JP3569158B2

JP3569158B2 - Billet for cold forging

Info

Publication number: JP3569158B2
Application number: JP17380599A
Authority: JP
Inventors: 省一安藤; 博史小野; 正小林; 満上川
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1999-06-21
Filing date: 1999-06-21
Publication date: 2004-09-22
Anticipated expiration: 2019-06-21
Also published as: JP2001003135A

Description

【０００１】
【発明の属する技術分野】
本発明は、中間焼鈍を必要とせず高い変形能の冷間鍛造が連続して可能で且つ焼入れ性を損なわない冷間鍛造用ビレットに関する。
【０００２】
【従来の技術】
従来、自動二輪車等のエンジンのクランク軸やコンロッド等の成形は熱間鍛造が主流であり、材料を再結晶温度以上に加熱して鍛錬成形するのが一般的である。
【０００３】
【発明が解決しようとする課題】
熱間鍛造による成形は、金型表面が摩耗しやすく、その結果鍛造品の精度が悪くなり、鍛造後の機械加工による取代が大きくなって加工効率が低下する。そして、レース加工代が大きい為に機械台数も多くなり初期投資が膨大になる。
また、熱間鍛造にあっては、加熱後に鍛造するためにスケールが発生し、更に離型剤等の塗布が必須になるので作業環境を最適に保つことが困難である。
【０００４】
【課題を解決するための手段】
そこで本発明は、冷間鍛造の連続成形によって成形可能で、焼入れ性が良く、更に連続成形の途中で軟化処理が不要な冷間鍛造用ビレットの成分組成を提供することを目的とする。
【０００５】
上記目的を達成するため、本発明は、連続して冷間鍛造を行うための鉄鋼材のビレットの材料組成として、Ｃが０．４６〜０．４８ｗｔ％、Ｓｉが０．１４ｗｔ％以下、Ｍｎが０．５５〜０．６５ｗｔ％、Ｐが０．０１５ｗｔ％以下、Ｓが０．０１５ｗｔ％以下、Ｃｕが０．１５ｗｔ％以下、Ｎｉが０．２０ｗｔ％以下、Ｃｒが０．３５ｗｔ％以下含まれるものとした。
【０００６】
ここで、機械構造用炭素鋼のＪＩＳＳ４８Ｃ（以下、単にＳ４８Ｃと記す）は熱間鍛造用素材として使用されており、材料成分は、Ｃが０．４５〜０．５１ｗｔ％、Ｓｉが０．１５〜０．３５ｗｔ％、Ｍｎが０．６〜０．９ｗｔ％、Ｐが０．０３ｗｔ％以下、Ｓが０．０３５ｗｔ％以下、Ｃｕが０．３０ｗｔ％以下、Ｎｉが０．２０ｗｔ％以下、Ｃｒが０．３５ｗｔ％以下が基準とされている。
【０００７】
一方、上記のようなＳ４８Ｃの冷間鍛造能力は、例えば据込み率が７０〜７５％程度であり、据込み率が９０％以上の変形量の多い冷間鍛造を行うと材料割れが生じて成形することができない。
【０００８】
そしてこの変形能に影響を及ぼす元素として、Ｓｉ、Ｐ、Ｓ或いはＣｕ等があり、Ｓｉは鋼の硬さや引張り強さ等を高め、熱処理にあたっては結晶粒の成長を早める作用があるものの、伸びや衝撃値を減じて鍛造性を害するようになり、またＰも、フェライト中に固溶すると硬さや引張り強さをやや増すが、衝撃値を減じて加工時に割れやすく、冷間脆さの原因となる。また、Ｓが多量に含まれると、冷間鍛造時にクラック発生の起点となる硫化マンガン（ＭｎＳ）が析出し、加工中に割れを生じやすくなり、Ｃｕの含有量が多くなるとフェライト硬さを増加させ、冷間鍛造性を損う原因となる。
【０００９】
一方、焼入れ性の確保の点から、Ｃの含有量は、上記熱間鍛造用素材と同等の量が望まれ、またＭｎもフェライト中に固溶すると、鋼の変態点を下げて焼きが入りやすくなるため、熱間鍛造用素材と同等の量が望まれる。
【００１０】
そこで、本発明にあっては、熱間鍛造用素材であるＳ４８Ｃの成分組成を基本にし、焼入れ性確保の点からＣの量をＳ４８Ｃと同等にした上で、冷間鍛造時の材料割れの原因になりやすいＳｉ、Ｐ、Ｓ及びＣｕの量をどの程度まで削減しなければならないかを検証し、前記組成の冷間鍛造用ビレットとした。
【００１１】
因みに、このような冷間鍛造用ビレットは、例えば、棒状素材を１回目の球状化焼鈍処理して内部の炭化物を球状化した後、所定の断面減少率で引抜き加工し、所望の寸法に切断した後、更に２回目の球状化焼鈍処理によって内部の炭化物の分散を促進し球状化率を高めるようにして製造すれば、硬度が低下して成形性が良くなり、また表層部の伸び率も良くなって好適である。また、冷間鍛造後の製品硬度は時効処理によって高めることができる。
【００１２】
上記の焼鈍処理にて、ビレットを構成する炭化物のアスペクト比を３００％以下にでき、結果として、上記組成のビレットは限界据込み率を９０％以上とすることが可能になる。
【００１３】
本発明に係るビレットは、連続した冷間鍛造により軸付きエンジン部品を成形するのに適している。
ここで、軸付きエンジン部品とは、例えばクランク軸等であり、従来、熱間鍛造で成形するのが主流であったため、高精度な製品形状のものを得ることが困難であったが、連続した冷間鍛造が適用可能となれば高精度な製品形状のものを生産性を高くして且つ安価に製造することができる。
【００１４】
【発明の実施の形態】
本発明の実施の形態について添付図面に基づき説明する。ここで、図１は本発明に係る材料成分の棒材から冷間鍛造用ビレットを製造する具体的方法の工程図、図２は冷間引抜き率と限界据込み率の関係を表すグラフ、図３は引抜き前に球状化焼鈍を行う効果を説明した図、図４（Ａ）〜（Ｃ）はビレットの金属組織を示すＳＥＭ写真（１０００倍）図５は据込み率の説明図、図６は本発明に係る冷間鍛造用ビレットを用いてクランク軸を連続鍛造する鍛造工程の一例の説明図である。
【００１５】
本発明に係るビレットの成分組成は、Ｃが０．４６〜０．４８ｗｔ％、Ｓｉが０．１４ｗｔ％以下、Ｍｎが０．５５〜０．６５ｗｔ％、Ｐが０．０１５ｗｔ％以下、Ｓが０．０１５ｗｔ％以下、Ｃｕが０．１５ｗｔ％以下、Ｎｉが０．２０ｗｔ％以下、Ｃｒが０．３５ｗｔ％以下含まれる鋼材である。この組成の基本的な構成は、熱間鍛造素材であるＳ４８Ｃの成分組成をベースにし、焼入れ性確保のためＣの量をＳ４８Ｃと同等にするとともに、材料割れの要因になりやすいＳｉとＰとＳの量を削減した成分組成にしている。
【００１６】
ここで、Ｃは単位％当り最も冷間鍛造性に大きな効果をもつ元素であり、機械的性質、特に材料強度、焼入れ性の面から重要である。即ち、クランク軸にあっては全体的に所定の機械的強度を必要とするとともに、ウォーム及びテーパ部など局部的に高硬度が要求される。このように局部的に高硬度が要求される部分を鍛造後の焼入れで硬度を上げるには、Ｃの割合を０．４６〜０．４８ｗｔ％とする必要がある。
【００１７】
またＳｉは原料の銑鉄中に存在し、製鋼の過程で殆ど除去されるが、製鋼過程の最後に脱酸剤として添加されることがあり、Ｓ４８Ｃでは０．１５〜０．３５ｗｔ％含まれ、一部は鋼中に入りフェライトに固溶するが、鍛造性を阻害するので冷間鍛造素材としては除去することが好ましい。
【００１８】
またＭｎは製鋼の過程でも多少残るが、脱酸剤として添加されるため、Ｓ４８Ｃには０．６０〜０．９０ｗｔ％含まれている。このＭｎはＳと結合して硫化マンガンとして鋼中に分散し、一部はフェライト中に固溶するが、Ｓに結合しやすいＭｎはＭｎＳとなり、鍛造成形時の割れの起点となりやすい為、低減させることが望ましいが、フェライト中に固溶するＭｎは焼きを入りやすくし、結晶粒の成長を抑える。このため、Ｍｎ量は０．５５〜０．６５ｗｔ％にする。
【００１９】
またＰはフェライト中に固溶し、多量に含まれる場合は鉄の一部と化合してリン化鉄になるが、Ｐがフェライト中に固溶するとフェライトは伸びが減じられるようになり、常温における衝撃値も減じられて加工時に割れが生じやすくなる。そしてこのＰはＳ４８Ｃでは０．０３ｗｔ％まで許容されており、冷間鍛造素材としては、この許容値が高すぎる。
【００２０】
またＳはＭｎの一部と化合してＭｎＳになり、このＭｎＳは冷間鍛造時に生じる表面割れの起点となるため、Ｓ４８Ｃでは０．０３５ｗｔ％まで許容されているが、冷間鍛造素材としては、許容値が高すぎる。
本発明にあっては、加工性を阻害する元素、Ｓｉ、Ｐ、Ｓの含有量をできるだけ減らして冷間鍛造性を高めるべく、Ｓｉを０．１４ｗｔ％以下、Ｐを０．０１５ｗｔ％以下、Ｓを０．０１５ｗｔ％以下にしている。
【００２１】
またＣｕは高温加熱ではＦｅより酸化が少ないため、表面に富化して赤熱脆性を起こすので、概ね当量のＮｉを添加して赤熱脆性を防止する。一方、ＣｕはＰと同様に微量の含有によりフェライト硬さを増加させ、冷間鍛造性を損うと考えられる為、０．１５ｗｔ％以下とする。
【００２２】
またＮｉは前記した効果の他に、焼入れ性を増し、低温脆性を防止し、耐食性を改善するため、Ｓ４８Ｃと同量添加する。更にＣｒは焼入性、焼戻し抵抗を大にし、耐食性を高め安定した炭化物を作りやすいため、Ｓ４８Ｃと同量程度含有せしめる。
【００２３】
そして、以上のような成分組成の鋼材からなる棒材に対して、図１に示すように、酸洗を行った後、第１回目の球状化焼鈍を行い、セメンタイトを球状化して素材全体の加工性を向上させ、内部まで歪みを与えることができるようにするとともに、パーライトの微細化を図る。
この球状化焼鈍は、実施形態では７４０℃で６時間保持した後、２０℃／ｈで６８０℃まで降温させ、その後、炉冷する手順で行う。
【００２４】
次に、酸洗、ボンデ処理を行って引抜き加工を行い、限界据込み率の向上を図る。
ここで図２は球状化焼鈍を施した鋼材の冷間引抜き率（断面減少率）と限界据込み率の関係を示すグラフであるが、冷間引抜き率（断面減少率）が２０％程度で限界据込み率を最大にすることができることが分る。このことは従来から知られている。
【００２５】
上記した引抜きを行うことによって限界据込み率が上がる理由は、引抜きを実施することで、第２回目の焼鈍時にオーステナイト粒が微細化し、球状化速度を速めることができるからと推定されるが、本実施形態では、最大の限界据込み率が得られるよう、冷間引抜き率（断面減少率）２０％で引抜いた。
因みに、冷間引抜き率（断面減少率）は、図２に示すように、加工前の径がＤで加工後の径がｄの場合、（Ｄ^２−ｄ^２）／Ｄ^２×１００で表される値である。
【００２６】
次に、この棒材を所望の寸法に切断し、これを酸洗した後、２回目の球状化焼鈍を行い、炭化物の分散を図るとともに球状化率を高める。
そして実施形態では、この２回目の球状化焼鈍は、７５０℃で２時間保持した後、２０℃／ｈで７３０℃まで降温させ、その後降温率を１５℃／ｈに下げて６８０℃まで降温させ、その後炉冷する手順で行う。
【００２７】
そしてこの２回目の球状化焼鈍を終えた後に、ショットブラスト、ボンデ処理を行って表面調整を行い、冷間鍛造用ビレットとした。
【００２８】
以上のような工程中、引抜き前と引抜き後にそれぞれ球状化焼鈍を行う（合計２回）場合と、引抜き後だけ１回行う場合の球状化レベルとビレット硬度（ＨＲＣ）と脱炭層深さ（ｍｍ）を比較すると図３の通りである。
ここで、球状化レベル（数値が少ないほど球形に近く良好）の向上とビレット硬度の低下は、成形性を良好にするために効果的であり、脱炭層深さ（表層のフェライト化）の浸透は、表層部の伸び率の向上に効果的である。
【００２９】
そこで、アスペクト比についての実験を行った。材料の成分組成は、Ｃが０．４６〜０．４８ｗｔ％、Ｓｉが０．１４ｗｔ％以下、Ｍｎが０．５５〜０．６５ｗｔ％、Ｐが０．０１５ｗｔ％以下、Ｓが０．０１５ｗｔ％以下、Ｃｕが０．１５ｗｔ％以下、Ｎｉが０．２０ｗｔ％以下、Ｃｒが０．３５ｗｔ％以下、残部はＦｅと不純物である。
【００３０】
引抜きを行わずに球状化焼鈍を行った場合（材料１）、引抜き前に球状化焼鈍を行わなずに引抜き率を２０％とした場合（材料２）、引抜き前と引抜き後に球状化焼鈍を行い引抜き率を２０％とした場合（材料３）の各材料の金属組織（１０００倍）を図４（Ａ）〜（Ｃ）に示す。
【００３１】
各材料の炭化物の球状化率を表すアスペクト比（ｂ／ａ×１００）は、（表１）に示すように、材料１については５０６％、材料２については３４７％、材料３については３００％であった。
そして、各材料を用いて、冷間鍛造（据込み）を行った。据込み率は図５に示すように（Ｌ_１−Ｌ_２）／Ｌ_１×１００＝９０（％）とした。この時の各材料の割れ発生率は、それぞれ３５％、５％及び０％であった。
したがって、球状化焼鈍を２回実施することにより、炭化物の結晶は、より球形に近づき、冷間鍛造の際に割れが生じにくくなることが判った。
【００３２】
【表１】

【００３３】
次に、本発明に係る材料成分の有効性を確認するため、据込み試験した結果を以下の（表２）に示す。据込み率はは前記同様９０％とし、据込み試験の材料（ビレット）はいずれも引抜きの前後に球状化焼鈍を行ったものを用いた。
【００３４】
【表２】

【００３５】
先ず、Ｓ４８Ｃを材料として据込み試験を行った場合には割れ発生は２０％（Ｎ＝１００）であった。割れ発生率が２０％ということは、冷間鍛造用の材料として不適である。
【００３６】
そこで、本発明者らはＭｎの割合を低減することを試みた。その結果、割れ発生は１２％に下がった。しかしながら、これでも冷間鍛造用の材料として不適である。尚、Ｓ４８Ｃでは、Ｍｎの割合は０．６０〜０．９０ｗｔ％であるので、０．６０〜０．６５ｗｔ％の範囲でＳ４８ＣとＭｎの割合を低減した材料とが重複することになる。これは、Ｍｎの割合を厳密に特定することはできず、ある程度のバラツキは不可避であることによる。これは、Ｓ４８Ｃを用いても割れが生じないものがあり、Ｍｎの割合を低減した材料を用いても割れが発生するものがあることからも是認できる。
【００３７】
Ｍｎの割合を低減しても十分ではないことが判明したので、本発明者らは、焼入れ性に悪影響を及ぼさない範囲でＣ（炭素）の量を低減してみた。その結果、割れ発生率は５％に下がった。しかしながら、これでも冷間鍛造用の材料としては不適である。
【００３８】
そこで、加工性を阻害する元素と考えられるＳｉ、Ｐ、Ｓ及びＣｕの含有量を減らして据込み試験を行った。即ちＳｉを０．１４ｗｔ％以下、Ｐを０．０１５ｗｔ％以下、Ｓを０．０１５ｗｔ％以下、Ｃｕを０．１５ｗｔ％以下とした。結果は、（表２）に示すように割れ発生率は０％であった。
【００３９】
そしてこのような冷間鍛造用ビレットを使用して、例えば図６に示すような複数段回の冷間鍛造を連続して行ってクランク軸を成形すれば、成形途中で中間焼鈍することなく連続成形することができる。また、焼入れ性も良好である。
【００４０】
【発明の効果】
以上のように本発明に係る冷間鍛造用ビレットは、連続して冷間鍛造を行うための鉄鋼材のビレットの材料組成として、所定の成分含有量にしたため、連続した冷間鍛造を行っても割れ等が発生することなく、高い据込み率で効率的に成形できるようになった。
【００４１】
そしてこのような冷間鍛造用ビレットで、軸付きエンジン部品を製造するようにすれば、従来の熱間鍛造のような複数の段取り換えを行う必要がなくなり、また後加工の切削工程等も省略できて好適である。
【図面の簡単な説明】
【図１】本発明に係る材料成分の棒材から冷間鍛造用ビレットを製造する方法の工程図
【図２】冷間引抜き率と限界据込み率の関係を表すグラフ
【図３】引抜き前の球状化焼鈍を行う効果を説明した図
【図４】（Ａ）〜（Ｃ）はビレットの金属組織を示すＳＥＭ写真（１０００倍）
【図５】据込み率の説明図
【図６】本発明に係る冷間鍛造用ビレットでクランク軸を連続鍛造する鍛造工程の一例の説明図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a billet for cold forging that can continuously perform cold forging with high deformability without requiring intermediate annealing and does not impair hardenability.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, forging of a crankshaft, a connecting rod, and the like of an engine of a motorcycle or the like is mainly performed by hot forging, and it is general that a material is heated to a recrystallization temperature or higher and forged.
[0003]
[Problems to be solved by the invention]
In the forming by hot forging, the mold surface is easily worn, and as a result, the accuracy of the forged product is deteriorated, the machining allowance after forging is increased, and the working efficiency is reduced. In addition, since the cost of lace processing is large, the number of machines increases, and initial investment becomes enormous.
Further, in hot forging, scale is generated due to forging after heating, and it is necessary to apply a release agent or the like. Therefore, it is difficult to keep the working environment optimal.
[0004]
[Means for Solving the Problems]
Accordingly, an object of the present invention is to provide a component composition of a billet for cold forging that can be formed by continuous cold forging, has good hardenability, and does not require a softening treatment during continuous forming.
[0005]
In order to achieve the above object, the present invention provides a steel composition having a C composition of 0.46 to 0.48 wt%, a Si content of 0.14 wt% or less, and a Mn of 0.55 to 0.65 wt%, P is 0.015 wt% or less, S is 0.015 wt% or less, Cu is 0.15 wt% or less, Ni is 0.20 wt% or less, and Cr is 0.35 wt% or less. It was assumed.
[0006]
Here, JIS S48C (hereinafter simply referred to as S48C) of carbon steel for machine structure is used as a material for hot forging, and the material components are 0.45 to 0.51 wt% of C and 0. 15 to 0.35 wt% , Mn is 0.6 to 0.9 wt%, P is 0.03 wt% or less, S is 0.035 wt% or less, Cu is 0.30 wt% or less, Ni is 0.20 wt% or less, The standard is that Cr is 0.35 wt% or less.
[0007]
On the other hand, the cold forging ability of S48C as described above is, for example, an upsetting ratio of about 70 to 75%, and a material crack occurs when the forging ratio is 90% or more and a large amount of deformation is performed. Cannot be molded.
[0008]
Elements that affect the deformability include Si, P, S, and Cu. Si increases the hardness and tensile strength of steel, and has the effect of accelerating the growth of crystal grains during heat treatment. And the impact value are reduced, impairing the forgeability. Also, when P forms a solid solution in ferrite, the hardness and tensile strength are slightly increased, but the impact value is reduced, so that it is easy to crack during processing and causes cold brittleness. It becomes. In addition, when S is contained in a large amount, manganese sulfide (MnS), which is a starting point of crack generation during cold forging, precipitates and easily cracks during processing, and as the Cu content increases, the ferrite hardness increases. This causes the cold forgeability to be impaired.
[0009]
On the other hand, from the viewpoint of ensuring hardenability, the content of C is desirably the same as that of the above-described hot forging material, and when Mn also forms a solid solution in ferrite, the transformation point of the steel is lowered to cause quenching. Therefore, an amount equivalent to the material for hot forging is desired.
[0010]
Therefore, in the present invention, based on the component composition of S48C which is a material for hot forging, the amount of C is made equal to that of S48C from the viewpoint of ensuring hardenability, and the material cracking during cold forging is performed. It was verified to what extent the amounts of Si, P, S and Cu, which are likely to cause the problem, had to be reduced, and a billet for cold forging having the above composition was obtained.
[0011]
By the way, such a billet for cold forging is, for example, after the rod-shaped material is subjected to a first spheroidizing annealing treatment to spheroidize the carbide therein, and then drawn at a predetermined cross-sectional reduction rate, and cut into desired dimensions. After that, if it is manufactured by further increasing the spheroidization rate by promoting the dispersion of carbides inside by a second spheroidizing annealing treatment, the hardness is reduced, the moldability is improved, and the elongation rate of the surface layer is also improved. It is better and better. Further, the product hardness after cold forging can be increased by aging treatment.
[0012]
By the above-mentioned annealing treatment, the aspect ratio of the carbide constituting the billet can be made 300% or less, and as a result, the billet having the above composition can have a critical upsetting ratio of 90% or more.
[0013]
The billet according to the present invention is suitable for forming an engine part with a shaft by continuous cold forging.
Here, the shaft-equipped engine part is, for example, a crankshaft, and conventionally, it has been difficult to obtain a high-precision product shape because it has conventionally been mainly formed by hot forging. If the forged cold forging can be applied, a product having a highly accurate product shape can be manufactured with high productivity and at low cost.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the accompanying drawings. Here, FIG. 1 is a process diagram of a specific method of manufacturing a billet for cold forging from a bar material having a material component according to the present invention, and FIG. 2 is a graph showing a relationship between a cold drawing rate and a limit upsetting rate. 3 is a view for explaining the effect of performing spheroidizing annealing before drawing, FIGS. 4A to 4C are SEM photographs (1000 times) showing the metal structure of the billet, FIG. 5 is an explanatory view of the upsetting ratio, and FIG. FIG. 3 is an explanatory view of an example of a forging process for continuously forging a crankshaft using the billet for cold forging according to the present invention.
[0015]
The component composition of the billet according to the present invention is such that C is 0.46 to 0.48 wt%, Si is 0.14 wt% or less, Mn is 0.55 to 0.65 wt%, P is 0.015 wt% or less, and S is The steel material contains 0.015 wt% or less, 0.15 wt% or less of Cu, 0.20 wt% or less of Ni, and 0.35 wt% or less of Cr. The basic composition of this composition is based on the component composition of S48C which is a hot forging material, and the amount of C is made equal to that of S48C in order to secure hardenability, and Si and P, which are likely to cause material cracking, are used. The composition of the component is such that the amount of S is reduced.
[0016]
Here, C is an element having the greatest effect on the cold forgeability per unit%, and is important in terms of mechanical properties, particularly, material strength and hardenability. That is, the crankshaft needs to have a predetermined mechanical strength as a whole and high hardness locally such as a worm and a tapered portion. In order to increase the hardness by quenching after forging a portion where high hardness is required locally as described above, it is necessary to set the proportion of C to 0.46 to 0.48 wt%.
[0017]
Also, Si is present in pig iron as a raw material and is almost removed in the steelmaking process, but may be added as a deoxidizing agent at the end of the steelmaking process. In S48C, 0.15 to 0.35 wt% is contained, Although a part of the steel enters the steel and forms a solid solution with the ferrite, it is preferable to remove it as a cold forging material because it inhibits the forgeability.
[0018]
Further, Mn slightly remains in the steelmaking process, but is added as a deoxidizing agent, so that S48C contains 0.60 to 0.90 wt%. This Mn combines with S and disperses in the steel as manganese sulfide and partially dissolves in ferrite. However, Mn that easily binds to S becomes MnS and tends to be a starting point of cracking during forging, so it is reduced. Although it is desirable that Mn dissolved in ferrite be easily quenched, growth of crystal grains is suppressed. For this reason, the Mn content is set to 0.55 to 0.65 wt%.
[0019]
P forms a solid solution in ferrite. When P is contained in a large amount, it is combined with a part of iron to form iron phosphide. However, when P forms a solid solution in ferrite, the ferrite decreases in elongation, and at room temperature. Is also reduced, and cracks are likely to occur during processing. And this P is allowed to 0.03 wt% in S48C, and this allowable value is too high for a cold forging material.
[0020]
Further, S is combined with a part of Mn to form MnS, and since this MnS becomes a starting point of a surface crack generated at the time of cold forging, S48C is allowed up to 0.035 wt%, but as a cold forging material, , The tolerance is too high.
In the present invention, in order to reduce the content of elements, Si, P, and S that inhibit workability as much as possible to enhance cold forgeability, Si is 0.14 wt% or less, P is 0.015 wt% or less, S is set to 0.015 wt% or less.
[0021]
Further, since Cu is less oxidized than Fe at high temperature heating, it is enriched on the surface and causes red hot embrittlement. Therefore, approximately equivalent amount of Ni is added to prevent red hot embrittlement. On the other hand, Cu is considered to increase the ferrite hardness by a small amount, like P, and impair the cold forgeability, so that Cu is set to 0.15 wt% or less.
[0022]
In addition to the above-mentioned effects, Ni is added in the same amount as S48C in order to increase quenchability, prevent low-temperature brittleness, and improve corrosion resistance. Further, Cr increases the hardenability and tempering resistance, increases the corrosion resistance, and easily produces a stable carbide. Therefore, Cr is contained in the same amount as S48C.
[0023]
Then, as shown in FIG. 1, a bar made of steel having the above-described composition is subjected to pickling, followed by a first spheroidizing annealing to spheroidize cementite to form a spheroid of cementite. Workability is improved, distortion can be given to the inside, and pearlite is miniaturized.
In this embodiment, the spheroidizing annealing is performed in such a manner that the temperature is kept at 740 ° C. for 6 hours, then the temperature is lowered to 680 ° C. at 20 ° C./h, and then the furnace is cooled.
[0024]
Next, a pickling process is performed by performing pickling and bonding, thereby improving the critical upsetting ratio.
Here, FIG. 2 is a graph showing the relationship between the cold drawing rate (cross-sectional reduction rate) and the critical upsetting rate of a steel material subjected to spheroidizing annealing. The cold drawing rate (cross-sectional reduction rate) is about 20%. It can be seen that the marginal upsetting rate can be maximized. This is conventionally known.
[0025]
Reason for limiting upsetting rate is increased by performing the pulling described above, by carrying out the drawing, the austenite grains are refined during the second round of annealing, but it is estimated that because it is possible to increase the spheroidization rate In the present embodiment, the sheet was drawn at a cold drawing rate (cross-section reduction rate) of 20% so as to obtain the maximum critical upsetting rate.
Incidentally, as shown in FIG. 2, when the diameter before processing is D and the diameter after processing is d, the cold drawing rate (cross-section reduction rate) is expressed as (D ² −d ² ) / D ² × 100. Value.
[0026]
Next, the bar is cut into a desired size, pickled, and then subjected to a second spheroidizing annealing to disperse carbides and increase the spheroidizing rate.
In the embodiment, in the second spheroidizing annealing, after maintaining at 750 ° C. for 2 hours, the temperature is decreased at 20 ° C./h to 730 ° C., and then the temperature is decreased to 15 ° C./h to 680 ° C. , Followed by furnace cooling.
[0027]
After the second round of spheroidizing annealing, shot blasting and bonding were performed to adjust the surface, thereby obtaining a billet for cold forging.
[0028]
In the above process, the spheroidizing annealing, the billet hardness (HRC), and the decarburized layer depth (mm) are performed before and after the drawing (two times in total) and in the case of performing only one time after the drawing. 3) is as shown in FIG.
Here, the improvement of the spheroidization level (the smaller the numerical value is, the better the sphere is closer to a sphere) and the reduction of the billet hardness are effective for improving the formability, and the penetration of the decarburized layer depth (ferrite formation of the surface layer) is effective. Is effective for improving the elongation of the surface layer.
[0029]
Therefore, an experiment on the aspect ratio was performed. The component composition of the material is as follows: C is 0.46 to 0.48 wt%, Si is 0.14 wt% or less, Mn is 0.55 to 0.65 wt%, P is 0.015 wt% or less, and S is 0.015 wt%. Hereinafter, Cu is 0.15 wt% or less, Ni is 0.20 wt% or less, Cr is 0.35 wt% or less, and the balance is Fe and impurities.
[0030]
When spheroidizing annealing was performed without drawing (Material 1), when spheroidizing annealing was not performed before drawing and the drawing rate was 20% (Material 2), spheroidizing annealing was performed before and after drawing. FIGS. 4A to 4C show the metallographic structures (1000 times) of the respective materials in the case where the drawing rate is set to 20% (material 3).
[0031]
As shown in Table 1, the aspect ratio (b / a × 100) representing the spheroidization ratio of carbide of each material is 506% for material 1, 347% for

material

2, and 300% for material 3. Met.
And cold forging (upsetting) was performed using each material. The upsetting ratio was (L ₁ −L ₂ ) / L ₁ × 100 = 90 (%) as shown in FIG. The crack occurrence rate of each material at this time was 35%, 5 %, and 0%, respectively.
Therefore, it was found that by performing the spheroidizing annealing twice, the carbide crystal became closer to a spherical shape, and cracking hardly occurred during cold forging.
[0032]
[Table 1]

[0033]
Next, in order to confirm the effectiveness of the material component according to the present invention, the results of an upsetting test are shown in Table 2 below. The upsetting ratio was set to 90% similarly to the above, and the material (billet) used for the upsetting test was subjected to spheroidizing annealing before and after drawing.
[0034]
[Table 2]

[0035]
First, when an upsetting test was performed using S48C as a material, the occurrence of cracks was 20% (N = 100). The fact that the crack occurrence rate is 20% is not suitable as a material for cold forging.
[0036]
Then, the present inventors tried to reduce the ratio of Mn. As a result, the occurrence of cracks was reduced to 12%. However, even this is not suitable as a material for cold forging. In S48C, the ratio of Mn is 0.60 to 0.90 wt%, In the range of 60 to 0.65 wt%, S48C and a material having a reduced proportion of Mn overlap. This is because the ratio of Mn cannot be strictly specified, and some variation is inevitable. This can be confirmed from the fact that some materials do not crack even when S48C is used, and some materials crack even when a material having a reduced Mn ratio is used.
[0037]
Since it was found that reducing the proportion of Mn was not sufficient, the present inventors tried to reduce the amount of C (carbon) within a range that did not adversely affect hardenability. As a result, the crack occurrence rate dropped to 5%. However, this is still unsuitable as a material for cold forging.
[0038]
Therefore, an upsetting test was performed by reducing the contents of Si, P, S, and Cu, which are considered to be elements that impair workability. That is, Si was set to 0.14 wt% or less, P was set to 0.015 wt% or less, S was set to 0.015 wt% or less, and Cu was set to 0.15 wt% or less. As a result, as shown in (Table 2), the crack occurrence rate was 0%.
[0039]
Then, using such a billet for cold forging, for example, by continuously performing cold forging in a plurality of stages as shown in FIG. 6 to form a crankshaft, a continuous process without intermediate annealing during forming is performed. Can be molded. Also, the hardenability is good.
[0040]
【The invention's effect】
As described above, the billet for cold forging according to the present invention has a predetermined component content as a material composition of a billet of a steel material for continuously performing cold forging, so that continuous cold forging is performed. Also, cracking and the like can be prevented and molding can be efficiently performed at a high upsetting ratio.
[0041]
If such a cold forging billet is used to manufacture an engine part with a shaft, there is no need to perform multiple setup changes as in the conventional hot forging, and the cutting process and the like for post-processing are also omitted. It is possible and suitable.
[Brief description of the drawings]
FIG. 1 is a process diagram of a method for producing a billet for cold forging from a rod material having a material component according to the present invention. FIG. 2 is a graph showing a relationship between a cold drawing rate and a limit upsetting rate. FIG. 4 (A) to 4 (C) are SEM photographs (× 1000) showing a metal structure of a billet.
FIG. 5 is an explanatory diagram of an upsetting ratio. FIG. 6 is an explanatory diagram of an example of a forging process for continuously forging a crankshaft with a billet for cold forging according to the present invention.

Claims

This is a billet of steel material for continuous cold forging and quenching after the cold forging , wherein C (carbon) is 0.46 to 0.48 wt% and Si (silicon) is 0.14 wt%. %, Mn (manganese) is 0.55 to 0.65 wt%, P (phosphorus) is 0.015 wt% or less, S (sulfur) is 0.015 wt% or less, Cu (copper) is 0.15 wt% or less, Ni (nickel) is contained at 0.20 wt% or less and Cr (chromium) is contained at 0.35 wt% or less , and the carbide constituting the billet has an aspect ratio of 300% or less and the upsetting ratio of the billet is 90%. A billet for cold forging characterized by the above .

2. The billet for cold forging according to claim 1, wherein the billet is a material for an engine part with a shaft such as a crankshaft.