RU2797277C1 - Steel material - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel material used for the manufacture of seamless pipes operating in contact with a medium containing hydrogen sulfide and gaseous carbon dioxide. The steel material has a chemical composition containing, wt.%: С: 0.035 or less, Si: 1.00 or less, Mn: 1.00 or less, P: 0.030 or less, S: 0.0050 or less, sol. Al: 0.005 to 0.100, N: 0.001 to 0.020, Ni: 5.00 to 7.00, Cr: 10.00 to 14.00, Cu: 1.50 to 3.50, Mo: from 1.00 to 4.00, V: 0.01 to 1.00, Ti: 0.02 to 0.30, Co: 0.01 to 0.50, Ca: 0.0003 to 0, 0030, O: 0.0050 or less, W: 0 to 1.50, Nb: 0 to 0.50, B: 0 to 0.0050, Mg: 0 to 0.0050, rare earth metals (REM): 0 up to 0.020, the rest is Fe and inevitable impurities. Among the inclusions present in the steel material, the total concentration of Mn sulfide particles having a Mn content of 10% or more and an S content of 10% or more, and having an equivalent circle diameter of 1.0 µm or more, and Ca sulfide particles having a calcium content of 20% or more and a sulfur content of 10% or more and an equivalent twist diameter of 2.0 μm or more is 0.50 particles/mm ² or less.

EFFECT: steel material has high resistance to sulfide stress corrosive effect.

8 cl, 3 tbl, 2 ex

Description

FIELD OF TECHNOLOGY

[0001] The present invention relates to a steel material, and more specifically, to a steel material suitable for use in an acidic environment containing hydrogen sulfide and carbon dioxide gas.

PRIOR ART

[0002] Oil and gas wells (hereinafter, oil and gas wells are generally referred to as "oil wells") contain an environment that contains a large amount of corrosive substances. Examples of the corrosive agent include corrosive gases such as hydrogen sulfide and carbon dioxide. In the present disclosure, an environment containing hydrogen sulfide and gaseous carbon dioxide is referred to as an "acidic environment". The temperature of the sour medium, although dependent on the depth of the well, ranges from normal temperature to approximately 200°C. The term "normal temperature" as used herein means 24±3°C.

[0003] Chromium (Cr) is known to be effective in improving carbon dioxide corrosion resistance of steel. Therefore, in an oil well in an environment containing a large amount of carbon dioxide, martensitic stainless steel materials containing about 13 wt. C reduced. The 13Cr steel material and the Super 13Cr steel material are mainly used in oil wells in moderately acidic environments in which the partial pressure of H ₂ S is 0.03 bar or less.

[0004] Meanwhile, in a highly corrosive acidic environment in which the partial pressure of H ₂ S exceeds 0.03 bar and 0.1 bar or less, the partial pressure of H ₂ S is higher than in a moderately acidic environment. For this reason, in such an acidic environment, duplex stainless steel material is used, which has a higher Cr content than 13Cr steel material and Super 13Cr steel material. However, duplex stainless steel material is more expensive than 13Cr steel material and Super 13Cr steel material. For this reason, there is a need for a steel material suitable for use in a highly corrosive acidic environment with an H ₂ S partial pressure of more than 0.03 to 0.1 bar, even if its Cr content is lower than that of duplex stainless steel material. .

[0005] Japanese Patent Application Unexamined Publication (PCT Application Translation) No. 10-503809 (Patent Document 1), Japanese Patent Application Publication No. 2000-192196 (Patent Document 2), Japanese Patent Application Publication No. 08-246107 (Patent Document 3) and Japanese Patent Application Publication No. 2012-136742 (patent document 4) provide steel materials with excellent resistance to sulfide stress corrosion cracking (SSC).

[0006] The steel material according to Patent Document 1 contains, in wt.%, C in the amount of 0.005 to 0.05%, Si ≤ 0.50%, Mn 0.1 to 1.0%, P ≤ 0.03 %, S ≤ 0.005%, Mo 1.0 to 3.0%, Cu 1.0 to 4.0%, Ni 5 to 8%, and Al ≤ 0.06%, the rest is Fe and impurities, and also corresponds to the condition Cr + 1.6Mo ≥ 13 and 40C + 34N + Ni + 0.3Cu - 1.1Cr - 1.8Mo ≥ -10.5. The microstructure of the martensitic stainless steel according to this document is the tempered martensite structure. Patent Document 1 states that a Mo content of 1.0 to 3.0% can improve SSC resistance.

[0007] The steel material of Patent Document 2 contains, in wt%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01% or less, Cr: 9% to 14%, Mo: 3.1% to 7%, Ni: 1% to 8%, Co: 0.5% to 7%, sol. Al: 0.001 to 0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0 to 5% and W: 0 to 5%, the rest is Fe and inevitable impurities. When Mo is contained, the Ms point decreases. Accordingly, the presence of Co together with Mo suppresses the lowering of the Ms point, so that the microstructure is formed as a single-phase martensite structure. Patent Document 2 claims that as a result, SSC resistance can be improved while maintaining a strength of 80 ksi. inch or more (552 MPa or more).

[0008] The chemical composition of martensitic stainless steel according to patent document 3 contains, in weight. %, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.1% to 1.0%, P: 0.025% or less, S: 0.015% or less , Cr: 12% to 15%, Ni: 4.5% to 9.0%, Cu: 1% to 3%, Mo: 2% to 3%, W: 0.1% to 3% , Al: from 0.005 to 0.2%, and N: from 0.005 to 0.1%, the rest is Fe and unavoidable impurities. The chemical composition disclosed above additionally satisfies the condition 40C + 34N + Ni + 0.3Cu + Co - 1.1Cr - 1.8Mo - 0.9W ≥ -10. Patent Document 3 claims that excellent SSC resistance can be obtained by selecting a steel material containing 12% to 15% Cr, with less than 0.05% C, 4.5% or more Ni, Cu: 1 to 3 %, Mo: 2 to 3% and W: 0.1 to 3%.

[0009] The martensitic stainless steel seamless pipe according to Patent Document 4 contains, in weight. %, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1% to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr : 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, the rest is Fe and unavoidable impurities. The martensitic stainless steel seamless pipe according to this document has a yield strength of 655 to 862 MPa and a stress ratio at yield strength to stress at tensile strength of 0.90 or more. Patent Document 4 claims that excellent SSC resistance can be achieved while maintaining a strength of 655 MPa or more by making the C content 0.01% or less, the content of Cr, Ni, and Mo being in proper ranges, and containing additional proper the amount of Cu and V or the proper amount of W.

LIST OF CONTRADING MATERIALS

PATENT LITERATURE

[0010] Patent Document 1: Japanese Patent Application Pending (PCT Application Translation) No. 10-503809

Patent Document 2: Japanese Patent Application Published No. 2000-192196

Patent Document 3: Japanese Patent Publication Publication No. 08-246107

Patent Document 4: Japanese Patent Application Published No. 2012-136742

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0011] Each of the above-described Patent Documents 1-4 proposes a means for improving the resistance to SSC in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar by adjusting the content of each element in the chemical composition . However, SSC resistance in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar can be improved by any means other than those suggested in the above patent literature.

[0012] It is an object of the present invention to provide a steel material excellent in SSC resistance.

THE SOLUTION OF THE PROBLEM

[0013] The steel material according to the present invention has a chemical composition containing (in wt.%),

C at 0.035% or less;

Si in the amount of 1.00% or less;

Mn 1.00% or less;

P 0.030% or less;

S in the amount of 0.0050% or less;

sol. Al in an amount from 0.005 to 0.100%;

N in an amount from 0.001 to 0.020%;

Ni in an amount from 5.00 to 7.00%;

Cr in an amount from 10.00 to 14.00%;

Cu in an amount from 1.50 to 3.50%;

Mo in an amount from 1.00 to 4.00%;

V in an amount from 0.01 to 1.00%;

Ti in an amount from 0.02 to 0.30%;

Co in an amount from 0.01 to 0.50%;

Ca in an amount from 0.0003 to 0.0030%;

O in an amount of 0.0050% or less;

W in an amount from 0 to 1.50%;

Nb in an amount from 0 to 0.50%;

B in an amount from 0 to 0.0050%;

Mg in an amount from 0 to 0.0050%; And

rare earth metals (REM) in an amount from 0 to 0.020%,

the rest is Fe and inevitable impurities, in which

among inclusions in steel material, the total concentration of Mn sulfide particles having a Mn content of 10% or more and an S content of 10% or more, and having an equivalent circle diameter of 1.0 µm or more, and Ca sulfide particles having a calcium content of 20% or more and a sulfur content of 10% or more and an equivalent circle diameter of 2.0 µm or more is 0.50 particles/mm ² or less.

TECHNICAL RESULT OF THE INVENTION

[0014] The steel material of the present invention has excellent SSC resistance.

DISCLOSURE OF EXAMPLES OF CARRYING OUT THE INVENTION

[0015] The inventors of the present invention have studied a steel material having excellent resistance to SSC, especially in an acidic environment having an H ₂ S partial pressure of more than 0.03 to 0.1 bar.

[0016] First, the inventors of the present invention investigated the chemical composition of a steel material that can exhibit excellent SSC resistance in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar. As a result, the inventors of the present invention concluded that a steel material having a chemical composition containing, in wt %, C of 0.035% or less, Si of 1.00% or less, Mn of 1.00%, or less, P in the amount of 0.030% or less, S in the amount of 0.0050% or less, sol. Al in an amount from 0.005 to 0.100%, N in an amount from 0.001 to 0.020%, Ni in an amount from 5.00 to 7.00%, Cr in an amount from 10.00 to 14.00%, Cu in an amount from 1, 50 to 3.50%, Mo from 1.00 to 4.00%, V from 0.01 to 1.00%, Ti from 0.02 to 0.30%, Co from 0.01 to 0.50%, O 0.0050% or less, W 0 to 1.50%, Nb 0 to 0.50%, B 0 to 0.0050 %, Mg in an amount of 0 to 0.0050% and rare earth metals (REM) in an amount of 0 to 0.020%, the rest is Fe and impurities, and can achieve sufficient resistance to SSC in an acidic environment with a partial pressure of H ₂ S ranging from 0.03 to 0.1 bar.

[0017] However, even in a steel material in which the content of each chemical composition element is within the above-described range, there are cases where sufficient SSC resistance cannot be achieved, for example, when the steel material has a high yield strength, such as 110 thousand pounds per sq. inch or more (758 MPa or more). Next, the present inventors investigated the reason for the deterioration of SSC resistance in a steel material having the above-described chemical composition. As a result, the authors of the present invention came to the following conclusions.

[0018] It is known that in a low alloy steel material with a Cr content of 2.00% or less, SSC is likely to occur in an acidic environment as a result of inclusion (oxide, sulfide, nitride, etc.) in the steel material serving as as the starting point of the crack. On the contrary, in high-alloy steel material with a Cr content of 10.00% or more, a strong passivation film is formed on the surface of the steel material due to the high Cr content compared to low-alloy steel material. For this reason, it is generally considered that SSC starting from inclusion is unlikely in high alloy steel material.

[0019] However, studies conducted by the present inventors have shown that even in a high alloy steel material, when its yield strength reaches 110 thousand psi. inch or more (758 MPa or more), SSC can occur in an acidic environment having a partial pressure of H ₂ S greater than a pressure in the range of 0.03 to 0.1 bar, by the following mechanism. When a particle of Mn sulfide is present in the outer layer of the steel material in an acidic environment with a partial pressure of H ₂ S in the range of 0.03 to 0.1 bar, the Mn sulfide in the surface layer will be dissolved due to the acidic environment. A depression forms on the surface as a trace of the dissolution of the manganese sulfide particle. The depression formed due to the dissolution of a large particle of manganese sulfide probably serves as the starting point for the occurrence of SSC.

[0020] In a low alloy steel material, all coarse grained inclusions (oxides, sulfides, nitrides, etc.) serve as the starting point of the SSC. On the other hand, in the case of a high alloy steel material, a particle of Mn sulfide, which is a special inclusion among the inclusions present in the surface layer of the steel material, dissolves, thereby forming a depression on the surface of the steel material to cause SSC. As already described, the present inventors have found that in the case of a high alloy steel material having a Cr content of 10.00% or more, SSC may occur by a different mechanism from that of a low alloy steel material.

[0021] Based on the above findings, the present inventors concluded that, in the case of a steel material having the above-described chemical composition, it is possible to suppress the formation of depressions on the surface, which are associated with the dissolution of Mn sulfide, by suppressing the formation of coarse particles of Mn sulfide, thereby improving the resistance of the steel material to SSC. Accordingly, the present inventors have concluded that an additional calcium content of 0.0003 to 0.0030 wt% in addition to the above chemical composition can suppress the formation of coarse Mn sulfide particles. As a result, the inventors of the present invention concluded that a steel material having a chemical composition containing, in wt %, C of 0.035% or less, Si of 1.00% or less, Mn of 1.00%, or less, P in the amount of 0.030% or less, S in the amount of 0.0050% or less, sol. Al in an amount from 0.005 to 0.100%, N in an amount from 0.001 to 0.020%, Ni in an amount from 5.00 to 7.00%, Cr in an amount from 10.00 to 14.00%, Cu in an amount from 1, 50 to 3.50%, Mo from 1.00 to 4.00%, V from 0.01 to 1.00%, Ti from 0.02 to 0.30%, Co from 0.01 to 0.50%, Ca 0.0003 to 0.0030%, O 0.0050% or less, W 0 to 1.50%, Nb 0 to 0 .50%, B in the amount of 0 to 0.0050%, Mg in the amount of 0 to 0.0050%, and rare earth metals (REM) in the amount of 0 to 0.020%, the rest is Fe and impurities, can suppress the formation of large particles Mn sulfide. In particular, as a result of the presence of Ca, Ca combines with S to form Ca sulfide. As a result of the formation of Ca sulfide, the amount of sulfur associated with manganese decreases. For this reason, the formation of large particles of manganese sulfide will be suppressed.

[0022] Accordingly, a steel material with the above-described chemical composition, which contains Ca, was obtained to study its resistance to SSC in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar. The result showed that although it is possible to suppress the formation of large particles of Mn sulfide, there may be a case where the resistance to SSC remains low. Accordingly, the inventors of the present invention carried out further investigation and study of the cause of low resistance to SSC. As a result, the present inventors have found that when there is the above-described Ca content, there may be a case where SSC resistance is degraded by the following mechanism.

[0023] At the above Ca content, Ca sulfide is formed and the formation of coarse particles of Mn sulfide is suppressed. However, the Ca sulfide itself, like the Mn sulfide, can also be dissolved in an acidic medium having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar. For this reason, when a large particle of Ca sulfide is present in the surface layer of the steel material, it dissolves, thereby forming a depression on the surface of the steel material, as in the case of Mn sulfide. This is the case where the SSC is due to a depression on the surface that is associated with calcium sulfide.

[0024] Based on the results described above, the inventors of the present invention concluded that if, in the steel material having the above-described chemical composition, it is possible not only to suppress the formation of coarse particles of Mn sulfide, but also to suppress the formation of large particles of Ca sulfide, excellent stability to SSC can be achieved even in an acidic environment with a partial pressure of H ₂ S in the range of 0.03 to 0.1 bar, even if the steel material has a high yield strength, 110 thousand psi. inch or more (758 MPa or more). Then, the present inventors further investigated to what level it is necessary to suppress the total concentration per unit area of Mn sulfide coarse particles and Ca sulfide coarse particles in order to achieve excellent SSC resistance even if the steel material has a yield strength of 110 kpsi. inch or more (758 MPa or more). As a result, the present inventors found that when the total concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 μm or more is 0.50 particles/ ^mm2 or less, excellent SSC resistance can be achieved even in an acidic environment with an H ₂ S partial pressure in the range of 0.03 to 0.1 bar, even if the steel material has a yield strength of 110 kpsi. inch or more (758 MPa or more).

[0025] As previously described, the steel material of the present invention is designed to suppress the formation of depressions on the surface of the steel material having a Cr content of 10.00% or more, and depressions were formed by Mn sulfide and Ca sulfide, which are inclusions soluble in acidic environment. The steel material according to the invention has the following configuration.

[0026] [1] A steel material having

chemical composition with the content, in wt.%,

C at 0.035% or less;

Si in the amount of 1.00% or less;

Mn 1.00% or less;

P 0.030% or less;

S in the amount of 0.0050% or less;

sol. Al in an amount from 0.005 to 0.100%;

N in an amount from 0.001 to 0.020%;

Ni in an amount from 5.00 to 7.00%;

Cr in an amount from 10.00 to 14.00%;

Cu in an amount from 1.50 to 3.50%;

Mo in an amount from 1.00 to 4.00%;

V in an amount from 0.01 to 1.00%;

Ti in an amount from 0.02 to 0.30%;

Co in an amount from 0.01 to 0.50%;

Ca in an amount from 0.0003 to 0.0030%;

O in an amount of 0.0050% or less;

W in an amount from 0 to 1.50%;

Nb in an amount from 0 to 0.50%;

B in an amount from 0 to 0.0050%;

Mg in an amount from 0 to 0.0050%; And

rare earth metals (REM) in an amount from 0 to 0.020%,

the rest is Fe and inevitable impurities, in which

among inclusions in steel material, the total concentration of Mn sulfide particles having a Mn content of 10% or more and an S content of 10% or more, and having an equivalent circle diameter of 1.0 µm or more, and Ca sulfide particles having a calcium content of 20% or more and a sulfur content of 10% or more and an equivalent circle diameter of 2.0 µm or more is 0.50 particles/mm ² or less.

[0027] [2] The steel material according to [1], wherein

chemical composition contains

W in an amount from 0.01 to 1.50%.

[0028] [3] The steel material according to [1] or [2], wherein

chemical composition contains

Nb in an amount from 0.01 to 0.50%.

[0029] [4] The steel material according to any one of paragraphs. [1] - [3], in which

the chemical composition contains one or more types of elements selected from the group consisting of:

B in an amount from 0.0001 to 0.0050%,

Mg in an amount from 0.0001 to 0.0050% and

rare earth metals (REM) in an amount from 0.001 to 0.020%.

[0030] [5] The steel material according to any one of paragraphs. [1] - [4], while

The steel material is a seamless steel pipe for oilfield tubular products.

[0031] Next, the steel material according to the invention will be disclosed in detail. The term "%" referring to an element means, unless otherwise indicated, wt%.

[0032] Chemical composition

The chemical composition of the steel material according to the invention contains the following elements.

[0033] C 0.035% or less

Carbon (C) is required. Those. the C content is greater than 0%. C improves hardenability, thereby increasing the strength of the steel material. However, when the C content exceeds 0.035%, the strength of the steel material becomes too high, which deteriorates the SSC resistance even if the content of other elements is within the range of the present invention. Therefore, the C content is 0.035% or less. As low a C content as possible is preferred. However, an excessive reduction in the C content will result in an increase in production costs. Therefore, considering industrial production, the lower limit of C content is preferably 0.001%. From the viewpoint of the strength of the steel material, the lower limit of the C content is preferably 0.002%, more preferably 0.005%, and even more preferably 0.007%. The upper limit of the C content is preferably 0.030%, more preferably 0.025%, even more preferably 0.020%, even more preferably 0.018%, even more preferably 0.016%, and even more preferably 0.015%.

[0034] Si 1.00% or less

Silicon (Si) is required. Those. the Si content is more than 0%. Si deoxidizes steel. However, when the Si content exceeds 1.00%, the deoxidation effect will be saturated and the hot workability of the steel material will deteriorate even if the content of other elements is within the range of the present invention. Therefore, the Si content is 1.00% or less. The lower limit of the Si content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Si content is preferably 0.70%, more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.45%.

[0035] Mn 1.00% or less

Manganese (Mn) is required. Those. the Mn content is more than 0%. Mn improves the hardenability, thereby increasing the strength of the steel material. However, when the Mn content is too high, Mn generates a large amount of coarse Mn sulfide particles. In an acidic environment, the coarse MnS particle present near the surface layer of the steel material can be dissolved. At this point, a depression is formed, which is a trace of dissolved MnS. This recess may serve as the starting point of the SSC, thereby causing the SSC. When the content of Mn exceeds 1.00%, depressions are formed, which are traces of dissolved MnS, which deteriorates the resistance to SSC, even if the content of other elements is within the range of the present invention. Therefore, the Mn content is 1.00% or less. The lower limit of the Mn content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Mn content is preferably 0.80%, more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0036] P 0.030% or less

Phosphorus (P) is an impurity that must be contained. Those. the P content is more than 0%. P segregates at grain boundaries, thereby promoting the occurrence of SSC. When the P content exceeds 0.030%, the SSC resistance of the steel material deteriorates significantly even if the content of other elements is within the range of the present invention. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.018%. As low a P content as possible is preferred. However, an excessive reduction in the P content will result in an increase in production costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0037] S at 0.0050% or less

Sulfur (S) is an impurity that must be contained. Those. the S content is more than 0%. Like P, S segregates at grain boundaries, thereby contributing to the occurrence of SSC. When the S content exceeds 0.0050%, the SSC resistance of the steel material deteriorates significantly even if the content of other elements is within the range of the present invention. Therefore, the S content is 0.0050% or less. The upper limit of the S content is preferably 0.0040%, more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%. As little S as possible is preferred. However, an excessive reduction in the S content will result in an increase in production costs. Therefore, considering industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0038] sol. Al in an amount from 0.005 to 0.100%

Aluminum (Al) deoxidizes steel. When the content of sol. Al is less than 0.005%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the content of sol. Al exceeds 0.100%, coarse-grained oxides are formed, which reduces the toughness of the steel material even if the content of other elements is within the range according to the present invention. Therefore, the content of sol. Al is from 0.005 to 0.100%. The lower limit of the content of sol. Al is preferably 0.010%, more preferably 0.013%, even more preferably 0.015% and even more preferably 0.018%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%, even more preferably 0.055%, and even more preferably 0.050%. The content of sol. Al used in the present description means the content of acid-soluble Al.

[0039] N in an amount of 0.001 to 0.020%

Nitrogen (N) combines with Ti to form fine Ti nitride. Finely dispersed TiN suppresses crystal grain enlargement due to the pinning effect. As a result, the strength of the steel material is increased. When the N content is less than 0.001%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the N content exceeds 0.020%, coarse nitrides will be generated, thereby degrading the SSC resistance of the steel material even if the content of other elements is within the range of the present invention. Therefore, the N content is from 0.001% to 0.020%. The lower limit of the N content is preferably 0.002%, more preferably 0.003%, even more preferably 0.004%, and even more preferably 0.005%. The upper limit of the N content is preferably 0.018%, more preferably 0.016%, even more preferably 0.014%, and even more preferably 0.012%.

[0040] Ni 5.00 to 7.00%

Nickel (Ni) is an austenite-forming element, and after quenching, the structure becomes martensitic. As a result, the strength of the steel material is increased. In addition, Ni forms sulfides on the passivating film in an acidic environment. Nickel sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from contacting the passivation film, thereby suppressing the destruction of the passivation film by chlorine ions and hydrogen sulfide ions. For this reason, the resistance of the steel material to SSC is increased. When the Ni content is less than 5.00%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the content of Ni exceeds 7.00%, the diffusion coefficient of hydrogen in the steel material decreases even if the content of other elements is within the range of the present invention. If the diffusion coefficient of hydrogen in the steel material decreases, the SSC resistance of the steel material deteriorates. Therefore, the Ni content is from 5.00 to 7.00%. The lower limit of the Ni content is preferably 5.10%, more preferably 5.20%, and even more preferably 5.30%. The upper limit of the Ni content is preferably 6.80%, more preferably 6.60%, even more preferably 6.50%, and even more preferably 6.40%.

[0041] Cr in an amount of 10.00 to 14.00%

Chromium (Cr) forms a passivating film on the surface of the steel material, thereby improving the SCC resistance of the steel material. When the Cr content is less than 10.00%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the Cr content exceeds 14.00%, δ (delta) ferrite is more likely to form in the steel material, thereby degrading the toughness of the steel material even if the content of other elements is within the range of the present invention. Therefore, the Cr content is from 10.00 to 14.00%. The lower limit of the Cr content is preferably 10.50%, more preferably 11.00%, even more preferably 11.50%, even more preferably 12.00%, even more preferably 12.20%. The upper limit of the Cr content is preferably 13.80%, more preferably 13.60%, even more preferably 13.50%, even more preferably 13.45%, even more preferably 13.40%.

[0042] 1.50% to 3.50% Cu

Copper (Cu) is dissolved in the steel material, thereby improving the resistance of the steel material to SSC. In addition, Cu forms sulfides on the passivating film in an acidic environment. The Cu sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from contacting the passivation film, thereby suppressing the destruction of the passivation film by chlorine ions and hydrogen sulfide ions. For this reason, the resistance of the steel material to SSC is improved. When the content of Cu is less than 1.50%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the content of Cu exceeds 3.50%, the hot workability of the steel material will deteriorate even if the content of other elements is within the range of the present invention. Therefore, the Cu content is from 1.50 to 3.50%. The lower limit of the Cu content is preferably 1.60%, more preferably 1.70%, and even more preferably 1.75%. The upper limit of the Cu content is preferably 3.40%, more preferably 3.30%, even more preferably 3.20%, and even more preferably 3.10%.

[0043] 1.00% to 4.00% Mo

Molybdenum (Mo) forms sulfides on the passivation film in an acidic environment. Mo sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from contacting the passivation film, thereby suppressing the destruction of the passivation film by chlorine ions and hydrogen sulfide ions. For this reason, the resistance of the steel material to SSC is increased. In addition, Mo is dissolved in the steel material, thereby increasing the strength of the steel material. When the content of Mo is less than 1.00%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the Mo content exceeds 4.00%, the austenite is unlikely to be stabilized even if the content of other elements is within the range of the present invention. As a result, a microstructure composed mainly of martensite will not be obtained in a stable form. Therefore, the Mo content is from 1.00 to 4.00%. The lower limit of the Mo content is preferably 1.20%, more preferably 1.50%, even more preferably 1.80%, even more preferably 2.10%, even more preferably 2.30%. The upper limit of the Mo content is preferably 3.80%, more preferably 3.60%, even more preferably 3.40%, even more preferably 3.30%, even more preferably 3.20%.

[0044] V in an amount of 0.01 to 1.00%

Vanadium (V) improves hardenability, thereby increasing the strength of the steel material. When the content of V is less than 0.01%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the V content exceeds 1.00%, the hardenability of the steel material will be excessively increased, resulting in deterioration of the SSC resistance of the steel material even if the content of other elements is within the range of the present invention. Therefore, the content of V is from 0.01 to 1.00%. The lower limit of the V content is preferably 0.02%, and more preferably 0.03%. The upper limit of the V content is preferably 0.70%, more preferably 0.50%, even more preferably 0.30%, even more preferably 0.20%, even more preferably 0.15% and even more preferably 0.10%.

[0045] Ti in an amount of 0.02 to 0.30%

Titanium (Ti) combines with C and/or N to form carbides and/or nitrides. In this case, coarsening of the crystal grain is suppressed due to the pinning effect, and thereby the strength of the steel material is increased. When the content of Ti is less than 0.02%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the Ti content exceeds 0.30%, δ ferrite is likely to form, thereby deteriorating the toughness of the steel material even if the content of other elements is within the range of the present invention. Therefore, the Ti content is from 0.02 to 0.30%. The lower limit of the Ti content is preferably 0.05%, and more preferably 0.07%. The upper limit of the Ti content is preferably 0.25%, more preferably 0.20%, even more preferably 0.18%, and even more preferably 0.16%.

[0046] 0.01 to 0.50% Co

Cobalt (Co) forms sulfides on a passivating film in an acidic environment. The Co sulfide prevents chloride ions (Cl ^- ) and hydrogen sulfide ions (HS ^- ) from contacting the passivation film, thereby suppressing degradation of the passivation film by chlorine ions and hydrogen sulfide ions. For this reason, the resistance of the steel material to SSC is improved. In addition, Co improves the hardenability of the steel material, ensures the stable high strength of the steel material, especially in industrial production. In particular, Co suppresses the formation of retained austenite, thereby suppressing the change in the strength of the steel material. When the content of Co is less than 0.01%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the content of Co exceeds 0.50%, the toughness of the steel material will deteriorate even if the content of other elements is within the range of the present invention. Therefore, the content of Co is from 0.01 to 0.50%. The lower limit of the Co content is preferably 0.02%, more preferably 0.04%, even more preferably 0.08%, and even more preferably 0.10%. The upper limit of the Co content is preferably 0.48%, more preferably 0.45%, even more preferably 0.40%, and even more preferably 0.35%.

[0047] 0.0003 to 0.0030% Ca

Calcium (Ca) combines with sulfur in the steel material to form Ca sulfide, thereby suppressing the formation of Mn sulfide. When Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more are present in the surface layer of the steel material, the Mn sulfide particles in the surface layer can be dissolved in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar. In this case, a depression is formed in the wake of the dissolution of the Mn sulfide particle. The depression that forms on the surface of the steel material is likely to serve as the starting point for the occurrence of SSC. Ca suppresses the generation of Mn sulfide particles, thereby reducing the concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more. For this reason, the resistance of the steel material to SSC is improved. When the content of Ca is less than 0.0003%, the above effect cannot be sufficiently achieved even if the content of other elements is within the range of the present invention. On the other hand, when the Ca content exceeds 0.0030%, Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more will be generated in excess even if the content of other elements is within the range of the present invention. When Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more, like the above-described Mn sulfide particles, are present in the surface layer of the steel material, they can be dissolved in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar, thereby forming depressions on the surface of the steel material. In this case, the SSC resistance of the steel material deteriorates. Therefore, the Ca content is from 0.0003 to 0.0030%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0007%, and even more preferably 0.0009%. The upper limit of the Ca content is preferably 0.0029%, more preferably 0.0028%, even more preferably 0.0027%, and even more preferably 0.0026%.

[0048] O 0.0050% or less

Oxygen (O) is an impurity that is necessarily contained. Those. the O content is more than 0%. O forms oxides, thereby degrading the toughness of the steel material. When the O content exceeds 0.0050%, the toughness of the steel material deteriorates significantly even if the content of other elements is within the range of the present invention. Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%. As low an O content as possible is preferred. However, an excessive reduction in the O content will result in an increase in production costs. Therefore, considering industrial production, the lower limit of O content is preferably 0.0001%, and more preferably 0.0002%.

[0049] The remaining constituents of the chemical composition of the steel material according to the present invention are Fe and impurities. In this case, impurities are considered to be elements which, during the industrial production of steel material, are mixed with ores and scrap metal as raw materials or come from the production environment, etc., and which are not intentionally contained, but are acceptable within the range if they do not adversely affect on the steel material according to the present invention.

[0050] With regard to optional elements

The chemical composition of the steel material according to the present invention may include W instead of the Fe part.

[0051] W in an amount of 0 to 1.50%

Tungsten (W) is an optional element and may not be included. Those. the W content can be 0%. When present, W stabilizes the passivation film in an acidic environment, thereby preventing the degradation of the passivation film by chloride ions and hydrogen sulfide ions. For this reason, the resistance of the steel material to SSC is increased. If W is contained even in a small amount, the above effect will be obtained to some extent. However, when the W content exceeds 1.50%, W combines with C to form coarse carbides. In this case, the toughness of the steel material will deteriorate even if the content of other elements is within the range according to the present invention. Therefore, the content of W is from 0 to 1.50%. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.30%, even more preferably 0.50%. The upper limit of the W content is preferably 1.45%, more preferably 1.40%, and even more preferably 1.37%.

[0052] The chemical composition of the steel material according to the present invention may include Nb instead of part of Fe.

[0053] Nb in an amount of 0 to 0.50%

Niobium (Nb) is an optional element and may not be included. Those. the Nb content can be 0%. When present, Nb combines with C and/or N to form Nb carbide and Nb carbonitride. In this case, coarsening of the crystal grain is suppressed due to the pinning effect, thereby increasing the strength of the steel material. If Nb is contained even in a small amount, the above effect will be obtained to some extent. However, when the Nb content exceeds 0.50%, Nb carbides and/or Nb carbonitrides will be generated in excess, thereby degrading the toughness of the steel material even if the content of other elements is within the range of the present invention. Therefore, the Nb content is from 0 to 0.50%. The lower limit of the Nb content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Nb content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0054] The chemical composition of the steel material according to the present invention may include B, Mg, and a rare earth metal (REM) instead of a portion of Fe.

[0055] B in an amount of 0 to 0.0050%

Boron (B) is an optional element and may not be included. Those. the content of B can be 0%. When B is present, B is dissolved in the steel material, thereby improving the hot workability of the steel material. If B is contained even in a small amount, the above effect will be obtained to some extent. However, when the content of B exceeds 0.0050%, coarse-grained B nitrides are generated, thereby deteriorating the toughness of the steel material even if the content of other elements is within the range of the present invention. Therefore, the content of B is from 0 to 0.0050%. The lower limit of B content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and even more preferably 0.0004%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0056] Mg in an amount of 0 to 0.0050%

Magnesium (Mg) is an optional element and may not be included. Those. the Mg content can be 0%. When Mg is present, it controls the morphology of the inclusions, thereby improving the hot workability of the steel material. If Mg is contained even in a small amount, the above effect will be obtained to some extent. However, when the Mg content exceeds 0.0050%, coarse-grained oxides are formed. In this case, the toughness of the steel material deteriorates even if the content of other elements is within the range according to the present invention. Therefore, the Mg content is from 0 to 0.0050%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0025%.

[0057] Rare earth metal (REM) in an amount from 0 to 0.020%

Rare earth metal (REM) is an optional element and may not be contained. Those. REM content can be 0%. When REM is present, it, like Mg, regulates the morphology of the inclusions, thereby improving the hot workability of the steel material. If the REM is contained even in a small amount, the above effect will be obtained to some extent. However, when the REM content exceeds 0.020%, coarse-grained oxides are formed. In this case, the toughness of the steel material deteriorates even if the content of other elements is within the range according to the present invention. Therefore, the REM content is from 0 to 0.020%. The lower limit of the REM content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the REM content is preferably 0.019%, more preferably 0.018%, and even more preferably 0.017%.

[0058] It should be noted that the term REM as used herein means elements of one or more element types selected from the group consisting of scandium (Sc), whose element number is 21, yttrium (Y), whose element number is 39 , a series of lanthanides, which include elements from lanthanum (La), whose element number is 57, to lutetium (Lu), whose element number is 71. The REM content used in this description means the total content of these elements.

[0059] Regarding Mn sulfide and Ca sulfide in steel material

In the steel material of the present invention, among the inclusions in the steel material, Mn sulfide particles and Ca sulfide particles are characterized as follows.

Mn sulfide particle: an inclusion having a wt% Mn content of 10% or more and an S content of 10% or more, and the inclusion wt% is 100%.

Ca sulfide particle: an inclusion having a wt% Ca content of 20% or more and an S content of 10% or more, and the wt% inclusion is 100%.

[0060] In the steel material of the invention, among the inclusions in the steel material, the total concentration (particles/mm ² ) of Mn sulfide particles and Ca sulfide particles such that each of the particles has a size to dissolve in an acidic environment and form a depression in the surface layer is reduced. The Mn sulfides in the steel material are present in such a form that they extend in the longitudinal direction (rolling direction) of the steel material. On the other hand, Ca sulfides in steel material are spherical. For this reason, Mn sulfides and Ca sulfides have a different size, which is likely to form a depression serving as a starting point for SSC. The diameter of the circle resulting from the transformation of the cross section of a particle of Mn sulfide or Ca sulfide into a circle of the same area is defined as the diameter of the equivalent circle. In the case of a steel material in which the content of each element in the chemical composition is within the range of the present invention, the amount per unit area of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2 0 µm or more correlates with resistance to SSC in an acidic environment having an H ₂ S partial pressure in the range of 0.03 to 0.1 bar.

[0061] In the present invention, the total number of particles of Mn sulfide and Ca sulfide per unit area (1 mm ² ) is defined as the total concentration (particles/mm ² ). Then, the total concentration of Mn sulfides having an equivalent circle diameter of 1.0 µm or more and calcium sulfides having an equivalent circle diameter of 2.0 µm or more is determined as the total concentration (ND). In this case, in the steel material of the present invention, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more is 0.50 particles/mm ² or less. Those. the total concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more is 0.50 particles/mm ² or less.

[0062] If the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more exceeds 0.50 particles/mm ² , the Mn sulfide particles and Ca sulfide in the surface layer of the steel material is likely to dissolve in an acidic environment with a partial pressure of H ₂ S in the range of 0.03 to 0.1 bar, and a depression may form on the surface of the steel material, which serves as the starting point for the occurrence of SSC, even if the content of each element in the chemical composition of the steel material is within the range according to the present invention. For this reason, the SSC resistance of the steel material deteriorates.

[0063] On the other hand, if the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more is 0.50 particles/ ^mm2 or less, the concentration of Mn sulfide particles and Ca sulfide particles having a size such that they are likely to dissolve in an acidic environment having a H ₂ S partial pressure in the range of 0.03 to 0.1 bar is sufficiently low assuming that the content of each element in the chemical composition of the steel material is within the range according to the present invention. For this reason, the formation of depressions on the surface of the steel material is unlikely even in an acidic environment. As a result, the SSC resistance of the steel material is greatly improved.

[0064] The upper limit of the total concentration ND of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more is preferably 0.48 particles/ ^mm2 , more preferably 0.47 particles/ ^mm2 , more preferably 0.46 particles/ ^mm2 , even more preferably 0.45 particles/ ^mm2 , even more preferably 0.44 particles/ ^mm2 , even more preferably 0.43 particles/mm2 ² and even more preferably 0.42 particles/mm ² .

[0065] A method for measuring the total concentration of ND particles

The total ND concentration of Mn particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more can be measured by the following method. In particular, the test specimen is taken from any position in the steel material. If the steel material is steel pipe, the test specimen is taken from a central position in the wall thickness. If the steel material is a steel rod, the test piece is taken from position R/2. In this case, the position R/2 means the position in the middle of the radius R in the cross section perpendicular to the longitudinal direction of the steel rod. If the steel material is a steel sheet, the test specimen is taken from a central position in thickness.

[0066] The test piece is cast with resin. When the steel material is a steel pipe, the surface of the test piece along the axial direction of the steel pipe and along the wall thickness direction of the steel pipe is defined as the observation surface. When the steel material is a steel rod, the surface of the test piece along the axial direction of the steel rod and along the radial direction of the steel rod is defined as the observation surface. When the steel material is a steel sheet, the surface of the test piece along the longitudinal direction (rolling direction) of the steel sheet and along the wall thickness direction of the steel sheet is defined as the observation surface. The observation surface is polished. After polishing, random 10 observation regions on the observation surface are examined. In each area of observation, the number of inclusions is determined. The area of each observation area is 36 mm ² (6 mm × 6 mm).

[0067] In particular, in order to determine the specific type of each inclusion, an analysis of the concentration of elements (EDS analysis) is carried out for each inclusion in each area of observation. In the EDS analysis, the accelerating voltage is set to 20 kV and the concentrations of the elements N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb are determined.

[0068] Based on the EDS analysis of each inclusion, it is determined whether the inclusion is Mn sulfide or Ca sulfide. When an inclusion has a Mn content of 10% or more and an S content of 10% or more, the inclusion is identified as "Mn sulfide". When an inclusion has a wt% Ca content of 20% or more and an S content of 10% or more, the inclusion is identified as "Ca sulfide".

[0069] For Mn sulfide particles identified in 10 observation areas, the total number of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more is obtained. In addition, for the Ca sulfide particles identified in 10 observation areas, the total number of Ca sulfide particles having an equivalent circle diameter of 2.0 μm or more is obtained. Based on the total number of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more, the total number of Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more, and the total area of 10 observation areas, the total concentration of ND (particles/mm ² ) Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more.

[0070] Measurement of the total concentration of ND particles can be carried out using a scanning electron microscope with a chemical composition analysis function (so-called SEM-EDS). For example, the FEI (ASPEX) Metal Quality Analyzer (product name) automatic inclusion analyzer can be used as the SEM-EDS.

[0071] Microstructure

The microstructure of the steel material according to the present invention mainly consists of martensite. In the present description, the term "martensite" includes not only hardened martensite, but also tempered martensite. Moreover, in the present specification, the phrase "mainly composed of martensite" means that the volume fraction of martensite in the microstructure is 80% or more. The rest of the microstructure is residual austenite. Those. the volume fraction of retained austenite is 0 to 20% in the steel material of the present invention. The volume fraction of retained austenite should preferably be as low as possible. The lower limit of the volume fraction of martensite in the microstructure of the steel material according to the present invention is preferably 85% and more preferably 90%. In addition, it is preferable that the microstructure of the steel material consists of a single phase of martensite.

[0072] In the microstructure, a small amount of retained austenite does not cause a significant decrease in strength and significantly increases the toughness of the steel material. However, if the volume fraction of retained austenite is too high, the strength of the steel material is greatly reduced. Therefore, as described above, in the microstructure of the steel material of the present invention, the volume fraction of retained austenite is 0 to 20%. From the viewpoint of ensuring strength, the upper limit of the volume fraction of retained austenite is preferably 15% and more preferably 10%. As described above, the microstructure of the steel material according to the present invention may be a single phase of martensite. Therefore, the volume fraction of retained austenite can be 0%. On the other hand, when even a small amount of retained austenite is present, the volume fraction of retained austenite is in the range of 0 to 20%, more preferably in the range of 0 to 15%, and even more preferably in the range of 0 to 10%.

[0073] Method for measuring the volume fraction of martensite

The volume fraction (vol.%) of martensite in the microstructure of the steel material according to the present invention is obtained by subtracting from 100% volume fraction (vol.%) of retained austenite, determined by the following method.

[0074] The volume fraction of retained austenite is determined by X-ray diffraction analysis. In particular, the test specimen is taken from any position in the steel material. If the steel material is steel pipe, the test specimen is taken from a central position in the wall thickness. If the steel material is a steel rod, the test piece is taken from position R/2. If the steel material is a steel sheet, the test specimen is taken from a central position in thickness. The size of the test sample is practically unlimited. The test specimen has dimensions of, for example, 15 mm x 15 mm x 2 mm thick. In this case, when the steel material is a steel pipe, the thickness direction of the test piece is the diameter direction of the steel pipe. When the steel material is a steel bar, the thickness direction of the test piece is the diameter direction of the steel bar. When the steel material is a steel sheet, the thickness direction of the test piece is the thickness direction of the steel sheet. Using the obtained test specimen to calculate the integral intensity of each plane, the X-ray diffraction intensity of each of the (200) α phase plane, (211) α phase plane, (200) γ phase plane, (220) γ phase plane, and plane ( 311) γ phases. When measuring the diffraction intensity of X-rays, the target of the X-ray diffraction apparatus is Mo (α radiation of MoK), and its output signal is 50 kV-40 mA. After the calculations, the volume fraction Vγ(%) of retained austenite is calculated by formula (I) for combinations (2×3=6 pairs) of each α phase plane and each γ phase plane. Then, the average value of the volume fractions Vγ of the retained austenite of the six pairs is determined as the volume fraction (%) of the retained austenite.

Vγ = 100/{1 + (Iα × Rγ)/(Iγ × Rα)}, (I)

where Iα is the integral intensity of the α phase, Rα is the crystallographic theoretical calculated value of the α phase, Iγ is the integral intensity of the γ phase, Rγ is the crystallographic theoretical calculated value of the γ phase. In the present description, Rα in the (200) plane of the α phase is 15.9, Rα in the (211) plane of the α phase is 29.2, and Rγ in the (200) plane of the γ phase is 35.5, Rγ in the (220) γ plane phase is 20.8, and Rγ in the (311) plane of the γ phase is 21.8. It should be noted that the volume fraction of retained austenite is obtained by rounding off the first decimal place of the obtained numerical value.

[0075] Using the volume fraction (%) of retained austenite obtained by the above X-ray diffraction analysis method, the volume fraction (vol.%) of martensite in the microstructure of the steel material is obtained by the following formula.

Volume fraction of martensite = 100 - volume fraction of retained austenite (%)

[0076] Yield strength

The yield strength of the steel material according to the present invention is practically unlimited. The yield strength of the steel material is preferably 758 MPa or more (110 thousand psi or more), and more preferably 862 MPa or more (125 thousand psi or more). Although the upper yield strength is practically unlimited, the upper yield strength of the steel material of the present invention is, for example, less than 1069 MPa (less than 155 psi). The upper yield strength of the steel material is more preferably 1000 MPa and even more preferably less than 965 MPa (less than 140 ksi).

[0077] As used herein, yield stress means 0.2% relative stress (MPa) obtained from tensile tests at normal temperature (24±3°C) in accordance with ASTM E8/E8M (2013). In particular, the yield strength is obtained in the following way. The test specimen is taken from any position in the steel material. When the steel material is a steel pipe, the test specimen is taken from a central position in the wall thickness. When the steel material is a steel rod, the test piece is taken from the R/2 position. When the steel material is a steel sheet, the test specimen is taken from a central position in thickness. The size of the test sample is practically unlimited. The tensile test specimen is, for example, a round bar tensile test specimen having a parallel section diameter of 8.9 mm and a parallel section length of 35.6 mm. Longitudinal direction of a parallel section of a tensile test specimen parallel to the longitudinal direction (rolling direction) of the steel material. Using a tensile test specimen, to obtain a value of 0.2% of the nominal relative stress (MPa), tensile tests are carried out at normal temperature (24 ± 3°C) in accordance with ASTM E8/E8M (2013). A value of 0.2% of the resulting nominal relative stress is defined as the yield point (MPa).

[0078] SSC resistance of steel material

The steel material of the present invention has excellent SSC resistance according to the yield strength that is obtained. The SSC resistance of the steel material of the present invention can be evaluated by SSC resistance evaluation tests at normal temperature at any yield point. Evaluation testing for resistance to SSC is carried out by the method in accordance with NACE TM0177-2005, method A.

[0079] SSC resistance at 110 ksi yield strength. inch

When the yield strength of the steel material according to the present invention is about 110 thousand psi. inch (758 to 862 MPa), the resistance of steel material to SSC can be evaluated in the following way.

[0080] A test specimen in the form of a round rod is taken from the steel material according to the present invention. When the steel material is a steel pipe, a round bar test specimen is taken from a central position in the wall thickness. When the steel material is a steel rod, the round rod test specimen is taken from the R/2 position. When the steel material is a steel sheet, a round bar test specimen is taken from a position central in thickness. The size of the test sample is practically unlimited. The round rod test piece is sized such that, for example, the diameter of the parallel section is 6.35 mm and the length of the parallel section is 25.4 mm. Longitudinal direction of a parallel section of a round bar test specimen parallel to the longitudinal direction (rolling direction) of the steel material.

[0081] The test solution is an aqueous solution containing 5 wt.% sodium chloride and having a pH of 3.5. The pH of the test solution is adjusted to 3.5 by adding acetic acid to an aqueous solution containing 5% by weight sodium chloride and 0.41 g/l sodium acetate. A stress equivalent to 90 % of the actual yield strength is applied to a test specimen in the form of a round rod. The test solution at 24°C is poured into the test vessel in such a way that the test specimen in the form of a round rod is immersed in it, thus forming a test bath. After degassing the test bath, H ₂ S gas at 0.10 bar and CO ₂ gas at 0.90 bar are injected into the test bath to saturate the test bath with H ₂ S gas. The test bath saturated with H ₂ S gas is held at 24 °C for 720 hours. For the test specimen, after soaking for 720 hours, the surface of a parallel section of the specimen is examined using a loupe having a magnification of 10 times to confirm the presence or absence of a crack. If a crack is suspected when examining through a loupe, a cross section of the suspected crack is examined using an optical microscope at 100x magnification to confirm the presence or absence of a crack.

[0082] In the steel material of the present invention, after 720 hours in the above SSC resistance evaluation test according to Method A, no cracks were confirmed. In the present specification, the phrase "no cracks were confirmed" means that the absence of cracks was confirmed when the test specimen was examined with a 10x magnifying glass and a 100x optical microscope after testing.

[0083] SSC resistance at a yield strength of 125 thousand psi. inch

When the yield strength of the steel material according to the present invention is 125 thousand psi. inch or more (862 MPa or more), the resistance of the steel material to SSC can be evaluated in the following way. The test solution is an aqueous solution containing 20 wt% sodium chloride and having a pH of 4.3. The pH of the test solution was adjusted to 4.3 by adding acetic acid to an aqueous solution containing 20% by weight sodium chloride and 0.41 g/l sodium acetate. A stress equivalent to 90 % of the actual yield strength is applied to a test specimen in the form of a round rod. The test solution at 24°C is poured into the test vessel in such a way that the test specimen in the form of a round rod is immersed in it, thus forming a test bath. After degassing the test bath, H ₂ S gas at 0.07 bar and CO ₂ gas at 0.93 bar are injected into the test bath to saturate the test bath with H ₂ S gas. The test bath saturated with H ₂ S gas is held at 24 °C for 720 hours. Other conditions are the same as in the 110 kpsi SSC tolerance test. inch.

[0084] Forms and use of steel material

The steel material of the present invention is a steel pipe, a round steel rod (solid material), or a steel sheet. The steel pipe can be both seamless steel pipe and welded steel pipe. The steel pipe is, for example, a steel pipe for oilfield tubulars. Steel pipe for oilfield tubular products means a steel pipe to be used for oilfield tubular products. Oilfield tubular products include casing, tubing, drill pipe, and the like, which are used for drilling an oil or gas well, collecting crude oil or natural gas, and the like. Preferably, the steel material of the present invention is a seamless steel pipe for oilfield tubulars.

[0085] As described above, in the steel material of the present invention, each element of the chemical composition is within the range of the present invention, and the total concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more and Ca sulfide particles having a diameter of equivalent circle of 2.0 µm or more is 0.50 particles/mm ² or less. For this reason, the steel material of the present invention exhibits excellent SSC resistance.

[0086] Method of production

Next, one example of the steel material production method according to the present invention will be described. It should be noted that the production method to be described below is an example, and the production method of the steel material according to the present invention will not be limited thereto. Those. provided that the steel material according to the present invention having the configuration described above can be produced, the production method will not be limited to the production method to be described below. However, the production method to be described below is the preferred production method for the steel material of the present invention.

[0087] One example of the method for producing a steel material according to the present invention includes a process (steel production process) for obtaining a raw material, a process (hot working process) for performing hot working of a raw material to obtain a billet of steel material, and a process (heat treatment process) for performing quenching and tempering of the steel material blank. Each stage will be described next.

[0088] Steel making process

The steel manufacturing process includes a process (refining process) for producing molten steel and a process (raw material manufacturing process) for producing raw material in a casting process using molten steel.

[0089] Refining Process

In the refining process, first, molten steel containing Cr is placed in a ladle, and the molten steel in the ladle is decarburized at atmospheric pressure (decarburization pre-refining process). The slag is formed as a result of decarburization in the decarburization pre-refining process. The slag resulting from the decarburization treatment floats on the liquid surface of the molten steel after the decarburization pre-refining process. In the decarburization pre-refining process, Cr in molten steel is oxidized to form Cr ₂ O ₃ . Cr ₂ O ₃ is absorbed by the slag. Then, a deoxidizer is added to the ladle to reduce Cr ₂ O ₃ in the slag, thereby reducing Cr in the molten steel (Cr reduction treatment process). The decarburization pre-refining process and the Cr reduction process are carried out, for example, by an electric furnace smelting process, a converter process, or an argon-oxygen decarburization (AOD) process. After the Cr reduction treatment process, the slag from the molten steel is removed (slag removal process).

[0090] In the case of chromium-containing steel, since the activity of carbon is reduced due to chromium, the decarburization reaction is suppressed. Accordingly, the molten steel after the slag removal process is subjected to a decarburization finishing process (decarburization finishing refining process). In the decarburization finish refining process, the decarburization treatment is carried out under reduced pressure. Performing the decarburization treatment under reduced pressure will lower the partial pressure of CO gas ( _PCO ) in the atmosphere, and thus the oxidation of Cr in the molten steel will be suppressed. For this reason, performing the decarburization treatment under reduced pressure will further reduce the concentration of C in the molten steel while suppressing the oxidation of Cr. After the decarburization finish refining process, a chromium reduction treatment is again performed in which a deoxidizer is added to the molten steel to reduce Cr ₂ O ₃ in the slag (chromium reduction treatment). The decarburization finish refining process and the Cr reduction process after decarburization finish refining are carried out by a vacuum oxygen decarburization (VOD) process.

[0091] After the Cr reduction processing process, the composition of the molten steel in the ladle is finally adjusted and the temperature of the molten steel is adjusted before the raw material production process (composition adjustment process). The process of adjusting the composition is carried out by the method of modification in the ladle (LT). In the second half of the composition adjustment process, Ca is added to the molten steel. Here, the time from when Ca is added to the molten steel to when Ca is uniformly distributed in the molten steel is defined as the "uniform stirring time" τ. The uniform mixing time τ is determined by the formula (A):

τ = 800 × ε ^-0.4 , (A)

where ε is the molten steel stirring power density in LT, which is determined by formula (B):

ε = 28.5(Q/W) × T × log(1+H/1.48), (B)

where Q is the gas flow rate during blowing from above (Nm ³ /min), W is the mass of molten steel (t), T is the temperature of molten steel (K), H is the depth of molten steel in the ladle (m).

[0092] In the process of adjusting the composition, the temperature of the molten steel in the ladle is maintained in the range from 1500 to 1700°C. In addition, the holding time from the elapse of the homogeneous mixing time after adding Ca to the molten steel is defined as "holding time t" (sec). In this case, in the present invention, the holding time t after the uniform stirring time has elapsed is set to 60 seconds or more.

[0093] If the holding time t is less than 60 seconds, Ca added to the molten steel cannot sufficiently convert Mn sulfide in the molten steel. In this case, larger particles of Mn sulfide remain in the steel material. For this reason, the amount per unit area of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more will become excessively large. As a result, the total concentration ND (particles/mm ² ) of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more can be more than 0.50 particles. /mm ² . Alternatively, although Mn sulfide reacts with Ca and continues to be converted, the number per unit area of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more will decrease, Ca sulfide that is formed when combined with S will remain in molten steel without sufficient slag absorption. As a result, the total concentration ND (particles/mm ² ) of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more can be more than 0.50 particles. /mm ² .

[0094] On the other hand, when the holding time t is at least 60 seconds, Ca added to the molten steel sufficiently converts the Mn sulfide in the molten steel, thereby reducing the coarse Mn sulfide particles. For this reason, the concentration per unit area of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more will be sufficiently reduced. In addition, sufficient time can be provided for large particles of calcium sulfide, which has been obtained by combining with sulfur, to float in the molten steel and be absorbed by the slag. For this reason, the concentration per unit area of Ca sulfide particles having an equivalent circle diameter of 2.0 μm or more will be sufficiently small. As a result, the total concentration ND (particles/mm ² ) of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more will be 0.50 particles/mm ² or less.

[0095] Raw material production process

When using molten steel obtained by the refining process described above, a starting material (cast or ingot) is obtained. In particular, when using molten steel, the casting is carried out in a continuous casting process. The casting may be a slab, bloom or billet. Alternatively, the ingot may be produced by a molten steel ingot manufacturing process. In addition, the casting or ingot may be subjected to a swaging or similar process to form a billet. The starting material is obtained by the methods described above.

[0096] Hot Working Process

In the hot working process, the raw material is hot worked to produce a billet of steel material. When the steel material is a steel pipe, the steel material blank corresponds to a hollow shell. First, the raw material is heated in a heating furnace. The heating temperature is, although not specifically limited to, for example, 1100 to 1300°C. The billet removed from the heating furnace is hot-worked to obtain a hollow shell (seamless steel pipe), which is a billet of steel material. The hot working method is not particularly limited, and may be any known method. For example, to produce a hollow shell, the Mannesmann process is performed as hot working. In this case, the round billet is subjected to processing on a piercing press. When piercing-rolling is performed, the piercing ratio is, for example, 1.0 to 4.0, although not particularly limited. The pierced-rolled round billet is further hot-worked with, for example, a continuous tube mill, a reduction tube mill, a sizing mill, and the like, to form a hollow shell. The total area reduction in the hot working step is, for example, 20 to 70%.

[0097] The hollow shell may be formed from the billet by another hot working process. For example, in the case of a short thick-walled steel material such as a sleeve, the hollow shell can be made by forging such as the Erhard method and the like. The hollow shell is obtained by the methods described above.

[0098] When the steel material is a steel rod, the raw material is first heated in a heating furnace. The heating temperature is, for example, 1100 to 1300°C, although not specifically limited. The raw material extracted from the heating furnace is hot-worked to obtain a steel rod, which is a steel material blank. Hot working is, for example, blooming reduction or hot rolling in a continuous rolling mill. The continuous rolling mill is configured such that a horizontal stand having a pair of vertically aligned grooved rolls and a vertical stand having a pair of horizontally aligned grooved rolls are arranged alternately.

[0099] When the steel material is a steel sheet, the raw material is first heated in a heating furnace. The heating temperature is, for example, 1100 to 1300°C, although not specifically limited. The raw material taken out from the heating furnace is subjected to hot rolling by blooming and continuous rolling mill to obtain a steel sheet, which is a steel material blank.

[0100] A billet of steel material obtained by hot working can be cooled in air (in a rolled state). A billet of steel material obtained by hot working may be subjected to direct quenching after hot working instead of air cooling to normal temperature, or may be subjected to quenching after additional heating (reheating) after hot working.

[0101] When direct quenching is performed after hot working or quenching is performed after hot working and then post-heating is performed, stress relief annealing (stress relief treatment) can be performed before the heat treatment process (quenching and tempering) in a subsequent process to relieve residual stresses.

[0102] Heat treatment process

The heat treatment process includes a quenching process and a tempering process.

[0103] Tempering process

In the heat treatment process, a billet of steel material obtained in the hot working process is first subjected to a quenching (quenching process). Hardening is carried out in a known manner. Specifically, the steel material after the hot working process is charged into a heat treatment furnace and kept at a quenching temperature. The quenching temperature is maintained at or above the A _C3 transformation temperature, for example in the range from 900 to 1000°C. After holding the steel material at the quenching temperature, it is rapidly cooled (quenched). The holding time at the quenching temperature is, for example, 10 to 60 minutes, although not specifically limited. The quenching method is, for example, cooling in water. The quenching method is not particularly limited. When the steel material blank is a hollow shell, the hollow shell can be quickly cooled by immersing it in a water bath or an oil bath, or the hollow shell can be quickly cooled by pouring or jet cooling water on the outer surface and/or the inner surface of the hollow shell by cooling spray or air-cooled.

[0104] It should be noted that, as described above, quenching (direct quenching) can be performed immediately after hot working without cooling the steel material blank to normal temperature after the hot working process, or quenching can be performed after holding the hollow shell at a temperature of hardening. by loading additional heat into the furnace before lowering the temperature of the hollow shell after hot working.

[0105] Vacation Process

The steel material blank after quenching is additionally subjected to a tempering process. During the tempering process, the yield strength of the steel material is corrected. In the present invention, the tempering temperature is maintained in the range of 540 to 620°C. The holding time at the tempering temperature is, for example, 10 to 180 minutes, although not specifically limited. It is well known to those skilled in the art that the yield strength of a steel material can be adjusted by appropriately adjusting the tempering temperature depending on the chemical composition. Preferably, the tempering conditions are controlled such that the yield strength of the steel material is 758 MPa or more (110 psi or more).

[0106] The steel material according to the present invention can be obtained by the methods described above. The production method of the steel material according to the present invention is not limited. Provided that the steel material in which the content of each element in the chemical composition is within the range of the present invention, and in which the total density ND (particles/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 µm or more, and Ca sulfides having an equivalent circle diameter of 2.0 µm or more is 0.50 particles/ ^mm2 or less, the production method of the steel material of the present invention will not be limited to the above-described production method.

EXAMPLES

EXAMPLE 1

[0107] Example 1 tested SSC resistance in steel materials having a yield strength of 125 ksi. inch or more (yield strength was 862 MPa or more). In particular, molten steels with the chemical composition shown in Table 1 were obtained.

[0108] Table 1

[0109] The symbol "-" in table 1 means that the content of the corresponding element was less than the limit of detection. For example, the W content in steel No. A was 0% as a result of rounding to the third decimal place. The content of Nb in steel No. A was 0% as a result of rounding to the third decimal place. The content of B in steel No. A was 0% as a result of rounding to the third decimal place. The Mg content in steel No. A was 0% as a result of rounding to the third decimal place. The REM content in steel No. A was 0% as a result of rounding to the fourth decimal place.

[0110] Molten steels of each steel number were made as follows. The Cr-containing molten steel was placed in a ladle and subjected to a well-known decarburization refining process and a Cr reduction treatment process by an AOD process. After the Cr reduction treatment process, a slag removal process was carried out from the molten steel. In addition, the finishing decarburization process and the Cr reduction treatment process were carried out in a well-known manner by a VOD process.

[0111] After the Cr reduction treatment process by the VOD process by LT, the final correction of the composition of the molten steel in the ladle and the correction of the temperature of the molten steel were performed before the raw material production process. The temperature of any molten steel ranged from 1500 to 1700°C. Next, Ca was added to the molten steel. The holding time t (s) from the point where the even mixing time elapsed after the addition of Ca was adjusted as shown in Table 2. Molten steels with the chemical composition shown in Table 1 were obtained by the above processes.

[0112] Table 2

Test No. Steel No. Process LT Martensite volume
Share
(%) Total concentration
ND (particles/mm2) YS (yield strength)
(MPa) SSC resistance holding time
t (seconds) 1 A 192 100 0.44 974 E 2 B 203 97 0.50 961 E 3 C 131 96 0.39 907 E 4 D 153 98 0.47 976 E 5 E 111 99 0.39 979 E 6 F 223 98 0.11 978 E 7 G 62 97 0.34 910 E 8 H 123 99 0.41 980 E 9 I 126 95 0.50 928 E 10 J 172 94 0.44 868 E eleven K 106 95 0.17 919 E 12 L 131 96 0.36 928 E 13 M 155 100 0.17 962 E 14 N 143 100 0.34 986 E 15 O 189 98 1.86 929 B 16 P 229 96 1.25 912 B 17 Q 137 99 0.69 948 B 18 R 112 96 0.66 873 B 19 U 32 95 0.69 866 B 20 V 49 96 0.61 906 B

[0113] Using the above-described molten steel, a billet with an outer diameter of 310 mm was obtained. The resulting billet was heated to 1250° C. and then subjected to hot rolling by the Mannesmann method to obtain a hollow shell (seamless steel pipe) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.

[0114] The hollow shell of each test number was quenched and tempered. In any test number, the quench temperature was 920° C. and the holding time at the quench temperature was 10 minutes. The hollow shell was tempered after quenching. The tempering temperature was controlled from 540 to 580°C for each test number so that the yield strength of the steel material (seamless steel pipe) after tempering was 862 MPa or more. The holding time at tempering temperature was 30 minutes for any number of tests.

[0115] Using the manufacturing processes described above, steel materials (seamless steel pipes) of each test number were produced.

[0116] Evaluation tests

For the steel materials of each test number after tempering, tests were carried out with the study of the microstructure, tests for measuring the total concentration of ND particles, tensile tests and tests for evaluating resistance to SSC.

[0117] Measurement of the volume fraction of martensite in the microstructure

The volume fraction (vol.%) of martensite in the microstructure of the steel material was obtained by the following method. First, the volume fraction of residual austenite in the microstructure of the steel material was obtained by X-ray diffraction analysis for each test number. In particular, the test piece was taken from the center position of the wall thickness of the steel material (seamless steel pipe) of each test number. The dimensions of the test specimen were 15 mm x 15 mm x 2 mm thick. The thickness direction of the test specimen corresponded to the direction of the diameter of the steel pipe. Using the obtained test specimen to calculate the integral intensity of each plane, the X-ray diffraction intensity of each of the (200) α phase plane, (211) α phase plane, (200) γ phase plane, (220) γ phase plane, and (311) plane ) γ phase. When measuring the intensity of X-ray diffraction, the target of the X-ray diffraction apparatus is Mo (α radiation of MoK), and its output signal is 50 kV-40 mA. After the calculations, the volume fraction Vγ(%) of retained austenite was calculated by formula (I) for combinations (2×3=6 pairs) of each α phase plane and each γ phase plane. Then, the average value of the volume fractions Vγ of the retained austenite of the six pairs was determined as the volume fraction (%) of the retained austenite.

Vγ = 100/{1 + (Iα × Rγ)/(Iγ × Rα)}, (I)

where Iα is the integral intensity of the α phase, Rα is the crystallographic theoretical calculated value of the α phase, Iγ is the integral intensity of the γ phase, Rγ is the crystallographic theoretical calculated value of the γ phase. In the present description, Rα in the (200) plane of the α phase is 15.9, Rα in the (211) plane of the α phase is 29.2, and Rγ in the (200) plane of the γ phase is 35.5, Rγ in the (220) plane of the γ phase is 20.8, and Rγ in the (311) plane of the γ phase is 21.8. It should be noted that the volume fraction of retained austenite was obtained by rounding off the first decimal place of the obtained numerical value.

[0118] Using the volume fraction (%) of retained austenite obtained by the above X-ray diffraction analysis method, the volume fraction (vol. %) of martensite in the microstructure of the steel material was obtained by the following formula.

Volume fraction of martensite = 100 - volume fraction of retained austenite (%)

The obtained volume fractions of martensite are shown in the column "Volume fraction of martensite (%)" in Table 2.

[0119] Tests with the measurement of the total concentration of ND particles

The total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was measured by the following method. The test specimen was taken from the center position of the wall thickness of the steel material (seamless steel pipe) of each test number. The test specimen was filled with resin. The surface of the test specimen along the axial direction of the steel pipe and along the wall thickness direction of the steel pipe were defined as the observation surface. The observation surface of the resin-filled steel material was polished. After polishing, arbitrary 10 observation regions on the observation surface were examined. In each observation area, the number of inclusions (particles) was determined. The area of each observation area was 36 mm ² (6 mm × 6 mm).

[0120] To determine the specific type of each inclusion, analysis of the concentration of elements (EDS analysis) was performed for each inclusion in each area of observation. In the EDS analysis, an accelerating voltage of 20 kV was set and the concentrations of the elements N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb were determined.

[0121] Based on the EDS analysis of each inclusion, it was determined whether the inclusion was a Mn sulfide particle or a Ca sulfide particle. When the inclusion had a wt% Mn content of 10% or more and an S content of 10% or more, the inclusion was identified as "Mn sulfide". When the inclusion had a wt% Ca content of 20% or more and an S content of 10% or more, the inclusion was identified as "Ca sulfide".

[0122] For Mn sulfide particles identified in each observation area, the total concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 μm or more was obtained. In addition, for the Ca sulfide particles identified in each observation area, the total concentration of Ca sulfide particles having an equivalent circle diameter of 2.0 μm or more was obtained. Based on the total concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more, the total concentration of Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more, and the total area of 10 observation areas, the total concentration of ND (particles/ mm ² ) Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more, and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more. It is noted that the measurement of the total concentration of ND was carried out using an automatic inclusion analyzer "Metal Quality Analyzer" (product name) manufactured by FEI (ASPEX). The total concentrations of ND (particles/mm ² ) obtained are shown in the column "Total ND concentration (particles/mm ² )" of Table 2.

[0123] Tensile testing

Tensile tests were carried out in accordance with ASTM E8 (2013). In particular, the round bar tensile test specimen was taken from the center position in the wall thickness of the steel material (seamless steel pipe) of each test number. The round bar tensile test specimen had a parallel section diameter of 8.9 mm and a parallel section length of 35.6 mm. The longitudinal direction of the round bar test piece was parallel to the longitudinal direction (rolling direction) of the steel material. Using a round bar tensile test specimen for each test number, tensile tests were performed at normal temperature (25°C) in an atmosphere to obtain a conditional yield strength of 0.2% (MPa). The resulting conditional yield strength of 0.2% was determined as the yield strength (MPa). The resulting yield strength is shown in the "YS (MPa)" column of Table 2.

[0124] Tests for the evaluation of resistance to SSC

Tests for evaluating the resistance to SSC steel material was carried out in the following way. A test specimen in the form of a round bar was taken from the central position of the wall thickness of the steel material of each test number. The test specimen in the form of a round rod had a diameter of 6.35 mm and a length of 25.4 mm. The longitudinal direction of the round bar test specimen was parallel to the longitudinal direction (pipe axis direction) of the steel material.

[0125] The test solution was an aqueous solution containing 20 wt.% sodium chloride and having a pH of 4.3. The pH of the test solution was adjusted to 4.3 by adding acetic acid to an aqueous solution containing 20 wt% sodium chloride and 0.41 g/l sodium acetate. A stress equivalent to 90% of the actual yield stress was applied to a test specimen in the form of a round rod. The test solution at 24°C was poured into the test vessel in such a way that the test specimen in the form of a round rod was immersed in it, thus forming a test bath. After degassing the test bath, H ₂ S gas at a pressure of 0.07 bar and CO ₂ gas at a pressure of 0.93 bar were injected into the test bath to saturate the test bath with H ₂ S gas. The test bath saturated with H ₂ S gas was kept at 24 °C for 720 hours. The surface of a parallel section of a round rod test specimen after soaking for 720 hours was examined with a loupe having a magnification of 10 to confirm the presence or absence of a crack. If a crack was suspected when examined through a loupe, a cross section of the suspected crack was examined using an optical microscope at 100x magnification to confirm the presence or absence of a crack.

[0126] When no cracks were observed even when examining the surface of a round rod specimen with a 10x magnifying glass and a 100x optical microscope, it was judged that excellent SSC resistance (denoted as "E (Excellent)" in the column " SSC resistance" in Table 2). When any crack was confirmed, the SSC resistance was judged to be low (indicated as "B (Bad, Bad)" in the "SSC resistance" column in Table 2).

[0127] Evaluation results

As shown in Table 2, the chemistry for Test Nos. 1-14 was acceptable. In addition, the volume fraction of martensite in the microstructure was 80% or more, and the yield strength was 862 MPa or more (125 thousand psi or more). In addition, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was 0.50 particles/mm ² or less. Thus, excellent resistance to SSC was achieved.

[0128] On the other hand, in trials No. 15 and 16, the S content was too high. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0129] In test No. 17, the Ca content was too low. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0130] In test No. 18, the Ca content was too high. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0131] In tests No. 19 and 20, the holding time t after the uniform mixing time was too short. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

EXAMPLE 2

[0132] Example 2 tested SSC resistance in steel materials having a yield strength of about 110 ksi. inch or more (yield strength ranged from 758 MPa to less than 862 MPa). In particular, molten steels Nos. G-R, U, and V shown in Table 1 were obtained by the same method. as in Example 1. The holding time t after the uniform mixing time in LT of the refining process in each test number had elapsed was as shown in Table 3.

[0133] Table 3

Test No. Steel No. Process LT Martensite volume
Share
(%) Total concentration
ND
(particles/mm ² ) YS (yield strength)
(MPa) SSC resistance for 110 kpsi inch holding time
t (seconds) 21 G 62 86 0.45 788 E 22 H 123 96 0.44 842 E 23 I 126 90 0.50 823 E 24 J 172 86 0.44 853 E 25 K 106 86 0.17 811 E 26 L 131 81 0.50 827 E 27 M 155 89 0.11 801 E 28 N 143 94 0.28 794 E 29 O 189 92 1.91 843 B thirty P 229 81 1.25 819 B 31 Q 137 92 0.75 842 B 32 R 112 91 0.56 802 B 33 U 32 90 0.64 837 B 34 V 49 88 0.55 816 B

[0134] Using the above-described molten steel, a billet with an outer diameter of 310 mm was obtained. The resulting billet was heated to 1250°C and then subjected to hot rolling by the Mannesmann method to obtain a hollow shell (seamless steel pipe) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.

[0135] The hollow shell of each test number was quenched and tempered. In any test number, the quench temperature was 920° C. and the holding time at the quench temperature was 10 minutes. The hollow shell was tempered after quenching. The tempering temperature was controlled from 580 to 620° C. in each test number so that the yield strength of the steel material (seamless steel pipe) after tempering was from 758 MPa to less than 862 MPa (110 ksi level). The holding time at tempering temperature was 30 minutes for any number of tests.

[0136] Using the manufacturing processes described above, steel materials (seamless steel pipes) of each test number were made.

[0137] Evaluation tests

For the steel materials of each test number, after said tempering, tests were carried out to study the microstructure, tests to measure the total concentration of ND particles, and tensile tests in the same way as in Example 1. In addition, subsequent tests were carried out to evaluate resistance to SSC.

[0138] Tests for the evaluation of resistance to SSC

Tests for evaluating the resistance to SSC steel material was carried out in the following way. A test specimen in the form of a round bar was taken from the middle of the wall thickness of the steel material of each test number. The test specimen in the form of a round rod had a diameter of 6.35 mm and a length of 25.4 mm. The longitudinal direction of the round bar test specimen was parallel to the longitudinal direction (pipe axis direction) of the steel material.

[0139] The test solution was an aqueous solution containing 5 wt.% sodium chloride and having a pH of 3.5. The pH of the test solution was adjusted to 3.5 by adding acetic acid to an aqueous solution containing 5 wt% sodium chloride and 0.41 g/l sodium acetate. A stress equivalent to 90% of the actual yield stress was applied to a test specimen in the form of a round rod. The test solution at 24°C was poured into the test vessel in such a way that the test specimen in the form of a round rod was immersed in it, thus forming a test bath. After degassing the test bath, H ₂ S gas at a pressure of 0.10 bar and CO ₂ gas at a pressure of 0.90 bar were injected into the test bath to saturate the test bath with H ₂ S gas. The test bath saturated with H ₂ S gas was kept at 24 °C for 720 hours. For the test specimen in the form of a round rod, after soaking for 720 hours, the surface of the specimen was examined using a loupe having a magnification of 10 times to confirm the presence or absence of a crack. If a crack was suspected when examined through a loupe, a cross section of the suspected crack was examined using an optical microscope at 100x magnification to confirm the presence or absence of a crack.

[0140] When no cracks were observed even when examining the surface of a round rod specimen with a 10x loupe and a 100x optical microscope, it was judged that excellent SSC resistance (denoted as "E (Excellent)" in the column " SSC resistance" in Table 3). When any crack was confirmed, the SSC resistance was judged to be low (indicated as "B (Poor)" in the "SSC Resistance" column in Table 3).

[0141] Evaluation results

As shown in Table 3, the chemistry for Test Nos. 21-28 was acceptable. In addition, the volume fraction of martensite in the microstructure was 80% or more, and the yield strength was from 758 MPa to less than 862 MPa (110 ksi level). In addition, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was 0.50 particles/mm ² or less. Thus, excellent resistance to SSC was achieved.

[0142] On the other hand, in trials No. 29 and 30, the S content was too high. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0143] In test No. 31, the Ca content was too low. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0144] In test No. 32, the Ca content was too high. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0145] In tests No. 33 and 34, the holding time t after the uniform mixing time was too short. For this reason, the total ND concentration of Mn sulfide particles having an equivalent circle diameter of 1.0 µm or more and Ca sulfide particles having an equivalent circle diameter of 2.0 µm or more was more than 0.50 particles/mm ² . As a result, resistance to SCC was low.

[0146] So far, embodiments of the present invention have been described. However, these embodiments are only examples of the practical application of the present invention. Thus, the present invention will not be limited to the exemplary embodiments, and can be implemented by appropriately changing the exemplary embodiments within a range without going beyond its spirit.

Claims (45)

1. Steel material having chemical composition with content in wt.%: C in an amount of 0.035% or less; Si in the amount of 1.00% or less; Mn in the amount of 1.00% or less; P in the amount of 0.030% or less; S in the amount of 0.0050% or less; sol. Al in an amount from 0.005 to 0.100%; N in an amount from 0.001 to 0.020%; Ni in an amount of 5.00 to 7.00%; Cr in an amount of 10.00 to 14.00%; Cu in an amount of 1.50 to 3.50%; Mo in an amount of from 1.00 to 4.00%; V in an amount of 0.01 to 1.00%; Ti in an amount of 0.02 to 0.30%; Co in an amount of 0.01 to 0.50%; Ca in an amount from 0.0003 to 0.0030%; O in an amount of 0.0050% or less; W in an amount from 0 to 1.50%; Nb in an amount of 0 to 0.50%; B in an amount from 0 to 0.0050%; Mg in an amount from 0 to 0.0050%; And rare earth metals (REM) in an amount from 0 to 0.020%, the rest is Fe and inevitable impurities, wherein, among the inclusions in the steel material, the total concentration of Mn sulfide particles having a Mn content of 10% or more and an S content of 10% or more and having an equivalent circle diameter of 1.0 µm or more, and Ca sulfide particles having a calcium content of 20% or more more and a sulfur content of 10% or more and an equivalent twist diameter of 2.0 μm or more is 0.50 particles/mm ² or less. 2. Steel material according to claim. 1, in which the chemical composition contains W in an amount of from 0.01 to 1.50%. 3. Steel material according to claim. 1, in which the chemical composition contains Nb in an amount of from 0.01 to 0.50%. 4. Steel material according to claim. 2, in which the chemical composition contains Nb in an amount of from 0.01 to 0.50%. 5. Steel material according to claim 1, in which the chemical composition contains one or more types of elements selected from the group consisting of: B in an amount from 0.0001 to 0.0050%, Mg in an amount from 0.0001 to 0.0050% and rare earth metals (REM) in an amount of 0.001 to 0.020%. 6. Steel material according to claim 2, wherein the chemical composition contains one or more types of elements selected from the group consisting of: B: from 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050% and rare earth metals (REM): from 0.001 to 0.020%. 7. Steel material according to claim 3, in which the chemical composition contains one or more types of elements selected from the group consisting of: B: from 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050% and rare earth metals (REM): from 0.001 to 0.020%. 8. Steel material according to claim 4, in which the chemical composition contains one or more types of elements selected from the group consisting of: B: from 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050% and rare earth metals (REM): from 0.001 to 0.020%. 9. Steel material according to any one of paragraphs. 1-8, wherein the steel material is a seamless steel pipe for oilfield tubulars.