EP4660344A1

EP4660344A1 - Hot-rolled steel sheet

Info

Publication number: EP4660344A1
Application number: EP24750337.8A
Authority: EP
Inventors: Takashi YASUTOMI; Yoshihiro Murai; Naoko KATOU; Genki ABUKAWA; Kunio Hayashi; Masafumi Azuma
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-31
Filing date: 2024-01-31
Publication date: 2025-12-10
Also published as: KR20250129033A; CN120641586A; JP7787475B2; WO2024162388A1; JPWO2024162388A1; MX2025008621A

Abstract

Disclosed is a hot rolled steel sheet excellent in balance of strength, elongation, and hole expansibility and excellent in impact property. The hot rolled steel sheet of the present disclosure has a predetermined chemical composition, has a prior austenite grain size of 25 µm or less, has an area ratio of regions with a GAM value of more than 0.6° and less than 2.0° of 50% or more and less than 100%, has an area ratio of regions with a GAM value of 0.6° or less of 0% or more and less than 50%, has an area ratio of regions with a GAM value of 2.0° or more of more than 0% and 50% or less, and satisfies the relations of 1.7≤LGr/LGt, and1.20≤(LGr/LGt)/(LMr/LMt). Here, LGr is the area average value of the rolling direction projected lengths of prior austenite grains, LGt is the area average value of the sheet thickness direction projected lengths of prior austenite grains, LMr is the area average value of the rolling direction projected lengths of regions with a GAM value of 2.0° or more, and LMt is the area average value of the sheet thickness direction projected lengths of regions with a GAM value of 2.0° or more.

Description

FIELD

The present application discloses a hot rolled steel sheet.

BACKGROUND

A hot rolled steel sheet is employed as the material for parts for automobiles such as suspension parts, structural parts, frames, and frame parts. For example, PTL 1 discloses high strength hot rolled steel sheet excellent in punchability having a predetermined composition and predetermined steel structure.

[CITATION LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 2012-062562

SUMMARY

[TECHNICAL PROBLEM]

Conventional hot rolled steel sheet has room for improvement in the balance of strength, elongation, and hole expansibility and in impact property.

[SOLUTION TO PROBLEM]

The inventors engaged in intensive research and as a result learned that by (1) using a slab having a suitable chemical composition, (2) modifying the finish rolling conditions at the time of hot rolling to control the form of the prior austenite grains etc., and (3) controlling the cooling speed at the time of cooling to control the transformation behavior (for example, controlling the cooling conditions at a run out table), it is possible to produce hot rolled steel sheet excellent in balance of strength, ductility, and hole expansibility and excellent in impact property. Further, they learned that hot rolled steel sheet produced in this way is excellent in balance of strength, ductility, and hole expansibility and excellent in impact property due to having a predetermined chemical composition and having a characteristic steel structure.
Based on the above findings, the present application discloses the following aspects as means for solving the problem.

A hot rolled steel sheet comprising, by 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,
Mo: 0% or more and 3.000% or less,
Ni: 0% or more and 0.500% or less,
B: 0% or more and 0.0100% or less,
Ca: 0% or more and 0.0500% or less,
Mg: 0% or more and 0.050% or less,
REM: 0% or more and 0.100% or less,
Bi: 0% or more and 0.100% or less,
Ta: 0% or more and 0.100% or less,
Zr: 0% or more and 0.500% or less,
Co: 0% or more and 3.000% or less,
Zn: 0% or more and 0.200% or less,
W: 0% or more and 0.200% or less,
Sb: 0% or more and 0.500% or less,
As: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less and
a balance: Fe and impurities, wherein
a prior austenite grain size is 25 µm or less,
an area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%,
an area ratio of regions with a GAM value of 0.6° or less is 0% or more and less than 50%,
an area ratio of regions with a GAM value of 0.2° or more is more than 0° and 50% or less, and
the following relations (1) and (2) are satisfied: $1.7 \leq LGr / LG t$ $1.20 \leq (LGr / LGt) / (LMr / LMt)$
where,
LGr: area average value of rolling direction projected lengths of prior austenite grains
LGt: area average value of sheet thickness direction projected lengths of prior austenite grains
LMr: area average value of rolling direction projected lengths of regions with GAM value of 2.0° or more
LMt: area average value of sheet thickness direction projected lengths of regions with GAM value of 2.0° or more

The hot rolled steel sheet of the aspect 1, wherein
an area ratio of regions with a GAM value of 0.6° or less is 0% or more and 45% or less.

The hot rolled steel sheet of the aspect 1 or 2, wherein
an area ratio of regions with a GAM value of 2.0° or more is more than 0% and 20% or less.

The hot rolled steel sheet of any of the aspects 1 to 3, wherein
the following relation (1-1) is satisfied: $1.7 \leq LGr / LGt \leq 10.0$

The hot rolled steel sheet of any of the aspects 1 to 4, wherein
the following relation (2-1) is satisfied: $1.20 \leq (LGr / LGt) / (LMr / LMt) \leq 5.00$

[ADVANTAGEOUS EFFECTS OF INVENTION]

The hot rolled steel sheet of the present disclosure is excellent in the balance of strength, elongation, and hole expansibility and excellent in impact property.

DESCRIPTION OF EMBODIMENTS

1. Hot Rolled Steel Sheet

Below, an embodiment of the hot rolled steel sheet will be explained, 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 contains, by 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,
Mo: 0% or more and 3.000% or less,
Ni: 0% or more and 0.500% or less,
B: 0% or more and 0.0100% or less,
Ca: 0% or more and 0.0500% or less,
Mg: 0% or more and 0.050% or less,
REM: 0% or more and 0.100% or less,
Bi: 0% or more and 0.100% or less,
Ta: 0% or more and 0.100% or less,
Zr: 0% or more and 0.500% or less,
Co: 0% or more and 3.000% or less,
Zn: 0% or more and 0.200% or less,
W: 0% or more and 0.200% or less,
Sb: 0% or more and 0.500% or less,
As: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less and
a balance: Fe and impurities, wherein
a prior austenite grain size is 25 µm or less,
an area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%,
an area ratio of regions with a GAM value of 0.6° or less is 0% or more and less than 50%,
an area ratio of regions with a GAM value of 0.2° or more and more than 0° and 50% or less, and
the following relations (1) and (2) are satisfied: $1.7 \leq LGr / LG t$ $1.20 \leq (LGr / LGt) / (LMr / LMt)$
where,
LGr: area average value of rolling direction projected lengths of prior austenite grains
LGt: area average value of sheet thickness direction projected lengths of prior austenite grains
LMr: area average value of rolling direction projected lengths of regions with GAM value of 2.0° or more
LMt: area average value of sheet thickness direction projected lengths of regions with GAM value of 2.0° or more

1.1. Chemical Composition

The reasons for limiting the chemical composition of the hot rolled steel sheet will be explained next. In the following explanation, the "%" of the constituents mean mass%.

(C: 0.045% or More and 0.120% or Less)

C is an element making the strength of the hot rolled steel sheet increase. If the C content is too small, the regions with a GAM value of 0.6° or less become excessive and the strength of the hot rolled steel sheet easily falls. On the other hand, if the C content is too great, the regions with a GAM value of 2.0° or more become excessive and the elongation of the hot rolled steel sheet and the hole expansibility easily fall. In the hot rolled steel sheet of the present disclosure, by the C content being 0.045% or more and 0.120% or less, the balance of strength, elongation, and hole expansibility of the hot rolled steel sheet is improved. The C content may also be 0.050% or more, 0.055% or more, or 0.060% or more and may also be 0.115% or less, 0.110% or less, 0.105% or less, or 0.100% or less.

(Si: 0% or More and 3.00% or Less)

Si is an element acting as a deoxidizer and affecting the form of carbides etc. and an element able to increase the tensile strength of hot rolled steel sheet. However, the hot rolled steel sheet of the present disclosure can secure sufficient tensile strength even if not containing Si. On the other hand, if the Si content is too great, the ductility becomes insufficient etc. and the hot rolling is liable to become difficult. In the hot rolled steel sheet of the present disclosure, by the Si content being 0% or more and 3.00% or less, the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The Si content may also be 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, 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more and may also be 2.50% or less, 2.00% or less, 1.80% or less, or 1.50% or less.

(Mn: 1.20% or More and 2.60% or Less)

Mn is an element able to make the tensile strength of the hot rolled steel sheet increase. If the Mn content is too small, regions with a GAM value of 0.6° or less become excessive and the strength of the hot rolled steel sheet easily falls. On the other hand, if the Mn content is too great, regions with a GAM value of 2.0° or more become excessive and the elongation of the hot rolled steel sheet easily falls. In the hot rolled steel sheet of the present disclosure, by the Mn content being 1.20% or more and 2.60% or less, the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The Mn content may also be 1.25% or more, 1.30% or more, 1.35% or more, or 1.40% or more and may also be 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

(Ti: 0.020% or More and 0.180% or Less)

Ti is a strengthening element and can contribute to the rise in the strength of the hot rolled steel sheet by precipitation strengthening, fine grain strengthening, and/or dislocation strengthening. Further, Ti is an element able to act as nuclei for transformation. Specifically, in the hot rolled steel sheet of the present disclosure, by making TiC precipitate at a high density, it can function as a nuclei for transformation. If the Ti content is too small, such a function is not exhibited, the prior austenite grains become coarse etc., and the hot rolled steel sheet easily becomes poor in balance of strength, elongation, and hole expansibility. On the other hand, if the Ti content is too great, the precipitates are excessively formed etc. and the hole expansibility of the hot rolled steel sheet easily falls. In the hot rolled steel sheet of the present disclosure, by the Ti content being 0.020% or more and 0.180% or less, the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The Ti content may also be 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more and may also be 0.175% or less, 0.170% or less, 0.165% or less, or 0.160% or less.

(Al: 0.010% or More and 0.400% or Less)

Al is an element acting as a deoxidizer. If the Al content is too small, the deoxidation easily becomes insufficient, inclusions are easily excessively formed, and the hole expansibility of the hot rolled steel sheet easily falls. On the other hand, if the Al content is too great, slab cracking etc. occur and hot rolling is liable to become difficult. In the hot rolled steel sheet of the present disclosure, by the Al content being 0.010% or more and 0.400% or less, slab cracking etc. are suppressed and the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The Al content may also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more and may also be 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less.

(P: 0% or More and 0.080% or Less)

P is an element segregating at the grain boundaries in the steel and promoting embrittlement of the grain boundaries. If the P content is too great, the elongation of the hot rolled steel sheet and the hole expansibility easily fall and, further, slab cracking etc. due to embrittlement occur and hot rolling is liable to become difficult. In the hot rolled steel sheet of the present disclosure, by the P content being 0% or more and 0.080% or less, slab cracking etc. are suppressed and the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The P content may also be 0.001% or more, 0.002% or more, 0.003% or more, or 0.004% or more and may also be 0.050% or less, 0.030% or less, 0.015% or less, or 0.010% or less.

(S: 0% or More and 0.0100% or Less)

S is an element forming MnS and other inclusions in the steel and causing the ductility of the hot rolled steel sheet to drop. If the S content is too great, the inclusions are excessively formed and the hole expansibility of the hot rolled steel sheet easily falls. In the hot rolled steel sheet of the present disclosure, by the S content being 0% or more and 0.0100% or less, the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The S content may also be 0.0001% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more and may also be 0.0090% or less, 0.0075% or less, 0.0060% or less, or 0.0050% or less.

(N: 0% or More and 0.0050% or Less)

N is an element forming coarse nitrides in the steel and causing the workability of the hot rolled steel sheet to fall. If the N content is too great, the nitrides are excessively formed etc., the elongation of the hot rolled steel sheet and the hole expansibility easily fall, and, further, slab cracking etc. due to embrittlement occur and hot rolling is liable to become difficult. In the hot rolled steel sheet of the present disclosure, by the N content being 0% or more and 0.0050% or less, slab cracking etc. are suppressed and the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The N content may also be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or more and may also be 0.0048% or less, 0.0045% or less, 0.0042% or less, or 0.0040% or less.

(O: 0% or More and 0.010% or Less)

O is an element forming oxides and causing the workability of the hot rolled steel sheet to fall. If the O content is too great, oxides are excessively formed etc. and the hole expansibility of the hot rolled steel sheet easily falls. In the hot rolled steel sheet of the present disclosure, by the O content being 0% or more and 0.010% or less, the hot rolled steel sheet is improved in the balance of strength, elongation, and hole expansibility. The O content may also be 0.001% or more and may also 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 of the present disclosure is as explained above. Further, the hot rolled steel sheet of the present disclosure may, in accordance with need, also contain at least one of the following elements. These elements need not be included, so the lower limits of the contents are 0%.

(Nb: 0% or More and 0.100% or Less)

Nb, like Ti, is an element effective for control of the form of carbides and is optionally added. On the other hand, if the Nb content is too great, the effect becomes saturated and precipitates are liable to be formed. In the hot rolled steel sheet of the present disclosure, the Nb content may also be 0% or more and 0.100% or less. The Nb content may also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.007% or more and may also 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.

(V: 0% or More and 1.000% or Less)

V is an element able to contribute to a rise in the strength of the hot rolled steel sheet by precipitation strengthening, fine grain strengthening, and/or dislocation strengthening and may be optionally added. On the other hand, if the V content is too great, the effect becomes saturated and precipitates are liable to 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 may also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.007% or more and may also be 0.900% or less, 0.700% or less, 0.500% or less, 0.300% or less, 0.250% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

(Cu: 0% or More and 1.000% or Less)

Cu is an element able to contribute to improvement of at least one of the strength and corrosion resistance and may be optionally added. On the other hand, if excessively containing Cu, a drop in the toughness etc. is liable to be invited. In the hot rolled steel sheet of the present disclosure, the Cu content is 0% or more and 1.000% or less. The Cu content may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also 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 and 2.000% or Less)

Cr is an element raising the quenchability of steel and able to contribute to improvement of at least one of strength and corrosion resistance and may be optionally added. On the other hand, if excessively containing Cr, in addition to an increase in alloy costs, a drop in toughness etc. are liable to be invited. 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 may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also 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.

(Mo: 0% or More and 3.000% or Less)

Mo is an element raising the quenchability of steel and able to contribute to improvement of at least one of strength and corrosion resistance and may be optionally added. On the other hand, if excessively containing Mo, the deformation resistance at the time of working is liable to increase. In the hot rolled steel sheet of the present disclosure, the Mo content is 0% or more and 3.000% or less. The Mo content may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also be 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 0.300% or less.

(Ni: 0% or More and 0.500% or Less)

Ni is an element raising the quenchability of steel and able to contribute to improvement of at least one of strength and corrosion resistance and may be optionally added. On the other hand, if excessively containing Ni, the effect becomes saturated and a rise in the production costs is liable to be invited. 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 may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also 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 0.150% or less.

(B: 0% or More and 0.0100% or Less)

B is an element beneficial to increasing the strength of steel and may be optionally added. In the hot rolled steel sheet of the present disclosure, the B content is 0% or more and 0.0100% or less. The B content may also be 0.0001% or more, 0.0003% or more, or 0.0005% or more and may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less.

(Ca: 0% or More and 0.0500% or Less)

Ca is an element able to control the form of sulfides and may be optionally added. On the other hand, if excessively containing Ca, the effect becomes saturated and a rise in the production costs is liable to be invited. 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 may also be 0.0001% or more, 0.0003% or more, or 0.0005% or more and may be 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% or less.

(Mg: 0% or More and 0.050% or Less)

Mg is an element able to contribute to control of the form of sulfides and may be optionally added. On the other hand, if excessively containing Mg, the toughness is liable to fall. In the hot rolled steel sheet of the present disclosure, the Mg content is 0% or more and 0.050% or less. The Mg content may also be 0.001% or more and may also be 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, 0.010% or less, or 0.005% or less.

(REM: 0% or More and 0.100% or Less)

An REM, in the same way as Ca, is an element enabling control of the form of sulfides by trace addition and may be optionally added. On the other hand, if excessively containing an REM, coarse inclusions are liable to be formed. 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 may also be 0.001% or more, 0.003% or more, or 0.005% or more and may also be 0.080% or less, 0.060% or less, or 0.040% or less. "REM" is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

(Bi: 0% or More and 0.100% or Less)

Bi is an element able to contribute to improvement of the corrosion resistance etc. and may be optionally added. On the other hand, if excessively containing Bi, the effect becomes saturated and a rise in the production costs is liable to be invited. In the hot rolled steel sheet of the present disclosure, the Bi content is 0% or more and 0.100% or less. The Bi content may also be 0.001% or more or 0.002% or more and may also be 0.070% or less, 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 and 0.100% or Less)

Ta is an element able to contribute to control of the form of carbides and increase of the strength and may be optionally added. On the other hand, if excessively containing Ta, the toughness is liable to fall due to 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 may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also be 0.080% or less, 0.060% or less, or 0.040% or less.

(Zr: 0% or More and 0.500% or Less)

Zr is an element able to contribute to control of the form of sulfides and may be optionally added. On the other hand, if excessively containing Zr, the effect becomes saturated and a rise in the production costs is liable to be invited. 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 may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also be 0.400% or less, 0.300% or less, or 0.200% or less.

(Co: 0% or More and 3.000% or Less)

Co is an element able to contribute to improvement of at least one of quenchability and heat resistance and may be optionally added. On the other hand, if excessively containing Co, the workability is liable to fall and an increase in material costs is led to. In the hot rolled steel sheet of the present disclosure, the Co content is 0% or more and 3.000% or less. The Co content may also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more and may also be 2.000% or less, 1.000% or less, 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, 0.180% or less, 0.160% or less, or 0.140% or less.

(Zn: 0% or More and 0.200% or Less)

Zn is an element able to control the form of inclusions and may be optionally added. On the other hand, if excessively containing Zn, precipitates and inclusions are liable to be formed in large amounts. In the hot rolled steel sheet of the present disclosure, the Zn content is 0% or more and 0.200% or less. The Zn content may also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more and may also be 0.180% or less, 0.160% or less, or 0.140% or less.

(W: 0% or More and 0.200% or Less)

W is an element raising the quenchability of steel and able to contribute to improvement of strength and may be optionally added. On the other hand, if excessively containing W, coarse inclusions are liable to form. 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 may also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more and may also be 0.180% or less, 0.160% or less, or 0.140% or less.

(Sb: 0% or More and 0.500% or Less)

Sb is an element able to contribute to improvement of the corrosion resistance and may be optionally added. On the other hand, if excessively containing Sb, a drop in toughness is liable to be invited. In the hot rolled steel sheet of the present disclosure, the Sb content is 0% or more and 0.500% or less. The Sb content may also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more and may also be 0.400% or less, 0.300% or less, or 0.200% or less.

(As: 0% or More and 0.050% or Less)

As is an element able to contribute to improvement of the machinability of steel and may be optionally added. On the other hand, if excessively containing As, the workability is liable to fall. In the hot rolled steel sheet of the present disclosure, the As content is 0% or more and 0.050% or less. The As content may also be 0.001% or more, or 0.005% or more and may also be 0.030% or less, 0.010% or less, 0.009% or less, 0.008% or less, or 0.007% or less.

(Sn: 0% or More and 0.050% or Less)

Sn is an element able to contribute to improvement of the corrosion resistance and may be freely added. On the other hand, if excessively containing Sn, a drop in the toughness is liable to be invited. In the hot rolled steel sheet of the present disclosure, the Sn content is 0% or more and 0.050% or less. The Sn content may also be 0.001% or more, 0.005% or more, or 0.010% or more and may also be 0.047% or less, 0.045% or less, or 0.043% or less.

(Balance: Fe and Impurities)

In the chemical composition of the hot rolled steel sheet of the present disclosure, the balance besides the above constituents is comprised of Fe and impurities. The "impurities" are constituents entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the hot rolled steel sheet.
The chemical composition of the above-mentioned hot rolled steel sheet may be analyzed using a spark discharge emission spectrometer etc. Note that, for the C and S, values identified by making them burn in an oxygen stream by using a gas component analyzer etc. and measuring them by infrared absorption are employed. Further, for the N, a value identified by making a test piece taken from the hot rolled steel sheet melt in a helium stream and measuring it by the thermal conductivity method is employed.

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. By the prior austenite grains being made finer in this way, the concentration of strain in the microstructure is eased, the mechanical properties of the hot rolled steel sheet are enhanced, and the hole expansibility and impact property of the hot rolled steel sheet are improved. In particular, if 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 the strength, elongation, and hole expansibility and impact property of the hot rolled steel sheet easily become excellent. The lower limit of the prior austenite grain size is not particularly prescribed and is more than 0 µm. It may also be 1 µm or more, 3 µm or more, 5 µm or more, or 7 µm or more.
Further, the "prior austenite grain size" of the hot rolled steel sheet is the average particle size of the prior austenite grains. The average grain size of the prior austenite grains is measured in the following way. First, at a 1/4 position from an end face of the hot rolled steel sheet in the sheet width direction, a sample is taken to enable examination of the metallostructure at the cross-section having the sheet width direction as its normal direction (sheet thickness direction×rolling direction cross-section). The size of the sample depends on the measurement device, but, for example, the sample may be made a block of the entire thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the examined surface is polished to a mirror surface, then corroded using a picric acid saturated aqueous solution by the Bechet-Beaujard method. The grains appearing black due to the corrosion are deemed the prior austenite grains. The examined surface with the prior austenite grains brought out is examined by an optical microscope and eight or more fields of area of 0.05 mm² or more (total 0.40 mm² or more) are photographed. Further, from the steel structural photographs captured by an optical microscope, the circle equivalent diameters of the prior austenite grains are calculated. The circle equivalent diameters of all of the prior austenite grains contained in the photographed fields except for the prior austenite grains where the prior austenite grains as a whole are not contained in the photographed fields such as at the end faces of the photograph fields are calculated as explained above. The area average value of the circle equivalent diameters of the prior austenite grains obtained in the photographed fields (average value weighted by area) is calculated to obtain the average grain size of the prior austenite grains. Here, regarding the area average value, when the circle equivalent diameter of a certain prior austenite grain G₁ is D₁ and the area is A₁ and when the circle equivalent diameter of a prior austenite grain G₂ separate from that is D₂ and the area is A₂, the area average value D of the circle equivalent diameters of these two prior austenite grains can be calculated so that D=(A₁ ×D₁ +A₂ ×D₂ )/(A₁ +A₂ ).
In this disclosure, the "x/y position from an end face (where, x, y are made natural numbers satisfying x<y)" means a position moved in the sheet width direction from an end face in the sheet width direction of the steel sheet toward the center part of the steel sheet exactly by the distance of x/y of the sheet width. For example, when the sheet width of the steel sheet is 1 m, the "1/4 position from an end face" means a position becoming a distance of 0.25 m from an end face in the sheet width direction of the steel sheet.
In the present disclosure, the "sheet thickness x/y position (where, x, y are made natural numbers satisfying x<y)" means a position moved from the surface (sheet surface) of the steel sheet in the sheet thickness direction toward a center part of the steel sheet in the sheet width direction exactly by the distance (depth) of x/y of the sheet thickness "t" . For example, if the sheet thickness "t" of the steel sheet is 2 mm, the "sheet thickness 1/8 position" means the position becoming a depth of 0.25 mm from the surface of the steel sheet in the sheet thickness direction. Note that if the steel sheet has a plating layer or other coating on its surface, the "surface of the steel sheet" means the interface of the steel sheet and the coating while the "sheet thickness 't' " means the sheet thickness of the sheet (base material) minus the coating.
In the present disclosure, the "sheet width direction" is the direction perpendicular to the rolling direction and sheet thickness direction.
If the rolling direction of the steel sheet is not clear, as the method for identifying the rolling direction of the steel sheet, it is possible to employ, for example, the following method. A sheet thickness cross-section of the steel sheet is polished to finish it to a mirror surface, then an electron probe micro analyzer (EPMA) is used to measure the S concentration. The measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 µm. An image of the distribution in a 500 µm square range at the center part of sheet thickness is measured. At this time, a stretched regions with a high S concentration is judged as an MnS or other inclusion. At the time of examination, a plurality of fields may also be examined. Next, based on the sheet thickness cross-section first examined by the above method, surfaces parallel to surfaces rotated at 5° increments in the 0° to 180° range about the sheet thickness direction are examined in cross-section by the above method. The average value of the lengths of the long axes of the plurality of inclusions at each obtained cross-section is calculated for each cross-section and the cross-section giving the largest average value of the long axes of the inclusions is identified. The direction parallel to the long axis directions of the inclusions at that cross-section is judged to be the rolling direction.

1.3. Area Ratio

In the hot rolled steel sheet of the present disclosure, the area ratios of the regions with a GAM (Grain Average Misorientation) value of more than 0.6° and less than 2.0°, regions with a GAM value of 0.6° or less, and regions with a GAM value of 2.0° or more are identified as follows: Further, in the hot rolled steel sheet, "regions with a GAM value of 0.6° or less" are relatively soft in many cases, "regions with a GAM value of 2.0° or more are relatively hard in many cases, and "regions with a GAM value of more than 0.6° and less than 2.0°" have intermediate degrees of hardness in many cases .

(Area Ratio of Regions With GAM Value of More Than 0.6° and Less Than 2.0°)

In the hot rolled steel sheet of the present disclosure, the area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%. If the area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more, the hot rolled steel sheet easily becomes excellent in balance of strength, elongation, and hole expansibility. The area ratio of regions with a GAM value of more than 0.6° and less than 2.0° may also be 55% or more, 60% or more, 70% or more, 75% or more, 80% or more, or 85% or more and may also be 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, if the area ratio of regions with 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 easily becomes excellent in balance of strength, elongation, and hole expansibility.

(Area Ratio of Regions With GAM Value of 0.6° or Less)

In the hot rolled steel sheet of the present disclosure, the area ratio of regions with a GAM value of 0.6° or less is 0% or more and less than 50%. If the area ratio of regions with a GAM value of 0.6° or less is less than 50%, the hot rolled steel sheet easily becomes excellent in balance of strength, elongation, and hole expansibility. The hot rolled steel sheet of the present disclosure need only be one having the above-mentioned regions with a GAM value of more than 0.6° and less than 2.0° and the later mentioned regions with a GAM value of 2.0° or more. The area ratio of the regions with a GAM value of 0.6° or less may also be 0%. The area ratio of the regions with a GAM value of 0.6° or less may also 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 and may also be 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, if the area ratio of regions with a GAM value of 0.6° or less is 0% or more and 45% or less, the hot rolled steel sheet easily becomes more excellent in balance of strength, elongation, and hole expansibility.

(Area Ratio of Regions With GAM Value of 2.0° or More)

In the hot rolled steel sheet of the present disclosure, the area ratio of regions with a GAM value of 2.0° or more is more than 0% and 50% or less. If the hot rolled steel sheet of the present disclosure has regions with a GAM value of 2.0° or more in addition to the above-mentioned regions with a GAM value of more than 0.6° and less than 2.0°, it easily becomes excellent in balance of strength, elongation, and hole expansibility. The area ratio of regions with a GAM value of 2.0° or more may also 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 and may also be 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, if the area ratio of regions with a GAM value of 2.0° or more is more than 0% and 20% or less, among that, 1% or more and 20% or less, in particular 1% or more and 10% or less, the hot rolled steel sheet easily becomes more excellent in balance of strength, elongation, and hole expansibility.
Further, the "GAM values" of the regions of the hot rolled steel sheet are values measured by EBSP (Electron Backscatter Pattern). In each measurement region (for example, in one crystal grain (defined as region surrounded by grain boundaries with orientation difference of 15° or more)), the average value of the orientation difference between adjoining pixels (measurement points) is made the GAM value of the measurement region (crystal grains). The area ratio of regions with a GAM value of more than 0.6° and less than 2.0°, the area ratio of regions with a GAM value of 0.6° or less, and the area ratio of regions with a GAM value of 2.0° or more are measured by the following method. First, at a 1/4 position from an end face in the sheet width direction of the hot rolled steel sheet, a sample is taken to enable examination of the metallostructure at the cross-section having the sheet width direction as its normal direction (sheet thickness direction×rolling direction cross-section). The size of the sample depends on the measurement device, but, for example, the sample may be made a block of the entire thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the examined surface is polished to a mirror surface, then polished at room temperature for 8 minutes using colloidal silica not containing an alkali solution to remove the strain introduced to the surface of the sample. A region of 200 µm in the sheet thickness direction centered at the 1/4 depth position from the surface in the sheet thickness direction of the sample and 400 µm or more at any position in the rolling direction (rectangular region centered about 1/4 depth position in sheet thickness direction, which rectangular region having a length (short side) of 200 µm in the sheet thickness direction and a length (long side) of 400 µm or more in the rolling direction) is measured by the EBSP method at 0.2 µm measurement intervals. For the measurement, an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and an EBSD detector (HIKARI detector made by TSL) is used. At that time, the vacuum degree inside the EBSD analysis apparatus is made 9.6×10^-5 Pa or less, the acceleration voltage is made 15 kV, the irradiation current levels are made 13, and the irradiation levels of the electron beam are made 62. The GAM values can be calculated using the software "OIM Analysis^™" attached to the EBSD analysis apparatus. Note that defined crystal grains with a circle equivalent diameter of 0.6 µm or less are excluded since there is a possibility of large measurement error.

1.4. Relation (1): 1.7≤LGr/LGt

The hot rolled steel sheet of the present disclosure satisfies the relation (1) of 1.7≤LGr/LGt. Here, LGr is the area average value of the rolling direction projected lengths of prior austenite grains and LGt is the area average value of the sheet thickness direction projected lengths of prior austenite grains. Further, the "area average value" means the average value weighted by the area. Here, "area" is the area in the cross-section having the sheet width direction as its normal direction (sheet thickness direction×rolling direction cross-section). For example, when the rolling direction projected length of a certain prior austenite grain G₁ is LGr₁, its sheet thickness direction projected length is LGt₁, and its area is A₁ and when the rolling direction projected length of a separate prior austenite grain G₂ is LGr₂, its sheet thickness direction projected length is LGt₂, and its area is A₂, the area average value LGr of the rolling direction projected lengths of these two prior austenite grains is LGr=(A₁ ×LGr₁ +A₂ ×LGr₂ )/(A₁ +A₂) and the area average value LGt of the sheet thickness direction projected lengths is LGt=(A₁ ×LGt₁ +A₂ xLGt₂ )/(A₁ +A₂). In the hot rolled steel sheet of the present disclosure, the above relation (1) being satisfied means in other words that the prior austenite grains are stretched in the rolling direction. Further, as explained above, in the hot rolled steel sheet of the present disclosure, the prior austenite grain sizes are small and the area ratio of regions with a GAM value of 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 excellent in balance of strength and ductility are stretched along the rolling direction. In conventional thinking, if the LGr/LGt of the hot rolled steel sheet was large, it was believed that the elongation and the hole expansibility of the hot rolled steel sheet would fall. In the past, LGr/LGt was controlled to become smaller, that is, to become equiaxed. As opposed to this, in the hot rolled steel sheet of the present disclosure, by the relation (1) and also the later explained relation (2) being satisfied, rather the balance of strength, elongation, and hole expansibility is improved. Further, in the hot rolled steel sheet of the present disclosure, due to the prior austenite grains being stretched along the rolling direction, crack growth in the sheet thickness direction becomes difficult and the impact property becomes excellent.
In the above relation (1), the upper limit of LGl/LGt is not particularly prescribed. In the hot rolled steel sheet of the present disclosure, the following relation (1-1) may also be satisfied. LGr/LGt may also 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. Further, LGr/LGt may also be 2.0 or more, 2.5 or more, 3.0 or more, or 4.0 or more. $1.7 \leq LGr / LGt \leq 10.0$
Further, "LGr/LGt" in the hot rolled steel sheet is measured in the following way. First, at a 1/4 position from an end face in the sheet width direction of the hot rolled steel sheet, a sample is taken so that the metallostructure of the cross-section having the sheet width direction as its normal direction (sheet thickness direction×rolling direction cross-section) can be examined. The size of the sample depends on the measurement device, but, for example, the sample may be made a block of the entire thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the examined surface is polished to a mirror surface, then corroded using a picric acid saturated aqueous solution by the Bechet-Beaujard method. The grains appearing black due to the corrosion are deemed the prior austenite grains. The examined surface with the prior austenite grains brought out is examined by an optical microscope and eight or more fields of area of 0.05 mm² or more (total 0.40 mm² or more) are photographed. Further, from the steel structural photographs captured by an optical microscope, the area ratios of the prior austenite grains are calculated and the rolling direction projected lengths and the sheet thickness direction projected lengths are respectively measured. The ratios of the area averages at these are made LGr/LGt. If the above method does not sufficiently bring out the prior austenite grains, the reconstruction method described in "Study Toward Increasing Precision of Method of Reconstruction of Austenite Structures of Steel" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kawano, Nippon Steel & Sumikin Technical Reports, No. 404 (2016), p. 24 to 30) is used to identify the prior austenite grains and find the LGr/LGt of the prior austenite grains.

1.5. Relation (2): 1.20≤(LGr/LGt)/(LMr/LMt)

The hot rolled steel sheet of the present disclosure satisfies the relation (2) of 1.20_<(LGr/LGt)/(LMr/LMt). Here, LGr and LGt are as explained above, LMr is the area average value of the rolling direction projected lengths of regions with a GAM value of 2.0° or more, and LMt is the area average value of the sheet thickness direction projected lengths of regions with a GAM value of 2.0° or more. That is, in the hot rolled steel sheet of the present disclosure, the LGr/LGt of the prior austenite grains becomes 1.20 times the LMr/LMt of the hard phases. In this way, by the LGr/LGt of the prior austenite grains being a certain extent or more larger than the LMr/LMt of the hard phases (the LMr/LMt of the hard phases being a certain extent or more smaller than the LGr/LGt of the prior austenite grains), the hard phases become present dispersed, the variation in strength and ductility of the steel sheet as a whole becomes smaller, and the hot rolled steel sheet becomes excellent in balance of strength, elongation, and hole expansibility.
In the above relation (2), the upper limit of (LGr/LGt)/(LMr/LMt) is not particularly prescribed. In the hot rolled steel sheet of the present disclosure, the following relation (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. Further, (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. $1.20 \leq (LGr / LGt) / (LMr / LMt) \leq 5.00$
Further, the "LMr/LMt" at the hot rolled steel sheet is measured in the following way. First, at a 1/4 position from an end face in the sheet width direction of the hot rolled steel sheet, a sample is taken to enable examination of the metallostructure at the cross-section having the sheet width direction as its normal direction (sheet thickness direction×rolling direction cross-section). The size of the sample depends on the measurement device, but, for example, the sample may be made a block of the entire thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the examined surface is polished to a mirror surface, then polished at room temperature for 8 minutes using colloidal silica not containing an alkali solution to remove the strain introduced to the surface of the sample. A region of 200 µm in the sheet thickness direction centered at the 1/4 depth position from the surface in the sheet thickness direction of the sample and 400 µm or more at any position in the rolling direction (rectangular region centered about 1/4 depth position in sheet thickness direction, which rectangular region having a length (short side) of 200 µm in the sheet thickness direction and a length (long side) of 400 µm or more in the rolling direction) is measured by the EBSP method at 0.2 µm measurement intervals. For the measurement, an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and an EBSD detector (HIKARI detector made by TSL) is used. At that time, the vacuum degree inside the EBSD analysis apparatus is made 9.6×10^-5 Pa or less, the acceleration voltage is made 15 kV, the irradiation current levels are made 13, and the irradiation levels of the electron beam are made 62. Next, the software "OIM Analysis^™" attached to the EBSD analysis apparatus is used to calculate the GAM values. Note that defined crystal grains with a circle equivalent diameter of 0.6 µm or less are excluded since there is a possibility of large measurement error. From the information of the GAM values calculated, regions with a GAM value of 2.0° or more are identified. The areas of the identified regions with a GAM value of 2.0° or more are calculated. From their shapes, the rolling direction projected lengths and sheet thickness direction projected lengths are respectively measured. The ratios of the area averages taken at these are made LMr/LMt.

1.6. Mechanical Properties Etc.

The hot rolled steel sheet of the present disclosure becomes excellent in balance of strength, elongation, and hole expansibility and excellent in impact property due to having the above-mentioned chemical composition and steel structure.

(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 960 MPa or more tensile strength TS. The tensile strength TS may also be 970 MPa or more or 980 MPa or more. The upper limit of the tensile strength TS is not particularly prescribed, but, for example, may be 1200 MPa or less, 1150 MPa or less, or 1100 MPa or less. Further, the tensile test for measuring the tensile strength TS of the hot rolled steel sheet is based on JIS Z 2241 and is performed taking a No. 5 test piece from an orientation where the long direction of the test piece becomes parallel with the rolling perpendicular direction (sheet width direction) of the steel sheet. If the above No. 5 test piece cannot be taken from the hot rolled steel sheet to be measured, as the test piece for measuring the tensile strength TS, a small test piece having the sheet width direction as its long direction can be used instead.

(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 4.0% or more and 12.0% or less uniform elongation uEL. The uniform elongation uEL may also 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 also 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. Further, the tensile test for measuring the uniform elongation uEL of the hot rolled steel sheet is based on JIS Z 2241 and is performed taking a No. 5 test piece from an orientation where the long direction of the test piece becomes parallel with the rolling perpendicular direction (sheet width direction) of the steel sheet.

(Hole Expansibility)

The hot rolled steel sheet of the present disclosure has excellent hole expansibility. For example, the hot rolled steel sheet of the present disclosure may have a 40% or more and 110% or less hole expansion rate λ. The hole expansion rate λ may also be 45% or more or 50% or more and may also be 100% or less, 90% or less, 80% or less, or 70% or less. Further, the hole expansibility of the hot rolled steel sheet is determined by punching a diameter 10 mm circular hole under conditions giving a clearance of 12.5% while making the burrs form at the die side using a 60° conical punch. The hole expansion rate (%) is used to evaluate it. A hole expansion test is conducted five times and the average value of the same is made the hole expansion rate λ.

(Impact Property)

The hot rolled steel sheet of the present disclosure has an excellent impact property. The impact property of hot rolled steel sheet can, for example, be evaluated by the crack growth resistance in the sheet thickness direction. The crack growth resistance in the sheet thickness direction is determined by the displacement-load curve when punching a hot rolled steel sheet. For example, it is determined by the ratio W2/W1 of the energy W2 and energy W1. Here, W2=∫Fds (maximum load and on) while W1=∫Fds (before 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.

(Sheet Thickness)

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

(Applications)

As explained above, the hot rolled steel sheet of the present disclosure is excellent in balance of strength, ductility, and hole expansibility and is excellent in impact property. Such a hot rolled steel sheet can, for example, be employed as the material for parts for automobiles such as suspension parts, structural parts, frames, and frame parts. In particular, it is optimal as a material for suspension parts of automobiles. As specific examples of suspension parts of automobiles, lower arms, upper arms, trailings, etc. may be mentioned.

2. Method of Production of Hot Rolled Steel Sheet

Below, one example of the method for producing the hot rolled steel sheet of the present disclosure will be explained, but the method of production of the hot rolled steel sheet is not limited to the one explained below. The method of production of the hot rolled steel sheet according to one embodiment comprises:

a heating step of heating a slab,
a hot rolling step of hot rolling the heated slab,
a cooling step of cooling the hot rolled steel sheet obtained by the hot rolling, and
a coiling step of coiling the cooled hot rolled steel sheet,
the slab containing, by 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,
- Mo: 0% or more and 3.000% or less,
- Ni: 0% or more and 0.500% or less,
- B: 0% or more and 0.0100% or less,
- Ca: 0% or more and 0.0500% or less,
- Mg: 0% or more and 0.050% or less,
- REM: 0% or more and 0.100% or less,
- Bi: 0% or more and 0.100% or less,
- Ta: 0% or more and 0.100% or less,
- Zr: 0% or more and 0.500% or less,
- Co: 0% or more and 3.000% or less,
- Zn: 0% or more and 0.200% or less,
- W: 0% or more and 0.200% or less,
- Sb: 0% or more and 0.500% or less,
- As: 0% or more and 0.050% or less,
- Sn: 0% or more and 0.050% or less and
- a balance: Fe and impurities,
the hot rolling step provided with rough rolling and finish rolling,
a start temperature ST of the finish rolling is 1000°C or more and 1150°C or less,
the finish rolling includes two or more high temperature difference rolling operations,
the high temperature difference rolling has a temperature difference ΔT between a rolling temperature of a rolling stand performing the high temperature difference rolling and a rolling temperature at the rolling stand right before it of 30°C or more,
in the finish rolling, a total rolling reduction after the second high temperature difference rolling is 50% or more,
an finish temperature FT of the finish rolling is 940°C or less,
a time from the end of the finish rolling to the start of cooling is within 2.0 seconds,
in the cooling step, after the start of the cooling, accelerated cooling is performed, a cooling stop temperature of the accelerated cooling being 520°C or more and 720°C or less, and,
in the cooling step, a slow cooling time in a 720°C to 470°C temperature region is 2.0 seconds or more.

2.1. Heating Step

In the heating step, a slab having the above-mentioned chemical composition is heated. If the heating temperature is too low, the carbides and nitrides insufficiently dissolve. On the other hand, if the heating temperature is too high, the amount of scale formed increases and the yield falls. On this point, the heating temperature of the slab at the heating step may be, for example, 1100°C or more and 1300°C or less. The heating temperature may also be 1150°C or more or 1200°C or more and may also be 1260°C or less. The heating time of the slab in the heating step may be any time enabling the slab as a whole to reach the target temperature. The heating time may also, for example, be 6000 seconds (100 minutes) or more or 9000 seconds (150 minutes) or more. In particular, a higher effect is easily obtained by holding at a 1150°C or more temperature for 6000 seconds (100 minutes) or more.

2.2. Hot Rolling Step

In the hot rolling step, the slab heated by the heating step is hot rolled. The hot rolling step is provided with rough rolling and finish rolling.

(Rough Rolling)

The rough rolling conditions are not particularly limited. It is sufficient that the slab be rolled at a predetermined temperature by predetermined rolling reduction. The temperature in the rough rolling may, for example, be the heating temperature of the heating step or less and the start temperature ST of the finish rolling or more. The rolling reduction in the rough rolling may, for example, also be a reduction in sheet thickness at 800 to 1150°C of 90% or more.

(Finish Rolling)

In the finish rolling, the rough rolled slab (rough bar) is rolled several times by a plurality of 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 produced hot rolled steel sheet does not satisfy the above requirement of the prior austenite grain size and the hole expansibility etc. easily fall. On the other hand, if the start temperature ST is too high, the structure of the steel can no longer be suitably controlled and the finally produced hot rolled steel sheet does not satisfy the above requirement of the prior austenite grain size and the relations (1) and (2) and the impact property easily falls. By the start temperature ST being 1000°C or more and 1150°C or less, these problems are eliminated. The start temperature ST may also be 1050°C or more 1150°C or less.
The finish rolling includes two or more high temperature difference rolling operations. In high temperature difference rolling, the temperature difference ΔT between the rolling temperature at the rolling stand performing the high temperature difference rolling and the rolling temperature at the rolling stand right before that becomes 30°C or more. Here, the rolling temperature is the temperature of the entry side of the rolling stand, that is, the surface temperature of the steel sheet measured right before the steel sheet is rolled at the rolling stand. Due to the first high temperature difference rolling, TiC, which act as nuclei for transformation, precipitates in the steel structure at a high density. Due to the second high temperature difference rolling, a drive force is obtained for realizing the desired steel structure. If high temperature difference rolling is performed one time or less, the desired steel structure cannot be realized, the finally produced hot rolled steel sheet does not satisfy the above (2), and the hole expansibility easily falls. If high temperature difference rolling is performed two times or more, the desired steel structure can be realized and the finally produced hot rolled steel sheet becomes excellent in balance of strength, elongation, and hole expansibility. The number of times of high temperature difference rolling may also be changed depending on the number of the rolling stands of the finish rolling. The number of times of the high temperature difference rolling may, for example, be 2 times or more and 10 times or less and may also be 3 times or more or 4 times or more and may also be 7 times or less or 6 times or less. The temperature difference ΔT of the high temperature difference rolling may be 30°C or more and may also be 35°C or more, 40°C or more, 45°C or more, or 50°C or more and may also be 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, for example, can be controlled by controlling the amount of spraying of water or other coolant from the cooling sprayers and other cooling devices right after rolling, by controlling the conveyance speed of the steel sheet between rolling operations, etc.
In the finish rolling, the total rolling reduction after the second high temperature difference rolling is 50% or more. If the total rolling reduction after the second high temperature difference rolling is too low, the structure of the steel can no longer be suitably controlled, the finally produced hot rolled steel sheet does not satisfy the above relation (2), and the hole expansibility easily falls. If the total rolling reduction after the second high temperature difference rolling is 50% or more, such problems are eliminated. The total rolling reduction after the second high temperature difference rolling may also be 55% or more, 60% or more, or 65% or more. If the total rolling reduction after the second high temperature difference rolling is too high, the anisotropy of the structure rises and the hole expansibility easily falls. If the total rolling reduction after the second high temperature difference rolling is 80% or less, such problems are eliminated, therefore the total rolling reduction after the second high temperature difference rolling is preferably 80% or less. The upper limit of the total rolling reduction after the second high temperature difference rolling may also be 75% or less or 70% or less. Note that even if performing high temperature difference rolling three times or more, the "total rolling reduction" means the total rolling reduction after the second high temperature difference rolling. Further, the total rolling reduction after the second high temperature difference rolling means the rate of reduction of sheet thickness due to the rolling after the second high temperature difference rolling (able to include high temperature difference rolling) from the sheet thickness after the second high temperature difference rolling. The second high temperature difference rolling of course does not become the final stage (final stand) of the finish rolling.
Due to the second high temperature difference rolling being performed at a predetermined temperature, it becomes easier for the TiC precipitating at a high density to suppress the reduction of dislocations of the steel structure, therefore this is preferable. Due to this, it is possible to control the value of (LGr/LGt)/(LMr/LMt) well and the impact property is improved. For example, in the case of steel where the Ti content is 0.1 to 0.13%, the Nb content is 0.008 to 0.02%, the V content is less than 0.01%, the Mo content is less than 0.01%, and the B content is less than 0.0001%, the rolling temperature of the second high temperature difference rolling is preferably made 980 to 1000°C.
The finish temperature FT of the finish rolling is 940°C or less. If the finish temperature FT is too high, it becomes no longer possible to suitably control the structure of the steel, the finally produced hot rolled steel sheet does not satisfy the above-mentioned requirement of the prior austenite grain size and the relatiosn (1) and (2) and the impact property easily falls. If the finish temperature FT is 940°C or less, these problems are eliminated. The finish temperature FT may also be 920°C or less or 900°C or less. The lower limit of the finish temperature FT is not particularly prescribed so long as the later explained requirement of the cooling step can be satisfied. For example, the finish temperature FT may also 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 the hot rolling is cooled. Here, the time from the end of the above finish rolling to the start of cooling is within 2.0 seconds. If the time is too long, coarsening of the crystal grains causes the prior austenite grain size to become more than 25 µm and the finally produced hot rolled steel sheet easily becomes inferior in balance of strength, elongation, and hole expansibility. By the time being within 2.0 seconds, such problems are eliminated. The time may also be within 1.8 seconds, within 1.6 seconds, within 1.4 seconds, or within 1.2 seconds.
In the cooling step, after the start of cooling, accelerated cooling is performed. "Accelerated cooling" means cooling under the cooling conditions of a cooling speed of 20°C/s or more and 200°C/s or less. It is important that the cooling stop temperature of the accelerated cooling be 520°C or more and 720°C or less. Transformation of the regions with a GAM value of 2.0° occurs mainly during the slow cooling after stopping cooling of the accelerated cooling. By the stop temperature of the accelerated cooling being 520°C or more and 720°C or less, the amount of formation of regions with a GAM value of less than 2.0° becomes suitable. At other than this temperature, the ratio of the regions with a GAM value of 2.0° or more excessively increases and sometimes the uniform elongation falls.
In the cooling step, the slow cooling time in the 720°C to 470°C temperature region is 2.0 seconds or more. "Slow cooling" means cooling under cooling conditions of a cooling speed of less than 20°C/s. If the slow cooling time in the 720°C to 470°C temperature region is 2.0 seconds or more, the area ratio of regions with a GAM value of more than 0.6° to less than 2.0° becomes 50% or more. For example, by slow cooling the hot rolled steel sheet at the run out table (ROT), the slow cooling time at the 720°C to 470°C temperature region can become 2.0 seconds or more. The slow cooling time may also 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, if the slow cooling time in the 680°C to 580 temperature region is 3.0 seconds or more, the amount of formation of regions with a GAM value of 2.0° or more can be more suitably controlled. The upper limit of the slow cooling time is not particularly prescribed. An optimal slow cooling time may be determined considering productivity etc. The slow cooling time may, for example, be 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, regions with a GAM value of 2.0° or more are easily excessively formed. Further, the above relation (2) is not satisfied and the finally produced hot rolled steel sheet easily becomes inferior in the balance of strength, elongation, and hole expansibility.
In the cooling step, the average cooling speed after the end of slow cooling until 300°C is preferably 30°C/s or more. If the average cooling speed is low, softening occurs due to tempering and the strength of the finally produced hot rolled steel sheet easily falls. If the average cooling speed is 30°C/s or more, such a problem can be more reliably eliminated. The average cooling speed may also 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 speed is not particularly limited. The average cooling speed 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. Further, if the coiling temperature is less than 300°C, the average cooling speed from 300°C to the coiling temperature is not particularly limited.

2.4. Coiling Step

At the coiling step, the hot rolled steel sheet cooled by the cooling step is coiled up. The conditions of the coiling are not particularly limited. The coiling temperature of the coiling step is for example 300°C or less. The coiling temperature may also be 200°C or less, 100°C or less, or 50°C or less and may also be 0°C or more or 20°C or more.
As explained above, by (1) using a slab having a suitable chemical composition, (2) modifying the finish rolling conditions at the time of the hot rolling to control the form of the prior austenite grains, and (3) controlling the cooling speed at the time of cooling to control the transformation behavior (for example, controlling the cooling conditions at a run out table (ROT)), it is possible to produce hot rolled steel sheet of the present disclosure. Further, when producing hot rolled steel sheet, it is also possible to perform steps other than the above in addition to the above steps. For example, after the coiling step, it is also possible to freely perform a tempering step.

EXAMPLES

1. Preparation of Hot Rolled Steel Sheet

Slabs having the chemical compositions shown in the following Tables 1 and 2 were subjected to a heating step and hot rolling step under the conditions shown in the following Table 3. After the end of the hot rolling, they were successively subjected to a cooling step and coiling step under the conditions shown in Table 3 to obtain sheet thickness 3.0 mm hot rolled steel sheets (steel strips). It was confirmed that the chemical compositions at the hot rolled steel sheets (steel strips) were also substantially the same as the chemical compositions at the slabs and the ones shown in Tables 1 and 2. Further, in Table 3, "ΔT≥30°C rolling performed 2 times or more" means "high temperature difference rolling performed 2 times or more". "High temperature difference rolling" means rolling so that the temperature difference ΔT between the rolling temperature at the rolling stand performing the high temperature difference rolling and the rolling temperature at the rolling stand right before it becomes 30°C or more. In Table 3, cases where the high temperature difference rolling is performed 2 times or more were indicated as "Yes" while cases where it was performed 1 time or less were indicated as "No". Further, in Table 3, "Total rolling reduction after 2nd ΔT temperature difference occurs" means "total rolling reduction after 2nd high temperature difference rolling". Further, in Table 3, "Accelerated cooling" means cooling after start of cooling which is performed by cooling speed of 20°C/s or more and 200°C/s or less.

[Table 1]

Steel type	Mass% (balance of Fe and impurities)
Steel type	C	Si	Mn	Ti	Al	P	S	N	O	Nb	v	Cu	Cr	Mo
A	0.044	0.90	2.02	0.111	0.187	0.008	0.0037	0.0023	0.002	0.007	-	-	-	-
B	0.121	0 88	1.30	0.125	0.108	0.008	0.0035	0.0034	0.001	0.012	-	-	-	-
C	0.072	3.05	1.70	0.097	0.081	0.009	0.0044	0.0034	0.003	0.013	-	-	-	-
D	0.060	0.60	2 80	0.105	0.096	0.008	0.0043	0.0030	0.003	0.011	-	-	-	-
E	0.085	1.00	1.10	0.094	0.070	0.009	0.0038	0.0024	0.002	0.014	-	-	0.100	-
F	0.080	088	1.70	0.210	0.119	0.008	0.0040	0.0017	0.001	0.011	-	-	-	-
G	0066	093	193	0.002	0.105	0.007	0.0038	0.0024	0.003	0..011	-	-	-	-
H	0.063	0.83	2.15	0.100	0.410	0.007	0.0044	0.0023	0.002	0.007	-	-	-	-
I	0.071	1.04	2.10	0.093	0.005	0.008	0.0043	0.0030	0.002	0.011	-	-	-	-
J	0 068	1.00	2.02	0.133	0.157	0.090	0.0038	0.0026	0.002	0.007	-	-	-	-
K	0070	084	1.95	0.125	0.056	0.009	0.0120	0.0016	0.002	0.012	-	0.180	-	-
L	0.077	097	2.09	0.097	0.132	0.007	0.0045	0.0100	0.001	0.015	-	-	0.200	-
M	0.072	0.85	2.02	0.111	0.199	0.008	0.0042	0.0025	0.012	-	-	-	-	0.110
N	0077	0.83	1.96	0.087	0.170	0.008	0.0040	0.0019	0.001	-	-	-	-	-
O	0071	0.77	2.20	0.114	0.068	0.007	0.0042	0.0029	0.003	0.009	0.013	-	0 100	-
P	0.071	0.97	2.02	0.086	0.168	0.007	0.0036	0.0035	0.002	0.007	0.009	-	-	-
Q	0110	0.90	1.25	0.118	0.181	0.009	0.0039	0.0024	0.001	0.008	-	-	-	-
R	0047	0.60	2.55	0.087	0.055	0.008	0.0040	0.0027	0.002	0.010	-	-	0.200	-
S	0.070	1 61	1.85	0.107	0.073	0.009	0.0044	0.0021	0.002	0.012	-	0.100	-	-
T	0.090	0.01	2.20	0.093	0.159	0.007	0.0039	0.0033	0.002	0.005	-	-	-	-
U	0.064	0.91	2.10	0.165	0.056	0.009	0.0045	0.0019	0 003	0.011	-	-	0.100	-
V	0.064	1.02	2.05	0.103	0.140	0.008	0.0045	0.0025	0.001	0.032	-	-	-	0.120
W	0077	1 01	2.12	0.121	0.203	0 008	0.0039	0 0028	0.002	0.008	0.290	-	0.100	-
X	0.070	0.87	2.11	0.130	0.350	0.007	0.0037	0.0018	0.003	0.010	-	-	-	-
Y	0.077	0.83	1.91	0.108	0.020	0.009	0.0042	0.0032	0.002	0.010	-	-	0.100	-
Z	0.076	0.85	1.60	0.111	0.108	0.008	0.0041	0.0018	0.003	0.010	-	-	-	-
AA	0.069	072	2.10	0.045	0.045	0.006	0.0022	0.0021	0.002	0.004	0.013	-	-	-

Underlines indicate outside predetermined scope.

[Table 2]

Steel type	Mass% (balance of Fe and impurities)
	Ni	B	Ca	Mg	REM	Bi	Ta	Zr	Co	Zn	w	Sb	As	Sn
A	-	-	-	-	-	-	-	-	-	-	-	-	-	-
B	-	-	-	-	-	-	-	-	-	-	-	-	-	-
C	-	-	0.0031	0.002	-	-	-	-	-	-	-	-	-	-
D	-	-	-	-	-	0.002	-	-	-	-	-	-	-	-
E	-	-	-	-	-	-	0010	-	-	-	-	-	-	-
F	-	-	-	-	-	-	-	-	-	0.160	-	-	-	-
G	-	0.0037	-	-	-	-	-	-	-	-	-	-	-	-
H	-	-	-	-	0.003	-	-	-	-	-	-	-	-	-
I	-	-	-	-	-	-	-	-	-	-	0170	-	-	-
J	-	-	-	-	-	-	-	0.340	-	-	-	-	-	-
K	-	-	-	-	-	-	-	-	0.190	-	-	-	-	-
L	-	0.0013	-	-	-	-	-	-	-	-	-	-	-	-
M	-	-	-	-	-	-	-	-	-	-	-	-	-	0.020
N	-	-	-	-	-	-	-	-	-	-	-	-	-	-
O	-	-	-	-	-	-	0.020	-	-	-	-	-	-	-
P	-	-	-	-	-	-	-	-	-	-	-	0.060	-	-
Q	-	-	-	-	-	-	-	0.300	-	-	-	-	-	-
R	-	-	-	-	-	-	-	-	-	-	-	-	-	0.040
S	-	-	-	-	-	-	-	-	-	-	-	-	-	-
T	0.050	-	-	-	-	-	-	-	-	-	-	-	0.006	-
U	-	-	00030	-	-	-	-	-	-	0.120	-	-	-	-
V	-	-	-	-	-	-	-	-	-	-	-	-	-	-
W	-	-	-	-	-	0.002	-	-	-	-	0.150	-	-	-
X	-	-	-	0.002	-	-	-	-	0.180	-	-	-	-	-
Y	-	-	-	-	0.100	-	-	-	-	-	-	-	-	-
Z	-	0.0010	-	-	-	-	-	-	-	-	-	-	-	-
AA	-	-	-	-	-	-	0015	-	-	-	-	-	-	-

Underlines indicate steel composition outside predetermined scope.

[Table 3]

Production no.	Steel type	Slab heating		Rough rolling	Finish rolling				Cooling				Coiling	Remarks
		Heating temp.	Holding time at 1150°C or mor temp. region	800 to 1150°C sheet thickness reduction	Finishing start temp.	Finishing end temp.	ΔT≥30°C rolling performed 2 times or more	Total rolling reduction after 2nd ΔT temp. difference occurs	Time from end of hot rolling to start o cooling	Accelerated cooling start temp.	720°C to 470°C temp region slow cooling time	Average cooling speed after low cooling end to 300°C	Coiling temp.
		°C	S	%	°C	°C	-	%	s	°C	s	°C/s	°C
1	A	1235	9391	92	1095	930	Yes	70	15	629	2.7	47	74	Comp ex.
2	B	1236	9412	92	1066	878	Yes	70	1.1	644	33	67	67	Comp ex.
3	C	1228	9404	Rolling not possible										Comp ex.
4	D	1229	9392	92	1075	899	Yes	54	1.2	620	28	52	56	Comp ex
5	E	1239	9409	94	1063	889	Yes	68	14	627	33	50	51	Comp ex.
6	F	1250	9397	93	1063	869	Yes	64	1.4	575	3.1	48	64	Comp ex.
7	G	1249	9395	94	1098	940	Yes	67	14	587	3 2	53	53	Comp ex.
8	H	Slab cracking												Comp ex
9	I	1250	9390	94	1072	891	Yes	57	13	642	2.5	73	62	Comp ex.
10	J	Slab cracking												Comp ex
11	K	1230	9387	95	1060	880	Yes	69	1.5	612	29	65	57	Comp ex
12	L	Slab cracking												Comp ex.
13	M	1254	9389	93	1091	925	Yes	56	1.4	640	2.9	50	69	Comp. ex.
14	N	1253	9391	95	1087	893	Yes	67	13	640	37	73	46	Inv ex
15	AA	1234	9391	95	990	840	Yes	54	1.1	656	32	51	34	Comp ex
16	AA	1241	9387	95	1180	990	Yes	62	1.1	614	25	50	33	Comp. ex.
17	P	1243	9414	94	1074	891	No	72	12	628	30	58	68	Comp. ex.
18	N	1247	9408	94	1057	889	Yes	30	1.4	587	2.9	54	42	Comp. ex.
19	AA	1251	9407	95	1058	884	Yes	59	3.0	633	2.6	61	52	Comp. ex.
20	P	1249	9398	94	1078	900	Yes	73	1.3	760	29	64	42	Comp. ex.
21	N	1227	9392	94	1088	897	Yes	58	1.1	450	29	65	34	Comp ex.
22	R	1235	9386	93	1057	870	Yes	62	1.5	574	30	31	47	Inv. ex.
23	P	1243	9404	94	1130	935	Yes	63	1.1	643	3.0	65	43	Inv. ex.
24	N	1254	9415	95	1080	875	Yes	67	14	710	2.5	48	71	Inv. ex
25	O	1242	9413	93	1010	850	Yes	55	1.3	530	3.6	69	39	Inv. ex
26	Q	1254	9405	94	1061	893	Yes	67	1.1	612	2.1	31	26	Inv. ex.
27	R	1247	9401	94	1066	874	Yes	62	15	604	3.3	70	43	Inv. ex.
28	S	1247	9400	93	1075	904	Yes	54	1.2	589	28	61	50	Inv. ex.
29	T	1247	9408	93	1079	887	Yes	64	14	578	26	53	48	Inv. ex
30	U	1232	9398	93	1061	887	Yes	58	1.1	647	3.0	73	70	Inv. ex.
31	V	1229	9405	94	1084	910	Yes	66	1.1	656	3.0	67	70	Inv. ex.
32	W	1244	9401	93	1070	889	Yes	60	1.1	602	2.8	50	73	Inv. ex.
33	X	1227	9393	93	1103	934	Yes	51	1.5	665	3.2	61	37	Inv. ex.
34	Y	1235	9391	95	1072	877	Yes	60	1.1	576	32	68	71	Inv. ex.
35	Z	1226	9406	94	1065	888	Yes	59	14	579	3 1	74	34	Inv ex
36	S	1244	9350	91	1066	860	Yes	59	1 1	670	3 2	56	85	Inv ex
37	O	1235	9408	92	1005	825	Yes	75	1.1	570	3.5	65	42	Inv. ex.
38	AA	1280	9381	92	1210	940	Yes	59	1.2	620	3.1	51	38	Comp ex
39	AA	1283	9380	91	1145	970	Yes	57	1 3	625	3 2	58	80	Comp ex
40	W	1240	9305	92	1065	891	Yes	57	1 2	610	0 2	100	40	Comp ex
41	AA	1255	9422	95	1030	860	Yes	53	1.5	560	3.2	72	51	Inv ex
42	Y	1232	9395	94	1075	852	Yes	72	1.3	616	3 4	69	70	Inv ex
43	Y	1237	9355	96	1070	862	Yes	85	1 2	609	3 1	66	72	Inv ex
44	G	1245	9384	92	1096	923	Yes	71	1 3	622	3 5	52	55	Comp ex

Underlines indicate steel composition and structure are outside predetermined ranges or the properties are not preferable.

2. Measurement of Prior Austenite Grain Size

Each hot rolled steel sheet was measured for prior austenite grain size. The method of measurement of the prior austenite grain size was as explained above. The results are shown in the following Table 4.

3. Measurement of GAM Values and Area Ratios

In each of the hot rolled steel sheets, the area ratio of the regions with a GAM value of more than 0.6° and less than 2.0°, the area ratio of the regions with a GAM value of 0.6° or less, and the area ratio of the regions with a GAM value of 2.0° or more were measured. The method of measurement of the GAM values and area ratios are as explained above. The results are shown in the following Table 4.

4. Measurement of LGr/LGt and (LGr/LGt)/(LMr/LMt)

Each hot rolled steel sheet was measured for LGr (area average value of rolling direction projected lengths of prior austenite grains) LGt (area average value of sheet thickness direction projected lengths of prior austenite grains), LMr (area average value of rolling direction projected lengths of regions with a GAM value of 2.0° or more), and LMt (area average value of sheet thickness direction projected lengths of regions with a GAM value of 2.0° or more). "LGr/LGt" and "(LGr/LGt)/(LMr/LMt)" were calculated. The methods of measurement and calculation of LGr/LGt and LMr/LMt were as explained above. The results are shown in the following Table 4.

5. Evaluation of Mechanical Properties

5.1. Tensile Strength TS, Uniform Elongation uEL, and Hole Expansion Rate λ

Each hot rolled steel sheet was measured for tensile strength TS, uniform elongation uEL, and hole expansion rate λ. The methods for measurement are as explained above. The results are shown in the following Table 4. Further, in the following Table 4, cases satisfying the mechanical properties particularly preferable envisioning application to suspension parts of automobiles etc., that is, "tensile strength TS: 960 MPa or more", "uniform elongation uEL: 4.0% or more and 12.0% or less", and "hole expansion rate λ: 40% or more and 110% or less", were judged excellent in balance of strength, elongation, and hole expansibility.

5.2. Impact Property

The impact property of each hot rolled steel sheet was evaluated based on the crack growth resistance in the sheet thickness direction of each hot rolled steel sheet. The crack growth resistance in the sheet thickness direction was determined by the displacement-load curve when punching hot rolled steel sheet. Specifically, it was determined by the ratio W2/W1 of the following energy W2 and energy W1. Here, W2=∫Fds (maximum load on) while W1=∫Fds (before maximum load). Further, F is the punching load (N), while S is the punching stroke (mm). In the present embodiment, cases satisfying 0.15≤W2/W1 were evaluated as being excellent in impact property, while cases satisfying 0.2<W2/W1 were evaluated as being particularly excellent in impact property. The results are shown in the following Table 4.

[Table 4]

Production no.	Steel type	Prior γ grain size	GAM value 0.6° or less	GAM value more than 0 6° to less than 2.0°	GAM value 2.0° or more	LGr /LGt	(LGr/LGt)/ (LMr/LMt)	TS	uEL	λ	W2/W 1	Remarks
Production no.	Steel type	Mm	Area%	Area%	Area%	-	-	MPa	%	%	-
1	A	13	52	33	15	4 5	2 41	862	10.3	102	0 17	Comp ex
2	B	12	6	42	52	64	2.41	1097	3.8	39	0.20	Comp. ex
3	C	Rolling not possible										Comp ex
4	D	13	4	45	51	52	2.13	1077	3 8	53	0 21	Comp ex
5	E	12	52	35	13	5.4	2.93	907	9.0	86	0.22	Comp ex
6	F	12	8	91	1	5.5	2.97	986	7.0	35	0.22	Comp. ex.
7	G	26	14	85	1	5 1	1.91	940	7 1	31	0 21	Comp ex
8	H	Slab cracking										Comp ex
9	I	12	4	74	22	5.2	3.04	1031	6.1	35	0.20	Comp. ex
10	J	Slab cracking										Comp ex
11	K	12	5	87	8	5.5	2.72	979	6.6	36	0.21	Comp ex
12	L	Slab cracking										Comp ex
13	M	12	6	92	2	4.9	2.44	994	7.1	37	0.16	Comp ex
14	N	14	21	72	7	5.0	2.32	1003	6.2	70	0.19	Inv. ex
15	AA	26	3	91	6	6.4	3.09	1021	5.7	37	0.21	Comp ex
16	AA	26	0	97	3	1.6	1.05	1019	6.1	78	0.09	Comp ex
17	I	12	8	83	9	5.6	1.16	1006	5.9	35	0.22	Comp ex.
1 8	N	13	2	93	5	5.5	1.05	1008	6.0	39	0.21	Comp. ex.
19	AA	26	5	92	3	5.5	2.40	1026	6.0	37	0.21	Comp ex
20	P	12	2	8	90	5.4	2.76	1100	3 0	60	0.19	Comp ex
21	N	12	2	11	87	5.1	2.14	1090	3.2	58	0.19	Comp. ex.
22	R	11	5	92	3	5.5	2.86	960	8.2	57	0.22	Inv. ex.
23	P	15	10	81	9	4.6	1.75	1005	6.8	76	0.19	Inv ex
24	N	12	5	94	1	6.1	3.20	1009	6 3	64	0.25	Inv ex
25	O	9	2	94	4	6.3	3.42	1026	6.0	68	0.25	Inv. ex.
26	Q	11	4	89	7	5.2	2.64	1046	5.7	54	0.21	Inv. ex.
27	R	12	4	91	5	5.4	2.70	978	6.7	70	0.22	Inv ex
28	S	13	34	56	10	5.2	2.26	1050	5.7	54	0.19	Inv ex
29	T	12	2	94	4	5.2	3.16	1024	6.3	60	0.22	Inv. ex.
30	U	11	8	87	5	5.4	2.96	977	7.3	68	0.22	Inv ex
31	V	12	20	67	13	4.9	1.93	979	7.0	70	0.16	Inv ex
32	W	11	1	82	17	5.2	2.36	1057	5.2	45	0.19	Inv ex
33	X	13	10	84	6	3.2	1.86	1010	6.6	73	0.16	Inv ex
34	Y	12	5	88	7	5.4	2.72	992	6.5	66	0.22	Inv. ex.
35	Z	13	10	80	10	5.3	2.67	990	7.1	63	0.22	Inv ex
36	S	12	43	50	7	5.4	2.45	1030	6.2	55	0.22	Inv ex
37	O	9	3	90	7	7.5	3.50	1024	6.1	52	0.24	Inv. ex.
38	AA	26	0	97	3	3.4	1.09	1015	6.2	79	0.09	Comp ex
39	AA	27	0	93	7	3.4	1.02	1005	6.3	75	0.09	Comp ex
40	W	8	1	20	79	5.2	1.03	1071	3.2	60	0.20	Comp. ex.
41	AA	12	3	93	4	5.6	3.22	1012	5.8	57	0.20	Inv. ex.
42	Y	13	4	89	7	3.5	4.50	994	7.2	68	0.25	Inv ex
43	Y	14	6	86	8	7.1	2.55	993	7.8	40	0.21	Inv ex
44	G	28	13	86	1	3.5	3.60	935	8.3	39	0.21	Comp ex

6. Results and Considerations

From the results shown in Tables 1 to 4, the following will be understood:
In No. 1, the C content of the hot rolled steel sheet was too small, therefore the regions with a GAM value of 0.6° or less became excessive and the strength of the hot rolled steel sheet fell.
In No. 2, the C content of the hot rolled steel sheet was too large, therefore the regions with a GAM value of 2.0° or more became excessive and the elongation and hole expansibility of the hot rolled steel sheet fell.
In No. 3, the Si content of the hot rolled steel sheet was too large, therefore the ductility became insufficient etc. and hot rolling became difficult.
In No. 4, the Mn content of the hot rolled steel sheet was too large, therefore the regions with a GAM value of 2.0° or more became excessive and the elongation of the hot rolled steel sheet fell.
In No. 5, the Mn content of the hot rolled steel sheet was too small, therefore the regions with a GAM value of 0.6° or less became excessive, the strength of the hot rolled steel sheet fell.
In No. 6, the Ti content of the hot rolled steel sheet was too large, therefore precipitates excessively formed etc. and the hole expansibility of the hot rolled steel sheet fell.
In No. 7, the Ti content of the hot rolled steel sheet was too small, therefore the effect of rise of strength due to precipitation strengthening, fine grain strengthening, and/or dislocation strengthening was not obtained and transformation nuclei could not be sufficiently formed, the prior austenite grain became coarser, and the strength and hole expansibility of the hot rolled steel sheet fell.
In No. 8, the Al content of the hot rolled steel sheet was too large, therefore cracking of the slab etc. occurred and hot rolling became difficult.
In No. 9, the content of the hot rolled steel sheet was too small, therefore deoxidation became insufficient and inclusions excessively formed etc. therefore the hole expansibility of the hot rolled steel sheet fell.
In No. 10, the P content of the hot rolled steel sheet was too large, therefore slab cracking etc. formed due to embrittlement and hot rolling became difficult.
In No. 11, the S content of the hot rolled steel sheet was too large, therefore inclusions excessively formed etc. therefore the hole expansibility of the hot rolled steel sheet fell.
In No. 12, the N content of the hot rolled steel sheet was too large, therefore cracking of the slab due to embrittlement etc. occurred and hot rolling became difficult.
In No. 13, the O content of the hot rolled steel sheet was too large, therefore oxides were excessively formed etc. and the hole expansibility of the hot rolled steel sheet fell.
In No. 15, the start temperature ST of the finish rolling was too low, therefore the prior austenite grains became coarser and the hole expansibility of the hot rolled steel sheet fell.
In No. 16, the start temperature ST of the finish rolling was too high and further the finish temperature FT of the finish rolling was also too high, therefore the structure of the steel could no longer be suitably controlled, the prior austenite grains of the hot rolled steel sheet became coarser, the predetermined relations (1) and (2) were not satisfied, and the impact property of the hot rolled steel sheet fell.
In No. 17, there was one or less high temperature difference rolling operations with a rolling temperature difference ΔT of 30°C or more, therefore it was not possible to realize the desired steel structure, the hot rolled steel sheet did not satisfy the predetermined relation (2), and the hole expansibility of the hot rolled steel sheet fell.
In No. 18, the total rolling reduction after the second high temperature difference rolling operation was too low, therefore it was no longer possible to suitably control the structure of the steel, the hot rolled steel sheet did not satisfy the predetermined relation (2), and the hole expansibility of the hot rolled steel sheet fell.
In No. 19, the time from the end of the finish rolling to the start of cooling was too long, therefore coarsening of the crystal grains caused the prior austenite grain size to become more than 25 µm and the hole expansibility of the hot rolled steel sheet fell.
In No. 20, the cooling stop temperature of the accelerated cooling was too high, therefore the regions with a GAM value of 2.0° or more became excessively greater and the elongation of the hot rolled steel sheet fell.
In No. 21, the cooling stop temperature at the time of accelerated cooling was too low, therefore regions with a GAM value of 2.0° or more became excessively greater and the elongation of the hot rolled steel sheet fell.
In No. 38, the start temperature ST of the finish rolling was too high, therefore it was no longer possible to suitably control the structure of the cooling, the prior austenite grains of the hot rolled steel sheet became coarser, the hot rolled steel sheet did not satisfy the predetermined relation (2), and the impact property of the hot rolled steel sheet fell.
In No. 39, the start temperature ST of the finish rolling was suitably controlled, but the finish rolling finish temperature FT was too high, therefore the structure of the steel could no longer be suitably controlled, the prior austenite grains of the hot rolled steel sheet became coarser, the hot rolled steel sheet did not satisfy the predetermined relation (2), and the impact property of the hot rolled steel sheet fell.
In No. 40, the slow cooling time at the 720°C to 470°C temperature region was too short, therefore in the hot rolled steel sheet, the regions with a GAM value of more than 0.6° and less than 2.0° were not sufficiently formed, the hot rolled steel sheet did not satisfy the predetermined relation (2), and the elongation of the hot rolled steel sheet fell.
In No. 44, like in No. 7, the Ti content of the hot rolled steel sheet was too small, therefore the effect of rise of strength due to precipitation strengthening, fine grain strengthening, and/or dislocation strengthening was not obtained and transformation nuclei could not be sufficiently formed, the prior austenite grain became coarser, and the strength and hole expansibility of the hot rolled steel sheet fell.
As opposed to this, in each of Nos. 14, 22 to 37, and 41 to 43, the hot rolled steel sheet was excellent in balance of strength, elongation, and hole expansibility and excellent in impact property. In No. 42, the second high temperature difference rolling was performed at 990°C. Since being performed at a predetermined temperature between 980 to 1000°C, even compared with No. 34 which performed the second time of high temperature difference rolling at 965°C, the impact property was particularly excellent. From the results of each of Nos. 14, 22 to 37, and 41 to 43, it can be said that the hot rolled steel sheet satisfying the following requirements (A) to (D) was excellent in balance of strength, elongation, and hole expansibility and was excellent in impact property.

(A) The hot rolled steel sheet contains, by mass%, C: 0.045% or more, 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, Mo: 0% or more and 3.000% or less, Ni: 0% or more and 0.500% or less, B: 0% or more and 0.0100% or less, Ca: 0% or more and 0.0500% or less, Mg: 0% or more and 0.050% or less, REM: 0% or more and 0.100% or less, Bi: 0% or more and 0.100% or less, Ta: 0% or more and 0.100% or less, Zr: 0% or more and 0.500% or less, Co: 0% or more and 3.000% or less, Zn: 0% or more and 0.200% or less, W: 0% or more and 0.200% or less, Sb: 0% or more and 0.500% or less, As: 0% or more and 0.050% or less, Sn: 0% or more and 0.050% or less and a balance of: Fe and impurities.
(B) A prior austenite grain size of the hot rolled steel sheet is 25 µm or less,
(C) An area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%, an area ratio of regions with a GAM value of 0.6° or less is 0% or more and less than 50%, and an area ratio of regions with a GAM value of 0.2° or more is more than 0° and 50% or less.
(D) The hot rolled steel sheet satisfies the following relations (1) and (2): $1.7 \leq LGr / LG t$ $1.20 \leq (LGr / LGt) / (LMr / LMt)$
- where, LGr: area average value of rolling direction projected lengths of prior austenite grains
- LGt: area average value of sheet thickness direction projected lengths of prior austenite grains
- LMr: area average value of rolling direction projected lengths of regions with GAM value of 2.0° or more
- LMt: area average value of sheet thickness direction projected lengths of regions with GAM value of 2.0° or more

Claims

A hot rolled steel sheet comprising, by 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,

Mo: 0% or more and 3.000% or less,

Ni: 0% or more and 0.500% or less,

B: 0% or more and 0.0100% or less,

Ca: 0% or more and 0.0500% or less,

Mg: 0% or more and 0.050% or less,

REM: 0% or more and 0.100% or less,

Bi: 0% or more and 0.100% or less,

Ta: 0% or more and 0.100% or less,

Zr: 0% or more and 0.500% or less,

Co: 0% or more and 3.000% or less,

Zn: 0% or more and 0.200% or less,

W: 0% or more and 0.200% or less,

Sb: 0% or more and 0.500% or less,

As: 0% or more and 0.050% or less,

Sn: 0% or more and 0.050% or less and

a balance: Fe and impurities, wherein

a prior austenite grain size is 25 µm or less,

an area ratio of regions with a GAM value of more than 0.6° and less than 2.0° is 50% or more and less than 100%,

an area ratio of regions with a GAM value of 0.6° or less is 0% or more and less than 50%,

an area ratio of regions with a GAM value of 0.2° or more is more than 0° and 50% or less, and

the following relations (1) and (2) are satisfied: $1.7 \leq LGr / LG t$ $1.20 \leq (LGr / LGt) / (LMr / LMt)$

where,

LGr: area average value of rolling direction projected lengths of prior austenite grains

LGt: area average value of sheet thickness direction projected lengths of prior austenite grains

LMr: area average value of rolling direction projected lengths of regions with GAM value of 2.0° or more

LMt: area average value of sheet thickness direction projected lengths of regions with GAM value of 2.0° or more
The hot rolled steel sheet of claim 1, wherein
an area ratio of regions with a GAM value of 0.6° or less is 0% or more and 45% or less.
The hot rolled steel sheet of claim 1 or 2, wherein
an area ratio of regions with a GAM value of 2.0° or more is more than 0% and 20% or less.
The hot rolled steel sheet of any of claims 1 to 3, wherein
the following relation (1-1) is satisfied: $1.7 \leq LGr / LGt \leq 10.0$
The hot rolled steel sheet of any of claims 1 to 4, wherein
the following relation (2-1) is satisfied: $1.20 \leq (LGr / LGt) / (LMr / LMt) \leq 5.00$