WO2023063010A1 - Hot-rolled steel plate - Google Patents

Abstract

This hot-rolled steel plate: has a prescribed chemical composition; has a metal structure having, in area%, retained austenite at less than 3.0%, ferrite at at least 15.0% to less than 60.0%, and pearlite at less than 5.0%; has, according to a gray level co-occurrence matrix method, an entropy value obtained by analyzing a SEM image of the metal structure, of 10.7% or more; an inverse differenced normalized value of at least 1.020; a cluster shade value of -8.0 × 105 - 8.0 × 105; an Mn concentration standard deviation of no more than 0.60 mass%; and a tensile strength of 980 MPa or more.

Description

hot rolled steel plate

The present invention relates to hot rolled steel sheets. Specifically, hot-rolled steel sheets that are used by being formed into various shapes by press working, etc., especially hot-rolled steel sheets that have high strength and critical rupture thickness reduction rate, as well as excellent ductility and shear workability. It relates to inter-rolled steel sheets.
This application claims priority based on Japanese Patent Application No. 2021-166958 filed in Japan on October 11, 2021, the content of which is incorporated herein.

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

In order to achieve both vehicle weight reduction and collision resistance, the use of high-strength steel plates to reduce the thickness of members is being considered. Therefore, there is a strong demand for a steel sheet having both high strength and excellent formability, and several techniques have been conventionally proposed to meet these demands. Since there are various processing methods for automobile parts, the required formability varies depending on the applied parts, but among them, the critical rupture thickness reduction rate and ductility are positioned as important indicators of formability. The critical rupture thickness reduction rate is a value obtained from the minimum thickness of the tensile test piece before fracture and the minimum thickness of the tensile test piece after fracture. If the critical rupture thickness reduction rate is low, it is not preferable because it tends to break early when a tensile strain is applied during press forming.

Automobile parts are formed by press molding, and the press-molded blank plates are often manufactured by highly productive shearing. A blank plate manufactured by shearing must be excellent in end face accuracy after shearing.

For example, if the appearance of the end face after shearing (sheared end face) is a secondary sheared face that becomes sheared face - fractured face - sheared face, the accuracy of the sheared end face will be significantly deteriorated.

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

WO2020/044445

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and critical rupture thickness reduction rate, as well as excellent ductility and shear workability. .

The gist of the present invention is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass% of
C: 0.050 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 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 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.00%,
Ni: 0 to 2.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.0200%,
As: 0 to 0.100%,
Zr: 0 to 1.00%,
Co: 0 to 1.00%,
Zn: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 0.05%, and the balance: Fe and impurities,
satisfying the following formulas (A) and (B),
The metal structure, in area %,
Retained austenite is less than 3.0%,
Ferrite is 15.0% or more and less than 60.0%,
Perlite is less than 5.0%,
The Entropy value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure by the gray-level co-occurrence matrix method is 10.7 or more,
The inverse differentiated normalized value represented by the following formula (2) is 1.020 or more,
The Cluster Shade value represented 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,
Tensile strength is 980 MPa or more.
0.060% ≤ Ti + Nb + V ≤ 0.500% (A)
Zr+Co+Zn+W≦1.00% (B)
However, each element symbol in the above formulas (A) and (B) indicates the content of the element in terms of mass %, and 0% is substituted when the element is not contained.
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 steel sheet described in (1) above may have an average crystal grain size of less than 3.0 μm in the surface layer.
(3) The hot-rolled steel sheet according to (1) or (2) above, wherein the chemical composition is, in mass%,
Ti: 0.001 to 0.500%,
Nb: 0.001 to 0.500%,
V: 0.001 to 0.500%,
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Ni: 0.01 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0001 to 0.0200%,
Mg: 0.0001-0.0200%,
REM: 0.0001 to 0.1000%,
Bi: 0.0001 to 0.0200%,
As: 0.001 to 0.100%,
Zr: 0.01 to 1.00%,
Co: 0.01 to 1.00%,
Zn: 0.01 to 1.00%,
W: 0.01-1.00%, and Sn: 0.01-0.05%
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 having high strength and critical rupture thickness reduction rate, as well as excellent ductility and shear workability. Further, according to the preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-described properties and further suppresses the occurrence of internal bending cracks, that is, has excellent resistance to internal bending cracks. can be done.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.

It is an example of a sheared edge surface of a hot-rolled steel sheet according to an example of the present invention. It is an example of a sheared end face of a hot-rolled steel plate according to a comparative example.

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

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

Chemical Composition Hereinafter, the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described in detail.

C: 0.050-0.250%
C increases the area ratio of the hard phase and 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 should be 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more or 0.090% or more.
On the other hand, if the C content exceeds 0.250%, the ductility of the hot-rolled steel sheet decreases due to the decrease in the area ratio of ferrite. Therefore, the C content should be 0.250% or less. The C content is preferably 0.200% or less, 0.150% or less or 0.120% or less.

Si: 0.05-3.00%
Si has the effect of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet and the effect of solid-solution strengthening the ferrite to increase the strength of the hot-rolled steel sheet. In addition, Si has the effect of making steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content should be 0.05% or more. The Si content is preferably 0.50% or more, more preferably 0.80% or more, 1.00% or more, 1.20% or more, or 1.40% or more.
However, if the Si content exceeds 3.00%, the surface properties and chemical conversion treatability of the steel sheet, as well as ductility and weldability, are significantly deteriorated, and the _A3 transformation point is significantly increased. This makes it difficult to stably perform hot rolling. In addition, austenite tends to remain after cooling, and the critical rupture thickness reduction rate decreases. Therefore, the Si content should be 3.00% or less. The Si content is preferably 2.70% or less, more preferably 2.50% or less, 2.20% or less, 2.00% or less or 1.80% 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%, the desired strength cannot be obtained. Therefore, the Mn content should be 1.00% or more. The Mn content is preferably 1.30% or more, more preferably 1.50% or more or 1.80% or more.
On the other hand, if the Mn content exceeds 4.00%, the form of the hard phase becomes periodic bands due to Mn segregation, making it difficult to obtain the desired shear workability. Therefore, the Mn content should be 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% or less, more preferably 3.20% or less, 3.00% or less or 2.60% 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 terms of mass %, and 0% is substituted when the element is not contained.
Ti, Nb and V are elements that precipitate finely in steel as carbides and nitrides and improve the strength of steel by precipitation strengthening. These effects cannot be obtained if the total content of Ti, Nb and V is less than 0.060%. Therefore, the total content of Ti, Nb and V is made 0.060% or more. That is, the value of the middle side of the formula (A) is set to 0.060% or more. It should be noted that Ti, Nb and V do not all need to be contained, and any one of them may be contained as long as the total content is 0.060% or more. Therefore, the lower limits of the contents of Ti, Nb and V are each 0%. The lower limits of the contents of Ti, Nb and V may be 0.001%, 0.010%, 0.030% or 0.050%, respectively. The total content of Ti, Nb and V is preferably 0.080% or more, more preferably 0.100% or more.
On the other hand, if the content of any one of Ti, Nb and V exceeds 0.500%, or if the total content of Ti, Nb and V exceeds 0.500%, the hot rolled steel sheet Machinability deteriorates. Therefore, the content of each of Ti, Nb and V is set to 0.500% or less, and 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. Each content of Ti, Nb and V is preferably 0.400% or less or 0.300% or less, more preferably 0.250% or less, and even more preferably 0.200% or less or 0 .100% or less. The total content of Ti, Nb and V is preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.200% or less.

sol. Al: 0.001-2.000%
Al, like Si, has the effect of deoxidizing the steel to make the steel sound, promotes the formation of ferrite, and has the effect of 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. The Al content is preferably 0.010% or more, 0.030% or more, or 0.050% or more, and more preferably 0.080% or more, 0.100% or more, or 0.150% or more.
On the other hand, sol. If the Al content exceeds 2.000%, the above effect saturates and is economically unfavorable. Al content is 2.000% or less. sol. The Al content is preferably 1.700% or less or 1.500% or less, more preferably 1.300% or less, even more preferably 1.000% or less.
In addition, sol. Al means acid-soluble Al, and indicates solid-solution Al present in steel in a solid-solution state.

P: 0.100% or less P is an element that is generally contained as an impurity, but it is also an element that increases the strength of hot-rolled steel sheets by solid-solution strengthening. Although the lower limit of the P content is 0%, P may be positively included. However, P is an element that easily segregates, and when the P content exceeds 0.100%, the ductility of the hot-rolled steel sheet and the critical rupture thickness reduction rate due to grain boundary segregation significantly decrease. Therefore, the P content should be 0.100% or less. The P content is preferably 0.050% or less, 0.030% or less, 0.020% or less or 0.015% or less. Although the lower limit of the P content does not have to be specified, the lower limit of the P content is 0%. From the viewpoint of refining cost, the lower limit of the P content may be 0.001%, 0.003% or 0.005%.

S: 0.0300% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel to reduce the ductility and critical rupture thickness reduction rate of hot-rolled steel sheets. When the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet and the critical rupture thickness reduction rate are remarkably lowered. Therefore, the S content should be 0.0300% or less. The S content is preferably 0.0100% or less, 0.0070% or less, or 0.0050% or less. The lower limit of the S content is 0%, but it may be 0.0001%, 0.0005%, 0.0010% or 0.0020% from the viewpoint of refining cost.

N: 0.1000% or less N is an element contained in steel as an impurity, and has the effect of reducing the ductility and critical rupture thickness reduction rate of hot-rolled steel sheets. If the N content exceeds 0.1000%, the ductility of the hot-rolled steel sheet and the critical rupture thickness reduction rate are remarkably lowered. Therefore, the N content should be 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less or 0.0300% or less, and even more preferably 0.0150% or less or 0.0100% or less. The lower limit of the N content is 0%, but when one or more of Ti, Nb and V are contained to refine the metal structure, N is added to promote the precipitation of carbonitrides. The content is preferably 0.0010% or more, more preferably 0.0015% or more or 0.0020% or more.

O: 0.0100% or less When contained in steel in a large amount, O forms coarse oxides that act as starting points for fracture, and causes brittle fracture 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 or 0.0030% or less. The lower limit of the O content is 0%, but the O content may be 0.0005% or more, or 0.0010% or more in order to disperse a large number of fine oxides during deoxidation of molten steel.

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

Cu: 0.01-2.00%
Cr: 0.01-2.00%
Mo: 0.01-1.00%
Ni: 0.01-2.00%
B: 0.0001 to 0.0100%
Cu, Cr, Mo, Ni and B all have the effect of increasing the hardenability of hot-rolled steel sheets. Moreover, Cu and Mo have the effect of increasing the strength of the hot-rolled steel sheet by precipitating as carbides in the steel. Furthermore, when Cu is contained, 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 described above, Cu has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by precipitating as carbide in the steel at low temperatures. 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, 0.70% or less or 0.50% or less.

As described above, Cr has the effect of increasing the hardenability of hot-rolled steel sheets. In order to more reliably obtain the effect of the above action, the Cr content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is remarkably lowered. Therefore, the Cr content should be 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less or 0.50% or less.

As described above, Mo has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by being precipitated as carbides in the steel. In order to more reliably obtain the effect 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 is saturated, which is economically unfavorable. Therefore, the Mo content should be 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less or 0.10% or less.

As described above, Ni has the effect of increasing the hardenability of hot-rolled steel sheets. In addition, when Cu is contained, 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 0.01% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less or 0.50% or less.

As described above, B has the effect of increasing the hardenability of hot-rolled steel sheets. 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 is significantly deteriorated, so the B content is made 0.0100% or less. The B content is preferably 0.0050% or less or 0.0025% or less.

Ca: 0.0001-0.0200%
Mg: 0.0001-0.0200%
REM: 0.0001 to 0.1000%
Bi: 0.0001 to 0.0200%
As: 0.001-0.100%
Ca, Mg and REM all have the effect of increasing the ductility of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a preferred shape. Moreover, Bi has the effect of increasing the ductility of the hot-rolled steel sheet by refining the solidified structure. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effect of the above action, it is preferable that the content of at least one of Ca, Mg, REM and Bi is 0.0001% or more. However, when the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and the ductility of the hot-rolled steel plate is rather reduced. may decrease. Moreover, even if the Bi content exceeds 0.0200%, the above effect is saturated, which is economically unfavorable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content to 0.1000% or less, and the Bi content to 0.0200% or less. The Ca content, Mg content and Bi content are preferably 0.0100% or less, more preferably 0.0070% or less or 0.0040% or less. The REM content is preferably 0.0070% or less or 0.0040% or less. As lowers the austenite single phase temperature, refines the prior austenite grains, and contributes to the improvement of the ductility of the hot-rolled steel sheet. In order to reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, even if a large amount of As is contained, the above effect is saturated, so the As content is made 0.100% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.

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)
Each element symbol in the formula (B) represents the content of the element in terms of mass %, and 0% is substituted when the element is not contained.
Sn: 0.01-0.05%
With regard to Zr, Co, Zn and W, the present inventors have confirmed that even if these elements are contained in a total of 1.00% or less, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. ing. 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 of the left side of the formula (B) may be 1.00% or less, 0.50% or less, 0.10% or less, or 0.05% or less. Each content of Zr, Co, Zn, W and Sn may be 0.50% or less, 0.10% or less, or 0.05% or less. Since Zr, Co, Zn and W do not have to be contained, the content of each may be 0%. The contents of Zr, Co, Zn and W may each be 0.01% or more in order to improve the strength by solid-solution strengthening of the steel sheet.
Moreover, the present inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained. However, if a large amount of Sn is contained, flaws may occur during hot rolling, so the Sn content is made 0.05% or less. Since Sn does not have 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 rest of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, impurities include ores used as raw materials, scraps, or impurities mixed in from the manufacturing environment, etc., and/or impurities that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. means.

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 can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid. C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-nondispersive 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 described.
The hot-rolled steel sheet according to the present embodiment has a metal structure, in terms of area %, of which retained austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is 5.0%. is less than 0%, and the Entropy value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure by the gray-level co-occurrence matrix method is 10.7 or more, and the following formula (2 ) is 1.020 or more, the Cluster Shade value represented by the following formula (3) is −8.0×10 ⁵ to 8.0×10 ⁵ , and the standard deviation of the Mn concentration is is 0.60% by mass or less.

Therefore, the hot-rolled steel sheet according to the present embodiment can obtain excellent ductility and shear workability while having high strength and critical rupture thickness reduction rate. In this embodiment, in a cross section parallel to the rolling direction, a depth position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) ) and defines the structure fraction, Entropy value, Inverse differentiated normalized value, Cluster Shade value and standard deviation of Mn concentration in the metal structure at the central position in the sheet width direction. The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.
In addition, the surface here means the interface of a coating layer and a steel plate, when a hot-rolled steel plate is provided with a coating layer.

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 inhibits stable crack generation and causes the formation of secondary shear planes and a decrease in the critical rupture thickness reduction rate. When the area ratio of retained austenite is 3.0% or more, the above effect becomes apparent, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite is set to less than 3.0%. The area fraction of retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since it is preferable that the amount of retained austenite is as small as possible, the area ratio of retained austenite may be 0%.

Known methods for measuring the area ratio of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. In this embodiment, it is not easily affected by polishing (when affected by polishing, retained austenite may change to other phases such as martensite, so the true area ratio may not be measured), The area ratio of retained austenite is measured by X-ray diffraction, which is relatively easy to obtain accurate measurement results and is not easily affected by polishing.

In the measurement of the retained austenite area ratio by X-ray diffraction in this embodiment, first, the 1/4 depth position of the plate thickness of the hot-rolled steel plate (1/8 depth of the plate thickness from the surface to 3 of the plate thickness from the surface /8 depth region), and in the plate thickness cross section parallel to the rolling direction at the center position in the plate width direction, using Co-Kα rays, α (110), α (200), α (211), γ (111), γ(200), and γ(220) are obtained, and the volume fraction of retained austenite is calculated using the intensity average 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 formed when fcc transforms to 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. It is preferably 20.0% or more, more preferably 25.0% or more, and even more preferably 30.0% or more.
On the other hand, since ferrite has a low strength, a desired strength cannot be obtained if the area ratio is excessive. Therefore, the ferrite area ratio is set to less than 60.0%. It is preferably 50.0% or less, more preferably 45.0% or less or 40.0% or less.

Area ratio of pearlite: less than 5.0% Pearlite is a lamellar metal structure in which cementite is deposited in layers between ferrite particles, and is softer than bainite and martensite. If the pearlite area ratio is 5.0% or more, carbon is consumed by the cementite contained in the pearlite, and the strength of the remaining structures, martensite and bainite, is lowered, and the desired strength cannot be obtained. Therefore, the area ratio of pearlite is set to less than 5.0%. The area ratio of pearlite is preferably 3.0% or less, 2.0% or less, or 1.0% or less.
In order to improve the stretch flangeability of the hot-rolled steel sheet, the area ratio of pearlite is preferably reduced as much as possible, and the area ratio of pearlite is more preferably 0%.

In addition, in the hot-rolled steel sheet according to the present embodiment, as residual structures other than retained austenite, ferrite and pearlite, bainite, martensite and tempered A hard structure composed of one or more martensite is included. The lower limits of these residual tissues may be 36.0%, 40.0%, 44.0%, 48.0%, 52.0% or 55.0%, and the upper limits are 82.0%. %, 78.0%, 74.0%, 70.0% or 66.0%. The residual structure other than retained austenite, ferrite and pearlite may be one or more of bainite, martensite and tempered martensite.

The area ratio of metallographic structure is measured by the following method. A plate thickness cross-section parallel to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced to the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross section, the length is 50 μm, and the depth position is 1/4 of the plate thickness from the surface (1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface. ), and the central region in the plate width direction is measured by the electron backscatter diffraction method at a measurement interval of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD analysis apparatus is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The observation area is 40000 μm ² .

In addition, a backscattered electron image is taken in the same field of view. First, crystal grains in which ferrite and cementite are deposited in layers are specified from a backscattered electron image, and the area ratio of the crystal grains is calculated to obtain the area ratio of pearlite. After that, among the crystal grains excluding the crystal grains determined to be pearlite, the crystal orientation information obtained for the crystal grains determined to have a body-centered cubic lattice structure is analyzed using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. )” is used to determine that the region with a grain average misorientation value of 1.0° or less is ferrite. At this time, the grain tolerance angle is set to 15°, and the area ratio of ferrite is obtained by calculating the area ratio of the region determined to be ferrite.

Subsequently, the area ratio of the regions excluding the regions discriminated as pearlite or ferrite is measured and taken as the area ratio of the residual structure (that is, bainite, martensite and tempered martensite). When the area ratio of bainite and the sum of the area ratios of martensite and tempered martensite are to be measured, these area ratios can be measured by the following method. Specifically, when the maximum value of the "Grain Average IQ" of the ferrite region with respect to the remaining region is Iα, a region exceeding Iα/2 is extracted (determined) as bainite, and is Iα/2 or less. The region is extracted (determined) as "martensite or tempered martensite". By calculating the area ratio of the region extracted (determined) as bainite, the area ratio of bainite is obtained. Also, the area ratio of the region extracted (determined) as martensite or tempered martensite is calculated to obtain the sum of the area ratios of martensite and tempered martensite.

In this embodiment, the area ratio of each tissue is measured by X-ray diffraction and EBSD analysis, so the total area ratio of each tissue obtained by measurement may not be 100.0%. If the total area ratio of each tissue obtained by the above method does not reach 100.0%, convert the area ratio of each tissue so that the total area ratio of each tissue becomes 100.0%. . For example, when the total area ratio of each structure is 103.0%, the area ratio of each structure is multiplied by "100.0/103.0" to obtain the area ratio of each structure.

Entropy value: 10.7 or higher, Inverse differentiated normalized value: 1.020 or higher It is necessary to suppress early cracking from the cutting edge of the tool during machining. For that purpose, it is important that the periodicity of the metallographic structure is low and the uniformity of the metallographic structure is high. In this embodiment, the entropy value (E value) indicating the periodicity of the metal structure and the inverse differentiated normalized value (I value) indicating the uniformity of the metal structure are controlled to suppress the occurrence of secondary shear planes.

The E value represents the periodicity of the metal structure. When the brightness is periodically arranged 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, it is necessary to increase the E value because the metal structure must have a low periodicity. When the E value is less than 10.7, secondary shear planes are likely to occur. Starting from the periodically arranged structure, cracks are generated from the cutting edge of the shearing tool at a very early stage of the shearing process to form a fractured surface, and then a sheared surface is formed again. It is presumed that this facilitates the generation of secondary shear planes. Therefore, the E value should be 10.7 or more. It is preferably 10.8 or more, more preferably 11.0 or more. The higher the E value, the better, and although the upper limit is not particularly defined, 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 the region with a certain brightness increases. A high I value means a high uniformity of the metal structure. The hot-rolled steel sheet according to the present embodiment, which has a metal structure with a ferrite area ratio of 15.0% or more and less than 60.0%, needs to have a highly uniform metal structure. Therefore, in this embodiment, it is necessary to increase the I value. If the I value is less than 1.020, a fracture surface is formed by cracking from the cutting edge of the shearing tool very early in the shearing process due to the influence of the hardness distribution caused by the precipitates in the crystal grains and the difference in element concentration. and then a shear plane is formed again. It is presumed that this facilitates the generation of secondary shear planes. Therefore, the I value should be 1.020 or more. It is preferably 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 defined, it may be 1.200 or less, 1.150 or less, or 1.100 or less.

Cluster Shade value: -8.0×10 ⁵ to 8.0×10 ⁵
The Cluster Shade value (CS value) indicates the skewness of the metallographic structure. The CS value becomes a positive value when there are many points with brightness exceeding the average value for the average value of brightness in the image obtained by photographing the metal structure, and when there are many points with brightness below the average value Negative value.

In the secondary electron image of the scanning electron microscope, the brightness is high where the surface unevenness of the observation object is large, and the brightness is low where the unevenness is small. The unevenness of the surface of the object to be observed is greatly affected by the grain size and intensity distribution in the metal structure. The CS value in this embodiment increases when the variation in strength of the metal structure is large or the organization unit is small, and decreases when the variation in strength is small or the organization unit is large.

In this embodiment, it is important to keep the CS value in the desired range close to zero. If the CS value is less than −8.0×10 ⁵ , the critical rupture thickness reduction rate of the hot rolled steel sheet is lowered. It is presumed that this is because crystal grains having a large grain size exist in the metal structure and the crystal grains are preferentially destroyed during the ultimate deformation. Therefore, the CS value should be -8.0×10 ⁵ or more. It is preferably −7.5×10 ⁵ or more, and more preferably −7.0×10 ⁵ or more.
On the other hand, if the CS value exceeds 8.0×10 ⁵ , the critical rupture thickness reduction rate of the hot-rolled steel sheet decreases. It is presumed that this is because the microscopic strength variation in the metallographic structure is large, and the strain during ultimate deformation concentrates locally, facilitating fracture. Therefore, the CS value should be 8.0×10 ⁵ or less. It is preferably 7.5×10 ⁵ or less, and still more preferably 7.0×10 ⁵ or less.

E value, I value and CS value can be obtained by the following method.
In the present embodiment, the photographing area of the SEM image (secondary electron image of a scanning electron microscope) photographed for calculating the E value, I value and CS value is a plate thickness cross section parallel to the rolling direction, from the surface A position at a depth of 1/4 of the plate thickness (a region from 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) and a central position in the plate width direction. SEM images are taken using an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation, with a tungsten emitter and an acceleration voltage of 1.5 kV. Based on the above settings, the SEM image is output at a magnification of 1000 and a gray scale of 256 gradations.

Next, the obtained SEM image is cut into an area of 880 × 880 pixels (observation area is 160 μm × 160 μm in actual size), and the limiting magnification for contrast enhancement described in Non-Patent Document 3 is set to 2.0. , smoothing processing with a tile grid size of 8×8 is performed. 179 images in total are obtained by rotating the SEM image after smoothing counterclockwise by 1 degree from 0 to 179 degrees except for 90 degrees and creating an image every 1 degree. . Next, for each of these 179 images, the GLCM method described in Non-Patent Document 1 is used to collect luminance frequency values between adjacent pixels in the form of a matrix.

A matrix of 179 frequency values obtained by the above method is expressed as p _k (k=0...89, 91,...179) where k is a rotation angle from the original image. For each image, 256 × 256 normalized so that the sum of each component is 1 after summing the generated p _k for all k (k = 0 ... 89, 91 ... 179) Calculate the matrix P of Furthermore, using the following formulas (1) to (5) described in Non-Patent Document 2, E value, I value and CS value are calculated respectively.
P(i, j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, and the value of the i-th row and j-th column of the matrix P is expressed as P(i, j). In addition, since it is calculated using the matrix P of 256 × 256 as described above, if you want to emphasize this point, the following formulas (1) to (5) are corrected 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). 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 represented by the following formulas (4) and (5), respectively. .
In the following equations (1′) to (5′), the value of the i-th row and j-th column of the matrix P is expressed as _Pij .

Standard deviation of Mn concentration: 0.60% by mass or less 1/4 depth position of the plate thickness from the surface of the hot-rolled steel plate according to the present embodiment (1/8 depth of the plate thickness from the surface to the thickness of the plate from the surface 3/8 depth region) and the center position in the sheet width direction is 0.60% by mass or less. As a result, the hard phase can be uniformly dispersed, and cracks can be prevented from occurring at the cutting edge of the shearing tool very early in the shearing process. As a result, the occurrence of secondary shear planes can be suppressed. The standard deviation of the Mn concentration is preferably 0.55% by mass or less or 0.50% by mass or less, more preferably 0.47% by mass or less or 0.45% by mass or less. From the viewpoint of suppressing excessive burrs, the lower limit of the standard deviation of the Mn concentration is preferably as small as possible. If necessary, the lower limit may be 0.20 mass % or 0.28 mass %.

After mirror-polishing the thickness cross-section of the hot-rolled steel sheet parallel to the rolling direction, the depth position of 1/4 of the plate thickness from the surface (1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface The depth area) and the center position in the plate width direction are measured with an electron probe microanalyzer (EPMA) to measure the standard deviation of the Mn concentration. The measurement conditions are an acceleration voltage of 15 kV, a magnification of 5000, and a distribution image in a range of 20 μm in the rolling direction of the sample and 20 μm in the thickness direction of the sample. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration is measured at 40,000 or more locations. Then, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.

Average crystal grain size of surface layer: less than 3.0 μm By making the crystal grain size of the surface layer finer, it is possible to suppress bending inner cracks in the hot-rolled steel sheet. As the strength of the hot-rolled steel sheet increases, cracks are more likely to occur from the inside of the bending during bending (hereinafter referred to as internal bending cracks). The mechanism of internal bending cracks is presumed as follows. During bending, compressive stress is generated inside the bend. At first, the inside of the bend is uniformly deformed as the work progresses, but as the amount of work increases, uniform deformation alone cannot support the deformation, and deformation progresses as the strain concentrates locally (the occurrence of shear deformation bands). As this shear deformation band grows further, a crack occurs and grows along the shear band from the inner surface of the bend. The reason why inner bending cracks are more likely to occur as the strength increases is that due to the decrease in work hardening ability that accompanies the increase in strength, it becomes difficult for uniform deformation to proceed, and uneven deformation tends to occur, which can lead to early ( Or under loose processing conditions) is presumed to be due to the occurrence of shear deformation bands.

The present inventors' research has revealed that internal bending cracks become prominent in steel sheets with a tensile strength of 980 MPa or higher. In addition, the present inventors have found that the finer the crystal grain size of the surface layer of the hot-rolled steel sheet, the more the local strain concentration is suppressed and the bending inner cracks are less likely to occur. In order to obtain the above effects, the average grain size of the surface layer of the hot-rolled steel sheet is preferably less than 3.0 μm. Therefore, in the present embodiment, the average grain size of the surface layer may be less than 3.0 μm. The average grain size of the surface layer is more preferably 2.7 μm or less or 2.5 μm or less. Although the lower limit of the average grain size of the surface layer region is not specified, it may be 0.5 μm or 1.0 μm.
In this embodiment, the surface layer is a region from the surface of the hot-rolled steel sheet to a depth of 50 μm from the surface. As described above, the surface here means the interface between the coating layer and the steel sheet when the hot-rolled steel sheet has a coating layer.

The grain size of the surface layer is measured using the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. The EBSP-OIM method is performed using an apparatus combining a scanning electron microscope and an EBSP analysis apparatus and OIM Analysis (registered trademark) manufactured by AMETEK. The analyzable area of the EBSP-OIM method is the area that can be observed with the SEM. Although it depends on the resolution of the SEM, the EBSP-OIM method enables analysis with a minimum resolution of 20 nm.

In the cross section parallel to the rolling direction of the hot-rolled steel sheet, in the area from the surface to the surface of the hot-rolled steel sheet at a depth of 50 μm from the surface and at the center position in the width direction, at a magnification of 1200 times, at least 5 in an area of 40 μm × 30 μm. Analysis is performed in the field of view. A place where the angle difference between adjacent measurement points is 5° or more is defined as a grain boundary, and the area-average grain size is calculated. The obtained area-average crystal grain size is taken as the average crystal grain size of the surface layer.

In addition, since retained austenite is not a structure generated by phase transformation at 600 ° C. or less and does not have the effect of dislocation accumulation, retained austenite is the object of analysis in this measurement method (method for measuring the average grain size of the surface layer). and not. When the area ratio of retained austenite is 0%, it is not necessary to exclude it from the analysis target. Retained austenite having a structure of fcc is excluded from analysis objects and measured.

Tensile Strength Characteristics Of the mechanical properties of hot-rolled steel sheets, tensile strength characteristics (tensile strength, total elongation) are evaluated according to JIS Z 2241:2011. The test piece shall be JIS Z 2241:2011 No. 5 test piece. A tensile test piece is taken from a quarter portion from the edge in the width direction of the sheet, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.

The hot-rolled steel sheet according to this embodiment has a tensile strength (TS) of 980 MPa or more. It is preferably 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited and the contribution to vehicle weight reduction is small. Although the upper limit is not particularly limited, it may be 1780 MPa from the viewpoint of mold wear suppression.

Also, 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, and even more preferably 13.0% or more. Further, the product of tensile strength and total elongation is more preferably 14000 MPa·% or more, and even more preferably 15000 MPa·% MPa or more. By setting the total elongation to 10.0% or more and the product of the tensile strength and the total elongation to 13000 MPa·% or more, it is possible to greatly contribute to weight reduction of the vehicle body without limiting applicable parts. Although it is not necessary to set an upper limit for the product of tensile strength and total elongation, it may be 22000 MPa·% or 18000 MPa·%. There is no need to set an upper limit for the total elongation, but it may be 30.0%, 25.0% or 22.0%.

Thickness The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 0.5 mm, it may become difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 0.5 mm or more. It is preferably 1.2 mm or more, 1.4 mm or more, or 1.8 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it becomes difficult to refine the metal structure, and it may be difficult to obtain the metal structure described above. Therefore, the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

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

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

In a preferred method for manufacturing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (10) are sequentially performed. 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, the stress refers to the tension applied in the rolling direction of the steel plate.

(1) After holding the slab in the temperature range of 700 to 850° C. for 900 seconds or longer, it is further heated and held in the temperature range of 1100° C. or higher for 6000 seconds or longer.
(2) Hot rolling is performed in a temperature range of 850 to 1100° C. so that the total thickness reduction is 90% or more.
(3) A stress of 170 kPa or more is applied to the steel sheet after the hot-rolling one stage before the final stage and before the final stage rolling.
(4) The rolling reduction at the final stage of hot rolling is 8% or more, and hot rolling is completed so that the rolling completion temperature Tf is 900°C or more and less than 1010°C.
(5) The stress applied to the steel sheet after the final stage of hot rolling and before the steel sheet is cooled to 800° C. is less than 200 kPa.
(6) Within 1 second after the completion of hot rolling, after cooling to a temperature range of hot rolling completion temperature Tf-50 ° C. or less, accelerate to a temperature range of 600 to 730 ° C. at an average cooling rate of 50 ° C./s or more. Cooling. However, cooling to a temperature range equal to or lower than the hot rolling completion temperature Tf-50° C. within 1 second after the completion of hot rolling is a more preferable cooling condition.
(7) In a temperature range of 600 to 730° C., slow cooling is performed for 2.0 seconds or more at an average cooling rate of less than 5° C./s.
(8) After 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.
(9) Cool so that the average cooling rate in the temperature range from the coiling temperature to 450°C is 50°C/s or more.
(10) Winding in a temperature range of 350°C or less.

By adopting the above manufacturing method, it is possible to stably manufacture a hot-rolled steel sheet having a metal structure with high strength, critical rupture thickness reduction rate, excellent ductility, and excellent shear workability. That is, by appropriately controlling the slab heating conditions and hot rolling conditions, the reduction of Mn segregation and equiaxed austenite before transformation are achieved, and together with the cooling conditions after hot rolling described later, A hot-rolled steel sheet having a desired metal structure can be stably produced.

(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 and blooming, etc. can be used. Moreover, if necessary, those obtained by adding hot working or cold working to them can be used.

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

In the 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 within the ferrite region. As a result, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. Further, by maintaining the temperature in the temperature range of 1100° C. or higher for 6000 seconds or longer, the austenite grains can be made uniform during heating of the slab.

For hot rolling, it is preferable to use a reverse mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity and the viewpoint of stress load to the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill in at least the last two stages.

(2) Reduction ratio of hot rolling: A total thickness reduction of 90% or more in the temperature range of 850 to 1100°C. By doing so, mainly the recrystallized austenite grains are refined, and the accumulation of strain energy in the non-recrystallized 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 carry out hot rolling in a temperature range of 850 to 1100° C. so that the total thickness reduction is 90% or more.

The thickness reduction in the temperature range of 850 to 1100 ° C. means that the inlet thickness before the first rolling in this temperature range is t ₀ , and the outlet thickness after the final stage rolling in this temperature range. is t ₁ , {(t ₀ −t ₁ )/t ₀ }×100(%).

(3) Stress after rolling at one stage before the final stage of hot rolling and before rolling at the final stage: 170 kPa or more Steel plate after rolling at one stage before the final stage of hot rolling and before rolling at the final stage It is preferable to set the stress applied to 170 kPa or more. This makes it possible to reduce the number of crystal grains having a crystal orientation of {110}<001> 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, it is possible to effectively promote recrystallization in the final reduction. 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.

If the stress applied to the steel sheet is less than 170 kPa, the E value may not be the desired value. The stress applied to the steel plate is more preferably 190 kPa or more.
The stress applied to a steel plate can be controlled by adjusting the roll rotation speed during tandem rolling, and can be obtained by dividing the load in the rolling direction measured at the rolling stand by the cross-sectional area of the plate being passed. can.

(4) Reduction ratio at the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900°C or more and less than 1010°C The reduction ratio at the final stage of hot rolling is 8% or more, and hot rolling is completed. The temperature Tf is preferably 900° C. or higher. By setting the rolling reduction in the final stage of hot rolling to 8% or more, it is possible to promote recrystallization due to the rolling reduction in the final stage. 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, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, it is possible to suppress the formation of ferrite in the final structure (the metal structure of the hot-rolled steel sheet after production) and obtain a high-strength hot-rolled steel sheet. Further, by setting the hot rolling completion temperature 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 a desired value.

(5) Stress after the final stage of hot rolling and until the steel sheet is cooled to 800 ° C.: less than 200 kPa Steel sheet after the final stage of hot rolling and until the steel sheet is cooled to 800 ° C. It is preferable that the stress applied to is less than 200 kPa. By setting the stress (tension) applied in the rolling direction of the steel sheet to less than 200 kPa, recrystallization of austenite preferentially progresses in the rolling direction, and an increase in the periodicity of the metal structure can be suppressed. As a result, the E value can be a desired value. The stress applied to the steel plate is more preferably 180 MPa or less. The stress applied in the rolling direction of the steel plate can be controlled by adjusting the rotation speed of the rolling stand and the winding device. can be found at

(6) Within 1 second after the completion of hot rolling, after cooling to a temperature range of hot rolling completion temperature Tf-50 ° C. or less, accelerate to a temperature range of 600 to 730 ° C. at an average cooling rate of 50 ° C./s or more. Cooling In order to suppress the growth of austenite grains refined by hot rolling, it is more preferable to cool the steel sheet by 50°C or more within 1 second after completion of hot rolling. In order to cool to a temperature range of the hot rolling completion temperature Tf-50 ° C. or less 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. should be injected into By cooling to a temperature range of Tf-50°C or less within 1 second after the completion of hot rolling, the crystal grain size of the surface layer can be refined, and the resistance to internal bending cracks of the hot-rolled steel sheet can be enhanced.

In addition, by performing accelerated cooling to a temperature range of 730°C or less at an average cooling rate of 50°C/s or more after the cooling described above, it is possible to suppress the formation of ferrite and pearlite with a small amount of precipitation strengthening. This improves the strength of the hot-rolled steel sheet. The average cooling rate here means the temperature drop width of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is taken out from the cooling equipment). It is the value divided by the required time from the start to the completion of accelerated cooling.

The upper limit of the cooling rate is not specified, but if the cooling rate is increased, the cooling equipment will be large-scaled and the equipment cost will increase. For this reason, considering the equipment cost, 300° C./s or less is preferable. 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.

(7) Slow cooling with an average cooling rate of less than 5°C/s in the temperature range of 600 to 730°C for 2.0 seconds or more In the temperature range of 600 to 730°C, the average cooling rate is less than 5°C/s By performing the 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.
The average cooling rate here is the temperature drop range of the steel sheet from the cooling stop temperature of accelerated cooling to the stop temperature of slow cooling divided by the time required from the stop of accelerated cooling to the stop of slow cooling. refers to value.

The time for slow cooling is preferably 3.0 seconds or longer. The upper limit of the slow cooling time is determined by the equipment layout, but it may be generally less than 10.0 seconds. Although the lower limit of the average cooling rate for slow cooling is not particularly set, it may be 0° C./s or more because increasing the temperature without cooling involves a large investment in equipment.

(8) After 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 cool so that the average cooling rate of the zone is 30° C./s or more and less than 50° C./s. By setting the average cooling rate in the temperature range to 30° C./s or more and less than 50° C./s, the CS value can be set to a desired value. If the average cooling rate exceeds 50° C./s, a flat lath-like structure with low brightness is likely to form, and the CS value will be less than −8.0×10 ⁵ . If the average cooling rate is less than 30° C./s, the concentration of carbon in the untransformed portion is promoted, the strength of the hard structure increases, and the difference in strength from the soft structure increases, so the CS value increases. 8.0×10 ⁵ is exceeded.
It should be noted that the average cooling rate here refers to the average cooling rate of 30 ° C. / s or more and less than 50 ° C. / s from the cooling stop temperature of slow cooling where the average cooling rate is less than 5 ° C. / s. The temperature drop range of the steel sheet to the stop temperature is 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. It means the value divided by the time required to

(9) Average cooling rate in the temperature range from coiling temperature to 450 ° C.: 50 ° C./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 to set the average cooling rate in the temperature range to 50° C./s or higher. Thereby, the matrix structure can be made hard.
The average cooling rate here means the temperature drop range of the steel sheet from the cooling stop temperature of cooling at an average cooling rate of 30 ° C./s or more and less than 50 ° C./s to the coiling temperature. It is a value obtained by dividing the time required from stopping cooling to winding, which is 30°C/s or more and less than 50°C/s.

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

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

Steels having chemical compositions shown in Tables 1 and 2 were melted and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 5 and 6 were obtained under the manufacturing conditions shown in Tables 3 and 4.
The average cooling rate of slow cooling was set to less than 5°C/s. Further, since 50° C. is the lower measurement limit for the winding temperature described in Table 4, the actual winding temperature in the example described as 50° C. is 50° C. or less.

The area ratio of the metal structure, the E value, the I value, the CS value, the standard deviation of the Mn concentration, the average crystal grain size of the surface layer, the tensile strength TS, and the total elongation were measured for the obtained hot-rolled steel sheet by the above-described methods. I asked for El. Tables 5 and 6 show the measurement results obtained.
The residual structure was one or more of bainite, martensite and tempered martensite.

Methods for evaluating properties of hot-rolled steel sheets Tensile properties 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 has high strength and excellent ductility, and is judged to be acceptable. If any one of the criteria was not satisfied, it was judged to be unacceptable because the hot-rolled steel sheet did not have high strength and excellent ductility.

Critical rupture thickness reduction rate The critical rupture thickness reduction rate of hot-rolled steel sheets was evaluated by a tensile test.
A tensile test was performed in the same manner as when evaluating tensile properties. By calculating the value of (t1-t2) × 100 / t1, where t1 is the plate thickness before the tensile test and t2 is the minimum value of the plate thickness at the center in the width direction of the tensile test piece after breaking, The critical rupture thickness reduction rate was obtained. The tensile test was performed 5 times, and the critical rupture thickness reduction rate was obtained by calculating the average value of 3 times excluding the maximum and minimum values of the critical rupture thickness reduction rate.

When the critical rupture thickness reduction rate was 60.0% or more, it was determined to be a hot-rolled steel sheet with a high critical rupture thickness reduction rate and judged as acceptable. On the other hand, when the critical rupture thickness reduction rate was less than 60.0%, it was determined that the hot-rolled steel sheet did not have a high critical rupture thickness reduction rate and was rejected.

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

Of the sheared end surfaces obtained, two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction, for example, as shown in FIG. determined that a secondary shear surface was formed. A total of 12 surfaces, 4 surfaces for each punched hole, were observed, and if there was no surface on which a secondary shear surface appeared, the hot-rolled steel sheet was judged to have excellent shear workability and was judged to be acceptable. "None" was written in On the other hand, when even one secondary shear plane was formed, the hot-rolled steel sheet did not have excellent shear workability and was judged to be unacceptable, and was described as "present" in the table.

Resistance to internal bending cracks Resistance to internal bending cracks was evaluated by the following bending test.
A bending test piece was obtained by cutting out a strip-shaped test piece of 100 mm×30 mm from a half position in the width direction of the hot-rolled steel sheet. For both bending (L-axis bending) in which the bending ridge is parallel to the rolling direction (L direction) and bending (C-axis bending) in which the bending ridge is parallel to the direction (C direction) perpendicular to the rolling direction, JIS Z 2248:2006 V-block method (bending angle θ is 90°). From this, find the minimum bending radius that does not cause cracks. The bending inner crack resistance was investigated. A value obtained by dividing the average value of the minimum bending radii of the L-axis and the C-axis by the plate thickness was defined as the limit bending R/t and was used as an index value of bending inner crack resistance. When R/t was 2.5 or less, it was determined that the hot-rolled steel sheet was excellent in resistance to internal bending cracks.

However, the presence or absence of cracks is determined by mirror-polishing the cross-section of the test piece after the test on a plane parallel to the bending direction and perpendicular to the plate surface, and then observing the cracks with an optical microscope. It was determined that there was a crack when the length of the crack exceeded 30 μm.

Tables 5 and 6 show that the hot-rolled steel sheets according to the examples of the present invention have excellent ductility and shear workability while having high strength and critical rupture thickness reduction rate. Further, among the examples of the present invention, the hot-rolled steel sheets having a surface layer with an average crystal grain size of less than 3.0 μm have the above-described properties and furthermore have excellent bending internal crack resistance.
On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples are degraded in at least one of strength, ductility, critical rupture thickness reduction rate, and shear workability.

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and critical rupture thickness reduction rate, as well as excellent ductility and shear workability. Further, according to the preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above-described properties and further suppresses the occurrence of internal bending cracks, that is, has excellent resistance to internal bending cracks. can be done.
INDUSTRIAL APPLICABILITY The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.

Claims (3)

1. The chemical composition, 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 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 to 2.00%,
   Cr: 0 to 2.00%,
   Mo: 0 to 1.00%,
   Ni: 0 to 2.00%,
   B: 0 to 0.0100%,
   Ca: 0 to 0.0200%,
   Mg: 0-0.0200%,
   REM: 0 to 0.1000%,
   Bi: 0 to 0.0200%,
   As: 0 to 0.100%,
   Zr: 0 to 1.00%,
   Co: 0 to 1.00%,
   Zn: 0 to 1.00%,
   W: 0 to 1.00%,
   Sn: 0 to 0.05%, and the balance: Fe and impurities,
   satisfying the following formulas (A) and (B),
   The metal structure, in area %,
   Retained austenite is less than 3.0%,
   Ferrite is 15.0% or more and less than 60.0%,
   Perlite is less than 5.0%,
   The Entropy value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure by the gray-level co-occurrence matrix method is 10.7 or more,
   The inverse differentiated normalized value represented by the following formula (2) is 1.020 or more,
   The Cluster Shade value represented 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,
   A hot-rolled steel sheet having a tensile strength of 980 MPa or more.
   0.060% ≤ Ti + Nb + V ≤ 0.500% (A)
   Zr+Co+Zn+W≦1.00% (B)
   However, each element symbol in the above formulas (A) and (B) indicates the content of the element in terms of % by mass, and 0% is substituted when the element is not contained.
   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 steel sheet according to claim 1, wherein the surface layer has an average grain size of less than 3.0 µm.
3. The chemical composition, in mass %,
   Ti: 0.001 to 0.500%,
   Nb: 0.001 to 0.500%,
   V: 0.001 to 0.500%,
   Cu: 0.01 to 2.00%,
   Cr: 0.01 to 2.00%,
   Mo: 0.01 to 1.00%,
   Ni: 0.01 to 2.00%,
   B: 0.0001 to 0.0100%,
   Ca: 0.0001 to 0.0200%,
   Mg: 0.0001-0.0200%,
   REM: 0.0001 to 0.1000%,
   Bi: 0.0001 to 0.0200%,
   As: 0.001 to 0.100%,
   Zr: 0.01 to 1.00%,
   Co: 0.01 to 1.00%,
   Zn: 0.01 to 1.00%,
   W: 0.01-1.00%, and Sn: 0.01-0.05%
   The hot-rolled steel sheet according to claim 1 or 2, containing one or more selected from the group consisting of:

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