JP7036237B2 - Steel material suitable for use in sour environment - Google Patents

Description

The present disclosure relates to steel materials, and more particularly to steel materials suitable for use in a sour environment.

By deepening oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as "oil wells"), it is required to increase the strength of steel pipes for oil wells. Specifically, 80 ksi class (yield strength of less than 80 to 95 ksi, that is, less than 552 to 655 MPa) and 95 ksi class (yield strength of less than 95 to 110 ksi, that is, less than 655 to 758 MPa) are widely used. Recently, steel pipes for oil wells of 110 ksi class (yield strength of 110 to 125 ksi, that is, 758 to 862 MPa) have begun to be sought.

Many deep wells are sour environments containing corrosive hydrogen sulfide. As used herein, the sour environment means an acidified environment containing hydrogen sulfide. In a sour environment, carbon dioxide may be contained. Steel pipes for oil wells used in such a sour environment are required to have not only high strength but also sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance).

Techniques for improving the SSC resistance of steel materials represented by steel pipes for oil wells are JP-A-62-253720 (Patent Document 1), JP-A-59-232220 (Patent Document 2), and JP-A-6-322478. Japanese Patent Application Laid-Open No. (Patent Document 3), Japanese Patent Application Laid-Open No. 8-3115151 (Patent Document 4), Japanese Patent Application Laid-Open No. 2000-25783 (Patent Document 5), Japanese Patent Application Laid-Open No. 2000-297344 (Patent Document 6), Japanese Patent Application Laid-Open No. 2005 -350754 (Patent Document 7), Japanese Patent Application Laid-Open No. 2012-591238 (Patent Document 8), and Japanese Patent Application Laid-Open No. 2012-26030 (Patent Document 9).

Patent Document 1 proposes a method of reducing impurities such as Mn and P to improve the SSC resistance of steel wells. Patent Document 2 proposes a method of performing quenching twice to refine crystal grains and improve the SSC resistance of steel.

Patent Document 3 proposes a method of refining a steel structure by induction heat treatment to improve the SSC resistance of a 125 ksi class steel material. Patent Document 4 proposes a method of improving the hardenability of steel by using a direct quenching method and further increasing the tempering temperature to improve the SSC resistance of 110 to 140 ksi class steel pipes.

Patent Document 5 and Patent Document 6 propose a method of controlling the morphology of carbides to improve the SSC resistance of steel for low alloy well pipes of 110 to 140 ksi class. Patent Document 7 proposes a method of controlling the dislocation density and the hydrogen diffusion coefficient to desired values to improve the SSC resistance of steel materials of 125 ksi class or higher. Patent Document 8 proposes a method for enhancing the SSC resistance of 125 ksi class steel by performing quenching a plurality of times on a low alloy steel containing 0.3 to 0.5% C. Patent Document 9 proposes a method of controlling the form and number of carbides by adopting a tempering step of a two-stage heat treatment. More specifically, in Patent Document 9, the number density of large M ₃ C or M ₂ C is suppressed to enhance the SSC resistance of 125 ksi class steel.

Japanese Unexamined Patent Publication No. 62-253720 Japanese Unexamined Patent Publication No. 59-232220 Japanese Unexamined Patent Publication No. 6-322478 Japanese Unexamined Patent Publication No. 8-311551 Japanese Unexamined Patent Publication No. 2000-256783 Japanese Unexamined Patent Publication No. 2000-297344 Japanese Unexamined Patent Publication No. 2005-350754 Special Table 2012-591238 Publication No. Japanese Unexamined Patent Publication No. 2012-26030

However, by a technique other than the techniques disclosed in Patent Documents 1 to 9, a steel material having a yield strength of 110 ksi class (758 to 862 MPa) and exhibiting excellent SSC resistance (for example, a steel pipe for an oil well) can be obtained. May be good.

An object of the present disclosure is to provide a steel material having a yield strength of 758 to 862 MPa (110 ksi class) and having excellent SSC resistance in a sour environment.

The steel material according to the present disclosure is C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less in terms of mass%. , S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0 .020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0020 %, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Cu: 0 to 0.50%, Ni : 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, the balance of which is Fe and impurities, and has a chemical composition satisfying the formula (1). In the steel material, the crystal grain size of the old austenite grains is 11.0 μm or less. In the steel material, the average area of the precipitates deposited at the former austenite grain boundaries is 10.0 × 10 ^-3 μm ² or less. The yield strength of the steel material is 758 to 862 MPa.
2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.45 x Cu + 0.8 x Cr + 2 x Mo ≧ 3.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol.

The steel material according to the present disclosure has a yield strength of 758 to 862 MPa (110 ksi class) and has excellent SSC resistance in a sour environment.

FIG. 1 is a diagram showing the relationship between the Mo content and the old γ particle size.

The present inventors investigated and investigated a method for obtaining excellent SSC resistance while maintaining a yield strength of 758 to 862 MPa (110 ksi class) in a steel material expected to be used in a sour environment, and obtained the following findings. Obtained.

If the dislocation density in the steel material is increased, the yield strength YS (Yield Strength) of the steel material is increased. On the other hand, dislocations in steel may occlude hydrogen. Therefore, if the dislocation density of the steel material increases, the amount of hydrogen occluded by the steel material may also increase. If the hydrogen concentration in the steel material is increased as a result of increasing the dislocation density, the SSC resistance of the steel material is lowered even if high strength is obtained. Therefore, in order to achieve both the yield strength of 110 ksi class and the excellent SSC resistance, it is not preferable to increase the strength by utilizing the dislocation density.

Therefore, the present inventors may obtain excellent SSC resistance even if the yield strength of the steel material is increased to 110 ksi class by increasing the yield strength of the steel material by a different method instead of increasing the dislocation density of the steel material. I wondered if there was one.

Specifically, the present inventors have a chemical composition of% by mass, C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Ti: 0.002 to 0.020%, V : 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0. If the steel material contains 50%, Co: 0 to 0.50%, and W: 0 to 0.50%, it is considered that there is a possibility that the yield strength of 110 ksi class and the SSC resistance can be compatible with each other. rice field.

The present inventors further thought that if Mo is contained in addition to the above-mentioned chemical composition, alloy carbides are formed, so that the yield strength can be increased without increasing the dislocation density too much. Therefore, the present inventors have produced various steel materials in which Mo is added to the above-mentioned chemical composition, and investigated their characteristics. As a result, the present inventors have newly found that in the steel material having the above-mentioned chemical composition, there is a dependence between the Mo content and the particle size of the former austenite grains (hereinafter, also referred to as “old γ grains”). I found out in.

Specifically, the relationship between the Mo content and the old γ grain size will be described with reference to the figure. FIG. 1 is a diagram showing the relationship between the Mo content and the old γ particle size. FIG. 1 shows the Mo content (% by mass) of a steel material having a chemical composition other than the Mo content satisfying the above-mentioned chemical composition range and manufactured by a preferable manufacturing method described later in the examples described later. , The old γ particle size (μm) obtained by microstructure observation described later was used. As used herein, the term "old γ grain size" means the crystal grain size of old γ grains determined by a method based on the comparative method specified in ASTM E112-10.

With reference to FIG. 1, as the Mo content increases, the old γ grain size sharply decreases. It has been clarified that in the steel material having the above-mentioned chemical composition, when the Mo content exceeds 1.00%, a remarkable effect of reducing the old γ grain size to 11.0 μm or less can be obtained. Further, if the old γ grains are fine, the steel material can enhance both the yield strength and the SSC resistance. Therefore, the chemical composition of the steel material according to the present embodiment contains more than 1.00 to 2.00% of Mo in addition to the above-mentioned chemical composition. In this case, the old γ grain size of the steel material is 11.0 μm or less.

The present inventors consider the reason for this as follows. When Mo is contained in an amount of more than 1.00 to 2.00% in a steel material having the above-mentioned chemical composition, Mo solid-solved in the steel material may segregate into austenite grain boundaries during quenching heating. Therefore, the movement of the crystal grain boundaries is suppressed by the solid solution Mo segregated at the austenite grain boundaries. As a result, the austenite grains are less likely to be coarsened during the heating of quenching, and it is considered that the old γ grains after tempering become finer.

On the other hand, in order for the steel material having the above-mentioned chemical composition to obtain a yield strength of 110 ksi class, it is preferable that the hardenability of the steel material is high. In the present specification, it is defined as F1 = 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.45 × Cu + 0.8 × Cr + 2 × Mo. F1 is an index of hardenability of steel materials. If F1 is too low, the hardenability of the steel material may not be sufficiently obtained, and the yield strength of 110 ksi class may not be obtained. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition and has an F1 of 3.90 or more.

Therefore, the steel material according to the present embodiment has a chemical composition of% by mass, C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, and so on. P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% Hereinafter, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Cu: 0. It contains ~ 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, and the balance is composed of Fe and impurities, and further described above. F1 is 3.90 or more. Further, the steel material according to the present embodiment has an old γ grain size of 11.0 μm or less in the microstructure.

On the other hand, in a steel material having the above-mentioned chemical composition and an old γ grain size of 11.0 μm or less, a large number of coarse carbides may be deposited in the steel material when trying to obtain a yield strength of 110 ksi class. As a result of further investigation by the present inventors, it has been found that in a steel material having the above-mentioned chemical composition, excellent SSC resistance may not be obtained in a sour environment when a large number of coarse carbides are deposited in the steel material. ..

Therefore, the present inventors have investigated in more detail the carbides that reduce the SSC resistance in the steel material having the above-mentioned chemical composition. As a result, the following findings were obtained. Coarse carbides are prone to stress concentration sources and facilitate the propagation of cracks generated by SSCs. Therefore, it has been considered that if the coarse carbides are reduced, the SSC resistance of the steel material is improved.

However, according to a detailed study by the present inventors, it has been found that, among the coarse carbides, the coarse carbides precipitated at the old γ grain boundary may reduce the SSC resistance of the steel material. Found. That is, the present inventors have found that the SSC resistance of a steel material can be enhanced by reducing the coarse carbides precipitated at the old γ grain boundaries, rather than simply reducing the coarse carbides.

In the steel material according to the present embodiment having the above-mentioned chemical composition, most of the precipitates deposited at the old γ grain boundaries are carbides. Therefore, if the coarse precipitates precipitated at the old γ grain boundaries are reduced, the coarse carbides precipitated at the old γ grain boundaries can be reduced.

Therefore, the steel material according to the present embodiment has the above-mentioned chemical composition, the old γ grain size is 11.0 μm or less, and further reduces the coarse carbides precipitated at the old γ grain boundaries. Specifically, the steel material according to the present embodiment has the above-mentioned chemical composition, the old γ grain size is 11.0 μm or less, and the average area of the precipitates at the old γ grain boundaries is 10.0 × 10 ⁻ . It is ³ μm ² or less. As a result, the steel material according to the present embodiment can achieve both a yield strength of 758 to 862 MPa (110 ksi class) and excellent SSC resistance in a sour environment.

The steel material according to the present embodiment completed based on the above findings has C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00 in mass%. %, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00. %, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0. 0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Cu : 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, and the balance consists of Fe and impurities, according to the formula ( It has a chemical composition that satisfies 1). In the steel material, the crystal grain size of the old austenite grains is 11.0 μm or less. In the steel material, the average area of the precipitates deposited at the former austenite grain boundaries is 10.0 × 10 ^-3 μm ² or less. The yield strength of the steel material is 758 to 862 MPa.
2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.45 x Cu + 0.8 x Cr + 2 x Mo ≧ 3.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol.

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 this embodiment exhibits a yield strength of 758 to 862 MPa (110 ksi class) and excellent SSC resistance.

The chemical composition is Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100. It may contain one or more selected from the group consisting of%.

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

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 steel material may be a steel pipe for an oil well.

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 shape of the steel pipe for oil wells is not limited, and may be, for example, a seamless steel pipe or a welded steel pipe. Oil country tubular goods are, for example, steel pipes used in casings and tubing applications.

The steel material may be a seamless steel pipe. If the steel material according to the present embodiment is a seamless steel pipe, it has a yield strength of 758 to 862 MPa (110 ksi class) even if the wall thickness is 15 mm or more, and has more stable SSC resistance in a sour environment. ..

The excellent SSC resistance can be specifically evaluated by a method based on NACE TM0177-2005 Method A and a 4-point bending test. In the method according to NACE TM0177-2005 Method A, a mixed aqueous solution (NACE solution A) of 5.0% by mass sodium chloride and 0.5% by mass acetic acid at 4 ° C. is used for the test bath. A stress corresponding to 90% of the actual yield stress is applied to the test piece collected from the steel material, and the test piece is immersed in the test bath. Subsequently, after degassing the test bath, 1 atm of _H2S gas is blown into the test bath to saturate it. A test bath saturated with H _2S gas is held at 4 ° C. for 720 hours.

On the other hand, in the 4-point bending test, the stress applied to the test piece is 90% of the actual yield stress of the steel material according to ASTM G39-99 (2011). Stress is applied by 4-point bending. A 5.0 mass% sodium chloride aqueous solution at 24 ° C. is used for the test bath. The stressed test piece is immersed in a test bath in an autoclave. After degassing the test bath, 20 atm of _H2S gas is pressurized and sealed in the autoclave. After sealing the autoclave, the test bath is stirred at 24 ° C. for 720 hours.

No cracks are confirmed in the steel material according to the present embodiment after 720 hours in any of the above methods according to Method A and the four-point bending test.

Hereinafter, the steel material according to this 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: 0.20 to 0.45%
Carbon (C) enhances the hardenability of the steel material and enhances the yield strength of the steel material. 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 yield 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 0.20 to 0.45%. The lower limit of the C content is preferably 0.21%, more preferably 0.22%, and even more preferably 0.25%. The preferred upper limit of the C content is 0.40%, more preferably 0.38%, and even more preferably 0.35%.

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 preferred Si content is 0.10%, more preferably 0.15%. The preferred upper limit of the Si content is 0.85%, more preferably 0.70%, and even more preferably 0.60%.

Mn: 0.01-1.00%
Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability of the steel material and enhances the yield strength of the steel material. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, Mn segregates at the grain boundaries together with impurities such as P and S. In this case, the SSC resistance of the steel material is lowered. Therefore, the Mn content is 0.01 to 1.00%. The preferred lower limit of the Mn content is 0.02%, more preferably 0.03%, and even more preferably 0.10%. The preferred upper limit of the Mn content is 0.80%, more preferably 0.70%, still more preferably 0.65%, still more preferably less than 0.60%, still more preferably 0. It is 55%.

P: 0.030% 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 SSC resistance of the steel material. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%. It is preferable that the P content is as low as possible. 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 0.0001%, more preferably 0.0003%, still more preferably 0.001%, still more preferably 0.002%. Is.

S: 0.0050% 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 SSC resistance of the steel material. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, and more preferably 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 generated, 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: 0.60 to 1.50%
Chromium (Cr) enhances the hardenability of the steel material and enhances the yield 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 improved. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, coarse carbides are generated at the old γ grain boundaries in the steel material. In this case, the SSC resistance of the steel material is lowered. Therefore, the Cr content is 0.60 to 1.50%. The preferred lower limit of the Cr content is 0.62%, more preferably 0.64%, still more preferably 0.65%, still more preferably 0.67%, still more preferably 0.70. %. The preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, still more preferably 1.20%, still more preferably 1.10%, still more preferably 1.00%. It is less than%, more preferably 0.95%.

Mo: Over 1.00 to 2.00%
Molybdenum (Mo) enhances the hardenability of the steel material and enhances the yield strength of the steel material. Mo further dissolves in the steel material, and a part of it segregates at the austenite grain boundaries during quenching heating. As a result, the old γ grain size of the tempered steel material becomes smaller due to the pinning effect. In this case, the SSC resistance of the steel material is increased. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, coarse carbides are generated at the old γ grain boundaries in the steel material. In this case, the SSC resistance of the steel material is lowered. Therefore, the Mo content is more than 1.00 to 2.00%. The preferred lower limit of the Mo content is 1.01%, more preferably 1.05%, still more preferably 1.10%, still more preferably 1.15%, still more preferably 1.20%. %. The preferred upper limit of the Mo content is 1.90%, more preferably 1.80%, still more preferably 1.75%, still more preferably 1.70%, still more preferably 1.65%. %.

In the chemical composition of the steel material according to the present embodiment, the Mo content is preferably less than 2.00 times the Cr content. If the Mo content is too high with respect to the Cr content, the old γ grains of the steel material may become coarse. The reason for this is not clear. However, in the steel material having the chemical composition according to the present embodiment, if the Mo content is less than 2.00 times the Cr content, the old γ grain size of the steel material can be stably reduced to 11.0 μm or less. Therefore, in the chemical composition of the steel material according to the present embodiment, the Mo content is preferably less than 2.00 times the Cr content.

A more preferable upper limit of the ratio of Mo content to Cr content (Mo / Cr ratio) is 1.98, more preferably 1.95, and even more preferably 1.90. The lower limit of the Mo / Cr ratio is not particularly limited, but the chemical composition of the steel material according to the present embodiment is substantially 0.67 or more.

Ti: 0.002 to 0.020%
Titanium (Ti) forms a nitride, and the structure of the steel material is miniaturized by the pinning effect. As a result, the SSC resistance of the steel material is improved. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, a large amount of Ti nitride is formed. As a result, the SSC resistance of the steel material is lowered. Therefore, the Ti content is 0.002 to 0.020%. The preferred lower limit of the Ti content is 0.003%, more preferably 0.004%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.015%.

V: 0.05 to 0.30%
Vanadium (V) combines with C and / or N to form carbides, nitrides or carbonitrides (hereinafter referred to as "carbonitrides and the like"). For carbonitrides and the like, the structure of the steel material is miniaturized by the pinning effect. As a result, the SSC resistance of the steel material is improved. V further combines with C to form fine carbides. As a result, the yield strength of the steel material is increased. If the V content is too low, these effects cannot be obtained. On the other hand, if the V content is too high, carbonitride and the like are excessively generated, and the SSC resistance of the steel material is lowered. Therefore, the V content is 0.05 to 0.30%. The lower limit of the V content is preferably more than 0.05%, more preferably 0.06%, still more preferably 0.07%, still more preferably 0.09%. The preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

Nb: 0.005 to 0.100%
Niobium (Nb) combines with C and / or N to form carbonitrides and the like. For carbonitrides and the like, the structure of the steel material is miniaturized by the pinning effect. As a result, the SSC resistance of the steel material is improved. Nb further combines with C to form fine carbides. As a result, the yield strength of the steel material is increased. If the Nb content is too low, these effects cannot be obtained. On the other hand, if the Nb content is too high, carbonitrides and the like are excessively generated, and the SSC resistance of the steel material is lowered. Therefore, the Nb content is 0.005 to 0.100%. The preferred lower limit of the Nb content is 0.007%, more preferably 0.010%, still more preferably 0.012%, still more preferably 0.015%. The preferred upper limit of the Nb content is 0.080%, more preferably 0.060%, still more preferably 0.050%, still more preferably 0.030%.

B: 0.0005 to 0.0040%
Boron (B) dissolves in steel to enhance the hardenability of the steel material and enhance the yield 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 0.0005 to 0.0040%. The lower limit of the B content is preferably 0.0007%, more preferably 0.0010%, and even more preferably 0.0012%. The preferred upper limit of the B content is 0.0035%, more preferably 0.0030%, still more preferably 0.0025%.

N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N combines with Ti to form a fine nitride, and the structure of the steel material is miniaturized by the pinning effect. As a result, the SSC resistance of the steel material is improved. On the other hand, if the N content is too high, coarse nitrides are generated and the SSC resistance of the steel material is lowered. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0080%, more preferably 0.0070%. The preferable lower limit of the N content for effectively obtaining the above effect is 0.0020%, more preferably 0.0025%, still more preferably 0.0030%, still more preferably 0.0035%. Yes, more preferably 0.0040%.

O: Less than 0.0020% Oxygen (O) is an impurity. That is, the O content is more than 0%. O forms a coarse oxide and lowers the SSC resistance of the steel material. Therefore, the O content is less than 0.0020%. The preferred upper limit of the O content is 0.0018%, more preferably 0.0015%. It is preferable that the O content is as low as possible. However, an extreme reduction in O content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the O content is 0.0001%, and more preferably 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 mixed from ore, scrap, or the manufacturing environment as a raw material when the steel material is industrially manufactured, and are within a range that does not adversely affect the steel material according to the present embodiment. Means what is acceptable.

[Arbitrary element]
The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Ca, Mg, Zr, and rare earth elements (REM) instead of a part of Fe. All of these elements are arbitrary elements and control the morphology of sulfides in the steel material to enhance the SSC resistance of the steel material.

Ca: 0-0.0100%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca detoxifies S in the steel material as a sulfide 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 SSC resistance of the steel material deteriorates. Therefore, the Ca content is 0 to 0.0100%. The lower limit of the Ca 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 Ca content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, still more preferably 0.0020%.

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 contained, Mg detoxifies S in the steel material as a 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 SSC resistance of the steel material deteriorates. 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 0.0030%, still more preferably 0.0025%, still more preferably 0.0020%.

Zr: 0 to 0.0100%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr detoxifies S in the steel material as a sulfide 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 in the steel material becomes coarse and the SSC resistance of the steel material deteriorates. Therefore, the Zr content is 0 to 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%, still more preferably 0.0010%. Is. The preferred upper limit of the Zr content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, still more preferably 0.0020%.

Rare earth element (REM): 0 to 0.0100%
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM detoxifies S in the steel material as a sulfide 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%, still more preferably 0.0010%. Is. The preferred upper limit of the REM content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, still more preferably 0.0020%.

The REM in the present specification refers to scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to 71, which is a lanthanoid. One or more elements selected from the group consisting of lutetium (Lu). Further, the REM content in the present specification is the total content of these elements.

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

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of the steel material and enhances the yield 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 deteriorates. Therefore, the Cu content is 0 to 0.50%. The lower limit of the Cu 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 Cu content is 0.35%, more preferably 0.25%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hardenability of the steel material and enhances the yield strength of the steel material. If even a small amount of Ni is contained, this effect 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 0.50%. The lower limit of the Ni 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 Ni content is 0.35%, more preferably 0.25%.

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 and suppress hydrogen intrusion. Thereby, these elements enhance 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 contained, Co forms a protective corrosive coating in a hydrogen sulfide environment and suppresses hydrogen intrusion. As a result, the SSC resistance of the steel material is improved. 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 yield 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 contained, W forms a protective corrosive coating in a hydrogen sulfide environment and suppresses hydrogen intrusion. As a result, the SSC resistance of the steel material is improved. 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 in the steel material, and the SSC resistance of the steel material is lowered. Therefore, the W content is 0 to 0.50%. The lower limit of the W 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 W content is 0.45%, more preferably 0.40%.

[About equation (1)]
The chemical composition of the steel material according to this embodiment further satisfies the formula (1).
2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.45 x Cu + 0.8 x Cr + 2 x Mo ≧ 3.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol.

F1 (= 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.45 × Cu + 0.8 × Cr + 2 × Mo) is an index showing the hardenability of the steel material having the above-mentioned chemical composition. When F1 is less than 3.90, the hardenability is not sufficiently obtained and the yield strength of the steel material cannot be obtained. Therefore, the steel material according to this embodiment has an F1 of 3.90 or more.

The preferred lower limit of F1 is 3.93, more preferably 4.00. The upper limit of F1 is not particularly limited. However, in the steel material according to the present embodiment having the above-mentioned chemical composition, the upper limit of F1 is, for example, 8.27. The preferred upper limit of F1 is 8.20, more preferably 8.10, and even more preferably 8.00.

[Old austenite particle size]
In the microstructure of the steel material according to the present embodiment, the old austenite grain size (former γ grain size) is 11.0 μm or less. As described above, in the present specification, the crystal grain size of the former austenite grain (former γ grain size) means the crystal grain size of the former austenite grain determined in accordance with the comparative method of ASTM E112-10. If the old γ grains of the steel material are fine, the yield strength and SSC resistance are stably enhanced. Therefore, in the present embodiment, the old γ grains of the steel material are made finer by containing more than 1.00% of Mo in the steel material.

If the old γ grain size of the steel material according to the present embodiment is 11.0 μm or less, both the yield strength of 110 ksi class and the excellent SSC resistance are compatible, provided that the other regulations of the steel material according to the present embodiment are satisfied. can do.

The preferable upper limit of the old γ grain size of the steel material according to the present embodiment is 10.5 μm, and more preferably 10.0 μm. The lower limit of the old γ grain size of the steel material according to this embodiment is not particularly limited. The lower limit of the old γ grain size of the steel material according to the present embodiment is, for example, 4.5 μm.

As described above, the old γ particle size can be determined according to the comparative method of ASTM E112-10. More specifically, it can be obtained by the following method. When the steel material is a steel plate, a test piece having an observation surface perpendicular to the rolling direction is cut out from the central portion of the plate thickness. When the steel material is a steel pipe, a test piece having an observation surface perpendicular to the pipe axis direction is cut out from the central portion of the wall thickness. After polishing the observation surface to a mirror surface, it is embedded in a resin, immersed in a 2% nital corrosive solution for about 10 seconds, and the old γ grain boundaries are exposed by etching.

A photographic image is generated by observing the etched observation surface with a secondary electron image in 10 fields using a scanning electron microscope (SEM). The observation magnification is, for example, 200 times. Using the generated photographic image, the crystal grain size number is evaluated by comparison with the crystal grain size standard diagram defined in ASTM E112-10. From the evaluated crystal grain size number, the average crystal grain size of the old γ grains in each field of view is obtained. The arithmetic average value of the average crystal grain size of the old γ grains obtained in 10 fields is defined as the crystal grain size of the old γ grains (former γ grain size) (μm).

[Precipitates deposited at the old γ grain boundaries]
In the steel material according to the present embodiment, the average area of the precipitates deposited at the former austenite grain boundaries (former γ grain boundaries) is 10.0 × 10 ^-3 μm ² or less. In the present specification, the precipitate deposited at the old γ grain boundary is also referred to as “specific precipitate”. If the average area of the specific precipitate is 10.0 × 10 ^-3 μm ² or less, the yield strength of 110 ksi class and the excellent SSC resistance are satisfied, provided that the other provisions of the steel material according to the present embodiment are satisfied. Can be compatible with each other.

As described above, in a steel material having the above-mentioned chemical composition and an old γ grain size of 11.0 μm or less, a large number of coarse carbides may be deposited in the steel material when trying to obtain a yield strength of 110 ksi class. Further, among the coarse carbides in the steel material, the carbides precipitated at the old γ grain boundaries reduce the SSC resistance of the steel material. Further, in the steel material according to the present embodiment, most of the precipitates deposited at the old γ grain boundaries are carbides.

Therefore, in the steel material according to the present embodiment, the average area of the precipitates (specific precipitates) deposited at the old γ grain boundaries is set to 10.0 × 10 ^-3 μm ² or less. If the average area of the specific precipitate exceeds 10.0 × 10 ^-3 μm ² , the SSC resistance of the steel material may decrease. If the average area of the specific precipitate exceeds 10.0 × 10 ^-3 μm ² , the yield strength of 758 to 862 MPa (110 ksi class) may not be obtained.

Therefore, in the steel material according to the present embodiment, the average area of the precipitates deposited at the old γ grain boundaries is 10.0 × 10 ^-3 μm ² or less. The preferred upper limit of the average area of the specific precipitate is 9.9 × 10 ^-3 μm ² , and more preferably 9.7 × 10 ^-3 μm ² .

The lower limit of the average area of the specific precipitate is not particularly limited and may be 0.0 × 10 ^-3 μm ² . However, in the steel material according to the present embodiment having the above-mentioned chemical composition, the lower limit of the average area of the specific precipitate is, for example, 3.0 × 10 ^-3 μm ² .

The average area of the specific precipitate can be obtained by the following method. A test piece is cut out from the steel material in the same manner as the above-mentioned method for measuring the old γ grain size. Specifically, when the steel material is a steel plate, a test piece having an observation surface perpendicular to the rolling direction is cut out from the central portion of the plate thickness. When the steel material is a steel pipe, a test piece having an observation surface perpendicular to the pipe axis direction is cut out from the central portion of the wall thickness. After polishing the observation surface to a mirror surface, it is embedded in a resin, immersed in a 2% nital corrosive solution for about 10 seconds, and the old γ grain boundaries are exposed by etching. The observation surface of the test piece is observed in 10 fields with a secondary electron image by SEM to generate a photographic image. The observation magnification is, for example, 10000 times.

From the generated photographic image, the old γ grain boundaries are identified based on the contrast. Precipitates are further identified from the generated photographic images based on contrast. In the present embodiment, in the observation of the specific precipitate, the observation magnification is, for example, 10,000 times. Therefore, if the precipitate has a diameter equivalent to a circle and is 50 nm or more, it can be specified from the observation field of view based on the contrast. On the other hand, in the present embodiment, the upper limit of the circle-equivalent diameter of the specified precipitate is not particularly limited. In the steel material having the above-mentioned chemical composition, the upper limit of the equivalent circle diameter of the specified precipitate is, for example, 1000 nm. Therefore, in the present embodiment, the circle-equivalent diameter of the specific precipitate is, for example, 50 to 1000 nm.

Precipitates that overlap with the specified old γ grain boundaries and / or come into contact with the specified old γ grain boundaries are designated as "specific precipitates". That is, the specific precipitate (precipitate deposited at the old γ grain boundary) means a precipitate whose part overlaps with and / or comes into contact with the old γ grain boundary. For the specified specific precipitate, the average area (μm ² ) is obtained by image analysis.

[Micro organization]
The microstructure of the steel material according to this embodiment mainly consists of tempered martensite and tempered bainite. More specifically, the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The rest of the microstructure is, for example, ferrite or pearlite.

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 758, provided that the other provisions of the present embodiment are satisfied. It becomes ~ 862 MPa (110 ksi class).

The total volume fraction of tempered martensite and tempered bainite can be determined by microstructure observation. For the microstructure, the photographic image generated when determining the old γ particle size described above is used. In each field of view, tempered martensite and tempered bainite and other phases (eg, ferrite or pearlite) can be distinguished from the contrast. Therefore, in each field of view, tempered martensite and tempered bainite are identified based on contrast.

Calculate the total area ratio of the identified tempered martensite and tempered bainite. In the present embodiment, the arithmetic mean value of the total area ratio of the tempered martensite and the tempered bainite obtained in all the visual fields is taken as the volume ratio of the tempered martensite and the tempered bainite.

[Yield strength of steel]
The yield strength of the steel material according to this embodiment is 758 to 862 MPa (110 ksi class). The yield strength as used herein means the stress at 0.7% elongation (0.7% proof stress) obtained in the tensile test. Even if the yield strength of the steel material according to the present embodiment is 110 ksi class, it has excellent SSC resistance by satisfying the above-mentioned chemical composition, old γ grain size, and average area of specific precipitates.

The yield strength of the steel material according to this embodiment can be obtained by the following method. Tensile tests are performed by a method according to ASTM E8 / E8M (2013). A round bar test piece is collected from the steel material according to the present embodiment. If the steel material is a steel plate, a round bar test piece is collected from the center of the plate thickness. If the steel material is a steel pipe, take a round bar test piece from the center of the wall thickness. The size of the round bar test piece is, for example, a parallel portion diameter of 8.9 mm and a parallel portion length of 35.6 mm. The axial direction of the round bar test piece is parallel to the rolling direction of the steel material. A tensile test is carried out in the air at room temperature (25 ° C.) using a round bar test piece, and the obtained stress at 0.7% elongation is defined as the yield strength (MPa).

[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 and a four-point bending test.

In the method according to NACE TM0177-2005 Method A, a round bar test piece is collected from the steel material according to the present embodiment. If the steel material is a steel plate, a round bar test piece is collected from the center of the plate thickness. If the steel material is a steel pipe, take a round bar test piece from the center of the wall thickness. The size of the round bar test piece is, for example, a diameter of 6.35 mm and a length of the parallel portion of 25.4 mm. The axial direction of the round bar test piece is parallel to the rolling direction of the steel material.

The test solution is a mixed aqueous solution (NACE solution A) of 5.0% by mass sodium chloride and 0.5% by mass acetic acid at 4 ° C. A stress corresponding to 90% of the actual yield stress is applied to the round bar test piece. A test solution at 4 ° C. is injected into the test container so that the stressed round bar test piece is immersed, and the test bath is used. After degassing the test bath, ₁ atm of H 2S gas is blown into the test bath to saturate the test bath with H _2S gas. A test bath saturated with H _2S gas is held at 4 ° C. for 720 hours.

On the other hand, in the 4-point bending test, a test piece is collected from the steel material according to the present embodiment. If the steel material is a steel plate, take a test piece from the center of the plate thickness. If the steel material is a steel pipe, take a test piece from the center of the wall thickness. The size of the test piece is, for example, 2 mm in thickness, 10 mm in width, and 75 mm in length. The length direction of the test piece is parallel to the rolling direction of the steel material.

The test solution is a 5.0 mass% sodium chloride aqueous solution at 24 ° C. According to ASTM G39-99 (2011), a stress corresponding to 90% of the actual yield stress is applied to the test piece by 4-point bending. The stressed test piece is enclosed in an autoclave together with the test jig. Inject the test solution into the autoclave, leaving the gas phase part, and use it as the test bath. After degassing the test bath, 20 atm of H ₂ S gas is pressurized and sealed in the autoclave, and the test bath is stirred to saturate the H ₂ S gas. After sealing the autoclave, the test bath is stirred at 24 ° C. for 720 hours.

No cracks are confirmed in the steel material according to the present embodiment after 720 hours in both the above Method A-compliant method and the four-point bending test. In addition, in this specification, "no crack is confirmed" means that no crack is confirmed when the test piece after the test is observed with the naked eye.

[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. More preferably, the steel material according to the present embodiment is suitable for use as a thick-walled seamless steel pipe. More specifically, even if the steel material according to the present embodiment is a seamless steel pipe having a thickness of 15 mm or more and a thickness of 20 mm or more, it exhibits a yield strength of 110 ksi class and excellent SSC resistance.

[Production method]
A method for manufacturing a steel material according to this embodiment will be described. The manufacturing method described below is a method for manufacturing a seamless steel pipe as an example of a steel material according to the present embodiment. The method for manufacturing a steel material according to this embodiment is not limited to the manufacturing method described below.

[Preparation process]
In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. The manufacturing 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 manufactured 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 manufactured by an ingot method using molten steel. If necessary, slabs, blooms or ingots may be block-rolled to produce steel pieces (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 oven. 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 may be carried out as hot working to manufacture a raw 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, forging such as the Erhard method may be carried out to manufacture a raw pipe. The raw tube is manufactured by the above process. The wall thickness of the raw tube to be produced 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). Alternatively, the raw tube produced by hot working may be directly quenched without being cooled to room temperature, or may be quenched after reheating (reheating).

When quenching is carried out directly or after quenching, cooling may be stopped or slow cooling may be carried out during the quenching. In this case, it is possible to prevent the raw pipe from being cracked. When quenching is carried out by direct quenching or after reheating Further, stress relief annealing (SR treatment) may be carried out after quenching and before heat treatment (tempering or the like) in the next step. In this case, the residual stress of the raw pipe is removed.

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, the 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 other business establishments. The intermediate steel material manufactured in the above may be prepared.

[Heat treatment process]
In the heat treatment step, the prepared intermediate steel material is heat-treated. Specifically, the prepared intermediate steel material is quenched and tempered. In the present specification, "quenching" means quenching an intermediate steel material having A ₃ points or more. In the present specification, "tempering" means that the intermediate steel material after quenching is reheated at 1 point or less of _Ac and held.

In the heat treatment step according to the present embodiment, quenching and tempering are preferably performed a plurality of times. Specifically, it is preferable to perform quenching and tempering twice or more each. More specifically, the prepared intermediate steel material is preferably quenched, then tempered, and further quenched to perform tempering.

In the heat treatment step according to this embodiment, quenching and tempering may be performed three or more times. However, even if quenching and tempering are repeated four or more times, the effect of the heat treatment is saturated. Therefore, in the heat treatment step according to the present embodiment, it is preferable to carry out quenching and tempering twice or three times. Hereinafter, quenching and tempering will be described in detail.

[Quenching]
Quenching is performed on the prepared intermediate steel material (raw pipe) and / or the tempered intermediate steel material. In the heat treatment step according to the present embodiment, the preferred quenching temperature is 800 to 1000 ° C. In the present specification, the "quenching temperature" corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed on the outlet side of the device for performing the final hot working when the quenching is performed directly after the hot working. do. The quenching temperature further corresponds to the temperature of the reheating furnace or the heat treatment furnace when quenching is performed using the reheating furnace or the heat treatment furnace after the hot working.

That is, the heat treatment step according to the present embodiment may be carried out by quenching the intermediate steel material at 800 to 1000 ° C. after hot working, or the intermediate steel material after hot working is used in a reheating furnace or a heat treatment furnace. It may be carried out by heating to 800 to 1000 ° C. and then quenching, or by heating the tempered intermediate steel material to 800 to 1000 ° C. using a heat treatment furnace and then quenching. good.

If the quenching temperature is too high, the old γ grains become coarse and the SSC resistance of the steel material may decrease. Therefore, the quenching temperature is preferably 800 to 1000 ° C. A more preferable upper limit of the quenching temperature is 950 ° C.

In the heat treatment step according to the present embodiment, when quenching is carried out using a reheating furnace or a heat treatment furnace after hot working, the preferred quenching time is 5 to 20 minutes. As used herein, the term "quenching time" means the time from when the intermediate steel material is charged into the heating or heat treatment furnace to when it is taken out.

When quenching is carried out using a reheating furnace or a heat treatment furnace after hot working, if the quenching time is too long, the old γ grains after the final tempering may become coarse. Therefore, in the heat treatment step according to the present embodiment, when quenching is carried out using a reheating furnace or a heat treatment furnace after hot working, the quenching time is preferably 5 to 20 minutes.

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 for cooling, or a method of accelerating cooling of 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 properties specified in this embodiment cannot be obtained. Therefore, in the method for manufacturing a steel material according to the present embodiment, the intermediate steel material (raw pipe) 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 (° C./sec). More specifically, the quenching cooling rate CR _800-500 is determined from the temperature measured on the surface of the intermediate steel to be quenched.

The preferred quenching cooling rate CR _800-500 is 8 ° C./sec or higher. In this case, the microstructure of the intermediate steel material (raw pipe) after quenching is stably composed mainly of martensite and bainite. A more preferable lower limit of the quenching cooling rate CR _800-500 is 10 ° C./sec. The preferred upper limit of the quenching cooling rate CR _800-500 is 500 ° C./sec.

[Tempering]
Tempering is performed on the intermediate steel material that has been quenched. In the tempering of steel materials, which is expected to be used in a sour environment, the tempering temperature and tempering time are adjusted according to the chemical composition of the steel materials and the yield strength to be obtained. In this case, only the final tempering is controlled, and for the non-final tempering, it has been conventionally considered that the tempering temperature should be A _c1 point or less.

On the other hand, in the steel material according to the present embodiment, the old γ grains are made finer by increasing the Mo content. Regarding this mechanism, as described above, it is considered that Mo solid-solved in the steel material segregates at the austenite grain boundaries during quenching heating, and the old γ grains after tempering become finer due to the pinning effect. Here, in the steel material having the above-mentioned chemical composition, Mo tends to form M ₂ C type carbides. Further, in the steel material having the above _- mentioned chemical composition, M2C type carbides are likely to precipitate in tempering.

Therefore, in the heat treatment step according to the present embodiment, Mo is sufficiently solid-solved in the second-to-final tempered steel material. Specifically, in the heat treatment step according to the present embodiment, in the second tempering from the final, the tempering parameter TMP ₂ (= (tempering temperature (° C.) +273) × (log (tempering time (minutes) / 60) +20)) is set. If controlled, the amount of Mo precipitated as M ₂ C type carbide can be reduced.

More specifically, in the steel material having the above-mentioned chemical composition, if the tempering parameter TMP ₂ of the penultimate tempering is 15,000 to 19000, the old γ grain size of the steel material after the final tempering is made finer. be able to. If the tempering parameter TMP ₂ in the penultimate tempering is less than 15,000, the effect of tempering may not be sufficiently obtained, and the steel material may be cracked or cracked. On the other hand, if the tempering parameter TMP ₂ in the penultimate tempering exceeds 19000, the amount of solid solution Mo cannot be sufficiently obtained during the heating of the final quenching, and the old γ grain size after the final tempering becomes coarse. There is.

Therefore, in the heat treatment step according to the present embodiment, the tempering parameter TMP ₂ of the penultimate tempering is preferably 15,000 to 19000. The more preferable lower limit of the tempering parameter TMP ₂ of the penultimate tempering is 15500, more preferably 16000. The more preferable upper limit of the tempering parameter TMP ₂ of the penultimate tempering is 18500, and more preferably 18000.

Preferably, in the penultimate tempering, the tempering temperature is less than 500-700 ° C. Further, preferably, in the second-to-final tempering, the tempering time (holding time) is set to 10 to 60 minutes. That is, in the present embodiment, in the second-to-final tempering, the tempering temperature is set to less than 500 to 700 ° C., the tempering time is set to 10 to 60 minutes, and the tempering parameter TMP ₂ is set to 15000 to 19000.

In the present specification, the "tempering temperature" corresponds to the temperature of the heat treatment furnace when the intermediate steel material after quenching is heated and held. In the present specification, the "tempering time (holding time)" means the time from charging the intermediate steel material into the heat treatment furnace for heating the intermediate steel material after quenching to taking it out.

Further, in the present specification, the "second-to-final tempering" means the final quenching and the tempering performed before the tempering. That is, when quenching and tempering are performed twice each in the heat treatment step, the penultimate tempering means the first tempering. When quenching and tempering are performed three times each in the heat treatment step, the penultimate tempering means the second tempering.

The steel material according to the present embodiment further reduces the coarse specific precipitates among the precipitates (specific precipitates) deposited at the old γ grain boundaries. As mentioned above, most of the specific precipitates are carbides. Therefore, most of the specific precipitates are precipitated in the final tempering. Therefore, in the heat treatment step according to the present embodiment, not only the tempering parameter TMP ₂ in the penultimate tempering but also the tempering parameter TMP ₁ in the final tempering (= (tempering temperature (° C.) +273) × (log (tempering time (baking time)). Minutes) / 60) +20)) are also controlled.

More specifically, in the steel material having the above-mentioned chemical composition, if the tempering parameter TMP ₁ of the final tempering is 19100 to 19600, it is possible to reduce coarse specific precipitates in the steel material after the final tempering. can. If the tempering parameter TMP ₁ in the final tempering is less than 19100, the effect of tempering may not be sufficiently obtained, and the yield strength of the steel material after tempering may become too high. If the tempering parameter TMP ₁ in the final tempering is less than 19100, a large number of coarse specific precipitates may be further deposited.

On the other hand, if the tempering parameter TMP ₁ in the final tempering exceeds 19,600, the yield strength of the tempered steel material may become too low. If the tempering parameter TMP ₁ in the final tempering exceeds 19,600, a large number of coarse specific precipitates may be further deposited.

Therefore, in the heat treatment step according to the present embodiment, the tempering parameter TMP ₁ of the final tempering is preferably 19100 to 19600. The more preferable lower limit of the tempering parameter TMP ₁ of the final tempering is 19200, more preferably 19300. A more preferable upper limit of the tempering parameter TMP ₁ of the final tempering is 19570, more preferably 19500.

Preferably, in the final tempering, the tempering temperature is 650-730 ° C. Further, preferably, in the final tempering, the tempering time (retention time) is set to 10 to 90 minutes. That is, in the present embodiment, in the final tempering, the tempering temperature is 650 to 730 ° C., the tempering time is 10 to 90 minutes, and the tempering parameter TMP ₁ is 19100 to 19600.

When the steel material is a steel pipe, the temperature of the steel pipe tends to vary during the holding of tempering as compared with other shapes. Therefore, when the steel material is a steel pipe, the tempering time is preferably 15 to 90 minutes. It is sufficiently possible for those skilled in the art to have a yield strength of 758 to 862 MPa (110 ksi class) by appropriately adjusting the tempering temperature and the tempering time of the steel material having the chemical composition of the present embodiment. ..

By the above manufacturing method, the steel material according to the present embodiment can be manufactured. In the above-mentioned manufacturing method, a method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to this embodiment may be a steel plate or another shape. Similar to the above-mentioned manufacturing method, a method for manufacturing a steel sheet or another shape also includes, for example, a preparation step and a heat treatment step. Further, the above-mentioned manufacturing method is an example, and may be manufactured by another manufacturing method.

A molten steel having the chemical composition shown in Table 1 was produced. Further, F1 was determined from the chemical compositions shown in Table 1. In addition, "-" in Table 1 means that the content of each element is an impurity level.

Billets were manufactured by a continuous casting method using the molten steel. After holding the billet of each test number manufactured at 1250 ° C. for 1 hour, hot rolling (hot working) by the Mannesmann-mandrel method was carried out to manufacture a raw pipe (seamless steel pipe) of each test number.

Heat treatment (quenching and tempering) was performed twice for each hot-worked raw tube of each test number. Specifically, the raw pipes of each test number were heat-treated by the following method.

The raw pipes of each test number manufactured by hot working were held in a heating furnace at 950 ° C. for 5 minutes, and then directly quenched (that is, the first quenching) was carried out. The cooling rate CR _800-500 at the time of quenching of the first quenching of each test number was in the range of 8 to 500 ° C./sec. The cooling rate CR _800-500 during quenching was determined by measuring the surface temperature of the raw tube of each test number.

Subsequently, the first tempering, that is, the penultimate tempering was carried out for the raw tubes of each test number. Specifically, the raw pipes of each test number were tempered by maintaining the tempering time (minutes) at the tempering temperature (° C.) described in the "second-to-final tempering" column of Table 2. Further, Table 2 shows the tempering parameter TMP ₂ (= (tempering temperature (° C.) +273) × (log (tempering time (minutes) / 60) +20)) in the penultimate tempering.

The second quenching, that is, the final quenching, was carried out for the raw tubes of each test number in which the above-mentioned first tempering was carried out. Specifically, the raw pipes of each test number were quenched at the quenching temperature (° C) described in the "final quenching" column of Table 2 for the quenching time (minutes). The cooling rate CR _800-500 during the second quenching of each test number was in the range of 8 to 500 ° C./sec.

Further, a second tempering, that is, a final tempering was performed on the raw pipes of each test number in which the final quenching was carried out. Specifically, the raw pipes of each test number were tempered by maintaining the tempering time (minutes) at the tempering temperature (° C) described in the "final tempering" column of Table 2. Table 2 shows the tempering parameters TMP ₁ (= (tempering temperature (° C.) +273) × (log (tempering time (minutes) / 60) +20)) in the final tempering.

In this embodiment, the temperature of the heating furnace and the heat treatment furnace used for heating the quenching was defined as "quenching temperature (° C.)". Further, the temperature of the heat treatment furnace used for tempering was defined as "tempering temperature (° C.)". Further, the time from when the raw pipe was charged into the reheating furnace or the heat treatment furnace at the time of heating for quenching until it was taken out was defined as "quenching time (minutes)". The time from when the raw tube was charged into the heat treatment furnace at the time of tempering until it was taken out was defined as "tempering time (minutes)".

[Evaluation test]
The microstructure observation, tensile test, and SSC resistance evaluation test described below were carried out on the seamless steel pipes of each test number after the tempering treatment.

[Microstructure observation]
For the seamless steel pipe of each test number, the old γ grain size was measured by the above method. Table 2 shows the old γ grain size (μm) of the seamless steel pipe of each test number. Further, for the seamless steel pipe of each test number, the average area of the precipitates (specific precipitates) deposited at the old γ grain boundaries was determined by the above method. Table 2 shows the average area (× 10 ^-3 μm ² ) of the specific precipitates of the seamless steel pipes of each test number.

[Tensile test]
The yield strength of the seamless steel pipe of each test number was measured by the above-mentioned method. Specifically, a tensile test was carried out in accordance with ASTM E8 / E8M (2013). More specifically, a round bar tensile test piece having a parallel portion diameter of 8.9 mm and a parallel portion length of 35.6 mm was produced from the central portion of the wall thickness of the seamless steel pipe of each test number. The axial direction of the round bar tensile test piece was parallel to the axial direction of the seamless steel pipe.

Tensile tests were carried out at room temperature (25 ° C.) in the air using the round bar test pieces of each test number to obtain the yield strength (MPa) of the seamless steel pipe of each test number. In this example, the stress at 0.7% elongation obtained in the tensile test was defined as the yield strength of each test number. Table 2 shows the obtained yield strength YS (MPa) and tensile strength TS (Tensile Strength) (MPa).

[SSC resistance evaluation test for steel materials]
Using the seamless steel pipe of each test number, a test conforming to NACE TM0177-2005 Method A and a 4-point bending test were carried out to evaluate the SSC resistance. Specifically, the test conforming to NACE TM0177-2005 Method A was carried out by the following method.

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 seamless steel pipe of each test number. The round bar test piece was collected so that its axial direction was parallel to the axial direction of the seamless steel pipe. A tensile stress was applied in the axial direction of the round bar test piece of each test number. At this time, the applied stress was adjusted to be 90% of the actual yield stress of the seamless steel pipe of each test number.

As the test solution, a mixed aqueous solution (NACE solution A) of 5.0% by mass sodium chloride and 0.5% by mass acetic acid was used. A test solution at 4 ° C. was injected into three test containers to prepare a test bath. The three stressed round bar test pieces were immersed in the test baths of different test containers one by one. After degassing each test bath, 1 atm of _H2S gas was blown into the test bath to saturate it. A test bath saturated with 1 atm of _H2S gas was kept at 4 ° C. for 720 hours.

On the other hand, the 4-point bending test was carried out by the following method. Three test pieces having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm were collected from the central portion of the wall thickness of the seamless steel pipe of each test number. The test piece was collected so that its longitudinal direction was parallel to the axial direction of the seamless steel pipe. For the test pieces of each test number, in accordance with ASTM G39-99 (2011), the stress applied to each test piece shall be 90% of the actual yield stress of the seamless steel pipe of each test number. Stress was applied by 4-point bending. The stressed test piece was enclosed in an autoclave together with the test jig.

As the test solution, a 5.0 mass% sodium chloride aqueous solution was used. The test solution was poured into the autoclave, leaving the gas phase part, and used as a test bath. After degassing the test bath, 20 atm of _H2S gas was pressurized and sealed, and the test bath was stirred to saturate the _H2S gas in the test bath. After sealing the autoclave, the test bath was stirred at 24 ° C. for 720 hours.

For each of the NACE TM0177-2005 Method A compliant test and the 4-point bending test, observe the presence or absence of sulfide stress cracking (SSC) in the test pieces of each test number after holding for 720 hours. did. Specifically, the test piece after holding for 720 hours was visually observed. As a result of observation, those in which no crack was confirmed in all the test pieces were judged as "E" (Excellent). On the other hand, those in which cracks were confirmed in the test piece were judged to be "NA" (Not Assistable).

[Test results]
Table 2 shows the test results. Regarding the SSC resistance test, the results of the NACE TM0177-2005 Method A-compliant test are shown in the "1 atmH ₂ S" column, and the results of the 4-point bending test are shown in the "20 atm H ₂ S" column.

With reference to Tables 1 and 2, the chemical composition of the seamless steel pipes of test numbers 1 to 8 is appropriate, the yield strength is 758 to 862 MPa, the old γ grain size is 11.0 μm or less, and The average area of the specific precipitate was 10.0 × 10 ^-3 μm ² or less. As a result, excellent SSC resistance was shown in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

On the other hand, in the seamless steel pipes of test numbers 9 and 10, the tempering parameter TMP ₂ in the penultimate tempering was too large. Therefore, the old γ particle size exceeded 11.0 μm. As a result, it did not show excellent SSC resistance in the test based on NACE TM0177-2005 Method A.

In the seamless steel pipe of test number 11, the tempering parameter TMP ₁ in the final tempering was too large. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, the yield strength was less than 758 MPa, and a yield strength of 110 ksi class could not be obtained.

In the seamless steel pipe of test number 12, the Cr content was too low. Therefore, the old γ particle size exceeded 11.0 μm. As a result, it did not show excellent SSC resistance in the 4-point bending test.

In the seamless steel pipe of test number 13, the Cr content was too low. In addition, the Mo content was too low. Moreover, F1 was too low. In addition, the temper parameter TMP ₁ in the final temper was too small. Therefore, the old γ particle size exceeded 11.0 μm. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

In the seamless steel pipe of test number 14, the O content was too high. As a result, it did not show excellent SSC resistance in the 4-point bending test.

In the seamless steel pipe of test number 15, the Mo content was too low. Moreover, F1 was too low. Therefore, the old γ particle size exceeded 11.0 μm. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

In the seamless steel pipe of test number 16, the Cr content was too high. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

In the seamless steel pipe of test number 17, the Mo content was too low. Therefore, the old γ particle size exceeded 11.0 μm. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

In the seamless steel pipe of test number 18, the Mo content was too high. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, it did not show excellent SSC resistance in the test based on NACE TM0177-2005 Method A.

In the seamless steel pipe of test number 19, the V content was too low. In addition, the temper parameter TMP ₁ in the final temper was too small. Therefore, the average area of the specific precipitate exceeded 10.0 × 10 ^-3 μm ² . As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

In the seamless steel pipe of test number 20, the tempering parameter TMP ₁ in the final tempering was too small. As a result, the yield strength exceeded 865 MPa, and a yield strength of 110 ksi class could not be obtained. As a result, they did not show excellent SSC resistance in both the NACE TM0177-2005 Method A compliant test and the 4-point bending test.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and implemented without departing from the spirit of the present disclosure.

The steel material according to the present disclosure can be widely applied to steel materials used in harsh environments such as polar regions, preferably can be used as steel materials used in oil well environments, and more preferably casings, tubing, line pipes and the like. It can be used as a steel material.

Claims (6)

By mass%,
C: 0.20 to 0.45%,
Si: 0.05 to 1.00%,
Mn: 0.01-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005 to 0.100%,
Cr: 0.60 to 1.50%,
Mo: Over 1.00 to 2.00%,
Ti: 0.002-0.020%,
V: 0.05 to 0.30%,
Nb: 0.005 to 0.100%,
B: 0.0005 to 0.0040%,
N: 0.0100% or less,
O: Less than 0.0020%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0-0.0100%,
Rare earth elements: 0-0.0100%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50% and
W: Contains 0 to 0.50%, the balance is composed of Fe and impurities, and has a chemical composition satisfying the formula (1).
In the steel material, the crystal grain size of the old austenite grains is 11.0 μm or less.
The average area of the precipitates deposited at the former austenite grain boundaries is 10.0 × 10 ^-3 μm ² or less.
A steel material having a yield strength of 758 to 862 MPa.
2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.45 x Cu + 0.8 x Cr + 2 x Mo ≧ 3.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol. The steel material according to claim 1.
The chemical composition is
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth element: A steel material containing one or more selected from the group consisting of 0.0001 to 0.0100%. The steel material according to claim 1 or 2.
The chemical composition is
Cu: 0.02 to 0.50%, and
Ni: 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 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 steel material is a steel material which is a steel pipe for an oil well. The steel material according to any one of claims 1 to 5.
The steel material is a steel material that is a seamless steel pipe.

Images