JP2004003009A - Bar for cold forging, cold forged product and manufacturing method - Google Patents

Abstract

Kind Code: A1 The present invention relates to a bar for cold forging having excellent cold workability, a cold forged product, and a manufacturing method.
SOLUTION: After rolling, the steel composition is ferrite single-phase structure, and in mass%, C ≦ 0.15, Si ≦ 0.3, Mn ≦ 2%, Ti: 0.03-0.35%, Mo: 0.05 to 0.8%, further Nb ≦ 0.08%, V ≦ 0.15%, one or more of W ≦ 1.5%, S: 0.03 to 0.1%, Pb: One or more of 0.2% or less, Ca: 0.005% or less, and B: 0.02% or less, preferably 0.5 ≦ (C / 12) / ｛(Ti / 48) + (Mo / 96) ) + (Nb / 93) + (V / 51) + (W / 192)｝ ≦ 1.5, steel bar of balance Fe and inevitable impurities. The steel is heated at 950 to 1250 ° C. or higher, rolled at an end temperature of 800 ° C., cooled at 700 ° C. to 550 ° C. at a rate of more than 0.5 ° C./sec. After a precipitation treatment at a temperature of about 700 ° C. for 10 minutes or more, a fine precipitate having a particle size of less than 10 nm is deposited.
[Selection diagram] Fig. 1

Description

但し、各元素は含有量（質量％）とする。
【００１２】
３．鋼組成として、更に質量％で、Ｎｂ≦０．０８％、Ｖ≦０．１５％、Ｗ≦１．５％の一種または二種以上を含有することを特徴とする１記載の冷間加工性および冷間加工後の被削性に優れた冷間鍛造用棒鋼。
【００１３】
４．鋼組成として更に式（２）を満足することを特徴とする３記載の冷間加工性および冷間加工後の被削性に優れた冷間鍛造用棒鋼。
０．５≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）＋（Ｎｂ／９３）
＋（Ｖ／５１）＋（Ｗ／１８４）｝≦１．５　　　　（２）
但し、各元素は含有量（質量％）とし、含まれないものは０とする。
【００１４】
５．鋼組成として更に質量％でＳ：０．０３〜０．１％、Ｐｂ：０．２％以下、Ｃａ：０．００５％以下、Ｂ：０．０２％以下の一種または二種以上を含有することを特徴とする１乃至４のいずれか一つに記載の冷間加工性および冷間加工後の被削性に優れた冷間鍛造用棒鋼。
【００１５】
６．フェライト単相組織を有し、フェライト相中に粒径１０ｎｍ未満の微細析出物が分散析出していることを特徴とする引張強さ７００ＭＰａ以上の冷間鍛造品。
【００１６】
７．鋼組成が、質量％で、Ｃ≦０．１５％、Ｓｉ≦０．３％、Ｍｎ≦２％、Ｔｉ：０．０３〜０．３５％、Ｍｏ：０．０５〜０．８％、残部Ｆｅ及び不可避的不純物よりなる６記載の引張強さ７００ＭＰａ以上の冷間鍛造品。
【００１７】
８．鋼組成として更に式（３）を満足することを特徴とする７記載の引張強さ７００ＭＰａ以上の冷間鍛造品。

但し、各元素は含有量（質量％）とする。
【００１８】
９．微細析出物がＴｉとＭｏの炭化物であることを特徴とする６乃至８のいずれか一つに記載の引張強さ７００ＭＰａ以上の冷間鍛造品。
【００１９】
１０．鋼組成として、更に質量％で、Ｎｂ≦０．０８％、Ｖ≦０．１５％、Ｗ≦１．５％の一種または二種以上を含有することを特徴とする７記載の引張強さ７００ＭＰａ以上の冷間鍛造品。
【００２０】
１１．鋼組成として、更に式（４）を満足することを特徴とする１０記載の引張強さ７００ＭＰａ以上の冷間鍛造品。
０．５≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）＋（Ｎｂ／９３）
＋（Ｖ／５１）＋（Ｗ／１８４）｝≦１．５　　　　　（４）
但し、各元素は含有量（質量％）とし、含まれないものは０とする。
【００２１】
１２．微細析出物がＴｉとＭｏとＮｂ、Ｖ、Ｗの内の少なくとも１種とを含む炭化物であることを特徴とする１０または１１記載の張強さ７００ＭＰａ以上の冷間鍛造品。
【００２２】
１３．鋼組成として、更に質量％でＳ：０．０３〜０．１％、Ｐｂ：０．２％以下、Ｃａ：０．００５％以下、Ｂ：０．０２％以下の一種または二種以上を含有することを特徴とする７乃至１２記載の引張強さ７００ＭＰａ以上の冷間鍛造品。
【００２３】
１４．１乃至５のいずれか一つに記載の組成の鋼を９５０〜１２５０℃に加熱後、８００℃以上で圧延を終了し、その後、７００〜５５０℃を０．５℃／ｓ超えで冷却することを特徴とする冷間加工性および被削性に優れた冷間鍛造用棒鋼の製造方法。
【００２４】
１５．１乃至５のいずれか一つに記載の組成の鋼を９５０〜１２５０℃に加熱し、８００℃以上で圧延を終了後、７００〜５５０℃を０．５℃／ｓｅｃ超えで冷却し、その後、冷間鍛造により所定の形状とした後、５５０〜７００℃で１０分以上保持することを特徴とする引張強さ７００ＭＰａ以上の冷間鍛造品の製造方法。
【００２５】
【発明の実施の形態】
本発明のミクロ組織、成分組成および製造条件について以下に詳細に説明する。
【００２６】
１．ミクロ組織
本発明に係る棒鋼はそのミクロ組織を圧延後においてフェライト単相組織とする。また、該棒鋼を素材とする冷間鍛造品のミクロ組織は冷間鍛造後、析出処理した後においてもフェライト単相組織であって粒径１０ｎｍ未満の微細析出物を含む組織とする。
【００２７】
圧延後、フェライト単相組織とした場合、球状化処理材と同等の優れた冷間加工性が得られるが、さらなる冷間加工性向上のためには、フェライト単相組織の結晶粒径は５０μｍ以下が望ましい。
【００２８】
冷間鍛造し、析出処理後の組織をフェライト単相に規定するのは、延性・靭性はフェライト組織が最も良好なためであり、その他の組織の場合、Ｃが消費され微細析出物による十分な析出強化が得られず引張強さ７００ＭＰａ以上の強度が得られないためである。
【００２９】
本発明においてフェライト単相組織とは、断面組織観察（２００倍の光学顕微鏡組織観察）でフェライト面積率９５％以上、好ましくは９８％以上とする。
【００３０】
冷間加工後の析出処理により、フェライト単相組織中に微細析出物を分散析出させた場合、圧延後の組織がフェライト単相組織であっても焼入れ焼戻し材と同等の優れた強度が得られる。
【００３１】
本発明では微細析出物は粒径１０ｎｍ未満とする。析出物の粒径が１０ｎｍ以上の場合、引張強さ７００ＭＰａ以上が得られず、冷間鍛造品として強度が低い。
【００３２】
微細析出物の粒径は小さいほど強度向上に有効で、望ましくは５ｎｍ、更に望ましくは３ｎｍ以下とし、そのような微細析出物としてＴｉ、Ｍｏを含む炭化物（Ｔｉ−Ｍｏ系炭化物）や、Ｔｉ、Ｍｏに更にＮｂ、Ｖ、Ｗの一種または二種以上を含む炭化物（Ｔｉ−Ｍｏ−（Ｎｂ、Ｖ、Ｗ）系炭化物）が好ましい。
【００３３】
Ｍｏは拡散速度が遅く、Ｔｉとともに析出する場合、析出物の成長速度が低下し、微細な析出物が得やすく、顕著な析出強化が得られる。
【００３４】
これらの微細析出物の分布形態は特に規定しないが、母相中に均一分散（分散析出）することが望ましい。
【００３５】
また、本発明において、微細析出物の大きさは、全析出物の９０％以上で満足すれば良く、目的とする引張強さ７００ＭＰａ以上が得られる。但し、１０ｎｍ以上の大きさの析出物は析出物形成元素を消費し、強度に悪影響をあたえるため、５０ｎｍ以下とすることが好ましい。
【００３６】
上述した析出物とは別に少量のＦｅ炭化物を含有しても本発明の効果は損なわれない。しかしながら、平均粒径が１μｍ以上のＦｅ炭化物を多量に含む場合、靭性を阻害する。
【００３７】
そこで本発明においては含有されるＦｅ炭化物の大きさの上限は１μｍで、含有率は全体の１％以下が望ましい。
【００３８】
微細析出物の全析出物に占める割合は、以下の方法により求める。電子顕微鏡試料を、ツインジェット法を用いた電解研磨法で作成し、加速電圧２００ｋＶで観察する。その際、微細析出物が母相に対して計測可能なコントラストになるように母相の結晶方位を制御し、析出物の数え落としを最低限にするために焦点を正焦点からずらしたデフォーカス法で観察を行う。
【００３９】
また、析出物粒子の計測を行った領域の試料の厚さは電子エネルギー損失分光法を用いて、弾性散乱ピークと非弾性散乱ピーク強度を測定することで評価する。
【００４０】
この方法により、粒子数の計測と試料厚さの計測を同じ領域について実行することができる。粒子数および粒子径の測定は試料の０．５×０．５μｍの領域４箇所について行い、１μｍ^２当たりに分布する析出物を粒径ごとの個数として算出する。
【００４１】
この値と試料厚さから、析出物の１μｍ^３当たりに分布する粒子径ごとの個数を算出し、径が１０ｎｍ未満の析出物について、測定した全析出物に占める割合を算出する。
【００４２】
冷間鍛造品は自動車その他の輸送機材、建機用としては、引張強さ７００ＭＰａ以上が要求されることが多く、また、７００ＭＰａ未満では、既存の機械構造用鋼であっても球状化焼鈍処理など要せずに冷間鍛造が可能なため本発明は引張強さ７００ＭＰａ以上とする。
【００４３】
２．成分組成
本発明鋼は上述したミクロ組織で目的とする性能が得られるが、以下の成分組成が好ましい。
【００４４】
Ｃ
Ｃは圧延後の組織をフェライト単相とするために０．１５％以下にする。また、０．１５％を超えて含有すると微細析出物が粗大化し、強度が低下する。より好ましくは、０．０３％以上、０．１２％以下である。
【００４５】
Ｓｉ
Ｓｉは脱酸のため添加するが、０．３％を越えるとフェライトに固溶し、冷間加工時の変形抵抗が増大するため０．３％以下とする。より好ましくは、０．１５％以下である。
【００４６】
Ｍｎ
Ｍｎは強度向上に有効なため添加するが、２％を超えると冷間加工性を劣化させるので２％以下とする。より好ましくは、０．５％以上、１．８％以下である。
【００４７】
Ｔｉ
ＴｉはＭｏとともにＴｉ−Ｍｏ系炭化物を含む析出物を微細に析出させ、強度を向上させるため添加する。引張強度７００ＭＰａ以上を確保するため０．０３％以上とし、一方、０．２０％を超えて添加すると析出物が粗大化し、強度が低下するため０．０３〜０．３５％とする。より好ましくは、０．０３〜０．２０％である。
【００４８】
Ｍｏ
ＭｏはＴｉとともにＴｉ−Ｍｏ系炭化物を含む析出物を微細に析出させ、強度を向上させるため添加する。引張強度７００ＭＰａ以上を確保するため０．０５％以上とし、一方、０．８％を超えて添加するとベイナイト等の低温変態相を形成し、微細析出物による析出強化が不足し、強度が低下するため０．０５〜０．８％とする。より好ましくは、０．１５〜０．４５％である。
【００４９】
（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）｝
本パラメータは、析出物の大きさに影響を与えるもので、０．５以上、１．５以下とした場合、粒径１０ｎｍ未満の微細析出物の形成が容易となる。さらに好ましくは、０．７以上１．２以下である。
【００５０】
微細なＴｉ−Ｍｏ系炭化物では、炭化物中のＴｉ、Ｍｏは原子比で２．０≧Ｔｉ／Ｍｏ≧０．２、更に微細な場合は１．５≧Ｔｉ／Ｍｏ≧０．７であることが観察された。
【００５１】
更に、特性を向上させる場合、Ｎｂ、Ｖ、Ｗの一種または二種以上を添加することが好ましい。
【００５２】
Ｎｂ
ＮｂはＴｉとともに微細析出物を形成して強度上昇に寄与する。また組織を微細化し、結晶粒の整粒により延性を向上させる。０．０８％を超えると析出物が粗大化するとともに、結晶粒が過度に微細化し、延性が低下するため０．０８％以下とする。より好ましくは、０．０４％以下である。
【００５３】
Ｖ
ＶはＴｉと微細析出物を形成するが、０．１５％を超えると析出物が粗大化するようになるため、０．１５％以下とする。より好ましくは、０．１０％以下である。
【００５４】
Ｗ
ＷはＴｉと微細析出物を形成するが、１．５％を超えると析出物が粗大化するようになるため、１．５％以下とする。より好ましくは、１．０％以下である。
【００５５】
（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｗ／１８４）｝
本パラメータはこれらの元素を添加した場合に析出物の大きさに影響を与えるもので、０．５以上、１．５以下とした場合、粒径１０ｎｍ未満の微細析出物の形成が容易となる。さらに好ましくは、０．７以上１．２以下である。
【００５６】
微細なＴｉ−Ｍｏ−（Ｎｂ、Ｖ、Ｗ）系炭化物では、炭化物中で２．０≧（Ｔｉ＋Ｎｂ＋Ｖ）／（Ｍｏ＋Ｗ）≧０．２、更に微細な炭化物では１．５≧（Ｔｉ＋Ｎｂ＋Ｖ）／（Ｍｏ＋Ｗ）≧０．７であった。
【００５７】
本発明において、被削性を向上させる場合、Ｓ：０．０３〜０．１％、Ｐｂ０．２％以下、Ｃａ：０．００５％以下、Ｂ：０．０２％以下の一種または二種以上を添加する。また、本発明鋼では上記添加元素以外の残部はＦｅ及び不可避不純物とするが、脱酸剤としてＡｌを０．１％以下添加することができる。
【００５８】
また、強度、延性を向上させる場合、Ｎｉ、Ｃｒの一種または二種をＮｉ≦２％、Ｃｒ≦２％の範囲で添加しても構わない。冷間加工性を更に向上させる場合には、不可避不純物であるＰ、ＮをＰ≦０．０４０％、Ｎ≦０．００８０％に規制することが望ましい。
【００５９】
これらの元素の添加の有無により本発明の効果が損なわれることはない。
【００６０】
３．製造条件
図１に本発明鋼による冷間鍛造部品の概略製造工程図を示す。Ｓ１は棒鋼製造工程、Ｓ２は搬送工程、Ｓ３は製品仕上げ過程で、製品仕上げ過程（Ｓ３）の析出処理で微細析出物を析出させ引張強さ７００ＭＰａ以上とする。
【００６１】
以下に上述した成分組成の鋼の望ましい製造工程について詳細に説明する。
【００６２】
圧延加熱温度
圧延加熱温度は９５０〜１２５０℃とする。圧延後、球状化処理をしない圧延ままの棒鋼（以下、圧延材）の冷間加工、切削において、従来鋼（Ｓ４５Ｃ）の球状化処理材と同等の特性が得られるよう、圧延時に溶解時から残存する炭化物を固溶させる。
【００６３】
圧延加熱温度を９５０℃未満とした場合、仕上までの圧延荷重が大きくなりすぎて圧延が不可能である。また、１２５０℃を超えると、析出物が再溶解するため、微細析出物が析出し、強度が上昇するため、冷間鍛造性が悪化する。よって、加熱温度は、９５０〜１２５０℃とする。
【００６４】
圧延終了温度
圧延終了温度は材質均一性に影響を与え、８００℃未満では圧延荷重が高く真円度が劣化するため８００℃以上とする。
【００６５】
冷却速度
圧延終了後の冷却速度は冷却中に微細析出物を析出させないよう析出温度範囲の７００〜５５０℃を微細析出物が得られる限界冷却速度である０．５℃／ｓｅｃ超えの速度で冷却する。
【００６６】
析出処理
圧延材を冷間鍛造や切削加工により部品形状とした後、析出処理により引張強さ７００ＭＰａ以上とする。析出処理においては母相をフェライト単相とし、強度向上に寄与する微細析出物を析出させることが必要で、加熱温度は５５０℃未満ではベイナイトが生成し、７００℃を超えると析出物が粗大化するため５５０〜７００℃とする。
【００６７】
また、微細なＴｉ、Ｍｏなどの炭化物を十分に生成、析出させるため該温度域において１０分以上保持する。
【００６８】
【実施例】
表１に示す組成の鋼（Ｎｏ．１〜１１）を１５０ｋｇ真空溶解炉にて溶製し、圧延を１１５０℃加熱、９２０℃仕上げで行い、その後１℃／ｓｅｃで室温まで冷却し３６ｍｍφの棒鋼とした。Ｎｏ．１１は従来材でＳ４５Ｃである。
【００６９】
Ｎｏ．１〜１０は圧延まま、Ｎｏ．１１は圧延後、球状化処理したものから冷間据えこみ加工用の試験片（直径１４ｍｍ、高さ／直径比：１．５）を採取し、同心円状溝突付きダイスを取りつけた拘束圧縮盤により圧縮加工時の変形抵抗、割れ発生限界加工率を平均歪速度０．０１／ｓｅｃで調査した。また引張試験により強度も調査した。
【００７０】
変形抵抗は平均歪が１．５（圧下率７０％）の荷重を拘束係数と変形前の初期面積で除して求めた。割れ発生限界加工率は実際に割れの発生した圧下率とした。
【００７１】
尚、変形抵抗、割れ発生限界加工率は部品鍛造時の鍛造工具寿命、割れ発生不良率と相関があることが知られている。
【００７２】
更に、被削性をドリル切削試験により評価した。上記圧縮試験採取材の残部を２５ｍｍ径まで冷間引抜き加工し、３０ｍｍ厚に切断したものを試験材として、ＪＩＳ高速度工具鋼ＳＫＨ５１の６ｍｍφのストレートドリルで送り０．１５ｍｍ／ｒｅｖ、回転数７４５ｒｐｍ、１断面当たり５箇所の貫通穴を開け、ドリルが切削不能になるまでの総穴数で評価した。
【００７３】
また、２５ｍｍ径まで冷間引抜き加工後、鋼Ｎｏ．１〜７のものは析出処理として５２５〜７２５℃で約１５分保持し、鋼Ｎｏ．８のものは焼入れ焼戻しし、引張試験、組織観察を行った。
【００７４】
組織観察は断面を光学顕微鏡で観察するとともに、析出物を透過型電子顕微鏡（ＴＥＭ）で観察し、組成をエネルギー分散型Ｘ線分光装置（ＥＤＸ）により求めた。
【００７５】
表２に試験結果を示す。Ｎｏ．１〜６が本発明例、Ｎｏ．７〜１０が比較例、Ｎｏ．１１が既存の冷間鍛造用鋼（従来例）である。
【００７６】
表から明らかなように、Ｎｏ．１〜６の圧延材の引張強さは従来材の球状化処理材と同等で、変形抵抗、限界加工率、ドリル切削加工性および析出処理後の強度の全てにおいてＮｏ．１１の従来材と同等の特性が得られている。
【００７７】
Ｎｏ．７は析出温度が本発明範囲外で高いため、析出処理後、フェライト＋パーライト組織となり、また、析出物の粒径も大きいため引張強さが７００ＭＰａ以下と本発明範囲外である。
【００７８】
Ｎｏ．８は析出温度が本発明範囲外で低いため、析出処理後、ベイナイト組織となり、Ｃが固溶されたため、析出強化が不足し引張強さが７００ＭＰａ以下と本発明範囲外である。
【００７９】
Ｎｏ．９はＣが本発明範囲外で高いため圧延材の引張強さが高く、変形抵抗、限界加工率、ドリル寿命が従来材に及ばない。また、析出処理後の析出物が大きく十分な析出強化が得られず、従来材より強度が低い。
【００８０】
Ｎｏ．１０はＳｉが本発明範囲外で高いため圧延材の引張強さが高く、変形抵抗、限界加工率、ドリル寿命が従来材に及ばない。また、Ｍｏを含まないため微細析出物が生成されず、従来材より強度が低い。
【００８１】
【表１】

【００８２】
【表２】

【００８３】
【発明の効果】
本発明によれば、圧延後の球状化処理や、冷間加工後の焼入れ焼戻し処理を行うことなく、球状化処理材と同等の優れた冷間加工性で且つ冷間加工後の析出処理により引張強さ７００ＭＰａ以上となる冷間鍛造品の素材となる棒鋼およびその製造方法が得られ、産業上極めて有用である。
【図面の簡単な説明】
【図１】本発明鋼の製造工程の一例を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel bar for cold forging, a cold forged part and a method for producing the same, and in particular, a steel bar for cold forging that does not require spheroidizing annealing for cold working required for part forming, and a material formed by using the same as a material. The present invention relates to a cold forged product that does not require a quenching and tempering treatment for securing strength later, and a method for manufacturing the same.
[0002]
[Prior art]
Cold forged products are made by rolling carbon steel for machine structure or alloy steel for machine structure, softening by spheroidizing annealing, forming by cold forging and cutting, and heat-treating according to the desired strength and toughness. Is done. Various proposals have been made for improving the cold workability and machinability of this cold forged product.
[0003]
Japanese Patent Application Laid-Open No. 1-100221 proposes that the microstructure of steel be ferrite + bainite in order to improve cold workability. According to the present proposal, the working limit during cold working is improved, but the deformation resistance during cold working is not sufficiently reduced because bainite is a low-temperature transformed structure having a high dislocation density.
[0004]
Japanese Patent Application Laid-Open No. Hei 11-246939 proposes that a part of carbon in steel is graphitized in order to improve machinability and cold workability, and that the microstructure is a three-phase structure of ferrite + graphite + cementite. I have.
[0005]
However, the graphitization treatment requiring a long time of heating instead of the spheroidization heat treatment and the quenching and tempering treatment for securing the strength are required, and the productivity is not improved.
[0006]
[Problems to be solved by the invention]
As described above, in the conventional technologies for improving workability and productivity during cold forging, since the strength of the final product is ensured by transformation strengthening such as quenching and tempering, the hardenability of steel is high, and after cold rolling, A softening treatment was required to improve the properties.
[0007]
Therefore, the present invention secures the strength as a cold forged product by precipitation strengthening even if the hardenability of the steel is reduced, so that it is possible to omit the softening treatment after rolling, and a method for manufacturing the same. The purpose is to provide.
[0008]
[Means for Solving the Problems]
The present inventors have found that steel hardening due to cooling after rolling is small, no softening treatment for cold forging or cold working is required, and the steel composition of the steel is ensured by precipitation strengthening to ensure strength as a final product. Intensive study was conducted on the manufacturing conditions.
[0009]
As a result, when the basic component composition is adjusted to obtain a ferrite single-phase structure after rolling to a low-carbon system, cold workability equivalent to that of a spheroidized material is obtained, while, as a cold-worked product It has been found that the strength can be ensured by strengthening the precipitation of fine precipitates by adding an appropriate amount of Ti and Mo and holding at a high temperature.
[0010]
The present invention has been made by further study based on the above findings, that is, the present invention,
1. In mass%, C ≦ 0.15%, Si ≦ 0.3%, Mn ≦ 2%, Ti: 0.03-0.35%, Mo: 0.05-0.8%, balance Fe and inevitable A bar for cold forging made of impurities and having a ferrite single phase structure and having excellent cold workability and machinability after cold work.
[0011]
2. 2. The steel bar for cold forging having excellent cold workability and machinability after cold working according to 1, wherein the steel composition further satisfies the formula (1).

However, each element is the content (% by mass).
[0012]
3. The cold workability according to 1, wherein the steel composition further contains one or more of Nb ≦ 0.08%, V ≦ 0.15%, and W ≦ 1.5% by mass%. Cold-forged steel bars with excellent machinability after cold working.
[0013]
4. 4. The steel bar for cold forging excellent in cold workability and machinability after cold working according to 3, wherein the steel composition further satisfies the formula (2).
0.5 ≦ (C / 12) / ｛(Ti / 48) + (Mo / 96) + (Nb / 93)
+ (V / 51) + (W / 184)｝ ≦ 1.5 (2)
However, each element is set to the content (% by mass), and those not included are set to 0.
[0014]
5. The steel composition further contains one or more of S: 0.03 to 0.1%, Pb: 0.2% or less, Ca: 0.005% or less, and B: 0.02% or less by mass%. 5. The steel bar for cold forging excellent in cold workability and machinability after cold work according to any one of 1 to 4 above.
[0015]
6. A cold forged product having a tensile strength of 700 MPa or more, which has a ferrite single-phase structure and in which fine precipitates having a particle size of less than 10 nm are dispersed and precipitated in the ferrite phase.
[0016]
7. Steel composition, in mass%, C ≦ 0.15%, Si ≦ 0.3%, Mn ≦ 2%, Ti: 0.03 to 0.35%, Mo: 0.05 to 0.8%, balance 6. A cold forged product having a tensile strength of 700 MPa or more according to 6, which comprises Fe and unavoidable impurities.
[0017]
8. 8. The cold forged product having a tensile strength of 700 MPa or more according to 7, wherein the steel composition further satisfies Expression (3).

However, each element is the content (% by mass).
[0018]
9. 9. The cold forged product having a tensile strength of 700 MPa or more according to any one of 6 to 8, wherein the fine precipitate is a carbide of Ti and Mo.
[0019]
10. The tensile strength 700 MPa according to 7, wherein the steel composition further contains one or more of Nb ≦ 0.08%, V ≦ 0.15%, and W ≦ 1.5% by mass%. More than cold forged products.
[0020]
11. 11. The cold forged product having a tensile strength of 700 MPa or more according to 10, wherein the steel composition further satisfies Expression (4).
0.5 ≦ (C / 12) / ｛(Ti / 48) + (Mo / 96) + (Nb / 93)
+ (V / 51) + (W / 184)｝ ≦ 1.5 (4)
However, each element is set to the content (% by mass), and those not included are set to 0.
[0021]
12. 12. The cold forged product having a tensile strength of 700 MPa or more according to 10 or 11, wherein the fine precipitate is a carbide containing Ti, Mo, Nb, V, and W.
[0022]
13. The steel composition further contains one or more of S: 0.03 to 0.1%, Pb: 0.2% or less, Ca: 0.005% or less, and B: 0.02% or less by mass%. 13. A cold forged product having a tensile strength of 700 MPa or more as described in 7 to 12.
[0023]
14. After heating the steel having the composition described in any one of 1 to 5 to 950 to 1250 ° C, finish rolling at 800 ° C or more, and then cool the steel at 700 to 550 ° C at a rate of more than 0.5 ° C / s. A method for producing a steel bar for cold forging having excellent cold workability and machinability.
[0024]
15. A steel having a composition according to any one of 1 to 5 is heated to 950 to 1250 ° C., and after rolling at 800 ° C. or higher, 700 to 550 ° C. is cooled at 0.5 ° C./sec or more, Thereafter, a method for producing a cold forged product having a tensile strength of 700 MPa or more, wherein the predetermined shape is formed by cold forging, and the temperature is maintained at 550 to 700 ° C. for 10 minutes or more.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
The microstructure, component composition and production conditions of the present invention will be described in detail below.
[0026]
1. Microstructure The steel bar according to the present invention has a microstructure of a ferrite single phase structure after rolling. Further, the microstructure of the cold forged product using the steel bar as a material is a ferrite single phase structure including a fine precipitate having a grain size of less than 10 nm even after the cold forging and after a precipitation treatment.
[0027]
When a ferrite single phase structure is obtained after rolling, excellent cold workability equivalent to that of a spheroidized material is obtained. However, in order to further improve the cold workability, the crystal grain size of the ferrite single phase structure is 50 μm. The following is desirable.
[0028]
The reason why the structure after the cold forging and precipitation treatment is defined as a ferrite single phase is that the ferrite structure is the best in ductility and toughness, and in other structures, C is consumed and sufficient due to fine precipitates. This is because precipitation strengthening cannot be obtained and a tensile strength of 700 MPa or more cannot be obtained.
[0029]
In the present invention, the ferrite single-phase structure is defined as having a ferrite area ratio of 95% or more, preferably 98% or more, as determined by observing a cross-sectional structure (observing a 200-fold optical microscope).
[0030]
By precipitation treatment after cold working, when fine precipitates are dispersed and precipitated in a ferrite single phase structure, even if the structure after rolling is a ferrite single phase structure, excellent strength equivalent to that of a quenched and tempered material is obtained. .
[0031]
In the present invention, the fine precipitate has a particle size of less than 10 nm. When the particle size of the precipitate is 10 nm or more, a tensile strength of 700 MPa or more cannot be obtained, and the strength is low as a cold forged product.
[0032]
The smaller the particle size of the fine precipitate is, the more effective it is to improve the strength, preferably 5 nm, more preferably 3 nm or less. Such fine precipitates include carbides containing Ti and Mo (Ti-Mo-based carbides), Ti, and Mo. A carbide (Ti-Mo- (Nb, V, W) -based carbide) further containing one or more of Nb, V, and W in Mo is preferable.
[0033]
Mo has a low diffusion rate, and when precipitated with Ti, the growth rate of the precipitates is reduced, so that fine precipitates are easily obtained and remarkable precipitation strengthening is obtained.
[0034]
Although the distribution form of these fine precipitates is not particularly defined, it is desirable that they are uniformly dispersed (dispersed and precipitated) in the parent phase.
[0035]
In the present invention, the size of the fine precipitates may be satisfied if it is 90% or more of the total precipitates, and the desired tensile strength of 700 MPa or more can be obtained. However, a precipitate having a size of 10 nm or more consumes a precipitate-forming element and adversely affects the strength.
[0036]
The effect of the present invention is not impaired even if a small amount of Fe carbide is contained in addition to the precipitates described above. However, when a large amount of Fe carbide having an average particle size of 1 μm or more is contained, toughness is impaired.
[0037]
Therefore, in the present invention, the upper limit of the size of the Fe carbide contained is 1 μm, and the content is desirably 1% or less of the whole.
[0038]
The ratio of the fine precipitates to the total precipitates is determined by the following method. An electron microscope sample is prepared by an electrolytic polishing method using a twin jet method, and observed at an acceleration voltage of 200 kV. At this time, the crystal orientation of the parent phase is controlled so that the fine precipitate has a measurable contrast with respect to the parent phase, and the defocus is shifted from the normal focus to minimize the number of precipitates. Observe by method.
[0039]
In addition, the thickness of the sample in the region where the precipitation particles are measured is evaluated by measuring the elastic scattering peak and the inelastic scattering peak intensity using electron energy loss spectroscopy.
[0040]
According to this method, the measurement of the number of particles and the measurement of the sample thickness can be performed for the same region. The measurement of the number of particles and the particle diameter is performed on four portions of a 0.5 × 0.5 μm region of the sample, and the number of precipitates distributed per 1 μm ² is calculated as the number for each particle size.
[0041]
From this value and the sample thickness, the number of precipitates per particle size distributed per 1 μm ³ is calculated, and the ratio of the precipitates having a diameter of less than 10 nm to the measured total precipitates is calculated.
[0042]
Cold forgings are often required to have a tensile strength of 700 MPa or more for use in automobiles and other transportation equipment and construction equipment. If the strength is less than 700 MPa, existing steel for machine structural use is subjected to spheroidizing annealing. In the present invention, the tensile strength is 700 MPa or more because cold forging can be performed without any need.
[0043]
2. Ingredient Composition The steel of the present invention can achieve desired performance with the above-mentioned microstructure, but the following ingredient composition is preferred.
[0044]
C
C is set to 0.15% or less to make the structure after rolling into a ferrite single phase. On the other hand, when the content exceeds 0.15%, the fine precipitates are coarsened and the strength is reduced. More preferably, it is 0.03% or more and 0.12% or less.
[0045]
Si
Si is added for deoxidation, but if it exceeds 0.3%, it is dissolved in ferrite and the deformation resistance at the time of cold working increases, so it is made 0.3% or less. More preferably, it is 0.15% or less.
[0046]
Mn
Mn is added because it is effective for improving the strength, but if it exceeds 2%, the cold workability deteriorates, so Mn is made 2% or less. More preferably, it is 0.5% or more and 1.8% or less.
[0047]
Ti
Ti is added in order to finely precipitate a precipitate containing Ti-Mo-based carbide together with Mo and improve the strength. To secure a tensile strength of 700 MPa or more, the content is made 0.03% or more. On the other hand, if added in excess of 0.20%, the precipitate becomes coarse and the strength is reduced, so that the content is made 0.03 to 0.35%. More preferably, it is 0.03 to 0.20%.
[0048]
Mo
Mo is added in order to finely precipitate a precipitate containing Ti-Mo-based carbide together with Ti and improve the strength. In order to secure a tensile strength of 700 MPa or more, the content is made 0.05% or more. On the other hand, if added in excess of 0.8%, a low-temperature transformation phase such as bainite is formed, precipitation strengthening by fine precipitates is insufficient, and strength is reduced. Therefore, it is set to 0.05 to 0.8%. More preferably, it is 0.15 to 0.45%.
[0049]
(C / 12) / {(Ti / 48) + (Mo / 96)}
This parameter affects the size of the precipitate. When the parameter is 0.5 or more and 1.5 or less, a fine precipitate having a particle size of less than 10 nm is easily formed. More preferably, it is 0.7 or more and 1.2 or less.
[0050]
In fine Ti-Mo based carbides, the atomic ratio of Ti and Mo in the carbide is 2.0 ≧ Ti / Mo ≧ 0.2, and in the case of finer particles, 1.5 ≧ Ti / Mo ≧ 0.7. Was observed.
[0051]
In order to further improve the characteristics, it is preferable to add one or more of Nb, V and W.
[0052]
Nb
Nb forms fine precipitates together with Ti and contributes to an increase in strength. Further, the structure is refined and ductility is improved by sizing the crystal grains. If it exceeds 0.08%, the precipitates become coarse, and the crystal grains become excessively fine, and the ductility decreases, so that the content is made 0.08% or less. More preferably, it is 0.04% or less.
[0053]
V
V forms fine precipitates with Ti, but if it exceeds 0.15%, the precipitates become coarse, so that V is set to 0.15% or less. More preferably, it is 0.10% or less.
[0054]
W
W forms fine precipitates with Ti, but if it exceeds 1.5%, the precipitates become coarse, so the content is made 1.5% or less. More preferably, it is 1.0% or less.
[0055]
(C / 12) / {(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)}
This parameter affects the size of the precipitate when these elements are added. When the parameter is 0.5 or more and 1.5 or less, the formation of a fine precipitate having a particle size of less than 10 nm becomes easy. . More preferably, it is 0.7 or more and 1.2 or less.
[0056]
For fine Ti-Mo- (Nb, V, W) -based carbides, 2.0 ≧ (Ti + Nb + V) / (Mo + W) ≧ 0.2 in the carbide, and 1.5 ≧ (Ti + Nb + V) / ( Mo + W) ≧ 0.7.
[0057]
In the present invention, when improving machinability, one or more of S: 0.03 to 0.1%, Pb 0.2% or less, Ca: 0.005% or less, B: 0.02% or less. Is added. In the steel of the present invention, the remainder other than the above-mentioned additional elements is Fe and inevitable impurities, but 0.1% or less of Al can be added as a deoxidizing agent.
[0058]
When improving the strength and ductility, one or two of Ni and Cr may be added in the range of Ni ≦ 2% and Cr ≦ 2%. In order to further improve the cold workability, it is desirable that P and N, which are unavoidable impurities, be restricted to P ≦ 0.040% and N ≦ 0.0080%.
[0059]
The effects of the present invention are not impaired by the presence or absence of these elements.
[0060]
3. Manufacturing Conditions FIG. 1 shows a schematic manufacturing process diagram of a cold forged part made of the steel of the present invention. S1 is a steel bar manufacturing process, S2 is a conveying process, S3 is a product finishing process, and fine precipitates are precipitated by a precipitation treatment in the product finishing process (S3) to have a tensile strength of 700 MPa or more.
[0061]
Hereinafter, a desirable production process of steel having the above-described composition will be described in detail.
[0062]
Rolling heating temperature The rolling heating temperature is 950 to 1250 ° C. After rolling, in cold working and cutting of as-rolled steel bars (hereinafter referred to as “rolled materials”) that have not been subjected to spheroidizing treatment, from the time of melting during rolling, the same characteristics as those of conventional steel (S45C) are obtained. The remaining carbides are dissolved.
[0063]
When the rolling heating temperature is lower than 950 ° C., the rolling load up to the finish becomes too large to perform rolling. On the other hand, when the temperature exceeds 1250 ° C., the precipitates are re-dissolved, so that fine precipitates are precipitated and the strength is increased, so that the cold forgeability deteriorates. Therefore, the heating temperature is 950 to 1250 ° C.
[0064]
Rolling end temperature The rolling end temperature affects the material uniformity. If the temperature is lower than 800 ° C, the rolling load is high and the roundness is deteriorated, so that the temperature is set to 800 ° C or higher.
[0065]
Cooling rate The cooling rate after the completion of rolling is to cool the precipitation temperature range of 700 to 550 ° C. at a rate exceeding 0.5 ° C./sec, which is the limit cooling rate at which fine precipitates can be obtained, so as not to precipitate fine precipitates during cooling. I do.
[0066]
Precipitation treatment After the rolled material is formed into a component shape by cold forging or cutting, the tensile strength is made 700 MPa or more by the precipitation treatment. In the precipitation treatment, it is necessary to use a ferrite single phase as the matrix and precipitate fine precipitates that contribute to strength improvement. If the heating temperature is lower than 550 ° C, bainite is formed, and if the heating temperature exceeds 700 ° C, the precipitates become coarse. 550-700 ° C.
[0067]
In order to sufficiently generate and precipitate fine carbides such as Ti and Mo, the temperature is maintained for 10 minutes or more in the temperature range.
[0068]
【Example】
A steel (No. 1 to 11) having the composition shown in Table 1 was melted in a 150 kg vacuum melting furnace, rolled at 1150 ° C. and finished at 920 ° C., and then cooled to room temperature at 1 ° C./sec to obtain a 36 mm φ steel bar. And No. 11 is a conventional material, which is S45C.
[0069]
No. Nos. 1 to 10 were rolled. Reference numeral 11 denotes a constrained compression plate on which a test piece (diameter: 14 mm, height / diameter ratio: 1.5) for cold upsetting was sampled from the spheroidized product after rolling, and a concentric grooved die was attached. The deformation resistance at the time of compression working and the limit working rate of crack occurrence were investigated at an average strain rate of 0.01 / sec. The strength was also investigated by a tensile test.
[0070]
The deformation resistance was determined by dividing a load having an average strain of 1.5 (a reduction ratio of 70%) by the constraint coefficient and the initial area before deformation. The crack generation limit working rate was defined as the rolling reduction at which cracks actually occurred.
[0071]
It is known that the deformation resistance and the cracking limit working rate have a correlation with a forging tool life and a crack occurrence defective rate at the time of forging a part.
[0072]
Further, the machinability was evaluated by a drill cutting test. The remaining part of the compression test specimen was cold-drawn to a diameter of 25 mm and cut to a thickness of 30 mm. The test specimen was used as a test specimen and fed with a 6 mmφ straight drill of JIS high speed tool steel SKH51 at 0.15 mm / rev at 745 rpm. 5. Five through holes were made per cross section, and the evaluation was made based on the total number of holes until the drill became uncuttable.
[0073]
After cold drawing to a diameter of 25 mm, steel No. Nos. 1 to 7 were held at 525 to 725 ° C. for about 15 minutes as a precipitation treatment. Eight samples were quenched and tempered, and were subjected to a tensile test and a structure observation.
[0074]
The structure was observed by observing the cross section with an optical microscope, observing the precipitate with a transmission electron microscope (TEM), and determining the composition with an energy dispersive X-ray spectrometer (EDX).
[0075]
Table 2 shows the test results. No. Nos. 1 to 6 are examples of the present invention; 7 to 10 are comparative examples; 11 is an existing cold forging steel (conventional example).
[0076]
As is clear from the table, No. The tensile strengths of the rolled materials of Nos. 1 to 6 are equivalent to those of the conventional spheroidized material, and all of the deformation resistance, the critical working rate, the drill workability and the strength after the precipitation treatment are No. 1. Properties equivalent to those of the eleventh conventional material are obtained.
[0077]
No. In No. 7, the precipitation temperature was high outside the range of the present invention, and after the precipitation treatment, a ferrite + pearlite structure was formed. Further, since the precipitate had a large particle size, the tensile strength was 700 MPa or less, out of the range of the present invention.
[0078]
No. In No. 8, the precipitation temperature was low outside the range of the present invention, so that a bainite structure was formed after the precipitation treatment, and C was dissolved as a solid solution. Therefore, precipitation strengthening was insufficient and the tensile strength was 700 MPa or less, which is outside the range of the present invention.
[0079]
No. In No. 9, since C is high outside the range of the present invention, the tensile strength of the rolled material is high, and the deformation resistance, critical working ratio, and drill life are inferior to those of the conventional material. Further, the precipitates after the precipitation treatment are large, and sufficient precipitation strengthening cannot be obtained, and the strength is lower than that of the conventional material.
[0080]
No. Sample No. 10 has a high tensile strength of the rolled material because Si is high outside the range of the present invention, and the deformation resistance, the critical working ratio, and the drill life are inferior to those of the conventional material. Further, since it does not contain Mo, fine precipitates are not generated, and the strength is lower than that of the conventional material.
[0081]
[Table 1]

[0082]
[Table 2]

[0083]
【The invention's effect】
According to the present invention, the spheroidizing treatment after rolling, without performing the quenching and tempering treatment after cold working, with excellent cold workability equivalent to the spheroidized material and by precipitation treatment after cold working A steel bar as a raw material of a cold forged product having a tensile strength of 700 MPa or more and a method for producing the same are obtained, and are extremely useful in industry.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a production process of the steel of the present invention.

Claims (15)

In mass%, C ≦ 0.15%, Si ≦ 0.3%, Mn ≦ 2%, Ti: 0.03-0.35%, Mo: 0.05-0.8%, balance Fe and inevitable A bar for cold forging made of impurities and having a ferrite single phase structure and having excellent cold workability and machinability after cold work. 2. The steel bar for cold forging according to claim 1, wherein the steel composition further satisfies the formula (1).

However, each element is the content (% by mass). 2. The steel according to claim 1, wherein the steel composition further contains one or more of Nb ≦ 0.08%, V ≦ 0.15%, and W ≦ 1.5% by mass%. A bar for cold forging with excellent workability and machinability after cold working. The steel bar for cold forging excellent in cold workability and machinability after cold working according to claim 3, wherein the steel composition further satisfies the formula (2).
0.5 ≦ (C / 12) / ｛(Ti / 48) + (Mo / 96) + (Nb / 93)
+ (V / 51) + (W / 184)｝ ≦ 1.5 (2)
However, each element is set to the content (% by mass), and those not included are set to 0. The steel composition further contains one or more of S: 0.03 to 0.1%, Pb: 0.2% or less, Ca: 0.005% or less, and B: 0.02% or less by mass%. 5. The steel bar for cold forging according to claim 1, which is excellent in cold workability and machinability after cold work. A cold forged product having a tensile strength of 700 MPa or more, which has a ferrite single-phase structure and in which fine precipitates having a particle size of less than 10 nm are dispersed and precipitated in the ferrite phase. Steel composition in mass%, C ≦ 0.1%, Si ≦ 0.3%, Mn ≦ 2%, Ti: 0.03 to 0.20%, Mo: 0.05 to 0.6%, balance The cold forged product having a tensile strength of 700 MPa or more according to claim 6, comprising a Fe and an unavoidable impurity. The cold forged product having a tensile strength of 700 MPa or more according to claim 7, wherein the steel composition further satisfies the formula (3).

However, each element is the content (% by mass). The cold forged product having a tensile strength of 700 MPa or more according to any one of claims 6 to 8, wherein the fine precipitate is a carbide of Ti and Mo. The tensile strength according to claim 7, wherein the steel composition further contains one or more of Nb ≤ 0.08%, V ≤ 0.15%, and W ≤ 1.5% by mass%. Cold forged product of 700MPa or more. The cold forged product having a tensile strength of 700 MPa or more according to claim 10, wherein the steel composition further satisfies Expression (4).
0.5 ≦ (C / 12) / ｛(Ti / 48) + (Mo / 96) + (Nb / 93)
+ (V / 51) + (W / 184)｝ ≦ 1.5 (4)
However, each element is set to the content (% by mass), and those not included are set to 0. The cold forged product having a tensile strength of 700 MPa or more according to claim 10 or 11, wherein the fine precipitate is a carbide containing Ti, Mo, Nb, V, and W. The steel composition further contains one or more of S: 0.03 to 0.1%, Pb: 0.2% or less, Ca: 0.005% or less, and B: 0.02% or less by mass%. 13. The cold forged product having a tensile strength of 700 MPa or more according to claim 7. After heating the steel having the composition according to any one of claims 1 to 5 to 950 to 1250 ° C, rolling is completed at 800 ° C or more, and then cooling is performed at 700 to 550 ° C at a rate exceeding 0.5 ° C / sec. A method for producing a steel bar for cold forging having excellent cold workability and machinability. A steel having a composition according to any one of claims 1 to 5 is heated to 950 to 1250 ° C, and after rolling at 800 ° C or higher, 700 to 550 ° C is cooled at a rate exceeding 0.5 ° C / sec. Thereafter, a cold forging is performed to obtain a predetermined shape, and the resultant is held at 550 to 700 ° C. for 10 minutes or more. A method of manufacturing a cold forged product having a tensile strength of 700 MPa or more.

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