WO2019112012A1 - High-mn steel and method for manufacturing same - Google Patents

High-mn steel and method for manufacturing same Download PDF

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Publication number
WO2019112012A1
WO2019112012A1 PCT/JP2018/044941 JP2018044941W WO2019112012A1 WO 2019112012 A1 WO2019112012 A1 WO 2019112012A1 JP 2018044941 W JP2018044941 W JP 2018044941W WO 2019112012 A1 WO2019112012 A1 WO 2019112012A1
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steel
austenite
temperature
toughness
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PCT/JP2018/044941
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French (fr)
Japanese (ja)
Inventor
孝一 中島
植田 圭治
茂樹 木津谷
亮 荒尾
大地 泉
聡 伊木
知宏 小野
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Jfeスチール株式会社
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Application filed by Jfeスチール株式会社 filed Critical Jfeスチール株式会社
Priority to BR112020011210-0A priority Critical patent/BR112020011210B1/en
Priority to JP2019518131A priority patent/JP6590117B1/en
Priority to MYPI2020002748A priority patent/MY192536A/en
Priority to SG11202005101PA priority patent/SG11202005101PA/en
Priority to EP18886695.8A priority patent/EP3722448B1/en
Priority to CN201880078458.5A priority patent/CN111433381B/en
Priority to KR1020207018443A priority patent/KR102405388B1/en
Priority to US16/768,884 priority patent/US20210164067A1/en
Publication of WO2019112012A1 publication Critical patent/WO2019112012A1/en
Priority to PH12020550830A priority patent/PH12020550830A1/en

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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/005Heat treatment of ferrous alloys containing Mn
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    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273Final recrystallisation annealing
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/005Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/001Austenite

Definitions

  • the present invention relates to a high Mn steel suitable for use in structures used in cryogenic environments, such as, for example, tanks for liquefied gas storage, and a method for producing the same.
  • the steel plate used for this type of structure is also required to have excellent toughness at very low temperature.
  • the steel plate used for this type of structure is also required to have excellent toughness at very low temperature.
  • austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy in which austenite which does not show brittleness at cryogenic temperature is used as the main structure of the steel plate has been used conventionally.
  • the alloy cost and the manufacturing cost are high, there is a demand for a steel material which is inexpensive and excellent in low temperature toughness.
  • Patent Literature Patent Document 1 and Patent Document 2 propose.
  • Patent Document 1 proposes to control the carbide coverage of austenite grain boundaries. Further, Patent Document 2 proposes that the austenite grain size be controlled by the addition of a carbide coating and Mg, Ca, and REM.
  • An object of the present invention is to provide a high Mn steel which is excellent not only in high strength and low temperature toughness but also in low temperature CTOD characteristics.
  • the "high strength” means that the yield strength is 400 MPa or more
  • the term “the excellent low temperature toughness” means that the absorbed energy vE-196 in the Charpy impact test at -196 ° C is 100 J or more.
  • excellent in low temperature CTOD characteristics means that the CTOD value at ⁇ 165 ° C. is 0.25 mm or more.
  • the inventors of the present invention earnestly studied about the way to solve the above-mentioned problems for high-Mn steel, and reached the following findings of a to b.
  • High Mn steels do not undergo brittle fracture even at very low temperatures, and they originate from grain boundaries if fracture occurs. From this, in order to improve the fracture resistance of the high-Mn steel, it is effective to control the diameter of the crystal grain by reducing the area of the grain boundary which is the starting point of the fracture.
  • b. Further, it is more effective to improve the fracture resistance of a high Mn steel by achieving the grain size adjustment in accordance with the regulation of the crystal grain size.
  • c. As a means to achieve the above a and b, it is appropriate to carry out hot rolling and cooling under appropriate manufacturing conditions.
  • the present invention has been made by further examining the above findings, and the summary thereof is as follows. 1. In mass%, C: 0.10% or more and 0.70% or less, Si: 0.05% or more and 0.50% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 0.5% or more and 7.0% or less, Ni: 0.01% or more and less than 0.1%, Ca: 0.0005% or more and 0.0050% or less, N: 0.0050% or more and 0.0050% or less, O: less than 0.0050%, Ti: less than 0.0050% and Nb: less than 0.0050%, and the balance has a component composition of Fe and unavoidable impurities, and a structure having austenite as a base phase, and the austenite has a particle size of High Mn steel having a diameter of 1 ⁇ m or more and a standard deviation of 9 ⁇ m or less.
  • the above component composition is, further, in mass%, Cu: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less,
  • the finish rolling finish temperature is 750 ° C. or more and less than 950 ° C.
  • the average rolling reduction per pass is 9% More than that, it is subjected to hot rolling, and then, the cooling treatment with an average cooling rate of 1.0 ° C./s or more from a temperature of (finish rolling finish temperature -100 ° C.) or more to a temperature range of 300 ° C. or more and 650 ° C. or less Manufacturing method of high Mn steel to do.
  • the present invention it is possible to provide a high Mn steel which is excellent in CTOD characteristics, particularly in the cryogenic temperature range, and low temperature toughness. Therefore, by using the high Mn steel of the present invention, it is possible to realize the improvement of the safety and the life of the steel structure used in the cryogenic environment such as the tank for liquefied gas storage tank, etc., and the industrial effect is remarkable. Play.
  • C 0.10% or more and 0.70% or less
  • C is an inexpensive austenite stabilizing element, and is an important element to obtain austenite. In order to obtain the effect, C needs to be contained at 0.10% or more.
  • the content exceeds 0.70%, Cr carbides are excessively formed and the low temperature toughness is lowered. Therefore, the C content is 0.10% or more and 0.70% or less, preferably 0.20% or more and 0.60% or less.
  • Si acts as a deoxidizer and is not only necessary for steel making, but also has the effect of strengthening the steel plate by solid solution strengthening and solid solution strengthening in steel. . In order to obtain these effects, it is necessary to contain Si at 0.05% or more. On the other hand, if the content is more than 0.50%, the weldability is deteriorated and the low temperature toughness, particularly the toughness at a very low temperature becomes low. Therefore, the amount of Si is 0.05% or more and 0.50% or less, 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 to achieve both strength and cryogenic toughness. In order to acquire the effect, it is necessary to contain Mn at 20% or more. On the other hand, even if the content is more than 30%, the effect of improving the low temperature toughness saturates, leading to an increase in alloy cost. In addition, weldability and cuttability are degraded. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the Mn content is set to 20% or more and 30% or less, preferably 23% or more and 28% or less.
  • P 0.030% or less
  • P is contained in excess of 0.030%, it segregates at grain boundaries and becomes a generation origin of stress corrosion cracking. For this reason, it is desirable to make it as upper limit 0.030%, and to reduce as much as possible. Therefore, P is made 0.030% or less.
  • S degrades the low temperature toughness and ductility of the base material, so the upper limit of 0.0070% is desirable, and it is desirable to reduce as much as possible. Therefore, S is 0.0070% or less. In addition, since excessive reduction of S raises the refining cost and is economically disadvantageous, it is desirable to make it 0.001% or more. Preferably, it is 0.0020% or more and 0.0060% or less.
  • Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheet. In order to acquire such an effect, it is necessary to contain Al by 0.01% or more. On the other hand, if the content is more than 0.07%, it is mixed with the weld metal at the time of welding to deteriorate the toughness of the weld metal, so the content is made 0.07% or less. Therefore, Al is set to 0.01% or more and 0.07% or less, preferably 0.02% or more and 0.06% or less.
  • Cr 0.5% or more and 7.0% or less
  • Cr is an element that stabilizes austenite with an appropriate amount of addition and is effective for improving low-temperature toughness and base material strength. In order to obtain such an effect, it is necessary to contain Cr at 0.5% or more. On the other hand, if the content is more than 7.0%, low temperature toughness and stress corrosion cracking resistance are reduced due to the formation of Cr carbides. Therefore, Cr is set to 0.5% or more and 7.0% or less. Preferably, it is 1.0% or more and 6.7% or less, more preferably 1.2% or more and 6.5% or less. Moreover, in order to further improve stress corrosion cracking, 2.0% or more and 6.0% or less is more preferable.
  • Ni has the effect of improving low-temperature toughness, but it is an important aspect in component design of the present invention to minimize the necessity from the point of alloy cost. From the viewpoint, the amount of Ni is 0.01% or more and less than 0.1%.
  • stainless steels such as SUS304 and SUS316 are available as austenitic steels that are excellent in low temperature toughness, but these steels are designed to optimize Ni equivalent and Cr equivalent as alloy design for obtaining austenitic structure. , A large amount of Ni is added.
  • the present invention is an austenitic material which has been reduced in cost by minimizing Ni. The necessary minimization of Ni was realized by optimizing the amount of addition of Mn.
  • the preferable amount of Ni is 0.03% or more and 0.07% or less.
  • Ca 0.0005% or more and 0.0050% or less
  • Ca improves ductility, toughness, and sulfide stress corrosion cracking resistance by controlling the form of inclusions described below, and suppresses a decrease in hot ductility and casts It works effectively to reduce the occurrence of cracking of pieces. In order to obtain such an effect, Ca needs to be 0.0005% or more. On the other hand, if it is added in excess of 0.0050%, the ductility, toughness, and resistance to sulfide stress corrosion cracking may decrease, and the effect of suppressing hot ductility also saturates. Therefore, the amount of Ca is set to 0.0005% or more and 0.0050% or less. Preferably, it is 0.0010% or more and 0.0045% or less.
  • N is an austenite stabilizing element and is an element effective for improving low-temperature toughness. In order to acquire such an effect, it is necessary to contain N by 0.0050% or more. On the other hand, if the content is more than 0.0300%, nitrides or carbonitrides become coarse and the toughness decreases. Therefore, N is set to be 0.0050% or more and 0.0050% or less, preferably 0.0060% or more and 0.0400% or less.
  • O degrades low temperature toughness by the formation of an oxide. Therefore, O is in the range of 0.0050% or less. Preferably, it is 0.0045% or less. In addition, since excessive reduction of O raises the refining cost and is economically disadvantageous, it is desirable to make it 0.0003% or more.
  • Ti and Nb content suppressed to less than 0.005% each Ti and Nb form high melting point carbonitride in steel and suppress coarsening of crystal grains, resulting in origin of fracture and crack propagation It becomes a route.
  • the low-temperature toughness is increased and the structure control for improving the ductility is hindered, and therefore, it is necessary to be intentionally suppressed. That is, Ti and Nb are components which are inevitably mixed from raw materials and the like, and it is usually mixed in the range of Ti: 0.005 to 0.010% and Nb: 0.005 to 0.010%. .
  • the content of Ti and Nb is made 0.003% or less.
  • the balance other than the above-mentioned essential components is iron and unavoidable impurities.
  • unavoidable impurities here, H etc. are mentioned, and it is acceptable if it is 0.01% or less in total.
  • the following elements can be contained as needed.
  • Mg 0.0005 to 0.0050%
  • REM 0.0010 to 0.0200%
  • Mg and REM are elements useful for controlling the form of inclusions and can be contained as necessary.
  • the form control of inclusions means that the spread sulfide inclusions are made into particulate inclusions.
  • the ductility, toughness and resistance to sulfide stress corrosion cracking are improved through the morphology control of the inclusions.
  • the amount of non-metallic inclusions may increase, and the ductility, the toughness, and the sulfide stress corrosion cracking resistance may decrease. In addition, it may be economically disadvantageous. Therefore, in the case of containing Mg, 0.0005 to 0.0050%, and in the case of containing REM, 0.0010% to 0.0200%.
  • the amount of Mg is 0.0010% to 0.0040%
  • the amount of REM is 0.0020% to 0.0150%.
  • the steel material may cause brittle fracture in a low temperature environment, so it is suitable for use in a low temperature environment Not.
  • the base phase of the steel material has an austenitic structure in which the crystal structure is a face-centered cubic structure (fcc).
  • “use austenite as a base phase” means that the austenite phase is 90% or more in area ratio.
  • the balance other than the austenite phase is a ferrite phase or a martensite phase, but it goes without saying that the austenite phase may be 100%.
  • Austenite grain size 1 ⁇ m or more Since a high Mn steel has a structure having austenite as a base phase, brittle fracture does not occur even at extremely low temperatures, and fracture occurs when it occurs from grain boundaries. It is advantageous to improve the fracture resistance of high Mn steels by reducing the area of grain boundaries that are the starting point of this fracture. For that purpose, it is important that the austenite grain size is 1 ⁇ m or more. This is because if the particle size is less than 1 ⁇ m, the amount of increase in the grain interface area becomes large, and the location of occurrence of breakage increases. Preferably, it is 2 ⁇ m or more
  • the standard deviation of austenite is 9 ⁇ m or less. It is effective to further improve the fracture resistance of a high-Mn steel by achieving the particle size regulation in accordance with the regulation of the crystal grain size. That is, in the case of mixed grain structure, a broad grain size distribution from coarse grains to fine grains forms a broad grain size distribution, and grain sizes less than 1 ⁇ m are included, especially when the standard deviation exceeds 9 ⁇ m. Mixed grain structures having a standard deviation of more than 9 ⁇ m should be avoided as they become noticeable.
  • a steel material can be melted and manufactured using a known melting method, such as a converter or an electric furnace, of a molten steel having the above-described component composition. Further, secondary refining may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which would interfere with favorable structure control, to the above-mentioned range, it is necessary to avoid the inevitable mixing from raw materials etc. and to take measures to reduce their content. . For example, by lowering the basicity of the slag in the refining stage, these alloys are concentrated into slag and discharged to reduce the concentration of Ti and Nb in the final slab product.
  • a known melting method such as a converter or an electric furnace
  • oxygen may be blown to oxidize, and an alloy of Ti and Nb may be floated and separated at the time of reflux.
  • the manufacturing conditions for forming the above-mentioned steel material into a steel material excellent in low temperature toughness are specified.
  • Steel material heating temperature 1100 ° C. or more and 1300 ° C. or less
  • the heating temperature before hot rolling is set to 1100 ° C. or more.
  • the upper limit of the heating temperature is 1300 ° C.
  • the temperature control here is based on the surface temperature of the steel material.
  • Finish rolling finish temperature 750 ° C. or more and less than 950 ° C.
  • the cumulative rolling reduction is set to 750 ° C.
  • the lower limit of the finish rolling end temperature is set to 750 ° C.
  • the crystal grain size becomes excessively large, and the desired yield strength can not be obtained. Therefore, one or more final finishing rolling is required at less than 950 ° C.
  • it is 900 ° C. or less.
  • Average rolling reduction in one pass 9% or more
  • the formation of precipitates is promoted and the low temperature toughness is deteriorated.
  • the formation of these precipitates can be suppressed by cooling at a cooling rate of 1.0 ° C./s or more.
  • excessive cooling may distort the steel sheet and reduce productivity. Therefore, the upper limit of the cooling start temperature is 900 ° C.
  • the cooling after hot rolling is the average cooling rate of the steel sheet surface from the temperature of (finish rolling finish temperature -100 ° C) or more to the temperature range of 300 ° C or more and 650 ° C or less to 1.0 ° C / s. And above. On the other hand, in terms of industrial production, it is preferable to set the average cooling rate to 200 ° C./s or less.
  • a steel slab having the composition shown in Table 1 was produced by a converter-ladle refining-continuous casting method. Then, the obtained steel slab was made into a steel plate with a thickness of 10 to 30 mm by slab rolling and hot rolling under the conditions shown in Table 2. The tensile properties, toughness and structure evaluations were carried out on the obtained steel sheets in the following manner.
  • CTOD test pieces were collected from a direction parallel to the rolling direction at a thickness 1/2 position of a steel plate, and two to three tests were conducted at -165 ° C, and the average value was evaluated.
  • a CTOD value of 0.25 mm or more is excellent in fracture resistance.
  • the high-Mn steel according to the present invention has the above-described target performance (yield strength of the base material is 400 MPa or more, low temperature toughness is 100 J or more in average of absorbed energy (vE-196), average CTOD value is 0.25 mm or more) It was confirmed to be satisfactory. On the other hand, in Comparative Examples outside the scope of the present invention, any one or more of the yield strength, the low temperature toughness, and the CTOD value can not satisfy the above-described target performance.

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  • Heat Treatment Of Steel (AREA)

Abstract

Provided is a high-Mn steel having exceptional high strength and low-temperature toughness, and furthermore having exceptional low-temperature CTOD characteristics. The high-Mn steel has: a component composition comprising, in percent by mass, 0.10-0.70% of C, 0.05-0.50% of Si, 20-30% of Mn, 0.030% or less of P, 0.0070% or less of S, 0.01-0.07% of Al, 0.5-7.0% of Cr, at least 0.01% and less than 0.1% of Ni, 0.0005-0.0050% of Ca, 0.0050-0.0500% of N, 0.0050% or less of O, less than 0.0050% of Ti, and less than 0.0050% of Nb, the balance being Fe and unavoidable impurities; and a structure having austenite as a matrix phase; the austenite having a grain size of 1 μm or greater and a standard deviation of 9 μm or less.

Description

高Mn鋼およびその製造方法High Mn steel and method of manufacturing the same
 本発明は、例えば液化ガス貯槽用タンク等の、極低温環境で使用される構造物に供して好適な高Mn鋼およびその製造方法に関する。 The present invention relates to a high Mn steel suitable for use in structures used in cryogenic environments, such as, for example, tanks for liquefied gas storage, and a method for producing the same.
 液化ガス貯槽用構造物は、その使用環境が極低温となるため、この種の構造物に用いる鋼板は高強度であることに加えて、極低温での靱性に優れることも要求される。例えば、液化天然ガスの貯槽に熱間圧延鋼板を使用する場合は、液化天然ガスの沸点:-164℃以下で優れた靱性が確保されている必要がある。鋼材の低温靱性が劣ると、極低温貯槽用構造物としての安全性を維持できなくなる可能性があるため、適用される鋼材に対する低温靱性の向上に対する要求は強い。 Since the use environment of the structure for liquefied gas storage tanks is extremely low temperature, in addition to high strength, the steel plate used for this type of structure is also required to have excellent toughness at very low temperature. For example, in the case of using a hot-rolled steel sheet for storage of liquefied natural gas, it is necessary to ensure excellent toughness at the boiling point of liquefied natural gas: -164 ° C or less. If the low temperature toughness of the steel material is poor, the safety as a cryogenic storage tank structure may not be maintained, so there is a strong demand for improvement of the low temperature toughness of the steel material to be applied.
 この要求に対して、従来、極低温で脆性を示さないオーステナイトを鋼板の主組織とするオーステナイト系ステンレス鋼や9%Ni鋼、もしくは5000系アルミニウム合金が使用されてきた。しかしながら、合金コストや製造コストが高いことから、安価で低温靱性に優れる鋼材に対する要望がある。 In response to this requirement, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy in which austenite which does not show brittleness at cryogenic temperature is used as the main structure of the steel plate has been used conventionally. However, since the alloy cost and the manufacturing cost are high, there is a demand for a steel material which is inexpensive and excellent in low temperature toughness.
 そこで、従来の極低温用鋼に代わる新たな鋼材として、比較的安価なオーステナイト安定化元素であるMnを多量に添加した高Mn鋼を極低温環境の構造用鋼として使用することが、特許文献1や特許文献2において提案されている。 Therefore, as a new steel material to replace conventional cryogenic steel, it is possible to use high Mn steel, to which a large amount of relatively inexpensive austenite stabilizing element Mn is added, as structural steel for cryogenic environment, Patent Literature Patent Document 1 and Patent Document 2 propose.
 すなわち、特許文献1には、オーステナイト結晶粒界の炭化物被覆率を制御することが提案されている。また、特許文献2には、炭化物被覆物、ならびにMg、Ca、REMの添加によりオーステナイト結晶粒径を制御することが提案されている。 That is, Patent Document 1 proposes to control the carbide coverage of austenite grain boundaries. Further, Patent Document 2 proposes that the austenite grain size be controlled by the addition of a carbide coating and Mg, Ca, and REM.
特開2016-84529号公報JP, 2016-84529, A 特開2016-196703号公報JP, 2016-196703, A
 ところで、液化ガス貯槽用タンクなどの使途では、該タンクの安全性確保の観点から、初期亀裂がより鋭利となる厳しい破壊条件下での耐破壊特性、具体的には低温域でのCTOD特性に優れることが求められている。上記した特許文献1および特許文献2では、シャルピー衝撃試験による低温靭性は評価されているが、優れたCTOD特性が保証されるに至ってはいない。 By the way, from the viewpoint of securing the safety of the tank for use as a tank for liquefied gas storage tanks, it is necessary to use the fracture resistance characteristics under severe destruction conditions where the initial cracks become sharper, specifically the CTOD characteristics in a low temperature range. It is required to be superior. Although the low temperature toughness by a Charpy impact test is evaluated in the above-mentioned patent documents 1 and patent documents 2, excellent CTOD characteristics have not come to be guaranteed.
 本発明は、高強度かつ低温靱性に優れることは勿論、さらに低温のCTOD特性にも優れた高Mn鋼を提供することを目的とする。ここで、前記「高強度」とは、降伏強度が400MPa以上であり、前記「低温靭性に優れた」とは、-196℃におけるシャルピー衝撃試験の吸収エネルギーvE-196が100J以上であり、前記「低温のCTOD特性に優れた」とは、-165℃におけるCTOD値が0.25mm以上であることをいう。 An object of the present invention is to provide a high Mn steel which is excellent not only in high strength and low temperature toughness but also in low temperature CTOD characteristics. Here, the "high strength" means that the yield strength is 400 MPa or more, and the term "the excellent low temperature toughness" means that the absorbed energy vE-196 in the Charpy impact test at -196 ° C is 100 J or more. The term “excellent in low temperature CTOD characteristics” means that the CTOD value at −165 ° C. is 0.25 mm or more.
 発明者らは、高Mn鋼を対象にして、上記課題を解決するための方途について鋭意研究を行った結果、以下のa~bの知見を得るに到った。
a.高Mn鋼は、極低温においても脆性破壊とならずに、破壊が生じる場合は結晶粒界から発生する。このことから、高Mn鋼の耐破壊特性を向上するには、破壊の起点となる結晶粒界の面積低減を所期して結晶粒の径を規制することが有効である。
b.さらに、上記結晶粒径の規制に併せて整粒化を図ることが、高Mn鋼の耐破壊特性向上に、より有効である。
c.上記のaおよびbを達成する手段として、適切な製造条件で熱間圧延および冷却を行うことが適切である。
The inventors of the present invention earnestly studied about the way to solve the above-mentioned problems for high-Mn steel, and reached the following findings of a to b.
a. High Mn steels do not undergo brittle fracture even at very low temperatures, and they originate from grain boundaries if fracture occurs. From this, in order to improve the fracture resistance of the high-Mn steel, it is effective to control the diameter of the crystal grain by reducing the area of the grain boundary which is the starting point of the fracture.
b. Further, it is more effective to improve the fracture resistance of a high Mn steel by achieving the grain size adjustment in accordance with the regulation of the crystal grain size.
c. As a means to achieve the above a and b, it is appropriate to carry out hot rolling and cooling under appropriate manufacturing conditions.
 本発明は、以上の知見にさらに検討を加えてなされたものであり、その要旨は次のとおりである。
1.質量%で、
 C:0.10%以上0.70%以下、
 Si:0.05%以上0.50%以下、
 Mn:20%以上30%以下、
 P:0.030%以下、
 S:0.0070%以下、
 Al:0.01%以上0.07%以下、
 Cr:0.5%以上7.0%以下、
 Ni:0.01%以上0.1%未満、
 Ca:0.0005%以上0.0050%以下、
 N:0.0050%以上0.0500%以下、
 O:0.0050%以下、
 Ti:0.0050%未満および
 Nb:0.0050%未満
を含有し、残部がFeおよび不可避的不純物の成分組成と、オーステナイトを基地相とする組織とを有し、前記オーステナイトは、粒径が1μm以上かつ標準偏差が9μm以下である高Mn鋼。
The present invention has been made by further examining the above findings, and the summary thereof is as follows.
1. In mass%,
C: 0.10% or more and 0.70% or less,
Si: 0.05% or more and 0.50% or less,
Mn: 20% or more and 30% or less,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 0.07% or less,
Cr: 0.5% or more and 7.0% or less,
Ni: 0.01% or more and less than 0.1%,
Ca: 0.0005% or more and 0.0050% or less,
N: 0.0050% or more and 0.0050% or less,
O: less than 0.0050%,
Ti: less than 0.0050% and Nb: less than 0.0050%, and the balance has a component composition of Fe and unavoidable impurities, and a structure having austenite as a base phase, and the austenite has a particle size of High Mn steel having a diameter of 1 μm or more and a standard deviation of 9 μm or less.
2.前記成分組成は、さらに、質量%で、
 Cu:1.0%以下、
 Mo:2.0%以下、
 V:2.0%以下、
 W:2.0%以下、
 Mg:0.0005%以上0.0050%以下および
 REM:0.0010%以上0.0200%以下
のうちから選ばれる1種または2種以上を含有する前記1に記載の高Mn鋼。
2. The above component composition is, further, in mass%,
Cu: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
The high Mn steel as described in 1 above, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.
3.前記1または2に記載の成分組成を有する鋼素材を1100℃以上1300℃以下の温度域に加熱した後、仕上圧延終了温度が750℃以上950℃未満かつ1パス当たりの平均圧下率が9%以上である、熱間圧延を施し、その後、(仕上圧延終了温度-100℃)以上の温度から300℃以上650℃以下の温度域までの平均冷却速度が1.0℃/s以上の冷却処理を行う高Mn鋼の製造方法。 3. After the steel material having the component composition described in 1 or 2 above is heated to a temperature range of 1100 ° C. or more and 1300 ° C. or less, the finish rolling finish temperature is 750 ° C. or more and less than 950 ° C., and the average rolling reduction per pass is 9% More than that, it is subjected to hot rolling, and then, the cooling treatment with an average cooling rate of 1.0 ° C./s or more from a temperature of (finish rolling finish temperature -100 ° C.) or more to a temperature range of 300 ° C. or more and 650 ° C. or less Manufacturing method of high Mn steel to do.
 本発明によれば、特に極低温域でのCTOD特性並びに低温靭性に優れた高Mn鋼を提供することができる。したがって、本発明の高Mn鋼を用いることによって、液化ガス貯槽用タンク等の、極低温環境で使用される鋼構造物の安全性や寿命の向上を実現することができ、産業上格段の効果を奏する。 According to the present invention, it is possible to provide a high Mn steel which is excellent in CTOD characteristics, particularly in the cryogenic temperature range, and low temperature toughness. Therefore, by using the high Mn steel of the present invention, it is possible to realize the improvement of the safety and the life of the steel structure used in the cryogenic environment such as the tank for liquefied gas storage tank, etc., and the industrial effect is remarkable. Play.
 以下、本発明の高Mn鋼について詳しく説明する。
[成分組成]
 まず、本発明の高Mn鋼の成分組成とその限定理由について説明する。なお、成分組成における「%」表示は、特に断らない限り「質量%」を意味するものとする。
C:0.10%以上0.70%以下
 Cは、安価なオーステナイト安定化元素であり、オーステナイトを得るために重要な元素である。その効果を得るには、Cを0.10%以上で含有する必要がある。一方、0.70%を超えて含有すると、Cr炭化物が過度に生成され、低温靱性が低下する。従って、C量は0.10%以上0.70%以下、好ましくは、0.20%以上0.60%以下とする。
Hereinafter, the high-Mn steel of the present invention will be described in detail.
[Component composition]
First, the component composition of the high-Mn steel of the present invention and the reason for limitation will be described. In addition, unless otherwise indicated, "%" display in a component composition shall mean "mass%."
C: 0.10% or more and 0.70% or less C is an inexpensive austenite stabilizing element, and is an important element to obtain austenite. In order to obtain the effect, C needs to be contained at 0.10% or more. On the other hand, if the content exceeds 0.70%, Cr carbides are excessively formed and the low temperature toughness is lowered. Therefore, the C content is 0.10% or more and 0.70% or less, preferably 0.20% or more and 0.60% or less.
Si:0.05%以上0.50%以下
 Siは、脱酸材として作用し、製鋼上必要であるだけでなく、鋼に固溶して固溶強化により鋼板を高強度化する効果も有する。これら効果を得るには、Siを0.05%以上で含有する必要がある。一方、0.50%を超えて含有すると、溶接性が劣化するとともに低温靭性、特に極低温での靭性が低位となる。従って、Si量は0.05%以上0.50%以下、好ましくは、0.07%以上0.50%以下とする。
Si: 0.05% or more and 0.50% or less Si acts as a deoxidizer and is not only necessary for steel making, but also has the effect of strengthening the steel plate by solid solution strengthening and solid solution strengthening in steel. . In order to obtain these effects, it is necessary to contain Si at 0.05% or more. On the other hand, if the content is more than 0.50%, the weldability is deteriorated and the low temperature toughness, particularly the toughness at a very low temperature becomes low. Therefore, the amount of Si is 0.05% or more and 0.50% or less, preferably 0.07% or more and 0.50% or less.
Mn:20%以上30%以下
 Mnは、比較的安価なオーステナイト安定化元素である。Mnは、本発明において、強度と極低温靱性を両立するために重要な元素である。その効果を得るためには、Mnを20%以上で含有する必要がある。一方、30%を超えて含有しても、低温靱性を改善する効果は飽和し、合金コストの上昇を招く。また、溶接性、切断性が劣化する。さらに、偏析を助長し、応力腐食割れの発生を助長する。従って、Mn量は20%以上30%以下、好ましくは23%以上28%以下とする。
Mn: 20% or more and 30% or less Mn is a relatively inexpensive austenite stabilizing element. In the present invention, Mn is an important element to achieve both strength and cryogenic toughness. In order to acquire the effect, it is necessary to contain Mn at 20% or more. On the other hand, even if the content is more than 30%, the effect of improving the low temperature toughness saturates, leading to an increase in alloy cost. In addition, weldability and cuttability are degraded. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the Mn content is set to 20% or more and 30% or less, preferably 23% or more and 28% or less.
P:0.030%以下
 Pは、0.030%を超えて含有すると、粒界に偏析し、応力腐食割れの発生起点となる。このため、0.030%を上限とし、可能なかぎり低減することが望ましい。したがって、Pは0.030%以下とする。尚、過度のP低減は精錬コストを高騰させ経済的に不利となるため、0.002%以上とすることが望ましい。好ましくは、0.005%以上0.028%以下、さらに好ましくは0.024%以下とする。
P: 0.030% or less When P is contained in excess of 0.030%, it segregates at grain boundaries and becomes a generation origin of stress corrosion cracking. For this reason, it is desirable to make it as upper limit 0.030%, and to reduce as much as possible. Therefore, P is made 0.030% or less. In addition, since excessive P reduction raises the refining cost and becomes economically disadvantageous, it is desirable to set it as 0.002% or more. Preferably, it is 0.005% or more and 0.028% or less, more preferably 0.024% or less.
S:0.0070%以下
 Sは、母材の低温靭性や延性を劣化させるため、0.0070%を上限とし、可能なかぎり低減することが望ましい。したがって、Sは0.0070%以下とする。尚、過度のSの低減は精錬コストを高騰させ経済的に不利となるため、0.001%以上とすることが望ましい。好ましくは0.0020%以上0.0060%以下とする。
S: 0.0070% or less S degrades the low temperature toughness and ductility of the base material, so the upper limit of 0.0070% is desirable, and it is desirable to reduce as much as possible. Therefore, S is 0.0070% or less. In addition, since excessive reduction of S raises the refining cost and is economically disadvantageous, it is desirable to make it 0.001% or more. Preferably, it is 0.0020% or more and 0.0060% or less.
Al:0.01%以上0.07%以下
 Alは、脱酸剤として作用し、鋼板の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。このような効果を得るためには、Alを0.01%以上で含有する必要がある。一方、0.07%を超えて含有すると、溶接時に溶接金属部に混入して、溶接金属の靭性を劣化させるため、0.07%以下とする。従って、Alは0.01%以上0.07%以下、好ましくは0.02%以上0.06%以下とする。
Al: 0.01% or more and 0.07% or less Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheet. In order to acquire such an effect, it is necessary to contain Al by 0.01% or more. On the other hand, if the content is more than 0.07%, it is mixed with the weld metal at the time of welding to deteriorate the toughness of the weld metal, so the content is made 0.07% or less. Therefore, Al is set to 0.01% or more and 0.07% or less, preferably 0.02% or more and 0.06% or less.
Cr:0.5%以上7.0%以下
 Crは、適量の添加でオーステナイトを安定化させ、低温靱性と母材強度の向上に有効な元素である。このような効果を得るためには、Crを0.5%以上で含有する必要がある。一方、7.0%を超えて含有すると、Cr炭化物の生成により、低温靭性および耐応力腐食割れ性が低下する。このため、Crは0.5%以上7.0%以下とする。好ましくは1.0%以上6.7%以下、より好ましくは1.2%以上6.5%以下とする。また、耐応力腐食割れをさらに向上させるためには2.0%以上6.0%以下がさらに好ましい。
Cr: 0.5% or more and 7.0% or less Cr is an element that stabilizes austenite with an appropriate amount of addition and is effective for improving low-temperature toughness and base material strength. In order to obtain such an effect, it is necessary to contain Cr at 0.5% or more. On the other hand, if the content is more than 7.0%, low temperature toughness and stress corrosion cracking resistance are reduced due to the formation of Cr carbides. Therefore, Cr is set to 0.5% or more and 7.0% or less. Preferably, it is 1.0% or more and 6.7% or less, more preferably 1.2% or more and 6.5% or less. Moreover, in order to further improve stress corrosion cracking, 2.0% or more and 6.0% or less is more preferable.
Ni:0.01%以上0.1%未満
 Niは、低温靱性を向上する効果を有するが、合金コストの点から必要最小限とすることが本発明の成分設計における重要な観点であり、この観点からNi量は0.01%以上0.1%未満とする。ここで、低温靱性に優れるオーステナイト鋼としてSUS304やSUS316などのステンレス鋼があるが、これらの鋼は、オーステナイト組織を得るための合金設計としてNi当量やCr当量を適正化が図られていることから、多量のNiが添加されている。これらの鋼に対して本発明は、Niを必要最小限とすることによって低廉化した、オーステナイト材料である。なお、このNiの必要最小限化は、Mn添加量の適正化により実現した。好ましいNi量は、0.03%以上0.07%以下である。
Ni: 0.01% or more and less than 0.1% Ni has the effect of improving low-temperature toughness, but it is an important aspect in component design of the present invention to minimize the necessity from the point of alloy cost. From the viewpoint, the amount of Ni is 0.01% or more and less than 0.1%. Here, stainless steels such as SUS304 and SUS316 are available as austenitic steels that are excellent in low temperature toughness, but these steels are designed to optimize Ni equivalent and Cr equivalent as alloy design for obtaining austenitic structure. , A large amount of Ni is added. For these steels, the present invention is an austenitic material which has been reduced in cost by minimizing Ni. The necessary minimization of Ni was realized by optimizing the amount of addition of Mn. The preferable amount of Ni is 0.03% or more and 0.07% or less.
Ca:0.0005%以上0.0050%以下
 Caは、下記に記載の介在物の形態制御により延性、靭性および耐硫化物応力腐食割れ性を向上させるとともに、熱間延性の低下を抑制し鋳片の割れ発生の低減に有効に作用する。このような効果を得るためには、Caは0.0005%以上必要である。一方、0.0050%を超えて添加すると、かえって延性、靭性、耐硫化物応力腐食割れ性が低下する場合があり、熱間延性の低下抑制効果も飽和する。このため、Ca量は0.0005%以上0.0050%以下とする。好ましくは、0.0010%以上0.0045%以下である。
Ca: 0.0005% or more and 0.0050% or less Ca improves ductility, toughness, and sulfide stress corrosion cracking resistance by controlling the form of inclusions described below, and suppresses a decrease in hot ductility and casts It works effectively to reduce the occurrence of cracking of pieces. In order to obtain such an effect, Ca needs to be 0.0005% or more. On the other hand, if it is added in excess of 0.0050%, the ductility, toughness, and resistance to sulfide stress corrosion cracking may decrease, and the effect of suppressing hot ductility also saturates. Therefore, the amount of Ca is set to 0.0005% or more and 0.0050% or less. Preferably, it is 0.0010% or more and 0.0045% or less.
N:0.0050%以上0.0500%以下
 Nは、オーステナイト安定化元素であり、低温靱性の向上に有効な元素である。このような効果を得るためには、Nを0.0050%以上で含有する必要がある。一方、0.0500%を超えて含有すると、窒化物または炭窒化物が粗大化し、靭性が低下する。従って、Nは0.0050%以上0.0500%以下、好ましくは0.0060%以上0.0400%以下とする。
N: 0.0050% or more and 0.0050% or less N is an austenite stabilizing element and is an element effective for improving low-temperature toughness. In order to acquire such an effect, it is necessary to contain N by 0.0050% or more. On the other hand, if the content is more than 0.0300%, nitrides or carbonitrides become coarse and the toughness decreases. Therefore, N is set to be 0.0050% or more and 0.0050% or less, preferably 0.0060% or more and 0.0400% or less.
O:0.0050%以下
 Oは、酸化物の形成により低温靱性を劣化させる。このため、Oは0.0050%以下の範囲とする。好ましくは、0.0045%以下である。尚、過度のOの低減は精錬コストを高騰させ経済的に不利となるため、0.0003%以上とすることが望ましい。
O: 0.0050% or less O degrades low temperature toughness by the formation of an oxide. Therefore, O is in the range of 0.0050% or less. Preferably, it is 0.0045% or less. In addition, since excessive reduction of O raises the refining cost and is economically disadvantageous, it is desirable to make it 0.0003% or more.
TiおよびNbの含有量を各々0.005%未満に抑制
 TiおよびNbは、鋼中で高融点の炭窒化物を形成し結晶粒の粗大化を抑制し、その結果破壊の起点や亀裂伝播の経路となる。特に、高Mn鋼においては低温靭性を高め、延性を向上するための組織制御の妨げとなるため、意図的に抑制する必要がある。すなわち、TiおよびNbは、原材料などから不可避的に混入する成分であり、Ti:0.005~0.010%およびNb:0.005~0.010%の範囲で混入するのが通例である。そこで、後述する手法に従って、TiおよびNbの不可避混入を回避し、TiおよびNbの含有量を各々0.005%未満に抑制する必要がある。TiおよびNbの含有量を各々0.005%未満に抑制することによって、上記した炭窒化物の悪影響を排除し、優れた低温靭性並びに延性を確保することができる。好ましくは、TiおよびNbの含有量を0.003%以下とする。
Ti and Nb content suppressed to less than 0.005% each Ti and Nb form high melting point carbonitride in steel and suppress coarsening of crystal grains, resulting in origin of fracture and crack propagation It becomes a route. In particular, in the case of a high-Mn steel, the low-temperature toughness is increased and the structure control for improving the ductility is hindered, and therefore, it is necessary to be intentionally suppressed. That is, Ti and Nb are components which are inevitably mixed from raw materials and the like, and it is usually mixed in the range of Ti: 0.005 to 0.010% and Nb: 0.005 to 0.010%. . Therefore, according to the method described later, it is necessary to avoid the inevitable mixing of Ti and Nb, and to control the content of each of Ti and Nb to less than 0.005%. By suppressing the contents of Ti and Nb to less than 0.005%, respectively, the above-mentioned adverse effects of carbonitrides can be eliminated, and excellent low temperature toughness and ductility can be ensured. Preferably, the content of Ti and Nb is made 0.003% or less.
 上記した必須成分以外の残部は鉄および不可避的不純物である。ここでの不可避的不純物としてはHなどが挙げられ、合計で0.01%以下であれば許容できる。 The balance other than the above-mentioned essential components is iron and unavoidable impurities. As unavoidable impurities here, H etc. are mentioned, and it is acceptable if it is 0.01% or less in total.
 本発明では、強度および低温靱性をさらに向上させることを目的として、上記の必須成分に加えて、必要に応じて下記の元素を含有することができる。
Cu:1.0%以下、Mo:2.0%以下、V:2.0%以下、W:2.0%以下、Mg:0.0005~0.0050%、REM:0.0010~0.0200%の1種または2種以上
Cu:1.0%以下、Mo、V、W:各々2.0%以下
 Cu、Mo、VおよびWは、オーステナイトの安定化に寄与するとともに母材強度の向上に寄与する。このような効果を得るためには、Cu、Mo、VおよびWは0.001%以上で含有することが好ましい。一方、Cuは1.0%、Mo、VおよびWは各々2.0%を超えて含有すると、粗大な炭窒化物が生成し、破壊の起点となることがある他、製造コストを圧迫する。このため、これらの合金元素を含有する場合は、その含有量は、Cuは1.0%以下、Mo、VおよびWは2.0%以下とする。好ましくは、0.003%以上である。さらに、Mo、VおよびWについては、好ましくは1.7%以下、より好ましくは1.5%以下とする。
In the present invention, for the purpose of further improving the strength and low temperature toughness, in addition to the above essential components, the following elements can be contained as needed.
Cu: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0 .0 200% of one or more Cu: 1.0% or less, Mo, V, W: 2.0% or less each Cu, Mo, V and W contribute to the stabilization of austenite and also the strength of the base material Contribute to the improvement of In order to acquire such an effect, it is preferable to contain Cu, Mo, V and W by 0.001% or more. On the other hand, if Cu is contained at 1.0% and Mo, V and W are each contained at more than 2.0%, coarse carbonitrides may be formed, which may become a starting point of breakage and pressurize the manufacturing cost . For this reason, when these alloy elements are contained, the content thereof is 1.0% or less for Cu, and 2.0% or less for Mo, V and W. Preferably, it is 0.003% or more. Furthermore, Mo, V and W are preferably at most 1.7%, more preferably at most 1.5%.
Mg:0.0005~0.0050%、REM:0.0010~0.0200%
 MgおよびREMは、介在物の形態制御に有用な元素であり、必要に応じて含有できる。介在物の形態制御とは、展伸した硫化物系介在物を粒状の介在物にすることをいう。この介在物の形態制御を介して、延性、靭性および耐硫化物応力腐食割れ性を向上させる。このような効果を得るためには、Ca、Mgは0.0005%以上、REMは0.0010%以上で含有することが好ましい。一方、いずれの元素も多く含有させると、非金属介在物量が増加し、かえって延性、靭性、耐硫化物応力腐食割れ性が低下する場合がある。また、経済的に不利になる場合がある。このため、Mgを含有する場合には、0.0005~0.0050%、REMを含有する場合には、0.0010%~0.0200%とする。好ましくは、Mg量は0.0010%以上0.0040%以下、REM量は0.0020%以上0.0150%以下とする。
Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200%
Mg and REM are elements useful for controlling the form of inclusions and can be contained as necessary. The form control of inclusions means that the spread sulfide inclusions are made into particulate inclusions. The ductility, toughness and resistance to sulfide stress corrosion cracking are improved through the morphology control of the inclusions. In order to acquire such an effect, it is preferable to contain Ca and Mg by 0.0005% or more and REM by 0.0010% or more. On the other hand, when any of the elements is contained in a large amount, the amount of non-metallic inclusions may increase, and the ductility, the toughness, and the sulfide stress corrosion cracking resistance may decrease. In addition, it may be economically disadvantageous. Therefore, in the case of containing Mg, 0.0005 to 0.0050%, and in the case of containing REM, 0.0010% to 0.0200%. Preferably, the amount of Mg is 0.0010% to 0.0040%, and the amount of REM is 0.0020% to 0.0150%.
[組織]
オーステナイトを基地相とするミクロ組織
 鋼材の結晶構造が体心立方構造(bcc)である場合、該鋼材は低温環境下で脆性破壊を起こす可能性があるため、低温環境下での使用には適していない。ここに、低温環境下での使用を想定したとき、鋼材の基地相は、結晶構造が面心立方構造(fcc)であるオーステナイト組織であることが必須となる。ここで、「オーステナイトを基地相とする」とは、オーステナイト相が面積率で90%以上であることを意味する。オーステナイト相以外の残部は、フェライト相またはマルテンサイト相であるが、オーステナイト相が100%であってもよいのは勿論である。
[Organization]
Austenite-based microstructure When the crystal structure of a steel material is a body-centered cubic structure (bcc), the steel material may cause brittle fracture in a low temperature environment, so it is suitable for use in a low temperature environment Not. Here, assuming use under a low temperature environment, it is essential that the base phase of the steel material has an austenitic structure in which the crystal structure is a face-centered cubic structure (fcc). Here, “use austenite as a base phase” means that the austenite phase is 90% or more in area ratio. The balance other than the austenite phase is a ferrite phase or a martensite phase, but it goes without saying that the austenite phase may be 100%.
オーステナイト粒径:1μm以上
 高Mn鋼は、オーステナイトを基地相とする組織を有するため、極低温においても脆性破壊とならずに、破壊が生じる場合は結晶粒界から発生する。この破壊の起点となる結晶粒界の面積を低減することが高Mn鋼の耐破壊特性を向上するのに有利である。そのためには、オーステナイト粒径は1μm以上であることが肝要である。なぜなら、粒径が1μm未満となると、粒界面積の増加量が大きくなり破壊の発生箇所が増大するためである。好ましくは、2μm以上である
Austenite grain size: 1 μm or more Since a high Mn steel has a structure having austenite as a base phase, brittle fracture does not occur even at extremely low temperatures, and fracture occurs when it occurs from grain boundaries. It is advantageous to improve the fracture resistance of high Mn steels by reducing the area of grain boundaries that are the starting point of this fracture. For that purpose, it is important that the austenite grain size is 1 μm or more. This is because if the particle size is less than 1 μm, the amount of increase in the grain interface area becomes large, and the location of occurrence of breakage increases. Preferably, it is 2 μm or more
オーステナイトの標準偏差が9μm以下
 上記結晶粒径の規制に併せて整粒化を図ることが、高Mn鋼の耐破壊特性の更なる向上に有効である。すなわち、混粒組織となった場合、粗大な結晶粒から微細な結晶粒まで幅広い粒径分布となって1μm未満の結晶粒を含むようになり、とくに標準偏差が9μmを超えると、その傾向が顕著となるため、標準偏差が9μmを超える混粒組織は避ける必要がある。
The standard deviation of austenite is 9 μm or less. It is effective to further improve the fracture resistance of a high-Mn steel by achieving the particle size regulation in accordance with the regulation of the crystal grain size. That is, in the case of mixed grain structure, a broad grain size distribution from coarse grains to fine grains forms a broad grain size distribution, and grain sizes less than 1 μm are included, especially when the standard deviation exceeds 9 μm. Mixed grain structures having a standard deviation of more than 9 μm should be avoided as they become noticeable.
[製造方法]
 本発明に係る高Mn鋼を製造するに当たり、まず、鋼素材は、上記した成分組成を有する溶鋼を転炉や電気炉等、公知の溶製方法で溶製することができる。また、真空脱ガス炉にて2次精錬を行ってもよい。その際、好適な組織制御の妨げとなるTiおよびNbを上述の範囲に制限するために、原料などから不可避的に混入することを回避し、これらの含有量を低減する措置を取る必要がある。例えば、精錬段階におけるスラグの塩基度を下げることによって、これらの合金をスラグへ濃化させて排出し最終的なスラブ製品におけるTiおよびNbの濃度を低減する。また、酸素を吹き込んで酸化させ、還流時にTiおよびNbの合金を浮上分離させるなどの方法でも良い。その後、連続鋳造法、造塊法等、公知の鋳造方法により、所定寸法のスラブ等の鋼素材とすることが好ましい。なお、連続鋳造後のスラブに分塊圧延を行って鋼素材としてもよい。
[Production method]
In producing the high-Mn steel according to the present invention, first, a steel material can be melted and manufactured using a known melting method, such as a converter or an electric furnace, of a molten steel having the above-described component composition. Further, secondary refining may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which would interfere with favorable structure control, to the above-mentioned range, it is necessary to avoid the inevitable mixing from raw materials etc. and to take measures to reduce their content. . For example, by lowering the basicity of the slag in the refining stage, these alloys are concentrated into slag and discharged to reduce the concentration of Ti and Nb in the final slab product. Alternatively, oxygen may be blown to oxidize, and an alloy of Ti and Nb may be floated and separated at the time of reflux. Then, it is preferable to set it as steel materials, such as a slab of a predetermined | prescribed dimension, by well-known casting methods, such as a continuous casting method and an ingot making method. Slab rolling may be performed on the slab after continuous casting to obtain a steel material.
 さらに、上記鋼素材を低温靭性に優れた鋼材へと造りこむための製造条件について規定する。
鋼素材加熱温度:1100℃以上1300℃以下
 鋼材のミクロ組織の結晶粒径を粗大にするために、熱間圧延前の加熱温度は1100℃以上とする。ただし、1300℃を超えると一部溶解が始まってしまう懸念があるため、加熱温度の上限は1300℃とする。ここでの温度制御は、鋼素材の表面温度を基準とする。
Furthermore, the manufacturing conditions for forming the above-mentioned steel material into a steel material excellent in low temperature toughness are specified.
Steel material heating temperature: 1100 ° C. or more and 1300 ° C. or less In order to make the grain size of the microstructure of the steel material coarse, the heating temperature before hot rolling is set to 1100 ° C. or more. However, since there is a concern that melting may partially start when the temperature exceeds 1300 ° C., the upper limit of the heating temperature is 1300 ° C. The temperature control here is based on the surface temperature of the steel material.
仕上圧延終了温度:750℃以上950℃未満
 鋼素材(鋼塊または鋼片)を加熱したのち、熱間圧延を行う。粗大な結晶粒を作りこむためには高温での累積圧下率を高めることが好ましい。すなわち、低温で熱間圧延を行うとミクロ組織は微細になり、また過度な加工ひずみが入るため低温靭性の低下を招く。そのため仕上圧延終了温度の下限は750℃とする。一方、950℃以上の温度領域で仕上げると、結晶粒径が過度に粗大となり所望の降伏強度が得られなくなる。そのため950℃未満で1パス以上の最終仕上圧延が必要である。好ましくは、900℃以下である。
Finish rolling finish temperature: 750 ° C. or more and less than 950 ° C. After heating a steel material (steel ingot or billet), hot rolling is performed. In order to form coarse crystal grains, it is preferable to increase the cumulative rolling reduction at high temperature. That is, when hot rolling is performed at a low temperature, the microstructure becomes finer, and excessive processing strain is introduced, resulting in a decrease in low temperature toughness. Therefore, the lower limit of the finish rolling end temperature is set to 750 ° C. On the other hand, when finishing in a temperature range of 950 ° C. or higher, the crystal grain size becomes excessively large, and the desired yield strength can not be obtained. Therefore, one or more final finishing rolling is required at less than 950 ° C. Preferably, it is 900 ° C. or less.
1パスでの平均圧下率:9%以上
 前記の熱間圧延に際して、オーステナイト粒径の整粒化を図り、かつ1μm以上の結晶粒径に制御するには、オーステナイトの再結晶を促進することが有効であり、熱間圧延時の1パス当たりの平均圧下率を9%以上とすることが重要となる。好ましくは11%以上である。
Average rolling reduction in one pass: 9% or more In order to achieve grain refinement of the austenite grain size and control to a grain size of 1 μm or more in the above-mentioned hot rolling, it is possible to promote austenite recrystallization. It is effective that it is important that the average rolling reduction per pass during hot rolling be 9% or more. Preferably it is 11% or more.
(仕上圧延終了温度-100℃)以上の温度から300℃以上650℃以下の温度域までの平均冷却速度:1.0℃/s以上
 熱間圧延終了後は速やかに冷却を行う。熱間圧延後の鋼板を緩やかに冷却させると析出物の生成が促進され低温靭性の劣化を招く。1.0℃/s以上の冷却速度で冷却することでこれら析出物の生成を抑制できる。また、過度な冷却を行うと鋼板が歪んでしまい、生産性を低下させる。そのため、冷却開始温度の上限は900℃とする。以上の理由から、熱間圧延後の冷却は、(仕上圧延終了温度-100℃)以上の温度から300℃以上650℃以下の温度域までの鋼板表面の平均冷却速度を1.0℃/s以上とする。一方、工業的生産の観点からは、前記平均冷却速度を200℃/s以下とすることが好ましい。
Average cooling rate from a temperature of (finish rolling finish temperature -100 ° C.) or more to a temperature range of 300 ° C. or more and 650 ° C. or less: 1.0 ° C./s or more After the end of hot rolling, cooling is performed promptly. When the steel sheet after hot rolling is gently cooled, the formation of precipitates is promoted and the low temperature toughness is deteriorated. The formation of these precipitates can be suppressed by cooling at a cooling rate of 1.0 ° C./s or more. In addition, excessive cooling may distort the steel sheet and reduce productivity. Therefore, the upper limit of the cooling start temperature is 900 ° C. From the above reasons, the cooling after hot rolling is the average cooling rate of the steel sheet surface from the temperature of (finish rolling finish temperature -100 ° C) or more to the temperature range of 300 ° C or more and 650 ° C or less to 1.0 ° C / s. And above. On the other hand, in terms of industrial production, it is preferable to set the average cooling rate to 200 ° C./s or less.
 以下、本発明を実施例により詳細に説明する。なお、本発明は以下の実施例に限定されない。
 転炉-取鍋精錬-連続鋳造法にて、表1に示す成分組成になる鋼スラブを作製した。次いで、得られた鋼スラブを表2に示す条件で分塊圧延および熱間圧延により10~30mm厚の鋼板とした。得られた鋼板について、引張特性、靭性および組織評価を下記の要領で実施した。
Hereinafter, the present invention will be described in detail by way of examples. The present invention is not limited to the following examples.
A steel slab having the composition shown in Table 1 was produced by a converter-ladle refining-continuous casting method. Then, the obtained steel slab was made into a steel plate with a thickness of 10 to 30 mm by slab rolling and hot rolling under the conditions shown in Table 2. The tensile properties, toughness and structure evaluations were carried out on the obtained steel sheets in the following manner.
(1)引張試験特性
 得られた各鋼板より、JIS5号引張試験片を採取し、JIS Z2241(1998年)の規定に準拠して引張試験を実施し、引張試験特性を調査した。本発明では、降伏強度400MPa以上および引張強度800MPa以上を引張特性に優れるものと判定した。さらに、伸び40%以上を延性に優れるものと判定した。
(1) Tensile test characteristics From each steel plate thus obtained, a JIS No. 5 tensile test specimen was collected, and a tensile test was conducted in accordance with the provisions of JIS Z 2241 (1998) to investigate the tensile test characteristics. In the present invention, it was determined that a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more are excellent in tensile properties. Furthermore, it was determined that the elongation of 40% or more was excellent in ductility.
(2)低温靭性
 板厚20mmを超える各鋼板の表面から板厚の1/4までの位置(以下、板厚1/4位置と示す)、もしくは板厚20mm以下の各鋼板の板厚の1/2までの位置(以下、板厚1/2位置と示す)の圧延方向と平行な方向から、JIS Z2202(1998年)の規定に準拠してシャルピーVノッチ試験片を採取し、JIS Z2242(1998年)の規定に準拠して各鋼板について3本のシャルピー衝撃試験を実施し、-196℃での吸収エネルギーを求め、母材靭性を評価した。本発明では、3本の吸収エネルギー(vE-196)の平均値が100J以上を母材靭性に優れるものとした。
(2) Low-temperature toughness Position from the surface of each steel plate exceeding 20 mm to 1/4 of the plate thickness (hereinafter referred to as 1/4 position of plate thickness), or 1 of the plate thickness of each steel plate having a plate thickness of 20 mm or less Charpy V-notch test pieces are collected from a direction parallel to the rolling direction at a position of up to 2 (hereinafter referred to as a plate thickness 1/2 position) in accordance with the provisions of JIS Z 2202 (1998). Three Charpy impact tests were carried out on each steel plate in accordance with the regulations of 1998), the absorbed energy at -196 ° C. was determined, and the base material toughness was evaluated. In the present invention, the average value of three absorbed energy (vE-196) is 100 J or more, which is excellent in the base material toughness.
(3)CTOD値の評価
 鋼板の板厚1/2位置の圧延方向と平行な方向からCTOD試験片を採取し、-165℃で2~3本の試験を行い、その平均値で評価した。本発明では、CTOD値が0.25mm以上を耐破壊特性に優れるものとした。
(3) Evaluation of CTOD value CTOD test pieces were collected from a direction parallel to the rolling direction at a thickness 1/2 position of a steel plate, and two to three tests were conducted at -165 ° C, and the average value was evaluated. In the present invention, a CTOD value of 0.25 mm or more is excellent in fracture resistance.
(4)組織評価
 鋼板の板厚1/4位置のL断面について、EBSD(Electron Backscatter Diffraction)解析により、200μm×200μmの視野を任意の2~3視野観察し、各視野内のオーステナイト結晶粒径の最小値を測定した。また、オーステナイト粒径の標準偏差は、前記のEBSP解析結果を用いて、各結晶粒径の面積割合の分布から評価した。上記で得られた全ての結晶粒径を母集団とし、それぞれの個値と平均値との差の2乗和である分散を求め、その分散の平方根を取って標準偏差を求めた。
 以上により得られた評価結果を、表3に示す。
(4) Evaluation of structure With respect to the L cross section at a 1/4 thickness position of the steel plate, EBSD (Electron Backscatter Diffraction) analysis observes a field of 200 μm × 200 μm for any two or three fields of view, and The minimum value of was measured. Moreover, the standard deviation of the austenite grain size was evaluated from the distribution of the area ratio of each crystal grain size using the above-mentioned EBSP analysis result. Taking all the crystal grain sizes obtained above as a population, the variance which is the sum of squares of the difference between each individual value and the mean value was determined, and the square root of the variance was taken to determine the standard deviation.
The evaluation results obtained by the above are shown in Table 3.
 本発明に従う高Mn鋼は、上述の目標性能(母材の降伏強度が400MPa以上、低温靭性が吸収エネルギー(vE-196)の平均値で100J以上、CTOD値の平均値で0.25mm以上)を満足することが確認された。一方、本発明の範囲を外れる比較例は、降伏強度および低温靭性、CTOD値のいずれか1つ以上が、上述の目標性能を満足できていない。 The high-Mn steel according to the present invention has the above-described target performance (yield strength of the base material is 400 MPa or more, low temperature toughness is 100 J or more in average of absorbed energy (vE-196), average CTOD value is 0.25 mm or more) It was confirmed to be satisfactory. On the other hand, in Comparative Examples outside the scope of the present invention, any one or more of the yield strength, the low temperature toughness, and the CTOD value can not satisfy the above-described target performance.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003

Claims (3)

  1.  質量%で、
     C:0.10%以上0.70%以下、
     Si:0.05%以上0.50%以下、
     Mn:20%以上30%以下、
     P:0.030%以下、
     S:0.0070%以下、
     Al:0.01%以上0.07%以下、
     Cr:0.5%以上7.0%以下、
     Ni:0.01%以上0.1%未満、
     Ca:0.0005%以上0.0050%以下、
     N:0.0050%以上0.0500%以下、
     O:0.0050%以下、
     Ti:0.0050%未満および
     Nb:0.0050%未満
    を含有し、残部がFeおよび不可避的不純物の成分組成と、オーステナイトを基地相とする組織とを有し、前記オーステナイトは、粒径が1μm以上かつ標準偏差が9μm以下である高Mn鋼。
    In mass%,
    C: 0.10% or more and 0.70% or less,
    Si: 0.05% or more and 0.50% or less,
    Mn: 20% or more and 30% or less,
    P: 0.030% or less,
    S: 0.0070% or less,
    Al: 0.01% or more and 0.07% or less,
    Cr: 0.5% or more and 7.0% or less,
    Ni: 0.01% or more and less than 0.1%,
    Ca: 0.0005% or more and 0.0050% or less,
    N: 0.0050% or more and 0.0050% or less,
    O: less than 0.0050%,
    Ti: less than 0.0050% and Nb: less than 0.0050%, and the balance has a component composition of Fe and unavoidable impurities, and a structure having austenite as a base phase, and the austenite has a particle size of High Mn steel having a diameter of 1 μm or more and a standard deviation of 9 μm or less.
  2.  前記成分組成は、さらに、質量%で、
     Cu:1.0%以下、
     Mo:2.0%以下、
     V:2.0%以下、
     W:2.0%以下、
     Mg:0.0005%以上0.0050%以下および
     REM:0.0010%以上0.0200%以下
    のうちから選ばれる1種または2種以上を含有する請求項1に記載の高Mn鋼。
    The above component composition is, further, in mass%,
    Cu: 1.0% or less,
    Mo: 2.0% or less,
    V: 2.0% or less,
    W: 2.0% or less,
    The high Mn steel according to claim 1, containing one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.
  3.  請求項1または2に記載の成分組成を有する鋼素材を1100℃以上1300℃以下の温度域に加熱した後、仕上圧延終了温度が750℃以上950℃未満かつ1パス当たりの平均圧下率が9%以上である、熱間圧延を施し、その後、(仕上圧延終了温度-100℃)以上の温度から300℃以上650℃以下の温度域までの平均冷却速度が1.0℃/s以上の冷却処理を行う高Mn鋼の製造方法。 A steel material having the component composition according to claim 1 or 2 is heated to a temperature range of 1100 ° C. or more and 1300 ° C. or less, and a finish rolling finish temperature is 750 ° C. or more and less than 950 ° C. and an average rolling reduction ratio per pass of 9 %, Hot rolling is applied, and then the average cooling rate is 1.0 ° C./s or more from the temperature of (finish rolling finish temperature -100 ° C.) or more to the temperature range of 300 ° C. or more and 650 ° C. or less Manufacturing method of high Mn steel which performs processing.
PCT/JP2018/044941 2017-12-07 2018-12-06 High-mn steel and method for manufacturing same WO2019112012A1 (en)

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