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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/085—Cooling or quenching
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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

• This disclosure relates to a steel pipe suitable for line pipes used for transporting crude oil and natural gas, particularly a steel pipe that is suitably used in sour environments containing hydrogen sulfide, and has excellent sulfide stress corrosion cracking resistance (SSCC resistance). This disclosure also relates to a method for producing the aforementioned steel pipe.
• SSCC resistance sulfide stress corrosion cracking resistance
• a steel pipe is produced by forming a steel plate produced by a plate mill or hot rolling mill into a steel pipe by, for example, UOE forming, press bend forming, and roll forming.
• Line pipes used for transporting oil and natural gas require various strengths, toughness, and weldability depending on the operating environment. Furthermore, line pipes used in sour environments of petroleum and natural gas containing hydrogen sulfide (H 2 S) are required to have properties that can suppress hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), commonly referred to as sour resistance.
• HIC hydrogen-induced cracking
• SSCC sulfide stress corrosion cracking
• HIC is a phenomenon in which hydrogen ions generated by corrosion reactions adsorb onto the surface of steel, enter the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS and hard secondary phase within the steel, recombine into molecular hydrogen, and cause cracking due to the internal pressure generated thereby.
• This HIC is considered as a problem in line pipes with relatively low strength levels relative to oil well pipes or tubes, and many countermeasure techniques have been disclosed.
• SSCC is known to occur in regions having high hardness in the welded portions of oil well pipes or tubes and line pipes, and has generally not been regarded as a significant issue in line pipes having relatively low hardness.
• SSCC originating from local corrosion in the base metal can occur. Therefore, it is considered important to suppress local corrosion in the base metal and improve SSCC resistance in line pipes used in sour environments.
• TMCP Thermo-Mechanical Control Process
• TMCP Thermo-Mechanical Control Process
• it is effective to increase the cooling rate during controlled cooling.
• controlled cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, resulting in an increase in the hardness of the surface layer compared to the inside of the steel plate.
• work hardening occurs when a steel plate is formed into a tubular shape to produce the steel pipe, resulting in an increase in the hardness of the surface layer of the steel pipe, which in turn leads to a reduction in SSCC resistance.
• WO2020/067209A1 discloses a high strength steel plate for a sour-resistant line pipe having a tensile strength of 520 MPa or more and improved SSCC resistance, in which the steel microstructure at a depth of 0.25 mm below the surface of the steel plate is a bainitic microstructure having a dislocation density of 1.0 ⁇ 10 14 (m -2 ) to 7.0 ⁇ 10 14 (m -2 ), and variations in Vickers hardness is controlled, thereby improving SSCC resistance.
• WO2021/020220A1 discloses a high strength steel plate for a sour-resistant line pipe having a tensile strength of 520 MPa or more and improved SSCC resistance, in which the steel microstructure at a depth of 0.25 mm below the surface of the steel plate is a bainitic microstructure, and the area ratio of crystal grains having a Kernel Average Misorientation (KAM) value of 0.4 or more in the bainite is set to 50 % or less, thereby improving SSCC resistance.
• KAM Kernel Average Misorientation
• steel pipes produced using the steel plate described in PTL 2, in which the KAM value is controlled in the bainitic microstructure at a depth of 0.25 mm below the steel plate surface exhibit excellent SSCC resistance.
• the steel pipe thus obtained possesses sufficient SSCC resistance, there remains room for improvement in terms of achieving this property consistently.
• high strength means a tensile strength of 520 MPa or more.
• the steel pipe in one of the disclosed embodiments is produced using a steel plate having a specific chemical composition and mechanical properties. Furthermore, the steel pipe obtained by forming the steel plate is characterized by having a specific microstructure in the outermost surface layer on the inside thereof. Below, the reasons for the limitations of the chemical composition, mechanical properties, and microstructure will be explained. In terms of chemical composition, since the steel pipe is produced using a steel plate, the chemical composition of the steel pipe is the same as that of the steel plate used.
• C is an element that contributes to improving the strength of the steel plate. If the C content is less than 0.020 %, sufficient strength cannot be ensured; therefore, the C content should be 0.020 % or more.
• the C content is preferably 0.025 % or more.
• the C content exceeds 0.080 %, the SSCC resistance and HIC resistance deteriorate due to increased hardness in the surface layer and central segregation area during accelerated cooling. In addition, the toughness of the steel plate also deteriorates. Therefore, the C content is set to 0.080 % or less.
• the C content is preferably 0.070 % or less.
• Mn 0.50 % or more and 1.80 % or less
• Mn is an element that contributes to improving the strength and toughness of the steel plate. If the Mn content is less than 0.50 %, these effects are not sufficiently exhibited. Therefore, the Mn content is set to 0.50 % or more. The Mn content is preferably 0.80 % or more. On the other hand, if the Mn content exceeds 1.80 %, the SSCC resistance and HIC resistance deteriorate due to an increase in the hardness of the surface layer and the central segregation area during accelerated cooling. Weldability is also degraded. Therefore, the Mn content is set to 1.80 % or less. The Mn content is preferably 1.70 % or less.
• Mo is an element that contributes to improving the strength and toughness of the steel plate, as well as enhancing SSCC resistance. If the Mo content is less than 0.01 %, these effects are not sufficiently exhibited. Therefore, the Mo content is set to 0.01 % or more. The Mo content is preferably 0.10 % or more. On the other hand, if the Mo content is too high, the hardenability becomes excessive, leading to an increase in hardness and a deterioration in SSCC resistance. Weldability is also degraded. Therefore, the Mo content is set to 0.50 % or less. The Mo content is preferably 0.40 % or less.
• N 0.0010 % or more and 0.0080 % or less
• N is an element that contributes to improving the strength of the steel plate, and for this purpose, it is contained in an amount of 0.0010 % or more.
• the N content is preferably 0.0015 % or more.
• the N content is set to 0.0080 % or less.
• the N content is preferably 0.0070 % or less.
• the chemical composition should include the above components along with the following components.
• Si 0.01 % or more and 0.50 % or less
• the Si is added for deoxidation, but when the Si content is less than 0.01 %, the deoxidizing effect is insufficient. Therefore, the Si content is set to 0.01 % or more.
• the Si content is preferably 0.05 % or more.
• the Si content is set to 0.50 % or less.
• the Si content is preferably 0.45 % or less.
• the P content is an inevitable impurity element, which degrades weldability and increases the hardness of the central segregation area, thereby degrading HIC resistance. This tendency becomes more pronounced if the P content exceeds 0.015 %. Therefore, the P content is set to be 0.015 % or less.
• the P content is preferably 0.008 % or less.
• the lower limit of the P content is not particularly restricted and may be 0 %. However, excessive reduction leads to higher refining costs, so from the standpoint of industrial production, it is preferable to set the P content to 0.001 % or more.
• the Al content is set to 0.010 % or more, and is preferably 0.015 % or more.
• the Al content is set to 0.080 % or less.
• the Al content is preferably 0.070 % or less.
• Ca is an effective element for improving HIC resistance through morphological control of sulfide inclusions, but its addition effect is not sufficient when the Ca content is less than 0.0005 %. Therefore, the Ca content is set to 0.0005 % or more.
• the Ca content is preferably 0.0008 % or more.
• the Ca content is set to 0.0050 % or less.
• the Ca content is preferably 0.0045 % or less.
• the chemical composition of the steel pipe in another embodiment of this disclosure may further contain at least one selected from the group consisting of Cu, Ni, Cr, Nb, V, Ti, Zr, Mg, and REM to further improve the properties as a steel pipe.
• Cu is an effective element for improving the toughness and increasing the strength of the steel plate.
• the Cu content be 0.05 % or more in order to obtain the aforementioned effect.
• the Cu content exceeds 0.30 %, fine cracks known as "fissures" are more likely to form in environments with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Cu is added, the upper limit is set to 0.30 %.
• the Cu content is preferably 0.20 % or less.
• Ni is an effective element for improving the toughness and increasing the strength of the steel plate.
• the Ni content be 0.01 % or more in order to obtain the aforementioned effect.
• the Ni content exceeds 0.10 %, fine cracks known as "fissures" are more likely to form in environments with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is added, the upper limit is set to 0.10 %.
• the Ni content is preferably 0.02 % or less.
• Cr like Mn
• Cr is an effective element for obtaining sufficient strength in the steel plate even with a low C content.
• the Cr content be 0.05 % or more in order to obtain the aforementioned effect.
• the Cr content exceeds 0.50 %, the hardenability becomes excessive, resulting in increased hardness and a deterioration in SSCC resistance. Weldability is also degraded. Therefore, when Cr is added, the upper limit is set to 0.50 %.
• Nb is an element that can be optionally added to increase the strength and toughness of the steel plate.
• Nb content be 0.005 % or more in order to obtain the aforementioned effect.
• the upper limit is set to 0.1 %.
• V 0.1 % or less
• V is an element that may be optionally added to improve the strength and toughness of the steel plate.
• V content be 0.005 % or more in order to obtain the aforementioned effect.
• the upper limit is set to 0.1 %.
• Ti like Nb and V, is an element that may be optionally added to increase the strength and toughness of the steel plate.
• Ti is added, it is preferable that the Ti content be 0.005 % or more in order to obtain the aforementioned effect.
• the Ti content exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, when Ti is added, the upper limit is set to 0.1 %.
• Zr is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions.
• Zr content be 0.0005 % or more in order to obtain the aforementioned effects.
• the Zr content exceeds 0.02 %, the effect becomes saturated. Therefore, when Zr is added, the upper limit is set to 0.02 %.
• Mg is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions.
• Mg content be 0.0005 % or more in order to obtain the aforementioned effects.
• the Mg content exceeds 0.02 %, the effect becomes saturated. Therefore, when Mg is added, the upper limit is set to 0.02 %.
• REM like Zr and Mg, is an element that can be optionally added to improve the toughness of the steel plate through crystal grain refinement and to enhance cracking resistance by controlling the characteristics of inclusions.
• the REM content is set to 0.0005 % or more in order to obtain the aforementioned effects.
• the REM content exceeds 0.02 %, the effect becomes saturated. Therefore, when REM is added, the upper limit is set to 0.02 %.
• Equation (2) above [%X] represents the content (mass%) of element X, and is taken to be 0 when the corresponding element is not contained.
• the above-mentioned CP value is an equation devised to estimate the material property of the central segregation area based on the content of each alloying element.
• the concentration of components in the central segregation zone increases, leading to an increase in the hardness of the central segregation zone. Therefore, by keeping the CP value determined by Equation (2) above at or below 1.00, it becomes possible to suppress crack formation in the HIC test. Since the hardness of the central segregation area decreases with a lower CP value, the upper limit for the CP value can be set to 0.95 when higher HIC resistance is required.
• O is an inevitable element in the steel plate, but is acceptable in this disclosure when its content is 0.0050 % or less, preferably 0.0040 % or less.
• the microstructure at a position 0.25 mm outward in a radial direction from an inner circumferential surface of the steel pipe (hereinafter referred to as at a depth of 0.25 mm below the inner surface of the steel pipe) is a bainitic microstructure mainly composed of bainite.
• "mainly composed of bainite” means that the area ratio of bainite is 95 % or more.
• the microstructure at 0.25 mm below the inner surface of the steel pipe In order to improve the SSCC resistance by keeping the maximum hardness at 0.25 mm below the inner surface of the steel pipe constant, it is necessary for the microstructure at 0.25 mm below the inner surface of the steel pipe to be a bainitic microstructure.
• hard phases such as martensite or martensite austenite constituent (MA) are formed in the surface layer up to a depth of 0.25 mm from the inner surface of the steel pipe, the hardness of the inner surface layer of the steel pipe increases, and the variation in the hardness of the inner surface layer increases, thereby hindering the material homogeneity. Therefore, the microstructure of the inner surface layer of the steel pipe should be a bainitic microstructure.
• the bainitic microstructure includes a microstructure referred to as lath-like bainite or granular bainite, which transforms during or after accelerated cooling and contributes to transformation strengthening.
• different microstructures such as ferrite, martensite, pearlite, martensite austenite constituent, and retained austenite
• a smaller area ratio of microstructures other than bainite is preferable.
• the area fraction of microstructures other than bainite is sufficiently low, their effects can be ignored, so a certain amount is acceptable.
• the total area ratio of microstructures other than bainite is less than 5 %.
• the area ratio of bainite with a crystal plane orientation of ⁇ 110 ⁇ aligned within 15° is 30.0 % or less.
• the reason for controlling the microstructure of the surface layer on the inner side of the steel pipe is that the inner surface is exposed to a sour environment and comes into contact with hydrogen sulfide.
• the reason for controlling the microstructure at a depth of 0.25 mm below the inner surface of the steel pipe is that the crack depth used to determine the presence or absence of SSCC occurs at this 0.25 mm position.
• the bainite with a crystal plane orientation of ⁇ 110 ⁇ aligned within 15° is explained by assuming the inner circumferential surface (curved surface) of the steel pipe as a flat plate, with reference to Fig. 1 . That is, the aforementioned bainite refers to bainite that has a crystal plane orientation in which the axis 1, which is perpendicular to the ⁇ 110 ⁇ plane, is aligned 15° or less with respect to the axis L perpendicular to the plate surface, as shown in Fig. 1 . Since the inner circumferential surface of the steel pipe is actually a curved surface, the axis L above can be considered as the normal to the inner peripheral surface.
• bainite with the specified crystal plane orientation contributes to suppressing the initiation of local corrosion, which is considered the initiation point for SSCC occurrence.
• bainite with a crystal plane orientation of ⁇ 110 ⁇ aligned within 15° has an impact on SSCC resistance, and by controlling the area ratio of such bainite to 30 % or less, as demonstrated in the examples described later, excellent and stable SSCC resistance can be consistently achieved.
• the tensile strength (TS) of the steel pipe according to this disclosure is not particularly limited as long as the steel pipe can be produced. However, in steel pipes with low tensile strength, SSCC resistance is generally not a concern. Therefore, it is preferable that the steel pipe of this disclosure has a tensile strength of 520 MPa or higher. In particular, steel pipes with a high tensile strength of 520 MPa or higher are particularly suitable for applications such as line pipes.
• the steel pipe of this disclosure can be produced by heating a steel material having the aforementioned chemical composition, followed by hot rolling to form a hot-rolled steel plate, then applying controlled cooling under predetermined conditions to the hot-rolled steel plate to form a steel plate, and finally forming the steel plate into a steel pipe.
• the steel material may be, for example, a steel slab.
• the steel material may be produced by any method, but for example, can be produced by smelting steel having the aforementioned chemical composition by a conventional method and subjecting the smelted steel to casting.
• the smelting can be performed by any method using a converter, an electric furnace, an induction furnace, and the like.
• the casting is preferably performed by continuous casting in terms of productivity, but may be performed by ingot casting.
• Heating temperature 1000 °C or higher and 1300 °C or lower
• the steel material is heated prior to hot rolling.
• the heating may be carried out after the steel material obtained by casting or other methods has been once cooled, or the steel material obtained may be directly subjected to the heating without cooling.
• the heating temperature of the steel material is lower than 1000 °C, carbides are not sufficiently dissolved, and the necessary strength as a steel plate cannot be obtained. Therefore, the heating temperature is set to 1000 °C or higher. On the other hand, if the heating temperature exceeds 1300 °C, excessive energy will be required, resulting in reduced productivity. In addition, the toughness of the steel plate also deteriorates. Therefore, the heating temperature is set to 1300 °C or lower. Note that the heating temperature refers to the temperature inside the heating furnace, and the steel material is heated to the aforementioned temperature throughout, including the center.
• the heated steel material is hot rolled to obtain a hot-rolled steel plate.
• the total rolling reduction during hot rolling in the austenite region is set to 95 % or less. If the total rolling reduction exceeds 95 %, shear deformation promotes the development of austenite with ⁇ 111 ⁇ crystal planes oriented along the plate surface. After hot rolling and subsequent cooling, this develops the formation of a bainitic texture with ⁇ 110 ⁇ crystal plane orientation.
• the total rolling reduction is set to 95 % or lower.
• the hot-rolling finish temperature should be as low as possible.
• the rolling finish temperature is measured as the surface temperature of the steel material, which can be determined using a radiation thermometer or the like.
• Equation (3) [%X] represents the content (mass%) of element X and is taken to be 0 when the corresponding element is not contained.
• cooling is applied to the hot-rolled steel plate after hot rolling, within the temperature range from the cooling start temperature to the cooling stop temperature, as described later.
• the cooling start temperature should be set to at least (Ar 3 - 10 °C).
• the cooling start temperature is also equal to or lower than the rolling finish temperature.
• the cooling start temperature is measured as the surface temperature of the hot-rolled steel plate, which can be determined using a radiation thermometer or the like.
• Cooling rate average cooling rate of 50 °C/s or less within a temperature range of 750 °C to 550 °C for a steel plate temperature at a depth of 0.25 mm below the steel plate surface
• the cooling rate of the steel plate surface layer To enhance the strength of the steel plate, i.e., the steel pipe, while reducing the hardness variation within the steel plate and improving the material homogeneity, it is important to control the cooling rate of the steel plate surface layer. In particular, to obtain the desired microstructure 0.25 mm below the steel plate surface, it is necessary to control the average cooling rate to 50 °C/s or less within the temperature range of 750 °C to 550 °C for the steel plate temperature at a depth of 0.25 mm below the steel plate surface.
• the average cooling rate By minimizing the average cooling rate, it is possible to produce bainite whose ⁇ 110 ⁇ crystal planes are not oriented within 15° Furthermore, by reducing the average cooling rate, it is possible to lower the maximum hardness, thereby improving the SSCC resistance. If the average cooling rate exceeds 50 °C/s, the area ratio of bainite with a crystal plane orientation of ⁇ 110 ⁇ aligned within 15° increases, and the maximum hardness HV 0.5 at 0.25 mm below the steel plate surface exceeds 230.
• the average cooling rate should be set to 50 °C/s or less. Preferably, it is 30 °C/s or less.
• the total rolling reduction in the austenite region and the average cooling rate within a temperature range of 750 °C to 550 °C for the steel plate temperature at a depth of 0.25 mm below the steel plate surface are critical.
• Cooling stop temperature 550 °C or lower for steel plate temperature at depth of 0.25 mm below steel plate surface
• the hot-rolled steel plate after hot rolling is cooled from the cooling start temperature to the cooling stop temperature by controlled cooling.
• the cooling stop temperature is set to 550 °C or lower.
• the cooling stop temperature is 250 °C or higher.
• the steel plate temperature at a depth of 0.25 mm below the steel plate surface cannot be measured directly due to physical limitations. However, based on the surface temperature of the hot-rolled steel plate measured by a radiation thermometer at the start of cooling and the surface temperature of the hot-rolled steel plate at the target cooling stop, the temperature distribution within the cross section along the thickness direction can be calculated in real-time using, for example, a process computer and differential calculation, and the temperature at the specified depth can be derived from this result.
• the temperature at a depth of 0.25 mm below the steel plate surface in the temperature distribution is referred to as "the steel plate temperature at a depth of 0.25 mm below the steel plate surface" in this specification.
• Steels with the chemical compositions listed in Table 1 were made into steel slabs as steel materials by continuous casting.
• the obtained steel slabs were heated to the heating temperatures listed in Table 2, and then hot rolled with a total rolling reduction in the austenite region listed in Table 2 to obtain hot-rolled steel plates with the thicknesses listed in Table 2.
• controlled cooling was performed on the obtained hot-rolled steel plates using a water-cooled controlled cooling system under the conditions listed in Table 2, resulting in steel plates.
• the edges of the obtained steel plates were subjected to groove machining, and the plates were formed into a pipe shape using C press, U-ing press, and O-ing press.

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