WO2011122134A1 - 高周波焼入れ用鋼、高周波焼入れ用粗形材、その製造方法、及び高周波焼入れ鋼部品 - Google Patents
高周波焼入れ用鋼、高周波焼入れ用粗形材、その製造方法、及び高周波焼入れ鋼部品 Download PDFInfo
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- C21D2211/00—Microstructure comprising significant phases
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Definitions
- the present invention relates to a steel for induction hardening, a rough shape material for induction hardening, a manufacturing method thereof, and a induction hardening steel component.
- Power transmission parts for example, gears, bearings, CVT sheaves, shafts, etc.
- a surface hardening treatment for the purpose of improving the quality.
- the carburizing treatment is superior to other surface hardening treatments in terms of surface hardness, hardened layer depth, productivity, and the like, and therefore, there are a large number of applied parts.
- a predetermined shape is usually obtained by hot forging, cold forging, cutting, or a combination thereof using medium carbon alloy steel such as JIS SCM420, SCR420, SNCM220. Is subjected to machining, followed by carburizing or carbonitriding. Fatigue fracture of gears is broadly divided into bending fatigue (tooth root fatigue) and tooth surface fatigue (pitting etc.). In order to increase the strength of gear parts, it is necessary to improve both of these two types of fatigue strength. A gear manufactured by carburizing has a feature that both the bending fatigue strength and the fatigue strength are excellent because the hardness of the hardened layer is extremely high.
- the carburizing process is a batch process in a gas atmosphere, and requires heating and holding for several hours or more near, for example, 930 ° C., so that a great amount of equipment costs and processing energy and costs are consumed.
- the carburizing process has a large amount of CO 2 emission, which is problematic in terms of environment.
- since it is a batch process there is a large room for variation in part accuracy due to heat treatment deformation due to the difference in the parts loading position during carburizing, and it is difficult to manage the part accuracy.
- great efforts have been made in terms of materials and operation, and a certain improvement effect has been obtained.
- no drastic solution has yet been found, and it cannot be said that it has reached a sufficient level.
- induction hardening electromagnettic induction hardening
- induction hardening is advantageous in terms of productivity and cost reduction because it can significantly reduce processing time and reduce energy required for carburizing. Furthermore less emission of CO 2, and because there is no discharge into the quenching oil environment, it is advantageous in environmental.
- the induction hardening treatment is limited in the vicinity of the surface because the part affected by the heat treatment is essentially not deformed by the heat treatment. Furthermore, since the processing time is short, continuous processing is easy, and there is an advantage that it is easy to manage variation in part accuracy due to heat treatment deformation.
- the 300 ° C. tempering hardness of the martensite structure obtained by carburizing and induction hardening increases as the carbon content of the surface layer increases.
- the tempering hardness at 300 ° C. is affected by the addition of alloy elements, but the effect of carbon content is greater.
- the effect of improving the tempering hardness at 300 ° C. by adding the alloy element increases as the amount of carbon increases. Therefore, in order to obtain surface fatigue strength equivalent to that of the carburized component, it is necessary to make the carbon amount (near 0.80%) equivalent to the surface layer portion of the carburized component.
- Patent Documents 1 to 6 describe techniques for producing parts by subjecting medium carbon steel (C: to 0.65%) to induction hardening.
- C medium carbon steel
- Patent Documents 7 to 13 describe a technique for obtaining a part having improved tooth surface fatigue strength by subjecting relatively high carbon steel (C: to 0.75%) to induction hardening.
- relatively high carbon steel C: to 0.75%
- the tooth surface fatigue strength comparable to the carburized part is not reached.
- these steels have a marked decrease in workability as the carbon content increases, but due to insufficient improvement technology, the tooth surface fatigue strength and workability are insufficient, eventually replacing carburization. I can't do it.
- Patent Documents 14 to 17 improve workability and the like by defining appropriate rolling conditions, forging conditions, and cooling conditions for relatively high carbon steel (C: ⁇ 0.75%). The technology intended for is described. However, as described above, since the carbon content is still smaller than that of the carburized component, the tooth surface fatigue strength comparable to that of the carburized component is not reached, and carburization cannot be substituted.
- Patent Documents 24 to 26 a steel containing a high carbon component comparable to the surface layer portion of a carburized part is subjected to heat treatment as necessary, and then induction hardening is performed to increase tooth surface fatigue strength. Techniques for obtaining improved parts are described. However, carburization cannot be substituted because the improvement technology for workability is insufficient.
- Patent Document 27 describes a technique intended to improve machinability by precipitating a certain amount of graphite using high carbon steel (C: 0.80 to 1.50%). Has been. Patent Document 27 also shows an example of application to induction-hardened steel parts. However, in such a steel material in which a large amount of graphite is dispersed, the graphite is hardly dissolved in the matrix and the graphite is present. There is a problem that voids are generated in places. For this reason, this method impairs various characteristics as a power transmission component requiring reliability. In order to dissolve graphite and eliminate voids, induction hardening must be performed under special conditions of high temperature and long time.
- the present invention ensures that the fatigue strength (tooth surface fatigue strength, root fatigue strength, etc.) of steel parts when induction-hardened is equal to or higher than that of carburized material, and ensures the fatigue strength of steel parts.
- An object of the present invention is to provide a steel for induction hardening, a rough shaped material for induction hardening, a method for producing the same, and a component for induction hardening steel that can achieve both the workability at the time of component molding.
- induction hardening steel steel that has been cast for the purpose of producing induction-hardened steel parts and that has been subjected to treatments such as soaking diffusion and ingot rolling as necessary is described as induction hardening steel.
- an intermediate material roughly formed by performing any one or more of processes such as warm forging, hot forging, hot rolling, cooling removal, annealing, etc. is for induction hardening. It is described as a rough shape material (also called a steel material; hereinafter simply referred to as a rough shape material).
- induction hardening steel parts are manufactured by subjecting this rough shaped material to processing such as cutting and / or cold forging and subjecting to induction hardening and other steps as required.
- the present inventors have studied to solve the above problems and found the following results.
- A The controlling factor of the strength of the high carbon steel material having a carbon content exceeding 0.75% is the strength of pearlite. Therefore, when manufacturing rough shapes before cutting and cold forging, the pearlite strength can be lowered and softened by annealing under appropriate conditions, machinability and cold forgeability. Can be improved.
- B Alternatively, in the case of producing a rough shaped material by hot working, the pearlite strength is lowered and softening can be realized by performing appropriate cooling in the cooling process after hot working.
- the steel for induction hardening according to one embodiment of the present invention is, in mass%, C: more than 0.75% to 1.20%, Si: 0.002 to 3.00%, Mn: 0.20 to 2.00%, S: 0.002 to 0.100%, Al: more than 0.050% to 3.00%, P: 0.050% or less, N: 0.0200% or less, O: The content is limited to 0.0030% or less, the balance is made of Fe and inevitable impurities, and the content by mass of Al and N satisfies Al- (27/14) ⁇ N> 0.050%.
- the induction hardening steel described in (1) above may further contain B: 0.0005 to 0.0050% by mass%.
- the induction-quenched steel according to (1) or (2) is in mass%, Cr: 0.05 to less than 0.30%, Mo: 0.01 to 1.00%, Cu: 0 One or more of 0.05 to 1.00% and Ni: 0.05 to 2.00% may be further contained.
- the induction-quenched steel according to any one of (1) to (3) above is, in mass%, V: 0.005 to less than 0.20%, Nb: 0.005 to 0.10%, One or more of Ti: 0.005 to 0.10% may be further contained.
- the induction-quenched steel according to any one of (1) to (4) described above is in mass%, Ca: 0.0005 to 0.0030%, Zr: 0.0005 to 0.0030%, Mg : It may further contain one or more of 0.0005 to 0.0030%.
- a rough shaped material for induction hardening according to one aspect of the present invention has the composition of the steel for induction hardening according to any one of (1) to (5), and is included in the rough shaped material for induction hardening.
- the number of graphite particles having an average particle size of 0.5 ⁇ m or more is 40 pieces / mm 2 or less.
- a method for producing a rough shaped material for induction hardening according to an aspect of the present invention uses a steel for induction hardening according to any one of (1) to (5) above, warm processing or hot processing, The cooling and annealing steps are sequentially performed, and the annealing is performed under the conditions of an annealing temperature of 680 to 800 ° C. and an annealing time of 10 to 360 minutes.
- an average cooling rate in a temperature range of 750 to 650 ° C. during the cooling may be 300 ° C./hour or less.
- the method of manufacturing a rough shaped material for induction hardening includes the steps of hot working and cooling using the induction hardening steel according to any one of (1) to (5) above.
- the average cooling rate in the temperature range of 750 to 650 ° C. during the cooling is 300 ° C./hour or less.
- An induction-hardened steel part according to an aspect of the present invention is manufactured using the steel for induction hardening according to any one of (1) to (5) above, and is 50 ⁇ m deep from the outermost surface of the induction-hardened steel part.
- the hardness of the surface hardened portion is HV650 or more
- the hardness of the non-inductively hardened portion is HV180 or more
- the number of graphite grains having an average particle size of 0.5 ⁇ m or more present in the non-inductively hardened portion is 40 pieces / mm 2 or less.
- the fatigue strength (tooth surface fatigue strength, tooth root fatigue) of the steel component subjected to induction hardening treatment Strength, etc.) is equal to or better than carburized material, and at the same time, the workability when molding parts is high. For this reason, it becomes possible to substitute the carburizing process by a high frequency process. Thereby, the surface hardening process can be continued, the burden on the environment can be reduced, and the component accuracy can be improved. For this reason, it is possible to greatly contribute to cost reduction, environmental load reduction, and performance enhancement of automobiles and the like through improvement of production methods of power transmission parts such as automobiles (for example, gears, bearings, shafts, CVT sheaves, etc.).
- the present inventors diligently studied various factors affecting the dispersion form of the carbide in the carburized layer in the high carbon carburizing process, and as a result of considering a method for realizing fatigue strength comparable to the carburized steel by induction hardening steel, the following knowledge was obtained.
- Got. (A) The tempering hardness at 300 ° C. increases as the carbon content of the rough material subjected to induction hardening increases, and when C exceeds 0.75%, a tempering hardness of 300 ° C. comparable to carburized parts is obtained. It is done. As a result, the tooth surface fatigue strength comparable to that of the carburized component can be secured even in the induction-hardened component.
- the rough shape material can be further softened, and the workability can be further improved or the annealing time can be shortened.
- the amount of Al added is significantly increased compared to conventional steel, and at the same time, the amount of N is suppressed and the amount of solute Al is secured, so that the tool life during cutting is greatly increased. Machinability can be improved.
- the carbon content of steel is increased, the hardness of the rough shape is increased and cutting cannot be performed.
- the present invention by securing a sufficient amount of solute Al, cutting can be performed even when the hardness of the rough shaped material is increased, and the carbon content of the steel can be increased.
- (G) Cr concentrates in the ⁇ carbide (cementite) to stabilize the ⁇ carbide, thereby inhibiting the carbide from dissolving into austenite during induction hardening, and causes hardness unevenness in the hardened layer. For this reason, when adding Cr, the addition amount is restrict
- content% of a component means the mass%.
- C Over 0.75% to 1.20% C is added to secure the surface hardness after induction hardening and the hardness of the core of the component.
- the surface carbon content of the carburized component is about 0.80%.
- tooth surface fatigue strength 300 ° C. tempering hardness
- C is added in excess of 1.20%, the workability when processing such as cutting and forging of parts through the increase in the hardness of the rough profile is significantly deteriorated. Therefore, it is necessary to set the content in the range of more than 0.75% to 1.20%.
- a preferred range for the amount of C is 0.76 to 0.90%.
- Si 0.002 to 3.00%
- Si When Si is added to high-carbon steel, it suppresses the transition from ⁇ carbide that precipitates during tempering to relatively coarse ⁇ carbide, and remarkably increases the temper softening resistance of low-temperature tempered martensitic steel. This improves the tooth surface fatigue strength of the steel.
- it is necessary to add 0.002% or more of Si to the induction hardening steel of the present invention. This effect increases as the amount of Si added increases, but if added over 3.00%, the workability when machining parts such as cutting and forging through a rise in the hardness of the rough profile is significantly deteriorated. .
- the Si amount needs to be in the range of 0.002 to 3.00%.
- a preferable range of the amount of Si is 0.20 to 1.50%.
- the amount of Si may be less than 0.50%.
- Mn 0.20 to 2.00% Since Mn has the effect of enhancing the hardenability of steel, it is effective for obtaining a martensite structure during carburizing and quenching. In order to obtain this effect, it is necessary to add 0.20% or more of Mn to the induction hardening steel of the present invention. On the other hand, when it exceeds 2.00%, the workability at the time of processing such as cutting and forging of parts through the increase in the hardness of the rough profile is remarkably deteriorated. Therefore, the amount of Mn needs to be in the range of 0.20 to 2.00%. A preferable range of the amount of Mn is 0.30 to 1.00%.
- S 0.002 to 0.100% S combines with Mn to form MnS, and has an effect of improving machinability as the addition amount increases. In order to obtain this effect, it is necessary to add 0.002% or more of S to the steel for induction hardening of the present invention. On the other hand, if added over 0.100%, MnS becomes a propagation path of fatigue cracks, so that the bending fatigue strength of products such as gears is lowered. Therefore, the S amount needs to be in the range of 0.002 to 0.100%. A preferable range of the amount of S is 0.010 to 0.050%.
- Al more than 0.050% to 3.00%
- Al has the effect of significantly improving the tool life in the cutting of the rough profile. This is because the solute Al of the rough profile reacts with oxygen during cutting to form a hard Al 2 O 3 coating, which suppresses tool wear.
- the Al 2 O 3 coating that protects the tool is formed by the solid solution Al of oxygen in the atmosphere, oxygen in the cutting oil, or oxygen in the homo-treated film (Fe 3 O 4 ) on the tool surface. Formed in reaction.
- Homo-treated film is also called steam treatment, and is an iron oxide film with a thickness of several ⁇ m produced by heat treatment in steam to give corrosion resistance to the tool (Reference: Japan Heat Treatment Technology Association) Edited by: “Handbook of Heat Treatment Technology”, Nikkan Kogyo Shimbun, Tokyo, 2000, P569).
- the presence of this coating that protects the tool prevents direct contact between the workpiece (rough profile) and the tool, and suppresses the adhesive wear of the tool.
- the tool wear increases remarkably as the hardness of the rough profile increases, so it is practically impossible to increase the carbon content of the rough profile.
- the present invention by adding a large amount of Al, the amount of increase in tool wear with respect to the increase in hardness of the rough shape material is suppressed, so even if the carbon content of induction hardening steel is increased compared to the prior art, Industrial production becomes possible.
- Al has the same effect as Si on the tempering behavior of low-temperature tempered martensitic steel, and is effective in improving the tooth surface fatigue strength by significantly increasing the temper softening resistance. In order to obtain this effect, it is necessary to add more than 0.050% of Al to the induction hardening steel of the present invention.
- Al stabilizes ferrite, so if added over 3.00%, ferrite remains during induction hardening and a uniform austenite phase cannot be obtained. As a result, a uniform martensite structure cannot be obtained after quenching. Therefore, the Al amount needs to be in the range of more than 0.050% to 3.00%. A preferable range of the Al content is 0.100 to 1.00%.
- P 0.050% or less
- P is an unavoidable impurity, and segregates at austenite grain boundaries to cause embrittlement of the prior austenite grain boundaries. For this reason, in this invention, it is necessary to make P amount of the steel for induction hardening into the range of 0.050% or less. Although there is no particular lower limit of the amount of P regarding the subject of the present invention, excessive cost is required to limit the amount of P to 0.001% or less. Therefore, a preferable range of the P content is 0.001 to 0.015%.
- N 0.0200% or less N combines with Al in steel to form AlN, and AlN functions to suppress grain growth by pinning austenite grain boundaries and prevent coarsening of the structure. .
- N may be added positively if it is desired to make the crystal grains finer.
- the ductility at a high temperature range of 1000 ° C. or higher is lowered, which causes a decrease in yield during continuous casting and rolling. For this reason, in this invention, it is necessary to restrict
- a preferable range of the N amount is 0.0050 to 0.0120%.
- O 0.0030% or less
- O forms oxide inclusions, and when the content is large, large inclusions that become the starting point of fatigue fracture increase, which causes deterioration of fatigue characteristics. It is desirable. For this reason, in this invention, it is necessary to restrict
- B is an optional component that can be added to the induction hardening steel of the present invention as required.
- B is an effective element for obtaining a martensitic structure at the time of carburizing and quenching because it has the effect of greatly increasing the hardenability of the steel in a small amount in a state in which it is dissolved in austenite.
- 0.0005% or more of B may be added to the induction hardening steel in the present invention.
- the effect is saturated even if added over 0.0050%. Therefore, when B is added, the B content is in the range of 0.0005 to 0.0050%.
- a preferable range of the B amount is 0.0010 to 0.0025%.
- Cr 0.05% to less than 0.30%
- Cr is an optional component that can be added to the induction hardening steel of the present invention as required.
- Cr has the effect of remarkably miniaturizing the lamella spacing during the pearlite transformation, so that the hardness of the coarse shaped material is greatly increased and the workability is deteriorated. Further, by concentrating and stabilizing in the ⁇ carbide, the penetration of the carbide into the austenite at the time of induction hardening is hindered, resulting in uneven hardness of the hardened layer. Therefore, when adding Cr, the Cr addition amount is limited to less than 0.30%.
- the ⁇ carbide may be graphitized and the induction hardenability may be reduced.
- a small amount of Cr may be added to the induction hardening steel.
- the lower limit of the amount of Cr necessary for preventing graphitization is 0.05%. Therefore, when adding Cr, the Cr addition amount is set to a range of 0.05% to less than 0.30%. A preferable range of the Cr content is 0.10 to 0.20%.
- Mo 0.01 to 1.00%
- Mo is an optional component that can be added to the induction hardening steel of the present invention as necessary. Since Mo has an effect of improving the hardenability of steel, it is an effective element for obtaining a martensite structure during carburizing and quenching. In order to acquire this effect, you may add 0.01% or more of Mo. On the other hand, if added over 1.00%, the addition cost becomes excessive, and the workability when machining parts such as cutting and forging through the increase in the hardness of the rough shape material is significantly deteriorated. Not desirable for production. Therefore, when Mo is added, the amount of Mo is set in the range of 0.01 to 1.00%. A preferable range of the Mo amount is 0.10 to 0.60%.
- Mo is an element that exhibits a relatively large hardenability improving effect even when added in a small amount.
- B is added in a composite manner, a large composite addition effect can be obtained with respect to the effect of improving the hardenability even with a small amount.
- Cu 0.05 to 1.00%
- Cu is an optional component that can be added to the induction hardening steel of the present invention as required. Since Cu has the effect of enhancing the hardenability of the steel, it is effective for obtaining a martensite structure during carburizing and quenching. In order to obtain this effect, 0.05% or more of Cu may be added. However, if it is added in excess of 1.00%, the ductility at a high temperature range of 1000 ° C. or higher is lowered, which causes a decrease in yield during continuous casting and rolling. Therefore, when Cu is added, the addition amount is made 0.05 to 1.00%. A preferable range of the amount of added Cu is 0.010 to 0.50%. In addition, in order to improve the ductility of a high temperature range, when adding Cu, it is desirable to add Ni more than 1/2 of Cu addition amount simultaneously.
- Ni is an optional component that can be added to the induction hardening steel of the present invention as required.
- Ni is an effective element for obtaining a martensite structure at the time of carburizing and quenching because it has the effect of enhancing the hardenability of the steel. In order to obtain this effect, 0.05% or more of Ni may be added.
- the addition amount is set in the range of 0.05 to 2.00%.
- a preferable range of the Ni content is 0.40 to 1.60%.
- V 0.005 to less than 0.20%
- V is an optional component that can be added to the steel for induction hardening according to the present invention as necessary.
- V combines with N and C in steel to form V (C, N), and V (C, N) coarsens the structure by suppressing grain growth by pinning the austenite grain boundaries. There is a function to prevent. In order to obtain this effect, 0.005% or more of V may be added.
- V (C, N) generated becomes excessive, causing unevenness in the hardness of the cured layer during induction hardening. Therefore, when V is added, the amount added is in the range of 0.005 to less than 0.20%.
- a preferable range of the V amount is 0.05 to 0.10%.
- Nb 0.005 to 0.10%
- Nb is an optional component that can be added to the induction hardening steel of the present invention as necessary.
- Nb combines with N and C in steel to form Nb (C, N), and Nb (C, N) suppresses grain growth by pinning austenite grain boundaries, thereby coarsening the structure There is a function to prevent.
- Nb may be added in an amount of 0.005% or more.
- the workability when processing such as cutting and forging of parts through the increase in the hardness of the rough profile is significantly deteriorated.
- the amount of Nb (C, N) generated becomes excessive, which causes uneven hardness of the hardened layer during induction hardening. Therefore, when Nb is added, the amount added is in the range of 0.005 to 0.10%.
- a preferable range of the Nb amount is 0.010 to 0.050%.
- Ti is an optional component that can be added to the induction hardening steel of the present invention as necessary. Ti combines with N and C in steel to form Ti (C, N), and Ti (C, N) suppresses grain growth by pinning austenite grain boundaries, thereby coarsening the structure There is a function to prevent. In order to obtain this effect, 0.005% or more of Ti may be added. On the other hand, if added over 0.10%, the workability when processing such as cutting and forging of parts through the increase in the hardness of the rough profile is significantly deteriorated. In addition, the amount of Ti (C, N) generated becomes excessive, causing unevenness in the hardness of the hardened layer during induction hardening. Therefore, when adding Ti, the addition amount is set in the range of 0.005 to 0.50%. A preferable range of the Ti content is 0.015 to 0.050%.
- Ca, Zr, Mg are optional components that can be added to the induction hardening steel of the present invention as necessary.
- Ca, Zr, and Mg all have a function of improving the machinability of steel through the form control of MnS and the formation of a protective film on the cutting tool surface during cutting. In order to obtain this effect, 0.0005% or more of Ca, Zr, or Mg may be added.
- the amount added is in the range of 0.0005 to 0.0030%.
- a preferable range of the total addition amount of Ca, Zr and Mg is 0.0008 to 0.0020%.
- Pb, Te, Zn, Sn and the like can be added within a range not impairing the effects of the present invention.
- Pb, Te, Zn, and Sn are optional components that can be added to the induction hardening steel of the present invention as necessary.
- the upper limit of the addition amount of these additive components is Pb: 0.50% or less, Te: 0.0030% or less, Zn: 0.50% or less, Sn: 0 .. 50% or less.
- Al when Al is in a solid solution state in steel, it has an effect of remarkably improving the tool life in cutting of steel parts, so it is added in the range of more than 0.050% to 3.00%.
- Al combines with N in the steel to form AlN, and may take the form of precipitates.
- Al present as a precipitate is not effective in improving the tool life.
- AlN is likely to precipitate as compared with a process of allowing to cool after hot forging.
- Al ⁇ (27/14) ⁇ N which is an index formula for the amount of solid solution Al
- the theoretical upper limit of “Al- (27/14) ⁇ N” for the induction hardening steel of the present invention is 3.00%, and the preferred range is 0.100 to 1.00%.
- the rough shaped material for induction hardening achieves both sufficient tooth surface fatigue strength and workability by adjusting the steel components and annealing conditions. Moreover, generation
- the number of graphite grains having an average grain size of 0.5 ⁇ m or more is set to 0 / mm 2 if annealing is performed under appropriate conditions.
- annealing is performed under appropriate conditions.
- CE C + Si / 3-Mn / 12 + Al / 6 + Cu / 9 + Ni / 9-Cr / 9-Mo / 9 + B (1)
- C, Si, Mn, Al, Cu, Ni, Cr, Mo, and B indicate mass% of each element included in the steel for induction hardening.
- the steps of warm working or hot working, cooling, and annealing are sequentially performed on the induction hardening steel having the above composition.
- the annealing temperature is 680 to 800 ° C. and the heating time is 10 to 360 minutes. The reason for using these conditions will be described below.
- Examples of warm working include warm forging, and examples of hot working include hot forging or hot rolling.
- warm working include warm forging
- hot working include hot forging or hot rolling.
- the structure of the rough shaped material mainly becomes a ferrite or pearlite structure (95% or more).
- the hardness of the rough shape is greatly affected by the amount of soft ferrite and the hardness of the ferrite itself.
- countermeasures for softening such a rough shape material there are a method of increasing the ferrite fraction by combining processing and heat treatment, a method of suppressing the addition amount of elements that solidify and strengthen ferrite.
- the induction hardening steel of the present invention has a carbon content exceeding 0.75%. For this reason, even if this steel is used to produce a rough profile by either warm forging, hot forging, or hot rolling, the coarse profile is mostly pearlite with a very small amount of ferrite. Including or substantially all (95% or more) becomes a pearlite structure. Therefore, the strength of the pearlite structure has a dominant influence on the strength of such a rough shape. The intensity of the pearlite structure is related to the lamella spacing of the pearlite.
- the higher the heating temperature the more fine pearlite lamella is broken and the coarser dispersion of the ⁇ carbide.
- the annealing temperature needs to be in the range of 680 to 800 ° C.
- the preferred annealing temperature range is 700-770 ° C. If the annealing heating time is too short, the shape of the pearlite lamella hardly changes and a sufficient softening effect cannot be obtained.
- the heating time for annealing needs to be in the range of 10 to 360 minutes.
- a preferable range of the heating time for annealing is 30 to 300 minutes.
- a preferable range of the average cooling rate in the temperature range of 750 to 650 ° C. is 300 ° C./hour or less.
- the steps of hot working and cooling are sequentially performed on the induction hardening steel having the above composition.
- the average cooling rate in the temperature range of 750 to 650 ° C. is set to 300 ° C./hour or less.
- annealing is not necessarily performed. The reason for using this cooling condition will be described below.
- the influence of the strength of the pearlite structure is dominant in the hardness of the coarse shaped material, and annealing is extremely effective for its softening.
- by performing such slow cooling after the pearlite transformation is completed, it stays in the high temperature region, so that the same effect as annealing can be obtained.
- the temperature range for performing slow cooling exceeds 750 ° C.
- the temperature range where pearlite transformation cannot occur is gradually cooled, so that the effect of softening cannot be obtained.
- the temperature range for slow cooling is less than 650 ° C.
- the pearlite transformation starts at a low temperature. For this reason, the increase in the pearlite lamella spacing becomes insufficient and the softening becomes insufficient, and the annealing effect after pearlite transformation by slow cooling is also reduced. Therefore, it is necessary to set the temperature range for slow cooling to a range of 750 to 650 ° C.
- a preferable range of the temperature range at which the slow cooling is performed is a range of 740 to 680 ° C.
- the average cooling rate in the temperature range where the slow cooling is performed is 300 ° C./hour or less.
- a preferable range of the average cooling rate in the temperature range in which the slow cooling is performed is 200 ° C./hour or less.
- the cooling rate limited above is an average cooling rate between 750 ° C. and 650 ° C., and it is not always necessary to perform continuous cooling. If the above conditions are satisfied, the cooling rate is kept constant during the cooling process. There may be a holding period.
- the lower limit of the average cooling rate is preferably 80 ° C./hour or more.
- annealing after cooling may not be performed, but annealing may be performed in combination with annealing under the above-described conditions. In this case, a greater softening effect can be obtained than when annealing and annealing are performed alone.
- the induction-hardened steel component according to one aspect of the present invention is subjected to cutting and / or cold working and induction hardening on a rough shaped material for induction hardening manufactured by any one of the above-described manufacturing methods. And then subjected to a low-temperature tempering treatment.
- This steel part is manufactured such that the hardness of the surface layer hardened portion having a depth of 50 ⁇ m from the outermost surface is HV650 or higher, and the hardness of the non-high frequency quenched portion is HV180 or higher.
- Examples of the steel parts of the present invention include power transmission parts (for example, gears, bearings, CVT sheaves, shafts) used in automobiles, construction machinery / agricultural machinery, wind turbines for power generation, and other industrial machines. .
- the induction hardening process corresponds to this surface hardening process.
- a hardness having a depth of 50 ⁇ m from the outermost surface was selected. If the hardness of this part is HV650 or more, it can be determined that the hardness is comparable to that of a normal carburized part. In this case, fatigue characteristics and wear resistance comparable to the carburized part can be obtained.
- the suitable upper limit of the hardness of the induction-hardened part of the part obtained by the steel composition and manufacturing method according to the present invention is about HV950.
- part is HV700 or more.
- induction hardening electromagnetic induction hardening
- a ring-shaped coil is used to perform quenching by electromagnetic induction under conditions of a frequency of 10 to 500 kHz and a processing time of 0.1 to 20 seconds, and then quenching by water cooling, Conditions for setting the curing depth to 0.2 to 2.5 mm can be used.
- the part to be processed may be rotated at 100 to 2000 rpm in order to homogenize the hardened layer depth and quench the contour of the gear.
- preheating may be performed in advance in the temperature range below the A1 point by low-frequency electromagnetic induction.
- Hardening by induction hardening can affect the depth of 0.1 to 3 mm from the surface of induction-hardened steel parts depending on processing conditions. Does not happen. Such a non-hardened part is set as a non-induction hardening part. Therefore, the hardness of the non-induction hardened portion is substantially equal to the hardness of the rough shaped material before induction hardening. Since the hardness of the non-induction hardened portion is related to the fatigue strength of the internal origin and the low cycle fatigue strength of the gear, it is not desirable that the hardness is too low.
- the internal hardness may be somewhat lower than that of a normal carburized part.
- the hardness of the non-high frequency quenched portion needs to be HV180 or more, and the preferred range is HV200 or more.
- the preferred upper limit of the hardness of the non-high frequency quenching part of the steel part according to the present invention is HV240.
- the shot peening treatment may be performed on the induction hardened steel part according to the above aspect of the present invention after the induction hardening treatment or after the induction hardening and the low temperature tempering (300 ° C. or less).
- the increase in compressive residual stress on the part surface layer introduced by shot peening treatment suppresses the occurrence and development of fatigue cracks, and therefore further improves the tooth root and tooth surface fatigue strength of parts manufactured with the steel of the present invention.
- the shot peening treatment is desirably performed using shot grains having a diameter of 0.7 mm or less and an arc height of 0.4 mm or more.
- a converter molten steel having the composition shown in Table 1 was manufactured by continuous casting, and if necessary, a rolling raw material of 162 mm square was obtained through a soaking diffusion treatment and a block rolling process. Next, steel for induction hardening in the form of a steel bar having a diameter of 45 mm was obtained by hot rolling.
- the shaded and underlined portions of the comparative steels in Table 1 indicate that they are outside the scope of the present invention.
- hot working or warm working simulation was performed on hot rolled steel (steel for induction hardening) under the conditions shown in Table 2.
- the heating temperature in the hot working simulation was 1250 ° C.
- the heating temperature in the warm working simulation was 750 ° C.
- annealing treatment was performed under the conditions shown in Table 2 as necessary. From the sample of the rough material thus prepared, a 45 ⁇ ⁇ 15 mm disk-shaped test piece for machinability evaluation and a roller pitching test piece having a large diameter portion (test portion) 26 ⁇ were prepared.
- Vickers hardness at a position of 1/4 part of the diameter in the cross section in the diameter direction was measured for each disk test piece at each test level. It was judged that the hardness of the rough shape material was over HV240, which was inferior in workability (cold forgeability, machinability).
- “Cooling rate after hot working or warm working” in Table 2 indicates an average cooling rate in a temperature range of 750 to 650 ° C.
- the underline of the cooling rate after thermal forging, the end temperature of annealing after thermal forging, the annealing conditions, the hardness of the hardened layer, and the hardness of the non-high frequency quenched portion is outside the scope of the present invention.
- Hardness of rough profile, tool life when cutting and processing rough profile, 300 ° C tempering hardness of hardened layer of induction-hardened steel parts, underline of roller pitting fatigue strength means that the target is not achieved. .
- the machinability evaluation test (measurement of tool life) was performed on the above disk specimen under the conditions shown in Table 3.
- the condition of the machinability evaluation test was used as an index of the tool life by obtaining the maximum cutting speed (m / min) at which the total depth of the hole by the drill reached 1000 mm. When this index did not reach 70 m / min, it was determined that the machinability was poor.
- roller pitching test piece is subjected to induction hardening treatment on the large diameter portion (test portion) under the condition that the hardened layer depth is 2 mm, and subsequently subjected to tempering treatment at 150 ° C. for 90 minutes.
- the grip was finished.
- the roller pitching test was conducted under the conditions of a large roller: SCM420 carburized product, crowning 150R, rotation speed: 2000 rpm, lubricating oil: transmission oil, oil temperature 80 ° C, slip rate: 40%, maximum 10 million cycles, SN line
- a diagram was created to determine the fatigue limit, which was defined as the roller pitting fatigue strength.
- the roller pitting fatigue strength that did not reach 2600 MPa was determined to be inferior in tooth surface fatigue strength.
- Each production No. First, one large diameter part of the roller pitching test piece of each test level subjected to the induction hardening and tempering treatment was cut, and a Vickers hardness measurement was performed on a section of 50 ⁇ m from the surface layer in the cross section. The measurement result was taken as the hardness of the hardened hardening layer.
- tempering is performed for another test piece one by one at 300 ° C. for 90 minutes, the large diameter portion is cut, and the Vickers hardness measurement is performed at 50 ⁇ m from the surface layer in the cross section. The hardness was determined. Those whose 300 ° C. tempering hardness did not reach HV630 were judged to be inferior to 300 ° C. tempering hardness, and thus inferior to tooth surface fatigue strength.
- Production No. The inventive examples 1 to 25 all achieved the target, had excellent workability, and had sufficient tooth surface fatigue strength.
- production No. No. 26 was within the scope of the present invention with respect to the steel component, but since neither slow cooling nor annealing after heat forging was performed, the hardness of the rough shape was high and the workability was inferior.
- Production No. No. 27 has an annealing temperature that is too low. In No. 28, since the annealing temperature was too high, the hardness of the rough shape was high and the workability was poor. Production No. In No. 29, the cooling rate after heat forging was too high, and the annealing temperature was too low, so the hardness of the rough shape was high and the workability was poor. Production No.
- the hardness of the induction hardening after-hardening layer and the 300 degreeC hardness of the hardening layer were insufficient.
- the hardness of the rough shape material decreased due to the precipitation of graphite, the hardness of the non-induction-hardened part after induction hardening was inevitably low, and the presence of voids in the hardened layer resulted in roller pitting fatigue. The strength was also low.
- Induction hardening steel, induction hardening rough shape material, manufacturing method thereof, and induction hardening steel parts according to each aspect of the present invention are used in automobiles, construction machinery / agricultural machinery, power generation wind turbines, other industrial machines, etc. It can be applied to existing power transmission parts (for example, gears, bearings, CVT sheaves, shafts) and the like, and it is possible to realize both the workability at the time of part molding and the fatigue strength of steel parts subjected to induction hardening. For this reason, it becomes possible to substitute the carburizing process by a high frequency process. Thereby, the surface hardening process can be continued, the burden on the environment can be reduced, and the component accuracy can be improved.
- existing power transmission parts for example, gears, bearings, CVT sheaves, shafts
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Abstract
Description
本願は、2010年3月30日に、日本に出願された特願2010-078232号に基づき優先権を主張し、その内容をここに援用する。
本発明者らは、上記のような課題を解決するため検討を行い、以下の結果を見出した。(a)炭素量が0.75%を超える高炭素鋼素材の強度の支配因子はパーライトの強度である。従って、切削加工や冷間鍛造前の粗形材を製造する際に、適正な条件で焼鈍を行うことによってパーライトの強度が低下して軟質化を図ることができ、被削性及び冷鍛性を向上することができる。(b)あるいは、粗形材を熱間加工によって製造する場合、熱間加工後の冷却過程において適切な冷却を行うことによってパーライトの強度が低下して軟質化が実現できる。(c)また、鋼成分としては、合金元素を過剰に添加せず、なおかつ従来鋼に比べてAl量を大幅に増加することにより、炭素量を増加して切削加工前の粗形材強度が増加しても被削性の低下が抑制できる。
本発明者らは、上記の技術を適切に活用することによって本発明を完成したものであり、その要旨は下記の通りである。
(a) 高周波焼入れ処理を行う粗形材の炭素量を増加するほど300℃焼戻し硬さが上昇し、0.75%を超えてCを添加すると浸炭部品に匹敵する300℃焼戻し硬さが得られる。これによって、高周波焼入れ処理された部品においても浸炭処理部品に匹敵する歯面疲労強度を確保することができる。
(b) 鋼の炭素量が0.75%を超える場合、部品成型加工(切削、冷鍛)前の粗形材の組織はほとんどがパーライト組織となる。このため、粗形材の硬さに対して、パーライト組織の強度(パーライトラメラ間隔に関係する)が支配的な影響を持つ。
(c) 部品成型加工前の粗形材の製造過程で適切な焼鈍を施すことによって、微細なパーライトラメラを崩して軟質化することができ、加工性を改善することができる。
(d) 一方、部品成型加工前の粗形材を熱間加工によって製造する場合、熱間加工後の冷却を適切に行うことによって、パーライトラメラ間隔を大きくし、軟質化することができ、加工性を改善することができる。
(e) 上記(c),(d)の組み合わせにより、粗形材を更に軟質化することができ、さらなる加工性の改善、あるいは焼鈍時間の短縮が可能となる。
(f) 従来鋼よりもAl添加量を大幅に増加し、同時にN量を抑制して、固溶Al量を確保することによって、切削加工時の工具寿命を大幅に増加し、粗形材の被削性を改善することができる。従来の技術では鋼の炭素量を増加すると粗形材の硬さが上昇して切削加工ができなくなっていた。一方、本発明によると、十分な固溶Al量を確保することによって、粗形材の硬さが上昇しても切削加工が可能となり、鋼の炭素量を増加することが可能となる。
(g) Crはθ炭化物(セメンタイト)中に濃化してθ炭化物を安定化することによって、高周波焼入れ時に炭化物のオーステナイトへの溶け込みを阻害し、硬化層の硬さムラの原因となる。このため、Crを添加する場合は、その添加量を制限する。V、Nb、Tiを添加する場合、過剰の添加はCrと同様に硬化層の硬さムラの原因となるのみならず、粗形材硬さも上昇し、加工性が低下するので添加量を制限する。
(h) 高周波焼入れ用鋼から粗形材を製造する際の焼鈍の条件によっては、粗形材内に黒鉛粒が発生する場合がある。上記粗形材に対して切削加工及び/または冷間加工を行う際に、粗形材内に一定以上のサイズの黒鉛粒が一定量以上存在していると、高周波焼入れの短時間加熱では黒鉛粒がオーステナイト中に十分に溶け込まないため、硬化層の硬さムラの原因となる。更に、黒鉛粒がオーステナイトに溶け込んだ場合でも、黒鉛粒の存在していた位置にボイドが残り、部品の特性を低下させる場合がある。これらの理由により、粗形材内の黒鉛の析出量を制限する必要がある。
Cは高周波焼入れ後の表面硬さを確保する作用と、部品の心部の硬さを確保するために添加する。通常、浸炭処理された部品の表面炭素量は0.80%程度である。高周波焼入れ鋼部品において浸炭部品と同等の歯面疲労強度(300℃焼戻し硬さ)を得るためには、高周波焼入れ用鋼の炭素量を従来の場合よりも増加する必要がある。添加量が少ないと浸炭部品に匹敵する歯面疲労強度が得られないので、炭素量は0.75%を超えて添加する必要がある。Cを1.20%を超えて添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。従って、0.75%超~1.20%の範囲にする必要がある。C量の好適な範囲は0.76~0.90%である。
Siは高炭素鋼に添加された場合、焼戻し時に析出するε炭化物から比較的粗大なθ炭化物への遷移を抑制し、低温焼戻しマルテンサイト鋼の焼戻し軟化抵抗を顕著に増加する。これによって鋼の歯面疲労強度が向上する。この効果を得るために、本発明の高周波焼入れ用鋼には、Siを0.002%以上添加する必要がある。この効果はSiの添加量が多いほど大きいが、3.00%を超えて添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。また、Siはフェライトを安定化するため、3.00%を超えて添加すると高周波焼入れ時にフェライトが残留し、均一なオーステナイト相が得られなくなり、この結果として、焼入れ後に均一なマルテンサイト組織が得られなくなる。従って、Si量を0.002~3.00%の範囲にする必要がある。Si量の好適な範囲は0.20~1.50%である。特に黒鉛量を規制する必要がある場合は、Si量を0.50%未満にしてもよい。
Mnは鋼の焼入性を高める効果があるので浸炭焼入れ時にマルテンサイト組織を得るために有効である。この効果を得るために、本発明の高周波焼入れ用鋼にはMnを0.20%以上添加する必要がある。一方、2.00%を超えて添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。従って、Mn量を0.20~2.00%の範囲にする必要がある。Mn量の好適な範囲は0.30~1.00%である。
SはMnと結合してMnSを形成し、添加量を増加するほど被削性を向上させる効果を持つ。この効果を得るために、本発明の高周波焼入れ用鋼にはSを0.002%以上添加する必要がある。一方、0.100%を超えて添加するとMnSが疲労亀裂の伝播経路となることによって歯車等の製品の曲げ疲労強度が低下する。従って、S量を0.002~0.100%の範囲にする必要がある。S量の好適な範囲は0.010~0.050%である。
Alが粗形材中において固溶状態にある場合、粗形材の切削加工において工具寿命を顕著に改善する効果を持つ。これは、粗形材の固溶Alが切削中に酸素と反応して硬質のAl2O3の被膜を形成し、この被膜が工具の摩耗を抑制するためである。この工具を保護するAl2O3の被膜は、粗形材の固溶Alが大気中の酸素、又は切削油中の酸素、又は工具表面のホモ処理膜(Fe3O4)中の酸素と反応して形成される。ホモ処理膜とは水蒸気処理とも言われ、工具に耐食性などを付与するために、水蒸気中で熱処理を行うことによって生成された、厚さ数μmの鉄酸化膜である(参考:日本熱処理技術協会編著:「熱処理技術便覧」日刊工業新聞社、東京、2000年発行、P569記載)。工具を保護するこの被膜が存在することにより、被切削物(粗形材)と工具との直接接触が妨げられ、工具の凝着摩耗が抑制される。従来技術においては、粗形材の硬さが上昇すると工具摩耗が顕著に増加するため、粗形材の炭素量の増加は実用上不可能であった。一方、本発明では、Alを多量に添加することによって粗形材の硬さの上昇に対する工具摩耗の増加量が抑制されるため、従来技術よりも高周波焼入れ用鋼の炭素量を増加しても工業生産が可能となる。また、Alは低温焼戻しマルテンサイト鋼の焼戻し挙動に対してSiと同様の効果を持ち、焼戻し軟化抵抗を顕著に増加することによって歯面疲労強度を向上するのに有効である。この効果を得るために、本発明の高周波焼入れ用鋼には、Alを0.050%超、添加する必要がある。一方、Alはフェライトを安定化するため、3.00%を超えて添加すると高周波焼入れ時にフェライトが残留し、均一なオーステナイト相が得られなくなる。この結果として、焼入れ後に均一なマルテンサイト組織が得られなくなる。従って、Al量を0.050%超~3.00%の範囲にする必要がある。Al量の好適な範囲は0.100~1.00%である。
Pは、不可避的不純物であり、オーステナイト粒界に偏析して、旧オーステナイト粒界を脆化させることによって粒界割れの原因となるので、できるだけ低減することが望ましい。このため、本発明では、高周波焼入れ用鋼のP量を0.050%以下の範囲にする必要がある。本発明の課題に関して特にP量の下限は無いが、P量を0.001%以下に制限するには過剰なコストがかかる。したがって、P量の好適な範囲は0.001~0.015%である。
Nは鋼中でAlと結合してAlNを形成し、AlNがオーステナイト結晶粒界をピン止めすることによって粒成長を抑制し、組織の粗大化を防止する働きがある。一般に、高周波加熱は加熱時間が極めて短時間であるため、積極的にAlNを利用しない場合でも結晶粒は粗大化しにくい。しかしながら結晶粒の微細化を積極的に図りたい場合はNを積極的に添加しても良い。一方、過剰に添加すると1000℃以上の高温域における延性が低下し、連続鋳造、圧延時の歩留まり低下の原因になる。このため、本発明では、高周波焼入れ用鋼のN量を0.0200%以下に制限する必要がある。N量の好適な範囲は0.0050~0.0120%である。
Oは酸化物系介在物を形成し、含有量が多い場合は疲労破壊の起点となる大きな介在物が増加し、疲労特性の低下の原因となるので、できるだけ低減することが望ましい。このため本発明では、高周波焼入れ用鋼のO量を0.0030%以下に制限する必要がある。本発明の課題に関して特にO量の下限は無いが、O量を0.0001%以下に制限するには過剰なコストがかかる。従って、O量の好適な範囲は0.0001~0.0015%以下である。
Bは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Bはオーステナイト中に固溶している状態において、微量で鋼の焼入性を大きく高める効果があるため、浸炭焼入れ時にマルテンサイト組織を得るために有効な元素である。この効果を得るために、本発明では、高周波焼入れ用鋼に0.0005%以上のBを添加してもよい。一方、0.0050%を超えて添加しても効果が飽和する。従ってBを添加する場合、B量を0.0005~0.0050%の範囲にする。B量の好適な範囲は0.0010~0.0025%である。なお、鋼中に一定量以上のNが存在している場合、BがNと結合してBNを形成し、固溶B量が減少することによって焼入性を高める効果が得られない場合があるため、Bを添加する場合にはNを固定するTiやAlを同時に適量添加することが望ましい。
Crは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Crはパーライト変態にあたって、ラメラ間隔を顕著に微細化させる効果があるので、粗形材の硬さが大きく増加し、加工性を劣化させる。また、θ炭化物中に濃化して安定化することによって、高周波焼入れ時の炭化物のオーステナイトへの溶け込みを阻害し、硬化層の硬さムラの原因となる。従って、Crを添加する場合は、Cr添加量を0.30%未満に制限する。一方、Si、Al添加量が多く、かつ焼鈍時間が長い場合にはθ炭化物が黒鉛化し、高周波焼入れ性が低下する場合がある。このため、これを防ぐ目的で、本発明では、高周波焼入れ用鋼にCrを少量添加しても良い。黒鉛化の防止に必要なCr量の下限値は0.05%である。従って、Crを添加する場合は、Cr添加量を0.05%~0.30%未満の範囲にする。Cr量の好適な範囲は0.10~0.20%である。
Moは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Moは鋼の焼入性を高める効果があるので、浸炭焼入れ時にマルテンサイト組織を得るために有効な元素である。この効果を得るために、Moを0.01%以上添加してもよい。一方、1.00%を超えて添加すると添加コストが過大となるとともに、粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化するため、工業生産上望ましくない。従ってMoを添加する場合、Mo量を0.01~1.00%の範囲にする。Mo量の好適な範囲は0.10~0.60%である。また、特に切削・鍛造時の加工性を少しでも劣化させずに、できるだけ焼入れ性を高めたいという場合は、Moを微量に添加することが好ましい。すなわち、添加量を0.01~0.05%未満の範囲にすれば、粗形材の硬さの上昇による加工性の低下は実質上無視できるほど小さなものとなり、なおかつ明確な焼入れ性向上効果も得られる。この理由は、Moは少量の添加でも比較的大きな焼入れ性向上効果を示す元素であるからである。特にBを複合添加すれば、微量の添加でも焼入れ性向上効果に対して大きな複合添加効果が得られる。
Cuは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Cuは鋼の焼入性を高める効果があるので、浸炭焼入れ時にマルテンサイト組織を得るために有効である。この効果を得るために、Cuを0.05%以上添加してもよい。しかしながら1.00%を超えて添加すると1000℃以上の高温域における延性が低下し、連続鋳造、圧延時の歩留まり低下の原因になる。従って、Cuを添加する場合、添加量を0.05~1.00%の範囲にする。添加Cu量の好適な範囲は0.010~0.50%である。なお、高温域の延性を改善するために、Cuを添加する場合にはCu添加量の1/2以上の量のNiを同時に添加することが望ましい。
Niは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Niは鋼の焼入性を高める効果があるので浸炭焼入れ時にマルテンサイト組織を得るために有効な元素である。この効果を得るために、Niを0.05%以上添加してもよい。一方、2.00%を超えて添加すると添加コストが過大となり、工業生産上望ましくない。従って、Niを添加する場合、添加量を0.05~2.00%の範囲にする。Ni量の好適な範囲は0.40~1.60%である。
Vは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Vは鋼中でN、Cと結合してV(C、N)を形成し、V(C、N)がオーステナイト結晶粒界をピン止めすることによって粒成長を抑制することによって組織の粗大化を防止する働きがある。この効果を得るために、Vを0.005%以上添加してもよい。一方、0.20%以上添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。また、V(C、N)の生成量が過大となり、高周波焼入れ時に硬化層の硬さムラの原因となる。従って、Vを添加する場合、添加量を0.005~0.20%未満の範囲にする。V量の好適な範囲は0.05~0.10%である。
Nbは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Nbは鋼中でN、Cと結合してNb(C、N)を形成し、Nb(C、N)がオーステナイト結晶粒界をピン止めすることによって粒成長を抑制することによって組織の粗大化を防止する働きがある。この効果を得るために、Nbを0.005%以上添加してもよい。一方、0.10%を超えて添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。また、Nb(C、N)の生成量が過大となり、高周波焼入れ時に硬化層の硬さムラの原因となる。従って、Nbを添加する場合、添加量を0.005~0.10%の範囲にする。Nb量の好適な範囲は0.010~0.050%である。
Tiは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Tiは鋼中でN、Cと結合してTi(C、N)を形成し、Ti(C、N)がオーステナイト結晶粒界をピン止めすることによって粒成長を抑制することによって組織の粗大化を防止する働きがある。この効果を得るために、Tiを0.005%以上添加してもよい。一方、0.10%を超えて添加すると粗形材の硬さの上昇を通じて部品の切削・鍛造等の加工を行うときの加工性が顕著に劣化する。また、Ti(C、N)の生成量が過大となり、高周波焼入れ時に硬化層の硬さムラの原因となる。従って、Tiを添加する場合、添加量を0.005~0.50%の範囲にする。Ti量の好適な範囲は0.015~0.050%である。
Ca、Zr、Mgは必要に応じて本発明の高周波焼入れ用鋼に添加可能な任意成分である。Ca、Zr、Mgは、共に、MnSの形態制御、及び切削時の切削工具表面における保護被膜形成を通じて鋼の被削性を向上する働きがある。この効果を得るために、Ca、Zr、Mgのいずれか1種又は2種以上をそれぞれ0.0005%以上添加してもよい。一方、0.0030%を超えて添加すると、粗大な酸化物や硫化物を形成して部品の疲労強度に悪影響を与える場合がある。従って、Ca、Zr、Mgを添加する場合、添加量は0.0005~0.0030%の範囲にする。Ca、Zr、Mg合計添加量の好適な範囲は0.0008~0.0020%である。
前述のように、Alは鋼中において固溶状態にある場合、鋼部品の切削加工において工具寿命を顕著に改善する効果を持つので、0.050%超~3.00%の範囲で添加する。一方、Alは鋼中のNと結びついてAlNを形成し、析出物の形態を取る場合がある。しかしながら析出物として存在しているAlは工具寿命の改善に有効ではない。特に本発明のように、熱間鍛造後に徐冷を行う場合や、切削加工前に焼鈍を施す場合は、熱間鍛造後に放冷する工程に比べてAlNが析出しやすい。従って、固溶状態のAl量を確実に確保するためには、AlをAlNを形成すると予測される量よりも過剰に添加する必要があり、そのためにはAlとNとの関係式を規定する必要がある。すなわち、固溶Al量の指標の式である「Al-(27/14)×N」の値が0.050%を超えていれば、確実に工具寿命の改善効果を得ることができる。本発明の高周波焼入れ用鋼について「Al-(27/14)×N」の理論的な上限値は3.00%であり、好適範囲は0.100~1.00%である。
高温、長時間の焼鈍を行う場合、高周波焼入れ用鋼の組成を調整し、下の(1)式で規定される黒鉛化定数CEを1.8以下にすることが好ましい。特に高温の焼鈍を行う場合は、CEを1.28以下にすることが更に好ましい。
CE=C+Si/3-Mn/12+Al/6+Cu/9+Ni/9-Cr/9-Mo/9+B … (1)
なお、式(1)で、C、Si、Mn、Al、Cu、Ni、Cr、Mo、Bは、高周波焼入れ用鋼に含まれる各元素の質量%を示す。
これに対して、本発明の高周波焼入れ用鋼では炭素量が0.75%を超える。このため、この鋼を用いて、粗形材を温間鍛造、熱間鍛造、または熱間圧延のいずれによって製造しても、粗形材の組織は大部分がパーライトでごく少量のフェライト組織を含むか、又は実質的に全て(95%以上)がパーライト組織となる。従って、このような粗形材の強度にはパーライト組織の強度が支配的な影響を持つ。パーライト組織の強度はパーライトのラメラ間隔に関係する。パーライトを主に含む鋼の軟質化のためには焼鈍によって微細なパーライトラメラの形態を変化させ、θ炭化物が粗分散した組織にすることが極めて有効である。すなわち、焼鈍による軟質化の効果は、低・中炭素鋼のフェライト及びパーライト組織の場合よりも、高炭素鋼のパーライト組織の場合の方が大きい。また、焼鈍の加熱温度が低いとパーライトラメラの形態がほとんど変化しないために十分な軟質化効果が得られない。このため、680℃以上の温度で焼鈍を行う必要がある。一般に、加熱温度が高いほど微細なパーライトラメラが崩れるとともにθ炭化物が粗分散する。しかし焼鈍温度が800℃を超える場合は、オーステナイトの生成量が多くなり、焼鈍温度から冷却されるときに再び微細なラメラを持つパーライトに変態するため、軟質化効果が得られなくなる。従って、焼鈍温度を680~800℃の範囲にする必要がある。好適な焼鈍温度の範囲は700~770℃である。焼鈍の加熱時間が短すぎるとパーライトラメラの形態がほとんど変化しないために十分な軟質化効果が得られないため、焼鈍の加熱を10分以上行う必要がある。他方、焼鈍の加熱を360分を超えて行う場合は生産性が低下するため、工業生産上望ましくない。従って、焼鈍の加熱時間は10~360分の範囲にする必要がある。焼鈍の加熱時間の好適な範囲は30~300分である。なお、焼鈍後の冷却条件については特に規定しないが、小さい冷却速度で冷却(徐冷)した方が鋼がより軟質化されるため、必要に応じて徐冷を行うことが望ましい。750~650℃の温度範囲の平均冷却速度の好適な範囲は300℃/時以下である。
上記のように徐冷を行う場合、冷却後の焼鈍を行わなくても良いが、徐冷に前述の条件の焼鈍を組み合わせて行っても良い。この場合、徐冷や焼鈍を単独で行う場合よりもさらに大きな軟質化の効果が得られる。
Claims (10)
- 質量%で、
C:0.75%超~1.20%、
Si:0.002~3.00%、
Mn:0.20~2.00%、
S:0.002~0.100%、
Al:0.050%超~3.00%を含有し、
P:0.050%以下、
N:0.0200%以下、
O:0.0030%以下に制限し、残部がFe及び不可避的不純物からなり、
AlおよびNの質量%の含有量が、Al-(27/14)×N>0.050%を満足することを特徴とする高周波焼入れ用鋼。 - 質量%で、B:0.0005~0.0050%を更に含有することを特徴とする請求項1に記載の高周波焼入れ用鋼。
- 質量%で、Cr:0.05~0.30%未満、Mo:0.01~1.00%、Cu:0.05~1.00%、Ni:0.05~2.00%の内の1種または2種以上を更に含有することを特徴とする請求項1または2に記載の高周波焼入れ用鋼。
- 質量%で、V:0.005~0.20%未満、Nb:0.005~0.10%、Ti:0.005~0.10%の内の1種または2種以上を更に含有することを特徴とする請求項1または2に記載の高周波焼入れ用鋼。
- 質量%で、Ca:0.0005~0.0030%、Zr:0.0005~0.0030%、Mg:0.0005~0.0030%の内の1種または2種以上を更に含有することを特徴とする請求項1または2に記載の高周波焼入れ用鋼。
- 請求項1または2に記載の高周波焼入れ用鋼の組成を持つ高周波焼入れ用粗形材であって、
前記高周波焼入れ用粗形材に含まれる平均粒径0.5μm以上の黒鉛粒の個数が40個/mm2以下であることを特徴とする高周波焼入れ用粗形材。 - 請求項1または2に記載の高周波焼入れ用鋼を用いて、温間加工または熱間加工、冷却、焼鈍の工程を順次行い、
前記焼鈍で焼鈍温度を680~800℃、焼鈍時間を10~360分の条件で行う、
ことを特徴とする高周波焼入れ用粗形材の製造方法。 - 請求項7に記載の高周波焼入れ用粗形材の製造方法であって、前記冷却中の、750~650℃の温度範囲の平均冷却速度が300℃/時以下であることを特徴とする高周波焼入れ用粗形材の製造方法。
- 請求項1または2に記載の高周波焼入れ用鋼を用いて、熱間加工、冷却の工程を順次行い、
前記冷却中の、750~650℃の温度範囲の平均冷却速度が300℃/時以下であることを特徴とする高周波焼入れ用粗形材の製造方法。 - 請求項1または2に記載の高周波焼入れ用鋼を用いて製造した高周波焼入れ鋼部品であって、
前記高周波焼入れ鋼部品の最表面から50μm深さの表層硬化部の硬さがHV650以上であり、
非高周波焼入れ部の硬さがHV180以上であり、
前記非高周波焼入れ部に存在している平均粒径0.5μm以上の黒鉛粒の個数が40個/mm2以下であることを特徴とする高周波焼入れ鋼部品。
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US9039962B2 (en) | 2015-05-26 |
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