WO2016052093A1 - ボルト用鋼およびボルト - Google Patents
ボルト用鋼およびボルト Download PDFInfo
- Publication number
- WO2016052093A1 WO2016052093A1 PCT/JP2015/075416 JP2015075416W WO2016052093A1 WO 2016052093 A1 WO2016052093 A1 WO 2016052093A1 JP 2015075416 W JP2015075416 W JP 2015075416W WO 2016052093 A1 WO2016052093 A1 WO 2016052093A1
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- WIPO (PCT)
- Prior art keywords
- less
- steel
- upper limit
- bolt
- preferable
- Prior art date
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 49
- 239000010959 steel Substances 0.000 title claims abstract description 49
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims abstract description 8
- 239000012535 impurity Substances 0.000 claims abstract description 6
- 229910052742 iron Inorganic materials 0.000 claims abstract description 4
- 229910001566 austenite Inorganic materials 0.000 claims description 29
- 239000002244 precipitate Substances 0.000 claims description 19
- 229910052758 niobium Inorganic materials 0.000 claims description 14
- 229910052719 titanium Inorganic materials 0.000 claims description 14
- 239000000203 mixture Substances 0.000 claims description 9
- 239000000126 substance Substances 0.000 claims description 9
- 229910052802 copper Inorganic materials 0.000 claims description 4
- 229910052749 magnesium Inorganic materials 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 4
- 229910052721 tungsten Inorganic materials 0.000 claims description 4
- 229910052726 zirconium Inorganic materials 0.000 claims description 4
- 238000005121 nitriding Methods 0.000 claims description 3
- 230000003111 delayed effect Effects 0.000 abstract description 45
- 239000000463 material Substances 0.000 abstract description 9
- 230000000694 effects Effects 0.000 description 26
- 238000005096 rolling process Methods 0.000 description 23
- 238000012360 testing method Methods 0.000 description 20
- 239000002436 steel type Substances 0.000 description 18
- 150000004767 nitrides Chemical class 0.000 description 17
- 238000010791 quenching Methods 0.000 description 16
- 230000000171 quenching effect Effects 0.000 description 16
- 239000013078 crystal Substances 0.000 description 14
- 238000001816 cooling Methods 0.000 description 13
- 238000005496 tempering Methods 0.000 description 11
- 230000007797 corrosion Effects 0.000 description 9
- 238000005260 corrosion Methods 0.000 description 9
- 238000004519 manufacturing process Methods 0.000 description 9
- 238000003303 reheating Methods 0.000 description 9
- 238000005259 measurement Methods 0.000 description 8
- 229920006395 saturated elastomer Polymers 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 6
- 239000003610 charcoal Substances 0.000 description 5
- 230000006866 deterioration Effects 0.000 description 5
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 4
- 238000011156 evaluation Methods 0.000 description 4
- 239000010410 layer Substances 0.000 description 4
- 229910000859 α-Fe Inorganic materials 0.000 description 4
- 239000002253 acid Substances 0.000 description 3
- 229910052782 aluminium Inorganic materials 0.000 description 3
- 150000001875 compounds Chemical class 0.000 description 3
- 238000007654 immersion Methods 0.000 description 3
- 229910000734 martensite Inorganic materials 0.000 description 3
- 150000001247 metal acetylides Chemical class 0.000 description 3
- 229910001562 pearlite Inorganic materials 0.000 description 3
- 238000002791 soaking Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 230000004580 weight loss Effects 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 238000000137 annealing Methods 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 238000005520 cutting process Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 238000010884 ion-beam technique Methods 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 238000000034 method Methods 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 229910052698 phosphorus Inorganic materials 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 229910052720 vanadium Inorganic materials 0.000 description 2
- 238000005491 wire drawing Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 235000008733 Citrus aurantifolia Nutrition 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 235000011941 Tilia x europaea Nutrition 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910001563 bainite Inorganic materials 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 238000010273 cold forging Methods 0.000 description 1
- 238000012790 confirmation Methods 0.000 description 1
- 230000001934 delay Effects 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000000605 extraction Methods 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- GNVXPFBEZCSHQZ-UHFFFAOYSA-N iron(2+);sulfide Chemical compound [S-2].[Fe+2] GNVXPFBEZCSHQZ-UHFFFAOYSA-N 0.000 description 1
- 239000004571 lime Substances 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 238000001556 precipitation 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
- 239000000243 solution Substances 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 238000010998 test method Methods 0.000 description 1
- 239000010409 thin film Substances 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
- F16B33/00—Features common to bolt and nut
- F16B33/008—Corrosion preventing means
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/32—Soft annealing, e.g. spheroidising
-
- 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
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
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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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
- C22C38/18—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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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
- F16B35/00—Screw-bolts; Stay-bolts; Screw-threaded studs; Screws; Set screws
-
- 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
- C21D2211/001—Austenite
-
- 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
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
-
- 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
Definitions
- the present invention relates to a steel for bolts used in automobiles, various industrial machines, and the like, and a bolt obtained by using this steel for bolts, and particularly excellent delayed fracture resistance and cold forging even when the tensile strength is 1100 MPa or more.
- the present invention relates to a steel for bolts and bolts that exhibit properties.
- Patent Documents 1 to 3 are technologies relating to high-strength steel materials and bolts excellent in delayed fracture resistance, and it is disclosed to ensure excellent delayed fracture resistance by increasing the nitrogen concentration of the surface layer.
- delayed fracture resistance is greatly reduced.
- special heat treatment is required to form the nitride layer, leaving problems in terms of productivity and cost.
- the present invention has been made in view of the circumstances as described above, and its purpose is for a bolt that has excellent cold forgeability and excellent delayed fracture resistance while maintaining the strength as a steel material.
- the object is to provide steel and bolts obtained from such bolt steel.
- the bolt steel according to the present invention is, in mass%, C: 0.20 to 0.40%, Si: 1.5 to 2.5%, Mn: 0.20 to 1.5%, P: 0 %, 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.05 to 1.5%, Al: 0.01 to 0.10%, B: 0.0003 to 0 .01%, N: 0.002 to 0.020%, and one selected from the group consisting of Ti: 0.02 to 0.10% and Nb: 0.02 to 0.10% or It contains two types, and the gist is that the balance is iron and inevitable impurities.
- the present invention also includes a bolt having the above chemical component composition and satisfying the relationship of the following formula (1). (L / L0) ⁇ 100 ⁇ 60 (1) Where L: the total length of precipitates having a thickness of 50 nm or more precipitated at the austenite grain boundaries, L0: Length of austenite grain boundary.
- the bolt according to the present invention does not have a nitriding layer.
- the bolt according to the present invention preferably has an austenite grain size number of 8 or more.
- the chemical composition of the steel is appropriately controlled, the cold forgeability and delayed fracture resistance of the bolt steel can be achieved at a high level.
- the present inventors have studied from the viewpoint of appropriately controlling the chemical composition, in particular, in order to realize a bolt steel that can achieve both cold forgeability and delayed fracture resistance. As a result, it has been found that if the Si content is relatively high and the amount of precipitates precipitated at the grain boundaries is reduced as much as possible, and the chemical composition is appropriately adjusted, a bolt steel suitable for the above purpose can be realized. Completed the invention.
- C 0.20 to 0.40%
- C is an element effective for ensuring the strength of steel.
- C In order to secure a target tensile strength of 1100 MPa or more, C needs to be contained by 0.20% or more.
- the preferable lower limit of the C amount is 0.23% or more, and the more preferable lower limit is 0.25% or more.
- the upper limit was made 0.40% or less.
- the upper limit with the preferable amount of C is 0.35% or less, and a more preferable upper limit is 0.32% or less.
- Si acts as a deoxidizer and is an effective element for ensuring the strength of steel.
- Si also suppresses the precipitation of coarse cementite that affects the G value, which will be described later, and also exhibits the effect of improving delayed fracture resistance.
- it is necessary to contain Si 1.5% or more.
- a preferable lower limit of the Si amount is 1.6% or more, and a more preferable lower limit is 1.7% or more.
- the upper limit needs to be 2.5% or less.
- the upper limit with preferable Si amount is 2.2% or less, and a more preferable upper limit is 2.0% or less.
- Mn 0.20 to 1.5%
- Mn is an element effective for ensuring the strength of the steel and forming an S compound with S and suppressing the formation of FeS, which degrades delayed fracture resistance.
- it is necessary to contain Mn in an amount of 0.20% or more.
- the minimum with the preferable amount of Mn is 0.30% or more, and a more preferable minimum is 0.40% or more.
- the upper limit must be 1.5% or less.
- the upper limit with the preferable amount of Mn is 1.3% or less, and a more preferable upper limit is 1.1% or less.
- P more than 0% and 0.03% or less
- P is an impurity element that lowers the toughness of steel by concentrating at grain boundaries and deteriorates delayed fracture resistance.
- the amount of P is preferably 0.015% or less, more preferably 0.010% or less. The smaller the P content, the better. However, it is difficult to make it zero, and about 0.003% is contained.
- S More than 0% and 0.03% or less S, like P, is an impurity element that reduces the toughness of steel and deteriorates delayed fracture resistance by concentrating on the grain boundaries.
- the amount of S is preferably 0.015% or less, more preferably 0.010% or less. The smaller the content of S, the better. However, it is difficult to make it zero, and about 0.003% is contained.
- Cr 0.05 to 1.5% Cr is an element effective for improving the corrosion resistance of steel and ensuring delayed fracture resistance. In order to exert these effects, it is necessary to contain 0.05% or more of Cr. A preferable lower limit of the Cr content is 0.10% or more, and a more preferable lower limit is 0.20% or more. On the other hand, when the Cr content is excessive, coarse carbides are generated, the cold forgeability is deteriorated, and the cost is increased. Therefore, the upper limit needs to be 1.5% or less. The upper limit with the preferable amount of Cr is 1.3% or less, and a more preferable upper limit is 1.0% or less.
- Al 0.01 to 0.10%
- Al is an effective element for acting as a deoxidizer and forming nitrides to improve crystal grain refinement and cold forgeability. In order to exhibit these effects, it is necessary to contain Al 0.01% or more.
- a preferable lower limit of the amount of Al is 0.03% or more, and a more preferable lower limit is 0.04% or more.
- the upper limit must be made 0.10% or less.
- the upper limit with preferable Al amount is 0.08% or less, and a more preferable upper limit is 0.06% or less.
- B 0.0003 to 0.01% B improves the hardenability of steel and suppresses the concentration of grain boundary segregating elements such as P and S by dispersing on the prior austenite grain boundaries, and delays resistance by cleaning the grain boundaries. It is an effective element for improving destructibility. In order to exert these effects, B needs to be contained by 0.0003% or more. A preferable lower limit of the amount of B is 0.0008% or more, and a more preferable lower limit is 0.001% or more. On the other hand, if the B content is excessive, a coarse compound is produced and delayed fracture resistance deteriorates, so the upper limit was made 0.01% or less. The upper limit with the preferable amount of B is 0.005% or less, and a more preferable upper limit is 0.003% or less.
- N 0.002 to 0.020%
- N is an effective element for forming nitrides with Al, Ti, and Nb and making the crystal grains finer. In order to exhibit such an effect, N needs to be contained by 0.002% or more.
- a preferable lower limit of the N amount is 0.003% or more, and a more preferable lower limit is 0.0035% or more.
- the upper limit was made 0.020% or less.
- the upper limit with preferable N amount is 0.010% or less, and a more preferable upper limit is 0.008% or less.
- Ti and Nb form nitrides with N to refine crystal grains It is an effective element to make it. Further, by forming nitrides of Ti and Nb, it becomes difficult to form nitrides of B, and the hardenability of steel is improved by increasing free B. In order to exert these effects, it is necessary to contain 0.02% or more of at least one of Ti and Nb.
- the preferable lower limit of the Ti amount and the Nb amount are both 0.03% or more, and the more preferable lower limit is 0.04% or more.
- the upper limit of each is 0.10% or less.
- the preferable upper limit of the Ti amount and the Nb amount are both 0.08% or less, and the more preferable upper limit is 0.06% or less.
- the basic components of the steel for bolts according to the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that steel contains inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like.
- Ni and Sn are effective elements for improving the corrosion resistance of steel and improving the delayed fracture resistance.
- the effect of these elements increases as the content increases.
- the upper limit when Cu is contained is preferably 0.5% or less.
- a more preferable upper limit of the amount of Cu is 0.4% or less, and a more preferable upper limit is 0.35% or less.
- the upper limit when Ni is contained is preferably 1.0% or less.
- a more preferable upper limit of the amount of Ni is 0.8% or less, and a more preferable upper limit is 0.7% or less.
- the upper limit when Sn is contained is preferably 0.5% or less.
- a more preferable upper limit of the Sn amount is 0.4% or less, and a more preferable upper limit is 0.3% or less.
- the minimum of Cu content is 0.03% or more.
- a more preferable lower limit of the amount of Cu is 0.1% or more, and a more preferable lower limit is 0.15% or more.
- a preferable lower limit is 0.1% or more, a more preferable lower limit is 0.2% or more, and a further preferable lower limit is 0.3% or more.
- the preferred lower limit is 0.03% or more, the more preferred lower limit is 0.1% or more, and the still more preferred lower limit is 0.15% or more.
- Mo more than 0% and 1.5% or less Mo is an element effective for improving the delayed fracture resistance by forming fine precipitates in the steel and increasing the strength of the steel. These effects increase as the content increases. However, if the Mo content is excessive, the manufacturing cost is deteriorated, so the upper limit is preferably 1.5% or less. A more preferable upper limit of the Mo amount is 1.2% or less, and a more preferable upper limit is 1.1% or less. In addition, in order to exhibit said effect, it is preferable that the minimum when containing Mo is 0.03% or more. A more preferable lower limit of the amount of Mo is 0.10% or more, and a more preferable lower limit is 0.15% or more.
- Ca one or more selected from the group consisting of more than 0% and not more than 0.01% V, W, Zr, Mg and Ca form carbonitrides and form austenite grains during quenching heating. It is effective in preventing coarsening, improving toughness, and improving delayed fracture resistance. The effect of these elements increases as the content increases. However, when each element is excessive, the following disadvantages occur. That is, when the V content is excessive, the above effect is saturated and the manufacturing cost is increased. From such a viewpoint, the upper limit when V is contained is preferably 0.5% or less. A more preferable upper limit of the V amount is 0.3% or less, and a more preferable upper limit is 0.2% or less.
- the upper limit when W is contained is preferably 0.5% or less.
- a more preferable upper limit of the amount of W is 0.3% or less, and a more preferable upper limit is 0.2% or less.
- the upper limit when Zr is contained is preferably 0.3% or less.
- a more preferable upper limit of the amount of Zr is 0.2% or less, and a more preferable upper limit is 0.1% or less.
- the upper limit when Mg is contained is preferably 0.01% or less.
- the upper limit with more preferable amount of Mg is 0.007% or less, and a more preferable upper limit is 0.005% or less.
- the upper limit when Ca is contained is preferably 0.01% or less.
- a more preferred upper limit of the Ca content is 0.007% or less, and a more preferred upper limit is 0.005% or less.
- the minimum of V content is 0.01% or more.
- a more preferable lower limit of the V amount is 0.03% or more, and a more preferable lower limit is 0.05% or more.
- a preferable lower limit is 0.01% or more, a more preferable lower limit is 0.03% or more, and a further preferable lower limit is 0.05% or more.
- the preferable lower limit when Zr is contained is 0.01% or more, the more preferable lower limit is 0.03% or more, and the still more preferable lower limit is 0.05% or more.
- a preferable lower limit when Mg is contained is 0.0003% or more, a more preferable lower limit is 0.0005% or more, and a further preferable lower limit is 0.001% or more.
- a preferable lower limit when Ca is contained is 0.0003% or more, a more preferable lower limit is 0.0005% or more, and a further preferable lower limit is 0.001% or more.
- the steel for bolts having the above chemical composition is heated to 950 ° C. or higher at the time of billet reheating before rolling (hereinafter, this temperature is referred to as “billet reheating temperature”), and the wire is heated in the temperature range of 800 to 1000 ° C.
- the structure after rolling is basically a mixed structure of ferrite and pearlite by cooling to a temperature of 600 ° C. or less at an average cooling rate of 3 ° C./second or less.
- the structure after rolling need not necessarily be a mixed structure of ferrite and pearlite.
- Billet reheating temperature 950 ° C or more
- Ti, Nb carbides, nitrides, and carbonitrides (hereinafter referred to as "charcoal / nitrides") effective for grain refinement are transformed into austenite.
- the billet reheating temperature is preferably 950 ° C. or higher.
- This temperature is more preferably 1000 ° C. or higher.
- the reheating temperature of the billet exceeds 1400 ° C., it becomes close to the melting temperature of steel, so the reheating temperature is preferably 1400 ° C. or less, more preferably 1300 ° C. or less, and further preferably 1250 ° C. or less.
- Finish rolling temperature 800-1000 ° C
- the finish rolling temperature is preferably 1000 ° C. or lower.
- the finish rolling temperature is more preferably 950 ° C. or lower.
- Ti and Nb charcoal / nitrides are difficult to precipitate, and the effect of grain refinement during quenching is reduced.
- the lower limit is preferably 800 ° C. or higher.
- the finish rolling temperature is more preferably 850 ° C. or higher.
- the finish rolling temperature is the average surface temperature that can be measured with a radiation thermometer before the final rolling pass or before the rolling roll group.
- the structure is preferably a mixed structure of ferrite and pearlite in order to improve formability in subsequent bolt processing.
- the average cooling rate is more preferably 2 ° C./second or less, and further preferably 1 ° C./second or less.
- a spheroidizing annealing treatment may be performed.
- quenching and tempering are performed to make the structure tempered martensite, thereby ensuring a predetermined tensile strength and having excellent delayed fracture resistance.
- Appropriate conditions for quenching and tempering at this time are as follows.
- the heating temperature (hereinafter, this temperature may be referred to as “quenching temperature”) is preferably 850 ° C. or higher in order to stably perform the austenitizing treatment.
- the quenching temperature is preferably 950 ° C. or lower in order to prevent coarsening of crystal grains.
- the more preferable upper limit of quenching temperature is 930 degrees C or less, More preferably, it is 920 degrees C or less.
- the more preferable minimum of quenching temperature is 870 degreeC or more, More preferably, it is 880 degreeC or more.
- As-quenched bolts have low toughness and ductility and cannot be used as bolt products as they are, they need to be tempered. For that purpose, it is effective to perform a tempering treatment at a temperature of at least 300 ° C. or higher.
- the bolt obtained in the present invention does not have a nitriding layer on the surface, but the proportion of precipitates having a thickness of 50 nm or more deposited on the austenite grain boundaries of the bolt shaft portion is 60% or less.
- the delayed fracture resistance can be further improved. That is, in the following formula (1), when the value on the left side, that is, the value of (L / L0) ⁇ 100 is set as the G value (%), this G value is 60% or less.
- Bolts having the above chemical component composition and satisfying the relationship of the following formula (1) have excellent delayed fracture resistance.
- the G value is more preferably 50% or less, and still more preferably 40% or less.
- the lower limit of the G value is preferably as low as possible, but is usually 10% or more.
- the “precipitate” that precipitates on the austenite grain boundary of the bolt shaft portion is mainly cementite, but is not limited to this. Carbides and carbonitrides including Cr, Ti, Nb, Al, V, etc. Is also included. (L / L0) ⁇ 100 ⁇ 60 (1)
- L the total length of precipitates having a thickness of 50 nm or more deposited on the austenite grain boundaries
- L0 the length of the austenite grain boundaries.
- the tempering temperature is important, and the tempering temperature is set to a temperature T (° C.) or less represented by the following formula (2), whereby the G value is reduced. It can be made 60% or less.
- T (° C.) 68.2 ⁇ Ln [Si] +480 (2)
- austenite crystal grains are preferred because they are more resistant to delayed fracture as they become finer. From such a viewpoint, it is preferable that the austenite crystal grain in the bolt shaft portion is 8 or more in crystal grain size number defined by JIS G 0551 (2006).
- the grain size number is more preferably 9 or more, and still more preferably 10 or more.
- the spheroidizing annealing conditions were as follows: soaking temperature: 760 ° C., soaking time: 5 hours, average cooling rate after soaking: 13 ° C./hour, extraction temperature: 685 ° C.
- soaking temperature 760 ° C.
- soaking time 5 hours
- average cooling rate after soaking 13 ° C./hour
- extraction temperature 685 ° C.
- ⁇ the portion represented by “ ⁇ ” means that no addition is made, and “tr.” Means that the measurement is below the measurement limit.
- a flange bolt of M12 mm ⁇ 1.25 Pmm and length: 100 mmL was produced from the obtained steel wire by cold heading using a multistage former.
- M represents the diameter of the shaft
- P represents the pitch.
- cold forgeability was evaluated by the presence or absence of the crack of a flange part. The cold forgeability was evaluated as OK when no cracks occurred and NG when cracks occurred.
- quenching and tempering were performed under the conditions shown in Tables 3 and 4 below.
- the other quenching and tempering conditions were as follows: quenching heating time: 20 minutes, quenching furnace atmosphere: air, quenching cooling condition: oil cooling 25 ° C., tempering heating time: 45 minutes.
- the crystal grain size, tensile strength, corrosion resistance, delayed fracture resistance, and G value of the shaft portion were evaluated in the following manner.
- the tensile strength of the bolt was determined by conducting a tensile test in accordance with JIS B 1051 (2009), and a tensile strength of 1100 MPa or more was determined to be acceptable. In addition, this measurement was not performed about the thing for which the cold forgeability failed.
- Delayed fracture resistance is determined by (a) immersing the jig together with 1% HCl for 15 minutes after tightening the bolt to the jig aiming at the yield point, (b) exposing in the atmosphere for 24 hours, (C) Confirmation of the presence or absence of breakage was defined as one cycle, and this was evaluated by repeating this cycle for 10 cycles. Ten bolts were evaluated for each level, and one that did not break was OK, and one that broke was NG. In addition, this evaluation was not performed about the thing from which the cold forgeability was rejected, or the thing whose tensile strength became less than 1100 Mpa.
- Precipitates precipitated at the austenite grain boundaries are observed by cutting the shaft portion of the bolt in a cross section and then using a focused ion beam processing apparatus (FIB: Focused Ion Beam Process, manufactured by Hitachi, Ltd .: trade name “FB”). -2000A ”) to produce a thin film specimen.
- FEMS-2100F transmission electron microscope
- three austenite grain boundaries were photographed at a magnification of 150,000 times for each sample.
- the length and thickness of the precipitate deposited on the substrate were calculated.
- the length of the precipitate means the length in the direction parallel to the austenite grain boundary.
- the thickness of the precipitate means the length in the direction perpendicular to the austenite grain boundary.
- the total length (L) calculated by adding the lengths of the precipitates having a thickness of 50 nm or more deposited on the austenite grain boundaries is divided by the length (L0) of the austenite grain boundaries and expressed as a percentage.
- the occupancy ratio (G value) of precipitates on the austenite grain boundaries was determined.
- the G value (%) was determined for each of the three photographs, and the average values are shown in Tables 3 and 4 below.
- Test No. Nos. 1 to 14 are invention examples that satisfy the requirements defined in the present invention, and it is found that they are excellent in cold forgeability, exhibit high delayed strength as well as high strength.
- test no. 15 to 33 are examples that do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. In No. 15, the tempering temperature was increased, the G value was increased, and the delayed fracture resistance was deteriorated.
- Test No. No. 16 used steel type A1 with a low C content, and could not secure a tensile strength of 1100 MPa or more.
- Test No. No. 17 uses steel type B1 having too much C content, toughness is lowered, and delayed fracture resistance is deteriorated.
- Test No. No. 18 uses a steel type C1 having a low Si content, and a large amount of coarse precipitates are precipitated to increase the G value, and the delayed fracture resistance is deteriorated.
- Test No. No. 19 uses steel type D1 having a large Si content, and the cold forgeability deteriorated.
- Test No. No. 20 uses a steel type E1 having a low Mn content, and a large amount of iron sulfide (FeS) is produced, resulting in deterioration of delayed fracture resistance.
- FeS iron sulfide
- Test No. No. 21 uses steel type F1 having a high Mn content, and the toughness deteriorates and the delayed fracture resistance deteriorates.
- Test No. No. 22 uses a steel type G1 having a high P content, which is concentrated on the grain boundary and deteriorated toughness and delayed fracture resistance.
- Test No. No. 23 uses steel type H1 with a high S content. As in the case of No. 22, the delayed fracture resistance deteriorated due to concentration on the grain boundaries and a decrease in toughness.
- Test No. No. 24 uses steel type I1 having a low Cr content, and the corrosion resistance is lowered and the delayed fracture resistance is deteriorated.
- Test No. No. 25 uses steel type J1 having a high Cr content, and coarse precipitates are generated, resulting in deterioration of cold forgeability.
- Test No. Nos. 26 and 28 use steel type K1 or steel type M1 that does not contain Ti and Nb, both of which have coarsened crystal grains and deteriorated delayed fracture resistance.
- Test No. No. 27 uses steel type L1 having a large Ti content, and coarse carbonitrides are produced, resulting in deterioration of cold forgeability.
- Test No. No. 29 uses steel type N1 having a high Nb content, and coarse carbonitrides are produced, resulting in deterioration of cold forgeability.
- Test No. No. 30 uses a steel type O1 that does not contain Al, and the ferrite crystal grains became coarse during rolling, and the cold forgeability deteriorated.
- Test No. No. 31 uses steel type P1 having a high Al content, and coarse nitrides are produced, resulting in deterioration of cold forgeability.
- Test No. No. 32 uses a steel type Q1 having a low N content, and nitrides are not sufficiently formed, so that crystal grains are expected to be coarsened, and cold forgeability deteriorates.
- Test No. No. 33 uses steel type R1 with a high N content, and it is expected that the amount of N in a solid solution state will increase, and the cold forgeability deteriorated.
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Abstract
Description
(a)Cu:0%超、0.5%以下、Ni:0%超、1.0%以下およびSn:0%超、0.5%以下よりなる群から選ばれる1種または2種以上、
(b)Mo:0%超、1.5%以下、
(c)V:0%超、0.5%以下、W:0%超、0.5%以下、Zr:0%超、0.3%以下、Mg:0%超、0.01%以下およびCa:0%超、0.01%以下よりなる群から選ばれる1種または2種以上、
等を含有させることも有用であり、これによって高強度ボルト用鋼の特性が更に改善される。
(L/L0)×100≦60 ・・・(1)
但し、L:オーステナイト結晶粒界に析出した厚さ50nm以上の析出物の合計長さ、
L0:オーステナイト結晶粒界の長さ、を示す。
Cは、鋼の強度を確保するために有効な元素である。目標とする引張強度:1100MPa以上を確保するため、Cは0.20%以上含有させる必要がある。C量の好ましい下限は0.23%以上であり、より好ましい下限は0.25%以上である。しかしながら、C含有量が過剰になると、耐遅れ破壊性が劣化するため、その上限を0.40%以下とした。C量の好ましい上限は0.35%以下であり、より好ましい上限は0.32%以下である。
Siは、脱酸剤として作用すると共に、鋼の強度を確保するために有効な元素である。また、Siは、後述するG値に影響を与える粗大なセメンタイトの析出を抑制し、耐遅れ破壊性を向上させる作用も発揮する。これらの効果を有効に発揮させるためには、Siは1.5%以上含有させる必要がある。Si量の好ましい下限は1.6%以上であり、より好ましい下限は1.7%以上である。一方、Si含有量が過剰になると、冷間鍛造性が劣化するため、その上限を2.5%以下とする必要がある。Si量の好ましい上限は2.2%以下であり、より好ましい上限は2.0%以下である。
Mnは、鋼の強度を確保すると共に、Sと化合物を形成し、耐遅れ破壊性を劣化させるFeSの生成を抑制する作用を発揮するのに有効な元素である。これらの効果を発揮させるためには、Mnは0.20%以上含有させる必要がある。Mn量の好ましい下限は0.30%以上であり、より好ましい下限は0.40%以上である。一方、Mn含有量が過剰になると、耐遅れ破壊性が劣化するため、その上限を1.5%以下とする必要がある。Mn量の好ましい上限は1.3%以下であり、より好ましい上限は1.1%以下である。
Pは、結晶粒界に濃化することで鋼の靭延性を低下させ、耐遅れ破壊性を劣化させる不純物元素である。Pの含有量を0.03%以下とすることで、耐遅れ破壊性が大きく向上する。P量は、好ましくは0.015%以下であり、より好ましくは0.010%以下である。Pの含有量は少なければ少ないほど好ましいが、ゼロとするのは製造上困難であり、0.003%程度は含有される。
SもPと同様、結晶粒界上に濃化することで鋼の靭延性を低下させ、耐遅れ破壊性を劣化させる不純物元素である。Sの含有量を0.03%以下とすることで、耐遅れ破壊性が大きく向上する。S量は、好ましくは0.015%以下であり、より好ましくは0.010%以下である。Sの含有量は少なければ少ないほど好ましいが、ゼロとするのは製造上困難であり、0.003%程度は含有される。
Crは、鋼の耐食性を向上させると共に、耐遅れ破壊性を確保するために有効な元素である。これらの効果を発揮させるためには、Crは0.05%以上含有させる必要がある。Cr量の好ましい下限は0.10%以上であり、より好ましい下限は0.20%以上である。一方、Cr含有量が過剰になると、粗大な炭化物が生成し冷間鍛造性が劣化すると共に、コスト増を招くため、その上限を1.5%以下とする必要がある。Cr量の好ましい上限は1.3%以下であり、より好ましい上限は1.0%以下である。
Alは、脱酸剤として作用すると共に、窒化物を形成して結晶粒の微細化や冷間鍛造性を向上させるために有効な元素である。これらの効果を発揮させるためには、Alは0.01%以上含有させる必要がある。Al量の好ましい下限は0.03%以上であり、より好ましい下限は0.04%以上である。一方、Al含有量が過剰になると、粗大な窒化物を生成し冷間鍛造性が劣化するため、その上限を0.10%以下とする必要がある。Al量の好ましい上限は0.08%以下であり、より好ましい上限は0.06%以下である。
Bは、鋼の焼入れ性を向上させると共に、旧オーステナイト結晶粒界上に分散することでPやS等の粒界偏析元素の濃化を抑制し、結晶粒界を清浄化することで耐遅れ破壊性を向上させるために有効な元素である。これらの効果を発揮させるためには、Bは0.0003%以上含有させる必要がある。B量の好ましい下限は0.0008%以上であり、より好ましい下限は0.001%以上である。一方、B含有量が過剰になると、粗大な化合物を生成し耐遅れ破壊性が劣化するため、その上限を0.01%以下とした。B量の好ましい上限は0.005%以下であり、より好ましい上限は0.003%以下である。
Nは、Al、TiおよびNbと窒化物を形成し、結晶粒を微細化させるために有効な元素である。こうした効果を発揮させるためには、Nは0.002%以上含有させる必要がある。N量の好ましい下限は0.003%以上であり、より好ましい下限は0.0035%以上である。一方、N含有量が過剰になると、化合物を形成しないで固溶状態となっているN量が増加し、冷間鍛造性が低下するため、上限を0.020%以下とした。N量の好ましい上限は0.010%以下であり、より好ましい上限は0.008%以下である。
TiとNbは、Nと窒化物を形成し、結晶粒を微細化させるのに有効な元素である。また、TiやNbの窒化物を形成することで、Bの窒化物が形成しにくくなり、フリーのBが増えることで鋼の焼入れ性が向上する。これらの効果を発揮させるためには、TiおよびNbの少なくとも1種を、0.02%以上含有させる必要がある。Ti量およびNb量の好ましい下限はいずれも0.03%以上であり、より好ましい下限は0.04%以上である。一方、Ti含有量およびNb含有量が過剰になると、粗大な炭窒化物が形成され、冷間鍛造性や耐遅れ破壊性が劣化する。こうした観点から、それらの上限をいずれも0.10%以下とした。Ti量およびNb量の好ましい上限はいずれも0.08%以下であり、より好ましい上限は0.06%以下である。
Cu、NiおよびSnは、鋼の耐食性を向上させると共に、耐遅れ破壊性を向上させるのに有効な元素である。これらの元素は、その含有量が増加するにつれてその効果が増大するが、それぞれの元素が過剰になると、下記のような不都合が生じる。即ち、Cu含有量が過剰になると、上記効果が飽和すると共に、熱間延性が低下して鋼の生産性が低下する。また、冷間鍛造性の低下や靭性の低下を招くことにもなる。こうした観点から、Cuを含有させるときの上限は0.5%以下であることが好ましい。Cu量のより好ましい上限は0.4%以下であり、更に好ましい上限は0.35%以下である。
Moは、鋼の強度を高めると共に、鋼中に微細な析出物を形成して耐遅れ破壊性を向上させるのに有効な元素である。これらの効果は、その含有量が増加するにつれて増大するが、Mo含有量が過剰になると製造コストを劣化させるため、その上限は1.5%以下が好ましい。Mo量のより好ましい上限は1.2%以下であり、更に好ましい上限は1.1%以下である。尚、上記の効果を発揮させるためには、Moを含有させるときの下限は0.03%以上であることが好ましい。Mo量のより好ましい下限は0.10%以上であり、更に好ましい下限は0.15%以上である。
V、W、Zr、MgおよびCaは、炭窒化物を形成し、焼入れ加熱時のオーステナイト結晶粒の粗大化を防止し、靭延性を向上させ、耐遅れ破壊性を向上させるのに有効である。これらの元素は、その含有量が増加するにつれてその効果が増大するが、それぞれの元素が過剰になると、下記のような不都合が生じる。即ち、V含有量が過剰になると上記効果が飽和して製造コストの増加を招く。こうした観点から、Vを含有させるときの上限は0.5%以下が好ましい。V量のより好ましい上限は0.3%以下であり、更に好ましい上限は0.2%以下である。
ビレット再加熱では、結晶粒微細化に有効なTiやNbの炭化物、窒化物および炭窒化物(以下、これを「炭・窒化物」と呼ぶ)を、オーステナイトに固溶させる必要があり、そのためにはビレットの再加熱温度を950℃以上にすることが好ましい。この温度が950℃未満になると炭・窒化物の固溶が不十分となり、後の熱間圧延で微細なTiやNbの炭・窒化物が生成しにくくなり、焼入れ時の結晶粒微細化の効果が減少する。この温度は、より好ましくは1000℃以上である。但し、ビレットの再加熱温度が1400℃を超えると鋼の溶解温度に近くなるため、再加熱温度は、1400℃以下が好ましく、より好ましくは1300℃以下、更に好ましくは1250℃以下である。
圧延では、ビレット再加熱時に固溶させたTiやNbを微細な炭・窒化物として鋼中に析出させる必要がある。そのためには、仕上げ圧延温度を1000℃以下とすることが好ましい。仕上げ圧延温度は、より好ましくは950℃以下である。仕上げ圧延温度が1000℃よりも高くなると、TiやNbの炭・窒化物が析出しにくくなるため、焼入れ時の結晶粒微細化の効果が減少する。
仕上げ圧延後の冷却では、後のボルト加工での成形性を向上させるため、組織をフェライト+パーライトの混合組織にすることが好ましい。そのためには、仕上げ圧延後の平均冷却速度を3℃/秒以下とし、この冷却速度で少なくとも600℃まで冷却することが好ましい。平均冷却速度が3℃/秒より速くなると、ベイトナイトやマルテンサイトが生成するため、ボルト成形性が大幅に劣化する。平均冷却速度は、より好ましくは2℃/秒以下であり、更に好ましくは1℃/秒以下である。
(L/L0)×100≦60 ・・・(1)
但し、L:オーステナイト結晶粒界に析出した厚さ50nm以上の析出物の合計長さ、L0:オーステナイト結晶粒界の長さ、を示す。
T(℃)=68.2×Ln[Si]+480 ・・・(2)
ボルトの軸部を、ボルトの軸に対して垂直な断面で切断後、軸部の直径をDとしたときのD/4位置の任意の0.039mm2の領域を、光学顕微鏡で観察し(倍率:400倍)、JIS G 0551(2006)に規定の「鋼-結晶粒度の顕微鏡試験方法」に従って旧オーステナイト結晶粒度番号を測定した。ボルトの軸に対して垂直な断面を、以下では「横断面」と呼ぶ。測定は4視野について行ない、これらの平均値をオーステナイト結晶粒度番号とした。尚、冷間鍛造性が不合格となったものについては、この測定は行なわなかった。
ボルトの引張強さは、JIS B 1051(2009)に従って引張試験を行って求め、引張強さが1100MPa以上のものを合格とした。尚、冷間鍛造性が不合格となったものについては、この測定は行なわなかった。
耐食性は、15%HCl水溶液にボルトを30分浸漬した際の浸漬前後の腐食減量(質量%)によって評価した。この腐食減量が、0.05質量%未満のものを合格と評価した。尚、冷間鍛造性が不合格となったもの、または引張強さが1100MPa未満になったものについては、この評価は行なわなかった。
腐食減量=[(酸浸漬前の質量-酸浸漬後の質量)/酸浸漬前の質量]×100
耐遅れ破壊性は、ボルトを冶具に降伏点狙いで締め付けた後、(a)冶具ごと1%HClに15分浸漬、(b)大気中で24時間暴露、(c)破断有無の確認、を1サイクルとし、これを10サイクル繰り返すことで評価した。ボルトは1水準に対し10本ずつ評価し、1本も破断しなかったものはOKとし、1本でも破断したものはNGとした。尚、冷間鍛造性が不合格となったもの、または引張強さが1100MPa未満になったものについては、この評価は行なわなかった。
ボルトに対し、オーステナイト結晶粒界に析出した析出物の観察を下記の通り行なった。
オーステナイト結晶粒界に析出した析出物の観察は、上記ボルトの軸部を横断面で切断後、集束イオンビーム加工装置(FIB:Focused Ion Beam Process、日立製作所製:商品名「FB-2000A」)により薄膜試験片を作製した。次いで、透過型電子顕微鏡(日立製作所製:商品名「FEMS-2100F」)を用いて1試料につき3枚ずつ、倍率:15万倍でオーステナイト結晶粒界を撮影し、画像解析で、結晶粒界に析出した析出物の長さと厚さを算出した。尚、析出物の長さは、オーステナイト結晶粒界に対して平行な方向の長さを意味する。析出物の厚さは、オーステナイト結晶粒界に対して垂直方向の長さを意味する。
Claims (6)
- 質量%で、
C :0.20~0.40%、
Si:1.5~2.5%、
Mn:0.20~1.5%、
P :0%超、0.03%以下、
S :0%超、0.03%以下、
Cr:0.05~1.5%、
Al:0.01~0.10%、
B :0.0003~0.01%、
N :0.002~0.020%
をそれぞれ含有すると共に、
Ti:0.02~0.10%および
Nb:0.02~0.10%よりなる群から選ばれる1種または2種を含有し、
残部が鉄および不可避的不純物であるボルト用鋼。 - 更に、下記(a)~(c)のいずれかに属する1種以上を含有する請求項1に記載のボルト用鋼。
(a)Cu:0%超、0.5%以下、Ni:0%超、1.0%以下およびSn:0%超、0.5%以下よりなる群から選ばれる1種または2種以上
(b)Mo:0%超、1.5%以下
(c)V:0%超、0.5%以下、W:0%超、0.5%以下、Zr:0%超、0.3%以下、Mg:0%超、0.01%以下およびCa:0%超、0.01%以下よりなる群から選ばれる1種または2種以上 - 請求項1または2に記載の化学成分組成を有し、下記(1)式の関係を満足するボルト。
(L/L0)×100≦60 ・・・(1)
但し、L:オーステナイト結晶粒界に析出した厚さ50nm以上の析出物の合計長さ、
L0:オーステナイト結晶粒界の長さ、を示す。 - 表面に窒化処理層を有さない請求項3に記載のボルト。
- オーステナイト結晶粒度番号が8以上である請求項3に記載のボルト。
- オーステナイト結晶粒度番号が8以上である請求項4に記載のボルト。
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CN107058868A (zh) * | 2017-03-29 | 2017-08-18 | 苏州浩焱精密模具有限公司 | 一种高硬度精密雕刻刀模 |
WO2018090682A1 (zh) * | 2016-11-15 | 2018-05-24 | 江阴兴澄特种钢铁有限公司 | 一种紧固件用高淬透性中碳低合金圆钢及其制造方法 |
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CN107109560B (zh) * | 2014-11-18 | 2019-01-29 | 新日铁住金株式会社 | 冷锻部件用轧制棒钢或轧制线材 |
JP6226086B2 (ja) * | 2014-11-18 | 2017-11-08 | 新日鐵住金株式会社 | 冷間鍛造部品用圧延棒鋼または圧延線材 |
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JP6897876B2 (ja) * | 2018-06-29 | 2021-07-07 | 日本製鉄株式会社 | 平鋼線 |
JP6988858B2 (ja) * | 2019-04-24 | 2022-01-05 | Jfeスチール株式会社 | ボルト用鋼材 |
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