Classifications

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  • 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
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  • 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
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  • 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 disclosure relates to a steel plate having excellent corrosion resistance particularly in saltwater corrosive environments and suitable for structural steel used in very-low-temperature environments such as liquefied gas storage tanks, and a method of producing the same.
• hot-rolled steel plates for structures such as liquefied gas storage tanks. Operating environments of such structures reach very low temperatures, and thus hot-rolled steel plates used for such structures are required to have excellent toughness at very low temperatures as well as high strength.
• a hot-rolled steel plate used for a liquefied natural gas storage needs to have excellent toughness at temperatures lower than or equal to -164 °C which is the boiling point of liquefied natural gas. If the low-temperature toughness of the steel material is insufficient, the safety of the very-low-temperature storage structure is likely to be undermined. There is thus strong need to improve the low-temperature toughness of the steel material used.
• austenitic stainless steel having austenite microstructure which is not embrittled at very low temperatures 9 % Ni steel, and 5000 series aluminum alloys have been conventionally used.
• high alloy costs or production costs of these metal materials there has been demand for a steel plate that is inexpensive and has excellent very-low-temperature toughness.
• studies have been conducted to use, as a new steel plate to replace conventional steels for very low temperature use, high-Mn steel containing a large amount of Mn which is a relatively inexpensive austenite-stabilizing element and having austenite microstructure, as a structural steel plate in very-low-temperature environments.
• Such hydrogen enters into the steel, causing hydrogen embrittlement.
• Conventionally studied high-Mn steel may be inferior in corrosion resistance to not only austenitic stainless steel but also 9 % Ni steel and typical low-alloy steel.
• it is important that the steel material used for the structure has excellent corrosion resistance in addition to high strength and very-low-temperature toughness.
• JP 2015-508452 A discloses a steel material that contains Mn: 15 % to 35 %, Cu: 5 % or less, and appropriate amounts of C and Cr to improve the machinability by cutting and the Charpy impact property of a heat-affected zone at -196 °C.
• JP 2016-84529 A discloses a high-Mn steel material that contains C: 0.25 % to 0.75 %, Si: 0.05 % to 1.0 %, Mn: more than 20 % and 35 % or less, Ni: 0.1 % or more and less than 7.0 %, and Cr: 0.1 % or more and less than 8.0 % to improve the low-temperature toughness.
• JP 2016-196703 A discloses a high-Mn steel material that contains C: 0.001 % to 0.80 %, Mn: 15 % to 35 %, and elements such as Cr, Ti, Si, Al, Mg, Ca, and REM to improve the very-low-temperature toughness of base metal and welds.
• “having excellent corrosion resistance” means that the fracture stress is 600 MPa or more in the case of immersing a sample in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 °C and conducting a constant-rate tensile test at a strain rate of 4 ⁇ 10 -7 inch/s in accordance with the NACE Standard TM0111-2011 Slow Strain Rate Test Method.
• a steel plate according to one of the disclosed embodiments will be described in detail below.
• the present disclosure is not limited to the embodiment described below.
• the chemical composition of the steel plate is defined as follows, in order to ensure excellent corrosion resistance.
• “%” used with regard to the chemical composition denotes “mass%” unless otherwise specified.
• the C content needs to be 0.20 % or more. If the C content is more than 0.70 %, excessive precipitation of Cr carbides and Nb-, V-, and Ti-based carbides is facilitated, and these precipitates form a corrosion initiation point. In addition, the low-temperature toughness decreases.
• the C content is therefore 0.20 % or more and 0.70 % or less.
• the C content is preferably 0.25 % or more and 0.60 % or less.
• Si 0.05 % or more and 1.00 % or less
• Si acts as a deoxidizer, and not only is necessary for steelmaking but also has an effect of strengthening the steel plate through solid solution strengthening by dissolving in the steel.
• the Si content needs to be 0.05 % or more. If the Si content is more than 1.00 %, the weldability and the surface characteristics degrade and the stress corrosion cracking resistance decreases in some cases. The Si content is therefore 0.05 % or more and 1.00 % or less. The Si content is preferably 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.
• Mn is an important element for achieving both the strength and the very-low-temperature toughness.
• the Mn content needs to be 15.0 % or more. If the Mn content is more than 35.0 %, the effect of improving the very-low-temperature toughness is saturated, and the alloy costs increase. Moreover, the weldability and the cuttability degrade. Furthermore, segregation of Mn is caused, and stress corrosion cracking is promoted.
• the Mn content is therefore 15.0 % or more and 35.0 % or less.
• the Mn content is preferably 18.0 % or more and 28.0 % or less.
• the P content is more than 0.030 %, P segregates to grain boundaries and decreases the grain boundary strength, and forms a stress corrosion cracking initiation point. It is therefore desirable to reduce the P content as much as possible, with its upper limit being set to 0.030 %. Since lower P content contributes to improved properties, the P content is preferably 0.024 % or less, and more preferably 0.020 % or less. Reducing the P content to less than 0.001 % requires considerable steelmaking costs and impairs the economic efficiency. Hence, P content of 0.001 % or more is allowable from the viewpoint of economic efficiency.
• S decreases the low-temperature toughness and the ductility of the base metal. It is therefore desirable to reduce the S content as much as possible, with its upper limit being set to 0.0200 %.
• the S content is therefore 0.0200 % or less, and preferably 0.0180 % or less. Reducing the S content to less than 0.0001 % requires considerable steelmaking costs and impairs the economic efficiency. Hence, S content of 0.0001 % or more is allowable from the viewpoint of economic efficiency.
• A1 0.010 % or more and 0.100 % or less
• A1 acts as a deoxidizer, and is most generally used in the molten steel deoxidation process. A1 also has an effect of suppressing coarsening of crystal grains by fixing solute N in the steel and forming A1N. A1 further has an effect of suppressing a decrease in toughness due to reduction of solute N. To achieve the effects, the A1 content needs to be 0.01 % or more. If the Al content is more than 0.100 %, coarse nitrides form and become a corrosion initiation point or a fracture origin to thus cause a decrease in stress corrosion cracking resistance in some cases. Moreover, A1 diffuses into a weld metal portion during welding and decreases the toughness of the weld metal. The A1 content is therefore 0.100 % or less. The A1 content is preferably 0.020 % or more and 0.070 % or less.
• Cr has an effect of delaying initial corrosion reaction on the steel plate surface in a saltwater corrosive environment when added in an appropriate amount. Cr is an important element that, by this effect, decreases the amount of hydrogen that enters into the steel plate and improves the stress corrosion cracking resistance. To achieve the effects, the Cr content needs to be 0.5 % or more. If the Cr content is more than 8.0 %, the effects are saturated, and the economic efficiency is impaired. The Cr content is therefore 0.5 % or more and 8.0 % or less. The Cr content is preferably 1.0 % or more.
• solute content in the added Cr contributes to improved stress corrosion cracking resistance, but the precipitate content in the added Cr has a possibility of hindering improvement in stress corrosion cracking resistance. Accordingly, it is important that at least 60 % of Cr is solute Cr. In other words, if solute Cr is 60 % or more of the Cr content, the foregoing effects can be achieved, and improvement in stress corrosion cracking resistance by addition of Cr can be achieved. Solute Cr is preferably 70 % or more of the Cr content, and more preferably 100 % of the Cr content.
• solute Cr denotes a state in which solute atoms exist in an atom state without forming a precipitate or the like.
• the amount of solute Cr can be calculated as follows: A test piece for electrolytic extraction is collected from the steel plate, and a precipitate is extracted by electrolytic extraction using a 10 % AA (10 % acetylacetone-1 % tetramethylammonium chloride-methanol) solution. The Cr content in the precipitate is measured by ICP optical emission spectrometry, and the measured Cr content is subtracted from all Cr in the test piece to yield the amount of solute Cr.
• N 0.0010 % or more and 0.0300 % or less
• N is an austenite-stabilizing element, and is effective in improving the very-low-temperature toughness. N also has an effect of combining with Nb, V, and Ti to form nitrides or carbonitrides which finely precipitate and suppress stress corrosion cracking as a diffusible hydrogen trapping site. To achieve the effects, the N content needs to be 0.0010 % or more. If the N content is more than 0.0300 %, excessive formation of nitrides or carbonitrides is facilitated, as a result of which not only the amount of solute element decreases and the corrosion resistance decreases but also the toughness decreases. The N content is therefore 0.0010 % or more and 0.0300 % or less. The N content is preferably 0.0020 % or more and 0.0150 % or less.
• the B is an element that strengthens the austenite grain boundaries, and is effective in improving the stress corrosion cracking resistance by suppressing cracking in the grain boundaries.
• the B content needs to be 0.0003 % or more.
• the B content is preferably 0.0005 % or more, further preferably more than 0.0007 %, and particularly preferably more than 0.0010 %. If the B content is more than 0.0100 %, the effects are saturated. The B content is therefore 0.0100 % or less.
• the B content is preferably 0.0070 % or less.
• the chemical composition of the steel plate according to one of the disclosed embodiments may optionally contain, in addition to the above-described essential elements, Nb: 0.003 % or more and 0.030 % or less, V: 0.01 % or more and 0.10 % or less, and Ti: 0.003 % or more and 0.040 % or less, for the purpose of further improving the corrosion resistance.
• Nb 0.003 % or more and 0.030 % or less
• Nb is an element that has an effect of suppressing stress corrosion cracking by forming carbonitrides that precipitate and function as a diffusible hydrogen trapping site.
• the Nb content is preferably 0.003 % or more. If the Nb content is more than 0.030 %, coarse carbonitrides may precipitate and form a fracture origin. In addition, precipitates may coarsen and cause a decrease in base metal toughness. Accordingly, in the case of containing Nb, the Nb content is preferably 0.003 % or more and 0.030 % or less.
• the Nb content is more preferably 0.005 % or more and 0.025 % or less, and further preferably 0.007 % or more and 0.022 % or less.
• V 0.01 % or more and 0.10 % or less
• V is an element that has an effect of suppressing stress corrosion cracking by forming carbonitrides that precipitate and function as a diffusible hydrogen trapping site.
• the V content is preferably 0.01 % or more. If the V content is more than 0.10 %, coarse carbonitrides may precipitate and form a fracture origin. In addition, precipitates may coarsen and cause a decrease in base metal toughness. Accordingly, in the case of containing V, the V content is preferably 0.01 % or more and 0.10 % or less. The V content is more preferably 0.02 % or more and 0.09 % or less, and further preferably 0.03 % or more and 0.08 % or less.
• Ti is an element that has an effect of suppressing stress corrosion cracking by forming nitrides or carbonitrides that precipitate and function as a diffusible hydrogen trapping site.
• the Ti content is preferably 0.003 % or more. If the Ti content is more than 0.040 %, precipitates may coarsen and cause a decrease in base metal toughness. In addition, coarse carbonitrides may precipitate and form a fracture origin. Accordingly, in the case of containing Ti, the Ti content is preferably 0.003 % or more and 0.040 % or less. The Ti content is more preferably 0.005 % or more and 0.035 % or less, and further preferably 0.007 % or more and 0.032 % or less.
• the chemical composition of the steel plate according to one of the disclosed embodiments may optionally further contain one or more selected from 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, Mo: 0.01 % or more and 2.0 % or less, and W: 0.01 % or more and 2.0 % or less, for the purpose of further improving the corrosion resistance.
• Cu, Ni, Sn, Sb, Mo, and W are each an element that, when added in combination with Cr, improves the corrosion resistance of the high-Mn steel in saltwater corrosive environments.
• Cu, Sn, and Sb each have an effect of suppressing hydrogen evolution reaction which is cathode reaction by increasing the hydrogen overvoltage of the steel material.
• Ni forms a precipitation coating on the steel material surface, and physically suppresses the permeation of corrosive anions such as Cl - into the steel substrate.
• Cu, Ni, Sn, Sb, Mo, and W each separate from the steel material surface as metal ions upon corrosion, and refine the corrosion product to thus suppress the permeation of corrosive anions through the steel interface (the interface between the rust layer and the steel substrate).
• Mo and W separate respectively as Mo 4 2- and WO 4 2- and are adsorbed in the corrosion product or on the steel plate surface, thereby imparting cation selective permeability and electrically suppressing the permeation of corrosive anions into
• the Cu content is 0.01 % or more and 0.50 % or less
• the Ni content is 0.01 % or more and 0.50 % or less
• the Sn content is 0.01 % or more and 0.30 % or less
• the Sb content is 0.01 % or more and 0.30 % or less
• the Mo content is 0.01 % or more and 2.0 % or less
• the W content is 0.01 % or more and 2.0 % or less.
• the Cu content is 0.02 % or more and 0.40 % or less
• the Ni content is 0.02 % or more and 0.40 % or less
• the Sn content is 0.02 % or more and 0.25 % or less
• the Sb content is 0.02 % or more and 0.25 % or less
• the Mo content is 0.02 % or more and 0.40 % or less
• the W content is 0.02 % or more and 0.40 % or less.
• the chemical composition of the steel plate according to one of the disclosed embodiments may optionally further contain one or more selected from 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, for the purpose of further improving the corrosion resistance.
• Ca, Mg, and REM are each an element useful for morphological control of inclusions, and may be optionally contained.
• Morphological control of inclusions means turning elongated sulfide-based inclusions into granular inclusions. Through such morphological control of inclusions, the ductility, the toughness, and the sulfide stress corrosion cracking resistance can be improved.
• the Ca content and the Mg content are each preferably 0.0005 % or more, and the REM content is preferably 0.0010 % or more. If the Ca content, the Mg content, and the REM content are each high, the amount of nonmetallic inclusions increases, which may decrease the ductility, the toughness, and the sulfide stress corrosion cracking resistance. Moreover, high contents of these elements are likely to be economically disadvantageous.
• the Ca content is preferably 0.0005 % or more and 0.0050 % or less.
• the Mg content is preferably 0.0005 % or more and 0.0100 % or less.
• the REM content is preferably 0.0010 % or more and 0.0200 % or less. More preferably, the Ca content is 0.0010 % or more and 0.0040 % or less, the Mg content is 0.0010 % or more and 0.0040 % or less, and the REM content is 0.0020 % or more and 0.0150 % or less.
• the temperature of the material to be rolled in hot rolling and the cooling rate in subsequent cooling are the temperature and the cooling rate measured on the surface of the material to be rolled.
• the steel plate is produced by: heating a steel raw material having the foregoing chemical composition to 1000 °C or more and 1300 °C or less; thereafter subjecting the steel raw material to hot rolling at a rolling reduction ratio of 3 or more and 30 or less and a rolling finish temperature of 750 °C or more, with the residence time during which the material to be rolled is in a temperature range of 950 °C or less and 600 °C or more being 30 min or less; and thereafter cooling the material at an average cooling rate of 3 °C/s or more in a temperature range of 700 °C or less and 600 °C or more.
• Heating temperature of steel raw material 1000 °C or more and 1300 °C or less
• the steel raw material is heated to 1000 °C or more in order to dissolve carbonitrides in the microstructure and homogenize the crystal grain size and the like. If the heating temperature is less than 1000 °C, carbonitrides do not dissolve sufficiently, making it impossible to obtain desired properties. If the heating temperature is more than 1300 °C, the material properties degrade due to coarsening of crystal grains. Moreover, excessive energy is required, and the productivity decreases. The upper limit of the heating temperature is therefore 1300 °C.
• the heating temperature is preferably 1050 °C or more and 1250 °C or less, and more preferably 1070 °C or more and 1250 °C or less.
• the steel raw material is preferably a continuously-cast slab, or a slab, a billet, or the like produced by a well-known method such as ingot casting.
• the molten steel may be additionally subjected to treatments such as ladle refining and vacuum degassing.
• the rolling finish temperature of the hot rolling is less than 750 °C, the amount of carbides precipitated during the rolling increases significantly, and the amount of solute Cr may be unable to be ensured even in the case where the residence time in a temperature range of 600 °C or more and 900 °C or less is less than 30 min as described later. This causes a decrease in corrosion resistance. If the rolling finish temperature of the hot rolling is less than 750 °C, the deformation resistance increases, and an excessive load is put on the production line. The rolling finish temperature is therefore 750 °C or more.
• the average cooling rate in a temperature range of 700 °C or less and 600 °C or more is less than 3 °C/s, precipitates such as Cr carbides form in large amounts.
• the average cooling rate is therefore limited to 3 °C/s or more.
• the average cooling rate is preferably 10 °C/s or more and 150 °C/s or less.
• the residence time during which the raw material to be rolled is in a temperature range of 950 °C or less and 600 °C or more is more than 30 min, carbonitrides and carbides precipitate in large amounts during the rolling, and the amount of solute Cr decreases to less than the necessary amount, which causes a decrease in corrosion resistance and a decrease in very-low-temperature toughness.
• the residence time in a temperature range of 950 °C or less and 600 °C or more is limited to 30 min or less.
• the residence time is preferably 5 min or more and 25 min or less.
• the rolling time can be shortened, and consequently the residence time in a range of 950 °C or less and 600 °C or more can be limited to 30 min or less.
• the rolling reduction ratio in the hot rolling is preferably 30 or less. If the rolling reduction ratio in the hot rolling is less than 3, there is a possibility that, as a result of lessening of the effect of facilitating recrystallization to achieve homogenization, coarse austenite grains remain and this part oxidizes preferentially, and consequently the corrosion resistance decreases. Accordingly, the rolling reduction ratio in the hot rolling is preferably 3 or more.
• the rolling reduction ratio is defined as "(the thickness of the raw material to be rolled in hot rolling)/(the thickness of the steel plate after the hot rolling)".
• the corrosion resistance test was conducted in accordance with the NACE Standard TM0111-2011 Slow Strain Rate Test Method (hereafter "SSRT test”).
• SSRT test Slow Strain Rate Test Method
• the test piece was immersed in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 °C, and a constant-rate tensile test was conducted at a strain rate of 4 ⁇ 10 -7 inch/s.
• the fracture stress was 600 MPa or more, the stress corrosion cracking resistance was evaluated as excellent.
• Each steel plate (samples No. 1 to No. 42) according to the present disclosure had corrosion resistance satisfying a fracture stress of 600 MPa or more in the SSRT test.
• Each Comparative Example (samples No. 43 to No. 65) outside the range according to the present disclosure failed to satisfy the foregoing target performance in stress corrosion cracking resistance.

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