WO2018056333A1 - 浸炭シャフト部品 - Google Patents
浸炭シャフト部品 Download PDFInfo
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- WO2018056333A1 WO2018056333A1 PCT/JP2017/033987 JP2017033987W WO2018056333A1 WO 2018056333 A1 WO2018056333 A1 WO 2018056333A1 JP 2017033987 W JP2017033987 W JP 2017033987W WO 2018056333 A1 WO2018056333 A1 WO 2018056333A1
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- hole
- carburized
- shaft component
- carburized shaft
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- 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
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- 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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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- 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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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/20—Carburising
- C23C8/22—Carburising of ferrous surfaces
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- 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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- F16C33/00—Parts of bearings; Special methods for making bearings or parts thereof
- F16C33/02—Parts of sliding-contact bearings
- F16C33/04—Brasses; Bushes; Linings
- F16C33/06—Sliding surface mainly made of metal
- F16C33/14—Special methods of manufacture; Running-in
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- 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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- 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
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D1/76—Adjusting the composition of the atmosphere
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C21D2261/00—Machining or cutting being involved
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Definitions
- the present invention relates to a carburized shaft component.
- shaft parts for example, transmission shafts
- induction 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 such as an oil hole is made by drilling or the like, and an intermediate member closer to the final product is manufactured. Finally, the intermediate member is quenched (induction quenching or carburizing quenching) to obtain a shaft component.
- Patent Document 1 discloses a method for manufacturing a shaft component with high torsional fatigue strength, in which steel material components and carburizing time are optimized.
- Patent Document 2 discloses a shaft excellent in fatigue resistance and a method for improving the fatigue characteristics, characterized in that the compressive residual stress in the surface layer of the oil hole is 50% to 90% of the tensile strength of the steel material. ing.
- shaft parts are required to have excellent static torsional strength in addition to further improvement in torsional fatigue strength.
- the shaft component obtained by the technique disclosed in Patent Document 1 has a high level of static torsional strength and torsional fatigue strength due to insufficient studies on hole processing and strength improvement, and further on the structure of the hole surface layer. It may be difficult to achieve both.
- the present invention has been made in view of the above circumstances, and an object thereof is to provide a carburized shaft component that is excellent in static torsional strength and torsional fatigue strength.
- the present inventors diligently studied carburized shaft parts that can achieve both excellent static torsional strength and torsional fatigue strength. As a result, the present inventors have found that by cutting the hole after carburizing and quenching, the retained austenite in the surface layer of the hole can be transformed into hard work-induced martensite during the cutting, and the hardness near the hole can be increased. I found it. Furthermore, the present inventors can improve the static torsional strength and torsional fatigue strength of the carburized shaft component because the occurrence of cracks from the site near the hole and the development thereof are suppressed by increasing the hardness near the hole. It has also been found that the static torsional strength and torsional fatigue strength of the carburized shaft component can be further improved by transforming more retained austenite during processing-induced martensite transformation during cutting.
- the present inventors have focused on the chemical composition of steel (carburized shaft parts) and heat treatment conditions and attempted to further improve the static torsional strength and torsional fatigue strength. As a result, it was found that by adopting specific steel material components and heat treatment conditions, work-induced martensitic transformation is likely to occur during cutting, and the static torsional strength and torsional fatigue strength of the carburized shaft parts are remarkably improved.
- the maximum retained austenite volume fraction (R1) in the range from the outer peripheral surface to the depth of 1 mm in the axial direction of the hole and from the surface of the hole to 200 ⁇ m is 10.0 to 30.0%, Residue obtained by the equation (A) from R1 and the residual austenite volume ratio (R2) at a depth position of 1 mm from the outer peripheral surface in the axial direction of the hole and at a depth position of 20 ⁇ m from the surface of the hole.
- a carburized shaft component having excellent static torsional strength and torsional fatigue strength can be obtained.
- FIG. 1A is a schematic diagram of a quenching material and a carburized shaft component
- FIG. 1B is a depth position of 1 mm in the axial direction of the hole from the outer periphery of the quenching material and the carburized shaft component, and It is a figure which shows the cross section AA 'perpendicular
- FIG. 2 is a diagram showing a reference position in the measurement of the retained austenite volume ratio of the carburized shaft component.
- FIG. 3 is a scanning electron microscope image of the hole surface layer at a cross-section A-A ′ at a depth position of 1 mm in the axial direction of the hole from the outer periphery of the carburized shaft component and perpendicular to the hole.
- FIG. 4 is a side view of a test piece used in the torsion test.
- FIG. 5 is a top view of the periphery of the hole in the carburized shaft component according to the present invention.
- the carburized shaft component according to the embodiment of the present invention has a depth of 3 mm from the outer peripheral surface or an inner portion deeper than that by mass%.
- C 0.10 to 0.30%, Si: 0.01 to 0.30%, Mn: 0.4 to 2.0%, P: 0.050% or less, S: 0.005 to 0.020%, Cr: 0.4 to 3.5%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, and O: 0.003% or less, with the balance being Fe and impurities,
- 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
- the C content (Cs) of the surface layer part is 0.60 to 1.00% by mass
- the maximum retained austenite volume fraction (R1) in the range from the outer peripheral surface to the depth of 1 mm in the axial direction of the hole and from the surface of the hole to 200 ⁇ m is 10.0 to 30.0%, Residue obtained by the equation (A) from R1 and the residual austenite volume ratio (R2) at a depth position of 1 mm from the outer peripheral surface in the axial direction of the hole and at a depth position of 20 ⁇ m from the surface of the hole.
- the austenite reduction rate ( ⁇ ) is 20% or more.
- the carburized shaft component according to the embodiment of the present invention includes any shaft component having at least one hole such as an oil hole on the outer peripheral surface and carburized, and is not particularly limited. And shaft parts used in industrial machines, such as transmission shafts.
- the carburized shaft component according to the embodiment of the present invention includes a shaft component having an arbitrary shape, and is not particularly limited.
- the diameter is about 150 mm or less, about 100 mm or less, or about 30 mm or less, and the length is It may be a hollow or solid cylindrical or rod-shaped shaft component that is 5 mm or more.
- the carburized shaft component has the following chemical composition.
- the ratio (%) of each element shown below means mass%.
- carbon is introduced into the surface layer portion by carburizing treatment, and strictly speaking, the chemical composition differs between the surface layer portion and the inside of the carburized shaft component. Therefore, the chemical composition shown below (including essential components, impurities, and optional components) is a region that is not affected by the carburizing process, that is, the carburized shaft component so as to match the chemical composition of the steel material before the carburizing process. This refers to the chemical composition at a depth of 3 mm or deeper from the outer peripheral surface.
- C 0.10 to 0.30% Carbon (C) increases the strength (particularly the strength of the core) of the carburized shaft component. C further generates retained austenite to increase static torsional strength and torsional fatigue strength. 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 carburized shaft component becomes too high. Therefore, the machinability of the steel material is reduced. Therefore, the C content is 0.10 to 0.30%. The minimum with preferable C content is 0.15% or more. The upper limit with preferable C content is less than 0.25%.
- Si 0.01 to 0.30%
- Silicon (Si) has an effect of improving hardenability, but increases the carburizing abnormal layer during the carburizing process. In particular, when the content exceeds 0.30%, the carburized abnormal layer greatly increases, so that a soft structure called an incompletely hardened structure is generated, and the torsional fatigue strength of the carburized shaft component is lowered.
- the Si content is preferably 0.25% or less, and more preferably 0.20% or less. However, it is difficult to make the Si content less than 0.01% in mass production. Therefore, the Si content is set to 0.01 to 0.30%. In view of the manufacturing cost in mass production, it is considered that the Si content in the actually manufactured product of the present invention is often 0.05% or more.
- Mn 0.4 to 2.0%
- Manganese (Mn) increases the hardenability of the steel and increases retained austenite in the steel. Compared with austenite not containing Mn, austenite containing Mn is likely to undergo work-induced martensitic transformation during cutting after carburizing and quenching. As a result, the static torsional strength and torsional fatigue strength of the carburized 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 carburizing and tempering becomes excessive. For this reason, sufficient work-induced martensite transformation does not occur during cutting, and residual austenite becomes excessive after cutting. Difficult to decrease.
- the Mn content is 0.4 to 2.0%.
- the minimum with preferable Mn content is 0.8%.
- the upper limit with preferable Mn content is 1.8%.
- 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 carburized 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 machinability. 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 and cold workability of steel and the torsional fatigue strength of the carburized 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%.
- Chromium (Cr) increases the hardenability of the steel and further increases the retained austenite. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the retained austenite after carburizing and tempering becomes excessive. In this case, sufficient machining-induced martensite transformation does not occur during the cutting process in the hole cutting process, and residual austenite is less likely to decrease after the cutting process. As a result, the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. Therefore, the Cr content is 0.4 to 3.5%. The minimum with preferable Cr content is 0.5%. The upper limit with preferable Cr content is 3.1%.
- Al 0.010 to 0.050%
- Aluminum (Al) 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 carburized 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 is lowered, and the torsional fatigue strength 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 carburized shaft components. 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 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. Oxide inclusions reduce the machinability of steel and reduce the torsional fatigue strength of carburized shaft components. 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 carburized shaft component is iron (Fe) and impurities.
- Impurities mean components that are mixed in from the ore and scrap used as a raw material for steel, the environment of the manufacturing process, and the like, but not intentionally contained in the carburized shaft parts. Even if impurities are mixed in the carburized shaft component, the object of the present invention can be achieved as long as the amount is small and the properties of the steel material are not impaired.
- the carburized shaft component according to the present invention can achieve the object of the present invention even if each of the following elements is included within a specified range.
- REM Rare earth element
- Ca Calcium
- Mg 0.0005% or less
- W 0.001% or less
- Sb Antimony
- Bismuth 0.001% or less
- Co Cobalt
- Tantalum Ta: 0.001% or less
- the carburized 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 contained, reduction in tool wear and improvement in chip disposal are realized. However, if the Pb content is too high, the strength and toughness of the steel are reduced, and the static torsional strength and torsional fatigue strength of the carburized shaft component are also reduced. Therefore, the Pb content is preferably 0.5% or less. A more preferable upper limit of the Pb content is 0.4%. In addition, in order to acquire said effect, it is preferable to make Pb content 0.03% or more.
- the carburized shaft component may further contain one or more selected from the group consisting of V, Nb, and Ti instead of 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. These elemental precipitates increase the static torsional strength and torsional fatigue strength of the carburized 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 carburized shaft component may further contain one or more selected from the group consisting of Mo and Ni instead of part of Fe. All of these elements increase the hardenability of the steel and increase the retained austenite.
- Mo Molybdenum
- Mo is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel and increases the retained austenite. Mo further increases the resistance to temper softening and increases the static torsional strength and torsional fatigue strength of the carburized shaft component. However, if the Mo content is too high, the retained austenite after carburizing and quenching 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 carburized 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 and increases the retained austenite. Ni further increases the toughness of the steel. However, if the Ni content is too high, residual austenite after carburizing and quenching becomes excessive. In this case, sufficient work-induced martensitic transformation does not occur during cutting after tempering. As a result, the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. Therefore, the Ni content is preferably 2.5% 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. However, if added over 0.020%, abnormal grain growth occurs during carburizing, and the torsional fatigue strength decreases. Therefore, 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.
- F1 1.54 * C + 0.81 * Si + 1.59 * Mn + 1.65 * Cr + 1.77 * Mo + 0.63 * Ni.
- F1 is a parameter representing the hardenability of steel. If F1 is too low, the hardenability of the steel will be low. In this case, low strength ferrite and pearlite are generated, and the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. Therefore, F1 is 2.35 or more. A more preferable lower limit of F1 is 3.0. In order to ensure the toughness of the carburized shaft component, the preferable upper limit of F1 is 8.0.
- F2 -0.1xSi + 15.2xMn + 7.0xCr + 6.7xMo + 6.2xNi.
- F2 is a parameter representing the stability of austenite. If F2 is too low, the ratio of retained austenite obtained after carburizing and quenching will be low. As a result, the hardening effect of the hole periphery due to the processing-induced martensite transformation cannot be obtained, and the static torsional strength and torsional fatigue strength of the carburized shaft component are lowered. On the other hand, if F2 is too high, the amount of retained austenite after carburizing and quenching and tempering becomes excessive, and the static torsional strength and torsional fatigue strength decrease.
- F2 is required to be 11.3 to 33.8.
- a preferred lower limit of F2 is 12.0.
- the preferable upper limit of F2 is 33.0.
- the carburized shaft component according to the embodiment of the present invention has one or a plurality of carburized shaft components that have a vertical or predetermined angle with respect to the longitudinal (axial) direction of the carburized shaft components and are opened from the outer peripheral surface of the carburized shaft components. It has a through hole or a non-through hole.
- the diameter of the hole is not particularly limited, but may be, for example, 0.2 mm to 10 mm.
- C content of surface layer portion (Cs): 0.60 to 1.00% C contained in the surface layer portion of the carburized shaft part increases the static torsional strength and torsional fatigue strength of the carburized shaft part.
- the C content of the carburized shaft component surface layer is measured by the following method.
- a part with a depth of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft part and a hole surface layer of 50 ⁇ m is cut by cutting, and the C content in the chip is quantitatively measured by emission spectroscopic analysis, and the value is obtained as a surface layer portion.
- the C concentration of the carburized shaft part surface layer portion can be quantitatively analyzed using EPMA (electron beam microanalyzer).
- the C content (Cs) contained in the surface layer portion is low, the hardness of the carburized layer will be low. As a result, the static torsional strength of the carburized shaft component decreases.
- (Cs) is high, hard pro-eutectoid cementite is generated in the surface layer portion of the carburized shaft component.
- cementite serves as a starting point of fracture, and static torsional strength and torsional fatigue strength are reduced.
- tool wear during cutting increases, and machinability decreases. Accordingly, the C content (Cs) in the surface layer portion is 0.60 to 1.00%.
- the preferable lower limit of Cs is 0.65%.
- a preferable upper limit of Cs is 0.90%.
- Total volume ratio of martensite and retained austenite ( ⁇ ′ + ⁇ ) in the structure at a depth of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft part and at a depth of 20 ⁇ m from the hole surface If a low-strength phase such as ferrite or pearlite exists as a structure at a depth of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft part and 20 ⁇ m from the surface of the hole, these phases As a starting point, cracks are likely to occur, and the static torsional strength and torsional fatigue strength of the carburized shaft parts are reduced.
- the total volume ratio ( ⁇ ′ + ⁇ ) of martensite and retained austenite in the structure at the above position is limited to 97% or more.
- the preferable range of the said total volume ratio is 99% or more.
- the total volume ratio ( ⁇ ′ + ⁇ ) of martensite and retained austenite corresponds to a depth position of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft component and a depth position of 20 ⁇ m from the hole surface.
- the reference position 21 (see FIG. 2) to be measured is measured by the following method by observing the structure. That is, the surface (cross section) perpendicular to the axial direction of the hole is the observation surface including the hole surface layer portion in a cross section perpendicular to the axial center of the hole at a depth of 1 mm from the outer periphery of the carburized shaft component. A test piece is collected (FIG. 1A-A ′).
- the mirror polished specimen is corroded with 5% nital solution.
- the corroded surface is observed with three optical fields using an optical microscope with a magnification of 1000 times.
- the reference position 21 is set to the center of the visual field (FIG. 1-11). 10 ⁇ m from the center of the field of view to the surface of the hardened material, 10 ⁇ m from the center of the field of view to the direction opposite to the surface of the hardened material, 50 ⁇ m from the center of the field to both directions perpendicular to the surface of the hardened material, 20 ⁇ m ⁇ 100 ⁇ m
- the area ratio of each phase is obtained by a normal image analysis method.
- the average value of the area ratio of each phase obtained for each of the three visual fields is defined as the volume ratio of each phase.
- the maximum retained austenite volume ratio (R1) in the range from the outer peripheral surface of the carburized shaft part to the depth of 1 mm in the axial direction of the hole and the depth of 200 ⁇ m from the surface of the hole is 10.
- the maximum retained austenite volume fraction (R1) is measured by the following method.
- the carburized shaft part is cut so as to divide the hole in two axial directions and through its center (FIG. 2B-B ′).
- masking is performed in which a hole having a diameter of 1 mm is formed around a position 1 mm deep from the outer peripheral surface, and electrolytic polishing is performed.
- the amount of polishing is adjusted by changing the time of electrolytic polishing, and a hole with a depth of 30 ⁇ m is dug.
- the electrolytic polishing is performed at a voltage of 20 V using an electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water.
- X-rays are irradiated around the reference position and analyzed by the X-ray diffraction method.
- the product name RINT-2500HL / PC manufactured by Rigaku Corporation is used for X-ray diffraction.
- 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.
- Data analysis uses AutoMATE software (manufactured by Rigaku Corporation).
- the K ⁇ 2 component is removed by the Rachinger method, and the residual austenite volume fraction (R1) is 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. calculate. Note that the spot size of X-rays to be irradiated is 0.5 mm or less.
- the residual austenite volume ratio (R2) at a depth position of 1 mm from the outer peripheral surface of the carburized shaft component in the axial direction of the hole and at a depth position of 20 ⁇ m from the hole surface is preferably 20% or less. If the volume ratio of the retained austenite after cutting is too high, hard martensite cannot be obtained, and static torsional strength and torsional fatigue strength are reduced.
- the retained austenite volume fraction (R2) is measured by the following method.
- the carburized shaft part is cut so as to divide the hole in two axial directions and through its center (FIG. 2B-B ′).
- masking is performed in which a hole having a diameter of 1 mm is formed around a position 1 mm deep from the outer peripheral surface, and electrolytic polishing 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 is irradiated with X-rays having a spot size of ⁇ 0.5 mm, and the residual austenite volume ratio (R2) is measured in the same manner as the residual austenite volume ratio (R1).
- 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 are improved. In order to obtain such an effect, ⁇ must be 20% or more. A preferable value of ⁇ is 25% or more.
- the carburized shaft component according to the embodiment of the present invention may have a plastic fluidized bed on the surface of the hole.
- This plastic fluidized bed is a layer formed by a large deformation occurring in the surface layer portion of the hole when the hole is cut.
- This plastic fluidized bed is hard, and when the thickness is 0.5 ⁇ m or more, the static torsional strength and torsional fatigue strength of the carburized shaft component can be improved.
- the plastic fluidized bed is fragile, it can be deformed to some extent when its thickness is thin. However, if the thickness exceeds 15 ⁇ m, cracking occurs and becomes the starting point of cracking, so the torsional fatigue strength is reversed. May fall.
- the thickness of the plastic fluidized layer on the surface layer of the carburized shaft component is preferably 0.5 to 15 ⁇ m.
- the thickness of the plastic fluidized layer on the surface layer of the carburized shaft component is preferably 1 ⁇ m or more, and more preferably 3 ⁇ m or more. .
- a preferable upper limit is 13 micrometers, More preferably, it is 10 micrometers.
- the thickness of the plastic fluidized bed on the hole surface is measured by the following method. It includes a hole surface layer portion in a cross section perpendicular to the hole at a depth of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft component, and a surface (cross section) perpendicular to the axial direction of the hole becomes an observation surface.
- a test piece is collected (AA ′ in FIG. 1).
- the mirror polished specimen is corroded with 5% nital solution.
- the corroded surface is observed with a scanning electron microscope (SEM) at a magnification of 5000 times. An example of the obtained SEM image is shown in FIG.
- the plastic fluidized bed 31 is a portion where the structure is curved along the surface of the hole with respect to the base material 32 (from the left direction to the right direction in FIG. 3), and is curved from the surface of the hole.
- the distance to the edge of the texture was defined as the thickness of the plastic fluidized bed 31.
- the carburized shaft component according to the embodiment of the present invention includes a layer hardened over a certain depth from the hole surface, including the plastic fluidized layer.
- a hardened layer includes a layer (working-induced martensite layer) formed by processing-induced martensite transformation of the retained austenite in the hole surface layer at the time of hole cutting, and has a thickness of about 200 to 300 ⁇ m, for example.
- the carburized shaft component according to the present invention has a plastic fluidized layer 31 and a work-induced martensite layer 51, particularly a hard material, around a hole 43 that may cause a reduction in static torsional strength and torsional fatigue strength.
- the carburized shaft component according to the embodiment of the present invention can be manufactured by cutting a hole after carburizing and quenching.
- the carburized shaft component can be manufactured by the method shown in the following modes 1 and 2.
- the carburizing shaft component manufacturing method includes a step of processing a steel material to obtain a rough member (rough member manufacturing step), a step of carburizing and quenching the rough member to obtain a carburized material (carburizing material manufacturing step), And a step of drilling a hole in the quenched material to obtain a carburized shaft component (hole cutting step).
- the method of manufacturing the carburized shaft component is in mass%, C: 0.10 to 0.30%, Si: 0.01 to 0.30%, Mn: 0.4 to 2.0%, P: 0.050% or less, S: 0.005 to 0.020%, Cr: 0.4 to 3.5%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, and O: 0.003% or less, with the balance being Fe and impurities,
- mass% Pb 0.5% or less
- a step of processing a steel material satisfying the formula (1) and the formula (2) to obtain a rough member (coarse member manufacturing step), Carburizing treatment, isothermal holding treatment, quenching treatment for the rough member to obtain a carburized material,
- the carbon potential (Cp1) during the carburizing process is set to 0.7% or higher and 1.1% or lower
- the carburizing time (t1) is set to 60 minutes or longer
- the constant temperature holding temperature. (T2) is 820 ° C. or higher and 870 ° C. or lower
- the carbon potential (Cp2) during the constant temperature holding treatment is 0.7% or more and 0.9% or less
- the constant temperature holding treatment time (t2) is 20 to 60 minutes.
- a structure at a reference position corresponding to a depth position of 1 ⁇ m from a position corresponding to the surface of the hole from a position corresponding to the surface of the hole is 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft component which is the final form.
- the volume ratio (RF) of residual austenite is 20% or less
- the volume ratio (RF) of residual austenite before cutting and the volume ratio (RF) of residual austenite after cutting is expressed by the formula (B).
- a steel member having the above chemical composition is processed to obtain a rough 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 part other than the hole of the rough member has the same shape as the carburized shaft part, and the diameter of the hole is smaller than the diameter of the hole of the carburized shaft part. Note that the difference between the radius of the hole of the carburized shaft component and the radius of the hole in the rough member corresponds to the notch (d) in the subsequent hole cutting step.
- carburizing and quenching In the carburizing and quenching step, first, carburizing treatment is performed, and then a constant temperature holding treatment is performed. The carburizing process and the constant temperature holding process are performed under the following conditions.
- Carburization treatment Carburizing temperature (T1): 900-1050 ° C If the carburizing temperature (T1) is too low, the surface layer of the coarse member is not sufficiently carburized. In this case, there is little retained austenite after carburizing and quenching, and the hardness of the surface layer is also low. Therefore, the static torsional strength and the torsional fatigue strength of the carburized shaft component are reduced. On the other hand, if the carburizing temperature (T1) is too high, the austenite grains become coarse, and the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. Accordingly, the carburizing temperature (T1) is 900 to 1050 ° C. The preferable lower limit of the carburizing temperature (T1) is 910 ° C, and the preferable upper limit is 1000 ° C.
- Carbon potential during carburization (Cp1): 0.7-1.1% If the carbon potential (Cp1) is too low, sufficient carburization is not performed. In this case, there is little retained austenite after carburizing and quenching, and the hardness of the surface layer is also low. For this reason, the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. On the other hand, if the carbon potential (Cp1) is too high, hard pro-eutectoid cementite precipitated during carburization remains over 3% even after carburizing and quenching. In this case, cracks are generated starting from proeutectoid cementite, and the torsional fatigue strength of the carburized shaft component is reduced.
- the carbon potential (Cp1) is 0.7 to 1.1%.
- the carbon potential (Cp1) may be varied within the above range during the carburizing process.
- Carburizing time (t1) 60 minutes or longer If the time of carburizing treatment (carburizing time) (t1) is too short, sufficient carburizing is not performed. Therefore, the carburizing time (t1) is 60 minutes or more. On the other hand, if the carburizing time (t1) is too long, the productivity is lowered. Therefore, the upper limit of the carburizing time (t1) is preferably 240 minutes.
- Constant temperature holding temperature (T2) 820 to 870 ° C If the constant temperature holding temperature (T2) is too low, it becomes difficult to control the atmosphere such as the carbon potential. In this case, it is difficult to adjust the volume ratio of retained austenite. On the other hand, if the constant temperature holding temperature (T2) is too high, the distortion generated during quenching may increase, and a crack may occur. Therefore, the constant temperature holding temperature (T2) is 820 to 870 ° C.
- Carbon potential (Cp2) during isothermal holding treatment 0.7-0.9% If the carbon potential (Cp2) during the constant temperature holding process is too low, C that has entered during carburizing is released to the outside again. In this case, there is little retained austenite after carburizing and quenching, and the surface layer hardness is also low. As a result, the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. On the other hand, if the carbon potential (Cp2) is too high, hard pro-eutectoid cementite precipitates. In this case, cracks are generated starting from proeutectoid cementite, and the torsional fatigue strength of the carburized shaft component is reduced. Moreover, the tool wear at the time of cutting increases, and the machinability of the carburized material decreases. Therefore, the carbon potential (Cp2) is 0.7 to 0.9%.
- Constant temperature holding time (t2) 20 to 60 minutes If the constant temperature holding time (t2) is too short, the temperature of the coarse member will not be uniform, and the distortion generated during quenching will increase. In this case, burn cracking may occur in the carburized material. On the other hand, if the constant temperature holding time (t2) is too long, productivity is lowered. Accordingly, the constant temperature holding time (t2) is 20 to 60 minutes.
- the quenching process After the constant temperature holding treatment, a quenching treatment is performed by a well-known method.
- the quenching process can be, for example, oil quenching.
- a tempering process may be performed after the carburizing and quenching process.
- the structure of the reference position 21 corresponding to the depth position of 1 mm from the outer peripheral surface of the shaft component which is the final form in the axial direction of the hole and the depth position of 20 ⁇ m from the position corresponding to the surface of the hole is Site and residual austenite (RI) of 12.0 to 35.0% by volume, and other phases other than the martensite and retained austenite are 3% or less by volume.
- the structure observation of the reference position 21 corresponding to a depth position of 1 ⁇ m from the outer peripheral surface of the carburized shaft component which is the final form in the quenching material in the axial direction of the hole and a depth position of 20 ⁇ m from the position corresponding to the surface of the hole. Is implemented in the following manner. That is, in the hardened material, a surface perpendicular to the axial direction of the hole, including the hole surface layer portion in a cross section perpendicular to the axial center of the hole at a depth of 1 mm in the axial direction of the hole from the outer periphery of the carburized shaft component which is the final form A test piece having a (cross section) as an observation surface is collected (FIG.
- the mirror polished specimen is corroded with 5% nital solution.
- the corroded surface is observed with three optical fields using an optical microscope with a magnification of 1000 times.
- the reference position is set to the center of the field of view (FIG. 1-11). 10 ⁇ m from the center of the field of view to the surface of the hardened material, 10 ⁇ m from the center of the field of view to the direction opposite to the surface of the hardened material, 50 ⁇ m from the center of the field to both directions perpendicular to the surface of the hardened material, 20 ⁇ m ⁇ 100 ⁇ m
- the area ratio of each phase is obtained by a normal image analysis method.
- the average value of the area ratio of each phase obtained for each of the three visual fields is defined as the volume ratio of each phase.
- 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, at a reference position (FIG. 2-21) corresponding to a depth position of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft component, which is the final form, and a depth position of 20 ⁇ m from the position corresponding to the surface of the hole.
- the retained austenite volume fraction (RI) is measured by the following method. The carburized material is cut so as to divide the hole into two in the axial direction and through the center of the hole (FIG. 2B-B ′).
- electrolytic polishing is performed on the surface of the hole.
- the amount of polishing is adjusted by changing the time of electrolytic polishing, and a hole having a depth reaching the reference position is dug.
- the electrolytic polishing is performed at a voltage of 20 V using an electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water.
- X-rays are irradiated around the reference position and analyzed by the X-ray diffraction method.
- the product name RINT-2500HL / PC manufactured by Rigaku Corporation is used for X-ray diffraction.
- 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.
- Data analysis uses AutoMATE software (manufactured by Rigaku Corporation).
- the K ⁇ 2 component is removed by the Rachinger method, and the residual austenite volume fraction (RI) is 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. calculate. Note that the spot size of X-rays to be irradiated is 0.5 mm or less.
- RI volume ratio of retained austenite at a reference position 21 corresponding to a depth position of 1 ⁇ m from the outer peripheral surface of the carburized shaft component as a final form in the axial direction of the hole and a position corresponding to the depth of 20 ⁇ m from the position corresponding to the surface of the hole.
- RI Volume ratio of retained austenite at a reference position 21 corresponding to a depth position of 1 ⁇ m from the outer peripheral surface of the carburized shaft component as a final form in the axial direction of the hole and a position corresponding to the depth of 20 ⁇ m from the position corresponding to the surface of the hole.
- Residual austenite undergoes work-induced martensi
- the volume ratio of phases other than martensite and retained austenite (for example, ferrite, pearlite, pro-eutectoid cementite) at the reference position of the carburized material is 3% or less. If low-strength phases such as ferrite and pearlite are present at the reference position of the carburized material, these phases are maintained even after cutting. Torsional strength and torsional fatigue strength are reduced. Moreover, if pro-eutectoid cementite exists, a crack will generate
- phases other than martensite and retained austenite for example, ferrite, pearlite, pro-eutectoid cementite
- Tool feed f More than 0.01 mm / rev (rotation) 0.1 mm / rev or less
- the feed f is more than 0.01 mm / rev and 0.1 mm / rev or less.
- a preferable lower limit of the feed f is 0.02 mm / rev.
- a preferable upper limit of the feed f is 0.08 mm / rev, and more preferably 0.05.
- Cutting speed v 10 to 50 m / min
- the tool is advanced from the outer periphery of the carburized material toward the center while rotating the tool along the hole.
- the speed at which the outer peripheral portion of the tool rotates is called a cutting speed v.
- the cutting speed v is 10 to 50 m / min.
- a preferable upper limit is 40 m / min, and more preferably 30 m / min.
- Cut (d) 0.05 to 0.25 mm
- the notch (d) is the difference between the radius of the hole in the carburized shaft component and the radius of the hole in the coarse member, and corresponds to the machining allowance by cutting. If the notch (d) is too small, the cutting resistance will be small. In this case, sufficient work-induced martensitic transformation does not occur. Therefore, the torsional fatigue strength of the carburized shaft component is not improved. On the other hand, if the notch (d) is too large, the carburized hardened layer becomes too thin, so that the static torsional strength and torsional fatigue strength of the carburized shaft component are reduced. Therefore, the cut (d) is 0.05 to 0.25 mm.
- the preferable lower limit of the cut (d) is 0.08 mm, and the preferable upper limit is 0.20 mm, more preferably 0.15 mm.
- Carburized shaft component structure Carburized shaft parts can be obtained by the hole machining described above.
- the volume ratio (RF) of retained austenite is 20% or less at the reference position 21 which is 1 mm deep from the outer peripheral surface of the carburized shaft part in the axial direction of the hole and 20 ⁇ m deep from the hole surface.
- the residual austenite reduction rate ( ⁇ ′) before and after cutting determined by the formula (B) from the volume ratio (RF) of the previous retained austenite (RI) and the retained austenite after cutting becomes 35% or more.
- the austenite volume fraction (RF) is measured by the following method. That is, the carburized shaft part is cut so as to divide the hole into two in the axial direction and through the center of the hole (FIG. 2B-B ′). On the surface of the hole, masking is performed in which a hole having a diameter of 1 mm is formed around a position 1 mm deep from the outer peripheral surface, and electrolytic polishing 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 is irradiated with X-rays having a spot size of ⁇ 0.5 mm, and the residual austenite volume ratio (RF) is measured by the same method as the above-mentioned residual austenite volume ratio (RI).
- volume reduction rate ( ⁇ ′) of the retained austenite before and after the cutting process is calculated by the formula (B) based on the obtained volume ratios (RI) and (RF).
- Reduction rate ⁇ ′ (RI ⁇ RF) / RI ⁇ 100 (B)
- the austenite volume ratio (RF) at a depth position of 1 mm in the axial direction of the hole from the outer peripheral surface of the carburized shaft component and at a depth position of 20 ⁇ m from the hole surface is 20% or less. If the volume ratio of the retained austenite after cutting is too high, hard martensite cannot be obtained, and static torsional strength and torsional fatigue strength are reduced.
- the volume reduction rate ( ⁇ ′) of retained austenite before and after cutting is 35% or more.
- the retained austenite undergoes work-induced martensitic transformation, thereby increasing the static torsional strength and fatigue strength. If the volume reduction rate ( ⁇ ′) is too low, this effect cannot be obtained sufficiently.
- the retained austenite volume ratio (RF) is the retained austenite volume ratio (R2) described in the column of the carburized shaft component.
- the retained austenite reduction rate ( ⁇ ′) obtained by the equation (B) is a value similar to the above-described ⁇ (described in the section of the carburized shaft part), and both are holes in the manufacturing process of the carburized shaft part. Describes the degree of work-induced transformation of austenite during cutting. Therefore, ⁇ ′ increases as ⁇ increases.
- the carburized shaft component according to the embodiment of the present invention can be manufactured by cutting a hole after performing a carburizing and quenching process without opening a pilot hole in a steel material.
- the time for the carburizing process that is, the carburizing time (t1) is made longer than that in the case of Mode 1 so as to surely carburize to a deeper position of the steel material. It is necessary. Therefore, according to aspect 2, it is preferable that the carburizing time (t1) is 300 minutes or longer, for example, 300 to 900 minutes. It is because sufficient carburization is not performed when t1 is less than 300 minutes.
- 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 rough bar which is the base of the torsion test piece 41 shown in FIG.
- the diameter of the hole is smaller than 3 mm in the state of the coarse member.
- a torsional test piece 41 corresponding to a carburized shaft part has a circular cross section, a cylindrical test part 42, a hole 43 arranged at the center of the test part 42, and a cylindrical large diameter part 44 arranged on both sides. And a pair of gripping portions 45 chamfered around the circumference of the large diameter portion.
- the center part of the test piece is a hollow hole 46 for weight reduction. As shown in FIG.
- the entire length of the torsion test piece 41 is 200 mm
- the outer diameter of the test part 42 is 20 mm
- the length of the test part 42 is 30 mm
- the diameter of the hole 43 is 3 mm
- the hollow hole The diameter of 46 is 6 mm.
- Carburizing and quenching was performed on the rough member of the torsion test piece 41 based on the conditions shown in Table 2.
- tempering was performed at 180 ° C. for 30 minutes.
- the thickness of the carburized hardened layer formed by carburizing and tempering under the heat treatment condition a in Table 2 using the steel type D in Table 1 is a measurement of the distance (thickness) from the surface and its Vickers hardness (HV). The value was about 1.0 mm.
- the structure observation at the reference position 21 corresponding to the depth position of 20 ⁇ m from the position corresponding to the hole surface from the position corresponding to the hole surface from the outer peripheral surface of the carburized shaft component which is the final form of the carburized material is 1 mm in the axial direction of the hole. It was carried out by the method. That is, the hardened material includes a hole surface layer portion in a cross section perpendicular to the axial center of the hole at a depth of 1 mm in the axial direction of the hole from the outer periphery of the test piece (torsion test piece 41) corresponding to the shaft component as the final form.
- a test piece was collected such that a surface (cross section) perpendicular to the axial direction of the hole was an observation surface (see reference numeral 12 in FIG. 1).
- the mirror polished specimen was corroded with a 5% nital solution.
- the corroded surface was observed with 3 optical fields using an optical microscope with a magnification of 1000 times. At this time, the reference position was set to the center of the visual field.
- the area ratio of each phase was determined by a normal image analysis method. The average value of the area ratio of each phase obtained for each of the three visual fields was defined as the volume ratio of each phase.
- electrolytic polishing On the surface of the hole, masking is performed in which a hole having a diameter of 1 mm is formed around a position 1 mm deep from the outer peripheral surface, and electrolytic polishing is performed. The amount of polishing is adjusted by changing the time of electrolytic polishing, and a hole having a depth reaching the reference position is dug.
- the electrolytic polishing was performed by using an electrolytic solution containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water at a voltage of 20V.
- X-rays were irradiated around the reference position and analyzed by the X-ray diffraction method.
- the product name RINT-2500HL / PC manufactured by Rigaku Corporation is used for X-ray diffraction.
- 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 is removed by the Rachinger method, and the residual austenite volume fraction (RI) is 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. Calculated.
- the spot size of the irradiated X-ray was set to ⁇ 0.5 mm or less.
- the torsion test piece 41 that became a carburized material was subjected to hole cutting under the conditions shown in Table 3 to obtain a torsion test piece 41 equivalent to a carburized shaft part.
- the cutting conditions ⁇ , ⁇ , ⁇ , ⁇ , and ⁇ in Table 3 are holes under the conditions of the tool feed f and the cutting speed v shown in Table 3 after drilling a pilot hole in the steel material and performing carburizing and quenching treatment. It means cutting.
- the cutting condition ⁇ means that after carburizing and quenching without drilling a pilot hole in the steel material, hole cutting is performed under the conditions of the tool feed f and the cutting speed v shown in Table 3, and the cutting condition ⁇ is a steel material. This means that after the pilot hole is drilled and the carburizing and quenching treatment is performed, no hole cutting is performed.
- a coated carbide drill having a diameter of 3 mm and having a ceramic coating on the surface of the cemented carbide was used as the cutting tool.
- the tip of a coated carbide drill with a tip angle of 90 ° and a diameter of 6 mm was used.
- the Vickers hardness near the hole surface formed by the heat treatment condition a in Table 2 and the cutting condition ⁇ in Table 3 using the steel type D in Table 1 is about 900 Hv and 20 ⁇ m at a distance of 10 ⁇ m in the depth direction from the hole surface. By the way, it was about 890 HV, about 860 HV at 40 ⁇ m, about 820 HV at 50 ⁇ m, about 770 HV at 100 ⁇ m, and about 740 HV at 300 ⁇ m.
- the maximum retained austenite volume ratio (R1) in the range from the outer peripheral surface of the test piece corresponding to the carburized shaft component (torsion test piece 41) to the depth of 1 mm in the axial direction of the hole and 200 ⁇ m from the surface of the hole is as follows: It measured by the method of. The carburized material was cut so as to divide the hole into two in the axial direction and through the center of the hole (FIG. 2). On the surface of the hole, masking with a hole having a diameter of 1 mm centered on a 1 mm depth position from the outer peripheral surface was performed, and electrolytic polishing was performed.
- the amount of polishing was adjusted by changing the time of electrolytic polishing, and a hole having a depth of 30 ⁇ m was dug.
- the surface was subjected to X-ray diffraction by the above-described method, and the volume fraction of retained austenite at a position of 30 ⁇ m from the surface was determined.
- the hole was deepened by 10 ⁇ m, and the volume ratio of retained austenite was measured each time until the hole depth reached 200 ⁇ m. And the largest retained austenite volume fraction obtained in that was made into (R1).
Abstract
Description
C:0.10~0.30%、
Si:0.01~0.30%、
Mn:0.4~2.0%、
P:0.050%以下、
S:0.005~0.020%、
Cr:0.4~3.5%、
Al:0.010~0.050%、
N:0.005~0.025%、及び
O:0.003%以下
を含有し、残部がFe及び不純物からなり、
任意選択で、さらに、質量%で、
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、
Mo:3.0%以下及びNi:2.5%以下からなる群から選択される1種以上、
Cu:0~0.50%、及び
B:0~0.020%
を含有し、式(1)及び式(2)を満たし、
表層部のC含有量(Cs)が質量%で0.60~1.00%であり、
前記外周表面に少なくとも1つの穴を有し、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での組織におけるマルテンサイトと残留オーステナイトの合計体積率(α’+γ)が97%以上であり、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から200μm深さまでの範囲における最大残留オーステナイト体積率(R1)が10.0~30.0%であり、
前記R1と、前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での残留オーステナイト体積率(R2)とから式(A)によって求められる残留オーステナイト減少率(Δγ)が20%以上であることを特徴とする、浸炭シャフト部品。
1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≧2.35 (1)
11.3≦-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≦33.8 (2)
ここで、式(1)及び式(2)中の各元素記号には、各元素の含有量(質量%)が代入され、元素を含まない場合は0が代入される。
Δγ=(R1-R2)/R1×100 (A)
[2]前記R2が20%以下であることを特徴とする、上記[1]に記載の浸炭シャフト部品。
[3]前記穴の表面に塑性流動層を有することを特徴とする、上記[1]又は[2]に記載の浸炭シャフト部品。
[4]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、上記[3]に記載の浸炭シャフト部品。
本発明の実施形態に係る浸炭シャフト部品は、外周表面から3mm深さ又はそれより深い内部が、質量%で、
C:0.10~0.30%、
Si:0.01~0.30%、
Mn:0.4~2.0%、
P:0.050%以下、
S:0.005~0.020%、
Cr:0.4~3.5%、
Al:0.010~0.050%、
N:0.005~0.025%、及び
O:0.003%以下
を含有し、残部がFe及び不純物からなり、
任意選択で、さらに、質量%で、
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、
Mo:3.0%以下及びNi:2.5%以下からなる群から選択される1種以上、
Cu:0~0.50%、及び
B:0~0.020%
を含有し、式(1)及び式(2)を満たし、
表層部のC含有量(Cs)が質量%で0.60~1.00%であり、
前記外周表面に少なくとも1つの穴を有し、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での組織におけるマルテンサイトと残留オーステナイトの合計体積率(α’+γ)が97%以上であり、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から200μm深さまでの範囲における最大残留オーステナイト体積率(R1)が10.0~30.0%であり、
前記R1と、前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での残留オーステナイト体積率(R2)とから式(A)によって求められる残留オーステナイト減少率(Δγ)が20%以上であることを特徴としている。
1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≧2.35 (1)
11.3≦-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≦33.8 (2)
ここで、式(1)及び式(2)中の各元素記号には、各元素の含有量(質量%)が代入され、元素を含まない場合は0が代入される。
Δγ=(R1-R2)/R1×100 (A)
浸炭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。浸炭シャフト部品では、浸炭処理によって表層部に炭素が導入されるため、厳密には浸炭シャフト部品の表層部と内部とで化学組成が異なる。そこで、以下に示す化学組成(必須成分、不純物及び任意選択的成分を含む)は、浸炭処理前の鋼材の化学組成と一致するように、浸炭処理の影響を受けない領域、すなわち浸炭シャフト部品の外周表面から3mm深さ又はそれより深い内部における化学組成について言うものである。
炭素(C)は、浸炭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、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、浸炭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。従って、C含有量は0.10~0.30%である。C含有量の好ましい下限は0.15%以上である。C含有量の好ましい上限は0.25%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、その含有量が0.30%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、浸炭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の含有量は0.25%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、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0.01~0.30%とした。なお、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、鋼の焼入れ性を高めるとともに、鋼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、浸炭焼入れ後の切削時に加工誘起マルテンサイト変態しやすい。その結果、浸炭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、浸炭焼入れ及び焼戻し後の残留オーステナイトが過剰に多くなる。そのため、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、切削加工後の浸炭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0.4~2.0%である。Mn含有量の好ましい下限は0.8%である。Mn含有量の好ましい上限は1.8%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、浸炭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、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、鋼の熱間加工性、冷間加工性、浸炭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。
クロム(Cr)は鋼の焼入れ性を高め、さらに、残留オーステナイトを増加させる。Cr含有量が低すぎれば、この効果が得られない。一方、Cr含有量が高すぎれば、浸炭焼入れ及び焼戻し後の残留オーステナイトが過剰となる。この場合、穴切削工程における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、浸炭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0.4~3.5%である。Cr含有量の好ましい下限は0.5%である。Cr含有量の好ましい上限は3.1%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を形成し、結晶粒を微細化する。その結果、浸炭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、鋼の被削性が低下し、さらに、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、浸炭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の被削性を低下させ、浸炭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。
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
浸炭シャフト部品はさらに、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、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、鋼の強度及び靱性が低下し、浸炭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、浸炭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。
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、浸炭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、浸炭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、浸炭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、鋼の靱性を高める。しかしながら、Ni含有量が高すぎれば、浸炭焼入れ後の残留オーステナイトが過剰となる。この場合、焼戻し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、浸炭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2.5%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%を超えて添加すると、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。
浸炭シャフト部品を構成する各元素の含有量の関係は、以下に示す式(1)及び式(2)を満たす。
1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≧2.35 (1)
11.3≦-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≦33.8 (2)
ここで、式(1)及び(2)中の各元素記号には、各元素の含有量(質量%)が代入され、元素を含まない場合は0が代入される。
F1=1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Niと定義する。F1は、鋼の焼入れ性を表すパラメータである。F1が低すぎれば、鋼の焼入れ性が低くなる。この場合、強度の低いフェライト及びパーライトが生成し、浸炭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2.35以上である。F1のより好ましい下限は3.0である。浸炭シャフト部品の靭性確保のためF1の好ましい上限は8.0である。
F2=-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Niと定義する。F2は、オーステナイトの安定度を表すパラメータである。F2が低すぎれば、浸炭焼き入れ後に得られる残留オーステナイト割合が低くなる。その結果、加工誘起マルテンサイト変態による穴周辺部の硬化作用を得られず、浸炭シャフト部品の静ねじり強度及びねじり疲労強度が低くなる。一方、F2が高すぎれば、浸炭焼入れ及び焼戻し後の残留オーステナイトの量が過剰となり、静ねじり強度及びねじり疲労強度が低下する。さらに、残留オーステナイトが安定であるために、切削加工時に得られる加工誘起マルテンサイト変態の割合も少なくなる。この観点からも、浸炭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、F2は11.3~33.8であることが求められる。F2の好ましい下限は12.0である。F2の好ましい上限は33.0である。
本発明の実施形態に係る浸炭シャフト部品は、当該浸炭シャフト部品の長手(軸)方向に対して垂直又は所定の角度を有し、かつ当該浸炭シャフト部品の外周表面から開けられた1個又は複数個の貫通穴又は非貫通穴を有する。穴の直径は、特に限定されないが、例えば0.2mm~10mmであってよい。
浸炭シャフト部品の表層部に含まれるCは、浸炭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。本発明において、浸炭シャフト部品表層部のC含有量は以下の手法で測定される。
浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面から20μmの深さ位置での組織として、フェライト、パーライト等の強度の低い相が存在すれば、これらの相を起点に亀裂が発生しやすく、浸炭シャフト部品の静ねじり強度及びねじり疲労強度が低くなる。また、初析セメンタイトが存在すれば、浸炭シャフト部品の製造工程における切削加工時の工具摩耗が増大するうえに、疲労破壊の起点となるためねじり疲労強度が低下する。従って、上記位置での組織におけるマルテンサイトと残留オーステナイトの合計体積率(α’+γ)を97%以上に限定する。なお、当該合計体積率の好ましい範囲は99%以上である。
浸炭焼き入れによって導入された残留オーステナイトは、浸炭シャフト部品の穴切削加工時に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の表面に近いほど強く、穴の表面から離れるほど弱くなる。
浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面から20μmの深さ位置での残留オーステナイト体積率(R2)は20%以下であることが好ましい。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ず、静ねじり強度及びねじり疲労強度が低下する。
R1とR2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20%以上である。
本発明の実施形態に係る浸炭シャフト部品は、穴の表面に塑性流動層を有していてもよい。この塑性流動層は、穴の切削加工時に、穴の表層部に大きな変形が生じることで形成される層である。この塑性流動層は硬質であり、厚さが0.5μm以上になると浸炭シャフト部品の静ねじり強度及びねじり疲労強度を向上させ得る。しかしながら、塑性流動層は脆いため、その厚さが薄い場合にはある程度変形が可能であるが、厚さが15μmを超えると、割れが生じて亀裂発生の起点となるため、ねじり疲労強度が逆に低下する場合がある。さらに、塑性流動層はその厚さが15μmを超えると、被削性が低下し、切削加工時の工具への負担が大きくなって工具寿命が著しく低下する場合がある。以上により、浸炭シャフト部品の表層の塑性流動層の厚さは好ましくは0.5~15μmである。なお、浸炭シャフト部品の静ねじり強度及びねじり疲労強度をさらに向上させるためには、浸炭シャフト部品の表層の塑性流動層の厚さは1μm以上とすることが好ましく、3μm以上とすることがさらに好ましい。また、好ましい上限は13μmであり、さらに好ましくは10μmである。
本発明の実施形態に係る浸炭シャフト部品は、上記の塑性流動層を含めて、穴表面から一定の深さにわたって硬化された層を有する。このような硬化層は、穴の切削加工時に穴表層部の残留オーステナイトが加工誘起マルテンサイト変態することで形成された層(加工誘起マルテンサイト層)を含み、例えば、約200~300μmの厚さを有する。本発明に係る浸炭シャフト部品は、図5に示すように、静ねじり強度及びねじり疲労強度を低下させる要因となりうる穴43の周辺に、塑性流動層31と加工誘起マルテンサイト層51、特に硬質な加工誘起マルテンサイト層51を含む硬化層を備えることにより、全体としては優れた静ねじり強度及びねじり疲労強度を実現したものである。
本発明の実施形態に係る浸炭シャフト部品は、浸炭焼入れ後に穴を切削加工することによって製造することができ、例えば、以下の態様1及び2に示す方法によって製造することができる。
浸炭シャフト部品の製造方法は、鋼材を加工して粗部材を得る工程(粗部材製造工程)と、粗部材に対して浸炭焼入れ処理を施して浸炭材を得る工程(浸炭材製造工程)と、焼入れ材に対して穴の切削加工を施して、浸炭シャフト部品を得る工程(穴切削工程)とを含む。より具体的には、浸炭シャフト部品の製造方法は、質量%で、
C:0.10~0.30%、
Si:0.01~0.30%、
Mn:0.4~2.0%、
P:0.050%以下、
S:0.005~0.020%、
Cr:0.4~3.5%、
Al:0.010~0.050%、
N:0.005~0.025%、及び
O:0.003%以下
を含有し、残部がFe及び不純物からなり、
任意選択で、さらに、質量%で、
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、
Mo:3.0%以下及びNi:2.5%以下からなる群から選択される1種以上、
Cu:0~0.50%、及び
B:0~0.020%
を含有し、式(1)及び式(2)を満たす鋼材を加工して粗部材を得る工程(粗部材製造工程)と、
前記粗部材に対して浸炭処理、恒温保持処理、焼入れ処理を施して浸炭材を得る工程であって、
浸炭温度(T1)を900°以上1050℃以下とし、浸炭処理時のカーボンポテンシャル(Cp1)を0.7%以上1.1%以下とし、浸炭時間(t1)を60分以上とし、恒温保持温度(T2)を820℃以上870℃以下とし、恒温保持処理時のカーボンポテンシャル(Cp2)を0.7%以上0.9%以下とし、恒温保持処理時間(t2)を20~60分とすることで、
前記浸炭材において、最終形態である浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面に相当する位置から20μmの深さ位置に相当する基準位置での組織が、マルテンサイトと体積率で12.0~35.0%の残留オーステナイト(RI)とを含むとともに、前記マルテンサイト及び残留オーステナイト以外の他の相が体積率で3%以下となる工程(浸炭材製造工程)と、
前記浸炭材の穴に対して切削加工を施して浸炭シャフト部品を得る工程であって、
切削時の工具送りを0.01mm/rev超0.1mm/rev以下とし、切削速度を10m/分以上50m/分以下とし、切り込み(d)を0.05mm以上0.25mm以下とすることで、
前記基準位置での組織において、残留オーステナイトの体積率(RF)が20%以下となり、切削前の残留オーステナイト体積率(RI)と切削後の残留オーステナイトの体積率(RF)から式(B)によって求められる残留オーステナイト減少率(Δγ’)が35%以上となる工程(穴切削工程)と
を含む。
1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≧2.35 (1)
11.3≦-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≦33.8 (2)
ここで、式(1)及び式(2)中の各元素記号には、各元素の含有量(質量%)が代入され、元素を含まない場合は0が代入される。
Δγ’=(RI-RF)/RI×100 (B)
本工程では、浸炭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、上記化学組成を有する鋼材を準備する。
上記化学組成を有する鋼材を加工して粗部材を得る。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、穴以外の部分は浸炭シャフト部品と同様の形状とし、穴の直径は、浸炭シャフト部品の穴の直径より小さくする。なお、浸炭シャフト部品の穴の半径と、粗部材における穴の半径の差が、後の穴切削工程における、切り込み(d)に相当する。
上記のようにして得られた粗部材に対して、浸炭処理、恒温保持処理、焼入れ処理を施して浸炭材を得る。これにより、浸炭材において、最終形態である浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さでかつ穴の表面に相当する位置から20μmの深さ位置に相当する基準位置21(図2参照)での組織が、マルテンサイトと体積率で12.0~35.0%の残留オーステナイト(RI)とを含むとともに、上記マルテンサイト及び残留オーステナイト以外の他の相が体積率で3%以下とする。
浸炭焼入れ工程は、初めに、浸炭処理を施し、その後、恒温保持処理を施す。浸炭処理及び恒温保持処理は、次の条件で行う。
浸炭温度(T1):900~1050℃
浸炭温度(T1)が低すぎれば、粗部材の表層が十分に浸炭されない。この場合、浸炭焼入れ後の残留オーステナイトが少なく、表層の硬さも低い。そのため、浸炭シャフト部品の静ねじり強度及びねじり疲労強度が低くなる。一方、浸炭温度(T1)が高すぎれば、オーステナイト粒が粗大化して浸炭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、浸炭温度(T1)は900~1050℃である。浸炭温度(T1)の好ましい下限は910℃であり、好ましい上限は1000℃である。
カーボンポテンシャル(Cp1)が低すぎれば、十分な浸炭がされない。この場合、浸炭焼入れ後の残留オーステナイトが少なく、表層の硬さも低い。そのため、浸炭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、カーボンポテンシャル(Cp1)が高すぎれば、浸炭時に析出した硬質な初析セメンタイトが浸炭焼入れ後にも3%を超えて残存する。この場合、初析セメンタイトを起点に亀裂が発生し、浸炭シャフト部品のねじり疲労強度が低下する。また、切削加工時の工具摩耗が増大し、浸炭材の被削性が低下する。従って、カーボンポテンシャル(Cp1)は0.7~1.1%である。カーボンポテンシャル(Cp1)は浸炭処理時に上記範囲内で変動させてもよい。
浸炭処理の時間(浸炭時間)(t1)が短すぎれば、十分な浸炭がされない。従って、浸炭時間(t1)は60分以上とする。一方、浸炭時間(t1)が長すぎれば、生産性が低下する。従って、浸炭時間(t1)の上限は240分とすることが好ましい。
浸炭処理後、恒温保持処理を施す。恒温保持処理は、次の条件で行う。
恒温保持温度(T2)が低すぎれば、カーボンポテンシャル等の雰囲気制御が困難になる。この場合、残留オーステナイトの体積率が調整しにくい。一方、恒温保持温度(T2)が高すぎれば、焼入れ時に生じる歪が増大して、焼割れが発生する場合がある。従って、恒温保持温度(T2)は820~870℃である。
恒温保持処理時におけるカーボンポテンシャル(Cp2)が低すぎれば、浸炭時に侵入したCが再度外部に放出される。この場合、浸炭焼入れ後の残留オーステナイトが少なく、表層硬さも低い。その結果、浸炭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、カーボンポテンシャル(Cp2)が高すぎれば、硬質な初析セメンタイトが析出する。この場合、初析セメンタイトを起点に亀裂が発生し、浸炭シャフト部品のねじり疲労強度が低下する。また、切削加工時の工具摩耗が増大し、浸炭材の被削性が低下する。従って、カーボンポテンシャル(Cp2)は0.7~0.9%である。
恒温保持時間(t2)が短すぎれば、粗部材の温度が均一にならず、焼入れ時に生じる歪が増大する。この場合、浸炭材に焼割れが発生する場合がある。一方、恒温保持時間(t2)が長すぎれば、生産性が低下する。従って、恒温保持時間(t2)は20~60分である。
恒温保持処理後、周知の方法で焼入れ処理を施す。焼入れ処理は、例えば、油焼入れとすることができる。
浸炭シャフト部品の靭性を高めたい場合、浸炭焼入れ処理を施した後、焼戻し処理を施してもよい。
上述の条件で最終形態であるシャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面に相当する位置から20μmの深さ位置に相当する基準位置21の組織は、マルテンサイトと体積率で12.0~35.0%の残留オーステナイト(RI)とを含むとともに、上記マルテンサイト及び残留オーステナイト以外の他の相が体積率で3%以下となる。
浸炭焼入れ処理を施した後、穴に切削加工を施す。切削加工により、穴を開けつつ、その表層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、浸炭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。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cBNのエンドミルを用いることができる。cBNのエンドミ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。また、工具コストを削減したい場合、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)を用いても良い。
浸炭後に油穴を切削加工する際には、浸炭材外周部から中心に向かって、油穴にそって工具を回転させながら進めていく。その際に、工具一回転あたりに進む距離を工具送りfという。工具送り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、浸炭シャフト部品の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0.01mm/rev超0.1mm/rev以下である。送りfの好ましい下限は0.02mm/revである。送りfの好ましい上限は0.08mm/revであり、より好ましくは0.05である。
浸炭後に穴を切削加工する際には、浸炭材外周部から中心に向かって、穴にそって工具を回転させながら進めていく。その際に、工具の外周部が回転する速度を切削速度vという。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、浸炭シャフト部品の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。
切り込み(d)は、浸炭シャフト部品の穴の半径と、粗部材における穴の半径の差であり、切削加工による削り代に相当する。切り込み(d)が小さすぎれば、切削抵抗が小さくなる。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、浸炭シャフト部品のねじり疲労強度が向上しない。一方、切り込み(d)が大きすぎれば、浸炭硬化層が薄くなりすぎるため、浸炭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、切り込み(d)は0.05~0.25mmである。切り込み(d)の好ましい下限は0.08mmであり、好ましい上限は0.20mmであり、より好ましくは0.15mmである。
以上に示す穴切削加工により浸炭シャフト部品が得られる。浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面から20μmの深さ位置である基準位置21において、残留オーステナイトの体積率(RF)が20%以下となり、切削前の残留オーステナイト体積率(RI)と切削後の残留オーステナイトの体積率(RF)から式(B)によって求められる切削前後の残留オーステナイト減少率(Δγ’)が35%以上となる。
減少率Δγ’=(RI-RF)/RI×100 (B)
本発明の実施形態に係る浸炭シャフト部品は、態様1の場合とは異なり、鋼材に下穴を開けずに浸炭焼入れ処理を行った後、穴を切削加工することによって製造することも可能である。しかしながら、この場合には、下穴を開ける場合と比較して、鋼材のより深い位置まで確実に浸炭されるように浸炭処理の時間、すなわち浸炭時間(t1)を態様1の場合よりも長くすることが必要である。したがって、態様2によれば、浸炭時間(t1)は300分以上、例えば300~900分とすることが好ましい。t1が300分未満であると、十分な浸炭がされないからである。なお、浸炭焼入れ処理における他の条件、すなわち浸炭温度(T1)、浸炭処理時のカーボンポテンシャル(Cp1)、恒温保持温度(T2)、恒温保持処理時のカーボンポテンシャル(Cp2)及び恒温保持時間(t2)は、態様1について上で記載された範囲内で適宜決定すればよい。また、穴切削工程における切削時の工具送り及び切削速度についても、態様1に関連して記載された範囲内で適宜決定すればよい。態様2は、下穴を開ける必要がないため、態様1より工程が簡単であるという点で有利である。しかしながら、態様2は、上記のとおり非常に長い浸炭時間(t1)を必要とする。したがって、生産性の観点からは態様1の製造方法を用いて本発明の実施形態に係る浸炭シャフト部品を製造することが好ましい。
浸炭材における最終形態である浸炭シャフト部品の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面に相当する位置から20μmの深さ位置に相当する基準位置21の組織観察を次の方法で実施した。即ち、焼入れ材において、最終形態であるシャフト部品相当の試験片(ねじり試験片41)の外周から穴の軸方向に1mmの深さ位置でかつ穴軸心に垂直な断面における穴表層部を含み、穴の軸方向に垂直な面(横断面)が観察面になるような試験片を採取した(図1の符号12参照)。鏡面研磨した試験片を、5%ナイタール溶液で腐食した。腐食された面を、倍率1000倍の光学顕微鏡にて3視野観察した。このとき、基準位置を視野の中心にした。視野の中心から焼入れ材の表面方向に10μm、視野の中心から焼入れ材の表面と反対の方向に10μm、視野の中心から焼入れ材の表面方向と垂直な両方向に各々50μmの、20μm×100μmの範囲の平面内において、各相の面積率を通常の画像解析方法によって求めた。3視野のそれぞれについて得られた各相の面積率の平均値を各相の体積率と定義した。
焼入れ材において、最終形態であるシャフト部品相当の試験片(ねじり試験片51)の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面に相当する位置から20μmの深さ位置に相当する基準位置21でのオーステナイト体積率(RI)を、次の方法で測定する。穴の軸方向でかつその中心を通って穴を2分割するように浸炭材を切断した(図2)。穴表面において、外周表面から1mm深さ位置を中心にφ1mmの穴が開いたマスキングを施し、電解研磨を施す。電解研磨の時間を変化させることで研磨量を調整し、基準位置に達する深さの穴を掘る。電解研磨は、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いて、電圧20Vで電解研磨を行った。
浸炭シャフト部品相当の試験片(ねじり試験片41)の穴の軸方向でかつその中心を通って穴を2分割するように浸炭シャフト部品を切断した(図2)。穴表面において、外周表面から1mm深さ位置を中心に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深さの穴を掘った。
浸炭シャフト部品相当の試験片(ねじり試験片41)の外周表面から穴の軸方向に1mmの深さ位置でかつ穴の表面から200μm深さまでの範囲における最大残留オーステナイト体積率(R1)を、次の方法で測定した。穴の軸方向でかつその中心を通って穴を2分割するように浸炭材を切断した(図2)。穴表面において、外周表面から1mm深さ位置を中心に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、30μm深さの穴を掘った。その表面に対して上述の方法でX線回折を実施し、表面から30μm位置の残留オーステナイトの体積率を求めた。この過程を繰り返すことで、10μmずつ穴を深くし、その都度残留オーステナイトの体積率を測定することを、穴の深さが200μmとなるまで繰り返した。そしてその中で得られた最大の残留オーステナイト体積率を(R1)とした。
図4に示すねじり試験片41を用いて、サーボパルサー式ねじり試験機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、いわゆる比例限を静ねじり強度とした。この比例限は、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520MPa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4に示すねじり試験片41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375MPa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するという点で合格である。
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。
12 走査型電子顕微鏡観察位置
21 基準位置
22 浸炭シャフト部品の穴表面
23 浸炭材の穴表面
31 塑性流動層
32 母材
41 ねじり試験片
42 試験部
43 穴
44 太径部
45 つかみ部
51 加工誘起マルテンサイト層
Claims (4)
- 外周表面から3mm深さ又はそれより深い内部が、質量%で、
C:0.10~0.30%、
Si:0.01~0.30%、
Mn:0.4~2.0%、
P:0.050%以下、
S:0.005~0.020%、
Cr:0.4~3.5%、
Al:0.010~0.050%、
N:0.005~0.025%、及び
O:0.003%以下
を含有し、残部がFe及び不純物からなり、
任意選択で、さらに、質量%で、
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、
Mo:3.0%以下及びNi:2.5%以下からなる群から選択される1種以上、
Cu:0~0.50%、及び
B:0~0.020%
を含有し、式(1)及び式(2)を満たし、
表層部のC含有量(Cs)が質量%で0.60~1.00%であり、
前記外周表面に少なくとも1つの穴を有し、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での組織におけるマルテンサイトと残留オーステナイトの合計体積率(α’+γ)が97%以上であり、
前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から200μm深さまでの範囲における最大残留オーステナイト体積率(R1)が10.0~30.0%であり、
前記R1と、前記外周表面から前記穴の軸方向に1mmの深さ位置でかつ前記穴の表面から20μmの深さ位置での残留オーステナイト体積率(R2)とから式(A)によって求められる残留オーステナイト減少率(Δγ)が20%以上であることを特徴とする、浸炭シャフト部品。
1.54×C+0.81×Si+1.59×Mn+1.65×Cr+1.77×Mo+0.63×Ni≧2.35 (1)
11.3≦-0.1×Si+15.2×Mn+7.0×Cr+6.7×Mo+6.2×Ni≦33.8 (2)
ここで、式(1)及び式(2)中の各元素記号には、各元素の含有量(質量%)が代入され、元素を含まない場合は0が代入される。
Δγ=(R1-R2)/R1×100 (A) - 前記R2が20%以下であることを特徴とする、請求項1に記載の浸炭シャフト部品。
- 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又は2に記載の浸炭シャフト部品。
- 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3に記載の浸炭シャフト部品。
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KR102161576B1 (ko) | 2020-10-05 |
US20200182286A1 (en) | 2020-06-11 |
JPWO2018056333A1 (ja) | 2019-06-24 |
CN109790593B (zh) | 2020-10-23 |
EP3517639A4 (en) | 2020-04-15 |
KR20190040012A (ko) | 2019-04-16 |
US11421727B2 (en) | 2022-08-23 |
JP6680361B2 (ja) | 2020-04-15 |
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