US20220259692A1

US20220259692A1 - Hot-rolled steel sheet and method of manufacturing same

Info

Publication number: US20220259692A1
Application number: US17/765,697
Authority: US
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: 2022-08-18
Also published as: WO2021090642A1; JP7243854B2; KR102715430B1; JPWO2021090642A1; EP4056723A4; CN114630917B; KR20220068250A; EP4056723A1; MX2022004885A; CN114630917A

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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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 Nov. 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 rand 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 ¼ 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 ¼ of the sheet thickness satisfy an expression of (Vαs>1.5 Vαq, where Vαs is an area ratio (%) of the polygonal ferrite a a position at a depth of 100 μm from the surface of the steel sheet, and Vαq is an area ratio of the polygonal ferrite at a position of a depth of ¼ 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 toted 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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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 I_d/S_dbetween a long axis I_dand a short axis S_dof 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 shed. 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 , 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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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 I_d/S_dbetween a long axis I_dand a short axis S_dof the prior austenite grains may be 2.0 or less.
In the present embodiment, the microstructure is defined at a depth of ¼ 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 ¼ 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 ⅛ of the sheet thickness from the surface to a depth of ⅜ 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 ¼ 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 9.6×10⁻⁵Pa 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 Orientation 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 L₇of Grain Boundary Having Crystal Orientation Difference of 7° and Length L₆₈of 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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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 L₇and the length L₆₈is 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 L₇and L₆₈is set to 0.35 μm/μm²or more. The total density thereof is preferably 0.40 μm/μm²or more. On the other hand, when the total density of the length L₇and the length L₆₈is 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 L₇and L₆₈is set to 0.60 μm/μm²or less. The total density thereof is preferably 0.55 μm/μm²or 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 L₇of a grain boundary and the length L₆₈as above are measured by using the electron back scatter diffraction pattern-orientation image microscopy (EBSP-OIM) method. In the EBSP-OLM 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 ¼ 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 L₇and L₆₈by the measurement area, the total density of the length L₇of the grain boundary having the crystal orientation difference of 7° and the length L₆₈of 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 I_d/S_dbetween Long Axis I_dand Short Axis S_dof 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 I_d/S_dbetween the long axis I_dand the short axis S_dof the prior austenite grains may be 2.0 or less. By controlling the average grain size of the prior austenite grains and the I_d/S_dwithin 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 I_d/S_dbetween the long axis I_dand the short axis S_dof 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 ¼ 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 ¼ 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 id and the short axis S_dof the prior austenite grain are obtained. By calculating these ratios, the ratio I_d/S_dbetween the long axis I_dand the short axis S_dof the prior austenite rains 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/cm²or More
The hot-rolled steel sheet according to the present embodiment may have an impact value of 60 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/cm²or less or 175 J/cm²or 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 (16 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_sand the sheet thickness t_rat 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 h₀.
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 L₇and L₆₈and 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 L₇and L₆₈in 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 roiling 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 L₇and L₆₈, the average grain size of the prior austenite grains, and the ratio I_d/S_dbetween the long axis I_dand the short, axis S_dof 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 (EI)
Among the mechanical properties of the obtained hot-rolled steel sheet, the tensile strength (TS) and total elongation (EI) 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 (EI) 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 as 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 (vE₄₀)
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 (vE₄₀) was 60 J/cm²or more, the toughness was determined excellent which was pass, and in a case where the impact value (vE₄₀) 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 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 um 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 I_d/S_dbetween the long axis I_dand the short axis S_dof 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.

TABLE 1

Kind of	Chemical composition (unit: mass %, remainder consisting of Fe and impurities)

steel	C	Si	Mn	sol. Al	Ti	P	S	N	Others	Remarks

A	0.070	0.90	2.20	0.050	0.120	0.010	0.001	0.003		Present Invention Steel
B	0.210	0.50	2.50	0.030	0.100	0.010	0.001	0.005		Comparative Steel
C	0.028	1.20	2.20	0.050	0.090	0.010	0.002	0.003		Comparative Steel
D	0.090	2.80	2.20	0.035	0.110	0.010	0.001	0.003		Comparative Steel
E	0.063	1.40	0.85	0.100	0.110	0.020	0.001	0.003		Comparative Steel
F	0.070	1.00	4.50	0.030	0.060	0.010	0.002	0.003		Comparative Steel
G	0.041	1.50	1.50	2.200	0.110	0.020	0.001	0.002		Comparative Steel
H	0.040	1.01	1.50	0.030	0.020	0.010	0.001	0.003		Comparative Steel
I	0.102	1.20	1.90	0.030	0.250	0.010	0.001	0.003		Comparative Steel
J	0.178	0.55	1.65	0.090	0.110	0.009	0.002	0.003		Present Invention Steel
K	0.033	1.67	2.43	0.030	0.130	0.010	0.001	0.003		Present Invention Steel
L	0.055	1.55	1.85	0.040	0.100	0.010	0.002	0.003	Nb: 0.020	Present Invention Steel
M	0.089	1.25	2.00	0.100	0.090	0.010	0.002	0.003	Nb: 0.230	Comparative Steel
N	0.065	1.20	2.10	0.020	0.110	0.005	0.001	0.002	Nb: 0.012, Cr: 0.50	Present Invention Steel
O	0.060	1.13	2.41	0.025	0.112	0.010	0.001	0.002	Nb: 0.020, B: 0.002	Present Invention Steel
P	0.049	1.20	1.80	0.060	0.090	0.010	0.003	0.003	B: 0.001	Present Invention Steel
Q	0.057	0.90	1.60	0.020	0.110	0.009	0.003	0.003	Cr: 0.70, B: 0.002	Present Invention Steel
R	0.060	0.53	1.95	0.050	0.150	0.010	0.002	0.002	V: 0.01	Present Invention Steel
S	0.101	0.30	1.67	0.020	0.120	0.007	0.001	0.003	Mo: 0.02	Present Invention Steel
T	0.070	1.20	1.50	0.030	0.100	0.010	0.002	0.003	Mo: 0.03, V: 0.03	Present Invention Steel
U	0.062	1.10	2.20	0.013	0.080	0.010	0.002	0.001	Cr: 0.61, Mo: 0.02,	Present Invention Steel
									V: 0.10
V	0.090	0.33	1.60	0.120	0.110	0.010	0.001	0.002	Cr: 0.45, Mo: 0.25,	Present Invention Steel
									V: 0.18, Ca: 0.003
W	0.081	0.63	1.55	0.130	0.099	0.011	0.003	0.003	Cr: 0.61, Mo: 0.20,	Present Invention Steel
									V: 0.24, B: 0.002
X	0.055	0.95	2.50	0.122	0.110	0.009	0.003	0.002	Mo: 1.30	Comparative Steel
Y	0.071	0.10	2.80	0.500	0.130	0.008	0.002	0.003	Cu: 0.05	Present Invention Steel
Z	0.053	1.00	1.50	0.050	0.130	0.002	0.003	0.003	Ni: 0.80	Present Invention Steel
AA	0.068	1.30	2.00	0.030	0.060	0.002	0.002	0.003	Ni: 1.30	Comparative Steel
AB	0.100	1.12	1.66	0.020	0.110	0.003	0.002	0.003	Co: 0.51	Present Invention Steel
AC	0.055	1.03	1.78	0.030	0.100	0.004	0.002	0.003	Ca: 0.040	Present Invention Steel
AD	0.081	0.90	2.03	0.030	0.120	0.010	0.001	0.003	Mg: 0.008	Present Invention Steel
AE	0.056	1.50	1.91	0.022	0.070	0.010	0.001	0.003	REM: 0.005	Present Invention Steel
AF	0.068	1.87	1.59	0.050	0.090	0.009	0.001	0.003	Zr: 0.003	Present Invention Steel
AG	0.071	1.00	1.72	0.030	0.110	0.008	0.001	0.002	Cr: 0.64	Present Invention Steel
AH	0.070	0.98	2.01	0.031	0.150	0.015	0.002	0.003	Nb: 0.050	Present Invention Steel
AI	0.180	1.71	2.58	0.016	0.091	0.015	0.002	0.003	B: 0.001	Present Invention Steel
AJ	0.071	1.20	2.10	0.040	0.090	0.007	0.002	0.003	W: 0.050	Present Invention Steel
AK	0.080	0.08	2.20	0.020	0.100	0.003	0.002	0.003		Present Invention Steel
AL	0.065	2.20	2.00	0.001	0.080	0.002	0.001	0.003		Present Invention Steel
AM	0.072	1.40	1.20	0.070	0.130	0.002	0.001	0.003		Present Invention Steel

An underline indicates that the value is outside a range of the present invention.

	TABLE 2

	Manufacture conditions

Heating step

Rough rolling step

Finish rolling step

Sample		Heating	Retention	Rolling completion	Total rolling	Rolling reduction	Rolling start	Rolling completion
material	Kind of	temperature	time	temperature	reduction	(%)	temperature	temperature

No.	steel	(° C.)	(h)	(° C.)	(%)	F1	F2	F3	(° C.)	(° C.)

1	N	1244	2.3	1146	82	28	25	9	1060	890
2	A	1250	3.0	1156	81	26	26	11	1055	899
3	U	1263	3.0	1145	81	26	26	11	1054	901
4	A	1255	2.6	1152	80	25	25	14	1063	879
5	L	1222	3.0	1159	83	25	26	13	1051	878
6	B	1254	3.1	1154	82	28	18	12	1055	904
7	C	1260	3.2	1163	81	30	13	12	1066	900
8	D	1222	3.5	1133	82	28	15	9	1079	878
9	E	1248	2.8	1121	80	20	25	25	1025	881
10	F	1253	2.5	1145	83	28	15	9	1034	894
11	G	1255	3.3	1146	82	30	11	9	1028	900
12	H	1247	2.5	1143	81	27	22	10	1053	892
13	I	1231	4.5	1090	80	28	20	10	1045	888
14	J	1244	3.1	1162	81	26	26	11	1049	901
15	K	1261	3.6	1141	84	26	18	10	1054	887
16	L	1268	3.5	1151	82	28	20	10	1061	893
17	M	1255	3.1	1100	85	26	20	8	1045	906
18	N	1262	2.1	1133	78	26	20	12	1080	910
19	O	1251	3.0	1143	80	30	11	9	1064	897
20	P	1231	1.7	1131	76	26	18	10	1045	899
21	Q	1222	1.5	1108	81	27	18	8	1056	901
22	R	1258	2.3	1121	80	30	13	12	1054	911
23	S	1255	3.2	1151	79	28	18	10	1056	904
24	T	1252	2.2	1109	80	26	26	11	1033	874
25	U	1266	3.1	1143	80	26	22	12	1043	889

Manufacture conditions

Cooling step

	Average cooling
Sample	rate after	Cooling stop	Retention start	Retention end	Retention	Cooling
material	finish rolling	temperature	temperature	temperature	time	method after
No.	(° C./s)	(° C.)	(° C.)	(° C.)	(h)	winding	Remarks

1	100	592	580	500	1.7	Air cooling	Comparative Example
2	80	596	580	500	3.1	Air cooling	Invention Example
3	87	589	580	500	5.3	Air cooling	Invention Example
4	100	596	580	500	11.1	Air cooling	Invention Example
5	100	599	580	500	14.2	Air cooling	Comparative Example
6	122	582	580	500	3.5	Air cooling	Comparative Example
7	125	592	580	500	4.3	Air cooling	Comparative Example
8	100	584	580	500	4.2	Air cooling	Comparative Example
9	45	593	580	500	2.2	Air cooling	Comparative Example
10	41	581	580	500	8.0	Air cooling	Comparative Example
11	67	595	580	500	4.6	Air cooling	Comparative Example
12	98	583	580	500	2.6	Air cooling	Comparative Example
13	80	585	580	500	3.5	Air cooling	Comparative Example
14	85	589	580	500	4.1	Air cooling	Invention Example
15	151	581	580	500	5.2	Air cooling	Invention Example
16	120	594	580	500	3.4	Air cooling	Invention Example
17	150	586	580	500	6.2	Air cooling	Comparative Example
18	120	589	580	500	4.0	Air cooling	Invention Example
19	130	588	580	500	7.1	Air cooling	Invention Example
20	75	583	580	500	2.6	Air cooling	Invention Example
21	89	598	580	500	4.5	Air cooling	Invention Example
22	110	595	580	500	6.5	Air cooling	Invention Example
23	120	593	580	500	4.7	Air cooling	Invention Example
24	44	590	580	500	7.4	Air cooling	Invention Example
25	130	591	580	500	5.1	Air cooling	Invention Example

An underline indicates that the value is outside a range of the present invention.

	TABLE 3

	Manufacture conditions

Heating step

Rough rolling step

Finish rolling step

No.	steel	(° C.)	(h)	(° C.)	(%)	F1	F2	F3	(° C.)	(° C.)

26	V	1265	2.8	1154	78	26	18	10	1055	901
27	W	1255	3.1	1143	81	28	18	15	1066	920
28	X	1241	1.6	1151	80	26	15	12	1065	895
29	Y	1255	2.5	1163	82	28	15	10	1054	901
30	Z	1266	1.8	1128	77	26	17	12	1061	899
31	AA	1263	2.3	1139	80	26	18	10	1045	879
32	AB	1231	1.9	1154	74	30	11	9	1053	931
33	AC	1261	3.7	1163	82	28	12	10	1045	904
34	AD	1255	3.2	1155	81	30	11	9	1051	879
35	AE	1224	3.1	1148	73	26	18	10	1059	911
36	AF	1245	2.3	1147	81	28	14	12	1061	893
37	AG	1248	1.7	1161	82	30	22	11	1056	897
38	A	1238	3.1	1158	81	22	22	18	1061	920
39	L	1255	2.1	1167	81	18	18	15	1053	910
40	P	1245	3.4	1151	82	20	20	22	1054	903
41	Q	1248	3.1	1133	81	19	19	12	1071	921
42	S	1247	2.7	1181	82	15	12	10	1061	910
43	T	1243	1.6	1136	81	18	18	12	1056	899
44	U	1251	2.6	1152	80	20	18	10	1051	900
45	V	1255	2.5	1148	83	22	16	12	1065	911
46	W	1260	3.5	1139	82	21	16	12	1055	915
47	AC	1248	3.1	1157	84	18	18	14	1042	909
48	AD	1251	3.0	1141	81	22	15	9	1050	912
49	R	1255	3.1	1152	80	20	15	11	1060	906
50	AG	1248	2.9	1144	82	18	18	17	1061	913

Manufacture conditions

Cooling step

	Average cooling
Sample	rate after	Cooling stop	Retention start	Retention end	Retention	Cooling
material	finish rolling	temperature	temperature	temperature	time	method after
No.	(° C./s)	(° C.)	(° C.)	(° C.)	(h)	winding	Remarks

26	100	591	580	500	4.3	Air cooling	Invention Example
27	150	585	580	500	2.8	Air cooling	Invention Example
28	120	598	580	500	3.4	Air cooling	Comparative Example
29	85	588	580	500	8.5	Air cooling	Invention Example
30	95	593	580	500	4.2	Air cooling	Invention Example
31	105	595	580	500	5.1	Air cooling	Comparative Example
32	65	588	580	500	6.2	Air cooling	Invention Example
33	80	588	580	500	4.6	Air cooling	Invention Example
34	180	582	580	500	3.9	Air cooling	Invention Example
35	49	595	580	500	9.8	Air cooling	Invention Example
36	120	588	580	500	7.3	Air cooling	Invention Example
37	80	598	580	500	3.2	Air cooling	Invention Example
38	141	588	580	500	5.5	Air cooling	Invention Example
39	108	587	580	500	3.1	Air cooling	Invention Example
40	72	590	580	500	2.9	Air cooling	Invention Example
41	54	588	580	500	5.2	Air cooling	Invention Example
42	100	591	580	500	3.9	Air cooling	Invention Example
43	70	593	580	500	4.5	Air cooling	Invention Example
44	150	595	580	500	2.9	Air cooling	Invention Example
45	120	589	580	500	3.5	Air cooling	Invention Example
46	110	593	580	500	3.1	Air cooling	Invention Example
47	100	587	580	500	5.1	Air cooling	Invention Example
48	150	598	580	500	7.9	Air cooling	Invention Example
49	112	591	580	500	4.5	Air cooling	Invention Example
50	121	589	580	500	2.9	Air cooling	Invention Example

An underline indicates that the value is outside a range of the present invention.

	TABLE 4

	Manufacture conditions

Heating step

Rough rolling step

Finish rolling step

No.	steel	(° C.)	(h)	(° C.)	(%)	F1	F2	F3	(° C.)	(° C.)

51	Y	1251	3.5	1147	80	24	20	18	1065	911
52	Z	1241	2.5	1161	74	20	20	20	1053	889
53	AB	1245	2.5	1138	79	22	18	20	1061	908
54	AE	1235	4.2	1157	81	20	20	16	1058	893
55	AF	1246	3.1	1148	77	21	20	18	1067	908
56	A	1138	1.5	1085	81	30	18	15	1023	899
58	A	1261	2.5	1153	81	31	16	10	1033	840
59	J	1256	2.7	1152	81	30	20	15	1091	995
60	K	1283	3.1	1148	80	28	21	13	1064	905
61	K	1260	2.8	1162	82	28	20	14	1071	894
62	K	1253	3.1	1183	81	30	22	11	1055	903
63	K	1265	2.5	1165	80	28	19	13	1068	901
64	J	1241	2.8	1156	85	30	17	12	1067	911
65	J	1263	2.2	1147	84	28	25	12	1076	899
66	AH	1259	1.8	1151	76	40	40	40	1058	910
67	AH	1261	2.1	1139	81	22	20	15	1056	915
68	AI	1251	3.5	1145	76	55	50	45	1095	950
69	AI	1261	4.0	1153	68	24	24	18	1065	901
70	AJ	1258	3.2	1161	81	22	20	19	1081	892
71	AJ	1249	2.9	1150	76	28	26	20	1045	883
72	A	1250	3.5	1154	62	30	18	14	1051	900
73	AH	1249	3.1	1152	83	30	22	11	1042	903
74	AK	1249	2.6	1161	72	24	15	13	1048	890
75	AL	1260	1.8	1158	75	30	18	15	1035	895
76	AM	1250	2.4	1155	74	28	20	16	1052	915

Manufacture conditions

Cooling step

	Average cooling
Sample	rate after	Cooling stop	Retention start	Retention end	Retention	Cooling
material	finish rolling	temperature	temperature	temperature	time	method after
No.	(° C./s)	(° C.)	(° C.)	(° C.)	(h)	winding	Remarks

51	102	591	580	500	6.1	Air cooling	Invention Example
52	100	596	580	500	3.5	Air cooling	Invention Example
53	120	587	580	500	4.6	Air cooling	Invention Example
54	80	590	580	500	4.8	Air cooling	Invention Example
55	101	587	580	500	6.2	Air cooling	Invention Example
56	130	586	580	500	5.1	Air cooling	Comparative Example
58	130	580	580	500	3.3	Air cooling	Comparative Example
59	89	598	580	500	4.2	Air cooling	Comparative Example
60	15	595	580	500	6.0	Air cooling	Comparative Example
61	35	596	580	500	3.7	Air cooling	Invention Example
62	80	630	580	500	7.2	Air cooling	Comparative Example
63	78	381*	568	500	3.5	Air cooling	Comparative Example
64	120	573	573	500	3.7	Air cooling	Invention Example
65	100	577	577	513	2.3	Water cooling	Invention Example
66	80	520	520	500	0.7	Air cooling	Comparative Example
67	150	589	580	500	4.1	Air cooling	Invention Example
68	120	550	550	500	1.4	Air cooling	Comparative Example
69	130	581	580	500	5.6	Air cooling	Invention Example
70	150	589	580	500	6.8	Air cooling	Invention Example
71	120	579	579	500	4.2	Air cooling	Invention Example
72	105	583	580	500	3.5	Air cooling	Comparative Example
73	110	560	560	500	2.1	Air cooling	Comparative Example
74	99	584	580	500	3.1	Air cooling	Invention Example
75	105	591	580	500	3.5	Air cooling	Invention Example
76	108	578	578	500	2.8	Air cooling	Invention Example

An underline indicates that the value is outside a range of the present invention.
*Reheating after stop of cooling

	TABLE 5

	Microstructure

					“Martensite +		Average
					tempered martensite”		grain size
Sample				Remainder in	in remainder in		of prior	l_d/S_d
material	Kind of	Bainite	Ferrite	microstructure	microstructure	L₇+ L₆₈	γ grains	of prior
No.	steel	(area %)	(area %)	(area %)	(area %)	(μm/μm²)	(μm)	γ grains

1	N	83.2	3.1	13.7	9.0	0.64	18	3.1
2	A	92.5	4.0	3.5	2.0	0.57	26	2.7
3	U	91.3	6.5	2.2	1.0	0.51	29	2.8
4	A	94.1	3.2	2.7	2.5	0.41	20	3.0
5	L	87.4	9.3	3.3	2.6	0.30	24	3.4
6	B	51.2	0.4	48.4	45.2	0.55	22	3.4
7	C	35.0	45.0	20.0	3.0	0.41	18	3.1
8	D	76.3	1.2	22.5	11.0	0.51	15	3.2
9	E	76.2	19.3	4.5	1.2	0.52	18	3.5
10	F	31.0	0.2	68.8	61.0	0.42	14	2.5
11	G	61.3	32.1	6.6	3.2	0.44	18	2.2
12	H	85.6	9.1	5.3	4.1	0.56	24	2.4
13	I	86.3	9.8	3.9	3.2	0.57	14	4.1
14	J	88.9	1.6	9.5	8.6	0.51	20	2.3
15	K	92.1	4.5	3.4	2.1	0.47	21	2.4
16	L	91.2	3.1	5.7	5.2	0.53	17	2.8
17	M	91.3	6.3	2.4	2.1	0.43	18	3.6
18	N	93.5	2.6	3.9	3.5	0.55	23	2.1
19	O	91.4	1.5	7.1	6.3	0.47	21	2.2
20	P	86.1	8.3	5.6	4.2	0.54	20	2.3
21	Q	94.5	1.5	4.0	2.1	0.51	22	2.1
22	R	87.3	8.2	4.5	3.1	0.47	20	2.7
23	S	91.2	1.2	7.6	6.7	0.53	16	2.8
24	T	91.0	4.3	4.7	4.2	0.45	17	2.6
25	U	90.5	2.1	7.4	5.1	0.54	19	2.5

Mechanical properties

Punching property

Sample

Strength

Workability

Toughness

Property of

material	TS	El	λ	vE₄₀	punched end
No.	(MPa)	(%)	(%)	(J/cm²)	surface	Remarks

1	1046	12.6	44	103	G	Comparative Example
2	999	14.6	70	89	G	Invention Example
3	990	16.0	73	68	G	Invention Example
4	983	17.0	74	67	G	Invention Example
5	901	19.0	72	35	B	Comparative Example
6	1251	10.0	35	65	G	Comparative Example
7	701	23.0	72	31	B	Comparative Example
8	1065	17.2	24	44	G	Comparative Example
9	756	21.3	65	41	B	Comparative Example
10	1089	10.2	48	81	G	Comparative Example
11	776	23.0	62	51	G	Comparative Example
12	738	25.0	71	40	G	Comparative Example
13	981	14.2	31	52	B	Comparative Example
14	996	15.2	61	121	G	Invention Example
15	803	20.8	72	78	G	Invention Example
16	993	15.2	61	89	G	Invention Example
17	1001	14.3	28	71	G	Comparative Example
18	1021	15.4	66	108	G	Invention Example
19	983	16.2	60	98	G	Invention Example
20	956	15.1	67	83	G	Invention Example
21	1035	16.2	67	72	G	Invention Example
22	1021	16.3	71	99	G	Invention Example
23	1098	14.8	54	102	G	Invention Example
24	1033	15.3	66	82	G	Invention Example
25	1100	15.4	61	120	G	Invention Example

An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

	TABLE 6

	Microstructure

					“Martensite +		Average
					tempered martensite”		grain size
Sample				Remainder in	in remainder in		of prior	l_d/S_d
material	Kind of	Bainite	Ferrite	microstructure	microstructure	L₇+ L₆₈	γ grains	of prior
No.	steel	(area %)	(area %)	(area %)	(area %)	(μm/μm²)	(μm)	γ grains

26	V	89.4	4.3	6.3	5.2	0.51	20	2.1
27	W	92.8	2.1	5.1	4.6	0.56	24	2.3
28	X	73.2	2.4	24.4	21.5	0.56	12	3.8
29	Y	83.1	8.1	8.8	6.2	0.38	19	2.3
30	Z	88.2	3.5	8.3	7.1	0.54	21	2.2
31	AA	72.5	0.6	26.9	24.1	0.49	17	2.7
32	AB	93.1	1.3	5.6	4.5	0.44	24	2.1
33	AC	88.4	4.1	7.5	5.6	0.51	22	2.4
34	AD	89.3	3.0	7.7	4.2	0.55	16	2.7
35	AE	90.5	6.1	3.4	3.1	0.48	21	2.2
36	AF	93.2	2.1	4.7	4.5	0.41	17	2.4
37	AG	92.5	4.2	3.3	3.1	0.52	19	2.3
38	A	88.3	2.6	9.1	6.3	0.54	25	1.8
39	L	93.2	2.4	4.4	4.1	0.57	25	1.6
40	P	88.5	5.3	6.2	4.2	0.58	23	1.5
41	Q	92.3	5.2	2.5	1.5	0.47	24	1.7
42	S	90.2	1.8	8.0	7.1	0.55	21	1.6
43	T	85.6	5.1	9.3	6.1	0.48	22	1.7
44	U	90.4	1.8	7.8	6.3	0.57	23	1.4
45	V	92.1	3.1	4.8	4.1	0.54	22	1.8
46	W	92.4	1.1	6.5	4.8	0.54	25	1.4
47	AC	88.3	6.2	5.5	3.9	0.49	24	1.5
48	AD	86.7	8.1	5.2	3.2	0.41	22	1.7
49	R	90.2	6.2	3.6	2.6	0.50	21	1.6
50	AG	93.1	2.1	4.8	3.1	0.56	24	1.7

Mechanical properties

					Punching property
	Sample	Strength	Workability	Toughness	Property of

material	TS	El	λ	vE₄₀	punched end
No.	(MPa)	(%)	(%)	(J/cm²)	surface	Remarks

26	1035	15.8	54	151	G	Invention Example
27	1065	15.1	67	115	G	Invention Example
28	1108	12.3	44	65	B	Comparative Example
29	1021	16.3	71	68	G	Invention Example
30	1002	14.8	55	102	G	Invention Example
31	1103	11.5	42	151	G	Comparative Example
32	1098	15.1	66	95	G	Invention Example
33	999	14.5	64	140	G	Invention Example
34	1054	16.9	68	100	G	Invention Example
35	1011	16.2	83	110	G	Invention Example
36	1045	15.8	66	100	G	Invention Example
37	993	15.1	71	81	G	Invention Example
38	998	16.2	63	70	E	Invention Example
39	981	16.3	72	85	E	Invention Example
40	941	17.3	65	78	E	Invention Example
41	975	16.7	79	81	E	Invention Example
42	999	15.3	62	94	E	Invention Example
43	1003	15.8	64	75	E	Invention Example
44	1081	16.1	59	100	E	Invention Example
45	1011	14.9	68	120	E	Invention Example
46	996	16.8	75	98	E	Invention Example
47	984	15.2	75	100	E	Invention Example
48	989	16.7	67	105	E	Invention Example
49	1007	15.6	75	100	E	Invention Example
50	1001	14.9	71	90	E	Invention Example

An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

	TABLE 7

	Microstructure

					“Martensite +		Average
					tempered martensite”		grain size
Sample				Remainder in	in remainder in		of prior	l_d/S_d
material	Kind of	Bainite	Ferrite	microstructure	microstructure	L₇+ L₆₈	γ grains	of prior
No.	steel	(area %)	(area %)	(area %)	(area %)	(μm/μm²)	(μm)	γ grains

51	Y	84.1	6.4	9.5	4.5	0.42	25	1.7
52	Z	84.6	6.1	9.3	7.5	0.55	24	1.6
53	AB	91.2	3.5	5.3	2.8	0.48	25	1.6
54	AE	90.2	4.2	5.6	5.1	0.55	25	1.7
55	AF	89.2	5.4	5.4	3.2	0.39	24	1.4
56	A	83.7	5.1	11.2	2.5	0.46	16	2.5
58	A	78.1	12.5	9.4	7.5	0.55	25	3.8
59	J	78.2	1.3	20.5	12.3	0.50	33	2.5
60	K	76.2	15.9	7.9	5.9	0.38	22	2.1
61	K	84.2	7.8	8.0	4.5	0.50	20	2.2
62	K	52.0	41.0	7.0	5.6	0.38	22	2.1
63	K	65.4	1.3	33.3	32.1	0.55	21	2.3
64	J	93.1	1.5	5.4	4.5	0.56	19	2.4
65	J	89.2	1.2	9.6	8.3	0.59	20	2.3
66	AH	85.1	8.1	6.8	4.0	0.71	21	2.6
67	AH	91.0	5.1	3.9	3.2	0.51	23	1.5
68	AI	77.5	14.1	8.4	6.2	0.63	22	3.2
69	AI	91.1	6.3	2.6	2.1	0.45	29	2.8
70	AJ	92.2	6.1	1.7	1.4	0.41	26	1.8
71	AJ	88.2	9.4	2.4	1.5	0.54	24	2.6
72	A	87.2	6.1	6.7	5.2	0.28	26	3.0
73	AH	91.0	2.1	6.9	2.8	0.25	26	2.6
74	AK	85.3	6.3	8.4	4.2	0.42	28	1.8
75	AL	86.2	6.1	7.7	3.4	0.43	25	2.6
76	AM	84.5	9.1	6.4	2.6	0.38	27	2.4

Mechanical properties

					Punching property
	Sample	Strength	Workability	Toughness	Property of

material	TS	El	λ	vE₄₀	punched end
No.	(MPa)	(%)	(%)	(J/cm²)	surface	Remarks

51	996	15.3	75	80	E	Invention Example
52	996	14.9	62	88	E	Invention Example
53	1055	16.2	71	86	E	Invention Example
54	1106	15.9	64	105	E	Invention Example
55	1061	16.2	75	93	E	Invention Example
56	771	23.0	45	70	G	Comparative Example
58	1023	14.2	35	75	B	Comparative Example
59	1013	14.5	56	35	B	Comparative Example
60	763	22.5	80	75	G	Comparative Example
61	791	23.0	77	65	G	Invention Example
62	705	25.0	90	64	G	Comparative Example
63	1105	10.5	50	120	G	Comparative Example
64	1101	14.5	68	132	G	Invention Example
65	1188	14.2	58	165	G	Invention Example
66	986	18.0	45	132	G	Comparative Example
67	902	21.0	71	95	E	Invention Example
68	956	15.2	48	85	B	Comparative Example
69	1021	15.3	71	82	G	Invention Example
70	1028	16.3	79	76	E	Invention Example
71	997	16.8	66	66	G	Invention Example
72	1015	14.1	55	45	B	Comparative Example
73	1084	14.6	51	52	G	Comparative Example
74	1045	15.3	62	105	E	Invention Example
75	1162	17.1	54	67	G	Invention Example
76	998	16.7	62	89	G	Invention Example

An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

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

1. A hot-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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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 claim 1,

wherein the hot-rolled steel sheet includes, as a chemical composition, by mass %, one or more selected from the group 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 claim 1,

wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 and

a ratio I_d/S_dbetween a long axis I_dand a short axis S_dof the prior austenite grains is 2.0 or less.

4. 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.

5. 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%.

6. The hot-rolled steel sheet according to claim 2,

wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 μm, and

7. A hot-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 comprising 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 L₇of a grain boundary having a crystal orientation difference of 7° and a length L₆₈of 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.