WO2021182110A1 - Steel material, manufacturing method therefor, and tank - Google Patents

Steel material, manufacturing method therefor, and tank Download PDF

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Publication number
WO2021182110A1
WO2021182110A1 PCT/JP2021/006963 JP2021006963W WO2021182110A1 WO 2021182110 A1 WO2021182110 A1 WO 2021182110A1 JP 2021006963 W JP2021006963 W JP 2021006963W WO 2021182110 A1 WO2021182110 A1 WO 2021182110A1
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Prior art keywords
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steel material
rolling
steel
plate thickness
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PCT/JP2021/006963
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French (fr)
Japanese (ja)
Inventor
大地 泉
佳子 竹内
倫教 石田
仲道　治郎
植田　圭治
聡 伊木
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Ｊｆｅスチール株式会社
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First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=77670492&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=WO2021182110(A1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to CN202180018720.9A priority Critical patent/CN115210400B/en
Priority to JP2021532003A priority patent/JP7272438B2/en
Priority to KR1020227029930A priority patent/KR20220131996A/en
Priority to EP21768230.1A priority patent/EP4089196A1/en
Publication of WO2021182110A1 publication Critical patent/WO2021182110A1/en

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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
    • 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
    • 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
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0226—Hot 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
    • 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
    • C22C38/22—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
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C13/00—Details of vessels or of the filling or discharging of vessels
    • F17C13/08—Mounting arrangements for vessels
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • 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
    • C21D2211/001—Austenite
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2203/00—Vessel construction, in particular walls or details thereof
    • F17C2203/06—Materials for walls or layers thereof; Properties or structures of walls or their materials
    • F17C2203/0634—Materials for walls or layers thereof
    • F17C2203/0636—Metals
    • F17C2203/0639—Steels
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2203/00—Vessel construction, in particular walls or details thereof
    • F17C2203/06—Materials for walls or layers thereof; Properties or structures of walls or their materials
    • F17C2203/0634—Materials for walls or layers thereof
    • F17C2203/0636—Metals
    • F17C2203/0648—Alloys or compositions of metals
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2221/00—Handled fluid, in particular type of fluid
    • F17C2221/03—Mixtures
    • F17C2221/032—Hydrocarbons
    • F17C2221/033—Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
    • F17C2223/0146—Two-phase
    • F17C2223/0153—Liquefied gas, e.g. LPG, GPL
    • F17C2223/0161—Liquefied gas, e.g. LPG, GPL cryogenic, e.g. LNG, GNL, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/03—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the pressure level
    • F17C2223/033—Small pressure, e.g. for liquefied gas

Definitions

  • the present invention relates to a steel material and a method for producing the same, which is suitable for structural steel used in an extremely low temperature environment such as a tank for a liquefied gas storage tank.
  • the present invention also relates to a tank using this steel material.
  • a hot-rolled steel sheet As a material for a structure for a liquefied gas storage tank, the operating environment is extremely low, so the steel sheet is required to have high strength and excellent toughness at low temperatures.
  • NS for example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary that excellent toughness is ensured at a boiling point of the liquefied natural gas: -164 ° C. or lower. If the low temperature toughness of the steel material is inferior, it may not be possible to maintain the safety of the structure for the cryogenic storage tank. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel material. In the following description, it is generically referred to as "low temperature" including the cryogenic region of -164 ° C. or lower.
  • Patent Document 1 Has been proposed to.
  • Patent Document 1 proposes a technique for ensuring low-temperature toughness in a weld heat-affected zone by reducing the surface integral of carbides to 5% or less.
  • the austenitic steel material described in Patent Document 1 is limited to a cooling rate of 10 ° C./s or more in the weld heat affected zone from the viewpoint of suppressing carbides.
  • a steel sheet having a thickness of less than 10 mm is cooled at 10 ° C./s or more, the steel sheet is liable to warp or distort, and extra steps such as shape correction are required, which hinders productivity.
  • the low temperature toughness in the rolling width direction (C direction) tends to be inferior to the low temperature toughness in the rolling direction (L direction), but the low temperature toughness in the C direction has not been verified at all in Patent Document 1.
  • the structure for a liquefied gas storage tank (for example, a tank for a liquefied gas storage tank) is manufactured by welding a steel material. Since the internal pressure from the liquefied natural gas is applied to the inner wall of the liquefied gas storage tank (hereinafter, also referred to as a tank), the steel material constituting the tank is in the rolling direction (L direction) and the plate width direction (C direction). ), Tensile stress is generated not only in the direction parallel to all the steel materials constituting the tank (hereinafter, may be referred to as "all directions"). Further, tensile stresses in the L and C directions are also generated in the welded portion of the tank.
  • the base material (base material part) and the welded part have characteristics that can withstand the load due to the tensile stress in all directions, especially the L direction and the C direction. be.
  • all directions refer to all directions including the direction perpendicular to the rolling direction and the direction parallel to the rolling direction.
  • the present invention has been made in view of the above problems, and an object of the present invention is to provide a steel material having excellent low temperature toughness, a method for producing the same, and a tank.
  • welding heat-affected zone refers to the weld heat-affected zone coarse grain region (CGHAZ), which is a portion of general steel in which toughness decreases.
  • CGHAZ weld heat-affected zone coarse grain region
  • the above-mentioned "excellent in low temperature toughness” means that the absorbed energy (vE -196 ) of the Charpy impact test at -196 ° C. in all directions at the plate thickness 1/2 position is 41 J or more in the steel material. Point to. Usually, the absorbed energy of the Charpy impact test in the C direction is the lowest value as compared with the L direction and the Z direction (plate thickness direction).
  • the C direction of the absorbed energy (vE -196) is 41J referred to as "superior in low temperature toughness.”
  • 41J is a draft specification of high Mn steel at -196 ° C in the L direction prepared by IACS (International Association of Classification Societies) as of 2019, and 27J is proposed as absorbed energy in the C direction.
  • IACS International Association of Classification Societies
  • austenite steel materials for example, high Mn steel materials
  • the composition of the steel material steel plate
  • the microstructure the manufacturing method
  • the characteristics of the welded portion to which the steel material is welded We conducted diligent research on various factors that determine. As a result, the following findings a to d were obtained.
  • the (110) [001] texture strength is less than 9.0.
  • the present invention has been made by further studying the above findings, and the gist thereof is as follows.
  • 95% or more of the area ratio is FCC.
  • the (110) [001] texture strength at the plate thickness 1/2 position is less than 10.0.
  • the hardness at the plate thickness 1/2 position is less than 300 HV,
  • the steel material according to [1], wherein the absorbed energy of the Charpy impact test at -196 ° C. in the C direction at the plate thickness 1/2 position after strain aging is 41 J or more.
  • the steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, the cross-rolling ratio calculated by Eq.
  • a method for producing a steel material in which hot rolling is performed under conditions of 750 ° C. or higher and then cooling is performed.
  • Cross rolling ratio rolling direction rolling ratio / rolling perpendicular rolling ratio ⁇ ⁇ ⁇ (1) [8]
  • a tank in which the absorbed energy of the Charpy impact test at -196 ° C. in the C direction in the coarse grain region of the weld heat affected zone is 41 J or more.
  • the steel material of the present invention is suitably used as a material for a steel structure (tank for liquefied gas storage tank, etc.) used in a low temperature environment, whereby the base material after welding and the heat-affected zone of welding are both excellent at low temperature.
  • a tank with toughness can be provided. Therefore, it can greatly contribute to the improvement of the safety and the life of the steel structure, and it is extremely effective in industry.
  • the production method of the present invention does not cause a decrease in productivity and an increase in production cost, it is possible to provide a production method excellent in economy.
  • austenite steel material for example, a high Mn steel material
  • an inexpensive steel material having excellent low temperature toughness.
  • the inner wall and welded part of the tank have characteristics that can withstand the internal pressure of the gas to be stored, especially in the L and C directions. It is required to have the property of being able to withstand the load due to tensile stress in all directions.
  • the high Mn steel material (here, a steel sheet having a Mn content of 20.0 to 40.0% by mass) is an austenite steel material, brittle fracture basically does not occur, and most of it is ductile fracture.
  • ordinary steel here, it refers to a low carbon steel sheet whose crystal structure at room temperature is BCC
  • the ductile fracture has nothing to do with the texture, and the shelf energy (maximum absorbed energy) of ordinary steel. Is 200J or more, and may exceed 300J depending on the conditions. That is, since the absorbed energy of ordinary steel is sufficiently large, in the case of ordinary steel, it was not necessary to consider the absorbed energy unless a brittle fracture surface was formed.
  • the absorbed energy in the L direction is about 100 J and the absorbed energy in the C direction is about 100 J, although it is ductile fracture. It was found that it may be less than 41J. This means that the base metal and the welded portion of the tank manufactured by welding the high Mn steel material are easily broken when a tensile impact stress is applied in the direction perpendicular to the rolling direction.
  • the internal pressure of the liquefied natural gas applied to the inner wall of the tank and the welded portion is generated in the L direction, the C direction, and the direction parallel to the inner surface (inner wall) of all the steel materials constituting the tank.
  • it is necessary to have a sufficient toughness value It is known that the rolled material has the lowest toughness when a Charpy impact test piece in the C direction with respect to the rolling direction is collected. Therefore, it is important to improve the toughness value of the Charpy impact test in the C direction.
  • the “C direction” refers to the direction perpendicular to the rolling direction (L direction).
  • the “Charpy impact test in the C direction” refers to a Charpy impact test piece whose longitudinal direction is parallel to the C direction and whose notch is oriented in the rolling direction.
  • the “rolling direction” of the present application refers to the rolling direction in which the total rolling reduction amount is the largest among the rolled materials rolled in various directions.
  • the present inventors have determined that the rolled texture (rolled texture) is caused by such a difference in absorbed energy, that is, the relationship between ductile fracture and the texture. I found a new one. The relationship between ductile fracture and aggregate structure will be described below.
  • the L-direction Charpy test piece (however, the notch faces the C direction) collected so that the longitudinal direction of the Charpy test piece is the rolling direction of the steel sheet, and the longitudinal direction of the Charpy test piece is perpendicular to the rolling direction of the steel sheet.
  • the C-direction Charpy test piece (however, the notch faces the L direction) collected so as to be in the direction of is considered as to the direction of striking.
  • the JIS standard stipulates that the absorbed energy value of the C-direction Charpy impact test is 27 J or more, and a low value is sufficient. However, when the tank is formed, as described above, the stress is applied in all directions, so it is preferable to have the same absorption energy in the C direction as in the L direction.
  • the base material does not change even when the temperature rises, so the texture of the welded part obtained by welding the austenite steel is almost the same as that of the base material, that is, it does not change. Therefore, it is important to create an aggregate structure at the time of manufacturing the austenite steel material as the base material.
  • the (110) [001] texture that is easily formed during normal rolling and the set that develops other orientations by cross rolling that is rolled by rotating 90 degrees. Mixing with the tissue as closely as possible reduces the strength of the (110) [001] texture (ie, does not develop the (110) [001] texture).
  • the surface having the lowest surface atomic density in the (110) plane and the surface having the lowest surface atomic density is the most brittle surface. In ductile fracture, it is considered that such a brittle surface is easily torn and the absorbed energy is lowered. Therefore, it is considered that the Charpy absorption energy in the L direction and the C direction can be equalized by not developing the (110) [001] texture.
  • the microstructure at normal pressure has an FCC structure in an area ratio of 95% or more, the (110) [001] texture strength at the plate thickness 1/2 position is less than 10.0, and the plate.
  • the hardness at the 1/2 thickness position is less than 300 HV, and the absorbed energy of the Charpy impact test at -196 ° C. in the C direction at the 1/2 thickness position is 41 J or more.
  • the steel material of the present invention can have an absorbed energy of 41 J or more in the Charpy impact test in the C direction at -196 ° C. after strain aging and in the welded heat-affected zone coarse grain region.
  • the microstructure can have a cleanliness of sulfide-based inclusions of less than 1.0%.
  • Microstructure at normal pressure FCC structure with an area ratio of 95% or more
  • microstructure at normal pressure refers to a microstructure in the temperature range from 1300 ° C. or lower to -273 ° C. under a pressure of 1 atm. ..
  • the microstructure in the temperature range of 1300 ° C. or lower for example, 1250 ° C.
  • the matrix phase of the steel material is required to have a face-centered cubic structure (FCC) in crystal structure.
  • FCC face-centered cubic structure
  • "using austenite as the base phase” means that the austenite phase has an area ratio of 95% or more with respect to the entire microstructure.
  • the austenite phase is preferably 97% or more.
  • the rest other than the austenite phase is the ferrite phase and / or the martensite phase.
  • the total area ratio of each phase is preferably 5% or less.
  • the surface integral of the austenite phase or the like can be measured by the method described in Examples described later.
  • the (110) [001] texture strength is 10.0 or more in the microstructure at the plate thickness 1/2 position, cracks are likely to propagate. As a result, the absorbed energy is reduced. Therefore, the above-mentioned (110) [001] texture strength is set to less than 10.0. It is preferably 9.0 or less. More preferably, it is 6.0 or less. Since the absorbed energy in the L direction decreases, the (110) [001] texture strength in the microstructure at the plate thickness 1/2 position is preferably 1.0 or more. More preferably, it is 4.0 or more.
  • the hardness at the plate thickness 1/2 position is 300 HV or more, the ductility is lowered and the absorbed energy is lowered. Therefore, the above hardness is set to less than 300 HV. Preferably, it is 280 HV or less. More preferably, it is 260 HV or less. Since the strength of the steel material is reduced, the hardness at the position where the plate thickness is 1/2 is preferably 200 HV or more. More preferably, it is 220 HV or more.
  • Cleanliness of sulfide-based inclusions less than 1.0% (optimal conditions)
  • the cleanliness of the sulfide-based inclusions in the microstructure at the plate thickness 1/2 position is 1.0% or more, it becomes the starting point of fracture. As a result, the absorbed energy may decrease. Therefore, the cleanliness of the sulfide-based inclusions is preferably less than 1.0%. More preferably, it is 0.8% or less. More preferably, it is 0.6% or less.
  • the lower limit of the above cleanliness is not particularly specified, but from the viewpoint of manufacturing cost, it is preferably 0.1% or more.
  • the cleanliness is calculated by the following equation (2).
  • d (n / p ⁇ f) ⁇ 100 ...
  • p the total number of grid points in the visual field
  • f the number of visual fields
  • n the number of grid point centers occupied by inclusions in the f visual fields. Therefore, the cleanliness is a value obtained by calculating the area percentage occupied by the sulfide-based inclusions at the position where the plate thickness of the steel material is 1/2, and indicates the sulfide-based inclusions in the C direction. Examples of the sulfide-based inclusions include MnS.
  • the above-mentioned texture strength, hardness, and cleanliness of sulfide-based inclusions can be measured by the methods described in Examples described later.
  • the steel material of the present invention having the above microstructure is excellent in low temperature toughness.
  • the absorbed energy of the Charpy impact test after strain aging and at -196 ° C of the weld heat affected zone was measured.
  • the microstructure at the 1/2 position of the plate thickness of the steel material is (110) [001]. If the texture strength is less than 10.0 and the hardness is less than 300 HV, the microstructure at the 1/2 position of the plate thickness of the steel material is in the C direction and at the 1/2 position. in all directions including the L direction, the absorbed energy (vE -196): 41J can be realized more. Accordingly, even in welds with a welded steel of the present invention, the heat affected zone coarse zone C in absorbed energy (vE -196): 41J can be realized more.
  • the steel material to a predetermined condition of the present invention (e.g., conditions described in the examples below) were subjected to aging treatment being pre-strained in, C directions of absorption energy after strain aging (vE -196): 41J or more Can be realized.
  • the welding conditions such as the preferable amount of heat are the same as the preferable welding conditions for the tank described later, and are therefore omitted here.
  • the structure (for example, tank) obtained by using the austenite steel material (for example, high Mn steel material) of the present invention as a material and welding this steel material has the same composition and microstructure as the base material and the welded portion. (However, the austenite particle size of the weld increases).
  • the austenite steel material and the steel material used for producing the austenite steel material have the above-mentioned composition.
  • the composition of the austenite steel material of the present invention and the reason for its limitation will be described.
  • the indication of "%” regarding the component composition means “mass%” unless otherwise specified.
  • C 0.100% or more and 0.700% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain this effect, C is preferably contained in an amount of 0.100% or more. On the other hand, if C is contained in an amount of more than 0.700%, Cr carbides may be excessively generated and the low temperature toughness may be lowered. Therefore, C is preferably 0.100% or more and 0.700% or less. C is more preferably 0.200% or more, and more preferably 0.600% or less. C is more preferably 0.250% or more, still more preferably 0.550% or less.
  • Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain such an effect, it is preferable that Si is contained in an amount of 0.05% or more. On the other hand, if Si is contained in an amount of more than 1.00%, the non-thermal stress is excessively increased, so that the low temperature toughness may be deteriorated. Therefore, Si is preferably 0.05% or more and 1.00% or less. Si is more preferably 0.07% or more, and more preferably 0.80% or less. Si is more preferably 0.10% or more, still more preferably 0.60% or less.
  • Mn 20.0% or more and 40.0% or less
  • Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and low temperature toughness. In order to obtain the effect, Mn is preferably contained in an amount of 20.0% or more. On the other hand, if Mn is contained in excess of 40.0%, the low temperature toughness may deteriorate. In addition, weldability and cutability may deteriorate. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is preferably 20.0% or more and 40.0% or less. Mn is more preferably 23.0% or more, still more preferably 24.0% or more. It is more preferably 35.0% or less, and further preferably 30.0% or less.
  • P 0.030% or less If P is contained in excess of 0.030%, it segregates excessively at the grain boundaries, resulting in a decrease in low temperature toughness. Therefore, it is desirable to limit the amount to 0.030% as much as possible. Therefore, P is set to 0.030% or less. It should be noted that excessive P reduction raises the refining cost and is economically disadvantageous, so it is desirable to set it to 0.002% or more. P is more preferably 0.005% or more, still more preferably 0.010% or more. It is more preferably 0.028% or less, and further preferably 0.024% or less.
  • S 0.0050% or less Since S deteriorates the low temperature toughness and ductility of the base material, it is desirable to limit it to 0.0050% and reduce it as much as possible. Therefore, S is set to 0.0050% or less. It is more preferably 0.0045% or less, and further preferably 0.0040% or less. It is desirable that S is 0.0010% or more because excessive reduction of S increases the refining cost and is economically disadvantageous. More preferably, it is 0.0012% or more.
  • Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. In addition, the yield strength and local elongation during the tensile test are improved. In order to obtain such an effect, Al preferably contains 0.01% or more. On the other hand, if Al is contained in an amount of more than 5.00%, a large amount of inclusions are present and the low temperature toughness is deteriorated, so the content is set to 5.00% or less. Al is more preferably 0.01% or more, still more preferably 0.02% or more. Al is more preferably 4.00% or less, still more preferably 3.50% or less.
  • Cr 7.0% or less
  • Cr is an element effective for improving low temperature toughness because it improves grain boundary strength.
  • Cr is preferably contained in an amount of 0.5% or more.
  • Cr is preferably 0.5% or more, more preferably 1.0% or more, and further preferably 1.2% or more.
  • Cr is more preferably 6.7% or less, still more preferably 6.5% or less.
  • it is further preferable that Cr is 2.0% or more and 6.0% or less.
  • N is an austenite stabilizing element, which is an effective element for improving low temperature toughness.
  • N is preferably contained in an amount of 0.0050% or more.
  • N is preferably 0.0500% or less.
  • N is preferably 0.0050% or more, more preferably 0.0060% or more, and further preferably 0.0070% or more.
  • N is more preferably 0.0400% or less, still more preferably 0.0300% or less.
  • O 0.0050% or less O deteriorates low temperature toughness due to the formation of oxides. Therefore, O is in the range of 0.0050% or less. It is preferably 0.0045% or less, more preferably 0.0040% or less, and further preferably 0.0035% or less. It is desirable that O is 0.0010% or more because excessive reduction of O increases the refining cost and is economically disadvantageous. More preferably, it is 0.0012% or more.
  • Ti and Nb form high melting point carbonitrides in steel, which reduces low temperature toughness. Since Ti and Nb are components that are inevitably mixed from raw materials, etc., Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less should be mixed. It is customary. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the content of Ti and Nb to less than 0.005%, respectively, according to the melting method described later. By suppressing the contents of Ti and Nb to less than 0.005%, respectively, the adverse effects of the above-mentioned carbonitride can be eliminated, and excellent low temperature toughness and ductility can be ensured.
  • the content of Ti and Nb is 0.003% or less. Of course, the content of Ti and Nb may be 0%. More preferably, it is 0.001% or more.
  • One or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less Ca, Mg and REM (rare earth metal) are used for morphological control of inclusions. It is a useful element. Morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Through morphological control of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferable that Ca and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, when a large amount of any of the elements is contained, the amount of non-metal inclusions increases, and on the contrary, the ductility, toughness, and sulfide stress corrosion cracking resistance decrease. It is also economically disadvantageous.
  • Ca and Mg are contained, it is preferably 0.0100% or less, and when REM is contained, it is preferably 0.0200% or less.
  • Ca is 0.0005% or more
  • Mg is 0.0005% or more
  • REM is 0.0010% or more.
  • Ca is 0.0010% or more and 0.0080% or less
  • Mg is 0.0010% or more and 0.0080% or less
  • REM is 0.0020% or more and 0.0150% or less. More preferably, Ca is 0.0050% or less and Mg is 0.0050% or less.
  • the balance other than the above-mentioned components is iron (Fe) and unavoidable impurities.
  • the unavoidable impurities include H and B, and if the total of each element is 0.01% or less, it is acceptable.
  • the above elements As the basic composition, the properties desired in the present invention can be obtained.
  • the following elements can be contained, if necessary, for the purpose of further improving the strength and low temperature toughness.
  • Cu and Ni are elements that not only increase the strength of steel sheets by solid solution strengthening, but also improve the mobility of dislocations and improve low temperature toughness. ..
  • Cu and Ni are preferably contained in an amount of 0.01% or more.
  • the content thereof is preferably 1.0% or less. It is more preferably 0.03% or more, and more preferably 0.7% or less. More preferably, it is 0.5% or less.
  • Mo 2.0% or less
  • V 2.0% or less
  • W 2.0% or less Mo
  • V and W contribute to the stabilization of austenite and the improvement of the base metal strength.
  • Mo, V and W each contain 0.001% or more.
  • Mo, V and W are contained in an amount of more than 2.0%, coarse carbonitrides may be formed, which may be a starting point of fracture and put pressure on the manufacturing cost. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less. It is more preferably 0.003% or more, and more preferably 1.7% or less. More preferably, it is 1.5% or less.
  • steel material refers to a steel plate having a plate thickness of 6 mm or more. From the viewpoint of preferably using it as a material for structural steel used in an extremely low temperature environment, the plate thickness is preferably more than 9 mm, more preferably 12 mm or more. The upper limit of the plate thickness is not particularly limited and may be any thickness, but it is preferably 40 mm or less.
  • the steel material (austenite steel material) of the present invention can melt molten steel having the above-mentioned composition by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace.
  • a known casting method such as a continuous casting method, an ingot-bulk rolling method, or the like to obtain a steel material such as a slab having a predetermined size.
  • the steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, a predetermined cross-rolling is performed, and the rolling reduction of the final pass of the finish rolling is 30% or less. It is important to perform hot rolling under the condition that the finish rolling end temperature is 750 ° C. or higher.
  • the temperature control here is based on the surface temperature of the steel material.
  • the "° C" indication regarding temperature is the surface temperature of the steel material or steel plate, respectively, unless otherwise specified.
  • the surface temperature can be measured with, for example, a radiation thermometer. Further, the temperature at the center of the thickness of the slab or the steel plate is measured by attaching a thermocouple to the center of the thickness of the steel plate, or the temperature distribution in the cross section of the steel plate is calculated by heat transfer analysis, and the result is the steel plate. It can be obtained by correcting with the surface temperature of.
  • Heating temperature of steel material 1100 ° C. or higher and 1300 ° C. or lower
  • the heating temperature of the steel material before hot rolling is set to 1100 ° C. or higher.
  • the stability of austenite can be ensured even in the Mn negative segregation part.
  • the stability of austenite can be ensured even in the coarse-grained region of the weld heat-affected zone obtained at the time of welding, and brittle fracture can be prevented.
  • the heating temperature exceeds 1300 ° C., there is a concern that the steel will start melting, so the upper limit of the heating temperature is set to 1300 ° C.
  • it is 1130 ° C. or higher and 1270 ° C. or lower.
  • Cross-rolling ratio calculated by equation (1): 20 or less
  • Cross-rolling ratio rolling direction rolling ratio / rolling perpendicular direction rolling ratio ...
  • the “rolling ratio in the rolling direction” refers to the rolling ratio in the rolling direction with respect to the total rolling.
  • Rolling right-angled rolling ratio refers to the rolling ratio in the direction perpendicular to rolling with respect to total rolling. Therefore, “rolling direction rolling ratio / rolling perpendicular direction rolling ratio” indicates the rolling ratio in the rolling direction with respect to rolling perpendicular direction rolling.
  • the cross-rolling ratio calculated by the equation (1) is 20 or less.
  • the cross-rolling ratio is preferably 18 or less, more preferably 15 or less.
  • the (110) [001] texture is developed by repeating rolling in the same direction, it is preferable to alternately repeat rolling in the rolling direction and rolling in the direction perpendicular to the rolling direction in order to make the texture uniform. .. It is preferable to repeat it twice or more. It is preferably 3 times or less.
  • Finish rolling final pass rolling rate 30% or less
  • finish rolling end temperature 750 ° C or higher If the rolling rate of the finish rolling final pass exceeds 30%, the dislocation density becomes excessively high and the low temperature toughness deteriorates.
  • the finish rolling end temperature is less than 750 ° C. (110) [001], the texture is excessively developed and the low temperature toughness is deteriorated. Therefore, the rolling reduction of the final pass for finish rolling is set to 30% or less.
  • the reduction rate is preferably less than 25%, more preferably 20% or less.
  • the finish rolling end temperature is 750 ° C. or higher.
  • the finish rolling end temperature is preferably 780 ° C. or higher, and more preferably 800 ° C. or higher.
  • the upper limit of the finish rolling end temperature is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 950 ° C. or lower, and more preferably 920 ° C. or lower.
  • the lower limit of the rolling reduction of the final pass for finish rolling is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 5% or more, and more preferably 10% or more.
  • the rolling start temperature is preferably 1100 to 1250 ° C. If the temperature is lower than 1100 ° C., the rolling temperature will be lower than 780 ° C., and the texture may be excessively developed. Above 1250 ° C, the texture may not change.
  • the rolling temperature (temperature during rolling) is preferably 780 to 1250 ° C. Below 780 ° C, the texture may develop excessively. Above 1250 ° C, the texture may not change.
  • the amount of reduction in the temperature range of 780 to 1250 ° C. is preferably 60 to 98%. If the reduction amount is less than 60%, the texture may not change. If the reduction amount exceeds 98%, the texture may be overdeveloped. The reduction amount indicates the total reduction rate in the temperature range of 780 to 1250 ° C.
  • the above-mentioned "temperature at the end of hot rolling” refers to the temperature at the end of finish rolling.
  • the tank of the present invention is a tank manufactured by welding the above-mentioned steel materials.
  • the steel material of the present invention inherits the microstructure before welding even after welding. Therefore, the composition and microstructure of the base material of the tank of the present invention are the same as those of the above-mentioned steel material (austenite steel material).
  • a tank having an absorbed energy of 41 J or more in the Charpy impact test at -196 ° C. at the plate thickness 1/2 position of the base material can be obtained. Be done. Further, the absorption energy of the Charpy impact test at -196 ° C. in the coarse grain region of the weld heat affected zone of the tank can be set to 41 J or more. Further, the absorbed energy of the Charpy impact test at -196 ° C. after strain aging can be increased to 41 J or more.
  • the tank of the present invention Since the tank of the present invention has the above characteristics, it can be used in an extremely low temperature environment such as a tank for a liquefied gas storage tank.
  • the tank of the present invention is manufactured by welding the above steel materials.
  • the method for producing the steel material (austenite steel material), which is the raw material, has already been described and will be omitted.
  • suitable welding conditions will be described.
  • the type of welding is preferably gas metal arc welding.
  • the heat input range is preferably 3.0 kJ / mm or less. Further, it is preferably 0.5 kJ / mm or more. By satisfying this heat input range, the above characteristics can be satisfied.
  • the average cooling rate in the temperature range of 500 to 800 ° C. is preferably 10 ° C./s or more. If the average cooling rate in this temperature range is less than 10 ° C./s, carbides are generated and the absorbed energy energy is reduced.
  • the absorbed energy of the Charpy impact test in all directions of the steel material can be equalized, the impact of the steel material (base material) and the welded portion can be equalized.
  • the orientation dependence of the characteristic can be reduced. This has improved the reliability of the material.
  • a steel slab having the composition shown in Table 1 was prepared by a converter-ladle refining-continuous casting method.
  • "-" shown in Table 1 indicates that it is not intentionally added, and means that it includes not only the case where it is not contained (0%) but also the case where it is unavoidably contained.
  • the obtained steel slab was hot-rolled under the conditions shown in Table 2 and then cooled to prepare a steel material (steel plate) having a plate thickness of 6 to 40 mm.
  • the temperature during rolling 780 to 1250 ° C.
  • the reduction amount at 780 to 1250 ° C. 60 to 98%
  • the cooling condition after the completion of rolling 1.0 ° C./s or more are appropriately controlled. I went there.
  • the above-mentioned "cooling condition after the end of rolling” refers to an average cooling rate from a temperature of (temperature at the end of hot rolling to -100 ° C) or higher to a temperature of 600 ° C or lower.
  • a joint test plate (size: 250 mm ⁇ 500 mm) was collected from the obtained steel plate, and the L direction and the C direction were welded to each other to prepare a welded joint.
  • welding was performed under welding conditions such as groove shape: re-shape, backing material: ceramics, shield gas: Ar-30% CO 2 , and torch receding angle: 5 to 10 °.
  • a Charpy V-notch test piece in the C direction was collected from the direction perpendicular to the rolling direction at a position 1/2 of the plate thickness from the surface of the steel sheet. Further, a Charpy V notch test piece in the L direction was collected from a direction parallel to the rolling direction at a position of 1/2 of the plate thickness from the surface of the obtained steel sheet. Further, at a position of 1/2 of the thickness of the obtained steel sheet from the surface of the steel sheet, tensile test pieces having a distance between the gauge points of 200 mm were sampled from the L direction and the C direction, respectively, and after 5% tensile prestrain, at 250 ° C. Charpy V notch test pieces in the L and C directions were collected from the tensile test pieces that had been aged for 1 hour.
  • a sub-size (5 mm) Charpy V-notch test piece was prepared in the C direction, and three Charpy impact tests were carried out for each test piece at -196 ° C.
  • Table 3 the sample carried out using the sub-sized Charpy V-notch test piece shows "* 1" in the item of absorbed energy.
  • the average value of three absorbed energy (vE -196) is, C direction: more than 27J was judged as "excellent in the base material toughness.”
  • the low temperature toughness of the welded joint was evaluated as follows.
  • Charpy V notch test pieces were collected from each welded joint with a plate thickness of more than 10 mm in accordance with JIS Z 2242 (2005), and three Charpy impact tests were conducted at -196 ° C for each welded joint. .. In this embodiment, it was determined that the average value of the absorbed energies of the three pieces was 41 J or more as "excellent in toughness of the welded portion".
  • EBSD analysis test pieces were collected from a cross section parallel to the rolling direction at a plate thickness of 1/2 of the obtained steel sheet, and EBSD analysis was performed in a measurement step of 0.3 ⁇ m in a field of view of 500 ⁇ m ⁇ 200 ⁇ m, and Phase map.
  • the values described in 1 were taken as the area ratios of the austenite phase, the ferrite phase, and the martensite phase.
  • the above-mentioned target performance ((110) [001] texture strength: less than 10.0, hardness: less than 300 HV, Charpy at position 1/2 of the plate thickness of the steel material. It was confirmed that the absorbed energy (vE -196) of the impact test satisfied (41J or more). Further, the welded joint obtained by welding the austenite steel material of the present invention can satisfy the above-mentioned target performance (the absorbed energy (vE -196 ) of the Charpy impact test in the coarse grain region of the weld heat affected zone is 41 J or more). confirmed. Furthermore, it was confirmed that even after the strain aging treatment, the above-mentioned performance (the absorbed energy (vE -196 ) of the Charpy impact test after the strain aging treatment is 41 J or more) is satisfied.
  • the austenite steel material could not satisfy the above target performance. Further, in the obtained welded joint, the absorbed energy could not satisfy the above-mentioned target performance. Furthermore, it was confirmed that the above-mentioned target performance was satisfied after the strain aging treatment.

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Abstract

Provided are a steel material, a manufacturing method for the steel material, and a tank. The steel material has a microstructure in which at least 95% is FCC by area ratio, the (110) [001] aggregate structural strength at the 1/2 plate thickness position is less than 10.0, the hardness at the 1/2 plate thickness position is less than 300 HV, and the absorption energy in a Charpy impact test at −196°C in the C direction at the 1/2 plate thickness position is at least 41 J.

Description

鋼材およびその製造方法、ならびにタンクSteel materials and their manufacturing methods, as well as tanks
　本発明は、例えば液化ガス貯槽用タンク等の、極めて低温の環境で使用される構造用鋼に供して好適な、鋼材およびその製造方法に関する。また、本発明は、この鋼材を用いたタンクに関する。 The present invention relates to a steel material and a method for producing the same, which is suitable for structural steel used in an extremely low temperature environment such as a tank for a liquefied gas storage tank. The present invention also relates to a tank using this steel material.
　液化ガス貯槽用構造物の素材として熱間圧延鋼板を用いるためには、使用環境が極めて低温となることから、鋼板は高強度であることに加えて、低温での靱性に優れることも要求される。例えば、液化天然ガスの貯槽に熱間圧延鋼板を使用する場合は、液化天然ガスの沸点：－１６４℃以下で優れた靱性が確保されている必要がある。鋼材の低温靱性が劣ると、極低温貯槽用構造物としての安全性を維持できなくなる可能性があるため、適用される鋼材に対する低温靱性向上の要求は強い。なお、以降の説明において、－１６４℃以下の極低温域を含めて「低温」と総称する。 In order to use a hot-rolled steel sheet as a material for a structure for a liquefied gas storage tank, the operating environment is extremely low, so the steel sheet is required to have high strength and excellent toughness at low temperatures. NS. For example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary that excellent toughness is ensured at a boiling point of the liquefied natural gas: -164 ° C. or lower. If the low temperature toughness of the steel material is inferior, it may not be possible to maintain the safety of the structure for the cryogenic storage tank. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel material. In the following description, it is generically referred to as "low temperature" including the cryogenic region of -164 ° C. or lower.
　この要求に対して、従来、低温で脆性を示さないオーステナイトを鋼板の組織とするオーステナイト系ステンレス鋼や９％Ｎｉ鋼、もしくは５０００系アルミニウム合金が使用されてきた。しかしながら、合金コストや製造コストが高いことから、安価で低温靱性に優れる鋼材に対する要望がある。 In response to this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy having austenite as a steel sheet structure that does not show brittleness at low temperature has been used. However, since the alloy cost and the manufacturing cost are high, there is a demand for a steel material that is inexpensive and has excellent low temperature toughness.
　そこで、従来の低温用鋼に代わる新たな鋼材として、比較的安価なオーステナイト安定化元素であるＭｎを多量に添加した高Ｍｎ鋼を低温環境の構造用鋼として使用することが、例えば特許文献１に提案されている。 Therefore, as a new steel material to replace the conventional low-temperature steel, it is possible to use a high-Mn steel to which a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is added as a structural steel in a low-temperature environment, for example, Patent Document 1. Has been proposed to.
　特許文献１には、炭化物の面積分率を５％以下にする等によって、溶接熱影響部において低温靱性を確保する技術が提案されている。 Patent Document 1 proposes a technique for ensuring low-temperature toughness in a weld heat-affected zone by reducing the surface integral of carbides to 5% or less.
特表２０１５－５０８４５２号公報Special Table 2015-508452
　特許文献１に記載のオーステナイト系鋼材は、炭化物抑制の観点から溶接熱影響部の冷却速度が１０℃／ｓ以上に限定されている。板厚１０ｍｍ未満の鋼板を１０℃／ｓ以上で冷却した場合、鋼板に反りや歪が発生しやすく、形状矯正などの余分な工程が必要となり生産性が阻害される。一般に、圧延幅方向（Ｃ方向）の低温靱性は、圧延方向（Ｌ方向）の低温靭性に比べて劣る傾向にあるが、このＣ方向の低温靭性について特許文献１では何ら検証されていない。 The austenitic steel material described in Patent Document 1 is limited to a cooling rate of 10 ° C./s or more in the weld heat affected zone from the viewpoint of suppressing carbides. When a steel sheet having a thickness of less than 10 mm is cooled at 10 ° C./s or more, the steel sheet is liable to warp or distort, and extra steps such as shape correction are required, which hinders productivity. Generally, the low temperature toughness in the rolling width direction (C direction) tends to be inferior to the low temperature toughness in the rolling direction (L direction), but the low temperature toughness in the C direction has not been verified at all in Patent Document 1.
　また、液化ガス貯槽用構造物（例えば、液化ガス貯槽用タンク）は、鋼材を溶接して製造される。液化ガス貯槽用タンク（以下、タンクと称する場合もある。）の内壁には液化天然ガスからの内圧が加わるため、タンクを構成する鋼材には圧延方向（Ｌ方向）および板幅方向（Ｃ方向）だけでなく、タンクを構成する全ての鋼材に対して平行な方向（以下、「全ての方向」と称する場合もある。）にも引張応力が発生する。さらにタンクの溶接部にもＬ方向およびＣ方向の引張応力が発生する。そのため、鋼材をタンクの素材に用いる場合、母材（母材部）および溶接部が、全ての方向、そのなかでもＬ方向およびＣ方向の引張応力による負荷に耐え得る特性を有することが必要である。なお上述したように、本発明では、上記「全ての方向」とは、圧延方向に対して垂直な方向、平行な方向を含む、あらゆる方向を指すものとする。 Further, the structure for a liquefied gas storage tank (for example, a tank for a liquefied gas storage tank) is manufactured by welding a steel material. Since the internal pressure from the liquefied natural gas is applied to the inner wall of the liquefied gas storage tank (hereinafter, also referred to as a tank), the steel material constituting the tank is in the rolling direction (L direction) and the plate width direction (C direction). ), Tensile stress is generated not only in the direction parallel to all the steel materials constituting the tank (hereinafter, may be referred to as "all directions"). Further, tensile stresses in the L and C directions are also generated in the welded portion of the tank. Therefore, when a steel material is used as the material of the tank, it is necessary that the base material (base material part) and the welded part have characteristics that can withstand the load due to the tensile stress in all directions, especially the L direction and the C direction. be. As described above, in the present invention, the above-mentioned "all directions" refer to all directions including the direction perpendicular to the rolling direction and the direction parallel to the rolling direction.
　そして、上述したような用途に用いられる鋼材は、素材段階のみならず、加工や不慮の事故などにより塑性変形を受けた場合に、歪時効脆化と呼ばれる靭性が劣化することが知られている。 It is known that steel materials used for the above-mentioned applications deteriorate toughness called strain aging embrittlement when they are subjected to plastic deformation not only at the material stage but also due to processing or accidents. ..
　本発明は、上記課題に鑑みてなされたものであり、低温靭性に優れた鋼材およびその製造方法、ならびにタンクを提供することを目的とする。 The present invention has been made in view of the above problems, and an object of the present invention is to provide a steel material having excellent low temperature toughness, a method for producing the same, and a tank.
　ここで、上記「溶接熱影響部」とは、一般的な鋼において靱性が低下する部分である溶接熱影響部粗粒域（ＣＧＨＡＺ）を指す。 Here, the above-mentioned "welding heat-affected zone" refers to the weld heat-affected zone coarse grain region (CGHAZ), which is a portion of general steel in which toughness decreases.
　また、上記「低温靭性に優れた」とは、鋼材において、板厚１／２位置における全ての方向での－１９６℃のシャルピー衝撃試験の吸収エネルギー（vＥ－１９６）が４１Ｊ以上であることを指す。通常、Ｌ方向およびＺ方向（板厚方向）と比較して、Ｃ方向におけるシャルピー衝撃試験の吸収エネルギーが一番低い値を示す。そのため、本発明では、Ｃ方向の吸収エネルギー（vＥ－１９６）が４１Ｊであれば「低温靭性に優れた」と称する。なお、上記「４１Ｊ」は、ＩＡＣＳ（国際船級協会連合）が２０１９年現在作成している高Ｍｎ鋼のＬ方向の－１９６℃のスペック案であり、Ｃ方向の吸収エネルギーとして２７Ｊが提案されている。本発明によれば、Ｃ方向のシャルピー衝撃試験においてもＬ方向のスペックを満足できる。 Further, the above-mentioned "excellent in low temperature toughness" means that the absorbed energy (vE -196 ) of the Charpy impact test at -196 ° C. in all directions at the plate thickness 1/2 position is 41 J or more in the steel material. Point to. Usually, the absorbed energy of the Charpy impact test in the C direction is the lowest value as compared with the L direction and the Z direction (plate thickness direction). Therefore, in the present invention, if the C direction of the absorbed energy (vE -196) is 41J referred to as "superior in low temperature toughness." The above "41J" is a draft specification of high Mn steel at -196 ° C in the L direction prepared by IACS (International Association of Classification Societies) as of 2019, and 27J is proposed as absorbed energy in the C direction. There is. According to the present invention, the specifications in the L direction can be satisfied even in the Charpy impact test in the C direction.
　本発明者らは、上記課題を達成するため、オーステナイト鋼材（例えば高Ｍｎ鋼材）を対象に、鋼材（鋼板）の成分組成、ミクロ組織、および製造方法、ならびにこの鋼材を溶接した溶接部の特性を決定する各種要因に関して鋭意研究を行った。その結果、以下のａ～ｄの知見を得た。 In order to achieve the above problems, the present inventors have targeted austenite steel materials (for example, high Mn steel materials), the composition of the steel material (steel plate), the microstructure, and the manufacturing method, and the characteristics of the welded portion to which the steel material is welded. We conducted diligent research on various factors that determine. As a result, the following findings a to d were obtained.
　ａ．－１９６℃でのシャルピー衝撃試験の吸収エネルギーを向上させるためには、面心立方構造（ＦＣＣ）において表面原子密度が最も小さい（１１０）[００１]の集合組織の発達を抑制し、硬さを３００ＨＶ未満にすることが重要である。適切な条件で熱間圧延を施し、（１１０）[００１]集合組織強度を１０．０未満に制御することが、吸収エネルギーの向上に有効である。好ましくは、（１１０）[００１]集合組織強度は９．０未満である。 A. In order to improve the absorbed energy of the Charpy impact test at -196 ° C, the development of the texture with the lowest surface atomic density (110) [001] in the face-centered cubic structure (FCC) is suppressed and the hardness is reduced. It is important to keep it below 300 HV. It is effective to improve the absorbed energy by performing hot rolling under appropriate conditions and controlling the (110) [001] texture strength to less than 10.0. Preferably, the (110) [001] texture strength is less than 9.0.
　ｂ．高Ｍｎのオーステナイト鋼は、Ｍｎを多量に含有することから、硫化物系介在物が炭素鋼に比べて多く存在する。さらに、硫化物系介在物は圧延方向に伸長するため、一般的にシャルピー衝撃試験のＣ方向破面はＬ方向破面に比べ、硫化物系介在物の面積率が高い。硫化物系介在物は破壊の起点の一要因のため、熱間圧延後、硫化物系介在物の清浄度が１．０％以上の場合、低温靱性の劣化を招く。このことから高Ｍｎ鋼の低温靱性向上には、硫化物系介在物の清浄度を低くすることが有効である。 B. Since austenitic steels with high Mn contain a large amount of Mn, sulfide-based inclusions are present in a larger amount than carbon steels. Further, since the sulfide-based inclusions extend in the rolling direction, the area ratio of the sulfide-based inclusions is generally higher in the C-direction fracture surface of the Charpy impact test than in the L-direction fracture surface. Since the sulfide-based inclusions are one of the starting points of fracture, if the cleanliness of the sulfide-based inclusions is 1.0% or more after hot rolling, the low temperature toughness deteriorates. Therefore, in order to improve the low temperature toughness of high Mn steel, it is effective to reduce the cleanliness of sulfide-based inclusions.
　ｃ．熱間圧延において、適切な条件でクロス圧延を行えば、Ｃ方向においても上記ｂを実現できる。 C. In hot rolling, if cross rolling is performed under appropriate conditions, the above b can be realized even in the C direction.
　ｄ．高Ｍｎ鋼は炭素鋼と異なり、溶接時に変態することがないため、溶接後も溶接前のミクロ組織を引き継ぐ。 D. Unlike carbon steel, high Mn steel does not deform during welding, so it inherits the microstructure before welding even after welding.
　本発明は、以上の知見にさらに検討を加えてなされたものであり、その要旨は次のとおりである。
［１］　ミクロ組織は、面積率で９５％以上がＦＣＣであり、
　板厚１／２位置の（１１０）［００１］集合組織強度が１０．０未満であり、
　板厚１／２位置の硬さが３００ＨＶ未満であり、
　板厚１／２位置におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、鋼材。
［２］　歪時効後における板厚１／２位置のＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、［１］に記載の鋼材。
［３］　溶接熱影響部粗粒域におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、［１］または［２］に記載の鋼材。
［４］　質量％で、
　Ｃ：０．１００％以上０．７００％以下、
　Ｓｉ：０．０５％以上１．００％以下、
　Ｍｎ：２０．０％以上４０．０％以下、
　Ｐ：０．０３０％以下、
　Ｓ：０．００５０％以下、
　Ａｌ：５．００％以下、
　Ｃｒ：７．０％以下、
　Ｎ：０．０５００％以下、
　Ｏ：０．００５０％以下、
　Ｔｉ：０．００５％未満、
　Ｎｂ：０．００５％未満を含有し、
　Ｃａ：０．０１００％以下、Ｍｇ：０．０１００％以下、ＲＥＭ：０．０２００％以下から選択される１種または２種以上を含有し、
残部が鉄および不可避不純物からなる成分組成と、
前記ミクロ組織は、硫化物系介在物の清浄度が１．０％未満である、［１］～［３］のいずれか１つに記載の鋼材。
［５］　前記成分組成は、さらに、質量％で、
　Ｃｕ：１．０％以下、
　Ｎｉ：１．０％以下、
　Ｍｏ：２．０％以下、
　Ｖ：２．０％以下、
　Ｗ：２．０％以下
から選択される１種または２種以上を含有する、［４］に記載の鋼材。
［６］　前記硫化物系介在物はＭｎＳである、［４］または［５］に記載の鋼材。
［７］　［１］～［６］のいずれか１つに記載の鋼材の製造方法であって、
鋼素材を、１１００℃以上１３００℃以下の温度域に加熱し、（１）式で算出されるクロス圧延比が２０以下、仕上圧延最終パスの圧下率が３０％以下、および仕上圧延終了温度が７５０℃以上となる条件で熱間圧延を行った後、冷却を行う、鋼材の製造方法。
クロス圧延比＝圧延方向圧延比／圧延直角方向圧延比　　・・・（１）
［８］　［１］～［６］のいずれか１つに記載の鋼材を溶接したタンクであって、
　溶接熱影響部粗粒域における、Ｃ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１J以上である、タンク。 The present invention has been made by further studying the above findings, and the gist thereof is as follows.
[1] In the microstructure, 95% or more of the area ratio is FCC.
The (110) [001] texture strength at the plate thickness 1/2 position is less than 10.0.
The hardness at the plate thickness 1/2 position is less than 300 HV,
A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196 ° C. in the C direction at a plate thickness of 1/2 position.
[2] The steel material according to [1], wherein the absorbed energy of the Charpy impact test at -196 ° C. in the C direction at the plate thickness 1/2 position after strain aging is 41 J or more.
[3] The steel material according to [1] or [2], wherein the absorption energy of the Charpy impact test at -196 ° C. in the C direction in the weld heat-affected zone coarse grain region is 41 J or more.
[4] By mass%
C: 0.100% or more and 0.700% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 20.0% or more and 40.0% or less,
P: 0.030% or less,
S: 0.0050% or less,
Al: 5.00% or less,
Cr: 7.0% or less,
N: 0.0500% or less,
O: 0.0050% or less,
Ti: less than 0.005%,
Nb: contains less than 0.005%,
Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less.
Ingredient composition with the balance consisting of iron and unavoidable impurities,
The steel material according to any one of [1] to [3], wherein the microstructure has a cleanliness of sulfide-based inclusions of less than 1.0%.
[5] The composition of the components is further increased by mass%.
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: The steel material according to [4], which contains one or more selected from 2.0% or less.
[6] The steel material according to [4] or [5], wherein the sulfide-based inclusion is MnS.
[7] The method for producing a steel material according to any one of [1] to [6].
The steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, the cross-rolling ratio calculated by Eq. A method for producing a steel material, in which hot rolling is performed under conditions of 750 ° C. or higher and then cooling is performed.
Cross rolling ratio = rolling direction rolling ratio / rolling perpendicular rolling ratio ・ ・ ・ (1)
[8] A tank obtained by welding the steel material according to any one of [1] to [6].
A tank in which the absorbed energy of the Charpy impact test at -196 ° C. in the C direction in the coarse grain region of the weld heat affected zone is 41 J or more.
　本発明によれば、低温靭性に優れた鋼材およびその製造方法を提供できる。また、本発明の鋼材は、低温環境で使用される鋼構造物（液化ガス貯槽用タンク等）の素材として好適に用いられ、これにより溶接後の母材および溶接熱影響部がともに優れた低温靭性を有するタンクを提供できる。よって、上記鋼構造物の安全性や寿命の向上に大きく寄与することができ、産業上格段の効果を奏する。また、本発明の製造方法は、生産性の低下および製造コストの増大を引き起こすことがないため、経済性にも優れた製造方法を提供することができる。 According to the present invention, it is possible to provide a steel material having excellent low temperature toughness and a method for producing the same. Further, the steel material of the present invention is suitably used as a material for a steel structure (tank for liquefied gas storage tank, etc.) used in a low temperature environment, whereby the base material after welding and the heat-affected zone of welding are both excellent at low temperature. A tank with toughness can be provided. Therefore, it can greatly contribute to the improvement of the safety and the life of the steel structure, and it is extremely effective in industry. Further, since the production method of the present invention does not cause a decrease in productivity and an increase in production cost, it is possible to provide a production method excellent in economy.
　以下、本発明について詳しく説明する。なお、本発明は以下の実施形態に限定されない。 Hereinafter, the present invention will be described in detail. The present invention is not limited to the following embodiments.
　まず、本発明の技術思想について詳細に説明する。 First, the technical idea of the present invention will be described in detail.
　上述したように、安価で低温靱性に優れる鋼材としてオーステナイト鋼材（例えば高Ｍｎ鋼材）がある。この高Ｍｎ鋼材を低温環境で使用される鋼構造物（例えばタンク）の素材として用いるためには、タンクの内壁および溶接部は貯槽するガスの内圧に耐えられる特性、特にＬ方向およびＣ方向だけでなく全ての方向での引張応力による負荷に耐えられる特性を有することが求められている。 As described above, there is an austenite steel material (for example, a high Mn steel material) as an inexpensive steel material having excellent low temperature toughness. In order to use this high Mn steel material as a material for steel structures (for example, tanks) used in a low temperature environment, the inner wall and welded part of the tank have characteristics that can withstand the internal pressure of the gas to be stored, especially in the L and C directions. It is required to have the property of being able to withstand the load due to tensile stress in all directions.
　高Ｍｎ鋼材（ここでは、Ｍｎ含有量が２０．０～４０．０質量％の鋼板を指す。）はオーステナイト鋼材であるため、脆性破壊は基本的に起こらず、ほとんどが延性破壊である。これに対し、普通鋼（ここでは、常温での結晶構造がＢＣＣである低炭素鋼板を指す。）では、延性破壊は集合組織とは関係なく、また、普通鋼のシェルフエネルギー（最大吸収エネルギー）は２００Ｊ以上、条件によっては３００Ｊを超えることもある。すなわち、普通鋼の吸収エネルギーは十分に大きいため、普通鋼の場合には、脆性破面が形成されなければ吸収エネルギーを問題にする必要がなかった。 Since the high Mn steel material (here, a steel sheet having a Mn content of 20.0 to 40.0% by mass) is an austenite steel material, brittle fracture basically does not occur, and most of it is ductile fracture. On the other hand, in ordinary steel (here, it refers to a low carbon steel sheet whose crystal structure at room temperature is BCC), the ductile fracture has nothing to do with the texture, and the shelf energy (maximum absorbed energy) of ordinary steel. Is 200J or more, and may exceed 300J depending on the conditions. That is, since the absorbed energy of ordinary steel is sufficiently large, in the case of ordinary steel, it was not necessary to consider the absorbed energy unless a brittle fracture surface was formed.
　本発明者らの研究の結果、高Ｍｎ鋼材は、－１９６℃の超低温でシャルピー衝撃試験を行った場合、延性破壊ではあるものの、Ｌ方向の吸収エネルギーが１００Ｊ程度となり、Ｃ方向の吸収エネルギーが４１Ｊを下回る場合があることが分かった。このことは、高Ｍｎ鋼材を溶接して製造されたタンクの母材および溶接部において、圧延方向に対して垂直の方向に引張の衝撃応力が働いた場合に、破壊しやすいことを意味する。 As a result of the research by the present inventors, when the high Mn steel material is subjected to the Charpy impact test at an ultra-low temperature of -196 ° C., the absorbed energy in the L direction is about 100 J and the absorbed energy in the C direction is about 100 J, although it is ductile fracture. It was found that it may be less than 41J. This means that the base metal and the welded portion of the tank manufactured by welding the high Mn steel material are easily broken when a tensile impact stress is applied in the direction perpendicular to the rolling direction.
　すなわち、タンクの内壁および溶接部に加わる液化天然ガスの内圧は、Ｌ方向、Ｃ方向、およびタンクを構成する全て鋼材の内側の面（内壁）に平行な方向に発生するので、全ての方向に対して十分な靱性値を有することが必要である。圧延材は、圧延方向に対してＣ方向のシャルピー衝撃試験片を採取した場合に最も靱性が低くなることが知られている。従って、Ｃ方向のシャルピー衝撃試験の靱性値を向上させることが重要である。 That is, the internal pressure of the liquefied natural gas applied to the inner wall of the tank and the welded portion is generated in the L direction, the C direction, and the direction parallel to the inner surface (inner wall) of all the steel materials constituting the tank. On the other hand, it is necessary to have a sufficient toughness value. It is known that the rolled material has the lowest toughness when a Charpy impact test piece in the C direction with respect to the rolling direction is collected. Therefore, it is important to improve the toughness value of the Charpy impact test in the C direction.
　なお、「Ｃ方向」とは圧延方向（Ｌ方向）に対して垂直の方向を指す。「Ｃ方向のシャルピー衝撃試験」とはシャルピー衝撃試験片の長手方向がＣ方向に平行であり、ノッチが圧延方向に向いているものを指す。本願の「圧延方向」とは、圧延材を種々の方向に圧延した中で、最も全圧下量が大きな圧延方向を指す。 The "C direction" refers to the direction perpendicular to the rolling direction (L direction). The "Charpy impact test in the C direction" refers to a Charpy impact test piece whose longitudinal direction is parallel to the C direction and whose notch is oriented in the rolling direction. The "rolling direction" of the present application refers to the rolling direction in which the total rolling reduction amount is the largest among the rolled materials rolled in various directions.
　そこで、本発明者らはこの原因を更に鋭意調査した結果、圧延集合組織（圧延による集合組織）がこのような吸収エネルギーの違いに起因していること、すなわち延性破壊と集合組織との関係を新たに見出した。以下に、延性破壊と集合組織との関係について説明する。 Therefore, as a result of further diligent investigation of this cause, the present inventors have determined that the rolled texture (rolled texture) is caused by such a difference in absorbed energy, that is, the relationship between ductile fracture and the texture. I found a new one. The relationship between ductile fracture and aggregate structure will be described below.
　本発明では、シャルピー衝撃試験におけるシャルピー試験片を打つ方向に着目した。シャルピー試験片の長手方向を鋼板の圧延方向となるように採取するＬ方向シャルピー試験片(ただし、ノッチはＣ方向を向いている。)と、シャルピー試験片の長手方向が鋼板の圧延方向に垂直の方向となるように採取するＣ方向シャルピー試験片（ただし、ノッチはＬ方向を向いている。）とを、打つ方向について考えた。 In the present invention, attention was paid to the direction in which the Charpy test piece is struck in the Charpy impact test. The L-direction Charpy test piece (however, the notch faces the C direction) collected so that the longitudinal direction of the Charpy test piece is the rolling direction of the steel sheet, and the longitudinal direction of the Charpy test piece is perpendicular to the rolling direction of the steel sheet. The C-direction Charpy test piece (however, the notch faces the L direction) collected so as to be in the direction of is considered as to the direction of striking.
　上述したように、（１１０）の集合組織が高くなると、より靱性が低くなる傾向にある。集合組織から吸収エネルギーを予測することはできないため、その理由は明らかではないが、後述するように恐らく（１１０）［００１］集合組織が影響していると考えられる。この集合組織は、Ｃ方向には（１００）面が、Ｌ方向には（１１０）面が、それぞれ配向する。そのため、Ｃ方向にノッチを持つＬ方向シャルピー衝撃試験では良い値を得られるが、Ｌ方向にノッチを持つＣ方向シャルピー衝撃試験では悪い値となる。ＪＩＳ規格では、Ｃ方向シャルピー衝撃試験の吸収エネルギー値は２７Ｊ以上と規定されており、低い値で良いことになっている。しかし、タンクを形成した場合には、上述したように、応力は全方向にかかるのでＬ方向と同程度の吸収エネルギーをＣ方向でも有することが好ましい。 As described above, the higher the texture of (110), the lower the toughness tends to be. Since the absorbed energy cannot be predicted from the texture, the reason is not clear, but it is considered that the (110) [001] texture is probably influential as described later. In this texture, the (100) plane is oriented in the C direction and the (110) plane is oriented in the L direction. Therefore, a good value can be obtained in the L-direction Charpy impact test having a notch in the C direction, but a bad value can be obtained in the C-direction Charpy impact test having a notch in the L direction. The JIS standard stipulates that the absorbed energy value of the C-direction Charpy impact test is 27 J or more, and a low value is sufficient. However, when the tank is formed, as described above, the stress is applied in all directions, so it is preferable to have the same absorption energy in the C direction as in the L direction.
　母材は、オーステナイト鋼材の場合には昇温しても変態がないため、オーステナイト鋼材を溶接して得られる溶接部の集合組織は母材とほぼ同じ状態、すなわち変化しない。従って、母材となるオーステナイト鋼材の製造時に集合組織を作り込んでおくことが重要となる。 In the case of austenite steel, the base material does not change even when the temperature rises, so the texture of the welded part obtained by welding the austenite steel is almost the same as that of the base material, that is, it does not change. Therefore, it is important to create an aggregate structure at the time of manufacturing the austenite steel material as the base material.
　そこで、本発明では後述する熱間圧延の工程において、通常の圧延時で形成されやすい（１１０）［００１］集合組織と、９０度回転して圧延するクロス圧延で他の方位を発達させた集合組織とをできるだけ同程度に混ぜることで、（１１０）［００１］集合組織の強度を落とす（すなわち、（１１０）［００１］集合組織を発達させない）。ここで、面心立方構造（ＦＣＣ）では、（１１０）面における表面原子密度が最も小さく、また表面原子密度の小さい面が最も脆い面である。延性破壊において、このような脆い面は千切れやすく、吸収エネルギーが低くなると考えられる。従って、（１１０）［００１］集合組織を発達させないことで、Ｌ方向およびＣ方向のシャルピー吸収エネルギーを均等化することができると考えている。 Therefore, in the present invention, in the hot rolling process described later, the (110) [001] texture that is easily formed during normal rolling and the set that develops other orientations by cross rolling that is rolled by rotating 90 degrees. Mixing with the tissue as closely as possible reduces the strength of the (110) [001] texture (ie, does not develop the (110) [001] texture). Here, in the face-centered cubic structure (FCC), the surface having the lowest surface atomic density in the (110) plane and the surface having the lowest surface atomic density is the most brittle surface. In ductile fracture, it is considered that such a brittle surface is easily torn and the absorbed energy is lowered. Therefore, it is considered that the Charpy absorption energy in the L direction and the C direction can be equalized by not developing the (110) [001] texture.
　さらに、本発明者らの研究の結果、高Ｍｎ鋼材は、硬さが３００ＨＶ以上の場合に、歪時効後におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ未満になることを知見した。詳細なメカニズムは不明であるが、高硬度の方が、転位密度が高いため、高Ｍｎ鋼において多量に含有されているＣがより多くの転位を固着したと考えられる。 Furthermore, as a result of the research by the present inventors, it was found that when the hardness of the high Mn steel material is 300 HV or more, the absorbed energy of the Charpy impact test at -196 ° C. in the C direction after strain aging is less than 41 J. I found out. Although the detailed mechanism is unknown, it is considered that C contained in a large amount in the high Mn steel fixed more dislocations because the dislocation density was higher in the higher hardness.
　次に、本発明の鋼材について説明する。 Next, the steel material of the present invention will be described.
　本発明の鋼材は、常圧におけるミクロ組織は、面積率で９５％以上がＦＣＣ構造であり、板厚１／２位置の（１１０）［００１］集合組織強度が１０．０未満であり、板厚１／２位置の硬さが３００ＨＶ未満であり、板厚１／２位置におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である。
また、本発明の鋼材は、歪時効後および溶接した溶接熱影響部粗粒域における－１９６℃でのＣ方向のシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上とすることができる。
また、ミクロ組織は、硫化物系介在物の清浄度が１．０％未満とすることができる。 In the steel material of the present invention, the microstructure at normal pressure has an FCC structure in an area ratio of 95% or more, the (110) [001] texture strength at the plate thickness 1/2 position is less than 10.0, and the plate. The hardness at the 1/2 thickness position is less than 300 HV, and the absorbed energy of the Charpy impact test at -196 ° C. in the C direction at the 1/2 thickness position is 41 J or more.
Further, the steel material of the present invention can have an absorbed energy of 41 J or more in the Charpy impact test in the C direction at -196 ° C. after strain aging and in the welded heat-affected zone coarse grain region.
In addition, the microstructure can have a cleanliness of sulfide-based inclusions of less than 1.0%.
　以下に、本発明においてミクロ組織を上記のように限定した理由を説明する。 The reason why the microstructure is limited as described above in the present invention will be described below.
　［鋼材のミクロ組織］
　常圧におけるミクロ組織：面積率で９５％以上がＦＣＣ構造
　本発明において「常圧におけるミクロ組織」とは、圧力１ａｔｍ下で１３００℃以下の温度から－２７３℃までの温度域におけるミクロ組織を指す。高Ｍｎ鋼材の場合、１３００℃以下の温度域（例えば、１２５０℃）におけるミクロ組織は、面積率で９５％以上がＦＣＣである。 [Microstructure of steel]
Microstructure at normal pressure: FCC structure with an area ratio of 95% or more In the present invention, "microstructure at normal pressure" refers to a microstructure in the temperature range from 1300 ° C. or lower to -273 ° C. under a pressure of 1 atm. .. In the case of high Mn steel material, the microstructure in the temperature range of 1300 ° C. or lower (for example, 1250 ° C.) has an area ratio of 95% or more of FCC.
　上述したように、鋼材の結晶構造が体心立方構造（ＢＣＣ）である場合、該鋼材は低温環境下で脆性破壊を起こす可能性があるため、低温環境下での使用には適していない。したがって、低温環境下での使用を想定したとき、鋼材の基地相は、結晶構造が面心立方構造（ＦＣＣ）であることが必要とされる。なお、本発明において「オーステナイトを基地相とする」とは、オーステナイト相がミクロ組織全体に対して面積率で９５％以上であることを意味する。オーステナイト相は、好ましくは９７％以上である。オーステナイト相以外の残部は、フェライト相および／またはマルテンサイト相である。オーステナイト相以外の残部は、各相の合計面積率が５％以下であることが好ましい。 As described above, when the crystal structure of the steel material is a body-centered cubic structure (BCC), the steel material may cause brittle fracture in a low temperature environment, so that it is not suitable for use in a low temperature environment. Therefore, when assumed to be used in a low temperature environment, the matrix phase of the steel material is required to have a face-centered cubic structure (FCC) in crystal structure. In the present invention, "using austenite as the base phase" means that the austenite phase has an area ratio of 95% or more with respect to the entire microstructure. The austenite phase is preferably 97% or more. The rest other than the austenite phase is the ferrite phase and / or the martensite phase. For the rest other than the austenite phase, the total area ratio of each phase is preferably 5% or less.
　なお、本発明では、オーステナイト相などの面積分率は、後述する実施例に記載の方法で測定することができる。 In the present invention, the surface integral of the austenite phase or the like can be measured by the method described in Examples described later.
　（１１０）［００１］集合組織強度：１０．０未満
　本発明では、上述したように、鋼材（母材）および溶接熱影響部の低温靭性を向上させるために、適正な条件で熱間圧延を行うことが重要である。これによりミクロ組織、特に（１１０）［００１］集合組織の強度を低下させ、Ｃ方向とＬ方向のシャルピー吸収エネルギーを均等化することができる。 (110) [001] Structure strength less than 10.0 In the present invention, as described above, hot rolling is performed under appropriate conditions in order to improve the low temperature toughness of the steel material (base material) and the weld heat affected zone. It is important to do. As a result, the strength of the microstructure, particularly the (110) [001] texture, can be reduced, and the Charpy absorption energy in the C direction and the L direction can be equalized.
　板厚１／２位置のミクロ組織における、（１１０）［００１］集合組織強度が１０．０以上では、亀裂が伝播しやすくなる。その結果、吸収エネルギーが低下する。このため、上記の（１１０）［００１］集合組織強度は１０．０未満とする。好ましくは９．０以下とする。より好ましくは６．０以下とする。Ｌ方向の吸収エネルギーが低下することから、板厚１／２位置のミクロ組織における（１１０）［００１］集合組織強度は、１．０以上とすることが好ましい。より好ましくは４．０以上とする。 When the (110) [001] texture strength is 10.0 or more in the microstructure at the plate thickness 1/2 position, cracks are likely to propagate. As a result, the absorbed energy is reduced. Therefore, the above-mentioned (110) [001] texture strength is set to less than 10.0. It is preferably 9.0 or less. More preferably, it is 6.0 or less. Since the absorbed energy in the L direction decreases, the (110) [001] texture strength in the microstructure at the plate thickness 1/2 position is preferably 1.0 or more. More preferably, it is 4.0 or more.
　硬さ：３００ＨＶ未満
　板厚１／２位置の硬さが３００ＨＶ以上では、延性が低下し、吸収エネルギーが低下する。このため、上記の硬さは３００ＨＶ未満とする。好ましくは、２８０ＨＶ以下とする。より好ましくは２６０ＨＶ以下とする。鋼材の強度が低下することから、板厚１／２位置の硬さは、２００ＨＶ以上とすることが好ましい。より好ましくは２２０ＨＶ以上とする。 Hardness: Less than 300 HV When the hardness at the plate thickness 1/2 position is 300 HV or more, the ductility is lowered and the absorbed energy is lowered. Therefore, the above hardness is set to less than 300 HV. Preferably, it is 280 HV or less. More preferably, it is 260 HV or less. Since the strength of the steel material is reduced, the hardness at the position where the plate thickness is 1/2 is preferably 200 HV or more. More preferably, it is 220 HV or more.
　硫化物系介在物の清浄度：１．０％未満（好適条件）
　板厚１／２位置のミクロ組織における、硫化物系介在物の清浄度が１．０％以上では、破壊の起点となる。その結果、吸収エネルギーが低下するおそれがある。このため、上記の硫化物系介在物の清浄度は１．０％未満とすることが好ましい。より好ましくは０．８％以下とする。さらに好ましくは０．６％以下とする。上記の清浄度の下限は特に規定しないが、製造コストの観点からは、０．１％以上とすることが好ましい。 Cleanliness of sulfide-based inclusions: less than 1.0% (optimal conditions)
When the cleanliness of the sulfide-based inclusions in the microstructure at the plate thickness 1/2 position is 1.0% or more, it becomes the starting point of fracture. As a result, the absorbed energy may decrease. Therefore, the cleanliness of the sulfide-based inclusions is preferably less than 1.0%. More preferably, it is 0.8% or less. More preferably, it is 0.6% or less. The lower limit of the above cleanliness is not particularly specified, but from the viewpoint of manufacturing cost, it is preferably 0.1% or more.
　なお、上記した清浄度とは、以下の（２）式で算出される。
ｄ＝（ｎ／ｐ×ｆ）×１００・・・（２）
ここで、上記（２）式における、ｐ：視野内の総格子点数、ｆ：視野数、ｎ：ｆ個の視野における介在物によって占められる格子点中心の数、とする。
よって、清浄度は、鋼材の板厚１／２位置における、硫化物系介在物が占める面積百分率を算出した値であり、Ｃ方向の硫化物系介在物を示す。硫化物系介在物として、例えばＭｎＳが挙げられる。 The above-mentioned cleanliness is calculated by the following equation (2).
d = (n / p × f) × 100 ... (2)
Here, in the above equation (2), p: the total number of grid points in the visual field, f: the number of visual fields, and n: the number of grid point centers occupied by inclusions in the f visual fields.
Therefore, the cleanliness is a value obtained by calculating the area percentage occupied by the sulfide-based inclusions at the position where the plate thickness of the steel material is 1/2, and indicates the sulfide-based inclusions in the C direction. Examples of the sulfide-based inclusions include MnS.
　上記した（１１０）［００１］集合組織強度：１０．０未満と、硬さ：３００ＨＶ未満と、硫化物系介在物の清浄度：１．０％未満は、後述する条件に従う熱間圧延を行うことによって、実現することができる。 For the above-mentioned (110) [001] texture strength: less than 10.0, hardness: less than 300 HV, and cleanliness of sulfide-based inclusions: less than 1.0%, hot rolling is performed according to the conditions described later. By doing so, it can be realized.
　なお、本発明では、上記した集合組織強度、硬さ、および硫化物系介在物の清浄度は、後述する実施例に記載の方法で測定することができる。 In the present invention, the above-mentioned texture strength, hardness, and cleanliness of sulfide-based inclusions can be measured by the methods described in Examples described later.
　以上のミクロ組織を有する本発明の鋼材は、低温靭性に優れる。 The steel material of the present invention having the above microstructure is excellent in low temperature toughness.
　ここで、上記したミクロ組織を有する鋼材（母材）に加え歪時効後および溶接熱影響部の－１９６℃における、シャルピー衝撃試験の吸収エネルギーを測定した。 Here, in addition to the steel material (base material) having the above-mentioned microstructure, the absorbed energy of the Charpy impact test after strain aging and at -196 ° C of the weld heat affected zone was measured.
　鋼材の板厚１／２位置におけるミクロ組織は、（１１０）［００１］集合組織強度を１０．０未満、かつ硬さ３００ＨＶ未満とすれば、鋼材の板厚１／２位置において、Ｃ方向およびＬ方向を含む全ての方向で、吸収エネルギー（ｖＥ－１９６）：４１Ｊ以上を実現することができる。これにより、本発明の鋼材を溶接した溶接部でも、溶接熱影響部粗粒域のＣ方向の吸収エネルギー（ｖＥ－１９６）：４１J以上を実現することができる。また、本発明の鋼材に所定の条件（例えば、後述の実施例に記載の条件）で予歪を与え時効処理を施した、歪時効後におけるＣ方向の吸収エネルギー（ｖＥ－１９６）：４１J以上を実現することができる。
なお、好ましい熱量等の溶接条件は、後述するタンクの好適な溶接条件と同様のため、ここでは省略する。 The microstructure at the 1/2 position of the plate thickness of the steel material is (110) [001]. If the texture strength is less than 10.0 and the hardness is less than 300 HV, the microstructure at the 1/2 position of the plate thickness of the steel material is in the C direction and at the 1/2 position. in all directions including the L direction, the absorbed energy (vE -196): 41J can be realized more. Accordingly, even in welds with a welded steel of the present invention, the heat affected zone coarse zone C in absorbed energy (vE -196): 41J can be realized more. Further, the steel material to a predetermined condition of the present invention (e.g., conditions described in the examples below) were subjected to aging treatment being pre-strained in, C directions of absorption energy after strain aging (vE -196): 41J or more Can be realized.
The welding conditions such as the preferable amount of heat are the same as the preferable welding conditions for the tank described later, and are therefore omitted here.
　また、上記の集合組織強度および硬さに加えて、鋼材の板厚１／２位置における硫化物系介在物の清浄度を１．０％未満とすれば、低値を示すＣ方向においても、より一層効果的に吸収エネルギー（ｖＥ－１９６）：４１Ｊ以上を得ることができる。 Further, in addition to the above-mentioned texture strength and hardness, if the cleanliness of the sulfide-based inclusions at the position of 1/2 of the plate thickness of the steel material is less than 1.0%, even in the C direction showing a low value, more effectively absorbed energy (vE -196): can be obtained than 41J.
　次に、本発明の鋼材（オーステナイト鋼材）における成分組成の好ましい範囲について説明する。なお、本発明のオーステナイト鋼材（例えば、高Ｍｎ鋼材）を素材として用い、この鋼材を溶接して得られた構造体（例えばタンク）は、母材および溶接部も同様の成分組成およびミクロ組織となる（ただし、溶接部のオーステナイト粒径は大きくなる）。 Next, a preferable range of the component composition in the steel material (austenite steel material) of the present invention will be described. The structure (for example, tank) obtained by using the austenite steel material (for example, high Mn steel material) of the present invention as a material and welding this steel material has the same composition and microstructure as the base material and the welded portion. (However, the austenite particle size of the weld increases).
　［成分組成］
　本発明では、オーステナイト鋼材およびその製造に用いられる鋼素材が、上記した成分組成を有する。本発明のオーステナイト鋼材の成分組成とその限定理由について説明する。なお、成分組成に関する「％」の表示は、特に断らない限り「質量％」を意味する。 [Ingredient composition]
In the present invention, the austenite steel material and the steel material used for producing the austenite steel material have the above-mentioned composition. The composition of the austenite steel material of the present invention and the reason for its limitation will be described. The indication of "%" regarding the component composition means "mass%" unless otherwise specified.
　Ｃ：０．１００％以上０．７００％以下
　Ｃは、安価なオーステナイト安定化元素であり、オーステナイトを得るために重要な元素である。この効果を得るために、Ｃは０．１００％以上の含有をすることが好ましい。一方、Ｃは０．７００％を超えて含有すると、Ｃｒ炭化物が過度に生成され、低温靱性が低下するおそれがある。このため、Ｃは０．１００％以上０．７００％以下とすることが好ましい。Ｃは、より好ましくは０．２００％以上とし、より好ましくは０．６００％以下とする。Ｃは、さらに好ましくは０．２５０％以上とし、さらに好ましくは０．５５０％以下とする。 C: 0.100% or more and 0.700% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain this effect, C is preferably contained in an amount of 0.100% or more. On the other hand, if C is contained in an amount of more than 0.700%, Cr carbides may be excessively generated and the low temperature toughness may be lowered. Therefore, C is preferably 0.100% or more and 0.700% or less. C is more preferably 0.200% or more, and more preferably 0.600% or less. C is more preferably 0.250% or more, still more preferably 0.550% or less.
　Ｓｉ：０．０５％以上１．００％以下
　Ｓｉは、脱酸材として作用し、製鋼上必要であるだけでなく、鋼に固溶して固溶強化により鋼板を高強度化する効果を有する。このような効果を得るために、Ｓｉは０．０５％以上の含有をすることが好ましい。一方、Ｓｉは１．００％を超えて含有すると、非熱的応力が過度に上昇するため、低温靱性が劣化するおそれがある。このため、Ｓｉは０．０５％以上１．００％以下とすることが好ましい。Ｓｉは、より好ましくは０．０７％以上とし、より好ましくは０．８０％以下とする。Ｓｉは、さらに好ましくは０．１０％以上とし、さらに好ましくは０．６０％以下とする。 Si: 0.05% or more and 1.00% or less Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain such an effect, it is preferable that Si is contained in an amount of 0.05% or more. On the other hand, if Si is contained in an amount of more than 1.00%, the non-thermal stress is excessively increased, so that the low temperature toughness may be deteriorated. Therefore, Si is preferably 0.05% or more and 1.00% or less. Si is more preferably 0.07% or more, and more preferably 0.80% or less. Si is more preferably 0.10% or more, still more preferably 0.60% or less.
　Ｍｎ：２０．０％以上４０．０％以下
　Ｍｎは、比較的安価なオーステナイト安定化元素である。本発明では、強度と低温靱性を両立するために重要な元素である。その効果を得るために、Ｍｎは２０．０％以上の含有をすることが好ましい。一方、Ｍｎは４０．０％を超えて含有した場合、低温靱性が劣化するおそれがある。また、溶接性、切断性が劣化するおそれがある。さらに、偏析を助長し、応力腐食割れの発生を助長する。このため、Ｍｎは２０．０％以上４０．０％以下とすることが好ましい。Ｍｎは、より好ましくは２３．０％以上とし、さらに好ましくは２４．０％以上とする。より好ましくは３５．０％以下とし、さらに好ましくは３０．０％以下とする。 Mn: 20.0% or more and 40.0% or less Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and low temperature toughness. In order to obtain the effect, Mn is preferably contained in an amount of 20.0% or more. On the other hand, if Mn is contained in excess of 40.0%, the low temperature toughness may deteriorate. In addition, weldability and cutability may deteriorate. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is preferably 20.0% or more and 40.0% or less. Mn is more preferably 23.0% or more, still more preferably 24.0% or more. It is more preferably 35.0% or less, and further preferably 30.0% or less.
　Ｐ：０．０３０％以下
　Ｐは、０．０３０％を超えて含有すると、過度に粒界に偏析するため、低温靱性が低下する。このため、０．０３０％を上限とし、可能なかぎり低減することが望ましい。したがって、Ｐは０．０３０％以下とする。尚、過度のＰ低減は精錬コストを高騰させ経済的に不利となるため、０．００２％以上とすることが望ましい。Ｐは、より好ましくは０．００５％以上とし、さらに好ましくは０．０１０％以上とする。より好ましくは０．０２８％以下とし、さらに好ましくは０．０２４％以下とする。 P: 0.030% or less If P is contained in excess of 0.030%, it segregates excessively at the grain boundaries, resulting in a decrease in low temperature toughness. Therefore, it is desirable to limit the amount to 0.030% as much as possible. Therefore, P is set to 0.030% or less. It should be noted that excessive P reduction raises the refining cost and is economically disadvantageous, so it is desirable to set it to 0.002% or more. P is more preferably 0.005% or more, still more preferably 0.010% or more. It is more preferably 0.028% or less, and further preferably 0.024% or less.
　Ｓ：０．００５０％以下
　Ｓは、母材の低温靭性や延性を劣化させるため、０．００５０％を上限とし、可能なかぎり低減することが望ましい。したがって、Ｓは０．００５０％以下とする。より好ましくは０．００４５％以下とし、さらに好ましくは０．００４０％以下とする。尚、過度のＳの低減は精錬コストを高騰させ経済的に不利となるため、Ｓは０．００１０％以上とすることが望ましい。より好ましくは０．００１２％以上とする。 S: 0.0050% or less Since S deteriorates the low temperature toughness and ductility of the base material, it is desirable to limit it to 0.0050% and reduce it as much as possible. Therefore, S is set to 0.0050% or less. It is more preferably 0.0045% or less, and further preferably 0.0040% or less. It is desirable that S is 0.0010% or more because excessive reduction of S increases the refining cost and is economically disadvantageous. More preferably, it is 0.0012% or more.
　Ａｌ：５．００％以下
　Ａｌは、脱酸剤として作用し、鋼板の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。また、引張試験時の降伏強度および局部伸びが向上する。このような効果を得るためには、Ａｌは０．０１％以上を含有することが好ましい。一方、Ａｌは５．００％を超えて含有すると、介在物が多量に存在し、低温靭性を劣化させるため、５．００％以下とする。Ａｌは、より好ましくは０．０１％以上とし、さらに好ましくは０．０２％以上とする。Ａｌは、より好ましくは４．００％以下とし、さらに好ましくは３．５０％以下とする。 Al: 5.00% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. In addition, the yield strength and local elongation during the tensile test are improved. In order to obtain such an effect, Al preferably contains 0.01% or more. On the other hand, if Al is contained in an amount of more than 5.00%, a large amount of inclusions are present and the low temperature toughness is deteriorated, so the content is set to 5.00% or less. Al is more preferably 0.01% or more, still more preferably 0.02% or more. Al is more preferably 4.00% or less, still more preferably 3.50% or less.
　Ｃｒ：７．０％以下
　Ｃｒは、粒界強度を向上させるため、低温靱性の向上に有効な元素である。このような効果を得るためには、Ｃｒは０．５％以上を含有することが好ましい。一方、Ｃｒは７．０％を超えて含有すると、Ｃｒ炭化物の生成により、低温靭性および耐応力腐食割れ性が低下するおそれがある。このため、Ｃｒは７．０％以下とすることが好ましい。Ｃｒは、好ましくは０．５％以上とし、より好ましくは１．０％以上とし、さらに好ましくは１．２％以上する。Ｃｒは、より好ましくは６．７％以下とし、さらに好ましくは６．５％以下とする。また、耐応力腐食割れをさらに向上させるためには、Ｃｒを２．０％以上６．０％以下とすることがさらに一層好ましい。 Cr: 7.0% or less Cr is an element effective for improving low temperature toughness because it improves grain boundary strength. In order to obtain such an effect, Cr is preferably contained in an amount of 0.5% or more. On the other hand, if Cr is contained in an amount of more than 7.0%, low temperature toughness and stress corrosion cracking resistance may decrease due to the formation of Cr carbides. Therefore, Cr is preferably 7.0% or less. Cr is preferably 0.5% or more, more preferably 1.0% or more, and further preferably 1.2% or more. Cr is more preferably 6.7% or less, still more preferably 6.5% or less. Further, in order to further improve the stress corrosion cracking resistance, it is further preferable that Cr is 2.0% or more and 6.0% or less.
　Ｎ：０．０５００％以下
　Ｎは、オーステナイト安定化元素であり、低温靱性の向上に有効な元素である。このような効果を得るためには、Ｎは０．００５０％以上を含有することが好ましい。一方、Ｎは０．０５００％を超えて含有すると、窒化物または炭窒化物が粗大化し、靭性が低下するおそれがある。このため、Ｎは０．０５００％以下とすることが好ましい。Ｎは、好ましくは０．００５０％以上とし、より好ましくは０．００６０％以上とし、さらに好ましくは０．００７０％以上とする。Ｎは、より好ましくは０．０４００％以下とし、さらに好ましくは０．０３００％以下とする。 N: 0.0500% or less N is an austenite stabilizing element, which is an effective element for improving low temperature toughness. In order to obtain such an effect, N is preferably contained in an amount of 0.0050% or more. On the other hand, if N is contained in an amount of more than 0.0500%, the nitride or carbonitride may be coarsened and the toughness may be lowered. Therefore, N is preferably 0.0500% or less. N is preferably 0.0050% or more, more preferably 0.0060% or more, and further preferably 0.0070% or more. N is more preferably 0.0400% or less, still more preferably 0.0300% or less.
　Ｏ：０．００５０％以下
　Ｏは、酸化物の形成により低温靱性を劣化させる。このため、Ｏは０．００５０％以下の範囲とする。好ましくは０．００４５％以下とし、より好ましくは０．００４０％以下とし、さらに好ましくは０．００３５％以下とする。尚、過度のＯの低減は精錬コストを高騰させ経済的に不利となるため、Ｏは０．００１０％以上とすることが望ましい。より好ましくは０．００１２％以上とする。 O: 0.0050% or less O deteriorates low temperature toughness due to the formation of oxides. Therefore, O is in the range of 0.0050% or less. It is preferably 0.0045% or less, more preferably 0.0040% or less, and further preferably 0.0035% or less. It is desirable that O is 0.0010% or more because excessive reduction of O increases the refining cost and is economically disadvantageous. More preferably, it is 0.0012% or more.
　Ｔｉ：０．００５％未満、Ｎｂ：０．００５％未満
　ＴｉおよびＮｂは、鋼中で高融点の炭窒化物を形成するため、低温靭性が低下する。ＴｉおよびＮｂは、原材料などから不可避的に混入する成分であるため、Ｔｉ：０．００５％以上０．０１０％以下およびＮｂ：０．００５％以上０．０１０％以下の範囲で混入するのが通例である。そこで、後述する溶製の手法に従って、ＴｉおよびＮｂの不可避混入を回避し、ＴｉおよびＮｂの含有量を各々０．００５％未満に抑制する必要がある。ＴｉおよびＮｂの含有量を各々０．００５％未満に抑制することによって、上記した炭窒化物の悪影響を排除し、優れた低温靭性並びに延性を確保することができる。好ましくは、ＴｉおよびＮｂの含有量を０．００３％以下とする。勿論、ＴｉおよびＮｂの含有量は０％であってもよい。より好ましくは０．００１％以上とする。 Ti: less than 0.005%, Nb: less than 0.005% Ti and Nb form high melting point carbonitrides in steel, which reduces low temperature toughness. Since Ti and Nb are components that are inevitably mixed from raw materials, etc., Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less should be mixed. It is customary. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the content of Ti and Nb to less than 0.005%, respectively, according to the melting method described later. By suppressing the contents of Ti and Nb to less than 0.005%, respectively, the adverse effects of the above-mentioned carbonitride can be eliminated, and excellent low temperature toughness and ductility can be ensured. Preferably, the content of Ti and Nb is 0.003% or less. Of course, the content of Ti and Nb may be 0%. More preferably, it is 0.001% or more.
　Ｃａ：０．０１００％以下、Ｍｇ：０．０１００％以下、ＲＥＭ：０．０２００％以下から選択される１種または２種以上
　Ｃａ、ＭｇおよびＲＥＭ（希土類金属）は、介在物の形態制御に有用な元素である。介在物の形態制御とは、展伸した硫化物系介在物を粒状の介在物とすることをいう。この介在物の形態制御を介して、延性、靭性および耐硫化物応力腐食割れ性を向上させる。このような効果を得るためには、ＣａおよびＭｇは０．０００５％以上、ＲＥＭは０．００１０％以上含有することが好ましい。一方、いずれの元素も多く含有させると、非金属介在物量が増加し、かえって延性、靭性、耐硫化物応力腐食割れ性が低下する。また、経済的に不利になる。 One or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less Ca, Mg and REM (rare earth metal) are used for morphological control of inclusions. It is a useful element. Morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Through morphological control of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferable that Ca and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, when a large amount of any of the elements is contained, the amount of non-metal inclusions increases, and on the contrary, the ductility, toughness, and sulfide stress corrosion cracking resistance decrease. It is also economically disadvantageous.
　このため、ＣａおよびＭｇを含有する場合には、それぞれ０．０１００％以下、ＲＥＭを含有する場合には、０．０２００％以下とすることが好ましい。好ましくは、Ｃａは０．０００５％以上、Ｍｇは０．０００５％以上、ＲＥＭは０．００１０％以上とする。より好ましくは、Ｃａは０．００１０％以上０．００８０％以下、Ｍｇは０．００１０％以上０．００８０％以下、ＲＥＭは０．００２０％以上０．０１５０％以下とする。さらに好ましくは、Ｃａは０．００５０％以下、Ｍｇは０．００５０％以下とする。 Therefore, when Ca and Mg are contained, it is preferably 0.0100% or less, and when REM is contained, it is preferably 0.0200% or less. Preferably, Ca is 0.0005% or more, Mg is 0.0005% or more, and REM is 0.0010% or more. More preferably, Ca is 0.0010% or more and 0.0080% or less, Mg is 0.0010% or more and 0.0080% or less, and REM is 0.0020% or more and 0.0150% or less. More preferably, Ca is 0.0050% or less and Mg is 0.0050% or less.
　本発明のオーステナイト鋼材は、上記した成分以外の残部が鉄（Ｆｅ）および不可避的不純物である。ここでの不可避的不純物としては、Ｈ、Ｂなどが挙げられ、各元素の合計で０．０１％以下であれば許容できる。 In the austenite steel material of the present invention, the balance other than the above-mentioned components is iron (Fe) and unavoidable impurities. Examples of the unavoidable impurities here include H and B, and if the total of each element is 0.01% or less, it is acceptable.
　上記の元素を基本の成分組成とすることが好ましい。この基本の成分組成によって本発明で目的とする特性は得られる。本発明では、強度および低温靱性をさらに向上させることを目的として、上記の元素に加えて、必要に応じて下記の元素を含有することができる。 It is preferable to use the above elements as the basic composition. With this basic composition, the properties desired in the present invention can be obtained. In the present invention, in addition to the above elements, the following elements can be contained, if necessary, for the purpose of further improving the strength and low temperature toughness.
　Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｍｏ：２．０％以下、Ｖ：２．０％以下、Ｗ：２．０％以下から選択される１種または２種以上
　Ｃｕ：１．０％以下、Ｎｉ：１．０％以下
　ＣｕおよびＮｉは、固溶強化により鋼板を高強度化するだけでなく、転位の易動度を向上させ、低温靱性も向上する元素である。このような効果を得るためには、ＣｕおよびＮｉは０．０１％以上で含有することが好ましい。一方、ＣｕおよびＮｉは１．０％を超えて含有すると、圧延時に表面性状が劣化する他、製造コストを圧迫する。このため、これらの合金元素を含有する場合は、その含有量は各々１.０％以下とすることが好ましい。より好ましくは０．０３％以上とし、より好ましくは０．７％以下とする。さらに好ましくは０．５％以下とする。 One or more Cu selected from Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less : 1.0% or less, Ni: 1.0% or less Cu and Ni are elements that not only increase the strength of steel sheets by solid solution strengthening, but also improve the mobility of dislocations and improve low temperature toughness. .. In order to obtain such an effect, Cu and Ni are preferably contained in an amount of 0.01% or more. On the other hand, if Cu and Ni are contained in an amount of more than 1.0%, the surface texture deteriorates during rolling and the manufacturing cost is reduced. Therefore, when these alloying elements are contained, the content thereof is preferably 1.0% or less. It is more preferably 0.03% or more, and more preferably 0.7% or less. More preferably, it is 0.5% or less.
　Ｍｏ：２．０％以下、Ｖ：２．０％以下、Ｗ：２．０％以下
　Ｍｏ、ＶおよびＷは、オーステナイトの安定化に寄与するとともに母材強度の向上に寄与する。このような効果を得るためには、Ｍｏ、ＶおよびＷは、各々０．００１％以上を含有することが好ましい。一方、Ｍｏ、ＶおよびＷは、各々２．０％を超えて含有すると、粗大な炭窒化物が生成し、破壊の起点となることがある他、製造コストを圧迫する。このため、これらの合金元素を含有する場合は、その含有量は各々２.０％以下とすることが好ましい。より好ましくは０．００３％以上とし、より好ましくは１．７％以下とする。さらに好ましくは１．５％以下とする。 Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less Mo, V and W contribute to the stabilization of austenite and the improvement of the base metal strength. In order to obtain such an effect, it is preferable that Mo, V and W each contain 0.001% or more. On the other hand, if Mo, V and W are contained in an amount of more than 2.0%, coarse carbonitrides may be formed, which may be a starting point of fracture and put pressure on the manufacturing cost. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less. It is more preferably 0.003% or more, and more preferably 1.7% or less. More preferably, it is 1.5% or less.
　なお、本発明において、「鋼材（オーステナイト鋼材）」は板厚６ｍｍ以上の鋼板を指すものとする。極めて低温の環境で使用される構造用鋼の素材として好適に用いる観点からは、板厚は９ｍｍ超えとすることが好ましく、１２ｍｍ以上とすることがさらに好ましい。板厚の上限は特に限定されず、任意の厚さとすることができるが、４０ｍｍ以下とすることが好ましい。 In the present invention, "steel material (austenite steel material)" refers to a steel plate having a plate thickness of 6 mm or more. From the viewpoint of preferably using it as a material for structural steel used in an extremely low temperature environment, the plate thickness is preferably more than 9 mm, more preferably 12 mm or more. The upper limit of the plate thickness is not particularly limited and may be any thickness, but it is preferably 40 mm or less.
　［鋼材の製造方法］
　次に、本発明の一実施形態における鋼材の製造方法について説明する。 [Manufacturing method of steel materials]
Next, a method for producing a steel material according to an embodiment of the present invention will be described.
　本発明の鋼材（オーステナイト鋼材）は、上記した成分組成を有する溶鋼を、転炉、電気炉等、公知の溶製方法で溶製することができる。また、真空脱ガス炉にて２次精錬を行ってもよい。 The steel material (austenite steel material) of the present invention can melt molten steel having the above-mentioned composition by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace.
　その際、組織制御の妨げとなるＴｉおよびＮｂを上述した数値範囲に制限するために、原料などから不可避的にＴｉおよびＮｂが混入することを回避し、これらの含有量を低減する措置を取る必要がある。例えば、精錬段階におけるスラグの塩基度を下げることによって、これらの合金をスラグへ濃化させて排出し、最終的なスラブ製品におけるＴｉおよびＮｂの濃度を低減する。あるいは、酸素を吹き込んで酸化させ、還流時にＴｉおよびＮｂの合金を浮上分離させる等の方法でも良い。 At that time, in order to limit Ti and Nb, which hinder tissue control, to the above-mentioned numerical range, measures are taken to prevent unavoidable contamination of Ti and Nb from raw materials and reduce their contents. There is a need. For example, by lowering the basicity of the slag during the refining step, these alloys are concentrated into the slag and discharged, reducing the concentration of Ti and Nb in the final slab product. Alternatively, a method such as blowing oxygen to oxidize and floating-separating the alloy of Ti and Nb at reflux may be used.
　その後、連続鋳造法、造塊－分塊圧延法等、公知の鋳造方法により、所定寸法のスラブ等の鋼素材とすることが好ましい。 After that, it is preferable to use a known casting method such as a continuous casting method, an ingot-bulk rolling method, or the like to obtain a steel material such as a slab having a predetermined size.
　以下に、上記鋼素材を低温靭性に優れた鋼材（オーステナイト鋼材）へと造りこむための製造条件について、詳細に説明する。 The manufacturing conditions for incorporating the above steel material into a steel material (austenite steel material) having excellent low temperature toughness will be described in detail below.
　上記した構成のオーステナイト鋼材を得るためには、鋼素材を１１００℃以上１３００℃以下の温度域に加熱し、所定のクロス圧延を実施し、かつ、仕上圧延最終パスの圧下率が３０％以下、仕上圧延終了温度が７５０℃以上となる条件の熱間圧延を行うことが重要である。ここでの温度制御は、鋼素材の表面温度を基準とする。 In order to obtain the austenite steel material having the above-mentioned structure, the steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, a predetermined cross-rolling is performed, and the rolling reduction of the final pass of the finish rolling is 30% or less. It is important to perform hot rolling under the condition that the finish rolling end temperature is 750 ° C. or higher. The temperature control here is based on the surface temperature of the steel material.
　なお、以下の製造方法の説明では、温度に関する「℃」表示は、特に断らない限り、それぞれ鋼素材または鋼板の表面温度である。表面温度は、例えば放射温度計等で測定することができる。また、スラブや鋼板の板厚中心位置の温度は、例えば、鋼板の板厚中心に熱電対を付けて測定することや、鋼板断面内の温度分布を伝熱解析により計算し、その結果を鋼板の表面温度によって補正することで求めることができる。 In the following description of the manufacturing method, the "° C" indication regarding temperature is the surface temperature of the steel material or steel plate, respectively, unless otherwise specified. The surface temperature can be measured with, for example, a radiation thermometer. Further, the temperature at the center of the thickness of the slab or the steel plate is measured by attaching a thermocouple to the center of the thickness of the steel plate, or the temperature distribution in the cross section of the steel plate is calculated by heat transfer analysis, and the result is the steel plate. It can be obtained by correcting with the surface temperature of.
　鋼素材の加熱温度：１１００℃以上１３００℃以下
　熱間圧延にてＭｎを拡散させるために、熱間圧延前の鋼素材の加熱温度は１１００℃以上とする。Ｍｎを拡散させることで、Ｍｎ負偏析部においてもオーステナイトの安定度を確保することができる。これにより、溶接したときに得られる溶接熱影響部粗粒領域においてもオーステナイトの安定度を確保することができ、脆性破壊を防ぐことができる。一方、加熱温度が１３００℃を超えると鋼の溶解が始まってしまう懸念があるため、加熱温度の上限は１３００℃とする。好ましくは、１１３０℃以上１２７０℃以下である。 Heating temperature of steel material: 1100 ° C. or higher and 1300 ° C. or lower In order to diffuse Mn in hot rolling, the heating temperature of the steel material before hot rolling is set to 1100 ° C. or higher. By diffusing Mn, the stability of austenite can be ensured even in the Mn negative segregation part. As a result, the stability of austenite can be ensured even in the coarse-grained region of the weld heat-affected zone obtained at the time of welding, and brittle fracture can be prevented. On the other hand, if the heating temperature exceeds 1300 ° C., there is a concern that the steel will start melting, so the upper limit of the heating temperature is set to 1300 ° C. Preferably, it is 1130 ° C. or higher and 1270 ° C. or lower.
　（１）式で算出されるクロス圧延比：２０以下
　クロス圧延比＝圧延方向圧延比／圧延直角方向圧延比　　・・・（１）
ここで、「圧延方向圧延比」とは、総圧延に対する圧延方向の圧延比を指す。「圧延直角方向圧延比」とは、総圧延に対する圧延直角方向の圧延比を指す。したがって、「圧延方向圧延比／圧延直角方向圧延比」は、圧延直角方向圧延に対する圧延方向の圧延比を示す。 Cross-rolling ratio calculated by equation (1): 20 or less Cross-rolling ratio = rolling direction rolling ratio / rolling perpendicular direction rolling ratio ... (1)
Here, the "rolling ratio in the rolling direction" refers to the rolling ratio in the rolling direction with respect to the total rolling. "Rolling right-angled rolling ratio" refers to the rolling ratio in the direction perpendicular to rolling with respect to total rolling. Therefore, "rolling direction rolling ratio / rolling perpendicular direction rolling ratio" indicates the rolling ratio in the rolling direction with respect to rolling perpendicular direction rolling.
　上述のように、オーステナイト鋼の圧延では（１１０）［００１］集合組織が発達しやすい。そのため、違う方向の圧延を入れることにより（１１０）［００１］集合組織の割合が小さくなり、（１１０）［００１］集合組織の強度を低下させることができる。（１１０）［００１］集合組織強度を１０．０未満とするためには、（１）式で算出されるクロス圧延比は２０以下とする。 As described above, in rolling austenitic steel, (110) [001] texture is likely to develop. Therefore, by rolling in different directions, the ratio of the (110) [001] texture can be reduced, and the strength of the (110) [001] texture can be reduced. (110) [001] In order to make the texture strength less than 10.0, the cross-rolling ratio calculated by the equation (1) is 20 or less.
　さらに、熱間圧延時にＣ方向で圧延を行うクロス圧延を実施し、クロス圧延比を２０以下とすることで、Ｃ方向の硫化物系介在物の面積分率を低減することも有効である。クロス圧延比は、好ましくは１８以下であり、さらに好ましくは１５以下である。 Furthermore, it is also effective to reduce the area fraction of sulfide-based inclusions in the C direction by performing cross rolling in which rolling is performed in the C direction during hot rolling and setting the cross rolling ratio to 20 or less. The cross-rolling ratio is preferably 18 or less, more preferably 15 or less.
　なお、同一方向に圧延を繰り返すことで、（１１０）［００１］集合組織が発達するため、圧延方向の圧延と圧延直角方向の圧延を交互に繰り返すことが、集合組織の均一化のために好ましい。好ましくは、２回以上繰り返すことが好ましい。好ましくは３回以下とする。 Since the (110) [001] texture is developed by repeating rolling in the same direction, it is preferable to alternately repeat rolling in the rolling direction and rolling in the direction perpendicular to the rolling direction in order to make the texture uniform. .. It is preferable to repeat it twice or more. It is preferably 3 times or less.
　仕上圧延最終パスの圧下率：３０％以下、仕上圧延終了温度：７５０℃以上
　仕上圧延最終パスの圧下率が３０％を超えると、過度に転位密度が高くなり、低温靱性が劣化する。仕上圧延終了温度が７５０℃未満になると（１１０）［００１］集合組織が過度に発達し、低温靱性が劣化する。このため、仕上圧延最終パスの圧下率は３０％以下とする。該圧下率は、２５％未満とすることが好ましく、２０％以下とすることがさらに好ましい。仕上圧延終了温度は７５０℃以上とする。仕上圧延終了温度は、７８０℃以上とすることが好ましく、８００℃以上とすることがさらに好ましい。仕上圧延終了温度は、その上限を特に規定しないが、強度確保の観点からは、９５０℃以下とすることが好ましく、９２０℃以下とすることがさらに好ましい。仕上圧延最終パスの圧下率は、その下限を特に規定しないが、強度確保の観点からは、５％以上が好ましく、１０％以上とすることがさらに好ましい。 Finish rolling final pass rolling rate: 30% or less, finish rolling end temperature: 750 ° C or higher If the rolling rate of the finish rolling final pass exceeds 30%, the dislocation density becomes excessively high and the low temperature toughness deteriorates. When the finish rolling end temperature is less than 750 ° C. (110) [001], the texture is excessively developed and the low temperature toughness is deteriorated. Therefore, the rolling reduction of the final pass for finish rolling is set to 30% or less. The reduction rate is preferably less than 25%, more preferably 20% or less. The finish rolling end temperature is 750 ° C. or higher. The finish rolling end temperature is preferably 780 ° C. or higher, and more preferably 800 ° C. or higher. The upper limit of the finish rolling end temperature is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 950 ° C. or lower, and more preferably 920 ° C. or lower. The lower limit of the rolling reduction of the final pass for finish rolling is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 5% or more, and more preferably 10% or more.
　なお、本発明では、強度および靱性の更なる向上を目的として、クロス圧延において更に以下の条件に制御することが好ましい。 In the present invention, it is preferable to further control the cross rolling under the following conditions for the purpose of further improving the strength and toughness.
　圧延開始温度（好適条件）
　圧延開始温度は、１１００～１２５０℃が好ましい。１１００℃未満では、圧延温度が７８０℃未満となり、集合組織が過度に発達する恐れがある。１２５０℃超えでは、集合組織が変化しない恐れがある。 Rolling start temperature (favorable conditions)
The rolling start temperature is preferably 1100 to 1250 ° C. If the temperature is lower than 1100 ° C., the rolling temperature will be lower than 780 ° C., and the texture may be excessively developed. Above 1250 ° C, the texture may not change.
　圧延温度（好適条件）
　圧延温度（圧延中の温度）は、７８０～１２５０℃が好ましい。７８０℃未満では、集合組織が過度に発達する恐れがある。１２５０℃超えでは、集合組織が変化しない恐れがある。 Rolling temperature (favorable conditions)
The rolling temperature (temperature during rolling) is preferably 780 to 1250 ° C. Below 780 ° C, the texture may develop excessively. Above 1250 ° C, the texture may not change.
　圧下量（好適条件）
　７８０～１２５０℃の温度域における圧下量は、６０～９８％が好ましい。該圧下量が６０％未満では、集合組織が変化しない恐れがある。該圧下量が９８％超えでは、集合組織が過度に発達する恐れがある。上記圧下量とは、７８０～１２５０℃の温度域における総圧下率を示す。 Reduction amount (favorable conditions)
The amount of reduction in the temperature range of 780 to 1250 ° C. is preferably 60 to 98%. If the reduction amount is less than 60%, the texture may not change. If the reduction amount exceeds 98%, the texture may be overdeveloped. The reduction amount indicates the total reduction rate in the temperature range of 780 to 1250 ° C.
　冷却
　熱間圧延が終了した後、冷却を行う。冷却条件は特に規定しない。（熱間圧延終了時の温度－１００℃）以上の温度から、１．０℃／ｓ以上の平均冷却速度で６００℃以下まで冷却することが好ましい。これにより、炭化物生成およびPの粒界偏析を抑制し、鋼材の特性がより高められる。なお、上記の「熱間圧延終了時の温度」とは仕上圧延終了温度を指す。 Cooling After hot rolling is completed, cooling is performed. Cooling conditions are not specified. It is preferable to cool from a temperature of (temperature at the end of hot rolling to −100 ° C.) or higher to 600 ° C. or lower at an average cooling rate of 1.0 ° C./s or higher. As a result, carbide formation and grain boundary segregation of P are suppressed, and the characteristics of the steel material are further enhanced. The above-mentioned "temperature at the end of hot rolling" refers to the temperature at the end of finish rolling.
　次に、本発明のタンクについて説明する。 Next, the tank of the present invention will be described.
　本発明のタンクは、上記した鋼材を溶接して製造されたタンクである。上記知見ｄに記載の通り、本発明の鋼材は溶接後も溶接前のミクロ組織を引き継ぐ。このため、本発明のタンクの母材における成分組成およびミクロ組織は、上記した鋼材（オーステナイト鋼材）と同様である。母材（鋼材）の成分組成およびミクロ組織を上記のように規定することにより、母材の板厚１／２位置における－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上であるタンクを得られる。また、タンクの溶接熱影響部粗粒域における、－１９６℃でのシャルピー衝撃試験の吸収エネルギーを４１Ｊ以上にできる。さらに、歪時効後における、－１９６℃でのシャルピー衝撃試験の吸収エネルギーを４１J以上にできる。 The tank of the present invention is a tank manufactured by welding the above-mentioned steel materials. As described in the above finding d, the steel material of the present invention inherits the microstructure before welding even after welding. Therefore, the composition and microstructure of the base material of the tank of the present invention are the same as those of the above-mentioned steel material (austenite steel material). By defining the composition and microstructure of the base material (steel material) as described above, a tank having an absorbed energy of 41 J or more in the Charpy impact test at -196 ° C. at the plate thickness 1/2 position of the base material can be obtained. Be done. Further, the absorption energy of the Charpy impact test at -196 ° C. in the coarse grain region of the weld heat affected zone of the tank can be set to 41 J or more. Further, the absorbed energy of the Charpy impact test at -196 ° C. after strain aging can be increased to 41 J or more.
　本発明のタンクは、上記特性を有するため、例えば液化ガス貯槽用タンク等の極めて低温の環境で使用することができる。 Since the tank of the present invention has the above characteristics, it can be used in an extremely low temperature environment such as a tank for a liquefied gas storage tank.
　続いて、上記タンクの製造方法の好適な一例について説明する。 Subsequently, a preferable example of the method for manufacturing the above tank will be described.
　本発明のタンクは、上記の鋼材を溶接して製造される。なお、素材である鋼材（オーステナイト鋼材）の製造方法については既に説明しているため省略する。ここでは、好適な溶接条件について説明する。 The tank of the present invention is manufactured by welding the above steel materials. The method for producing the steel material (austenite steel material), which is the raw material, has already been described and will be omitted. Here, suitable welding conditions will be described.
　［好適な溶接条件］
　溶接の種類は、ガスメタルアーク溶接が好ましい。 [Suitable welding conditions]
The type of welding is preferably gas metal arc welding.
　入熱範囲は、３．０ｋJ／ｍｍ以下が好ましい。また、好ましくは０．５ｋJ／ｍｍ以上である。この入熱範囲を満たすことにより、上記の特性を満足することができる。 The heat input range is preferably 3.0 kJ / mm or less. Further, it is preferably 0.5 kJ / mm or more. By satisfying this heat input range, the above characteristics can be satisfied.
　５００～８００℃の温度範囲での平均冷却速度は、１０℃／ｓ以上とすることが好ましい。この温度範囲での平均冷却速度が１０℃／ｓ未満では、炭化物が生成して、吸収エネエネルギーが低下する。 The average cooling rate in the temperature range of 500 to 800 ° C. is preferably 10 ° C./s or more. If the average cooling rate in this temperature range is less than 10 ° C./s, carbides are generated and the absorbed energy energy is reduced.
　以上説明したように、本発明によれば、鋼材の全ての方向、そのなかでもＬ方向およびＣ方向のシャルピー衝撃試験の吸収エネルギーを均等化がでるため、鋼材（母材）および溶接部の衝撃特性の方位依存性を小さくできる。これにより材料（素材）の信頼性が向上した。 As described above, according to the present invention, since the absorbed energy of the Charpy impact test in all directions of the steel material, especially the L direction and the C direction, can be equalized, the impact of the steel material (base material) and the welded portion can be equalized. The orientation dependence of the characteristic can be reduced. This has improved the reliability of the material.
　以下、本発明を実施例に基づいて、更に詳細に説明する。なお、以下の実施例は本発明の好適な一例を示すものであり、本発明はこの実施例に限定されない。 Hereinafter, the present invention will be described in more detail based on examples. The following examples show a suitable example of the present invention, and the present invention is not limited to this example.
　転炉－取鍋精錬－連続鋳造法によって、表１に示す成分組成の鋼スラブを作製した。なお、表１に示す「－」は、意図的に添加しないことを表しており、含有しない（０％）場合だけでなく、不可避的に含有する場合も含むことを意味する。次いで、得られた鋼スラブを表２に示す条件で熱間圧延を行い、その後冷却を行い、板厚が６～４０ｍｍの鋼材（鋼板）を作製した。
なお、クロス圧延では、圧延中の温度：７８０～１２５０℃、７８０～１２５０℃の圧下量：６０～９８％、圧延終了後の冷却条件：１．０℃／ｓ以上となるように、適宜制御して行った。上記「圧延終了後の冷却条件」とは、（熱間圧延終了時の温度－１００℃）以上の温度から６００℃以下の温度までの平均冷却速度を指す。 A steel slab having the composition shown in Table 1 was prepared by a converter-ladle refining-continuous casting method. In addition, "-" shown in Table 1 indicates that it is not intentionally added, and means that it includes not only the case where it is not contained (0%) but also the case where it is unavoidably contained. Next, the obtained steel slab was hot-rolled under the conditions shown in Table 2 and then cooled to prepare a steel material (steel plate) having a plate thickness of 6 to 40 mm.
In cross-rolling, the temperature during rolling: 780 to 1250 ° C., the reduction amount at 780 to 1250 ° C.: 60 to 98%, and the cooling condition after the completion of rolling: 1.0 ° C./s or more are appropriately controlled. I went there. The above-mentioned "cooling condition after the end of rolling" refers to an average cooling rate from a temperature of (temperature at the end of hot rolling to -100 ° C) or higher to a temperature of 600 ° C or lower.
　また、得られた鋼板から継手用試験板（大きさ：２５０ｍｍ×５００ｍｍ）を採取し、それらのＬ方向同士及びＣ方向同士を溶接することで、溶接継手を作製した。ここでは、開先の形状：レ形、裏当て材：セラミックス、シールドガス：Ａｒ－３０％ＣＯ２、トーチ後退角：５～１０°の溶接条件で溶接した。 Further, a joint test plate (size: 250 mm × 500 mm) was collected from the obtained steel plate, and the L direction and the C direction were welded to each other to prepare a welded joint. Here, welding was performed under welding conditions such as groove shape: re-shape, backing material: ceramics, shield gas: Ar-30% CO 2 , and torch receding angle: 5 to 10 °.
　得られた鋼板と溶接継手を用いて、鋼板については引張試験特性、低温靭性、およびミクロ組織の評価を、溶接継手の溶接熱影響部粗粒域については低温靭性の評価を、それぞれ下記の要領で実施した。 Using the obtained steel sheet and welded joint, evaluate the tensile test characteristics, low temperature toughness, and microstructure of the steel sheet, and evaluate the low temperature toughness of the coarse grain area of the weld heat-affected zone of the welded joint as follows. It was carried out in.
　（１）引張試験特性
　得られた鋼板を用いて、鋼板の長手方向および幅方向の中央位置における、板厚１／２位置から次に示す引張試験片を採取した。板厚１５ｍｍを超える鋼板ではＪＩＳ４号引張試験片を採取し、板厚１５ｍｍ以下の鋼板では丸棒引張試験片を採取した。各引張試験片を用いて、ＪＩＳ　Ｚ２２４１（２０１１年）の規定に準拠した引張試験を行い、引張強度（ＴＳ）、降伏応力（ＹＳ）を評価した。本実施例では、降伏応力が４００ＭＰａ以上の特性を有するものを「母材強度に優れる」と判定した。 (1) Tensile test characteristics Using the obtained steel sheet, the following tensile test pieces were collected from the plate thickness 1/2 position at the center positions in the longitudinal direction and the width direction of the steel sheet. A JIS No. 4 tensile test piece was collected for a steel plate having a plate thickness of more than 15 mm, and a round bar tensile test piece was collected for a steel plate having a plate thickness of 15 mm or less. Each tensile test piece was used to perform a tensile test in accordance with JIS Z2241 (2011), and tensile strength (TS) and yield stress (YS) were evaluated. In this example, a material having a yield stress of 400 MPa or more was determined to be "excellent in base material strength".
　（２）低温靭性
　鋼板の低温靭性の評価は、以下の通り行った。 (2) Low temperature toughness The low temperature toughness of the steel sheet was evaluated as follows.
　得られた鋼板を用いて、鋼板の表面から板厚の１／２位置において、圧延方向に垂直な方向から、Ｃ方向のシャルピーＶノッチ試験片を採取した。また、得られた鋼板の鋼板表面から板厚の１／２位置において、圧延方向に平行な方向から、Ｌ方向のシャルピーＶノッチ試験片を採取した。さらに、得られた鋼板の鋼板表面から板厚の１／２位置において、標点間距離２００ｍｍの引張試験片をＬ方向およびＣ方向からそれぞれ採取し、５％の引張り予歪後、２５０℃で１時間の時効処理を行った引張試験片から、Ｌ方向およびＣ方向のシャルピーＶノッチ試験片を採取した。 Using the obtained steel sheet, a Charpy V-notch test piece in the C direction was collected from the direction perpendicular to the rolling direction at a position 1/2 of the plate thickness from the surface of the steel sheet. Further, a Charpy V notch test piece in the L direction was collected from a direction parallel to the rolling direction at a position of 1/2 of the plate thickness from the surface of the obtained steel sheet. Further, at a position of 1/2 of the thickness of the obtained steel sheet from the surface of the steel sheet, tensile test pieces having a distance between the gauge points of 200 mm were sampled from the L direction and the C direction, respectively, and after 5% tensile prestrain, at 250 ° C. Charpy V notch test pieces in the L and C directions were collected from the tensile test pieces that had been aged for 1 hour.
　次いで、ＪＩＳ　Ｚ　２２４２（２００５年）の規定に準拠して、各鋼板について３本のシャルピー衝撃試験を実施し、－１９６℃での吸収エネルギーを求め、鋼材（母材）靭性を評価した。上述したように鋼板Ｃ方向が靭性の低値を示す。そのため、本実施例では、３本の吸収エネルギー（ｖＥ－１９６）の平均値が、Ｃ方向：４１Ｊ以上を「母材靭性に優れる」と判定した。 Next, in accordance with the provisions of JIS Z 2242 (2005), three Charpy impact tests were carried out on each steel sheet, the absorbed energy at -196 ° C. was determined, and the toughness of the steel material (base material) was evaluated. As described above, the steel sheet C direction shows a low toughness value. Therefore, in this embodiment, the average value of three absorbed energy (vE -196) is, C direction: more than 41J was judged as "excellent in the base material toughness."
　なお、板厚１０ｍｍ以下の鋼板については、サブサイズ(５ｍｍ)のシャルピーＶノッチ試験片をＣ方向で作製し、各試験片について３本のシャルピー衝撃試験を－１９６℃で実施した。表３中、サブサイズのシャルピーＶノッチ試験片を用いて実施したサンプルには、吸収エネルギーの項目に「＊１」を示す。サブサイズの場合、３本の吸収エネルギー（ｖＥ－１９６）の平均値が、Ｃ方向：２７Ｊ以上を「母材靭性に優れる」と判定した。 For a steel plate having a thickness of 10 mm or less, a sub-size (5 mm) Charpy V-notch test piece was prepared in the C direction, and three Charpy impact tests were carried out for each test piece at -196 ° C. In Table 3, the sample carried out using the sub-sized Charpy V-notch test piece shows "* 1" in the item of absorbed energy. For sub-size, the average value of three absorbed energy (vE -196) is, C direction: more than 27J was judged as "excellent in the base material toughness."
　溶接継手の低温靭性の評価は、以下の通り行った。 The low temperature toughness of the welded joint was evaluated as follows.
　板厚が１０ｍｍを超える各溶接継手から、ＪＩＳ　Ｚ　２２４２（２００５年）の規定に準拠してシャルピーＶノッチ試験片を採取し、各溶接継手について３本のシャルピー衝撃試験を－１９６℃で実施した。本実施例では、３本の吸収エネルギーの平均値が４１Ｊ以上を「溶接部の靭性に優れる」と判定した。 Charpy V notch test pieces were collected from each welded joint with a plate thickness of more than 10 mm in accordance with JIS Z 2242 (2005), and three Charpy impact tests were conducted at -196 ° C for each welded joint. .. In this embodiment, it was determined that the average value of the absorbed energies of the three pieces was 41 J or more as "excellent in toughness of the welded portion".
　なお、板厚が１０ｍｍ未満の各溶接継手については、ＪＩＳ　Ｚ　２２４２（２００５年）の規定に準拠して５ｍｍサブサイズのシャルピーＶノッチ試験片を採取し、各溶接継手について３本のシャルピー衝撃試験を－１９６℃で実施した。表３中、サブサイズのシャルピーＶノッチ試験片を用いて実施したサンプルには、吸収エネルギーの項目に「＊１」を示す。サブサイズの場合、３本の吸収エネルギーの平均値が２７Ｊ以上を「溶接部の靭性に優れる」に優れると判定した。
ここでは、上記と同様、最も低値を示す鋼板Ｃ方向での測定値を用いて評価を行った。 For each welded joint with a plate thickness of less than 10 mm, a 5 mm sub-sized Charpy V notch test piece was collected in accordance with the provisions of JIS Z 2242 (2005), and three Charpy impact tests were conducted for each welded joint. Was carried out at -196 ° C. In Table 3, the sample carried out using the sub-sized Charpy V-notch test piece shows "* 1" in the item of absorbed energy. In the case of the sub-size, it was judged that the average value of the absorbed energies of the three pieces was 27 J or more, which was excellent in "excellent toughness of the welded portion".
Here, as in the above, the evaluation was performed using the measured value in the steel plate C direction showing the lowest value.
　（３）組織評価
　［ミクロ組織の観察］
　ミクロ組織の各相の面積率は、ＥＢＳＤ解析のＰｈａｓｅ　ｍａｐから求めた。 (3) Tissue evaluation [Observation of microstructure]
The area ratio of each phase of the microstructure was determined from the Phase map of EBSD analysis.
　得られた鋼板の板厚１／２位置で、圧延方向に平行な断面から、ＥＢＳＤ解析用試験片を採取し、５００μｍ×２００μｍの視野において、測定ステップ０．３μｍでＥＢＳＤ解析を行い、Ｐｈａｓｅ　ｍａｐに記載の値をオーステナイト相、フェライト相、マルテンサイト相の面積率とした。 EBSD analysis test pieces were collected from a cross section parallel to the rolling direction at a plate thickness of 1/2 of the obtained steel sheet, and EBSD analysis was performed in a measurement step of 0.3 μm in a field of view of 500 μm × 200 μm, and Phase map. The values described in 1 were taken as the area ratios of the austenite phase, the ferrite phase, and the martensite phase.
　なお、表３中、「その他の相」には、オーステナイト相以外の残部、すなわち、フェライト相および／またはマルテンサイト相の合計面積率を示す。 In Table 3, "other phases" indicates the total area ratio of the rest other than the austenite phase, that is, the ferrite phase and / or the martensite phase.
　［集合組織強度］
　得られた鋼板を用いて、鋼板の長手方向および幅方向の中央位置における、板厚１／２位置から、測定用試験片を採取した。各測定用試験片を用いて、ＮＤ面の集合組織強度をＸ線回折で測定した。得られたＯＤＦ（Orientation Determination Function：３次元結晶方位分布関数）から集合組織強度の最大値を求めた。なお、ＯＤＦは、化学研磨で鋼板表面の残留応力を除去した後、Ｘ線回折（内部規格化）により測定した極点図（（１１０）［００１］、（１００）［０１１］、（１００）［０１０］、（１１０）［１１２］、（１１２）［１１１］）より得ることができる。 [Strength of texture]
Using the obtained steel sheet, a test piece for measurement was taken from a plate thickness 1/2 position at the center position in the longitudinal direction and the width direction of the steel sheet. Using each measurement test piece, the texture strength of the ND surface was measured by X-ray diffraction. The maximum value of the texture strength was obtained from the obtained ODF (Orientation Determination Function). The ODF is a pole figure ((110) [001], (100) [011], (100) [ 010], (110) [112], (112) [111]).
　［硬さ］
　得られた鋼板を用いて、鋼板の長手方向および幅方向の中央位置における、板厚１／２位置において、ＨＶ１０ｋｇで１００点測定した。その最大値を最高硬さ値として用いた。 [Hardness]
Using the obtained steel sheet, 100 points were measured at HV 10 kg at the center position in the longitudinal direction and the width direction of the steel sheet at the plate thickness 1/2 position. The maximum value was used as the maximum hardness value.
　［硫化物系介在物の清浄度］
　得られた鋼板を用いて、鋼板の長手方向および幅方向の中央位置における、板厚１／２位置から、圧延方向断面の光学顕微鏡サンプルを切り出し、ＪＩＳ　Ｇ　０５５５付属書１の「点算法による非金属介在物の顕微鏡試験方法」により算出した。ここでは、Ｃ方向の硫化物系介在物の清浄度を算出した。顕微鏡の倍率×４００で６０視野測定し、以下の式を用いて清浄度（％）を算出した。
ｄ＝（ｎ／ｐ×ｆ）×１００・・・（２）
ここで、上記（２）式における、ｐ：視野内の総格子点数、ｆ：視野数、ｎ：ｆ個の視野における介在物によって占められる格子点中心の数、とする。
なお、硫化物系介在物としてＭｎＳの清浄度を算出した。 [Cleanliness of sulfide-based inclusions]
Using the obtained steel sheet, an optical microscope sample with a cross section in the rolling direction was cut out from the plate thickness 1/2 position at the center position in the longitudinal direction and the width direction of the steel sheet. It was calculated by "Microscopic test method of metal inclusions". Here, the cleanliness of the sulfide-based inclusions in the C direction was calculated. 60 visual fields were measured at a magnification of a microscope × 400, and the cleanliness (%) was calculated using the following formula.
d = (n / p × f) × 100 ... (2)
Here, in the above equation (2), p: the total number of grid points in the visual field, f: the number of visual fields, and n: the number of grid point centers occupied by inclusions in the f visual fields.
The cleanliness of MnS was calculated as a sulfide-based inclusion.
　以上により得られた結果を、表３に示す。 The results obtained from the above are shown in Table 3.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表３に示すように、本発明のオーステナイト鋼材では、上述の目標性能（（１１０）［００１］集合組織強度：１０．０未満、硬さ：３００ＨＶ未満、鋼材の板厚１／２位置のシャルピー衝撃試験の吸収エネルギー（ｖＥ-１９６）が４１J以上）を満足することが確認された。また、本発明のオーステナイト鋼材を溶接して得られる溶接継手では、上述の目標性能（溶接熱影響部粗粒域のシャルピー衝撃試験の吸収エネルギー（ｖＥ-１９６）が４１J以上）を満足することが確認された。さらに、歪時効処理後においても、上述の性能（歪時効後におけるシャルピー衝撃試験の吸収エネルギー（ｖＥ-１９６）が４１J以上）を満足することが確認された。 As shown in Table 3, in the austenite steel material of the present invention, the above-mentioned target performance ((110) [001] texture strength: less than 10.0, hardness: less than 300 HV, Charpy at position 1/2 of the plate thickness of the steel material. It was confirmed that the absorbed energy (vE -196) of the impact test satisfied (41J or more). Further, the welded joint obtained by welding the austenite steel material of the present invention can satisfy the above-mentioned target performance (the absorbed energy (vE -196 ) of the Charpy impact test in the coarse grain region of the weld heat affected zone is 41 J or more). confirmed. Furthermore, it was confirmed that even after the strain aging treatment, the above-mentioned performance (the absorbed energy (vE -196 ) of the Charpy impact test after the strain aging treatment is 41 J or more) is satisfied.
　これに対し、本発明の範囲を外れる比較例では、オーステナイト鋼材が上記目標性能を満足できなかった。また、得られる溶接継手では、吸収エネルギーが上述の目標性能を満足できなかった。さらに、歪時効処理後において上述の目標性能を満足することが確認された。 On the other hand, in the comparative example outside the scope of the present invention, the austenite steel material could not satisfy the above target performance. Further, in the obtained welded joint, the absorbed energy could not satisfy the above-mentioned target performance. Furthermore, it was confirmed that the above-mentioned target performance was satisfied after the strain aging treatment.

Claims (8)

  1. 　ミクロ組織は、面積率で９５％以上がＦＣＣであり、
    　板厚１／２位置の（１１０）［００１］集合組織強度が１０．０未満であり、
    　板厚１／２位置の硬さが３００ＨＶ未満であり、
    　板厚１／２位置におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、鋼材。     The microstructure is FCC with an area ratio of 95% or more.
    The (110) [001] texture strength at the plate thickness 1/2 position is less than 10.0.
    The hardness at the plate thickness 1/2 position is less than 300 HV,
    A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196 ° C. in the C direction at a plate thickness of 1/2 position.
  2. 　歪時効後における板厚１／２位置のＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、請求項１に記載の鋼材。 The steel material according to claim 1, wherein the absorbed energy of the Charpy impact test at -196 ° C. in the C direction at the plate thickness 1/2 position after strain aging is 41 J or more.
  3. 　溶接熱影響部粗粒域におけるＣ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、請求項１または２に記載の鋼材。 The steel material according to claim 1 or 2, wherein the absorbed energy of the Charpy impact test at -196 ° C. in the C direction in the weld heat-affected zone coarse grain region is 41 J or more.
  4. 　質量％で、
    　Ｃ：０．１００％以上０．７００％以下、
    　Ｓｉ：０．０５％以上１．００％以下、
    　Ｍｎ：２０．０％以上４０．０％以下、
    　Ｐ：０．０３０％以下、
    　Ｓ：０．００５０％以下、
    　Ａｌ：５．００％以下、
    　Ｃｒ：７．０％以下、
    　Ｎ：０．０５００％以下、
    　Ｏ：０．００５０％以下、
    　Ｔｉ：０．００５％未満、
    　Ｎｂ：０．００５％未満を含有し、
    　Ｃａ：０．０１００％以下、Ｍｇ：０．０１００％以下、ＲＥＭ：０．０２００％以下から選択される１種または２種以上を含有し、
    残部が鉄および不可避不純物からなる成分組成と、
    　前記ミクロ組織は、硫化物系介在物の清浄度が１．０％未満である、請求項１～３のいずれか１項に記載の鋼材。     By mass%
    C: 0.100% or more and 0.700% or less,
    Si: 0.05% or more and 1.00% or less,
    Mn: 20.0% or more and 40.0% or less,
    P: 0.030% or less,
    S: 0.0050% or less,
    Al: 5.00% or less,
    Cr: 7.0% or less,
    N: 0.0500% or less,
    O: 0.0050% or less,
    Ti: less than 0.005%,
    Nb: contains less than 0.005%,
    Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less.
    Ingredient composition with the balance consisting of iron and unavoidable impurities,
    The steel material according to any one of claims 1 to 3, wherein the microstructure has a cleanliness of sulfide-based inclusions of less than 1.0%.
  5. 　前記成分組成は、さらに、質量％で、
    　Ｃｕ：１．０％以下、
    　Ｎｉ：１．０％以下、
    　Ｍｏ：２．０％以下、
    　Ｖ：２．０％以下、
    　Ｗ：２．０％以下
    から選択される１種または２種以上を含有する、請求項４に記載の鋼材。     The composition of the components is further increased by mass%.
    Cu: 1.0% or less,
    Ni: 1.0% or less,
    Mo: 2.0% or less,
    V: 2.0% or less,
    W: The steel material according to claim 4, which contains one or more selected from 2.0% or less.
  6. 　前記硫化物系介在物はＭｎＳである、請求項４または５に記載の鋼材。 The steel material according to claim 4 or 5, wherein the sulfide-based inclusion is MnS.
  7. 　請求項１～６のいずれか１項に記載の鋼材の製造方法であって、
    鋼素材を、１１００℃以上１３００℃以下の温度域に加熱し、（１）式で算出されるクロス圧延比が２０以下、仕上圧延最終パスの圧下率が３０％以下、および仕上圧延終了温度が７５０℃以上となる条件で熱間圧延を行った後、冷却を行う、鋼材の製造方法。
    クロス圧延比＝圧延方向圧延比／圧延直角方向圧延比　　・・・（１）     The method for producing a steel material according to any one of claims 1 to 6.
    The steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, the cross-rolling ratio calculated by Eq. A method for producing a steel material, in which hot rolling is performed under conditions of 750 ° C. or higher and then cooling is performed.
    Cross rolling ratio = rolling direction rolling ratio / rolling perpendicular rolling ratio ・ ・ ・ (1)
  8. 　請求項１～６のいずれか１項に記載の鋼材を溶接したタンクであって、
    　溶接熱影響部粗粒域における、Ｃ方向の－１９６℃でのシャルピー衝撃試験の吸収エネルギーが４１Ｊ以上である、タンク。     A tank obtained by welding the steel material according to any one of claims 1 to 6.
    A tank in which the absorbed energy of the Charpy impact test at -196 ° C. in the C direction in the coarse grain region of the weld heat affected zone is 41 J or more.
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