WO2024053701A1 - Hot-rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a specific chemical composition; the metal structure of this hot-rolled steel sheet at a position where the depth from the surface is 1/4 in the sheet thickness direction comprises, in area%, less than 3.0% of residual austenite, not less than 15.0% but less than 60.0% of ferrite and less than 5.0% of pearlite, while having an E value of 10.7 or more, an I value of 1.020 or more, a CS value of -8.0 × 10⁵ to 8.0 × 10⁵, and a standard deviation of the Mn concentration of 0.60% by mass or less; the average solid solution Cr concentration in the outermost layer region is 0.10% by mass or more; and the average number density of Cr oxides having a sphere equivalent radius of 0.1 µm or more in the surface is 1.0 × 10⁴ per cm² or less.

Description

hot rolled steel plate

The present invention relates to a hot rolled steel plate. Specifically, hot-rolled steel sheets are used after being formed into various shapes by press working, etc., and in particular, have high strength and critical thickness reduction rate at break, as well as excellent ductility and shear workability. The present invention relates to a hot rolled steel sheet having excellent fatigue properties after press forming.
This application claims priority based on Japanese Patent Application No. 2022-142994 filed in Japan on September 8, 2022, the contents of which are incorporated herein.

In recent years, efforts have been made to reduce carbon dioxide emissions in many fields from the perspective of protecting the global environment. Automobile manufacturers are also actively developing technologies to reduce the weight of vehicle bodies with the aim of improving fuel efficiency. However, it is not easy to reduce the weight of the vehicle body, as emphasis is placed on improving collision resistance to ensure passenger safety.

In order to reduce the weight of the car body and improve crash resistance, the use of high-strength steel plates to make the parts thinner is being considered. For this reason, there is a strong desire for a steel plate that has both high strength and excellent formability, and several techniques have been proposed to meet these demands. Since there are various processing methods for automobile parts, the required formability differs depending on the part to which it is applied. Among these, the critical thickness reduction rate at break and ductility are positioned as important indicators of formability. The critical rupture plate thickness reduction rate is a value determined from the plate thickness of the tensile test piece before rupture and the minimum value of the plate thickness of the tensile test piece after rupture. If the critical thickness reduction rate at rupture is low, it is not preferable because it is likely to break early when tensile strain is applied during press forming.

Automotive parts are formed by press forming, and blank plates for press forming are often manufactured by shearing, which is highly productive. Blank plates manufactured by shearing must have excellent end face accuracy after shearing.

For example, if a secondary shear surface occurs in which the end surface (sheared end surface) after shearing is sheared surface-fractured surface-sheared surface, the accuracy of the sheared end surface will be significantly degraded.

Furthermore, steel sheets used for automobile parts are required to have excellent fatigue properties after press forming.

For example, Patent Document 1 discloses a hot-rolled steel sheet that is a material for a cold-rolled steel sheet with excellent surface properties after press working, in which the degree of Mn segregation and the degree of P segregation in the central part of the sheet thickness are controlled.
However, Patent Document 1 does not consider the critical thickness reduction rate at rupture, shear workability, and fatigue properties after press forming of the hot rolled steel sheet.

International Publication No. 2020/044445

The present invention was made in view of the above-mentioned circumstances, and the present invention has been made in view of the above-mentioned circumstances. The purpose is to provide rolled steel sheets.

The gist of the invention is as follows.
(1) The hot rolled steel sheet according to one aspect of the present invention has a chemical composition in mass %,
C: 0.050-0.250%,
Si: 0.05-3.00%,
Mn: 1.00-4.00%,
sol. Al: 0.001-0.500%,
Cr: 0.060-2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0-2.00%,
Mo: 0-1.00%,
Ni: 0-2.00%,
B: 0 to 0.0100%,
Ca: 0-0.0200%,
Mg: 0 to 0.0200%,
REM: 0-0.1000%,
Bi: 0 to 0.0200%,
As: 0 to 0.100%,
Zr: 0 to 1.00%,
Co: 0-1.00%,
Zn: 0 to 1.00%,
Contains W: 0 to 1.00% and Sn: 0 to 0.05%,
The remainder consists of Fe and impurities,
The following formulas (A) and (B) are satisfied,
The metal structure at the 1/4 depth position from the surface in the thickness direction is
In area%,
The retained austenite is less than 3.0%,
Ferrite is 15.0% or more and less than 60.0%,
Pearlite is less than 5.0%,
The Entropy value expressed by the following formula (1) obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method is 10.7 or more,
The Inverse difference normalized value shown by the following formula (2) is 1.020 or more,
The Cluster Shade value shown by the following formula (3) is -8.0×10 ⁵ to 8.0×10 ⁵ ,
The standard deviation of the Mn concentration is 0.60% by mass or less,
The solid solution Cr concentration in the outermost layer region, which is a region starting from the surface and ending at a position 5 μm deep in the plate thickness direction, is 0.10% by mass or more,
The number density of Cr oxides having an equivalent sphere radius of 0.1 μm or more on the surface is 1.0×10 ⁴ pieces/cm ² or less.
0.060%≦Ti+Nb+V≦0.500%…(A)
Zr+Co+Zn+W≦1.00%…(B)
However, each element symbol in the formulas (A) and (B) indicates the content in mass % of the element, and when the element is not contained, 0% is substituted.
Here, P (i, j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, and L in the following formula (2) is a normalization constant of the brightness value that the SEM image can take. , i and j in the following formulas (2) and (3) are natural numbers from 1 to the above L, and μ _x and μ _y in the following formula (3) are the following formulas (4) and (5), respectively. It is indicated by.

(2) The hot-rolled steel sheet according to (1) above has an average grain size of less than 3.0 μm in the surface layer region, which is a region starting from the surface and ending at a position 20 μm deep in the sheet thickness direction. It may be.
(3) The hot rolled steel sheet according to (1) or (2) above has the chemical composition in mass%,
Ti: 0.001 to 0.500%,
Nb: 0.001-0.500%,
V: 0.001-0.500%,
Cu: 0.01-2.00%,
Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000%,
Bi: 0.0005-0.0200%,
As: 0.001 to 0.100%,
Zr: 0.01-1.00%,
Co: 0.01 to 1.00%,
Zn: 0.01-1.00%,
W: 0.01 to 1.00%, and Sn: 0.01 to 0.05%
It may contain one or more selected from the group consisting of:

According to the above aspect of the present invention, it is possible to obtain a hot-rolled steel sheet that has high strength, critical thickness reduction rate at rupture, excellent ductility and shear workability, and has excellent fatigue properties after press forming. can.
Further, according to the above-described preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-mentioned properties and further suppresses the occurrence of cracking in bending, that is, has excellent resistance to cracking in bending. can.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, mechanical structural parts, and even building parts.

1 is an example of a sheared end surface of a hot rolled steel plate according to an example of the present invention. It is an example of a sheared end surface of a hot rolled steel plate according to a comparative example. FIG. 3 is a diagram for explaining press molding performed in an example. It is a figure for demonstrating the punch shape used for the said press forming.

The chemical composition and metal structure of the hot rolled steel sheet according to the present embodiment will be explained in more detail below. However, the present invention is not limited to only the configuration disclosed in this embodiment, and various changes can be made without departing from the spirit of the present invention.

The numerically limited ranges described below with "~" in between include the lower limit and the upper limit. Numerical values indicated as "less than" or "greater than" do not include the value within the numerical range. In the following description, percentages regarding chemical compositions are percentages by mass unless otherwise specified.

Chemical composition The chemical composition of the hot rolled steel sheet according to this embodiment is, in mass%, C: 0.050 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00. %, sol. Al: 0.001 to 0.500%, Cr: 0.060 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0. 0100% or less, and the remainder: contains Fe and impurities, and satisfies formula (A) (0.060%≦Ti+Nb+V≦0.500%).
Each element will be explained in detail below.

C: 0.050-0.250%
C increases the area ratio of the hard phase and also increases the strength of ferrite by combining with precipitation-strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.055% or more, more preferably 0.060% or more, even more preferably 0.065% or more.
On the other hand, when the C content exceeds 0.250%, the area ratio of ferrite decreases, and the ductility of the hot rolled steel sheet decreases. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.150% or less.

Si: 0.05-3.00%
Si has the function of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet, and the function of solid solution strengthening of ferrite to increase the strength of the hot-rolled steel sheet. Further, Si has the effect of making the steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in the steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.40% or more, more preferably 0.60% or more.
However, if the Si content exceeds 3.00%, the surface quality and chemical conversion treatability of the hot-rolled steel sheet, as well as the ductility and weldability are significantly deteriorated, and the _A3 transformation point is significantly increased. This makes it difficult to perform hot rolling stably. In addition, ferrite tends to be produced excessively, which lowers the strength. In addition, austenite tends to remain after cooling, and the critical thickness reduction rate at rupture decreases. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, more preferably 2.00% or less.

Mn: 1.00-4.00%
Mn has the effect of suppressing ferrite transformation and increasing the strength of the hot rolled steel sheet. If the Mn content is less than 1.00%, desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.10% or more, more preferably 1.20% or more.
On the other hand, if the Mn content exceeds 4.00%, the hard phase becomes periodic band-like due to segregation of Mn, making it difficult to obtain desired shearing workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less, more preferably 3.00% or less, even more preferably 2.50% or less.

Ti: 0~0.500%
Nb: 0-0.500%
V: 0~0.500%
0.060%≦Ti+Nb+V≦0.500%…(A)
However, each element symbol in the formula (A) indicates the content of the element in mass %, and when the element is not contained, 0% is substituted.
Ti, Nb, and V are elements that finely precipitate in steel as carbides and nitrides and improve the strength of steel through precipitation strengthening. If the total content of Ti, Nb and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb, and V is set to 0.060% or more. That is, the value of the middle side of the formula (A) is set to 0.060% or more. Note that it is not necessary that all of Ti, Nb, and V be contained; it is sufficient that at least one of them is contained, and the total content thereof may be 0.060% or more. The total content of Ti, Nb and V is preferably 0.080% or more, more preferably 0.100% or more. The contents of Ti, Nb and V are each preferably 0.001% or more.
On the other hand, when the total content of Ti, Nb and V exceeds 0.500%, the workability of the hot rolled steel sheet deteriorates. Therefore, the total content of Ti, Nb and V is set to 0.500% or less. That is, the value of the middle side of the formula (A) is set to 0.500% or less. Preferably it is 0.300% or less, more preferably 0.250% or less, even more preferably 0.200% or less.

sol. Al: 0.001-0.500%
Like Si, Al has the effect of deoxidizing the steel and making it sound, and also has the effect of promoting the formation of ferrite and increasing the ductility of the hot rolled steel sheet. 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. Al content is preferably 0.010% or more.
On the other hand, sol. If the Al content exceeds 0.500%, the above effects are saturated and it is economically unfavorable, so sol. Al content shall be 0.500% or less. sol. The Al content is preferably 0.450% or less, more preferably 0.400% or less, even more preferably 0.350% or less.
In addition, sol. Al means acid-soluble Al, and indicates solid solution Al that exists in steel in a solid solution state.

Cr:0.060~2.000%
Cr has the effect of increasing the hardenability of hot rolled steel sheets. Further, when combined with desired manufacturing conditions, Cr is concentrated in the outermost layer region of the hot rolled steel sheet, thereby suppressing scale growth and having the effect of reducing the arithmetic mean roughness Ra after press forming. If the Cr content is less than 0.060%, the above effects cannot be obtained. Therefore, the Cr content is set to 0.060% or more. The Cr content is preferably 0.200% or more, more preferably 0.400% or more, even more preferably 0.600% or more.
On the other hand, if the Cr content exceeds 2.000%, the chemical conversion treatability of the hot rolled steel sheet will be significantly reduced. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.800% or less, more preferably 1.600% or less.

P: 0.100% or less P is also an element that has the effect of increasing the strength of the hot rolled steel sheet through solid solution strengthening. Therefore, P may be actively included. However, P is an element that tends to segregate, and when the P content exceeds 0.100%, the ductility and critical thickness reduction rate of the hot rolled steel sheet due to grain boundary segregation decrease significantly. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be particularly specified and may be 0%, but from the viewpoint of refining cost, it is preferably 0.001%.

S: 0.0300% or less S forms sulfide inclusions in the steel and reduces the ductility and critical thickness reduction rate at break of the hot rolled steel sheet. When the S content exceeds 0.0300%, the ductility and critical thickness reduction rate at break of the hot rolled steel sheet decrease significantly. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content does not need to be particularly specified and may be 0%, but from the viewpoint of refining cost, it is preferably 0.0001%.

N: 0.1000% or less N has the effect of reducing the ductility and critical thickness reduction rate at break of the hot rolled steel sheet. When the N content exceeds 0.1000%, the ductility and critical thickness reduction rate at break of the hot rolled steel sheet are significantly reduced. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, even more preferably 0.0100% or less. The lower limit of the N content does not need to be particularly specified and may be 0%, but if one or more of Ti, Nb and V is contained to make the metal structure finer, carbonitriding In order to promote the precipitation of substances, the N content is preferably 0.0010% or more, more preferably 0.0020% or more.

O: 0.0100% or less When O is contained in large amounts in steel, it forms coarse oxides that serve as starting points for fractures, causing brittle fractures and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, more preferably 0.0050% or less. The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

The remainder of the chemical composition of the hot rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities refer to things that are mixed in from ore as a raw material, scrap, or the manufacturing environment, and/or things that are allowed within a range that does not adversely affect the hot rolled steel sheet according to this embodiment. do.

The hot rolled steel sheet according to this embodiment may contain the following elements as optional elements in place of a part of Fe. When these arbitrary elements are not contained, the lower limit of the content is 0%. The arbitrary elements will be explained in detail below.

Cu: 0.01-2.00%
Mo: 0.01~1.00%
Ni: 0.02-2.00%
B: 0.0001-0.0100%
Cu, Mo, Ni, and B all have the effect of improving the hardenability of the hot rolled steel sheet. Further, Cu and Mo precipitate as carbides in the steel and have the effect of increasing the strength of the hot rolled steel sheet. Furthermore, when containing Cu, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.

As mentioned above, Cu has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of precipitating as carbides in the steel at low temperatures to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effects of the above action, the Cu content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.

As described above, Mo has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of precipitating as carbides in the steel to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effects of the above action, the Mo content is preferably 0.01% or more, more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the effect of the above action will be saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less. Mo content is preferably 0.50% or less, more preferably 0.20% or less.

As mentioned above, Ni has the effect of increasing the hardenability of the hot rolled steel sheet. Further, when containing Cu, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the effects of the above action, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically undesirable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.

As mentioned above, B has the effect of increasing the hardenability of the hot rolled steel sheet. In order to more reliably obtain the effect of this action, the B content is preferably 0.0001% or more, more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot rolled steel sheet will be significantly reduced, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

Ca: 0.0005-0.0200%
Mg: 0.0005-0.0200%
REM: 0.0005-0.1000%
Bi: 0.0005-0.0200%
Ca, Mg, and REM all have the effect of increasing the ductility of the hot rolled steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. Furthermore, Bi has the effect of increasing the ductility of the hot rolled steel sheet by making the solidification structure finer. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects of the above action, the content of any one or more of Ca, Mg, REM and Bi is preferably 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, inclusions will be excessively generated in the steel, which will actually reduce the ductility of the hot rolled steel sheet. There may be cases where Furthermore, even if the Bi content exceeds 0.0200%, the effect of the above action will be saturated, which is not economically preferable. Therefore, the Ca content and Mg content should be 0.0200% or less, the REM content should be 0.1000% or less, and the Bi content should be 0.0200% or less. Bi content is preferably 0.0100% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are added industrially in the form of mischmetal.

As: 0.001-0.100%
By lowering the austenite single phase temperature, As makes prior austenite grains finer and contributes to improving the ductility of the hot rolled steel sheet. In order to reliably obtain this effect, it is preferable that the As content be 0.001% or more.
On the other hand, since the above effect is saturated even if a large amount of As is contained, the As content is set to 0.100% or less.

Zr: 0.01~1.00%
Co:0.01~1.00%
Zn: 0.01-1.00%
W: 0.01~1.00%
Zr+Co+Zn+W≦1.00%…(B)
Sn: 0.01~0.05%
However, each element symbol in the formula (B) indicates the content of the element in mass %, and when the element is not contained, 0% is substituted.
Regarding Zr, Co, Zn and W, the present inventors have confirmed that the effect of the hot rolled steel sheet according to the present embodiment is not impaired even if these elements are contained in a total of 1.00% or less. There is. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total amount of 1.00% or less. That is, the value on the left side of the equation (B) may be 1.00% or less. Since Zr, Co, Zn, and W do not need to be contained, their respective contents may be 0%. In order to solid solution strengthen the steel sheet and improve its strength, the contents of Zr, Co, Zn, and W may each be 0.01% or more.
Moreover, the present inventors have confirmed that even if a small amount of Sn is contained, the effect of the hot rolled steel sheet according to the present embodiment is not impaired. However, if a large amount of Sn is contained, defects may occur during hot rolling, so the Sn content is set to 0.05% or less. Since Sn does not need to be contained, the Sn content may be 0%. In order to improve the corrosion resistance of the hot rolled steel sheet, the Sn content may be 0.01% or more.

The chemical composition of the hot-rolled steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol. Al may be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid. C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.
When the hot-rolled steel sheet has a plating layer on its surface, the chemical composition may be analyzed after removing the plating layer by mechanical grinding or the like, if necessary.

Metal structure of hot rolled steel sheet Next, the metal structure of the hot rolled steel sheet according to the present embodiment will be explained.
The hot-rolled steel sheet according to the present embodiment has a metal structure at a 1/4 depth position from the surface in the thickness direction, in terms of area%, residual austenite is less than 3.0%, ferrite is 15.0% or more, 60.0%, pearlite is less than 5.0%, and the entropy value is obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method and is expressed by the following formula (1). is 10.7 or more, the Inverse difference normalized value shown by the following formula (2) is 1.020 or more, and the Cluster Shade value shown by the following formula (3) is -8.0×10 ⁵ to 8. ^0x105 , the standard deviation of the Mn concentration is 0.60% by mass or less, and the solid solution in the outermost layer region is a region starting from the surface and ending at a depth of 5 μm in the plate thickness direction. The Cr concentration is 0.10% by mass or more, and the number density of Cr oxides having an equivalent sphere radius of 0.1 μm or more on the surface is 1.0×10 ⁴ pieces/cm ² or less.

Therefore, the hot rolled steel sheet according to the present embodiment has high strength and critical thickness reduction rate at rupture, as well as excellent ductility and shear workability, and can obtain excellent fatigue properties after press forming.
In addition, in this embodiment, the microstructure fraction of the metallographic structure, the entropy value, the inverse difference normalized value, the Cluster Shade value, and the standard deviation of the Mn concentration in a region at a depth of 1/4 from the surface in the plate thickness direction are defined. . The reason is that the metal structure at this position shows a typical metal structure of a steel plate.
In addition, the "surface" here refers to the interface between the plating layer and the steel sheet when the hot-rolled steel sheet has a plating layer, and the "1/4 depth position from the surface" refers to the hot-rolled steel sheet. A position at a depth of 1/4 of the thickness of the plate in the thickness direction from the surface of the plate.

Area ratio of retained austenite: less than 3.0% Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the effect of increasing the ductility of hot rolled steel sheets through transformation induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, which becomes the starting point for cracks during deformation and causes a decrease in the rate of thickness reduction at critical rupture. When the area ratio of retained austenite is 3.0% or more, the above-mentioned effect becomes obvious and the critical plate thickness reduction rate of the hot rolled steel sheet decreases. Therefore, the area ratio of retained austenite is set to be less than 3.0%. The area percentage of retained austenite is preferably less than 1.5%, more preferably less than 1.0%.
Since it is preferable to have as little retained austenite as possible, the area ratio of retained austenite may be 0%.

Methods for measuring the area ratio of retained austenite include methods using X-ray diffraction, EBSP (electron back scattering diffraction pattern) analysis, and magnetic measurement. In this embodiment, the area ratio of retained austenite is measured by X-ray diffraction.

In the measurement of the retained austenite area ratio by X-ray diffraction in this embodiment, first, in a cross section at a depth of 1/4 from the surface in the thickness direction of a hot rolled steel sheet, 1 mm or more at any position in the rolling direction, A sample is taken so that the metal structure can be observed in an area of 1 mm or more centered on the center in the direction perpendicular to the plate thickness direction. For the above sample, use Co-Kα rays to determine the integrated intensity of a total of 6 peaks: α(110), α(200), α(211), γ(111), γ(200), and γ(220). . Next, the volume fraction of retained austenite is calculated from the integrated intensity using an intensity averaging method. The obtained volume fraction of retained austenite is regarded as the area fraction of retained austenite.

Area ratio of ferrite: 15.0% or more and less than 60.0% Ferrite is a structure that is generated when fcc is transformed into bcc at a relatively high temperature. Since ferrite has a high work hardening rate, it has the effect of increasing the strength-ductility balance of hot rolled steel sheets. In order to obtain the above effect, the area ratio of ferrite is set to 15.0% or more. Preferably it is 20.0% or more, more preferably 25.0% or more, even more preferably 30.0% or more.
On the other hand, since ferrite has low strength, if the area ratio is excessive, desired strength cannot be obtained. Therefore, the ferrite area ratio is set to less than 60.0%. Preferably it is 50.0% or less, more preferably 45.0% or less.

Area ratio of pearlite: less than 5.0% Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite particles, and is a soft metal structure compared to bainite and martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite and bainite, which are the remaining structures, decreases, making it impossible to obtain the desired strength. Therefore, the area ratio of pearlite is made less than 5.0%. The area ratio of pearlite is preferably 3.0% or less.
In order to improve the stretch flangeability of the hot rolled steel sheet, it is preferable to reduce the area ratio of pearlite as much as possible, and it is even more preferable that the area ratio of pearlite is 0%.

In addition, the hot rolled steel sheet according to the present embodiment includes bainite, martensite, and tempered marten with a total area ratio of more than 32.0% and 85.0% or less as residual structures other than retained austenite, ferrite, and pearlite. A hard tissue consisting of one or more types of sites is included.

The area ratio of ferrite and pearlite is measured by the following method. First, at the center in the direction orthogonal to the rolling direction and the plate thickness direction, the sample was prepared so that the metal structure in a region 1/4 depth from the surface in the plate thickness direction could be observed in a plate thickness section parallel to the rolling direction. Collect. The sample has a size that allows observation of about 10 mm in the rolling direction. Next, the cross section of the sample is polished to a mirror finish, and then polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove the strain introduced into the surface layer of the sample. . An area of 200 μm or more at any position in the rolling direction of the above sample cross section, and a region of 200 μm or more centering on a 1/4 depth position from the surface in the plate thickness direction, is measured using an electron beam at measurement intervals of 0.1 μm in the rolling direction and the plate thickness direction. Obtain crystal orientation information by measuring by backscattering diffraction method. For the above measurements, 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 degree of vacuum in the EBSD analyzer 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. Note that the number of observation fields is 5.

Further, backscattered electron images are taken in each of the same observation fields from which the crystal orientation information described above was obtained. Note that in photographing this image, the accelerating voltage is 15 kV, the irradiation current level is 12 to 13, and the electron beam irradiation level is 62. The focal length (WD: Working Distance) is 5 mm. The area ratio of ferrite and pearlite is identified from the backscattered electron image and the crystal orientation information. First, crystal grains in which cementite is precipitated in a lamellar shape are identified in a backscattered electron image. In the backscattered electron image, cementite is observed as a white contrast. The cementite in this pearlite has a lamellar form, and the crystal grains in which the white contrast of the lamellar form is observed at intervals of 1.0 μm or less are defined as pearlite crystal grains. By calculating the area ratio of the crystal grains, the area ratio of pearlite is obtained.
After that, the obtained crystal orientation information for the crystal grains excluding the crystal grains determined to be pearlite was used using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer. Then, a region where the Grain Average Misorientation value is 1.0° or less is determined to be ferrite. At this time, the Grain Tolerance Angle is set to 15 degrees, and the area ratio of ferrite is obtained by determining the area ratio of the region determined to be ferrite.

The area ratio of the residual structure is obtained by subtracting the area ratio of retained austenite, ferrite, and pearlite from 100%.
Furthermore, in this embodiment, the rolling direction of the hot rolled steel sheet is determined by the following method.
First, a test piece is taken so that a cross section parallel to the surface of the hot rolled steel sheet can be observed. After finishing the cross section of the test piece taken in the plate pressure direction with mirror polishing, it is observed using an optical microscope. The observation plane is a plane parallel to the surface of the plate at an arbitrary depth in the range of 1/4 to 1/2 in the plate thickness direction, and on the observation plane, the direction parallel to the stretching direction of the crystal grains is determined as the rolling direction. do.

Entropy value: 10.7 or more, Inverse difference normalized value: 1.020 or more In order to suppress the generation of secondary shear surfaces, it is important to form a fracture surface after a sufficient shear surface has been formed. It is necessary to suppress early crack formation from the cutting edge of the tool during machining. For this purpose, it is important that the periodicity of the metal structure is low and the uniformity of the metal structure is high. In this embodiment, the generation of secondary shear planes is suppressed by controlling the entropy value (E value) indicating the periodicity of the metal structure and the inverse difference normalized value (I value) indicating the uniformity of the metal structure.

The E value represents the periodicity of the metal structure. When the brightness is arranged periodically due to the formation of a band-like structure, that is, when the periodicity of the metal structure is high, the E value decreases. In this embodiment, since it is necessary to have a metal structure with low periodicity, it is necessary to increase the E value. If the E value is less than 10.7, secondary shear surfaces are likely to occur. Starting from the periodically arranged structure, cracks occur from the cutting edge of the shearing tool very early in the shearing process, forming a fractured surface, and then a sheared surface is formed again. It is estimated that this makes secondary shear planes more likely to occur. Therefore, the E value is set to 10.7 or more. Preferably it is 10.8 or more, more preferably 11.0 or more. The higher the E value is, the more preferable it is, and although the upper limit is not particularly specified, it may be 13.0 or less, 12.5 or less, or 12.0 or less.

The I value represents the uniformity of the metal structure, and increases as the area of a region with a certain brightness increases. A high I value means that the uniformity of the metal structure is high. In this embodiment, since it is necessary to have a highly uniform metal structure, it is necessary to increase the I value. If the I value is less than 1.020, cracks will occur from the cutting edge of the shearing tool at the very early stage of shearing and a fractured surface will form due to the influence of hardness distribution caused by precipitates and element concentration differences within the crystal grains. The shear plane is then formed again. It is estimated that this makes secondary shear planes more likely to occur. Therefore, the I value is set to 1.020 or more. Preferably it is 1.025 or more, more preferably 1.030 or more. The higher the I value, the better, and although the upper limit is not particularly specified, it may be 1.200 or less, 1.150 or less, or 1.100 or less.

Cluster Shade value: -8.0×10 ⁵ ~8.0×10 ⁵
The Cluster Shade value (CS value) indicates the degree of distortion of the metal structure. The CS value is a positive value if there are many points with brightness above the average value of the average value of brightness in the image obtained by photographing the metal structure, and a positive value if there are many points with brightness below the average value. It becomes a negative value.

In a secondary electron image of an electron microscope, the brightness increases where the surface of the object to be observed has large irregularities, and the brightness decreases where the irregularities are small. The unevenness of the surface of an object to be observed is greatly affected by the grain size and intensity distribution within the metal structure. The CS value in this embodiment increases when the strength variation of the metal structure is large or the structure unit is small, and becomes small when the strength variation is small or the structure unit is large.

In this embodiment, it is important to keep the CS value within a desired range close to zero. When the CS value is less than −8.0×10 ⁵ , the critical thickness reduction rate at break of the hot rolled steel sheet decreases. This is presumed to be because crystal grains with large grain sizes exist in the metal structure, and these crystal grains are preferentially destroyed during ultimate deformation. Therefore, the CS value is set to -8.0×10 ⁵ or more. It is preferably -7.5×10 ⁵ or more, and even more preferably -7.0×10 ⁵ or more.
On the other hand, when the CS value is more than 8.0×10 ⁵ , the critical thickness reduction rate at break of the hot rolled steel sheet decreases. This is presumed to be because the microscopic strength variation in the metal structure is large, and the strain during ultimate deformation is locally concentrated, making it easier to break. Therefore, the CS value is set to 8.0×10 ⁵ or less. It is preferably 7.5×10 ⁵ or less, and even more preferably 7.0×10 ⁵ or less.

The E value, I value and CS value can be obtained by the following method.
In this embodiment, the imaging area of the SEM image taken to calculate the E value, I value, and CS value is a cross section parallel to the rolling direction at the center in the direction perpendicular to the rolling direction and the sheet thickness direction. The area is 160 μm×160 μm centered at a 1/4 depth position from the surface in the thickness direction, and the number of observation fields is 5. To take the SEM images, an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, with a tungsten emitter and an accelerating voltage of 1.5 kV. Based on the above settings, a SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.

Next, the obtained SEM image was cut out into an area of 880 x 880 pixels (the observation area is 160 μm x 160 μm in actual size), and the limit magnification of contrast enhancement described in Non-Patent Document 3 was set to 2.0. , performs smoothing processing with a tile grid size of 8×8. By rotating the smoothed SEM image counterclockwise in 1 degree increments from 0 degrees to 179 degrees, excluding 90 degrees, and creating images in 1 degree increments, a total of 179 images are obtained. . Next, for each of these 179 images, frequency values of brightness between adjacent pixels are collected in the form of a matrix using the GLCM method described in Non-Patent Document 1.

The matrix of 179 frequency values collected by the above method is expressed as p _k (k=0...89, 91,...179), where k is the rotation angle from the original image. For each image, the generated p _k is summed for all k (k = 0...89, 91...179), and then normalized so that the sum of each component is 1. Calculate the matrix P of . Furthermore, using the following formulas (1) to (5) described in Non-Patent Document 2, the E value, I value, and CS value are calculated, respectively. Note that the average value obtained by measuring the entire visual field is calculated.
P(i,j) in the following equations (1) to (5) is a gray level co-occurrence matrix, and the value at the i-th row and j-th column of the matrix P is expressed as P(i,j). As mentioned above, it is calculated using the 256 x 256 matrix P, so if you want to emphasize this point, modify the following formulas (1) to (5) to the following formulas (1') to (5'). be able to. Here, L in the following formula (2) is the number of grayscale levels that the SEM image can take (Quantization levels of grayscale), and in this embodiment, as described above, the SEM image is output with a grayscale of 256 gradations. Therefore, L is 256. i and j in the following formulas (2) and (3) are natural numbers from 1 to the above L, and μ _x and μ _y in the following formula (3) are shown by the following formulas (4) and (5), respectively. .
In formulas (1') to (5') below, the value at the i-th row and j-th column of the matrix P is expressed as P _ij .

Standard deviation of Mn concentration: 0.60% by mass or less The standard deviation of Mn concentration of the hot rolled steel sheet according to the present embodiment is 0.60% by mass or less. Thereby, the hard phase can be uniformly dispersed, and it is possible to prevent cracks from forming at the cutting edge of the shearing tool very early in the shearing process. As a result, the generation of secondary shear planes can be suppressed. The standard deviation of the Mn concentration is preferably 0.50% by mass or less, more preferably 0.47% by mass or less. The lower limit of the standard deviation of the Mn concentration is preferably as small as possible from the viewpoint of suppressing excessive burrs, but due to manufacturing process constraints, the actual lower limit is 0.10% by mass.

The standard deviation of Mn concentration can be obtained by the following method. First, a sample is taken at the center in a direction perpendicular to the rolling direction and the thickness direction so that a region at a depth of 1/4 from the surface in the thickness direction can be observed in a cross section parallel to the rolling direction. The size of the sample is such that it can be observed about 10 mm in the rolling direction, although it depends on the measuring device. Next, after mirror polishing the sample, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions were an accelerating voltage of 15 kV, a magnification of 5000 times, and a distribution image of Mn concentration in a range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction centered on a 1/4 depth position from the surface in the plate thickness direction. Measure. More specifically, the Mn concentration is measured at 40,000 or more locations with a measurement interval of 0.1 μm. Next, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentration obtained from all measurement points.

Solid solution Cr concentration in the outermost layer region: 0.10% by mass or more The present inventors found that when the solid solution Cr concentration in the outermost layer region (a region 5 μm deep from the surface in the plate thickness direction) is high, It has been found that by reducing the number density of Cr oxides with a sphere equivalent radius of 0.1 μm or more, it is possible to suppress deterioration of fatigue properties of hot rolled steel sheets after press forming. If the solid solution Cr concentration in the outermost layer region is less than 0.10% by mass, deterioration of fatigue properties after press forming of the hot rolled steel sheet cannot be suppressed. Therefore, the solid solution Cr concentration in the outermost layer region is set to 0.10% by mass or more. The solid solution Cr concentration in the outermost layer region is preferably 0.20% by mass or more, more preferably 0.40% by mass or more.
The solid solution Cr concentration in the outermost layer region may be 5.00% by mass or less.
In this embodiment, the region 5 μm deep from the surface in the thickness direction is the depth in the thickness direction of the range starting from the surface of the hot rolled steel sheet and ending at a position 5 μm deep in the thickness direction. A layered region with As mentioned above, the "surface" here refers to the interface between the plating layer and the steel sheet when the hot rolled steel sheet includes a plating layer.

The solid solution Cr concentration in the outermost layer region can be analyzed by GD-MS (Glow Discharge-Mass Spectrometry) analysis. GD-MS analysis is an analysis method that tracks changes in composition from the surface of a hot rolled steel sheet in the depth direction as the discharge time progresses.
In this embodiment, when a sample is taken from an arbitrary position of a hot rolled steel sheet and GD-MS analysis is performed on the sample in the sheet thickness direction, a region 5 μm deep from the surface in the sheet thickness direction (starting from the surface) The average value of the Cr concentration in mass % in a region whose end point is a position 5 μm deep in the plate thickness direction is determined. This operation is performed at any three or more locations (preferably five or more locations) and the average value of the obtained values is calculated to obtain the solid solution Cr concentration in the outermost layer region.
When the hot-rolled steel sheet has a plating layer on the surface, the depth position where the Fe concentration is 90% by mass when analyzed by GD-MS is the interface between the plating layer and the hot-rolled steel sheet, that is, the surface of the hot-rolled steel sheet. regarded as.

The number density of Cr oxides with a sphere equivalent radius of 0.1 μm or more on the surface: 1.0 × 10 ⁴ pieces/cm ² or less The number density of Cr oxides with a sphere equivalent radius of 0.1 μm or more on the surface is 1 When the number exceeds .0×10 ⁴ pieces/cm ² , the surface roughness becomes large when the hot rolled steel sheet is press-formed. Since this surface roughness deteriorates the fatigue characteristics of the pressed part, it is preferable that the surface roughness is small. The Cr oxide defined here has an equivalent sphere radius of 0.1 μm or more, and is relatively coarse. It is thought that this coarse Cr oxide inhibits sliding between the hot rolled steel sheet and the mold, causing an increase in surface roughness. Generally, in addition to being present at the bottom of the scale, Cr oxides have high adhesion to the base iron, so it is difficult to reduce the number density of Cr oxides on the surface of Cr-containing steel. In this embodiment, this problem is solved by controlling the temperature during descaling and the temperature and rolling reduction rate during rough rolling. Therefore, the number density of Cr oxides having an equivalent sphere radius of 0.1 μm or more on the surface is set to be 1.0×10 ⁴ pieces/cm ² or less. The number density of Cr oxides with a sphere equivalent radius of 0.1 μm or more on the surface is preferably 0.8×10 ⁴ pieces/cm ² or less, more preferably 0.6×10 ⁴ pieces/cm ² or less. It is.
The number density of Cr oxides having an equivalent sphere radius of 0.1 μm or more on the surface may be 0.1×10 ⁴ pieces/cm ² or more.

The number density of Cr oxides on the surface is measured by the following method.
A sample is cut out from a hot-rolled steel plate so that the surface in the thickness direction is the observation surface. After degreasing the observation surface at 60° C. for 60 seconds using FC-E6403 manufactured by Nippon Parkerizing Co., Ltd., it is immersed in acetone and subjected to ultrasonic cleaning for 90 seconds. Thereafter, 10 or more fields of view are observed at a magnification of 3000 times. The composition of the precipitate can be measured by EDS (energy dispersive X-ray spectrometer). The number of regions containing Cr and O with a former equivalent radius of 0.1 μm or more is counted in each field of view and divided by the measurement area to obtain the number density of Cr oxides. Note that if the precipitate is subjected to EDS analysis and 20 atomic percent or more of each of Cr and O is detected, the precipitate is considered to be a Cr oxide.
In addition, when a hot-rolled steel sheet is provided with a plating layer on the surface, the above-mentioned measurement is performed after removing the plating layer by pickling with fuming nitric acid.

Average crystal grain size in the surface layer region: less than 3.0 μm By reducing the grain size in the surface layer region (area 20 μm deep from the surface in the thickness direction), cracking in bending of hot rolled steel sheets can be suppressed. . The higher the strength of the hot-rolled steel sheet, the more likely cracks will occur from the inside of the bend during bending (hereinafter referred to as cracks within bending). The mechanism of cracking in bending is estimated as follows. During bending, compressive stress is generated on the inside of the bend. Initially, processing progresses while the entire inside of the bend deforms uniformly, but as the amount of processing increases, uniform deformation alone is no longer sufficient to support the deformation, and the deformation progresses as strain concentrates locally (generation of shear deformation bands). As this shear deformation band further grows, a crack is generated along the shear band from the inner surface of the bend and grows. The reason why cracks in bending are more likely to occur as strength increases is that due to the decrease in work hardening ability associated with increase in strength, it becomes difficult for uniform deformation to progress, and deformation tends to occur unevenly. This is presumed to be due to the formation of shear deformation bands (or under loose processing conditions).
In this embodiment, the region 20 μm deep from the surface in the thickness direction is the depth in the thickness direction of the range starting from the surface of the hot rolled steel sheet and ending at a position 20 μm deep in the thickness direction. A layered region with

Through research conducted by the present inventors, it has been found that cracking in bending becomes noticeable in steel plates with a tensile strength of 980 MPa class or higher. Furthermore, the present inventors have discovered that the finer the crystal grain size in the surface layer region of a hot rolled steel sheet, the more local strain concentration is suppressed and the occurrence of in-bending cracks becomes less likely. In order to obtain the above effect, the average grain size in the surface layer region of the hot rolled steel sheet is preferably less than 3.0 μm. Therefore, in this embodiment, the average grain size in the surface layer region may be less than 3.0 μm. The average crystal grain size in the surface layer region is more preferably 2.5 μm or less. Although the lower limit of the average crystal grain size in the surface layer region is not particularly specified, it may be 0.5 μm.

The crystal grain size in the surface layer region is measured using the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. The EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer, and OIM Analysis (registered trademark) manufactured by AMETEK. The analyzable area of the EBSP-OIM method is the area that can be observed with SEM. Depending on the resolution of the SEM, the EBSP-OIM method allows analysis with a minimum resolution of 20 nm.

A cross section made in the direction parallel to the rolling direction and the plate thickness direction of a hot rolled steel plate, in the surface layer region 20 μm deep from the surface in the plate thickness direction, at a magnification of 1200 times, at least 5 fields of view, and adjacent A location where the angular difference between the measurement points is 5° or more is defined as a grain boundary, and the area average grain size is calculated.

Note that retained austenite is not a structure generated by phase transformation at 600° C. or lower and does not have the effect of dislocation accumulation, so retained austenite is not analyzed in this measurement method. In the EBSP-OIM method, retained austenite having an fcc crystal structure can be excluded from the analysis target.

Tensile Strength Properties Among the mechanical properties of hot rolled steel sheets, tensile strength properties (tensile strength, total elongation) are evaluated in accordance with JIS Z 2241:2011. The test piece is a JIS Z 2241:2011 No. 5 test piece. The test piece may be collected at a 1/4 position from the end face in a direction perpendicular to the rolling direction and the plate thickness direction, and the plate width direction may be the longitudinal direction of the test piece.

The hot rolled steel sheet according to this embodiment has a tensile strength of 980 MPa or more. Preferably it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts will be limited and the contribution to reducing the weight of the vehicle body will be small. Although there is no need to specifically limit the upper limit, it may be set to 1780 MPa from the viewpoint of suppressing mold wear.

Further, the total elongation of the hot rolled steel sheet according to the present embodiment is preferably 10.0% or more, and the product of tensile strength and total elongation (TS×El) is preferably 13000 MPa·% or more. The total elongation is more preferably 11.0% or more, even more preferably 13.0% or more. Further, the product of tensile strength and total elongation is more preferably 14,000 MPa·% or more, and even more preferably 15,000 MPa·% MPa or more. By setting the total elongation to 10.0% or more and the product of tensile strength and total elongation to 13000 MPa·% or more, the applicable parts are not limited and it can greatly contribute to reducing the weight of the vehicle body.

Plate Thickness The thickness of the hot rolled steel plate according to this embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the thickness of the hot rolled steel plate is less than 0.5 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot rolled steel plate according to this embodiment may be 0.5 mm or more. Preferably it is 1.2 mm or more or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to refine the metal structure and obtain the above-mentioned metal structure. 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-mentioned chemical composition and metallographic structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing, electrolytic Zn--Ni alloy plating, and the like. Examples of the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, 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. Ru. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, 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 liquid) after plating.

Manufacturing Conditions A preferred method for manufacturing the hot rolled steel sheet according to this embodiment having the above-mentioned chemical composition and metallographic structure is as follows.

In a preferred method for manufacturing a hot-rolled steel sheet according to the present embodiment, the following steps (1) to (11) are sequentially performed. Note that 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. Moreover, stress refers to the tension applied to the steel plate in the rolling direction.

(1) After holding the slab in a temperature range of 700 to 850°C for 900 seconds or more, it is further heated and held in a temperature range of 1100°C or more for 6000 seconds or more.
(2) Descaling is performed at least once in a temperature range of 1150°C or higher before rough rolling, and descaling is performed at least twice in a temperature range of 1130°C or higher during rough rolling, and the temperature is 1130°C or higher. The maximum value of the total rolling reduction rate between each descaling in the area shall be less than 40%.
(3) Hot rolling is carried out in a temperature range of 850 to 1100°C with a total rolling reduction of 90% or more.
(4) A stress of 170 kPa or more is applied to the steel plate from the end of the last stage of hot rolling until the start of the final stage of rolling.
(5) The rolling reduction in the final stage of hot rolling is 8% or more, and the hot rolling is completed so that the rolling completion temperature Tf is 900°C or more and less than 1010°C.
(6) A stress of less than 200 kPa is applied to the steel plate after the final stage of hot rolling until the steel plate is cooled to 800°C.
(7) Within 1 second after completion of hot rolling, cool to a temperature range below hot rolling completion temperature Tf - 50°C, and then cool to a temperature range of 600 to 780°C at an average cooling rate of 50°C/s or more. do. However, a more preferable cooling condition is to cool to a temperature range below the hot rolling completion temperature Tf - 50° C. within 1 second after the completion of hot rolling.
(8) Perform slow cooling at a temperature range of 600 to 780°C with an average cooling rate of less than 5°C/s for 2.0 seconds or more.
(9) After completion of slow cooling, cool so that the average cooling rate in the temperature range of 450 to 600°C is 30°C/s or more and less than 50°C/s.
(10) Cooling is performed so that the average cooling rate in the temperature range from the winding temperature to 450°C is 50°C/s or more.
(11) Wind up in a temperature range of 350°C or less.

By adopting the above manufacturing method, we can stably produce hot-rolled steel sheets that have high strength, critical thickness reduction at rupture, excellent ductility and shear workability, and have excellent fatigue properties after press forming. can do.

(1) Slab, slab temperature and holding time when subjected to hot rolling As the slab subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting/blowing, etc. can be used. Moreover, if necessary, those obtained by hot working or cold working can be used.

The slab to be subjected to hot rolling is preferably held at a temperature range of 700 to 850°C for 900 seconds or more during slab heating, and then further heated and held at a temperature range of 1100°C or higher for 6000 seconds or more. Note that when maintaining the temperature in a temperature range of 700 to 850°C, the steel plate temperature may be varied within this temperature range or may be kept constant. Further, when maintaining the temperature at 1100°C or higher, the steel plate temperature may be varied in the temperature range of 1100°C or higher, or may be kept constant.

During austenite transformation in the temperature range of 700 to 850°C, Mn is distributed between ferrite and austenite, and by lengthening the transformation time, Mn can diffuse into the ferrite region. This eliminates the Mn micro-segregation that is unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration. Further, by holding the temperature in a temperature range of 1100° C. or higher for 6000 seconds or more, the standard deviation of the Mn concentration can be significantly reduced.

For hot rolling, it is preferable to use a liver mill or a tandem mill as multi-pass rolling. Particularly from the viewpoint of industrial productivity and the stress load on the steel plate during rolling, it is more preferable that at least the last two stages are hot rolled using a tandem mill. Hot rolling includes rough rolling and finish rolling, each of which is performed multiple times (stages). Rough rolling is a process of rolling a slab to a minimum thickness of 25 mm, and finish rolling is a process of rolling a plate after rough rolling to a target thickness.

(2) Descaling before rough rolling: once or more in a temperature range of 1150°C or higher, descaling during rough rolling: 2 or more times in a temperature range of 1130°C or higher, at a temperature of 1130°C or higher during rough rolling Maximum value of total rolling reduction between each descaling in the area: less than 40% By controlling the descaling conditions before rough rolling, the descaling conditions during rough rolling, and the rolling conditions between each descaling, the The number density of Cr oxides on the surface can be preferably controlled. Descaling can be done by water jets.

It is preferable to perform descaling at least once in a temperature range of 1150° C. or higher before rough rolling. By performing descaling one or more times in a temperature range of 1150° C. or higher before rough rolling, it is possible to remove the primary scale formed in the heating furnace and suppress the occurrence of subsequent descaling defects. As a result, the number density of Cr oxides on the surface of the hot rolled steel sheet can be preferably controlled. The upper limit of the number of times of descaling in the temperature range of 1150° C. or higher is not particularly limited, but may be 5 times or less.

In rough rolling, rolling and descaling are performed multiple times. During rough rolling, descaling is performed between rollings or after multiple rollings. In this embodiment, during rough rolling, descaling is performed twice or more in a temperature range of 1130°C or higher, and the maximum value of the total rolling reduction between each descaling in a temperature range of 1130°C or higher is set to 40 It is preferable to set it as less than %. By performing descaling two or more times in a temperature range of 1130°C or higher, the thickness of the scale formed in the first stage of rough rolling is reduced or the scale is removed, thereby reducing the number density of Cr oxides on the surface of the hot rolled steel sheet. can be preferably controlled.
By setting the maximum value of the total rolling reduction during each descaling in a temperature range of 1130°C or higher to less than 40%, scale will not get caught between the base steel during rolling between descaling, and the surface of the hot rolled steel sheet will be reduced. The number density of Cr oxides can be preferably controlled.

The total rolling reduction rate between each descaling in the temperature range of 1130°C or higher is t0, where the plate thickness before the nth descaling in the temperature range of 1130°C or higher is _t0 , and the n+1th descaling in the temperature range of 1130°C or higher is When the subsequent exit plate thickness is t ₁ , it can be expressed as {(t ₀ −t ₁ )/t ₀ }×100(%). Between the n-th descaling and the (n+1)-th descaling, rolling may be performed only once, or rolling may be performed multiple times.

(3) Hot rolling reduction: 90% or more in the temperature range of 850 to 1100°C By performing hot rolling such that the total reduction in the temperature range of 850 to 1100°C is 90% or more, Mainly, the recrystallized austenite grains are made finer, and the accumulation of strain energy in the unrecrystallized austenite grains is promoted. Then, the recrystallization of austenite is promoted and the atomic diffusion of Mn is promoted, so that the standard deviation of the Mn concentration can be reduced. Therefore, it is preferable to perform hot rolling such that the total rolling reduction in the temperature range of 850 to 1100° C. is 90% or more.
Note that hot rolling here includes rough rolling and finish rolling.

Note that the total rolling reduction in the temperature range of 850 to 1100°C is defined as the inlet plate _thickness before the first rolling in this temperature range, and the outlet plate thickness after the final rolling in this temperature range. When t 1 is t ₁ , it can be expressed as {(t ₀ −t ₁ )/t ₀ }×100(%).

(4) Stress applied to the steel plate from the time of rolling before the final stage of hot rolling until the start of rolling of the final stage: 170 kPa or more From the end of rolling of the first stage before the final stage of hot rolling until the start of rolling of the final stage In addition, it is preferable to apply a stress of 170 kPa or more to the steel plate. This makes it possible to reduce the number of crystal grains having the {110}<001> crystal orientation in the recrystallized austenite after rolling one stage before the final stage. Since {110}<001> is a crystal orientation that is difficult to recrystallize, by suppressing the formation of this crystal orientation, recrystallization by the final stage of rolling can be effectively promoted. As a result, the band-like structure of the hot rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased.
Note that the term "rolling one stage before the final stage of hot rolling" as used herein refers to rolling one stage before the final stage of finish rolling. For example, when finish rolling is performed in seven passes of F1, F2...F6, and F7, the sixth pass (F6) is referred to as the pass of the sixth stage (F6).

If the stress applied to the steel plate is less than 170 kPa, it may not be possible to set the E value to the desired value. The stress applied to the steel plate is more preferably 190 kPa or more.
The stress applied to a steel plate refers to the tension applied in the longitudinal direction of the steel plate, and can be controlled by adjusting the roll rotation speed during tandem rolling. It can be calculated by dividing by the cross-sectional area of the steel plate.

(5) Reduction ratio in the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900°C or more, less than 1010°C The rolling reduction ratio in the final stage of hot rolling is 8% or more, and hot rolling is completed. Preferably, the temperature Tf is 900°C or higher. By setting the reduction ratio in the final stage of hot rolling to 8% or more, recrystallization due to the reduction in the final stage can be promoted. As a result, the band-like structure of the hot rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased. By setting the hot rolling completion temperature Tf to 900° C. or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite. As a result, the formation of ferrite in the final structure (metal structure of the hot-rolled steel sheet after production) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained. Further, by setting Tf to less than 1010° C., coarsening of the austenite grain size can be suppressed, the periodicity of the metal structure can be reduced, and the E value can be set to a desired value.

(6) Stress applied to the steel plate after the final stage of hot rolling until the steel plate is cooled to 800°C: Less than 200 kPa After the final stage of hot rolling, the stress applied to the steel plate until the steel plate is cooled to 800°C It is preferable to apply a stress of less than 200 kPa to the steel plate until the steel plate is heated. By applying a stress of less than 200 kPa to the steel sheet, recrystallization of austenite proceeds preferentially in the rolling direction, and an increase in periodicity of the metal structure can be suppressed. As a result, the E value can be set to a desired value. The stress applied to the steel plate is more preferably 180 kPa or less.

(7) Within 1 second after completion of hot rolling, cool to a temperature range below hot rolling completion temperature Tf - 50°C, and then accelerate to a temperature range of 600 to 780°C at an average cooling rate of 50°C/s or more. Cooling In order to suppress the growth of austenite crystal grains refined by hot rolling, cooling should be performed at 50°C or higher within 1 second after the completion of hot rolling, that is, the amount of cooling within 1 second after completing hot rolling should be It is more preferable that the temperature is at least ℃. In order to cool the steel plate to a temperature range below the hot rolling completion temperature Tf - 50°C within 1 second after the completion of hot rolling, cooling with a high average cooling rate is performed immediately after the completion of hot rolling, for example, cooling water is applied to the surface of the steel sheet. Just spray it on. By cooling to a temperature range of Tf-50° C. or lower within 1 second after completion of hot rolling, the grain size of the surface layer can be made finer, and the resistance to internal cracking in bending of the hot rolled steel sheet can be improved.

Further, by performing accelerated cooling after the above cooling to a temperature range of 780°C or less at an average cooling rate of 50°C/s or more, the formation of ferrite and pearlite, which have a small amount of precipitation strengthening, can be suppressed. This improves the strength of the hot rolled steel sheet. Note that the average cooling rate here refers to the temperature drop range of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the end of accelerated cooling (when the steel plate is taken out from the cooling equipment). This is the value divided by the time required from the start to the completion of accelerated cooling.

Although there is no particular upper limit for the cooling rate, increasing the cooling rate requires larger cooling equipment and increases equipment costs. Therefore, considering the equipment cost, the temperature is preferably 300° C./s or less. Further, the cooling stop temperature of accelerated cooling is preferably set to 600° C. or higher in order to perform slow cooling, which will be described later.

(8) Perform slow cooling with an average cooling rate of less than 5°C/s in the temperature range of 600 to 780°C for 2.0 seconds or more. In the temperature range of 600 to 780°C, perform slow cooling with an average cooling rate of less than 5°C/s. By performing slow cooling for 2.0 seconds or more, precipitation-strengthened ferrite can be sufficiently precipitated. This makes it possible to achieve both strength and ductility of the hot rolled steel sheet.
Note that the average cooling rate here is the width of the temperature drop of the steel plate from the cooling stop temperature of accelerated cooling to the stop temperature of slow cooling divided by the time required from the time of stopping accelerated cooling to the time of stopping slow cooling. Refers to value.

The time for performing slow cooling is preferably 3.0 seconds or more. The upper limit of the time for slow cooling is determined by the equipment layout, but may be approximately less than 10.0 seconds. Further, although there is no particular lower limit to the average cooling rate of slow cooling, since increasing the temperature without cooling involves a large investment in equipment, it may be set to 0° C./s or more.

(9) After completion of slow cooling, cool so that the average cooling rate in the temperature range of 450 to 600 °C is 30 °C/s or more and less than 50 °C/s. It is preferable to perform cooling so that the average cooling rate in the area is 30° C./s or more and less than 50° C./s. By setting the average cooling rate in the above temperature range to 30° C./s or more and less than 50° C./s, the CS value can be set to a desired value. When the average cooling rate is 50° C./s or more, a flat lath-like structure with low brightness is likely to be formed, and the CS value becomes less than −8.0×10 ⁵ . When the average cooling rate is less than 30°C/s, the concentration of carbon in the untransformed part is promoted, the strength of the hard structure increases, and the strength difference with the soft structure increases, so the CS value decreases. It becomes more than 8.0×10 ⁵ .
Note that the average cooling rate here refers to the range from the cooling stop temperature of slow cooling with an average cooling rate of less than 5°C/s to the cooling of cooling with an average cooling rate of 30°C/s or more and less than 50°C/s. The temperature drop range of the steel plate to the stop temperature is determined from the time when slow cooling is stopped when the average cooling rate is less than 5°C/s to the time when cooling is stopped when the average cooling rate is 30°C/s or more and less than 50°C/s. This is the value divided by the time required to reach the destination.

(10) Average cooling rate in the temperature range from coiling temperature to 450℃: 50℃/s or more In order to suppress the area ratio of pearlite and retained austenite and obtain the desired strength and formability, It is preferable that the average cooling rate in the temperature range is 50° C./s or more. Thereby, the matrix structure can be made hard.
Note that the average cooling rate here refers to the range of temperature drop of the steel plate from the cooling stop temperature to the coiling temperature when the average cooling rate is 30°C/s or more and less than 50°C/s. It refers to the value divided by the time required from the time of stopping cooling to winding, which is 30° C./s or more and less than 50° C./s.

(11) Winding temperature: 350°C or less The winding temperature shall be 350°C or less. By setting the winding temperature to 350° C. or lower, it is possible to reduce the amount of iron carbide precipitation and to reduce variations in hardness distribution within the hard phase. As a result, the I value can be increased and the generation of secondary shear planes can be suppressed.

Next, the effects of one aspect of the present invention will be explained in more detail with reference to Examples. The conditions in the Examples are examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of one condition. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

Steel having the chemical composition shown in Tables 1 and 2 was melted, and slabs with a thickness of 240 to 300 mm were manufactured by continuous casting. Using the obtained slabs, hot rolled steel plates shown in Tables 5A to 6B were obtained under the manufacturing conditions shown in Tables 3A to 4B.
Note that the average cooling rate of the slow cooling was less than 5° C./s. Furthermore, since the lower measurement limit of the winding temperature listed in Table 4A and Table 4B is 50°C, the actual winding temperature of the example described as 50°C is 50°C or lower.

The obtained hot-rolled steel sheet was subjected to the above-mentioned method to determine the area ratio of the metallographic structure, E value, I value, CS value, standard deviation of Mn concentration, solid solution Cr concentration in the outermost layer region, and equivalent sphere radius at the surface. The number density of Cr oxides of 0.1 μm or more, the average crystal grain size in the surface layer region, the tensile strength TS, and the total elongation El were determined. The measurement results obtained are shown in Tables 5A to 6B.
The remaining structure was one or more of bainite, martensite, and tempered martensite.

Method for evaluating properties of hot rolled steel sheets Tensile properties High strength when tensile strength TS is 980 MPa or more, total elongation El is 10.0% or more, and tensile strength TS x total elongation El is 13000 MPa・% or more. The hot-rolled steel sheet was judged to have passed the test because it had the following characteristics and excellent ductility. If any one of the conditions was not satisfied, it was determined that the hot rolled steel sheet did not have high strength and excellent ductility and was rejected.

Limiting rate of thickness reduction at break The limit rate of thickness reduction at break of hot-rolled steel sheets was evaluated by a tensile test.
A tensile test was conducted in the same manner as when the tensile properties were evaluated. When the plate thickness before the tensile test is t ₁ and the minimum value of the plate thickness at the center in the width direction (short side direction) of the tensile test piece after rupture is t ₂ , (t ₁ - _{t 2} )×100/ By calculating the value of _t1 , the critical plate thickness reduction rate at rupture was obtained. The tensile test was carried out five times, and the average value of the three tests, excluding the maximum and minimum values of the limit thickness reduction rate at break, was calculated to obtain the limit thickness reduction rate at break.

If the critical thickness reduction rate at rupture was 60.0% or more, the hot rolled steel sheet was judged to have passed the test as having a high critical rupture thickness reduction rate. On the other hand, when the critical thickness reduction rate at break was less than 60.0%, it was determined that the hot rolled steel sheet did not have a high critical failure plate thickness reduction rate and was rejected.

Shearing workability (secondary shear surface evaluation)
The shear workability of the hot-rolled steel sheet was evaluated by a punching test.
Three punched holes were made for each example using a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m/s. Next, the cross section perpendicular to the rolling direction and the cross section parallel to the rolling direction of the punched hole were respectively embedded in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. In the obtained observation photograph, a sheared end surface as shown in FIG. 1 or 2 can be observed. Note that FIG. 1 is an example of a sheared end surface of a hot rolled steel sheet according to an example of the present invention, and FIG. 2 is an example of a sheared end surface of a hot rolled steel sheet according to a comparative example. In FIG. 1, the diagram shows a sag, a sheared surface, a fractured surface, and a sheared end surface of a burr. On the other hand, FIG. 2 shows the sheared end surface of the burr - sheared surface - fractured surface - sheared surface - fractured surface. Here, the sag is the area of the smooth R-shaped surface, the sheared surface is the area of the punched end face separated by shear deformation, and the fractured surface is the area of the punched end face separated by a crack generated near the cutting edge. A burr is a surface having protrusions protruding from the lower surface of a hot-rolled steel sheet.

Among the obtained sheared end faces, if a sheared surface-fractured surface-sheared surface is observed on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction, as shown in FIG. determined that a secondary shear plane was formed. We observed 4 surfaces for each punched hole, 12 surfaces in total, and if no surface showed a secondary shear surface, it was determined that the hot rolled steel sheet had excellent shear workability and passed, and the It was written as "None". On the other hand, if even one secondary shear plane was formed, it was determined that the hot-rolled steel sheet did not have excellent shearing workability, and was judged to be rejected, and "present" was written in the table.

Fatigue properties after press forming The fatigue properties after press forming were evaluated by the arithmetic mean roughness Ra of the surface of the hot rolled steel sheet after press forming.
As shown in FIG. 3, one surface A of the hot-rolled steel sheet in the thickness direction was ground to give a thickness of 1.6 mm or less to the other surface B of the test piece 10. As shown in FIG. Press molding was performed by pulling out in one direction D while pressing with a load. The shape of the punch is shown in FIG. 4 (FIG. 9 of Patent No. 5,655,394). After the shape of the hot rolled steel plate after press forming was corrected using a leveler, the arithmetic mean roughness Ra of the surface was measured by the following method.
Regarding the sample surface of 1000 mm x 1000 mm, measurement points were set at 200 mm intervals in the rolling direction and a direction orthogonal to the rolling direction and the plate thickness direction, and the surface roughness was measured at each measurement point. However, the measurement length at each measurement point was 5 mm. A roughness curve was obtained by sequentially applying contour curve filters with cutoff values λc and λs to the measured cross-sectional curve. Specifically, from the obtained measurement results, components with a wavelength λc of 0.8 mm or less and components with a wavelength λs of 2.5 mm or more were removed to obtain a roughness curve. Based on the obtained roughness curve, the arithmetic mean roughness Ra of each measurement location was calculated in accordance with JIS B 0601:2013. By calculating the average value of the obtained values, the arithmetic mean roughness Ra of the surface of the hot rolled steel sheet after press forming was obtained.

If the arithmetic mean roughness Ra of the surface of the hot-rolled steel sheet after press-forming was 3.0 μm or less, the hot-rolled steel sheet had excellent fatigue properties after press-forming and was judged to be acceptable. On the other hand, when the arithmetic mean roughness Ra of the surface was more than 3.0 μm, it was determined that the hot rolled steel sheet did not have excellent fatigue properties after press forming and was rejected.

Resistance to internal cracking in bending The resistance to internal cracking in bending was evaluated by the following bending test.
A bending test piece was obtained by cutting out a rectangular test piece of 100 mm x 30 mm at a 1/2 position from the end face of the hot rolled steel plate in a direction perpendicular to the rolling direction and the plate thickness direction. Both bending where the bending ridgeline is parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridgeline is parallel to the direction perpendicular to the rolling direction and the plate thickness direction (C-direction) (C-axis bending) A test was conducted in accordance with the V-block method (bending angle θ is 90°) of JIS Z 2248:2022. As a result, the minimum bending radius without cracking was determined, and the resistance to internal cracking in bending was investigated. The value obtained by dividing the average value (R) of the minimum bending radius of the L axis and the C axis by the plate thickness (t) was taken as the limit bending R/t and used as an index value of resistance to internal cracking in bending. When R/t was 2.5 or less, it was determined that the hot rolled steel sheet had excellent resistance to internal cracking in bending.

However, the presence or absence of cracks can be determined by mirror-polishing a cross section of the test piece cut along a plane parallel to the bending direction and perpendicular to the plate surface, and then observing cracks under an optical microscope. It was determined that a crack was present when the length of the crack exceeded 30 μm.

Looking at Tables 5A to 6B, the hot rolled steel sheets according to the examples of the present invention have high strength and critical thickness reduction rate at rupture, as well as excellent ductility and shear workability, as well as excellent fatigue properties after press forming. It can be seen that it has Furthermore, it can be seen that among the examples of the present invention, the hot-rolled steel sheets in which the average grain size in the surface layer region is less than 3.0 μm have excellent internal bending cracking resistance in addition to having the above-mentioned properties.
On the other hand, it can be seen that the hot rolled steel sheet according to the comparative example has deteriorated in one or more properties.

According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet that has high strength, critical thickness reduction rate at rupture, excellent ductility and shear workability, and has excellent fatigue properties after press forming. Can be done. Further, according to the above-described preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-mentioned properties and further suppresses the occurrence of cracking in bending, that is, has excellent resistance to cracking in bending. can.
The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile parts, mechanical structural parts, and even building parts.

Claims (3)

1. The chemical composition is in mass%,
   C: 0.050-0.250%,
   Si: 0.05-3.00%,
   Mn: 1.00-4.00%,
   sol. Al: 0.001-0.500%,
   Cr: 0.060-2.000%,
   P: 0.100% or less,
   S: 0.0300% or less,
   N: 0.1000% or less,
   O: 0.0100% or less,
   Ti: 0 to 0.500%,
   Nb: 0 to 0.500%,
   V: 0 to 0.500%,
   Cu: 0-2.00%,
   Mo: 0-1.00%,
   Ni: 0-2.00%,
   B: 0 to 0.0100%,
   Ca: 0-0.0200%,
   Mg: 0 to 0.0200%,
   REM: 0-0.1000%,
   Bi: 0 to 0.0200%,
   As: 0 to 0.100%,
   Zr: 0 to 1.00%,
   Co: 0-1.00%,
   Zn: 0 to 1.00%,
   Contains W: 0 to 1.00% and Sn: 0 to 0.05%,
   The remainder consists of Fe and impurities,
   The following formulas (A) and (B) are satisfied,
   The metal structure at the 1/4 depth position from the surface in the thickness direction is
   In area%,
   The retained austenite is less than 3.0%,
   Ferrite is 15.0% or more and less than 60.0%,
   Pearlite is less than 5.0%,
   The Entropy value expressed by the following formula (1) obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method is 10.7 or more,
   The Inverse difference normalized value shown by the following formula (2) is 1.020 or more,
   The Cluster Shade value shown by the following formula (3) is -8.0×10 ⁵ to 8.0×10 ⁵ ,
   The standard deviation of the Mn concentration is 0.60% by mass or less,
   The solid solution Cr concentration in the outermost layer region, which is a region starting from the surface and ending at a position 5 μm deep in the plate thickness direction, is 0.10% by mass or more,
   A hot-rolled steel sheet characterized in that the number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more on the surface is 1.0×10 ⁴ pieces/cm ² or less.
   0.060%≦Ti+Nb+V≦0.500%…(A)
   Zr+Co+Zn+W≦1.00%…(B)
   However, each element symbol in the formulas (A) and (B) indicates the content in mass % of the element, and when the element is not contained, 0% is substituted.
   Here, P (i, j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, and L in the following formula (2) is the number of gray scale levels that the SEM image can take. i and j in the following formulas (2) and (3) are natural numbers from 1 to the above L, and μ _x and μ _y in the following formula (3) are represented by the following formulas (4) and (5), respectively. shown.
   

   

   

   

   

   
2. The hot rolled sheet according to claim 1, wherein the average crystal grain size in a surface layer region, which is a region starting from the surface and ending at a position 20 μm deep in the plate thickness direction, is less than 3.0 μm. steel plate.
3. The chemical composition is in mass%,
   Ti: 0.001 to 0.500%,
   Nb: 0.001-0.500%,
   V: 0.001-0.500%,
   Cu: 0.01-2.00%,
   Mo: 0.01-1.00%,
   Ni: 0.02-2.00%,
   B: 0.0001 to 0.0100%,
   Ca: 0.0005-0.0200%,
   Mg: 0.0005-0.0200%,
   REM: 0.0005-0.1000%,
   Bi: 0.0005-0.0200%,
   As: 0.001 to 0.100%,
   Zr: 0.01 to 1.00%,
   Co: 0.01 to 1.00%,
   Zn: 0.01-1.00%,
   W: 0.01 to 1.00%, and Sn: 0.01 to 0.05%
   The hot rolled steel sheet according to claim 1 or 2, characterized in that it contains one or more selected from the group consisting of:

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