JP2009299147A

JP2009299147A - Method for manufacturing high-strength carburized component

Info

Publication number: JP2009299147A
Application number: JP2008156013A
Authority: JP
Inventors: Motohiro Nishikawa; 元裕西川
Original assignee: Sanyo Special Steel Co Ltd
Current assignee: Sanyo Special Steel Co Ltd
Priority date: 2008-06-13
Filing date: 2008-06-13
Publication date: 2009-12-24

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carburized component having excellent impact strength and bending strength which have not been obtained heretofore by performing carburizing and quenching, also refining crystal grains, and removing a carburized abnormal layer on the surface. <P>SOLUTION: A steel having a composition comprising, by mass, 0.10-0.45% C, 0.05-2.0% Si, 0.1-2.0% Mn, ≤0.030% P, ≤0.20% S, 0.30-3.0% Cr, ≤0.30% Cu, 0.001-0.1% Al, <0.01% N, <0.05% Ti, and 0.0010-0.0050% B, and further comprising one or two selected from 0.02-0.50% Nb and 0.02-0.50% V, and the balance Fe with inevitable impurities is formed into a component shape by machining or forging, is thereafter subjected to gas carburizing and quenching, is subsequently subjected to whole quenching for one or more times, and thereafter tempered, and after that, a carburized abnormal layer is removed, so as to manufacture a carburized component. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

この発明は、はだ焼鋼から浸炭焼入れ・焼戻し処理してなる浸炭部品の製造、例えば、自動車、建設機械、工作機械などのギア、ＣＶＪやシャフトなどのはだ焼鋼から浸炭処理してなる浸炭部品の製造に関する。 In the present invention, carburized parts are manufactured by carburizing and tempering from case-hardened steel, for example, carburized from carburized steel such as gears for automobiles, construction machines, machine tools, and CVJs and shafts. It relates to the manufacture of carburized parts.

近年、自動車用部品の高出力・小型軽量化に伴い、ギア、等速ジョイント部品やシャフトなどの浸炭焼入れ・焼戻し処理してなる自動車用部品では、一層の高強度化、長寿命化が要求されている。そこで、ＪＩＳ規格のＳＮＣＭなどのニッケルクロムモリブデン鋼のように合金元素を添加して高強度化を図っている。しかし、このように合金元素を添加して高強度化を図った場合、素材コストが高くなり、冷間加工性が劣るため冷間鍛造ができず、さらに熱間鍛造後そのままでは切削の際に切削バイト寿命が短くなるため、焼鈍などの熱処理が必要となる問題がある。 In recent years, with higher output, smaller size, and lighter parts for automobiles, car parts that are carburized and tempered, such as gears, constant velocity joint parts and shafts, are required to have higher strength and longer life. ing. Therefore, an alloying element is added to increase the strength as in nickel chromium molybdenum steel such as JIS standard SNCM. However, when alloying elements are added in this way to increase the strength, the material cost becomes high and cold workability is poor, so cold forging cannot be performed. Since the cutting tool life is shortened, there is a problem that heat treatment such as annealing is required.

一方、結晶粒微細化により鋼の強度が向上することが知られているが、この方法は合金元素の添加なしに高強度化でき、素材の鍛造性や切削性といった加工性を低下させず、延性や靭性を損なわず高強度化できることから極めて有効な方法であると言える。 On the other hand, it is known that the strength of steel is improved by crystal grain refinement, but this method can increase the strength without the addition of alloy elements, and does not reduce workability such as forgeability and machinability of the material, It can be said that this is an extremely effective method because the strength can be increased without impairing the ductility and toughness.

結晶粒を微細化させる方法として加工熱処理による方法があるが、この場合、成形加工と熱処理を組み合わせるため、成形加工の難しいものには適用できないなど、部品形状が限定され、自動車のギア、ＣＶＪやシャフトなどには適用が難しいという問題がある。 There is a method by thermomechanical treatment as a method for refining crystal grains, but in this case, since the molding process and the heat treatment are combined, the shape of the parts is limited, such as being difficult to apply to those that are difficult to mold, automotive gear, CVJ, There is a problem that it is difficult to apply to shafts.

これらの問題点を解消するために、浸炭後に繰返し焼入れを行なうことにより、積極的に旧オーステナイト粒を微細化して強度を向上させることができる高強度はだ焼鋼が提案されている（例えば、特許文献１、特許文献２参照）。 In order to solve these problems, a high strength hardened steel that can actively refine the prior austenite grains and improve the strength by repeatedly quenching after carburizing has been proposed (for example, (See Patent Document 1 and Patent Document 2).

しかし、特許文献１の方法では、浸炭前のはだ焼鋼をＪＩＳＧ０５５１で規定されている粒度番号Ｎｏ．１１以上まで微細化したマルテンサイト組織とする必要があり、コストが高いという問題がある。 However, in the method of Patent Document 1, the case-hardened steel before carburizing is classified into a grain number No. defined in JIS G0551. There is a problem that the martensite structure must be refined to 11 or more and the cost is high.

また、特許文献１および特許文献２では、浸炭焼入れ後、繰返し焼入れを行なうことによって旧オーステナイト粒径を微細化しているが、これらの方法では強度向上が十分とはいえないという問題がある。 In Patent Document 1 and Patent Document 2, the prior austenite grain size is refined by repeated quenching after carburizing and quenching, but there is a problem that the strength cannot be improved sufficiently by these methods.

特開２００３−３４８４３号公報JP 2003-34843 A 特開平８−９２６９０号公報JP-A-8-92690

上記の特許文献１あるいは特許文献２に記載の技術に対し、鋼材をより一層高強度化する方法を鋭意検討したところ、従来のガス浸炭を行った鋼において結晶粒径を小さくしても、ある粒径で強度は飽和してしまうことを見いだした。これはガス浸炭時に部品表面に浸炭異常層や粒界酸化層が生成し、その深さ以上に旧オーステナイト粒径を微細化しても、浸炭異常層や粒界酸化層が初期欠陥として作用し、旧オーステナイト粒の微細化効果がなくなったことによると推定された。そこで、表面の浸炭異常層を除去したところ、強度の飽和現象は見られず、結晶粒径が小さくなればなるほど強度は向上することを発明者は見いだした。 With respect to the technique described in Patent Document 1 or Patent Document 2, a method for further strengthening the steel material has been intensively studied, and even if the crystal grain size is reduced in the conventional gas carburized steel, It was found that the strength was saturated with the particle size. This is because an abnormal carburization layer or grain boundary oxide layer is generated on the part surface during gas carburization, and even if the prior austenite grain size is refined beyond that depth, the carburization abnormal layer and grain boundary oxide layer act as initial defects, It is presumed that the refinement effect of the prior austenite grains disappeared. Thus, when the surface carburized abnormal layer was removed, the inventors found that the strength saturation phenomenon was not observed, and that the strength improved as the crystal grain size became smaller.

すなわち、本発明が解決しようとする課題は、浸炭した鋼部品において、従来に比して結晶粒を超微細化するとともに、浸炭異常層である表面欠陥層を除去することによる相乗作用により、従来にまして優れた衝撃強度および曲げ強度を有する浸炭部品を製造する方法を提供することである。 That is, the problem to be solved by the present invention is that, in carburized steel parts, the crystal grains are made ultrafine compared to the conventional one, and the synergistic action by removing the surface defect layer which is a carburizing abnormal layer is conventionally achieved. It is another object of the present invention to provide a method for manufacturing a carburized part having excellent impact strength and bending strength.

上記の課題を解決するための本発明の手段は、請求項１の発明では、質量％で、Ｃ：０．１０〜０．４５％、Ｓｉ：０．０５〜２．０％、Ｍｎ：０．１０〜２．０％、Ｐ：０．０３０％以下、Ｓ：０．２０％以下、Ｃｒ：０．３０〜３．０％、Ｃｕ：０．３０％以下、Ａｌ：０．００１〜０．１％、Ｎ：０．０１％未満、Ｔｉ：０．０５％未満、Ｂ：０．００１０〜０．００５０％を含有し、さらにＮｂ：０．０２〜０．５０％、Ｖ：０．０２〜０．５０％のいずれか１種もしくは２種を含有し、ただし、ＴｉおよびＮは３．４Ｎ［％］＜Ｔｉ［％］の関係を有し、残部Ｆｅおよび不可避不純物からなる鋼を用い、機械加工もしくは鍛造によって部品形状に成形した後、ガス浸炭焼入れを行い、その後に１回以上のズブ焼入れを行なった後、これを焼戻し、その後に浸炭異常層を除去することにより浸炭部品を製造することを特徴とする衝撃強度および曲げ強度に優れた浸炭部品の製造方法である。 The means of the present invention for solving the above-mentioned problems is that, in the invention of claim 1, in mass%, C: 0.10 to 0.45%, Si: 0.05 to 2.0%, Mn: 0 .10 to 2.0%, P: 0.030% or less, S: 0.20% or less, Cr: 0.30 to 3.0%, Cu: 0.30% or less, Al: 0.001 to 0 0.1%, N: less than 0.01%, Ti: less than 0.05%, B: 0.0010 to 0.0050%, Nb: 0.02 to 0.50%, V: 0.00. Any one or two of 02 to 0.50%, provided that Ti and N have a relationship of 3.4N [%] <Ti [%], and the steel is composed of the balance Fe and inevitable impurities. Use, after forming into a part shape by machining or forging, perform gas carburizing quenching, and then perform one or more times quenching, Les tempering, is a manufacturing method of subsequently carburized component having excellent impact strength and flexural strength, characterized in that to produce a carburized part by removing the carburized abnormal layer.

請求項２の発明では、請求項１の鋼成分に加え、さらに質量％で、Ｎｉ：０．２０〜５．０％、Ｍｏ：０．０５〜３．０％のいずれか１種もしくは２種を含有し、残部Ｆｅおよび不可避不純物からなる鋼を用い、機械加工もしくは鍛造によって部品形状に成形した後、ガス浸炭焼入れを行い、その後に１回以上のズブ焼入れを行なった後、これを焼戻し、その後に浸炭異常層を除去することにより浸炭部品を製造することを特徴とする衝撃強度および曲げ強度に優れた浸炭部品の製造方法である。 In the invention of claim 2, in addition to the steel component of claim 1, in addition to mass percent, any one or two of Ni: 0.20-5.0%, Mo: 0.05-3.0% Containing steel and the balance Fe and inevitable impurities, after forming into a part shape by machining or forging, gas carburizing and quenching, and then performing one or more sub-quenching, then tempering, Thereafter, the carburized part is manufactured by removing the carburized abnormal layer, and this is a method for manufacturing a carburized part excellent in impact strength and bending strength.

請求項３の発明では、浸炭異常層を除去する部位は浸炭部品であるギアの歯元やスプライン軸の底や段付き部品などの応力集中部であることを特徴とする請求項１または２の手段の衝撃強度および曲げ強度に優れた浸炭部品の製造方法である。 According to a third aspect of the present invention, the part from which the carburized abnormal layer is removed is a stress concentration portion such as a gear tooth base, a spline shaft bottom or a stepped part which is a carburized part. This is a method for producing a carburized part having excellent impact strength and bending strength.

上記の方法における鋼材の成分を限定した理由を以下に説明する。なお、％は質量％を示す。 The reason which limited the component of the steel materials in said method is demonstrated below. In addition,% shows the mass%.

Ｃ：０．１０〜０．４５％、望ましくは、Ｃ：０．１０〜０．２５％
Ｃは機械構造用部品として浸炭処理後の芯部強度を確保するために必要な元素である。Ｃが０．１０％未満では、その効果は十分に得られず、０．４５％を超えると加工性を低下し、かつ靱性を低下させる。そこでＣは０．１０〜０．４５％、望ましくは０．１０〜０．２５％とする。 C: 0.10 to 0.45%, desirably C: 0.10 to 0.25%
C is an element necessary for securing the core strength after carburizing as a machine structural component. If C is less than 0.10%, the effect is not sufficiently obtained, and if it exceeds 0.45%, workability is lowered and toughness is lowered. Therefore, C is 0.10 to 0.45%, preferably 0.10 to 0.25%.

Ｓｉ：０．０５〜２．０％
Ｓｉは脱酸に必要な元素で、０．０５％未満では脱酸が十分に得られず、２．０％を超えると加工性を低下させる。そこでＳｉは０．０５〜２．０％とする。 Si: 0.05-2.0%
Si is an element necessary for deoxidation. If it is less than 0.05%, sufficient deoxidation cannot be obtained, and if it exceeds 2.0%, workability is deteriorated. Therefore, Si is set to 0.05 to 2.0%.

Ｍｎ：０．１０〜２．０％
Ｍｎは焼入性を確保するために必要な元素であるが、０．１０％未満ではその効果は十分に得られず、２．０％を超えると加工性を低下させる。そこでＭｎは０．１０〜２．０％とする。 Mn: 0.10 to 2.0%
Mn is an element necessary for ensuring hardenability, but if it is less than 0.10%, the effect cannot be sufficiently obtained, and if it exceeds 2.0%, workability is lowered. Therefore, Mn is set to 0.10 to 2.0%.

Ｐ：０．０３０％以下
Ｐはスクラップから含有される不可避な元素であるが、オーステナイト粒界に偏析して衝撃強度や曲げ強度などの靱性を低下するので、含有量の上限を０．０３０％とする。 P: 0.030% or less P is an unavoidable element contained in scrap, but segregates at the austenite grain boundaries and lowers toughness such as impact strength and bending strength, so the upper limit of content is 0.030%. And

Ｓ：０．２０％以下
Ｓは被削性を向上させる元素であるが、非金属介在物であるＭｎＳを生成して、横方向の靱性および疲労強度を低下する。そこで、Ｓは０．２０％以下とする。なお、Ｓはなくても良いが、被削性を要する場合にはＳは０．００１〜０.２０％の範囲で添加する。 S: 0.20% or less S is an element that improves machinability, but generates non-metallic inclusions, MnS, and lowers the toughness and fatigue strength in the lateral direction. Therefore, S is set to 0.20% or less. S may be omitted, but when machinability is required, S is added in a range of 0.001 to 0.20%.

Ｎｉ：０．２０〜５．０％
Ｎｉは焼入性および靱性を向上させる元素であるが、０．２０％未満ではその効果が十分ではなく、５．０％を超えて含有すると加工性を著しく低下させ、かつ、コストアップとなる。そこでＮｉは０．２０〜５．０％とする。 Ni: 0.20 to 5.0%
Ni is an element that improves hardenability and toughness, but if it is less than 0.20%, the effect is not sufficient, and if it exceeds 5.0%, the workability is remarkably lowered and the cost is increased. . Therefore, Ni is set to 0.20 to 5.0%.

Ｃｒ：０．３０〜３．０％
Ｃｒは焼入性および浸炭性を向上させる元素であるが、０．３０％未満ではその効果が十分ではなく、３．０％を超えて含有すると加工性を低下する。そこでＣｒは０．３０〜３．０％とする。 Cr: 0.30 to 3.0%
Cr is an element that improves hardenability and carburization, but if it is less than 0.30%, its effect is not sufficient, and if it exceeds 3.0%, the workability decreases. Therefore, Cr is set to 0.30 to 3.0%.

Ｍｏ：０．０５〜３．０％
Ｍｏは焼入性および靱性を向上させる元素であるが、０．０５％未満ではその効果が十分ではなく、３．０％を超えて含有すると加工性を低下させる。そこでＭｏは０．０５〜３．０％とする。 Mo: 0.05-3.0%
Mo is an element that improves hardenability and toughness, but if it is less than 0.05%, its effect is not sufficient, and if it exceeds 3.0%, workability is lowered. Therefore, Mo is set to 0.05 to 3.0%.

Ｃｕ：０．３０％以下
Ｃｕはスクラップから含有される不可避な元素で、時効性を有し強度を上昇させる。しかし、Ｃｕは０．３０％を超えると熱間加工性を低下する。そこで、Ｃｕは０．３０％以下とする。 Cu: 0.30% or less Cu is an unavoidable element contained in scrap, has aging properties and increases strength. However, when Cu exceeds 0.30%, the hot workability decreases. Therefore, Cu is made 0.30% or less.

Ａｌ：０．００１〜０．０５０％、望ましくは０．０２〜０．０５０％
Ａｌは脱酸材として使用される元素であり、０．００１％未満では脱酸効果は不十分であり、０．０５０％を超えるとアルミナ系酸化物が増加し疲労特性、加工性を低下する。そこでＡｌは０．００１〜０．０５０％、望ましくは０．０２〜０．０５０％とする。 Al: 0.001 to 0.050%, desirably 0.02 to 0.050%
Al is an element used as a deoxidizing material, and if it is less than 0.001%, the deoxidation effect is insufficient, and if it exceeds 0.050%, alumina-based oxides increase and fatigue characteristics and workability deteriorate. . Therefore, Al is 0.001 to 0.050%, preferably 0.02 to 0.050%.

Ｔｉ：３．４Ｎ［％］＜Ｔｉ［％］でかつ＜０．０５％
Ｂを含有する鋼材について、Ｂの焼入性、強度向上効果を増すためには、Ｔｉで鋼中のｆｒｅｅ−Ｎを固定しなければならず、鋼中のｆｒｅｅ−Ｎを固定するためには、３．４Ｎ［％］＜Ｔｉ［％］の関係式を満たさなければならない。この場合、Ｔｉが多すぎると、ＴｉＮが多くなり、被削性、疲労強度を低下させる。また、Ｔｉは、Ｔｉ炭化物、Ｔｉを含有する複合炭化物、Ｔｉ窒化物を微細に析出させることによって、浸炭時のオーステナイト結晶粒度の粗大化を抑制するために必要な元素である特に、鋼中に微細分散したナノオーダーのＴｉＣが結晶粒の成長を抑制するが、Ｔｉが多すぎると、鋼の切削性を低下する。そこで、Ｔｉは、３．４Ｎ［％］＜Ｔｉ［％］でかつ＜０．０５％とする。 Ti: 3.4N [%] <Ti [%] and <0.05%
In order to increase the hardenability and strength improvement effect of B for steel containing B, free-N in steel must be fixed with Ti, and in order to fix free-N in steel The relational expression of 3.4 N [%] <Ti [%] must be satisfied. In this case, if there is too much Ti, TiN will increase and machinability and fatigue strength will be reduced. Further, Ti is an element necessary for suppressing coarsening of austenite crystal grain size during carburizing by finely depositing Ti carbide, composite carbide containing Ti, and Ti nitride, particularly in steel. Although finely dispersed nano-order TiC suppresses the growth of crystal grains, too much Ti decreases the machinability of the steel. Therefore, Ti is set to 3.4 N [%] <Ti [%] and <0.05%.

Ｎ：０．０１％未満、望ましくは０．００５％未満
Ｂを含有する鋼材では、Ｔｉで鋼中のｆｒｅｅ−Ｎを固定しなければならないが、Ｎが多すぎるとＴｉＮが多くなり、被削性、疲労強度を低下させる。そこで、Ｎは０．０１％未満、望ましくは０．００５％未満とする。 N: Less than 0.01%, desirably less than 0.005% In steel materials containing B, free-N in the steel must be fixed with Ti, but if there is too much N, TiN will increase and the work will be cut Reduces fatigue and fatigue strength. Therefore, N is less than 0.01%, desirably less than 0.005%.

Ｂ：０．００１０〜０．００５０％
Ｂは極小量の含有によって鋼の焼入性を著しく向上させ、浸炭部品の強度を向上させる元素であるが、０．００１０％未満では焼入性、強度の向上効果が十分ではなく、０．００５０％を超えると強度を低下する。そこで、Ｂは０．００１０〜０．００５０％とする。 B: 0.0010 to 0.0050%
B is an element that remarkably improves the hardenability of the steel by containing a minimum amount and improves the strength of the carburized part, but if it is less than 0.0010%, the effect of improving the hardenability and strength is not sufficient. If it exceeds 0050%, the strength decreases. Therefore, B is 0.0010 to 0.0050%.

Ｖ：０．０２〜０．５０％
Ｖは炭化物あるいは炭窒化物を形成し、オーステナイト結晶粒度の粗大化を抑制する効果を有するが０．０２％未満ではその効果が十分得られず、０．５０％を超えると析出物の量が過剰となり加工性を低下する。そこで、Ｖは０．０２〜０．５０％とする。 V: 0.02-0.50%
V forms carbides or carbonitrides and has the effect of suppressing the coarsening of the austenite grain size. However, if less than 0.02%, the effect is not sufficiently obtained, and if it exceeds 0.50%, the amount of precipitates is reduced. Excessive workability decreases. Therefore, V is 0.02 to 0.50%.

Ｎｂ：０．０２〜０．５０％
Ｎｂは炭化物あるいは炭窒化物を形成し、オーステナイト結晶粒度の粗大化を抑制する効果を有するが０．０２％未満ではその効果が十分得られず、０．５０％を超えると析出物の量が過剰となり加工性を低下する。そこで、Ｎｂは０．０２〜０．５０％とする。 Nb: 0.02 to 0.50%
Nb forms carbides or carbonitrides and has the effect of suppressing the coarsening of the austenite grain size. However, if it is less than 0.02%, the effect cannot be sufficiently obtained. Excessive workability decreases. Therefore, Nb is made 0.02 to 0.50%.

Ｎｂ、Ｖを含有せしめる理由
本発明の工程では、浸炭焼入れ後の繰返し焼入れによって結晶粒を微細化する。ところで、繰返し焼入れの際の加熱時に非常に微細なオーステナイト初期粒が生成するが、ＪＩＳＳＣＭ４２０のような鋼では、その後の焼入れ温度までの加熱時に結晶粒が粗大化してしまい微細化しない。この結晶粒の粗大化を防止するために、Ｎｂ、Ｖといったピンニング力の高い元素を含有させる。 Reason for including Nb and V In the process of the present invention, crystal grains are refined by repeated quenching after carburizing and quenching. By the way, very fine austenite initial grains are generated during heating during repeated quenching. However, in a steel such as JIS SCM420, crystal grains are coarsened during heating up to the quenching temperature and are not refined. In order to prevent the coarsening of the crystal grains, elements having high pinning power such as Nb and V are included.

ズブ焼入れによる繰返し焼入れして結晶粒微細化し、さらに表面研削による浸炭異常層の除去、すなわち、結晶粒微細化と浸炭異常層の低減または削減の組合せ、などの工程の限定理由について以下に説明する。 The reason for limiting the process, such as repeated quenching by sub-quenching to refine crystal grains and further removal of abnormal carburization layer by surface grinding, that is, combination of grain refinement and reduction or reduction of carburization abnormal layer will be described below. .

先ず、繰返し焼入れについて説明する。本発明は結晶粒の微細化手法としてズブ焼入れによる繰返し焼入れ法を用いる。しかし、１回の焼入れよりも繰返し２回の焼入れの方がその効果は大きい。ただし、鋼種によっては、３回以上の繰返し焼入れを行うと逆に混粒が発生し、強度も低下するという問題がある。 First, repeated quenching will be described. In the present invention, a repetitive quenching method by sub-quenching is used as a method for refining crystal grains. However, the effect is greater when the quenching is repeated twice than when the quenching is performed once. However, depending on the type of steel, when repeated quenching is performed three times or more, there is a problem that mixed grains are generated and the strength is also lowered.

次いで、繰返し焼入れによる結晶粒微細化処理と浸炭異常層の低減あるいは削減処理の二つの処理の組合せについて説明する。ガス浸炭処理を行なう場合、雰囲気中に含まれている酸素が鋼材表面から侵入し、結晶粒界近傍のＳｉ、Ｍｎ、Ｃｒと結びつき酸化物を形成する。これらの固溶合金成分が少なくなった近傍では、焼入れ性が低下し、焼入れ時にマルテンサイトが生成せずに、トルースタイトやベイナイトが生成する。特に酸素は結晶粒界に沿って侵入し易く、結晶粒界にそって浸炭異常層が生成する。この結晶粒界にそった浸炭異常層は特に粒界酸化層と呼ばれている。鋼材表面に粒界酸化層が生成すると、粒界酸化層は欠陥として作用するため、その深さが深いほど強度が低下することが知られている。 Next, a description will be given of a combination of two treatments, a crystal grain refining treatment by repeated quenching and a carburizing abnormal layer reduction or reduction treatment. When performing a gas carburizing process, oxygen contained in the atmosphere enters from the surface of the steel material and forms oxides by combining with Si, Mn, and Cr in the vicinity of the grain boundaries. In the vicinity where these solid solution alloy components are reduced, the hardenability is lowered, and martensite is not generated at the time of quenching, and troostite and bainite are generated. In particular, oxygen easily penetrates along the crystal grain boundary, and an abnormal carburization layer is generated along the crystal grain boundary. The carburized abnormal layer along the grain boundary is particularly called a grain boundary oxide layer. It is known that when a grain boundary oxide layer is formed on the surface of a steel material, the grain boundary oxide layer acts as a defect, so that the strength decreases as the depth increases.

ところで、ガス浸炭した材料は結晶粒径を小さくしていった場合、ある程度までは、結晶粒径が小さくなるほど強度は向上するが、ある粒径以下に小さくしても、強度は飽和して向上しない。この理由としては、粒界酸化層が影響していると推定される。すなわち、結晶粒径が粒界酸化層より大きい場合は、結晶粒径が小さくなればなるほど強度は向上する。しかし、結晶粒径が粒界酸化層より小さくなると、粒界酸化層の方が欠陥として大きくなり、結晶粒微細化の効果が得られないと考えられる。したがって、結晶粒微細化の効果を最大限に発揮させようとすれば、研削に限らないが、浸炭異常層を除去することが必須である。一方、浸炭異常層を低減して強度を向上させる方法も知られている。しかし、この方法でも結晶粒が大きければ浸炭異常層の低減の効果が十分に得られず、結晶粒微細化と組み合わせることで強度を大きく向上させることができる。つまり、ガス浸炭を行なう場合、浸炭部品の高強度化のためには、「結晶粒微細化」と「浸炭異常層の低減あるいは削減」のいずれか片方ではそれぞれの効果は十分発揮できないが、これらの二つを組み合わせることによって大きな効果が得られる。 By the way, when the crystal grain size of the gas carburized material is reduced, the strength is improved as the crystal grain size is reduced to a certain extent, but the strength is saturated and improved even if the crystal grain size is reduced below a certain grain size. do not do. It is estimated that this is because the grain boundary oxide layer has an influence. That is, when the crystal grain size is larger than the grain boundary oxide layer, the strength improves as the crystal grain size becomes smaller. However, if the crystal grain size is smaller than the grain boundary oxide layer, the grain boundary oxide layer becomes larger as a defect, and it is considered that the effect of crystal grain refinement cannot be obtained. Therefore, if the effect of crystal grain refinement is to be maximized, it is essential to remove the carburized abnormal layer, although not limited to grinding. On the other hand, a method of improving the strength by reducing the carburized abnormal layer is also known. However, even in this method, if the crystal grains are large, the effect of reducing the carburizing abnormal layer cannot be sufficiently obtained, and the strength can be greatly improved by combining with the refinement of crystal grains. In other words, when gas carburizing is performed, in order to increase the strength of carburized parts, either of “grain refinement” and “reduction or reduction of abnormal carburizing layer” cannot exert their respective effects sufficiently. A great effect can be obtained by combining the two.

本発明は、表層部の浸炭異常層などの欠陥を除去することと、さらに結晶粒を微細化する両方法の手段でもって、自動車、建設機械、工作機械などのギアやシャフトなどの機械部品の浸炭鋼材による高強度浸炭部品を、従来の鋼材に比して、加工性を低下することなく、低コストで製造可能とすることができるなど、本発明の方法は従来にない優れた効果を奏するものである。 The present invention eliminates defects such as a carburized abnormal layer in the surface layer part, and further uses both means for refining crystal grains, and is used for gears and shafts of automobiles, construction machines, machine tools, etc. The high strength carburized parts made of carburized steel can be manufactured at a low cost without degrading workability as compared with conventional steel, and the method of the present invention has an excellent effect that has never been achieved. Is.

本発明の方法を実施するための最良の形態について表および図面を参照して説明する。先ず、表１に示す比較例のＮｏ．１〜６と本発明の実施例のＮｏ．１〜１５の化学成分を含有するそれぞれの鋼を、１００ｋｇ真空誘導溶解炉で溶製してインゴットに鋳造した。これらの鋼において、Ａｌ、Ｎｂ、Ｖ、Ｔｉの析出物をいったん固溶させ、その後に熱処理で微細に析出させるため、このインゴットを１２５０℃に加熱し、５時間保持して溶体化処理を行い、析出物を微細に析出させた鋼材を得た。 The best mode for carrying out the method of the present invention will be described with reference to tables and drawings. First, No. of the comparative example shown in Table 1. 1 to 6 and Nos. Of Examples of the present invention. Each steel containing 1 to 15 chemical components was melted in a 100 kg vacuum induction melting furnace and cast into an ingot. In these steels, in order to once precipitate Al, Nb, V, Ti precipitates and then finely precipitate them by heat treatment, this ingot is heated to 1250 ° C. and held for 5 hours for solution treatment. Thus, a steel material on which precipitates were finely precipitated was obtained.

上記の溶体化処理した鋼材を角４０ｍｍの素材に鍛伸した。この素材を９００℃に加熱し、１時間保持した後、空冷することにより焼きならしを行ない、図１に示す２ｍｍ１０ＲＣノッチ２のシャルピー衝撃性試験片１と、図２に示す２ｍｍＶノッチ４の静曲げ試験片３を作製した。これらの試験片を、それぞれ図３に示すように９３０℃に加熱して０．５時間予熱しガス浸炭を３時間行い、２．５時間保持して拡散し、８３０℃に下げて０．５時間保持し、次いで６０℃の油に焼入れし、次いで１８０℃に加熱して１．５時間保持して焼戻した。さらに繰返し焼入れを行なうものは、浸炭焼入れ後に図４に示す８５０℃に０．５時間保持して６０℃に油焼入れを１〜２回繰り返した後、１８０℃に加熱して１．５時間保持して焼戻す条件により、ズブ焼入れをして結晶粒を微細化し、焼戻しを行った。すなわち、ガス浸炭焼入れし、このままのものである浸炭焼入れままのものと、さらに、これに加えて１回または２回のズブ焼入れと焼戻しを行った。これらの場合、（１）浸炭焼入れ・焼戻して、これを表２に「浸炭焼入れまま」と示したものと、さらに（１）の浸炭焼入れに加えて、（２）のズブ焼入れ１回の繰返し焼入れをした後に焼戻したものと、または（１）の浸炭焼入れに加え、（３）の２回の繰返し焼入れをした後に焼戻したものとを、それぞれ表２に「浸炭焼入れまま」、「ズブ焼入れ１回」、および「ズブ焼入れ２回」と示し、これらの３種の焼入れ・焼戻しを実施した。 The solution-treated steel material was forged into a 40 mm square material. This material was heated to 900 ° C., held for 1 hour, and then air-cooled to normalize, and a 2 mm 10 RC notch 2 Charpy impact test piece 1 shown in FIG. 1 and a 2 mm V notch 4 static piece shown in FIG. A bending test piece 3 was produced. As shown in FIG. 3, each of these test pieces was heated to 930 ° C., preheated for 0.5 hours, gas carburized for 3 hours, held for 2.5 hours to diffuse, lowered to 830 ° C. and 0.5%. Hold for hours, then quench into oil at 60 ° C., then heat to 180 ° C. and hold for 1.5 hours to temper. In the case of further quenching, after carburizing and quenching, hold at 850 ° C. shown in FIG. 4 for 0.5 hours and repeat oil quenching at 60 ° C. once or twice, then heat to 180 ° C. and hold for 1.5 hours. Then, under the conditions of tempering, the crystal grains were refined by tempering and tempered. That is, gas carburizing and quenching was performed, and the carburizing and quenching as it was, and in addition to this, one or two times of quenching and tempering were performed. In these cases, (1) Carburizing and tempering, which is shown in Table 2 as “Carburizing and quenching”, and in addition to (1) Carburizing and quenching, (2) Submerged quenching once. Table 2 shows the products that were tempered after quenching, or those that were tempered after two repeated quenchings in (3) in addition to (1) carburizing and quenching. These were indicated as “once” and “submerged twice”, and these three types of quenching and tempering were performed.

以上のように、焼入れ・焼戻し条件を３種に変化させることによって、結晶粒の異なる試験片を作製し、その衝撃強度および静曲げ強度と、それらに及ぼす結晶粒径の影響を調査した。上記の熱処理終了後に、浸炭層を完全に除去するために、試験片の側面およびノッチ面の反対面を２ｍｍ研削した。さらに浸炭異常層除去の効果を確認するために、試験片のノッチ面を０．１ｍｍ研削したものと、異常層を除去しない試験片を作製した。 As described above, test pieces having different crystal grains were prepared by changing the quenching and tempering conditions into three types, and the impact strength and static bending strength, and the influence of the crystal grain size on them were investigated. After completion of the heat treatment, in order to completely remove the carburized layer, the side surface of the test piece and the opposite surface of the notch surface were ground by 2 mm. Further, in order to confirm the effect of removing the carburizing abnormal layer, a test piece having a notched surface of 0.1 mm ground and a test piece not removing the abnormal layer were prepared.

上記のように作製したシャルピー衝撃試験片を、シャルピー衝撃試験機を用いて衝撃試験し、その亀裂発生エネルギーにより衝撃値を評価し、この評価をシャルピー衝撃試験片の浸炭層表面の平均結晶粒径とあわせて、表２に衝撃試験結果として示した。表２で、シャルピー衝撃試験片の浸炭層表面の平均結晶粒径はμｍを単位として示し、衝撃値は比較例のＮｏ．１の浸炭焼入れままで浸炭異常層を除去していない試験片の亀裂発生エネルギーを１．０とし、この値を基準として対比したそれぞれの亀裂発生エネルギーの値により示した。なお、衝撃試験は室温で行った。 The Charpy impact test piece produced as described above is subjected to an impact test using a Charpy impact tester, and the impact value is evaluated by the crack initiation energy. This evaluation is the average grain size of the carburized layer surface of the Charpy impact test piece. The results are shown in Table 2 as impact test results. In Table 2, the average grain size of the carburized layer surface of the Charpy impact test piece is shown in μm, and the impact value is No. of the comparative example. The crack initiation energy of the test piece in which the carburized abnormal layer was not removed while being carburized and quenched was set to 1.0, and the crack initiation energy was compared with this value as a reference. The impact test was performed at room temperature.

表２に示すように、比較例の鋼は浸炭焼入れ後のズブ焼入れ１回では旧オーステナイト粒がやや小さくなるが、ズブ焼入れ２回でもそれ以上はほとんど小さくならなかった。一方、実施例の鋼は比較例の鋼に比して浸炭焼入れ後の１回のズブ焼入れで旧オーステナイト粒径が大幅に小さくなり、２回のズブ焼入れを繰り返すとさらに小さくなった。 As shown in Table 2, in the steel of the comparative example, the prior austenite grains were slightly reduced by one sub-quenching after carburizing and quenching, but even after two sub-quenching, the steel was hardly reduced any more. On the other hand, in the steel of the example, the prior austenite grain size was significantly reduced by one sub-quenching after carburizing and quenching as compared with the steel of the comparative example, and became smaller when the sub-quenching was repeated twice.

以上のように、比較例の鋼は浸炭焼入れ後にズブ焼入れを繰り返しても旧オーステナイト粒径は小さくなっておらず、熱処理後に浸炭異常層を除去したものも、除去していないものも、衝撃強度はほとんど向上しなかった。これに対し、実施例の鋼は、熱処理後に浸炭異常層を除去したものも除去していないものも、浸炭焼入れ後のズブ焼入れを繰り返すことで、旧オーステナイト粒径は小さくなっているが、浸炭異常層を除去していないものは、旧オーステナイト粒径が小さくなっても衝撃強度は大きく向上していない。これに対し、熱処理後に浸炭異常層を除去したものは、旧オーステナイト粒径の微細化により衝撃強度が大きく向上した。以上の様に、実施例の鋼を用いて、浸炭焼入れ後に繰返し焼入れを行なった後浸炭異常層を除去することにより衝撃値が大幅に向上した。 As described above, the steel of the comparative example does not reduce the prior austenite grain size even after repeated quenching after carburizing and quenching. There was little improvement. On the other hand, the steel of the example has the old austenite grain size reduced by repeating the submerged quenching after carburizing and quenching, both of which the carburized abnormal layer has been removed after heat treatment and which has not been removed. In the case where the abnormal layer is not removed, the impact strength is not greatly improved even when the prior austenite grain size is reduced. On the other hand, in the case where the carburized abnormal layer was removed after the heat treatment, the impact strength was greatly improved by the refinement of the prior austenite grain size. As described above, the impact value was significantly improved by removing the carburizing abnormal layer after repeatedly quenching after carburizing and quenching using the steel of the example.

さらに、上記で熱処理を行った静曲げ試験片３を、図５に示すように、支点間距離５０ｍｍの３点曲げにより、中心のクロスヘッドを２ｍｍ／ｍｉｎの移動速度で、すなわち静曲げ試験片３の両端部を下方から支持して中心５を下方の矢印方向に荷重を掛けて押し、静曲げ試験を実施した。この試験により、静曲げ試験片３の表面層に亀裂が生じた時点における荷重を亀裂発生荷重として静曲げ強度を評価し、浸炭層表面の平均結晶粒径とあわせて、表３に静曲げ試験の結果を示す。表３において、比較例のＮｏ．１の浸炭焼入れままの亀裂発生荷重を１．０とし、この値を基準に対比した値でそれぞれの亀裂発生荷重を示した。なお、この静曲げ試験は室温で行った。 Further, as shown in FIG. 5, the static bending test piece 3 subjected to the heat treatment as described above is subjected to three-point bending with a fulcrum distance of 50 mm, and the central crosshead is moved at a moving speed of 2 mm / min, that is, the static bending test piece. The both ends of 3 were supported from below, and the center 5 was pushed by applying a load in the downward arrow direction, and a static bending test was performed. By this test, the static bending strength was evaluated using the load at the time when a crack occurred in the surface layer of the static bending specimen 3 as a crack generation load, and the static bending test is shown in Table 3 together with the average grain size of the surface of the carburized layer. The results are shown. In Table 3, No. of the comparative example. The crack initiation load of 1 carburizing and quenching was set to 1.0, and each crack initiation load was shown as a value compared with this value. This static bending test was performed at room temperature.

表３に示すように、比較例の鋼は衝撃試験片と同様に、浸炭焼入れ後のズブ焼入れ１回では、旧オーステナイト粒がやや小さくなるが、ズブ焼入れ２回でもそれ以上はほとんど小さくならなかった。一方、実施例の鋼は、比較例の鋼に比して、浸炭焼入れ後の１回のズブ焼入れで、旧オーステナイト粒径が大幅に小さくなり、２回のズブ焼入れを繰り返すとさらに小さくなった。 As shown in Table 3, the steel of the comparative example, like the impact test piece, is slightly smaller in the prior austenite grains after one quenching after carburizing and quenching, but it does not become much smaller even after two times quenching. It was. On the other hand, compared with the steel of the comparative example, the steel of the example was greatly reduced in the prior austenite grain size by one sub-quenching after carburizing and quenching, and further reduced by repeating the sub-quenching twice. .

以上のように、比較例の鋼は、浸炭焼入れ後にズブ焼入れを繰り返しても、旧オーステナイト粒径は小さくなっておらず、熱処理後に浸炭異常層を除去したものも、除去していないものも、静曲げ強度はほとんど向上しなかった。これに対し、実施例の鋼は、熱処理後に浸炭異常層を除去したものも、除去していないものも、浸炭焼入れ後のズブ焼入れを繰り返すことで、旧オーステナイト粒径は小さくなっている。さらに浸炭異常層を除去していないものは、旧オーステナイト粒径が小さくなっても静曲げ強度は大きく向上していないのに対し、熱処理後に浸炭異常層を除去したものは、旧オーステナイト粒径の微細化により静曲げ強度が大きく向上した。以上の様に、実施例の鋼を用いて、浸炭焼入れ後に繰返し焼入れを行なった後、浸炭異常層を除去することにより大幅に静曲げ強度が向上した。 As described above, the steel of the comparative example, even if repeated quenching after carburizing and quenching, the prior austenite grain size is not reduced, both the carburized abnormal layer removed after the heat treatment, those that have not been removed, The static bending strength was hardly improved. On the other hand, in the steels of the examples, both the case where the abnormal carburizing layer was removed after the heat treatment and the case where the carburized abnormal layer was not removed were repeatedly subjected to sub-quenching after carburizing and quenching, thereby reducing the prior austenite grain size. In addition, those in which the carburized abnormal layer is not removed do not significantly improve the static bending strength even when the prior austenite grain size is reduced, whereas those in which the carburized abnormal layer is removed after the heat treatment are those of the old austenite grain size The static bending strength is greatly improved by miniaturization. As described above, the static bending strength was greatly improved by removing the carburized abnormal layer after repeatedly quenching after carburizing and quenching using the steel of the example.

以上に説明したように、本発明の方法の繰返し焼入れを行って、焼戻し後に浸炭異常層を除去することで、結晶粒の微細な衝撃強度および静曲げ強度に優れた浸炭部品を製造することができた。 As described above, by repeatedly quenching the method of the present invention and removing the carburizing abnormal layer after tempering, it is possible to produce a carburized part having excellent fine impact strength and static bending strength of crystal grains. did it.

シャルピー衝撃試験片の形状・大きさを示す図である。It is a figure which shows the shape and magnitude | size of a Charpy impact test piece. 静曲げ試験片の形状・大きさを示す図である。It is a figure which shows the shape and magnitude | size of a static bending test piece. 浸炭焼入れ・焼戻し条件を示す図である。It is a figure which shows carburizing quenching and tempering conditions. 繰返し焼入れ・焼戻し条件を示す図である。It is a figure which shows repeated hardening and tempering conditions. 試験片に静曲げ試験方法を示す図である。It is a figure which shows the static bending test method to a test piece.

符号の説明Explanation of symbols

１シャルピー衝撃試験片
２１０Ｒ２ｍｍＣノッチ
３静曲げ試験片
４２ｍｍＶノッチ
５中心 1 Charpy impact test piece 2 10R2mmC notch 3 Static bending test piece 4 2mmV notch 5 Center

Claims

質量％で、Ｃ：０．１０〜０．４５％、Ｓｉ：０．０５〜２．０％、Ｍｎ：０．１０〜２．０％、Ｐ：０．０３０％以下、Ｓ：０．２０％以下、Ｃｒ：０．３０〜３．０％、Ｃｕ：０．３０％以下、Ａｌ：０．００１〜０．１％、Ｎ：０．０１％未満、Ｔｉ：０．０５％未満、Ｂ：０．００１０〜０．００５０％を含有し、さらにＮｂ：０．０２〜０．５０％、Ｖ：０．０２〜０．５０％のいずれか１種もしくは２種を含有し、ただし、ＴｉおよびＮは３．４Ｎ［％］＜Ｔｉ［％］の関係を有し、残部Ｆｅおよび不可避不純物からなる鋼を用い、機械加工もしくは鍛造によって部品形状に成形した後、ガス浸炭焼入れを行ない、その後に１回以上のズブ焼入れを行なった後、これを焼戻し、その後に浸炭異常層を除去することにより浸炭部品を製造することを特徴とする衝撃強度および曲げ強度に優れた浸炭部品の製造方法。 In mass%, C: 0.10 to 0.45%, Si: 0.05 to 2.0%, Mn: 0.10 to 2.0%, P: 0.030% or less, S: 0.20 %: Cr: 0.30-3.0%, Cu: 0.30% or less, Al: 0.001-0.1%, N: less than 0.01%, Ti: less than 0.05%, B : 0.0010 to 0.0050%, Nb: 0.02 to 0.50%, V: 0.02 to 0.50% any one or two of them, Ti And N have a relationship of 3.4 N [%] <Ti [%], and the steel composed of the remaining Fe and inevitable impurities is used. After forming into a part shape by machining or forging, gas carburizing and quenching is performed. Carburized parts by performing tempering at least once and then tempering, and then removing the abnormal carburizing layer Impact strength and bending method for producing superior carburized component strength, characterized in that to manufacture.

請求項１の鋼成分に加え、さらに質量％で、Ｎｉ：０．２０〜５．０％、Ｍｏ：０．０５〜３．０％のいずれか１種もしくは２種を含有し、残部Ｆｅおよび不可避不純物からなる鋼を用い、機械加工もしくは鍛造によって部品形状に成形した後、ガス浸炭焼入れを行ない、その後に１回以上のズブ焼入れを行なった後、これを焼戻し、その後に浸炭異常層を除去することにより浸炭部品を製造することを特徴とする衝撃強度および曲げ強度に優れた浸炭部品の製造方法。 In addition to the steel component of claim 1, further containing, by mass%, any one or two of Ni: 0.20 to 5.0% and Mo: 0.05 to 3.0%, and the balance Fe and Using steel consisting of inevitable impurities, after forming into a part shape by machining or forging, gas carburizing and quenching is performed, followed by tempering one or more times and then tempering, and then removing the carburizing abnormal layer A method of manufacturing a carburized part excellent in impact strength and bending strength, characterized in that a carburized part is manufactured.

浸炭異常層を除去する部位は浸炭部品であるギアの歯元やスプライン軸の底や段付き部品などの応力集中部であることを特徴とする請求項１または２に記載の衝撃強度および曲げ強度に優れた浸炭部品の製造方法。 The impact strength and bending strength according to claim 1 or 2, wherein the part where the abnormal carburizing layer is removed is a stress concentration part such as a gear tooth base, a spline shaft bottom or a stepped part which is a carburized part. An excellent carburized part manufacturing method.