WO2024162388A1

WO2024162388A1 - Hot-rolled steel sheet

Info

Publication number: WO2024162388A1
Application number: PCT/JP2024/003062
Authority: WO
Inventors: 隆安富; 良洋村井; 菜緒子加藤; 玄紀虻川; 邦夫林; 昌史東
Original assignee: 日本製鉄株式会社
Priority date: 2023-01-31
Filing date: 2024-01-31
Publication date: 2024-08-08

Abstract

Disclosed is a hot-rolled steel sheet that excels in a balance between strength, elongation, and hole expandability, and that also excels in a collision characteristic. The hot-rolled steel sheet according to the present disclosure has a specified chemical composition, and: has a prior austenite grain size of 25 μm or less; has an area ratio of a region having a GAM value greater than 0.6° and less than 2.0° of 50% or greater, and less than 100%; has an area ratio of a region having a GAM value of 0.6° or less of 0% or greater and less than 50%; has an area ratio of a region having a GAM value of 2.0° or greater of greater than 0% and 50% or less; and satisfies the relationships 1.7≤LGr/LGt and 1.20≤(LGr/LGt)/(LMr/LMt). Here, LGr is the area average value for the rolling-direction projection length of the prior austenite grains, LGt is the area average value for the sheet-thickness-direction projection length of the prior austenite grains, LMr is the area average value for the rolling-direction projection length of the region having the GAM value of 2.0° or greater, and LMt is the area average value for the sheet-thickness-direction projection length of the region having the GAM value of 2.0° or greater.

Description

Hot-rolled steel

This application discloses a hot-rolled steel sheet.

Hot-rolled steel sheets are used as materials for automobile suspension parts, structural parts, frameworks, frame parts, etc. For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent punching workability, which has a specified composition and a specified steel structure.

JP 2012-062562 A

Conventional hot-rolled steel sheets have room for improvement in terms of the balance of strength, elongation and hole expansion, as well as crash properties.

As a result of intensive research, the inventors have discovered that by (1) using a slab with an appropriate chemical composition, (2) controlling the morphology of prior austenite grains by adjusting the finish rolling conditions during hot rolling, and (3) controlling the transformation behavior by controlling the cooling rate during cooling (for example, controlling the cooling conditions on the runout table), it is possible to manufacture a hot-rolled steel sheet that has an excellent balance of strength, ductility, and hole expandability, and also has excellent impact properties. Furthermore, the inventors have discovered that the hot-rolled steel sheet manufactured in this manner has a predetermined chemical composition and a characteristic steel structure, and thus has an excellent balance of strength, ductility, and hole expandability, and also has excellent impact properties.

Based on the above findings, the present application discloses the following aspects as means for solving the above problems.
<Aspect 1>
In mass percent,
C: 0.045% or more, 0.120% or less,
Si: 0% or more, 3.00% or less,
Mn: 1.20% or more, 2.60% or less,
Ti: 0.020% or more, 0.180% or less,
Al: 0.010% or more, 0.400% or less,
P: 0% or more, 0.080% or less,
S: 0% or more, 0.0100% or less,
N: 0% or more, 0.0050% or less,
O: 0% or more, 0.010% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Cr: 0% or more, 2.000% or less,
Mo: 0% or more, 3.000% or less,
Ni: 0% or more, 0.500% or less,
B: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0500% or less,
Mg: 0% or more, 0.050% or less,
REM: 0% or more, 0.100% or less,
Bi: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Zr: 0% or more, 0.500% or less,
Co: 0% or more, 3.000% or less,
Zn: 0% or more, 0.200% or less,
W: 0% or more, 0.200% or less,
Sb: 0% or more, 0.500% or less,
As: 0% or more, 0.050% or less, and Sn: 0% or more, 0.050% or less;
The balance is composed of Fe and impurities.
The prior austenite grain size is 25 μm or less,
The area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%,
The area ratio of the region having a GAM value of 0.6° or less is 0% or more and less than 50%,
The area ratio of the region having a GAM value of 2.0° or more is more than 0% and 50% or less,
The following relationships (1) and (2):
1.7≦LGr/LGt…(1)
1.20≦(LGr/LGt)/(LMr/LMt)…(2)
LGr: average area of the projection length in the rolling direction of the prior austenite grains LGt: average area of the projection length in the sheet thickness direction of the prior austenite grains LMr: average area of the projection length in the rolling direction of the region having a GAM value of 2.0° or more LMt: average area of the projection length in the sheet thickness direction of the region having a GAM value of 2.0° or more
Hot-rolled steel sheet.
<Aspect 2>
The area ratio of the region having a GAM value of 0.6° or less is 0% or more and 45% or less.
The hot-rolled steel sheet of embodiment 1.
<Aspect 3>
The area ratio of the region having a GAM value of 2.0° or more is more than 0% and 20% or less.
The hot-rolled steel sheet according to embodiment 1 or 2.
<Aspect 4>
The following relationship (1-1):
1.7≦LGr/LGt≦10.0…(1-1)
is satisfied,
The hot-rolled steel sheet according to any one of aspects 1 to 3.
<Aspect 5>
The following relationship (2-1):
1.20≦(LGr/LGt)/(LMr/LMt)≦5.00…(2-1)
is satisfied,
The hot-rolled steel sheet according to any one of aspects 1 to 4.

The hot-rolled steel sheet disclosed herein has an excellent balance of strength, elongation, and hole expansion properties, and also has excellent impact properties.

1. Hot-rolled steel sheet Hereinafter, one embodiment of a hot-rolled steel sheet will be described, but the hot-rolled steel sheet of the present disclosure is not limited to the following embodiment.

The hot-rolled steel sheet of the present disclosure has, in mass%,
C: 0.045% or more, 0.120% or less,
Si: 0% or more, 3.00% or less,
Mn: 1.20% or more, 2.60% or less,
Ti: 0.020% or more, 0.180% or less,
Al: 0.010% or more, 0.400% or less,
P: 0% or more, 0.080% or less,
S: 0% or more, 0.0100% or less,
N: 0% or more, 0.0050% or less,
O: 0% or more, 0.010% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Cr: 0% or more, 2.000% or less,
Mo: 0% or more, 3.000% or less,
Ni: 0% or more, 0.500% or less,
B: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0500% or less,
Mg: 0% or more, 0.050% or less,
REM: 0% or more, 0.100% or less,
Bi: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Zr: 0% or more, 0.500% or less,
Co: 0% or more, 3.000% or less,
Zn: 0% or more, 0.200% or less,
W: 0% or more, 0.200% or less,
Sb: 0% or more, 0.500% or less,
As: 0% or more, 0.050% or less, and Sn: 0% or more, 0.050% or less;
The balance is composed of Fe and impurities.
In the heat-rolled steel sheet of the present disclosure, the prior austenite grain size is 25 μm or less.
In the hot-rolled steel sheet disclosed herein, the area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%, the area ratio of the region having a GAM value of 0.6° or less is 0% or more and less than 50%, and the area ratio of the region having a GAM value of 2.0° or more is more than 0% and less than 50%.
In the hot-rolled steel sheet of the present disclosure, the following relationships (1) and (2):
1.7≦LGr/LGt…(1)
1.20≦(LGr/LGt)/(LMr/LMt)…(2)
LGr: average area of the projection length in the rolling direction of prior austenite grains LGt: average area of the projection length in the sheet thickness direction of prior austenite grains LMr: average area of the projection length in the rolling direction of regions having a GAM value of 2.0° or more LMt: the average area of the projection length in the sheet thickness direction of regions having a GAM value of 2.0° or more is satisfied.

1.1 Chemical Composition The reasons for limiting the chemical composition of the hot-rolled steel sheet will be described below. In the following description, "%" for each component means mass %.

(C: 0.045% or more, 0.120% or less)
C is an element that increases the strength of a hot-rolled steel sheet. If the C content is too low, the region in which the GAM value is 0.6° or less becomes excessive, and the strength of the hot-rolled steel sheet is likely to decrease. If the content is too high, the region in which the GAM value is 2.0° or more becomes excessive, and the elongation and hole expandability of the hot-rolled steel sheet are likely to decrease. By setting the C content to 0.045% or more and 0.120% or less, the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet is improved. It may be 0.060% or more, or 0.115% or less, 0.110% or less, 0.105% or less, or 0.100% or less.

(Si: 0% or more, 3.00% or less)
Silicon acts as a deoxidizer, affects the morphology of carbides, and increases the tensile strength of the hot-rolled steel sheet. On the other hand, if the Si content is too high, there is a risk that hot rolling may become difficult due to insufficient ductility. In the steel sheet, when the Si content is 0% or more and 3.00% or less, the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet is improved. The Si content is more than 0%, 0.001% or more, 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.100% or more, and 0.200% or more. , 0.300% or more, 0.400% or more, or 0.500% or more, and 2.50% or less, 2.00% or less, 1.80% or less, or 1.50% or less. Good too.

(Mn: 1.20% or more, 2.60% or less)
Mn is an element that can increase the tensile strength of a hot-rolled steel sheet. If the Mn content is too low, the region with a GAM value of 0.6° or less becomes excessive, and the strength of the hot-rolled steel sheet is likely to decrease. On the other hand, if the Mn content is too high, the region in which the GAM value is 2.0° or more becomes excessive, and the elongation of the hot-rolled steel sheet is likely to decrease. By setting the Mn content to 20% or more and 2.60% or less, the balance of strength, elongation and hole expandability of the hot rolled steel sheet is improved. It may be 35% or more, or 1.40% or more, and may be 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

(Ti: 0.020% or more, 0.180% or less)
Ti is a strengthening element and can contribute to increasing the strength of the hot-rolled steel sheet by precipitation strengthening, fine grain strengthening, and/or dislocation strengthening. Ti is also an element that can act as a nucleus for transformation. In other words, in the hot-rolled steel sheet of the present disclosure, TiC is precipitated at a high density, and this can function as a nucleus for transformation. If the Ti content is too low, this function is not exerted, and the old On the other hand, if the Ti content is too high, excessive precipitates are formed, and the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheet is easily deteriorated. In the hot-rolled steel sheet of the present disclosure, the Ti content is 0.020% or more and 0.180% or less, so that the strength, elongation and hole expandability of the hot-rolled steel sheet are easily deteriorated. The spreadability balance is improved. The Ti content may be 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more, and may be 0.175% or less, 0.170% or less, 0.165% or less. It may be 0.160% or less or 0.160% or less.

(Al: 0.010% or more, 0.400% or less)
Al is an element that acts as a deoxidizer. If the Al content is too low, deoxidization is likely to be insufficient, inclusions are likely to be excessively generated, and the hole expandability of the hot-rolled steel sheet is likely to decrease. On the other hand, if the Al content is too high, cracks in the slab may occur, making hot rolling difficult. By setting the Al content to 400% or less, cracks in the slab are suppressed, and the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheet is improved. % or more, 0.040% or more, or 0.050% or more, and may be 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less.

(P: 0% or more, 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, the embrittlement may occur. In the hot-rolled steel sheet of the present disclosure, the P content is 0% or more and 0.080% or less, so that the slab can be easily cracked and hot-rolled. The P content is 0.001% or more, 0.002% or more, 0.003% or more, or 0.004% or more. It may be 0.004% or more, or 0.050% or less, 0.030% or less, 0.015% or less, or 0.010% or less.

(S: 0% or more, 0.0100% or less)
S is an element that generates inclusions such as MnS in steel and reduces the ductility of the hot-rolled steel sheet. If the S content is too high, excessive inclusions are generated, which reduces the hole expansion properties of the hot-rolled steel sheet. In the hot-rolled steel sheet of the present disclosure, the S content is 0% or more and 0.0100% or less, so that the balance of the strength, elongation and hole expandability of the hot-rolled steel sheet is improved. The S content may be 0.0001% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more, and may be 0.0090% or less, 0.0075% or less, 0. It may be 0.0060% or less or 0.0050% or less.

(N: 0% or more, 0.0050% or less)
N is an element that forms coarse nitrides in steel and reduces the workability of hot-rolled steel sheets. If the N content is too high, excessive nitrides are generated, and the workability of the hot-rolled steel sheets is reduced. The elongation and hole expandability are likely to decrease, and furthermore, there is a risk that cracks in the slab due to embrittlement may occur, making hot rolling difficult. By setting the N content to 0.0050% or less, cracking of the slab is suppressed, and the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheet is improved. , 0.0005% or more, 0.0010% or more, or 0.0015% or more, and 0.0048% or less, 0.0045% or less, 0.0042% or less, or 0.0040% or less. Good too.

(O: 0% or more, 0.010% or less)
O is an element that forms oxides and reduces the workability of hot-rolled steel sheets. If the O content is too high, oxides are generated in excess, and the hole expandability of the hot-rolled steel sheet is reduced. In the hot-rolled steel sheet of the present disclosure, the O content is 0% or more and 0.010% or less, so that the balance of the strength, elongation, and hole expandability of the hot-rolled steel sheet is improved. The content may be 0.001% or more, and may be 0.008% or less, 0.006% or less, 0.005% or less, or 0.004% or less.

The basic chemical composition of the hot-rolled steel sheet disclosed herein is as described above. Furthermore, the hot-rolled steel sheet disclosed herein may contain at least one of the following elements as necessary. These elements do not necessarily need to be contained, so the lower limit of their content is 0%.

(Nb: 0% or more, 0.100% or less)
Nb is an element effective for controlling the morphology of carbides, similar to Ti, and may be added as desired. On the other hand, if the Nb content is too high, the effect becomes saturated and there is a risk of precipitation. In the hot-rolled steel sheet of the present disclosure, the Nb content is 0% or more and 0.100% or less. The Nb content is 0.001% or more, 0.003% or more, 0.005% or more, or It may be 0.007% or more, and may be 0.090% or less, 0.070% or less, 0.050% or less, 0.045% or less, 0.040% or less, 0.035% or less, or 0.030% or less. % or less.

(V: 0% or more, 1.000% or less)
V is an element that can contribute to increasing the strength of a hot-rolled steel sheet by precipitation strengthening, fine grain strengthening, and/or dislocation strengthening, and may be added as desired. On the other hand, if the V content is too high, In addition, the effect of the present invention may become saturated, and precipitates may be formed. In the hot-rolled steel sheet of the present disclosure, the V content is 0% or more and 1.000% or less. The V content is 0.001% or more. , 0.003% or more, 0.005% or more, or 0.007% or more, and may be 0.900% or less, 0.700% or less, 0.500% or less, 0.300% or less, 0. It may be 250% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

(Cu: 0% or more, 1.000% or less)
Cu is an element that can contribute to improving at least one of strength and corrosion resistance, and may be added as desired. On the other hand, excessive Cu content may lead to a decrease in toughness, etc. In the hot-rolled steel sheet, the Cu content is 0% or more and 1.000% or less. The Cu content may be 0.001% or more, 0.005% or more, or 0.010% or more, It may be 0.800% or less, 0.600% or less, 0.400% or less, 0.350% or less, 0.250% or less, or 0.150% or less.

(Cr: 0% or more, 2.000% or less)
Cr is an element that can improve the hardenability of steel and contribute to improving at least one of strength and corrosion resistance, and may be added as desired. On the other hand, an excessive Cr content increases the alloy cost and In the hot-rolled steel sheet of the present disclosure, the Cr content is 0% or more and 2.000% or less. The Cr content is 0.001% or more and 0.005% or less. or 0.010% or more, and may be 1.500% or less, 1.000% or less, 0.800% or less, 0.700% or less, 0.600% or less, or 0.500% or less. This is also fine.

(Mo: 0% or more, 3.000% or less)
Mo is an element that can improve the hardenability of steel and contribute to improving at least one of strength and corrosion resistance, and may be added as desired. On the other hand, if Mo is contained in excess, deformation resistance during processing increases. In the hot-rolled steel sheet of the present disclosure, the Mo content is 0% or more and 3.000% or less. % or more, 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.600% or less, 0.500% or less, 0.400% or less, or It may be 0.300% or less.

(Ni: 0% or more, 0.500% or less)
Ni is an element that can improve the hardenability of steel and contribute to improving at least one of strength and heat resistance, and may be added as desired. On the other hand, if Ni is contained in an excessive amount, the effect becomes saturated and There is a risk of an increase in manufacturing costs. In the hot-rolled steel sheet of the present disclosure, the Ni content is 0% or more and 0.500% or less. The Ni content is 0.001% or more and 0.005% or less. or 0.010% or more, and may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, 0.200% or less, or 0 It may be 150% or less.

(B: 0% or more, 0.0100% or less)
B is an element beneficial for increasing the strength of steel and may be added as desired. In the hot-rolled steel sheet of the present disclosure, the B content is 0% or more and 0.0100% or less. The amount may be 0.0001% or more, 0.0003% or more, or 0.0005% or more, and may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, It may be 0.0030% or less, or 0.0025% or less.

(Ca: 0% or more, 0.0500% or less)
Ca is an element capable of controlling the morphology of sulfides and may be added as desired. However, if an excessive amount of Ca is added, the effect becomes saturated and there is a risk of an increase in the manufacturing cost. In the hot-rolled steel sheet of the present disclosure, the Ca content is 0% or more and 0.0500% or less. The Ca content is 0.0001% or more, 0.0003% or more, or 0.0005% or more. 0.0300% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% It may be the following.

(Mg: 0% or more, 0.050% or less)
Mg is an element that can contribute to controlling the morphology of sulfides and may be added as desired. On the other hand, if Mg is contained in an excessive amount, there is a risk that the toughness will decrease. The Mg content is 0% or more and 0.050% or less. The Mg content may be 0.001% or more and 0.040% or less, 0.030% or less, 0.020% or less. It may be 0.015% or less, 0.010% or less, or 0.005% or less.

(REM: 0% or more, 0.100% or less)
REM is an element that can control the morphology of sulfides by adding a small amount of it, similar to Ca, and may be added as desired. On the other hand, if REM is contained in an excessive amount, there is a risk of coarse inclusions being generated. In the hot-rolled steel sheet of the present disclosure, the REM content is 0% or more and 0.100% or less. The REM content is 0.001% or more, 0.003% or more, or 0.005% or more. The REM content may be 0.080% or less, 0.060% or less, or 0.040% or less. The "REM content" is the total content of these elements. It is.

(Bi: 0% or more, 0.100% or less)
Bi is an element that can contribute to improving corrosion resistance and the like, and may be added as desired. On the other hand, if Bi is contained in an excessive amount, the effect becomes saturated and there is a risk of an increase in manufacturing costs. In the disclosed hot-rolled steel sheet, the Bi content is 0% or more and 0.100% or less. The Bi content may be 0.001% or more or 0.002% or more, and may be 0.070% or less. It may be 0.050% or less, 0.030% or less, 0.010% or less, 0.008% or less, 0.006% or less, or 0.004% or less.

(Ta: 0% or more, 0.100% or less)
Ta is an element that can contribute to controlling the morphology of carbides and increasing strength, and may be added as desired. However, if Ta is contained in excess, there is a risk of the toughness decreasing due to the precipitation of Ta carbides, etc. In the hot-rolled steel sheet of the present disclosure, the Ta content is 0% or more and 0.100% or less. The Ta content is 0.001% or more, 0.005% or more, or 0.010% or more. Alternatively, it may be 0.080% or less, 0.060% or less, or 0.040% or less.

(Zr: 0% or more, 0.500% or less)
Zr is an element that can contribute to controlling the morphology of sulfides and may be added as desired. However, if Zr is contained in an excessive amount, the effect becomes saturated and there is a risk of an increase in production costs. In the hot-rolled steel sheet of the present disclosure, the Zr content is 0% or more and 0.500% or less. The Zr content is 0.001% or more, 0.005% or more, or 0.010% or more. Alternatively, it may be 0.400% or less, 0.300% or less, or 0.200% or less.

(Co: 0% or more, 3.000% or less)
Co is an element that can contribute to improving at least one of hardenability and heat resistance, and may be added as desired. On the other hand, if Co is contained in an excessive amount, there is a risk that workability will decrease and the raw material cost will increase. In the hot rolled steel sheet of the present disclosure, the Co content is 0% or more and 3.000% or less. It may be 0.030% or more or 0.050% or more, and may be 2.000% or less, 1.000% or less, 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less. , 0.180% or less, 0.160% or less, or 0.140% or less.

(Zn: 0% or more, 0.200% or less)
Zn is an element that can control the morphology of inclusions and may be added as desired. On the other hand, if Zn is contained in an excessive amount, there is a risk that a large amount of precipitates or inclusions will be generated. In the hot-rolled steel sheet, the Zn content is 0% or more and 0.200% or less. The Zn content is 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more. or may be 0.180% or less, 0.160% or less, or 0.140% or less.

(W: 0% or more, 0.200% or less)
W is an element that can improve the hardenability of steel and contribute to improving strength, and may be added as desired. However, if W is contained in an excessive amount, there is a risk of coarse inclusions being generated. In the hot-rolled steel sheet of the present disclosure, the W content is 0% or more and 0.200% or less. The W content is 0.001% or more, 0.010% or more, 0.030% or more, or 0. It may be 0.050% or more, or 0.180% or less, 0.160% or less, or 0.140% or less.

(Sb: 0% or more, 0.500% or less)
Sb is an element that can contribute to improving corrosion resistance and may be added as desired. On the other hand, if Sb is contained in an excessive amount, it may cause a decrease in toughness. The Sb content is 0% or more and 0.500% or less. The Sb content may be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more. It may be 0.400% or less, 0.300% or less, or 0.200% or less.

(As: 0% or more, 0.050% or less)
As is an element that can contribute to improving the machinability of steel and may be added as desired. On the other hand, if As is contained in an excessive amount, there is a risk that the workability will decrease. In the steel sheet, the As content is 0% or more and 0.050% or less. The As content may be 0.001% or more or 0.005% or more, and may be 0.030% or less, 0. It may be 0.010% or less, 0.009% or less, 0.008% or less, or 0.007% or less.

(Sn: 0% or more, 0.050% or less)
Sn is an element that can contribute to improving corrosion resistance and may be added as desired. On the other hand, if Sn is contained in an excessive amount, it may cause a decrease in toughness. The Sn content is 0% or more and 0.050% or less. The Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more, and 0.047% or less. It may be 0.045% or less, or 0.043% or less.

(balance: Fe and impurities)
The chemical composition of the hot-rolled steel sheet of the present disclosure includes the balance other than the above-mentioned components, namely Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing the hot-rolled steel sheet.

The chemical composition of the above-mentioned hot-rolled steel sheet may be analyzed using a spark discharge optical emission spectrometer or similar. Values identified for C and S are determined by burning the sheet in an oxygen stream using a gas composition analyzer or similar and measuring using an infrared absorption method. Values identified for N are determined by melting a test piece taken from the hot-rolled steel sheet in a helium stream and measuring using a thermal conductivity method.

1.2 Prior austenite grain size The prior austenite grain size in the hot-rolled steel sheet of the present disclosure is 25 μm or less. In this way, by refining the prior austenite grains, the concentration of strain in the microstructure is alleviated, the mechanical properties of the hot-rolled steel sheet are improved, and the hole expandability and impact properties of the hot-rolled steel sheet are improved. In particular, when the prior austenite grain size is 20 μm or less, 18 μm or less, 15 μm or less, 12 μm or less, 10 μm or less, or 8 μm or less, the balance of strength, elongation, hole expandability, and impact properties of the hot-rolled steel sheet is likely to be good. The lower limit of the prior austenite grain size is not particularly limited, and may be more than 0 μm, 1 μm or more, 3 μm or more, 5 μm or more, or 7 μm or more.

The "prior austenite grain size" of the hot-rolled steel sheet is the average grain size of the prior austenite grains. The average grain size of the prior austenite grains is measured as follows. First, a sample is taken at a quarter position from the end face in the sheet width direction of the hot-rolled steel sheet so that the metal structure of the cross section (sheet thickness direction x rolling direction cross section) with the sheet width direction as the normal direction can be observed. The size of the sample depends on the measuring device, but for example, it may be a rectangular parallelepiped with the full thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-polished, and then corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. The grains that appear black due to corrosion are considered to be prior austenite grains. The observation surface on which the prior austenite grains are revealed is observed by an optical microscope, and eight or more fields of view with an area of 0.05 ^mm2 or more (total of 0.40 ^mm2 or more) are photographed. Then, the circle-equivalent diameter is calculated for each prior austenite grain from the steel structure photograph taken by the optical microscope. The circle-equivalent diameter is calculated as described above for all prior austenite grains included in each photographed field of view, except for prior austenite grains not entirely included in the photographed field of view, such as the end of the photographed field of view. The average grain size of the prior austenite grains is obtained by calculating the area-average value (average value weighted by area) of the circle-equivalent diameters of the prior austenite grains obtained in each photographed field of view. Here, when a prior austenite grain _G1 has a circle-equivalent diameter _D1 and an area _A1 , and another prior austenite grain _G2 has a circle-equivalent diameter _D2 and an area _A2 , the area-average value D of the circle-equivalent diameters of these two prior austenite grains can be calculated as follows: D=( _A1 × _D1 + _A2 × _D2 )/( _A1 + _A2 ).

In this disclosure, "x/y position from the end face (where x and y are natural numbers such that x<y)" refers to a position that is moved in the width direction from the end face of the steel plate by a distance of x/y of the plate width toward the center of the steel plate. For example, if the width of the steel plate is 1 m, "1/4 position from the end face" refers to a position that is 0.25 m away from the end face of the steel plate in the width direction.

In this disclosure, "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, if the plate thickness t of the steel plate is 2 mm, "plate thickness 1/8 position" refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate. Note that, if the steel plate has a coating such as a plating layer on its surface, "the surface of the steel plate" refers to the interface between the steel plate and the coating, and "plate thickness t" refers to the thickness of the steel plate (base material) excluding the coating.

In this disclosure, the plate width direction is the direction perpendicular to the rolling direction and plate thickness direction.

When the rolling direction of the steel plate is unclear, the following method can be adopted, for example, to identify the rolling direction of the steel plate. After mirror polishing the thickness cross section of the steel plate, the S concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 μm, and a distribution image is measured in a 500 μm square range in the center of the plate thickness. At this time, the extended area with a high S concentration is determined to be an inclusion such as MnS. When observing, observation may be performed in multiple fields of view. Next, using the plate thickness cross section first observed by the above method as a reference, a surface parallel to the surface rotated in 5° increments in the range of 0° to 180° around the plate thickness direction is observed by the above method. The average value of the long axis length of the multiple inclusions in each obtained cross section is calculated for each cross section, and the cross section with the largest average long axis length of the inclusions is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

1.3 Area ratio In the hot-rolled steel sheet of the present disclosure, the area ratios of the region with a GAM (Grain Average Misorientation) value of more than 0.6° and less than 2.0°, the region with a GAM value of 0.6° or less, and the region with a GAM value of 2.0° or more are specified as follows. In the hot-rolled steel sheet, the "region with a GAM value of 0.6° or less" is often relatively soft. The "region with a GAM value of 2.0° or more" is often relatively hard. The "region with a GAM value of more than 0.6° and less than 2.0°" often has an intermediate hardness.

(Area ratio of region having GAM value greater than 0.6° and less than 2.0°)
In the hot-rolled steel sheet of the present disclosure, the area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%. When the area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 50% or more, the hot-rolled steel sheet is likely to have an excellent balance of strength, elongation and hole expandability. The area ratio of the region having a GAM value of more than 0.6° and less than 2.0° may be 55% or more, 60% or more, 70% or more, 75% or more, 80% or more or 85% or more, or 99% or less, 95% or less, 90% or less, 85% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less or 55% or less. In particular, when the area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 55% or more and 95% or less, the hot-rolled steel sheet is likely to have an excellent balance of strength, elongation and hole expandability.

(Area ratio of region having GAM value of 0.6° or less)
In the hot-rolled steel sheet of the present disclosure, the area ratio of the region having a GAM value of 0.6° or less is 0% or more and less than 50%. When the area ratio of the region having a GAM value of 0.6° or less is less than 50%, the hot-rolled steel sheet is likely to have an excellent balance of strength, elongation, and hole expandability. The hot-rolled steel sheet of the present disclosure may have the above-mentioned region having a GAM value of more than 0.6° and less than 2.0° and the below-described region having a GAM value of 2.0° or more, and the area ratio of the region having a GAM value of 0.6° or less may be 0%. The area ratio of the region having a GAM value of 0.6° or less may be 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, or 5% or less, or more than 0%, 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, or 45% or more. In particular, when the area ratio of the region having a GAM value of 0.6° or less is 0% or more and 45% or less, the heat-rolled steel sheet is likely to have a better balance of strength, elongation, and hole expandability.

(Area ratio of region having GAM value of 2.0° or more)
In the hot-rolled steel sheet of the present disclosure, the area ratio of the region having a GAM value of 2.0° or more is more than 0% and not more than 50%. When the hot-rolled steel sheet of the present disclosure has a region having a GAM value of 2.0° or more in addition to the above-mentioned region having a GAM value of more than 0.6° and less than 2.0°, it is likely to have an excellent balance of strength, elongation and hole expandability. The area ratio of the region having a GAM value of 2.0° or more may be 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, or 5% or less, or 1% or more, 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, or 45% or more. In particular, when the area ratio of the region having a GAM value of 2.0° or more is more than 0% and not more than 20%, in particular, 1% or more and not more than 20%, and particularly 1% or more and not more than 10%, the heat-rolled steel sheet is more likely to have a better balance of strength, elongation, and hole expandability.

The "GAM value" of each region of the hot-rolled steel sheet is measured by the EBSP (Electron Backscatter Pattern) method, and the average value of the orientation difference between adjacent pixels (measurement points) in each measurement region (for example, within one crystal grain (defined as a region surrounded by grain boundaries with an orientation difference of 15° or more)) is taken as the GAM value of the measurement region (crystal grain). The area ratio of the region with a GAM value of more than 0.6° and less than 2.0°, the area ratio of the region with a GAM value of 0.6° or less, and the area ratio of the region with a GAM value of 2.0° or more are measured by the following method. First, a sample is taken at a quarter position from the end face in the sheet width direction of the hot-rolled steel sheet so that the metal structure of the cross section (sheet thickness direction x rolling direction cross section) normal to the sheet width direction can be observed. The size of the sample depends on the measuring device, but 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. A region of 200 μm in the thickness direction and 400 μm or more at any position in the rolling direction from the surface in the thickness direction of the sample (a rectangular region having a center at a 1/4 depth position in the thickness direction, with 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 by the EBSP method at measurement intervals of 0.2 μm. For the above measurement, 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 analyzer is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The GAM value can be calculated using the software "OIM Analysis (registered trademark)" that comes with the EBSD analyzer. Note that crystal grains with a defined circle equivalent diameter of 0.6 μm or less are excluded because there is a possibility of a large measurement error.

1.4 Relationship (1): 1.7≦LGr/LGt
The hot-rolled steel sheet of the present disclosure satisfies the relationship (1) of 1.7≦LGr/LGt. Here, LGr is the area average value of the projected length of the prior austenite grains in the rolling direction, and LGt is the area average value of the projected length of the prior austenite grains in the sheet thickness direction. The "area average value" means an average value weighted by area. Here, the "area" is the area in a cross section (sheet thickness direction × rolling direction cross section) normal to the sheet width direction. For example, when a certain prior austenite grain _G1 has a projected length in the rolling direction _LGr1 , a projected length in the sheet thickness direction _LGt1 , and an area _A1 , and another prior austenite grain _G2 has a projected length in the rolling direction _LGr2 , a projected length in the sheet thickness direction _LGt2 , and an area _A2 , the area average value LGr of the projected lengths in the rolling direction of these two prior austenite grains is LGr = ( _A1 x _LGr1 + _A2 x _LGr2 ) / ( _A1 + _A2 ), and the area average value LGt of the projected lengths in the sheet thickness direction is LGt = ( _A1 x _LGt1 + _A2 x _LGt2 ) / ( _A1 + _A2 ). In the hot-rolled steel sheet of the present disclosure, the fact that the above relationship (1) is satisfied means, in other words, that the prior austenite grains are elongated in the rolling direction. As described above, in the hot-rolled steel sheet of the present disclosure, the area ratio of the region in which the prior austenite grain size is small and the GAM value is more than 0.6° and less than 2.0° is 50% or more and less than 100%. That is, in the hot-rolled steel sheet of the present disclosure, it can be said that fine crystal grains having an excellent balance between strength and ductility are elongated along the rolling direction. In the conventional common sense, it has been thought that when LGr/LGt in a hot-rolled steel sheet is large, the elongation and hole expandability of the hot-rolled steel sheet are reduced. Therefore, in the past, LGr/LGt was controlled to be small, that is, to be equiaxed. In contrast, in the hot-rolled steel sheet of the present disclosure, the balance between strength, elongation and hole expandability is improved by satisfying the relationship (2) described below together with the relationship (1). In addition, in the hot-rolled steel sheet of the present disclosure, the prior austenite grains are elongated along the rolling direction, so that cracks are less likely to progress in the sheet thickness direction and the impact characteristics are excellent.

In the above relationship (1), the upper limit of LGl/LGt is not particularly limited. In the hot-rolled steel sheet of the present disclosure, the following relationship (1-1) may be satisfied. LGr/LGt may be 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, or 5.0 or less. In addition, LGr/LGt may be 2.0 or more, 2.5 or more, 3.0 or more, or 4.0 or more.
1.7≦LGr/LGt≦10.0…(1-1)

The "LGr/LGt" in the hot-rolled steel sheet is measured as follows. First, a sample is taken at a quarter position from the end face in the sheet width direction of the hot-rolled steel sheet so that the metal structure of the cross section (sheet thickness direction x rolling direction cross section) with the sheet 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 the full thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-polished, and then corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. The grains that appear black due to corrosion are considered to be prior austenite grains. The observation surface where prior austenite grains are revealed is observed by an optical microscope, and eight or more fields of view with an area of 0.05 ^mm2 or more (total of 0.40 ^mm2 or more) are photographed. Then, from the steel structure photograph taken by the optical microscope, the area of each prior austenite grain is calculated, and the projected length in the rolling direction and the projected length in the sheet thickness direction are measured, and the ratio of the area averages is defined as LGr/LGt. If the prior austenite grains cannot be sufficiently revealed by the above method, the prior austenite grains are identified by the reconstruction method described in "Study on High-Precision Reconstruction Method of Austenite Structure of Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the LGr/LGt of the prior austenite grains is determined.

1.5 Relationship (2): 1.20≦(LGr/LGt)/(LMr/LMt)
The hot-rolled steel sheet of the present disclosure satisfies the relationship (2) of 1.20≦(LGr/LGt)/(LMr/LMt). Here, LGr and LGt are as described above, LMr is the average area of the rolling direction projected length of the region having a GAM value of 2.0° or more, and LMt is the average area of the thickness direction projected length of the region having a GAM value of 2.0° or more. That is, in the hot-rolled steel sheet of the present disclosure, LGr/LGt of the prior austenite grains is 1.20 times or more of LMr/LMt of the hard phase. In this way, since the LGr/LGt of the prior austenite grains is larger than the LMr/LMt of the hard phase by a certain amount (the LMr/LMt of the hard phase is smaller than the LGr/LGt of the prior austenite grains by a certain amount), the hard phase is dispersed and the variations in strength and ductility of the steel sheet as a whole are reduced, resulting in a hot-rolled steel sheet with an excellent balance of strength, elongation and hole expandability.

In the above relationship (2), the upper limit of (LGr/LGt)/(LMr/LMt) is not particularly limited. In the hot-rolled steel sheet of the present disclosure, the following relationship (2-1) may be satisfied. (LGr/LGt)/(LMr/LMt) may be 5.00 or less, 4.80 or less, 4.50 or less, 4.30 or less, 4.00 or less, 3.80 or less, 3.60 or less, or 3.40 or less. In addition, (LGr/LGt)/(LMr/LMt) may be 1.40 or more, 1.60 or more, 1.80 or more, 2.00 or more, 2.20 or more, or 2.40 or more.
1.20≦(LGr/LGt)/(LMr/LMt)≦5.00…(2-1)

The "LMr/LMt" of the hot-rolled steel sheet is measured as follows. First, a sample is taken at a quarter position from the end face in the sheet width direction of the hot-rolled steel sheet so that the metal structure of the cross section (sheet thickness direction x rolling direction cross section) normal to the sheet width direction can be observed. The size of the sample depends on the measuring device, but may be, for example, a rectangular parallelepiped with the full thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet 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 in the thickness direction and 400 μm or more at any position in the rolling direction (a rectangular region having a center at 1/4 depth position in the thickness direction and having 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 by the EBSP method at a measurement interval of 0.2 μm. For the above measurement, an EBSD analyzer 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 analyzer is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. Next, the GAM value is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. In addition, crystal grains having a defined equivalent circle diameter of 0.6 μm or less are excluded because there is a possibility of a large measurement error. From the calculated GAM value information, a region having a GAM value of 2.0° or more is identified. The area of each of the identified regions having a GAM value of 2.0° or more is calculated, and from the shape, the rolling direction projected length and the plate thickness direction projected length are measured, and the ratio of the area averages in each region is defined as LMr/LMt.

1.6 Mechanical Properties, etc. The hot-rolled steel sheet according to the present disclosure has the above-mentioned chemical composition and steel structure, and therefore has an excellent balance of strength, elongation, and hole expandability, and also has excellent impact properties.

(Tensile strength TS)
The hot-rolled steel sheet of the present disclosure has excellent strength. For example, the hot-rolled steel sheet of the present disclosure may have a tensile strength TS of 960 MPa or more. The tensile strength TS may be 970 MPa or more or 980 MPa or more. The upper limit of the tensile strength TS is not particularly limited, and may be, for example, 1200 MPa or less, 1150 MPa or less, or 1100 MPa or less. The tensile test for measuring the tensile strength TS of the hot-rolled steel sheet is performed in accordance with JIS Z 2241, by taking a No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling transverse direction (sheet width direction) of the steel sheet. If the No. 5 test piece cannot be taken from the hot-rolled steel sheet to be measured, a small test piece with the sheet width direction as the longitudinal direction can be used as the test piece for measuring the tensile strength TS.

(Uniform elongation uEL)
The hot-rolled steel sheet of the present disclosure has excellent ductility. For example, the hot-rolled steel sheet of the present disclosure may have a uniform elongation uEL of 4.0% or more and 12.0% or less. The uniform elongation uEL may be 5.0% or more, 6.0% or more, 6.5% or more, 7.0% or more, 7.5% or more, or 8.0% or more, and may be 11.5% or less, 11.0% or less, 10.5% or less, 10.0% or less, 9.5% or less, or 9.0% or less. The tensile test for measuring the uniform elongation uEL of the hot-rolled steel sheet is performed in accordance with JIS Z 2241, with the test piece No. 5 taken from a direction in which the longitudinal direction of the test piece is parallel to the rolling transverse direction (sheet width direction) of the steel sheet.

(Hole expansion ability)
The hot-rolled steel sheet of the present disclosure has excellent hole expansion properties. For example, the hot-rolled steel sheet of the present disclosure may have a hole expansion ratio λ of 40% or more and 110% or less. The hole expansion ratio λ may be 45% or more or 50% or more, and may be 100% or less, 90% or less, 80% or less, or 70% or less. The hole expansion properties of the hot-rolled steel sheet are evaluated by punching a circular hole with a diameter of 10 mm under conditions where the clearance is 12.5%, forming the burr on the die side, and forming with a 60° conical punch. Five hole expansion tests are performed, and the average value is taken as the hole expansion ratio λ.

(Collision characteristics)
The hot-rolled steel sheet of the present disclosure has excellent impact properties. The impact properties of the hot-rolled steel sheet can be evaluated, for example, by the crack growth resistance in the thickness direction. The crack growth resistance in the thickness direction is determined by the displacement-load curve when the hot-rolled steel sheet is punched. For example, it is determined by the ratio W2/W1 of the energy W2 to the energy W1. Here, W2=∫Fds (after the maximum load) and W1=∫Fds (before the maximum load). The hot-rolled steel sheet of the present disclosure may satisfy 0.15≦W2/W1. The value of the ratio W2/W1 may be 0.17 or more, 0.18 or more, 0.19 or more, or 0.20 or more.

(Thickness)
The thickness of the hot-rolled steel sheet is not particularly limited, and may be, for example, 0.5 mm or more and 10.0 mm or less. The upper limit of the thickness may be 8.0 mm, 6.0 mm, or 4.0 mm.

(Application)
As described above, the hot-rolled steel sheet of the present disclosure has an excellent balance of strength, ductility, and hole expandability, and also has excellent collision properties. Such a hot-rolled steel sheet can be used, for example, as a material for automobile suspension parts, structural parts, frameworks, frame parts, etc. In particular, it is suitable as a material for automobile suspension parts. Specific examples of automobile suspension parts include lower arms, upper arms, trail links, etc.

2. Manufacturing method of hot-rolled steel sheet Hereinafter, an example of a method for manufacturing a hot-rolled steel sheet according to the present disclosure will be described, but the manufacturing method of the hot-rolled steel sheet is not limited to the one described below. The manufacturing method of the hot-rolled steel sheet according to one embodiment is as follows:
a heating step of heating the slab;
a hot rolling step of hot rolling the heated slab;
A cooling step of cooling the hot-rolled steel sheet obtained by hot rolling;
A winding process of winding the cooled hot-rolled steel sheet,
The slab comprises, in mass %,
C: 0.045% or more, 0.120% or less,
Si: 0% or more, 3.00% or less,
Mn: 1.20% or more, 2.60% or less,
Ti: 0.020% or more, 0.180% or less,
Al: 0.010% or more, 0.400% or less,
P: 0% or more, 0.080% or less,
S: 0% or more, 0.0100% or less,
N: 0% or more, 0.0050% or less,
O: 0% or more, 0.010% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Cr: 0% or more, 2.000% or less,
Mo: 0% or more, 3.000% or less,
Ni: 0% or more, 0.500% or less,
B: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0500% or less,
Mg: 0% or more, 0.050% or less,
REM: 0% or more, 0.100% or less,
Bi: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Zr: 0% or more, 0.500% or less,
Co: 0% or more, 3.000% or less,
Zn: 0% or more, 0.200% or less,
W: 0% or more, 0.200% or less,
Sb: 0% or more, 0.500% or less,
As: 0% or more, 0.050% or less, and Sn: 0% or more, 0.050% or less;
The balance is composed of Fe and impurities.
The hot rolling step includes rough rolling and finish rolling,
The start temperature ST of the finish rolling is 1000° C. or more and 1150° C. or less,
The finish rolling includes high temperature difference rolling two or more times,
The high temperature difference rolling is such that a temperature difference ΔT between the rolling temperature in a rolling stand performing the high temperature difference rolling and the rolling temperature in a rolling stand immediately preceding the high temperature difference rolling is 30° C. or more;
In the finish rolling, the total rolling reduction after the second high temperature difference rolling is 50% or more,
The completion temperature FT of the finish rolling is 940° C. or less,
The time from the completion of the finish rolling to the start of the cooling is within 2.0 seconds,
In the cooling step, accelerated cooling is performed after the start of the cooling, and the cooling stop temperature of the accelerated cooling is 520° C. or more and 720° C. or less;
In the cooling step, the slow cooling time in the temperature range of 720 ° C. to 470 ° C. is 2.0 seconds or more.
It is characterized by:

2.1 Heating process In the heating process, the slab having the above-mentioned chemical composition is heated. If the heating temperature is too low, the dissolution of carbides and nitrides is insufficient. On the other hand, if the heating temperature is too high, the amount of scale generated increases and the yield decreases. In this regard, the heating temperature of the slab in the heating process may be, for example, 1100°C or higher and 1300°C or lower. The heating temperature may be 1150°C or higher or 1200°C or higher, or 1260°C or lower. The heating time of the slab in the heating process may be a time that allows the entire slab to reach the target temperature. The heating time may be, for example, 6000 seconds (100 minutes) or more or 9000 seconds (150 minutes) or more. In particular, a higher effect is likely to be obtained by holding the temperature of 1150°C or higher for 6000 seconds (100 minutes) or more.

2.2 Hot Rolling Step In the hot rolling step, the slab heated in the heating step is hot rolled. The hot rolling step includes rough rolling and finish rolling.

(Rough rolling)
The conditions of the rough rolling are not particularly limited, and the slab may be rolled at a predetermined temperature and a predetermined reduction ratio. The temperature in the rough rolling may be, for example, equal to or lower than the heating temperature in the heating step and equal to or higher than the start temperature ST of the finish rolling described later. The reduction ratio in the rough rolling may be, for example, such that the thickness reduction at 800 to 1150 ° C. is 90% or more.

(Finish rolling)
In the finish rolling, the slab (rough bar) subjected to the rough rolling is rolled multiple times by multiple stands. The start temperature ST of the finish rolling is 1000°C or more and 1150°C or less. If the start temperature ST is too low, the finally manufactured hot-rolled steel sheet does not satisfy the above-mentioned requirements of the prior austenite grain size, and the hole expandability and the like are likely to be reduced. On the other hand, if the start temperature ST is too high, the structure of the steel cannot be appropriately controlled, and the finally manufactured hot-rolled steel sheet does not satisfy the above-mentioned requirements of the prior austenite grain size, the relationships (1) and (2), and the collision characteristics are likely to be reduced. These problems are solved by the start temperature ST being 1000°C or more and 1150°C or less. The start temperature ST may be 1050°C or more and 1150°C or less.

The finishing rolling includes two or more high temperature difference rollings. In the high temperature difference rolling, the temperature difference ΔT between the rolling temperature in the rolling stand where the high temperature difference rolling is performed and the rolling temperature in the rolling stand immediately before that is 30°C or more. Here, the rolling temperature is the temperature at the entry side of the rolling stand, that is, the surface temperature of the steel plate measured immediately before the steel plate is rolled in the rolling stand. The first high temperature difference rolling causes TiC, which is the nucleus of transformation, to precipitate in the steel structure at a high density, and the second high temperature difference rolling provides the driving force for creating the steel structure. If the high temperature difference rolling is performed once or less, the steel structure cannot be created, and the finally manufactured hot rolled steel plate does not satisfy the above relationship (2), and the hole expandability is likely to decrease. If the high temperature difference rolling is performed twice or more, the steel structure can be created, and the finally manufactured hot rolled steel plate has an excellent balance of strength, elongation, and hole expandability. The number of high temperature difference rolling may vary depending on the number of rolling stands in the finish rolling. The number of high temperature difference rolling may be, for example, 2 to 10 times, 3 to 4 times, or 7 to 6 times. The temperature difference ΔT of the high temperature difference rolling may be 30°C or more, 35°C or more, 40°C or more, 45°C or more, or 50°C or more, or 150°C or less, 100°C or less, 80°C or less, 60°C or less, or 50°C or less. The temperature difference ΔT of the high temperature difference rolling can be controlled, for example, by controlling the amount of coolant such as water sprayed from a cooling device such as a cooling spray immediately after rolling, or by controlling the conveying speed of the steel sheet during rolling.

In the finish rolling, the total reduction ratio after the second high temperature difference rolling is 50% or more. If the total reduction ratio after the second high temperature difference rolling is too low, the structure of the steel cannot be properly controlled, and the finally manufactured hot-rolled steel sheet does not satisfy the above relationship (2), and the hole expandability is likely to decrease. If the total reduction ratio after the second high temperature difference rolling is 50% or more, this problem is solved. The total reduction ratio after the second high temperature difference rolling may be 55% or more, 60% or more, or 65% or more. If the total reduction ratio after the second high temperature difference rolling is too high, the anisotropy of the structure increases and the hole expandability is likely to decrease. If the total reduction ratio after the second high temperature difference rolling is 80% or less, this problem is solved, so it is preferable that the total reduction ratio after the second high temperature difference rolling is 80% or less. The upper limit of the total reduction rate after the second high temperature difference rolling may be 75% or less or 70% or less. Even if high temperature difference rolling is performed three or more times, the above total reduction rate means the total reduction rate after the second high temperature difference rolling. In addition, the total reduction rate after the second high temperature difference rolling means the thickness reduction rate due to rolling (which may include high temperature difference rolling) after the second high temperature difference rolling, relative to the thickness after the second high temperature difference rolling. It goes without saying that the second high temperature difference rolling is not the final stage (final stand) of the finishing rolling.

The second high temperature differential rolling is preferably performed at a specified temperature, since the densely precipitated TiC makes it easier to suppress the reduction of dislocations in the steel structure. This allows the value of (LGr/LGt)/(LMr/LMt) to be controlled in a favorable manner, improving the collision properties. For example, in the case of steel with a Ti content of 0.1-0.13%, an Nb content of 0.008-0.02%, a V content of less than 0.01%, an Mo content of less than 0.01%, and a B content of less than 0.0001%, it is preferable to set the rolling temperature of the second high temperature differential rolling to 980-1000°C.

The completion temperature FT of the finish rolling is 940°C or less. If the completion temperature FT is too high, the structure of the steel cannot be properly controlled, and the hot-rolled steel sheet finally produced does not satisfy the above-mentioned requirements for the prior austenite grain size, relationships (1) and (2), and the collision properties are likely to deteriorate. If the completion temperature FT is 940°C or less, such problems are eliminated. The completion temperature FT may be 920°C or less or 900°C or less. The lower limit of the completion temperature FT is not particularly limited as long as the requirements for the cooling process described below can be achieved. For example, the completion temperature FT may be 750°C or more, 770°C or more, 800°C or more, 830°C or more, or 850°C or more.

2.3 Cooling Step In the cooling step, the hot-rolled steel sheet obtained by hot rolling is cooled. Here, the time from the completion of the above-mentioned finish rolling to the start of cooling is within 2.0 seconds. If the time is too long, the prior austenite grain size exceeds 25 μm due to the coarsening of the crystal grains, and the finally manufactured hot-rolled steel sheet is likely to have a poor balance of strength, elongation, and hole expandability. By setting the time to within 2.0 seconds, such a problem is solved. The time may be within 1.8 seconds, within 1.6 seconds, within 1.4 seconds, or within 1.2 seconds.

In the cooling process, accelerated cooling is performed after the start of cooling. "Accelerated cooling" refers to cooling under conditions where the cooling rate is 20°C/s or more and 200°C/s or less. It is important that the accelerated cooling stop temperature is 520°C or more and 720°C or less. Transformation of the region with a GAM value of 2.0° mainly occurs during slow cooling after the accelerated cooling is stopped. By stopping the accelerated cooling at a temperature of 520°C or more and 720°C or less, the amount of regions with a GAM value of less than 2.0° is appropriate. Outside this temperature range, the proportion of regions with a GAM value of 2.0° or more may increase excessively, resulting in a decrease in uniform elongation.

In the cooling process, the slow cooling time in the temperature range from 720°C to 470°C is 2.0 seconds or more. "Slow cooling" means cooling under cooling conditions where the cooling rate is less than 20°C/s. When the slow cooling time in the temperature range from 720°C to 470°C is 2.0 seconds or more, the area ratio of the region with a GAM value of more than 0.6° and less than 2.0° is 50% or more. For example, by slowly cooling the hot-rolled steel sheet on a run-out table (ROT), the slow cooling time in the temperature range from 720°C to 470°C can be 2.0 seconds or more. The slow cooling time may be 2.2 seconds or more, 2.4 seconds or more, 2.6 seconds or more, 2.8 seconds or more, or 3.0 seconds or more. In particular, when the slow cooling time in the temperature range from 680°C to 580°C is 3.0 seconds or more, the amount of the region with a GAM value of 2.0° or more can be more appropriately controlled. The upper limit of the slow cooling time is not particularly limited, and the optimal slow cooling time may be determined taking into consideration productivity, etc. The slow cooling time may be, for example, 5.0 seconds or less, 4.5 seconds or less, 4.0 seconds or less, or 3.5 seconds or less. If the slow cooling time is too short, the region with a GAM value of 2.0° or more is likely to be excessively generated, and the above relationship (2) will not be satisfied, and the finally manufactured hot-rolled steel sheet will likely have a poor balance of strength, elongation, and hole expandability.

In the cooling process, after the slow cooling is completed, the average cooling rate until the temperature reaches 300°C is preferably 30°C/s or more. If the average cooling rate is slow, softening occurs due to tempering, and the strength of the hot-rolled steel sheet finally manufactured is likely to decrease. By setting the average cooling rate to 30°C/s or more, such problems can be more reliably solved. The average cooling rate may be 35°C/s or more, 40°C/s or more, 45°C/s or more, or 50°C/s or more. The upper limit of the average cooling rate is not particularly limited. The average cooling rate may be, for example, 120°C/s or less, 110°C/s or less, 100°C/s or less, 90°C/s or less, or 80°C/s or less. In addition, when the coiling temperature is less than 300°C, the average cooling rate from 300°C to the coiling temperature is not particularly limited.

2.4 Winding process In the winding process, the hot-rolled steel sheet cooled in the cooling process is wound. There is no particular limitation on the winding conditions. The winding temperature in the winding process is, for example, 300°C or less. The winding temperature may be 200°C or less, 100°C or less, or 50°C or less, and may be 0°C or more, or 20°C or more.

As described above, the hot-rolled steel sheet of the present disclosure can be manufactured by (1) using a slab with an appropriate chemical composition, (2) controlling the morphology of the prior austenite grains by adjusting the finish rolling conditions during hot rolling, and (3) controlling the transformation behavior by controlling the cooling rate during cooling (for example, controlling the cooling conditions at the run-out table (ROT)). Note that, when manufacturing the hot-rolled steel sheet, in addition to the above steps, other steps may be performed. For example, a tempering step may be optionally performed after the coiling step.

1. Preparation of hot-rolled steel sheet A slab having the chemical components shown in Tables 1 and 2 below was subjected to a heating step and a hot-rolling step under the conditions shown in Table 3 below, and after hot rolling, a cooling step and a coiling step were sequentially performed under the conditions shown in Table 3 to obtain a hot-rolled steel sheet (steel strip) having a thickness of 3.0 mm. It was confirmed that the chemical components in the hot-rolled steel sheet (steel strip) were substantially the same as those in the slab, and were those shown in Tables 1 and 2. In Table 3, "rolling at ΔT≧30° C. twice or more" means "high temperature difference rolling twice or more". "High temperature difference rolling" refers to a rolling stand in which the rolling temperature in the rolling stand performing the high temperature difference rolling and the rolling temperature in the rolling stand immediately before it are 30° C. or more. In Table 3, the cases in which high temperature difference rolling was performed twice or more are indicated as "○", and the cases in which high temperature difference rolling was performed once or less are indicated as "×". In addition, in Table 3, "total reduction rate after the second temperature difference of ΔT occurs" means "total reduction rate after the second high temperature difference rolling." In addition, in Table 3, "accelerated cooling" means cooling after the start of cooling at a cooling rate of 20° C./s or more and 200° C./s or less.

2. Measurement of Prior Austenite Grain Size The prior austenite grain size was measured for each of the hot-rolled steel sheets. The method for measuring the prior austenite grain size was as described above. The results are shown in Table 4 below.

3. Measurement of GAM value and area ratio For each hot-rolled steel sheet, the area ratio of the region with a GAM value of more than 0.6° and less than 2.0°, the area ratio of the region with a GAM value of 0.6° or less, and the area ratio of the region with a GAM value of 2.0° or more were measured. The measurement method of the GAM value and the area ratio is as described above. The results are shown in Table 4 below.

4. Measurement of LGr/LGt and (LGr/LGt)/(LMr/LMt) For each hot-rolled steel sheet, LGr (area average value of the projection length in the rolling direction of the prior austenite grains), LGt (area average value of the projection length in the thickness direction of the prior austenite grains), LMr (area average value of the projection length in the rolling direction of the region having a GAM value of 2.0° or more), and LMt (area average value of the projection length in the thickness direction of the region having a GAM value of 2.0° or more) were measured, and "LGr/LGt" and "(LGr/LGt)/(LMr/LMt)" were calculated. The measurement and calculation method of LGr/LGt and LMr/LMt is as described above. The results are shown in Table 4 below.

5. Evaluation of mechanical properties 5.1 Tensile strength TS, uniform elongation uEL and hole expansion ratio λ
The tensile strength TS, uniform elongation uEL, and hole expansion ratio λ were measured for each hot-rolled steel sheet. The measurement methods for each were as described above. The results are shown in Table 4 below. In Table 4 below, those that satisfy the mechanical properties "tensile strength TS: 960 MPa or more", "uniform elongation uEL: 4.0% to 12.0%", and "hole expansion ratio λ: 40% to 110%", which are particularly preferable when applying to automobile suspension parts, etc., were judged to have an excellent balance of strength, elongation, and hole expansion property.

5.2 Impact characteristics The impact characteristics of each hot-rolled steel sheet were evaluated based on the crack propagation resistance in the thickness direction of each hot-rolled steel sheet. The crack propagation resistance in the thickness direction is determined by the displacement-load curve when the hot-rolled steel sheet is punched. Specifically, it is determined by the ratio W2/W1 of the energy W2 to the energy W1 below. Here, W2=∫Fds (after the maximum load) and W1=∫Fds (before the maximum load), F is the punching load (N), and S is the punching stroke (mm). In this embodiment, those satisfying 0.15≦W2/W1 were evaluated as having excellent impact characteristics, and those satisfying 0.2<W2/W1 were evaluated as having particularly excellent impact characteristics. The results are shown in Table 4 below.

6. Results and Discussion The results shown in Tables 1 to 4 reveal the following:
In the case of No. 1, since the C content of the hot-rolled steel sheet was too low, the region in which the GAM value was 0.6° or less was excessive, and the strength of the hot-rolled steel sheet was reduced.
In No. 2, the C content of the hot-rolled steel sheet was too high, so the region with a GAM value of 2.0° or more was excessive, and the elongation and hole expandability of the hot-rolled steel sheet were reduced.
In the case of No. 3, the Si content of the hot-rolled steel sheet was too high, so that the ductility was insufficient, etc., making hot rolling difficult.
In No. 4, since the Mn content of the hot-rolled steel sheet was too high, the region in which the GAM value was 2.0° or more was excessive, and the elongation of the hot-rolled steel sheet was reduced.
In No. 5, the Mn content of the hot-rolled steel sheet was too low, so the region with a GAM value of 0.6° or less was excessive, and the strength of the hot-rolled steel sheet was reduced.
In the case of No. 6, the Ti content of the hot-rolled steel sheet was too high, so that the hole expandability of the hot-rolled steel sheet was deteriorated due to the excessive generation of precipitates, etc.
For No. 7, the Ti content of the hot-rolled steel sheet was too low, so the strength increasing effect due to precipitation strengthening, fine grain strengthening and/or dislocation strengthening could not be obtained, and the transformation nuclei could not be sufficiently generated, so that the prior austenite grains became coarse, and the strength and hole expandability of the hot-rolled steel sheet were reduced.
In the case of No. 8, the Al content of the hot-rolled steel sheet was too high, which caused cracks in the slab and made hot rolling difficult.
In the case of No. 9, the Al content of the hot-rolled steel sheet was too low, so deoxidation was insufficient, and inclusions were excessively generated, resulting in a decrease in the hole expandability of the hot-rolled steel sheet.
In the case of No. 10, the P content of the hot-rolled steel sheet was too high, which caused embrittlement and resulted in cracking of the slab, making hot rolling difficult.
In the case of No. 11, the S content of the hot-rolled steel sheet was too high, so that inclusions were generated excessively, and the hole expandability of the hot-rolled steel sheet was deteriorated.
In the case of No. 12, the N content of the hot-rolled steel sheet was too high, which caused embrittlement and cracking of the slab, making hot rolling difficult.
In the case of No. 13, the O content of the hot-rolled steel sheet was too high, so that oxides were generated in excess, and the hole expandability of the hot-rolled steel sheet was reduced.
In the case of No. 15, the finish rolling start temperature ST was too low, so that the prior austenite grains became coarse and the hole expandability of the hot-rolled steel sheet was deteriorated.
For No. 16, the finish rolling start temperature ST was too high, and further, the finish rolling completion temperature FT was also too high, so that the structure of the steel could not be appropriately controlled, the prior austenite grains of the hot-rolled steel sheet became coarse, and the predetermined relationships (1) and (2) were not satisfied, and the collision properties of the hot-rolled steel sheet were deteriorated.
For No. 17, the high temperature difference rolling in which the rolling temperature difference ΔT was 30 ° C. or more was performed once or less, so the steel structure could not be created, and the hot-rolled steel sheet did not satisfy the predetermined relationship (2), and the hole expandability of the hot-rolled steel sheet was reduced.
For No. 18, the total reduction rate after the second high temperature difference rolling was too low, so the structure of the steel could not be appropriately controlled, and the hot-rolled steel sheet did not satisfy the predetermined relationship (2), and the hole expandability of the hot-rolled steel sheet was reduced.
For No. 19, the time from the completion of finish rolling to the start of cooling was too long, so that the prior austenite grain size exceeded 25 μm due to the coarsening of the crystal grains, and the hole expandability of the hot-rolled steel sheet was reduced.
For No. 20, the cooling stop temperature of the accelerated cooling was too high, so the region with a GAM value of 2.0° or more became excessively large, and the elongation of the hot-rolled steel sheet decreased.
For No. 21, the cooling stop temperature of the accelerated cooling was too low, so the region with a GAM value of 2.0° or more became excessively large, and the elongation of the hot-rolled steel sheet decreased.
For No. 38, the finish rolling start temperature ST was too high, so that the structure of the steel could not be appropriately controlled, the prior austenite grains of the hot-rolled steel sheet became coarse, the hot-rolled steel sheet did not satisfy the predetermined relationship (2), and the collision properties of the hot-rolled steel sheet were deteriorated.
For No. 39, although the finish rolling start temperature ST was appropriately controlled, the finish rolling completion temperature FT was too high, so that the structure of the steel could not be appropriately controlled, the prior austenite grains of the hot-rolled steel sheet became coarse, the hot-rolled steel sheet did not satisfy the predetermined relationship (2), and the collision properties of the hot-rolled steel sheet were deteriorated.
For No. 40, the slow cooling time in the temperature range from 720 ° C. to 470 ° C. was too short, so the region in which the GAM value was more than 0.6 ° and less than 2.0 ° was not sufficiently generated in the hot-rolled steel sheet, and the hot-rolled steel sheet did not satisfy the predetermined relationship (2), and the elongation of the hot-rolled steel sheet was reduced.
As for No. 44, like No. 7, the Ti content of the hot-rolled steel sheet was too low, so that the strength increasing effect by precipitation strengthening, fine grain strengthening and/or dislocation strengthening could not be obtained, and the transformation nuclei could not be sufficiently generated, so that the prior austenite grains became coarse, and the strength and hole expandability of the hot-rolled steel sheet were deteriorated.
In contrast, for Nos. 14, 22 to 37, and 41 to 43, the hot-rolled steel sheets had an excellent balance of strength, elongation, and hole expandability, and also had excellent impact properties. In No. 42, the second high-temperature differential rolling was performed at 990°C, and at a predetermined temperature between 980 and 1000°C, so that the impact properties were particularly excellent, even compared to No. 34, in which the second high-temperature differential rolling was performed at 965°C. From the results of Nos. 14, 22 to 37, and 41 to 43, it can be said that the hot-rolled steel sheets satisfying the following requirements (A) to (D) have an excellent balance of strength, elongation, and hole expandability, and also have excellent impact properties.

(A) The hot-rolled steel sheet has, in mass%, C: 0.045% or more and 0.120% or less, Si: 0% or more and 3.00% or less, Mn: 1.20% or more and 2.60% or less, Ti: 0.020% or more and 0.180% or less, Al: 0.010% or more and 0.400% or less, P: 0% or more and 0.080% or less, S: 0% or more and 0.0100% or less, N: 0% or more and 0.0050% or less, O: 0% or more and 0.010% or less, Nb: 0% or more and 0.100% or less, V: 0% or more and 1.000% or less, Cu: 0% or more and 1.000% or less, Cr: 0% or more and 2.000% or less, and Mo: 0% or more and 3.000% or less. , Ni: 0% or more, 0.500% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0500% or less, Mg: 0% or more, 0.050% or less, REM: 0% or more, 0.100% or less, Bi: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Zr: 0% or more, 0.500% or less, Co: 0% or more, 3.000% or less, Zn: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Sb: 0% or more, 0.500% or less, As: 0% or more, 0.050% or less, and Sn: 0% or more, 0.050% or less, with the balance being Fe and impurities.
(B) The prior austenite grain size of the hot-rolled steel sheet is 25 μm or less.
(C) The area ratio of the area in the hot-rolled steel sheet having a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%, the area ratio of the area having a GAM value of 0.6° or less is 0% or more and less than 50%, and the area ratio of the area having a GAM value of 2.0° or more is more than 0% and less than 50%.
(D) The hot-rolled steel sheet has the following relationships (1) and (2):
1.7≦LGr/LGt…(1)
1.20≦(LGr/LGt)/(LMr/LMt)…(2)
LGr: average area of the projection length in the rolling direction of prior austenite grains LGt: average area of the projection length in the sheet thickness direction of prior austenite grains LMr: average area of the projection length in the rolling direction of regions having a GAM value of 2.0° or more LMt: average area of the projection length in the sheet thickness direction of regions having a GAM value of 2.0° or more.

Claims

In mass percent,
C: 0.045% or more, 0.120% or less,
Si: 0% or more, 3.00% or less,
Mn: 1.20% or more, 2.60% or less,
Ti: 0.020% or more, 0.180% or less,
Al: 0.010% or more, 0.400% or less,
P: 0% or more, 0.080% or less,
S: 0% or more, 0.0100% or less,
N: 0% or more, 0.0050% or less,
O: 0% or more, 0.010% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Cr: 0% or more, 2.000% or less,
Mo: 0% or more, 3.000% or less,
Ni: 0% or more, 0.500% or less,
B: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0500% or less,
Mg: 0% or more, 0.050% or less,
REM: 0% or more, 0.100% or less,
Bi: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Zr: 0% or more, 0.500% or less,
Co: 0% or more, 3.000% or less,
Zn: 0% or more, 0.200% or less,
W: 0% or more, 0.200% or less,
Sb: 0% or more, 0.500% or less,
As: 0% or more, 0.050% or less, and Sn: 0% or more, 0.050% or less;
The balance is composed of Fe and impurities.
The prior austenite grain size is 25 μm or less,
The area ratio of the region having a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%,
The area ratio of the region having a GAM value of 0.6° or less is 0% or more and less than 50%,
The area ratio of the region having a GAM value of 2.0° or more is more than 0% and 50% or less,
The following relationships (1) and (2):
1.7≦LGr/LGt…(1)
1.20≦(LGr/LGt)/(LMr/LMt)…(2)
LGr: average area of the projection length in the rolling direction of the prior austenite grains LGt: average area of the projection length in the sheet thickness direction of the prior austenite grains LMr: average area of the projection length in the rolling direction of the region having a GAM value of 2.0° or more LMt: average area of the projection length in the sheet thickness direction of the region having a GAM value of 2.0° or more
Hot-rolled steel sheet.
The area ratio of the region having a GAM value of 0.6° or less is 0% or more and 45% or less.
The hot rolled steel sheet according to claim 1.
The area ratio of the region having a GAM value of 2.0° or more is more than 0% and 20% or less.
The hot-rolled steel sheet according to claim 1 or 2.
The following relationship (1-1):
1.7≦LGr/LGt≦10.0…(1-1)
is satisfied,
The hot-rolled steel sheet according to any one of claims 1 to 3.
The following relationship (2-1):
1.20≦(LGr/LGt)/(LMr/LMt)≦5.00…(2-1)
is satisfied,
The hot-rolled steel sheet according to any one of claims 1 to 4.