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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/38—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling sheets of limited length, e.g. folded sheets, superimposed sheets, pack rolling
• • 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
• • 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
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/42—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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B1/024—Forging or pressing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B1/026—Rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B2001/028—Slabs
• • 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
  • 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
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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
  • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
• • 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

Definitions

• the present invention relates to a thick-walled high-toughness high-strength steel plate for use in steel structures in construction, bridges, shipbuilding, offshore structures, construction and industrial machinery, tanks, penstocks, and the like and to a method for manufacturing the thick-walled high-toughness high-strength steel plate.
• the surface of the steel plate has high toughness
• the inner part of the steel plate has high strength and toughness.
• the steel plate has a thickness of 100 mm or more and a yield strength of 620 MPa or more.
• steel for use in construction, bridges, shipbuilding, offshore structures, construction and industrial machinery, tanks, penstocks, and other fields is welded to have a desired shape.
• the strength and thickness of steel to be used have also been greatly increased.
• the cooling rate is higher on the surface of a steel plate than in the half-thickness portion.
• a martensite structure having low toughness is formed on the surface of the steel plate.
• a high-strength steel plate having a thickness of 100 mm or more rarely has both high surface toughness and high strength and toughness of the inner part thereof.
• Non Patent Literature 1 describes a material having a thickness of 210 mm
• Non Patent Literature 2 describes a material having a thickness of 180 mm.
• the present invention has been made to solve such problems and aims to provide a thick-walled high-toughness high-strength steel plate that has high surface toughness and high strength and toughness of the inner part thereof and a method for manufacturing the thick-walled high-toughness high-strength steel plate.
• the present inventors have extensively studied the microstructure control factors that satisfy high toughness of the surface of a thick-walled steel plate having a yield strength of 620 MPa or more and a thickness of 100 mm or more and also satisfy high strength and toughness of the half-thickness portion of the thick-walled steel plate, and have found the following.
• the present invention provides the following.
• the present invention provides a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more and having a yield strength of 620 MPa or more and high toughness.
• the thick-walled high-toughness high-strength steel plate can be used to manufacture steel structures having high safety.
• a thick-walled high-toughness high-strength steel plate according to the present invention has a composition containing, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less (including 0%), Mn: 0.5% to 5.0%, P: 0.010% or less (including 0%), S: 0.0050% or less (including 0%), Cr: 3.0% or less (including 0%), Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less (including 0%), and O: 0.0025% or less (including 0%).
• the symbol "%" in the component content refers to "% by mass”.
• C is an element useful for achieving the strength necessary for structural steel at low cost. This effect requires a C content of 0.08% or more. In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, however, a C content of more than 0.20% significantly deteriorates toughness of the base metal and weld. Thus, the C content has an upper limit of 0.20%. The C content preferably ranges from 0.08% to 0.14%.
• Si is added for deoxidation.
• a steel plate according to the present invention does not necessarily contain Si.
• a Si content of more than 0.40% significantly deteriorates toughness of the base metal and heat-affected zone.
• the Si content is 0.40% or less, preferably 0.05% to 0.3%, more preferably 0.1% to 0.3%.
• Mn is added to ensure high strength of the base metal. This effect is insufficient at a Mn content of less than 0.5%.
• a Mn content of more than 5.0% promotes center segregation, results in a larger casting defect of the slab, and deteriorates mechanical properties of the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• the Mn content has an upper limit of 5.0%.
• the Mn content preferably ranges from 0.6% to 2%, more preferably 0.6% to 1.6%.
• a P content of more than 0.010% significantly deteriorates toughness of the base metal and heat-affected zone.
• the P content is preferably minimized (may be zero) and is limited to 0.010% or less.
• a S content of more than 0.0050% significantly deteriorates toughness of the base metal and heat-affected zone.
• the S content is preferably minimized (may be zero) and is 0.0050% or less.
• the Cr is an element effective in strengthening the base metal. However, an excessively high Cr content deteriorates weldability. Thus, the Cr content is 3.0% or less, preferably 0.1% to 2%, more preferably 0.7% to 1.7%. The Cr content may be 0%.
• Ni is an element useful for improving the strength of steel and the toughness of the heat-affected zone. This effect requires a Ni content of 0.1% or more. However, a Ni content of more than 5.0% significantly deteriorates economic efficiency. Thus, the Ni content has an upper limit of 5.0%.
• the Ni content preferably ranges from 0.4% to 4%, more preferably 0.8% to 3.8%.
• Al is added for sufficient deoxidation of molten steel.
• An Al content of less than 0.010% is insufficient for the effect.
• an Al content of more than 0.080% deteriorates toughness of the base metal due to an increased dissolved Al content in the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• the Al content is 0.080% or less, preferably 0.030% to 0.080%, more preferably 0.030% to 0.070%.
• N together with Ti, forms a nitride and thereby performs refinement of the structure and improves the toughness of the base metal and heat-affected zone in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• the toughness can be improved by a constituent other than N.
• a steel plate according to the present invention does not necessarily contain N.
• the N content is preferably 0.0015% or more.
• a N content of more than 0.0070% deteriorates toughness of the base metal due to an increased dissolved N content in the base metal and deteriorates toughness of the heat-affected zone due to the formation of coarse carbonitride.
• the N content is 0.0070% or less, preferably 0.006% or less, more preferably 0.005% or less.
• O content of more than 0.0025% significantly deteriorates toughness due to the formation of a hard oxide in steel.
• the O content is preferably minimized (may be zero) and is 0.0025% or less.
• a thick-walled high-toughness high-strength steel plate according to the present invention can contain at least one of Cu, Mo, V, Nb, and Ti in order to further improve strength and/or toughness.
• Cu can improve the strength of steel without reducing toughness.
• a Cu content of more than 0.50% may cause a crack on the surface of a steel plate during hot working.
• the Cu content, if any, is 0.50% or less.
• Mo contributes to high strength of the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• a Mo content of more than 1.50% results in increased hardness and deteriorates toughness due to the precipitation of alloy carbide.
• the Mo content if any, has an upper limit of 1.50%.
• the Mo content preferably ranges from 0.2% to 0.8%.
• V 0.400% or less
• V contributes to improved strength and toughness of the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• V precipitates as VN and is effective in decreasing the amount of dissolved N.
• a V content of more than 0.400% deteriorates toughness due to the precipitation of hard VC.
• the V content, if any, is preferably 0.400% or less, more preferably 0.01% to 0.1%.
• Nb is effective in improving the strength of the base metal.
• a Nb content of more than 0.100% deteriorates toughness of the base metal.
• the Nb content has an upper limit of 0.100%.
• the Nb content is preferably 0.025% or less.
• Ti forms TiN during heating and effectively suppresses the coarsening of austenite.
• Ti improves the toughness of the base metal and heat-affected zone.
• a Ti content of more than 0.020% results in coarsening of Ti nitride and deteriorates toughness of the base metal.
• the Ti content if any, ranges from 0.005% to 0.020%, preferably 0.008% to 0.015%.
• a thick-walled high-toughness high-strength steel plate according to the present invention can further contain at least one of Mg, Ta, Zr, Y, B, Ca, and REM to improve the material quality.
• Mg forms a stable oxide at high temperatures, effectively suppresses the coarsening of prior ⁇ grains in the heat-affected zone, and is effective in improving the toughness of the weld.
• These effects require a Mg content of 0.0001% or more.
• a Mg content of more than 0.0050% results in an increased number of inclusions and deteriorates toughness.
• the Mg content, if any, is preferably 0.0050% or less, more preferably 0.0001% to 0.015%.
• Ta 0.01% to 0.20%
• Ta content 0.01% or more is effective.
• a Ta content of more than 0.20% deteriorates toughness due to formation of precipitates.
• the Ta content if any, ranges from 0.01% to 0.20%.
• Zr is an element effective in improving strength.
• a Zr content of 0.005% or more is effective in producing this effect.
• a Zr content of more than 0.1% deteriorates toughness due to the formation of a coarse precipitate.
• the Zr content, if any, ranges from 0.005% to 0.1%.
• Y forms a stable oxide at high temperatures, effectively suppresses the coarsening of prior ⁇ grains in the heat-affected zone, and is effective in improving the toughness of the weld.
• An Y content of 0.001% or more is effective in producing these effects.
• an Y content of more than 0.01% results in an increased number of inclusions and deteriorates toughness.
• the Y content, if any, ranges from 0.001% to 0.01%.
• B segregates at austenite grain boundaries, suppresses ferrite transformation from the grain boundaries, and improves hardenability.
• a B content of more than 0.0030% deteriorates hardenability and toughness due to the precipitation of B as a carbonitride.
• the B content is 0.0030% or less.
• the B content, if any, preferably ranges from 0.0003% to 0.0030%, more preferably 0.0005% to 0.002%.
• Ca is an element useful for the morphology control of a sulfide inclusion. This effect requires a Ca content of 0.0005% or more. However, a Ca content of more than 0.0050% deteriorates cleanliness and toughness. Thus, the Ca content, if any, is preferably 0.0050% or less, more preferably 0.0005% to 0.0025%.
• REM forms an oxide and a sulfide in steel and is effective in improving the material quality. This effect requires a REM content of 0.0005% or more. However, the effect levels off at a REM content of 0.0100% or more. Thus, the REM content, if any, is 0.0100% or less, preferably 0.0005% to 0.005%.
• the present invention provides a steel plate having desirable characteristics even when the steel plate is manufactured from steel casted under conditions where the cooling rate of a slab surface during solidification is 1°C/s or less.
• microsegregation needs to be reduced to achieve high toughness (vE-40 ⁇ 70 J) of the surface of a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more, particularly manufactured from steel casted under conditions where the cooling rate of a slab surface during solidification is 1°C/s or less.
• C L 0.2 ⁇ ⁇ 0.1 ⁇ 0.2 ⁇ Si ⁇ 0.03 ⁇ 1.1 ⁇ Mn ⁇ 0.12 ⁇ 0.2 ⁇ Cu ⁇ 0.11 ⁇ 3 ⁇ Ni + 0.025 ⁇ 1.2 ⁇ Cr + 0.1 ⁇ 0.5 ⁇ Mo + 0.2 ⁇ 0.04 ⁇ V ⁇ 0.05 ⁇ 0.06 ⁇ Al
• the element symbols denote the respective alloy component contents (% by mass), and in the absence of an element, the element symbol is denoted by 0.
• the C content needs to be specified depending on each component other than C, such as Si or Mn.
• the effects of an alloying element on the C solid solubility limit (C L ) of the ⁇ phase were calculated using thermodynamic calculation software "Thermo-Calc". The result was used to determine the factor.
• the factor "-0.1" for "Si” means that 1% Si decreases the C solid solubility limit of the ⁇ phase by 0.1%, and the C content of the base metal needs to be decreased to achieve the required percentage of the ⁇ phase.
• the calculation of C L was based on the component of C: 0.12%, Si: 0.2%, Mn: 1.1%, Cu: 0.2%, Cr: 1.2%, Ni: 3%, Mo: 0.5%, V: 0.04% and Al: 0.06%, and the factors for the calculation of C L were determined by calculating a variation from the dissolved C content caused by a variation in each alloying element content.
• the percentage (C L - C)/C L x 100 of C to be added relative to the C solid solubility limit in the ⁇ phase thus calculated is 30% or more, the percentage of the ⁇ phase at the beginning of the formation of the ⁇ phase can be 30% or more.
• the reduction of area in the thickness direction at half the thickness of the plate is preferably 40% or more when measured by a method described in the example.
• the temperature "°C” refers to the temperature in the half-thickness portion except for the quenching temperature in the case of quenching without leaving to cool after rolling.
• the quenching temperature in the case of quenching without leaving to cool after rolling is the surface temperature of the steel plate. This is because the temperature distribution of the steel plate in the thickness direction increases during rolling, and a decrease in the surface temperature of the steel plate needs to be considered.
• the temperature of the half-thickness portion is determined, for example, by simulation calculation from the thickness, surface temperature, and cooling conditions. For example, the temperature of the half-thickness portion is determined by calculating the temperature distribution in the thickness direction using finite difference methods.
• a molten steel having the composition described above is produced by a conventional method, such as with a converter, an electric furnace, or a vacuum melting furnace, and is formed into a piece of steel, such as a slab or billet, by a conventional casting method, such as a continuous casting process or an ingot casting process.
• the cooling rate during solidification is determined by direct measurement with a thermocouple or by simulation calculation, such as heat-transfer calculation.
• steel manufactured under conditions where the cooling rate of a surface during solidification is 1°C/s or less can preferably be used.
• the thickness of the material may be reduced by slabbing.
• a cast bloom or steel bloom having the composition described above is heated to a temperature in the range of 1200°C to 1350°C.
• a reheating temperature of less than 1200°C results in not only an insufficient rolling reduction due to an increased load to achieve a predetermined cumulative rolling reduction in hot working but also low production efficiency due to additional heating as required during working.
• the reheating temperature is 1200°C or more.
• a large amount of additive alloying element as steel having a carbon equivalent of 0.65% or more according to the present invention results in a casting defect such as a center porosity or porous shrinkage cavity, having a much increased size in steel. In order to make them harmless by pressure bonding, the cumulative rolling reduction needs to be 25% or more.
• a reheating temperature of more than 1350°C results in excessive energy consumption, increased likelihood of occurrence of surface flaws due to scales during heating, and increased repair load after hot forging.
• the upper limit is 1350°C.
• a cast bloom or steel bloom having the composition described above is heated to a temperature in the range of 1200°C to 1350°C.
• a reheating temperature of less than 1200°C results in not only an insufficient rolling reduction due to an increased load to achieve a predetermined cumulative rolling reduction in hot working but also low production efficiency due to additional heating as required during working.
• the reheating temperature is 1200°C or more.
• the cumulative rolling reduction is 30% or more, preferably 40% or more in terms of good reduction of area (RA).
• a reheating temperature of more than 1350°C results in excessive energy consumption, increased likelihood of surface flaws due to scales during heating, and increased repair load after hot forging.
• the upper limit is 1350°C.
• the heating temperature preferably ranges from 1000°C to 1200°C.
• the Ac3 transformation temperature is calculated using the following formula (4).
• Ac 3 937.2 ⁇ 476.5 C + 56 Si ⁇ 19.7 Mn ⁇ 16.3 Cu ⁇ 26.6 Ni ⁇ 4.9 Cr + 38.1 Mo + 124.8 V + 136.3 Ti + 198.4 Al + 3315 B
• the element symbols in the formula (4) denote the respective alloy component contents (% by mass).
• a steel plate is left to cool (for example, air cooling) after hot rolling or is rapidly cooled from the Ar3 temperature or more to 350°C or less without leaving to cool after hot rolling.
• the steel plate is reheated to the Ac3 temperature to 1050°C and is rapidly cooled from the Ac3 temperature or more to 350°C or less.
• the reason for the reheating temperature of 1050°C or less is that reheating at a high temperature of more than 1050°C results in coarsening of austenite grains and significantly deteriorates toughness of the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
• the reheating temperature is the Ac3 temperature or more in order that the steel plate may entirely have an austenite structure.
• the quenching temperature is the Ac3 temperature or more because the desirable characteristics are not obtained at a temperature below the Ac3 temperature due to the formation of a nonuniform structure composed of ferrite and austenite.
• the quenching temperature is the Ar3 temperature or more for quenching from the austenite single phase region.
• the rapid cooling stop temperature is a lower temperature selected from 350°C or less and the Ar3 temperature or less in order to ensure that the steel plate entirely has a transformed structure.
• the stop temperature should be the Ar3 temperature or less and 350°C or less.
• the Ar3 transformation temperature is calculated using the following formula (5).
• Ar 3 910 ⁇ 310 C ⁇ 80 Mn ⁇ 20 Cu ⁇ 15 Cr ⁇ 55 Ni ⁇ 80 Mo
• the element symbols in the formula (5) denote the respective alloy component contents (% by mass).
• the rapid cooling method is industrially water cooling. It is desirable that the cooling rate be as high as possible.
• the cooling method is not necessarily water cooling and may be gas cooling, for example.
• quenching is sometimes repeated to strengthen steel. Although quenching may be repeated also in the present invention, final quenching requires rapid cooling to 350°C or less after heating to the Ac3 temperature to 1050°C and requires subsequent tempering at 450°C to 700°C.
• Steel plate samples No. 1 to No. 38 were manufactured by melting and casting steel No. 1 to No. 30 listed in Table 1 under the conditions listed in Table 2, performing hot forging (except for the samples No. 5, No. 6, and No. 41) or slabbing (the samples No. 5, No. 6, and No. 41), hot-rolling the steel to form a steel plate having a thickness listed in Table 2, and subjecting the steel plate to water quenching and tempering.
• the steel plate samples No. 1 to No. 38 were subjected to the following tests. In reheating and quenching in this example, the reheating temperature corresponds to the quenching temperature.
• the percentage of the ⁇ phase is calculated using the formula (2) from C L calculated using the formula (3) with each base metal component and the C content of the base metal.
• the cooling rate during solidification in the manufacture of steel is determined by heat-transfer calculation from the mold surface temperature data measured with a radiation thermometer.
• a round bar tensile test piece ( ⁇ 12.5 mm, GL 50 mm) was taken from the half-thickness portion of each steel plate in the direction perpendicular to the rolling direction and was measured in terms of yield strength (YS) and tensile strength (TS).
• Three 2-mm V-notched Charpy impact test specimens were taken from each surface and half-thickness portion of the steel plates.
• the rolling direction was the longitudinal direction.
• the absorbed energies of the test specimens were measured at a test temperature of -40°C in a Charpy impact test and were averaged (the average value for the test specimens taken from the half-thickness portion and the average value for the test specimens taken from the surface).
• a round bar tensile test piece ( ⁇ 10 mm) was taken from a region including the half-thickness portion of each steel plate in the thickness direction and was measured in terms of reduction of area (RA).
• the reduction of area is the percentage of the difference between the minimum cross-sectional area after the test specimen was broken and the original cross-sectional area relative to the original cross-sectional area.
• Table 2 shows the test results.
• the results showed that the steel plates of the examples having a steel composition according to the present invention (samples No. 1 to No. 21 and No. 41) had YS of 620 MPa or more, TS of 720 MPa or more, and toughness (vE-40) of 70 J or more at -40°C in the surface and half-thickness portion of the base metal, showing high strength and toughness of the base metal.
• a comparison between Nos. 5 and 6 and No. 41 showed that reduction of area (RA) was also satisfactory under particular slabbing conditions.
• the base metal had at least one of YS of less than 620 MPa, TS of less than 720 MPa, and toughness (vE-40) of less than 70 J, thus deteriorating characteristics.

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• 2016
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