JP2001323321A - Method for producing steel excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently produce steel having excellent strength and toughness without the excess addition of alloys causing cost increase and deterioration in weldability, and complicated hot working or heat treating process inferior in productivity. SOLUTION: In this method, a slab in which as the grains having specified components, the grains having a grain size of 0.005 to 2.0 μm, and having a composition, containing at least Ca, Al and O, wherein, as for the elements other than O, Ca is contained in >=5% by mass, and the balance Al with inevitable impurities are contained by the grain number of 100 to 3,000 pieces/mm2 is heated to the temperature region of 1,100 to 1,300 deg.C to control the austenite grain size to <=200 μm, is thereafter rolled at 800 to 1,100 deg.C at a cumulative draft of >=40% to recrystallize austenite and to control the grain size to <=40 μm and is further cooled from >=700 to <=600 deg.C at a cooling rate of >=1 deg.C/s.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently producing a steel material having excellent toughness used for ships, bridges, middle and high-rise buildings, marine structures and the like.

[0002]

2. Description of the Related Art In recent years, with the increase in size of bridges, middle and high-rise buildings, marine structures, and the like, demands for toughness as well as the use of thicker steel materials have increased.

As means for improving toughness, TMCP is used.
(Thermo-Mechanical Contro
l Process) is well known. This is a method in which the microstructure of the steel material is achieved by a suitable combination of heating, rolling, cooling, and heat treatment steps, and the steel material is toughened.

[0004] In steels having a strength of 500 N / mm ² or less, the main structures are ferrite (α) and pearlite (P), and toughness is mainly controlled by the α grain size. Various methods have been conventionally proposed as a method for reducing the α particle size. As a typical method, as disclosed in Japanese Patent Publication No. 49-7291, for example, controlled rolling is performed in the non-recrystallization temperature range of austenite (γ), followed by accelerated cooling to reduce γ to α. There is a method of reducing α during transformation to α. Further, a technique for improving strength and toughness by so-called two-phase rolling, in which the temperature range of controlled rolling is expanded to a γ / α two-phase range, has also been proposed. For example, Japanese Patent Publication No. 58-5967
As disclosed in Japanese Patent Application Laid-Open Publication No. H11-260, a method for improving toughness by devising components and rolling conditions has been proposed.

[0005] However, as the required characteristics of steel materials become stricter, high-pressure rolling at a low temperature and rapid cooling are required, resulting in a decrease in productivity due to temperature waiting, and a load on a refining process due to shape instability. Problems such as an increase may occur. Therefore, in order to efficiently produce a tough steel, it is necessary to refine γ grains before transformation, but the conventional technique has the following disadvantages. For example, suppression of γ coarsening due to low-temperature heating of slab may cause material deterioration due to segregation,
The refinement of recrystallized γ by large rolling under the γ high temperature range is naturally limited by the capacity of the rolling mill.

There is also disclosed a method of reducing the α grain size by heat treatment without using hot working such as rolling. For example, "Iron and Steel", 77th, No. 1, 1991, 171
~ 178 pages, V and N were added in larger amounts than usual to achieve a finer γ,
A method for producing a fine α-structure by normalizing treatment has been developed. However, in order to obtain a fine α structure by this method, it is necessary to add V in an amount of 0.1% or more and N in an amount of 0.01% or more, and deterioration of weldability and toughness of a heat affected zone is inevitable. .

A high-strength steel having a strength of 600 N / mm ² or higher has a structure mainly composed of bainite (B) and martensite (M). The basic structural unit that governs the base material toughness of such steel is not the old γ grain size, but the size of the region called a packet or a block. Is the most effective method.

[0008] The basic manufacturing method is a tempering process including quenching and tempering. The rolling temperature range and rolling reduction are devised to reduce the γ grain size, and the strength level is ensured to ensure hardenability. A typical method is to adjust the amount of alloy addition in accordance with the conditions. For example, JP-A-49-37814 discloses a strength of 600 N / m characterized by low C-Ti-B addition.
m ² Class High tensile steel has been proposed. However, the use of B to increase the strength has the drawback that the properties are unstable due to variations in components and manufacturing conditions, and that the HAZ hardness is significantly increased.

[0009] As a technique without B addition, there is Japanese Patent Publication No. 60-9 / 1985.
Although disclosed in Japanese Patent Application Laid-Open No. 086, from the examples, the applicable plate thickness range of this technique is about 30 mm. Japanese Patent Application Laid-Open No. 53-119219 aims to provide a thick high-tensile steel by a reheating quenching and tempering process. This is intended to leave undissolved Nb carbonitride at the time of reheating by adding Nb exceeding 0.02%, to prevent coarsening of crystal grains and improve base metal toughness. This technique cannot sufficiently utilize the effect of solid-solution Nb to refine the rolling structure, improve hardenability, and strengthen precipitation. Therefore, addition of Ni and Mo in addition to Nb and V is substantially essential. At a disadvantage.

As described above, most of the prior art of tempered high-strength steel of 600 N / mm ² class or higher is achieved by securing hardenability by adding B, and when B is not added, thin material is used. Limited.

Japanese Patent Application Laid-Open No. Hei 6-93332 discloses a technique relating to a method of rolling high strength steel for the purpose of improving low temperature toughness. This includes C, Si, Mn, Nb, Ti, B, sol. Immediately after the controlled rolling of Al and N regulated material steel, accelerated cooling to a predetermined temperature range and then isothermally maintaining the temperature range for a certain time or gradually cooling the temperature range for a certain time to obtain a fine bainite structure However, the waiting time for temperature adjustment becomes extremely long, which causes a reduction in rolling efficiency, an increase in cost associated with isothermal holding and slow cooling, and a significant decrease in productivity. In Japanese Patent Application Laid-Open No. Hei 6-128638, a slab to which Nb and V are added is heated to an Ac ₃ point or higher, and hot-rolled while being cooled at an Ar ₃ point or higher.
A method for producing a high-strength, high-toughness thick steel plate, characterized in that accelerated cooling to a temperature of 650 ° C. or less at a cooling rate of at least / s is disclosed. According to this, in order to improve low-temperature toughness, the temperature at which hot rolling is completed is set near the Ar ₃ point (700 to 8).
(00 ° C.). As a result, the deformation resistance of the rolled steel increases, and a large load is applied to the rolling mill.

Japanese Patent Application Laid-Open No. 4-358023 discloses a technique for producing a tough steel by tempering. This is a technique for rapidly increasing the temperature by charging a steel sheet into a furnace set at a temperature higher than the tempering temperature, suppressing quenching dislocations and suppressing coarsening of precipitates, and achieving toughness. However, this requires strict temperature control, and it is difficult to suppress material variations in the plate.

[0013]

Excellent strength and toughness without the need for excessive addition of alloys which cause cost increase and deterioration of weldability, and without the need for complicated hot working or heat treatment steps which do not have good productivity. It is intended to provide a method for efficiently producing a steel material having the following.

[0014]

The feature of the present invention is that, unlike the conventional idea, the fine oxides of Ca, Al and Mg are dispersed to achieve a finer structure by an efficient and inexpensive method. In that it has realized a steel material with excellent toughness. The summary is as follows.

(1) In mass%, C: 0.03 to 0.1
8%, Si: 0.01-0.5%, Mn: 0.4-2.
0%, P: ≦ 0.02%, S: ≦ 0.02%, Al:
0.005 to 0.04%, Ti: 0.005 to 0.03
%, Ca: 0.0005 to 0.003%, N: 0.00
0.05 to 0.006%, the balance being Fe and unavoidable impurities, and having a particle size of 0.005 to 2.0 μm.
m, the composition contains at least Ca, Al, and O, and the element excluding O contains at least 5% by mass of Ca, and the balance is composed of Al and other unavoidable impurities. / Mm ² containing 110%
After heating to a temperature range of 0 to 1300 ° C. to reduce the austenite grain size to 200 μm or less, austenite is recrystallized by rolling at 800 to 1100 ° C. at a cumulative reduction of 40% or more to further reduce the grain size to 40 μm or less. , 70
A method for producing a steel material having excellent toughness, comprising cooling from a temperature of 0 ° C. or more to a temperature of 600 ° C. or less at a cooling rate of 1 ° C./s or more.

(2) C: 0.03-0.1% by mass
8%, Si: 0.01 to 0.5%, Mn: 0.40 to
2.0%, P: ≦ 0.02%, S: ≦ 0.02%, A
l: 0.005 to 0.04%, Ti: 0.005 to 0.
03%, Ca: 0.0005 to 0.003%, Mg:
0.0001-0.002%, N: 0.0005-0.
006%, the balance being Fe and unavoidable impurities, and having a particle diameter of 0.005 to 2.0 μm, containing at least Ca, Al, Mg, and O as components and excluding O in mass ratio, A slab containing 5% or more of Ca and 1% or more of Mg and the balance of Al and other unavoidable impurities contains 100 to 3000 particles / mm ^{2 in} a temperature range of 1100 to 1300 ° C. After heating to reduce the austenite particle size to 200 μm or less, 800 to 110
Austenite is recrystallized by rolling at 0 ° C. with a cumulative reduction ratio of 40% or more to a grain size of 40 μm or less, and further cooled from a temperature of 700 ° C. or more to a temperature of 600 ° C. or less at a cooling rate of 1 ° C./s or more. A method for producing a steel material having excellent toughness, characterized in that:

(3) After performing the cooling, an additional 65
The method for producing a steel material having excellent toughness according to the above (1) or (2), wherein tempering is performed at a temperature of 0 ° C. or lower.

(4) In mass%, Cu: ≦ 1.5%, N
i: ≦ 2.0%, Nb: ≦ 0.05%, V: ≦ 0.1
%, Cr: ≦ 0.6%, Mo: ≦ 0.6%, B: 0.0
The method for producing a steel material having excellent toughness according to any one of the above (1) to (3), comprising one or more of 002 to 0.002%.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The present inventors have focused on the γ grain size at the time of slab reheating as a metallographic factor for improving the toughness, and studied to achieve fine γ fineness using an oxide. This is based on the recognition that the reheated γ structure is the origin of the material fabrication in the steps after rolling. That is, if γ is coarsened or mixed during reheating, no matter how much the rolling conditions are adjusted in the recrystallization temperature range to refine γ grains, the This is because the particle size does not reach.

If the recrystallized γ is fine, it is advantageous not only for α-granulation using the γ / α transformation, but also for improving the toughness of high-tensile steel using B or M. Generally γ
It is known that the hardenability decreases as the grain size decreases, but on the other hand, in addition to the miniaturization of packets and blocks, the fine α partially generated also has the effect of dividing effective crystal grains. Therefore, the toughness is significantly improved. α
The decrease in strength due to the formation does not appear remarkably unless growth and coarsening of α occur.

In order to refine the reheated γ grains, it is necessary to suppress the growth of γ grains at a high temperature. The most effective means for that purpose is a method of pinning the γ grain boundary with dispersed particles and stopping the movement of the grain boundary. As one of the dispersed particles having such an action, conventionally, Ti or A
It was believed that 1 nitrides were effective. However, since these nitrides are inferior in stability at high temperatures to oxides, the dissolution of the nitrides is started particularly at a temperature higher than 1200 ° C., and some coarse particles are formed. On the other hand, by utilizing an oxide stable at a high temperature as pinning particles, coarsening of γ grains is suppressed, and the final structure can be refined without imposing a load on a process after rolling.

Furthermore, since the vicinity of the stable oxide becomes a non-uniform deformation region at the time of processing, it becomes a preferential nucleation site for recrystallization together with the γ grain boundary, and has the effect of promoting the miniaturization of recrystallized γ.

The pinning effect of the crystal grain boundaries by the dispersed particles increases as the volume ratio of the dispersed particles increases and as the diameter of one particle increases. However, since the volume fraction of dispersed particles has an upper limit depending on the concentration of the elements constituting the particles contained in the steel, if the volume fraction is assumed to be constant, a smaller particle diameter is more effective for pinning. . From such a viewpoint, the present inventors have conducted various studies to increase the volume fraction of the oxide and to obtain an appropriate particle size.

As one of means for increasing the volume fraction of the oxide, there is a case where the amount of oxygen is increased. However, since an increase in the amount of oxygen causes generation of a large number of coarse inclusions harmful to the material, It is not an effective means. Therefore, the present inventors have studied the use of an element having a small solubility product with oxygen in order to make maximum use of oxygen. Al is generally used as a material having a small solubility product with oxygen, that is, as a strongly deoxidizing element. However, since oxygen cannot be sufficiently utilized by Al alone, it is necessary to utilize Ca, which is a deoxidizing element stronger than Al, and utilization of Mg is also effective. As a result of conducting an experiment using Ca and Mg as deoxidizing elements, the composition of oxide particles generated in steel was C
It has been found that when the content of a is 5% or more and the content of Mg is 1% or more, the volume fraction of the oxide, that is, the amount of the oxide can be increased. On the basis of this result, the composition of the particles contained in the steel was changed to include at least Ca, Al, O, and O
Elements except for the mass ratio of Ca: 5% or more,
When Mg is contained in addition to a, Al and O, Ca: 5%
Above, Mg: 1% or more.

Next, the particle size effective for pinning will be described. As described above, the pinning effect of the crystal grain boundaries by the dispersed particles, the larger the volume ratio of the dispersed particles,
The larger the particle size of one particle, the larger. Assuming that the volume ratio of the particles is constant, the smaller the size of a single particle, the larger the number of particles and the larger the pinning effect.However, if the particles are too small, the proportion of the particles present at the grain boundaries will be small, The effect was thought to be reduced. Therefore, using a test piece in which the size of the oxide particles was changed variously, as a result of a detailed investigation of the austenite particle size when heated to a high temperature,
It has been determined that particles having a size of 0.005 to 2.0 μm are effective for pinning. 0.005μm
Few smaller oxide particles were observed. From these results, the required particle diameter was set to 0.005 to 2.0 μm.

Further, the number of pinning particles required for suppressing the growth of γ grains was examined. As the number of particles increases, the structure unit becomes finer, and thus the toughness improves. The toughness required for the steel material varies depending on the application, but in order to stably obtain a reheat γ having a particle size of 200 μm or less, which is advantageous for the final structure and material fabrication, the number of particles is 100 particles / m 2.
It was found that m ² or more was required. However, as the number of particles increases, the effect of improving toughness saturates, and increasing the number of particles more than necessary increases the possibility of generating coarse particles that are harmful to toughness. Considering this, the upper limit of the number of particles is 3000 particles / m.
m ² is appropriate.

The size and number of the oxide are measured, for example, in the following manner. An extract replica is prepared from a steel material, and the size and the number of the oxide are measured by observing at least 20 fields of view at a magnification of 10000 with an electron microscope at an observation area of at least 1000 μm ² . At this time, an extracted replica collected from any part from the surface layer to the center of the steel material may be used. If the particles can be properly observed, the observation magnification may be reduced.

Oxide particles are generated when deoxidizing molten steel. This is called a primary oxide. Furthermore, during casting and solidification, Ti-Al-Ca oxide is generated as the temperature of the molten steel decreases. This is called a secondary oxide. In the present invention, either a primary oxide or a secondary oxide may be used.

Next, the reasons for limiting the range of the basic components of the present invention will be described.

C is an effective component for improving the strength of steel, with the lower limit being 0.03%, and an excessive addition significantly reduces the weldability and HAZ toughness of the steel material, so the upper limit is 0.18%. did.

Si is an element necessary for deoxidation during melting,
When an appropriate amount is added, the matrix is solid-solution strengthened.
Add 1% or more. On the other hand, if over 0.5% is added, HA
Since the toughness is reduced by the hardening of Z, the upper limit is set to 0.5%.

Mn needs to be added in an amount of 0.4% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is 2.0% in the allowable range of the toughness and cracking property of the welded portion. And

The lower the content of P is, the more desirable it is. However, in order to reduce this industrially, a large cost is required. Therefore, the upper limit is set to 0.02%.

The lower the content of S is, the more desirable it is. However, in order to reduce this industrially, a large cost is required. Therefore, the upper limit is set to 0.02%.

Al is an important deoxidizing element, and the lower limit is set to 0.005%. Also, when Al is present in a large amount,
Since the surface quality of the slab deteriorates, the upper limit is set to 0.04%.

At the same time, Ti is a deoxidizing element and is added in an amount of 0.005% or more in order to exert a certain effect on the heating γ and the grain refinement of HAZ by forming a Ti nitride by combining with N. However, when the amount of solid solution Ti increases, the HAZ toughness decreases. Therefore, the upper limit is set to 0.03%.

Ca must be added in an amount of 0.0005% or more in order to generate a Ca-based oxide. However, excessive addition produces coarse inclusions,
The upper limit was 03%.

N has an effect of improving HAZ toughness by precipitating as TiN, so the lower limit is made 0.0005%. However, an increase in solid solution N causes a decrease in HAZ toughness, so the upper limit was made 0.006%.

Cu is effective for improving the strength of the steel material, but if it exceeds 1.5%, the HAZ toughness is reduced. Therefore, the upper limit is set to 1.5%.

Although Ni is effective for improving the strength and toughness of the steel material, the upper limit is 2.0% because an increase in the amount of Ni increases the production cost.

Nb is an element effective for improving the strength and toughness of the base material by improving the hardenability, but in the HAZ portion, an excessive addition significantly reduces the toughness, so the upper limit is 0.05%. And

Since V, Cr and Mo also have the same effect as Nb, 0.1%, 0.6%,
The upper limit was 0.6%.

B is grain boundary ferrite harmful to HAZ toughness,
0.0002% or more from the suppression of the growth of the ferrite side plate and the fixation of the solute N in the HAZ by the precipitation of BN.
002% or less.

Next, the manufacturing process of the steel of the present invention will be described. The slab having the above-described component composition and oxide composition / number is heated to a temperature range of 1100 to 1300 ° C.
When the heating temperature is lower than 1100 ° C., homogenization of alloy elements cannot be achieved, which causes material instability. On the other hand, if the heating temperature exceeds 1300 ° C., the heating γ does not become coarse, but in addition to the increase in the heating intensity, a temperature wait occurs in order to make the rolling temperature appropriate.

After the slab is heated, the slab is rolled at a temperature of 800 to 1100 ° C. with a cumulative reduction of 40% or more in order to reduce the recrystallization γ to about 40 μm or less. If the rolling is performed at a temperature lower than 800 ° C., there is a possibility that a process α that causes embrittlement is generated, and the productivity is significantly reduced. If the rolling temperature exceeds 1100 ° C., there is a concern that the recrystallized γ grains may become coarse. Reduction rate of 40
%, The γ grains are not refined to 40 μm or less, and the refinement of the final structure and improvement in toughness are not achieved.

After the completion of the rolling in the γ region, the cooling from a temperature of 700 ° C. or more to a temperature of 600 ° C. or less at a cooling rate of 1 ° C./s or more is for refining the final structure. If the cooling start is lower than 700 ° C., α may be generated and coarsened before cooling. If the cooling rate is lower than 1 ° C./s or the cooling stop temperature is higher than 600 ° C., sufficient strength cannot be secured.

After the above cooling, tempering may be performed at a temperature of 650 ° C. or less. Tempering above 650 ° C. must be avoided because it results in significant strength reduction.

As described above, by fabricating under a predetermined condition using a slab having a predetermined oxide composition and a predetermined number, a fine structure can be achieved, and as shown in FIG. It becomes possible.

[0049]

EXAMPLES Trial production was carried out using the chemical components shown in Table 1. Numbers 1 to 8 are inventive examples, and 9 to 14 are comparative examples. The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. Adjust the dissolved oxygen of molten steel with Si before adding Ti,
Thereafter, Ti, Al, Ca or Ti, Al, M
g and Ca were sequentially added to perform deoxidation, and continuous casting was performed.
Then, through heating, rolling, cooling and heat treatment steps, steel plates of various thicknesses were obtained.

[0050]

[Table 1]

Table 2 shows the composition of the oxide particles and the particle diameter of the oxide particles.
The number of particles of 005 to 2.0 μm, the manufacturing conditions of the steel sheet, and the mechanical properties of the base material are shown. Yield strength of each steel plate (YP or Y
S) and tensile strength (TS) were evaluated using a JIS No. 4 tensile test piece, and the fracture surface transition temperature (vTrs) was determined by JI.
It was determined from a 2 mm V notch Charpy test using an S4 impact test piece. The test piece was sampled from the center of the thickness in a direction perpendicular to the rolling direction.

[0052]

[Table 2]

As is clear from Table 2, Examples 1 to 8 of the present invention have a particle diameter of 0.005 to 2.0 μm, Ca, M
g, the number of particles of the oxide containing Al in a predetermined composition is 100 to
Since it is in the range of 3000 pieces / mm ² , the reheating γ and the rolling γ are finer, and the toughness at the center of the sheet thickness is extremely excellent.

On the other hand, all of Comparative Examples 9 to 14 are inferior in toughness at a transition temperature of −40 ° C. or higher. These were caused by the fact that 12 did not have the prescribed oxide composition of the present invention and had a small amount of oxide, and the others had a smaller number of oxide particles than the prescribed number. And the fine structure of the final structure could not be achieved.

[0055]

According to the present invention, ships, bridges, middle and high-rise buildings,
This is an efficient method for producing steel that meets strict toughness requirements and is applied to marine structures, etc., and has a great effect on this kind of industrial field, and also contributes to society from the viewpoint of structural safety. Very large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between the number of particles having a predetermined particle size and composition and a fracture surface transition temperature.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshihiko Koseki 20-1 Shintomi, Futtsu-shi Nippon Steel Corporation Technology Development Headquarters (72) Inventor Jun Otani 1-chome, Nishinoshu, Oita City Nippon Steel Corporation Inside the Oita Works (72) Inventor Tomohiko Hata 1st in Nishinoshi, Oita City Nippon Steel Corporation F-term inside the Oita Works (reference) AA23 AA24 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CA03 CB01 CB02 CC03 CC04 CD02 CF01 CF02

Claims (4)

[Claims] C: 0.03 to 0.18% by mass%,
Si: 0.01 to 0.5%, Mn: 0.4 to 2.0%,
P: 0.02%, S: 0.02%, Al: 0.00
5 to 0.04%, Ti: 0.005 to 0.03%, C
a: 0.0005 to 0.003%, N: 0.0005 to 0.005%
0.006%, the balance being Fe and unavoidable impurities, and having a particle size of 0.005 to 2.0 μm, containing at least Ca, Al, O as a composition, and excluding O in mass ratio. Particles containing 5% or more of Ca, the balance being Al and other unavoidable impurities, having a particle number of 100
Slabs containing 3000 pieces / mm ² , 1100-13
Heat to a temperature range of 00 ° C to reduce the austenite grain size to 200
μm or less, and at 800-1100 ° C., the cumulative rolling reduction 4
The toughness is characterized in that austenite is recrystallized by rolling at 0% or more to reduce the grain size to 40 μm or less, and further cooled from a temperature of 700 ° C. or more to a temperature of 600 ° C. or less at a cooling rate of 1 ° C./s or more. Method of manufacturing excellent steel products. 2. C: 0.03 to 0.18% by mass%
Si: 0.01 to 0.5%, Mn: 0.4 to 2.0%,
P: 0.02%, S: 0.02%, Al: 0.00
5 to 0.04%, Ti: 0.005 to 0.03%, C
a: 0.0005% to 0.003%, Mg: 0.0001
０．００0.002%, N: 0.0005 to 0.006%, the balance being Fe and unavoidable impurities, and having a particle size of 0.005 to 2.0 μm and a composition of at least Ca, Al, Mg, Particles containing O and containing elements other than O in a mass ratio of Ca: 5% or more and Mg: 1% or more, with the balance being Al and other unavoidable impurities, are 100 to 3000 particles / mm ^2. 1100
After heating to a temperature range of １1300 ° C. to reduce the austenite grain size to 200 μm or less, austenite is recrystallized by rolling at 800 to 1100 ° C. with a cumulative reduction ratio of 40% or more to further reduce the grain size to 40 μm or less. 700
A method for producing a steel material having excellent toughness, characterized in that the steel is cooled from a temperature of at least 1 ° C to a temperature of at most 600 ° C at a cooling rate of at least 1 ° C / s. 3. The method for producing a steel material having excellent toughness according to claim 1, wherein after the cooling, tempering is further performed at a temperature of 650 ° C. or less. 4. In mass%, Cu: ≦ 1.5%, Ni: ≦
2.0%, Nb: ≦ 0.05%, V: ≦ 0.1%, C
r: ≦ 0.6%, Mo: ≦ 0.6%, B: 0.0002
The method for producing a steel material having excellent toughness according to any one of claims 1 to 3, comprising one or more of 0.002% to 0.002%.