JP6947012B2 - Steel materials, steel pipes for oil wells, and manufacturing methods for steel materials - Google Patents

Description

The present invention relates to steel materials, steel pipes for oil wells, and methods for manufacturing steel materials.

Higher strength of steel pipes for oil wells is required by deepening wells in oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells"). Specifically, steel pipes for oil wells of 80 ksi class (yield strength of 80 to 95 ksi, that is, 551 to 655 MPa) and 95 ksi class (yield strength of 95 to 110 ksi, that is, 655 to 758 MPa) are widely used. More recently, 110 ksi class (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa), 125 ksi class (yield strength is 125 to 140 ksi, that is, 862-965 MPa), 140 ksi class (yield strength is 140 to 155 ksi, that is, 965 to 965). Steel pipes for oil wells of 1069 MPa) and 155 ksi class (yield strength of 155 to 170 ksi, that is, 1069 to 1172 MPa) are beginning to be sought.

A harsh environment where such high strength is required is, for example, a polar region. Steel pipes for oil wells used in cold regions such as polar regions are required to have not only high strength but also low temperature toughness. However, if the yield strength of the oil country steel pipe becomes excessively high, there is a concern that the low temperature toughness of the oil country steel pipe may decrease.

In addition, many deep wells are sour environments containing corrosive hydrogen sulfide. That is, steel pipes for oil wells used in such a sour environment are required to have not only high strength and low temperature toughness but also sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance).

Techniques for enhancing the strength and low temperature toughness of steel materials represented by steel pipes for oil wells are described in JP-A-61-272351 (Patent Document 1), JP-A-59-74221 (Patent Document 2), and It is proposed in Kai 2001-2711134 (Patent Document 3).

The steel pipes for high-strength and high-toughness oil wells disclosed in Patent Document 1 have a C: 0.25 to 0.40%, Si: 0.10 to 0.50%, Mn: 0.40 to 2 in weight%. 0.0%, Cr: 0.5 to 2.0%, Mo: 0.5 to 2.0%, Nb: 0.05% or less, V: 0.03 to 0.08%, Al: 0.03 It contains ~ 0.1%, P as an impurity is 0.015% or less, S is 0.015% or less, and the balance is composed of unavoidable impurities other than Fe, P, and S. Patent Document 1 describes that this high-strength, high-toughness steel pipe for oil wells can achieve both high strength and high toughness at the same time.

The high-strength seamless steel pipe disclosed in Patent Document 2 has a weight% of C: 0.1 to 0.5%, Si: 0.1 to 0.3%, and Mn: 0.2 to 0.8. %, Cr: 1.0 to 4.0%, Al: 0.005 to 0.1%, both P and S reduced to 0.005% or less, and N: 0.004% or less, respectively. And Mo: 0.2-1.0%, and Nb: 0.01-0.1%, along with Zr and / or Ti: 0.005-0.1%, and if necessary, further. The component composition contains at least one of V: 0.1% or less and B: 0.005% or less. It is described in Patent Document 2 that this high-strength seamless steel pipe has both excellent sulfide corrosion cracking resistance and low-temperature toughness at a 0.6% proof stress of 70 to 120 kgf / mm ^2. There is.

The low alloy steel material disclosed in Patent Document 3 has a mass% of C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.1 to 1.2%, Mo: 0.1 to 1%, B: 0.0001 to 0.005%, Al: 0.005 ~ 0.1%, V: 0.05 ~ 0.5%, Ni: 0.1% or less, N: 0.01% or less, O (oxygen): 0.01% or less, and the balance Fe and Chemistry composed of impurities and having Mo and V contents satisfying the formulas (1) (0.03 ≦ Mo × V ≦ 0.3) and the formula (2) (0.5 × Mo−V + GS / 10 ≧ 1). Has a composition. The low alloy steel material further has a yield stress of 1060 MPa (155 ksi) or more. Patent Document 3 describes that this low alloy steel material is excellent in SSC resistance and toughness even if it has high strength.

Japanese Unexamined Patent Publication No. 61-272351 Japanese Unexamined Patent Publication No. 59-074221 Japanese Unexamined Patent Publication No. 2001-2711134

However, even if the techniques disclosed in Patent Documents 1 to 3 are applied, in the case of a steel pipe for an oil well having a yield strength of more than 155 ksi (yield strength of 1069 MPa), excellent low temperature toughness and excellent SSC resistance are stable. May not be obtained.

An object of the present disclosure is to provide a steel material and a steel pipe for an oil well, which have a high yield strength of more than 1069 to 1172 MPa (more than 155 to 170 ksi, 155 ksi class), excellent low temperature toughness, and excellent SSC resistance. That is.

The steel material according to the present disclosure has C: 0.15 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less in mass%. , S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.25 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0. 050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.10. %, Cu: 0 to 0.50%, and rare earth element: 0 to 0.0100%, and the balance has a chemical composition of Fe and impurities. The steel material according to the present disclosure further contains 0.010 to 0.060% by mass of solid solution C. Further, the steel material according to the present disclosure has a grain size number of 8.0 or more for the former austenite crystal grains. Further, the steel material according to the present disclosure has a yield strength of more than 1069 to 1172 MPa and a yield ratio of 85% or more.

The method for producing a steel material according to the present disclosure includes a preparation step, a quenching step, and a tempering step. In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. In the quenching step, after the preparatory step, the intermediate steel material at 800 to 1000 ° C. is cooled at a cooling rate of 300 ° C./min or more. In the tempering step, the intermediate steel material after quenching is held at 580 to 720 ° C. for 10 to 180 minutes, and then the average cooling rate between 580 ° C. and 200 ° C. is cooled at 4 to 300 ° C./sec.

The steel materials and steel pipes for oil wells according to the present disclosure have high yield strength of more than 1069 to 1172 MPa (155 ksi class), excellent low temperature toughness, and excellent SSC resistance.

FIG. 1 is a diagram showing the relationship between the amount of solid solution C of each test number, the absorbed energy at −40 ° C., and the SSC resistance.

The present inventors investigated and investigated a method for obtaining high strength with a yield strength of more than 1069 to 1172 MPa (155 ksi class), excellent low temperature toughness, and excellent SSC resistance in steel materials and steel pipes for oil wells. , The following findings were obtained.

(A) In a steel material having high strength, the dislocation density of the steel material increases as the strength increases. When the dislocation density of the steel material is increased, the yield strength YS (Yield Strength) of the steel material is increased, while the low temperature toughness and SSC resistance of the steel material are lowered.

When the dislocation of steel material is a movable dislocation, the dislocation may disappear. In this case, the strength of the steel material decreases. Therefore, if the dislocations of the steel material are prevented from becoming movable dislocations, the disappearance of dislocations can be suppressed and the decrease in dislocation density can be suppressed. In this case, the strength of the steel material can be maintained. Therefore, the present inventors have considered to increase the yield strength of the steel material by making the dislocation of the steel material an immovable dislocation.

Specifically, the present inventors have studied making dislocations immobile dislocations by C (hereinafter, also referred to as “solid solution C”) that is solid-solved in a steel material. Hereinafter, immovable dislocations due to solid solution C are also referred to as "solid solution C immovable dislocations". As a result of the examination, when the amount of solid solution C in the steel material was adjusted, the decrease in the yield strength of the steel material was suppressed. The present inventors have found that adjusting the amount of solid solution C in the steel material may further increase the low temperature toughness and SSC resistance of the steel material.

That is, if the amount of solid solution C is increased, the low temperature toughness of the steel material and the SSC resistance of the steel material can be increased. Therefore, in order to obtain a steel material having improved yield strength, low temperature toughness, and SSC resistance, it is necessary to increase the amount of solid solution C and increase the solid solution C immobile dislocation density with respect to the overall dislocation density. The present inventors thought that this might be the case.

As described above, the present inventors can improve the low temperature toughness of the steel material and the SSC resistance of the steel material while maintaining the yield strength of 155 ksi class by appropriately adjusting the amount of solid solution C in the steel material. I thought I could do it. Therefore, the present inventors, in terms of mass%, C: 0.15 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025. % Or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.25 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0 Using a steel material containing 10%, Cu: 0 to 0.50%, and rare earth elements: 0 to 0.0100% and having a chemical composition with the balance consisting of Fe and impurities, the amount of solid solution C and The relationship between yield strength, low temperature toughness and SSC resistance was investigated.

[Relationship between solid solution C amount, low temperature toughness and SSC resistance]
FIG. 1 is a diagram showing the relationship between the amount of solid solution C, the absorbed energy at −40 ° C., and the SSC resistance. FIG. 1 was obtained by the following method. Among the examples to be described in detail later, for a steel material in which conditions other than the solid solution C amount satisfy the range of the present embodiment, the obtained solid solution C amount (mass%) and the absorbed energy E (-) at −40 ° C. FIG. 1 was created using 40 ° C.) (J) and the evaluation results of SSC resistance determined by the method described later.

The yield strength YS of the steel materials shown in FIG. 1 was in the range of more than 1069 to 1172 MPa (155 ksi class). The yield strength YS was adjusted by adjusting the tempering temperature. Further, regarding the low temperature toughness, when the absorbed energy E (-40 ° C.) at −40 ° C., which is an index of the low temperature toughness, is 74.0 J or more, it is judged that the low temperature toughness of the steel material is excellent. In addition, "◯" in FIG. 1 indicates a steel material which obtained excellent SSC resistance. On the other hand, “●” in FIG. 1 indicates a steel material for which excellent SSC resistance could not be obtained.

With reference to FIG. 1, in a steel material satisfying the above chemical composition, if the amount of solid solution C is 0.010% by mass or more, the absorption energy E (-40 ° C.) is 74.0J or more, and the steel material has an excellent low temperature. Showed toughness. The present inventors consider the reason for this as follows. Coarse carbides are a source of stress concentration. That is, the coarse carbide becomes the starting point of cracking and lowers the low temperature toughness of the steel material. On the other hand, in a steel material satisfying the above chemical composition, precipitation of carbides is suppressed by increasing the amount of solid solution C. In this case, the formation of coarse carbides is suppressed in the process of precipitation of carbides. That is, by increasing the amount of solid solution C, the formation of coarse carbides is suppressed, and the carbides are finely dispersed. Therefore, cracks originating from coarse carbides are suppressed. As a result, the low temperature toughness of the steel material is increased.

With reference to FIG. 1, in the steel material satisfying the above chemical composition, when the solid solution C amount was 0.010% by mass or more, the steel material further exhibited excellent SSC resistance. The present inventors consider the reason for this as follows. Movable dislocations tend to occlude hydrogen. Therefore, if the dislocation density of the steel material increases, the amount of hydrogen occluded by the steel material tends to increase. As a result, the hydrogen concentration of the steel material increases, and the SSC resistance of the steel material decreases. On the other hand, in a steel material satisfying the above chemical composition, by increasing the amount of solid solution C, movable dislocations in the steel material are fixed by C, resulting in solid solution C immovable dislocations. Therefore, the amount of hydrogen occluded in the steel material is reduced. As a result, the SSC resistance of the steel material is increased.

On the other hand, referring to FIG. 1, in a steel material satisfying the above chemical composition, if the amount of solid solution C exceeds 0.060% by mass, the steel material does not show excellent SSC resistance. The reason for this is not clear. However, in a steel material satisfying the above chemical composition, if the amount of solid solution C is 0.010% by mass or more and the amount of solid solution C is 0.060% by mass or less, excellent SSC resistance can be obtained. If the amount of solid solution C exceeds 0.060% by mass, the absorbed energy E (-40 ° C.) becomes less than 74.0J, and the steel material may not exhibit excellent low temperature toughness.

Based on the above, if the above-mentioned chemical composition is satisfied and the amount of solid-dissolved C is 0.010 to 0.060% by mass, the steel material has a yield strength YS of more than 1069 to 1172 MPa, provided that the conditions described below are satisfied. Even if there is, the absorbed energy E (-40 ° C) at −40 ° C. is 74.0 J or more, showing excellent low temperature toughness, and further showing excellent SSC resistance. Therefore, in the present embodiment, the amount of solid solution C is 0.010 to 0.060% by mass.

(B) If the old austenite crystal grains (hereinafter, also referred to as "old γ grains") are coarse, that is, if the crystal grain size number of the old γ grains is too low, stress is concentrated at the old γ grain boundaries. Therefore, cracks occur and progress from the old γ grain boundaries. In this case, the low temperature toughness and SSC resistance of the steel material are lowered. If the old γ grains are coarse, the elements are more likely to segregate at the old γ grain boundaries. When an element segregates at the old γ grain boundary, the segregated element increases the crack susceptibility of the old γ grain boundary. As a result, the low temperature toughness and SSC resistance of the steel material are further reduced.

On the other hand, if the old γ grains are fine, that is, if the crystal grain size number of the old γ grains is high, the grain boundary area per unit volume increases. In this case, the stress concentration is relaxed, and the low temperature toughness and SSC resistance of the steel material are enhanced. In this case, it is possible to further suppress the segregation of elements into the old γ grain boundaries of the steel material. As a result, the low temperature toughness and SSC resistance of the steel material are enhanced. Therefore, the old γ grains of the steel material according to the present embodiment have a crystal grain size number of 8.0 or more. In this case, the low temperature toughness and SSC resistance of the steel material can be improved. In addition, in this specification, the crystal particle size number means the particle size number measured by the method conforming to JIS G0551 (2013).

The microstructure of the steel material according to this embodiment is mainly tempered martensite and tempered bainite. The term "tempered martensite and tempered bainite" means that the total volume ratio of tempered martensite and tempered bainite is 90% or more. If the microstructure of the steel material is mainly tempered martensite and tempered bainite, the yield strength YS is more than 1069 to 1172 MPa (155 ksi class) and the yield ratio YR (yield strength with respect to tensile strength TS (Tensil Strength)) in the steel material according to the present embodiment. The ratio of YS, that is, the yield ratio YR = yield strength YS / tensile strength TS (%)) is 85% or more.

The steel material according to the present embodiment completed based on the above findings has a mass% of C: 0.15 to 0.50%, Si: 0.05 to 1.00%, and Mn: 0.05 to 1.00. %, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.25 to 1.50%, Mo: 0.25 to 1.50% , Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50 %, Ni: 0 to 0.10%, Cu: 0 to 0.50%, and rare earth element: 0 to 0.0100%, and the balance has a chemical composition of Fe and impurities. The steel material according to this embodiment further contains 0.010 to 0.060% by mass of solid solution C. Further, the steel material according to the present embodiment has a grain size number of 8.0 or more for the former austenite crystal grains. Further, the steel material according to the present embodiment has a yield strength of more than 1069 to 1172 MPa and a yield ratio of 85% or more.

In the present specification, the steel material is not particularly limited, but is, for example, a steel pipe or a steel plate.

The steel material according to the present embodiment exhibits excellent strength, excellent low temperature toughness, and excellent SSC resistance.

The chemical composition may contain one or more selected from the group consisting of V: 0.01 to 0.60% and Nb: 0.002 to 0.030%.

The chemical composition is one or 2 selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and Zr: 0.0001 to 0.0100%. It may contain more than a seed.

The chemical composition may contain one or more selected from the group consisting of Co: 0.02 to 0.50% and W: 0.02 to 0.50%.

The chemical composition may contain one or more selected from the group consisting of Ni: 0.01 to 0.10% and Cu: 0.01 to 0.50%.

The chemical composition may contain a rare earth element: 0.0001 to 0.0100%.

The steel material has the above chemical composition, contains 0.010 to 0.060% by mass of solid solution C, has a grain size number of 8.0 or more for the former austenite crystal grains, and has a yield strength of more than 1069 to 1172 MPa. It may be a steel pipe for an oil well having a yield ratio of 85% or more.

In the present specification, the steel pipe for an oil well may be a steel pipe for a line pipe or an oil well pipe. The steel pipe for oil wells may be a seamless steel pipe or a welded steel pipe. The well pipe is, for example, a steel pipe used for casing and tubing applications.

The above-mentioned excellent low-temperature toughness specifically means that the absorbed energy E (-40 ° C.) at −40 ° C. is 74 in the Charpy impact test conforming to JIS Z 2242 (2005) conducted at −40 ° C. It means that it is 0.0J or more.

The excellent SSC resistance is specifically defined as 5% sodium chloride +0.5 in which _{0.003 bar of H 2} S is sealed by applying a stress corresponding to 85% of the yield stress to the steel material. % aqueous solution of acetic acid, and, immersed in 5% NaCl + 0.5% acetic acid aqueous solution encapsulating _{H 2} S of 0.005Bar, in Korutesuto test according to NACE TM0177-2005 Method a, also break 720 hours either steel Means not.

Further, the solid solution C amount means the difference between the C amount (mass%) in the carbide in the steel material and the C content in the chemical composition of the steel material. The amount of C in the carbide in the steel material is Fe concentration <Fe> a, Cr concentration <Cr> a in the carbide (cementite and MC type carbide) obtained as a residue by performing extraction residue analysis on the steel material. For Mn concentration <Mn> a, Mo concentration <Mo> a, V concentration <V> a, Nb concentration <Nb> a, and cementite identified by TEM observation of the replica film obtained by the extraction replica method. Using the Fe concentration <Fe> b, Cr concentration <Cr> b, Mn concentration <Mn> b, and Mo concentration <Mo> b in cementite obtained by performing point analysis by EDS, the formula (1) ) ~ Eq. (5).
<Mo> c = (<Fe> a + <Cr> a + <Mn> a) × <Mo> b / (<Fe> b + <Cr> b + <Mn> b) (1)
<Mo> d = <Mo> a- <Mo> c (2)
<C> a = (<Fe> a / 55.85 + <Cr> a / 52 + <Mn> a / 53.94 + <Mo> c / 95.9) / 3 × 12 (3)
<C> b = (<V> a / 50.94 + <Mo> d / 95.9 + <Nb> a / 92.9) × 12 (4)
(Amount of solid solution C) = <C>-(<C> a + <C> b) (5)
In the present specification, cementite means a carbide having an Fe content of 50% by mass or more.

The method for producing a steel material according to the present embodiment includes a preparation step, a quenching step, and a tempering step. In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. In the quenching step, after the preparatory step, the intermediate steel material at 800 to 1000 ° C. is cooled at a cooling rate of 300 ° C./min or more. In the tempering step, the intermediate steel material after quenching is held at 580 to 720 ° C. for 10 to 180 minutes, and then the average cooling rate between 580 ° C. and 200 ° C. is cooled at 4 to 300 ° C./sec.

The preparation step of the above-mentioned manufacturing method may include a material preparation step of preparing a material having the above-mentioned chemical composition and a hot-working step of hot-working the material to produce an intermediate steel material.

Hereinafter, the steel material and the steel pipe for oil wells according to the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition]
The chemical composition of the steel material according to this embodiment contains the following elements.

C: 0.15 to 0.50%
Carbon (C) enhances hardenability and enhances the strength of steel materials. When the C content is 0.15% or more, the yield strength can be increased to more than 1069 MPa, provided that the content of other elements is within the range of the present embodiment. C further promotes spheroidization of carbides during tempering during the manufacturing process and enhances the SSC resistance of the steel material. If the carbides are dispersed, the strength of the steel material is further increased. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, the toughness of the steel material is lowered and shrinkage is likely to occur. Therefore, the C content is 0.15 to 0.50%. The preferable lower limit of the C content is 0.20%. The preferred upper limit of the C content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

Si: 0.05 to 1.00%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the SSC resistance of the steel material is lowered. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.20%. The preferable upper limit of the Si content is 0.85%.

Mn: 0.05 to 1.00%
Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability of steel materials. If the Mn content is too low, these effects cannot be obtained. On the other hand, Mn facilitates segregation of impurities such as P and S at the old γ grain boundaries. Therefore, if the Mn content is too high, the low temperature toughness of the steel material decreases. In this case, the SSC resistance of the steel material is further reduced. Therefore, the Mn content is 0.05 to 1.00%. The preferred lower limit of the Mn content is 0.25%, more preferably 0.30%. The preferred upper limit of the Mn content is 0.90%, more preferably 0.80%.

P: 0.025% or less Phosphorus (P) is an impurity. That is, the P content is more than 0%. P segregates at the grain boundaries and lowers the low temperature toughness and SSC resistance of the steel material. Therefore, the P content is 0.025% or less. The preferred upper limit of the P content is 0.020%, more preferably 0.015%. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the P content is 0.003%, more preferably more than 0.005%.

S: 0.0100% or less Sulfur (S) is an impurity. That is, the S content is more than 0%. S segregates at the grain boundaries and lowers the low temperature toughness and SSC resistance of the steel material. Therefore, the S content is 0.0100% or less. The preferred upper limit of the S content is 0.0050%, more preferably 0.0030%. The S content is preferably as low as possible. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the S content is, for example, 0.0003%.

Al: 0.005 to 0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained and the SSC resistance of the steel material is lowered. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed and the SSC resistance of the steel material is lowered. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.015%, more preferably 0.020%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%. The "Al" content as used herein means "acid-soluble Al", that is, the content of "sol.Al".

Cr: 0.25 to 1.50%
Chromium (Cr) enhances the hardenability of the steel material and enhances the strength of the steel material. Cr further increases temper softening resistance and enables high temperature tempering. As a result, the SSC resistance of the steel material is increased. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the low temperature toughness and SSC resistance of the steel material are lowered. Therefore, the Cr content is 0.25 to 1.50%. The lower limit of the Cr content is preferably 0.25%, more preferably 0.35%, and even more preferably 0.40%. The preferable upper limit of the Cr content is 1.30%.

Mo: 0.25 to 1.50%
Molybdenum (Mo) enhances the hardenability of steel materials. Mo also produces fine carbides to increase the temper softening resistance of the steel material. As a result, Mo enhances the SSC resistance of the steel material by high-temperature tempering. Mo further suppresses the segregation of P at the grain boundaries. As a result, Mo enhances the low temperature toughness and SSC resistance of the steel material. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the above effect will be saturated. Therefore, the Mo content is 0.25 to 1.50%. The preferred lower limit of the Mo content is 0.50%, more preferably 0.60%. The preferred upper limit of the Mo content is 1.30%, more preferably 1.25%, and even more preferably 1.10%.

Ti: 0.002 to 0.050%
Titanium (Ti) forms a nitride, and the crystal grains of the steel material are refined by the pinning effect. This increases the strength of the steel material. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, the Ti nitride becomes coarse and the SSC resistance of the steel material deteriorates. Therefore, the Ti content is 0.002 to 0.050%. The preferred lower limit of the Ti content is 0.003%, more preferably 0.005%. The preferred upper limit of the Ti content is 0.030%, more preferably 0.020%.

B: 0.0001 to 0.0050%
Boron (B) dissolves in the steel material to enhance the hardenability of the steel material and increase the strength of the steel material. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, coarse nitrides are formed and the SSC resistance of the steel material is lowered. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0003%, more preferably 0.0007%. The preferred upper limit of the B content is 0.0035%, more preferably 0.0030%, still more preferably 0.0025%, still more preferably 0.0015%.

N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N forms a coarse nitride and lowers the low temperature toughness and SSC resistance of the steel material. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0050%, more preferably 0.0045%. The N content is preferably as low as possible. However, when a small amount of Ti is contained to make the crystal grains finer by the precipitation of fine nitrides, N may be contained in an amount of 0.0020% or more.

O: 0.0100% or less Oxygen (O) is an impurity. That is, the O content is more than 0%. O forms a coarse oxide and lowers the low temperature toughness and corrosion resistance of the steel material. Therefore, the O content is 0.0100% or less. The preferred upper limit of the O content is 0.0050%, more preferably 0.0030%, and even more preferably 0.0020%. The O content is preferably as low as possible. However, an extreme reduction in O content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the O content is, for example, 0.0003%.

The balance of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities are those mixed from ore, scrap, or the manufacturing environment as a raw material when the steel material is industrially manufactured, and are allowed as long as they do not adversely affect the steel material according to the present embodiment. Means what is done.

[About arbitrary elements]
The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of V and Nb instead of a part of Fe. All of these elements are optional elements and enhance the SSC resistance of steel materials.

V: 0 to 0.60%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is contained, V combines with C or N to form carbides, nitrides, carbonitrides, etc. (hereinafter referred to as "carbonitrides, etc."). These carbonitrides and the like refine the substructure of the steel material by the pinning effect and enhance the SSC resistance of the steel material. V also forms fine carbides during tempering. Fine carbides increase the temper softening resistance of the steel material and increase the strength of the steel material. V further, since the spherical MC type carbide, and suppress the formation of needle-like M _{2 C-type} carbide, enhance the SSC resistance of the steel. If even a small amount of V is contained, these effects can be obtained to some extent. However, if the V content is too high, the low temperature toughness of the steel material will decrease. If the V content is too high, the SSC resistance of the steel material may further decrease. Therefore, the V content is 0 to 0.60%. The preferable lower limit of the V content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the V content is 0.40%, more preferably 0.20%, still more preferably 0.10%, still more preferably less than 0.05%.

Nb: 0 to 0.030%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, Nb forms a carbonitride or the like. These carbonitrides and the like refine the substructure of the steel material by the pinning effect and enhance the SSC resistance of the steel material. Nb further, since the spherical MC type carbide, and suppress the formation of needle-like M _{2 C-type} carbide, enhance the SSC resistance of the steel. If even a small amount of Nb is contained, these effects can be obtained to some extent. However, if the Nb content is too high, carbonitrides and the like are excessively generated, and the low temperature toughness and SSC resistance of the steel material are lowered. Therefore, the Nb content is 0 to 0.030%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.007%. The preferred upper limit of the Nb content is 0.025%, more preferably 0.020%.

The total content of V and Nb is preferably 0.60% or less, more preferably 0.40% or less, and further preferably 0.20% or less.

The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Ca, Mg, and Zr instead of a part of Fe. All of these elements are optional elements and enhance the SSC resistance of steel materials.

Ca: 0 to 0.0100%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, Ca refines the sulfide in the steel material and enhances the SSC resistance of the steel material. If even a small amount of Ca is contained, this effect can be obtained to some extent. However, if the Ca content is too high, the oxide in the steel material becomes coarse, and the low temperature toughness and SSC resistance of the steel material deteriorate. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%. The preferred upper limit of the Ca content is 0.0040%, more preferably 0.0025%.

Mg: 0 to 0.0100%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When Mg is contained, Mg detoxifies S in the steel material as sulfide and enhances the SSC resistance of the steel material. If even a small amount of Mg is contained, this effect can be obtained to some extent. However, if the Mg content is too high, the oxide in the steel material becomes coarse, and the low temperature toughness and SSC resistance of the steel material deteriorate. Therefore, the Mg content is 0 to 0.0100%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%, still more preferably 0.0010%. Is. The preferred upper limit of the Mg content is 0.0040%, more preferably 0.0025%, and even more preferably 0.0020%.

Zr: 0-0.0100%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When Zr is contained, Zr refines the sulfide in the steel material and enhances the SSC resistance of the steel material. If even a small amount of Zr is contained, this effect can be obtained to some extent. However, if the Zr content is too high, the oxide becomes coarse and the low temperature toughness and SSC resistance of the steel material deteriorate. Therefore, the Zr content is 0-0.0100%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%. The preferred upper limit of the Zr content is 0.0040%, more preferably 0.0025%, and even more preferably 0.0020%.

When two or more kinds selected from the group consisting of Ca, Mg, and Zr are compounded and contained, the total amount is preferably 0.0100% or less, preferably 0.0050% or less. Is even more preferable.

The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Co and W instead of a part of Fe. All of these elements are optional elements and form a protective corrosive film in a hydrogen sulfide environment to suppress hydrogen intrusion. This enhances the SSC resistance of the steel material.

Co: 0 to 0.50%
Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When Co is contained, Co forms a protective corrosive coating in a hydrogen sulfide environment and suppresses hydrogen intrusion. This enhances the SSC resistance of the steel material. If even a small amount of Co is contained, this effect can be obtained to some extent. However, if the Co content is too high, the hardenability of the steel material is lowered and the strength of the steel material is lowered. Therefore, the Co content is 0 to 0.50%. The lower limit of the Co content is preferably more than 0%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.05%. The preferred upper limit of the Co content is 0.45%, more preferably 0.40%.

W: 0 to 0.50%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When W is contained, W forms a protective corrosive coating in a hydrogen sulfide environment and suppresses hydrogen intrusion. This enhances the SSC resistance of the steel material. If W is contained even in a small amount, this effect can be obtained to some extent. However, if the W content is too high, coarse carbides are generated, and the low temperature toughness and SSC resistance of the steel material are lowered. Therefore, the W content is 0 to 0.50%. The preferable lower limit of the W content is more than 0%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.05%. The preferred upper limit of the W content is 0.45%, more preferably 0.40%.

The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Ni and Cu instead of a part of Fe. All of these elements are optional elements and enhance the hardenability of steel materials.

Ni: 0 to 0.10%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, Ni enhances the hardenability of the steel material and enhances the strength of the steel material. Ni also enhances the low temperature toughness of steel materials. If even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, local corrosion is promoted and the SSC resistance of the steel material is lowered. Therefore, the Ni content is 0 to 0.10%. The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%. The preferred upper limit of the Ni content is 0.09%, more preferably 0.08%.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, Cu enhances the hardenability of the steel material and enhances the strength of the steel material. If even a small amount of Cu is contained, this effect can be obtained to some extent. However, if the Cu content is too high, the hardenability of the steel material becomes too high, and the SSC resistance of the steel material decreases. Therefore, the Cu content is 0 to 0.50%. The lower limit of the Cu content is more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%.

Rare earth element (REM): 0-0.0100%
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When REM is contained, REM refines the sulfide in the steel material and enhances the SSC resistance of the steel material. REM further binds to P in the steel material and suppresses segregation of P at the grain boundaries. Therefore, the decrease in low temperature toughness and SSC resistance of the steel material due to the segregation of P is suppressed. If even a small amount of REM is contained, these effects can be obtained to some extent. However, if the REM content is too high, the oxide becomes coarse and the low temperature toughness and SSC resistance of the steel material deteriorate. Therefore, the REM content is 0 to 0.0100%. The preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%. The preferred upper limit of the REM content is 0.0040%, more preferably 0.0025%.

In addition, REM in this specification means ittium (Y) of atomic number 39, lantern (La) of atomic number 57 which is a lanthanoid to lutetium (Lu) of atomic number 71, and atomic number 89 which is an actinium. It is one or more elements selected from the group consisting of actinium (Ac) No. 103 and lawrencium (Lr) No. 103. Further, the REM content in the present specification is the total content of these elements.

[Amount of solid solution C]
The steel material according to this embodiment contains 0.010 to 0.060% by mass of solid solution C. If the amount of solid solution C is less than 0.010% by mass, the carbides precipitated in the steel material become coarse and the low temperature toughness of the steel material decreases. If the amount of solid solution C is less than 0.010% by mass, the dislocations are not sufficiently fixed and the SSC resistance of the steel material is lowered. On the other hand, if the amount of solid solution C exceeds 0.060% by mass, the SSC resistance of the steel material is rather lowered. If the amount of solid solution C exceeds 0.060% by mass, the low temperature toughness of the steel material may further decrease. Therefore, the amount of solid solution C is 0.010 to 0.060% by mass. The preferable lower limit of the amount of solid solution C is 0.015% by mass, and more preferably 0.020% by mass. The preferable upper limit of the amount of solid solution C is 0.054% by mass, and more preferably 0.050% by mass.

The amount of solid solution C in the above range can be obtained, for example, by controlling the holding time of tempering and controlling the cooling rate after tempering. The reason for this is as follows.

In the tempering step, if the tempering holding time is short, the tempering is insufficient. In this case, the precipitation of carbides in the steel material is insufficient, and the amount of solid solution C becomes too high. As a result, the SSC resistance of the steel material is lowered. On the other hand, if the tempering retention time is too long, these effects saturate. Therefore, the tempering retention time is 10 to 180 minutes.

In the tempering step, in the cooling after tempering, if the cooling rate is slow, the solid-solved C reprecipitates during the temperature decrease. In the conventional method for manufacturing steel materials, the cooling rate after tempering is slow because the cooling is performed by allowing cooling. Therefore, the amount of solid solution C was almost 0% by mass. Therefore, in the present embodiment, the cooling rate after tempering is increased to obtain a solid solution C amount of 0.010 to 0.060% by mass.

As a cooling method, for example, the steel material is continuously forcibly cooled from the tempering temperature, and the temperature of the steel material is continuously lowered. As such continuous cooling treatment, for example, there are a method of immersing the steel material in a water tank to cool it, and a method of accelerating cooling of the steel material by shower water cooling, mist cooling or forced air cooling.

The cooling rate after tempering is measured at the slowest cooling site in the cross section of the tempered steel (for example, in the case of forced cooling on both surfaces, the center of the steel thickness). Specifically, when the steel material is a steel plate, the cooling rate after tempering can be measured by charging a sheath-type thermocouple in the center of the thickness of the steel plate and measuring the temperature. When the steel material is a steel pipe, the cooling rate after tempering can be measured by charging a sheath-type thermocouple in the center of the wall thickness of the steel pipe and measuring the temperature. Further, when only one side surface of the steel material is forcibly cooled, the surface temperature on the non-forced cooling side of the steel material can be measured by a non-contact infrared thermometer.

The temperature range from 600 ° C. to 200 ° C. is a temperature range in which the diffusion of C is relatively fast. On the other hand, the preferred tempering temperature of this embodiment is 580 to 720 ° C. Therefore, if the average cooling rate between 580 ° C. and 200 ° C. is 4 ° C./sec or more, the amount of solid solution C in the steel material can be increased. The preferred lower limit of the cooling rate after tempering is 5 ° C./sec, more preferably 10 ° C./sec, and even more preferably 15 ° C./sec.

On the other hand, if the cooling rate after tempering is too fast, C that has been solid-solved after the soaking heat of tempering is hardly precipitated. As a result, the amount of solid solution C may be excessive. Therefore, the cooling rate after tempering is 300 ° C./sec or less. The preferred upper limit of the cooling temperature after tempering is 150 ° C./sec, more preferably 100 ° C./sec, and even more preferably 50 ° C./sec.

The above method is an example, but according to this method, the amount of solid solution C can be set to 0.010 to 0.060% by mass.

When tempering is carried out by the above method, the amount of P segregated at the old γ grain boundaries (hereinafter, also referred to as “grain boundary segregation P”) is further reduced. Specifically, if tempering is carried out by the above method, the amount of grain boundary segregation P is 3.0 mol. % Or less. The present inventors consider the reason for this as follows.

Between 550 ° C and 500 ° C, grain boundary segregation of P is likely to occur. On the other hand, the preferred tempering temperature of this embodiment is 580 to 720 ° C. That is, the tempering temperature is higher than the temperature at which P grain boundary segregation is likely to occur. Therefore, at the time of cooling after tempering, grain boundary segregation of P occurs. Therefore, if the cooling between 550 ° C. and 500 ° C. is accelerated, the segregation of P to the grain boundaries can be suppressed. Specifically, if the average cooling rate between 580 ° C. and 200 ° C. is 4 ° C./sec or more, the amount of grain boundary segregation P is 3.0 mol. It can be less than or equal to%.

As described above, when an element segregates at the old γ grain boundary, the old γ grain boundary is likely to be cracked. As a result, the low temperature toughness and SSC resistance of the steel material are lowered. On the other hand, the amount of grain boundary segregation P was 3.0 mol. If it is less than%, the occurrence of cracks at the old γ grain boundaries is suppressed, and the low temperature toughness and SSC resistance of the steel material are further enhanced.

The preferable upper limit of the grain boundary segregation P amount is 2.5 mol. %, More preferably 2.0 mol. %. It is preferable that the amount of grain boundary segregation P is as low as possible. The lower limit of the grain boundary segregation P amount is, for example, 0.01 mol. %.

[Calculation method of solid solution C amount]
The solid solution C amount means the difference between the C amount (mass%) in the carbide in the steel material and the C content in the chemical composition of the steel material. The amount of C in the carbide in the steel material is Fe concentration <Fe> a, Cr concentration <Cr> a in the carbide (cementite and MC type carbide) obtained as a residue by performing extraction residue analysis on the steel material. For Mn concentration <Mn> a, Mo concentration <Mo> a, V concentration <V> a, Nb concentration <Nb> a, and cementite identified by TEM observation of the replica film obtained by the extraction replica method. Using the Fe concentration <Fe> b, Cr concentration <Cr> b, Mn concentration <Mn> b, and Mo concentration <Mo> b in cementite obtained by performing point analysis by EDS, the formula (1) ) ~ Eq. (5).
<Mo> c = (<Fe> a + <Cr> a + <Mn> a) × <Mo> b / (<Fe> b + <Cr> b + <Mn> b) (1)
<Mo> d = <Mo> a- <Mo> c (2)
<C> a = (<Fe> a / 55.85 + <Cr> a / 52 + <Mn> a / 53.94 + <Mo> c / 95.9) / 3 × 12 (3)
<C> b = (<V> a / 50.94 + <Mo> d / 95.9 + <Nb> a / 92.9) × 12 (4)
(Amount of solid solution C) = <C>-(<C> a + <C> b) (5)
In the present specification, cementite means a carbide having an Fe content of 50% by mass or more. Hereinafter, a method for calculating the amount of solid solution C will be described in detail.

[Quantification of C content of steel materials]
When the steel material is a plate material, a chip-like analysis sample is collected from the central portion of the plate thickness, and when the steel material is a pipe material, a chip-like analysis sample is collected from the central portion of the wall thickness. The C content (% by mass) is analyzed by the combustion in oxygen stream-infrared absorption method. This is defined as the C content (<C>) of the steel material.

[Calculation of the amount of C precipitated as carbide (precipitated C amount)]
The amount of precipitated C is calculated by the following steps 1 to 4. Specifically, the extraction residue analysis is carried out in step 1. By the extraction replica method using a transmission electron microscope (hereinafter referred to as "TEM") and the energy dispersive X-ray analysis method (hereinafter referred to as "EDS") in step 2. An element concentration analysis in cementite (hereinafter referred to as "EDS analysis") is performed. The Mo content is adjusted in step 3. The amount of precipitated C is calculated in step 4.

[Procedure 1. Quantification of Fe, Cr, Mn, Mo, V, and Nb residues by extraction residue analysis]
In step 1, carbides in the steel material are captured as a residue, and the Fe, Cr, Mn, Mo, V, and Nb contents in the residue are determined. Here, "carbides", cementite (M 3 _C type carbide) and is a generic name of MC-type carbides. The specific procedure is as follows. When the steel material is a plate material, a columnar test piece having a diameter of 6 mm and a length of 50 mm is collected from the central portion of the plate thickness. When the steel material is a steel pipe, a columnar test piece having a diameter of 6 mm and a length of 50 mm is collected from the central portion of the wall thickness of the steel pipe so that the center of the wall thickness is the center of the cross section. The surface of the collected test piece is polished by preliminary electropolishing to about 50 μm to obtain a new surface. The electropolished test piece is electrolyzed with an electrolytic solution of 10% acetylacetone + 1% tetraammonium + methanol. The electrolytic solution after electrolysis is passed through a 0.2 μm filter to capture the residue. The obtained residue is acid-decomposed, and the concentrations of Fe, Cr, Mn, Mo, V, and Nb are quantified in mass% units by ICP (inductively coupled plasma) emission spectrometry. This concentration is defined as <Fe> a, <Cr> a, <Mn> a, <Mo> a, <V> a, and <Nb> a, respectively.

[Procedure 2. Quantification of Fe, Cr, Mn, and Mo contents in cementite by extraction replica method and EDS]
In step 2, the Fe, Cr, Mn, and Mo contents in cementite are determined. The specific procedure is as follows. A micro test piece is cut out from the central part of the plate thickness when the steel material is a plate material, and from the central part of the wall thickness when the steel material is a steel pipe, and the surface is finished by mirror polishing. The test piece is immersed in a 3% nital corrosive solution for 10 minutes to corrode the surface. The surface is covered with a carbon vapor deposition film. The test piece whose surface is covered with the vapor-deposited film is immersed in a 5% nital corrosive solution and held for 20 minutes to peel off the vapor-deposited film. After washing the peeled thin-film film with ethanol, scoop it with a sheet mesh and dry it. This vapor-deposited film (replica film) is observed by TEM, and 20 cementites are point-analyzed by EDS. The Fe, Cr, Mn, and Mo concentrations are quantified in mass% units when the total of the alloying elements excluding carbon in cementite is 100%. The concentrations of 20 cementites are quantified, and the arithmetic mean values of each element are defined as <Fe> b, <Cr> b, <Mn> b, and <Mo> b.

[Procedure 3. Adjustment of Mo amount]
Subsequently, the Mo concentration in the carbide is determined. Here, Fe, Cr, Mn, and Mo are concentrated in cementite. On the other hand, V, Nb, and Mo are concentrated in MC-type carbides. That is, Mo is concentrated in both cementite and MC-type carbides by tempering. Therefore, the amount of Mo is calculated individually for cementite and MC-type carbides. In addition, a part of V may be concentrated in cementite. However, the amount of V concentrated in cementite is negligibly small as compared with the amount of concentrated in MC-type carbides. Therefore, in determining the amount of solid solution C, V is considered to be concentrated only in MC-type carbides.

Specifically, the amount of Mo precipitated as cementite (<Mo> c) is calculated by the formula (1).
<Mo> c = (<Fe> a + <Cr> a + <Mn> a) × <Mo> b / (<Fe> b + <Cr> b + <Mn> b) (1)

On the other hand, the amount of Mo precipitated as MC-type carbide (<Mo> d) is calculated in units of mass% by the formula (2).
<Mo> d = <Mo> a- <Mo> c (2)

[Procedure 4. Calculation of the amount of precipitated C]
The amount of precipitated C is calculated as the sum of the amount of C precipitated as cementite (<C> a) and the amount of C precipitated as MC-type carbide (<C> b). <C> a and <C> b are calculated in mass% units by the formulas (3) and (4), respectively. Incidentally, Equation (3), the structure of cementite _{M 3} C-type (M includes Fe, Cr, Mn, and Mo) is an expression derived from it is.
<C> a = (<Fe> a / 55.85 + <Cr> a / 52 + <Mn> a / 53.94 + <Mo> c / 95.9) / 3 × 12 (3)
<C> b = (<V> a / 50.94 + <Mo> d / 95.9 + <Nb> a / 92.9) × 12 (4)

From the above, the amount of precipitated C is <C> a + <C> b.

[Calculation of solid solution C amount]
The solid solution C amount (hereinafter, also referred to as <C> c) is calculated in mass% units by the formula (5) as the difference between the C content (<C>) of the steel material and the precipitated C amount.
<C> c = <C>-(<C> a + <C> b) (5)

[Measurement method of grain boundary segregation P amount]
The amount of grain boundary segregation P can be calculated by the following method. When the steel material is a plate material, the test piece is collected from the central part of the plate thickness, and when the steel material is a pipe material, the test piece is collected from the central part of the wall thickness. The test piece is cooled in liquid nitrogen and broken in vacuum. Ten planes broken at the grain boundaries are identified, Auger electron spectroscopy is performed, and the P concentration is measured. The average value of the obtained 10 P concentrations is defined as the grain boundary segregation P amount (mol.%).

[Micro tissue]
The microstructure of the steel material according to this embodiment mainly consists of tempered martensite and tempered bainite. More specifically, the microstructure consists of tempered martensite and / or tempered bainite having a volume fraction of 90% or more. That is, in the microstructure, the total volume fraction of tempered martensite and tempered bainite is 90% or more. The rest of the microstructure is, for example, retained austenite. If the microstructure of the steel material having the above-mentioned chemical composition contains 90% or more of the total volume fraction of tempered martensite and tempered bainite, the yield strength is more than 1069 to 1172 MPa (155 ksi class) and the yield ratio is 85. % Or more. Preferably, the yield ratio is 90% or more.

In the present embodiment, when the yield strength is more than 1069 to 1172 MPa (155 ksi class) and the yield ratio is 85% or more, the microstructure has a total volume ratio of tempered martensite and tempered bainite of 90% or more. Suppose there is. Preferably, the microstructure consists only of tempered martensite and / or tempered bainite.

When the total volume fraction of tempered martensite and tempered bainite is obtained by observation, it can be obtained by the following method. When the steel material is a plate material, a small piece having an observation surface of 10 mm in the rolling direction and 10 mm in the plate thickness direction is cut out from the central portion of the plate thickness. When the steel material is a steel pipe, a small piece having an observation surface of 10 mm in the pipe axis direction and 8 mm in the wall thickness direction is cut out from the central portion of the wall thickness. After polishing the observation surface to a mirror surface, it is immersed in a nital corrosive solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed with a secondary electron image using a scanning electron microscope (SEM). Observe 10 fields of view with a magnification of about ^{400 μm 2} per field of view (magnification of 5000 times). In each field of view, tempered martensite and tempered bainite are identified from the contrast. Calculate the total surface integral of the identified tempered martensite and tempered bainite. In the present embodiment, the arithmetic average value of the total area fraction of the tempered martensite and the tempered bainite obtained in all the visual fields is taken as the volume fraction of the tempered martensite and the tempered bainite.

[Crystal grain size of old austenite grains]
In the steel material according to the present embodiment, the grain size number of the former austenite crystal grains is 8.0 or more. If the crystal grain size number of the old γ grains is less than 8.0, an impurity element such as P segregates at the old γ grain boundaries. In this case, the amount of grain boundary segregation P is 3.0 mol. Over%. As a result, the old γ grain boundaries become brittle and the low temperature toughness of the steel material decreases. In this case, the SSC resistance of the steel material is further reduced. Therefore, in the steel material according to the present embodiment, the crystal grain size number of the old γ grains is 8.0 or more. The preferable lower limit of the crystal particle size number of the old γ grains is 8.5, more preferably 9.0. The upper limit of the crystal particle size number of the old γ grains is not particularly defined, but the upper limit of the crystal particle size number of the old γ grains is, for example, 16.0.

The crystal grain size number of the old γ grain can be determined by the following method. When the steel material is a plate material, a test piece for microscopic observation is collected from the central portion of the plate thickness, and when the steel material is a pipe material, a test piece for microscopic observation is collected from the central portion of the wall thickness. Using the collected test pieces, the microscopic test method of the crystal size specified in JIS G0551 (2013) is carried out, and the austenite crystal size number is evaluated. Specifically, the test piece is embedded with resin, polished, and then immersed in a nital corrosive solution for about 10 seconds to reveal the grain boundaries of old austenite on the surface. In 10 fields of view on the corroded surface, the crystal grain size number of each field of view is obtained. The area of each field of view is, for example, 0.066 mm ² . The crystal grain size number in each field of view is evaluated by comparison with the crystal grain size standard diagram specified in 7.2 of JIS G0551 (2013). The arithmetic mean value of the particle size numbers evaluated in 10 fields of view is defined as the crystal particle size number of the old γ grains.

[Shape of steel]
The shape of the steel material according to this embodiment is not particularly limited. The steel material is, for example, a steel pipe or a steel plate. When the steel material is a steel pipe for an oil well, the preferable wall thickness is 9 to 60 mm. This embodiment is particularly suitable for use as a thick-walled steel pipe for oil wells. More specifically, even if the steel material according to the present embodiment is a steel pipe for oil wells having a thickness of 15 mm or more and a thickness of 20 mm or more, it exhibits excellent strength, excellent low temperature toughness, and excellent SSC resistance. ..

[YS and YR of steel materials]
The yield strength YS of the steel material according to the present embodiment is more than 1069 to 1172 MPa (155 ksi class), and the yield ratio YR is 85% or more. The yield strength YS as used herein means the stress at 0.2% elongation obtained in the tensile test. In short, the strength of the steel material according to this embodiment is 155 ksi class. Even with such high strength, the steel material according to the present embodiment has excellent low temperature toughness and excellent SSC resistance by satisfying the above-mentioned chemical composition, solid solution C amount, and microstructure.

[Low temperature toughness of steel]
The low temperature toughness of the steel material according to this embodiment can be evaluated by a method according to JIS Z 2242 (2005). As the test piece, a V-notch test piece having a width of 10 mm and a length of 55 mm is used. Perform a Charpy impact test on the test piece cooled to -40 ° C. Under the above conditions, the steel material according to the present embodiment has an absorption energy E (-40 ° C.) at −40 ° C. of 74.0 J or more.

[SSC resistance of steel materials]
The SSC resistance of the steel material according to this embodiment can be evaluated by a method based on NACE TM0177-2005 Method A. Test bath, 5% sodium chloride + 0.5% acetic acid aqueous solution encapsulating _{H 2} S of 0.003Bar, and, and 5% sodium chloride + 0.5% acetic acid aqueous solution encapsulating _{H 2} S of 0.005Bar. A stress corresponding to 85% of the yield stress is applied to the steel material, and the steel material is immersed in a test bath. The steel material according to the present embodiment does not break for 720 hours or more under any of the above conditions.

[Production method]
The method for producing a steel material according to the present embodiment includes a preparation step, a quenching step, and a tempering step. The preparatory step may include a material preparatory step and a hot working step. In this embodiment, a method for manufacturing a steel pipe for an oil well will be described as an example of a method for manufacturing a steel material. The method for manufacturing a steel pipe for an oil well includes a step of preparing a raw pipe (preparation step) and a step of quenching and tempering the raw pipe to obtain a steel pipe for an oil well (quenching step and tempering step). Hereinafter, each step will be described in detail.

[Preparation process]
In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. The production method of the intermediate steel material is not particularly limited as long as it has the above chemical composition. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate, and is a raw pipe when the final product is a steel pipe.

Preferably, the preparation step may include a step of preparing the material (material preparation step) and a step of hot-working the material to produce an intermediate steel material (hot-working step). Hereinafter, the case where the material preparation step and the hot working step are included will be described in detail.

[Material preparation process]
In the material preparation step, a material is produced using molten steel having the above-mentioned chemical composition. Specifically, slabs (slabs, blooms, or billets) are manufactured by a continuous casting method using molten steel. An ingot may be produced by an ingot method using molten steel. If necessary, slabs, blooms or ingots may be lump-rolled to produce billets. The material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
In the hot working process, the prepared material is hot-worked to produce an intermediate steel material. When the steel material is a steel pipe, the intermediate steel material corresponds to a raw pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300 ° C. Hot working is performed on the billets extracted from the heating furnace to manufacture raw pipes (seamless steel pipes). For example, the Mannesmann method is carried out as hot working to manufacture a bare tube. In this case, the round billet is drilled and rolled by a drilling machine. In the case of drilling and rolling, the drilling ratio is not particularly limited, but is, for example, 1.0 to 4.0. The perforated round billet is further hot-rolled with a mandrel mill, reducer, sizing mill or the like to form a raw pipe. The cumulative surface reduction rate in the hot working process is, for example, 20 to 70%.

A raw tube may be manufactured from a billet by another hot working method. For example, in the case of a short thick-walled steel material such as a coupling, the raw pipe may be manufactured by forging such as the Erhard method. A bare tube is manufactured by the above process. The wall thickness of the raw tube is not particularly limited, but is, for example, 9 to 60 mm.

The raw tube produced by hot working may be air-cooled (As-Rolled). Steel pipes manufactured by hot working can also be directly hardened after hot pipe making without cooling to room temperature, or can be hardened after reheating (reheating) after hot pipe making. good. However, when direct quenching or quenching is carried out after quenching, it is preferable to stop cooling during quenching or to carry out slow cooling for the purpose of suppressing quench cracking.

When quenching is performed directly after hot pipe making or after heating after hot pipe making, stress relief annealing is performed after quenching and before heat treatment in the next step for the purpose of removing residual stress. It is preferable to carry out the treatment (SR treatment).

As described above, the intermediate steel material is prepared in the preparation process. The intermediate steel material may be manufactured by the above-mentioned preferable process, an intermediate steel material manufactured by a third party, or a factory other than the factory where the quenching process and the tempering process described later are carried out, or another business establishment. The intermediate steel material manufactured in the above may be prepared. Hereinafter, the quenching process will be described in detail.

[Quenching process]
In the quenching process, the prepared intermediate steel material (bare pipe) is quenched. As used herein, "quenching" means to quench the _three or more points A of the intermediate steel. The preferred quenching temperature is 800-1000 ° C. The quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a temperature gauge installed on the outlet side of the apparatus for performing the final hot working when the quenching is performed directly after the hot working. The quenching temperature further corresponds to the temperature of the furnace in which the heating is performed when the quenching is performed after the heat is supplemented after the hot working.

If the quenching temperature is too high, the crystal grain size number of the old γ grains will be less than 8.0. In this case, P segregates at the old γ grain boundaries, and the low temperature toughness and SSC resistance of the steel material are lowered. Therefore, the quenching temperature is 800 to 1000 ° C. The preferred upper limit of the quenching temperature is 950 ° C.

In the quenching method, for example, the raw pipe is continuously cooled from the quenching start temperature, and the temperature of the raw pipe is continuously lowered. The method of continuous cooling treatment is not particularly limited, and a well-known method may be used. The method of continuous cooling treatment is, for example, a method of immersing the raw pipe in a water tank to cool it, or a method of accelerating cooling the raw pipe by shower water cooling or mist cooling.

If the cooling rate at the time of quenching is too slow, the microstructure will not be mainly composed of martensite and bainite, and the mechanical performance specified in this embodiment cannot be obtained. Therefore, as described above, in the method for producing a steel material according to the present embodiment, the intermediate steel material is rapidly cooled at the time of quenching. Specifically, in the quenching step, the average cooling rate in the range where the temperature of the intermediate steel material (bare pipe) during quenching is in the range of 800 to 500 ° C. is defined _{as the quenching cooling rate CR 800-500.} More specifically, the quenching cooling rate CR _800-500 is the slowest cooling portion in the cross section of the intermediate steel material to be hardened (for example, the central part of the intermediate steel material thickness when both surfaces are forcibly cooled). Determined from the temperature measured in.

The quenching cooling rate CR _800-500 is 300 ° C./min or higher. The lower limit of the preferred quenching cooling rate CR _800-500 is 450 ° C./min, more preferably 600 ° C./min. The upper limit of the quenching cooling rate CR _800-500 is not particularly specified, but is, for example, 60,000 ° C./min.

Preferably, the raw tube is heated in the austenite region a plurality of times, and then the quenching treatment is performed. In this case, the austenite grains before quenching are made finer, so that the low temperature toughness is further enhanced. The heating in the austenite region may be repeated a plurality of times by performing the quenching treatment a plurality of times, or the heating in the austenite region may be repeated a plurality of times by performing the normalizing treatment and the quenching treatment. Hereinafter, the tempering process will be described in detail.

[Tempering process]
In the tempering step, the tempering treatment is carried out after the above-mentioned quenching treatment is carried out. As used herein, "tempering" means reheating and retaining an intermediate steel material after quenching. The tempering temperature is appropriately adjusted according to the chemical composition of the steel material and the yield strength YS to be obtained. That is, the tempering temperature of the intermediate steel material (raw pipe) having the chemical composition of the present embodiment is adjusted to adjust the yield strength YS of the steel material to more than 1069 to 1172 MPa (155 ksi class). Here, the tempering temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held.

The preferred tempering temperature is 580 to 720 ° C. When the tempering temperature is 580 ° C. or higher, the carbides are sufficiently spheroidized and the low temperature toughness of the steel material is further enhanced. A more preferable lower limit of the tempering temperature is 600 ° C, and even more preferably 610 ° C. A more preferable upper limit of the tempering temperature is 710 ° C, and even more preferably 700 ° C.

If the tempering holding time (tempering time) is too short, the precipitation of carbides does not proceed, so that the amount of solid solution C becomes excessive. Even if the tempering time is too long, the effect of solid-solving C is saturated. Therefore, the tempering time for controlling the amount of solid solution C in an appropriate range is 10 to 180 minutes. The preferred lower limit of the tempering time is 15 minutes. The preferred upper limit of the tempering time is 120 minutes, more preferably 90 minutes. When the steel material is a steel pipe, the temperature variation of the steel pipe is likely to occur during tempering soaking heat retention as compared with other shapes. Therefore, when the steel material is a steel pipe, the tempering time is preferably 15 to 180 minutes. It is sufficiently possible for those skilled in the art to bring the yield strength YS within the range of more than 1069 to 1172 MPa by appropriately adjusting the tempering temperature and the holding time of the steel material having the chemical composition of the present embodiment. be.

[About quenching after tempering]
Cooling after tempering has traditionally been uncontrolled. However, the temperature range from 600 ° C. to 200 ° C. is a temperature range in which the diffusion of C is relatively fast. Therefore, if the cooling rate of the steel material after tempering (that is, after holding the holding time at the tempering temperature) is slow, most of the solid-dissolved C will reprecipitate during the temperature decrease. That is, the amount of solid solution C becomes almost 0% by mass. Further, the temperature range from 550 ° C. to 500 ° C. is a temperature range in which P is likely to segregate into the old γ grain boundaries. Therefore, if the cooling rate after tempering is slow, impurity elements such as P are further segregated at the old γ grain boundaries. That is, the amount of grain boundary segregation P is 3.0 mol. Over%. Therefore, in the present embodiment, the intermediate steel material (bare pipe) after tempering is rapidly cooled.

Specifically, in the tempering step, the average cooling rate in the range where the temperature of the intermediate steel material (bare pipe) after tempering is in the range of 580 to 200 ° C. is defined as the _{cooling rate after tempering CR 580-200.} In the method for producing a steel material according to the present embodiment, the cooling rate after tempering CR _580-200 is 4 ° C./sec or more. On the other hand, if the cooling rate after tempering is too fast, the solid solution C is hardly precipitated, and the amount of solid solution C may be excessive. In this case, the SSC resistance of the steel material is rather lowered. In this case, the low temperature toughness of the steel material may further decrease. Therefore, in the method for producing a steel material according to the present embodiment, the cooling rate CR _580-200 after tempering is 300 ° C./sec or less.

From the above, the cooling rate CR _580-200 after tempering is 4 to 300 ° C./sec. As a result, in the steel material according to the present embodiment, the amount of solid solution C is 0.010 to 0.060% by mass, and the amount of grain boundary segregation P is 3.0 mol. It becomes less than%. The preferred lower limit of the post-tempering cooling rate CR _580-200 is 5 ° C./sec, more preferably 10 ° C./sec, and even more preferably 15 ° C./sec. The preferred upper limit of the post-tempering cooling rate CR _580-200 is 150 ° C./sec, more preferably 100 ° C./sec, and even more preferably 50 ° C./sec.

The cooling method in which the cooling rate CR _580-200 after tempering is set to 4 to 300 ° C./sec is not particularly limited, and a well-known method may be used. As a cooling method, for example, the raw pipe is continuously forcibly cooled from the tempering temperature, and the temperature of the raw pipe is continuously lowered. As such continuous cooling treatment, for example, there are a method of immersing the raw pipe in a water tank to cool it, and a method of accelerating cooling the raw pipe by shower water cooling, mist cooling or forced air cooling. The post-tempering cooling rate CR _580-200 is measured at the slowest cooling portion in the cross section of the intermediate steel material to be tempered (for example, in the case of forced cooling of both surfaces, the central portion of the intermediate steel material thickness).

In the above-mentioned manufacturing method, a method for manufacturing a steel pipe has been described as an example. However, the steel material according to the present embodiment may have a steel plate or another shape. An example of a method for manufacturing a steel sheet or another shape also includes, for example, a preparation step, a quenching step, and a tempering step, as in the manufacturing method described above.

180 kg of molten steel having the chemical composition shown in Table 1 was produced.

An ingot was manufactured using the molten steel. The ingot was hot-rolled to produce a steel plate having a thickness of 15 mm.

The steel sheet of each steel number after hot rolling was allowed to cool, and the steel sheet temperature was set to room temperature (25 ° C.).

After allowing to cool, the steel sheets of each test number were reheated, adjusted so that the steel sheet temperature became the quenching temperature, and kept evenly heated for 20 minutes. The steel sheets of each test number kept at equal heat were immersed in a water tank and quenched. Quenching was repeated once or twice. Further, a sheath-type K thermocouple was charged in advance in the central portion of the thickness of the steel plate, and the temperature was measured for quenching and cooling during quenching. Table 2 shows the quenching temperature (° C.), the number of quenching times (times), and the average cooling rate between 800 ° C. and 500 ° C., that is, the cooling rate during quenching (CR _{800-500) (° C./min).}

After quenching, the steel sheets of each test number were tempered. In the tempering treatment, the tempering temperature was adjusted so as to be 155 ksi class (yield strength exceeds 1069 to 1172 MPa). After performing heat treatment at each tempering temperature, it was cooled. For cooling, mist water cooling was performed from both sides of the steel sheet. A sheath-type K thermocouple was previously charged in the central portion of the thickness of the steel sheet, and the temperature was measured for tempering and subsequent cooling. Table 2 shows the tempering temperature (° C.), the tempering time (minutes), and the subsequent average cooling rate between 580 ° C. and 200 ° C., that is, the post-tempering cooling rate (CR _580-200 ) (° C./sec). _{The Acc1} points of the steel materials of steel numbers 1 to 24 were all 750 ° C.

[Evaluation test]
[YS and TS test]
The tensile test was performed in accordance with ASTM E8. A round bar tensile test piece having a diameter of 6.35 mm and a parallel portion length of 35 mm was prepared from the center of the thickness of the steel plate of each test number after the above quenching and tempering treatment. The axial direction of the tensile test piece was parallel to the rolling direction of the steel sheet. Tensile tests were carried out at room temperature (25 ° C.) in the air using each round bar test piece to obtain yield strength YS (MPa) and tensile strength TS (MPa) at each position. In this example, the stress at 0.2% elongation obtained in the tensile test was defined as YS of each test number. The maximum stress during uniform elongation was taken as TS. The ratio of YS to TS (= YS / TS) was defined as the yield ratio YR (%).

[Microstructure judgment test]
Regarding the microstructure of the steel sheet of each test number, except for test number 14, YS was more than 1069 to 1172 MPa (155 ksi class) and YR was 85% or more, so the total volume fraction of tempered martensite and tempered bainite was It was judged to be 90% or more. In test number 14, it is considered that ferrite was formed.

[Solid solution C amount measurement test]
For the steel sheet of each test number, the amount of solid solution C (mass%) was measured and calculated by the above-mentioned measuring method. The TEM was JEM-2010 manufactured by JEOL Ltd., the acceleration voltage was 200 kV, the EDS point analysis was performed with an irradiation current of 2.56 nA, and each point was measured for 60 seconds. The observation area by TEM was 8 μm × 8 μm, and observation was performed in any 10 visual fields. Table 3 shows the residual amount of each element and the concentration in cementite used in the calculation of the amount of solid solution C.

[Grain boundary segregation P amount measurement test]
For the steel sheet of each test number, the grain boundary segregation P amount (mol.%) Was measured by the above-mentioned measuring method. The Auger electron spectroscopic analyzer used was PHI680 manufactured by ULVAC-PHI, Inc. The test conditions were an acceleration voltage of 10 kV and a sample current of 10 nA.

[Charpy impact test]
Using each steel sheet, a Charpy impact test conforming to JIS Z 2242 (2005) was carried out to evaluate low temperature toughness. Specifically, five V-notch test pieces having a width of 10 mm and a length of 55 mm were collected from the central portion of each steel plate. The longitudinal direction of the test piece was parallel to the plate width direction. The collected test piece was cooled to −40 ° C., and a Charpy impact test conforming to JIS Z 2242 (2005) was carried out to determine the absorbed energy (J). The arithmetic mean value of the absorbed energy obtained was defined as the absorbed energy E (-40 ° C.) (J).

When the absorbed energy E (-40 ° C.) was 74.0 J or more, it was judged that excellent low temperature toughness was exhibited. On the other hand, if the absorbed energy E (-40 ° C.) was less than 74.0 J, it was judged that excellent low temperature toughness was not exhibited.

[SSC resistance of steel materials]
SSC resistance was evaluated using the steel sheets of each test number by a method compliant with NACE TM0177-2005 Method A. Specifically, three round bar test pieces having a diameter of 6.35 mm and a parallel portion having a length of 25.4 mm were collected from the central portion of the wall thickness of the steel plate of each test number. The longitudinal direction of the test piece was parallel to the rolling direction. Tensile stress was applied in the axial direction of each test piece. At this time, the stress applied to each test piece was adjusted to be 85% of the yield stress (actual measurement) of each steel sheet in accordance with NACE TM0177-2005 Method A.

Test bath, 5% sodium chloride + 0.5% acetic acid aqueous solution encapsulating _{H 2} S of 0.003Bar, and, using 5% sodium chloride + 0.5% acetic acid aqueous solution encapsulating _{H 2} S of 0.005bar .. The temperature of each test bath was 25 ° C. One round bar test piece loaded with tensile stress was immersed in each of the above test baths for 720 hours. The presence or absence of sulfide stress cracking (SSC) was observed in the test piece after immersion for 720 hours. Specifically, the test piece after being immersed for 720 hours was visually observed. As a result of observation, those in which the test piece did not break were judged to be "E" (Excellent). On the other hand, the one in which the test piece was broken was judged to be "NA" (Not Accessable).

[Test results]
Table 2 shows the test results.

With reference to Tables 1 and 2, the chemical composition of the steel sheets of test numbers 1 to 13 is appropriate, the yield strength YS is more than 1069 to 1172 MPa (155 ksi class), and the yield ratio YR is 85% or more. rice field. The crystal particle size number of the old γ grains was 8.0 or more, and the amount of solid solution C was 0.010 to 0.060% by mass. As a result, the absorbed energy E (-40 ° C.) was 74.0 J or more, and excellent low temperature toughness was exhibited. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it exhibited excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Was less than%.

On the other hand, in the steel sheet of test number 14, the cooling rate at the time of quenching was too slow. Therefore, YR was less than 85%. As a result, in each of SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. It is considered that ferrite was mixed in the microstructure.

The Ti content of the steel sheet of test number 15 was too low. Moreover, the B content was too low. In addition, the cooling rate after tempering was too slow. Therefore, the amount of solid solution C was less than 0.010% by mass. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

For the steel sheet of test number 16, the cooling rate after tempering was too slow. Therefore, the amount of solid solution C was less than 0.010% by mass. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Furthermore, although showed excellent SSC resistance in SSC resistance test at 0.003barH ₂ S, it showed excellent SSC resistance in SSC resistance test at 0.005barH ₂ S. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

For the steel sheets of test numbers 17 and 18, the cooling rate after tempering was too slow. Therefore, the amount of solid solution C was less than 0.010% by mass. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

The quenching temperature was too high for the steel sheets of test numbers 19 and 20. Therefore, the crystal grain size number of the old γ grains was less than 8.0. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

In the steel sheet of test number 21, the cooling rate after tempering was too fast. Therefore, the amount of solid solution C exceeded 0.060% by mass. As a result, in each of SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance.

For the steel sheet of test number 22, the tempering time was too short. Therefore, the amount of solid solution C exceeded 0.060% by mass. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance.

The Cr content of the steel sheet of Test No. 23 was too low. As a result, in each of SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance.

In the steel sheet of test number 24, the Mo content was too low. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

In the steel sheet of test number 25, the Mn content was too high. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

The N content of the steel sheet of test number 26 was too high. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance.

In the steel sheet of test number 27, the P content was too high. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance. The amount of grain boundary segregation P was 3.0 mol. Exceeded%.

The V content of the steel sheet of test number 28 was too high. As a result, the absorbed energy E (-40 ° C.) was less than 74.0 J, and did not show excellent low temperature toughness. Further, in any of the SSC resistance test at SSC resistance test and 0.005barH ₂ S in 0.003barH ₂ S, it showed excellent SSC resistance.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

The steel material according to the present invention can be widely applied to a steel material used in a sour environment, preferably can be used as a steel material used in an oil well environment, and more preferably as a steel material such as a casing, tubing, and line pipe. It is available.

Claims (9)

By mass%
C: 0.15 to 0.50%,
Si: 0.05 to 1.00%,
Mn: 0.05 to 1.00%,
P: 0.025% or less,
S: 0.0100% or less,
Al: 0.005 to 0.100%,
Cr: 0.25 to 1.50%,
Mo: 0.25 to 1.50%,
Ti: 0.002 to 0.050%,
B: 0.0001 to 0.0050%,
N: 0.0100% or less,
O: 0.0100% or less,
V: 0-0.60%,
Nb: 0 to 0.030%,
Ca: 0-0.0100%,
Mg: 0-0.0100%,
Zr: 0-0.0100%,
Co: 0-0.50%,
W: 0 to 0.50%,
Ni: 0 to 0.10%,
Cu: 0 to 0.50%, and
Rare earth element: Contains 0 to 0.0100% and has a chemical composition with the balance consisting of Fe and impurities.
Containing 0.010 to 0.060% by mass of solid solution C,
The grain size number of the former austenite crystal grains is 8.0 or more, and
A steel material having a yield strength of more than 1069 to 1172 MPa and a yield ratio of 85% or more. The steel material according to claim 1.
The chemical composition is
V: 0.01 to 0.60% and
Nb: A steel material containing at least one selected from the group consisting of 0.002 to 0.030%. The steel material according to claim 1 or 2.
The chemical composition is
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100% and
Zr: A steel material containing one or more selected from the group consisting of 0.0001 to 0.0100%. The steel material according to any one of claims 1 to 3.
The chemical composition is
Co: 0.02 to 0.50%, and
W: A steel material containing at least one selected from the group consisting of 0.02 to 0.50%. The steel material according to any one of claims 1 to 4.
The chemical composition is
Ni: 0.01 to 0.10% and
Cu: A steel material containing at least one selected from the group consisting of 0.01 to 0.50%. The steel material according to any one of claims 1 to 5.
The chemical composition is
Rare earth element: A steel material containing 0.0001 to 0.0100%. It has the chemical composition according to any one of claims 1 to 6.
Containing 0.010 to 0.060% by mass of solid solution C,
The grain size number of the former austenite crystal grains is 8.0 or more, and
A steel pipe for oil wells having a yield strength of more than 1069 to 1172 MPa and a yield ratio of 85% or more. A preparatory step for preparing an intermediate steel material having the chemical composition according to any one of claims 1 to 6.
After the preparatory step, a quenching step of cooling the intermediate steel material at 800 to 1000 ° C. at a cooling rate of 300 ° C./min or more, and a quenching step.
Said intermediate steel after quenching, after holding for 10 to 180 minutes at five hundred and eighty to seven hundred twenty ° C., and a tempering step of cooling the average cooling rate between 200 ° C. from 580 ° C. at 4 to 300 ° C. / sec, claim The method for producing a steel material according to any one of 1 to 6. The method for producing a steel material according to claim 8.
The preparation step includes a material preparation step of preparing a material having the chemical composition according to any one of claims 1 to 6.
A method for manufacturing a steel material, which comprises a hot working step of hot-working the material to produce an intermediate steel material.

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