JPH10306316A - Production of low yield ratio high tensile-strength steel excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel having sufficient performance as a steel for welding structures, low in a yield ratio, excellent in plastic deformation and moreover excellent in low temp. toughness by heating a slab having a specified compsn. to a specified temp. range, executing rough rolling, accelerated cooling and finish rolling under specified conditions and thereafter executing accelerated cooling again. SOLUTION: A slab having a compsn. contg., by weight, 0.01 to 0.20% C, 0.01 to 1.0% Si, 0.1 to 2.0% Mn, 0.001 to 0.1% Al and 0.001 to 0.01% N, in which the content of P as impurities is regulated to <=0.025% and S to <=0.015%, and the balance Fe with inevitable impurities is heated at the Ac3 point to 1250 deg.C and is subjected to rough rolling at a cumulative draft of 10 to 80% in the range of the heating temp. to 900 deg.C. After that, accelerated cooling at 2 to 40 deg.C/sec is executed to the Ar3 , point +50 deg.C to the Ar3 -50 deg.C in the cooling rate to supercool γ phases, and after the accelerated cooling, finish rolling at a cumulated draft of 30 to 90% is completed at 650 deg.C. Furthermore, accelerated cooling is again executed to 20 to 450 deg.C at a cooling rate of 5 to 40 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel with a low yield ratio, which has sufficient performance as a welded structural steel, has a low yield ratio, is excellent in plastic deformability, and is also excellent in low-temperature toughness. Things. For example, steel produced in this way can be used in offshore structures, pressure vessels, shipbuilding, bridges, buildings,
Although it can be used in general for welded steel structures such as line pipes, since it is a low yield ratio steel, it is particularly useful as a structural steel material for buildings and bridges that require earthquake resistance.
Further, although the form of the steel material is not particularly limited, it is used as a structural member, and a steel sheet requiring low-temperature toughness, particularly a thick plate,
It is particularly useful for steel pipe stock or shaped steel.

[0002]

2. Description of the Related Art A conventional method for producing a low yield ratio steel is a method of performing an intermediate heat treatment for heating a ferrite (α) + austenite (γ) two-phase region between quenching and tempering heat treatment (hereinafter referred to as QLT treatment). Basically, the purpose is to mix α as a soft phase and bainite or martensite as a hard phase.
The overall strength level and yield ratio have been controlled by changing the mixture ratio of these phases. Various production methods for obtaining a mixed structure of the soft phase and the hard phase have been conventionally proposed, for example, Japanese Patent Application Laid-Open No. 53-23817.
Japanese Patent Application Laid-Open Publication No. Hei 4 (1994) discloses a method in which a steel sheet is reheated and quenched, then reheated between an Ac ₁ transformation point and an Ac ₃ transformation point to form two phases of γ and α and then air-cooled. -314
No. 824 also discloses a method of quenching after reheating to a two-phase region. In addition, as a method of online production without performing reheating treatment, for example, Japanese Unexamined Patent Publication No.
Japanese Patent Application Laid-Open No. 3-286517 discloses a method in which after hot rolling is performed from the γ region to the two-phase region, air cooling is performed to a temperature 20 to 100 ° C. lower than the Ar ₃ transformation point to generate an α phase, and then quenching is performed. Have been.

After reheating and quenching, the material is reheated between the Ac ₁ transformation point and the Ac ₃ transformation point to form two phases of γ and α and then air-cooled or water-cooled. The QLT processing is relatively easy to control the structure. However, since the process is complicated, the problem of a large decrease in productivity cannot be solved.

On the other hand, Japanese Patent Application Laid-Open No.
After performing hot rolling from the γ region to the two-phase region, as shown in -286517, air cooling to a temperature lower by 20 to 100 ° C. than the Ar ₃ transformation point to generate an α phase,
Then, in the case of the so-called DLT processing,
The productivity can be improved because the intermediate heat treatment indispensable in the T treatment can be omitted, but the waiting time from rolling to quenching becomes longer, so that the productivity is still higher than that of the normal hot rolling and thermomechanical processing (TMCP) process. Sex is low. Further, since the waiting time for the generation of the α phase is long, the temperature in the plate is likely to be non-uniform, so that the variation of the material in the plate tends to increase. Further, α and the hard second phase generated in such a process tend to be coarse, and it is difficult to secure toughness.

[0005]

SUMMARY OF THE INVENTION In view of the problems of current low yield ratio steels, the present invention simultaneously solves the problem of low productivity in QLT processing and the problem of ensuring toughness in DLT processing. Specifically, in a method of manufacturing in a hot rolling step without performing reheating treatment to a two-phase region, a soft phase α is conventionally used in a cooling step after hot rolling. It is an object of the present invention to solve the problem that α and the size of the hard phase are coarsened and the productivity is reduced due to the slow cooling step for sufficiently generating the.

[0006]

In order to sufficiently produce α, which is a soft phase, in the cooling step after hot rolling, in the conventional method, after the hot rolling, the two-phase region is cooled at a slow cooling rate such as cooling. After cooling to a temperature to generate a certain amount of bulk α, accelerated cooling is performed to generate martensite or bainite which is a hard phase. Due to the slow cooling step for generating α, there was a problem that the size of α and the hard phase became coarse, and the productivity was reduced.

[0007] The present inventors have developed α which is formed after hot rolling.
Investigating the methods for miniaturization of α, we have found a completely new means that can simultaneously satisfy the miniaturization of α and the improvement of productivity. That is, the γ phase is subcooled by accelerated cooling at an appropriate cooling rate from the γ region to the transformation temperature region, and then the supercooled γ is subjected to appropriate hot rolling to obtain a fine from the supercooled γ. Is generated, and a structure capable of achieving both a low yield ratio and low-temperature toughness is formed. In addition, according to this method, the temperature range where cooling is normally performed is accelerated, so that productivity can be improved at the same time.

The present invention has been made based on the above-described new tissue control method, and the gist thereof is as follows. (1) By weight%, C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N: 0 0.001 to 0.010%, the content of P and S as impurities is P: 0.025% or less, S: 0.015% or less, and the steel slab composed of the balance of iron and unavoidable impurities is transformed into Ac ₃ To a temperature of not less than 1250 ° C.
After performing rough rolling at a cumulative reduction rate of 10 to 80% in the range of ° C, accelerated cooling at a cooling rate of 2 to 40 ° C / s is performed at the cooling rate at the Ar ₃ transformation point + 50 ° C to the Ar ₃ transformation point -50. ° C
To accelerate the supercooling of the γ phase. After accelerated cooling, finish rolling at a cumulative rolling reduction of 30 to 90% is completed at 650 ° C. or higher, and after finishing rolling is completed, cooling is performed at a cooling rate of 5 to 40 ° C./s.
A method for producing a low-yield-ratio high-strength steel excellent in low-temperature toughness, characterized by accelerated cooling to 0 ° C to 450 ° C again. (2) The method for producing a low-yield-ratio high-strength steel excellent in low-temperature toughness according to (1), wherein tempering is performed at a temperature of 450 ° C. or more and an Ac ₁ transformation point after rolling. (3) By weight%: Cr: 0.01 to 1.0% Ni: 0.01 to 3.0% Mo: 0.01 to 1.0% Cu: 0.01 to 1.5% Ti: 0 0.003 to 0.10% V: 0.005 to 0.50% Nb: 0.003 to 0.10% Zr: 0.003 to 0.10% Ta: 0.005 to 0.20% W: 0 Low yielding excellent in low temperature toughness as described in (1) or (2) above, wherein one or more of B: 0.0003% to 0.0020% is contained. Manufacturing method for high tensile strength steel. (4) By weight%, Mg: 0.0005 to 0.01% Ca: 0.0005 to 0.01% REM: 0.005 to 0.10% The method for producing a low-yield-ratio high-tensile steel excellent in low-temperature toughness according to any one of the above (1) to (3).

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The most important requirement of the present invention is that the γ phase is supercooled by accelerated cooling at an appropriate cooling rate from the γ range to the transformation temperature range, It is to generate fine α from supercooled γ by adding hot rolling. In order to satisfy the desired properties, it is necessary to optimize both the chemical composition and the manufacturing method,
For miniaturization of α, which is the most important for achieving both toughness and yield ratio, optimization of the manufacturing method is most important. Therefore, reasons for limiting the manufacturing conditions for that purpose will be described first.

The first requirement for the production method of the present invention is as follows.
After coarse cooling, accelerated cooling is performed to supercool γ, then finish rolling is performed to promote the transformation of α, and further accelerated cooling is performed after finish rolling to make the soft phase finer and softer. An object of the present invention is to appropriately control the ratio of the phase to the hard phase to achieve both toughness and a low yield ratio.

The accelerated cooling conditions after the rough rolling are 2 to 4
The cooling is performed at a cooling rate of 0 ° C./s from the Ar ₃ transformation point at the cooling rate + 50 ° C. to the Ar ₃ transformation point −50 ° C.
At the desired ratio and fine α
When the cooling rate is less than 2 ° C./s, the supercooling is not sufficient, and when the cooling rate is more than 40 ° C./s, the temperature difference between the surface and the inside of the steel material becomes excessive and the structure becomes uneven. , Α production amount is difficult to control, so the accelerated cooling rate is limited to 2 to 40 ° C./s.

While γ is supercooled in the cooling rate range, in order to generate a desired ratio and fine α by the subsequent finish rolling, accelerated cooling after the rough rolling is performed at the transformation starting temperature at the cooling rate. Is the Ar ₃ transformation point, and the Ar ₃ transformation point +
It is necessary to carry out from 50 ° C. to the Ar ₃ transformation point −50 ° C. This is because, when accelerated cooling is completed at a temperature higher than the Ar ₃ transformation point + 50 ° C, γ supercooling due to accelerated cooling is not sufficient, and
₍₃₎ Transformation point: When accelerated cooling to a low temperature of less than −50 ° C., most of the transformation is completed before starting the finish rolling after the accelerated cooling, so that α becomes finer and the hardness of α and the hard phase becomes smaller. This is because the difference becomes so small that both toughness and a low yield ratio cannot be achieved.

After the rough rolling and accelerated cooling, finish rolling is performed.
To generate fine α, for that purpose, after accelerated cooling,
Finish rolling at a cumulative rolling reduction of 30 to 90% needs to be completed at 650 ° C. or higher. If the cumulative rolling reduction is less than 30%, the refinement of α is insufficient, and if it exceeds 90%, the degree of refinement of α saturates, but the temperature decreases during rolling before accelerated cooling after finish rolling due to a decrease in temperature during rolling. Transformation occurs, and the hardness and ratio of the hard phase become insufficient, and the reduction of the yield ratio becomes insufficient. For the same reason, finish rolling needs to be completed at 650 ° C. or higher. That is, if the finish of the finish rolling is less than 650 ° C., the untransformed γ that should become a hard phase undergoes transformation before accelerated cooling after the finish rolling, and is likely to become a coarse bainite phase with low hardness and poor toughness. Becomes In addition, toughness is deteriorated by rolling after the transformation.

After finish rolling, accelerated cooling is performed to transform γ into a hard phase mainly composed of martensite. At that time, 5-4
It accelerates and cools from 20 ° C to 450 ° C at a cooling rate of 0 ° C / s. If the cooling rate is less than 5 ° C./s, the hardness and toughness of the hard phase are not sufficient. The higher the cooling rate, the higher the hardness of the hard phase.However, if the entire martensite phase is used, the hardness does not increase even if the cooling rate is further increased. An upper limit of 40 ° C./s is set as a cooling rate sufficient to obtain a hard phase required for the ratio. The accelerated cooling is performed from 20 ° C to 450 ° C. That is, even if accelerated cooling to less than 20 ° C., no structural change occurs,
The reason why it is difficult to cool the temperature to less than ° C is that the upper limit is set to 450 ° C because when accelerated cooling is stopped at a temperature exceeding 450 ° C, a coarse bainite phase having insufficient hardness and toughness is generated. Accelerated cooling should be started as soon as possible after finish rolling. However, since untransformed γ after α is generated from supercooled γ, C is concentrated and relatively stable. Interval between cooling and 60s-150
s is the upper limit. In order to more reliably suppress the transformation before accelerated cooling, the interval between finish rolling and accelerated cooling is 60 seconds.
It is preferable to set the following.

The above is the main reason for limiting the main requirements relating to the manufacturing method in the present invention. Other reasons for limiting the manufacturing conditions will be described below. First, the heating temperature of the steel slab prior to rolling is set to be higher than the Ac ₃ transformation point and lower than or equal to 1250 ° C. If the heating temperature is lower than the Ac ₃ transformation point, the solution becomes insufficient and a coarse structure remains after rolling, so the lower limit is defined as the Ac ₃ transformation point. If the heating temperature is higher than 1250 ° C., the heating γ particle size becomes excessively coarse, and the microstructure may not be sufficiently refined by subsequent rolling. Therefore, the upper limit of the heating temperature is 1250 ° C. And

Rough rolling is performed before accelerated cooling for supercooling the γ phase. The main purpose of rough rolling is accelerated cooling, adjusting the sheet thickness before finish rolling, so that it can be cooled at the desired cooling rate to the center of the sheet thickness, and auxiliary γ is refined. This is for making the final structure finer. The temperature range of the rough rolling is from the heating temperature to 900 ° C., because if the end of the rough rolling is lower than 900 ° C., there is a concern that it becomes difficult to secure the temperature until the subsequent accelerated cooling and finish rolling. The rolling reduction is determined by the thickness of the steel slab, the finishing rolling reduction, and the finished plate thickness. In order to exert the effect of accelerated cooling to the center of the plate thickness, a cumulative rolling reduction of 10% or more is required. Also, as the cumulative rolling reduction in rough rolling is larger, the effect of accelerated cooling can be extended to the center of the sheet thickness and the structure can be refined, which is preferable, but disadvantageous in that the cumulative rolling reduction of finish rolling cannot be secured. The upper limit is set to 80% as a range in which the cumulative rolling reduction of the finish rolling can be secured.

In the present invention, a tempering treatment can be performed after hot rolling, if necessary, for adjusting strength, toughness and yield ratio. In this case, if the tempering temperature is lower than 450 ° C., the effect of the tempering is not clear, and if the temperature exceeds the Ac ₁ transformation point, α becomes coarse and the strength and toughness deteriorate, so the tempering temperature is from 450 ° C. to the Ac ₁ transformation point. Limit to the range.

The reasons for the limitation of the present invention relating to the production method are as follows. The production of a steel for welded structures is demonstrated, and a low yield ratio high tensile strength steel sheet having a low low yield ratio and excellent plastic deformability is produced. Therefore, it is necessary to define the chemical components as well. The reasons for limiting each chemical component are described below.

First, C is added as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and more than 0.20%. Excessive addition significantly reduces toughness, weld cracking resistance, etc., and is therefore in the range of 0.01 to 0.20%.

Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. Addition of less than 0.01% results in insufficient deoxidation and is disadvantageous for securing strength.
Conversely, an excessive addition exceeding 1.0% forms a coarse oxide and causes deterioration in ductility and toughness. Therefore, the range of Si is 0.
01 to 1.0%.

Mn is an element necessary for securing the strength and toughness of the base material, and it is necessary to add at least 0.1% or more. The upper limit was 2.0%.

Al is an element effective for deoxidation, grain refinement of the γ particle size, etc., and it is necessary to contain 0.001% or more in order to exhibit the effect. , A coarse oxide is formed and the ductility is extremely deteriorated, so it is necessary to limit the range to 0.001% to 0.1%.

N works effectively with Al and Ti to reduce the size of γ grains.
On the other hand, it is necessary to contain 1% or more. On the other hand, if it is added excessively, the amount of dissolved N increases, leading to an increase in yield ratio and deterioration in toughness. The upper limit is set to 0.01% as an allowable range from the viewpoint of securing the toughness of the weld heat affected zone.

P is an impurity element and is preferably reduced as much as possible, but the upper limit is set to 0.025% as an allowable amount from the viewpoint of securing the toughness of the heat affected zone.

Since S forms MnS and degrades the ductility value, S is an element that needs to be particularly reduced in a steel sheet that is required to ensure plastic deformability, as the object of the present invention.
However, the content is set to 0.015% or less as a practically allowable upper limit where ductility is not significantly deteriorated.

The basic components of the steel according to the present invention have been described above, but Cr, Ni, Mo, Cu, Ti, V, Nb, Z
One, two or more of r, Ta, W, and B can be contained.

First, Cr and Mo are both effective elements for improving the strength of the base material. However, in order to produce a clear effect, 0.1% or more is required. When added, the toughness tends to deteriorate.
１．０1.0%.

Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but must be contained at 0.01% or more in order to exert its effect. When the content is increased, the strength and toughness are improved, but the effect is saturated even if added over 3.0%. Therefore, the upper limit is set to 3.0% in consideration of economy.

Next, Cu has almost the same effect as Ni, but if added in excess of 1.5%, causes a problem in hot workability, so it is limited to the range of 0.01 to 1.5%.

Ti is an element effective for improving the toughness by refining γ grains by forming TiN. However, in order to exhibit the effect, it is necessary to add 0.003% or more. on the other hand,
If it exceeds 0.10%, as in the case of Al, a coarse oxide is formed to deteriorate toughness and ductility.
And

Both V and Nb mainly contribute to the improvement of the strength of the base material by precipitation strengthening, but the toughness is deteriorated by excessive addition. Therefore, V is 0.005 to 0.50% and Nb is a range in which the effect can be exhibited without inducing the toughness.
Is set to 0.003 to 0.10%.

Zr is an element that exerts an effect on precipitation strengthening and grain refinement, but in order to exhibit the effect, it is necessary to add 0.003% or more. On the other hand, an excessive addition of more than 0.10% causes the deterioration of toughness due to coarsening of precipitates.
It is limited to the range of 0.003% to 0.10%.

Although Ta is also effective for precipitation strengthening and grain refinement, it must be 0.005% or more in order to exhibit its effect, and if it exceeds 0.20%, on the contrary, toughness deteriorates. The range is 0.005% to 0.20%.

W is an element effective for improving the strength of the base material by solid solution strengthening and precipitation strengthening. The effect is exhibited when the addition is 0.01% or more, and since addition of more than 2.0% causes precipitation embrittlement and deterioration of weldability, it is limited to the range of 0.01% to 2.0%.

B is very effective in increasing the strength by increasing the hardenability of steel by adding a very small amount of 0.0003% or more.
If added in excess, BN is formed, and conversely, the hardenability is lowered and the toughness is greatly deteriorated.
0%.

Further, in the present invention, in order to improve the toughness (HAZ toughness) of the welded portion, Mg,
One or more of Ca and REM can be contained. In any case, the microstructure of the weld heat affected zone (HAZ) is refined by the fine dispersion of oxides and sulfides to improve the HAZ toughness.
Must be contained 0.0005% or more, and REM must be contained 0.005% or more. On the other hand, if added in excess, oxides,
Since the sulfide coarsens and itself becomes a starting point of brittle fracture and deteriorates the HAZ toughness, the upper limit is set to Mg, Ca
Is limited to 0.01% and REM is limited to 0.10%.

[0037]

Next, the effects of the present invention will be described more specifically with reference to examples. Table 1 shows the chemical components of the test steels used in the examples. After ingot casting, each test steel was formed into a slab by slab rolling or continuous casting. In Table 1, steel numbers 1 to 15 satisfy the chemical composition defined by the present invention, and steel numbers 16 to 21 do not satisfy the chemical composition range of the present invention.

The slabs shown in Table 1 were manufactured into steel sheets under the conditions shown in Table 2, and tensile properties and Charpy impact properties were examined.
All test pieces were collected in the rolling direction from the center of the thickness. The tensile test was performed using a round bar tensile test piece having a parallel part diameter of 14 mm and a parallel part length of 60 mm, and yield stress (YP) and tensile strength (T
S), yield ratio and total elongation were investigated. The Charpy impact test was performed using a JIS No. 4 standard test piece, and the characteristics were evaluated at a 50% fracture surface transition temperature (vTrs). Table 2 also shows the strength and toughness test results.

In Table 2, Test No. A1 to A20 are steel sheets manufactured according to the present invention, all having good elongation and toughness, and under the same manufacturing conditions as those of the comparative example,
The yield ratio is reduced as compared with that at the tensile strength level. On the other hand, Test No. B1 to B12 are comparative examples,
Since any of the conditions are out of the range of limitation of the present invention, when compared at the same tensile strength level, the yield ratio is high, and the ductility and toughness are not necessarily sufficient as steel for welded structures.

First, in Test No. B1 to B6 are examples in which sufficient properties cannot be obtained because the chemical composition does not satisfy the present invention. That is, the test No. B1 is inferior in ductility and toughness because the amount of C is excessively added outside the range of the present invention. No. B2 does not have sufficient toughness because Mn is excessive. No. B3 has an excessive amount of N, resulting in poor toughness and a high transformation element temperature due to a large amount of alloying elements. Therefore, it is difficult to secure a rolling finish temperature when performing the processing after supercooling, which is the main point of the present invention. No. In the example of B3, since the finishing temperature was set to 650 ° C. or higher, the accelerated cooling conditions before the finish rolling could not satisfy the conditions of the present invention, and a low yield ratio could not be achieved. No. B4 is inferior in toughness and ductility due to excessive P content. No. B5 is inferior in ductility due to an excessive amount of S. No. B6 has an excessive amount of Cr and Ni, so that the toughness is inferior and at the same time No. B6. As in B3, the transformation start temperature is too low, so that the accelerated cooling conditions before finish rolling cannot satisfy the conditions of the present invention, and the yield ratio is high.

No. B7 to B13 are examples in which the chemical composition satisfies the present invention, but the yield ratio is high or the ductility and toughness are inferior when compared at the same tensile strength level because the requirements for the production method are not satisfied. is there. That is, the test No. B7 does not have accelerated cooling between rough rolling and finish rolling, and is manufactured by a manufacturing method close to general DLT processing. Unlike the present invention, B7 has significantly deteriorated toughness. N
o. B8 is obtained by a production method equivalent to ordinary DQT processing under the condition that there is no accelerated cooling in the middle and there is no α generation step, and a low yield ratio cannot be achieved. No. In B9, since the heating temperature of the steel slab is too high, although the subsequent manufacturing conditions are in accordance with the method of the present invention, the refinement of the crystal grains is not achieved, and sufficient toughness cannot be secured. No. In B10, since the cumulative rolling reduction of the finish rolling is too small, the refinement of the α grain size is not sufficient, and the toughness is poor. No. In B11, since the stop temperature of the accelerated cooling between the rough rolling and the finish rolling is too high, the supercooling before the finish rolling is insufficient, so that the α grain size is not sufficiently refined and the toughness is poor. No. In B12, since the finishing temperature is too low, the hardness of the hard phase is not sufficient, so that the overall strength level is lower than in other production methods, and the yield ratio is not sufficiently reduced. Further, the development of the processed structure is remarkable and the toughness is poor. No. In B13, the accelerated cooling between the rough rolling and the finish rolling is performed, but the cooling rate is low and the supercooling is not sufficient. Therefore, the toughness is not sufficiently improved.

From the above, according to the present invention, when compared at the same tensile strength level, lower yield ratio and improved toughness can be simultaneously achieved without lowering ductility and productivity as compared with the prior art. It is clear that this is done.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

According to the present invention, based on thermomechanical treatment (TMCP), it has sufficient performance as a steel for welded structure without complicated reheating treatment, and simultaneously has both characteristics of toughness and low yield ratio. This is an epoch-making method for producing a high yield steel sheet with a low yield ratio, which can be improved. Its industrial effects are extremely large, such as reduction in production cost and improvement in safety as a structure.

Claims (4)

[Claims] C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N by weight% : 0.001 to 0.010%, the content of P and S as impurities is P: 0.025% or less, S: 0.015% or less, and the steel slab consisting of iron and unavoidable impurities is Ac Heat to a temperature between ₃ transformation point and 1250 ° C, heating temperature ~ 900
After performing rough rolling at a cumulative rolling reduction of 10 to 80% in the range of ° C, accelerated cooling at a cooling rate of 2 to 40 ° C / s is performed at the cooling rate at the Ar ₃ transformation point + 50 ° C to the Ar ₃ transformation point -50. ° C
To accelerate the supercooling of the γ phase. After accelerated cooling, finish rolling at a cumulative rolling reduction of 30 to 90% is completed at 650 ° C. or higher, and after finishing rolling is completed, cooling is performed at a cooling rate of 5 to 40 ° C./s.
A method for producing a low-yield-ratio high-strength steel excellent in low-temperature toughness, characterized by accelerated cooling to 0 ° C to 450 ° C again. 2. The method according to claim 1, wherein tempering is performed at a temperature of 450 ° C. or more and an Ac ₁ transformation point after rolling. 3. In% by weight, Cr: 0.01 to 1.0% Ni: 0.01 to 3.0% Mo: 0.01 to 1.0% Cu: 0.01 to 1.5% Ti : 0.003 to 0.10% V: 0.005 to 0.50% Nb: 0.003 to 0.10% Zr: 0.003 to 0.10% Ta: 0.005 to 0.20% W : 0.01 to 2.0% B: 0.0003 to 0.0020% The low yield ratio excellent in low-temperature toughness according to claim 1 or 2, characterized by containing one or more of B: 0.0003 to 0.0020%. A method for manufacturing high-tensile steel. 4. The composition contains one or more of Mg: 0.0005 to 0.01% Ca: 0.0005 to 0.01% REM: 0.005 to 0.10% by weight% The method for producing a low-yield-ratio high-strength steel excellent in low-temperature toughness according to any one of claims 1 to 3, characterized in that: