WO2020166668A1

WO2020166668A1 - Steel material for use in sour environments

Info

Publication number: WO2020166668A1
Application number: PCT/JP2020/005617
Authority: WO
Inventors: 裕紀神谷; 陽平乙▲め▼; 貴志相馬; 大江　太郎; 伸明小松原; 晋士吉田; 勇次荒井
Original assignee: 日本製鉄株式会社
Priority date: 2019-02-15
Filing date: 2020-02-13
Publication date: 2020-08-20
Also published as: AR118070A1; MX2021009588A; US20220098712A1; JP7036237B2; JPWO2020166668A1; BR112021013441A2; EP3926059A1; EP3926059A4

Abstract

Provided is a steel material that has 110-ksi-grade yield strength and excellent SSC resistance. This steel material has a chemical composition that is, by mass%, 0.20%–0.45% C, 0.05%–1.00% Si, 0.01%–1.00% Mn, no more than 0.030% P, no more than 0.0050% S, 0.005%–0.100% Al, 0.60%–1.50% Cr, more than 1.00% but no more than 2.00% Mo, 0.002%–0.020% Ti, 0.05%–0.30% V, 0.005%–0.100% Nb, 0.0005%–0.0040% B, no more than 0.0100% N, and less than 0.0020% O, the remainder being Fe and impurities. The chemical composition also satisfies formula (1) set forth in the description. The crystal grain size of prior austenite grains is no more than 11.0 μm, and the average area of precipitates that precipitate at prior austenite grain boundaries is no more than 10.0×10-3 μm2. The yield strength of the steel material s 758–862 MPa.

Description

Steel suitable for use in sour environments

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

Demand for higher strength steel pipes for oil wells is required due to the deeper wells of oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as "oil wells"). Specifically, steel pipes for oil wells of 80 ksi class (yield strength of 80 to less than 95 ksi, that is, 552 to 655 MPa) and 95 ksi class (yield strength of 95 to less than 110 ksi, that is, 655 to 758 MPa) are widely used. Recently, further, 110 ksi class (yield strength of 110 to 125 ksi, that is, 758 to 862 MPa) oil well steel pipes have begun to be demanded.

Most of the deep wells are in a sour environment containing corrosive hydrogen sulfide. As used herein, the sour environment means an environment that contains hydrogen sulfide and is acidified. Note that carbon dioxide may be included in the sour environment. Oil well steel pipes 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 increasing the SSC resistance of steel materials represented by oil well steel pipes are disclosed in JP-A-62-253720 (Patent Document 1), JP-A-59-232220 (Patent Document 2), and JP-A-6-322478. Japanese Unexamined Patent Application Publication (Patent Document 3), Japanese Unexamined Patent Publication (Kokai) No. 8-311551 (Patent Document 4), Japanese Unexamined Patent Application Publication No. 2000-256783 (Patent Document 5), Japanese Unexamined Patent Application Publication 2000-297344 (Patent Document 6), and Japanese Unexamined Patent Application Publication 2005 -350754 (Patent Document 7), Japanese Patent Laid-Open No. 2012-510238 (Patent Document 8) and Japanese Patent Laid-Open No. 2012-26030 (Patent Document 9).

Patent Document 1 proposes a method of reducing impurities such as Mn and P to enhance SSC resistance of oil well steel. Patent Document 2 proposes a method in which quenching is performed twice to make crystal grains finer and enhance the SSC resistance of steel.

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

Patent Documents 5 and 6 propose a method of controlling the morphology of carbides to enhance the SSC resistance of 110-140 ksi grade low alloy oil country tubular goods steels. Patent Document 7 proposes a method of controlling the dislocation density and the hydrogen diffusion coefficient to desired values to enhance the SSC resistance of steel materials of 125 ksi class or higher. Patent Document 8 proposes a method of increasing 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 process 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 and the SSC resistance of 125 ksi grade steel is increased.

JP 62-253720 JP-A-59-232220 JP-A-6-322478 JP-A-8-311551 Japanese Patent Laid-Open No. 2000-256783 Japanese Patent Laid-Open No. 2000-297344 JP 2005-350754 A Special table 2012-518238 Japanese Patent Laid-Open No. 2012-26030

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

The steel material according to the present disclosure has a mass% of C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, and 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 to 0 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 element: 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 being Fe and impurities, and having a chemical composition satisfying the formula (1). In the steel material, the crystal grain size of the former austenite grains is 11.0 μm or less. In the steel material, the average area of the precipitates precipitated at the former austenite grain boundaries is 10.0×10 ⁻³ μm ² or less. The yield strength of steel is 758 to 862 MPa.
2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≧3.90 (1)
Here, the content (mass %) of the corresponding element is substituted for the element symbol in the formula (1). When 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 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 have 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 that is supposed to be used in a sour environment, and made 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 the steel material may occlude hydrogen. Therefore, if the dislocation density of the steel material increases, the amount of hydrogen stored in the steel material may increase. If the hydrogen concentration in the steel material increases as a result of increasing the dislocation density, the SSC resistance of the steel material deteriorates even if high strength is obtained. Therefore, in order to achieve both 110 ksi class yield strength and excellent SSC resistance, it is not preferable to increase the strength using the dislocation density.

Therefore, if the yield strength of the steel material is increased by a different method instead of increasing the dislocation density of the steel material, the inventors of the present invention may obtain excellent SSC resistance even if the yield strength of the steel material is increased to 110 ksi class. I thought it might be.

Specifically, the present inventors have a chemical composition, in mass%, of 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 : 0.05-0.30%, Nb: 0.005-0.100%, B: 0.0005-0.0040%, 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 element: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0. A steel material containing 50%, Co: 0 to 0.50%, and W: 0 to 0.50% is considered to be able to achieve both 110 ksi class yield strength and SSC resistance. It was

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

Specifically, the relationship between the Mo content and the old γ particle size will be explained using 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 (mass %) of a steel material manufactured by a preferred manufacturing method described later, in which the chemical composition other than the Mo content satisfies the above-mentioned chemical composition range in Examples described later. , And the former γ grain size (μm) obtained by microstructure observation described later. In the present specification, the "old γ grain size" means the crystal grain size of the old γ grain obtained by the method based on the comparison method defined in ASTM E112-10.

Referring to FIG. 1, as the Mo content increases, the old γ particle size sharply decreases. In the steel material having the above-mentioned chemical composition, it became clear that when the Mo content exceeds 1.00%, the remarkable effect that the old γ particle diameter becomes 11.0 μm or less is obtained. Further, if the old γ-grains are fine, the steel material can have improved yield strength and SSC resistance. Therefore, the chemical composition of the steel material according to the present embodiment contains Mo in excess of 1.00 to 2.00% in addition to the above-described 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. In the steel material having the above-described chemical composition, when Mo is contained in the range of more than 1.00 to 2.00%, Mo dissolved in the steel material may be segregated to the austenite grain boundaries during heating during quenching. Therefore, the solid solution Mo segregated at the austenite grain boundaries suppresses the movement of the crystal grain boundaries. As a result, the austenite grains are less likely to coarsen during heating during quenching, and it is considered that the old γ grains after tempering become fine.

On the other hand, in order for the steel material having the above chemical composition to obtain a yield strength of 110 ksi class, it is preferable that the hardenability of the steel material is high. In this 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 the hardenability of the steel material. If F1 is too low, the hardenability of the steel material may not be sufficiently obtained and 110 ksi class yield strength may not be obtained. Therefore, the steel material according to the present embodiment has the above-described chemical composition and further has F1 of 3.90 or more.

Therefore, the chemical composition of the steel material according to the present embodiment is, in 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%, 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% Below, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth element: 0 to 0.0100%, Cu: 0 .About.0.50%, Ni:0.about.0.50%, Co:0.about.0.50%, and W:0.about.0.50%, the balance consisting of Fe and impurities. F1 is 3.90 or more. Further, the steel material according to the present embodiment has a microstructure with a former γ grain size of 11.0 μm or less.

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

Therefore, the present inventors examined in more detail the carbides that reduce the SSC resistance in the steel materials having the above-described chemical composition. As a result, the following findings were obtained. Coarse carbides tend to be a source of stress concentration and promote the propagation of cracks caused by SSC. Therefore, it has been considered that if the coarse carbides are reduced, the SSC resistance of the steel material is enhanced.

However, according to the detailed study by the present inventors, it was found that, among the coarse carbides, the coarse carbides precipitated in the old γ grain boundaries may reduce the SSC resistance of the steel material. Found out. That is, the present inventors have found that the SSC resistance of the steel material can be enhanced by not simply reducing the coarse carbides but by reducing the coarse carbides precipitated at the old γ grain boundaries.

Note that, in the steel material according to the present embodiment having the above-described chemical composition, most of the precipitates that precipitate at the old γ grain boundaries are carbides. Therefore, by reducing the coarse precipitates that precipitate at the old γ grain boundaries, it is possible to reduce the coarse carbides that precipitate at the old γ grain boundaries.

Therefore, the steel material according to the present embodiment has the above-described 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-described chemical composition, the old γ grain size is 11.0 μm or less, and the average area of the precipitates of the old γ grain boundary 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 on the basis of the above findings is, in 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%, Mo: over 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 element: 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 consisting of Fe and impurities, and the formula ( It has a chemical composition satisfying 1). In the steel material, the crystal grain size of the former austenite grains is 11.0 μm or less. In the steel material, the average area of the precipitates precipitated at the former austenite grain boundaries is 10.0×10 ⁻³ μm ² or less. The yield strength of steel is 758 to 862 MPa.
2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≧3.90 (1)
Here, the content (mass %) of the corresponding element is substituted for the element symbol in the formula (1). When 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 the present embodiment exhibits a yield strength of 758 to 862 MPa (110 ksi class) and excellent SSC resistance.

The above 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: 0.0001 to 0.0100%. You may contain 1 type(s) or 2 or more types selected from the group which consists of %.

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

The above 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 above steel material may be a steel pipe for oil wells.

In the present specification, the oil well steel pipe may be a line pipe steel pipe or an oil well pipe. The shape of the oil well steel pipe is not limited, and may be, for example, a seamless steel pipe or a welded steel pipe. The oil country tubular goods are, for example, steel tubes used for casings and tubing applications.

The above steel material may be a seamless steel pipe. If the steel material according to the present embodiment is a seamless steel tube, 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 evaluated specifically by a method based on NACE TM0177-2005 Method A and a 4-point bending test. In the method based on NACE TM0177-2005 Method A, a mixed aqueous solution of 5.0 mass% sodium chloride and 0.5 mass% acetic acid (NACE solution A) at 4° C. is used for the test bath. A test piece taken from a steel material is immersed in a test bath under a stress equivalent to 90% of the actual yield stress. Subsequently, the test bath is degassed, and 1 atm of H ₂ S gas is blown into the test bath to saturate it. The test bath saturated with H ₂ S gas is kept at 4° C. for 720 hours.

On the other hand, in the four-point bending test, according to ASTM G39-99 (2011), the stress applied to the test piece taken from the steel material should be 90% of the actual yield stress of the steel material. Stress is applied by four-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 deaeration of the test bath, 20 atm of H ₂ S gas is pressurized and sealed in the autoclave. After sealing the autoclave, the test bath is stirred at 24° C. for 720 hours.

In the steel material according to the present embodiment, cracking is not confirmed after 720 hours in any of the method based on the above Method A and the four-point bending test.

The steel material according to this embodiment will be described in detail below. "%" regarding an element means mass% unless otherwise specified.

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

C: 0.20 to 0.45%
Carbon (C) enhances the hardenability of steel and enhances the yield strength of steel. C further promotes spheroidization of carbides and enhances SSC resistance of steel during tempering during the manufacturing process. If the carbide is 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 decreases and quench cracking easily occurs. Therefore, the C content is 0.20 to 0.45%. The preferable lower limit of the C content is 0.21%, more preferably 0.22%, and further preferably 0.25%. The preferable upper limit of the C content is 0.40%, more preferably 0.38%, and further preferably 0.35%.

Si: 0.05-1.00%
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 deteriorates. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%. The preferable upper limit of the Si content is 0.85%, more preferably 0.70%, and further 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 decreases. Therefore, the Mn content is 0.01 to 1.00%. The preferable lower limit of the Mn content is 0.02%, more preferably 0.03%, and further preferably 0.10%. The preferable upper limit of the Mn content is 0.80%, more preferably 0.70%, further preferably 0.65%, further preferably less than 0.60%, further preferably 0. 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 reduces the SSC resistance of the steel material. Therefore, the P content is 0.030% or less. The preferable 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, the extreme reduction of the P content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of P content is 0.0001%, more preferably 0.0003%, further preferably 0.001%, and further 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 reduces the SSC resistance of the steel material. Therefore, the S content is 0.0050% or less. The preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, and further preferably 0.0020%. It is preferable that the S content is as low as possible. However, the extreme reduction of the S content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, 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 deteriorates. 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 deteriorates. Therefore, the Al content is 0.005 to 0.100%. The preferable lower limit of the Al content is 0.015%, more preferably 0.020%. The preferable upper limit of the Al content is 0.080%, more preferably 0.060%. The “Al” content as used herein means the content of “acid-soluble Al”, that is, “sol.Al”.

Cr: 0.60 to 1.50%
Chromium (Cr) enhances the hardenability of steel and enhances the yield strength of steel. Cr further increases the temper softening resistance and enables high temperature tempering. As a result, the SSC resistance of the steel material is enhanced. 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 formed at the old γ grain boundaries in the steel material. In this case, the SSC resistance of the steel material decreases. Therefore, the Cr content is 0.60 to 1.50%. The preferable lower limit of the Cr content is 0.62%, more preferably 0.64%, further preferably 0.65%, further preferably 0.67%, further preferably 0.70. %. The preferable upper limit of the Cr content is 1.40%, more preferably 1.30%, further preferably 1.20%, further preferably 1.10%, further preferably 1.00. %, and more preferably 0.95%.

Mo: over 1.00 to 2.00%
Molybdenum (Mo) enhances the hardenability of steel and enhances the yield strength of steel. Further, Mo is solid-dissolved in the steel material, and a part thereof is segregated at the austenite grain boundaries during heating during quenching. As a result, the old γ grain size of the steel material after tempering becomes small due to the pinning effect. In this case, the SSC resistance of the steel material is enhanced. 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 decreases. Therefore, the Mo content is more than 1.00 to 2.00%. The preferable lower limit of the Mo content is 1.01%, more preferably 1.05%, further preferably 1.10%, further preferably 1.15%, further preferably 1.20. %. The preferable upper limit of the Mo content is 1.90%, more preferably 1.80%, further preferably 1.75%, further preferably 1.70%, further 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 relative to the Cr content, the old γ grains of the steel material may become coarse. The reason for this is unknown. 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.

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

Ti: 0.002-0.020%
Titanium (Ti) forms a nitride and refines the structure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is enhanced. 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 decreases. Therefore, the Ti content is 0.002 to 0.020%. The preferable lower limit of the Ti content is 0.003%, more preferably 0.004%. The preferable 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 a carbide, a nitride, or a carbonitride (hereinafter, referred to as “carbonitride or the like”). Carbonitride and the like refine the structure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is enhanced. V further combines with C to form fine carbides. As a result, the yield strength of the steel material increases. If the V content is too low, these effects cannot be obtained. On the other hand, if the V content is too high, carbonitrides and the like are excessively generated, and the SSC resistance of the steel material deteriorates. Therefore, the V content is 0.05 to 0.30%. The preferable lower limit of the V content is more than 0.05%, more preferably 0.06%, further preferably 0.07%, and further preferably 0.09%. The preferable upper limit of the V content is 0.25%, more preferably 0.20%, and further preferably 0.15%.

Nb: 0.005 to 0.100%
Niobium (Nb) combines with C and/or N to form a carbonitride or the like. Carbonitride and the like refine the structure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is enhanced. Nb further combines with C to form fine carbides. As a result, the yield strength of the steel material increases. 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 produced, and the SSC resistance of the steel material deteriorates. Therefore, the Nb content is 0.005 to 0.100%. The preferable lower limit of the Nb content is 0.007%, more preferably 0.010%, further preferably 0.012%, further preferably 0.015%. The preferable upper limit of the Nb content is 0.080%, more preferably 0.060%, further preferably 0.050%, and further preferably 0.030%.

B: 0.0005 to 0.0040%
Boron (B) forms a solid solution 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 generated, and the SSC resistance of the steel material deteriorates. Therefore, the B content is 0.0005 to 0.0040%. The preferable lower limit of the B content is 0.0007%, more preferably 0.0010%, and further preferably 0.0012%. The preferable upper limit of the B content is 0.0035%, more preferably 0.0030%, and further 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 refines the structure of the steel material by the pinning effect. As a result, the SSC resistance of the steel material is enhanced. On the other hand, if the N content is too high, coarse nitrides are generated, and the SSC resistance of the steel material deteriorates. Therefore, the N content is 0.0100% or less. The preferable 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%, further preferably 0.0030%, further preferably 0.0035%. Yes, and 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 reduces the SSC resistance of the steel material. Therefore, the O content is less than 0.0020%. The preferable upper limit of the O content is 0.0018%, and more preferably 0.0015%. The O content is preferably as low as possible. However, the extreme reduction of the O content significantly increases the manufacturing cost. 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 is Fe and impurities. Here, the impurities are ores as raw materials when industrially manufacturing a steel material, scrap, or those that are mixed from the manufacturing environment, etc. within a range that does not adversely affect the steel material according to the present embodiment. Means acceptable.

[Arbitrary element]
The chemical composition of the above-described steel material may further contain, in place of part of Fe, one or more selected from the group consisting of Ca, Mg, Zr, and a rare earth element (REM). Each of these elements is an arbitrary element and controls the form of sulfides in the steel material to enhance the SSC resistance of the steel material.

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 included, Ca renders S in the steel material harmless as a sulfide and enhances the SSC resistance of the steel material. This effect can be obtained to some extent if even a small amount of Ca is contained. However, if the Ca content is too high, the oxides in the steel material become coarse, and the SSC resistance of the steel material decreases. 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%, further preferably 0.0003%, further preferably 0.0006%, further preferably 0.0010%. Is. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further 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 makes S in the steel material harmless as a sulfide and enhances the SSC resistance of the steel material. This effect can be obtained to some extent if Mg is contained at all. However, if the Mg content is too high, the oxides in the steel material become coarse, and the SSC resistance of the steel material decreases. 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%, further preferably 0.0003%, further preferably 0.0006%, further preferably 0.0010%. Is. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further 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 renders S in the steel material harmless as a sulfide and enhances the SSC resistance of the steel material. This effect can be obtained to some extent if Zr is contained even in a small amount. However, if the Zr content is too high, the oxides in the steel material become coarse, and the SSC resistance of the steel material decreases. 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%, further preferably 0.0003%, further preferably 0.0006%, further preferably 0.0010%. Is. The preferable upper limit of the Zr content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further preferably 0.0020%.

Rare earth element (REM): 0-0.0100%
The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, REM renders S in the steel material harmless as a sulfide and enhances the SSC resistance of the steel material. REM further binds to P in the steel material and suppresses the segregation of P at the grain boundaries. Therefore, the deterioration of the low temperature toughness and SSC resistance of the steel material due to the segregation of P is suppressed. These effects can be obtained to some extent if REM is contained in any amount. However, if the REM content is too high, the oxide is coarsened, 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%, further preferably 0.0003%, further preferably 0.0006%, further preferably 0.0010%. Is. The preferable upper limit of the REM content is 0.0040%, more preferably 0.0030%, further preferably 0.0025%, and further preferably 0.0020%.

Note that REM in this specification means scandium (Sc) having an atomic number of 21, an yttrium (Y) having an atomic number of 39, and a lanthanide (lanthanum (La) having an atomic number of 57 to an atomic number of 71). It is one or more elements selected from the group consisting of lutetium (Lu). In addition, 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 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. This effect can be obtained to some extent if Cu is contained in a small amount. 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 preferable lower limit of the Cu content is more than 0%, more preferably 0.02%, further preferably 0.03%, further preferably 0.05%. The preferable 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. This effect can be obtained to some extent if Ni is contained at all. However, if the Ni content is too high, localized corrosion is promoted and the SSC resistance of the steel material deteriorates. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content is more than 0%, more preferably 0.02%, further preferably 0.03%, further preferably 0.05%. The preferable 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 part of Fe. Each of these elements is an arbitrary element and forms a protective corrosive film in a hydrogen sulfide environment to suppress hydrogen invasion. 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 corrosion film in a hydrogen sulfide environment and suppresses hydrogen invasion. As a result, the SSC resistance of the steel material is enhanced. This effect can be obtained to some extent if Co is contained in a small amount. However, if the Co content is too high, the hardenability of the steel material deteriorates, and the yield strength of the steel material decreases. Therefore, the Co content is 0 to 0.50%. The preferable lower limit of the Co content is more than 0%, more preferably 0.02%, further preferably 0.03%, further preferably 0.05%. The preferable upper limit of the Co content is 0.45%, and 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 corrosion film in a hydrogen sulfide environment and suppresses hydrogen invasion. As a result, the SSC resistance of the steel material is enhanced. If W is contained even a little, 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 deteriorates. 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%, further preferably 0.03%, further preferably 0.05%. The preferable upper limit of the W content is 0.45%, more preferably 0.40%.

[About Formula (1)]
The chemical composition of the steel material according to the present embodiment further satisfies Expression (1).
2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≧3.90 (1)
Here, the content (mass %) of the corresponding element is substituted for the element symbol in the formula (1). When 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 chemical composition. When F1 is less than 3.90, sufficient hardenability cannot be obtained and yield strength of steel cannot be obtained. Therefore, the steel material according to the present embodiment has F1 of 3.90 or more.

The preferable lower limit of F1 is 3.93, and 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 chemical composition, the upper limit of F1 is, for example, 8.27. The preferable upper limit of F1 is 8.20, more preferably 8.10, and further preferably 8.00.

[Old austenite grain size]
In the microstructure of the steel material according to the present embodiment, the former 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 old austenite grains (old γ grain size) means the crystal grain size of old austenite grains, which is obtained in accordance with the comparison 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 fine by including Mo in the steel material in an amount of more than 1.00%.

If the old γ grain size of the steel material according to the present embodiment is 11.0 μm or less, both 110 ksi class yield strength and excellent SSC resistance are satisfied, provided that the other requirements 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, more preferably 10.0 μm. The lower limit of the old γ grain size of the steel material according to the present 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 mentioned above, the old γ particle size can be determined according to the comparison method of ASTM E112-10. More specifically, it can be determined 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 center part 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 the observation surface is mirror-polished, it is embedded in a resin and immersed in a 2% Nital etchant for about 10 seconds to expose old γ grain boundaries by etching.

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

[Precipitate that precipitates on the old γ grain boundary]
In the steel material according to the present embodiment, the average area of the precipitates precipitated 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 on 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, 110 ksi class yield strength and excellent SSC resistance are provided, provided that the other requirements of the steel material according to the present embodiment are satisfied. Can be compatible.

As described above, with steel materials having the above chemical composition and old γ grain size of 11.0 μm or less, when trying to obtain a yield strength of 110 ksi class, many coarse carbides may precipitate in the steel material. Furthermore, 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 precipitated 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) that precipitate at the old γ grain boundaries is 10.0×10 ⁻³ μm ² or less. If the average area of the specific precipitates exceeds 10.0×10 ⁻³ μm ² , the SSC resistance of the steel material may deteriorate. If the average area of the specific precipitates 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 precipitated at the old γ grain boundary is 10.0×10 ⁻³ μm ² or less. The preferable 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-described chemical composition, the lower limit of the average area of specific precipitates is, for example, 3.0×10 ⁻³ μm ² .

The average area of specific precipitates can be obtained by the following method. A test piece is cut out from the steel material in the same manner as the above-described method for measuring the old γ particle 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 center part 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 the observation surface is mirror-polished, it is embedded in a resin and immersed in a 2% Nital etchant for about 10 seconds to expose old γ grain boundaries by etching. The observation surface of the test piece is observed in 10 fields of view with a secondary electron image by SEM to generate a photographic image. The observation magnification is, for example, 10,000 times.

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

A precipitate that overlaps with the specified old γ grain boundary and/or is in contact with the specified old γ grain boundary is specified as a “specific precipitate”. That is, the specific precipitate (precipitate that precipitates at the old γ grain boundary) means a precipitate that partially overlaps with and/or contacts the old γ grain boundary. The average area (μm ² ) of the identified specific precipitate is determined by image analysis.

[Microstructure]
The microstructure of the steel material according to the present embodiment mainly includes tempered martensite and tempered bainite. More specifically, the microstructure has a total volume ratio 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-described 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 regulations of this embodiment are satisfied. Up to 862 MPa (110 ksi class).

The total volume ratio of tempered martensite and tempered bainite can be obtained by microstructure observation. As the microstructure, a photographic image generated when obtaining the above-mentioned old γ grain size is used. In each field of view, the tempered martensite and tempered bainite and the other phases (for example ferrite or pearlite) can be distinguished from the contrast. Therefore, tempered martensite and tempered bainite are specified based on the contrast in each visual field.

Calculate the total area ratio of the specified tempered martensite and tempered bainite. In the present embodiment, the arithmetic average value of the total area ratios of tempered martensite and tempered bainite obtained from all fields of view is taken as the volume ratio of tempered martensite and 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 a tensile test. The steel material according to the present embodiment has excellent SSC resistance even if the yield strength is 110 ksi class, by satisfying the above-mentioned chemical composition, old γ grain size, and average area of the specific precipitate.

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

[Steel resistance to SSC]
The SSC resistance of the steel material according to the present embodiment can be evaluated by a method based on NACE TM0177-2005 Method A and a 4-point bending test.

In the method based on NACE TM0177-2005 Method A, a round bar test piece is collected from the steel material according to this embodiment. When the steel material is a steel plate, a round bar test piece is taken from the center part of the plate thickness. If the steel material is a steel pipe, collect 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 parallel portion length 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 equivalent to 90% of the actual yield stress is applied to the round bar test piece. A test solution at 4° C. is poured into a test container so that a stress-loaded round bar test piece is immersed therein to form a test bath. After deaeration of the test bath, 1 atm of H ₂ S gas is blown into the test bath to saturate the test bath with H ₂ S gas. The test bath saturated with H ₂ S gas is kept at 4° C. for 720 hours.

On the other hand, in the 4-point bending test, test pieces are taken from the steel material according to the present embodiment. When the steel material is a steel plate, a test piece is taken from the center part of the plate thickness. If the steel material is a steel pipe, collect 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. The test solution is injected into the autoclave, leaving the gas phase part, to prepare a test bath. After deaeration of the test bath, 20 atm of H ₂ S gas was pressurized and sealed in the autoclave, and the test bath was stirred to saturate the H ₂ S gas. After sealing the autoclave, the test bath is stirred at 24° C. for 720 hours.

In the steel material according to the present embodiment, cracks are not confirmed after 720 hours in both the method according to Method A and the four-point bending test. In the present specification, "no crack is confirmed" means that no crack is confirmed when the test piece after the test is visually observed.

[Shape of steel material]
The shape of the steel material according to the present 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 oil wells, 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 further 20 mm or more, it exhibits a yield strength of 110 ksi class and excellent SSC resistance.

[Production method]
The 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 the steel material according to the present embodiment. In addition, the manufacturing method of the steel material according to the present embodiment is not limited to the manufacturing method described below.

[Preparation process]
In the preparing 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 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 a material (material preparation step) and a step of hot working the material to produce an intermediate steel material (hot working step). Hereinafter, the case of including the material preparing step and the hot working step will be described in detail.

[Material preparation process]
In the material preparing step, the material is manufactured using the molten steel having the above chemical composition. Specifically, a slab (slab, bloom, or billet) is manufactured by a continuous casting method using molten steel. You may manufacture an ingot by the ingot making method using molten steel. If necessary, a slab, bloom or ingot may be slab-rolled to produce a billet. A material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
In the hot working step, the prepared raw 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. The billet extracted from the heating furnace is subjected to hot working to manufacture a raw pipe (seamless steel pipe).

For example, the Mannesmann method may be carried out as hot working to manufacture a raw tube. In this case, a round billet is perforated and rolled by a perforator. When piercing and rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot-rolled by a mandrel mill, reducer, sizing mill or the like to obtain a raw tube. The cumulative area reduction rate in the hot working step is, for example, 20 to 70%.

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

　The raw pipe manufactured by hot working may be air-cooled (As-Rolled). Alternatively, the blank produced by hot working may be directly quenched without cooling to room temperature, or may be supplemented (reheated) and then quenched.

When directly quenching or quenching after supplementing heat, cooling may be stopped or slow cooling may be performed during quenching. In this case, it is possible to suppress the occurrence of quench cracks in the raw pipe. In the case where the quenching is performed directly or after the supplementary heat treatment, the stress relief annealing (SR treatment) may be performed after the quenching and before the heat treatment (tempering etc.) in the next step. In this case, the residual stress of the raw pipe is removed.

As mentioned above, the intermediate steel is prepared in the preparation process. The intermediate steel material may be manufactured by the above-described preferable process, or 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 below are carried out, other establishments. You may prepare the intermediate steel material manufactured by.

[Heat treatment process]
In the heat treatment step, heat treatment is performed on the prepared intermediate steel material. Specifically, quenching and tempering are performed on the prepared intermediate steel material. In the present specification, “quenching” means quenching of an intermediate steel material having a point A ₃ or higher. In the present specification, “tempering” means to reheat and hold the intermediate steel material after quenching at the A _c1 point or less.

In the heat treatment process according to the present embodiment, preferably, quenching and tempering are performed multiple times. Specifically, it is preferable that the quenching and the tempering be performed twice or more each. More specifically, preferably, the prepared intermediate steel material is preferably quenched, then tempered, further quenched, and then tempered.

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

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

That is, the heat treatment step according to the present embodiment may be performed by rapidly cooling the intermediate steel material at 800 to 1000° C. after hot working, or using the auxiliary steel furnace or the heat treatment furnace as the intermediate steel material after hot working. It may be carried out by heating to 800 to 1000° C. and then quenching, or by heating the intermediate steel material after tempering 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 may become coarse and the SSC resistance of the steel material may deteriorate. 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 performed using a supplementary heating furnace or a heat treatment furnace after hot working, a preferable quenching time is 5 to 20 minutes. In the present specification, the "quenching time" means the time from the charging of the intermediate steel material into the supplementary heat treatment furnace or the heat treatment furnace to the removal thereof.

When performing quenching using a supplementary heating furnace or 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 process according to the present embodiment, when quenching is performed using a supplementary heating furnace or a heat treatment furnace after hot working, the quenching time is preferably 5 to 20 minutes.

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

If the cooling rate at the time of quenching is too slow, a microstructure mainly composed of martensite and bainite will not be obtained, and the mechanical properties defined in this embodiment cannot be obtained. Therefore, in the method for manufacturing a steel material according to the present embodiment, the intermediate steel material (base pipe) is rapidly cooled during quenching. Specifically, in the quenching step, the average cooling rate in the range of 800 to 500° C. of the intermediate steel material (base pipe) during quenching 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 at the surface of the intermediate steel to be quenched.

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

[Tempering]
The tempered intermediate steel material is tempered. In the tempering of a steel material that was supposed to be used in a sour environment, the tempering temperature and the tempering time were adjusted according to the chemical composition of the steel material 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 fine by increasing the Mo content. Regarding this mechanism, as described above, it is considered that Mo solid-dissolved in the steel material segregates to the austenite grain boundaries during heating during quenching, and the pinning effect makes the old γ grains after tempering fine. Here, in the steel material having the above chemical composition, Mo easily forms M ₂ C type carbide. Further, in the steel material having the above-mentioned chemical composition, M ₂ C type carbide is likely to precipitate during tempering.

Therefore, in the heat treatment step according to the present embodiment, Mo is sufficiently dissolved in the steel material after the second to final tempering. Specifically, in the heat treatment process according to the present embodiment, the tempering parameter TMP ₂ (=(tempering temperature (° C.)+273)×(log (tempering time (min)/60)+20)) is set in the second to last tempering process. 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 second} to last tempering is 15000 to 19000, the old γ grain size of the steel material after the final tempering is made fine. be able to. If the tempering parameter TMP ₂ in the penultimate tempering is less than 15000, the tempering effect may not be sufficiently obtained, and the steel material may be subject to temper cracking or set cracking. On the other hand, if the tempering parameter TMP ₂ in the penultimate tempering exceeds 19000, the amount of solute Mo is not sufficiently obtained during heating in the final quenching, and the old γ grain size after the final tempering becomes coarse. There is.

Therefore, in the heat treatment process according to the present embodiment, it is preferable that the tempering parameter TMP ₂ of the second-to-last tempering is 15,000 to 19000. The more preferable lower limit of the tempering parameter TMP _{2 for} the second temper from the last is 15500, and more preferably 16000. The more preferable upper limit of the tempering parameter TMP _{2 for} the second-to-last tempering is 18500, and more preferably 18000.

Preferably, in the penultimate tempering, the tempering temperature is 500 to less than 700°C. Further, preferably, in the second to last tempering, the tempering time (holding time) is set to 10 to 60 minutes. That is, in the second to last tempering in the present embodiment, the tempering temperature is 500 to less than 700° C., the tempering time is 10 to 60 minutes, and the tempering parameter TMP ₂ is 15,000 to 19000.

In the present specification, the "tempering temperature" corresponds to the temperature of the heat treatment furnace when heating and holding the intermediate steel after quenching. In the present specification, the "tempering time (holding time)" means the time from heating the intermediate steel after quenching and holding the intermediate steel in the heat treatment furnace until holding it.

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

The steel material according to the present embodiment further reduces coarse specific precipitates among the precipitates (specific precipitates) precipitated 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 process 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 ( Min)/60)+20)) is also controlled.

More specifically, in the steel material having the above-described chemical composition, if the tempering parameter TMP ₁ of the final tempering is 19100 to 19600, coarse specific precipitates can be reduced in the steel material after the final tempering. it can. If the tempering parameter TMP ₁ in the final tempering is less than 19100, the tempering effect may not be sufficiently obtained, and the yield strength of the steel material after tempering may be 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 precipitated.

On the other hand, if the tempering parameter TMP ₁ in the final tempering exceeds 19600, the yield strength of the steel material after tempering may be too low. If the tempering parameter TMP ₁ in the final tempering exceeds 19600, a large number of coarse specific precipitates may be further precipitated.

Therefore, in the heat treatment process according to the present embodiment, the tempering parameter TMP ₁ of the final tempering is preferably set to 19100 to 19600. The more preferable lower limit of the tempering parameter TMP _{1 for} the final tempering is 19200, and more preferably 19300. The more preferable upper limit of the tempering parameter TMP _{1 in} the final tempering is 19570, and more preferably 19500.

Preferably, in the final tempering, the tempering temperature is 650 to 730°C. Further, preferably, in the final tempering, the tempering time (holding 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.

Note that when the steel material is a steel pipe, the temperature of the steel pipe tends to fluctuate during holding tempering as compared to 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 set the yield strength to 758 to 862 MPa (110 ksi class) by appropriately adjusting the tempering temperature and the tempering time in the steel material having the chemical composition of the present embodiment. ..

The steel material according to the present embodiment can be manufactured by the above manufacturing method. In addition, in the above-mentioned manufacturing method, the manufacturing method of the seamless steel pipe has been described as an example. However, the steel material according to the present embodiment may be a steel plate or another shape. The manufacturing method of a steel plate or another shape also includes, for example, a preparation step and a heat treatment step, similar to the above-described manufacturing method. Furthermore, the above-described manufacturing method is an example, and the manufacturing method may be performed by another manufacturing method.

Molten steel having the chemical composition shown in Table 1 was manufactured. Further, F1 was obtained from the chemical composition shown in Table 1. In addition, "-" in Table 1 means that the content of each element is at the impurity level.

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

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

The raw pipe of each test number manufactured by hot working was held in a supplementary heating furnace at 950° C. for 5 minutes and then directly quenched (that is, the first quench). The cooling rate during quenching CR _{800-500 in} the first quenching of each test number was in the range of 8 to 500° _C./sec in all cases. The cooling rate during quenching, CR _800-500 , was determined by measuring the surface temperature of the raw pipe of each test number.

Then, the first tempering, that is, the second tempering from the last, was carried out for the raw tubes of the respective test numbers. Specifically, tempering was performed on the raw tubes of each test number at the tempering temperature (° C.) shown in the “second tempering from the last” column in Table 2 for the tempering time (minutes). 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 performed on the raw tubes of the respective test numbers for which the first tempering was performed. Specifically, for the test tubes of each test number, quenching was carried out after holding for the quenching time (minutes) at the quenching temperature (°C) described in the "final quenching" column of Table 2. The cooling rate during quenching CR _800-500 of the second quenching of each test number was in the range of 8 to 500° _C./sec in all cases.

Furthermore, the second tempering, that is, the final tempering, was performed on the raw pipe of each test number on which the final quenching was performed. Specifically, tempering was performed on the raw tubes of each test number at the tempering temperature (° C.) described in the “final tempering” column of Table 2 for the tempering time (minutes). Table 2 shows the tempering parameter TMP ₁ (=(tempering temperature (° C.)+273)×(log (tempering time (min)/60)+20)) in the final tempering.

In this example, the temperature of the auxiliary heating furnace and the heat treatment furnace used for heating the quenching was set to the "quenching temperature (°C)". Further, the temperature of the heat treatment furnace used for tempering was defined as "tempering temperature (°C)". In addition, the time from the time the raw tube was charged into the supplementary heating furnace or the heat treatment furnace to the time it was taken out at the time of heating during quenching was defined as "quenching time (minutes)". The time from the time the raw tube was put into the heat treatment furnace during tempering to the time it was taken out was defined as "tempering time (minutes)".

[Evaluation test]
After the tempering treatment, the seamless steel pipe of each test number was subjected to the microstructure observation, tensile test, and SSC resistance evaluation test described below.

[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 particle size (μm) of the seamless steel pipe of each test number. With respect to the seamless steel pipe of each test number, the average area of the precipitates (specific precipitates) precipitated at the old γ grain boundaries by the above method was obtained. Table 2 shows the average area (×10 ⁻³ μm ² ) of the specific precipitate in the seamless steel pipe of each test number.

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

Using a round bar test piece of each test number, a tensile test was performed at room temperature (25° C.) in the atmosphere 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).

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

③Three round bar test pieces with a diameter of 6.35 mm and a parallel length of 25.4 mm were taken from the center of the wall thickness of the seamless steel pipe of each test number. The round bar test piece was sampled so that its axial direction was parallel to the axial direction of the seamless steel pipe. 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 of 5.0 mass% sodium chloride and 0.5 mass% acetic acid (NACE solution A) was used. The test solution at 4° C. was poured into three test containers to prepare a test bath. Three stress-loaded round bar test pieces were immersed in the test baths of different test containers one by one. After deaeration of each test bath, 1 atm of H ₂ S gas was blown thereinto to saturate the test bath. The test bath saturated with 1 atm of H ₂ S 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 each 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 sampled so that its longitudinal direction was parallel to the axial direction of the seamless steel pipe. In accordance with ASTM G39-99 (2011), the stress applied to each test piece of each test number is 90% of the actual yield stress of the seamless steel pipe of each test number. Stress was applied by 4-point bending. The stress-loaded test piece was enclosed together with the test jig in an autoclave.

As the test solution, a 5.0 mass% sodium chloride aqueous solution was used. The test solution was injected into the autoclave, leaving the gas phase, to prepare a test bath. After deaeration of the test bath, 20 atm of H ₂ S gas was pressure-filled, and the test bath was stirred to saturate the test bath with H ₂ S gas. 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 piece 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 the observation, when no crack was confirmed in all the test pieces, it was judged as "E" (Excellent). On the other hand, the one in which cracks were confirmed in the test piece was judged to be "NA" (Not Acceptable).

[Test results]
Table 2 shows the test results. Regarding the SSC resistance test, the result of the test based on NACE TM0177-2005 Method A is shown in the “1 atmH ₂ S” column, and the result of the four-point bending test is shown in the “20 atmH ₂ S” column.

With reference to Table 1 and Table 2, the seamless steel pipes of test numbers 1 to 8 have appropriate chemical compositions, yield strength of 758 to 862 MPa, old γ grain size of 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 exhibited in both the test based on NACE TM0177-2005 Method A 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, in the test based on NACE TM0177-2005 Method A, excellent SSC resistance was not shown.

In the seamless steel pipe of Test No. 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 110 ksi class yield strength was not 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, excellent SSC resistance was not exhibited in the 4-point bending test.

In the seamless steel pipe of Test No. 13, the Cr content was too low. Furthermore, the Mo content was too low. Furthermore, F1 was too low. Furthermore, the tempering parameter TMP ₁ in the final tempering was too small. Therefore, the old γ particle size exceeded 11.0 μm. Therefore, the average area of the specific precipitate further exceeded 10.0×10 ⁻³ μm ² . As a result, excellent SSC resistance was not exhibited in both the test based on NACE TM0177-2005 Method A and the 4-point bending test.

In the seamless steel pipe of test No. 14, the O content was too high. As a result, excellent SSC resistance was not exhibited in the 4-point bending test.

In the seamless steel pipe of Test No. 15, the Mo content was too low. Furthermore, F1 was too low. Therefore, the old γ particle size exceeded 11.0 μm. Therefore, the average area of the specific precipitate further exceeded 10.0×10 ⁻³ μm ² . As a result, excellent SSC resistance was not exhibited in both the test based on NACE TM0177-2005 Method A and the 4-point bending test.

In the seamless steel pipe of Test No. 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, excellent SSC resistance was not exhibited in both the test based on NACE TM0177-2005 Method A and the 4-point bending test.

In the seamless steel pipe of Test No. 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 further exceeded 10.0×10 ⁻³ μm ² . As a result, excellent SSC resistance was not exhibited in both the test based on NACE TM0177-2005 Method A and the 4-point bending test.

In the seamless steel pipe of Test No. 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, in the test based on NACE TM0177-2005 Method A, excellent SSC resistance was not shown.

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

In the seamless steel pipe of Test No. 20, the tempering parameter TMP ₁ in the final tempering was too small. As a result, the yield strength exceeded 865 MPa, and 110 ksi class yield strength was not obtained. As a result, further, excellent SSC resistance was not shown in both the test based on NACE TM0177-2005 Method A and the four-point bending test.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.

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

Claims

In 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%,
Mo: over 1.00 to 2.00%,
Ti: 0.002 to 0.020%,
V: 0.05-0.30%,
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 element: 0-0.0100%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%, and
W: 0 to 0.50%, the balance consisting of Fe and impurities, and having a chemical composition satisfying the formula (1),
In the steel material, the crystal grain size of the former austenite grains is 11.0 μm or less,
The average area of the precipitates deposited on 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×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≧3.90 (1)
Here, the content (mass %) of the corresponding element is substituted for the element symbol in the formula (1). When 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 member 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 tube for oil wells.
The steel material according to any one of claims 1 to 5,
A steel material, wherein the steel material is a seamless steel pipe.