EP4613894A1

EP4613894A1 - Hot-rolled steel plate

Info

Publication number: EP4613894A1
Application number: EP23885307.1A
Authority: EP
Inventors: Takashi Shimada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-11-02
Filing date: 2023-06-30
Publication date: 2025-09-10
Also published as: MX2025004682A; US20260085391A1; KR20250067186A; CN120129764A; EP4613894A4; JPWO2024095534A1; WO2024095534A1

Abstract

Provided is a hot rolled steel sheet having a predetermined chemical composition, and a microstructure comprising, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%, wherein TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³, and the hot rolled steel sheet has a tensile strength of 780 MPa or more tensile strength.

Description

FIELD

The present invention relates to a hot rolled steel sheet.

BACKGROUND

In recent years, in the automotive industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel efficiency. To achieve both lighter weight of car bodies and collision safety, increasing the strength of the steel sheet used is one effective method. A high strength steel sheet is being developed from this background.
On the other hand, most auto parts are being made by press forming. In general, along with higher strength, the formability of a steel sheet falls. For example, it is known that the ductility and other properties fall.
In relation to this, for example, PTL 1 describes a high strength thin steel sheet having a steel structure comprised of a ferrite phase and martensite phase, having, in the ferrite phase, carbonitrides precipitated by interphase precipitation, and having a spacing of precipitated surfaces of the interphase precipitation in 40% or more of the regions of the ferrite phase of 20 nm or more and 60 nm or less. Further, PTL 1 teaches that by precipitation strengthening by precipitates in the ferrite phase, sufficient strength can be secured and, in addition, that by forming a mixed structure of the martensite phase, high fatigue properties can be secured while high ductility unique to mixed structures can be obtained.

[CITATIONS LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 2011-225935

SUMMARY

[TECHNICAL PROBLEM]

As explained above, it is known that along with higher strength, the formability of a steel sheet falls and, aside from ductility, the hole expandability and other properties fall. If the hole expandability falls, for example, sometimes it is not possible to form a steel sheet into the desired shapes in the suspension parts of automobiles etc. For this reason, in development of a high strength steel sheet, securing the properties for the specific application to a certain extent or more while raising the strength is important. Further, a high strength steel sheet is produced by hot rolling a cast slab, but it is known that sometimes there is anisotropy in strength between the strength in the rolling direction (L direction) relating to the hot rolling and the strength in the width direction perpendicular to the same (C direction). If the anisotropy of strength becomes greater, in general, the workability of the steel sheet falls, and therefore this becomes a problem.
The present invention was made in consideration of the above and has as its object the provision of a hot rolled steel sheet which, despite being high in strength, is improved in hole expandability and is reduced in anisotropy of strength, by a novel constitution.

[SOLUTION TO PROBLEM]

The inventors engaged in studies focusing on the microstructure of a hot rolled steel sheet so as to achieve the above object. As a result, the inventors discovered that by forming the microstructure of a hot rolled steel sheet having a predetermined chemical composition from a triplex structure containing ferrite, bainite, and martensite in specific ratios, it is possible to achieve a high strength of a tensile strength of 780 MPa or more while reducing the anisotropy of strength and that by making TiC precipitates having a suitable diameter in the ferrite be present in a predetermined number density, it is possible to strengthen the ferrite by precipitates and thereby reduce the hardness difference in the triplex structure and improve the hole expandability and thereby completed the present invention.
The present invention able to achieve this object is as follows:

(1) A hot rolled steel sheet having a chemical composition comprising, by mass%,
- C: 0.010 to 0.100%,
- Si: 0.01 to 0.10%,
- Mn: 0.50 to 3.00%,
- Ti: 0.050 to 0.200%,
- Nb: 0.010 to 0.020%,
- Al: 0.100 to 1.000%,
- P: 0.1000% or less,
- S: 0.0100% or less,
- N: 0.0100% or less,
- O: 0.0100% or less,
- Ni: 0 to 2.000%,
- Mo: 0 to 1.000%,
- Cr: 0 to 2.000%,
- B: 0 to 0.0100%,
- Co: 0 to 2.000%,
- V: 0 to 1.000%,
- Cu: 0 to 2.000%,
- W: 0 to 1.0000%,
- Ta: 0 to 1.0000%,
- Sn: 0 to 1.0000%,
- Sb: 0 to 1.0000%,
- As: 0 to 0.0100%,
- Mg: 0 to 0.0100%,
- Ca: 0 to 0.0100%,
- Zr: 0 to 0.0100%,
- Hf: 0 to 0.0100%,
- Bi: 0 to 0.0100%,
- REM: 0 to 0.0100%, and
- balance: Fe and impurities, and
- a microstructure comprising, by area ratio,
- ferrite: 60 to 80%,
- bainite: 15 to 30%, and
- martensite: 3 to 10%, wherein
- TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶ /cm³ , and
- the hot rolled steel sheet has a tensile strength of 780 MPa or more.
(2) The hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass%, at least one of:
- Ni: 0.001 to 2.000%,
- Mo: 0.001 to 1.000%,
- Cr: 0.001 to 2.000%,
- B: 0.0001 to 0.0100%,
- Co: 0.001 to 2.000%,
- V: 0.001 to 1.000%,
- Cu: 0.001 to 2.000%,
- W: 0.0001 to 1.0000%,
- Ta: 0.0001 to 1.0000%,
- Sn: 0.0001 to 1.0000%,
- Sb: 0.0001 to 1.0000%,
- As: 0.0001 to 0.0100%,
- Mg: 0.0001 to 0.0100%,
- Ca: 0.0001 to 0.0100%,
- Zr: 0.0001 to 0.0100%,
- Hf: 0.0001 to 0.0100%,
- Bi: 0.0001 to 0.0100%, and
- REM: 0.0001 to 0.0100%.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the present invention, it is possible to provide a hot rolled steel sheet which, despite being high in strength, is improved in hole expandability and is reduced in anisotropy of strength.

DESCRIPTION OF EMBODIMENTS

The hot rolled steel sheet according to an embodiment of the present invention is characterized by having a chemical composition comprising, by mass%,

C: 0.010 to 0.100%,
Si: 0.01 to 0.10%,
Mn: 0.50 to 3.00%,
Ti: 0.050 to 0.200%,
Nb: 0.010 to 0.020%,
Al: 0.100 to 1.000%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0100% or less,
O: 0.0100% or less,
Ni: 0 to 2.000%,
Mo: 0 to 1.000%,
Cr: 0 to 2.000%,
B: 0 to 0.0100%,
Co: 0 to 2.000%,
V: 0 to 1.000%,
Cu: 0 to 2.000%,
W: 0 to 1.0000%,
Ta: 0 to 1.0000%,
Sn: 0 to 1.0000%,
Sb: 0 to 1.0000%,
As: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
REM: 0 to 0.0100%, and
balance: Fe and impurities, and
a microstructure comprising, by area ratio,
ferrite: 60 to 80%,
bainite: 15 to 30%, and
martensite: 3 to 10%, wherein
TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶ /cm³, and
the hot rolled steel sheet has a tensile strength of 780 MPa or more.

As explained previously, it is known that hole expandability and other properties fall along with higher strength of a steel sheet and that sometimes there is strong anisotropy in strength between the strength in the rolling direction (L direction) and strength in the width direction (C direction) perpendicular to the same due to the hot rolling at the time of production of a steel sheet. Explaining the anisotropy of strength in more detail, due to the anisotropic microstructure obtained by the hot rolling at the time of production of a steel sheet, the tensile strength tends to differ between the rolling direction (L direction) and the width direction (C direction) perpendicular to that. In general, there is a tendency for anisotropy of strength to be exhibited where the L direction tensile strength in a hot rolled steel sheet becomes lower than the C direction tensile strength. By improving the expandability and reducing such anisotropy of strength, it is possible to greatly improve the workability of a high strength steel sheet used in applications such as automobiles, but achieving both higher strength of a steel sheet and improvement of these properties is generally extremely difficult. Therefore, the inventors engaged in studies focusing on the microstructure of a hot rolled steel sheet in addition to making the chemical composition of a hot rolled steel sheet more suitable. First, the inventors discovered that by making the microstructure of the hot rolled steel sheet having a predetermined chemical composition a triplex structure containing ferrite, bainite, and martensite in specific ratios, more specifically a triplex structure comprised of, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%, it is possible to achieve a high strength of a tensile strength of 780 MPa or more while remarkably reducing the anisotropy of strength compared with the case of DP steel (duplex steel) mainly comprised of soft ferrite and hard martensite. Further, the inventors studied improvement of the hole expandability from the viewpoint of reduction of the hardness difference in the triplex structure since in the case of a triplex structure comprised of ferrite, bainite, and martensite, the hardness difference of the phases is relatively large and there is a possibility of a drop in the hole expandability due to such a hardness difference. As a result, the inventors discovered that by strengthening the ferrite, which is the softest in the triplex structure, by precipitates more specifically establishing the presence of diameter 1.0 to 5.0 nm TiC precipitates in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³, so as to strengthen the ferrite by precipitates, it is possible to reduce the hardness difference in the triplex structure. As a result, the inventors discovered that regardless of forming the microstructure by a triplex structure in which a relative hardness difference easily becomes greater so as to reduce the anisotropy of strength, by raising the hardness of ferrite by precipitation strengthening using TiC precipitates having specific diameter and number density, it is possible to simultaneously achieve both reduction of the anisotropy of strength and improvement of the hole expandability.
While not intending to be bound by any specific theory, it is believed that in the case of the above such triplex structure, due to the dispersion of the different hard structures bainite and martensite in the soft ferrite, a more isotropic structure results compared with the case of a duplex structure such as DP steel. Further, the hot rolled steel sheet according to an embodiment of the present invention contains Nb, and therefore this formed carbides, nitrides, and/or carbonitrides in the steel. Due to the pinning effects of these, it is believed that ferrite transformation is promoted and ferrite grains are refined. Refinement of the ferrite grains forming the main phase of the triplex structure is believed to not only contribute to improvement of the strength of the hot rolled steel sheet as a whole, but to also contribute to reduction of the hardness difference of the triplex structure comprised of ferrite, bainite, and martensite. As a result, according to an embodiment of the present invention, despite the high strength of a tensile strength of 780 MPa or more, improvement of the hole expandability and reduction of the anisotropy of strength can be achieved. Therefore, the hot rolled steel sheet according to an embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and therefore is particularly useful in use in the automotive field where achievement of both of these properties is sought.
Below, the hot rolled steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the "%" of the units of contents of the elements, unless otherwise indicated, means "mass%". Further, in this Description, the "to" showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.

[C: 0.010 to 0.100%]

C is an element effective for raising the strength of a steel sheet. Further, C forms carbides and/or carbonitrides with Ti and Nb in the ferrite and also contributes to the precipitation strengthening of ferrite based on the precipitates formed and the refinement of the ferrite grains by the pinning effect of the precipitates. To sufficiently obtain these effects, the C content is 0.010% or more. The C content may also be 0.012% or more, 0.015% or more, 0.018% or more, 0.020% or more, or 0.022% or more. On the other hand, if excessively containing C, sometimes the elongation falls. Further, sometimes the desired triplex structure is not obtained and the anisotropy of strength cannot be sufficiently reduced. Therefore, the C content is 0.100% or less. The C content may also be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[Si: 0.01 to 0.10%]

Si is an element effective as a solution strengthening element for raising the strength. To sufficiently obtain such an effect, the Si content is 0.01% or more. The Si content may also be 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, if excessively containing Si, defects in surface quality called "Si scale" sometimes are formed. Further, due to the Si scale, the surface roughness of the hot rolled steel sheet sometimes increases. In addition, by excessively containing Si, the amount of ferrite increases and sometimes the desired triplex structure cannot be obtained. Due to these, the anisotropy of strength in the L direction and C direction tensile strengths sometimes becomes remarkable. Therefore, the Si content is 0.10% or less. The Si content may also be 0.09% or less, 0.08% or less, 0.07% or less, or 0.06% or less.

[Mn: 0.50 to 3.00%]

Mn is an element effective for hardenability and raising strength as a solution strengthening element. To sufficiently obtain these effects, the Mn content is 0.50% or more. The Mn content may also be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if excessively including Mn, the C dispersion coefficient falls, and therefore the TiC precipitates become smaller in diameter and sometimes the effect of improvement of hardness of ferrite due to precipitation strengthening based on the TiC precipitates cannot be sufficiently obtained. Therefore, the Mn content is 3.00% or less. The Mn content may also be 2.70% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[Ti: 0.050 to 0.200%]

Ti has the action of forming the carbide TiC precipitates in the ferrite to raise the hardness of the ferrite by precipitation strengthening. To sufficiently obtain such an effect, the Ti content is 0.050% or more. The Ti content may also be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if excessively containing Ti, the TiC precipitates become coarser and sometimes it is not possible to obtain the desired precipitation strengthening in the ferrite. In addition, along with the coarsening of TiC precipitates, the number density of TiC precipitates also falls, and therefore in this case, it is no longer possible to sufficiently raise the hardness of ferrite by precipitation strengthening. Therefore, the Ti content is 0.200% or less. The Ti content may also be 0.190% or less, 0.180% or less, 0.160% or less, or 0.140% or less.

[Nb: 0.010 to 0.020%]

Nb is an element forming carbides, nitrides, and/or carbonitrides in steel to contribute to refining of the structure by the pinning effect. Due to the pinning effect, coarsening of the austenite grains is suppressed, ferrite transformation is promoted, and ferrite grains can be refined. Refining of ferrite grains not only raises the strength of a steel sheet, but also contributes to reduction of the hardness difference of the triplex structure. If the Nb content is small, sometimes it is not possible to sufficiently obtain the effects of these. Therefore, the Nb content is 0.010% or more. The Nb content may also be 0.012% or more, 0.013% or more, or 0.015% or more. On the other hand, if excessively containing Nb, sometimes coarse carbides, etc., are formed in the steel and the strength of the steel sheet falls. In addition, since coarse carbides are formed, the pinning effect cannot be sufficiently exhibited and sometimes the desired triplex structure cannot be obtained. Therefore, the Nb content is 0.020% or less. The Nb content may also be 0.018% or less or 0.016% or less.

[Al: 0.100 to 1.000%]

Al is an element acting as a deoxidizer. If the Al content is small, sometimes such an effect cannot be sufficiently obtained and/or the desired triplex structure cannot be obtained. Therefore, the Al content is 0.100% or more. The Al content may also be 0.120% or more, 0.150% or more, or 0.200% or more. On the other hand, if excessively containing Al, sometimes coarse oxides are formed and the strength falls and/or the desired triplex structure cannot be obtained. Therefore, the Al content is 1.000% or less. The Al content may also be 0.800% or less, 0.600% or less, or 0.400% or less.

[P: 0.1000% or Less]

If P is excessively contained, it sometimes detrimentally affects the weldability, etc. Therefore, the P content is 0.1000% or less. The P content may also be 0.0800% or less, 0.0500% or less, 0.0300% or less, or 0.0250% or less. The lower limit of the P content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.0001% or more, 0.0010% or more, or 0.0050% or more.

[S: 0.0100% or Less]

If S is excessively contained, a large amount of MnS is formed and sometimes the toughness is made to decrease. Therefore, the Si content is 0.0100% or less. The S content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less. The lower limit of the S content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.0001% or more or 0.0005% or more.

[N: 0.0100% or Less]

If excessively containing N, coarse nitrides are formed and sometimes the toughness is made to fall. Therefore, the N content is 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less. The lower limit of the N content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may be 0.0001% or more or 0.0005% or more.

[O: 0.0100% or Less]

O is an element entering in the production process. If excessively containing O, sometimes coarse inclusions are formed and the toughness of the steel sheet is made to fall. Therefore, the O content is 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly prescribed and may also be 0%, but for reducing it to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more or 0.0005% or more.
The basic chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is as explained above. Further, the hot rolled steel sheet may, in accordance with need, contain at least one of the following optional elements in place of part of the balance of Fe. For example, the hot rolled steel sheet may also contain at least one of Ni: 0 to 2.000%, Mo: 0 to 1.000%, Cr: 0 to 2.000%, B: 0 to 0.0100%, Co: 0 to 2.000%, V: 0 to 1.000%, Cu: 0 to 2.000%, W: 0 to 1.0000%, and Ta: 0 to 1.0000%. Further, the hot rolled steel sheet may also contain at least one of Sn: 0 to 1.0000%, Sb: 0 to 1.0000%, and As: 0 to 0.0100%. Further, the hot rolled steel sheet may also contain at least one of Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, and Hf: 0 to 0.0100%. Further, the hot rolled steel sheet may also contain Bi: 0 to 0.0100%. Further, the hot rolled steel sheet may also contain REM: 0 to 0.0100%. Below, these optional elements will be explained in detail.

[Ni: 0 to 2.000%]

Ni is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more and may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if excessively containing Ni, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Ni content is preferably 2.000% or less and may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[Mo: 0 to 1.000%]

Mo is an element raising the hardenability of steel and contributing to improvement of the strength and is an element also contributing to improvement of the corrosion resistance. The Mo content may be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes great. Therefore, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[Cr: 0 to 2.000%]

Cr is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more and may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if excessively containing Cr, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Cr content is preferably 2.000% or less and may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[B: 0 to 0.0100%]

B is an element raising the hardenability of steel and contributing to improvement of the strength. The B content may be 0%, but to obtain these effects, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if excessively containing B, the toughness and/or weldability sometimes falls. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[Co: 0 to 2.000%]

Co is an element raising the hardenability and/or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing Co, sometimes the hot workability falls and an increase in material costs is also led to. Therefore, the Co content is preferably 2.000% or less. The Co content may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[V: 0 to 1.000%]

V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if excessively containing V, a large amount of precipitates is formed and sometimes the toughness is lowered. Therefore, the V content is preferably 1.000% or less. The V content may also be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.

[Cu: 0 to 2.000%]

Cu is an element contributing to improvement of the strength and/or corrosion resistance. The Cu content may also be 0%, but to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing Cu, sometimes deterioration of the toughness or weldability is invited. Therefore, the Cu content is preferably 2.000% or less. The Cu content may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.150% or less, or 0.100% or less.

[W: 0 to 1.0000%]

W is an element raising the hardenability of steel and contributing to improvement of strength. The W content may be 0%, but to obtain these effects, the W content is preferably 0.0001% or more. The W content may also be 0.0010% or more, 0.0020% or more, or 0.0050% or more. On the other hand, if excessively containing W, the weldability sometimes falls. Therefore, the W content is preferably 1.0000% or less. The W content may also be 0.8000% or less, 0.5000% or less, 0.2000% or less, 0.1000% or less, or 0.0500% or less.

[Ta: 0 to 1.0000%]

Ta is an element effective for controlling the form of the carbides and improving the strength of the steel sheet. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.0001% or more. The Ta content may be 0.0010% or more, 0.0020% or more, or 0.0050% or more. On the other hand, even if excessively containing Ta, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. For this reason, the Ta content is preferably 1.0000% or less. The Ta content may also be 0.8000% or less, 0.5000% or less, 0.2000% or less, 0.1000% or less, or 0.0500% or less.

[Sn: 0 to 1.0000%]
[Sb: 0 to 1.0000%]

Sn and Sb are elements effective for improving the corrosion resistance. The Sn and Sb contents may be 0%, but to obtain these effects, the Sn and Sb contents are preferably 0.0001% or more and may be 0.0010% or more, 0.0020% or more, or 0.0050% or more. On the other hand, if excessively containing Sn and Sb, sometimes a drop in toughness is invited. Therefore, the Sn and Sb contents are preferably 1.0000% or less and may be 0.8000% or less, 0.5000% or less, 0.3000% or less, 0.1000% or less, or 0.0500% or less.

[As: 0 to 0.0100%]

As is an element effective for improvement of the corrosion resistance. The As content may be 0%, but to obtain these effects, the As content is preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing As, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the As content is preferably 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Mg: 0 to 0.0100%]
[Ca: 0 to 0.0100%]
[Zr: 0 to 0.0100%]
[Hf: 0 to 0.0100%]

Mg, Ca, Zr, and Hf are elements enabling control of the form of sulfides. The Mg, Ca, Zr, and Hf contents may be 0%, but to obtain these effects, the contents of these elements are preferably respectively 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Mg, Ca, Zr, and Hf contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Bi: 0 to 0.0100%]

Bi is an element effective for improvement of the corrosion resistance. The Bi content may be 0%, but to obtain these effects, the Bi content is preferably 0.0001% or more and may be 0.0005% or more, 0.0010% or more or 0.0015% or more. On the other hand, even if excessively containing Bi, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi content is preferably 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[REM: 0 to 0.0100%]

An REM is an element enabling control of the form of sulfides. The REM content may be 0%, but to obtain these effects, the REM content is preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing REM, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the REM content is preferably 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The "REM" in this Description 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.
In the hot rolled steel sheet according to an embodiment of the present invention, the balance aside from the above elements is comprised of Fe and impurities. The "impurities" are constituents, etc., 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 hot rolled steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.

[Microstructure]

The microstructure of the hot rolled steel sheet according to an embodiment of the present invention is comprised of, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%. By configuring the microstructure of the hot rolled steel sheet by such a triplex structure, for example, it becomes possible to remarkably reduce the anisotropy of strength in the L direction and C direction tensile strengths of the hot rolled steel sheet compared with the case of DP steel mainly comprised of soft ferrite and hard martensite. In addition, by including these three structures in the above such specific area ratios, it becomes possible to maintain the high tensile strength of 780 MPa or more while suitably reducing the hardness difference of the phases explained in detail later relating to the hole expandability. For example, if the area ratio of ferrite is small, the ratios of the hard phases bainite and martensite become higher and sometimes it no longer becomes possible to suitably reduce the hardness difference of the phases, more specifically the hardness difference of ferrite and martensite, by the precipitation strengthening of ferrite explained later in detail and becomes impossible to achieve the desired hole expandability. Therefore, the area ratio of ferrite is 60% or more, for example, may also be 62% or more, 65% or more, or 68% or more. On the other hand, if the area ratio of ferrite becomes higher, the ratios of the hard phases bainite and martensite become lower and as a result sometimes a tensile strength of 780 MPa or more cannot be achieved. Therefore, the area ratio of ferrite is 80% or less, for example, may also be 78% or less, 75% or less, or 72% or less.
From the viewpoint of improving the tensile strength, higher area ratios of the hard phase bainite and martensite are preferable. From such a viewpoint, for example, the area ratio of bainite may be 18% or more, 20% or more, or 22% or more. Similarly, the area ratio of martensite may be more than 3%, 3.1% or more, 3.2% or more, 3.3% or more, 3.5% or more, 3.8% or more, 4% or more, 4.5% or more, 5% or more, or 6% or more. On the other hand, from the viewpoint of reducing the hardness difference of the phases and improving the hole expandability, lower area ratios of bainite and martensite are preferable. From such a viewpoint, for example, the area ratio of bainite may be 28% or less, 26% or less, or 24% or less. Similarly, the area ratio of martensite may be 9% or less, 8% or less, or 7% or less.

[Balance Structures]

The microstructure of the hot rolled steel sheet according to an embodiment of the present invention, as explained above, is comprised of ferrite, bainite, and martensite and either does not contain or substantially does not contain structures other than these (balance structures). "Substantially does not contain" means the area ratio of the balance structures other than ferrite, bainite, and martensite is 3% or less. Therefore, the area ratio of the balance structures is 0 to 3%, for example, may be 0 to 1.5%, 0 to 1%, or 0 to 0.5%. In other words, the total area ratio of the ferrite, bainite, and martensite is 97 to 100%, for example, may be 98.5 to 100%, 99 to 100%, or 99.5 to 100%. If there are balance structures present, the balance structures are, for example, pearlite.

[Identification of Microstructure and Calculation of Area Ratios]

The structures are observed by a scan type electron microscope. Before observation, the sample for observation of the structures is wet polished by emery paper and polished by a diamond abrasive having an average grain size of 1 µm so as to finish the observed surface to a mirror finish, then the structures are etched by a 3% nitric acid alcohol solution. The power of the observation is 3000X. Ten 30 µm×40 µm fields at a 1/4 position of sheet thickness from the surface are randomly photographed. The ratios of the structures are found by the point count method. Over the obtained structural image, a total of 100 lattice points are arranged at intervals of vertically 3 µm and horizontally 4 µm, the structures present under the lattice points are judged, and the ratios of structures contained in the steel material are found from the average values of the 10 photos. Ferrite is a clumpy crystal grain not including inside it long axis 100 nm or more iron-based carbides. Bainite is a collection of lath shaped crystal grains not including inside it long axis 20 nm or more iron-based carbides or containing inside it long axis 20 nm or more iron-based carbides but the carbides belonging to a single variant, i.e., a group of iron-based carbides stretching in the same direction. Here, "group of iron-based carbides stretching in the same direction" means a difference in stretching direction of the group of iron-based carbides of within 5°. In bainite, bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as a single bainite grain. Further, martensite, which contains a large amount of dissolved carbon, has a smaller reduction by corrosion at the time of etching compared with other structures and becomes relatively high in height compared with other structures in the observed field after etching. For this reason, it appears relatively whiter than other structures, and therefore martensite and other structures can be differentiated. If there are structures other than ferrite, bainite, and martensite, the area ratio of balance structures is determined by subtracting from 100% the total area ratio of ferrite, bainite, and martensite. There is no need to specifically identify the balance structures, but if the balance structures include pearlite etc., since pearlite has a unique structure of cementite precipitated in a lamellar manner, it can be discriminated by a scan type electron microscope.

[Number Density of TiC Precipitates Having Diameter of 1.0 to 5.0 nm in Ferrite: 1.0×10¹⁶ to 100.0×10¹⁶/cm³]

In the hot rolled steel sheet according to an embodiment of the present invention, TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³. By making TiC precipitates having a diameter of 1.0 to 5.0 nm be present in the ferrite by such a number density, it is possible to raise the hardness of ferrite by precipitation strengthening. More specifically, by raising the hardness of the ferrite and reducing the hardness difference with the hardest martensite, it is possible to reduce the hardness difference of the phases in a triplex structure comprised of ferrite, bainite, and martensite and, as a result, becomes possible to remarkably improve the hole expandability of the hot rolled steel sheet. If the diameter of TiC precipitates is smaller than 1.0 nm, the TiC precipitates cannot sufficiently act as obstacles to dislocation motion and therefore it is not possible to sufficiently obtain the effect of improvement of hardness of ferrite by precipitation strengthening. On the other hand, if the diameter of TiC precipitates is too large as well, sometimes it is not possible to obtain the desired precipitation strengthening at the ferrite.
While not intending to be bound by any specific theory, it is believed that this is because by the TiC precipitates becoming coarser, the strengthening mechanism changes in relation to dislocation motion and that, for example, the dislocation line does not pass cutting across the TiC precipitates but passes leaving behind a loop of dislocation line around the coarse TiC precipitates and therefore the amount of precipitation strengthening becomes small. In addition, along with coarsening of the TiC precipitates, the number density of the TiC precipitates also greatly falls, and therefore it becomes no longer possible to sufficiently raise the hardness of the ferrite by precipitation strengthening. Therefore, to effectively raise the hardness of ferrite by precipitation strengthening, it is effective to control the diameter of TiC precipitates to a range of 1.0 to 5.0 nm. To increase the hardness of ferrite by precipitation strengthening to the desired level, it becomes important to control the number density of TiC precipitates having such a diameter to within a predetermined range. From such a viewpoint, it is necessary to establish the presence of TiC precipitates having a diameter of 1.0 to 5.0 nm in the ferrite, as explained above, by a number density of 1.0×10¹⁶/cm³ or more. To further enhance the effect or improvement of hardness of the ferrite, the higher the number density, the more preferable. For example, it may be 2.0×10¹⁶/cm³ or more, 5.0×10¹⁶/cm³ or more, 10.0×10¹⁶/cm³ or more, or 20.0×10¹⁶/cm³ or more. On the other hand, if the number density becomes too high, it becomes difficult to control the diameter of the TiC precipitates to within the desired range. Therefore, the number density is 100.0×10¹⁶/cm³ or less and, for example, may also be 80.0×10¹⁶/cm³ or less or 50.0×10¹⁶/cm³ or less. In the hot rolled steel sheet according to an embodiment of the present invention, if performing measurement by the 3D atom probe measurement method explained in detail later, it is sufficient that TiC precipitates having a diameter of 1.0 to 5.0 nm be present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³. Therefore, so long as satisfying the above diameter and number density requirements, for example, coarse TiC precipitates may also be present in the ferrite.

[Calculation of Diameter and Number Density of TiC Precipitates]

The diameter and number density of TiC precipitates are calculated by the 3D atom probe measurement method as follows: First, from a sample for measurement, a needle-shaped sample is prepared by cutting and electrolytic polishing making use of, if necessary, the focused ion beam processing method along with electrolytic polishing. In the 3D atom probe measurement method, it is possible to reconstruct cumulative data to find an image of distribution of atoms in real space. In the case of fine TiC precipitates of Na-Cl structures, the unit lattice is 4.33Å, and therefore the distance between atoms of Ti and Ti is deemed to be 4.33×√2=6.1Å. Therefore, if there are a plurality of Ti atoms at substantially the same coordinate position (7Å or less), it is judged that these Ti atoms are in the same precipitates and the number of Ti atoms judged to be in the same precipitates is counted. If the number is 50 or more, these precipitates are defined as "fine TiC precipitates". The diameter of the fine TiC precipitates is made the circle equivalent diameter calculated from the number of atoms of Ti forming the observed fine Ti precipitates and the lattice constant of the fine Ti precipitates assuming the fine Ti precipitates to be spherical. The method of using the number of Ti atoms of the fine TiC precipitates obtained by the 3D atom probe measurement method so as to find the diameter of the fine TiC precipitates (circle equivalent diameter) R is shown below: The number N of all of the atoms of the sample covered is measured by the 3D atom probe measurement method, but in actuality, it is not possible to detect the number N of all of the atoms of the sample covered by the 3D atom probe measurement method. There is a detection rate α (=number of atoms detected/total number of atoms) of atoms inherent to each device, and therefore the number N of atoms probably present is calculated from the actual measurement value "n". That is, the total number N of atoms=n/α. Next, for the total number N of atoms, Na-Cl structure TiC precipitates are assumed to have eight Ti atoms present in a unit lattice. Further, the lattice constant "a" of the Na-Cl structure was deemed 4.33Å and the following formula is used to calculate the diameter R (circle equivalent diameter) of the TiC precipitates. $Diameter R of TiC precipitates = {\{(6 / 8) \cdot (1 / π) \cdot N \cdot a^{3}\}}^{(1 / 3)}$
Finally, the number density of TiC precipitates is calculated using the measurement field as the denominator and the number of fine TiC precipitates as the numerator.

[Sheet Thickness]

The hot rolled steel sheet according to an embodiment of the present invention is not particularly limited, but generally has a 1.0 to 6.0 mm sheet thickness. For example, the sheet thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and/or may be 5.0 mm or less or 4.0 mm or less.

[Mechanical Properties]

[Tensile Strength: TS]

According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a tensile strength of 780 MPa or more. The tensile strength is preferably 850 MPa or more, 900 MPa or more, or 980 MPa or more. According to the hot rolled steel sheet according to an embodiment of the present invention, despite having such an extremely high tensile strength, by the specific combination of the chemical composition and microstructure explained above, it is possible to achieve both improvement of the hole expandability and reduction of the anisotropy of strength. The upper limit of the tensile strength is not particularly prescribed, but for example the tensile strength of the hot rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, or 1080 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.

[Total Elongation: EL]

According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to improve the total elongation in addition to the high tensile strength, more specifically it is possible to reach a 16.0% or more total elongation. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, most preferably 22.0% or more. The upper limit is not particularly prescribed, but, for example, the total elongation may be 30.0% or less or 25.0% or less. The total elongation is measured by taking a JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.

(Hole Expansion Rate: λ)

According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to realize a high hole expandability, specifically a 50% or more hole expansion rate. The hole expansion rate is preferably 55% or more, 60% or more, or 65% or more. The upper limit of the hole expansion rate is not particularly prescribed, but, for example, the hole expansion rate may be 120% or less, 110% or less, or 100% or less. The hole expansion rate is determined in the following way: First, a circular hole having a diameter of 10 mm (initial hole: hole size d0=10 mm) is punched out of the test piece under conditions giving a clearance of 12.5%. With the raised edge (burr) becoming the die side, the initial hole is pushed open by a conical punch of a vertex angle of 60° until a crack is formed passing through the sheet thickness. The hole size d1 mm at the time of cracking is measured to find the hole expansion rate λ (%) of each test piece by the following formula. This hole expansion test is conducted five times and the average value of these is determined as the hole expansion rate λ. $λ = 100 \times \{(d 1 - d 0) / d 0\}$

Next, a preferable method of production of the hot rolled steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for production of the hot rolled steel sheet according to an embodiment of the present invention and is not intended to limit the hot rolled steel sheet to one produced by the method of production explained below.
The method of production of the hot rolled steel sheet according to an embodiment of the present invention is characterized by comprising

hot rolling including heating a slab having a chemical composition explained above in relation to the hot rolled steel sheet to a temperature of 1100 to 1300°C, then finish rolling it, wherein the end temperature of the finish rolling is 900 to 1000°C,
cooling the finish rolled steel sheet by an average cooling rate of 10°C/s or more to an intermediate air cooling temperature of 620 to 700°C, then air cooling it by an average cooling rate of 2°C/s or more and less than 10°C/s for 5 to 10 seconds, and
primary cooling the intermediate cooled steel sheet by an average cooling rate of 10 to 20°C/s for 1 to 3 seconds, then secondary cooling it by an average cooling rate of 25°C/s or more to 200°C or less and coiling it. Below, the steps will be explained in detail.

[Hot Rolling Step]

[Heating of Slab]

First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but may also be produced by the ingot making method or thin slab casting method. The slab used contains a relatively large amount of alloy elements for obtaining a high strength steel sheet. For this reason, before supplying the slab to hot rolling, it must be heated to make the alloy elements dissolve in the slab. If the heating temperature is less than 1100°C, the alloy elements will not sufficiently dissolve in the slab but will remain as coarse alloy carbides and will sometimes cause brittle fracture during hot rolling. Therefore, the heating temperature is preferably 1100°C or more. The upper limit of the heating temperature is not particularly prescribed, but is 1300°C or less from the viewpoint of the capacity of the heating facilities and productivity.

[Rough Rolling]

In the present method, for example, the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc. The rough rolling need only secure the desired sheet bar dimensions. The conditions are not particularly limited.

[Finish Rolling]

The heated slab or the slab additionally rough rolled according to need is next finish rolled. The slab used in the above way contains a relatively large amount of alloy elements, and therefore at the time of hot rolling, the rolling load has to be made greater. For this reason, the hot rolling is preferably performed at a high temperature. In particular, the end temperature of the finish rolling is important on the point of control of the metallostructure of the steel sheet. If the end temperature of the finish rolling is low, recrystallization is suppressed, the metallostructure becomes uneven, and sometimes the strength and/or hole expandability falls. For this reason, the end temperature of the finish rolling is 900°C or more. On the other hand, if the end temperature of the finish rolling is high, the austenite becomes coarser, the ratio of ferrite becomes smaller, and the desired triplex structure cannot be obtained. Therefore, the end temperature of the finish rolling is 1000°C or less.

[Intermediate Cooling Step]

The finish rolled steel sheet is cooled in the next intermediate cooling step by an average cooling rate of 10°C/s or more down to an intermediate air cooling temperature of 620 to 700°C, then is air cooled by an average cooling rate of 2°C/s or more and less than 10°C/s for 5 to 10 seconds. By cooling by an average cooling rate of 10°C/s or more down to an intermediate air cooling temperature of 620 to 700°C, it is possible to make ferrite precipitate in a desired ratio and form TiC precipitates having the desired size in the ferrite. On the other hand, if the intermediate air cooling temperature is more than 700°C or the average cooling rate until the intermediate air cooling temperature is less than 10°C/s, ferrite transformation proceeds too much and it becomes no longer possible to obtain a triplex structure containing ferrite, bainite, and martensite in specific ratios at the finally obtained hot rolled steel sheet. In addition, the intermediate air cooling temperature is more than 700°C, coarse TiC precipitates form in a relatively large amount and sometimes obtaining the desired precipitation strengthening in the ferrite is not possible. The average cooling rate to the intermediate air cooling temperature of 620 to 700°C is preferably 20°C/s or more. The upper limit is not particularly prescribed, but, for example, the average cooling rate may also be 30°C/s or less. On the other hand, if the intermediate air cooling temperature is less than 600°C, ferrite cannot be made to sufficiently precipitate and similarly in the finally obtained hot rolled steel sheet, it becomes no longer possible to form a triplex structure containing ferrite, bainite, and martensite in specific ratios. In addition, if the intermediate air cooling temperature is less than 600°C, it is not possible to form TiC precipitates having the desired size in a sufficient number density even by the subsequent air cooling and as a result it is no longer possible to sufficiently obtain the effect of improvement of the hardness of ferrite by precipitation strengthening.
The average cooling rate and time in the air-cooling after cooling down to the intermediate air cooling temperature of 620 to 700°C are also similarly important for making ferrite precipitate in the desired ratio and forming TiC precipitates having the desired size in the ferrite. First, if the temperature region of the ferrite transformation is in the relatively high temperature region of 620 to 700°C, it is possible to make the ferrite precipitate in the desired ratio and make the TiC precipitates precipitating in the ferrite grow in grain size. Making the TiC precipitates grow in grain size to a certain extent is advantageous in that it is possible to increase the amount of precipitation strengthening of ferrite. However, even at the intermediate air cooling temperature of 620 to 700°C, if the average cooling rate of the air-cooling is less than 2°C/s, coarse TiC precipitates form relatively greatly and it is no longer possible to obtain the desired precipitation strengthening at the ferrite. On the other hand, if performing not air-cooling, but water cooling or other forced cooling, it is not possible to form TiC precipitates having the desired size in a sufficient number density and, as a result, it is no longer possible to sufficiently obtain the effect of improvement of hardness of ferrite by precipitation strengthening. Further, if the air-cooling time is less than 5 seconds, the ferrite cannot be made to sufficiently precipitate and it is no longer possible to form a triplex structure containing ferrite, bainite, and martensite in specific ratios in the finally obtained hot rolled steel sheet. On the other hand, if the air-cooling time is more than 10 seconds, ferrite excessively precipitates and the ratio of the hard phases becomes lower and, as a result, sometimes the desired triplex structure is not obtained, the anisotropy of strength becomes remarkable, and the desired tensile strength cannot be achieved. As opposed to this, by cooling steel sheet, cooled to the intermediate air cooling temperature of 620 to 700°C, by an average cooling rate of 2°C/s or more for 5 to 10 seconds, it becomes possible to make ferrite precipitate by the desired ratio and to make TiC precipitate in the ferrite and make it suitably grow in grain size to establish the presence of TiC precipitates finally having a diameter of 1.0 to 5.0 nm in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³. As a result, it is possible to raise the hardness of ferrite by precipitation strengthening and possible to reduce the hardness difference in the triplex structure to remarkably improve the hole expandability. In addition, the above air-cooling control is an extremely important operation not only for precipitation strengthening of ferrite by TiC precipitates, but also for making carbides, nitrides, and/or carbonitrides of Nb precipitate in the ferrite and sufficiently bringing out the pinning effect by such precipitates and achieving refining of ferrite grains and in turn higher strength of the hot rolled steel sheet.

[Cooling Step]

After the intermediate cooling, in the next cooling step, the steel sheet is primary cooled by an average cooling rate of 10 to 20°C/s for 1 to 3 seconds, then is secondary cooled by an average cooling rate of 25°C/s or more down to 200°C or less and coiled up. By such two-stage cooling, in the relatively slow average cooling rate primary cooling, mainly bainite is made to precipitate while similarly in the relatively fast average cooling rate secondary cooling, mainly martensite is made to suitably precipitate, and therefore in the finally obtained hot rolled steel sheet, it becomes possible to form a triplex structure containing ferrite, bainite and martensite in specific ratios. As opposed to this, if the average cooling rate of the primary cooling is made less than 10°C/s or the average cooling rate is made more than 20°C/s to substantially eliminate such primary cooling and perform single stage cooling comprised of just secondary cooling, bainite cannot be made to sufficiently precipitate and therefore it no longer becomes possible to form a triplex structure containing ferrite, bainite and martensite in the finally obtained hot rolled steel sheet. In such a case, the tendency for the L direction tensile strength to become lower than the C direction tensile strength at the hot rolled steel sheet becomes remarkable, i.e., the anisotropy of strength at the L direction and C direction tensile strengths becomes remarkable. Preferably, the average cooling rate of the primary cooling is 12 to 18°C/s.
On the other hand, if the average cooling rate of the secondary cooling is less than 25°C/s or the coiling temperature is more than 200°C, martensite cannot be made to precipitate in the desired amount and/or precipitation of bainite becomes greater and it becomes no longer possible to obtain the desired triplex structure. Preferably, the average cooling rate of the secondary cooling is 27°C/s or more. The upper limit of the average cooling rate of the secondary cooling is not particularly prescribed, but, for example, the average cooling rate may also be 50°C/s or less or 40°C/s or less. Further, the lower limit of the coiling temperature also is not particularly prescribed, but if the coiling temperature is too low, excessive water cooling, etc., become necessary and the productivity falls. Therefore, the coiling temperature is preferably, for example, 100°C or more.
According to the hot rolled steel sheet produced by the above method of production, the microstructure is comprised of a triplex structure containing, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%, and therefore it is possible to achieve a high strength of a tensile strength of 780 MPa or more while remarkably reducing the anisotropy of strength at the L direction and C direction tensile strengths of the hot rolled steel sheet. In addition, since size 1.0 to 5.0 nm TiC precipitates are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³, it is possible to raise the hardness of the ferrite by precipitation strengthening and reduce the hardness difference in the phases in the triplex structure. As a result, it becomes possible to remarkably improve the hole expandability of the hot rolled steel sheet. Therefore, according to the hot rolled steel sheet produced by the above method of production, it is possible to reliably achieve both the contradictory properties of high strength and excellent workability, and therefore the invention is particularly useful in use in the automotive field where realization of both of these properties is sought.
Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.

EXAMPLES

In the following examples, hot rolled steel sheets according to an embodiment of the present invention were produced under various conditions and were investigated for the tensile strength (TS), total elongation (EL), hole expansion rate (λ), and anisotropy of strength of the obtained hot rolled steel sheets.
First, slabs having various chemical compositions shown in Table 1 were formed by casting molten steels by the continuous casting method. These slabs were heated under the conditions shown in Table 2, then were hot rolled. The hot rolling was performed by rough rolling and finish rolling. The end temperatures of the finish rolling were as shown in Table 2. Next, the finish rolled steel sheets were intermediately cooled under the conditions shown in Table 2, the intermediately cooled steel sheets were primary cooled under the conditions shown in Table 2 over 2 seconds, then were secondary cooled and coiled to obtain hot rolled steel sheets having 3.2 mm sheet thicknesses.

[Table 1-1]

Table 1-1

Steel no.	Chemical composition (mass%), balance: Fe and impurities
Steel no.	C	Si	Mn	Ti	Nb	Al	P	S	N	O	Ni	Mo	Cr	B
A	0.034	0.03	1.67	0.144	0.017	0.284	0.0938	0.0006	0.0006	0.0088
B	0.091	0.02	1.40	0.150	0.014	0.336	0.0450	0.0072	0.0073	0.0023
C	0.032	0.08	1.12	0.063	0.010	0.282	0.0118	0.0043	0.0073	0.0080
D	0.057	0.06	2.08	0.187	0.011	0.329	0.0068	0.0094	0.0089	0.0100
E	0.094	0.06	2.00	0.067	0.018	0.311	0.0515	0.0019	0.0065	0.0100
F	0.081	0.02	1.27	0.128	0.019	0.259	0.0075	0.0082	0.0008	0.0011
G	0.061	0.04	0.83	0.053	0.018	0.301	0.0823	0.0047	0.0061	0.0001	0.009
H	0.061	0.02	1.14	0.115	0.013	0.241	0.0899	0.0014	0.0085	0.0085			0.008
I	0.092	0.04	1.21	0.160	0.010	0.253	0.0682	0.0044	0.0048	0.0003
J	0.071	0.04	1.45	0.063	0.017	0.311	0.0218	0.0004	0.0015	0.0046				0.0019
K	0.073	0.07	2.21	0.172	0.013	0.315	0.0789	0.0093	0.0051	0.0014			0.008	0.0018
L	0.093	0.06	0.88	0.080	0.015	0.330	0.0262	0.0026	0.0024	0.0025			0.005
M	0.054	0.06	2.60	0.106	0.015	0.248	0.0646	0.0064	0.0030	0.0006
N	0.047	0.05	0.66	0.078	0.019	0.285	0.0328	0.0078	0.0035	0.0043
O	0.094	0.01	0.75	0.152	0.019	0.239	0.0695	0.0070	0.0056	0.0050		0.007		0.0091
P	0.070	0.06	1.90	0.144	0.018	0.303	0.0414	0.0050	0.0095	0.0075			0.008	0.0050
Q	0.061	0.04	0.97	0.086	0.012	0.230	0.0975	0.0021	0.0056	0.0031
R	0.087	0.03	2.24	0.155	0.013	0.295	0.0509	0.0054	0.0014	0.0057	0.002		0.005	0.0053
S	0.060	0.08	1.61	0.155	0.011	0.297	0.0219	0.0008	0.0066	0.0086			0.003
T	0.074	0.04	1.69	0.128	0.015	0.325	0.0486	0.0050	0.0015	0.0075	0.008	0.007
U	0.042	0.10	0.60	0.137	0.016	0.332	0.0336	0.0063	0.0081	0.0001		0.002
V	0.096	0.05	1.22	0.076	0.011	0.239	0.0297	0.0041	0.0054	0.0032				0.0026
W	0.091	0.04	0.91	0.105	0.017	0.304	0.0673	0.0081	0.0080	0.0013	1.932
X	0.041	0.02	0.72	0.110	0.015	0.268	0.0048	0.0087	0.0028	0.0077		0.961
Y	0.077	0.08	2.25	0.073	0.013	0.342	0.0438	0.0087	0.0014	0.0097			1.894
Z	0.075	0.04	2.53	0.060	0.015	0.353	0.0831	0.0030	0.0050	0.0002				0.0094
AA	0.059	0.09	2.26	0.188	0.011	0.281	0.0651	0.0032	0.0055	0.0026	0.008			0.0051
AB	0.097	0.02	2.43	0.113	0.016	0.309	0.0814	0.0025	0.0086	0.0034		0.010	0.004	0.0016
AC	0.056	0.08	2.76	0.174	0.013	0.274	0.0148	0.0023	0.0027	0.0016		0.000		0.0089
AD	0.096	0.01	1.42	0.195	0.012	0.253	0.0887	0.0029	0.0043	0.0077	0.000
AE	0.062	0.07	2.47	0.130	0.012	0.357	0.0107	0.0027	0.0037	0.0082	0.003	0.001
AF	0.039	0.09	1.09	0.095	0.012	0.265	0.0301	0.0077	0.0020	0.0016	0.003	0.010		0.0018
AG	0.045	0.07	1.67	0.165	0.014	0.320	0.0584	0.0015	0.0085	0.0026
AH	0.060	0.05	1.98	0.185	0.019	0.321	0.0319	0.0091	0.0038	0.0075			0.006
AI	0.093	0.03	2.44	0.065	0.013	0.280	0.0606	0.0090	0.0093	0.0083	0.003
AJ	0.063	0.07	2.92	0.098	0.018	0.322	0.0250	0.0062	0.0058	0.0028
AK	0.091	0.09	2.43	0.146	0.011	0.278	0.0170	0.0044	0.0068	0.0046	0.006	0.008	0.005
AL	0.053	0.09	1.81	0.175	0.015	0.280	0.0768	0.0059	0.0065	0.0099	0.000	0.005		0.0088
AM	0.028	0.06	1.83	0.090	0.014	0.297	0.0434	0.0065	0.0087	0.0050	0.003	0.009		0.0014
AN	0.073	0.10	1.53	0.123	0.012	0.352	0.0404	0.0021	0.0076	0.0087		0.006	0.004
AO	0.084	0.03	1.54	0.159	0.018	0.295	0.0071	0.0082	0.0046	0.0022
AP	0.063	0.09	2.57	0.191	0.017	0.280	0.0809	0.0052	0.0009	0.0011				0.0026
AQ	0.054	0.05	2.61	0.172	0.018	0.308	0.0776	0.0056	0.0023	0.0069	0.003
AR	0.035	0.08	1.21	0.096	0.015	0.314	0.0592	0.0007	0.0075	0.0086		0.005
AS	0.036	0.05	2.74	0.121	0.016	0.298	0.0315	0.0066	0.0037	0.0080			0.002
AT	0.084	0.07	2.89	0.088	0.014	0.267	0.0702	0.0092	0.0079	0.0042		0.000
AU	0.043	0.04	0.72	0.078	0.014	0.341	0.0178	0.0038	0.0022	0.0080		0.004
AV	0.088	0.02	1.90	0.113	0.019	0.306	0.0522	0.0014	0.0006	0.0005		0.010
AW	0.081	0.07	2.14	0.178	0.017	0.352	0.0512	0.0076	0.0040	0.0057				0.0045
AX	0.044	0.08	2.86	0.122	0.016	0.265	0.0572	0.0039	0.0091	0.0071		0.009	0.006
AY	0.076	0.09	1.82	0.171	0.020	0.238	0.0922	0.0039	0.0018	0.0064			0.001
AZ	0.008	0.08	2.48	0.153	0.015	0.328	0.0618	0.0072	0.0026	0.0068		0.008
BA	0.140	0.07	2.08	0.074	0.016	0.311	0.0332	0.0087	0.0074	0.0099	0.002			0.0082
BB	0.022	0.18	2.10	0.059	0.019	0.266	0.0926	0.0028	0.0007	0.0041			0.002
BC	0.077	0.07	0.38	0.110	0.011	0.322	0.0516	0.0053	0.0009	0.0086		0.007	0.001
BD	0.058	0.09	3.28	0.097	0.010	0.304	0.0612	0.0094	0.0071	0.0081
BE	0.084	0.06	2.12	0.035	0.012	0.358		0.0033	0.0027	0.0022	0.003			0.0070
BF	0.071	0.08	1.10	0.240	0.018	0.274	0.0043	0.0040	0.0094	0.0086		0.010
BG	0.093	0.02	0.75	0.100	0.006	0.225	0.0473		0.0088	0.0100	0.009
BH	0.058	0.02	2.72	0.142	0.023	0.324	0.0286	0.0048	0.0085	0.0020
BI	0.041	0.02	1.64	0.063	0.014	0.079	0.0068	0.0012	0.0067	0.0014			0.001	0.0070
BJ	0.091	0.03	1.38	0.084	0.019	1.230	0.0360	0.0005	0.0048	0.0085	0.009			0.0058

Underlines indicate outside scope of present invention.

[Table 1-2]

Table 1-2

Steel no.	Chemical composition (mass%), balance: Fe and impurities
Steel no.	Co	V	Cu	W	Ta	Sn	Sb	As	Mg	Ca	Zr	Hf	Bi	REM
A
B
C
D
E
F
G		0.002	0.008			0.0050		0.0068			0.0040	0.0015	0.0019
H		0.009	0.003		0.0070						0.0019
I				0.0066					0.0059	0.0002	0.0070		0.0042
J	0.006	0.007	0.003	0.0016		0.0017	0.0005		0.0019					0.0004
K		0.007	0.005				0.0005				0.0011
L	0.002						0.0073						0.0002	0.0056
M			0.005	0.0043	0.0012	0.0028						0.0079	0.0084	0.0012
N					0.0088			0.0001		0.0055			0.0089
O					0.0089		0.0039	0.0072		0.0044	0.0022
P		0.005					0.0044	0.0013		0.0088	0.0022		0.0089	0.0057
Q				0.0074		0.0055				0.0013	0.0022	0.0022
R	0.001													0.0042
S	0.007				0.0030						0.0082	0.0063
T	0.003					0.0018	0.0011	0.0021				0.0031
U	0.001					0.0086			0.0091				0.0011
V	0.004							0.0026				0.0013
W						0.0053	0.0023		0.0038				0.0084
X					0.0072	0.0085				0.0022
Y				0.0066		0.0052	0.0039	0.0048	0.0048		0.0060
Z			0.000		0.0019				0.0000	0.0015				0.0007
AA	1.981			0.0057						0.0034	0.0064
AB		0.946	0.008											0.0027
AC			1.911			0.0031			0.0076		0.0092			0.0027
AD		0.006		0.9680	0.0009		0.0076	0.0016	0.0090					0.0000
AE				0.0083	0.9190					0.0009
AF		0.002		0.0057		0.9330	0.0093	0.0093
AG							0.9370					0.0043
AH	0.009			0.0071			0.0045	0.0094	0.0023
AI	0.001		0.002					0.0043	0.0097
AJ	0.003	0.009	0.001		0.0088	0.0085			0.0012	0.0090		0.0031
AK			0.001								0.0098	0.0050
AL				0.0072						0.0054		0.0093	0.0049
AM	0.008	0.007	0.009		0.0095			0.0024		0.0050			0.0091
AN							0.0013		0.0049			0.0031		0.0093
AO
AP		0.004				0.0088						0.0069		0.0054
AQ					0.0027			0.0007	0.0055					0.0039
AR	0.009	0.004						0.0054
AS				0.0027			0.0032		0.0081
AT	0.000			0.0071	0.0066					0.0055			0.0077
AU			0.002	0.0052					0.0086		0.0044
AV			0.001			0.0088					0.0049			0.0009
AW						0.0026			0.0080			0.0020
AX	0.007								0.0024		0.0006
AY				0.0033	0.0004		0.0026		0.0082		0.0092		0.0021
AZ				0.0090				0.0016					0.0023
BA							0.0076
BB							0.0005
BC		0.001					0.0091				0.0081
BD	0.008		0.001									0.0013
BE			0.009		0.0023						0.0003
BF	0.002				0.0021		0.0025	0.0058						0.0047
BG
BH	0.003	0.004		0.0020				0.0095		0.0078	0.0044	0.0068		0.0066
BI											0.0029	0.0018		0.0030
BJ					0.0040	0.0056	0.0068

Underlines indicate outside scope of present invention.

[Table 2]

Table 2

Prod. no.	Steel no.	Hot rolling step		Intarmediate cooling step				Cooling step			Remarks
		Slab heating temp.	Finish rolling end temp.	Average cooling rate until inter. air cooling temp.	Inter. air cooling temp.	Air cooling time	Average cooling rate of inter. air cooling	Average cooling rate of primary cooling	Average cooling rate of secondary cooling	Coiling temp.
		°C	°C	°C/s	°C	s	°C/s	°C/s	°C/s	°C
1	A	1171	942	25	632	9	5	15	33	67	Inv. ex.
2	B	1183	937	26	661	7	5	17	42	132	Inv. ex.
3	C	1225	975	28	628	6	4	16	33	91	Inv. ex.
4	D	1230	911	22	693	5	5	17	27	85	Inv. ex.
5	E	1249	922	25	657	7	5	14	27	29	Inv. ex.
6	F	1150	918	26	650	6	5	17	39	53	Inv. ex.
7	G	1222	928	27	638	9	5	13	42	159	Inv. ex.
8	H	1199	963	23	667	8	5	15	36	43	Inv. ex.
9	I	1177	959	28	658	8	5	18	34	181	Inv. ex.
10	J	1249	912	28	655	7	4	16	39	76	Inv. ex.
11	K	1214	955	28	668	6	5	18	34	66	Inv. ex.
12	L	1180	937	23	674	7	5	16	37	146	Inv. ex.
13	M	1153	910	27	655	9	5	18	34	52	Inv. ex.
14	N	1164	988	21	640	6	5	14	34	36	Inv. ex.
15	O	1194	939	25	653	5	5	15	41	140	Inv. ex.
16	P	1224	931	25	640	5	5	15	29	193	Inv. ex.
17	Q	1223	946	25	688	6	6	17	30	82	Inv. ex.
18	R	1177	971	21	678	6	6	15	37	197	Inv. ex.
19	S	1209	910	26	632	9	6	14	36	163	Inv. ex.
20	T	1173	901	26	650	7	5	16	44	181	Inv. ex.
21	U	1162	917	26	672	7	5	16	40	183	Inv. ex.
22	V	1237	946	20	688	5	6	14	44	72	Inv. ex.
23	W	1164	976	23	670	5	5	14	37	87	Inv. ex.
24	X	1249	919	25	692	8	4	16	38	32	Inv. ex.
25	Y	1208	933	23	692	5	5	17	30	72	Inv. ex.
26	Z	1188	942	23	663	5	6	14	31	161	Inv. ex.
27	AA	1215	959	23	649	6	6	16	33	46	Inv. ex.
28	AB	1212	905	27	627	7	5	17	33	94	Inv. ex.
29	AC	1231	900	21	685	6	6	17	38	136	Inv. ex.
30	AD	1190	925	26	673	6	5	14	37	175	Inv. ex.
31	AE	1227	943	29	664	8	5	14	36	114	Inv. ex.
32	AF	1160	919	24	624	9	5	16	40	131	Inv. ex.
33	AG	1246	912	24	682	6	5	17	29	66	Inv. ex.
34	AH	1233	992	23	645	7	5	16	36	47	Inv. ex.
35	AI	1160	950	22	644	7	5	15	45	98	Inv. ex.
36	AJ	1215	935	28	634	8	6	18	34	108	Inv. ex.
37	AK	1208	945	20	628	8	4	14	33	113	Inv. ex.
38	AL	1176	951	22	671	5	4	14	40	39	Inv. ex.
39	AM	1164	906	24	636	8	5	16	38	124	Inv. ex.
40	AN	1235	981	23	694	9	5	17	31	159	Inv. ex.
41	AO	1207	973	24	640	7	5	15	31	59	Inv. ex.
42	AP	1183	1038	25	680	9	5	15	38	174	Comp. ex.
43	AQ	1153	872	29	687	6	5	18	29	133	Comp. ex.
44	AR	1171	924	22	730	7	5	15	39	109	Comp. ex.
45	AS	1228	920	23	591	8	5	13	31	172	Comp. ex.
46	AT	1230	922	27	687	12	5	17	43	128	Comp. ex.
47	AU	1191	981	26	662	2	5	14	32	182	Comp. ex.
48	AV	1150	970	22	644	8	5	30	29	117	Comp. ex.
49	AW	1156	954	28	698	6	5	3	27	137	Comp. ex.
50	AX	1151	926	27	626	6	6	15	8	147	Comp. ex.
51	AY	1200	942	22	650	8	6	14	31	239	Comp. ex.
52	AZ	1218	905	22	691	6	4	16	33	127	Comp. ex.
53	BA	1169	956	20	698	5	5	18	32	142	Comp. ex.
54	BB	1206	908	27	639	5	5	15	30	106	Comp. ex.
55	BC	1231	919	27	658	7	4	16	33	191	Comp. ex.
56	BD	1202	940	21	688	10	4	18	38	88	Comp. ex.
57	BE	1153	925	28	644	6	5	13	38	100	Comp. ex.
58	BF	1194	936	20	692	5	5	18	36	147	Comp. ex.
59	BG	1228	938	26	643	8	5	15	29	34	Comp. ex.
60	BH	1210	952	24	626	5	5	17	41	163	Comp. ex.
61	BI	1152	991	28	637	9	5	17	34	52	Comp. ex.
62	BJ	1165	905	24	665	6	5	13	38	168	Comp. ex.

Underlines indicate outside scope of present invention or production conditions not preferable.

The properties of the hot rolled steel sheets obtained were measured and evaluated by the following methods.

[Calculation of Size and Number Density of TiC Precipitates]

The size and number density of TiC precipitates were calculated by the 3D atom probe measurement method described in detail in this Description with a device-specific atom detection rate α of 0.35.

[Tensile Strength (TS) and Total Elongation (EL)]

The tensile strength (TS) and total elongation (EL) were measured by taking a JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011. Here, the obtained tensile strength will also be referred to as the "C direction TS".

[Hole Expansion Rate (λ)]

The hole expansion rate (λ) was determined as follows: First, a circular hole having a diameter of 10 mm (initial hole: hole size d0=10 mm) was punched out of the test piece under conditions given a clearance of 12.5%. The initial hole was pushed open by a conical punch of a vertex angle of 60° so that the burr becomes the die side until a crack was formed passing through the sheet thickness. The hole size d1 mm at the time of cracking was measured to find the hole expansion rate λ (%) of each test piece by the following formula. This hole expansion test was conducted five times and the average value of these was determined as the hole expansion rate λ. $λ = 100 \times \{(d 1 - d 0) / d 0\}$

[Anisotropy of Strength]

The anisotropy of strength was determined by, first, taking a JIS No. 5 test piece from an orientation of the long direction of the test piece becoming parallel to the rolling direction of the hot rolled steel sheet (L direction) and conducting a tensile test based on JIS Z 2241: 2011 to measure the L direction tensile strength, i.e., L direction TS. Next, if the obtained L direction TS and the previously found C direction TS satisfied the following formula, it was deemed that the anisotropy of strength was reduced and the sample was evaluated as passing ("Good") and if they did not satisfy the following formula, the sample was evaluated as failing ("Poor").

0.95≤L direction TS/C direction TS≤1.00

A case where the tensile strength of the hot rolled steel sheet (TS) was 780 MPa or more, the hole expansion rate (λ) was 50% or more, and the anisotropy of strength was evaluated as passing was evaluated as a hot rolled steel sheet which is high in strength, but despite this is improved in hole expandability and reduced in anisotropy of strength. The result is shown in Table 3.

[Table 3]

Table 3

Prod. no.	Steel no.	Ferrite	Bainite	Martensite	Balance structures	Diameter of TiC precipitates in ferrite	Number density of TiC precipitates	Tensile strength TS	Total elongation EL	Hole expandability λ	Anisotropy of strength	Remarks
Prod. no.	Steel no.	Area%	Area%	Area%	Area%	nm	×10¹⁶/cm³	MPa	%	%	-	Remarks
1	A	67	28	5	0	2.2	30.0	800	21.9	61	Good	Inv. ex.
2	B	72	24	4	0	3.5	57.7	797	22.3	59	Good	Inv. ex.
3	C	65	26	9	0	2.0	13.6	824	17.8	84	Good	Inv. ex.
4	D	79	18	3	0	4.9	96.8	787	23.9	67	Good	Inv. ex.
5	E	68	26	6	0	3.1	48.3	810	20.1	67	Good	Inv. ex.
6	F	67	27	6	0	2.9	45.3	808	20.5	67	Good	Inv. ex.
7	G	65	29	6	0	2.3	26.0	812	19.9	69	Good	Inv. ex.
8	H	66	25	8	1	3.6	62.4	814	19.6	84	Good	Inv. ex.
9	I	65	27	8	0	3.1	57.7	824	17.9	76	Good	Inv. ex.
10	J	73	24	3	0	3.0	47.8	788	23.8	53	Good	Inv. ex.
11	K	67	25	8	0	3.8	70.0	815	19.4	80	Good	Inv. ex.
12	L	69	24	7	0	3.8	70.2	812	19.8	74	Good	Inv. ex.
13	M	69	26	5	0	3.2	49.0	800	21.7	68	Good	Inv. ex.
14	N	61	30	9	0	2.5	31.1	826	17.5	87	Good	Inv. ex.
15	O	66	27	7	0	3.1	51.7	817	19.1	71	Good	Inv. ex.
16	P	69	27	4	0	2.6	35.8	797	22.2	56	Good	Inv. ex.
17	Q	71	22	7	0	4.6	83.9	808	20.5	85	Good	Inv. ex.
18	R	67	24	9	0	4.2	81.0	820	18.5	85	Good	Inv. ex.
19	S	70	27	3	0	2.1	30.3	792	23.2	65	Good	Inv. ex.
20	T	69	26	5	0	3.1	46.3	801	21.6	63	Good	Inv. ex.
21	U	71	23	6	0	4.0	69.5	797	22.2	77	Good	Inv. ex.
22	V	75	20	5	0	4.4	82.2	800	21.7	65	Good	Inv. ex.
23	W	66	25	9	0	3.8	65.1	824	17.8	84	Good	Inv. ex.
24	X	78	19	3	0	4.8	91.6	788	23.7	68	Good	Inv. ex.
25	Y	77	19	4	0	4.8	87.2	794	22.7	67	Good	Inv. ex.
26	Z	71	24	5	0	3.5	53.1	797	22.2	61	Good	Inv. ex.
27	AA	64	28	8	0	2.9	52.8	813	19.6	82	Good	Inv. ex.
28	AB	65	29	5	1	1.7	17.5	814	19.6	58	Good	Inv. ex.
29	AC	74	21	5	0	4.6	91.1	795	22.7	75	Good	Inv. ex.
30	AD	74	22	4	0	4.1	80.1	793	23.0	57	Good	Inv. ex.
31	AE	71	24	5	0	3.4	62.0	795	22.5	64	Good	Inv. ex.
32	AF	68	28	3	1	1.7	15.8	795	22.5	53	Good	Inv. ex.
33	AG	72	22	6	0	4.2	83.4	798	22.0	75	Good	Inv. ex.
34	AH	61	29	10	0	2.8	45.2	825	17.7	89	Good	Inv. ex.
35	AI	64	28	7	1	2.5	29.3	821	18.3	74	Good	Inv. ex.
36	AJ	68	28	4	0	2.3	24.6	801	21.7	56	Good	Inv. ex.
37	AK	65	28	7	0	1.9	22.4	828	17.2	70	Good	Inv. ex.
38	AL	67	25	8	0	3.7	72.4	810	20.2	82	Good	Inv. ex.
39	AM	71	26	3	0	2.2	24.3	787	23.9	52	Good	Inv. ex.
40	AN	69	22	9	0	4.8	93.9	818	18.8	95	Good	Inv. ex.
41	AO	62	29	9	0	2.4	36.0	828	17.2	80	Good	Inv. ex.
42	AP	56	32	12	0	4.3	87.6	832	16.6	68	Poor	Comp. ex.
43	AQ	83	15	2	0	4.6	91.7	778	25.4	46	Poor	Comp. ex.
44	AR	83	13	4	0	6.4	130.4	764	27.7	95	Poor	Comp. ex.
45	AS	51	46	3	0	0.2	0.2	863	11.4	49	Poor	Comp. ex.
46	AT	85	9	6	0	4.5	81.8	759	28.5	75	Poor	Comp. ex.
47	AU	57	34	9	0	3.5	56.1	832	16.6	52	Poor	Comp. ex.
48	AV	63	13	24	0	2.6	39.4	867	10.8	38	Poor	Comp. ex.
49	AW	83	9	8	0	5.6	86.0	772	26.4	85	Poor	Comp. ex.
50	AX	64	32	4	0	1.9	16.6	808	20.4	69	Poor	Comp. ex.
51	AY	75	24	1	0	2.9	52.0	801	21.7	74	Poor	Comp. ex.
52	AZ	85	15	0	0	4.7	0.8	543	38.7	110	Poor	Comp. ex.
53	BA	57	28	10	5	4.2	92.8	843	14.7	62	Poor	Comp. ex.
54	BB	81	13	6	0	4.2	67.0	801	21.6	96	Poor	Comp. ex.
55	BC	81	15	4	0	3.2	55.7	770	26.7	54	Poor	Comp. ex.
56	BD	60	28	12	0	0.5	83.2	820	18.4	47	Poor	Comp. ex.
57	BE	58	36	6	0	2.6	0.7	798	22.2	49	Poor	Comp. ex.
58	BF	81	12	7	0	7.9	0.8	795	22.7	42	Poor	Comp. ex.
59	BG	58	38	4	0	2.7	35.6	829	17.1	53	Poor	Comp. ex.
60	BH	81	11	8	0	1.9	18.7	765	27.4	74	Poor	Comp. ex.
61	BI	56	34	10	0	2.3	24.8	837	15.7	89	Poor	Comp. ex.
62	BJ	83	12	5	0	3.6	60.6	779	25.2	64	Poor	Comp. ex.

Referring to Tables 1 to 3, in Comparative Example 42, the end temperature of the finish rolling was high, and therefore it is believed the austenite became coarser and the ratio of ferrite became smaller. As a result, the desired triplex structure was not obtained, and the anisotropy of strength became large. In Comparative Example 43, the end temperature of the finish rolling was low, and therefore the metallostructure became uneven and TS and λ fell. In Comparative Example 44, the intermediate air cooling temperature was high, and therefore ferrite transformation proceeded too much, the desired triplex structure was not obtained, and the anisotropy of strength became large. In Comparative Example 45, the intermediate air cooling temperature was low, and therefore ferrite could not be made to sufficiently precipitate and similarly the desired triplex structure was not obtained and the anisotropy of strength became large. In addition, the intermediate air cooling temperature was low, and therefore it was not possible to for TiC precipitates having the desired diameter by a sufficient number density even by the later air cooling. As a result, the effect of improvement of hardness of ferrite by precipitation strengthening could not be sufficiently obtained and λ dropped. In Comparative Example 46, the time period of intermediate cooling was long, and therefore ferrite excessively precipitated, the ratio of the hard phase became lower, and as a result, the desired triplex structure was not obtained, the anisotropy of strength became remarkable, and the desired tensile strength could not be achieved. In Comparative Example 47, the time period of the intermediate cooling was short, and therefore ferrite could not be made to sufficiently precipitate, and as a result, the desired triplex structure was not obtained, and the anisotropy of strength became large. In Comparative Example 48, the average cooling rate of the primary cooling in the cooling step was fast, and therefore bainite could not be made to sufficiently precipitate, and as a result, the desired triplex structure was not obtained, and the anisotropy of strength became large. Further, in Comparative Example 48, the ratio of martensite became high, and therefore it is believed it was not possible to suitably reduce the hardness difference of the phases by precipitation strengthening of the ferrite. As a result, λ also fell. In Comparative Example 49, the average cooling rate of the primary cooling in the cooling step was slow, and therefore similarly bainite could not be made to sufficiently precipitate and, as a result, the desired triplex structure was not obtained and the anisotropy of strength became large. In Comparative Example 50, the average cooling rate of the secondary cooling in the cooling step was slow, and therefore bainite greatly precipitated and, as a result, the desired triplex structure was not obtained and the anisotropy of strength became large. In Comparative Example 51, the coiling temperature was high, and therefore martensite could not be made to precipitate in a sufficient amount and, as a result, the desired triplex structure was not obtained and the anisotropy of strength became large.
In each of Comparative Examples 52 and 55, the C and Mn contents were low, and therefore the TS fell. In Comparative Example 53, the C content was high, and therefore the desired triplex structure was not obtained and the anisotropy of strength became large. In Comparative Example 54, the Si content was high, and therefore Si scale caused the surface roughness of the hot rolled steel sheet to increase and, further, since the desired triplex structure could not be obtained, the anisotropy of strength became large. In Comparative Example 56, the Mn content was high, and therefore it is believed the dispersion coefficient of C fell and the diameter of the TiC precipitates became smaller. As a result, the effect of improvement of hardness of ferrite by precipitation strengthening based on the TiC precipitates could not be sufficiently obtained and λ fell. In Comparative Example 57, the Ti content was low, and therefore TiC precipitates could not be formed in a sufficient number density and λ fell. In Comparative Example 58, the Ti content was high, and therefore the TiC precipitates coarsened and along with this, the number density of the TiC precipitates fell and similarly λ also fell. In each of Comparative Examples 59 and 61, the Nb and Al contents were low, and therefore the desired triplex structure was not obtained, and the anisotropy of strength became large. In particular, in Comparative Example 59, the Nb content was low, and therefore the pinning effect by carbides, etc., could not be sufficiently obtained and, in relation to this, it is believed ferrite transformation was not promoted. In Comparative Example 60, the Nb content was high, and therefore it is believed coarse carbides, etc., were formed in the steel and, further, the desired triplex structure could not be obtained. As a result, the TS fell and the anisotropy of strength became large. In Comparative Example 62, the Al content was high, and therefore it is believed coarse oxides were formed. Further, the desired triplex structure also could not be obtained. As a result, the TS fell and the anisotropy of strength became large.
In contrast to this, in the hot rolled steel sheets according to all of the invention examples, by having the predetermined chemical composition and, further, suitably controlling the conditions in the method of production, it was possible to form the microstructure by a triplex structure comprised of, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10% and achieve a high strength of a tensile strength of 780 MPa or more while remarkably reducing the anisotropy of strength. In addition, by establishing the presence in the ferrite of TiC precipitates having a diameter of 1.0 to 5.0 nm in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³ for precipitation strengthening of ferrite, it was possible to reduce the hardness difference in the triplex structure and remarkably improve the hole expandability. Further, in many of the invention examples, the area ratio of the balance structures was 0%, but if there were balance structures, the balance structures were pearlite.

Claims

A hot rolled steel sheet having a chemical composition comprising, by mass%,
C: 0.010 to 0.100%,

Si: 0.01 to 0.10%,

Mn: 0.50 to 3.00%,

Ti: 0.050 to 0.200%,

Nb: 0.010 to 0.020%,

Al: 0.100 to 1.000%,

P: 0.1000% or less,

S: 0.0100% or less,

N: 0.0100% or less,

O: 0.0100% or less,

Ni: 0 to 2.000%,

Mo: 0 to 1.000%,

Cr: 0 to 2.000%,

B: 0 to 0.0100%,

Co: 0 to 2.000%,

V: 0 to 1.000%,

Cu: 0 to 2.000%,

W: 0 to 1.0000%,

Ta: 0 to 1.0000%,

Sn: 0 to 1.0000%,

Sb: 0 to 1.0000%,

As: 0 to 0.0100%,

Mg: 0 to 0.0100%,

Ca: 0 to 0.0100%,

Zr: 0 to 0.0100%,

Hf: 0 to 0.0100%,

Bi: 0 to 0.0100%,

REM: 0 to 0.0100%, and

balance: Fe and impurities, and

a microstructure comprising, by area ratio,

ferrite: 60 to 80%,

bainite: 15 to 30%, and

martensite: 3 to 10%, wherein

TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×10¹⁶ to 100.0×10¹⁶/cm³, and

the hot rolled steel sheet has a tensile strength of 780 MPa or more.
The hot rolled steel sheet according to claim 1, wherein the chemical composition contains, by mass%, at least one of:
Ni: 0.001 to 2.000%,

Mo: 0.001 to 1.000%,

Cr: 0.001 to 2.000%,

B: 0.0001 to 0.0100%,

Co: 0.001 to 2.000%,

V: 0.001 to 1.000%,

Cu: 0.001 to 2.000%,

W: 0.0001 to 1.0000%,

Ta: 0.0001 to 1.0000%,

Sn: 0.0001 to 1.0000%,

Sb: 0.0001 to 1.0000%,

As: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0100%,

Ca: 0.0001 to 0.0100%,

Zr: 0.0001 to 0.0100%,

Hf: 0.0001 to 0.0100%,

Bi: 0.0001 to 0.0100%, and

REM: 0.0001 to 0.0100%.