JP2012125838A - Method for producing hot-forged product for case hardened steel - Google Patents

Method for producing hot-forged product for case hardened steel Download PDF

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JP2012125838A
JP2012125838A JP2010294734A JP2010294734A JP2012125838A JP 2012125838 A JP2012125838 A JP 2012125838A JP 2010294734 A JP2010294734 A JP 2010294734A JP 2010294734 A JP2010294734 A JP 2010294734A JP 2012125838 A JP2012125838 A JP 2012125838A
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forging
steel
ferrite
forged
pearlite
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Kiyoshi Uchida
清 内田
Tomohiro Kawakami
朋弘 川上
Shoji Hayashi
祥次 林
Kunihiko Yamamoto
邦彦 山本
Mineo Kimura
峯男 木村
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Kawakami Tekkosho:Kk
株式会社川上鉄工所
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PROBLEM TO BE SOLVED: To provide a forging method which has excellent machinability in machining and has reduced generation of strains in carburizing heat treatment even if normalizing is not performed after hot forging.SOLUTION: In the hot forging method, an alloy steel for structural use such as a Cr steel and a Cr-Mo steel used as a case hardening steel is heated at 1,150 to 1,200°C, is subjected to hot forging in such a manner that the temperature of the final working of the forging is controlled to 900 to 1,100°C and the forging ratio is controlled to ≥1.5, is thereafter subjected to forced air-cooling to 650 to 750°C to make austenite crystal grain size into the fine one so as to control its multiplying factor (Di value) to ≤95 and is also annealed in the range of 700 to 600°C at a cooling rate of 5 to 20°C/min, thus its structure is transformed into a fine-grained (ferritte+pearlite) structure having a ferrite fraction of ≥50% and whose crystal grain size number is ≥5.

Description

本発明は、主として自動車のカウンターギアシャフト、メインシャフト、インプットシャフト、アウトプットシャフトなどに採用される肌焼鋼(構造用合金鋼)の熱間鍛造品の製造方法に関する。  The present invention relates to a method for manufacturing a hot forged product of case-hardened steel (structural alloy steel) employed mainly for counter gear shafts, main shafts, input shafts, output shafts and the like of automobiles.
肌焼鋼の熱間鍛造品は、Cr、Mo、Mnなどの合金元素量が高く、かつ熱間鍛造でオーステナイト粒が粗大化するため、焼入性が非常に高くなっている。そのため、鍛造後に鍛造品を放冷すると、図1に示すように、粗大なベイナイト〜パーライトの硬質組織に変態する。肌焼鋼熱間鍛造品には、機械加工、浸炭、焼入れが施されるが、ベイナイトを含んだ硬質組織は被削性が悪く、粗大な硬質組織は浸炭、焼入れ時の歪の原因になる。そのため、従来より、熱間鍛造後に焼準を施し、図2に示すような、軟質で微細な(フェライト+パーライト)組織に整えることによって、被削性を改善し、その後の熱処理時の歪発生を防止している。  The hot forged product of case-hardened steel has a high amount of alloy elements such as Cr, Mo, Mn, and austenite grains are coarsened by hot forging, so that the hardenability is very high. Therefore, when the forged product is allowed to cool after forging, it is transformed into a coarse bainite to pearlite hard structure as shown in FIG. Case-hardened steel hot forged products are machined, carburized and quenched, but hard structures containing bainite have poor machinability, and coarse hard structures cause distortion during carburizing and hardening. . Therefore, conventionally, after hot forging, normalization is performed, and a soft and fine (ferrite + pearlite) structure as shown in FIG. 2 is prepared, thereby improving machinability and generating strain during subsequent heat treatment. Is preventing.
近年、製造コスト低減、省エネの観点から、焼準の省略が求められており、熱間鍛造のままで良好な被削性、耐歪性をもつ鍛造方法が提案されつつある。例えば、特開2002−316231には、V、Nb、Taなどを含む非調質型機械構造用炭素鋼を熱間鍛造後、500〜700℃で30〜60分保持することによって、微細な(フェライト+パーライト)組織に変態して機械的性質を高め、焼入れ焼戻しを省略する技術が示されている。しかしながら、当技術はV、Nb、Taなどを含む非調質型機械構造用炭素鋼を用いた鍛造法であり、これらの鋼材は基本的には炭素鋼でありCr、Moなど合金元素量をほとんど含んでないので、本発明の肌焼鋼に要求される高い強度、靭性が得られない。また、V、Nb、Taなどの炭窒化物を析出させて強化するために、500〜700℃で30〜60分の長時間保持を行っているが、このような長時間の保持は省エネ、生産性の観点で好ましくない。  In recent years, from the viewpoint of manufacturing cost reduction and energy saving, there is a demand for omission of normalization, and a forging method having good machinability and strain resistance while hot forging is being proposed. For example, in JP-A No. 2002-316231, a non-tempered mechanical structural carbon steel containing V, Nb, Ta, etc. is finely retained by holding at 500 to 700 ° C. for 30 to 60 minutes after hot forging. (Ferrite + pearlite) A technology that improves the mechanical properties by transformation into a structure and omits quenching and tempering is shown. However, this technology is a forging method using non-tempered carbon steel for mechanical structure containing V, Nb, Ta, etc., and these steel materials are basically carbon steel, and the amount of alloying elements such as Cr, Mo is reduced. Since it is hardly contained, the high strength and toughness required for the case hardening steel of the present invention cannot be obtained. In addition, in order to precipitate and strengthen carbonitride such as V, Nb, Ta, etc., holding is performed for a long time at 500 to 700 ° C. for 30 to 60 minutes. It is not preferable from the viewpoint of productivity.
また、特開2001−303174にも、Al、Nb、Nなどを含んだ特定の鋼材を、1150℃以上の温度で加熱し熱間鍛造した後、800〜500℃間を徐冷する鍛造方法が示されている。当技術は加熱温度を1150℃以上と規定しているが、好適範囲を1200〜1300℃としており、実施例でもAlN、Nb(CN)を固溶するため加熱温度のほとんどが1225〜1350℃の高温で加熱している。しかし、このような高温で加熱を行った場合は結晶粒が著しく粗大化するので、この粗大粒を鍛造で細粒化することは困難である。オーステナイト粒度の粗い肌焼鋼(構造用合金鋼)は、焼入性が高いため、当技術の実施例である冷却速度0.1〜1℃/秒で徐冷しても、軟質の細粒の(フェライト+パーライト)組織に変態させることはできない。さらに、当技術の規定するミクロ組織によれば、パーライト分率が75%以下と示されているが、これはフェライト分率に換算すると25%以上となる。しかし、このように少ないフェライト分率では、所望の低い硬さが得られないので良好な被削性は得られない。以上のとおり、いずれも従来の熱処理を省こうとするものではあるが、本発明の課題とする熱間鍛造のままで良好な被削性と耐熱処理歪性を兼備させるという技術ではなく、またその製造方法も異なる。  Japanese Patent Laid-Open No. 2001-303174 also discloses a forging method in which a specific steel material containing Al, Nb, N, etc. is heated at a temperature of 1150 ° C. or higher and hot forged, and then gradually cooled between 800 and 500 ° C. It is shown. Although this technology defines the heating temperature as 1150 ° C. or higher, the preferred range is 1200 to 1300 ° C., and AlN and Nb (CN) are dissolved in the examples, so most of the heating temperature is 1225 to 1350 ° C. Heated at high temperature. However, when heating is performed at such a high temperature, the crystal grains become extremely coarse, and it is difficult to make the coarse grains fine by forging. Since austenitic coarse case-hardened steel (structural alloy steel) has high hardenability, even if it is gradually cooled at a cooling rate of 0.1 to 1 ° C./second, which is an example of this technology, soft fine particles The (ferrite + pearlite) structure cannot be transformed. Furthermore, according to the microstructure defined by the present technology, the pearlite fraction is shown to be 75% or less, but this is 25% or more when converted to the ferrite fraction. However, with such a small ferrite fraction, the desired low hardness cannot be obtained, so that good machinability cannot be obtained. As described above, both of them are intended to omit the conventional heat treatment, but it is not a technique that combines good machinability and heat-treatment distortion with hot forging as the subject of the present invention. The manufacturing method is also different.
特開2002−316231号公報JP 2002-316231 A 特開2001−303174号公報JP 2001-303174 A
本発明は、熱間鍛造の後に焼準を施さなくとも、切削加工における被削性に優れ、浸炭熱処理での歪の発生が少ない鍛造方法を提供するものである。  The present invention provides a forging method that is excellent in machinability in cutting without causing normalization after hot forging and that generates less strain in carburizing heat treatment.
本発明の要旨は以下のとおりである。肌焼鋼である構造用合金鋼のCr鋼、CrMo鋼を、下記(1)〜(5)の工程で処理することにより、50%以上のフェライト分率で結晶粒度番号が5番以上の細粒の(フェライト+パーライト)組織を得ることを特徴とする熱間鍛造品の製造方法。
(1)鋼材を1100〜1200℃に加熱する工程。
(2)加熱された鍛造素材を熱間鍛造し、その最終加工における温度を900〜1100℃の範囲で、鍛錬比を1.5以上で鍛造する工程。
(3)熱間鍛造された高温の鍛造品を(フェライト+パーライト)変態の開始直前の650〜750℃まで強制空冷する工程。
(4)(フェライト+パーライト)変態の開始直前の温度まで強制空冷された鍛造品の焼入性倍率(Di値)が95以下になるよう、合金元素量に応じて、加熱温度と最終加熱温度を設定し熱間鍛造する工程。
但し、Di値=(炭素鋼のDi)×fSi×fMn×fNi×fCr×fMo
炭素鋼のDiは図4のγ粒度から求め、各合金元素の焼入倍数(fSi、fMn、fNi、fCr、fMo)は表6より読み取り求める。
(5)Di値が95以下に調整された鍛造品を引き続いて冷却する際、700〜600℃の(フェライト+パーライト)変態域を5〜20℃/分の冷却速度で徐冷する工程。
The gist of the present invention is as follows. By treating the structural alloy steels Cr and CrMo, which are case hardening steels, in the following steps (1) to (5), fine grains having a grain size number of 5 or more with a ferrite fraction of 50% or more are obtained. A method for producing a hot forged product characterized by obtaining a grain (ferrite + pearlite) structure.
(1) The process of heating steel materials to 1100-1200 degreeC.
(2) A step of hot forging the heated forging material, forging the final processing at a temperature in the range of 900 to 1100 ° C. and a forging ratio of 1.5 or more.
(3) A step of forcibly air-cooling the hot forged product forged to 650 to 750 ° C. immediately before the start of (ferrite + pearlite) transformation.
(4) The heating temperature and the final heating temperature according to the amount of alloying elements so that the hardenability magnification (Di value) of the forged product cooled to the temperature immediately before the start of the (ferrite + pearlite) transformation is 95 or less. Is a process for hot forging.
However, Di value = (Di of carbon steel) × fSi × fMn × fNi × fCr × fMo
The Di of carbon steel is obtained from the γ grain size in FIG. 4 and the quenching multiples (fSi, fMn, fNi, fCr, fMo) of each alloy element are obtained from Table 6.
(5) A step of gradually cooling a (ferrite + pearlite) transformation region of 700 to 600 ° C. at a cooling rate of 5 to 20 ° C./min when the forged product whose Di value is adjusted to 95 or less is subsequently cooled.
本発明方法によれば、熱間鍛造、強制空冷および徐冷の工程だけで、切削加工において良好な被削性が得られ、かつ浸炭、熱処理時において歪の発生が小さい、肌焼鋼(構造用合金鋼)の熱間鍛造品が製造できる。よって、従来、鍛造後に施していた、焼準が省略できるので、製造コストを大きく低減することができる。また、この方法は、熱間鍛造の後に連続して強制空冷および徐冷を行うので、生産性が高い。さらに、徐冷においては、鍛造品の保有熱と徐冷ラインの保温だけで賄うので、熱エネルギー投入が不要の省エネ処理である。  According to the method of the present invention, a case-hardened steel (structure) in which good machinability is obtained in cutting by only hot forging, forced air cooling, and gradual cooling, and distortion is small during carburizing and heat treatment. Alloy forged steel) can be manufactured. Therefore, since the normalization that has been conventionally performed after forging can be omitted, the manufacturing cost can be greatly reduced. In addition, this method has high productivity because forced air cooling and slow cooling are continuously performed after hot forging. Furthermore, in slow cooling, it is an energy-saving process that does not require the input of thermal energy because it is provided only by the heat retained in the forged product and the warming of the slow cooling line.
従来の熱間鍛造後に放冷した場合の粗大なベイナイト組織Coarse bainite structure when allowed to cool after conventional hot forging 従来の熱間鍛造後に焼準を施した場合の微細な(フェライト+パーライト)組織Fine (ferrite + pearlite) structure when normalizing after conventional hot forging 本発明の熱間鍛造法で得られた(フェライト+パーライト)組織(Ferrite + pearlite) structure obtained by the hot forging method of the present invention 炭素鋼のDiに及ぼすc含有量、結晶粒度の影響Effect of c content and grain size on Di of carbon steel 鍛造結晶粒度に及ぼす加熱温度、仕上温度の影響Effect of heating temperature and finishing temperature on forged crystal grain size 各種合金元素の含有量と焼入性倍数Content and hardenability multiple of various alloy elements HB硬さに及ぼす焼入性倍数、冷却速度の影響Effects of hardenability multiple and cooling rate on HB hardness
発明者らは、熱間鍛造後の冷却時に鍛造品を焼準する技術を確立するために、肌焼鋼の焼入性、鍛造時の細粒化および冷却時の変態特性を調べ、以下のことを明らかにした。
1)熱間鍛造条件を制御し、その後の高温域を強制空冷することによって、従来、粗大であった、オーステナイト結晶粒度をかなりの細粒に調整することができる。
2)オーステナイト結晶粒度を細かくすることによって、合金元素量の高い肌焼鋼(構造用合金鋼)でも焼入性(Di値)を小さく抑えることができる。
3)さらに、鍛造後の冷却速度を徐冷することにより、軟質かつ微細な(フェライト+パーライト)組織に変態させることができることが判った。
すなわち、熱間鍛造(および冷却)の工程だけであっても、上記のとおり、鍛造時のオーステナイト粒度を細かく制御し、かつ鍛造後の冷却を徐冷することによって、図3に示すように、焼準で得られる(フェライト+パーライト)組織と同様の組織に変態できることが可能になる。
The inventors investigated the hardenability of case-hardened steel, fine graining during forging, and transformation characteristics during cooling in order to establish a technique for normalizing a forged product during cooling after hot forging. It revealed that.
1) By controlling the hot forging conditions and forcibly air-cooling the subsequent high-temperature region, the austenite crystal grain size, which has been conventionally coarse, can be adjusted to a considerably fine grain size.
2) By making the austenite grain size finer, hardenability (Di value) can be kept small even in case-hardened steel (structural alloy steel) having a high alloy element amount.
3) Furthermore, it has been found that a soft and fine (ferrite + pearlite) structure can be transformed by gradually cooling the cooling rate after forging.
That is, even in the hot forging (and cooling) step only, as described above, by finely controlling the austenite grain size during forging and gradually cooling the cooling after forging, as shown in FIG. It becomes possible to transform into a structure similar to the (ferrite + pearlite) structure obtained by normalization.
まず、熱間鍛造条件を限定した理由を説明する。
鋼材の加熱温度は、従来よりも低い1100〜1200℃とする。加熱温度を低くするのは、加熱時の結晶粒の粗大化を防止するためである。加熱温度が1200℃以上になると、オーステナイト粒度の粗大化が顕著となり、粒度番号で1番以下の粗大粒となってしまうため、鍛造での再結晶細粒化が困難になり、鍛造後に粒度番号で3番以上の細粒が得難くなくなる。一方、1100℃より低温での加熱では素材の変形抵抗が大きくなり、ハンマー鍛造のブロー数(時間)の増加、金型の磨耗増加を招くことになる。よって、加熱温度は1100〜1200℃に限定する。
First, the reason for limiting the hot forging conditions will be described.
The heating temperature of the steel material is 1100 to 1200 ° C., which is lower than the conventional temperature. The reason for lowering the heating temperature is to prevent coarsening of crystal grains during heating. When the heating temperature is 1200 ° C. or higher, the coarsening of the austenite grain size becomes remarkable and the grain size number becomes 1 or less coarse grain. Therefore, recrystallization refinement in forging becomes difficult, and the grain size number after forging becomes difficult. It becomes difficult to obtain fine grains of 3 or more. On the other hand, heating at a temperature lower than 1100 ° C. increases the deformation resistance of the material, leading to an increase in the number of blows (time) of hammer forging and increased wear of the mold. Therefore, heating temperature is limited to 1100-1200 degreeC.
熱間鍛造は通常、再結晶温度域で行うが、細粒化のためにとくに重要になるのは最終加工温度である。 最終加工温度が高いと、鍛造で再結晶細粒化を図っても、高温であるために粒成長を起こして粗大粒になってしまう。粒成長を抑制し粒度番号で3番以上の細粒を得るためには、最終加工温度を1100℃以下に抑える必要がある。粒成長抑制の観点からは、最終加工温度は低い方が望ましいが、900℃以下になると、素材の変形抵抗が大きくなりハンマー鍛造成形が困難になる。したがって、鍛造仕上温度は900〜1100℃に限定する。  Hot forging is usually performed in the recrystallization temperature range, but the final processing temperature is particularly important for grain refinement. When the final processing temperature is high, even if the recrystallization is refined by forging, the grains are grown and become coarse grains because of the high temperature. In order to suppress grain growth and obtain fine grains having a grain size number of 3 or more, it is necessary to suppress the final processing temperature to 1100 ° C. or less. From the viewpoint of suppressing grain growth, it is desirable that the final processing temperature is low. However, when the temperature is 900 ° C. or less, the deformation resistance of the material increases and hammer forging is difficult. Therefore, the forging finishing temperature is limited to 900 to 1100 ° C.
最終加工での鍛錬比を大きくとるほど、再結晶が活発になるので細粒化が図れる。しかし、鍛錬比が1.5未満の小さい加工量では、再結晶が起こり難いので細粒化がほとんど図れない。再結晶細粒化のために、鍛錬比を1.5以上に限定する。最終加工で所定の鍛造品(シャフト)形状に仕上げる際の鍛造法として、一般に鍛伸および型打ちがある。最終加工における鍛錬比は、(1)式に示すように、鍛伸と型打ちの両鍛錬比の積で求める。まず、鍛伸では多数回の鍛造ブローを加えるが、その鍛錬比は、各ブロー毎の圧下比(直径比D0/D1)の積の1/2乗とし、(2)式で求める。次に、型打ちの鍛錬比は、型打ち前の断面積A0と型打ち後の断面積A1との比(A0/A1)より求める。段付きシャフトでは部位によって直径(断面積)が異なるので鍛錬比も異なるが、この場合は鍛錬比が最も小さくなる部位で、鍛錬比1.5以上を確保する。
最終加工の鍛錬比=鍛伸の鍛錬比×型打ちの鍛錬比・・・・・・・・・・(1)式
型打ちの鍛錬比=型打ち前の断面積A0÷型打ち後の断面積A1・・・・(3)式
The larger the forging ratio in the final processing, the more active the recrystallization, and the finer the grain. However, at a small processing amount with a forging ratio of less than 1.5, recrystallization hardly occurs, so that fine graining can hardly be achieved. For recrystallization refinement, the forging ratio is limited to 1.5 or more. As forging methods for finishing a predetermined forged product (shaft) shape in the final processing, there are generally forging and stamping. The training ratio in the final machining is obtained by the product of both the training ratio of forging and stamping, as shown in equation (1). First, forging blow is applied many times in forging, and the forging ratio is determined by the formula (2), which is a half power of the product of the reduction ratio (diameter ratio D0 / D1) for each blow. Next, the punching forging ratio is obtained from the ratio (A0 / A1) of the cross-sectional area A0 before stamping and the cross-sectional area A1 after stamping. In a stepped shaft, the diameter (cross-sectional area) varies depending on the part, so the training ratio also varies.
Forging ratio of final processing = Forging ratio of forging × Forging ratio of stamping ... (1)
Stamping training ratio = sectional area A0 before stamping ÷ sectional area A1 after stamping (3)
次に、鍛造後の冷却を強制空冷する理由を説明する。鍛造後を強制空冷する理由は3つある。
1つ目は、鍛造後の高温からを強制空冷することによって、フェライト変態温度までの冷却時間を極力短くするためである。本発明では700〜600℃間を徐冷して(フェライト+パーライト)変態を進行完了させるが、それより高温域(鍛造後〜750℃)の冷却は変態特性にほとんど関与しないので、この間の冷却は強制空冷して時間短縮を図るためである。
2つ目の理由は、鍛造品の温度差を小さくすることにある。段付き形状のシャフト鍛造品を大気中で放冷した場合、鍛造品の小径部、端部は冷却が速いのに対し、大径部、中央部は冷却が遅くなり、鍛造品の部位による温度差が生じる。この部位による温度差は、その後の徐冷時の変態特性(ミクロ組織、硬さ)にバラツキを生じさせるので、冷却の遅い(温度の高い)大径部、中央部を強制空冷することによって、鍛造品の温度差を揃えることができる。
3つ目の理由は、結晶粒の粗大化抑止にある。本発明では、鍛造の最終加工温度を900〜1100℃に規定することで、結晶粒の粗大化抑止を図っているが、最終加工温度が規定範囲内であっても高め側の場合は粗大化が若干起きている。この若干の粗大化抑止に対しても、強制空冷が有効に作用する。以上の3つの理由で、鍛造後を強制空冷している。強制空冷の冷却速度は、各部位の温度を揃えたり、停止温度を制御するのに、150℃/分程度が望ましい。
Next, the reason why forced air cooling is performed after forging will be described. There are three reasons for forced air cooling after forging.
The first is to shorten the cooling time to the ferrite transformation temperature as much as possible by forced air cooling from a high temperature after forging. In the present invention, the transformation is completed by gradually cooling between 700 and 600 ° C. (ferrite + pearlite), but cooling in a higher temperature range (up to 750 ° C. after forging) is hardly involved in the transformation characteristics. Is for forced air cooling to shorten the time.
The second reason is to reduce the temperature difference of the forged product. When a stepped shaft forged product is allowed to cool in the air, the small-diameter portion and end of the forged product are cooled quickly, whereas the large-diameter portion and the central portion are cooled slowly. There is a difference. The temperature difference due to this part causes variations in the transformation characteristics (microstructure, hardness) during subsequent slow cooling, so by forced air cooling of the large-diameter part and the central part of the slow cooling (high temperature), The temperature difference of forged products can be made uniform.
The third reason is the suppression of crystal grain coarsening. In the present invention, the final processing temperature for forging is regulated to 900 to 1100 ° C. to suppress the coarsening of the crystal grains. However, if the final processing temperature is within the specified range, it is coarsened if it is on the higher side. Is happening slightly. Forced air cooling works effectively against this slight coarsening suppression. For the above three reasons, forced air cooling is performed after forging. The cooling rate of forced air cooling is preferably about 150 ° C./min in order to equalize the temperature of each part and control the stop temperature.
強制空冷の停止温度を750〜650℃に限定した理由を説明する。強制空冷を750℃以上の高温で停止してしまえば、変態開始までの徐冷にかなりの時間を要することになる。徐冷時間の増加は、生産性を低下させるし、設備的に徐冷のコンベアを非常に長くしなければならないという欠点を招く。一方、強制空冷の停止温度が600℃以下になると、被削性の悪いベイナイトが変態生成するとともに、フェライト変態が緩慢になり十分に低い硬さが得られなくなるので、強制空冷の停止温度下限は過冷却を考慮して650℃に止めるのが望ましい。したがって、強制冷却の停止温度は650〜750℃に限定する。  The reason why the forced air cooling stop temperature is limited to 750 to 650 ° C. will be described. If forced air cooling is stopped at a high temperature of 750 ° C. or higher, a considerable time is required for slow cooling until the start of transformation. Increasing the slow cooling time reduces productivity and causes the disadvantage that the slow cooling conveyor must be very long in terms of equipment. On the other hand, when the forced air cooling stop temperature is 600 ° C. or lower, bainite having poor machinability is transformed and ferrite transformation becomes slow and sufficiently low hardness cannot be obtained. It is desirable to stop at 650 ° C. in consideration of supercooling. Therefore, the forced cooling stop temperature is limited to 650-750 ° C.
次に、焼入性倍率(Di値)を限定した理由を説明する。オーステナイト化温度域から冷却した際に得られる変態組織は、鋼材の焼入性に依存する。焼入性は、鋼材の化学組成とオーステナイト結晶粒度で決まる、焼入性倍数(Di値)で示されるが、熱間鍛造時のDi値と変態組織との関係についてはよく判っていない。本発明者らは、熱間鍛造時のDi値とその後の冷却で得られる変態組織について調査した。その結果、通常の熱間鍛造では、結晶粒が粗いので必然的にDi値が大となり、その後の空冷でベイナイトを含んだ硬質組織に変態してしまう。しかし、熱間鍛造条件を制御し結晶粒を細かくすれば、鍛造時のDi値を小さく抑えることができ、その後の冷却でベイナイト変態を抑制できることがわかった。さらに、適正Di値について調査した結果、Di値が95以下になるように鍛造条件を制御すれば、その後の冷却を徐冷することで、軟質の(フェライト+パーライト)組織に変態できることが判った。すなわち、細粒のフェライトを適量確保し、硬質のベイナイトおよび粗大なパーライトの生成を抑止するために、Di値を95以下に抑える必要がある。よって、鍛造時のDi値は95以下に限定した。  Next, the reason why the hardenability magnification (Di value) is limited will be described. The transformation structure obtained when cooled from the austenitizing temperature range depends on the hardenability of the steel material. The hardenability is indicated by a hardenability multiple (Di value) determined by the chemical composition of the steel material and the austenite grain size, but the relationship between the Di value and the transformation structure during hot forging is not well understood. The present inventors investigated the Di value at the time of hot forging and the transformation structure obtained by subsequent cooling. As a result, in normal hot forging, since the crystal grains are coarse, the Di value inevitably increases, and the subsequent air cooling transforms into a hard structure containing bainite. However, it was found that if the hot forging conditions are controlled and the crystal grains are made finer, the Di value during forging can be kept small, and the bainite transformation can be suppressed by subsequent cooling. Furthermore, as a result of investigating the proper Di value, it was found that if the forging conditions are controlled so that the Di value is 95 or less, the subsequent cooling can be gradually cooled to transform into a soft (ferrite + pearlite) structure. . That is, in order to secure an appropriate amount of fine ferrite and to suppress the formation of hard bainite and coarse pearlite, the Di value needs to be suppressed to 95 or less. Therefore, the Di value during forging is limited to 95 or less.
なお、Di値は以下の式より計算できる。
Di値=(炭素鋼のDi)×fSi×fMn×fNi×fCr×fMo
炭素鋼のDiは、オーステナイト結晶粒度とC含有量で決まるもので、図4より読み取り求める。図中の結晶粒度4〜8のデータは一般に示されているものであるが、結晶粒度1〜3の粗粒のデータは発明者らが調査して明らかにし、図中に追記したものである。
The Di value can be calculated from the following equation.
Di value = (Di of carbon steel) × fSi × fMn × fNi × fCr × fMo
Di of carbon steel is determined by the austenite grain size and C content, and is obtained from FIG. The data of the crystal grain size 4-8 in the figure is generally shown, but the data of the coarse grain of the crystal grain size 1-3 is clarified through investigation by the inventors and added to the figure. .
鍛造時のオーステナイト結晶粒度についても、発明者らが調査して明らかにした、図5の加熱温度と最終鍛造温度の関係から簡易的に求めることができる。合金元素の含有量による焼入倍数(fSi、fMn、fNi、fCr、fMo)は、一般に示されている、図6より求めることができる。  The austenite grain size at the time of forging can also be easily determined from the relationship between the heating temperature and the final forging temperature in FIG. The quenching multiple (fSi, fMn, fNi, fCr, fMo) depending on the alloy element content can be obtained from FIG. 6, which is generally shown.
鍛造後の冷却条件および変態組織を限定した理由を説明する。
徐冷する温度域を700〜600℃間に規定したのは、(フェライト+パーライト)変態を効率的に短時間で完結させるためである。肌焼鋼の(フェライト+パーライト)変態は、650℃程度の温度で最も速く進行する。温度が700℃よりも高かかったり、600℃よりも低くかったりすると、(フェライト+パーライト)変態の開始、完了が著しく遅いので、そこを徐冷しても(フェライト+パーライト)変態はほとんど進行しないので、限られた徐冷時間では変態が完結できない。その場合、未変態オーステナイトはその後の冷却で硬質のベイナイトに変態する。したがって、徐冷域は700〜600℃範囲に限定した。
The reason for limiting the cooling conditions and transformation structure after forging will be described.
The reason why the temperature range for slow cooling is defined between 700 and 600 ° C. is to efficiently complete the (ferrite + pearlite) transformation in a short time. The (ferrite + pearlite) transformation of case-hardened steel proceeds the fastest at a temperature of about 650 ° C. If the temperature is higher than 700 ° C or lower than 600 ° C, the (ferrite + pearlite) transformation starts and completes very slowly, so even if it is slowly cooled (ferrite + pearlite), the transformation is almost advanced. Therefore, the transformation cannot be completed with a limited slow cooling time. In that case, untransformed austenite is transformed into hard bainite by subsequent cooling. Therefore, the slow cooling region was limited to a range of 700 to 600 ° C.
700〜600℃間を5〜20℃/分の冷却速度で徐冷するのは、(フェライト+パーライト)変態を完了させるためである。発明者らは、軟質の(フェライト+パーライト)が得られる、焼入倍数(Di値)と冷却速度の関係を調査し、図7の結果を得た。それによると、鍛造品のDi値を70〜95程度に調整し、かつその後の冷却を20℃/分以下の徐冷却にすることにより、硬さがHB180以下の軟質の(フェライト+パーライト)に変態できることが判る。この徐冷却は、鍛造品の保有熱と徐冷コンベアの保温とで賄うもので、基本的には自然冷却であるので、徐冷可能な速度範囲が存在する。遅い側の冷却速度の下限は5℃/分程度である。また、これより冷却が遅くなると処理時間が長くなり作業性が低下する。よって、徐冷の範囲を5〜20℃/分に限定した。  The reason for slow cooling between 700 and 600 ° C. at a cooling rate of 5 to 20 ° C./min is to complete the (ferrite + pearlite) transformation. The inventors investigated the relationship between the quenching multiple (Di value) and the cooling rate at which soft (ferrite + pearlite) was obtained, and obtained the result of FIG. According to this, by adjusting the Di value of the forged product to about 70 to 95 and gradually cooling the subsequent cooling to 20 ° C./min or less, the hardness becomes soft (ferrite + pearlite) of HB 180 or less. You can see that it can be transformed. This slow cooling is provided by the heat retained in the forged product and the heat retention of the slow cooling conveyor, and is basically natural cooling, so there is a speed range in which slow cooling is possible. The lower limit of the slow cooling rate is about 5 ° C./min. Further, if the cooling is slower than this, the processing time becomes longer and workability is lowered. Therefore, the range of slow cooling was limited to 5 to 20 ° C./min.
このようにして得た(フェライト+パーライト)組織は、フェライトの分率が50%以上で、その結晶粒度は粒度番号で5番以上の細粒となる。また、フェライト分率の増加とともに、パーライト、ベイナイトの分率が減り、硬さが低くなる。フェライトの分率を50%以上にすることによって、ベイナイトの生成が抑止でき、被削性に優れた軟質の(フェライト+パーライト)組織に変態させることができる。したがって、フェライトの分率を50%以上に限定した。  The (ferrite + pearlite) structure thus obtained has a ferrite fraction of 50% or more, and its crystal grain size is a fine grain having a grain size number of 5 or more. In addition, as the ferrite fraction increases, the fraction of pearlite and bainite decreases and the hardness decreases. By setting the ferrite fraction to 50% or more, the formation of bainite can be suppressed, and transformation into a soft (ferrite + pearlite) structure excellent in machinability can be achieved. Therefore, the ferrite fraction is limited to 50% or more.
前述のとおり、鍛造時のオーステナイト粒度を3番以上の細粒とし、その粒界に微細なフェライトを50%以上 生成させることにより、パーライト粒(未変態オーステナイト粒)を5番以上の細粒に縮小できる。結晶粒度で5番以上の細粒、すなわち粗大パーライトを含まない、均一かつ細粒の(フェライト+パーライト)組織を得ることができる。この細粒かつ均一な(フェライト+パーライト)組織は、浸炭熱処理時の歪の発生が少ないと考えられている。したがって、(フェライト+パーライト)組織の結晶粒度を5番以上の細粒に限定した。  As mentioned above, the austenite grain size at the time of forging is made as fine as 3 or more, and by forming fine ferrite at 50% or more at the grain boundary, pearlite grains (untransformed austenite grains) are made as fine as 5 or more. Can be reduced. A fine grain having a grain size of 5 or more, that is, a uniform and fine (ferrite + pearlite) structure free of coarse pearlite can be obtained. This fine and uniform (ferrite + pearlite) structure is considered to generate less strain during carburizing heat treatment. Therefore, the crystal grain size of the (ferrite + pearlite) structure was limited to fine particles of 5th or larger.
Cr鋼、CrMo鋼の主要合金元素であるCr、Mo含有量の上下限を説明する。Cr、Moは焼入性、機械的性質の確保に有効な合金元素であるが、その含有量が高くなり過ぎるとDi値が大となり所望の(フェライト+パーライト)組織が得難くなる。
Cr鋼におけるCr含有量は、0.85%未満では十分な焼入性、機械的性質が得られず、1.25%を超えると構造用合金鋼として通常用いられるCr含有量の範囲を外れるので、本発明の対象外とした。以上より、Cr鋼のCr含有量を0.85〜1.25%に限定した。
CrMo鋼におけるMo含有量についても同様に、0.15%未満では十分な焼入性、機械的性質が得られず、0.45%を超えると適正Di値が得難くなるとともに、構造用合金鋼として通常用いられるMo含有量の範囲を外れるので、本発明の対象外とした。以上より、CrMo鋼のCr含有量を0.85〜1.25%に、Mo含有量を0.15〜0.45%に、限定した。
The upper and lower limits of Cr and Mo contents, which are main alloy elements of Cr steel and CrMo steel, will be described. Cr and Mo are effective alloy elements for securing hardenability and mechanical properties. However, if the content is excessively high, the Di value becomes large and it is difficult to obtain a desired (ferrite + pearlite) structure.
If the Cr content in the Cr steel is less than 0.85%, sufficient hardenability and mechanical properties cannot be obtained, and if it exceeds 1.25%, it is outside the range of the Cr content normally used as a structural alloy steel. Therefore, it was excluded from the scope of the present invention. From the above, the Cr content of Cr steel was limited to 0.85 to 1.25%.
Similarly, with respect to the Mo content in CrMo steel, if it is less than 0.15%, sufficient hardenability and mechanical properties cannot be obtained, and if it exceeds 0.45%, it is difficult to obtain an appropriate Di value, and a structural alloy Since it is outside the range of Mo content normally used as steel, it was excluded from the scope of the present invention. As mentioned above, Cr content of CrMo steel was limited to 0.85-1.25%, and Mo content was limited to 0.15-0.45%.
φ50〜90mm丸棒鋼材を熱間鍛造用素材に使用した。鍛造用素材の化学組成は表1に示すとおり、JIS G4052構造用合金鋼に相当する鋼材である。  A φ50-90 mm round steel bar was used as a material for hot forging. As shown in Table 1, the chemical composition of the forging material is a steel material corresponding to JIS G4052 structural alloy steel.
鍛造用素材を1120〜1180℃に加熱し、1/4トンのエアハンマーで素材を予備成形した後、1.5トンのエアドロップハンマーで仕上げ鍛造を行った。 仕上げ鍛造、すなわち、最終加工を900〜1100℃で鍛錬比1.5以上の加工を加えて所定の製品形状に鍛錬成形した後、鍛造品の表面を700℃前後まで強制空冷した。その際、鍛造素材の焼入倍数(Di値)が95以下になるように、加熱温度と最終加工温度を調節し鍛造した。  The forging material was heated to 1120 to 1180 ° C., preformed with a 1/4 ton air hammer, and then forged with a 1.5 ton air drop hammer. After finishing forging, that is, final processing was performed at 900 to 1100 ° C. with a forging ratio of 1.5 or more and forged into a predetermined product shape, the surface of the forged product was forcibly cooled to about 700 ° C. At that time, the forging was performed by adjusting the heating temperature and the final processing temperature so that the quenching multiple (Di value) of the forging material was 95 or less.
次に、保温材でカバーしたコンベア上に鍛造品を搬送することによって、700〜600℃間を5〜20℃/分の冷却速度で徐冷した。600℃以降は、製品缶の中に投入し室温まで放冷した。これらの熱間鍛造条件は、本発明規定の範囲で行ったものである。  Next, the forged product was conveyed onto a conveyor covered with a heat insulating material, and was gradually cooled between 700 and 600 ° C. at a cooling rate of 5 to 20 ° C./min. After 600 ° C., it was put into a product can and allowed to cool to room temperature. These hot forging conditions are performed within the scope of the present invention.
鍛造冷却後の鍛造品についてミクロ組織を観察し、ブリネル硬さ(HB)を測定した。これまでの肌焼鋼鍛造品の調査結果から、熱処理歪の防止に関しては、フェライト分率が50%以上で、結晶粒度が粒度番号で5番以上の細粒であることが有効と判断している。被削性については、硬さがHB180を超えるもの、ミクロ組織が粘っこいベイナイトを含むものは、被削性が劣ると判定した。  The microstructure of the forged product after forging cooling was observed, and the Brinell hardness (HB) was measured. From the investigation results of the case-hardened steel forgings so far, regarding the prevention of heat treatment distortion, it is judged that it is effective that the ferrite fraction is 50% or more and the crystal grain size is grain size number 5 or more. Yes. As for machinability, it was determined that the machinability was inferior when the hardness exceeded HB180 or the microstructure contained bainite.
これらの調査結果を整理して表2に示す。本発明により製造した鍛造品は、ミクロ組織はベイナイトをほとんど含まない(フェライト+パーライト)組織で、その結晶粒度は粒度番号で5番以上の細粒であった。かつ、その硬さはいずれもHB180以下に調節されていた。本発明によれば、熱間鍛造のままでも良好な耐歪性、被削性を有していると判断される。従って、従来より、熱間鍛造の後に施していた焼準が省略できる。  These survey results are organized and shown in Table 2. The forged product produced according to the present invention has a microstructure containing almost no bainite (ferrite + pearlite), and its crystal grain size is a fine grain having a grain size number of 5 or more. And all the hardness was adjusted to HB180 or less. According to the present invention, it is judged that even with hot forging, it has good strain resistance and machinability. Therefore, conventionally, normalization performed after hot forging can be omitted.
一方、本発明規定の範囲から外れた比較例では、ミクロ組織にベイナイトを含んだり、硬さが高くなり過ぎていた。また、粗大なベイナイトもしくはパーライトを含む、不均一なミクロ組織となっていた。そのため、被削性が劣り、耐歪性も悪いと判断された。これらの比較例では、鍛造ままでの被削性、耐歪性が劣るので、焼準を省略することができないのは明白である。  On the other hand, in the comparative example outside the scope of the present invention, bainite was included in the microstructure or the hardness was too high. Moreover, it had a non-uniform microstructure including coarse bainite or pearlite. Therefore, it was judged that machinability was inferior and strain resistance was poor. In these comparative examples, since machinability and strain resistance in the forged state are inferior, it is clear that normalization cannot be omitted.

Claims (3)

  1. 肌焼鋼用構造用合金鋼の熱間鍛造素材を、下記(1)〜(5)の工程で処理することにより、50%以上のフェライト分率で結晶粒度番号が5番以上の細粒の(フェライト+パーライト)組織を得ることを特徴とする熱間鍛造品の製造方法。
    (1)鋼材を1100〜1200℃に加熱する工程、
    (2)加熱された鍛造素材を熱間鍛造し、その最終加工における温度を900〜1100℃の範囲で、鍛錬比を1.5以上で鍛造する工程、
    (3)熱間鍛造された高温の鍛造品を(フェライト+パーライト)変態の開始直前の650〜750℃まで強制空冷する工程、
    (4)(フェライト+パーライト)変態の開始直前の温度まで強制空冷された鍛造品の焼入性倍率(Di値)が95以下になるよう、合金元素量に応じて、加熱温度と最終加熱温度を設定し熱間鍛造する工程、
    但し、Di値=(炭素鋼のDi)×fSi×fMn×fNi×fCr×fMo
    炭素鋼のDiは図4のγ粒度から求め、各合金元素の焼入倍数(fSi、fMn、fNi、fCr、fMo)は表6より読み取り求める。
    (5)Di値が95以下に調整された鍛造品を引き続いて冷却する際、700〜600℃の(フェライト+パーライト)変態域を5〜20℃/分の冷却速度で徐冷する工程。
    By processing the hot forging material of the structural alloy steel for case-hardened steel in the following steps (1) to (5), fine grains having a grain size number of 5 or more with a ferrite fraction of 50% or more A method for producing a hot forged product characterized by obtaining a (ferrite + pearlite) structure.
    (1) A step of heating the steel material to 1100 to 1200 ° C,
    (2) Hot forging the heated forging material, forging at a final ratio of 900 to 1100 ° C. and forging at a forging ratio of 1.5 or more,
    (3) a step of forcibly air-cooling a hot forged product that has been hot forged to 650-750 ° C. immediately before the start of (ferrite + pearlite) transformation;
    (4) The heating temperature and the final heating temperature according to the amount of alloying elements so that the hardenability magnification (Di value) of the forged product cooled to the temperature immediately before the start of the (ferrite + pearlite) transformation is 95 or less. Setting and hot forging process,
    However, Di value = (Di of carbon steel) × fSi × fMn × fNi × fCr × fMo
    The Di of carbon steel is obtained from the γ grain size in FIG. 4 and the quenching multiples (fSi, fMn, fNi, fCr, fMo) of each alloy element are obtained from Table 6.
    (5) A step of gradually cooling a (ferrite + pearlite) transformation region of 700 to 600 ° C. at a cooling rate of 5 to 20 ° C./min when the forged product whose Di value is adjusted to 95 or less is subsequently cooled.
  2. 肌焼鋼である構造用合金鋼が0.85〜1.25質量%のクロムを含有するクロム鋼である請求項1に記載の熱間鍛造品の製造方法。  The method for producing a hot forged product according to claim 1, wherein the structural alloy steel, which is case-hardened steel, is chromium steel containing 0.85 to 1.25% by mass of chromium.
  3. 肌焼鋼である構造用合金鋼が0.85〜1.25質量%のクロムと0.15〜0.45質量%のモリブデンを含有するクロムモリブデン鋼である請求項1に記載の熱間鍛造品の製造方法。  The hot forging according to claim 1, wherein the structural alloy steel which is a case hardening steel is a chromium molybdenum steel containing 0.85 to 1.25 mass% chromium and 0.15 to 0.45 mass% molybdenum. Product manufacturing method.
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Publication number Priority date Publication date Assignee Title
CN102989953A (en) * 2012-08-22 2013-03-27 昌利锻造有限公司 Forging method of automotive eccentric shaft
CN103350173A (en) * 2013-06-24 2013-10-16 钢铁研究总院 Production method of integral special-shaped heavy forging made of austenitic stainless steel
JP2014155944A (en) * 2013-02-15 2014-08-28 Kawakami Tekkosho:Kk Forging and production method thereof
JP2014155943A (en) * 2013-02-15 2014-08-28 Kawakami Tekkosho:Kk Temperature regulator
CN105458135A (en) * 2015-11-29 2016-04-06 郑臣钏 Alloy piece forging technology
CN107470530A (en) * 2017-08-28 2017-12-15 西北有色金属研究院 A kind of forging method of radio frequency superconducting cavity High-purity Niobium ingot
CN107747032A (en) * 2017-09-30 2018-03-02 张家港中环海陆特锻股份有限公司 Aviation high-toughness long-life large rotor forged shaft manufacturing process
CN107937703A (en) * 2017-11-01 2018-04-20 沈阳透平机械股份有限公司 The conditioning treatment technique of compressor 35CrMoV ionic nitriding gears

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102989953A (en) * 2012-08-22 2013-03-27 昌利锻造有限公司 Forging method of automotive eccentric shaft
JP2014155944A (en) * 2013-02-15 2014-08-28 Kawakami Tekkosho:Kk Forging and production method thereof
JP2014155943A (en) * 2013-02-15 2014-08-28 Kawakami Tekkosho:Kk Temperature regulator
CN103350173A (en) * 2013-06-24 2013-10-16 钢铁研究总院 Production method of integral special-shaped heavy forging made of austenitic stainless steel
CN105458135A (en) * 2015-11-29 2016-04-06 郑臣钏 Alloy piece forging technology
CN107470530A (en) * 2017-08-28 2017-12-15 西北有色金属研究院 A kind of forging method of radio frequency superconducting cavity High-purity Niobium ingot
CN107470530B (en) * 2017-08-28 2018-11-23 西北有色金属研究院 A kind of forging method of the high-purity niobium ingot of radio frequency superconducting cavity
CN107747032A (en) * 2017-09-30 2018-03-02 张家港中环海陆特锻股份有限公司 Aviation high-toughness long-life large rotor forged shaft manufacturing process
CN107937703A (en) * 2017-11-01 2018-04-20 沈阳透平机械股份有限公司 The conditioning treatment technique of compressor 35CrMoV ionic nitriding gears

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