JP6845855B2 - Low yield ratio type high strength steel and its manufacturing method - Google Patents
Low yield ratio type high strength steel and its manufacturing method Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 69
- 239000010959 steel Substances 0.000 title claims description 69
- 238000004519 manufacturing process Methods 0.000 title claims description 24
- 239000000463 material Substances 0.000 claims description 54
- 229910001563 bainite Inorganic materials 0.000 claims description 25
- 238000005096 rolling process Methods 0.000 claims description 22
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 21
- 238000001816 cooling Methods 0.000 claims description 21
- 229910000859 α-Fe Inorganic materials 0.000 claims description 20
- 239000011572 manganese Substances 0.000 claims description 15
- 239000010936 titanium Substances 0.000 claims description 15
- 239000011575 calcium Substances 0.000 claims description 14
- 239000011651 chromium Substances 0.000 claims description 14
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 12
- 239000010949 copper Substances 0.000 claims description 11
- 239000013078 crystal Substances 0.000 claims description 10
- 229910000734 martensite Inorganic materials 0.000 claims description 10
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 9
- 238000005098 hot rolling Methods 0.000 claims description 7
- 239000012535 impurity Substances 0.000 claims description 7
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 claims description 6
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 claims description 6
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 6
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 6
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 6
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 6
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 6
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 6
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 claims description 6
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 6
- 229910052782 aluminium Inorganic materials 0.000 claims description 6
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 6
- 229910052796 boron Inorganic materials 0.000 claims description 6
- 229910052791 calcium Inorganic materials 0.000 claims description 6
- 229910052799 carbon Inorganic materials 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 6
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims description 6
- 229910052757 nitrogen Inorganic materials 0.000 claims description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims description 6
- 239000011574 phosphorus Substances 0.000 claims description 6
- 229910052710 silicon Inorganic materials 0.000 claims description 6
- 239000010703 silicon Substances 0.000 claims description 6
- 229910052717 sulfur Inorganic materials 0.000 claims description 6
- 239000011593 sulfur Substances 0.000 claims description 6
- 229910052719 titanium Inorganic materials 0.000 claims description 6
- 229910052804 chromium Inorganic materials 0.000 claims description 5
- 238000010438 heat treatment Methods 0.000 claims description 5
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims description 4
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 claims description 4
- 229910052802 copper Inorganic materials 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 239000011733 molybdenum Substances 0.000 claims description 4
- 230000009467 reduction Effects 0.000 claims description 4
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 claims description 4
- 230000000694 effects Effects 0.000 description 14
- 239000010955 niobium Substances 0.000 description 12
- 229910001566 austenite Inorganic materials 0.000 description 8
- 229910045601 alloy Inorganic materials 0.000 description 7
- 239000000956 alloy Substances 0.000 description 7
- 230000007423 decrease Effects 0.000 description 6
- 239000000203 mixture Substances 0.000 description 6
- 238000010276 construction Methods 0.000 description 5
- 238000003466 welding Methods 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- 230000007797 corrosion Effects 0.000 description 4
- 238000005260 corrosion Methods 0.000 description 4
- 238000001953 recrystallisation Methods 0.000 description 4
- 239000010953 base metal Substances 0.000 description 3
- 238000005520 cutting process Methods 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 238000005553 drilling Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 238000003754 machining Methods 0.000 description 2
- 229910052755 nonmetal Inorganic materials 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 230000035882 stress Effects 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 230000006378 damage Effects 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000009977 dual effect Effects 0.000 description 1
- 238000001887 electron backscatter diffraction Methods 0.000 description 1
- 230000006355 external stress Effects 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 230000008961 swelling Effects 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- RMLPZKRPSQVRAB-UHFFFAOYSA-N tris(3-methylphenyl) phosphate Chemical compound CC1=CC=CC(OP(=O)(OC=2C=C(C)C=CC=2)OC=2C=C(C)C=CC=2)=C1 RMLPZKRPSQVRAB-UHFFFAOYSA-N 0.000 description 1
Classifications
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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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- 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
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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
- 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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- 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
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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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- 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
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
-
- 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
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/46—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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- 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
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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
- C21D2211/005—Ferrite
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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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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
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- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Steel (AREA)
Description
本発明は、低降伏比型高強度鋼材及びその製造方法に関し、より詳しくは、低い降伏比及び高い引張強度を有し、建設用鋼材として好適に用いることができる低降伏比型高強度鋼材及びその製造方法に関する。 The present invention relates to a low yield ratio type high strength steel material and a method for producing the same. More specifically, the low yield ratio type high strength steel material having a low yield ratio and a high tensile strength and can be suitably used as a construction steel material and a low yield ratio type high strength steel material. Regarding the manufacturing method.
最近、国内外のビルや橋梁などの構造物は、超高層化、長スパン化が進むにつれて、極厚・高強度鋼材の開発が求められている。高強度鋼を用いると、高い許容応力を有するため、建築及び橋梁構造を合理化、軽量化することができ、経済的な建設が可能であるのみならず、板厚を薄くすることができるため、切断や穿孔などの機械加工と溶接作業が容易になる。 Recently, for structures such as buildings and bridges in Japan and overseas, the development of ultra-thick and high-strength steel materials is required as the skyscrapers and spans increase. When high-strength steel is used, since it has a high allowable stress, it is possible to rationalize and reduce the weight of construction and bridge structure, and not only economical construction is possible, but also the plate thickness can be reduced. Machining and welding operations such as cutting and drilling become easier.
一方、鋼材の強度を高くすると、引張強度と降伏強度との比である降伏比（降伏強度／引張強度）が上昇する場合が多いが、降伏比が上昇すると、塑性変形が起こる時点（降伏点）から破壊が起こる時点までの応力差が大きくないため、建築物が変形によってエネルギーを吸収して破壊を防止するのが困難になり、地震などの巨大な外力が作用したときに安全性を担保するのが困難であるという問題がある。したがって、構造用鋼材は高強度及び低降伏比をいずれも満たさなければならない。 On the other hand, when the strength of the steel material is increased, the yield ratio (yield strength / yield strength), which is the ratio of the tensile strength to the yield strength, is often increased, but when the yield ratio is increased, the time point at which plastic deformation occurs (yield point). Since the stress difference from) to the time of failure is not large, it becomes difficult for the building to absorb energy due to deformation and prevent destruction, ensuring safety when a huge external force such as an earthquake acts. There is a problem that it is difficult to do. Therefore, structural steels must meet both high strength and low yield ratios.
一般に鋼材の降伏比は、鋼材の金属組織においてフェライト（ｆｅｒｒｉｔｅ）のような軟質相（ｓｏｆｔ ｐｈａｓｅ）を主組織とし、ベイナイト（ｂａｉｎｉｔｅ）やマルテンサイト（ｍａｒｔｅｎｓｉｔｅ）などの硬質相（ｈａｒｄ ｐｈａｓｅ）が適度に分散した組織を実現することにより低くすることが知られている。 Generally, the yield ratio of a steel material is mainly composed of a soft phase such as ferrite in the metal structure of the steel material, and an appropriate hard phase such as bainite or martensite. It is known to lower it by realizing a bainite-distributed organization.
このような軟質相ベースの微細組織に硬質相が適度に分散した組織を得るため、特許文献１には、フェライトとオーステナイト（ａｕｓｔｅｎｉｔｅ）の２相域（ｄｕａｌ ｐｈａｓｅ ｒｅｇｉｏｎ）で適切な焼き入れ（ｑｕｅｎｃｈｉｎｇ）と焼き戻し（ｔｅｍｐｅｒｉｎｇ）とによって降伏比を低くする方法が開示されている。しかし、上記の方法は圧延製造工程以外に熱処理工程数が追加されるため、生産性の低下はもちろん製造単価の増加も不可避であるという問題がある。 In order to obtain a structure in which the hard phase is appropriately dispersed in such a soft phase-based microstructure, Patent Document 1 describes appropriate quenching in a dual phase region of ferrite and austenite. ) And tempering to reduce the yield ratio. However, since the above method adds the number of heat treatment steps in addition to the rolling manufacturing step, there is a problem that not only the productivity is lowered but also the manufacturing unit price is inevitably increased.
したがって、生産性の低下と製造単価の上昇などの問題をすべて解決し、かつ超高強度及び低降伏比が確保される低降伏比型高強度鋼材及びその製造方法の開発が求められている。 Therefore, it is required to develop a low-yield ratio type high-strength steel material and a method for manufacturing the same, which solves all the problems such as a decrease in productivity and an increase in a manufacturing unit price and secures an ultra-high strength and a low yield ratio.
本発明は、上記従来の問題点に鑑みてなされたものであって、本発明の目的は、低降伏比型高強度鋼材及びその製造方法を提供することにある。より詳細には、生産性の低下や製造単価の上昇なしに、超高強度及び低降伏比が確保された低降伏比型高強度鋼材及びその製造方法を提供することにある。 The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a low yield ratio type high-strength steel material and a method for producing the same. More specifically, it is an object of the present invention to provide a low yield ratio type high strength steel material in which ultra high strength and low yield ratio are ensured, and a method for producing the same, without reducing productivity or increasing the production unit price.
上記目的を達成するためになされた本発明の一態様による低降伏比型高強度鋼材は、炭素（Ｃ）を０．０２重量％〜０．１１重量％、シリコン（Ｓｉ）を０．１重量％〜０．５重量％、マンガン（Ｍｎ）を１．５重量％〜２．５重量％、アルミニウム（Ａｌ）を０．０１重量％〜０．０６重量％、ニッケル（Ｎｉ）を０．１重量％〜０．６重量％、チタン（Ｔｉ）を０．０１重量％〜０．０３重量％、ニオブ（Ｎｂ）を０．００５重量％〜０．０８重量％、クロム（Ｃｒ）を０．１重量％〜０．５重量％、リン（Ｐ）を０．０１重量％以下（０重量％は除く）、硫黄（Ｓ）を０．０１重量％以下（０重量％は除く）、ボロン（Ｂ）を５重量ｐｐｍ〜３０重量ｐｐｍ、窒素（Ｎ）を２０重量ｐｐｍ〜７０重量ｐｐｍ、カルシウム（Ｃａ）を５０重量ｐｐｍ以下（０重量ｐｐｍは除く）、スズ（Ｓｎ）を５重量ｐｐｍ〜５０重量ｐｐｍ含み、残りは鉄（Ｆｅ）及びその他の不可避不純物からなることを特徴とする。 The low yield ratio type high-strength steel material according to one aspect of the present invention made to achieve the above object contains 0.02% by weight to 0.11% by weight of carbon (C) and 0.1% by weight of silicon (Si). % To 0.5% by weight, manganese (Mn) 1.5% to 2.5% by weight, aluminum (Al) 0.01% to 0.06% by weight, nickel (Ni) 0.1% Weight% to 0.6% by weight, titanium (Ti) 0.01% to 0.03% by weight, niobium (Nb) 0.005% to 0.08% by weight, chromium (Cr) 0. 1% to 0.5% by weight, phosphorus (P) 0.01% by weight or less (excluding 0% by weight), sulfur (S) 0.01% by weight or less (excluding 0% by weight), boron (excluding 0% by weight) B) is 5 wt ppm to 30 wt ppm, nitrogen (N) is 20 wt ppm to 70 wt ppm, calcium (Ca) is 50 wt ppm or less (excluding 0 wt ppm), tin (Sn) is 5 wt ppm to It is characterized by containing 50 wt ppm and the rest consisting of iron (Fe) and other unavoidable impurities.
上記目的を達成するためになされた本発明の一態様による低降伏比型高強度鋼材の製造方法は、炭素（Ｃ）を０．０２重量％〜０．１１重量％、シリコン（Ｓｉ）を０．１重量％〜０．５重量％、マンガン（Ｍｎ）を１．５重量％〜２．５重量％、アルミニウム（Ａｌ）を０．０１重量％〜０．０６重量％、ニッケル（Ｎｉ）を０．１重量％〜０．６重量％、チタン（Ｔｉ）を０．０１重量％〜０．０３重量％、ニオブ（Ｎｂ）を０．００５重量％〜０．０８重量％、クロム（Ｃｒ）を０．１重量％〜０．５重量％、リン（Ｐ）を０．０１重量％以下（０重量％は除く）、硫黄（Ｓ）を０．０１重量％以下（０重量％は除く）、ボロン（Ｂ）を５重量ｐｐｍ〜３０重量ｐｐｍ、窒素（Ｎ）を２０重量ｐｐｍ〜７０重量ｐｐｍ、カルシウム（Ｃａ）を５０重量ｐｐｍ以下（０重量ｐｐｍは除く）、スズ（Ｓｎ）を５重量ｐｐｍ〜５０重量ｐｐｍ含み、残りは鉄（Ｆｅ）及びその他の不可避不純物からなるスラブを１０５０℃〜１２５０℃に加熱する段階と、前記加熱されたスラブを９５０℃〜１１５０℃で粗圧延してバー（Ｂａｒ）を得る段階と、前記バー（Ｂａｒ）を仕上げ圧延温度７００℃〜９５０℃で熱間圧延して熱延鋼板を得る段階と、前記熱延鋼板を２５℃／ｓ〜５０℃／ｓの冷却速度でＢｓ温度以下の冷却終了温度まで冷却する段階と、を有することを特徴とする。 In the method for producing a low yield ratio type high-strength steel material according to one aspect of the present invention, which has been made to achieve the above object, carbon (C) is 0.02% by weight to 0.11% by weight, and silicon (Si) is 0. .1% by weight to 0.5% by weight, manganese (Mn) from 1.5% by weight to 2.5% by weight, aluminum (Al) from 0.01% by weight to 0.06% by weight, nickel (Ni) 0.1% by weight to 0.6% by weight, titanium (Ti) from 0.01% by weight to 0.03% by weight, niobium (Nb) from 0.005% by weight to 0.08% by weight, chromium (Cr) 0.1% by weight to 0.5% by weight, phosphorus (P) is 0.01% by weight or less (excluding 0% by weight), sulfur (S) is 0.01% by weight or less (excluding 0% by weight) , Boron (B) is 5 wt ppm to 30 wt ppm, Nitrogen (N) is 20 wt ppm to 70 wt ppm, Calcium (Ca) is 50 wt ppm or less (excluding 0 wt ppm), Tin (Sn) is 5. A step of heating a slab containing ppm to 50 wt ppm by weight and the rest consisting of iron (Fe) and other unavoidable impurities to 1050 ° C to 1250 ° C, and a rough rolling of the heated slab at 950 ° C to 1150 ° C. A step of obtaining a bar, a step of hot rolling the bar at a finish rolling temperature of 700 ° C. to 950 ° C. to obtain a hot-rolled steel sheet, and a step of hot-rolling the hot-rolled steel sheet at 25 ° C./s to 50 ° C./ It is characterized by having a step of cooling to a cooling end temperature equal to or lower than the Bs temperature at a cooling rate of s.
本発明によれば、生産性の低下や製造単価の上昇なしに超高強度及び低降伏比が確保された低降伏比型高強度鋼材及びその製造方法を提供することができる。 According to the present invention, it is possible to provide a low yield ratio type high strength steel material in which ultra-high strength and a low yield ratio are ensured without a decrease in productivity or an increase in a production unit price, and a method for producing the same.
以下では、本発明の好ましい実施形態を説明する。しかし、本発明の実施形態は多様に変形実施することが可能であり、本発明の技術範囲は以下で説明する実施形態に限定されない。また、本発明の実施形態は、当該技術分野における通常の知識を有する者に本発明をより完全に説明するために提供されるものである。 Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be variously modified and implemented, and the technical scope of the present invention is not limited to the embodiments described below. Also, embodiments of the present invention are provided to more fully explain the present invention to those who have conventional knowledge in the art.
以下、本発明の一実施形態による低降伏比型高強度鋼材について詳細に説明する。 Hereinafter, a low yield ratio type high-strength steel material according to an embodiment of the present invention will be described in detail.
本発明の一実施形態による低降伏比型高強度鋼材は、炭素（Ｃ）を０．０２重量％〜０．１１重量％、シリコン（Ｓｉ）を０．１重量％〜０．５重量％、マンガン（Ｍｎ）を１．５重量％〜２．５重量％、アルミニウム（Ａｌ）を０．０１重量％〜０．０６重量％、ニッケル（Ｎｉ）を０．１重量％〜０．６重量％、チタン（Ｔｉ）を０．０１重量％〜０．０３重量％、ニオブ（Ｎｂ）を０．００５重量％〜０．０８重量％、クロム（Ｃｒ）を０．１重量％〜０．５重量％、リン（Ｐ）を０．０１重量％以下（０重量％は除く）、硫黄（Ｓ）を０．０１重量％以下（０重量％は除く）、ボロン（Ｂ）を５重量ｐｐｍ〜３０重量ｐｐｍ、窒素（Ｎ）を２０重量ｐｐｍ〜７０重量ｐｐｍ、カルシウム（Ｃａ）を５０重量ｐｐｍ以下（０重量ｐｐｍは除く）、スズ（Ｓｎ）を５重量ｐｐｍ〜５０重量ｐｐｍ含み、残りは鉄（Ｆｅ）及びその他の不可避不純物からなる。 The low yield ratio type high-strength steel material according to one embodiment of the present invention contains 0.02% by weight to 0.11% by weight of carbon (C) and 0.1% by weight to 0.5% by weight of silicon (Si). Manganese (Mn) is 1.5% by weight to 2.5% by weight, aluminum (Al) is 0.01% by weight to 0.06% by weight, and nickel (Ni) is 0.1% by weight to 0.6% by weight. , Titanium (Ti) 0.01% by weight to 0.03% by weight, Niob (Nb) 0.005% by weight to 0.08% by weight, Chrome (Cr) 0.1% by weight to 0.5% by weight. %, Phosphorus (P) in 0.01% by weight or less (excluding 0% by weight), Sulfur (S) in 0.01% by weight or less (excluding 0% by weight), Boron (B) in 5% by weight ppm to 30 It contains weight ppm, nitrogen (N) 20 weight ppm to 70 weight ppm, calcium (Ca) 50 weight ppm or less (excluding 0 weight ppm), tin (Sn) 5 weight ppm to 50 weight ppm, and the rest is iron. Consists of (Fe) and other unavoidable impurities.
炭素（Ｃ）：０．０２重量％〜０．１１重量％
Ｃは、ベイナイト又はマルテンサイトを形成し、このベイナイト又はマルテンサイトの大きさ及び分率を決定する重要な元素である。
Carbon (C): 0.02% by weight to 0.11% by weight
C is an important element that forms bainite or martensite and determines the size and fraction of this bainite or martensite.
Ｃ含有量が０．１１重量％を超えると、低温靱性を低下させ、Ｃ含有量が０．０２重量％未満の場合、ベイナイト又はマルテンサイトの形成を妨げ、強度の低下をもたらす。したがって、Ｃ含有量は０．０２重量％〜０．１１重量％であることが好ましい。 If the C content exceeds 0.11% by weight, the low temperature toughness is lowered, and if the C content is less than 0.02% by weight, the formation of bainite or martensite is hindered, resulting in a decrease in strength. Therefore, the C content is preferably 0.02% by weight to 0.11% by weight.
一方、溶接用鋼構造物として用いられる板材の場合には、より良い溶接性のためにＣ含有量の上限を０．０８重量％とすることが好ましい。 On the other hand, in the case of a plate material used as a steel structure for welding, the upper limit of the C content is preferably 0.08% by weight for better weldability.
シリコン（Ｓｉ）：０．１重量％〜０．５重量％
Ｓｉは、脱酸剤として用いられ、強度及び靱性を向上させる元素である。
Silicon (Si): 0.1% by weight to 0.5% by weight
Si is an element that is used as a deoxidizer and improves strength and toughness.
Ｓｉ含有量が０．５重量％を超えると、低温靱性及び溶接性が低下するのみならず、板材の表面にスケールが厚く形成され、ガス切断性不良及びその他の表面クラックなどを誘発する可能性がある。これに対し、Ｓｉ含有量が０．１重量％未満の場合、脱酸効果が十分でない。したがって、Ｓｉ含有量は０．１重量％〜０．５重量％である。より好ましくは０．１５重量％〜０．３５重量％である。 If the Si content exceeds 0.5% by weight, not only the low temperature toughness and weldability are deteriorated, but also a thick scale is formed on the surface of the plate material, which may induce poor gas cutting property and other surface cracks. There is. On the other hand, when the Si content is less than 0.1% by weight, the deoxidizing effect is not sufficient. Therefore, the Si content is 0.1% by weight to 0.5% by weight. More preferably, it is 0.15% by weight to 0.35% by weight.
マンガン（Ｍｎ）：１．５重量％〜２．５重量％
Ｍｎは、固溶強化によって強度を向上させる有用な元素であるため、１．５重量％以上添加される必要がある。しかし、Ｍｎ含有量が２．５重量％を超えると、過度な硬化能の増加によって溶接部の靱性が大きく低下する。したがって、Ｍｎの含有量は１．５重量％〜２．５重量％であることが好ましい。
Manganese (Mn): 1.5% by weight to 2.5% by weight
Since Mn is a useful element for improving the strength by strengthening the solid solution, it is necessary to add 1.5% by weight or more. However, when the Mn content exceeds 2.5% by weight, the toughness of the welded portion is greatly reduced due to an excessive increase in hardening ability. Therefore, the Mn content is preferably 1.5% by weight to 2.5% by weight.
アルミニウム（Ａｌ）：０．０１重量％〜０．０６重量％
Ａｌは、溶鋼を安価に脱酸することができ、また、フェライトを安定化する元素である。Ａｌ含有量が０．０１重量％未満の場合、上述の効果が十分でない。これに対し、Ａｌ含有量が０．０６重量％を超えると、連続鋳造時にノズル詰まりが発生する。したがって、Ａｌ含有量は０．０１重量％〜０．０６重量％であることが好ましい。
Aluminum (Al): 0.01% by weight to 0.06% by weight
Al is an element that can inexpensively deoxidize molten steel and stabilize ferrite. If the Al content is less than 0.01% by weight, the above effects are not sufficient. On the other hand, if the Al content exceeds 0.06% by weight, nozzle clogging occurs during continuous casting. Therefore, the Al content is preferably 0.01% by weight to 0.06% by weight.
ニッケル（Ｎｉ）：０．１重量％〜０．６重量％
Ｎｉは、母材の強度と靱性を同時に向上させる元素である。上述の効果を十分に奏するためには０．１重量％以上添加することが好ましい。しかし、Ｎｉは高価な元素であるため、添加量が０．６重量％を超えると、経済性が低下し、また溶接性が低下する。したがって、Ｎｉ含有量は０．１重量％〜０．６重量％であることが好ましい。
Nickel (Ni): 0.1% by weight to 0.6% by weight
Ni is an element that simultaneously improves the strength and toughness of the base metal. In order to sufficiently exert the above-mentioned effects, it is preferable to add 0.1% by weight or more. However, since Ni is an expensive element, if the addition amount exceeds 0.6% by weight, the economic efficiency is lowered and the weldability is lowered. Therefore, the Ni content is preferably 0.1% by weight to 0.6% by weight.
チタン（Ｔｉ）：０．０１重量％〜０．０３重量％
Ｔｉは、再加熱時の結晶粒の成長を抑制し、低温靱性を大きく向上させるため、０．０１重量％以上添加することが好ましい。しかし、Ｔｉ含有量が０．０３重量％を超えると、連続鋳造ノズルの詰まりや中心部の晶出による低温靱性の減少などの問題を発生させる。したがって、Ｔｉ含有量は０．０１重量％〜０．０３重量％であることが好ましい。
Titanium (Ti): 0.01% by weight to 0.03% by weight
Ti is preferably added in an amount of 0.01% by weight or more in order to suppress the growth of crystal grains during reheating and greatly improve the low temperature toughness. However, if the Ti content exceeds 0.03% by weight, problems such as clogging of the continuous casting nozzle and reduction of low temperature toughness due to crystallization of the central portion occur. Therefore, the Ti content is preferably 0.01% by weight to 0.03% by weight.
ニオブ（Ｎｂ）：０．００５重量％〜０．０８重量％
Ｎｂは、ＴＭＣＰ鋼の製造において重要な元素であり、ＮｂＣ又はＮｂＣＮの形で析出し、母材及び溶接部の強度を大きく向上させる。また、高温に再加熱される時、固溶したＮｂはオーステナイトの再結晶及びフェライト又はベイナイトの変態を抑制して組織が微細化する効果を奏する。さらに、粗圧延後スラブが冷却される時、低い冷却速度でもベイナイトを形成させるのみならず、最終圧延後の冷却時にもオーステナイトの安定性を高め、低速の冷却でもマルテンサイトの生成を促進させる役割も果たす。
Niobium (Nb): 0.005% by weight to 0.08% by weight
Nb is an important element in the production of TMCP steel and precipitates in the form of NbC or NbCN, which greatly improves the strength of the base metal and the welded portion. Further, when reheated to a high temperature, the solid-solved Nb has the effect of suppressing the recrystallization of austenite and the transformation of ferrite or bainite to make the structure finer. Furthermore, when the slab is cooled after rough rolling, it not only forms bainite even at a low cooling rate, but also enhances the stability of austenite during cooling after final rolling and promotes the formation of martensite even at low speed cooling. Also fulfills.
上述の効果を十分に得るためにはＮｂ含有量が０．００５重量％以上であることが好ましい。しかし、Ｎｂ含有量が０．０８重量％を超えると、鋼材のエッジに脆性クラックが発生する。したがって、Ｎｂ含有量は０．００５重量％〜０．０８重量％であることが好ましい。 In order to obtain the above-mentioned effects sufficiently, the Nb content is preferably 0.005% by weight or more. However, if the Nb content exceeds 0.08% by weight, brittle cracks occur at the edges of the steel material. Therefore, the Nb content is preferably 0.005% by weight to 0.08% by weight.
クロム（Ｃｒ）：０．１重量％〜０．５重量％
Ｃｒは、強度を確保するために添加される元素であり、焼き入れ性を増加させる役割も果たす。上述の効果を十分に得るためには０．１重量％以上添加する必要がある。しかし、Ｃｒ含有量が０．５重量％を超えると、溶接部の硬度を過度に増加させ、靱性を阻害する。したがって、Ｃｒ含有量は０．１重量％〜０．５重量％であることが好ましい。
Chromium (Cr): 0.1% by weight to 0.5% by weight
Cr is an element added to ensure strength and also plays a role in increasing hardenability. In order to obtain the above-mentioned effects sufficiently, it is necessary to add 0.1% by weight or more. However, when the Cr content exceeds 0.5% by weight, the hardness of the welded portion is excessively increased and the toughness is impaired. Therefore, the Cr content is preferably 0.1% by weight to 0.5% by weight.
リン（Ｐ）：０．０１重量％以下
Ｐは、強度向上及び耐食性に有利な元素であるが、衝撃靱性を大きく阻害するため、できる限り低く維持するのがよい。したがって、その上限を０．０１重量％とすることが好ましい。
Phosphorus (P): 0.01% by weight or less P is an element that is advantageous for strength improvement and corrosion resistance, but it should be kept as low as possible because it greatly inhibits impact toughness. Therefore, the upper limit is preferably 0.01% by weight.
硫黄（Ｓ）：０．０１重量％以下
Ｓは、ＭｎＳなどを形成して衝撃靱性を大きく阻害する元素であるため、できる限り低く維持するのがよい。したがって、その上限を０．０１重量％とすることが好ましい。
Sulfur (S): 0.01% by weight or less Since S is an element that forms MnS and the like and greatly inhibits impact toughness, it is preferable to keep it as low as possible. Therefore, the upper limit is preferably 0.01% by weight.
ボロン（Ｂ）：５重量ｐｐｍ〜３０重量ｐｐｍ
Ｂは、非常に安価な添加元素であり、強力な硬化能を示し、粗圧延後の冷却において低速冷却でもベイナイトの形成に大きく寄与する有益な元素である。
Boron (B): 5 wt ppm to 30 wt ppm
B is a very inexpensive additive element, exhibits a strong curing ability, and is a beneficial element that greatly contributes to the formation of bainite even at low speed cooling in cooling after rough rolling.
少量の添加のみでも強度を大きく向上させることができるため、５重量ｐｐｍ以上添加する。しかし、Ｂ含有量が３０重量ｐｐｍを超えると、Ｆｅ２３（ＣＢ）６を形成し、逆に硬化能を低下させ、低温靱性も大きく低下させる。したがって、Ｂ含有量は５重量ｐｐｍ〜３０重量ｐｐｍであることが好ましい。 Since the strength can be greatly improved by adding only a small amount, 5 ppm by weight or more is added. However, when the B content exceeds 30 ppm by weight, Fe 23 (CB) 6 is formed, and conversely, the curability is lowered and the low temperature toughness is also greatly lowered. Therefore, the B content is preferably 5 ppm by weight to 30 ppm by weight.
窒素（Ｎ）：２０重量ｐｐｍ〜７０重量ｐｐｍ
Ｎは、強度を増加させるが、靱性を大きく減少させるため、７０重量ｐｐｍ以下に制御することが好ましい。但し、Ｎ含有量を２０重量ｐｐｍ未満に制御することは製鋼負荷を増加させるため、Ｎ含有量の下限は２０重量ｐｐｍであることが好ましい。
Nitrogen (N): 20 wt ppm to 70 wt ppm
N increases the strength but greatly reduces the toughness, so it is preferably controlled to 70 wt ppm or less. However, controlling the N content to less than 20 ppm by weight increases the steelmaking load, so the lower limit of the N content is preferably 20 ppm by weight.
カルシウム（Ｃａ）：５０重量ｐｐｍ以下（０重量ｐｐｍは除く）
Ｃａは、主にＭｎＳの非金属介在物を抑制し、低温靱性を向上させる元素として用いられる。しかし、Ｃａを過剰に添加すると、鋼中に含有された酸素と反応し、非金属介在物であるＣａＯを生成するため、その上限値は５０重量ｐｐｍであることが好ましい。
Calcium (Ca): 50 ppm by weight or less (excluding 0 ppm by weight)
Ca is mainly used as an element that suppresses non-metal inclusions of MnS and improves low temperature toughness. However, when Ca is added in an excessive amount, it reacts with oxygen contained in the steel to form CaO which is a non-metal inclusion, so that the upper limit is preferably 50 ppm by weight.
スズ（Ｓｎ）：５重量ｐｐｍ〜５０重量ｐｐｍ
Ｓｎは、耐食性を確保するのに有用な元素である。
Tin (Sn): 5% by weight ppm to 50% by weight ppm
Sn is an element useful for ensuring corrosion resistance.
耐食性確保の面で５ｐｐｍ以上添加することが好ましい。しかし、Ｓｎ含有量が５０ｐｐｍ重量％を超えると、耐食性向上に対する寄与効果よりも鋼材の表面に水泡のようにスケールが膨れたり割れたりする形の欠陥が多量に発生する。また、Ｓｎは鋼の強度を増加させるが、延伸率と低温衝撃靱性を低下させるため、その上限は５０重量ｐｐｍであることが好ましい。 It is preferable to add 5 ppm or more from the viewpoint of ensuring corrosion resistance. However, when the Sn content exceeds 50 ppm by weight, a large amount of defects in the form of scale swelling or cracking like blisters occur on the surface of the steel material rather than the effect of contributing to the improvement of corrosion resistance. Further, Sn increases the strength of the steel, but lowers the draw ratio and the low temperature impact toughness, so that the upper limit thereof is preferably 50 ppm by weight.
本発明の低降伏比型高強度鋼材において、残りの成分は鉄（Ｆｅ）である。但し、通常の製造工程では原料又は周囲環境から意図しない不純物が不可避的に混入し、これを排除することはできない。これらの不純物は、当該技術分野における通常の知識を有する技術者であれば容易に理解されるものであるため、本明細書ではその全ての内容を詳細には説明しない。 In the low yield ratio type high strength steel material of the present invention, the remaining component is iron (Fe). However, in a normal manufacturing process, unintended impurities are inevitably mixed from the raw material or the surrounding environment, and this cannot be eliminated. Since these impurities are easily understood by an engineer having ordinary knowledge in the technical field, all the contents thereof are not described in detail in this specification.
本発明による有利な鋼組成を有する低降伏比型高強度鋼材は、上述した含有量範囲の合金元素を含むだけでも十分な効果が得られるが、０．１重量％〜０．５重量％の銅（Ｃｕ）、０．１５重量％〜０．３重量％のモリブデン（Ｍｏ）、及び０．００５重量％〜０．３重量％のバナジウム（Ｖ）のうちの一つ以上をさらに含むことにより鋼材の強度、靱性、溶接熱影響部の靱性、溶接性などの特性をより向上させることができる。 The low yield ratio type high-strength steel material having an advantageous steel composition according to the present invention can obtain a sufficient effect even if it contains an alloy element in the above-mentioned content range, but it is 0.1% by weight to 0.5% by weight. By further comprising one or more of copper (Cu), 0.15% to 0.3% by weight molybdenum (Mo), and 0.005% to 0.3% by weight vanadium (V). It is possible to further improve characteristics such as strength and toughness of steel materials, toughness of weld heat affected parts, and weldability.
銅（Ｃｕ）：０．１重量％〜０．５重量％
Ｃｕは、母材の靱性低下を最小化させるとともに強度を高める元素である。上述の効果を十分に得るためには０．１重量％以上添加することが好ましい。しかし、Ｃｕ含有量が０．５重量％を超えると、製品の表面品質を大きく阻害する。したがって、Ｃｕ含有量は０．１重量％〜０．５重量％であることが好ましい。
Copper (Cu): 0.1% by weight to 0.5% by weight
Cu is an element that minimizes the decrease in toughness of the base metal and increases the strength. In order to obtain the above-mentioned effects sufficiently, it is preferable to add 0.1% by weight or more. However, if the Cu content exceeds 0.5% by weight, the surface quality of the product is greatly impaired. Therefore, the Cu content is preferably 0.1% by weight to 0.5% by weight.
モリブデン（Ｍｏ）：０．１５重量％〜０．３重量％
Ｍｏは、少量の添加のみでも硬化能を大きく向上させる効果があり、強度を大きく向上させるため、０．１５重量％以上添加する必要があるが、０．３重量％を超えて添加すると、溶接部の硬度を過度に増加させ、靱性を阻害する。したがって、Ｍｏ含有量は０．１５重量％〜０．３重量％であることが好ましい。
Molybdenum (Mo): 0.15% by weight to 0.3% by weight
Mo has the effect of greatly improving the curing ability even with a small amount of addition, and in order to greatly improve the strength, it is necessary to add 0.15% by weight or more, but if it is added in excess of 0.3% by weight, welding It excessively increases the hardness of the part and inhibits toughness. Therefore, the Mo content is preferably 0.15% by weight to 0.3% by weight.
バナジウム（Ｖ）：０．００５重量％〜０．３重量％
Ｖは、他の微細合金に比べて固溶する温度が低く、溶接熱影響部に析出して強度の低下を防止する効果がある。上述の効果を十分に得るためには０．００５重量％以上添加することが好ましい。しかし、Ｖ含有量が０．３重量％を超えると、逆に靱性を低下させる。したがって、Ｖ含有量は０．００５重量％〜０．３重量％であることが好ましい。
Vanadium (V): 0.005% by weight to 0.3% by weight
V has a lower solid solution temperature than other fine alloys, and has an effect of preventing a decrease in strength by precipitating in a heat-affected zone of welding. In order to obtain the above-mentioned effects sufficiently, it is preferable to add 0.005% by weight or more. However, when the V content exceeds 0.3% by weight, the toughness is conversely lowered. Therefore, the V content is preferably 0.005% by weight to 0.3% by weight.
また、本発明の低降伏比型高強度鋼材の微細組織は、ベイニティックフェライト及びグラニュラーベイナイトを主相として含み、Ｍ−Ａ（島状マルテンサイト）を二次相として含む。 Further, the microstructure of the low yield ratio type high-strength steel material of the present invention contains bainitic ferrite and granular bainite as the main phase and MA (island martensite) as the secondary phase.
ベイニティックフェライトは、初期オーステナイト結晶粒界を維持しながら粒内に多くの高傾角粒界を含んでいるため、結晶粒の微細化の効果による強度と衝撃靱性の向上に有用である。 Since bainitic ferrite contains many highly inclined grain boundaries in the grains while maintaining the initial austenite grain boundaries, it is useful for improving the strength and impact toughness due to the effect of grain refinement.
グラニュラーベイナイトは、ベイニティックフェライトと同様に初期オーステナイト結晶粒を維持しているが、粒内又は粒界にＭ−Ａのような二次相が存在する。粒内に高傾角粒界が存在しておらず、衝撃靱性に多少不利な影響を及ぼすが、粒内転位のような低傾角粒界が多量に存在することにより強度は多少増加する。 Granular bainite retains the initial austenite grains, similar to bainite ferrite, but has a secondary phase such as MA in the grains or at the grain boundaries. Highly inclined grain boundaries do not exist in the grains, which has a somewhat unfavorable effect on impact toughness, but the presence of a large amount of lowly inclined grain boundaries such as intragranular dislocations increases the strength to some extent.
ベイニティックフェライト及びグラニュラーベイナイトを主相として含むことにより低降伏比と高強度を確保することができる。 By including bainitic ferrite and granular bainite as the main phase, a low yield ratio and high strength can be ensured.
この際、面積分率で、ベイニティックフェライトは８０％〜９５％であり、グラニュラーベイナイトは５％〜２０％であり、Ｍ−Ａは３％以下（０％を含む）である。 At this time, in terms of surface integral, bainitic ferrite is 80% to 95%, granular bainite is 5% to 20%, and MA is 3% or less (including 0%).
ベイニティックフェライトの面積分率が８０％未満の場合、高い引張強度を確保するのが困難であり、９５％を超えると、降伏比が増加するという問題がある。 When the surface integral of bainitic ferrite is less than 80%, it is difficult to secure a high tensile strength, and when it exceeds 95%, there is a problem that the yield ratio increases.
グラニュラーベイナイトの面積分率が５％未満の場合、引張強度のみならず降伏強度も増加して、低い降伏比を確保することができず、２０％を超えると、粗大な初期オーステナイト結晶粒を効果的に微細化させることができず、引張強度が劣る。 When the area fraction of granular bainite is less than 5%, not only the tensile strength but also the yield strength increases, and a low yield ratio cannot be secured. When it exceeds 20%, coarse initial austenite grains are effective. It cannot be finely divided and its tensile strength is inferior.
Ｍ−Ａのような二次相は、低降伏比の実現に有用な微細組織として３％以下の面積分率を有することが好ましい。Ｍ−Ａの面積分率が３％を超えると、降伏比は減少するが、相対的に外部応力に対するクラック（ｃｒａｃｋ）の起点として作用するため、引張強度を高く確保するのが困難になる。 A secondary phase such as MA preferably has a surface integral of 3% or less as a microstructure useful for achieving a low yield ratio. When the surface integral of MA exceeds 3%, the yield ratio decreases, but it acts as a starting point of cracks relative to external stress, so that it becomes difficult to secure a high tensile strength.
一方、本発明による低降伏比型高強度鋼材は、ＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）が１．０以上１．８以下である。ＰＩｍａｘ．（１１１）はＸ線回折又は電子後方散乱回折などの方法で得られた（１１１）結晶面の極点強度（ｐｏｌｅ ｉｎｔｅｎｓｉｔｙ、ＰＩｍａｘ．）であり、ＰＩｍａｘ．（１００）は（１００）結晶面の極点強度である。 On the other hand, the low yield ratio type high-strength steel material according to the present invention has PImax. (111) / PImax. (100) is 1.0 or more and 1.8 or less. PImax. (111) is the pole intensity (PImax.) Of the (111) crystal plane obtained by a method such as X-ray diffraction or electron backscatter diffraction. (100) is the extreme point strength of the (100) crystal plane.
結晶面の極点強度は、本発明の一実施形態による低降伏比型高強度鋼材の最終微細組織によって決定される。ベイニティックフェライト及びグラニュラーベイナイトを主相とする場合、ベイニティックフェライトの分率が高いほどＰＩｍａｘ．（１１１）の値が大きくなり、グラニュラーベイナイトの分率が高いほどＰＩｍａｘ．（１００）の値が大きくなる。本実施形態による低降伏比型高強度鋼材の最終微細組織は、ベイニティックフェライトがグラニュラーベイナイトよりも面積分率が高くＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）が１．８以下の場合、低降伏比型高強度鋼材の製造が可能である。ＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）が１．８を超えると、低降伏比を満たすことができないため、その上限値を１．８以下とすることが好ましい。より好ましいＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）は１．６以下である。 The extreme point strength of the crystal plane is determined by the final microstructure of the low yield ratio type high strength steel material according to the embodiment of the present invention. When bainitic ferrite and granular bainite are the main phases, the higher the fraction of bainitic ferrite, the more PImax. The larger the value of (111) and the higher the fraction of granular bainite, the more PImax. The value of (100) becomes large. In the final microstructure of the low yield ratio type high-strength steel material according to the present embodiment, bainitic ferrite has a higher area fraction than granular bainite, and PImax. (111) / PImax. When (100) is 1.8 or less, a low yield ratio type high-strength steel material can be manufactured. PImax. (111) / PImax. If (100) exceeds 1.8, the low yield ratio cannot be satisfied, so the upper limit thereof is preferably 1.8 or less. More preferable PImax. (111) / PImax. (100) is 1.6 or less.
ＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）が１．０未満の場合にはグラニュラーベイナイトの分率が２０％超と高くなり、高強度を確保するのが困難であるという問題がある。したがって、ＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）の下限値は１．０以上とすることが好ましく、より好ましい下限値は１．２以上である。 PImax. (111) / PImax. When (100) is less than 1.0, the fraction of granular bainite becomes as high as more than 20%, and there is a problem that it is difficult to secure high strength. Therefore, PImax. (111) / PImax. The lower limit of (100) is preferably 1.0 or more, and a more preferable lower limit is 1.2 or more.
本発明による低降伏比型高強度鋼材は、降伏比が０．８５以下であり、引張強度８００ＭＰａ以上を確保することで、建設用鋼材などとして好適に用いることができる。 The low yield ratio type high-strength steel material according to the present invention can be suitably used as a construction steel material or the like by ensuring a yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.
また、本発明による鋼材の厚さは６０ｍｍ以下である。 Further, the thickness of the steel material according to the present invention is 60 mm or less.
本発明による低降伏比型高強度鋼材は、高強度及び低降伏比を確保することができ、板厚を６０ｍｍ以下と薄くすることができるため、切断や穿孔などの機械加工と溶接作業が容易になる。したがって、鋼材の厚さは６０ｍｍ以下であることが好ましい。より好ましくは４０ｍｍ以下、さらに好ましくは３０ｍｍ以下である。 The low-yield ratio type high-strength steel material according to the present invention can secure high strength and low yield ratio, and can reduce the plate thickness to 60 mm or less, so that machining such as cutting and drilling and welding work are easy. become. Therefore, the thickness of the steel material is preferably 60 mm or less. It is more preferably 40 mm or less, still more preferably 30 mm or less.
下限は特に限定する必要はないが、建設構造用鋼材として用いるためには１５ｍｍ以上であればよい。 The lower limit is not particularly limited, but it may be 15 mm or more for use as a steel material for construction structure.
以下、本発明の一実施形態による低降伏比型高強度鋼材の製造方法について詳細に説明する。 Hereinafter, a method for producing a low yield ratio type high-strength steel material according to an embodiment of the present invention will be described in detail.
本発明の一実施形態による低降伏比型高強度鋼材の製造方法は、上述の合金組成を有するスラブを１０５０℃〜１２５０℃に加熱する段階と、加熱されたスラブを９５０℃〜１１５０℃で粗圧延してバー（Ｂａｒ）を得る段階と、バー（Ｂａｒ）を仕上げ圧延温度７００℃〜９５０℃で熱間圧延して熱延鋼板を得る段階と、熱延鋼板を２５℃／ｓ〜５０℃／ｓの冷却速度でＢｓ温度以下の冷却終了温度まで冷却する段階と、を有する。 The method for producing a low-yield ratio type high-strength steel material according to one embodiment of the present invention includes a step of heating a slab having the above-mentioned alloy composition to 1050 ° C to 1250 ° C and a roughing of the heated slab at 950 ° C to 1150 ° C. The stage of rolling to obtain a bar, the stage of hot rolling the bar at a finishing rolling temperature of 700 ° C to 950 ° C to obtain a hot-rolled steel sheet, and the stage of hot-rolling the hot-rolled steel sheet at 25 ° C./s to 50 ° C. It has a step of cooling to a cooling end temperature of Bs temperature or less at a cooling rate of / s.
＜スラブ加熱段階＞
上述の合金組成を有するスラブを１０５０℃〜１２５０℃に加熱する。
<Slab heating stage>
The slab having the above alloy composition is heated to 1050 ° C to 1250 ° C.
＜粗圧延段階＞
加熱されたスラブを９５０℃〜１０５０℃で粗圧延してバー（Ｂａｒ）を得る。
<Rough rolling stage>
The heated slab is roughly rolled at 950 ° C to 1050 ° C to obtain a bar.
粗圧延温度が９５０℃未満の場合、再結晶が起こらない状態でオーステナイトが変形するため、粒子が粗大化し、１０５０℃を超えると、再結晶が起こると同時に粒子が成長し、同様に、オーステナイト粒子が粗大になる。 When the rough rolling temperature is less than 950 ° C, the austenite is deformed without recrystallization, so that the particles become coarse, and when the temperature exceeds 1050 ° C, the particles grow at the same time as the recrystallization occurs, and similarly, the austenite particles Becomes coarse.
＜熱間圧延段階＞
バー（Ｂａｒ）を仕上げ圧延温度７００℃〜９５０℃で熱間圧延して熱延鋼板を得る。
<Hot rolling stage>
The bar is hot-rolled at a finish rolling temperature of 700 ° C. to 950 ° C. to obtain a hot-rolled steel sheet.
仕上げ圧延温度が７００℃未満の場合、板材の温度が低く、圧延機に負荷が発生し、最終厚さまで圧延を行うことができず、９５０℃を超えると、圧延中に再結晶が起こる。 If the finish rolling temperature is less than 700 ° C., the temperature of the plate material is low, a load is generated on the rolling mill, and rolling cannot be performed to the final thickness. If the temperature exceeds 950 ° C., recrystallization occurs during rolling.
この際、熱間圧延の圧下率は５０％〜８０％であればよい。 At this time, the rolling reduction ratio of hot rolling may be 50% to 80%.
仕上げ圧延の圧下率が５０％未満の場合、圧延中に素材に作用する荷重が増加して、設備事故の危険があり、８０％を超えると、圧延パス数が増加して、圧延終了温度まで最終厚さを確保することができない。 If the rolling reduction is less than 50%, the load acting on the material during rolling increases and there is a risk of equipment accidents. If it exceeds 80%, the number of rolling passes increases and the rolling end temperature is reached. The final thickness cannot be secured.
＜冷却段階＞
熱延鋼板を２５℃／ｓ〜５０℃／ｓの冷却速度でＢｓ温度以下の冷却終了温度まで冷却する。
<Cooling stage>
The hot-rolled steel sheet is cooled to a cooling end temperature equal to or lower than the Bs temperature at a cooling rate of 25 ° C./s to 50 ° C./s.
熱延鋼板の冷却がＢｓ温度を超える温度で終了すると、ベイニティックフェライト及びグラニュラーベイナイトが十分に相変態することができず、強度を確保することができない。冷却速度の場合、板材の厚さによって物理的な制約があるが、２５℃／ｓ未満の冷却速度では軟質のフェライトが生成されることにより引張強度８００ＭＰａ以上を満たすのが困難である。また、５０℃／ｓを超える冷却速度では低温変態組織であるマルテンサイトが生成される確率が高くなるにつれ、引張強度のみならず降伏強度も増加し、降伏比０．８５以下を満たすのが困難である。 When the cooling of the hot-rolled steel sheet is completed at a temperature exceeding the Bs temperature, the bainitic ferrite and the granular bainite cannot be sufficiently phase-transformed, and the strength cannot be ensured. In the case of the cooling rate, there are physical restrictions depending on the thickness of the plate material, but at a cooling rate of less than 25 ° C./s, it is difficult to satisfy the tensile strength of 800 MPa or more due to the formation of soft ferrite. Further, at a cooling rate exceeding 50 ° C./s, as the probability that martensite, which is a low temperature transformation structure, is formed increases, not only the tensile strength but also the yield strength increases, and it is difficult to satisfy the yield ratio of 0.85 or less. Is.
以下、実施例を挙げて本発明をより具体的に説明する。但し、下記の実施例は本発明の一例を具体的に示すものに過ぎず、本発明の技術範囲を限定するものではない。 Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are merely specific examples of the present invention, and do not limit the technical scope of the present invention.
下記の表１に示す成分系を満たすスラブを１１６０℃に加熱し、１０００℃で粗圧延した後、表２に示す製造条件に合うように熱間圧延及び冷却して鋼材を得た。この鋼材の降伏強度、引張強度、降伏比、及び微細組織を測定して表３に示す。 A slab satisfying the component system shown in Table 1 below was heated to 1160 ° C., roughly rolled at 1000 ° C., and then hot-rolled and cooled so as to meet the production conditions shown in Table 2 to obtain a steel material. The yield strength, tensile strength, yield ratio, and microstructure of this steel material are measured and shown in Table 3.
また、鋼材の（１００）結晶面、（１１０）結晶面の極点強度を測定し、ＰＩｍａｘ．（１１１）／ＰＩｍａｘ．（１００）値を表３に示す。 Further, the pole strengths of the (100) crystal plane and the (110) crystal plane of the steel material were measured, and PImax. (111) / PImax. (100) The values are shown in Table 3.
降伏強度及び引張強度は万能引張実験機を用いて測定した。 Yield strength and tensile strength were measured using a universal tensile tester.
微細組織は、鋼材を鏡面研磨して、化学的に腐食させた後、光学顕微鏡で観察した。 The microstructure was observed with an optical microscope after the steel material was mirror-polished and chemically corroded.
極点強度及び集合組織強度は、Ｘ線回折器及び電子後方散乱回折器を用いて測定した。 Extreme point strength and texture strength were measured using an X-ray diffractometer and an electron backscatter diffractometer.
表１において各元素含有量の単位は重量％である。 In Table 1, the unit of each element content is% by weight.
上記の表３において、ＢＦはベイニティックフェライト、ＧＢはグラニュラーベイナイト、ＭＡは島状マルテンサイト、ＡＦはアシキュラーフェライト、Ｂはベイナイトを意味し、単位は面積％である。 In Table 3 above, BF means bainite ferrite, GB means granular bainite, MA means island-like martensite, AF means acicular ferrite, B means bainite, and the unit is area%.
本発明の合金組成及び製造条件を満たす発明例１〜９は、０．８５以下の低降伏比及び８００ＭＰａ以上の引張強度を確保することが分かる。 It can be seen that Invention Examples 1 to 9 satisfying the alloy composition and production conditions of the present invention secure a low yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.
これに対し、比較例１〜３は、本発明の合金組成は満たしているが、製造条件を満たしておらず、低降伏比を確保することができなかったり引張強度が劣ったりすることが確認できる。 On the other hand, it was confirmed that Comparative Examples 1 to 3 satisfy the alloy composition of the present invention but do not satisfy the production conditions, and the low yield ratio cannot be ensured or the tensile strength is inferior. it can.
また、比較例４、７、及び８は、本発明の製造条件は満たしているが、合金組成を満たしておらず、低降伏比を確保することができないことが確認できる。 Further, it can be confirmed that Comparative Examples 4, 7, and 8 satisfy the production conditions of the present invention, but do not satisfy the alloy composition, and the low yield ratio cannot be ensured.
以上、実施例を参照しながら説明したが、当該技術分野に熟練した当業者であれば本発明の技術範囲から逸脱しない範囲内で本発明を多様に変形実施することができる。 Although the above description has been made with reference to Examples, a person skilled in the art can carry out various modifications of the present invention within a range that does not deviate from the technical scope of the present invention.
Claims (7)
炭素（Ｃ）を０．０２重量％〜０．１１重量％、シリコン（Ｓｉ）を０．１重量％〜０．５重量％、マンガン（Ｍｎ）を１．５重量％〜２．５重量％、アルミニウム（Ａｌ）を０．０１重量％〜０．０６重量％、ニッケル（Ｎｉ）を０．１重量％〜０．６重量％、チタン（Ｔｉ）を０．０１重量％〜０．０３重量％、ニオブ（Ｎｂ）を０．００５重量％〜０．０８重量％、クロム（Ｃｒ）を０．１重量％〜０．５重量％、リン（Ｐ）を０．０１重量％以下（０重量％は除く）、硫黄（Ｓ）を０．０１重量％以下、ボロン（Ｂ）を５重量ｐｐｍ〜３０重量ｐｐｍ、窒素（Ｎ）を２０重量ｐｐｍ〜７０重量ｐｐｍ、カルシウム（Ｃａ）を５０重量ｐｐｍ以下（０重量ｐｐｍは除く）、スズ（Ｓｎ）を５重量ｐｐｍ〜５０重量ｐｐｍ含み、残りは鉄（Ｆｅ）及びその他の不可避不純物からなり、
微細組織として、ベイニティックフェライト及びグラニュラーベイナイトを主相として含み、
前記ベイニティックフェライトは８０面積％〜９５面積％であり、前記グラニュラーベイナイトは５面積％〜２０面積％であり、Ｍ−Ａ（島状マルテンサイト）は３面積％以下（０面積％を含む）であり、
降伏比が０．８５以下であり、引張強度が８００ＭＰａ以上であることを特徴とする低降伏比型高強度鋼材。 Low yield ratio type high strength steel material
Carbon (C) is 0.02% by weight to 0.11% by weight, silicon (Si) is 0.1% by weight to 0.5% by weight, and manganese (Mn) is 1.5% by weight to 2.5% by weight. , Aluminum (Al) 0.01% to 0.06% by weight, Nickel (Ni) 0.1% to 0.6% by weight, Titanium (Ti) 0.01% to 0.03% by weight %, Niob (Nb) 0.005% by weight to 0.08% by weight, Chromium (Cr) 0.1% to 0.5% by weight, Phosphorus (P) 0.01% by weight or less (0% by weight) %), Sulfur (S) in 0.01% by weight or less, Boron (B) in 5% by weight to 30% by weight, Nitrogen (N) in 20% by weight to 70% by weight, Calcium (Ca) in 50% by weight. It contains less than ppm (excluding 0 ppm by weight), 5 wt ppm to 50 wt ppm of tin (Sn), and the rest consists of iron (Fe) and other unavoidable impurities.
As a microstructure, bainitic ferrite and granular bainite are contained as main phases, and
The bainitic ferrite is 80 area% to 95 area%, the granular bainite is 5 area% to 20 area%, and MA (island martensite) is 3 area% or less (including 0 area%). ) der is,
A low yield ratio type high strength steel material having a yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.
（ここで、前記ＰＩｍａｘ．（１１１）は（１１１）結晶面の極点強度であり、前記ＰＩｍａｘ．（１００）は（１００）結晶面の極点強度である。） PImax. Is the ratio of the pole intensity (PImax.) Of the (100) crystal plane and the (111) crystal plane of the low yield ratio type high-strength steel material. (111) / PImax. The low yield ratio type high-strength steel material according to claim 1, wherein (100) is 1.0 or more and 1.8 or less.
(Here, the PImax. (111) is the pole strength of the (111) crystal plane, and the PImax. (100) is the pole strength of the (100) crystal plane.)
前記加熱されたスラブを９５０℃〜１０５０℃で粗圧延してバー（Ｂａｒ）を得る段階と、前記バー（Ｂａｒ）を仕上げ圧延温度７００℃〜９５０℃で熱間圧延して熱延鋼板を得る段階と、
前記熱延鋼板を２５℃／ｓ〜５０℃／ｓの冷却速度でＢｓ温度以下の冷却終了温度まで冷却する段階と、を有し、
前記熱延鋼板は、微細組織として、ベイニティックフェライト及びグラニュラーベイナイトを主相として含み、
前記ベイニティックフェライトは８０面積％〜９５面積％であり、前記グラニュラーベイナイトは５面積％〜２０面積％であり、Ｍ−Ａ（島状マルテンサイト）は３面積％以下（０面積％を含む）であり、
前記熱延鋼板は、降伏比が０．８５以下であり、引張強度が８００ＭＰａ以上であることを特徴とする低降伏比型高強度鋼材の製造方法。 Carbon (C) is 0.02% by weight to 0.11% by weight, silicon (Si) is 0.1% by weight to 0.5% by weight, and manganese (Mn) is 1.5% by weight to 2.5% by weight. , Aluminum (Al) 0.01% to 0.06% by weight, Nickel (Ni) 0.1% to 0.6% by weight, Titanium (Ti) 0.01% to 0.03% by weight %, Niob (Nb) 0.005% by weight to 0.08% by weight, Chromium (Cr) 0.1% to 0.5% by weight, Phosphorus (P) 0.01% by weight or less (0% by weight) %), Sulfur (S) in 0.01% by weight or less, Boron (B) in 5% by weight to 30% by weight, Nitrogen (N) in 20% by weight to 70% by weight, Calcium (Ca) in 50% by weight. A step of heating a slab consisting of ppm or less (excluding 0 wt ppm), tin (Sn) of 5 wt ppm to 50 wt ppm, and the rest consisting of iron (Fe) and other unavoidable impurities to 1050 ° C to 1250 ° C.
The heated slab is roughly rolled at 950 ° C to 1050 ° C to obtain a bar, and the bar is hot-rolled at a finish rolling temperature of 700 ° C to 950 ° C to obtain a hot-rolled steel sheet. Stages and
It has a step of cooling the hot-rolled steel sheet to a cooling end temperature of Bs temperature or less at a cooling rate of 25 ° C./s to 50 ° C./s.
The hot-rolled steel sheet contains bainitic ferrite and granular bainite as main phases as a microstructure.
The bainitic ferrite is 80 area% to 95 area%, the granular bainite is 5 area% to 20 area%, and MA (island martensite) is 3 area% or less (including 0 area%). ) And
A method for producing a low yield ratio type high-strength steel material, wherein the hot-rolled steel sheet has a yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.
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