Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a martensitic stainless steel as OCTG used in oil wells or gas wells, and to a method for manufacturing thereof, specifically the present invention relates to a martensitic stainless steel for inexpensive seamless pipes having 655 MPa yield strength and high toughness, suitable for the uses in high CO 2 environments, and a method for manufacturing thereof.
• OCTG materials are 410 Steel or 420 Steel specified by the American Iron and Steel Institute (AISI). Although these grades of steels are relatively inexpensive and achieve 552 MPa or higher yield strength by heat treatment, they do not have satisfactory corrosion resistance and toughness. Furthermore, since these steels contain carbon by about 0.1% or more by weight, they cannot be treated by water-cooling in the manufacturing process, which degrades the production efficiency.
• AISI American Iron and Steel Institute
• Patent Literature 1 Japanese Patent No. 2665009 discloses a steel containing 0.005 to 0.04% C, 12.0 to 17.0% Cr, and 1.5 to 6.0% Ni, by weight, and a method for manufacturing thereof. Although the steel has 784 to 1078 MPa yield strength (proof stress), which is higher than that of general-use 552 MPa grade and 655 MPa grade steels, and although the steel gives favorable corrosion resistance in a 65% nitric acid corrosion test, the steel is not examined for corrosion resistance under high CO 2 environments.
• Patent Literature 2 Japanese Patent No. 2091532 discloses a steel containing 0.15% or less C, 9 to 16.0% Cr, and 0.2 to 2.5% Ni, by weight, and a method for manufacturing thereof.
• Patent Literature 3 Japanese Patent No. 2995524. discloses a steel containing 0.03% or less C, 11 to 17% Cr, and 3.5 to 7.0% Ni, by weight, and a method for manufacturing thereof. Since, however, the steel needs to add 3.5% or more Ni, the steel is not advantageous in economy.
• Patent Literature 4 JP-A-2004-115890
• JP-A Japanese Patent Laid-Open No.”
• JP-A Japanese Patent Laid-Open No.
• the steel is, however, limited to 552 MPa grade strength because these materials as shown above rapidly degrade properties such as toughness and SSC resistance if the strength exceeds 552 MPa grade.
• Patent Literature 5 JP-A-2004-99964 discloses a low Ni—Nb steel containing 0.02 to 0.05% Cr 10 to 12% Cr, 1.5 to 3.0% Ni, and 0.005 to 0.10% Nb, by weight, and a method for manufacturing thereof.
• the steel is, however, limited to 758 MPa grade of strength.
• Patent Literature 1 Japanese Patent No. 2665009
• Patent Literature 2 Japanese Patent No. 2091532
• Patent Literature 3 Japanese Patent No. 2995524
• the inventors of the present invention improved corrosion resistance and toughness by suppressing the precipitation of carbide through the control of C content to a low level, thereby allowed to apply water-cooling in the manufacturing process.
• C content the inventors of the present invention found that the corrosion resistance is maintained to the level of conventional steels even when the Cr content is reduced to below 13% which is the content in conventional 420 Steel and the like.
• the Ni content is reduced to about 2% to reduce the cost.
• the inventors of the present invention found that the addition of small amount of Mo provides the steel with stable toughness durable for the use in cold: region, for example, at ⁇ 40° C.
• the inventors of the present invention investigated the optimum balance of chemical compositions and heat treatment conditions in terms of strength-toughness-corrosion resistance under high CO 2 environment by varying the heat treatment conditions of quenching and tempering after the hot-rolling of steel having different chemical compositions.
• a martensitic stainless steel for seamless pipes having 655 MPa grade of yield strength suitable for the use under high CO 2 environments was obtained by controlling the chemical compositions and the heat treatment conditions in a specific range according to the present invention.
• the 655 MPa grade martensitic stainless steel having high toughness and the method for manufacturing thereof according to the present invention have been completed on the basis of the above-described findings, and the essence thereof is the following.
• a 655 MPa grade martensitic stainless steel having high toughness is a steel having the chemical compositions of the first aspect thereof, further comprising at least one chemical composition selected from the group consisting of Cu: ⁇ 0. 5%, Nb: ⁇ 0.05%, V: ⁇ 0.1%, B: ⁇ 0.005%, Ca: ⁇ 0.005%, by weight.
• a method for manufacturing 655 MPa grade martensitic stainless steel having high toughness comprising the steps of: hot-rolling the steel having the chemical compositions of the third aspect thereof further comprising at least one chemical composition selected from the group consisting of Cu: ⁇ 0.5%, Nb: ⁇ 0.05%, V: ⁇ 0.1%, B: ⁇ 0.005%, Ca: ⁇ 0.005%, by weight, cooling the hot-rolled steel; reheating the cooled steel at temperatures from 780° C. to 960° C.; quenching the reheated steel; and then tempering the quenched steel at temperatures from 550° C. to 650° C.
• Carbon increases the strength of steel by enhancing solid solution, by hardening the transformed martensite, and by precipitation hardening as carbide. If the C content is in this range, the effect of increasing the strength of steel is attained, and both the toughness and the corrosion resistance are maintained to a favorable level. In particular, decrease in the C content to the level of the present invention suppresses the precipitation of carbide in the steel so that the corrosion resistance under high CO 2 environments becomes high.
• Sulfur is an impurity element similar with P, and induces degradation of toughness and hot-workability. Accordingly, lower S content is more preferable. If the S content is in this range, the degradation of toughness and hot-workability is suppressed.
• Chromium has an effect of improving the corrosion resistance. If the Cr content is in this range, satisfactory corrosion resistance is attained even when C content is low, from 0.005% to 0.05%. If the Cr content exceeds this range, the effect of increasing the corrosion resistance saturates, which is not advantageous in terms of economy.
• the Cr content is in a range from 10.0% or more to less than 12.0%, and more preferably from 11.0% or more to less than 12.0%.
• Molybdenum has effects to limit the precipitation sites and the kinds of precipitates and to improve the toughness.
• the quantity of precipitate of Cr 2 (C, N) and M 7 C 3 is less than the quantity of precipitate of M 23 C 6 , effective increase in toughness is available.
• the Mo content is in this range, the effect of improving toughness is attained. Excess addition of Mo above the range is unfavorable in terms of ⁇ -ferrite formation and of economy.
• the Mo content is from 0.15 to 0.40%.
• Nickel has an effect to improve the corrosion resistance and the toughness. If the Ni content is in this range, the ⁇ -ferrite formation is suppressed, which gives favorable hot-workability. Excess addition of Ni above the range is not advantageous because the effect of improving the toughness saturates.
• the Ni content is in a range from, 2.0 to 3.0%.
• Nitrogen has an effect of strength-increase by the enhancement of solid solution and by the precipitation hardening. Excess addition of N above the range induces binding N with V, Nb, Ti, and the like to form coarse precipitates, which degrades the toughness and hot-workability. Therefore, excess addition of N is not advantageous.
• Aluminum inevitably exists in steel as a deoxidizing element. If the Al content is in this range, the effect of deoxidization is attained, and the formation of AlN which precipitates in grain boundaries to decrease the grain boundary strength is suppressed, thus to degrade the toughness.
• the FT is defined by the following formula:
• the FI is a parameter of formation of ⁇ -ferrite.
• the coefficient is selected in accordance with Schaefler diagram. If the FI value is not more than 8.49%, toughness becomes favorable because the formation of ⁇ -ferrite is suppressed.
• substantially Fe referred to herein means that the steel of the present invention may allow the existence of gas components of O and H, and impurities such as Sn, As, and Sb, both of which are inevitably contained in the steel during melting and refining process in the range not affect the purpose of the present invention.
• the steel may further contain at least one chemical composition selected from the group consisting of Cu, Nb, V, B, and Ca to improve strength, toughness, corrosion resistance, and hot-workability.
• chemical compositions selected from the group consisting of Cu, Nb, V, B, and Ca to improve strength, toughness, corrosion resistance, and hot-workability. The reasons to limit these chemical compositions are described in the following.
• Copper has an effect of increasing the corrosion resistance. If the Cu content is in this range, no problem of degradation of hot-workability and other characteristics occurs.
• Niobium refines austenitic grains by the Nb-carbide precipitation during quenching thus to improve the toughness, and can increase the strength by fine Nb-carbide precipitated during tempering. If the Nb content is in this range, the increase in the strength by the precipitation of Nb-carbide is controlled to an appropriate range.
• Vanadium has an effect of forming nitride with N, thus increasing the strength. If the V content is in this range, the effect of increasing the strength does not saturate, and the degradation of toughness caused by the formation of coarse precipitates is not induced, so the strength increases without degrading the toughness.
• B content is in this ranger no low-melting compound is formed at grain boundaries so that the grain boundaries are strengthened while maintaining the hot-workability.
• Calcium has an effect to control the morphology of manganese sulfide inclusion and to improve the toughness and the corrosion resistance. If the Ca content is in this range, the formation of Ca-based precipitate is suppressed, thereby to improve the toughness and the corrosion resistance.
• Temperature of heating for quenching 780° C. to 960° C.
• the heating temperature for quenching is in this range, a fully austenitic single-phase microstructure is attained during heating so that the succeeding cooling (quenching) gives martensite-single phase microstructure to provide a stable quenched microstructure, as well as suppressing the formation of coarse austenitic grains, thus attaining favorable toughness.
• Tempering Temperature 550° C. to 650° C.
• the steel according to the present invention has high strength and insufficient toughness in as-quenched state, appropriate tempering is required. If the tempering temperature is in this range, desirable strength is attained and the toughness becomes favorable.
• the martensitic stainless steel according to the present invention may be prepared by any melting and refining process such as LD converter or electric furnace, which can control the chemical compositions within the range of the present invention.
• LD converter low-density polyethylene
• the steel is formed into billet or other required shape by casting or rolling thereof, and then is subjected to a process such as piercing using a piercer with extrusion stem or a piercing mill with inclined roll, or rolling to form seamless steel pipes followed by specified heat treatment.
• the martensitic stainless steel according to the present invention is applicable to other usage other than OCTG.
• the steel may be used as transportation steel pipes such as line pipes.
• the ingoted steel is formed into slab shape by casting or rolling, and it is rolled into steel plate using a plate mill or a hot-strip mill, and then is subjected to a specified heat treatment, followed by welding to manufacture the steel pipes.
• the rolled steel plate may be formed into steel pipe by welding thereof, followed by a specified heat treatment such as quenching and tempering.
• the martensitic stainless steel according to the present invention is subjected to reheating and quenching after hot-rolling. If, however, a direct quenching apparatus which can apply direct quenching to the steel after hot-rolling is available, direct quenching may be applied instead of reheating and quenching, followed by specified tempering.
• Table 1 shows the chemical compositions of the working examples (Nos. 1-17) according to the present invention.
• Table 2 shows the results of mechanical properties thereof.
• Table 3 shows the chemical compositions of the comparative examples (Nos. 18-35).
• Table 4 shows the results of mechanical properties.
• All of the steels of working examples according to the present invention and the steels of comparative examples were vacuum melted using a laboratory furnace, and the obtained ingots were hot rolled to 12 mm-thick plates. Each of plates was heat-treated, and was tested to determine strength, toughness, and corrosion resistance. Regarding the strength, round bar samples of ASTM Type F were cut from the center portion in the thickness direction of the plate, and the samples were tested by tensile tester to determine the yield strength. As for the toughness, full-size V-notch Charpy test samples were cut from the center portion in the thickness direction of the plate, and the samples were tested by impact tester at ⁇ 40° C. to evaluate the absorbed energy. For the corrosion resistance, samples were immersed in a 10% NaCl aqueous solution equilibrated with 30 bar carbon dioxide gas at 100° C. for 336 hours, and the corrosion loss was evaluated.
• the acceptable target values were 655 to 758 MPa of yield strength for the strength, 200 J or higher absorbed energy (vE ⁇ 40 ) at ⁇ 40° C. for the toughness, and 0.3 mm/year or less of corrosion rate for the corrosion resistance.
• the present invention improves the properties required for the seamless pipes by specifying the chemical compositions and the manufacturing conditions. As a result, the present invention provides a martensitic stainless steel of 655 MPa grade for seamless steel pipes suitable for use under high CO 2 environments and having high toughness, with low cost.

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• 2004
  • 2004-12-15 WO PCT/JP2004/018710 patent/WO2006064553A1/ja active Application Filing
  • 2004-12-15 JP JP2006548607A patent/JP4975448B2/ja active Active
  • 2004-12-15 US US11/720,351 patent/US20080112839A1/en not_active Abandoned
• 2007
  • 2007-05-31 NO NO20072779A patent/NO20072779L/no not_active Application Discontinuation
• 2009
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