WO2023022222A1 - 鋼材 - Google Patents
鋼材 Download PDFInfo
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- WO2023022222A1 WO2023022222A1 PCT/JP2022/031349 JP2022031349W WO2023022222A1 WO 2023022222 A1 WO2023022222 A1 WO 2023022222A1 JP 2022031349 W JP2022031349 W JP 2022031349W WO 2023022222 A1 WO2023022222 A1 WO 2023022222A1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 388
- 239000010959 steel Substances 0.000 title claims abstract description 388
- 239000000463 material Substances 0.000 title claims abstract description 304
- 238000000605 extraction Methods 0.000 claims abstract description 60
- 238000005868 electrolysis reaction Methods 0.000 claims abstract description 35
- 239000012535 impurity Substances 0.000 claims abstract description 21
- 150000001247 metal acetylides Chemical class 0.000 claims description 46
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 24
- 238000005554 pickling Methods 0.000 abstract description 88
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 abstract description 84
- 229910052739 hydrogen Inorganic materials 0.000 abstract description 84
- 239000001257 hydrogen Substances 0.000 abstract description 84
- 239000000314 lubricant Substances 0.000 abstract description 44
- 229910052804 chromium Inorganic materials 0.000 abstract description 17
- 229910052750 molybdenum Inorganic materials 0.000 abstract description 17
- 229910052757 nitrogen Inorganic materials 0.000 abstract description 11
- 229910052799 carbon Inorganic materials 0.000 abstract description 9
- 229910052698 phosphorus Inorganic materials 0.000 abstract description 8
- 229910052717 sulfur Inorganic materials 0.000 abstract description 8
- 229910052782 aluminium Inorganic materials 0.000 abstract description 7
- 229910052748 manganese Inorganic materials 0.000 abstract description 7
- 229910052710 silicon Inorganic materials 0.000 abstract description 6
- 239000011651 chromium Substances 0.000 description 119
- 238000012360 testing method Methods 0.000 description 75
- 238000000034 method Methods 0.000 description 68
- 239000000126 substance Substances 0.000 description 42
- 239000000203 mixture Substances 0.000 description 37
- 238000000137 annealing Methods 0.000 description 36
- 239000000243 solution Substances 0.000 description 36
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 31
- 238000011156 evaluation Methods 0.000 description 27
- 230000000694 effects Effects 0.000 description 25
- 238000004519 manufacturing process Methods 0.000 description 25
- 239000002344 surface layer Substances 0.000 description 25
- 239000002253 acid Substances 0.000 description 24
- 230000001050 lubricating effect Effects 0.000 description 24
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 24
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 22
- 239000010949 copper Substances 0.000 description 22
- 239000010955 niobium Substances 0.000 description 22
- 238000005096 rolling process Methods 0.000 description 22
- 239000010936 titanium Substances 0.000 description 22
- 239000011572 manganese Substances 0.000 description 21
- 239000011575 calcium Substances 0.000 description 20
- 239000011248 coating agent Substances 0.000 description 20
- 238000000576 coating method Methods 0.000 description 20
- 239000011777 magnesium Substances 0.000 description 20
- 238000005406 washing Methods 0.000 description 19
- 238000007654 immersion Methods 0.000 description 17
- 239000007789 gas Substances 0.000 description 16
- 238000010273 cold forging Methods 0.000 description 12
- 238000005491 wire drawing Methods 0.000 description 12
- 150000004767 nitrides Chemical class 0.000 description 11
- 238000005336 cracking Methods 0.000 description 10
- 229910001567 cementite Inorganic materials 0.000 description 9
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 9
- 229910052719 titanium Inorganic materials 0.000 description 9
- 239000003929 acidic solution Substances 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 8
- 229910052758 niobium Inorganic materials 0.000 description 8
- 239000000344 soap Substances 0.000 description 8
- 229910052802 copper Inorganic materials 0.000 description 7
- 229910052749 magnesium Inorganic materials 0.000 description 7
- 238000005259 measurement Methods 0.000 description 7
- 239000002994 raw material Substances 0.000 description 7
- 229910052714 tellurium Inorganic materials 0.000 description 7
- 229910052720 vanadium Inorganic materials 0.000 description 7
- 229910052797 bismuth Inorganic materials 0.000 description 6
- 229910052791 calcium Inorganic materials 0.000 description 6
- 239000013078 crystal Substances 0.000 description 6
- 238000005098 hot rolling Methods 0.000 description 6
- 229910052759 nickel Inorganic materials 0.000 description 6
- 238000009864 tensile test Methods 0.000 description 6
- 229910019142 PO4 Inorganic materials 0.000 description 5
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 5
- 239000010410 layer Substances 0.000 description 5
- 229910052760 oxygen Inorganic materials 0.000 description 5
- 239000001301 oxygen Substances 0.000 description 5
- NBIIXXVUZAFLBC-UHFFFAOYSA-K phosphate Chemical compound [O-]P([O-])([O-])=O NBIIXXVUZAFLBC-UHFFFAOYSA-K 0.000 description 5
- 239000010452 phosphate Substances 0.000 description 5
- 229910052726 zirconium Inorganic materials 0.000 description 5
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 4
- 229910052796 boron Inorganic materials 0.000 description 4
- 238000010586 diagram Methods 0.000 description 4
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 4
- 229910001562 pearlite Inorganic materials 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 3
- 238000005266 casting Methods 0.000 description 3
- 238000000921 elemental analysis Methods 0.000 description 3
- 238000009776 industrial production Methods 0.000 description 3
- 239000013067 intermediate product Substances 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000002360 preparation method Methods 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 229910052721 tungsten Inorganic materials 0.000 description 3
- 238000004506 ultrasonic cleaning Methods 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- YRKCREAYFQTBPV-UHFFFAOYSA-N acetylacetone Chemical compound CC(=O)CC(C)=O YRKCREAYFQTBPV-UHFFFAOYSA-N 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 238000007739 conversion coating Methods 0.000 description 2
- 238000005520 cutting process Methods 0.000 description 2
- 238000004090 dissolution Methods 0.000 description 2
- 238000005242 forging Methods 0.000 description 2
- 230000035515 penetration Effects 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 238000005482 strain hardening Methods 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 229910052725 zinc Inorganic materials 0.000 description 2
- 239000011701 zinc Substances 0.000 description 2
- 229910000859 α-Fe Inorganic materials 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 239000004215 Carbon black (E152) Substances 0.000 description 1
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229910052765 Lutetium Inorganic materials 0.000 description 1
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 206010057175 Mass conditions Diseases 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- OKIZCWYLBDKLSU-UHFFFAOYSA-M N,N,N-Trimethylmethanaminium chloride Chemical compound [Cl-].C[N+](C)(C)C OKIZCWYLBDKLSU-UHFFFAOYSA-M 0.000 description 1
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 description 1
- 229910002091 carbon monoxide Inorganic materials 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 description 1
- 238000010622 cold drawing Methods 0.000 description 1
- 238000009841 combustion method Methods 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000006356 dehydrogenation reaction Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- AWJWCTOOIBYHON-UHFFFAOYSA-N furo[3,4-b]pyrazine-5,7-dione Chemical compound C1=CN=C2C(=O)OC(=O)C2=N1 AWJWCTOOIBYHON-UHFFFAOYSA-N 0.000 description 1
- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- OHSVLFRHMCKCQY-UHFFFAOYSA-N lutetium atom Chemical compound [Lu] OHSVLFRHMCKCQY-UHFFFAOYSA-N 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
- 239000000155 melt Substances 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 229910017604 nitric acid Inorganic materials 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical compound OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 238000007788 roughening Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- RYYKJJJTJZKILX-UHFFFAOYSA-M sodium octadecanoate Chemical compound [Na+].CCCCCCCCCCCCCCCCCC([O-])=O RYYKJJJTJZKILX-UHFFFAOYSA-M 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 238000010301 surface-oxidation reaction Methods 0.000 description 1
- PORWMNRCUJJQNO-UHFFFAOYSA-N tellurium atom Chemical compound [Te] PORWMNRCUJJQNO-UHFFFAOYSA-N 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
- LRXTYHSAJDENHV-UHFFFAOYSA-H zinc phosphate Chemical compound [Zn+2].[Zn+2].[Zn+2].[O-]P([O-])([O-])=O.[O-]P([O-])([O-])=O LRXTYHSAJDENHV-UHFFFAOYSA-H 0.000 description 1
- 229910000165 zinc phosphate Inorganic materials 0.000 description 1
- 229910052845 zircon Inorganic materials 0.000 description 1
- GFQYVLUOOAAOGM-UHFFFAOYSA-N zirconium(iv) silicate Chemical compound [Zr+4].[O-][Si]([O-])([O-])[O-] GFQYVLUOOAAOGM-UHFFFAOYSA-N 0.000 description 1
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- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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- 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
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
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- 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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- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
- C25F5/00—Electrolytic stripping of metallic layers or coatings
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- This disclosure relates to steel materials.
- the manufacturing process for machine structural parts is as follows.
- a descaling treatment for the purpose of removing scale from the steel material is performed on the steel material that has been spheroidized and annealed.
- the steel is pickled.
- Lubricating coating treatment is applied to the steel material after descaling treatment to apply a lubricant to the surface of the steel material.
- a steel wire is manufactured by drawing a steel material to which a lubricant has been applied.
- Steel wires are forged to produce intermediate products.
- the intermediate product is subjected to heat treatment (such as thermal refining treatment) to manufacture a machine structural part. In some cases, the intermediate product after forging is cut.
- processing such as wire drawing (cold drawing) may be performed before forging is performed.
- a lubricating coating treatment is performed on the steel material before wire drawing.
- a lubricating film treatment a lubricating film is formed on the surface of the steel material.
- a chemical conversion coating is formed on the surface of steel.
- soap metallic soap, etc.
- Patent Document 1 Steel materials that can be used as materials for machine structural parts have been proposed in International Publication No. 2015/189978 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2013-237903 (Patent Document 2).
- the steel material disclosed in Patent Document 1 has, in mass%, C: 0.005 to 0.60%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.80%, Al: 0 .01 to 0.06%, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Cr: 0 to 1.50%, Mo: 0 to 0.50%, It contains Ni: 0 to 1.00%, V: 0 to 0.50%, B: 0 to 0.0050%, Ti: 0 to 0.05%, and the balance consists of Fe and impurities.
- the metallographic structure of this steel contains pearlite.
- the value obtained by dividing the Mn content in atomic % contained in cementite in pearlite by the Mn content in atomic % contained in ferrite in pearlite is more than 0 and 5.0 or less.
- the chemical composition and metallographic structure are controlled, and furthermore, the Mn distribution ratio with respect to cementite and ferrite in pearlite is adjusted.
- Patent Document 1 describes that this can shorten the spheroidizing annealing treatment time.
- the steel material for bolts disclosed in Patent Document 2 has, in mass%, C: 0.30 to 0.40%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P : 0.030% or less, S: 0.030% or less, Al: 0.005-0.10%, Cr: 0.90-1.8%, Mo: 0.10-2.0%, N: 0.003 to 0.030%, Nb: 0 to 0.10%, the balance being Fe and impurities.
- the number ratio of carbides having an equivalent circle diameter of 1.0 ⁇ m or more among carbides having an equivalent circle diameter of 0.5 ⁇ m or more is 10% or less. In this steel material, the number ratio of coarse carbides is reduced. Patent Document 2 describes that this allows the carbides to be sufficiently solid-dissolved during quenching, thereby reducing variations in the tensile strength of the bolt product.
- Patent Documents 1 and 2 as the descaling treatment, the hydrogen embrittlement resistance of the steel material pickled before wire drawing, and the lubricating agent for the steel material in the lubricating film treatment before wire drawing Adhesion is not considered.
- the object of the present disclosure is to provide a steel material with excellent hydrogen embrittlement resistance after pickling treatment for descaling and excellent lubricant adhesion.
- the steel material according to the present disclosure has the following configuration.
- the steel material according to the present disclosure has excellent hydrogen embrittlement resistance after pickling treatment for descaling, and also has excellent lubricant adhesion.
- FIG. 1 is a diagram showing a region to be electrolyzed and removed by preliminary constant current electrolysis.
- FIG. 2 is a diagram showing a region electrolyzed by constant current electrolysis after preliminary constant current electrolysis.
- the inventors examined steel materials that can be applied as materials for machine structural parts, typified by cold forged parts such as bolts, from the viewpoint of chemical composition.
- the present inventors further investigated and examined the factors that cause hydrogen embrittlement in the steel material after the pickling process when the steel material is subjected to the pickling process for the purpose of descaling. As a result, the present inventors obtained the following findings.
- the following means can be considered from the viewpoint of chemical composition.
- A) Increase the strength of the grains of steel. Specifically, the contents of Mn, P, and S, which are elements that segregate at grain boundaries and lower the grain boundary strength, are suppressed as much as possible.
- the inventors of the present invention found that the chemical composition of steel material applicable to the material of mechanical structural parts is C: 0.30 to 0.50%, Si: 0.5% by mass %. 40% or less, Mn: 0.10-0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90-1.80%, Mo: 0.30-1.
- the present inventors further investigated means for improving the hydrogen embrittlement resistance of steel materials after pickling treatment from the standpoints other than the chemical composition. As a result, the inventors obtained the following knowledge.
- the steel portion of the surface layer region which is the region from the surface of the steel material to a depth of about 100 ⁇ m to 200 ⁇ m, is dissolved. Therefore, if excessive dissolution of the steel portion of the surface region can be suppressed, excessive generation of hydrogen can be suppressed.
- Cr and Mo form dense oxides on the surface of the steel material.
- oxides containing Cr and/or Mo are referred to as "specific oxides". If the specific oxide is formed on the surface of the steel material during the pickling treatment, it is possible to suppress direct contact of the acid solution with the surface layer of the steel material.
- the specific oxide formed on the surface of the steel material further suppresses penetration of hydrogen generated on the surface of the steel material. Therefore, by setting the Cr concentration and the Mo concentration in the surface layer region of the steel material within appropriate ranges, it is possible to suppress the generation of hydrogen during the pickling treatment and the penetration of hydrogen into the steel material.
- the Cr and Mo are contained in carbides and carbonitrides and are concentrated.
- the Cr concentration in the extraction residue obtained by electrolyzing the surface layer region of the steel material having the above chemical composition by the electrolytic extraction method is defined as [Cr] (% by mass).
- the Mo concentration in the extraction residue is defined as [Mo] (% by mass).
- the main types of extraction residues (inclusions and precipitates) in the surface region are carbides and carbonitrides.
- carbides and carbonitrides are also referred to as “carbides and the like”. Therefore, the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue in the surface layer region are indicators of the Cr concentration and Mo concentration in the carbide or the like.
- the present inventors investigated and examined the relationship between the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue in the surface layer region and the hydrogen embrittlement resistance of the steel material after the pickling treatment. .
- the total amount of Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the surface layer region is 10.0% or more in the steel material in which the content of each element in the chemical composition is within the above range, It was found that the hydrogen embrittlement resistance of the steel material after pickling is enhanced.
- the present inventors further investigated the total amount of Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the surface region.
- the total amount of Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue in the surface layer region is 10.0% or more, and , 30.0% or less, it is possible to achieve both excellent hydrogen embrittlement resistance of the steel material after the pickling treatment and excellent lubricant adhesion.
- the steel material of this embodiment was completed based on the above technical concept.
- the steel material of this embodiment has the following configuration.
- [1] is steel, in % by mass, C: 0.30 to 0.50%, Si: 0.40% or less, Mn: 0.10-0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90 to 1.80%, Mo: 0.30 to 1.00%, Al: 0.005 to 0.100%, N: 0.003 to 0.030%, and the balance consists of Fe and impurities, After the region to a depth of 100 ⁇ 20 ⁇ m from the surface of the steel is electrolyzed and removed by preliminary constant current electrolysis, the region to a depth of 100 ⁇ 20 ⁇ m from the surface of the steel is further removed by constant current electrolysis.
- the steel material according to [1] The number ratio RN of carbides with an equivalent circle diameter of 0.8 ⁇ m or more to the number of carbides with an equivalent circle diameter of 0.5 ⁇ m or more is 5 to 20%. steel.
- the steel material of this embodiment satisfies the following feature 1 and feature 2.
- the chemical composition is, in mass %, C: 0.30 to 0.50%, Si: 0.40% or less, Mn: 0.10 to 0.60%, P: 0.030% or less, S: 0.5%.
- C 0.30-0.50% Carbon (C) enhances the hardenability of the steel material and enhances the strength of the steel material. If the C content is less than 0.30%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.50%, the toughness of the steel material is lowered even if the content of other elements is within the range of the present embodiment. In this case, in the process of manufacturing cold forged parts using steel as a raw material, the cold forging cracking resistance of the steel deteriorates. Therefore, the C content is 0.30-0.50%. A preferred lower limit for the C content is 0.31%, more preferably 0.32%, and still more preferably 0.33%. A preferable upper limit of the C content is 0.48%, more preferably 0.46%, and still more preferably 0.44%.
- Si 0.40% or less
- Silicon (Si) is an impurity. Si lowers the toughness of the steel material. If the Si content exceeds 0.40%, even if the content of other elements is within the range of the present embodiment, the toughness of the steel material is significantly lowered, and the cold forging cracking resistance of the steel material is lowered. Therefore, the Si content is 0.40% or less. It is preferable that the Si content is as low as possible. However, excessive reduction of the Si content reduces productivity and increases manufacturing costs. Therefore, when considering normal industrial production, the lower limit of the Si content is preferably more than 0%, more preferably 0.01%, more preferably 0.02%, and still more preferably 0.03%. %. A preferable upper limit of the Si content is 0.38%, more preferably 0.36%, and still more preferably 0.34%.
- Mn 0.10-0.60%
- Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability of the steel material and increases the strength of the steel material. If the Mn content is less than 0.10%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.60%, even if the contents of other elements are within the range of the present embodiment, Mn excessively segregates at the grain boundaries and reduces the grain boundary strength. As a result, the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Mn content is 0.10-0.60%. A preferred lower limit for the Mn content is 0.12%, more preferably 0.14%, and still more preferably 0.16%. A preferable upper limit of the Mn content is 0.58%, more preferably 0.56%, and still more preferably 0.54%.
- Phosphorus (P) is an impurity. P segregates at the grain boundaries of the steel material and lowers the grain boundary strength. If the P content exceeds 0.030%, even if the content of other elements is within the range of the present embodiment, the grain boundary strength will decrease, and the hydrogen embrittlement resistance of the steel material after the pickling treatment will occur. Decrease in properties. Therefore, the P content is 0.030% or less. The lower the P content is, the better. However, excessive reduction of the P content reduces productivity and increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the P content is more than 0%, more preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. %. A preferable upper limit of the P content is 0.028%, more preferably 0.026%, and still more preferably 0.024%.
- S 0.030% or less Sulfur (S) is an impurity. S segregates at the grain boundaries of the steel material and lowers the grain boundary strength. If the S content exceeds 0.030%, the hydrogen embrittlement resistance of the pickled steel deteriorates even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.030% or less. It is preferable that the S content is as low as possible. However, excessive reduction of the S content reduces productivity and increases manufacturing costs. Therefore, when considering normal industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, more preferably 0.002%, still more preferably 0.003 %. A preferable upper limit of the S content is 0.028%, more preferably 0.026%, and still more preferably 0.024%.
- Chromium (Cr) dissolves in carbides and forms specific oxides containing Cr and Mo on the steel material surface during pickling. Formation of this specific oxide suppresses generation of hydrogen due to excessive pickling. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. Cr further enhances the hardenability of the steel and increases the strength of the steel. If the Cr content is less than 0.90%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 1.80%, even if the content of other elements is within the range of the present embodiment, the toughness of the steel material is lowered, and the cold forging cracking resistance of the steel material is lowered.
- the Cr content is 0.90-1.80%.
- a preferable lower limit of the Cr content is 0.91%, more preferably 0.92%, and still more preferably 0.93%.
- a preferable upper limit of the Cr content is 1.75%, more preferably 1.70%, further preferably 1.65%.
- Mo 0.30-1.00% Molybdenum (Mo) dissolves in carbides and forms specific oxides containing Cr and Mo on the surface of the steel during pickling. Formation of this specific oxide suppresses generation of hydrogen due to excessive pickling. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. Mo further enhances the hardenability of the steel material and enhances the strength of the steel material. If the Mo content is less than 0.30%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
- the Mo content is 0.30-1.00%.
- a preferable lower limit of the Mo content is 0.31%, more preferably 0.32%, and still more preferably 0.33%.
- a preferable upper limit of the Mo content is 0.95%, more preferably 0.90%, and still more preferably 0.85%.
- Al 0.005-0.100%
- Aluminum (Al) deoxidizes steel. Al further combines with N to form Al nitrides. Al nitride suppresses coarsening of crystal grains due to the pinning effect. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.100%, coarse Al nitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse Al nitride serves as a starting point for fracture.
- the Al content is 0.005-0.100%.
- a preferable lower limit of the Al content is 0.006%, more preferably 0.007%, and still more preferably 0.008%.
- a preferable upper limit of the Al content is 0.090%, more preferably 0.080%, and still more preferably 0.070%.
- the Al content means the total Al (Total-Al) content.
- N 0.003-0.030%
- Nitrogen (N) combines with Al to form nitrides.
- Al nitride suppresses coarsening of crystal grains due to the pinning effect.
- the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced.
- the N content is less than 0.003%, the above effect cannot be sufficiently obtained even if the other element content is within the range of the present embodiment.
- the N content exceeds 0.030%, coarse nitrides are formed even if the content of other elements is within the range of the present embodiment.
- Coarse nitrides serve as starting points for fracture. Therefore, the cold forging cracking resistance of the steel material is lowered. Therefore, the N content is 0.003-0.030%.
- a preferable lower limit of the N content is 0.004%, more preferably 0.005%, and still more preferably 0.006%.
- a preferable upper limit of the N content is 0.029%, more preferably 0.028%, and still more preferably 0.027%.
- the remainder of the chemical composition of the steel material of this embodiment consists of Fe and impurities.
- the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material is industrially manufactured, and are within a range that does not adversely affect the steel material of the present embodiment. means acceptable.
- the chemical composition of the steel material of the present embodiment may further contain one or more selected from the following Groups 1 to 5 in place of part of Fe.
- Cu 0.40% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%.
- Cu forms a dense oxide during the pickling treatment. This suppresses generation of hydrogen due to excessive pickling. Therefore, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.40%, descaling of the steel material after the pickling treatment becomes insufficient even if the content of other elements is within the range of the present embodiment. As a result, the lubricant adhesion of the steel is reduced.
- the Cu content is 0-0.40%, and if included, the Cu content is 0.40% or less.
- the lower limit of the Cu content is preferably over 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.03%.
- a preferable upper limit of the Cu content is 0.35%, more preferably 0.30%, and still more preferably 0.25%.
- Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%.
- Ni forms a dense oxide during the pickling treatment. This suppresses generation of hydrogen due to excessive pickling. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. If Ni is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.40%, descaling of the steel material after the pickling treatment becomes insufficient even if the content of other elements is within the range of the present embodiment. As a result, the lubricant adhesion of the steel is reduced.
- the Ni content is 0 to 0.40%, and if included, the Ni content is 0.40% or less.
- the lower limit of the Ni content is preferably over 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.03%.
- a preferable upper limit of the Ni content is 0.35%, more preferably 0.30%, and still more preferably 0.25%.
- V, Ti and Nb The chemical composition of the steel material of the present embodiment is further replaced by part of Fe, selected from the group consisting of V: 0.50% or less, Ti: 0.100% or less, and Nb: 0.100% or less. may contain one or more of the All of these elements are optional elements and may not be contained.
- V, Ti, and Nb combine with C and N to form carbonitrides. These carbonitrides suppress coarsening of crystal grains due to the pinning effect. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced.
- V, Ti and Nb are described below.
- V 0.50% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%.
- V combines with C and N to form carbonitrides and suppress coarsening of crystal grains.
- the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. If even a small amount of V is contained, the above effect can be obtained to some extent.
- the V content exceeds 0.50%, coarse carbonitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse carbonitrides serve as starting points for fracture. Therefore, the cold forging cracking resistance of the steel material is lowered.
- the V content is 0 to 0.50%, and when included, the V content is 0.50% or less.
- the lower limit of the V content is preferably over 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.03%.
- a preferable upper limit of the V content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.
- Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%.
- Ti When Ti is contained, that is, when the Ti content is more than 0%, Ti combines with C and N to form carbonitrides, suppressing grain coarsening. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.100%, coarse carbonitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse carbonitrides serve as starting points for fracture. Therefore, the cold forging cracking resistance of the steel material is lowered.
- the Ti content is 0-0.100%, and if included, the Ti content is 0.100% or less.
- the lower limit of the Ti content is preferably over 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferable upper limit of the Ti content is 0.080%, more preferably 0.060%, and still more preferably 0.040%.
- the chemical composition of the steel material of the present embodiment may further contain B: 0.0100% or less instead of part of Fe.
- B is an optional element and may not be contained.
- the B content is 0 to 0.0100%, and if included, the B content is 0.0100% or less.
- the lower limit of the B content is preferably over 0%, more preferably 0.0001%, still more preferably 0.0002%, still more preferably 0.0003%.
- a preferable upper limit of the B content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.
- the chemical composition of the steel material of the present embodiment may further contain W: 0.500% or less instead of part of Fe.
- W is an optional element and may not be contained.
- W 0.500% or less
- Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%.
- W enhances the hardenability of the steel material and enhances the strength of the steel material. If even a small amount of W is contained, the above effect can be obtained to some extent.
- the W content exceeds 0.500%, the toughness of the steel material is lowered, and the cold forging crack resistance of the steel material is lowered. Therefore, the W content is 0 to 0.500%, and when included, the W content is 0.500% or less.
- a preferable lower limit of the W content is more than 0%, more preferably 0.005%, and still more preferably 0.010%.
- a preferable upper limit of the W content is 0.480%, more preferably 0.460%, and still more preferably 0.440%.
- the chemical composition of the steel material of the present embodiment further includes Ca: 0.010% or less, Mg: 0.100% or less, rare earth elements (REM): 0.100% or less, and Bi: 0% instead of part of Fe. .300% or less, Te: 0.300% or less, and Zr: 0.300% or less. All of these elements are optional elements and may not be contained. When included, Ca, Mg, REM, Bi, Te and Zr all enhance the machinability of the steel. Ca, Mg, REM, Bi, Te and Zr are described below.
- Ca 0.010% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%.
- Ca enhances the machinability of the steel material. If even a little Ca is contained, the above effect can be obtained to some extent.
- the Ca content is 0-0.010%, and when included, the Ca content is 0.010% or less.
- a preferable lower limit of the Ca content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferable upper limit of the Ca content is 0.008%, more preferably 0.006%, and still more preferably 0.004%.
- Mg 0.100% or less
- Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%.
- Mg enhances the machinability of the steel material. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.100%, the hot ductility of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mg content is 0-0.100%, and if included, the Mg content is 0.100% or less.
- a preferable lower limit of the Mg content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferable upper limit of the Mg content is 0.090%, more preferably 0.085%, and still more preferably 0.080%.
- Rare earth elements 0.100% or less
- Rare earth elements are optional elements and may not be contained. That is, the REM content may be 0%.
- REM enhances the machinability of steel. The above effect can be obtained to some extent if REM is contained even in a small amount. However, if the REM content exceeds 0.100%, the hot ductility of the steel is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the REM content is 0-0.100%, and if included, the REM content is 0.100% or less.
- a preferable lower limit of the REM content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferred upper limit for the REM content is 0.090%, more preferably 0.085%, and even more preferably 0.080%.
- Bi 0.300% or less Bismuth (Bi) is an optional element and may not be contained. That is, the Bi content may be 0%. When Bi is contained, Bi enhances the machinability of the steel material. If even a little Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.300%, the hot ductility of the steel is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Bi content is 0-0.300%, and if included, the Bi content is 0.300% or less. A preferable lower limit of the Bi content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%. A preferable upper limit of the Bi content is 0.280%, more preferably 0.260%, and still more preferably 0.240%.
- Te 0.300% or less
- Tellurium (Te) is an optional element and may not be contained. That is, the Te content may be 0%.
- Te enhances the machinability of the steel material. If even a little Te is contained, the above effect can be obtained to some extent.
- the Te content is 0-0.300%, and if included, the Te content is 0.300% or less.
- the lower limit of the Te content is preferably over 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferable upper limit of the Te content is 0.280%, more preferably 0.260%, and still more preferably 0.240%.
- Zr 0.300% or less
- Zircon (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%.
- Zr enhances the machinability of the steel material. If even a small amount of Zr is contained, the above effect can be obtained to some extent.
- the Zr content exceeds 0.300%, the hot ductility of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Zr content is 0-0.300%, and if included, the Zr content is 0.300% or less.
- the lower limit of the Zr content is preferably over 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%.
- a preferable upper limit of the Zr content is 0.280%, more preferably 0.260%, and still more preferably 0.240%.
- the chemical composition of the steel material of this embodiment can be measured by a well-known component analysis method (JIS G 0321:2017). Specifically, chips are collected from the R/2 portion of the steel material using a drill. Here, the R/2 portion means the central portion of the radius R of the steel material in a cross section perpendicular to the axial direction (rolling direction) of the steel material. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas fusion-thermal conductivity method.
- JIS G 0321:2017 well-known component analysis method
- a region from the surface to a depth of 100 ⁇ 20 ⁇ m means a region between the surface and a depth of D ⁇ m from the surface.
- a position at a depth of 100 ⁇ 20 ⁇ m from the surface means that the depth D from the surface is within the range of 80 to 120 ⁇ m.
- FIG. 1 is a diagram showing a region to be electrolyzed and removed by preliminary constant-current electrolysis.
- FIG. 2 is a diagram showing a region electrolyzed by constant current electrolysis after preliminary constant current electrolysis.
- An extraction residue is obtained. That is, the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue described above are the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue obtained in the substantial surface region RE1.
- the outermost layer region RE0 removed by preliminary constant-current electrolysis includes scales formed on the steel material surface and impurities adhering to the steel material surface. Therefore, the outermost surface layer region RE0 was not used to measure the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue, and the Cr concentration in the extraction residue in the substantial surface layer region RE1 where the influence of scale and impurities was extremely small. [Cr] and Mo concentration [Mo] are measured.
- the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue in the outermost layer region RE0 are the Cr concentration [Cr] and Mo concentration in the extraction residue in the substantial surface layer region RE1 It is considered that almost the same numerical value as the Mo concentration [Mo] is obtained.
- a method for measuring the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue will be described below.
- Galvanostatic electrolysis was performed on the cut surface coated sample using a 10% AA-based solution (solution containing 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol solution in volume fraction). implement.
- preliminary constant-current electrolysis is performed to remove the outermost layer region RE0 of the sample steel.
- the sample steel is immersed in an alcohol solution.
- ultrasonic cleaning is performed to remove deposits on the surface of the sample steel material. The mass of the sample steel material from which deposits have been removed, that is, the mass of the sample steel material before the constant current electrolysis is measured.
- the 10% AA-based solution used in this constant-current electrolysis and the alcohol solution used in subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 ⁇ m to extract residues. That is, the extraction residue in the substantial surface layer region RE1 electrolyzed by the constant current electrolysis is obtained.
- the extraction residue is dissolved in acid to obtain a solution.
- a chemical elemental analysis using ICP-AES is performed on the solution to obtain the Cr mass in the extraction residue and the Mo mass in the extraction residue.
- the Cr concentration [Cr] (% by mass) in the extraction residue is obtained by dividing the Cr mass by the total mass of the extraction residue.
- the Mo mass is divided by the total mass of the extraction residue to obtain the Mo concentration [Mo] (% by mass) in the extraction residue.
- F1 [Cr] + [Mo].
- the extraction residue of the substantial surface layer region RE1 obtained by the method described above contains inclusions and precipitates. Precipitates include carbides, carbonitrides and nitrides. However, the main types of extraction residues are carbides and carbonitrides. Therefore, although F1 indicates the total amount of Cr concentration and Mo concentration in the extraction residue, F1 can actually be an index of Cr concentration and Mo concentration in carbides and carbonitrides. If the Cr concentration and Mo concentration in the carbides and carbonitrides are high, the Cr concentration and Mo concentration dissolved in the steel material are also considered to be high. Therefore, F1 is also an index of the concentration of Cr and Mo dissolved in the surface layer of the steel material.
- F1 is less than 10.0, the total amount of Cr concentration and Mo concentration in the extraction residue of the steel material surface layer is insufficient. In this case, the dissolved Cr concentration and the dissolved Mo concentration in the surface layer of the steel are insufficient. Therefore, during the pickling treatment, the specific oxides containing Cr and Mo are not sufficiently formed on the surface of the steel material. Therefore, even if the content of each element in the chemical composition of the steel is within the above range, hydrogen is excessively generated on the surface of the steel by pickling, and the generated hydrogen tends to penetrate into the steel. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is lowered.
- the total amount of Cr concentration and Mo concentration in the extraction residue of the surface layer of the steel is excessively high.
- the solid solution Cr concentration and the solid solution Mo concentration in the surface layer of the steel material are too high. Therefore, during the pickling of the steel material, an excessive amount of specific oxides are formed on the surface of the steel material.
- the lubricating coating reacts with Fe on the surface of the steel material to enhance adhesion to the surface of the steel material.
- the specific oxide when the specific oxide is excessively generated on the steel material surface, the specific oxide makes it difficult for the lubricating coating to react with Fe on the steel material surface. As a result, the adhesion of the lubricant to the surface of the steel material is reduced.
- the total amount of Cr concentration and Mo concentration in the extraction residue of the surface layer of the steel material is an appropriate amount.
- the solid solution Cr concentration and the solid solution Mo concentration in the surface layer of the steel are also appropriate amounts. Therefore, an appropriate amount of specific oxide is formed on the surface of the steel material during the pickling treatment. As a result, the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced. Furthermore, during the pickling treatment, the specific oxide is not excessively formed on the surface of the steel material. Therefore, in the lubricating coating treatment before wire drawing, the lubricating coating tends to react with Fe on the surface of the steel material. As a result, the adhesion of the lubricating coating to the surface of the steel material increases, and the lubricant adherence of the steel material increases.
- a preferable lower limit of F1 is 11.0, more preferably 12.0, and still more preferably 13.0.
- a preferable upper limit of F1 is 29.0, more preferably 28.0, and still more preferably 27.0.
- the steel material of the present embodiment Preferably, the steel material of the present embodiment further satisfies characteristics 1 and 2, and further satisfies characteristic 3. (Feature 3)
- the ratio of the number of carbides having an equivalent circle diameter of 0.8 ⁇ m or more to the number of carbides having an equivalent circle diameter of 0.5 ⁇ m or more is 5 to 20%. Feature 3 will be described below.
- RN coarse carbide number ratio RN
- carbides having an equivalent circle diameter of 0.8 ⁇ m or more are defined as “coarse carbides”.
- the number ratio of coarse carbides to the number of carbides having an equivalent circle diameter of 0.5 ⁇ m or more is defined as coarse carbide number ratio RN (%).
- carbides with an equivalent circle diameter of 0.5 ⁇ m or more are substantially cementite (Fe 3 C), and other carbides (including carbonitrides) can be ignored.
- the steel material satisfies the features 1 and 2, there is no particular limitation on the coarse carbide number ratio RN, and the hydrogen embrittlement resistance of the steel material after the pickling treatment is enhanced, as well as the lubricant adhesion.
- the coarse carbide number ratio RN in the steel material that satisfies feature 1 and feature 2 is 5 to 20%. If the coarse carbide number ratio RN is 5% or more, the hydrogen embrittlement resistance of the steel material after the pickling treatment is further enhanced. Further, if the coarse carbide number ratio RN is 20% or less, the lubricant adhesion of the steel material is further enhanced. Therefore, the preferred coarse carbide number ratio RN is 5 to 20%. A more preferable lower limit of the coarse carbide number ratio RN is 6%, more preferably 7%, and still more preferably 8%. A more preferable upper limit of the coarse carbide number ratio RN is 19%, more preferably 18%, and still more preferably 17%.
- the coarse carbide number ratio RN of the steel material can be measured by the following method.
- the steel material is cut perpendicularly to the axial direction (rolling direction) of the steel material at six different positions in the longitudinal direction of the steel material, and six sample steel materials are collected.
- a cross section perpendicular to the axial direction of the sample steel corresponds to the cross section of the steel.
- a cut surface perpendicular to the axial direction of the surface of each sample steel material is used as an observation surface.
- the viewing surface is etched with a picral etchant to reveal the carbide.
- the observation area is the area from the surface of the steel material to a depth of 100 ⁇ m to 200 ⁇ m (substantial surface region RE1).
- a scanning electron microscope is used to generate photographic images (secondary electron images) of any six fields of view at a magnification of 5000 out of the observation area.
- the area of each field of view is 19 ⁇ m ⁇ 25 ⁇ m.
- the chars are identified by contrast in the photographic image of each field. Calculate the equivalent circle diameter of the specified carbide.
- carbides having an equivalent circle diameter of 0.5 ⁇ m or more are to be measured.
- the number of carbides with an equivalent circle diameter of 0.5 ⁇ m or more and the number of carbides with an equivalent circle diameter of 0.8 ⁇ m or more (coarse carbides) in each field of view are determined.
- the ratio (%) of the total number of coarse carbides with respect to the total number of carbides with an equivalent circle diameter of 0.5 ⁇ m or more in all fields of view (6 ⁇ 6 36 fields of view: total area 17400 ⁇ m 2 ), the coarse carbide number ratio RN (%).
- the microstructure of the steel material according to this embodiment is not particularly limited.
- the steel material of this embodiment is used as a material for mechanical structural parts. Then, heat treatment such as refining treatment is performed during the manufacturing process of the mechanical structural parts. In other words, the structure of the steel material used as the raw material undergoes a phase transformation due to heat treatment such as refining treatment. Therefore, as described above, the microstructure itself of the steel material used as the material for the machine structural parts is not particularly limited.
- the microstructure of the steel material of this embodiment is, for example, a structure containing a BCC phase, which is a phase whose crystal structure is a body-centered cubic (BCC), and carbides arranged in the BCC phase. .
- BCC structure A structure consisting of a BCC phase and carbides dispersed in the BCC phase is referred to herein as a "BCC structure.”
- Carbide contained in the BCC structure is, for example, cementite.
- the cementite may be lamellar cementite or spherical cementite.
- Cementite may be present in the BCC phase in the form of dots.
- Microstructures can be identified by the following methods.
- a test piece including the R/2 portion is taken from a section perpendicular to the axial direction (rolling direction) of the steel material.
- the surface corresponding to the cross section perpendicular to the axial direction of the steel material is used as the observation surface.
- the observation surface is mirror-polished, the observation surface is etched using 2% nitric acid alcohol (nital etchant).
- the R/2 portion in the etched observation surface is observed using a 400x optical microscope.
- the area of the observation field is 500 ⁇ m ⁇ 500 ⁇ m.
- the BCC phase and carbide can be identified from the contrast and morphology.
- the steel material of this embodiment may be a steel bar or a wire rod.
- the diameter of the steel material is not particularly limited.
- the diameter of the steel material is, for example, 5-50 mm.
- the steel material of this embodiment is excellent in hydrogen embrittlement resistance and lubricant adhesion after pickling treatment when descaling treatment is performed by pickling treatment. Therefore, it is suitable as a steel material for cold working applications such as wire drawing and cold forging. However, the steel material of this embodiment can of course be used for applications other than cold working applications.
- An example of the steel manufacturing method according to the present embodiment includes the following steps.
- (Step 1) Material preparation step (Step 2) Hot working step (Step 3) Descaling treatment step (Step 4) Spheroidizing annealing step Each step will be described below.
- Step 1 Material preparation step
- a material is prepared in which the content of each element in the chemical composition is within the range of the present embodiment.
- the material is manufactured, for example, by the following method.
- a molten steel whose chemical composition satisfies feature 1 is produced.
- molten steel a raw material (slab or ingot) is produced by casting.
- molten steel is used to produce a slab (bloom) by a well-known continuous casting method.
- an ingot is produced by a well-known ingot casting method using molten steel.
- Hot working is performed on the prepared material to produce an intermediate steel material.
- hot rolling is performed as hot working, for example, there are the following methods.
- the hot working process which is based on hot rolling, includes a rough rolling process in which a raw material is roughly rolled into a billet, and a finish rolling process in which the billet is finish-rolled into an intermediate steel material.
- the rough rolling step includes the following steps. After heating the raw material (ingot or cast piece), it is bloomed using a blooming mill. If necessary, after blooming, it is further rolled by a continuous rolling mill to produce a billet. In a continuous rolling mill, horizontal roll stands and vertical roll stands are alternately arranged in a row. The raw material is rolled into a billet using grooves formed on rolling rolls of each stand of the continuous rolling mill.
- the finish rolling process includes the following processes.
- the billet is put into a heating furnace and heated.
- the heated billet is subjected to finish rolling (hot rolling) in a row of finishing rolling mills to produce an intermediate steel product.
- a finishing mill train includes a plurality of stands arranged in a row. Each stand includes multiple rolls arranged around the pass line.
- a billet is rolled using grooves formed on rolling rolls of each stand to produce an intermediate steel material.
- Step 3 Descaling treatment step
- oxide scale formed on the surface of the intermediate steel material produced in the hot working step is removed.
- the descaling process includes a pickling treatment process and a water washing process. Each step will be described below.
- the intermediate steel material is immersed in an acid solution to remove oxide scale on the surface of the intermediate steel material.
- the pickling treatment step is performed, for example, under the following conditions 1 to 3.
- Condition 1 Acid solution temperature T1 (°C): 30 to 60°C
- Condition 2 Hydrochloric acid concentration C1 (% by mass) of acidic solution: 5.0 to 20.0% by mass
- Condition 3 Immersion time t1 (minutes) in acidic solution: 2.0 to 10.0 minutes
- Conditions 1 to 3 are described below.
- the amounts of Cr and Mo that migrate (diffuse) from the carbides in the intermediate steel material to the surface of the steel material and are absorbed by the oxide scale increase. Therefore, in the steel material, the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue become too low.
- the surface of the intermediate steel material after the pickling process will: Oxidized scale is not sufficiently removed. Therefore, in the subsequent spheroidizing annealing, the oxide scale formed on the surface of the intermediate steel is insufficient. In this case, the amounts of Cr and Mo that migrate (diffuse) from the carbides in the intermediate steel material to the surface of the steel material and are absorbed by the oxide scale become insufficient. Therefore, in the steel material, the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue become too high.
- the acid solution temperature T1 is 30 to 60° C.
- the hydrochloric acid concentration C1 of the acid solution is 5.0 to 20.0% by mass
- the immersion time t1 is 2.0 to 10.0 minutes, other production
- the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue of the steel are within appropriate ranges.
- a preferable lower limit of the acidic solution temperature T1 is 33°C, and a preferable upper limit is 57°C.
- a preferable lower limit of the hydrochloric acid concentration C1 of the acidic solution is 5.3% by mass, and a preferable upper limit is 19.7% by mass.
- a preferred lower limit for the immersion time t1 is 2.3 minutes, and a preferred upper limit is 9.7 minutes.
- the intermediate steel material after the pickling treatment process is immersed in a water tank to remove the acid solution adhering to the surface of the intermediate steel material.
- the washing process is performed under the following condition 4.
- Condition 4 Immersion time tw in water tank: 1.0 to 5.0 minutes
- the immersion time tw is too long, the acid solution remaining on the surface of the intermediate steel material after the pickling process will be insufficient.
- the surface of the intermediate steel material is less likely to oxidize during the subsequent spheroidizing annealing process. Therefore, during spheroidizing annealing, it becomes difficult for Cr and Mo to migrate from the carbides in the intermediate steel material to the surface of the steel material. As a result, the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue of the steel become too high.
- the immersion time tw in the water tank is 1.0 to 5.0 minutes, the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the steel are Appropriate range.
- a preferable lower limit of the immersion time tw in the water tank is 1.3 minutes, and a preferable upper limit is 4.7 minutes.
- the temperature of the water in the water tank is, for example, 10-50.degree.
- the temperature of the water is normal temperature (5-35°C).
- Step 4 Spheroidizing annealing step
- the intermediate steel material after the descaling treatment process is subjected to spheroidizing annealing to produce the steel material of the present embodiment.
- carbides typified by cementite are spheroidized to enhance the cold workability of the steel material.
- the spheroidizing annealing step is performed, for example, under conditions 5 to 7 below.
- Condition 6 Annealing temperature T2: 680-840°C
- Condition 7 Annealing time t2: 0.1 to 3.0 hours
- Conditions 5 to 7 will be described below.
- a reducing gas is introduced into the atmosphere in order to suppress surface oxidation of the intermediate steel material during annealing.
- the reducing gas is, for example, one or more selected from the group consisting of CO, H2 and hydrocarbon gases. If the reducing gas concentration in the atmosphere is too low compared to the oxygen concentration in the atmosphere, the surface of the intermediate steel material will be excessively oxidized. In this case, excessive Cr and Mo migrate from the carbides in the intermediate steel to the surface of the steel. As a result, the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue of the steel material become low.
- Annealing temperature T2 in the spheroidizing annealing step is, for example, 680 to 840° C.
- annealing time t2 is, for example, 0.1 to 3.0 hours. If the annealing temperature T2 and the annealing time t2 are within the ranges described above, the Cr concentration [Cr] and the Mo concentration [Mo] in the extraction residue of the steel are within appropriate ranges.
- Preferred annealing temperature T2 and preferred annealing time t2 are as follows.
- Annealing temperature T2 700-800°C
- Annealing time t2 0.5 to 2.0 hours
- the number of coarse carbides in the surface layer region of the steel material is The ratio RN becomes 5 to 20%.
- the hydrogen embrittlement resistance of the steel material during pickling is further enhanced, and the lubricant adhesion is further enhanced.
- the steel material according to the present embodiment is manufactured through the manufacturing process described above.
- the steel material of the present embodiment is used as a material for structural machine parts.
- descaling treatment including pickling treatment may be performed on the steel material during the manufacturing process of the structural machine component.
- the descaling-treated steel material is subjected to lubricating coating treatment and then to wire drawing.
- the steel material of the present embodiment is excellent after the pickling treatment. It is possible to achieve both excellent hydrogen embrittlement resistance and excellent lubricant adhesion.
- the effect of one aspect of the steel material of this embodiment will be explained more specifically by way of examples.
- the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one condition example.
- “-" in Tables 1-1 and 1-2 means that the content of the corresponding element is 0% in significant figures (values up to the least significant digit) specified in the embodiment. In other words, it means that the corresponding element content is 0% when rounded off to the specified significant digits (values up to the least significant digit) in the above embodiment.
- the Cu content specified in the present embodiment is specified by a numerical value up to the second decimal place. Therefore, for test number 1 in Table 1-1, it means that the measured Cu content was 0% when rounded to the third decimal place.
- the Ni content specified in the present embodiment is specified by a numerical value up to the second decimal place.
- test number 1 in Table 1-1 it means that the measured Ni content was 0% when rounded to the third decimal place.
- Rounding off means rounding down if the digit (fraction) below the defined minimum digit is less than 5, and rounding up if it is 5 or more.
- Blooms were manufactured by continuously casting each of the molten steels in Tables 1-1 and 1-2.
- the bloom was subjected to hot working steps (rough rolling step and finish rolling step). Specifically, in the rough rolling step, after heating the bloom to 1200° C., hot rolling was performed to produce a billet having a cross-sectional shape of 160 mm ⁇ 160 mm.
- the descaling process was performed on the intermediate steel.
- Table 2 shows the temperature T1 (° C.) of the acid solution, the concentration C1 (mass %) of hydrochloric acid in the acid solution, and the immersion time t1 (minutes) of the acid solution in the pickling process.
- Table 2 shows the immersion time tw (minutes) of the water tank in the water washing step. The temperature of the water in the water tank used in the washing process was 25°C.
- a spheroidizing annealing process was performed on the steel bar after the descaling process.
- Table 2 shows the gas concentration ratio RG, annealing temperature T2 (°C), and annealing time t2 (hours) in the spheroidizing annealing.
- a steel material (steel bar) was manufactured by the manufacturing process described above. The diameter of the steel material was 10-40 mm.
- test specimens with a diameter of 10 mm and a length of 500 mm were obtained by cutting perpendicularly to the axial direction (rolling direction) from four different locations in the steel material after the water washing process.
- the shape of the test piece was JIS Z 2241:2011 stipulated No. 14A test piece.
- the four specimens were divided into two groups of two (group 1 and group 2).
- a tensile test was performed 1 hour after the completion of the water washing process.
- the tensile test was performed on the group 1 test pieces in a state in which they may have been embrittled by hydrogen that has penetrated into the steel material during the pickling process.
- the two test pieces of group 2 were left in the air at room temperature for 168 hours (one week) after the completion of the water washing step, and dehydrogenated from the test pieces. Then, a tensile test was performed on the test piece after dehydrogenation. That is, the specimens of Group 2 were subjected to the tensile test without the possibility of hydrogen embrittlement.
- a tensile test conforming to JIS B 1051:2014 was carried out at room temperature (25°C) in the atmosphere to determine the tensile strength (MPa) of two test pieces. Then, the arithmetic mean value of the tensile strength (MPa) of the two pieces was defined as the tensile strength (MPa) of each group (group 1 or group 2). Specifically, the arithmetic mean value of the two tensile strengths of group 1 is defined as tensile strength 1 (MPa), and the arithmetic mean value of the tensile strengths of the two test pieces of group 2 is tensile strength 2 (MPa). defined as
- a lubricating coating was applied to the steel material after the water washing process. Specifically, the steel material was subjected to chemical conversion treatment to form a phosphate coating on the surface of the steel material.
- the bath temperature of the phosphate bath was 70° C., and the treatment time was 10 minutes.
- the phosphate was zinc phosphate.
- the steel material was immersed for 10 minutes in a soap treatment liquid containing a soap lubricant containing sodium stearate as a main component to adhere soap (metallic soap and unreacted soap) onto the phosphate coating.
- Lubricants (soap and phosphate coating) were applied to the surface of the steel material through the above steps.
- test pieces each having a diameter of 10 mm and a length of 200 mm were obtained by cutting perpendicularly to the axial direction from five different points in the axial direction of the steel material to which the lubricant was applied.
- the total weight 1 of the five test pieces was determined.
- the five test pieces were immersed in an aqueous chromic acid solution at 70° C. for 15 minutes to completely remove the lubricant.
- the total weight 2 of the 5 test pieces after immersion was determined.
- the value obtained by subtracting the total weight 2 from the total weight 1 was defined as the lubricant adhesion amount (g).
- Evaluation A Lubricating adhesion amount LA is 10 g/m 2 or more Evaluation B: Lubricating adhesion amount LA is 8 to less than 10 g/m 2 Evaluation C: Lubricating adhesion amount LA is 6 to 8 g/m 2 Evaluation D: Lubricating adhesion amount LA is 4 to 6 g/m 2 or less Evaluation E: Lubricant adhesion amount LA is 2 to 4 g/m 2 or less Evaluation X: Lubricant adhesion amount LA is less than 2 g/m 2 Evaluation A to Evaluation E, excellent lubricant adhesion I decided. In the case of evaluation X, it was determined that the lubricant adhesion of the steel material was low. Table 2 shows the evaluation results.
- test numbers 1 to 44 and 47 to 50 the coarse carbide number ratio RN was 5 to 20%. Therefore, compared with Test Nos. 45, 46, 51 and 52, they exhibited better hydrogen embrittlement resistance or better lubricant adhesion.
- test number 54 The P content of test number 54 was too high. Therefore, the hydrogen embrittlement resistance of the steel material was low.
- the S content of test number 55 was too high. Therefore, the hydrogen embrittlement resistance of the steel material was low.
- test number 56 The Al content of test number 56 was too low. Therefore, the hydrogen embrittlement resistance of the steel material was low.
- the N content of test number 57 was too low. Therefore, the hydrogen embrittlement resistance of the steel material was low.
- test number 58 the temperature T1 of the acid solution in the pickling process was low. Therefore, the F1 value exceeded the upper limit of formula (1). As a result, the lubricant adhesion of the steel material was low.
- test number 59 the hydrochloric acid concentration C1 of the acid solution in the pickling process was low. Therefore, the F1 value exceeded the upper limit of formula (1). As a result, the lubricant adhesion of the steel material was low.
- test number 60 the immersion time t1 in the pickling process was short. Therefore, the F1 value exceeded the upper limit of formula (1). As a result, the lubricant adhesion of the steel material was low.
- test number 61 the temperature T1 of the acid solution in the pickling process was high. Therefore, the F1 value was less than the lower limit of formula (1). As a result, the hydrogen embrittlement resistance of the steel material was low.
- the acid solution had a high hydrochloric acid concentration C1 in the pickling process. Therefore, the F1 value was less than the lower limit of formula (1). As a result, the hydrogen embrittlement resistance of the steel material was low.
- test number 63 the immersion time t1 in the pickling process was long. Therefore, the F1 value was less than the lower limit of formula (1). As a result, the hydrogen embrittlement resistance of the steel material was low.
- test number 65 although the chemical composition was appropriate, the water washing time tw in the water washing process was too short. Therefore, F1 was less than the lower limit of Formula (1). As a result, the hydrogen embrittlement resistance of the steel material was low.
- test number 66 Although the chemical composition was appropriate, the gas concentration ratio RG in the atmosphere in the spheroidizing annealing process was too high. Therefore, F1 exceeded the upper limit of formula (1). As a result, the lubricant adhesion of the steel material was low.
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Abstract
Description
質量%で、
C:0.30~0.50%、
Si:0.40%以下、
Mn:0.10~0.60%、
P:0.030%以下、
S:0.030%以下、
Cr:0.90~1.80%、
Mo:0.30~1.00%、
Al:0.005~0.100%、
N:0.003~0.030%、及び、
残部はFe及び不純物からなり、
予備定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域を電解して除去した後、本定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域をさらに電解して得られた抽出残渣中のCr濃度を[Cr](質量%)と定義し、前記抽出残渣中のMo濃度を[Mo](質量%)と定義したとき、式(1)を満たす、
鋼材。
10.0≦[Cr]+[Mo]≦30.0 (1)
(A)鋼材の結晶粒の強度を高める。具体的には、粒界に偏析して粒界強度を下げる元素であるMn、P、Sの含有をなるべく抑制する。
(B)鋼材の結晶粒の粗大化を抑制して、水素の局所的な集積の集中を抑制する。具体的には、AlNによるピンニング効果を利用する。そのため、Al及びNを適正量含有する。
鋼材であって、
質量%で、
C:0.30~0.50%、
Si:0.40%以下、
Mn:0.10~0.60%、
P:0.030%以下、
S:0.030%以下、
Cr:0.90~1.80%、
Mo:0.30~1.00%、
Al:0.005~0.100%、
N:0.003~0.030%、及び、
残部はFe及び不純物からなり、
予備定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域を電解して除去した後、本定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域をさらに電解して得られた抽出残渣中のCr濃度を[Cr](質量%)と定義し、前記抽出残渣中のMo濃度を[Mo](質量%)と定義したとき、式(1)を満たす、
鋼材。
10.0≦[Cr]+[Mo]≦30.0 (1)
[1]に記載の鋼材であって、
円相当径が0.5μm以上の炭化物の個数に対する、円相当径が0.8μm以上の炭化物の個数割合RNは、5~20%である、
鋼材。
[1]又は[2]に記載の鋼材であってさらに、
Feの一部に代えて、
Cu:0.40%以下、
Ni:0.40%以下、
V:0.50%以下、
Ti:0.100%以下、
Nb:0.100%以下、
B:0.0100%以下、
W:0.500%以下、
Ca:0.010%以下、
Mg:0.100%以下、
希土類元素:0.100%以下、
Bi:0.300%以下、
Te:0.300%以下、及び、
Zr:0.300%以下、
からなる群から選択される1種以上を含有する、
鋼材。
本実施形態の鋼材は、次の特徴1及び特徴2を満たす。
(特徴1)
化学組成が、質量%で、C:0.30~0.50%、Si:0.40%以下、Mn:0.10~0.60%、P:0.030%以下、S:0.030%以下、Cr:0.90~1.80%、Mo:0.30~1.00%、Al:0.005~0.100%、N:0.003~0.030%、Cu:0~0.40%、Ni:0~0.40%、V:0~0.50%、Ti:0~0.100%、Nb:0~0.100%、B:0~0.0100%、W:0~0.500%、Ca:0~0.010%、Mg:0~0.100%、希土類元素:0~0.100%、Bi:0~0.300%、Te:0~0.300%、Zr:0~0.300%、及び残部はFe及び不純物からなる。
(特徴2)
予備定電流電気分解により鋼材の表面から100±20μm深さ位置までの領域を電解して除去した後、本定電流電気分解により鋼材の表面から100±20μm深さ位置までの領域をさらに電解して得られた抽出残渣中のCr濃度を[Cr](質量%)と定義し、抽出残渣中のMo濃度を[Mo](質量%)と定義したとき、式(1)を満たす。
10.0≦[Cr]+[Mo]≦30.0 (1)
以下、特徴1及び特徴2について説明する。
本実施形態による鋼材の化学組成は、次の元素を含有する。
炭素(C)は、鋼材の焼入れ性を高めて鋼材の強度を高める。C含有量が0.30%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、C含有量が0.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の靱性が低下する。この場合、鋼材を素材として冷間鍛造部品を製造する工程において、鋼材の耐冷間鍛造割れ性が低下する。
したがって、C含有量は0.30~0.50%である。
C含有量の好ましい下限は0.31%であり、さらに好ましくは0.32%であり、さらに好ましくは0.33%である。
C含有量の好ましい上限は0.48%であり、さらに好ましくは0.46%であり、さらに好ましくは0.44%である。
シリコン(Si)は不純物である。Siは鋼材の靱性を低下する。Si含有量が0.40%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の靱性が顕著に低下し、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Si含有量は0.40%以下である。
Si含有量はなるべく低い方が好ましい。しかしながら、Si含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、Si含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。
Si含有量の好ましい上限は0.38%であり、さらに好ましくは0.36%であり、さらに好ましくは0.34%である。
マンガン(Mn)は鋼を脱酸する。Mnはさらに、鋼材の焼入れ性を高めて鋼材の強度を高める。Mn含有量が0.10%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Mn含有量が0.60%を超えれば、他の元素含有量が本実施形態の範囲内であっても、Mnが結晶粒界に過剰に偏析して、粒界強度を低下させる。その結果、鋼材の耐水素脆化特性が低下する。
したがって、Mn含有量は0.10~0.60%である。
Mn含有量の好ましい下限は0.12%であり、さらに好ましくは0.14%であり、さらに好ましくは0.16%である。
Mn含有量の好ましい上限は0.58%であり、さらに好ましくは0.56%であり、さらに好ましくは0.54%である。
リン(P)は不純物である。Pは鋼材の結晶粒界に偏析して、粒界強度を低下させる。P含有量が0.030%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粒界強度の低下に起因して、酸洗処理後の鋼材の耐水素脆化特性が低下する。
したがって、P含有量は0.030%以下である。
P含有量はなるべく低い方が好ましい。しかしながら、P含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、P含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
P含有量の好ましい上限は0.028%であり、さらに好ましくは0.026%であり、さらに好ましくは0.024%である。
硫黄(S)は不純物である。Sは鋼材の結晶粒界に偏析して、粒界強度を低下させる。S含有量が0.030%を超えれば、他の元素含有量が本実施形態の範囲内であっても、酸洗処理後の鋼材の耐水素脆化特性が低下する。
したがって、S含有量は0.030%以下である。
S含有量はなるべく低い方が好ましい。しかしながら、S含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、S含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
S含有量の好ましい上限は0.028%であり、さらに好ましくは0.026%であり、さらに好ましくは0.024%である。
クロム(Cr)は、炭化物に固溶して、酸洗処理時において、Cr及びMoを含有する特定酸化物を鋼材表面に形成する。この特定酸化物の形成により、過剰酸洗による水素の発生が抑制される。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Crはさらに、鋼材の焼入れ性を高めて鋼材の強度を高める。Cr含有量が0.90%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Cr含有量が1.80%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の靱性が低下して、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Cr含有量は0.90~1.80%である。
Cr含有量の好ましい下限は0.91%であり、さらに好ましくは0.92%であり、さらに好ましくは0.93%である。
Cr含有量の好ましい上限は1.75%であり、さらに好ましくは1.70%であり、さらに好ましくは1.65%である。
モリブデン(Mo)は、炭化物に固溶して、酸洗処理時において、Cr及びMoを含有する特定酸化物を鋼材表面に形成する。この特定酸化物の形成により、過剰酸洗による水素の発生が抑制される。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Moはさらに、鋼材の焼入れ性を高めて鋼材の強度を高める。Mo含有量が0.30%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Mo含有量が1.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の靱性が低下して、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Mo含有量は0.30~1.00%である。
Mo含有量の好ましい下限は0.31%であり、さらに好ましくは0.32%であり、さらに好ましくは0.33%である。
Mo含有量の好ましい上限は0.95%であり、さらに好ましくは0.90%であり、さらに好ましくは0.85%である。
アルミニウム(Al)は、鋼を脱酸する。Alはさらに、Nと結合してAl窒化物を形成する。Al窒化物は、ピンニング効果により結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Al含有量が0.005%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Al含有量が0.100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大なAl窒化物が生成する。粗大なAl窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Al含有量は0.005~0.100%である。
Al含有量の好ましい下限は0.006%であり、さらに好ましくは0.007%であり、さらに好ましくは0.008%である。
Al含有量の好ましい上限は0.090%であり、さらに好ましくは0.080%であり、さらに好ましくは0.070%である。
本実施形態の鋼材の化学組成において、Al含有量は、全Al(Total-Al)含有量を意味する。
窒素(N)は、Alと結合して窒化物を形成する。Al窒化物は、ピンニング効果により結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。N含有量が0.003%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、N含有量が0.030%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大な窒化物が生成する。粗大な窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、N含有量は0.003~0.030%である。
N含有量の好ましい下限は0.004%であり、さらに好ましくは0.005%であり、さらに好ましくは0.006%である。
N含有量の好ましい上限は0.029%であり、さらに好ましくは0.028%であり、さらに好ましくは0.027%である。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、次の第1群~第5群から選択される1種以上を含有してもよい。
[第1群]
Cu:0.40%以下、及び、
Ni:0.40%以下、からなる群から選択される1種以上
[第2群]
V:0.50%以下、
Ti:0.100%以下、及び、
Nb:0.100%以下、からなる群から選択される1種以上
[第3群]
B:0.0100%以下
[第4群]
W:0.500%以下
[第5群]
Ca:0.010%以下、
Mg:0.100%以下、
希土類元素:0.100%以下、
Bi:0.300%以下、
Te:0.300%以下、及び、
Zr:0.300%以下、からなる群から選択される1種以上
以下、これらの任意元素について説明する。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、Cu:0.40%以下、及び、Ni:0.40%以下、からなる群から選択される1種以上を含有してもよい。これらの元素はいずれも任意元素であり、含有されなくてもよい。含有される場合、Cu及びNiは、酸洗処理時において、緻密な酸化物を形成する。そのため、過剰酸洗による水素の発生が抑制される。その結果、本実施形態の鋼材の、酸洗処理時の耐水素脆化特性が高まる。以下、Cu及びNiについて説明する。
銅(Cu)は任意元素であり、含有されなくてもよい。つまり、Cu含有量は0%であってもよい。
Cuが含有される場合、酸洗処理時において、Cuは緻密な酸化物を形成する。これにより、過剰酸洗による水素の発生が抑制される。そのため、酸洗処理後の鋼材の耐水素脆化特性が高まる。Cuが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Cu含有量が0.40%を超えれば、他の元素含有量が本実施形態の範囲内であっても、酸洗処理後の鋼材の脱スケールが不十分となる。その結果、鋼材の潤滑剤付着性が低下する。
したがって、Cu含有量は0~0.40%であり、含有される場合、Cu含有量は0.40%以下である。
Cu含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。
Cu含有量の好ましい上限は0.35%であり、さらに好ましくは0.30%であり、さらに好ましくは0.25%である。
ニッケル(Ni)は任意元素であり、含有されなくてもよい。つまり、Ni含有量は0%であってもよい。
Niが含有される場合、酸洗処理時において、Niは緻密な酸化物を形成する。これにより、過剰酸洗による水素の発生が抑制される。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Niが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Ni含有量が0.40%を超えれば、他の元素含有量が本実施形態の範囲内であっても、酸洗処理後の鋼材の脱スケールが不十分となる。その結果、鋼材の潤滑剤付着性が低下する。
したがって、Ni含有量は0~0.40%であり、含有される場合、Ni含有量は0.40%以下である。
Ni含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。
Ni含有量の好ましい上限は0.35%であり、さらに好ましくは0.30%であり、さらに好ましくは0.25%である。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、V:0.50%以下、Ti:0.100%以下、及び、Nb:0.100%以下、からなる群から選択される1種以上を含有してもよい。これらの元素はいずれも任意元素であり、含有されなくてもよい。含有される場合、V、Ti、及びNbは、C及びNと結合して炭窒化物を形成する。これらの炭窒化物はピンニング効果により、結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。以下、V、Ti及びNbについて説明する。
バナジウム(V)は任意元素であり、含有されなくてもよい。つまり、V含有量は0%であってもよい。
Vが含有される場合、VはC及びNと結合して炭窒化物を形成して、結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Vが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、V含有量が0.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大な炭窒化物が生成する。粗大な炭窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、V含有量は0~0.50%であり、含有される場合、V含有量は0.50%以下である。
V含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。
V含有量の好ましい上限は0.45%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%である。
チタン(Ti)は任意元素であり、含有されなくてもよい。つまり、Ti含有量は0%であってもよい。
Tiが含有される場合、つまり、Ti含有量が0%超である場合、TiはC及びNと結合して炭窒化物を形成して、結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Tiが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Ti含有量が0.100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大な炭窒化物が生成する。粗大な炭窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Ti含有量は0~0.100%であり、含有される場合、Ti含有量は0.100%以下である。
Ti含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Ti含有量の好ましい上限は0.080%であり、さらに好ましくは0.060%であり、さらに好ましくは0.040%である。
ニオブ(Nb)は任意元素であり、含有されなくてもよい。つまり、Nb含有量は0%であってもよい。
Nbが含有される場合、つまり、Nb含有量が0%超である場合、NbはC及びNと結合して炭窒化物を形成して、結晶粒の粗大化を抑制する。その結果、酸洗処理後の鋼材の耐水素脆化特性が高まる。Nbが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Nb含有量が0.100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大な炭窒化物が生成する。粗大な炭窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、Nb含有量は0~0.100%であり、含有される場合、Nb含有量は0.100%以下である。
Nb含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Nb含有量の好ましい上限は0.080%であり、さらに好ましくは0.060%であり、さらに好ましくは0.040%である。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、B:0.0100%以下、を含有してもよい。Bは任意元素であり、含有されなくてもよい。
ボロン(B)は任意元素であり、含有されなくてもよい。つまり、B含有量は0%であってもよい。
Bが含有される場合、Bは鋼材の焼き入れ性を高める。Bが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、B含有量が0.0100%を超えれば、鋼材の焼き入れ性は飽和し、製造コストが高くなる。さらに、他の元素含有量が本実施形態の範囲内であっても、粗大な窒化物が生成する。粗大な窒化物は破壊の起点となる。そのため、鋼材の耐冷間鍛造割れ性が低下する。
したがって、B含有量は0~0.0100%であり、含有される場合、B含有量は0.0100%以下である。
B含有量の好ましい下限は0%超であり、さらに好ましくは0.0001%であり、さらに好ましくは0.0002%であり、さらに好ましくは0.0003%である。
B含有量の好ましい上限は0.0080%であり、さらに好ましくは0.0060%であり、さらに好ましくは0.0040%である。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、W:0.500%以下、を含有してもよい。Wは任意元素であり、含有されなくてもよい。
タングステン(W)は任意元素であり、含有されなくてもよい。つまり、W含有量は0%であってもよい。
Wが含有される場合、Wは、鋼材の焼入れ性を高めて鋼材の強度を高める。Wが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、W含有量が0.500%を超えれば、鋼材の靱性が低下して、鋼材の耐冷間鍛造割れ性が低下する。
したがって、W含有量は0~0.500%であり、含有される場合、W含有量は0.500%以下である。
W含有量の好ましい下限は0%超であり、さらに好ましくは0.005%であり、さらに好ましくは0.010%である。
W含有量の好ましい上限は0.480%であり、さらに好ましくは0.460%であり、さらに好ましくは0.440%である。
本実施形態の鋼材の化学組成はさらに、Feの一部に代えて、Ca:0.010%以下、Mg:0.100%以下、希土類元素(REM):0.100%以下、Bi:0.300%以下、Te:0.300%以下、及び、Zr:0.300%以下、からなる群から選択される1種以上を含有してもよい。これらの元素はいずれも任意元素であり、含有されなくてもよい。含有される場合、Ca、Mg、REM、Bi、Te及びZrはいずれも、鋼材の被削性を高める。以下、Ca、Mg、REM、Bi、Te及びZrについて説明する。
カルシウム(Ca)は任意元素であり、含有されなくてもよい。つまり、Ca含有量は0%であってもよい。
Caが含有される場合、Caは鋼材の被削性を高める。Caが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Ca含有量が0.010%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、Ca含有量は0~0.010%であり、含有される場合、Ca含有量は0.010%以下である。
Ca含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Ca含有量の好ましい上限は0.008%であり、さらに好ましくは0.006%であり、さらに好ましくは0.004%である。
マグネシウム(Mg)は任意元素であり、含有されなくてもよい。つまり、Mg含有量は0%であってもよい。
Mgが含有される場合、Mgは鋼材の被削性を高める。Mgが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Mg含有量が0.100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、Mg含有量は0~0.100%であり、含有される場合、Mg含有量は0.100%以下である。
Mg含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Mg含有量の好ましい上限は0.090%であり、さらに好ましくは0.085%であり、さらに好ましくは0.080%である。
希土類元素(REM)は任意元素であり、含有されなくてもよい。つまり、REM含有量は0%であってもよい。
REMが含有される場合、REMは鋼材の被削性を高める。REMが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、REM含有量が0.100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、REM含有量は0~0.100%であり、含有される場合、REM含有量は0.100%以下である。
REM含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
REM含有量の好ましい上限は0.090%であり、さらに好ましくは0.085%であり、さらに好ましくは0.080%である。
ビスマス(Bi)は任意元素であり、含有されなくてもよい。つまり、Bi含有量は0%であってもよい。
Biが含有される場合、Biは鋼材の被削性を高める。Biが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Bi含有量が0.300%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、Bi含有量は0~0.300%であり、含有される場合、Bi含有量は0.300%以下である。
Bi含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Bi含有量の好ましい上限は0.280%であり、さらに好ましくは0.260%であり、さらに好ましくは0.240%である。
テルル(Te)は任意元素であり、含有されなくてもよい。つまり、Te含有量は0%であってもよい。
Teが含有される場合、Teは鋼材の被削性を高める。Teが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Te含有量が0.300%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、Te含有量は0~0.300%であり、含有される場合、Te含有量は0.300%以下である。
Te含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Te含有量の好ましい上限は0.280%であり、さらに好ましくは0.260%であり、さらに好ましくは0.240%である。
ジルコン(Zr)は任意元素であり、含有されなくてもよい。つまり、Zr含有量は0%であってもよい。
Zrが含有される場合、Zrは鋼材の被削性を高める。Zrが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Zr含有量が0.300%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間延性が低下する。
したがって、Zr含有量は0~0.300%であり、含有される場合、Zr含有量は0.300%以下である。
Zr含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
Zr含有量の好ましい上限は0.280%であり、さらに好ましくは0.260%であり、さらに好ましくは0.240%である。
本実施形態の鋼材の化学組成は、周知の成分分析法(JIS G 0321:2017)で測定できる。具体的には、ドリルを用いて、鋼材のR/2部から切粉を採取する。ここで、R/2部とは、鋼材の軸方向(圧延方向)に垂直な断面において、鋼材の半径Rの中央部分を意味する。採取された切粉を酸に溶解させて溶液を得る。溶液に対して、ICP-AES(Inductively Coupled Plasma Atomic Emission Spectrometry)を実施して、化学組成の元素分析を実施する。C含有量及びS含有量については、周知の高周波燃焼法(燃焼-赤外線吸収法)により求める。N含有量については、周知の不活性ガス溶融-熱伝導度法を用いて求める。
本実施形態の鋼材ではさらに、予備定電流電気分解により鋼材の表面から100±20μm深さ位置までの領域を電解して除去した後、本定電流電気分解により鋼材の表面から100±20μm深さ位置までの領域をさらに電解して得られた抽出残渣中のCr濃度を[Cr](質量%)と定義し、抽出残渣中のMo濃度を[Mo](質量%)と定義したとき、式(1)を満たす。
10.0≦[Cr]+[Mo]≦30.0 (1)
ここで、「表面から100±20μm深さ位置までの領域」とは、表面と、表面からの深さDμmとの間の領域を意味する。「表面から100±20μm深さ位置」とは、表面からの深さDが80~120μmの範囲内であることを意味する。
実質表層領域RE1の抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]は次の方法で求める。
鋼材を、鋼材の軸方向(圧延方向)に対して垂直に切断して、サンプル鋼材を採取する。サンプル鋼材の軸方向に垂直な断面は、鋼材の断面に相当する。サンプル鋼材の切断面に、絶縁性樹脂をコーティングする。
F1の好ましい上限は29.0であり、さらに好ましくは28.0であり、さらに好ましくは27.0である。
好ましくは、本実施形態の鋼材はさらに、特徴1及び特徴2を満たし、さらに、特徴3を満たす。
(特徴3)
円相当径が0.5μm以上の炭化物の個数に対する、円相当径が0.8μm以上の炭化物の個数割合は、5~20%である。
以下、特徴3について説明する。
鋼材中の炭化物のうち、円相当径が0.8μm以上の炭化物を「粗大炭化物」と定義する。円相当径が0.5μm以上の炭化物の個数に対する、粗大炭化物の個数割合を、粗大炭化物個数割合RN(%)と定義する。粗大炭化物個数割合RNは以下の式で定義できる。
RN=粗大炭化物の個数/円相当径が0.5μm以上の炭化物の個数×100
粗大炭化物個数割合RNのさらに好ましい下限は6%であり、さらに好ましくは7%であり、さらに好ましくは8%である。
粗大炭化物個数割合RNのさらに好ましい上限は19%であり、さらに好ましくは18%であり、さらに好ましくは17%である。
鋼材の粗大炭化物個数割合RNは次の方法で測定できる。
鋼材の長手方向の異なる6つの位置で、鋼材を、鋼材の軸方向(圧延方向)に垂直に切断し、6つのサンプル鋼材を採取する。サンプル鋼材の軸方向に垂直な断面は、鋼材の断面に相当する。各サンプル鋼材の表面のうち、軸方向に垂直な切断面を、観察面とする。観察面をピクラール腐食液でエッチングして、炭化物を現出させる。
本実施形態による鋼材のミクロ組織は特に限定されない。本実施形態の鋼材は、機械構造用部品の素材として用いられる。そして、機械構造用部品の製造工程中で、調質処理等の熱処理が施される。つまり、素材として用いられる鋼材の組織は、調質処理等の熱処理により、相変態する。そのため、上述のとおり、機械構造用部品の素材として用いられる鋼材のミクロ組織自体は特に限定されない。
ミクロ組織は、次の方法で特定することができる。鋼材の軸方向(圧延方向)に垂直な断面のうち、R/2部を含む試験片を採取する。試験片の表面のうち、鋼材の軸方向に垂直な断面に相当する表面を、観察面とする。
観察面を鏡面研磨した後、2%硝酸アルコール(ナイタール腐食液)を用いて観察面をエッチングする。エッチングされた観察面中のR/2部を、400倍の光学顕微鏡を用いて観察する。観察視野の面積は500μm×500μmとする。
本実施形態の鋼材は、棒鋼であってもよいし、線材であってもよい。鋼材の直径は特に限定されない。鋼材の直径は例えば、5~50mmである。
本実施形態による鋼材の製造方法の一例を説明する。以降に説明する鋼材の製造方法は、本実施形態による鋼材を製造するための一例である。したがって、上述の構成を有する鋼材は、以降に説明する製造方法以外の他の製造方法により製造されてもよい。しかしながら、以降に説明する製造方法は、本実施形態による鋼材の製造方法の好ましい一例である。
(工程1)素材準備工程
(工程2)熱間加工工程
(工程3)脱スケール処理工程
(工程4)球状化焼鈍工程
以下、各工程について説明する。
素材準備工程では、化学組成中の各元素含有量が本実施形態の範囲内である素材を準備する。素材は例えば、次の方法により製造される。化学組成が特徴1を満たす溶鋼を製造する。溶鋼を用いて、鋳造法により素材(鋳片又はインゴット)を製造する。例えば、溶鋼を用いて周知の連続鋳造法により鋳片(ブルーム)を製造する。又は、溶鋼を用いて周知の造塊法によりインゴットを製造する。
準備された素材に対して熱間加工を実施して、中間鋼材を製造する。熱間加工として、熱間圧延を実施する場合、例えば、次の方法がある。熱間圧延を前提とした熱間加工工程は、素材を粗圧延してビレットにする粗圧延工程と、ビレットを仕上げ圧延して中間鋼材にする仕上げ圧延工程とを含む。
粗圧延工程は例えば、次の工程を実施する。素材(インゴット又は鋳片)を加熱後、分塊圧延機を用いて分塊圧延する。必要に応じて、分塊圧延後に連続圧延機でさらに圧延して、ビレットを製造する。連続圧延機では、水平ロールスタンド、垂直ロールスタンドが交互に一列に配列されている。連続圧延機の各スタンドの圧延ロールに形成された孔型を用いて素材を圧延して、ビレットにする。
仕上げ圧延工程は例えば、次の工程を実施する。ビレットを加熱炉に装入して加熱する。加熱されたビレットを用いて、仕上げ圧延機列で仕上げ圧延(熱間圧延)を実施して、中間鋼材を製造する。仕上げ圧延機列は、一列に配列された複数のスタンドを含む。各スタンドは、パスライン周りに配置された複数のロールを含む。各スタンドの圧延ロールに形成された孔型を用いてビレットを圧延して、中間鋼材を製造する。
脱スケール処理工程では、熱間加工工程で製造された中間鋼材の表面に形成されている酸化スケールを除去する。脱スケール工程は、酸洗処理工程と、水洗工程とを含む。以下、各工程について説明する。
酸洗処理工程では、中間鋼材を酸性溶液に浸漬して、中間鋼材表面の酸化スケールを除去する。酸洗処理工程は例えば、次の条件1~条件3で実施する。
条件1:酸性溶液の温度T1(℃) :30~60℃
条件2:酸性溶液の塩酸濃度C1(質量%):5.0~20.0質量%
条件3:酸性溶液での浸漬時間t1(分) :2.0~10.0分
以下、条件1~条件3について説明する。
酸性溶液の温度T1が高すぎる場合、又は、酸性溶液の塩酸濃度C1が高すぎる場合、又は、酸性溶液での浸漬時間t1が長すぎる場合、酸洗処理工程後の中間鋼材の表面が酸の腐食により過剰に荒れて、凹凸が多くなる。この場合、中間鋼材の表面積が増大する。そのため、後工程の球状化焼鈍工程での加熱時に、中間鋼材の表面に形成される酸化スケールが厚くなる。酸化スケールが厚くなれば、中間鋼材中の炭化物から鋼材表面に移動(拡散)して酸化スケールに吸収されるCr量及びMo量が多くなる。そのため、鋼材において、抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]が低くなりすぎる。
水洗工程では、酸洗処理工程後の中間鋼材を水槽に浸漬して、中間鋼材の表面に付着している酸性溶液を除去する。水洗工程は例えば、次の条件4で実施する。
条件4:水槽での浸漬時間tw:1.0~5.0分
水槽中での浸漬時間twが短すぎる場合、酸洗処理工程後の中間鋼材の表面に酸性溶液が過剰に残存する。この場合、後工程の球状化焼鈍時に、中間鋼材の表面が酸化しやすくなる。そのため、球状化焼鈍時において、中間鋼材中の炭化物からCr及びMoが過剰に鋼材表面に移動して、酸化する。その結果、鋼材の抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]が低くなりすぎる。
球状化焼鈍工程では、脱スケール処理工程後の中間鋼材に対して球状化焼鈍を実施して、本実施形態の鋼材を製造する。球状化焼鈍では、セメンタイトに代表される炭化物を球状化して、鋼材の冷間加工性を高める。球状化焼鈍工程は例えば、次の条件5~条件7で実施する。
条件5:ガス濃度比RG=雰囲気中の還元性ガス濃度/酸素濃度:100~1000
条件6:焼鈍温度T2:680~840℃
条件7:焼鈍時間t2:0.1~3.0時間
以下、条件5~条件7について説明する。
球状化焼鈍では、焼鈍中の中間鋼材の表面酸化を抑制するために、雰囲気中に還元性ガスを導入する。還元性ガスは例えば、CO、H2及び炭化水素ガスからなる群から選択される1種以上である。雰囲気中の酸素濃度と比較して、雰囲気中の還元性ガス濃度が低すぎれば、中間鋼材の表面が過剰に酸化される。この場合、中間鋼材中の炭化物から鋼材表面にCr及びMoが過剰に移動する。その結果、鋼材の抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]が低くなる。
ガス濃度比RG=雰囲気中の還元性ガス濃度/酸素濃度
ガス濃度比RGが100~1000であれば、他の製造工程の条件を満たすことを前提として、鋼材の抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]が適切な範囲となる。
球状化焼鈍工程での焼鈍温度T2は例えば、680~840℃であり、焼鈍時間t2は例えば、0.1~3.0時間である。焼鈍温度T2及び焼鈍時間t2が上述の範囲内であれば、鋼材の抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]が適切な範囲となる。
焼鈍温度T2:700~800℃
焼鈍時間t2:0.5~2.0時間
焼鈍温度T2が700~800℃であり、かつ、焼鈍時間t2が0.5~2.0時間であれば、鋼材の表層領域での粗大炭化物個数割合RNが5~20%となる。この場合、酸洗時における鋼材の耐水素脆化特性がさらに高まり、かつ、潤滑剤付着性がさらに高まる。
本実施形態の鋼材が構造用機械部品の素材となる場合を想定する。この場合、構造用機械部品の製造工程中において、鋼材に対して酸洗処理を含む脱スケール処理が実施される場合がある。そして、脱スケール処理が実施された鋼材に対して、潤滑被膜処理が実施され、その後、伸線加工が実施される場合がある。本実施形態の鋼材に対して、上述の製造工程(酸洗処理を含む脱スケール処理、及び、その後の潤滑被膜処理)が実施されたとき、本実施形態の鋼材は、酸洗処理後の優れた耐水素脆化特性と、優れた潤滑剤付着性とを両立できる。
例えば、本実施形態で規定されたCu含有量は小数第二位までの数値で規定されている。したがって、表1-1中の試験番号1では、測定されたCu含有量が、小数第三位で四捨五入した場合に、0%であったことを意味する。
また、本実施形態で規定されたNi含有量は小数第二位までの数値で規定されている。したがって、表1-1中の試験番号1では、測定されたNi含有量が、小数第三位で四捨五入した場合に、0%であったことを意味する。
なお、四捨五入とは、規定された最小桁の下の桁(端数)が5未満であれば切り捨て、5以上であれば切り上げることを意味する。
各試験番号の鋼材に対して、次の評価試験を実施した。
(試験1)鋼材の化学組成測定試験
(試験2)抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]測定試験
(試験3)粗大炭化物個数割合RN測定試験
(試験4)ミクロ組織観察試験
(試験5)耐水素脆化特性評価試験
(試験6)潤滑剤付着性評価試験
以下、試験1~試験6について説明する。
上述の[鋼材の化学組成の測定方法]に記載の方法に基づいて、各試験番号の鋼材の化学組成を次の方法で求めた。測定の結果、いずれの試験番号の鋼材も表1-1及び1-2の化学組成のとおりであった。
上述の[抽出残渣中のCr濃度[Cr]及びMo濃度[Mo]の測定方法]に記載の方法に基づいて、各試験番号の鋼材の表層領域のCr濃度[Cr](質量%)及びMo濃度[Mo](質量%)の総量であるF1(=[Cr]+[Mo])を求めた。求めたF1を表2に示す。
上述の[粗大炭化物個数割合RNの測定方法]に記載の方法に基づいて、各試験番号の鋼材の粗大炭化物個数割合RN(%)を求めた。求めた粗大炭化物個数割合RNを表2に示す。
上述の[ミクロ組織の特定方法]に記載の方法に基づいて、各試験番号の鋼材のミクロ組織観察を行った。その結果、いずれの試験番号においても、鋼材のミクロ組織は、炭化物が分散したBCC相からなる組織(BCC組織)であった。
各試験番号の鋼材に対して、脱スケール処理工程を想定して、次の酸洗処理工程及び水洗工程を実施した。酸洗処理工程では、各試験番号の鋼材を、40℃の酸性溶液に5.0分浸漬した。酸性溶液中の塩酸濃度は、15.0質量%であった。水洗工程では、酸洗処理工程後の鋼材を25℃の水が貯留された水槽に1.0分浸漬した。
耐水素脆化指標HI=引張強度1/引張強度2
得られた耐水素脆化指標HIに応じて、耐水素脆化特性を次のとおり評価した。
評価S:耐水素脆化指標HIが0.95~1.00
評価A:耐水素脆化指標HIが0.90~0.95未満
評価B:耐水素脆化指標HIが0.85~0.90未満
評価C:耐水素脆化指標HIが0.80~0.85未満
評価D:耐水素脆化指標HIが0.75~0.80未満
評価E:耐水素脆化指標HIが0.70~0.75未満
評価X:耐水素脆化指標HIが0.70未満
評価S~評価Eの場合、耐水素脆化特性に優れると判断した。一方、評価Xの場合、鋼材の耐水素脆化特性が低いと判断した。評価結果を表2に示す。
各試験番号の潤滑剤付着性を、次の方法で評価した。
各試験番号の鋼材に対して、脱スケール処理工程を想定して、次の酸洗処理工程及び水洗工程を実施した。酸洗処理工程では、各試験番号の鋼材を、40℃の酸性溶液に5.0分浸漬した。酸性溶液中の塩酸濃度は、15.0質量%であった。水洗工程では、酸洗処理工程後の鋼材を25℃の水が貯留された水槽に1.0分浸漬した。
評価A:潤滑付着量LAが10g/m2以上
評価B:潤滑付着量LAが8~10g/m2未満
評価C:潤滑付着量LAが6~8g/m2未満
評価D:潤滑付着量LAが4~6g/m2未満
評価E:潤滑付着量LAが2~4g/m2未満
評価X:潤滑付着量LAが2g/m2未満
評価A~評価Eの場合、潤滑剤付着性に優れると判断した。評価Xの場合、鋼材の潤滑剤付着性が低いと判断した。評価結果を表2に示す。
表1-1、表1-2及び表2を参照して、試験番号1~52の鋼材の化学組成は適切であり、さらに、F1は式(1)を満たした。そのため、試験番号1~52の鋼材では、酸洗処理後の耐水素脆化特性に優れ、かつ、潤滑剤付着性にも優れた。
Claims (3)
- 鋼材であって、
質量%で、
C:0.30~0.50%、
Si:0.40%以下、
Mn:0.10~0.60%、
P:0.030%以下、
S:0.030%以下、
Cr:0.90~1.80%、
Mo:0.30~1.00%、
Al:0.005~0.100%、
N:0.003~0.030%、及び、
残部はFe及び不純物からなり、
予備定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域を電解して除去した後、本定電流電気分解により前記鋼材の表面から100±20μm深さ位置までの領域をさらに電解して得られた抽出残渣中のCr濃度を[Cr](質量%)と定義し、前記抽出残渣中のMo濃度を[Mo](質量%)と定義したとき、式(1)を満たす、
鋼材。
10.0≦[Cr]+[Mo]≦30.0 (1) - 請求項1に記載の鋼材であって、
円相当径が0.5μm以上の炭化物の個数に対する、円相当径が0.8μm以上の炭化物の個数割合は、5~20%である、
鋼材。 - 請求項1又は請求項2に記載の鋼材であってさらに、
Feの一部に代えて、
Cu:0.40%以下、
Ni:0.40%以下、
V:0.50%以下、
Ti:0.100%以下、
Nb:0.100%以下、
B:0.0100%以下、
W:0.500%以下、
Ca:0.010%以下、
Mg:0.100%以下、
希土類元素:0.100%以下、
Bi:0.300%以下、
Te:0.300%以下、及び、
Zr:0.300%以下、
からなる群から選択される1種以上を含有する、
鋼材。
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JP2011047010A (ja) * | 2009-08-27 | 2011-03-10 | Kobe Steel Ltd | 耐遅れ破壊性の改善された高強度ボルト及びその製造方法 |
JP2011117035A (ja) * | 2009-12-03 | 2011-06-16 | Sumitomo Metal Ind Ltd | 高強度ボルト用鋼 |
JP2013237903A (ja) * | 2012-05-16 | 2013-11-28 | Nippon Steel & Sumitomo Metal Corp | ボルト用鋼材 |
JP2019183218A (ja) * | 2018-04-06 | 2019-10-24 | 日本製鉄株式会社 | 高圧水素容器、及び、高圧水素用鋼材 |
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WO2015189978A1 (ja) | 2014-06-13 | 2015-12-17 | 新日鐵住金株式会社 | 冷間鍛造用鋼材 |
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JP2011047010A (ja) * | 2009-08-27 | 2011-03-10 | Kobe Steel Ltd | 耐遅れ破壊性の改善された高強度ボルト及びその製造方法 |
JP2011117035A (ja) * | 2009-12-03 | 2011-06-16 | Sumitomo Metal Ind Ltd | 高強度ボルト用鋼 |
JP2013237903A (ja) * | 2012-05-16 | 2013-11-28 | Nippon Steel & Sumitomo Metal Corp | ボルト用鋼材 |
JP2019183218A (ja) * | 2018-04-06 | 2019-10-24 | 日本製鉄株式会社 | 高圧水素容器、及び、高圧水素用鋼材 |
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