WO2023063350A1 - Hot-rolled steel plate - Google Patents

Abstract

This hot-rolled steel plate has a prescribed chemical composition and metal structure, wherein: the pole density of the ｛001｝＜１１０＞, ｛１１１｝＜１１０＞, and ｛１１２｝＜１１０＞ orientation group is 2.0 to 8.0 in the texture of a region from a surface to a depth of 1/8 of the thickness from the surface; the pole density of the ｛１１０｝＜１１２＞ orientation is 2.0 to 4.0 in the texture of a region from the depth of 1/8 of the thickness from the surface to a depth of 1/2 of the thickness from the surface; and the tensile strength is 980 MPa.

Description

hot rolled steel plate

The present invention relates to hot rolled steel sheets.
This application claims priority based on Japanese Patent Application No. 2021-168627 filed in Japan on October 14, 2021, the content of which is incorporated herein.

From the perspective of protecting the global environment, weight reduction of automobile bodies is being promoted with the aim of improving fuel efficiency of automobiles. In order to further reduce the weight of automobile bodies, it is necessary to increase the strength of steel sheets applied to automobile bodies. However, in general, increasing the strength of a steel sheet lowers its formability.

As a method of improving the formability of steel sheets, there is a method of adding retained austenite to the metal structure of steel sheets. However, when the metal structure of the steel sheet contains retained austenite, the ductility is improved, but the isotropy of the ductility may be deteriorated and the hole expansibility may be deteriorated. When performing bending, hole expanding and burring, it is required that ductility anisotropy be reduced, that is, ductility isotropy be excellent. Furthermore, when performing the above processing, it is also required to have excellent hole expandability.

In Patent Document 1, the microstructure is mainly composed of bainite, the hard phase composed of martensite and / or austenite is 3% or more and less than 20% in terms of area fraction, and the hard phase present in the center of the plate thickness Among the phases, those having an aspect ratio of 3 or more account for 60% or more, the length of the hard phase present in the center of the plate thickness in the rolling direction is less than 20 μm, and the <011> orientation and <111> orientation viewed from the rolling direction > orientation X-ray random intensity ratio is 3.5 or more, and the X-ray random intensity ratio of <001> orientation viewed from the rolling direction is 1.0 or less. .

WO2016/010004

However, in Patent Document 1, it is necessary to further improve the strength in order to further reduce the weight of the automobile body. Moreover, in Patent Document 1, isotropy of ductility is not taken into consideration.

An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductility isotropy and hole expandability.

In view of the above-mentioned problems, the present inventors have made intensive studies on the relationship between the chemical composition and metallographic structure of hot-rolled steel sheets and the mechanical properties, and as a result, have obtained the following knowledge and completed the present invention.

The present inventors have found that it is important to control the textures of the surface layer region and the internal region of the hot-rolled steel plate in order to increase the isotropy of ductility and the hole expansibility of the hot-rolled steel plate. . Moreover, the present inventors have found that it is particularly effective to control the finish rolling conditions in order to control the textures of the surface layer region and the internal region of the hot-rolled steel sheet.

The gist of the present invention made based on the above knowledge is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass% of
C: 0.100 to 0.350%,
Si: 0.010 to 3.00%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
Si+sol. Al: 1.00% or more,
Ti: 0.010 to 0.380%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Nb: 0 to 0.100%,
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 0.020%,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%,
The balance consists of Fe and impurities,
The metal structure in the region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface is area%,
retained austenite: 10-20%,
Fresh martensite: 10% or less, and bainite: 70 to 90%,
In the texture of the region from the surface to the depth of 1/8 of the plate thickness from the surface,
{001} <110>, {111} <110> and {112} <110> orientation groups have a pole density of 2.0 to 8.0,
In the texture in the region from the surface to the depth of 1/8 of the plate thickness to the depth of 1/2 of the plate thickness from the surface,
{110} <112> orientation pole density is 2.0 to 4.0,
Tensile strength is 980 MPa or more.
(2) The hot-rolled steel sheet according to (1) above has the chemical composition, in mass%,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.500%,
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Ni: 0.02 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000% and Bi: 0.0005-0.020%
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 provide a hot-rolled steel sheet having high strength and excellent ductility isotropy and hole expansibility.

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 The hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.100 to 0.350%, Si: 0.010 to 3.00%, Mn: 1.00 to 4. .00%, sol. Al: 0.001-2.000%, Si+sol. Al: 1.00% or more, Ti: 0.010 to 0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less , and the balance: Fe and impurities.
Each element will be described in detail below.

C: 0.100-0.350%
C is an element necessary to obtain desired strength. If the C content is less than 0.100%, it becomes difficult to obtain the desired strength. Therefore, the C content should be 0.100% or more. The C content is preferably 0.120% or more and 0.150% or more.
On the other hand, when the C content is more than 0.350%, the transformation rate slows down, making it easier to generate MA (mixed phase of fresh martensite and retained austenite), resulting in excellent ductility isotropy and hole expandability. difficult to obtain. Therefore, the C content should be 0.350% or less. The C content is preferably 0.330% or less and 0.310% or less.

Si: 0.010-3.00%
Si has the effect of delaying the precipitation of cementite. This action can increase the amount of untransformed austenite remaining, that is, the area ratio of retained austenite. In addition, the strength can be increased by maintaining a large amount of dissolved C in the hard phase and preventing cementite from coarsening. Si itself also has the effect of increasing the strength of the hot-rolled steel sheet through solid-solution strengthening. 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 0.010%, the above effects cannot be obtained. Therefore, the Si content should be 0.010% or more. The Si content is preferably 0.50% or more, 1.00% or more, 1.20% or more, 1.50% or more.
On the other hand, if the Si content exceeds 3.00%, the precipitation of cementite is significantly retarded and the amount of retained austenite becomes excessive, which is not preferable. In addition, the hot-rolled steel sheet has significantly deteriorated surface properties, chemical conversion treatability, ductility and weldability, and the _A3 transformation point is significantly increased. This makes it difficult to stably perform hot rolling. Therefore, the Si content should be 3.00% or less. The Si content is preferably 2.70% or less and 2.50% 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 1.50% or more and 1.80% or more.
On the other hand, if the Mn content exceeds 4.00%, the ductility isotropy and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the Mn content should be 4.00% or less. The Mn content is preferably 3.70% or less and 3.50% or less.

sol. Al: 0.001-2.000%
sol. Al, like Si, has the effect of deoxidizing the steel to make it sound and suppressing the precipitation of cementite from austenite, thereby promoting the formation of retained austenite. sol. If the Al content is less than 0.001%, the above effects cannot be obtained. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.010% or more.
On the other hand, sol. If the Al content exceeds 2.000%, the above effect is saturated and it is economically unfavorable. Furthermore, the _A3 transformation point rises significantly, making it difficult to perform stable hot rolling. Therefore, sol. Al content is 2.000% or less. sol. The Al content is preferably 1.500% or less and 1.300% or less.
In addition, in this embodiment, sol. Al means acid-soluble Al, and indicates solid-solution Al present in steel in a solid-solution state.

Si+sol. Al: 1.00% or more Si and sol. Al has the effect of delaying the precipitation of cementite, and this effect can increase the amount of untransformed austenite remaining, that is, the area ratio of retained austenite. Si and sol. If the total Al content is less than 1.00%, the above effects cannot be obtained. Therefore, Si and sol. The total content of Al is set to 1.00% or more. It is preferably 1.20% or more and 1.50% or more.
In addition, Si of "Si+sol.Al" shows content in mass % of Si, and sol. Al is sol. Content of Al in mass % is shown.

Ti: 0.010-0.380%
Ti is an element effective for suppressing recrystallization and grain growth of austenite between stands of hot rolling. By suppressing the recrystallization of austenite between the stands, more strain can be accumulated. As a result, the texture of the hot-rolled steel sheet can be preferably controlled. If the Ti content is less than 0.010%, the above effect cannot be obtained. Therefore, the Ti content is set to 0.010% or more. Preferably, it is 0.050% or more, 0.070% or more, or 0.080% or more.
On the other hand, when the Ti content exceeds 0.380%, inclusions originating from TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.380% or less. Preferably, it is 0.320% or less or 0.300% or less.

P: 0.100% or less P is an element generally contained in steel as an impurity. Therefore, P may be positively contained. However, P is an element that easily segregates, and when the P content exceeds 0.100%, the isotropic deterioration of hole expansibility and ductility due to grain boundary segregation becomes significant. Therefore, the P content should be 0.100% or less. The P content is preferably 0.030% or less.
Although the lower limit of the P content does not have to be specified, it is preferably 0.001% from the viewpoint of refining cost.

S: 0.0300% or less S is an element contained in steel as an impurity, and forms sulfide-based inclusions in steel, deteriorating the isotropy of hole expansibility and ductility of hot-rolled steel sheets. Let When the S content exceeds 0.0300%, the hole expansibility and ductility isotropy of the hot-rolled steel sheet are remarkably deteriorated. Therefore, the S content should be 0.0300% or less. The S content is preferably 0.0050% or less.
Although the lower limit of the S content does not have to be specified, it is preferably 0.0001% 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 deteriorating the hole expansibility and ductility isotropy of the hot-rolled steel sheet. If the N content exceeds 0.1000%, the isotropy of the hole expansibility and ductility of the hot-rolled steel sheet is significantly deteriorated. Therefore, the N content should be 0.1000% or less. The N content is preferably 0.0800% or less and 0.0700% or less.
The lower limit of the N content does not need to be specified, but in order to promote the precipitation of carbonitrides, the N content is preferably 0.0010% or more, and preferably 0.0020% or more. more preferred.

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 and 0.0050% or less.
The O content may be 0.0005% or more and 0.0010% or more in order to disperse a large number of fine oxides when deoxidizing molten steel.

The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment consists of Fe and impurities. In the present embodiment, impurities refer to elements that are mixed from ore, scrap, or the manufacturing environment as raw materials, or elements that are intentionally added in small amounts, and have an adverse effect on the hot-rolled steel sheet according to the present embodiment. It means what is permissible within the range not given.

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

Nb: 0.005-0.100% and V: 0.005-0.500%
Both Nb and V are elements that, like Ti, suppress recrystallization and grain growth of austenite between hot rolling stands. 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 to set the Nb content to 0.005% or more or the V content to 0.005% or more.
However, even if these elements are excessively contained, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Nb content is set to 0.100% or less, and the V content is set to 0.500% or less.

Cu: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00% and B: 0.0001- 0.0100%
Cu, Cr, Mo, Ni and B all have the effect of increasing the hardenability of hot-rolled steel sheets. Moreover, Cr and Ni have the effect of stabilizing retained austenite, and Cu and Mo have the effect of precipitating carbides in the steel to increase the strength of the hot-rolled steel sheet. 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 steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by being precipitated as carbides in the steel at low temperatures. The Cu content is preferably 0.01% or more in order to more reliably obtain the effects of the above action.
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.

As described above, Cr has the effect of increasing the hardenability of steel sheets and the effect of stabilizing retained austenite. In order to more reliably obtain the effects of the above action, the Cr content is preferably 0.01% 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.

As described above, Mo has the effect of increasing the hardenability of the steel sheet and the effect of precipitating carbides in the steel to increase the strength. In order to more reliably obtain the effects of the above action, the Mo content is preferably 0.01% 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.

As described above, Ni has the effect of enhancing the hardenability of the steel sheet. 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 0.02% 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.

As described above, B has the effect of enhancing the hardenability of the steel sheet. In order to more reliably obtain the effect of this action, the B content is preferably 0.0001% or more.
However, if the B content exceeds 0.0100%, the hole expansibility and ductility isotropy of the hot-rolled steel sheet are remarkably deteriorated, so the B content is made 0.0100% or less.

Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005-0.1000% and Bi: 0.0005-0.020%
Ca, Mg and REM all have the effect of improving the formability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferred shape. Moreover, Bi has the effect of increasing the formability of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects of the above action, it is preferable that at least one of Ca, Mg, REM and Bi is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and the hole expansion of the hot-rolled steel plate is rather reduced. may degrade the isotropy of toughness and ductility. Moreover, even if the Bi content exceeds 0.020%, the above effect is saturated, which is economically unfavorable. 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 0.020% or less. The Bi content is preferably 0.010% or less.

Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the content of REM above refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.

One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%
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.
In addition, the present inventors have confirmed that even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired, but flaws may occur during hot rolling. , the Sn content is 0.050% or less.

The chemical composition of the hot-rolled steel sheet mentioned above can 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.

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.
In the hot-rolled steel sheet according to the present embodiment, the metal structure in the region from the surface to the depth of ⅛ of the plate thickness from the surface to the depth of ⅛ of the plate thickness is area%, retained austenite: 10 to 20%. , fresh martensite: 10% or less, and bainite: 70 to 90%. 110> and {112} <110> orientation groups have a pole density of 2.0 to 8.0, and a region from a depth of 1/8 the plate thickness from the surface to a depth of 1/2 the plate thickness from the surface In the texture of the {110}<112> orientation, the pole density is 2.0 to 4.0.

In this embodiment, in the plate thickness cross section parallel to the rolling direction, the 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 region) defines the area ratio of retained austenite, fresh martensite and bainite. The reason is that the metallographic structure at this position shows the typical metallographic structure of hot-rolled steel sheets.

Retained austenite: 10-20%
Retained austenite is a structure that improves the hole expansibility and ductility isotropy of the hot-rolled steel sheet. If the area ratio of retained austenite is less than 10%, desired isotropic hole expandability and ductility cannot be obtained. Therefore, the area ratio of retained austenite is set to 10% or more. Preferably, it is 12% or more or 13% or more.
On the other hand, if the area ratio of retained austenite exceeds 20%, desired strength cannot be obtained. Therefore, the area ratio of retained austenite is set to 20% or less. Preferably, it is 18% or less, 17% or less.

Fresh martensite: 10% or less Since fresh martensite is a hard structure, it contributes to improvement in the strength of the hot-rolled steel sheet. However, fresh martensite is also a structure with poor isotropy in expansibility and ductility. If the area ratio of fresh martensite exceeds 10%, desired isotropy of hole expandability and ductility cannot be obtained. Therefore, the area ratio of fresh martensite is set to 10% or less. Preferably, it is 8% or less, 6% or less, 4% or less, or 2% or less. The area ratio of fresh martensite may be 0%.

Bainite: 70-90%
Bainite is a structure that improves the strength and ductility isotropy of hot-rolled steel sheets. If the area ratio of bainite is less than 70%, the desired strength cannot be obtained. Therefore, the area ratio of bainite is set to 70% or more. Preferably, it is 73% or more, 75% or more or 77% or more.
On the other hand, if the area ratio of bainite exceeds 90%, the strength becomes too high, and the desired hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 90% or less. Preferably less than 90%, no more than 88% or no more than 85%.

Among the structures described above, the area ratio of structures other than retained austenite is measured by the following method.
From the hot-rolled steel sheet, in the thickness cross section parallel to the rolling direction, 1/4 depth 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) ) Take a test piece so that the metal structure can be observed. Next, after polishing the thickness cross-section, the polished surface is subjected to nital corrosion, and a 30 μm×30 μm region is structurally observed using an optical microscope and a scanning electron microscope (SEM). At least three observation areas are provided. The area ratio of bainite is obtained by performing image analysis on the structure photograph obtained by this structure observation. After that, after performing repeller corrosion on the same observation position, the structure was observed using an optical microscope and a scanning electron microscope, and the image analysis was performed on the obtained structure photograph. get rate.

In the tissue observation described above, each tissue is identified by the following method.
Fresh martensite has a high dislocation density and substructures such as blocks and packets within the grains, so it can be distinguished from other metal structures according to electron channeling contrast images using a scanning electron microscope. is possible.

A structure that is an aggregate of lath-shaped crystal grains and is not fresh martensite in a structure that does not contain Fe-based carbides with a major axis of 20 nm or more in the interior of the structure, or contains Fe-based carbides with a major axis of 20 nm or more in the interior of the structure, and A structure in which Fe-based carbides have a single variant, that is, Fe-based carbides elongated in the same direction is considered bainite. Here, the term "Fe-based carbides elongated in the same direction" refers to Fe-based carbides with a difference of 5° or less in the orientation of the Fe-based carbides.

The area ratio of retained austenite is measured by the following method.
In this embodiment, the area ratio of retained austenite is measured by X-ray diffraction. First, in the thickness cross section parallel to the rolling direction of the hot-rolled steel sheet, 1/4 depth 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) region), using Co-Kα rays, obtain the integrated intensity of a total of 6 peaks α(110), α(200), α(211), γ(111), γ(200), γ(220) , the volume fraction of retained austenite is calculated using the intensity average method. This volume fraction of retained austenite is regarded as the area fraction of retained austenite.

Polar density of {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the region from the surface to the depth of 1/8 of the plate thickness from the surface: 2.0 to 8. 0
{001}<110>, {111}<110>, and {112}<110> in the texture of the region from the surface to the depth of 1/8 of the plate thickness from the surface (hereinafter sometimes referred to as the surface layer region) If the pole density of the orientation group is less than 2.0, the ductility isotropy and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the pole density of {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface layer region is set to 2.0 or more. It is preferably 2.2 or more, 2.5 or more, or 2.7 or more.
When the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface region is greater than 8.0, the ductility isotropy of the hot-rolled steel sheet And the hole expansibility deteriorates. Therefore, the pole density of {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface layer region is set to 8.0 or less. Preferably, it is 7.5 or less or 7.0 or less.

Polar density of {110} <112> orientation in the texture of the region from the surface to the depth of 1/8 of the plate thickness to the depth of 1/2 of the plate thickness from the surface: 2.0 to 4.0
The extreme density of {110} <112> orientation in the texture of the region from the surface to the depth of 1/8 the plate thickness to the depth of 1/2 the plate thickness from the surface (hereinafter sometimes referred to as the internal region) If it exceeds 4.0, the ductility isotropy and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the pole density of the {110}<112> orientation in the texture of the inner region is set to 4.0 or less. It is preferably 3.6 or less, 3.2 or less, or 3.0 or less.
The extreme density of the {110}<112> orientation in the texture of the inner region is set to 2.0 or more from the viewpoint of suppressing strength deterioration. It is preferably 2.3 or more or 2.5 or more.

For extreme density, a device that combines a scanning electron microscope and an EBSD analysis device and AMETEK's OIM Analysis (registered trademark) are used. From the crystal orientation distribution function (ODF: Orientation Distribution Function) that displays the three-dimensional texture, calculated using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and the spherical harmonic function, the surface region Determine the extreme densities of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture and {110}<112> in the texture of the inner region.

The measurement range for the surface layer area is from the surface to 1/8 of the plate thickness from the surface, and for the internal area, from the surface to 1/8 of the plate thickness from the surface to 1/8 of the plate thickness. Let it be a region of 2 depths. A measurement pitch is 5 μm/step.

{hkl} represents a crystal plane parallel to the rolling plane, and <uvw> represents a crystal direction parallel to the rolling direction. That is, {hkl}<uvw> indicates a crystal in which {hkl} is oriented in the plate surface normal direction and <uvw> is oriented in the rolling direction.

The rolling direction of the hot-rolled steel sheet can be determined by the following method.
First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. After mirror-polishing the plate thickness cross-section of the sampled test piece, it is observed using an optical microscope. The observation range is the entire plate thickness, and areas with dark luminance are determined to be inclusions. Among the inclusions, the direction parallel to the extending direction of the inclusions having a major axis length of 40 μm or more is determined as the rolling direction.

Mechanical Properties The hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 980 MPa or more. By setting the tensile strength to 980 MPa or more, it is possible to contribute to the weight reduction of the vehicle body. More preferably, the tensile strength is 1180 MPa or higher. The upper limit is not particularly limited, but may be 1470 MPa, 1300 MPa or less, or 1200 MPa or less.
The difference between the total elongation in the C direction and the total elongation in the L direction ((total elongation in the L direction - total elongation in the C direction)/total elongation in the C direction), which is an index of ductility isotropy, is ±3.0. % or less.
It is preferable that the hole expansion ratio, which is an index of the hole expandability, is 40% or more.

The tensile strength TS and total elongation EL are measured according to JIS Z 2241:2011 using 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 (C direction) is taken as the longitudinal direction. Regarding the total elongation EL, the total elongation in the L direction is measured by also performing a tensile test on a tensile test piece whose longitudinal direction is parallel to the rolling direction (L direction).

The hole expansion ratio λ is measured according to JIS Z 2256:2010 using a No. 5 test piece of JIS Z 2241:2011. The hole-expanding test piece may be sampled at a 1/4 portion from the edge of the hot-rolled steel sheet in the width direction.

Thickness The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. By setting the thickness of the hot-rolled steel sheet to 1.2 mm or more, it becomes easy to secure the rolling completion temperature, the rolling load can be reduced, and hot rolling can be easily performed. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. Preferably, it is 1.4 mm or more. Also, if the plate thickness is 8.0 mm or less, it becomes difficult to control the texture, and it may be difficult to obtain the texture described above. 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 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.

Next, a suitable method for manufacturing the hot-rolled steel sheet according to this embodiment will be described. A preferred method for manufacturing the hot-rolled steel sheet according to the present embodiment includes the following steps (a) to (d). In addition, the temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.

(a) A heating step of heating the slab having the chemical composition described above to a temperature range of 1100°C or higher and lower than 1350°C.
(b) A finish rolling step in which the heated slab is finish rolled using a rolling mill having a plurality of stands, satisfying the following conditions (I) to (V).
(I) The finish rolling start temperature is 850° C. or higher.
(II) Rolling is performed so that σ represented by the following formula (1) is 40 to 80 in each of the last four stands out of the plurality of stands.
σ=exp(0.753+3000/T)× ^ε0.21 × ^ε′0.13 (1)
Here, T is the temperature (° C.) just before entering each stand, ε is the equivalent plastic strain, and ε′ is the strain rate.
(III) The interpass time between each of the last four stands is 0.1-10.0 seconds.
(IV) The cumulative rolling reduction of the last four stands shall be 60% or more.
(V) The finish rolling completion temperature is set to 850 to 1000°C.
(c) A cooling step in which air cooling is performed for 2.0 to 4.0 seconds after completion of finish rolling, and then the average cooling rate to the temperature range of 450 to 550° C. is 100° C./s or more.
(d) A winding step in which winding is performed after cooling.
Each step will be described below.

(a) Heating Step In the heating step, the slab having the chemical composition described above is preferably heated to a temperature range of 1100°C or higher and lower than 1350°C. The method for producing the slab is not particularly limited, and a commonly used method of melting molten steel having the chemical composition described above in a converter or the like and forming a slab by a casting method such as continuous casting can be applied. In addition, an agglomeration-blooming method may be used.

In slabs, most carbonitride-forming elements such as Ti exist as coarse carbonitrides with uneven distribution in the slab. Coarse precipitates (carbonitrides) present in a non-uniform distribution deteriorate various properties (eg, tensile strength, ductility, hole expansibility, etc.) of hot-rolled steel sheets. Therefore, the slab before hot rolling is heated in a desired temperature range to dissolve coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the slab is preferably 1100° C. or higher. However, if the heating temperature of the slab becomes too high, surface defects will occur and the yield will decrease due to scale off. Therefore, the heating temperature of the steel material is preferably less than 1350°C.

The slab is heated to a temperature range of 1100°C or more and less than 1350°C and held for a predetermined time. As a result, in the subsequent finish rolling process, entrapment of scale and the like is likely to occur, and the surface quality of the hot-rolled steel sheet may deteriorate. Therefore, the holding time in the temperature range of 1100° C. or more and less than 1350° C. is preferably 4800 seconds or less.

Rough Rolling Step Between the heating step and the finish rolling step, the slab may be subjected to rough rolling. Conditions for the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be obtained.

(b) Finish Rolling Step In the finish rolling step, the heated slab is finish rolled using a rolling mill having a plurality of stands. At this time, it is preferable to satisfy the following conditions (I) to (V).
Descaling is preferably performed before finish rolling or during rolling between rolling stands for finish rolling.

(I) Finish rolling start temperature: 850°C or higher The finish rolling start temperature (entry side temperature of the first pass of finish rolling) is preferably 850°C or higher. If the finish rolling start temperature is less than 850° C., rolling in some of the plurality of rolling stands (especially the first half of the stands) will be performed at the two-phase region temperature of ferrite+austenite. As a result, the worked structure may remain after finish rolling, deteriorating the strength and ductility of the hot-rolled steel sheet. Therefore, the finishing rolling start temperature is preferably 850° C. or higher.
In addition, the finish rolling start temperature may be set to 1100° C. or less in order to suppress coarsening of austenite.

(II) σ: 40 to 80 represented by the following formula (1) in each of the last four stands
σ=exp(0.753+3000/T)·ε ^0.21 ·ε′ ^0.13 (1)
Here, T is the temperature (° C.) just before entering each stand (that is, entry temperature), ε is the equivalent plastic strain, and ε′ is the strain rate.
σ of 40 to 80 at each of the last four stands gives σ for the fourth to last stand, σ for the third to last stand, σ for the second to last stand, In other words, σ of the final stand and σ are all 40-80.

If there is even one stand with σ less than 40, each of the last four stands may not adequately impart the strain necessary for texture development in the superficial region. As a result, the pole density of the {001} <110>, {111} <110> and {112} <110> orientation groups is preferably controlled in the texture in the region from the surface to the depth of 1/8 of the plate thickness from the surface. may not be possible. Also, the texture of the inner region may not be controlled favorably. Therefore, σ in each of the last four stands is preferably 40 or more.
Also, if there is even one stand with σ exceeding 80, the texture may not develop and the texture may become randomized due to dynamic recrystallization. As a result, the ductility isotropy and hole expansibility of the hot-rolled steel sheet may deteriorate. Therefore, σ in each of the last four stands is preferably 80 or less.

The equivalent plastic strain ε can be obtained by ε=(2/√3)×(h/H), where h is the plate thickness on the entry side and H is the plate thickness on the delivery side. Further, ε', which is the strain rate, can be obtained by ε'=ε/t, where t(s) is the rolling time.
Further, the rolling time t is the time during which the steel sheet is in contact with the rolling rolls and strain is applied to the steel sheet.

(III) Inter-pass time between each of the last four stands: 0.1 to 10.0 seconds Any inter-pass time exceeding 10.0 seconds between each of the last four stands , recovery and recrystallization progresses between passes. As a result, accumulation of strain becomes difficult, and a desired texture may not be obtained in the surface layer region and the inner region. Therefore, the interpass time between each of the last four stands is preferably 10.0 seconds or less.
It is preferable that the interpass time between the last four stands is short, but there are restrictions on the installation space of each stand and the rolling speed in terms of shortening the interpass time, so it should be 0.1 seconds or more. is preferred.
In addition, if the inter-pass time between each of the last four stands is 0.1 to 10.0 seconds, the inter-pass time between the fourth to last stand and the third to last stand, The inter-pass time between the third to last stand and the second to last stand, and the inter-pass time between the second to last stand and the last stand are all 0.1 to 10.0 seconds. can be rephrased as

(IV) Cumulative rolling reduction of the last four stands: 60% or more If the cumulative rolling reduction of the last four stands is less than 60%, the dislocation density introduced into the unrecrystallized austenite may become small. When the density of dislocations introduced into unrecrystallized austenite becomes small, it becomes difficult to obtain a desired texture, and the isotropy of hole expandability and ductility of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last four stands is preferably 60% or more.
Note that if the cumulative rolling reduction of the last four stands exceeds 97%, the shape of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last four stands may be 97% or less.

The cumulative rolling reduction of the last four stands is {1−(t1/t0)}×100 when the thickness t0 at the entrance of the fourth stand from the end and the thickness t1 at the exit of the final stand are taken. It can be expressed in (%).

(V) finish rolling completion temperature 850 ~ 1000 ° C
If the finish rolling end temperature (the delivery side temperature of the final stand) is less than 850°C, the rolling is performed at the two-phase temperature of ferrite + austenite. As a result, a worked structure may remain after rolling, deteriorating isotropy of strength and ductility of the hot-rolled steel sheet. Therefore, the finish rolling completion temperature is preferably 850° C. or higher.
In addition, in the slab having the chemical composition according to the present embodiment, the non-recrystallized austenite region is generally a temperature region of 1000° C. or lower. Therefore, when the finish rolling completion temperature exceeds 1000°C, the austenite grains grow and the grain length of martensite in the hot-rolled steel sheet obtained after cooling increases. As a result, it becomes difficult to obtain a desired texture, and the strength and ductility isotropy of the hot-rolled steel sheet may deteriorate. Therefore, the finish rolling completion temperature is preferably 1000° C. or lower.

(c) Cooling step In the cooling step, air cooling is performed for 2.0 to 4.0 seconds after completion of finish rolling, and then cooling is performed so that the average cooling rate to the temperature range of 550 to 450 ° C. is 100 ° C./s or more. is preferred.

Air cooling time: 2.0 to 4.0 seconds Air cooling is preferably performed for 2.0 to 4.0 seconds after completion of finish rolling. If the air cooling time is less than 2.0 seconds or more than 4.0 seconds, the desired amount of bainite may not be obtained. Therefore, air cooling is preferably performed for 2.0 to 4.0 seconds.
In this embodiment, air cooling refers to cooling at an average cooling rate of less than 10°C/s.

In the present embodiment, it is preferable to install a cooling facility in the rear stage of the finish rolling facility, and perform cooling after the air cooling while allowing the steel sheet after finish rolling to pass through the cooling facility. The cooling referred to here does not include the air cooling described above.
The cooling equipment is preferably equipment capable of cooling the steel sheet at an average cooling rate of 100° C./s or more. As such cooling equipment, for example, a water cooling equipment using water as a cooling medium can be exemplified.

The average cooling rate in the cooling process is a value obtained by dividing the temperature drop width of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling. The start of cooling is when the steel plate is introduced into the cooling equipment, and the end of cooling is when the steel plate is taken out of the cooling equipment.
Cooling equipment includes equipment that has no air-cooling section in the middle and equipment that has one or more air-cooling sections in the middle. In this embodiment, any cooling equipment may be used. Even when using a cooling facility having an air-cooling section, the average cooling rate from the start of cooling to the end of cooling should be 100° C./s or more.

Average cooling rate from the air cooling end temperature to the temperature range of 450 to 550 ° C.: 100 ° C./s or more Ferrite tends to be formed, and the desired amount of bainite may not be obtained. Therefore, the average cooling rate from the air-cooling end temperature to the temperature range of 450 to 550° C. is preferably 100° C./s or more.

(d) Winding Step In the winding step, it is preferable to coil the steel sheet cooled to a temperature range of 450 to 550°C. Since the steel sheet is coiled immediately after cooling, the coiling temperature is approximately equal to the cooling stop temperature. If the coiling temperature is less than 450°C, the desired amount of bainite cannot be obtained, and the isotropy of hole expansibility and ductility may deteriorate. Moreover, when the winding temperature is higher than 550° C., a large amount of ferrite and pearlite are generated, and the desired strength may not be obtained. Therefore, the winding temperature is preferably in the temperature range of 450-550°C.

After winding, it should be air-cooled. After coiling, the hot-rolled steel sheet may be temper-rolled or pickled to remove scales formed on the surface. Alternatively, further, plating treatment such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot dip galvanizing, electrogalvanizing, alloyed hot dip galvanizing, etc., or chemical conversion treatment may be applied.

The hot-rolled steel sheet according to the present embodiment can be stably manufactured by the suitable manufacturing method described above.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. 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.

Molten steel having the chemical composition shown in Table 1 was melted in a converter, and slabs were obtained by continuous casting. Next, these slabs were heated under the conditions shown in Tables 2A and 2B, subjected to rough rolling, and then subjected to finish rolling under the conditions shown in Tables 2A and 2B. After completion of the finish rolling, the hot-rolled steel sheets having the thicknesses shown in Tables 3A and 3B were obtained by cooling and winding under the conditions shown in Tables 3A and 3B.
In the heating step, the holding time at the heating temperatures listed in Tables 2A and 2B was 4800 seconds or less.

Cooling after finish rolling (excluding air cooling) was by water cooling, and the steel sheet was passed through water cooling equipment that does not have an air cooling section on the way. The average cooling rate in Tables 3A and 3B is a value obtained by dividing the temperature drop width of the steel plate from the time when the water cooling equipment is introduced to the time when the water cooling equipment is taken out by the time required for the steel plate to pass through the water cooling equipment.

A test piece was taken from the obtained hot-rolled steel sheet, and the area ratio of each structure, the extreme density of the texture, the tensile strength, the total elongation in the C and L directions, and the hole expansion ratio were measured by the method described above. It was measured.
The results obtained are shown in Tables 4A and 4B.

When the obtained tensile strength was 980 MPa or more, it was judged to have high strength and to be accepted. On the other hand, when the obtained tensile strength was less than 980 MPa, it was determined to be unsatisfactory because it did not have high strength.

When the difference between the obtained total elongation in the C direction and the total elongation in the L direction was ±3.0% or less, it was judged to have excellent ductility isotropy and to pass. On the other hand, when the difference between the total elongation in the C direction and the total elongation in the L direction was more than ±3.0%, it was determined to be unsatisfactory because it did not have excellent ductility isotropy.

When the obtained hole expansion rate was 40% or more, it was determined to be acceptable as having excellent hole expansion properties. On the other hand, when the hole expansion rate was less than 40%, it was judged to be unsatisfactory because it did not have excellent hole expandability.

As can be seen from Tables 4A and 4B, hot-rolled steel sheets having high strength and excellent ductility isotropy and hole expansibility were obtained in the examples of the present invention.
On the other hand, the comparative examples whose chemical composition and/or metallographic structure were not within the range defined by the present invention were inferior in at least one of the above properties.

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility isotropy and hole expansibility.

Claims (2)

The chemical composition, in mass %,
C: 0.100 to 0.350%,
Si: 0.010 to 3.00%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
Si+sol. Al: 1.00% or more,
Ti: 0.010 to 0.380%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Nb: 0 to 0.100%,
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 0.020%,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%,
The balance consists of Fe and impurities,
The metal structure in the region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface is area%,
retained austenite: 10-20%,
Fresh martensite: 10% or less, and bainite: 70 to 90%,
In the texture of the region from the surface to the depth of 1/8 of the plate thickness from the surface,
{001} <110>, {111} <110> and {112} <110> orientation groups have a pole density of 2.0 to 8.0,
In the texture of the region from the depth of 1/8 the plate thickness from the surface to the depth of 1/2 the plate thickness from the surface,
{110} <112> orientation pole density is 2.0 to 4.0,
A hot-rolled steel sheet having a tensile strength of 980 MPa or more. The chemical composition, in mass %,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.500%,
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Ni: 0.02 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000% and Bi: 0.0005-0.020%
The hot-rolled steel sheet according to claim 1, containing one or more selected from the group consisting of: