WO2011115255A1 - ばね用鋼および鋼材の表面処理方法 - Google Patents
ばね用鋼および鋼材の表面処理方法 Download PDFInfo
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- WO2011115255A1 WO2011115255A1 PCT/JP2011/056570 JP2011056570W WO2011115255A1 WO 2011115255 A1 WO2011115255 A1 WO 2011115255A1 JP 2011056570 W JP2011056570 W JP 2011056570W WO 2011115255 A1 WO2011115255 A1 WO 2011115255A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- 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
- C23C8/24—Nitriding
- C23C8/26—Nitriding of ferrous surfaces
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/06—Surface hardening
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
- C21D1/76—Adjusting the composition of the atmosphere
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- 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
- C23C8/28—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
- C23C8/30—Carbo-nitriding
- C23C8/32—Carbo-nitriding of ferrous surfaces
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- C—CHEMISTRY; METALLURGY
- 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
- C23C8/80—After-treatment
Definitions
- the present invention relates to a spring steel and a surface treatment method for a steel material, and more particularly, to a technique for forming a thick high-hardness layer without generating nitrogen compounds as much as possible on a surface layer by performing a nitriding treatment.
- valve springs for automobiles are required for steel for valve springs due to demands for reducing friction loss in valve systems and securing shock absorbing space in engine rooms, and to reduce the size and weight.
- the strength is increased.
- the fatigue strength of steel increases with hardness when the material hardness is about 400 HV or less, but the increase width becomes dull in a high hardness region exceeding about 400 HV, and tends to be saturated when the hardness further increases.
- One reason for this is thought to be an increase in notch sensitivity of steel due to increased hardness. Since the hardness of spring steel and springs in recent years exceeds 500 HV, it is considered that a significant increase in fatigue strength due to an increase in the hardness of the entire steel material cannot be expected. Therefore, the following methods have been proposed to solve such problems.
- Patent Document 1 proposes a spring excellent in fatigue characteristics in which the crystal grain size is refined by adding V, Nb, or the like, and the toughness of the steel material is improved.
- the spring described in this document has an effect of improving fatigue characteristics, there is a problem that the price of the steel material is increased because the additive elements as described above are expensive.
- Patent Document 2 high fatigue is achieved by quenching and tempering using high-frequency induction heating to refine the structure, finely disperse and precipitate carbides, suppress precipitation at grain boundaries, and improve the toughness of the steel material. Techniques for imparting strength have been proposed. However, in general, it is not easy to control the temperature of each part of the steel using high-frequency induction heating for a spring having a three-dimensional shape, resulting in high cost due to the complexity of management of heating conditions, etc. There is a problem of becoming.
- the composition of oxide-based inclusions in the steel material is SiO 2 : 30 to 60% by weight, Al 2 O 3 : 10 to 30%, CaO: 10 to 30%, MgO: 3 to 15%.
- spring steels having excellent fatigue characteristics with a grain size of 15 ⁇ m or less in equivalent circle diameter have been proposed.
- Patent Document 4 proposes a nitrogen-quenched product that has a nitrogen diffusion layer in which nitrogen is solid-solved from the surface to a predetermined depth without a brittle nitrogen compound in the surface layer, and has been subjected to quenching treatment, and a method for manufacturing the same. ing. According to this technique, even after nitriding, a nitrogen compound that can be a starting point of fracture is not formed, and the surface layer has high hardness, so that improvement in fatigue strength can be expected. However, in the example described in Patent Document 4, the thickness of the high hardness layer of the surface layer is very thin at about 0.06 mm at the maximum, which is insufficient for improving the fatigue strength.
- Patent Document 5 discloses a carburized gear part having a carburized layer having a surface hardness of 700 to 900 HV by carburizing in a reduced pressure atmosphere of 1 to 30 hPa.
- Such reduced pressure carburizing is a method for obtaining a thick high hardness layer by utilizing the fact that it can suppress grain boundary oxidation which can be a fracture starting point and has a high processing temperature (950 ° C. or higher), unlike gas carburizing conventionally widely used. It is said that.
- the carburization under reduced pressure uses the non-equilibrium decomposition reaction of acetylene, the carbon penetration amount cannot be controlled as in the case of gas carburizing.
- the present invention has been made in view of the above circumstances, and can be manufactured at low cost without the need for expensive additive elements or complicated temperature control, and the thickness of the surface nitrogen compound layer and carbon compound layer. It is an object of the present invention to provide a spring steel and a surface treatment method for a steel material that can reduce the thickness as much as possible and obtain the necessary hardness of the central portion and the thickness of the high hardness layer.
- the inventors of the present invention have intensively studied a method for forming a thick high-hardness layer without damaging the toughness of the central portion and hardly generating brittle nitrogen compounds and carbon compounds with respect to the spring ropes and springs.
- general carbon steel to which no special element is added is subjected to a chemical surface treatment at a predetermined temperature, followed by quenching and tempering, so that almost no brittle nitrogen and carbon compounds are formed on the surface layer. It was found that a steel material having a hardness with which a thick surface high hardness layer is formed and the central portion also maintains toughness can be obtained.
- the surface treatment method of the steel material of the present invention was made based on the above knowledge, and in terms of weight%, C: 0.27 to 0.48%, Si: 0.01 to 2.2%, Mn: 0.30
- a steel material satisfying ⁇ 1.0%, P: 0.035% or less, S: 0.035% or less, and the balance being iron and inevitable impurities, is heated to 3 to 1100 ° C or less of steel A, 50 a nitriding step of thickening the nitrogen and carbon in the surface layer by vol% or more of the NH 3 and the balance is contacted with the mixed gas atmosphere composed of inert gas and inevitable impurities, then to room temperature at 20 ° C. / sec or faster It has a quenching step for cooling and a tempering step for heating to 100 to 400 ° C.
- the spring steel and spring of the present invention are obtained by the above method, and by weight, C: 0.27 to 0.48%, Si: 0.01 to 2.2%, Mn: 0.00. 30 to 1.0%, P: 0.035% or less, S: 0.035% or less, the balance being the entire composition of Fe and inevitable impurities, and the thickness of the surface nitrogen compound layer and carbon compound layer being 2 ⁇ m or less, and the hardness of the central portion in the cross section is 500 to 700 HV, and the portion having a thickness of 0.1 to 1.0 mm inside the nitrogen compound and carbon compound layer is 100 to 100 from the hardness of the central portion. It is characterized by being a high hardness layer 500 HV larger. Further, the equivalent circle diameter of the transverse section is preferably 1.5 to 15.0 mm.
- the “cross section” is a section orthogonal to the longitudinal direction of the spring steel and the spring.
- C is an element necessary for ensuring the strength of steel that can withstand the load required for the spring by quenching and tempering.
- the hardness of the steel material tends to increase with an increase in the concentration of C.
- C it is necessary to make the concentration of 0.27% or more.
- the concentration of C is excessive, the hardness of the center portion after quenching exceeds 700 HV, and the toughness is significantly reduced.
- the hardness of the central portion can be lowered by tempering at a high temperature exceeding 400 ° C., but at the same time, a nitrogen compound or a carbon compound is generated in a solid solution layer of nitrogen and carbon. Therefore, even if low temperature tempering that does not generate a nitrogen compound or a carbon compound is performed, the C concentration is set to 0.48% or less so that the hardness of the center portion of the steel material is 700 HV or less.
- Si 0.01-2.2%
- Si is a deoxidizing element useful in steel refining, and it is necessary to add 0.01% or more.
- Si is also a solid solution strengthening element and is an effective element for obtaining high strength.
- the Si concentration is excessive, the workability is deteriorated, so that it is set to 2.2% or less.
- Mn 0.30 to 1.0%
- Mn is added as a deoxidizing element, but contributes to solid solution strengthening and hardenability improvement, so 0.30% or more is added.
- concentration of Mn is excessive, segregation occurs and the workability is liable to be lowered.
- P 0.035% or less
- S 0.035% or less
- the high hardness layer of the present invention is heated to a temperature of 3 points or more and 1100 ° C. or less of steel, and is brought into contact with a mixed gas atmosphere consisting of 50% by volume of NH 3 and the balance of inert gas and inevitable impurities. Is formed by performing a nitriding step for concentrating the surface layer, a quenching step for cooling to room temperature at a rate of 20 ° C./second or more, and a tempering step for heating to 100 to 400 ° C.
- a The structure of the steel material before heating to 3 points or more is not particularly limited. For example, it is possible to use a hot-forged or drawn steel bar material as a raw material. Below, the reason for limitation in each process is demonstrated.
- a mixed gas atmosphere consisting of 50% by volume or more of NH 3 and the balance of an inert gas.
- the concentration of NH 3 exceeds 90% by volume, a nitrogen compound and a carbon compound are likely to be formed, so 80 to 90% by volume is preferable.
- the heating time in the nitriding process is preferably 15 to 110 minutes. If the heating time is less than 15 minutes, the desired thickness of the high hardness layer may not be obtained. If the heating time exceeds 110 minutes, a nitrogen compound or carbon compound thicker than 2 ⁇ m is easily formed on the surface layer, which is the origin of cracks. It becomes. Further, when nitrogen compounds or carbon compounds thicker than 2 ⁇ m are formed, it is very difficult to completely remove these compounds even if shot pinning is performed.
- the temperature, mixed gas composition and time of the nitriding process are such that nitrogen and carbon that have infiltrated the surface of the steel material quickly diffuse into the interior, thereby suppressing the formation of nitrogen compounds and carbon compounds on the surface layer, and thick high hardness This is an important condition for forming the layer.
- Atomic N is expected to be in a radical state with unpaired electrons.
- This radical N maintains the radical state for some reason even if it enters and dissolves in the steel, and is an electron beam microanalyzer (EPMA manufactured by Shimadzu Corporation) used for elemental analysis described in the embodiment for carrying out the invention. 1600), it is considered that some change occurs in the wavelength of characteristic X-rays originally obtained from N, and this may be detected as carbon in the analysis.
- the cooling rate to room temperature is preferably 50 ° C./second or more.
- Tempering process In the martensitic structure after quenching at the center of the steel material, tempering is performed because defects such as setting cracks occur due to strain caused by quenching, and the toughness is extremely low and the load resistance as a spring cannot be secured. Tempering needs to be performed at 100 ° C. or higher in order to reduce strain at the center of the steel material. On the other hand, when the tempering temperature exceeds 400 ° C., the hardness of the central portion of the steel material becomes low and cannot withstand the load of the spring. Moreover, since nitrogen and carbon which were dissolved are forming a compound, it is not preferable.
- the hardness of the steel center portion needs to be 500 HV or more in order to ensure the strength that can withstand the load required for the spring.
- the hardness is excessively high, the notch sensitivity of the steel material itself is increased and the fatigue strength is lowered, so that it is suppressed to 700 HV or less.
- the surface high-hardness layer is very effective for suppressing the occurrence of cracks, and the hardness of the high-hardness layer needs to be 100 HV or more larger than the hardness of the central portion. However, if the hardness of the high hardness layer is too high, it becomes extremely brittle, so the upper limit of the increase width is 500 HV or less.
- the thickness of the high hardness layer is required to be 0.1 mm or more in order to suppress the occurrence of cracks, and preferably 0.3 mm or more. On the other hand, if the thickness of the high hardness layer is too large, the toughness of the steel material itself is reduced, so the thickness is suppressed to 1.0 mm or less.
- the nitrogen compound and the carbon compound layer have a nitrogen-containing layer in which the sum of the average concentration of nitrogen and carbon is 0.1 to 1.5% by weight more than the total concentration of the entire composition of nitrogen and carbon. It is desirable. Further, the thickness of the nitriding layer is preferably 0.1 mm or more, and more preferably 0.1 to 0.5 mm. Note that the nitrous layer does not necessarily coincide with the high hardness layer. By increasing the total concentration of nitrogen and carbon in the nitrous layer by 0.1% or more from the average concentration of the steel material, the hardness is increased by solid solution strengthening of nitrogen and carbon.
- One feature of the present invention is that solid nitrogen and carbon are dissolved in the surface layer, but since nitrogen has a higher maximum solid solution concentration in steel than carbon, a high hardness layer containing more nitrogen is thicker. To be efficient. On the other hand, an increase in nitrogen and carbon concentration cause a decrease in the martensite transformation start temperature (M S point), thereby the excess residual Oh after quenching - generates austenite to lower the hardness. For this reason, the increase in the total concentration of nitrogen and carbon in the surface layer is desirably 1.5% or less. If the thickness of the nitrous layer is less than 0.1 mm, it is difficult to obtain the above effect sufficiently. If the thickness of the nitriding layer exceeds 0.5 mm, the steel material itself tends to be deteriorated in toughness, so 0.5 mm or less is more desirable.
- the present invention can be manufactured at low cost without requiring expensive additive elements and complicated temperature control, and the thickness of the surface nitrogen compound layer and carbon compound layer is reduced as much as possible.
- the effect that the hardness of a center part and the thickness of a high-hardness layer can be obtained is acquired.
- Nitrogen treatment was performed on round steel bars with a diameter of 4 mm of A and B having the average chemical composition described in Table 1 under the conditions described in Table 2, and then cooled to room temperature at a rate of 20 ° C./second or more. Quenching followed by tempering for 60 minutes.
- Various properties of the steel material thus obtained were investigated in the following manner.
- the carbon concentration of the steel material exceeds the range of the present invention, and since tempering is not performed, the hardness of the central portion is too high and the toughness of the steel material is inferior. Further, since the nitriding time exceeds 100 minutes, a nitrogen compound and a carbon compound are formed in a surface layer of 8 ⁇ m or more, which promotes the generation of cracks.
- the present invention can be widely applied to valve springs and suspension springs for automobiles or springs for uses other than automobiles.
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Abstract
Description
Cは焼入れ焼戻しによりばねに必要な荷重に耐え得る鋼の強度を確保するために必要な元素である。鋼材の硬さはCの濃度の増加に伴って高くなる傾向があるが、本発明の表面処理方法において400℃の焼戻し後でも鋼材の中心部の硬さを500HV以上とするためには、Cの濃度を0.27%以上とすることが必要である。一方、Cの濃度が過剰になると、焼入れ後の中心部の硬さが700HVを超え、靭性が著しく低下する。これに対し400℃を超える高温の焼戻しにより中心部の硬さ低下させることはできるが、同時に窒素および炭素の固溶層において窒素化合物や炭素化合物が生じてしまう。そこで、窒素化合物や炭素化合物を生じさせない程度の低温焼戻しを行っても鋼材の中心部の硬さを700HV以下とするためにCの濃度は0.48%以下とする。
Siは鋼精錬において有用な脱酸元素であり0.01%以上を添加する必要がある。また、Siは固溶強化元素でもあり、高強度を得るために有効な元素であるが、Siの濃度が過剰であると加工性の低下を招くため2.2%以下とする。
Mnは脱酸元素として添加されるが、固溶強化や焼入れ性の向上にも寄与するため、0.30%以上を添加する。一方、Mnの濃度が過剰であると偏析が生じ加工性が低下し易くなるため、1.0%以下とする。
PおよびSは、粒界偏析による粒界破壊を助長する元素であるため、濃度は低いほうが望ましく、上限は0.035%とする。PおよびSの濃度は、0.01%以下であることが好ましい。
鋼材の中心部をオーステナイト化するため、鋼材をA3点以上に加熱する必要がある。一方、加熱温度が高過ぎると、NH3ガスが導入後直ちに分解され、被処理材への窒素および炭素の侵入(炭素の侵入については後述する)が著しく抑制されるため、加熱温度は1100℃を上限とする。望ましくは850~1000℃が良い。
浸窒工程後の焼入れ工程では、室温までの冷却速度は速いほど良く、20℃/秒以上の冷却速度で行う必要がある。冷却速度が20℃/秒未満では、冷却途中でパ-ライトが生成し、焼入れが不完全となって所望の硬さを得ることができない。室温までの冷却速度は、50℃/秒以上であることが好ましい。
鋼材の中心部が焼入れ後のマルテンサイト組織では、焼入れによって生じたひずみにより置き割れなどの不具合が生じるとともに、靭性が著しく低くばねとしての耐荷重を確保できないため焼戻しを行う。焼戻しは、鋼材の中心部のひずみを低減するため100℃以上で行う必要がある。一方、焼戻し温度が400℃を超えると、鋼材の中心部の硬度が低くなりばねの荷重に耐えられない。また、固溶していた窒素や炭素が化合物を形成するため好ましくない。
[表面の窒素化合物層および炭素化合物層の厚さ]
窒素化合物や炭素化合物は脆く靭性が乏しいため、それらを表面に形成すると亀裂の発生を促進する。したがって、窒素化合物および炭素化合物はある程度許容されるものの、それらの厚さの上限は2μmであり、1μm以下が好ましい。
鋼材中心部の硬さは、ばねに必要な荷重に耐え得る強度を確保するために500HV以上必要である。一方、硬さが過剰に高い場合は鋼材自体の切欠き感受性が増加し、疲労強度が低下するため、700HV以下に抑える。
表層の高硬度層は亀裂の発生を抑制するために非常に効果的であり、高硬度層の硬さは中心部の硬さより100HV以上大きいことが必要である。しかしながら、高硬度層の硬さが高過ぎると著しく脆くなるため、増加幅の上限は500HV以下である。
高硬度層の厚さは、亀裂の発生を抑制するため0.1mm以上必要であり、望ましくは0.3mm以上が良い。一方、上記高硬度層の厚さが厚過ぎると鋼材自体の靭性低下を招くため厚さは1.0mm以下に抑制する。
本発明では、窒素化合物および炭素化合物層より内側に、窒素および炭素の平均含有濃度の合計が窒素および炭素の全体組成の合計濃度よりも0.1~1.5重量%多い浸窒層を有することが望ましい。また、この浸窒層の厚さは、0.1mm以上であることが望ましく、0.1~0.5mmであることがより望ましい。なお、この浸窒層は、高硬度層と必ずしも一致する必要はない。浸窒層における窒素および炭素の合計濃度を鋼材の平均濃度より0.1%以上多くすることにより、窒素および炭素の固溶強化による硬さの増加をもたらす。本発明は表層に窒素および炭素を固溶していることが特徴の一つであるが、窒素は炭素よりも鋼に対する最大固溶濃度が高いため、窒素をより多く含むことが厚い高硬度層を得るために効率的である。一方、窒素および炭素濃度の増加はマルテンサイト変態開始温度(MS点)の低下を招き、これにより焼入れ後に過剰な残留オ-ステナイトを生成し硬さを低下させる。このため、表層における窒素および炭素の合計濃度の増加は1.5%以下が望ましい。浸窒層の厚さは、0.1mm未満であると、上記効果を十分に得難い。なお、浸窒層の厚さが0.5mmを超えると、鋼材自体の靭性の低下を招き易くなるため、0.5mm以下がより望ましい。
丸棒の外周側面に対しX線回折プロファイルを測定し、窒素化合物および炭素化合物に相当するピ-クの有無から鋼材の表面における化合物の有無を判定した。また、その厚さは、EPMAを用いた窒素および炭素の元素分布から測定した。
横断面において、鋼材の中心の位置で硬さを測定した。
横断面において、鋼材の外周表面から中心に向かって硬さを測定し、中心部の硬さより100~500HV大きい高硬度層に対し、表面からの深さを測定した。
上記高硬度層について、鋼材の外周表面から中心に向かって0.025mm間隔で硬さを測定しその平均値を求めた。
鋼材の横断面において、EPMAを用いて窒素および炭素の元素分布(定量値)をそれぞれ測定し、両元素の合計量が窒素および炭素の全体組成の合計(鋼材中心部の合計濃度)よりも0.1~1.5%多くなる領域(浸窒層)に対し厚さを測定した。
Claims (9)
- 重量%で、C:0.27~0.48%、Si:0.01~2.2%、Mn:0.30~1.0%、P:0.035%以下、S:0.035%以下を満たし、残部が鉄及び不可避不純物からなる全体組成の鋼材に対し、鋼のA3点以上1100℃以下に加熱し、50体積%以上のNH3と残部が不活性ガスおよび不可避不純物からなる混合ガス雰囲気に接触させることで窒素および炭素を表層に濃化させる浸窒工程と、次いで20℃/秒以上の速度で室温まで冷却する焼入れ工程と、次いで100~400℃に加熱する焼戻し工程を有することを特徴とする鋼材の表面処理方法。
- 前記浸窒工程において、加熱温度が850~1000℃であり、かつ加熱時間が15~110分であることを特徴とする請求項1に記載の鋼材の表面処理方法。
- 前記浸窒工程において、前記混合ガス雰囲気のNH3濃度が80~90体積%であることを特徴とする請求項1または2に記載の鋼材の表面処理方法。
- 重量%で、C:0.27~0.48%、Si:0.01~2.2%、Mn:0.30~1.0%、P:0.035%以下、S:0.035%以下、残部がFe及び不可避不純物からなる全体組成を有し、表面の窒素化合物層および炭素化合物層の厚さが2μm以下であり、かつ横断面において中心部の硬さが500~700HVであり、前記窒素化合物および前記炭素化合物層より内側の厚さ0.1~1.0mmの部分が、前記中心部の硬さより100~500HV大きい高硬度層とされていることを特徴とするばね用鋼。
- 前記高硬度層の厚さが0.3mm~1.0mmであることを特徴とする請求項4に記載のばね用鋼。
- 前記窒素化合物および前記炭素化合物層より内側に、窒素および炭素の平均含有濃度の合計が窒素および炭素の全体組成の合計濃度よりも0.1~1.5重量%多い浸窒層を厚さ0.1mm以上有することを特徴とする請求項4または5に記載のばね用鋼。
- 前記浸窒層の厚さが0.1~0.5mmであることを特徴とする請求項4~6のいずれかに記載のばね用鋼。
- 横断面の円相当径が1.5~15.0mmであることを特徴とする請求項4~7のいずれかに記載のばね用鋼。
- 請求項4~8のいずれかに記載のばね用鋼で製造されたばね。
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US13/581,962 US9469895B2 (en) | 2010-03-18 | 2011-03-18 | Spring steel and surface treatment method for steel material |
CN201180014482.0A CN102791890B (zh) | 2010-03-18 | 2011-03-18 | 弹簧用钢及钢材的表面处理方法 |
JP2012505766A JP5632454B2 (ja) | 2010-03-18 | 2011-03-18 | ばね用鋼および鋼材の表面処理方法 |
EP11756438.5A EP2548976A4 (en) | 2010-03-18 | 2011-03-18 | SPRING STEEL AND SURFACE TREATMENT METHOD FOR A STEEL MATERIAL |
KR1020127026523A KR20130005286A (ko) | 2010-03-18 | 2011-03-18 | 스프링용 강 및 강재의 표면 처리 방법 |
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Cited By (2)
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US9080233B2 (en) | 2010-04-14 | 2015-07-14 | Nhk Spring Co., Ltd. | Spring and method for producing same |
KR20230093723A (ko) | 2021-12-20 | 2023-06-27 | 주식회사 포스코 | 내구성이 우수한 고탄소 강판 및 그 제조방법, 산업용 또는 자동차용 부품 |
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EP2942413B1 (en) * | 2013-03-12 | 2018-08-08 | Honda Motor Co., Ltd. | Steel wire for spring and method for manufacturing same |
WO2015073098A2 (en) * | 2013-08-27 | 2015-05-21 | University Of Virginia Patent Foundation | Three-dimensional space frames assembled from component pieces and methods for making the same |
CN105950834A (zh) * | 2016-06-13 | 2016-09-21 | 张家港阿斯恩表面工程科技有限公司 | 一种不锈钢机加工制品的表面处理工艺 |
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CN102791890B (zh) | 2014-05-28 |
JPWO2011115255A1 (ja) | 2013-07-04 |
EP2548976A4 (en) | 2014-10-01 |
US9469895B2 (en) | 2016-10-18 |
KR20130005286A (ko) | 2013-01-15 |
US20120318407A1 (en) | 2012-12-20 |
JP5632454B2 (ja) | 2014-11-26 |
CN102791890A (zh) | 2012-11-21 |
EP2548976A1 (en) | 2013-01-23 |
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