EP4613886A1

EP4613886A1 - Hot-rolled coil

Info

Publication number: EP4613886A1
Application number: EP23885827.8A
Authority: EP
Inventors: Daisuke Ito; Tatsuo Yokoi; Eisaku Sakurada; Akifumi SAKAKIBARA; Jun Ando; Takaaki TSUTSUMI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-11-02
Filing date: 2023-11-01
Publication date: 2025-09-10
Also published as: KR20250075682A; CN120153111A; WO2024096073A1; EP4613886A4; JPWO2024096073A1; MX2025004595A

Abstract

Provided a hot rolled coil having a predetermined chemical composition and a microstructure, wherein at a center part in a longitudinal direction, when defining a region surrounded by boundaries with an orientation difference of 15° or more as a "crystal grain", a mean grain size of crystal grains is 5.0 to 8.0 µm, a mean grain size of precipitates is 3.0 to 9.5 nm, and a difference of a maximum value and minimum value in grain size of the precipitates in a width direction is 15.0% or less of the mean grain size of the precipitates, and at a center part in the width direction, a difference of a maximum value and minimum value in grain size of the precipitates in the longitudinal direction is 15.0% or less of the mean grain size of the precipitates in the longitudinal direction.

Description

FIELD

The present invention relates to a hot rolled coil.

BACKGROUND

In recent years, in the automobile industry, lighter weight of car bodies is being sought from the viewpoint of improving fuel economy. To achieve both lighter weight of car bodies and collision safety, making the steel sheet used higher in strength is one effective method. With this as a background, high strength steel sheets are being developed. On the other hand, along with the higher strength, the workability and formability of steel sheet have generally fallen. For this reason, in the development of high strength steel sheet, it is important to maintain the workability, etc., at a certain level or more while pursuing higher strength.
In relation to this, for example, PTL 1 describes high strength hot rolled steel sheet having a predetermined composition and having a structure having, by area ratio, 95% or more of a ferrite phase, having fine carbides precipitate in the ferrite phase in 1.0×10²²/m³ or more and cementite grains precipitate in 10/10000 µm² or more, having a mean grain size of the fine carbides of 10 nm or less, and having a difference ΔHV_0.025 of hardness HV_1/2t of the sheet thickness center position and hardness HV_1/4t of the sheet thickness 1/4 position or hardness HV_3/4t of the sheet thickness 3/4 position of 20HV or less. Further, PTL 1 describes that, according to the above constitution, it is possible to easily produce high strength hot rolled steel sheet having a high strength of a tensile strength of 780 MPa or more and excellent in ductility, hole expandability, and other workability and exhibiting a particularly advantageous effect in industry. Furthermore, PTL 1 teaches that Ti contributes to the formation of 10 nm or less fine carbides (Ti carbides) and that to secure the desired high strength by precipitation strengthening and secure the desired hole expandability, the Ti content should be in a range of 0.070 to 0.220%.
PTL 2 describes high strength hot rolled steel sheet comprised of steel sheet having a predetermined chemical composition and having a ratio of grains of 10 to 70% with a mean grain size of 8 µm or less, with an amount of segregation of C at large angle boundaries with an orientation difference of 15° or more of 4 to 15 atoms/nm², with a ratio of grains with a number of equivalent spherical diameter 3 nm or more TiC precipitates on the large angle boundaries of less than 0.01/nm² , and with a number density of equivalent spherical diameter 0.8 nm or more and 2 nm or less TiC precipitates in the grains of 8×10¹⁶/cm³ or more. Further, PTL 2 describes that, according to the above constitution, it is possible to provide high strength hot rolled steel sheet excellent in formability and low temperature toughness and with a tensile strength of 740 MPa or more and that the contribution to industry is extremely remarkable. Furthermore, PTL 2 teaches that by making the equivalent spherical diameter of the TiC precipitates in the crystal grains 0.8 nm or more and 2 nm or less, it is possible to efficiently impart precipitation strengthening and that this is effective for raising the strength.
PTL 3 describes high formability, high strength steel sheet with a tensile strength of 550 MPa or more and excellent strength stability containing, by wt%, C: 0.03 to 0.15%, Mn≥0.2%, N≤0.01%, and Ti: 0.05 to 0.35%, containing one or more elements selected from Mo≤0.6% and W≤1.5%, when Mo and W are independently contained, Mo≥0.1% andW≥0.2%, having an Ex. C shown by the formula Ex. C=C-{Ti-N×(48/14)-S×(48/32)}×(12/48)-Mo×(12/96)-W×(12/184) of 0.015% or less, satisfying Mn≤1.7-30×Ex. C, and substantially containing in the ferrite structures less than 10 nm precipitates containing at least one of Ti, Mo, and W dispersed therein.

[CITATIONS LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 2015-063748
[PTL 2] Japanese Unexamined Patent Publication No. 2015-218352
[PTL 3] Japanese Unexamined Patent Publication No. 2003-321735

SUMMARY

[TECHNICAL PROBLEM]

As explained above, PTLs 1 and 2 teach utilization of precipitation strengthening by Ti carbides so as to improve the workability, etc., while achieving a more than 700 MPa high strength. On the other hand, in making a steel material high in strength utilizing precipitation strengthening, for example, the strength sometimes will vary due to the states of precipitation of precipitates differing in the longitudinal direction and width direction of a hot rolled coil.
In relation to this, PTL 3 teaches that in steel with a ferrite single phase structure which is strengthened by fine precipitates containing one or more of Ti, Mo, and W, by making the Ex. C of C not bonding with Ti, Mo, and W 0.015% or less and making Mn 0.2≤Mn1.7-30×Ex. C, variations in quality in the longitudinal direction of a coil, in particular variations in strength, are reduced. However, while PTL 3, in the above way, studies reduction of variations in strength from mainly the viewpoint of the chemical composition of the steel sheet, it does not necessarily sufficiently study this from the viewpoint of making the state of precipitation of the fine precipitates in the steel sheet a suitable one. Therefore, in the invention described in PTL 3, there was still room for improvement relating to suppression of variations in strength.
The present invention was made in consideration of the above situation and has as its object the provision of a hot rolled coil which is high in strength and is reduced in variations in strength by a novel constitution.

[SOLUTION TO PROBLEM]

The inventors engaged in studies to achieve the above object and as a result discovered that by utilizing fine grain reinforcement and precipitation strengthening, it is possible to realize higher strength and that by suitably controlling the grain size of the precipitates contributing to precipitation strengthening of the hot rolled coil in the width direction at the center part in the longitudinal direction and in the longitudinal direction at the center part in the width direction, it is possible to remarkably suppress or reduce variations in strength in the longitudinal direction and width direction of the hot rolled coil and thereby completed the present invention.
The present invention able to achieve this object is as follows:

(1) A hot rolled coil having a chemical composition comprising, by mass%,
- C: 0.050 to 0.100%,
- Si: 0.01 to 0.30%,
- Mn: 1.30 to 2.10%,
- Ti: 0.080 to 0.150%,
- Nb: 0.020 to 0.050%,
- Al: 0.001 to 0.050%,
- P: 0.100% or less,
- S: 0.050% or less,
- N: 0.0050% or less,
- O: 0.0050% or less,
- B: 0 to 0.0050%,
- Cu: 0 to 0.20%,
- Ni: 0 to 0.20%,
- Sn: 0 to 0.10%,
- Cr: 0 to 0.40%,
- Mo: 0 to 0.200%,
- V: 0 to 0.100%,
- As: 0 to 0.100%,
- Zr: 0 to 0.100%,
- Ca: 0 to 0.0050%,
- Mg: 0 to 0.100%,
- Bi: 0 to 0.020%,
- Co: 0 to 0.20%,
- W: 0 to 0.20%,
- Zn: 0 to 0.20%,
- REM: 0 to 0.1000%, and
- balance: Fe and impurities, and
- a microstructure, wherein
  - at a center part in a longitudinal direction,
  - when defining a region surrounded by boundaries with an orientation difference of 15° or more as a "crystal grain", a mean grain size of crystal grains is 5.0 to 8.0 µm,
  - a mean grain size of precipitates is 3.0 to 9.5 nm, and
  - a difference of a maximum value and minimum value in grain size of the precipitates in a width direction is 15.0% or less of the mean grain size of the precipitates, and
  - at a center part in the width direction, a difference of a maximum value and minimum value in grain size of the precipitates in the longitudinal direction is 15.0% or less of the mean grain size of the precipitates in the longitudinal direction.
(2) The hot rolled coil according to (1), wherein the chemical composition comprises, by mass%, at least one of
- B: 0.0001 to 0.0050%,
- Cu: 0.01 to 0.20%,
- Ni: 0.01 to 0.20%,
- Sn: 0.01 to 0.10%,
- Cr: 0.01 to 0.40%,
- Mo: 0.001 to 0.200%,
- V: 0.001 to 0.100%,
- As: 0.001 to 0.100%,
- Zr: 0.001 to 0.100%,
- Ca: 0.0001 to 0.0050%,
- Mg: 0.001 to 0.100%,
- Bi: 0.001 to 0.020%,
- Co: 0.01 to 0.20%,
- W: 0.01 to 0.20%,
- Zn: 0.01 to 0.20%, and
- REM: 0.0001 to 0.1000%.
(3) The hot rolled coil according to (1) or (2), wherein the hot rolled coil has an effective Ti amount of 0.070% or more.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the present invention, it is possible to provide a hot rolled coil which is high in strength and is reduced in variations in strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a hot rolled coil in a coiled up state.

DESCRIPTION OF EMBODIMENTS

A hot rolled coil according to an embodiment of the present invention has a chemical composition comprising, by mass%,

C: 0.050 to 0.100%,
Si: 0.01 to 0.30%,
Mn: 1.30 to 2.10%,
Ti: 0.080 to 0.150%,
Nb: 0.020 to 0.050%,
Al: 0.001 to 0.050%,
P: 0.100% or less,
S: 0.050% or less,
N: 0.0050% or less,
O: 0.0050% or less,
B: 0 to 0.0050%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%,
Cr: 0 to 0.40%,
Mo: 0 to 0.200%,
V: 0 to 0.100%,
As: 0 to 0.100%,
Zr: 0 to 0.100%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.100%,
Bi: 0 to 0.020%,
Co: 0 to 0.20%,
W: 0 to 0.20%,
Zn: 0 to 0.20%,
REM: 0 to 0.1000%, and
balance: Fe and impurities, and
a microstructure, wherein
- at a center part in a longitudinal direction,
- when defining a region surrounded by boundaries with an orientation difference of 15° or more as a "crystal grain", a mean grain size of crystal grains is 5.0 to 8.0 µm,
- a mean grain size of precipitates is 3.0 to 9.5 nm, and
- a difference of a maximum value and minimum value in grain size of the precipitates in a width direction is 15.0% or less of the mean grain size of the precipitates, and
- at a center part in the width direction, a difference of a maximum value and minimum value in grain size of the precipitates in the longitudinal direction is 15.0% or less of the mean grain size of the precipitates in the longitudinal direction.

As explained before, in making the strength of a steel material higher utilizing precipitation strengthening, for example, sometimes variations in strength occur due to the difference in the states of precipitation of precipitates in the longitudinal direction and the width direction of a hot rolled coil. This is believed to be because due to the cooling speed after coiling differing in a hot rolled coil, the states of precipitation of precipitates in the longitudinal direction and/or width direction of the hot rolled coil change. The tip end part and tail end part in the longitudinal direction of a hot rolled coil respectively correspond to an innermost circumference part and outermost circumference part of the hot rolled coil and therefore are exposed to the atmosphere. For this reason, in general, the tip end part and the tail end part of a hot rolled coil are quick to cool. Sometimes the precipitates do not sufficiently proceed to precipitate. Therefore, as the state of precipitation, a sub-aged state is reached. The strength tends to easily fall. If a drop in strength occurs at the tip end part and the tail end part in the longitudinal direction of a hot rolled coil, these are cut off and only the range having the desired strength is utilized as the product, and therefore the yield falls and the productivity drops. Further, even if a hot rolled coil has the desired strength at the tip end part and the tail end part in the longitudinal direction, if the state of precipitation of the precipitates differs in the longitudinal direction, there is the problem that if variations in strength occur, for example, cracking and other shaping defects easily occur at the time of press-forming. On the other hand, the center part of a hot rolled coil in the longitudinal direction is not directly exposed to the atmosphere, therefore is difficult to cool and is held in a relatively high temperature state. Therefore, in some cases, sometimes the precipitates coarsen and peak aging where a high effect of precipitation strengthening is obtained passes, the overaged state is reached, and similarly the strength falls. In this case as well, in the same way, sometimes the desired strength is not obtained, variations in strength occur, and the productivity falls or shape defects are caused at the time of press-forming.
In addition, the states of precipitation of precipitates in the longitudinal direction and width direction of a hot rolled coil are believed to be closely related to each other. For this reason, for example, even if suitably cooling only the center part in the longitudinal direction of a hot rolled coil after coiling, if not suitably cooling the tip end part and/or the tail end part, due to the effects of these, it becomes no longer possible to reliably obtain the desired state of precipitation of precipitates even at the center part. Similarly, for example, even if having suitably cooled the tip end part, the center part, and the tail end part in the longitudinal direction of the hot rolled coil after coiling, if not cooling in the width direction, more specifically cooling in the width direction from after hot rolling to before coiling or cooling uniformly in the width direction after coiling, the state of precipitation of precipitates in the longitudinal direction is also affected by temperature deviation in the width direction. As a result, similarly in the finally obtained hot rolled coil, sometimes the desired strength is not obtained or the variations in strength become remarkable. In this way, the state of precipitation of precipitates changes due to the temperature history in the total length of the hot rolled coil (overall length of hot rolled coil in rolling direction) and total width (overall length of hot rolled coil in width direction), so effectively utilizing the precipitation strengthening by precipitates to achieve the desired strength while suppressing or reducing the variations in strength in the longitudinal direction and width direction of a hot rolled coil is generally extremely difficult.
Here, in this Description, if referring to the "longitudinal direction" in relation to a hot rolled coil, the "longitudinal direction", as shown in FIG. 1, means the "rolling direction". Similarly, in in this Description, if referring to the "width direction" in relation to a hot rolled coil, the "width direction", as shown in FIG. 1, means the "direction perpendicular to the rolling direction and sheet thickness direction".
Therefore, the inventors engaged in studies to make the chemical composition of the hot rolled coil more suitable and, in addition, particularly focused on the microstructure of that hot rolled coil. Explained more specifically, first the inventors discovered that by utilizing precipitation strengthening by Ti carbides and other precipitates and additionally fine grain reinforcement by addition of Nb, etc., it is possible to achieve the desired high strength. More specifically, the inventors discovered that by controlling the mean grain size of Ti carbides and other precipitates in the center part in the longitudinal direction of a hot rolled coil to a range of 3.0 to 9.5 nm, it is possible to sufficiently realize the effect of improvement of strength by precipitation strengthening and that by controlling the mean grain size of crystal grains due to addition of Nb, etc., to a range of 5.0 to 8.0 µm, it is possible to add the effect of improvement of strength by fine grain reinforcement and as a result realize high strength, for example, high strength of a tensile strength of 780 MPa or more. In addition, since the states of precipitation of precipitates in the longitudinal direction and width direction of a hot rolled coil are probably closely related, the inventors, as explained above, engaged in further studies focusing in particular on the states of precipitation of precipitates in the width direction at the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction at the center part in the width direction. As a result, the inventors discovered that, as explained later in detail regarding the method of production of a hot rolled coil, by making the cooling treatment in the cooling step after hot rolling and the cooling treatment in the subsequent coiling step suitable ones, it is possible to control the variations in grain size of the precipitates in the width direction of the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction of the center part in the width direction to within a predetermined range. More specifically, the inventors discovered that by making the cooling step after hot rolling and the cooling treatment in the coiling step suitable, it is possible to control the difference of the maximum value and minimum value of grain size of the precipitates in the width direction of the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction of the center part in the width direction to 15.0% or less of the mean grain size of the precipitates and as a result it is possible to remarkably suppress or reduce the variations in strength in the longitudinal direction and width direction of a hot rolled coil.
In general, the grain size of precipitates is on the nano order. For observation of this, a transmission type electron microscope (TEM), 3D atom probe, or other high precision measuring device is necessary. Therefore, to reduce the variations in strength over the total length and total width of a hot rolled coil, for example, analyzing the grain size of precipitates over the total length and total width of the hot rolled coil and feeding this back to the production conditions would require tremendous time and cost and is not necessarily practical. Therefore, the fact that by making the cooling step after the hot rolling and the cooling treatment in the coiling step suitable, as explained above, it is possible to control the variations of grain size of precipitates in the width direction of the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction of the center part in the width direction to within a predetermined range and thereby remarkably suppress or reduce the variations in strength in the longitudinal direction and width direction of a hot rolled coil is extremely unexpected and further is surprising. Further, according to a hot rolled coil according to an embodiment of the present invention, as explained above, since the variations of strength in the longitudinal direction and width direction of a hot rolled coil are remarkably suppressed or reduced, it is possible to reduce the risk of shaping defects at the time of press forming and possible to remarkably improve the productivity. Therefore, a hot rolled coil according to an embodiment of the present invention is particularly useful in use of the automobile field naturally and can be extremely effectively used in other fields. In this Description, "hot rolled coil" is not necessarily limited to a completely coiled up coil shape such as shown in FIG. 1. For example, it may be one which is partially or completely uncoiled. Cases including at least partial sheet shapes (i.e., hot rolled steel sheets) are also covered.
Below, the hot rolled coil 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: 0.050 to 0.100%]

C is an element effective for raising the strength of steel sheet. To sufficiently obtain this effect, the C content is 0.050% or more. The C content may also be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if excessively containing C, sometimes the weldability falls. Therefore, the C content is 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.

[Si: 0.01 to 0.30%]

Si is an element effective for raising strength as a solid solution strengthening element. To sufficiently obtain such an effect, the Si content is 0.01% or more. The Si content may also be 0.03% or more, 0.05% or more, 0.08% or more, 0.12% or more, or 0.15% or more. On the other hand, if excessively containing Si, sometimes defects in surface quality called "Si scale" are formed. Therefore, the Si content is 0.30% or less. The Si content may also be 0.28% or less, 0.25% or less, 0.22% or less, or 0.20% or less.

[Mn: 1.30 to 2.10%]

Mn is an element effective for raising hardenability and strength as a solid solution strengthening element. To sufficiently obtain these effects, the Mn content is 1.30% or more. The Mn content may also be 1.40% or more, 1.50% or more, 1.60% or more, or 1.70% or more. On the other hand, if excessively containing Mn, a large amount of MnS is formed and sometimes the toughness is made to fall. Therefore, the Mn content is 2.10% or less. The Mn content may also be 2.00% or less, 1.90% or less, or 1.80% or less.

[Ti: 0.080 to 0.150%]

Ti is an element which finely precipitates as TiC or other Ti carbides in the steel and contributes to improvement of strength by precipitation strengthening. To sufficiently obtain such an effect, the Ti content is 0.080% or more. The Ti content may also be 0.090% or more, 0.095% or more, 0.100% or more, 0.105% or more, or 0.110% or more. On the other hand, if excessively containing Ti, the precipitates become coarser and the effect of improvement of strength by precipitation strengthening cannot be sufficiently exhibited. Therefore, the Ti content is 0.150% or less. The Ti content may also be 0.140% or less, 0.135% or less, 0.130% or less, 0.125% or less, or 0.120% or less.

[Nb: 0.020 to 0.050%]

Nb is an element which forms carbides, nitrides, and/or carbonitrides in the steel to refine the crystal grains by the pinning effect and contributes to higher strength of steel sheet by grain strengthening. To sufficiently obtain such an effect, the Nb content is 0.020% or more. The Nb content may also be 0.025% or more, 0.028% or more, 0.030% or more, or 0.032% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the toughness of the steel sheet is made to fall. Therefore, the Nb content is 0.050% or less. The Nb content may also be 0.045% or less, 0.042% or less, 0.040% or less, or 0.038% or less.

[Al: 0.001 to 0.050%]

Al is an element acting as a deoxidizer. To sufficiently obtain such an effect, the Al content is 0.001% or more. The Al content may also be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if excessively containing Al, coarse oxides are formed and sometimes the toughness is made to fall. Therefore, the Al content is 0.050% or less. The Al content may also be 0.045% or less or 0.040% or less.

[P: 0.100% or Less]

P, if excessively contained, sometimes disadvantageously affects the weldability, etc. Therefore, the P content is 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% 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 cost. Therefore, the P content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[S: 0.050% or Less]

S, if excessively contained, forms a large amount of MnS and sometimes causes a drop in toughness. Therefore, the Si content is 0.050% 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 cost. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[N: 0.0050% or Less]

N, if excessively contained, forms coarse nitrides and sometimes cause the toughness to fall. Further, N bonds with the Ti in the steel to form titanium nitride (TiN) and thereby reduce the effective Ti amount able to form Ti carbides or other precipitates and sometimes reduce the effect of improvement of strength by precipitation strengthening. Therefore, the lower the N content, the better and is made 0.0050% or less. The N content may also be 0.0045% or less, 0.0040% or less, or 0.0035% 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, 0.0005% or more, or 0.0010% or more.

[O: 0.0050% or Less]

O, if excessively contained, forms coarse inclusions and sometimes causes the toughness to fall. Therefore, the O content is 0.0050% or less. The O content may also be 0.0040% or less, 0.0035% or less, or 0.0030% or less. The lower limit of the O content is not particularly prescribed and may also be 0%, but for reduction of the O content 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, 0.0005% or more, or 0.0010% or more.
The basic chemical composition of the hot rolled coil according to an embodiment of the present invention is as explained above. Further, the hot rolled coil may, in accordance with need, contain at least one of the following optional elements in place of part of the balance of Fe.

[B: 0 to 0.0050%]

B is an element raising the hardenability of steel and contributing to improvement of the strength. 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, if excessively containing B, sometimes the toughness and/or weldability falls. Therefore, the B content is preferably 0.0050% or less. The B content may also be 0.0030% or less, 0.0015% or less, 0.0012% or less, or 0.0008% or less.

[Cu: 0 to 0.20%]

Cu is an element contributing to the improvement of strength and/or corrosion resistance. The Cu content may also be 0%, but to obtain these effects, the Cu content is preferably 0.01% or more. The Cu content may also be 0.03% or more or 0.05% or more. On the other hand, if excessively containing Cu, deterioration of toughness and weldability is sometimes invited. Therefore, the Cu content is preferably 0.20% or less. The Cu content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[Ni: 0 to 0.20%]

Ni is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.01% or more. The Ni content may also be 0.03% or more, or 0.05% or more. On the other hand, even if excessively containing Ni, the effect becomes saturated and a rise in production costs is invited. Therefore, the Ni content is preferably 0.20% or less. The Ni content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[Sn: 0 to 0.10%]

Sn is an element effective for improvement of the 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. On the other hand, if excessively containing Sn, a drop in the toughness is sometimes invited. Therefore, the Sn content is preferably 0.10% or less. The Sn content may also be 0.08% or less, 0.06% or less, or 0.04% or less.

[Cr: 0 to 0.40%]

Cr is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively containing Cr, the effect becomes saturated and a rise in the production costs is invited. Therefore, the Cr content is preferably 0.40% or less. The Cr content may also be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.

[Mo: 0 to 0.200%]

Mo is an element raising the hardenability of steel and contributing to improvement of the strength and an element contributing also to improvement of the corrosion resistance. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.030% 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 0.200% or less. The Mo content may also be 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, or 0.080% or less.

[V: 0 to 0.100%]

V is an element contributing to improvement of strength by 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 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if excessively containing V, a large amount of precipitates is formed and sometimes the toughness is made to drop. Therefore, the V content is preferably 0.100% or less. The V content may also be 0.080% or less, 0.060% or less, or 0.040% or less.

[As: 0 to 0.100%]

As is an element effective for improvement of the corrosion resistance. The As content may also be 0%, but to obtain such an effect, the As content is preferably 0.001% or more. The As content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if excessively containing As, the effect becomes saturated and a rise in production costs is invited. Therefore, the As content is preferably 0.100% or less. The As content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[Zr: 0 to 0.100%]

Zr is an element able to control the form of sulfides. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content may also be 0.005% or more or 0.010% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zr content is preferably 0.100% or less. The Zr content may also be 0.050% or less, 0.030% or less, or 0.020% or less.

[Ca: 0 to 0.0050%]

Ca is an element able to control the form of sulfides. The Ca content may also be 0%, but to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca 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 Ca, the effect becomes saturated and a rise in production costs is invited. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may also be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[Mg: 0 to 0.100%]

Mg is an element able to control the form of sulfides. The Mg content may also be 0%, but to obtain such an effect, the Mg content is preferably 0.001% or more and may also be 0.005% or more or 0.008% or more. On the other hand, even if excessively containing Mg, the effect becomes saturated and a rise in production costs is invited. Therefore, the Mg content is preferably 0.100% or less. The Mg content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[Bi: 0 to 0.020%]

Bi is an element effective for improvement of the corrosion resistance. The Bi content may also be 0%, but to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may also be 0.002% or more or 0.003% or more. On the other hand, even if excessively containing Bi, the effect becomes saturated and a rise in production costs is invited. Therefore, the Bi content is preferably 0.020% or less. The Bi content may also be 0.010% or less, 0.008% or less, or 0.005% or less.

[Co: 0 to 0.20%]

Co is an element contributing to the 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 0.03% or more or 0.05% or more. On the other hand, if excessively containing Co, sometimes the hot workability falls. An increase in the cost of materials is also led to. Therefore, the Co content is preferably 0.20% or less. The Co content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[W: 0 to 0.20%]

W is an element raising the hardenability of steel and contributing to improvement of the 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 0.03% or more or 0.05% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 0.20% or less. The W content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[Zn: 0 to 0.20%]

Zn is an element effective for controlling the form of inclusions. To obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may also be 0.03% or more, or 0.05% or more. On the other hand, even if excessively containing Zn, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zn content is preferably 0.20% or less. The Zn content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[REM: 0 to 0.1000%]

An REM (rare earth metal) is an element enabling control of the form of sulfides. 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 a REM, the effect becomes saturated and a rise in production costs is invited. Therefore, the REM content is preferably 0.1000% or less. The REM content may also be 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.
In the hot rolled coil according to an embodiment of the present invention, the balance aside from the above elements is comprised of Fe and impurities. The "impurities" are constituents, etc., entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the hot rolled coil.
The chemical composition of a hot rolled coil 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 coil 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.

[Effective Ti Amount]

In a hot rolled coil according to an embodiment of the present invention, from the viewpoint of sufficiently realizing the effect of improvement of strength by precipitation strengthening, the effective Ti amount is preferably 0.070% or more. The effective Ti amount corresponds to the dissolved amount of Ti right before aging, i.e., corresponds to the value of the total amount of Ti in the hot rolled coil minus the amount of Ti fixed by N. Explained in more detail, TiN is extremely small in solubility, therefore once precipitated TiN cannot be redissolved at a normal solubilization temperature. For this reason, the effective Ti amount for age hardening of Ti carbides and other precipitates becomes the value of the total amount of Ti minus the amount able to be fixed as TiN and is calculated by the following formula: $Effective Ti amount (%) = [Ti] - 48 / 14 [N]$
Where, [Ti] and [N] is the content (mass%) of elements in a hot rolled coil. By making the effective Ti amount 0.070% or more, it is possible to form precipitates sufficient for obtaining the desired effect of improvement of strength by precipitation strengthening. For example, the effective Ti amount may also be 0.075% or more, 0.080% or more, 0.085% or more, or 0.090% or more. The upper limit is not particularly prescribed, but, for example, the effective Ti amount may also be 0.150% or less, 0.145% or less, 0.140% or less, or 0.135% or less.

[Microstructure]

[Mean Grain Size of Crystal Grains Surrounded by Grain Boundaries With Orientation Difference of 15° or More: 5.0 to 8.0 µm]

In a hot rolled coil according to an embodiment of the present invention, at the center part in the longitudinal direction, the mean grain size of crystal grains is controlled to 5.0 to 8.0 µm when defining a region surrounded by boundaries with an orientation difference of 15° or more as a "crystal grain". By controlling the mean grain size of crystal grains to within a range of 5.0 to 8.0 µm by the pinning effect due to carbides, nitrides, and/or carbonitrides of Nb, it is possible to sufficiently obtain the effect of improvement of strength by fine grain reinforcement. By combination with the effect of improvement of strength by precipitation strengthening by Ti carbides or other precipitates explained in detail later, it is possible to reliably achieve a high tensile strength, for example, a 780 MPa or more tensile strength, at the finally obtained hot rolled coil. If the above mean grain size of crystal grains is less than 5.0 µm or more than 8.0 µm, such an effect of improvement of strength by fine grain reinforcement cannot be sufficiently obtained and the desired high strength cannot be achieved at the hot rolled coil. The mean grain size of crystal grains may, for example, be 5.5 µm or more, 6.0 µm or more, or 6.5 µm or more. Similarly, the mean grain size of crystal grains may, for example, be 7.5 µm or less or 7.0 µm or less.

[Measurement of Mean Grain Size of Crystal Grains Surrounded by Boundaries With Orientation Difference of 15° or More]

The mean grain size of crystal grains surrounded by boundaries with an orientation difference of 15° or more is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the center part in the longitudinal direction and the center part in the width direction of a hot rolled coil 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 of 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 surrounded by boundaries with an orientation difference of 15° or more is defined as a crystal grain, the circle equivalent diameters of the crystal grains are analyzed, and the mean value of these is found and determined as the mean grain size. The grain size of crystal grains defined as explained above can be determined utilizing the value of the area mean calculated by the grain size (diameter) by the function loaded in the software "OIM Analysis^™ Version 7.01" attached to the EBSD analysis apparatus.

[Mean Grain Size of Precipitates: 3.0 to 9.5 nm]

In a hot rolled coil according to an embodiment of the present invention, at the center part in the longitudinal direction, the mean grain size of precipitates is controlled to a range of 3.0 to 9.5 nm. By controlling the mean grain size of precipitates to within a range of 3.0 to 9.5 nm, it is possible to sufficiently realize the effect of improvement of strength by precipitation strengthening. By combining this with the previously explained effect of improvement of strength by fine grain reinforcement, it becomes possible to reliably achieve a high tensile strength, for example, a 780 MPa or more tensile strength, in the finally obtained hot rolled coil. The precipitates may also include Ti carbides or may be Ti carbides. The Ti carbides are not particularly limited, but, for example, may be TiC or may be composite carbides including Ti and other elements besides Ti, for example, Nb. If the grain size of the precipitates is small, the precipitates cannot sufficiently act as obstacles to dislocation motion and therefore the effect of improvement of strength by precipitation strengthening cannot be sufficiently obtained. On the other hand, even if the grain size of the precipitates is too large, similarly sometimes it is not possible to obtain the desired precipitation strengthening.
While not intending to be bound by any specific theory, it is believed that this is because by the precipitates becoming coarser, the strengthening mechanism changes in relation to dislocation motion and that, for example, the dislocation line does not pass cutting across the precipitates but passes leaving behind a loop of dislocation line around the coarse precipitates and therefore the amount of precipitation strengthening becomes small. In addition, along with coarsening of the precipitates, the number density of the precipitates also greatly falls, therefore it becomes no longer possible to sufficiently raise the strength by precipitation strengthening. Therefore, to effectively raise the strength of hot rolled coil by precipitation strengthening, it is effective to control the mean grain size of precipitates to a range of 3.0 to 9.5 nm. The mean grain size of precipitates, for example, may also be 4.0 nm or more, 5.0 nm or more, or 6.0 nm or more. Similarly, the mean grain size of crystal grains may, for example, be 9.0 nm or less, 8.0 nm or less, or 7.0 nm or less.

[Difference of Maximum Value and Minimum Value of Grain Size of Precipitates in Width Direction of 15.0% or Less of Mean Grain Size of Precipitates]

In a hot rolled coil according to an embodiment of the present invention, the difference of the maximum value and minimum value of the grain size of precipitates in the width direction of the center part in the longitudinal direction is controlled to 15.0% or less of the mean grain size of precipitates. By controlling the difference of the maximum value and minimum value of the grain size of precipitates in the width direction of the center part in the longitudinal direction to 15.0% or less of the mean grain size of precipitates, it is possible to reduce the variations of strength of the hot rolled coil in the width direction. As explained previously, it is believed that the states of precipitation of precipitates in the longitudinal direction and width direction of a hot rolled coil are closely related with each other. For this reason, for example, even if suitably cooling just the center part in the longitudinal direction of a hot rolled coil, if not suitably cooling the tip end part and/or the tail end part, it is no longer possible to reliably obtain the desired state of precipitation of precipitates at the center part affected by these as well. Similarly, for example, even if suitably cooling the tip end part, the center part, and the tail end part of the hot rolled coil in the longitudinal direction, if the cooling in the width direction is not uniformly performed, the state of precipitation of precipitates in the longitudinal direction is affected due to temperature deviation in the width direction. Therefore, as explained later in detail regarding the method of production of a hot rolled coil, it is necessary to suitably perform cooling in the longitudinal direction and width direction. Due to this, it is possible to control the variation of grain size of the precipitates at the width direction of the center part in the longitudinal direction of a hot rolled coil to within a predetermined range. More specifically, it is possible to control the difference of the maximum value and minimum value of the grain size of precipitates in the width direction of the center part in the longitudinal direction of the hot rolled coil to 15.0% or less of the mean grain size of precipitates. As a result, variation of strength of a hot rolled coil in the width direction can be remarkably suppressed or reduced.
From the viewpoint of suppressing or reducing variations of strength, the difference of the maximum value and minimum value of the grain size of precipitates in the width direction at the center part in the longitudinal direction of the hot rolled coil is preferably smaller than the mean grain size of precipitates. Therefore, at the center part in the longitudinal direction of the hot rolled coil, the difference of the maximum value and minimum value of the grain size of precipitates in the width direction is preferably 12.0% or less, 10.0% or less, or 8.0% or less of the mean grain size of precipitates. On the other hand, the lower limit is not particularly prescribed, but, for example, at the center part in the longitudinal direction of the hot rolled coil, the difference of the maximum value and minimum value of the grain size of precipitates in the width direction may be 2.0% or more, 3.0% or more, or 5.0% or more of the mean grain size of precipitates.

[Determination of Mean Grain Size of Precipitates of Center Part in Longitudinal Direction and Ratio of Difference of Maximum Value and Minimum Value of Grain Size of Precipitates in Width Direction to Mean Grain Size]

The mean grain size of precipitates of the center part in the longitudinal direction of the hot rolled coil, etc., are determined in the following way. First, when designating the total width of the hot rolled coil as "W", samples are taken by the replica method at the positions of the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from an end part in the width direction at the center part in the longitudinal direction of that hot rolled coil. Next, the obtained samples were examined using a transmission type electron microscope (TEM, for example, "JEM-2100" made by JEOL can be used) for 50 to 100 precipitates, the grain sizes of the precipitates were calculated as circle equivalent diameters, the mean values of all of the circle equivalent diameters calculated were determined as the grain sizes of precipitates at the different width direction positions, and the arithmetic mean of the obtained five grain sizes was determined as the mean grain size of precipitates at the center part in the longitudinal direction. Finally, the difference of the maximum value and minimum value among the obtained five grain sizes is calculated and the calculated value is divided by the mean grain size of precipitates to determine the ratio of the difference of the maximum value and minimum value of the grain size of precipitates in the width direction to the mean grain size. The component elements of the precipitates can be identified by EDS analysis.

[Difference of Maximum Value and Minimum Value of Grain Size of Precipitates in Longitudinal Direction of 15.0% or Less of Mean Grain Size of Precipitates]

In a hot rolled coil according to an embodiment of the present invention, the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction at the center part in the width direction is controlled to 15.0% or less of the mean grain size of precipitates. By controlling the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction at the center part in the width direction to 15.0% or less of the mean grain size of precipitates, it is possible to reduce the variation of strength of the hot rolled coil in the longitudinal direction. In relation to this, as explained later in detail regarding the method of production of a hot rolled coil, it is particularly important to suitably perform cooling the tip end part, the center part, and the tail end part at the longitudinal direction of the hot rolled coil after coiling. Due to this, it becomes possible to control the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction at the center part of the hot rolled coil in the width direction to 15.0% or less of the mean grain size of precipitates in the longitudinal direction.
From the viewpoint of suppression or reduction of the variations of strength, at the center part in the width direction of the hot rolled coil, the difference of the maximum value and minimum value of the grain size of precipitates at the longitudinal direction is preferably as small as possible with respect to the mean grain size of precipitates in the longitudinal direction. Therefore, at the center part of the hot rolled coil in the width direction, the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction is preferably 12.0% or less, 10.0% or less, or 8.0% or less of the mean grain size of precipitates in the longitudinal direction. On the other hand, the lower limit is not particularly prescribed, but for example, at the center part in the width direction of the hot rolled coil, the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction may be 1.0% or more, 2.0% or more, or 3.0% or more of the mean grain size of precipitates in the longitudinal direction.

[Determination of Mean Grain Size of Precipitates in Longitudinal Direction at Center Part in Width Direction and Ratio of Difference of Maximum Value and Minimum Value of Grain Size of Precipitates in Longitudinal Direction With Respect to That Mean Grain Size]

The mean grain size of precipitates in the longitudinal direction at the center part of the hot rolled coil in the width direction, etc., are determined in the following way. First, when designating the total length of the hot rolled coil as "L", samples are taken by the replica method at the positions of the 1/10L position, 5/10L position, and 9/10L position from an end part in the longitudinal direction at the center part in the width direction of the hot rolled coil. Next, the obtained samples are examined using a transmission type electron microscope (TEM) for 50 to 100 precipitates, the grain sizes of the precipitates are calculated as circle equivalent diameters, the mean values of all of the circle equivalent diameters calculated are determined as the grain sizes of precipitates at the different longitudinal direction positions, and the arithmetic mean of the obtained three grain sizes is determined as the mean grain size of precipitates at the longitudinal direction at the center part in the width direction. Finally, the difference of the maximum value and minimum value among the obtained three grain sizes is calculated and the calculated value is divided by the mean grain size of precipitates in the longitudinal direction to determine the ratio of the difference of the maximum value and minimum value of the grain size of precipitates in the longitudinal direction to the mean grain size of precipitates in the longitudinal direction. The component elements of the precipitates can be identified by EDS analysis.

[Ferrite: 50% or More]

The microstructure of a hot rolled coil according to an embodiment of the present invention is not particularly limited, but, for example, may include an area% of ferrite of 50% or more. The present invention, as explained above, has as its object the provision of a hot rolled coil which is high in strength and which is reduced in variations of strength. By having a predetermined chemical composition, utilizing fine grain reinforcement and precipitation strengthening, and suitably controlling the grain size of precipitates contributing to that precipitation strengthening in the width direction at the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction at the center part in the width direction, that object is achieved. Therefore, other features relating to the microstructure clearly are not technical features essential in achieving the object of the present invention. In actuality, in a hot rolled coil according to an embodiment of the present invention, even if a microstructure is comprised of a soft ferrite single phase structure, by satisfying the previously explained requirements relating to chemical composition, crystal grains, and precipitates, for example, it is possible to reliably achieve a 780 MPa or more tensile strength. The area ratio of ferrite may also, for example, be 55% or more, 60% or more, 70% or more, 80% or more, or 90% or more. The upper limit is not particularly prescribed, but, for example, the area ratio of ferrite may also be 100% and may also be 95% or less. If there are remaining structures besides ferrite present, the remaining structures basically include structures harder than ferrite, for example, martensite, bainite, pearlite, and retained austenite. For this reason, it is clear that the specific structures of the remaining structures are not limited from the viewpoint of providing a high strength hot rolled coil.
The area ratio of ferrite is determined in the following way. First, a sample having a sheet thickness cross-section in a direction parallel to the rolling direction of the hot rolled coil and vertical to the sheet surface. That cross-section is made the examined surface. It is found by observing, in the examined surface, a region of 100 µm×100 µm in the range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position in an electron channeling contrast image by an FE-SEM (field emission scan electron microscope). More specifically, it is possible to identify parts in the above region shown by even contrast as ferrite and calculate the area ratio of the same by the image analysis software Image J.

[Tensile Strength]

In a hot rolled coil according to an embodiment of the present invention, by having the chemical composition and microstructure explained above, it is possible to achieve a high tensile strength, for example, a 780 MPa or more tensile strength. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. According to a hot rolled coil according to an embodiment of the present invention, despite having such an extremely high tensile strength, by suitably controlling the grain size of precipitates in the width direction at the center part of the hot rolled coil in the longitudinal direction and the longitudinal direction at the center part in the width direction, it is possible to remarkably suppress or reduce the variations in strength in the longitudinal direction and width direction of a hot rolled coil. The upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength of the hot rolled coil may also be 980 MPa or less, 950 MPa or less, or 900 MPa or less. The tensile strength is determined in the following way. First, a No. 5 tensile test piece of JIS Z2241: 2011 having a direction parallel to the rolling direction as a test direction is taken from the center part in the longitudinal direction of the hot rolled coil and the center part in the width direction. Next, the tensile test piece is used for a tensile test compliant with JIS Z2241: 2011 so as to determine the tensile strength of a hot rolled coil according to an embodiment of the present invention.

[Total Width W]

A hot rolled coil according to an embodiment of the present invention has any total width W. While not particularly limited, for example, the total width W may also be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. The upper limit is not particularly prescribed, but, for example, the total width may also be 2500 mm or less, 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.

[Sheet Thickness]

The hot rolled coil 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 be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and/or may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

Next, a preferable method of production of a hot rolled coil 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 coil according to an embodiment of the present invention and is not intended to limit the hot rolled coil to one produced by the method of production explained below.
The method of production of a hot rolled coil according to an embodiment of the present invention comprises:

hot rolling including heating a slab having a chemical composition explained above relating to a hot rolled coil to a temperature of 1230 to 1260°C and rough rolling and finish rolling the same, wherein an exit side temperature of the rough rolling is 1070 to 1140°C, an entry side temperature (F0) of the finish rolling is 980 to 1050°C, an exit side temperature (FT) of the finish rolling is 850 to 920°C, and a total rolling reduction of the finish rolling is 85 to 95%,
cooling of primary cooling the finish rolled steel sheet in a temperature region from the exit side temperature (FT) of the finish rolling to a temperature T1 in a range of 650 to 720°C by a mean cooling speed of 60 to 100°C/s, then secondary cooling it in a temperature region from the temperature T1 to a coiling temperature CTf at a 1/10L position of a coil total length L in the longitudinal direction by a mean cooling speed of 5 to 10°C/s, wherein a top/bottom cooling ratio of a top surface of the steel sheet to the bottom surface in the primary cooling is 0.8 to 1.2 and a top/bottom cooling ratio of a top surface of the steel sheet to the bottom surface in the secondary cooling is 0.8 to 1.2, and
coiling of coiling the secondary cooled steel sheet, then soaking an edge part of the coiled steel sheet in the width direction, wherein the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the respective 1/10L position, 5/10L position, and 9/10L position of the coil total length L in the longitudinal direction satisfy the following formulas 1 to 3: $550 \leq CTm \leq 620$ $CTm + 15 \leq CTf \leq CTm + 30$ $CTm + 30 \leq CTt \leq CTm + 50$

Below, the individual steps will be explained in detail.

[Hot Rolling Step]

[Heating of Slab]

First, a slab having a chemical composition explained above in relation to a hot rolled coil is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but it may also be produced by the ingot making method or thin slab casting method. The slab used contains a relatively large amount of alloy elements, in particular contains Ti. For this reason, the alloy elements have to be made to dissolve in the slab. In particular, Ti has to be made to sufficiently dissolve. If the Ti does not sufficiently dissolve at the time of heating the slab, in the coiling step, it becomes difficult to make Ti finely precipitate in the steel as carbides (TiC), etc., to improve the strength of the steel by precipitation strengthening. Therefore, to make Ti sufficiently dissolve, the heating temperature of the slab has to be 1230°C or more. On the other hand, if the heating temperature of the slab is more than 1260°C, scale off causes the yield to drop. Therefore, the heating temperature of the slab is 1260°C or less.

[Rough Rolling]

In the present method, the heated slab is rough rolled before finish rolling for adjusting the sheet thickness, etc. In order for the rough rolling to secure the desired sheet bar dimensions and enable adjustment of the total rolling reduction in the finish rolling in a 850°C or more temperature region to within the desired range, the exit side temperature of the rough rolling is 1070 to 1140°C, preferably 1100 to 1140°C. If the exit side temperature of the rough rolling is less than 1070°C, it becomes difficult to obtain a 850°C or more exit side temperature at the finish rolling following the rough rolling. Further, if the exit side temperature of the rough rolling is more than 1140°C, the crystal grains become coarser and sometimes the toughness of the obtained hot rolled coil falls.

[Finish Rolling]

The rough rolled slab is then finish rolled. As explained above, the slab used contains relatively large amounts of alloy elements, therefore it is necessary to increase the rolling load at the time of hot rolling. For this reason, the hot rolling is performed at a high temperature under high pressure. Specifically, the entry side temperature (F0) of the finish rolling is 980 to 1050°C, the exit side temperature (FT) of the finish rolling is 850 to 920°C, and the total rolling reduction of the finish rolling is 85 to 95%. In particular, the exit side temperature (FT) of the finish rolling is important in the point of control of the microstructure of the steel sheet. More specifically, if the exit side temperature (FT) of the finish rolling is low, the microstructure becomes uneven and sometimes the formability falls and/or at the center part in the longitudinal direction, the mean grain size of crystal grains surrounded by boundaries with an orientation difference of 15° or more becomes less than 5.0 µm and the strength falls. For this reason, the exit side temperature (FT) of the finish rolling is 850°C or more. On the other hand, if the exit side temperature (FT) of the finish rolling is more than 920°C, the austenite grains coarsen and it becomes no longer possible to control the mean grain size of crystal grains obtained by subsequent cooling, more specifically the crystal grains surrounded by boundaries with an orientation difference of 15° or more, to 8.0 µm or less.

[Cooling Step]

[Primary Cooling]

In the next cooling step, the finish rolled steel sheet is first primary cooled in the temperature range from the exit side temperature (FT) of the finish rolling to the temperature T1 in a range of 650 to 720°C by a mean cooling speed of 60 to 100°C/s. By primary cooling in this temperature region by a mean cooling speed of 60 to 100°C/s, coarsening of the crystal grains is suppressed while the finally obtained microstructure can be made uniform in the width direction. In relation to this, it becomes possible to remarkably suppress variation in grain size in the width direction of TiC or other precipitates precipitating in the subsequent coiling step. If the mean cooling speed of the primary cooling is less than 60°C/s, the mean grain size of crystal grains surrounded by boundaries with an orientation difference of 15° or more sometimes cannot be controlled to within the desired range. On the other hand, if the mean cooling speed of the primary cooling is more than 100°C/s, since the cooling speed is fast, it becomes difficult to uniformly cool the steel sheet in the width direction and uneven cooling (temperature deviation) occurs in the width direction. In this case, in the finally obtained hot rolled coil, it is not possible to sufficiently suppress variation in the grain size of TiC or other precipitates in the width direction, more specifically it becomes no longer possible to control the difference of the maximum value and minimum value of the grain size of TiC or other precipitates in the width direction to 15.0% or less of the mean grain size of precipitates. Therefore, the mean cooling speed of the primary cooling is 60 to 100°C/s, preferably 65 to 85°C/s.
In the primary cooling, in addition to control of the mean cooling speed, even cooling of the steel sheet at the top surface and bottom surface is extremely important. Such cooling is performed so that the top/bottom cooling ratio of the top surface of the steel surface to the bottom surface becomes 0.8 to 1.2, more specifically so that the amount of cooling water sprayed at the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed at the bottom surface of the steel sheet. By evenly cooling the top and bottom surfaces of the steel sheet in such a way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. In relation to this, it is possible to remarkably suppress variation in the grain size in the width direction of TiC or other precipitates precipitating at the later coiling step. As a result, it becomes possible to remarkably suppress or reduce variation of strength of the finally obtained hot rolled coil in the longitudinal direction and width direction of the hot rolled coil. If the top/bottom cooling ratio is less than 0.8 or more than 1.2, it becomes difficult to perform uniform cooling even at the later coiling step due to uneven cooling in the width direction, i.e., temperature deviation. In this case, it is not possible to sufficiently suppress variation in the grain size of TiC or other precipitates in the width direction in the finally obtained hot rolled coil, i.e., to control the difference of the maximum value and minimum value of the grain size of TiC or other precipitates in the width direction to 15.0% or less of the mean grain size of precipitates. As a result, it is no longer possible to sufficiently suppress or reduce variations of strength of a hot rolled coil in a longitudinal direction and/or width direction.
Here, the "top/bottom cooling ratio" does not mean the ratio of the amount of cooling water at the top surface as a whole and the amount of cooling water at the bottom surface as a whole at the FT to T1°C span. More specifically, in the present method of production, the FT to T1°C span is divided into sections of 10 m each, the top/bottom cooling ratio is calculated for each of these sections from the amount of cooling water at the top surface and the amount of cooling water at the bottom surface, and the thus calculated top/bottom cooling ratio of each section is controlled to within a range of 0.8 to 1.2 in all cases. With control of the top/bottom cooling ratio of the span as a whole rather than the divided sections, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top/bottom cooling ratio for each section, it becomes possible to reduce local overcooling, etc., and reliably suppress uneven cooling. Further, the control of the top/bottom cooling ratio at each section can be performed by any suitable means. While not particularly limited to this, for example, since each section has a plurality of cooling water nozzles arranged at the top side and bottom side of the steel sheet along the direction of progression of the steel sheet, by suitably spraying the cooling water from these based on on/off control, it is possible relatively easily control the top/bottom cooling ratio of each section to within a range of 0.8 to 1.2.

[Secondary Cooling]

The primary cooled steel sheet is next secondary cooled in the temperature region from the temperature T1 to the coiling temperature CTf at the 1/10L position from the tip end part in the coil total length L in the longitudinal direction by a mean cooling speed of 5 to 10°C/s. By secondary cooling the temperature region from the temperature T1 to the coiling temperature CTf by a mean cooling speed of 5 to 10°C/s, it is possible to suitably form crystal grains surrounded by boundaries with an orientation difference of 15° or more and control the mean grain size to within the desired range. If the mean cooling speed of the secondary cooling is less than 5°C/s, the crystal grains surrounded by boundaries with an orientation difference of 15° or more coarsen and the mean grain size can no longer be controlled to 8.0 µm or less. In this case, the effect of improvement of strength by fine grain reinforcement cannot be sufficiently obtained and therefore it is no longer possible to reliably achieve the desired high strength at the finally obtained hot rolled coil. On the other hand, if the mean cooling speed of the secondary cooling is more than 10°C/s, sometimes crystal grains surrounded by boundaries with an orientation difference of 15° or more cannot be suitably formed or that the mean grain size of crystal grains cannot be controlled to 5.0 µm or more. In this case as well, in the same way, the effect of improvement of strength by fine grain reinforcement can no longer be sufficiently obtained. In addition, if the mean cooling speed of the secondary cooling is more than 10°C/s, sometimes variations of strength remarkably occur in the hot rolled coil in the longitudinal direction and/or width direction due to the excessive formation of hard structures.
Note that, in this Description, the X/10L position (X is a natural number of 1 to 9) at the total length L of the hot rolled coil in the longitudinal direction means a position of the hot rolled coil separated by exactly a distance "X/10L" from the tip end part in the longitudinal direction (rolling direction) toward the tail end part of the longitudinal direction. For example, if the total length L of the hot rolled coil in the longitudinal direction is 1000 m, the "1/10L position" is the position separated by exactly a distance "100 m" from the tip end part in the longitudinal direction toward the tail end part in the longitudinal direction.
In the secondary cooling, in the same way as the case of the primary cooling, in addition to control of the mean cooling speed, even cooling of the steel sheet at the top surface and bottom surface is extremely important. Such cooling, in the same way as the primary cooling, is performed so that the top/bottom cooling ratio of the top surface of the steel surface to the bottom surface becomes 0.8 to 1.2, more specifically so that the amount of cooling water sprayed at the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed at the bottom surface of the steel sheet. By evenly cooling the top and bottom surfaces of the steel sheet in such a way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. In relation to this, it is possible to remarkably suppress variation in the grain size in the width direction of TiC or other precipitates precipitating at the later coiling step. As a result, it becomes possible to remarkably suppress or reduce variation of strength of the finally obtained hot rolled coil in the longitudinal direction and width direction of the hot rolled coil. If the top/bottom cooling ratio is less than 0.8 or more than 1.2, it becomes difficult to perform uniform cooling even at the later coiling step due to uneven cooling in the width direction, i.e., temperature deviation. In this case, it is not possible to sufficiently suppress variation of the grain size of TiC or other precipitates in the finally obtained hot rolled coil, i.e., it is no longer possible to control the difference of the maximum value and minimum value of the grain size of TiC and other precipitates in the width direction to 15.0% or less of the mean grain size of precipitates. As a result, it is no longer possible to sufficiently suppress or reduce variation of strength of a hot rolled coil in the longitudinal direction and/or width direction.
Here, the "top/bottom cooling ratio" does not mean the ratio of the amount of cooling water at the top surface as a whole and the amount of cooling water at the bottom surface as a whole at the T1 to CTf°C span. More specifically, in the present method of production, the T1 to CTf°C span is divided into sections of 10 m each, the top/bottom cooling ratio is calculated for each of these sections from the amount of cooling water at the top surface and the amount of cooling water at the bottom surface, and the thus calculated top/bottom cooling ratio of each section is controlled to within a range of 0.8 to 1.2 in all cases. With control of the top/bottom cooling ratio of the span as a whole rather than the divided sections, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top/bottom cooling ratio for each section, it becomes possible to reduce local overcooling, etc., and reliably suppress uneven cooling. Further, the control of the top/bottom cooling ratio at each section can be performed by any suitable means. While not particularly limited to this, in the same way as the case of primary cooling, for example, since each section has a plurality of cooling water nozzles arranged at the top side and bottom side of the steel sheet along the direction of progression of the steel sheet, by suitably spraying the cooling water from these based on on/off control, it is possible relatively easily control the top/bottom cooling ratio of each section to within a range of 0.8 to 1.2.

[Coiling Step]

The secondary cooled steel sheet is finally coiled up in the coiling step, then the edge parts of the coiled up steel sheet in the width direction are soaked. In addition, in the coiling step, the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at respectively the 1/10L position, 5/10L position, and 9/10L position of the coil total length L in the longitudinal direction have to satisfy the following formulas 1 to 3: $550 \leq CTm \leq 620$ $CTm + 15 \leq CTf \leq CTm + 30$ $CTm + 30 \leq CTt \leq CTm + 50$
The state of precipitation of precipitates is greatly affected by the cooling history after coiling. For example, the tip end part and the tail end part of the hot rolled coil in the longitudinal direction respectively correspond to the innermost circumference part and outermost circumference part of the hot rolled coil and therefore are exposed to the atmosphere. For this reason, in general, the tip end part and the tail end part of the hot rolled coil are quick to cool. Sometimes the precipitates do not sufficiently proceed to precipitate. Therefore, as the state of precipitation, a sub-aged state is reached. The strength tends to easily fall. In this case, sometimes the desired strength is not obtained or the state of precipitation of precipitates differs in the longitudinal direction whereby variations in strength occur. On the other hand, the center part in the longitudinal direction of the hot rolled coil is not directly exposed to the atmosphere, therefore is hard to cool and is held in a relatively high temperature state. Therefore, in some cases, the precipitates coarsen and peak aging where a high effect of precipitation strengthening is obtained passes resulting in an overaged state and similarly the strength falls. Similarly in this case as well, sometimes the desired strength is not obtained or variations in strength occurs. In addition, if the cooling in the width direction after coiling is not uniformly performed, the state of precipitation of precipitates in the longitudinal direction also is affected due to temperature deviation in the width direction. As a result, similarly in the finally obtained hot rolled coil, sometimes the desired strength is not obtained or the variations in strength become remarkable.
Therefore, in the present method of production, first, by soaking the edge parts of coiled steel sheet in the width direction further directly exposed to the atmosphere, it is possible to reduce the temperature deviation of the coiled hot rolled coil in the width direction. If not treating the edge parts by soaking, the state of precipitation of precipitates in the longitudinal direction and width direction also is affected by the temperature deviation in the width direction. As a result, sometimes the mean grain size of the precipitates at the center part in the longitudinal direction and variation of the grain size of precipitates at the center part in the longitudinal direction cannot be controlled to within the desired range. Such soaking treatment of the edge parts can be performed by any suitable means known to persons skilled in the art. While not particularly limited to this, for example, the soaking treatment of the edge parts can be performed by arranging a plurality of hot rolled coils adjacent to thereby prevent cooling of the edge parts by the atmosphere. Here, "arranging a plurality of hot rolled coils adjacent" includes arranging the hot rolled coils so that the end faces (edge parts) in the width direction face each other. Further, if the diameters of the plurality of hot rolled coils are equal, the hot rolled coils are preferably arranged so that the center axes are aligned, i.e., coaxially. Further, when arranging a plurality of hot rolled coils adjacent, the distance between the end faces (edge parts) is preferably 200 to 800 mm, more preferably 200 to 600 mm, further preferably 200 to 500 mm.
In addition, in the present method of production, the hot rolled coil is also suitably cooled in the longitudinal direction. Specifically, to make the state of precipitation of precipitates suitable at the center part in the longitudinal direction of the hot rolled coil, the coiling temperature CTm (°C) at the 5/10L position of the coil total length L of the longitudinal direction is controlled to satisfy the following formula 1: $550 \leq CTm \leq 620$
If CTm is less than 550°C, the state of precipitation of precipitates at the center part becomes the sub-aged state, while if CTm is more than 620°C, the state of precipitation of precipitates at the center part becomes the overaged state. In both cases, sometimes it is not possible to sufficiently obtain the effect of improvement of strength by precipitation strengthening and/or not possible to sufficiently suppress or reduce variations of strength of the hot rolled coil in the longitudinal direction and width direction. Further, even if CTm satisfies formula 1, if CTf does not satisfy formula 2 and/or CTt does not satisfy formula 3, the state of precipitation of precipitates at the center part in the longitudinal direction is greatly affected. In this case, sometimes it is not possible to control the mean grain size of the precipitates at the center part in the longitudinal direction and the variation of grain size of the precipitates at the width direction at the center part in the longitudinal direction to within the desired range.
The tip end part of the hot rolled coil in the longitudinal direction, as explained above, corresponds to the innermost circumference part of the hot rolled coil, therefore is exposed to the atmosphere and is readily easily cooled. Therefore, the coiling temperature CTf (°C) at the 1/10L of the coil total length L in the longitudinal direction is controlled to a temperature range 15 to 30°C higher than CTm so as to satisfy formula 2: $CTm + 15 \leq CTf \leq CTm + 30$
By controlling CTf to within a temperature range 15 to 30°C higher than CTm such as shown in formula 2 considering cooling by the atmosphere, the state of precipitation of precipitates at the tip end part can be maintained in a state of precipitation similar to the center part. If CTf is less than CTm+15°C, the state of precipitation of precipitates at the tip end part becomes a sub-aged state, while if CTf is more than CTm+30°C, sometimes the state of precipitation of precipitates of the tip end part becomes the overaged state. In both cases, sometimes it is not possible to sufficiently obtain the effect of improvement of strength by precipitation strengthening and/or not possible to sufficiently suppress or reduce variations of strength of the hot rolled coil in the longitudinal direction and width direction.
The tail end part of the hot rolled coil in the longitudinal direction, as explained above, corresponds to the outermost circumference part of the hot rolled coil, therefore is further easier to cool compared with the tip end part corresponding to the innermost circumference part. Therefore, the coiling temperature CTt (°C) at the 9/10L position of the coil total length L in the longitudinal direction is controlled to a temperature range 30 to 50°C higher than CTm so as to satisfy the following formula 3: $CTm + 30 \leq CTt \leq CTm + 50$
By controlling CTm to a temperature region 30 to 50°C higher than CTm so as to satisfy formula 3 considering cooling by the atmosphere, it becomes possible to maintain the state of precipitation of precipitates at the tail end part at the state of precipitation similar to the center part. If CTt is less than CTm+30°C, the state of precipitation of precipitates at the tail end part becomes a sub-aged state. On the other hand, if CTt is more than CTm+50°C, sometimes the state of precipitation of precipitates at the tail end part becomes an overaged state. In either case, sometimes the effect of improvement of strength by precipitation strengthening cannot be sufficiently obtained and/or the variations of strength in the longitudinal direction and width direction of a hot rolled coil cannot be sufficiently suppressed or reduced.
The coiling temperature can be controlled based on the above formulas 1 to 3 by any suitable means. The method is not particularly limited, but, for example, it is possible to perform control relatively easily by suitably controlling the amount of cooling water in the secondary cooling. In the past, coiling up the hot rolled steel sheet within a predetermined temperature range is the general practice, but no operation is performed for changing the range of control of the coiling temperature at the tip end part, the center part, and the tail end part in the longitudinal direction of the hot rolled coil. Therefore, the fact that by making the cooling history of the tip end part, the center part, and the tail end part in the longitudinal direction of the hot rolled coil a suitable one and further by soaking the hot rolled coil in the width direction, it is possible to remarkably suppress or reduce the variation of strength due to precipitation strengthening was first clarified by the inventors.
According to the hot rolled coil produced by the above method of production, by controlling the mean grain size of crystal grains surrounded by boundaries with an orientation difference of 15° or more to a range of 5.0 to 8.0 µm in the center part in the longitudinal direction for fine grain reinforcement and by controlling the mean grain size of precipitates to a range of 3.0 to 9.5 nm for precipitation strengthening in combination, it becomes possible to reliably achieve a high tensile strength, for example, a 780 MPa or more tensile strength, in the finally obtained hot rolled coil. In addition, it is possible to control the differences of the maximum value and minimum value of grain size of precipitates in the width direction at the center part in the longitudinal direction of the hot rolled coil and in the longitudinal direction at the center part in the width direction to 15.0% or less of the mean grain size of precipitates. As a result, it is possible to remarkably suppress or reduce variations in strength in the longitudinal direction and width direction of the hot rolled coil. Therefore, the hot rolled coil produced by the above method of production can achieve remarkably suppressed or reduced variation in strength regardless of being high in strength. For this reason, it is possible to reduce the risk of shaping defects occurring at the time of press forming steel sheet. The productivity can also be remarkably improved. Therefore, the hot rolled coil is particularly useful in use in the automobile field naturally and can be extremely effectively used in other fields.
Below, examples will be illustrated to explain the present invention more specifically, but the present invention is not limited to these examples in any way.

EXAMPLES

First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. These slabs were heated under the conditions shown in Table 2, then were hot rolled. The hot rolling was performed by rough rolling and finish rolling. The exit side temperature of the rough rolling and the entry side temperature (F0), exit side temperature (FT), and total rolling reduction of the finish rolling were as shown in Table 2. Next, the finish rolled steel sheets were primary cooled under the conditions shown in Table 2 first in a temperature region from the exit side temperature (FT) of the finish rolling to a temperature T1 in a range of 650 to 720°C, then was secondary cooled in a temperature region from the temperature T1 to the coiling temperature CTf at the 1/10L position of the coil total length L in the longitudinal direction.
In the primary cooling and secondary cooling, the FT to T1°C span and the T1 to CTf°C span were divided into sections of 10 m each, the top/bottom cooling ratio was calculated for each of these sections from the amount of cooling water at the top surface and the amount of cooling water at the bottom surface, and cooling was performed so that the thus calculated top/bottom cooling ratio of each section was controlled to within a predetermined range. The top/bottom cooling ratios in the primary cooling and secondary cooling of Table 2 show top/bottom cooling ratios of the largest absolute values of differences from the cooling ratio 1 among those of the different sections in the primary cooling and secondary cooling. Finally, the secondary cooled steel sheets were coiled. The coiling temperature CTf (°C), CTm (°C), and CTt (°C) at the 1/10L position, 5/10L position, and 9/10L position of the coil total length L in the longitudinal direction were as shown in Table 2. Further, the soaking treatment of the edge parts after coiling was performed by placing a plurality of hot rolled coils adjacent to each other. The distance between edge parts of the adjacently arranged hot rolled coils was 300 mm. The entry "Adjacent" in Table 2 indicates soaking treatment was performed after coiling. On the other hand, the entry "Alone" in Table 2 means the hot rolled coil was air-cooled alone, i.e., indicates soaking treatment was not performed on the edge parts after coiling. The obtained hot rolled coils had about 2.3 to 4.0 mm sheet thicknesses, about 800 to 1500 mm total widths W, and about 500 to 1200 m total lengths L.

[Table 2]

Table 2

Test no.	Steel no.	Hot rolling step					Cooling step					Coiling step				Remarks
		Heating	Rough rolling	Finish rolling			Primary cooling			Secondary cooling		Length 1/10L position coiling temp. CTf	Length 5/10L position coiling temp CTm	Length 9/10L position coiling temp. CTt	Soaking after coiling
		Heating temp.	Exit side temp.	Entry side temp. F0	Exit side temp. FT	Total rolling reduction	Temp. T1	FT to T1 mean cooling speed	FT to T1 top/bottom cooling ratio	T1 to CT mean cooling speed	T1 to CTf top/bottom cooling ratio	Length 1/10L position coiling temp. CTf	Length 5/10L position coiling temp CTm	Length 9/10L position coiling temp. CTt	Soaking after coiling
		°C	°C	°C	°C	%	°C	°C/s	-	°C/s	-	°C	°C	°C	-
1	A	1235	1115	1020	901	90	680	80	1.0	8	1.1	592	571	609	Adjacent	Inv. ex.
2	B	1232	1117	1027	910	91	685	76	1.1	7	1.1	611	591	621	Adjacent	Inv. ex.
3	C	1230	1115	1022	897	87	670	82	1.1	7	1.1	596	581	616	Adjacent	Inv. ex.
4	D	1246	1124	1031	912	90	680	77	1.2	7	1.1	594	575	605	Adjacent	Inv. ex.
5	E	1250	1125	1030	920	92	675	65	1.1	8	1.2	630	610	647	Adjacent	Inv. ex.
6	F	1246	1126	1012	881	88	662	71	1.2	8	1.0	581	562	599	Adjacent	Inv. ex.
7	G	1240	1110	1028	850	90	661	81	1.2	5	1.0	582	561	602	Adjacent	Inv. ex.
8	H	1234	1123	1030	919	88	675	75	1.2	8	1.0	597	575	613	Adjacent	Inv. ex.
9	I	1248	1128	1027	906	89	688	81	1.2	6	1.0	613	588	633	Adjacent	Inv. ex.
10	J	1258	1130	997	885	87	706	75	1.1	10	1.0	625	606	645	Adjacent	Inv. ex.
11	K	1239	1110	1028	881	90	659	82	0.9	10	1.0	577	559	597	Adjacent	Inv. ex.
12	L	1245	1129	998	887	92	680	75	1.0	10	1.0	596	580	615	Adjacent	Inv. ex.
13	M	1236	1112	1015	889	90	703	82	1.2	10	1.0	619	603	635	Adjacent	Inv. ex.
14	N	1256	1128	1034	912	90	671	85	1.1	8	1.1	586	571	602	Adjacent	Inv. ex.
15	O	1246	1126	1012	885	90	680	83	1.1	5	1.1	599	580	618	Adjacent	Inv. ex.
16	A	1233	1111	1015	910	90	681	80	1.1	9	1.2	610	603	605	Alone	Comp. ex.
17	A	1244	1118	1036	917	90	678	81	1.1	7	1.1	621	601	638	Alone	Comp. ex.
18	A	1235	1105	1013	898	90	682	81	1.1	8	1.1	602	599	603	Adjacent	Comp. ex.
19	A	1256	1126	1009	887	90	682	131	1.4	5	1.1	622	602	639	Adjacent	Comp. ex.
20	P	1252	1140	1020	910	90	680	80	1.1	8	1.1	589	574	611	Adjacent	Comp. ex.
21	A	1235	1130	1041	921	90	680	85	1.1	8	1.1	596	575	611	Adjacent	Comp. ex.
22	A	1235	1120	1023	900	90	681	82	1.1	8	1.1	610	577	632	Adjacent	Comp. ex.
23	A	1234	1117	1013	888	90	667	81	1.1	8	1.1	565	545	583	Adjacent	Comp. ex.
24	A	1237	1115	1021	848	93	653	65	1.1	8	1.1	568	550	583	Adjacent	Comp. ex.
25	A	1236	1110	1028	890	91	670	55	1.1	8	1.1	584	567	604	Adjacent	Comp. ex.
26	A	1231	1111	1015	905	90	651	102	1.1	8	1.1	601	582	615	Adjacent	Comp. ex.
27	A	1232	1116	1022	899	90	655	85	1.3	8	1.1	602	583	613	Adjacent	Comp. ex.
28	A	1232	1113	1021	901	90	680	80	0.7	8	1.1	599	578	610	Adjacent	Comp. ex.
29	A	1231	1114	1022	908	90	700	75	1.1	8	1.1	644	621	661	Adjacent	Comp. ex.

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

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

[Tensile Strength of Hot Rolled Coil]

First, a No. 5 tensile test piece of JIS Z2241: 2011 having a direction parallel to the rolling direction as a test direction was taken from the center part in the longitudinal direction and center part in the width direction of the hot rolled coil. Next, the tensile test piece was used to perform a tensile test based on JIS Z2241: 2011 to thereby determine the tensile strength of the hot rolled coil.

[Variation of Strength in Longitudinal Direction]

First, No. 5 tensile test pieces of JIS Z2241: 2011 having a direction parallel to the rolling direction as a test direction were taken from the 1/10L, 5/10L, and 9/10L positions of the coil total length L in the longitudinal direction at the center part in the width direction of the hot rolled coil. Next, these tensile test pieces were used to perform tensile tests based on JIS Z2241: 2011 to thereby obtain three values of tensile strength, then the difference of the maximum value and minimum value of these was calculated and the obtained value was determined as the value of the variation of strength in the longitudinal direction. A case where this value of the variation of strength was 15.0 MPa or less was evaluated as having a passing variation of strength in the longitudinal direction while a case where it was more than 15.0 MPa was evaluated as having a failing variation of strength in the longitudinal direction.

[Variation of Strength in Width Direction]

First, No. 5 tensile test pieces of JIS Z2241: 2011 having a direction parallel to the rolling direction as a test direction were taken from positions of the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the end part of the width direction at the 1/10L position of the coil total length L in the longitudinal direction of the hot rolled coil. Next, these tensile test pieces were used to perform tensile tests based on JIS Z2241: 2011 to thereby obtain five values of tensile strength, then the difference of the maximum value and minimum value of these was calculated and the obtained value was determined as the value of the variation of strength in the width direction. Similarly, tensile tests were performed at the 5/10L and 9/10L positions of the coil total length L at the longitudinal direction of the hot rolled coil and the values of the variation of strength in the width direction at the different positions were determined. A case where all of the values of variation of strength at the 1/10L, 5/10L, and 9/10L positions of the longitudinal direction were 15.0 MPa or less was evaluated as having a passing variation of strength in the width direction while a case where it was more than 15.0 MPa was evaluated as having a failing variation of strength in the width direction.
A case where the tensile strength of the hot rolled coil was 780 MPa or more and the variations of strength in both of the longitudinal direction and the width direction were "passing" was evaluated a hot rolled coil which was high in strength and was reduced in variation of strength. The results are shown in Table 3.

[Table 3]

Table 3

Test no.	Steel no.	Microstructure				Tensile strength	Strength variation				Remarks
		Longitudinal direction center part			Width direction center part	Width center (5/10L)	Longitudinal direction variation	Width direction variation
		Mean grain size of crystal grains	Mean grain size of precipitates	Width direction variation of precipitate size	Longitudinal direction variation of precipitate size	Width center (5/10L)	Longitudinal direction variation	Coil tip end part (1/10L)	Coil center part (5/10L)	Coil tail end part (9/10L)
		µm	nm	%	%	MPa	MPa	MPa	MPa	MPa
1	A	6.1	6.2	8.3	5.2	801	3.9	8.5	8.5	8.0	Inv. ex.
2	B	6.6	8.0	14.5	14.7	818	11.7	12.0	13.1	7.2	Inv. ex.
3	C	5.9	8.3	9.5	11.7	851	12.2	11.4	12.2	4.2	Inv. ex.
4	D	6.7	6.0	5.5	3.5	786	1.6	3.3	4.8	4.8	Inv. ex.
5	E	7.2	9.5	10.0	4.6	882	4.2	11.9	11.6	13.7	Inv. ex.
6	F	5.4	4.6	8.3	7.1	794	7.4	7.9	11.6	11.3	Inv. ex.
7	G	5.0	5.3	9.9	7.2	795	7.5	7.8	11.4	3.9	Inv. ex.
8	H	7.1	5.8	10.8	7.1	801	7.1	13.1	12.1	12.1	Inv. ex.
9	I	6.4	9.4	12.9	2.2	846	1.4	10.6	13.6	7.4	Inv. ex.
10	J	5.5	9.5	13.1	3.2	887	2.4	10.3	12.8	9.1	Inv. ex.
11	K	5.4	6.1	7.9	6.1	805	7.2	11.6	11.2	14.8	Inv. ex.
12	L	5.6	7.7	12.6	8.1	837	7.9	13.4	14.3	11.0	Inv. ex.
13	M	5.6	9.4	8.1	9.1	834	3.7	4.1	3.7	2.2	Inv. ex.
14	N	6.7	7.5	10.6	3.3	836	2.2	14.3	14.1	14.3	Inv. ex.
15	O	5.5	7.6	11.4	2.5	845	1.7	13.1	13.4	13.4	Inv. ex.
16	A	6.6	9.9	56.2	27.1	837	20.4	34.1	54.5	34.1	Comp. ex.
17	A	7.0	10.0	59.9	5.4	830	2.8	29.8	56.8	13.5	Comp. ex.
18	A	6.0	8.7	15.1	86.2	848	84.8	10.4	11.4	6.8	Comp. ex.
19	A	5.6	8.8	41.2	4.9	857	2.8	49.4	36.2	20.4	Comp. ex.
20	P	8.6	6.1	7.8	5.0	752	3.4	7.4	7.4	7.0	Comp. ex.
21	A	9.2	6.2	8.4	5.3	756	3.9	8.5	8.5	8.0	Comp. ex.
22	A	6.1	7.2	10.3	18.8	808	17.6	9.0	9.9	9.0	Comp. ex.
23	A	5.6	2.9	2.9	9.2	731	9.9	9.9	10.4	10.9	Comp. ex.
24	A	4.9	3.4	14.5	10.0	763	11.1	10.1	10.3	12.3	Comp. ex.
25	A	8.2	5.7	10.1	7.1	772	10.3	10.2	11.1	9.8	Comp. ex.
26	A	5.9	7.9	23.0	11.1	802	7.0	15.3	21.0	19.0	Comp. ex.
27	A	6.0	7.8	31.0	9.9	805	8.3	21.3	23.4	23.1	Comp. ex.
28	A	6.1	7.9	16.0	8.9	803	6.8	15.9	16.8	15.3	Comp. ex.
29	A	6.2	12.0	15.3	8.0	745	11.3	11.2	12.3	13.1	Comp. ex.

Underlines indicate outside scope of present invention or properties which are not preferable.

If referring to Tables 1 to 3, in Comparative Example 16, the coiling temperatures CTf and CTt were low and soaking was not performed after coiling, therefore it was not possible to control the mean grain size of the precipitates at the center part in the longitudinal direction, the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction, and the variation of grain size of the precipitates in the longitudinal direction at the center part in the longitudinal direction to within the desired ranges. As a result, the variations of strength in the longitudinal direction and width direction became remarkable. In Comparative Example 17, soaking was not performed after coiling, therefore it was not possible to control the mean grain size of the precipitate at the center part in the longitudinal direction and the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction to within the desired ranges. As a result, the variation of strength in the width direction became remarkable. In Comparative Example 18, the coiling temperatures CTf and CTt were low, therefore it was not possible to control the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction and the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction to within the desired ranges. As a result, the variation of strength in the longitudinal direction became remarkable. In Comparative Example 19, the mean cooling speed of the primary cooling was high and, further, the top/bottom cooling ratio of the primary cooling was not suitable, therefore uneven cooling occurred and due to this it was not possible to control the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction to within the desired range. As a result, the variation of strength in the width direction became remarkable. In Comparative Example 20, the Nb content was low, therefore the crystal grains were insufficiently refined and the effect of improvement of strength by fine grain reinforcement could not be sufficiently obtained. As a result, it was not possible to achieve the desired tensile strength. In Comparative Example 21, the exit side temperature (FT) of the finish rolling was high, therefore it is believed the austenite grains coarsened. As a result, even with subsequent cooling, it was not possible to sufficiently refine the mean grain size of crystal grains and was not possible to achieve the desired tensile strength. In Comparative Example 22, the coiling temperatures CTf and CTt were high, therefore it was not possible to control the variation of grain size of the precipitates in the longitudinal direction at the center part in the longitudinal direction to the desired range. As a result, the variation of strength in the longitudinal direction became remarkable. In Comparative Example 23, the coiling temperature CTm was low, therefore the mean grain size of precipitates at the center part in the longitudinal direction became small and the desired tensile strength could not be achieved.
In Comparative Example 24, the exit side temperature (FT) of the finish rolling was low, therefore the mean grain size of crystal grains at the center part in the longitudinal direction became small and the desired tensile strength could not be achieved. In Comparative Example 25, the mean cooling speed of the primary cooling was low, therefore it was not possible to control the mean grain size of crystal grains at the center part in the longitudinal direction to within the desired range and similarly the desired tensile strength could not be achieved. In Comparative Example 26, the mean cooling speed of the primary cooling was high, therefore uneven cooling occurred and due to this it was not possible to control the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction to within the desired range. As a result, the variation of strength in the width direction became remarkable. In each of Comparative Examples 27 and 28, the top/bottom cooling ratio of the primary cooling was not suitable, therefore uneven cooling occurred and due to this it was not possible to control the variation of grain size of the precipitates in the width direction at the center part in the longitudinal direction to within the desired range. As a result, the variation of strength in the width direction became remarkable. In Comparative Example 29, the coiling temperature CTm was high, therefore the precipitates at the center part in the longitudinal direction became coarser, the mean grain size increased, and the desired tensile strength could not be achieved.
In contrast to this, in the hot rolled coils according to all of the invention examples, it was possible to achieve a 780 MPa or more tensile strength by having a predetermined chemical composition and further by suitably controlling the conditions in the method of production to control the mean grain size of crystal grains surrounded by boundaries with an orientation difference of 15° or more at the center part in the longitudinal direction to within a range of 5.0 to 8.0 µm for fine grain reinforcement and to control the mean grain size of precipitates to within a range of 3.0 to 9.5 nm for precipitation strengthening in combination. In addition, it was possible to control the difference of the maximum value and minimum value of the grain size of precipitates in the width direction of the center part in the longitudinal direction of the hot rolled coil and the longitudinal direction of the center part in the width direction to 15.0% or less of the main grain size of the precipitates (in Table 3, "width direction variation of precipitate grain size" and "longitudinal direction variation of precipitate grain size") and, as a result, remarkably suppress or reduce the variations of strength in the longitudinal direction and width direction of a hot rolled coil. Further, the inventors analyzed the microstructure of the obtained hot rolled coils and as a result, in the hot rolled coils according to all of the invention examples, the area ratio of ferrite was 90% or more.

Claims

A hot rolled coil having a chemical composition comprising, by mass%,
C: 0.050 to 0.100%,

Si: 0.01 to 0.30%,

Mn: 1.30 to 2.10%,

Ti: 0.080 to 0.150%,

Nb: 0.020 to 0.050%,

Al: 0.001 to 0.050%,

P: 0.100% or less,

S: 0.050% or less,

N: 0.0050% or less,

O: 0.0050% or less,

B: 0 to 0.0050%,

Cu: 0 to 0.20%,

Ni: 0 to 0.20%,

Sn: 0 to 0.10%,

Cr: 0 to 0.40%,

Mo: 0 to 0.200%,

V: 0 to 0.100%,

As: 0 to 0.100%,

Zr: 0 to 0.100%,

Ca: 0 to 0.0050%,

Mg: 0 to 0.100%,

Bi: 0 to 0.020%,

Co: 0 to 0.20%,

W: 0 to 0.20%,

Zn: 0 to 0.20%,

REM: 0 to 0.1000%, and

balance: Fe and impurities, and

a microstructure, wherein
at a center part in a longitudinal direction,

when defining a region surrounded by boundaries with an orientation difference of 15° or more as a "crystal grain", a mean grain size of crystal grains is 5.0 to 8.0 µm,

a mean grain size of precipitates is 3.0 to 9.5 nm, and

a difference of a maximum value and minimum value in grain size of the precipitates in a width direction is 15.0% or less of the mean grain size of the precipitates, and

at a center part in the width direction, a difference of a maximum value and minimum value in grain size of the precipitates in the longitudinal direction is 15.0% or less of the mean grain size of the precipitates in the longitudinal direction.
The hot rolled coil according to claim 1, wherein the chemical composition comprises, by mass%, at least one of
B: 0.0001 to 0.0050%,

Cu: 0.01 to 0.20%,

Ni: 0.01 to 0.20%,

Sn: 0.01 to 0.10%,

Cr: 0.01 to 0.40%,

Mo: 0.001 to 0.200%,

V: 0.001 to 0.100%,

As: 0.001 to 0.100%,

Zr: 0.001 to 0.100%,

Ca: 0.0001 to 0.0050%,

Mg: 0.001 to 0.100%,

Bi: 0.001 to 0.020%,

Co: 0.01 to 0.20%,

W: 0.01 to 0.20%,

Zn: 0.01 to 0.20%, and

REM: 0.0001 to 0.1000%.
The hot rolled coil according to claim 1 or 2, wherein the hot rolled coil has an effective Ti amount of 0.070% or more.