JP2006348321A - Steel for nitriding treatment - Google Patents

Steel for nitriding treatment Download PDF

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JP2006348321A
JP2006348321A JP2005173112A JP2005173112A JP2006348321A JP 2006348321 A JP2006348321 A JP 2006348321A JP 2005173112 A JP2005173112 A JP 2005173112A JP 2005173112 A JP2005173112 A JP 2005173112A JP 2006348321 A JP2006348321 A JP 2006348321A
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JP4737601B2 (en
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Yasushi Hiraoka
泰 平岡
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Daido Steel Co Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for nitriding treatment, which can be subjected to high temperature nitriding treatment without the addition of a large quantity of Ti, to obtain high hardening depth, and has high surface hardness and satisfactory fatigue properties. <P>SOLUTION: The steel for nitriding treatment contains, by mass, 0.05 to 0.60% C, 0.03 to 3.0% Si, 0.01 to 3.5% Mn, 0.10 to 5.00% Cr, 0.05 to 3.0% Mo, 0.1 to 3.0% V, ≤0.5% Ti, 0.001 to 3.0% Al and 0.005 to 0.025% N, and the balance Fe with inevitable impurities, and satisfies inequality 1 of [Si%]+[Mo%]+2×([V%]+[Ti%]+[Al%])≥2.3 and inequality 2 of 242×[Ti%]+230×[Al%]+100×[V%]≥150. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は、窒化処理を施すことにより、例えば歯車部品といった高面圧が負荷された状態で長時間使用される自動車等の部品に好適な窒化処理用鋼に関する。   The present invention relates to a steel for nitriding which is suitable for parts such as automobiles which are used for a long time in a state where high surface pressure is applied, for example, gear parts by performing nitriding.

自動車用の変速機に使用される歯車部品は、高面圧が負荷された状態で長時間継続して使用されるため、耐摩耗性,耐ピッチング特性,歯元曲げ疲労強度など厳しい特性が要求される。このような要求を満たすため、従来から、JIS規格のSCr420HやSCM420Hのような肌焼き鋼を歯車形状に成形した後、浸炭焼入れ焼戻ししたものが用いられている。浸炭処理を実施するのは、高い表面硬さ(表面硬度Hv700以上)と、高面圧に耐えるに十分な硬化深さ(Hv550以上で0.40mm以上)を容易に得ることができるからである。しかしながら、浸炭処理は、変態点を超えた温度での加熱が必須となるため、処理後に熱歪や変態歪が発生して、部品の接触部の形状が不均一となってノイズの発生原因となる。このため、浸炭処理後の仕上げ加工作業が必要になり、品質や生産性の低下、コストアップを招くという問題が生ずる。   Gear parts used in automobile transmissions require strict characteristics such as wear resistance, anti-pitting characteristics, and root bending fatigue strength because they are used continuously for a long time under high surface pressure. Is done. In order to satisfy these requirements, conventionally, case-hardened steel such as JIS standard SCr420H and SCM420H is formed into a gear shape and then carburized, quenched and tempered. Carburizing treatment is performed because high surface hardness (surface hardness Hv 700 or more) and a curing depth sufficient to withstand high surface pressure (Hv 550 or more and 0.40 mm or more) can be easily obtained. . However, since the carburizing process requires heating at a temperature exceeding the transformation point, thermal distortion or transformation strain occurs after the treatment, and the shape of the contact part of the part becomes non-uniform, causing noise. Become. For this reason, finishing work after the carburizing process is required, resulting in a problem that quality and productivity are reduced and costs are increased.

この浸炭処理における歪の問題を解決するための表面硬化方法として、従来から窒化処理が検討されている。窒化処理は、浸炭処理と異なり変態温度以下(500〜600℃程度)の加熱で処理することから、歪については浸炭処理と比較して小さく抑えられるため、歪の問題を重視しなければならない部品に対して従来から積極的に利用されている。窒化処理による硬化原理は、母相フェライト相中にNが侵入拡散して、Nによる固溶強化や、合金元素との窒化物の形成によってひずみを形成し、硬化層が形成されるためと考えられている。そこで、かかる硬化原理を利用するため、窒化処理は、変態点以下の温度で実施され、高温で生成するγ相や、窒化処理後の冷却によって生ずるマルテンサイト相の生成を避けている。変態点を超える温度で窒化処理を行って、表面にγ相が生成したり、窒化処理後の冷却によってマルテンサイト相が生成すると、窒化処理による硬化原理が得られず、適正な表面硬さが得られないうえ、熱処理歪みの増大や機械的特性の低下を招いてしまう。   Conventionally, nitriding has been studied as a surface hardening method for solving the problem of distortion in the carburizing treatment. Unlike carburizing, nitriding is processed by heating at a transformation temperature or lower (about 500 to 600 ° C.), so distortion can be suppressed to a small level compared to carburizing. However, it has been actively used in the past. The principle of hardening by nitriding is considered to be that N penetrates and diffuses into the parent phase ferrite phase and strain is formed by solid solution strengthening by N or formation of nitrides with alloy elements, thereby forming a hardened layer. It has been. Therefore, in order to utilize such a curing principle, the nitriding treatment is performed at a temperature below the transformation point, and the generation of the γ phase generated at a high temperature and the martensite phase generated by cooling after the nitriding treatment are avoided. When nitriding is performed at a temperature exceeding the transformation point and a γ phase is generated on the surface or a martensite phase is generated by cooling after nitriding, the hardening principle by nitriding cannot be obtained, and an appropriate surface hardness is obtained. In addition to this, the heat treatment distortion increases and the mechanical properties decrease.

しかしながら、通常広く行われている窒化処理方法であるガス窒化処理やガス軟窒化処理は、浸炭処理に比較して歪を小さく抑えられるという利点がある一方で、処理温度が低いことから、硬化深さが表面から0.15mm程度(全硬化深さ;ガス軟窒化処理で通常の処理温度である約580℃×4hrで処理した場合)と浸炭処理と比べてかなり浅く、高面圧が継続して負荷される環境には好ましくない。また、高面圧環境での使用を可能とするために、例えば0.3mmを超える深い硬化深さを得ようとすると、処理時間を大幅に長くする必要があり(10時間以上)、浸炭処理に比べて長時間の処理となって、生産性が著しく阻害される。このような問題点があるため、浸炭処理で歪が発生して問題となっている部品に窒化処理用鋼を適用できない現状にある。   However, gas nitriding treatment and gas soft nitriding treatment, which are usually widely used nitriding methods, have the advantage that distortion can be kept small compared to carburizing treatment. Is about 0.15mm from the surface (total hardening depth; when processed at about 580 ° C x 4hr, which is the usual processing temperature in gas soft nitriding), which is considerably shallower than the carburizing process and the high surface pressure continues. It is not preferable for an environment that is loaded with Further, in order to enable use in a high surface pressure environment, for example, when trying to obtain a deep curing depth exceeding 0.3 mm, it is necessary to significantly increase the processing time (10 hours or more), and carburizing treatment. As a result, the productivity is significantly inhibited. Because of such problems, the steel for nitriding cannot be applied to parts that are problematic due to distortion caused by carburizing.

特開2004−300472号公報JP 2004-300472 A

特許文献1には、窒化処理の迅速化を実現する窒化鋼が提案されている。これは、軟窒化処理の短時間化を図るため、Tiを多量に添加して変態点を高くし、高温で窒化処理中のCおよびNの拡散促進して、高い表面硬さを得ることを特徴とするものである。しかしながら、このような鋼種では高Ti成分であるため、真空溶解法など特殊な溶解方法を必要とし量産鋼としては使用困難である。さらに、このような高Ti鋼を用いるとTiCの析出量が増えるため、疲労強度の著しい低下を招く。   Patent Document 1 proposes a nitrided steel that realizes rapid nitriding treatment. In order to shorten the time of soft nitriding treatment, a large amount of Ti is added to increase the transformation point, and diffusion of C and N during nitriding treatment at high temperature is promoted to obtain high surface hardness. It is a feature. However, since such a steel type has a high Ti component, it requires a special melting method such as a vacuum melting method and is difficult to use as a mass-produced steel. Furthermore, when such high Ti steel is used, the amount of TiC deposited increases, resulting in a significant decrease in fatigue strength.

本発明は、上記問題を鑑みて為されたものであり、多量のTiを添加することなしに、高温による窒化処理が可能で深い硬化深さを得ることができ、高い表面硬さと良好な疲労特性を有する窒化処理用鋼を提供することを目的とする。   The present invention has been made in view of the above problems, and without adding a large amount of Ti, it is possible to perform a nitriding treatment at a high temperature and obtain a deep hardening depth, and a high surface hardness and a good fatigue. An object is to provide a nitriding steel having characteristics.

課題を解決するための手段及び発明の効果Means for Solving the Problems and Effects of the Invention

上記課題を解決するため、本発明の窒化処理用鋼は、質量%で、C:0.05%以上0.60%以下,Si:0.03%以上3.0%以下,Mn:0.01%以上3.5%以下,Cr:0.10%以上5.00%以下,Mo:0.05%以上3.0%以下,V:0.1%以上3.0%以下,Ti:0.5%以下,Al:0.001%以上3.0%以下,N:0.005%以上0.025%以下を含有し、残部がFe及び不可避不純物からなり、下記式1および式2を満たすことを特徴とする。
[Si%]+[Mo%]+2×([V%]+[Ti%]+[Al%])≧2.3・・・式1
242×[Ti%]+230×[Al%]+100×[V%]≧150・・・式2
なお、本明細書において“[X%]”とは、Xに表される元素の含有量を表す。
In order to solve the above problems, the nitriding steel of the present invention is, in mass%, C: 0.05% to 0.60%, Si: 0.03% to 3.0%, Mn: 0.00. 01% to 3.5%, Cr: 0.10% to 5.00%, Mo: 0.05% to 3.0%, V: 0.1% to 3.0%, Ti: 0.5% or less, Al: 0.001% or more and 3.0% or less, N: 0.005% or more and 0.025% or less, with the balance being Fe and inevitable impurities. It is characterized by satisfying.
[Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2
In the present specification, “[X%]” represents the content of the element represented by X.

本発明者等は、窒化による窒素侵入に対する鋼の共析温度を上げる効果があり、且つ、特殊な溶解方法等が必要とならない合金元素の添加によって、高温での窒化処理を可能とすることについて鋭意研究を重ねたところ、上記式1および式2を満たす合金元素の組合せによって、高温での窒化処理が可能で従来の窒化処理用鋼よりも深い硬化深さを得ることができ、且つ、高い表面硬さを得ることができる窒化処理用鋼を見出した。また、本発明の窒化処理用鋼では、Tiの添加量が0.5%以下に抑えられているため、特殊な溶解方法を必要とせず量産が可能で、且つ、TiCの析出が抑制されて疲労特性に優れる。   The present inventors have the effect of increasing the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and enabling the nitriding treatment at high temperature by adding an alloy element that does not require a special melting method or the like. As a result of extensive research, it is possible to perform nitriding treatment at a high temperature by using a combination of alloy elements satisfying the above formulas 1 and 2, and to obtain a deeper hardening depth than conventional nitriding steel, and high. The steel for nitriding which can obtain surface hardness was discovered. Further, in the nitriding steel of the present invention, the amount of Ti added is suppressed to 0.5% or less, so that mass production is possible without requiring a special melting method, and precipitation of TiC is suppressed. Excellent fatigue properties.

以下、本発明の窒化処理用鋼における数値限定理由について説明する。   Hereinafter, the reason for the numerical limitation in the nitriding steel of the present invention will be described.

(1)[Si%]+[Mo%]+2×([V%]+[Ti%]+[Al%])≧2.3・・・式1
式1を満たす合金元素の組合せによって、窒化時のN侵入に対する鋼の共析温度が上がり、高温(例えば600℃以上750℃以下)での窒化処理が可能となる。これによって、従来の窒化処理用鋼よりも深い硬化深さ(例えば0.3mm以上)を得ることができる。
(1) [Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
The combination of alloy elements satisfying Equation 1 increases the eutectoid temperature of steel against N intrusion during nitriding, and enables nitriding at a high temperature (for example, 600 ° C. or more and 750 ° C. or less). As a result, it is possible to obtain a hardening depth (for example, 0.3 mm or more) deeper than the conventional nitriding steel.

(2)242×[Ti%]+230×[Al%]+100×[V%]≧150・・・式2
式2を満たす合金元素の組合せによって、高温(例えば、600℃以上750℃以下)での窒化処理でも硬い拡散層を形成することができる。これによって、高い表面硬さ(例えば650HV以上)を得ることができる。
(2) 242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2
By a combination of alloy elements satisfying Equation 2, a hard diffusion layer can be formed even by nitriding at a high temperature (for example, 600 ° C. or higher and 750 ° C. or lower). Thereby, high surface hardness (for example, 650 HV or more) can be obtained.

(3)C:0.05%以上0.60%以下
Cは、強度確保のための内部硬さを得るために必要な元素であり、この効果を得るには0.05%以上の添加が必要である。他方、過度の添加は、鍛造または圧延,溶体化処理後の硬さが増加して加工性を劣化させるため、0.60%以下の添加とする。
(3) C: 0.05% or more and 0.60% or less C is an element necessary for obtaining internal hardness for securing strength. To obtain this effect, 0.05% or more must be added. is necessary. On the other hand, excessive addition increases the hardness after forging, rolling, or solution treatment, and deteriorates workability, so the addition is made 0.60% or less.

(4)Si:0.03%以上3.0%以下
Siは、鋼の溶製時における脱酸剤として使用されるとともに、鋼の疲労強度を向上させる効果もある。また、式1に含まれるMo,V,Ti,AlやCrと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素でもある。これらの効果を得るには0.03%以上の添加が必要である。他方、過度の添加は、鋼の脆化を促進し、表面脱炭を伴って被削性等の加工性を劣化させてしまうので、3.0%以下の添加とする。
(4) Si: 0.03% or more and 3.0% or less Si is used as a deoxidizer during the melting of steel and has an effect of improving the fatigue strength of steel. Further, like Mo, V, Ti, Al, and Cr included in Formula 1, it is also an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. To obtain these effects, 0.03% or more must be added. On the other hand, excessive addition promotes embrittlement of steel and deteriorates workability such as machinability with surface decarburization, so the addition is made 3.0% or less.

(5)Mn:0.01%以上3.5%以下
Mnは、固溶強化により硬さ向上に寄与する元素であるとともに、靭性向上に効果のある元素である。これらの効果を得るには0.01%以上の添加が必要である。他方、過度の添加は、被削性が低下して機械加工性を劣化させるとともに、窒化処理後の硬化深さが低下する原因となるため、3.5%以下の添加とする。
(5) Mn: 0.01% or more and 3.5% or less Mn is an element that contributes to improvement in hardness by solid solution strengthening and an element that is effective in improving toughness. To obtain these effects, 0.01% or more must be added. On the other hand, excessive addition reduces machinability and deteriorates machinability, and causes a decrease in the depth of hardening after nitriding, so it is added at 3.5% or less.

(6)Cr:0.10%以上5.00%以下
Crは、式1に含まれるSi,Mo,V,Ti,Alと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素であり、この効果を得るには0.10%以上の添加が必要である。他方、過度の添加は、鍛造または圧延,溶体化処理後に硬くなりすぎ、被削性を低下させるため、5.00%以下の添加とする。
(6) Cr: 0.10% or more and 5.00% or less Cr, like Si, Mo, V, Ti, and Al included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. In order to obtain this effect, addition of 0.10% or more is necessary. On the other hand, excessive addition becomes too hard after forging, rolling, or solution treatment, and reduces machinability, so the addition is made 5.00% or less.

(7)Mo:0.05%以上3.0%以下
Moは、式1に含まれるSi,V,Ti,AlやCrと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素であり、この効果を得るには0.05%以上の添加が必要である。他方、過度の添加は、鍛造または圧延,溶体化処理後に硬くなりすぎて被削性を低下させるとともに、コストの増加を招くため、3.0%以下の添加とする。
(7) Mo: 0.05% or more and 3.0% or less Mo, like Si, V, Ti, Al and Cr included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. In order to obtain this effect, it is necessary to add 0.05% or more. On the other hand, excessive addition becomes too hard after forging, rolling, or solution treatment, thereby reducing machinability and increasing the cost. Therefore, the addition is made 3.0% or less.

(8)V:0.1%以上3.0%以下
Vは、式1に含まれるSi,Mo,Ti,AlやCrと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素であり、その傾向は他の元素よりも優れる。また、拡散層の硬さを上げる元素でもある。これら効果を得るには、0.1%以上の添加が必要である。他方、過度の添加は、鍛造または圧延,溶体化処理後に炭化物を析出して、マトリックス中のCを低下させるとともに、窒化時に有効に作用するVが減少してしまうため、3.0%以下の添加とする。
(8) V: 0.1% or more and 3.0% or less V is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding, like Si, Mo, Ti, Al and Cr included in Formula 1. Yes, the trend is superior to other elements. It is also an element that increases the hardness of the diffusion layer. In order to obtain these effects, addition of 0.1% or more is necessary. On the other hand, excessive addition causes carbide to precipitate after forging, rolling, or solution treatment, lowers C in the matrix, and decreases V that effectively acts during nitriding. Addition.

(9)Ti:0.5%以下
Tiは、式1に含まれるSi,Mo,V,AlやCrと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素であり、その傾向は他の元素よりも優れる。また、拡散層の硬さを上げる元素でもある。しかしながら、上述したように、過度の添加は生産性の困難さを招き、且つ、TiCの析出量が増えて疲労強度の著しい低下を招いてしまう。また、窒化後の化合物層の形状が針状となって機械的性質が損なわれる。このため、0.5%以下の添加とする。
(9) Ti: 0.5% or less Ti, like Si, Mo, V, Al and Cr included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and its tendency is Superior to other elements. It is also an element that increases the hardness of the diffusion layer. However, as described above, excessive addition causes difficulty in productivity and increases the precipitation amount of TiC, leading to a significant decrease in fatigue strength. Further, the shape of the compound layer after nitriding becomes a needle shape, and mechanical properties are impaired. For this reason, 0.5% or less is added.

(10)Al:0.001%以上3.0%以下
Alは、溶製時の脱酸剤として添加される。また、式1に含まれるSi,Mo,V,TiやCrと同様に、窒化による窒素侵入に対する鋼の共析温度を上げる元素であり、且つ、高温で拡散層の硬さを上げる元素である。これらの効果を得るには0.001%以上の添加が必要である。他方、過度の添加は、母材がα相となる傾向が強くなって芯部の靭性が低下してしまうため、3.0%以下の添加とする。
(10) Al: 0.001% to 3.0% Al is added as a deoxidizer during melting. In addition, similar to Si, Mo, V, Ti and Cr included in Formula 1, it is an element that increases the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and an element that increases the hardness of the diffusion layer at high temperatures. . To obtain these effects, 0.001% or more must be added. On the other hand, excessive addition tends to cause the base material to become an α phase and the toughness of the core portion is reduced, so the addition is made 3.0% or less.

(11)N:0.005%以上0.025%以下
Nは、Alと微細な窒化物を生成し、熱間鍛造時における結晶粒径の成長を抑制して鋼の靭性向上に資する成分である。この効果を得るには0.005%以上の添加が必要である。他方、過度の添加は、鍛造時にブローホールなどが発生して鋼塊の健全性が損なわれるので、0.025%以下の添加とする。
(11) N: 0.005% or more and 0.025% or less N is a component that produces fine nitrides with Al and contributes to improving the toughness of steel by suppressing the growth of crystal grain size during hot forging. is there. To obtain this effect, 0.005% or more must be added. On the other hand, excessive addition causes blowholes or the like during forging and impairs the soundness of the steel ingot, so the addition is made 0.025% or less.

次に、本発明の窒化処理用鋼は、鋼成分として更に、以下の任意成分を含有させることができる。   Next, the nitriding steel of the present invention can further contain the following optional components as steel components.

(12)Nb:0.5%以下,Zr:0.5%以下
これらの成分は、鋼の溶製時に溶鋼中に生成するこれらの酸化物によって、鋼中に存在するMnS等の硫化物を微細化して分散させるので、鋼の被削性の向上に寄与する。また、微細化した硫化物は、鋼の鍛造時や焼ならし後の組織を微細化して鋼の疲労強度を向上させる。しかし、過度に添加してもその効果は飽和するので、それぞれ0.5%以下とするのが好ましい。
(12) Nb: 0.5% or less, Zr: 0.5% or less These components are used to form sulfides such as MnS present in the steel due to these oxides generated in the molten steel when the steel is melted. Since it is refined and dispersed, it contributes to improving the machinability of steel. In addition, the refined sulfide refines the structure after forging or normalizing the steel to improve the fatigue strength of the steel. However, even if added excessively, the effect is saturated, so each content is preferably 0.5% or less.

(13)Cu:1.0%以下,Ni:1.0%以下
これらの成分は、鋼の芯部強度を向上させる。しかし、過度に添加しても経済的に得策ではないので、それぞれ1.0%以下とすることが好ましい。
(13) Cu: 1.0% or less, Ni: 1.0% or less These components improve the core strength of the steel. However, even if added excessively, it is not economically advantageous.

(14)S:0.01%以上0.20%以下,Ca:0.0005%以上0.0030%以下,Pb:0.3%以下,Bi:0.3%以下
これらの成分は、鋼の被削性を向上させるため、時効処理前に行う機械加工時に高い被削性が要求される場合には、これらの成分の少なくとも1種を添加することが好ましい。しかし、過度に添加すると、鋼の熱間加工性や疲れ限度を劣化させるので、Sは0.2%以下,Caは0.010%以下,Pbは0.3%以下,Biは0.3%以下とすることが好ましい。
(14) S: 0.01% or more and 0.20% or less, Ca: 0.0005% or more and 0.0030% or less, Pb: 0.3% or less, Bi: 0.3% or less In order to improve the machinability, it is preferable to add at least one of these components when high machinability is required during machining performed before the aging treatment. However, excessive addition degrades the hot workability and fatigue limit of steel, so S is 0.2% or less, Ca is 0.010% or less, Pb is 0.3% or less, and Bi is 0.3. % Or less is preferable.

次に、本発明の窒化処理用鋼の製造および窒化処理について説明する。   Next, the production and nitriding of the nitriding steel of the present invention will be described.

本発明の窒化処理用鋼は、溶解して所定の精錬を行った後に、熱間圧延,鍛造,焼入れ焼戻し等の各種熱処理おいて十分に溶体化処理を行い、窒化時に有効に作用するSi,Cr,Mo,V,Ti,Alなどの元素をマトリクス中に十分に固溶させる。その後の冷却では、これらの元素を炭化物や窒化物として析出させずにマトリックス中に残すように、冷却速度を製品種類や合金成分の組合せに応じて適宜設定する。一例を挙げると、冷却後にフェライトが50%以上生成される成分組成の場合、800℃から500℃までの領域を0.05℃/sec以上10℃/sec以下の冷却速度で行う必要がある。   The steel for nitriding treatment of the present invention is subjected to a sufficient solution treatment in various heat treatments such as hot rolling, forging, quenching and tempering after being melted and subjected to predetermined refining, and Si that effectively acts during nitriding, Elements such as Cr, Mo, V, Ti, and Al are sufficiently dissolved in the matrix. In the subsequent cooling, the cooling rate is appropriately set according to the combination of product types and alloy components so that these elements remain in the matrix without being precipitated as carbides or nitrides. As an example, in the case of a component composition in which 50% or more of ferrite is generated after cooling, it is necessary to perform the region from 800 ° C. to 500 ° C. at a cooling rate of 0.05 ° C./sec to 10 ° C./sec.

本発明の窒化処理用鋼が得られた後は、各種製品となるように切断や成型などの加工を行うが、かかる加工は窒化処理前に実施するため、炭窒化物が析出しない温度で実施する。具体的には、500℃以下での実施が好ましい。   After the nitriding steel of the present invention is obtained, processing such as cutting and molding is performed so as to become various products, but since such processing is performed before nitriding, it is performed at a temperature at which carbonitride is not precipitated. To do. Specifically, implementation at 500 ° C. or lower is preferable.

窒化処理は600℃以上で行う。通常の鋼では、窒化による窒素侵入に対する鋼の共析温度が600℃未満であり、それ以上の温度で処理するとγ相やマルテンサイト相を生成してしまう。しかし、本発明の窒化処理用鋼は、式1によって窒化時のN侵入に対する鋼の共析温度が600℃以上となっているため、600℃以上の高温で窒化処理が可能である。窒化処理温度の上限は、例えば750℃以下とすることができる。また、硬化深さについては、要求特性に応じて窒化処理時間により適宜調整することができる。ここで、本発明の窒化処理用鋼は、高温での窒化処理が可能であるため処理時間の短縮化を図ることができ、従来の窒化鋼よりも生産性に優れる。なお、窒化処理には、ガス窒化,ガス軟窒化,塩浴窒化,塩浴軟窒化,イオン窒化,プラズマ窒化,ラジカル窒化等の方法があり、いずれを適用してもよい。   Nitriding is performed at 600 ° C. or higher. In ordinary steel, the eutectoid temperature of the steel against nitrogen intrusion due to nitriding is less than 600 ° C., and if it is treated at a temperature higher than that, a γ phase and a martensite phase are generated. However, the steel for nitriding treatment of the present invention can be nitrided at a high temperature of 600 ° C. or higher because the eutectoid temperature of the steel against N intrusion during nitriding is 600 ° C. or higher according to Equation 1. The upper limit of the nitriding temperature can be set to 750 ° C. or lower, for example. Further, the curing depth can be appropriately adjusted according to the nitriding time according to the required characteristics. Here, the nitriding steel of the present invention can be nitrided at a high temperature, so that the processing time can be shortened, and the productivity is superior to the conventional nitriding steel. The nitriding treatment includes methods such as gas nitriding, gas soft nitriding, salt bath nitriding, salt bath soft nitriding, ion nitriding, plasma nitriding, radical nitriding, and any of them may be applied.

次に、本発明の効果を確認するために行った試験について説明する。
試験は、表1に示す鋼成分について、5kg真空誘導溶解炉によって溶解した鋼塊を用い、1200℃以上に加熱して直径22mmの丸棒に鍛伸したものを使用した。その後、切断して所定の大きさに成形後(回転曲げ疲労試験片および摩耗試験片)、全ての鋼材について850〜1200℃で30分保持し、油冷及び空冷した。なお、X鋼については、その後に650℃で2hr時効処理を行い、窒化時に有効に作用する元素のマトリックス中からの低減を図った。
Next, tests conducted to confirm the effects of the present invention will be described.
The test used the steel components shown in Table 1 and a steel ingot melted by a 5 kg vacuum induction melting furnace, heated to 1200 ° C. or higher and forged into a round bar having a diameter of 22 mm. Then, after cutting and forming to a predetermined size (rotary bending fatigue test piece and wear test piece), all steel materials were held at 850 to 1200 ° C. for 30 minutes, and then oil-cooled and air-cooled. In addition, about X steel, the aging treatment for 2 hours was performed at 650 degreeC after that, and the reduction | restoration from the matrix of the element which acts effectively at the time of nitriding was aimed at.

表1において、A〜P鋼は本発明の成分範囲内にある発明鋼であり、R〜X鋼はそれに対する比較鋼である。比較鋼Rは、汎用鋼であるSCM440相当の鋼について通常の窒化条件で軟窒化したものである。比較鋼Xは、製造時の成分が本発明で規定する範囲に含まれているが、時効処理によって窒化時に有効に作用する元素のマトリックス中からの低減を図っているので、ここでは比較鋼として扱う。
なお、表1中の比較鋼の組成において、本発明で規定する組成範囲を逸脱しているものには、下限を下回る場合は下向矢印(↓)、上限を上回る場合は上向矢印(↑)を付している。
In Table 1, A to P steels are invention steels within the compositional range of the present invention, and R to X steels are comparative steels. The comparative steel R is obtained by soft nitriding a steel equivalent to SCM440, which is a general-purpose steel, under normal nitriding conditions. In Comparative Steel X, the components at the time of manufacture are included in the range defined in the present invention. However, since the aging treatment is intended to reduce the elements that act effectively at the time of nitriding from the matrix, deal with.
In addition, in the composition of the comparative steel in Table 1, those that deviate from the composition range defined in the present invention include a downward arrow (↓) when below the lower limit, and an upward arrow (↑) when exceeding the upper limit. ) Is attached.

以上の発明鋼(A〜P鋼)および比較鋼(R〜X鋼)に対して、表2に示す各条件で窒化処理を実施した。窒化処理は、焼入を行った試験片について、通常の窒化処理温度に比べて高温の620〜750℃の範囲でガス軟窒化処理を行った(但し、R鋼については通常の窒化処理温度である550℃でガス軟窒化処理を行った)。次に、窒化処理後の試験片に対して、回転曲げ疲労試験および摩耗試験を実施した。以下に試験方法について説明する。   Nitriding treatment was carried out under the conditions shown in Table 2 for the above invention steels (A to P steel) and comparative steels (R to X steel). The nitriding treatment was performed on the specimens that had been quenched by gas soft nitriding in the range of 620 to 750 ° C., which is higher than the normal nitriding temperature (however, the R steel was subjected to normal nitriding temperature) A gas soft nitriding treatment was performed at a certain 550 ° C.). Next, a rotating bending fatigue test and a wear test were performed on the test piece after the nitriding treatment. The test method will be described below.

・回転曲げ疲労試験
回転曲げ疲労試験は、小野式回転曲げ疲れ試験機を用いて行った。以下に試験条件を示す。
回転数:3500rpm
温度:25℃
-Rotating bending fatigue test The rotating bending fatigue test was conducted using an Ono type rotating bending fatigue tester. The test conditions are shown below.
Rotation speed: 3500rpm
Temperature: 25 ° C

・摩耗試験
摩耗試験は、大越式磨耗試験機を用いて行った。以下に試験条件を示す。
相手材:SUJ2
摩擦距離:400m
摩擦速度:0.94m/s
非潤滑
・ Abrasion test The abrasion test was performed using an Ogoshi type abrasion tester. The test conditions are shown below.
Opponent material: SUJ2
Friction distance: 400m
Friction speed: 0.94 m / s
Non-lubricated

以上の回転曲げ疲労試験および磨耗試験の試験結果を表2に示す。なお、これらの試験結果は、発明鋼1を基準とする相対評価で表している。また、EPMAにより観察したN拡散深さも表2に示す。   Table 2 shows the results of the above rotating bending fatigue test and wear test. In addition, these test results are represented by relative evaluation based on Invention Steel 1. Table 2 also shows the N diffusion depth observed by EPMA.

表2によると、発明鋼(A〜P鋼)は、高温の窒化処理を施しているため、比較鋼Rのような汎用鋼の通常窒化条件では得られないN拡散深さ(N拡散深さは、一般的に窒化後の硬化深さと対応する。)が得られている。また、発明鋼(A〜P鋼)は式1を満たすため、高温で窒化してもγ相が生成しないことがX線回折から確認されている。さらに、発明鋼(A〜P鋼)は式2を満たすため、高温窒化により優れた疲労特性と耐摩耗性が得られていることが比較鋼Rと比べて明らかである。   According to Table 2, since the inventive steels (A to P steel) are subjected to high-temperature nitriding treatment, N diffusion depth (N diffusion depth) that cannot be obtained under the normal nitriding conditions of general-purpose steel such as comparative steel R Generally corresponds to the hardening depth after nitriding.). In addition, since the inventive steels (A to P steels) satisfy Formula 1, it has been confirmed from X-ray diffraction that no γ phase is generated even when nitriding at a high temperature. Furthermore, since the inventive steels (AP steels) satisfy the formula 2, it is clear that excellent fatigue characteristics and wear resistance are obtained by high temperature nitriding compared to the comparative steel R.

比較鋼S,Tは、発明鋼(A〜P鋼)と比べて、疲労強度および摩耗量が劣化していることがわかる。これは、式1を満たすものの式2を満たしていないために、拡散層の硬さが十分に得られなかったことによると考えられる。   It can be seen that the comparison steels S and T have deteriorated fatigue strength and wear compared to the invention steels (AP steels). This is considered to be because the hardness of the diffusion layer was not sufficiently obtained because Expression 1 was satisfied but Expression 2 was not satisfied.

比較鋼U,Vは、発明鋼(A〜P鋼)と比べて、疲労強度および摩耗量が劣化していることがわかる。これは、式2を満たすものの式1を満たしていないために、化合物層直下にγ相が生成したことによると考えられる(X線回折により確認)。   It can be seen that the comparative steels U and V have deteriorated fatigue strength and wear compared to the inventive steels (AP steels). This is considered to be due to the fact that the γ phase was formed immediately below the compound layer because the formula 2 was satisfied but the formula 1 was not satisfied (confirmed by X-ray diffraction).

比較鋼Wは、発明鋼(A〜P鋼)と比べて、疲労強度および摩耗量ともに低くなっていることがわかる。これは、高Ti鋼であることから、マトリクス中のTiC量が多く、かかる介在物を起点とする疲労破壊が起こることによると考えられる(破面観察により確認)。よって、このような高Ti鋼は、高温窒化処理に適していないものと考えられる。   It can be seen that the comparative steel W is lower in both fatigue strength and wear compared to the inventive steel (AP steel). This is considered to be due to the fact that because of the high Ti steel, the amount of TiC in the matrix is large and fatigue fracture occurs starting from such inclusions (confirmed by fracture surface observation). Therefore, it is considered that such high Ti steel is not suitable for high temperature nitriding treatment.

比較鋼Xは、発明鋼(A〜P鋼)と比べて、疲労特性および耐摩耗特性がともに低下していることがわかる。これは、式1および式2を満たすものの、窒化前に時効処理を実施したため、窒化時に有効に働く元素量がマトリックス中から低減した結果、表面にγ層が生成したことによると考えられる(X線回折および顕微鏡観察により確認)。   It can be seen that the comparative steel X has both fatigue characteristics and wear resistance characteristics that are lower than those of the inventive steels (AP steels). This is considered to be due to the fact that the γ layer was formed on the surface as a result of the reduction of the amount of elements that worked effectively during nitriding from the matrix because aging treatment was performed before nitriding, although Expressions 1 and 2 were satisfied (X Confirmed by line diffraction and microscopic observation).

Claims (4)

質量%で、C:0.05%以上0.60%以下,Si:0.03%以上3.0%以下,Mn:0.01%以上3.5%以下,Cr:0.10%以上5.00%以下,Mo:0.05%以上3.0%以下,V:0.1%以上3.0%以下,Ti:0.5%以下,Al:0.001%以上3.0%以下,N:0.005%以上0.025%以下を含有し、残部がFe及び不可避不純物からなり、下記(1)式および(2)式を満たすことを特徴とする窒化処理用鋼。
[Si%]+[Mo%]+2×([V%]+[Ti%]+[Al%])≧2.3・・・式1
242×[Ti%]+230×[Al%]+100×[V%]≧150・・・式2
In mass%, C: 0.05% to 0.60%, Si: 0.03% to 3.0%, Mn: 0.01% to 3.5%, Cr: 0.10% or more 5.00% or less, Mo: 0.05% to 3.0%, V: 0.1% to 3.0%, Ti: 0.5% or less, Al: 0.001% to 3.0% % Or less, N: 0.005% or more and 0.025% or less, the balance being Fe and inevitable impurities, satisfying the following formulas (1) and (2).
[Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2
鋼成分として更に、Nb:0.5%以下,Zr:0.5%以下から選ばれる1種または2種を含有する請求項1に記載の窒化処理用鋼。   The steel for nitriding according to claim 1, further comprising one or two kinds selected from Nb: 0.5% or less and Zr: 0.5% or less as a steel component. 鋼成分として更に、Cu:1.0%以下,Ni:1.0%以下から選ばれる1種または2種を含有する請求項1または2に記載の窒化処理用鋼。   The steel for nitriding according to claim 1 or 2, further comprising one or two kinds selected from Cu: 1.0% or less and Ni: 1.0% or less as a steel component. 鋼成分として更に、S:0.01%以上0.20%以下,Ca:0.0005%以上0.0030%以下,Pb:0.3%以下,Bi:0.3%以下から選ばれる1種または2種以上を含有する請求項1ないし3のいずれか1項に記載の窒化処理用鋼。   The steel component is further selected from S: 0.01% to 0.20%, Ca: 0.0005% to 0.0030%, Pb: 0.3% or less, Bi: 0.3% or less 1 The steel for nitriding according to any one of claims 1 to 3, comprising seeds or two or more kinds.
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