EP3467130A1 - Stahlplatte mit hoher bruchfestigkeit und hervorragender beständigkeit gegen niedrige temperaturen - Google Patents

Stahlplatte mit hoher bruchfestigkeit und hervorragender beständigkeit gegen niedrige temperaturen Download PDF

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EP3467130A1
EP3467130A1 EP16903957.5A EP16903957A EP3467130A1 EP 3467130 A1 EP3467130 A1 EP 3467130A1 EP 16903957 A EP16903957 A EP 16903957A EP 3467130 A1 EP3467130 A1 EP 3467130A1
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comparative example
steel plate
content
steel
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French (fr)
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EP3467130A4 (de
EP3467130B1 (de
Inventor
Fumitoshi Takamine
Mitsuru Sawamura
Norimasa Kawabata
Toshiaki NAMBA
Naoki Saitoh
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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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
    • 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
    • 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
    • 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
    • C22C38/005—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
    • C22C38/02—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
    • C22C38/18—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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—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
    • C22C38/54—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
    • 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
    • C21D2211/002—Bainite
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high tensile strength steel plate having excellent low temperature toughness and a large plate thickness. More specifically, the present invention relates to a steel plate having a plate thickness of more than 200 mm, a tensile strength of 780 MPa or more, and an absorbed energy of a thickness middle portion at -60°C of 69 J or more.
  • the steel plate is suitably used for structures such as offshore structures, pressure vessels, penstocks, and large cranes for a ship.
  • a steel plate used as a base metal is generally required to have low temperature toughness in order to ensure the safety of the structures.
  • the scale of the structures in recent years has been significantly increased, and there is a trend toward using a steel plate having a large plate thickness and has a high strength for such structures.
  • a 780 MPa class high tensile strength steel plate is generally used.
  • a microstructure primarily containing low temperature transformation products such as bainite and martensite is formed by quenching such as a direct quenching method.
  • quenching such as a direct quenching method.
  • alloying elements such as C, Mn, Cr, Mo, and V for improving hardenability have been added to the steel in an appropriate amount so that sufficient low temperature transformation products can be obtained even when the cooling rate is decreased.
  • Patent Document 1 discloses a high tensile strength steel plate in which Ceq is 0.80 or less, the C content, the P content, the Mn content, the Ni content, and the Mo content satisfy a predetermined equation, and the ratio (HVmax/HVave) of the hardness of a center segregation portion of the steel plate to the average value of the hardness of a certain region of the steel plate, the C content, and the plate thickness satisfy a predetermined equation.
  • the plate thickness of the steel plate is 60 mm to 150 mm.
  • Patent Document 2 discloses a high tensile strength steel plate in which Ceq is CeqM or less, and the plate thickness is 75 mm to 200 mm.
  • Patent Document 3 discloses a high tensile strength steel plate with high toughness, in which a parameter x determined by the amounts of elements is 26 to 42 and the plate thickness is 75 mm to 200 mm. However, in these three patent documents, if the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
  • Patent Document 4 discloses a high tensile strength steel plate in which the C content is 0.005% to 0.02%, and the plate thickness is 50 mm to 200 mm.
  • Patent Document 5 discloses a high tensile strength steel plate in which the C content is 0.02% to 0.05%, and the plate thickness is 75 mm to 200 mm.
  • Patent Documents 4 and 5 disclose a method of requiring rapid cooling at which the cooling rate of a thickness middle portion during quenching is 1.1 °C/s or more.
  • the plate thickness of the steel plate exceeds 200 mm, it is industrially impossible to increase the cooling rate of the thickness middle portion to 1.1 °C/s or more. Therefore, when the plate thickness of the steel plate exceeds 200 mm, the method disclosed in Patent Documents 4 and 5 cannot be realized.
  • Patent Document 6 discloses a method in which, in order to obtain fine austenite grains, the cumulative rolling reduction in a temperature range of Ar 3 point to 900°C during hot rolling is increased to 50% or more, and the heating temperature for quenching is limited to a temperature range of Ac 3 point to (Ac 3 point + 100°C).
  • Patent Document 6 discloses a high tensile strength steel plate having a plate thickness of 40 mm to 65 mm. However, as the plate thickness of the steel plate is increased, the effect of rolling is decreased at the center in a plate thickness direction of the steel plate. Therefore, when the plate thickness of the steel plate exceeds 100 mm, the effect of low temperature rolling on grain refinement is small.
  • low temperature rolling causes an increase in deformation resistance and makes it difficult to bury voids inside the steel plate. Therefore, low temperature rolling is not suitable for the manufacturing of a steel plate having a plate thickness of more than 200 mm.
  • Patent Document 7 discloses a high tensile strength steel plate in which Ceq is 0.50 to 0.80, a parameter ⁇ determined by the amounts of elements is 8.45 to 15.2, the average grain size in a thickness middle portion of the steel plate is 35 ⁇ m or less, and the plate thickness is 25 mm to 200 mm.
  • Patent Document 7 discloses a method of increasing the cumulative rolling reduction in a temperature range of 900°C to 1150°C to 50% or more so as to achieve an average grain size of 35 ⁇ m or less.
  • the effect of rolling is decreased at the center in a plate thickness direction of the steel plate.
  • Patent Document 7 when the plate thickness of the steel plate exceeds 200 mm, the cooling rate of the thickness middle portion significantly is decreased, resulting in grain coarsening. Therefore, in Patent Document 7, when the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
  • Patent Document 8 discloses a method of performing a quenching treatment twice or more so that fine and uniform austenite grains can be obtained by recrystallization.
  • Patent Document 1 and Non-Patent Document 2 in a low-alloy steel, as the heating speed is decreased, the effect of reheating on grain refinement is decreased.
  • Patent Document 8 discloses a high tensile strength steel plate having a plate thickness of 50 mm.
  • the heating speed is decreased. Therefore, in the manufacturing of a steel plate having a plate thickness of more than 200 mm, grains are rarely refined even when two or more quenching treatments are performed, and only an increase in the manufacturing cost is caused. Therefore, in the method disclosed in Patent Document 8, when the plate thickness of the steel plate exceeds 200 mm, the desired effect cannot be exerted on the steel plate.
  • Patent Document 9 and Patent Document 10 disclose a high tensile strength steel plate in which the plate thickness is 150 mm to 200 mm, the amount of residual austenite is 1% to 10%, and a property of stopping propagation of brittle fracture (crack) is good.
  • a method of tempering the steel plate in a temperature range (a temperature range higher than Ac1) in which austenitic transformation can be achieved so as to form fine residual austenite.
  • Patent Document 9 discloses a method in which the temperature range of finish rolling is limited to 700°C to 850°C to obtain fine austenite and the cumulative rolling reduction in this temperature range is limited to 25% to 75%. As described above, in Patent Document 9, since low temperature rolling is used, the method of Patent Document 9 is not suitable for the manufacturing of a steel plate having a plate thickness of more than 200 mm.
  • the present invention has been made taking the foregoing problems into consideration, and an object of the present invention is to provide a steel plate having a plate thickness of more than 200 mm, excellent low temperature toughness, and high strength.
  • the inventors found a novel chemical composition and a microstructure capable of imparting high strength and high low temperature toughness to a thickness middle portion of a steel plate even when the plate thickness of the steel plate exceeds 200 mm.
  • this novel chemical composition is different from the chemical composition that imparts high strength and high low temperature toughness to a steel plate of the related art, and it is suitable to apply a novel method which is different from the method of the related art to steel having the novel chemical composition.
  • the present invention has been made based on the above findings, and the gist of thereof is as follows.
  • the present invention it is possible to provide a steel plate having a plate thickness of more than 200 mm and having excellent low temperature toughness and high strength. Therefore, the safety of a structure having a larger scale can be further increased.
  • the C content increases the hardness of the microstructure of a steel plate after quenching and is thus effective for improving strength. Therefore, the C content is needed to be 0.08% or more.
  • the C content is preferably 0.09% or more or 0.10% or more.
  • the C content is preferably 0.14% or less, more preferably 0.13% or less, or more preferably 0.12% or less.
  • Mn is effective for both deoxidation and improvement in hardenability.
  • the Mn content is needed to be 0.80% or more.
  • the Mn content may be set to 0.85% or more, 0.90% or more, 0.95% or more, 1.00% or more, 1.05% or more, or 1.10% or more.
  • the Mn content is excessive, the hardenability is excessive, and the microstructure becomes hard.
  • tempering embrittlement is promoted when the Mn content is excessive, the toughness of steel is decreased due to the synergistic effect between the hard microstructure and the tempering embrittlement. Therefore, the Mn content is needed to be 1.60% or less.
  • the Mn content is 0.80% to 1.60%.
  • the Mn content is preferably 1.50% or less, more preferably 1.40% or less, and most preferably 1.35% or less or 1.30% or less. If necessary, the Mn content may be set to 1.25% or less or 1.20% or less.
  • Ni is effective for improving the strength and toughness of steel, and the Ni content is needed to be 3.00% or more.
  • the Ni content is excessive, it is necessary to lower the tempering temperature by decreasing Ac1, resulting in an increase in the tempering time.
  • Ni stabilizes austenite there is concern that residual austenite may be remained.
  • Ni is expensive. Therefore, when the Ni content is excessive, the manufacturing cost is deteriorated. Therefore, the Ni content is needed to be 4.50% or less. Accordingly, the Ni content is 3.00% to 4.50%.
  • the Ni content is preferably 3.15% or more, 3.30% or more, 3.40% or more, or 3.50% or more, and more preferably 3.60% or more.
  • the Ni content may be set to 4.30% or less, 4.15% or less, 4.00% or less, 3.90% or less, or 3.80% or less.
  • the Cr content is needed to be 0.50% or more, and the Mo content is needed to be 0.50% or more.
  • the Cr content is needed to be 1.00% or less, and the Mo content is needed to be 1.00% or less. Therefore, the Cr content is 0.50% to 1.00%, and the Mo content is 0.50% to 1.00%.
  • the Cr content is preferably 0.60% or more, and more preferably 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more.
  • the Cr content may be set to 0.96% or less, 0.94% or less, or 0.91% or less.
  • the Mo content is preferably 0.60% or more, and more preferably 0.70% or more, 0.75% or more, 0.80% or more, or 0.85% or more.
  • the Mo content may be 0.96% or less, 0.94% or less, 0.92% or less, or 0.90% or less.
  • Al is effective for deoxidation, and is bonded to solute N in steel to form AlN.
  • This AlN makes grains fine and reduces the amount of solute N in steel, whereby the effect of B on the hardenability of the steel is stabilized. Therefore, the Al content is needed to be 0.020% or more.
  • the Al content is 0.020% to 0.085%.
  • the Al content may be set to 0.030% or more, 0.040% or more, or 0.045% or more.
  • the upper limit of the Al content may be 0.070%, 0.065% or 0.060%.
  • N is bonded to alloying elements to form compounds (nitrides and carbonitrides), thereby making grains fine. Therefore, the N content is needed to be 0.0020% or more.
  • the N content is needed to be 0.0070% or less. Therefore, the N content is 0.0020% to 0.0070%.
  • the N content may be set to 0.0025% or more, 0.0030% or more, or 0.0040% or more, and may be set to 0.0065% or less, or 0.0060% or less.
  • the B content is needed to be 0.0005% or more.
  • the B content may be set to 0.0007% or more or 0.0008% or more.
  • the B content may be set to 0.0018% or less, 0.0016% or less, or 0.0014% or less.
  • the steel plate of the embodiment contains the above eight elements (C, Mn, Ni, Cr, Mo, Al, N, and B) as essential elements.
  • the steel may contain the following elements as optional elements.
  • P is an impurity in the steel and decreases the toughness by promoting intergranular embrittlement.
  • the P content is preferably as small as possible. Therefore, the P content is needed to be 0.010% or less.
  • the P content may be 0.000%. Therefore, the P content is 0.000% to 0.010%.
  • the P content may be set to 0.007% or less or 0.005% or less.
  • the P content may be set to 0.0005% or more, or 0.001% or more.
  • the S content is an impurity in the steel, and segregation of S and sulfides reduce the toughness. Therefore, the S content is preferably as small as possible. Therefore, the S content is needed to be 0.003% or less.
  • the S content may be 0.000%. Therefore, the S content is 0.000% to 0.003%.
  • the S content may be set to 0.002% or less. When the S content is reduced, the refining cost is increased or the productivity is decreased. Therefore, the S content may be set to 0.0004% or more, or 0.0006% or more.
  • the Si content When the Si content is excessive, S promotes the tempering embrittlement and decreases the toughness. Therefore, the Si content is needed to be 0.30% or less. On the other hand, the Si content may be 0.00%. Therefore, the Si content is 0.00% to 0.30%.
  • the steel since Si is effective for both deoxidation and improvement in strength, the steel may optionally contain Si.
  • the Si content may be 0.01% or more, 0.02% or more, or 0.03% or more in order to increase the deoxidation efficiency in refining molten steel.
  • the Si content in order to more stably increase the toughness, the Si content is preferably 0.25% or less, and more preferably 0.20% or less, or 0.15% or 0.10% or less.
  • the Cu content is needed to be 0.50% or less.
  • the strength of the steel can be increased without impairing low temperature toughness.
  • the steel may optionally contain Cu.
  • the effect of Cu on the strength of steel and Ceq can be obtained even when Cu is replaced by another alloying element. Therefore, the Cu content may be 0.00%. Accordingly, the Cu content is 0.00% to 0.50%.
  • the Cu content may be set to 0.01% or more, 0.02% or more, or 0.06% or more.
  • the Cu content may be set to 0.45% or less, 0.40% or less, 0.35% or less, or 0.030% or less.
  • V 0.000% to 0.050%
  • the V content When the V content is excessive, the toughness is decreased due to the formation of alloy carbides. Therefore, the V content is needed to be 0.050% or less. On the other hand, V forms carbides or improves hardenability, thereby improving the strength of steel. In addition, when the V content is increased, Ceq is increased, and thus the formation of ferrite at the time of quenching can be more stably suppressed. Therefore, the steel may optionally contain V. However, the effect of V on the strength of steel and Ceq can be obtained even when V is replaced by another alloying element. Therefore, the V content may be 0.000%. Accordingly, the V content is 0.000% to 0.050%.
  • the V content In a case where V is contained in molten steel used as crude material, it is difficult to reduce the V content to 0.000% by refining, and thus the V content may be set to 0.003% or more, or 0.005% or more. In order to stably increase the strength of the steel, the V content is more preferably 0.010% or more, and the V content is most preferably 0.020% or more. The upper limit of the V content may be 0.045%, 0.040% or 0.035%.
  • the steel may optionally contain Nb.
  • the Nb content may be 0.000%.
  • the Nb content may be 0.001%.
  • the upper limit of the Nb content may be 0.040%, 0.035%, 0.030% or 0.025%.
  • the intentional addition of Nb may not be performed.
  • the steel may optionally contain Ti.
  • the Ti content may be 0.000%.
  • the Ti content may be 0.001% or more.
  • the Ti content may be 0.010% or less, 0.004% or less, or 0.002% or less. In a case where the effect for refining grains by Ti is unnecessary or the like, the intentional addition of Ti may not be performed.
  • the steel may optionally contain at least one selected from the group consisting of Ca, Mg and REM.
  • the Ca content, the Mg content, and the REM content may all be 0.0000%.
  • any of the Ca content, the Mg content, and the REM content is needed to be 0.0030% or less. Therefore, any of the Ca content, the Mg content, and the REM content is 0.0000% to 0.0030%.
  • any of the Ca content, the Mg content, and the REM content is 0.0001% or more. This effect is saturated when the amount of each element is reached 0.0030%.
  • the intentional addition of Ca, Mg, REM may not be performed.
  • the W content is 0.00% to 0.10%
  • the Co content is 0.00% to 0.10%
  • the Sb content is 0.000% to 0.010%
  • the As content is 0.000% To 0.010%
  • the Sn content is 0.000% to 0.010%
  • the Pb content is 0.000% to 0.050%.
  • These elements may be incorporated into the molten steel from, for example, scrap.
  • Each of the W content and the Co content may be set to 0.05% or less, 0.02% or less, 0.01% or less, or 0.005% or less.
  • the steel plate of the embodiment has a chemical composition containing the above eight essential elements and the remainder including Fe and impurities or a chemical composition containing the above eight essential elements, at least one selected from the group consisting of the above optional elements, and the remainder including Fe and impurities.
  • the chemical composition is needed to satisfy the following conditions.
  • Ts is defined by Equation 5 below and has a relatively strong correlation with the microstructure of the steel plate after the steel plate having a thickness of more than 200 mm is quenched by water cooling.
  • Ts is excessively low, the microstructure primarily contains martensite, and the toughness of the steel plate is decreased. Therefore, as shown in FIG. 1 , Ts is needed to be 380 or more.
  • Ts is needed to be 430 or less. Accordingly, the range of Ts is 380 to 430.
  • Ts As described above, since the range of Ts is defined as 380 to 430, Ts itself is a dimensionless quantity. Therefore, there is no need to limit the unit of Ts. If a unit is given to Ts, the unit of Ts is mm -1.4. %. In order to more stably increase the toughness of the steel plate, Ts is preferably 385 or more, 390 or more, 395 or more, or 400 or more. For the same reason, Ts is preferably 425 or less, 420 or less, 415 or less, or 412 or less.
  • Ts 750 ⁇ 4240 ⁇ t / 2 ⁇ 1.4 ⁇ 80 ⁇ C + 10 ⁇ Mn + 7 ⁇ Ni + 13 ⁇ Cr + 13 ⁇ Mo ⁇ 40 ⁇ Si
  • t is the plate thickness mm of the steel plate, and each element symbol is the amount % of the corresponding element.
  • Ceq is defined by Equation 6 below and represents the hardenability of the steel. When Ceq is too low, ferrite is crystallized, and the strength and low temperature toughness of the steel plate are not sufficient. Therefore, as shown in FIG. 2 , Ceq is needed to be 0.80 or more. On the other hand, when the Ceq is too high, the strength of the steel plate becomes too high and the toughness of the steel plate significantly is decreased. Therefore, as shown in FIG. 2 , Ceq is needed to be 1.05 or less. Therefore, the range of Ceq is 0.80 to 1.05. As described above, since the range of Ceq is defined as 0.80 to 1.05, Ceq itself is a dimensionless quantity. Therefore, there is no need to limit the unit of Ceq.
  • Ceq the unit of Ceq is %.
  • Ceq is preferably more than 0.80, and Ceq is more preferably 0.85 or more, 0.86 or more, 0.87 or more, or 0.89 or more.
  • the upper limit of Ceq may be 1.02, 0.99, 0.96, or 0.94.
  • Ceq C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5
  • each element symbol is the amount % of the corresponding element.
  • x is defined by Equation 7 below and represents the hardenability of the steel.
  • x is needed to be 46 or more.
  • x is needed to be 90 or less. Therefore, the range of x is 46 to 90.
  • the range of x is 46 to 90.
  • x itself is a dimensionless quantity. Therefore, there is no need to limit the unit of x.
  • the unit of x is % 6.5 .
  • the lower limit of x may be 50, 53, 56, 59, 61, or 63, and the upper limit of x may be 85, 82, 79, 76, or 73.
  • x C 1 / 2 ⁇ 1 + 0.64 ⁇ Si ⁇ 1 + 4.10 ⁇ Mn ⁇ 1 + 0.27 ⁇ Cu ⁇ 1 + 0.52 ⁇ Ni ⁇ 1 + 2.33 ⁇ Cr ⁇ 1 + 3.14 ⁇ Mo
  • each element symbol is the amount % of the corresponding element.
  • ⁇ is defined by Equation 8 below and represents the hardenability of the steel.
  • ⁇ is too low, the quenched structure primarily contains upper bainite, and the strength and low temperature toughness of the steel plate are not sufficient. Therefore, ⁇ is needed to be 22 or more.
  • ⁇ is needed to be 60 or less. Therefore, the range of ⁇ is 22 to 60.
  • the Si content is 0.00% to 0.30% and x is 46 to 90, the range of ⁇ is always 22 to 60. Therefore, there is no need to limit the range of ⁇ .
  • ⁇ 0.65 ⁇ C 1 / 2 ⁇ 1 + 0.27 ⁇ Si ⁇ 1 + 4.10 ⁇ Mn ⁇ 1 + 0.27 ⁇ Cu ⁇ 1 + 0.52 ⁇ Ni ⁇ 1 + 2.33 ⁇ Cr ⁇ 1 + 3.14 ⁇ Mo
  • each element symbol is the amount % of the corresponding element.
  • Ac1 represents the temperature at which austenitic transformation starts when the steel is heated and is defined by Equation 9 below.
  • Ac1 is needed to be 580 or more.
  • Ac1 is 647 or less. Therefore, the range of Ac1 is 580 to 647. As described above, since the range of Ac1 is defined as 580 to 647, Ac1 itself is a dimensionless quantity.
  • the unit of Ac1 is °C.
  • the upper limit of Ac1 may be set to 640, 635, 630, or 625, and the lower limit thereof may be set to 585, 590, or 595.
  • Ac 1 720 ⁇ 25 ⁇ C + 22 ⁇ Si ⁇ 40 ⁇ Mn ⁇ 30 ⁇ Ni + 20 ⁇ Cr + 25 ⁇ Mo
  • each element symbol is the amount % of the corresponding element.
  • Ti is added to steel
  • Ti is bonded to N to form TiN.
  • the ratio of Ti to N is smaller than the stoichiometric ratio (3.4)
  • Ti can be prevented from being bonded to an element other than N (for example, C). Therefore, the effect of TiN on grain refinement can be stably obtained, and the low temperature toughness can be further increased. Therefore, it is preferable that the chemical composition of the steel satisfies Ti/N ⁇ 3.4.
  • Martensite and bainite increase the strength of the steel plate. Therefore, the total amount of martensite and bainite is needed to be 99% to 100%.
  • the amount of the remainder (the total amount of ferrite, pearlite, and residual austenite) is 0% to 1%.
  • the amount of the remainder may be 0.5% or less, 0.2% or less, or 0.1% or less. That is, the total amount of martensite and bainite may be 99.5% or more, 99.8% or more, or 99.9% or more. It is most preferable that the amount of the remainder is 0%, that is, the total amount of martensite and bainite is 100%.
  • a microstructure may contain martensite, bainite, pearlite, ferrite, and residual austenite.
  • the amount of the remainder that is, the total amount of ferrite, pearlite, and residual austenite is previously determined by the following method. Thereafter, the total amount of martensite and bainite is calculated by subtracting the total amount of these three structures from 100%.
  • the amount of ferrite and the amount of pearlite are expressed in area fraction (area%) and are determined from a photograph taken with an optical microscope at a magnification of 500-fold.
  • a sample is taken from a thickness middle portion at a position more than 100 mm away from the edge of the steel plate.
  • the longitudinal section of this sample (a plane including a plate thickness direction and a rolling direction; a plane perpendicular to a width direction) is etched by Nital and the three visual fields are taken from this etched surface.
  • the three visual fields are determined such that there is no overlapping region.
  • the amount of ferrite is determined by integrating white regions (regions of ferrite) in an optical micrograph, thereafter dividing the integrated area by a measurement area, and averaging the obtained area fractions.
  • the amount of residual austenite is expressed in volume fraction (volume%) and is measured by an X-ray diffraction method.
  • the sample is taken from the thickness middle portion at a position more than 100 mm away from the edge of the steel plate. X-rays are caused to incident on the longitudinal section of this sample (the plane including the plate thickness direction and the rolling direction; the plane perpendicular to the width direction), and the volume fraction of residual austenite is determined from the obtained data.
  • the volume fraction (volume%) of the austenite is identified with the area fraction (area%) of the residual austenite such that the area fraction of the residual austenite is determined. In a case where the amount of the residual austenite is a trace amount and cannot be quantified, the amount of the residual austenite is regarded as 0%.
  • the total amount of martensite and bainite is also expressed in area fraction (area%).
  • area fraction area%.
  • the measurement according to the X-ray diffraction method can be omitted.
  • the thickness middle portion means a position in the steel plate which is half the plate thickness in the plate thickness direction away from the surface of the steel plate. It is the most difficult for martensite and bainite to be generated at the thickness middle portion. Therefore, when the total amount of martensite and bainite is in the range of 99% to 100% at the thickness middle portion, the total amount of martensite and bainite can be regarded as 99% to 100% over the entire steel plate excluding a decarburized layer having a depth (thickness) of about 1 mm from the surface of the steel plate. Therefore, it is sufficient to evaluate the structure only for the thickness middle portion.
  • FIG. 4 shows an example of the microstructure of the steel plate according to the embodiment.
  • ferrite and pearlite are not observed.
  • the total amount of ferrite, pearlite, and residual austenite is 0%, so that the total amount of martensite and bainite is 100%.
  • the tensile strength of the steel plate is needed to be 780 MPa to 930 MPa and the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is needed to be 69 J or more. The reason for this will be described below.
  • Tempered lower bainite most effectively increases the strength and low temperature toughness of the steel plate. Tempered martensite also increases the strength and low temperature toughness of the steel plate. The tempered martensite further increases the strength of the steel plate compared to the tempered lower bainite and does not increase the low temperature toughness of the steel plate as much as the tempered lower bainite does. Therefore, it is most preferable that the steel plate has a microstructure containing tempered lower bainite or a microstructure containing tempered lower bainite and tempered martensite. When the total amount of the tempered lower bainite and the tempered martensite is sufficient, the steel plate may include tempered upper bainite.
  • the tempered upper bainite does not increase the strength and low temperature toughness of the steel plate as much as the tempered lower bainite or tempered martensite does. Therefore, it is preferable that the amount of the tempered upper bainite is as small as possible.
  • virgin (untempered) martensite, virgin (untempered) upper bainite, and virgin (untempered) lower bainite greatly decrease the low temperature toughness. Therefore, there is a need to reduce as much as possible the untempered martensite, the untempered upper bainite, and the untempered lower bainite.
  • the untempered martensite, the untempered upper bainite, and the untempered lower bainite are not present. That is, in order not to generate the untempered martensite, the untempered upper bainite, and the untempered lower bainite, a heat treatment (tempering) may be performed so as not to cause the tempering temperature, which will be described later, to exceed Ac1. It is preferable that the total amount of the untempered martensite, the untempered upper bainite, and the untempered lower bainite is 0%.
  • the amounts of the tempered martensite, the tempered upper bainite, the tempered lower bainite, the untempered martensite, the untempered upper bainite, and the untempered lower bainite are substantially impossible to appropriately measure the amounts of the tempered martensite, the tempered upper bainite, the tempered lower bainite, the untempered martensite, the untempered upper bainite, and the untempered lower bainite.
  • the tensile strength of the steel plate is 780 MPa to 930 MPa
  • the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is 69 J or more
  • Ts has a relatively strong correlation with the quenched structure, and as shown in FIG. 5 , a considerable part of the quenched structure (the amounts of the martensite, the lower bainite, and the upper bainite) is achieved by adjusting Ts.
  • Ts alone does not completely represents the quenched structure or determine the structure after being tempered.
  • the morphology of precipitates for example, carbides or nitrides
  • the microstructure after being tempered final structure
  • the embodiment since there may be cases where precipitates are extremely fine and the grain size distribution is very wide, measurement of the precipitates is extremely difficult.
  • the amounts of the six structures and the morphology of the precipitates are expressed by a combination of the chemical composition, the tensile strength, and the charpy impact test. Accordingly, as described above, the tensile strength of the steel plate is needed to be 780 MPa to 930 MPa and the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is needed to be 69 J or more. The upper limit of the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is not needed to be limited, and may be 400 J or less.
  • the tempered martensite and the untempered martensite are subordinate concepts of martensite, and the tempered upper bainite, the tempered lower bainite, the untempered upper bainite, and the untempered lower bainite are subordinate concepts of bainite.
  • the tensile strength of the steel plate is preferably less than 930 MPa.
  • Preferable upper limits of the tensile strength are 900 MPa, 880 MPa, and 870 MPa, which are arranged in order toward the most preferable strength.
  • the yield strength of the steel plate is preferably 880 MPa or less.
  • Preferable upper limits of the yield strength are 850 MPa, 830 MPa, and 810 MPa, which are arranged in order toward the most preferable strength.
  • the yield strength of the steel plate is preferably 665 MPa or more, or 685 MPa or more.
  • the tensile strength is measured by a tensile test specified in JIS Z 2241.
  • a No. 14 tensile test piece specified in JIS Z 2201 is taken from a t/4 portion.
  • the longitudinal direction (tensile direction) of the No. 14 tensile test piece is a transverse direction (T direction), that is, a direction (C direction) perpendicular to a rolling direction.
  • the t/4 portion means a position in the steel plate, which is 1/4 of the plate thickness away from the surface of the steel plate in the plate thickness direction.
  • the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is measured by the charpy impact test specified in JIS Z 2242.
  • a charpy impact test piece specified in JIS Z 2242 is taken from the thickness middle portion.
  • the longitudinal direction of the charpy impact test piece is a transverse direction (T direction), that is, a direction (C direction) perpendicular to the rolling direction.
  • the depth direction of a V notch is the rolling direction.
  • the absorbed energy of the thickness middle portion obtained by the charpy impact test at -60°C is sometimes abbreviated to vE -60°C .
  • the plate thickness of the steel plate is as large as possible as long as the steel plate can be produced and handled. Therefore, the plate thickness is needed to be more than 200 mm, and preferable lower limits of the plate thickness are 210 mm, 215 mm, 220 mm, 225 mm, and 230 mm, which are arranged in order toward the most preferable thickness.
  • the plate thickness becomes too large, it becomes more difficult to produce a steel plate having high strength and excellent low temperature toughness, and moreover, the effect of the chemical composition described above on high strength and excellent low temperature toughness is decreased.
  • the plate thickness is needed to be 300 mm or less, and preferable upper limits of the plate thickness are 290 mm, 280 mm, 270 mm, and 260 mm, which are arranged in order toward the most preferable thickness. For the above reasons, the plate thickness is needed to be more than 200 mm and not more than 300 mm.
  • the steel plate according to the embodiment is suitably manufactured by a manufacturing method of a steel plate according to an embodiment described below from the viewpoint of reducing the manufacturing cost.
  • molten steel having the chemical composition described above is cast to obtain a slab.
  • the slab may also be obtained by continuous casting or by blooming an ingot using a blooming mill.
  • the slab is not soaked at a temperature of 1200°C or higher before hot rolling, coarse AlN (AlN of 1.5 ⁇ m or more) is remained in the steel, and this coarse AlN lowers the toughness of the steel plate. Therefore, the slab is soaked at 1200°C to 1380°C before hot rolling.
  • the soaking temperature is preferably 1250°C or higher. In order to further improve the productivity, the soaking temperature is preferably 1300°C or lower. It is extremely difficult to determine that AlN of 1.5 ⁇ m or more is rarely present.
  • AlN of 1.5 ⁇ m or more can be observed with a transmission electron microscope, the region observed by the transmission electron microscope is very small. Therefore, it is impossible to determine that AlN of 1.5 ⁇ m or more is rarely present with a realistic number of measurements. On the other hand, it can be confirmed by the absorbed energy (69 J or more) of the thickness middle portion obtained by the charpy impact test at -60°C that AlN of 1.5 ⁇ m or more is rarely present.
  • the slab After the soaking, the slab is hot rolled to obtain a hot rolled steel plate having a plate thickness of more than 200 mm and not more than 300 mm as an intermediate product. Except for the target plate thickness, the hot rolling conditions are not limited. In order to sufficiently add the effect of reduction on the grain size and the like to the thickness middle portion while properly maintaining the quality of the surface of the steel plate, it is preferable to start the hot rolling from a temperature of 950°C to 1250°C.
  • the steel plate in a quenching treatment, is reheated to a temperature of Ac3°C or higher and is water cooled to a temperature of lower than 300°C.
  • the microstructure of the steel plate is transformed into a single phase of austenite.
  • austenite is transformed into martensite or bainite such that the microstructure of the steel plate becomes uniform.
  • the average water cooling rate in the thickness middle portion in order to obtain a sufficient amount of martensite and lower bainite, is needed to be 0.4 °C/s to 0.8 °C/s.
  • the temperature and the water cooling rate in the thickness middle portion can be determined by heat transfer calculation.
  • Ac3 is defined by Equation 10 below.
  • the steel plate after the quenching is heated to a temperature of 580°C to Ac1°C, and thereafter water cooled from the temperature of 580°C to Ac1°C to a temperature of lower than 300°C.
  • austenite is generated in the steel plate, and untempered bainite remains after the tempering treatment, so that the toughness of the steel plate is decreased.
  • the tempering temperature is lower than 580°C, a sufficient amount of tempered structure cannot be obtained, or tempering embrittlement occurs. Therefore, the toughness of the steel plate is not sufficient. Accordingly, the tempering temperature is needed to be 580°C to Ac1°C.
  • Ac1 is defined by Equation 9 described above.
  • the plate thickness of the hot rolled steel plate exceeds 200 mm, segregation proceeds and embrittlement occurs during cooling in the tempering treatment.
  • the temperature range in which the embrittlement occurs is mainly 300°C to 500°C. Therefore, the steel plate is needed to pass through this temperature range as rapidly as possible after hot rolling. Accordingly, in the tempering treatment, the average water cooling rate in the thickness middle portion, while the temperature of the thickness middle portion decreases from 500°C to 300°C, is needed to be set to 0.3 °C/s to 0.7 °C/s.
  • the temperature and the water cooling rate in the thickness middle portion can be determined by heat transfer calculation.
  • the temperature of the surface of the steel plate is needed to be set to 580°C or higher when water cooling is started.
  • the temperature of the surface of the steel plate is measured with a radiation-type thermometer.
  • Tables 5 to 6 show the temperatures at which the steel pieces were soaked, the temperatures at which the steel plates were heated for quenching, the average water cooling rates from 800°C to 500°C during quenching, the tempering temperatures, and the temperatures at which water cooling started immediately after tempering (the temperatures of the surfaces of the steel plates), and the average water cooling rates from 500°C to 300°C during water cooling immediately after tempering.
  • the plate thickness of the hot rolled steel plate was 210 mm to 270 mm.
  • test pieces were taken from the thickness middle portions, and the test pieces were etched with Nital.
  • the etched test piece was observed in the width direction perpendicular to the rolling direction using an optical microscope.
  • the magnification of the optical microscope was 500-fold, and the measurement visual fields were three in number.
  • the samples were moved only in the rolling direction so that the visual fields did not overlap, and optical micrographs of the three visual fields were taken.
  • the area fractions of ferrite and pearlite were determined from the optical micrographs. As a result, no pearlite was detected in all of Nos. 1 to 50, and the amount of pearlite was 0%. In Nos. 12, 29, 35, and 41, the amount of ferrite was 0.5% or more and less than 1.0%, and in Nos. 37 and 38, the amount of ferrite was 4.5% or more and less than 5.0%.
  • Table 4 shows the amount of ferrite rounded off to the first decimal place.
  • a test piece was taken from a separate thickness middle portion, the volume fraction of austenite was measured by an X-ray diffraction method, and the volume fraction was assumed to be equal to the area fraction.
  • the X-ray diffraction method X-rays were caused to be incident in the width direction of the test piece. Residual austenite was detected in all of Nos. 1 to 50. However, the amount of the residual austenite was a trace amount, and could not be quantified. Therefore, the amount of residual austenite was 0% in all of Nos. 1 to 50.
  • the final products had the chemical composition and microstructure of the present invention and had excellent low temperature toughness and high strength. As can be seen from Nos. 1 to 11, when Ti/N is reduced to 3.4 or less, the low temperature toughness can be further enhanced.
  • the present invention since a high tensile strength steel plate having excellent low temperature toughness and a plate thickness of more than 200 mm is provided, the safety of a structure having a larger scale can be further increased. Therefore, the industrial applicability of the present invention is great.

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