US20170369958A1 - Thick-walled high-toughness high-strength steel plate and method for manufacturing the same - Google Patents
Thick-walled high-toughness high-strength steel plate and method for manufacturing the same Download PDFInfo
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- US20170369958A1 US20170369958A1 US15/543,364 US201615543364A US2017369958A1 US 20170369958 A1 US20170369958 A1 US 20170369958A1 US 201615543364 A US201615543364 A US 201615543364A US 2017369958 A1 US2017369958 A1 US 2017369958A1
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- steel
- temperature
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- toughness
- thick
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 255
- 239000010959 steel Substances 0.000 title claims abstract description 255
- 238000004519 manufacturing process Methods 0.000 title claims description 25
- 238000000034 method Methods 0.000 title claims description 22
- 238000001816 cooling Methods 0.000 claims abstract description 40
- 238000003303 reheating Methods 0.000 claims description 56
- 230000009467 reduction Effects 0.000 claims description 46
- 238000005096 rolling process Methods 0.000 claims description 46
- 238000010438 heat treatment Methods 0.000 claims description 40
- 230000001186 cumulative effect Effects 0.000 claims description 34
- 238000005496 tempering Methods 0.000 claims description 19
- 238000005098 hot rolling Methods 0.000 claims description 17
- 238000005242 forging Methods 0.000 claims description 15
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 9
- 229910045601 alloy Inorganic materials 0.000 claims description 8
- 239000000956 alloy Substances 0.000 claims description 8
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052796 boron Inorganic materials 0.000 claims description 4
- 229910052791 calcium Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 229910052727 yttrium Inorganic materials 0.000 claims description 4
- 229910052726 zirconium Inorganic materials 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 3
- 229910052757 nitrogen Inorganic materials 0.000 claims description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims description 3
- 229910052715 tantalum Inorganic materials 0.000 claims description 3
- 238000007711 solidification Methods 0.000 abstract description 12
- 230000008023 solidification Effects 0.000 abstract description 12
- 239000000203 mixture Substances 0.000 abstract description 11
- 238000010791 quenching Methods 0.000 description 58
- 230000000171 quenching effect Effects 0.000 description 58
- 239000010953 base metal Substances 0.000 description 28
- 230000000694 effects Effects 0.000 description 13
- 238000003466 welding Methods 0.000 description 13
- 230000015572 biosynthetic process Effects 0.000 description 10
- 239000000463 material Substances 0.000 description 8
- 229910001566 austenite Inorganic materials 0.000 description 7
- 238000012360 testing method Methods 0.000 description 7
- 238000005275 alloying Methods 0.000 description 6
- 238000004364 calculation method Methods 0.000 description 6
- 238000005266 casting Methods 0.000 description 6
- 229910000734 martensite Inorganic materials 0.000 description 5
- 238000010276 construction Methods 0.000 description 4
- 238000001556 precipitation Methods 0.000 description 4
- 238000009864 tensile test Methods 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 3
- 229910001563 bainite Inorganic materials 0.000 description 3
- 229910052799 carbon Inorganic materials 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 230000003247 decreasing effect Effects 0.000 description 3
- 230000007547 defect Effects 0.000 description 3
- 238000009863 impact test Methods 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 239000007787 solid Substances 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229910000859 α-Fe Inorganic materials 0.000 description 3
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 238000005265 energy consumption Methods 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 230000008439 repair process Effects 0.000 description 2
- 238000004088 simulation Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 238000012546 transfer Methods 0.000 description 2
- VZSRBBMJRBPUNF-UHFFFAOYSA-N 2-(2,3-dihydro-1H-inden-2-ylamino)-N-[3-oxo-3-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)propyl]pyrimidine-5-carboxamide Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)C(=O)NCCC(N1CC2=C(CC1)NN=N2)=O VZSRBBMJRBPUNF-UHFFFAOYSA-N 0.000 description 1
- 229910000746 Structural steel Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000003749 cleanliness Effects 0.000 description 1
- ZPUCINDJVBIVPJ-LJISPDSOSA-N cocaine Chemical compound O([C@H]1C[C@@H]2CC[C@@H](N2C)[C@H]1C(=O)OC)C(=O)C1=CC=CC=C1 ZPUCINDJVBIVPJ-LJISPDSOSA-N 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000002542 deteriorative effect Effects 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- NRNCYVBFPDDJNE-UHFFFAOYSA-N pemoline Chemical compound O1C(N)=NC(=O)C1C1=CC=CC=C1 NRNCYVBFPDDJNE-UHFFFAOYSA-N 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
Classifications
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- 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
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- C—CHEMISTRY; METALLURGY
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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- C—CHEMISTRY; METALLURGY
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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
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- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
- 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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- 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
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- 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
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C—ALLOYS
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- C22C—ALLOYS
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- C22C—ALLOYS
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C—ALLOYS
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C—ALLOYS
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- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
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- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- 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
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- 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
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- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- 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
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- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- 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
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- 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
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- 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
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- 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
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- 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
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- 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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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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
- 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
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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
- C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
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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
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 alloy components should be appropriately selected, and in particular the carbon equivalent (Ceq) should be 0.65% or more.
- the carbon equivalent (Ceq) should be 0.65% or more.
- Refinement of prior ⁇ grain size is effective in improving toughness. Refinement of prior ⁇ grain size before heat treatment, that is, refinement of prior ⁇ grain size just after hot working is important for refinement of prior ⁇ grain size after heat treatment. Thus, it is important to select appropriate hot working conditions and rolling conditions.
- the present invention provides for the following embodiments.
- a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more, containing, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, formulae (1) and (2) being satisfied, the remainder being Fe and incidental impurities, a surface of the steel plate having a toughness (vE-40) of 70 J or more,
- Ceq IIW C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ⁇ 0.65 (1)
- C L is defined by the following formula:
- the surface of the steel plate has high toughness, and the inner part of the steel plate has high strength and toughness.
- heating steel to 1200° C. to 1350° C. hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200° C. or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, rapidly cooling the steel from the Ar3 temperature or more to a lower temperature selected from 350° C. or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450° C. to 700° C.
- the present invention provides for 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 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%.
- an appropriate alloy component needs to be added to ensure that a half-thickness portion of a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more has a yield strength of 620 MPa or more and high toughness. More specifically, as represented by the following formula (1), alloying element contents need to be adjusted such that the carbon equivalent (Ceq IIW ) is 0.65% or more.
- Ceq IIW C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ⁇ 0.65 (1)
- embodiments of 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.
- primary crystals during solidification need to form a ⁇ phase, and the percentage ((C L ⁇ C)/C L ⁇ 100) of the ⁇ phase at the beginning of the formation of a ⁇ phase needs to be 30% or more.
- 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 ⁇ 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 embodiments of 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).
- 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.
- 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).
- 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 embodiments of 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 ( ⁇ 2.5 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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Abstract
Description
- This is the U.S. National Phase application of PCT/JP2016/000197, filed Jan. 15, 2016, which claims priority to Japanese Patent Application No. 2015-006670, filed Jan. 16, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.
- 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. In particular, the surface of the steel plate has high toughness, and 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.
- In general, 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. In recent years, with significantly increasing in size of steel structures, the strength and thickness of steel to be used have also been greatly increased.
- Despite trying to manufacture a thick-walled high-strength steel plate having a thickness of 100 mm or more and having high strength and toughness in a half-thickness portion (the central portion in the thickness direction), a structure having relatively low strength, such as ferrite, tends to be formed in the half-thickness portion due to a decreased cooling rate. Thus, the addition of large amounts of alloying elements is required to reduce the formation of such a structure.
- In particular, in order to achieve high strength and toughness of a half-thickness portion of a thick-walled material (a thick-walled steel plate having a thickness of 100 mm or more), it is important to form bainite or a mixed structure of bainite and martensite in the half-thickness portion during quenching. This requires the addition of large amounts of alloying elements, such as Mn, Ni, Cr, and/or Mo.
- The cooling rate is higher on the surface of a steel plate than in the half-thickness portion. Thus, a martensite structure having low toughness is formed on the surface of the steel plate. Thus, 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.
- A steel plate is described in the following two pieces of non-patent literature. Non Patent Literature 1 describes a material having a thickness of 210 mm, and Non Patent Literature 2 describes a material having a thickness of 180 mm.
-
- NPL 1: Nippon Steel Technical Report, 348 (1993), 10-16
- NPL 2: Nippon Kokan Technical Report, 107 (1985), 21-30
- These pieces of non-patent literature describe high strength and toughness of the half-thickness portion. However, these pieces of non-patent literature do not describe the toughness (Charpy impact characteristics) of the surface of a steel plate. In general, thick-walled materials are manufactured by a quenching and tempering process. The formation of a martensite structure on the surface of a steel plate, on which the cooling rate is higher than in the half-thickness portion, deteriorates the toughness (Charpy impact characteristics) of the surface of the steel plate. However, these pieces of non-patent literature do not describe the manufacture of a steel plate consistently having a tough surface.
- 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.
- In order to solve the problems, 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.
- 1. When the cooling rate during the solidification of a raw material steel exceeds 1° C./s, microsegregation competes with the solidification reaction. This reduces microsegregation. In the manufacture of a large piece of steel, the cooling rate during the solidification of the steel decreases to 1° C./s or less, and consequently microsegregation becomes noticeable. Even in such a case, in order to achieve high toughness of the surface of a steel plate, on which a martensite structure is formed during quenching, it is important to reduce the P content and microsegregation during solidification. When primary crystals during solidification form a δ phase, and the percentage of the δ phase at the beginning of the formation of a γ phase is 30% or more, this results in reduced microsegregation and improved toughness. The percentage % described above refers to % by volume.
- 2. In order to achieve high strength and toughness of the half-thickness portion, in which the cooling rate is much lower than on the surface of a steel plate during cooling after hot working, it is important to appropriately select the steel composition (components) so as to form a martensite and/or bainite microstructure even at a low cooling rate. To this end, the alloy components should be appropriately selected, and in particular the carbon equivalent (Ceq) should be 0.65% or more. In addition to the appropriate component design, it is also important to form the desired structure by hot working and heat treatment.
- 3. Refinement of prior γ grain size is effective in improving toughness. Refinement of prior γ grain size before heat treatment, that is, refinement of prior γ grain size just after hot working is important for refinement of prior γ grain size after heat treatment. Thus, it is important to select appropriate hot working conditions and rolling conditions.
- As a result of further investigation of these findings, the present invention provides for the following embodiments.
- [1] A thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more, containing, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, formulae (1) and (2) being satisfied, the remainder being Fe and incidental impurities, a surface of the steel plate having a toughness (vE-40) of 70 J or more,
-
CeqIIW=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5≧0.65 (1) -
(CL−C)/CL×100≧30 (2) - wherein CL is defined by the following formula:
-
CL=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)) (3) - wherein element symbols in the formulae denote the respective alloy component contents (% by mass), and, in the absence of an element, the element symbol is denoted by 0.
- The surface of the steel plate has high toughness, and the inner part of the steel plate has high strength and toughness.
- [2] The thick-walled high-toughness high-strength steel plate according to [1], further containing, on a mass percent basis, one or two or more of Cu: 0.50% or less, Mo: 1.50% or less, V: 0.400% or less, Nb: 0.100% or less, and Ti: 0.005% to 0.020%.
- [3] The thick-walled high-toughness high-strength steel plate according to [1] or [2], further containing, on a mass percent basis, at least one of Mg: 0.0001% to 0.0050%, Ta: 0.01% to 0.20%, Zr: 0.005% to 0.1%, Y: 0.001% to 0.01%, B: 0.0030% or less, Ca: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0100%.
- [4] The thick-walled high-toughness high-strength steel plate according to any one of [1] to [3], wherein the thick-walled high-toughness high-strength steel plate has a yield strength of 620 MPa or more.
- [5] The thick-walled high-toughness high-strength steel plate according to any one of [1] to [4], wherein the reduction of area in the thickness direction at half the thickness of the plate is 40% or more.
- [6] The method for manufacturing the thick-walled high-toughness high-strength steel plate according to any one of [1] to [5],
- heating steel to 1200° C. to 1350° C., hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200° C. or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool, reheating the steel to the Ac3 temperature or more and 1050° C. or less, rapidly cooling the steel from the Ac3 temperature or more to a lower temperature selected from 350° C. or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450° C. to 700° C.
- [7] The method for manufacturing the thick-walled high-toughness high-strength steel plate according to any one of [1] to [5],
- heating steel to 1200° C. to 1350° C., hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200° C. or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, rapidly cooling the steel from the Ar3 temperature or more to a lower temperature selected from 350° C. or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450° C. to 700° C.
- [8] The method for manufacturing the thick-walled high-toughness high-strength steel plate according to any one of [1] to [5],
- heating steel to 1200° C. to 1350° C., slabbing the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200° C. or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool, reheating the steel to the Ac3 temperature or more and 1050° C. or less, rapidly cooling the steel from the Ac3 temperature or more to a lower temperature selected from 350° C. or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450° C. to 700° C.
- [9] The method for manufacturing the thick-walled high-toughness high-strength steel plate according to any one of [1] to [5],
- heating steel to 1200° C. to 1350° C., slabbing the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200° C. or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, rapidly cooling the steel from the Ar3 temperature or more to a lower temperature selected from 350° C. or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450° C. to 700° C.
- In certain embodiments, the present invention provides for 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.
- Embodiments of the present invention will be described below. The present invention is not limited to these embodiments.
- 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%). Each of the components will be described below. 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. When another element is added for deoxidation, however, a steel plate according to the present invention does not necessarily contain Si. In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, a Si content of more than 0.40% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, 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. Thus, 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%.
- In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, a P content of more than 0.010% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, the P content is preferably minimized (may be zero) and is limited to 0.010% or less.
- In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, a S content of more than 0.0050% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, the S content is preferably minimized (may be zero) and is 0.0050% or less.
- 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. On the other hand, 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. Thus, 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. Thus, a steel plate according to the present invention does not necessarily contain N. When trying to produce this effect with N, the N content is preferably 0.0015% or more. In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, however, 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. Thus, the N content is 0.0070% or less, preferably 0.006% or less, more preferably 0.005% or less.
- An O content of more than 0.0025% significantly deteriorates toughness due to the formation of a hard oxide in steel. Thus, the O content is preferably minimized (may be zero) and is 0.0025% or less.
- In addition to these elements, 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. Thus, 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. However, a Mo content of more than 1.50% results in increased hardness and deteriorates toughness due to the precipitation of alloy carbide. Thus, the Mo content, if any, has an upper limit of 1.50%. The Mo content preferably ranges from 0.2% to 0.8%.
- 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. However, a V content of more than 0.400% deteriorates toughness due to the precipitation of hard VC. Thus, 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. Thus, 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. In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, Ti improves the toughness of the base metal and heat-affected zone. However, a Ti content of more than 0.020% results in coarsening of Ti nitride and deteriorates toughness of the base metal. Thus, the Ti content, if any, ranges from 0.005% to 0.020%, preferably 0.008% to 0.015%.
- In addition to these components, 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 γ 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. However, a Mg content of more than 0.0050% results in an increased number of inclusions and deteriorates toughness. Thus, the Mg content, if any, is preferably 0.0050% or less, more preferably 0.0001% to 0.015%.
- The addition of an adequate amount of Ta is effective in improving strength. More specifically, a Ta content of 0.01% or more is effective. However, a Ta content of more than 0.20% deteriorates toughness due to formation of precipitates. Thus, 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. However, a Zr content of more than 0.1% deteriorates toughness due to the formation of a coarse precipitate. Thus, 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 γ 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. However, an Y content of more than 0.01% results in an increased number of inclusions and deteriorates toughness. Thus, 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. However, a B content of more than 0.0030% deteriorates hardenability and toughness due to the precipitation of B as a carbonitride. Thus, 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%.
- Like Ca, 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%.
- These optional elements in amounts below the lower limits do not reduce the advantages of the present invention. Thus, the optional elements in amounts below the lower limits are considered to be contained as incidental impurities.
- In embodiments of the present invention, an appropriate alloy component needs to be added to ensure that a half-thickness portion of a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more has a yield strength of 620 MPa or more and high toughness. More specifically, as represented by the following formula (1), alloying element contents need to be adjusted such that the carbon equivalent (CeqIIW) is 0.65% or more.
-
CeqIIW=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5≧0.65 (1) - The element symbols in the formula denote the corresponding element contents (% by mass). In the absence of an element, the element symbol is denoted by 0.
-
(CL−C)/CL×100≧30 (2) - As described later, embodiments of 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. In embodiments of the present invention, 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. To this end, primary crystals during solidification need to form a δ phase, and the percentage ((CL−C)/CL×100) of the δ phase at the beginning of the formation of a γ phase needs to be 30% or more.
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CL=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)) (3) - In the formula (3), 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.
- In order to form a δ phase, 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 (CL) of the δ phase were calculated using thermodynamic calculation software “Thermo-Calc”. The result was used to determine the factor. For example, the factor “−0.1” for “Si” means that 1% Si decreases the C solid solubility limit of the δ phase by 0.1%, and the C content of the base metal needs to be decreased to achieve the required percentage of the δ phase. In embodiments of the present invention, the calculation of CL 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 CL were determined by calculating a variation from the dissolved C content caused by a variation in each alloying element content. When the percentage (CL−C)/CL×100 of C to be added relative to the C solid solubility limit in the δ phase thus calculated is 30% or more, the percentage of the δ phase at the beginning of the formation of the γ phase can be 30% or more.
- In embodiments of the present invention, in order to ensure the safety of steel during use, 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 manufacturing conditions in embodiments of the present invention will be described below. In the description, 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. As described above, in embodiments of the present invention, steel manufactured under conditions where the cooling rate of a surface during solidification is 1° C./s or less can preferably be used.
- When the loads of a forging machine and a rolling mill and so on are restricted, 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. Thus, 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 embodiments of 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. On the other hand, 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. Thus, 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. Thus, the reheating temperature is 1200° C. or more. In order to make casting defects harmless by pressure bonding and to provide the advantages of embodiments of the present invention, the cumulative rolling reduction is 30% or more, preferably 40% or more in terms of good reduction of area (RA). On the other hand, 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. Thus, the upper limit is 1350° C.
- Reheating of Steel after Forging or after Slabbing
- Steel after forging is heated to an Ac3 transformation temperature or more and 1200° C. or less in order that the steel may have a uniform austenite structure alone. The heating temperature preferably ranges from 1000° C. to 1200° C.
- The Ac3 transformation temperature is calculated using the following formula (4).
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Ac3=937.2−476.5C+56Si−19.7Mn−16.3Cu−26.6Ni−4.9Cr+38.1Mo+124.8V+136.3Ti+198.4Al+3315B (4) - The element symbols in the formula (4) denote the respective alloy component contents (% by mass).
- Steel is hot-rolled to form a plate having a desired thickness. In order to ensure desirable mechanical properties of a half-thickness portion of a thick-walled steel plate having a thickness of 100 mm or more, it is necessary to adjust the steel in the rolling step in order to sufficiently elicit an effect of adjusting and refining of the prior γ grain size. More specifically, rolling at a cumulative rolling reduction of 40% or more can adjust the grain size in the rolling step even in the half-thickness portion in which recrystallization rarely occurs in processing.
- In order to achieve high strength and toughness of a half-thickness portion, in embodiments of the present invention, 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. When the steel plate is left to cool, 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. In the case of rapid cooling without leaving to cool, 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).
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Ar3=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo (5) - The element symbols in the formula (5) denote the respective alloy component contents (% by mass).
- In general, the rapid cooling method is industrially water cooling. It is desirable that the cooling rate be as high as possible. Thus, the cooling method is not necessarily water cooling and may be gas cooling, for example.
- The reason for tempering at a temperature in the range of 450° C. to 700° C. after rapid cooling is described below. Residual stress is not effectively relieved at less than 450° C. On the other hand, a temperature of more than 700° C. results in precipitation of various carbides and coarsens the structure of the base metal and deteriorates strength and toughness of the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding.
- Industrially, quenching is sometimes repeated to strengthen steel. Although quenching may be repeated also in embodiments of 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 δ phase is calculated using the formula (2) from CL 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 (φ2.5 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 (φ10 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.
- In contrast, in the steel plates according to comparative examples having a composition outside the scope of the present invention (samples No. 22 to No. 32), 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.
- As in samples No. 33 to No. 40, even if steel plates had a steel composition according to the present invention, steel plates manufactured under the conditions outside the scope of the present invention had at least one deterioration in YS, TS, and toughness (vE-40) (No. 41, which had a cumulative rolling reduction of 30% and satisfied the minimum conditions required to provide the advantages of the present invention, is not outside the scope of the present invention).
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TABLE 1 Steel ingot Chemical composition (mass %) Category No. C Si Mn P S Cr Ni Ti Al N B Cu Mo Example 1 0.085 0.18 1.61 0.006 0.0012 0.66 0.88 0.063 0.0041 0.0013 0.25 0.45 2 0.095 0.22 1.15 0.005 0.0006 1.80 1.95 0.045 0.0022 3 0.128 0.21 0.98 0.005 0.0011 0.81 1.01 0.010 0.045 0.0040 0.0011 0.28 0.54 4 0.120 0.22 1.14 0.006 0.0006 0.94 1.94 0.062 0.0049 0.0011 0.18 0.49 5 0.126 0.20 1.15 0.003 0.0008 0.95 2.05 0.010 0.043 0.0028 0.0009 0.22 0.51 6 0.123 0.20 1.18 0.004 0.0007 0.87 2.44 0.066 0.0050 0.0012 0.19 0.52 7 0.109 0.17 0.85 0.003 0.0004 1.13 3.56 0.071 0.0049 0.0010 0.08 0.53 8 0.121 0.22 0.88 0.006 0.0011 1.49 2.93 0.068 0.0046 0.0008 0.21 0.55 9 0.142 0.21 0.69 0.006 0.0009 1.51 2.87 0.071 0.0038 0.0013 0.20 0.50 10 0.113 0.11 0.81 0.006 0.0005 1.53 3.61 0.069 0.0052 0.0010 0.02 0.56 11 0.178 0.09 0.60 0.006 0.0006 1.14 2.84 0.078 0.0054 0.0009 0.11 0.55 12 0.196 0.06 0.53 0.009 0.0007 1.35 3.65 0.062 0.0046 1.43 13 0.095 0.23 2.21 0.006 0.0004 0.74 0.035 0.0039 0.0011 0.20 0.65 14 0.118 0.21 1.13 0.006 0.0008 1.44 3.49 0.065 0.0045 0.12 0.57 15 0.142 0.05 0.57 0.009 0.0012 2.41 0.15 0.009 0.026 0.0050 0.0009 0.14 1.07 16 0.161 0.22 1.03 0.005 0.0013 1.55 0.98 0.011 0.045 0.0033 0.0011 0.32 17 0.103 0.20 0.98 0.006 0.0018 0.56 2.21 0.049 0.0063 0.0013 0.18 0.55 18 0.132 0.13 1.01 0.005 0.0006 0.86 1.44 0.012 0.065 0.0041 0.0009 0.22 0.44 19 0.162 0.16 0.59 0.006 0.0019 1.41 3.98 0.063 0.0038 0.0018 0.16 1.47 Comparative 20 0.113 0.22 1.14 0.006 0.0006 0.81 3.57 0.070 0.0050 0.0012 0.20 0.51 example 21 0.083 0.16 1.45 0.007 0.0011 0.67 0.45 0.006 0.0043 0.0014 0.21 0.45 22 0.221 0.55 1.02 0.006 0.0006 0.91 0.89 0.010 0.044 0.0028 0.0011 0.12 0.46 23 0.058 0.39 1.41 0.010 0.0018 1.23 1.96 0.045 0.0056 0.0012 0.23 0.41 24 0.129 0.63 1.33 0.009 0.0009 0.98 0.65 0.058 0.0049 0.0009 0.21 0.55 25 0.153 0.18 6.11 0.009 0.0009 1.04 1.21 0.066 0.0051 0.0009 0.21 0.44 26 0.124 0.22 1.41 0.018 0.0007 0.91 2.11 0.056 0.0048 0.0007 0.19 0.38 27 0.135 0.21 1.14 0.005 0.0069 1.21 1.48 0.065 0.0044 0.0009 0.48 28 0.124 0.26 0.65 0.005 0.0006 4.10 1.38 0.056 0.0038 0.0007 0.25 29 0.131 0.24 1.15 0.009 0.0008 1.05 1.95 0.006 0.098 0.0069 0.0009 0.29 0.48 30 0.133 0.26 1.13 0.009 0.0018 0.96 1.99 0.063 0.0123 0.0008 0.22 0.43 Steel δ ingot Chemical composition (mass %) Ac3 Ar3 fraction Category No. V O Nb Mg Ta Zr Y Ca REM CeqIIW ° C. ° C. % Example 1 0.035 0.0021 0.0017 0.658 883 656 78 2 0.0015 0.777 830 654 68 3 0.045 0.0018 0.0019 0.0021 0.656 873 675 69 4 0.039 0.0016 0.0017 0.745 850 618 61 5 0.042 0.0018 0.0015 0.769 840 607 58 6 0.041 0.0019 0.0020 0.781 836 585 52 7 0.040 0.0022 0.0012 0.833 819 551 34 8 0.038 0.0021 0.0019 0.893 828 570 46 9 0.037 0.0019 0.0018 0.871 823 586 39 10 0.040 0.0023 0.0016 0.916 813 543 39 11 0.041 0.0024 0.821 807 587 31 12 0.059 0.0021 1.095 807 471 31 13 0.0022 0.0022 0.656 874 607 76 14 0.045 0.0021 0.0013 0.958 809 521 30 15 0.328 0.0022 0.031 1.018 934 688 80 16 0.043 0.0009 0.0013 0.0019 0.738 833 694 56 17 0.036 0.0022 0.048 0.0022 0.655 855 622 63 18 0.028 0.0019 0.015 0.0018 0.677 853 657 64 19 0.115 0.0018 0.005 0.0016 1.135 831 452 31 Comparative 20 0.040 0.0020 0.0016 0.826 813 530 9 example 21 0.038 0.0022 0.0018 0.600 887 693 82 22 0.0013 0.732 844 658 43 23 0.041 0.0018 0.0023 0.775 875 616 79 24 0.039 0.0021 0.0016 0.722 900 665 68 25 0.041 0.0016 0.0014 1.570 751 252 36 26 0.0026 0.0016 0.770 827 595 52 27 0.026 0.0019 0.767 854 639 65 28 0.0016 0.0026 1.194 846 662 72 29 0.0016 0.778 845 610 54 30 0.0022 0.0018 0.747 836 616 53 CeqIIW, Ac3, Ar3, and δ fraction were calculated using the formulae (1), (2), (4), and (5), respectively. -
TABLE 2 Cooling Hot forging rate or slabbing Thick- Thick- during Cumulative Hot rolling ness Steel ness of solidi- rolling Cumulative of Sample ingot material fication Heating reduction Heating rolling product Type of Category No. No. (mm) ° C./s (° C.) (%) (° C.) reduction (%) (mm) heat treatment Example 1 1 1000 0.32 1270 80 1130 43 100 Direct quenching 2 2 500 0.64 1230 65 1130 43 100 Reheating quenching 3 3 300 0.90 1200 30 1130 50 100 Reheating quenching 4 4 1000 0.33 1270 70 1160 43 150 Reheating quenching 5 4 1000 0.33 1320 55 1200 53 210 Reheating quenching 6 4 1000 0.33 1300 55 1180 67 150 Direct quenching 7 5 500 0.61 1270 45 1160 43 150 Reheating quenching 8 6 1000 0.32 1270 55 1130 60 180 Reheating quenching 9 7 1000 0.31 1270 55 1130 53 210 Reheating quenching 10 8 1000 0.31 1300 55 1130 53 210 Reheating quenching 11 9 1000 0.30 1270 55 1130 53 210 Reheating quenching 12 10 1200 0.25 1270 55 1160 50 250 Reheating quenching 13 11 1000 0.29 1270 70 1160 43 150 Reheating quenching 14 12 1300 0.19 1270 60 1160 50 250 Reheating quenching 15 13 1200 0.32 1270 85 1130 43 100 Reheating quenching 16 14 1000 0.31 1270 55 1160 53 210 Reheating quenching 17 15 1000 0.32 1270 55 1160 53 210 Reheating quenching 18 16 500 0.64 1230 65 1130 43 100 Reheating quenching 19 17 500 0.65 1230 65 1160 43 100 Reheating quenching 20 18 300 0.91 1200 40 1160 43 100 Direct quenching 21 19 1000 0.29 1350 50 1160 50 250 Reheating quenching Compar- 22 20 1000 0.29 1270 55 1160 53 210 Reheating quenching ative 23 21 300 0.92 1200 40 1160 43 100 Reheating quenching example 24 22 1000 0.31 1270 70 1160 43 150 Reheating quenching 25 23 1000 0.29 1270 70 1160 53 210 Reheating quenching 26 24 1000 0.29 1270 55 1130 50 250 Reheating quenching 27 25 1200 0.24 1270 55 1160 43 150 Reheating quenching 28 26 1000 0.28 1270 70 1160 53 210 Reheating quenching 29 27 1000 0.31 1270 55 1130 53 210 Reheating quenching 30 28 1000 0.29 1270 55 1130 53 210 Reheating quenching 31 29 1000 0.32 1270 55 1130 43 150 Reheating quenching 32 30 1000 0.31 1270 70 1160 60 100 Reheating quenching 33 3 300 0.91 1200 15 1130 20 100 Reheating quenching 34 3 300 0.89 1200 55 1130 50 100 Reheating quenching 35 3 300 0.90 1200 30 1130 50 100 Reheating quenching 36 3 300 0.92 1200 30 1130 50 100 Reheating quenching 37 3 300 0.90 1200 30 1130 50 100 Direct quenching 38 3 300 0.91 1200 30 1130 50 100 Reheating quenching 39 3 300 0.90 1200 30 1130 50 100 Reheating quenching 40 3 300 0.91 1200 30 1130 50 100 Reheating quenching Example 41 5 500 0.61 1300 30 1180 40 210 Reheating quenching Mechanical properties of base metal (1/2t) Tough- Tensile ness Final heat RA in of base treatment conditions thickness metal Quenching Cooling direction (surface) Sample temperature stop Tempering YS TS vE-40 RA vE-40 Category No. (° C.) (° C.) (° C.) (MPa) (MPa) (J) (%) (J) Example 1 850 150 630 695 786 108 73 98 2 930 100 650 698 795 167 63 105 3 930 100 630 725 793 102 70 116 4 900 100 630 785 856 205 65 221 5 900 100 630 743 829 145 71 124 6 860 100 630 751 832 126 58 159 7 900 100 630 725 821 153 63 205 8 880 100 630 705 816 238 61 211 9 880 100 650 756 864 216 63 163 10 880 100 650 741 836 193 60 121 11 880 100 650 759 857 186 62 101 12 900 100 630 748 852 221 59 146 13 880 100 630 728 839 169 71 101 14 880 100 660 793 912 221 68 145 15 930 100 650 665 756 103 73 126 16 880 100 650 723 814 203 69 163 17 1050 150 670 641 738 145 70 101 18 930 100 650 732 841 167 77 83 19 900 100 650 703 789 196 73 113 20 880 150 650 752 846 138 65 106 21 900 100 650 746 863 231 62 93 Compar- 22 880 100 640 716 815 196 61 43 ative 23 930 100 650 589 695 22 69 121 example 24 900 100 630 845 956 19 62 96 25 930 100 630 547 663 21 68 124 26 930 100 650 728 823 41 64 39 27 880 100 630 802 915 23 51 96 28 880 100 630 731 829 32 57 23 29 900 100 650 741 854 35 63 31 30 900 100 650 796 921 43 61 89 31 900 100 650 732 816 48 72 56 32 900 100 630 698 784 61 70 52 33 930 100 630 716 804 43 23 115 34 930 100 630 722 816 45 35 96 35 1100 100 630 735 824 36 53 49 36 840 100 630 536 646 46 49 52 37 623 100 630 596 683 52 51 61 38 930 400 630 503 641 32 64 103 39 930 100 720 569 657 153 68 146 40 930 100 400 793 902 41 63 53 Example 41 900 100 600 683 766 105 29 192
Claims (20)
CeqIIW=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5≧0.65 (1);
(CL−C)/CL×100≧30 (2);
CL=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)) (3);
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