EP4613892A1

EP4613892A1 - Hot rolled steel sheet

Info

Publication number: EP4613892A1
Application number: EP23885305.5A
Authority: EP
Inventors: Mutsumi YOSHITAKE; Hiroshi Shuto; Daisuke Ito; Jun Ando; Kensuke Nagai
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: WO2024095532A1; KR20250071276A; CN120129765A; JPWO2024095532A1; MX2025004679A; EP4613892A4

Abstract

Provided is a hot rolled steel sheet having a predetermined chemical composition and having a microstructure comprising, by area%, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, wherein when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, having a ratio of crystal grains with an orientation difference in grains of 5 to 14° of, by area%, 10 to 60%, an average value of pole densities of {110}<111> and {112}<111> orientations in a region from a surface down to a 1/6 position in sheet thickness is 2.50 or more, and an average value of pole densities of {100}<011>, {211}<011>, and {332}<113> orientations in a region from a 2/5 position in sheet thickness to a 3/5 position in sheet thickness is 7.00 or less.

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, along with higher strength, the workability of a steel sheet generally falls. For this reason, in development of a high strength steel sheet, it is important to secure a certain level or more of workability while raising the strength.
In relation to this, PTL 1 describes a hot rolled steel sheet having a predetermined chemical composition and having a structure containing, by area ratio, a total of 80 to 98% of ferrite and bainite and 2 to 10% of martensite and, where, in that structure, when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area%, 10 to 60%. Further, PTL 1 teaches that by making the ratio of the above crystal grains with an orientation difference in grains of 5 to 14° an area ratio of 10 to 60%, it is possible to improve the stretch flangeability and ductility while maintaining high strength and further that by controlling the total area ratio of ferrite and bainite in the structure and the area ratio of martensite to within predetermined ranges, it is possible to improve the notch fatigue property.

[CITATIONS LIST]

[PATENT LITERATURE]

[PTL 1] WO2016/133222

SUMMARY

[TECHNICAL PROBLEM]

A high strength steel sheet is produced by hot rolling a cast slab, but it is known that sometimes along with the hot rolling, an anisotropy of strength is formed between the rolling direction (L direction) strength and that perpendicular width direction (C direction) strength. If the anisotropy of strength becomes larger, in general the workability of a steel sheet falls, and therefore this becomes a problem. Therefore, to improve the workability of a steel sheet, in addition to the stretch flangeability, ductility, and notch fatigue property such as described in PTL 1, there is a high need for high strength steel sheet reduced in anisotropy of strength.
Therefore, the present invention has as its object the provision of a hot rolled steel sheet which, despite being high in strength, is improved in stretch flangeability, ductility, and notch fatigue property and reduced in anisotropy of strength.

[SOLUTION TO PROBLEM]

To inventors engaged in studies to achieve the above object focusing in particular on the microstructure of the hot rolled steel sheet. As a result, the inventors discovered that by making the microstructure of the hot rolled steel sheet having a predetermined chemical composition contain at least one of ferrite and bainite and martensite in specific ratios and further controlling the ratio of crystal grains to within a predetermined range, it is possible to improve the stretch flangeability, ductility, and notch fatigue property while by additionally suitably controlling the texture of the sheet thickness surface layer part and sheet thickness center part of the steel sheet, it is possible to reduce the anisotropy of strength, 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: 0.020 to 0.070%,
- Si: 0.010 to 2.000%,
- Mn: 0.60 to 2.00%,
- Ti: 0.015 to 0.200%,
- sol. Al: 0.010 to 1.000%,
- P: 0.100% or less,
- S: 0.030% or less,
- N: 0.0060% or less,
- O: 0.0100% or less,
- Nb: 0 to 0.050%,
- V: 0 to 0.300%,
- Cr: 0 to 2.00%,
- Ni: 0 to 2.00%,
- Cu: 0 to 2.00%,
- Mo: 0 to 1.000%,
- B: 0 to 0.0100%,
- Sb: 0 to 1.00%,
- Ca: 0 to 0.0100%,
- Mg: 0 to 0.0100%,
- Hf: 0 to 0.0100%,
- REM: 0 to 0.1000%,
- Bi: 0 to 0.0100%,
- As: 0 to 0.0100%,
- Zr: 0 to 1.00%,
- Co: 0 to 1.00%,
- Zn: 0 to 1.00%,
- W: 0 to 1.00%,
- Sn: 0 to 1.00%, and
- balance: Fe and impurities and
- satisfying 0.100≤[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are contents (mass%) of the elements, and
- a microstructure comprising, by area%,
- at least one of ferrite and bainite: 80 to 98% in total, and
- martensite: 2 to 10%, wherein
- when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area%, 10 to 60%,
- an average value of pole densities of {110}<111> and {112}<111> orientations in a region from a surface down to a 1/6 position in sheet thickness is 2.50 or more, and
- an average value of pole densities of {100}<011>, {211}<011>, and {332}<113> orientations in a region from a 2/5 position in sheet thickness to a 3/5 position in sheet thickness is 7.00 or less.
(2) The hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass%, at least one of
- Nb: 0.001 to 0.050%,
- V: 0.001 to 0.300%,
- Cr: 0.01 to 2.00%,
- Ni: 0.01 to 2.00%,
- Cu: 0.01 to 2.00%,
- Mo: 0.001 to 1.000%,
- B: 0.0001 to 0.0100%,
- Sb: 0.01 to 1.00%,
- Ca: 0.0001 to 0.0100%,
- Mg: 0.0001 to 0.0100%,
- Hf: 0.0001 to 0.0100%,
- REM: 0.0001 to 0.1000%,
- Bi: 0.0001 to 0.0100%,
- As: 0.0001 to 0.0100%,
- Zr: 0.01 to 1.00%,
- Co: 0.01 to 1.00%,
- Zn: 0.01 to 1.00%,
- W: 0.01 to 1.00%, and
- Sn: 0.01 to 1.00%.

[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 stretch flangeability, ductility, and notch fatigue property and reduced in anisotropy of strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a shape of a saddle shaped article used in a saddle type stretch flanging test method.
FIG. 2 is a view showing a shape of a fatigue test piece used for evaluating a notch fatigue property.

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: 0.020 to 0.070%,
Si: 0.010 to 2.000%,
Mn: 0.60 to 2.00%,
Ti: 0.015 to 0.200%,
sol. Al: 0.010 to 1.000%,
P: 0.100% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0100% or less,
Nb: 0 to 0.050%,
V: 0 to 0.300%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.000%,
B: 0 to 0.0100%,
Sb: 0 to 1.00%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0100%,
REM: 0 to 0.1000%,
Bi: 0 to 0.0100%,
As: 0 to 0.0100%,
Zr: 0 to 1.00%,
Co: 0 to 1.00%,
Zn: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%, and
balance: Fe and impurities, and
satisfying 0.100≤[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are contents (mass%) of the elements, and
a microstructure comprising, by area%,
at least one of ferrite and bainite: 80 to 98% in total, and
martensite: 2 to 10%, wherein
when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area%, 10 to 60%,
an average value of pole densities of {110}<111> and {112}<111> orientations in a region from a surface down to a 1/6 position in sheet thickness is 2.50 or more, and
an average value of pole densities of {100}<011>, {211}<011>, and {332}<113> orientations in a region from a 2/5 position in sheet thickness to a 3/5 position in sheet thickness is 7.00 or less.

As explained above, it is known that the stretch flangeability and other properties fall along with higher strength of a steel sheet and that due to hot rolling at the time of a steel sheet production, sometimes anisotropy of strength occurs between the strength in the rolling direction (L direction) and strength in the width direction perpendicular to the same (C direction). First, in an embodiment of the present invention, by making the microstructure of the hot rolled steel sheet having a predetermined chemical composition include at least one of ferrite and bainite and martensite in specific ratios, more specifically by making it include, by area%, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, it is possible to improve the strength and the stretch flangeability, ductility, and notch fatigue property with a good balance. In addition, if defining a region surrounded by grain boundaries with an orientation difference of 15° or more and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, crystal grains with an orientation difference in grains of 5 to 14° are effective for improving the strength and the stretch flangeability and ductility. For this reason, by suitably controlling the ratios of the crystal grains, more specifically by controlling it within a range of, by area%, 10 to 60%, it becomes possible to further improve the balance of the strength and the stretch flangeability and ductility.
On the other hand, if explaining the anisotropy of strength in more detail, there is a tendency for the tensile strength to differ between the rolling direction (L direction) and the width direction perpendicular to that (C direction) due to the anisotropic microstructure obtained by hot rolling at the time of steel sheet production. In general, there is a tendency for anisotropy of strength to be exhibited where the L direction tensile strength at the hot rolled steel sheet becomes lower than the C direction tensile strength. By improving the stretch flangeability, ductility, and notch fatigue property and by reducing such anisotropy of strength, it is possible to greatly improve the workability of the high strength steel sheet used in automobiles and other applications. However, achieving both higher strength of the steel sheet and improvement of these properties is generally extremely difficult. Therefore, the inventors studied reduction of the anisotropy of strength in addition to improvement of the stretch flangeability, ductility, and notch fatigue property so as to simultaneously realize higher strength of the steel sheet, in particular focusing on the texture of the hot rolled steel sheet. As a result, the inventors discovered that by controlling the average value of the pole densities of the {110}<111> and {112}<111> orientations at the region from the surface of the hot rolled steel sheet down to a 1/6 position in sheet thickness to 2.50 or more and controlling the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness to 7.00 or less, it is possible to remarkably reduce the anisotropy of strength at the L direction and C direction tensile strengths of the hot rolled steel sheet.
If explained in more detail, if analyzing the sheet thickness cross-section of the hot rolled steel sheet, the orientations of the crystals differ between the sheet thickness surface layer part directly receiving the effect of rolling (i.e., the region from the surface of the hot rolled steel sheet down to a 1/6 position in sheet thickness) and the sheet thickness center part (i.e., the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness). More specifically, at the sheet thickness surface layer part, the texture of the {110}<111> and {112}<111> orientations grows. Due to such growth of the texture, it is believed that the L direction strength is raised. On the other hand, it is believed that at the sheet thickness center part, the texture at the {100}<011>, {211}<011> and {332}<113> orientations grows and the C direction strength is raised due to such growth of the texture. However, at the sheet thickness surface layer part and the sheet thickness center part, the effect of the sheet thickness center part easily strongly appears. Therefore, it is believed that anisotropy of strength is exhibited where the L direction tensile strength becomes lower than the C direction tensile strength. Here, for the crystal orientation of the rolled sheet, usually a crystal orientation vertical to the rolling surface is expressed by {hkl} or (hkl) while a crystal orientation parallel to the rolling direction is expressed by <uvw> or [uvw]. {hkl}and <uvw> are general terms for equivalent planes and orientations, while (hkl) and [uvw] indicate individual crystal planes. In the hot rolled steel sheet according to an embodiment of the present invention, mainly body centered cubic structures (bcc structures) are covered, so, for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1), and (0-1-1) are equivalent and cannot be differentiated. In an embodiment of the present invention, these orientations are expressed overall as {110}.
Therefore, the inventors discovered that by making the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part larger to a predetermined value or more to raise the L direction strength and on the other hand reducing the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the sheet thickness center part to a predetermined value or less to lower the C direction strength, more specifically by controlling the average value of the pole densities of the {110}<111> and {112}<111> orientations at the region from the surface of the hot rolled steel sheet down to a 1/6 position in sheet thickness to 2.50 or more and controlling the average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations at the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness to 7.00 or less, it is possible to remarkably reduce the anisotropy of strength in the L direction and C direction tensile strengths of the hot rolled steel sheet. Here, the "pole density" means the ratio of degree of control to a specific orientation of the supplied sample with respect to a standard sample not control to a specific orientation. In the hot rolled steel sheet according to an embodiment of the present invention, as previously explained, to improve the stretch flangeability, ductility, and notch fatigue property, the microstructure is made one including at least one of ferrite and bainite, and martensite in specific ratios and the specific crystal grains with an orientation difference in grains of 5 to 14° is controlled within a range of, by area%, 10 to 60%. Therefore, maintaining the configuration of the microstructure controlled in this way while further controlling the pole density of a specific texture at the sheet thickness surface layer part and the pole density of a specific texture at the sheet thickness center part respectively to within desired ranges is extremely difficult. As opposed to this, as explained in detail later in relation to the method of production of the hot rolled steel sheet, in an embodiment of the present invention, by making the rolling conditions in the hot rolling step suitable ones, it is possible to maintain the configuration of the microstructure for improving the stretch flangeability, ductility, and notch fatigue property while realizing a microstructure with an average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part of 2.50 or more and an average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations at the sheet thickness center part of 7.00 or less. As a result, according to an embodiment of the present invention, despite being high strength, for example, a high strength of a tensile strength of 540 MPa or more, it is possible to achieve an improvement of the stretch flangeability, ductility, and notch fatigue property and a reduction of the anisotropy of strength. Therefore, since 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, it 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: 0.020 to 0.070%]

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 steel and also contributes to the precipitation strengthening based on the precipitates formed and the refinement of the structure by the pinning effect of the precipitates. To sufficiently obtain these effects, the C content is 0.020% or more. The C content may also be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more. On the other hand, if excessively containing C, sometimes the stretch flangeability and weldability fall. Therefore, the C content is 0.070% or less. The C content may also be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.

[Si: 0.010 to 2.000%]

Si is an element effective for raising strength as a solid solution strengthening element. To sufficiently obtain such an effect, the Si content is 0.010% or more. The Si content may also be 0.100% or more, more than 0.100%, 0.110% or more, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more. On the other hand, if excessively containing Si, sometimes defects in surface quality called "Si scale" are caused. Therefore, the Si content is 2.000% or less. The Si content may also be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.

[Mn: 0.60 to 2.00%]

Mn is an element effective for hardenability and for raising strength as a solid solution strengthening element. To sufficiently obtain these effects, the Mn content is 0.60% or more. The Mn content may also be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if excessively containing Mn, sometimes the stretch flangeability falls. Therefore, the Mn content is 2.00% or less. The Mn content may also be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.

[Ti: 0.015 to 0.200%]

Ti is an element finely precipitating in steel as a carbide (TiC) and improving the strength of the steel by precipitation strengthening. Further, Ti is an element forming carbides to fix C and suppress the formation of the cementite harmful for the stretch flangeability. To sufficiently obtain these effects, the Ti content is 0.015% or more. The Ti content may also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if excessively containing Ti, the carbides become coarser and sometimes the ductility falls. Therefore, the Ti content is 0.200% or less. The Ti content may also be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.

[sol. Al: 0.010 to 1.000%]

sol. Al is an element acting as a deoxidizer of molten steel. To sufficiently obtain such an effect, the sol. Al content is 0.010% or more. The sol. Al content may also be 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if excessively containing sol. Al, coarse oxides are formed, the toughness and ductility fall, and sometimes fracture results during rolling. Therefore, the sol. Al content is 1.000% or less. The sol. Al content may also be 0.800% or less, 0.600% or less, or 0.400% or less. Note that, "sol. Al" means acid soluble Al and indicates the dissolved Al present in the steel in the dissolved state.

[P: 0.100% or Less]

P, if excessively contained, sometimes affects the weldability detrimentally. Therefore, the P content is 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% 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 0.001% or more, 0.003% or more, or 0.005% or more.

[S: 0.030% or Less]

S, if excessively contained, forms a large amount of MnS and sometimes causes a drop in toughness. Therefore, the S content is 0.030% or less. The S content may also be 0.020% or less, 0.010% or less, or 0.005% 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 0.001% or more, 0.002% or more, or 0.003% or more.

[N: 0.0060% or Less]

N forms precipitates with Ti more preferentially than C and sometimes reduces the Ti effective for fixing the C. Therefore, the N content is 0.0060% or less. The N content may also be 0.0050% or less, 0.0040% or less, or 0.0030% 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 also 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, coarse inclusions are formed and sometimes the toughness of the steel sheet falls. 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 reduction 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.

[Nb: 0 to 0.050%]

Nb is an element forming carbides, nitrides, and/or carbonitrides in steel to contribute to refinement of the structure due to the pinning effect and in turn higher strength of the steel sheet. Further, Nb is an element forming carbides and/or carbonitrides to fix C and suppress the formation of the cementite harmful to the stretch flangeability. The Nb content may also be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the ductility of the steel sheet falls. Therefore, the Nb content is 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.020% or less.

[V: 0 to 0.300%]

V is an element contributing to improvement of strength due to precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, 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, even if excessively including V, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the V content is preferably 0.300% or less. The V content may also be 0.200% or less, 0.100% or less, or 0.080% or less.

[Cr: 0 to 2.00%]

Cr is an element raising the hardenability of steel and contributing to improvement of strength. The Cr content may also be 0%, but to obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including Cr, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Cr content is preferably 2.00% or less. The Cr content may also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[Ni: 0 to 2.00%]

[Cu: 0 to 2.00%]

Ni and Cu are elements contributing to improvement of strength by precipitation strengthening and solid solution strengthening. The Ni and Cu contents may also be 0%, but to obtain such effects, the contents of these elements are preferably respectively 0.01% or more and may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including these elements, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Ni and Cu content are preferably respectively 2.00% or less and may also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[Mo: 0 to 1.000%]

Mo is an element raising the hardenability of steel and contributing to improvement of the strength. The Mo content may also be 0%, but to obtain such an effect, 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 greater. 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.

[B: 0 to 0.0100%]

B segregates at the grain boundaries to raise the intergranular strength and thereby improve the low temperature toughness. The B content may also be 0%, but to obtain such an effect, 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, even if excessively containing B, the effect becomes saturated and a rise in production cost is liable to be invited. 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.

[Sb: 0 to 1.00%]

Sb is an element effective for improvement of the corrosion resistance. The Sb content may also be 0%, but to obtain such an effect, the Sb content is preferably 0.01% or more. The Sb content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively containing Sb, sometimes a drop in toughness is invited. Therefore, the Sb content is preferably 1.00% or less. The Sb content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[Ca: 0 to 0.0100%]

[Mg: 0 to 0.0100%]

[Hf: 0 to 0.0100%]

Ca, Mg, and Hf are elements enabling control of the form of the nonmetallic inclusions. The Ca, Mg, and Hf contents may also be 0%, but to obtain such an effect, 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 Ca, Mg, 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.

[REM: 0 to 0.1000%]

An REM is an element enabling control of the form of nonmetallic inclusion. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content 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, REM content is preferably 0.1000% or less. The REM content may also be 0.0500% or less, 0.0100% or less, 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.

[Bi: 0 to 0.0100%]

[As: 0 to 0.0100%]

Bi and As are elements effective for improvement of the corrosion resistance. The Bi and As content may also be 0%, but to obtain such an effect, 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 including these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi and As 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.

[Zr: 0 to 1.00%]

Zr is an element enabling control of the form of nonmetallic inclusions. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.01% or more. The Zr content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Zr content is preferably 1.00% or less. The Zr content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[Co: 0 to 1.00%]

Co is an element contributing to improvement of the hardenability and/or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.01% or more. The Co content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing Co, the hot workability sometimes falls. This also leads to an increase in material costs. Therefore, the Co content is preferably 1.00% or less. The Co content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[Zn: 0 to 1.00%]

Zn is an element effective for control of the shape of inclusions. To obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively Zn, the effect becomes saturated and invites a rise in production costs. Therefore, the Zn content is preferably 1.00% or less. The Zn content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[W: 0 to 1.00%]

W is an element raising the hardenability of steel and contributes to improvement of strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.01% or more. The W content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[Sn: 0 to 1.00%]

Sn is an element effective for improvement of corrosion resistance. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.01% or more. The Sn content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively containing Sn, sometimes a fall in toughness is invited. Therefore, the Sn content is preferably 1.00% or less. The Sn content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
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 entering, etc., 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.

[0.100≤[Si]+[sol. Al]≤2.500]

The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention must satisfy the following formula: $0.100 \leq [Si] + [sol . Al] \leq 2.500$ where, in the formula, [Si] and [sol. Al] are contents (mass%) of the elements. As explained above, if defining a region surrounded by grain boundaries with an orientation difference of 15° or more and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, crystal grains with an orientation difference in grains of 5 to 14° are effective for improving the strength and the stretch flangeability. For this reason, in the hot rolled steel sheet according to an embodiment of the present invention, as explained in detail later, the ratio of those crystal grains is controlled within a range of, by area%, 10 to 60% to improve the balance of strength and stretch flangeability. In addition to the effects explained for the individual elements, Si and sol. Al are elements effective for controlling the ratio of crystal grains with an orientation difference in grains of 5 to 14° within a range of 10 to 60%. This is believed to be due to the inclusion of Si and sol. Al causing the temperature of the Ar3 point to rise and transformation strain introduced into the grains becoming smaller. To sufficiently obtain these effects, the chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is controlled so that the contents of the elements are controlled to within the ranges previously explained while the total content of Si and sol. Al is 0.100% or more, i.e., [Si]+[sol. Al]≥0.100 is satisfied. The total content of Si and sol. Al may also be 0.120% or more, 0.150% or more, 0.200% or more, or 0.300% or more. On the other hand, if the total content of Si and sol. Al is too high, sometimes formation of ferrite is promoted and the strength falls. Therefore, the total content of Si and sol. Al is 2.500% or less, i.e., [Si]+[sol. Al]≤2.500. The total content of Si and sol. Al may also be 2.000% or less, 1.500% or less, 1.000% or less, or 0.000% or less.
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]

[At Least One of Ferrite and Bainite: 80 to 98% in Total and Martensite: 2 to 10%]

The microstructure of the hot rolled steel sheet according to an embodiment of the present invention contains, by area%, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%. By configuring the microstructure of the hot rolled steel sheet by these structures, it is possible to improve the strength and the stretch flangeability, ductility, and notch fatigue property with a good balance. If the total area ratio of the at least one of ferrite and bainite is low or the area ratio of the martensite is high, sometimes in particular the balance of the strength and the stretch flangeability falls and the desired properties are not obtained. Therefore, the total area ratio of the at least one of ferrite and bainite is 80% or more and, for example, may also be 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area ratio of martensite is 10% or less and, for example, may also be 9% or less, 8% or less, 7% or less, or 6% or less. On the other hand, if the total area ratio of the at least one of ferrite and bainite is high or the area ratio of the martensite is low, sometimes in particular the balance of the strength and the notch fatigue property falls and the desired properties are not obtained. Therefore, the total area ratio of the at least one of ferrite and bainite is 98% or less and, for example, may be 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite is 2% or more and, for example, may be 3% or more, 4% or more, or 5% or more.
The microstructure of the hot rolled steel sheet may include either of ferrite and bainite, preferably includes both ferrite and bainite. Therefore, either of the area ratios of ferrite and bainite may be 0%. For example, they may respectively also be 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more. Similarly, the area ratios of ferrite and bainite may, for example, also be 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less. From the viewpoint of improvement of ductility of the hot rolled steel sheet, the area ratio of bainite is preferably 80% or less, more preferably 70% or less.

[Balance Structures]

The balance structures other than ferrite, bainite, and martensite may be an area% of 0%, but if there are balance structures present, the balance structures may also be at least one of retained austenite and pearlite. The area ratio of the balance structures is not particularly limited, but, for example, may be 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the balance structures is, for example, preferably 10% or less and may also be 8% or less, 6% or less, or 5% or less.

[Identification of Microstructure and Calculation of Area Ratios]

The microstructure at the hot rolled steel sheet is identified and the area ratios are calculated by observation under an optical microscope after corrosion using a Nital reagent or LePera solution and by X-ray diffraction. The structure is observed under an optical microscope at a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface. Specifically, first, a sample is taken from the hot rolled steel sheet and the observed surface of the sample is etched by Nital. Next, an optical microscope is used to obtain a structural photograph of a 300 µm×300 µm field at a 1/4 depth position in sheet thickness. The image is analyzed to calculate the area ratios of the ferrite and pearlite and calculate the total area ratio of bainite and martensite. Next, the observed surface of the sample is corroded by a LePera solution. In the same way, an optical microscope is used to obtain a structural photograph of a 300 µm×300 µm field at a 1/4 depth position in sheet thickness. The image is analyzed to calculate the total area ratio of the retained austenite and martensite. Next, the sample is face cut from the rolling surface normal direction down to a 1/4 depth of the sheet thickness. This is then measured by X-ray diffraction to calculate the volume ratio of the retained austenite. The volume ratio of retained austenite is equivalent to the area ratio, so this is made the area ratio of the retained austenite. The area ratio of the retained austenite obtained is subtracted from the total area ratio of the residual austenite and martensite calculated previously to calculate the area ratio of the martensite. Finally, the area ratio of the obtained martensite is subtracted from the total area ratio of the bainite and martensite calculated previously in the same way to calculate the area ratio of the bainite.

[Ratio of Crystal Grains With Orientation Difference in Grains of 5 to 14°: Area% of 10 to 60%]

In the microstructure of the hot rolled steel sheet according to an embodiment of the present invention, if deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is controlled within a range of, by area%, 10 to 60%. Crystal grains having such an orientation difference in grains are effective for raising the strength and the stretch flangeability. While not intending to be constrained to any specific theory, it is believed that the crystal orientation difference in the grains is correlated with the dislocation density contained in the crystal grains. In general, an increase in the dislocation density in grains causes improvement of strength, but lowers the workability. However, in crystal grains with an orientation difference in grains controlled to 5 to 14°, it is believed possible to raise the strength without lowering the workability. As opposed to this, crystal grains with an orientation difference in grains of less than 5° are excellent in workability, but difficult to make high in strength. On the other hand, crystal grains with an orientation difference in grains of more than 14° differ in deformation ability in the crystal grains, and therefore do not necessarily contribute to improvement of the stretch flangeability. Therefore, in the hot rolled steel sheet according to an embodiment of the present invention, it becomes possible to suitably control the ratio of crystal grains with an orientation difference in grains of 5 to 14°, more specifically to control it within a range of, by area%, 10 to 60%, to achieve the desired steel sheet strength while improving the stretch flangeability and to further improve the balance of strength and the stretch flangeability. If the ratio of crystal grains with an orientation difference in grains of 5 to 14° is small, sometimes the stretch flangeability falls. Therefore, from the viewpoint of improvement of the stretch flangeability, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 15% or more, 18% or more, or 20% or more. On the other hand, if the ratio of crystal grains with an orientation difference in grains of 5 to 14° is large, sometimes the ductility falls. Therefore, from the viewpoint of improvement of ductility, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 55% or less, 50% or less, 45% or less, or 40% or less.

[Measurement of Ratio of Crystal Grains With Orientation Difference in Grains of 5 to 14°]

The ratio of crystal grains with an orientation difference in grains of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel sheet so that a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface becomes the examined surface. Next, a region of 200 µm in the rolling direction of the steel sheet and 100 µm in the rolling surface normal direction at a 1/4 depth position in sheet thickness from the steel sheet surface is analyzed by EBSD at 0.2 µm measurement intervals so as to acquire crystal orientation information. Here, the EBSD analysis is performed using an 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) by a 50 to 300 points/s analysis speed. Next, for the obtained crystal orientation information, a region with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 µm or more is defined as a crystal grain, the average orientation difference in the grain of each crystal grain is calculated, and the ratio of crystal grains with an orientation difference in grains of 5 to 14° is found. Such a defined crystal grain or average orientation difference in grains can be calculated using the software "OIM Analysis^™" attached to the EBSD analysis apparatus. In the present invention, the "orientation difference in grains" expresses the grain orientation spread (GOS). The value of the orientation difference in grains, as described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-Ray Diffraction Method", Hidehiko Kimura et al., Transactions of the JSME (A Edition), vol. 71, no. 712, 2005, p. 1722-1728, is found as the average value of the misorientation among all measurement points from the crystal orientation becoming the reference in the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is the orientation of the average of all measurement points in the same crystal grain. The value of GOS can be calculated using the software "OIM Analysis^™ Version 7.0.1" attached to the EBSD analysis apparatus.

[Average Value of Pole Densities of Specific Orientations at Sheet Thickness Surface Layer Part: 2.50 or More and Average Value of Pole Densities of Specific Orientations at Sheet Thickness Center Part: 7.00 or Less]

In the microstructure of the hot rolled steel sheet according to an embodiment of the present invention, the average value of the pole densities of the {110}<111> and {112}<111> orientations at the region from the surface of the hot rolled steel sheet down to a 1/6 position in sheet thickness (i.e., at the sheet thickness surface layer part) is controlled to 2.50 or more and the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness (i.e., at the sheet thickness center part) is controlled to 7.00 or less. By controlling the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part to 2.50 or more to raise L direction strength while controlling the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the sheet thickness center part to 7.00 or less to lower the C direction strength, it is possible to reduce the difference in the L direction and C direction tensile strengths of the obtained hot rolled steel sheet and as a result remarkably reduce the anisotropy of strength at the L direction and C direction tensile strengths. From the viewpoint of reducing the anisotropy of strength more, the greater the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part, the more preferable. For example, it may also be 2.80 or more, 3.00 or more, 3.20 or more, or 3.50 or more. The upper limit is not particularly prescribed, but, for example, the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part may also be 5.00 or less, 4.80 or less, 4.70 or less, 4.50 or less, 4.20 or less, 4.00 or less, or 3.80 or less. Similarly, from the viewpoint of better reducing the anisotropy of strength, the smaller the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the sheet thickness center part, the more preferable. For example, it may also be 6.80 or less, 6.50 or less, 6.20 or less, or 6.00 or less. The lower limit is not particularly prescribed, but, for example, the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the sheet thickness center part may also be 3.50 or more, 4.00 or more, 4.20 or more, 4.40 or more, 4.50 or more, or 5.00 or more.

[Measurement of Average Values of Pole Densities of Specific Orientations at Sheet Thickness Surface Layer Part and Sheet Thickness Center Part]

The average value of the pole densities of the {110}<111> and {112}<111> orientation at the sheet thickness surface layer part and the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the sheet thickness center part are measured by EBSD. More specifically, for measurement of the sheet thickness surface layer part, first, a sample is taken from the steel sheet so that the sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface becomes the examined surface. EBSD analysis is performed at 1 µm measurement intervals for a rectangular region of 1000 µm in the rolling direction of the steel sheet and 100 µm in the rolling surface normal direction centered at a 1/2 depth position in sheet thickness from the steel sheet surface so as to acquire crystal orientation information of this rectangular region. The EBSD analysis is performed using an apparatus configured by a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and an EBSD detector (HIKARI detector made by TSL) by a 50 to 300 point/s analysis speed. Next, from the crystal orientation information of this rectangular region, software "OIM Analysis ^™" attached to the EBSD analysis apparatus is used to calculate the ODF (orientation distribution function) of this rectangular region. As the method for calculation of the ODF, harmonic series expansion was used to make the expansion order 16. Further, calculation was performed symmetrically (orthotropically). Due to this, it is possible to find the pole densities of the crystal orientations of {110}<111> and {112}<111>. The arithmetic average of these is determined as "the average value of the pole densities of the {110}<111> and {112}<111> orientations at the region from the surface of the hot rolled steel sheet down to a 1/6 position in sheet thickness (sheet thickness surface layer part)". For measurement of the sheet thickness center part, other than performing the EBSD analysis for a rectangular region of 1000 µm in the rolling direction of the steel sheet and 100 µm in the rolling surface normal direction centered at a 1/2 depth position in sheet thickness from the steel sheet surface so as to acquire crystal orientation information of this rectangular region, measurement can be performed in the same way as the case of measurement of the sheet thickness surface layer part to find the pole densities of the crystal orientations of {100}<011>, {211}<011>, and {332}<113>. The arithmetic average of these is determined as the "average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations at the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness (sheet thickness center part)". Note that, the "crystal orientation" expresses the crystal orientation in a direction vertical to the steel sheet surface, and therefore at the time of analysis, it is necessary to consider the direction of the sample set in the measurement and combine the measurement coordinate system and sample coordinate system of the crystal orientation data.

[Sheet Thickness]

The hot rolled steel sheet according to an embodiment of the present invention is not particularly limited, but in general has a 1.0 to 6.0 mm sheet thickness. For example, the sheet thickness may also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and/or may also 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-mentioned chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a 540 MPa or more tensile strength. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. According to the hot rolled steel sheet according to the present embodiment, despite having such an extremely high tensile strength, the stretch flangeability, ductility, and notch fatigue property can be improved and the anisotropy of strength can be reduced by the specific combination of the chemical composition and microstructure explained above. 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, 1080 MPa or less, or 980 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011. The thus obtained tensile strength is also referred to as the "C direction TS" (TSC) in this Description.

[Total Elongation: El]

According to the hot rolled steel sheet having the above-mentioned chemical composition and microstructure, in addition to the high tensile strength, it is possible to improve the total elongation, more specifically possible to achieve a 15.0% 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 is 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.

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 comprises

(A) hot rolling including heating a slab having a chemical composition explained above in relation to the hot rolled steel sheet, then finish rolling it, and satisfying the following (A1) to (A5) conditions:
- (A1) a heating temperature of the slab is a solubilization temperature (SRTmin)°C expressed by the following formula 1 or more and 1260°C or less,
- (A2) a cumulative strain (εeff.) expressed by the following formula 2 of the final three stages of finish rolling is 0.50 to 0.60,
- (A3) an end temperature of the finish rolling is Ar3+30°C or more,
- (A4) a rolling pass with a shape ratio (X) expressed by the following formula 3 of 2.3 or more in finish rolling is performed for two passes or more at 1100°C or less, and
- (AS) a rolling temperature of the initial three stages of the finish rolling is the entry side temperature of the finish rolling (FT0)-50°C or more $SRTmin = 7000 / \{2.75 - \log ([Ti] \times [C])\} - 273$
- where, [Ti] and [C] are the contents (mass%) of the elements in the steel. $εeff . = \sum εi (t, T)$ where, $εi (t, T) = εi 0 / \exp \{{(t / τR)}^{2 / 3}\}$ $τR = τ 0 \cdot \exp (Q / RT)$ $τ 0 = 8.46 \times 10^{- 6}$
- Q=183200J
- R=8.314J/K·mol
- εi0 indicates the logarithmic strain at the time of rolling reduction, "t" indicates the cumulative time (s) up right before cooling at the pass, and T indicating the rolling temperature (°C) at the pass. $X = 2 \sqrt (R (h_{0} - h_{1})) / (h_{0} + h_{1})$
- where,
- R: roll radius of rolling machine (mm)
- h₀ : entry side sheet thickness (mm)
- h₁ : exit side sheet thickness (mm)
and
(B) cooling including primary cooling the finish rolled steel sheet by an average cooling rate of 10°C/s or more down to a temperature region of 650 to 750°C, holding at the temperature region for 3.0 to 10.0 seconds, then secondary cooling down to 100°C or less by an average cooling rate of 30°C/s or more. Below, the steps will be explained in detail.

[(A) Hot Rolling Step]

[(A1) Heating Temperature 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 a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot making method or a thin slab casting method. The heating temperature of the slab has to be made the solubilization temperature (SRTmin)°C expressed by the following formula 1 or more and 1260°C or less. $SRTmin = 7000 / \{2.75 - \log ([Ti] \times [C])\} - 273$ where, [Ti] and [C] are the contents of the elements in the steel (mass%).
The hot rolled steel sheet according to an embodiment of the present invention contains Ti. If the heating temperature of the slab is less than the solubilization temperature (SRTmin)°C, the Ti will not sufficiently dissolve. If Ti does not sufficiently dissolve at the time of slab heating, in the cooling step after the hot rolling step, it becomes difficult for Ti to be finely made to precipitate in the steel as carbides (TiC) and for the strength of the steel to be increased by precipitation strengthening. In addition, by forming carbides (TiC), C is fixed and formation of the cementite harmful to stretch flangeability also becomes difficult. On the other hand, if the heating temperature of the slab is more than 1260°C, the yield falls due to detachment of scale.

[Rough Rolling]

In the present method of production, 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.

[(A2) Cumulative Strain (εeff.) of Final Three Stages of Finish Rolling: 0.50 to 0.60]

The heated slab or the slab additionally rough rolled according to need is next finish rolled. In the present method of production, it is preferable to use a tandem rolling machine comprised of five or more rolling stands to perform the finish rolling. If using a tandem rolling machine comprising five rolling stands, there is a partial overlap in rolling passes between the final three stages of the finish rolling of (A2) and the prior three stages of finish rolling of (A5) explained later. However, so long as satisfying (A2) and (A5), the rolling passes under the conditions of (A2) and the rolling pass under the condition of (A5) may partially overlap with each other. In the present method of production, to control the ratio of crystal grains with an orientation difference in grains of 5 to 14° within a range of, by area%, 10 to 60%, in the finish rolling performed for the heated slab, it is necessary to make the cumulative strain (εeff.) of the final three stages (final three passes) 0.50 to 0.60 and then perform the later explained cooling step. This is due to the following reason, crystal grains with an orientation difference in grains of 5 to 14°are formed by transformation at a relatively low temperature in a paraequilibrium state. For this reason: By limiting the dislocation density of austenite before transformation in the hot rolling step to a certain range and limiting the cooling speed in the subsequent cooling step to a certain range, it becomes possible to control the formation of crystal grains with an orientation difference in grains of 5 to 14°. That is, by controlling the cumulative strain of the final three stages of finish rolling and the subsequent cooling, it is possible to control the frequency of nucleation of crystal grains with an orientation difference in grains of 5 to 14°and subsequent speed of growth. As a result, it is possible to control the area ratio of crystal grains with an orientation difference in grains of 5 to 14°at the hot rolled steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by finish rolling mainly relates to the frequency of nucleation and the cooling speed after finish rolling mainly relates to the speed of growth.
If the cumulative strain of the final three stages of the finish rolling is less than 0.50, the dislocation density of the austenite introduced is not sufficient and the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. On the other hand, the cumulative strain of the final three stages of the finish rolling is more than 0.60, recrystallization of austenite occurs during the hot rolling, and the stored dislocation density at the time of transformation falls. As a result, in the same way, the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. In the present method of production, the cumulative strain of the final three stages of the finish rolling (εeff.) is found by the following formula 2: $εeff . = \sum εi (t, T)$ where, $εi (t, T) = εi 0 / \exp \{{(t / τR)}^{2 / 3}\}$ $τR = τ 0 \cdot \exp (Q / RT)$ $τ 0 = 8.46 \times 10^{- 6}$

Q=183200J
R=8.314J/K·mol
εi0 indicates the logarithmic strain at the time of rolling reduction, "t" indicates the cumulative time (s) up to right before cooling in the pass, and T indicates the rolling temperature (°C) at that pass.

[(A3) End Temperature of Finish Rolling: Ar3+30°C or More]

In the present method of production, the end temperature of the finish rolling has to be Ar3+30°C or more. If the end temperature of the finish rolling is less than Ar3+30°C, if ferrite is formed in part of the structure due to variations in the constituents and rolling temperature in the steel sheet, the ferrite is liable to be worked. The worked ferrite sometimes becomes a cause of a drop in the ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30°C, sometimes the ratio of crystal grains with an orientation difference in grains of 5 to 14° will become more than 60% or excessively high. In the present method of production, Ar3 (°C) is found by the following formula 4 based on the chemical composition of the hot rolled steel sheet: $Ar 3 = 901 - 325 \times [C] + 33 \times [Si] + 287 \times [P] + 40 \times [sol . Al] - 92 \times ([Mn] + [Mo] + [Cu]) - 46 \times ([Cr] + [Ni])$ where, [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are the contents (mass%) of the elements in the steel. Cases where the elements are not contained are indicated as 0.

[(A4) Two or More Rolling Passes With Shape Ratio (X) of 2.3 or More at 1100°C or Less]

To reduce the anisotropy of strength, as explained before, it is necessary to increase the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part to a predetermined value or more to raise the L direction strength while reducing the average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations to a predetermined value or less at the sheet thickness center part to lower the C direction strength. Therefore, the inventors discovered that by increasing the shear strain introduced to the sheet thickness surface layer part of the steel sheet at the finish rolling, it is possible to raise the degree of control to the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part and thereby control the average value of the pole densities of these orientations to within the desired range. More specifically, by performing two or more rolling passes with a shape ratio (X) shown in the following formula 3 of 2.3 or more at 1100°C or less in the finish rolling, it becomes possible to raise the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part to 2.50 or more. $X = 2 \sqrt (R (h_{0} - h_{1})) / (h_{0} + h_{1})$
Here,

R: roll radius of rolling machine (mm)
h₀ : entry side sheet thickness (mm)
h₁ : exit side sheet thickness (mm)

The "shape ratio (X)" means the roll contact arc length (√(R(h₀ -h₁ ))) divided by the average sheet thickness ((h₀ +h₁ )/2). In the present method of production, by using rolls having suitable roll radii in the rolling machine to perform rolling by a suitable rolling reduction, it is possible to achieve a 2.3 or more shape ratio (X) and increase the shear strain introduced to the sheet thickness surface layer part of the steel sheet. Further, by limiting the rolling temperature at that time to 1100°C or less, it is possible to keep the introduced shear strain from recovering. Therefore, by performing such a rolling pass by two passes or more, it is possible to introduce sufficient shear strain in the sheet thickness surface layer part and reliably raise the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part to 2.50 or more. From the viewpoint of further reducing the anisotropy of strength, it is preferable that X be 2.3 or more and a 1100°C or less rolling pass be performed by three passes or more. The upper limit of the number of such rolling passes is not particularly prescribed. For example, the number of rolling passes may be five passes or less. On the other hand, if X is less than 2.3, the rolling temperature is more than 1100°C, or X is 2.3 or more and there is one or less 1100°C or less rolling pass, it is not possible to introduce sufficient shear strain to the sheet thickness surface layer part. As a result, it becomes no longer possible to raise the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part to 2.50 or more. As the roll radius of a roll used in the rolling machine, it is possible to select a suitable value in a range giving an X of 2.3 or more. While not particularly limited, for example, the roll radius can be selected from a range of 150 to 400 mm.

[(A5) Rolling Temperature of Initial Three Stages of Finish Rolling: FT0-50°C or More]

At the sheet thickness center part, opposite to the sheet thickness surface layer part, it is necessary to reduce the strain introduced by the finish rolling to limit the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations to 7.00 or less. Therefore, in the present method of production, by controlling the rolling temperatures of the initial three stages of the finish rolling, i.e., the rolling temperature (FT1) of the first stage of finish rolling, the rolling temperature (FT2) of the second stage of finish rolling, and the rolling temperature (FT3) of the third stage of finish rolling, to the entry side temperature (FT0) of the finish rolling minus 50°C or more, the shear strain introduced to the sheet thickness center part is lightened and thereby the average value of the pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations of 7.00 or less is achieved. The sheet thickness center part is smaller in shear strain introduced compared with the case of the sheet thickness surface layer part directly contacting the rolls, and therefore by controlling the rolling temperatures of the initial three stages of the finish rolling to relatively high temperatures such as explained above, it is possible to sufficiently lighten the shear strain introduced. From the viewpoint of further reducing the anisotropy of strength, the rolling temperatures of the initial three stages of the finish rolling are preferably controlled to FT0-45°C or more. On the other hand, if the rolling temperature at even one stage among the initial three stages of rolling becomes less than FT0-50°C, a sufficient effect of lightening of the shear strain is not obtained and the average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations at the sheet thickness center part can no longer be reduced to 7.00 or less. The upper limits of the rolling temperatures of the initial three stages of the finish rolling are not particularly prescribed, but, for example, the rolling temperatures of the initial three stages of the finish rolling may also be 1100°C or less or 1000°C or less.
In the present method of production, so long as satisfying the conditions of (A4) and (A5), one or more rolling passes under the condition of (A4) and one or more rolling passes under the condition of (A5) may overlap with each other.

[(B) Cooling Step]

In the present method of production, the finish rolled steel sheet is cooled in two stages in the next cooling step. Specifically, first, the finish rolled steel sheet is primary cooled by an average cooling speed of 10°C/s or more down to a temperature region of 650 to 750°C, is held at the temperature region for 3.0 to 10.0 seconds, then is secondary cooled down to 100°C or less by an average cooling speed of 30°C/s or more. By combining with the conditions of (A2) and (A3), etc., in the hot rolling step and performing such two stage cooling, transformation by paraequilibrium occurs at the desired relatively low temperature region and thereby it becomes possible to reliably control the ratio of crystal grains with an orientation difference in grains of 5 to 14° within a range of, by area%, 10 to 60%. As opposed to this, if the average cooling speed of the primary cooling is less than 10°C/s or the cooling stop temperature of primary cooling is more than 750°C, transformation occurs by paraequilibrium at a relatively high temperature and the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Further, if the cooling stop temperature of primary cooling is less than 650°C, transformation occurs by paraequilibrium at a temperature lower than the desired temperature region and similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Furthermore, even if the holding time at 650 to 750°C is less than 3.0 seconds, similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. On the other hand, if the holding time at 650 to 750°C becomes more than 10.0 seconds or the average cooling speed of the secondary cooling is less than 30°C/s, the cementite harmful to the stretch flangeability is easily formed. Further, if the cooling stop temperature of the secondary cooling is more than 100°C, the area ratio of martensite becomes less than 2%. The upper limits of the average cooling speeds of the primary and secondary cooling are not particularly prescribed, but, for example, the average cooling speeds of the primary and secondary cooling may be 200°C/s or less considering the capacity of the cooling facilities.
According to the hot rolled steel sheet produced by the above method of production, if including by area% the at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, deeming a boundary with an orientation difference of 15° or more as a grain boundary, and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, it is possible to obtain a microstructure with a ratio of crystal grains with an orientation difference in grains of 5 to 14° of 10 to 60%. As a result, despite the high strength, it becomes possible to remarkably improve the stretch flangeability, ductility, and notch fatigue property. In addition, in the microstructure, the average value of the pole densities of the {110}<111> and {112}<111> orientations at the region from the surface down to a 1/6 position in sheet thickness is controlled to 2.50 or more and the average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations at the region from a 2/5 position in sheet thickness down to a 3/5 position in sheet thickness is controlled to 7.00 or less, so it is possible to remarkably reduce the anisotropy of strength of the L direction and C direction tensile strengths 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 the contradictory properties of high strength and excellent workability, so this is particularly useful in use in the automotive field where achievement of both 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. The obtained hot rolled steel sheets were investigated for tensile strength, stretch flangeability, ductility, notch fatigue property, and anisotropy of strength.
First, slabs having various chemical compositions shown in Tables 1 and 2 were formed by casting molten steels by the continuous casting method. These slabs were heated under the conditions shown in Table 3, then were hot rolled. The hot rolling was performed by rough rolling and finish rolling. More specifically, rough rolling was performed under the same conditions in all of the examples and comparative examples while finish rolling was performed using a tandem rolling machine comprised of seven rolling stands. The entry side temperature (F0) of the finish rolling, the rolling temperature (FT1) of the first finish rolling stage, the rolling temperature (FT2) of the second finish rolling stage, the rolling temperature (FT3) of the third finish rolling stage, the end temperature of the finish rolling, and the cumulative strain (εeff.) of the final three stages of the finish rolling were as shown in Table 2. Further, the finish rolling was performed using rolls having the roll radii shown in Table 3 by rolling passes giving a shape ratio (X) of 2.3 or more at 1100°C or less for the number of times shown in Table 3. Next, the finish rolled steel sheet was cooled by primary cooling and secondary cooling under the conditions shown in Table 3 to obtain the hot rolled steel sheet having the sheet thickness shown in Table 2.

[Table 1]

Table 1

Steel no.	Chemical com position (mass%), balance: Fe and impurities
Steel no.	C	Si	Mn	Ti	sol. Al	P	S	N	O	Nb	V	Cr	Ni	Cu
A	0.043	0.030	1.07	0.110	0.280	0.013	0.002	0.0034	0.0034	0.016		0.09
B	0.048	0.300	1.35	0.152	0.340	0.012	0.003	0.0027	0.0030	0.014		0.11
c	0.068	0.029	1.07	0.100	0.091	0.017	0.002	0.0024	0.0031
D	0.062	0.050	1.23	0.110	0.240	0.012	0.002	0.0024	0.0035			0.12
E	0.023	0.022	1.07	0.110	0.300	0.012	0.003	0.0028	0.0029
F	0.045	1.354	1.03	0.097	0.420	0.015	0.003	0.0030	0.0031	0.017		0.09
G	0.052	1.820	1.06	0.102	0.030	0.010	0.003	0.0030	0.0031	0.020
H	0.045	0.100	1.96	0.099	0.350	0.015	0.002	0.0029	0.0033	0.022
I	0.049	0.320	0.76	0.101	0.190	0.017	0.003	0.0031	0.0028	0.021		0.11
J	0.051	0.280	1.12	0.098	0.610	0.010	0.003	0.0034	0.0028
K	0.038	0.240	1.04	0.192	0.320	0.015	0.002	0.0034	0.0033			0.12	0.08	0.09
L	0.046	0.140	0.98	0.016	0.270	0.013	0.002	0.0024	0.0030	0.038		0.08
M	0.044	0.120	1.21	0.124	0.260	0.018	0.002	0.0024	0.0034		0.090
N	0.042	0.045	0.98	0.102	0.240	0.018	0.002	0.0030	0.0030	0.017		0.12
O	0.043	0.320	1.01	0.110	0.320	0.016	0.003	0.0027	0.0028			0.09
P	0.082	0.310	1.02	0.152	0.350	0.011	0.003	0.0029	0.0033	0.020		0.10
Q	0.018	1.010	1.07	0.105	0.320	0.015	0.003	0.0029	0.0032	0.021		0.10
R	0.042	0.410	2.32	0.120	0.250	0.018	0.003	0.0026	0.0029	0.018
S	0.040	1.050	0.50	0.112	0.030	0.018	0.002	0.0030	0.0032	0.018
T	0.041	0.150	1.00	0.101	1.160	0.018	0.002	0.0026	0.0028	0.019		0.11
U	0.046	1.680	1.23	0.150	0.920	0.008	0.003	0.0029	0.0028			0.12
V	0.046	0.051	1.15	0.104	0.035	0.008	0.003	0.0029	0.0031	0.021		0.12
W	0.038	0.010	1.32	0.209	0.220	0.012	0.002	0.0035	0.0032	0.020
X	0.051	0.720	1.21	0.012	0.210	0.021	0.002	0.0025	0.0033	0.020

Underlines indicate outside scope of present invention.

[Table 2]

Table 2

Steel no.	Chemical composition (mass%), balance: Fe and impurities
Steel no.	Mo	B	Sb	Ca	Mg	Hf	REM	Bi	As	Zr	Co	Zn	W	Sn	Si+sol.Al
A															0.310
B															0.640
C															0.120
D							0.0081								0.290
E			0.15											0.15	0.322
F									0.0023						1.774
G										0.02					1.850
H												0.02			0.450
I											0.03				0.510
J						0.0011		0.0007							0.890
K															0.560
L															0.410
M													0.04		0.380
N	0.127	0.0015													0.285
O				0.0043	0.0021										0.640
P															0.660
Q															1.330
R															0.660
S															1.080
T															1.310
U															2.600
V															0.086
W															0.230
X															0.930

Underlines indicate outside scope of present invention.

[Table 3]

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

[Tensile Strength (TSC) and Total Elongation (El)]

The tensile strength (TSC) and the 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.

[Evaluation of Stretch Flangeability]

The stretch flangeability was evaluated by the saddle shaped stretch flangeability test method using a saddle shaped article. Specifically, a saddle shaped article simulating a stretched flange shape comprised of a straight part and curved part such as shown in FIG. 1 was press-formed. The stretch flangeability was evaluated by the limit shaped height at that time. In the saddle shaped stretch flangeability test method, a saddle shaped article having a radius of curvature R of the corners of 50 to 60 mm and an opening angle θ of 120° was used to measure the limit shaped height H (mm) when making the clearance 11% at the time of punching the corner parts. Here, the "clearance" indicates the ratio of the gap between the punching die and punch and the thickness of the test piece. The clearance is actually determined by the combination of the punching tool and sheet thickness, so "11%" means the 10.5 to 11.5% range is satisfied. The limit shaped height H was judged by examining for the presence of any cracks having lengths of 1/3 or more of the sheet thickness by visual observation after shaping. This was made the limit shaped height with no cracks present. The product (TSC×H) of the tensile strength TSC (MPa) and limit shaped height H (mm) was evaluated as an indicator of the stretch flangeability. If TSC×H≥19500 MPa·mm, it was evaluated that the stretch flangeability was improved.

[Evaluation of Ductility]

If the product of TSC (MPa) and El (%) (TSC×El) satisfies TSC×El≥13500 MPa·%, it was evaluated that the ductility was improved.

[Evaluation of Notch Fatigue Property]

The notch fatigue property was evaluated as follows: Specifically, from a similar position as the position where the tensile test piece was taken, a fatigue test piece of a shape shown in FIG. 2 was taken so that the orientation (C direction) becoming parallel to the rolling perpendicular direction becomes the long side and a fatigue test was performed. The fatigue test piece was ground down to a 0.05 mm or so depth from the surfacemost layer. A stress controlled fatigue test was conducted with a stress ratio R=0.1 and a frequency 5Hz. The stress with no fracture after 10 million cycles was defined as the notch fatigue limit (FL) and notch fatigue property was evaluated. As a result of the test, if satisfying FL/TSC≥0.25, it was evaluated that the notch fatigue property was improved.

[Evaluation of Anisotropy of Strength]

The anisotropy of strength was measured by taking a JIS No. 5 test piece from an orientation (L direction) of the long direction of the test piece becoming parallel to the rolling direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011 to thereby measure the L direction tensile strength, i.e., the L direction TS (TSL). Next, if the obtained L direction TS and the previously found C direction TS (TSC) satisfy TSL/TSC≥0.95, it was evaluated that the anisotropy of strength was reduced.
A case where the tensile strength TSC was 540 MPa or more and TSC×H≥19500 MPa·mm, TSC×El≥13500 MPa·%, FL/TSC≥0.25, and TSL/TSC≥0.95 was evaluated as a hot rolled steel sheet which, despite being high in strength, was improved in stretch flangeability, ductility, and notch fatigue property and reduced in anisotropy of strength. The results are shown in Tables 4 and 5.

[Table 4]

Table 4

Test no.	Steel no.	Ferrite area ratio (%)	Bainite area ratio (%)	Ferrite and bainite total area ratio (%)	Martensite area ratio (%)	Ratio of crystal grains with orientation difference in grains of 5 to 14° (%)	Average value of pole densities of the {110}<111> and {112}<111> orientations at sheet thickness surface layer part	Average value of pole densities of the {100}<011>,{211}<001>, and {332}<113> orientations at sheet thickness center part	Remarks
1	A	52	34	86	6	26	3.25	5.28	Inv. ex.
2	A	42	48	90	8	26	2.52	4.60	Inv. ex.
3	B	22	68	90	5	20	3.27	6.13	Inv. ex.
4	B	19	66	85	8	8	325	6.23	Comp. ex.
5	B	16	72	88	5	9	3.21	6.14	Comp. ex.
6	B	72	19	91	7	75	3.58	6.89	Comp. ex.
7	B	23	65	88	5	27	2.20	5.72	Comp. ex.
8	B	24	67	91	5	31	2.42	5.98	Comp. ex.
9	B	22	65	87	5	30	3.27	7.25	Comp. ex.
10	B	85	3	88	5	9	3.25	5.74	Comp. ex.
11	B	52	29	81	7	5	3.21	6.80	Comp. ex.
12	B	15	72	87	6	6	3.54	6.63	Comp. ex.
13	B	10	72	82	7	8	3.46	5.93	Comp. ex.
14	B	25	70	95	1	20	2.87	5.88	Comp. ex.
15	C	4	82	86	10	15	3.55	6.38	Inv. ex.
16	C	0	89	89	10	11	3.32	5.01	Inv. ex.
17	C	12	79	91	7	16	4.62	6.82	Inv. ex.
18	D	18	69	87	5	23	2.95	5.73	Inv. ex.
19	E	71	25	96	3	23	2.69	5.80	Inv. ex.
20	F	32	52	84	7	17	3.52	6.28	Inv. ex.
21	G	78	8	86	6	19	3.05	6.24	Inv. ex.
22	G	85	0	85	8	12	3.21	4.95	Inv. ex.
23	H	8	81	89	10	18	3.49	5.72	Inv. ex.
24	I	68	25	93	3	24	2.62	5.68	Inv. ex.
25	J	72	19	91	3	19	2.98	5.30	Inv. ex.
26	K	25	67	92	5	23	3.14	6.63	Inv. ex.
27	L	24	66	90	5	25	2.62	5.30	Inv. ex.
28	M	25	65	90	5	21	2.97	6.41	Inv. ex.
29	N	19	66	85	6	27	2.88	5.30	Inv. ex.
30	O	18	68	86	8	19	2.65	6.58	Inv. ex.
31	P	15	67	82	11	26	2.68	6.25	Comp. ex.
32	Q	85	12	97	1	25	2.54	5.12	Comp. ex.
33	R	12	65	77	22	13	3.54	6.82	Comp. ex.
34	S	87	5	92	5	27	2.82	5.42	Comp. ex.
35	T	Cracking during rolling							Comp. ex.
36	U	92	5	97	1	30	2.62	5.14	Comp. ex.
37	V	10	72	82	10	9	3.55	6.38	Comp. ex.
38	W	27	58	85	5	22	2.74	5.67	Comp. ex.
39	X	76	16	92	5	18	2.62	5.21	Comp. ex.

Underlines indicate outside scope of present invention.

[Table 5]

Table 5

Test no.	Steel no.	Tensile strength in rolling perpendicular direction TSC (MPa)	Total elongation El (%)	Limit shaped height H (mm)	TSC×El (MPa.%)	TSC×H (MPa·mm)	Notch fatigue limit FL (MPa)	FL/TSC	TSL/TSC	Remarks
1	A	625	28.5	38.3	17813	23938	223	0.36	0.96	Inv. ex.
2	A	658	27.0	36.4	17766	23951	243	0.37	0.96	Inv. ex.
3	B	812	20.5	27.2	16646	22086	252	0.31	0.99	Inv. ex.
4	B	792	19.1	23.1	15127	18295	234	0.30	0.96	Comp. ex.
5	B	789	19.2	22.8	15149	17989	235	0.30	0.98	Comp. ex.
6	B	612	21.5	32.5	13158	19890	217	0.35	0.97	Comp. ex.
7	B	794	19.2	27.3	15245	21676	235	0.30	0.94	Comp. ex.
8	B	795	19.3	27.1	15344	21545	237	0.30	0.93	Comp. ex.
9	B	802	19.1	24.8	15318	19890	239	0.30	0.93	Comp. ex.
10	B	611	28.4	31.5	17352	19247	196	0.32	0.97	Comp. ex.
11	B	601	31.2	31.3	18751	18811	211	0.35	1.02	Comp. ex.
12	B	624	22.9	31.2	14290	19469	211	0.34	0.97	Comp. ex.
13	B	635	29.1	28.8	18479	18288	214	0.34	0.96	Comp. ex.
14	B	535	32.4	38.1	17334	20384	128	0.24	0.98	Comp. ex.
15	C	847	19.7	257	16686	21768	261	0.31	0.96	Inv. ex.
16	C	912	18.3	24.1	16690	21979	256	0.28	0.96	Inv. ex.
17	C	884	18.9	24.8	16708	21923	260	0.29	0.96	Inv. ex.
18	D	892	19.5	26.8	17394	23906	268	0.30	0.97	Inv. ex.
19	E	582	32.3	38.7	18799	22523	172	0.30	0.98	Inv. ex.
20	F	812	19.5	26.8	15834	21762	242	0.30	0.98	Inv. ex.
21	G	632	27.2	342	17190	21614	199	0.31	0.97	Inv. ex.
22	G	601	29.2	35.8	17549	21516	200	0.33	0.96	Inv. ex.
23	H	832	18.9	24.6	15725	20467	262	0.31	0.97	Inv. ex.
24	I	598	29.2	37.8	17462	22604	178	0.30	0.96	Inv. ex.
25	J	725	22.8	30.4	16530	22040	218	0.30	0.96	Inv. ex.
26	K	835	18.7	27.5	15615	22963	248	0.30	0.96	Inv. ex.
27	L	754	22.3	28.6	16814	21564	227	0.30	0.97	Inv. ex.
28	M	811	19.8	28.2	16058	22870	242	0.30	0.99	Inv. ex.
29	N	812	19.5	276	15834	22411	243	0.30	0.99	Inv. ex.
30	O	835	19.2	276	16032	23046	247	0.30	0.98	Inv. ex.
31	P	862	19.2	20.1	16550	17326	262	0.30	0.96	Comp. ex.
32	Q	501	35.9	48.7	17986	24399	117	0.23	0.99	Comp. ex.
33	R	875	15.7	18.6	13738	16275	262	0.30	0.96	Comp. ex.
34	S	532	35.2	44.9	18726	23887	202	0.38	0.95	Comp.
35	T	Cracking during rolling								Comp. ex.
36	U	516	28.1	32.3	14500	16667	115	0.22	0.97	Comp. ex.
37	V	609	30.8	23.5	18757	14312	189	0.31	0.96	Comp. ex.
38	W	807	16.7	27	13477	21789	251	0.31	0.99	Comp. ex.
39	X	578	324	334	18727	19305	172	0.30	0.98	Comp. ex.

Underlines indicate outside scope of present invention.

Referring to Tables 1 to 5, In Comparative Example 4, the cumulative strain (εeff.) of the final three stages of the finish rolling was high, and therefore it is believed that recrystallization of austenite occurred during hot rolling and the stored dislocation density at the time of transformation fell. As a result, the ratio of crystal grains with an orientation difference in grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 5, the εeff. was low, and therefore it is believed that the dislocation density of the austenite introduced was not sufficient. As a result, in the same way, the ratio of crystal grains with an orientation difference in grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 6, the end temperature of the finish rolling was low, and therefore the ratio of crystal grains with an orientation difference in grains of 5 to 14° became more than 60% and the ductility fell. In each of Comparative Examples 7 and 8, the number of rolling passes with a shape ratio (X) at 1100°C or less of 2.3 or more was small, and therefore it is believed it was not possible to introduce sufficient shear strain at the sheet thickness surface layer part. As a result, the average value of the pole densities of the {110}<111> and {112}<111> orientations at the sheet thickness surface layer part became less than 2.50, the L direction strength could not be raised, and the anisotropy of strength became remarkable. In particular, in Comparative Example 7, finish rolling was performed using rolls having roll radii similar to the other examples, but the sheet thickness was relatively large, and therefore a sufficient shape ratio could not be secured. On the other hand, in Comparative Example 8, while the sheet thickness was similar to the other examples, the roll radius was a comparatively small 150 mm, and therefore a sufficient shape ratio could not be secured. In Comparative Example 9, the rolling temperature (FT3) of the third stage of the finish rolling was low, and therefore it is believed it was not possible to lighten the shear strain introduced to the sheet thickness center part. As a result, the average value of the pole densities of the {100}<011>, {211 }<011>, and {332}<113> orientations became more than 7.00, the C direction strength could not be reduced, and the anisotropy of strength became remarkable. In Comparative Example 10, the average cooling speed of the primary cooling in the cooling step was low, and therefore it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of crystal grains with an orientation difference in grains of 5 to 14°became less than 10% and the stretch flangeability fell. In Comparative Example 11, the cooling stop temperature of primary cooling was high, so, similarly it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of crystal grains with an orientation difference in grains of 5 to 14°became less than 10% and the stretch flangeability fell. In Comparative Example 12, the cooling stop temperature of primary cooling was low, and therefore it is believed that transformation by paraequilibrium occurred at a lower temperature than the desired temperature region. As a result, similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 13, the holding time at 650 to 750°C in the primary cooling was short, and therefore similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 14, the cooling stop temperature of the secondary cooling at the cooling step was high, and therefore the area ratio of martensite became less than 2%. As a result, the TSC and notch fatigue property fell.
In each of Comparative Examples 31 and 33, the C and Mn contents were high, and therefore the stretch flangeability fell. In each of Comparative Examples 32 and 34, the C and Mn contents were low, and therefore a sufficient strength could not be obtained. In Comparative Example 35, the Al content was high, and therefore cracks formed during rolling and the subsequent tests could not be performed. In Comparative Example 36, the total content of Si and sol. Al was high, and therefore formation of ferrite was promoted and the TSC fell. In Comparative Example 37, the total content of Si and sol. Al was low, and therefore the ratio of crystal grains with an orientation difference in grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 38, the Ti content was high, and therefore the carbides (TiC) became coarser and the ductility fell. In Comparative Example 39, the Ti content was low, and therefore it is believed that the formation of cementite could not be sufficiently suppressed. As a result, the stretch flangeability fell.
In contrast to this, in the hot rolled steel sheets according to all of the invention examples, it was possible to obtain the hot rolled steel sheet having a predetermined chemical composition and a microstructure obtained by further suitably controlling the conditions in the method of production, the microstructure containing, by area%, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, wherein a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area%, 10 to 60%, an average value of pole densities of the {110}<111> and {112}<111> orientations in a region from the surface down to a 1/6 position in sheet thickness is 2.50 or more, and an average value of pole densities of the {100}<011>, {211}<011>, and {332}<113> orientations in a region from a 2/5 position in sheet thickness to a 3/5 position in sheet thickness is 7.00 or less. Further, as a result, despite having a high strength of a tensile strength of 540 MPa or more, it was possible to improve the stretch flangeability, ductility, and notch fatigue property and remarkably reduce the anisotropy of strength at the L direction and C direction tensile strengths.

Claims

A hot rolled steel sheet having a chemical composition comprising, by mass%,
C: 0.020 to 0.070%,

Si: 0.010 to 2.000%,

Mn: 0.60 to 2.00%,

Ti: 0.015 to 0.200%,

sol. Al: 0.010 to 1.000%,

P: 0.100% or less,

S: 0.030% or less,

N: 0.0060% or less,

O: 0.0100% or less,

Nb: 0 to 0.050%,

V: 0 to 0.300%,

Cr: 0 to 2.00%,

Ni: 0 to 2.00%,

Cu: 0 to 2.00%,

Mo: 0 to 1.000%,

B: 0 to 0.0100%,

Sb: 0 to 1.00%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%,

Hf: 0 to 0.0100%,

REM: 0 to 0.1000%,

Bi: 0 to 0.0100%,

As: 0 to 0.0100%,

Zr: 0 to 1.00%,

Co: 0 to 1.00%,

Zn: 0 to 1.00%,

W: 0 to 1.00%,

Sn: 0 to 1.00%, and

balance: Fe and impurities, and

satisfying 0.100≤[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are contents (mass%) of the elements, and

a microstructure comprising, by area%,

at least one of ferrite and bainite: 80 to 98% in total, and

martensite: 2 to 10%, wherein

when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 µm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area%, 10 to 60%,

an average value of pole densities of {110}<111> and {112}<111> orientations in a region from a surface down to a 1/6 position in sheet thickness is 2.50 or more, and

an average value of pole densities of {100}<011>, {211}<011>, and {332}<113> orientations in a region from a 2/5 position in sheet thickness to a 3/5 position in sheet thickness is 7.00 or less.
The hot rolled steel sheet according to claim 1, wherein the chemical composition contains, by mass%, at least one of
Nb: 0.001 to 0.050%,

V: 0.001 to 0.300%,

Cr: 0.01 to 2.00%,

Ni: 0.01 to 2.00%,

Cu: 0.01 to 2.00%,

Mo: 0.001 to 1.000%,

B: 0.0001 to 0.0100%,

Sb: 0.01 to 1.00%,

Ca: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0100%,

Hf: 0.0001 to 0.0100%,

REM: 0.0001 to 0.1000%,

Bi: 0.0001 to 0.0100%,

As: 0.0001 to 0.0100%,

Zr: 0.01 to 1.00%,

Co: 0.01 to 1.00%,

Zn: 0.01 to 1.00%,

W: 0.01 to 1.00%, and

Sn: 0.01 to 1.00%.