WO2013031587A1 - 熱間鍛造用圧延棒鋼又は線材 - Google Patents
熱間鍛造用圧延棒鋼又は線材 Download PDFInfo
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- WO2013031587A1 WO2013031587A1 PCT/JP2012/071118 JP2012071118W WO2013031587A1 WO 2013031587 A1 WO2013031587 A1 WO 2013031587A1 JP 2012071118 W JP2012071118 W JP 2012071118W WO 2013031587 A1 WO2013031587 A1 WO 2013031587A1
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- steel
- fatigue strength
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- ferrite
- test
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 188
- 239000010959 steel Substances 0.000 title claims abstract description 188
- 238000005242 forging Methods 0.000 title claims abstract description 39
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 82
- 239000000126 substance Substances 0.000 claims abstract description 51
- 239000000203 mixture Substances 0.000 claims abstract description 50
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 28
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 17
- 239000012535 impurity Substances 0.000 claims abstract description 8
- 229910052760 oxygen Inorganic materials 0.000 claims description 8
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 5
- 239000001301 oxygen Substances 0.000 claims description 5
- 229910052719 titanium Inorganic materials 0.000 claims description 5
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 238000005452 bending Methods 0.000 abstract description 86
- 239000002245 particle Substances 0.000 abstract description 51
- 229910052750 molybdenum Inorganic materials 0.000 abstract description 8
- 229910052804 chromium Inorganic materials 0.000 abstract description 7
- 238000005299 abrasion Methods 0.000 abstract description 5
- 229910052799 carbon Inorganic materials 0.000 abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 abstract description 4
- 229910052782 aluminium Inorganic materials 0.000 abstract description 3
- 229910052748 manganese Inorganic materials 0.000 abstract description 3
- 229910052710 silicon Inorganic materials 0.000 abstract description 3
- 229910052717 sulfur Inorganic materials 0.000 abstract description 3
- 238000012360 testing method Methods 0.000 description 147
- 238000010438 heat treatment Methods 0.000 description 42
- 238000004519 manufacturing process Methods 0.000 description 37
- 239000011651 chromium Substances 0.000 description 36
- 238000005520 cutting process Methods 0.000 description 27
- 238000001816 cooling Methods 0.000 description 23
- 238000005096 rolling process Methods 0.000 description 23
- 239000010955 niobium Substances 0.000 description 21
- 238000005255 carburizing Methods 0.000 description 18
- 239000011572 manganese Substances 0.000 description 17
- 239000013078 crystal Substances 0.000 description 15
- 238000000034 method Methods 0.000 description 15
- 230000000007 visual effect Effects 0.000 description 13
- 239000013067 intermediate product Substances 0.000 description 11
- 238000010791 quenching Methods 0.000 description 11
- 230000000171 quenching effect Effects 0.000 description 11
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 11
- 239000000463 material Substances 0.000 description 10
- 239000010936 titanium Substances 0.000 description 9
- 238000005496 tempering Methods 0.000 description 7
- 238000005256 carbonitriding Methods 0.000 description 5
- 238000009661 fatigue test Methods 0.000 description 5
- 229910000734 martensite Inorganic materials 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000009749 continuous casting Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 239000011159 matrix material Substances 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- 238000010606 normalization Methods 0.000 description 4
- 230000002159 abnormal effect Effects 0.000 description 3
- 238000005098 hot rolling Methods 0.000 description 3
- 238000003754 machining Methods 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 230000006698 induction Effects 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 239000003921 oil Substances 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 238000005480 shot peening Methods 0.000 description 2
- 238000007711 solidification Methods 0.000 description 2
- 230000008023 solidification Effects 0.000 description 2
- 229910000851 Alloy steel Inorganic materials 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-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
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
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- 239000000446 fuel Substances 0.000 description 1
- 239000000314 lubricant Substances 0.000 description 1
- 239000003595 mist 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
- 238000005121 nitriding Methods 0.000 description 1
- 230000002093 peripheral effect Effects 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/06—Surface hardening
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a steel bar or wire, and more particularly to a rolled steel bar or wire for hot forging.
- Mechanical parts such as gears and pulleys are used in automobiles or industrial machines. Many of these mechanical parts are manufactured in the following manner.
- the material has a chemical composition corresponding to, for example, JIS standard SCr420, SCM420, or SNCM420.
- the material is, for example, hot rolled steel bar or wire.
- Hot forging is performed on the material to produce intermediate products. Normalize the intermediate product as necessary.
- cutting is performed on the intermediate product.
- Surface hardening treatment is performed on the cut intermediate product.
- the surface hardening treatment is, for example, carburizing quenching, carbonitriding quenching, or induction quenching. Tempering is carried out at a tempering temperature of 200 ° C. or lower on the surface-cured intermediate product.
- a shot peening treatment is performed on the intermediate product after tempering as necessary.
- a machine part is manufactured by the above process.
- JP-A-60-21359 In the steel for gears disclosed in JP-A-60-21359, it is specified that Si: 0.1% or less and P: 0.01% or less. According to such regulations, JP-A-60-21359 describes that gear steel has high strength, is strong and has high reliability.
- the gear steel disclosed in Japanese Patent Application Laid-Open No. 7-242994 contains Cr: 1.50 to 5.0%, and if necessary, 7.5%> 2.2 ⁇ Si (%) + 2 0.5 ⁇ Mn (%) + Cr (%) + 5.7 ⁇ Mo (%) is satisfied, and Si: 0.40 to 1.0% is contained.
- JP-A-7-242994 discloses that the gear steel has excellent tooth surface strength by having such a chemical composition.
- the steel for carburized gears disclosed in JP-A-7-126803 contains Si: 0.35 to 3.0% or less, V: 0.05 to 0.5%, and the like.
- Japanese Patent Laid-Open No. 7-126803 discloses that gear steel has high bending fatigue strength and high surface fatigue strength by having such a chemical composition.
- JP-A-60-21359 does not discuss surface fatigue strength. For this reason, the surface fatigue strength of the gear steel disclosed in JP-A-60-21359 may be low.
- Japanese Patent Laid-Open No. 7-242994 does not discuss bending fatigue strength. For this reason, the bending fatigue strength of the gear steel disclosed in JP-A-7-242994 may be low.
- the gear steel disclosed in JP-A-7-126803 contains V. V increases the hardness of the steel after hot forging. Therefore, the machinability of the steel after hot forging may be reduced.
- JP-A-60-21359, JP-A-7-242994, and JP-A-7-126803 have excellent bending fatigue strength, surface fatigue strength and wear resistance, and Steel with excellent machinability is not disclosed.
- An object of the present invention is to provide a rolled steel bar or wire rod for hot forging having excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability even after hot forging.
- the rolled steel bar or wire rod for hot forging according to the present invention has a chemical composition of mass%, C: 0.1 to 0.25%, Si: 0.30 to 0.60%, Mn: 0.50 to 1.0%, S: 0.003 to 0.05%, Cr: 1.50 to 2.00%, Mo: 0.10% or less (including 0%), Al: 0.025 to 0.05 %, N: 0.010 to 0.025%, the balance is made of Fe and impurities, and P, Ti and O in the impurities are respectively P: 0.025% or less, Ti: 0.003% or less , O (oxygen): 0.002% or less, and fn1 defined by the formula (1) is 1.60 to 2.10.
- the structure of the rolled steel bar or wire rod for hot forging described above is composed of a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure.
- the maximum value / minimum value of the average ferrite particle diameter obtained by measuring 15 visual fields with an area of 62500 ⁇ m 2 per visual field is 2.0 or less.
- fn1 Cr + 2 ⁇ Mo (1)
- the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).
- the steel bar or wire for hot forging according to the present invention has excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability.
- the rolled steel bar or wire rod for hot forging according to the present invention may contain Nb: 0.08% or less in mass% instead of part of Fe.
- FIG. 1 is a side view of a small roller test piece for a roller pitching test produced in the example.
- FIG. 2 is a side view of the Ono rotary bending fatigue test piece with a notch produced in the example.
- FIG. 3 is a diagram illustrating carburizing and quenching conditions in the examples.
- FIG. 4 is a front view of a large roller for a roller pitching test in the embodiment.
- the present inventors investigated and studied the bending fatigue strength, surface fatigue strength, wear resistance, and machinability of rolled steel bars or wires for hot forging (hereinafter simply referred to as bars or wires). As a result, the present inventors obtained the following knowledge.
- the ferrite average grain size ratio is defined as follows. From the cross section of the steel bar or the wire rod, 15 visual fields having an area of each visual field of 62500 ⁇ m 2 are selected from the region excluding the surface decarburized layer. Image analysis is performed for each of the selected 15 fields of view. Specifically, the ferrite average particle diameter is measured in each visual field. The ferrite average particle diameter of each visual field is measured according to the cutting method defined in JIS G0551 (2005).
- the maximum value and the minimum value are selected from the average ferrite grain sizes determined in each of the 15 visual fields. Then, the maximum value / minimum value is obtained.
- % of the content of elements constituting the chemical composition means “mass%”.
- C 0.1 to 0.25%
- Carbon (C) enhances carburizing and quenching or carbonitriding and quenching. Therefore, C increases the strength of the steel. In particular, C increases the strength of the core part of the machine part after carburizing or carbonitriding. On the other hand, if C is contained excessively, the deformation amount of the machine part after carburizing and quenching or carbonitriding is significantly increased. Therefore, the C content is 0.1 to 0.25%.
- the lower limit of the preferable C content is higher than 0.1%, more preferably 0.15% or more, and further preferably 0.18% or more.
- the upper limit of the preferable C content is less than 0.25%, more preferably 0.23% or less, and still more preferably 0.20% or less.
- Si 0.30 to 0.60%
- Silicon (Si) enhances the hardenability of the steel. Si further increases the temper softening resistance of the steel. Therefore, Si increases the surface fatigue strength and wear resistance of steel.
- Si is excessively contained, the strength of the steel after hot forging becomes excessively high. As a result, the machinability of steel decreases. If Si is excessively contained, the bending fatigue strength further decreases. Therefore, the Si content is 0.30 to 0.60%.
- the minimum of preferable Si content is higher than 0.30%, More preferably, it is 0.40% or more, More preferably, it is 0.45% or more.
- the upper limit of the Si content is preferably less than 0.60%, more preferably 0.57% or less, and further preferably 0.55% or less.
- Mn 0.50 to 1.0%
- Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. Therefore, Mn increases the strength of the core of machine parts that have been carburized or carbonitrided.
- Mn increases the strength of the core of machine parts that have been carburized or carbonitrided.
- Mn is contained excessively, the machinability of the steel after hot forging is lowered.
- Mn oxide is generated on the surface of the steel.
- the carburizing abnormal layer is, for example, a grain boundary oxide layer and an incompletely quenched layer. If the depth of the carburized abnormal layer is increased, the bending fatigue strength and the pitting strength of the steel are lowered.
- the Mn content is 0.50 to 1.0%.
- the minimum of preferable Mn content is higher than 0.50%, More preferably, it is 0.55% or more, More preferably, it is 0.60% or more.
- the upper limit with preferable Mn content is less than 1.0%, More preferably, it is 0.95% or less, More preferably, it is 0.9% or less.
- S 0.003 to 0.05% Sulfur (S) combines with Mn to form MnS.
- MnS increases the machinability of steel.
- coarse MnS is formed.
- Coarse MnS lowers the bending fatigue strength and surface fatigue strength of steel. Therefore, the S content is 0.003 to 0.05%.
- the lower limit of the preferable S content is higher than 0.003%, more preferably 0.005% or more, and still more preferably 0.01% or more.
- the upper limit of the preferable S content is less than 0.05%, more preferably 0.03% or less, and further preferably 0.02% or less.
- Chromium (Cr) increases the hardenability of the steel and the temper softening resistance of the steel. Therefore, Cr increases the bending fatigue strength, surface fatigue strength, and wear resistance of steel. On the other hand, if Cr is excessively contained, the formation of bainite is promoted in the steel after hot forging or after normalization. Therefore, the machinability of the steel is reduced. Therefore, the Cr content is 1.50 to 2.00%.
- the lower limit of the preferable Cr content is higher than 1.50%, more preferably 1.70% or more, and further preferably 1.80% or more.
- the upper limit of preferable Cr content is less than 2.00%, More preferably, it is 1.95% or less, More preferably, it is 1.90% or less.
- Mo Molybdenum
- Mo Molybdenum
- Mo may not be contained. Mo increases the hardenability and temper softening resistance of the steel. Therefore, Mo increases the bending fatigue strength, surface fatigue strength, and wear resistance of steel. On the other hand, if Mo is contained excessively, bainite generation is promoted in steel after hot forging or after normalization. Therefore, the machinability of the steel is reduced. Therefore, the Mo content is 0.10% or less (including 0%). The minimum of preferable Mo content is 0.02% or more. The upper limit of the preferable Mo content is less than 0.10%, more preferably 0.08% or less, and still more preferably 0.05% or less.
- Al 0.025 to 0.05%
- Aluminum (Al) deoxidizes steel. Al further combines with N to form AlN. AlN suppresses the coarsening of austenite crystal grains due to carburizing heating. On the other hand, if Al is contained excessively, a coarse Al oxide is formed. Coarse Al oxide reduces the bending fatigue strength of steel. Therefore, the Al content is 0.025 to 0.05%.
- the lower limit of the preferable Al content is higher than 0.025%, more preferably 0.027% or more, and further preferably 0.030% or more.
- the upper limit of the preferable Al content is less than 0.05%, more preferably 0.045% or less, and further preferably 0.04% or less.
- N 0.010 to 0.025%
- Nitrogen (N) combines with Al or Nb to form AlN or NbN.
- AlN or NbN suppresses the coarsening of austenite crystal grains due to carburizing heating.
- the N content is 0.010 to 0.025%.
- the minimum of preferable N content is higher than 0.010%, More preferably, it is 0.012% or more, More preferably, it is 0.013% or more.
- the upper limit of the preferable N content is less than 0.025%, more preferably 0.020% or less, and still more preferably 0.018% or less.
- the balance of the chemical composition of the steel bar or wire according to the present invention consists of Fe and impurities.
- the impurity in this specification means the element mixed from the ore and scrap utilized as a raw material of steel, or the environment of a manufacturing process.
- the contents of P, Ti, and O (oxygen) as impurities are limited as follows.
- P 0.025% or less Phosphorus (P) segregates at the grain boundaries and embrittles the grain boundaries. Therefore, P reduces the fatigue strength of steel. Therefore, the P content is preferably as low as possible.
- the P content is 0.025% or less.
- P content is preferably less than 0.025%, more preferably 0.020% or less.
- Ti 0.003% or less Titanium (Ti) combines with N to form coarse TiN. Coarse TiN reduces the fatigue strength of steel. Therefore, the Ti content is preferably as low as possible. Ti content is 0.003% or less. A preferable Ti content is less than 0.003%, more preferably 0.002% or less.
- Oxygen (O) combines with Al to form oxide inclusions. Oxide inclusions reduce the bending fatigue strength of steel. Therefore, it is preferable that the O content is as low as possible.
- the O content is 0.002% or less.
- the preferable O content is less than 0.002%, and more preferably 0.001% or less.
- the chemical composition of the steel bar or wire according to the invention further satisfies the formula (2). 1.60 ⁇ Cr + 2 ⁇ Mo ⁇ 2.10 (2) Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2).
- fn1 is less than 1.60, at least one of the bending fatigue strength, surface fatigue strength, and wear resistance of the steel becomes low. On the other hand, if fn1 exceeds 2.10, the formation of bainite is promoted in the steel after hot forging or normalization. Therefore, the machinability of the steel is reduced. If fn1 is 1.60 to 2.10, it is possible to increase the bending fatigue strength, surface fatigue strength, and wear resistance of the steel while suppressing the deterioration of the machinability of the steel. A preferred lower limit of fn1 is 1.80 or more. A preferable upper limit of fn1 is less than 2.00.
- the chemical composition of the rolled steel bar or wire rod for hot forging according to the present invention may contain Nb instead of a part of Fe.
- Niobium (Nb) is a selective element. Nb combines with C and N to form Nb carbide, Nb nitride or Nb carbonitride. Nb carbide, Nb nitride, and Nb carbonitride suppress the coarsening of austenite crystal grains during carburizing heating, as with Al nitride. If Nb is contained even a little, the above effect can be obtained. On the other hand, if Nb is contained excessively, Nb carbonitride, Nb nitride and Nb carbonitride become coarse. Therefore, coarsening of austenite crystal grains cannot be suppressed during carburizing heating. Therefore, the Nb content is 0.08% or less. The minimum with preferable Nb content is 0.01% or more. The upper limit of the preferable Nb content is less than 0.08%, and more preferably 0.05% or less.
- the microstructure of the steel bar or wire according to the present invention comprises a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure.
- the “ferrite / pearlite structure” means a two-phase structure in which a matrix (matrix) is composed of ferrite and pearlite.
- the “ferrite / pearlite / bainite structure” means a three-phase structure in which the matrix is composed of ferrite, pearlite, and bainite.
- the “ferrite bainite structure” means a two-phase structure in which the matrix is composed of ferrite and bainite.
- the microstructure of the steel bar or wire according to the present invention does not contain martensite. Martensite is hard and reduces the ductility of the steel. Accordingly, when a steel bar or wire containing martensite is transported or corrected, cracks are likely to occur in the steel bar or wire. Since the microstructure of the steel bar or wire according to the present invention does not contain martensite, cracking is unlikely to occur during correction or conveyance.
- Each phase described above is identified by the following method.
- a sample including the center of a cross section (cross section) perpendicular to the longitudinal direction of the steel bar or wire is cut out.
- the surface (including the center part) of the cut sample is mirror-polished. Corrodes the polished surface with nital.
- the corroded surface is observed with a microstructure using an optical microscope with a magnification of 400 times.
- 15 visual fields are arbitrarily selected from the region excluding the steel bar or the surface decarburized layer of the wire rod. Each field of view is observed to identify the microstructure. If bainite is included in any of the 15 visual fields, it is determined that bainite is included in the microstructure of the steel. The same judgment is made for ferrite and pearlite.
- the ferrite average particle size ratio defined by the formula (3) is 2.0 or less in the cross section.
- Execute image analysis for each of the above 15 fields of view Specifically, the ferrite phase is identified in each visual field. Measure the ferrite grain size in the identified ferrite phase. The average ferrite grain size in each field of view is measured according to the cutting method defined in JIS G0551 (2005).
- a ferrite average particle diameter ratio (the maximum value of a ferrite average particle diameter / the minimum value of a ferrite average particle diameter) is calculated
- the crystal grain size is non-uniform in the steel material after hot rolling (that is, as-rolled steel), the crystal grain size remains non-uniform even after hot forging, which is a subsequent process, or after carburizing and quenching. It is. If the crystal grain size is not uniform, bending fatigue strength and surface fatigue strength are reduced. Therefore, it is preferable that the crystal grain size in the hot-rolled material is as uniform as possible. In order to evaluate the degree of uniformity of the crystal grain size, it is preferable to evaluate the ferrite average grain size ratio. The ferrite particle diameter can be easily observed by etching as compared with pearlite or bainite.
- the degree of uniformity of the average ferrite grain size (that is, the ratio of average ferrite grain diameter) is examined, it is easy to evaluate the degree of crystal grain uniformity in the structure. Furthermore, fatigue fracture occurs starting from the lowest strength part. Therefore, using the maximum value / minimum value of the ferrite average particle size as an index is more suitable for evaluating the bending fatigue strength and the surface fatigue strength than using the standard deviation of the ferrite average particle size as an index.
- the microstructure is composed of various mixed structures including the above-mentioned ferrite and the ferrite average particle size ratio is 2.0 or less, the variation in crystal particle size in the steel bar or wire is small. Therefore, the bending fatigue strength and surface fatigue strength of the steel after hot forging or after carburizing and quenching are increased.
- the ferrite average particle size ratio is preferably 1.6 or less.
- the ferrite average particle size ratio exceeds 2.0, one or more of the bending fatigue strength and surface fatigue strength of the steel will be low.
- a molten steel having the above chemical composition and satisfying the formula (2) is manufactured.
- a slab (slab or bloom) is produced by continuous casting using molten steel. In the continuous casting method, a reduction is applied to the slab in the middle of solidification. Next, the slab is heated. The heating temperature at this time is 1250 to 1300 ° C., and the heating time is 10 hours or more. The heated cast slab is subjected to block rolling with a block mill to produce a steel slab (billet).
- the steel slab is hot-rolled to produce hot forging bar or wire. Specifically, the steel piece is heated. The heating temperature at this time is 1150 to 1200 ° C., and the heating time is 1.5 hours or more.
- the heated steel slab is hot-rolled to produce a steel bar or wire.
- the finishing temperature in hot rolling is set to 900 to 1000 ° C. Water cooling is not performed before finish rolling.
- the steel bar or wire is cooled at a cooling rate equal to or lower than that in the air (hereinafter simply referred to as “cooling”) until the surface temperature reaches 600 ° C. or lower.
- the cross-section reduction rate (%) defined by the formula (4) is set to 87.5% or more.
- Cross-section reduction rate ⁇ 1 ⁇ (section area of bar, wire rod / section area of steel slab) ⁇ ⁇ 100 (4)
- the bar or wire rod may be cooled at a higher cooling rate than that of cooling, such as air cooling, mist cooling, or water cooling.
- the above-mentioned heating temperature means an average value of the furnace temperature of the heating furnace.
- the above heating time means the in-furnace time at the above heating temperature.
- the finishing temperature means the surface temperature of the steel bar and wire immediately after finish rolling.
- finish rolling means rolling at the last stand among a plurality of stands used for rolling in a continuous mill.
- the cooling rate after finishing means the surface cooling rate of steel bars and wire rods.
- An example of a method of manufacturing a machine part using hot-rolled steel bar or wire rod for hot forging is as follows.
- Hot forging is performed on rolled steel bars or wire rods for hot forging to produce coarse intermediate products.
- a tempering treatment may be performed on the intermediate product.
- the tempering process is, for example, normalizing.
- the intermediate product is machined into a predetermined shape. Machining is, for example, cutting or drilling.
- Surface hardening treatment may be performed on the intermediate product after machining.
- the surface hardening treatment is, for example, carburizing treatment, nitriding treatment or induction hardening treatment. Finishing is performed on the intermediate product that has been subjected to the surface hardening treatment to produce a machine part.
- the steel bar or wire manufactured by the above process has excellent bending fatigue strength, surface fatigue strength, wear resistance and excellent machinability even after hot forging.
- a 400 mm ⁇ 300 mm slab (bloom) was manufactured by continuous casting using molten steel of steels A to C. The produced bloom was allowed to cool to 600 ° C. in the atmosphere. In the continuous casting process, the slab in the middle of solidification was reduced.
- the “heating temperature” column in the “slab” column of Table 2 indicates the heating temperature (° C.) of the slab under each condition.
- the “heating time” column in the “slab” column of Table 2 shows the heating time (minutes) of the slab in each condition.
- the “heating temperature” column in the “steel” column of Table 2 shows the heating temperature (° C.) of the steel slab under each condition.
- the “heating time” column in the “steel” column shows the heating time (minutes) of the steel slab under each condition.
- the “water cooling before finish rolling” column in the “rolling conditions” column indicates whether or not the steel slabs are water cooled before finish rolling in each condition. “Yes” in the column indicates that water cooling was performed.
- “None” indicates that water cooling was not performed.
- the “finishing temperature” column in the “rolling conditions” column indicates the finishing temperature (° C.) under each condition.
- the “cooling condition” column in the “rolling condition” column shows the cooling condition after finish rolling in each condition.
- the steel slab shown in Table 3 was heated under the manufacturing conditions shown in Table 3 (heating temperature and heating time of the slab). The heated slab was rolled into blocks to produce a steel slab of 180 mm ⁇ 180 mm. The produced steel slab was cooled to room temperature (25 ° C.).
- the steel slab was heated under the manufacturing conditions shown in Table 3 (heating temperature and heating time of the steel slab).
- the heated steel slab was hot-rolled under the production conditions shown in Table 3 (water cooling before finish rolling, finishing temperature, cooling conditions) to produce steel bars having a diameter of 50 mm and a diameter of 70 mm.
- the rolled steel bar was allowed to cool to room temperature in the atmosphere. In other words, the steel bar was a hot rolled material.
- the microstructure of any test number did not contain martensite.
- the microstructure of each test number was either a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure.
- the microstructure observation results are shown. “F + P” in the table indicates that the microstructure of the corresponding test number is a ferrite pearlite structure.
- F + P + B indicates a ferrite / pearlite / bainite structure.
- F + B indicates a ferrite bainite structure.
- Carburizing and quenching was performed on the prepared test pieces under the conditions shown in FIG. 3 using a gas carburizing furnace. After quenching, tempering was performed at 150 ° C. for 1.5 hours. For the small roller test piece and the Ono type rotating bending fatigue test piece, the gripping part was finished for the purpose of removing heat treatment strain.
- the large roller shown in FIG. 4 is made of steel that satisfies the standard of JIS standard SCM420H, and is a general manufacturing process, that is, normalization, test piece processing, eutectoid carburization with a gas carburizing furnace, low temperature tempering and polishing. Made by.
- the rotation speed of the small roller test piece was 1000 rpm
- the slip rate was ⁇ 40%
- the contact surface pressure between the large roller and the small roller test piece under test was 4000 MPa
- the number of repetitions was 2.0 ⁇ 10. It was set to 7 cycles.
- a lubricant commercial oil for automatic transmission
- the number of tests in the roller pitching test was six.
- an SN diagram was prepared with the surface pressure on the vertical axis and the number of repetitions until the occurrence of pitching on the horizontal axis.
- the highest surface pressure was defined as the surface fatigue strength of the test number.
- the area of the largest thing became 1 mm ⁇ 2 > or more among the places where the surface of a small roller test piece was damaged, it defined as generating pitting.
- Table 3 shows the surface fatigue strength obtained by the test. With respect to the surface fatigue strength in Table 3, the surface fatigue strength of Test No. 1 was set as a reference value (100%). And the surface fatigue strength of each test number was shown by ratio (%) with respect to a reference value. If the surface fatigue strength was 120% or more, it was judged that excellent surface fatigue strength was obtained.
- the bending fatigue strength test was determined by an Ono type rotating bending fatigue test. The number of tests in the Ono rotary bending fatigue test was 8 for each test number. The rotational speed at the time of the test was 3000 rpm, and the others were tested by ordinary methods. Among those that did not break until the number of repetitions of 1.0 ⁇ 10 4 and 1.0 ⁇ 10 7 , the highest stress was defined as medium cycle and high cycle rotational bending fatigue strength, respectively.
- Table 3 shows the bending fatigue strength of medium and high cycles.
- the bending fatigue strength of test number 1 in the middle cycle and high cycle was defined as a reference value (100%).
- the bending fatigue strength of the middle cycle and the high cycle of each test number was shown by ratio (%) with respect to a reference value. It was judged that an excellent bending fatigue strength was obtained when the bending fatigue strength was 115% or more in both the middle cycle and the high cycle.
- a cutting test was conducted to evaluate machinability.
- a cutting specimen was obtained by the following method.
- a steel bar having a diameter of 70 mm for each test number was heated at a heating temperature of 1250 ° C. for 30 minutes.
- the heated steel bar was hot forged at a finishing temperature of 950 ° C. or higher to obtain a round bar having a diameter of 60 mm.
- a cutting test piece having a diameter of 55 mm and a length of 450 mm was obtained from this round bar by machining.
- a cutting test was performed using the cutting test piece under the following conditions.
- Cutting test (turning) Insert Base material: Carbide P20 grade, coating None Conditions: peripheral speed 200m / min, feed 0.30mm / rev, cutting 1.5mm, water-soluble cutting oil used Measurement item: flank after 10 minutes of cutting time Main cutting edge wear amount
- Table 3 shows the amount of main cutting edge wear obtained.
- the main cutting edge wear amount of test number 2 (using steel B) was set as a reference value (100%).
- the amount of main cutting edge abrasion of each test number was shown by ratio (%) with respect to a reference value. If the main cutting edge wear amount was 80% or less, it was judged that excellent machinability was obtained.
- the chemical composition (steel A) of the steel bar of test number 1 corresponded to JIS standard SCr420H. Therefore, the Si content and the Cr content of Test No. 1 were less than the lower limits of the Si content and the Cr content of the present invention. Furthermore, fn1 of test number 1 was less than the lower limit of formula (2). Therefore, the bending fatigue strength, surface fatigue strength, and wear resistance of Test No. 1 were low.
- the chemical composition (steel B) of the steel bar of test number 2 corresponded to JIS standard SCM420H. Therefore, the Si content and the Cr content of Test No. 2 were less than the lower limits of the Si content and the Cr content of the present invention. Furthermore, the Mo content of Test No. 2 exceeded the upper limit of the Mo content of the present invention. Furthermore, fn1 of test number 2 was less than the lower limit of formula (2). Therefore, the bending fatigue strength of Test No. 2 was as low as less than 115%, and the machinability was also low.
- test number 3 was within the range of the chemical composition of the present invention. Further, fn1 also satisfied the formula (2). However, since the heating time of the slab was too short (see production condition 1 in Table 2), the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and high cycle of test number 3 was less than 115% and was low.
- test number 5 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2).
- water cooling was performed before finish rolling (see production condition 3 in Table 2). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 5 was less than 115% and was low.
- test number 6 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2).
- the steel bar after finish rolling was water-cooled to 800 ° C. (see production condition 4 in Table 2). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 6 was both less than 115% and low. Furthermore, the surface fatigue strength was less than 120% and was low. Further, the wear amount exceeded 80%, and the wear resistance was low.
- test number 7 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2).
- the heating time of the slab was too short, and the heating time of the steel slab was too short (see manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 7 was both less than 115% and low.
- test number 8 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2).
- the heating temperature of the steel slab was too high, and the finishing temperature was too high (see production condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0.
- the bending fatigue strength of the middle and high cycles of test number 8 was both less than 115% and low.
- the surface fatigue strength was less than 120% and was low. Further, the wear amount exceeded 80%, and the wear resistance was low.
- test number 10 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2).
- the heating temperature of the slab was too low (see production condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.
- Example 2 Under the same production conditions as in Example 1, steel bars with test numbers 11 to 42 shown in Table 6 were produced. The diameters of the steel bars were 50 mm and 70 mm. The test similar to Example 1 was implemented using the manufactured steel bar. Then, the bending fatigue strength, surface fatigue strength, wear resistance, and main cutting edge wear amount of medium cycle and high cycle were determined.
- Table 6 shows the results obtained.
- the chemical compositions of test numbers 17, 19, 21, 23, 31, 33, 41 and 42 were within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2).
- the ferrite average particle size ratios of these test numbers were all 2.0 or less. Therefore, the middle and high cycle bending fatigue strengths of these test numbers were 115% or more, and the surface fatigue strength was 120% or more. Furthermore, the amount of wear was 80% or less. Further, the main cutting edge wear amount was 80% or less.
- test number 11 the Si content and the Cr content of the chemical composition of test number 11 (steel D) were less than the lower limits of the Si content and the Cr content of the present invention. Therefore, the surface fatigue strength of Test No. 11 was less than 120%, and the wear amount was higher than 80%.
- Test number 12 used the same steel D as test number 11. Therefore, the surface fatigue strength and wear resistance were low.
- the slab heating time was too short (manufacturing condition 1). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.
- test number 13 (steel E) was within the range of the chemical composition of the present invention, fn1 was less than the lower limit of formula (2). Therefore, the high cycle bending fatigue strength was less than 115%, which was low.
- Test number 14 used the same steel E as test number 13. Therefore, the bending fatigue strength at a high cycle was low. In test No. 14, water cooling was further performed before finish rolling (manufacturing condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was lower than the test number 13.
- the Si content of the chemical composition of test number 15 exceeded the upper limit of the Si content of the present invention. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low. Further, the amount of wear of the main cutting edge was higher than 80%, and the machinability was low.
- Test number 16 used the same steel F as test number 15. Therefore, bending fatigue strength and machinability were low.
- the steel bar after finish rolling was further water-cooled to 800 ° C. (production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength was lower than test number 15. Furthermore, the surface fatigue strength was less than 120%, and the wear amount was higher than 80%.
- test number 18 was within the scope of the present invention, and fn1 satisfied the formula (2).
- the heating time of the slab was too short (manufacturing condition 1). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Furthermore, the surface fatigue strength was less than 120% and was low.
- test number 20 was within the scope of the present invention, and fn1 satisfied the formula (2).
- water cooling was performed before the finish rolling (production condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.
- the chemical composition (steel I) of test number 22 is within the scope of the present invention, and fn1 satisfies the formula (2).
- the steel bar after finish rolling was water-cooled to 800 ° C. (Production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.
- test number 24 (steel J) was within the scope of the present invention, and fn1 satisfied the formula (2).
- the slab heating time and the steel slab heating time were too short (manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.
- the Cr content of the chemical composition of test number 25 exceeded the upper limit of the Cr content of the present invention. Therefore, the main cutting edge wear amount was higher than 80% and the machinability was low. This is probably because the Cr content was too high and bainite was excessively generated in the steel.
- Test No. 26 used the same steel K as Test No. 25. Therefore, machinability was low. In test number 26, the heating time of the slab and the heating time of the steel slab were too short (manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low.
- the Cr content of the chemical composition of test number 27 was less than the lower limit of the Cr content of the present invention. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low. Furthermore, the surface fatigue strength was also low, less than 120%.
- Test number 28 used the same steel L as test number 27. Therefore, the bending fatigue strength was low. Furthermore, in the test number 28, the heating temperature of the steel slab was too high, and the finishing temperature was too high (manufacturing condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low. Furthermore, the surface fatigue strength was also low, less than 120%.
- the Mo content of the chemical composition of test number 29 exceeded the upper limit of the Mo content of the present invention. Therefore, the main cutting edge wear amount of test number 29 exceeded 80%, and the machinability was low. This is probably because the Mo content was too high and bainite was excessively produced in the steel.
- Test No. 30 used the same steel M as the test No. 29. Therefore, machinability was low. In test number 30, the heating temperature of the slab was too low (manufacturing condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low.
- test number 32 (steel N) is within the scope of the present invention, and fn1 satisfies the formula (2).
- the heating temperature and finishing temperature of the steel slab were too high (manufacturing condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.
- test number 34 was within the scope of the present invention, and fn1 satisfied the formula (2).
- the heating temperature of the slab was too low (manufacturing condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.
- the Mn content and Al content of the chemical composition of test number 35 were less than the lower limits of the Mn content and Al content of the present invention. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Furthermore, the surface fatigue strength was less than 120% and was low.
- the test No. 36 used the same steel P as the test No. 35. Therefore, the bending fatigue strength and surface fatigue strength in the middle cycle were low.
- the steel bar after finish rolling was further water-cooled to 800 ° C. (Production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength at a high cycle was less than 115% and was low. Furthermore, the bending fatigue strength in the middle cycle was lower than the test number 35.
- the Mn content and Al content of the chemical composition of test number 37 exceeded the upper limits of the Mn content and Al content of the present invention. Therefore, the bending fatigue strength at a high cycle was less than 115% and was low. Furthermore, the main cutting edge wear amount exceeded 80%, and the machinability was low.
- the same steel Q as the test number 37 was used for the test number 38. Therefore, the bending fatigue strength at a high cycle was low and the machinability was also low. Furthermore, in the test number 38, the heating temperature and finishing temperature of the steel slab were too high. Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Further, the high cycle bending fatigue strength was lower than the test number 37.
- test number 39 (steel R) is within the range of the chemical composition of the present invention, fn1 exceeded the upper limit of the formula (2). Therefore, the machinability of the steel of test number 39 was low.
- the test No. 40 used the same steel R as the test No. 39. Therefore, the machinability of the steel of test number 40 was low.
- water cooling was further performed before rolling (Production Condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength at the middle cycle and the high cycle was lower than the test number 39.
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Abstract
Description
fn1=Cr+2×Mo (1)
ここで、式(1)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
1.60≦Cr+2×Mo≦2.10 ・・・(2)
ここで、式(2)中の各元素記号には、対応する元素の含有量(質量%)が代入される。
フェライト平均粒径比=15視野で得られたフェライト平均粒径のうちの最大値/15視野で得られたフェライト平均粒径のうちの最小値 (3)
フェライト平均粒径比が2.0以下である場合、鋼中の結晶粒のばらつきが小さい。そのため、鋼の曲げ疲労強度及び面疲労強度が高い。
本発明による棒鋼又は線材の化学組成は、以下の元素を含有する。
炭素(C)は、浸炭焼入れ、又は浸炭窒化焼入れ性を高める。そのため、Cは、鋼の強度を高める。特に、Cは、浸炭焼入れ又は浸炭窒化焼入れ後の機械部品の芯部の強度を高める。一方、Cが過剰に含有されれば、浸炭焼入れ、又は浸炭窒化焼入れ後の機械部品の変形量が顕著に増加する。したがって、C含有量は0.1~0.25%である。好ましいC含有量の下限は0.1%よりも高く、さらに好ましくは、0.15%以上であり、さらに好ましくは、0.18%以上である。好ましいC含有量の上限は、0.25%未満であり、さらに好ましくは、0.23%以下であり、さらに好ましくは、0.20%以下である。
珪素(Si)は、鋼の焼入れ性を高める。Siはさらに、鋼の焼戻し軟化抵抗を高める。したがって、Siは、鋼の面疲労強度及び耐摩耗性を高める。一方、Siが過剰に含有されれば、鋼の熱間鍛造後の強度が過剰に高くなる。その結果、鋼の被削性が低下する。Siが過剰に含有されればさらに、曲げ疲労強度が低下する。したがって、Si含有量は0.30~0.60%である。好ましいSi含有量の下限は0.30%よりも高く、さらに好ましくは0.40%以上であり、さらに好ましくは0.45%以上である。好ましいSi含有量の上限は0.60%未満であり、さらに好ましくは0.57%以下であり、さらに好ましくは0.55%以下である。
マンガン(Mn)は、鋼の焼入れ性を高め、鋼の強度を高める。したがって、Mnは、浸炭焼入れ又は浸炭窒化焼入れされた機械部品の芯部の強度を高める。一方、Mnが過剰に含有されれば、熱間鍛造後の鋼の被削性が低下する。さらに、Mnが過剰に含有されれば、鋼の表面にMn酸化物が生成される。その結果、浸炭焼入れ又は浸炭窒化焼入れ後の浸炭異常層の深さが大きくなる。浸炭異常層はたとえば、粒界酸化層及び不完全焼入れ層である。浸炭異常層の深さが大きくなれば、鋼の曲げ疲労強度及びピッチング強度が低下する。ピッチングは、面疲労の破壊形態の一つである。したがって、ピッチング強度が低ければ、面疲労強度も低くなる。したがって、Mn含有量は、0.50~1.0%である。好ましいMn含有量の下限は0.50%よりも高く、さらに好ましくは0.55%以上であり、さらに好ましくは0.60%以上である。Mn含有量の好ましい上限は1.0%未満であり、さらに好ましくは0.95%以下であり、さらに好ましくは0.9%以下である。
硫黄(S)はMnと結合してMnSを形成する。MnSは鋼の被削性を高める。一方、Sが過剰に含有されれば、粗大なMnSが形成される。粗大なMnSは鋼の曲げ疲労強度及び面疲労強度を低下する。したがって、S含有量は、0.003~0.05%である。好ましいS含有量の下限は0.003%よりも高く、さらに好ましくは0.005%以上であり、さらに好ましくは0.01%以上である。好ましいS含有量の上限は0.05%未満であり、さらに好ましくは0.03%以下であり、さらに好ましくは0.02%以下である。
クロム(Cr)は、鋼の焼入れ性、及び、鋼の焼戻し軟化抵抗を高める。そのため、Crは鋼の曲げ疲労強度、面疲労強度及び耐摩耗性を高める。一方、Crが過剰に含有されれば、熱間鍛造後、又は、焼きならし後の鋼でベイナイトの生成が促進される。そのため、鋼の被削性が低下する。したがって、Cr含有量は1.50~2.00%である。好ましいCr含有量の下限は1.50%よりも高く、さらに好ましくは1.70%以上であり、さらに好ましくは1.80%以上である。好ましいCr含有量の上限は2.00%未満であり、さらに好ましくは1.95%以下であり、さらに好ましくは1.90%以下である。
モリブデン(Mo)は、含有されなくてもよい。Moは鋼の焼入れ性及び焼戻し軟化抵抗を高める。そのため、Moは鋼の曲げ疲労強度、面疲労強度及び耐摩耗性を高める。一方、Moが過剰に含有されれば、熱間鍛造後、又は、焼きならし後の鋼でベイナイト生成が促進される。そのため、鋼の被削性が低下する。したがって、Mo含有量は0.10%以下(0%を含む)である。好ましいMo含有量の下限は0.02%以上である。好ましいMo含有量の上限は0.10%未満であり、さらに好ましくは0.08%以下であり、さらに好ましくは0.05%以下である。
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AlNを形成する。AlNは、浸炭加熱によるオーステナイト結晶粒の粗大化を抑制する。一方、Alが過剰に含有されれば、粗大なAl酸化物を形成する。粗大なAl酸化物は、鋼の曲げ疲労強度を低下する。したがって、Al含有量は0.025~0.05%である。好ましいAl含有量の下限は0.025%よりも高く、さらに好ましくは0.027%以上であり、さらに好ましくは0.030%以上である。好ましいAl含有量の上限は0.05%未満であり、さらに好ましくは0.045%以下であり、さらに好ましくは0.04%以下である。
窒素(N)は、Al又はNbと結合して、AlN又はNbNを形成する。AlN又はNbNは、浸炭加熱によるオーステナイト結晶粒の粗大化を抑制する。一方、Nが過剰に含有されれば、製鋼工程において安定して製造しにくくなる。したがって、N含有量は0.010~0.025%である。好ましいN含有量の下限は0.010%よりも高く、さらに好ましくは0.012%以上であり、さらに好ましくは0.013%以上である。好ましいN含有量の上限は0.025%未満であり、さらに好ましくは0.020%以下であり、さらに好ましくは0.018%以下である。
燐(P)は粒界に偏析して粒界を脆化する。そのため、Pは鋼の疲労強度を低下する。したがって、P含有量はなるべく低い方が好ましい。P含有量は0.025%以下である。好ましいP含有量は0.025%未満であり、さらに好ましくは0.020%以下である。
チタン(Ti)は、Nと結合して粗大なTiNを形成する。粗大なTiNは、鋼の疲労強度を低下する。したがって、Ti含有量はなるべく低い方が好ましい。Ti含有量は0.003%以下である。好ましいTi含有量は0.003%未満であり、さらに好ましくは0.002%以下である。
酸素(O)は、Alと結合して酸化物系介在物を形成する。酸化物系介在物は、鋼の曲げ疲労強度を低下する。したがって、O含有量はなるべく低い方が好ましい。O含有量は0.002%以下である。好ましいO含有量は0.002%未満であり、さらに好ましくは0.001%以下である。
1.60≦Cr+2×Mo≦2.10 (2)
ここで、式(2)中の元素記号には、対応する元素の含有量(質量%)が代入される。
ニオブ(Nb)は選択元素である。Nbは、C、Nと結合してNb炭化物、Nb窒化物又はNb炭窒化物を形成する。Nb炭化物、Nb窒化物及びNb炭窒化物は、Al窒化物と同様に、浸炭加熱時においてオーステナイト結晶粒が粗大化するのを抑制する。Nbが少しでも含有されれば、上記効果が得られる。一方、Nbが過剰に含有されれば、Nb炭窒化物、Nb窒化物及びNb炭窒化物が粗大化する。そのため、浸炭加熱時においてオーステナイト結晶粒の粗大化を抑制できない。したがって、Nb含有量は、0.08%以下である。好ましいNb含有量の下限は0.01%以上である。好ましいNb含有量の上限は、0.08%未満であり、さらに好ましくは、0.05%以下である。
本発明による棒鋼又は線材のミクロ組織は、フェライト・パーライト組織、フェライト・パーライト・ベイナイト組織、又は、フェライト・ベイナイト組織からなる。ここで、「フェライト・パーライト組織」とは、マトリックス(母相)が、フェライトとパーライトとからなる2相組織を意味する。「フェライト・パーライト・ベイナイト組織」は、マトリックスが、フェライトと、パーライトと、ベイナイトとからなる3相組織を意味する。「フェライト・ベイナイト組織」は、マトリックスが、フェライトとベイナイトとからなる2相組織を意味する。
本発明の棒鋼または線材の製造方法の一例、及び、歯車及びプーリに代表される機械部品の製造方法の一例を説明する。なお、製造方法は下記に限定されない。
断面減少率={1-(棒鋼、線材の断面積/鋼片の断面積)}×100 (4)
直径50mmの棒鋼を長手方向に垂直に切断した。切断面の中心部を含むサンプルを切り出した。サンプルの表面のうち、上述の中心部に相当する表面を鏡面に研磨した。研磨面をナイタールで腐食した。腐食面を倍率400倍の光学顕微鏡で、15視野観察した。15視野は、表層の脱炭層を除いた領域から任意に選択された。各視野の大きさは250μm×250μmであった。各視野においてミクロ組織を観察した。
上述の15視野のフェライト平均粒径を、JIS G0551(2005)に規定された切断法に準拠して測定した。
各試験番号の棒鋼を、1200℃で30分加熱した。次に、仕上げ温度を950℃以上として熱間鍛造し、直径35mmの丸棒を製造した。直径35mmの丸棒を機械加工して、図1に示すローラピッチング小ローラ試験片(以下、単に小ローラ試験片という)と、図2に示す切欠き付き小野式回転曲げ疲労試験片(図1及び図2ともに、図中の寸法の単位はmm)を作製した。図1に示す小ローラ試験片は、中央に試験部(直径26mm、幅28mmの円柱部)を備えた。
ローラピッチング試験では、上記の小ローラ試験片と、図4に示す形状の大ローラ(図中の寸法の単位はmm)とを組合せた。図4に示す大ローラは、JIS規格SCM420Hの規格を満たす鋼からなり、一般的な製造工程、つまり、焼きならし、試験片加工、ガス浸炭炉による共析浸炭、低温焼戻し及び研磨、の工程によって作製された。
すべり率=(V2-V1)/V2×100
ローラピッチング試験において、繰り返し数が1.0×106回となった小ローラ試験片の試験部の摩耗量を測定した。具体的には、JIS B0601(2001)に準拠して、最大高さ粗さ(Rz)を求めた。Rz値が小さいほど、耐摩耗性が高いことを示す。摩耗量の測定には、粗さ計を用いた。表3に、摩耗量を示す。表3中の摩耗量では、試験番号1の摩耗量を基準値(100%)とした。そして、各試験番号の摩耗量を基準値に対する比(%)で示した。摩耗量が80%以下であれば、優れた耐摩耗性が得られたと判断した。
曲げ疲労強度は、小野式回転曲げ疲労試験により求めた。小野式回転曲げ疲労試験での試験数は各試験番号ごとに8個とした。試験時の回転数は3000rpmとし、その他は通常の方法により試験を行った。繰り返し数1.0×104回、および1.0×107回まで破断しなかったもののうち、最も高い応力をそれぞれ中サイクル、および高サイクル回転曲げ疲労強度と定義した。
切削試験を実施し、被削性を評価した。以下の方法により切削試験片を得た。各試験番号の直径70mmの棒鋼を1250℃の加熱温度で30分加熱した。加熱された棒鋼を950℃以上の仕上げ温度で熱間鍛造し、直径60mmの丸棒を得た。この丸棒から機械加工によって、直径55mm、長さ450mmの切削試験片を得た。切削試験片を用いて、下記の条件で切削試験を行った。
チップ:母材材質 超硬P20種グレード、コーティング なし
条件:周速200m/分、送り0.30mm/rev、切り込み1.5mm、水溶性切削油を使用
測定項目:切削時間10分後の逃げ面の主切刃摩耗量
表3を参照して、試験番号4及び9の棒鋼の化学組成(鋼C)は本発明の範囲内であり、かつ、fn1は、式(2)を満たした。さらに、試験番号4及び9のフェライト平均粒径比はいずれも2.0以下であった。そのため、試験番号4及び9の中サイクル及び高サイクルの曲げ疲労強度は115%以上であり、面疲労強度は120%以上であった。さらに、摩耗量は80%以下であった。さらに、主切刃摩耗量は80%以下であった。したがって、試験番号4及び9の棒鋼は、優れた曲げ疲労強度、面疲労強度、耐摩耗性及び被削性を有した。
Claims (3)
- 化学組成が、質量%で、
C:0.1~0.25%、
Si:0.30~0.60%、
Mn:0.50~1.0%、
S:0.003~0.05%、
Cr:1.50~2.00%、
Mo:0.10%以下(0%を含む)、
Al:0.025~0.05%、
N:0.010~0.025%を含有し、残部がFeおよび不純物からなり、
前記不純物中のP、TiおよびOがそれぞれ、
P:0.025%以下、
Ti:0.003%以下、
O(酸素):0.002%以下であり、
式(1)で定義されるfn1が1.60~2.10であり、
組織が、フェライト・パーライト組織、フェライト・パーライト・ベイナイト組織、またはフェライト・ベイナイト組織からなり、
横断面において、1視野あたりの面積62500μm2で15視野観察測定して得られたフェライト平均粒径の最大値/最小値が2.0以下である、熱間鍛造用圧延棒鋼又は線材。
fn1=Cr+2×Mo (1)
ここで、式(1)中の各元素記号には、対応する元素の含有量(質量%)が代入される。 - fn1が1.80以上である、請求項1に記載の熱間鍛造用圧延棒鋼又は線材。
- Feの一部に代えて、質量%で、Nb:0.08%以下を含有する、請求項1又は2に記載の熱間鍛造用圧延棒鋼又は線材。
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JP2017106079A (ja) * | 2015-12-10 | 2017-06-15 | 山陽特殊製鋼株式会社 | 耐結晶粒粗大化特性、耐曲げ疲労強度および耐衝撃強度に優れた機械構造用鋼 |
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JPH1150191A (ja) * | 1997-08-05 | 1999-02-23 | Nippon Steel Corp | 浸炭軸状部品とその製造方法 |
JP2001303174A (ja) * | 2000-04-26 | 2001-10-31 | Nippon Steel Corp | 結晶粒粗大化防止特性に優れた高温浸炭部品用素形材とその製造方法 |
JP2008189989A (ja) * | 2007-02-05 | 2008-08-21 | Sumitomo Metal Ind Ltd | 高温浸炭用鋼材 |
JP2009052062A (ja) * | 2007-08-24 | 2009-03-12 | Sumitomo Metal Ind Ltd | 熱間圧延棒鋼または線材 |
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JP2016183399A (ja) * | 2015-03-26 | 2016-10-20 | 新日鐵住金株式会社 | 浸炭機械構造部品 |
WO2016159392A1 (ja) * | 2015-03-31 | 2016-10-06 | 新日鐵住金株式会社 | 熱間圧延棒線材、部品および熱間圧延棒線材の製造方法 |
JPWO2016159392A1 (ja) * | 2015-03-31 | 2018-02-08 | 新日鐵住金株式会社 | 熱間圧延棒線材、部品および熱間圧延棒線材の製造方法 |
EP3279361A4 (en) * | 2015-03-31 | 2018-10-24 | Nippon Steel & Sumitomo Metal Corporation | Hot-rolled bar member, part, and hot-rolled bar member manufacturing method |
US20180355455A1 (en) * | 2015-03-31 | 2018-12-13 | Nippon Steel & Sumitomo Metal Corporation | Hot rolled bar or hot rolled wire rod, component, and manufacturing method of hot rolled bar or hot rolled wire rod |
JP2017106079A (ja) * | 2015-12-10 | 2017-06-15 | 山陽特殊製鋼株式会社 | 耐結晶粒粗大化特性、耐曲げ疲労強度および耐衝撃強度に優れた機械構造用鋼 |
WO2022071420A1 (ja) * | 2020-09-30 | 2022-04-07 | 日本製鉄株式会社 | 鋼材 |
WO2022071419A1 (ja) * | 2020-09-30 | 2022-04-07 | 日本製鉄株式会社 | 鋼材 |
JP7385160B2 (ja) | 2020-09-30 | 2023-11-22 | 日本製鉄株式会社 | 鋼材 |
JP7417171B2 (ja) | 2020-09-30 | 2024-01-18 | 日本製鉄株式会社 | 鋼材 |
Also Published As
Publication number | Publication date |
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JP5561436B2 (ja) | 2014-07-30 |
US20140363329A1 (en) | 2014-12-11 |
IN2014DN02151A (ja) | 2015-05-15 |
CN103797144A (zh) | 2014-05-14 |
KR101552449B1 (ko) | 2015-09-10 |
CN103797144B (zh) | 2016-07-06 |
KR20140056378A (ko) | 2014-05-09 |
JPWO2013031587A1 (ja) | 2015-03-23 |
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