WO2018105510A1 - High mn steel sheet and method for producing same - Google Patents

High mn steel sheet and method for producing same Download PDF

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Publication number
WO2018105510A1
WO2018105510A1 PCT/JP2017/043245 JP2017043245W WO2018105510A1 WO 2018105510 A1 WO2018105510 A1 WO 2018105510A1 JP 2017043245 W JP2017043245 W JP 2017043245W WO 2018105510 A1 WO2018105510 A1 WO 2018105510A1
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Prior art keywords
steel sheet
temperature
steel
less
austenite
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PCT/JP2017/043245
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French (fr)
Japanese (ja)
Inventor
植田　圭治
長谷　和邦
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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 CN201780075814.3A priority Critical patent/CN110050082B/en
Priority to KR1020217017558A priority patent/KR102309644B1/en
Priority to EP17879107.5A priority patent/EP3553195B1/en
Priority to JP2018512637A priority patent/JP6418358B1/en
Priority to KR1020197015408A priority patent/KR20190077470A/en
Priority to BR112019010870-0A priority patent/BR112019010870B1/en
Publication of WO2018105510A1 publication Critical patent/WO2018105510A1/en
Priority to PH12019501270A priority patent/PH12019501270A1/en

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    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • the present invention relates to a high Mn steel sheet suitable for structural steel used in a cryogenic environment, such as a tank for a liquefied gas storage tank, and particularly excellent in stress corrosion cracking resistance in a salt water corrosive environment and a method for producing the same. .
  • Patent Document 1 discloses that when Mn is 15 to 35%, Cu is 5% or less, and C and Cr are added in appropriate amounts, the machinability and Charpy impact characteristics at ⁇ 196 ° C. of the heat and heat affected zone are shown. A steel material with improved is disclosed.
  • Patent Document 2 C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, Ni: 0.1% or more and 7.0 %, And Cr: 0.1% or more and less than 8.0% is added, and a high Mn steel material with improved low temperature toughness is disclosed.
  • Patent Documents 1 and 2 are intended to have strength and low-temperature toughness, and the Charpy impact characteristics at ⁇ 196 ° C. in the heat and heat affected zone are 60 to 135 J ( Only Patent Document 1 is displayed).
  • the cryogenic toughness of the base material is still insufficient, and the cryogenic toughness and the stress corrosion cracking resistance have not been achieved at the same time.
  • the present invention has been made in view of the above problems, and an object thereof is to provide a high-Mn steel sheet excellent in cryogenic toughness and stress corrosion cracking resistance and a method for producing the same.
  • the present inventors have conducted intensive research on various factors that determine the component composition, manufacturing method, and microstructure of a steel sheet for ensuring excellent stress corrosion cracking resistance performance for high-Mn steel sheets. The following findings were obtained.
  • Nb, V, Ti carbides, nitrides, and composite carbonitrides in steel sheets can be further improved in stress corrosion cracking resistance by properly managing their dispersion state. it can.
  • the carbides, nitrides, and composite carbonitrides of Nb, V, and Ti act as diffusible hydrogen trap sites in the steel sheet. That is, it acts as a trap site for diffusible hydrogen generated by the corrosion reaction of the steel material, and has the effect of suppressing stress corrosion cracking.
  • the heating, rolling, and cooling conditions in the hot rolling process affect the dispersion state of Nb, V, and Ti carbides, nitrides, and composite carbonitrides in austenite. Therefore, it is important to manage these manufacturing conditions.
  • P is an element that easily co-segregates with Mn in the solidification process of the steel slab, and lowers the grain boundary strength that intersects the micro-segregation part. Therefore, it is necessary to reduce impurity elements such as P.
  • the present invention has been made by further studying the above knowledge, and the gist thereof is as follows.
  • C 0.20 to 0.70%
  • Si 0.05 to 1.0%
  • Mn 15 to 30%
  • P 0.028% or less
  • S 0.02%
  • Al 0.01 to 0.1%
  • Cr 0.5 to 7.0%
  • Ni 0.03 to 0.30%
  • N 0.0010 to 0.0200%
  • Nb 0.003 to 0.030%
  • V 0.03 to 0.10%
  • Ti 0.003 to 0.040%, or one or more of them
  • It has a component composition, and a microstructure of 0.5 mm below the surface of the steel sheet has austenite as a base phase, of which austenite has an area ratio of 25% or more, a circle equivalent diameter of 10 ⁇ m or more, and a major axis and a minor axis.
  • a high Mn steel sheet having an aspect ratio of 3 or more.
  • the microstructure further includes carbides, nitrides, and carbonitrides containing one or more of Nb, V, and Ti having an equivalent circle diameter of 0.01 to 0.5 ⁇ m in the microstructure.
  • T Nb (° C.) 7500 / ⁇ 3.0 ⁇ log 10 ([% Nb] ⁇ [% C]) ⁇ ⁇ 273 (1)
  • T V (° C.) 10800 / ⁇ 7.2-log 10 ([% V] ⁇ [% C]) ⁇ ⁇ 273 (2)
  • T Ti (° C.) 7000 / ⁇ 2.8 ⁇ log 10 ([% Ti] ⁇ [% C]) ⁇ ⁇ 273 (3)
  • [% Nb], [% V], [% Ti] and [% C] indicate the contents (mass%) of Nb, V, Ti and C in the steel, respectively. In the case of an element not included, the element symbol in the formula is calculated as 0.
  • “high strength” means a material having a yield strength of 400 MPa or more.
  • “very low temperature toughness” means low temperature toughness, that is, the absorbed energy vE ⁇ 196 of the Charpy impact test at ⁇ 196 ° C. is 50 J or more.
  • “excellent in stress corrosion cracking resistance” is a test in accordance with the Narrow Standard TM0111-2011 standard Slow Strain Rate Test Method, which is artificial seawater (chloride ion concentration) at a temperature of 23 ° C. immersed in 18000ppm), strain rate:. when performing a constant velocity tensile test at 4 ⁇ 10 -7 inch / sec, refers to breaking stress is more than 500 MPa.
  • a high Mn steel sheet having excellent cryogenic toughness and stress corrosion cracking resistance can be obtained.
  • the high Mn steel plate of this invention contributes greatly to the improvement of the safety
  • C 0.20 to 0.70%
  • carbonized_material and Nb, V Ti type carbide
  • C is 0.25% or more.
  • C is 0.60% or less. More preferably, C is 0.30% or more. More preferably, C is 0.55% or less.
  • Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of increasing the strength of the steel sheet by solid solution and solid solution strengthening. In order to acquire such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.0%, weldability will deteriorate. It also affects the SCC resistance. For this reason, Si is made 0.05 to 1.0%. Preferably, Si is 0.07% or more. Preferably, Si is 0.50% or less. More preferably, Si is 0.15% or more. More preferably, Si is 0.45% or less.
  • Mn 15-30% Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to acquire the effect, Mn needs to contain 15% or more. On the other hand, even if the content exceeds 30%, the effect of improving the cryogenic toughness is saturated, leading to an increase in alloy cost. In addition, the weldability and cutability are deteriorated. Furthermore, segregation is promoted and stress corrosion cracking is promoted. Therefore, Mn is set to 15 to 30%. Preferably, Mn is 18% or more. Preferably, Mn is 28% or less. More preferably, Mn is 20% or more. More preferably, Mn is 27% or less.
  • P 0.028% or less
  • P 0.028% or less
  • P is 0.005% or more.
  • P is 0.024% or less.
  • S 0.02% or less Since S deteriorates the low-temperature toughness and ductility of the base material, 0.02% is the upper limit and it is desirable to reduce it as much as possible. Therefore, S is set to 0.02% or less. In addition, since excessive S reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.001% or more. Preferably, S is 0.002% or more. Preferably, S is 0.018% or less. More preferably, S is 0.010% or less.
  • Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheets. Moreover, it has the effect which suppresses the coarsening of a crystal grain by fixing the solid solution N in steel and forming AlN. At the same time, it has the effect of suppressing toughness deterioration due to the reduction of solute N. In order to acquire such an effect, Al needs to contain 0.01% or more. On the other hand, if Al is contained in an amount exceeding 0.1%, it is mixed into the weld metal part during welding and deteriorates the toughness of the weld metal. For this reason, Al is made 0.01 to 0.1%. Preferably, Al is 0.02% or more. Preferably, Al is 0.07% or less.
  • Cr 0.5 to 7.0% Cr is an element that stabilizes austenite by addition of an appropriate amount and is effective in improving cryogenic toughness and base material strength. Further, in the present invention, it is an important element that improves the stress corrosion cracking resistance by reducing the amount of hydrogen penetration into the steel sheet through the effect of densifying rust generated on the surface of the base material in a salt water environment. . In order to acquire such an effect, Cr needs to contain 0.5% or more. On the other hand, if the content exceeds 7.0%, the low temperature toughness and stress corrosion cracking resistance decrease due to the formation of Cr carbide. Therefore, Cr is 0.5 to 7.0%. Cr is preferably 1.0% or more, more preferably 1.2% or more, and further preferably 2.5% or more. Cr is preferably 6.0% or less, more preferably 5.7% or less, and still more preferably 5.5% or less.
  • Ni 0.03-0.30%
  • Ni is a typical austenite stabilizing element, and is an element effective for improving cryogenic toughness and base metal strength. In the present invention, it is an important element that improves the stress corrosion cracking resistance by reducing the amount of hydrogen entering the steel sheet through the effect of densifying the rust generated on the surface of the base material in a salt water environment. . In order to acquire such an effect, Ni needs to contain 0.03% or more. On the other hand, when the content exceeds 0.30%, the alloy cost increases and the effect of improving the stress corrosion cracking resistance is saturated. For this reason, Ni is made 0.03 to 0.30%.
  • Ni is 0.25% or less.
  • it is 0.04% or more. More preferably, Ni is 0.23% or less. More preferably, Ni is 0.05% or more. More preferably, Ni is 0.21% or less.
  • N 0.0010 to 0.0200%
  • N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. Moreover, it combines with Nb, V, and Ti, precipitates as nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a diffusible hydrogen trap site. In order to acquire such an effect, N needs to contain 0.0010% or more. On the other hand, if the content exceeds 0.0200%, the nitride or carbonitride becomes coarse and the toughness decreases. For this reason, N is made 0.0010 to 0.0200%.
  • N is 0.0020% or more.
  • N is 0.0150% or less. More preferably, N is 0.0030% or more. More preferably, N is 0.0170% or less.
  • Nb 0.003 to 0.030%
  • V 0.03 to 0.10%
  • Ti 0.003 to 0.040%
  • Nb is an element that precipitates as carbonitride (including carbide), and the produced carbonitride is effective at the trap site of diffusible hydrogen and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, Nb needs to contain 0.003% or more.
  • Nb exceeds 0.030%, coarse carbonitride precipitates, which may be the starting point of destruction. Further, the precipitates may become coarse and the base material toughness may be deteriorated. Therefore, when Nb is contained, the content is made 0.003 to 0.030%.
  • Nb is preferably 0.005% or more, more preferably 0.007% or more.
  • Nb is preferably 0.025% or less, more preferably 0.022% or less.
  • V 0.03-0.10%
  • V is an element which precipitates as carbonitride and the produced carbonitride is effective at the trap site of diffusible hydrogen and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, V needs to contain 0.03% or more.
  • V exceeds 0.10%, coarse carbonitride precipitates and may become a starting point of fracture. Further, the precipitates may become coarse and the base material toughness may be deteriorated. Therefore, when V is contained, the content is made 0.03 to 0.10%.
  • V is preferably 0.04% or more, more preferably 0.05% or more.
  • V is preferably 0.09% or less, more preferably 0.08% or less, and still more preferably 0.07% or less.
  • Ti 0.003-0.040%
  • Ti precipitates as nitride or carbonitride, and the generated nitride or carbonitride is an element effective for diffusible hydrogen trap sites and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, Ti needs to contain 0.003% or more.
  • Ti is contained in an amount exceeding 0.040%, the precipitates are coarsened and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and become the starting point of fracture. Therefore, when Ti is contained, the content is made 0.003 to 0.040%.
  • Ti is preferably 0.005% or more, more preferably 0.007% or more.
  • Ti is preferably 0.035% or less, more preferably 0.032% or less.
  • the balance is iron and inevitable impurities.
  • Inevitable impurities include O and H, and a total of 0.01% or less is acceptable.
  • O and S are preferably defined as follows.
  • O 0.0005 to 0.0070% If O is contained in an amount exceeding 0.0070%, it forms coarse inclusions with Al and lowers the low temperature toughness. Therefore, the upper limit of O is 0.0070%, and it is desirable to reduce it as much as possible. Preferably, O is 0.0060% or less. In addition, excessive O reduction increases the refining cost and is economically disadvantageous, so it is 0.0005% or more. Preferably, O is 0.0008% or more.
  • O / S ⁇ 1 The balance of O and S forms oxides, sulfides, and composite precipitates thereof with Al, Ti, and Mn, and effectively acts as a trapping site for diffusible hydrogen to improve stress corrosion cracking. In order to obtain this effect, O / S ⁇ 1.
  • O / S ⁇ 1 When O / S ⁇ 1, coarse oxysulfides are formed, and the low-temperature toughness may be lowered. Therefore, in the present invention, O / S ⁇ 1 is set to ensure low temperature toughness.
  • the target characteristics of the present invention can be obtained.
  • the following elements can be contained as required in addition to the above essential elements.
  • Mo 0.05 to 2.0%
  • W 0.05 to 2.0%
  • Mo 0.05 to 2.0%
  • Mo is an element useful for increasing the strength of the base material, and can be contained as necessary.
  • Mo preferably contains 0.05% or more.
  • Mo is preferably made 2.0% or less.
  • Mo is 0.07% or more.
  • Mo is 1.7% or less.
  • W 0.05-2.0% W is an element useful for increasing the strength of the base material, and can be contained as necessary. In order to obtain such an effect, W preferably contains 0.05% or more. On the other hand, if the content exceeds 2.0%, the toughness and weld crack resistance may be adversely affected, so W is preferably set to 2.0% or less. Therefore, when W is contained, the content is made 0.05 to 2.0%. More preferably, the content is 0.07% or more. More preferably, it is 1.5% or less.
  • Ca 0.0005 to 0.0050%
  • Mg 0.0005 to 0.0050%
  • REM 0.0010 to 0.0200%
  • Ca 0.0005 to 0.0050%
  • the inclusion shape control means that the expanded sulfide inclusion is a granular inclusion. Ductility, toughness, and resistance to sulfide stress corrosion cracking are improved through shape control of the inclusions.
  • Ca preferably contains 0.0005% or more.
  • the content is made 0.0005 to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.
  • Mg 0.0005 to 0.0050%
  • Mg is useful as an element that contributes to the improvement of resistance to sulfide stress corrosion cracking, and can be contained if necessary.
  • Mg preferably contains 0.0005% or more.
  • the content is made 0.0005 to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.
  • REM 0.0010 to 0.0200% REM is useful as an element that contributes to the improvement of resistance to sulfide stress corrosion cracking, and can be contained as necessary.
  • REM preferably contains 0.0010% or more.
  • the content is 0.0020% or more. More preferably, it is 0.0150% or less.
  • the microstructure of 0.5 mm below the surface of the steel sheet has austenite as a base phase, and the area ratio of the austenite is 25% or more, the equivalent circle diameter is 10 ⁇ m or more, and the aspect ratio of major axis and minor axis is 3 or more.
  • the base phase having a microstructure of 0.5 mm below the surface of the steel sheet is austenite.
  • the grain boundary in the vicinity of the steel sheet surface layer is obtained by having an area ratio of 25% or more of austenite having an equivalent circle diameter of 10 ⁇ m or more and an aspect ratio of major axis to minor axis of 3 or more.
  • the deformation zone in the crystal grains effectively acts as a diffusible hydrogen trap site and effectively acts on the stress corrosion cracking property. Thereby, suppression of stress corrosion cracking can be remarkably improved.
  • the yield strength is improved.
  • the area ratio is 30% or more.
  • the area ratio exceeds 95%, the strength of the steel material becomes excessive, and the base material toughness may be deteriorated.
  • it is 95% or less, More preferably, you may be 94% or less. More preferably, it is 90% or less. More preferably, it is 85% or less.
  • the equivalent circle diameter is less than 10 ⁇ m, or the aspect ratio of the major axis to the minor axis is less than 3, the desired yield strength cannot be obtained, and a deformation band within the crystal grains that effectively acts as a trapping site for diffusible hydrogen can be obtained. Therefore, the stress corrosion cracking property is lowered, and the above-described effects cannot be obtained.
  • the circle equivalent diameter of the above-mentioned austenite, an area ratio, and an aspect ratio can be measured by the method as described in the Example mentioned later.
  • 0.5 mm below the surface of the steel sheet means a cross section parallel to the rolling direction at a position 0.5 mm in the thickness direction from the front and back surfaces of the steel sheet.
  • 0.5 mm below the surface of the steel sheet means that the above-mentioned micro is formed in a cross section parallel to the rolling direction in any of ⁇ 5% from the position of 0.5 mm from the front and back surfaces of the steel sheet in the thickness direction.
  • the microstructure 0.5 mm below the surface of the steel sheet further includes carbides and nitrides containing one or more of Nb, V, and Ti having an equivalent circle diameter of 0.01 to 0.5 ⁇ m in the structure. And a total of 2 ⁇ 10 2 pieces / mm 2 or more of carbonitrides Carbide containing one or more of Nb, V, Ti in the microstructure at 0.5 mm below the surface of the steel sheet of the present invention.
  • Nb, V, and Ti-based precipitates The existence state of nitrides and carbonitrides (hereinafter referred to as Nb, V, and Ti-based precipitates) will be described.
  • Carbides, nitrides, and carbonitrides containing one or more of Nb, V, and Ti are carbides containing one or more of Nb, V, and Ti, Nb, V, A nitride containing one or more of Ti, and a carbonitride containing one or more of Nb, V, and Ti.
  • the particle diameter of the Nb, V, and Ti-based precipitates is 0.01 to 0.5 ⁇ m in terms of equivalent circle diameter.
  • the thickness is less than 0.01 ⁇ m, the effect of suppressing hydrogen embrittlement cracking as a diffusible hydrogen trap site is saturated.
  • a manufacturing load will increase extremely and manufacturing cost will rise.
  • it exceeds 0.5 ⁇ m the low temperature toughness is lowered.
  • the effect of suppressing hydrogen embrittlement cracking as a trapping site for diffusible hydrogen cannot be obtained.
  • it is 0.03 ⁇ m or more.
  • it is 0.4 micrometer or less.
  • the total of the Nb, V, and Ti-based precipitates having the above particle diameter is less than 2 ⁇ 10 2 pieces / mm 2 in a microstructure of 0.5 mm below the surface of the steel sheet, it acts as a diffusible hydrogen trap site. Since the precipitates are insufficient, the effect of suppressing hydrogen embrittlement cracking as a diffusible hydrogen trap site cannot be obtained. For this reason, it is set to 2 ⁇ 10 2 pieces / mm 2 or more. Preferably, it is 5 ⁇ 10 2 pieces / mm 2 or more.
  • the number density and equivalent circle diameter of the Nb, V, and Ti-based precipitates described above can be measured by the method described in the examples described later.
  • the low temperature toughness decreases. For this reason, austenite shall be 90% or more.
  • the area ratio of the structure such as martensite is small.
  • the structures such as martensite are martensite, bainite, ferrite, and pearlite.
  • the total area ratio of each structure with respect to the entire steel sheet is preferably 10% or less.
  • the steel sheet is manufactured by hot rolling at a temperature of 1000 ° C. or less and then the average cooling rate on the surface of the steel plate from the lower one of (finishing finish temperature ⁇ 50 ° C.) or the cooling start temperature to 650 ° C. is 1.0. It can be obtained by cooling at a temperature of ° C / s or higher.
  • the “° C.” display relating to the temperature means the temperature on the surface of the steel plate or the surface of the steel material.
  • the high Mn steel sheet according to the present invention can be produced by melting a molten steel having the above-described composition by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, a steel material such as a slab having a predetermined size is preferably formed by a known casting method such as a continuous casting method or an ingot-bundling rolling method.
  • Tx (° C) defined by the formulas (1) to (3) and is heated to a temperature range of (Tx-50) ° C to (Tx + 200) ° C.
  • Nb (°C) 7500 / ⁇ 3.0-log 10 ([% Nb] ⁇ [% C]) ⁇ - 273 ⁇ (1)
  • T V (° C.) 10800 / ⁇ 7.2-log 10 ([% V] ⁇ [% C]) ⁇ ⁇ 273 (2)
  • T Ti (° C.) 7000 / ⁇ 2.8 ⁇ log 10 ([% Ti] ⁇ [% C]) ⁇ ⁇ 273 (3)
  • [% Nb], [% V], [% Ti] and [% C] indicate the contents (mass%) of Nb, V, Ti and C in the steel, respectively. In the case of an element not included, the element symbol in the formula is calculated as 0.
  • the heating temperature is less than (Tx-50) ° C.
  • the deformation resistance in hot rolling becomes high and the amount of reduction per pass cannot be made large, so the number of rolling passes increases and the rolling efficiency decreases.
  • a casting defect in a steel material (slab) cannot be crimped.
  • the crystallized material containing Nb, V and Ti that has been crystallized non-uniformly in the steel in the melting stage remains in the steel plate after the rolling, and a precipitate containing the desired Nb, V and Ti is obtained.
  • the stress corrosion cracking resistance decreases.
  • the heating temperature exceeds (Tx + 200) ° C.
  • Tx + 200 surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases.
  • the surface of the steel material is excessively decarburized, the surface of the steel sheet after rolling becomes martensite, and bendability and hydrogen embrittlement are reduced.
  • the target microstructure cannot be obtained due to coarsening of austenite grains.
  • the heating temperature of the steel material is set to (Tx-50) ° C. or higher and (Tx + 200) ° C. or lower.
  • the temperature is (Tx-30) ° C. or higher.
  • it is set to (Tx + 180) ° C. or lower.
  • hot rolling is started when the steel material is (Tx-50) ° C. or higher and (Tx + 200) ° C. or lower.
  • any of Tx (° C.) defined by the formulas (1) to (3) “When heated to a temperature range where the surface temperature of the steel material is (Tx ⁇ 50) ° C. or higher and (Tx + 200) ° C. or lower” with two or more of Nb and V as the above component composition, for example.
  • the heating temperature should satisfy at least one of (T Nb ⁇ 50) ° C. or more and (T Nb +200) ° C. or (T V ⁇ 50) ° C. or more and (T V +200) ° C. or less. Means. That is, either heating temperature may be selected.
  • the finish rolling finish temperature in finish rolling is set to 750 ° C. or more and 1000 ° C. or less to obtain a steel plate having a desired thickness.
  • the finish rolling finish temperature of hot rolling exceeds 1000 ° C., the steel plate surface The recrystallization of the nearby austenite proceeds easily, the desired microstructure cannot be obtained, and the stress corrosion cracking resistance is lowered.
  • the finish rolling finish temperature is less than 750 ° C., the hot deformation resistance becomes excessively high, and the load on the rolling mill increases. In addition, the rolling efficiency is reduced, and the manufacturing cost is increased.
  • the finish rolling finish temperature of hot rolling shall be 750 degreeC or more and 1000 degrees C or less.
  • the temperature is 800 ° C. or higher.
  • the temperature is preferably 950 ° C. or lower. More preferably, it is set to 940 ° C. or lower.
  • Cumulative rolling reduction in the temperature range of 850 ° C. or higher (Tx ⁇ 50) ° C. or lower in finish rolling 10% to 50% (preferred condition) If the cumulative rolling reduction in the temperature range of 850 ° C. or higher (Tx-50) ° C. or lower is less than 10%, the target microstructure may not be obtained. On the other hand, if it exceeds 50%, the efficiency during rolling decreases. Moreover, there exists a possibility that intensity may become excessive and low-temperature toughness may fall.
  • the cumulative reduction ratio is the sum of the reduction ratios in each rolling pass in the temperature range of 850 ° C. or more (Tx ⁇ 50) ° C. in finish rolling.
  • Cumulative rolling reduction of 5% to 60% (more suitable conditions) in the non-recrystallization temperature range (960 ° C. or less) in finish rolling If the cumulative rolling reduction in the non-recrystallization temperature range is less than 5%, the target strength may not be obtained. On the other hand, if it exceeds 60%, the yield strength becomes excessive and the low-temperature toughness may be lowered.
  • the cumulative reduction ratio is the sum of the reduction ratios in each rolling pass that is in the non-recrystallization temperature range in finish rolling.
  • the average cooling rate is preferably 1.0 ° C./s or more. More preferably, it is set to 2.0 ° C./s or more.
  • the average cooling rate is preferably 150.0 ° C./s or less.
  • the average cooling rate is more preferably 120.0 ° C./s or less. More preferably, it shall be 100.0 degrees C / s or less.
  • the average cooling rate is the average of the cooling rates from the lower temperature of (finishing rolling finishing temperature ⁇ 50 ° C.) or the cooling start temperature to 650 ° C. after finishing rolling.
  • controlling the average cooling rate in cooling is effective in suppressing Cr carbide precipitation during cooling, thereby improving the stress corrosion cracking resistance.
  • the average cooling rate in the temperature range from the finish rolling finish temperature to (finish finish finish temperature ⁇ 50 ° C.) is not particularly specified, but is 1.0 ° C./s because precipitation of Nb, V, and Ti-based precipitates can be promoted. The following is preferable.
  • the average cooling rate below 650 degreeC is not prescribed
  • regulated it is preferable to set it as less than 100.0 degreeC / s from a viewpoint of preventing the distortion of a steel plate. More preferably, it is 80.0 ° C./s or less.
  • the obtained hot-rolled steel sheet having a thickness of 12 mm to 80 mm was subjected to a microstructure investigation, a base material tensile test, a base material toughness, and a stress corrosion cracking test in the following manner.
  • the microstructure was examined by taking a sample for microstructural observation of a cross section parallel to the rolling direction at a position 0.5 mm below the surface of the thickness of each steel sheet, and adding a sodium pyrosulfite aqueous solution (10 g Na After immersion corrosion with 2 S 2 O 5 +95 ml water solution, 5 fields of the optical microscope tissue were photographed at a magnification of 500 times. Thereafter, the area ratio, equivalent circle diameter, and aspect ratio of the austenite were obtained from the obtained tissue image using an image analysis apparatus.
  • Austenite area ratio The austenite area ratio is austenite etched, the structure is photographed at 500 times, the austenite grain boundary is traced, and by image analysis, the area of austenite is 10 ⁇ m or more relative to the total area of austenite The percentage was determined.
  • Austenite equivalent circle diameter The crystal grain size of austenite, that is, the equivalent circle diameter of austenite, was measured for the area of each austenite using image analysis on the above-described structure image. The equivalent circle diameter was calculated from each area.
  • Austenite grain aspect ratio The austenite grain aspect ratio is determined by observing the structure of the austenite grain boundary by the above-mentioned corrosion using an optical microscope. The ratio of the widest width (minor axis) was calculated.
  • Equivalent circle diameter of Nb, V, and Ti-based precipitates The investigation of equivalent circle diameter of Nb, V, and Ti-based precipitates was conducted on a cross section parallel to the rolling direction at a position 0.5 mm below the surface of each steel plate. Ten fields of view were taken with a scanning electron microscope at a magnification of 50000 times, and the area of each Nb, V, Ti-based precipitate was measured using image analysis on the tissue image. The equivalent circle diameter of the Nb, V, and Ti-based precipitates was calculated from each area.
  • Number density of Nb, V, and Ti-based precipitates The number density of Nb, V, and Ti-based precipitates was determined by examining transmission electron for a cross section parallel to the rolling direction at a position 0.5 mm below the surface of each steel plate. Ten fields of view were taken with a microscope at 50000 times, and the number of Nb, V, and Ti-based precipitates having an equivalent circle diameter of 0.01 to 0.5 ⁇ m per 1 mm 2 was examined. The total number density of precipitates was determined.
  • (2) Base material tensile properties JIS No. 5 tensile test specimens were collected from each of the obtained steel plates, and subjected to a tensile test in accordance with the default of JIS Z 2241 (1998) to investigate the tensile properties.
  • a yield strength of 400 MPa or more is assumed to have excellent base material tensile properties (within the scope of the present invention).
  • what was excellent in the base material tensile characteristics of this invention was the tensile strength of 800 MPa or more and the total elongation of 30% or more.
  • Base material toughness JIS Z 2202 (1998) from the direction perpendicular to the rolling direction of the plate thickness 1/4 position of each steel plate exceeding 20 mm thickness or the plate thickness 1/2 position of each steel plate thickness 20 mm or less.
  • Charpy V-notch test specimens were collected in accordance with the provisions of JIS Z 2242, and three Charpy impact tests were performed on each steel sheet in accordance with the provisions of JIS Z 2242 (1998), and the absorbed energy at -196 ° C And the toughness of the base material was evaluated.
  • the average value of three absorbed energy (vE -196) is more than 50J was excellent in base metal toughness (within the scope of the present invention). More preferably, the average value of absorbed energy (vE ⁇ 196 ) is 100 J or more.
  • Stress Corrosion Cracking Test The stress corrosion cracking test was carried out in accordance with the Narrow Standard TM0111-2011 standard Slow Strain Rate Test Method.
  • the shape of the specimen is a Type A round bar notched specimen, immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C., and subjected to a constant velocity tensile test at a strain rate of 4 ⁇ 10 ⁇ 7 inch / sec. Carried out.
  • the fracture stress is 500 MPa or more and is excellent in stress corrosion cracking resistance (within the scope of the present invention). More preferably, the breaking stress is 600 MPa or more.
  • the present invention is, target performance of the above (yield strength of the base material is more than 400 MPa, 50 J or more in the average value of the low-temperature toughness absorbed energy (vE -196), more 500MPa stress corrosion cracking resistance in breaking stress) satisfies Confirmed to do.
  • Yield strength of the base material is more than 400 MPa, 50 J or more in the average value of the low-temperature toughness absorbed energy (vE -196), more 500MPa stress corrosion cracking resistance in breaking stress
  • any one or more of the base material strength, the low temperature toughness, and the stress corrosion cracking resistance cannot satisfy the above target performance.
  • the steel plate No. Nos. 12 and 36 have a stable austenite because C is out of the range of the present invention, but there are a lot of unstable austenites. Therefore, the average equivalent circle diameter is 10 ⁇ m or more and the aspect ratio of major axis to minor axis is 3 The area ratio of the above austenite was 70%.

Abstract

Provided are a high Mn steel sheet and a method for producing this high Mn steel sheet. This high Mn steel sheet has a component composition which contains, in mass%, 0.20-0.70% of C, 0.05-1.0% of Si, 15-30% of Mn, 0.028% or less of P, 0.02% or less of S, 0.01-0.1% of Al, 0.5-7.0% of Cr, 0.03-0.30% of Ni and 0.0010-0.0200% of N, while containing one or more of 0.003-0.030% of Nb, 0.03-0.10% V and 0.003-0.040% of Ti, with the balance made up of Fe and unavoidable impurities. With respect to this high Mn steel sheet, the microstructure at 0.5 mm below the steel sheet surface has a matrix phase of austenite; and 25% or more of the austenite in terms of area ratio has a circle-equivalent diameter of 10 μm or more, while having an aspect ratio of the length to the breadth of 3 or more.

Description

高Ｍｎ鋼板およびその製造方法High Mn steel sheet and method for producing the same
　本発明は、液化ガス貯槽用タンク等、極低温環境で使用される構造用鋼に供して好適な、特に、塩水腐食環境での耐応力腐食割れ性に優れた高Ｍｎ鋼板およびその製造方法に関する。 The present invention relates to a high Mn steel sheet suitable for structural steel used in a cryogenic environment, such as a tank for a liquefied gas storage tank, and particularly excellent in stress corrosion cracking resistance in a salt water corrosive environment and a method for producing the same. .
　液化ガス貯槽用構造物に熱間圧延鋼板が用いられる際には、使用環境が極低温となるため、鋼板の強度のみならず、極低温での靱性が要求される。例えば、液化天然ガスの貯槽に熱間圧延鋼板が使用される場合には、液化天然ガスの沸点：－１６４℃以下で優れた靱性を確保する必要がある。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危険性があるため、適用される鋼材に対する低温靱性向上に対する要求は強い。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、安価で極低温靱性に優れる鋼材に対する要望がある。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鋼板を極低温環境の構造用鋼として使用することが検討されている。 When a hot-rolled steel sheet is used for a structure for a liquefied gas storage tank, since the use environment is extremely low temperature, not only the strength of the steel sheet but also toughness at a very low temperature is required. For example, when a hot-rolled steel sheet is used in a liquefied natural gas storage tank, it is necessary to ensure excellent toughness at a boiling point of liquefied natural gas: −164 ° C. or lower. If the low-temperature toughness of the steel material is inferior, there is a risk that the safety as a cryogenic storage tank structure cannot be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material. In response to this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy having an austenite that does not show brittleness at a very low temperature as a steel sheet structure has been used. However, since alloy costs and manufacturing costs are high, there is a demand for a steel material that is inexpensive and excellent in cryogenic toughness. Therefore, as a new steel material to replace the conventional cryogenic steel, the use of a high Mn steel sheet with a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is being used as a structural steel in a cryogenic environment. Yes.
　一方、オーステナイト鋼が腐食環境で使用される場合、オーステナイト結晶粒界が腐食により侵食され、引張応力が付加された場合に、応力腐食割れが発生しやすい問題がある。特に、液化ガス貯槽用構造物などの製作段階には、鋼板の地鉄表面が露出する場合があり、鋼材表面が塩分など腐食性の物質を含む水蒸気や、水分や油分などと接触すると鋼材の腐食が発生する。従来より検討されている高Ｍｎ鋼板では、オーステナイト系ステンレス鋼は勿論のこと、９％Ｎｉ鋼や通常の低合金鋼と比較しても、耐食性に劣る場合がある。この際、高Ｍｎ鋼板の表面での腐食反応においては、鉄がアノード反応により酸化物（さび）を生成する一方で、水分のカソード反応により水素が発生して、水素脆化により応力腐食割れを助長する。このように発生した応力腐食割れが、製作時の曲げ加工や溶接などでの残留応力、あるいは使用環境での負荷応力の存在下において、構造物が破壊に至る危険性がある。そのため、安全性の観点から、使用される鋼材の強度と極低温靱性は勿論のこと、耐応力腐食割れ性に優れることが重要である。 On the other hand, when austenitic steel is used in a corrosive environment, there is a problem that stress corrosion cracking is likely to occur when austenite grain boundaries are eroded by corrosion and tensile stress is applied. In particular, in the manufacturing stage of structures for liquefied gas storage tanks, the steel plate surface of the steel sheet may be exposed, and if the steel surface comes into contact with water vapor containing salt or other corrosive substances, moisture or oil, etc. Corrosion occurs. High Mn steel plates that have been studied in the past may be inferior in corrosion resistance as compared with austenitic stainless steel as well as 9% Ni steel and ordinary low alloy steel. At this time, in the corrosion reaction on the surface of the high Mn steel plate, iron generates oxide (rust) by the anode reaction, while hydrogen is generated by the cathode reaction of moisture, and stress corrosion cracking is caused by hydrogen embrittlement. To encourage. The stress corrosion cracks generated in this way may cause the structure to break down in the presence of residual stress in bending or welding during production or load stress in the usage environment. Therefore, from the viewpoint of safety, it is important to have excellent stress corrosion cracking resistance as well as strength and cryogenic toughness of the steel used.
　例えば、特許文献１には、Ｍｎを１５～３５％、Ｃｕ：５％以下、さらにＣとＣｒを適量添加することで、被削性および溶熱熱影響部の－１９６℃でのシャルピー衝撃特性を改善した鋼材が開示されている。 For example, Patent Document 1 discloses that when Mn is 15 to 35%, Cu is 5% or less, and C and Cr are added in appropriate amounts, the machinability and Charpy impact characteristics at −196 ° C. of the heat and heat affected zone are shown. A steel material with improved is disclosed.
　また、特許文献２には、Ｃ：０．２５～０．７５％、Ｓｉ：０．０５～１．０％、Ｍｎ：２０％を超え３５％以下、Ｎｉ：０．１％以上７．０％未満、Ｃｒ：０．１％以上８．０％未満を添加する低温靱性を改善した高Ｍｎ鋼材が開示されている。 In Patent Document 2, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, Ni: 0.1% or more and 7.0 %, And Cr: 0.1% or more and less than 8.0% is added, and a high Mn steel material with improved low temperature toughness is disclosed.
特表２０１５－５０８４５２号公報JP-T-2015-508452 特開２０１６－８４５２９号公報JP 2016-84529 A
　しかしながら、特許文献１、２に記載の高Ｍｎ鋼板は、強度と低温靭性を備えることを目的とするものであり、溶熱熱影響部での－１９６℃でのシャルピー衝撃特性は６０～１３５Ｊ(特許文献１のみ表示あり)である。しかし、母材の極低温靱性は未だ不十分であり、極低温靱性と耐応力腐食割れ性を両立するには至っていない。 However, the high Mn steel sheets described in Patent Documents 1 and 2 are intended to have strength and low-temperature toughness, and the Charpy impact characteristics at −196 ° C. in the heat and heat affected zone are 60 to 135 J ( Only Patent Document 1 is displayed). However, the cryogenic toughness of the base material is still insufficient, and the cryogenic toughness and the stress corrosion cracking resistance have not been achieved at the same time.
　本発明は係る問題に鑑み、極低温靱性、耐応力腐食割れ性に優れた高Ｍｎ鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above problems, and an object thereof is to provide a high-Mn steel sheet excellent in cryogenic toughness and stress corrosion cracking resistance and a method for producing the same.
　本発明者らは、上記課題を達成するため、高Ｍｎ鋼板を対象に、優れた耐応力腐食割れ性能を確保するための鋼板の成分組成、製造方法およびミクロ組織を決定する各種要因に関して鋭意研究を行い、以下の知見を得た。 In order to achieve the above-mentioned problems, the present inventors have conducted intensive research on various factors that determine the component composition, manufacturing method, and microstructure of a steel sheet for ensuring excellent stress corrosion cracking resistance performance for high-Mn steel sheets. The following findings were obtained.
　１．極低温靱性と優れた耐応力腐食割れ性を両立するためには、腐食反応を介した鋼板中への水素侵入量を低下させることが有効である。塩水腐食環境で、鋼板表面の耐食性を向上することが重要であり、このためには、高Ｍｎ鋼板をベースに厳格に成分組成を管理することが重要である。特に、ＣｒとＮｉを同時添加し、その添加量を適正に制御することにより、鋼板表面での腐食反応の初期に形成される錆が微細になる。そして、その後の腐食反応を遅延させることにより、鋼中に侵入する水素量を低減することができる。 1. In order to achieve both cryogenic toughness and excellent stress corrosion cracking resistance, it is effective to reduce the amount of hydrogen entering the steel sheet through the corrosion reaction. In a salt water corrosive environment, it is important to improve the corrosion resistance of the steel sheet surface. To this end, it is important to strictly control the composition of components based on high Mn steel sheets. In particular, by simultaneously adding Cr and Ni and appropriately controlling the addition amount, rust formed at the initial stage of the corrosion reaction on the steel sheet surface becomes fine. And the amount of hydrogen which penetrate | invades in steel can be reduced by delaying subsequent corrosion reaction.
　２．さらに、鋼板表面近傍のミクロ組織を厳格に管理することも耐応力腐食割れ性を改善する上で有効であることを見出した。すなわち、オーステナイトのうち面積率で２５％以上が、円相当直径で１０μｍ以上であり、かつ長径と短径のアスペクト比が３以上であることが応力腐食割れ性を向上するうえで重要となる。これは、腐食反応で鋼板内部に侵入した水素が、未再結晶オーステナイトの結晶粒内にトラップされることにより、オーステナイト粒界上の水素量が相対的に低下し、オーステナイト粒界での応力腐食割れ感受性が低下するためと考えられる。 2. Furthermore, it was found that strictly controlling the microstructure near the surface of the steel sheet is also effective in improving the stress corrosion cracking resistance. That is, it is important for improving the stress corrosion cracking property that the area ratio of austenite is 25% or more, the equivalent circle diameter is 10 μm or more, and the aspect ratio of the major axis to the minor axis is 3 or more. This is because the hydrogen intruded into the steel sheet by the corrosion reaction is trapped in the crystal grains of unrecrystallized austenite, so that the amount of hydrogen on the austenite grain boundary relatively decreases, and stress corrosion at the austenite grain boundary occurs. This is thought to be due to a decrease in crack sensitivity.
　３．上記１、２に加えて、鋼板中のＮｂ、Ｖ、Ｔｉの炭化物、窒化物および複合炭窒化物は、その分散状態を適正に管理することにより、さらに耐応力腐食割れ性を向上することができる。Ｎｂ、Ｖ、Ｔｉの炭化物、窒化物および複合炭窒化物は、鋼板中で拡散性水素のトラップサイトとして作用する。すなわち、鋼材の腐食反応により生成された拡散性水素のトラップサイトとして作用し、応力腐食割れを抑制する効果を有する。オーステナイト中のＮｂ、Ｖ、Ｔｉの炭化物、窒化物および複合炭窒化物の分散状態に対しては、熱間圧延工程の加熱、圧延、および冷却条件などが影響を及ぼす。そのため、これらの製造条件を管理することが重要である。 3. In addition to 1 and 2 above, Nb, V, Ti carbides, nitrides, and composite carbonitrides in steel sheets can be further improved in stress corrosion cracking resistance by properly managing their dispersion state. it can. The carbides, nitrides, and composite carbonitrides of Nb, V, and Ti act as diffusible hydrogen trap sites in the steel sheet. That is, it acts as a trap site for diffusible hydrogen generated by the corrosion reaction of the steel material, and has the effect of suppressing stress corrosion cracking. The heating, rolling, and cooling conditions in the hot rolling process affect the dispersion state of Nb, V, and Ti carbides, nitrides, and composite carbonitrides in austenite. Therefore, it is important to manage these manufacturing conditions.
　４．さらに、オーステナイト粒界破壊を効果的に抑制するためには、結晶粒界強度を高める対策が有効である。Ｐは、鋼片の凝固過程において、Ｍｎとともに共偏析しやすい元素であり、ミクロ偏析部に交わる結晶粒界強度を低下させる。そのため、Ｐなどの不純物元素を低減する必要がある。 4. Furthermore, in order to effectively suppress the austenite grain boundary breakdown, a measure for increasing the grain boundary strength is effective. P is an element that easily co-segregates with Mn in the solidification process of the steel slab, and lowers the grain boundary strength that intersects the micro-segregation part. Therefore, it is necessary to reduce impurity elements such as P.
　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、その要旨は以下のとおりである。
［１］　質量％で、Ｃ：０．２０～０．７０％、Ｓｉ：０．０５～１．０％、Ｍｎ：１５～３０％、Ｐ：０．０２８％以下、Ｓ：０．０２％以下、Ａｌ：０．０１～０．１％、Ｃｒ：０．５～７．０％、Ｎｉ：０．０３～０．３０％、Ｎ：０．００１０～０．０２００％を含有し、Ｎｂ：０．００３～０．０３０％、Ｖ：０．０３～０．１０％、Ｔｉ：０．００３～０．０４０％の１種または２種以上を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、鋼板表面下０．５ｍｍのミクロ組織が、オーステナイトを基地相とし、当該オーステナイトのうち面積率で２５％以上が、円相当直径で１０μｍ以上であり、かつ長径と短径のアスペクト比が３以上である高Ｍｎ鋼板。
［２］　前記成分組成に加えて、さらに下記のグループＡまたはＢのうちから選択された少なくとも一つのグループの元素を含有する［１］に記載の高Ｍｎ鋼板。 The present invention has been made by further studying the above knowledge, and the gist thereof is as follows.
[1] By mass%, C: 0.20 to 0.70%, Si: 0.05 to 1.0%, Mn: 15 to 30%, P: 0.028% or less, S: 0.02% Hereinafter, Al: 0.01 to 0.1%, Cr: 0.5 to 7.0%, Ni: 0.03 to 0.30%, N: 0.0010 to 0.0200%, Nb : 0.003 to 0.030%, V: 0.03 to 0.10%, Ti: 0.003 to 0.040%, or one or more of them, and the balance consisting of Fe and inevitable impurities It has a component composition, and a microstructure of 0.5 mm below the surface of the steel sheet has austenite as a base phase, of which austenite has an area ratio of 25% or more, a circle equivalent diameter of 10 μm or more, and a major axis and a minor axis. A high Mn steel sheet having an aspect ratio of 3 or more.
[2] The high-Mn steel sheet according to [1], further containing at least one element selected from the following group A or B in addition to the component composition.
　　　　　　　　　　　　記
グループＡ：質量％で、Ｍｏ：０．０５～２．０％、Ｗ：０．０５～２．０％のうちから選んだ１種または２種
グループＢ：質量％で、Ｃａ：０．０００５～０．００５０％、Ｍｇ：０．０００５～０．００５０％、ＲＥＭ：０．００１０～０．０２００％のうちから選んだ１種または２種以上
［３］　鋼板表面下０．５ｍｍの前記ミクロ組織が、さらに、前記ミクロ組織中に、円相当直径が０．０１～０．５μｍである、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する炭化物、窒化物および炭窒化物を、合計で２×１０２個／ｍｍ２以上を有する［１］または［２］に記載の高Ｍｎ鋼板。
［４］　［１］～［３］のいずれかに記載の成分組成を有する鋼素材をＴｘ（ｘ＝Ｎｂ、ＶまたはＴｉ）を式（１）～（３）に示す温度とするとき、式（１）～（３）で定義されるＴｘ（℃）のいずれか１つ以上で、鋼素材の表面温度が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下の温度域に加熱し、仕上圧延終了温度が７５０℃以上１０００℃以下の熱間圧延し、鋼板を製造し、その後、（仕上圧延終了温度－５０℃）または冷却開始温度のいずれか低い温度から６５０℃までの鋼板表面の平均冷却速度が１．０℃／ｓ以上で冷却する高Ｍｎ鋼板の製造方法。
ＴＮｂ（℃）＝７５００／｛３．０－ｌｏｇ１０（[％Ｎｂ]×[％Ｃ]）｝－２７３　・・・（１）
ＴＶ（℃）＝１０８００／｛７．２－ｌｏｇ１０（[％Ｖ]×[％Ｃ]）｝－２７３　・・・（２）
ＴＴｉ（℃）＝７０００／｛２．８－ｌｏｇ１０（[％Ｔｉ]×[％Ｃ]）｝－２７３　・・・（３）
　ここで、[％Ｎｂ]、[％Ｖ]、[％Ｔｉ]および[％Ｃ]は、それぞれ鋼中のＮｂ、Ｖ、ＴｉおよびＣの含有量（質量％）を示す。含まない元素の場合は、式中の元素記号を０として計算する。 Group A:% by mass, Mo: 0.05 to 2.0%, W: 0.05 to 2.0%, or one selected from Group B:% by mass, Ca: 0.0. One or more selected from 0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200% [3] The above 0.5 mm below the steel sheet surface The microstructure further includes carbides, nitrides, and carbonitrides containing one or more of Nb, V, and Ti having an equivalent circle diameter of 0.01 to 0.5 μm in the microstructure. The high Mn steel sheet according to [1] or [2], having a total of 2 × 10 2 pieces / mm 2 or more.
[4] When the steel material having the component composition according to any one of [1] to [3] is set to Tx (x = Nb, V or Ti) at a temperature represented by the formulas (1) to (3), the formula Finish rolling by heating any one or more of Tx (° C) defined in (1) to (3) to a temperature range where the surface temperature of the steel material is (Tx-50) ° C or higher and (Tx + 200) ° C or lower. Hot rolling is performed at an end temperature of 750 ° C. or more and 1000 ° C. or less to produce a steel plate, and then the average cooling of the steel plate surface from the lower one of (finishing finish temperature −50 ° C.) or the cooling start temperature to 650 ° C. The manufacturing method of the high Mn steel plate cooled at a speed | rate of 1.0 degree-C / s or more.
T Nb (° C.) = 7500 / {3.0−log 10 ([% Nb] × [% C])} − 273 (1)
T V (° C.) = 10800 / {7.2-log 10 ([% V] × [% C])} − 273 (2)
T Ti (° C.) = 7000 / {2.8−log 10 ([% Ti] × [% C])} − 273 (3)
Here, [% Nb], [% V], [% Ti] and [% C] indicate the contents (mass%) of Nb, V, Ti and C in the steel, respectively. In the case of an element not included, the element symbol in the formula is calculated as 0.
　なお、本発明において、「高強度」とは、降伏強度が４００ＭＰａ以上の強度を有するものをいう。また、本発明において、「極低温靭性」とは、低温靭性、すなわち－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５０Ｊ以上であることをいう。さらに、本発明において、「耐応力腐食割れ性に優れた」とは、ＮＡＣＥ Ｓｔａｎｄａｒｄ　ＴＭ０１１１－２０１１基準のＳｌｏｗ Ｓｔｒａｉｎ Ｒａｔｅ Ｔｅｓｔ Ｍｅｔｈｏｄに準拠した試験であって、温度２３℃で人工海水（塩化物イオン濃度１８０００ｐｐｍ）に浸漬し、ひずみ速度：４×１０－７ｉｎｃｈ／ｓｅｃ.で等速引張試験を行った場合に、破断応力が５００ＭＰa以上であることをいう。 In the present invention, “high strength” means a material having a yield strength of 400 MPa or more. In the present invention, “very low temperature toughness” means low temperature toughness, that is, the absorbed energy vE −196 of the Charpy impact test at −196 ° C. is 50 J or more. Furthermore, in the present invention, “excellent in stress corrosion cracking resistance” is a test in accordance with the Narrow Standard TM0111-2011 standard Slow Strain Rate Test Method, which is artificial seawater (chloride ion concentration) at a temperature of 23 ° C. immersed in 18000ppm), strain rate:. when performing a constant velocity tensile test at 4 × 10 -7 inch / sec, refers to breaking stress is more than 500 MPa.
　本発明によれば、極低温靱性、耐応力腐食割れ性に優れた高Ｍｎ鋼板が得られる。そして、本発明の高Ｍｎ鋼板は、液化ガス貯槽用タンク等、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に大きく寄与し、産業上格段の効果を奏する。また、生産性の低下および製造コストの増大を引き起こすことがないため、経済性に優れる。 According to the present invention, a high Mn steel sheet having excellent cryogenic toughness and stress corrosion cracking resistance can be obtained. And the high Mn steel plate of this invention contributes greatly to the improvement of the safety | security and lifetime of steel structures used in cryogenic environments, such as a tank for liquefied gas storage tanks, and has a remarkable industrial effect. Moreover, since it does not cause a decrease in productivity and an increase in manufacturing cost, it is excellent in economic efficiency.
　以下、本発明の実施形態について説明する。なお、本発明は以下の実施形態に限定されない。 Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.
　［成分組成］

　まず、本発明の鋼板の成分組成と、その限定理由について説明する。本発明では、優れた耐応力腐食割れ性を確保するため、以下のように鋼板の成分組成を規定する。なお、成分組成を表す％は、特に断らない限り質量％を意味するものとする。　Ｃ：０．２０～０．７０％
　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。その効果を得るためには、Ｃは０．２０％以上の含有を必要とする。一方、０．７０％を超えて含有すると、Ｃｒ炭化物およびＮｂ、Ｖ、Ｔｉ系炭化物が過度に生成され、低温靱性および耐応力腐食割れ性が低下する。このため、Ｃは０．２０～０．７０％とする。好ましくは、Ｃは０．２５％以上とする。好ましくは、Ｃは０．６０％以下とする。より好ましくは、Ｃは０．３０％以上とする。より好ましくは、Ｃは０．５５％以下とする。 [Ingredient composition]

First, the component composition of the steel plate of this invention and the reason for limitation are demonstrated. In this invention, in order to ensure the outstanding stress corrosion cracking resistance, the component composition of a steel plate is prescribed | regulated as follows. In addition, unless otherwise indicated,% showing a component composition shall mean the mass%. C: 0.20 to 0.70%
C is an inexpensive austenite stabilizing element and an important element for obtaining austenite. In order to acquire the effect, C needs to contain 0.20% or more. On the other hand, when it contains exceeding 0.70%, Cr carbide | carbonized_material and Nb, V, Ti type carbide | carbonized_material will be produced | generated excessively, and low temperature toughness and stress corrosion cracking resistance will fall. Therefore, C is set to 0.20 to 0.70%. Preferably, C is 0.25% or more. Preferably, C is 0.60% or less. More preferably, C is 0.30% or more. More preferably, C is 0.55% or less.
　Ｓｉ：０．０５～１．０％
　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を有する。このような効果を得るためには、Ｓｉは０．０５％以上の含有を必要とする。一方、１．０％を超えて含有すると、溶接性が劣化する。また、耐ＳＣＣ性にも影響する。このため、Ｓｉは０．０５～１．０％とする。好ましくは、Ｓｉは０．０７％以上とする。好ましくは、Ｓｉは０．５０％以下とする。より好ましくは、Ｓｉは０．１５％以上とする。より好ましくは、Ｓｉは０．４５％以下とする。 Si: 0.05 to 1.0%
Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of increasing the strength of the steel sheet by solid solution and solid solution strengthening. In order to acquire such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.0%, weldability will deteriorate. It also affects the SCC resistance. For this reason, Si is made 0.05 to 1.0%. Preferably, Si is 0.07% or more. Preferably, Si is 0.50% or less. More preferably, Si is 0.15% or more. More preferably, Si is 0.45% or less.
　Ｍｎ：１５～３０％
　Ｍｎは、比較的安価なオーステナイト安定化元素である。本発明では、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は１５％以上の含有を必要とする。一方、３０％を超えて含有しても、極低温靱性を改善する効果が飽和し、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、耐応力腐食割れの発生を助長する。このため、Ｍｎは１５～３０％とする。好ましくは、Ｍｎは１８％以上とする。好ましくは、Ｍｎは２８％以下とする。より好ましくは、Ｍｎは２０％以上とする。より好ましくは、Ｍｎは２７％以下とする。 Mn: 15-30%
Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to acquire the effect, Mn needs to contain 15% or more. On the other hand, even if the content exceeds 30%, the effect of improving the cryogenic toughness is saturated, leading to an increase in alloy cost. In addition, the weldability and cutability are deteriorated. Furthermore, segregation is promoted and stress corrosion cracking is promoted. Therefore, Mn is set to 15 to 30%. Preferably, Mn is 18% or more. Preferably, Mn is 28% or less. More preferably, Mn is 20% or more. More preferably, Mn is 27% or less.
　Ｐ：０．０２８％以下
　Ｐは、０．０２８％を超えて含有すると、粒界に偏析し、耐応力腐食割れの発生起点となる。このため、０．０２８％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０．０２８％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、Ｐは０．００５％以上とする。好ましくは、Ｐは０．０２４％以下とする。 P: 0.028% or less When P exceeds 0.028%, it segregates at the grain boundary and becomes the starting point of stress corrosion cracking. For this reason, it is desirable to make 0.028% an upper limit and to reduce as much as possible. Therefore, P is set to 0.028% or less. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.002% or more. Preferably, P is 0.005% or more. Preferably, P is 0.024% or less.
　Ｓ：０．０２％以下
　Ｓは母材の低温靭性や延性を劣化させるため、０．０２％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０．０２％以下とする。尚、過度のＳ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、Ｓは０．００２％以上とする。好ましくは、Ｓは０．０１８％以下とする。より好ましくは、Ｓは０．０１０％以下とする。 S: 0.02% or less Since S deteriorates the low-temperature toughness and ductility of the base material, 0.02% is the upper limit and it is desirable to reduce it as much as possible. Therefore, S is set to 0.02% or less. In addition, since excessive S reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.001% or more. Preferably, S is 0.002% or more. Preferably, S is 0.018% or less. More preferably, S is 0.010% or less.
　Ａｌ：０．０１～０．１％
　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。また、鋼中の固溶Ｎを固定してＡｌＮを形成することにより、結晶粒の粗大化を抑制する効果を有する。これとともに、固溶Ｎ低減による靱性劣化を抑制する効果を有する。このような効果を得るためには、Ａｌは０．０１％以上の含有を必要とする。一方、Ａｌは０．１％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、０．１％以下とする。このため、Ａｌは０．０１～０．１％とする。好ましくは、Ａｌは０．０２％以上とする。好ましくは、Ａｌは０．０７％以下とする。 Al: 0.01 to 0.1%
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheets. Moreover, it has the effect which suppresses the coarsening of a crystal grain by fixing the solid solution N in steel and forming AlN. At the same time, it has the effect of suppressing toughness deterioration due to the reduction of solute N. In order to acquire such an effect, Al needs to contain 0.01% or more. On the other hand, if Al is contained in an amount exceeding 0.1%, it is mixed into the weld metal part during welding and deteriorates the toughness of the weld metal. For this reason, Al is made 0.01 to 0.1%. Preferably, Al is 0.02% or more. Preferably, Al is 0.07% or less.
　Ｃｒ：０．５～７．０％
　Ｃｒは、適量の添加でオーステナイトを安定化させ、極低温靱性と母材強度の向上に有効な元素である。また、本発明では、塩水環境における母材表面に生成する錆を緻密にする効果を介して、鋼板中への水素侵入量を低下させて、耐応力腐食割れ性を向上する重要な元素である。このような効果を得るためには、Ｃｒは０．５％以上の含有を必要とする。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０．５～７．０％とする。Ｃｒは、好ましくは１．０％以上、より好ましくは１．２％以上、さらに好ましくは２．５％以上とする。Ｃｒは、好ましくは６．０％以下、より好ましくは５．７％以下、さらに好ましくは５．５％以下とする。 Cr: 0.5 to 7.0%
Cr is an element that stabilizes austenite by addition of an appropriate amount and is effective in improving cryogenic toughness and base material strength. Further, in the present invention, it is an important element that improves the stress corrosion cracking resistance by reducing the amount of hydrogen penetration into the steel sheet through the effect of densifying rust generated on the surface of the base material in a salt water environment. . In order to acquire such an effect, Cr needs to contain 0.5% or more. On the other hand, if the content exceeds 7.0%, the low temperature toughness and stress corrosion cracking resistance decrease due to the formation of Cr carbide. Therefore, Cr is 0.5 to 7.0%. Cr is preferably 1.0% or more, more preferably 1.2% or more, and further preferably 2.5% or more. Cr is preferably 6.0% or less, more preferably 5.7% or less, and still more preferably 5.5% or less.
　Ｎｉ：０．０３～０．３０％
　Ｎｉは、代表的なオーステナイト安定化元素であり、極低温靱性と母材強度の向上に有効な元素である。また、本発明では、塩水環境における母材表面に生成する錆を緻密にする効果を介して、鋼板中への水素侵入量を低下させて、耐応力腐食割れ性を向上する重要な元素である。このような効果を得るためには、Ｎｉは０．０３％以上の含有を必要とする。一方、０．３０％を超えて含有すると、合金コストが上昇する上、耐応力腐食割れ性の向上効果が飽和する。このため、Ｎｉは０．０３～０．３０％とする。好ましくは、Ｎｉは０．２５％以下とする。好ましくは０．０４％以上とする。より好ましくは、Ｎｉは０．２３％以下とする。より好ましくは、Ｎｉは０．０５％以上とする。さらに好ましくは、Ｎｉは０．２１％以下とする。 Ni: 0.03-0.30%
Ni is a typical austenite stabilizing element, and is an element effective for improving cryogenic toughness and base metal strength. In the present invention, it is an important element that improves the stress corrosion cracking resistance by reducing the amount of hydrogen entering the steel sheet through the effect of densifying the rust generated on the surface of the base material in a salt water environment. . In order to acquire such an effect, Ni needs to contain 0.03% or more. On the other hand, when the content exceeds 0.30%, the alloy cost increases and the effect of improving the stress corrosion cracking resistance is saturated. For this reason, Ni is made 0.03 to 0.30%. Preferably, Ni is 0.25% or less. Preferably it is 0.04% or more. More preferably, Ni is 0.23% or less. More preferably, Ni is 0.05% or more. More preferably, Ni is 0.21% or less.
　Ｎ：０．００１０～０．０２００％
　Ｎは、オーステナイト安定化元素であり、極低温靱性向上に有効な元素である。また、Ｎｂ、Ｖ、Ｔｉと結合し、窒化物または炭窒化物として析出して、拡散性水素のトラップサイトとして応力腐食割れを抑制する効果を有する。このような効果を得るためには、Ｎは０．００１０％以上の含有を必要とする。一方、０．０２００％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。このため、Ｎは０．００１０～０．０２００％とする。好ましくは、Ｎは０．００２０％以上とする。好ましくは、Ｎは０．０１５０％以下とする。より好ましくは、Ｎは０．００３０％以上とする。より好ましくは、Ｎは０．０１７０％以下とする。 N: 0.0010 to 0.0200%
N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. Moreover, it combines with Nb, V, and Ti, precipitates as nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a diffusible hydrogen trap site. In order to acquire such an effect, N needs to contain 0.0010% or more. On the other hand, if the content exceeds 0.0200%, the nitride or carbonitride becomes coarse and the toughness decreases. For this reason, N is made 0.0010 to 0.0200%. Preferably, N is 0.0020% or more. Preferably, N is 0.0150% or less. More preferably, N is 0.0030% or more. More preferably, N is 0.0170% or less.
　Ｎｂ：０．００３～０．０３０％、Ｖ：０．０３～０．１０％、Ｔｉ：０．００３～０．０４０％の１種または２種以上
　Ｎｂ：０．００３～０．０３０％
　Ｎｂは、炭窒化物（炭化物を含む）として析出し、生成した炭窒化物が拡散性水素のトラップサイトに有効であり、応力腐食割れ抑制の効果を有する元素である。このような効果を得るためには、Ｎｂは０．００３％以上の含有を必要とする。一方、Ｎｂは０．０３０％を超えて含有すると、粗大な炭窒化物が析出し、破壊の起点となることがある。また、析出物が粗大化し、母材靱性を劣化させることがある。このため、Ｎｂを含有する場合は、０．００３～０．０３０％とする。Ｎｂは、好ましくは０．００５％以上、より好ましくは０．００７％以上とする。Ｎｂは、好ましくは０．０２５％以下、より好ましくは０．０２２％以下とする。 Nb: 0.003 to 0.030%, V: 0.03 to 0.10%, Ti: 0.003 to 0.040%, one or more Nb: 0.003 to 0.030%
Nb is an element that precipitates as carbonitride (including carbide), and the produced carbonitride is effective at the trap site of diffusible hydrogen and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, Nb needs to contain 0.003% or more. On the other hand, if Nb exceeds 0.030%, coarse carbonitride precipitates, which may be the starting point of destruction. Further, the precipitates may become coarse and the base material toughness may be deteriorated. Therefore, when Nb is contained, the content is made 0.003 to 0.030%. Nb is preferably 0.005% or more, more preferably 0.007% or more. Nb is preferably 0.025% or less, more preferably 0.022% or less.
　Ｖ：０．０３～０．１０％
　Ｖは、炭窒化物として析出し、生成した炭窒化物が拡散性水素のトラップサイトに有効であり、応力腐食割れ抑制の効果を有する元素である。このような効果を得るためには、Ｖは０．０３％以上の含有を必要とする。一方、Ｖは０．１０％を超えて含有すると、粗大な炭窒化物が析出し、破壊の起点となることがある。また、析出物が粗大化し、母材靱性を劣化させることがある。このため、Ｖを含有する場合は、０．０３～０．１０％とする。Ｖは、好ましくは０．０４％以上、より好ましくは０．０５％以上とする。Ｖは、好ましくは０．０９％以下、より好ましくは０．０８％以下、さらに好ましくは０．０７％以下とする。 V: 0.03-0.10%
V is an element which precipitates as carbonitride and the produced carbonitride is effective at the trap site of diffusible hydrogen and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, V needs to contain 0.03% or more. On the other hand, when V exceeds 0.10%, coarse carbonitride precipitates and may become a starting point of fracture. Further, the precipitates may become coarse and the base material toughness may be deteriorated. Therefore, when V is contained, the content is made 0.03 to 0.10%. V is preferably 0.04% or more, more preferably 0.05% or more. V is preferably 0.09% or less, more preferably 0.08% or less, and still more preferably 0.07% or less.
　Ｔｉ：０．００３～０．０４０％
　Ｔｉは、窒化物もしくは炭窒化物として析出し、生成した窒化物もしくは炭窒化物が拡散性水素のトラップサイトに有効であり、応力腐食割れ抑制の効果を有する元素である。このような効果を得るためには、Ｔｉは０．００３％以上の含有を必要とする。一方、Ｔｉは０．０４０％を超えて含有すると、析出物が粗大化し、母材靱性を劣化させることがある。また、粗大な炭窒化物が析出し、破壊の起点となることがある。このため、Ｔｉを含有する場合は、０．００３～０．０４０％とする。Ｔｉは、好ましくは０．００５％以上、より好ましくは０．００７％以上とする。Ｔｉは、好ましくは０．０３５％以下、より好ましくは０．０３２％以下とする。 Ti: 0.003-0.040%
Ti precipitates as nitride or carbonitride, and the generated nitride or carbonitride is an element effective for diffusible hydrogen trap sites and has the effect of suppressing stress corrosion cracking. In order to acquire such an effect, Ti needs to contain 0.003% or more. On the other hand, if Ti is contained in an amount exceeding 0.040%, the precipitates are coarsened and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and become the starting point of fracture. Therefore, when Ti is contained, the content is made 0.003 to 0.040%. Ti is preferably 0.005% or more, more preferably 0.007% or more. Ti is preferably 0.035% or less, more preferably 0.032% or less.
　残部は鉄および不可避的不純物である。不可避的不純物としては、Ｏや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 is iron and inevitable impurities. Inevitable impurities include O and H, and a total of 0.01% or less is acceptable.
　さらに、低温靱性を低下させる観点から、ＯとＳは以下のように規定することが好ましい。 Further, from the viewpoint of lowering the low temperature toughness, O and S are preferably defined as follows.
　Ｏ：０．０００５～０．００７０％
　Ｏは、０．００７０％超えて含有するとＡｌと粗大な介在物を形成し、低温靱性を低下させる。このため、Ｏは０．００７０％を上限とし、可能なかぎり低減することが望ましい。好ましくは、Ｏは０．００６０％以下とする。尚、過度のＯ低減は、精錬コストを高騰させ経済的に不利となるため、０．０００５％以上とする。好ましくは、Ｏは０．０００８％以上とする。 O: 0.0005 to 0.0070%
If O is contained in an amount exceeding 0.0070%, it forms coarse inclusions with Al and lowers the low temperature toughness. Therefore, the upper limit of O is 0.0070%, and it is desirable to reduce it as much as possible. Preferably, O is 0.0060% or less. In addition, excessive O reduction increases the refining cost and is economically disadvantageous, so it is 0.0005% or more. Preferably, O is 0.0008% or more.
　Ｏ／Ｓ＜１
　ＯおよびＳのバランスはＡｌ、ＴｉおよびＭｎと酸化物、硫化物およびこれらの複合析出物を形成し、拡散性水素のトラップサイトとして有効に作用し応力腐食割れ性を向上する。この効果を得るためにはＯ／Ｓ＜１とする。Ｏ／Ｓ≧１では、粗大な酸硫化物が形成し、低温靱性が低下する恐れがある。よって、本発明では、低温靱性確保のため、Ｏ／Ｓ＜１とする。 O / S <1
The balance of O and S forms oxides, sulfides, and composite precipitates thereof with Al, Ti, and Mn, and effectively acts as a trapping site for diffusible hydrogen to improve stress corrosion cracking. In order to obtain this effect, O / S <1. When O / S ≧ 1, coarse oxysulfides are formed, and the low-temperature toughness may be lowered. Therefore, in the present invention, O / S <1 is set to ensure low temperature toughness.
　以上の必須元素で、本発明の目的とする特性が得られる。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、上記の必須元素に加えて、必要に応じて下記の元素を含有することができる。 With the above essential elements, the target characteristics of the present invention can be obtained. In the present invention, for the purpose of further improving the strength and the low temperature toughness, the following elements can be contained as required in addition to the above essential elements.
　Ｍｏ：０．０５～２．０％、Ｗ：０．０５～２．０％の１種または２種
　Ｍｏ：０．０５～２．０％
　Ｍｏは、母材の高強度化に有用な元素であり、必要に応じて含有できる。このような効果を得るためには、Ｍｏは０．０５％以上を含有することが好ましい。一方、２．０％を超えて含有すると、靭性および耐溶接割れ性に悪影響を及ぼす場合があるため、Ｍｏは２．０％以下とすることが好ましい。このため、Ｍｏを含有する場合には、０．０５～２．０％とする。より好ましくは、Ｍｏは０．０７％以上とする。より好ましくは、Ｍｏは１．７％以下とする。 One or two types of Mo: 0.05 to 2.0%, W: 0.05 to 2.0% Mo: 0.05 to 2.0%
Mo is an element useful for increasing the strength of the base material, and can be contained as necessary. In order to obtain such an effect, Mo preferably contains 0.05% or more. On the other hand, if the content exceeds 2.0%, the toughness and weld crack resistance may be adversely affected, so Mo is preferably made 2.0% or less. For this reason, when it contains Mo, it is made 0.05 to 2.0%. More preferably, Mo is 0.07% or more. More preferably, Mo is 1.7% or less.
　Ｗ：０．０５～２．０％
　Ｗは、母材の高強度化に有用な元素であり、必要に応じて含有できる。このような効果を得るためには、Ｗは０．０５％以上を含有することが好ましい。一方、２．０％を超えて含有すると、靭性および耐溶接割れ性に悪影響を及ぼす場合があるため、Ｗは２．０％以下とすることが好ましい。このため、Ｗを含有する場合には、０．０５～２．０％とする。より好ましくは０．０７％以上とする。より好ましくは１．５％以下とする。 W: 0.05-2.0%
W is an element useful for increasing the strength of the base material, and can be contained as necessary. In order to obtain such an effect, W preferably contains 0.05% or more. On the other hand, if the content exceeds 2.0%, the toughness and weld crack resistance may be adversely affected, so W is preferably set to 2.0% or less. Therefore, when W is contained, the content is made 0.05 to 2.0%. More preferably, the content is 0.07% or more. More preferably, it is 1.5% or less.
　Ｃａ：０．０００５～０．００５０％、Ｍｇ：０．０００５～０．００５０％、ＲＥＭ：０．００１０～０．０２００％の１種または２種以上
　Ｃａ：０．０００５～０．００５０％
　Ｃａ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とすることをいう。この介在物の形態制御を介して、延性、靭性、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は０．０００５％以上を含有することが好ましい。一方、０．００５０％を超えて含有すると、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、Ｃａを含有する場合には、０．０００５～０．００５０％とする。より好ましくは０．００１０％以上とする。より好ましくは０．００４０％以下とする。 One or more of Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200% Ca: 0.0005 to 0.0050%
Ca is an element useful for controlling the form of inclusions, and can be contained as necessary. The inclusion shape control means that the expanded sulfide inclusion is a granular inclusion. Ductility, toughness, and resistance to sulfide stress corrosion cracking are improved through shape control of the inclusions. In order to obtain such an effect, Ca preferably contains 0.0005% or more. On the other hand, if the content exceeds 0.0050%, the amount of non-metallic inclusions may increase, and the ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. Moreover, it may become economically disadvantageous. Therefore, when Ca is contained, the content is made 0.0005 to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.
　Ｍｇ：０．０００５～０．００５０％
　Ｍｇは、耐硫化物応力腐食割れ性の改善に寄与する元素として有用であり、必要に応じて含有できる。このような効果を得るためには、Ｍｇは０．０００５％以上を含有することが好ましい。一方、０．００５０％を超えて含有しても、上述の効果は飽和し、含有量に見合う効果が期待できない場合がある。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％とする。より好ましくは０．００１０％以上とする。より好ましくは０．００４０％以下とする。 Mg: 0.0005 to 0.0050%
Mg is useful as an element that contributes to the improvement of resistance to sulfide stress corrosion cracking, and can be contained if necessary. In order to obtain such an effect, Mg preferably contains 0.0005% or more. On the other hand, even if it contains exceeding 0.0050%, the above-mentioned effect is saturated and the effect commensurate with the content may not be expected. Moreover, it may become economically disadvantageous. Therefore, when Mg is contained, the content is made 0.0005 to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.
　ＲＥＭ：０．００１０～０．０２００％
　ＲＥＭは、耐硫化物応力腐食割れ性の改善に寄与する元素として有用であり、必要に応じて含有できる。このような効果を得るためには、ＲＥＭは０．００１０％以上を含有することが好ましい。一方、０．０２００％を超えて含有しても、上述の効果は飽和し、含有量に見合う効果が期待できない場合がある。このため、ＲＥＭを含有する場合には、０．００１０～０．０２００％とする。より好ましくは０．００２０％以上とする。より好ましくは０．０１５０％以下とする。
［ミクロ組織］
　次に、本発明の鋼板の重要な要件である、鋼板表面近傍のミクロ組織について説明する。 REM: 0.0010 to 0.0200%
REM is useful as an element that contributes to the improvement of resistance to sulfide stress corrosion cracking, and can be contained as necessary. In order to obtain such an effect, REM preferably contains 0.0010% or more. On the other hand, even if it contains exceeding 0.0200%, the above-mentioned effect is saturated and the effect commensurate with the content may not be expected. For this reason, when it contains REM, it is 0.0010 to 0.0200%. More preferably, the content is 0.0020% or more. More preferably, it is 0.0150% or less.
[Microstructure]
Next, the microstructure near the steel sheet surface, which is an important requirement of the steel sheet of the present invention, will be described.
　鋼板表面下０．５ｍｍのミクロ組織が、オーステナイトを基地相とし、当該オーステナイトのうち面積率で２５％以上が、円相当直径で１０μｍ以上であり、かつ長径と短径のアスペクト比が３以上
　本発明では、鋼板表面下０．５ｍｍのミクロ組織の基地相をオーステナイトとする。そして、当該オーステナイトのうち、円相当直径が１０μｍ以上であり、かつ長径と短径のアスペクト比が３以上であるオーステナイトを、面積率で、２５％以上有することにより、鋼板表層近傍の結晶粒界に加えて結晶粒内の変形帯も拡散性水素のトラップサイトとして有効に作用し、応力腐食割れ性に有効に作用する。これにより、応力腐食割れの抑制を格段に向上できる。また、降伏強度も向上する。好ましくは、面積率で３０％以上とする。一方、面積率で９５％を超えると、鋼材の強度が過多となり、母材靭性の劣化を生じる場合がある。好ましくは９５％以下であり、より好ましくは９４％以下とする。さらに好ましくは９０％以下とする。よりさらに好ましくは８５％以下とする。 The microstructure of 0.5 mm below the surface of the steel sheet has austenite as a base phase, and the area ratio of the austenite is 25% or more, the equivalent circle diameter is 10 μm or more, and the aspect ratio of major axis and minor axis is 3 or more. In the invention, the base phase having a microstructure of 0.5 mm below the surface of the steel sheet is austenite. And among the austenites, the grain boundary in the vicinity of the steel sheet surface layer is obtained by having an area ratio of 25% or more of austenite having an equivalent circle diameter of 10 μm or more and an aspect ratio of major axis to minor axis of 3 or more. In addition, the deformation zone in the crystal grains effectively acts as a diffusible hydrogen trap site and effectively acts on the stress corrosion cracking property. Thereby, suppression of stress corrosion cracking can be remarkably improved. In addition, the yield strength is improved. Preferably, the area ratio is 30% or more. On the other hand, if the area ratio exceeds 95%, the strength of the steel material becomes excessive, and the base material toughness may be deteriorated. Preferably it is 95% or less, More preferably, you may be 94% or less. More preferably, it is 90% or less. More preferably, it is 85% or less.
　円相当直径で１０μｍ未満、または長径と短径のアスペクト比が３未満では、所望の降伏強度が得られないうえ、拡散性水素のトラップサイトとして有効に作用する結晶粒内の変形帯が得られず応力腐食割れ性が低下し、上述の効果が得られない。なお、上述のオーステナイトの円相当直径、面積率、アスペクト比は、後述する実施例に記載の方法にて測定することができる。 If the equivalent circle diameter is less than 10 μm, or the aspect ratio of the major axis to the minor axis is less than 3, the desired yield strength cannot be obtained, and a deformation band within the crystal grains that effectively acts as a trapping site for diffusible hydrogen can be obtained. Therefore, the stress corrosion cracking property is lowered, and the above-described effects cannot be obtained. In addition, the circle equivalent diameter of the above-mentioned austenite, an area ratio, and an aspect ratio can be measured by the method as described in the Example mentioned later.
　本発明では、鋼板表面下０．５ｍｍとは、鋼板の表裏面から板厚方向に０.５ｍｍの位置における圧延方向に平行な断面を意味する。また、本発明では、鋼板表面下０．５ｍｍの位置から±５％の範囲における圧延方向に平行な断面に、上述のミクロ組織が存在していても、同様に、上述の効果は得られる。そのため、本発明において、鋼板表面下０．５ｍｍとは、鋼板の表裏面から板厚方向に０.５ｍｍの位置から±５％の範囲のいずれかにおいて、圧延方向に平行な断面に上述のミクロ組織が存在していることを意味する。なお、成品の単純な表面だけではなく、鋼板表面を結晶の集積度が測定可能な面に処理した後の面、例えば、鋼板の最表面がスケールで覆われている時などは、それを取り除いた面を意味する。 In the present invention, 0.5 mm below the surface of the steel sheet means a cross section parallel to the rolling direction at a position 0.5 mm in the thickness direction from the front and back surfaces of the steel sheet. In the present invention, even if the above-described microstructure exists in a cross section parallel to the rolling direction in a range of ± 5% from a position 0.5 mm below the surface of the steel plate, the above-described effects can be obtained similarly. Therefore, in the present invention, 0.5 mm below the surface of the steel sheet means that the above-mentioned micro is formed in a cross section parallel to the rolling direction in any of ± 5% from the position of 0.5 mm from the front and back surfaces of the steel sheet in the thickness direction. Means that an organization exists. Not only the simple surface of the product, but also the surface after processing the steel plate surface to a surface where the degree of crystal accumulation can be measured, for example, when the outermost surface of the steel plate is covered with a scale, remove it It means the surface.
　鋼板表面下０．５ｍｍのミクロ組織は、さらに、組織中に、円相当直径が０．０１～０．５μｍである、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する炭化物、窒化物および炭窒化物を合計で２×１０２個／ｍｍ２以上
　本発明の鋼板表面下０．５ｍｍにおける、ミクロ組織中の、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する、炭化物、窒化物、炭窒化物（以下、Ｎｂ、Ｖ、Ｔｉ系析出物と称する）の存在状態について説明する。なお、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する、炭化物、窒化物、炭窒化物とは、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する炭化物、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する窒化物、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する炭窒化物をいう。 The microstructure 0.5 mm below the surface of the steel sheet further includes carbides and nitrides containing one or more of Nb, V, and Ti having an equivalent circle diameter of 0.01 to 0.5 μm in the structure. And a total of 2 × 10 2 pieces / mm 2 or more of carbonitrides Carbide containing one or more of Nb, V, Ti in the microstructure at 0.5 mm below the surface of the steel sheet of the present invention, The existence state of nitrides and carbonitrides (hereinafter referred to as Nb, V, and Ti-based precipitates) will be described. Carbides, nitrides, and carbonitrides containing one or more of Nb, V, and Ti are carbides containing one or more of Nb, V, and Ti, Nb, V, A nitride containing one or more of Ti, and a carbonitride containing one or more of Nb, V, and Ti.
　Ｎｂ、Ｖ、Ｔｉ系析出物の粒径は、円相当直径で０．０１～０．５μｍとする。０．０１μｍ未満では、拡散性水素のトラップサイトとして水素脆化割れを抑制する効果が飽和する。また、実製造で０．０１μｍ未満に管理するには、製造負荷が極度に増大し、製造コストが上昇する。一方、０．５μｍを超えると、低温靱性が低下する。また、拡散性水素のトラップサイトとして水素脆化割れを抑制する効果が得られない。好ましくは０．０３μｍ以上とする。好ましくは０．４μｍ以下である。 The particle diameter of the Nb, V, and Ti-based precipitates is 0.01 to 0.5 μm in terms of equivalent circle diameter. When the thickness is less than 0.01 μm, the effect of suppressing hydrogen embrittlement cracking as a diffusible hydrogen trap site is saturated. Moreover, in order to manage to less than 0.01 micrometer by actual manufacture, a manufacturing load will increase extremely and manufacturing cost will rise. On the other hand, when it exceeds 0.5 μm, the low temperature toughness is lowered. Moreover, the effect of suppressing hydrogen embrittlement cracking as a trapping site for diffusible hydrogen cannot be obtained. Preferably, it is 0.03 μm or more. Preferably it is 0.4 micrometer or less.
　上記粒径のＮｂ、Ｖ、Ｔｉ系析出物の合計が、鋼板表面下０．５ｍｍのミクロ組織中で、２×１０２個／ｍｍ２未満であると、拡散性水素のトラップサイトとして作用する析出物が不足するため、拡散性水素のトラップサイトとして水素脆化割れを抑制する効果が得られない。このため、２×１０２個／ｍｍ２以上とする。好ましくは５×１０２個／ｍｍ２以上とする。なお、上述のＮｂ、Ｖ、Ｔｉ系析出物の個数密度および円相当直径は、後述する実施例に記載の方法にて測定することができる。 When the total of the Nb, V, and Ti-based precipitates having the above particle diameter is less than 2 × 10 2 pieces / mm 2 in a microstructure of 0.5 mm below the surface of the steel sheet, it acts as a diffusible hydrogen trap site. Since the precipitates are insufficient, the effect of suppressing hydrogen embrittlement cracking as a diffusible hydrogen trap site cannot be obtained. For this reason, it is set to 2 × 10 2 pieces / mm 2 or more. Preferably, it is 5 × 10 2 pieces / mm 2 or more. The number density and equivalent circle diameter of the Nb, V, and Ti-based precipitates described above can be measured by the method described in the examples described later.
　なお、鋼板表面下０．５ｍｍのミクロ組織中に、オーステナイトの他に、マルテンサイト等の組織が混在すると、低温靱性が低下する。このため、オーステナイトは９０％以上とする。なお、低温靱性が低下する観点より、マルテンサイト等の組織の面積率は少ない方が良い。上記マルテンサイト等の組織とは、マルテンサイト、ベイナイト、フェライト、パーライトである。マルテンサイト等の組織が混在する場合には、各組織の鋼板全体に対する面積率の合計で１０％以下とすることが望ましい。
［製造条件］
　次に、本発明の鋼板の製造方法について説明する。なお、本発明に係る鋼板は、板厚４ｍｍ以上の高Ｍｎ鋼板に好適である。 In addition, when a structure such as martensite is mixed in addition to austenite in the 0.5 mm microstructure below the steel sheet surface, the low temperature toughness decreases. For this reason, austenite shall be 90% or more. From the viewpoint of lowering the low temperature toughness, it is preferable that the area ratio of the structure such as martensite is small. The structures such as martensite are martensite, bainite, ferrite, and pearlite. When structures such as martensite coexist, the total area ratio of each structure with respect to the entire steel sheet is preferably 10% or less.
[Production conditions]
Next, the manufacturing method of the steel plate of this invention is demonstrated. The steel plate according to the present invention is suitable for a high Mn steel plate having a thickness of 4 mm or more.
　本発明の鋼板は、上記した成分組成を有する鋼素材を、Ｔｘ（ｘ＝Ｎｂ、ＶまたはＴｉ）を後述する式（１）～（３）に示す温度とするとき、式（１）～（３）で定義されるＴｘ（℃）のいずれか１つ以上で、鋼素材の表面温度が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下の温度域に加熱し、仕上圧延終了温度が７５０℃以上１０００℃以下の熱間圧延し、鋼板を製造し、その後、（仕上圧延終了温度－５０℃）または冷却開始温度のいずれか低い温度から６５０℃までの鋼板表面の平均冷却速度が１．０℃／ｓ以上で冷却することで得られる。 In the steel sheet of the present invention, when the steel material having the above-described component composition is Tx (x = Nb, V or Ti) is set to a temperature shown in the following expressions (1) to (3), the expressions (1) to ( The surface temperature of the steel material is heated to a temperature range of (Tx−50) ° C. to (Tx + 200) ° C. at one or more of Tx (° C.) defined in 3), and the finish rolling finish temperature is 750 ° C. The steel sheet is manufactured by hot rolling at a temperature of 1000 ° C. or less and then the average cooling rate on the surface of the steel plate from the lower one of (finishing finish temperature −50 ° C.) or the cooling start temperature to 650 ° C. is 1.0. It can be obtained by cooling at a temperature of ° C / s or higher.
　以下、詳細に説明する。なお、説明において、温度に関する「℃」表示は、鋼板表面あるいは鋼素材の表面における温度を意味するものとする。 The details will be described below. In the description, the “° C.” display relating to the temperature means the temperature on the surface of the steel plate or the surface of the steel material.
　本発明に係る高Ｍｎ鋼板は、上記した成分組成を有する溶鋼を、転炉、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。その後、連続鋳造法あるいは造塊－分塊圧延法等、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。 The high Mn steel sheet according to the present invention can be produced by melting a molten steel having the above-described composition by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, a steel material such as a slab having a predetermined size is preferably formed by a known casting method such as a continuous casting method or an ingot-bundling rolling method.
　鋳造後スラブ：得られた鋼素材を、室温まで冷却することなく、または、室温まで冷却した後に、Ｔｘ（ｘ＝Ｎｂ、ＶもしくはＴｉ）を式（１）～（３）で示す温度とするとき、式（１）～（３）で定義されるＴｘ（℃）のいずれか１つ以上で、鋼素材の表面温度が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下の温度域に加熱
ＴＮｂ（℃）＝７５００／｛３．０－ｌｏｇ１０（[％Ｎｂ]×[％Ｃ]）｝－２７３　　・・・（１）
ＴＶ（℃）＝１０８００／｛７．２－ｌｏｇ１０（[％Ｖ]×[％Ｃ]）｝－２７３　　・・・（２）
ＴＴｉ（℃）＝７０００／｛２．８－ｌｏｇ１０（[％Ｔｉ]×[％Ｃ]）｝－２７３　　・・・（３）
　ここで、[％Ｎｂ]、[％Ｖ]、[％Ｔｉ]および[％Ｃ]は、それぞれ鋼中のＮｂ、Ｖ、ＴｉおよびＣの含有量（質量％）を示す。含まない元素の場合は、式中の元素記号を０として計算する。 Slab after casting: Tx (x = Nb, V, or Ti) is set to a temperature represented by the formulas (1) to (3) without cooling the obtained steel material to room temperature or after cooling to room temperature. When the surface temperature of the steel material is at least one of Tx (° C) defined by the formulas (1) to (3) and is heated to a temperature range of (Tx-50) ° C to (Tx + 200) ° C. Nb (℃) = 7500 / { 3.0-log 10 ([% Nb] × [% C])} - 273 ··· (1)
T V (° C.) = 10800 / {7.2-log 10 ([% V] × [% C])} − 273 (2)
T Ti (° C.) = 7000 / {2.8−log 10 ([% Ti] × [% C])} − 273 (3)
Here, [% Nb], [% V], [% Ti] and [% C] indicate the contents (mass%) of Nb, V, Ti and C in the steel, respectively. In the case of an element not included, the element symbol in the formula is calculated as 0.
　加熱温度が（Ｔｘ－５０）℃未満では、熱間圧延での変形抵抗が高くなり、１パス当たりの圧下量が大きく取れなくなることから、圧延パス数が増加し、圧延能率の低下を招くとともに、鋼素材（スラブ）中の鋳造欠陥を圧着することができない場合がある。さらに、溶製段階で鋼中に不均一に晶出したＮｂ、ＶおよびＴｉを含む晶出物が圧延終了後の鋼板中にも残存し、所望のＮｂ、ＶおよびＴｉを含む析出物が得られず、耐応力腐食割れ性が低下する。 If the heating temperature is less than (Tx-50) ° C., the deformation resistance in hot rolling becomes high and the amount of reduction per pass cannot be made large, so the number of rolling passes increases and the rolling efficiency decreases. In some cases, a casting defect in a steel material (slab) cannot be crimped. Further, the crystallized material containing Nb, V and Ti that has been crystallized non-uniformly in the steel in the melting stage remains in the steel plate after the rolling, and a precipitate containing the desired Nb, V and Ti is obtained. The stress corrosion cracking resistance decreases.
　一方、加熱温度が（Ｔｘ＋２００）℃を超えると、加熱時のスケールによって表面疵が生じやすく、圧延後の手入れ負荷が増大する。また、鋼素材の表面が過度に脱炭し、圧延後の鋼板表面がマルテンサイトになり、曲げ性や水素脆性が低下する。さらに、オーステナイト粒粗大化により、目的とするミクロ組織を得られない。 On the other hand, if the heating temperature exceeds (Tx + 200) ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. Moreover, the surface of the steel material is excessively decarburized, the surface of the steel sheet after rolling becomes martensite, and bendability and hydrogen embrittlement are reduced. Furthermore, the target microstructure cannot be obtained due to coarsening of austenite grains.
　このため、鋼素材の加熱温度は、（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下とする。好ましくは、（Ｔｘ－３０）℃以上とする。好ましくは（Ｔｘ＋１８０）℃以下とする。なお、直送圧延する場合は、鋼素材が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下で熱間圧延を開始する。 For this reason, the heating temperature of the steel material is set to (Tx-50) ° C. or higher and (Tx + 200) ° C. or lower. Preferably, the temperature is (Tx-30) ° C. or higher. Preferably, it is set to (Tx + 180) ° C. or lower. In the case of direct rolling, hot rolling is started when the steel material is (Tx-50) ° C. or higher and (Tx + 200) ° C. or lower.
　なお、本発明の「Ｔｘ（ｘ＝Ｎｂ、ＶもしくはＴｉ）を式（１）～（３）に示す温度とするとき、式（１）～（３）で定義されるＴｘ（℃）のいずれか１つ以上で、鋼素材の表面温度が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下の温度域に加熱し」とは、例えば、上記成分組成としてＮｂとＶの２種を含有する場合、加熱温度は、（ＴＮｂ－５０）℃以上（ＴＮｂ＋２００）℃以下または（ＴＶ－５０）℃以上（ＴＶ＋２００）℃以下のいずれか１つ以上を満足していれば良いことを意味する。すなわち、どちらの加熱温度を選択してもよい。 In the present invention, when “Tx (x = Nb, V or Ti) is a temperature represented by the formulas (1) to (3), any of Tx (° C.) defined by the formulas (1) to (3) “When heated to a temperature range where the surface temperature of the steel material is (Tx−50) ° C. or higher and (Tx + 200) ° C. or lower” with two or more of Nb and V as the above component composition, for example. The heating temperature should satisfy at least one of (T Nb −50) ° C. or more and (T Nb +200) ° C. or (T V −50) ° C. or more and (T V +200) ° C. or less. Means. That is, either heating temperature may be selected.
　熱間圧延：粗圧延後、仕上圧延での仕上圧延終了温度を７５０℃以上１０００℃以下とし、所望の板厚の鋼板とする
　熱間圧延の仕上圧延終了温度が１０００℃を超えると、鋼板表面近傍のオーステナイトの再結晶が容易に進行し、所望のミクロ組織が得られず、耐応力腐食割れ性の低下を招く。一方、仕上圧延終了温度を７５０℃未満にすると熱間変形抵抗が過度に高くなり、圧延機への負荷が大きくなる。また、圧延能率が低下して製造コストの上昇を招く。このため、熱間圧延の仕上圧延終了温度は、７５０℃以上１０００℃以下とする。好ましくは８００℃以上とする。好ましくは９５０℃以下とする。より好ましくは９４０℃以下とする。 Hot rolling: After rough rolling, the finish rolling finish temperature in finish rolling is set to 750 ° C. or more and 1000 ° C. or less to obtain a steel plate having a desired thickness. When the finish rolling finish temperature of hot rolling exceeds 1000 ° C., the steel plate surface The recrystallization of the nearby austenite proceeds easily, the desired microstructure cannot be obtained, and the stress corrosion cracking resistance is lowered. On the other hand, when the finish rolling finish temperature is less than 750 ° C., the hot deformation resistance becomes excessively high, and the load on the rolling mill increases. In addition, the rolling efficiency is reduced, and the manufacturing cost is increased. For this reason, the finish rolling finish temperature of hot rolling shall be 750 degreeC or more and 1000 degrees C or less. Preferably, the temperature is 800 ° C. or higher. The temperature is preferably 950 ° C. or lower. More preferably, it is set to 940 ° C. or lower.
　仕上圧延での、８５０℃以上（Ｔｘ－５０）℃以下の温度域での累積圧下率１０％以上５０％以下（好適条件）
　８５０℃以上（Ｔｘ－５０）℃以下の温度域での累積圧下率は、１０％未満では、目標とするミクロ組織が得られないおそれがある。一方、５０％超えでは、圧延時の能率が低下してしまう。また、強度が過多になり低温靱性が低下するおそれがある。なお、累積圧下率は、仕上圧延において、８５０℃以上（Ｔｘ－５０）℃以下の温度域となる各圧延パスでの圧下率をそれぞれ加算して合計したものとする。 Cumulative rolling reduction in the temperature range of 850 ° C. or higher (Tx−50) ° C. or lower in finish rolling 10% to 50% (preferred condition)
If the cumulative rolling reduction in the temperature range of 850 ° C. or higher (Tx-50) ° C. or lower is less than 10%, the target microstructure may not be obtained. On the other hand, if it exceeds 50%, the efficiency during rolling decreases. Moreover, there exists a possibility that intensity may become excessive and low-temperature toughness may fall. The cumulative reduction ratio is the sum of the reduction ratios in each rolling pass in the temperature range of 850 ° C. or more (Tx−50) ° C. in finish rolling.
　仕上圧延での、未再結晶温度域（９６０℃以下）での累積圧下率５％以上６０％以下（より好適条件）
　未再結晶温度域での累積圧下率は、５％未満では、目標の強度が得られないおそれがある。一方、６０％超えでは、降伏強度が過多になり低温靱性が低下するおそれがある。なお、累積圧下率は、仕上圧延において、未再結晶温度域となる各圧延パスでの圧下率をそれぞれ加算して合計したものとする。 Cumulative rolling reduction of 5% to 60% (more suitable conditions) in the non-recrystallization temperature range (960 ° C. or less) in finish rolling
If the cumulative rolling reduction in the non-recrystallization temperature range is less than 5%, the target strength may not be obtained. On the other hand, if it exceeds 60%, the yield strength becomes excessive and the low-temperature toughness may be lowered. The cumulative reduction ratio is the sum of the reduction ratios in each rolling pass that is in the non-recrystallization temperature range in finish rolling.
　仕上圧延終了後、（仕上圧延終了温度－５０℃）または冷却開始温度のいずれか低い温度から６５０℃まで、鋼板表面の平均冷却速度１．０℃／ｓ以上で冷却
　鋼板表面の平均冷却速度が１．０℃／ｓ未満では、高温で長時間滞留するため炭化物が粗大化するため、強度が低下する。それだけでなく、Ｃｒ炭化物が形成され、靱性および応力腐食割れ性が低下する。よって、平均冷却速度は１．０℃／ｓ以上とすることが好ましい。より好ましくは２.０℃／ｓ以上とする。一方、平均冷却速度が１５０．０℃／ｓを超えると、鋼板形状の確保が困難になる。よって、平均冷却速度は１５０．０℃／ｓ以下とすることが好ましい。平均冷却速度は１２０．０℃／ｓ以下がより好ましい。さらに好ましくは１００．０℃／ｓ以下とする。ここでの平均冷却速度とは、仕上圧延終了後、（仕上圧延終了温度－５０℃）または冷却開始温度のいずれか低い温度から６５０℃までの冷却速度の平均である。 After finishing rolling, cooling is performed at an average cooling rate of 1.0 ° C./s or more from the lower temperature of (finishing finishing temperature −50 ° C.) or the cooling start temperature to 650 ° C. The average cooling rate of the steel plate surface is If it is less than 1.0 ° C./s, the carbide is coarsened because it stays at a high temperature for a long time, and the strength is lowered. In addition, Cr carbide is formed, and toughness and stress corrosion cracking properties are reduced. Therefore, the average cooling rate is preferably 1.0 ° C./s or more. More preferably, it is set to 2.0 ° C./s or more. On the other hand, if the average cooling rate exceeds 150.0 ° C./s, it becomes difficult to ensure the shape of the steel sheet. Therefore, the average cooling rate is preferably 150.0 ° C./s or less. The average cooling rate is more preferably 120.0 ° C./s or less. More preferably, it shall be 100.0 degrees C / s or less. Here, the average cooling rate is the average of the cooling rates from the lower temperature of (finishing rolling finishing temperature−50 ° C.) or the cooling start temperature to 650 ° C. after finishing rolling.
　本発明では、冷却での平均冷却速度を制御することが、冷却中のＣｒ炭化物析出を抑制し、これにより、耐応力腐食割れ性を向上することに有効であることを新たに見出した。 In the present invention, it has been newly found that controlling the average cooling rate in cooling is effective in suppressing Cr carbide precipitation during cooling, thereby improving the stress corrosion cracking resistance.
　なお、仕上圧延終了温度～（仕上圧延終了温度－５０℃）の温度域の平均冷却速度は、特に規定しないが、Ｎｂ、Ｖ、Ｔｉ系析出物の析出を促進できることから１.０℃／ｓ以下であることが好ましい。また、６５０℃未満の平均冷却速度も特に規定しないが、鋼板の歪を防止する観点から、１００.０℃／ｓ未満とすることが好ましい。より好ましくは８０．０℃／ｓ以下とする。 The average cooling rate in the temperature range from the finish rolling finish temperature to (finish finish finish temperature−50 ° C.) is not particularly specified, but is 1.0 ° C./s because precipitation of Nb, V, and Ti-based precipitates can be promoted. The following is preferable. Moreover, although the average cooling rate below 650 degreeC is not prescribed | regulated, it is preferable to set it as less than 100.0 degreeC / s from a viewpoint of preventing the distortion of a steel plate. More preferably, it is 80.0 ° 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。 Hereinafter, the present invention will be described in detail with reference to examples. The present invention is not limited to the following examples.
　転炉－取鍋精錬－連続鋳造法で、表１－１、表１－２に示す種々の成分組成に調製した鋼スラブ（素材厚：２５０～３００ｍｍ）を、（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下（ｘ＝Ｎｂ、ＶまたはＴｉ）に加熱した後、表２－１、表２－２に示す製造条件で熱間圧延を施した後、表２－１、表２－２に示す製造条件で冷却を行った。なお、Ｎｂ、ＶまたはＴｉにおける（Ｔｘ－５０）℃、（Ｔｘ＋２００）℃は、それぞれ表１－１、表１－２に示す。 Steel slabs (material thickness: 250 to 300 mm) prepared in various components and compositions shown in Table 1-1 and Table 1-2 by the converter-ladder refining-continuous casting method (Tx-50) ° C. or higher ( After heating to Tx + 200) ° C. or lower (x = Nb, V or Ti), hot rolling was performed under the production conditions shown in Table 2-1 and Table 2-2, and then Table 2-1 and Table 2-2 Cooling was performed under the production conditions shown. Note that (Tx-50) ° C. and (Tx + 200) ° C. in Nb, V or Ti are shown in Table 1-1 and Table 1-2, respectively.
　得られた板厚１２ｍｍ～８０ｍｍの熱延鋼板について、ミクロ組織調査、母材引張試験、母材靭性、応力腐食割れ性試験を下記の要領で実施した。
（１）ミクロ組織
　ミクロ組織の調査は、得られた各鋼板の板厚表面下０．５ｍｍの位置における圧延方向に平行な断面について、ミクロ組織観察用サンプルを採取し、ピロ亜硫酸ナトリウム水溶液（１０ｇＮａ２Ｓ２Ｏ５＋９５ｍｌ ｗａｔｅｒ ｓｏｌｕｔｉｏｎ)で浸漬腐食の後、倍率５００倍で光学顕微鏡組織を５視野撮影した。その後、得られた組織画像に対して画像解析装置を用いて、オーステナイトの面積率、円相当径およびアスペクト比を求めた。 The obtained hot-rolled steel sheet having a thickness of 12 mm to 80 mm was subjected to a microstructure investigation, a base material tensile test, a base material toughness, and a stress corrosion cracking test in the following manner.
(1) Microstructure The microstructure was examined by taking a sample for microstructural observation of a cross section parallel to the rolling direction at a position 0.5 mm below the surface of the thickness of each steel sheet, and adding a sodium pyrosulfite aqueous solution (10 g Na After immersion corrosion with 2 S 2 O 5 +95 ml water solution, 5 fields of the optical microscope tissue were photographed at a magnification of 500 times. Thereafter, the area ratio, equivalent circle diameter, and aspect ratio of the austenite were obtained from the obtained tissue image using an image analysis apparatus.
　オーステナイトの面積率
　オーステナイトの面積率は、オーステナイトエッチングをし、５００倍で組織を写真撮影して、オーステナイト粒界をトレースし、画像解析により、オーストナイト面積の全体面積に対する１０μｍ以上のオーステナイトの面積の割合を求めた。 Austenite area ratio The austenite area ratio is austenite etched, the structure is photographed at 500 times, the austenite grain boundary is traced, and by image analysis, the area of austenite is 10 μm or more relative to the total area of austenite The percentage was determined.
　オーステナイトの円相当直径
　オーステナイトの結晶粒径、すなわちオーステナイトの円相当直径は、上述の組織画像に対して画像解析を用いて、個々のオーステナイトの面積を測定した。個々の面積から円相当直径を算出した。 Austenite equivalent circle diameter The crystal grain size of austenite, that is, the equivalent circle diameter of austenite, was measured for the area of each austenite using image analysis on the above-described structure image. The equivalent circle diameter was calculated from each area.
　オーステナイト粒のアスペクト比
　オーステナイト粒のアスペクト比は、上述の腐食によってオーステナイト粒界を現出させた組織を光学顕微鏡で観察し、個々のオーステナイト粒について、一番長い径（長径）に対する、長径と直行するもっとも広い幅（短径）の比を算出した。 Austenite grain aspect ratio The austenite grain aspect ratio is determined by observing the structure of the austenite grain boundary by the above-mentioned corrosion using an optical microscope. The ratio of the widest width (minor axis) was calculated.
　Ｎｂ、Ｖ、Ｔｉ系析出物の円相当直径
　Ｎｂ、Ｖ、Ｔｉ系析出物の円相当直径の調査は、各鋼板の板厚表面下０．５ｍｍの位置における圧延方向に平行な断面について、透過型電子顕微鏡にて５００００倍の撮影を１０視野行い、この組織画像に対して画像解析を用いて、個々のＮｂ、Ｖ、Ｔｉ系析出物の面積を測定した。個々の面積からＮｂ、Ｖ、Ｔｉ系析出物の円相当直径を算出した。 Equivalent circle diameter of Nb, V, and Ti-based precipitates The investigation of equivalent circle diameter of Nb, V, and Ti-based precipitates was conducted on a cross section parallel to the rolling direction at a position 0.5 mm below the surface of each steel plate. Ten fields of view were taken with a scanning electron microscope at a magnification of 50000 times, and the area of each Nb, V, Ti-based precipitate was measured using image analysis on the tissue image. The equivalent circle diameter of the Nb, V, and Ti-based precipitates was calculated from each area.
　Ｎｂ、Ｖ、Ｔｉ系析出物の個数密度
　Ｎｂ、Ｖ、Ｔｉ系析出物の個数密度の調査は、各鋼板の板厚表面下０．５ｍｍの位置における圧延方向に平行な断面について、透過型電子顕微鏡にて５００００倍の撮影を１０視野行い、１ｍｍ２当たりの、円相当直径が０．０１～０．５μｍであるＮｂ、Ｖ、Ｔｉ系析出物の個数を調べて、Ｎｂ、Ｖ、Ｔｉ系析出物の合計の個数密度を求めた。
（２）母材引張特性
　得られた各鋼板より、ＪＩＳ５号引張試験片を採取し、ＪＩＳ　Ｚ　２２４１（１９９８年）の既定に準拠して引張試験を実施し、引張特性を調査した。本発明では、降伏強度４００ＭＰａ以上を母材引張特性に優れるもの（本発明範囲内）とした。なお、本発明の母材引張特性に優れるものは、引張強度８００ＭＰａ以上、全伸び３０％以上であった。
（３）母材靭性
　板厚２０ｍｍを超える各鋼板の板厚１／４位置、もしくは板厚２０ｍｍ以下の各鋼板の板厚１／２位置の圧延方向と垂直な方向から、ＪＩＳ　Ｚ　２２０２（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　Ｚ　２２４２（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５０Ｊ以上を母材靭性に優れるもの（本発明範囲内）とした。さらに好ましくは、吸収エネルギー（ｖＥ－１９６）の平均値が１００Ｊ以上とした。
（４）応力腐食割れ性
　応力腐食割れ性試験は、ＮＡＣＥ Ｓｔａｎｄａｒｄ　ＴＭ０１１１－２０１１基準のＳｌｏｗ Ｓｔｒａｉｎ Ｒａｔｅ Ｔｅｓｔ Ｍｅｔｈｏｄに準拠して実施した。試験片形状はＴｙｐｅＡ丸棒切欠き付き試験片を用い、温度２３℃で人工海水（塩化物イオン濃度１８０００ｐｐｍ）に浸漬し、ひずみ速度：４×１０－７ｉｎｃｈ／ｓｅｃ.で等速引張試験を実施した。本発明では、破断応力が５００ＭＰa以上を耐応力腐食割れ性に優れるもの（本発明範囲内）とした。さらに好ましくは破断応力が６００ＭＰa以上とした。 Number density of Nb, V, and Ti-based precipitates The number density of Nb, V, and Ti-based precipitates was determined by examining transmission electron for a cross section parallel to the rolling direction at a position 0.5 mm below the surface of each steel plate. Ten fields of view were taken with a microscope at 50000 times, and the number of Nb, V, and Ti-based precipitates having an equivalent circle diameter of 0.01 to 0.5 μm per 1 mm 2 was examined. The total number density of precipitates was determined.
(2) Base material tensile properties JIS No. 5 tensile test specimens were collected from each of the obtained steel plates, and subjected to a tensile test in accordance with the default of JIS Z 2241 (1998) to investigate the tensile properties. In the present invention, a yield strength of 400 MPa or more is assumed to have excellent base material tensile properties (within the scope of the present invention). In addition, what was excellent in the base material tensile characteristics of this invention was the tensile strength of 800 MPa or more and the total elongation of 30% or more.
(3) Base material toughness JIS Z 2202 (1998) from the direction perpendicular to the rolling direction of the plate thickness 1/4 position of each steel plate exceeding 20 mm thickness or the plate thickness 1/2 position of each steel plate thickness 20 mm or less. Charpy V-notch test specimens were collected in accordance with the provisions of JIS Z 2242, and three Charpy impact tests were performed on each steel sheet in accordance with the provisions of JIS Z 2242 (1998), and the absorbed energy at -196 ° C And the toughness of the base material was evaluated. In the present invention, the average value of three absorbed energy (vE -196) is more than 50J was excellent in base metal toughness (within the scope of the present invention). More preferably, the average value of absorbed energy (vE −196 ) is 100 J or more.
(4) Stress Corrosion Cracking Test The stress corrosion cracking test was carried out in accordance with the Narrow Standard TM0111-2011 standard Slow Strain Rate Test Method. The shape of the specimen is a Type A round bar notched specimen, immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C., and subjected to a constant velocity tensile test at a strain rate of 4 × 10 −7 inch / sec. Carried out. In the present invention, the fracture stress is 500 MPa or more and is excellent in stress corrosion cracking resistance (within the scope of the present invention). More preferably, the breaking stress is 600 MPa or more.
　以上により得られた結果を、表３－１、表３－２に示す。 The results obtained as described above are shown in Tables 3-1 and 3-2.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
　本発明例は、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５０Ｊ以上、耐応力腐食割れ性が破断応力で５００ＭＰa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、母材強度、低温靭性、および耐応力腐食割れ性のいずれか１つ以上が、上述の目標性能を満足できない。なお、表３－１、３－２において、比較例である鋼板Ｎｏ．１２、３６は、成分組成のうちＣが本発明の範囲を外れるため安定なオーステナイトが少ないが、不安定なオーステナイトが多いため、平均円相当直径が１０μｍ以上かつ長径と短径のアスペクト比が３以上のオーステナイトの面積率が７０％であった。 Examples The present invention is, target performance of the above (yield strength of the base material is more than 400 MPa, 50 J or more in the average value of the low-temperature toughness absorbed energy (vE -196), more 500MPa stress corrosion cracking resistance in breaking stress) satisfies Confirmed to do. On the other hand, in a comparative example that is out of the scope of the present invention, any one or more of the base material strength, the low temperature toughness, and the stress corrosion cracking resistance cannot satisfy the above target performance. In Tables 3-1, 3-2, the steel plate No. Nos. 12 and 36 have a stable austenite because C is out of the range of the present invention, but there are a lot of unstable austenites. Therefore, the average equivalent circle diameter is 10 μm or more and the aspect ratio of major axis to minor axis is 3 The area ratio of the above austenite was 70%.

Claims (4)

  1. 　質量％で、
    Ｃ：０．２０～０．７０％、
    Ｓｉ：０．０５～１．０％、
    Ｍｎ：１５～３０％、
    Ｐ：０．０２８％以下、
    Ｓ：０．０２％以下、
    Ａｌ：０．０１～０．１％、
    Ｃｒ：０．５～７．０％、
    Ｎｉ：０．０３～０．３０％、
    Ｎ：０．００１０～０．０２００％
    を含有し、
    Ｎｂ：０．００３～０．０３０％、
    Ｖ：０．０３～０．１０％、
    Ｔｉ：０．００３～０．０４０％
    の１種または２種以上を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、
    鋼板表面下０．５ｍｍのミクロ組織が、
    オーステナイトを基地相とし、
    当該オーステナイトのうち面積率で２５％以上が、円相当直径で１０μｍ以上であり、かつ長径と短径のアスペクト比が３以上
    である高Ｍｎ鋼板。     % By mass
    C: 0.20 to 0.70%,
    Si: 0.05 to 1.0%,
    Mn: 15-30%
    P: 0.028% or less,
    S: 0.02% or less,
    Al: 0.01 to 0.1%,
    Cr: 0.5 to 7.0%,
    Ni: 0.03-0.30%,
    N: 0.0010 to 0.0200%
    Containing
    Nb: 0.003 to 0.030%,
    V: 0.03-0.10%,
    Ti: 0.003-0.040%
    One or more of the following, having a component composition consisting of the balance Fe and inevitable impurities,
    The microstructure of 0.5mm below the steel sheet surface
    With austenite as the base phase,
    A high-Mn steel sheet having an area ratio of 25% or more of the austenite, an equivalent circle diameter of 10 μm or more, and an aspect ratio of a major axis to a minor axis of 3 or more.
  2. 　前記成分組成に加えて、さらに下記のグループＡまたはＢのうちから選択された少なくとも一つのグループの元素を含有する請求項１に記載の高Ｍｎ鋼板。
    　　　　　　　　　　　　記
    グループＡ：質量％で、
    Ｍｏ：０．０５～２．０％、
    Ｗ：０．０５～２．０％
    のうちから選んだ１種または２種
    グループＢ：質量％で、
    Ｃａ：０．０００５～０．００５０％、
    Ｍｇ：０．０００５～０．００５０％、
    ＲＥＭ：０．００１０～０．０２００％
    のうちから選んだ１種または２種以上     The high Mn steel sheet according to claim 1, further comprising at least one element selected from the following group A or B in addition to the component composition.
    Group A:% by mass
    Mo: 0.05-2.0%,
    W: 0.05-2.0%
    1 type or 2 types selected from the group B: mass%,
    Ca: 0.0005 to 0.0050%,
    Mg: 0.0005 to 0.0050%,
    REM: 0.0010 to 0.0200%
    One or more selected from
  3. 　鋼板表面下０．５ｍｍの前記ミクロ組織が、さらに、前記ミクロ組織中に、円相当直径が０．０１～０．５μｍである、Ｎｂ、Ｖ、Ｔｉの１種または２種以上を含有する炭化物、窒化物および炭窒化物を、合計で２×１０２個／ｍｍ２以上を有する請求項１または２に記載の高Ｍｎ鋼板。 Carbide containing one or more of Nb, V and Ti, wherein the microstructure 0.5 mm below the surface of the steel sheet further has an equivalent circle diameter of 0.01 to 0.5 μm in the microstructure. The high Mn steel sheet according to claim 1, having a total of 2 × 10 2 pieces / mm 2 or more of nitride, carbonitride.
  4. 　請求項１～３のいずれか１項に記載の成分組成を有する鋼素材を
    Ｔｘ（ｘ＝Ｎｂ、ＶまたはＴｉ）を式（１）～（３）に示す温度とするとき、式（１）～（３）で定義されるＴｘ（℃）のいずれか１つ以上で、鋼素材の表面温度が（Ｔｘ－５０）℃以上（Ｔｘ＋２００）℃以下の温度域に加熱し、
    仕上圧延終了温度が７５０℃以上１０００℃以下の熱間圧延し、鋼板を製造し、
    その後、（仕上圧延終了温度－５０℃）または冷却開始温度のいずれか低い温度から６５０℃までの鋼板表面の平均冷却速度が１．０℃／ｓ以上で冷却する
    高Ｍｎ鋼板の製造方法。
    ＴＮｂ（℃）＝７５００／｛３．０－ｌｏｇ１０（[％Ｎｂ]×[％Ｃ]）｝－２７３　・・・（１）
    ＴＶ（℃）＝１０８００／｛７．２－ｌｏｇ１０（[％Ｖ]×[％Ｃ]）｝－２７３　・・・（２）
    ＴＴｉ（℃）＝７０００／｛２．８－ｌｏｇ１０（[％Ｔｉ]×[％Ｃ]）｝－２７３　・・・（３）
    　ここで、[％Ｎｂ]、[％Ｖ]、[％Ｔｉ]および[％Ｃ]は、それぞれ鋼中のＮｂ、Ｖ、ＴｉおよびＣの含有量（質量％）を示す。含まない元素の場合は、式中の元素記号を０として計算する。     When the steel material having the component composition according to any one of claims 1 to 3 is set to Tx (x = Nb, V or Ti) at a temperature represented by formulas (1) to (3), formula (1) Heat to a temperature range where the surface temperature of the steel material is (Tx−50) ° C. or more and (Tx + 200) ° C. at any one or more of Tx (° C.) defined in (3),
    Hot rolling at a finish rolling finishing temperature of 750 ° C. or higher and 1000 ° C. or lower to produce a steel plate,
    Then, a method for producing a high Mn steel sheet, in which the average cooling rate of the steel sheet surface from the lower temperature of (finishing finish temperature −50 ° C.) or the cooling start temperature to 650 ° C. is 1.0 ° C./s or more.
    T Nb (° C.) = 7500 / {3.0−log 10 ([% Nb] × [% C])} − 273 (1)
    T V (° C.) = 10800 / {7.2-log 10 ([% V] × [% C])} − 273 (2)
    T Ti (° C.) = 7000 / {2.8−log 10 ([% Ti] × [% C])} − 273 (3)
    Here, [% Nb], [% V], [% Ti] and [% C] indicate the contents (mass%) of Nb, V, Ti and C in the steel, respectively. In the case of an element not included, the element symbol in the formula is calculated as 0.
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