WO2018043570A1 - 鋼材及び油井用鋼管 - Google Patents
鋼材及び油井用鋼管 Download PDFInfo
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- WO2018043570A1 WO2018043570A1 PCT/JP2017/031180 JP2017031180W WO2018043570A1 WO 2018043570 A1 WO2018043570 A1 WO 2018043570A1 JP 2017031180 W JP2017031180 W JP 2017031180W WO 2018043570 A1 WO2018043570 A1 WO 2018043570A1
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Definitions
- the present invention relates to a steel material and an oil well steel pipe, and more particularly to a steel material and an oil well steel pipe suitable for use in a sour environment.
- oil wells and gas wells By making deep wells in oil wells and gas wells (hereinafter, oil wells and gas wells are simply referred to as “oil wells”), it is required to increase the strength of steel pipes for oil wells.
- steel pipes for oil wells of 80 ksi class yield stress is 80 to 95 ksi, that is, 551 to 655 MPa
- 95 ksi class yield stress is 95 to 110 ksi, that is, 655 to 758 MPa
- yield stress is 110 to 125 ksi, that is, 758 to 862 MPa
- 125 ksi class yield strength is 125 to 140 ksi, that is, 862 to 965 MPa
- Patent Document 1 Steels with improved hydrogen embrittlement resistance have been proposed in JP-A-56-5949 (Patent Document 1) and JP-A-57-35622 (Patent Document 2).
- the steel disclosed in these documents enhances the hydrogen embrittlement resistance (SSC resistance, delayed fracture resistance) by containing Co.
- the high-tensile steel disclosed in Patent Document 1 is C: 0.05 to 0.50%, Si: 0.10 to 0.28%, Mn: 0.10 to 2.0%, Co: 0.05 to 1.50%, Al: 0.01 to 0.10%, the balance is made by quenching and tempering steel having a chemical composition consisting of Fe and inevitable impurities, 60 kg / mm 2 or more Has proof stress.
- the high-strength oil well steel disclosed in Patent Document 2 is C: 0.27 to 0.50%, Si: 0.08 to 0.30%, Mn: 0.90 to 1.30%, Cr: 0 0.5 to 0.9%, Ni: 0.03% or less, V: 0.04 to 0.11%, Nb: 0.01 to 0.10%, Mo: 0.60 to 0.80%, Al : Steel containing 0.1% or less and Co: 3% or less, the balance being Fe and unavoidable impurities, P having a chemical composition of P: 0.005% or less, S: 0.003% or less Is quenched at 880 to 980 ° C. and then tempered at 650 to 700 ° C.
- Patent Document 3 proposes an oil well steel pipe with an increased C content in order to obtain high strength.
- the oil well steel pipe disclosed in Patent Document 3 is mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Al : 0.005 to 0.10%, Cr + Mo: 1.5 to 3.0%, provided that Mo is 0.5% or more, V: 0.05 to 0.3%, the balance is Fe and impurities, P in the impurity has a chemical composition containing 0.025% or less, S is 0.01% or less, B is 0.0010% or less, and O (oxygen) is 0.01% or less.
- a low alloy steel having a metal structure is manufactured by oil-cooling or austempering and then tempering.
- Patent Document 3 describes that the above manufacturing method can suppress quench cracking that is likely to occur during quenching of high-C low alloy steel, and can provide oil well steel or oil well steel pipe having excellent SSC resistance. Yes.
- the evaluation of the SSC resistance of conventional steel materials was 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 TM0177. Since these tests use smooth test pieces, the propagation stop characteristics of SSC are not considered. For this reason, even in a steel material evaluated as having excellent SSC resistance in these tests, SSC may occur due to propagation of a latent crack in the steel.
- Patent Documents 1 to 3 the fracture toughness value in the DCB test is not examined.
- JP-A-56-5949 Japanese Patent Laid-Open No. 57-35622 JP 2006-265657 A
- An object of the present invention is to provide a steel material and oil well steel pipe having high strength of 862 MPa or more and excellent SSC resistance.
- the steel material according to the present invention is, in mass%, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, P: 0.050% or less.
- the prior austenite grain size of the microstructure is less than 5 ⁇ m.
- the block diameter of the microstructure is less than 2 ⁇ m.
- the microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite.
- Effective B B-11 (N-Ti / 3.4) / 14 (3)
- ⁇ in the formula (1) is 0.250 when the effective B (% by mass) defined by the formula (3) is 0.0003% or more, and the effective B is less than 0.0003%. Is 0.
- the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3).
- the steel material and oil well steel pipe according to the present invention have high strength and excellent SSC resistance.
- FIG. 1 is a diagram showing the relationship between the yield strength and the fracture toughness value K ISSC of each test number steel.
- FIG. 2A is a side view and a cross-sectional view of a DCB test piece used in the DCB test of the example. The numerical value in FIG. 2A shows the dimension (a unit is mm) of a corresponding site
- FIG. 2B is a perspective view of a wedge used in the DCB test of the example. The numerical value in FIG. 2B shows the dimension (a unit is mm) of a corresponding site
- Co enhances SSC resistance.
- C 0.15 to 0.45%
- Si 0.10 to 1.0%
- Mn 0.10 to 0.8%
- P 0.050% or less
- S 0 0.010% or less
- Al 0.01 to 0.1%
- N 0.010% or less
- Cr 0.1 to 2.5%
- Mo 0.35 to 3.0%
- Co 0.0.
- Co unlike other alloy elements (C, Mn, Cr, V, Cu, Ni, etc.), raises the Ms point and lowers the hardenability of the steel. Therefore, if the Co content is higher than the C, Mn, Cr, V, Cu, and Ni contents, the hardenability decreases. In this case, if a manufacturing method similar to steel containing no Co is used, the microstructure becomes not only tempered martensite and tempered bainite but also a heterogeneous structure containing residual austenite. Therefore, the SSC resistance decreases due to the microstructure. Then, as a result of examining the relationship between Co and other alloy elements in the SSC resistance, the present inventors obtained the following knowledge.
- F1 C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 ⁇ Co / 6 + ⁇ .
- F1 is an index of hardenability.
- C, Mn, Cr, Mo, V, Cu and a predetermined amount of effective B (solid solution B) enhance the hardenability of the steel.
- B solid solution B
- Co lowers the hardenability of steel. If F1 is 0.70 or more, even if it contains Co, excellent hardenability can be obtained, and the volume ratio of tempered martensite and tempered bainite in the microstructure can be increased.
- the microstructure When the microstructure is substantially composed of tempered martensite and tempered bainite, excellent SSC resistance can be obtained. On the other hand, when the microstructure is a non-uniform structure composed of tempered martensite and tempered bainite and other phases (residual austenite or the like), the SSC resistance is lowered. When F1 satisfies the formula (1), the volume ratio of tempered martensite and tempered bainite in the microstructure is 90% or more in total, and excellent SSC resistance is obtained.
- F2 (3C + Mo + 3Co) / (3Mn + Cr).
- F2 is an index of SSC resistance.
- F2 is 1.0 or more, that is, when the ratio of the SSC resistance improving elements (C, Mo and Co) content to the Mn and Cr content is large, excellent SSC resistance can be obtained.
- the present inventors examined a method for further improving the SSC resistance in addition to containing Co. Therefore, the present inventors focused on the metal structure, and thought that if the old ⁇ grain size and block diameter were refined, the boundary increased and the resistance to fracture increased, so that the SSC resistance increased. The present inventors have further investigated and investigated the relationship between the old ⁇ particle size and block diameter and the strength and SSC resistance.
- prior austenite particle size (hereinafter also referred to as prior ⁇ particle size) was measured for each steel based on the test method described later.
- the prior ⁇ particle size was 16 ⁇ m under Test Condition I, 9.8 ⁇ m under Test Condition II, 2.6 ⁇ m under Test Condition III, and 4.1 or 4.2 ⁇ m under Test Condition IV, respectively.
- Each steel was tempered under the conditions shown in Table 3.
- the block diameter was measured for each steel after tempering based on the test method described later.
- a test piece was prepared from each steel, a yield strength test and a DCB test were performed based on the test method described later, and a yield strength and a fracture toughness value K ISSC of each steel were obtained.
- FIG. 1 is a diagram showing the relationship between the yield strength and the fracture toughness value K ISSC of each test number steel.
- ⁇ , ⁇ , ⁇ and ⁇ indicate the results of test condition I, test condition II, test condition III and test condition IV in Table 2, respectively.
- test numbers 1 to 24 if the old ⁇ grains are refined, the structure after quenching and tempering is refined. That is, the block becomes finer. In this case, the SSC resistance is increased.
- the steel transforms from austenite to martensite and bainite during quenching.
- austenite grains are fine
- the martensite block and the bainite block that are transformed therefrom are also refined.
- a martensite block is a sub-organization of martensite.
- a bainite block is a substructure of bainite.
- a boundary between martensite grains and bainite grains having an orientation difference of 15 ° or more is defined as a block boundary.
- An area surrounded by block boundaries is defined as one block.
- the SSC resistance may be low. More specifically, even if the old ⁇ grains are as fine as less than 5 ⁇ m, when the block diameter is 2 ⁇ m or more, the SSC resistance is low.
- the old ⁇ grains of the microstructure are fine and the block diameter is also fine, high SSC resistance can be obtained even if the strength of the steel is increased. Specifically, if the average crystal grain size of the old ⁇ grains in the microstructure is less than 5 ⁇ m and the average block diameter of the blocks is less than 2 ⁇ m, both the strength of the steel and the SSC resistance can be achieved.
- Co increases the block diameter. Therefore, if Co is contained, the block diameter may be coarsened even if the old ⁇ grains are fine. The reason for this is not clear, but it is thought that Co increases the Ms point and lowers the hardenability, so that the block diameter is increased.
- the present inventors further examined a method capable of suppressing the coarsening of the block diameter even when the above chemical composition contains a specific amount of Co. As a result, the following knowledge was obtained.
- the average crystal grain size of the old ⁇ grains in the microstructure can be made less than 5 ⁇ m.
- the average crystal grain size of the old ⁇ grains in the microstructure is less than 5 ⁇ m, the block diameter becomes as fine as less than 2 ⁇ m.
- the block diameter when Co is contained, Co coarsens the block diameter as described above. Therefore, even if the average crystal grain size of the old ⁇ grains in the microstructure is less than 5 ⁇ m, the block diameter may be 2 ⁇ m or more. In this case, the SSC resistance is low.
- the cooling to the Ms point is rapid cooling. More specifically, if the cooling rate of 500 to 200 ° C. is set to 5 ° C./s or more, the coarsening of crystal grains in the quenching process can be sufficiently suppressed, and the block diameter can be made less than 2 ⁇ m. The inventors have found.
- the steel material according to the present invention completed based on the above knowledge is, in mass%, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%. , P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0 .35 to 3.0%, Co: 0.05 to 2.0%, Ti: 0.003 to 0.040%, Nb: 0.003 to 0.050%, Cu: 0.01 to 0.50 %, Ni: 0.01 to 0.50%, V: 0 to 0.5%, B: 0 to 0.003%, W: 0 to 1.0%, Ca: 0 to 0.004%, Mg : 0 to 0.004% and rare earth element: 0 to 0.004%, the balance is Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2).
- the prior austenite grain size of the microstructure is less than 5 ⁇ m.
- the block diameter of the microstructure is less than 2 ⁇ m.
- the microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite.
- Effective B B-11 (N-Ti / 3.4) / 14 (3)
- ⁇ in the formula (1) is 0.250 when the effective B (% by mass) defined by the formula (3) is 0.0003% or more, and the effective B is less than 0.0003%. Is 0.
- the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3).
- the chemical composition may contain V: 0.015 to 0.5%.
- the chemical composition may contain one or more selected from the group consisting of B: 0.0003 to 0.003% and W: 0.05 to 1.0%.
- the chemical composition is selected from the group consisting of Ca: 0.0003 to 0.004%, Mg: 0.0003 to 0.004%, and rare earth elements: 0.0003 to 0.004%, or You may contain 2 or more types.
- the oil well steel pipe according to the present invention exhibits excellent strength and SSC resistance even if the wall thickness is 15 mm or more, if it has the above chemical composition and microstructure.
- C 0.15-0.45% Carbon (C) increases the hardenability and increases the strength of the steel. C further promotes the spheroidization of carbides during tempering during the manufacturing process and enhances SSC resistance. C further combines with Mo or V to form carbides and increases temper softening resistance. If the carbide is dispersed, the strength of the steel 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 is lowered and fire cracks are likely to occur. Therefore, the C content is 0.15 to 0.45%. The minimum with preferable C content is 0.20%, More preferably, it is 0.25%. The upper limit with preferable C content is 0.35%, More preferably, it is 0.30%.
- Si 0.10 to 1.0% 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, excessive austenite is generated and the SSC resistance is lowered. Therefore, the Si content is 0.10 to 1.0%.
- the minimum of preferable Si content is 0.15%, More preferably, it is 0.20%.
- the upper limit of the preferable Si content is 0.85%, more preferably 0.50%.
- Mn 0.10 to 0.8%
- Manganese (Mn) deoxidizes steel. Mn further increases the hardenability and increases the strength of the steel. 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 grain boundaries together with impurities such as P and S. In this case, the SSC resistance of the steel decreases. Therefore, the Mn content is 0.10 to 0.8%.
- the minimum of preferable Mn content is 0.25%, More preferably, it is 0.28%.
- the upper limit of the preferable Mn content is 0.65%.
- P 0.050% 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 0.050% or less. A preferable P content is 0.020% or less. The P content is preferably as low as possible.
- S 0.010% 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 0.010% or less. A preferable S content is 0.005% or less, and more preferably 0.003% or less. The S content is preferably as low as possible.
- Al 0.01 to 0.1%
- Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained and the SSC resistance of the steel decreases. On the other hand, if the Al content is too high, coarse oxide inclusions are generated and the SSC resistance of the steel is lowered. Therefore, the Al content is 0.01 to 0.1%.
- the minimum with preferable Al content is 0.015%, More preferably, it is 0.020%.
- the upper limit with preferable Al content is 0.08%, More preferably, it is 0.05%.
- Al content means “acid-soluble Al”, that is, the content of “sol. Al”.
- N 0.010% or less Nitrogen (N) is inevitably contained. N forms coarse nitrides and lowers the SSC resistance of the steel. Therefore, the N content is 0.010% or less. A preferable N content is 0.005% or less, and more preferably 0.004% or less. The N content is preferably as low as possible. However, in the case where a slight amount of Ti is contained and the aim is to refine crystal grains by precipitation of fine nitride, it is preferable to contain N in an amount of 0.002% or more.
- Chromium (Cr) increases the hardenability of the steel and increases the strength of the 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 SSC resistance of the steel decreases. Therefore, the Cr content is 0.1 to 2.5%.
- the minimum with preferable Cr content is 0.25%, More preferably, it is 0.30%.
- the upper limit with preferable Cr content is 1.5%, More preferably, it is 1.3%.
- Mo 0.35 to 3.0% Molybdenum (Mo) increases the hardenability of the steel. Mo further generates fine carbides, increases the temper softening resistance of the steel, and increases the SSC resistance in a high-pressure H 2 S environment. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, the above effect is saturated. Therefore, the Mo content is 0.35 to 3.0%.
- the minimum with preferable Mo content is 0.40%, More preferably, it is 0.50%.
- the upper limit with preferable Mo content is 2.0%, More preferably, it is 1.75%.
- Co 0.05-2.0% Cobalt (Co) increases the SSC resistance of steel in a high H 2 S environment. The reason is not clear, but the following reasons are possible. Co is concentrated on the surface of steel in a sour environment, and suppresses the penetration of hydrogen into the steel. This increases the SSC resistance of the steel. If the Co content is too low, this effect cannot be obtained. On the other hand, if the Co content is too high, the hardenability of the steel decreases and the strength of the steel decreases. If the Co content is too high, the block diameter is further coarsened and the SSC resistance is lowered. Therefore, the Co content is 0.05 to 2.0%. The minimum with preferable Co content is more than 0.05%, More preferably, it is 0.10%, More preferably, it is 0.25%. The upper limit with preferable Co content is 1.5%, More preferably, it is 1.25%.
- Ti 0.003-0.040% Titanium (Ti) forms a nitride and refines crystal grains by a pinning effect. This increases the strength of the steel. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, the Ti nitride becomes coarse and the SSC resistance of the steel decreases. Therefore, the Ti content is 0.003 to 0.040%. A preferable lower limit of the Ti content is 0.005%. The upper limit with preferable Ti content is 0.020%, More preferably, it is 0.015%.
- Niobium (Nb) combines with C and / or N to form a carbide, nitride or carbonitride (hereinafter referred to as carbonitride). These carbonitrides etc. refine crystal grains and increase the strength of steel. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, coarse precipitates are generated and the SSC resistance of the steel is lowered. Therefore, the Nb content is 0.003 to 0.050%. The minimum with preferable Nb content is 0.007%, More preferably, it is 0.010%. The upper limit with preferable Nb content is 0.025%.
- Cu 0.01 to 0.50% Copper (Cu) increases the hardenability of the steel and increases the strength of the steel. If the Cu content is too low, these effects cannot be obtained. On the other hand, if the Cu content is too high, hydrogen is trapped, and the SSC resistance decreases. Therefore, the Cu content is 0.01 to 0.50%.
- the minimum with preferable Cu content is 0.02%, More preferably, it is 0.05%.
- the upper limit with preferable Cu content is 0.35%, More preferably, it is 0.25%.
- Ni 0.01 to 0.50%
- Nickel (Ni) increases the hardenability of the steel and increases the strength of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, local corrosion is promoted and the SSC resistance is lowered. Therefore, the Ni content is 0.01 to 0.50%.
- the minimum with preferable Ni content is 0.02%, More preferably, it is 0.05%.
- the upper limit with preferable Ni content is 0.45%, More preferably, it is 0.25%.
- the balance of the chemical composition of the steel material according to the present invention consists of Fe and impurities.
- the impurities are mixed from ore as a raw material, scrap, or production environment when industrially producing steel materials, and are allowed within a range that does not adversely affect the steel materials of the present invention. Means something.
- the chemical composition of the steel material described above may further contain V instead of a part of Fe.
- V 0 to 0.5%
- Vanadium (V) is an optional element and may not be contained. When contained, V forms carbonitride and the like, refines the crystal grains, and increases the strength of the steel. However, if the V content is too high, the toughness of the steel decreases. Therefore, the V content is 0 to 0.5%.
- the minimum with preferable V content is 0.015%, More preferably, it is 0.030%.
- the upper limit with preferable V content is 0.30%, More preferably, it is 0.20%.
- the chemical composition of the steel material described above may further include one or more selected from the group consisting of B and W instead of part of Fe.
- B 0 to 0.003%
- 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 the strength. However, if the B content is too high, coarse nitrides are produced and the SSC resistance of the steel is reduced. Therefore, the B content is 0 to 0.003%.
- the minimum with preferable B content is 0.0003%, More preferably, it is 0.0007%.
- the upper limit with preferable B content is 0.0015%, More preferably, it is 0.0012%.
- W 0 to 1.0%
- Tungsten (W) is an optional element and may not be contained. When contained, W dissolves in the steel to increase the hardenability of the steel and increase the strength. However, if the W content is too high, coarse carbides are generated and the SSC resistance of the steel is reduced. Therefore, the W content is 0 to 1.0%.
- the minimum with preferable W content is 0.05%, More preferably, it is 0.10%.
- the upper limit with preferable W content is 0.75%, More preferably, it is 0.5%.
- the chemical composition of the steel material described above may further include one or more selected from the group consisting of Ca, Mg, and rare earth elements instead of part of Fe. Any of these elements is an arbitrary element, and improves the SSC resistance of the steel by improving the shape of the sulfide.
- Ca 0 to 0.004%
- Calcium (Ca) is an optional element and may not be contained.
- Ca combines with S in the steel. Thereby, S in steel is made harmless as a sulfide, and SSC resistance of steel is improved.
- the Ca content is 0 to 0.004%.
- the minimum with preferable Ca content is 0.0003%, More preferably, it is 0.0006%.
- the upper limit with preferable Ca content is 0.0025%, More preferably, it is 0.0020%.
- Mg 0 to 0.004%
- Magnesium (Mg) is an optional element and may not be contained. When contained, Mg renders S in the steel harmless as a sulfide and improves the SSC resistance of the steel. However, if the Mg content is too high, the oxide in the steel becomes coarse and the SSC resistance of the steel decreases. Therefore, the Mg content is 0 to 0.004%.
- the minimum with preferable Mg content is 0.0003%, More preferably, it is 0.0006%, More preferably, it is 0.0010%.
- the upper limit with preferable Mg content is 0.0025%, More preferably, it is 0.0020%.
- the rare earth element is an optional element and may not be contained.
- REM refines sulfides in the steel to increase the SSC resistance of the steel.
- REM further combines with P in the steel to suppress P segregation at the grain boundaries. For this reason, a decrease in the SSC resistance of the steel due to the segregation of P is suppressed.
- the REM content is 0 to 0.004%.
- the minimum with preferable REM content is 0.0003%, More preferably, it is 0.0006%, More preferably, it is 0.0010%.
- the upper limit with preferable REM content is 0.0025%, More preferably, it is 0.0020%.
- REM means yttrium (Y) having an atomic number of 39, lanthanum (La) having an atomic number of 57, which is a lanthanoid, lutetium (Lu) having an atomic number of 71, and an atomic number having an atomic number of 89.
- Y yttrium
- La lanthanum
- Lu lutetium
- Lu lutetium
- REM content in this specification is the total content of these elements.
- F1 C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 ⁇ Co / 6 + ⁇ .
- F1 is an index of hardenability. If F1 is 0.70 or more, even if it contains Co, excellent hardenability is obtained, and the total volume ratio of tempered martensite and tempered bainite in the microstructure is 90% or more. As a result, excellent SSC resistance can be obtained.
- the minimum with preferable F1 is 0.75, More preferably, it is 0.85, More preferably, it is 1.0.
- a preferable upper limit of F1 is 1.5.
- F2 (3C + Mo + 3Co) / (3Mn + Cr).
- F2 is an index of SSC resistance.
- the content of SSC resistance improving elements C, Mo and Co
- the content of Mn and Cr distributed to hardenability, but excessive content decreases SSC resistance
- the ratio to the element obtained is increased.
- excellent SSC resistance in a high-pressure H 2 S environment can be obtained.
- the preferable upper limit of F2 is 3.0.
- the old ⁇ grain size of the microstructure of the steel material of the present invention is less than 5 ⁇ m. Furthermore, the block diameter of the microstructure is less than 2 ⁇ m. As a result, it is possible to achieve both high strength with a yield strength of 862 MPa or more and SSC resistance.
- a lath group of martensite sub-organizations with almost the same orientation is called a martensite block.
- a bainite lath group having a bainite substructure and substantially the same orientation is called a bainite block.
- the martensite block and the bainite block are collectively referred to as a block.
- a boundary between martensite grains and bainite grains having an orientation difference of 15 ° or more is defined as a block boundary.
- An area surrounded by block boundaries is defined as one block.
- the block is made finer, the hardness of martensite and bainite can be increased. If the hardness of martensite and bainite is increased, the strength of martensite and bainite can be increased. If the block is miniaturized, the SSC resistance can be further improved. As a result, it is possible to achieve both high strength with a yield strength of 862 MPa or more and SSC resistance.
- a preferable lower limit of the block diameter is 0.2 ⁇ m.
- the heating rate during quenching is set to 10 ° C./s or more.
- Co increases the Ms point. Therefore, if Co is contained, the block diameter may be coarsened even if the old ⁇ grains are fine.
- the cooling rate of 500 to 200 ° C. is set to 5 ° C./s or more in the quenching step.
- the block diameter can be made less than 2 ⁇ m.
- the prior ⁇ particle size is determined by the following method. Collect specimens from as-quenched steel. In the case of a steel pipe, the cross section is a plane perpendicular to the axis, and a specimen is taken from the center of the wall thickness. After mirror-polishing the test piece, the old ⁇ grain boundary is exposed using a saturated aqueous solution of picric acid. In the test piece, the old ⁇ grain size (average crystal grain size of the old ⁇ grain) is measured with an arbitrary 10 fields of view. The measurement is performed with a cutting method shown in JIS G0551 (2005) while observing with a 1000 ⁇ optical microscope. Calculate the old ⁇ grain size number in each field of view.
- the average of the 10 old ⁇ grain size numbers calculated (average old ⁇ grain size number) is obtained. Based on the average old ⁇ grain size number, the average area of each crystal grain is calculated. The equivalent circle diameter is calculated from the average area, and the obtained equivalent circle diameter is defined as the old ⁇ particle diameter.
- the block diameter ( ⁇ m) is obtained from a crystal orientation map obtained by the FESEM-EBSP method without distinguishing between martensite blocks and bainite blocks.
- EBSP measurement is performed with a 50 ⁇ m ⁇ 50 ⁇ m visual field at a pitch of 0.1 ⁇ m.
- the orientation of ⁇ Fe is identified from the Kikuchi line pattern collected by EBSP measurement.
- a crystal orientation map is obtained from the orientation of ⁇ Fe. From the crystal orientation diagram, a region surrounded by an orientation difference of 15 ° or more from an adjacent crystal is identified to obtain a crystal orientation map. A region surrounded by an orientation difference of 15 ° or more is defined as one grain of the block.
- the equivalent circle diameter is obtained from the area of each block. The average value of the equivalent circle diameters in the field of view is calculated and used as the block diameter.
- the steel material of the present invention contains Co. Co raises the Ms point. Therefore, the microstructure of the steel material of the present invention mainly comprises tempered martensite and tempered bainite. More specifically, the microstructure consists of 90% by volume or more of tempered martensite and tempered bainite in total. The balance of the microstructure is, for example, retained austenite. If the microstructure contains tempered martensite and tempered bainite of 90% by volume or more in total, the SSC resistance is enhanced. Preferably, the microstructure consists of a tempered martensite single phase. Preferably, the content of tempered bainite is 2 to 10% by volume.
- the volume ratio of tempered martensite and tempered bainite in the microstructure correlates with the difference between the maximum value and the minimum value of Rockwell hardness (HRC) in the steel material after quenching and tempering.
- HRC Rockwell hardness
- the maximum value of Rockwell hardness after quenching and tempering is defined as HRCmax.
- the minimum value of Rockwell hardness after quenching and tempering is defined as HRCmin.
- the difference between HRCmax and HRCmin is defined as ⁇ HRC.
- ⁇ HRC HRCmax ⁇ HRCmin If ⁇ HRC is less than 2.0, the volume ratio of tempered martensite and tempered bainite in the microstructure of the steel material is considered to be 90% or more in total.
- the Rockwell hardness at the steel surface is HRCmax
- the Rockwell hardness at the steel thickness central portion (hereinafter referred to as the steel central portion) is HRCmin.
- the reason for this is as follows.
- the cooling rate during quenching cooling is fast on the steel surface and slows at the center of the steel. Therefore, in the as-quenched steel material, the difference in the volume ratio of martensite and bainite may be large between the steel material surface and the steel material central part. Since the volume ratio of martensite and bainite in the microstructure correlates with the Rockwell hardness, in this case, the difference in the as-quenched Rockwell hardness increases between the steel surface and the steel center.
- the hardness decreases on both the steel surface and the steel center, and the difference in Rockwell hardness between the steel surface and the steel center decreases, but the steel surface and the steel center The difference in Rockwell hardness between the parts remains. Therefore, the Rockwell hardness at the steel surface is HRCmax, and the Rockwell hardness at the steel center is HRCmin. If ⁇ HRC is 2.0 or more, the hardness of the steel material center is too low. If ⁇ HRC is less than 2.0, sufficient hardness is obtained even in the central part of the steel material. In this case, the volume ratio of tempered martensite and tempered bainite in the central part of the steel material is 90% or more in total. I reckon.
- ⁇ HRC is measured by the following method. 2.0 mm depth position from the surface (outer surface in the case of a steel pipe) of the steel material after quenching and tempering treatment, 2.0 mm depth position from the back surface (inner surface in the case of a steel pipe) of the steel material, and the thickness direction center of the steel material At each of the positions, a Rockwell hardness test (C scale) in accordance with JIS Z2245 (2011) is performed at any three locations to determine Rockwell hardness (HRC). If the maximum value of the obtained hardness is HRCmax, the minimum value is HRCmin, and ⁇ HRC is less than 2.0, it is determined that the volume ratio of tempered martensite and tempered bainite is 90% or more in total. If ⁇ HRC is 2.0 or more, it is determined that the volume ratio of tempered martensite and tempered bainite is less than 90% in total at the position of HRCmin.
- the shape of the steel material is not particularly limited.
- the steel material is, for example, a steel pipe or a steel plate.
- the preferred wall thickness is 9 to 60 mm.
- the present invention is particularly suitable for use as a thick-walled oil well steel pipe. More specifically, even if the steel material according to the present invention is a steel pipe for oil wells having a thickness of 15 mm or more, and further 20 mm or more, high strength and excellent SSC resistance are exhibited.
- the yield strength of the steel material of this embodiment is 862 MPa or more.
- the yield strength as used in this specification means a lower yield point (MPa).
- the yield strength of the steel material of this embodiment is 125 ksi class.
- the steel material of this embodiment has excellent SSC resistance by using the above-described chemical composition and microstructure even with such high strength.
- the manufacturing method of steel pipes for oil wells includes the process of preparing the raw material (preparation process), the process of hot working the raw material to manufacture the raw pipe (hot working process), and quenching and tempering the raw pipe And a step (quenching step and tempering step) for making the oil well steel pipe.
- preparation process the process of preparing the raw material
- hot working process the process of hot working the raw material to manufacture the raw pipe
- quenching and tempering the raw pipe quenching and tempering step
- fills Formula (1) and Formula (2) is manufactured.
- the material is manufactured using molten steel.
- a slab slab, bloom, or billet
- the billet may be produced by rolling the slab, bloom or ingot into pieces.
- the material (slab, bloom, or billet) is manufactured by the above process.
- the raw material is manufactured by hot working the prepared material.
- the billet is heated in a heating furnace.
- the billet extracted from the heating furnace is hot-worked to produce a raw pipe (seamless steel pipe).
- the Mannesmann method is performed as hot working to manufacture a raw tube.
- the round billet is pierced and rolled by a piercing machine.
- the round billet that has been pierced and rolled is further hot-rolled by a mandrel mill, a reducer, a sizing mill, or the like into a blank tube.
- the blank tube may be manufactured from the billet by other hot working methods.
- the raw pipe may be manufactured by forging. Through the above steps, a blank tube having a thickness of 9 to 60 mm is manufactured.
- the raw tube manufactured by hot working may be air-cooled (As-Rolled). Steel pipes manufactured by hot working can also be directly quenched after hot pipe making without cooling to room temperature, or after being reheated after hot pipe making and quenching. Good. However, when quenching directly after quenching or after supplementary heating, it is preferable to stop cooling during quenching or to perform slow cooling for the purpose of suppressing quench cracking.
- the purpose of removing residual stress is to perform stress relief annealing after quenching and before heat treatment in the next process. It is preferable to perform processing (SR processing).
- SR processing processing
- Quenching is performed on the blank after hot working.
- the old ⁇ particle diameter is adjusted to less than 5 ⁇ m and the block diameter is adjusted to less than 2 ⁇ m according to the quenching conditions.
- Quenching is performed by, for example, a high frequency induction heating furnace.
- the heating rate to the ultimate temperature and the ultimate temperature are controlled.
- a preferred heating start temperature is room temperature. In this case, it is easy to make finer.
- a preferable heating rate is 10 ° C./s or more, and a preferable reached temperature is 850 to 920 ° C. If the ultimate temperature is 1000 ° C. or lower, the coarsening of the old ⁇ particle size can be suppressed.
- the ultimate temperature is preferably maintained for 5 to 180 seconds. If the holding time is 180 seconds or less, the coarsening of the old ⁇ particle size can be suppressed. If other conditions are satisfied and the heating rate is 10 ° C./s or more, the old ⁇ particle size can be made less than 5 ⁇ m.
- forced cooling at a cooling rate of 5 ° C./s or more is started before the temperature at the position where the cooling rate is the smallest among the positions in the thickness direction becomes Ar 3 points or less. In this case, it is easy to further increase the yield strength.
- the cooling rate of 500 to 200 ° C. is 5 ° C./s or more.
- a block diameter can be made into less than 2 micrometers.
- the heating rate at the time of quenching is 10 ° C./s or more
- the old ⁇ particle size can be adjusted to less than 5 ⁇ m, but the cooling rate of 500 to 200 ° C. is less than 5 ° C./s.
- the block diameter is 2 ⁇ m or more.
- a more preferable lower limit of the cooling rate of 500 to 200 ° C. is 10 ° C./s.
- the cooling rate of 500 to 200 ° C. can be set to 5 ° C./s or more, for example, by setting the water density of spray water to 0.15 m 3 / min ⁇ m 2 or more in spray cooling.
- Quenching treatment may be performed multiple times.
- SR treatment can prevent the occurrence of cracks after quenching.
- a preferable processing temperature is 600 degrees C or less. In this case, coarsening of austenite can be suppressed.
- the cooling rate of 500 to 200 ° C. may be set to 5 ° C./s or more only during the last quenching. Thereby, a block diameter can be made into less than 2 micrometers.
- Quenching may be performed in a gas-fired furnace.
- the preferable heating rate is 1 ° C./s or more, and the preferable temperature is 850 ° C. to 1000 ° C. At the ultimate temperature, it is preferably held for 10 minutes or more.
- the cooling rate of 500 to 200 ° C. may be set to 5 ° C./s or more only during the last quenching. Thereby, a block diameter can be made into less than 2 micrometers.
- Tempeering process A tempering process is implemented after implementing the above-mentioned hardening process.
- the yield strength of the steel material is adjusted to 862 to 965 MPa by tempering treatment.
- a preferred lower limit of the tempering temperature is 650 ° C.
- the upper limit with preferable tempering temperature is 730 degreeC.
- a preferred holding time at the tempering temperature is 5 to 180 minutes.
- the manufacturing method described above a method for manufacturing a steel pipe has been described as an example.
- the steel material of this invention is a steel plate and another shape
- the manufacturing method of a steel plate is similarly equipped with a preparation process, a hot working process, a quenching process, and a tempering process.
- An ingot was manufactured using the above molten steel. Referring to Table 6, in Test No. 1 to Test No. 20 and Test No. 26 to Test No. 28, the ingot was hot-rolled to produce a steel plate having a plate thickness of 15 mm. The quenching conditions for test number 20 were the same for all three times.
- each steel plate of each steel after hot rolling was allowed to cool to bring the steel plate temperature to room temperature.
- Each steel plate was reheated under the quenching conditions shown in Table 6 and then quenched at a cooling rate shown in Table 6 at 500 to 200 ° C.
- the holding time at the ultimate temperature was 5 seconds.
- the holding time at the ultimate temperature was 10 minutes.
- each steel plate was tempered at the tempering temperatures shown in Table 6.
- the tempering temperature was adjusted to be an API standard 125 ksi class. In any steel plate, the holding time at the tempering temperature was 60 minutes.
- Each steel plate was manufactured by the above manufacturing process.
- Test No. 21 to Test No. 25 were tempered twice. Specifically, the ingot was rolled to a thickness of 35 mm at a finish of 1000 ° C., then cooled with water and subjected to first quenching, and tempered at the same temperature as the hot rolling finish temperature (described in Table 6) of the next step. Further, hot rolling was performed at the hot rolling finishing temperature shown in Table 6 to produce a steel plate having a plate thickness of 15 mm. This refined the structure.
- the subsequent steps, that is, the second quenching step and after, were the same as test numbers 1 to 20 and test numbers 26 to 28.
- Yield Strength (YS) and Tensile Strength (TS) Test A round bar tensile test piece having a diameter of 6.35 mm and a parallel part length of 35 mm was prepared from the center of the thickness of each steel plate after the quenching and tempering treatments. The axial direction of the tensile specimen was parallel to the rolling direction of the steel plate. Using each round bar test piece, a tensile test was carried out at normal temperature (25 ° C.) and in the atmosphere to obtain a yield strength (YS) (MPa) and a tensile strength (TS) at each position. In this example, the yield point obtained by the tensile test was defined as the yield strength (YS) of each test number.
- a Rockwell hardness (HRC) test based on JIS Z2245 (2011) was performed on each steel plate after the final quenching and tempering treatment. Specifically, in each of the 2.0 mm depth position from the surface of the steel sheet, the 2.0 mm depth position from the back surface of the steel sheet, and the central position in the thickness direction of the steel sheet, three arbitrary Rockwell hardnesses ( HRC). The difference ⁇ HRC between the maximum value and the minimum value of the Rockwell hardness at 9 points was less than 2.0 except for test number 14. Therefore, in the microstructure of this embodiment, the volume ratio of tempered martensite and tempered bainite was considered to be 90% or more in total even at the position of HRCmin.
- Block diameter measurement test Test pieces were sampled from the plate thickness center of each steel plate after the above quenching and tempering treatments, and the average block diameter of the blocks was measured by the method described above.
- DCB test Each steel plate was subjected to a DCB test in accordance with NACE TM0177-96 Method D to evaluate SSC resistance. Specifically, three DCB test pieces shown in FIG. 2A were collected from the thickness center of each steel plate. The DCB specimen was collected so that the longitudinal direction was parallel to the rolling direction. Further, the wedge shown in FIG. 2B was produced from the steel plate. The wedge thickness t was 2.92 mm.
- a wedge was driven between the arms of the DCB test piece. Thereafter, the DCB test piece into which the wedge was driven was sealed in an autoclave. A solution adjusted to pH 3.5 by mixing degassed 5% saline, acetic acid, and Na acetate was poured into the autoclave so that a gaseous portion remained in the autoclave. Thereafter, 10 atm hydrogen sulfide gas was pressurized and sealed in the autoclave, the liquid phase was stirred, and the high-pressure hydrogen sulfide gas was saturated with the solution.
- the solution was kept at 25 ° C. for 336 hours while stirring. Thereafter, the autoclave was decompressed and the DCB test piece was taken out.
- h is the height (mm) of each arm of the DCB specimen
- B is the thickness (mm) of the DCB specimen
- Bn is the web thickness (mm) of the DCB specimen. It is.
- Fracture toughness values K ISSC (MPa ⁇ m) of three DCB test pieces were determined for each test number.
- the average of the fracture toughness values of the three DCB specimens was defined as the fracture toughness value K ISSC (MPa ⁇ m) of the steel plate.
- the obtained fracture toughness value K ISSC is shown in Table 7.
- interval of the arm at the time of driving a wedge before being immersed in a test tank influences a KISSC value. Therefore, the distance between the arms was measured with a micrometer, and it was confirmed that it was within the API standard range.
- the chemical compositions of the steel plates of Test No. 1 to Test No. 11 and Test No. 20 were appropriate and satisfied the formulas (1) and (2). Furthermore, since ⁇ HRC was less than 2.0, the microstructure determination was acceptable, and the total microstructure was 90% by volume or more of tempered martensite and tempered bainite. Furthermore, the old ⁇ particle size was less than 5 ⁇ m. Further, since the cooling rate at 500 to 200 ° C. during quenching was 5 ° C./s or more, the block diameter was less than 2 ⁇ m. As a result, the K ISSC values of Test No. 1 to Test No. 11 and Test No. 20 were 35 MPa ⁇ m or more, indicating excellent SSC resistance. The yield strengths of Test No. 1 to Test No. 11 and Test No. 20 were 900 MPa or more and had high yield strength.
- the chemical compositions of the steel plates of Test No. 21 to Test No. 25 are appropriate and satisfy the formulas (1) and (2). Furthermore, since ⁇ HRC was less than 2.0, the microstructure determination was acceptable, and the total microstructure was 90% by volume or more of tempered martensite and tempered bainite. For the steel plates of test number 21 to test number 25, the old ⁇ grain size was reduced to 3 ⁇ m by performing quenching and tempering twice and reducing the finishing temperature during hot rolling (before quenching) to refine the structure. It was possible to refine the following. Furthermore, since the cooling rate at 500 to 200 ° C. during quenching was 10 ° C./s or more, the block diameter could be made extremely fine to 1 ⁇ m or less. As a result, even when the yield strength exceeded 930 MPa, the K ISSC value was 35 MPa ⁇ m or more, indicating excellent SSC resistance.
- the steel plate of test number 12 did not contain Co, and F2 was less than the lower limit of formula (2).
- the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the Co content was low.
- the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- Mn exceeded the upper limit and F2 was less than the lower limit of formula (2).
- the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the ratio of the content of the SSC resistance improving elements (C, Mo and Co) to the Mn and Cr content is too low, and as a result, the SSC resistance is considered to be low.
- the C content was too low. Therefore, tempering was performed at a low temperature to obtain the desired strength. As a result, the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the Nb content was too low, and F2 was less than the lower limit of formula (2). Therefore, the old ⁇ grains were 5 ⁇ m or more during quenching, and the block diameter after tempering was 2 ⁇ m or more. As a result, the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the chemical composition of the steel was appropriate, and although the formulas (1) and (2) were satisfied, an appropriate quenching treatment was not performed. Therefore, the old ⁇ particle size became 5 ⁇ m or more during quenching, and the block diameter after tempering also became 2 ⁇ m or more. As a result, the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the chemical composition of the steel is appropriate, and satisfies the formulas (1) and (2), and the heating rate at the time of quenching is 10 ° C./s or more.
- the cooling rate at 0 ° C. was less than 5 ° C./s. Therefore, although the old ⁇ particle diameter was less than 5 ⁇ m, the block diameter after tempering was 2 ⁇ m or more. As a result, the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the steel plate of test number 28 did not contain Co. Therefore, even when the cooling rate at 500 to 200 ° C. during quenching is less than 5 ° C./s, the block diameter after tempering is less than 2 ⁇ m. However, since it did not contain Co, the K ISSC value was less than 35 MPa ⁇ m, and the SSC resistance was low.
- the steel material according to the present invention can be widely applied to steel materials used in sour environments, preferably as steel materials for oil wells used in oil well environments, and more preferably steel pipes for oil wells such as casings and tubing. Is available as
Abstract
Description
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
有効B=B-11(N-Ti/3.4)/14 (3)
ここで、式(1)のαは、式(3)で定義される有効B(質量%)が0.0003%以上の場合は0.250であり、有効Bが0.0003%未満の場合は0である。式(1)~式(3)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
(1)Coは耐SSC性を高める。特に、質量%で、C:0.15~0.45%、Si:0.10~1.0%、Mn:0.10~0.8%、P:0.050%以下、S:0.010%以下、Al:0.01~0.1%、N:0.010%以下、Cr:0.1~2.5%、Mo:0.35~3.0%、Co:0.05~2.0%、Ti:0.003~0.040%、Nb:0.003~0.050%、Cu:0.01~0.50%、Ni:0.01~0.50%、V:0~0.5%、B:0~0.003%、W:0~1.0%、Ca:0~0.004%、Mg:0~0.004%、及び、希土類元素:0~0.004%を含有する化学組成を有する鋼材においては、Coを0.05~2.0%含有すれば、優れた耐SSC性が得られる。その理由は定かではないが、次の理由が考えられる。サワー環境下での使用中において、Coは鋼材の表層に濃化する。表層に濃化したCoにより、鋼中への水素の侵入が抑制される。これにより、耐SSC性が高まると考えられる。
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
有効B=B-11(N-Ti/3.4)/14 (3)
ここで、式(1)のαは、式(3)で定義される有効B(質量%)が0.0003%以上の場合は0.250であり、有効Bが0.0003%未満の場合は0である。式(1)~式(3)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
F1=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+αと定義する。F1は焼入れ性の指標である。C、Mn、Cr、Mo、V、Cu、及び、所定量の有効B(固溶しているB)は、鋼の焼入れ性を高める。一方、上述のとおり、Coは、鋼の焼入れ性を低くする。F1が0.70以上であれば、Coを含有していても、優れた焼入れ性が得られ、ミクロ組織中の焼戻しマルテンサイト及び焼戻しベイナイトの体積率を高めることができる。
F1が式(1)を満たせば、ミクロ組織が実質的に焼戻しマルテンサイト及び焼戻しベイナイトとなる。しかしながら、合金元素が過剰に含有されれば、鋼材中に水素をトラップする(溜め込む)ため、耐SSC性がかえって低下する。焼入れ性を高める元素のうち、特にMn及びCrは、焼入れ性を高めるものの、耐SSC性を低下しうる。一方、上述のCoとともに、C及びMoは、鋼の耐SSC性を高める元素である。
表1に示す化学組成を有する鋼に対し、表2に示す条件で圧延及び焼入れ工程を施した。
本願発明者らはさらに、上記の表3において、旧γ粒が微細であっても、耐SSC性が低い場合がある点について、Coとブロック径との関係に着目して、検討した。その結果、以下の知見を得た。
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
有効B=B-11(N-Ti/3.4)/14 (3)
ここで、式(1)のαは、式(3)で定義される有効B(質量%)が0.0003%以上の場合は0.250であり、有効Bが0.0003%未満の場合は0である。式(1)~式(3)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
本発明による鋼材の化学組成は、次の元素を含有する。
炭素(C)は、焼入れ性を高め、鋼の強度を高める。Cはさらに、製造工程中の焼戻し時において、炭化物の球状化を促進し、耐SSC性を高める。Cはさらに、Mo又はVと結合して炭化物を形成し、焼戻し軟化抵抗を高める。炭化物が分散されればさらに、鋼の強度が高まる。C含有量が低すぎれば、これらの効果が得られない。一方、C含有量が高すぎれば、鋼の靭性が低下し、焼割れが発生しやすくなる。したがって、C含有量は0.15~0.45%である。C含有量の好ましい下限は0.20%であり、さらに好ましくは0.25%である。C含有量の好ましい上限は0.35%であり、さらに好ましくは0.30%である。
シリコン(Si)は、鋼を脱酸する。Si含有量が低すぎれば、この効果が得られない。一方、Si含有量が高すぎれば、残留オーステナイトが過剰に生成して耐SSC性が低下する。したがって、Si含有量は、0.10~1.0%である。好ましいSi含有量の下限は、0.15%であり、さらに好ましくは0.20%である。好ましいSi含有量の上限は、0.85%であり、さらに好ましくは0.50%である。
マンガン(Mn)は、鋼を脱酸する。Mnはさらに、焼入れ性を高め、鋼の強度を高める。Mn含有量が低すぎれば、これらの効果が得られない。一方、Mn含有量が高すぎれば、Mnは、P及びS等の不純物とともに、粒界に偏析する。この場合、鋼の耐SSC性が低下する。したがって、Mn含有量は、0.10~0.8%である。好ましいMn含有量の下限は、0.25%であり、さらに好ましくは0.28%である。好ましいMn含有量の上限は、0.65%である。
燐(P)は不純物である。Pは、粒界に偏析して鋼の耐SSC性を低下する。したがって、P含有量は、0.050%以下である。好ましいP含有量は0.020%以下である。P含有量はなるべく低い方が好ましい。
硫黄(S)は不純物である。Sは、粒界に偏析して鋼の耐SSC性を低下する。したがって、S含有量は0.010%以下である。好ましいS含有量は0.005%以下であり、さらに好ましくは0.003%以下である。S含有量はなるべく低い方が好ましい。
アルミニウム(Al)は、鋼を脱酸する。Al含有量が低すぎれば、この効果が得られず、鋼の耐SSC性が低下する。一方、Al含有量が高すぎれば、粗大な酸化物系介在物が生成して鋼の耐SSC性が低下する。したがって、Al含有量は0.01~0.1%である。Al含有量の好ましい下限は0.015%であり、さらに好ましくは0.020%である。Al含有量の好ましい上限は0.08%であり、さらに好ましくは0.05%である。本明細書にいう「Al」含有量は「酸可溶Al」、つまり、「sol.Al」の含有量を意味する。
窒素(N)は不可避に含有される。Nは粗大な窒化物を形成して、鋼の耐SSC性を低下する。したがって、N含有量は、0.010%以下である。好ましいN含有量は0.005%以下であり、さらに好ましくは0.004%以下である。N含有量はなるべく低い方が好ましい。ただし、若干量のTiを含有させて、微細窒化物の析出による結晶粒の微細化を狙う場合は、Nを0.002%以上含有させることが好ましい。
クロム(Cr)は、鋼の焼入れ性を高め、鋼の強度を高める。Cr含有量が低すぎれば、上記効果が得られない。一方、Cr含有量が高すぎれば、鋼の耐SSC性が低下する。したがって、Cr含有量は0.1~2.5%である。Cr含有量の好ましい下限は0.25%であり、さらに好ましくは0.30%である。Cr含有量の好ましい上限は1.5%であり、さらに好ましくは1.3%である。
モリブデン(Mo)は、鋼の焼入れ性を高める。Moはさらに、微細な炭化物を生成し、鋼の焼戻し軟化抵抗を高め、高圧H2S環境における耐SSC性を高める。Mo含有量が低すぎれば、この効果が得られない。一方、Mo含有量が高すぎれば、上記効果が飽和する。したがって、Mo含有量は0.35~3.0%である。Mo含有量の好ましい下限は0.40%であり、さらに好ましくは0.50%である。Mo含有量の好ましい上限は2.0%であり、さらに好ましくは1.75%である。
コバルト(Co)は、高H2S環境において、鋼の耐SSC性を高める。その理由は定かではないが、次の理由が考えられる。Coはサワー環境において、鋼の表面に濃化して、鋼中への水素の侵入を抑制する。これにより、鋼の耐SSC性が高まる。Co含有量が低すぎれば、この効果が得られない。一方、Co含有量が高すぎれば、鋼の焼入れ性が低下して、鋼の強度が低くなる。Co含有量が高すぎればさらに、ブロック径が粗大化して、耐SSC性が低くなる。したがって、Co含有量は0.05~2.0%である。Co含有量の好ましい下限は0.05%超であり、さらに好ましくは0.10%であり、さらに好ましくは0.25%である。Co含有量の好ましい上限は1.5%であり、さらに好ましくは1.25%である。
チタン(Ti)は窒化物を形成し、ピンニング効果により、結晶粒を微細化する。これにより、鋼の強度が高まる。Ti含有量が低すぎれば、この効果が得られない。一方、Ti含有量が高すぎれば、Ti窒化物が粗大化して鋼の耐SSC性が低下する。したがって、Ti含有量は0.003~0.040%である。Ti含有量の好ましい下限は0.005%である。Ti含有量の好ましい上限は0.020%であり、さらに好ましくは0.015%である。
ニオブ(Nb)は、C及び/又はNと結合して炭化物、窒化物又は炭窒化物(以下、炭窒化物等という)を形成する。これらの炭窒化物等は結晶粒を微細化して鋼の強度を高める。Nb含有量が低すぎれば、この効果が得られない。一方、Nb含有量が高すぎれば、粗大な析出物が生成して鋼の耐SSC性が低下する。したがって、Nb含有量は0.003~0.050%である。Nb含有量の好ましい下限は0.007%であり、さらに好ましくは0.010%である。Nb含有量の好ましい上限は0.025%である。
銅(Cu)は、鋼の焼入れ性を高め、鋼の強度を高める。Cu含有量が低すぎれば、これらの効果が得られない。一方、Cu含有量が高すぎれば、水素をトラップするようになり、耐SSC性が低下する。したがって、Cu含有量は0.01~0.50%である。Cu含有量の好ましい下限は0.02%であり、さらに好ましくは0.05%である。Cu含有量の好ましい上限は0.35%であり、さらに好ましくは0.25%である。
ニッケル(Ni)は、鋼の焼入れ性を高め、鋼の強度を高める。Ni含有量が低すぎれば、これらの効果が得られない。一方、Ni含有量が高すぎれば、局部的な腐食を促進させ、耐SSC性が低下する。したがって、Ni含有量は0.01~0.50%である。Ni含有量の好ましい下限は0.02%であり、さらに好ましくは0.05%である。Ni含有量の好ましい上限は0.45%であり、さらに好ましくは0.25%である。
上述の鋼材の化学組成はさらに、Feの一部に代えて、Vを含有してもよい。
バナジウム(V)は任意元素であり、含有されなくてもよい。含有される場合、Vは炭窒化物等を形成し、結晶粒を微細化して鋼の強度を高める。しかしながら、V含有量が高すぎれば、鋼の靭性が低下する。したがって、V含有量は0~0.5%である。V含有量の好ましい下限は0.015%であり、さらに好ましくは0.030%である。V含有量の好ましい上限は0.30%であり、さらに好ましくは0.20%である。
ボロン(B)は任意元素であり、含有されなくてもよい。含有される場合、Bは鋼に固溶して鋼の焼入れ性を高め、強度を高める。しかしながら、B含有量が高すぎれば、粗大な窒化物が生成して鋼の耐SSC性が低下する。したがって、B含有量は0~0.003%である。B含有量の好ましい下限は0.0003%であり、さらに好ましくは0.0007%である。B含有量の好ましい上限は0.0015%であり、さらに好ましくは0.0012%である。
タングステン(W)は任意元素であり、含有されなくてもよい。含有される場合、Wは鋼に固溶して鋼の焼入れ性を高め、強度を高める。しかしながら、W含有量が高すぎれば、粗大な炭化物が生成して鋼の耐SSC性が低下する。したがって、W含有量は0~1.0%である。W含有量の好ましい下限は0.05%であり、さらに好ましくは0.10%である。W含有量の好ましい上限は0.75%であり、さらに好ましくは0.5%である。
カルシウム(Ca)は任意元素であり、含有されなくてもよい。含有される場合、Caは、鋼中のSと結合する。これにより、鋼中のSを硫化物として無害化し、鋼の耐SSC性を高める。しかしながら、Ca含有量が高すぎれば、鋼中の酸化物が粗大化して、鋼の耐SSC性が低下する。したがって、Ca含有量は0~0.004%である。Ca含有量の好ましい下限は0.0003%であり、さらに好ましくは0.0006%である。Ca含有量の好ましい上限は0.0025%であり、さらに好ましくは0.0020%である。
マグネシウム(Mg)は任意元素であり、含有されなくてもよい。含有される場合、Mgは、鋼中のSを硫化物として無害化し、鋼の耐SSC性を高める。しかしながら、Mg含有量が高すぎれば、鋼中の酸化物が粗大化して鋼の耐SSC性が低下する。したがって、Mg含有量は0~0.004%である。Mg含有量の好ましい下限は0.0003%であり、さらに好ましくは0.0006%であり、さらに好ましくは0.0010%である。Mg含有量の好ましい上限は0.0025%であり、さらに好ましくは0.0020%である。
希土類元素(REM)は任意元素であり、含有されなくてもよい。含有される場合、REMは、鋼中の硫化物を微細化して鋼の耐SSC性を高める。REMはさらに、鋼中のPと結合して、結晶粒界におけるPの偏析を抑制する。そのため、Pの偏析に起因した鋼の耐SSC性の低下が抑制される。しかしながら、REM含有量が高すぎれば、酸化物が粗大化して鋼の耐SSC性が低下する。したがって、REM含有量は0~0.004%である。REM含有量の好ましい下限は0.0003%であり、さらに好ましくは0.0006%であり、さらに好ましくは0.0010%である。REM含有量の好ましい上限は0.0025%であり、さらに好ましくは0.0020%である。
上記化学組成はさらに、式(1)及び式(2)を満たす。
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
有効B=B-11(N-Ti/3.4)/14 (3)
ここで、式(1)のαは、式(3)で定義される有効B(質量%)が0.0003%以上の場合は0.250であり、有効Bが0.0003%未満の場合は0である。式(1)~式(3)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
F1=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+αと定義する。F1は焼入れ性の指標である。F1が0.70以上であれば、Coを含有していても、優れた焼入れ性が得られ、ミクロ組織中の焼戻しマルテンサイト及び焼戻しベイナイトの体積率が合計で90%以上となる。その結果、優れた耐SSC性が得られる。F1の好ましい下限は0.75であり、さらに好ましくは0.85であり、さらに好ましくは1.0である。F1の好ましい上限は1.5である。
F2=(3C+Mo+3Co)/(3Mn+Cr)と定義する。F2は、耐SSC性の指標である。F2が1.0以上である場合、耐SSC性向上元素(C、Mo及びCo)の含有量の、Mn及びCr含有量(焼入れ性に寄与するものの、過剰な含有が耐SSC性を低下し得る元素)に対する比が大きくなる。その結果、高圧H2S環境での優れた耐SSC性が得られる。F2の好ましい上限は3.0である。
[旧γ粒径及びブロック径]
本発明の鋼材のミクロ組織の旧γ粒径は5μm未満である。さらに、ミクロ組織のブロック径は2μm未満である。その結果、降伏強度が862MPa以上の高強度と耐SSC性とを両立させることができる。
旧γ粒径は、次の方法で求められる。焼入れままの鋼材から試験片を採取する。鋼管の場合、横断面は軸に対して垂直な面とし、肉厚中央部から試験片を採取する。試験片を鏡面研磨した後、ピクリン酸飽和水溶液を用いて旧γ粒界を現出させる。試験片において、任意の10視野で旧γ粒径(旧γ粒の平均結晶粒径)を測定する。測定は、1000倍の光学顕微鏡により観察し、JIS G0551(2005)に示される切断法により行う。各視野における旧γ粒度番号を算出する。算出した10個の旧γ粒度番号の平均(平均旧γ粒度番号)を求める。平均旧γ粒度番号に基づいて各結晶粒の平均面積を算出する。平均面積から円相当径を算出し、得られた円相当径を旧γ粒径とする。
本実施形態において、ブロック径(μm)は、マルテンサイトブロック及びベイナイトブロックの区別なく、FESEM-EBSP法により得られる結晶方位マップから求める。具体的には、50μm×50μmの視野を0.1μmピッチでEBSP測定を行う。EBSP測定により採取した菊池線パターンから、αFeの方位を同定する。αFeの方位から結晶方位図を求める。結晶方位図から、隣接する結晶との方位差が15°以上で囲まれる領域を識別し、結晶方位マップを得る。15°以上の方位差で囲まれた領域をブロックのひとつの粒と定義する。それぞれのブロックの面積から円相当径を求める。視野内の円相当径の平均値を算出して、それをブロック径とする。
本発明の鋼材はCoを含有する。CoはMs点を高める。したがって、本発明の鋼材のミクロ組織は、主として焼戻しマルテンサイト及び焼戻しベイナイトからなる。より具体的には、ミクロ組織は合計で90体積%以上の焼戻しマルテンサイト及び焼戻しベイナイトからなる。ミクロ組織の残部はたとえば、残留オーステナイト等である。ミクロ組織が合計で90体積%以上の焼戻しマルテンサイト及び焼戻しベイナイトを含有すれば、耐SSC性が高まる。好ましくは、ミクロ組織は焼戻しマルテンサイト単相からなる。好ましくは、焼戻しベイナイトの含有量は、2~10体積%である。
ΔHRC=HRCmax-HRCmin
ΔHRCが2.0未満であれば、鋼材のミクロ組織中の焼戻しマルテンサイト及び焼戻しベイナイトの体積率が合計で90%以上であるとみなす。
鋼材の形状は特に限定されない。鋼材はたとえば鋼管、鋼板である。鋼材が油井用鋼管の場合、好ましい肉厚は9~60mmである。本発明は特に、厚肉の油井用鋼管としての使用に適する。より具体的には、本発明による鋼材が15mm以上、さらに、20mm以上の厚肉の油井用鋼管であっても、高い強度及び優れた耐SSC性を示す。
本実施形態の鋼材の降伏強度は862MPa以上である。本明細書でいう降伏強度は、下降伏点(MPa)を意味する。要するに、本実施形態の鋼材の降伏強度は125ksi級である。本実施形態の鋼材は、このような高強度であっても、上述の化学組成及びミクロ組織とすることで優れた耐SSC性を有する。
上述の鋼材の製造方法の一例として、油井用鋼管の製造方法を説明する。油井用鋼管の製造方法は、素材を準備する工程(準備工程)と、素材を熱間加工して素管を製造する工程(熱間加工工程)と、素管に対して焼入れ及び焼戻しを実施して、油井用鋼管とする工程(焼入れ工程及び焼戻し工程)とを備える。以下、各工程について詳述する。
上述の化学組成を有し、式(1)及び式(2)を満たす溶鋼を製造する。溶鋼を用いて素材を製造する。具体的には、溶鋼を用いて連続鋳造法により鋳片(スラブ、ブルーム、又は、ビレット)を製造する。溶鋼を用いて造塊法によりインゴットを製造してもよい。必要に応じて、スラブ、ブルーム又はインゴットを分塊圧延して、ビレットを製造してもよい。以上の工程により素材(スラブ、ブルーム、又は、ビレット)を製造する。
準備された素材を熱間加工して素管を製造する。始めに、ビレットを加熱炉で加熱する。加熱炉から抽出されたビレットに対して熱間加工を実施して、素管(継目無鋼管)を製造する。たとえば、熱間加工としてマンネスマン法を実施し、素管を製造する。この場合、穿孔機により丸ビレットを穿孔圧延する。穿孔圧延された丸ビレットをさらに、マンドレルミル、レデューサ、サイジングミル等により熱間圧延して素管にする。
熱間加工後の素管に対して、焼入れを実施する。焼入れ条件により、旧γ粒径を5μm未満、かつブロック径を2μm未満に調整する。焼入れはたとえば、高周波誘導加熱炉により行う。高周波誘導加熱炉にて行う場合、到達温度までの加熱速度及び到達温度を制御する。好ましい加熱開始温度は室温である。この場合、さらに細粒化されやすい。好ましい加熱速度は10℃/s以上であり、好ましい到達温度は850~920℃である。到達温度が1000℃以下であれば、旧γ粒径の粗大化を抑制できる。到達温度では、好ましくは5~180秒保持する。保持時間が180秒以下であれば、旧γ粒径の粗大化を抑制できる。その他の条件を満たし、かつ、加熱速度が10℃/s以上であれば、旧γ粒径を5μm未満とすることができる。
上述の焼入れ処理を実施した後、焼戻し処理を実施する。焼戻し処理により、鋼材の降伏強度を862~965MPaに調整する。焼戻し温度の好ましい下限は650℃である。焼戻し温度の好ましい上限は730℃である。焼戻し温度での好ましい保持時間は、5~180分である。
[旧γ粒径測定試験]
最終焼入れままの板材の肉厚中央部から試験片を採取し、上述の方法で旧γ粒の平均粒径を測定した。
上記の焼入れ及び焼戻し処理後の各鋼板の板厚中央から、直径6.35mm、平行部長さ35mmの丸棒引張試験片を作製した。引張試験片の軸方向は、鋼板の圧延方向と平行であった。各丸棒試験片を用いて、常温(25℃)、大気中にて引張試験を実施して、各位置における降伏強度(YS)(MPa)及び引張強度(TS)を得た。なお、本実施例では、引張試験により得られた下降伏点を、各試験番号の降伏強度(YS)と定義した。
上記の最終の焼入れ及び焼戻し処理後の各鋼板に対して、JIS Z2245(2011)に準拠したロックウェル硬さ(HRC)試験を実施した。具体的には、鋼板の表面から2.0mm深さ位置、鋼板の裏面から2.0mm深さ位置、及び、鋼板の厚さ方向中央位置の各々において、任意の3箇所のロックウェル硬さ(HRC)を求めた。9点のロックウェル硬さの最大値と最小値の差ΔHRCは、試験番号14を除き、すべて2.0未満であった。したがい、本実施形態のミクロ組織において、HRCminの位置でも焼戻しマルテンサイト及び焼戻しベイナイトの体積率が合計で90%以上であるとみなした。
上記の焼入れ及び焼戻し処理後の各鋼板の板厚中央部から試験片を採取し、上述の方法でブロックの平均ブロック径を測定した。
各鋼板を用いて、NACE TM0177-96 Method Dに準拠したDCB試験を実施し、耐SSC性を評価した。具体的には、各鋼板の厚さ中央部から、図2Aに示すDCB試験片を3本ずつ採取した。DCB試験片の長手方向が圧延方向と平行となるよう採取した。鋼板からさらに、図2Bに示すクサビを作製した。クサビの厚さtは2.92mmであった。
表6及び表7に試験結果を示す。
Claims (5)
- 質量%で、
C:0.15~0.45%、
Si:0.10~1.0%、
Mn:0.10~0.8%、
P:0.050%以下、
S:0.010%以下、
Al:0.01~0.1%、
N:0.010%以下、
Cr:0.1~2.5%、
Mo:0.35~3.0%、
Co:0.05~2.0%、
Ti:0.003~0.040%、
Nb:0.003~0.050%、
Cu:0.01~0.50%、
Ni:0.01~0.50%、
V:0~0.5%、
B:0~0.003%、
W:0~1.0%、
Ca:0~0.004%、
Mg:0~0.004%、及び、
希土類元素:0~0.004%を含有し、
残部がFe及び不純物からなり、式(1)及び(2)を満たす化学組成を有し、
ミクロ組織の旧オーステナイト粒径が5μm未満であり、
前記ミクロ組織のブロック径が2μm未満であり、
前記ミクロ組織が合計で90体積%以上の焼戻しマルテンサイト及び焼戻しベイナイトを含有する、鋼材。
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
有効B=B-11(N-Ti/3.4)/14 (3)
ここで、式(1)のαは、式(3)で定義される有効B(質量%)が0.0003%以上の場合は0.250であり、前記有効Bが0.0003%未満の場合は0である。式(1)~式(3)中の各元素記号には、対応する元素の含有量(質量%)が代入される。 - 請求項1に記載の鋼材であって、
前記化学組成は、
V:0.015~0.5%を含有する、鋼材。 - 請求項1又は請求項2に記載の鋼材であって、
前記化学組成は、
B:0.0003~0.003%、及び、
W:0.05~1.0%からなる群から選択される1種以上を含有する、鋼材。 - 請求項1~請求項3のいずれか1項に記載の鋼材であって、
前記化学組成は、
Ca:0.0003~0.004%、
Mg:0.0003~0.004%、及び、
希土類元素:0.0003~0.004%からなる群から選択される1種又は2種以上を含有する、鋼材。 - 請求項1~請求項4のいずれか1項に記載の化学組成及びミクロ組織と、
15mm以上の肉厚とを有する、油井用鋼管。
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CA3035163A CA3035163A1 (en) | 2016-09-01 | 2017-08-30 | Steel material and oil-well steel pipe |
BR112019002925-7A BR112019002925B1 (pt) | 2016-09-01 | 2017-08-30 | Material de aço e tubo de aço de poço de petróleo |
RU2019109029A RU2707845C1 (ru) | 2016-09-01 | 2017-08-30 | Стальной материал и стальная труба для нефтяной скважины |
US16/328,777 US10655200B2 (en) | 2016-09-01 | 2017-08-30 | Steel material and oil-well steel pipe |
EP17846574.6A EP3508603A4 (en) | 2016-09-01 | 2017-08-30 | STEEL AND OIL HOLE PIPE |
CN201780053014.1A CN109642293A (zh) | 2016-09-01 | 2017-08-30 | 钢材以及油井用钢管 |
MX2019002291A MX2019002291A (es) | 2016-09-01 | 2017-08-30 | Material de acero y tuberia de acero para pozos de petroleo. |
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EP (1) | EP3508603A4 (ja) |
JP (1) | JP6677310B2 (ja) |
CN (1) | CN109642293A (ja) |
BR (1) | BR112019002925B1 (ja) |
CA (1) | CA3035163A1 (ja) |
MX (1) | MX2019002291A (ja) |
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JPWO2021131461A1 (ja) * | 2019-12-26 | 2021-12-23 | Jfeスチール株式会社 | 高強度継目無鋼管およびその製造方法 |
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JP7364993B1 (ja) | 2022-04-22 | 2023-10-19 | 日本製鉄株式会社 | 鋼材 |
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Also Published As
Publication number | Publication date |
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EP3508603A1 (en) | 2019-07-10 |
US20190226063A1 (en) | 2019-07-25 |
CA3035163A1 (en) | 2018-03-08 |
RU2707845C1 (ru) | 2019-11-29 |
MX2019002291A (es) | 2019-07-04 |
US10655200B2 (en) | 2020-05-19 |
JPWO2018043570A1 (ja) | 2019-06-24 |
BR112019002925B1 (pt) | 2022-09-20 |
JP6677310B2 (ja) | 2020-04-08 |
CN109642293A (zh) | 2019-04-16 |
BR112019002925A2 (pt) | 2019-05-21 |
EP3508603A4 (en) | 2020-06-03 |
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