JP6859835B2 - Seamless steel pipe for steel materials and oil wells - Google Patents

Description

The present invention relates to seamless steel pipes for steel materials and oil wells, and more particularly to steel materials and seamless steel pipes for oil wells suitable for use in a sour environment.

By deepening oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as “oil wells”), it is required to increase the strength of seamless steel pipes for oil wells. Specifically, a seamless steel pipe for oil wells having a yield strength YS of 700 MPa or more is required.

Many deep wells are sour environments containing corrosive hydrogen sulfide. Seamless steel pipes for oil wells used in such a sour environment are required to have not only high strength but also sour resistance. One of the sour resistance properties is sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance).

Steels with enhanced strength and SSC resistance have been proposed in Japanese Patent Application Laid-Open No. 2013-076125 (Patent Document 1) and International Publication No. 2015/01/917 (Patent Document 2). The steels disclosed in these documents enhance SSC resistance by refining the crystal grains of tempered martensite.

Specifically, the steel material for machine structure disclosed in Patent Document 1 has a mass% of C: 0.18 to 0.30%, Si: 0.10 to 0.30%, and Mn: 0.10 to 10. 0.40%, P: 0.015% or less, S: 0.003% or less, N: 0.0040% or less, Ti: 0.005 to 0.015%, Mo: 0.13 to 0.40% , B: 0.0005 to 0.0020%, Cu: 0.8 to 1.2%, Cr: 0.15 to 0.40%, and Cr and Mn are Cr (mass%) / Mn (mass). %) Exceeds 0.94, Mo and B are contained so that (Mo (mass%) /8.7)/B (mass%) exceeds 17.5, and the balance is Fe and unavoidable impurities. It has a chemical composition consisting of. Furthermore, it has a structure mainly composed of the tempered martensite phase. Further, the yield strength YS is 490 MPa or more. The steel material for machine structure disclosed in Patent Document 1 is obtained by heating the above steel from 900 to 1000 ° C. at a heating rate of 30 ° C./s or more at the time of quenching, holding the steel at that temperature for 5 seconds or more, and then quenching. Patent Document 1 discloses that it can be obtained.

The seamless steel pipe for low alloy oil wells disclosed in Patent Document 2 has a mass% of C: 0.40 to 0.65%, Si: 0.05 to 0.50%, and Mn: 0.10 to 1. 00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr: 0.40 to 1.50%, Mo: 0.50 to 2.50%, V : 0.05 to 0.25%, Ti: 0 to less than 0.01%, Nb: 0.01 to 0.2%, sol. Al: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0 to 0.003%, and the balance is from Fe and unavoidable impurities. Has a chemical composition of Further, it comprises a structure composed of tempered martensite and retained austenite having a volume fraction of less than 0 to 2%. Furthermore, it has a yield strength of 965 MPa or more. The crystal grain size number of the former austenite grains in the above structure is 9.0 or more. In the tempered martensite, among the boundaries of packets, blocks and laths, the equivalent circle diameter of the substructure surrounded by the boundary with a crystal orientation difference of 15 ° or more is 3 μm or less. The seamless steel pipe for low alloy oil wells disclosed in Patent Document 2 is subjected to a heat treatment for reheating the raw pipe after hot working at least once, and the temperature is 500 to 100 ° C. in the cooling step of the heat treatment. Patent Document 2 discloses that it is obtained by a quenching treatment in which the cooling rate between them is 1 to 15 ° C./sec or less.

Japanese Unexamined Patent Publication No. 2013-076125 International Publication No. 2015/011917

T. Ungar, 3 outsiders, Journal of Applied Crystallography, Wiley, 1999, Vol. 32, pp. 992-1002

By the way, the evaluation of the SSC resistance of the conventional steel material is based on, for example, a tensile test or a bending test such as a Method A test or a Method B test specified in NACE (National Association of Corrosion Engineers) TM0177. These tests are tests for confirming the occurrence of SSC using a smoothing test piece. That is, the suppression of crack extension is not considered. Therefore, even if the steel material is evaluated to have excellent SSC resistance in these tests, SSC may occur due to the extension of latent cracks in the steel. That is, the sour resistance may be low.

Although Patent Document 1 defines the heating rate of the heat treatment and the subsequent holding time, the metal structure of the material used for the heat treatment is not specified. Therefore, the miniaturization effect may not be sufficient. Therefore, even if the generation of SSC can be suppressed, the suppression of crack extension may not be sufficient and the sour resistance may be low.

Since Patent Document 2 refines tempered martensite crystal grains, the miniaturization effect may not be sufficient. Therefore, even if the generation of SSC can be suppressed, the suppression of crack extension may not be sufficient and the sour resistance may be low.

Due to the deepening of oil wells in recent years, oil country tubular goods are required to have better sour resistance than before. Therefore, in order to further improve the sour resistance, it is preferable not only to prevent the generation of SSC but also to suppress the extension of cracks. From this point of view, the Method D DCB (Double Cantilever Beam) test specified in NACE TM0177 has come to be imposed. Oil well pipe steel materials used in a highly corrosive environment are required to have _{a high stress expansion coefficient (hereinafter referred to as K ISSC) in the DCB test.}

An object of the present invention is to provide a steel material having high strength and excellent sour resistance and a seamless steel pipe for an oil well.

The steel material according to the present invention has C: 0.10 to 0.45%, Si: 1.0% or less, Mn: 0.1 to 3.0%, Cr: 0.1 to 3.0% in mass%. , Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Ni: 0 to 3.0%, Cu: 0 to 3 .0%, Ti: 0-0.3%, Nb: 0-0.3%, V: 0-0.5%, Mo: 0-2.0%, W: 0-1.0%, Co : 0 to 2.0%, B: 0 to 0.01%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, and rare earth elements: 0 to 0.01%. The balance consists of Fe and impurities. The metallographic structure contains tempered martensite of 90.0% or more in area ratio. The old austenite particle size of tempered martensite is 5.0 μm or less. Tempering martensite contains crystal grains having an area ratio of 50% or more and an aspect ratio of less than 3.0. The ratio YS / TS of the yield strength YS to the tensile strength TS is 0.95 or more. Yield point elongation is 2.5% or more.

The steel material and the seamless steel pipe for oil wells according to the present invention have high strength and excellent sour resistance.

FIG. 1A is a TEM photograph of crystal grains having a low aspect ratio (aspect ratio of less than 3.0) in tempered martensite. FIG. 1B is a TEM photograph of crystal grains having a high aspect ratio (aspect ratio of 3.0 or more) in tempered martensite. FIG. 2 is a diagram showing the aspect ratios of the crystal grains in the former austenite grains in the tempered martensite structure and the area ratios of the crystal grains having each aspect ratio. FIG. 3 is a side view and a cross-sectional view of the DCB test piece used in the DCB test of the example. The numerical values in the figure indicate the dimensions (unit: mm).

In general, the higher the strength of steel, the lower the sour resistance. Therefore, the present inventors have investigated various methods for achieving both high strength with a yield strength of 700 MPa or more and sour resistance in steel materials and seamless steel pipes for oil wells. As a result, the following findings were obtained.

[Metal structure]
If the metal structure is mainly tempered martensite, high strength can be obtained. In the present embodiment, the tempered martensite-based material means that the metal structure contains tempered martensite having an area ratio of 90.0% or more.

[Old austenite particle size]
Tempered martensite contains multiple austenite granules. The present inventors have stated that by refining the particle size of the former austenite grains in the tempered martensite structure (hereinafter, simply referred to as the former austenite particle size), it is possible to achieve both unprecedented high strength and excellent sour resistance. I found it. Therefore, in the present invention, the old austenite particle size of the metal structure is set to 5.0 μm or less. As a result, the sour resistance characteristics of the steel material according to the present embodiment are enhanced.

If the grain size of the old austenite in the tempered martensite structure is fine, the strength of the steel is increased. Therefore, the strength of the steel is unlikely to decrease in the tempering heat treatment process. As a result, heat treatment can be performed at a high temperature for a long time. If heat treatment is performed at a high temperature for a long time, the dislocation density in the steel material decreases. If the dislocation density decreases, the amount of hydrogen entering the steel material decreases. If the amount of hydrogen entering the steel material is reduced, the embrittlement of the material in a sulfide corrosive environment can be suppressed. As a result, both the generation of SSC and the extension of cracks can be suppressed.

[Crystal grains in tempered martensite]
In steel materials according to conventional manufacturing methods, tempered martensite contains a plurality of austenite grains, and each austenite grain consists of a plurality of packets. Each packet consists of a plurality of plate-shaped blocks, and each block consists of a plurality of laths. In this case, when SSC is generated in the steel material, cracks are extended, so that the sour resistance may be low.

Therefore, the present inventors have found that if the tempered martensite structure contains spherical crystal grains, excellent sour resistance can be obtained. Specifically, if the tempered martensite structure contains 50 area% or more of crystal grains having an aspect ratio of less than 3.0, the sour resistance of the steel material is further enhanced. The aspect ratio of the crystal grains is the ratio between the maximum length and the minimum length of the crystal grains, which will be described later. The closer the aspect ratio is to 1, the greater the degree of spheroidization of the crystal grains. That is, a crystal grain having an aspect ratio of less than 3.0 means that the crystal grain is spherical. Hereinafter, crystal grains having an aspect ratio of less than 3.0 are also referred to as specific crystal grains.

FIG. 1A is a TEM photograph of crystal grains having a low aspect ratio (aspect ratio of less than 3.0) in tempered martensite. FIG. 1A was obtained by observing the structure of the steel material of Test No. 3 in the examples described later by TEM. In FIG. 1A, the crystal grains in the old austenite grains are spherical.

FIG. 1B is a TEM photograph of crystal grains having a high aspect ratio (aspect ratio of 3.0 or more) in tempered martensite. FIG. 1B was obtained by observing the structure of the steel material of test number 23 in the examples described later by TEM. In FIG. 1B, the crystal grains in the old austenite grains are columnar.

FIG. 2 is a diagram showing the aspect ratios of the crystal grains in the former austenite grains in the tempered martensite structure and the area ratios of the crystal grains having each aspect ratio. FIG. 2 was obtained from test number 3 and test number 23 in the examples described later. With reference to FIG. 2, the former austenite grains in the tempered martensite structure according to the present embodiment contain 50% or more of specific crystal grains in an area ratio.

In the SSC resistance test described below, no SSC occurred at test number 3. On the other hand, in test number 23, SSC occurred. Further, in the examples described later, in the DCB test, the stress expansion coefficient K _ISSC of test number 3 was 31.6 MPa√m. On the other hand, the stress expansion coefficient K _ISSC of test number 23 was 23.5 MPa√m, which was lower than that of test number 3. That is, the steel material of the present embodiment has better sour resistance. The reason for this is not obvious, but it is thought to be as follows.

The specific crystal grains in the tempered martensite structure of the steel material of the present embodiment can be obtained by tempering martensite containing fine old austenite grains. That is, the specific crystal grains are produced by recovering the lath structure and dislocations of martensite during the tempering heat treatment step. The dislocation density inside the specific crystal grain is lower than the dislocation density in the non-spherical crystal grain. Therefore, the dislocation density of the steel material of the present embodiment decreases. If the dislocation density is reduced, both SSC generation and crack extension can be suppressed as described above.

On the other hand, when the martensite containing coarse austenite grains is tempered and heat-treated, a large number of original lath structures remain. In this case, the area ratio of the specific crystal grains decreases. Therefore, the dislocation density of the steel material is increased, and the sour resistance is lowered.

[Ratio of yield strength YS to tensile strength TS YS / TS (yield ratio)]
If the structure of the steel is made finer, the ratio YS / TS (hereinafter referred to as the yield ratio) of the yield strength YS measured in the tensile test to the tensile strength TS (maximum stress) increases. If the yield ratio is high, the tensile strength TS can be suppressed to be lower than the yield strength YS required for the product. If the tensile strength TS is low, the sensitivity of SSC decreases. As a result, SSC resistance is improved. The metal structure of the steel material according to the present embodiment has been miniaturized as before. Therefore, in the steel material of the present embodiment, the yield ratio can be set to 0.95 or more.

[Yield point extension]
If the structure of steel is miniaturized, the yield point elongation will be further increased. If the yield point elongation is increased, plastic deformation can be achieved by the yield point elongation when a crack occurs. If plastic deformation is possible, stress concentration at the crack tip is relaxed. As a result, the extension of the crack is suppressed. Miniaturizing the structure of steel further complicates the expansion path of cracks. As a result, the extension of the crack is suppressed. The metal structure of the steel material according to the present embodiment has been miniaturized as before. Therefore, in the steel material of the present embodiment, the yield point elongation can be set to 2.5% or more.

The above metal structure can be obtained, for example, by producing a steel material by the production method described later using the raw material having the chemical composition of the present embodiment.

The steel material according to the present invention completed based on the above findings has a mass% of C: 0.10 to 0.45%, Si: 1.0% or less, Mn: 0.1 to 3.0%, Cr: 0.1 to 3.0%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Ni: 0 to 3. 0%, Cu: 0-3.0%, Ti: 0-0.3%, Nb: 0-0.3%, V: 0-0.5%, Mo: 0-2.0%, W: 0-1.0%, Co: 0-2.0%, B: 0-0.01%, Ca: 0-0.01%, Mg: 0-0.01%, and rare earth elements: 0- It contains 0.01% and the balance consists of Fe and impurities. The metallographic structure contains tempered martensite of 90.0% or more in area ratio. The old austenite particle size of tempered martensite is 5.0 μm or less. Tempering martensite contains crystal grains having an area ratio of 50% or more and an aspect ratio of less than 3.0. The ratio YS / TS of the yield strength YS to the tensile strength TS is 0.95 or more. Yield point elongation is 2.5% or more.

The chemical composition may contain one or more selected from the group consisting of Ni: 0.1 to 3.0% and Cu: 0.1 to 3.0%.

The chemical composition is Ti: 0.01 to 0.3%, Nb: 0.01 to 0.3%, V: 0.01 to 0.5%, Mo: 0.05 to 2.0%, W. It may contain one or more selected from the group consisting of: 0.05 to 1.0% and Co: 0.05 to 2.0%.

The chemical composition may contain B: 0.0003 to 0.01%.

The chemical composition is one selected from the group consisting of Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and rare earth elements: 0.0001 to 0.01%. Two or more kinds may be contained.

The steel material according to the present invention preferably has a yield strength YS of 700 MPa or more. Further, it is preferable that the yield strength YS (MPa) and the dislocation density ρ (m- ² ) satisfy the relationship of the formula (1) in the steel material according to the present invention.
YS / ρ> 1.1 × 10 ^-11 (1)

In this case, the sour resistance is further enhanced.

The seamless steel pipe for oil wells according to the present invention has the above chemical composition, and if it has the above metal structure, it exhibits excellent strength and sour resistance.

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

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

C: 0.10 to 0.45%
Carbon (C) enhances hardenability and enhances the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, shrinkage will occur when quenching is performed in the cooling step after the hot working step. Therefore, the C content is 0.10 to 0.45%. The lower limit of the C content is preferably 0.15%, more preferably 0.20%. The preferable upper limit of the C content is 0.43%.

Si: 1.0% or less Silicon (Si) is inevitably contained. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability is lowered and the material is easily cracked during rolling. Therefore, the Si content is 1.0% or less. The lower limit of the Si content preferable for obtaining the above effect is 0.05%, more preferably 0.1%. The preferred upper limit of the Si content is 0.8%, more preferably 0.6%.

Mn: 0.1 to 3.0%
Manganese (Mn) enhances hardenability and enhances the strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the hot workability is lowered and cracks are likely to occur during rolling. Therefore, the Mn content is 0.1 to 3.0%. The preferred lower limit of the Mn content is 0.2%, more preferably 0.3%. The upper limit of the preferred Mn content is 2.5%, more preferably 2.0%.

Cr: 0.1 to 3.0%
Chromium (Cr) enhances the hardenability of steel and enhances the strength of steel. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the toughness of the steel material is lowered and it becomes easy to crack during rolling. Therefore, the Cr content is 0.1 to 3.0%. The lower limit of the Cr content is preferably 0.2%, more preferably 0.3%. The preferred upper limit of the Cr content is 2.5%, more preferably 2.0%.

Al: 0.001 to 1.0%
Aluminum (Al) suppresses the formation of cementite in the cooling step of the austenitic heat treatment of steel materials. As a result, Al enhances the hardenability of the steel material. Al further deoxidizes the steel. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, the hot workability is lowered and the material is easily cracked during rolling. Therefore, the Al content is 0.001 to 1.0%. The lower limit of the Al content is preferably 0.005%, more preferably 0.01%. The preferred upper limit of the Al content is 0.8%, more preferably 0.6%. The "Al" content as used herein means "acid-soluble Al", that is, the content of "sol.Al".

P: 0.05% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the SSC resistance of the steel. Therefore, the P content is 0.05% or less. The preferred P content is 0.02% or less. The P content is preferably as low as possible.

S: 0.01% or less Sulfur (S) is an impurity. S segregates at the grain boundaries and lowers the SSC resistance of the steel. Therefore, the S content is 0.01% or less. The S content is preferably 0.005% or less, more preferably 0.003% or less. The S content is preferably as low as possible.

N: 0.01% or less Nitrogen (N) is inevitably contained. N forms a coarse nitride and reduces the SSC resistance of the steel. Therefore, the N content is 0.01% or less. The preferred N content is 0.005% or less, more preferably 0.004% or less. The N content is preferably as low as possible. However, when a small amount of Ti is contained and aiming at finer crystal grains by precipitation of fine nitrides, it is preferable to contain N in an amount of 0.002% or more.

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

[About arbitrary elements]
The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Ni and Cu instead of a part of Fe. All of these elements are optional elements and refine the structure of the steel to enhance the strength and sour resistance of the steel.

Ni: 0-3.0%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni _{lowers the A 1} transformation point and lowers the austenite formation temperature range. As a result, Ni suppresses the growth (coarseness) of the austenite structure. However, if the Ni content is too high, this effect will be saturated. Therefore, the Ni content is 0 to 3.0%. The lower limit of the Ni content is preferably 0.1%, more preferably 0.2%. The preferred upper limit of the Ni content is 2.0%, more preferably 1.5%.

Cu: 0-3.0%
Copper (Cu) is an optional element and may not be contained. When contained, Cu _{lowers the A 1} transformation point and lowers the austenite formation temperature range. As a result, Cu suppresses the growth (coarseness) of the austenite structure. However, if the Cu content is too high, the hot workability is lowered and the material is easily cracked during rolling. Therefore, the Cu content is 0 to 3.0%. The lower limit of the Cu content is preferably 0.1%, more preferably 0.3%. The preferred upper limit of the Cu content is 2.5%, more preferably 2.0%.

The chemical composition of the above-mentioned steel material may further contain one or more selected from the group consisting of Ti, Nb, V, Mo, W and Co instead of a part of Fe. All of these elements are optional elements and suppress the growth (coarseness) of the austenite structure to increase the strength of the steel.

Ti: 0-0.3%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti suppresses the growth (coarseness) of the austenite structure by the particle pinning effect or the solute drag effect. However, if the Ti content is too high, the steel will be embrittled. Therefore, the Ti content is 0 to 0.3%. The preferable lower limit of the Ti content is 0.01%. The preferred upper limit of the Ti content is 0.2%, more preferably 0.15%.

Nb: 0 to 0.3%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb suppresses the growth (coarseness) of the austenite structure by the particle pinning effect or the solute drug effect. However, if the Nb content is too high, the steel will be embrittled. Therefore, the Nb content is 0 to 0.3%. The preferable lower limit of the Nb content is 0.01%. The preferred upper limit of the Nb content is 0.2%, more preferably 0.15%.

V: 0-0.5%
Vanadium (V) is an optional element and may not be contained. When contained, V suppresses the growth (coarseness) of the austenite structure by the particle pinning effect or the solute drug effect. However, if the V content is too high, the steel will be embrittled. Therefore, the V content is 0 to 0.5%. The lower limit of the V content is preferably 0.01%, more preferably 0.03%. The preferred upper limit of the V content is 0.3%, more preferably 0.25%.

Mo: 0-2.0%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo suppresses the growth (coarseness) of the austenite structure by the particle pinning effect or the solute drug effect. However, if the Mo content is too high, this effect will be saturated. Therefore, the Mo content is 0 to 2.0%. The lower limit of the Mo content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the Mo content is 1.5%, more preferably 1.0%.

W: 0-1.0%,
Tungsten (W) is an optional element and may not be contained. When contained, W suppresses the growth (coarseness) of the austenite structure by the particle pinning effect or the solute drug effect. However, if the W content is too high, this effect will be saturated. Therefore, the W content is 0 to 1.0%. The lower limit of the W content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the W content is 0.8%, more preferably 0.6%.

Co: 0-2.0%
Cobalt (Co) is an optional element and may not be contained. When contained, Co suppresses the growth (coarseness) of austenite tissue. Co also suppresses the invasion of hydrogen into the steel material. This suppresses the embrittlement of the steel and enhances the SSC resistance of the steel. However, if the Co content is too high, the alloying cost of the steel material will be high. Therefore, the Co content is 0 to 2.0%. The lower limit of the Co content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the Co content is 1.5%, more preferably 1.0%.

The chemical composition of the above-mentioned steel material may further contain B instead of a part of Fe.

B: 0-0.01%
Boron (B) is an optional element and may not be contained. When contained, B dissolves in the steel to increase the hardenability of the steel and increase its strength. However, if the B content is too high, the toughness of the steel will decrease. Therefore, the B content is 0 to 0.01%. The preferred lower limit of the B content is 0.0003%, more preferably 0.0005%. The preferred upper limit of the B content is 0.005%, more preferably 0.004%.

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

Ca: 0-0.01%,
Calcium (Ca) is an optional element and may not be contained. When contained, Ca binds to P and S in the steel. This detoxifies P and S in the steel and enhances the hot workability of the steel. However, if the Ca content is too high, the steel becomes brittle and the workability is rather lowered. Therefore, the Ca content is 0 to 0.01%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%. The preferred upper limit of the Ca content is 0.008, more preferably 0.006%.

Mg: 0-0.01%,
Magnesium (Mg) is an optional element and may not be contained. When contained, Mg combines with P and S in the steel. This detoxifies P and S in the steel and enhances the hot workability of the steel. However, if the Mg content is too high, the steel becomes brittle and the workability is rather lowered. Therefore, the Mg content is 0 to 0.01%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%. The preferred upper limit of the Mg content is 0.008%, more preferably 0.006%.

Rare earth elements: 0-0.01%,
Rare earth elements (REM) are optional elements and may not be contained. When contained, REM combines with P and S in steel. This detoxifies P and S in the steel and enhances the hot workability of the steel. However, if the REM content is too high, the steel becomes brittle and the workability is rather lowered. Therefore, the REM content is 0-0.01%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0003%. The preferred upper limit of the REM content is 0.008%, more preferably 0.006%.

The REM in the present specification refers to ittium (Y) having an atomic number of 39, lancium (La) having an atomic number of 57 which is a lanthanoid, lutetium (Lu) having an atomic number of 71, and atomic number 89 which is an actinoid. It is one or more elements selected from the group consisting of actinium (Ac) to lawrencium (Lr) having an atomic number of 103. Further, the REM content in the present specification is the total content of these elements.

Since the steel material of the present embodiment has a metal structure described in detail below, a large amount of alloying elements (Mo, Co, Cu, Ni, Ti, Nb, V and W) having an effect of miniaturizing the structure can be added. Even if it is not, the structure can be miniaturized. More specifically, it is preferable that the alloying element having the effect of refining the structure satisfies the formula (2).
Mo + Co + Cu + Ni + Ti + Nb + V + W <1.5 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).
It is defined as F2 = Mo + Co + Cu + Ni + Ti + Nb + V + W. When F2 is less than 1.5, high strength and excellent sour resistance can be obtained while reducing the cost of the alloy component.

[Metal structure]
[Tempering martensite: 90.0 area% or more]
The metallographic structure of the steel material of the present invention mainly consists of tempered martensite. More specifically, the metallographic structure consists of tempered martensite having an area ratio of 90.0% or more. This increases the strength of the steel material.

The metal structure of the present embodiment may contain other structures in an area of less than 10.0 area%. Other structures are, for example, bainite, ferrite, pearlite and retained austenite. Preferably, the metallographic structure consists of a tempered martensite single phase.

Tempering martensite is a metallographic structure obtained by performing tempering heat treatment on martensite. In tempered martensite, carbon that has been supersaturated in martensite precipitates as carbides such as cementite. In tempered martensite, the recovery further reduces the dislocation density.

[Measurement method of area ratio of tempered martensite]
The area ratio (%) of tempered martensite in the metallographic structure is measured by the following method.

Cut the steel material perpendicular to the rolling direction. A sample for observing the metallographic structure including this cut surface is collected. The sample is embedded in resin and mirror-polished so that the cut surface becomes the observation surface. After polishing, the observation surface is etched with a nital solution. Observe the secondary electron image with a scanning electron microscope (SEM, Scanning Electron Microscope (observation magnification 2000 times, acceleration voltage of scanning electron beam 20 kV)) in any three fields (viewing area = 70 μm × 40 μm) of the etched observation surface. To do. Tempered martensite is observed as a structure in which cementite particles are deposited at the old γ grain boundaries, packet boundaries, block boundaries, and lath boundaries of the original martensite. Martensite is distinguished by the fact that cementite is not precipitated to an equilibrium amount. The ferrite-pearlite structure is distinguished by the lamellar morphology of cementite. This makes it possible to confirm the presence or absence of tempered martensite and tissues other than tempered martensite.

The area ratio (%) of the tempered martensite and the tissue other than the tempered martensite in each visual field is measured by a point calculation method based on JIS G0555. The average area ratio of tempered martensite in each field of view is defined as the area ratio (%) of tempered martensite. The average area ratio of tissues other than tempered martensite in each visual field is defined as the area ratio (%) of tissues other than tempered martensite.

[Old austenite particle size: 5.0 μm or less]
In the steel material of the present embodiment, the grain size of the old austenite in the tempered martensite structure of the metal structure is 5.0 μm or less. As a result, the steel material of the present embodiment can obtain a high strength with a yield strength YS of 700 MPa or more.

Further, when the old austenite particle size is 5.0 μm or less, excellent sour resistance can be obtained for the above-mentioned reason. The preferred upper limit of the old austenite particle size is 4.0 μm.

[Measuring method of old austenite particle size]
The old austenite particle size is determined by the following method. Austenitized Specimens are collected from the heat-treated steel material. In the case of a steel pipe, the cross section shall be a plane perpendicular to the axis, and the test piece shall be collected from the center of the wall thickness. After the test piece is mirror-polished, the former austenite grain boundaries are revealed using a saturated aqueous solution of picric acid. In the test piece, the old austenite grain size (average crystal grain size of the old austenite grains) is measured in any 10 fields of view. The measurement is carried out by the section method shown in JIS G0551 (2005) by observing with a 1000x optical microscope. Calculate the old austenite particle size number in each field of view. The average of the calculated 10 old austenite particle size numbers (average old austenite particle size number) is obtained. Average The average area of each crystal grain is calculated based on the old austenite particle size number. The equivalent circle diameter is calculated from the average area, and the obtained equivalent circle diameter is used as the old austenite particle size.

As described above, the steel material according to the present embodiment has a fine metal structure. If the metal structure of the steel material is fine, the crack extension path is complicated. Therefore, in a sulfide corrosive environment, even when fine cracks occur inside the steel material, the expansion of the cracks is suppressed. Therefore, the sour resistance of the steel material is enhanced.

The magnitude of the effect of suppressing the extension of the crack can be evaluated by the stress expansion coefficient at the tip of the crack. The stress expansion factor at the crack tip can be evaluated, for example, by a DCB test according to NACE TM0177-96 Method D.

[Crystal grains with aspect ratio less than 3.0: 50 area% or more]
In the steel material of the present embodiment, the tempered martensite structure contains 50 area% or more of crystal grains (specific crystal grains) having an aspect ratio of less than 3.0. This further enhances the sour resistance of the steel material. A crystal grain having an aspect ratio of less than 3.0 means that the crystal grain is spherical. The specific crystal grains are preferably contained in an area% of 60 area% or more.

[Measuring method of aspect ratio of crystal grains and measuring method of area ratio of specific crystal grains]
The aspect ratio is obtained by the following method. From the SEM image obtained by measuring the area ratio of tempered martensite, crystal grains surrounded by a boundary (large angle grain boundary) having a crystal orientation difference of 15 ° or more are identified. Obtain the aspect ratio of the specified crystal grains. Specifically, in the field of view of the above SEM image, the crystal orientation map is measured by an electron backscatter diffraction (EBSD, Electron Backscattering Diffraction) method. Program processing is performed using the obtained data. By program processing, the average coordinates (center of gravity) of the crystal grains, the straight line passing through the center of gravity, and the length of the intercept crossing the crystal grains are obtained. In the program processing, the average X coordinate of the crystal grains, the average Y coordinate of the crystal grains, and a straight line having an arbitrary slope a passing through the center of gravity are obtained by the following equations.

Of the obtained section lengths, the ratio of the maximum length to the minimum length (= maximum length / minimum length) is defined as the aspect ratio of the crystal grains.

The ratio of the total area of crystal grains (specific crystal grains) having an aspect ratio of less than 3.0 obtained by the above method to the total area of tempered martensite is defined as the area ratio of the specific crystal grains. For each of the three visual fields observed when determining the area ratio of tempered martensite, the area ratio of specific crystal grains is calculated. The average of the area ratios of the specific crystal grains obtained from each field of view is defined as the area ratio of the specific crystal grains.

[Dislocation density]
As described above, if the tempered martensite structure contains specific crystal grains having an area ratio of 50% or more, the sour resistance of the steel material is further enhanced. The dislocation density of specific crystal grains is low. Therefore, the dislocation density of the steel material decreases.

When the dislocation density is low, embrittlement of the material due to sulfide corrosion can be suppressed. Therefore, both the generation of SSC and the extension of cracks can be suppressed. As a result, excellent sour resistance can be obtained.

The dislocation density ρ of the steel material of the present invention preferably satisfies the following formula (1). In this case, both the generation of SSC and the extension of cracks can be further suppressed.
YS / ρ> 1.1 × 10 ^-11 (1)
The unit of dislocation density is m- ² , and the unit of YS is MPa. The larger the value of YS / ρ, the higher the strength is obtained with the lower dislocation density. A more preferable lower limit of YS / ρ is 1.2 × 10 ^-11 , and more preferably 1.3 × 10 ^-11 .

A dislocation density ρ satisfying the formula (1) can be obtained by performing a tempering heat treatment on a material having the above-mentioned chemical composition and metal structure by the production method described later.

[Measurement of dislocation density ρ]
The dislocation density ρ can be measured by the modified Williamson-Hall method. Specifically, X-ray diffraction is performed on the steel material. Multiple diffraction peaks of BCC-iron on the diffraction profile obtained by X-ray diffraction are identified. The dislocation density ρ is analyzed from the half width of these diffraction peaks. For details on the modified Williamson-Hall method, refer to T.I. Ungar, 3 outsiders, Journal of Applied Crystallography, Wiley, 1999, Vol. 32, pp. 992-1002 (Non-Patent Document 1).

[Ratio of yield strength YS to tensile strength TS YS / TS (yield ratio): 0.95 or more]
In the steel material of the present embodiment, the yield ratio is 0.95 or more. In this case, the tensile strength TS can be suppressed to be lower than the yield strength YS required for the product. If the tensile strength TS is low, the sensitivity of SSC decreases. As a result, SSC resistance is improved. The metal structure of the steel material according to the present embodiment has been miniaturized as before. Therefore, in the steel material of the present embodiment, the yield ratio can be set to 0.95 or more.

[Yield point elongation: 2.5% or more]
In the steel material of the present embodiment, the yield point elongation is 2.5% or more. In this case, when a crack occurs in the steel material, it can be plastically deformed by the yield point elongation. If plastic deformation is possible, stress concentration at the crack tip is relaxed. As a result, the extension of the crack is suppressed. As a result, excellent sour resistance can be obtained. If the metal structure of the steel material is fine, the yield point elongation increases. By having the above-mentioned metal structure, the steel material of the present embodiment can obtain a yield point elongation of 2.5% or more. The preferred lower limit of yield point elongation is 3.0%, more preferably 3.5%.

[Shape of steel]
The shape of the steel material is not particularly limited. The steel material is, for example, a steel pipe or a steel plate. When the steel material is a seamless steel pipe for oil wells, the preferable wall thickness is 9 to 60 mm. The present invention is particularly suitable for use as a seamless steel pipe for thick oil wells. More specifically, even if the steel material according to the present invention is a seamless steel pipe for oil wells having a thickness of 15 mm or more and a thickness of 20 mm or more, it exhibits excellent strength and sour resistance.

[Strength of steel]
The yield strength YS of the steel material of the present embodiment is 700 MPa or more. The yield strength YS as used herein means a yield point (MPa).

YS x K _ISSC > 20000 (3)
Equation (3) shows the relationship between the yield strength YS (MPa) and the fracture toughness value K _ISSC value (MPa√m). When YS × K _ISSC exceeds 20000 (MPa ² √m), it has excellent sour resistance while having the yield strength YS required in this embodiment.

The steel material of the present embodiment can satisfy the formula (3) by having the above-mentioned chemical composition and metal structure even if the yield strength YS is as high as 700 MPa or more. That is, the steel material of the present embodiment has high strength and excellent sour resistance.

[Production method]
As an example of the above-mentioned method for manufacturing a steel material, a method for manufacturing a seamless steel pipe for an oil well will be described. The manufacturing method of the seamless steel pipe for oil wells is the process of preparing the material (preparation process), the process of hot-working the material to manufacture the raw pipe (hot working process), and the austenite heat treatment for the raw pipe. And a step of performing a tempering heat treatment to form a seamless steel pipe for an oil well (austenizing heat treatment step and a tempering heat treatment step) is provided. Hereinafter, each step will be described in detail.

[Preparation process]
A molten steel having the above-mentioned chemical composition is produced. The material is manufactured using molten steel. Specifically, slabs (slabs, blooms, or billets) are manufactured by a continuous casting method using molten steel. Ingots may be produced by the ingot method using molten steel. If desired, slabs, blooms or ingots may be block-rolled to produce billets. The material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
The prepared material is hot-processed to manufacture a bare tube. First, the billet is heated in a heating furnace. Hot working is performed on the billets extracted from the heating furnace to manufacture raw pipes (seamless steel pipes). For example, the Mannesmann method is carried out as hot working to manufacture a bare tube. In this case, the round billet is drilled and rolled by a drilling machine. The perforated round billet is further hot-rolled with a mandrel mill, reducer, sizing mill or the like to form a raw pipe.

A raw tube may be manufactured from a billet by another hot working method. For example, in the case of a short seamless steel pipe for a thick-walled oil well such as a coupling, the raw pipe may be manufactured by forging. Through the above steps, a bare tube having a wall thickness of 9 to 60 mm is manufactured.

The temperature during hot working _{may be Ar 3} points or more.

The raw tube produced by hot working is cooled to a temperature range of 400 ° C. or lower at a cooling rate of more than 1 ° C./sec. The cooling method is, for example, mist cooling. As a result, the steel material before the austenitic heat treatment, which is the next step, has one or more structures selected from the group consisting of martensite, bainite, processed martensite and processed bainite. can do. These early tissues are rich in austenite nucleation sites such as former austenite grains, packets, blocks, and laths. Therefore, in the austenitic heat treatment of the next step, extremely fine old austenite grains can be obtained by setting the austenitic heat treatment conditions described later.

Further, it is preferable that the time until the start of cooling after the hot working is short. The short time is, for example, 15 seconds or less, more preferably 10 seconds or less in the case of a steel material having a wall thickness of 15 mm. If the time until the start of cooling after hot working is short, the structure becomes martensite or bainite transformed from unrecrystallized austenite (referred to as austenated martensite and austenite bainite, respectively). If the structure is ausformed martensite or ausformed bainite, the crystal grains become finer. As a result, the number of nucleation sites increases, and the nucleation site becomes finely dispersed in the metal structure. If such a structure is used as a steel material, a finer metal structure can be obtained.

[Austenitic heat treatment process]
Austenitizing heat treatment is performed on the raw tube after hot working. The austenite particle size is adjusted to 5.0 μm or less according to the austenitic heat treatment conditions. The austenitizing heat treatment is performed, for example, in a high frequency induction heating furnace or a gas burning furnace.

The preferred heating rate is 50 ° C./s or higher. The preferred temperature range is Ac ₃ points to _{Ac 3} points + 150 ° C. When the heating rate is 50 ° C./s or more, coarsening of crystal grains and cementite in the initial structure is suppressed. This secures an austenite nucleation site. As a result, the effect of micronizing the structure is enhanced, and very fine austenite can be produced.

On the other hand, if the heating rate is less than 50 ° C./s, crystal grains and cementite grow and coarsen during heating. If the heating rate is less than 50 ° C./s, further dislocation recovery and recrystallization occur. In this case, austenite nucleation sites are reduced.

If the ultimate temperature range is _{less than Ac 3} , an austenite single-phase structure cannot be obtained. As a result, the area ratio of tempered martensite is less than 90.0% in the structure of the steel material. When the reached temperature range _{exceeds Ac 3} points + 150 ° C., austenite grains grow and coarsen in a short time.

Therefore, the preferable heating rate is 50 ° C./s or more, and the preferable ultimate temperature range is Ac ₃ points to _{Ac 3} points + 150 ° C. A more preferable lower limit of the heating rate is 80 ° C./s, and even more preferably 100 ° C./s.

In this embodiment, the Ac ₃ points are obtained from the thermal expansion measurement when heated at a heating rate of 50 ° C./s.

After heating and holding under the above conditions, cooling is started. The holding time at a predetermined temperature is preferably within 60 seconds. If the holding time in the above temperature range is 60 seconds or less, coarsening of austenite particles can be prevented, and fine austenite particles having a particle size of 5.0 μm or less can be obtained. A more preferable upper limit of the holding time in the above temperature range is 30 seconds, and even more preferably 20 seconds.

In the cooling step, the average cooling rate from 750 ° C. to 400 ° C. or lower is preferably 10 ° C./s or higher. When the average cooling rate from 750 ° C. to 400 ° C. or lower is 10 ° C./s or more, the area ratio of tempered martensite can be 90.0% or more in the structure of the steel material. Cooling is carried out, for example, by water spray, mist or immersion.

By carrying out the above-mentioned austenitic heat treatment, the steel material of the present embodiment contains tempered martensite having an area ratio of 90.0% or more without adding a large amount of alloying elements. A fine metallographic structure having an austenite particle size of 5.0 μm or less can be obtained.

Further, by performing tempering heat treatment on the fine martensite obtained by the above-mentioned austenitizing heat treatment under the conditions described later in the next step, tempered martensite containing 50% or more of specific crystal grains in an area ratio. Can be obtained.

[Tempering heat treatment process]
After performing the above-mentioned austenitizing heat treatment, a tempering heat treatment is performed. The yield strength YS of the steel material is adjusted to 700 MPa or more by tempering heat treatment.

In the tempering heat treatment step, the tempering heat treatment temperature is _{preferably 600 ° C. to Ac 1} point. During the tempering heat treatment process, the solid-dissolved carbon precipitates as cementite in the metal structure. During the tempering heat treatment step, dislocation recovery occurs, the area ratio of specific crystal grains increases, and the dislocation density in the steel material decreases. As a result, the toughness and sour resistance of the steel material are further enhanced. The tempering heat treatment temperature is preferably carried out at a high temperature as long as the required strength is secured. When the tempering heat treatment temperature is less than 600 ° C., the precipitation of cementite and the recovery of dislocations are difficult to proceed, and the sour resistance may be deteriorated. When the tempering heat treatment temperature _{exceeds Ac 1} , the area ratio of tempered martensite is less than 90.0% in the structure of the steel material. As a result, the sour resistance may deteriorate. A more preferable lower limit of the tempering heat treatment temperature is 650 ° C.

In the tempering heat treatment step, the preferable lower limit of the holding time is preferably 10 minutes or more. If the holding time is less than 10 minutes, the crystal grains in the old austenite grains are not sufficiently spheroidized. Therefore, the dislocation density increases. As a result, the sour resistance is reduced. The preferred upper limit of the holding time is 500 minutes.

If the old austenite grain size is coarse, dislocation reinforcement must be used to obtain the required strength. If tempering heat treatment is performed at a high temperature for a long time, the dislocation density decreases. In this case, the required strength cannot be obtained. Therefore, it is necessary to perform the tempering heat treatment at a relatively low temperature and in a short time.

The steel material according to the present embodiment has a finer structure and is strengthened to increase its strength. Therefore, even if the tempering heat treatment is performed at a high temperature for a long time, the strength is unlikely to decrease. From this, the steel material according to the present embodiment can be tempered at a high temperature for a long time as described above. Dislocation density in steel can be reduced by tempering heat treatment at high temperature for a long time. If the dislocation density is reduced, embrittlement of the steel material due to SSC can be suppressed. As a result, both the generation of SSC and the extension of cracks can be suppressed, and excellent sour resistance can be obtained.

Through the above steps, the steel material of the present embodiment contains tempered martensite having a metal structure of 90.0% or more in area ratio, the former austenite grain size of the tempered martensite is 5.0 μm or less, and the tempered martensite. However, it can contain crystal grains having an area ratio of 50% or more and an aspect ratio of less than 3.0. Further, by the above steps, the yield ratio can be set to 0.95 or more and the yield point elongation can be set to 2.5% or more.

In the above-mentioned manufacturing method, a method for manufacturing a steel pipe has been described as an example. However, the steel material of the present invention also includes a preparation step, a hot working step, an austenitizing heat treatment step, and a tempering heat treatment step, even if the steel material has a steel plate or another shape.

180 kg of molten steel having the chemical composition shown in Table 1 was produced in a high-frequency vacuum melting furnace. Table 1 also shows ₁ point of Ac and ₃ points of Ac of each material. Acc ₁ point and Ac ₃ point were obtained by measuring the coefficient of thermal expansion when each steel material was heated at 50 ° C./sec by energization heating using a differential transformer type displacement meter. Acc ₁ point was the temperature at which the coefficient of thermal expansion deviates from the proportional relationship with temperature. Acc ₃ points are the temperatures at which the coefficient of thermal expansion again converges in proportion to the temperature. The F2 of each steel material was less than 1.5.

Using the molten steel, a steel piece having a thickness of 30 mm was produced by hot forging. The produced steel pieces were hot-rolled using a hot-rolling tester to produce a hot-rolled steel sheet having a plate thickness of 15 mm.

The hot-rolled steel sheets of each steel after hot rolling were cooled under the following three conditions. The metallographic structure of each of the obtained hot-rolled steel sheets was as follows. This condition is also shown in Table 2.
I. After 20 seconds or more passed after hot rolling, the mixture was cooled to 400 ° C. or lower at a cooling rate of 30 ° C./sec or 50 ° C./sec. The metallographic structure of the hot-rolled steel sheet was martensite and / or bainite (M + B in Table 2).
II. Within 10 seconds after hot rolling, the mixture was cooled to 400 ° C. or lower at a cooling rate of 50 ° C./sec or 100 ° C./sec. The metallographic structure of the hot-rolled steel sheet was ausformed martensite and / or bainite (AF (M + B) in Table 2).
III. After 20 seconds or more passed after hot rolling, the mixture was cooled to 400 ° C. or lower at a cooling rate of 1 ° C./sec. The metal structure of the hot-rolled steel sheet was a mixed structure of ferrite and pearlite (F + P in Table 2).

Each hot-rolled steel sheet was subjected to austenitizing heat treatment under the conditions shown in Table 2. Specifically, each hot-rolled steel sheet was reheated to a soaking temperature using a high-frequency induction heating furnace or an electric furnace. After heating, the heat was kept uniform. Then, it was immersed in a water tank and cooled to 100 ° C. or lower at a cooling rate of 50 ° C./s. In the hot-rolled steel sheet of Test No. 31, the cooling rate was 2 ° C./s because the cooling was allowed to cool in the air.

After the austenitizing heat treatment, each hot-rolled steel sheet was subjected to a tempering heat treatment under the tempering heat treatment conditions shown in Table 2. The tempering heat treatment was carried out using an electric furnace. Specifically, it was held at a predetermined temperature in an electric furnace, then allowed to cool outside the furnace, and cooled to room temperature.

Each hot-rolled steel sheet was manufactured by the above manufacturing process.

[Evaluation test]
[Metallic structure test]
[Measurement of tempered martensite area ratio]
After the tempering heat treatment, test pieces having a width of 20 mm and a length of 20 mm were collected from each steel material while maintaining the plate thickness. Structures other than tempered martensite and tempered martensite were identified in 1/2 part of the plate thickness of each test piece by the above method. Furthermore, the area ratio (%) of the tempered martensite and the tissues other than the tempered martensite was determined.

[Old austenite particle size measurement]
A test piece was collected from the central part of the wall thickness of the steel material as it was austenitized and heat-treated, and the average particle size of the austenite grains was measured by the above method.

[Measurement of aspect ratio of crystal grains and measurement of specific grain area ratio]
From the SEM image in which the area ratio of tempered martensite was measured, the aspect ratio of the crystal grains was determined by the above method.

[Measurement of dislocation density]
The dislocation density was measured by the method described above.

[Yield strength (YS), yield point elongation, tensile strength (TS) and yield ratio test]
Two JIS14A (diameter 6.35 mm, parallel portion length 35 mm) round bar tensile test pieces were prepared from the center of the thickness of each steel sheet after the austenitizing heat treatment and the tempering heat treatment. The axial direction of the tensile test piece was parallel to the rolling direction of the steel sheet. Using each round bar test piece, a tensile test was carried out at room temperature (25 ° C.) in the air, and a stress-strain curve was measured. As a result, yield strength (YS), yield point elongation, and tensile strength (TS) were obtained. Each value was taken as the average value of the results of the two tensile tests. In this example, the yield point obtained by the tensile test was defined as the yield strength (YS) of each test number. The obtained YS / TS value was used as the yield ratio.

The results obtained are shown in Table 3.

[Sour resistance test]
As a sour resistance test, an SSC resistance test and a DCB test were performed.

[SSC resistance test]
From each steel material, smooth 4-point bending test pieces having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm were collected. A 4-point bending test was performed in a test solution containing hydrogen sulfide using the collected 4-point bending test piece. Specifically, an aqueous solution (Solution A defined by NACE-TM0177) containing 5% by mass of NaCl and 0.5% by mass of glacial acetic acid (CH _{3 COOH) was prepared as a test solution.} The H ₂ S gas partial pressure 1atm was dissolved in the test solution. The applied stress to the 4-point bending test piece during the test was 90.0% of the actual YS by the strain gauge method. The test temperature was room temperature (25 ± 1 ° C.), and the test time was 336 hours.

After the test, the presence or absence of SSC in the test piece was visually observed. The results obtained are shown in Table 4. “SSC” in the “Presence / absence of SSC” column in Table 4 indicates that SSC has occurred, and “No SSC” indicates that SSC has not occurred.

[DCB test]
A DCB test was carried out using each steel sheet. Specifically, the DCB test piece shown in FIG. 3 was collected from the central portion of the thickness of each steel plate. The DCB test piece was sampled so that the longitudinal direction was parallel to the rolling direction.

A 2 mm fatigue crack was formed in the notch of the DCB test piece. Then, with elastic stress applied to the notch, the DCB test piece was subjected to Solution A test solution (5% by mass NaCl and 0.5% by mass glacial acetic acid (CH ₃ COOH)) specified by NACE-TM0177. Soaked in an aqueous solution containing and. 1 atm of hydrogen sulfide gas was dissolved in the test solution. It was kept at 25 ° C. for 336 hours. Then, the DCB test piece was taken out.

The distant elastic stress σ (MPa) was measured in each DCB test piece taken out. Further, in the DCB test piece, the crack extension length a (m), which is the crack extension distance starting from the tip of the initial fatigue crack, was measured. The crack growth length a was visually measured using a caliper. Based on the obtained distant elastic stress σ and the crack growth length a, the fracture toughness value K _ISSC (MPa√m) was determined using the equation (4).
K _ISSC = σ × (πa) ^1/2 (4)

The obtained fracture toughness value K _ISSC is shown in Table 4. When the fracture toughness value K _ISSC value defined above satisfies the above formula (3), it is judged that the sour resistance characteristics are good.
YS x K _ISSC > 20000 (3)

[Test results]
The chemical composition of the hot-rolled steel sheets of Test Nos. 1 to 22 was appropriate. Further, the metallographic structure was tempered martensite in an area ratio of 90.0% or more. Further, the old austenite particle size of the tempered martensite was 5.0 μm or less, and the area ratio of the specific crystal grains was 50% or more. Further, the yield ratio was 0.95 or more, and the yield elongation was 2.5% or more. As a result, the yield strengths of Test Nos. 1 to 22 were 700 MPa or more and had a high yield strength YS. Furthermore, since YS / ρ also ^{exceeded 1.1 × 10 -11} MPa · m ² , it had an appropriate dislocation density. Furthermore, no SSC was generated, and YS × K _ISSC exceeded 20000, showing excellent sour resistance.

On the other hand, in the hot-rolled steel sheet of Test No. 23, the heating rate in the austenitic heat treatment step was less than 50 ° C./s. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 24, the soaking temperature in the austenitizing heat treatment step was too high. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 25, the soaking temperature in the austenitizing heat treatment step was too low. Therefore, the tempered martensite area ratio was less than 90.0%, and the area ratio of the specific crystal grains was also less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 26, the soaking time in the austenitizing heat treatment step exceeded 60 seconds. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 27, the cooling rate in the hot working process was too low. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 28, the temperature in the tempering heat treatment was too high. Therefore, the tempered martensite area ratio was less than 90.0%, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 29, the time for tempering heat treatment was too short. Therefore, the area ratio of the specific crystal grains was also less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 30, the cooling rate in the hot working process was too low. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of Test No. 31, the cooling rate in the austenitic heat treatment step was too low. Therefore, the tempered martensite area ratio was less than 90.0%, the old austenite particle size exceeded 5.0 μm, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield strength YS was less than 700 MPa, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheets of test numbers 32 and 33, the cooling rate in the hot working process was too low. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. Therefore, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheets of test numbers 34 to 36, the heating rate in the austenitizing heat treatment step was too low. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. Therefore, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of test number 37, the C content was too low, and the cooling rate in the hot working step was too low. Therefore, the tempered martensite area ratio was less than 90.0%, the old austenite particle size exceeded 5.0 μm, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield strength YS was less than 700 MPa, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of Test No. 38, the C content was too low, the cooling rate in the hot working step was too low, and the heating rate in the austenitic heat treatment step was too low. Therefore, the tempered martensite area ratio was less than 90.0%, the old austenite particle size exceeded 5.0 μm, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield strength YS was less than 700 MPa, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

The Mn content of the hot-rolled steel sheet of Test No. 39 was too high. Therefore, the tempered martensite area ratio was less than 90.0%, the old austenite particle size exceeded 5.0 μm, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of Test No. 40, the Mn content was too high, and the heating rate in the austenitizing heat treatment step was too low. Therefore, the tempered martensite area ratio was less than 90.0%, the old austenite particle size exceeded 5.0 μm, and the area ratio of the specific crystal grains was also less than 50%. Therefore, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

In the hot-rolled steel sheet of Test No. 41, the heating rate in the austenitic heat treatment step was less than 50 ° C./s. Therefore, the particle size of the old austenite exceeded 5.0 μm, and the area ratio of the specific crystal grains was less than 50%. As a result, the yield point elongation was less than 2.5%, the yield ratio was less than 0.95, and the YS / ρ was 1.1 × 10 ^-11 MPa · m ² or less. As a result, SSC was generated, YS × K _ISSC was less than 20000 MPa ² · m ^1/2 , and the sour resistance was low.

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

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

Claims (7)

By mass%
C: 0.10 to 0.45%,
Si: 1.0% or less,
Mn: 0.1 to 3.0%,
Cr: 0.1 to 3.0%,
Al: 0.001 to 1.0%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.01% or less,
Ni: 0-3.0%,
Cu: 0-3.0%,
Ti: 0-0.3%,
Nb: 0-0.3%,
V: 0-0.5%,
Mo: 0-2.0%,
W: 0-1.0%,
Co: 0-2.0%,
B: 0-0.01%
Ca: 0-0.01%,
Mg: 0 to 0.01% and
Rare earth element: Contains 0-0.01%,
The balance has a chemical composition consisting of Fe and impurities,
The metallographic structure contains tempered martensite with an area ratio of 90.0% or more.
The old austenite particle size of the tempered martensite is 5.0 μm or less.
The tempered martensite contains crystal grains having an area ratio of 50% or more and an aspect ratio of less than 3.0.
The ratio YS / TS relative to the tensile strength TS of the yield strength YS is not less than 0.95 state, and are yield point elongation of 2.5% or more,
The yield strength YS is Ru der more than 700MPa, steel. The steel material according to claim 1.
The chemical composition is
Ni: 0.1 to 3.0% and
Cu: A steel material containing at least one selected from the group consisting of 0.1 to 3.0%. The steel material according to claim 1 or 2.
The chemical composition is
Ti: 0.01-0.3%,
Nb: 0.01-0.3%,
V: 0.01-0.5%,
Mo: 0.05-2.0%,
W: 0.05 to 1.0% and
Co: A steel material containing one or more selected from the group consisting of 0.05 to 2.0%. The steel material according to any one of claims 1 to 3.
The chemical composition is
B: A steel material containing 0.0003 to 0.01%. The steel material according to any one of claims 1 to 4.
The chemical composition is
Ca: 0.0001-0.01%,
Mg: 0.0001 to 0.01% and
Rare earth element: A steel material containing one or more selected from the group consisting of 0.0001 to 0.01%. The steel material according to any one of claims 1 to 5.
A steel material in which the yield strength YS and the dislocation density ρ satisfy the relationship of the formula (1).
YS / ρ> 1.1 × ^10-11 (1) A seamless steel pipe for oil wells made of the steel material according to any one of claims 1 to 6.

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