JP2020094231A - Manufacturing method of carburized steel part - Google Patents
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本発明は、浸炭鋼部品の製造方法に関する。 The present invention relates to a method for manufacturing a carburized steel part.
例えば、自動車のエンジンからの出力をドライブシャフトへと伝えるデフリングギヤは、静的な強度と、低サイクルから高サイクルにおける疲労強度という幅広い強度特性を必要とする駆動系部品である。すなわち、デフリングギヤは、長期間の自動車の使用中において、突発的な急発進等で瞬間的に高応力が負荷されるため、高い低サイクル疲労強度が要求される。その一方で、突発的な急発進時ほど高い応力負荷はかからない状態で長期間安心して使用可能とするためには、優れた高サイクル疲労強度の確保が必要である。従って、両者の疲労強度を両立できる技術開発が強く求められていた。 For example, a diff ring gear that transmits the output from an automobile engine to a drive shaft is a drive system component that requires a wide range of strength characteristics such as static strength and fatigue strength in low cycles to high cycles. That is, the diff ring gear is required to have a high low cycle fatigue strength because a high stress is momentarily applied due to a sudden sudden start or the like during long-term use of the vehicle. On the other hand, it is necessary to ensure excellent high cycle fatigue strength in order to be able to use the product for a long period of time in a state where a high stress load is not applied as in a sudden sudden start, for a long time. Therefore, there has been a strong demand for technological development capable of achieving both fatigue strengths.
ここで、本願明細書においては、低サイクル疲労強度とは、１００〜１００００回程度の繰り返し数まで耐えられる応力値のことを言い、高サイクル疲労強度とは、従来から普通に疲労強度と言われている耐久限度であって、例えば繰り返し数１０7回程度まで耐えられる応力値のことをいう。 Here, in the specification of the present application, low cycle fatigue strength refers to a stress value that can withstand up to 100 to 10,000 cycles, and high cycle fatigue strength is conventionally referred to as fatigue strength. This is the endurance limit, which is, for example, the stress value that can withstand up to about 10 7 repetitions.
従来の浸炭鋼部品に関する文献においては、サイクル数について特に言及のない場合が多く、この場合は、１０7回といった高サイクルでの耐久限について検討されたものであることが一般的である。そして、後述の一部の特許文献において、低サイクルに注目した発明が提案されているにすぎない。さらに、低サイクルに加え、高サイクルの疲労特性まで両立できる発明は、ほとんど提案されていない。 In the literatures on the conventional carburized steel parts, the number of cycles is often not mentioned in particular, and in this case, it is general that the durability limit in a high cycle such as 10 7 is examined. And, in some patent documents described later, only the invention focusing on the low cycle is proposed. Furthermore, almost no invention has been proposed that can achieve both high cycle fatigue characteristics in addition to low cycle fatigue.
例えば、特許文献１には、鋼材の化学成分および浸炭後の表面の硬さおよび心部の硬さを適切に制御することにより、繰り返し荷重の付加が１０2回〜１０4回の低サイクル疲労強度を高めることが示されている。特に、心部硬さをＨＶ４００〜５００とすることを特徴としている。しかしながら、本特許文献では、個々の実施例に熱処理条件に関する記載が全くなく、得られた低サイクル疲労強度と具体的な熱処理条件との関係を把握することができない。また、高サイクル疲労強度については、全く記載されていない。従って、いかなる条件とした場合に、２つの疲労強度を両立できるのか判断できない。 For example, in Patent Document 1, by appropriately controlling the chemical composition of the steel material and the hardness of the surface and the hardness of the core portion after carburization, low cycle fatigue in which repeated load is applied is 10 2 to 10 4 times. It has been shown to increase strength. In particular, the core hardness is set to HV 400 to 500. However, in this patent document, there is no description of heat treatment conditions in each example, and it is not possible to grasp the relationship between the obtained low cycle fatigue strength and specific heat treatment conditions. Further, high cycle fatigue strength is not described at all. Therefore, it cannot be determined under what conditions the two fatigue strengths can be compatible.
また、特許文献２には、鋼材の化学成分および浸炭後の表面近傍の浸炭異常層、あるいは部品芯部の不完全焼入れ相の制御をＣ、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉの添加量により規定することで繰り返し荷重の付加が１００回〜５００回の低サイクル疲労強度を高めることが示されている。しかしながら、この例でも、高サイクル疲労強度については何ら言及されておらず、特許文献１と同様に２つの疲労強度を両立できる条件の明確化が成されていない。 Further, in Patent Document 2, the chemical composition of the steel material and the abnormal carburizing layer near the surface after carburization or the incompletely hardened phase of the core of the component are controlled by adding C, Si, Mn, Cr, Mo, Cu, and Ni. It is shown that the cyclic load is increased by increasing the low cycle fatigue strength of 100 to 500 times by specifying the amount. However, even in this example, no reference is made to the high cycle fatigue strength, and as in Patent Document 1, no clarification of the condition for achieving the two fatigue strengths is made.
なお、過去に特許文献３に示す通り、幅広いサイクル数において、優れた疲労強度の得られる鋼が開発されているが、この鋼は、Ｔｉ、Ｂの必須添加を前提とする発明である。しかし、Ｔｉ、Ｂ添加鋼の場合、浸炭焼入れ処理によって生じる浸炭歪が非常に大きい点で、未添加鋼に比べて問題があるため、実部品への適用は十分にできていないのが現状である。 Note that, as shown in Patent Document 3, a steel has been developed in the past that provides excellent fatigue strength over a wide range of cycle numbers, but this steel is an invention premised on the essential addition of Ti and B. However, in the case of Ti and B-added steel, the carburizing strain caused by the carburizing and quenching treatment is very large, and there is a problem as compared with unadded steel, so that it cannot be applied to actual parts at present. is there.
本発明は、かかる背景に鑑みてなされたものであり、浸炭歪を悪化させることなく、高サイクルと低サイクルの両方において高い疲労強度特性を発揮する浸炭鋼部品の製造方法を提供しようとするものである。 The present invention has been made in view of such a background, and an object of the present invention is to provide a method for manufacturing a carburized steel component that exhibits high fatigue strength characteristics in both high cycle and low cycle without deteriorating carburizing strain. Is.
本発明の一態様は、質量比において、Ｃ：０．１０％〜０．３０％、Ｓｉ：０．７０％〜１．２０％、Ｍｎ：０．３０％〜１．００％、Ｃｒ：０．１５％〜１．２５％、Ｍｏ：０．８０％以下、Ａｌ：０．０２０％〜０．０５０％、Ｎ：０．００３０％〜０．０２００％を含有し、残部がＦｅ及び不可避的不純物からなる化学成分を有し、表面に浸炭層を備えた浸炭鋼部品を製造する方法であって、
前記化学成分を有する鋼部材に浸炭焼入れ処理を施した後、焼戻し処理を行うに当たり、当該焼戻し処理は、処理温度Ｔ（℃）が以下の式（１）及び式（２）を満足する条件で行う、浸炭鋼部品の製造方法である。
式（１）：Ｔ（℃）≧２００−４［Ｓｉ］−２９［Ｍｏ］
式（２）：１００［Ｓｉ］＋４５［Ｍｏ］＋１８０≧Ｔ（℃）
（ここで、［Ｓｉ］及び［Ｍｏ］は、それぞれＳｉの含有率（％）の値及びＭｏの含有率（％）の値を意味する）
One aspect of the present invention is C: 0.10% to 0.30%, Si: 0.70% to 1.20%, Mn: 0.30% to 1.00%, and Cr:0 in mass ratio. 0.1% to 1.25%, Mo: 0.80% or less, Al: 0.020% to 0.050%, N: 0.0030% to 0.0200%, the balance being Fe and unavoidable. A method for producing a carburized steel part having a carburized layer on the surface, which has a chemical component consisting of impurities,
After performing the carburizing and quenching treatment on the steel member having the chemical composition, the tempering treatment is performed under the condition that the treatment temperature T (°C) satisfies the following equations (1) and (2). It is a method of manufacturing a carburized steel part.
Formula (1): T (° C.)≧200-4 [Si]-29 [Mo]
Formula (2): 100 [Si]+45 [Mo]+180≧T (° C.)
(Here, [Si] and [Mo] mean the value of Si content (%) and the value of Mo content (%), respectively)
上記浸炭鋼部品の製造方法においては、上記特定の化学成分を有することを前提として、焼入れ処理後の焼戻し処理の処理温度条件を、上記化学成分のうちのＳｉとＭｏの含有率との関係で整理した式（１）及び式（２）で規定される範囲に制限することによって、確実に、高サイクルと低サイクル（極低サイクルを含む）の両方において高い疲労強度特性を発揮する浸炭鋼部品を製造することが可能となる。 In the method for manufacturing a carburized steel component, on the premise that the carburized steel component has the specific chemical component, the treatment temperature condition of the tempering treatment after the quenching treatment is related to the content ratios of Si and Mo among the chemical components. Carburized steel parts that reliably exhibit high fatigue strength characteristics in both high cycles and low cycles (including ultra-low cycles) by limiting the range defined by formulas (1) and (2) Can be manufactured.
すなわち、低サイクル領域における破損モードは、負荷の上昇に伴い、浸炭表層において初期クラック（クラックインとよび、その応力をクラックイン応力という。）を生じ、繰返し応力付与の過程で、クラックが進展し、最終的に破断に至るというものである。本願の上記製造方法においては、鋼材の化学成分の制御および浸炭処理後の焼戻し処理温度を、式（１）を満足する範囲で、従来の浸炭処理後に行われる焼戻し温度である１５０℃前後に比較して高温化することにより、浸炭層および芯部の靭性を向上させることで、クラックイン応力や最終破断応力を高め、高強度化を達成することができ、これにより、低サイクル疲労強度の向上を図ることができる。後述の実施例にて示す通り、焼戻し温度の高温化で大きな低サイクル疲労強度の改善がみられており、この効果は非常に顕著なものであり、当業者には予測できない大きな効果が得られることが見出された。 That is, the failure mode in the low cycle region causes an initial crack (called crack-in, the stress is called crack-in stress) in the carburized surface layer as the load increases, and the crack progresses in the process of applying repeated stress. That is, it will eventually break. In the above-mentioned manufacturing method of the present application, the control of the chemical composition of the steel material and the tempering temperature after carburizing treatment are compared with the tempering temperature of about 150° C. which is the tempering temperature performed after the conventional carburizing treatment within the range satisfying the formula (1). By increasing the toughness of the carburized layer and the core by increasing the temperature to high temperature, it is possible to increase the crack-in stress and final fracture stress and achieve high strength, which improves the low cycle fatigue strength. Can be planned. As shown in Examples described later, a large improvement in low cycle fatigue strength is observed with increasing tempering temperature, and this effect is very remarkable, and a large effect that cannot be predicted by those skilled in the art is obtained. It was found.
また、焼戻し温度の高温化は、浸炭表層の硬さが低下し、高サイクル疲労強度の低下を招く背反が懸念される。しかし、上記特定の化学成分を採用すると共に式（２）により焼戻し温度の上限を定めることにより、高Ｓｉ化による熱軟化抵抗性の効果を考慮した、優れた高サイクル疲労強度の得られる範囲を明確化することができる。その結果、従来の背反的な特性を打破し、高サイクルと低サイクル（極低サイクル）の両方において高い疲労強度特性を得ることが可能となる。 In addition, there is a concern that increasing the tempering temperature may reduce the hardness of the carburized surface layer and reduce the high cycle fatigue strength. However, by adopting the above-mentioned specific chemical components and setting the upper limit of the tempering temperature by the formula (2), the range in which excellent high cycle fatigue strength can be obtained in consideration of the effect of heat softening resistance due to the high Si content. Can be clarified. As a result, it becomes possible to break the conventional contradictory characteristics and obtain high fatigue strength characteristics in both high cycle and low cycle (extremely low cycle).
上記浸炭鋼部品の化学成分について、その限定理由を説明する。
Ｃ：０．１０％〜０．３０％、
Ｃ（炭素）は、内部硬さを確保するために重要な元素であり、その効果を得るためにＣ含有率は０．１０％以上とする。一方、Ｃ含有率が高すぎると、被削性劣化、靭性劣化、冷間鍛造性劣化等のおそれがあるため、上限値は０．３０％とする。
The reason for limiting the chemical composition of the carburized steel part will be described.
C: 0.10% to 0.30%,
C (carbon) is an important element for ensuring the internal hardness, and in order to obtain its effect, the C content is 0.10% or more. On the other hand, if the C content is too high, machinability deterioration, toughness deterioration, cold forgeability deterioration, etc. may occur, so the upper limit value is made 0.30%.
Ｓｉ：０．７０％〜１．２０％、
Ｓｉ（ケイ素）は、焼戻し温度を従来より高温化しても焼戻し軟化抵抗を高めることにより、高サイクル疲労強度の低下を抑制するとともに、浸炭層における粒界強化、および、浸炭異常層の形態悪化の抑制（ひげ状の粒界酸化層の抑制）効果を得るため、含有率の下限値を０．７０％とする。ここで、浸炭異常層の形態悪化とは、特開平１０−２５９４７０号公報に記載されているように、浸炭異常層の最大深さ位置から表面までの所定範囲の断面における浸炭異常層の占める面積が低下することを意味する。一方、Ｓｉ含有率が高すぎると、ガス浸炭性の劣化、被削性の劣化、冷間鍛造性の劣化等のおそれがあるため、上限値は１．２０％とする。
Si: 0.70% to 1.20%,
Si (silicon) suppresses the deterioration of high cycle fatigue strength by increasing the temper softening resistance even when the tempering temperature is higher than that of the conventional one, and at the same time, strengthens the grain boundaries in the carburized layer and deteriorates the morphology of the abnormal carburized layer. In order to obtain the suppression effect (suppression of whisker-shaped grain boundary oxide layer), the lower limit of the content rate is set to 0.70%. Here, as described in Japanese Patent Laid-Open No. 10-259470, the morphological deterioration of the abnormal carburized layer means the area occupied by the abnormal carburized layer in a predetermined range from the maximum depth position of the abnormal carburized layer to the surface. Means that it will decrease. On the other hand, if the Si content is too high, the gas carburizing property may deteriorate, the machinability may deteriorate, and the cold forgeability may deteriorate. Therefore, the upper limit value is set to 1.20%.
Ｍｎ：０．３０％〜１．００％、
Ｍｎ（マンガン）は、内部硬さ（強度）確保のため、０．３０％以上含有させる。一方、Ｍｎ含有率が高すぎると、浸炭異常層の深さが増大すると共に、残留オーステナイトが増加するおそれがあり、上限値は１．００％とする。
Mn: 0.30% to 1.00%,
Mn (manganese) is contained at 0.30% or more in order to secure internal hardness (strength). On the other hand, if the Mn content is too high, the depth of the carburized abnormal layer may increase and the retained austenite may increase, so the upper limit is made 1.00%.
Ｃｒ：０．１５％〜１．２５％、
Ｃｒ（クロム）は、内部硬さ（強度）確保のため、０．１５％以上含有させる。一方、Ｃｒ含有率が高すぎると、浸炭異常層の深さが増大すると共に、ガス浸炭性が劣化するおそれがあるため、上限値は１．２５％とする。
Cr: 0.15% to 1.25%,
Cr (chrome) is contained in an amount of 0.15% or more in order to secure the internal hardness (strength). On the other hand, if the Cr content is too high, the depth of the abnormal carburizing layer may increase and the gas carburizing property may deteriorate, so the upper limit is set to 1.25%.
Ｍｏ：０．８０％以下（０％を含む）、
Ｍｏ（モリブデン）は、必要に応じて添加できる任意添加元素であり、添加した場合には、内部硬さ（強度）確保及び浸炭異常層の抑制に有効な元素である。また、Ｍｏは、Ｓｉと同様に浸炭異常層の形態悪化の抑制に効果的な元素である。しかし、Ｍｏは、合金鉄の価格変動によって高価な元素になることがあるため、本発明では、任意添加元素としている。しかし、添加する場合でもコストと得られる効果のバランスを考慮して、上限値を０．８０％とする。
Mo: 0.80% or less (including 0%),
Mo (molybdenum) is an optional additive element that can be added as necessary, and when added, it is an element effective for ensuring internal hardness (strength) and suppressing the abnormal carburized layer. Further, Mo is an element effective in suppressing the deterioration of the morphology of the abnormal carburization layer, like Si. However, Mo may be an expensive element due to the price fluctuation of the ferroalloy, so in the present invention, it is an optional addition element. However, even when added, the upper limit is set to 0.80% in consideration of the balance between the cost and the effect obtained.
Ａｌ：０．０２０％〜０．０５０％、
Ａｌ（アルミニウム）は、ＡｌＮ生成による結晶粒微細化に有効であるため、０．０２０％以上含有させる。一方、Ａｌ含有率が高すぎると、アルミナを起点とする強度低下が生じるおそれがあるため、上限値は０．０５０％とする。
Al: 0.020% to 0.050%,
Al (aluminum) is effective in refining crystal grains due to AlN generation, so 0.020% or more is contained. On the other hand, if the Al content is too high, the strength starting from alumina may decrease, so the upper limit is made 0.050%.
Ｎ：０．００３０％〜０．０２００％、
Ｎ（窒素）は、Ａｌと共にＡｌＮを生成することによる結晶粒微細化に有効であるため、０．００３０％以上含有させる。一方、Ｎ含有率が高すぎる場合には、Ｎのガス化によって、製造性が劣化するおそれがあり、上限値は０．０２００％とする。
N: 0.0030% to 0.0200%,
N (nitrogen) is effective in refining the crystal grains by forming AlN together with Al, so N is contained in an amount of 0.0030% or more. On the other hand, if the N content is too high, the gasification of N may deteriorate the productivity, so the upper limit is made 0.0200%.
次に、上記浸炭鋼部品の製造方法における焼戻し処理は、処理温度Ｔ（℃）が以下の式（１）及び式（２）を満足する条件で行う。
式（１）：Ｔ（℃）≧２００−４［Ｓｉ］−２９［Ｍｏ］
式（２）：１００［Ｓｉ］＋４５［Ｍｏ］＋１８０≧Ｔ（℃）
（ここで、［Ｓｉ］及び［Ｍｏ］は、それぞれＳｉの含有率（％）の値及びＭｏの含有率（％）の値を意味する）
Next, the tempering treatment in the method for manufacturing a carburized steel component is performed under the condition that the treatment temperature T (°C) satisfies the following equations (1) and (2).
Formula (1): T (° C.)≧200-4 [Si]-29 [Mo]
Formula (2): 100 [Si]+45 [Mo]+180≧T (° C.)
(Here, [Si] and [Mo] mean the value of Si content (%) and the value of Mo content (%), respectively)
式（１）及び式（２）は、本発明の特徴である低サイクル疲労強度と高サイクル疲労強度を両立するための焼戻し処理温度の条件を式で表現したものである。ＳｉとＭｏの含有率で式を定めているのは、前記した成分からなる鋼について、多数の実験を繰返した結果、従来の１５０℃程度で焼戻し処理した場合に得られる性能に比較して、優れた特性の得られる焼戻し処理温度の範囲が特にＳｉ、Ｍｏ含有率によって変化することが見出されたためである。 Formulas (1) and (2) express the conditions of tempering temperature for achieving both low cycle fatigue strength and high cycle fatigue strength, which are the features of the present invention, by formulas. The content of Si and Mo defines the formula, as a result of repeating a number of experiments for the steel composed of the above-mentioned components, as compared with the performance obtained when the conventional tempering treatment is performed at about 150° C., This is because it has been found that the range of tempering temperature at which excellent characteristics are obtained varies depending on the Si and Mo contents.
すなわち、Ｓｉ、Ｍｏは前記したように、共に浸炭異常層の形態を改善し、これが低サイクル疲労強度及び高サイクル疲労強度改善に寄与する。また、Ｍｏは浸炭異常層抑制に効果的であるとともに、Ｓｉは焼戻し軟化抵抗向上により、優れた高サイクル疲労強度の得られる焼戻し処理温度の上限値拡大に寄与する。以上、説明した効果を多数のデータに基づき表現した式が、式（１）及び式（２）である。 That is, Si and Mo both improve the morphology of the abnormal carburized layer as described above, which contributes to the improvement of low cycle fatigue strength and high cycle fatigue strength. Further, Mo is effective in suppressing the abnormal carburizing layer, and Si contributes to the expansion of the upper limit of the tempering temperature at which excellent high cycle fatigue strength can be obtained by improving the temper softening resistance. The expressions expressing the effects described above based on a large number of data are the expressions (1) and (2).
そして、式（１）と式（２）の両方を満足する条件で焼戻し処理を実施することにより、低サイクル疲労強度と高サイクル疲労強度の両方に優れた特性を得ることが可能となる。一方、式（１）を満足しない場合には、低サイクル疲労強度を確保することが困難となり、式（２）を満足しない場合には、高サイクル疲労強度を確保することが困難となる。 Then, by performing the tempering treatment under the conditions that satisfy both the formula (1) and the formula (2), it becomes possible to obtain excellent characteristics in both low cycle fatigue strength and high cycle fatigue strength. On the other hand, when the formula (1) is not satisfied, it is difficult to secure low cycle fatigue strength, and when the formula (2) is not satisfied, it is difficult to secure high cycle fatigue strength.
なお、前記した通り、従来でも焼戻しを行なう点では違いがないが、従来の焼戻しは１５０〜１６０℃程度であり、式（１）で定まる下限温度より明確に低い。本発明は、成分の最適化と焼戻し処理温度の高温化を特定の条件範囲内で行うことにより、低サイクル疲労強度と高サイクル疲労強度の両立が可能となることを見出したものである。 As described above, although there is no difference in the conventional tempering, the conventional tempering is about 150 to 160° C., which is clearly lower than the lower limit temperature determined by the formula (1). The present invention has found that both low cycle fatigue strength and high cycle fatigue strength can be achieved by optimizing the components and increasing the tempering temperature within a specific condition range.
上記浸炭鋼部品の製造方法の実施例について説明する。
本例では、表１に示すごとく、１０種類の鉄鋼材料（鋼種Ａ〜Ｊ）を用意し、鍛造加工及び切削加工を施して、３種類の鋼部材、つまり、低サイクル疲労試験用の第１試験片、高サイクル疲労試験用の第２試験片、及び浸炭焼入れ歪評価用の歯車を作製した。このうち、Ａ〜Ｅ鋼が本発明の条件を満足する鋼であり、Ｆ鋼は従来のＪＩＳのＣｒ−Ｍｏ鋼、Ｇ〜Ｉ鋼は、一部の成分が本発明の条件を満足しない比較鋼、Ｊ鋼は特許文献３で開示されている過去の開発鋼である。そして、これらの３種類の鋼部材に所定の条件で浸炭焼入れ処理及び焼戻し処理を行って浸炭鋼部材とした。なお、Ａ鋼はＭｏを不純物として含有する鋼であり、Ｐ、Ｓ、Ｃｕ、Ｎｉについては請求項に記載はないが、不純物として含有していた分析値を示したものである。
An example of the method for manufacturing the carburized steel component will be described.
In this example, as shown in Table 1, ten types of steel materials (steel types A to J) are prepared, forged and cut, and subjected to three types of steel members, that is, the first for low cycle fatigue test. A test piece, a second test piece for high cycle fatigue test, and a gear for carburizing and quenching strain evaluation were produced. Among them, A to E steels are steels satisfying the conditions of the present invention, F steels are conventional JIS Cr-Mo steels, and GI steels are some components not satisfying the conditions of the present invention. Steel and J steel are past developed steels disclosed in Patent Document 3. Then, these three types of steel members were subjected to carburizing and quenching treatment and tempering treatment under predetermined conditions to obtain carburized steel members. Steel A is a steel containing Mo as an impurity, and P, S, Cu, and Ni are not described in the claims, but show the analysis values contained as impurities.
浸炭処理は、ガス浸炭法を用いた。浸炭処理条件は、浸炭ガス雰囲気において９５０℃に２．５時間保持することを基本条件とし、鋼種ごとに温度、時間、ＣＰ（カーボンポテンシャル）等の条件を調整した。浸炭処理直後の焼入れ処理は、１３０℃の油によって油冷する条件で行った。 A gas carburizing method was used for the carburizing treatment. The carburizing conditions were basically maintained at 950° C. for 2.5 hours in a carburizing gas atmosphere, and the conditions such as temperature, time and CP (carbon potential) were adjusted for each steel type. The quenching treatment immediately after the carburizing treatment was performed under the condition of oil cooling with 130° C. oil.
浸炭焼入れ処理後の焼戻し処理は、表２に示す焼戻し温度に１時間保持することを基本条件とした。そして、得られた種類の浸炭鋼部材に対して、次のようにして各評価を行った。 The tempering treatment after the carburizing and quenching treatment was performed under the basic condition that the tempering temperature shown in Table 2 was maintained for 1 hour. Then, each of the obtained types of carburized steel members was evaluated as follows.
＜低サイクル疲労試験＞
低サイクル疲労試験は、形状及びサイズが縦横１７．８ｍｍの正方形断面の角柱状の試験片であって、ノッチ角度６０°、ノッチ深さ１．８ｍｍ、ノッチ底部の曲率半径１．５ｍｍというノッチを設けた第１試験片を用いて行った。試験は、株式会社島津製作所製の３点曲げ試験装置を用い、２箇所の支点に支えられた第１試験片に対して、ノッチを設けた面と反対側の面から圧子を当接させて、１Ｈｚあるいは５Ｈｚの周波数で繰り返し所定応力を負荷して行った。評価は、繰り返し回数１００回において、折損せずに耐えうる負荷応力の値が、前記の通り過去に幅広いサイクル数で優れた性能が得られるとされたＪ鋼の値である１５４０ＭＰａ以上の場合を合格（○）とし、それ未満の場合を不合格（×）とした。
<Low cycle fatigue test>
The low cycle fatigue test is a square columnar test piece having a shape and size of 17.8 mm in length and width, and a notch angle of 60°, a notch depth of 1.8 mm and a notch bottom with a radius of curvature of 1.5 mm. It carried out using the 1st test piece provided. The test was carried out by using a three-point bending tester manufactured by Shimadzu Corporation, and an indenter was brought into contact with the first test piece supported by two fulcrums from the surface opposite to the surface provided with the notch. A predetermined stress was repeatedly applied at a frequency of 1 Hz or 5 Hz. In the evaluation, when the number of repetitions is 100, the value of the load stress that can be endured without breaking is 1540 MPa or more, which is the value of the J steel which is said to be excellent in a wide number of cycles in the past as described above. The test was judged as pass (○), and the case of less than that was judged as fail (x).
＜高サイクル疲労試験＞
高サイクル疲労試験は、形状及びサイズが直径φ１０ｍｍの円形断面の円柱状の試験片であって、ノッチ深さ１．０ｍｍ、ノッチ曲率半径１．０ｍｍというノッチを設けた第２試験片を用いて行った。試験は、株式会社島津製作所製の小野式回転曲げ試験装置（型番：Ｈ６型）を用い、回転数１８００ｒｐｍで繰り返し曲げ応力を付与して行った。評価は、繰り返し回数１０7回において、折損せずに耐えうる負荷応力の値が、従来のＣｒ−Ｍｏ鋼に通常温度である１５０℃で焼戻し処理を行って得られたＦ鋼の値である５２５ＭＰａ以上の場合を合格（○）とし、それ未満の場合を不合格（×）とした。
<High cycle fatigue test>
The high cycle fatigue test uses a second test piece having a notch with a notch depth of 1.0 mm and a notch curvature radius of 1.0 mm, which is a cylindrical test piece with a circular cross section having a diameter and a diameter of 10 mm. went. The test was performed by using an Ono-type rotary bending tester (model number: H6 type) manufactured by Shimadzu Corporation, and repeatedly applying bending stress at a rotation speed of 1800 rpm. In the evaluation, the value of the load stress that can be endured without breaking at the number of repetitions of 10 7 is the value of the F steel obtained by performing the tempering treatment on the conventional Cr-Mo steel at 150°C which is a normal temperature. The case of 525 MPa or more was passed (◯), and the case of less than 525 MPa was rejected (x).
＜焼入れ歪み評価＞
焼入れ歪み評価は、歯車を用いて行った。試験に用いた歯車は、外径：１６０ｍｍφ、軸方向厚み：２０ｍｍであり、歯数：５１、モジュール：２．５４、圧力角：１５°、ねじれ角：右３４°という諸元の歯を設けたはすば歯車を用いた。歪みの評価は、浸炭焼入れ処理前後における歯溝のふれ変化量、具体的には、歯車の歯溝に測定子を挿入し、半径方向の位置の最大値と最小値の差を求め、その値が従来のＣｒ−Ｍｏ鋼であるＦ鋼の値と同等あるいはそれよりも優れる場合を合格（○）、それよりも劣る場合を不合格（×）とした。
<Quenching strain evaluation>
Quenching distortion was evaluated using gears. The gear used for the test has an outer diameter of 160 mmφ, an axial thickness of 20 mm, a number of teeth of 51, a module of 2.54, a pressure angle of 15°, and a helix angle of 34° to the right. It uses a helical gear. The strain is evaluated by the amount of change in the tooth groove runout before and after carburizing and quenching, specifically, by inserting a probe into the tooth groove of the gear and finding the difference between the maximum and minimum radial positions. When the value is equal to or better than the value of the F steel, which is the conventional Cr-Mo steel, it is judged as pass (◯), and when it is lower than that, it is judged as fail (x).
＜表面硬さ及び内部硬さ＞
表面硬さ及び内部硬さは、低サイクル疲労試験と同様の第１試験片を用い、その表面と、任意断面の厚み方向中央部とにおいて、ビッカース硬さを測定した。
<Surface hardness and internal hardness>
Regarding the surface hardness and the internal hardness, the same first test piece as in the low cycle fatigue test was used, and the Vickers hardness was measured on the surface and the center portion in the thickness direction of the arbitrary cross section.
表２に示されているように、Ａ〜Ｅの全ての鋼種において、式１の値以上の温度条件で焼戻し処理を行った場合に、低サイクル疲労強度が合格し、式２の値以下の温度条件で焼戻し処理を行った場合に、高サイクル疲労強度が合格した。そして、式１の値以上、かつ、式２の値以下の温度条件で焼戻し処理を行った場合に、低サイクル疲労強度と高サイクル疲労強度の両方において合格することがわかった。また、Ａ〜Ｅの全ての鋼種は、焼入れ歪みが従来鋼と比較して同等以下であって小さく、問題のない形状維持性を有していることがわかった。 As shown in Table 2, in all the steel types A to E, when the tempering treatment was performed under the temperature condition of the value of the formula 1 or more, the low cycle fatigue strength passed and the value of the formula 2 or less was obtained. The high cycle fatigue strength passed when tempered under temperature conditions. Then, it was found that when the tempering treatment was performed under the temperature condition of the value of the expression 1 or more and the value of the expression 2 or less, both the low cycle fatigue strength and the high cycle fatigue strength passed. Further, it was found that all of the steel types A to E had a quenching strain equivalent to or less than that of the conventional steel and small, and had a shape-retaining property with no problem.
より具体的には、焼入歪み評価試験の結果、本発明であるＡ〜Ｅ鋼の歯溝のふれ変化量は、焼戻し温度に関係なく、２７〜３３μｍとなっており、本発明の条件を満足する焼戻し温度で行った場合についても、従来から肌焼鋼として普通に用いられているＣｒ−Ｍｏ鋼であるＦ鋼について１５０℃で焼戻し（従来の温度）を行った場合の結果である３１μｍとほぼ同等で問題のない値となっていた。
これに対し、Ｆ鋼は、従来のＣｒ−Ｍｏ鋼であり、Ｓｉ含有率が低いことが影響して、焼戻し温度を高温化した場合に高サイクル疲労強度の低下が大きくなり、２つの疲労強度を両立できる焼戻し温度の範囲が存在しないものである。Ｇ、Ｈ鋼は、Ｃ含有率が範囲外である影響で、低サイクル疲労強度が低下した。
More specifically, as a result of the quenching strain evaluation test, the amount of change in the tooth groove deflection of the A to E steels of the present invention is 27 to 33 μm regardless of the tempering temperature. Even when the tempering is performed at a satisfactory temperature, it is the result when tempering (conventional temperature) is performed at 150° C. for the F steel, which is a Cr-Mo steel that has been conventionally commonly used as case hardening steel, and is 31 μm. It was almost the same as and there was no problem.
On the other hand, the F steel is a conventional Cr-Mo steel, and its low Si content has an effect, and when the tempering temperature is increased, the decrease in high cycle fatigue strength becomes large and the two fatigue strengths are increased. There is no tempering temperature range that can satisfy both requirements. In the G and H steels, the low cycle fatigue strength was reduced due to the influence of the C content outside the range.
なお、鋼種Ｊは、前記の通り幅広いサイクル数で優れた疲労強度の得られる開発鋼であるため、低サイクル疲労強度と高サイクル疲労強度の両方において合格するものであるが、Ｔｉ、Ｂを含有している影響で焼入れによる歪みが発生しやすく、例えば歪の条件に厳しい歯車等の製品に使用することは困難である。より、具体的には、歪の評価値が５１μｍと明らかに本発明であるＡ〜Ｅ鋼を用いた歯車に比較して劣る結果となっていた。 In addition, since the steel type J is a developed steel capable of obtaining excellent fatigue strength over a wide number of cycles as described above, it passes both low cycle fatigue strength and high cycle fatigue strength, but contains Ti and B. Due to this, distortion due to quenching is likely to occur, and it is difficult to use it in products such as gears that are severe in distortion conditions. More specifically, the evaluation value of the strain was 51 μm, which was clearly inferior to the gear using the A to E steels of the present invention.
次に、上述した鋼種Ａ〜Ｅまでの結果を用い、式（１）、焼戻し温度（℃）及び、低サイクル疲労強度及び高サイクル疲労強度との関係をよりわかりやすくするために、図１において、横軸に式（１）の計算値を取り、縦軸に焼戻し温度（℃）を取り、式（１）の値に対して実際に採用した焼戻し温度（℃）をプロットし、サイクル疲労強度の結果を○×△によって表した。低サイクル疲労強度と高サイクル疲労強度の両方に優れる合格の場合を「○」、高サイクル疲労強度が合格であるものの低サイクル疲労強度が不合格の場合を「×」、低サイクル疲労強度が合格であるものの高サイクル疲労強度が不合格の場合を「△」として示した。また、式（１）の値＝焼戻し温度（℃）の関係を示す破線ａを示した。 Next, in order to make it easier to understand the relationship between the formula (1), the tempering temperature (° C.), and the low cycle fatigue strength and the high cycle fatigue strength using the results of the steel types A to E described above, in FIG. , The abscissa is the calculated value of the formula (1), the ordinate is the tempering temperature (°C), the tempering temperature (°C) actually adopted is plotted against the value of the formula (1), and the cycle fatigue strength is plotted. The results are shown by XX. Excellent in both low-cycle fatigue strength and high-cycle fatigue strength, "○" when passing, "X" when high-cycle fatigue strength passed but low-cycle fatigue strength failed, and low-cycle fatigue strength passing However, the case where the high cycle fatigue strength was unacceptable was shown as “Δ”. Further, a broken line a showing the relationship of the value of the formula (1)=tempering temperature (° C.) is shown.
また、上述した鋼種Ａ〜Ｅまでの結果を用い、式（２）、焼戻し温度（℃）及び、低サイクル疲労強度及び高サイクル疲労強度との関係をよりわかりやすくするために、図２において、横軸に式（２）を取り、縦軸に焼戻し温度（℃）を取り、式（２）の値に対して実際に採用した焼戻し温度（℃）をプロットし、サイクル疲労強度の結果を○×△によって表した。○×△の表記の意味は、図１の場合と同じとした。また、式（２）の値＝焼戻し温度（℃）の関係を示す破線ｂを示した。 In addition, in order to make it easier to understand the relationship between the formula (2), the tempering temperature (°C), and the low cycle fatigue strength and the high cycle fatigue strength using the results of the above-described steel types A to E, in FIG. The formula (2) is taken on the horizontal axis and the tempering temperature (°C) is taken on the vertical axis, and the tempering temperature (°C) actually adopted is plotted against the value of the formula (2), and the result of cycle fatigue strength is It is represented by ×Δ. The meaning of the notation ○×△ is the same as in the case of FIG. Further, a broken line b showing the relationship of the value of the formula (2)=tempering temperature (° C.) is shown.
図１から理解できるように、式（１）の値よりも高い温度に焼戻し温度を設定した場合には、確実に、低サイクル疲労強度を向上させることができる。また、図２から理解できるように、式（２）の値よりも低い温度に焼戻し温度を設定した場合には、確実に、高サイクル疲労強度を向上させることができることがわかる。そして、式（１）及び式（２）を満足することによって、低サイクル疲労強度と高サイクル疲労強度の両方を同時に向上させることができることがわかる。 As can be understood from FIG. 1, when the tempering temperature is set to a temperature higher than the value of the formula (1), the low cycle fatigue strength can be surely improved. Further, as can be understood from FIG. 2, when the tempering temperature is set to a temperature lower than the value of the formula (2), it is possible to surely improve the high cycle fatigue strength. Then, it is understood that both the low cycle fatigue strength and the high cycle fatigue strength can be simultaneously improved by satisfying the expressions (1) and (2).
ここで、注目すべきことは、従来普通に行われていた焼戻し処理温度は、式（１）、式（２）の満足する温度範囲より低く、本発明は、前記の通り、従来よりも焼戻し処理温度を高温化した際に、前記両方の強度を向上できる最適条件範囲が存在することを見出したものである。
また、前記した実施例から明らかなように、図１、図２のように、合格（○）となる範囲を大きく確保できるのは、本発明で指定した成分範囲の場合に限定されるのであり、成分が適切でない場合には合格（○）となる領域を確保することが難しくなるということである。
Here, it should be noted that the tempering temperature which is conventionally performed is lower than the temperature range satisfied by the formulas (1) and (2), and thus the present invention, as described above, is tempered more than before. It was discovered that there is an optimum condition range in which the strength of both of the above can be improved when the treatment temperature is raised.
Further, as is clear from the above-mentioned examples, as shown in FIG. 1 and FIG. 2, it is only in the case of the component range designated in the present invention that a large range of passing (◯) can be secured. If the components are not appropriate, it is difficult to secure the area that passes (○).
以上説明したように、本発明により、特定の成分範囲内の鋼に対し、特定の範囲内の条件で焼戻し処理した場合に、低サイクル疲労強度と高サイクル疲労強度が両立できることが新規に見出されたものであり、産業への貢献は極めて大きいものである。 As described above, according to the present invention, it is newly found that low cycle fatigue strength and high cycle fatigue strength can be compatible with each other in the case of tempering the steel within a specific composition range under the conditions within a specific range. The contribution to the industry is extremely large.
ａ 式（１）の値＝焼戻し温度を示す破線
ｂ 式（２）の値＝焼戻し温度を示す破線
a Value of formula (1)=broken line showing tempering temperature b Value of formula (2)=broken line showing tempering temperature
Claims (2)
前記化学成分を有する鋼部材に浸炭焼入れ処理を施した後、焼戻し処理を行うに当たり、当該焼戻し処理は、処理温度Ｔ（℃）が以下の式（１）及び式（２）を満足する条件で行う、浸炭鋼部品の製造方法。
式（１）：Ｔ（℃）≧２００−４［Ｓｉ］−２９［Ｍｏ］
式（２）：１００［Ｓｉ］＋４５［Ｍｏ］＋１８０≧Ｔ（℃）
（ここで、［Ｓｉ］及び［Ｍｏ］は、それぞれＳｉの含有率（％）の値及びＭｏの含有率（％）の値を意味する） In terms of mass ratio, C: 0.10% to 0.30%, Si: 0.70% to 1.20%, Mn: 0.30% to 1.00%, Cr: 0.15% to 1.25. %, Mo: 0.80% or less (including 0%), Al: 0.020% to 0.050%, N: 0.0030% to 0.0200%, with the balance being Fe and inevitable impurities. A method of manufacturing a carburized steel part having a chemical composition consisting of
After performing the carburizing and quenching treatment on the steel member having the chemical composition, the tempering treatment is performed under the condition that the treatment temperature T (° C.) satisfies the following equations (1) and (2). A method of manufacturing a carburized steel part.
Formula (1): T (° C.)≧200-4 [Si]-29 [Mo]
Formula (2): 100 [Si]+45 [Mo]+180≧T (° C.)
(Here, [Si] and [Mo] mean the value of Si content (%) and the value of Mo content (%), respectively)
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2003231943A (en) * | 2002-02-12 | 2003-08-19 | Nippon Steel Corp | Case hardening steel superior in temper softening resistance |
| JP2009249700A (en) * | 2008-04-08 | 2009-10-29 | Kobe Steel Ltd | Steel component having excellent bending fatigue strength, and method for producing the same |
| JP2012017499A (en) * | 2010-07-08 | 2012-01-26 | Jfe Bars & Shapes Corp | Gear with excellent fatigue resistance and method of manufacturing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2003231943A (en) * | 2002-02-12 | 2003-08-19 | Nippon Steel Corp | Case hardening steel superior in temper softening resistance |
| JP2009249700A (en) * | 2008-04-08 | 2009-10-29 | Kobe Steel Ltd | Steel component having excellent bending fatigue strength, and method for producing the same |
| JP2012017499A (en) * | 2010-07-08 | 2012-01-26 | Jfe Bars & Shapes Corp | Gear with excellent fatigue resistance and method of manufacturing the same |
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