Classifications

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  • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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 steel material according to [1], wherein the absorbed energy of the Charpy impact test at -196 ° C. in the weld heat-affected zone coarse grain region is 41 J or more.
• composition with the balance consisting of iron and unavoidable impurities The steel material according to [1] or [2], wherein the microstructure has a cleanliness of sulfide-based inclusions of less than 1.0%.
• the composition of the components is further increased by mass%.
• the steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and hot rolling is performed under the conditions that the cross-rolling ratio calculated by Eq. (1) is 20 or lower and the finish rolling end temperature is 750 ° C. or higher.
• a method for manufacturing steel materials in which the material is cooled after rolling.
• Cross rolling ratio rolling direction rolling ratio / rolling perpendicular rolling ratio ⁇ ⁇ ⁇ (1) [7]
• a tank in which the absorbed energy of the Charpy impact test at -196 ° C. in the weld heat-affected zone coarse grain area 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 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 absorbed energy of the Charpy impact test at -196 ° C. at the thickness 1/2 position is 41 J or more.
• the steel material of the present invention can have an absorption energy of 41 J or more in the Charpy impact test at -196 ° C. in the welded weld 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. 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.
• 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.
• 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 texture strength and the cleanliness of the sulfide-based inclusions described above 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 microstructure at the 1/2 position of the plate thickness of the steel material if the texture strength is less than 10.0 in (110) [001], all the microstructures at the 1/2 position of the plate thickness of the steel material including the C direction and the L direction. in the direction, it 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 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.
• C is preferably contained in an amount of 0.100% or more.
• 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.
• 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.
• 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. 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. 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. More preferably, it is 0.0045% 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.
• 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.
• 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, and more preferably 0.0060% or more.
• N is more preferably 0.0400% 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. It is desirable that O is 0.0010% or more because excessive reduction of O increases the refining cost and is economically disadvantageous.
• 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%.
• 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 finish rolling end temperature is 750 ° C. or higher. It is important to perform inter-rolling.
• 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 end temperature 750 ° C. or higher
• the finish rolling end temperature is set to 750 ° C. or higher.
• the temperature is preferably 780 ° C. or higher.
• the upper limit of the finish rolling end temperature is not particularly specified, but it is preferably 950 ° C. or lower from the viewpoint of ensuring strength.
• 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%. Below 60%, the texture may not change. Above 98%, there is a risk of overdeveloped aggregate tissue.
• the reduction amount indicates the total reduction rate in the temperature range of 780 to 1250 ° C.
• 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 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).
• 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.
• 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.
• a joint test plate (size: 250 mm ⁇ 500 mm) was collected from the obtained steel plate and welded to each other in the C direction 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 a direction parallel to the rolling direction at a position 1/2 of the plate thickness from the surface of the steel sheet.
• 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 2 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.
• Table 2 shows the results obtained from the above.
• the above-mentioned target performance ((110) [001] texture strength: less than 10.0, absorbed energy of the Charpy impact test at the position where the plate thickness of the steel material is 1/2 ( It was confirmed that vE -196 ) satisfies 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.
• 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.

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