JPH09137253A - Ultra-high-strength steel excellent in stress corrosion cracking resistance and low temperature toughness and method for producing the same - Google Patents

Abstract

(57) An object of the present invention is to provide an ultra high strength steel having a yield strength of 1270 MPa or more, which is excellent in stress corrosion cracking resistance and low temperature toughness, and a manufacturing method thereof. SOLUTION: C: 0.06-0.12%, Si: 0.
02-0.25%, Mn: 0.05-0.50%, N
i: more than 11.0 to 13.0%, Mo: 0.8 to 2.5
%, Cr: more than 1.0 to 3.0%, V: 0.05 to 0.2
0%, Al: 0.01 to 0.10% is contained, and if necessary, a steel slab containing one or more of Cu, Nb, Ti, Ca, and REM is heated to 1000 to 1250 ° C., subjected to inter-sectional reduction 50% or more of the thermal processing, and ends the processing at 700 ° C. or higher, then cooled to 0.99 ° C. or less, then, after heating to a temperature range of Ac less than ₃ points -40 ° C. to Ac ₃ point, quenching After that, tempering is performed at a temperature of Ac ₁ point or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel having high strength despite having a low carbon content, and excellent in stress corrosion cracking resistance and low temperature toughness in a stress corrosion environment such as seawater or salt water, and particularly, The present invention relates to a method for producing an ultra high strength steel having a yield strength of 1270 MPa or more.

[0002]

2. Description of the Related Art In recent years, energy demand has increased year by year, and in order to secure a stable supply of the energy, interest in marine development such as submarine resource development and submarine crust geological survey has increased, and marine structures and submarine research work vessels that lead to this submarine development. The construction concept for the construction of a submarine oil production base, etc. is becoming active.

It is desired that the material used for the above-mentioned purpose has structurally very high strength and high toughness and resistance to stress corrosion cracking even under use environment conditions such as seawater. In order to meet the demand for such safe, reliable, high-strength and high-toughness materials, Ni-containing low alloy steels have been developed and their quality has been improved. As an ultrahigh-strength steel material having high reliability in the deep sea, for example, as disclosed in JP-B-64-11105,
Ni-containing steel reduces N and O, Al (%) × N (%) ×
Ni-Cr-Mo satisfying the relation of 10 ⁴ <1.5
A -V type high toughness ultra high strength steel has been proposed, and a great effect can be seen.

Further, as disclosed in Japanese Patent Publication No. 1-51526, Ni-Mo-containing 5 to 8% of Ni.
A method for producing an ultra-high strength steel having excellent stress corrosion cracking resistance by directly quenching and tempering Nb-based steel has been proposed. However, none of the proposals has satisfied the above-mentioned object. On the other hand, 10Ni-8Co developed in the United States
Ultra-high strength steel, which is a type of steel, has sufficient strength, but its low temperature toughness and stress corrosion cracking resistance are not sufficient.
The problem is that the cost is very high because it contains 8% of Co.

[0005]

As described above, according to the prior art, it is possible to sufficiently satisfy both stress corrosion cracking resistance and low temperature toughness in an ultra high strength steel, particularly in a high strength steel having a yield strength of 1270 MPa or more. It is difficult, and at present, steel materials that meet the above-mentioned purpose have not yet been developed. The present invention provides an ultrahigh-strength steel having a yield strength of 1270 MPa or more, which is excellent in stress corrosion cracking resistance and low temperature toughness, and which solves the above problems, and a method for producing the same.

[0006]

DISCLOSURE OF THE INVENTION The present inventors have for the purpose of developing a Ni-containing low alloy steel having stress corrosion cracking resistance in seawater or salt water and having higher strength and toughness. As a result of various studies on steel components and manufacturing methods thereof, in particular, hot working-reheating quenching / tempering, as a result, Ni-containing steel with reduced C, Si, and Mn, Mo,
V, Cr, etc. are added, and these elements are sufficiently solid-solved and processed in the hot working step, and then in the reheating and quenching step,
Solid solution was Mo, V, Cr and the like precipitated in the heating as a carbide, in the temperature range of Ac less than ₃ points -40 ° C. to Ac ₃ point still has precipitated, thereby prior austenite grain boundary and intragranular Form dense non-diffusive acicular austenite groups with high dislocation density, and suppress the formation of general massive diffusion type austenite grains (dislocation density lower than non-diffusion austenite), resulting in high strength. And high toughness can be achieved. Furthermore, at this heating temperature, the former austenite grain boundaries disappear, the susceptibility to intergranular cracking due to stress corrosion is significantly reduced, and the stress corrosion cracking resistance is improved, and the target steel can be obtained. Found out.

The present invention is constructed on the basis of such knowledge, and the gist thereof is (1)% by weight, and C: 0.06.
~ 0.12%, Si: 0.02-0.25%, Mn:
0.05 to 0.50%, Ni: more than 11.0 to 13.0%
Carbide mainly composed of a mixed tempering structure of acicular martensite produced from non-diffusion type reverse transformation austenite and lumpy martensite produced from diffusion type reverse transformation austenite and containing at least one of Cr, Mo and V An ultra high strength steel excellent in stress corrosion cracking resistance and low temperature toughness, which is characterized by having (2)% by weight, C:
0.06 to 0.12%, Si: 0.02 to 0.25%,
Mn: 0.05 to 0.50%, Ni: more than 11.0 to 1
3.0%, Mo: 0.8-2.5%, Cr: over 1.0-
3.0%, V: 0.05 to 0.20%, Al: 0.01
Ultra-high-strength steel excellent in stress corrosion cracking resistance and low-temperature toughness, characterized by containing 0.10% to 0.10% and the balance being Fe and unavoidable impurity elements. (3)% by weight, C:
0.06 to 0.12%, Si: 0.02 to 0.25%,
Mn: 0.05 to 0.50%, Ni: more than 11.0 to 1
3.0%, Mo: 0.8-2.5%, Cr: over 1.0-
3.0%, V: 0.05 to 0.20%, Al: 0.01
.About.0.10%, and further Cu: 0.2 to 2.0%,
Nb: 0.005-0.10%, Ti: 0.005-
Ca having a strength improving element group consisting of 0.05% and / or an inclusion morphology controlling effect: 0.0005 to 0.0050
%, REM: 0.0005 to 0.0050% of one kind or two or more kinds, and the balance is Fe and inevitable impurity elements, which is excellent in stress corrosion cracking resistance and low temperature toughness. Tensile steel. (4) C in weight%: 0.06
~ 0.12%, Si: 0.02-0.25%, Mn:
0.05 to 0.50%, Ni: more than 11.0 to 13.0
%, Mo: 0.8 to 2.5%, Cr: more than 1.0 to 3.0
%, V: 0.05 to 0.20%, Al: 0.01 to 0.
A steel slab containing 10% and the balance consisting of Fe and unavoidable impurities is heated between 1000 and 1250 ° C., hot working with a cross-sectional area reduction of 50% or more is performed, and the processing ends at 700 ° C. or more. And cool to a temperature below 150 ° C, then
After a further re-heating to a temperature range of Ac less than ₃ points -40 ° C. to Ac ₃ point,
A method for producing an ultra-high strength steel excellent in stress corrosion cracking resistance and low temperature toughness, which comprises performing quenching treatment and then tempering treatment at a temperature of Ac ₁ point or less. (5) C: 0.06 to 0.12% and Si: 0.02 to 0.
25%, Mn: 0.05 to 0.50%, Ni: 11.0
Super-13.0%, Mo: 0.8-2.5%, Cr: 1.
0 to 3.0%, V: 0.05 to 0.20%, Al:
0.01 to 0.10%, further Cu: 0.2 to
2.0%, Nb: 0.005 to 0.10%, Ti: 0.
005 to 0.05% of a strength improving element group, and / or Ca having an effect of controlling inclusion morphology: 0.0005 to 0.
0050%, REM: 0.0005 to 0.0050% of a steel slab containing one kind or two or more kinds and the balance consisting of Fe and unavoidable impurities is heated between 1000 to 1250 ° C. to reduce the cross-sectional area. 70% after hot working 50% or more
Quit working at 0 ℃ or higher, cooled to a temperature of 0.99 ° C. or less, then, after a further reheated to a temperature range of Ac less than ₃ points -40 ° C. to Ac ₃ point, performs quenching, then Ac ₁ point A method for producing an ultra-high-strength steel excellent in stress corrosion cracking resistance and low-temperature toughness, characterized by performing tempering treatment at the following temperature.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the reasons for limiting the steel components of the present invention will be described. C: C is an element effective for improving hardenability and easily increasing strength, but if it is less than 0.06, the strength of the steel of the present invention cannot be secured. If it exceeds 0.12%, the heat-affected zone of welding is significantly hardened and the stress corrosion cracking resistance is lowered. Therefore, the C content is set to 0.06 to 0.12%.

Si: Si is effective for improving strength. It is also a necessary deoxidizing element for steelmaking, and if it is less than 0.02%, the stress corrosion cracking resistance of the base material decreases due to the increase of non-metallic inclusions. If it exceeds 0.25%, in the case of Ni-containing steel, temper embrittlement becomes large and the low temperature toughness deteriorates. Therefore, the Si content is set to 0.02 to 0.25%. Mn: Mn improves hardenability and is effective in securing strength and toughness, but if it is less than 0.05%, it is not effective. On the other hand, in the case of the Ni-containing steel of the present invention, if Mn is high, the temper embrittlement becomes large like Si, and the stress corrosion cracking resistance of the base material and the weld heat affected zone is lowered, so that it is 0.50.
% Or less. Therefore, the Mn content is set to 0.
It was set to 05 to 0.50%.

Ni: Ni increases stacking fault energy,
It is effective in increasing cross-slip, making stress relaxation easier, increasing impact absorption energy, and improving low temperature toughness. Further N
i is most effective in coexistence with Cr, Mo, V, etc. included in the present invention, but at the reheating temperature during reheating and quenching treatment, it is a diffusion type reverse alloy consisting of massive austenite formed by melting of carbides. Mixed austenite is formed between the transformed austenite and the non-diffusion type reverse transformation austenite, which is a dense formation of needle-shaped austenite without the dissolution of carbide, but this non-diffusion type reverse transformation austenite is higher than the diffusion type reverse transformation austenite. It has a dislocation density and acts extremely effectively to increase the strength. That is,
Ni has the effect of delaying the dissolution of carbides such as Cr, Mo and V, and can hold acicular austenite stably up to a high temperature. Therefore, in order to secure the strength of the present invention due to the high temperature stability of non-diffusion type austenite, it is necessary to add more than 11.0%. Further, if added in excess of 13.0%, retained austenite precipitates during tempering to lower strength / toughness. Therefore, the Ni content exceeds 11.0
It was set to 13.0%.

Mo: Mo is an element effective for precipitation strengthening by tempering and suppression of temper embrittlement, and at the same time, it is an important element of the present invention like Ni. That is, at the reheating temperature of the reheating and quenching treatment, Mo carbide precipitated in the heating process partially remains at a high temperature, which promotes the formation of acicular austenite having a high dislocation density. If it is less than 0.8%, the Mo carbides are dissolved at the reheating quenching temperature, the generation of diffusion-type reverse transformation austenite composed of massive austenite is promoted, and the strength is remarkably lowered. The effect of improving strength exceeding 2.5% is not recognized, and rather coarse alloy carbides increase and the toughness decreases. Therefore, the content of Mo is set to 0.8 to 2.5%.

Cr: Cr is effective in improving the hardenability and ensuring the strength. Further, as with Mo, at the reheating temperature of the reheating quenching treatment, Cr carbide precipitated in the heating process partially remains at a high temperature, which promotes the formation of needle-like austenite having a high dislocation density and increases to a high temperature. Can hold,
It is easy to secure strength without impairing toughness. If it is 1.0% or less, the strength of the steel cannot be obtained, and if it exceeds 3.0%, the increase in strength is saturated and the toughness decreases. Therefore, the Cr content is 1.
It was set to 0 to 3.0%.

V: V is necessary for forming a carbonitride during the tempering treatment to secure the strength. Also, Mo, Cr
Similarly to the above, during the reheating and quenching treatment, V is precipitated during the heating and remains, which promotes the formation of acicular austenite having a high dislocation density and is effective for securing the strength. 0.0
If less than 5%, the target strength cannot be obtained, and 0.20%
If it exceeds, the toughness will decrease. Therefore, if the V content is 0.
It was set to 05 to 0.20%.

Al: Al is an element necessary for deoxidation, and at the same time, forms a nitride during heating of a steel piece and is effective for making austenite grains fine. However, if it is less than 0.01%, its effect is small, and if it exceeds 0.08%, alumina-based inclusions increase to impair the toughness. Therefore, Al
Content of 0.01 to 0.10%. The above are the basic components of the steel in the present invention, but in the present invention, one or more of Cu, Nb, Ti and Ca can be added in addition to the above components. Cu, Nb, and Ti components have an equal effect of improving the strength of steel.
Also, the Ti component is effective for making the austenite grains finer, and in order to secure the desired effect, the respective lower limit contents are Cu: 0.2%, Nb: 0.005%, Ti:
It is necessary to set it to 0.005%. But each C
If the content of u: 2.0%, Nb: 0.10% and Ti: 0.05% is exceeded, the low temperature toughness is lowered and the stress corrosion cracking resistance is enhanced. .

Ca: Ca is extremely effective in spheroidizing non-metallic inclusions, and is effective in improving low temperature toughness and reducing toughness anisotropy. It requires 0.0005%, but if it exceeds 0.005%, the inclusions increase and the toughness and stress corrosion cracking resistance deteriorate. Therefore, the content of Ca is set to 0.0005 to 0.0050%. Similar to Ca, REM: REM has the effect of improving the shape of non-metallic inclusions, and improves the low temperature toughness and reduces the anisotropy of the toughness. It takes 0.0005%, but 0.00
If it exceeds 5%, toughness and stress corrosion cracking resistance are reduced due to the increase of inclusions. Therefore, the content of REM is 0.0
It was set to 005 to 0.0050%.

In addition to the above-mentioned components, P, S, N, O and the like as unavoidable impurities are harmful elements which lower the low temperature toughness and the stress corrosion cracking resistance which are the characteristics of the present invention.
The smaller the amount, the better. Preferably, P: 0.005% or less, S: 0.003% or less, N: 0.0050% or less,
O: Adjust to 0.0030% or less. Next, a manufacturing method which is another skeleton of the present invention will be described.

That is, with the above component system, sufficient stress corrosion cracking resistance and low temperature toughness can be obtained with an ultra high strength steel having a yield strength of 1270 MPa or more. Further, as shown in FIG. With the microstructure distribution, the strength, stress corrosion cracking resistance and low temperature toughness are sufficiently satisfied. In order to obtain this texture distribution, it is preferable that the production method is under the following conditions. The processing and heat treatment of steel will be described by taking heating, rolling and heat treatment of a thick plate as an example.

First, 1000 pieces of steel pieces having the above-mentioned steel composition were prepared.
Heat to ~ 1250 ° C. In this heating, in addition to refining the heated austenite grains, use is made of the above-mentioned needle-like austenite non-diffusion type reverse transformation austenite generation in the reheating quenching treatment after hot rolling and strengthening by fine precipitation in the tempering treatment. In order to do so, it is necessary to heat the steel slab to 1000 ° C. or higher and sufficiently dissolve Mo, Cr, V and the like. At a low heating temperature of less than 1000 ° C., this solid solution action becomes insufficient, the undissolved alloy carbide becomes coarse, and on the contrary, sufficient precipitation strengthening cannot be expected at the time of tempering, which also causes a decrease in toughness. On the other hand, 1250 ° C
At a heating temperature exceeding 1, although alloyed carbides such as Mo, Cr, and V sufficiently form a solid solution, in the Ni-containing steel of the present invention, oxides increase on the surface of the billet, and finally surface flaws after rolling are removed. Occurs. Further, the heated austenite grains become coarse, and the austenite grains are less likely to become finer in the subsequent rolling, which causes a decrease in toughness. Therefore, in consideration of these problems, the heating temperature of the billet is set to 1000 to 1250 ° C.
And

Next, the thus heated steel slab is hot-rolled at a reduction of the cross-sectional area of 50% or more, finished at a temperature of 700 ° C. or higher, and cooled to a temperature of 150 ° C. or lower. This is because the rolled microstructure has a martensite structure composed of fine austenite grains. The austenite grains formed by hot rolling inherit the as-rolled austenite grains even in the subsequent reheating and quenching treatment, so in order to secure a higher level of low temperature toughness, the austenite grains are refined by rolling-recrystallization. Is important, and therefore a reduction rate of 50% or more in cross-sectional area reduction is required. Also,
At a finishing temperature of 700 ° C or lower, rolling in the non-recrystallized region increases the anisotropy of strength and toughness. Further, the cooling after the completion of rolling is specified to be 150 ° C. or less, because when the temperature from 150 ° C. is exceeded to the next reheating quenching treatment temperature range,
This is because in the steel of the present invention, the martensitic transformation may not be completed, and the strength and toughness are greatly affected. still,
Since the steel of the present invention has a sufficiently high hardenability, cooling after rolling produces a martensitic structure even with air cooling. Therefore, the cooling method (air cooling or water cooling) is not particularly limited.

Next, the hot-rolled and cooled steel had Ac ₃ points-
After reheating to a temperature range of 40 ° C. to less than Ac ₃ points, quenching treatment is performed. FIG. 1 schematically shows the concept of the austenite formation process as the quenching temperature rises. In FIG.
(A) Ac ₃ points When quenched at a temperature below -40 ° C,
(B) If the quenched at a temperature of Ac less than ₃ points -40 ° C. to Ac ₃ point, also (c) are respectively shown schematically austenite formation form of reheating temperature when quenched in Ac ₃ point or higher There is. In the process of heating the martensite of the pre-structure to Ac ₁ to Ac ₃ points, acicular austenite is generated from the former austenite grain boundaries and the martensite lath boundaries within the grains, and agglomerate austenite from some grain boundaries. (Diffusion-type reverse transformation austenite) is also generated and coexists with carbide and ferrite. Since acicular austenite is generated by non-diffusion (martensite type) reverse transformation, it has a large amount of dislocations as compared with diffusion type reverse transformation austenite and contributes to strengthening. Then, as shown in (b), Ac ₃ points -40 ° C
When heated to less than ~ Ac ₃ point, ferrite decreases, the area of the acicular austenite group increases, and the area of partially agglomerated austenite also increases. When quenching from this temperature range,
In particular, acicular austenite has a martensite structure in which a larger number of dislocations are introduced, and high strength can be achieved. At this heating temperature, the former austenite grain boundaries disappear, stress susceptibility to stress corrosion cracking is significantly reduced, and stress corrosion cracking resistance is improved. On the other hand, as shown in (c), non-diffusion type reverse transformation austenite which contributes to strengthening after quenching is generally used due to solid solution and coarsening of agglomerates of Mo, Cr and V carbides when heated to the Ac ₃ point or more. It changes to diffusion-type reverse transformation austenite and the dislocation density decreases rapidly, and when quenching from this temperature range, the hardness of the martensitic structure also decreases. As a result, the strength is lowered and the diffusion-type austenite grain boundaries are clearly formed, so that the stress corrosion cracking resistance is lowered. Also,
As shown in (a), at a temperature below Ac ₃ point -40 ° C, the ferrite area between acicular austenite is large and the strength is reduced.

Next, the reheat-quenched steel is then tempered at a temperature below the Ac ₁ point. At temperatures above the Ac ₁ point, toughness decreases due to the formation of unstable austenite. Therefore, the tempering temperature is limited to the Ac ₁ point or lower in order to sufficiently strengthen the carbide forming elements such as Mo, Cr and V by precipitation strengthening and to obtain strength and toughness. The steel obtained by such a manufacturing process has high strength and high toughness, and the stress corrosion cracking resistance is improved.

[0022]

EXAMPLES Steel pieces obtained by smelting steel having the composition shown in Table 1 were manufactured into steel plates having a thickness of 25 to 60 mm based on the respective manufacturing conditions of the method of the present invention and the comparative method shown in Table 2. did. For these, the mechanical properties of the base material and the KIscc value (critical fracture toughness value for stress corrosion cracking resistance) of the base material and the weld heat affected zone were investigated. The welding was performed by TIG welding with a heat input of 2.5 kJ / mm.

Steels having the chemical compositions shown in Table 1 and Table 2
And the mechanical properties of the base metal obtained according to the manufacturing conditions shown in Table 3 and ASTM E 3 in artificial seawater with 3.5% NaCl.
Table 3 shows the KIscc test results of the base metal portion and the welding heat affected zone using the test piece (1 TCT) shown in FIG. In the table, the underlined numerical values indicate component values, temperature conditions and characteristics that are outside the scope of the present invention.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

In the examples of the present invention (1-A to 16-P in which the steel of the present invention and the method of the present invention are combined), the mechanical properties of the base material are high strength and high toughness, and the intent of the present invention is The stress corrosion cracking resistance is also a high KIscc value for both the base material and the heat-affected zone. On the other hand, even with the method of the present invention, comparative steels (Q, R,
S, T) in a comparative example in combination with Example 17-Q
In, the C content is high and the KIscc value in the heat affected zone is low.
In Example 18-R, since the Mn is high, the KIscc value of the base material and the weld heat affected zone is low. In Example 19-S, since the amount of Ni is low, acicular austenite having a high dislocation density is less generated and γ
Since m is small, the strength is insufficient. In Example 20-T, since the amounts of addition of Mo and Cr are low, acicular austenite having a high dislocation density is not formed and γm is small, so that the strength is insufficient.

Next, even in the case of the steel of the present invention, in the comparative example combined with the comparative method (21 to 26) deviating from the scope of the method of the present invention, Example 21-D has a low heating temperature of the billet. Undissolved alloy carbide is coarsened, precipitation strengthening during tempering is reduced, and strength and toughness are reduced. In Example 22-D, the cumulative rolling reduction during rolling was small, the austenite grains were not finely pulverized by rolling-recrystallization, and the toughness was lowered because γd was the main component. In Example 23-D, since the cooling stop temperature after rolling was 150 ° C. or higher, the temperature was increased to the next reheating quenching treatment temperature, so that the generation of needle-like austenite was decreased, and the retained austenite was increased, resulting in a decrease in strength and toughness. doing. In Example 24-E, the reheating and quenching temperature is low, and a large amount of ferrite mixed between acicular austenite is mixed, resulting in a decrease in strength. Example 25-E has a reheating and quenching temperature of A.
c Higher than ₃ points and dislocation density is lower than that of acicular austenite. Only bulk austenite (diffusion type austenite) is available, and strength is insufficient, and austenite grain boundaries become clear, and crack susceptibility due to hydrogen is high, and the limit KIscc value decreases. doing. In Example 26-E, the tempering temperature exceeds Ac ₁ point, and strength and toughness are deteriorated due to the formation of unstable austenite and α.

[0029]

According to the range of components and the production method of the present invention,
It has become possible to manufacture an ultra-high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness (for example, a steel having a yield strength of 1270 MPa or more). As a result, it can be applied even in a more severe usage environment.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the concept of austenite formation process with increase in reheating and quenching temperature, (a) Ac ₃ points and quenching at a temperature below -40 ° C., (b) Ac according to the present invention. Quenching at a temperature of ₃ points -40 ℃ ~ Ac ₃ points, (c) is A
FIG. 3 is a diagram schematically showing austenite formation at the reheating temperature when quenching is performed at ₃ points or more.

Claims (5)

[Claims] 1. By weight%, C: 0.06 to 0.12% Si: 0.02 to 0.25% Mn: 0.05 to 0.50% Ni: more than 11.0 to 13.0% Carbide mainly composed of a mixed tempering structure of acicular martensite produced from non-diffusion type reverse transformation austenite and lumpy martensite produced from diffusion type reverse transformation austenite and containing at least one of Cr, Mo and V An ultra high strength steel excellent in stress corrosion cracking resistance and low temperature toughness, which is characterized by having 2. By weight%, C: 0.06 to 0.12% Si: 0.02 to 0.25% Mn: 0.05 to 0.50% Ni: more than 11.0 to 13.0% Mo: 0.8 to 2.5% Cr: more than 1.0 to 3.0% V: 0.05 to 0.20% Al: 0.01 to 0.10%, with the balance being Fe and unavoidable High-strength steel excellent in stress corrosion cracking resistance and low-temperature toughness, which is characterized by comprising a mechanical impurity element. 3. By weight%, C: 0.06 to 0.12% Si: 0.02 to 0.25% Mn: 0.05 to 0.50% Ni: more than 11.0 to 13.0% Mo: 0.8 to 2.5% Cr: more than 1.0 to 3.0% V: 0.05 to 0.20% Al: 0.01 to 0.10% and further Cu: 0. 2 to 2.0% Nb: 0.005 to 0.10% Ti: 0.005 to 0.05% Strength-improving element group, and / or inclusion morphology controlling action Ca: 0.0005 to 0 .0050% REM: 0.0005 to 0.0050% of one or more, and the balance being Fe and unavoidable impurity elements, which is excellent in stress corrosion cracking resistance and low temperature toughness. High strength steel. 4. By weight%, C: 0.06-0.12% Si: 0.02-0.25% Mn: 0.05-0.50% Ni: more than 11.0-13.0% Mo : 0.8 to 2.5% Cr: more than 1.0 to 3.0% V: 0.05 to 0.20% Al: 0.01 to 0.10%, balance Fe and unavoidable A steel slab made of impurities is heated between 1000 and 1250 ° C., hot working with a cross-sectional area reduction of 50% or more is performed, the processing is completed at 700 ° C. or more, and the temperature is cooled to 150 ° C. or less, and thereafter. , Further A
c ₃ points −40 ° C. to less than Ac ₃ points, reheat, quenching treatment, then tempering treatment at a temperature of Ac ₁ point or less, stress corrosion cracking resistance and low temperature toughness A method for producing super high strength steel with excellent durability. 5. By weight%, C: 0.06 to 0.12% Si: 0.02 to 0.25% Mn: 0.05 to 0.50% Ni: more than 11.0 to 13.0% Mo : 0.8 to 2.5% Cr: more than 1.0 to 3.0% V: 0.05 to 0.20% Al: 0.01 to 0.10%, and further Cu: 0.2 To 2.0% Nb: 0.005 to 0.10% Ti: 0.005 to 0.03%, and / or Ca: 0.0005 to 0. 0050% REM: A steel slab containing 0.0005 to 0.0050% of one kind or two or more kinds, the balance of which is Fe and unavoidable impurities, is heated between 1000 to 1250 ° C. to reduce the cross-sectional area by 50. 7% after hot working
Quit working at 00 ° C. or higher, cooled to a temperature of 0.99 ° C. or less, then, after a further reheated to a temperature range of Ac less than ₃ points -40 ° C. to Ac ₃ point, performs quenching, then Ac ₁ point A method for producing an ultra-high-strength steel excellent in stress corrosion cracking resistance and low-temperature toughness, characterized by performing tempering treatment at the following temperature.