JP2002256389A - High tensile strength hot rolled steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a relatively inexpensive high tensile strength steel sheet which has excellent strength, ductility, hole expandability and weldability. SOLUTION: The high tensile strength steel sheet has a composition containing, by mass, 0.05 to 0.25% C, 0.5 to 3.0% Si, 0.8 to 2.5% Mn, 0.020 to 1.5% sol.Al and 0.0002 to 0.0050% B and the balance Fe with inevitable impurities, and in has the value of (Si+sol.Al+255B) of 1.2 to 3.5. The steel sheet further has a structure essentially consisting of polygonal ferrite and containing >=5% retained austenite by volume. Steel having the above composition is subjected to hot rolling which is started from the temperature region of the Ac3 point or higher and finished at 780 to 840 deg.C. The steel is next cooled to 300 to 500 deg.C at a cooling rate of 10 to 50 deg.C/s and is coiled to produce the steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet suitable as a steel sheet for high-strength members used in automobiles, industrial machines, and the like, and a method for producing the same. In particular, the high-tensile hot-rolled steel sheet according to the present invention is suitable as a steel sheet for high-strength members requiring ductility and hole-expandability.

[0002]

2. Description of the Related Art Today, hot-rolled steel sheets manufactured by continuous hot rolling are widely used in various fields as relatively inexpensive structural materials. Since these materials are often applied to members formed by press working or the like, they are materials that have high requirements for both strength and workability.

[0003] However, the strength and the workability are contradictory. When the strength of the steel sheet is increased, the workability decreases, and it is generally difficult to achieve both the strength and the workability. As a steel sheet having high strength and high ductility, for example, Japanese Patent Application Laid-Open
Dual P as described in US Pat.
Hase steel (hereinafter, referred to as “DP steel”) is known. DP steel is a two-phase structure steel composed of a ferrite phase and a martensite phase, and is characterized by a low yield ratio and high ductility.

However, DP having excellent strength-ductility balance
Even in the case of steel, in the case of a 590 MPa class high-strength steel sheet (tensile strength (TS): 590 MPa), the elongation (El) is about 30% and the value of “TS × El” is less than 20,000, 78
0MPa class high-strength steel sheet (tensile strength (TS): 780MP)
In a), the elongation (El) is about 20% and “TS × E”
l "is less than 18000, and DP
No higher ductility can be expected for steel.

On the other hand, apart from DP steel, as a means for greatly improving the ductility of a high-strength steel sheet, steel utilizing transformation-induced plasticity (TRIP) of retained austenite (hereinafter referred to as “residual austenitic steel”). Has been developed, and a manufacturing method thereof is disclosed in, for example, Japanese Patent Application Laid-Open No. 55-145121. According to the production method disclosed in the above publication, tensile strength (TS): 1080 MPa
As described above, the elongation (El) is 22% or more, and “TS × E”
It is considered that it is possible to manufacture a high-ductility and high-strength steel sheet in which the value of “l” exceeds 24000. However, in the above method, since the C content needs to be as high as 0.40 to 0.85% by weight, the obtained steel sheet has poor weldability and can be applied to applications requiring weldability. However, the range in which the steel plate can be applied to, for example, automobile members has been limited.

As an attempt to solve the above problems, Japanese Patent Application Laid-Open No. 63-4017 discloses a method in which C: 0.2% by weight.
%, Si: 1.5%, and Mn: 1.5%, hot-rolled, and finish-rolled near the Ar ₃ point, then to 40 ° C.
A method is disclosed in which a steel is cooled at a cooling rate of at least / s and then wound around 400 ° C. to secure retained austenite under a low C content and to increase the ductility of steel.
However, although the hot-rolled steel sheet obtained by the manufacturing method disclosed in the above publication has a higher elongation (El) than the DP steel, the hole expandability (HEL), which is one of the important indexes of workability, is used. Is not improved and its value is 30
%. Therefore, it has been difficult to apply the method to a member requiring stretch flangeability.

On the other hand, the applicant of the present invention has disclosed in
No. 2846, a method of improving the hole expandability of a retained austenite steel by including a proper amount of Al and Si in a steel and forming a structure mainly composed of polygonal ferrite containing a retained austenite in a volume ratio of 5% or more. Is disclosed. The hot-rolled steel sheet disclosed in the above publication has strength,
Although it has excellent properties in terms of ductility, weldability, and hole expandability, it needs to contain a large amount of Al, so there is room for improvement in terms of economy.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel which contains a sufficient amount of austenite to obtain high ductility due to the TRIP effect within a range of a C content which can ensure weldability, and provides a good hole. An object of the present invention is to provide a high-strength hot-rolled steel sheet which has expandability and is also economical, and a method for producing the same.

[0009]

Means for Solving the Problems In order to achieve the above object, the present inventors have studied in detail how to improve the economic efficiency of the retained austenitic steel disclosed in Japanese Patent Application Laid-Open No. 5-112846. As a result, the following new knowledge was obtained.

(A) B is considered to be an undesirable element from the viewpoint of austenite stabilization required for retained austenitic steel, since B has the effect of improving hardenability and suppresses ferrite formation. By adding an appropriate amount of B to the steel, the action of promoting the formation of polygonal ferrite by Si and Al is promoted, the retention of austenite is promoted, the ductility is improved, and the second phase is uniformly dispersed, and Expandability is improved.

Although the mechanism of promoting the formation of polygonal ferrite by Si and Al by promoting the inclusion of B is not clear, the recrystallization of austenite in the finish rolling process is suppressed and the accumulation of strain is promoted. It is considered that the ferrite transformation in the cooling process after finish rolling is promoted by the induction of strain.

(B) Further, by adding an appropriate amount of B, stabilization of austenite is promoted and ductility is improved. The mechanism by which B is added to stabilize austenite is not clear, but in addition to the above-mentioned polygonal ferrite formation promoting action, bainite transformation is promoted in the cooling process after winding to stabilize austenite. It is thought that.

(C) Accordingly, by adding a suitable amount of Si, Al and B to a steel having a C content within a range in which weldability can be ensured, the residual austenite disclosed in JP-A-5-112846 is disclosed. High tensile hot rolled steel with a lower Al content and a structure mainly composed of polygonal ferrite containing retained austenite with a volume fraction of 5% or more, and having excellent strength, ductility, weldability and hole expandability. Steel plates can be obtained at lower cost.

(D) Further, the high-tensile hot-rolled steel sheet can be stably manufactured with good reproducibility by rolling a steel slab having a predetermined chemical composition under specific conditions. The present invention has been completed based on these findings, and the gist of the present invention is to provide a high-tensile hot-rolled steel sheet described in the following items (1) to (4) and a steel sheet described in the items (5) and (6). The method is for manufacturing a high-tensile hot-rolled steel sheet.

(1) In mass%, C: 0.05 to 0.25
%, Si: 0.50 to 3.0%, Mn: 0.8 to 2.5
%, Sol. Al: 0.020 to 1.5%, B: 0.0
002-0.0050%, and has a chemical composition in which a value a defined by the following formula (i) is 1.2-3.5, and further contains 5% or more of retained austenite by volume ratio. A high-strength hot-rolled steel sheet characterized by having a structure mainly composed of polygonal ferrite. a value = Si + sol. Al + 255B (i) Here, Si, sol. Al and B are numerical values when the respective chemical compositions are represented by mass%.

(2) Further, in mass%, Ca: 0.00
02 to 0.010%, Zr: 0.01 to 0.10%, R
EM: The high tensile strength hot-rolled steel sheet according to the above (1), which is a chemical composition containing one or more of 0.002 to 0.10% of EM.

(3) Further, in mass%, Nb: 0.05
0 to 0.10%, Ti: 0.005 to 0.10%, V:
The high-tensile hot-rolled steel sheet according to the above item (1) or (2), which has a chemical composition containing one or more kinds of 0.005 to 0.20%.

(4) Further, in mass%, Cr: 0.05
-1.0%, Mo: one or two of 0.05-1.0%
The high tensile strength hot-rolled steel sheet according to the above (1), (2) or (3), wherein the high tensile strength steel sheet has a chemical composition containing at least one species.

(5) The steel having the chemical composition described in any one of the above items (1) to (4) is applied to a steel starting from a temperature range of ₃ or more Ac and ending at 780 to 840 ° C. Hot rolling, and then at a cooling rate of 10 to 50 ° C./s for 3 hours.
A method for producing a high-tensile hot-rolled steel sheet, comprising cooling the sheet to a temperature of from 00 to 500 ° C and winding the sheet.

(6) The steel having the chemical composition described in any one of the above items (1) to (4) is applied to a steel having a temperature range of _three or more Ac and ending at 780 to 940 ° C. Hot rolling at 60 ° C. at a cooling rate of 10 ° C./s or more.
Cool to a temperature range of 0 to 700 ° C, and in that temperature range,
Air-cool for 0 seconds, and further at a cooling rate of 20 ° C / s or more for 30 seconds.
A method for producing a high-tensile hot-rolled steel sheet, comprising cooling to 0 to 500 ° C and winding.

Here, the term "structure mainly composed of polygonal ferrite" means that a structure other than retained austenite (eg, bainite) does not significantly affect the properties of polygonal ferrite. Organization. "

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below. In addition,% of the chemical composition described below
The indication means% by mass.

A. Chemical composition C: C is an element effective in concentrating in untransformed austenite with the progress of ferrite transformation in the cooling process after hot rolling to stabilize austenite and increasing the strength of a steel sheet. If the C content is less than 0.05%, the effect of improving the strength of the steel sheet and the effect of stabilizing austenite are not sufficient. On the other hand, if the C content exceeds 0.25%,
In addition to deterioration in weldability, the amount of polygonal ferrite decreases and the amount of bainite increases excessively, so that hole expandability also deteriorates. Therefore, the C content is 0.05 to 0.25.
%. Preferably it is 0.08 to 0.20%.

Si: Since Si has the effect of promoting the formation of ferrite, assisting the enrichment of C into untransformed austenite and delaying the precipitation of cementite,
It is an extremely effective element for retaining austenite. Further, it is an element effective for increasing the strength of the steel sheet by solid solution strengthening of polygonal ferrite. If the Si content is less than 0.5%, the effect of the above-described action will be insufficient. If the Si content is more than 3.0%, the effect of the above-described effect is saturated, and hard martensite is generated, which causes deterioration of hole expandability. Therefore, the Si content is in the range of 0.5% to 3.0%. Preferably 1.0 to
2.5%.

Sol. Al: sol. Al is a ferrite stabilizing element similar to Si, and has an effect of promoting the formation of polygonal ferrite to assist the concentration of C into untransformed austenite and delaying the precipitation of cementite. It is an extremely effective element for causing sol. Al content of 0.020
If the amount is less than%, the effect obtained by the above-mentioned action will be insufficient. so
l. When the Al content is more than 1.5%, the effect of the above-mentioned action is saturated, and economic efficiency is impaired. Therefore, sol. The Al content is set to 0.020 to 1.5%. Preferably 0.1
~ 1.0%.

B: B promotes the effect of promoting the formation of ferrite by Si and Al and has the effect of stabilizing austenite. If the B content is less than 0.0002%, the effect of the above-mentioned action will be insufficient. If the B content is more than 0.0050%, the effect of the above-mentioned action is saturated, and the generation of carboides such as Fe ₃ (C, B) inhibits austenite from remaining. Therefore, the B content is set to 0.0002 to 0.0050%. Preferably it is more than 0.0005% and 0.0035% or less.

A value: as described above, Si and Al
Has the effect of promoting the retention of austenite, and B
Has an effect of promoting the above-mentioned action and further stabilizing austenite. In order to exhibit these effects effectively, the value a defined by the following formula (i) is set to 1.2 or more. On the other hand, if the value of a exceeds 3.5, the effect of the above operation is saturated and the economic efficiency is impaired. Therefore, the upper limit of the value a is set to 3.5. a value = Si + sol. Al + 25
5B (i) where Si, sol. Al and B are numerical values when the respective chemical compositions are represented by mass%.

In the range satisfying the above range, the Si content is preferably set to 1.0% or more in order to improve the workability of the steel sheet. In order to improve the economic efficiency, sol.
Preferably, the Al content is less than 0.8%.

Mn: Mn is an austenite stabilizing element and has an effect of lowering the Ms point of untransformed austenite, improving hardenability, and suppressing the transformation of untransformed austenite into pearlite. But,
If the Mn content is less than 0.8%, the effect of the above-described action may not be sufficiently obtained. On the other hand, if the content exceeds 2.5%, it becomes difficult to sufficiently produce polygonal ferrite in the cooling process after hot rolling, and therefore, the concentration of C in the untransformed austenite becomes insufficient and the austenite May not be stabilized. Therefore, the content of Mn is set to 0.8 to 2.5%. More preferably, it is 0.8 to 2.0%.

As elements other than the above elements, as described below, Ca: 0.0002 to 0.01%, Z:
r: 0.01 to 0.10%, rare earth element (REM):
0.002 to 0.10%, Ti: 0.005 to 0.10
%, Nb: 0.005 to 0.10%, V: 0.005 to
0.20%, Cr: 0.05 to 1.0%, Mo: 0.0
One or more of 5 to 1.0% may be contained as necessary.

Ca, Zr, rare earth element (REM): Since these components all have the effect of adjusting the shape of inclusions and improving the cold workability, one or more of these components may be used as necessary. It is preferable to contain two or more kinds. If the content is less than 0.0002% in the case of Ca, less than 0.01% in the case of Zr, and less than 0.01% in the case of the rare earth element, the effect of the above-mentioned effect is obtained. In some cases, it cannot be obtained sufficiently. On the other hand, the Ca content is 0.01%
If the content is too high, the Zr content is more than 0.10%, or the rare earth element is more than 0.10%, the inclusions in the steel become excessive and workability deteriorates. Therefore, when including
Ca content is 0.0002 to 0.01%, Zr content is 0.01 to 0.10%, and rare earth element content is 0.01 to 0.01%.
It is good to be 0.10%.

Ti, Nb, V: Ti, Nb, and V all precipitate as carbonitride on ferrite ground and are effective elements for further increasing the strength of the steel sheet. Therefore, it is preferable to include one or more kinds as necessary. If the content of any of the elements is less than 0.005%, the effect of the above-mentioned effect may not be sufficiently obtained. Also, Nb and Ti each exceed 0.10%, and V is 0.1%.
Even if the content exceeds 20%, the above effect is saturated and is not economical. Therefore, when it is contained, T
i content 0.005 to 0.10%, Nb content 0.1%.
005 to 0.10%, V content 0.005 to 0.20
It is good to be%.

Cr, Mo: Cr and Mo have the effect of improving hardenability and leaving austenite. Therefore, one or two kinds may be contained as needed. If the content of any of the elements is less than 0.05%, the effect of the above-mentioned action may not be sufficiently obtained. Also,
Even if the content exceeds 1.0%, the effect of the above-mentioned action is saturated, and economic efficiency is impaired. Therefore, the contents of Cr and Mo are preferably set to 0.05 to 1.0%, respectively.

The balance is Fe and inevitable impurities.
The unavoidable impurities include P, S, N, Cu, N
Although i, Sn and the like can be mentioned, for example, the contents of P, S and N are desirably set as follows.

P: Since P is an impurity element which has an adverse effect on weldability, its content is desirably suppressed to 0.05% or less, and 0.010% when polygonal ferrite is more uniformly dispersed. It is more desirable to set the following.

S: S is an impurity element that forms sulfide-based inclusions and lowers workability.
It is desirably suppressed to 0.05% or less, and more preferably 0.003% or less when more excellent workability is required.

N: Since N is an impurity element which lowers the workability, its content is preferably suppressed to 0.01% or less. Steel of the chemical composition described above, for example, converter, electric furnace,
Alternatively, it is melted in a flat furnace or the like. The steel type may be any of rimed steel, capped steel, semi-killed steel, and killed steel. Also, the production of the billet may be performed by any of ingot-bulking rolling and continuous casting.

B. Manufacturing method The hot-rolled steel sheet of the present invention having the chemical composition described in the section a and having a structure mainly composed of polygonal ferrite containing 5% or more of retained austenite by volume ratio is a steel having the above chemical composition. , Starting from the temperature range of Ac ₃ points, 780-840 ° C.
, Followed by cooling to 300 to 500 ° C. at a cooling rate of 10 to 50 ° C./s and winding.

That is, in order to start from a state in which the alloy element is completely dissolved in austenite, hot rolling is started from a temperature range of _three or more Ac. In addition, when heating in a heating furnace in order to set the steel to a temperature range of _three or more Ac,
The steel to be charged into the heating furnace may be a slab kept at a high temperature after casting or a slab left at room temperature.

By ending hot rolling at 780 to 840 ° C., austenite can be refined and work hardening can be caused in austenite to promote the formation of polygonal ferrite.
A sufficient amount of polygonal ferrite can be generated during cooling after finish rolling at a cooling rate of ° C / s.

Here, the temperature at which the hot rolling is completed (hereinafter, referred to as “hot rolling”)
Also referred to as “hot rolling end temperature”. ) Is less than 780 ° C., ferrite is formed during hot rolling to become a processed ferrite, and thus the workability of the hot-rolled steel sheet is deteriorated. On the other hand, when the hot-rolling end temperature exceeds 840 ° C., work hardening of austenite becomes insufficient, so that polygonal ferrite is not sufficiently generated, and retained austenite decreases.

The cooling rate after hot rolling is 10 ° C./s.
If it is less than 1, pearlite is generated during cooling, and no austenite remains. On the other hand, if the cooling rate exceeds 50 ° C./s, a sufficient amount of polygonal ferrite is not formed, and no austenite remains.

Further, if the winding temperature exceeds 500 ° C., pearlite is formed and austenite does not sufficiently remain. On the other hand, when the winding is performed in a temperature range lower than 300 ° C., the formation of martensite is promoted, and the ductility and the hole expanding property are deteriorated.

The high-strength hot-rolled steel sheet according to the present invention can be used in addition to the above-described method,
Ac hot rolling is performed starting from a temperature range of ₃ points or more and ending at 780 to 940 ° C., and then cooled to a temperature range of 600 to 700 ° C. at a cooling rate of 10 ° C./s or more. It can also be manufactured by air cooling for 10 to 10 seconds, cooling to 300 to 500 ° C. at a cooling rate of 20 ° C./s or more, and winding.

That is, by finishing hot rolling at 780 to 940 ° C., austenite can be refined, and then cooled to a temperature range of 600 to 700 ° C., and air-cooled at that temperature range for 2 to 10 seconds. In addition, the formation of polygonal ferrite is promoted, and the concentration of C in untransformed austenite is promoted, so that a sufficient amount of austenite can be left.

In this case, the temperature at which air cooling is performed (hereinafter, also referred to as “air cooling temperature”) is less than 600 ° C., and the time for performing air cooling (hereinafter, also referred to as “air cooling time”) is less than 2 seconds. If the air cooling temperature exceeds 700 ° C. or the air cooling time exceeds 10 seconds, pearlite is generated and austenite does not remain.

The reason for cooling at a cooling rate of 10 ° C./s or more after hot rolling is to secure an air cooling time of 2 to 10 seconds on a limited hot runtable. Further, the cooling at 20 ° C./s or more after the air cooling is necessary in order to suppress the generation of pearlite.

Regarding the winding temperature, as in the case of the above alternative method, if the temperature exceeds 500 ° C., pearlite is formed,
If the austenite does not sufficiently remain, and if the winding is performed in a temperature range lower than 300 ° C., the formation of martensite is promoted, and the ductility and hole-expandability deteriorate.

The high-strength hot-rolled steel sheet according to the present invention has a level of weldability required for a steel sheet for high-strength members because the C content is 0.05 to 0.25%, and further has a TR.
Contains sufficient austenite to utilize IP. Further, hole expandability is sufficiently improved. In addition, by containing an appropriate amount of B, the Al content can be suppressed as compared with the conventional case, so that it is also excellent in economic efficiency.

In the hot rolling, the entire rough bar may be reheated or maintained at a temperature by a rough bar heating device, a heat insulating cover, or the like for the purpose of suppressing a temperature drop of the rough bar.

The high-strength hot-rolled steel sheet according to the present invention is subjected to a surface treatment such as hot-dip galvanizing, galvannealed galvanizing, and electroplating, so that a surface-treated steel sheet having excellent ductility and hole-expanding properties is obtained. Can be obtained.

[0052]

EXAMPLE A slab composed of steel types A to AB having the chemical compositions shown in Table 1 was subjected to heating, hot rolling, controlled cooling and winding under the conditions shown in Tables 2 and 3 to obtain a sheet having a thickness of 2.3 mm. A hot rolled steel sheet was obtained. Next, a JIS No. 5 tensile test piece was sampled from the obtained steel sheet, and its mechanical properties were examined. In addition, the size is 2.
14mm on a steel plate specimen of 3mm thickness x 120mm width x 120mm length
After punching out a hole of φ with a clearance of 15%, the hole expandability was examined using a conical punch. Further, a test piece for an X-ray test was sampled from the center of the steel sheet, and the amount of retained austenite was investigated. The results are shown in Tables 2 and 4.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

As shown in Tables 2 and 4, the hot-rolled steel sheet manufactured by the manufacturing method of the present invention has a tensile strength (TS):
High strength of 490 MPa or more and elongation (El): 30%
With the above excellent ductility, the hole expansion rate (HE
L): It shows excellent stretch flangeability of 80% or more, and satisfies both high strength and high workability at the same time.

On the other hand, in Test Nos. 26 and 60 using the steel type V having a C content exceeding the specified range of the present invention, the elongation is relatively high but the hole expandability is poor. Si, sol. Al,
Steel types W, Z, A whose B content is outside the specified range of the present invention
Test numbers 27, 30 to 32, 61, 6 using A and AB
In the case of 4 to 66, elongation or hole expanding property is inferior. Test No. 28 using steel grade X having a C content below the specified range of the present invention
And 62, the elongation is relatively high, but the strength-elongation balance is poor. In Test Nos. 29 and 63 using the steel type Y whose Mn content is below the specified range of the present invention, both the strength and the elongation are inferior.

Test Nos. 22 and 59 under conditions where the winding temperature is outside the specified range of the present invention, the hot-rolling end temperature, the cooling rate after hot rolling, the air cooling temperature, the air cooling time, and the cooling rate after air cooling are specified by the present invention. Test numbers 23-25, 54-58 outside the range
In any of the steel sheets obtained in the above, retained austenite was not sufficiently generated, the elongation was low, and the hole expandability was not good.

[0059]

According to the present invention, a high-tensile hot-rolled steel sheet excellent in strength, ductility, hole expandability and weldability can be obtained at a relatively low steel material cost. In addition, by applying the high-tensile hot-rolled steel sheet according to the present invention to industrial equipment members such as automobile undercarriage parts, the performance of those products can be improved.

Claims (6)

[Claims] 1. A mass% of C: 0.05 to 0.25%,
Si: 0.50 to 3.0%, Mn: 0.8 to 2.5%,
sol. Al: 0.020 to 1.5%, B: 0.000
2 to 0.0050%, and has a chemical composition having an a value defined by the following formula (i) of 1.2 to 3.5, and further contains 5% or more of retained austenite by volume. A high-strength hot-rolled steel sheet characterized by having a structure mainly composed of polygonal ferrite. a value = Si + sol. Al + 255B (i) Here, Si, sol. Al and B are numerical values when the respective chemical compositions are represented by mass%. 2. In addition, Ca: 0.0002 by mass%.
-0.010%, Zr: 0.01-0.10%, RE
The high-strength hot-rolled steel sheet according to claim 1, wherein the chemical composition contains one or more of M: 0.002 to 0.10%. 3. Nb: 0.050% by mass%
0.10%, Ti: 0.005 to 0.10%, V: 0.
3. A chemical composition containing 005 to 0.20% of one or more kinds.
The high-tensile hot-rolled steel sheet according to 1. 4. The method according to claim 1, further comprising:
The high tensile strength according to claim 1, wherein the chemical composition contains one or more of 1.0% and Mo: 0.05 to 1.0%. 5. Hot rolled steel sheet. 5. Hot rolling of a steel having the chemical composition according to any one of claims 1 to 4 starting from a temperature range of _three or more Ac and ending at 780 to 840 ° C. And then at a cooling rate of 10 to 50 ° C./s for 300 to 5
A method for producing a high-tensile hot-rolled steel sheet, comprising cooling to 00 ° C and winding. 6. Hot rolling of a steel having the chemical composition according to any one of claims 1 to 4 starting from a temperature range of _three or more Ac and ending at 780 to 940 ° C. And then at a cooling rate of 10 ° C./s or more for 600 to 70
It is cooled to a temperature range of 0 ° C., air-cooled in the temperature range for 2 to 10 seconds, and further cooled at a cooling rate of 20 ° C./s or more for 300 to 50 ° C.
A method for producing a high-tensile hot-rolled steel sheet, comprising cooling to 0 ° C and winding.