JP7243854B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet and a method for 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 for producing the same.
This application claims priority based on Japanese Patent Application No. 2019-201427 filed in Japan on November 6, 2019, the contents of which are incorporated herein.

In recent years, efforts have been made to reduce carbon dioxide emissions in many fields from the viewpoint of protecting the global environment. Automobile manufacturers are actively developing technologies to reduce the weight of automobile bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of the car body because the emphasis is placed on improving crashworthiness in order to ensure the safety of passengers.

In order to achieve both weight reduction of the vehicle body and collision resistance, thinning of members using high-strength steel plates is being studied. Therefore, there is a strong demand for a steel sheet that has both high strength and excellent formability. There is a demand for a steel sheet that is particularly excellent in ductility and hole expansibility among formability. In addition, steel sheets applied to automobile bodies are also required to have excellent toughness in order to sufficiently absorb the impact at the time of collision.

For example, in Patent Document 1, the bainite fraction is 80% or more, and the average grain size r (nm) of the precipitates is calculated by the formula (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 the precipitate fraction f satisfy the formula (r / f ≤ 12000 ) is disclosed as a high-strength hot-rolled steel sheet having excellent fatigue properties and stretch-flangeability.

In Patent Document 2, the steel structure at a depth position of 1/4 of the plate thickness from the steel plate surface has, in area%, bainite: 60% or more, polygonal ferrite: 5% or more and less than 30%, retained austenite: less than 3%. , The balance excluding bainite, retained austenite and polygonal ferrite: 10% or less, and the polygonal ferrite area ratio at a depth of 100 μm from the steel plate surface and the polygonal ferrite area ratio at a depth of 1/4 of the plate thickness is the formula (Vαs>1.5Vαq, where Vαs is the polygonal ferrite area ratio (%) at a depth of 100 μm from the steel plate surface, and Vαq is a polygonal ferrite at a depth of 1/4 of the plate thickness from the steel plate surface A hot-rolled steel sheet characterized by satisfying the area ratio of ferrite is disclosed.

Japanese Patent Application Laid-Open No. 2009-84637 Japanese Patent Application Laid-Open No. 2016-50335

However, Patent Documents 1 and 2 do not consider toughness. The present inventors have found that in order to achieve both vehicle weight reduction and collision performance, it is necessary not only to improve ductility and hole expansibility, but also to ensure toughness.

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 for producing the same.

In addition to the above properties, steel sheets used for automobile bodies are sometimes required to have excellent punching properties. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a hot-rolled steel sheet and a method for producing the same, which are preferably excellent in punching properties in addition to the above properties.

In view of the above problems, the present inventors have made intensive studies on the relationship between the chemical composition and metallographic structure of hot rolled steel sheets and the mechanical properties. As a result, the following findings (a) to (e) have been obtained. completed the invention.

(a) In order to obtain excellent ductility and hole expansibility, the total area ratio of bainite must be 80.0% or more.

(b) By controlling the density of grain boundaries with specific orientations in bainite, ductility, hole expansibility and toughness can be further improved.

(c) It is necessary to control the coiling temperature and the holding temperature and holding time after coiling in order to make the density of grain boundaries having a specific orientation in the bainite fall within the desired range.

(d) In order to improve punching properties, it is necessary to control the average grain size and aspect ratio of prior austenite grains.

(e) In order to obtain the desired average grain size and aspect ratio of the prior austenite grains, it is necessary to control the hot rolling conditions more strictly. Specifically, it is necessary to strictly control the total rolling reduction in rough rolling and the rolling reduction in the last three stages of finish rolling in the hot rolling process.

The gist of the present invention made based on the above knowledge is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in 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: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010% and Zr: 0-0.010%
with the balance consisting of iron and impurities,
The metal structure, in area %,
Bainite: 80.0% or more,
Ferrite: 10.0% or less,
Residual tissue: 10.0% or less,
In the bainite, the sum of the densities of the grain boundary length _L7 with a crystal misorientation of 7° and the grain boundary length L68 with a crystal misorientation of ₆₈ ° with the <110> direction as an axis is 0 .35 to 0.60 μm/μm ² ,
Tensile strength is 780 MPa or more.
(2) In the hot-rolled steel sheet according to (1) above, the chemical composition is, in mass%,
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: 0.0005-0.010%,
REM: 0.0005-0.010% and Zr: 0.0005-0.010%
You may contain 1 type(s) or 2 or more types out of the group which consists of.
(3) The hot-rolled steel sheet according to (1) or (2) above has, in the metal structure,
The average grain size of the prior austenite grains is 10 to 30 μm,
A ratio l _d /S _d of the long axis l _d to the short axis S _d of the prior austenite grains may be 2.0 or less.
(4) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention comprises:
a heating step of holding a slab having the chemical composition described in (1) above at a heating temperature of 1200° C. or higher for 1.0 hour or longer;
A hot rolling step in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total reduction ratio is more than 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C. ,
After cooling to a temperature range of 570 to 620 ° C. at an average cooling rate of 20 ° C./s or more and winding, it is held in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cooled to room temperature. and a cooling step.
(5) The method for producing a hot-rolled steel sheet according to (4) above,
In the hot rolling step,
The total rolling reduction in the rough rolling is 70% or more,
The finish rolling may be performed so that the reduction ratios of the three subsequent stages of the finish rolling are all less than 25%.

According to the aspect of 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 for producing the same. According to the above preferred aspects of the present invention, it is possible to provide a hot-rolled steel sheet and a method for producing the same that are excellent in punching properties in addition to the above properties.

The chemical composition and metallographic structure of the hot-rolled steel sheet (hereinafter sometimes simply referred to as steel sheet) according to the present embodiment will be described more specifically below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.

The numerical limits described below with "-" in between include the lower limit and the upper limit. Any numerical value indicated as "less than" or "greater than" excludes that value from the numerical range. In the following description, % regarding chemical composition is mass % unless otherwise specified.

Chemical composition The hot-rolled steel sheet according to the present embodiment is 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 the balance: Contains Fe and impurities. Each element will be described in detail below.

C: 0.030-0.200%
C is an element that promotes the formation of bainite by improving the strength of the hot-rolled steel sheet and improving the hardenability. To obtain this effect, the C content should be 0.030% or more. Preferably, the C content is 0.040% or more.
On the other hand, if the C content exceeds 0.200%, it becomes difficult to control the formation of bainite, a large amount of martensite is formed, and both or one of the ductility and hole expandability of the hot rolled steel sheet is reduced. descend. Therefore, the C content should be 0.200% or less. The C content is preferably 0.180% or less.

Si: 0.05-2.50%
Si is an element that contributes to solid-solution strengthening, and is an element that contributes to improving the strength of a hot-rolled steel sheet. In addition, Si has the effect of making steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content should be 0.05% or more. The Si content is preferably 0.50% or more, more preferably 1.00% or more.
On the other hand, Si is an element that promotes the formation of a mixture (MA) of hard martensite (hereinafter simply referred to as martensite means fresh martensite) and retained austenite. If the Si content exceeds 2.50%, MA is generated and the hole expansibility of the hot-rolled steel sheet is lowered. Therefore, the Si content should be 2.50% or less. The Si content is preferably 2.30% or less, more preferably 2.00% or less.

Mn: 1.00-4.00%
Mn forms a solid solution in the steel and contributes to increasing the strength of the hot-rolled steel sheet. At the same time, it promotes the formation of bainite by improving hardenability, thereby improving the hole expansibility of the hot-rolled steel sheet. In order to obtain such effects, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.30% or more.
On the other hand, if the Mn content exceeds 4.00%, it becomes difficult to control the formation of bainite, it becomes impossible to obtain the desired amount of bainite, and both or either of the ductility and hole expandability of the hot rolled steel sheet descend. Therefore, the Mn content is set to 4.00% or less. Preferably, the Mn content is 3.50% or less.

sol. Al: 0.001-2.000%
Al, like Si, has the effect of deoxidizing the steel and making the steel sound. sol. If the Al content is less than 0.001%, the above effects cannot be obtained. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.010% or more.
On the other hand, sol. If the Al content exceeds 2.000%, oxide-based inclusions increase and the hole expandability of the hot-rolled steel sheet decreases. Therefore, sol. Al content is 2.000% or less. sol. The Al content is preferably 1.500% or less, more preferably 1.300% or less.
In addition, in this embodiment, sol. Al means acid-soluble Al, and indicates solid-solution Al present in steel in a solid-solution state.

Ti: 0.030-0.200%
Ti precipitates in steel as carbides or nitrides, and has the effect of refining the metal structure due to the pinning effect and improving the strength of the hot-rolled steel sheet. If the Ti content is less than 0.030%, the above effects cannot be obtained, so the Ti content is made 0.030% or more. The Ti content is preferably 0.050% or more, more preferably 0.080% or more.
On the other hand, if the Ti content exceeds 0.200%, it becomes difficult for the prior austenite grains to recrystallize, and the rolling texture develops, which reduces the hole expansibility of the hot-rolled steel sheet. Therefore, the Ti content should be 0.200% or less. The Ti content is preferably 0.170% or less, more preferably 0.150% or less.

P: 0.020% or less P is an element that forms a solid solution in steel and contributes to increasing the strength of the hot-rolled steel sheet. However, P is also an element that segregates at grain boundaries, particularly prior austenite grain boundaries, and promotes intergranular fracture due to intergranular segregation, thereby deteriorating the workability of hot-rolled steel sheets. Although it is preferable to keep the P content as low as possible, a P content of up to 0.020% is acceptable. Therefore, the P content is set to 0.020% or less. Preferably, the P content is 0.015% or less.
The P content is preferably 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases, so 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 the occurrence of delayed fracture. The S content is preferably as low as possible, but S content up to 0.020% is acceptable. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.015% or less.
The S content is preferably 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases and is economically disadvantageous, so the S content may be 0.0001% or more.

N: 0.010% or less N is an element that forms coarse nitrides in steel. This nitride deteriorates the bendability and expandability of the hot-rolled steel sheet. Therefore, the N content is made 0.010% or less. Preferably, the N content is 0.008% or less.
Since reducing the N content to less than 0.0001% causes a significant increase in manufacturing costs, the N content may be 0.0001% or more.

The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment consists of Fe and impurities. In the present embodiment, the term “impurities” refers to ores used as raw materials, scraps, or impurities that are mixed from the manufacturing environment, etc., and/or those that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.

The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in addition to part of Fe. The lower limit of the content when the following optional elements are not included is 0%. Each arbitrary element will be described in detail below.

Nb: 0-0.200%
Nb is an element that forms carbides during hot rolling and contributes to improving the strength of the hot-rolled steel sheet by precipitation strengthening. In order to reliably obtain this effect, the Nb content is preferably 0.005% or more.
On the other hand, if the Nb content exceeds 0.200%, the recrystallization temperature of the prior austenite grains becomes too high, the texture develops, and the hole expansibility of the hot rolled steel sheet may deteriorate. Therefore, the Nb content is set to 0.200% or less.

B: 0-0.010%
B is an element that segregates at prior austenite grain boundaries, suppresses the formation and growth of ferrite, and contributes to improving the strength and hole expansibility of hot-rolled steel sheets. In order to obtain these effects, the B content is preferably 0.001% or more.
On the other hand, even if the content of B exceeds 0.010%, the above effect is saturated. Therefore, the B content is set to 0.010% or less.

V: 0-1.00%
V is an element that forms carbonitrides during hot rolling and contributes to the improvement of the strength of the hot-rolled steel sheet by precipitation strengthening. In order to reliably obtain this effect, the V content is preferably 0.005% or more.
On the other hand, if the V content exceeds 1.00%, coarse carbides are formed in the slab, which causes cracks during the heating process. Therefore, the V content is set to 1.00% or less.

Mo: 0-1.00%
Mo is an element that promotes the formation of bainite by improving the hardenability of steel and contributes to improving the strength and hole expansion of hot-rolled steel sheets. In order to reliably obtain this effect, the Mo content is preferably 0.005% or more.
On the other hand, when the Mo content exceeds 1.00%, martensite tends to form, and both or one of the elongation and hole expansibility of the hot-rolled steel sheet may decrease. Therefore, Mo content shall be 1.00% or less.

Cu: 0-1.00%
Cu is an element that is effective for stably ensuring the strength of the hot-rolled steel sheet. Therefore, Cu may be contained. However, even if the content exceeds 1.00%, the effect of the above action tends to saturate, which may be economically disadvantageous. Therefore, the Cu content should be 1.00% or less. The Cu content is preferably 0.80% or less, more preferably 0.50% or less. In order to more reliably obtain the effects of the above action, the Cu content is preferably 0.005% or more.

W: 0-1.00%
W is an element effective in improving the strength of a hot-rolled steel sheet as a solid or by precipitation. However, even if the content exceeds 1.00%, the effect of the above action tends to saturate, which may be economically disadvantageous. Therefore, the W content should be 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. It should be noted that the W content is preferably 0.005% or more in order to more reliably obtain the effects of the above action.

Cr: 0-1.00%
Cr is an element effective in improving hardenability and strength of the hot-rolled steel sheet. However, even if the content exceeds 1.00%, the effect of the above action tends to saturate, which may be economically disadvantageous. Therefore, the Cr content should be 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. In order to more reliably obtain the effects of the above action, the Cr content is preferably 0.005% or more.

Ni: 0-1.00%
Ni is an element effective in improving hardenability and strength of the hot-rolled steel sheet. However, when the content exceeds 1.00%, the hardenability is excessively increased, and the fraction of the martensite structure is increased, which may deteriorate the hole expansibility of the hot-rolled steel sheet. Therefore, the Ni content should be 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. It should be noted that the Ni content is preferably 0.005% or more in order to more reliably obtain the effects of the above action.

Co: 0-1.00%
Co is an element effective in improving the strength of hot-rolled steel sheets through solid-solution strengthening. However, even if the content exceeds 1.00%, the effect of the above action tends to saturate, which may be economically disadvantageous. Therefore, the Co content should be 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. Note that the Co content is preferably 0.005% or more in order to more reliably obtain the effects of the above action.

Ca: 0-0.010%
Mg: 0-0.010%
REM: 0-0.010%
Zr: 0-0.010%
Ca (calcium), Mg (magnesium), REM (rare earth elements), and Zr (zirconium) are all elements that contribute to inclusion control, particularly fine dispersion of inclusions, and have the effect of increasing the toughness of hot-rolled steel sheets. is. Therefore, these elements may be contained. However, if any of the elements is contained in an amount exceeding 0.010%, deterioration of the surface properties may become apparent. Therefore, the content of each of these elements should be 0.010% or less. The content of each of these elements is preferably 0.005% or less, more preferably 0.003% or less. In order to more reliably obtain the effects of the above action, it is preferable that the content of each element is 0.0005% or more.

In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of mischmetals.

The chemical composition of the hot-rolled steel sheet can be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) or OES (Optical Emission Spectroscopy). Incidentally, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

Metal structure of hot-rolled steel sheet Next, the metal structure of the hot-rolled steel sheet according to the present embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has a metal structure, in terms of area %, of bainite: 80.0% or more, ferrite: 10.0% or less, residual structure: 10.0% or less, and in the bainite, With the <110> direction as the axis, the sum of the densities of the grain boundary length _L7 with a crystal misorientation of 7° and the grain boundary length _L68 with a crystal misorientation of 68° is 0.35 to 0.35. 60 μm/μm ² .

In the metal structure of 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 between the major axis l _d and the minor axis S _d of the prior austenite grains is l _d / _Sd may be 2.0 or less.

In the present embodiment, the metallographic structure is defined in a section parallel to the rolling direction at a depth of 1/4 of the plate thickness from the surface and at the central position in the plate width direction. The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.

Bainite: 80.0% or More Bainite means lath-like bainitic ferrite and a structure having Fe-based carbides between and/or inside the bainitic ferrite. Unlike polygonal ferrite, bainitic ferrite has a lath-like shape and has a relatively high dislocation density inside, so it can be easily distinguished from other structures using SEM or TEM.

If the area ratio of bainite is less than 80.0%, the toughness and hole expansibility of the hot-rolled steel sheet are remarkably lowered. Therefore, the area ratio of bainite is set to 80.0% or more. Preferably, it is 85.0% or more, more preferably 90.0% or more. The higher the area ratio of bainite, the better, but the presence of ferrite, cementite, or MA (a mixture of retained austenite and martensite) makes it difficult to achieve an area ratio of 97.5% or more. It may be 97.5%.

Ferrite: 10.0% or less Ferrite is polygonal ferrite, and bainitic ferrite is not included in ferrite. If the ferrite area ratio exceeds 10.0%, the desired tensile strength cannot be obtained. Therefore, the area ratio of ferrite is set to 10.0% or less. Preferably, it is 5.0% or less. From the viewpoint of ensuring ductility, the area ratio of ferrite may be 1.0% or more.

Residual structure (cementite, pearlite, martensite, tempered martensite and retained austenite): 10.0% or less in total Cementite, pearlite, martensite, tempered martensite and retained austenite all serve as starting points for voids during deformation, It is a structure that deteriorates the hole expansibility of the hot-rolled steel sheet. If the total area ratio of these residual structures exceeds 10.0%, desired ductility and hole expansibility cannot be obtained. Therefore, the area ratio of residual structures (cementite, pearlite, martensite, tempered martensite and retained austenite) is set to 10.0% or less. Preferably, it is 5.0% or less.

On the other hand, in texture control, since it is practically difficult to control the area ratio of the residual tissue to less than 1.0%, the area ratio of the residual tissue may be set to 1.0% or more.
In addition, the smaller the total area ratio of martensite and tempered martensite in the residual structure, the more stably excellent hole expandability can be obtained, so the total area ratio of martensite and tempered martensite is 5. It is preferably 0% or less. More preferably, it is 3.0% or less.

The method for measuring the area ratio of each tissue will be described below.
A test piece is taken from a hot-rolled steel sheet in a cross section parallel to the rolling direction so that the metal structure can be observed at a depth of 1/4 of the thickness from the surface and at the central position in the width direction.
After polishing the cross section of the above test piece using silicon carbide paper of #600 to #1500, diamond powder with a particle size of 1 to 6 μm is applied to a mirror surface using a diluted solution such as alcohol or a liquid dispersed in pure water. Finish. Next, the strain introduced into the surface layer of the sample is removed by polishing with colloidal silica that does not contain an alkaline solution at room temperature. At an arbitrary position in the longitudinal direction of the sample cross section, a length of 50 μm, a depth of 1/8 of the plate thickness from the surface to 3 of the plate thickness from the surface, so that the position of 1/4 of the plate thickness from the surface is the center. /8 depth regions are measured by electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystallographic orientation information.

For the measurement, an EBSD analyzer composed 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. At this time, the degree of vacuum in the EBSD analysis apparatus is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kv, the irradiation current level is 13, and the electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. It should be noted that a crystal structure of fcc is determined to be retained austenite.

Next, those with a crystal structure of bcc are judged to be bainite, ferrite, and “residual structures other than retained austenite (cementite, pearlite, martensite and tempered martensite)”. For these regions, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, under the condition that the 15 ° grain boundary is defined as the grain boundary, A region where "Grain Orientation Spread" is 1° or less is extracted as ferrite. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained.

Subsequently, the maximum value of the "Grain Average IQ" of the ferrite region under the condition that the 5° grain boundary is defined as the grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is Iα , the region exceeding Iα/2 is extracted as bainite, and the region below Iα/2 is extracted as “residual structures other than retained austenite (cementite, pearlite, martensite and tempered martensite)”. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained. In addition, by calculating the area ratio of the extracted "residual structures other than retained austenite (cementite, pearlite, martensite and tempered martensite)" and adding the area ratios of the retained austenite, the remaining structures (cementite, pearlite , martensite, tempered martensite and retained austenite) are obtained.

Regarding the above-extracted "residual structures other than retained austenite (cementite, pearlite, martensite and tempered martensite)", 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 with an SEM, a Vickers indentation is stamped in the vicinity of the observation position. After that, leaving the structure of the observation surface, contamination on the surface layer is removed by polishing, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with a SEM at a magnification of 3000 times.

In the EBSD measurement, among the regions determined to be residual structures, those regions having substructures within grains and having multiple variants of cementite precipitated are determined to be tempered martensite. A region in which cementite is precipitated in a lamellar shape is determined to be pearlite. Spherical particles with high brightness and a particle size (equivalent circle diameter) of 2 μm or less are judged to be cementite. A region with high brightness and in which the substructure is not revealed by etching is judged as "martensite and retained austenite". By calculating the respective area ratios, the area ratios of tempered martensite, pearlite, martensite, and "martensite and retained austenite" are obtained. The area ratio of martensite can be obtained by subtracting the area ratio of retained austenite obtained by the above-described EBSD from the obtained area ratio of "martensite and retained austenite".

For removing contaminants from the surface layer of the observation surface, buffing using alumina particles having a particle size of 0.1 μm or less, Ar ion sputtering, or the like may be used.

The sum of the densities of the grain boundary length _L7 with a crystal misorientation of 7° and the grain boundary length L68 with a crystal misorientation of ₆₈ ° about the <110> direction in bainite: 0. 35-0.60 μm/μm ²
In bainite, the sum of the densities of the grain boundary length _L7 with a crystal misorientation of 7° and the grain boundary length _L68 with a crystal misorientation of 68° with the <110> direction as an axis is 0. The ductility, hole expansibility and toughness of the hot-rolled steel sheet can be improved by setting it to 35 to 0.60 μm/μm ² .

If the sum of the densities of _L7 and _L68 is less than 0.35 μm/μm ² , the toughness of bainite is significantly reduced, and the desired toughness cannot be obtained in the hot-rolled steel sheet. Therefore, the sum of the densities of _L7 and _L68 should be 0.35 μm/μm ² or more. Preferably, it is 0.40 μm/μm ² or more. On the other hand, if the sum of the densities of _L7 and _L68 exceeds 0.60 μm/μm ² , the ductility of bainite is lowered, and excellent ductility and hole expansibility cannot be obtained in hot-rolled steel sheets. Therefore, the sum of the densities of _L7 and _L68 should be 0.60 μm/μm ² or less. Preferably, it is 0.55 μm/μm ² or less.

A grain boundary having a crystal orientation difference of X° with the <110> direction as an axis is defined as two crystal grains A and B that are adjacent to each other at a certain grain boundary. It refers to a grain boundary having a crystallographic relationship in which the crystal orientations of the crystal grain A and the crystal grain B match when rotated by X degrees about the 110> axis. However, considering the measurement accuracy of the crystal orientation, a misorientation of ±4° from the matching orientation relationship is allowed.

In this embodiment, the grain boundary lengths L ₇ and L ₆₈ as described above are measured using EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. In the EBSP-OIM method, an electron beam is irradiated to a highly tilted sample in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattering is photographed with a high-sensitivity camera, and the photographed photograph is image-processed by a computer. By doing so, the crystal orientation of the irradiation point can be measured in a short time. The EBSP-OIM method is performed using an apparatus combining a scanning electron microscope and an EBSP analysis apparatus and OIM Analysis (registered trademark) manufactured by AMETEK.

Since the EBSP-OIM method can analyze the microstructure and crystal orientation of the sample surface, it is possible to quantitatively determine the length of the grain boundary having a specific crystal orientation difference. Also, the analyzable area of the EBSP-OIM method is the area that can be observed with the SEM. Although it depends on the resolution of the SEM, the EBSP-OIM method enables analysis with a minimum resolution of 20 nm.

In the cross section parallel to the rolling direction, when measuring the density of the length of the specific grain boundary in the metal structure at the depth of 1/4 of the plate thickness from the surface and the center position in the plate width direction, 1000 times magnification, 50 μm × 50 μm _L7 is obtained by performing analysis in at least 5 fields of view in the region and calculating the average length of grain boundaries with a crystal misorientation of 7° around the <110> direction in bainite. Similarly, L ₆₈ is obtained by calculating the average length of grain boundaries with a crystal misorientation of 68° with the <110> direction in bainite as an axis. In addition, as described above, a misorientation of ±4° is allowed.

By dividing the obtained L ₇ and L ₆₈ by the measured area, the length L ₇ of the grain boundary with a crystal misorientation of 7° and the crystal misorientation of 68 with the <110> direction as the axis in the bainite Obtain the sum of the densities of the grain boundary length L ₆₈ which is °. In order to extract only bainite and measure the density of the length of a specific grain boundary, a region exceeding Iα/2 may be extracted as bainite in the same manner as when obtaining the area ratio of bainite.

Average grain size of prior austenite grains: 10 to 30 μm
The ratio l _d /S _d of the major axis l _d and the minor axis S _d of the 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, 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 prior austenite grains and l _d /S _d within the above ranges, the punching properties of the hot-rolled steel sheet can be improved.

A method for measuring the average grain size of the prior austenite grains and the ratio l _d /S _d between the major axis _ld and the minor axis S _d of the prior austenite grains will be described below.
A test piece is taken from a hot-rolled steel sheet in a cross section parallel to the rolling direction so that the metal structure can be observed at a depth of 1/4 of the thickness from the surface and at the central position in the width direction. Prior austenite grain boundaries are exposed by etching the observed surface with a saturated aqueous solution of picric acid. Enlarged photographs of the corroded cross section parallel to the rolling direction at a depth of 1/4 of the plate thickness from the surface and the center position in the plate width direction are taken with a scanning electron microscope (SEM) at a magnification of 1000 times and 5 or more fields of view. . The circle equivalent diameter (diameter) of at least 20 prior austenite grains having an equivalent circle diameter (diameter) of 2 μm or more, which is included in each SEM photograph, is obtained by image processing, and the average value of these is calculated to obtain the old Obtain the average grain size of the austenite grains. When prior austenite grains having an equivalent circle diameter of less than 2 μm are included, the above measurement is performed excluding them.

In addition, the major axis and minor 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 _ld 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. If the tensile strength is less than 780 MPa, the applicable parts are limited and the contribution to vehicle weight reduction is small. It is preferable that the tensile strength is 980 MPa or more. Although the upper limit is not particularly limited, it may be 1800 MPa from the viewpoint of mold wear suppression.

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. Although the upper limit of the total elongation is not particularly limited, it may be 30.0% or less or 25.0% or less.

Tensile strength and total elongation are measured according to JIS Z 2241:2011 using JIS Z 2241:2011 No. 5 test piece. The tensile test piece is taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction. The crosshead speed shall be 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. Although the upper limit of the hole expansion ratio is not particularly limited, it may be 90% or less or 85% or less.
A hole expansion rate is obtained by performing a hole expansion test based on JISZ2256:2010.

Impact value at −40° C.: 60 J/cm ² or more The hot-rolled steel sheet according to the present embodiment may have an impact value of 60 J/cm ² or more at −40° C. The upper limit of the impact value at −40° C. is not particularly limited, but may be 180 J/cm ² or less or 175 J/cm ² or less.
A sub-sized Charpy impact test piece is taken from an arbitrary position of the hot-rolled steel sheet, and the impact value at -40°C is determined according to the test method described in JIS Z 2242:2005.

Thickness: 0.6-8.0mm
The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 0.6 to 8.0 mm. If the thickness of the steel sheet is less than 0.6 mm, it may become difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the plate thickness of the steel plate according to this embodiment may be 0.6 mm or more. It is preferably 1.2 mm or more, or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it becomes difficult to refine the metal structure, particularly the prior austenite grains, and it may be difficult to ensure the above-described metal structure in terms of the structure fraction. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

Plating Layer The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallographic structure may be provided with a plating layer on the surface thereof for the purpose of improving corrosion resistance, etc., to serve as a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy. Examples of hot-dip coating layers 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. be. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described.
A preferred method for manufacturing a hot-rolled steel sheet according to this embodiment includes the following steps. The temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.

A heating step of holding 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 in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total reduction rate is more than 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C.
After cooling to a temperature range of 570 to 620 ° C. at an average cooling rate of 20 ° C./s or more and winding, hold in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cool to room temperature. cooling process.
In the hot rolling step, the finish rolling may be performed so that the total rolling reduction in the rough rolling is 70% or more, and the rolling reduction in the last three stages of the finish rolling is all less than 25%.
Each step will be described in detail below.

Heating Step In the heating step, the slab having the chemical composition described above is heated to a heating temperature of 1200° C. or higher and held for 1.0 hour. Since coarse precipitates present in the slab stage cause cracks during rolling and deterioration of material properties, it is preferable to heat the steel material before hot rolling to dissolve coarse carbides. Therefore, the heating temperature is set to 1200° C. or higher, and the holding time is set to 1.0 hour or longer. A preferable heating temperature is 1230° C. or higher, and a preferable holding time is 3.0 hours or longer.

On the other hand, if the heating temperature is too high or the holding time is too long, the amount of scale generated may increase and the yield may decrease. It may be 0 hours or less.
It should be noted that the slab to be heated is preferably produced by continuous casting from the viewpoint of production cost, but may be produced by other casting methods (for example, ingot casting method).

Hot Rolling Step If rough rolling is performed at less than 1000° C., the prior austenite grains are not sufficiently recrystallized, so 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. Preferably, it is 1050° C. or higher. On the other hand, if the rough rolling is performed at a temperature exceeding 1300° C., the amount of scale generated increases and the yield may decrease. Therefore, the rough rolling completion temperature may be 1300° C. or lower.

If the total rolling reduction in rough rolling is low, the crystal grain size of the prior austenite grains becomes non-uniform, causing a decrease in toughness. The total rolling reduction in rough rolling is preferably 68% or more, more preferably 70% or more, and even more preferably 80% or more. Although the upper limit of the total rolling reduction in rough rolling is not particularly limited, it may be 90% or less.
The total rolling reduction in rough rolling is expressed by (1−t _r /t _s )×100 (%) using the slab thickness: t _s and the plate thickness t _r at the end of rough rolling. .

Realize the above average grain size and aspect ratio of prior austenite grains by setting the total rolling reduction in rough rolling to 70% or more and strictly controlling the rolling reduction in the latter three stages of finish rolling as described later. can be done.

When the finish rolling completion temperature is lower than 860°C, the prior austenite grains are not sufficiently recrystallized, so the texture develops and the hole expansibility deteriorates. Therefore, finish rolling completion temperature shall be 860 degreeC or more. Preferably, the temperature is 900° C. or higher. On the other hand, if the finish rolling completion temperature is higher than 980°C, the prior austenite grains are significantly coarsened, and desired toughness cannot be obtained. Therefore, finish rolling completion temperature shall be 980 degrees C or less. It is preferably 950° C. or less.

In this embodiment, in order to realize the above-described average grain size and aspect ratio of the prior austenite grains and improve the punching characteristics of the hot-rolled steel sheet, the total rolling reduction in the rough rolling and the final three stages of the finish rolling The rolling reduction may be strictly controlled. Specifically, as described above, the total rolling reduction in the rough rolling may be 70% or more, and the rolling reduction in the latter three stages of the finish rolling may all be less than 25%.

When even one of the reduction ratios of the last three stages of finish rolling, that is, the reduction ratio of the final pass of finish rolling, the second pass from the final pass, and the third pass from the final pass is 25% or more, the rolling , the prior austenite grains become flattened, forming prior austenite grains with a large aspect ratio that act as starting points for crack generation during punching. Therefore, the rolling reduction in the last three stages of the finish rolling (the rolling reduction in the final pass of the finish rolling, the second pass from the final pass, and the third pass from the final pass) may be less than 25%. Both are preferably 20% or less. The rolling reduction can be expressed by (1−h/h ₀ )×100(%), where h is the plate thickness after rolling in one pass, and h ₀ is the plate thickness before rolling.

Cooling process After the hot rolling process, the steel sheet is cooled to a temperature range of 570 to 620°C at an average cooling rate of 20°C/s or more. In this 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.

If the average cooling rate is less 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 more. It is preferably 30° C./s or more, more preferably 50° C./s or more. From the viewpoint of suppressing an increase in cooling equipment, the average cooling rate may be 200° C./s or less.

Cooling at an average cooling rate of 20°C/s or more is performed to a temperature range of 570 to 620°C. If the cooling stop temperature is higher than 620°C, the 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 620° C. or less and a temperature that can be maintained in the temperature range of 500 to 580° C., but in order to maintain the temperature range of 500 to 580° C. for 2.0 hours or more, the cooling stop temperature is A temperature of 550° C. or higher is preferable. In order to preferably control the sum of the densities of _L7 and _L68 and obtain excellent toughness, the cooling stop temperature is preferably 570°C or higher.

The desired amount of bainite cannot be obtained even if the cooling stop temperature is lower than 500 ° C. and is held in the temperature range of 500 to 580 ° C. after heating again, so heating after cooling is not desirable. .

After cooling at an average cooling rate of 20° C./s or more, winding is performed. After winding, the film is held in a temperature range of 500 to 580°C for 2.0 to 12.0 hours. When the holding temperature is outside the temperature range of 500 to 580° C., or the holding time is less than 2.0 hours or more than 12.0 hours, the desired amount of the total density of L ₇ and L ₆₈ in the bainite is can't get Therefore, the holding temperature is set to a temperature range of 500 to 580° C., and the holding time is set to 2.0 to 12.0 hours. The lower limit of the holding temperature is preferably 530°C. The upper limit of the holding temperature is preferably 560°C. The lower limit of the retention time is preferably 4.0 hours, more preferably 6.0 hours. The upper limit of the retention time is preferably 10.0 hours, more preferably 8.0 hours.

In the holding in the temperature range of 500 to 580°C, the steel sheet temperature may be varied within the temperature range of 500 to 580°C, or may be kept constant. In addition, even if the cooling stop temperature of cooling with an average cooling rate of 20 ° C./s or more is less than 580 ° C., it is sufficient if a holding time of 2.0 to 12.0 hours can be secured in the temperature range of 500 to 580 ° C. .

After the above-mentioned holding in the temperature range of 500 to 580° C., it is cooled to room temperature. Any cooling method may be used as the cooling method to room temperature, and air cooling, mist cooling, rapid cooling using a water cooling bath, or other appropriate methods may be used. The room temperature referred to here is a temperature range of 20 to 30°C.

Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

Steels having chemical compositions shown in steel grades A to AM in Table 1 were melted and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 5-7 were obtained under the manufacturing conditions shown in Tables 2-4. In addition, "F1", "F2" and "F3" in Tables 2 to 4 respectively represent the reduction ratio of the final pass of finish rolling, the reduction ratio of the second pass from the final pass, and the reduction ratio of the third pass from the final pass. show. In addition, test material No. in Table 4. 63 was reheated after cooling was stopped, and then held in the temperature range of 500 to 580°C.

For the obtained hot-rolled steel sheet, by the above-described method, the structure fraction, the sum of the densities of _L7 and _L68 , the average grain size of the prior austenite grains, and the major axis l _d and minor axis S _d of the prior austenite grains The ratio l _d /S _d of was determined. The results obtained are shown in Tables 5-7.

Method for evaluating properties of hot-rolled steel sheets Tensile strength (TS) and total elongation (El)
Among the mechanical properties of the obtained hot-rolled steel sheet, tensile strength (TS) and total elongation (El) were measured according to JIS Z 2241: 2011 using JIS Z 2241: 2011 No. 5 test piece. bottom. The tensile test piece was taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction was taken as the longitudinal direction. The crosshead speed was 3 mm/min.

When the tensile strength (TS) was 780 MPa or more, it was judged to be excellent in strength, and it was judged to be acceptable. In addition, when the total elongation (El) was 14.0% or more, the ductility was judged to be excellent, and it was judged to be acceptable.

Hole expansion ratio (λ)
The hole expansion rate (λ) was evaluated by performing a hole expansion test in accordance with JIS Z 2256:2010.
When the hole expansion rate (λ) was 50% or more, the hole expandability was judged to be excellent, and it was judged to be acceptable.

Impact value (vE _-40 )
The toughness was evaluated by performing a Charpy impact test at -40°C to obtain an impact value. A sub-sized Charpy impact test piece was taken from an arbitrary position of the hot-rolled steel sheet, and toughness was evaluated by determining the impact value at -40°C according to the test method described in JIS Z 2242:2005.
When ^the impact value (vE ₋₄₀ ) was 60 J/cm ² or more, the toughness was judged to be excellent, and it was judged to be acceptable.

Punching characteristics Punching characteristics were evaluated by conducting a punching test and observing the properties of the punched end surface. First, a punched hole was produced with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm/s. Next, a section perpendicular to the rolling direction of the punched hole was embedded in resin, and the punched end face was photographed with a scanning electron microscope. The observation photographs obtained were observed, and when no edge roughness was observed, the punching characteristics were considered to be particularly good, and were described as "E (Excellent)" in Tables 5 to 7. In addition, when a small chipping of less than 100 μm is observed, it is indicated as “G (Good)” in Tables 5 to 7 because the punching characteristics are good, and when a large chipping of 100 μm or more is observed, punching It was described as "B (Bad)" in Tables 5 to 7 as being inferior in properties.

Looking at Tables 5-7, it can be seen that the inventive examples have high strength as well as excellent ductility, hole expandability and toughness. In addition, the average grain size of the prior austenite grains is 10 to 30 μm, and the ratio l _d /S _d between the major axis _ld and the minor axis S _d of the prior austenite grains is 2.0 or less. It can be seen that the characteristics are particularly good.
On the other hand, it can be seen that the comparative examples are inferior in at least one of strength, ductility, hole expansibility and toughness.

ADVANTAGE OF THE INVENTION According to this invention, while having high intensity|strength, the hot-rolled steel plate which has the outstanding ductility, hole expandability, and toughness, and its manufacturing method can be provided. According to the above preferred aspects of the present invention, it is possible to provide a hot-rolled steel sheet and a method for producing the same that are excellent in punching properties in addition to the above properties.

Claims (5)

The chemical composition, in 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: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010% and Zr: 0-0.010%
with the balance consisting of iron and impurities,
The metal structure, in area %,
Bainite: 80.0% or more,
Ferrite: 10.0% or less,
Residual tissue: 10.0% or less,
In the bainite, the sum of the densities of the grain boundary length _L7 with a crystal misorientation of 7° and the grain boundary length L68 with a crystal misorientation of ₆₈ ° with the <110> direction as an axis is 0 .35 to 0.60 μm/μm ² ,
A hot-rolled steel sheet having a tensile strength of 780 MPa or more. The chemical composition, in mass %,
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: 0.0005-0.010%,
REM: 0.0005-0.010% and Zr: 0.0005-0.010%
The hot-rolled steel sheet according to claim 1, containing one or more of the group consisting of: In the metal structure,
The average grain size of the prior austenite grains is 10 to 30 μm,
The hot-rolled steel sheet according to claim 1 or 2, wherein the ratio _ld / _Sd of the major axis _ld to the minor axis _Sd of the prior austenite grains is 2.0 or less. A method for manufacturing a hot-rolled steel sheet according to claim 1,
a heating step of holding 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 in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total reduction ratio is more than 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C. ,
After cooling to a temperature range of 570 to 620 ° C. at an average cooling rate of 20 ° C./s or more and winding, hold in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cool to room temperature. A method for manufacturing a hot-rolled steel sheet, comprising a cooling step. In the hot rolling step,
The total rolling reduction in the rough rolling is 70% or more,
5. The method for producing a hot-rolled steel sheet according to claim 4, wherein the finish rolling is performed so that the reduction ratios of the three subsequent stages of the finish rolling are all less than 25%.