JP6794012B2 - Mechanical structural steel with excellent grain coarsening resistance, bending fatigue resistance, and impact resistance - Google Patents

Mechanical structural steel with excellent grain coarsening resistance, bending fatigue resistance, and impact resistance Download PDF

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JP6794012B2
JP6794012B2 JP2015241171A JP2015241171A JP6794012B2 JP 6794012 B2 JP6794012 B2 JP 6794012B2 JP 2015241171 A JP2015241171 A JP 2015241171A JP 2015241171 A JP2015241171 A JP 2015241171A JP 6794012 B2 JP6794012 B2 JP 6794012B2
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bainite
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武 宮崎
武 宮崎
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Sanyo Special Steel Co Ltd
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本発明は、浸炭(ガス浸炭、真空浸炭、高濃度浸炭、プラズマ浸炭など)や浸炭窒化、浸窒などの表面硬化処理を施して使用される自動車もしくは建設機械、産業機械用の動力伝達部品(例えばギヤ、シャフトなど)に用いる鋼であって、特に耐結晶粒粗大化特性および耐曲げ疲労強度、耐衝撃強度に優れた機械構造用部品に関するものである。 The present invention is a power transmission component for automobiles, construction machines, and industrial machines used by subjecting surface hardening treatment such as carburizing (gas carburizing, vacuum carburizing, high concentration carburizing, plasma carburizing, etc.), carburizing nitriding, and carburizing. For example, it is a steel used for gears, shafts, etc.), and particularly relates to mechanical structural parts having excellent crystal grain coarsening resistance, bending fatigue resistance, and impact resistance.

自動車、建設機械、産業機械などのギヤ用やシャフト用には、代表的な機械構造用鋼としてJIS規格鋼のSCr420のクロム鋼、SCM420のクロムモリブデン鋼、SNCM420のニッケルクロムモリブデン鋼などが広く使用されている。これら機械構造用鋼の製造工程は、主に熱間鍛造や冷間鍛造、場合によっては切削加工などの何れかが選択されて実施されることになる。 For gears and shafts of automobiles, construction machinery, industrial machinery, etc., JIS standard steel SCr420 chrome steel, SCM420 chrome molybdenum steel, SNCM420 nickel chrome molybdenum steel, etc. are widely used as typical machine structural steels. Has been done. The manufacturing process of these mechanical structural steels is mainly carried out by selecting one of hot forging, cold forging, and in some cases, cutting.

すなわち、熱間鍛造の工程は、機械構造用鋼を準備した上で、鋼材切断→熱間鍛造→焼ならし→粗加工→浸炭焼入れ・焼戻し→仕上げ加工の工程順からなるのが一般的である。この機械構造用鋼の熱間鍛造後のミクロ組織は、結晶粒度特性に有利であるフェライトやパーライトだけでなく、結晶粒度特性に対して不利といわれるベイナイト組織を生成する場合もある。そのために、通常は、粗加工時の被削性や浸炭時の結晶粒度特性の改善を目的として、熱間鍛造後に焼ならしが行われる場合が多い。しかし、熱間鍛造後に焼ならしを行っても化学成分の適正化がなされていない鋼材の場合には、ベイナイト組織を多く生成してしまうため、浸炭時に結晶粒粗大化を引き起こし、浸炭部品の曲げ疲労強度や衝撃強度の低下だけでなく、熱処理時にひずみが増加してしまう事例があった。 In other words, the hot forging process generally consists of the process of steel cutting → hot forging → normalizing → roughing → carburizing quenching / tempering → finishing after preparing steel for machine structure. is there. The microstructure of this mechanical structural steel after hot forging may form not only ferrite and pearlite, which are advantageous for grain size characteristics, but also bainite structure, which is said to be disadvantageous for crystal size characteristics. Therefore, in many cases, normalizing is usually performed after hot forging for the purpose of improving machinability during roughing and crystal grain size characteristics during carburizing. However, in the case of steel materials whose chemical composition has not been optimized even after hot forging and then normalizing, a large amount of bainite structure is generated, which causes coarsening of crystal grains during carburizing and causes carburized parts. In addition to the decrease in bending fatigue strength and impact strength, there were cases where strain increased during heat treatment.

また、昨今、CO2排出量の削減や、製造コスト低減の観点から、熱間鍛造後の焼ならし省略のニーズが高まっている。熱間鍛造後に結晶粒度特性に有利といわれるフェライト+パーライト組織になるように、熱間鍛造条件や化学成分の適正化を行っても、極端に微細なフェライト・パーライト粒径や粗大なフェライト・パーライト粒径の場合には、その後の浸炭時に結晶粒粗大化を招き、部品の要求特性を満たすことができない問題もあった。 Recently, from the viewpoint of reducing CO 2 emissions and manufacturing costs, there is an increasing need for omitting normalizing after hot forging. Even if the hot forging conditions and chemical components are optimized so that the ferrite + pearlite structure is said to be advantageous for crystal grain size characteristics after hot forging, extremely fine ferrite pearlite grain size and coarse ferrite pearlite In the case of the particle size, there is also a problem that the crystal grains become coarse during the subsequent carburizing and the required characteristics of the parts cannot be satisfied.

一方、冷間鍛造工程は、機械構造用鋼を準備した上で、鋼材切断→軟化焼なまし→冷間鍛造→焼ならし→粗加工→浸炭焼入れ・焼戻し→仕上げ加工の工程順からなるのが一般的である。このように、冷間鍛造は室温付近で加工を行うため歩留まりが良好なこと、また成形時の寸法精度が高いことから、後の粗工程時の工程負荷を軽減できる利点がある。ところで、冷間鍛造を行うには変形能を向上させる必要があるため、鋼材に軟化焼なましが施される。冷間鍛造後に直接浸炭処理を行った場合、昇温時に再結晶フェライト粒を経ることで微細なオーステナイト粒を形成し、粒の合体・成長を駆動力にして結晶粒粗大化を引き起こし易くなる。そこで、冷間鍛造で導入された加工ひずみを解消させる目的で焼ならしが通常行われる。しかし、焼ならしを行った場合であっても、化学成分の適正化がなされていない鋼材の場合には、結晶粒度特性に対して不利といわれるベイナイト組織を多く生成してしまうため浸炭時に結晶粒粗大化を引き起こし、浸炭部品の強度低下だけでなく、熱処理ひずみ増加を招いてしまう事例があった。 On the other hand, the cold forging process consists of the process of steel cutting → softening annealing → cold forging → normalizing → roughing → carburizing quenching / tempering → finishing after preparing steel for machine structure. Is common. As described above, cold forging has an advantage that the yield is good because the processing is performed at around room temperature and the dimensional accuracy at the time of molding is high, so that the process load in the subsequent roughing process can be reduced. By the way, since it is necessary to improve the deformability in order to perform cold forging, the steel material is softened and annealed. When the carburizing treatment is performed directly after cold forging, fine austenite grains are formed by passing through recrystallized ferrite grains at the time of temperature rise, and the coalescence and growth of the grains are used as a driving force to easily cause grain coarsening. Therefore, normalizing is usually performed for the purpose of eliminating the processing strain introduced in the cold forging. However, even when normalizing is performed, in the case of steel materials for which the chemical composition has not been optimized, many bainite structures, which are said to be disadvantageous to the grain size characteristics, are generated, so crystals are formed during carburizing. In some cases, grain coarsening was caused, which not only reduced the strength of carburized parts but also increased heat treatment strain.

冷間鍛造工程の場合でもCO2排出量削減や製造コスト低減を指向して、冷間鍛造後の焼ならし省略が多く試みられてきた。しかし、冷間鍛造前の軟化焼なまし組織が結晶粒度特性に有効といわれるフェライト+球状炭化物組織やフェライト+パーライト組織であっても、フェライトやパーライト粒径が極端に微細であったり、粗大であった場合にも浸炭時に結晶粒粗大化を引き起こし、浸炭部品の曲げ疲労や衝撃強度の低下だけでなく、熱処理ひずみ増加を招く問題があった。 Even in the cold forging process, many attempts have been made to omit normalizing after cold forging with the aim of reducing CO 2 emissions and manufacturing costs. However, even if the softened annealed structure before cold forging is a ferrite + spherical carburized structure or a ferrite + pearlite structure, which is said to be effective for crystal grain size characteristics, the ferrite and pearlite grain size are extremely fine or coarse. Even in such a case, there is a problem that coarsening of crystal grains is caused at the time of carburizing, which causes not only bending fatigue and a decrease in impact strength of the carburized parts but also an increase in heat treatment strain.

浸炭時の結晶粒粗大化を防止する技術として、AlやNb、Tiなどの元素を適切量添加し、これらの析出物を微細にかつ大量にマトリックス中に分散させ、これらの析出物のピンニング効果を利用する技術が提案されている。さらに鋼塊またはブルームから鋼片またはビレットへの圧延工程で加熱温度の適正化を行い、製造条件を適切に調整することによって結晶粒粗大化のピンニング粒子として寄与する析出物の制御方法が出願されている(例えば、特許文献2参照。)。しかし、この先行技術では鋼塊やブルームの冷却速度や、鋼片(ビレット)の加熱温度に制限が必要なため、生産性の点から制約が多い問題があった。また、幅広い寸法の圧延を行う実際のラインでは安定的に製造ができない問題もあった。 As a technique for preventing grain coarsening during carburizing, an appropriate amount of elements such as Al, Nb, and Ti are added, and these precipitates are finely and in large quantities dispersed in the matrix, and the pinning effect of these precipitates is achieved. Technology has been proposed. Furthermore, a method for controlling precipitates that contributes as pinning particles for grain coarsening by optimizing the heating temperature in the rolling process from a steel ingot or bloom to a steel piece or billet and appropriately adjusting the manufacturing conditions has been filed. (See, for example, Patent Document 2). However, this prior art has a problem that there are many restrictions from the viewpoint of productivity because it is necessary to limit the cooling rate of steel ingots and blooms and the heating temperature of steel pieces (billets). In addition, there is a problem that stable production cannot be performed on an actual line for rolling a wide range of dimensions.

特開平08−199316号公報Japanese Unexamined Patent Publication No. 08-199316 特開平09−78184号公報Japanese Unexamined Patent Publication No. 09-78184 特開2007−031787号公報Japanese Unexamined Patent Publication No. 2007-031787 特開2007−162128号公報JP-A-2007-162128 特開2010−222634号公報JP-A-2010-22634

本発明は、上記問題点を鑑みて開発されたものであり、本願発明が解決しようとする課題は、熱間鍛造後に焼ならし処理して浸炭する工程もしくは直接浸炭する工程、または、冷間鍛造後に焼ならし処理して浸炭する工程もしくは直接浸炭する工程のいずれを選択した場合であっても、耐結晶粒粗大化特性に優れ、かつ浸炭部品の耐曲げ疲労強度、耐衝撃強度に優れた機械構造用鋼を提供することである。 The present invention has been developed in view of the above problems, and the problem to be solved by the present invention is a step of normalizing and carburizing after hot forging, a step of direct carburizing, or a cold. Regardless of whether the step of normalizing and carburizing after forging or the step of direct carburizing is selected, the crystal grain coarsening resistance is excellent, and the carburized parts are excellent in bending fatigue resistance and impact resistance. It is to provide steel for machine structural use.

上記の課題を解決するための本発明の手段は、請求項1の手段では、化学成分が質量%で、C:0.13〜0.33%、Si:0.10〜0.80%、Mn:0.20〜1.00%、P:0.030%以下、S:0.030%以下、Cr:1.30〜2.80%、Al:0.010〜0.040%、N:0.0250%以下、O:0.0020%以下を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼である。 In the means of the present invention for solving the above problems, the chemical component is mass%, C: 0.13 to 0.33%, Si: 0.10 to 0.80%, in the means of claim 1. Mn: 0.25 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.30 to 2.80%, Al: 0.010 to 0.040%, N A state in which steel containing 0.0250% or less and O: 0.0020% or less and the balance of which is composed of Fe and unavoidable impurities is further hot-processed, and its microstructure is a structure composed of ferrite and pearlite. It is either a structure composed of ferrite, pearlite and bainite, or a structure composed of ferrite and bainite. In these microstructures, the ferrite particle size of the ferrite structure is 10 to 70 μm, and the pearlite particle size of the pearlite structure is 20 to 200 μm. The bainite particle size of the bainite structure is 20 to 200 μm, the area ratio of the bainite structure is 85% or less when the bainite structure is present, and the hardness of the above-mentioned microstructure is 130 to 350 HV. Bainite steel.

請求項2の手段では、請求項1に記載の化学成分に加えて、質量%で、Ni:0.25〜0.50%、Mo:0.08〜0.50%の1種または2種を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼である。 In the means of claim 2, in addition to the chemical component according to claim 1, one or two kinds of Ni: 0.25 to 0.50% and Mo: 0.08 to 0.50% in mass%. A state in which steel containing Fe and unavoidable impurities is further hot-worked, and its microstructure is composed of ferrite and pearlite, a structure consisting of ferrite, pearlite and bainite, or ferrite and bainite. The ferrite particle size of the ferrite structure in these microstructures is 10 to 70 μm, the pearlite particle size of the pearlite structure is 20 to 200 μm, and the bainite particle size of the bainite structure is 20 to 200 μm. When a bainite structure is present, the area ratio is 85% or less, and the hardness of the above-mentioned microstructure is 130 to 350 HV, which is a mechanical structural steel.

請求項3の手段では、請求項1又は2に記載の化学成分に加えて、質量%で、Nb:0.01〜0.08%を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼である。 In the means of claim 3, in addition to the chemical component according to claim 1 or 2, a steel containing Nb: 0.01 to 0.08% in mass% and the balance of Fe and unavoidable impurities is used. Further hot-worked, the microstructure is either a structure composed of ferrite and pearlite, a structure composed of ferrite, pearlite and bainite, or a structure composed of ferrite and bainite, and in these microstructures, The ferrite particle size of the ferrite structure is 10 to 70 μm, the pearlite particle size of the pearlite structure is 20 to 200 μm, the bainite particle size of the bainite structure is 20 to 200 μm, and the area ratio is 85% or less when the bainite structure is present. It is a bainite steel having a hardness of 130 to 350 HV as described above .

請求項4の手段では、請求項1〜3のいずれかに1項に記載の化学成分の鋼が冷間加工前の軟化焼きなましされた状態であって、そのミクロ組織はフェライトと球状炭化物からなる組織またはフェライトとパーライトからなる組織で、ベイナイト組織は存在せず、その時のフェライト粒径が10〜70μm、パーライト粒径が10〜70μmで、かつ前述のミクロ組織の硬さが65〜88HRBであることを特徴とする機械構造用鋼である。 In the means of claim 4, the steel having the chemical component according to any one of claims 1 to 3 is in a softened and annealed state before cold working, and its microstructure is composed of ferrite and spherical carbide. A structure or a structure composed of ferrite and pearlite, in which a bainite structure does not exist, the ferrite particle size at that time is 10 to 70 μm, the pearlite particle size is 10 to 70 μm, and the hardness of the above-mentioned microstructure is 65 to 88 HRB. It is a mechanical structural steel characterized by this.

請求項5の手段では、請求項1から3のいずれか1項に記載のミクロ組織、および該ミクロ組織の硬さを有する熱間加工された鋼がさらに浸炭された状態であって、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号が7.0以上であることを特徴とする機械構造用鋼である。 In the means of claim 5, the microstructure according to any one of claims 1 to 3 and the hot-worked steel having the hardness of the microstructure are further carburized, and the carburized layer. It is a steel for machine structure characterized in that the former austenite crystal grain size number of the surface and the core of the non-carburized layer is 7.0 or more .

請求項6の手段では、冷間加工された請求項4に記載のミクロ組織、および該ミクロ組織の硬さを有する鋼がさらに浸炭された状態であって、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号が7.0以上であることを特徴とする機械構造用鋼である。 In the means of claim 6, the microstructure according to claim 4 which has been cold-worked and the steel having the hardness of the microstructure are further carburized, and the carburized layer surface and the non-carburized layer core portion. The former austenite crystal grain size number is 7.0 or more, which is a mechanical structural steel .

本発明の機械構造用鋼は、鋼中の化学成分のうちAl、N、Nbなどの量を適正化下だけでなく、その他のC、Si、Mn、Cr、Ni、Moなどの量を適正化し、かつ浸炭前のフェライト、パーライト、ベイナイトなどの組織の粒径のサイズを規定することで、熱間加工または冷間加工であれ、またその後の焼ならしの有無によらず、従来の浸炭用鋼に比べて耐結晶粒度粗大化特性に優れているだけでなく、耐曲げ疲労強度や耐衝撃強度に優れた浸炭部品が得られる。したがって、本発明の機械構造用鋼を用いると、自動車、建設機械、産業機械などに用いるギヤ、シャフトなどの動力伝達部品を高温浸炭する際の、浸炭時間の短縮や焼ならし工程の省略により、製造コストの低減、部品強度の向上による軽量化およびコンパクト化、ならびに熱処理ひずみの減少による切削加工の負荷の低減が可能になるなど、従来にない優れた効果を有する。 In the mechanical structural steel of the present invention, not only the amounts of Al, N, Nb, etc. among the chemical components in the steel are optimized, but also the amounts of other C, Si, Mn, Cr, Ni, Mo, etc. are appropriate. By defining the size of the grain size of the structure of ferrite, pearlite, bainite, etc. before carburizing, conventional carburizing can be performed whether it is hot or cold, and with or without subsequent normalizing. It is possible to obtain carburized parts having excellent bending fatigue resistance and impact resistance as well as excellent crystal grain size coarsening characteristics as compared with bainite. Therefore, when the mechanical structural steel of the present invention is used, the carburizing time is shortened and the normalizing process is omitted when the power transmission parts such as gears and shafts used in automobiles, construction machines, industrial machines, etc. are carburized at high temperature. It has unprecedented excellent effects such as reduction of manufacturing cost, weight reduction and compactness by improving component strength, and reduction of cutting load by reducing heat treatment strain.

浸炭焼入焼戻し条件の処理工程を示す図である。It is a figure which shows the processing process of the carburizing, quenching and tempering conditions. 軟化焼なまし条件の処理工程を示す図である。It is a figure which shows the processing process of the softening annealing condition. (a)は曲げ試験を示し、(b)は試験片の形状を示す図で、数値の単位はmmである。(A) shows a bending test, (b) is a figure showing the shape of a test piece, and the unit of numerical value is mm. 実験で採用したシャルピー衝撃試験片の形状を示す図である。It is a figure which shows the shape of the Charpy impact test piece adopted in an experiment.

本願の請求項1〜6の手段の発明を実施するための形態の説明に先立って、本願の請求項1〜3の手段における鋼中の化学成分の限定理由、ならびに本願の請求項〜6の鋼の熱間加工後のミクロ組織、ミクロ組織の大きさ、その硬さ、ならびに浸炭した際の浸炭層表面および非浸炭層芯部の旧オーステナイト結晶粒度番号の大きさなどの限定理由を以下に順次説明する。 Prior to the description of the embodiment for carrying out the invention of the means of claims 1 to 6 of the present application, the reasons for limiting the chemical components in the steel in the means of claims 1 to 3 of the present application, and claims 1 to 6 of the present application. The reasons for the limitation such as the microstructure after hot working of the steel, the size of the microstructure, its hardness, and the size of the former austenite crystal grain number of the carburized layer surface and the non-carburized layer core when carburized are as follows. Will be described in order.

上記の課題を解決するための手段における耐結晶粒粗大化特性および耐曲げ疲労強度、耐衝撃強度に優れた鋼成分の限定理由を以下に記載する。なお、以下の各成分の%は、質量%で示す。 The reasons for limiting the steel components having excellent grain coarsening resistance, bending fatigue resistance, and impact resistance in the means for solving the above problems will be described below. In addition,% of each component below is shown by mass%.

C:0.13〜0.33%
Cは、機械構造用部品として浸炭処理後の浸炭層面の強度ならびに芯部の強度を確保するために、機械構造用鋼として必要な元素である。しかし、Cが0.13%未満では芯部の強度を十分に得られず、0.33%を超えると芯部の靭性を低下させるとともに、冷間加工性も劣化させる。そこで、Cは0.13〜0.33%とし、望ましくは0.15〜0.27%とする。
C: 0.13 to 0.33%
C is an element required as a mechanical structural steel in order to secure the strength of the carburized layer surface and the strength of the core portion after the carburizing treatment as a mechanical structural component. However, if C is less than 0.13%, the strength of the core portion cannot be sufficiently obtained, and if it exceeds 0.33%, the toughness of the core portion is lowered and the cold workability is also deteriorated. Therefore, C is 0.13 to 0.33%, preferably 0.15 to 0.27%.

Si:0.10〜0.80%
Siは、鋼の溶製時の脱酸に必要な元素であるとともに、鋼に必要な強度や焼入性を与え、かつ耐焼戻し軟化抵抗性を向上して耐ピッチング特性も向上させるために有効な元素である。しかし、Siが0.10%未満では上記した効果を十分に得られない。一方、Siが0.80%を超えると、焼ならしや軟化焼なまし後の鋼の硬さが上昇して加工性が劣化し、かつ、浸炭時の粒界酸化の助長を引き起し、さらに鋼の表面層のC濃度を狙いよりも低くする浸炭阻害を引き起こす。そこで、Siは0.10〜0.80%とし、望ましくは0.20〜0.65%とする。
Si: 0.10 to 0.80%
Si is an element required for deoxidation during melting of steel, and is effective for imparting the strength and hardenability required for steel, improving tempering and softening resistance, and improving pitching resistance. Element. However, if Si is less than 0.10%, the above-mentioned effect cannot be sufficiently obtained. On the other hand, when Si exceeds 0.80%, the hardness of the steel after normalizing or softening and annealing increases, the workability deteriorates, and the intergranular oxidation during carburizing is promoted. In addition, it causes carburizing inhibition that lowers the C concentration of the steel surface layer than intended. Therefore, Si is 0.10 to 0.80%, and preferably 0.25 to 0.65%.

Mn:0.20〜1.00%
Mnは、鋼の溶製時の脱酸に必要な元素であるとともに、鋼の焼入性や強度のを向上に有効な元素である。しかし、Mnが0.20%未満では鋼の焼入性向上の効果を確保できない。また、Mnが1.00%を超えると鋼の被削性や冷間加工性が劣化する。そこで、Mnは0.20〜1.00%とし、望ましくは0.20〜0.85%、より望ましくは0.25〜0.65%とする
Mn: 0.20-1.00%
Mn is an element necessary for deoxidation during melting of steel and is an element effective for improving the hardenability and strength of steel. However, if Mn is less than 0.20%, the effect of improving the hardenability of steel cannot be ensured. Further, if Mn exceeds 1.00%, the machinability and cold workability of the steel deteriorate. Therefore, Mn is set to 0.25 to 1.00%, preferably 0.25 to 0.85%, and more preferably 0.25 to 0.65%.

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り、オーステナイト粒界に偏析して鋼の曲げ疲労強度や衝撃強度などを低下させる元素である。一方、Pはフェライトの固溶強化元素であり、0.030%より多いと、冷間加工前の軟化焼なまし後の鋼の硬さの上昇を招く。そこで、Pは0.030%以下とする。
P: 0.030% or less P is an unavoidable element contained from scrap, and is an element that segregates at the austenite grain boundaries to reduce the bending fatigue strength and impact strength of steel. On the other hand, P is a solid solution strengthening element of ferrite, and if it is more than 0.030%, the hardness of the steel after softening and annealing before cold working is increased. Therefore, P is set to 0.030% or less.

S:0.030%以下
Sは、非金属介在物であるMnSの形成によって鋼の被削性を向上させる元素である。しかし、MnSを多く生成すると、鋼の冷間加工性や、曲げ疲労強度および衝撃強度などを劣化させる。そこで、Sは0.030%以下とする。
S: 0.030% or less S is an element that improves the machinability of steel by forming MnS, which is a non-metal inclusion. However, if a large amount of MnS is produced, the cold workability of steel, bending fatigue strength, impact strength, and the like are deteriorated. Therefore, S is set to 0.030% or less.

Cr:1.30〜2.80%
Crは、鋼の焼入れ性や強度向上を与えるだけでなく、焼戻し軟化抵抗を向上してこの耐ピッチング特性を確保するために重要な元素である。しかし、Crが1.30%未満では、それらの効果が十分に得られず、2.80%を超えると、鋼の被削性や加工性を劣化する。また、Crは2.80%を超えて含有されると、浸炭雰囲気によっては鋼に網目状セメンタイトが析出し、曲げ疲労強度や衝撃強度を大きく低下させることが調査から分かった。そこで、Crは1.30〜2.80%とし、望ましくは1.60〜2.50%とする。
Cr: 1.30 to 2.80%
Cr is an important element not only for improving hardenability and strength of steel, but also for improving temper softening resistance and ensuring this pitching resistance property. However, if Cr is less than 1.30%, those effects cannot be sufficiently obtained, and if it exceeds 2.80%, the machinability and workability of the steel are deteriorated. Further, it was found from the investigation that when Cr is contained in excess of 2.80%, reticulated cementite is deposited on the steel depending on the carburized atmosphere, and the bending fatigue strength and the impact strength are greatly reduced. Therefore, Cr is set to 1.30 to 2.80%, preferably 1.60 to 2.50%.

Al:0.010〜0.040%
Alは、鋼の溶製時の脱酸元素として必要であり、固溶Nと結合してAlNとして析出し、浸炭時の結晶粒粗大化を抑制する効果のある元素である。Alが0.010%未満ではその効果は十分ではなく、Alが0.050%を超えると鋼中にアルミナ系酸化物が増加し、曲げ疲労特性や加工性を低下する。そこで、Alは0.010〜0.050%とし、望ましくは0.015〜0.035%とする。
Al: 0.010 to 0.040%
Al is necessary as a deoxidizing element at the time of melting steel, and is an element having an effect of combining with a solid solution N and precipitating as AlN to suppress grain coarsening at the time of carburizing. If Al is less than 0.010%, the effect is not sufficient, and if Al exceeds 0.050%, alumina-based oxides increase in the steel, and bending fatigue characteristics and workability are lowered. Therefore, Al is set to 0.010 to 0.050%, and preferably 0.015 to 0.035%.

O:0.0020%以下
Oは、鋼の耐曲げ疲労強度や耐衝撃特性に対して有害な酸化系介在物を形成する元素である。そこで、Oは0.0020%以下とする。
O: 0.0020% or less O is an element that forms oxidative inclusions that are harmful to the bending fatigue strength and impact resistance of steel. Therefore, O is set to 0.0020% or less.

N:0.0250%以下
Nは、鋼中のAlやNbと結合してAlNやNbCNを形成することで、浸炭時の結晶粒粗大化の抑制に寄与する有効な元素である。しかし、その効果はNが0.0250%で飽和し、それより多く含有させると熱間加工性を劣化させる。そこで、Nは0.0250%以下とする。
N: 0.0250% or less N is an effective element that contributes to the suppression of grain coarsening during carburizing by combining with Al and Nb in steel to form AlN and NbCN. However, the effect is that N is saturated at 0.0250%, and if it is contained in a larger amount, the hot workability is deteriorated. Therefore, N is set to 0.0250% or less.

Nb:0.01〜0.08%
Nbは、炭化物あるいは炭窒化物を形成し、微細に分散したナノオーダーのNbCまたはNb(CN)が浸炭時の結晶粒粗大化の抑制効果をもたらす。Nbが0.01%未満ではその効果が十分に得られず、0.08%を超えると析出物の量が過剰になり加工性を低下させる。また浸炭雰囲気や浸炭部品の表面状態によっては0.08%を超えて添加すると浸炭阻害が発生する場合もある。そこで、Nbは0.01〜0.08%、望ましくは0.03〜0.07%とする。
Nb: 0.01 to 0.08%
Nb forms carbides or carbonitrides, and finely dispersed nano-order NbC or Nb (CN) has an effect of suppressing grain grain coarsening during carburizing. If Nb is less than 0.01%, the effect cannot be sufficiently obtained, and if it exceeds 0.08%, the amount of precipitates becomes excessive and the workability is lowered. Further, depending on the carburized atmosphere and the surface condition of the carburized parts, addition of more than 0.08% may cause carburizing inhibition. Therefore, Nb is set to 0.01 to 0.08%, preferably 0.03 to 0.07%.

Ni:0.25〜0.50%
Niは、添加しなくてもよい元素である。しかし、通常の精錬では原料から0.25%未満程度のNiが精錬時に含有される。ところで、Niは0.25%以上に添加することで、鋼の焼入性や強度を向上させ得る元素でもある。しかし、Niの含有量が0.50%を超えると、熱間鍛造後の冷却時に容易にベイナイトを形成して、結晶粒度特性や加工性や被削性を劣化する。また、冷間加工前の軟化焼なまし時にNi添加の影響によって、軟質なフェライトやパーライトを形成しないで、ベイナイトを生成し易くなるため、冷間加工性や被削性に悪影響を及ぼす。そこで、Niは0.25〜0.50%とする。
Ni: 0.25 to 0.50%
Ni is an element that does not need to be added. However, in normal refining, less than 0.25% of Ni is contained from the raw material during refining. By the way, Ni is also an element that can improve the hardenability and strength of steel by adding it to 0.25% or more. However, if the Ni content exceeds 0.50%, bainite is easily formed during cooling after hot forging, and the crystal grain size characteristics, processability, and machinability deteriorate. In addition, due to the effect of Ni addition during softening and annealing before cold working, bainite is easily generated without forming soft ferrite and pearlite, which adversely affects cold workability and machinability. Therefore, Ni is set to 0.25 to 0.50%.

Mo:0.08〜0.50%
Moは、添加しなくてもよい元素である。しかし、通常の精錬では原料から0.08%未満程度のMoが精錬時に含有される。ところで、Moは0.08%以上に添加することで、鋼の焼入性や強度を向上させ得る元素でもある。しかし、Moの含有量が0.50%を超えると、熱間鍛造後の冷却時にベイナイトを形成し、結晶粒度特性や加工性を劣化する。また、冷間加工前の軟化焼なまし時にMo添加の影響によってベイナイトを生成するため、冷間加工性や被削性に悪影響を及ぼす。そこで、Moは0.08〜0.50%とする。
Mo: 0.08 to 0.50%
Mo is an element that does not need to be added. However, in normal refining, less than 0.08% of Mo is contained from the raw material during refining. By the way, Mo is also an element that can improve the hardenability and strength of steel by adding it to 0.08% or more. However, if the Mo content exceeds 0.50%, bainite is formed during cooling after hot forging, and the crystal grain size characteristics and processability are deteriorated. In addition, bainite is generated by the effect of Mo addition during softening and annealing before cold working, which adversely affects cold workability and machinability. Therefore, Mo is set to 0.08 to 0.50%.

NiおよびMoは、請求項2の手段または請求項2を引用する請求項3の手段で、上記のそれぞれの成分の範囲で1種または2種を選択的に含有できる。 Ni and Mo can selectively contain one or two kinds in the range of each of the above-mentioned components by the means of claim 2 or the means of claim 3 which cites claim 2.

請求項1〜3の手段の鋼成分を有する鋼の熱間加工後のミクロ組織は、フェライト組織とパーライト組織、フェライト組織とパーライト組織とベイナイト組織、もしくはフェライト組織とベイナイト組織からなるミクロ組織であって、その時のフェライト粒径は10〜70μm、パーライト粒径は20〜200μm、ベイナイト粒径は20〜200μmであり、さらにベイナイト組織を有する場合、ベイナイト組織の面積率は85%以下であり、かつ、これらのミクロ組織の硬さは130HV〜350HVであるとした理由
フェライト粒径は10μm未満になると、浸炭加熱時に微細な逆変態オーステナイトを多く生成するため、結晶粒粗大化を招いてしまう。逆にフェライト粒径が70μmより大きくなると、浸炭加熱時の逆変態オーステナイトの生成数が少なくなるため、浸炭後の結晶粒径が粗大(結晶粒度番号は小さくなる)になってしまう。そこで、フェライト粒径は10〜70μmとした。パーライト粒径も同じく20μm未満になると、浸炭加熱時にオーステナイトの核生成数が多くなるため粗大化が起こり、パーライト粒径が200μmより大きくなると、浸炭後の結晶粒径が粗大になってしまう。そこで、パーライト粒径は20〜200μmにした。また、ベイナイト粒径も同様の考え方から20〜200μmとした。また熱間加工後のミクロ組織にベイナイト組織が認められる場合は、その面積率が85%を超えると浸炭加熱時の逆変態オーステナイトが多く形成されて結晶粒の粗大化を招き易いことが分かったため、熱間加工後のミクロ組織にベイナイト組織が認められる場合に、その面積率を85%以下と規定している。さらに、これらのミクロ組織の硬さについては、請求項1〜3の手段に記載の化学成分によって、または前述のいずれかのミクロ組織の形成によって、130〜350HVが実現できるため、いずれかのミクロ組織の硬さを130〜350HVと規定した。
The microstructure of the steel having the steel component of the means of claims 1 to 3 after hot working is a microstructure composed of a ferrite structure and a pearlite structure, a ferrite structure and a pearlite structure and a bainite structure, or a ferrite structure and a bainite structure. At that time, the ferrite particle size is 10 to 70 μm, the pearlite particle size is 20 to 200 μm, the bainite particle size is 20 to 200 μm, and when it has a bainite structure, the area ratio of the bainite structure is 85% or less, and The reason why the hardness of these microstructures is 130 HV to 350 HV When the ferrite particle size is less than 10 μm, a large amount of fine reverse-transformed austenite is generated during carburizing and heating, which causes coarsening of crystal grains. On the contrary, when the ferrite grain size is larger than 70 μm, the number of reverse-transformed austenites produced during carburizing heating is reduced, so that the crystal grain size after carburizing becomes coarse (the crystal grain size number becomes smaller). Therefore, the ferrite particle size was set to 10 to 70 μm. Similarly, if the pearlite grain size is less than 20 μm, the number of austenite nucleations increases during carburizing and heating, resulting in coarsening. If the pearlite grain size is larger than 200 μm, the crystal grain size after carburizing becomes coarse. Therefore, the pearlite particle size was set to 20 to 200 μm. The bainite particle size was also set to 20 to 200 μm based on the same concept. In addition, when a bainite structure is observed in the microstructure after hot working, it was found that if the area ratio exceeds 85%, a large amount of reverse-transformed austenite is formed during carburizing and heating, which tends to cause coarsening of crystal grains. When a bainite structure is found in the microstructure after hot working, the area ratio is specified as 85% or less. Further, regarding the hardness of these microstructures, 130 to 350 HV can be realized by the chemical composition according to the means of claims 1 to 3 or by the formation of any of the above-mentioned microstructures, and therefore any of the microstructures. Tissue hardness was defined as 130-350 HV.

請求項4の手段では、請求項1〜3の手段の鋼成分に加えて、冷間加工前に軟化焼なましを施した時のミクロ組織をフェライト組織と球状炭化物組織、もしくはフェライト組織とパーライト組織とし、これらにはベイナイト組織は存在せず、その時のフェライト粒径は10〜70μm、パーライト粒径は10〜70μmで、かつ前述のいずれかのミクロ組織の硬さは65〜88HRBであるとする理由
フェライト粒径10〜70μmとする理由は上記の請求項4の手段と同じで、パーライト粒径10〜70μmの限定理由は10μm未満になると、浸炭加熱時にオーステナイトの核生成数が多くなるため粗大化が起こり、パーライト粒径が70μmより大きくなると、浸炭後の結晶粒径が粗大になってしまう。そこで、パーライト粒径は10〜70μmとした。またベイナイト組織が存在すると、浸炭加熱時に逆変態オーステナイトが多く生成されることによって結晶粒が粗大化してしまうため、この組織は存在しないことと規定した。硬さについては、請求項1〜3の手段の化学成分、かつ前述のいずれかのミクロ組織の形成によって硬さ65〜85HRBが実現できるので、硬さ65〜85HRBと規定した。
In the means of claim 4, in addition to the steel components of the means of claims 1 to 3, the microstructure when softened and annealed before cold working is a ferrite structure and a spherical carbide structure, or a ferrite structure and pearlite. As a structure, there is no baynite structure in these, the ferrite particle size at that time is 10 to 70 μm, the pearlite particle size is 10 to 70 μm, and the hardness of any of the above-mentioned microstructures is 65 to 88 HRB. Reason for this The reason for setting the ferrite particle size to 10 to 70 μm is the same as the means of claim 4 above, and the reason for limiting the pearlite particle size to 10 to 70 μm is that if it is less than 10 μm, the number of austenite nucleated during carburizing and heating increases. When coarsening occurs and the pearlite particle size becomes larger than 70 μm, the crystal particle size after carbonization becomes coarse. Therefore, the pearlite particle size was set to 10 to 70 μm. In addition, if a bainite structure is present, a large amount of reverse-transformed austenite is generated during carburizing and heating, which coarsens the crystal grains. Therefore, it is defined that this structure does not exist. Regarding the hardness, since the hardness of 65 to 85 HRB can be realized by the chemical composition of the means of claims 1 to 3 and the formation of any of the above-mentioned microstructures, the hardness is defined as 65 to 85 HRB.

請求項5および請求項6の手段において、請求項1〜3の手段に記載の鋼の化学成分、ならびに請求項1〜4の手段に記載のミクロ組織、硬さに調整した機械構造用鋼を浸炭した際に、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号が7.0以上であるとした理由
耐曲げ疲労強度や耐衝撃強度は、浸炭後の旧オーステナイト粒が整細な(すなわち旧オーステナイト結晶粒度番号が大きい)程、優れることが分かっている。しかし、一部に粗大化した結晶粒が存在するとその部分を起点として破壊に至りやすくなり、曲げ疲労強度や衝撃強度が低下してしまう。曲げ疲労強度や衝撃強度の確保には、機械構造用鋼を浸炭した際の、浸炭後の旧オーステナイト粒度番号7.0以上が必要であることが判ったので、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号は7.0以上であると規定した。
In the means of claims 5 and 6, the chemical composition of the steel according to the means of claims 1 to 3 and the microstructure and hardness of the mechanical structural steel adjusted to the means of claims 1 to 4 are used. The reason why the old austenite crystal grain size number on the surface of the carburized layer and the core of the non-carburized layer was 7.0 or more when carburized, the bending fatigue resistance and impact resistance are as follows. It has been found that the higher the old austenite grain size number (that is, the higher the grain size number), the better. However, if coarsened crystal grains are present in a part of the crystal grains, the crystal grains are likely to be broken starting from that part, and the bending fatigue strength and the impact strength are lowered. In order to secure bending fatigue strength and impact strength, it was found that the old austenite particle size number 7.0 or higher after carburizing when carburizing steel for machine structure is required, so the carburized layer surface and non-carburized layer core The former austenite crystal grain size number of the part was specified to be 7.0 or more.

次いで、本発明を実施するための形態について、表および図を参照しながら、以下に説明する。 Next, a mode for carrying out the present invention will be described below with reference to tables and figures.

表1に示す実施例のNo.1〜30および比較例のNo.1〜10の各化学成分と残部がFeおよび不可避的不純物からなる鋼の各100kgを真空溶解炉で溶製した。この表1の実施例および比較例では、Feおよび不可避的不純物を除いて含有される全ての化学成分を示している。ただし、比較例の網かけ部は本願の請求項の範囲を外れていることを示す。 No. of Examples shown in Table 1. Nos. 1 to 30 and Comparative Examples. 100 kg each of steel having each of the chemical components 1 to 10 and the balance consisting of Fe and unavoidable impurities was melted in a vacuum melting furnace. In the examples and comparative examples of Table 1, all the chemical components contained except Fe and unavoidable impurities are shown. However, the shaded portion of the comparative example indicates that it is out of the scope of the claims of the present application.

上記で溶製して得た実施例および比較例の鋼を熱間鍛伸により、径40mmの棒鋼とした。さらに、鍛伸組織の均質化を目的としてこれらの焼ならし処理を行った。その後、熱間加工を想定して1200℃に再加熱し、空冷を行い、径40mmの棒鋼の中周部から組織観察用試験片、曲げ試験片、およびシャルピー衝撃試験片を採取した。組織観察用試験片は鋼材径中周部の長手方向に切出し、この試験片用いてミクロ組織観察や、フェライト粒径、パーライト粒径、ベイナイト粒径、ベイナイト面積率、およびビッカース硬さ(荷重300g)の各測定を行った。曲げ試験片とシャルピー衝撃試験片は、1200℃から空冷後の棒鋼の中周部から試験片を採取した後、図1に示す浸炭焼入焼戻し処理工程を実施した。 The steels of Examples and Comparative Examples obtained by melting above were hot-forged to obtain steel bars having a diameter of 40 mm. Furthermore, these normalizing treatments were carried out for the purpose of homogenizing the forged structure. Then, assuming hot working, it was reheated to 1200 ° C. and air-cooled, and a test piece for structure observation, a bending test piece, and a Charpy impact test piece were collected from the middle circumference of a steel bar having a diameter of 40 mm. The test piece for structure observation is cut out in the longitudinal direction of the middle circumference of the steel material diameter, and this test piece is used for microstructure observation, ferrite grain size, pearlite grain size, bainite grain size, bainite area ratio, and Vickers hardness (load 300 g). ) Was carried out. The bending test piece and the Charpy impact test piece were subjected to the charcoal-burning and tempering treatment step shown in FIG. 1 after the test pieces were collected from the middle circumference of the steel bar after air cooling from 1200 ° C.

さらに上記の溶製した鋼から熱間鍛伸して得た径32mmの棒鋼では、1200℃から空冷した際の中周部の冷却速度は0.5℃/s程度であることが分かっている。実際に熱間鍛伸を行った部品のミクロ組織や硬さの調査を行ったところ、冷却速度は約0.5℃/sに相当することが分かったことから、前述の棒鋼径、再加熱温度を選定した。 Further, it is known that the cooling rate of the middle peripheral portion of the steel bar having a diameter of 32 mm obtained by hot forging from the above-mentioned molten steel when air-cooled from 1200 ° C. is about 0.5 ° C./s. .. As a result of investigating the microstructure and hardness of the parts that were actually hot-stretched, it was found that the cooling rate was equivalent to about 0.5 ° C / s, so the steel bar diameter and reheating mentioned above were found. The temperature was selected.

冷間加工を想定した場合も、実施例および比較例の各鋼を、熱間鍛伸により径32mmの棒鋼とし、さらに、鍛伸組織の均質化を目的として焼ならし処理を行った。その後、図2に示す軟化焼なまし処理工程を施した後、引抜きによって径20mmとなるまで冷間加工を行った。組織観察用の試験片は鋼材の径中周部を長手方向に切出し、この試験片を用いてミクロ組織の観察や、フェライト粒径、パーライト粒径、ベイナイト組織の有無およびブリネル硬さなどの測定を行った。曲げ試験片とシャルピー衝撃試験片は軟化焼なまし後の棒鋼の中周部から試験片を採取し、図1に示す浸炭焼入焼戻し処理工程を実施した。 Even in the case of assuming cold working, each of the steels of Examples and Comparative Examples was made into steel bars having a diameter of 32 mm by hot forging, and further subjected to normalizing treatment for the purpose of homogenizing the forged structure. Then, after performing the softening annealing treatment step shown in FIG. 2, cold working was performed until the diameter became 20 mm by drawing. The test piece for microstructure observation is made by cutting out the middle circumference of the steel material in the longitudinal direction, and using this test piece, observation of microstructure and measurement of ferrite grain size, pearlite grain size, presence or absence of bainite structure, Brinell hardness, etc. Was done. For the bending test piece and the Charpy impact test piece, the test piece was taken from the middle circumference of the steel bar after softening and annealing, and the carburizing, quenching and tempering treatment step shown in FIG. 1 was carried out.

表2に請求項1〜3の実施例および比較例として、熱間加工を想定した焼ならし後の径32mmの棒鋼を1200℃に再加熱した時のミクロ組織や、フェライト粒径、パーライト粒径、ベイナイト粒径、ベイナイト面積率、硬さの測定結果を示す。ミクロ組織における各粒径は光学顕微鏡で観察した写真から各20〜30個を測定し、その平均値から求めた。ベイナイト面積率の測定は光学顕微鏡で観察した写真を画像解析して算出した。実施例はフェライト粒径、パーライト粒径、ベイナイト粒径、ベイナイト面積率、硬さのいずれも請求項1〜3の構成の内容を満たしている。一方、比較例についてはNo.3〜5、No.7を除いて請求項1〜3の構成から外れている。 In Table 2, as examples and comparative examples of claims 1 to 3 , the microstructure, ferrite grain size, and pearlite grains when a steel bar having a diameter of 32 mm after normalizing assuming hot working is reheated to 1200 ° C. The measurement results of diameter, bainite particle size, bainite area ratio, and hardness are shown. Each particle size in the microstructure was determined from the average value of 20 to 30 particles each measured from photographs observed with an optical microscope. The measurement of the bainite area ratio was calculated by image analysis of photographs observed with an optical microscope. In the examples, the ferrite particle size, the pearlite particle size, the bainite particle size, the bainite area ratio, and the hardness all satisfy the contents of the configurations of claims 1 to 3 . On the other hand, for comparative examples, No. 3-5, No. Except for 7, it is out of the configuration of claims 1 to 3 .

さらに、表2に請求項4の実施例および比較例として、冷間加工を想定した焼ならし後の径32mmの棒鋼を軟化焼なましを行った時のミクロ組織や、フェライト粒径、パーライト粒径、ベイナイト粒径、ベイナイト組織の有無、硬さの測定結果を示す。各組織の粒径は上記の熱間加工を想定した場合のものと同じである。実施例はいずれも請求項4に記載の内容を満たしている。一方、比較例についてはNo.1とNo.4を除いて請求項4の構成から外れている。 Further, as an example and a comparative example of claim 4 in Table 2, the microstructure, ferrite grain size, and pearlite when softened and annealed a steel bar having a diameter of 32 mm after normalizing assuming cold working is performed. The measurement results of particle size, bainite particle size, presence / absence of bainite structure, and hardness are shown. The particle size of each structure is the same as that assuming the above hot working. All of the embodiments satisfy the contents of claim 4 . On the other hand, for comparative examples, No. 1 and No. Except for 4 , it is out of the configuration of claim 4 .

表3に請求項5の実施例および比較例として、熱間加工を想定した曲げ試験や、シャルピー衝撃試験などの試験片に対して、浸炭焼入焼戻処理の熱処理を行った後、直接浸炭して、浸炭層ならびに芯部の結晶粒度番号(旧オーステナイト結晶粒度番号)、103曲げ疲労強度、シャルピー衝撃値(J/cm3)の試験結果を示す。結晶粒度番号はJIS G 0551に則って切断法で測定した。曲げ試験は図3の(b)に示す寸法の試験片に(a)に示すように正弦波10Hzの荷重を及ぼして実施し、シャルピー衝撃試験は図4に示す形状の試験片を用いて、共に常温で試験を行った。実施例の浸炭層の結晶粒度番号は7.1〜10.8、芯部のそれは7.3〜10.7であり、いずれも請求項5に記載の旧オーステナイト結晶粒度番号7以上を満たした。また3曲げ疲労強度は15〜25kN、シャルピー衝撃値は19〜36J/cm2であった。一方、比較例では、No.3〜5、No.7〜8を除くNo.1〜2、No.6、No.9〜10の結晶粒度番号7以下の曲げ疲労強度は8〜14kN、シャルピー衝撃値は8〜16J/cm2であり、実施例と比較していずれも劣る結果であった。 In Table 3, as examples and comparative examples of claim 5 , the test pieces such as the bending test assuming hot working and the Charpy impact test are heat-treated by carburizing, quenching and tempering, and then directly carburized. to, carburized layer and the grain size number of the core portions (prior austenite grain size number), 10 3 bending fatigue strength, the test results of the Charpy impact value (J / cm 3). The crystal grain size number was measured by a cutting method according to JIS G 0551. The bending test was carried out by applying a load of a sine wave of 10 Hz as shown in (a) to the test piece having the dimensions shown in FIG. 3 (b), and the Charpy impact test was carried out using the test piece having the shape shown in FIG. Both tests were conducted at room temperature. The crystal grain size number of the carburized layer of the example was 7.1 to 10.8, and that of the core portion was 7.3 to 10.7, both satisfying the former austenite crystal grain size number 7 or higher according to claim 5 . .. The 3- bending fatigue strength was 15 to 25 kN, and the Charpy impact value was 19 to 36 J / cm 2 . On the other hand, in the comparative example, No. 3-5, No. No. except 7-8. 1-2, No. 6, No. The bending fatigue strength of 9 to 10 having a crystal grain size number of 7 or less was 8 to 14 kN, and the Charpy impact value was 8 to 16 J / cm 2 , which were inferior to those of the examples.

上記の熱間加工を想定した試験において、比較例では、結晶粒度番号が7以上であったにも関わらず、曲げ疲労強度がNo.3で11kN、No.4で10kN、No.7で14kNと低く、同じく結晶粒度番号が7以上であったにも関わらず、衝撃値がNo.4で13J/cm2、No.7で16J/cm2と低かったものがあるので、この理由を次に説明する。比較例のNo.3については、C量が0.12%で低かったため、浸炭焼入焼戻後の芯部硬さが低下したことによって、曲げ疲労強度が11kNであったと考えられる。なお、衝撃値は低Cの効果によって34J/cm2で高かった。比較例のNo.4については、Cr量が2.95%で高かったことにより、浸炭時に強度低下を招く網状炭化物を形成したことで、曲げ疲労強度や衝撃値が低位であったと思われる。また比較例のNo.7については、O量が23ppmで高位であったことにより、比較的大きなAl23の非金属介在物を形成し、曲げ疲労強度や衝撃値が低下したと考えられる。なお、比較例のNo.5とNo.7は熱間加工を想定したものにおいては、実施例と同様の曲げ疲労強度や衝撃値を有したが、冷間加工を想定した試験では優位な結果は得られなかった。 In the above test assuming hot working, in the comparative example, the bending fatigue strength was No. 1 even though the crystal grain size number was 7 or more. 3 is 11 kN, No. 4 is 10 kN, No. Although it was as low as 14 kN at 7 and the crystal grain size number was 7 or more, the impact value was No. 4 is 13 J / cm 2 , No. Some of them were as low as 16 J / cm 2 in 7, so the reason for this will be explained next. Comparative example No. Regarding No. 3, since the C amount was as low as 0.12%, it is probable that the bending fatigue strength was 11 kN because the core hardness after charcoal-burning and tempering decreased. The impact value was as high as 34 J / cm 2 due to the effect of low C. Comparative example No. Regarding No. 4, it is considered that the bending fatigue strength and the impact value were low because the amount of Cr was high at 2.95% and the reticulated carbide that caused the strength decrease at the time of carburizing was formed. In addition, No. of Comparative Example. Regarding No. 7, it is considered that the O amount was as high as 23 ppm, so that relatively large non-metal inclusions of Al 2 O 3 were formed, and the bending fatigue strength and the impact value were lowered. In addition, No. of the comparative example. 5 and No. No. 7 had the same bending fatigue strength and impact value as in the examples in the case of assuming hot working, but no superior result was obtained in the test assuming cold working.

請求項6の実施例および比較例として、冷間加工を想定した曲げ試験や、シャルピー衝撃試験などの試験片に対して、浸炭焼入焼戻処理の熱処理を行った後、直接滲炭して、浸炭層ならびに芯部の結晶粒度番号(旧オーステナイト結晶粒度番号)、103曲げ疲労強度、シャルピー衝撃値(J/cm3)の試験結果を表3に示す。実施例の浸炭層の結晶粒度番号は8.0〜11.0、芯部の結晶粒度番号は8.1〜10.8であり、いずれも請求項6に記載の旧オーステナイト結晶粒度番号7以上を満たした。熱間加工を想定したものと比べて結晶粒が小さかった理由として、径32mmから径20mmまでの冷間引抜き時に組織の微細化が図られ、かつ浸炭時に結晶粒粗大化が起こらなかったためである。また、実施例の103曲げ疲労強度は18〜27kN、シャルピー衝撃値は21〜38J/cm2であった。一方、比較例については、No.3〜4を除く結晶粒度番号7以下の曲げ疲労強度は7〜15kN、シャルピー衝撃値は7〜18J/cm2であり、実施例と比較していずれも劣る結果であった。 As an example and a comparative example of claim 6, a test piece such as a bending test assuming cold working or a Charpy impact test is heat-treated by carburizing, quenching and tempering, and then directly bleached. Table 3 shows the test results of the crystal grain size number of the carburized layer and the core (former austenite crystal grain size number), 10 3 bending fatigue strength, and Charpy impact value (J / cm 3 ). The crystal grain size number of the carburized layer of the example is 8.0 to 11.0, and the crystal grain size number of the core portion is 8.1 to 10.8, both of which are the former austenite crystal grain size numbers 7 or higher according to claim 6. Satisfied. The reason why the crystal grains were smaller than those assuming hot working was that the structure was miniaturized during cold drawing from a diameter of 32 mm to 20 mm, and the crystal grains did not become coarse during carburizing. .. In addition, the 10 3 bending fatigue strength of Example was 18 to 27 kN, and the Charpy impact value was 21 to 38 J / cm 2 . On the other hand, for comparative examples, No. The bending fatigue strength of the crystal grain size number 7 or less excluding 3 to 4 was 7 to 15 kN, and the Charpy impact value was 7 to 18 J / cm 2 , which were inferior to those of the examples.

冷間加工を想定した試験で、比較例のNo.3、No.4は結晶粒度番号が7以上であるにも関わらず、曲げ疲労強度および衝撃値が低かった理由を次に説明する。比較例のNo.3はC量が0.12%であることで芯部硬さが低かったことで曲げ疲労強度が低く、比較例のNo.4はCr量が2.95%で高いことによって浸炭時に網状炭化物を形成したことで曲げ疲労強度の低下およびシャルピー衝撃値の低下を招いたと考えられる。なお、比較施例8はNbを0.09%添加しているにも関わらず結晶粒が粗大化した理由として、軟化焼なましのフェライト粒径が微細であったことにより、浸炭加熱時に微細な逆変態オーステナイトを多く生成したためと思われる。 In the test assuming cold working, No. of Comparative Example. 3, No. The reason why the bending fatigue strength and the impact value of No. 4 were low even though the crystal grain size number was 7 or more will be described next. Comparative example No. In No. 3, the bending fatigue strength was low because the C amount was 0.12% and the core hardness was low. In No. 4, it is considered that the high Cr content of 2.95% caused the formation of reticulated carbide during carburizing, which resulted in a decrease in bending fatigue strength and a decrease in Charpy impact value. In Comparative Example 8, the reason why the crystal grains were coarsened despite the addition of 0.09% of Nb was that the ferrite grain size of the softened annealing was fine, so that it was fine during carburizing and heating. It is thought that this is because a large amount of reverse metamorphosis austenite was produced.

以上の説明から明らかなように、本発明による機械構造用鋼は化学成分を適正化し、かつ浸炭前のフェライト組織、パーライト組織、ベイナイト組織のサイズや面積率を規定することで、熱間加工または冷間加工であれ、またその後の焼ならしの有無によらず、従来の浸炭用鋼に比べて耐結晶粒粗大化特性だけでなく耐曲げ疲労強度や耐衝撃強度に優れた浸炭部品が得られる。したがって、本発明の機械構造用鋼を用いると自動車、建設機械、産業機械などにおけるギヤ、シャフトなどの動力伝達部品の高温浸炭化による浸炭時間の短縮や焼ならしの工程省略による製造コスト低減、部品強度の向上による軽量かつコンパクト化、熱処理ひずみ減による切削加工の負荷低減などが可能になり、従来にない優れた機械構造用鋼とすることができる。 As is clear from the above description, the mechanical structural steel according to the present invention is hot-worked or hot-worked by optimizing the chemical composition and defining the size and area ratio of the ferrite structure, pearlite structure, and bainite structure before carburizing. Carburized parts with excellent bending fatigue resistance and impact resistance as well as crystal grain coarsening characteristics can be obtained compared to conventional carburized steel, regardless of whether it is cold-worked or with or without subsequent normalizing. Be done. Therefore, when the mechanical structural steel of the present invention is used, the carburizing time is shortened by high-temperature carburizing of power transmission parts such as gears and shafts in automobiles, construction machines, industrial machines, etc., and the manufacturing cost is reduced by omitting the normalizing process. It is possible to reduce the weight and size by improving the strength of parts, and to reduce the load of cutting by reducing the heat treatment strain, which makes it possible to obtain an excellent steel for machine structure that has never existed before.

Claims (6)

化学成分が質量%で、C:0.13〜0.33%、Si:0.10〜0.80%、Mn:0.20〜1.00%、P:0.030%以下、S:0.030%以下、Cr:1.30〜2.80%、Al:0.010〜0.040%、N:0.0250%以下、O:0.0020%以下を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、
そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼。
The chemical composition is mass%, C: 0.13 to 0.33%, Si: 0.10 to 0.80%, Mn: 0.25 to 1.00%, P: 0.030% or less, S: It contains 0.030% or less, Cr: 1.30 to 2.80%, Al: 0.010 to 0.040%, N: 0.0250% or less, O: 0.0020% or less, and the balance is Fe. And steel consisting of unavoidable impurities is further hot-worked.
The microstructure is either a structure composed of ferrite and pearlite, a structure composed of ferrite, pearlite and bainite, or a structure composed of ferrite and bainite, and the ferrite particle size of the ferrite structure in these microstructures is 10 to 70 μm. The pearlite particle size of the pearlite structure is 20 to 200 μm, the bainite particle size of the bainite structure is 20 to 200 μm, the area ratio of the bainite structure is 85% or less when the bainite structure is present, and the hardness of the above-mentioned microstructure. Is a machine structural steel characterized by being 130 to 350 HV .
請求項1に記載の化学成分に加えて、質量%で、Ni:0.25〜0.50%、Mo:0.08〜0.50%の1種または2種を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、
そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼。
In addition to the chemical component according to claim 1, 1 or 2 types of Ni: 0.25 to 0.50% and Mo: 0.08 to 0.50% are contained in mass%, and the balance is Fe. And steel consisting of unavoidable impurities is further hot-worked.
The microstructure is either a structure composed of ferrite and pearlite, a structure composed of ferrite, pearlite and bainite, or a structure composed of ferrite and bainite, and the ferrite particle size of the ferrite structure in these microstructures is 10 to 70 μm. The pearlite particle size of the pearlite structure is 20 to 200 μm, the bainite particle size of the bainite structure is 20 to 200 μm, the area ratio of the bainite structure is 85% or less when the bainite structure is present, and the hardness of the above-mentioned microstructure. Is a machine structural steel characterized by being 130 to 350 HV .
請求項1又は2に記載の化学成分に加えて、質量%で、Nb:0.01〜0.08%を含有し、残部がFeおよび不可避的不純物からなる鋼をさらに熱間加工した状態であって、
そのミクロ組織はフェライトとパーライトからなる組織、フェライトとパーライトとベイナイトからなる組織、またはフェライトとベイナイトからなる組織のいずれかの組織であり、これらミクロ組織における、フェライト組織のフェライト粒径は10〜70μm、パーライト組織のパーライト粒径は20〜200μm、ベイナイト組織のベイナイト粒径は20〜200μmであり、ベイナイト組織が存在する場合はその面積率を85%以下であり、かつ前述のミクロ組織の硬さは130〜350HVであることを特徴とする機械構造用鋼。
In a state where steel containing Nb: 0.01 to 0.08% in mass% in addition to the chemical component according to claim 1 or 2 and the balance being Fe and unavoidable impurities is further hot-worked. There,
The microstructure is either a structure composed of ferrite and pearlite, a structure composed of ferrite, pearlite and bainite, or a structure composed of ferrite and bainite, and the ferrite particle size of the ferrite structure in these microstructures is 10 to 70 μm. The pearlite particle size of the pearlite structure is 20 to 200 μm, the bainite particle size of the bainite structure is 20 to 200 μm, the area ratio of the bainite structure is 85% or less when the bainite structure is present, and the hardness of the microstructure described above. Is a machine structural steel characterized by being 130 to 350 HV .
請求項1〜3のいずれかに1項に記載の化学成分の鋼が冷間加工前の軟化焼きなましされた状態であって、
そのミクロ組織はフェライトと球状炭化物からなる組織またはフェライトとパーライトからなる組織で、ベイナイト組織は存在せず、その時のフェライト粒径が10〜70μm、パーライト粒径が10〜70μmで、かつ前述のミクロ組織の硬さが65〜88HRBであることを特徴とする機械構造用鋼。
The steel having the chemical component according to any one of claims 1 to 3 is in a state of being softened and annealed before cold working.
The microstructure is a structure composed of ferrite and spherical carbide or a structure composed of ferrite and pearlite, and there is no bainite structure. At that time, the ferrite particle size is 10 to 70 μm, the pearlite particle size is 10 to 70 μm, and the above-mentioned micro structure is used. A steel for machine structural use, characterized in that the hardness of the structure is 65 to 88 HRB.
請求項1から3のいずれか1項に記載のミクロ組織、および該ミクロ組織の硬さを有する熱間加工された鋼がさらに浸炭された状態であって、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号が7.0以上であることを特徴とする機械構造用鋼 The microstructure according to any one of claims 1 to 3 and the hot-worked steel having the hardness of the microstructure are further carburized, and the carburized layer surface and the non-carburized layer core portion. Former austenite crystal grain size number of 7.0 or higher . 冷間加工された請求項4に記載のミクロ組織、および該ミクロ組織の硬さを有する鋼がさらに浸炭された状態であって、浸炭層表面ならびに非浸炭層芯部の旧オーステナイト結晶粒度番号が7.0以上であることを特徴とする機械構造用鋼 The cold-worked microstructure according to claim 4 and the steel having the hardness of the microstructure are further carburized, and the former austenite crystal grain size number of the carburized layer surface and the non-carburized layer core portion is Machine structural steel characterized by being 7.0 or higher .
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