, promotes the formation of bainite by improving the hardenability, and improves the hole expansibility of the hot-rolled steel sheet. In order to obtain such an effect, a Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more.
On the other hand, when the Mn content is more than 4.00%, it becomes difficult to control the formation of the bainite, a desired amount of bainite cannot be obtained, and one or both of the ductility and the hole expansibility of the hot-rolled steel sheet is decreased. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less.

sol. Al: 0.001% to 2.000%

Similar to Si, Al has an action of deoxidizing steel to make the steel soundness. When a sol. Al content is less than 0.001%, an effect by the action cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more.
On the other hand, when the sol. Al content is more than 2.000%, an increase of an oxide-based inclusion is caused, and the hole expansibility of the hot-rolled steel sheet is decreased. Therefore, the sol. Al content is set to 2.000% or less. The sol. Al content is preferably 1.500% or less and more preferably 1.300% or less.
The sol. Al in the present embodiment means acid-soluble Al, and refers to solid solution Al present in steel in a solid solution state.

Ti: 0.030% to 0.200%

Ti precipitates as a carbide or a nitride in steel, and has an action of refining the microstructure by an austenite pinning effect and improving the strength of the hot-rolled steel sheet. When a Ti content is less than 0,030%, an effect by the action cannot be obtained. Therefore, the Ti content is set to 0.030% or more. The Ti content is preferably 0.050% or more and more preferably 0.080% or more.
On the other hand, when the Ti content is more than 0.200%, the prior austenite grains are less likely to recrystallize, and a rolled texture develops, resulting in decrease in the hole expansibility of the hot-rolled steel sheet. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.170% or less, and more preferably 0.150% or less.

P: 0.020% or less

P is an element that dissolves in steel and contributes to an increase of the strength of the hot-rolled steel sheet. However, P is also an element that segregates at a grain boundary, particularly at a prior austenite grain boundary, and promotes a grain boundary fracture due to a boundary segregation, thereby causing a decrease in the workability of the hot-rolled steel sheet. A P content is preferably as low as possible, and containing of P is acceptable up to 0.020%. Therefore, the P content is set to 0.020% or less. The P content is preferably 0.015% or less.
The P content is preferably set to 0%. However, when the P content is reduced to less than 0.0001%, the manufacturing costs increase. Therefore, the P content may be 0.0001% or more.

S: 0.020% or less

S is an element that adversely affects weldability and manufacturability during casting and hot rolling. S combines with Mn to form coarse MnS. This MnS deteriorates the bendability and hole expansibility of the hot-rolled steel sheet, and promotes an initiation of a delayed fracture. A S content is preferably as low as possible, and containing of S is acceptable up to 0.020%. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.015% or less.
The S content is preferably set to 0%. However, when the S content is reduced to less than 0.0001%, the manufacturing cost increases and it is economically disadvantageous. Therefore, the S content may be set to 0.0001% or more.

N: 0.010% or less

N is an element that forms a coarse nitride in steel. This nitride deteriorates the bendability and the hole expansibility of the hot-rolled steel sheet. Therefore, a N content is set to 0.010% or less. The N content is preferably 0.008% or less.
When the N content is reduced to less than 0.0001%, a significant increase in manufacturing cost is caused. Therefore, the N content may be set to 0.0001% or more.
A remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment consists of Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, and the like, and/or those allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
The hot-rolled steel sheet according to the present embodiment may contain the following elements as an optional element in addition to a part of Fe. In a case where the above optional element is not contained, a lower limit of a content thereof is 0%. Hereinafter, each optional element will be described in detail.

Nb: 0% to 0.200%

Nb is an element that forms a carbide during hot rolling and contributes to improvement in the strength of hot-rolled steel sheet by precipitation hardening. In order to reliably obtain the effect, a Nb content is preferably set to 0.005% or more.
On the other hand, when the Nb content is more than 0.200%, a recrystallization temperature of the prior austenite grains becomes too high and a texture develops, and the hole expansibility of the hot-rolled steel sheet may be decreased in some cases. Therefore, the Nb content is set to 0.200% or less.

B: 0% to 0.010%

B is an element that segregates into the prior austenite grain boundary, suppresses the formation and growth of ferrite, and contributes to improvement in the strength and hole expansibility of the hot-rolled steel sheet. In order to reliably obtain these effects, a B content is preferably set to 0.001% or more.
On the other hand, even when B is contained in an amount more than 0.010%, the above effects are saturated. Therefore, the B content is set to 0.010% or less.

V: 0% to 1.00%

V is an element that forms a carbonitride during hot rolling and contributes to improvement in the strength of hot-rolled steel sheet by precipitation hardening. In order to reliably obtain the effect, a V content is preferably set to 0.005% or more.
On the other hand, when the V content is more than 1.00%, a coarse carbide is formed in the slab, which causes an initiation of cracking in the heating step. Therefore, the V content is set to 1.00% or less.

Mo: 0% to 1.00%

Mo is an element that promotes the formation of bainite by improving the hardenability of steel and contributes to the improvement in the strength and the hole expansibility of the hot-rolled steel sheet. In order to reliably obtain the effect, a Mo content is preferably set to 0.005% or more.
On the other hand, when the Mo content is more than 1.00%, martensite is likely to be formed, and one or both of elongation and the hole expansibility of the hot-rolled steel sheet may be decreased in some cases. Therefore, the Mo content is set to 1.00% or less.

Cu: 0% to 1.00%

Cu is an element that has an effect for stably securing the strength of the hot-rolled steel sheet. Therefore, Cu may also be contained. However, even when containing Cu in an amount more than 1.00%, the effect of the action is likely to be saturated and may be economically disadvantageous in some cases. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Cu content is preferably 0.005% or more.

W: 0% to 1.00%

W is an element that is effective in improving the strength of the hot-rolled steel sheet by solid or precipitation. However, even when containing W in an amount more than 1.00%, the effect of the action is likely to be saturated and may be economically disadvantageous in some cases. Therefore, a W content is set to 1.00% or less. The W content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the W content is preferably 0.005% or more.

Cr: 0% to 1.00%

Cr is an element that is effective in improving the hardenability and improving the strength of the hot-rolled steel sheet. However, even when containing Cr in an amount more than 1.00%, the effect of the action is likely to be saturated and may be economically disadvantageous in some cases. Accordingly, a Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Cr content is preferably 0.005% or more.

Ni: 0% to 1.00%

Ni is an element that is effective in improving the hardenability and improving the strength of the hot-rolled steel sheet. However, when containing Ni in an amount more than 1.00%, the hardenability is excessively increased and a microstructural fraction of martensite is increased, so that the hole expansibility of the hot-rolled steel sheet may deteriorate in some cases. Therefore, a Ni content is set to 1.00% or less. The Ni content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Ni content is preferably 0.005% or more.

Co: 0% to 1.00%

Co is an element that is effective in improving the strength of the hot-rolled steel sheet by solid solution strengthening. However, even when containing Co in an amount more than 1.00%, the effect of the action is likely to be saturated and may be economically disadvantageous in some cases. Accordingly, a Co content is set to 1.00% or less. The Co content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Co content is preferably 0.005% or more.

Ca: 0% to 0.010%
Mg: 0% to 0.010%
REM: 0% to 0.010%

Zr: 0% to 0.010%

All of calcium (Ca), magnesium (Mg), a rare earth element (REM), and zirconium (Zr) are elements that contribute to inclusion control, especially fine dispersion of an inclusion, and has an action of enhancing the toughness of the hot-rolled steel sheet. Therefore, these elements may be contained. However, when each of the elements is contained in an amount of more than 0.010%, deterioration of surface properties may become apparent in some cases. Therefore, the amount of each of these elements is set to 0.010% or less. Each amount of these elements is preferably 0.005% or less and more preferably 0.003% or less. In order to obtain the effect by the action more reliably, each amount of the elements is preferably 0.0005% or more.
REM in the present embodiment refers to a total of 17 elements including Sc, Y, and lanthanoid, and the REM content refers to a total amount of these elements. In a case of the lanthanoid, lanthanoid is industrially added in the form of misch metal.
The chemical composition of the hot-rolled steel sheet may be measured by a general analytical method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) or optical emission spectroscopic (OES). Noted that, C and S may be measured by using a combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method.
Microstructure of Hot-Rolled Steel Sheet
Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, a microstructure contains, by area%, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, and a total density of a length L₇ of a grain boundary having a crystal orientation difference of 7° and a length L₆₈ of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 µm/µm².
In addition, in the hot-rolled steel sheet according to the present embodiment, in the microstructure, an average grain size of prior austenite grains is 10 to 30 µm, and a ratio ld/Sd between a long axis l_d and a short axis S_d of the prior austenite grains may be 2.0 or less.
In the present embodiment, the microstructure is defined at a depth of 1/4 of the sheet thickness from a surface and a center position in a sheet width direction in a cross section parallel to a rolling direction. The reason is that the microstructure at this position is a typical microstructure of a steel sheet.

Bainite: 80.0% or more

Bainite means a lath-shaped bainitic ferrite and a structure having a Fe-based carbide between and/or inside the bainitic ferrite. Unlike polygonal ferrite, the bainitic ferrite has a lath shape and has a relatively high dislocation density inside, and therefore can be easily distinguished from other structures using SEM or TEM.
When an area ratio of the bainite is less than 80.0%, the toughness and the hole expansibility of the hot-rolled steel sheet are significantly decreased. Therefore, the area ratio of the bainite is set to 80.0% or more. The area ratio of the bainite is preferably 85.0% or more and more preferably 90.0% or more. The higher the area ratio of the bainite, the more preferable. However, since it is difficult to achieve an area ratio of 97.5% or more due to the presence of ferrite, cementite, or MA (mixture of residual austenite and martensite), a practical upper limit may be 97.5%.

Ferrite: 10.0% or less

Ferrite is polygonal ferrite, and the bainitic ferrite is not included in ferrite. When an area ratio of the ferrite is more than 10.0%, a desired tensile strength cannot be obtained. Therefore, the area ratio of the ferrite is set to 10.0% or less. The area ratio of the ferrite is preferably 5.0% or less. From the viewpoint of securing ductility, the area ratio of the ferrite may be 1.0% or more.

Remainder in Microstructure (Cementite, Pearlite, Martensite, Tempered martensite, and residual austenite): 10.0% or Less in Total

All of Cementite, pearlite, martensite, tempered martensite, and residual austenite are starting points of voids during distortion, and are structures that deteriorate the hole expansibility of the hot-rolled steel sheet. When a total area ratio of these remainder in the microstructure is more than 10.0%, the desired ductility and the hole expansibility cannot be obtained. Therefore, the area ratio of the remainder in the microstructure (cementite, pearlite, martensite, tempered martensite, and residual austenite) is set to 10.0% or less. The area ratio thereof is preferably 5.0% or less.
On the other hand, in a microstructure control, since it is practically difficult to control the area ratio of the remainder in the microstructure to less than 1.0%, the area ratio of the remainder in the microstructure may be 1.0% or more.
In addition, the smaller the total area ratio of the martensite and the tempered martensite in the remainder in the microstructure, the more stable and excellent hole expansibility can be obtained. Therefore, the total area ratio of the martensite and the tempered martensite is preferably 5.0% or less. The total area fraction thereof is more preferably 3.0% or less.
A method of measuring an area ratio of each structure will be described below.
A test piece is taken from the hot-rolled steel sheet so that a microstructure at a depth of 1/4 of the sheet thickness from the surface and a center position in a sheet width direction in the cross section parallel to the rolling direction can be observed.
After polishing the cross section of the test piece with silicon carbide paper of #600 to #1500, finishing is performed to a mirror surface using a diamond powder having a grain size of 1 to 6 µm using a diluted solution such as alcohol or a liquid dispersed in pure water. Next, polishing is performed with colloidal silica without containing an alkaline solution at a room temperature to remove a strain introduced into a surface layer of a sample. A region with a length of 50 µm and between a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 µm, so that a position at the depth of 1/4 of the sheet thickness from the surface is the center in a random position of the sample cross section in a longitudinal direction, to obtain crystal orientation information.
For the measurement, an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. In this case, the EBSD analyzer is set such that the degree of vacuum inside is 9.6 × 10^-5 Pa or less, an acceleration voltage is 15 kV, an irradiation current level is 13, and an electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of the residual austenite using a "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Those having a crystal structure of fcc are determined to be residual austenite.
Next, those having a crystal structure of bcc are determined to be bainite, ferrite, and the "remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than residual austenite". In these regions, a region where the "Grain Orientation Spread" is 1° or less is extracted as ferrite, by using a "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, under the condition in which 15° grain boundary is defined as the grain boundary. By calculating the area ratio of the extracted ferrite, the area ratio of the ferrite is obtained.
Subsequently, under the condition in which 5° grain boundary is defined as the grain boundary in the residual area (a region where the "Grain Orienttion Spread" is more than 1°), when a maximum value of the "Grain Age IQ" of the ferrite region is set to Iα, a region exceeding Iα/2 is extracted as bainite, and a region of equal to or less than Iα/2 is extracted as "remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than the residual austenite". By calculating the area ratio of the extracted bainite, the area ratio of the bainite is obtained. In addition, the area ratio of the extracted "remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than residual austenite" is calculated, and the area ratio of the above residual austenite is added to obtain the area ratio of the remainder in the microstructure (cementite, pearlite, martensite, tempered martensite, and residual austenite).
Regarding the extracted "remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than residual austenite", cementite, pearlite, martensite, and tempered martensite can be distinguished by the following method. First, in order to observe the same region as the EBSD measurement region by SEM, a Vickers indentation is imprinted in the vicinity of an observation position. Thereafter, a contamination on the surface layer is removed by polishing, leaving the structure of the observed section, and nital etching is performed. Next, the same visual field as the EBSD observed section is observed by SEM at a magnification of 3000 times.
In the EBSD measurement, among the regions determined as the remainder in the microstructure, a region having a substructure in the grain and where cementite precipitates with a plurality of variants is determined to be tempered martensite. A region where the cementite precipitates in a lamellar shape is determined to be pearlite. Spherical particles with high brightness and grain size (circle equivalent diameter) of 2 µm or less are determined to be cementite. A region where the brightness is high and the substructure is not exposed by etching is determined as "martensite and residual austenite". By calculating the area ratio of each structure, the area ratio of the tempered martensite, the pearlite, the martensite, and the "martensite and residual austenite" is obtained. The area ratio of the martensite can be obtained by subtracting the area ratio of the residual austenite obtained by the above-mentioned EBSD from the area ratio of the obtained "martensite and residual austenite".
For removing contamination on the surface layer of the observed section, a method such as buffing using alumina particles having a particle diameter of 0.1 µm or less or Ar ion sputtering may be used.
Total Density of Length L₇ of Grain Boundary Having Crystal Orientation Difference of 7° and Length L₆₈ of Grain Boundary Having Crystal Orientation Difference of 68° about <110> Direction in Bainite: 0.35 to 0.60 µm/µm²
When a total density of a length L₇ of a grain boundary having a crystal orientation difference of 7° and a length L₆₈ of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is set to 0.35 to 0.60 µm/µm², the ductility, the hole expansibility, and the toughness of the hot-rolled steel sheet can be improved.
When the total density of the length L₇ and the length L₆₈ is less than 0.35 µm/µm², the toughness of the bainite is significantly decreased, and the desired toughness cannot be obtained in the hot-rolled steel sheet. Therefore, the total density of L₇ and L₆₈ is set to 0.35 µm/µm² or more. The total density thereof is preferably 0.40 µm/µm² or more. On the other hand, when the total density of the length L₇ and the length L₆₈ is more than 0.60 µm/µm², the ductility of the bainite is significantly decreased, and the excellent ductility and the hole expansibility cannot be obtained in the hot-rolled steel sheet. Therefore, the total density of L₇ and L₆₈ is set to 0.60 µm/µm² or less. The total density thereof is preferably 0.55 µm/µm² or less.
The grain boundary having a crystal orientation difference of X° about the <110> direction refers to a grain boundary having a crystallographic relationship in which the crystal orientations of the crystal grain A and the crystal grain B are the same by rotating one crystal grain B by X° about the <110> axis, when two adjacent crystal grain A and crystal grain B are specified at a certain grain boundary. However, considering the measurement accuracy of the crystal orientation, an orientation difference of ±4° is allowed from the matching orientation relationship.
In the present embodiment, the length L₇ of a grain boundary and the length L₆₈ as above are measured by using the electron back scatter diffraction pattern-orientation image microscopy (EBSP-OIM) method. In the EBSP-OIM method, a crystal orientation of an irradiation point can be measured for a short time period in such manner that a highly inclined sample in a scanning electron microscope (SEM) is irradiated with electron beams, a Kikuchi pattern formed by back scattering is photographed by a high sensitive camera, and the photographed image is processed by a computer. The EBSP-OIM method is performed using a device in which a scanning electron microscope and an EBSP analyzer are combined and an OIM Analysis (registered trademark) manufactured by AMETEK Inc.
In the EBSP-OIM method, since the fine structure of the sample surface and the crystal orientation can be analyzed, the length of the grain boundary having a specific crystal orientation difference can be quantitatively determined. The analyzable area of the EBSP-OIM method is a region that can be observed by the SEM. The EBSP-OIM method makes it possible to analyze a region with a minimum resolution of 20 nm, which varies depending on the resolution of the SEM.
When measuring the density of the length of specific grain boundary of the microstructure at the depth of 1/4 of the sheet thickness from the surface and at the center position in the sheet width direction in the cross section parallel to the rolling direction, an analysis is performed in at least 5 visual fields of a region of 50 µm × 50 µm at a magnification of 1000 times and an average value of the lengths of the grain boundary having a crystal orientation difference of 7° about the <110> direction in the bainite is calculated to obtain L₇. Similarly, an average value of the lengths of the grain boundary having a crystal orientation difference of 68° about the <110> direction in the bainite is calculated to obtain L₆₈. As described above, the orientation difference of ±4° is allowed.
By dividing the obtained L₇ and L₆₈ by the measurement area, the total density of the length L₇ of the grain boundary having the crystal orientation difference of 7° and the length L₆₈ of the grain boundary having the crystal orientation difference of 68° about the <110> direction in the bainite is obtained. In order to extract only the bainite and measure the density of the length of a specific grain boundary, a region exceeding Iα/2 may be extracted as the bainite, as in the case of determining the area ratio of the bainite.

Average grain size of prior austenite grains: 10 to 30 µm

Ratio l_d/S_d between Long Axis l_d and Short Axis S_d of Prior Austenite Grains: 2.0 or less

In the hot-rolled steel sheet according to the present embodiment, the average grain size of the prior austenite grains is 10 to 30 µm, and the ratio l_d/S_d between the long axis l_d and the short axis S_d of the prior austenite grains may be 2.0 or less. By controlling the average grain size of the prior austenite grains and the l_d/S_d within the above range, the punching properties of the hot-rolled steel sheet can be improved.
A method of measuring the average grain size of the prior austenite grains and the ratio l_d/S_d between the long axis l_d and the short axis S_d of the prior austenite grains will be described below.
A test piece is taken from the hot-rolled steel sheet so that a microstructure at a depth of 1/4 of the sheet thickness from the surface and a center position in a sheet width direction in the cross section parallel to the rolling direction can be observed. The prior austenite grain boundary is exposed by corroding the observed section with a saturated aqueous solution of picric acid. A magnified photograph of a cross section parallel to the rolling direction that has been corroded, at a depth of 1/4 of the sheet thickness from the surface and at the center position in the sheet width direction is photographed with a scanning electron microscope (SEM) at a magnification of 1000 times and 5 or more visual fields. The equivalent circle diameters (diameters) of at least 20 prior austenite grains having an equivalent circle diameter (diameter) of 2 µm or more, which are included in each SEM photograph, are determined by image processing, and an average value thereof is calculated to obtain the average grain size of the prior austenite grains. In a case where the prior austenite grains having an equivalent circle diameter of less than 2 µm are included, the above measurement is performed by excluding these grains.
In addition, the long axis and the short axis of at least 20 prior austenite grains having an equivalent circle diameter (diameter) of 2 µm or more, which are included in each of the above SEM photographs, are measured. By calculating the average value of the long axis and the short axis obtained by measuring each prior austenite grain, the long axis l_d and the short axis S_d of the prior austenite grain are obtained. By calculating these ratios, the ratio l_d/S_d between the long axis l_d and the short axis S_d of the prior austenite grains is obtained.

Tensile Strength: 780 MPa or More

The hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 780 MPa or more. When the tensile strength is less than 780 MPa, an applicable component is limited, and the contribution of weight reduction of the vehicle body is small. The tensile strength is preferably 980 MPa or more. An upper limit is not particularly limited, and may be 1800 MPa from the viewpoint of suppressing wearing of a die.

Total Elongation: 14.0% or More

The hot-rolled steel sheet according to the present embodiment may have a total elongation of 14.0% or more. An upper limit of the total elongation is not particularly limited, and may be 30.0% or less or 25.0% or less.
The tensile strength and the total elongation are measured according to JIS Z 2241: 2011 using a No. 5 test piece of JIS Z 2241: 2011. A sampling position of the tensile test piece is set to the center position in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction. The cross-head speed is set to 3 mm/min.

Hole Expansion Rate: 50% or More

The hot-rolled steel sheet according to the present embodiment may have a hole expansion rate of 50% or more. It is not necessary to particularly limit an upper limit of the hole expansion rate, and the upper limit thereof may be 90% or less or 85% or less.
The hole expansion rate is obtained by performing a hole expanding test in accordance with JIS Z 2256: 2010.

Impact Value at -40°C: 60 J/cm² or More

The hot-rolled steel sheet according to the present embodiment may have an impact value of 60 J/cm² or more at -40°C. It is not necessary to particularly limit an upper limit of the impact value at -40°C, and the upper limit thereof may be 180 J/cm² or less or 175 J/cm² or less.
A sub-sized Charpy impact test piece is taken from a predetermined position of the hot-rolled steel sheet, and the impact value at -40°C is determined in accordance with a test method described in JIS Z 2242: 2005.

Sheet Thickness: 0.6 to 8.0 mm

The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be 0.6 to 8.0 mm. When the sheet thickness of the steel sheet is less than 0.6 mm, it becomes difficult to secure the rolling completion temperature and the rolling force becomes excessive, which may make hot rolling difficult. Therefore, the sheet thickness of the steel sheet according to the present embodiment may be set to 0.6 mm or more. The sheet thickness is preferably 1.2 mm or more or 1.4 mm or more. On the other hand, when the sheet thickness is more than 8.0 mm, it becomes difficult to refine the microstructure, particularly, the prior austenite grains, and it may become difficult to secure the microstructure described above from the viewpoint of the microstructural fraction. Therefore, the sheet thickness may be set to 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.

Plating Layer

The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be an electro plating layer or a hot-dip plating layer. Examples of the electro plating layer include electrogalvanizing and electro Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The plating adhesion amount is not particularly limited and may be the same as before. Further, it is also possible to further enhance the corrosion resistance by applying an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
Next, a preferred method of manufacturing the hot-rolled steel sheet according to the present embodiment will be described.
The preferred method of manufacturing the hot-rolled steel sheet according to the present embodiment includes the following steps. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
A heating step of retaining a slab having a predetermined chemical composition at a heating temperature of 1200°C or higher for 1.0 hour or longer.
A hot rolling step of performing rough rolling so that a rough rolling completion temperature is 1000°C or higher and a total rolling reduction is more than 65%, and performing finish rolling so that a finish rolling completion temperature is 860°C to 980°C.
A cooling step of performing cooling to a temperature range of 570°C to 620°C at an average cooling rate of 20 °C/s or higher and performing winding, then, performing retaining at a temperature range of 500°C to 580°C for 2.0 to 12.0 hours, and then performing cooling to a room temperature.
In the hot rolling step, a total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling may be performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.

Each step will be described in detail below.

Heating step

In the heating step, the slab having the above-mentioned chemical composition is heated to a heating temperature of 1200°C or higher and retained for 1.0 hour. Since a coarse precipitate present at a slab stage causes cracking during rolling and a decrease in material properties, it is preferable to heat a steel material before the hot rolling to dissolve the coarse carbide. Therefore, the heating temperature is set to 1200°C or higher, and the retention time is set to 1.0 hour or longer. The preferred heating temperature is 1230°C or higher, and the preferred retention time is 3.0 hours or longer.
On the other hand, when the heating temperature becomes too high or the retention time becomes too long, the yield may decrease due to the large amount of scale generated. Therefore, the heating temperature may be set to 1400°C or lower, and the retention time may be set to 20.0 hours or shorter.
The slab to be heated is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by another casting method (for example, ingot-making method).

Hot Rolling Step

When the rough rolling is performed at a temperature lower than 1000°C, the prior austenite grains are not sufficiently recrystallized. Therefore, the texture develops and the desired hole expansibility cannot be obtained. Therefore, rough rolling is performed so that the rough rolling completion temperature is 1000°C or higher. The rough rolling completion temperature is preferably 1050°C or higher. On the other hand, when the rough rolling is performed at a temperature higher than 1300°C, the yield may decrease in some cases due to an increase in the amount of scale generated. Therefore, the rough rolling completion temperature may be 1300°C or lower.
In a case where the total rolling reduction in the rough rolling is low, grain sizes of the prior austenite grains becomes non-uniform, which causes a decrease in toughness. Therefore, the total rolling reduction in the rough rolling is set to more than 65%. The total rolling reduction in the rough rolling is preferably 68% or more, more preferably 70% or more, and even more preferably 80% or more. An upper limit of the total rolling reduction in the rough rolling is not particularly limited, and may be set to 90% or less.
The total rolling reduction in the rough rolling is represented by (1-t_r/t_s) × 100 (%) using the slab thickness: t_s and the sheet thickness t_r at the end of the rough rolling.
By setting the total rolling reduction in the rough rolling to 70% or more and strictly controlling the rolling reduction in the final three stages of finish rolling as described later, the average grain size and an aspect ratio of the prior austenite grains described above can be realized.
When the finish rolling completion temperature is lower than 860°C, the prior austenite grains are not sufficiently recrystallized. Therefore, the texture develops and the hole expansibility deteriorates. Therefore, the finish rolling completion temperature is set to 860°C or higher. The finish rolling completion temperature is preferably set to 900°C or higher. On the other hand, when the finish rolling completion temperature is higher than 980°C, the prior austenite grains become significantly coarse and the desired toughness cannot be obtained. Therefore, the finish rolling completion temperature is set to 980° C or lower. The finish rolling completion temperature is preferably 950° C or lower.
In the present embodiment, in order to realize the above-mentioned average grain size and the aspect ratio of the prior austenite grains and improve the punching properties of the hot-rolled steel sheet, the total rolling reduction in the rough rolling and rolling reduction of final three stages of the finish rolling may be strictly controlled. Specifically, as described above, the total rolling reduction in the rough rolling may be set to 70% or more, and the rolling reduction in the final three stages of the finish rolling may be set to less than 25%.
When even one rolling reduction among the rolling reductions of the final three stages of the finish rolling, that is, among the rolling reductions of the final pass of the finish rolling, second pass from the final pass, and the third pass from the final pass is 25% or more, the prior austenite grains become flat due to the rolling, and the prior austenite grains having a large aspect ratio, which is the starting point of a crack during punching, are formed. Therefore, all of the rolling reductions of the final three stages of the finish rolling (the rolling reductions of the final pass of the finish rolling, the second pass from the final pass, and the third pass from the final pass) may be set to less than 25%. All of the rolling reductions are 20% or less. The rolling reduction can be represented by (1-h/h₀) × 100 (%) when the sheet thickness after rolling in one pass is h and the sheet thickness before rolling is ho.

Cooling Step

After the hot rolling step, cooling is performed to a temperature range of 570°C to 620°C at an average cooling rate of 20 °C/s or higher. In the present embodiment, the average cooling rate is a value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.
When the average cooling rate is lower than 20 °C/s, a large amount of ferrite precipitates and a desired amount of bainite cannot be obtained. Therefore, the average cooling rate is set to 20 °C/s or higher. The average cooling rate is preferably 30 °C/s or higher, and more preferably 50 °C/s or higher. From the viewpoint of suppressing the increase in cooling equipment, the average cooling rate may be 200 °C/s or lower.
Cooling at the average cooling rate of 20 °C/s or higher is performed to a temperature range of 570°C to 620°C. When a cooling stop temperature is higher than 620° C, a desired amount of bainite cannot be obtained. Therefore, the cooling stop temperature is set to 620°C or lower. The cooling stop temperature may be any temperature as long as it can be retained in a temperature range of 620°C or lower and 500°C to 580°C, and in order to retain in the temperature range of 500°C to 580°C for 2.0 hours or more, the cooling stop temperature is preferably set to 550°C or higher. In addition, in order to preferably control the total density of L₇ and L₆₈ and obtain excellent toughness, the cooling stop temperature is preferably set to 570°C or higher.
Although the cooling stop temperature is lower than 500°C and retention is performed in the temperature range of 500°C to 580°C by reheating, a desired amount of bainite cannot be obtained. Therefore, heating after the stop of cooling is not desirable.
After cooling with an average cooling rate of 20 °C/s or higher, winding is performed. After winding, retaining is performed in a temperature range of 500°C to 580°C for 2.0 to 12.0 hours. When the retention temperature is outside the temperature range of 500°C to 580°C, or when the retention time is shorter than 2.0 hours or longer than 12.0 hours, a total density of L₇ and L₆₈ in the desired amount of bainite cannot be obtained. Therefore, the retention temperature is set to a temperature range of 500°C to 580°C, and the retention time is set to 2.0 to 12.0 hours. A lower limit of the retention temperature is preferably 530°C. An upper limit of the retention temperature is preferably 560°C. A lower limit of the retention time is preferably 4.0 hours, and more preferably 6.0 hours. An upper limit of the retention time is preferably 10.0 hours, and more preferably 8.0 hours.
During retaining in the temperature range of 500°C to 580°C, the steel sheet temperature may be fluctuated or be constant in the temperature range of 500°C to 580°C. In addition, even when the cooling stop temperature of cooling having an average cooling rate of 20 °C/s or higher is lower than 580°C, it is sufficient that the retention time of 2.0 to 12.0 hours can be secured in the temperature range of 500°C to 580°C.
After performing the above-mentioned retaining in the temperature range of 500°C to 580°C, cooling is performed to a room temperature. As the method of cooling to a room temperature, any method may be used, and cooling may be performed by an appropriate method such as mist cooling or rapid cooling using a water cooling tank, in addition to air cooling. The room temperature referred to here is a temperature range of 20°C to 30°C.

[Examples]

Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Steels having chemical compositions shown in Steel Nos. A to AM in Table 1 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Tables 5 to 7 under the manufacturing conditions shown in Tables 2 to 4. In addition, "F1", "F2", and "F3" in Tables 2 to 4 represent a rolling reduction of the final pass of finish rolling, a rolling reduction of the second pass from the final pass, and a rolling reduction of the third pass from the final pass, respectively. In addition, a test material No. 63 in Table 4 was reheated after the stop of cooling, and then retained in the temperature range of 500°C to 580°C.
With respect to the obtained hot-rolled steel sheet, the microstructural fraction, the total density of L₇ and L₆₈, the average grain size of the prior austenite grains, and the ratio ld/Sd between the long axis l_d and the short axis S_d of the prior austenite grains were determined. The results obtained are shown in Tables 5 to 7.

Evaluation Method of Properties of Hot-Rolled Steel Sheet

Tensile strength (TS) and Total Elongation (El)

Among the mechanical properties of the obtained hot-rolled steel sheet, the tensile strength (TS) and total elongation (El) were measured by using a test piece No 5 of JIS Z 2241: 2011, in accordance with JIS Z 2241: 2011. A sampling position of the tensile test piece was set to the center position in the sheet width direction, and the direction perpendicular to the rolling direction was set to the longitudinal direction. The cross-head speed was set to 3 mm/min.
In a case where the tensile strength (TS) was 780 MPa or more, the strength was determined excellent which was pass, and in a case where the tensile strength was less than 780 MPa, the strength was determined poor which was fail. In addition, in a case where the total elongation (El) was 14.0% or more, the ductility was determined excellent which was pass, and in a case where the total elongation was less than 14.0%, the ductility was determined poor which was fail.

Hole Expansion Rate (λ)

The hole expansion rate (λ) was evaluated by performing a hole expanding test in accordance with JIS Z 2256: 2010.
In a case where the hole expansion rate (λ) was 50% or more, the hole expansibility was determined excellent which was pass, and in a case where hole expansion rate (λ) was less than 50%, the hole expansibility was determined poor which was fail.

Impact Value (vE_-40)

The toughness was evaluated by performing a Charpy impact test at -40°C and determining the impact value. A sub-sized Charpy impact test piece was taken from a predetermined position of the hot-rolled steel sheet, and the impact value at -40°C was determined in accordance with a test method described in JIS Z 2242: 2005 to evaluate the toughness.
In a case where the impact value (vE_-40) was 60 J/cm² or more, the toughness was determined excellent which was pass, and in a case where the impact value (vE_-40) was less than 60 J/cm², the toughness was determined poor which was fail.

Punching Properties

The punching properties were evaluated by performing a punching test and observing the properties of the punched end surface. First, a punched hole was prepared with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm/s. Next, a cross section of the punched hole perpendicular to the rolling direction was embedded in a resin, and the punched end surface was imaged with a scanning electron microscope. In a case where the obtained observation photographs were observed and end surface roughness was not observed, "E (Excellent)" was noted in Tables 5 to 7 as having particularly good punching properties. In addition, in a case where a small elopement of less than 100 µm was observed, "G (Good)" is noted in Tables 5 to 7 as having good punching properties, and in a case where a large elopement of 100 µm or more is observed, "B (Bad)" is noted in Tables 5 to 7 as having poor punching properties.
When referring to Tables 5 to 7, it can be seen that Invention Examples have high strength and excellent ductility, hole expansibility, and toughness. In addition, it can be seen that Invention Examples in which the average grain size of the prior austenite grains is 10 to 30 µm, and the ratio l_d/Sd between the long axis l_d and the short axis S_d of the prior austenite grains is 2.0 or less have particularly good punching properties.
On the other hand, it can be seen that Comparative Example is poor in any one or more of the strength, the ductility, the hole expansibility, and toughness.

[Industrial Applicability]

According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same. According to the above preferred aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.

Claims

Ahot-rolled steel sheet comprising, as a chemical composition, by mass%:
C: 0.030% to 0.200%;

Si: 0.05% to 2.50%;

Mn: 1.00% to 4.00%;

sol. Al: 0.001% to 2.000%;

Ti: 0.030% to 0.200%,

P: 0.020% or less;

S: 0.020% or less;

N: 0.010% or less;

Nb: 0% to 0.200%;

B: 0% to 0.010%;

V: 0% to 1.00%;

Mo: 0% to 1.00%;

Cu: 0% to 1.00%;

W: 0% to 1.00%;

Cr: 0% to 1.00%;

Ni: 0% to 1.00%;

Co: 0% to 1.00%;

Ca: 0% to 0.010%;

Mg: 0% to 0.010%;

REM: 0% to 0.010%;

Zr: 0% to 0.010%; and

a remainder consisting of iron and impurities,

wherein a microstructure contains, by area%,
bainite: 80.0% or more,

ferrite: 10.0% or less, and

a remainder in the microstructure: 10.0% or less,

a total density of a length L₇ of a grain boundary having a crystal orientation difference of 7° and a length L₆₈ of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 µm/µm², and

a tensile strength is 780 MPa or more.
The hot-rolled steel sheet according to claim 1,
wherein the hot-rolled steel sheet includes, as a chemical composition, by mass%, one or two or more selected from the group consisting of:
Nb: 0.005% to 0.200%;

B: 0.001% to 0.010%;

V: 0.005% to 1.00%;

Mo: 0.005% to 1.00%;

Cu: 0.005% to 1.00%;

W: 0.005% to 1.00%;

Cr: 0.005% to 1.00%;

Ni: 0.005% to 1.00%;

Co: 0.005% to 1.00%;

Ca: 0.0005% to 0.010%;

Mg: 0.0005% to 0.010%;

REM: 0.0005% to 0.010%; and

Zr: 0.0005% to 0.010%.
The hot-rolled steel sheet according to claim 1 or 2,
wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 µm, and

a ratio l_d/S_d between a long axis l_d and a short axis S_d of the prior austenite grains is 2.0 or less.
A method of manufacturing the hot-rolled steel sheet according to claim 1, comprising:
a heating step of retaining a slab having the chemical composition according to claim 1, at a heating temperature of 1200°C or higher for 1.0 hour or longer;

a hot rolling step of performing rough rolling so that a rough rolling completion temperature is 1000°C or higher and a total rolling reduction is more than 65%, and performing finish rolling so that a finish rolling completion temperature is 860°C to 980°C; and

a cooling step of performing cooling to a temperature range of 570°C to 620°C at an average cooling rate of 20 °C/s or higher and performing winding, then, performing retaining at a temperature range of 500°C to 580°C for 2.0 to 12.0 hours, and then performing cooling to a room temperature.
The method of manufacturing the hot-rolled steel sheet according to claim 4,
wherein in the hot rolling step, the total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling is performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.