EP4019656A1 - Steel and method for manufacturing same - Google Patents

Steel and method for manufacturing same Download PDF

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EP4019656A1
EP4019656A1 EP20853747.2A EP20853747A EP4019656A1 EP 4019656 A1 EP4019656 A1 EP 4019656A1 EP 20853747 A EP20853747 A EP 20853747A EP 4019656 A1 EP4019656 A1 EP 4019656A1
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content
steel
toughness
reheating
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German (de)
English (en)
French (fr)
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EP4019656A4 (en
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Daichi Izumi
Koichi Nakashima
Keiji Ueda
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JFE Steel Corp
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JFE Steel Corp
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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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • 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
    • 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
    • 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
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C38/02—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
    • C22C38/06—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
    • C22C38/48—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
    • 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

Definitions

  • the present disclosure relates to steel suitable for structural steel used in environments at cryogenic temperatures, such as tanks for storing liquid hydrogen, liquid helium, liquefied gas, and the like, in particular having excellent toughness at cryogenic temperatures, and a method of manufacturing the same.
  • the hot-rolled steel sheet or plate In order to use a hot-rolled steel sheet or plate for structures for liquid hydrogen, liquid helium and liquefied gas storage, the hot-rolled steel sheet or plate requires to have excellent toughness at cryogenic temperatures because the structures are used at cryogenic temperatures. For example, when the hot-rolled steel sheet or plate is used for liquid helium storage, it is necessary to ensure excellent toughness at a temperature of -269 °C or lower, which is the boiling point of helium. If the toughness at cryogenic temperatures of the steel material is inferior, the safety as the structure for cryogenic storage may not be maintained. Therefore, there is a strong demand for improving the toughness at cryogenic temperatures of the steel material for this purpose.
  • austenitic stainless steel where austenite, which does not exhibit brittleness at cryogenic temperatures, is the microstructure of the steel sheet or plate, have conventionally been used.
  • austenite which does not exhibit brittleness at cryogenic temperatures
  • JP 2018-104792 A proposes using a high-Ni steel containing a large amount of Ni, which is an austenite-stabilizing element, as a structural steel for environments at -253 °C, as a new steel material which replaces conventional steel for low temperature.
  • PTL 1 proposes a technique to ensure toughness at cryogenic temperatures by controlling the grain size and morphology of prior austenite.
  • the technique described in PTL 1 can provide high-Ni steel with excellent toughness at cryogenic temperatures, but the high-Ni steel must contain 12.5 % or more Ni from the viewpoint of ensuring toughness at cryogenic temperatures, and thus a reduction in material cost has been required.
  • the "excellent toughness at cryogenic temperatures” means that the absorbed energy of a Charpy impact test at -196 °C, or even -269 °C, is 150 J or more.
  • the "excellent tensile properties” refer to a total elongation of 30 % or more in a tensile test at -269 °C.
  • the main form of brittle fracture in the austenite steel is intergranular fracture originating from crystal grain boundaries. Therefore, coarsening the crystal grain size is effective to improve the toughness at cryogenic temperatures of the steel.
  • the toughness at cryogenic temperatures and tensile properties can be improved with a minimum number of heat treatments, which can reduce the manufacturing cost.
  • the above-mentioned temperatures refer to a surface temperature of the steel material or steel sheet or plate.
  • the steel of the present disclosure makes a significant contribution to improving the safety and product life of the steel structure used in cryogenic environments, such as a tank for liquefied hydrogen, liquid helium, and liquefied gas storage, which exhibits remarkable industrial effects.
  • the manufacturing method of the present disclosure does not cause a decrease in productivity and an increase in manufacturing cost, thus providing a method with excellent economic efficiency.
  • FIG. 1 is a graph illustrating the relationship between the average grain size and the absorbed energy at -269 °C for a steel satisfying the chemical composition of the present disclosure.
  • the C is an inexpensive austenite-stabilizing element and is an important element for obtaining austenite. In order to achieve the effect, the C content needs to be 0.100 % or more. On the other hand, when the C content exceeds 0.700 %, Cr carbides are excessively formed and the toughness at cryogenic temperatures is deteriorated. Therefore, the C content is set to 0.100 % or more and 0.700 % or less.
  • the C content is preferably 0.200 % or more.
  • the C content is preferably 0.600 % or less.
  • the C content is more preferably 0.200 % or more and 0.600 % or less.
  • Si acts as a deoxidizer and is a necessary element in steelmaking, so it is preferable to add 0.05 % or more.
  • Si content exceeds 1.00 %, the non-thermal stress (internal stress) increases excessively, resulting in deterioration of toughness at cryogenic temperatures. For this reason, Si is set to 1.00 % or less.
  • the Si content is preferably 0.80 % or less.
  • Mn 20.0 % or more and 40.0 % or less
  • Mn is a relatively inexpensive austenite-stabilizing element and is important in the present disclosure to ensure low-temperature toughness. In order to achieve this effect, the Mn content needs to be 20.0 % or more. On the other hand, when the Mn content is more than 40.0 %, the toughness at cryogenic temperatures is deteriorated. Therefore, the Mn content is set to 20.0 % or more and 40.0 % or less.
  • the Mn content is preferably 23.0 % or more.
  • the Mn content is preferably 38.0 % or less.
  • the Mn content is more preferably 23.0 % or more and 38.0 % or less.
  • the Mn content is further preferably 36.0 % or less.
  • the P content When the P content exceeds 0.030 %, it excessively segregates at grain boundaries, resulting in a decrease in toughness at cryogenic temperatures. Therefore, the P content is desirably as low as possible with the upper limit being 0.030 %. Therefore, the P content is set to 0.030 % or less.
  • the P content is desirably 0.002 % or more because excessive reduction of P content increases refining cost and is economically disadvantageous.
  • the P content is more preferably 0.005 % or more.
  • the P content is more preferably 0.028 % or less.
  • the P content is further preferably 0.005 % or more and 0.028 % or less.
  • the P content is still more preferably 0.024 % or less.
  • the S content is desirably as low as possible with the upper limit being 0.0070 %. Therefore, the S content is set to 0.0070 % or less.
  • the S content is desirably 0.0010 % or more because excessive reduction of S content increases refining cost and is economically disadvantageous.
  • the S content is preferably 0.0050 % or less.
  • Al 0.01 % or more and 5.00 % or less
  • Al acts as a deoxidizer and is most commonly used in a molten steel deoxidation process of a steel sheet or plate. In order to achieve this effect, the Al content needs to be 0.01 % or more. On the other hand, an Al content exceeding 5.00 % produces a large amount of inclusions, which results in deterioration in toughness at cryogenic temperatures. Therefore, the Al content is set to 5.00 % or less. For this reason, the Al content is set to 0.01 % or more and 5.00 % or less.
  • the Al content is preferably 0.02 % or more.
  • the Al content is preferably 4.00 % or less.
  • the Al content is more preferably 0.02 % or more and 4.00 % or less.
  • the Cr content is an effective element to improve the toughness at cryogenic temperatures because it improves the grain boundary strength. In order to achieve this effect, the Cr content needs to be 0.5 % or more. On the other hand, when the Cr content exceeds 7.0 %, the toughness at cryogenic temperatures is deteriorated due to formation of Cr carbides. Therefore, the Cr content is set to 0.5 % or more and 7.0 % or less.
  • the Cr content is preferably 1.0 % or more and more preferably 1.2 % or more.
  • the Cr content is preferably 6.7 % or less and more preferably 6.5 % or less.
  • the Cr content is further preferably 1.0 % or more and 6.7 % or less.
  • the Cr content is still more preferably 1.2 % or more and 6.5 % or less.
  • N 0.0050 % or more and 0.0500 % or less
  • N is an austenite-stabilizing element and is an effective element for improving toughness at cryogenic temperatures. In order to achieve this effect, the N content needs be 0.0050 % or more. On the other hand, when the content exceeds 0.0500 %, nitrides or carbonitrides are coarsened and the toughness is deteriorated. Therefore, the N content is set to 0.0050 % or more and 0.0500 % or less.
  • the N content is preferably 0.0060 % or more.
  • the N content is preferably 0.0400 % or less.
  • the N content is more preferably 0.0060 % or more and 0.0400 % or less.
  • the O content is set to 0.0050 % or less.
  • the O content is preferably 0.0045 % or less.
  • the O content is desirably 0.0010 % or more because excessive reduction of O content increases refining cost and is economically disadvantageous.
  • Ti and Nb contents reduce toughness at cryogenic temperatures because Ti and Nb form carbonitride with a high melting point in the steel.
  • Ti and Nb are elements that are inevitably mixed in from raw materials, etc.
  • Ti of more than 0.005 % and 0.010 % or less and Nb of more than 0.005 % and 0.010 % or less are mixed in. Therefore, it is necessary to intentionally limit the mixed content of Ti and Nb to suppress the content of each of Ti and Nb to 0.005 % or less according to the method described below.
  • By suppressing the content of each of Ti and Nb to 0.005 % or less it is possible to eliminate the above-mentioned adverse effects of carbonitrides and to ensure excellent toughness at cryogenic temperatures.
  • the content of each of Ti and Nb is preferably set to 0.003 % or less.
  • the content of each of Ti and Nb may be 0 %, but is desirably 0.001 % or more because excessive reduction is not preferable from the viewpoint of steelmaking cost.
  • the following elements can be contained as necessary in addition to the above essential elements in the present disclosure: at least one selecting from the group consisting of Cu: 1.0 % or less, Ni: 1.0 % or less, Mo: 2.0 % or less, V: 2.0 % or less, W: 2.0 % or less, Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0010 % or more and 0.0002 % or less.
  • Cu and Ni are elements that improve low-temperature toughness.
  • the content of each of Cu and Ni is preferably 0.01 % or more, and more preferably 0.03 % or more.
  • the content of each element is preferably 1.0 % or less, more preferably 0.7 % or less, and further preferably 0.5 % or less.
  • the content of each of Mo, V and W contribute to stabilization of austenite.
  • the content of each of Mo, V and W is preferably 0.001 % or more and more preferably 0.003 % or more.
  • the content of each of Mo, V, and W exceeds 2.0 %, coarse carbonitrides are formed, which may be an initiation point of fractures and increases manufacturing cost.
  • the content of each element is preferably 2.0 % or less, and more preferably 1.7 % or less.
  • the content of each of Mo, V and W is further preferably 0.003 % or more.
  • the content of each of Mo, V, and W is further preferably 1.7 % or less.
  • the content of each of Mo, V, and W is still more preferably 1.5 % or less.
  • Ca, Mg and REM are useful elements for controlling the morphology of inclusions and can be contained as required.
  • Controlling the morphology of inclusions means making expanded sulfide-based inclusions (mainly MnS) into granular inclusions.
  • MnS which is an initiation point of fracture, is reduced through the morphological control of inclusions to thereby improve toughness.
  • the content of each of Ca and Mg is preferably 0.0005 % or more and REM content is preferably 0.0010 % or more.
  • the amount of nonmetallic inclusions increases, and the toughness may rather be deteriorated. In addition, it may be economically disadvantageous.
  • the Ca and Mg contents are each preferably 0.0005 % or more.
  • the Ca and Mg contents are each preferably 0.0050 % or less.
  • the REM content is preferably 0.0010 % or more.
  • the REM content is preferably 0.0200 % or less.
  • the Ca content is more preferably 0.0010 % or more.
  • the Ca content is more preferably 0.0040 % or less.
  • the Ca content is further preferably 0.0010 % or more and 0.0040 % or less.
  • the Mg content is more preferably 0.0010 % or more.
  • the Mg content is more preferably 0.0040 % or less.
  • the Mg content is further preferably 0.0010 % or more and 0.0040 % or less.
  • the REM content is more preferably 0.0020 % or more.
  • the REM content is more preferably 0.0150 % or less.
  • the REM content is further preferably 0.0020 % or more and 0.0150 % or less.
  • REM refers to rare earth metals, and is a generic term for 17 elements, which are the 15 elements of lanthanides plus Y and Sc. At least one of these elements can be contained.
  • the REM content means the total content of these elements.
  • the balance other than the above components is a chemical composition having iron and inevitable impurities.
  • the inevitable impurities include H and B, and a total of 0.01 % or less is acceptable.
  • the base phase of the steel material is preferably an austenite microstructure where the crystal structure is a face-centered cubic structure (fcc).
  • austenite as a base phase means that the austenite phase has an area ratio of 90 % or more, preferably 95 % or more. The remainder other than the austenite phase is a ferrite phase or martensite phase.
  • Average grain size in microstructure being 80 ⁇ m or more
  • the absorption energy can be 150 J or more.
  • the crystal grain in this specification mainly refers to an austenite grain, and the average grain size can be determined by randomly selecting 100 crystal grains from an image taken at 200x magnifications using an optical microscopy, calculating an equivalent circle diameter for each of the crystal grains, and obtaining the average value of the equivalent circle diameters.
  • the average grain size can be achieved by performing hot rolling and heat treatment according to the conditions described below under the chemical composition described above.
  • the steel of the present disclosure can be obtained from a molten steel having the above-described chemical composition obtained by steelmaking using a publicly-known smelting method such as a converter or an electric furnace.
  • secondary refinement may be performed in a vacuum degassing furnace.
  • Ti and Nb which hinder the control of a preferable microstructure, to the above-described ranges
  • a method of blowing oxygen to oxidize the Ti and Nb and floating and separating the alloy of Ti and Nb in reflux may also be used. Subsequently, it is preferable to obtain a steel material such as a slab having a predetermined size with a known casting method such as a continuous casting method, ingot casting and blooming, or the like.
  • the following specifies the manufacturing conditions for making the above steel material into a steel material having excellent toughness at cryogenic temperatures.
  • the heating temperature of the steel material before hot rolling is set to 1100 °C or higher.
  • the heating temperature of the steel material is preferably 1130 °C or higher.
  • Thee heating temperature of the steel material is preferably 1270 °C or lower.
  • the heating temperature of the steel material is more preferably 1130 °C or higher and 1270 °C or lower.
  • hot rolling is performed.
  • the method of hot rolling is not particularly limited, but it is preferable to set the finish temperature of finish rolling to 700 °C or higher because when the finish temperature is lower, rolling efficiency decreases.
  • the finish temperature is more preferably 750 °C or higher.
  • a predetermined heat treatment is performed.
  • reheating is executed to the temperature range of 1100 °C to 1300 °C and the product of a reheating temperature (°C) and a reheating time (h: hour) is set to 100 °C/h or more.
  • a reheating temperature °C
  • h reheating time
  • the temperature range for reheating is set to 1100 °C or higher and 1300 °C or lower for the following reasons.
  • the heating temperature during reheating in the heat treatment is set to 1100 °C or higher.
  • the upper limit of the reheating temperature is set to 1300 °C.
  • the reason why the product of the reheating temperature (°C) and the reheating time (h) is specified is that there is a correlation between crystal grain growth and dislocation recovery.
  • the upper limit of the product of the reheating temperature and the reheating time is preferably 650 °C ⁇ h for manufacturing cost, and the lower limit thereof is preferably 208 °C ⁇ h in order to coarsen all crystal grains.
  • the reheating temperature during reheating in the heat treatment is preferably 1130 °C or higher.
  • the reheating temperature is preferably 1270 °C or lower.
  • the reheating temperature is more preferably 1130 °C or higher and 1270 °C or lower.
  • the reheating time is preferably 0.1 h or more in order to promote grain growth.
  • the reheating time is preferably 0.5 h or less in order to suppress a decrease in manufacturing efficiency.
  • the reheating time is more preferably 0.1 h or more and 0.5 h or less.
  • the cooling treatment may be performed after either or both of the hot rolling and subsequent heat treatment. This is to inhibit carbide precipitation.
  • the cooling temperature after the hot rolling is preferably 300 °C or higher.
  • the cooling temperature after the hot rolling is preferably 650 °C or lower.
  • the cooling temperature after the heat treatment is preferably 300 °C or higher.
  • the cooling temperature after the heat treatment is preferably 900 °C or lower.
  • the average cooling rate is preferably 1.0 °C/s or more.
  • Steel slabs (steel materials) having the chemical compositions listed in Table 1 were prepared by a process for refining with converter and ladle and continuous casting. Next, the steel slabs thus obtained were hot rolled under the conditions listed in Table 2 to obtain steel plates having a thickness of 6 mm to 30 mm.
  • the reheating temperature during reheating in the heat treatment was set to the same temperature as the heating temperature of the steel material for each sample.
  • the steel plates thus obtained were evaluated as follows for microstructure and mechanical properties of toughness at cryogenic temperatures and tensile properties.
  • finish temperature of finish rolling refers to the rolling finish temperature
  • the area ratio of each phase of the microstructure was obtained from the Phase map of electron backscatter diffraction (EBSD) analysis.
  • EBSD electron backscatter diffraction
  • the area ratio of the austenite phase was 90 % or more in all the examples and comparative examples, confirming that the base phase was austenite.
  • the cross section along the rolling direction was polished, and 100 crystal grains were randomly selected from an image taken at a position of plate thickness ⁇ 1/2 at 200x magnification using an optical microscopy, and the average grain size was determined from the equivalent circle diameters of the crystal grains.
  • the steel according to the present disclosure satisfies the above-mentioned target performances (the average value of absorbed energy in the Charpy impact test of 150 J or more and the total elongation in the tensile test of 30 % or more).
  • Comparative examples which are outside the scope of the present disclosure, do not satisfy at least one of the above-mentioned desired performances of absorbed energy and total elongation.

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  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
EP20853747.2A 2019-08-21 2020-08-18 Steel and method for manufacturing same Pending EP4019656A4 (en)

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JPS5896853A (ja) * 1981-11-17 1983-06-09 Sumitomo Metal Ind Ltd 耐食性および機械加工性に優れた極低温用高Ｍｎ鋼
JPS58197256A (ja) * 1982-05-12 1983-11-16 Kawasaki Steel Corp 耐候性および耐銹性にすぐれる高靭性高Ｍｎ鋼
KR100320959B1 (ko) 1996-12-30 2002-06-20 전선기 극저온충격특성이우수한고망간강및그제조방법
JP5003785B2 (ja) * 2010-03-30 2012-08-15 Ｊｆｅスチール株式会社 延性に優れた高張力鋼板およびその製造方法
JP6645103B2 (ja) * 2014-10-22 2020-02-12 日本製鉄株式会社 高Ｍｎ鋼材及びその製造方法
JP6693217B2 (ja) * 2015-04-02 2020-05-13 日本製鉄株式会社 極低温用高Ｍｎ鋼材
JP6589535B2 (ja) * 2015-10-06 2019-10-16 日本製鉄株式会社 低温用厚鋼板及びその製造方法
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WO2018104984A1 (ja) 2016-12-08 2018-06-14 Ｊｆｅスチール株式会社 高Ｍｎ鋼板およびその製造方法
JP6760055B2 (ja) 2016-12-28 2020-09-23 日本製鉄株式会社 液体水素用Ｎｉ鋼
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