WO2021117382A1 - Steel sheet and method for manufacturing same - Google Patents

Steel sheet and method for manufacturing same Download PDF

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Publication number
WO2021117382A1
WO2021117382A1 PCT/JP2020/041265 JP2020041265W WO2021117382A1 WO 2021117382 A1 WO2021117382 A1 WO 2021117382A1 JP 2020041265 W JP2020041265 W JP 2020041265W WO 2021117382 A1 WO2021117382 A1 WO 2021117382A1
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steel sheet
steel
contained
average
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PCT/JP2020/041265
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French (fr)
Japanese (ja)
Inventor
佐藤　祐也
植田　圭治
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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 JP2021507720A priority Critical patent/JP7156500B2/en
Priority to CN202080085120.XA priority patent/CN114829646A/en
Priority to KR1020227009500A priority patent/KR20220048031A/en
Publication of WO2021117382A1 publication Critical patent/WO2021117382A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • 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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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 steel sheet having excellent stress corrosion cracking resistance, which is suitable for structural steel used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, and a method for producing the same.
  • Patent Documents 1, 2 and 3 propose a low-temperature steel sheet having a Ni content lower than 9% and having performance equal to or higher than that of a 9% Ni steel sheet.
  • Ni-containing steel materials described in Patent Documents 1, 2 and 3 are excellent in low-temperature toughness, stress corrosion cracking caused by hydrogen is not mentioned, and there is still room for examination. That is, for example, in the case of a marine LNG tank, since sulfide and chloride are contained in the usage environment, there is a high possibility that stress corrosion cracking due to hydrogen will occur, so that it is durable against stress corrosion cracking. It is also required to have stress corrosion cracking resistance.
  • the present invention has been made in view of the above problems, and an object of the present invention is to provide a steel sheet having excellent stress corrosion cracking resistance, which is particularly suitable for use in a low temperature environment.
  • the present inventors have conducted intensive studies on the composition and structure of the steel sheet, and obtained the following findings.
  • the structure up to 1 mm below the surface of the steel sheet can be oriented. It is a microstructure with a grain size of 5 ⁇ m or less surrounded by large-angle grain boundaries with a difference of 15 ° or more. Then, the microstructure disperses hydrogen trap sites, so that crack growth due to hydrogen embrittlement can be reduced.
  • the present invention has been made by further studying the above findings, and the gist thereof is as follows. 1.
  • C By mass% C: 0.01% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 3.00% or less, Al: 0.002% or more and 0.100% or less, Ni: 5.0% or more and 10.0% or less, N: 0.0010% or more and 0.0080% or less, Co: More than 0% and less than 1.50%, It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition of the balance Fe and unavoidable impurities.
  • the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 ⁇ m or less, and the maximum circle equivalent diameter of the retained austenite grains is 1 ⁇ m.
  • composition of the components is further increased by mass%.
  • Nb 0.001% or more and 0.030% or less
  • V 0.01% or more and 0.10% or less
  • Ti 0.003% or more and 0.050% or less
  • B 0.0003% or more and 0.0100% or less
  • Cu 0.01% or more and 1.00% or less
  • Cr 0.01% or more and 1.50% or less
  • Sn 0.01% or more and 0.50% or less
  • Sb 0.01% or more and 0.50% or less
  • the steel sheet according to 1 above which contains one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less.
  • composition of the components is further increased by mass%.
  • Ca 0.0005% or more and 0.0050% or less
  • Zr 0.0005% or more and 0.0050% or less
  • a method for producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment.
  • the present invention it is possible to provide a steel sheet having high durability against stress corrosion cracking due to hydrogen.
  • a steel structure used in a low temperature environment such as a tank for a liquefied gas storage tank, the safety of the steel structure can be improved, which brings about a remarkable industrial effect.
  • C 0.01% or more and 0.15% or less C is effective for increasing the strength, and in order to obtain the effect, it is necessary to contain C at 0.01% or more. On the other hand, if it is contained in excess of 0.15%, the low temperature toughness is lowered. Therefore, C is set to 0.01% or more and 0.15% or less. Preferably, it is 0.03% or more. Preferably, it is 0.10% or less.
  • Si acts as an antacid and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, if it is contained in excess of 1.00%, the low temperature toughness deteriorates. Therefore, Si is set to 0.01% or more and 1.00% or less. Preferably, it is 0.03% or more. Preferably, it is 0.5% or less.
  • Mn 0.10% or more and 3.00% or less
  • Mn is an element effective for enhancing the hardenability of steel and increasing the strength of steel sheets. In order to obtain the effect, Mn needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 3.00%, the corrosion cracking resistance is lowered. Therefore, Mn is set in the range of 0.10% or more and 3.00% or less. Preferably, it is 0.20% or more. It is preferably 2.00% or less, more preferably 1.00% or less.
  • Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. Further, it has the effect of fixing the solid solution N in the steel to form AlN and suppressing the deterioration of toughness due to the reduction of the solid solution N. On the other hand, if it is contained in excess of 0.100%, the toughness is deteriorated, so the content is set to 0.100% or less. Preferably, it is 0.010% or more. Preferably, it is 0.070% or less. More preferably, it is 0.020% or more. More preferably, it is 0.060% or less.
  • Ni 5.0% or more and 10.0% or less Ni is an extremely effective element for improving the low temperature toughness of steel sheets.
  • the Ni content is set to 10.0% or less.
  • the Ni content is set to 5.0% or more and 10.0% or less.
  • it is 9.5% or less.
  • it is 6.0% or more.
  • N 0.0010% or more and 0.0080% or less N forms a precipitate in steel, and when the content exceeds 0.0080%, when the steel plate is welded to form a welded structure, the base material And it causes a decrease in toughness of the weld heat affected zone.
  • N is also an element that contributes to the refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. It is preferably 0.0020% or more. More preferably, it is 0.0060% or less.
  • Co More than 0% and less than 1.50% Co is an important element that concentrates on the surface layer of steel sheets in a corrosive environment and contributes to suppressing corrosion cracking by reducing the invasion of hydrogen. Therefore, it needs to be contained in excess of 0%.
  • the amount of Co is preferably 0.05% or more, more preferably 0.1% or more. However, even if it is contained in excess of 1.50%, the effect is saturated and Co is an expensive element, so the maximum addition amount is 1.50%.
  • P 0.030% or less
  • the content of P is preferably 0.025% or less, and more preferably 0.020% or less. It goes without saying that the content of P may be 0%, but since removing P requires a high cost, it is preferably 0.002% or more from the viewpoint of cost.
  • S 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to limit S to 0.0050% and reduce it as much as possible. It is preferably 0.0020% or less. It goes without saying that the S content may be 0%, but since removing S requires a high cost, it is preferably 0.0005% or more from the viewpoint of cost.
  • Nb 0.001% or more and 0.030% or less
  • Nb is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add Nb in an amount of 0.001% or more. On the other hand, if it is contained in excess of 0.030%, coarse carbonitride may be precipitated and the toughness of the base metal may be deteriorated. Therefore, when Nb is contained, it is set to 0.001% or more and 0.030% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.
  • V 0.01% or more and 0.10% or less
  • V is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add V at 0.01% or more.
  • V is set to 0.01% or more and 0.10% or less. It is preferably 0.02% or more, more preferably 0.03% or more. It is preferably 0.09% or less, more preferably 0.08% or less.
  • Ti 0.003% or more and 0.050% or less
  • Ti is an element that precipitates as a nitride or carbonitride and is effective for improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to add Ti at 0.003% or more. On the other hand, if it is contained in excess of 0.050%, the precipitate may become coarse and the toughness of the base metal may be deteriorated. In addition, coarse carbonitride may precipitate and serve as a starting point for fracture. Therefore, when Ti is contained, it is set to 0.003% or more and 0.050% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.
  • B 0.0003% or more and 0.0100% or less B is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add B at 0.0003% or more. On the other hand, if it is contained in excess of 0.0100%, coarse B precipitates may be formed and the toughness may be lowered. Therefore, B is in the range of 0.0003% or more and 0.0100% or less. Preferably, it is 0.0030% or less.
  • Cu 0.01% or more and 1.00% or less
  • Cu is an element effective for increasing the strength of a steel sheet by improving hardenability, but when the content exceeds 1.00%, the low temperature toughness of the steel sheet decreases. There is a risk of Therefore, when Cu is contained, the content thereof is preferably 1.00% or less. On the other hand, if it is less than 0.01%, the effect of increasing the strength cannot be obtained. Therefore, when it is added, it is preferably 0.01% or more. More preferably, it is 0.10% or more and 0.30% or less.
  • Cr 0.01% or more and 1.50% or less Cr is an element that contributes to the improvement of low temperature toughness and corrosion resistance of high Mn steel.
  • the amount of Cr is preferably 0.01% or more.
  • Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates causes corrosion and fracture to reduce low temperature toughness. It is preferably 1.50%. More preferably, it is 1.00% or less.
  • Mo 0.03% or more and 1.00% or less
  • Mo is an element effective for suppressing the temper embrittlement sensitivity of a steel sheet, and is also an element for increasing the strength of a steel sheet without impairing low temperature toughness.
  • the Mo content is preferably 0.03% or more.
  • Mo exceeds 1.00%, the low temperature toughness may decrease. Therefore, when Mo is contained, the content is set to 0.03% or more and 1.00% or less. More preferably, it is more than 0.05% and 0.30% or less.
  • Sn 0.01% or more and 0.50% or less
  • Sb 0.01% or more and 0.50% or less
  • W 0.05% or more and 2.00% or less
  • Sn, Sb and W are elements effective for improving corrosion resistance. is there. These effects are exhibited when Sn and Sb are 0.01% or more and W is 0.05% or more.
  • the Sn amount is in the range of 0.01% or more and 0.50% or less
  • the Sb amount is in the range of 0.01% or more and 0.50% or less
  • the W amount is in the range of 0.05% or more and 2.00% or less.
  • the Sn amount is 0.02% or more and 0.25% or less
  • the Sb amount is 0.02% or more and 0.25% or less
  • the W amount is 0.10% or more and 1.00% or less.
  • the following elements can be contained, if necessary.
  • the following Ca, Zr, Mg and REM are elements useful for morphological control of inclusions such as MnS, and can be added as needed.
  • the morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Toughness and sulfide stress corrosion cracking resistance are improved through morphological control of the inclusions.
  • Ca, Zr and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more.
  • the content is 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, the content is 0.0010% or more and 0.0100% or less.
  • the Ca amount is 0.0010% or more and 0.0040% or less
  • the Zr amount is 0.0010% or more and 0.0040% or less
  • the Mg amount is 0.0010% or more and 0.0040% or less
  • the REM amount is 0. It shall be 0020% or more and 0.0100% or less.
  • the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries having an orientation difference of 15 ° or more is 5 ⁇ m or less.
  • the maximum circular equivalent diameter of the retained austenite grains is 1 ⁇ m or less.
  • the average equivalent circle diameter of the crystal grains is preferably 4 ⁇ m or less, more preferably 3 ⁇ m or less.
  • the crystal grains surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more and the average circle-equivalent diameter of the crystal grains can be specified by the measurement method in the examples described later.
  • the temperature is determined after hot rolling or, if heat treatment is performed after hot rolling, after the heat treatment. Perform cooling treatment with an average cooling rate of 1 ° C./s or higher in the region.
  • the maximum circle-equivalent diameter of the retained austenite grains needs to be 1 ⁇ m or less. This is because by setting the diameter corresponding to the maximum circle to 1 ⁇ m or less, the hydrogen traps on the retained austenite are dispersed and the local concentration of the hydrogen traps is avoided, and as a result, the crack growth due to hydrogen embrittlement can be reduced.
  • the amount of retained austenite in the surface layer structure is preferably 15% or less in terms of area ratio, and more preferably 10% or less.
  • the structure of the steel sheet is preferably martensite and / or bainite. At that time, the area ratio of martensite and / or bainite is preferably 80% or more.
  • the conditions for manufacturing the steel sheet of the present invention will be described. That is, it can be produced by heating a steel material having the above-mentioned composition and cooling it by hot rolling, or by further heat-treating it after hot rolling and cooling it. At that time, in the cooling after hot rolling, or when the heat treatment is performed after the hot rolling, the average cooling rate in a predetermined temperature range is set to 1 ° C./s or more in the cooling after the heat treatment. Needed to get.
  • the manufacturing conditions will be described in the order of steps. In the following description, the temperature (° C.) means the temperature at the center of the plate thickness.
  • the reheating temperature of the steel material in hot rolling is preferably 1000 ° C. or higher and 1300 ° C. or lower.
  • Heating temperature of steel material 1000 ° C or higher and 1300 ° C or lower.
  • the heating temperature is preferably 1000 ° C. or higher and 1300 ° C. or lower. .. That is, if the heating temperature is less than 900 ° C., the precipitate may not be sufficiently solid-solved, so that the desired characteristics may not be obtained.
  • the material may be deteriorated due to the coarsening of the crystal grain size, and excessive energy may be required for production to reduce the productivity. More preferably, it is in the range of 1050 ° C. or higher and 1250 ° C. or lower, and further preferably 1100 ° C. or higher and 1250 ° C. or lower.
  • cooling treatment is performed after hot rolling.
  • the average cooling rate of the surface layer structure in the temperature range of 600 ° C. or lower and 200 ° C. or higher is set to 1 ° C./s or higher. That is, when the cooling rate in this cooling treatment is less than 1 ° C./s, the surface layer structure becomes an upper bainite structure, the large-angle grain boundaries contained in the structure are reduced, the structure is not sufficiently finely divided, and stress corrosion cracking resistance is deteriorated. I can't get it.
  • the upper limit of the average cooling rate does not need to be particularly limited. When the heat treatment described later is performed after the hot rolling, it is not necessary to set the cooling rate after the hot rolling to 1 ° C./s or more.
  • Heat treatment after hot rolling After hot rolling, the following heat treatment may be performed without cooling. As described above, in order to preferably make the surface layer structure of the steel sheet a martensite and / or bainite structure and increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, it is hot.
  • heat treatment is performed after rolling, it is preferable to perform hot rolling by heating to Acc 3 points or more and 900 ° C. or less for quenching (primary quenching). That is, if the heating temperature is less than 3 points of Ac or exceeds 900 ° C., the equivalent circle diameter of the large-angle grain boundary becomes coarse, and there is a possibility that the desired characteristics cannot be obtained.
  • the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
  • the low temperature toughness of the base metal can be improved.
  • the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
  • the retained austenite grains in the surface layer structure In order to obtain properties such as high strength and excellent low temperature toughness, it is effective to make the retained austenite grains in the surface layer structure into fine grains having a diameter of 1 ⁇ m or less.
  • steel plates (Sample Nos. 1 to 26) having a thickness of 30 to 50 mm are manufactured according to the manufacturing conditions shown in Table 2, and each sample is prepared. It was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, for each sample, the spacing between large-angle grain boundaries and the residual austenite grain size in the surface structure were investigated.
  • the large-angle grain boundary was defined as a grain boundary with a grain boundary orientation difference of 15 ° or more, and this was specified using EBSD. Then, the crystal grain size was measured in an arbitrary range of 500 ⁇ 500 ⁇ m at a depth of 1 mm from the surface of the steel sheet, and the average value of the equivalent circle diameters of the crystal grains surrounded by the large angle grain boundaries was obtained. The range surrounded by the large-angle grain boundaries of less than 0.1 ⁇ m was excluded from the calculation.
  • the retained austenite grains existing in the same EBSD measurement region were specified from the crystal structure, and the circle-equivalent diameter of the largest of the crystal grains recognized as austenite was used.
  • Sample No. according to the present invention It was confirmed that Nos. 1 to 14, 23, and 26 ensure low temperature toughness and have excellent stress corrosion cracking resistance.
  • the absorbed energy is lower than 34J, or the DCB test is less than 25MPa ⁇ m%, and the above-mentioned target performance is achieved. I wasn't satisfied.

Abstract

Provided is a steel sheet that is suitable for use in low-temperature environments and has superior stress corrosion cracking resistance. The steel sheet has the following component composition in terms of mass%: C: 0.01% or more and 0.15% or less; Si: 0.01% or more and 1.00% or less; Mn: 0.10% or more and 3.00% or less; Al: 0.002% or more and 0.100% or less; Ni: 5.0% or more and 10.0% or less; N: 0.0010% or more and 0.0080% or less; Co: greater than 0% and 1.50% or less; P: 0.030% or less; and S: 0.0050% or less, with the remainder being constituted by Fe and inevitable impurities. In the structure of the steel sheet from the surface thereof to a location having a depth of 1 mm, the average equivalent circle diameter of crystal grains surrounded by high-angle grain boundaries having a misorientation of 15° or greater is 5 μm or less, and the maximum equivalent circle diameter of residual austenite grains is 1 μm or less.

Description

鋼板およびその製造方法Steel plate and its manufacturing method
　本発明は、例えば液化ガス貯槽用タンク等の、極低温環境下で使用される構造用鋼に供して好適な、特に耐応力腐食割れ性に優れる鋼板およびその製造方法に関する。 The present invention relates to a steel sheet having excellent stress corrosion cracking resistance, which is suitable for structural steel used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, 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は、液化天然ガスの沸点である－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虞があるため、適用される鋼板に対する低温靱性向上の要求は強い。この要求に対して、従来は、７％Ｎｉ鋼板や９％Ｎｉ鋼板が使用されている。 When a hot-rolled steel sheet is used for a structure for a liquefied gas storage tank, the operating environment is extremely low, so not only the strength of the steel sheet but also the toughness at the extremely low temperature is required. For example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary to secure excellent toughness at -164 ° C. or lower, which is the boiling point of liquefied natural gas. If the low temperature toughness of the steel material is inferior, the safety of the structure for the cryogenic storage tank may not be maintained. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel sheet. Conventionally, a 7% Ni steel sheet or a 9% Ni steel sheet has been used in response to this requirement.
　例えば、特許文献１、２および３には、９％より低いＮｉ含有量にて９％Ｎｉ鋼板と同等以上の性能を有する低温用鋼板について提案されている。 For example, Patent Documents 1, 2 and 3 propose a low-temperature steel sheet having a Ni content lower than 9% and having performance equal to or higher than that of a 9% Ni steel sheet.
国際公開第２００７／０３４５７６号International Publication No. 2007/034576 国際公開第２００７／０８０６４６号International Publication No. 2007/080646 特開２０１１－２４１４１９号公報Japanese Unexamined Patent Publication No. 2011-241419
　しかしながら、特許文献１、２および３に記載のＮｉ含有鋼材は、低温靱性に優れるものの、水素起因の応力腐食割れについて言及されておらず、未だ検討の余地があった。すなわち、例えば船舶用LNGタンクの場合は、その使用環境に硫化物や塩化物が含まれることから、水素起因の応力腐食割れが発生する可能性が高いために、応力腐食割れに対する耐久性である、耐応力腐食割れ性も兼備することが求められている。 However, although the Ni-containing steel materials described in Patent Documents 1, 2 and 3 are excellent in low-temperature toughness, stress corrosion cracking caused by hydrogen is not mentioned, and there is still room for examination. That is, for example, in the case of a marine LNG tank, since sulfide and chloride are contained in the usage environment, there is a high possibility that stress corrosion cracking due to hydrogen will occur, so that it is durable against stress corrosion cracking. It is also required to have stress corrosion cracking resistance.
　本発明は係る問題に鑑みなされたものであり、特に低温環境下での使用に適合する、耐応力腐食割れ性に優れる鋼板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 of the present invention is to provide a steel sheet having excellent stress corrosion cracking resistance, which is particularly suitable for use in a low temperature environment.
　本発明者らは、上記課題を解決するため、鋼板の成分組成および組織に関して鋭意研究を行い、以下の知見を得た。
（１）Ｃｏを添加することによって、鋼板表面で腐食が進行した際に鋼板表面にＣｏが濃化し、鋼中への水素侵入を低減し、水素脆化によるき裂進展を低減できる。 In order to solve the above problems, the present inventors have conducted intensive studies on the composition and structure of the steel sheet, and obtained the following findings.
(1) By adding Co, when corrosion progresses on the surface of the steel sheet, Co is concentrated on the surface of the steel sheet, hydrogen invasion into the steel can be reduced, and crack growth due to hydrogen embrittlement can be reduced.
（２）熱間圧延後の冷却、もしくは熱処理(焼入または２相域焼入)後の冷却における、冷却速度を１℃/s以上とすることによって、鋼板表面下１mmまでの組織は、方位差１５°以上の大角粒界で囲まれた結晶粒の径が５μm以下である微細組織となる。そして、この微細組織が水素のトラップサイトを分散させることにより、水素脆化によるき裂進展を軽減できる。 (2) By setting the cooling rate to 1 ° C./s or higher in cooling after hot rolling or cooling after heat treatment (quenching or two-phase region quenching), the structure up to 1 mm below the surface of the steel sheet can be oriented. It is a microstructure with a grain size of 5 μm or less surrounded by large-angle grain boundaries with a difference of 15 ° or more. Then, the microstructure disperses hydrogen trap sites, so that crack growth due to hydrogen embrittlement can be reduced.
（３）鋼板表層における残留オーステナイトの最大円相当径を１μm以下とすることにより、残留オーステナイトへの水素トラップの局所集中を分散でき、水素脆化によるき裂進展を軽減できる。 (3) By setting the maximum circular equivalent diameter of retained austenite on the surface layer of the steel sheet to 1 μm or less, the local concentration of hydrogen traps on retained austenite can be dispersed, and crack growth due to hydrogen embrittlement can be reduced.
　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：０．０１％以上０．１５％以下、
　Ｓｉ：０．０１％以上１．００％以下、
　Ｍｎ：０．１０％以上３．００％以下、
　Ａｌ：０．００２％以上０．１００％以下、
　Ｎｉ：５．０％以上１０．０％以下、
　Ｎ：０．００１０％以上０．００８０％以下、
　Ｃｏ：０％超１．５０％以下、
　Ｐ：０．０３０％以下および
　Ｓ：０．００５０％以下
を含有し、残部Ｆｅおよび不可避的不純物である成分組成を有し、
　鋼板の表面から深さが１ｍｍの位置までの組織は、方位差１５°以上の大角粒界で囲まれた結晶粒の平均円相当径が５μm以下、かつ残留オーステナイト粒の最大円相当径が１μm以下である鋼板。 The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. By mass%
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 3.00% or less,
Al: 0.002% or more and 0.100% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.0010% or more and 0.0080% or less,
Co: More than 0% and less than 1.50%,
It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition of the balance Fe and unavoidable impurities.
In the structure from the surface of the steel sheet to the position of 1 mm in depth, the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 μm or less, and the maximum circle equivalent diameter of the retained austenite grains is 1 μm. The following steel plate.
２．前記成分組成は、さらに質量％で、
　Ｎｂ：０．００１％以上０．０３０％以下、
　Ｖ：０．０１％以上０．１０％以下、
　Ｔｉ：０．００３％以上０．０５０％以下、
　Ｂ：０．０００３％以上０．０１００％以下、
　Ｃｕ：０．０１％以上１．００％以下、
　Ｃｒ：０．０１％以上１．５０％以下、
　Ｓｎ：０．０１％以上０．５０％以下、
　Ｓｂ：０．０１％以上０．５０％以下、
　Ｍｏ：０．０３％以上１．００％以下および
　Ｗ：０．０５％以上２．００％以下
から選択される１種または２種以上を含有する前記１に記載の鋼板。 2. The composition of the components is further increased by mass%.
Nb: 0.001% or more and 0.030% or less,
V: 0.01% or more and 0.10% or less,
Ti: 0.003% or more and 0.050% or less,
B: 0.0003% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.50% or less,
Sn: 0.01% or more and 0.50% or less,
Sb: 0.01% or more and 0.50% or less,
The steel sheet according to 1 above, which contains one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less.
３．前記成分組成は、さらに質量％で、
　Ｃａ：０．０００５％以上０．００５０％以下、
　Ｚｒ：０．０００５％以上０．００５０％以下、
　Ｍｇ：０．０００５％以上０．００５０％以下および
　ＲＥＭ：０．００１０％以上０．０１００％以下
から選択される１種または２種以上を含有する前記１または２に記載の鋼板。 3. 3. The composition of the components is further increased by mass%.
Ca: 0.0005% or more and 0.0050% or less,
Zr: 0.0005% or more and 0.0050% or less,
The steel sheet according to 1 or 2 above, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% or less.
４．前記１から３のいずれかに記載の成分組成を有する鋼素材を加熱し、熱間圧延を施した後に冷却処理を行う鋼板の製造方法において、前記冷却処理における６００℃以下２００℃以上の温度域での平均冷却速度を１℃/s以上とする、鋼板の製造方法。 4. In the method for producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then cooled, the temperature range of 600 ° C. or lower and 200 ° C. or higher in the cooling treatment is performed. A method for manufacturing a steel sheet, in which the average cooling rate in the steel sheet is 1 ° C./s or more.
５．前記１から３のいずれかに記載の成分組成を有する鋼素材を加熱し、熱間圧延を施し、さらに熱処理を施した後に冷却処理を行う鋼板の製造方法において、前記冷却処理における６００℃以下２００℃以上の温度域での平均冷却速度を１℃/s以上とする、鋼板の製造方法。 5. In a method for producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment. A method for manufacturing a steel sheet, in which the average cooling rate in a temperature range of ° C or higher is 1 ° C / s or higher.
　本発明によれば、水素による応力腐食割れに対して高い耐久性を有する、鋼板を提供することができる。この鋼板を、液化ガス貯槽用タンク等の、低温環境で使用される鋼構造物に供することによって、該鋼構造物の安全性が向上することができ、産業上格段の効果をもたらす。 According to the present invention, it is possible to provide a steel sheet having high durability against stress corrosion cracking due to hydrogen. By applying this steel sheet to a steel structure used in a low temperature environment such as a tank for a liquefied gas storage tank, the safety of the steel structure can be improved, which brings about a remarkable industrial effect.
　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。
［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、優れた耐食性を確保するため、以下のように鋼板の成分組成を規定する。なお、成分組成における％表示は、特に断らない限り質量％を意味するものとする。 Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
[Ingredient composition]
First, the component composition of the steel sheet of the present invention and the reason for its limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the composition of the steel sheet is defined as follows. The% indication in the component composition shall mean mass% unless otherwise specified.
Ｃ：０．０１％以上０．１５％以下
　Ｃは、高強度化に有効であり、その効果を得るためには、Ｃを０．０１％以上にて含有する必要がある。一方、０．１５％を超えて含有すると、低温靱性が低下する。このため、Ｃは０．０１％以上０．１５％以下とする。好ましくは、０．０３％以上とする。好ましくは、０．１０％以下とする。 C: 0.01% or more and 0.15% or less C is effective for increasing the strength, and in order to obtain the effect, it is necessary to contain C at 0.01% or more. On the other hand, if it is contained in excess of 0.15%, the low temperature toughness is lowered. Therefore, C is set to 0.01% or more and 0.15% or less. Preferably, it is 0.03% or more. Preferably, it is 0.10% or less.
Ｓｉ：０．０１％以上１．００％以下
　Ｓｉは、脱酸剤として作用し、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。この効果を得るためには、Ｓｉを０．０１％以上にて含有する必要がある。一方、１．００％を超えて含有すると、低温靱性が劣化する。このため、Ｓｉは０．０１％以上１．００％以下とする。好ましくは、０．０３％以上とする。好ましくは、０．５％以下とする。 Si: 0.01% or more and 1.00% or less Si acts as an antacid and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, if it is contained in excess of 1.00%, the low temperature toughness deteriorates. Therefore, Si is set to 0.01% or more and 1.00% or less. Preferably, it is 0.03% or more. Preferably, it is 0.5% or less.
Ｍｎ：０．１０％以上３．００％以下
　Ｍｎは、鋼の焼き入れ性を高め、鋼板の高強度化に有効な元素である。その効果を得るためには、Ｍｎは０．０１％以上の含有を必要とする。一方、３．００％を超えて含有すると、耐腐食割れ性が低下する。このため、Ｍｎは０．１０％以上３．００％以下の範囲とする。好ましくは、０．２０％以上とする。好ましくは、２．００％以下、より好ましくは１．００％以下とする。 Mn: 0.10% or more and 3.00% or less Mn is an element effective for enhancing the hardenability of steel and increasing the strength of steel sheets. In order to obtain the effect, Mn needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 3.00%, the corrosion cracking resistance is lowered. Therefore, Mn is set in the range of 0.10% or more and 3.00% or less. Preferably, it is 0.20% or more. It is preferably 2.00% or less, more preferably 1.00% or less.
Ａｌ：０．００２％以上０．１００％以下
　Ａｌは、脱酸剤として作用し、鋼板の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。また、鋼中の固溶Ｎを固定してＡｌＮを形成し固溶Ｎ低減による靱性劣化を抑制する効果を有する。一方、０．１００％を超えて含有すると、靱性を劣化させるため、０．１００％以下とする。好ましくは、０．０１０％以上とする。好ましくは、０．０７０％以下とする。より好ましくは０．０２０％以上とする。より好ましくは、０．０６０％以下とする。 Al: 0.002% or more and 0.100% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. Further, it has the effect of fixing the solid solution N in the steel to form AlN and suppressing the deterioration of toughness due to the reduction of the solid solution N. On the other hand, if it is contained in excess of 0.100%, the toughness is deteriorated, so the content is set to 0.100% or less. Preferably, it is 0.010% or more. Preferably, it is 0.070% or less. More preferably, it is 0.020% or more. More preferably, it is 0.060% or less.
Ｎｉ：５．０％以上１０．０％以下
　Ｎｉは、鋼板の低温靭性の向上に極めて有効な元素である。一方で、高価な元素であるため、その含有量が高くなるにつれて鋼板コストが高騰する。従って、本発明においては、Ｎｉ含有量を１０．０％以下とする。但し、Ｎｉ含有量が５．０％未満になると、鋼板強度が低下するほか、低温で安定した残留オーステナイトが得られなくなる結果、鋼板の低温靭性や強度が低下する。従って、Ｎｉ含有量を５．０％以上１０．０％以下とする。好ましくは、９．５％以下とする。好ましくは、６．０％以上とする。 Ni: 5.0% or more and 10.0% or less Ni is an extremely effective element for improving the low temperature toughness of steel sheets. On the other hand, since it is an expensive element, the cost of the steel sheet rises as its content increases. Therefore, in the present invention, the Ni content is set to 10.0% or less. However, if the Ni content is less than 5.0%, the strength of the steel sheet is lowered, and stable retained austenite cannot be obtained at a low temperature. As a result, the low temperature toughness and strength of the steel sheet are lowered. Therefore, the Ni content is set to 5.0% or more and 10.0% or less. Preferably, it is 9.5% or less. Preferably, it is 6.0% or more.
Ｎ：０．００１０％以上０．００８０％以下
　Ｎは、鋼中で析出物を形成し、その含有量が０．００８０％を超えると、鋼板を溶接して溶接構造物とした際、母材および溶接熱影響部の靭性低下の原因となる。但し、Ｎは、AlNを形成することにより母材の細粒化に寄与する元素でもあり、このような効果はＮ含有量を０．００１０％以上とすることにより得られる。したがって、Ｎ含有量は０．００１０％以上０．００８０％以下とする。好ましくは０．００２０％以上とする。より好ましくは０．００６０％以下とする。 N: 0.0010% or more and 0.0080% or less N forms a precipitate in steel, and when the content exceeds 0.0080%, when the steel plate is welded to form a welded structure, the base material And it causes a decrease in toughness of the weld heat affected zone. However, N is also an element that contributes to the refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. It is preferably 0.0020% or more. More preferably, it is 0.0060% or less.
Ｃｏ：０％超１．５０％以下
　Ｃｏは、腐食環境下で鋼板の表層に濃化し、水素の侵入を低減することによって腐食割れを抑制するのに寄与する重要な元素である。従って、０％を超えて含有している必要がある。好ましくは、Ｃｏ量を０．０５％以上、より好ましくは０．１％以上とする。しかし、１．５０％を超えて含有しても効果は飽和する上に、Ｃｏは高価な元素であることから、最大添加量は１．５０％とする。 Co: More than 0% and less than 1.50% Co is an important element that concentrates on the surface layer of steel sheets in a corrosive environment and contributes to suppressing corrosion cracking by reducing the invasion of hydrogen. Therefore, it needs to be contained in excess of 0%. The amount of Co is preferably 0.05% or more, more preferably 0.1% or more. However, even if it is contained in excess of 1.50%, the effect is saturated and Co is an expensive element, so the maximum addition amount is 1.50%.
Ｐ：０．０３０％以下
　Ｐは、０．０３０％を超えて含有すると、耐腐食割れ性を低下させる。そのため、０．０３０％を上限とし、可能なかぎり低減することが望ましい。したがって、Ｐは０．０３０％以下とする。Ｐは含有量が少ないほど特性が向上するため、好ましくは０．０２５％以下とし、より好ましくは０．０２０％以下とする。なお、Ｐの含有量は０％でよいことは勿論であるが、脱Ｐには高コストを要するため、コストの観点からは０．００２％以上とすることが好ましい。 P: 0.030% or less When P is contained in excess of 0.030%, the corrosion cracking resistance is lowered. Therefore, it is desirable to set 0.030% as the upper limit and reduce it as much as possible. Therefore, P is set to 0.030% or less. Since the characteristics of P improve as the content is smaller, the content of P is preferably 0.025% or less, and more preferably 0.020% or less. It goes without saying that the content of P may be 0%, but since removing P requires a high cost, it is preferably 0.002% or more from the viewpoint of cost.
Ｓ：０．００５０％以下
　Ｓは、鋼中でＭｎＳを形成し低温靭性を著しく劣化させるため、０．００５０％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０．００２０％以下とする。なお、Ｓの含有量は０％でよいことは勿論であるが、脱Ｓには高コストを要するため、コストの観点からは０．０００５％以上とすることが好ましい。 S: 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to limit S to 0.0050% and reduce it as much as possible. It is preferably 0.0020% or less. It goes without saying that the S content may be 0%, but since removing S requires a high cost, it is preferably 0.0005% or more from the viewpoint of cost.
　以上の各元素を含み、残部がＦｅおよび不可避不純物である成分組成を基本とする。
　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。 It is based on a component composition containing each of the above elements and the balance being Fe and unavoidable impurities.
In the present invention, in addition to the above essential elements, the following elements can be contained, if necessary, for the purpose of further improving the strength and low temperature toughness.
Ｎｂ：０．００１％以上０．０３０％以下
　Ｎｂは、鋼板の強度の向上に有効な元素である。このような効果を得るためには、Ｎｂを０．００1％以上で添加することが好ましい。一方、０．０３０％を超えて含有すると、粗大な炭窒化物が析出し、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、０．００１％以上０．０３０％以下とする。好ましくは０．００５％以上、より好ましくは０．００７％以上とする。好ましくは０．０２５％以下、より好ましくは０．０２２％以下とする。 Nb: 0.001% or more and 0.030% or less Nb is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add Nb in an amount of 0.001% or more. On the other hand, if it is contained in excess of 0.030%, coarse carbonitride may be precipitated and the toughness of the base metal may be deteriorated. Therefore, when Nb is contained, it is set to 0.001% or more and 0.030% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.
Ｖ：０．０１％以上０．１０％以下
　Ｖは、鋼板の強度向上に有効な元素である。このような効果を得るためには、Ｖを０．０１％以上で添加することが好ましい。一方、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、０．０１％以上０．１０％以下とする。好ましくは０．０２％以上、より好ましくは０．０３％以上とする。好ましくは０．０９％以下、より好ましくは０．０８％以下とする。 V: 0.01% or more and 0.10% or less V is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add V at 0.01% or more. On the other hand, if it is contained in excess of 0.10%, coarse carbonitride may be precipitated and may become a starting point of fracture. In addition, the precipitate may become coarse and the toughness of the base metal may be deteriorated. Therefore, when V is contained, it is set to 0.01% or more and 0.10% or less. It is preferably 0.02% or more, more preferably 0.03% or more. It is preferably 0.09% or less, more preferably 0.08% or less.
Ｔｉ：０．００３％以上０．０５０％以下
　Ｔｉは、窒化物もしくは炭窒化物として析出し、鋼板の強度向上に有効な元素である。このような効果を得るためには、Ｔｉを０．００３％以上で添加することが好ましい。一方、０．０５０％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％以上０．０５０％以下とする。好ましくは０．００５％以上、より好ましくは０．００７％以上とする。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% or more and 0.050% or less Ti is an element that precipitates as a nitride or carbonitride and is effective for improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to add Ti at 0.003% or more. On the other hand, if it is contained in excess of 0.050%, the precipitate may become coarse and the toughness of the base metal may be deteriorated. In addition, coarse carbonitride may precipitate and serve as a starting point for fracture. Therefore, when Ti is contained, it is set to 0.003% or more and 0.050% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.
Ｂ：０．０００３％以上０．０１００％以下
　Ｂは、鋼板の強度向上に有効な元素である。このような効果を得るためには、Ｂを０．０００３％以上で添加することが好ましい。一方、０．０１００％を超えて含有すると、粗大なＢ析出物を生成し、靭性が低下することがある。このため、Ｂは０．０００３％以上０．０１００％以下の範囲とする。好ましくは、０．００３０％以下である。 B: 0.0003% or more and 0.0100% or less B is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add B at 0.0003% or more. On the other hand, if it is contained in excess of 0.0100%, coarse B precipitates may be formed and the toughness may be lowered. Therefore, B is in the range of 0.0003% or more and 0.0100% or less. Preferably, it is 0.0030% or less.
Ｃｕ：０．０１％以上１．００％以下
　Ｃｕは、焼入れ性向上により鋼板強度を高めるのに有効な元素であるが、その含有量が１．００％を超えると、鋼板の低温靭性が低下するおそれがある。したがって、Ｃｕを含有させる場合には、その含有量を１．００％以下とすることが好ましい。一方、０．０１％未満では、強度を高める効果が得られないため、添加する場合は０．０１％以上とすることが好ましい。より好ましくは、０．１０％以上０．３０％以下とする。 Cu: 0.01% or more and 1.00% or less Cu is an element effective for increasing the strength of a steel sheet by improving hardenability, but when the content exceeds 1.00%, the low temperature toughness of the steel sheet decreases. There is a risk of Therefore, when Cu is contained, the content thereof is preferably 1.00% or less. On the other hand, if it is less than 0.01%, the effect of increasing the strength cannot be obtained. Therefore, when it is added, it is preferably 0.01% or more. More preferably, it is 0.10% or more and 0.30% or less.
Ｃｒ：０．０１％以上１．５０％以下
　Ｃｒは、高Mn鋼の低温靭性および耐食性向上に寄与する元素である。そのためには、Ｃｒ量を０．０１％以上とすることが好ましい。一方、Ｃｒは圧延中に窒化物、炭化物、炭窒化物等の形態で析出する場合があり、このような析出物の形成により腐食や破壊の起点となって低温靭性が低下するため、上限を１．５０％とすることが好ましい。より好ましくは、１．００％以下である。 Cr: 0.01% or more and 1.50% or less Cr is an element that contributes to the improvement of low temperature toughness and corrosion resistance of high Mn steel. For that purpose, the amount of Cr is preferably 0.01% or more. On the other hand, Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates causes corrosion and fracture to reduce low temperature toughness. It is preferably 1.50%. More preferably, it is 1.00% or less.
Ｍｏ：０．０３％以上１．００％以下
　Ｍｏは、鋼板の焼戻し脆化感受性を抑制するのに有効な元素であり、また、低温靭性を損なうことなく鋼板強度を高める元素でもある。このような効果を得るためには、Ｍｏ含有量を０．０３％以上とすることが好ましい。一方、Ｍｏが１．００％を超えると、低温靭性が低下する、おそれがある。したがって、Ｍｏを含有させる場合には、その含有量を０．０３％以上１．００％以下とする。より好ましくは０．０５％超０．３０％以下である。 Mo: 0.03% or more and 1.00% or less Mo is an element effective for suppressing the temper embrittlement sensitivity of a steel sheet, and is also an element for increasing the strength of a steel sheet without impairing low temperature toughness. In order to obtain such an effect, the Mo content is preferably 0.03% or more. On the other hand, if Mo exceeds 1.00%, the low temperature toughness may decrease. Therefore, when Mo is contained, the content is set to 0.03% or more and 1.00% or less. More preferably, it is more than 0.05% and 0.30% or less.
Ｓｎ：０．０１％以上０．５０％以下
Ｓｂ：０．０１％以上０．５０％以下
Ｗ：０．０５％以上２．００％以下
　Ｓｎ、ＳｂおよびＷは、耐食性向上に有効な元素である。これらの効果は、ＳｎおよびＳｂが０．０１％以上並びにＷが０．０５％以上にて発現する。しかし、いずれの元素も多く含有させると、溶接性や靱性を劣化させ、コストの観点からも不利になる、おそれがある。従って、Ｓｎ量は０．０１％以上０．５０％以下の範囲、Ｓｂ量は０．０１％以上０．５０％以下の範囲、Ｗ量は０．０５％以上２．００％以下の範囲とする。好ましくは、Ｓｎ量は０．０２％以上０．２５％以下、Ｓｂ量は０．０２％以上０．２５％以下、Ｗ量は０．１０％以上１．００％以下である。 Sn: 0.01% or more and 0.50% or less Sb: 0.01% or more and 0.50% or less W: 0.05% or more and 2.00% or less Sn, Sb and W are elements effective for improving corrosion resistance. is there. These effects are exhibited when Sn and Sb are 0.01% or more and W is 0.05% or more. However, if a large amount of any of the elements is contained, the weldability and toughness are deteriorated, which may be disadvantageous from the viewpoint of cost. Therefore, the Sn amount is in the range of 0.01% or more and 0.50% or less, the Sb amount is in the range of 0.01% or more and 0.50% or less, and the W amount is in the range of 0.05% or more and 2.00% or less. To do. Preferably, the Sn amount is 0.02% or more and 0.25% or less, the Sb amount is 0.02% or more and 0.25% or less, and the W amount is 0.10% or more and 1.00% or less.
　さらに、本発明では、必要に応じて次の元素を含有することができる。
Ｃａ：０．０００５％以上０．００５０％以下、Ｚｒ：０．０００５％以上０．００５０％以下、Ｍｇ：０．０００５％以上０．００５０％以下およびＲＥＭ：０．００１０％以上０．０１００％以下の１種または２種以上
　Ｃａ、Ｚｒ、ＭｇおよびＲＥＭは、ＭｎＳ等の介在物の形態制御に有用な元素であり、必要に応じて添加できる。ここで、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、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、ＺｒおよびＭｇは０．０００５％以上、ＲＥＭは０．００１０％以上にて含有することが好ましい。一方、いずれの元素も多く含有させると、非金属介在物量が増加し、かえって低温靱性が低下する場合がある。このため、Ｃａ、Ｚｒ、Ｍｇを含有する場合には、それぞれ０．０００５％以上０．００５０％以下、ＲＥＭを含有する場合には、０．００１０％以上０．０１００％以下とする。より好ましくは、Ｃａ量を０．００１０％以上０．００４０％以下、Ｚｒ量を０．００１０％以上０．００４０％以下、Ｍｇ量を０．００１０％以上０．００４０％以下、ＲＥＭ量を０．００２０％以上０．０１００％以下とする。 Furthermore, in the present invention, the following elements can be contained, if necessary.
Ca: 0.0005% or more and 0.0050% or less, Zr: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% One or more of the following Ca, Zr, Mg and REM are elements useful for morphological control of inclusions such as MnS, and can be added as needed. Here, the morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Toughness and sulfide stress corrosion cracking resistance are improved through morphological control of the inclusions. In order to obtain such an effect, it is preferable that Ca, Zr and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, if a large amount of any of the elements is contained, the amount of non-metal inclusions may increase, and the low temperature toughness may decrease. Therefore, when Ca, Zr, and Mg are contained, the content is 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, the content is 0.0010% or more and 0.0100% or less. More preferably, the Ca amount is 0.0010% or more and 0.0040% or less, the Zr amount is 0.0010% or more and 0.0040% or less, the Mg amount is 0.0010% or more and 0.0040% or less, and the REM amount is 0. It shall be 0020% or more and 0.0100% or less.
［表層組織］
　次に、鋼板の表面から深さが１ｍｍの位置までの組織（以下、表層組織ともいう）は、方位差１５°以上の大角粒界で囲まれた結晶粒の平均円相当径が５μm以下、かつ残留オーステナイト粒の最大円相当径が１μm以下である、ことが肝要である。
　まず、方位差１５°以上の大角粒界で囲まれた結晶粒の平均円相当径を５μm以下とする必要がある。なぜなら、水素のトラップサイトとなる方位差１５°以上の結晶粒界の量が増え、かつ分散させることになるため、水素脆化によるき裂進展を軽減できるからである。さらに、当該結晶粒の平均円相当径は、４μm以下であることが好ましく、さらに好ましくは３μm以下である。 [Surface structure]
Next, in the structure from the surface of the steel sheet to the position of 1 mm in depth (hereinafter, also referred to as the surface layer structure), the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries having an orientation difference of 15 ° or more is 5 μm or less. Moreover, it is important that the maximum circular equivalent diameter of the retained austenite grains is 1 μm or less.
First, it is necessary to set the average equivalent circle diameter of the crystal grains surrounded by the large-angle grain boundaries having an orientation difference of 15 ° or more to 5 μm or less. This is because the amount of grain boundaries having an orientation difference of 15 ° or more, which is a trap site for hydrogen, increases and is dispersed, so that crack growth due to hydrogen embrittlement can be reduced. Further, the average equivalent circle diameter of the crystal grains is preferably 4 μm or less, more preferably 3 μm or less.
　なお、方位差１５°以上の大角粒界で囲まれた結晶粒の特定並びに、該結晶粒の平均円相当径の特定は、後述する実施例における測定手法によって行うことができる。 The crystal grains surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more and the average circle-equivalent diameter of the crystal grains can be specified by the measurement method in the examples described later.
　この方位差１５°以上の大角粒界で囲まれた結晶粒の平均円相当径を５μm以下にするには、熱間圧延後または、熱間圧延後に熱処理を施す場合は該熱処理後に、所定温度域での平均冷却速度が１℃/ｓ以上の冷却処理を行う。 In order to reduce the average circle-equivalent diameter of crystal grains surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more to 5 μm or less, the temperature is determined after hot rolling or, if heat treatment is performed after hot rolling, after the heat treatment. Perform cooling treatment with an average cooling rate of 1 ° C./s or higher in the region.
　さらに、表層組織において、残留オーステナイト粒の最大円相当径を１μm以下とする必要がある。なぜなら、前記最大円相当径を１μm以下とすることによって、残留オーステナイトへの水素トラップが分散されて水素トラップの局所集中が回避される結果、水素脆化によるき裂進展を軽減できるからである。なお、表層組織における残留オーステナイト量は、面積率で１５％以下であることが好ましく、さらに好ましくは１０％以下である。 Furthermore, in the surface layer structure, the maximum circle-equivalent diameter of the retained austenite grains needs to be 1 μm or less. This is because by setting the diameter corresponding to the maximum circle to 1 μm or less, the hydrogen traps on the retained austenite are dispersed and the local concentration of the hydrogen traps is avoided, and as a result, the crack growth due to hydrogen embrittlement can be reduced. The amount of retained austenite in the surface layer structure is preferably 15% or less in terms of area ratio, and more preferably 10% or less.
　なお、鋼板の組織は、マルテンサイトおよび／またはベイナイトであることが好ましい。その際、マルテンサイトおよび／またはベイナイトの面積率は８０％以上であることが好ましい。 The structure of the steel sheet is preferably martensite and / or bainite. At that time, the area ratio of martensite and / or bainite is preferably 80% or more.
　次に、本発明の鋼板を製造する条件について説明する。すなわち、上記した成分組成を有する鋼素材を加熱し、熱間圧延を施し冷却するか、あるいは熱間圧延後にさらに熱処理を施し冷却する、ことによって製造することができる。その際、熱間圧延後の冷却、あるいは熱間圧延後に熱処理を行う場合は該熱処理後の冷却、において所定温度域の平均冷却速度を１℃/s以上とすることが、上記した表層組織を得るために必要である。以下、製造条件について、工程順に説明する。なお、以下の説明において、温度（℃）は、板厚中心部における温度を意味するものとする。 Next, the conditions for manufacturing the steel sheet of the present invention will be described. That is, it can be produced by heating a steel material having the above-mentioned composition and cooling it by hot rolling, or by further heat-treating it after hot rolling and cooling it. At that time, in the cooling after hot rolling, or when the heat treatment is performed after the hot rolling, the average cooling rate in a predetermined temperature range is set to 1 ° C./s or more in the cooling after the heat treatment. Needed to get. Hereinafter, the manufacturing conditions will be described in the order of steps. In the following description, the temperature (° C.) means the temperature at the center of the plate thickness.
　まず、熱間圧延における鋼素材の再加熱温度は、１０００℃以上１３００℃以下とすることが好ましい。
［鋼素材の再加熱温度：１０００℃以上１３００℃以下］
　鋼素材を１０００℃以上に加熱するのは、組織中の析出物を固溶させ、結晶粒径等を均一化するためであり、加熱温度としては、１０００℃以上１３００℃以下とすることが好ましい。すなわち、加熱温度が９００℃未満の場合、析出物が十分に固溶しない場合があるため、所望の特性が得られない、おそれがある。一方、１３００℃を超えて加熱すると、結晶粒径の粗大化によって材質が劣化する場合があり、また製造に過剰なエネルギーが必要となり生産性が低下する、おそれがある。より好ましくは１０５０℃以上１２５０℃以下、さらには１１００℃以上１２５０℃以下の範囲である。 First, the reheating temperature of the steel material in hot rolling is preferably 1000 ° C. or higher and 1300 ° C. or lower.
[Reheating temperature of steel material: 1000 ° C or higher and 1300 ° C or lower]
The reason why the steel material is heated to 1000 ° C. or higher is to dissolve the precipitates in the structure to make the crystal grain size uniform, and the heating temperature is preferably 1000 ° C. or higher and 1300 ° C. or lower. .. That is, if the heating temperature is less than 900 ° C., the precipitate may not be sufficiently solid-solved, so that the desired characteristics may not be obtained. On the other hand, if it is heated to exceed 1300 ° C., the material may be deteriorated due to the coarsening of the crystal grain size, and excessive energy may be required for production to reduce the productivity. More preferably, it is in the range of 1050 ° C. or higher and 1250 ° C. or lower, and further preferably 1100 ° C. or higher and 1250 ° C. or lower.
［熱間圧延後の冷却］
　鋼板の表層組織を好ましくはマルテンサイトおよび／またはベイナイトの組織とし、かつ該組織に含まれる大角粒界を増加させて、優れた耐応力腐食割れ性を確保するには、熱間圧延後に冷却処理を施し、表層組織における６００℃以下２００℃以上の温度域での平均冷却速度を１℃/s以上とする。すなわち、この冷却処理における冷却速度が１℃/s未満の場合、表層組織が上部ベイナイト組織となり、組織に含まれる大角粒界が減少し、組織が十分微細化されず、耐応力腐食割れ性を得られない。平均冷却速度の上限は特に制限する必要はない。
　なお、熱間圧延後に後述の熱処理を施す場合は、この熱間圧延後の冷却における速度を１℃/s以上とする必要はない。 [Cooling after hot rolling]
In order to make the surface structure of the steel plate preferably martensite and / or bainite structure and increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, cooling treatment is performed after hot rolling. The average cooling rate of the surface layer structure in the temperature range of 600 ° C. or lower and 200 ° C. or higher is set to 1 ° C./s or higher. That is, when the cooling rate in this cooling treatment is less than 1 ° C./s, the surface layer structure becomes an upper bainite structure, the large-angle grain boundaries contained in the structure are reduced, the structure is not sufficiently finely divided, and stress corrosion cracking resistance is deteriorated. I can't get it. The upper limit of the average cooling rate does not need to be particularly limited.
When the heat treatment described later is performed after the hot rolling, it is not necessary to set the cooling rate after the hot rolling to 1 ° C./s or more.
［熱間圧延後の熱処理］
　熱間圧延後に冷却せずに以下の熱処理を施してもよい。上述の通り、鋼板の表層組織を好ましくはマルテンサイトおよび／またはベイナイトの組織とし、かつ該組織に含まれる大角粒界を増加させて、優れた耐応力腐食割れ性を確保するには、熱間圧延後に熱処理を施す場合は、熱間圧延後にＡｃ3点以上９００℃以下に加熱して焼入れ（一次焼入れ）することが好ましい。すなわち、加熱温度がＡｃ3点未満あるいは９００℃を超えると、大角粒界の円相当径が粗大となり、所望の特性が得られない、おそれがある。 [Heat treatment after hot rolling]
After hot rolling, the following heat treatment may be performed without cooling. As described above, in order to preferably make the surface layer structure of the steel sheet a martensite and / or bainite structure and increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, it is hot. When heat treatment is performed after rolling, it is preferable to perform hot rolling by heating to Acc 3 points or more and 900 ° C. or less for quenching (primary quenching). That is, if the heating temperature is less than 3 points of Ac or exceeds 900 ° C., the equivalent circle diameter of the large-angle grain boundary becomes coarse, and there is a possibility that the desired characteristics cannot be obtained.
　なお、熱間圧延後に上記した熱処理を施す場合は、この熱処理後の冷却における速度を制御する必要があるのは、上述のとおりである。すなわち、表層組織における６００℃以下２００℃以上の温度域での平均冷却速度を１℃/s以上とする。 As described above, when the above heat treatment is performed after hot rolling, it is necessary to control the cooling rate after the heat treatment. That is, the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
　さらに、熱間圧延後の熱処理として、上記した焼入れ（一次焼入れ）に替えて、または一次焼入れして冷却した後に、Ａｃ1変態点以上Ａｃ3変態点未満に加熱して冷却する熱処理（二次焼入れ）を施してもよい。この二次焼入れを行うことによって、母材低温靱性を向上させることができる。
　なお、上記した熱処理（二次焼入れ）を施す場合は、この熱処理後の冷却における速度を制御する必要があるのは、上述のとおりである。すなわち、表層組織における６００℃以下２００℃以上の温度域での平均冷却速度を１℃/s以上とする。 Further, as a heat treatment after hot rolling, a heat treatment (secondary) in which the above-mentioned quenching (primary quenching) is performed, or after the primary quenching is performed to cool the material, the heat treatment is performed by heating to an Ac 1 transformation point or more and less than an Ac 3 transformation point. (Quenching) may be applied. By performing this secondary quenching, the low temperature toughness of the base metal can be improved.
As described above, when the above-mentioned heat treatment (secondary quenching) is performed, it is necessary to control the speed of cooling after the heat treatment. That is, the average cooling rate in the temperature range of 600 ° C. or lower and 200 ° C. or higher in the surface layer structure is set to 1 ° C./s or higher.
　高強度および優れた低温靭性などの特性を得るには、表層組織における残留オーステナイト粒を径が１μｍ以下の微細粒とすることが有効である。そのためには、上記の最終の冷却後に５００℃以上６５０℃以下の温度に加熱して焼戻しすることが好ましい。すなわち、焼戻し温度が５００℃未満では、低温靭性を確保することが難しくなる、おそれがある。一方、焼戻し温度が６５０℃を超えると、粗大な残留オーステナイトとなり、所望の特性が得られない、おそれがある。 In order to obtain properties such as high strength and excellent low temperature toughness, it is effective to make the retained austenite grains in the surface layer structure into fine grains having a diameter of 1 μm or less. For that purpose, it is preferable to heat and temper at a temperature of 500 ° C. or higher and 650 ° C. or lower after the final cooling. That is, if the tempering temperature is less than 500 ° C., it may be difficult to secure low temperature toughness. On the other hand, if the tempering temperature exceeds 650 ° C., coarse retained austenite may be formed and desired characteristics may not be obtained.
　表１に示したＡ～Ｗの鋼を溶製し、スラブとした後、表２に示す製造条件により板厚が３０～５０ｍｍの鋼板（試料No.１～２６）を製造し、各試料を以下のシャルピー衝撃試験および応力腐食割れ試験に供した。また、各試料について、表層組織における大角粒界の間隔および残留オーステナイト粒径を調査した。 After the steels A to W shown in Table 1 are melted and made into slabs, steel plates (Sample Nos. 1 to 26) having a thickness of 30 to 50 mm are manufactured according to the manufacturing conditions shown in Table 2, and each sample is prepared. It was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, for each sample, the spacing between large-angle grain boundaries and the residual austenite grain size in the surface structure were investigated.
　大角粒界は、粒界方位差が１５°以上の粒界と定義し、これを、ＥＢＳＤを用いて特定した。そして、結晶粒径は、鋼板の表面から１ｍｍの深さ位置における任意の５００×５００μmの範囲を測定し、大角粒界に囲まれる結晶粒の円相当径の平均値を求めた。なお、大角粒界に囲まれている範囲が０．１μm未満のものは計算から除外した。 The large-angle grain boundary was defined as a grain boundary with a grain boundary orientation difference of 15 ° or more, and this was specified using EBSD. Then, the crystal grain size was measured in an arbitrary range of 500 × 500 μm at a depth of 1 mm from the surface of the steel sheet, and the average value of the equivalent circle diameters of the crystal grains surrounded by the large angle grain boundaries was obtained. The range surrounded by the large-angle grain boundaries of less than 0.1 μm was excluded from the calculation.
　また、残留オーステナイトの最大円相当径は、同様のＥＢＳＤ測定領域において存在する残留オーステナイト粒を結晶構造から特定し、オーステナイトと認識された結晶粒のうちの最大のものの円相当径とした。 For the maximum circle-equivalent diameter of retained austenite, the retained austenite grains existing in the same EBSD measurement region were specified from the crystal structure, and the circle-equivalent diameter of the largest of the crystal grains recognized as austenite was used.
［シャルピー衝撃試験（低温靭性）］
　各試料について、JIS Ｚ2242に規定のＶノッチ試験片を準備し、試験温度：－１９６℃にてJIS Ｚ2242に準拠してシャルピー衝撃試験を実施し、吸収エネルギーを測定した。各試料につき３本の試験片での試験を実施し、それらの平均値が３４Ｊ以上である場合を合格とした。 [Charpy impact test (low temperature toughness)]
For each sample, a V-notch test piece specified in JIS Z2242 was prepared, and a Charpy impact test was carried out in accordance with JIS Z2242 at a test temperature of -196 ° C., and the absorbed energy was measured. The test was carried out with three test pieces for each sample, and the case where the average value thereof was 34 J or more was regarded as a pass.
［応力腐食割れ試験（耐応力腐食割れ性）］
　NACE　TM0177-96 2003版に準拠した、ＤＣＢ（ Double-Cantilever-Beam）試験を実施した。試験環境は、NACE TM0177 sol.Ａ(初期pH2.7)×100％Ｈ2Ｓガス飽和(0.1MPa) 浸漬時間は３３６時間とした。浸漬終了後、Wedge load とcrack lengthからＫISSCを導出した。各試料につき３本の試験片での試験を実施し、それらの平均値が25MPa√m以上である場合を合格とした。
　以上により得られた結果を、表２に示す。 [Stress corrosion cracking test (stress corrosion cracking resistance)]
A DCB (Double-Cantilever-Beam) test was conducted in accordance with NACE TM0177-96 2003 version. Test environment, NACE TM0177 sol.A (initial pH2.7) × 100% H 2 S gas saturation (0.1 MPa) immersion time was 336 hours. After the immersion was completed, KISSC was derived from the Wedge load and crack length. The test was carried out with three test pieces for each sample, and the case where the average value thereof was 25 MPa√m or more was regarded as acceptable.
The results obtained as described above are shown in Table 2.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　本発明に従う試料Ｎｏ．１～１４、２３、および２６は、低温靭性が確保されるとともに、優れた耐応力腐食割れ性を有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(試料Ｎｏ．１５～２２および２４、２５)は、吸収エネルギーが３４Ｊより低い、もしくはＤＣＢ試験が25MPa√m ％未満となっており、上述の目標性能を満足できなかった。 Sample No. according to the present invention. It was confirmed that Nos. 1 to 14, 23, and 26 ensure low temperature toughness and have excellent stress corrosion cracking resistance. On the other hand, in the comparative examples (Sample Nos. 15 to 22 and 24, 25) outside the scope of the present invention, the absorbed energy is lower than 34J, or the DCB test is less than 25MPa√m%, and the above-mentioned target performance is achieved. I wasn't satisfied.

Claims (5)

  1. 　質量％で、
    　Ｃ：０．０１％以上０．１５％以下、
    　Ｓｉ：０．０１％以上１．００％以下、
    　Ｍｎ：０．１０％以上３．００％以下、
    　Ａｌ：０．００２％以上０．１００％以下、
    　Ｎｉ：５．０％以上１０．０％以下、
    　Ｎ：０．００１０％以上０．００８０％以下、
    　Ｃｏ：０％超１．５０％以下、
    　Ｐ：０．０３０％以下および
    　Ｓ：０．００５０％以下
    を含有し、残部Ｆｅおよび不可避的不純物である成分組成を有し、
    　鋼板の表面から深さが１ｍｍの位置までの組織は、方位差１５°以上の大角粒界で囲まれた結晶粒の平均円相当径が５μm以下、かつ残留オーステナイト粒の最大円相当径が１μm以下である鋼板。     By mass%
    C: 0.01% or more and 0.15% or less,
    Si: 0.01% or more and 1.00% or less,
    Mn: 0.10% or more and 3.00% or less,
    Al: 0.002% or more and 0.100% or less,
    Ni: 5.0% or more and 10.0% or less,
    N: 0.0010% or more and 0.0080% or less,
    Co: More than 0% and less than 1.50%,
    It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition of the balance Fe and unavoidable impurities.
    In the structure from the surface of the steel sheet to the position of 1 mm in depth, the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 μm or less, and the maximum circle equivalent diameter of the retained austenite grains is 1 μm. The following steel plate.
  2. 　前記成分組成は、さらに質量％で、
    　Ｎｂ：０．００１％以上０．０３０％以下、
    　Ｖ：０．０１％以上０．１０％以下、
    　Ｔｉ：０．００３％以上０．０５０％以下、
    　Ｂ：０．０００３％以上０．０１００％以下、
    　Ｃｕ：０．０１％以上１．００％以下、
    　Ｃｒ：０．０１％以上１．５０％以下、
    　Ｓｎ：０．０１％以上０．５０％以下、
    　Ｓｂ：０．０１％以上０．５０％以下、
    　Ｍｏ：０．０３％以上１．００％以下および
    　Ｗ：０．０５％以上２．００％以下
    から選択される１種または２種以上を含有する請求項１に記載の鋼板。     The composition of the components is further increased by mass%.
    Nb: 0.001% or more and 0.030% or less,
    V: 0.01% or more and 0.10% or less,
    Ti: 0.003% or more and 0.050% or less,
    B: 0.0003% or more and 0.0100% or less,
    Cu: 0.01% or more and 1.00% or less,
    Cr: 0.01% or more and 1.50% or less,
    Sn: 0.01% or more and 0.50% or less,
    Sb: 0.01% or more and 0.50% or less,
    The steel sheet according to claim 1, which contains one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less.
  3. 　前記成分組成は、さらに質量％で、
    　Ｃａ：０．０００５％以上０．００５０％以下、
    　Ｚｒ：０．０００５％以上０．００５０％以下、
    　Ｍｇ：０．０００５％以上０．００５０％以下および
    　ＲＥＭ：０．００１０％以上０．０１００％以下
    から選択される１種または２種以上を含有する請求項１または２に記載の鋼板。     The composition of the components is further increased by mass%.
    Ca: 0.0005% or more and 0.0050% or less,
    Zr: 0.0005% or more and 0.0050% or less,
    The steel sheet according to claim 1 or 2, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% or less.
  4. 　請求項１から３のいずれかに記載の成分組成を有する鋼素材を加熱し、熱間圧延を施した後に冷却処理を行う鋼板の製造方法において、前記冷却処理における６００℃以下２００℃以上の平均冷却速度を１℃/s以上とする、鋼板の製造方法。 In a method for producing a steel sheet in which a steel material having the component composition according to any one of claims 1 to 3 is heated, hot-rolled, and then cooled, the average of 600 ° C. or lower and 200 ° C. or higher in the cooling treatment is performed. A method for manufacturing a steel sheet with a cooling rate of 1 ° C./s or higher.
  5. 　請求項１から３のいずれかに記載の成分組成を有する鋼素材を加熱し、熱間圧延を施し、さらに熱処理を施した後に冷却処理を行う鋼板の製造方法において、前記冷却処理における６００℃以下２００℃以上の平均冷却速度を１℃/s以上とする、鋼板の製造方法。 In a method for producing a steel sheet in which a steel material having the component composition according to any one of claims 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment. A method for manufacturing a steel sheet, wherein an average cooling rate of 200 ° C. or higher is 1 ° C./s or higher.
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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007034576A1 (en) * 2005-09-21 2007-03-29 Sumitomo Metal Industries, Ltd. Steel product usable at low temperature and method for production thereof
WO2014203347A1 (en) * 2013-06-19 2014-12-24 新日鐵住金株式会社 Steel material, process for producing same, and lng tank
JP2016176141A (en) * 2015-03-18 2016-10-06 Ｊｆｅスチール株式会社 Steel material for low temperature and production method therefor
CN109280848A (en) * 2018-10-17 2019-01-29 东北大学 A kind of low-nickel type LNG tank steel plate and preparation method thereof
WO2019087318A1 (en) * 2017-10-31 2019-05-09 新日鐵住金株式会社 Nickel-containing steel sheet for low-temperature applications and tank using nickel-containing steel sheet for low-temperature applications
KR20190056783A (en) * 2017-11-17 2019-05-27 주식회사 포스코 Low temperature steeel plate having excellent impact toughness property and method for manufacturing the same

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007080646A1 (en) 2006-01-13 2007-07-19 Sumitomo Metal Industries, Ltd. Cryogenic steel
JP5513254B2 (en) 2010-05-17 2014-06-04 新日鐵住金株式会社 Low temperature steel plate and method for producing the same
JP6007847B2 (en) * 2013-03-28 2016-10-12 Ｊｆｅスチール株式会社 Wear-resistant thick steel plate having low temperature toughness and method for producing the same
JP6068314B2 (en) * 2013-10-22 2017-01-25 株式会社神戸製鋼所 Hot-rolled steel sheet with excellent cold workability and surface hardness after carburizing heat treatment

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007034576A1 (en) * 2005-09-21 2007-03-29 Sumitomo Metal Industries, Ltd. Steel product usable at low temperature and method for production thereof
WO2014203347A1 (en) * 2013-06-19 2014-12-24 新日鐵住金株式会社 Steel material, process for producing same, and lng tank
JP2016176141A (en) * 2015-03-18 2016-10-06 Ｊｆｅスチール株式会社 Steel material for low temperature and production method therefor
WO2019087318A1 (en) * 2017-10-31 2019-05-09 新日鐵住金株式会社 Nickel-containing steel sheet for low-temperature applications and tank using nickel-containing steel sheet for low-temperature applications
KR20190056783A (en) * 2017-11-17 2019-05-27 주식회사 포스코 Low temperature steeel plate having excellent impact toughness property and method for manufacturing the same
CN109280848A (en) * 2018-10-17 2019-01-29 东北大学 A kind of low-nickel type LNG tank steel plate and preparation method thereof

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