WO2018056332A1 - シャフト部品 - Google Patents
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- WO2018056332A1 WO2018056332A1 PCT/JP2017/033984 JP2017033984W WO2018056332A1 WO 2018056332 A1 WO2018056332 A1 WO 2018056332A1 JP 2017033984 W JP2017033984 W JP 2017033984W WO 2018056332 A1 WO2018056332 A1 WO 2018056332A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/06—Surface hardening
- C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
- C21D1/10—Surface hardening by direct application of electrical or wave energy; by particle radiation by electric induction
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/28—Normalising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/28—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—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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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
- F16C3/00—Shafts; Axles; Cranks; Eccentrics
- F16C3/02—Shafts; Axles
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2261/00—Machining or cutting being involved
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/30—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
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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
- Y02P10/25—Process efficiency
Definitions
- the present invention relates to a shaft component, and more particularly to a shaft component subjected to induction hardening.
- shaft parts for example, transmission shafts
- induction hardening or carburizing and quenching which is a kind of surface hardening treatment.
- a method for manufacturing a shaft part to be quenched for example, the following method can be given. That is, first, a rough member having a shape close to the final product is manufactured. Next, a hole is drilled or the like to produce an intermediate member that is closer to the final product. Finally, the intermediate member is quenched (induction quenching or carburizing quenching) to obtain a shaft component.
- Patent Document 1 discloses a crankshaft having a high torsional fatigue strength in which a hardened hardened layer is formed in an oil hole opening by induction hardening.
- Patent Document 2 discloses a shaft having excellent fatigue resistance and a method for improving the fatigue strength thereof, wherein the compressive residual stress in the surface layer of the oil hole is 50% to 90% of the tensile strength of the steel material. Has been.
- the burning boundary is strengthened by generating a compressive residual stress on the inner surface of the oil hole by striking the inner surface of the oil hole with an ultrasonic vibration terminal.
- it is difficult to uniformly treat the entire oil hole by hitting with an ultrasonic vibration terminal, and the target strength may not always be obtained.
- it may be difficult to achieve both static torsional strength and torsional fatigue strength due to the reason that the steel components and the surface layer structure are inappropriate.
- the present invention has been made in view of the above circumstances, and an object thereof is to provide a shaft component that is excellent in static torsional strength and torsional fatigue strength.
- the inventors of the present invention diligently studied a shaft component that can achieve both static torsional strength and torsional fatigue strength, and a method for manufacturing the shaft component. As a result, the present inventors do not perform the drilling process before induction hardening, which is usually performed, but the hole is drilled by cutting after induction hardening, thereby increasing the hardness near the hole and generating cracks. It has been found that the static torsional strength and torsional fatigue strength of the shaft components are improved in order to suppress the development. It has also been found that the static torsional strength and torsional fatigue strength of the shaft parts can be further improved if more retained austenite is subjected to processing-induced martensite transformation during cutting.
- the present inventors have focused on the chemical composition and heat treatment conditions of the steel material and attempted to further improve the static torsional strength and torsional fatigue strength. As a result, it has been found that by adopting specific steel material components and heat treatment conditions, processing-induced martensitic transformation is likely to occur at the time of cutting, and the static torsional strength and torsional fatigue strength of shaft parts are remarkably improved.
- the present inventors have appropriately controlled the structure after induction hardening and the structure after cutting by organic optimization of the chemical composition, heat treatment conditions and cutting conditions of the steel material. It was found that a shaft component with improved torsional fatigue strength can be obtained in a well-balanced manner. Based on the above findings, the present inventors have completed a shaft component excellent in static torsional strength and torsional fatigue strength.
- the summary is as follows.
- FIG. 1A is a schematic view of a hardened material or a shaft component in a final form
- FIG. 1B is a view showing a cross section A-A ′ cut perpendicular to the longitudinal direction of the shaft component
- FIG. 2 is a diagram showing a measurement position 21 of the retained austenite volume ratio at a depth of 2 mm from the outer peripheral surface and a depth position of 20 ⁇ m from the hole surface
- Fig. 3 (a) is a schematic view of the shaft component
- Fig. 3 (b) is a cut at a depth of 2 mm in the axial direction of the hole from the outer periphery of the shaft component and perpendicular to the axial direction of the hole.
- FIG. 1A is a schematic view of a hardened material or a shaft component in a final form
- FIG. 1B is a view showing a cross section A-A ′ cut perpendicular to the longitudinal direction of the shaft component.
- FIG. 2 is a diagram showing a measurement
- FIG. 6 is a view showing a cross section CC ′.
- FIG. 4 is a scanning electron microscope image of the surface layer of the hole in a cross section cut at a depth of 2 mm from the outer periphery of the shaft component in the axial direction of the hole and perpendicular to the axial direction of the hole.
- FIG. 5 is a side view of a test piece used in the torsion test.
- FIG. 6 is a top view of a hole drilled in the shaft component.
- the shaft component according to the embodiment of the present invention is, in mass%, C: 0.35 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.5 to 2.6%, S: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, As an impurity element, P: 0.050% or less, O: 0.003% or less Further, as an optional element, Pb: 0.5% or less, One or more selected from the group consisting of V, Nb and Ti with a total content of 0.1% or less, One or more selected from the group consisting of Cr: 3.0% or less, Mo: 3.0% or less, and Ni: 3.0% or less, Cu: 0 to 0.50%, B: 0 to 0.020% is contained, And the balance consists of Fe and impurities, and has a chemical composition satisfying the formula (1): 15.0 ⁇ 25.9C + 6.35Mn + 2.88Cr + 3.09Mo + 2.73Ni ⁇ 27.2, Having at
- the shaft parts according to the embodiment of the present invention include shaft parts used in automobiles and industrial machines, for example, transmission shafts.
- a preferable shape of the shaft component is a hollow or solid cylindrical or rod-shaped component having a diameter of 150 mm or less and a length of 5 mm or more.
- the shaft component has the following chemical composition.
- the ratio (%) of each element shown below means mass%.
- Carbon (C) increases the strength of the shaft component (particularly the strength of the core). C further generates retained austenite to increase the static torsional strength and torsional fatigue strength of the shaft component. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the strength of the steel material processed into the shaft component becomes too high. Therefore, the machinability of the steel material is reduced. In addition, the strain generated during induction hardening increases and quench cracks occur. Therefore, the C content is 0.35% or more and 0.70% or less. The minimum with preferable C content is 0.40% or more. The upper limit with preferable C content is less than 0.65%.
- Si 0.01-0.40 % Silicon (Si) has an effect of improving hardenability, but increases the carburizing abnormal layer during the carburizing process.
- the Si content is preferably 0.30% or less, and more preferably 0.20% or less.
- the Si content is set to 0.01 to 0.40%. In consideration of the manufacturing cost in the mass production of steel, it is considered that the Si content is often 0.05% or more in the actually manufactured product of the present invention.
- Mn 0.5 to 2.6%
- Manganese (Mn) increases the hardenability of the steel material processed into the shaft component and increases the retained austenite in the steel material.
- austenite containing Mn is likely to undergo work-induced martensitic transformation during cutting of a hole after induction hardening. As a result, the static torsional strength and torsional fatigue strength of the shaft component are increased. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the retained austenite after induction hardening becomes excessive.
- the Mn content is 0.5 to 2.6%.
- the minimum with preferable Mn content is 0.8%, More preferably, it is 1.4%.
- the upper limit with preferable Mn content is 2.0%.
- P 0.050% or less Phosphorus (P) is an impurity. P segregates at the grain boundary and lowers the grain boundary strength. As a result, the static torsional strength and torsional fatigue strength of the shaft component are reduced. Therefore, the P content is 0.050% or less. The upper limit with preferable P content is 0.030%. The P content should be as low as possible. The minimum with preferable P content is 0.0002%.
- S 0.005 to 0.020%
- Sulfur (S) combines with Mn to form MnS and enhances the machinability of the steel material. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, coarse MnS is formed, and the hot workability, cold workability, and torsional fatigue strength of the shaft parts are reduced. Therefore, the S content is 0.005 to 0.020%. A preferable lower limit of the S content is 0.008%. The upper limit with preferable S content is 0.015%.
- Al 0.010 to 0.050%
- Aluminum (Al) is an element that deoxidizes steel. Al is further combined with N to form AlN, and the crystal grains are refined. As a result, the static torsional strength and torsional fatigue strength of the shaft component are increased. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, hard and coarse Al 2 O 3 is generated, the machinability of the steel material is lowered, and the torsional fatigue strength of the shaft part is also lowered. Therefore, the Al content is 0.010 to 0.050%. The minimum with preferable Al content is 0.020%. The upper limit with preferable Al content is 0.040%.
- N 0.005 to 0.025%
- Nitrogen (N) forms nitrides to refine crystal grains, and increases the static torsional strength and torsional fatigue strength of the shaft component. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, coarse nitrides are generated and the toughness of the steel material is reduced. Therefore, the N content is 0.005 to 0.025%.
- the minimum with preferable N content is 0.010%.
- the upper limit with preferable N content is 0.020%.
- Oxygen (O) is an impurity. O combines with Al to form hard oxide inclusions. The oxide inclusions reduce the machinability of the steel material and also reduce the torsional fatigue strength of the shaft component. Accordingly, the O content is 0.003% or less. The lower the O content, the better. A preferable lower limit of the O content is 0.0001%.
- the balance of the chemical composition of the steel material is iron (Fe) and impurities.
- Impurities mean components that are mixed in from the ore and scrap used as a raw material for steel, or the environment of the manufacturing process, and are not intentionally contained in the steel.
- the steel material processed into the shaft component may further contain Pb instead of a part of Fe.
- Pb 0.5% or less
- Lead (Pb) is an optional element and may not be contained. When it contains, the fall of the tool wear at the time of a cutting process and the improvement of chip disposal property are implement
- the steel material processed into the shaft component may further contain one or more selected from the group consisting of V, Nb, and Ti instead of a part of Fe.
- V, Nb and Ti 0.1% or less in total content
- Vanadium (V), niobium (Nb) and titanium (Ti) are optional elements and may not be contained. These elements combine with C and N to form precipitates. Precipitates of these elements complement the grain refinement of the quenched part by AlN. Precipitates of these elements increase the static torsional strength and torsional fatigue strength of the shaft component. However, if the total content of these elements exceeds 0.1%, the precipitates become coarse and the torsional fatigue strength decreases. Therefore, the total content of V, Nb and Ti is preferably 0.1% or less. If any one or more of V, Nb, and Ti is contained as an optional element, the above effect can be obtained. A more preferable upper limit of the total content of V, Nb and Ti is 0.08%. In order to acquire said effect by V, Nb, and Ti, containing 0.01% or more is preferable.
- the steel material processed into the shaft component may further contain one or more selected from the group consisting of Cr, Mo, and Ni instead of a part of Fe. All of these elements increase the hardenability of the steel material and increase the retained austenite.
- Chromium (Cr) is an optional element and may not be contained. Cr increases the hardenability of the steel material and further increases the retained austenite. However, if the Cr content is too high, the retained austenite after induction hardening becomes excessively high. In this case, sufficient work-induced martensite transformation does not occur during cutting, and residual austenite is less likely to decrease after cutting. As a result, the static torsional strength and torsional fatigue strength of the shaft component are reduced. Therefore, the Cr content is preferably 3.0% or less. In order to acquire the said effect by Cr, containing 0.1% or more is preferable. The upper limit with preferable Cr content is 2.0%.
- Mo Molybdenum
- Mo is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel material and increases retained austenite. Mo further increases the temper softening resistance and increases the static torsional strength and torsional fatigue strength of the shaft component. However, if the Mo content is too high, the retained austenite after induction hardening becomes excessive. In this case, sufficient work-induced martensitic transformation does not occur during cutting. As a result, the static torsional strength and torsional fatigue strength of the shaft component are reduced. Therefore, the Mo content is preferably 3.0% or less. A more preferable upper limit of the Mo content is 2.0%. In order to acquire the said effect by Mo, containing 0.1% or more is preferable.
- Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the hardenability of the steel material and increases retained austenite. Ni further increases the toughness of the steel material. However, if the Ni content is too high, the retained austenite after induction hardening becomes excessive. In this case, sufficient work-induced martensitic transformation does not occur during cutting after quenching. As a result, the static torsional strength and torsional fatigue strength of the shaft component are reduced. Therefore, the Ni content is preferably 3.0% or less. A more preferable upper limit of the Ni content is 2.0%. In order to acquire the said effect by Ni, containing 0.1% or more is preferable.
- Cu 0 to 0.50% Cu is dissolved in martensite to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. However, if the Cu content is too high, it segregates at the grain boundaries of steel during hot forging and induces hot cracking. Therefore, the Cu content is 0.50% or less. Note that the Cu content is preferably 0.40% or less, and more preferably 0.25% or less. In order to acquire the said effect by Cu, containing 0.10% or more is preferable.
- B 0 to 0.020% B has the effect of suppressing the grain boundary segregation of P and increasing the toughness.
- the B content is 0.020% or less.
- the B content is preferably 0.015% and more preferably 0.010% or less. In order to acquire the said effect by B, containing 0.0005% or more is preferable.
- the shaft component according to the present invention may contain a trace amount of elements other than the above as impurities in its chemical composition. Even in this case, the object of the present invention can be achieved.
- the shaft component according to the present invention can include the following elements within the prescribed ranges.
- the F1 value is a parameter representing the stability of austenite.
- Formula (1) is an empirical formula obtained by multiple regression analysis from the measured values of the residual ⁇ volume fraction of hardened steel having various chemical components. If the F1 value is too low, the austenite becomes thermodynamically unstable, the retained austenite is not sufficiently generated after induction hardening, and the static torsional strength and torsional fatigue strength of the shaft component are reduced. On the other hand, if the F1 value is too high, the stability of austenite increases, and the retained austenite after induction hardening becomes excessive.
- F1 is 15.0 to 27.2.
- the preferable lower limit of F1 is 16.5, and the preferable upper limit is 27.0 or less.
- the shaft component according to the embodiment of the present invention has a through hole or a non-through hole opened from the outer peripheral surface of the shaft at a perpendicular or constant angle with respect to the longitudinal direction of the shaft component.
- the diameter of the hole is 0.2 mm to 10 mm.
- the shaft component has one or more of these holes.
- This retained austenite undergoes work-induced martensitic transformation in the vicinity of the hole during drilling after quenching of the shaft part. Specifically, at the time of drilling, a part of the retained austenite near the surface layer of the hole is transformed into work-induced martensite due to the frictional force between the cutting tool and the base material. On the other hand, the occurrence of processing-induced martensitic transformation due to this action is limited to the vicinity of the hole.
- the processing-induced martensitic transformation associated with the drilling process no longer occurs. Therefore, the retained austenite volume ratio (R1) at a depth of 2 mm from the outer peripheral surface and 2 mm from the surface of the hole is a portion not affected by the drilling after quenching, and is the residual austenite volume ratio before cutting. Can think.
- the strength of the shaft component increases, and the static torsional strength and torsional fatigue strength increase.
- the maximum retained austenite volume fraction (R1) after quenching must be 4% or more.
- the residual austenite volume ratio (R2) at a depth position of 2 mm from the outer peripheral surface to the hole axial direction and a depth position of 20 ⁇ m from the hole surface is the residual austenite near the surface created by drilling. It is a volume ratio, and can be considered as a retained austenite volume ratio after cutting. If the volume fraction of retained austenite after cutting is too high, hard martensite is not obtained, and static torsional strength and torsional fatigue strength are reduced.
- the retained austenite reduction rate ( ⁇ ) represents the degree of work-induced martensitic transformation during cutting.
- ⁇ When ⁇ is large, it means that more work-induced martensitic transformation has occurred during cutting, and the static torsional strength and torsional fatigue strength of the shaft parts are improved. In order to obtain such an effect, ⁇ must be 40% or more. A preferable value of ⁇ is 42% or more.
- the plastic fluidized layer is a layer formed on the surface of the hole by deformation caused by friction generated between the cutting tool and the base material when the hole is cut.
- the thickness of the plastic fluidized bed on the surface of the hole is measured by the following method. Observe the surface (cross section) perpendicular to the axial direction of the hole, including the hole surface layer in a cross section perpendicular to the axial direction of the hole at a depth of 2 mm from the outer periphery of the shaft component to the axial direction of the hole.
- a test piece that becomes a surface is collected (see reference numeral 31 in FIG. 3B).
- the mirror polished specimen is corroded with 5% nital solution.
- the position (31) including the surface of the corroded surface hole is observed with a scanning electron microscope (SEM) at a magnification of 5000 times.
- SEM scanning electron microscope
- An example of the obtained SEM image is shown in FIG.
- the plastic fluidized bed 41 is a portion where the plastic fluidized structure is curved in the circumferential direction of the surface of the hole of the shaft part (from the left to the right in FIG. 4) with respect to the center part 42 of the base material. It is.
- a plastic fluidized bed is formed by causing a large deformation in the surface layer of the hole due to friction between the cutting tool and the base material.
- This plastic fluidized bed is more resistant to deformation than the base material. Therefore, when a plastic fluidized bed having a thickness of 0.5 ⁇ m or more is present, the torsional strength and torsional fatigue strength of the shaft component are improved.
- the plastic fluidized bed is brittle, if its thickness exceeds 15 ⁇ m, cracking occurs due to deformation and becomes the starting point of crack generation. Therefore, a plastic fluidized bed that is too thick reduces the torsional fatigue strength of the shaft component.
- the thickness of the plastic fluidized bed of the shaft part is limited to 0.5 to 15 ⁇ m.
- the thickness of the plastic fluidized layer on the surface layer of the shaft component is preferably 1 ⁇ m or more, and more preferably 3 ⁇ m or more.
- a preferable upper limit is 13 micrometers.
- the shaft component according to the present invention includes a portion having excellent strength around the hole that may cause a reduction in static torsional strength and torsional fatigue strength.
- the shaft component includes a region (also referred to as a “processing induced martensite layer”) in which the ratio of the processing induced martensite structure is high at the periphery of the hole.
- work-induced martensite increases the strength of the structure. Therefore, the strength of the periphery of the hole in this shaft part (20 ⁇ m deep position from the hole surface) ) Higher than strength. Therefore, this shaft component is excellent in static torsional strength and torsional fatigue strength.
- this shaft component may be provided with a plastic fluidized layer having an appropriate thickness on the surface layer portion of the hole.
- This plastic fluidized bed is also superior in strength compared to the base material.
- the shaft component according to the embodiment of the present invention is manufactured by the following method. In mass%, C: 0.35 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.5 to 2.6%, S: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, As an impurity element, P: 0.050% or less, O: 0.003% or less Further, as an optional element, Pb: 0.5% or less, One or more selected from the group consisting of V, Nb and Ti with a total content of 0.1% or less, One or more selected from the group consisting of Cr: 3.0% or less, Mo: 3.0% or less, and Ni: 3.0% or less, Cu: 0 to 0.50%, B: 0 to 0.020% is contained, And the balance consists of Fe and impurities, and processing the steel material having the chemical composition satisfying the formula (2) into the shape of the shaft component to obtain the shaft component rough member, A step of subjecting the rough member to induction hardening to obtain a hard
- the shaft component manufacturing method of the present embodiment includes a step of processing a steel material into a shape close to the shape of the shaft component to obtain a rough shaft component member (coarse member manufacturing step), and subjecting the rough member to induction hardening. It includes a step of obtaining a quenching material (quenching material manufacturing step) and a step of drilling a hole in the quenching material to form a hole to obtain a shaft component (drilling step).
- the steel material has the same chemical composition with the same content as the shaft component according to the embodiment of the present invention described above.
- the steel material having the above chemical composition is processed into a shape close to the shape of the shaft component to obtain a rough shaft component member.
- a known method can be adopted as the processing method. Examples of the processing method include hot processing, cold processing, cutting processing, and the like.
- the rough member has the same shape as that of the shaft component according to the embodiment of the present invention except for the hole. At this stage, the hole is not opened.
- Induction hardening In the induction hardening process, first, (i) induction heating is performed, and then (ii) quenching is performed. Induction heating and quenching are performed under the following conditions.
- Heating time during high-frequency heating 1.0 to 40 seconds
- the heating time is the time from the start of heating of the rough member to the start of water cooling at an output of 40 KW. If the heating time at the time of high-frequency heating is too long, austenite grains become coarse, and the static torsional strength and torsional fatigue strength of the shaft component are reduced. On the other hand, if the heating time is too short, cementite is not sufficiently dissolved, and the stability of austenite is lowered. Therefore, a structure composed of martensite and retained austenite having a volume ratio of 4 to 20% cannot be obtained after induction hardening. Accordingly, the heating time of the rough member during high-frequency heating is 1.0 to 40 seconds. By controlling both the heating frequency and the heating time, a region deeper than 2 mm from the outer peripheral surface is heated to a temperature of A 3 point or higher.
- Quenching After the constant temperature holding treatment, quenching is performed by a well-known method. Quenching can be, for example, water quenching. Thereby, the area
- the tempering process When it is desired to increase the toughness of the shaft part, after the induction hardening process, the tempering process may be performed.
- test piece 1 including a position of 2 mm from the outer periphery toward the center is prepared.
- the structure at a depth of 2 mm from the outer peripheral surface in the quenched material is retained austenite and martensite, and there is no phase other than these.
- retained austenite is contained in martensite. That is, it is impossible to distinguish between martensite and retained austenite by microstructure observation with an optical microscope. Therefore, the depth position of 2 mm from the outer peripheral surface and the residual austenite volume ratio (R1) of 2 mm from the hole surface are measured by the following method.
- Electrolytic polishing is performed on the test piece 1.
- An electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water is prepared. Using this electrolytic solution, electrolytic polishing is performed at a voltage of 20 V on the surface of the test piece including the reference position.
- X-rays are irradiated around a depth position of 2 mm from the outer peripheral surface, and analysis is performed by the X-ray diffraction method.
- the product name RINT-2500HL / PC manufactured by Rigaku Corporation is used.
- a Cr tube is used as the light source.
- the tube voltage is 40 kV
- the tube current is 40 mA
- the collimator diameter is 0.5 mm.
- K ⁇ rays were removed by a V filter and K ⁇ rays were used.
- AutoMATE software manufactured by Rigaku Corporation was used for data analysis.
- the K ⁇ 2 component was removed by the Rachinger method, and the residual austenite volume fraction was calculated based on the integrated intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure using the profile of the K ⁇ 1 component. Note that the spot size of the irradiated X-rays was set to 1 mm or less.
- the volume ratio (R1) of retained austenite at a depth position of 2 mm from the outer peripheral surface of the quenched material after completion of the quenched material manufacturing process and 2 mm from the surface of the hole is 4 to 20%.
- This retained austenite undergoes processing-induced martensitic transformation during the cutting process in the next drilling process.
- the reduction in static torsional strength and torsional fatigue strength of the shaft component due to the presence of the hole is suppressed by the processing-induced martensite formed around the hole.
- This effect cannot be obtained when the volume ratio of retained austenite at a depth of 2 mm from the outer peripheral surface is lower than 4%.
- the volume ratio of retained austenite is higher than 20%, a lot of soft austenite remains even after cutting. For this reason, the overall shaft component cannot obtain excellent static torsional strength and torsional fatigue strength.
- the shaft component according to the embodiment of the present invention has one or a plurality of through-holes or non-through-holes that are opened at a perpendicular or constant angle with respect to the longitudinal direction of the shaft component.
- drilling is performed by cutting.
- the machining-induced martensitic transformation is generated in the surface layer portion of the hole while making a hole by cutting.
- the reduction in static torsional strength and torsional fatigue strength of the shaft component due to the formation of the hole is suppressed, and a shaft component having excellent static torsional strength and torsional fatigue strength is created.
- Cutting is performed under the following conditions.
- a cemented carbide drill in which the surface of a cemented carbide is coated with carbide, nitride, oxide, diamond or the like (Coated as defined in JIS B 0171: 2014, 1003, 1004) (Carbide drill) can be used.
- a coated carbide drill is effective in suppressing tool wear and improving machining efficiency.
- Tool feed f More than 0.02 mm / rev (rotation), 0.2 mm / rev or less If the feed f is too small, the cutting resistance, that is, the force with which the tool is pressed against the work material is too small. In this case, sufficient work-induced martensitic transformation does not occur. Therefore, the static torsional strength and the torsional fatigue strength of the shaft component are not improved. On the other hand, if the feed is too large, the cutting resistance becomes too large. In this case, the tool may be damaged during cutting. Accordingly, the feed f is more than 0.02 mm / rev and less than 0.2 mm / rev. A preferable lower limit of the feed f is 0.03 mm / rev. A preferable upper limit of the feed f is 0.15 mm / rev, and more preferably 0.1 mm / rev.
- Cutting speed v 10 to 50 m / min If the cutting speed v is too large, the cutting temperature rises and martensitic transformation is difficult to occur. Therefore, the static torsional strength and the torsional fatigue strength of the shaft component are not improved. On the other hand, if the cutting speed is too low, the cutting efficiency is lowered and the production efficiency is lowered. Accordingly, the cutting speed v is 10 to 50 m / min. A preferable upper limit is 40 m / min, and more preferably 30 m / min.
- a shaft component is obtained by the drilling process described above.
- the retained austenite volume ratio (R1) of 2 mm from the outer peripheral surface of the obtained shaft component and 2 mm from the surface of the hole is 4 to 20%
- the measurement of the retained austenite volume ratio (R2) at a depth position of 2 mm from the outer peripheral surface in the axial direction of the hole and a depth position of 20 ⁇ m from the surface of the hole is carried out by the following method. That is, the shaft part is cut perpendicularly to the longitudinal direction of the shaft part and through the center of the hole so that the hole is vertically divided into two (B-B ′ line in FIG. 2). On the surface of the hole, masking with a hole having a diameter of 1 mm is applied around the position of a depth of 2 mm from the outer peripheral surface, and electropolishing is performed. The amount of polishing is adjusted by changing the electrolytic polishing time, and a hole having a depth of 20 ⁇ m is formed.
- the center of the hole (reference numeral 21 in FIG. 2) is irradiated with X-rays having a spot size of 0.5 mm, and the retained austenite volume ratio (R1) is 2 mm deep from the aforementioned outer peripheral surface and 2 mm from the hole surface.
- the residual austenite volume fraction is measured using a method similar to the measurement method of
- the residual austenite volume ratio (R1) at a depth of 2 mm from the outer peripheral surface and 2 mm from the surface of the hole is not affected by the drilling after quenching, and is considered as the residual austenite volume ratio before cutting. be able to.
- the residual austenite volume ratio (R2) at a depth position of 2 mm from the outer peripheral surface in the axial direction of the hole and a depth position of 20 ⁇ m from the hole surface is the residual austenite near the surface formed by drilling. It is a volume ratio, and can be considered as a retained austenite volume ratio after cutting.
- the retained austenite reduction rate ⁇ of the retained austenite before and after the cutting is calculated by the equation (A) based on the obtained volume ratios (R1) and (R2).
- Reduction rate ⁇ [(R1 ⁇ R2) / R1] ⁇ 100 (A)
- the residual austenite volume ratio (R2) at a depth position of 2 mm from the outer peripheral surface of the shaft part in the axial direction of the hole and a depth position of 20 ⁇ m from the hole surface is too high for the residual austenite after cutting. In this case, hard martensite cannot be obtained, and static torsional strength and torsional fatigue strength are reduced.
- the retained austenite reduction rate ⁇ of the retained austenite before and after cutting is 40% or more. Residual austenite undergoes machining-induced martensite transformation by cutting, thereby increasing static torsional strength and torsional fatigue strength. If the volume reduction rate ⁇ is too low, this effect cannot be obtained sufficiently.
- an Example is one aspect
- surface shown below the asterisk (*) was provided about the item which does not satisfy the requirements of this invention, and the item which does not satisfy the desirable manufacturing conditions of this invention.
- a vacuum melting furnace 150 kg of molten steels A to P having chemical compositions shown in Table 1 were obtained.
- Ingots were obtained by the ingot-making method using molten steel of each steel type. Each ingot was heated at 1250 ° C. for 4 hours and then hot forged to obtain a round bar having a diameter of 35 mm. The finishing temperature during hot forging was 1000 ° C.
- the normalizing process was performed on each round bar.
- the normalizing treatment temperature was 925 ° C., and the normalizing treatment time was 2 hours. After the normalizing treatment, the round bar was allowed to cool to room temperature (25 ° C.).
- the round bar after being allowed to cool is machined, and the torsional test piece 51 shown in FIG. 5 is a test piece for static torsion test and torsional fatigue test (hereinafter referred to as “torsion test piece”).
- the original rough member was produced. In the state of the rough member, the hole of ⁇ 3 mm is not opened.
- a torsional test piece 51 corresponding to a shaft part has a circular cross section, a cylindrical test part 52, a hole 53 arranged at the center of the test part 52, and cylindrical large diameter parts 54 arranged on both sides. And a pair of gripping portions 55 chamfered around the circumference of the large diameter portion.
- the center part of the test piece is a hollow hole 56 for weight reduction. As shown in FIG.
- the total length of the torsion test piece 51 is 200 mm
- the outer diameter of the test part 52 is 20 mm
- the length of the test part 52 is 30 mm
- the diameter of the hole 53 is 3 mm
- the hollow hole The diameter of 56 is 6 mm.
- the induction hardening was performed on the rough member of the torsion test piece 51 at an output of 40 KW based on the conditions shown in Table 2.
- the thickness of the surface hardened layer formed by induction hardening under the condition a in Table 2 using the steel type A in Table 1 is a measured value of the distance (thickness) from the surface and its Vickers hardness (HV). From about 2.5 mm.
- the rough member of the quenched torsion test piece 51 was subjected to drilling under the conditions shown in Table 3 to obtain a torsion test piece 51 corresponding to the shaft component.
- a coated carbide drill having a diameter of 3 mm and a ceramic coating on the surface of the cemented carbide was used as the cutting tool. Further, using a tip of a coated carbide drill having a tip angle of 90 ° and a diameter of 6 mm, chamfering of C 0.5 mm was performed at the entrance of the hole.
- the Vickers hardness near the hole surface formed by induction hardening condition a in Table 2 and cutting condition ⁇ in Table 3 using steel type A in Table 1 is 840HV at a distance of 50 ⁇ m in the thickness direction from the hole surface. 760 HV at 100 ⁇ m, 710 HV at 200 ⁇ m, and 695 HV at 300 ⁇ m.
- test piece 1 A position 2 mm from the surface of the hole of the test portion 52 of the torsion test piece 51 was cut perpendicularly to the longitudinal direction of the test piece 51. On the cut surface, a test piece (test piece 1) including a position of 2 mm from the outer periphery toward the center is prepared (FIG. 1B). Electrolytic polishing was performed on the cut surface. An electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water is prepared. Using this electrolytic solution, electrolytic polishing was performed at a voltage of 20 V on the surface including the reference position.
- the surface of the electrolytically polished test piece was subjected to X-ray diffraction by the above-described method, and the volume ratio (R1) of retained austenite at a position of 2 mm from the outer peripheral surface and 2 mm from the surface of the hole was obtained.
- Table 4 shows the retained austenite volume ratio (R2) at a depth position of 2 mm from the outer peripheral surface in the axial direction of the hole and at a depth position (10 ⁇ m and 50 ⁇ m) other than 20 ⁇ m from the hole surface.
- R2 retained austenite volume ratio
- each condition for the method of manufacturing a shaft component according to the embodiment of the present invention is satisfied (that is, on the premise that the chemical composition of the rough member is adjusted, in particular, the quenching material after induction quenching. And the structure of the hardened material (remaining ⁇ volume ratio (R1)) and the structure of the shaft part (residual ⁇ volume). It can be seen that excellent results are obtained for any of the ratios (R2)). Therefore, according to the manufacturing methods of these test examples, it was proved that a shaft component excellent in static torsional strength and torsional fatigue strength can be obtained.
- the conditions for the method of manufacturing the shaft component according to the embodiment of the present invention are not satisfied (that is, the chemical composition of the rough member is adjusted, and the structure of the quenching material after induction hardening)
- the structure of the shaft part after drilling is not improved, the structure of the hardened material (residual ⁇ volume ratio (R1)) and the structure of the shaft part (residual ⁇ volume ratio)
- R1 residual ⁇ volume ratio
- R2 residual ⁇ volume ratio
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Abstract
Description
質量%で、C:0.35~0.70%、Si:0.01~0.40%、Mn:0.5~2.6%、S:0.005~0.020%、Al:0.010~0.050%、N:0.005~0.025%を含有し、
不純物元素として、
P:0.050%以下、
O:0.003%以下
さらに、任意選択の元素として、
Pb:0.5%以下、
V、Nb及びTiからなる群から選択される1種以上を総含有量で0.1%以下、
Cr:3.0%以下、Mo:3.0%以下、及び、Ni:3.0%以下からなる群から選択される1種以上
Cu:0~0.50%、
B:0~0.020%を含有し
かつ、残部がFe及び不純物からなり、式(1)を満たす化学組成を有し、
外周表面に少なくとも1つの穴を有し、
前記外周表面から2mmの深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)が4~20%であり、
前記R1と前記外周表面から前記穴の軸方向に2mmの深さ位置で、かつ前記穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)とから式(A):Δγ=[(R1-R2)/R1]×100によって求められる残留オーステナイト減少率Δγが40%以上である、
ことを特徴とする、シャフト部品。
15.0≦25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≦27.2 (1)
ここで、式(1)中の各元素記号には、各元素の含有量(質量%)が代入される。
[2]前記穴の表面に厚さ0.5~15μmの塑性流動層を有することを特徴とする前記[1]に記載のシャフト部品。
本発明の実施形態に係るシャフト部品は、質量%で、C:0.35~0.70%、Si:0.01~0.40%、Mn:0.5~2.6%、S:0.005~0.020%、Al:0.010~0.050%、N:0.005~0.025%を含有し、
不純物元素として、P:0.050%以下、
O:0.003%以下
さらに、任意選択の元素として、
Pb:0.5%以下、
V、Nb及びTiからなる群から選択される1種以上を総含有量で0.1%以下,
Cr:3.0%以下、Mo:3.0%以下、及び、Ni:3.0%以下からなる群から選択される1種以上、
Cu:0~0.50%、
B:0~0.020%を含有し、
かつ、残部がFe及び不純物からなり、式(1):15.0≦25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≦27.2を満たす化学組成を有し、
外周表面に少なくとも1つの穴を有し、
外周表面から2mmの深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)が4~20%であり、
前記R1と前記外周表面から前記穴の軸方向に2mmの深さ位置で、かつ前記穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)とから式(A):Δγ=[(R1-R2)/R1]×100によって求められる残留オーステナイト減少率Δγが40%以上である。
シャフト部品は以下の化学組成を有する。なお、以下に示す各元素の割合(%)は全て質量%を意味する。
炭素(C)は、シャフト部品の強度(特に芯部の強度)を高める。Cはさらに、シャフト部品の静ねじり強度及びねじり疲労強度を高めるための残留オーステナイトを生成する。C含有量が低すぎれば、この効果が得られない。一方、C含有量が高すぎれば、シャフト部品に加工する鋼材の強度が高くなりすぎる。そのため、鋼材の被削性が低下する。さらに、高周波焼入れ時に発生するひずみが大きくなり、焼割れが発生する。従って、C含有量は0.35%以上、0.70%以下である。C含有量の好ましい下限は0.40%以上である。C含有量の好ましい上限は0.65%未満である。
シリコン(Si)は、焼入れ性を高める作用を有するが、浸炭処理の際、浸炭異常層を増加させてしまう。特に、Si含有量が0.40%を超えると、浸炭異常層が大幅に増加するために不完全焼入れ組織とよばれる軟質組織が生成して、シャフト部品のねじり疲労強度が低下する。浸炭異常層の生成を防止するには、Siの含有量を0.30%以下とすることが好ましく、0.20%以下とすることがより好ましい。しかし、鋼の量産においてSiの含有量を0.01%未満にすることは困難である。したがって、Siの含有量を0.01~0.40%とした。なお、鋼の量産における製造コストを考慮すると、実際に製造される本発明品では、Si含有量は0.05%以上含まれることが多いと思われる。
マンガン(Mn)は、シャフト部品に加工する鋼材の焼入れ性を高めるとともに、鋼材中の残留オーステナイトを増加させる。Mnを含有するオーステナイトは、Mnを含有しないオーステナイトと比較して、高周波焼入れ後の穴の切削加工時に、加工誘起マルテンサイト変態しやすい。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が高まる。Mn含有量が低すぎれば、この効果が得られない。一方、Mn含有量が高すぎれば、高周波焼入れ後の残留オーステナイトが過剰に多くなる。そのため、穴の切削加工時に十分な加工誘起マルテンサイト変態が発生せず、切削加工後も残留オーステナイトが過剰となり、ひいては切削加工時に十分な加工誘起マルテンサイト変態が発生せず、切削加工後も残留オーステナイトが減少しにくい。その結果、切削加工後のシャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、Mn含有量は0.5~2.6%である。Mn含有量の好ましい下限は0.8%であり、さらに好ましくは1.4%である。Mn含有量の好ましい上限は2.0%である。
燐(P)は不純物である。Pは、粒界に偏析して粒界強度を下げる。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、P含有量は0.050%以下である。P含有量の好ましい上限は0.030%である。P含有量はなるべく低い方がよい。P含有量の好ましい下限は0.0002%である。
硫黄(S)は、Mnと結合してMnSを形成し、鋼材の被削性を高める。S含有量が低すぎれば、この効果が得られない。一方、S含有量が高すぎれば、粗大なMnSを形成して、鋼材の熱間加工性、冷間加工性、シャフト部品のねじり疲労強度が低下する。従って、S含有量は0.005~0.020%である。S含有量の好ましい下限は0.008%である。S含有量の好ましい上限は0.015%である。
アルミニウム(Al)は鋼を脱酸する元素である。Alはさらに、Nと結合してAlNを形成し、結晶粒を微細化する。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が高まる。Al含有量が低すぎれば、この効果は得られない。一方、Al含有量が高すぎれば、硬質で粗大なAl2O3が生成して、鋼材の被削性が低下し、さらに、シャフト部品のねじり疲労強度も低下する。従って、Al含有量は0.010~0.050%である。Al含有量の好ましい下限は0.020%である。Al含有量の好ましい上限は0.040%である。
窒素(N)は窒化物を形成して結晶粒を微細化し、シャフト部品の静ねじり強度及びねじり疲労強度を高める。N含有量が低すぎれば、この効果が得られない。一方、N含有量が高すぎれば、粗大な窒化物が生成して、鋼材の靱性が低下する。従って、N含有量は0.005~0.025%である。N含有量の好ましい下限は0.010%である。N含有量の好ましい上限は0.020%である。
酸素(O)は不純物である。OはAlと結合して硬質な酸化物系介在物を形成する。酸化物系介在物は、鋼材の被削性を低下させ、シャフト部品のねじり疲労強度も低下させる。従って、O含有量は0.003%以下である。O含有量はなるべく低い方がよい。O含有量の好ましい下限は0.0001%である。
シャフト部品に加工する鋼材はさらに、Feの一部に代えて、Pbを含有してもよい。
鉛(Pb)は任意選択的元素であり、含有されていなくてもよい。含有される場合、切削加工時の工具摩耗の低下及び切り屑処理性の向上が実現される。しかしながら、Pb含有量が高すぎれば、鋼材の強度及び靱性が低下し、シャフト部品の静ねじり強度及びねじり疲労強度も低下する。従って、Pb含有量は0.5%以下とすることが好ましい。Pb含有量のさらに好ましい上限は0.4%である。なお、上記の効果を得るためにはPb含有量を0.03%以上とすることが好ましい。
バナジウム(V)、ニオブ(Nb)及びチタン(Ti)は任意選択的元素であり、含有されていなくてもよい。これらの元素は、C及びNと結合して、析出物を形成する。これらの元素の析出物は、AlNによる焼入れ部の結晶粒微細化を補完する。これらの元素の析出物は、シャフト部品の静ねじり強度及びねじり疲労強度を高める。しかしながら、これらの元素の総含有量が0.1%を超えれば、析出物が粗大化し、ねじり疲労強度が低下する。従って、V、Nb及びTiの総含有量は0.1%以下であることが好ましい。任意選択的元素として、V、Nb及びTiのいずれか1種以上が含有されれば、上記効果が得られる。V、Nb及びTiの総含有量のさらに好ましい上限は0.08%である。V、Nb及びTiによる上記の効果を得るためには、0.01%以上の含有が好ましい。
クロム(Cr)は任意選択的元素であり、含有されなくてもよい。Crは鋼材の焼入れ性を高め、さらに、残留オーステナイトを増加させる。しかしながら、Cr含有量が高すぎれば、高周波焼入れ後の残留オーステナイトが過剰に高くなる。この場合、切削加工時に十分な加工誘起マルテンサイト変態が生じず、切削加工後も残留オーステナイトが減少しにくい。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、Cr含有量は3.0%以下であることが好ましい。Crによる上記の効果を得るためには、0.1%以上の含有が好ましい。Cr含有量の好ましい上限は、2.0%である。
モリブデン(Mo)は任意選択的元素であり、含有されていなくてもよい。含有される場合、Moは鋼材の焼入れ性を高め、残留オーステナイトを増加させる。Moはさらに、焼戻し軟化抵抗を高め、シャフト部品の静ねじり強度及びねじり疲労強度を高める。しかしながら、Mo含有量が高すぎれば、高周波焼入れ後の残留オーステナイトが過剰となる。この場合、切削加工時に十分な加工誘起マルテンサイト変態が発生しない。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、Mo含有量は3.0%以下とすることが好ましい。Mo含有量のさらに好ましい上限は2.0%である。Moによる上記の効果を得るためには、0.1%以上の含有が好ましい。
ニッケル(Ni)は任意選択的元素であり、含有されていなくてもよい。含有される場合、Niは鋼材の焼入れ性を高め、残留オーステナイトを増加させる。Niはさらに、鋼材の靱性を高める。しかしながら、Ni含有量が高すぎれば、高周波焼入れ後の残留オーステナイトが過剰となる。この場合、焼入れ後の切削加工時に十分な加工誘起マルテンサイト変態が発生しない。その結果、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、Ni含有量は3.0%以下であることが好ましい。Ni含有量のさらに好ましい上限は2.0%である。Niによる上記の効果を得るためには、0.1%以上の含有が好ましい。
Cuは、マルテンサイトに固溶して鋼材の強度を高める。そのため、鋼材の疲労強度が高まる。しかしながら、Cu含有量が高すぎれば、熱間鍛造時に鋼の粒界に偏析して熱間割れを誘起する。したがって、Cu含有量は0.50%以下である。なお、Cu含有量は0.40%以下であることが好ましく、0.25%以下であることが一層好ましい。Cuによる上記の効果を得るためには、0.10%以上の含有が好ましい。
Bは、Pの粒界偏析を抑制して靭性を高める効果がある。しかしながら、0.020%以上添加すると、浸炭時に異常粒成長が生じ、ねじり疲労強度が低下する。したがって、B含有量は0.020%以下である。なお、B含有量は、0.015%あることが好ましく、0.010%以下であることが一層好ましい。Bによる上記の効果を得るためには、0.0005%以上の含有が好ましい。
希土類元素(REM):0.0005%以下、
カルシウム(Ca):0.0005%以下、
マグネシウム(Mg):0.0005%以下、
タングステン(W):0.001%以下、
アンチモン(Sb):0.001%以下、
ビスマス(Bi):0.001%以下、
コバルト(Co):0.001%以下、
タンタル(Ta):0.001%以下、
シャフト部品に加工する鋼材を構成する各元素の含有量の関係は、以下に示す式(1)を満たす。
15.0≦25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≦27.2 (1)
ここで、式(1)中の元素記号には、対応する元素の鋼材中の含有量(質量%)が代入される。
式(1)で、F1=25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Niと定義する。F1値は、オーステナイトの安定性を表すパラメータである。式(1)は、種々の化学成分の焼入れ鋼の残留γ体積率の測定値から重回帰分析によって求めた経験式である。F1値が低すぎれば、オーステナイトが熱力学的に不安定になり、高周波焼入れ後に、残留オーステナイトが十分に生成されず、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。一方、F1値が高すぎれば、オーステナイトの安定性が高まり、高周波焼入れ後の残留オーステナイトが過剰に多くなる。この場合、切削加工時に加工誘起マルテンサイト変態が生じにくい。そのため、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。従って、F1は15.0~27.2である。F1の好ましい下限は16.5であり、好ましい上限は27.0以下である。
本発明の実施形態に係るシャフト部品は、シャフト部品の長手方向に対して垂直又は一定の角度を有して、シャフト外周表面から開けられた貫通穴又は非貫通穴を有する。穴の直径は、0.2mm~10mmである。シャフト部品はこれらの穴を1個又は複数個有している。
シャフト部品への高周波焼き入れにより、シャフト部品の表層(外周表面から2mm深さ位置を含む)に、残留オーステナイトが発生する。この残留オーステナイトは、シャフト部品の焼入れ後の穴開け加工時に、穴付近において、加工誘起マルテンサイト変態する。具体的には、穴開け加工時に、切削工具と母材との間の摩擦力により、穴の表層付近にある残留オーステナイトの一部が、加工誘起マルテンサイトに変態する。一方、この作用による加工誘起マルテンサイト変態の発生は穴付近に限定される。穴の表面から2mm程度離れると、もはや穴開け加工に伴う加工誘起マルテンサイト変態は起こらない。そのため、外周表面から2mm深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)は、焼入れ後の穴開け加工の影響を受けていない部分であり、切削前の残留オーステナイト体積率と考えることができる。
穴開け加工にともなう加工誘起マルテンサイト変態の結果、シャフト部品の強度が上昇し、静ねじり強度及びねじり疲労強度が上昇する。このような効果を得るためには、焼入れ後の最大残留オーステナイト体積率(R1)が4%以上でなければならない。
シャフト部品の外周表面から穴の軸方向に2mmの深さ位置で、かつ穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)は、穴開け加工によって創成された表面付近の残留オーステナイト体積率であり、切削後の残留オーステナイト体積率と考えることができる。切削加工後の残留オーステナイトの体積率が高すぎれば、硬質なマルテンサイトが得られておらず、静ねじり強度及びねじり疲労強度が低下する。
R1とR2とから、上記式(A)によって求められる残留オーステナイト減少率(Δγ)が40%以上である。
塑性流動層は、穴を切削する際に、切削工具と母材との間に生じる摩擦による変形で穴の表面に形成される層である。穴の表面の塑性流動層の厚さは、次の方法で測定される。シャフト部品の外周から穴の軸方向に2mmの深さ位置で、かつ穴の軸方向に対して垂直な断面における穴表層部を含み、穴の軸方向対して垂直な面(横断面)が観察面になるような試験片を採取する(図3(b)の符号31参照)。鏡面研磨した試験片を、5%ナイタール溶液で腐食する。腐食された面の穴の表面を含む位置(31)を、倍率5000倍の走査型電子顕微鏡(SEM)にて観察する。得られたSEM像の一例を図4に示す。同図において、塑性流動層41は、母材中心部42に対して塑性流動組織がシャフト部品の穴の表面の円周方向(図4において紙面の左方向から右方向)に湾曲している部分である。
質量%で、C:0.35~0.70%、Si:0.01~0.40%、Mn:0.5~2.6%、S:0.005~0.020%、Al:0.010~0.050%、N:0.005~0.025%を含有し、
不純物元素として、P:0.050%以下、
O:0.003%以下
さらに、任意選択の元素として、
Pb:0.5%以下、
V、Nb及びTiからなる群から選択される1種以上を総含有量で0.1%以下、
Cr:3.0%以下、Mo:3.0%以下、及び、Ni:3.0%以下からなる群から選択される1種以上、
Cu:0~0.50%、
B:0~0.020%を含有し、
かつ、残部がFe及び不純物からなり、式(2)を満たす化学組成を有する鋼材をシャフト部品の形状に加工してシャフト部品粗部材を得る工程と、
前記粗部材に対して高周波焼入れ処理を施して焼入れ材を得る工程であって、高周波加熱時の周波数を10KHz以上、300KHz以下とし、高周波加熱時の加熱時間を1秒以上40秒以下として、その後焼入れすることで、焼入れ材の外周表面から2mmの深さ位置の組織が、マルテンサイトと、体積率で4~20%の残留オーステナイトとを含む組織となる焼入れ材を得る工程と、
前記焼入れ材に対して切削による穴開け加工を施して、シャフト部品を得る工程であって、穴開け加工時の工具送りを0.02mm/rev超、0.2mm/rev以下とし、切削速度を10m/分以上、50m/分以下とし、
シャフト部品の外周表面から2mmの深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)と、シャフト部品の外周表面から穴の軸方向に2mmの深さ位置で、かつ穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)とから、式(A):Δγ=[(R1-R2)/R1]×100によって求められる残留オーステナイト減少率Δγが40%以上である、シャフト部品が製造される。
式(1):15.0≦25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≦27.2
ここで、式(1)中の各元素記号には、各元素の含有量(質量%)が代入される。
本実施形態のシャフト部品の製造方法は、鋼材をシャフト部品の形状に近い形状に加工してシャフト部品粗部材を得る工程(粗部材製造工程)と、粗部材に対して高周波焼入れ処理を施して焼入れ材を得る工程(焼入れ材製造工程)と、焼入れ材に対して切削による穴開け加工を施して穴を開け、シャフト部品を得る工程(穴開け工程)とを含む。
本工程では、シャフト部品の形状に近い所望の形状を有する粗部材を製造する。初めに、鋼材を準備する。
鋼材は上述した本発明の実施形態に係るシャフト部品と同じ含有量の同じ化学組成を有する。
上記化学組成を有する鋼材をシャフト部品の形状に近い形状に加工してシャフト部品粗部材を得る。加工方法は周知の方法を採用することができる。加工方法としては、例えば、熱間加工、冷間加工、切削加工等が挙げられる。粗部材は、穴以外の部分は本発明の実施形態に係るシャフト部品と同様の形状とし、この段階では、穴は開けない。
上記のようにして得られた粗部材に対して、高周波焼入れ処理を施して焼入れ材を得る。これにより、焼入れ材において、最終形態であるシャフト部品の外周表面から2mmの深さ位置の組織を、マルテンサイトと体積率で4~20%の残留オーステナイトとする。
高周波焼入れ処理は、初めに、(i)高周波加熱を施し、その後、(ii)焼入れを施す。高周波加熱及び焼入れは次の条件で行う。
高周波加熱時の周波数:10~300KHz
周波数が低すぎれば、加熱範囲が広がる。そのため、焼入れ時の歪みが大きくなる。一方、周波数が高すぎれば、加熱範囲が表層のみに集中する。この場合、表面の硬化層が薄くなり、静ねじり強度及びねじり疲労強度が低下する。従って、高周波加熱時の周波数は10~300KHzである。
加熱時間とは、出力40KWで、粗部材の加熱が開始されてから水冷が開始されるまでの時間である。高周波加熱時の加熱時間が長すぎれば、オーステナイト粒が粗大化し、シャフト部品の静ねじり強度及びねじり疲労強度が低下する。一方、加熱時間が短すぎれば、セメンタイトが十分に固溶せず、オーステナイトの安定性が低下する。そのため、高周波焼入れ後に、マルテンサイトと、体積率で4~20%の残留オーステナイトとからなる組織が得られない。従って、高周波加熱時の粗部材の加熱時間は1.0~40秒である。
加熱周波数と加熱時間の両者をコントロールして、外周表面から2mm以上深い領域までがA3点以上の温度に加熱されるようにする。
恒温保持処理後、周知の方法で焼入れを施す。焼入れは、例えば、水焼入れとすることができる。これにより、A3点以上に加熱された領域が、マルテンサイトと残留オーステナイトとを含有する組織に変化する。
シャフト部品の靭性を高めたい場合、高周波焼入れ処理を施した後、焼戻し処理を施してもよい。
上述の条件で高周波焼入れ処理を施して得られた焼入れ材について、焼入れ材の外周表面(最終形態のシャフト部品の外周表面と同じ)から2mmの深さ位置の組織は、マルテンサイトと、体積率で4~20%の残留オーステナイトとを含有する。
なお、照射するX線のスポットサイズは1mm以下とした。
本発明の実施形態に係るシャフト部品は、シャフト部品の長手方向に対して垂直又は一定の角度を有して開けられた、貫通穴又は非貫通穴を、1個又は複数有している。
高周波焼入れ処理を施した後、切削による穴開け加工を施す。切削加工により、穴を開けつつ、穴の表層部で加工誘起マルテンサイト変態を発生させる。これにより、穴の形成によるシャフト部品の静ねじり強度及びねじり疲労強度の低下が抑制され、優れた静ねじり強度及びねじり疲労強度のシャフト部品が作成される。切削加工は、次の条件で行う。なお、切削工具としては、例えば超硬合金の表面に、炭化物,窒化物,酸化物,ダイヤモンドなどのコーティングが施された超硬ドリル(JIS B 0171:2014年、1003、1004番に規定するコーテッド超硬ドリル)を用いることができる。コーテッド超硬ドリルを使用することは、工具摩耗の抑制及び加工能率向上の点で有効である。
送りfが小さすぎれば、切削抵抗、つまり、工具が被削材に押し付けられる力が小さすぎる。この場合、十分な加工誘起マルテンサイト変態が発生しない。そのため、シャフト部品の静ねじり強度及びねじり疲労強度が向上しない。一方、送りが大きすぎれば、切削抵抗が大きくなり過ぎる。この場合、切削時に工具が破損する恐れがある。従って、送りfは0.02mm/rev超、0.2mm/rev以下である。送りfの好ましい下限は0.03mm/revである。送りfの好ましい上限は0.15mm/revであり、より好ましくは0.1mm/revである。
切削速度vが大きすぎれば、切削温度が上昇し、マルテンサイト変態が生じ難くなる。そのため、シャフト部品の静ねじり強度及びねじり疲労強度が向上しない。一方、切削速度が小さすぎれば、切削能率が低下し、製造効率が低下する。従って、切削速度vは10~50m/分である。好ましい上限は40m/分であり、より好ましくは30m/分である。
以上に示す穴開け加工によりシャフト部品が得られる。得られたシャフト部品の外周表面から2mmの深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)は、4~20%であり、
上記R1と外周表面から穴の軸方向に2mmの深さ位置で、かつ穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)とから式(A):Δγ=[(R1-R2)/R1]×100によって求められる残留オーステナイト減少率Δγは、40%以上である。
減少率Δγ=[(R1-R2)/R1]×100 (A)
真空溶解炉を用いて、表1に示す化学組成を有する150kgの溶鋼A~Pを得た。
ねじり試験片51の試験部52の穴の表面から2mmの位置を試験片51の長手方向に対して垂直に切断した。切断面において、外周から中心に向かって2mmの位置を含む試験片(試験片1)を用意する(図1(b))。切断面に対して電解研磨を行った。11.6%の塩化アンモニウムと、35.1%のグリセリンと、53.3%の水とを含有する電解液を用意する。この電解液を用いて、基準位置を含む表面に対して、電圧20Vで電解研磨を行った。
ねじり試験片51の長手方向に対して垂直、かつ穴の中心を通り、穴を縦に2分割するようにシャフト部品を切断した(図2のB-B’線)。穴の表面において、外周表面から2mmの深さ位置を中心として、φ1mmの穴が開いたマスキングを施し、電解研磨を施した。電解研磨の時間を変化させることで研磨量を調整し、20μmの深さの穴を開けた。
図5に示すねじり試験片51を用いて、サーボパルサー式ねじり試験機(株式会社島津製作所製のEHF-TB2KNM)でねじり試験を行い、応力とねじり角の関係を取得した。次いで、応力とねじり角が比例関係を保つ最大のせん断応力τ、いわゆる比例限度を静ねじり強度とした。この比例限度は、引張試験でいう降伏応力に相当する。本試験においては、静ねじり強度が530MPa以上の場合が、従来技術に対して優れた静ねじり強度を有するという点で合格である。
図5に示すねじり試験片51を用いて、負荷最大せん断応力τを50MPaピッチで変化させて、繰り返し周波数4Hzで両振りのねじり疲労試験を行った。そして、繰り返し数105回に達する前に破断した最大せん断応力の最小値(τf,min)と、(τf,min)より低い応力で最大の未破断点の最大せん断応力(σr,max)との中間点を疲労限度とした。なお、試験機には前記サーボパルサー式ねじり試験機を用いた。本試験においては、ねじり疲労強度が325MPa以上の場合が、従来技術に対して優れたねじり疲労強度を有しているので合格である。
以上に説明した各試験等に関する結果を表4、表5に示す。
21 R2測定位置
31 走査型電子顕微鏡観察位置
41 塑性流動層
42 母材
51 ねじり試験片
52 試験部
53 穴
54 太径部
55 つかみ部
56 中空穴
61 加工誘起マルテンサイト層
Claims (3)
- 質量%で、C:0.35~0.70%、Si:0.01~0.40%、Mn:0.5~2.6%、S:0.005~0.020%、Al:0.010~0.050%、N:0.005~0.025%を含有し、
不純物元素として、
P:0.050%以下、
O:0.003%以下
さらに、任意選択の元素として、
Pb:0.5%以下、
V、Nb及びTiからなる群から選択される1種以上を総含有量で0.1%以下,
Cr:3.0%以下、Mo:3.0%以下、及び、Ni:3.0%以下からなる群から選択される1種以上、
Cu:0~0.50%、
B:0~0.020%を含有し、
かつ、残部がFe及び不純物からなり、式(1)を満たす化学組成を有し、
外周表面に少なくとも1つの穴を有し、
前記外周表面から2mmの深さ位置、かつ穴の表面から2mmの残留オーステナイト体積率(R1)が4~20%であり、
前記R1と前記外周表面から前記穴の軸方向に2mmの深さ位置で、かつ前記穴の表面から20μmの深さ位置の残留オーステナイト体積率(R2)とから式(A):Δγ=[(R1-R2)/R1]×100によって求められる残留オーステナイト減少率Δγが40%以上である、
ことを特徴とする、シャフト部品。
15.0≦25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≦27.2 (1)
ここで、式(1)中の各元素記号には、各元素の含有量(質量%)が代入される。 - 前記穴の表面に塑性流動層を有することを特徴とする請求項1に記載のシャフト部品。
- 前記塑性流動層の厚さが0.5~15μmである、ことを特徴とする請求項2に記載のシャフト部品。
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