WO2020036090A1 - Steel sheet and method for manufacturing same - Google Patents

Steel sheet and method for manufacturing same Download PDF

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Publication number
WO2020036090A1
WO2020036090A1 PCT/JP2019/030769 JP2019030769W WO2020036090A1 WO 2020036090 A1 WO2020036090 A1 WO 2020036090A1 JP 2019030769 W JP2019030769 W JP 2019030769W WO 2020036090 A1 WO2020036090 A1 WO 2020036090A1
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steel
steel sheet
content
toughness
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PCT/JP2019/030769
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French (fr)
Japanese (ja)
Inventor
博司 池田
茂樹 木津谷
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 JP2019568801A priority Critical patent/JP6904438B2/en
Priority to KR1020217005489A priority patent/KR102497359B1/en
Priority to EP19850397.1A priority patent/EP3825436A1/en
Priority to CN201980051501.3A priority patent/CN112513309B/en
Publication of WO2020036090A1 publication Critical patent/WO2020036090A1/en
Priority to PH12021550314A priority patent/PH12021550314A1/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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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

Definitions

  • the present invention relates to a steel sheet which is suitable for structural steel used in a cryogenic environment, such as a tank for a liquefied gas storage tank, and which has excellent corrosion resistance particularly in a salt water corrosion environment, and a method for producing the same.
  • hot-rolled steel sheets for structures such as liquefied gas storage tanks. Since such a structure is used at an extremely low temperature, the hot-rolled steel sheet applied to the structure is required to have not only high strength but also excellent toughness at an extremely low temperature. For example, when a hot-rolled steel sheet is used for a liquefied natural gas storage tank, 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, there is a risk that the safety as a structure for a cryogenic storage tank may not be maintained.
  • an austenitic stainless steel, a 9% Ni steel, or a 5000-type aluminum alloy having an austenitic structure that does not exhibit brittleness at cryogenic temperatures has been used.
  • these metal materials have high alloy costs and high production costs, there is a demand for steel plates that are inexpensive and have excellent cryogenic toughness. Therefore, as a new steel sheet that replaces the conventional cryogenic steel, a high-Mn steel is used as a structural steel sheet in a cryogenic environment by adding a large amount of relatively inexpensive austenitic stabilizing element Mn to form an austenitic structure. That is being considered.
  • Patent Document 1 discloses that the machinability and the Charpy impact characteristics at -196 ° C. of the heat-affected zone due to heat and heat are affected by adding Mn of 15 to 35%, Cu of 5% or less, and C and Cr in appropriate amounts. Are disclosed.
  • Patent Document 2 discloses that C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, and Ni: 0.1% to 7.0. %, Cr: 0.1% or more and less than 8.0% are added to improve the low-temperature toughness, and a high Mn steel material is disclosed.
  • Patent Document 3 discloses that the content of C is 0.001 to 0.80% and Mn is 15 to 35%, and the elements such as Cr, Ti, Si, Al, Mg, Ca, and REM are added.
  • a high Mn steel material with improved cryogenic toughness of the material and welds is disclosed.
  • Patent Literatures 1, 2, and 3 still have room for improvement in terms of manufacturing cost for achieving strength and low-temperature toughness, and corrosion resistance when the above-described austenitic steel material is placed in a salt corrosion environment. There is.
  • the present invention has been made in view of the above problems, and has as its object to provide a high Mn steel having excellent corrosion resistance, particularly in a salt corrosion environment.
  • excellent in corrosion resistance refers to a test based on the Slow Strain Rate Test Method based on NACE Standard TM0111-2011, which is immersed in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 ° C. : The breaking stress is 600 MPa or more when a constant velocity tensile test is performed at 4 ⁇ 10 ⁇ 7 inch / s.
  • the initial corrosion reaction on the steel sheet surface in a saltwater corrosion environment can be delayed by adding Cr and appropriately controlling the addition amount and the solid solution amount based on the high Mn steel. Thereby, the amount of hydrogen penetrating into the steel can be reduced, and the above-described stress corrosion cracking of the austenitic steel is suppressed.
  • P is an element that tends to segregate together with Mn in the process of solidifying the steel slab, and lowers the grain boundary strength at a portion where such a segregated portion intersects. Therefore, it is necessary to reduce impurity elements such as P.
  • B is an element that enhances the strength of the austenite grain boundary. By adding B in addition to reducing impurity elements such as P, it is possible to further effectively suppress grain boundary destruction.
  • the present invention has been made by further studying the above findings, and the gist thereof is as follows. 1. In mass%, C: 0.20% or more and 0.70% or less, Si: 0.05% or more and 1.00% or less, Mn: 15.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0200% or less, Al: 0.010% to 0.100%, Cr: 0.5% or more and 8.0% or less, N: 0.0010% or more and 0.0300% or less and B: 0.0003% or more and 0.0100% or less, and has a component composition of the balance of Fe and unavoidable impurities. Steel plate that is molten Cr.
  • the component composition further includes, in mass%, Nb: 0.003% or more and 0.030% or less, 2.
  • the component composition further includes, in mass%, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.30% or less, Sb: 0.01% or more and 0.30% or less, 3.
  • the component composition further includes, in mass%, Ca: 0.0005% or more and 0.0050% or less, 4.
  • hot rolling is performed at a finishing temperature: 750 ° C. or more and a material to be rolled: 950 ° C. or less.
  • a method for producing a steel sheet in which the residence time at 600 ° C. or more is set to 30 minutes or less, and then cooling is performed at a cooling rate of 3 ° C./s or more in a temperature range of 700 ° C. to 600 ° C.
  • the present invention it is possible to provide a steel sheet having excellent corrosion resistance, especially in a salt corrosion environment. Therefore, by using the steel sheet of the present invention for a steel structure used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, for example, the safety and life of the steel structure are greatly improved. Will bring about the effect. Further, since the steel sheet of the present invention is inexpensive as compared with existing materials, it also has the advantage of being economical.
  • the steel sheet of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.
  • the component composition of the steel sheet of the present invention in order to ensure excellent corrosion resistance, the component composition of the steel sheet is specified as follows. Note that “%” representing the component composition means “% by mass” unless otherwise specified.
  • C 0.20% or more and 0.70% or less C is effective for increasing the strength, is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite. To obtain the effect, C needs to be contained at 0.20% or more. On the other hand, when the content exceeds 0.70%, excessive precipitation of Cr carbide and Nb, V, and Ti-based carbides is promoted, and these precipitates become a starting point of corrosion and lower the low-temperature toughness. Therefore, C is set to 0.20% or more and 0.70% or less. Preferably, the content is 0.25% or more and 0.60% or less.
  • Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of forming a solid solution in steel and strengthening the steel sheet by solid solution strengthening. .
  • the content of Si needs to be 0.05% or more.
  • Si is set to 0.05% or more and 1.00% or less. Preferably, it is 0.07% or more and 0.50% or less.
  • Mn 15.0% or more and 35.0% or less
  • Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and toughness at extremely low temperatures. In order to obtain the effect, Mn needs to be contained at 15.0% or more. On the other hand, when the content exceeds 35.0%, the effect of improving the toughness at a very low temperature is saturated, which causes an increase in alloy cost. Also, the weldability and the cuttability deteriorate. Further, it causes the segregation of Mn to promote the occurrence of stress corrosion cracking. Therefore, Mn is set to 15.0% or more and 35.0% or less. Preferably, it is in the range of 18.0% or more and 28.0% or less.
  • P 0.030% or less
  • the content of P is preferably 0.024% or less, more preferably 0.020% or less.
  • the content of P is less than 0.001%, a large cost is required for steel making and the economic efficiency is impaired. Therefore, the content of 0.001% or more is allowable from the viewpoint of economic efficiency.
  • S 0.0200% or less Since S deteriorates the low-temperature toughness and ductility of the base material, the upper limit is set to 0.0200%, and it is desirable to reduce as much as possible. Therefore, S is set to 0.0200% or less, preferably 0.0180% or less. On the other hand, if the content is less than 0.0001%, a large cost is required for steel making and the economic efficiency is impaired. Therefore, the content of 0.0001% or more is allowable from the viewpoint of economic efficiency.
  • Al acts as a deoxidizing agent and is most commonly used in a molten steel deoxidizing process.
  • solid solution N in steel to form AlN
  • Al needs to be contained at 0.01% or more.
  • the content exceeds 0.100%, a coarse nitride is formed, which becomes a starting point of corrosion and destruction, and the stress corrosion cracking resistance may be reduced.
  • Al is set to 0.100% or less.
  • it is 0.020% or more and 0.070% or less.
  • Cr 0.5% or more and 8.0% or less and 60% or more of Cr is a solid solution
  • Cr Cr has an effect of delaying an initial corrosion reaction on the surface of a steel sheet in a saltwater corrosion environment when contained in an appropriate amount. This effect is an important element that reduces the amount of hydrogen penetrating into the steel sheet and improves stress corrosion cracking resistance. In order to obtain such an effect, the content needs to be 0.5% or more. On the other hand, when the content of Cr exceeds 8.0%, the above-mentioned effect obtained is saturated, and on the contrary, economic efficiency is impaired. Therefore, the Cr content is set to 0.5% or more and 8.0% or less. Preferably, it is at least 1.0%.
  • the solid solution component of the added Cr contributes to the improvement of the stress corrosion cracking resistance, but the precipitated component may hinder the improvement of the stress corrosion cracking resistance. It is important that at least 60% be solid solution Cr. That is, when the amount of the solid solution Cr is 60% or more of the contained Cr amount, the above-described effects can be obtained, and the improvement of the stress corrosion cracking resistance by adding Cr can be realized.
  • the solute Cr is preferably at least 70% of the Cr content, more preferably 100%.
  • Solute solution Cr refers to a state in which a solute atom exists in an atomic state without forming a precipitate or the like. Specifically, the amount of solute Cr was determined by extracting a test piece for electrolytic extraction from a steel sheet and extracting it by an electrolytic extraction method using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution. The precipitate can be determined by measuring the amount of Cr in the precipitate by ICP emission spectrometry and subtracting it from the total Cr in the test piece.
  • N 0.0010% or more and 0.0300% or less
  • N is an austenite stabilizing element and is an element effective for improving the cryogenic toughness.
  • Nb, V and Ti combines with Nb, V and Ti and finely precipitates as nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a trap site for diffusible hydrogen.
  • N needs to be contained at 0.0010% or more.
  • N is set to 0.0010% or more and 0.0300% or less.
  • it is 0.0020% or more and 0.0150% or less.
  • B 0.0003% or more and 0.0100% or less
  • B is an element that increases the strength of the austenite grain boundary, and is an element that suppresses cracking at the grain boundary and is effective in improving stress corrosion cracking resistance.
  • B needs to be contained at 0.0003% or more.
  • it is at least 0.0005%, more preferably more than 0.0007% and more than 0.0010%.
  • the content exceeds 0.0100%, this effect is saturated. Therefore, B is limited to the range of 0.0100% or less. Preferably, it is 0.0070% or less.
  • Nb 0.003% to 0.030%
  • V 0.01% to 0.10.
  • Ti 0.003% to 0.040%.
  • Nb is an element having the effect of suppressing stress corrosion cracking because it precipitates as carbonitride and the deposited carbonitride functions as a trap site for diffusible hydrogen. .
  • Nb is preferably contained at 0.003% or more.
  • the content is preferably set to 0.003% or more and 0.030% or less. More preferably, it is 0.005% or more and 0.025% or less, furthermore, 0.007% or more and 0.022% or less.
  • V 0.01% or more and 0.10% or less
  • V is an element having an effect of suppressing stress corrosion cracking because V is precipitated as carbonitride and the generated carbonitride functions as a trap site for diffusible hydrogen. .
  • V is preferably contained at 0.01% or more.
  • the content exceeds 0.10%, coarse carbonitrides may precipitate and become a starting point of fracture. Further, the precipitates may be coarsened and the toughness of the base material may be deteriorated.
  • the content is preferably set to 0.01% or more and 0.10% or less. More preferably, it is 0.02% or more and 0.09% or less, and further more preferably 0.03% or more and 0.08% or less.
  • Ti 0.003% or more and 0.040% or less Ti precipitates as nitride or carbonitride, and the generated nitride or carbonitride functions as a trap site for diffusible hydrogen. It is an element that has an effect. In order to obtain such an effect, it is preferable that Ti is contained at 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitates may be coarsened and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and serve as starting points for destruction. Therefore, when Ti is contained, the content is preferably set to 0.003% or more and 0.040% or less. More preferably, it is 0.005% or more and 0.035% or less, furthermore, 0.007% or more and 0.032% or less.
  • Cu 0.01% to 0.50%
  • Ni 0.01% to 0.50%
  • Sn 0.01% to 0.30%
  • Sb 0.01% to 0.30%
  • Mo 0.01% or more and 2.0% or less
  • W 0.01% or more and 2.0% or less
  • Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of a high Mn steel in a saltwater corrosion environment by being combined with Cr.
  • Cu, Sn, and Sb have an effect of suppressing a hydrogen generation reaction, which is a cathode reaction, by increasing a hydrogen overvoltage of a steel material.
  • Ni forms a precipitate coating on the steel material surface, Cl - physically inhibit the transmission of the corrosive anions such as base steel.
  • Cu, Ni, Sn, Sb, Mo and W are liberated as metal ions from the surface of the steel material during the corrosion, and the corrosion products are densified, whereby the corrosion to the steel interface (the interface between the rust layer and the ground iron) is reduced.
  • Mo and W are liberated as Mo 4 2 ⁇ and WO 4 2 ⁇ , respectively, and imparted to the cation selective permeability by being adsorbed in the corrosion product or on the steel sheet surface, and the permeation of corrosive anions to the iron base Is electrically suppressed.
  • the Cu content is in the range of 0.01% to 0.50%
  • the Ni content is in the range of 0.01% to 0.50%
  • the Sn content is in the range of 0.01% to 0.30%
  • the Sb content is in the range of 0.01% to 0.30%
  • the Mo content is in the range of 0.01% to 2.0%
  • the W content is in the range of 0.01% to 2.0%. Is preferred.
  • the Cu content is 0.02% to 0.40%
  • the Ni content is 0.02% to 0.40%
  • the Sn content is 0.02% to 0.25%
  • the Sb content is 0%.
  • Mo amount is 0.02% or more and 0.40% or less
  • W amount is 0.02% or more and 0.40% or less.
  • Ca 0.0005% or more and 0.0050% or less
  • Mg 0.0005% or more and 0.0100% or less
  • REM 0.0010% or more and 0.0200% or less.
  • Ca, Mg and REM are useful elements for controlling the morphology of inclusions, and can be contained as necessary.
  • the morphological control of inclusions means that expanded sulfide-based inclusions are used as granular inclusions. Through control of the inclusion morphology, ductility, toughness and sulfide stress corrosion cracking resistance can be improved.
  • Ca and Mg are contained at 0.0005% or more and REM is contained at 0.0010% or more.
  • REM is contained at 0.0010% or more.
  • the Ca content is 0.0010% to 0.0040%
  • the Mg content is 0.0010% to 0.0040%
  • the REM content is 0.0020% to 0.0150%.
  • the temperature of the material to be rolled in the hot rolling step and the cooling rate in the subsequent cooling step mean the temperature and the cooling rate measured on the surface of the rolled material. That is, after the steel material having the above-described composition is heated to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed at a reduction ratio of 3 or more and 30 or less and a finishing temperature of 750 ° C. or more and a material to be rolled: 950 ° C.
  • the steel sheet is manufactured by performing the cooling at a temperature of 700 ° C. or more and 600 ° C. or more at an average cooling rate of 3 ° C./s or more.
  • Step material heating temperature 1000 ° C to 1300 ° C
  • the reason why the steel material is heated to 1000 ° C. or higher is to make the carbonitride in the structure form a solid solution and make the crystal grain size and the like uniform. That is, if the heating temperature is lower than 1000 ° C., desired characteristics cannot be obtained because the carbonitride does not sufficiently form a solid solution. Further, when heating is performed at more than 1300 ° C., in addition to material deterioration due to coarsening of the crystal grain size, excessive energy is required and productivity is reduced. Therefore, the upper limit of the heating temperature is 1300 ° C.
  • the steel material it is preferable to use a steel material such as a slab or a billet by a conventionally known method such as an ingot-making method, in addition to the continuous casting slab. Needless to say, a process such as ladle refining and vacuum degassing may be added to the molten steel.
  • Cooling after hot rolling is performed when the average cooling rate at 700 ° C or lower and 600 ° C or higher is less than 3 ° C / s, since a large amount of precipitates such as Cr carbides are generated, the average cooling rate is 3 ° C / s or higher. limit. Preferably, it is in the range of 10 ° C / s or more and 150 ° C / s or less.
  • the residence time in a temperature range of 950 ° C. or lower and 600 ° C. or higher is restricted to 30 minutes or less, since it decreases to cause a decrease in corrosion resistance and a decrease in cryogenic toughness.
  • it is in the range from 5 minutes to 25 minutes.
  • the length of the material to be rolled is 5000 mm or less, and the reduction ratio in hot rolling from the material to be rolled is 30 or less. Is preferable. That is, if the length of the material to be rolled is 5000 mm or less and the rolling reduction is 30 or less, the rolling time is shortened, and as a result, the residence time in the range of 950 ° C. or less and 600 ° C. or more can be 30 minutes or less. .
  • the upper limit of the reduction ratio in hot rolling is preferably 30 or less.
  • the reduction ratio in hot rolling is less than 3, the effect of promoting recrystallization and reducing the size of the grains is reduced, so that coarse austenite grains remain, and the parts are preferentially oxidized, so that the corrosion resistance is reduced. There is a risk of deterioration. Therefore, it is preferable to set the reduction ratio in hot rolling to 3 or more.
  • the reduction ratio is defined as (the thickness of the rolled material to be subjected to hot rolling) / (the thickness of the steel sheet after hot rolling).
  • the corrosion resistance test was performed in accordance with the Slow Strain Rate Test Method (hereinafter, SSRT test) based on NACE Standard TM0111-2011. That is, the test piece was a Type A round bar notched test piece, 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 / s. Was carried out.
  • a rupture stress of 600 MPa or more is considered to be excellent in stress corrosion cracking resistance.
  • Table 2 shows the results obtained as described above.
  • the steel sheet according to the present invention (sample Nos. 1 to 42) had a corrosion resistance satisfying 600 MPa or more as a breaking stress in the SSRT test.
  • the comparative examples (sample Nos. 43 to 65) out of the range of the present invention do not satisfy the above-mentioned target performance in the stress corrosion cracking resistance.

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Abstract

Provided is steel that has a high Mn content and excellent in corrosion resistance, particularly corrosion resistance in a corrosive salt environment.　The steel has a composition containing 0.20 to 0.70% of C, 0.05 to 1.00% of Si, 15.0 to 35.0% of Mn, 0.030% or less of P, 0.0200% or less of S, 0.010 to 0.100% of Al, 0.5 to 8.0% of Cr, and 0.0010 to 0.0300% of N, with the remainder comprising Fe and unavoidable impurities, and 60% or more of the Cr being solid Cr.

Description

鋼板およびその製造方法Steel plate and manufacturing method thereof
　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食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 which is suitable for structural steel used in a cryogenic environment, such as a tank for a liquefied gas storage tank, and which has excellent corrosion resistance particularly in a salt water corrosion environment, and a method for producing the same.
　液化ガス貯槽用タンク等の構造物に熱間圧延鋼板を用いることが試みられている。かような構造物は、その使用環境が極低温となるため、該構造物に適用する熱延鋼板は高強度のみならず、極低温での靱性に優れることも要求される。例えば、液化天然ガスの貯槽に熱間圧延鋼板が使用される場合には、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。 It has been attempted to use hot-rolled steel sheets for structures such as liquefied gas storage tanks. Since such a structure is used at an extremely low temperature, the hot-rolled steel sheet applied to the structure is required to have not only high strength but also excellent toughness at an extremely low temperature. For example, when a hot-rolled steel sheet is used for a liquefied natural gas storage tank, 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, there is a risk that the safety as a structure for a cryogenic storage tank may not be maintained.
　この要求に対して、従来は、極低温で脆性を示さないオーステナイト組織を有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、これらの金属材料は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板への需要がある。そこで、従来の極低温用鋼に代わる新たな鋼板として、比較的安価なオーステナイト安定化元素であるＭｎを多量に添加しオーステナイト組織とした、高Ｍｎ鋼を極低温環境の構造用鋼板として適用することが検討されている。 To meet this requirement, conventionally, an austenitic stainless steel, a 9% Ni steel, or a 5000-type aluminum alloy having an austenitic structure that does not exhibit brittleness at cryogenic temperatures has been used. However, since these metal materials have high alloy costs and high production costs, there is a demand for steel plates that are inexpensive and have excellent cryogenic toughness. Therefore, as a new steel sheet that replaces the conventional cryogenic steel, a high-Mn steel is used as a structural steel sheet in a cryogenic environment by adding a large amount of relatively inexpensive austenitic stabilizing element Mn to form an austenitic structure. That is being considered.
　しかし、オーステナイト組織を有する鋼板が腐食環境に置かれる場合、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際に、応力腐食割れが発生しやすいという問題がある。液化ガス貯槽用の構造物などの製作段階では、鋼板の地鉄が表面に露出することがあり、鋼材表面が塩分など腐食性の物質を含む水蒸気および水分や油分などと接触すると、鋼材に腐食が発生する。この際、鋼板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（錆）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、鋼中に水素が侵入することによる水素脆化が生じる。そこに、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が作用すると、応力腐食割れが発生し、構造物が破壊に至る危険性がある。従来検討されている高Ｍｎ鋼では、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。そのため、安全性の観点から、構造物に使用される鋼材が高強度かつ極低温での靱性を有するのは勿論のこと、耐食性に優れることが重要になる。 However, when a steel sheet having an austenitic structure is placed in a corrosive environment, there is a problem that austenitic crystal grain boundaries are eroded by corrosion and stress corrosion cracking is likely to occur when a tensile stress is applied. At the stage of manufacturing structures for liquefied gas storage tanks, the steel sheet steel may be exposed on the surface, and if the steel material surface comes into contact with water vapor or water or oil containing corrosive substances such as salt, the steel will corrode Occurs. At this time, in the corrosion reaction on the steel sheet surface, iron generates an oxide (rust) by an anodic reaction, while hydrogen is generated by a cathodic reaction of water, and hydrogen intrusion occurs due to intrusion of hydrogen into the steel. Transformation occurs. If there is a residual stress due to bending or welding at the time of manufacturing, or a load stress in a use environment, stress corrosion cracking occurs and there is a risk that the structure may be broken. Conventionally studied high Mn steels are sometimes inferior in corrosion resistance to 9% Ni steel and ordinary low alloy steels as well as austenitic stainless steels. Therefore, from the viewpoint of safety, it is important not only that the steel material used for the structure has high strength and toughness at extremely low temperatures, but also that it has excellent corrosion resistance.
　例えば、特許文献１には、Ｍｎを１５～３５％、Ｃｕを５％以下、さらにＣとＣｒを適量添加することによって、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 the machinability and the Charpy impact characteristics at -196 ° C. of the heat-affected zone due to heat and heat are affected by adding Mn of 15 to 35%, Cu of 5% or less, and C and Cr in appropriate amounts. Are 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％未満を添加して低温靱性を改善した、高Ｍｎ鋼材が開示されている。 Patent Document 2 discloses that C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, and Ni: 0.1% to 7.0. %, Cr: 0.1% or more and less than 8.0% are added to improve the low-temperature toughness, and a high Mn steel material is disclosed.
　さらに、特許文献３には、Ｃを０．００１～０．８０％、Ｍｎを１５～３５％含有し、Ｃｒ、Ｔｉ、Ｓｉ、Ａｌ、Ｍｇ、Ｃａ、ＲＥＭといった元素を添加することにより、母材および溶接部の極低温靱性を改善した、高Ｍｎ鋼材が開示されている。 Further, Patent Document 3 discloses that the content of C is 0.001 to 0.80% and Mn is 15 to 35%, and the elements such as Cr, Ti, Si, Al, Mg, Ca, and REM are added. A high Mn steel material with improved cryogenic toughness of the material and welds is disclosed.
特表２０１５－５０８４５２号公報JP-T-2015-508452 A 特開２０１６－８４５２９号公報JP 2016-84529 A 特開２０１６－１９６７０３号公報JP 2016-196703 A
　しかしながら、特許文献１、２および３に記載の鋼材は、強度と低温靱性を達成するための製造コストの観点並びに、上述したオーステナイト鋼材が塩分腐食環境に置かれる際の、耐食性について未だ改善の余地がある。 However, the steel materials described in Patent Literatures 1, 2, and 3 still have room for improvement in terms of manufacturing cost for achieving strength and low-temperature toughness, and corrosion resistance when the above-described austenitic steel material is placed in a salt corrosion environment. There is.
　本発明は係る問題に鑑みなされたものであり、耐食性、特に塩分腐食環境における耐食性に優れる高Ｍｎ鋼を提供することを目的とする。ここで、「耐食性に優れる」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／ｓで等速引張試験を行った場合に、破断応力が６００ＭＰa以上であることをいう。 The present invention has been made in view of the above problems, and has as its object to provide a high Mn steel having excellent corrosion resistance, particularly in a salt corrosion environment. Here, “excellent in corrosion resistance” refers to a test based on the Slow Strain Rate Test Method based on NACE Standard TM0111-2011, which is immersed in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 ° C. : The breaking stress is 600 MPa or more when a constant velocity tensile test is performed at 4 × 10 −7 inch / s.
　本発明者らは、上記課題を達成するために、高Ｍｎ鋼を対象にして、その成分組成や製造条件を決定する各種要因に関して鋭意究明したところ、以下の知見を得るに到った。
ａ．高Ｍｎ鋼をベースにして、ここにＣｒを添加し、かつ添加量および固溶量を適正に制御することにより、塩水腐食環境における鋼板表面での初期の腐食反応を遅延させることができる。これにより、鋼中に侵入する水素量を低減することができ、上述したオーステナイト鋼の応力腐食割れは抑制される。 Means for Solving the Problems In order to achieve the above object, the present inventors have made intensive studies on various factors that determine the component composition and production conditions of a high Mn steel, and have obtained the following knowledge.
a. The initial corrosion reaction on the steel sheet surface in a saltwater corrosion environment can be delayed by adding Cr and appropriately controlling the addition amount and the solid solution amount based on the high Mn steel. Thereby, the amount of hydrogen penetrating into the steel can be reduced, and the above-described stress corrosion cracking of the austenitic steel is suppressed.
ｂ．さらに、オーステナイトの結晶粒界からの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。特に、Ｐは、鋼片の凝固過程において、Ｍｎとともに偏析しやすい元素であり、このような偏析部と交わる部分の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。一方、Ｂは、オーステナイト粒界の強度を高める元素であり、Ｐなどの不純物元素の低減に加えて、Ｂを添加することによって、さらに粒界破壊を効果的に抑制することが可能になる。 b. Furthermore, in order to effectively suppress the austenite from breaking from the crystal grain boundaries, measures to increase the crystal grain boundary strength are effective. In particular, P is an element that tends to segregate together with Mn in the process of solidifying the steel slab, and lowers the grain boundary strength at a portion where such a segregated portion intersects. Therefore, it is necessary to reduce impurity elements such as P. On the other hand, B is an element that enhances the strength of the austenite grain boundary. By adding B in addition to reducing impurity elements such as P, it is possible to further effectively suppress grain boundary destruction.
　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％以上０．７０％以下、
　Ｓｉ：０．０５％以上１．００％以下、
　Ｍｎ：１５.０％以上３５.０％以下、
　Ｐ：０．０３０％以下、
　Ｓ：０．０２００％以下、
　Ａｌ：０．０１０％以上０．１００％以下、
　Ｃｒ：０．５％以上８．０％以下、
　Ｎ：０．００１０％以上０．０３００％以下および
　Ｂ：０.０００３％以上０．０１００％以下
を含有し、残部Ｆｅおよび不可避的不純物の成分組成を有し、前記Ｃｒの６０％以上が固溶Ｃｒ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. In mass%,
C: 0.20% or more and 0.70% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% or more and 35.0% or less,
P: 0.030% or less,
S: 0.0200% or less,
Al: 0.010% to 0.100%,
Cr: 0.5% or more and 8.0% or less,
N: 0.0010% or more and 0.0300% or less and B: 0.0003% or more and 0.0100% or less, and has a component composition of the balance of Fe and unavoidable impurities. Steel plate that is molten Cr.
２．前記成分組成は、さらに、質量％で、
　Ｎｂ：０．００３％以上０．０３０％以下、
　Ｖ：０．０１％以上０．１０％以下および
　Ｔｉ：０．００３％以上０．０４０％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に記載の耐食性に優れる鋼板。 2. The component composition further includes, in mass%,
Nb: 0.003% or more and 0.030% or less,
2. The steel sheet having excellent corrosion resistance as described in 1 above, containing one or more selected from V: 0.01% or more and 0.10% or less and Ti: 0.003% or more and 0.040% or less.
３．前記成分組成は、さらに、質量％で、
　Ｃｕ：０．０１％以上０．５０％以下、
　Ｎｉ：０．０１％以上０．５０％以下、
　Ｓｎ：０．０１％以上０．３０％以下、
　Ｓｂ：０．０１％以上０．３０％以下、
　Ｍｏ：０．０１％以上２．０％以下および
　Ｗ：０．０１％以上２．０％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. The component composition further includes, in mass%,
Cu: 0.01% or more and 0.50% or less,
Ni: 0.01% or more and 0.50% or less,
Sn: 0.01% or more and 0.30% or less,
Sb: 0.01% or more and 0.30% or less,
3. The steel sheet according to 1 or 2 above, comprising one or more selected from Mo: 0.01% or more and 2.0% or less and W: 0.01% or more and 2.0% or less.
４．前記成分組成は、さらに、質量％で、
　Ｃａ：０．０００５％以上０．００５０％以下、
　Ｍｇ：０．０００５％以上０．０１００％以下および
　ＲＥＭ：０．００１０％以上０．０２００％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、２または３に記載の鋼板。 4. The component composition further includes, in mass%,
Ca: 0.0005% or more and 0.0050% or less,
4. The steel sheet according to the above 1, 2 or 3, containing one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.
５．前記１から４のいずれかに記載の成分組成を有する鋼素材を、１０００℃以上１３００℃以下に加熱後、熱間圧延を、仕上げ温度：７５０℃以上にて、被圧延材温度：９５０℃以下６００℃以上における滞在時間を３０分以下として施し、次いで７００℃以下６００℃以上の温度範囲における平均冷却速度：３℃／ｓ以上の冷却を行う鋼板の製造方法。 5. After heating the steel material having the composition of any one of the above items 1 to 4 to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed at a finishing temperature: 750 ° C. or more and a material to be rolled: 950 ° C. or less. A method for producing a steel sheet in which the residence time at 600 ° C. or more is set to 30 minutes or less, and then cooling is performed at a cooling rate of 3 ° C./s or more in a temperature range of 700 ° C. to 600 ° C.
　本発明によれば、耐食性、特に塩分腐食環境における耐食性に優れる鋼板を提供することができる。従って、本発明の鋼板を、例えば液化ガス貯槽用タンク等の、極低温環境で使用される鋼構造物に用いることによって、該鋼構造物の安全性や寿命が大きく向上する結果、産業上格段の効果をもたらすことになる。また、本発明の鋼板は、既存の材料に比べて安価であるため、経済性に優れる利点も有する。 According to the present invention, it is possible to provide a steel sheet having excellent corrosion resistance, especially in a salt corrosion environment. Therefore, by using the steel sheet of the present invention for a steel structure used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, for example, the safety and life of the steel structure are greatly improved. Will bring about the effect. Further, since the steel sheet of the present invention is inexpensive as compared with existing materials, it also has the advantage of being economical.
　以下、本発明の鋼板について詳しく説明する。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、成分組成を表す「％」は、特に断らない限り「質量％」を意味するものとする。 Hereinafter, the steel sheet of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.
[Component composition]
First, the component composition of the steel sheet of the present invention and the reasons for the limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the component composition of the steel sheet is specified as follows. Note that “%” representing the component composition means “% by mass” unless otherwise specified.
Ｃ：０．２０％以上０．７０％以下
　Ｃは、高強度化に有効であり、さらに、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系炭化物の過度な析出を促し、これら析出物が腐食の発生起点となり、また低温靱性を低下する。このため、Ｃは０．２０％以上０．７０％以下とする。好ましくは、０．２５％以上０．６０％以下とする。 C: 0.20% or more and 0.70% or less C is effective for increasing the strength, is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite. To obtain the effect, C needs to be contained at 0.20% or more. On the other hand, when the content exceeds 0.70%, excessive precipitation of Cr carbide and Nb, V, and Ti-based carbides is promoted, and these precipitates become a starting point of corrosion and lower the low-temperature toughness. Therefore, C is set to 0.20% or more and 0.70% or less. Preferably, the content is 0.25% or more and 0.60% or less.
Ｓｉ：０．０５％以上１．００％以下
　Ｓｉは、脱酸材として作用し、製鋼上必要であるだけでなく、鋼に固溶して固溶強化により鋼板を高強度化する効果を有する。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、１．００％を超えて含有すると、溶接性および表面性状が劣化し耐応力腐食割れ性が低下する場合がある。このため、Ｓｉは０．０５％以上１．００％以下とする。好ましくは、０．０７％以上０．５０％以下とする。 Si: 0.05% or more and 1.00% or less Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of forming a solid solution in steel and strengthening the steel sheet by solid solution strengthening. . To obtain such an effect, the content of Si needs to be 0.05% or more. On the other hand, if the content exceeds 1.00%, weldability and surface properties may be deteriorated, and stress corrosion cracking resistance may be reduced. Therefore, Si is set to 0.05% or more and 1.00% or less. Preferably, it is 0.07% or more and 0.50% or less.
Ｍｎ：１５.０％以上３５.０％以下
　Ｍｎは、比較的安価なオーステナイト安定化元素である。本発明では、強度と極低温での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１５.０％以上の含有を必要とする。一方、３５.０％を超えて含有する場合、極低温での靱性を改善する効果は飽和し、合金コストの上昇を招く。また、溶接性および切断性が劣化する。さらに、Ｍｎの偏析を招いて、応力腐食割れの発生を助長する。このため、Ｍｎは１５.０％以上３５.０％以下とする。好ましくは、１８.０％以上２８.０％以下の範囲とする。 Mn: 15.0% or more and 35.0% or less Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and toughness at extremely low temperatures. In order to obtain the effect, Mn needs to be contained at 15.0% or more. On the other hand, when the content exceeds 35.0%, the effect of improving the toughness at a very low temperature is saturated, which causes an increase in alloy cost. Also, the weldability and the cuttability deteriorate. Further, it causes the segregation of Mn to promote the occurrence of stress corrosion cracking. Therefore, Mn is set to 15.0% or more and 35.0% or less. Preferably, it is in the range of 18.0% or more and 28.0% or less.
Ｐ：０．０３０％以下
　Ｐは、０．０３０％を超えて含有すると、粒界に偏析し粒界強度を低下させ、応力腐食割れの発生起点となる。このため、０．０３０％を上限とし、可能なかぎり低減することが望ましい。Ｐは含有量が低いほど特性が向上するため、好ましくは０．０２４％以下とし、より好ましくは０．０２０％以下とする。一方、Ｐを０．００１％未満とするには製鋼に多大なコストを要し経済性が損なわれるため、経済性の観点からは０．００１％以上の含有は許容される。 P: 0.030% or less When P is contained in excess of 0.030%, it segregates at the grain boundary, lowers the grain boundary strength, and becomes a starting point of stress corrosion cracking. For this reason, it is desirable to set the upper limit to 0.030% and reduce as much as possible. Since the lower the content of P, the better the properties, the content of P is preferably 0.024% or less, more preferably 0.020% or less. On the other hand, if the content of P is less than 0.001%, a large cost is required for steel making and the economic efficiency is impaired. Therefore, the content of 0.001% or more is allowable from the viewpoint of economic efficiency.
Ｓ：０．０２００％以下
　Ｓは、母材の低温靭性や延性を劣化させるため、０．０２００％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０．０２００％以下、好ましくは０．０１８０％以下とする。一方、０．０００１％未満とするには製鋼に多大なコストを要し経済性が損なわれるため、経済性の観点からは０．０００１％以上の含有は許容される。 S: 0.0200% or less Since S deteriorates the low-temperature toughness and ductility of the base material, the upper limit is set to 0.0200%, and it is desirable to reduce as much as possible. Therefore, S is set to 0.0200% or less, preferably 0.0180% or less. On the other hand, if the content is less than 0.0001%, a large cost is required for steel making and the economic efficiency is impaired. Therefore, the content of 0.0001% or more is allowable from the viewpoint of economic efficiency.
Ａｌ：０．０１０％以上０．１００％以下
　Ａｌは、脱酸剤として作用し、溶鋼脱酸プロセスに於いて最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。さらに、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、０．１００％を超えて含有すると、粗大な窒化物を形成し腐食や破壊の起点となって耐応力腐食割れ性が低下する場合がある。また、溶接時に溶接金属部に拡散して、溶接金属の靭性を劣化させる。そのため、Ａｌは０．１００％以下とする。好ましくは、０．０２０％以上０．０７０％以下とする。 Al: 0.010% or more and 0.100% or less Al acts as a deoxidizing agent and is most commonly used in a molten steel deoxidizing process. In addition, by fixing solid solution N in steel to form AlN, there is an effect of suppressing coarsening of crystal grains. Further, it has the effect of suppressing the degradation of toughness due to the reduction of dissolved N. To obtain such an effect, Al needs to be contained at 0.01% or more. On the other hand, when the content exceeds 0.100%, a coarse nitride is formed, which becomes a starting point of corrosion and destruction, and the stress corrosion cracking resistance may be reduced. In addition, it diffuses into the weld metal during welding and deteriorates the toughness of the weld metal. Therefore, Al is set to 0.100% or less. Preferably, it is 0.020% or more and 0.070% or less.
Ｃｒ：０．５％以上８．０％以下かつＣｒの６０％以上が固溶Ｃｒ
　Ｃｒは、適量の含有によって塩水腐食環境における鋼板表面での初期の腐食反応を遅延させる効果を有する。この効果により鋼板中への水素侵入量を低下させ、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には、０．５％以上の含有が必要である。一方、Ｃｒは８．０％を超えると得られる上記効果は飽和し、却って経済性を損ねることになる。従って、Ｃｒ量は０．５％以上８．０％以下とする。好ましくは、１.０％以上である。 Cr: 0.5% or more and 8.0% or less and 60% or more of Cr is a solid solution Cr
Cr has an effect of delaying an initial corrosion reaction on the surface of a steel sheet in a saltwater corrosion environment when contained in an appropriate amount. This effect is an important element that reduces the amount of hydrogen penetrating into the steel sheet and improves stress corrosion cracking resistance. In order to obtain such an effect, the content needs to be 0.5% or more. On the other hand, when the content of Cr exceeds 8.0%, the above-mentioned effect obtained is saturated, and on the contrary, economic efficiency is impaired. Therefore, the Cr content is set to 0.5% or more and 8.0% or less. Preferably, it is at least 1.0%.
　ここで、添加したＣｒのうち固溶分は耐応力腐食割れ性の向上に寄与するが、析出分は逆に耐応力腐食割れ性の向上を阻害する可能性があることから、上記したＣｒの少なくとも６０％は固溶Ｃｒであることが肝要である。すなわち、固溶Ｃｒが含有Ｃｒ量の６０％以上であれば、上記した効果を享受することができ、Ｃｒ添加による耐応力腐食割れ性の向上を実現することできる。固溶Ｃｒは含有Ｃｒ量の７０％以上であることが好ましく、１００％であることがより好ましい。 Here, the solid solution component of the added Cr contributes to the improvement of the stress corrosion cracking resistance, but the precipitated component may hinder the improvement of the stress corrosion cracking resistance. It is important that at least 60% be solid solution Cr. That is, when the amount of the solid solution Cr is 60% or more of the contained Cr amount, the above-described effects can be obtained, and the improvement of the stress corrosion cracking resistance by adding Cr can be realized. The solute Cr is preferably at least 70% of the Cr content, more preferably 100%.
　なお、固溶Ｃｒとは、溶質原子が析出物等を形成せずに原子の状態で存在している状態のことである。具体的には、固溶Ｃｒ量は、鋼板から電解抽出用試験片を採取して、１０％ＡＡ（１０％アセチルアセトン－１％塩化テトラメチルアンモニウム－メタノール）溶液を利用した電解抽出法で抽出した析出物について、ＩＣＰ発光分析法で析出物中のＣｒ量を測定して、試験片中の全Ｃｒから差し引くことによって求めることができる。 固 Solute solution Cr refers to a state in which a solute atom exists in an atomic state without forming a precipitate or the like. Specifically, the amount of solute Cr was determined by extracting a test piece for electrolytic extraction from a steel sheet and extracting it by an electrolytic extraction method using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution. The precipitate can be determined by measuring the amount of Cr in the precipitate by ICP emission spectrometry and subtracting it from the total Cr in the test piece.
Ｎ：０．００１０％以上０．０３００％以下
　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およびＴｉと結合し、窒化物または炭窒化物として微細に析出して、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、０．０３００％を超えて含有すると、過剰な窒化物または炭窒化物の生成を促し、固溶元素量が低下し耐食性が低下するだけでなく、靭性も低下する。このため、Ｎは０．００１０％以上０．０３００％以下とする。好ましくは０．００２０％以上０．０１５０％以下とする。 N: 0.0010% or more and 0.0300% or less N is an austenite stabilizing element and is an element effective for improving the cryogenic toughness. In addition, it combines with Nb, V and Ti and finely precipitates as nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a trap site for diffusible hydrogen. In order to obtain such an effect, N needs to be contained at 0.0010% or more. On the other hand, when the content exceeds 0.0300%, the generation of excessive nitride or carbonitride is promoted, so that not only the amount of solid solution elements is reduced and the corrosion resistance is reduced, but also the toughness is reduced. Therefore, N is set to 0.0010% or more and 0.0300% or less. Preferably it is 0.0020% or more and 0.0150% or less.
Ｂ：０．０００３％以上０．０１００％以下
　Ｂは、オーステナイト粒界の強度を高める元素であり、粒界での割れを抑制する、耐応力腐食割れ性の向上に有効な元素である。このような効果を得るためには、Ｂは０．０００３％以上の含有を必要とする。好ましくは、０．０００５％以上であり、さらに好ましくは０．０００７％超、そして０．００１０％超である。一方、０．０１００％を超えて含有すると、この効果が飽和する。そのため、Ｂは０．０１００％以下の範囲に限定した。好ましくは、０．００７０％以下である。 B: 0.0003% or more and 0.0100% or less B is an element that increases the strength of the austenite grain boundary, and is an element that suppresses cracking at the grain boundary and is effective in improving stress corrosion cracking resistance. In order to obtain such an effect, B needs to be contained at 0.0003% or more. Preferably it is at least 0.0005%, more preferably more than 0.0007% and more than 0.0010%. On the other hand, if the content exceeds 0.0100%, this effect is saturated. Therefore, B is limited to the range of 0.0100% or less. Preferably, it is 0.0070% or less.
　本発明では、耐食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、Ｎｂ：０．００３％以上０．０３０％以下、Ｖ：０．０１％以上０．１０％以下およびＴｉ：０．００３％以上０．０４０％以下を含有することができる。 In the present invention, in order to further improve the corrosion resistance, in addition to the above essential elements, if necessary, Nb: 0.003% to 0.030%, V: 0.01% to 0.10. % And Ti: 0.003% to 0.040%.
Ｎｂ：０．００３％以上０．０３０％以下
　Ｎｂは、炭窒化物として析出し、析出した炭窒化物が拡散性水素のトラップサイトとして機能するため、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％以上で含有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、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％以上０．０３０％以下とすることが好ましい。より好ましくは０．００５％以上０．０２５％以下、さらには０．００７％以上０．０２２％以下である。 Nb: 0.003% or more and 0.030% or less Nb is an element having the effect of suppressing stress corrosion cracking because it precipitates as carbonitride and the deposited carbonitride functions as a trap site for diffusible hydrogen. . In order to obtain such effects, Nb is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.030%, coarse carbonitrides may precipitate and become a starting point of fracture. Further, the precipitates may be coarsened and the toughness of the base material may be deteriorated. Therefore, when Nb is contained, the content is preferably set to 0.003% or more and 0.030% or less. More preferably, it is 0.005% or more and 0.025% or less, furthermore, 0.007% or more and 0.022% or less.
Ｖ：０．０１％以上０．１０％以下
　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として機能するため、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０．０１％以上で含有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％以下とすることが好ましい。より好ましくは０．０２％以上０．０９％以下、さらには０．０３％以上０．０８％以下である。 V: 0.01% or more and 0.10% or less V is an element having an effect of suppressing stress corrosion cracking because V is precipitated as carbonitride and the generated carbonitride functions as a trap site for diffusible hydrogen. . In order to obtain such effects, V is preferably contained at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitrides may precipitate and become a starting point of fracture. Further, the precipitates may be coarsened and the toughness of the base material may be deteriorated. For this reason, when V is contained, the content is preferably set to 0.01% or more and 0.10% or less. More preferably, it is 0.02% or more and 0.09% or less, and further more preferably 0.03% or more and 0.08% or less.
Ｔｉ：０．００３％以上０．０４０％以下
　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として機能するため、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％以上で含有することが好ましい。一方、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％以上０．０４０％以下とすることが好ましい。より好ましくは０．００５％以上０．０３５％以下、さらには０．００７％以上０．０３２％以下である。 Ti: 0.003% or more and 0.040% or less Ti precipitates as nitride or carbonitride, and the generated nitride or carbonitride functions as a trap site for diffusible hydrogen. It is an element that has an effect. In order to obtain such an effect, it is preferable that Ti is contained at 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitates may be coarsened and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and serve as starting points for destruction. Therefore, when Ti is contained, the content is preferably set to 0.003% or more and 0.040% or less. More preferably, it is 0.005% or more and 0.035% or less, furthermore, 0.007% or more and 0.032% or less.
　さらに、本発明では、耐食性を一層向上させることを目的として、必要に応じて、
　Ｃｕ：０．０１％以上０．５０％以下、Ｎｉ：０．０１％以上０．５０％以下、Ｓｎ：０．０１％以上０．３０％以下、Ｓｂ：０．０１％以上０．３０％以下、Ｍｏ：０．０１％以上２．０％以下、Ｗ：０．０１％以上２．０％以下の１種または２種以上を含有することができる。 Furthermore, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30% Hereinafter, one or more of Mo: 0.01% or more and 2.0% or less, and W: 0.01% or more and 2.0% or less can be contained.
　すなわち、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、ＭｏおよびＷは、Ｃｒと複合添加することによって、高Ｍｎ鋼の塩水腐食環境における耐食性を向上させる元素である。ここで、Ｃｕ、ＳｎおよびＳｂは、鋼材の水素過電圧を増大することによって、カソード反応である水素発生反応を抑制する効果を有する。Ｎｉは、鋼材表面に沈殿皮膜を形成し、Ｃｌ－等の腐食性アニオンの地鉄への透過を物理的に抑制する。また、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、ＭｏおよびＷは、腐食に際し、鋼材表面から金属イオンとして遊離し、腐食生成物を緻密にすることによって、鋼界面（錆層と地鉄の界面）への腐食性アニオンの透過を抑制する。ＭｏおよびＷはそれぞれ、Ｍｏ４ ２－およびＷＯ４ ２－として遊離し、腐食生成物中または鋼板表面に吸着されることにより、カチオン選択透過性を付与し、腐食性アニオンの地鉄への透過を電気的に抑制する。 That is, Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of a high Mn steel in a saltwater corrosion environment by being combined with Cr. Here, Cu, Sn, and Sb have an effect of suppressing a hydrogen generation reaction, which is a cathode reaction, by increasing a hydrogen overvoltage of a steel material. Ni forms a precipitate coating on the steel material surface, Cl - physically inhibit the transmission of the corrosive anions such as base steel. Further, Cu, Ni, Sn, Sb, Mo and W are liberated as metal ions from the surface of the steel material during the corrosion, and the corrosion products are densified, whereby the corrosion to the steel interface (the interface between the rust layer and the ground iron) is reduced. Suppresses the transmission of corrosive anions. Mo and W are liberated as Mo 4 2− and WO 4 2− , respectively, and imparted to the cation selective permeability by being adsorbed in the corrosion product or on the steel sheet surface, and the permeation of corrosive anions to the iron base Is electrically suppressed.
　以上の効果は、高Ｍｎ鋼において、Ｃｒと共存した場合において顕在化し、それぞれの元素を０．０１％以上添加した際に発現する。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。
　従って、Ｃｕ量は０．０１％以上０．５０％以下の範囲、Ｎｉ量は０．０１％以上０．５０％以下の範囲、Ｓｎ量は０．０１％以上０．３０％以下の範囲、Ｓｂ量は０．０１％以上０．３０％以下の範囲、Ｍｏ量は０．０１％以上２．０％以下の範囲、Ｗ量は０．０１％以上２．０％以下の範囲とすることが好ましい。
　より好ましくは、Ｃｕ量は０．０２％以上０．４０％以下、Ｎｉ量は０．０２％以上０．４０％以下、Ｓｎ量は０．０２％以上０．２５％以下、Ｓｂ量は０．０２％以上０．２５％以下、Ｍｏ量は０．０２％以上０．４０％以下、Ｗ量は０．０２％以上０．４０％以下である。 The above effects are evident when coexisting with Cr in a high Mn steel, and are manifested when each element is added by 0.01% or more. However, when any of these elements is contained in a large amount, weldability and toughness are degraded, which is disadvantageous from the viewpoint of cost.
Therefore, the Cu content is in the range of 0.01% to 0.50%, the Ni content is in the range of 0.01% to 0.50%, the Sn content is in the range of 0.01% to 0.30%, The Sb content is in the range of 0.01% to 0.30%, the Mo content is in the range of 0.01% to 2.0%, and the W content is in the range of 0.01% to 2.0%. Is preferred.
More preferably, the Cu content is 0.02% to 0.40%, the Ni content is 0.02% to 0.40%, the Sn content is 0.02% to 0.25%, and the Sb content is 0%. 0.02% or more and 0.25% or less, Mo amount is 0.02% or more and 0.40% or less, and W amount is 0.02% or more and 0.40% or less.
　同様に、本発明では、耐食性を一層向上させることを目的として、必要に応じて、
　Ｃａ：０．０００５％以上０．００５０％以下、Ｍｇ：０．０００５％以上０．０１００％以下およびＲＥＭ：０．００１０％以上０．０２００％以下の１種または２種以上を含有することができる。
　すなわち、Ｃａ、ＭｇおよびＲＥＭは、介在物の形態制御に有用な元素であり、必要に応じて含有できる。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、延性、靭性および耐硫化物応力腐食割れ性を向上させ得る。このような効果を得るためには、ＣａおよびＭｇは０．０００５％以上、ＲＥＭは０．００１０％以上で含有することが好ましい。一方、いずれの元素も多く含有させると、非金属介在物量が増加し、かえって延性、靭性、耐硫化物応力腐食割れ性が低下する場合がある。また、経済的に不利になる場合がある。 Similarly, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
Ca: 0.0005% or more and 0.0050% or less; Mg: 0.0005% or more and 0.0100% or less; and REM: 0.0010% or more and 0.0200% or less. it can.
That is, Ca, Mg and REM are useful elements for controlling the morphology of inclusions, and can be contained as necessary. Here, the morphological control of inclusions means that expanded sulfide-based inclusions are used as granular inclusions. Through control of the inclusion morphology, ductility, toughness and sulfide stress corrosion cracking resistance can be improved. In order to obtain such effects, it is preferable that Ca and Mg are contained at 0.0005% or more and REM is contained at 0.0010% or more. On the other hand, when a large amount of any of the elements is contained, the amount of nonmetallic inclusions increases, and on the contrary, ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. In addition, it may be economically disadvantageous.
　このため、Ｃａを含有する場合には０．０００５％以上０．００５０％以下、Ｍｇを含有する場合には０．０００５％以上０．０１００％以下、ＲＥＭを含有する場合には０．００１０％以上０．０２００％以下とすることが好ましい。より好ましくは、Ｃａ量は０．００１０％以上０．００４０％以下、Ｍｇ量は０．００１０％以上０．００４０％以下、ＲＥＭ量は０．００２０％以上０．０１５０％以下である。 Therefore, 0.0005% or more and 0.0050% or less when Ca is contained, 0.0005% or more and 0.0100% or less when Mg is contained, and 0.0010% or less when REM is contained. It is preferable to set it to 0.0200% or less. More preferably, the Ca content is 0.0010% to 0.0040%, the Mg content is 0.0010% to 0.0040%, and the REM content is 0.0020% to 0.0150%.
　次に、本発明の製造条件について説明する。なお、熱間圧延工程における被圧延材の温度およびその後の冷却工程における冷却速度は、当該圧延材の表面において測定した温度ならびに冷却速度を意味する。すなわち、上記した成分組成を有する鋼素材を１０００℃以上１３００℃以下に加熱後、熱間圧延を、圧下比：３以上３０以下かつ仕上げ温度：７５０℃以上にて、被圧延材温度：９５０℃以下６００℃以上における滞在時間を３０分以下として施し、次いで７００℃以下６００℃以上の温度範囲における平均冷却速度：３℃／ｓ以上の冷却を行うことによって、鋼板を製造する。 Next, the manufacturing conditions of the present invention will be described. The temperature of the material to be rolled in the hot rolling step and the cooling rate in the subsequent cooling step mean the temperature and the cooling rate measured on the surface of the rolled material. That is, after the steel material having the above-described composition is heated to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed at a reduction ratio of 3 or more and 30 or less and a finishing temperature of 750 ° C. or more and a material to be rolled: 950 ° C. The steel sheet is manufactured by performing the cooling at a temperature of 700 ° C. or more and 600 ° C. or more at an average cooling rate of 3 ° C./s or more.
［鋼素材の加熱温度：１０００℃以上１３００℃以下］
　鋼素材を１０００℃以上に加熱するのは、組織中の炭窒化物を固溶させ、結晶粒径等を均一化するためである。すなわち、加熱温度が１０００℃未満の場合、炭窒化物が十分に固溶しないため所望の特性が得られない。また、１３００℃を超えて加熱すると、結晶粒径の粗大化による材質劣化に加えて、過剰なエネルギーが必要となり生産性が低下するため、加熱温度の上限は１３００℃とする。好ましくは１０５０℃以上１２５０℃以下、より好ましくは１０７０℃以上１２５０℃以下の範囲である。なお、鋼素材としては、連続鋳造スラブのほか、造塊法等の通常公知の方法でスラブやビレット等の鋼素材とするのが好ましい。なお、溶鋼に、取鍋精錬や真空脱ガス等の処理を付加しても良いことは言うまでもない。 [Steel material heating temperature: 1000 ° C to 1300 ° C]
The reason why the steel material is heated to 1000 ° C. or higher is to make the carbonitride in the structure form a solid solution and make the crystal grain size and the like uniform. That is, if the heating temperature is lower than 1000 ° C., desired characteristics cannot be obtained because the carbonitride does not sufficiently form a solid solution. Further, when heating is performed at more than 1300 ° C., in addition to material deterioration due to coarsening of the crystal grain size, excessive energy is required and productivity is reduced. Therefore, the upper limit of the heating temperature is 1300 ° C. It is preferably in the range of 1050 ° C to 1250 ° C, more preferably in the range of 1070 ° C to 1250 ° C. As the steel material, it is preferable to use a steel material such as a slab or a billet by a conventionally known method such as an ingot-making method, in addition to the continuous casting slab. Needless to say, a process such as ladle refining and vacuum degassing may be added to the molten steel.
［熱間圧延の仕上げ温度：７５０℃以上］
　熱間圧延の仕上げ温度が７５０℃未満の場合、該圧延中の炭化物析出量が著しく増大し、後述のように６００℃以上９００℃以下における滞在時間を３０分未満としても固溶Ｃｒ量が確保できなくなる場合があり、耐食性が低下する。また、７５０℃未満で圧延する場合、変形抵抗が大きくなり製造設備に過大な負荷がかかるため、圧延仕上げ温度は７５０℃以上とする。 [Hot rolling finish temperature: 750 ° C or higher]
When the finishing temperature of the hot rolling is lower than 750 ° C., the amount of carbide precipitation during the rolling is remarkably increased, and the amount of solid-dissolved Cr is ensured even when the residence time at 600 ° C. to 900 ° C. is less than 30 minutes as described later. In some cases, corrosion resistance may be reduced. Further, when rolling is performed at a temperature lower than 750 ° C., since deformation resistance increases and an excessive load is applied to manufacturing equipment, the rolling finishing temperature is set to 750 ° C. or higher.
［７００℃以下６００℃以上における平均冷却速度：３℃/s以上］
　熱間圧延後の冷却は、７００℃以下６００℃以上における平均冷却速度が３℃/s未満の場合、Ｃｒ炭化物などの析出物が大量に生成するため、平均冷却速度を３℃/s以上に限定する。好ましくは、１０℃/s以上１５０℃/s以下の範囲である。  [Average cooling rate at 700 ° C or lower and 600 ° C or higher: 3 ° C / s or higher]
Cooling after hot rolling is performed when the average cooling rate at 700 ° C or lower and 600 ° C or higher is less than 3 ° C / s, since a large amount of precipitates such as Cr carbides are generated, the average cooling rate is 3 ° C / s or higher. limit. Preferably, it is in the range of 10 ° C / s or more and 150 ° C / s or less.
［９５０℃以下６００℃以上の温度域における滞在時間：３０分以下］
　熱間圧延において被圧延素材が９５０℃以下６００℃以上の温度域に滞在する時間は、３０分を超えると、圧延中から炭窒化物や炭化物が大量に析出し、必要な固溶Ｃｒ量が減少し耐食性の低下および極低温靭性の低下を引き起こすため、９５０℃以下６００℃以上の温度域における滞在時間を３０分以下に規制する。好ましくは、５分以上２５分以下の範囲である。  [Dwell time in the temperature range of 950 ° C or lower and 600 ° C or higher: 30 minutes or less]
When the material to be rolled stays in a temperature range of 950 ° C. or lower and 600 ° C. or higher in hot rolling, if it exceeds 30 minutes, a large amount of carbonitrides and carbides precipitate during rolling, and the required amount of solute Cr is reduced. Therefore, the residence time in a temperature range of 950 ° C. or lower and 600 ° C. or higher is restricted to 30 minutes or less, since it decreases to cause a decrease in corrosion resistance and a decrease in cryogenic toughness. Preferably, it is in the range from 5 minutes to 25 minutes.
　ここで、９５０℃以下６００℃以上の温度域における滞在時間を３０分以下とするには、被圧延材の長さを５０００ｍｍ以下にし、かつ被圧延材からの熱間圧延における圧下比を３０以下にすることが好ましい。すなわち、被圧延材の長さを５０００ｍｍ以下にして圧下比を３０以下にすれば、圧延時間は短くなり、結果として９５０℃以下６００℃以上の範囲における滞在時間を３０分以下にすることができる。 Here, in order to make the staying time in the temperature range of 950 ° C. or less and 600 ° C. or more 30 minutes or less, the length of the material to be rolled is 5000 mm or less, and the reduction ratio in hot rolling from the material to be rolled is 30 or less. Is preferable. That is, if the length of the material to be rolled is 5000 mm or less and the rolling reduction is 30 or less, the rolling time is shortened, and as a result, the residence time in the range of 950 ° C. or less and 600 ° C. or more can be 30 minutes or less. .
　上記のとおり、熱間圧延における圧下比の上限は３０以下にすることが好ましい。一方、熱間圧延における圧下比が３未満になると、再結晶を促進し整粒化を図る効果が少なくなる結果、粗大なオーステナイト粒が残存し、その部分が優先的に酸化することで耐食性が劣化する、虞がある。したがって、熱間圧延における圧下比を３以上にすることが好ましい。
　ここで、圧下比とは、（熱間圧延に供する圧延素材の板厚）／（熱間圧延後の鋼板の板厚）で定義されるものである。 As described above, the upper limit of the reduction ratio in hot rolling is preferably 30 or less. On the other hand, if the reduction ratio in hot rolling is less than 3, the effect of promoting recrystallization and reducing the size of the grains is reduced, so that coarse austenite grains remain, and the parts are preferentially oxidized, so that the corrosion resistance is reduced. There is a risk of deterioration. Therefore, it is preferable to set the reduction ratio in hot rolling to 3 or more.
Here, the reduction ratio is defined as (the thickness of the rolled material to be subjected to hot rolling) / (the thickness of the steel sheet after hot rolling).
　表１に示すＮｏ．１から５７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６ｍｍ以上５０ｍｍ以下にある、試料Ｎｏ．１～６５の鋼板を製造した。次いで、得られた鋼板について、下記の耐食性試験に供した。また、固溶Ｃｒ量を、前述した電解抽出法によって測定した結果についても表２に併記する。 Ｎｏ No. shown in Table 1 After slabs of steels Nos. 1 to 57 were melted into slabs, the sample thickness was 6 mm or more and 50 mm or less under the manufacturing conditions shown in Table 2. 1 to 65 steel plates were produced. Next, the obtained steel sheet was subjected to the following corrosion resistance test. Table 2 also shows the results of measuring the amount of solute Cr by the above-described electrolytic extraction method.
　耐食性試験を、ＮＡＣＥ Ｓｔａｎｄａｒｄ　ＴＭ０１１１－２０１１基準のＳｌｏｗ Ｓｔｒａｉｎ Ｒａｔｅ Ｔｅｓｔ Ｍｅｔｈｏｄ(以下、ＳＳＲＴ試験)に準拠して実施した。すなわち、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／ｓで等速引張試験を実施した。ここでは、破断応力が６００ＭＰa以上を耐応力腐食割れ性に優れるものとした。
　以上により得られた結果を、表２に示す。 The corrosion resistance test was performed in accordance with the Slow Strain Rate Test Method (hereinafter, SSRT test) based on NACE Standard TM0111-2011. That is, the test piece was a Type A round bar notched test piece, 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 / s. Was carried out. Here, a rupture stress of 600 MPa or more is considered to be excellent in stress corrosion cracking resistance.
Table 2 shows the results obtained as described above.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　本発明に従う鋼板（試料Ｎｏ．１から４２）は、耐食性がＳＳＲＴ試験の破断応力で６００ＭＰa以上を満足することを確認した。一方、本発明の範囲を外れる比較例(試料Ｎｏ．４３から６５)は、耐応力腐食割れ性が、上述の目標性能を満足できていない。
  It was confirmed that the steel sheet according to the present invention (sample Nos. 1 to 42) had a corrosion resistance satisfying 600 MPa or more as a breaking stress in the SSRT test. On the other hand, the comparative examples (sample Nos. 43 to 65) out of the range of the present invention do not satisfy the above-mentioned target performance in the stress corrosion cracking resistance.

Claims (5)

  1. 　質量％で、
    　Ｃ：０．２０％以上０．７０％以下、
    　Ｓｉ：０．０５％以上１．００％以下、
    　Ｍｎ：１５.０％以上３５.０％以下、
    　Ｐ：０．０３０％以下、
    　Ｓ：０．０２００％以下、
    　Ａｌ：０．０１０％以上０．１００％以下、
    　Ｃｒ：０．５％以上８．０％以下、
    　Ｎ：０．００１０％以上０．０３００％以下および
    　Ｂ：０.０００３％以上０．０１００％以下
    を含有し、残部Ｆｅおよび不可避的不純物の成分組成を有し、前記Ｃｒの６０％以上が固溶Ｃｒである鋼板。     In mass%,
    C: 0.20% or more and 0.70% or less,
    Si: 0.05% or more and 1.00% or less,
    Mn: 15.0% or more and 35.0% or less,
    P: 0.030% or less,
    S: 0.0200% or less,
    Al: 0.010% to 0.100%,
    Cr: 0.5% or more and 8.0% or less,
    N: 0.0010% or more and 0.0300% or less and B: 0.0003% or more and 0.0100% or less, and has a component composition of the balance of Fe and unavoidable impurities. Steel plate that is molten Cr.
  2. 　前記成分組成は、さらに、質量％で、
    　Ｎｂ：０．００３％以上０．０３０％以下、
    　Ｖ：０．０１％以上０．１０％以下および
    　Ｔｉ：０．００３％以上０．０４０％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 component composition further includes, in mass%,
    Nb: 0.003% or more and 0.030% or less,
    2. The steel sheet according to claim 1, comprising one or more selected from V: 0.01% to 0.10% and Ti: 0.003% to 0.040%. 3.
  3. 　前記成分組成は、さらに、質量％で、
    　Ｃｕ：０．０１％以上０．５０％以下、
    　Ｎｉ：０．０１％以上０．５０％以下、
    　Ｓｎ：０．０１％以上０．３０％以下、
    　Ｓｂ：０．０１％以上０．３０％以下、
    　Ｍｏ：０．０１％以上２．０％以下および
    　Ｗ：０．０１％以上２．０％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 component composition further includes, in mass%,
    Cu: 0.01% or more and 0.50% or less,
    Ni: 0.01% or more and 0.50% or less,
    Sn: 0.01% or more and 0.30% or less,
    Sb: 0.01% or more and 0.30% or less,
    The steel sheet according to claim 1, comprising one or more selected from Mo: 0.01% to 2.0% and W: 0.01% to 2.0%.
  4. 　前記成分組成は、さらに、質量％で、
    　Ｃａ：０．０００５％以上０．００５０％以下、
    　Ｍｇ：０．０００５％以上０．０１００％以下および
    　ＲＥＭ：０．００１０％以上０．０２００％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、２または３に記載の鋼板。     The component composition further includes, in mass%,
    Ca: 0.0005% or more and 0.0050% or less,
    4. The steel sheet according to claim 1, containing at least one selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%. 5.
  5. 　請求項１から４のいずれかに記載の成分組成を有する鋼素材を、１０００℃以上１３００℃以下に加熱後、熱間圧延を、仕上げ温度：７５０℃以上にて、被圧延材温度：９５０℃以下６００℃以上における滞在時間を３０分以下として施し、次いで７００℃以下６００℃以上の温度範囲における平均冷却速度：３℃／ｓ以上の冷却を行う鋼板の製造方法。
     
          After heating the steel material having the component composition according to any one of claims 1 to 4 to 1000 ° C or higher and 1300 ° C or lower, hot rolling is performed at a finishing temperature: 750 ° C or higher, and a material to be rolled: 950 ° C. A method for producing a steel sheet in which a staying time at 600 ° C. or higher is performed for 30 minutes or less, and then cooling is performed at an average cooling rate of 3 ° C./s or higher in a temperature range of 700 ° C. to 600 ° C.

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