EP3604592A1 - Hochfestes stahlblech für sauergasbeständiges leitungsrohr, verfahren zur herstellung davon und hochfestes stahlrohr mit hochfestem stahlblech für sauergasbeständiges leitungsrohr - Google Patents

Hochfestes stahlblech für sauergasbeständiges leitungsrohr, verfahren zur herstellung davon und hochfestes stahlrohr mit hochfestem stahlblech für sauergasbeständiges leitungsrohr Download PDF

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EP3604592A1
EP3604592A1 EP18774336.4A EP18774336A EP3604592A1 EP 3604592 A1 EP3604592 A1 EP 3604592A1 EP 18774336 A EP18774336 A EP 18774336A EP 3604592 A1 EP3604592 A1 EP 3604592A1
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Prior art keywords
steel plate
high strength
temperature
sour
less
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French (fr)
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EP3604592B1 (de
EP3604592A4 (de
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Shusaku Ota
Tomoyuki Yokota
Kazukuni Hase
Yuta Tamura
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JFE Steel Corp
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JFE Steel Corp
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    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 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
    • 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
    • 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0231—Warm rolling
    • 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
    • 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
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium

Definitions

  • This disclosure relates to a high strength steel plate for a sour-resistant line pipe that is excellent in material homogeneity in the steel plate and that is suitable for use in line pipes in the fields of construction, marine structure, shipbuilding, civil engineering, and construction industry machinery, and to a method for manufacturing the same.
  • This disclosure also relates to a high strength steel pipe using the high strength steel plate for a sour-resistant line pipe.
  • a line pipe is manufactured by forming a steel plate manufactured by a plate mill or a hot rolling mill into a steel pipe by UOE forming, press bend forming, roll forming, or the like.
  • the line pipe used to transport crude oil and natural gas containing hydrogen sulfide is required to have so-called sour resistance such as resistance to hydrogen-induced cracking (HIC resistance) and resistance to sulfide stress corrosion cracking (SSCC resistance), in addition to strength, toughness, weldability, and so on.
  • HIC resistance hydrogen-induced cracking
  • SSCC resistance resistance to sulfide stress corrosion cracking
  • HIC resistance hydrogen-induced cracking
  • SSCC resistance resistance to sulfide stress corrosion cracking
  • SSCC is generally known to occur in high strength seamless steel pipes for oil wells and in high hardness regions of welds, and has not been regarded as a problem in line pipes with relatively low hardness.
  • SSCC also occurs in the base metal of line pipes in environments where oil and natural gas mining environments have become increasingly severe and environments with high hydrogen sulfide partial pressure or low pH. It is also pointed out that it is important to control the hardness of the surface layer of the inner surface of a steel pipe to improve the SSCC resistance under more severe corrosion environments.
  • TMCP Thermo-Mechanical Control Process
  • TMCP Thermo-Mechanical Control Process
  • it is effective to increase the cooling rate during controlled cooling.
  • the control cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, and the hardness of the surface layer becomes higher than that of the inside of the steel plate, and the hardness distribution in the plate thickness direction becomes uneven. Therefore, it is a problem in terms of ensuring the material homogeneity in the steel plate.
  • JP3951428B (PTL 1) and JP3951429B (PTL 2) describe methods for manufacturing steel plates with a reduced material property difference in the plate thickness direction by interrupting accelerated cooling after rolling, leaving the surface recuperated, and then performing accelerated cooling again.
  • JP2002-327212A (PTL 3) and JP3711896B (PTL 4) describe methods for manufacturing steel plates for line pipes in which the hardness of the surface layer is reduced by heating the surface of a steel plate after accelerated cooling to a higher temperature than the inside using a high frequency induction heating device.
  • the cooling rate of the surface layer in accelerated cooling is so high that the hardness of the surface layer may not be sufficiently reduced only by heating the steel plate surface.
  • the methods of PTLs 5 and 6 apply descaling to reduce the surface characteristics defects due to the scale indentation during hot leveling and to reduce the variation in the cooling stop temperature of the steel plate to improve the steel plate shape.
  • no consideration is given to the cooling conditions for obtaining a uniform material property. That is, in the techniques described in PTLs 5 and 6, the cooling rate of the surface layer in accelerated cooling is not considered at all. Therefore, there is a possibility that the hardness of the surface layer can not be sufficiently reduced at the cooling rate for securing the tensile characteristics at mid-thickness part, and as a result, the variation in hardness occurs in the plate thickness direction.
  • the present inventors repeated many experiments and examinations about the chemical compositions, microstructures, and manufacturing conditions of steel materials in order to ensure proper HIC resistance and SSCC resistance under more severe corrosion environments.
  • the inventors discovered that in order to further improve the SSCC resistance of a high strength steel pipe, it is not sufficient to merely suppress the surface layer hardness as conventionally found, and in particular, that it is possible to reduce the increase in hardness in the coating process after pipe making by forming the outermost surface layer of the steel plate, specifically at 0.5 mm below the surface of the steel plate, with a bainite microstructure having a dislocation density of 0.5 ⁇ 10 14 to 7.0 ⁇ 10 14 (m -2 ), and as a result the SSCC resistance of the steel pipe is improved.
  • the inventors also discovered that it is important to strictly control both the thermal hysteresis at 0.5 mm below the surface of the steel plate in controlled cooling and the average thermal hysteresis of the steel plate, and then reduce excess dislocations introduced by controlled cooling by induction heating.
  • the inventors also discovered that the variation in hardness in the plate thickness direction can be remarkably reduced by performing induction heating under predetermined conditions taking into account T 1 and T 2 in controlled cooling, where T 1 denotes a temperature of the surface of the steel plate at the start of cooling and T 2 denotes a cooling stop temperature in terms of an average temperature of the steel plate.
  • T 1 denotes a temperature of the surface of the steel plate at the start of cooling
  • T 2 denotes a cooling stop temperature in terms of an average temperature of the steel plate.
  • the high strength steel plate for a sour-resistant line pipe and the high strength steel pipe using the high strength steel plate for a sour-resistant line pipe disclosed herein are excellent in HIC resistance and SSCC resistance under more severe corrosion environments, and excellent in hardness uniformity in the thickness direction.
  • FIG. 1 is a schematic view illustrating a method for obtaining test pieces for evaluation of SSCC resistance in Examples.
  • the C effectively contributes to the improvement in strength.
  • the content is less than 0.02 %, sufficient strength can not be secured, while if it exceeds 0.08 %, the hardness of the surface layer increases during accelerated cooling, causing deterioration in HIC resistance and SSCC resistance. The toughness also deteriorates. Therefore, the C content is in a range of 0.02 % to 0.08 %.
  • Si is added for deoxidation. However, if the content is less than 0.01 %, the deoxidizing effect is not sufficient, while if it exceeds 0.50 %, the toughness and weldability are degraded. Therefore, the Si content is in a range of 0.01 % to 0.50 %.
  • Mn effectively contributes to the improvement in strength and toughness. However, if the content is less than 0.50 %, the addition effect is poor, while if it exceeds 1.80 %, the hardness of the central segregation area increases during accelerated cooling, causing deterioration in HIC resistance. The weldability also deteriorates. Therefore, the Mn content is in a range of 0.50 % to 1.80 %.
  • the P is an inevitable impurity element that degrades the weldability and increases the hardness of the central segregation area, causing deterioration in HIC resistance. Since this tendency becomes remarkable when the P content exceeds 0.015 %, the upper limit is set at 0.015 %. Preferably, the P content is 0.008 % or less. Although a lower P content is preferable, the P content is set to 0.001 % or more from the viewpoint of the refining cost.
  • S is an inevitable impurity element that forms MnS inclusions in the steel and degrades the HIC resistance, and hence a lower S content is preferable. However, up to 0.0015 % is acceptable. Although a lower S content is preferable, the S content is set to 0.0002 % or more from the viewpoint of the refining cost.
  • Al is added as a deoxidizing agent.
  • an Al content below 0.01 % provides no addition effect, while an Al content beyond 0.08 % lowers the cleanliness of the steel and deteriorates the toughness. Therefore, the Al content is in a range of 0.01 % to 0.08 %.
  • Ca is an element effective for improving the HIC resistance by morphological control of sulfide inclusions.
  • the content is less than 0.0005 %, its addition effect is not sufficient.
  • the content exceeds 0.005 %, not only the addition effect saturates, but also the HIC resistance is deteriorated due to the reduction in the cleanliness of the steel. Therefore, the Ca content is in a range of 0.0005 % to 0.005 %.
  • the chemical composition of the present disclosure may also contain at least one selected from the group consisting of Cu, Ni, Cr, and Mo to further improve the strength and toughness of the steel plate.
  • the Cu is an element effective for improving the toughness and increasing the strength.
  • the Cu content is preferably 0.05 % or more, yet if the content is too large, the weldability deteriorates. Therefore, when Cu is added, the Cu content is up to 0.50 %.
  • Ni is an element effective for improving the toughness and increasing the strength. To obtain this effect, the Ni content is preferably 0.05 % or more, yet excessive addition of Ni is not only economically disadvantageous but also deteriorates the toughness of the heat-affected zone. Therefore, when Ni is added, the Ni content is up to 0.50 %.
  • the Cr content is preferably 0.05 % or more, yet if the content is too large, the weldability deteriorates. Therefore, when Cr is added, the Cr content is up to 0.50 %.
  • Mo is an element effective for improving the toughness and increasing the strength.
  • the Mo content is preferably 0.05 % or more, yet if the content is too large, the weldability deteriorates. Therefore, when Mo is added, the Mo content is up to 0.50 %.
  • the chemical composition according to the present disclosure may further optionally contain one or more selected from the group consisting of Nb, V, and Ti in the following range.
  • Nb 0.005 % to 0.1 %
  • V 0.005 % to 0.1 %
  • Ti 0.005 % to 0.1 %
  • Nb, V, and Ti are all elements that can be optionally added to enhance the strength and toughness of the steel plate. If the content of each added element is less than 0.005 %, the addition effect is poor, while if it exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, the content of each added element is preferably in a range of 0.005 % to 0.1 %.
  • the present disclosure discloses a technique for improving the SSCC resistance of the high strength steel pipe using the high strength steel plate for a sour-resistant line pipe
  • the technique disclosed herein needs to satisfy the HIC resistance at the same time as the sour resistant performance. Therefore, the CP value obtained by the following Expression (1) is set to 1.00 or less.
  • CP 4.46 % C + 2.37 % Mn / 6 + 1.74 % Cu + 1.7 % Ni / 15 + 1.18 % Cr + 1.95 % Mo + 1.74 % V / 5 + 22.36 % P
  • [%X] indicates the content by mass% of the element X in steel.
  • the CP value is a formula devised to estimate the material property at the central segregation area from the content of each alloying element, and the component concentrations of the central segregation area are higher as the CP value of Expression (1) is higher, causing a rise in the hardness of the central segregation area. Therefore, by setting the CP value obtained in Expression (1) to 1.00 or less, it is possible to suppress the occurrence of cracking in the HIC test. In addition, since the hardness of the central segregation area is lower as the CP value is lower, the upper limit for the CP value may be set to 0.95 when higher HIC resistance is required.
  • the steel microstructure of the high strength steel plate for a sour-resistant line pipe disclosed herein will be described.
  • the steel microstructure needs to be a bainite microstructure.
  • a hard phase such as martensite or martensite austenite constituent (MA)
  • MA martensite or martensite austenite constituent
  • the surface layer hardness is increased, the variation in hardness in the steel plate is increased, and the material homogeneity is impaired.
  • the surface layer is formed with a bainite microstructure as the steel microstructure.
  • the bainite microstructure includes a microstructure called bainitic ferrite or granular ferrite which contributes to transformation strengthening. These microstructures appear through transformation during or after accelerated cooling. If different microstructures such as ferrite, martensite, pearlite, martensite austenite constituent, retained austenite, and the like are mixed in the bainite microstructure, a decrease in strength, a deterioration in toughness, a rise in surface hardness, and the like occur. Therefore, it is preferable that microstructures other than the bainite phase have smaller proportions. However, when the volume fraction of such microstructures other than the bainitic phase is sufficiently low, their effects are negligible, and up to a certain amount is acceptable.
  • the total of the steel microstructures other than bainite (such as ferrite, martensite, pearlite, martensite austenite constituent, and retained austenite) is less than 5 % by volume fraction, there is no adverse effect, and this is acceptable.
  • the bainite microstructure takes various forms according to the cooling rate, it is important for the present disclosure that the outermost surface layer of the steel plate, specifically at 0.5 mm below the surface of the steel plate, is formed with a bainite microstructure having a dislocation density of 0.5 ⁇ 10 14 to 7.0 ⁇ 10 14 (m -2 ). Since the dislocation density decreases in the coating process after pipe making, the hardness increase due to age hardening can be minimized if the dislocation density at 0.5 mm below the surface of the steel plate is 7.0 ⁇ 10 14 (m -2 ) or less.
  • the dislocation density at 0.5 mm below the surface of the steel plate exceeds 7.0 ⁇ 10 14 (m -2 )
  • the dislocation density does not decrease in the coating process after pipe making, and the hardness is significantly increased due to age hardening, causing deterioration in the SSCC resistance.
  • the range of dislocation density is preferably 6.0 ⁇ 10 14 (m -2 ) or less in order to obtain good SSCC resistance after pipe making.
  • the dislocation density at 0.5 mm below the surface of the steel plate is less than 0.5 ⁇ 10 14 (m -2 )
  • the strength of the steel plate deteriorates.
  • dislocation density 1.0 ⁇ 10 14 (m -2 ) or more.
  • the outermost surface layer ranging from the surface of the steel plate to a depth of 0.5 mm has an equivalent dislocation density, and as a result, the above-described SSCC resistance improving effect is obtained.
  • the HV 0.1 at 0.5 mm below the surface is 230 or less. From the viewpoint of securing the SSCC resistance of the steel pipe, it is important to suppress an increase in the surface hardness of the steel plate. However, by setting the HV 0.1 at 0.5 mm below the surface of the steel plate to 230 or less, the HV 0.1 at 0.5 mm below the surface following the coating process after pipe making can be suppressed to 260 or less, and the SSCC resistance can be secured.
  • the high strength steel plate disclosed herein in addition to the average value of Vickers hardness at 0.5 mm below the surface of the steel plate being 230 HV or less, it is also important from the viewpoint of securing the material property in the mid-thickness part while suppressing an increase in the hardness of the surface layer that the difference ⁇ HV between the average value of Vickers hardness at 0.5 mm below the surface of the steel plate and the average value of Vickers hardness at the mid-thickness part of the steel plate is 25 HV or less. More preferably, ⁇ HV is 20 HV or less.
  • the high strength steel plate disclosed herein is a steel plate for steel pipes having a strength of X60 grade or higher in API 5L, and thus has a tensile strength of 520 MPa or more.
  • the manufacturing method according to the present disclosure comprises: heating a slab having the above-described chemical composition, and then hot rolling the slab to form a steel plate; then subjecting the steel plate to controlled cooling under predetermined conditions; and then reheating the steel plate by induction heating.
  • the slab heating temperature is set to 1000 °C to 1300 °C. This temperature is the temperature in the heating furnace, and the slab is heated to this temperature to the center.
  • the rolling finish temperature in terms of a temperature of a surface of the steel plate needs to be set in consideration of the required toughness for base metal and rolling efficiency. From the viewpoint of improving the strength and the HIC resistance, it is preferable to set the rolling finish temperature at or above the Ar 3 transformation temperature in terms of a temperature of the surface of the steel plate.
  • the Ar 3 transformation temperature means the ferrite transformation start temperature during cooling, and can be determined, for example, from the components of steel according to the following equation.
  • the rolling reduction ratio in a temperature range of 950 °C or lower corresponding to the austenite non-recrystallization temperature range to 60 % or more.
  • the temperature of the surface of the steel plate can be measured by a radiation thermometer or the like.
  • Ar 3 ° C 910 ⁇ 310 % C ⁇ 80 % Mn ⁇ 20 % Cu ⁇ 15 % Cr ⁇ 55 % Ni ⁇ 80 % Mo , where [%X] indicates the content by mass% of the element X in steel.
  • T 1 is (Ar 3 - 10 °C) or higher, where T 1 denotes a temperature of the surface of the steel plate at the start of cooling.
  • the temperature of the surface of the steel plate at the start of cooling is set to (Ar 3 - 10 °C) or higher.
  • Average cooling rate in a temperature range from 750 °C to 550 °C in terms of a temperature at 0.5 mm below the surface of the steel plate 100 °C/s or lower
  • the average cooling rate in a temperature range from 750 °C to 550 °C in terms of a temperature at 0.5 mm below the surface of the steel plate exceeds 100 °C/s
  • the dislocation density at 0.5 mm below the surface of the steel plate exceeds 7.0 ⁇ 10 14 (m -2 ).
  • the average cooling rate is set to 100 °C/s or lower. Preferably, it is 80 °C/s or lower.
  • the lower limit of the average cooling rate is not particularly limited, yet if the cooling rate is excessively low, ferrite and pearlite are generated and the strength is insufficient. Therefore, from the viewpoint of preventing this, 10 °C/s or higher is preferable.
  • Average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate 15 °C/s or higher If the average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate is lower than 15 °C/s, a bainite microstructure can not be obtained, causing deterioration in the strength and HIC resistance, and also causing more variations in the hardness in the plate thickness direction, and the like. Therefore, the cooling rate in terms of an average temperature of the steel plate is set to 15 °C/s or higher. From the viewpoint of variations in the strength and hardness of the steel plate, the steel plate average cooling rate is preferably 20 °C/s or higher. The upper limit of the average cooling rate is not particularly limited, yet is preferably 80 °C/s or lower such that excessive low-temperature transformation products will not be generated.
  • a temperature distribution in a cross section in the plate thickness direction can be determined in real time by difference calculation using a process computer on the basis of the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling.
  • the temperature at 0.5 mm below the surface of the steel plate in the temperature distribution is referred to as the "temperature at 0.5 mm below the surface of the steel plate", and the average value of temperatures in the thickness direction in the temperature distribution as the "average temperature of the steel plate”.
  • T 2 is 250 °C to 550 °C, where T 2 is a cooling stop temperature in terms of an average temperature of the steel plate.
  • a bainite phase is generated by performing controlled cooling to quench the steel plate to a temperature range of 250 °C to 550 °C which is the temperature range of bainite transformation.
  • the cooling stop temperature exceeds 550 °C, bainite transformation is incomplete and sufficient strength can not be obtained.
  • the cooling stop temperature is lower than 250 °C, martensite and martensite austenite constituent (MA) are formed, and in particular, the variation in hardness in the plate thickness direction becomes significant. Therefore, in order to suppress deterioration of material homogeneity in the steel plate, the cooling stop temperature of the controlled cooling is set to 250 °C to 550 °C in terms of an average temperature of the steel plate.
  • T 3 is 550 °C to 750 °C, where T 3 denotes an induction heating temperature in terms of a temperature of the surface of the steel plate.
  • the dislocation density at 0.5 mm below the steel plate surface becomes 7.0 ⁇ 10 14 (m -2 ) or less, and excellent SSCC resistance is obtained.
  • the difference ⁇ HV between the average value of Vickers hardness at 0.5 mm below the surface of the steel plate and the average value of Vickers hardness at the mid-thickness part of the steel plate can be set to 25 HV or less.
  • the induction heating temperature when the induction heating temperature is below 550 °C, sufficient tempering effect can not be obtained, and even if the dislocation density of the surface layer can be set to 7.0 ⁇ 10 14 (m -2 ) or less, ⁇ HV can not be set to 25 HV or lower.
  • the induction heating temperature exceeds 750 °C, the mid-thickness part is also tempered, in which case a predetermined strength may not be obtained. Therefore, in order to secure the strength at the mid-thickness part while suppressing the deterioration of the material homogeneity in the steel plate, the end-point temperature of on-line induction heating is set to 550 °C to 750 °C in terms of a temperature of the surface of the steel plate. In this embodiment, it is important not to temper the inside of the steel plate as much as possible in order to suppress a decrease in strength, and it is important to temper only the surface layer. Therefore, heating is performed using an on-line induction heating device.
  • TP defined by the following Expression (2) is preferably 0.50 or more and 1.50 or less. More preferably, it is 0.60 or more and 1.00 or less.
  • TP T 3 ⁇ T 2 ⁇ T 2 / T 1 ⁇ T 2 2 TP is a relational expression of tempering to the degree of supercooling of controlled cooling.
  • ⁇ HV can be set to 20 or less.
  • a high strength steel pipe for a sour-resistant line pipe (such as a UOE steel pipe, an electric-resistance welded steel pipe, and a spiral steel pipe) that has excellent material homogeneity in the steel plate and that is suitable for transporting crude oil and natural gas can be manufactured.
  • an UOE steel pipe is manufactured by groove machining the ends of a steel plate, forming the steel plate into a steel pipe shape by C press, U-ing press, and O-ing press, then seam welding the butting portions by inner surface welding and outer surface welding, and optionally subjecting it to an expansion process.
  • Any welding method may be applied as long as sufficient joint strength and joint toughness are guaranteed, yet it is preferable to use submerged arc welding from the viewpoint of excellent weld quality and manufacturing efficiency.
  • the steels (Steels A to I) having the chemical compositions listed in Table 1 are made into slabs by continuous casting, heated to the temperatures listed in Table 2, and then hot rolled at the rolling finish temperatures and rolling reduction ratios listed in Table 2 to obtain the steel plates of the thicknesses listed in Table 2. Then, the steel plates were subjected to controlled cooling using a water-cooled controlled cooling device under the conditions listed in Table 2. Immediately thereafter, each steel plate was reheated by the method presented in "Heating method" in Table 2 such that the temperature of the surface of the steel plate reached "Maximum temperature in reheating" in Table 2.
  • each obtained steel plate was observed by an optical microscope and a scanning electron microscope.
  • the microstructure at a position of 0.5 mm below the surface of each steel plate and the microstructure at the mid-thickness part are listed in Table 2.
  • HV 0.1 Vickers hardness
  • HV 0.1 Vickers hardness
  • a sample for X-ray diffraction was taken from a position having an average hardness, the sample surface was polished to remove scale, and X-ray diffraction measurement was performed at a position of 0.5 mm below the surface of the steel plate.
  • the dislocation density was converted from the strain obtained from the half width ⁇ of X-ray diffraction measurement.
  • K ⁇ 1 and K ⁇ 2 rays having different wavelengths overlap, and are thus separated by the Rachinger's method.
  • the Williamson-Hall method described below is used for extraction of strain.
  • the strain ⁇ is calculated from the slope of the straight line by plotting ⁇ cos ⁇ / ⁇ relative to sin ⁇ / ⁇ .
  • the diffraction lines used for the calculation are (110), (211), and (220).
  • ⁇ 14.4 ⁇ 2 /b 2 .
  • ⁇ means the peak angle calculated by the ⁇ -2 ⁇ method for X-ray diffraction
  • ⁇ means the wavelength of the X-ray used in the X-ray diffraction
  • b is a Burgers vector of Fe( ⁇ ), which is 0.25 nm in this embodiment.
  • the SSCC resistance was evaluated for a pipe made from a part of each steel plate.
  • Each pipe was manufactured by groove machining the ends of a steel plate, and forming the steel plate into a steel pipe shape by C press, U-ing press, and O-ing press, then seam welding the butting portions on the inner and outer surfaces by submerged arc welding, and subjecting it to an expansion process.
  • FIG. 1 after a coupon cut out from each obtained steel pipe was flattened, an SSCC test piece of 5 mm ⁇ 15 mm ⁇ 115 mm was collected from the inner surface of the steel pipe. At this time, the inner surface to be tested was left intact without removing the scale in order to leave the state of the outermost layer.
  • Each collected SSCC test piece was loaded with 90 % stress of the actual yield strength (0.5 % YS) of the corresponding steel pipe, and evaluation was made using a NACE standard TM0177 Solution A solution, at a hydrogen sulfide partial pressure of 1 bar, in accordance with the 4-point bending SSCC test specified by the EFC 16 standard. After immersion for 720 hours, the SSCC resistance was judged as "Good” when no cracks were observed, or "Poor" when cracking occurred. The results are listed in Table 2.
  • HIC resistance was determined by performing HIC test with an immersion time of 96 hours in accordance with the NACE Standard TM-02-84. The HIC resistance was judged as "Good” when no cracks were observed, or "Poor” when cracking occurred. The results are listed in Table 2.
  • Nos. 1 to 9 are our examples in which the chemical composition and the production conditions satisfy the appropriate ranges of the present disclosure.
  • the tensile strength as a steel plate was 520 MPa or more
  • the microstructure at both positions of 0.5 mm below the surface and of t/2 was a bainite microstructure
  • the HV 0.1 at 0.5 mm below the surface was 230 or less
  • ⁇ HV was 25 or less
  • the SSCC resistance and HIC resistance were also good in each high strength steel pipe produced using the steel plate.
  • Nos. 10 to 16 are comparative examples whose chemical compositions are within the scope of the present disclosure but whose production conditions are outside the scope of the present disclosure.
  • No. 10 since the cooling stop temperature was low, the difference in hardness between the surface layer and the mid-thickness part was large.
  • Nos. 11 and 12 the controlled cooling conditions were outside the scope of the present disclosure, and the dislocation density was significantly increased in the surface layer of the steel plate, with the result that the surface hardness increased and SSCC occurred.
  • the steel plate average cooling rate was not sufficiently secured, and ferrite was formed at the mid-thickness part, resulting in a decrease in strength.
  • No. 10 to 16 are comparative examples whose chemical compositions are within the scope of the present disclosure but whose production conditions are outside the scope of the present disclosure.
  • the difference in hardness between the surface layer and the mid-thickness part was large.
  • Nos. 11 and 12 the controlled cooling conditions were outside the scope of the present disclosure, and the dislocation density was significantly increased in the surface layer of
  • the heating temperature in the on-line induction heating was not optimal, and a difference occurred in the hardness in the plate thickness direction.
  • No. 15 although tempering was carried out by furnace heating, the temperature rise rate was so slow that the entire plate thickness was evenly tempered, resulting in a low strength.
  • No. 16 reheating was not performed, the surface layer was not softened by tempering, and thus the dislocation density of the surface layer was high and SSCC occurred. The variation in hardness in the thickness direction was also large.
  • the chemical compositions of the steel plates were out of the range of the present disclosure, causing deterioration in the HIC resistance.
  • steel pipes such as electric-resistance welded steel pipes, spiral steel pipes, and UOE steel pipes
  • steel pipes manufactured by cold-forming the disclosed steel plate can be suitably used for transportation of crude oil and natural gas that contain hydrogen sulfide and require sour resistance.

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EP18774336.4A 2017-03-30 2018-03-28 Hochfeste stahlplatte für sauergasbeständiges leitungsrohr, verfahren zur herstellung davon und hochfestes stahlrohr aus hochfester stahlplatte für sauergasbeständiges leitungsrohr Active EP3604592B1 (de)

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CN110475894B (zh) 2022-03-22
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