WO2024162382A1 - Hot-rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a prescribed chemical composition. In the orientation distribution function of the aggregate structure in the surface layer region, the maximum value A in the range of Φ=0-60° and φ₁=50-90° in the φ₂=45° section is no higher than 6.0, the maximum value B in the range of Φ=120-180° and φ₁=50-90° in the φ₂=45° section is no higher than 6.0, ｜Φ_A-35°｜ is no higher than 10°, and ｜Φ_B-145°｜ is no higher than 10°. In the metal structure at a position at 1/4 depth from the surface in the sheet thickness direction, the area ratio of a region in which the GAM value is greater than 0.6° is 50% or higher, and the total of the area ratio of a region in which the GAM value is greater than 3.0° and the area ratio of retained austenite is less than 15%.

Description

Hot-rolled steel

The present invention relates to a hot rolled steel sheet. In particular, the present invention relates to a hot rolled steel sheet having high strength, excellent ductility and hole expansion properties, and excellent tensile bending properties in the rolling direction.
This application claims priority based on Japanese Patent Application No. 2023-013128, filed on January 31, 2023, the contents of which are incorporated herein by reference.

In recent years, there has been progress in reducing the weight of automotive parts. It is possible to reduce the weight of automotive parts by ensuring rigidity through optimal design of the part shape. Furthermore, in the case of blank formed parts such as press molded parts, weight can be reduced by reducing the plate thickness of the part material.

When trying to maintain the strength properties of parts, such as static fracture strength and yield strength, while reducing plate thickness, it is necessary to use high-strength materials. Automotive parts are manufactured by subjecting steel plate to various processes, so steel plate used in automotive parts is required to have excellent formability, especially ductility and hole expansion properties.

When manufacturing automotive parts, steel sheets may be bent while tension is applied. Bending while tension is applied is often performed along the rolling direction of the steel sheet. Therefore, steel sheets used in automotive parts are required to have excellent tensile bending properties, especially in the rolling direction.

For example, Patent Document 1 discloses a hot-rolled steel sheet having a structure consisting of 70% or more ferrite and pearlite in terms of area fraction, a sheet thickness T0 of 6 to 25 mm, an average grain size GC of the ferrite grains inside the sheet thickness of 5 to 15 μm, the hot-rolled steel sheet having a fine grain layer formed from the surface in the sheet thickness direction and having an average grain size of the ferrite grains less than 1.0 times the average grain size GC, the fine grain layer including a specific fine grain layer having an average grain size of the ferrite grains of 0.1 to 0.4 times the average grain size GC, and satisfying a predetermined formula when the thickness of the specific fine grain layer is TF0 and the thickness of an ultrafine grain layer of the fine grain layer in which the average grain size of the ferrite grains is less than 0.1 times the average grain size GC is TF1, and the average grain size of the ferrite grains of the specific fine grain layer and the ultrafine grain layer is 0.1 to 0.4 times the average grain size GC.

Patent Document 2 discloses a high-strength hot-rolled steel sheet with excellent hole expandability and weld fatigue properties, characterized in that the random strength ratio of the {110}<111> to {110}<001> orientation group in the thickness cross section in the region from the outermost layer to 1/6 of the sheet thickness is 3.5 or less.

Japanese Patent No. 6477020 Japanese Patent No. 6701954

However, Patent Documents 1 and 2 do not take into consideration the tensile bending characteristics in the rolling direction.

In view of the above circumstances, the present invention aims to provide a hot-rolled steel sheet that has high strength, excellent ductility and hole expansion properties, and excellent tensile bending properties in the rolling direction.

The present inventors have found that, in the crystal orientation distribution function of the texture in the surface region of a hot-rolled steel sheet (the region from the surface to a depth of 500 μm in the sheet thickness direction), by controlling the maximum value A in the range of Φ= ₀ to 60° and _Φ1 =50 to 90° at a _φ2 =45° cross section, and the maximum value B in the range of Φ=120 to 180° and _Φ1 =50 to 90° at a φ2=45° cross section, and by setting the peak positions of Φ where these maximum values are located within desired ranges, the bendability of the hot-rolled steel sheet can be further improved and the tensile bending characteristics can be improved.

The inventors also discovered that controlling the rough rolling conditions and finish rolling conditions of hot rolling is effective in controlling the texture in the surface region of a hot-rolled steel sheet.

The gist of the present invention, which has been made based on the above findings, is as follows.
(1) A hot-rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-2.60%,
Ti: 0.020 to 0.180%,
Al: 0.010-0.400%,
P: 0.080% or less,
S: 0.0100% or less,
N: 0.0050% or less,
O: 0.010% or less,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.0500%,
REM: 0-0.100%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
Contains As: 0 to 0.050% and Sn: 0 to 0.050%;
The balance is Fe and impurities,
In the crystal orientation distribution function of the texture in the region from the end face to a 1/4 position in the width direction and from the surface to a depth of 500 μm in the sheet thickness direction,
The maximum value A in the range of Φ=0 to 60° and Φ= ₅₀ to 90° at the cross section of φ ₂ =45° is 6.0 or less;
the maximum value B in the range of Φ=120 to 180° and _φ1 =50 to 90° at the _φ2 =45° cross section is 6.0 or less;
When the peak position of Φ where the maximum value A is located is defined as Φ _A , and the peak position of Φ where the maximum value B is located is defined as Φ _B , |Φ _A -35°| is 10° or less, and |Φ _B -145°| is 10° or less;
In the metal structure at a position of 1/4 depth from the surface in the plate thickness direction,
The area ratio of the region having a GAM value exceeding 0.6° is 50% or more,
The sum of the area ratio of the region in which the GAM value exceeds 3.0° and the area ratio of retained austenite is less than 15%.
(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition is, in mass%,
Nb: 0.001 to 0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 0.500%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.0001-0.0500%,
REM: 0.001-0.100%,
Bi: 0.001-0.100%,
Ta: 0.001 to 0.100%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%
may contain one or more selected from the group consisting of:
(3) The hot-rolled steel sheet according to (1) or (2) above,
In the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 μm in the sheet thickness direction,
The absolute value of the difference between the maximum value A and the maximum value B may be 3.0 or less at each of a 1/4 position from the end face in the width direction, a 1/4-15 mm position from the end face in the width direction, and a 1/4+15 mm position from the end face in the width direction.
(4) The hot-rolled steel sheet according to any one of (1) to (3) above,
In the metal structure at a position of ¼ depth from the surface in the plate thickness direction,
The area ratio of the region in which the GAM value is more than 0.6° and less than 2.0° may be 50% or more.
(5) The hot-rolled steel sheet according to any one of (1) to (3) above,
In the metal structure at a position of ¼ depth from the surface in the plate thickness direction,
The area ratio of the region having a GAM value of 2.0° or more may be 50% or more.

According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent tensile bending properties in the rolling direction.
Furthermore, according to a preferred embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having not only the above-mentioned properties but also excellent tensile bending properties in the width direction.

FIG. 13 is a diagram for explaining a tension bending test.

The hot-rolled steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention.

Note that the numerical ranges described below with "~" include the lower and upper limits. Values marked "less than" and "greater than" are not included in the numerical range. All "%" in chemical composition refers to "% by mass."

The chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 2.60%, Ti: 0.020 to 0.180%, Al: 0.010 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, and the balance: Fe and impurities.
Each element will be described in detail below.

C: 0.045-0.120%
C is an element necessary for obtaining a desired tensile strength of the hot-rolled steel sheet. If the C content is less than 0.045%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, more preferably 0.060% or more, and even more preferably 0.080% or more. be.
On the other hand, if the C content exceeds 0.120%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.110% or less. and more preferably 0.100% or less.

Si: 0-3.00%
Si is an element that improves the tensile strength of a hot-rolled steel sheet by solid solution strengthening. However, the hot-rolled steel sheet according to the present embodiment ensures sufficient tensile strength even without containing Si. The Si content may be 0%. The Si content is preferably 0.01% or more, and more preferably 0.03% or more.
On the other hand, if the Si content is too high, hot rolling may become difficult due to insufficient ductility, etc. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less. In the hot-rolled steel sheet according to the present embodiment, the Si content is set to 0 to 3.00%, thereby improving the strength and It is possible to achieve a good balance between elongation and hole expandability.

Mn: 1.20-2.60%
Mn is an element necessary for improving the strength of a hot-rolled steel sheet. If the Mn content is less than 1.20%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. The Mn content is 1.20% or more, preferably 1.40% or more, and more preferably 1.60% or more.
On the other hand, if the Mn content exceeds 2.60%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 2.60% or less. The Mn content is preferably 2.30% or less. % or less, and more preferably 2.20% or less.

Ti: 0.020-0.180%
Ti is an element that forms fine nitrides in steel to increase the strength of the hot-rolled steel sheet. If the Ti content is less than 0.020%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the Ti content is set to 0.020% or more, preferably 0.050% or more, and more preferably 0.080% or more.
On the other hand, if the Ti content exceeds 0.180%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0. It is preferably 160% or less, and more preferably 0.150% or less.

Al: 0.010-0.400%
Al is an element that acts as a deoxidizer and improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of Al inclusions in the steel are generated. Such inclusions deteriorate the workability, particularly the hole expandability, of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is , preferably 0.020% or more, and more preferably 0.030% or more.
On the other hand, if the Al content exceeds 0.400%, casting becomes difficult. Therefore, the Al content is set to 0.400% or less. The Al content is preferably set to 0.300% or less, and more preferably 0.400% or less. It is preferably 0.200% or less, and even more preferably 0.100% or less.

P: 0.080% or less P is an element that segregates at grain boundaries in steel and promotes embrittlement of the grain boundaries. If the P content is too high, the elongation and hole expandability of the hot-rolled steel sheet are likely to decrease, and further, cracks in the slab due to embrittlement may occur, making hot rolling difficult. Therefore, the P content is set to 0.080% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.
The lower the P content, the better, and 0% is preferable. However, if the P content is excessively reduced, the dephosphorization cost increases significantly, so the P content may be 0.001% or more.

S: 0.0100% or less S is an element that embrittles the slab when present as a sulfide. S is also an element that deteriorates the workability of the hot-rolled steel sheet. If the S content exceeds 0.0100%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, more preferably 0.0050% or less.
The lower the S content, the better, and 0% is preferable. However, if the S content is excessively reduced, the desulfurization cost increases significantly, so the S content may be 0.0005% or more.

N: 0.0050% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expandability of hot-rolled steel sheets. If the N content is too high, excessive nitrides are generated, which tends to reduce the elongation and hole expandability of the hot-rolled steel sheet, and furthermore, cracks in the slab due to embrittlement may occur, making hot rolling difficult. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, and more preferably 0.0035% or less.
The lower the N content, the better, and 0% is preferable. However, if the N content is excessively reduced, the cost of denitrification increases significantly, so the N content may be 0.0005% or more.

O: 0.010% or less O is an element that forms oxides and reduces the workability of hot-rolled steel sheets. If the O content exceeds 0.010%, the hole expandability of the hot-rolled steel sheet is likely to decrease due to excessive generation of oxides, etc. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.008% or less, and more preferably 0.006% or less.
The lower the O content, the better, and 0% is preferable. However, if the O content is excessively reduced, the cost of deoxidization increases significantly, so the O content may be 0.001% or more.

The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities refer to substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment, or substances that are acceptable to the extent that they do not adversely affect the hot-rolled steel sheet according to this embodiment.

The chemical composition of the hot-rolled steel sheet according to this embodiment may contain the following optional elements instead of a portion of Fe. When no optional element is contained, the lower limit of the content is 0%.
Each optional element will be described below.

Nb: 0.001-0.100%
Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of hot-rolled steel sheets by forming fine carbides. In order to obtain this, the Nb content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.
On the other hand, if the Nb content exceeds 0.100%, the toughness of the cast slab deteriorates, and hot rolling may become difficult. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, and more preferably 0.060% or less.

V:0.001~1.000%
V is an element that forms fine carbides in steel to increase the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, if the V content exceeds 1.000%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.500 % or less, and more preferably 0.300% or less.

Cu: 0.001-1.000%
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 carbides in the steel at low temperatures. The Cu content is preferably 0.001% or more, more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, if the Cu content exceeds 1.000%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably set to 0.500% or less. and more preferably 0.300% or less.

Cr:0.001~2.000%
Cr is an element that exerts an effect similar to that of Mn. In order to reliably obtain the effect of increasing the strength of the hot-rolled steel sheet by including Cr, the Cr content is preferably 0.001% or more. The content is more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, even if the Cr content exceeds 2.000%, the above effect is saturated. Therefore, the Cr content is set to 2.000% or less. From the viewpoint of reducing the alloy cost, the Cr content is preferably is 1.000% or less, and more preferably 0.500% or less.

Mo: 0.001~3.000%
Mo is an element that increases the strength of a hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, if the Mo content exceeds 3.000%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.000% or less. % or less, and more preferably 1.000% or less.

Ni: 0.001-0.500%
Ni is an element that enhances the hardenability of hot-rolled steel sheets. In addition, when Cu is contained, Ni has the effect of effectively suppressing grain boundary cracking of slabs caused by Cu. In order to reliably obtain the effect, the Ni content is preferably 0.001% or more, more preferably 0.050% or more, and even more preferably 0.100% or more. be.
On the other hand, Ni is an expensive element, so it is economically undesirable to include a large amount of it. Therefore, the Ni content is set to 0.500% or less. From the viewpoint of reducing the alloy cost, the Ni content is , preferably 0.300% or less, and more preferably 0.200% or less.

B: 0.0001-0.0100%
B is an element that increases the strength of a hot-rolled steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more. % or more, and more preferably 0.0010% or more.
On the other hand, even if the B content exceeds 0.0100%, the above effect is saturated. Therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0070% or less. More preferably, it is 0.0050% or less.

Ca: 0.0001-0.0500%
Ca is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferred shape. In order to reliably obtain this effect, the Ca content is set to 0.0001% or more. The Ca content is preferably 0.0010% or more, and more preferably 0.0050% or more.
On the other hand, if the Ca content exceeds 0.0500%, excessive inclusions are generated in the steel, which may deteriorate the ductility and hole expandability of the hot-rolled steel sheet. The Ca content is preferably 0.0300% or less, and more preferably 0.0100% or less.

Mg: 0.0001-0.0500%
Mg is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferred shape. In order to reliably obtain this effect, the Mg content should be 0.0001% or more. The Mg content is preferably 0.0010% or more, and more preferably 0.0020% or more.
On the other hand, if the Mg content exceeds 0.0500%, excessive inclusions are generated in the steel, which may deteriorate the ductility and hole expandability of the hot-rolled steel sheet. The Mg content is preferably 0.0300% or less, and more preferably 0.0100% or less.

REM: 0.001~0.100%
REM is an element that improves the ductility and hole expandability of hot-rolled steel sheets by controlling the shape of inclusions to a preferred shape. To reliably obtain this effect, the REM content should be 0.001% or more. The REM content is preferably 0.003% or more, and more preferably 0.005% or more.
On the other hand, if the REM content exceeds 0.100%, excessive inclusions are generated in the steel, which may deteriorate the ductility and hole expandability of the hot-rolled steel sheet. The REM content is preferably 0.050% or less, and more preferably 0.030% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of the REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal. will be done.

Bi:0.001~0.100%
Bi is an element that refines the solidification structure and thereby improves the ductility and hole expandability of the hot-rolled steel sheet. To reliably obtain this effect, the Bi content must be 0.001% or more. The Bi content is preferably 0.002% or more, and more preferably 0.003% or more.
On the other hand, even if the Bi content exceeds 0.100%, the above effects are saturated, which is economically undesirable. Therefore, the Bi content is set to 0.100% or less. From the viewpoint of reducing alloy costs, The Bi content is preferably 0.050% or less, and more preferably 0.030% or less.

Ta: 0.001~0.100%
Ta, like V, is an element that increases the strength of hot-rolled steel sheets by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is set to 0.001% or more. The Ta content is more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, if the Ta content exceeds 0.100%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080 % or less, and more preferably 0.050% or less.

Zr: 0.001-0.500%
Zr is an element that increases the strength of a hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably is 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Zr content exceeds 0.500%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0. It is preferably 0.300% or less, and more preferably 0.100% or less.

Co:0.001~3.000%
Co is an element that increases the strength of a hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably is 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Co content exceeds 3.000%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1. It is preferably 0.000% or less, and more preferably 0.500% or less.

Zn: 0.001-0.200%
Zn is an element that increases the strength of a hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Zn content is preferably 0.001% or more. The Zn content is more preferably is 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the Zn content exceeds 0.200%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0. .150% or less, more preferably 0.100% or less.

W: 0.001-0.200%
W is an element that increases the strength of a hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably is 0.005% or more, and more preferably 0.010% or more.
On the other hand, if the W content exceeds 0.200%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0. .150% or less, more preferably 0.100% or less.

Sb: 0.001-0.500%
Sb is an element that suppresses the generation of oxides that are the starting points of fracture, thereby improving the ductility and hole expandability of the hot-rolled steel sheet. To reliably obtain this effect, the Sb content is set to 0.001 The Sb content is preferably 0.005% or more, and more preferably 0.10% or more.
On the other hand, even if Sb is contained in a large amount, the above effect is saturated, so the Sb content is set to 0.500% or less. The Sb content is preferably 0.300% or less, and more preferably 0.100% or less. % or less.

As: 0.001-0.050%
As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of the hot-rolled steel sheet. To reliably obtain this effect, the As content should be kept at 0. The As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, since the above effect is saturated even if As is contained in a large amount, the As content is set to 0.050% or less. The As content is preferably set to 0.040% or less, and more preferably set to 0.030% or less. % or less.

Sn: 0.001-0.050%
Sn is an element that suppresses the generation of oxides that are the starting points of fracture, thereby improving the ductility and hole expandability of hot-rolled steel sheets. To reliably obtain this effect, the Sn content is 0.001% or more. The Sn content is more preferably 0.005% or more, and even more preferably 0.010% or more.
On the other hand, even if a large amount of Sn is contained, the above effect is saturated, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less, and more preferably 0.030% or less. % or less.

The chemical composition of the above-mentioned hot-rolled steel sheet may be analyzed using a spark discharge optical emission spectrometer or the like. Note that, for C and S, values identified by burning in an oxygen stream using a gas composition analyzer or the like and measuring by an infrared absorption method are adopted. For N, values identified by melting a test piece taken from the steel sheet in a helium stream and measuring by a thermal conductivity method are adopted.
When the hot-rolled steel sheet has a plating layer on the surface, the plating layer may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.

Next, the texture of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to this embodiment, in a crystal orientation distribution function of the texture in a region from the surface to a depth of 500 μm in the sheet thickness direction, a maximum value A in the range of Φ= ₀ to 60° and Φ= ₅₀ to 90° at a _Φ2 =45° cross section is 6.0 or less, a maximum value B in the range of Φ=120 to 180° and _Φ1 =50 to 90° at the Φ2=45° cross section is 6.0 or less, and when a peak position of Φ where the maximum value A is located is defined as _ΦA and a peak position of Φ where the maximum value B is located is defined as _ΦB , | _ΦA -35°| is 10° or less and | _ΦB -145°| is 10° or less.

In this embodiment, the texture is defined in a region from the end face to a 1/4 position in the width direction and from the surface to a depth of 500 μm in the sheet thickness direction. The 1/4 position in the width direction from the end face here means a w/4 position from the end face in the width direction, where w is the length in the width direction.
That is, "x/y position from the end face (here, x and y are natural numbers satisfying x<y)" means a position moved in the width direction from the end face in the width direction of the steel plate toward the center of the steel plate by a distance of x/y of the plate width. For example, when the width of the steel plate is 1 m, "1/4 position from the end face" means a position that is 0.25 m away from the end face in the width direction of the steel plate.
The term "plate thickness x/y position (where x and y are natural numbers satisfying x<y)" refers to a position moved in the plate thickness direction from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t. For example, when the plate thickness t of the steel plate is 2 mm, the "plate thickness 1/8 position" refers to a position that is 0.25 mm deep from the surface of the steel plate in the plate thickness direction.
In addition, when the steel sheet has a coating such as a plating layer on its surface, the "surface of the steel sheet" means the interface between the steel sheet and the coating, and the "sheet thickness t" means the thickness of the steel sheet (base material) excluding the coating.
Each provision will be explained below.

Maximum value A
In the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 μm in the sheet thickness direction (hereinafter sometimes referred to as the surface region), if the maximum pole density A in the range of Φ=0 to 60° and _Φ1 =50 to 90° at the _φ2 =45° cross section exceeds 6.0, excellent tensile bending properties cannot be obtained in the rolling direction. Therefore, the maximum value A in the range of Φ=0 to 60° and _Φ1 =50 to 90° at the _φ2 =45° cross section is set to 6.0 or less. The maximum value A in the range of Φ=0 to 60° and _Φ1 =50 to 90° at the _φ2 =45° cross section is preferably 5.0 or less, more preferably 4.0 or less. In addition, φ ₂ , Φ, and φ ₁ in the crystal orientation distribution function are rotation angles in each of Bunge's Euler notations shown in FIG. 4 of Light Metals 60 (2010), 12, pp. 666-675.

If the maximum value A in the range of Φ=0 to 60° and Φ= ₅₀ to 90° at the _φ2 =45° cross section is less than 1.0, the pole density in other orientations increases, and the tensile bending properties in the rolling direction may deteriorate. Therefore, the maximum value A in the range of Φ=0 to 60° and _Φ1 =50 to 90° at the _φ2 =45° cross section may be 1.0 or more.

When the peak position of Φ at which the maximum value A is located in the range of Φ=0 to 60° and _Φ1 =50 to 90° at the cross section of _Φ2 =45° is defined as _ΦA , if | _ΦA -35°| exceeds 10°, excellent tensile bending properties cannot be obtained in the rolling direction. Therefore, | _ΦA -35°| is set to 10° or less. | _ΦA -35°| is preferably 5° or less.

Maximum value B
In the crystal orientation distribution function of the texture in the surface region, if the maximum value B in the range of Φ=120-180° and _Φ1 =50-90° at the _Φ2 =45° cross section exceeds 6.0, excellent tensile bending properties cannot be obtained in the rolling direction. Therefore, the maximum value B in the range of Φ=120-180° and _Φ1 =50-90° at the _Φ2 =45° cross section is set to 6.0 or less. The maximum value B in the range of Φ=120-180° and _Φ1 =50-90° at the _Φ2 =45° cross section is preferably 5.0 or less, more preferably 4.0 or less.

If the maximum value B in the ranges of Φ=120 to 180° and Φ= ₅₀ to 90° at the _φ2 =45° cross section is less than 1.0, the pole density in other orientations increases, and the tensile bending properties in the rolling direction may deteriorate. Therefore, the maximum value B in the ranges of Φ=120 to 180° and _Φ1 =50 to 90° at the _φ2 =45° cross section may be 1.0 or more.

When the peak position of Φ at which the maximum value B is located in the range of Φ=120 to 180° and _Φ1 =50 to 90° at the cross section of _Φ2 =45° is defined as _ΦB , if | _ΦB -145°| exceeds 10°, excellent tensile bending properties cannot be obtained in the rolling direction. Therefore, | _ΦB -145°| is set to 10° or less. | _ΦB -145°| is preferably 5° or less.

The maximum values A and B, and the peak positions of Φ at which these maximum values are located, are measured by the following method.
First, a sample is taken at a quarter position in the width direction from the end face of the hot-rolled steel sheet so that the metal structure of the cross section (thickness direction x rolling direction cross section) normal to the width direction can be observed. The size of the sample depends on the measuring device, but it may be, for example, a rectangular parallelepiped with the full thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface of the sample is mirror-polished, and then polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the strain introduced into the surface of the sample. The region from the surface of the polished sample to a depth of 500 μm in the thickness direction and the region of 2000 μm or more at any position in the rolling direction are measured at measurement intervals of 5.0 μm.

For the measurements, a device combining a scanning electron microscope and an EBSD analyzer and an OIM Analysis (registered trademark) manufactured by TSL are used. The above sample is analyzed using the EBSD (Electron Back Scattering Diffraction) method. The crystal orientation distribution function (ODF: Orientation Distribution Function) is calculated from the obtained orientation data.

From the obtained crystal orientation distribution function, a maximum value A in the range of Φ=0 to 60° and Φ= ₅₀ to 90° on the _Φ2 =45° cross section, a maximum value B in the range of Φ= ₁₂₀ to 180° and Φ=50 to 90° on the _Φ2 =45° cross section, and the peak positions of Φ where these maximum values are located are obtained.

The rolling direction of the hot-rolled steel sheet is determined by the following method.
A test piece is taken so that a cross section parallel to the plate surface of the hot-rolled steel sheet can be observed. In the taken test piece, a cross section at a position where the distance from the surface is 1/4 of the plate thickness is mirror-polished and then observed using an optical microscope. The observation range is 500 μm × 500 μm or more, and the direction parallel to the extension direction of the crystal grains is determined to be the rolling direction. In the observed cross section, the direction perpendicular to the determined rolling direction is determined to be the width direction of the hot-rolled steel sheet.

Difference between maximum value A and maximum value B In the crystal orientation distribution function of the texture in the surface region, by setting the absolute value of the difference between maximum value A and maximum value B at each of the positions 1/4 from the end face in the width direction, 1/4-15 mm from the end face in the width direction, and 1/4+15 mm from the end face in the width direction to 3.0 or less, it is possible to obtain excellent tensile bending properties not only in the rolling direction but also in the width direction. Therefore, it is preferable that the absolute value of the difference between maximum value A and maximum value B at the position 1/4 from the end face in the width direction, the absolute value of the difference between maximum value A and maximum value B at the position 1/4-15 mm from the end face in the width direction, and the absolute value of the difference between maximum value A and maximum value B at the position 1/4+15 mm from the end face in the width direction are all set to 3.0 or less.

Note that the position 1/4+15 mm in the width direction from the end face here refers to a position 15 mm away from the "w/4 position from the end face in the width direction" in the opposite direction to the end face, where w is the length in the width direction. Additionally, the position 1/4-15 mm in the width direction from the end face here refers to a position 15 mm away from the "w/4 position from the end face in the width direction" in the direction of the end face, where w is the length in the width direction.

The absolute value of the difference between maximum value A and maximum value B at each position is obtained by performing EBSD analysis using the method described above at a position 1/4 of the way from the end face in the width direction, a position 1/4-15 mm in the width direction from the end face, and a position 1/4+15 mm in the width direction from the end face, and calculating the crystal orientation distribution function.

Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to this embodiment, in the metal structure at a position of 1/4 depth from the surface in the sheet thickness direction, the area ratio of the region where the GAM value exceeds 0.6° is 50% or more, and the sum of the area ratio of the region where the GAM value exceeds 3.0° and the area ratio of retained austenite is less than 15%.
In this embodiment, the metal structure is defined at a position that is 1/4 the width from the end face and 1/4 the depth from the surface in the plate thickness direction.

Area ratio of the region where the GAM value is more than 0.6°: 50% or more If the area ratio of the region where the GAM value is more than 0.6° is less than 50%, the desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of the region where the GAM value is more than 0.6° is set to 50% or more. The area ratio of the region where the GAM value is more than 0.6° is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more.
The area ratio of the region where the GAM value exceeds 0.6° may be set to 100%.

Sum of the area ratio of the region where the GAM value is more than 3.0° and the area ratio of the retained austenite: less than 15% If the sum of the area ratio of the region where the GAM value is more than 3.0° and the area ratio of the retained austenite is 15% or more, the desired hole expandability may not be obtained in the hot-rolled steel sheet. Therefore, the sum of the area ratio of the region where the GAM value is more than 3.0° and the area ratio of the retained austenite is less than 15%. The sum of the area ratio of the region where the GAM value is more than 3.0° and the area ratio of the retained austenite is preferably 10% or less, more preferably 5% or less.
The sum of the area ratio of the region where the GAM value exceeds 3.0° and the area ratio of the retained austenite may be 0% or may be 1% or more.

Here, the desired strength, ductility, and tensile bending properties vary depending on the automobile part to which it is applied. The hot-rolled steel sheet according to this embodiment has the above-mentioned chemical composition, texture, and metal structure, and may have either the first or second metal structure described below, depending on the desired strength, ductility, and tensile bending properties.

(First aspect) Area ratio of the region where the GAM value is more than 0.6° and less than 2.0°: 50% or more The first aspect is a metal structure suitable for cases where a higher level of strength and ductility is required. In this embodiment, by setting the area ratio of the region where the GAM value is more than 0.6° and less than 2.0° to 50% or more, it is possible to achieve a higher level of strength and ductility in the hot-rolled steel sheet. In the first aspect, the area ratio of the region where the GAM value is more than 0.6° and less than 2.0° is preferably 60% or more, more preferably 70% or more.
The area ratio of the region where the GAM value is more than 0.6° and less than 2.0° may be 100%.

In the first embodiment, the remaining structure other than the areas where the GAM value is greater than 0.6° and less than 2.0° may include areas where the GAM value is 2.0° or more and areas where the GAM value is 0.6° or less, with a total area ratio of 0 to 50%.

(Second aspect) Area ratio of the region where the GAM value is 2.0° or more: 50% or more The second aspect is a metal structure suitable for cases where relatively higher strength is required. In this embodiment, by setting the area ratio of the region where the GAM value is 2.0° or more to 50% or more, higher strength can be obtained in the hot-rolled steel sheet. The area ratio of the region where the GAM value is 2.0° or more is preferably 60% or more, more preferably 70% or more.
The area ratio of the region having a GAM value of 2.0° or more may be 100%.

In the second embodiment, the remaining structure other than the area where the GAM value is 2.0° or more may include an area ratio of 0 to 50% where the GAM value is less than 2.0°.

The area ratio of the region where the GAM value exceeds 0.6°, the area ratio of the region where the GAM value is greater than 0.6° and less than 2.0°, the area ratio of the region where the GAM value is 2.0° or more, and the area ratio of the region where the GAM value is greater than 3.0° are measured by the following method.
The "GAM value" of each structure of the hot-rolled steel sheet is measured by the EBSP (Electron Backscatter Pattern) method.

First, a sample is taken at a 1/4 position in the width direction from the end face of the hot-rolled steel sheet so that the metal structure of the cross section (thickness direction x rolling direction cross section) with the width direction as the normal direction can be observed. The size of the sample depends on the measuring device, but it may be, for example, a rectangular parallelepiped with a total thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface of the sample is mirror-polished, and then polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the strain introduced into the surface of the sample. A region of 200 μm from the surface of the sample after the above polishing at a 1/4 depth position in the thickness direction and 400 μm or more at any position in the rolling direction (a rectangular region having a center at a 1/4 depth position in the thickness direction, a length (short side) of 200 μm in the thickness direction, and a length (long side) of 400 μm or more in the rolling direction) is measured at a measurement interval of 0.2 μm to obtain crystal orientation information. For the measurement by the EBSP method, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD analysis device 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.

From the obtained crystal orientation information, the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with fcc crystal structure and regions with bcc crystal structure. In the region with bcc crystal structure, a region surrounded by grain boundaries with an orientation difference of 15° or more is regarded as one crystal grain, and the GAM value of the crystal grain is calculated by calculating the average orientation difference between adjacent pixels within the crystal grain. The area ratio of each region is obtained by calculating the area ratio of crystal grains with the obtained GAM value exceeding 0.6°, the area ratio of crystal grains with an orientation difference of more than 0.6° and less than 2.0°, the area ratio of crystal grains with an orientation difference of 2.0° or more, and the area ratio of crystal grains with an orientation difference of more than 3.0°.

Note that defined crystal grains with an equivalent circular diameter of 0.6 μm or less are excluded from the measurement because there is a possibility of large measurement error.

The area ratio of retained austenite is measured by the following method.
In the measurement of the area ratio of retained austenite by X-ray diffraction in this embodiment, first, a sample is taken so that the metal structure can be observed in a region of 1 mm or more at any position in the rolling direction and 1 mm or more from the end face in the cross section at 1/4 position in the sheet thickness direction from the surface of the hot-rolled steel sheet. The sample is subjected to Co-Kα radiation to obtain the integrated intensity of a total of six peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220). Next, the volume ratio of retained austenite is calculated from the integrated intensity using the intensity averaging method. The obtained volume ratio of retained austenite is regarded as the area ratio of retained austenite.

Mechanical properties Tensile strength (TS): 940 MPa or more The tensile strength may be 940 MPa or more. By making the tensile strength 940 MPa or more, the contribution to weight reduction of the vehicle body can be increased, and the material can be suitably applied to automobile parts. The upper limit of the tensile strength does not need to be particularly limited, but from the viewpoint of suppressing die wear, it may be 1400 MPa or less.

Uniform elongation (uEl): 3.0% or more The uniform elongation may be 3.0% or more. By setting the uniform elongation to 3.0% or more, it can be suitably applied to automobile parts. There is no need to particularly limit the upper limit of the uniform elongation, but it may be 10.0% or less.

The tensile strength and uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241: 2022 using a No. 5 test piece of JIS Z 2241: 2022. The tensile test piece is taken from the center position in the width direction, and the direction perpendicular to the rolling direction and the plate thickness direction (width direction) is defined as the longitudinal direction.
When the above No. 5 test piece cannot be taken from the hot-rolled steel sheet to be measured, a minute test piece with the width direction as the longitudinal direction can be used instead as the test piece for measuring the tensile strength.

Hole expansion ratio (λ): 40% or more The hole expansion ratio may be 40% or more. By setting the hole expansion ratio to 40% or more, it can be suitably applied to automobile parts. There is no need to particularly limit the upper limit of the hole expansion ratio, but it may be 80% or less.
The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

Tensile bending properties can be evaluated by performing a tension bending test by the method shown in Fig. 1. In this embodiment, when the length of the steel sheet before the test is _L0 and the length of the steel sheet at the time of fracture is _LMAX , _LMAX / _L0 when the tension bending test is performed in the rolling direction or width direction is used as an index of the tension bending properties.
In the tension bending test in the rolling direction, the rolling direction is arranged in the direction of L ₀ in FIG. 1, the punch is pressed down, and the amount of pressing h when the steel sheet breaks is measured. Here, the amount of pressing h is the stroke amount (mm) of the punch from when the punch contacts the steel sheet to when it breaks. L _MAX is calculated as twice the length L in FIG. 1 (L _MAX =2L). The length L is calculated by the formula "L=h/cos θ", and θ, which is the angle between the pressing direction of the punch and the sheet surface of the steel sheet at the time of breaking, is calculated by the formula "θ=arctan(h/(L _o /2))". It should be noted that the steel sheet is judged to have broken if the pressing load of the punch is reduced by 20% or more within 1 second. The tension bending test in the width direction is the same as the tension bending test in the rolling direction, except that the width direction is arranged in the direction of L _o .

When the tensile strength is less than 1040 MPa, _LMAX / _L0 may be 1.028 or more when a tension bending test is performed in the rolling direction. When the tensile strength is less than 1040 MPa, if _LMAX / _L0 is 1.028 or more when a tension bending test is performed in the rolling direction, it can be determined that the steel sheet has excellent tension bending properties in the rolling direction.
Furthermore, when the tensile strength is less than 1040 MPa, if L _MAX /L ₀ is 1.028 or more when a tension bending test is carried out in the width direction, it can be determined that the material has excellent tension bending properties also in the width direction.

When the tensile strength is 1040 MPa or more, _LMAX / _L0 may be 1.018 or more when a tension bending test is performed in the rolling direction. When the tensile strength is 1040 MPa or more, if _LMAX / _L0 is 1.018 or more when a tension bending test is performed in the rolling direction, it can be determined that the steel sheet has excellent tension bending properties in the rolling direction.
Furthermore, when the tensile strength is 1040 MPa or more, if L _MAX /L ₀ is 1.018 or more when a tension bending test is carried out in the width direction, it can be determined that the material has excellent tension bending properties also in the width direction.

The tension bending test shown in Fig. 1 is performed under the following conditions. The pressing force applied by the blank holder may be such that the hot-rolled steel sheet does not move. The initial thickness _t0 of the hot-rolled steel sheet before the test is 1.5 mm. When the thickness of the hot-rolled steel sheet is thicker than 1.5 mm, one surface of the hot-rolled steel sheet is mechanically ground to a thickness of 1.5 mm, and the mechanically ground surface is then placed on the punch side to perform the test.
Furthermore, when the thickness of the hot-rolled steel plate is thinner than 1.5 mm, the test is performed without mechanical grinding. However, when the thickness of the hot-rolled steel plate is thinner than 1.5 mm, the tensile bending properties are evaluated using the index L _MAX /L ₀ - (1.5 - t ₀ ) x 0.0242 instead of L _MAX /L ₀ .

L ₀ : Length of steel plate before test Punch speed P: 3 mm/min
Die span Dd: 100 mm
Punch tip radius Rp: 20 mm
Die tip radius Rd: 20 mm

In addition, since the desired strength, ductility, and tensile bending properties differ in the first and second embodiments described above, each embodiment may have the following strength, ductility, and tensile bending properties. Note that the desired hole expansion properties are the same in both embodiments, so explanations are omitted.

(First embodiment) Tensile strength: 940 MPa or more, uniform elongation: 4.0% or more In the first embodiment, the tensile strength may be 940 MPa or more, and the uniform elongation may be 4.0% or more. In the first embodiment, the tensile strength may be 980 MPa or less. In addition, in the first embodiment, the uniform elongation may be 5.0% or less.

(First Aspect) Tension Bending Property In the first aspect, L _MAX /L ₀ may be set to 1.028 or more when a tension bending test is carried out in the rolling direction.
In the first aspect, L _MAX /L ₀ may be 1.028 or more when a tension bending test is performed in the width direction.

(Second embodiment) Tensile strength: 1040 MPa or more, uniform elongation: 3.0% or more In the second embodiment, the tensile strength may be 1040 MPa or more, and the uniform elongation may be 3.0% or more. In the second embodiment, the tensile strength may be 1080 MPa or less. In the second embodiment, the uniform elongation may be 4.0% or less.

(Second Aspect) Tension Bending Property In the second aspect, L _MAX /L ₀ may be set to 1.018 or more when a tension bending test is carried out in the rolling direction.
In the second aspect, L _MAX /L ₀ may be 1.018 or more when a tension bending test is performed in the width direction.

The hot-rolled steel sheet according to this embodiment may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of electroplating layers include electrogalvanizing and electrogalvanizing Zn-Ni alloy plating. Examples of hot-dip plating layers include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. There is no particular restriction on the amount of plating applied, and it may be the same as in the past. In addition, it is also possible to further improve corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

Next, a preferred manufacturing method for the hot-rolled steel sheet according to this embodiment will be described. According to the manufacturing method described below, the hot-rolled steel sheet according to this embodiment can be stably manufactured. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

Steps (1) to (3) described below are common to the first and second aspects. As for the subsequent steps, steps (4) and (5) correspond to the first aspect, and step (6) corresponds to the second aspect.

A preferred method for producing a hot-rolled steel sheet according to this embodiment is as follows:
(1) before rough rolling, a step of applying strain to a slab having the above-mentioned chemical composition one or more times so that the width direction strain is 3 to 15% in total;
(2) performing rough rolling on the strained slab;
(3) performing finish rolling so that the difference in the entry temperature between the immediately preceding final pass and the final pass is 30° C. or more and the finish rolling completion temperature is in a temperature range of 920° C. or more;
Furthermore, the method includes one or more of the following steps (4) to (6).
(4) After the completion of finish rolling, accelerated cooling is performed to a temperature range of 580 to 680°C at an average cooling rate of 30°C/s or more, and slow cooling (air cooling) is performed in this temperature range for 2.0 seconds or more.
(5) After the slow cooling is completed, a step of accelerated cooling down to 300°C at an average cooling rate of 30°C/s or more.
(6) After the completion of the finish rolling, a step of accelerated cooling down to 300°C at an average cooling rate of 30°C/s or more.
Each step will be described below.

(1) Imparting strain before rough rolling: common to the first and second embodiments Before rough rolling, a slab having the above-mentioned chemical composition is subjected to one or more strains so that the total width direction strain is 3 to 15%. This can reduce the unevenness of the surface layer of the slab while increasing the uniformity of the texture. The imparting of strain may be performed after the slab is heated for rough rolling.
If the strain imparted in the width direction of the slab is less than 3% or more than 15% in total, the peak positions of the maximum values A and B in the texture of the hot-rolled steel sheet may not be controlled within a preferred range. Here, the "width direction of the slab" refers to a direction perpendicular to the transport direction and thickness direction of the slab, and the transport direction of the slab corresponds to the rolling direction in the subsequent process.

Furthermore, by applying strain to the slab in the width direction not just once but multiple times, it is possible to suppress significant changes in the texture of the hot-rolled steel sheet in the width direction. As a result, it is possible to reduce the absolute value of the difference between maximum value A and maximum value B at a position 1/4 of the way from the end face of the hot-rolled steel sheet in the width direction, a position 1/4-15 mm from the end face in the width direction, and a position 1/4+15 mm from the end face in the width direction.

The total strain imparted to the slab in the width direction can be expressed as (1-w1/w0) x ₁₀₀ (%), where _{w0 is} the width direction length of the slab before the first strain is imparted, and _w1 is the width direction length of the slab after the final strain is imparted.
A method for imparting strain in the width direction of a slab includes, for example, passing the slab between rolls whose rotation axes are perpendicular to the plate surface and conveying direction of the slab to impart strain in the width direction to the slab (pressing down in the width direction).

The slab to which strain is applied is not particularly limited except for the chemical composition described above. For example, a slab produced by melting molten steel of the above chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may be used. When heating the slab before rough rolling, the heating temperature may be in the range of 1100 to 1300°C.

(2) Rough rolling: common to the first and second aspects The conditions of rough rolling are not particularly limited, and the rough rolling can be, for example, a process in which rolling is performed multiple times at a temperature of 1100 ° C. or higher to reduce the plate thickness to 30 to 60 mm.

(3) Finish rolling: common to the first and second aspects In the finish rolling process, finish rolling is performed so that the difference in the entry temperature between the pass immediately before the final pass and the final pass is 30°C or more, and the finish rolling completion temperature is in a temperature range of 920°C or more. If the difference in the entry temperature between the pass immediately before the final pass and the final pass is less than 30°C, the peak positions of maximum value A and maximum value B in the texture of the hot-rolled steel sheet may not be controlled to a preferred range. Also, if the finish rolling completion temperature is less than 920°C, maximum value A and maximum value B cannot be controlled to a preferred value.
Examples of a method for making the difference in the inlet temperature between the immediately preceding final pass and the final pass 30°C or more include controlling the amount of coolant such as water sprayed from a cooling device such as a cooling spray immediately after rolling, or controlling the conveying speed of the steel sheet during rolling.

The pass immediately before the final pass refers to the pass immediately before the final pass. For example, if the finish rolling is performed in passes F1, F2, ... F6, and F7, the pass immediately before the final pass refers to the pass F6.
The finish rolling completion temperature refers to the temperature at the outlet of the final pass of finish rolling.

(4) Slow cooling (air cooling) in a temperature range of 580 to 680°C: corresponds to the first embodiment. After the completion of finish rolling, accelerated cooling is performed to a temperature range of 580 to 680°C at an average cooling rate of 30°C/s or more, and slow cooling (air cooling) is performed in this temperature range for 2.0 seconds or more. By performing slow cooling (air cooling) in a temperature range of 580 to 680°C for 2.0 seconds or more, the area ratio of the region where the GAM value is more than 0.6° and less than 2.0° can be increased.
In this embodiment, slow cooling (air cooling) refers to cooling at an average cooling rate of 20° C./s or less.

(5) Accelerated cooling after completion of slow cooling (air cooling): corresponds to the first embodiment After completion of slow cooling (air cooling) in the temperature range of 580 to 680° C. (first embodiment), accelerated cooling is performed at an average cooling rate of 30° C./s or more until the temperature reaches 300° C. After completion of slow cooling (air cooling), accelerated cooling is performed at an average cooling rate of 30° C./s or more until the temperature reaches 300° C., whereby a desired metal structure can be obtained.
After accelerated cooling to 300° C., the wire may be left to cool to room temperature or may be wound into a coil and then water-cooled.

(6) Accelerated cooling down to 300° C.: Corresponding to the second embodiment After the completion of finish rolling, accelerated cooling is performed at an average cooling rate of 30° C./s or more down to 300° C. By performing accelerated cooling down to 300° C. at an average cooling rate of 30° C./s or more without performing slow cooling (air cooling) during the accelerated cooling, the area ratio of the region having a GAM value of 2.0° or more can be increased.
After accelerated cooling to 300° C., the wire may be left to cool to room temperature or may be wound into a coil and then water-cooled.

In this embodiment, the average cooling rate is the temperature difference between the start and end points of the set range divided by the elapsed time from the start point to the end point.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Slabs having the chemical compositions shown in Tables 1A to 2B were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets having a thickness of 3.0 mm were produced under the conditions shown in Tables 3A to 3C.
In addition, blank cells in Tables 1A to 2B indicate that the element was not intentionally added.

In Tables 3A to 3C, if the difference in inlet temperature between the pass immediately before the final pass and the final pass was 30°C or more, the column for that condition was marked "OK," and if that condition was not met, it was marked "NG."

　For production numbers 1 to 20 and 29 to 49, after the completion of finish rolling, accelerated cooling was performed at an average cooling rate of 30°C/s or more to the "start temperature of slow cooling" in the table. Slow cooling was performed by air cooling, and the average cooling rate during slow cooling was 20°C/s or less. After slow cooling, accelerated cooling was performed at the "average cooling rate after completion of slow cooling until the temperature reaches 300°C" in the table. After accelerated cooling was stopped, coiling was performed immediately.

For production numbers 21 to 28 and 50 to 67, after the completion of finish rolling, accelerated cooling was not performed without slow cooling, but rather at the "average cooling rate after the completion of finish rolling until the temperature reaches 300°C" in the table. After the accelerated cooling was stopped, coiling was performed immediately.

The obtained hot-rolled steel sheets were evaluated for texture, metal structure, tensile strength (TS), uniform elongation (uEl), hole expansion ratio (λ) and tensile bending properties (L _MAX /L ₀ in the rolling direction (L direction) and L _MAX /L ₀ in the width direction (C direction)) by the above-mentioned methods.
The results obtained are shown in Tables 4A to 5C. However, for examples in which the thickness of the hot-rolled steel sheet was thinner than 1.5 mm, the value L _MAX /L ₀ - (1.5 - t ₀ ) x 0.0242 was recorded instead of L _MAX /L ₀ .

In addition, if "in the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 μm in the sheet thickness direction, the absolute value of the difference between maximum value A and maximum value B at the position 1/4 in the width direction from the end face, the position 1/4-15 mm in the width direction from the end face, and the position 1/4+15 mm in the width direction from the end face was 3.0 or less," then "OK" was entered in the column in the table that reads "The absolute value of the difference between maximum value A and maximum value B at each of the three positions in the width direction is 3.0 or less." On the other hand, if the above absolute value exceeded 3.0, then "NG" was entered in that column.

If the tensile strength (TS) was 940 MPa or more, it was judged to have high strength and to have passed. On the other hand, if the tensile strength (TS) was less than 940 MPa, it was judged to have low strength and to have failed.

If the uniform elongation (uEl) was 3.0% or more, it was judged to have excellent ductility and to have passed. On the other hand, if the uniform elongation (uEl) was less than 3.0%, it was judged to have no excellent ductility and to have failed.

If the hole expansion ratio (λ) was 40% or more, it was judged to have excellent hole expansion properties and to pass. On the other hand, if the hole expansion ratio (λ) was less than 40%, it was judged to have no excellent hole expansion properties and to fail.

The tensile bending properties were evaluated according to the following criteria depending on the tensile strength.
When the tensile strength is less than 1,040 MPa, the tensile bending property (L _MAX /L ₀ in the rolling direction (L direction)): 1.028 or more is pass, and less than 1.028 is fail.
Furthermore, when L _MAX /L ₀ in the width direction (C direction) was 1.028 or more, it was determined that the sheet had excellent tensile bending properties in the width direction as well.
When the tensile strength is 1040 MPa or more, the tensile bending property (L _MAX /L ₀ in the rolling direction (L direction)): 1.018 or more is pass, and less than 1.018 is fail.
When L _MAX /L ₀ in the width direction (C direction) was 1.018 or more, it was determined that the sheet had excellent tensile bending properties also in the width direction.

From Tables 4A to 5C, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength, as well as excellent ductility and hole expandability, and also have excellent tensile bending properties in the rolling direction.
On the other hand, it is seen that the steel sheets according to the comparative examples are inferior in at least one of the characteristics.

According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent tensile bending properties in the rolling direction.
Furthermore, according to a preferred embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having not only the above-mentioned properties but also excellent tensile bending properties in the width direction.

Claims (6)

The chemical composition, in mass%, is
C: 0.045-0.120%,
Si: 0-3.00%,
Mn: 1.20-2.60%,
Ti: 0.020 to 0.180%,
Al: 0.010-0.400%,
P: 0.080% or less,
S: 0.0100% or less,
N: 0.0050% or less,
O: 0.010% or less,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Cu: 0 to 1.000%,
Cr: 0-2.000%,
Mo: 0-3.000%,
Ni: 0 to 0.500%,
B: 0 to 0.0100%,
Ca: 0-0.0500%,
Mg: 0 to 0.0500%,
REM: 0-0.100%,
Bi: 0-0.100%,
Ta: 0-0.100%,
Zr: 0 to 0.500%,
Co: 0-3.000%,
Zn: 0-0.200%,
W: 0-0.200%,
Sb: 0 to 0.500%,
Contains As: 0 to 0.050% and Sn: 0 to 0.050%;
The balance is Fe and impurities,
In the crystal orientation distribution function of the texture in the region from the end face to a 1/4 position in the width direction and from the surface to a depth of 500 μm in the sheet thickness direction,
The maximum value A in the range of Φ=0 to 60° and Φ= ₅₀ to 90° at the cross section of φ ₂ =45° is 6.0 or less,
the maximum value B in the range of Φ=120 to 180° and Φ= ₅₀ to 90° at the φ ₂ =45° cross section is 6.0 or less;
When the peak position of Φ where the maximum value A is located is defined as Φ _A , and the peak position of Φ where the maximum value B is located is defined as Φ _B , |Φ _A -35°| is 10° or less, and |Φ _B -145°| is 10° or less;
In the metal structure at a position of 1/4 depth from the surface in the plate thickness direction,
The area ratio of the region having a GAM value exceeding 0.6° is 50% or more,
A hot-rolled steel sheet, characterized in that the sum of an area ratio of the region in which the GAM value exceeds 3.0° and an area ratio of retained austenite is less than 15%. The chemical composition, in mass%,
Nb: 0.001-0.100%,
V: 0.001 to 1.000%,
Cu: 0.001 to 1.000%,
Cr: 0.001-2.000%,
Mo: 0.001-3.000%,
Ni: 0.001 to 0.500%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0500%,
Mg: 0.0001-0.0500%,
REM: 0.001-0.100%,
Bi: 0.001-0.100%,
Ta: 0.001-0.100%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 3.000%,
Zn: 0.001-0.200%,
W: 0.001-0.200%,
Sb: 0.001 to 0.500%,
As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%
The hot-rolled steel sheet according to claim 1, further comprising at least one selected from the group consisting of: In the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 μm in the sheet thickness direction,
The hot-rolled steel sheet according to claim 1, characterized in that an absolute value of a difference between the maximum value A and the maximum value B is 3.0 or less at a 1/4 position from the end face in the width direction, a 1/4-15 mm position from the end face in the width direction, and a 1/4+15 mm position from the end face in the width direction. In the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 μm in the sheet thickness direction,
The hot-rolled steel sheet according to claim 2, characterized in that an absolute value of a difference between the maximum value A and the maximum value B is 3.0 or less at a 1/4 position from the end face in the width direction, a 1/4-15 mm position from the end face in the width direction, and a 1/4+15 mm position from the end face in the width direction. In the metal structure at a position of ¼ depth from the surface in the plate thickness direction,
The hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that an area ratio of the region in which the GAM value is more than 0.6° and less than 2.0° is 50% or more. In the metal structure at a position of ¼ depth from the surface in the plate thickness direction,
The hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that an area ratio of the region having the GAM value of 2.0° or more is 50% or more.

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