JP2004100016A - Rolled wire rod for bearing and drawn wire rod - Google Patents
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Abstract
Description
【０００１】
【発明が属する技術分野】
本発明は、伸線加工された軸受用伸線材およびその素材となる圧延線材に係り、特に伸線加工前に１次球状化焼鈍を省略して伸線することができる、生引き可能な軸受用圧延線材に関する。
【０００２】
【従来の技術】
軸受用線材は、球軸受の鋼球やコロ軸受のコロ等の素材として用いられており、一般的に、鋳塊から熱間圧延した圧延線材を１次球状化焼鈍した後、伸線加工を施し、さらに２次球状化焼鈍した後、仕上伸線することによって製造される。前記鋼球、コロ等の転動部品は、前記伸線材を適宜のサイズに切断し、切断片を概略部品形状に鍛造した後、熱処理が施され、仕上加工されて製造される。
【０００３】
軸受用線材を形成する軸受用鋼は、過共析鋼であり、難加工材であるため、圧延線材を直接伸線加工すること（「生引き」という。）は困難であり、材質を軟化し、加工性を改善するため、伸線加工する前に上記のように１次球状化焼鈍が施される。
【０００４】
しかし、この球状化焼鈍は、処理時間、コストがかかるため、近年、コスト削減のために１次球状化焼鈍を省略することができる圧延線材が望まれている。
圧延線材を生引き可能にするには、加工性を向上させる必要がある。従来、粗大な初析セメンタイトが破壊の起点になり、圧延線材の加工性を劣化させるものと考えられていた。かかる観点から、初析セメンタイトの面積率、サイズを抑え、加工性を向上させた圧延線材が、例えば特開平８−２６００４６号公報、特開平９−２６３８８７号公報に開示され、さらにパーライトのラメラ間隔を０．０５〜０．２μｍ に規制した圧延線材が特開２００１−２３４２８６号公報に記載されている。
【０００５】
【特許文献１】
特開平８−２６００４６号公報（特許請求の範囲）
【特許文献２】
特開平９−２６３８８７号公報（特許請求の範囲）
【特許文献３】
特開２００１−２３４２８６号公報（特許請求の範囲）
【０００６】
【発明が解決しようとする課題】
しかしながら、このような圧延線材を用いて伸線し、球状化焼鈍された伸線材を素材として製作された転動部品は、転動疲労特性が劣化し、軸受の耐久性が低下するという問題がある。
本発明は、かかる問題に鑑みなされたもので、最終製品である転動部品に優れた転動疲労特性を付与することができる、生引きが可能な圧延線材および伸線材を提供することを目的とする。
【０００７】
【課題を解決するための手段】
本発明者は、生引きを可能とする鋼の組織を鋭意研究したところ、加工性劣化の原因は、従来信じられていたように粗大な初析セメンタイトの生成にあるのではなく、パーライト内から生じていることを知見した。本発明は、かかる知見を基に、加工性を改善するには、パーライト内におけるマイクロクラックの進展が可能な最小単位であるコロニーを微細化することによって、球状化焼鈍を省略しても、伸線加工によって破壊し難い過共析鋼組織を得ることができるとの着想の下に完成されたものである。
【０００８】
すなわち、本発明の軸受用圧延線材は、 ｍａｓｓ％で、
Ｃ：０．８〜１．３％、
Ｓｉ：０．１〜１．０％、
Ｍｎ：０．２〜２．０％、
Ｃｒ：０．８〜２．０％
を含み、パーライトのコロニーが６μｍ 以下とされたものである。この線材において、初析セメンタイトは面積率で３％超とすることができる。また、本発明の伸線材は、圧延線材を用いて伸線加工した軸受用伸線材であって、鋼材密度が７．７０ｇ／ｃｍ３ 以上であり、かつ平均炭化物粒径が０．６５μｍ 未満とされたものである。
【０００９】
【発明の実施の形態】
まず、本発明の圧延線材および伸線材の鋼組成について説明する。以下、単位はｍａｓｓ％である。
【００１０】
Ｃ：０．８〜１．３％
Ｃは軸受鋼に必要な強度を確保するために必要な元素であり、０．８％未満では強度不足を招来し、一方１．３％超では強度が過大となり、伸線加工性が劣化するようになる。このため、Ｃ量の下限を０．８％、上限を１．３％とする。
【００１１】
Ｓｉ：０．１〜１．０％
Ｓｉは脱酸剤として添加され、０．１％未満では脱酸作用が過小であり、一方１．０％超添加すると強度が過大となり、伸線加工性が低下する。このため、Ｓｉ量の下限を０．１％、上限を１．０％とする。
【００１２】
Ｍｎ：０．２〜２．０％
Ｍｎは脱酸の効果と焼入れ性確保のために添加される。０．２％未満では十分な効果が得られず、一方２．０％超では圧延後の冷却時に過冷組織が発生して、延性が劣化するようになる。このため、Ｍｎ量の下限を０．２％、上限を２．０％とする。
【００１３】
Ｃｒ：０．８〜２．０％
Ｃｒは焼入れ性の向上と、オーステナイト中のセメンタイトの安定性を向上させ、セメンタイトを球状化を促進するために添加される。０．８％未満では十分な効果が得られず、一方２．０％超では焼入れ性が過大となり、圧延後の冷却時に過冷組織が発生し、強度が過大となり、延性が低下する。このため、Ｃｒ量の下限を０．８％、上限を２．０％とする。
【００１４】
本発明の線材は、上記合金成分の他、典型的には残部Ｆｅおよび不可避的不純物で形成されるが、上記作用効果を害しない元素およびその含有範囲として、例えば、Ｍｏ：０．２％以下、Ｖ：０．１％以下、Ｂ：０．００３０％以下の１種以上を単独で、あるいは複合して添加することができる。なお、不純物であるＰ、Ｓは伸線加工性を阻害するため、少ないほどよい。
【００１５】
本発明にかかる過共析鋼の組織は、通常、図１に示すように、旧オーステナイト粒界１に沿って初析セメンタイト２が生成し、その内側にパーライト組織が形成されている。パーライト組織には、旧オーステナイト粒内のフェライトの方位が同じ領域（ノジュールという。）３がいくつか形成され、その一つのノジュール３にはラメラが平行に揃った領域（コロニーという。）４がいくつか形成されている。
【００１６】
前記コロニー４は破壊組織単位であり、本発明ではコロニーのサイズを６μｍ 以下に微細化するので、破壊組織単位が微細化され、延性が向上し、生引き伸線しても、断線しないことはもちろん、密度低下が起こらないようになる。６μｍ 超では組織が粗大なために延性が劣化し、生引き伸線することができても、伸線材の密度低下が起こり、この伸線材から製造した転動部品の転動疲労特性が悪化する。このため、本発明ではコロニーサイズの上限を６μｍ とする。コロニーは微細なほどよく、好ましくは２〜５μｍ とするのがよい。
【００１７】
また、初析セメンタイトの析出量が増えるほどパーライト中のＣ量が低下し、強度が低下するので、生引き伸線性がより向上する。初析セメンタイトの面積率が３％以下では強度が１３００ＭＰａ以上になり、伸線加工性が低下するので、３％超とすることが好ましい。初析セメンタイトの析出量は多ければ多いほど強度が低下するが、熱力学的には９％が限界である。望ましくは５％以上にすることで、強度を１２００ＭＰａ以下に低下させることができる。
【００１８】
次に、本発明の圧延線材の推奨される製造条件について説明する。
本発明の圧延線材は、前記組成の鋼を溶製して鋳造し、図２に示すように、▲１▼鋳造片に１１００〜１２００℃程度で１０〜２０ｈｒ保持するソーキング処理（鋳造により発生した偏析を軽減するために高温で長時間保持する熱処理）を施した後、鋳造片の温度を制御して、圧延終了温度７００〜８５０℃にて分塊圧延を終了し、線材圧延に適した大きさの鋼片（線材圧延前の鋼塊であり、ビレットと呼ばれる。）を製造する。そして、▲２▼この鋼片を、８５０〜１０００℃に加熱し、圧延温度を制御し、圧延終了温度を７００〜８５０℃として線材圧延（熱間圧延）を行い、▲３▼圧延後、３℃／ｓ以下の冷却速度にて５００℃以下まで連続冷却することによって製造される。なお、従来の分塊圧延では、鋳造片の温度コントロールは行われず、鋳造片の温度はソーキングを行った均熱炉を出た段階で１１００〜１２００℃程度、鋳造片の圧延終了時の温度は９００〜１０００℃程度である。
【００１９】
前記分塊圧延の圧延終了温度を７５０〜８５０℃と低温域に設定するのは、線材圧延前の鋼片組織を微細化するためである。７５０℃未満では鋳造片の強度が高くなるため、圧延に必要な荷重が過大となり、一方８５０℃超では圧延後にオーステナイトが再結晶し、粗大に成長してしまうため、線材圧延前の組織を微細化することができない。好ましくは７７５〜８２５℃である。
【００２０】
分塊圧延によって得られた鋼片は、線材圧延に際し、その強度を低下させることが必要であるために加熱されるが、８５０〜１０００℃に加熱することによって、強度を低下させつつ、分塊圧延によって微細化された組織の粗大化を抑制することができる。８５０℃未満の加熱では粗圧延をするために十分な軟らかさを確保することができず、一方１０００℃超に加熱すると、加熱により形成されたオーステナイトが粗大化し、最終組織のパーライトコロニーを微細化することができず、延性が得られないようになる。
線材圧延の圧延終了温度を７００〜８５０℃とするのは、未再結晶温度域での圧延を行い、旧オーステナイト粒を圧延方向にパンケーキ状に伸展させ、単位体積あたりの旧オーステナイト粒界面積を増大させ、パーライトの核生成サイトを多量に導入し、形成されるパーライトのコロニーサイズを微細化するためである。８５０℃超では、圧延後、パーライト変態が起こるまでに再結晶が起こるため旧オーステナイト粒が等軸状になり、十分な核生成サイトが得られず、一方７００℃未満では圧延前にパーライト変態が生じ、変形抵抗が高くなる。好ましくは７５０〜８００℃である。
【００２１】
線材圧延後の冷却速度は、オーステナイトからパーライトへの変態を制御する因子である。冷却速度が３℃／ｓ超ではでは冷却中にパーライト変態が完全に終了せずにベイナイトやマルテンサイトが形成され、延性が劣化するおそれがある。冷却速度が小さい分には問題はないが、製造に時間がかかり過ぎるようになるので、１〜２℃／ｓとするのが好ましい。この冷却速度は、パーライト変態が完全に終了している５００℃以下まで維持することが必要である。もっとも、冷却終了温度を５００℃以下の範囲で高温にした方が温度を管理する時間が短時間になるので、不必要に低温まで前記冷却速度を維持する必要はない。
【００２２】
また、既述のとおり、圧延線材の組織において、初析セメンタイトを３面積％超析出させることによって線材の強度を低下させることができる。このように初析セメンタイトを多量に析出させるには、線材圧延終了後、７００℃までの冷却速度を０．５℃／ｓ以下で徐冷するとよい。０．５℃／ｓ超では、初析セメンタイトが十分に析出しないため、強度低下に寄与しないようになるからである。冷却速度が遅いほど強度低下の効果は大きいので、冷却速度は遅ければ遅いほど望ましい。もっとも、生産性を考慮すると、０．０１℃／ｓ以上とするのがよい。好ましくは０．１〜０．３℃／ｓとするのがよい。７００℃未満の冷却については、前記のとおり、５００℃以下までを３℃／ｓ以下で冷却すればよい。
【００２３】
上記のようにして製造された圧延線材は、酸洗後、１次球状化焼鈍が施されることなく、要求される線径に応じて総減面率が１０〜８０％程度となるように直接伸線加工され、球状化焼鈍されて、本発明の伸線材とされる。前記球状化焼鈍は、常法のとおり、７５０〜８５０℃で保持後、５５０〜７００℃までを５〜３０℃／ｈで徐冷すればよい。
【００２４】
このようにして製造された伸線材の密度および炭化物粒径について説明する。
伸線材の密度は、伸線時に導入された欠陥量の指標になり、欠陥量が多いと、き裂が既に発生していることになり、伸線材から加工される転動部材の転動疲労特性が劣化する。なお、転動疲労の過程は、▲１▼き裂の発生、▲２▼き裂の進展、▲３▼マクロなき裂の進展と剥離の各段階に分けられ、き裂の発生が転動疲労の発端となる。
本発明の伸線材では、密度は７．７０ｇ／ｃｍ３ 以上であり、球状化焼鈍後に伸線した線材と同程度の欠陥導入量に抑えられており、球状化焼鈍材と同等の転動疲労寿命が得られる。密度は高いほど好ましいが、上限は鉄の密度である７．７８６ｇ／ｃｍ３ に止まる。７．７０ｇ／ｃｍ３ 未満では、欠陥量が多く、高振動下、高疲労下での繰り返し疲労において、き裂発生の起点が多く、破壊の進行が早くなるため、転勤疲労寿命が低下する。
【００２５】
また、本発明の伸線材の炭化物平均粒径は、０．６５μｍ 未満であり、炭化物粒径が細かいほど耐転動疲労特性は向上する。炭化物粒径は、球状化焼鈍の回数が多いほど大きくなり、従来のように球状化焼鈍を２回行ったものでは、平均粒径が１．０μｍ 程度になるが、本発明のように球状化焼鈍回数が１回になることで炭化物粒径が均一微細になり、０．６５μｍ 未満に止まる。なお、球状化焼純が終了した段楷の炭化物の面積率は炭素量に比例し、本発明では２０〜３０％となる。炭化物の単位面積あたりの個数は面積率と炭化物の平均粒径が決まれば計算により算出することができる。
【００２６】
次に、実施例を挙げて本発明をより具体的に説明するが、本発明はかかる実施例によって限定的に解釈されるものではない。
【００２７】
【実施例】
表１に示す鋼組成（残部Ｆｅ）を有する鋼を連続鋳造し、その鋳造片（２００ｍｍ角）を１１５０℃で１５ｈｒ保持するソーキング処理を行った後、同表に示す分塊圧延終了温度にて鋳造片を分塊圧延し、５５ｍｍ角の鋼片を得た。次いで同表に示す鋼片加熱温度、線材圧延終了温度にて線材圧延を行い、同表に示す冷却条件にて冷却し、直径８ｍｍの圧延線材を得た。
【００２８】
得られた圧延線材を酸洗後、伸線加工した。伸線の総減面率は約３４％であり、パススケジュールは、１パス目：８．０ｍｍから７．５ｍｍ、２パス目：７．５ｍｍから７．０ｍｍ、３パス目：７．０ｍｍから６．５ｍｍとした。その後、球状化焼鈍を施した。焼鈍条件は８０００℃、３時間保持後、６００℃まで１０℃／ｓで冷却した。さらに、この伸線材を切断、鍛造、研磨して直径６．３５ｍｍの軸受鋼球を製作し、８２０℃で３０分保持後油冷し、１５０℃で３０分保持する焼き戻しを施した。
【００２９】
下記の方法にて、上記圧延線材の引張強度、初析セメンタイト（初析θ）面積率、コロニーサイズを調べた。また、上記伸線材の線材密度、炭化物平均粒径を調べた。さらに、前記鋼球を用いて転動疲労寿命を測定した。これらの測定結果を表２に示す。同表中には、生引伸線性についても示した。生引き伸線性は、直径８ｍｍの線材１００ｋｇを伸線し、破断した回数を測定し、断線ゼロを○、断線１〜５回を△、断線６回以上を×として示す。なお、表中の試料Ｎｏ． １は、１次球状化焼鈍後に伸線した従来例を示し、生引伸線性が×のものについては伸線材の密度等および転動疲労寿命の測定は行わなかった。
【００３０】
引張強度は、ＪＩＳＺ２２０１（金属材料引張試験片）９号試験片を用いてＪＩＳＺ２２４１（金属材料引張試験方法）によって測定された。
【００３１】
コロニーサイズおよび初析セメンタイト面積率は、組織観察片を鏡面に研磨した後、２％ナイタールで腐食した後、走査型電子顕微鏡（ＪＥＯＬ製ＪＳＭ−５４１０）で倍率５０００倍で１０視野を観察することとし、コロニー（ラメラが平行に並んだ領域）サイズは切片法により測定し、初析セメンタイト（旧オーステナイト粒界に沿って生成したセメンタイト）の面積を画像解析用ソフトウェア（商品名Ｉｍａｇｅ−Ｐｒｏ）を用いて測定した。
【００３２】
球状炭化物平均粒径は、抽出レプリカ法を用いてセメンタイトを抽出し、前記走査型電子顕微鏡でランダムに１０視野観察し、全炭化物の直径を測定し、それらの平均値を求めた。
【００３３】
線材の密度は、表面スケールを取り除くため酸洗を施した後、ＪＩＳＺ８８０７の固体密度測定方法を用いて測定した。
【００３４】
転動疲労寿命は、図３に示すように、スラスト式寿命試験機を用い、１鋼種につき２０個測定し、メジアン寿命を採用した。前記試験機は、円板状の基盤（材質ＳＵＪ２）１１に鋼球１２を載置し、これに荷重（１個当たり１０００Ｎ）をかけながら基盤を１０００ｒｐｍで回転させるものであり、鋼球表面に剥離（フレーキング）が発生した時点での総回転数を転動疲労寿命とした。なお、表２では、試料Ｎｏ． １（従来例）の寿命を１とし、これとの寿命比を示した。
【００３５】
【表１】
【表２】
表１および表２より、成分および組織条件を満足する試料Ｎｏ． ２，５，７，９，１０（実施例）は、生引伸線性に優れ、しかも伸線材から製作した鋼球の転動寿命も従来例（Ｎｏ． １）に比して、１．６倍のものが１点あるものの、他は２倍超と優れている。Ｎｏ． ３，４，６，８，１１は本発明成分範囲を満足するものの、製造条件が適切でないため組織条件を満足しておらず、生引伸線性に劣り、Ｎｏ． ３については転動疲労寿命が著しく劣化している。また、Ｎｏ． １２〜１９は製造条件は適切であるものの、本発明成分範囲を満足しておらず、生引伸線性が概ね劣り、良好なもの（Ｎｏ． １２，１９）では転動疲労寿命が悪化している。
前記比較例について具体的に見ると、Ｎｏ． ３は分塊圧延終了温度が高いため、Ｎｏ． ４は線材圧延の際の鋼片加熱温度が高いため、またＮｏ． ６は線材圧延終了温度が高いため、各々コロニーサイズが粗大化し、生引伸線性が劣化している。また、Ｎｏ． ８およびＮｏ． １１は、冷却速度が過大であるため、過冷組織が形成され、やはり生引伸線性が劣化している。
また、試料Ｎｏ． １２はＣ量が過少であるため、十分な硬さを確保することができず、転動寿命が低下している。Ｎｏ． １３はＣ量が過多であるため、強度が過大となり、延性が劣化して生引伸線性が劣化している。Ｎｏ． １４および１６はＳｉ量、Ｍｎ量が過少であるため、介在物に起因した破断が発生し、生引伸線性に問題がある。Ｎｏ． １５はＳｉ量が過多のため、強度が過大となり、延性が劣化するため、生引伸線性が劣化している。Ｎｏ． １７，１８はＭｎ量、Ｃｒ量が過多のため強度が過大となり、生引伸線性に問題がある。また、Ｎｏ． １９はＣｒ量が過少なため、炭化物の球状化ができず、転動疲労寿命が低下している。
【００３８】
【発明の効果】
本発明の軸受用圧延線材は、所定の成分の下、破壊の単位となるパーライトのコロニーサイズを６μｍ 以下としたので、良好な生引き伸線性が得られると共に、この圧延線材を伸線加工し、得られた伸線材を素材として加工することによって優れた転動疲労特性を有する転動部品を得ることができる。
【図面の簡単な説明】
【図１】過共析鋼の組織を示す模式図である。
【図２】本発明の圧延線材の製造方法を示す加工熱処理線図である。
【図３】転動寿命の測定要領を示す説明図である。
【符号の説明】
１ 旧オーステナイト粒界
２ 初析セメンタイト
３ ノジュール
４ コロニー[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a drawn wire for a bearing that has been drawn and a rolled wire as a material thereof, and in particular, a drawable bearing that can be drawn without primary spheroidizing annealing before drawing. Related to rolled wire rods.
[0002]
[Prior art]
Wire for bearings is used as a material for steel balls for ball bearings and rollers for roller bearings. Generally, after rolling a hot rolled wire from an ingot into primary spheroids, wire drawing is performed. And then subjected to secondary spheroidizing annealing, followed by finish wire drawing. The rolling parts such as the steel balls and rollers are manufactured by cutting the drawn wire into an appropriate size, forging the cut pieces into a roughly part shape, heat-treating and finishing.
[0003]
Since the bearing steel forming the bearing wire is a hypereutectoid steel and is difficult to process, it is difficult to directly draw a rolled wire (referred to as “raw drawing”), and the material is softened. In order to improve workability, primary spheroidizing annealing is performed as described above before wire drawing.
[0004]
However, since the spheroidizing annealing requires processing time and cost, a rolled wire rod capable of omitting the primary spheroidizing annealing has recently been desired for cost reduction.
In order to make the rolled wire rods raw, it is necessary to improve workability. Conventionally, it has been considered that coarse pro-eutectoid cementite becomes a starting point of fracture and deteriorates workability of a rolled wire. From such a viewpoint, a rolled wire rod having reduced area ratio and size of proeutectoid cementite and improved workability is disclosed in, for example, JP-A-8-260046 and JP-A-9-263887, and further, lamella spacing of pearlite. Is regulated to 0.05 to 0.2 μm is described in JP-A-2001-234286.
[0005]
[Patent Document 1]
JP-A-8-260046 (Claims)
[Patent Document 2]
Japanese Patent Application Laid-Open No. 9-268887 (Claims)
[Patent Document 3]
JP 2001-234286 A (Claims)
[0006]
[Problems to be solved by the invention]
However, rolling parts manufactured using such a rolled wire and drawn using a spheroidized annealed wire have a problem in that rolling fatigue characteristics deteriorate and durability of the bearing decreases. is there.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a rolled wire and a drawn wire capable of imparting excellent rolling fatigue characteristics to a rolling component as a final product, and capable of being drawn. And
[0007]
[Means for Solving the Problems]
The present inventor has conducted intensive studies on the structure of steel that allows for the production of raw material, and found that the cause of workability deterioration is not due to the formation of coarse pro-eutectoid cementite as conventionally believed, but from within pearlite. It was found that it had occurred. The present invention is based on this finding, in order to improve the processability, by minimizing the colony, which is the smallest unit in which microcracks can develop in pearlite, even if the spheroidizing annealing is omitted, It has been completed under the idea that a hypereutectoid steel structure that is difficult to break by wire working can be obtained.
[0008]
That is, the rolled wire rod for a bearing of the present invention has a mass% of
C: 0.8-1.3%,
Si: 0.1 to 1.0%,
Mn: 0.2-2.0%,
Cr: 0.8-2.0%
And the size of the pearlite colony is 6 μm or less. In this wire, the proeutectoid cementite can have an area ratio of more than 3%. The drawn wire of the present invention is a drawn wire for a bearing drawn by using a rolled wire, and has a steel material density of 7.70 g / cm 3 or more and an average carbide particle size of less than 0.65 μm. It was done.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the steel composition of the rolled wire and the drawn wire of the present invention will be described. Hereinafter, the unit is mass%.
[0010]
C: 0.8-1.3%
C is an element necessary for ensuring the necessary strength of the bearing steel. If the content is less than 0.8%, the strength is insufficient. On the other hand, if the content is more than 1.3%, the strength is excessive, and the wire drawing workability is deteriorated. Become like For this reason, the lower limit of the C content is set to 0.8% and the upper limit is set to 1.3%.
[0011]
Si: 0.1 to 1.0%
Si is added as a deoxidizing agent, and if it is less than 0.1%, the deoxidizing effect is too small, while if it is more than 1.0%, the strength becomes too large and the wire drawing workability is reduced. For this reason, the lower limit of the amount of Si is set to 0.1%, and the upper limit is set to 1.0%.
[0012]
Mn: 0.2-2.0%
Mn is added for the purpose of deoxidizing and ensuring hardenability. If it is less than 0.2%, a sufficient effect cannot be obtained, while if it exceeds 2.0%, a supercooled structure is generated at the time of cooling after rolling, and ductility is deteriorated. For this reason, the lower limit of the amount of Mn is set to 0.2% and the upper limit is set to 2.0%.
[0013]
Cr: 0.8-2.0%
Cr is added to improve the quenchability, improve the stability of cementite in austenite, and promote the spheroidization of cementite. If it is less than 0.8%, a sufficient effect cannot be obtained, while if it exceeds 2.0%, the hardenability becomes excessive, a supercooled structure is generated at the time of cooling after rolling, the strength becomes excessive, and the ductility decreases. For this reason, the lower limit of the Cr content is set to 0.8% and the upper limit is set to 2.0%.
[0014]
The wire of the present invention is typically formed of the balance of Fe and unavoidable impurities in addition to the above alloy components. , V: 0.1% or less, B: 0.0030% or less can be added alone or in combination. In addition, since P and S, which are impurities, impair the drawability, the smaller the better, the better.
[0015]
In the structure of the hypereutectoid steel according to the present invention, as shown in FIG. 1, usually,
[0016]
The colony 4 is a fractured tissue unit, and in the present invention, the size of the colony is reduced to 6 μm or less. Therefore, the fractured tissue unit is refined, ductility is improved, and even if the raw wire is drawn, it does not break. Of course, the density does not decrease. If it exceeds 6 μm, the structure is coarse and the ductility is deteriorated. Even if the wire can be drawn and drawn, the density of the drawn wire decreases, and the rolling fatigue characteristics of the rolling component manufactured from the drawn wire deteriorate. . Therefore, in the present invention, the upper limit of the colony size is set to 6 μm. The finer the colonies, the better, and preferably 2 to 5 μm.
[0017]
Further, as the amount of precipitation of the proeutectoid cementite increases, the amount of C in the pearlite decreases and the strength decreases, so that the drawing and drawing properties are further improved. If the area ratio of proeutectoid cementite is 3% or less, the strength becomes 1300 MPa or more, and the wire drawing workability decreases. The greater the amount of proeutectoid cementite, the lower the strength, but the thermodynamic limit is 9%. Desirably, the strength can be reduced to 1200 MPa or less by setting it to 5% or more.
[0018]
Next, recommended production conditions for the rolled wire rod of the present invention will be described.
As shown in FIG. 2, the rolled wire rod of the present invention is made by melting and casting steel having the above composition, and as shown in FIG. 2, (1) a soaking process (formed by casting) in which a cast piece is held at about 1100 to 1200 ° C. for 10 to 20 hours. After performing a heat treatment for maintaining segregation at a high temperature for a long time to reduce segregation), the temperature of the cast piece is controlled, the slab rolling is completed at a rolling end temperature of 700 to 850 ° C., and a size suitable for wire rod rolling is obtained. A steel billet (a steel ingot before wire rod rolling, called a billet) is manufactured. Then, (2) this steel slab is heated to 850 to 1000 ° C., the rolling temperature is controlled, the rolling end temperature is set to 700 to 850 ° C., and wire rod rolling (hot rolling) is performed. It is manufactured by continuously cooling to 500 ° C. or less at a cooling rate of not more than 500 ° C./s. In the conventional bulk rolling, the temperature of the cast piece is not controlled, and the temperature of the cast piece is about 1100 to 1200 ° C. when it leaves the soaking soaking furnace. It is about 900 to 1000 ° C.
[0019]
The reason why the rolling end temperature of the bulk rolling is set to a low temperature range of 750 to 850 ° C. is to refine the steel slab structure before wire rod rolling. If it is lower than 750 ° C., the strength of the cast piece becomes high, and the load required for rolling becomes excessive. On the other hand, if it exceeds 850 ° C., austenite recrystallizes after rolling and grows coarsely. Cannot be transformed. Preferably it is 775-825 degreeC.
[0020]
The slab obtained by slab rolling is heated because it is necessary to reduce the strength during wire rod rolling, but by heating to 850 to 1000 ° C., the slab is The coarsening of the structure refined by rolling can be suppressed. Heating at less than 850 ° C. cannot ensure sufficient softness for rough rolling, while heating at more than 1000 ° C. results in coarsening of austenite formed by heating and refinement of pearlite colonies in the final structure. And ductility cannot be obtained.
The reason for setting the rolling end temperature of wire rod rolling to 700 to 850 ° C. is that rolling is performed in a non-recrystallization temperature range, the old austenite grains are expanded in a pancake shape in the rolling direction, and the old austenite grain boundary area per unit volume. To increase the number of pearlite nucleation sites and reduce the size of the formed pearlite colonies. If the temperature exceeds 850 ° C., recrystallization occurs before the pearlite transformation occurs after rolling, so that the old austenite grains become equiaxed and sufficient nucleation sites cannot be obtained. On the other hand, if the temperature is less than 700 ° C., the pearlite transformation occurs before rolling. And deformation resistance increases. Preferably it is 750-800 degreeC.
[0021]
The cooling rate after wire rod rolling is a factor that controls the transformation from austenite to pearlite. If the cooling rate exceeds 3 ° C./s, bainite or martensite is formed without complete completion of pearlite transformation during cooling, and ductility may be deteriorated. Although the cooling rate is small, there is no problem, but the production takes too much time. It is necessary to maintain this cooling rate to 500 ° C. or less at which the pearlite transformation is completely completed. However, when the cooling end temperature is set to a high temperature within the range of 500 ° C. or less, the time for controlling the temperature is shortened, so that it is not necessary to maintain the cooling rate unnecessarily to a low temperature.
[0022]
Further, as described above, in the structure of the rolled wire rod, the strength of the wire rod can be reduced by precipitating more than 3 area% of proeutectoid cementite. In order to precipitate a large amount of proeutectoid cementite in this way, it is preferable to gradually cool the cooling rate up to 700 ° C. at a rate of 0.5 ° C./s or less after the completion of wire rod rolling. If it exceeds 0.5 ° C./s, the pro-eutectoid cementite does not sufficiently precipitate, so that it does not contribute to the decrease in strength. The slower the cooling rate, the greater the effect of reducing the strength. Therefore, the slower the cooling rate, the better. However, in consideration of productivity, the temperature is preferably set to 0.01 ° C./s or more. Preferably, the temperature is 0.1 to 0.3 ° C./s. As for the cooling at a temperature lower than 700 ° C., as described above, the cooling to 500 ° C. or lower may be performed at 3 ° C./s or lower.
[0023]
The rolled wire rod manufactured as described above is not subjected to the primary spheroidizing annealing after the pickling, so that the total area reduction rate is about 10 to 80% depending on the required wire diameter. It is directly drawn and spheroidized to obtain a drawn material of the present invention. The spheroidizing annealing may be carried out at a temperature of 750 to 850 ° C and then gradually cooled to 550 to 700 ° C at a rate of 5 to 30 ° C / h as usual.
[0024]
The density and carbide particle size of the wire drawn in this way will be described.
The density of the drawn wire is an indicator of the amount of defects introduced at the time of wire drawing.If the amount of defects is large, a crack has already occurred and the rolling fatigue of the rolling member processed from the drawn wire The characteristics deteriorate. The rolling fatigue process can be divided into (1) crack initiation, (2) crack propagation, and (3) macro-crack growth and delamination stages. The beginning of
In the drawn material of the present invention, the density is 7.70 g / cm 3 or more, and the amount of defects introduced is as low as that of the drawn wire after the spheroidizing annealing. Life is obtained. The higher the density, the better, but the upper limit is 7.786 g / cm 3 which is the density of iron. If it is less than 7.70 g / cm 3 , the amount of defects is large, and in repeated fatigue under high vibration and high fatigue, the number of crack initiation points is large and the progress of fracture is accelerated, so that the transfer fatigue life is reduced.
[0025]
Further, the carbide average particle diameter of the drawn wire of the present invention is less than 0.65 μm, and the smaller the carbide particle diameter, the better the rolling contact fatigue resistance. The carbide grain size increases as the number of times of spheroidizing annealing increases, and when the spheroidizing annealing is performed twice as in the prior art, the average grain size becomes about 1.0 μm. When the number of times of annealing becomes one, the grain size of carbide becomes uniform and fine, and it is less than 0.65 μm. In addition, the area ratio of the step-shaped carbide after the spheroidizing and sintering is completed is proportional to the carbon content, and is 20 to 30% in the present invention. The number of carbides per unit area can be calculated by calculation once the area ratio and the average particle size of the carbides are determined.
[0026]
Next, the present invention will be described more specifically with reference to examples. However, the present invention is not construed as being limited to such examples.
[0027]
【Example】
A steel having a steel composition (remainder Fe) shown in Table 1 was continuously cast, and a soaking treatment was performed by holding the cast piece (200 mm square) at 1150 ° C. for 15 hours. The cast piece was subjected to slab rolling to obtain a 55 mm square steel piece. Next, the wire rod was rolled at the billet heating temperature and the wire rod end temperature shown in the table, and cooled under the cooling conditions shown in the table to obtain a rolled wire rod having a diameter of 8 mm.
[0028]
The obtained rolled wire was pickled and then drawn. The total area reduction rate of wire drawing is about 34%, and the pass schedule is from 8.0 mm to 7.5 mm for the first pass, from 7.5 mm to 7.0 mm for the second pass, and from 7.0 mm for the third pass. 6.5 mm. Thereafter, spheroidizing annealing was performed. After annealing at 8000 ° C. for 3 hours, the sample was cooled to 600 ° C. at 10 ° C./s. Further, the drawn wire material was cut, forged, and polished to produce a bearing steel ball having a diameter of 6.35 mm. After being held at 820 ° C. for 30 minutes, it was oil-cooled and tempered at 150 ° C. for 30 minutes.
[0029]
The tensile strength, pro-eutectoid cementite (pro-eutectoid θ) area ratio, and colony size of the rolled wire rod were examined by the following methods. In addition, the wire density and carbide average particle diameter of the drawn wire were examined. Further, rolling fatigue life was measured using the steel balls. Table 2 shows the measurement results. The table also shows the raw drawability. The raw drawability was determined by drawing 100 kg of a wire rod having a diameter of 8 mm and measuring the number of breaks. O indicates zero disconnection, Δ indicates 1 to 5 disconnections, and X indicates 6 or more disconnections. In addition, the sample No. in the table was used. No. 1 shows a conventional example in which wire was drawn after primary spheroidizing annealing, and the density and the rolling fatigue life of the drawn material were not measured for those having a raw drawability of x.
[0030]
The tensile strength was measured according to JISZ2241 (metallic material tensile test method) using a JISZ2201 (metallic material tensile test piece) No. 9 test piece.
[0031]
The colony size and proeutectoid cementite area ratio should be determined by polishing a tissue observation piece to a mirror surface and then corroding it with 2% nital, and then observing 10 visual fields with a scanning electron microscope (JSM-5410 manufactured by JEOL) at a magnification of 5000 times. The size of the colony (region in which the lamellas are arranged in parallel) is measured by the slice method, and the area of proeutectoid cementite (cementite generated along the former austenite grain boundary) is determined using image analysis software (product name Image-Pro). It measured using.
[0032]
The average particle diameter of the spherical carbide was obtained by extracting cementite by using an extraction replica method, observing 10 visual fields randomly with the above-mentioned scanning electron microscope, measuring the diameter of all the carbides, and calculating the average value thereof.
[0033]
The density of the wire was measured using a solid density measurement method according to JISZ8807 after performing pickling to remove surface scale.
[0034]
As shown in FIG. 3, the rolling contact fatigue life was measured using a thrust type life tester, and 20 pieces were measured for each steel type, and the median life was adopted. In the tester, a
[0035]
[Table 1]
[Table 2]
From Tables 1 and 2, it can be seen that Sample No. satisfying the components and the microstructure conditions. Examples 2, 5, 7, 9, and 10 (Examples) have excellent raw drawability, and the rolling life of a steel ball manufactured from a drawn material is 1.6 times that of the conventional example (No. 1). There is one point, but the others are more than twice as good. No. Nos. 3, 4, 6, 8 and 11 satisfy the component range of the present invention, but do not satisfy the structural conditions due to improper production conditions, are inferior in raw drawability, and With regard to No. 3, the rolling fatigue life was significantly deteriorated. No. Nos. 12 to 19 are suitable for the production conditions, but do not satisfy the range of the components of the present invention, are generally inferior in raw drawability, and deteriorate in rolling fatigue life in good (Nos. 12, 19). .
Looking specifically at the comparative example, No. 3 has a high end-of-bulking-rolling temperature. No. 4 has a high billet heating temperature at the time of wire rod rolling. In No. 6, the wire rod ending temperature was high, so that the colony size was coarsened and the raw drawability was deteriorated. No. 8 and No. In No. 11, since the cooling rate was excessive, a supercooled structure was formed, and the raw drawability was also deteriorated.
Further, the sample No. In No. 12, since the C amount is too small, sufficient hardness cannot be secured, and the rolling life is shortened. No. No. 13 has an excessive C content, so that the strength becomes excessive, the ductility is deteriorated, and the raw drawability is deteriorated. No. In Nos. 14 and 16, since the amounts of Si and Mn are too small, breakage due to inclusions occurs, and there is a problem in the drawability. No. In No. 15, since the Si content is excessive, the strength becomes excessive, and the ductility is deteriorated, so that the raw drawability is deteriorated. No. Nos. 17 and 18 have excessive strengths due to excessive amounts of Mn and Cr, and have a problem in the drawability. No. In No. 19, since the amount of Cr was too small, the carbide could not be spheroidized, and the rolling fatigue life was reduced.
[0038]
【The invention's effect】
The rolled wire for bearings of the present invention has a pearlite colony size of 6 μm or less, which is a unit of destruction, under a predetermined component, so that good raw drawability can be obtained and this rolled wire is drawn. A rolling component having excellent rolling fatigue characteristics can be obtained by processing the obtained drawn wire as a raw material.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the structure of a hypereutectoid steel.
FIG. 2 is a working heat treatment diagram showing a method for producing a rolled wire rod of the present invention.
FIG. 3 is an explanatory view showing a procedure for measuring a rolling life.
[Explanation of symbols]
1 old
Claims (3)
Ｃ：０．８〜１．３％、
Ｓｉ：０．１〜１．０％、
Ｍｎ：０．２〜２．０％、
Ｃｒ：０．８〜２．０％
を含み、パーライトのコロニーが６μｍ 以下である軸受用圧延線材。mass%,
C: 0.8-1.3%,
Si: 0.1 to 1.0%,
Mn: 0.2-2.0%,
Cr: 0.8-2.0%
And a pearlite colony of 6 μm or less.
鋼材密度が７．７０ｇ／ｃｍ３ 以上であり、かつ平均炭化物粒径が０．６５μｍ 未満である軸受用伸線材。A drawn wire for a bearing drawn by using the rolled wire according to claim 1 or 2,
A drawn wire rod for a bearing having a steel material density of 7.70 g / cm 3 or more and an average carbide particle diameter of less than 0.65 μm.
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| JP2002266368A JP4008320B2 (en) | 2002-09-12 | 2002-09-12 | Rolled and drawn wire rods for bearings |
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| JP2002266368A JP4008320B2 (en) | 2002-09-12 | 2002-09-12 | Rolled and drawn wire rods for bearings |
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| JP4008320B2 JP4008320B2 (en) | 2007-11-14 |
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