Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a high-Mn steel plate that is suitable for structural steel used in a cryogenic environment, such as a liquefied gas storage tank, in particular, to a high-Mn steel plate having excellent resistance to stress corrosion cracking in a salt water corrosive environment and to a manufacturing method therefor.
• cryogenic temperatures are encountered in the usage environment.
• the steel plate is required to have not only strength, but also toughness.
• excellent toughness at the boiling point of liquefied natural gas of -164°C or lower is required.
• low-temperature toughness of a steel material is poor, there is a risk of failure in maintaining the safety of a cryogenic storage structure. Accordingly, there is a high demand for enhanced low-temperature toughness of a steel material to be employed.
• oxides are formed by the anodic reaction of iron while hydrogen is generated by the cathodic reaction of moisture, thereby promoting stress corrosion cracking due to hydrogen embrittlement.
• Consequent stress corrosion cracking could result in breakage of a structure in the presence of stress applied in the usage environment or in the presence of residual stress through bending or welding during manufacture. Accordingly, in view of safety, it is important to have excellent resistance to stress corrosion cracking, not to mention strength and cryogenic toughness of a steel material to be used.
• Patent Literature 1 discloses a steel material having improved machinability and Charpy impact characteristics at -196°C in weld heat affected zones through addition of 15 to 35% of Mn, 5% or less of Cu, and appropriate amounts of C and Cr.
• Patent Literature 2 discloses a high-Mn steel material having improved low-temperature toughness through addition of 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%.
• Patent Literature 1 and 2 are intended to have strength and low-temperature toughness.
• the Charpy impact characteristics at -196°C in weld heat affected zones are 60 to 135 J (disclosed only in Patent Literature 1).
• the cryogenic toughness of the base metals is still unsatisfactory without achieving both cryogenic toughness and resistance to stress corrosion cracking.
• an object of the present invention is to provide a high-Mn steel plate having excellent resistance to stress corrosion cracking and cryogenic toughness and to provide a manufacturing method therefor.
• the present inventors intensively studied, for high-Mn steel plates, various factors that determine the microstructure, a manufacturing method, and a component composition of a steel plate for ensuring excellent resistance to stress corrosion cracking, and found the following.
• high strength means the strength of 400 MPa or higher in yield stress.
• cryogenic toughness means low-temperature toughness, in other words, an absorbed energy vE -196 in a Charpy impact test at -196°C of 50 J or higher.
• excellent resistance to stress corrosion cracking means a fracture stress of 500 MPa or higher when a test in accordance with a slow strain rate test method based on NACE Standard TM0111-2011 is performed by immersing in artificial seawater (chloride ion concentration of 18,000 ppm) at 23°C and performing a constant-rate tensile test at a strain rate of 4 ⁇ 10 -7 inch/sec.
• a high-Mn steel plate having excellent resistance to stress corrosion cracking and cryogenic toughness is obtained.
• a high-Mn steel plate of the present invention contributes greatly to enhanced safety and lifetime of steel structures used in a cryogenic environment, such as liquefied gas storage tanks, and exerts industrially remarkable effects.
• the high-Mn steel plate has excellent economic efficiency without causing lowered productivity or increased manufacturing costs.
• the component composition of a steel plate of the present invention and reasons for limiting the component composition will be described.
• the component composition of a steel plate is specified as follows.
• the symbol % that represents the component composition means mass% unless otherwise indicated.
• C is an inexpensive austenite stabilizing element and an important element for obtaining austenite. To achieve such an effect, C content of 0.20% or more is required. Meanwhile, when the content exceeds 0.70%, Cr carbide and a Nb-, V-, and/or Ti-based carbide are formed excessively, thereby impairing low-temperature toughness and resistance to stress corrosion cracking. Accordingly, C is set to 0.20 to 0.70%, preferably 0.25% or more and 0.60% or less, and more preferably 0.30% or more and 0.55% or less.
• Si is essential in steel making due to its action as a deoxidizing agent and further has an effect of increasing strength of a steel plate by solid solution strengthening through dissolution in steel. To obtain such an effect, Si content of 0.05% or more is required. Meanwhile, when the content exceeds 1.0%, weldability deteriorates and SCC resistance is also affected. Accordingly, Si is set to 0.05 to 1.0%, preferably 0.07% or more and 0.50% or less, and more preferably 0.15% or more and 0.45% or less.
• Mn is a relatively inexpensive austenite stabilizing element.
• Mn is an important element for achieving both strength and cryogenic toughness.
• Mn content of 15% or more is required.
• Mn content exceeds 30%, an effect of improving cryogenic toughness levels off and increased alloying costs result.
• weldability and cutting properties deteriorate. Further, segregation is promoted, and the occurrence of stress corrosion cracking is thus promoted.
• Mn is set to 15 to 30%, preferably 18% or more and 28% or less, and more preferably 20% or more and 27% or less.
• P content exceeds 0.028%, P is segregated at grain boundaries and becomes initiation sites of stress corrosion cracking. Accordingly, the upper limit is set to 0.028% and P is desirably decreased as much as possible. P is thus set to 0.028% or less. Meanwhile, an excessive decrease in P results in soaring refining costs and economic disadvantages. Accordingly, P is desirably set to 0.002% or more. Preferably, P is set to 0.005% or more and 0.024% or less.
• S impairs low-temperature toughness and ductility of a base metal
• the upper limit is set to 0.02% and S is desirably decreased as much as possible. Accordingly, S is set to 0.02% or less. Meanwhile, an excessive decrease in S results in soaring refining costs and economic disadvantages. Accordingly, S is set to desirably 0.001% or more and preferably 0.002% or more. S is set to preferably 0.018% or less and more preferably 0.010% or less.
• Al acts as a deoxidizing agent and is most widely used in a deoxidation process of molten steel for steel plates.
• Al has an effect of suppressing coarsening of crystal grains by fixing N dissolved in steel to form AlN. Together with such an effect, Al also has an effect of suppressing deterioration in toughness due to a decrease in dissolved N.
• Al content of 0.01% or more is required. Meanwhile, when Al content exceeds 0.1%, Al is mixed into a weld metal portion during welding, thereby impairing toughness of the weld metal. Al is thus set to 0.1% or less. Accordingly, Al is set to 0.01 to 0.1% and preferably 0.02% or more and 0.07% or less.
• Cr is an element effective for stabilizing austenite through its addition in an appropriate amount and for enhancing cryogenic toughness and base metal strength. Moreover, in the present invention, Cr is an important element that enhances resistance to stress corrosion cracking through a decreased amount of hydrogen that penetrates a steel plate through its effect of closely forming rust on a base metal surface in a salt water environment. To obtain such effects, Cr content of 0.5% or more is required. Meanwhile, when the content exceeds 7.0%, low-temperature toughness and resistance to stress corrosion cracking deteriorate due to formation of Cr carbide. Cr is thus set to 0.5 to 7.0%. Cr is set to preferably 1.0% or more, more preferably 1.2% or more, and further preferably 2.5% or more. Meanwhile, Cr is set to preferably 6.0% or less, more preferably 5.7% or less, and further preferably 5.5% or less.
• Ni is a representative austenite stabilizing element and is an element effective for enhancing cryogenic toughness and base metal strength. Moreover, in the present invention, Ni is an important element that enhances resistance to stress corrosion cracking through a decreased amount of hydrogen that penetrates a steel plate through its effect of closely forming rust on a base metal surface in a salt water environment. To obtain such effects, Ni content of 0.03% or more is required. Meanwhile, when the content exceeds 0.30%, the alloying costs increase, and further, an effect of enhancing resistance to stress corrosion cracking levels off. Ni is thus set to 0.03 to 0.30%. Preferably, Ni is set to 0.25% or less and 0.04% or more. More preferably, Ni is set to 0.23% or less and 0.05% or more. Further preferably, Ni is set to 0.21% or less.
• N is an austenite stabilizing element and is an element effective for enhancing cryogenic toughness. Moreover, N has an effect of suppressing stress corrosion cracking as trapping sites of diffusible hydrogen through bonding with Nb, V, and/or Ti to precipitate as a nitride or a carbonitride. To obtain such effects, N content of 0.0010% or more is required. Meanwhile, when the content exceeds 0.0200%, such a nitride or a carbonitride coarsens, thereby impairing toughness. Accordingly, N is set to 0.0010 to 0.0200%. Preferably, N is set to 0.0020% or more and 0.0150% or less. More preferably, N is set to 0.0030% or more and 0.0170% or less.
• Nb 0.003 to 0.030%
• V 0.03 to 0.10%
• Ti 0.003 to 0.040%
• Nb is an element that has an effect of suppressing stress corrosion cracking through precipitation as a carbonitride (including a carbide), which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, Nb content of 0.003% or more is required. Meanwhile, when Nb content exceeds 0.030%, a coarse carbonitride is precipitated and becomes the origin of breakage in some cases. Moreover, coarsened precipitates impair base metal toughness in some cases. Accordingly, if contained, Nb is set to 0.003 to 0.030%. Nb is set to preferably 0.005% or more and more preferably 0.007% or more. Meanwhile, Nb is set to preferably 0.025% or less and more preferably 0.022% or less.
• V is an element that has an effect of suppressing stress corrosion cracking through precipitation as a carbonitride, which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, V content of 0.03% or more is required. Meanwhile, when V content exceeds 0.10%, a coarse carbonitride is precipitated and becomes the origin of breakage in some cases. Moreover, coarsened precipitates impair base metal toughness in some cases. Accordingly, if contained, V is set to 0.03 to 0.10%. V is set to preferably 0.04% or more and more preferably 0.05% or more. Meanwhile, V is set to preferably 0.09% or less, more preferably 0.08% or less, and further preferably 0.07% or less.
• Ti is an element that has an effect of suppressing stress corrosion cracking through precipitation as a nitride or a carbonitride, which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, Ti content of 0.003% or more is required. Meanwhile, when Ti content exceeds 0.040%, a precipitate coarsens, thereby impairing base metal toughness in some cases. In addition, a coarse carbonitride is precipitated and becomes the origin of breakage in some cases. Accordingly, if contained, Ti is set to 0.003 to 0.040%. Ti is set to preferably 0.005% or more and more preferably 0.007% or more. Meanwhile, Ti is set to preferably 0.035% or less and more preferably 0.032% or less.
• the balance is iron and incidental impurities.
• incidental impurities include O and H, and the total of 0.01% or less is tolerable.
• O and S are preferably specified as follows.
• O content exceeds 0.0070%, coarse inclusions are formed with Al, thereby impairing low-temperature toughness. Accordingly, the upper limit is set to 0.0070% and O is desirably decreased as much as possible. Preferably, O is set to 0.0060% or less. Meanwhile, an excessive decrease in O results in soaring refining costs and economic disadvantages. Accordingly, O is set to 0.0005% or more and preferably 0.0008% or more.
• O/S is set to less than 1.
• O/S is set to less than 1 to ensure low-temperature toughness.
• the characteristics intended to achieve by the present invention can be obtained from the above-described essential elements.
• the following elements may be contained as necessary, in addition to the above-described essential elements.
• Mo is a useful element for increasing strength of a base metal and may be contained as necessary. To obtain such an effect, Mo is preferably contained at 0.05% or more. Meanwhile, the content exceeding 2.0% adversely affects toughness and resistance to weld cracking in some cases. Mo is thus preferably set to 2.0% or less. Accordingly, if contained, Mo is set to 0.05 to 2.0%. More preferably, Mo is set to 0.07% or more and 1.7% or less.
• W is a useful element for increasing strength of a base metal and may be contained as necessary. To obtain such an effect, W is preferably contained at 0.05% or more. Meanwhile, the content exceeding 2.0% adversely affects toughness and resistance to weld cracking in some cases. W is thus preferably set to 2.0% or less. Accordingly, if contained, W is set to 0.05 to 2.0%. More preferably, W is set to 0.07% or more and 1.5% or less.
• Ca is a useful element for morphology control of an inclusion and may be contained as necessary.
• Morphology control of an inclusion herein means making an elongated sulfide inclusion into a granular inclusion. Through such morphology control of an inclusion, ductility, toughness, and resistance to sulfide stress corrosion cracking are enhanced.
• Ca is preferably contained at 0.0005% or more. Meanwhile, when the content exceeds 0.0050%, the amount of nonmetal inclusions increases. Consequently, ductility, toughness, and resistance to sulfide stress corrosion cracking rather deteriorate in some cases. In addition, economic disadvantages result in some cases. Accordingly, if contained, Ca is set to 0.0005 to 0.0050%. More preferably, Ca is set to 0.0010% or more and 0.0040% or less.
• Mg is useful as an element that contributes to improved resistance to sulfide stress corrosion cracking and may be contained as necessary. To obtain such an effect, Mg is preferably contained at 0.0005% or more. Meanwhile, when the content exceeds 0.0050%, the above-mentioned effect levels off and the effect commensurate with the content cannot be expected in some cases. In addition, economic disadvantages result in some cases. Accordingly, if contained, Mg is set to 0.0005 to 0.0050%. More preferably, Mg is set to 0.0010% or more and 0.0040% or less.
• REM is useful as an element that contributes to improved resistance to sulfide stress corrosion cracking and may be contained as necessary. To obtain such an effect, REM is preferably contained at 0.0010% or more. Meanwhile, when the content exceeds 0.0200%, the above-mentioned effect levels off and the effect commensurate with the content cannot be expected in some cases. Accordingly, if contained, REM is set to 0.0010 to 0.0200%. More preferably, REM is set to 0.0020% or more and 0.0150% or less.
• the base phase of the microstructure 0.5 mm under the steel plate surface is austenite.
• the austenite 25% or more, in area ratio, is austenite having an equivalent circle diameter of 10 ⁇ m or more and an aspect ratio of a major axis to a minor axis of 3 or more. Because of this, deformation bands inside crystal grains, in addition to grain boundaries near a steel plate surface layer, effectively act as trapping sites of diffusible hydrogen, thereby effectively acting against stress corrosion cracking. Consequently, suppression of stress corrosion cracking can be enhanced remarkably. Moreover, yield stress is also enhanced.
• the area ratio is set to 30% or more.
• the area ratio is set to preferably 95% or less, more preferably 94% or less, further preferably 90% or less, and still further preferably 85% or less.
• an equivalent circle diameter is less than 10 ⁇ m or an aspect ratio of a major axis to a minor axis is less than 3, a desirable yield stress cannot be achieved.
• deformation bands inside crystal grains that effectively act as trapping sites of diffusible hydrogen cannot be obtained, and resistance to stress corrosion cracking deteriorates. Consequently, the above-described effects cannot be obtained.
• the above-mentioned equivalent circle diameter, area ratio, and aspect ratio of austenite can be measured by the methods in the Examples section described hereinafter.
• 0.5 mm under a steel plate surface means cross-sections parallel to the rolling direction at positions 0.5 mm from the front and rear surfaces of a steel plate in the thickness direction. Moreover, in the present invention, even when the above-described microstructure exists in a cross-section parallel to the rolling direction within a ⁇ 5% range of a position 0.5 mm under a steel plate surface, the above-described effects can similarly be obtained. Accordingly, in the present invention, 0.5 mm under a steel plate surface means that the above-described microstructure exists in a cross-section parallel to the rolling direction anywhere within a ⁇ 5% range of positions 0.5 mm from the front and rear surfaces of the steel plate in the thickness direction.
• the front and rear surfaces refer to not only intact surfaces of a finished product, but also steel plate surfaces that have been treated such that a cumulative degree of a crystal can be measured. For example, when the outermost surfaces of a steel plate are covered with scale, surfaces after scale have been removed are meant.
• Nb-, V-, and/or Ti-based precipitates a carbide, a nitride, and a carbonitride (hereinafter, referred to as Nb-, V-, and/or Ti-based precipitates) containing one or two or more of Nb, V, and Ti in the microstructure 0.5 mm under a steel plate surface of the present invention
• a carbide, a nitride, and a carbonitride containing one or two or more of Nb, V, and Ti refer to: a carbide containing one or two or more of Nb, V, and Ti; a nitride containing one or two or more of Nb, V, and Ti; and a carbonitride containing one or two or more of Nb, V, and Ti.
• the particle size of the Nb-, V-, and/or Ti-based precipitates is set to 0.01 to 0.5 ⁇ m in equivalent circle diameter.
• the particle size is set to 0.01 to 0.5 ⁇ m in equivalent circle diameter.
• the particle size is set to 0.03 ⁇ m or more and 0.4 ⁇ m or less.
• the total number of Nb-, V-, and/or Ti-based precipitates having the above-described particle size is less than 2 ⁇ 10 2 /mm 2 in the microstructure 0.5 mm under a steel plate surface, precipitates that act as trapping sites of diffusible hydrogen are insufficient. Consequently, an effect of suppressing hydrogen embrittlement cracking as trapping sites of diffusible hydrogen cannot be obtained. Accordingly, the total number is set to 2 ⁇ 10 2 /mm 2 or more and preferably 5 ⁇ 10 2 /mm 2 or more.
• the above-mentioned number density and equivalent circle diameter of the Nb-, V-, and/or Ti-based precipitates can be measured by the methods in the Examples section described hereinafter.
• austenite is set to 90% or more.
• the area ratio of martensite and other microstructures is preferably small.
• the above-mentioned martensite and other microstructures herein refer to martensite, bainite, ferrite, and pearlite.
• the total area ratio of each microstructure is desirably set to 10% or less based on the entire steel plate.
• a steel plate according to the present invention is suitable for a high-Mn steel plate having a thickness of 4 mm or more.
• °C a temperature on a steel plate surface or a steel surface.
• molten steel having the above-described component composition can be refined by a publicly known refining method, such as by using a converter or an electric furnace. Moreover, secondary refining may be performed in a vacuum degasser. Subsequently, steel, such as a slab of a predetermined size, is preferably formed by a continuous casting method or a publicly known casting method, such as an ingot casting/slabbing method.
• T Nb ° C 7500 / 3.0 ⁇ log 10 % Nb ⁇ % C ⁇ 273
• V ° C 10800 / 7.2 ⁇ log 10 % V ⁇ % C ⁇ 273
• Ti ° C 7000 / 2.8 ⁇ log 10 % Ti ⁇ % C ⁇ 273
• [%Nb], [%V], [%Ti], and [%C] represent contents (mass%) of Nb, V, Ti, and C, respectively, in steel; and when an element is not contained, calculation is performed
• the heating temperature of steel is set to (Tx - 50)°C or higher and (Tx + 200)°C or lower.
• the heating temperature is set to (Tx - 30)°C or higher and (Tx + 180)°C or lower.
• hot rolling is started while steel is at (Tx - 50)°C or higher and (Tx + 200)°C or lower.
• Hot Rolling Steel Plate Having Desirable Thickness is Obtained by Setting Finishing Temperature to 750°C or Higher and 1,000°C or Lower in Finish Rolling After Roughening
• a finishing temperature in hot rolling exceeds 1,000°C, recrystallization of austenite near a steel plate surface readily progresses and the desirable microstructure cannot be obtained. Consequently, resistance to stress corrosion cracking deteriorates. Meanwhile, when a finishing temperature is set to lower than 750°C, hot deformation resistance increases excessively, thereby increasing a load on a rolling mill. In addition, low rolling efficiency and increased manufacturing costs result. Accordingly, a finishing temperature in hot rolling is set to 750°C or higher and 1,000°C or lower, preferably 800°C or higher and 950°C or lower, and more preferably 940°C or lower.
• the cumulative reduction is a total reduction obtained by adding up a reduction in each rolling pass in the temperature range of 850°C or higher and (Tx - 50)°C or lower in finish rolling.
• the cumulative reduction is a total reduction obtained by adding up a reduction in each rolling pass in the non-recrystallization region in finish rolling.
• the average cooling rate is set to preferably 1.0°C/s or more and more preferably 2.0°C/s or more.
• the average cooling rate is set to preferably 150.0°C/s or less, more preferably 120.0°C/s or less, and further preferably 100.0°C/s or less.
• the average cooling rate is an average cooling rate to 650° from a lower temperature of either (finishing temperature - 50°C) or a cooling start temperature after the end of finish rolling.
• controlling an average cooling rate in cooling is effective for suppressing precipitation of Cr carbide during cooling and thereby enhancing resistance to stress corrosion cracking.
• an average cooling rate in the temperature range from a finishing temperature to (finishing temperature - 50°C) is not particularly specified, but is preferably 1.0°C/s or less since formation of Nb-, V-, and/or Ti-based precipitates can be promoted.
• an average cooling rate at lower than 650°C is not particularly specified, but is set to preferably less than 100.0°C/s from a viewpoint of preventing strain of a steel plate and more preferably 80.0°C/s or less.
• Steel slabs (slab thickness: 250 to 300 mm) were prepared to have various component compositions shown in Table 1-1 and Table 1-2 by a converter/ladle refining/continuous casting method.
• (Tx - 50)°C and (Tx + 200)°C for Nb, V, or V are each shown in Table 1-1 and Table 1-2.
• the obtained 12 mm to 80 mm-thick hot-rolled steel plates underwent microstructure examination, a base metal tensile test, a base metal toughness test, and a stress corrosion cracking test in the following manner.
• microstructure examination a specimen for microstructure observation was taken from each of the obtained steel plates on a cross-section parallel to the rolling direction at a position 0.5 mm under the surface in the thickness direction, etched with an aqueous solution of sodium pyrosulfite (10 g Na 2 S 2 O 5 + 95 mL water solution), and imaged for the optical microscopic structure in five fields of view at a magnification of 500 ⁇ . Subsequently, an area ratio of austenite, an equivalent circle diameter, and an aspect ratio were obtained from each of the obtained microstructure images by using an image analyzer.
• the area ratio of austenite was obtained as a ratio of the area of austenite of 10 ⁇ m or more to the total area of austenite by performing austenite etching, imaging the microstructure at a magnification of 500 ⁇ , tracing austenite grain boundaries, and performing image analysis.
• the equivalent circle diameter of austenite As for the grain size of austenite, in other words, the equivalent circle diameter of austenite, the individual areas of austenite were first determined through image analysis of the above-mentioned microstructure images. The equivalent circle diameter was then calculated from individual areas.
• the aspect ratio of austenite grains was calculated as a ratio of the longest diameter (major axis) to the largest width orthogonal to the major axis (minor axis) for each austenite grain through observation under an optical microscope of the microstructure in which austenite grain boundaries were exposed by the above-mentioned etching.
• Nb-, V-, and/or Ti-based precipitates In examination of the number density of Nb-, V-, and/or Ti-based precipitates, ten fields of view were imaged under a transmission electron microscope at a magnification of 50,000 ⁇ on the cross-section parallel to the rolling direction at a position 0.5 mm under the surface of each steel plate in the thickness direction, the number of Nb-, V-, and/or Ti-based precipitates having an equivalent circle diameter of 0.01 to 0.5 ⁇ m was counted per 1 mm 2 , and a total number density of Nb-, V-, and/or Ti-based precipitates was obtained.
• the tensile characteristics were examined by taking JIS No. 5 tensile specimens from each of the obtained steel plates and performing a tensile test in accordance with JIS Z 2241 (1998).
• a specimen having a yield stress of 400 MPa or higher is evaluated as excellent base metal tensile characteristics (within the scope of the present invention).
• Specimens having excellent base metal tensile characteristics of the present invention had a tensile strength of 800 MPa or higher and total elongation of 30% or more.
• the base metal toughness was evaluated by: taking Charpy V-notch specimens in accordance with JIS Z 2202 (1998) in a direction perpendicular to the rolling direction at a position of 1/4 thickness for each steel plate having a thickness of more than 20 mm or at a position 1/2 thickness for each steel plate having a thickness of 20 mm or less; performing a Charpy impact test for three specimens for each steel plate in accordance with JIS Z 2242 (1998); and obtaining an absorbed energy at -196°C.
• a steel plate having an average absorbed energy (vE -196 ) of three specimens of 50 J or higher is evaluated as excellent base metal toughness (within the scope of the present invention). More preferably, the average absorbed energy (vE -196 ) is 100 J or higher.
• a stress corrosion cracking test was performed in accordance with a slow strain rate test method based on NACE Standard TM0111-2011.
• a test piece having a shape of notched Type A round bar was used.
• the test piece was immersed in artificial seawater (chloride ion concentration of 18,000 ppm) at 23°C and subjected to a constant-rate tensile test at a strain rate of 4 ⁇ 10 -7 inch/sec.
• a test piece having a fracture stress of 500 MPa or higher is evaluated as excellent resistance to stress corrosion cracking (within the scope of the present invention). More preferably, a fracture stress is 600 MPa or higher.

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