Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C21D2211/002—Bainite
• • 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
  • C21D2211/005—Ferrite
• • 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 seamless steel pipe excellent in hydrogen-induced cracking resistance (hereinafter referred to as "HIC resistance”), which is used as a line pipe having 5L-X70 grade or higher of American Petroleum Institute (API) Standard in strength level.
• HIC resistance hydrogen-induced cracking resistance
• API American Petroleum Institute
• oil well and the hke In recent years, well conditions of an oil well for crude oil and a gas well for natural gas (hereinafter referred to as only "oil well and the hke" generally) become severe and the transportation of the crude oil and natural gas has been performed under a severe environment. As the depth of water is increased, the well condition of the oil well and the like tends to contain C0 2 ,, H2S, Cl ⁇ , and the like in the ambient, and H 2 S is often contained in the crude oil and natural gas.
• HIC hydrogen-induced cracking or hydrogen-induced blistering or the hke
• the above-mentioned HIC is a steel material fracture phenomenon that inclusions such as MnS, AI2O3, CaO, CaS and the like existing in steel are changed, during the rolling of a steel material, to elongated ones in the rolling direction or crushed cluster-like ones, hydrogen absorbed into the interfaces between these inclusions and matrix steel is accumulated and gasified, cracks are generated by the gas pressure of the accumulated hydrogen, and these cracks propagate in steel.
• various steel materials for a line pipe has been proposed. For example, Japanese Patent Application Laid-open No.
• S50-97515 proposes steel for a line pipe in which Cu: 0.2 - 0.8 % is added to steel having strength of X42 - X80 grade in the API standard to form an anticorrosive film thereby preventing hydrogen from absorbing into the matrix steel.
• Japanese Patent Application Laid-open No. S53-106318 proposes a steel material for a line pipe in which Ca: excess 0.005 % - 0.020 or less %, which is comparatively a large amount, is added to steel and inclusion (MnS) in steel is spheroidized by a shape control by Ca treatment thereby reducing cracking sensitivity. Even at present HIC resistant steel has been produced based on these proposed technologies.
• the HIC resistant steel since the principal use of the HIC resistant steel is a transporting pipeline for crude oil and natural gas, weldability is important. Thus a low-carbon steel is applied to the HIC resistant steel, but high strength steel is difficult to obtain due to the low C content of the steel. On the other hand, as mentioned above, consumers require for high strength materials. Thus, to satisfy the requirement, the following steps are often performed: after finish rolling a steel pipe by hot rolling, the steel pipe is heated and quenched, and subsequently tempered.
• Such quenching and tempering treatment of a rolled steel pipe is effective for avoiding a ferrite and pearlite band-shaped microstructure in which HIC is liable to occur.
• the weldability is important and high strength is required.
• a rolled steel pipe is often subjected to be quenched and tempered.
• a seamless steel pipe of a high strength material was produced by quenching and tempering after soaking without cooling the rolled steel pipe to Ar 3 point after hot rolling by the use of a previously proposed steel in which inclusions (MnS) are shape-controlled by Ca treatment.
• MnS inclusions
• the occurrence of HIC exhibiting a form of an intergranular fracture was observed.
• the HIC resistant steel proposed in the above-described Japanese Patent Application Laid-open No. S53-106318 and the like was applied to a high strength steel, the HIC resistance is not necessarily improved.
• the present invention was made in consideration to the production of a seamless steel pipe having high strength and HIC resistance, and an object of the present invention is to provide a high strength seamless steel pipe, which can exhibit excellent HIC resistance and its production method.
• the present inventors have collated the knowledge about behaviors of HIC, which occurs in a line pipe, to solve the above-mentioned problem.
• HIC is a breakage of steel by hydrogen-induced cracking or hydrogen-induced blistering, which is generated by the facts that hydrogen generated by corrosion absorbs into the steel and accumulates at the interface between the inclusions in the steel and the matrix steel and gasifies, and that the gas pressure is increased more than the yield strength of the steel to generate cracks, which propagate in the steel.
• an inclusion shape control and the hke were performed so that the absorbed hydrogen hardly gasificates.
• all of starting point of HIC is not at inclusions, and an HIC fracture exhibits a fracture hke sulfide stress -corrosion cracking and can exhibit a form of intergranular fracture.
• the relationships between HIC resistance of steel and a quenched microstructure thereof were further reviewed.
• the present invention has been completed based on the above-mentioned knowledge and the gist of the present invention is the following high strength seamless steel pipes (l) and (2) and the following production method of the high strength seamless steel pipe (3).
• a high strength seamless steel pipe excellent in HIC resistance characterized by consisting of, by mass %, C: 0.03 - 0.11 %, Si: 0.05 - 0.5 %, Mn: 0.8 - 1.6 %, P: 0.025 % or less, S: 0.003 % or less, Ti: 0.002 - 0.017 %, Al: 0.001 - 0.1 %, Cr: 0.05 - 0.5 %, Mo: 0.02 - 0.3 %, V 0.02 - 0.20 %, Ca: 0.0005 - 0.005 %, N: 0.008 % or less and O (Oxygen): 0.004 % or less, and the balance Fe and impurities, and also characterized in that the microstructure of steel is bainite and/or martensite, ferrite is precipitated on grain boundaries and yield stress is 483 MPa or more.
• a production method of a high strength seamless steel pipe excellent in HIC resistance characterized in that after rolling a billet having a composition described in the above-mentioned (l) or (2) to a seamless steel pipe by hot rolling, said seamless steel pipe is immediately soaked and then cooled at a starting temperature of quenching of (Ar 3 point + 50 °C) to 1100 °C and at a coohng rate of 5 °C/sec or more, and then said seamless steel pipe is tempered at 550 °C to Aci points, whereby a seamless steel pipe in which the microstructure of steel is bainite and/or martensite, ferrite is precipitated at grain boundaries and yield stress is 483 MPa or more is produced.
• FIG. 1 is a view showing a microstructure photograph of a seamless steel pipe inferior in HIC resistance.
• FIG. 2 is a view showing a microstructure photograph of a seamless steel pipe excellent in HIC resistance.
• C Carbon is an element necessary to enhance hardenability and to increase the strength of the steel.
• the content of C is less than 0.03 %, the hardenability are lowered, and high strength is difficult to ensure.
• the content of C exceeds 0.11 %, in a case where QT is applied, the steel tends to have a fully quenched microstructure such as bainite and/or martensite or the hke, whereby the HIC resistance of the steel is not only lowered but also weldability is lowered.
• Si Silicon
• Si is added to steel for the purpose of deoxidation of steel, and contributes to an increase in strength and enhancing a softening resistance during tempering the steel.
• the addition of 0.05 % or more Si is needed.
• the Si content was set to 0.5 % or less.
• Mn (Manganese) is an effective element for increasing hardenability of the steel to increase strength thereof and for enhancing hot workabihty of the steel. Particularly, to enhance the hot workabihty of steel 0.8 % or more Mn is needed.
• the Mn content was set to 1.6 % or less.
• P 0.025 % or less P (Phosphorus) exists in the steel as impurities. Since the segregation of P in grain boundaries deteriorates toughness of steel, the P content was set to 0.025 % or less. The P content is preferably 0.015 % or less, and more preferably 0.009 % or less. S: 0.003 % or less
• S sulfur
• MnS manganese
• HIC resistance HIC resistance
• Ti is an element effective to prevent cracking of the billet. To exhibit the effect the Ti content of 0.002 % or more is needed. On the other hand, since excessive addition of Ti deteriorates toughness of the steel, the TL content was set to 0.017 % or less, and preferably 0.010 % or less. Al: 0.001 - 0.10 %
• Al is an indispensable element for deoxidation of the steel.
• the Al content was set to 0.001 % or more.
• the Al content was set to 0.10 % or less, and preferably 0.040 % or less.
• Cr 0.05 - 0.5 %
• Cr Chromium
• Cr Chromium
• Mo Mo
• Mo Mo
• Mo Mo
• V (Vanadium) is an element for enhancing the strength of the steel. The significant effect can be obtained by addition of 0.02 % or more V. However, since even excessive addition of V saturates the effect, the V content was set to 0.20 % or less and preferably 0.09 % or less. Ca: 0.0005 - 0.005 %
• Ca (Calcium) is used for the shape controlling of inclusion. To enhance the HIC resistance by sphering the MnS inclusions, the Ca content of 0.0005 % or more is needed. On the other hand, when the Ca content exceeds 0.005 %, the effect is saturated and further effects cannot be exhibited. Additionally, Ca inclusions tend to be clusters so that the HIC resistance is lowered. Accordingly, the upper hmit of Ca content was set to 0.005%. N: 0.008 % or less
• N (Nitrogen) exists in the steel as impurities. When the N content is increased, cracks are generated in the billet so that the steel property deteriorates. Thus the N content was set to 0.008 % or less. Preferably, the N content is 0.006 % or less. O (Oxygen): 0.004 % or less
• the O content means a total content of soluble oxygen in the steel and oxygen in oxide inclusions. This O content is substantially the same as the O content in oxide inclusions in the sufficiently deoxidized steel. Therefore, as the O content is increased, there exist increased oxide inclusions in the steel thereby decreasing HIC resistance. Accordingly, smaller O content is better and the O content was set to 0.004 % or less.
• Nb The Nb (Niobium) content does not influence on the HIC resistance and strength of the steel.
• the Nb element can be cared as an impurity element and its content is not be defined in the present invention.
• the Nb content range is preferably 0.1 % or less.
• a steel pipe microstructure In the seamless steel pipe of the present invention, a steel pipe microstructure must be a quenched microstructure such as bainite and/or martensite to ensure the strength of 5L - X70 grade or more by use of a comparatively low C steel as shown by the above-mentioned chemical compositions.
• the inline QT is preferably apphed.
• the precipitation of ferrite on the bainite and/or martensite grain boundary has an effect to prevent the generation of HIC, which exhibits a form of a intergranular fracture such as sulfide stress-corrosion cracking, while ensuring the strength of 5L - X70 grade or more.
• FIG. 1 is a view showing a microstructure photograph of a seamless steel pipe inferior in HIC resistance.
• the microstructure in FIG. 1 is a structure etched by a nital and exhibits a bainite and/or martensite fully quenched microstructure in which prior austenite grain boundaries can be clearly recognized.
• an HIC which exhibits a form of intergranular fracture such as sulfide stress-corrosion cracking, tends to generate.
• FIG. 2 is a view showing a microstructure photograph of a seamless steel pipe excellent in HIC resistance relating to the present invention.
• FIG. 2 shows a microstructure etched by a nital as in FIG. 1. Because a ferrite phase is generated in the grain boundary, the prior austenitic grain boundaries are not clear in the microstructure. In a case of such a microstructure, the HIC, which shows a form of intergranular fracture, is not occurred.
• a seamless steel pipe excellent in an aimed performance i.e. HIC resistance can be obtained.
• a preferable production method for obtaining a seamless steel pipe, which satisfies the microstructure and the high strength simultaneously, is shown as follows.
• the obtained steel pipe is soaked immediately to a temperature of (A_3 point + 50 °C) or more by use of soaking furnace without coohng it to the Ar 3 point and is quenched.
• the starting temperature of quenching is less than (Are point + 50 °C)
• variation is generated in strength.
• the starting temperature of quenching is increased, toughness of the steel pipe is significantly lowered.
• the starting temperature of quenching must be 1100 °C or less. Therefore, the starting temperature of quenching is set to (Ar 3 point + 50 °C) to 1100 °C.
• the quenching of the finish rolled steel pipe is performed by cooling it to room temperature, for example, while keeping the cooling rate of 5 °C/sec.
• the cooling rate of 5 °C/sec or more should be kept.
• a tempering temperature of 550 °C or more is needed.
• the tempering temperature exceeds Aci point, the strength of the steel pipe is decreased. Accordingly, the tempering must be performed under a temperature condition of 550 °C to Aci point.
• the present invention does not limit production steps until finish rolling a steel pipe from a billet, which is a starting material.
• a Mannesmann-mandrel mill process a billet cast by a continuous casting machine or a billet obtained by rolling in a blooming mill after casting is heated and a hollow shell is obtained by a piercer such as a inclined rolling mill. After that a mandrel bar is inserted into the pipe to roll it, a finish rolling is performed by use of a sizer or reducer.
• Some kinds of steels having chemical compositions shown in Table 1, were melted by a converter. Billets produced by continuous casting were heated to 1100 C C or more and hollow shells were obtained by use of a tilting roller piercer. These hollow shells were finish rolled to steel pipes by a mandrel mill and a sizer. After that without cooling the steel pipes to Ar 3 point or less, they were soaked at 950 °C and subjected to quenching and tempering treatment to produce seamless steel pipes. The steel pipe sizes and heat treatment conditions are shown in Table 2. In this case the coohng rate was set to 30 °C/sec.
• the steel of No. 3 in Table 1 was melted by converter, and an billet produced by continuous casting was heated to 1100 °C or more and a hollow shell was obtained by use of a inclined rolling mill.
• the hollow shell was finish rolled to a steel pipe by a mandrel mill and a sizer. After that the steel pipe was cooled in a range of 920 °C to 20 °C, and seamless steel pipes were produced by changing the cooling starting temperature, coohng rate and tempering temperature.
• the sizes of the produced steel pipes and heat treatment conditions are shown in Table 3.
• the Ar 3 point of the tested steel of No. 3 was 768 °C
• the Act point thereof was 745 °C.
• Example 2 tensile test specimens of JIS 12 were taken and as tensile tests, tensile strength (TS) and yield strength (YS) were measured. Further, HIC resistance tests were performed under the same conditions as in Example 1, and crack area ratios (CAR (%)) were measured. Further, after HIC resistance testing, cross-sections of HIC test specimens were cut off and microstructure observation was performed by an optical microscope. These results are shown in Table 3.
• the steel of test No. 30 adopts a tempering temperature, which is outside the specified values of the present invention, and the strength could not satisfy 5L - X70 grade.
• the steel of test No. 31 adopts a coohng rate outside the specified values of the present invention and the microstructure of the steel is a ferrite-pearlite microstructure whereby the strength of the steel could not satisfy 5L - X70 grade. Further, since in the steel of test No. 32 the starting temperature of quenching was less than (Ar3 point + 50 °C), the strength of the steel could not satisfy 5L - X70 grade.
• the steel of test No. 33 could not ensure a tempering temperature of 550 °C or more, an additional welding test was performed and it was found that the strength was decreased in a welding heat affected zone.
• the chemical compositions of the steels, the microstructure of the steel, and the precipitation of ferrite at grain boundaries in the steels are specified. Accordingly, the steel can obtain high strength and stable, excellent
• the seamless steel pipe and its production method of the present invention can be utilized widely in technical fields requiring for a high strength seamless steel pipe excellent in HIC resistance.

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• 2003
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