US20220154303A1 - Steel plate and method for manufacturing the same - Google Patents

Steel plate and method for manufacturing the same Download PDF

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Publication number
US20220154303A1
US20220154303A1 US17/437,505 US202017437505A US2022154303A1 US 20220154303 A1 US20220154303 A1 US 20220154303A1 US 202017437505 A US202017437505 A US 202017437505A US 2022154303 A1 US2022154303 A1 US 2022154303A1
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steel plate
thickness direction
rolling
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Yuya Sato
Shigeki Kitsuya
Shusaku Ota
Tomoyuki Yokota
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: OTA, SHUSAKU, KITSUYA, Shigeki, SATO, YUYA, YOKOTA, TOMOYUKI
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • 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
    • C21D6/001—Heat treatment of ferrous alloys containing Ni
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
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    • 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
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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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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
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    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • 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
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    • 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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
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    • 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
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
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    • C21—METALLURGY OF IRON
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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    • 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
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
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    • 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

Definitions

  • the present application relates to a steel plate which can preferably be used as structural steel used in a cryogenic environment for, for example, a liquefied gas storage tank and to a method for manufacturing the steel plate.
  • the disclosed embodiments relate to a steel plate which is excellent in terms of mechanical properties and, especially, deformability in the central portion in the thickness direction and to a method for manufacturing the steel plate.
  • the term “steel plate” denotes a steel plate having a thickness of 6 mm to 80 mm.
  • a steel plate which is used in a cryogenic environment for, for example, a liquefied gas storage tank is required to have not only satisfactory strength as a steel plate but also toughness at cryogenic temperatures.
  • a steel plate is used for a liquefied natural gas (LNG) storage tank
  • LNG liquefied natural gas
  • the steel material is poor in terms of low-temperature toughness, there is a risk in that it is not possible to maintain the safety of a structure used as a cryogenic storage tank. Therefore, the steel plate to be used is strongly required to have improved low-temperature toughness.
  • a Ni-containing steel plate having a retained austenite microstructure such as a 7% Ni steel plate or a 9% Ni steel plate, in which embrittlement does not occur at cryogenic temperatures, is used.
  • Patent Literature 1 discloses a method for stabilizing a retained austenite microstructure, which tends to be unstable at low temperatures, by decreasing the grain size of untransformed austenite and by generating lattice defects to decrease the Mf temperature.
  • Patent Literature 2 discloses steel for cryogenic temperatures having an excellent CTOD property in a welded heat affected zone including a weld toe obtained by adjusting the contents of Si, Al, and N and by controlling the Fe content in residue after a simulated thermal recycle test has been performed.
  • Patent Literature 3 discloses a steel plate whose yield strength, tensile strength, and toughness at cryogenic temperatures are equal to or higher than predetermined values and which is excellent in terms of safety against fracturing and a method for manufacturing the steel plate.
  • a T-shaped joint is formed around a weld zone between a bottom panel and a side panel. Since stress applied to a steel material increases with an increase in the size of the tank, the steel material is required to have satisfactory deformability in the thickness direction from the viewpoint of safety. Therefore, achieving satisfactory deformability in the central portion in the thickness direction, in which such a property tends to be particularly poor, is required.
  • the disclosed embodiments have been completed in view of such a problem, and an object of the disclosed embodiments is to provide a steel plate having excellent deformability in the central portion in the thickness direction and a method for manufacturing the steel plate.
  • the present inventors diligently conducted investigations regarding the chemical composition and manufacturing method of a Ni— containing steel plate which can preferably be used as structural steel for use in a cryogenic environment and, as a result, established the following knowledge.
  • the percentage reduction of area decreases with an increase in the number of casting defects and coarse MnS particles having a major axis of 100 ⁇ m or more in the central portion in the thickness direction.
  • the percentage reduction of area decreases with an increase in the number of coarse prior austenite grains having a circle-equivalent diameter of 100 ⁇ m or more. This is because, since stress concentration occurs at the positions of casting defects, coarse MnS particles, and coarse prior austenite grains, such positions may become fracture origins.
  • a method for manufacturing a steel plate comprising heating a slab having the chemical composition according to any one of [1] to [3] to a temperature of 1000° C. or higher and 1300° C. or lower and performing hot rolling on the heated slab in such a manner that finish rolling is performed with a reduction ratio of 3 or more and that each of at least two rolling passes of final three rolling passes is performed with a rolling shape factor of 0.7 or more.
  • the steel plate according to the disclosed embodiments contributes significantly to improving the safety of steel structures used in a cryogenic environment such as a liquefied gas storage tank, which has a significant effect on the industry.
  • the C content is effective for increasing strength, and it is necessary that the C content be 0.01% or more to realize such an effect. It is preferable that the C content be 0.03% or more.
  • the C content is set to be 0.15% or less. It is preferable that the C content be 0.10% or less.
  • Si functions as a deoxidizing agent, Si is necessary for a steel-making process.
  • Si is effective for increasing the strength of a steel plate through solid solution strengthening by forming a solid solution in steel.
  • the Si content it is necessary that the Si content be 0.01% or more.
  • the Si content is set to be 1.00% or less. It is preferable that the Si content be 0.5% or less or more preferably 0.3% or less.
  • Mn is an element which is effective for increasing strength by improving the hardenability of a steel plate. To realize such an effect, it is necessary that the Mn content be 0.10% or more. It is preferable that the Mn content be 0.40% or more. On the other hand, in the case where the Mn content is more than 2.00%, since central segregation is promoted, there is a deterioration in cryogenic toughness and percentage reduction of area in a tensile test in the thickness direction performed on the central portion in the thickness direction, and stress corrosion cracking occurs.
  • the Mn content is set to be 2.00% or less. It is preferable that the Mn content be 1.00% or less.
  • the P content is more than 0.010%, since P causes a deterioration in grain-boundary strength as a result of being segregated at grain boundaries, thereby becoming a fracture origin, there is a deterioration in percentage reduction of area in a tensile test in the thickness direction performed on the central portion in the thickness direction. Therefore, it is preferable that the P content be as small as possible, and the P content is set to be 0.010% or less.
  • the S content be as small as possible, and the S content is set to be 0.0050% or less. It is preferable that the S content be 0.0020% or less.
  • Al functions as a deoxidizing agent, Al is most commonly used in a deoxidizing process performed on molten steel.
  • Al is effective for inhibiting a deterioration in toughness by decreasing the amount of solid solution N.
  • the Al content be 0.002% or more. It is preferable that the Al content be 0.010% or more or more preferably 0.020% or more.
  • the Al content is set to be 0.100% or less. It is preferable that the Al content be 0.070% or less or more preferably 0.060% or less.
  • Ni is an element which increases the strength of a steel plate and which is significantly effective for improving the low-temperature toughness of a steel plate by stabilizing retained austenite. Since Ni is an expensive element, steel plate cost increases significantly with an increase in the Ni content. Therefore, the Ni content is set to be 10.0% or less. On the other hand, in the case where the Ni content is less than 5.0%, there is a deterioration in the strength of a steel plate, and it is not possible to form stable retained austenite at low temperatures, which results in a deterioration in the low-temperature toughness and strength of a steel plate. Therefore, the Ni content is set to be 5.0% or more. It is preferable that the Ni content be 6.0% to 9.0%.
  • N is an austenite-stabilizing element
  • N is an element which is effective for improving cryogenic toughness.
  • Nb, V, and Ti to be finely precipitated in the form of nitrides or carbonitrides, which function as trap sites of diffusive hydrogen, N is effective for inhibiting stress corrosion cracking.
  • the N content it is necessary that the N content be 0.0010% or more. It is preferable that the N content be 0.0020% or more.
  • the N content is set to be 0.0080% or less. It is preferable that the N content be 0.0060% or less.
  • one, two, or more selected from Cr: 0.01% to 1.50%, Mo: 0.03% to 1.0%, Nb: 0.001% to 0.030%, V: 0.01% to 0.10%, Ti: 0.003% to 0.050%, B: 0.0003% to 0.0100%, and Cu: 0.01% to 1.00% may be added as needed in addition to the indispensable constituents described above.
  • Cr is an element which is effective for increasing strength. To realize such an effect, the Cr content is set to be 0.01% or more, in the case where Cr is added. On the other hand, since Cr may be precipitated in the form of nitrides, carbides, carbonitrides, and the like when rolling is performed, such precipitates become origins at which corrosion and fracturing occur, which results in a deterioration in low-temperature toughness. Therefore, in the case where Cr is added, the Cr content is set to be 1.50% or less. It is preferable that the Cr content be 1.00% or less.
  • Mo is an element which is effective for decreasing the temper embrittlement susceptibility of a steel plate and for increasing the strength of a steel plate without causing a deterioration in low-temperature toughness.
  • the Mo content is set to be 0.03% or more, in the case where Mo is added. It is preferable that the Mo content be more than 0.05%.
  • the Mo content is 1.0% or less or more preferably 0.30% or less.
  • Nb is an element which is effective for improving the strength of a steel plate.
  • the Nb content is set to be 0.001% or more, in the case where Nb is added. It is preferable that the Nb content be 0.005% or more or more preferably 0.007% or more.
  • the Nb content is set to be 0.030% or less. It is preferable that the Nb content be 0.025% or less or more preferably 0.022% or less.
  • V 0.01% to 0.10%
  • V is an element which is effective for improving the strength of a steel plate.
  • the V content is set to be 0.01% or more, in the case where V is added. It is preferable that the V content be 0.02% or more or more preferably 0.03% or more.
  • the V content is set to be 0.10% or less. It is preferable that the V content be 0.09% or less or more preferably 0.08% or less.
  • Ti is an element which is effective for improving the strength of a steel plate as a result of being precipitated in the form of nitrides or carbonitrides.
  • the Ti content is set to be 0.003% or more, in the case where Ti is added. It is preferable that the Ti content be 0.005% or more or more preferably 0.007% or more.
  • the Ti content is set to be 0.050% or less. It is preferable that the Ti content be 0.035% or less or more preferably 0.032% or less.
  • the B is an element which is effective for improving the strength of a base material.
  • the B content be 0.0003% or more, in the case where B is added.
  • the B content is set to be 0.0100% or less. It is preferable that the B content be 0.0030% or less.
  • the Cu is an element which is effective for improving the strength of a steel plate because of an improvement in hardenability.
  • the Cu content is set to be 0.01% or more, in the case where Cu is added.
  • the Cu content is set to be 1.00% or less. It is preferable that the Cu content be 0.30% or less.
  • one, two, or more selected from Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30%, W: 0% (not inclusive) to 2.00%, Co: 0% (not inclusive) to 2.00%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM: 0.0010% to 0.0100% may be added as needed.
  • Sn is an element which is effective for improving corrosion resistance. Although such an element is effective, even in the case where its content is low, the Sn content is set to be 0.01% or more, in the case where Sn is added. However, in the case where the Sn content is high, there is a deterioration in weldability and toughness, and there is also a disadvantage from a cost point of view. Therefore, in the case where Sn is added, the Sn content is set to be 0.30% or less. It is preferable that the Sn content be 0.25% or less.
  • Sb is, like Sn, an element which is effective for improving corrosion resistance. Although such an element is effective, even in the case where its content is low, the Sb content is set to be 0.01% or more, in the case where Sb is added. However, in the case where the Sb content is high, there is a deterioration in weldability and toughness, and there is also a disadvantage from a cost point of view. Therefore, in the case where Sb is added, the Sb content is set to be 0.30% or less. It is preferable that the Sb content be 0.25% or less.
  • W is, like Sn and Sb, an element which is effective for improving corrosion resistance. Since such an element is effective, even in the case where its content is low, W may be added in an amount of more than 0%. However, in the case where the W content is high, there is a deterioration in weldability and toughness, and there is also a disadvantage from a cost point of view. Therefore, in the case where W is added, the W content is set to be 2.00% or less. It is preferable that the W content be 0.50% or less.
  • Co is, like Sn, Sb, and W, an element which is effective for improving corrosion resistance. Since such an element is effective, even in the case where its content is low, Co may be added in an amount of more than 0%. It is preferable that the Co content be 0.10% or more. However, in the case where the Co content is high, there is a deterioration in weldability and toughness, and there is also a disadvantage from a cost point of view. Therefore, in the case where Co is added, the Co content is set to be 2.00% or less. It is preferable that the Co content be 1.50% or less.
  • Ca is an element which is effective for the morphological control of inclusions such as MnS
  • Ca may be added as needed.
  • the expression “morphological control of inclusions” denotes a case where elongated sulfide-based inclusions are made into granular inclusions. Through such morphological control of inclusions, it is possible to improve a tensile property in the thickness direction, toughness, and sulfide stress corrosion cracking resistance in the central portion in the thickness direction.
  • the Ca content is set to be 0.0005% or more, in the case where Ca is added. It is preferable that the Ca content be 0.0010% or more.
  • the Ca content is set to be 0.0050% or less. It is preferable that the Ca content be 0.0040% or less.
  • Mg is, like Ca, an element which is effective for the morphological control of inclusions such as MnS, Mg may be added as needed. Through such morphological control of inclusions, it is possible to improve a tensile property in the thickness direction, toughness, and sulfide stress corrosion cracking resistance in the central portion in the thickness direction. To realize such effects, the Mg content is set to be 0.0005% or more, in the case where Mg is added. It is preferable that the Mg content be 0.0010% or more.
  • the Mg content is set to be 0.0050% or less. It is preferable that the Mg content be 0.0040% or less.
  • the Zr content is set to be 0.0005% or more. It is preferable that the Zr content be 0.0010% or more.
  • the Zr content is set to be 0.0050% or less. It is preferable that the Zr content be 0.0040% or less.
  • REM are, like Ca, Mg, and Zr, elements which are effective for the morphological control of inclusions such as MnS, REM may be added as needed. Through such morphological control of inclusions, it is possible to improve a tensile property in the thickness direction, toughness, and sulfide stress corrosion cracking resistance in the central portion in the thickness direction. To realize such effects, the REM content is set to be 0.0010% or more. It is preferable that the REM content be 0.0020% or more.
  • the REM content is set to be 0.0100% or less.
  • the remainder is Fe and incidental impurities.
  • the steel plate according to the disclosed embodiments has deformability represented by a percentage reduction of area of 30% or more in a tensile test in the thickness direction performed on the central portion in the thickness direction.
  • the term “percentage reduction of area” denotes, when a tensile test is performed, the ratio ( ⁇ S/S (%)) of the amount of reduction in the cross-sectional area ⁇ S of a test specimen after the test to the cross-sectional area S of the test specimen before the test.
  • the number of MnS particles having a major axis of 100 pm or more be 10 pieces/mm 2 or less and that prior austenite grains have a circle-equivalent diameter of less than 100 ⁇ m. This is because, since stress concentration occurs at the positions of casting defects, coarse MnS particles, and coarse prior austenite grains, such positions may become fracture origins.
  • it is possible to form the desired MnS by controlling the soft reduction condition applied for continuous casting described below.
  • the expression “central portion in the thickness direction” denotes a position located at 1 ⁇ 2 of the thickness, and the percentage reduction of area and the sizes of MnS particles and prior austenite grains are determined by using the determination methods described in EXAMPLES below.
  • temperature (° C.) denotes the temperature in the central portion in the thickness direction.
  • the method for manufacturing the steel plate according to the disclosed embodiments includes heating a slab having the desired chemical composition to a temperature of 1000° C. or higher and 1300° C. or lower and performing hot rolling on the heated slab in such a manner that finish rolling is performed with a reduction ratio of 3 or more and that each of at least two rolling passes of the final three rolling passes is performed with a rolling shape factor of 0.7 or more.
  • Reheating temperature of steel material 1000° C. or higher and 1300° C. or lower
  • the steel material is reheated to dissolve precipitates in a microstructure and to homogenize a grain size distribution and the like, and the reheating temperature is set to be 1000° C. or higher and 1300° C. or lower.
  • the reheating temperature is lower than 1000° C.
  • precipitates such as A1N are not sufficiently dissolved, and since there is coarsening of such precipitates when reheating is performed, such precipitates become fracture origins, which results in the desired percentage reduction of area in a tensile test in the thickness direction not being achieved.
  • the reheating temperature is higher than 1300° C., there is a deterioration in toughness due to an increase in grain size, and there is an increase in manufacturing costs.
  • the reheating temperature is set to be 1300° C. or lower. It is preferable that the reheating temperature be 1250° C. or lower or more preferably 1200° C. or lower. Here, it is preferable that the reheating time be 1 hour to 10 hours.
  • Reduction ratio in finish rolling 3 or more
  • the reduction ratio ((slab thickness)/(final thickness)) 3 or more
  • the reduction ratio is set to be 3 or more. It is preferable that the reduction ratio be 4 or more or more preferably 5 or more.
  • Rolling shape factor in each of at least two rolling passes of final three rolling passes in finish rolling 0.7 or more
  • rollering shape factor (ld/h m ) denotes ⁇ (contact length between rolling roll and steel plate (roll contact arc length: 1d) ⁇ / ⁇ average thickness calculated from thickness on the roll entry side and thickness on the roll exit side: h m ⁇ , which is calculated by using equation (1).
  • the number of passes having a rolling shape factor of 0.7 or more is less than 2, since it is not possible to form the desired microstructure, for example, due to coarse grains remaining in the microstructure and due to the above-described casting defects being insufficiently rendered harmless, it is not possible to achieve the desired percentage reduction of area in a tensile test in the thickness direction performed on the central portion in the thickness direction. Therefore, the number of rolling passes having a rolling shape factor of 0.7 or more is set to be at least 2.
  • to increase the rolling shape factor it is sufficient that the diameters of rolling rolls be increased or that the rolling reduction be increased.
  • slab be subjected to soft reduction when continuous casting is performed.
  • soft reduction it is possible to further inhibit coarse MnS particles having a major axis of 100 ⁇ m or more and coarse prior austenite grains having a circle-equivalent diameter of 100 ⁇ m or more from remaining in the central portion in the thickness direction.
  • the reduction gradient be 0.1 mm/m or more on the upstream side of the final solidification position of a slab.
  • the cooling start temperature after hot rolling there is no particular limitation on the cooling start temperature after hot rolling has been performed, and it is preferable that the cooling start temperature be 1000° C. or lower and 500° C. or higher.
  • cooling method used after hot rolling has been performed there is no particular limitation on the cooling method used after hot rolling has been performed, and an appropriate method such as an air cooling method or a water cooling method may be used.
  • an appropriate method such as an air cooling method or a water cooling method may be used.
  • water cooling such as spray cooling, mist cooling, or laminar cooling may be performed after hot rolling has been performed.
  • a final product may be obtained by performing cooling after hot rolling has been performed, it is preferable that a heat treatment be further performed to achieve necessary properties such as low-temperature toughness.
  • a heating treatment it is preferable that a tempering treatment be performed after hot rolling has been performed.
  • a quenching-tempering treatment in which a quenching treatment is also performed before a tempering treatment is performed, may be performed.
  • an inter-critical quenching-tempering treatment in which a tempering treatment is performed after an inter-critical quenching has been performed, may be performed.
  • a quenching-inter-critical quenching-tempering treatment in which an inter-critical quenching is performed between a quenching treatment and a tempering treatment, may be performed. It is preferable that at least one of the manufacturing treatments described above be performed.
  • the quenching temperature be equal to or higher than the Ac 3 transformation temperature and 1000° C. or lower. It is preferable that the inter-critical quenching temperature be equal to or higher than the M I transformation temperature and lower than the Ac 3 transformation temperature. It is preferable that the tempering temperature be 500° C. to 650° C.
  • the Ac 1 transformation temperature and the Ac 3 transformation temperature are respectively calculated by using equations (2) and (3) below.
  • each of the atomic symbols in equations (2) and (3) above denotes the content (mass %) of the corresponding element and is assigned a value of 0 in the case where the corresponding element is not contained.
  • Molten steels having the chemical compositions given in Table 1 were prepared and made into slabs, and the slabs were then made into steel plates having a thickness of 12 mm to 70 mm under the manufacturing conditions given in Table 2.
  • the reduction gradient was 0.20 mm/m in the case of sample Nos. 1 to 30, 0.07 mm/m in the case of sample No. 31, and 0.10 mm/m in the case of sample No. 32.
  • the steel plate was processed into a test specimen having the shape of Type A so that the thickness direction of the steel plate was the tensile direction, and a tensile test was performed in accordance with JIS G 3199.
  • a test specimen which had been taken so that the thickness direction of the steel plate was the tensile direction was cooled to a temperature of ⁇ 196° C. in liquefied nitrogen, and then subjected to a Charpy impact test in accordance with JIS Z 2242 to derive absorbed energy vE ⁇ 196 at a temperature of ⁇ 196° C.
  • YS yield strength
  • TS tensile strength
  • test specimen for microstructure observation was taken from the obtained steel plate so that a position located at 1 ⁇ 2 of the thickness was an observation position.
  • the test specimen was embedded in a resin so that the cross section perpendicular to the rolling direction was an observation surface and subjected to mirror polishing.
  • the test specimen was etched in picric acid and observed by using a SEM at a magnification of 200 times to obtain the SEM image of a microstructure at the position located at 1 ⁇ 2 of the thickness.
  • the photographic images obtained in five fields of view were analyzed by using an image analyzer to derive the number density of MnS particles having a major axis of 100 ⁇ m or more and the maximum value of the circle-equivalent diameter of prior austenite grains.
  • the examples of the disclosed embodiments (sample Nos. 1 to 15, 27 to 29, 31, and 32) had a percentage reduction of area of 30% or more and excellent strength and low-temperature toughness.
  • the comparative examples (sample Nos. 16 to 26 and 30), which were outside the scope of the disclosed embodiments, were poor in terms of at least one of percentage reduction of area, strength, and low-temperature toughness.

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