JPH09279233A - Production of high tension steel excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart a high strength and high toughness by rolling with controlling a low C and low Mn steel in a specific composition and micronizing the structure. SOLUTION: The composition is, by weight%, 0.03 to 0.45% C, 0.01 to 0.50% Si, 0.02 to 5.0% Mn, 0.001 to 0.1% Al, and the remainder part is Fe, B and inevitable impurity. The cast billet in this composition is executed with hot machining of rolling, etc., without cooling after casting, or it is once cooled to the room temperature, re-heated to, the temperature range of Ac3 point to 1250 deg.C and it is hot machined to be a steel. In this case, one pass or more than two passes, with continuing interval within 20sec., is executed at a temp. of 500 to 700 deg.C, the rolling strain rate of 0.1 to 20sec, the total strain amount is made as 0.8 to 5 and then it is cooled. Further, it is preferred to forcibly cool to the room temperature to <=500 deg.C, within 90sec. with the cooling velocity of 2 to <=20sec., after finishing of hot rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel product (a thin steel plate, a thick steel plate, a wire rod, a shaped steel, a steel bar, etc.) produced by hot rolling, which is a high-strength steel excellent in basic strength and toughness as basic characteristics. The present invention relates to a manufacturing method of.

[0002]

2. Description of the Related Art In recent years, with the weight reduction of steel products and the severer conditions of use of steel structures, there has been a demand for the development of tougher and safer steel. In order to meet such demands, conventionally, a rolling method has been developed for improving the method for producing a steel sheet, for refining the crystal grains of the metal structure, and for improving the strength and toughness of the steel. An example of such a method is a so-called controlled rolling method, and as a manufacturing method combined with an accelerated cooling method, JP-A-63-223124 and JP-A-63-1 are known.
No. 28117 is disclosed.

In the controlled rolling method shown in these conventional methods, the static recrystallization that occurs between rolling passes in the recrystallization temperature range of a relatively high temperature austenite (hereinafter abbreviated as γ) is utilized, and Atomize. Then, after waiting for the temperature of the steel sheet to decrease, it is possible to introduce defects such as dislocations into the γ crystal by rolling again in a temperature range in which γ recrystallization does not occur (unrecrystallized temperature range). Has been done. Such a defect, when γ is transformed into ferrite or the like, serves as a nucleation site for a transformation generation structure of ferrite or the like, similarly to the γ grain boundary, so that a large number of crystal grains are simultaneously produced during cooling,
This is because it is possible to make the metal structure finer.

[0004]

However, even if the ferrite obtained by such a method has a small grain size, it is at most about 5 μm, and there is a demand for a method of further refining the crystal grains.

[0005]

The present invention uses a processing and cooling method capable of obtaining remarkable fine grains that cannot be obtained by the conventional grain refining means such as controlled rolling and accelerated cooling. An object is to provide a method for manufacturing steel. As a means for solving the above problems, the following manufacturing method was created. (1)% by weight, C: 0.03 to 0.45%, Si:
0.01 to 0.50%, Mn: 0.02 to 5.0%, A
l: containing 0.001 to 0.1%, the balance being Fe and unavoidable impurities, and then hot working by rolling etc. without cooling after casting, or after cooling once to room temperature When re-heating to a temperature of Ac ₃ points to 1250 ° C. again and performing hot working to manufacture a steel material, in one series of hot working, one pass or two or more consecutive passes within 20 seconds of the time between passes Is processed at a temperature of 700 ° C. or lower and a temperature of 500 ° C. or higher and the strain rate of rolling is 0.1 to 20 /
Second, a method for producing a high-strength steel excellent in toughness, in which processing is performed so that the total strain amount is 0.8 or more and 5 or less and then allowed to cool.

(2) C: 0.03 to 0.4 by weight%
5%, Si: 0.01 to 0.50%, Mn: 0.02
A steel slab containing 5.0% and Al: 0.001 to 0.1%, the balance of which is Fe and unavoidable impurities, is hot-worked or hot-worked as it is without cooling after casting. Without cooling once to a temperature of 600 ° C to room temperature, or after cooling once to a room temperature after casting and then reheating to a temperature of Ac ₃ point to 1250 ° C and hot working or without hot working 600 After cooling to a temperature from ℃ to room temperature, it is then heated to a temperature of 700 ° C or less and 500 ° C or more, and one pass or two or more consecutive passes within 20 seconds between passes are 700 ° C or less and 500 ° C or less. Above temperature,
And the strain rate of rolling is set to 0.1 to 20 / sec, and processing is performed under the condition that the total strain amount is 0.8 or more and 5 or less.
A method for producing a high-strength steel excellent in toughness, which is allowed to cool.

(3) After performing the hot working described in (1) or (2) above, at room temperature to within 90 seconds after the end of the hot working.
A method for producing a high-strength steel having excellent strength and toughness, which comprises forcibly cooling to a temperature of less than 500 ° C at a cooling rate of 2 ° C / sec to 200 ° C / sec or less. (4) A method for producing a high-strength steel excellent in strength and toughness, which comprises performing hot working and cooling as described in (3) above, followed by tempering at a temperature of 300 ° C to Ac ₁ .

(5) Furthermore, in% by weight, Nb: 0.00
1 to 0.05%, Ti: 0.001 to 0.1%, V:
0.001 to 0.1% of any one kind or two or more kinds is contained, and the strength and toughness are excellent and high. Method of manufacturing tensile steel.

(6) Further, in weight%, Mo: 0.01
~ 1%, Ni: 0.01-5%, Cr: 0.01-3
%, Cu: 0.01 to 3%, B: 0.0001 to 0.
Any one kind of 003% or 2 kinds or more is contained, The manufacturing method of the high strength steel excellent in strength and toughness as described in any one of said (1)-(5) characterized by the above-mentioned.

(7) Further, in% by weight, REM: 0.0
02 to 0.10%, Ca: 0.0003 to 0.003
The method for producing a high-strength steel excellent in strength and toughness according to any one of the above items (1) to (6), characterized by containing 0% of any one kind or two kinds or more. is there.

Hereinafter, the present invention will be described in detail. First, when the conventional method of controlling grain refining by controlled rolling is examined from a metallurgical point of view, as described above, it is considered that the main effects are as follows. In the recrystallization temperature range of a relatively high temperature austenite (hereinafter, abbreviated as γ), static crystallization that occurs between rolling passes is used to refine γ grains. Further, by rolling in a temperature range (non-recrystallization temperature range) where γ recrystallization does not occur at a relatively low temperature, many defects such as dislocations are introduced into γ crystals. The defects such as γ grain boundaries and dislocations described above serve as nucleation sites of the transformation-generated structure when γ is transformed into ferrite or the like, and thus the metal structure is made fine.

[0012] Of these, each of (1) to (3) provides a nucleation site for a transformation forming structure of ferrite or the like, which makes the final ferrite crystal grain size finer, and all of them are made of austenite. The number of ferrite grains generated during transformation into ferrite is increased to achieve fineness. However, in the refinement utilizing such transformation, the transformation start temperature of ordinary steel from austenite to ferrite is 750 ° C to 800 ° C, and the diffusion of iron atoms in the steel is relatively fast. The growth is fast, and only crystal grains of about 5 μm can be obtained. Further, the temperature of the ferrite transformation can be forcibly lowered by forced cooling, but in such a case, the resulting ferrite has an acicular shape or bainite is formed, which is an unfavorable structure from the viewpoint of toughness.

Therefore, the inventors of the present invention have overcome the limitation of grain refinement utilizing the above-mentioned transformation to obtain a ferrite structure of extremely fine grain, as a method of obtaining ferrite and austenite or ferrite and pearlite, bainite and martensite. By processing and cooling the mixed structure under appropriate conditions and causing recrystallization (dynamic recrystallization or recovery) under the roll of the ferrite grains during rolling, the ferrite grains are made extremely fine as shown in FIG. We found that we could, and investigated and analyzed these phenomena.
Invented is a method for producing a high-strength steel having excellent toughness of ~ 7.

The main points of the technique which is the basis of the present invention are as follows. The crystal grains can be made finer to 1 μm or less by the dynamic recrystallization (or recovery) of ferrite. At this time, in order to obtain fine and uniform ferrite grains, the following processing and cooling conditions are required. First, the structure before processing that causes recrystallization is ferrite and austenite or ferrite and pearlite, bainite,
It is a mixed structure with a second phase such as martensite. This is because the ferrite is more easily processed due to the difference in deformation resistance between the second phase and the ferrite, and the ferrite is elongated by the processing, and the ferrite is divided by the second phase when recrystallized, so that the elongation is extremely thin, It is considered that grain growth and coalescence after recrystallization can be suppressed.

Next, the processing temperature must be a temperature at which the ferrite and the second phase coexist in order to obtain the above-described structure before processing, and it is necessary that the temperature is between Ac ₃ and room temperature.
Since the main purpose of the present invention is to dynamically recrystallize the structure of ferrite mainly containing a small amount of the second phase, a certain amount of ferrite structure fraction (at least about 60%) is required, but this is stably achieved. In order to achieve this, the temperature must be 700 ° C. or lower. Further, the processing temperature is preferably 700 ° C. or lower from the viewpoint of suppressing grain growth after dynamic recrystallization due to processing. However, if the processing temperature is too low, the diffusion of atoms is significantly delayed, and dynamic recrystallization cannot be stably generated. From such a viewpoint, it is necessary to process at a temperature of 500 ° C. or higher.

Next, the strain amount and strain rate in processing must be set so that dynamic recrystallization can be stably generated and the crystal grain size after recrystallization can be made fine.
FIG. 2 shows 0.14% C-0.25% Si-1.2% Mn-
Stress when a steel containing 0.01% Nb-0.01% Ti is processed under the conditions of the present invention to obtain a fine grain structure as shown in FIG.
It is a distortion curve. It is presumed that softening occurs during processing, which is considered to be due to dynamic recrystallization, and further softening occurs repeatedly, and recrystallization repeatedly occurs. In such a case, it is considered that in the first softening, the metal structure becomes fine-grained and uniform, and thereafter it is in a substantially steady state.

Therefore, it has been experimentally confirmed that a certain amount of strain is required to obtain a preferable structure by dynamic recrystallization, and that strain of 0.8 or more is required in the temperature range of the present invention. . Regarding the strain rate, the smaller the strain rate, the more easily dynamic recrystallization occurs, but the grain size cannot be reduced. It was also found that if the strain rate is too high, dynamic recrystallization does not occur and the load during processing becomes extremely large. Then, as a result of examining the range in which the dynamic recrystallization is stably generated and the crystal grains can be made finer, it has been found that the appropriate strain rate range is 0.1 to 20.

Next, whether the above-mentioned processing is performed in one pass or in two or more passes, the effect is basically the same if the time between passes is short. This is because, in the temperature range of the present invention, the processing temperature is relatively low and the recovery between passes is not so fast, and if the interpass time is within 20 seconds, the recovery during that period is small and the strains in multiple passes are almost accumulated. . Finally, by the above processing, it is possible to obtain an extremely fine ferrite structure immediately after processing, but it is possible to suppress the crystal grain growth of ferrite by cooling it rapidly after processing and obtain more excellent properties. Is possible. When austenite is present as the second phase during processing, water cooling transforms austenite into martensite and bainite, which can strengthen the steel but may deteriorate ductility and toughness. In such a case, water cooling can be stopped during cooling, or tempering can be performed after water cooling, whereby steel with excellent strength, toughness, and ductility can be manufactured.

The reasons for limiting each component and manufacturing conditions will be described below. C is an element effective for strengthening steel, and if it is less than 0.03%, sufficient strength cannot be obtained and the second phase essential to the present invention is not stably generated. On the other hand, if its content exceeds 0.45%, the weldability is deteriorated. Si is effective as a deoxidizing element and as a strengthening element for steel, but it is not effective when the content is less than 0.01%. On the other hand, if it exceeds 0.5%, the surface properties of steel are impaired.

Mn is an element effective for strengthening steel, and M.
If it is less than 02%, a sufficient effect cannot be obtained. On the other hand, if its content exceeds 5.0%, the workability of steel deteriorates. A
l is added as a deoxidizing element and as crystal grains at the time of reheating to the austenite temperature range, but if the content is less than 0.001%, there is no effect, and if it exceeds 0.1%, the surface properties of steel deteriorate. Let Each of Ti, V and Nb effectively functions in terms of grain refinement and precipitation strengthening when added in a small amount, and thus may be used in a range that does not deteriorate the toughness of the welded portion. From this point of view, the upper limits of the amounts added are 0.1% for Ti and V, and 0.05% for Nb. Further, the lower limit of the amount added is 0.001% because if it is less than this, there is no effect.

Cu, Ni. Cr, Mo, and B are all elements that improve the hardenability of steel, and in the case of the present invention, the addition of Cr can increase the strength of steel. But,
Since excessive addition impairs the toughness and weldability of steel,
01% ≤ Cu ≤ 3.0%, 0.01% ≤ Ni ≤ 5.0
%, 0.01% ≦ Cr ≦ 3.0%, 0.01% ≦ Mo ≦
It is limited to 1.0% and 0.0001% ≦ B ≦ 0.003%. The lower limit of each of Cu, Ni, Cr and Mo is set to 0.0
The lower limit of 1% and B is set to 0.0001% because the effect is not achieved below this range.

REM and Ca are effective for detoxifying S, but if the addition amount is small, the effect is not obtained, and excessive addition impairs toughness, so REM is 0.002 to 0.002.
0.10%, 0.0003 to 0.003 for Ca
Limited to 0%. In addition, inevitable impurities such as P and S
The content of each is preferably 0.02% or less and 0.008% or less.

Next, the manufacturing conditions in the present invention will be described. Since the present invention is effective for a steel slab cast under any casting condition, it is not necessary to specify the casting condition. Further, the processing in the state in which the second phase, which is the basis of the present invention, exists is once heated to a complete austenite region (a temperature of Ac ₃ or higher) and then 700 ° C. to 500 ° C. in the cooling process.
When processing is carried out between ℃ (Claim 1) and once 600 ℃ ~
A case may be considered in which the temperature is cooled to room temperature and the processing is performed in the subsequent temperature rising process (claim 2).

In the former case, after casting the cast slab, even if the hot working is started as it is without cooling, the cast slab once cooled to room temperature is reheated to Ac ₃ point to 1250 ° C. and then rolled. You may. Here, the reheating temperature is set to Ac ₃ or higher because the metal structure of the steel at the time of rolling does not become a complete austenite structure below this. Further, the upper limit of the reheating temperature is set to 1250 ° C. because the metal structure of steel becomes coarse at a temperature higher than this temperature and desired toughness cannot be obtained.

In the latter case, the steel slab is not hot-worked after casting but is once cooled to a temperature of 600 ° C. to room temperature and then reheated to a temperature of 700 ° C. or lower and 500 ° C. or higher. Or, after the casting, hot working as it is, once cooled to a temperature of 600 ° C. to room temperature and then reheated to a temperature of 700 ° C. or less and 500 ° C. or higher, or once cooled to room temperature after casting. The slab is reheated to Ac ₃ point to 1250 ° C and then hot-worked or cooled to a temperature of 600 ° C to room temperature without hot-working and then reheated to 700 ° C or lower and 500 ° C or higher. You can do it.

The metallographic structure of steel can be obtained by directly cooling from the austenite region to a temperature of 600 ° C. to room temperature, directly after casting, after performing post-casting processing, after cooling after casting, and after reheating to the austenite region. Is mainly composed of ferrite and the second phase, such as pearlite, bainite, and martensite, and satisfies the main requirements of the present invention. It should be noted that the processing and reheating prior to cooling to a temperature of 600 ° C. to room temperature make the ferrite crystal grains fine when cooled to 600 ° C. to room temperature, which is advantageous for the present invention.

Next, the processing for causing the dynamic recrystallization of ferrite must be carried out at 700 ° C to 500 ° C. This is because at 700 ° C. or higher, the volume fraction of austenite is too large, and even if the ferrite portion can be made fine, it is only a part of it and the desired structure cannot be obtained.
This is because the ferrite part processed under the same conditions and the part processed by austenite and transformed into ferrite have a much smaller crystal grain size. At the same time, as the processing temperature becomes higher, the crystal grains also tend to become larger in the dynamically recrystallized ferrite portion, and the processing temperature is preferably as low as possible. However, if the processing temperature is too low, the diffusion of atoms is less likely to occur and recrystallization is less likely to occur. In such a case, the processed ferrite grains are simply flattened, a fine grain size control structure cannot be obtained, and anisotropy occurs in the characteristics of the steel. Therefore, in order for ferrite recrystallization to occur stably, it is necessary to perform processing in a temperature range of 500 ° C. or higher.

The strain amount during processing at 700 ° C. to 500 ° C. requires that recrystallization occurs in the entire structure during processing in this temperature range and the crystal grain size after recrystallization is fine. In order to cause recrystallization in the entire structure, the processing amount is required to be a certain amount or more. From this viewpoint, the total strain amount due to the series of processing is required to be 0.8 or more. Also,
If the strain amount is secured at 0.8 or more, the larger the strain amount is, the better. However, it is difficult to secure the strain amount of 5 or more in the ordinary processing such as rolling. Therefore, in the present invention, the upper limit of the strain applied is set to 5.

Further, as the strain rate during processing is smaller, dynamic recrystallization is more likely to occur, and when the strain rate is higher, it is less likely to occur. On the other hand, when the strain rate is low, the dislocations during processing are reduced (recovered) quickly, and as a result, the crystal grain size obtained after recrystallization is large, and when the strain rate is high, the crystal grain size is small. Considering both the easiness of such dynamic recrystallization and the crystal grain size after recrystallization, there is an appropriate region for the strain rate.
From this point of view, the strain rate during processing is limited to 0.1 / sec or more and 20 / sec or less. If it is less than 0.1 / sec, the time required for rolling is too long, and dislocation recovery occurs during this period.
This is because a large number of dislocations cannot be introduced into α and fine crystal grains cannot be obtained even if dynamic recrystallization occurs. The strain rate during rolling is set to 20 / sec or less because it is 20 / sec.
This is because it is difficult to cause dynamic recrystallization in the temperature range of 700 ° C. to 500 ° C. for more than a second. When the above rolling is performed in one pass or in multiple passes, the time between passes needs to be 20 seconds or less. This is because if the interpass time is set to 20 seconds or more, the ferrite recovery progresses between the passes, and the cumulative effect of strain cannot be obtained.

Next, a method of performing forced cooling following a series of hot rolling that causes dynamic recrystallization will be described. First, the following two points can be considered regarding the effect of forced cooling. First, by forcibly cooling, the fine ferrite structure obtained after processing grows due to the growth of crystal grains during subsequent cooling, and the reduction of its effectiveness is suppressed. In addition, the second reason is that a trace amount of austenite existing as the second phase when the processing is performed in the cooling process from austenite is not more than the transformation start point (Ac ₃ point) in the equilibrium state without transformation. By maintaining the temperature and causing transformation from a state of high supercooling, the driving force for nucleation of the transformation structure is significantly increased and a large number of crystal grains are simultaneously generated to transform the transformed structure such as ferrite and pearlite. The first purpose is to achieve fine graining, and such fine graining effect is prominent when austenite undergoes processing and many defects such as dislocations are introduced into the austenite grains. . Further, by causing the transformation to occur at a low temperature by cooling, the austenite portion has fine ferrite and bainite or martensite structure having a relatively high strength, and the strength of the steel can be improved.

In the present invention, from the above viewpoints, the effect of grain refining of ferrite by forced cooling is exerted, and in some cases, bainite and martensite are slightly generated to strengthen the ferrite. It is specified that, following hot rolling that causes static recrystallization, forced cooling is started within 90 seconds, and cooling is performed at room temperature to less than 500 ° C. to room temperature or more at 2 to 200 ° C./second. . The reasons for limitation will be described below.

First, the reason why cooling was started within 90 seconds from the end of processing was to prevent the fine ferrite structure formed by dynamic recrystallization during rolling from coarsening due to grain growth. This is because the effect is not maximized when cooling is started after the temperature exceeds the limit, and the effect of water cooling does not appear remarkably as it is when it is left to cool after processing. Next, the reason why the cooling end temperature is set to room temperature to less than 500 ° C. is that the temperature is too high at a temperature of 500 ° C. or higher and grain growth cannot be suppressed in the fine ferrite structure obtained by processing, and the temperature is set to room temperature or higher. This is because cooling to a temperature below this is difficult to carry out with ordinary water cooling or the like.

Further, according to claim 4, when the austenite portion after processing is transformed into martensite having a very high strength when the above-mentioned cooling is performed and the strength of the steel is excessively increased, At the same time, significant deterioration of toughness may occur. In such a case, after cooling, the temperature is further increased to 300 ° C to Ac
By tempering at a temperature of ₁ , a steel having excellent strength and toughness can be obtained. The tempering is carried out for the purpose of improving the toughness that is significantly deteriorated by the formation of martensite. The tempering temperature is set to 300 ° C. or higher because if the temperature is lower than this, the temperature is too low and solid solution carbon is formed in a short time. is because it is impossible to easily precipitated, the tempering temperature was less than Ac ₁ point is rather toughness is deteriorated due to non-uniformity and a decrease in tissue strength will occur is transformed exceeds Ac ₁ point This is because it will end up.

[0034]

EXAMPLES The effectiveness of the present invention will be shown by the examples of the present invention. Table 1 shows the components of the steels of the examples. In the table, the steel marked with a circle indicates that it is a comparative steel, and the items that do not correspond to the present invention are underlined. Next, Table 2 and Table 3 show the strength and toughness obtained for the steel plates manufactured under various manufacturing conditions using the steels having such components. As the strength, the yield strength (YS (k
gf / mm ² )) and tensile strength (TS (kgf / mm
² )) is shown. Moreover, the toughness showed the ductility-brittleness transition temperature (vTrs (° C)) in the Charpy impact test. In the case of any steel, the main requirement of the present invention is 500 to
It can be seen that the processing at 700 ° C. significantly improves the strength and toughness as compared with the case where the same processing is not performed. Also,
The manufacturing conditions shown in the method of the present invention all show clearly better characteristics than the comparative method. The method of the present invention makes it possible to produce high-strength steel excellent in strength and toughness, and the present invention is effective.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

As described above, according to the present invention, it is possible to inexpensively provide a high-strength steel sheet having excellent strength and toughness.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a fine ferrite structure obtained by dynamic recrystallization (or recovery).

FIG. 2 is an explanatory diagram of changes in a stress-strain curve due to dynamic recrystallization (or recovery).

Claims (7)

[Claims] 1. By weight%, C: 0.03 to 0.45%, Si: 0.01 to 0.50%, Mn: 0.02 to 5.0%, Al: 0.001 to 0. A steel slab containing 1% of which the balance is Fe and unavoidable impurities is hot-worked by rolling without casting after cooling after casting, or once cooled to room temperature and then once again at an AC ₃ point to 1250 ° C. When reheating to temperature and performing hot working to produce steel, one of the series of hot working
Processing of two or more consecutive passes within 20 seconds between passes or passes is performed at a temperature of 700 ° C or lower and a strain rate of rolling of 0.1 to 20 / sec, and the total strain amount is 0.8 or more. A method for producing a high-strength steel excellent in toughness, which is processed so as to be 5 or less and then allowed to cool. 2. C .: 0.03 to 0.45%, Si: 0.01 to 0.50%, Mn: 0.02 to 5.0%, Al: 0.001 to 0. After casting, a steel slab containing 1% and the balance Fe and unavoidable impurities is hot-worked without cooling after casting, or is once cooled to a temperature of 600 ° C to room temperature without hot-working. Or, after cooling once to room temperature after casting, it is reheated to a temperature of Ac ₃ point to 1250 ° C. and hot-worked, or it is cooled to a temperature of 600 ° C.-room temperature without hot working, After that, 700 ℃ or less 500
Heat to a temperature of ℃ or more and pass for 1 pass or time between passes.
Processing of two or more consecutive passes within seconds is 700 ° C or less 50
It is processed at a temperature of 0 ° C. or higher, a strain rate of rolling of 0.1 to 20 / sec, and a total strain amount of 0.8 or more and 5 or less, and then allowed to cool. Method of manufacturing tensile steel. 3. After performing hot working according to claim 1 or 2, room temperature to 500 within 90 seconds after completion of hot working.
A method for producing a high-strength steel having excellent strength and toughness, which comprises forcibly cooling to a temperature of less than 0 ° C at a cooling rate of 2 ° C / sec to 200 ° C / sec or less. 4. A method for producing a high-strength steel excellent in strength and toughness, which comprises performing hot working and cooling according to claim 3 and then tempering at a temperature of 300 ° C. to Ac ₁ . 5. Further, in weight%, any one of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.1%, V: 0.001 to 0.1%, or The method for producing a high-strength steel excellent in strength and toughness according to any one of claims 1 to 4, characterized by containing two or more kinds. 6. Further, by weight%, Mo: 0.01 to 1%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Cu: 0.01 to 3%, B: 0. 1.0001-0.003% of any one kind, or two or more kinds are contained, The manufacture of the high strength steel excellent in strength and toughness according to any one of claims 1 to 5. Method. 7. The composition further comprises, by weight, any one or more of REM: 0.002 to 0.10% and Ca: 0.0003 to 0.0030%. A method for producing a high-strength steel excellent in strength and toughness according to any one of claims 1 to 6.