KR102715430B1 - Hot rolled steel sheet and its manufacturing method - Google Patents

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, a metal structure in which, in area %, bainite: 80.0% or more, ferrite: 10.0% or less, and residual structure: 10.0% or less, a total density of length _L7 of grain boundaries having a crystal orientation difference of 7° and length _L68 of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction as an axis in the bainite is 0.35 to 0.60 µm/ ^µm2 , and a tensile strength is 780 MPa or more.

Description

Hot rolled steel sheet and its manufacturing method

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 expandability and toughness, and a method for manufacturing the same.

This application claims priority from Japanese Patent Application No. 2019-201427, filed in Japan on November 6, 2019, the contents of which are incorporated herein.

In recent years, from the perspective of protecting the global environment, reduction of carbon dioxide emissions has been promoted in many fields. Automobile manufacturers are also actively developing technologies to reduce vehicle weight for the purpose of reducing fuel consumption. However, because the focus is also on improving collision characteristics to ensure passenger safety, vehicle weight reduction is not easy.

In order to achieve both weight reduction and collision characteristics, the use of high-strength steel plates to thin the parts is being considered. For this purpose, steel plates that combine high strength and excellent formability are strongly desired. Among the formability characteristics, steel plates that are particularly excellent in ductility and hole expandability are desired. In addition, steel plates applied to automobile bodies are also required to have excellent toughness in order to sufficiently absorb impact during collisions.

For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet having excellent fatigue properties and elongation flangeability, characterized in that the bainite fraction is 80% or more, the average grain size r (nm) of precipitates satisfies 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).

Patent Document 2 discloses a hot rolled steel sheet characterized in that the steel structure at a 1/4 depth from the steel sheet surface includes, in area %, bainite: 60% or more, polygonal ferrite: 5% or more and less than 30%, retained austenite: less than 3%, and the remainder excluding bainite, retained austenite, and polygonal ferrite: 10% or less, and the area ratio of polygonal ferrite at a 100 ㎛ depth from the steel sheet surface and the area ratio of polygonal ferrite at a 1/4 depth from the steel sheet surface satisfy the formula (Vαs>1.5Vαq, where Vαs is the area ratio (%) of polygonal ferrite at a 100 ㎛ depth from the steel sheet surface, and Vαq is the area ratio of polygonal ferrite at a 1/4 depth from the steel sheet surface of the sheet thickness).

Japanese Patent Publication No. 2009-84637 Japanese Patent Publication No. 2016-50335

However, in patent documents 1 and 2, toughness is not considered. The inventors of the present invention have found that in order to achieve both weight reduction of the body and collision characteristics, it is necessary to secure toughness as well as improve ductility and hole expandability.

The present invention has been made in consideration of the above problems, and aims to provide a hot-rolled steel sheet having high strength, excellent ductility, hole expandability and toughness, and a method for manufacturing the same.

In addition, steel sheets applied to automobile bodies are sometimes required to have excellent punching characteristics in addition to the above-described characteristics. To this end, the present invention aims to provide a hot-rolled steel sheet and a method for manufacturing the same, which preferably has excellent punching characteristics in addition to the above-described characteristics.

In consideration of the above-described problems, the inventors of the present invention have conducted extensive research on the relationship between the chemical composition and metal structure of hot-rolled steel sheets and mechanical properties, and as a result, obtained the following findings (a) to (e), thereby completing the present invention.

(a) In order to obtain excellent ductility and hole expandability, it is necessary to make the area ratio of bainite 80.0% or more in total.

(b) By controlling the grain boundary density with a specific orientation within bainite, ductility, pore expandability and toughness can be further improved.

(c) In order to keep the grain boundary density within a specific orientation within the bainite within a desired range, it is necessary to control the coiling temperature and the holding temperature and holding time after coiling.

(d) In order to improve the punching characteristics, it is necessary to control the average particle size and aspect ratio of the old austenite particles.

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

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

(1) A 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 to 0.010%,

Mg: 0 to 0.010%,

REM: 0 to 0.010%, and

Zr: 0 to 0.010%

, and the remainder is composed of iron and impurities,

Metal structure, in area %,

Bainite: 80.0% or more,

Ferrite: 10.0% or less,

Residual tissue: 10.0% or less,

In the above bainite, the sum of the densities of the length _L7 of grain boundaries having a crystal orientation difference of 7° and the length _L68 of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction as the axis is 0.35 to 0.60 ㎛/ ^㎛2 ,

The tensile strength is 780 MPa or more.

(2) The hot-rolled steel plate described in (1) above has 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 to 0.010%,

REM: 0.0005 to 0.010%, and

Zr: 0.0005 to 0.010%

It may contain one or more types among the groups consisting of:

(3) The hot-rolled steel plate described in (1) or (2) above has, in the metal structure,

The average particle size of the old austenite particles is 10 to 30 ㎛,

The ratio of the major axis l _d to the minor axis S _d of the above austenite particles l _d /S _d may be 2.0 or less.

(4) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention,

A heating process in which a slab having the chemical composition described in (1) above is maintained at a heating temperature of 1200°C or higher for 1.0 hour or longer,

A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000℃ or higher and the total reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980℃,

A cooling process is provided in which the coil is cooled to a temperature range of 570 to 620°C at an average cooling rate of 20°C/s or higher, maintained at a temperature range of 500 to 580°C for 2.0 to 12.0 hours, and then cooled to room temperature.

(5) The method for manufacturing the hot-rolled steel plate described in (4) above is as follows:

In the above hot rolling process,

In the above rolling process, the total reduction ratio is set to 70% or more,

The above finishing rolling may be performed so that the reduction ratios of the last three stages of the above finishing rolling are all less than 25%.

According to the above-described aspect of the present invention, a hot-rolled steel sheet having high strength, excellent ductility, hole expandability and toughness, and a method for producing the same can be provided. According to the above-described preferred aspect of the present invention, in addition to the above-described characteristics, a hot-rolled steel sheet having excellent punching characteristics and a method for producing the same can be provided.

The chemical composition and metal structure of the hot-rolled steel sheet (hereinafter, sometimes simply referred to as the steel sheet) according to the present embodiment will be described in more detail below. However, the present invention is not limited to the composition disclosed in the present embodiment, and various changes are possible without departing from the spirit of the present invention.

The numerical ranges indicated below with "within" include the lower and upper limits. The numerical values indicated as "less than" or "exceeding" are not included in 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 contains, 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, and the remainder: Fe and impurities. Each element is described in detail below.

C: 0.030 to 0.200%

C is an element that promotes the formation of bainite by improving the strength of hot-rolled steel plates and enhancing ??quenching properties. To obtain this effect, the C content is set to 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 either or both of the ductility and hole expandability of the hot-rolled steel sheet deteriorates. 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 the strengthening of the solid solution and to the improvement of the strength of hot-rolled steel sheets. In addition, Si has the effect of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 0.05%, the effect of the above effect 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.

Meanwhile, Si is an element that promotes the formation of a mixture (MA) of hard martensite (hereinafter, when simply referred to as martensite, fresh martensite is indicated) and retained austenite. When the Si content exceeds 2.50%, MA is formed, and the hole expandability of the hot-rolled steel sheet deteriorates. 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%

Manganese contributes to the increase in strength of hot-rolled steel sheets by being employed in steel, and also promotes the formation of bainite by improving quenching properties, thereby improving the hole expandability of hot-rolled steel sheets. To obtain these effects, the manganese content is 1.00% or more. Preferably, the manganese 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, so that the desired amount of bainite cannot be obtained, and either or both of the ductility and hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 4.00% or less. Preferably, the Mn content is 3.50% or less.

sol.Al: 0.001 to 2.000%

Al, like Si, has the function of deoxidizing steel and making the steel sound. If the sol.Al content is less than 0.001%, the effect of the above function 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, if the sol.Al content exceeds 2,000%, it causes an increase in oxide inclusions, which reduces the hole expandability of the hot-rolled steel sheet. 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.

In addition, in the present embodiment, sol.Al means acid-soluble Al, and represents dissolved Al that exists in the steel in a solid solution state.

Ti: 0.030 to 0.200%

Ti is precipitated as carbide or nitride in steel, and has the function of improving the strength of hot-rolled steel sheets by refining the metal structure through the peening effect. Since the effect of the above function cannot be obtained if the Ti content is less than 0.030%, 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, if the Ti content exceeds 0.200%, it becomes difficult for the old austenite particles to recrystallize, and the rolled aggregate structure develops, thereby reducing the hole expandability 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 is employed in steel and contributes to the increase in strength of hot-rolled steel sheets. However, P is also an element that causes a decrease in the workability of hot-rolled steel sheets by segregating at grain boundaries, especially at old austenite grain boundaries, and promoting grain boundary destruction due to grain boundary segregation. It is desirable to reduce the P content as much as possible, but a P content of up to 0.020% is permissible. Therefore, the P content is set to 0.020% or less. Preferably, the P content is 0.015% or less.

It is desirable to have the P content at 0%, but since manufacturing costs increase when the P content is reduced to less than 0.0001%, 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 expandability of hot-rolled steel sheets or promotes the occurrence of delayed fracture. It is desirable to reduce the S content as much as possible, but an S content of up to 0.020% is permissible. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.015% or less.

It is desirable to have the S content at 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases, which 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. These nitrides deteriorate the bendability and hole expandability of hot-rolled steel sheets. Therefore, the N content is set to 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 cost, the N content may be 0.0001% or more.

The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment is composed of Fe and impurities. In the present embodiment, impurities mean those mixed in from ores, scrap, or the manufacturing environment as raw materials, and/or those permitted 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 optional elements in addition to a portion of Fe. The lower limit of the content when the following optional elements are not contained is 0%. Hereinafter, each optional element will be described in detail.

Nb: 0 to 0.200%

Nb is an element that forms carbides during hot rolling and contributes to improving the strength of hot-rolled steel sheets through precipitation strengthening. In order to reliably obtain this effect, it is preferable that the Nb content be 0.005% or more.

On the other hand, if the Nb content exceeds 0.200%, the recrystallization temperature of the old austenite particles becomes too high, the aggregate structure develops, and the hole expandability of the hot-rolled steel sheet may be reduced. Therefore, the Nb content is set to 0.200% or less.

B: 0 to 0.010%

B is an element that segregates in the old austenite grain boundaries, suppresses the formation and growth of ferrite, and contributes to improving the strength and hole expandability of hot-rolled steel sheets. In order to obtain these effects, it is preferable that the B content be 0.001% or more.

On the other hand, even if the B content exceeds 0.010%, the above effect becomes saturated. Therefore, the B content is set to 0.010% or less.

V: 0 to 1.00%

V is an element that forms carbonitride during hot rolling and contributes to improving the strength of hot-rolled steel sheets through precipitation strengthening. In order to reliably obtain this effect, it is preferable that the V content be 0.005% or more.

On the other hand, if the V content exceeds 1.00%, coarse carbides are generated in the slab, which becomes a factor in the occurrence of cracking during the heating process. 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 ??quenching property of steel, thereby contributing to the improvement of strength and hole expansion of hot-rolled steel sheets. In order to obtain this effect reliably, the Mo content is preferably 0.005% or more.

On the other hand, if the Mo content exceeds 1.00%, martensite is likely to be formed, and either or both of the elongation and hole expandability of the hot-rolled steel sheet may deteriorate. Therefore, the Mo content is set to 1.00% or less.

Cu: 0 to 1.00%

Cu is an effective element for stably securing the strength of hot-rolled steel sheets. Therefore, Cu may be contained. However, even if it is contained in excess of 1.00%, the effect due to the above action tends to be saturated, which may be economically disadvantageous. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, more preferably 0.50% or less. Furthermore, in order to more reliably obtain the effect due to the above 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 hot-rolled steel sheets by solid or precipitation. However, even if it is contained in excess of 1.00%, the effect by the above action tends to be saturated, which may be economically disadvantageous. Therefore, the W content is set to 1.00% or less. Preferably, it is 0.80% or less, more preferably 0.50% or less. In addition, in order to obtain the effect by the above action more reliably, the W content is preferably 0.005% or more.

Cr: 0 to 1.00%

Cr is an element that is effective in improving the strength of hot-rolled steel sheets, as well as improving the ??quenching property. However, even if it is contained in excess of 1.00%, the effect due to the above action tends to be saturated, which may be economically disadvantageous. Therefore, the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less, more preferably 0.50% or less. In addition, in order to obtain the effect due to the above action more reliably, the Cr content is preferably 0.005% or more.

Ni: 0 to 1.00%

Ni is an element that is effective in improving the strength of hot-rolled steel sheets, as well as improving the quenching property. However, if it is contained in excess of 1.00%, the quenching property may be excessively increased, and the microstructure fraction of martensite may increase, thereby deteriorating the hole expandability of the hot-rolled steel sheet. Therefore, the Ni content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. In addition, in order to more reliably obtain the effect by the above 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 hot-rolled steel sheets by solidification. However, even if it is contained in excess of 1.00%, the effect by the above action tends to be saturated, which may be economically disadvantageous. Therefore, the Co content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. In addition, in order to obtain the effect by the above action more reliably, 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%

Ca (calcium), Mg (magnesium), REM (rare earth element), and Zr (zirconium) are all elements that contribute to inclusion control, especially to fine dispersion of inclusions, and have the function of increasing the toughness of hot-rolled steel sheets. Therefore, these elements may be contained. However, if any of the elements is contained in excess of 0.010%, deterioration of the surface appearance may become apparent. Therefore, the content of each of these elements is set to 0.010% or less. The content of each of these elements is preferably 0.005% or less, more preferably 0.003% or less. Furthermore, in order to more reliably obtain the effect by the above function, it is preferable that any of the elements is 0.0005% or more.

In addition, in the present embodiment, REM refers to a total of 17 elements composed of Sc, Y, and lanthanoids, and the content of the REM refers to the total content of these elements. In the case of lanthanoids, they are added industrially in the form of mischmetal.

The chemical composition of hot-rolled steel sheets can be measured by general analysis methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) or OES (Optical Emission Spectroscopy). In addition, C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas melting-thermal conductivity method.

Metallographic structure of hot rolled steel plate

Next, the metal structure of the hot-rolled steel plate according to the present embodiment will be described.

A hot-rolled steel sheet according to the present embodiment has a metal structure in which, in area %, bainite: 80.0% or more, ferrite: 10.0% or less, and residual structure: 10.0% or less, and a total density of a length _L7 of grain boundaries having a crystal orientation difference of 7° and a length _L68 of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction as an axis in the bainite is 0.35 to 0.60 µm/ ^µm2 .

In addition, the hot-rolled steel sheet according to the present embodiment may have, in the metal structure, an average particle size of prior austenite particles of 10 to 30 µm, and a ratio of the major axis l _d to the minor axis S _d of the prior austenite particles l _d /S _d of 2.0 or less.

In addition, in this embodiment, with respect to the metal structure, the metal structure is defined at a depth of 1/4 of the plate thickness from the surface in a cross section parallel to the rolling direction and also at the center position in the plate width direction. This is because the metal structure at this position represents a representative metal structure of the steel plate.

Bainite: 80.0% or more

Bainite refers to a structure having bainitic ferrite in the lath shape and Fe-based carbides between and/or inside the bainitic ferrite. Unlike polygonal ferrite, bainitic ferrite has a lath shape and also has a relatively high dislocation density inside, so it can be easily distinguished from other structures using SEM or TEM.

When the area ratio of bainite is less than 80.0%, the toughness and hole expandability of the hot-rolled steel sheet are significantly reduced. Therefore, the area ratio of bainite is set to 80.0% or more. Preferably, it is 85.0% or more, and more preferably, it is 90.0% or more. The higher the area ratio of bainite, the more preferable it is, but since it is difficult to achieve an area ratio of 97.5% or more due to the presence of ferrite, cementite, or MA (a mixture of retained austenite and martensite), the practical upper limit may be 97.5%.

Ferrite: 10.0% or less

Ferrite refers to polygonal ferrite, and bainitic ferrite is not included in ferrite. If the area ratio of ferrite 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 securing ductility, the area ratio of ferrite may be 1.0% or more.

Residual structure (cementite, pearlite, martensite, tempered martensite and retained austenite): Total of 10.0% or less

Cementite, pearlite, martensite, tempered martensite, and retained austenite are all structures that form voids during deformation and deteriorate the hole expandability of the hot-rolled steel sheet. If the area ratio of these residual structures exceeds 10.0% in total, the desired ductility and hole expandability cannot be obtained. Therefore, the area ratio of the residual structures (cementite, pearlite, martensite, tempered martensite, and retained austenite) is set to 10.0% or less. Preferably, it is 5.0% or less.

Meanwhile, in terms of tissue control, it is practically difficult to control the area ratio of the residual tissue to less than 1.0%, so the area ratio of the residual tissue may be 1.0% or more.

In addition, among the residual structures, the smaller the sum of the area ratios of martensite and tempered martensite, the more stably excellent hole expandability is obtained. Therefore, the sum of the area ratios of martensite and tempered martensite is preferably 5.0% or less. More preferably, it is 3.0% or less.

Below, the method of measuring the area ratio of each organization is explained.

From a hot-rolled steel plate, a test piece is taken so that the metal structure can be observed at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction in a cross section parallel to the rolling direction.

The cross-section of the above test piece is polished using silicon carbide paper of #600 to #1500, and then finished into a mirror-like surface using a liquid in which diamond powder having a particle size of 1 to 6 ㎛ is dispersed in a diluted solution such as alcohol or in pure water. Next, the polishing is performed using colloidal silica that does not contain an alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At an arbitrary position in the long side direction of the cross-section of the sample, an area having a length of 50 ㎛ and a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface is measured at a measurement interval of 0.1 ㎛ by electron backscatter diffraction so that the center is a position at a depth of 1/4 of the plate thickness from the surface, to obtain crystal orientation information.

For the measurement, an EBSD analysis device consisting 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 vacuum inside the EBSD analysis device is set to 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. In addition, it is determined that the crystal structure is fcc.

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

Next, among the residual regions (regions where the "Grain Orientation Spread" exceeds 1°), under the condition that a 5° grain boundary is defined as a grain boundary, when the maximum value of the "Grain Average IQ" of the ferrite region is Iα, the region exceeding Iα/2 is extracted as bainite, and the region less than or equal to Iα/2 is extracted as "residual structure 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 structure other than retained austenite (cementite, pearlite, martensite, and tempered martensite)" and adding the area ratio of the residual austenite, the area ratio of the residual structure (cementite, pearlite, martensite, tempered martensite, and retained austenite) is obtained.

Regarding the above extracted "residual structures other than residual 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 area as the EBSD measurement area with SEM, a Vickers indentation is made near the observation position. Then, the structure of the observation surface is left, and surface contaminants are polished away, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed at a magnification of 3000 times with SEM.

In EBSD measurement, among the areas determined to be residual structures, an area having a lower structure within the grains and in which cementite is precipitated in multiple variants is determined to be tempered martensite. An area in which cementite is precipitated in a lamellar shape is determined to be pearlite. Spherical particles having high brightness and a particle size (circular diameter) of 2 ㎛ or less are determined to be cementite. An area having high brightness and in which the lower structure is not exposed by etching is determined to be “martensite and retained austenite.” By calculating the area ratios of each, 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 the retained austenite obtained by the above-described EBSD from the area ratio of the obtained “martensite and retained austenite.”

In addition, for removing contaminants on the surface of the observation surface, a method such as buffing using alumina particles with a particle size of 0.1 ㎛ or less or Ar ion sputtering can be used.

The sum of the density of the length L ₇ of grain boundaries with a crystal orientation difference of 7° and the length L ₆₈ of grain boundaries with a crystal orientation difference of 68°, with the <110> direction as the axis, in bainite: 0.35 to 0.60 μm/μm ²

By setting the sum of the densities of the length _L7 of grain boundaries having a crystal orientation difference of 7° and the length _L68 of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction in bainite to 0.35 to 0.60 ㎛/ ^㎛2 , the ductility, hole expandability, and toughness of a hot-rolled steel sheet can be improved.

If the sum of the densities of L ₇ and L ₆₈ 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 L ₇ and L ₆₈ is set to 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 L ₇ and L ₆₈ exceeds 0.60 μm/μm ² , the ductility of bainite is reduced, and excellent ductility and hole expandability cannot be obtained in the hot-rolled steel sheet. Therefore, the sum of the densities of L ₇ and L ₆₈ is set to 0.60 μm/μm ² or less. Preferably, it is 0.55 μm/μm ² or less.

In addition, a grain boundary with a crystal orientation difference of X° with respect to the <110> direction as an axis means a grain boundary in which, when two adjacent grains A and B are specified at a grain boundary, the crystallographic relationship in which the crystal orientations of grains A and B match is established by rotating one of the grains B by X° about the <110> axis. However, considering the measurement precision of crystal orientations, an orientation difference of ±4° from the matching orientation relationship is permitted.

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

In the EBSP-OIM method, since the microstructure and crystal orientation of the sample surface can be analyzed, the length of grain boundaries with specific crystal orientation differences can be quantitatively obtained. In addition, the analyzable area of the EBSP-OIM method is the area that can be observed with a SEM. Depending on the resolution of the SEM, the EBSP-OIM method can be analyzed with a resolution of at least 20 nm.

When measuring the density of the length of a specific grain boundary in the metal structure at a depth of 1/4 of the plate thickness from the surface in a cross-section parallel to the rolling direction and at the center position in the plate width direction, analysis is performed in at least 5 fields of view in an area of 50 µm × 50 µm at a magnification of 1000 times, and the average value of the length of the grain boundary having a crystal orientation difference of 7° about the <110> direction in bainite is calculated, thereby obtaining L _7. Similarly, L ₆₈ is obtained by calculating the average value of the length of the grain boundary having a crystal orientation difference of 68° about the <110> direction in bainite. In addition, as described above, an orientation difference of ±4° is permitted.

By dividing the obtained L ₇ and L ₆₈ by the measured area, the sum of the densities of the length L ₇ of grain boundaries having a crystal orientation difference of 7° and the length L ₆₈ of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction in bainite is obtained. In order to measure the density of the length of a specific grain boundary by extracting only bainite, the area exceeding Iα/2 can be extracted as bainite, similarly to the case where the area ratio of bainite is obtained.

Average particle size of old austenite particles: 10 to 30 ㎛

The ratio of the major axis l _d to the minor axis S _d of the austenite grain l _d /S _d : 2.0 or less

In the hot-rolled steel sheet according to the present embodiment, the average particle size of the prior austenite particles may be 10 to 30 µm, and the ratio of the major axis l _d to the minor axis S _d of the prior austenite particles l _d /S _d may be 2.0 or less. By controlling the average particle size of the prior austenite particles and l _d /S _d within the above ranges, the punching characteristics of the hot-rolled steel sheet can be improved.

Below, the average particle size of the old austenite particles and the measurement method of the ratio of the major axis l _d and minor axis S _d of the old austenite particles l _d /S _d are described.

A test piece is taken from a hot-rolled steel sheet so that the metal structure can be observed at a depth of 1/4 of the plate thickness from the surface and at the center of the plate width direction in a cross-section parallel to the rolling direction. The observation surface is corroded with a saturated picric acid aqueous solution to reveal the old austenite grain boundaries. An enlarged photograph of the cross-section parallel to the rolling direction that has been subjected to corrosion treatment at a depth of 1/4 of the plate thickness from the surface and at the center of the plate width direction is taken with a scanning electron microscope (SEM) at a magnification of 1000 times and a field of view of 5 or more. The equivalent diameters (diameters) of at least 20 old austenite particles having an equivalent diameter (diameter) of 2 ㎛ or more included in each SEM photograph are obtained by image processing, and the average value of these is calculated to obtain the average particle diameter of the old austenite particles. When old austenite particles having an equivalent diameter of less than 2 ㎛ are included, the above-described measurement is performed excluding them.

In addition, the major and minor axes of at least 20 austenite particles having an equivalent circular diameter (diameter) of 2 ㎛ or more, which are included in each of the above-described SEM photographs, are measured. By calculating the average value of the major and minor axes obtained by measuring each austenite particle, the major axis l _d and minor axis S _d of the austenite particle are obtained. By calculating the ratio of these, the ratio l _d /S _d of the major axis l _d and minor axis S _d of the austenite particle is obtained.

Tensile strength: 780MPa 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 weight reduction of the car body is small. The tensile strength is preferably 980 MPa or more. There is no particular need to limit the upper limit, but from the viewpoint of suppressing mold wear, it may be 1800 MPa.

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. The upper limit of the total elongation need not be specifically limited, but may be 30.0% or less or 25.0% or less.

Tensile strength and total elongation are measured in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece may be taken from the center of the plate width direction, and the direction perpendicular to the rolling direction may be the long side direction. The crosshead speed shall be 3 mm/min.

Hole expansion ratio: 50% or more

The hot-rolled steel sheet according to the present embodiment may have a hole expansion ratio of 50% or more. The upper limit of the hole expansion ratio need not be specifically limited, but may be 90% or less or 85% or less.

The hole expansion ratio is obtained by conducting a hole expansion test in accordance with JIS Z 2256:2010.

Impact value at -40℃: 60J/cm2 or more

The hot-rolled steel sheet according to the present embodiment may have an impact value of 60 J/cm2 or more at -40°C. The upper limit of the impact value at -40°C need not be specifically limited, but may be 180 J/cm2 or less or 175 J/cm2 or less.

A sub-sized Charpy impact test piece is collected from an arbitrary location on a hot-rolled steel plate, and the impact value at -40°C is obtained according to the test method described in JIS Z 2242:2005.

Plate thickness: 0.6 to 8.0 mm

The plate thickness of the hot-rolled steel plate according to the present embodiment is not particularly limited, but may be 0.6 to 8.0 mm. If the plate thickness of the steel plate is less than 0.6 mm, it becomes difficult to secure the rolling completion temperature and the rolling load becomes excessive, which may make hot rolling difficult. Therefore, the plate thickness of the steel plate according to the present 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 particles, and it may become difficult to secure the above-described metal structure in terms of the structure fraction. Therefore, the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.

Plating layer

The hot-rolled steel sheet according to the present embodiment having the chemical composition and metal structure described above may be used as a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrozinc plating, electrozinc Zn-Ni alloy plating, etc. Examples of the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, etc. The amount of plating adhesion is not particularly limited, and may be used in the same manner as in the past. Furthermore, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, applying and drying a silicate-based chromium-free chemical conversion treatment solution) after plating.

Next, a preferred method for manufacturing a hot-rolled steel sheet according to the present embodiment will be described.

A preferred method for manufacturing a hot-rolled steel plate according to the present embodiment comprises the following processes. In addition, the temperature of the slab and the temperature of the steel plate in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.

A heating process in which a slab having a predetermined chemical composition is maintained at a heating temperature of 1200℃ or higher for 1.0 hour or longer.

A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000℃ or higher and the total reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980℃.

A cooling process in which the coil is cooled to a temperature range of 570 to 620°C at an average cooling rate of 20°C/s or higher, maintained in a temperature range of 500 to 580°C for 2.0 to 12.0 hours, and then cooled to room temperature.

In the above hot rolling process, the finishing rolling may be performed so that the total reduction ratio in the rough rolling is 70% or more, and the reduction ratios of the three stages after the finishing rolling are all less than 25%.

Below, each process is described in detail.

Heating process

In the heating process, the slab having the chemical composition described above is heated to a heating temperature of 1200°C or higher and maintained for 1.0 hour. Since coarse precipitates present at the slab stage cause cracking during rolling or 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. The preferable heating temperature is 1230°C or higher and the preferable holding time is 3.0 hour or longer.

On the other hand, since there are cases where the yield is reduced due to an increase in scale generation when the heating temperature is too high or the holding time is too long, the heating temperature may be set to 1400°C or lower, and the holding time may be set to 20.0 hours or lower.

In addition, the heating slab is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by other casting methods (e.g., ingot casting).

Hot rolling process

If rough rolling is performed at a temperature lower than 1000℃, the austenite particles are not sufficiently recrystallized, so the aggregate structure develops and the desired hole expandability cannot be obtained. Therefore, rough rolling is performed so that the rough rolling completion temperature is 1000℃ or higher. It is preferably 1050℃ or higher. On the other hand, if rough rolling is performed at a temperature higher than 1300℃, the yield may be reduced because the amount of scale generated increases. Therefore, the rough rolling completion temperature may be 1300℃ or lower.

When the total reduction ratio in rough rolling is low, the crystal grain size of the old austenite particles becomes uneven, which becomes a factor in reducing the toughness, so the total reduction ratio in rough rolling is set to exceed 65%. The total reduction ratio in rough rolling is preferably 68% or more, more preferably 70% or more, and still more preferably 80% or more. The upper limit of the total reduction ratio in rough rolling is not particularly limited, but may be 90% or less.

In addition, the total reduction ratio in rough rolling is expressed as (1-t _r /t _s ) × 100(%) using the slab thickness t _s and the plate thickness t _r at the end of rough rolling.

By setting the total reduction ratio in rough rolling to 70% or more and strictly controlling the reduction ratios in the three subsequent stages of finishing rolling as described later, the average particle size and aspect ratio of the prior austenite particles described above can be achieved.

If the finish rolling completion temperature is lower than 860°C, the old austenite particles do not sufficiently recrystallize, so the aggregate structure develops and the hole expandability deteriorates. Therefore, the finish rolling completion temperature is set to 860°C or higher. Preferably, it is set to 900°C or higher. On the other hand, if the finish rolling completion temperature exceeds 980°C, the old austenite particles significantly coarsen, and the desired toughness cannot be obtained. Therefore, the finish rolling completion temperature is set to 980°C or lower. Preferably, it is 950°C or lower.

In this embodiment, in order to realize the average particle size and aspect ratio of the austenite particles described above and to improve the punching characteristics of the hot rolled steel sheet, the total reduction ratio in the rough rolling and the reduction ratio in the three stages of the latter part of the finishing rolling may be strictly controlled. Specifically, as described above, the total reduction ratio in the rough rolling may be 70% or more, and further, the reduction ratios in the three stages of the latter part of the finishing rolling may all be less than 25%.

If at least one of the reduction ratios of the three latter stages of the finishing rolling, that is, the final pass, the second pass from the final pass, and the third pass from the final pass of the finishing rolling, is 25% or more, the prior austenite particles become flat due to the rolling, and prior austenite particles having a large aspect ratio, which become the starting point of crack occurrence during punching, are formed. Therefore, the reduction ratios of the three latter stages of the finishing rolling (the reduction ratios of the final pass, the second pass from the final pass, and the third pass from the final pass of the finishing rolling) may all be less than 25%. Preferably, they are all 20% or less. In addition, the reduction ratio can be expressed by (1-h/ _h0 )×100(%), where h is the plate thickness after rolling in one pass and _h0 is the plate thickness before rolling.

Cooling process

After the hot rolling process, cooling is performed to a temperature range of 570 to 620°C at an average cooling rate of 20°C/s or more. In addition, in the present embodiment, the average cooling rate is a value obtained by dividing the temperature difference between the starting point and the end point of the set range by the elapsed time from the starting point to the end point.

If the average cooling rate is less than 20°C/s, a large amount of ferrite is precipitated, and the desired amount of bainite cannot be obtained. Therefore, the average cooling rate is set to 20°C/s or more. Preferably, it is 30°C/s or more, and more preferably, it is 50°C/s or more. From the viewpoint of suppressing the increase in cooling equipment, the average cooling rate may be set to 200°C/s or less.

In addition, 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 exceeds 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 any temperature that can be maintained in a temperature range of 620°C or lower and 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 preferably set to a temperature of 550°C or higher. In addition, in order to preferably control the total density of L ₇ and L ₆₈ and obtain excellent toughness, the cooling stop temperature is preferably set to 570°C or higher.

Since the cooling stop temperature is lower than 500℃, the desired amount of bainite cannot be obtained even if the temperature is maintained in the temperature range of 500 to 580℃ after reheating, so heating after cooling stop is not desirable.

After cooling at an average cooling rate of 20°C/s or more, coiling is performed. After coiling, it is maintained in a temperature range of 500 to 580°C for 2.0 to 12.0 hours. If the holding temperature is outside the temperature range of 500 to 580°C, or if the holding time is less than 2.0 hours or more than 12.0 hours, the desired amount of the sum of the densities of _L7 and _L68 in bainite cannot be obtained. 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 holding time is preferably 4.0 hours, more preferably 6.0 hours. The upper limit of the holding time is preferably 10.0 hours, more preferably 8.0 hours.

In addition, in the maintenance in the temperature range of 500 to 580°C, the steel plate temperature may be varied in 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 maintenance time of 2.0 to 12.0 hours can be secured in the temperature range of 500 to 580°C.

After the maintenance described above is performed in the temperature range of 500 to 580℃, it is cooled to room temperature. Any method may be used for cooling to room temperature, and in addition to air cooling, any appropriate method such as mist cooling or rapid cooling using a water cooling tank may be used. In addition, the room temperature referred to here is a temperature range of 20 to 30℃.

Example

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 examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the purpose of the present invention.

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

For the obtained hot-rolled steel sheet, the total density of the tissue fraction, L ₇ and L ₆₈ , the average particle diameter of the old austenite particles, and the ratio of the major axis l _d to the minor axis S _d of the old austenite particles (l _d /S _d) were obtained by the above-described method. The obtained results are shown in Tables 5 to 7.

Method for evaluating the properties of hot rolled steel plates

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 in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece was taken from the center of the plate width direction, and the direction perpendicular to the rolling direction was the long side direction. The crosshead speed was 3 mm/min.

When the tensile strength (TS) was 780 MPa or more, it was judged as having excellent strength and passed, and when it was less than 780 MPa, it was judged as having poor strength and failed. In addition, when the total elongation (El) was 14.0% or more, it was judged as having excellent ductility and passed, and when it was less than 14.0%, it was judged as having poor ductility and failed.

Hole expansion ratio (λ)

The hole expansion ratio (λ) was evaluated by conducting a hole expansion test in accordance with JIS Z 2256:2010.

When the hole expansion ratio (λ) is 50% or more, the hole expandability is judged to be excellent and passed, and when it is less than 50%, the hole expandability is judged to be poor and failed.

Shock value (vE _-40 )

The toughness was evaluated by obtaining the impact value through a Charpy impact test at -40℃. Sub-sized Charpy impact test pieces were collected from arbitrary locations of the hot-rolled steel sheet, and the toughness was evaluated by obtaining the impact value at -40℃ according to the test method described in JIS Z 2242:2005.

When the impact value (vE _-40 ) is 60J/cm2 or more, it is judged as having excellent toughness and passing, and when it is less than 60J/cm2, it is judged as having poor toughness and failing.

Punching characteristics

The punching characteristics were evaluated by performing a punching test and observing the punching end surface appearance. First, a punching hole was made with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm/s. Next, a cross-section perpendicular to the rolling direction of the punching hole was embedded in resin, and the punching end surface was photographed with a scanning electron microscope. The obtained observation photographs were observed, and when no end surface roughness was observed, the punching characteristics were considered particularly good and were described as "E (Excellent)" in Tables 5 to 7. In addition, when a small defect of less than 100 ㎛ was observed, the punching characteristics were considered good and were described as "G (Good)" in Tables 5 to 7, and when a large defect of 100 ㎛ or more was observed, the punching characteristics were considered poor and were described as "B (Bad)" in Tables 5 to 7.

Looking at Tables 5 to 7, it can be seen that the examples of the present invention have high strength, as well as excellent ductility, hole expandability, and toughness. In addition, it can be seen that the examples of the present invention, in which the average particle diameter of the prior austenite particles is 10 to 30 ㎛ and the ratio of the major axis l _d to the minor axis S _d of the prior austenite particles l _d /S _d is 2.0 or less, have particularly good punching characteristics.

Meanwhile, it can be seen that the comparative example has a decrease in at least one of strength, ductility, hole expandability, and toughness.

<Industrial applicability>

According to the present invention, a hot rolled steel sheet having high strength, excellent ductility, hole expandability and toughness, and a method for producing the same can be provided. According to the above preferred aspect of the present invention, in addition to the above-described properties, a hot rolled steel sheet having excellent punching properties and a method for producing the same can be provided.

Claims (5)

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 to 0.010%,
Mg: 0 to 0.010%,
REM: 0 to 0.010%, and
Zr: 0 to 0.010%
, and the remainder is composed of iron and impurities,
Metal structure, in area %,
Bainite: 80.0% or more,
Ferrite: 10.0% or less,
Residual tissue: 10.0% or less,
In the above bainite, the sum of the densities of the length _L7 of grain boundaries having a crystal orientation difference of 7° and the length _L68 of grain boundaries having a crystal orientation difference of 68° with respect to the <110> direction as the axis is 0.35 to 0.60 ㎛/ ^㎛2 ,
Tensile strength of 780MPa or more
Hot rolled steel plate characterized by: In the first paragraph, 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 to 0.010%,
REM: 0.0005 to 0.010%, and
Zr: 0.0005 to 0.010%
A hot-rolled steel sheet characterized by containing one or more types among the groups consisting of: In claim 1 or 2, in the metal structure,
The average particle size of the old austenite particles is 10 to 30 ㎛,
The ratio of the major axis l _d and minor axis S _d of the above austenite particles l _d /S _d is 2.0 or less.
Hot rolled steel plate characterized by: A method for manufacturing a hot-rolled steel plate as described in Article 1,
A heating process in which a slab having the chemical composition described in Article 1 is maintained at a heating temperature of 1200°C or higher for 1.0 hour or longer,
A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000℃ or higher and the total reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980℃,
A cooling process in which the coil is cooled to a temperature range of 570 to 620°C at an average cooling rate of 20°C/s or more, then maintained in a temperature range of 500 to 580°C for 2.0 to 12.0 hours, and then cooled to room temperature.
A method for manufacturing a hot-rolled steel sheet, characterized by comprising: In the fourth paragraph, in the hot rolling process,
In the above rolling process, the total reduction ratio is set to 70% or more,
A method for manufacturing a hot rolled steel sheet, characterized in that the finishing rolling is performed so that the reduction ratios of the last three stages of the finishing rolling are all less than 25%.