WO2014017074A1 - 軟窒化用鋼および軟窒化部品ならびにこれらの製造方法 - Google Patents
軟窒化用鋼および軟窒化部品ならびにこれらの製造方法 Download PDFInfo
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- 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
- C23C8/28—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 more than one element being applied in one step
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- 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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- 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
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
Definitions
- the present invention relates to a nitrocarburizing steel, a nitrocarburized component obtained from the nitrocarburized steel, and a method for producing the same. is there.
- Mechanical structure parts such as automobile gears are usually required to have excellent fatigue characteristics and are subjected to surface hardening treatment.
- surface hardening treatment carburizing treatment, induction hardening treatment, nitriding treatment and the like are well known.
- the carburizing treatment allows C to penetrate and diffuse in the high-temperature austenite region, so that a deep hardening depth is obtained and effective in improving fatigue strength.
- heat treatment distortion occurs due to the carburizing treatment, it has been difficult to apply to parts that require strict dimensional accuracy from the viewpoint of quietness.
- the induction hardening process is a process in which the surface layer portion is quenched by induction heating, heat treatment distortion is generated and the dimensional accuracy is inferior as in the carburizing process.
- the nitriding treatment is a treatment for increasing the surface hardness by intruding and diffusing nitrogen in a relatively low temperature range below the Ac 1 transformation point, and thus there is no possibility that the heat treatment strain described above occurs.
- the treatment time is as long as 50 to 100 hours, and it is necessary to remove a brittle compound layer on the surface layer after the treatment.
- the carburizing process described above can increase the core hardness by quench hardening, the soft nitriding process is performed at a temperature below the transformation point of the steel, so the core hardness is low.
- the nitrocarburized material does not increase, and the fatigue strength is inferior to the carburized material.
- Patent Document 1 proposes a nitrocarburizing steel that can obtain high bending fatigue strength after nitrocarburizing treatment by including Ni, Al, Cr, Ti, or the like in the steel. ing. That is, this steel is age-hardened with Ni-Al, Ni-Ti intermetallic compound or Cu compound at the core by soft nitriding, while Cr, Al, Ti, etc. are contained in the nitrided layer at the surface layer. Bending fatigue strength is improved by precipitation hardening of nitrides and carbides.
- Patent Document 2 steel containing 0.5 to 2% of Cu is forged by hot forging and then air-cooled to obtain a ferrite-based structure in which Cu is a solid solution.
- a steel for soft nitriding in which excellent bending fatigue properties can be obtained after soft nitriding treatment by precipitation hardening of Cu therein and further using precipitation hardening of Ti, V, and Nb carbonitride.
- Patent Document 3 proposes a steel for soft nitriding in which Ti—Mo carbides and further carbides containing one or more of Nb, V, and W are dispersed.
- the steel for soft nitriding described in Patent Document 1 has a bending fatigue strength improved by precipitation hardening of Ni—Al, Ni—Ti intermetallic compounds and Cu, but it is difficult to say that workability is sufficient.
- the steel for soft nitriding described in Patent Document 2 has a problem of high production cost because it is necessary to add a relatively large amount of Cu, Ti, V, and Nb.
- the steel for soft nitriding described in Patent Document 3 contains a relatively large amount of Ti and Mo, there is still a problem of high cost.
- the present invention advantageously solves the above-described problems, and an object of the present invention is to provide a nitrocarburizing steel that is ensured in machinability by suppressing hardening before nitrocarburizing treatment, along with its manufacturing method. . Another object of the present invention is to provide a nitrocarburized component that can increase the core hardness by soft nitriding after machining and thereby improve the fatigue characteristics together with its manufacturing method.
- the gist configuration of the present invention is as follows. 1. % By mass C: 0.01% or more and less than 0.10%, Si: 1.0% or less, Mn: 0.5-3.0% P: 0.02% or less, S: 0.06% or less, Cr: 0.3-3.0% Mo: 0.005-0.4%, V: 0.02-0.5% Nb: 0.003-0.15%, Al: 0.005-0.2% and Sb: 0.0005-0.02%
- the balance is made of Fe and inevitable impurities, and the bainite phase satisfies an area ratio of more than 50% with respect to the entire structure.
- a soft nitriding part obtained by finishing the soft nitriding steel according to 1 above into a desired shape and then performing a soft nitriding treatment.
- the steel for soft nitriding obtained by the production method described in 4 above is finished into a desired shape and then subjected to soft nitriding at a soft nitriding temperature of 550 to 700 ° C. and a soft nitriding time of 10 minutes or more.
- a method for producing a soft nitrided component is described in detail.
- the present invention it is possible to obtain a soft nitriding steel with an inexpensive component system and excellent machinability, and after nitrocarburizing treatment, fatigue characteristics equal to or higher than those of JIS SCr420 material subjected to carburizing treatment are obtained. It is possible to obtain a soft nitrided part having the same.
- the soft nitriding component of the present invention is extremely useful when applied to machine structural components such as automobiles.
- C 0.01% or more and less than 0.10% C is added to form a bainite phase and ensure strength.
- the amount of C is less than 0.01%, not only a sufficient amount of bainite phase cannot be obtained, but also the amount of V and Nb precipitates becomes insufficient after nitrocarburizing treatment, making it difficult to ensure strength. To do.
- the hardness of the generated bainite phase is increased, and the machinability is lowered.
- it is 0.03% or more and less than 0.10% of range.
- Si 1.0% or less Si is added because it is effective for deoxidation and bainite phase generation. However, if it exceeds 1.0%, the machinability and cold workability deteriorate due to solid solution hardening in the ferrite phase and bainite phase. Therefore, 1.0% or less. Preferably it is 0.5% or less, More preferably, it is 0.3% or less. In order to effectively contribute Si to deoxidation, the Si addition amount is preferably 0.01% or more.
- Mn 0.5-3.0% Mn is added because it is effective in generating a bainite phase and improving the strength.
- the amount of Mn is less than 0.5%, the amount of bainite phase produced is reduced, and V and Nb precipitates are produced in the bainite phase before the soft nitriding treatment, so that the hardness before the soft nitriding treatment increases.
- the Mn content is 0.5% or more.
- it exceeds 3.0% the machinability and the cold workability are deteriorated.
- it is in the range of 0.5 to 2.5%, more preferably in the range of 0.6 to 2.0%.
- P 0.02% or less P segregates at austenite grain boundaries and lowers the grain boundary strength, thereby lowering strength and toughness. Therefore, it is desirable to suppress the P content as much as possible, but 0.02% is allowed. In addition, since it requires high cost to make P less than 0.001%, it may be industrially reduced to 0.001%.
- S 0.06% or less
- S is a useful element that forms MnS in steel and improves the machinability, but if it exceeds 0.06%, the toughness is impaired, so it is limited to 0.06% or less. Preferably it is 0.04% or less.
- S content 0.002% or more.
- Cr 0.3-3.0% Cr is added because it is effective for forming a bainite phase.
- the Cr content is less than 0.3%, the amount of bainite phase produced is reduced, and V and Nb precipitates are produced in the bainite phase before the soft nitriding treatment, so that the hardness before the soft nitriding treatment increases.
- the Cr content is 0.3% or more.
- it exceeds 3.0% the machinability and the cold workability are deteriorated.
- it is in the range of 0.5 to 2.0%, more preferably in the range of 0.5 to 1.5%.
- Mo 0.005-0.4%
- Mo has the effect of precipitating V and Nb precipitates finely and improving the strength of the nitrocarburized material, and is an important element in the present invention. It is also effective for the generation of a bainite phase.
- Mo is added in an amount of 0.005% or more.
- the range is 0.005 to 0.4%. Preferably it is in the range of 0.01 to 0.3%, more preferably in the range of 0.04 to 0.2%.
- V 0.02 to 0.5%
- V is an important element that improves the strength by forming fine precipitates with Nb and increasing the core hardness due to the temperature rise during soft nitriding. If the amount of V is less than 0.02%, it is difficult to obtain the desired effect, so the content is made 0.02% or more. On the other hand, if it exceeds 0.5%, the precipitates become coarse, and a sufficient strength improvement effect cannot be obtained. Preferably it is 0.03 to 0.3% of range, more preferably 0.03 to 0.25% of range.
- Nb 0.003-0.15%
- Nb is extremely effective in improving fatigue strength because fine precipitates are formed together with V to increase the core hardness due to temperature rise during soft nitriding. If the Nb content is less than 0.003%, it is difficult to obtain a desired effect, so the content is made 0.003% or more. On the other hand, if it exceeds 0.15%, the precipitates become coarse and a sufficient strength improvement effect cannot be obtained, so the content is made 0.15% or less. Preferably it is 0.02 to 0.12% of range.
- Al 0.005-0.2%
- Al is an element useful for improving the surface hardness and effective hardened layer depth after soft nitriding, and is positively added. Moreover, it is an element useful also for refine
- Sb 0.0005-0.02%
- Sb has the effect of promoting the formation of a bainite phase. If the amount added is less than 0.0005%, the effect of addition is poor, while adding more than 0.02% saturates the effect, leading to an increase in component cost and also causing a decrease in base material toughness due to segregation.
- Sb is limited to a range of 0.0005 to 0.02%. Preferably it is 0.0010 to 0.01% of range.
- components other than those described above are Fe and inevitable impurities.
- Ti adversely affects the precipitation strengthening of V and Nb and lowers the core hardness. Therefore, Ti should be avoided as much as possible. Preferably it is less than 0.010%, more preferably less than 0.005%.
- N is contained as an inevitable impurity, but when the amount of N increases, coarse VN is generated and toughness is lowered. Therefore, the upper limit is preferably made 0.02%.
- the reason why the steel structure of the soft nitriding steel in the present invention is limited to the above range will be described.
- the present invention attempts to disperse and precipitate V and Nb precipitates in the core portion other than the surface nitriding portion after the soft nitriding treatment, thereby increasing the core hardness and improving the fatigue strength after the soft nitriding treatment. Is.
- V and Nb precipitates exist before the nitrocarburizing treatment, it is disadvantageous from the viewpoint of machinability at the time of cutting usually performed before the nitrocarburizing treatment.
- the steel structure of the nitrocarburizing steel of the present invention that is, the steel structure before the nitronitriding treatment is mainly composed of a bainite phase.
- the bainite phase is more than 50% in terms of the area ratio with respect to the entire structure. Preferably it is more than 60%, more preferably more than 80%. Further, it may be 100%.
- a structure other than the bainite phase a ferrite phase, a pearlite phase, or the like can be considered, but it goes without saying that the smaller the structure, the better.
- the area ratio of each phase is determined by taking a test piece from the obtained soft nitriding steel, and corroding it with nital after polishing for a vertical cross section (L cross section) parallel to the rolling direction, and using an optical microscope or scanning electron Using a microscope (SEM), the type of phase is identified by cross-sectional structure observation (200 times optical microscope structure observation), and the area ratio of each phase is obtained.
- the soft nitriding steel of the present invention is subjected to soft nitriding treatment, and precipitates containing V and Nb are dispersed and precipitated in the bainite phase.
- the particle size of the precipitate containing V and Nb in the bainite phase is preferably less than 10 nm in order to contribute to precipitation strengthening after the soft nitriding treatment.
- the measurement limit of the particle size of the precipitate is about 1 nm.
- the number of precipitates is preferably 500 or more per 1 ⁇ m 2 for sufficiently strengthening the precipitation.
- the upper limit is preferably 10,000 per 1 ⁇ m 2 .
- FIG. 1 shows a typical manufacturing process for manufacturing a soft nitrided part using the soft nitriding steel (bar steel) according to the present invention.
- S1 is a steel bar manufacturing process as a raw material
- S2 is a conveying process
- S3 is a product (soft-nitriding component) finishing process.
- the steel ingot is hot-rolled into a steel bar in the steel bar manufacturing process (S1), and shipped after quality inspection.
- the steel bar is cut into a predetermined dimension, hot forging or cold forging is performed, and drilling or turning is performed as necessary.
- hot forging or cold forging is performed to obtain a product.
- soft nitriding is performed to obtain a product.
- the hot rolled material may be finished as it is by a cutting process such as turning or drilling, and then subjected to soft nitriding to obtain a product.
- cold correction may be performed after hot forging.
- the final product may be subjected to a coating treatment such as paint or plating.
- hot working mainly means hot rolling and hot forging, but hot forging may be further performed after hot rolling. Needless to say, cold forging may be performed after hot rolling.
- the hot working process immediately before the soft nitriding treatment is a hot rolling process, that is, when hot forging is not performed after hot rolling, the following conditions are satisfied in the hot rolling process.
- Rolling heating temperature 950-1250 ° C
- carbides remaining from the time of dissolution are dissolved so that fine precipitates are not deposited on the rolled material (bar steel used as a material for the hot forged parts) and the forgeability is not impaired.
- the rolling heating temperature is less than 950 ° C., the remaining carbides from the time of melting are not easily dissolved.
- the rolling heating temperature is 950 ° C. to 1250 ° C.
- Rolling finish temperature 800 ° C or more
- the rolling finish temperature is less than 800 ° C, a ferrite phase is generated, which is disadvantageous in generating a bainite phase that satisfies more than 50% of the area ratio of the entire structure before soft nitriding. Become. Also, the rolling load is increased. Therefore, the rolling finishing temperature is 800 ° C. or higher.
- the upper limit is preferably about 1100 ° C.
- the cooling rate after rolling is set to a rate exceeding 0.5 ° C./s, which is the critical cooling rate for obtaining fine precipitates.
- the upper limit is preferably about 200 ° C./s.
- the hot working process immediately before the soft nitriding process is a hot forging process, that is, when only hot forging is performed or when hot forging is performed after hot rolling, the hot forging process will be described below. Satisfy the conditions. When hot rolling is performed before hot forging, the hot rolling conditions do not necessarily satisfy the above hot rolling conditions.
- Hot forging conditions In this hot forging, fine precipitates do not precipitate from the viewpoint of cold straightening and machinability after hot forging because the bainite phase is more than 50% in terms of the area ratio with respect to the entire structure. Therefore, the heating temperature during hot forging is 950 to 1250 ° C., the forging finishing temperature is 800 ° C. or higher, and the cooling rate after forging is at least 0.5 ° C./s in the temperature range of at least 700 to 550 ° C. The upper limit is preferably about 200 ° C./s.
- soft nitriding treatment precipitation treatment
- the soft nitriding treatment is preferably performed at a soft nitriding temperature of 550 to 700 ° C. and a soft nitriding time of 10 minutes or more so as to precipitate fine precipitates.
- the soft nitriding temperature is in the range of 550 to 700 ° C.
- a sufficient amount of precipitates cannot be obtained unless the soft nitriding temperature is lower than 550 ° C., and if it exceeds 700 ° C., it becomes an austenitic region and soft nitriding becomes difficult. Because. More preferably, it is in the range of 550 to 630 ° C.
- a nitriding gas such as NH 3 and N 2
- a carburizing gas such as CO 2 and CO
- the hot forged material thus obtained was evaluated by a drill cutting test for machinability, particularly drillability. Using a hot forged material cut to 20 mm thickness as a test material, drill it with a JIS high-speed tool steel SKH51 6 mm ⁇ straight drill at 0.15 mm / rev, rotation speed 795 rpm, 5 holes per cross section, drill was evaluated by the total number of holes until cutting became impossible.
- hardness HV hardness HV
- the steel types A to O were further subjected to soft nitriding after the above hot forging.
- the hot forging material of steel type P was carburized for comparison.
- the carburizing treatment was performed by carburizing at 930 ° C. for 3 hours, holding at 850 ° C. for 40 minutes, oil cooling, and tempering at 170 ° C. for 1 hour.
- the heat treated material thus obtained was subjected to structure observation, hardness measurement, precipitate observation and fatigue property evaluation.
- phase type was identified and the area ratio of each phase was determined by the method described above, as before soft nitriding.
- the surface hardness of the heat-treated material was measured at a position of 0.05 mm from the surface, and the core hardness was measured at the center (core).
- the surface hardness and core hardness were both measured using a Vickers hardness tester in accordance with JIS Z 2244 with a test load of 2.94N (300gf), and the average value was measured for each surface hardness.
- HV and core hardness HV were measured using a Vickers hardness tester in accordance with JIS Z 2244 with a test load of 2.94N (300gf), and the average value was measured for each surface hardness.
- HV and core hardness HV was defined as the depth from the surface which becomes HV400, and was measured.
- a specimen for transmission electron microscope observation was prepared from the core of nitrocarburized material and carburized material by an electropolishing method using a twin jet method, and the obtained sample was a transmission type with an acceleration voltage of 200 kV.
- the precipitate was observed using an electron microscope.
- the composition of the observed precipitate was determined by an energy dispersive X-ray spectrometer (EDX).
- Fatigue property evaluation was performed by an Ono-type rotating bending fatigue test to determine fatigue strength.
- the fatigue test was carried out by collecting notched test pieces (notch R: 1.0 mm, notch diameter: 8 mm, stress concentration factor: 1.8) as test pieces from the above heat treated material.
- Table 2 shows the structure observation and hardness measurement results before and after soft nitriding, and the fatigue property evaluation results after soft nitriding.
- Nos. 1 to 6 are invention examples
- Nos. 7 to 16 are comparative examples
- No. 17 is a conventional example in which JIS-SCr420 equivalent steel is carburized.
- Invention Examples Nos. 1 to 6 are all excellent in fatigue strength as compared with Conventional Example No. 17 subjected to carburizing treatment. Further, the drilling workability before soft nitriding treatment of No. 1 to 6 is equal to or higher than that of the conventional example No. 17. Furthermore, as a result of observation of precipitates with a transmission electron microscope and investigation of the precipitate composition with an energy dispersive X-ray spectrometer (EDX), the nitrocarburized materials No. 1 to 6 contained V, It was confirmed that 500 or more fine precipitates containing Nb and having a particle diameter of less than 10 nm were dispersed and deposited per 1 ⁇ m 2 . From this result, it is considered that the nitrocarburized material according to the present invention exhibited high fatigue strength by precipitation strengthening due to the fine precipitates.
- EDX energy dispersive X-ray spectrometer
- Comparative Examples Nos. 7 to 16 are inferior in fatigue strength or drill workability because the component composition or the obtained steel structure is outside the scope of the present invention.
- No. 7 has a slow cooling rate after hot forging, so an appropriate amount of bainite phase cannot be obtained, and since the amount of fine precipitates produced by nitrocarburizing treatment is small, precipitation strengthening is insufficient. Compared with low fatigue strength.
- No. 8 since the C amount exceeds the appropriate range, the hardness of the hot forged material before the soft nitriding treatment is increased, and the drill workability is lowered.
- the amount of Si and Mn exceeds the appropriate ranges, so the hardness of the hot forging before soft nitriding increases, and the drillability is up to about 1/5 of the conventional No.17. It is falling.
- the steel structure of the hot forged material before soft nitriding is mainly composed of ferrite phase and pearlite phase. For this reason, V and Nb precipitates are precipitated in the structure, the hardness before the soft nitriding treatment is increased, and drill workability is lowered.
- the Cr content is less than the proper range, the steel structure of the hot forged material before soft nitriding is mainly composed of ferrite phase and pearlite phase. For this reason, V and Nb precipitates are precipitated in the structure, the hardness before the soft nitriding treatment is increased, and drill workability is lowered.
Abstract
Description
また、高周波焼入処理は、高周波誘導加熱により表層部を焼入れする処理であるため、やはり熱処理歪が発生し、浸炭処理と同様に寸法精度が劣る。
すなわち、この鋼は、軟窒化処理により、芯部についてはNi-Al、Ni-Ti系の金属間化合物あるいはCu化合物で時効硬化させる一方、表層部については窒化層中にCr,Al,Ti等の窒化物や炭化物を析出硬化させることで、曲げ疲労強度を向上させている。
特許文献3では、Ti-Mo炭化物、またそれらに更にNb,V,Wの一種または二種以上を含む炭化物を分散させた軟窒化用鋼が提案されている。
また、本発明は、機械加工後、軟窒化処理により芯部硬さを高め、もって疲労特性を向上させることができる、軟窒化部品を、その製造方法とともに提供することを目的とする。
その結果、鋼の成分組成としてVおよびNbを適正量含有させ、さらに鋼組織としてベイナイト相を面積率で50%超とすることにより、TiやCuといった比較的高価な元素を含有させずとも、優れた機械加工性が得られ、また軟窒化処理後には、芯部にVおよびNbを含む微細な析出物を分散析出させて芯部硬さを上昇させることにより、優れた疲労特性が得られることの知見を得た。
本発明は、上記の知見に基づき、さらに検討を加えた末に完成されたものである。
1.質量%で、
C:0.01%以上0.10%未満、
Si:1.0%以下、
Mn:0.5~3.0%、
P:0.02%以下、
S:0.06%以下、
Cr:0.3~3.0%、
Mo:0.005~0.4%、
V:0.02~0.5%、
Nb:0.003~0.15%、
Al:0.005~0.2%および
Sb:0.0005~0.02%
を含有し、残部はFeおよび不可避的不純物からなり、ベイナイト相が組織全体に対する面積率で50%超を満足することを特徴とする軟窒化用鋼。
C:0.01%以上0.10%未満、
Si:1.0%以下、
Mn:0.5~3.0%、
P:0.02%以下、
S:0.06%以下、
Cr:0.3~3.0%、
Mo:0.005~0.4%、
V:0.02~0.5%、
Nb:0.003~0.15%、
Al:0.005~0.2%および
Sb:0.0005~0.02%
を含有し、残部はFeおよび不可避的不純物からなる成分組成の鋼を、加熱温度:950~1250℃、仕上げ温度:800℃以上として熱間加工し、加工後、少なくとも700~550℃の温度域における冷却速度を0.5℃/s超として冷却することを特徴とする軟窒化用鋼の製造方法。
そして、本発明の軟窒化部品は、自動車等の機械構造部品に適用して極めて有用である。
まず、本発明において、成分組成を前記の範囲に限定した理由について説明する。なお、以下の成分組成を表す「%」は、特に断らない限り「質量%」を意味するものとする。
C:0.01%以上0.10%未満
Cは、ベイナイト相の生成および強度確保のために添加する。しかしながら、C量が0.01%未満の場合、十分な量のベイナイト相が得られないだけでなく、軟窒化処理後にVおよびNb析出物量が不足し、強度確保が困難となるため、0.01%以上とする。一方、0.10%以上添加すると、生成したベイナイト相の硬さが増加し、機械加工性が低下するため、0.10%未満とする。好ましくは0.03%以上0.10%未満の範囲である。
Siは、脱酸ならびにベイナイト相の生成に有効なため添加するが、1.0%を超えるとフェライト相およびベイナイト相に対する固溶硬化により、機械加工性および冷間加工性を劣化させるため1.0%以下とする。好ましくは0.5%以下、より好ましくは0.3%以下である。
なお、Siを脱酸に有効に寄与させるためには、Si添加量を0.01%以上とすることが好ましい。
Mnは、ベイナイト相の生成ならびに強度向上に有効なため添加する。しかしながら、Mn量が0.5%未満の場合、ベイナイト相の生成量が少なくなり、軟窒化処理前にVおよびNb析出物がベイナイト相で生成するため、軟窒化処理前の硬さが増加する。加えて、軟窒化処理後におけるVおよびNb析出物の絶対量が減少するため、軟窒化処理後の硬さが低下して強度確保が困難となる。従って、Mn量は0.5%以上とする。一方、3.0%を超えると機械加工性および冷間加工性を劣化させるので、3.0%以下とする。好ましくは0.5~2.5%の範囲、より好ましくは0.6~2.0%の範囲である。
Pは、オーステナイト粒界に偏析し、粒界強度を低下させることにより強度、靭性を低下させる。従って、Pの含有は極力抑制することが望ましいが、0.02%までは許容される。
なお、Pを0.001%未満とするには高いコストを要することから、工業的には0.001%まで低減すればよい。
Sは、鋼中でMnSを形成し、被削性を向上させる有用元素であるが、0.06%を超えて含有させると靭性を損なうため、0.06%以下に制限する。好ましくは0.04%以下である。
なお、Sによる被削性向上効果を発現させるためには、S含有量を0.002%以上とすることが好ましい。
Crは、ベイナイト相の生成に有効なため添加する。しかしながら、Cr量が0.3%未満の場合、ベイナイト相の生成量が少なくなり、軟窒化処理前にVおよびNb析出物がベイナイト相で生成するため、軟窒化処理前の硬さが増加する。加えて、軟窒化処理後におけるVおよびNb析出物の絶対量が減少するため、軟窒化処理後の硬さが低下して強度確保が困難となる。従って、Cr量は0.3%以上とする。一方、3.0%を超えると機械加工性および冷間加工性を劣化させるので、3.0%以下とする。好ましくは0.5~2.0%の範囲、より好ましくは0.5~1.5%の範囲である。
Moは、VおよびNb析出物を微細に析出させ、軟窒化処理材の強度を向上させる効果があり、本発明において重要な元素である。また、ベイナイト相の生成にも有効である。強度向上のため、Moは0.005%以上を添加するが、高価な元素のため0.4%を超えて添加すると、成分コストの上昇を招く。このため、0.005~0.4%の範囲とする。好ましくは0.01~0.3%の範囲、より好ましくは0.04~0.2%の範囲である。
Vは、軟窒化処理時の温度上昇により、Nbとともに微細析出物を形成して芯部硬さを増加させ、強度を向上させる重要な元素である。V量が0.02%未満では、所望の効果が得難いので、0.02%以上とする。一方、0.5%を超えると析出物が粗大化し、十分な強度向上効果が得られないため、0.5%以下とする。好ましくは0.03~0.3%の範囲、より好ましくは0.03~0.25%の範囲である。
Nbは、軟窒化処理時の温度上昇により、Vとともに微細析出物を形成して芯部硬さを増加させるため、疲労強度向上に極めて有効である。Nb量が0.003%未満では所望の効果が得難いので、0.003%以上とする。一方、0.15%を超えると析出物が粗大化し、十分な強度向上効果が得られないため、0.15%以下とする。好ましくは0.02~0.12%の範囲である。
Alは、軟窒化処理後の表面硬さおよび有効硬化層深さの向上に有用な元素であり、積極的に添加する。また、熱間鍛造時におけるオーステナイト粒成長を抑制することによって、組織を微細化し靭性を向上させる上でも有用な元素である。このような観点から、Alは0.005%以上添加する。一方、0.2%を超えて含有させてもその効果は飽和し、むしろ成分コストの上昇を招く不利が生じるので、0.2%以下に限定する。好ましくは0.020%以上0.1%以下の範囲であり、より好ましくは0.020%以上0.040%以下の範囲である。
Sbは、ベイナイト相の生成を促進する効果を有する。その添加量が0.0005%に満たないと添加効果に乏しく、一方0.02%を超えて添加しても効果が飽和し、成分コストの上昇を招くだけでなく、偏析により母材靭性の低下も生じるため、Sbは0.0005~0.02%の範囲に限定する。好ましくは0.0010~0.01%の範囲である。
なお、特にTiは、VおよびNbの析出強化に悪影響を及ぼし、芯部硬さを低下させるので、極力含有させないようにする。好ましくは0.010%未満、より好ましくは0.005%未満である。
また、Nは、不可避的不純物として含有されるが、N量が増大すると粗大なVNを生成し、靭性が低下するため、上限を0.02%とすることが好ましい。
ベイナイト相を組織全体に対する面積率で50%超
本発明では、ベイナイト相を組織全体に対する面積率で50%超とすることが、極めて重要である。
本発明は、軟窒化処理後に表層窒化部以外の芯部にはVおよびNb析出物を分散析出させ、これによって芯部硬さを上昇させ、軟窒化処理後の疲労強度を向上させようとするものである。
ここで、軟窒化処理前にVおよびNb析出物が存在していると、通常軟窒化処理前に行われる切削加工時の被削性の観点からは不利である。また、ベイナイト変態過程では、フェライト-パーライト変態過程に比べ、母相中へのVおよびNb析出物が生成しにくい。
従って、本発明の軟窒化用鋼の鋼組織、すなわち軟窒化処理前の鋼組織はベイナイト相を主体とする。具体的には、ベイナイト相を組織全体に対する面積率で50%超とする。好ましくは60%超、より好ましくは80%超である。また、100%であってもよい。
なお、ベイナイト相以外の組織としては、フェライト相やパーライト相等が考えられるが、これらの組織は少ないほど好ましいのは言うまでもない。
この理由は、表層軟窒化部以外の芯部組織中にVおよびNb析出物を分散析出させることで、芯部硬さが上昇し、軟窒化処理後の疲労強度が顕著に向上するからである。
ここに、ベイナイト相中のVおよびNbを含む析出物の粒径は10nm未満とすることが、軟窒化処理後の析出強化に寄与させる上で好ましい。なお、析出物の粒径の測定限界は、1nm程度である。
また、析出物の個数としては、1μm2当り500個以上存在することが十分に析出強化させる上で好ましい。一方、上限は1μm2当り10000個とすることが好ましい。
図1に、本発明に係る軟窒化用鋼(棒鋼)を用いて軟窒化部品を製造する代表的な製造工程を示す。ここで、S1は素材となる棒鋼製造工程、S2は搬送工程、S3は製品(軟窒化部品)仕上げ工程である。
まず、棒鋼製造工程(S1)で鋼塊を熱間圧延して棒鋼とし、品質検査後、出荷する。
そして、搬送(S2)後、製品(軟窒化部品)仕上げ工程(S3)で、該棒鋼を所定の寸法に切断し、熱間鍛造あるいは冷間鍛造を行い、必要に応じてドリル穿孔や旋削等の切削加工で所望の形状(例えば、ギア部品やシャフト部品)とした後、軟窒化処理を行って、製品とする。
また、熱間圧延材をそのまま旋削やドリル穿孔等の切削加工で所望の形状に仕上げ、その後軟窒化処理を行い製品とすることもある。なお、熱間鍛造の場合、熱間鍛造後に冷間矯正が行われる場合がある。また、最終製品にペンキやメッキ等の皮膜処理がなされる場合もある。
ここに、熱間加工とは、主に熱間圧延、熱間鍛造を意味するが、熱間圧延後さらに熱間鍛造を行ってもよい。なお、熱間圧延後、冷間鍛造を行ってもよいのは言うまでもない。
圧延加熱温度:950~1250℃
熱間圧延工程では、圧延材(熱間鍛造部品の素材となる棒鋼)に微細析出物が析出し鍛造性を損なわないよう、溶解時から残存する炭化物を固溶させる。
ここで、圧延加熱温度が950℃に満たないと、溶解時から残存する炭化物が固溶しづらくなる。一方1250℃を超えると、結晶粒が粗大化して鍛造性が悪化しやすくなる。このため、圧延加熱温度は950℃~1250℃とする。
圧延仕上げ温度が800℃未満の場合、フェライト相が生成するため、軟窒化処理前に組織全体に対する面積率で50%超を満足するベイナイト相を生成させる上で不利となる。また、圧延負荷も高くなる。従って、圧延仕上げ温度は800℃以上とする。なお、上限値については、1100℃程度とすることが好ましい。
鍛造前に微細析出物が析出し、鍛造性を損なわないようにするため、微細析出物の析出温度範囲である少なくとも700~550℃の温度域においては、圧延後の冷却速度を、微細析出物が得られる限界冷却速度である0.5℃/sを超える速度とする。なお、上限値については、200℃/s程度とすることが好ましい。
なお、熱間鍛造前に熱間圧延を行う場合には、熱間圧延条件としては必ずしも上記した熱延条件を満足していなくてもよい。
この熱間鍛造では、ベイナイト相を組織全体に対する面積率で50%超とするため、および、熱間鍛造後の冷間矯正や被削性の観点から微細析出物が析出しないようにするため、熱間鍛造時の加熱温度を950~1250℃、鍛造仕上げ温度を800℃以上、鍛造後の冷却速度を少なくとも700~550℃の温度域において0.5℃/s超とする。なお、上限値については、200℃/s程度とすることが好ましい。
軟窒化処理(析出処理)条件
軟窒化処理は、微細析出物を析出させるように、軟窒化処理温度を550~700℃、軟窒化処理時間を10分以上として行うことが好ましい。ここに、軟窒化処理温度を550~700℃の範囲とするのは、550℃に満たないと十分な量の析出物が得られず、700℃を超えるとオーステナイト域となり軟窒化が困難となるからである。より好ましくは550~630℃の範囲である。
なお、軟窒化処理ではNとCを同時に侵入・拡散させるので、NH3やN2といった浸窒性ガスと、CO2やCOといった浸炭性ガスの混合雰囲気、例えばNH3:N2:CO2=50:45:5の雰囲気で軟窒化処理を行えばよい。
表1に示す成分組成の鋼(鋼種A~P)を150kg、真空溶解炉にて溶製し、1150℃に加熱後、圧延仕上げ温度:970℃の条件で熱間圧延し、その後0.9℃/sの速度で室温まで冷却し、50mmφの棒鋼とした。なお、鋼種PはJIS SCr420に相当する鋼である。
なお、表1中の全鋼について、PおよびNは積極的に添加してはいない。よって、表1中のPおよびN含有量は、不可避的不純物として混入している値を示している。また、Tiについては、表1中の鋼種Nは添加したものであるが、その他の鋼種については積極的に添加していない。よって、表1中、鋼種A,B,C,D,E,F,G,H,I,J,K,L,M,OおよびPのTi含有量は、いずれも不可避的不純物として混入している値を示している。
これらの素材をさらに、1200℃に加熱後、仕上げ温度:1100℃の条件で熱間鍛造して、30mmφの棒鋼とし、その後、700~550℃の範囲を0.8℃/sの速度として、室温まで冷却した。なお、一部については、比較のため700~550℃の範囲を0.1℃/sの速度として、室温まで冷却した。
組織観察では、前述した方法により、相の種類を同定するとともに、各相の面積率を求めた。
軟窒化処理は、NH3:N2:CO2=50:45:5の雰囲気で525~620℃に加熱し、3.5時間保持することによって行った。
一方、浸炭処理は、930℃で3時間浸炭し、850℃に40分保持後、油冷し、さらに170℃、1時間焼戻すことにより行った。
かくして得られた熱処理材について、組織観察、硬度測定、析出物の観察および疲労特性評価を行った。
また、軟窒化材ならびに浸炭材の芯部から、透過型電子顕微鏡観察用の試料を、ツインジェット法を用いた電解研磨法により作成し、得られた試料について、加速電圧を200kVとした透過型電子顕微鏡を用いて析出物の観察を行った。さらに、観察される析出物の組成をエネルギー分散型X線分光装置(EDX)により求めた。
さらに、透過型電子顕微鏡による析出物の観察およびエネルギー分散型X線分光装置(EDX)による析出物組成の調査の結果、No.1~6の軟窒化処理材には、ベイナイト相中にV,Nbを含む粒径:10nm未満の微細な析出物が1μm2当り500個以上分散析出していることが確認できた。この結果から、本発明に従う軟窒化処理材は、上記微細な析出物による析出強化により、高い疲労強度を示したものと考えられる。
No.7は、熱間鍛造後の冷却速度が遅いため、適正量のベイナイト相が得られず、また軟窒化処理による微細析出物の生成量が少ないため、析出強化が不足し、発明例に比べ疲労強度が低い。
No.8は、C量が適正範囲を超えているため、軟窒化処理前の熱間鍛造材の硬さが増加し、ドリル加工性が低下している。
No.10は、Mn量が適正範囲に満たないため、軟窒化処理前の熱間鍛造材の鋼組織がフェライト相-パーライト相主体となっている。このため、組織中にVおよびNb析出物が析出して軟窒化処理前の硬さが増加し、ドリル加工性が低下している。
No.11は、Cr量が適正範囲に満たないため、軟窒化処理前の熱間鍛造材の鋼組織がにフェライト相-パーライト相主体となっている。このため、組織中にVおよびNb析出物が析出して軟窒化処理前の硬さが増加し、ドリル加工性が低下している。
No.13は、V量およびNb量が適正範囲に満たないため、軟窒化処理後の微細析出物の生成量が少なく、十分な芯部硬さが得られていない。このため、従来例No.17に比べて疲労強度が低い。
No.14は、Nb量が適正範囲に満たないため、軟窒化処理後の微細析出物の生成量が少なく、十分な芯部硬さが得られていない。このため、従来例No.17に比べて疲労強度が低い。
No.15は、本発明では不純物成分となるTiを多量に含むため、軟窒化処理後の微細析出物の生成量が少なく、十分な芯部硬さが得られていない。このため、従来例No.17に比べて疲労強度が低い。
No.16は、Al量が適正範囲に満たないため、十分な軟窒化処理後の表面硬さおよび有効硬化層深さが得られず、従来例No.17に比べて疲労強度が低い。
Claims (5)
- 質量%で、
C:0.01%以上0.10%未満、
Si:1.0%以下、
Mn:0.5~3.0%、
P:0.02%以下、
S:0.06%以下、
Cr:0.3~3.0%、
Mo:0.005~0.4%、
V:0.02~0.5%、
Nb:0.003~0.15%、
Al:0.005~0.2%および
Sb:0.0005~0.02%
を含有し、残部はFeおよび不可避的不純物からなり、ベイナイト相が組織全体に対する面積率で50%超を満足することを特徴とする軟窒化用鋼。 - 請求項1に記載の軟窒化用鋼を、所望の形状に仕上げたのち、軟窒化処理を施して得たことを特徴とする軟窒化部品。
- 前記軟窒化処理後、ベイナイト相中にVおよびNbを含む析出物が分散析出していることを特徴とする請求項2に記載の軟窒化部品。
- 質量%で、
C:0.01%以上0.10%未満、
Si:1.0%以下、
Mn:0.5~3.0%、
P:0.02%以下、
S:0.06%以下、
Cr:0.3~3.0%、
Mo:0.005~0.4%、
V:0.02~0.5%、
Nb:0.003~0.15%、
Al:0.005~0.2%および
Sb:0.0005~0.02%
を含有し、残部はFeおよび不可避的不純物からなる成分組成の鋼を、加熱温度:950~1250℃、仕上げ温度:800℃以上として熱間加工し、加工後、少なくとも700~550℃の温度域における冷却速度を0.5℃/s超として冷却することを特徴とする軟窒化用鋼の製造方法。 - 請求項4に記載の製造方法にて得られた軟窒化用鋼を、所望の形状に仕上げたのち、軟窒化処理温度:550~700℃、軟窒化処理時間:10分以上として軟窒化処理を施すことを特徴とする軟窒化部品の製造方法。
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