Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C22C38/18—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
  • 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
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—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
  • C22C38/04—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
  • C22C38/40—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
  • 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

• This disclosure relates to a high-Mn steel suitable for a structure used in cryogenic environments, such as a tank for liquefied gas storage, and a method for manufacturing the same.
• a structure for liquefied gas storage is used at cryogenic temperatures. Therefore, a steel sheet used for this type of structure is required to have not only high strength but also excellent toughness at cryogenic temperatures.
• a hot rolled steel sheet is used for liquefied natural gas storage, it is necessary to ensure excellent toughness at a temperature of -164 °C, which is the boiling point of the liquefied natural gas, or lower. If the low-temperature toughness of the steel material is inferior, the safety as a structure for cryogenic storage may not be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material.
• austenitic stainless steel, 9 % Ni steel, and 5000 series aluminum alloy, where austenite, which does not exhibit brittleness at cryogenic temperatures, is the main structure of the steel sheet have conventionally been used.
• austenite which does not exhibit brittleness at cryogenic temperatures
• JP 2016-84529 A (PTL 1) and JP 2016-196703 A (PTL 2) propose using a high-Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite-stabilizing element, as a structural steel in cryogenic environments, as a new steel material to replace conventional cryogenic steels.
• PTL 1 proposes controlling the carbide coverage of austenite crystal grain boundaries
• PTL 2 proposes controlling the austenite crystal grain size by a carbide coating as well as addition of Mg, Ca, and REM.
• the "high strength” means that the yield strength is 400 MPa or more
• the "excellent low-temperature toughness” means that the absorbed energy vE-196 of a Charpy impact test at -196 °C is 100 J or more
• the "excellent CTOD property at low temperatures” means that the CTOD value at -165 °C is 0.25 mm or more.
• the present disclosure it is possible to provide a high-Mn steel having excellent CTOD property and low-temperature toughness especially at cryogenic temperatures. Therefore, by using the high-Mn steel of the present disclosure, it is possible to realize an improvement in safety and product life of a steel structure used in cryogenic environments, such as a tank for liquefied gas storage, which exhibits remarkable industrial effects.
• the C is an inexpensive austenite-stabilizing element and is an important element in obtaining austenite. To obtain this effect, the C content needs to be 0.10 % or more. On the other hand, when the C content exceeds 0.70 %, Cr carbides are excessively formed, and the low-temperature toughness is deteriorated. Therefore, the C content is 0.10 % or more and 0.70 % or less. The C content is preferably 0.20 % or more. The C content is preferably 0.60 % or less.
• Si 0.05 % or more and 0.50 % or less
• Si is an element that acts as a deoxidizing material. It not only is necessary for steelmaking but also dissolves in steel to increase the strength of a steel sheet by solid solution strengthening. To obtain these effects, the Si content needs to be 0.05 % or more. On the other hand, when the Si content exceeds 0.50 %, the weldability is deteriorated and the low-temperature toughness, especially the toughness at cryogenic temperatures is lowered. Therefore, the Si content is 0.05 % or more and 0.50 % or less. The Si content is preferably 0.07 % or more and 0.50 % or less.
• Mn 20 % or more and 30 % or less
• Mn is a relatively inexpensive austenite-stabilizing element. Mn is an important element for the present disclosure to achieve both the strength and the toughness at cryogenic temperatures. To obtain this effect, the Mn content needs to be 20 % or more. On the other hand, when the content exceeds 30 %, the effect of improving low-temperature toughness saturates, leading to an increase in alloy cost. In addition, the weldability and the cuttability deteriorate. Further, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the Mn content is 20 % or more and 30 % or less. The Mn content is preferably 23 % or more. The Mn content is preferably 28 % or less.
• the upper limit is set to 0.030 %, and the P content is desirably as low as possible.
• the P content is 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 preferably 0.005 % or more.
• the P content is preferably 0.028 % or less.
• the P content is more preferably 0.024 % or less.
• the upper limit is set to 0.0070 %, and the S content is desirably as low as possible.
• the S content is 0.0070 % or less.
• the S content is desirably 0.001 % or more, because excessive reduction of S content increases refining cost and is economically disadvantageous.
• the S content is preferably 0.0020 % or more.
• the S content is preferably 0.0060 % or less.
• Al 0.01 % or more and 0.07 % or less
• Al acts as a deoxidizer and is most commonly used in a molten steel deoxidation process of a steel sheet. To obtain this effect, the Al content needs to be 0.01 % or more. On the other hand, when the Al content exceeds 0.07 %, Al is mixed into a weld metal part during welding and deteriorates the toughness of the weld metal. Therefore, the Al content is 0.07 % or less. Thus, the Al content is 0.01 % or more and 0.07 % or less. The Al content is preferably 0.02 % or more. The Al content is preferably 0.06 % or less.
• the Cr content is an element that stabilizes austenite when added in an appropriate amount and is an element effective in improving low-temperature toughness and base metal strength. To obtain these effects, the Cr content needs to be 0.5 % or more. On the other hand, when the content exceeds 7.0 %, the low-temperature toughness and the stress corrosion cracking resistance are deteriorated due to formation of Cr carbides. Therefore, the Cr content is 0.5 % or more and 7.0 % or less.
• the Cr content is preferably 1.0 % or more.
• the Cr content is preferably 6.7 % or less.
• the Cr content is more preferably 1.2 % or more.
• the Cr content is more preferably 6.5 % or less. In order to further improve the stress corrosion cracking resistance, the content is still more preferably 2.0 % or more and 6.0 % or less.
• Ni 0.01 % or more and less than 0.1 %
• Ni has the effect of improving low-temperature toughness.
• minimizing the necessary alloy cost is an important viewpoint in designing the composition of the present disclosure, and from this viewpoint, the Ni content is 0.01 % or more and less than 0.1 %.
• austenitic steels having excellent low-temperature toughness include stainless steels such as SUS304 and SUS316.
• a large amount of Ni is added in these steels to optimize the Ni equivalent and the Cr equivalent as an alloy design for obtaining an austenitic structure.
• the present disclosure is an austenitic material whose price is lowered by minimizing necessary Ni. Note that the minimization of necessary Ni is realized by optimizing the addition amount of Mn.
• the Ni content is preferably 0.03 % or more.
• the Ni content is preferably 0.07 % or less.
• Ca improves ductility, toughness and sulfide stress corrosion cracking resistance by controlling the morphology of inclusions described below.
• Ca suppresses the deterioration of hot ductility and is effective in reducing the occurrence of cracks in cast steel.
• the Ca content needs to be 0.0005 % or more.
• the Ca content exceeds 0.0050 %, the ductility, toughness, and sulfide stress corrosion cracking resistance may be rather deteriorated, and the effect of suppressing the deterioration of hot ductility may saturate. Therefore, the Ca content is 0.0005 % or more and 0.0050 % or less.
• the Ca content is preferably 0.0010 % or more.
• the Ca content is preferably 0.0045 % or less.
• N 0.0050 % or more and 0.0500 % or less
• N is an austenite-stabilizing element and is an element effective in improving low-temperature toughness. To obtain these effects, the N content needs to 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 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 O content is in the range of 0.0050 % or less.
• the O content is preferably 0.0045 % or less.
• the O content is desirably 0.0003 % or more, because excessive reduction of O content increases refining cost and is economically disadvantageous.
• Ti and Nb form high-melting carbonitrides in steel and suppress the coarsening of crystal grains, and as a result, they become a starting point of fractures and propagation path of cracks. In particular, they hinder the microstructure control for enhancing the low-temperature toughness and improving the ductility in the high-Mn steel. Therefore, the contents of Ti and Nb must be suppressed intentionally. That is, Ti and Nb are components inevitably mixed from raw materials and the like, and they are generally mixed in the ranges of Ti: 0.005 % to 0.010 % and Nb: 0.005 % to 0.010 %.
• Ti and Nb are preferably 0.003 % or less.
• the balance other than the above essential components is iron and inevitable impurities.
• the inevitable impurities include H, and a total of 0.01 % or less is acceptable.
• Cu, Mo, V and W contribute to the stabilization of austenite and to the improvement of base metal strength.
• the contents of Cu, Mo, V and W are preferably 0.001 % or more.
• the contents of Mo, V and W are preferably 0.003 % or more.
• the contents of Mo, V and W are preferably 1.7 % or less.
• the contents of Mo, V and W are more preferably 1.5 % or less.
• Mg and REM are useful elements for controlling the morphology of inclusions and can be contained as necessary. Controlling the morphology of inclusions means making expanded sulfide-based inclusions into granular inclusions. By controlling the morphology of inclusions, the ductility, toughness and sulfide stress corrosion cracking resistance are improved. To obtain these effects, the Ca and Mg contents are preferably 0.0005 % or more, and the REM content is preferably 0.0010 % or more. On the other hand, when any of these elements is contained in a large amount, the amount of nonmetallic inclusions increases, and the ductility, toughness, and sulfide stress corrosion cracking resistance may rather be deteriorated. In addition, it may be economically disadvantageous.
• the content when Mg is contained, the content is 0.0005 % to 0.0050 %, and when REM is contained, the content is 0.0010 % to 0.0200 %.
• the Mg content is preferably 0.0010 % or more.
• the Mg content is preferably 0.0040 % or less.
• the REM content is preferably 0.0020 % or more.
• the REM content is preferably 0.0150 % or less.
• the steel material is not suitable for use in low-temperature environments because it may cause brittle fractures in low-temperature environments.
• the base phase of the steel material should be an austenitic structure 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. The remainder other than the austenite phase is a ferrite phase or a martensite phase. Of course, the austenite phase may be 100 %.
• Austenite grain size 1 ⁇ m or more
• the high-Mn steel has a microstructure having austenite as a base phase, brittle fractures do not occur even at cryogenic temperatures, and, if a fracture occurs, it is generated from crystal grain boundaries. It is advantageous to reduce the area of crystal grain boundaries, which are the starting point of fractures, to improve the fracture resistance of the high-Mn steel. Therefore, it is important that the austenite grain size be 1 ⁇ m or more. This is because, when the grain size is less than 1 ⁇ m, the increasing amount of grain boundary area increases, which increases the number of locations where fractures occur. It is preferably 2 ⁇ m or more.
• Standard deviation of austenite 9 ⁇ m or less
• Realizing homogenization in conjunction with the regulation of crystal grain size is effective in further improving the fracture resistance of the high-Mn steel. That is, in a mixed-grain-size microstructure, a wide grain size distribution from coarse crystal grains to fine crystal grains results in containing of crystal grains of less than 1 ⁇ m, and especially when the standard deviation exceeds 9 ⁇ m, the tendency is remarkable. Therefore, it is necessary to avoid a mixed-grain-size microstructure having a standard deviation of more than 9 ⁇ m.
• the steel material may be a molten steel having the above-described chemical composition obtained with a 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.
• a steel material such as a slab having a predetermined size with a known casting method such as a continuous casting method or an ingot casting method. It is also acceptable to subject the slab after continuous casting to blooming to obtain a steel material.
• the following specifies the manufacturing conditions for making the above steel material into a steel material having excellent low-temperature toughness.
• the heating temperature before hot rolling is 1100 °C or higher to increase the crystal grain size of the microstructure of the steel material.
• the upper limit of the heating temperature is set to 1300 °C.
• the temperature control here is based on the surface temperature of the steel material.
• Rolling finish temperature 750 °C or higher and lower than 950 °C
• the steel material (steel ingot or slab) is subjected to hot rolling after the heating.
• the finish temperature is in the range of 950 °C or higher, the crystal grain size becomes excessively coarse, and a desired yield strength cannot be obtained. Therefore, it is necessary to perform the final finish rolling of one or more passes at a temperature of lower than 950 °C. It is preferably 900 °C or lower.
• Average rolling reduction for one pass 9 % or more
• the hot rolling in order to realize the homogenization of austenite grain size and control the crystal grain size to 1 ⁇ m or more, it is effective to promote the recrystallization of austenite, and it is important to have an average rolling reduction for one pass of 9 % or more during the hot rolling. It is preferably 11 % or more.
• Cooling is immediately performed after the hot rolling. If the steel sheet after hot rolling is cooled slowly, formation of precipitates is promoted and the low-temperature toughness is deteriorated. The formation of these precipitates can be suppressed by cooling the steel sheet at a cooling rate of 1.0 °C/s or more. Excessive cooling distorts the steel sheet and lowers the productivity. Therefore, the upper limit of the cooling start temperature is set to 900 °C.
• the average cooling rate at the steel sheet surface from a temperature of (rolling finish temperature - 100 °C) or higher to a temperature range of 300 °C or higher and 650 °C or lower is 1.0 °C/s or more. On the other hand, from the viewpoint of industrial production, the average cooling rate is preferably 200 °C/s or less.
• Steel slabs having the chemical composition 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 subjected to blooming and hot rolling under the conditions listed in Table 2 to obtain steel sheets having a thickness of 10 mm to 30 mm. The steel sheets thus obtained were subjected to tensile property, toughness and microstructure evaluation as described below.
• a JIS No. 5 tensile test piece was collected from each steel sheet thus obtained, and the tensile test piece was subjected to a tensile test according to the provisions of JIS Z2241 (1998) to investigate the tensile test property.
• a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more were determined to be excellent in tensile properties. Further, elongation of 40 % or more was determined to be excellent in ductility.
• a Charpy V-notch test piece was collected from a direction parallel to the rolling direction at a position at a depth of one-fourth of the sheet thickness from the surface of each steel sheet having a thickness of more than 20 mm (hereinafter referred to as "position of sheet thickness ⁇ 1/4"), or a position at a depth of half of the sheet thickness of each steel sheet having a thickness of 20 mm or less (hereinafter referred to as "position of sheet thickness ⁇ 1/4) according to the provisions of JIS Z2202 (1998).
• Three Charpy impact tests were performed on each steel sheet according to the provisions of JIS Z2242 (1998) to determine the absorbed energy at -196 °C, thereby evaluating the base metal toughness. In the present disclosure, when the average of three absorbed energy (vE-196) values was 100 J or more, it was determined to be excellent in base metal toughness.
• a CTOD test piece was collected from a direction parallel to the rolling direction at the position of sheet thickness ⁇ 1/2 of the steel sheet, and two or three tests were conducted at -165 °C to evaluate the average value.
• a CTOD value of 0.25 mm or more was determined to be excellent in fracture resistance.
• Electron backscatter diffraction (EBSD) analysis was performed on a L cross section at the position of sheet thickness ⁇ 1/4 of the steel sheet. Two or three visual fields of 200 ⁇ m ⁇ 200 ⁇ m were selected arbitrarily and observed, and the minimum value of austenite crystal grain size in each visual field was measured. In addition, the standard deviation of the austenite grain size was evaluated from the distribution of the area ratio of each crystal grain size using the results of the EBSP analysis. All the crystal grain sizes thus obtained were taken as a population, a variance that is a sum of squares of the difference between each individual value and the average value was obtained, and a square root of the variance was obtained to determine the standard deviation.
• EBSD Electron backscatter diffraction
• the high-Mn steel of the present disclosure satisfies the above-mentioned desired performance (the yield strength of base metal is 400 MPa or more, the average value of absorbed energy (vE-196) is 100 J or more with respect to the low-temperature toughness, and the average value of CTOD value is 0.25 mm or more).
• Comparative Examples which are outside the scope of the present disclosure, do not satisfy at least one of the above-mentioned desired performance of yield strength, low-temperature toughness, and CTOD value.
• Table 2 Sample No. Steel No.

Applications Claiming Priority (2)

Publications (3)

Family

ID=66750519

Family Applications (1)

Country Status (10)

Families Citing this family (4)

Family Cites Families (14)

• 2018
  • 2018-12-06 BR BR112020011210-0A patent/BR112020011210B1/pt active IP Right Grant
  • 2018-12-06 EP EP18886695.8A patent/EP3722448B1/en active Active
  • 2018-12-06 KR KR1020207018443A patent/KR102405388B1/ko active IP Right Grant
  • 2018-12-06 JP JP2019518131A patent/JP6590117B1/ja active Active
  • 2018-12-06 US US16/768,884 patent/US20210164067A1/en not_active Abandoned
  • 2018-12-06 MY MYPI2020002748A patent/MY192536A/en unknown
  • 2018-12-06 CN CN201880078458.5A patent/CN111433381B/zh active Active
  • 2018-12-06 SG SG11202005101PA patent/SG11202005101PA/en unknown
  • 2018-12-06 WO PCT/JP2018/044941 patent/WO2019112012A1/ja unknown
• 2020
  • 2020-06-05 PH PH12020550830A patent/PH12020550830A1/en unknown

Also Published As

Similar Documents

Legal Events