Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties 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
• • 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
  • C22C38/14—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
  • 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

Definitions

• the present invention relates to a high-strength steel plate having excellent low-temperature toughness and a yield strength of 555 MPa or more before and after long-term aging in the medium temperature range, a method for manufacturing the same, and a steel pipe made from the high-strength steel plate and a method for manufacturing the same.
• the high-strength steel plate of the present invention is suitable for use as a material for high-strength steel pipes for steam piping.
• Methods for recovering oil sands from oil reservoirs in Canada and elsewhere include open-cut mining and the steam injection method, in which high-temperature, high-pressure steam is pumped into the reservoir through steel pipes. Because open-cut mining is not feasible in many areas, the steam injection method is used in many regions. In the steam injection method, the steam pumped into the reservoir is in the 300-400°C temperature range (hereinafter referred to as the medium-temperature range). In the steam injection method, medium-temperature steam is pumped into the reservoir at high pressure. As mentioned above, steel pipes are used to pump this steam. In recent years, with the rise in energy demand, there has been a demand for larger diameter and higher strength steel pipes to improve heavy oil recovery rates and reduce construction costs. Furthermore, when laying the steel pipes, construction work and hydraulic pressure testing are typically carried out from early spring onward when temperatures are high. However, due to increased demand, there is also a demand for pipes with excellent low-temperature toughness to enable construction work during the cooler winter months.
• Patent Documents 1 and 2 are examples of prior art steel pipes for transporting steam that can be used in the steam injection method. These patent documents disclose seamless steel pipes equivalent to API X80 grade, with a maximum outer diameter of 16 inches. It is difficult to further increase the diameter of seamless steel pipes. Furthermore, in order to achieve strength equal to or greater than API X80 grade, the addition of large amounts of alloying elements is required.
• Patent Documents 3 and 4 disclose manufacturing technologies for high-strength steel pipes that can be made larger in diameter and are manufactured by welding. More specifically, Patent Documents 3, 4, and 5 relate to manufacturing technologies for high-strength steel pipes that are produced by TMCP (thermo-mechanical control process) and have strengths of API X80 grade or higher.
• TMCP thermo-mechanical control process
• Patent Document 3 Steel pipes manufactured using the steel pipe manufacturing method disclosed in Patent Document 3 satisfy API X80 grade high-temperature properties in the medium temperature range. However, Patent Document 3 does not disclose the strength properties of the steel pipes when used for long periods of time in the medium temperature range.
• excellent low-temperature toughness means that the ductile fracture surface area ratio (DWTTSA-40°C) obtained by DWTT (test temperature: -40°C) in accordance with API 5L is 85% or more, and the fracture surface transition temperature is -40°C or less.
• the test temperature for DWTT is set at -40°C to take into account the reduction in toughness due to work hardening during pipe manufacturing.
• the present invention was made based on the above findings and further studies, and is therefore comprised of the following: [1]
• the component composition is, in mass%, C: 0.04-0.09%, Si: 0.03-0.25%, Mn: 1.5-2.5%, P: 0.020% or less, S: 0.002% or less, Mo: 0.10-0.50%, Nb: 0.010-0.055%, Ti: 0.005-0.020%, Ca: 0.0040% or less, Al: 0.01-0.04%, N: 0.006% or less, the balance being Fe and unavoidable impurities;
• X (%) represented by formula (1) is 0.65% or more, the microstructure has bainite in an area ratio of 80% or more at the center of the plate thickness, the average grain size of the bainite is 25 ⁇ m or less, and the minimum grain size of the top 20% of the bainite grains with the largest grain size is 60 ⁇ m or less, the toughness of the steel plate is such that the ductile fracture area ratio obtained by DWTT at -40
• the high-strength steel sheet of the present invention may contain one or more of Cr, V, Cu, and Ni to further improve its properties.
• Cr 0.50% or less Cr is one of the elements effective in increasing temper softening resistance and high-temperature strength.
• a Cr content exceeding 0.50% adversely affects weldability. Therefore, when Cr is contained, the Cr content is set to 0.50% or less.
• the Cr content is preferably set to 0.48% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.
• the lower limit of the Cr content is not particularly limited and may be 0%. To achieve the above effect, the Cr content is preferably 0.05% or more, more preferably 0.08% or more, and even more preferably 0.10% or more.
• V 0.070% or less
• a small amount of V refines grains and contributes to increased strength. It also increases temper softening resistance and is one of the elements effective in increasing strength in the medium temperature range.
• the V content exceeds 0.070%, the toughness of the weld heat-affected zone deteriorates. Therefore, when V is contained, the V content is set to 0.070% or less.
• the V content is preferably 0.060% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less. Note that, as long as high-temperature strength can be increased by, for example, ensuring that X is within the desired range, the lower limit of the V content is not particularly limited and may be 0%. To achieve the above effect, the V content is preferably 0.005% or more, more preferably 0.010% or more.
• Ni 0.50% or less
• Ni is one of the elements effective in improving toughness and increasing strength.
• the Ni content is preferably 0.05% or more, and more preferably 0.10% or more. If the Ni content exceeds 0.50%, the effect saturates and production costs increase. Therefore, when Ni is contained, the Ni content is set to 0.50% or less.
• the Ni content is preferably 0.48% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.
• Cu + Ni + Cr + Mo the element symbols represent the content (mass%) of each element
• Cu + Ni + Cr + Mo the element symbols represent the content (mass%) of each element
• Cu + Ni + Cr + Mo is preferably 1.00% or less, more preferably 0.98% or less, even more preferably 0.95% or less, and most preferably 0.90% or less.
• Cu + Ni + Cr + Mo is preferably 0.10% or more, more preferably 0.15% or more, even more preferably 0.20% or more, and most preferably 0.25% or more.
• the remainder of the components is Fe and unavoidable impurities. These impurities are unavoidably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are permitted to be present to the extent that they do not impair the objectives of the present invention.
• Raw materials include iron ore, reduced iron, and scrap.
• unavoidable impurities include Pb, Zn, Sn, As, B, Sb, Bi, Co, H, O, and REM.
• Bainite is an important structure for achieving both strength and low-temperature toughness. Bainite effectively contributes to improving the strength of steel plates by strengthening the transformation structure. Microstructural uniformity is necessary for increasing the initial dislocation density and for improving the strength of high-strength steels, particularly in the mid-temperature range. Therefore, the structure of the high-strength steel of the present invention must be predominantly bainite. Specifically, the area fraction of bainite relative to the entire steel structure at the thickness center must be 80% or more. The area fraction of bainite is preferably 83% or more, more preferably 85% or more, even more preferably 88% or more, and most preferably 90% or more. While there are no particular limitations on the upper limit of the bainite fraction, from the viewpoint of improving deformability, the area fraction of bainite is preferably 98% or less, more preferably 95% or less.
• Ferrite area ratio of 10% or less (optimal condition)
• the area fraction of ferrite at the center of the sheet thickness is 10% or less.
• the area fraction of ferrite is more preferably 9% or less, even more preferably 8% or less, and most preferably 7% or less.
• the area fraction of ferrite is preferably 1% or more.
• Island martensite (MA: Martensite-Austenite constituent) is a very hard phase and may act as a fracture initiation point, thereby reducing the low-temperature toughness of the steel plate. Therefore, the area fraction of island martensite (MA) at the center of the plate thickness is preferably 10% or less.
• the area fraction of island martensite is more preferably 8% or less, even more preferably 5% or less, and most preferably 3% or less. There is no particular restriction on the lower limit of the area fraction of island martensite, but the area fraction is preferably 1% or more, and more preferably 2% or more.
• the steel structure of the steel plate serving as the base material must basically be composed of the above-mentioned bainite, but examples of the remaining structure other than bainite, ferrite, and island martensite (MA) include pearlite, martensite, cementite, and retained austenite.
• the average grain size of bainite is 25 ⁇ m or less, and the minimum grain size of the top 20% of bainite grains with the largest grain size is 60 ⁇ m or less. Because bainite grain boundaries provide resistance to brittle crack propagation, grain refinement contributes to improving low-temperature toughness. Therefore, the average grain size of all bainite is 25 ⁇ m or less.
• the average grain size of all bainite is preferably 23 ⁇ m or less, more preferably 20 ⁇ m or less, even more preferably 19 ⁇ m or less, and most preferably 18 ⁇ m or less. There is no particular lower limit, but 5 ⁇ m or more is preferred, and 6 ⁇ m or more is more preferred.
• the average grain size improves low-temperature toughness
• Coarse bainite is likely to become the origin of fracture, and if the top 20% of bainite grains have large grain sizes, low-temperature toughness deteriorates.
• the minimum grain size of the top 20% of grains with the largest grain size exceeds 60 ⁇ m, they are likely to become the origin of fracture.
• the minimum grain size of the top 20% of grains with the largest grain size at the center of the plate thickness must be 60 ⁇ m or less, preferably 55 ⁇ m or less, more preferably 50 ⁇ m or less, even more preferably 45 ⁇ m or less, and most preferably 40 ⁇ m or less.
• the lower limit is not particularly limited, but is preferably 10 ⁇ m or more, and more preferably 15 ⁇ m or more.
• the average grain size of bainite is determined as follows. That is, after mirror polishing the L cross section of the steel sheet (a cross section parallel to the rolling direction and parallel to the normal direction of the rolling surface), the crystal orientation of a randomly selected 1 mm x 1 mm2 region at the center of the sheet thickness was measured by electron backscatter diffraction (EBSD), and regions where the angle difference between adjacent pixels was 15° or more were determined as grain boundaries by image analysis.
• the EBSD measurement conditions were an acceleration voltage of 17 kV and a measurement pitch of 0.8 ⁇ m.
• the number of fine precipitates with a diameter of 100 nm or less in the steel at the center of the plate thickness is 50,000 or more per mm2. More preferably, it is 80,000 or more, even more preferably, it is 100,000 or more, and most preferably, it is 130,000 or more.
• the number of fine precipitates with a diameter of 100 nm or less exceeds 1,000,000 per mm2 , the fine precipitates may aggregate and coarsen, thereby weakening the austenite grain growth inhibitory effect and increasing strength degradation in the intermediate temperature range. Therefore, it is preferable that the number of fine precipitates with a diameter of 100 nm or less in the steel be 1,000,000 or less per mm2. More preferably, it is 950,000 or less, even more preferably, 900,000 or less, and most preferably, 850,000 or less.
• the corroded surface at any location on the head cross section is observed using a scanning electron microscope (SEM), or an extracted replica sample or thin film sample is prepared and observed using a transmission electron microscope (TEM).
• SEM scanning electron microscope
• TEM transmission electron microscope
• the number of precipitates with a size of 100 nm or less is measured in an area of at least 100 ⁇ m2 . For example, when observing at a magnification of 100,000 times with one field of view being 2000 nm x 2000 nm, the observation area per field is 4 ⁇ m2 , so 25 fields of view are observed randomly. This measurement result is converted to the number per unit area.
• the density of precipitates can be converted to 50,000 precipitates per mm2 .
• the density of the precipitates is called the number per mm2 of the observation area.
• the diameter of the precipitate is the average value of the major axis (long side) and minor axis (short side).
• Aging treatment with an LMP of 15700 refers to aging treatment performed under conditions of a heat treatment temperature and a heat treatment time such that the LMP, expressed by the following formula (2), becomes 15700.
• An LMP of 15700 corresponds to a condition of heat treatment at 350°C, which is a medium temperature range, for 20 years.
• the condition of Larson Miller Parameter (LMP) of 15700 means that the Larson Miller Parameter (LMP) is equal to or greater than 15650 and less than 15750.
• LMP (T+273) ⁇ (20+log(t))...(2)
• T Heat treatment temperature (°C)
• t heat treatment time (hours)
• the high-strength steel sheet of the present invention has a yield strength of 555 MPa or more before and after the aging treatment.
• a yield strength of 555 MPa or more has the effect of enabling stable operation as a steel pipe for steam piping.
• the yield strength means the yield strength measured in a high-temperature tensile test at 350°C.
• the yield strength before and after the aging treatment is preferably 560 MPa or more, more preferably 565 MPa or more, even more preferably 570 MPa or more, and most preferably 575 MPa or more. Although there is no particular upper limit, the yield strength is preferably 840 MPa or less. As described in the Examples, in the present invention, the test specimens taken from both the steel plate and the steel pipe have a yield strength of 555 MPa or more both before and after the aging treatment.
• the difference in yield strength before and after aging calculated by subtracting the yield strength after aging from the yield strength before aging, is 50 MPa or less (suitable requirement).
• the yield strength before and after aging is an index for evaluating the decrease in yield strength when held for a long time in the intermediate temperature range. If this difference is 50 MPa or less, the decrease in yield strength after long-term holding in the intermediate temperature range is within a range that is practically acceptable.
• the difference in yield strength before and after aging be 50 MPa or less.
• the difference in yield strength before and after aging is more preferably 45 MPa or less, even more preferably 40 MPa or less, and most preferably 35 MPa or less.
• the lower limit is not particularly limited and may be a negative value, such as -100 MPa or more.
• the aging treatment conditions under the above conditions include, for example, heat treatment at 400° C. for 2335 hours.
• the toughness of the steel plate is such that the ductile fracture surface area ratio (DWTT) obtained by DWTT at -40°C is 85% or more.
• the toughness of the steel plate of the present invention is such that the ductile fracture surface area ratio (DWTTSA-40°C) obtained by DWTT (test temperature: -40°C) in accordance with API 5L is 85% or more. If the ductile fracture surface area ratio is less than 85%, the steel plate is prone to brittle fracture at low temperatures, making it difficult to install the steel plate throughout the year, including in winter when the temperature drops below 0°C, or to use it in areas with very low ambient temperatures. Therefore, the ductile fracture surface area ratio must be 85% or more.
• a ductile fracture surface area ratio of 85% or more obtained by DWTT at -40°C means that the fracture transition temperature is -40°C or less.
• the ductile fracture surface area ratio is preferably 86% or more, more preferably 87% or more, even more preferably 88% or more, and most preferably 89% or more.
• the reason why the test temperature in the DWTT is set to ⁇ 40° C. is to take into account the decrease in toughness due to work hardening during pipe making.
• the upper limit of the ductile fracture area ratio obtained by the DWTT at ⁇ 40° C. is not particularly limited, and may be 100% or less.
• the steel pipe of the present invention is manufactured using the high-strength steel plate of the present invention, and therefore has the strength characteristics and low-temperature toughness required for high-strength welded steel pipe for steam transportation, even when it has a large diameter.
• the outer diameter (diameter) of the steel pipe is 400 mm or more.
• the outer diameter of the steel pipe is preferably 500 mm or more, more preferably 600 mm or more, and even more preferably 700 mm or more.
• the thickness of the steel pipe is not particularly limited, but in the case of steam transport, it is 12 to 30 mm. That is, the thickness of the steel pipe is preferably 12 mm or more, more preferably 13 mm or more, even more preferably 14 mm or more, and most preferably 15 mm or more. The thickness of the steel pipe is preferably 30 mm or less, more preferably 29 mm or less, even more preferably 28 mm or less, and most preferably 27 mm or less.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=96847166

Family Applications (1)

Country Status (2)

Citations (4)

• 2025
  • 2025-02-17 JP JP2025533659A patent/JP7758256B1/ja active Active
  • 2025-02-17 WO PCT/JP2025/005169 patent/WO2025177987A1/ja active Pending

Patent Citations (4)

Also Published As

Similar Documents

Legal Events