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EP4249621A1 - Matériau d'acier et son procédé de production, ainsi que réservoir et son procédé de production - Google Patents

Matériau d'acier et son procédé de production, ainsi que réservoir et son procédé de production Download PDF

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Publication number
EP4249621A1
EP4249621A1 EP22749560.3A EP22749560A EP4249621A1 EP 4249621 A1 EP4249621 A1 EP 4249621A1 EP 22749560 A EP22749560 A EP 22749560A EP 4249621 A1 EP4249621 A1 EP 4249621A1
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EP
European Patent Office
Prior art keywords
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steel material
steel
temperature
content
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EP22749560.3A
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German (de)
English (en)
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EP4249621A4 (fr
Inventor
Daichi Izumi
Toshinori Ishida
Keiji Ueda
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JFE Steel Corp
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JFE Steel Corp
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Publication of EP4249621A1 publication Critical patent/EP4249621A1/fr
Publication of EP4249621A4 publication Critical patent/EP4249621A4/fr
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    • 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
    • C22C38/18—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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D51/00—Making hollow objects
    • B21D51/16—Making hollow objects characterised by the use of the objects
    • B21D51/18—Making hollow objects characterised by the use of the objects vessels, e.g. tubs, vats, tanks, sinks, or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D8/0221—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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/22—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
    • C22C38/40—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
    • C22C38/58—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
    • F17C3/00—Vessels not under pressure
    • 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
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the present invention relates to a steel material suitable for use in structural steels used in extremely low temperature environments such as liquid helium and liquefied gases, including, for example, tanks for storing liquid hydrogen, and to a method for producing the steel material.
  • the present invention also relates to a tank using this steel material and a method for producing the tank.
  • the hot-rolled steel plate is required to have excellent toughness at a low temperature because it is used in an extremely low-temperature environment.
  • a hot-rolled steel plate is used for a storage tank for liquid helium, it is necessary to ensure excellent toughness at a temperature equal to or lower than a boiling point of helium of -269°C.
  • the low-temperature toughness of the steel material is poor, there is a possibility that the safety as a structure for a cryogenic storage tank cannot be maintained.
  • austenitic stainless steels in the form of steel plates with austenite microstructures that do not exhibit brittleness at low temperatures, 9%-Ni steels, and 5000 series aluminum alloys have been used.
  • the alloying costs and production costs are high; thus, a steel material that is inexpensive and excellent in low-temperature toughness is required.
  • Patent Literature 1 discloses the use of a high-Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite-stabilizing element, as a structural steel for a low-temperature environment, as a new steel material in place of a conventional low-temperature service steel.
  • Patent Literature 1 discloses a technique for ensuring low-temperature toughness in a welded heat affected zone by controlling grain size, coverage by carbides, and the like.
  • a liquefied gas storage structure (such as a liquefied gas storage tank) is produced by line heating of a steel material.
  • Line heating is a processing method that uses plastic deformation due to local thermal stress to form a curved surface.
  • the line heating condition for high tensile strength steel having an equivalent carbon content (Ceq) of more than 0.38% in shipbuilding is 650°C or lower in terms of the maximum heating temperature of a surface during water cooling immediately after heating. If it is higher than that, it is specified that the maximum surface heating temperature is 900°C or lower, and water cooling is performed after natural cooling to 500°C.
  • the low-temperature toughness is reduced.
  • Patent Literature 1 does not verify the low-temperature toughness after the line heating.
  • the present invention has been made in view of the above disadvantages, and aims to provide a steel material having excellent low-temperature toughness after line heating, a method for producing the steel material, a tank composed of the steel material, and a method for producing the tank.
  • the phrase "excellent low-temperature toughness after line heating" described above indicates that, in a tank obtained by subjecting a steel material to line heat treatment described below, the absorbed energy in a Charpy impact test at -269°C or higher at a position 1 mm below the surface of the steel material (a position 1 mm from the surface of the steel material in the thickness direction) in a line-heated portion is 41 J or more.
  • the "line-heated portion” refers to a region thermally affected after the steel material is subjected to line heating.
  • the absorbed energy in the line-heated portion in the Charpy impact test can be measured by a method described in Examples below. Solution to Problem
  • the inventors have conducted intensive studies on an austenitic steel material (for example, a high-Mn steel material) with respect to the chemical composition, microstructure, and production method of the steel material (steel plate), various factors that determine the properties of the steel material, and a structure produced by the line heating of the steel material, and have found the following findings a to c.
  • the term "high-Mn steel material” refers to a steel material having a Mn content of 20% to 40% by mass.
  • the present invention has been made by further studying the above-described findings, and the gist thereof is described below.
  • the steel material having excellent low-temperature toughness after line heating and a method for producing the steel material can be provided.
  • the steel material of the present invention is suitably used as a material for a steel structure (for example, a tank for a liquefied gas storage tank) used in a low-temperature environment, and thus it is possible to provide the tank having excellent low-temperature toughness even after line heating and a method for producing the tank. Accordingly, it is possible to greatly contribute to the improvement of the safety and the life of the steel structure, and industrially significant effects are exhibited.
  • the production method of the present invention does not cause a decrease in productivity or an increase in production cost; thus, it is possible to provide a production method that is also excellent in economy.
  • Fig. 1 is a schematic view illustrating a line-heating specimen used in Examples of the present invention.
  • an austenitic steel material for example, a high-Mn steel material
  • a high-Mn steel material a steel material that is inexpensive and excellent in low-temperature toughness.
  • the high-Mn steel material is required to have excellent low-temperature toughness even at a portion thermally affected in a step of subjecting the material to line heating.
  • the inventors have conducted intensive investigation of the cause and have newly found that the C concentration at grain boundaries is responsible for a decrease in absorbed energy.
  • the relationship between the decrease in absorbed energy and the C concentration at the grain boundaries will be described below.
  • One of the origins of fracture of high-Mn steels is a grain boundary.
  • the low-temperature toughness is improved by reducing the grain boundaries, that is, by coarsening the grains.
  • C around the carbide is depleted, and the grain boundary strength decreases.
  • a self-healing phenomenon occurs in which C having a high diffusion rate is sufficiently supplied from the inside of grains away from grain boundaries during the formation and growth of carbides at the grain boundaries. This can suppress steep C depletion at grain boundaries.
  • the crystal grains are excessively coarse, the supply of C from the insides of the grains is not timely, resulting in a depletion of C at the grain boundaries.
  • the maximum grain size is set to less than 200 ⁇ m in a hot rolling step described below, so that a C concentration of 0.100% or more can be ensured even when carbides are formed, thereby inhibiting a reduction in absorbed energy.
  • a steel material of the present invention will be described below.
  • the steel material of the present invention has a chemical composition described below, and the microstructure has a maximum grain size of less than 200 ⁇ m at a position 1 mm below a surface of the steel material.
  • the C concentration at the grain boundaries can be 0.100% or more.
  • the symbol "%" regarding the C concentration indicates "% by mass”.
  • an austenitic steel material for example, a high-Mn steel material
  • a raw steel material used for the production thereof have the above-described chemical composition.
  • the chemical composition of the austenitic steel material of the present invention and the reasons for its limitation will be described.
  • the symbol “%” regarding the chemical composition indicates “% by mass” unless otherwise specified.
  • C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite.
  • C is contained in an amount of 0.200% or more.
  • Cr carbide is excessively formed, thereby deteriorating low-temperature toughness (low-temperature toughness after line heating).
  • the C content is 0.200% or more and 0.700% or less.
  • the C content is preferably 0.250% or more, more preferably 0.300% or more.
  • the content of C is preferably 0.600% or less, more preferably 0.550% or less.
  • Si 0.05% or more and 1.00% or less
  • Si acts as a deoxidizing agent and is necessary for steelmaking, and is also effective in strengthening the steel plate by solid solution strengthening when dissolved in the steel.
  • Si is contained in an amount of 0.05% or more.
  • a Si content of more than 1.00% results in an excessively high non-thermal stress, thereby deteriorating the low-temperature toughness.
  • the Si content is 0.05% or more and 1.00% or less.
  • the Si content is preferably 0.07% or more, more preferably 0.10% or more, still more preferably 0.15% or more.
  • the Si content is preferably 0.80% or less, more preferably 0.75% or less, still more preferably 0.70% or less.
  • Mn 20.0% or more and 40.0% or less
  • Mn is a relatively inexpensive austenite-stabilizing element.
  • Mn is an important element for achieving both good strength and low-temperature toughness.
  • Mn is contained in an amount of 20.0% or more.
  • the low-temperature toughness deteriorates.
  • weldability and cuttability deteriorate.
  • segregation is promoted, and the occurrence of stress corrosion cracking is promoted.
  • the Mn content is 20.0% or more and 40.0% or less.
  • the Mn content is preferably 23.0% or more, more preferably 23.3% or more, still more preferably 23.5% or more.
  • the Mn content is preferably 35.0% or less, more preferably 30.0% or less.
  • the upper limit is 0.030%, and it is desirable to reduce the content as much as possible.
  • the P content is 0.030% or less.
  • the P content is preferably 0.002% or more.
  • the P content is more preferably 0.005% or more, still more preferably 0.007% or more.
  • the P content is preferably 0.028% or less, more preferably 0.024% or less, still more preferably 0.020% or less.
  • the upper limit is 0.0050%, and it is desirable to reduce it as much as possible. Accordingly, the S content is 0.0050% or less.
  • the S content is preferably 0.0045% or less, more preferably 0.0043% or less.
  • An excessive reduction in S content causes an increase in refining cost, which is economically disadvantageous.
  • the S content is preferably 0.0010% or more.
  • the S content is more preferably 0.0012% or more.
  • Al acts as a deoxidizing agent and is most commonly used in a molten steel deoxidizing process of a steel plate. In addition, the yield strength and the local elongation in a tensile test are improved. To provide such effects, Al is preferably contained in an amount of 0.01% or more. When 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. Thus, the Al content is 5.00% or less. The Al content is preferably 0.01% or more, more preferably 0.02% or more. The Al content is preferably 4.00% or less, more preferably 3.00% or less.
  • Cr is an element effective in improving the low-temperature toughness because it improves the grain boundary strength.
  • Cr is preferably contained in an amount of 0.5% or more.
  • the Cr content is 7.0% or less.
  • the Cr content is preferably 0.5% or more, more preferably 1.0% or more, still more preferably 1.2% or more.
  • the Cr content is preferably 6.7% or less, more preferably 6.5% or less.
  • the Cr content is still more preferably 2.0% or more and 6.0% or less.
  • N is an austenite-stabilizing element and is an element effective in improving low-temperature toughness. To provide such effects, N is preferably contained in an amount of 0.0050% or more. When N is contained in an amount of more than 0.0500%, nitrides or carbonitrides may be coarsened to deteriorate the low-temperature toughness. Thus, the N content is 0.0500% or less. The N content is preferably 0.0050% or more, more preferably 0.0060% or more. The N content is preferably 0.0400% or less, more preferably 0.0300% or less.
  • the O content forms oxides to deteriorate the low-temperature toughness.
  • the O content is in the range of 0.0050% or less.
  • the O content is preferably 0.0045% or less, more preferably 0.0040% or less.
  • An excessive reduction in O content increases the refining cost, which is economically disadvantageous.
  • the O content is desirably 0.0010% or more.
  • the O content is more preferably 0.0012% or more.
  • Ti and Nb form high-melting-point carbonitrides in steel, thereby deteriorating the low-temperature toughness.
  • Ti and Nb are components that are inevitably incorporated from raw materials and so forth. Typically, Ti is incorporated in the range of 0.005% or more to 0.010% or less, and Nb is incorporated in ranges of 0.005% or more to 0.010% or less. It is thus necessary to avoid the inevitable incorporation of Ti and Nb and to reduce each of the Ti content and the Nb content to less than 0.005% in accordance with a steelmaking method described below. A reduction in each of the Ti content and the Nb content to less than 0.005% can eliminate the above-described adverse effects of carbonitrides to ensure excellent low-temperature toughness and ductility.
  • Each of the Ti content and the Nb content is preferably 0.003% or less, more preferably 0.002% or less. Of course, each of the Ti content and the Nb content may be 0%.
  • Ca, Mg, and rare earth metals (REM) are elements useful for morphological control of inclusions.
  • the morphological control of the inclusions indicates that flattened inclusions are formed into granular inclusions.
  • Ductility, low-temperature toughness, and stress corrosion cracking resistance are improved by the morphological control of the inclusions.
  • Ca and Mg are preferably contained in an amount of 0.0005% or more
  • REM is preferably contained in an amount of 0.0010% or more. If any of the elements is contained in a large amount, the amount of non-metallic inclusion increases, which in turn deteriorate the ductility, the low-temperature toughness, and the stress corrosion cracking resistance. Moreover, it is economically disadvantageous.
  • each of the Ca content and the Mg content is 0.0100% or less, and when REM is contained, the REM content is 0.0200% or less.
  • the Ca content is 0.0005% or more and 0.0090% or less
  • the Mg content is 0.0005% or more and 0.0090% or less
  • the REM content is 0.0010% or more and 0.0180% or less.
  • the Ca content is 0.0010% or more and 0.0080% or less
  • the Mg content is 0.0010% or more and 0.0080% or less
  • the REM content is 0.0020% or more and 0.0150% or less.
  • the Ca content is 0.0015% or more and 0.0050% or less
  • the Mg content is 0.0015% or more and 0.0050% or less
  • the REM content is 0.0030% or more and 0.0100% or less.
  • the balance other than the above-described components is iron (Fe) and incidental impurities.
  • the incidental impurities include H and B, and are acceptable if the total amount of impurity elements is 0.01% or less.
  • the above elements are contained as a basic chemical composition.
  • the target properties of the present invention can be obtained by this basic chemical composition.
  • the following elements can be contained as necessary.
  • Each component of Cu, Ni, Mo, V, and W described below can be contained as necessary, and thus these components may be 0%.
  • Cu and Ni are elements that increase the strength of the steel plate by solid solution strengthening, and also improve the mobility of dislocations and improve the low-temperature toughness. To provide such effects, Cu and Ni are preferably contained in an amount of 0.01% or more. When Cu and Ni are contained in an amount of more than 1.0%, the surface quality deteriorates during rolling, and in addition, the production cost is increased. For these reasons, when these alloy elements are contained, the amount of each element is preferably 1.0% or less. Each of the Cu content and the Ni content is more preferably 0.03% or more and more preferably 0.7% or less, still more preferably 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 to an improvement in the strength of the base material.
  • each of Mo, V, and W is preferably contained in an amount of 0.001% or more.
  • coarse carbonitrides may be formed to act as starting points of fracture, and in addition, the production cost is increased.
  • each element is preferably contained in an amount of 2.0% or less.
  • Each of the Mo content, the V content, and the W content is more preferably 0.003% or more, and more preferably 1.7% or less. The content is still more preferably 0.1% or more, and still more preferably 1.5% or less.
  • the grain size of the steel material (base metal) is large, C is deficient at the time of carbide formation.
  • the maximum grain size at a position 1 mm below a surface of the steel material is less than 200 pm, the C concentration at the grain boundaries can be 0.100% or more even after the steel material has been subjected to line heating. That is, it is possible to produce a steel material having excellent low-temperature toughness in which absorbed energy in a Charpy impact test at -269°C or higher is 41 J or more in a line-heated portion of a structure (for example, a tank) obtained after line heating.
  • the maximum grain size is preferably 150 pm or less, more preferably 100 pm or less, still more preferably 80 pm or less.
  • the lower limit of the maximum grain size is not particularly specified.
  • the maximum grain size is preferably 50 pm or more, more preferably 60 pm or more.
  • the above-mentioned grains refer to grains exposed by etching. In the present invention, the above-mentioned maximum grain size can be measured by a method described in Examples described below.
  • the maximum grain size of the steel material can be controlled within the above numerical range by performing hot rolling under the conditions described below.
  • the C concentration at the grain boundaries can be sufficiently ensured even after the line heating, and the above-described absorbed energy can be achieved.
  • the origins of fracture of a high-Mn steel are grain boundaries. Cracks propagate through grain boundaries. Thus, the presence of coarse crystal grains can inhibit the propagation of a crack to further improve the low-temperature toughness.
  • the number of austenite grains, per 1 mm 2 having a grain size of 50 pm or more, is preferably 1.0 or more, more preferably 2.0 or more. When the number of the austenite grains is more than 10.0 per 1 mm 2 , the strength deteriorates. Thus, the number per 1 mm 2 is preferably 10.0 or less, more preferably 9.0 or less.
  • the number of austenite grains, per 1 mm 2 , having a grain size of 50 pm or more can be measured by a method described in Examples below.
  • the number density can be controlled within the above-mentioned numerical range by performing hot rolling described below.
  • the stress corrosion cracking resistance deteriorates. It has been found that when the inclusion particle size in the top 10% of an inclusion particle size distribution (top 10% inclusion particle size) at the position 1 mm below the surface of the steel material is more than 3.5 ⁇ m, the stress corrosion cracking resistance deteriorates.
  • the top 10% inclusion particle size is preferably 3.5 pm or less, more preferably 3.0 pm or less. A smaller top 10% inclusion grain size is more preferable. From the viewpoint of productivity, the top 10% inclusion particle size is preferably 1.5 pm or more, more preferably 2.0 pm or more.
  • top 10% inclusion particle size is a particle size corresponding to a 10% position when the inclusion particle sizes are arranged in descending order in the inclusion particle size distribution.
  • the above-mentioned inclusion particle size can be measured by a method described in Examples below.
  • the term "steel material (austenitic steel material)” refers to a steel plate having a thickness of 6 mm or more. From the viewpoint of suitable use as a material for structural steel used in extremely low-temperature environments, 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, can be any thickness, and is preferably 40 mm or less.
  • a molten steel having the chemical composition described above can be produced by a steelmaking method using, for example, a converter or an electric arc furnace.
  • secondary refining may be performed in a vacuum degassing furnace.
  • a raw steel material such as a slab having a predetermined size
  • a casting method such as a continuous casting method or an ingot-making and blooming method.
  • an austenitic steel material having the above-described composition it is important to heat the raw steel material having the above-described chemical composition to a temperature range of 1,100°C or higher and 1,300°C or lower and then perform hot rolling in which rolling is performed at a total rolling reduction of 40% or more at 950°C or higher, then one or more hot rolling passes is performed at lower than 950°C, and finish rolling is performed at a finishing temperature of 750°C or higher. Then, after the hot rolling is finished, cooling is performed.
  • the temperature control here is based on the surface temperature of the raw steel material.
  • the symbol "°C” regarding the temperature indicates the surface temperature of the raw steel material or the steel plate, unless otherwise specified.
  • the surface temperature can be measured with, for example, a radiation thermometer.
  • the temperature at the center of the thickness of the slab or steel plate can be determined, for example, by attaching a thermocouple to the center of the thickness of the steel plate and measuring the temperature, or by calculating the temperature distribution in the cross-section of the steel plate using heat transfer analysis and correcting the result using the surface temperature of the steel plate.
  • Heating Temperature of Raw Steel Material 1,100°C or more and 1,300°C or lower
  • the heating temperature of the raw steel material before hot rolling is set to 1,100°C or higher.
  • the stability of austenite can be secured even in the Mn negative segregation zone. This can ensure austenite stability even in a line-heated portion and prevent brittle fracture. That is, the absorbed energy at -269°C can be ensured.
  • the heating temperature is higher than 1,300°C, the steel may begin to melt; thus, the upper limit of the heating temperature is 1,300°C.
  • the heating temperature of the raw steel material is preferably 1,130°C or higher and preferably 1,270°C or lower.
  • the heating temperature is more preferably 1,150°C or higher and more preferably 1,250°C or lower.
  • the maximum grain size at the position 1 mm below the surface of the steel material be less than 200 ⁇ m. If the equiaxed grains cannot be obtained by the rolling in the recrystallization region, the grains remain as coarse grains even in the subsequent rolling in the non-recrystallization region, resulting in a maximum grain size of 200 pm or more.
  • the number density of grains having a grain size of 50 pm or more is more than 10.0 grains/mm 2 .
  • the total rolling reduction in the recrystallization region is preferably 50% or more, more preferably 52% or more.
  • the upper limit of the total rolling reduction in the recrystallization region is not particularly specified. For the reason of ensuring the strength, the total rolling reduction in the recrystallization region is preferably 85% or less, more preferably 70% or less.
  • the number of hot rolling passes at lower than 950°C be one or more.
  • the number of hot rolling passes is two or more.
  • the maximum grain size is 200 pm or more.
  • the number density of grains having a grain size of 50 pm or more is more than 10.0 grains/mm 2 .
  • the upper limit of the number of hot rolling passes is not particularly specified. From the viewpoint of productivity, the number of hot rolling passes is preferably 10 or less, more preferably 8 or less.
  • the finishing temperature is 750°C or higher.
  • the finishing temperature is 775°C or lower, the grain size is small, and as a result, the maximum grain size is sometimes less than 50 ⁇ m.
  • the finishing temperature is preferably higher than 775°C, more preferably 780°C or higher.
  • the upper limit of the finishing temperature is not particularly specified. From the viewpoint of ensuring the strength, the finishing temperature is preferably 930°C or lower, more preferably 900°C or lower.
  • cooling is performed.
  • the cooling conditions are not particularly specified.
  • the above "temperature at the end of hot rolling” refers to the finishing temperature.
  • the upper limit of the average cooling rate is not particularly specified. From the viewpoint of controlling the finish cooling temperature, the cooling rate is preferably 30.0 °C/s or less.
  • a steel structure for example, a tank
  • the steel material of the present invention as a raw material and subjecting the raw material to line heating will be described below.
  • the tank of the present invention is produced by subjecting the above-described steel material to line heating under specific line heating conditions to form a curved surface, and welding the curved steel materials.
  • the tank of the present invention produced as described above has the same chemical composition and microstructure in the base metal zone as the steel material (austenitic steel material) described above.
  • the C concentration at the grain boundaries at a position 1 mm below a surface of the base metal zone after the line heating is 0.100% or more.
  • the C concentration at the grain boundaries at the above-described position of the base metal zone after the line heating is 0.100% or more, preferably 0.200% or more, more preferably 0.250% or more.
  • the upper limit of the C concentration at the grain boundaries at the above-described position of the base metal zone after the line heating is not particularly specified. From the viewpoint of a deterioration in low-temperature toughness due to excessive formation of Cr carbide, the C concentration is preferably 0.600% or less, more preferably 0.550% or less.
  • the absorbed energy in a Charpy impact test at -269°C or higher at a position 1 mm below a surface of the line-heated portion after the line heating can be 41 J or more.
  • the absorbed energy in the Charpy impact test can be measured by a method described in Examples below. That is, the absorbed energy in the Charpy impact test at -269°C or higher in the line-heated portion is 41 J or more in the case of a full-size specimen, and is 27 J or more in the case of a 5-mm sub-size specimen.
  • stress corrosion cracking resistance can also be provided.
  • the tank of the present invention is produced by subjecting the above steel material to line heating under the following conditions to form a curved surface, and welding the curved steel materials.
  • the method for producing the steel material (austenitic steel material) as a raw material has already been described, and thus description thereof is omitted.
  • preferable line heating conditions and welding conditions will be described.
  • the steel material is subjected to line heating at a target heating temperature (heating target temperature) of a surface of the steel material of 900°C or lower. After heating, the steel material is subjected to natural cooling to a surface temperature of 500°C or lower and then water cooling.
  • the line heat treatment including heating and natural cooling may be performed once, or may be repeated one or more times.
  • the number of repetitions is preferably one or more in order to modify the microstructure.
  • the number of repetitions is preferably five or less because the local thermal cycle history is complicated.
  • the heating temperature is preferably higher than 800°C.
  • the resulting hot-rolled steel plate (steel plate) was used to evaluate the grain size and the inclusion particle size in the following procedures.
  • the resulting steel plate was subjected to line heating.
  • the steel plate after the line heating was used to evaluate the C concentration, the low-temperature toughness, and the stress corrosion cracking resistance in the following procedures.
  • a line-heating specimen having a length of 1,000 mm and a width of 500 mm was prepared from the resulting steel plate.
  • the specimen was fixed by restraint plates at a 1/2 position in the width direction (rolling direction).
  • the plate line heating was performed under the following conditions.
  • the target heating temperature of the surface of the steel material was 900°C, and the steel material was heated to this temperature, naturally cooled to a surface temperature of the steel material of 500°C or lower, and then water-cooled.
  • the line heating of the same region was repeated under the conditions illustrated in Table 2-2.
  • a cross section in the rolling direction was polished and etched.
  • a position 1 mm below a surface of the steel plate was photographed at a magnification of 200 times with an optical microscope. From the photographed image, 100 grains exposed by etching were randomly selected. The equivalent circular diameters of the grains were taken as the grain size. The maximum grain size (pm) at the position 1 mm below the surface of the steel plate was determined. The total area of 100 grains and the number of grains having a size of 50 pm or more were determined. The number density of grains having a grain size of 50 pm or more per 1 mm 2 (mm 2 /grain) was determined. Aqua regia was used as an etchant.
  • the resulting hot-rolled steel plate was examined for the inclusion particle size using a scanning electron microscope (SEM). The evaluation region of 200 mm 2 was used. The top 10% inclusion particle size (pm) at a position 1 mm below the surface of the steel plate was determined.
  • a TEM sample of 12 mm ⁇ 10 mm was prepared from the resulting hot-rolled steel plate after the steel plate was subjected to line heating.
  • the sample was subjected to composition analysis across carbide-free grain boundaries using an EDS detector attached to a transmission electron microscope (TEM), and the resulting C concentration was evaluated.
  • TEM transmission electron microscope
  • a position 1 mm below the surface of the steel plate was used as an observation target. The analysis was performed on 10 grain boundaries, and the average value was determined.
  • the low-temperature toughness of the line-heated portion was evaluated as described below.
  • a line-heating specimen illustrated in Fig. 1 was produced from the resulting hot-rolled steel plate.
  • the low-temperature toughness of the line-heated portion was evaluated using a steel plate obtained by subjecting the test specimen to plate line heating under the above-described conditions.
  • Charpy V-notch test specimens (full-size Charpy V-notch test specimens) were taken from the line-heated portion having a plate thickness of 10 mm or more in accordance with JIS Z 2242 (2005).
  • a Charpy impact test was conducted at -196°C and -269°C using the three Charpy V-notch test specimens. An average absorbed energy value of three test specimens at each temperature was determined. In this example, in the case of the full-size Charpy V-notch test specimens, when the average absorbed energy value of the three test specimens at -269°C was 41 J or more, the steel plate was determined to have excellent low-temperature toughness.
  • sub-size Charpy V-notch test specimens of 5 mm were taken in accordance with JIS Z 2242 (2005).
  • a Charpy impact test was conducted at -196°C and - 269°C using the three Charpy V-notch test specimens.
  • the average absorbed energy value of the three test specimens at each temperature was determined.
  • "*1" is attached to each value of absorbed energy for the samples tested using the sub-sized Charpy V-notch test specimens.
  • the steel plate was determined to have excellent low-temperature toughness.
  • the stress corrosion cracking resistance was evaluated by a stress corrosion cracking test in accordance with ASTM G36.
  • a test specimen having a thickness of 2.5 mm, a width of 20 mm, and a length of 80 mm was taken from a position 1 mm below the surface of the resulting hot-rolled steel plate.
  • a boiling MgCl 2 solution was used.
  • the bend radius was 5 mm.
  • the test specimen to which stress was applied was immersed in the above-described solution for 400 hours. Thereafter, whether cracking occurred was checked. When no cracking occurred, the sample was evaluated as "o (pass)" in Table 2-2. When cracking occurred, the sample was evaluated as " ⁇ (fail)" in Table 2-2.

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EP22749560.3A 2021-02-08 2022-01-25 Matériau d'acier et son procédé de production, ainsi que réservoir et son procédé de production Pending EP4249621A4 (fr)

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