JP2004156764A - Bearing unit with flange and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the fatigue strength of a neck of a flange for fitting to a wheel without increasing high frequency hardening parts, and to facilitate the lightening of a bearing unit with the flange by making the thickness of the flange for fitting to the wheel thin. <P>SOLUTION: A hub wheel 2 of the bearing unit with flange is made of carbon steel with the carbon content of more than 0.45 weight % but less than 0.65 weight %. The hub wheel 2 is made so that the area rate of the pro-eutectoid ferrite per unit area (10mm<SP>2</SP>) at least on the outside neck part 14 of the flange 6 for fitting to the wheel is 3 to 15 %, the maximum length of the pro-eutectoid ferrite is made to be less than 200μm, and it makes less than five pieces of pro-eutectoid ferrite with the length of more than 180μm. <P>COPYRIGHT: (C)2004,JPO

Description

【００５６】
熱間鍛造は棒鋼を切断後、９５０〜１２００°Ｃの間で各種温度まで高周波誘導加熱し、組織の微細レベルを変化させ、据えこみ加工を主体とした熱間鍛造を施し、さらに、種々の冷却速度で冷却し、多様の初析フェライト組織の析出状態を作り出している。
ミクロ組織はピラクール腐食液でエッチングを行い、組織を電子顕微鏡で撮影し、画像解折装置によって、その電子顕微鏡画像の素地から初析フェライト組織だけを取り出してその面積率を算出した。
【００５７】
電子顕微鏡：日本電子社製、ＪＳＭ−Ｔ２２０Ａ
画像解析装置：カールツァイス社製、ＩＢＡＳ２０００
熱間鍛造冷却後にショットブラストによる酸化膜除去をした後、旋削加工により各種試験片を作成し、切削工具寿命試験、回転曲げ疲労試験、冷間加工試験、異物混入潤滑下寿命試験を行い、各種評価を行った。各試験条件は次の通りである。
〈切削工具寿命試験〉
切削機械：高速旋盤
工具：Ｐ１０（ＪＩＳ Ｂ ４０５３）
切り込み速度：１８０〜２２０ｍ／ｓｅｃ
送り量：０．２〜０．３ｍｍ／ｒｅｖ
切り込み深さ：０．６〜１．０ｍｍ
ＪＩＳ Ｂ ４０１１のバイト切削試験法に従って、上記条件で各試料を研削し、バイトの逃げ面摩耗量が０．２ｍｍに達するまの時間を工具寿命とした。
〈回転曲げ疲労試験〉
試験機：小野式回転曲げ疲労試験機
試験片：ＪＩＳ１−８号試験片（ＪＩＳ Ｚ ２２７４）
回転速度：３７００ｍｉｎ^−１
停止回転数は１０^７ 回とし、試験荷重を変えて破損しなくなった条件を疲労限強度とした。
〈冷間加工試験〉
φ２０×３０ｍｍの円柱試験片を旋削加工で作成し、据込率８０％で円柱試験片の上下から冷間圧縮（鍛造）加工を行い、円周上に割れが発生したかどうかの確認を行った。試験は各々１０個加工し、割れの発生した確率を調査した。
〈異物混入潤滑下寿命試験〉
「特殊鋼便覧」第一版（電気製鋼研究所編、理工学社、１９６９年５月２５日発行）第１０〜２１頁に記載のスラスト型軸受鋼寿命試験機を用いて、転動体にはＳＵＪ２のボールを用い、各サンプル２０個試験を行い、フレーキングが発生した時点までの累積応力繰り返し回数（寿命）を調査してワイブルプロットを作成し、各ワイブル分布の結果から各々のＬ_１０寿命を求めた。また、応力繰り近し回数が１０^７ を超えたものは中断とし、全数１０^７ を超えた場合にはＬ_１０寿命を１０^７ とした。
【００５８】
試験面圧：最大４９００ＭＰａ
回転速度：１０００ｍｉｎ^−１
潤滑油：♯６８タービン油
寿命試験片の軌道部は高周波加熱による焼入、焼戻を施している。
各試験結果を表２に纏めて示す。また、図３にＳ量（重量％）×１０００＋α面積率（％）と工具寿命との関係を、図４にＳ量（重量％）／α面積率（％）と疲労限強度との関係をそれぞれ示す。なお、表２及び図３、図４においては、便宜上、初析フェライト組織の面積率をα面積率として表示している。
【００５９】
【表２】

【００６０】
表２、図３、図４から明らかなように、実施例１〜２１ではＳ添加量と初析フェライト組織の面積率及び旋削加工性係数（Ｓ含有量（重量％）×１０００＋初析フェライト組織の面積率（％））が本発明範囲内にあるため、旋削加工性（工具寿命）や疲労限強度、冷間割れ発生率及び焼入硬化後の転がり寿命すべてに優れた良好なフランジ付軸受装置用の材料を得ることができ、特に、フランジ疲労限係数＝（Ｓ含有量（重量％）×１０００）／初析フェライト組織の面積率（％）が１以上２以下の実施例５、６、８、９、１７〜１９は疲労限強度が優れているのが判る。
【００６１】
一方、材料成分でＳ添加量が高すぎる比較例１は、冷間圧縮加工でＭｎＳを起点とした割れが多発している。材料成分でＣ添加量が低すぎる比較例２では、高周波焼入による硬さが低下したため、転がり寿命が著しく低下している。材料成分でＣ添加量が高すぎる比較例３は初析フェライト組織の面積率が低くなりすぎるため、また、旋削加工性係数（Ｓ含有量（重量％）×１０００＋初析フェライト組織の面積率（％））が低すぎる比較例５は旋削加工性が著しく低下してしまう。逆に初析フェライト組織の面積率が高めのため旋削加工性係数（Ｓ含有量（重量％）×１０００＋初析フェライト組織の面積率（％））が高すぎる比較例４や初析フェライト組織の面積率自体が高すぎる比較例６は初析フェライト組織の微細分散が不十分のため、疲労限強度が低下してしまう。
（第２実施例）
本発明の効果を確認するために、以下の実験を行った。
【００６２】
まず、表３のＡ〜Ｄの成分の棒状素材を用い、各棒状素材を切断後、それぞれについて９５０〜１２００°Ｃの温度にまで高周波誘導加熱、熱間鍛造により所定の形状に仕上げた。その後、強制空冷または放冷し、ショットブラストによる酸化膜除去をした後、旋削、軌道面の高周波焼入・研磨を行って図６に示すハブ輪２を製作した。
【００６３】
なお、ハブ輪２は表４の実施例２２〜３０、比較例７〜１１のものを用意した。また、実施例２２〜３０のハブ輪２については、上述した初析フェライトの分布制御を行ったものを用いた。
【００６４】
【表３】

【００６５】
【表４】

【００６６】
次いで、各ハブ輪２について、車輪取付用フランジ６の外側付け根部１４を研磨した後にピクラールで表面を腐食してミクロ組織を観察した。
この観察は、まず、顕微鏡倍率２００〜５００倍で視野内のミクロ組織を２値化して、初析フェライト粒のみを抽出し、図５に示すように、連続したフェライト粒の絶対最大長（輪郭線上の任意の２点間の距離の最大値で与えられ、これをフェライト長さとする）の分布およびフェライト面積率を画像解析によって求めた。
【００６７】
次に、各ハブ輪２を用いて図６に示す試験用のフランジ付軸受装置（駆動輪支持用）を作成した。なお、この軸受装置は、図１で説明したものと基本的構成が同一であるため、図１と同一符号を付して説明を省略する。また、この軸受装置の複列の転動体５の軸方向ピッチは５９ｍｍ、玉数は１２個とし、外輪４の素材はＳ５３Ｃ、内輪３及び転動体５の素材はＳＵＪ２を用いた。図６において符号１５はアキシアル荷重を受け止めるべく外輪４の懸架装置取付用フランジ１１側の端部に嵌め込まれた受け部材である。
【００６８】
そして、実施例２２〜３０及び比較例７〜１１の各ハブ輪２を組み込んだ図６のフランジ付軸受装置を用いて、それぞれ車輪取付用フランジ６の外側付け根部１４の耐久荷重試験を行った。
試験は、上記フランジ付軸受装置を、アキシアル荷重Ｆａ＝５０００Ｎ、ラジアル荷重Ｆｒ＝６０００〜９０００Ｎ、回転速度１００ｍｉｎ^−１で５０時間回転試験を行い、試験後にハブ輪２の車輪取付用フランジ６の外側付け根部１４にクラックが発生したラジアル荷重Ｆｒを耐久荷重と判定した。試験結果は、比較例７に対する比で表した。
【００６９】
次に、ドリル穿孔試験によってハブ輪２の車輪取付用フランジ６の加工性を評価した。
試験条件は次の通りである。
試験部位 ：車輪取付用フランジ６
ドリル工具 ：φ８ｍｍ、ＳＫＨ５１
穿孔方式 ：乾式
穿孔深さ ：１０ｍｍ
切削速度 ：２１ｍ／ｍｉｎ（８４０ｍｉｎ^−１）
推力 ：６８６Ｎ（７０ｋｇｆ）
試験結果は、従来品レベルの工具寿命となる比較例７に対して、同等以上のものを「○」、それ以下のものを「×」とした。
【００７０】
以上の各試験結果を表４に纏めて示す。
表４から明らかなように、ハブ輪２の素材である炭素鋼の炭素の含有量が０．４５重量％以上０．６５重量％以下とされ、且つ車輪取付用フランジ６の外側付け根部１４の単位面積（１０ｍｍ^２ ）あたりの初析フェライトの面積率が３〜１５％とされると共に、該初析フェライトの最大長さが２００μｍ以下とされ、更に、１８０μｍ以上の長さの初析フェライトが５個以下とされた実施例２２〜３０は、比較例７〜１１に比べて、車輪取付用フランジ６の外側付け根部１４の耐久荷重および車輪取付用フランジ６の加工性において良好な結果が得られた。また、ハブ輪２の素材にＶを添加した実施例２２，２４は車輪取付用フランジ６の外側付け根部１４の耐久荷重性能が他の実施例２３，２５〜３０より増加しているのが判る。
【００７１】
一方、初析フェライトの最大長さが２００μｍを超えている比較例１０は、長さ１８０μｍ以上のフェライト粒が５個以上存在した比較例７、フェライト面積率が本発明の上限を越えている比較例８、ハブ輪２の素材の炭素鋼のＣ含有量が高く、初析フェライトの最大長さは、本発明範囲内であったが、鍛造放冷ままでは、加工性が悪く焼鈍を行わざるを得なかった比較例１１はいずれも車輪取付用フランジ６の外側付け根部１４の耐久荷重が非常に低くなった。さらに、比較例９は車輪取付用フランジ６の外側付け根部１４の耐久荷重が増加しているが、車輪取付用フランジ６の加工性の低下が問題となった。
（第２の実施の形態：請求項６〜９、１２に対応）
また、図１を参照して、０．４５〜０．６５重量％のＣを含む中炭素鋼で構成されるフランジを有する、内方部材（ハブ輪２）あるいは外方部材（外輪４）の、熱間鍛造後の金属組織は、初析フェライトが旧オーステナイ結晶粒界に沿って網目状に析出したフェライト・パーライト組織になる。フェライト組織は、パーライト組織と比較して、強度が低いため、初析フェライトが粗大に析出した金属組織は、疲労強度が低くなる可能性がある。本発明者らは、フランジ付軸受装置の内方部材（ハブ輪２）あるいは外方部材（外輪４）の、前記フェライト・パーライト組織からなる部位の疲労強度を向上させるためには、網目状に析出する初析フェライトを微細に分散析出させることが有効であるという知見を得た。
【００７２】
旧オーステナイト粒径を微細化すると、結晶粒界での応力集中を抑制する効果や、オーステナイト／フェライト変態時の核生成のサイトを増やし、初析フェライト粒を微細に析出させる効果がある。初祈フェライトが微細に分断すると亀裂発生の最小単位が小さくなるので、疲労破壊を効果的に抑制することができる。
網目状に析出する初祈フェライトを微細に分散析出させるためには、熱間鍛造条件が大きな影響を与える。
【００７３】
熱間鍛造のために、材料を加熱すると、金属組織はオーステナイト組織になる。この際、オーステナイト粒径は、熱間鍛造の加熱温度を高温にするほど、原子の拡散が活発になり粒成長しやすい。また、熱間鍛造時の塑性加工量が大きくなるほど、再結晶の際の核生成エネルギーおよび核生成サイト数が大きくなるため、オーステナイト粒径は小さくなる。
【００７４】
本発明によれば、フランジを有する、内方部材あるいは外方部材においては、フランジにおける熱間鍛造時の塑性変形量が大きいため、熱間鍛造時の加熱温度を、従来温度より低くすることにより、効果的にオーステナイト粒の成長が抑制され、疲労強度が向上することができる。鍛造時の加熱温度が１０５０°Ｃ以上になると、オーステナイト粒径が粗大化し、疲労強度向上の効果が小さい。また、９００°Ｃ未満になると、変形抵抗が大きくなり、プレス成形機および金型の寿命を低下させる。よって、本発明の熱間鍛造の加熱温度は、９００〜１０５０°Ｃとする。
【００７５】
しかしながら、熱間鍛造温度を従来温度より低くすると、鍛造割れが生じやすくなる。これは、温度が低下した際に、金属組織の一部がオーステナイト状態からフェライト・パーライト組織に変態し、その状態で鍛造すると、金属組織が不均一に塑性変形し、鍛造割れが生じる。本発明は、鍛造割れを防止するために、鍛造が打ち終わる際に、金属組織を均一なオーステナイト状態にするように、鍛造打ち終わり時の温度を規定している。鍛造時の打ち終わり温度が８００°Ｃ未満になると、金属組織の塑性変形が不均―になり鍛造割れを生じやすくなる。よって、本発明の鍛造打ち終わり時の温度は、８００°Ｃ以上とする。
【００７６】
以下に、本発明で使用する鋼の合金成分の規定理由を述べる。
［Ｃ：０．４５重量％以上０．６５重量％以下］
ハブ輪２の素材の鋼中のＣが０．４５重量％未満であると、転動部の高周波焼入硬さが低く、転動疲労寿命を与えるのに必要な強度ＨＲＣ５８以上とすることができない。さらに、車輪取付用フランジ６の付け根部の硬さが十分に得られなくなり、特に、外側付け根部１４の回転曲げ応力に対する疲労強度が低くなる。一方、Ｃが０．６５重量％を超えると、初析フェライト組織の面積率の低下と硬さの上昇により旋削加工性が低下するだけでなく、転動疲労寿命および疲労強度を大きく改善できないため、ハブ輪２の素材の炭素鋼中のＣは０．４５重量％以上、０．６５重量％以下とした。
［ Ｍｎ：０．５〜１．５重量％］
Ｍｎは、鋼の焼入れ性を向上させる元素であり、０．５重量％未満であると、高周波焼入れ時の硬化層が浅くなるため、軌道部の転がり疲労が低下する。しかしながら、１．５重量％を超えると、加工性が低下する。よって、本発明のＭｎ量は、０．５重量％以上１．５重量％以下とする。
［ Ｓｉ：０．１〜１．０重量％］
Ｓｉは、焼入れ性を向上させ、しかもマルテンサイトを強化し、転がり疲労寿命を向上させる元素である。また、非調質部のフェライトに固溶し、フェライト組織の強度を向上させることにより、非調質部の疲労強度を向上させる。０．１重量％未満では前記の効果が不足する。しかしながら、１．０重量％を超えると、熱間鍛造性が低下する。更に、鍛造後の脱炭が大きくなるため、熱間鍛造後切削加工を行わなわず、鍛造したままの表面で使用する部位の疲労強度を低下させる。よって、本発明のＳｉ量は、０．１重量％以上１．０重量％以下とする。
［ Ｃｒ：０．０１〜０．５重量％］
Ｃｒは、焼入れ性を向上させる効果があり、さらに焼入れ後のマルテンサイト組織を強化し、転がり疲労寿命を向上させる。０．０１重量％未満では高周波焼入れ時の硬化層が浅くなり、また、マルテンサイト組織の強度も低下するので転がり疲労寿命が低下する。しかしながら、０．５重量％を超えると、熱間鍛造性および切削性が低下する。よって、本発明のＣｒ量は、０．０１重量％以上０．５重量％以下とする。
［ Ｓ≦０．０２５重量％］
Ｓは、鋼中でＭｎＳなどの非金属介在物を形成し、鋼中のＭｎＳが鍛造割れの起点になる場合がある。また、ハブ輪２に内輪３を加締め（図１の符号９参照）により固定するタイプのフランジ付軸受装置において、非調質部のＭｎＳは加締め部９の割れの起点になる場合がある。
【００７７】
以上２つの理由からＳ量は少ないほうが好ましく、Ｓ量が０．０２５重量％を超えると、鍛造割れあるいは加締め部９の割れなどが増加する場合があるため、本発明のＳ量は、０．０２５重量％以下とする。好ましくは、安定的な鍛造割れ防止と加締め部９の割れ防止を考慮して、Ｓ量は０．０１５重量％以下とする。
［Ｏ≦１５ｐｐｍ］
Ｏは、高周波焼入れされた軌道部の転がり疲労に大きな影響を与える元素である。Ｏは鋼中でＡｌ_２ Ｏ_３ などの非金属介在部を形成し、転がり疲労によるはくりの起点になり転がり疲労寿命を低下させる。よって、転がり疲労寿命を向上させるためには、Ｏ量は少ないほうが好ましい。Ｏ量は、１５ｐｐｍを超えると、転がり疲労寿命を低下させる場合があるため、本発明のＯ量は、１５ｐｐｍ以下とする。
【００７８】
更に、非調質部の疲労強度と、高周波焼入れをした軌道部の転がり寿命を向上させるためには、熱間鍛造後の冷却速度を、所定の範囲に規定することが効果的である。
熱間鍛造が打ち終わった際の金属組織は、オーステナイト状態であるが、冷却すると変態が生じ、初析フェライトとパーライト組織が生成する。この変態は、約６００°Ｃでほぼ終了し、冷却後の組織はフェライト・パーライト組織になる。この際、冷却速度が遅いと、初析フェライトの成長が促進され、初析フェライトの粗大な塊が生成する。この場合、フェライトはパーライトと比較して、強度が低いため、粗大な初析フェライトの塊は、疲労きれつの起点および伝播経路になりやすく、非調質部の疲労強度が低下する。また、軌道部周辺に高周波焼入れをする際、粗大な初析フェライトの塊が存在すると、焼入れ性の低下、あるいは硬さの不均一が生じる場合がある。
【００７９】
そこで、平均冷却速度は以下の式で規定する。
平均冷却速度（°Ｃ／秒）＝（鍛造打ち終わり時の温度（°Ｃ）−６００（°Ｃ））／（鍛造打ち終わり時から６００°Ｃになるまでの冷却時間（秒））
鍛造打ち終わり時の温度から、６００°Ｃになるまでの平均冷却速度が０．５°Ｃ／秒未満であると、前述した疲労強度の低下、高周波焼入れ性の低下あるいは高周波焼入れ後の硬さの不均一などが生じる場合がある。
【００８０】
一方、冷却速度が速すぎると、初析フェライト量が少なくなるため、硬さが高くなり、切削加工性が低下する。また、一部に不完全焼入れ組織が生じた場合には、著しく加工性が低下する。前記平均冷却速度が５°Ｃ／秒を超えると加工性が低下する。よって、本発明の平均冷却速度は０．５°Ｃ／秒以上５°Ｃ／秒以下が好ましい。さらに好ましくは、安定的な疲労強度向上と、切削加工時の生産性を考慮して、平均冷却速度は１°Ｃ／秒以上３°Ｃ／秒以下とする。
【００８１】
また、鋼中に炭化物あるいは窒化物を微細に分散させるとそれらのピン止め効果によって、結晶粒の成長を効果的に抑制することができ、鍛造温度の低下と同様の効果を得ることができる。添加する合金元素としては、後述のようにＶ，ＴｉあるいはＮｂが好ましい。
本発明で使用されるＶ，ＮｂあるいはＴｉは、鋼中で微細な炭化物あるいは窒化物を生成し、熱間鍛造時に鋼材を加熱した際に生ずるオーステナイト結晶粒の粗大化を抑制する効果がある。また、Ｖ，Ｎｂ、あるいはＴｉの微細な炭化物あるいは炭窒化物は、熱間鍛造後の冷却時に、初析フェライトの析出サイトになるという効果もあり、フェライトの微細分散析出を促進する。よって、Ｖ，Ｎｂ、あるいはＴｉの添加によって、フェライトが微細に分散した金属組織を得ることができ、フランジ周辺部の非調質部の疲労強度を向上させる。
【００８２】
また、高周波焼入れによって形成した硬化層を有する軌道部は、転がり疲労寿命が要求される。高周波焼入れがされた金属組織は、主にマルテンサイト組織になるが、本発明で使用される鋼には、Ｖ，ＮｂあるいはＴｉが添加されているため、マルテンサイト中にも微細な炭化物あるいは炭窒化物が分散している。前記炭化物あるいは炭窒化物が微細分散すると、耐摩耗性および硬度が向上するため、転がり疲労寿命が向上する。
【００８３】
以下に、合金成分の限定理由を示す。
Ｖは、鋼中で炭化物あるいは窒化物を形成し、熱間鍛造時に、オーステナイト粒が成長するのを抑制し、旧オーステナイト粒を小さくする。また、Ｖの炭化物、窒化物自体も初析フェライトの析出サイトになるため、微細に分散された炭化物、窒化物から初析フェライトが析出し、フェライトの微細分散析出を促進する。特に、旧オーステナイト粒界に存在するＶの炭化物あるいは窒化物は、それぞれの炭化物粒子あるいは窒化物粒子から、初析フェライトを析出するため、旧オーステナイト粒界に網目状に析出するフェライトを分断し、疲労き裂がフェライト組織中を伝播するのを防ぎ、フェライト・パーライト組織を有する非調質部の疲労強度を向上させる効果が大きい。
【００８４】
また、Ｖの炭化物あるいは炭窒化物は非常に硬度が高いため、高周波焼入れされた軌道部のマルテンサイト組織内に微細に分散させると、耐摩耗性が向上し、転がり疲労寿命が向上する効果がある。Ｖが０．０１重量％未満であると、上記の効果が発揮されない。Ｖが０．２重量％を超えると、熱間鍛造性、切削性、および研削性が低下する。よって、本発明のＶ量は０．０１重量％以上０．２重量％以下が好ましい。さらに好ましくは、疲労強度の安定的な向上と生産性を考慮して、０．０２重量％以上０．１０重量％以下とする。
【００８５】
ＮｂもＶと同様、鋼中で炭化物あるいは窒化物を形成し、旧オーステナイト粒の成長を抑制させる効果および初祈フェライトの析出サイトになる効果を有するため、フェライト・パーライト組織を有する非調質部の初析フェライトを微細分散させ、疲労強度を向上させる効果がある。特にＮｂは、旧オーステナイト粒の成長を抑制させる効果が大きい。Ｎｂが０．０１重量％未満であると、上記の効果が発揮されない。Ｎｂが０．１５重量％を超えると、熱間鍛造性、切削性、および研削性が低下する。よって、本発明のＮｂ量は０．０１重量％以上０．１５重量％以下が好ましい。
【００８６】
ＴｉもＶと同様、鋼中で炭化物あるいは窒化物を形成し、旧オーステナイト粒の成長を抑制させる効果および初析フェライトの析出サイトになる効果を有するため、フェライト・パーライト組織を有する非調質部の初析フェライトを微細分散させ、疲労強度を向上させる効果がある。特にＴｉは、旧オーステナイト粒の成長を抑制させる効果が大きい。Ｔｉが０．０１重量％未満であると、上記の効果が発揮されない。Ｔｉが０．１５重量％を超えると、熱間鍛造性、切削性、および研削性が低下する。よって、本発明のＴｉ量は０．０１重量％以上０．１５重量％以下が好ましい。
【００８７】
前述したように、旧オーステナイト粒径の大きさを小さく抑制することは、非調質部の疲労強度向上に効果が大きい。結晶粒度が４未満であると、疲労強度向上の効果が小さい。
よって、本発明では、フランジ付軸受装置の応力集中が高くなるフランジ付け根部の旧オーステナイト結晶粒度を４以上とすることが好ましい。
【００８８】
本発明のフランジ付軸受装置に用いられる転動体には、ＳＵＪ２などの高炭素クロム軸受鋼、あるいは前記高炭素クロム軸受鋼に浸炭窒化処理をしたもののなどを使用することが好ましい。また、本発明で用いられる転動体の形状は、用途に応じて、玉あるいはころを使用することができる。
更に、ハブ輪２に内輪３を加締め（図１の符号９参照）により固定するタイプのフランジ付軸受装置では、内輪３は、ＳＵＪ２などの高炭素クロム軸受鋼を用いることが好ましい。
【００８９】
【実施例Ｂ】
図６に示す前述のフランジ付軸受装置を製作した。
上記ハブ輪２は 表５のＡ〜Ｈに示す合金成分を有する鋼を用いて、表６および表７に示す各条件で熱間鍛造し、強制空冷又は放冷を行った。その後、切削加工で所定の形状に加工し、内輪軌道面７ｂの周辺部から小径段部８の周辺部まで高周波焼入れを行い、表面に硬化層を形成し、その後、研削加工を行って仕上げ形状にした。表６に、高周波焼入れをした軌道部表面硬さ及び高周波焼入れをしていない非調質部の硬さを併記する。結晶粒度はＪＩＳ Ｇ０５５１により測定した。
【００９０】
【表５】

【００９１】
【表６】

【００９２】
【表７】

【００９３】
外輪４は従来材Ｓ５３Ｃを用いて、１１００〜１１５０°Ｃで熱間鍛造を行った。その後、切削加工を行い、外輪軌道面１０ａ周辺と外輪軌道面１０ｂの周辺は高周波焼入れを行った。その後、研削加工を行い最終形状にした。内輪３および転動体５はＳＵＪ２製であり、通常の焼入れ処理により、表面から芯部まで硬化している。
【００９４】
製作したフランジ付軸受装置の軸受形式は、転動体ピッチ径４９ｍｍの複列玉軸受であり、各列の玉数１２個である。この軸受装置を用いて、外輪４側のフランジ１１を固定側に、ハブ輪２側のフランジ６を回転側に取り付け、下記の条件で回転試験を行った。よって、この条件で回転試験を行った場合には、ハブ輪２のフランジ６の付け根部に繰り返し曲げ応力が負荷される。
【００９５】
ラジアル荷重Ｆｒ：５０００〜１５０００Ｎ
アキシアル荷重Ｆａ：５０００Ｎ
回転速度：４００ｍｉｎ^−１
上記範囲内の所定のラジアル荷重で、４５ｈ回転試験を行い、軸受振動の増加の有無、あるいは、フランジ周辺部の疲労クラックの有無を確認した。振動の増加およびフランジ周辺部のクラックが無かった場合には、段階的にラジアル荷重を増加させて４０ｈ回転試験を行い、軸受の振動の増加あるいはフランジ周辺部の疲労クラックが発生した時の、ラジアル荷重を耐久荷重とした。回転試験結果を表６に併記する。なお、表６に記載した耐久荷重は、比較例１−９の回転試験結果を１．０として、比で表したものである。
【００９６】
表６に示す実施例１−１〜１−８は、合金成分および熱間鍛造条件が、本発明で規定する範囲内であるため、良好な金属組織が得られており、非調質部の回転曲げ疲労強度および高周波焼入れをした軌道部の転がり疲労寿命が優れており、良好な回転試験結果になっている。
―方、比較例１−９および１−１０は、熱間鍛造時の材料加熱温度（加熱保持温度）が本発明で規定する範囲より高いため、非調質部の疲労強度が劣り、回転試験の耐久荷重は低い結果となった。また、比較例１−１１は、熱間鍛造時の材料加熱温度が本発明で規定する範囲より低いため、変形抵抗が大きく、プレス成形機および金型への負荷が大きいため、加工を中止した。比較例１−１２は、鍛造後の平均冷却速度が、本発明で規定する範囲より遅いため、粗大な初析フェライトが生成したため、回転試験の耐久荷重は低い結果となった。比較例１−１３は、鍛造後の平均冷却速度が、本発明で規定する範囲より速いため、非調質部の硬さが大きくなり、切削性が著しく低下したため、加工を中止した。
【００９７】
図７は、鍛造時の材料加熱温度（加熱保持温度）と回転試験の耐久荷重との関係を示したものである。なお、図７では、平均冷却速度は同程度のもので比較を行ってる。
図８は、鍛造打ち終わり時から６００°Ｃまでの平均冷却速度と回転試験の耐久荷重との関係を示したものである。なお、図８では、材料加熱温度を１０００°Ｃと一定にしたもので比較を行っている。
【００９８】
以上より、熱間鍛造条件を最適化することにより、フランジ周辺の非調質部の回転曲げ疲労強度および高周波焼入れした軌道部の転がり疲労強度が優れたフランジ付軸受装置を得ることができる。
次に、鋼種を変えて、同様にハブ輪２を製作してフランジ付軸受装置を組立て、以下の条件で回転試験を行った。
【００９９】
ラジアル荷重：５０００〜１５０００Ｎ
アキシアル荷重：７０００Ｎ
回転速度：４００ｍｉｎ^−１
表７に回転試験結果を併記する。なお、表７に記載した耐久荷重は、比較例２−８の回転試験結果を１．０として、比で表したものである。
【０１００】
表７に示す実施例２−１〜２−７は、合金成分および熱間鍛造条件が、本発明で規定する範囲内であるため、良好な金属組織が得られており、非調質部の回転曲げ疲労強度および高周波焼入れをした軌道部の転がり疲労寿命が優れており、良好な回転試験結果になっている。特に、実施例２−４〜２−７は、Ｖ、ＮｂあるいはＴｉを添加しているため、非調質部の組織が微細化して該非調質部の疲労強度が一段と高く、また、炭化物および窒化物の析出により、高周波焼入れされた軌道部の転がり疲労も良好である。
【０１０１】
一方、比較例２−８は、合金成分は本発明で規定する範囲であるが、熱間鍛造時の材料加熱温度が、本発明で規定する範囲より高いため、非調質部の疲労強度が劣り、回転試験の耐久荷重は低い結果となった。また、比較例２−９は、合金元素中に含まれるＳ量が、本発明で規定する範囲より高いため、鍛造割れが生じやすく、鍛造後にクラックが生じていたものがあったため加工を中止した。
【０１０２】
以上より、本発明で規定する合金成分の鋼を用いて、本発明で規定する熱間鍛造条件で鍛造することにより、フランジ周辺の非調質部の回転曲げ疲労強度および高周波焼入れした軌道部の転がり疲労強度が優れたフランジ付軸受装置を得ることができる。さらにＶ、ＮｂあるいはＴｉを添加することによって、一段と前記疲労強度向上の効果が高くなる。
（第３の実施の形態：請求項１０、１１、１２に対応））
フランジを有する内方部材あるいは外方部材は、熱間鍛造により成形された後、切削加工、穴あけ加工により所定の形状にする。その後、所定の部位に高周波焼入れを行い、硬化層を形成し、軌道部などは研削加工によって仕上げ加工を行う。
【０１０３】
中炭素鋼からなるフランジを有する内方部材あるいは外方部材の熱間鍛造後の金属組織は、初析フェライトが旧オーステナイ結晶粒界に沿って網目状に析出したフェライト・パーライト組織になる。前記金属組織において、切削性および穴あけ加工性を向上させるためには、初析フェライトの面積率を増加させ、かつ、初析フェライトを微細に分散析出させることが有効である。
【０１０４】
本発明は、鋼のＣ量を、従来よりも低減させることによって、初析フェライトの面積率を増加させている。また、Ｖを添加すると、Ｖの炭化物あるいは窒化物のピン止め効果により、オーステナイト結晶粒が微細化するため、旧オーステナイト結晶粒に沿って析出する初析フェライトの析出単位が微細になる。さらに、Ｖの炭化物あるいは窒化物自身が初析フェライトの析出核になるため、旧オーステナイト粒界に沿って析出する初析フェライトを分断し、より一層、初析フェライトを微細分散させる効果がある。
【０１０５】
以上の効果により、初析フェライトの面積率の増加および初析フェライトの微細分散により、良好な切削性および穴あけ加工性を得ることができる。好ましくは、初析フェライトの面積率を５％以上１５％以下とする。なお、初析フェライトの面積率は、Ｃ量、熱間鍛造時の鍛造温度、あるいは、熱間鍛造後の冷却速度を制御することによって、所望の面積率に制御することができる。
【０１０６】
また、高周波焼入れがされていない非調質部は、熱間鍛造後の金属組織のまま使用されるが、前述したＶ添加によるフェライトの微細分散効果は、非調質部の疲労強度の向上にも寄与する。理由を以下に述べる。
旧オーステナイト粒径を微細化すると、結晶粒界での応力集中を抑制する効果がある。また、フェライトは、パーライトと比較して強度が小さいため、疲労き裂の起点あるいは伝播経路になる可能性が高い。よって、初析フェライトが微細に分断することによって、疲労き裂の最小単位が小さくなる。以上より、疲労破壊を効果的に抑制することができる。
【０１０７】
また、もう一つの効果として、Ｖはフェライトの析出硬化にも寄与し、さらに、本発明で添加するＳｉは、フェライトの固溶強化に寄与する。よって、フェライト・パーライト組織の中で、強度が小さい部位である初析フェライト相を強化するため、最弱部の強度が向上し、疲労強度が向上する。
以上の効果により、旧オーステナイト結晶粒の微細化、初析フェライトの微細分散、およびフェライトの強化によって、非調質部の疲労強度が向上する。
【０１０８】
高周波焼入れによって形成した硬化層を有する軌道部の金属組織は、主にマルテンサイト組織になり、転がり疲労寿命が要求される。一般にＣ量を低減すると、炭化物が減少したり、素地のマルテンサイトの強度が低下したりするため、転がり疲労寿命は低下する。しかしながら、本発明で使用される鋼には、Ｖが添加されているため、マルテンサイト中に、微細なＶの炭化物あるいは窒化物が分散している。前記Ｖの炭化物あるいは窒化物は非常に硬度が高く、微細分散すると、耐摩耗性および硬度が向上するため、転がり疲労寿命が向上する。
【０１０９】
また、Ｓｉはマルテンサイトに固溶して、マルテンサイトの素地を強化するため、転がり疲労寿命を向上させる効果がある。さらに、焼戻し抵抗性を著しく向上させるため、Ｃ量を低減した鋼を焼入れ後に、焼戻しを行っても、硬さの低下が少ないため、良好な硬さを保持でき、転がり疲労寿命を良好に保つ。
以上の効果により、Ｃ量を低減しても、ＶおよびＳｉを所定の量添加することにより、転がり疲労寿命を良好に保持できる。
【０１１０】
以下に、本発明で使用する鋼の合金成分と硬さの限定理由を示す。
Ｃは、熱間鍛造後の硬さおよび、焼入れ焼き戻し後の硬さに大きな影響を与える元素であり、０．４５重量％未満では、焼入れ時の硬さが不足し、軌道部の転がり疲労寿命が低下する。さらに、熱間鍛造後の硬さも不足するため、非調質部の曲げ疲労強度も低下する。しかしながら、Ｃが０．５重量％を超えると、熱間鍛造後の硬さが大きくなり、切削性および穴あけ加工性が低下し、加工精度を向上させるには、加工時間を要する。よって、Ｓｉが０．３〜１．５重量％、Ｖが０．０３〜０．３重量％の場合は、本発明のＣ量は、０ ．４５重量％以上０．５０重量％以下とする。
【０１１１】
Ｓｉは、前述したように、マルテンサイトを強化し、さらに焼き戻し抵抗性を高めるため、転がり疲労寿命を向上させる元素である。また、非調質部のフェライトに固溶し、フェライト組織の強度を向上させることにより、非調質部の疲労強度も向上させる。Ｃが０．４５〜０．５０重量％の場合、Ｓｉが０．３重量％未満では前記の効果が不足する。しかしながら、Ｓｉが１．５重量％を超えると、熱間鍛造性が低下する。よって、本発明のＳｉ量は、０．３重量％以上１．５重量％以下とする。好ましくは、転がり疲労寿命および非調質部の疲労強度の安定的な向上と、熱間鍛造時の生産性を考慮して、Ｓｉ量は０．６５重量％以上１．０重量％以下とする。
【０１１２】
Ｖは、前述したように、非調質部の疲労強度と高周波焼入れ部の転がり疲労寿命を向上させる重要な元素である。Ｖは、旧オーステナイト粒を小さくし、さらに、初析フェライトの微細分散に寄与するため、非調質部の疲労強度を向上させる。また、Ｖの炭化物あるいは窒化物は非常に硬度が高いため、高周波焼入れされた軌道部のマルテンサイト組織内に微細に分散させると、耐摩耗性が向上し、転がり疲労寿命が向上する効果がある。Ｖが０．０３重量％未満であると、上記の効果が発揮されない。Ｖが０．３重量％を超えると、熱間鍛造性、切削性、および研削性が低下する。よって、本発明のＶ量は０．０３重量％以上０．３重量％以下とする。好ましくは、前記効果とコストを考慮して、Ｖ量は０．０３重量％以上０．１重量％以下とし、より好ましくは、０．０５重量％以上０．１重量％以下とする。
【０１１３】
Ｍｎは、鋼の焼入れ性を向上させる元素であるが、１．５重量％を超えると、切削性および穴あけ加工性が低下する。よって、本発明のＭｎ量は１．５重量％以下とする。好ましくは、焼入れ時の生産性と切削性および穴あけ加工性を考慮して、Ｍｎ量は０．５重量％以上１．０重量％以下とする。
Ｃｒは、鋼の焼入れ性を向上させ、さらに焼入れ後のマルテンサイト組織を強化し、転がり疲労寿命を向上させる元素であるが、１．０重量％を超えると、熱間鍛造性および切削性が低下する。よって、本発明のＣｒ量は、１．０重量％以下とする。好ましくは、焼入れ時の生産性および転がり疲労寿命の向上と、加工性を考慮して、Ｃｒ量は０．１重量％以上０．５重量％以下とする。
【０１１４】
Ｓは、鋼中でＭｎＳなどの非金属介在物を形成する。高周波焼入れされた軌道部に存在するＭｎＳは、転がり疲労によるはくりの起点になり転がり疲労寿命を低下させる。また、ハブ輪２に内輪３を加締め（図１の符号９参照）により固定するタイプのフランジ付軸受装置において、非調質部のＭｎＳは加締め部９の割れの起点になる場合がある。
【０１１５】
以上２つの理由からＳ量は少ないほうが好ましく、Ｓ量が０．０３５重量％を超えると、転がり疲労寿命の低下あるいは加締め部９の割れなどが増加する場合があるため、本発明のＳ量は、０．０３５重量％以下とする。好ましくは、安定的な転がり疲労寿命確保と加締め部の割れ防止を考慮して、Ｓ量は０．０２０重量％以下とする。
【０１１６】
Ｏは、高周波焼入れされた軌道部の転がり疲労に大きな影響を与える元素である。Ｏは鋼中でＡｌ_２ Ｏ_３ などの非金属介在物を形成し、転がり疲労によるはくりの起点になり転がり疲労寿命を低下させる。よって、転がり疲労寿命を向上させるためには、Ｏ量は少ないほうが好ましい。Ｏ量は、１５ｐｐｍを超えると、転がり疲労寿命を低下させる場合があるため、本発明のＯ量は、１５ｐｐｍ以下とする。
【０１１７】
Ｃ＋０．２Ｓｉ＋０．５Ｖの値は、Ｃを低減した場合の、ＳｉおよびＶの転がり疲労寿命への寄与を表したものである。Ｃを低減すると、転がり疲労寿命は低下するが、ＳｉおよびＶを添加することによって、転がり疲労寿命の低下を抑制することができる。ただし、Ｃ＋０．２Ｓｉ＋０．５Ｖの値が０．５５未満であると、転がり疲労寿命が低下する。一方、０．７５を超えると、切削性および穴あけ加工性が低下する。よって、本発明におけるＣ＋０．２Ｓｉ＋０．５Ｖの値は、０．５５以上０．７５以下とする。好ましくは、転がり寿命の安定的な向上と生産性を考慮して、Ｃ＋０．２Ｓｉ＋０．５Ｖの値は、０．６０以上０．７０以下とする。
【０１１８】
また、内方部材あるいは外方部材の軌道部は、転動体から高面圧を受けるため、転がり疲労寿命向上のためには、高面圧に耐え得る高い硬度が必要になる。よって、高周波焼入れによって形成された硬化層の軌道部表面硬さはＨｖ６３０〜Ｈｖ７５０とするのが好ましい。軌道部表面の硬さがＨｖ６３０未満であると、硬度が不足することにより、転がり疲労寿命が低下する。一方、本発明で規定する合金成分でＨｖ７５０を超えると、靭性が低下するため、耐衝撃性が低下する。よって、本発明の高周波焼入れによって形成された硬化層の軌道部表面硬さは、Ｈｖ６３０以上Ｈｖ７５０以下とする。より好ましくは、転がり寿命向上のため、Ｈｖ７００以上とする。
【０１１９】
フランジ周辺部の非調質部は、回転曲げ疲労強度が必要である。本発明のフランジ付軸受装置に用いられる部材は、金属組織中のフェライトが微細に分散析出しているため、疲労強度が向上している。しかしながら、Ｈｖ２２０未満であると、非調質部の疲労強度が低下する。一方、Ｈｖ３００を超えると切削性および穴あけ加工性が低下する。よって、本発明の高周波焼入れによる硬化処理がされていない非調質部の硬さはＨｖ２２０以上Ｈｖ３００以下とする。より好ましくは、非調質部の安定的な疲労強度向上と、切削加工時および穴あけ加工時における生産性を考慮して、Ｈｖ２４０以上Ｈｖ２８０以下とする。
【０１２０】
本発明で用いられる転動体には、ＳＵＪ２等の高炭素クロム軸受鋼、あるいは前記高炭素クロム軸受鋼に浸炭窒化処理をしたものなどを使用することが好ましい。また、本発明で用いられる転動体の形状は、用途に応じて、玉あるいはころを使用することができる。
更に、ハブ輪２に内輪３を加締め（図１の符号９参照）により固定するタイプのフランジ付軸受装置では、内輪３は、ＳＵＪ２などの高炭素クロム軸受鋼を用いることが好ましい。
【０１２１】
【実施例Ｃ】
図６に示す前述のフランジ付軸受装置を製作した。
上記ハブ輪２は、表８に示す合金成分を有する鋼を用い、１０００〜１１５０°Ｃで熱間鍛造後、切削加工および穴あけ加工を行い、所定の形状にした。その後、内輪軌道面７ｂの周辺部から小径段部８の周辺部まで高周波焼入れおよび焼き戻しを行うことにより、表面に硬化層を形成し、研削により仕上げ加工を行った。表８に、高周波焼入れをした軌道部表面硬さ、および、高周波焼入れをしていない非調質部の硬さを併記する。
【０１２２】
【表８】

【０１２３】
外輪４はＳ５３Ｃを用いて製作し、外輪軌道面１０ａの周辺と外輪軌道面１０ｂの周辺は高周波焼入れを行い、表面硬化層を有している。内輪３および転動体５はＳＵＪ２製であり、通常の焼入れ処理により、表面から芯部まで硬化している。
［回転試験］
製作したフランジ付軸受装置の軸受形式は、転動体ピッチ径４９ｍｍの複列玉軸受であり、各列の玉数１２個である。このフランジ付軸受装置を試験軸受装置として、下記の条件で回転試験を行った。
【０１２４】
ラジアル荷重Ｆｒ：９８００Ｎ
アキシアル荷重Ｆａ：４９００Ｎ
回転速度：３００ｍｉｎ^−１
軌道部にはくりが生じた時点、あるいは、フランジ周辺部のクラックを確認した時点を寿命と判定した。回転試験から得られた寿命試験結果を表８に併記する。なお、表８に記載した試験寿命は、比較例３−１に示す従来品の試験寿命を１．０として、比で表したものである。図９はＣ＋０．２Ｓｉ＋０．５Ｖと試験寿命との関係を示したものである。
［ 加工試験］
製作したフランジ付軸受装置のハブ輪２を用いて、ドリル穴あけ試験を行った。下記の条件で、フランジ６にφ８ｍｍ、深さ１３ｍｍの穴あけ加工を行った。
【０１２５】
ドリル材質：ＳＫＨ５１
切削速度：１８ｍｍ／ｍｉｎ
送り速度：０．１５ｍｍ／ｒｅｖ
試験後、ドリル刃先の逃げ面摩耗量を測定した。表８に結果を併記する。なお、表８の摩耗量は、比較例３−１に示す従来品の寿命を１．０として、比で表したものである。
【０１２６】
なお、表８に併記するフェライト面積率（％）は、ハブ輪２が備えるフランジ６の外側付け根部１４の非調質部の断面を、鏡面研磨およびエッチングすることによって金属ミクロ組織を現出させ、金属顕微鏡の観察写真０．１〜０．３ｍｍ^２ を画像解析することによって測定した。
同じく表８に併記する非調質部硬さ（Ｈｖ）は、ハブ輪２が備えるフランジ６の外側付け根部１４の非調質部の断面を、ビッカース硬さ測定機で測定して求め、表８に併記する軌道部硬さ（Ｈｖ）は、ハブ輪２が備える軌道部の断面を切り出し、軌道溝表面から深さ０．２ｍｍの部位をビッカース硬さ測定機で測定して求めた。
【０１２７】
表８に示す実施例３−１〜３−１０は、合金成分が、本発明で規定する範囲内であるため、回転試験結果が従来品と同等以上であり、かつ、ドリル穴あけ試験の加工性が向上している。特に、実施例３−２、３−３、３−８、３−９および３−１０は、従来品よりも回転試験結果が優れており、かつ、加工性が特段に向上している。
【０１２８】
一方、比較例３−２および３−３は、鋼に含まれるＣあるいはＳｉ量が、本発明で規定する範囲より少なく、Ｃ＋０．２Ｓｉ＋０．５Ｖも本発明で規定する範囲より少ないため、回転試験結果が著しく劣る。
比較例３−４は、Ｖの添加量が少ないため、回転試験結果が従来品よりやや劣り、さらにフェライトの析出量が少ないため、加工性が劣る。
【０１２９】
比較例３−５および３−６は、鋼に含まれるＳｉあるいはＶの添加量が、本発明で規定する範囲より多いため、回転試験の結果は良好であるが、加工性が著しく低下している。
以上より、合金成分を本発明で規定する範囲にし、フェライト面積率、非調質部の硬さ、および高周波焼入れした軌道部の硬さを所定の範囲に規定することによって、非調質部の疲労強度、高周波焼入れした軌道部の転がり寿命を良好に保持しながら、加工性に優れたフランジ付軸受装置を得ることができる。
【０１３０】
なお、本発明は上記各実施の形態に限定されるものではなく、本発明の要旨を逸脱しない範囲において適宜変更可能である。
例えば、上記各実施の形態では、内方部材をハブ輪２とした場合を例に採ったが、外方部材をハブ輪として該ハブ輪のフランジに本発明を適用してもよい。
また、上記各実施の形態では、ハブ輪２のみに本発明を適用した場合を例に採ったが、これに限定されず、ハブ輪２に加えて、内輪３や外輪４にも本発明を適用してもよい。
【０１３１】
【発明の効果】
上記の説明から明らかなように、本発明によれば、少なくともフランジのミクロ組織を微細化することにより、フランジの形状・寸法を変更することなく、且つ高周波焼入れ部の増加によるコストアップを招くことなく、回転曲げ疲労の最弱部であるフランジを高強度化することが可能になる。この結果、フランジの薄肉化が可能となり、フランジ付軸受装置の軽量化を実現できる。
【０１３２】
また、鋼の成分と熱間鍛造条件を最適化することによって、フランジ周辺の非調質部の回転曲げ疲労強度および高周波焼入れした軌道部の転がり疲労強度が優れたフランジ付軸受装置を得ることができ、該フランジ付軸受装置の軽量化が可能になる。
更に、合金成分を所定範囲に規定し、フェライト面積率、非調質部の硬さ、および高周波焼入れした軌道部の硬さを所定の範囲に規定することによって、非調質部の疲労強度、高周波焼入れした軌道部の転がり寿命を良好に保持しながら、加工性に優れたフランジ付軸受装置を得ることができる。
【図面の簡単な説明】
【図１】フランジ付軸受装置の一例を示す断面図である。
【図２】結晶粒界の交点の説明図である。
【図３】Ｓ量（重量％）×１０００＋α面積率（％）と工具寿命との関係を示すグラフ図である。
【図４】Ｓ量（重量％）／α面積率（％）と疲労限強度との関係を示すグラフ図である。
【図５】視野内のミクロ組織を２値化して初析フェライト粒のみを抽出した顕微鏡写真である。
【図６】実施例に用いた試験機としてのフランジ付軸受装置の断面図である。
【図７】鍛造時の材料加熱温度（加熱保持温度）と回転試験の耐久荷重との関係を示すグラフ図である。
【図８】鍛造打ち終わり時から６００°Ｃまでの平均冷却速度と回転試験の耐久荷重との関係を示すグラフ図である。
【図９】Ｃ＋０．２Ｓｉ＋０．５Ｖと試験寿命との関係を示すグラフ図である。
【符号の説明】
１…フランジ付軸受装置
２…ハブ輪（内方部材）
３…内輪
４…外輪（外方部材）
５…転動体
６…車輪取付用フランジ
７ａ，７ｂ…内輪軌道面
８…小径段部
１０ａ，１０ｂ…外輪軌道面
１１…懸架装置取付用フランジ
１４…外側付け根部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a flanged bearing device and a method of manufacturing the device, and more particularly to a flanged bearing device suitable for supporting wheels of automobiles and the like, and a method of manufacturing the device.
[0002]
[Prior art]
The rolling bearing includes an inner member having a raceway surface on an outer surface, an outer member having a raceway surface facing the raceway surface of the inner member on the inner surface, and an outer member arranged outside the inner member, and the two raceway surfaces. It is provided with a plurality of rolling elements arranged so as to be rotatable between them, and a retainer for holding the rolling elements at equal intervals in the circumferential direction.
[0003]
Similarly, a flanged bearing device used for supporting a wheel of an automobile includes an inner member, an outer member, a rolling element, a retainer, and the like. Many types have at least one of the outer members provided with a flange and a bolt hole for attachment to the rotating side or the fixed side, and have a complicated shape as compared with a normal rolling bearing.
[0004]
In a bearing device, a high surface pressure is repeatedly applied to a raceway groove portion on which a rolling element rolls, so that hardness and wear resistance required for prolonging the life of rolling fatigue are required. Therefore, high-carbon chromium steel such as SUJ2 is used as a material of a normal rolling bearing, and the entire bearing member is hardened by quenching and tempering.
[0005]
On the other hand, a bearing device with a flange has a complicated shape as compared with a normal bearing, and is formed of medium carbon steel such as S53C from the viewpoint of hot forging, cutting, and drilling. In many cases, a surface hardened layer is formed around the raceway portion by induction hardening in order to secure the rolling fatigue life and used.
In this case, the portions that are not subjected to induction hardening are used in a heat-treated state while hot forged (hereinafter, portions that are not subjected to induction hardening and are used in a heat-treated state while hot forged are not heat-treated). Part.).
[0006]
Conventionally, when hot forging a member provided with a flange (inner member and / or outer member) of a flanged bearing device, the amount of plastic working by forging of the flange is large, so the material is about 1200 ° C. And forging is performed in a state where the deformation resistance is lowered.
FIG. 1 shows an example of a conventional bearing device with a flange for supporting a wheel.
[0007]
The bearing device 1 with a flange includes a hub wheel 2, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5, and an outer end portion of an outer peripheral surface of the hub wheel 2 (the outer end portion in the vehicle width direction when assembled to an automobile). At the end (left end in FIG. 1), a wheel mounting flange 6 for supporting the wheel is provided.
A small-diameter step 8 is formed at the inner end (the right end in FIG. 1) of the hub wheel 2, and the outer peripheral surface of the inner ring 3 fitted into the small-diameter step 8 and the axial direction of the hub wheel 2. A raceway surface is formed on the outer peripheral surface of the intermediate portion to form a double-row inner raceway surface 7a, 7b. The inner end of the hub wheel 2 is formed in a cylindrical shape, and the inner ring 3 is fixed to the hub wheel 2 by caulking and expanding the cylindrical portion 9 radially outward.
[0008]
Double-row outer raceway surfaces 10a, 10b corresponding to the double-row inner raceway surfaces 7a, 7b are formed on the inner peripheral surface of the outer race 4, and are separated from the wheel mounting flange 6 of the outer race 4. A suspension device mounting flange 11 is provided at the side end. A plurality of rolling elements 5 are arranged between the double-row inner raceway surfaces 7a and 7b and the double-row outer raceway surfaces 10a and 10b, respectively.
[0009]
Although balls are used as the rolling elements 5 in the illustrated example, tapered rollers may be used as the rolling elements 5 in the case of a heavy flanged bearing device.
In order to assemble the bearing device 1 with a flange as described above to an automobile, the suspension mounting flange 11 of the outer race 4 is fixed to the suspension, and the wheel is fixed to the wheel mounting flange 6 of the hub wheel 2. Thus, the wheels can be supported rotatably with respect to the suspension device.
[0010]
The hub wheel 2 is made of medium carbon steel such as S53C from the viewpoint of hot forgeability and machinability, and the cut bar is subjected to high frequency induction heating to perform hot working in an austenite region of about 1200 ° C. Forge and let cool. At this time, pro-eutectoid ferrite precipitates from the austenite grain boundaries, and a pro-eutectoid ferrite + pearlite structure is obtained at room temperature by the subsequent pearlite transformation.
[0011]
Most of them are used without quenching and tempering, but the inner ring raceway surface (hereinafter referred to as the inner root portion) 12 at the inner side in the vehicle width direction (plate thickness direction) of the wheel mounting flange 6. In the region from 7b to the small-diameter stepped portion 8, a hardened layer 13 is formed by induction hardening to secure the rolling fatigue life and to prevent fretting of the inner ring fitting portion.
[0012]
By the way, in recent years, in order to improve the fuel efficiency of automobiles, it is required to reduce the weight of flanged bearing devices used for supporting wheels and the like. For the purpose of weight reduction, the thickness of the wheel mounting flange 6 is being reduced. However, when the thickness of the wheel mounting flange 6 is reduced, the strength of the root portion of the wheel mounting flange 6 is reduced. Has a limit.
In particular, at the root portion (hereinafter, referred to as an outer root portion) 14 outside the wheel mounting flange 6 in the vehicle width direction (plate thickness direction), the rotational bending stress is concentrated by the load received by the bearing device, so that fatigue cracks occur. there is a possibility.
[0013]
On the other hand, the inner root portion 12 of the wheel mounting flange 6 has a higher strength due to the formation of the hardened layer 13 by induction hardening as described above, and therefore has a lower fatigue strength than the non-heat-treated outer root portion 14. It is high and the possibility of cracking is low.
Therefore, by forming a hardened surface layer by induction hardening on the outer root portion 14 of the wheel mounting flange 6, a flanged bearing device capable of reducing the thickness by increasing the strength of the entire root portion of the wheel mounting flange 6 is provided. Proposed (for example, see Patent Document 1)
In addition, for the purpose of improving the rolling fatigue life of a raceway portion having a hardened layer, a flanged bearing device formed of an alloy steel containing more C than in normal S53C is disclosed (for example, Patent Document 1). 2).
[0014]
[Patent Document 1]
JP 2002-87008 A
[Patent Document 2]
JP 2001-200314 A
[0015]
[Problems to be solved by the invention]
However, in the wheel bearing device described in Patent Literature 1, an increase in the frequency of the induction hardening portion at the outer root portion 14 increases the cost, and the inner root portion 12 of the wheel mounting flange 6 has a higher cost. There is a concern that the impact resistance may be reduced due to quenching and hardening of both sides of the outer root portion 14.
[0016]
Therefore, in order to promote the thinning of the wheel mounting flange 6 without quenching and hardening the outer root portion 14 of the wheel mounting flange 6, the durability ratio (fatigue ratio) is considered in consideration of the fatigue strength after forging and the machinability. (Limited strength / tensile strength) must be improved.
Further, in recent years, in a bearing device with a flange for supporting a wheel, for the purpose of suppressing vibration during traveling and uneven wear of a brake, a more precise machining of a brake rotor fixing surface of a flange and the like has been required. . Since the processing of the flange is performed by turning and drilling, the demand for the machinability and drilling property of the material is becoming stronger, but in the flanged bearing device described in Patent Document 2 described above, However, there has been no solution to the problems of material machinability and drilling workability. Poor machinability and drilling workability lead to a reduction in productivity and a reduction in tool life, leading to an increase in cost.
[0017]
Further, as a method of improving the machinability and drilling workability of the material, it is effective to reduce the amount of C contained in steel, but when the amount of C is reduced, the rolling fatigue life of the induction hardened raceway portion is reduced. There is a problem.
Furthermore, since the rotating wheel (wheel) side rotates while receiving a load, repeated rotational bending stress is generated at the root of the flange. Since the root portion of the flange includes a non-tempered part that has not been induction hardened, the fatigue strength of the non-tempered part is also required.
[0018]
The present invention has been made in view of such a technical background, and by improving the fatigue strength of a flange without increasing the induction hardened portion, a flanged bearing capable of reducing the weight by reducing the thickness of the flange. It is an object to provide an apparatus and a method for manufacturing the apparatus.
[0019]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes an inner member having a raceway surface on an outer surface, and a raceway surface facing the raceway surface of the inner member on the inner surface, the inner member having a raceway surface. With an outer member disposed on the outside, a rolling element rotatably disposed between the two raceway surfaces, and a flange provided with a flange provided on at least one of the inner member and the outer member In the bearing device,
At least the member provided with the flange is made of carbon steel having a carbon content of 0.45% by weight or more and 0.65% by weight or less, and at least the area ratio of proeutectoid ferrite of the flange is 3 to 15%. Characterized by
In the invention according to claim 2, in claim 1, the carbon steel has an S content of 0.003% by weight or more and 0.020% by weight or less, and the S content and an area ratio of the proeutectoid ferrite structure. The relationship is 10 ≦ (S content (% by weight) × 1000 + area ratio (%) of pro-eutectoid ferrite structure) ≦ 30.
[0020]
The invention according to claim 3 is the method according to claim 1 or 2, wherein the area ratio of the pro-eutectoid ferrite structure to the S content is 1 ≦ (S content (% by weight) × 1000) / pro-eutectoid ferrite structure. The area ratio (%) ≦ 2.
The invention according to claim 4, according to any one of claims 1 to 3, wherein the unit area (10 mm²), The area ratio of pro-eutectoid ferrite is 3 to 15%, the maximum length of the pro-eutectoid ferrite is 200 μm or less, and the number of pro-eutectoid ferrites having a length of 180 μm or more is five or less. Features.
[0021]
The invention according to claim 5 is characterized in that, in any one of claims 1 to 4, the member provided with the flange contains 0.05 to 0.3% by weight of V (vanadium).
In addition, the invention according to claim 6 is effective in uniformly improving the metal structure after hot forging in order to improve the rotational bending fatigue strength of the non-heat-treated part where induction hardening is not performed. By optimizing the hot forging method, it has been made based on the knowledge that the object can be achieved, and a plurality of rolling elements are disposed between an inner member and an outer member, A method for producing a flanged bearing device having a flange for attaching to a fixed side or a rotating side to at least one member of the member and the outer member, and having a hardened layer by induction hardening at least around a track portion.
At least the member provided with the flange is made of C: 0.45 to 0.65% by weight, Mn: 0.5 to 1.5% by weight, Si: 0.1 to 1.0% by weight, Cr: 0. 01 to 0.5% by weight, S ≦ 0.025% by weight, O ≦ 15 ppm, the balance being formed by hot forging using alloy steel composed of Fe and unavoidable impurities, and the material heating temperature during the hot forging Is 900 to 1050 ° C, and the temperature at the end of forging is 800 ° C or more.
[0022]
The invention according to claim 7 is characterized in that in claim 6, the average cooling rate from the temperature at the end of forging to 600 ° C. is 0.5 to 5 ° C./sec.
The invention according to claim 8 is the method according to claim 6 or 7, wherein V: 0.01 to 0.2% by weight, Nb: 0.01 to 0.15% by weight, and Ti: 0.01 to 0.15% by weight. Characterized in that an alloy steel containing at least one of the above is used.
[0023]
According to a ninth aspect of the present invention, a plurality of rolling elements are disposed between the inner member and the outer member, and are attached to at least one of the inner member and the outer member on a fixed side or a rotating side. A flanged bearing device having a flange for and having a hardened layer by induction hardening at least around the raceway portion,
It is manufactured using the manufacturing method according to any one of claims 6 to 8, wherein the metal structure at the base of the flange includes a ferrite-pearlite structure, and the prior-austenite grain size of the ferrite-pearlite structure is: 4 or more.
[0024]
Furthermore, the invention according to claim 10 optimizes the alloying element, that is, reduces the C contained in the steel and adds Si and V at a predetermined ratio to thereby reduce the rolling fatigue of the raceway portion subjected to induction hardening. In order to maintain high service life and at the same time to enable high-precision machining without lowering productivity, improve the machinability and drilling workability, and also improve the fatigue strength of non-tempered parts that have not been induction hardened. It is based on the knowledge that it can be improved, a plurality of rolling elements are disposed between the inner member and the outer member, at least one of the inner member and the outer member is provided on the vehicle body side Or with a flange for mounting on the wheel side, in a flanged bearing device for wheel support having a hardened layer by induction hardening at least around the track portion,
At least the member provided with the flange is made up of C: 0.45 to 0.50% by weight, Si: 0.3 to 1.5% by weight, V: 0.03 to 0.3% by weight, Mn ≦ 1. 5% by weight, Cr ≦ 1.0% by weight, S ≦ 0.035% by weight, O ≦ 15ppm, the balance being made of alloy steel comprising Fe and unavoidable impurities, and the value of C + 0.2Si + 0.5V being 0 0.55 to 0.75, the surface hardness of the raceway portion of the hardened layer by the induction hardening is Hv630 to Hv750, and the hardness of the portion not hardened by the induction hardening is Hv220 to Hv300. And
[0025]
The invention according to claim 11 is characterized in that, in claim 10, the proeutectoid ferrite area ratio at least in the non-heat-treated portion at the root of the flange attached to the wheel side is 5% or more and 15% or less.
According to a twelfth aspect of the present invention, in any one of the first to fifth and ninth to eleventh aspects, the inner member is a hub wheel, and a wheel mounting flange as the flange is provided at one end of the hub wheel. In addition, the inner ring is fitted into a small-diameter step formed at the other end of the hub wheel, and a track surface is formed on each of the outer peripheral surface of the inner ring and the outer peripheral surface of the intermediate portion in the axial direction of the hub wheel to form a double row. And the outer member is an outer ring, and a double-row outer raceway surface corresponding to the double-row inner raceway surface is formed on an inner peripheral surface of the outer race, and the wheel mounting of the outer race is performed. A suspension mounting flange is formed at an end on the side away from the use flange, and the plurality of rolling elements are rotatably disposed between the double-row inner raceway surface and the double-row outer raceway surface. It is characterized by having done.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described. The basic configuration of the flanged bearing device of this embodiment is the same as that described with reference to FIG.
(First Embodiment: Corresponding to Claims 1 to 5, 12)
In the bearing device 1 with a flange, a radial load is generated on the bearing from the road surface while the hub wheel 2 is rotating, so that a rotational bending stress is generated at the root of the wheel mounting flange 6. In particular, the outer root portion 14 of the wheel mounting flange 6 is not subjected to heat treatment such as quenching and hardening, and since rotational bending stress is concentrated, there is a possibility that breakage may occur depending on use conditions and design conditions. .
[0027]
On the other hand, as described above, since the hub wheel 2 is required to be reduced in weight, it is desired to reduce the thickness of the wheel mounting flange 6. However, in order to reduce the thickness of the wheel mounting flange 6, it is necessary to improve the fatigue strength of the outer base 14 of the wheel mounting flange 6.
In the outer root portion (non-heat-treated steel) 14 of the wheel mounting flange 6 which is not subjected to heat treatment such as quenching and hardening after hot forging of medium carbon steel, generally, fatigue strength is improved due to an increase in hardness. (See "Metal Fatigue: Influence of Microdefects and Inclusions", Takanori Murakami, Yokendo, 1993, p8). However, since the wheel mounting flange 6 of the hub wheel 2 is subjected to turning and drilling after hot forging, if the hardness is increased more than necessary, the workability is significantly reduced.
[0028]
Thus, the present inventors have earnestly studied a method of increasing the fatigue strength without unnecessarily increasing the hardness and reducing the workability, and as a result, have obtained the following findings.
(1) Improvement in fatigue strength is not limited to hardness.
Although the fatigue strength tends to increase with an increase in hardness, the fatigue strength varied even with the same hardness. As a result of comparison between the two, differences were found in the structures of the materials. In medium carbon steel, cracks occur in the pro-eutectoid ferrite structure, which is the weakest structure against rotational bending stress, and mainly propagate in the pro-eutectoid ferrite grains and the boundary between the pro-eutectoid ferrite and pearlite.
[0029]
That is, it was found that the variation in the fatigue strength was caused by the amount and form of the pro-eutectoid ferrite structure.
(2) Decrease in turning workability is not limited to hardness.
The turning workability tends to decrease as the hardness increases, but the workability varies even with the same hardness. As a result of comparison between the two, differences were found in the amount and shape of inclusions, particularly MnS (manganese sulfide), present in the structure of the material.
[0030]
The hardness of the non-heat treated medium carbon steel is determined by the amount of the proeutectoid ferrite structure. Since the proeutectoid ferrite structure has low hardness and is easily plastically deformed, if the precipitation amount of the proeutectoid ferrite structure is large, the hardness decreases and the workability improves. Similarly, although a small amount, MnS is also easily plastically deformed, so that the turning property is improved, but the hardness is hardly affected.
That is, it was found that the variation in turning workability was caused by the amount and shape of inclusions such as MnS.
[0031]
The present inventors have found a method for improving the fatigue strength of a medium-carbon steel without heat treatment, without unnecessarily increasing hardness and reducing workability, based on the above findings.
As a method of improving the fatigue strength, optimization of the amount and morphology of the proeutectoid ferrite structure is effective. In order to improve the workability, the larger the amount of precipitation of the pro-eutectoid ferrite structure and the amount of dispersion of MnS, the better.
[0032]
The proeutectoid ferrite structure precipitates and grows using fine precipitates such as crystal grain boundaries and carbides as nuclei. Furthermore, there are cases where MnS, which is a nonmetallic inclusion in steel, precipitates and grows from the starting point. As described above, the proeutectoid ferrite structure is easily plastically deformed and weak, so that a proeutectoid ferrite structure that has grown greatly tends to concentrate stress and break easily. In addition, since the pro-eutectoid ferrite structure precipitates and grows along the crystal grain boundary, if it grows large, the pro-eutectoid ferrite structure grows in a mesh-like manner at the crystal grain boundary (hereinafter simply referred to as a net) and surrounds the crystal. In such a state, the structure becomes extremely weak against the rotational bending stress.
[0033]
Therefore, in the present invention, the proeutectoid ferrite structure is finely dispersed and precipitated, whereby the growth of the proeutectoid ferrite structure can be suppressed, and the precipitation in the net state can be prevented. Further, since the pro-eutectoid ferrite structure is dispersed, the stress concentration is also dispersed, so that the structure becomes strong against rotational bending stress. The pro-eutectoid ferrite structure dispersed and precipitated increases fatigue strength and is also effective in workability. Therefore, by finely dispersing the pro-eutectoid ferrite structure, it is possible to simultaneously improve both fatigue strength and workability.
[0034]
Specific production methods for finely dispersing the proeutectoid ferrite structure include (1) adding an alloying element to the material, (2) refining the crystal structure by optimizing hot forging conditions, and (3) MnS And the like in an appropriate amount. Hereinafter, description will be made in the order of (1), (2) and (3).
{Circle around (1)} When precipitates such as carbides are finely dispersed in steel, the growth of crystal grains can be suppressed and the crystal structure can be refined. As described above, the proeutectoid ferrite structure precipitates and grows with fine precipitates such as crystal grain boundaries and carbides as nuclei. In particular, at the intersection (triple point) of the crystal grain boundary as shown in FIG. 2, a pro-eutectoid ferrite structure easily precipitates. Therefore, when the crystal structure is refined, the intersection of the grain boundary increases, and as a result, the pro-eutectoid ferrite structure becomes fine. Can be dispersed. In order to finely disperse precipitates such as carbides in steel, it is necessary to add an appropriate amount of C (% by weight), and it is also effective to add Cr or V, which is a carbide forming element, in a small amount.
{Circle around (2)} As described above, when the crystal structure is refined, the proeutectoid ferrite structure can be finely dispersed as a result. Heating in hot forging activates the atomic diffusion of the metal structure and grows crystal grains. Therefore, it is preferable to lower the hot forging temperature in order to reduce the crystal grain size. The crystal grain size becomes finer as the forging degree increases. However, if the hot forging temperature is too low, the deformation resistance of the material increases, the processing load increases, and when performing forging of a complicated shape at a low temperature, a shape defect or a crack may occur.
[0035]
On the other hand, with regard to cooling after forging, the growth of the proeutectoid ferrite structure is suppressed by increasing the cooling rate. However, if the cooling rate is too high, quench hardening (martensite transformation) occurs, which significantly affects the turning workability after forging or, in some cases, cracks. By optimizing forging conditions including heating and cooling, the crystal grain size can be reduced, and as a result, the proeutectoid ferrite structure can be finely dispersed.
{Circle around (3)} MnS tends to serve as a nucleus for the precipitation of a pro-eutectoid ferrite structure, tends to promote fine dispersion of the pro-eutectoid ferrite structure, and has an effect of improving turning workability. However, for example, in the manufacturing process of the hub wheel, since there is a cold-worked portion indicated by reference numeral 9 in FIG. 1, if MnS is dispersed more than necessary, it may be a starting point of a crack during the cold-working. By dispersing an appropriate amount of MnS, fine dispersion of the proeutectoid ferrite structure can be promoted.
In the present invention, the amount of carbon as a carbide is limited, and the amount of precipitation of a pro-eutectoid ferrite structure is limited by an area ratio, the amount of S added to control the precipitation state of MnS is limited, and furthermore, the pro-eposition which affects the turning workability is determined. By limiting the sum of the amount of precipitation of the ferrite structure and the amount of dispersion of MnS, the medium carbon steel is not heat-treated, does not unnecessarily increase hardness, and improves fatigue strength without reducing workability. Was made possible.
[0036]
According to the literature (“Effects of thermomechanical treatment on fatigue strength of non-heat treated steel for machine structural use”, Heat Treatment Vol. 39, No. 6: Kazue Nomura, edited by Japan Heat Treatment Technology Association), “Ferrite of medium carbon + pearlite type With respect to the rotational bending stress of the non-heat treated steel, fatigue cracks are generated by the stress concentration generated by the pro-eutectoid ferrite on the surface, and mainly propagate within the pro-eutectoid ferrite grains and the pro-eutectoid ferrite / pearlite boundary. In order to improve the strength, it is effective to make the ferrite grains of the crack generation unit finer. "
[0037]
At the base of the wheel mounting flange 6 of the conventional flanged bearing device 1, long and large pro-eutectoid ferrite is precipitated along the crystal grain boundaries, and the frequency of ferrite nucleation during transformation is low, or This is because those having the same crystal orientation were united by the growth.
A ferrite structure precipitated in the form of a film at such austenite grain boundaries easily causes fatigue cracks due to rotational bending stress. In order to reduce the size of ferrite grains, it is effective to reduce the austenite grain size, which determines the number of ferrite nuclei, or to finely disperse carbides and nitrides.
[0038]
However, by merely reducing the average grain size of austenite, the distribution of pro-eutectoid ferrite could not be completely controlled, and cracks at the root of the wheel mounting flange 6 could not be completely suppressed.
The inventors of the present invention have conducted intensive studies for the purpose of further improving the fatigue strength at the base of the wheel mounting flange 6 of the flanged bearing device 1, and as a result, have obtained the following knowledge.
[0039]
First, the distribution state of proeutectoid ferrite in the outer root portion 14 of the wheel mounting flange 6 was examined by an image analysis method described later, and the relationship with the fatigue strength of the outer root portion 14 was examined. As a result, they found that the length of the proeutectoid ferrite and the area ratio of the proeutectoid ferrite dominated the fatigue strength.
Specifically, the unit area (10 mm) of the surface of the outer root 14 of the wheel mounting flange 6²), The area ratio of pro-eutectoid ferrite was 3 to 15%, the maximum length of pro-eutectoid ferrite was 200 μm or less, and the number of pro-eutectoid ferrites having a length of 180 μm or more was 5 or less. The maximum length of the proeutectoid ferrite is preferably 150 μm or less, more preferably 100 μm or 50 μm or less.
[0040]
Next, the reasons for limiting the numerical values of the present invention will be described.
[Area ratio of proeutectoid ferrite structure is 3% or more and 15% or less]
Proeutectoid ferrite is, as described in the above-mentioned literature, a starting point for cracking, and it is preferable that the amount is as small as possible from the viewpoint of induction hardening.However, according to the drilling test described below, Since it also has an effect of providing, the area ratio of proeutectoid ferrite is required to be 3% or more. However, when the area ratio of the proeutectoid ferrite is 15% or more, the fatigue strength decreases due to an increase in soft ferrite. Therefore, the area ratio of the proeutectoid ferrite is set to 3 to 15%.
[0041]
In order to obtain the distribution of proeutectoid ferrite as described above, it is necessary to generate many nuclei at the time of austenite / ferrite transformation in the heat treatment step of the hub wheel 2. It is effective to reduce the austenite grain size by lowering the forging temperature to such an extent as not to hinder the reduction of the austenite grain size by alloying carbide such as V (vanadium), or to form nuclei of ferrite on the alloying carbide.
[0042]
Further, the distribution of proeutectoid ferrite can be controlled by cooling control after forging. For example, when the cooling rate after forging is increased, the growth of ferrite can be suppressed, and the ferrite can be hardly connected in a net shape.
As described above, controlling the distribution of large ferrite grains in the direction that causes stress concentration at the outer root 14 of the wheel mounting flange 6 of the hub wheel 2 results in a smaller minimum unit of crack generation. Fatigue fracture at the base 14 can be effectively suppressed.
[C: 0.45% by weight or more and 0.65% by weight or less]
When the C in the steel of the material of the hub wheel 2 is less than 0.45% by weight, the high frequency quenching hardness of the rolling portion is low, and the strength HRC 58 or more required to provide the rolling fatigue life is obtained. I can't. Further, the hardness of the root portion of the wheel mounting flange 6 cannot be sufficiently obtained, and in particular, the fatigue strength of the outer root portion 14 against rotational bending stress decreases. On the other hand, when C exceeds 0.65% by weight, not only the turning property is reduced due to the decrease in the area ratio of the proeutectoid ferrite structure and the increase in hardness, but also the rolling fatigue life and fatigue strength cannot be significantly improved. C in the carbon steel of the material of the hub wheel 2 was set to 0.45% by weight or more and 0.65% by weight or less.
[S: 0.003% to 0.020% by weight]
S is an element causing sulfide-based nonmetallic inclusions such as MnS, and nonmetallic inclusions tend to increase in proportion to the added amount. As described above, sulfide-based nonmetallic inclusions such as MnS are easily plastically deformed, so that the turning property is improved. In addition, although there is a tendency to promote the fine dispersion of the pro-eutectoid ferrite structure, if it is added more than necessary, MnS becomes too large, and there is a possibility that it becomes a starting point of cracking during cold working.
[0043]
Further, in order to improve the workability and obtain the effect of finely dispersing the pro-eutectoid ferrite structure, the addition amount is required to be 0.003% by weight or more. On the other hand, if it exceeds 0.020% by weight, MnS increases. It is too high and may be a starting point of cracking during cold working.
[10 ≦ (S content (% by weight) × 1000 + area ratio (%) of proeutectoid ferrite structure) ≦ 30]
The turning workability is determined by the amount of precipitate of the pro-eutectoid ferrite structure and the amount of MnS, but the amount of precipitation of the pro-eutectoid ferrite structure varies depending on the hot forging conditions and precipitates even if the amount of carbon is the same. Further, MnS serves as a starting point of precipitation of a pro-eutectoid ferrite structure and also has a function of promoting dispersion precipitation. Therefore, simply specifying the carbon amount and the S addition amount does not serve as a guide for turning workability.
[0044]
Therefore, the present inventors measured the amount of precipitation of the pro-eutectoid ferrite structure by the area ratio, and added the area ratio of the pro-eutectoid ferrite structure by multiplying a constant by the S addition amount in consideration of the effect of MnS. The turning workability coefficient was derived. If the turning workability coefficient is less than 10, the precipitation amount of the proeutectoid ferrite structure and the amount of MnS are insufficient, and sufficient workability cannot be obtained. On the other hand, when the turning workability coefficient exceeds 30, the precipitation amount of the pro-eutectoid ferrite structure becomes too large, and the fatigue limit is reduced.
[0045]
Therefore, the turning workability coefficient is set to 10 or more and 30 or less.
[V: 0.05 to 0.3% by weight]
Further, when the austenite particle size is reduced by V (vanadium) alloy carbide, the amount of V added is preferably 0.05 to 0.3% by weight, and the oxygen concentration of the material of the hub wheel 2 is preferably set to 0.05 to 0.3% by weight. In consideration of the rolling fatigue life of the rolling portion, the content is preferably 12 ppm or less.
[0046]
As described above, in this embodiment, the microstructure of at least the wheel mounting flange 6 of the hub wheel 2 is refined, so that the shape and dimensions of the wheel mounting flange 6 are not changed, and the induction hardened portion is not changed. Without increasing the cost due to the increase, it is possible to increase the strength of the outer base 14 of the wheel mounting flange 6 which is the weakest part of the rotating bending fatigue. As a result, the thickness of the wheel mounting flange 6 can be reduced, and the weight of the flanged bearing device can be reduced.
[0047]
As described above, the amount of carbon to be carbide is limited, the amount of precipitation of the pro-eutectoid ferrite structure is limited by the area ratio, the amount of S added to control the amount of MnS is limited, and further, the pro-eutectoid ferrite structure which affects the turning property By limiting the sum of the amount of precipitation and the amount of MnS, it was possible to improve the fatigue strength of medium-carbon steel without refining, without unnecessarily increasing hardness, and without reducing workability. .
[0048]
Here, the present inventors have found that the fatigue strength of the outer root portion 14 of the wheel mounting flange 6 has a correlation with the ratio between the amount of the pro-eutectoid ferrite structure and the amount of MnS. The sum of the amount of pro-eutectoid ferrite microstructure and the amount of MnS indicates turning properties, but when the balance between the amount of pro-eutectoid ferrite micro-structure and the amount of MnS is lost, the turning property is maintained, but the wheel mounting property is maintained. In some cases, the fatigue strength of the outer root portion 14 of the flange 6 may be reduced.
[0049]
When the amount of precipitation of the pro-eutectoid ferrite structure is too large relative to the amount of MnS, the amount of precipitation increases even if the starting point of precipitation of the pro-eutectoid ferrite structure is reduced. That is, it is understood that the pro-eutectoid ferrite structure has grown greatly and the area ratio has been increased. If the amount of the proeutectoid ferrite structure is too large relative to the amount of MnS, the fine dispersion of the proeutectoid ferrite structure becomes insufficient, and the fatigue strength of the outer root portion 14 of the wheel mounting flange 6 tends to decrease.
[0050]
On the other hand, if the amount of pro-eutectoid ferrite microstructure is too small relative to the amount of MnS, the amount of pro-eutectoid ferrite micro-structure precipitated from the precipitates and the intersections of grain boundaries other than the pro-eutectoid ferrite micro-structure precipitated from MnS is abnormal. Less. There are few places where pro-eutectoid ferrite structure precipitates. That is, the microstructure of the proeutectoid ferrite is insufficiently finely dispersed and the precipitates are insufficient, and the fatigue strength tends to decrease.
[0051]
Therefore, in order to obtain sufficient fatigue strength of the outer root portion 14 of the wheel mounting flange 6 in addition to the turning property, the balance between the amount of pro-eutectoid ferrite structure and the amount of MnS (flange fatigue limit coefficient) is important. . Since the amount of MnS is proportional to the amount of S added, the flange fatigue limit coefficient is expressed by the following formula using the ratio of the amount of S added to the area ratio of the proeutectoid ferrite structure.
[0052]
Flange fatigue limit coefficient = (S content (% by weight) × 1000) / area ratio of proeutectoid ferrite structure (%)
Here, when the flange fatigue limit coefficient exceeds 2, the precipitation amount of the pro-eutectoid ferrite structure is too large with respect to the MnS amount, so that the fatigue strength tends to decrease. Since the precipitation amount of the ferrite structure was too small, the fatigue strength tended to decrease.
[0053]
Therefore, in order to obtain sufficient fatigue limit strength of the outer root portion 14 of the wheel mounting flange 6 in addition to turning property, flange fatigue limit coefficient = (S content (% by weight) × 1000) / proeutectoid ferrite It is desirable that the area ratio (%) of the tissue is 1 or more and 2 or less.
[0054]
[Example A]
(First embodiment)
In order to confirm the effects of the present invention, the materials shown in Table 1 were processed under various hot forging conditions, the microstructure was confirmed, and the amount of proeutectoid ferrite structure was measured by image analysis.
[0055]
[Table 1]

[0056]
In hot forging, after cutting a steel bar, high-frequency induction heating is performed to various temperatures between 950 and 1200 ° C. to change the microscopic level of the structure, and to perform hot forging mainly based on upsetting, It cools at a cooling rate to create various proeutectoid ferrite microstructures.
The microstructure was etched with Pyracool's corrosive solution, the structure was photographed with an electron microscope, and only the proeutectoid ferrite structure was taken out of the base material of the electron microscope image using an image analyzer to calculate the area ratio.
[0057]
Electron microscope: JSM-T220A, manufactured by JEOL Ltd.
Image analyzer: IBAS2000, manufactured by Carl Zeiss
After removing the oxide film by shot blasting after hot forging cooling, various test pieces are created by turning, and cutting tool life test, rotary bending fatigue test, cold working test, foreign material mixed lubrication life test are performed, An evaluation was performed. Each test condition is as follows.
<Cutting tool life test>
Cutting machine: High-speed lathe
Tool: P10 (JIS B 4053)
Cutting speed: 180 to 220 m / sec
Feed amount: 0.2 to 0.3 mm / rev
Cutting depth: 0.6-1.0mm
Each sample was ground under the above conditions according to the cutting tool test method of JIS B 4011, and the time until the flank wear of the cutting tool reached 0.2 mm was defined as the tool life.
<Rotating bending fatigue test>
Testing machine: Ono-type rotating bending fatigue testing machine
Test piece: JIS No. 1-8 test piece (JIS Z 2274)
Rotation speed: 3700min^-1
Stop rotation speed is 10⁷And the condition at which the test load was no longer damaged by changing the test load was defined as the fatigue limit strength.
<Cold work test>
A cylindrical test piece of φ20 × 30 mm is made by turning, cold compression (forging) processing is performed from above and below the cylindrical test piece at an upsetting rate of 80%, and it is checked whether cracks have occurred on the circumference. Was. In each test, ten pieces were processed, and the probability of occurrence of cracks was investigated.
<Life test under contaminated lubrication>
Using the thrust type bearing steel life tester described on pages 10 to 21 of "Special Steel Handbook", 1st edition (edited by Denki Steel Institute, Rigaku Corp., published on May 25, 1969), Using a SUJ2 ball, 20 samples of each sample were tested, and the number of cumulative stress repetitions (life) up to the point of occurrence of flaking was investigated to create a Weibull plot.₁₀Life was determined. Also, the number of stress repetitions is 10⁷If the number exceeds the limit, the total is 10⁷L if exceeds₁₀10 life⁷And
[0058]
Test surface pressure: Maximum 4900MPa
Rotation speed: 1000min^-1
Lubricating oil: $ 68 turbine oil
The track portion of the life test specimen is quenched and tempered by high frequency heating.
Table 2 summarizes the test results. FIG. 3 shows the relationship between the S amount (% by weight) × 1000 + α area ratio (%) and the tool life, and FIG. 4 shows the relationship between the S amount (% by weight) / α area ratio (%) and the fatigue limit strength. Shown respectively. In Table 2 and FIGS. 3 and 4, the area ratio of the pro-eutectoid ferrite structure is indicated as α area ratio for convenience.
[0059]
[Table 2]

[0060]
As is clear from Table 2, FIG. 3 and FIG. 4, in Examples 1 to 21, the amount of S added, the area ratio of the proeutectoid ferrite structure and the turning workability coefficient (S content (% by weight) × 1000 + proeutectoid ferrite structure) Bearing ratio is within the range of the present invention, so that a good flanged bearing is excellent in all of the turning workability (tool life), fatigue limit strength, cold crack occurrence rate and rolling life after quench hardening. Materials for the apparatus can be obtained, and in particular, Examples 5 and 6 in which the flange fatigue limit coefficient = (S content (% by weight) × 1000) / area ratio (%) of proeutectoid ferrite structure are 1 or more and 2 or less. , 8, 9, 17 to 19 have excellent fatigue limit strength.
[0061]
On the other hand, in Comparative Example 1 in which the added amount of S is too high as a material component, cracks starting from MnS occur frequently in the cold compression working. In Comparative Example 2 in which the amount of C added was too low as a material component, the hardness due to induction hardening was reduced, and the rolling life was significantly reduced. In Comparative Example 3 in which the amount of C added was too high as a material component, the area ratio of the pro-eutectoid ferrite structure was too low, and the turning workability coefficient (S content (% by weight) × 1000 + the area ratio of the pro-eutectoid ferrite structure ( %)) Is too low, the turning workability is significantly reduced. Conversely, since the area ratio of the proeutectoid ferrite structure is high, the turning workability coefficient (S content (% by weight) × 1000 + area ratio (%) of the proeutectoid ferrite structure) is too high. In Comparative Example 6 in which the area ratio itself is too high, the microdispersion of the pro-eutectoid ferrite structure is insufficient, so that the fatigue limit strength decreases.
(Second embodiment)
The following experiment was performed to confirm the effects of the present invention.
[0062]
First, using the rod-shaped materials of the components A to D in Table 3, each rod-shaped material was cut, and each was finished to a predetermined shape by high-frequency induction heating to a temperature of 950 to 1200 ° C and hot forging. Then, after forced air cooling or cooling, the oxide film was removed by shot blasting, turning, and induction hardening / polishing of the raceway surface were performed to produce the hub wheel 2 shown in FIG.
[0063]
The hub wheels 2 of Examples 22 to 30 and Comparative Examples 7 to 11 shown in Table 4 were prepared. The hub wheels 2 of Examples 22 to 30 used were subjected to the above-described proeutectoid ferrite distribution control.
[0064]
[Table 3]

[0065]
[Table 4]

[0066]
Next, with respect to each hub wheel 2, the outer root portion 14 of the wheel mounting flange 6 was polished, and then the surface was corroded with Picral to observe the microstructure.
In this observation, first, the microstructure in the visual field was binarized at a microscope magnification of 200 to 500 times to extract only pro-eutectoid ferrite grains, and as shown in FIG. This is given by the maximum value of the distance between any two points on the line, which is referred to as the ferrite length), and the ferrite area ratio was determined by image analysis.
[0067]
Next, a test bearing device (for driving wheel support) for a test shown in FIG. 6 was prepared using each hub wheel 2. Since the basic configuration of this bearing device is the same as that described with reference to FIG. 1, the same reference numerals as in FIG. The axial pitch of the double row rolling elements 5 of this bearing device was 59 mm, the number of balls was 12, the outer ring 4 was made of S53C, and the inner ring 3 and the rolling elements 5 were made of SUJ2. In FIG. 6, reference numeral 15 denotes a receiving member fitted to an end of the outer race 4 on the side of the suspension device mounting flange 11 to receive an axial load.
[0068]
Using the flanged bearing device of FIG. 6 incorporating the hub wheels 2 of Examples 22 to 30 and Comparative Examples 7 to 11, endurance load tests of the outer root portions 14 of the wheel mounting flanges 6 were performed. .
In the test, the bearing device with the flange was subjected to an axial load Fa = 5000 N, a radial load Fr = 6000 to 9000 N, and a rotation speed of 100 min.^-1A rotation test was performed for 50 hours, and after the test, the radial load Fr in which cracks occurred at the outer base 14 of the wheel mounting flange 6 of the hub wheel 2 was determined to be a durable load. The test results were expressed as a ratio to Comparative Example 7.
[0069]
Next, the workability of the wheel mounting flange 6 of the hub wheel 2 was evaluated by a drilling test.
The test conditions are as follows.
Test area: Wheel mounting flange 6
Drill tool: φ8mm, SKH51
Perforation method: Dry type
Drilling depth: 10mm
Cutting speed: 21m / min (840min)^-1)
Thrust: 686N (70kgf)
As for the test results, in comparison with Comparative Example 7 in which the tool life was at the level of the conventional product, those of equal to or greater than "○" and those of less than "X" were evaluated.
[0070]
Table 4 summarizes the above test results.
As is clear from Table 4, the carbon content of the carbon steel as the material of the hub wheel 2 is set to 0.45% by weight or more and 0.65% by weight or less, and the outer root portion 14 of the wheel mounting flange 6 is formed. Unit area (10mm²), The area ratio of the proeutectoid ferrite is 3 to 15%, the maximum length of the proeutectoid ferrite is 200 μm or less, and the number of proeutectoid ferrites having a length of 180 μm or more is five or less. In Examples 22 to 30, favorable results were obtained in the durability load of the outer root portion 14 of the wheel mounting flange 6 and the workability of the wheel mounting flange 6 as compared with Comparative Examples 7 to 11. In addition, in Examples 22 and 24 in which V was added to the material of the hub wheel 2, it can be seen that the durability load performance of the outer root portion 14 of the wheel mounting flange 6 was increased compared to the other Examples 23 and 25 to 30. .
[0071]
On the other hand, Comparative Example 10 in which the maximum length of proeutectoid ferrite exceeds 200 μm is Comparative Example 7 in which five or more ferrite grains having a length of 180 μm or more are present, and Comparative Example 7 in which the ferrite area ratio exceeds the upper limit of the present invention. Example 8, the carbon content of the carbon steel as the material of the hub wheel 2 was high, and the maximum length of the proeutectoid ferrite was within the range of the present invention. However, if left forged, the workability was poor and annealing had to be performed. In each of Comparative Examples 11 in which no wheel was obtained, the endurance load of the outer root portion 14 of the wheel mounting flange 6 was extremely low. Further, in Comparative Example 9, although the durable load of the outer root portion 14 of the wheel mounting flange 6 was increased, the workability of the wheel mounting flange 6 was reduced.
(Second Embodiment: Corresponding to Claims 6 to 9 and 12)
Referring to FIG. 1, the inner member (hub ring 2) or the outer member (outer ring 4) having a flange made of medium carbon steel containing 0.45 to 0.65% by weight of C is used. After hot forging, the metal structure becomes a ferrite-pearlite structure in which proeutectoid ferrite is precipitated in a network along the prior austenite crystal grain boundaries. Since the ferrite structure has a lower strength than the pearlite structure, a metal structure in which pro-eutectoid ferrite is coarsely precipitated may have a low fatigue strength. In order to improve the fatigue strength of the ferrite-pearlite structure of the inner member (hub ring 2) or the outer member (outer ring 4) of the flanged bearing device, the present inventors have proposed a mesh-like structure. It has been found that it is effective to finely disperse the precipitated pro-eutectoid ferrite.
[0072]
Refining the prior austenite grain size has the effect of suppressing stress concentration at the grain boundaries, increasing the number of nucleation sites during austenite / ferrite transformation, and precipitating finely precipitated pro-ferrite grains. When the first ferrite is finely divided, the minimum unit of crack generation becomes smaller, so that fatigue fracture can be effectively suppressed.
Hot forging conditions have a great effect on finely dispersing and precipitating the fermentation ferrite that precipitates in a network.
[0073]
When the material is heated for hot forging, the metal structure becomes an austenitic structure. At this time, the higher the heating temperature of the hot forging, the higher the austenite grain size, the more the diffusion of atoms becomes, and the more the grains grow. In addition, as the amount of plastic working during hot forging increases, the nucleation energy and the number of nucleation sites during recrystallization increase, and the austenite grain size decreases.
[0074]
According to the present invention, having a flange, in the inner member or the outer member, since the amount of plastic deformation during hot forging in the flange is large, the heating temperature during hot forging, by lowering the conventional temperature In addition, the growth of austenite grains is effectively suppressed, and the fatigue strength can be improved. When the heating temperature during forging is 1050 ° C. or higher, the austenite grain size becomes coarse, and the effect of improving fatigue strength is small. On the other hand, when the temperature is lower than 900 ° C., the deformation resistance increases, and the life of the press molding machine and the mold is shortened. Therefore, the heating temperature of the hot forging of the present invention is 900 to 1050 ° C.
[0075]
However, if the hot forging temperature is lower than the conventional temperature, forging cracks are likely to occur. This is because when the temperature is lowered, a part of the metal structure is transformed from an austenitic state to a ferrite-pearlite structure, and when forging is performed in that state, the metal structure is non-uniformly plastically deformed and forging cracks occur. The present invention defines the temperature at the end of forging to prevent the forging crack, so that the metal structure is made to have a uniform austenite state when the forging is completed. If the forging end temperature is less than 800 ° C., the plastic deformation of the metal structure becomes uneven, and forging cracks easily occur. Therefore, the temperature at the end of forging of the present invention is 800 ° C. or more.
[0076]
The reasons for defining the alloy components of the steel used in the present invention are described below.
[C: 0.45% by weight or more and 0.65% by weight or less]
When C in the steel of the material of the hub wheel 2 is less than 0.45% by weight, the induction hardening hardness of the rolling portion is low, and the strength HRC 58 or more required to provide the rolling fatigue life can be obtained. Can not. Further, the hardness of the root portion of the wheel mounting flange 6 cannot be sufficiently obtained, and in particular, the fatigue strength of the outer root portion 14 against rotational bending stress decreases. On the other hand, when C exceeds 0.65% by weight, not only the turning property is reduced due to the decrease in the area ratio of the proeutectoid ferrite structure and the increase in hardness, but also the rolling fatigue life and fatigue strength cannot be significantly improved. C in the carbon steel of the material of the hub wheel 2 was set to 0.45% by weight or more and 0.65% by weight or less.
[Mn: 0.5 to 1.5% by weight]
Mn is an element that improves the hardenability of steel, and if it is less than 0.5% by weight, the hardened layer at the time of induction hardening becomes shallow, so that rolling fatigue of the raceway portion decreases. However, when the content exceeds 1.5% by weight, workability is reduced. Therefore, the Mn content of the present invention is 0.5% by weight or more and 1.5% by weight or less.
[Si: 0.1 to 1.0% by weight]
Si is an element that improves hardenability, strengthens martensite, and improves rolling fatigue life. Further, by dissolving in the ferrite in the non-heat-treated part and improving the strength of the ferrite structure, the fatigue strength of the non-heat-treated part is improved. If the amount is less than 0.1% by weight, the above effect is insufficient. However, when the content exceeds 1.0% by weight, hot forgeability decreases. Furthermore, since decarburization after forging becomes large, cutting work after hot forging is not performed, and the fatigue strength of a portion used on the surface as forged is reduced. Therefore, the amount of Si of the present invention is set to 0.1% by weight or more and 1.0% by weight or less.
[Cr: 0.01 to 0.5% by weight]
Cr has the effect of improving the quenchability, further strengthens the martensite structure after quenching, and improves the rolling fatigue life. If the content is less than 0.01% by weight, the hardened layer at the time of induction hardening becomes shallow, and the strength of the martensite structure also decreases, so that the rolling fatigue life decreases. However, when the content exceeds 0.5% by weight, hot forgeability and machinability deteriorate. Therefore, the Cr content of the present invention is set to 0.01% by weight or more and 0.5% by weight or less.
[S ≦ 0.025% by weight]
S forms nonmetallic inclusions such as MnS in steel, and MnS in steel may be a starting point of forging cracking. Further, in a flanged bearing device in which the inner ring 3 is fixed to the hub wheel 2 by crimping (see reference numeral 9 in FIG. 1), MnS in the non-heat-treated portion may be a starting point of cracking of the crimping portion 9. .
[0077]
For the above two reasons, it is preferable that the amount of S is small. If the amount of S exceeds 0.025% by weight, forging cracks or cracks of the caulked portion 9 may increase. 0.025% by weight or less. Preferably, the amount of S is set to 0.015% by weight or less in consideration of stable prevention of forging cracking and cracking of the caulked portion 9.
[O ≦ 15ppm]
O is an element that greatly affects the rolling fatigue of the induction hardened raceway portion. O is Al in steel₂O₃A non-metallic interposed portion such as the one described above is formed, and serves as a starting point of peeling due to rolling fatigue and shortens rolling fatigue life. Therefore, in order to improve the rolling fatigue life, it is preferable that the O content is small. If the O content exceeds 15 ppm, the rolling fatigue life may be reduced. Therefore, the O content of the present invention is 15 ppm or less.
[0078]
Further, in order to improve the fatigue strength of the non-heat treated part and the rolling life of the raceway part subjected to induction hardening, it is effective to regulate the cooling rate after hot forging within a predetermined range.
The metal structure at the end of hot forging is in an austenitic state, but when cooled, transformation takes place to produce proeutectoid ferrite and a pearlite structure. This transformation is substantially completed at about 600 ° C., and the structure after cooling becomes a ferrite-pearlite structure. At this time, if the cooling rate is low, the growth of pro-eutectoid ferrite is promoted, and a coarse mass of pro-eutectoid ferrite is generated. In this case, since ferrite has a lower strength than pearlite, a coarse pro-eutectoid ferrite lump is likely to be a starting point and a propagation path of fatigue cracks, and the fatigue strength of the non-heat treated portion is reduced. In addition, when induction hardening is performed around the raceway portion, if a coarse pro-eutectoid ferrite lump is present, the hardenability may be reduced or the hardness may be uneven.
[0079]
Therefore, the average cooling rate is defined by the following equation.
Average cooling rate (° C / sec) = (Temperature at end of forging (° C) −600 (° C)) / (Cooling time from end of forging to 600 ° C (second))
If the average cooling rate from the temperature at the end of the forging to 600 ° C. is less than 0.5 ° C./sec, the aforementioned fatigue strength decreases, the induction hardenability decreases, or the hardness after induction hardening. May occur.
[0080]
On the other hand, if the cooling rate is too high, the amount of pro-eutectoid ferrite decreases, so that the hardness increases and the machinability decreases. Further, when an incompletely quenched structure is partially generated, workability is significantly reduced. If the average cooling rate exceeds 5 ° C./sec, the workability is reduced. Therefore, the average cooling rate of the present invention is preferably 0.5 ° C / sec or more and 5 ° C / sec or less. More preferably, the average cooling rate is 1 ° C./sec or more and 3 ° C./sec or less in consideration of stable improvement in fatigue strength and productivity during cutting.
[0081]
Further, when carbides or nitrides are finely dispersed in steel, the growth of crystal grains can be effectively suppressed by the pinning effect thereof, and the same effect as lowering the forging temperature can be obtained. As the alloying element to be added, V, Ti or Nb is preferable as described later.
V, Nb or Ti used in the present invention produces fine carbides or nitrides in steel, and has an effect of suppressing coarsening of austenite crystal grains generated when a steel material is heated during hot forging. The fine carbide or carbonitride of V, Nb, or Ti also has an effect of becoming a precipitation site of proeutectoid ferrite during cooling after hot forging, and promotes fine dispersion of ferrite. Therefore, by adding V, Nb, or Ti, a metal structure in which ferrite is finely dispersed can be obtained, and the fatigue strength of the non-heat treated portion around the flange can be improved.
[0082]
In addition, a raceway portion having a hardened layer formed by induction hardening requires a rolling fatigue life. The metal structure subjected to induction hardening mainly becomes a martensite structure. However, since V, Nb or Ti is added to the steel used in the present invention, fine carbide or carbonaceous material is also contained in the martensite. Nitride is dispersed. When the carbide or carbonitride is finely dispersed, the wear resistance and the hardness are improved, so that the rolling fatigue life is improved.
[0083]
The reasons for limiting the alloy components are shown below.
V forms carbides or nitrides in the steel, suppresses the growth of austenite grains during hot forging, and reduces prior austenite grains. In addition, since the carbides and nitrides of V themselves also serve as precipitation sites for proeutectoid ferrite, proeutectoid ferrite precipitates from the finely dispersed carbides and nitrides, thereby promoting the finely dispersed precipitation of ferrite. In particular, the carbides or nitrides of V present in the former austenite grain boundaries separate the ferrite that precipitates in the former austenite grain boundaries in a network-like manner in order to precipitate pro-eutectoid ferrite from the respective carbide particles or nitride particles. The effect of preventing the fatigue crack from propagating in the ferrite structure and improving the fatigue strength of the non-heat-treated portion having the ferrite-pearlite structure is great.
[0084]
In addition, since the carbide or carbonitride of V is very high in hardness, if it is finely dispersed in the martensite structure of the induction hardened raceway, the wear resistance is improved and the rolling fatigue life is improved. is there. If V is less than 0.01% by weight, the above effects cannot be obtained. When V exceeds 0.2% by weight, hot forgeability, machinability, and grindability decrease. Therefore, the V content of the present invention is preferably from 0.01% by weight to 0.2% by weight. More preferably, the content is 0.02% by weight or more and 0.10% by weight or less in consideration of stable improvement of fatigue strength and productivity.
[0085]
Nb also forms carbides or nitrides in the steel similarly to V, has the effect of suppressing the growth of old austenite grains and the effect of becoming a precipitation site for ferrite ferrite, and therefore has a non-heat treated part having a ferrite-pearlite structure. Has the effect of finely dispersing the pro-eutectoid ferrite and improving the fatigue strength. In particular, Nb has a large effect of suppressing the growth of old austenite grains. If Nb is less than 0.01% by weight, the above effects cannot be obtained. If Nb exceeds 0.15% by weight, hot forgeability, machinability, and grindability decrease. Therefore, the Nb content of the present invention is preferably 0.01% by weight or more and 0.15% by weight or less.
[0086]
Since Ti also forms carbides or nitrides in steel similarly to V, it has the effect of suppressing the growth of old austenite grains and the effect of becoming a precipitation site for pro-eutectoid ferrite, and thus has a non-heat treated part having a ferrite-pearlite structure. Has the effect of finely dispersing the pro-eutectoid ferrite and improving the fatigue strength. In particular, Ti has a large effect of suppressing the growth of old austenite grains. If the Ti content is less than 0.01% by weight, the above effects cannot be exhibited. If Ti exceeds 0.15% by weight, hot forgeability, machinability, and grindability decrease. Therefore, the Ti content of the present invention is preferably 0.01% by weight or more and 0.15% by weight or less.
[0087]
As described above, suppressing the size of the prior austenite particle size to a small value is highly effective in improving the fatigue strength of the non-heat-treated portion. When the crystal grain size is less than 4, the effect of improving the fatigue strength is small.
Therefore, in the present invention, it is preferable that the prior austenite crystal grain size at the root of the flange at which the stress concentration of the bearing device with the flange becomes high is 4 or more.
[0088]
As the rolling elements used in the bearing device with a flange of the present invention, it is preferable to use high-carbon chromium bearing steel such as SUJ2, or the above-mentioned high-carbon chromium bearing steel subjected to carbonitriding. Further, as the shape of the rolling element used in the present invention, a ball or a roller can be used according to the application.
Furthermore, in a flanged bearing device of a type in which the inner ring 3 is fixed to the hub wheel 2 by caulking (see reference numeral 9 in FIG. 1), the inner ring 3 is preferably made of a high carbon chromium bearing steel such as SUJ2.
[0089]
[Example B]
The aforementioned bearing device with flange shown in FIG. 6 was manufactured.
The hub wheel 2 was subjected to hot forging using steel having alloy components shown in Tables A to H under the conditions shown in Tables 6 and 7, and was subjected to forced air cooling or cooling. Then, it is processed into a predetermined shape by cutting, and induction hardening is performed from the peripheral portion of the inner raceway surface 7b to the peripheral portion of the small-diameter stepped portion 8 to form a hardened layer on the surface, and then the finished shape is formed by grinding. I made it. Table 6 also shows the surface hardness of the raceway portion subjected to induction hardening and the hardness of the non-heat treated portion not subjected to induction hardening. The crystal grain size was measured according to JIS G0551.
[0090]
[Table 5]

[0091]
[Table 6]

[0092]
[Table 7]

[0093]
The outer ring 4 was subjected to hot forging at 1100 to 1150 ° C using the conventional material S53C. Thereafter, cutting was performed, and induction hardening was performed around the outer raceway surface 10a and around the outer raceway surface 10b. Thereafter, a grinding process was performed to obtain a final shape. The inner ring 3 and the rolling elements 5 are made of SUJ2, and are hardened from the surface to the core by a normal quenching process.
[0094]
The bearing type of the manufactured bearing device with a flange is a double-row ball bearing having a rolling element pitch diameter of 49 mm, and has 12 balls in each row. Using this bearing device, the flange 11 on the outer ring 4 side was attached to the fixed side, and the flange 6 on the hub wheel 2 side was attached to the rotating side, and a rotation test was performed under the following conditions. Therefore, when a rotation test is performed under these conditions, a bending stress is repeatedly applied to the root of the flange 6 of the hub wheel 2.
[0095]
Radial load Fr: 5000-15000N
Axial load Fa: 5000N
Rotation speed: 400min^-1
A 45-hour rotation test was performed with a predetermined radial load within the above range, and it was confirmed whether or not the bearing vibration had increased or whether or not there was a fatigue crack around the flange. If there was no increase in vibration and no cracks around the flange, the radial load was increased stepwise and a 40-hour rotation test was performed. The radial vibration was increased when the vibration of the bearing increased or fatigue cracks occurred around the flange. The load was taken as the durable load. The results of the rotation test are also shown in Table 6. In addition, the durable load described in Table 6 is represented by a ratio with the rotation test result of Comparative Example 1-9 set to 1.0.
[0096]
In Examples 1-1 to 1-8 shown in Table 6, since the alloy components and hot forging conditions were within the ranges specified in the present invention, a good metallographic structure was obtained, and The rotating bending fatigue strength and the rolling fatigue life of the raceway part subjected to induction hardening are excellent, and good rotation test results are obtained.
On the other hand, in Comparative Examples 1-9 and 1-10, since the material heating temperature (heating holding temperature) during hot forging was higher than the range specified in the present invention, the fatigue strength of the non-heat treated portion was inferior, and the rotation test Resulted in low endurance load. In Comparative Example 1-11, since the material heating temperature during hot forging was lower than the range specified in the present invention, the deformation resistance was large, and the load on the press molding machine and the mold was large, so the processing was stopped. . In Comparative Example 1-12, since the average cooling rate after forging was lower than the range specified in the present invention, coarse proeutectoid ferrite was generated, and the endurance load in the rotation test was low. In Comparative Example 1-13, since the average cooling rate after forging was higher than the range specified in the present invention, the hardness of the non-heat-treated portion was increased, and the machinability was significantly reduced.
[0097]
FIG. 7 shows the relationship between the material heating temperature during forging (heating holding temperature) and the endurance load in the rotation test. In FIG. 7, the comparison is made with the same average cooling rate.
FIG. 8 shows the relationship between the average cooling rate from the end of forging and 600 ° C. to the endurance load in the rotation test. In FIG. 8, the comparison is made with the material heating temperature kept constant at 1000 ° C.
[0098]
As described above, by optimizing the hot forging conditions, it is possible to obtain a flanged bearing device which is excellent in the rotational bending fatigue strength of the non-heat-treated portion around the flange and the rolling fatigue strength of the induction hardened raceway portion.
Next, the hub wheel 2 was similarly manufactured by changing the steel type to assemble the flanged bearing device, and a rotation test was performed under the following conditions.
[0099]
Radial load: 5000-15000N
Axial load: 7000N
Rotation speed: 400min^-1
Table 7 also shows the results of the rotation test. In addition, the durable load described in Table 7 is represented by a ratio with the rotation test result of Comparative Example 2-8 set to 1.0.
[0100]
In Examples 2-1 to 2-7 shown in Table 7, since the alloy components and hot forging conditions were within the ranges specified in the present invention, a good metallographic structure was obtained, and The rotating bending fatigue strength and the rolling fatigue life of the raceway part subjected to induction hardening are excellent, and good rotation test results are obtained. In particular, in Examples 2-4 to 2-7, since V, Nb or Ti was added, the structure of the non-heat-treated portion was refined, and the fatigue strength of the non-heat-treated portion was further increased. Due to the precipitation of nitride, the rolling fatigue of the induction hardened raceway portion is also good.
[0101]
On the other hand, in Comparative Example 2-8, the alloy component is in the range specified in the present invention, but the material heating temperature during hot forging is higher than the range specified in the present invention, so that the fatigue strength of the non-heat treated part is low. Inferior, the endurance load of the rotation test was low. In Comparative Example 2-9, since the amount of S contained in the alloy element was higher than the range specified in the present invention, forging cracks were likely to occur, and processing was stopped because there were cracks after forging. .
[0102]
From the above, by using the steel of the alloy component specified in the present invention, forging under the hot forging conditions specified in the present invention, the rotating bending fatigue strength of the non-heat treated portion around the flange and the raceway portion subjected to induction hardening. A flanged bearing device with excellent rolling fatigue strength can be obtained. Further, by adding V, Nb or Ti, the effect of improving the fatigue strength is further enhanced.
(Third Embodiment: Corresponding to Claims 10, 11, and 12)
The inner member or the outer member having a flange is formed into a predetermined shape by cutting and drilling after being formed by hot forging. Thereafter, induction hardening is performed on a predetermined portion to form a hardened layer, and the raceway portion and the like are finished by grinding.
[0103]
The metal structure of the inner member or the outer member having the flange made of medium carbon steel after hot forging is a ferrite-pearlite structure in which proeutectoid ferrite is precipitated in a network along the prior austenite crystal grain boundaries. In the metallographic structure, it is effective to increase the area ratio of the proeutectoid ferrite and to finely disperse and precipitate the proeutectoid ferrite in order to improve the machinability and drilling workability.
[0104]
According to the present invention, the area ratio of proeutectoid ferrite is increased by reducing the C content of steel as compared with the related art. Further, when V is added, the austenite crystal grains are refined due to the pinning effect of the carbide or nitride of V, so that the precipitation unit of proeutectoid ferrite precipitated along the old austenite crystal grains is refined. Further, since the carbide or nitride of V itself becomes a precipitation nucleus of pro-eutectoid ferrite, the pro-eutectoid ferrite precipitated along the old austenite grain boundary is divided, and the effect of finely dispersing the pro-eutectoid ferrite is obtained.
[0105]
Due to the above effects, good machinability and drilling workability can be obtained due to an increase in the area ratio of proeutectoid ferrite and fine dispersion of proeutectoid ferrite. Preferably, the area ratio of proeutectoid ferrite is 5% or more and 15% or less. The area ratio of proeutectoid ferrite can be controlled to a desired area ratio by controlling the amount of carbon, the forging temperature during hot forging, or the cooling rate after hot forging.
[0106]
In addition, the non-heat treated portion that has not been subjected to induction hardening is used as it is in the metal structure after hot forging. However, the fine dispersion effect of ferrite by the addition of V described above improves the fatigue strength of the non-heat treated portion. Also contributes. The reason is described below.
Refining the prior austenite grain size has the effect of suppressing stress concentration at the crystal grain boundaries. In addition, since ferrite has lower strength than pearlite, there is a high possibility of becoming a starting point or a propagation path of a fatigue crack. Therefore, the minimum unit of the fatigue crack is reduced by finely dividing the proeutectoid ferrite. As described above, fatigue fracture can be effectively suppressed.
[0107]
As another effect, V contributes to precipitation hardening of ferrite, and Si added in the present invention contributes to solid solution strengthening of ferrite. Accordingly, in the ferrite-pearlite structure, the pro-eutectoid ferrite phase, which is a low-strength portion, is strengthened, so that the strength of the weakest part is improved and the fatigue strength is improved.
Due to the above effects, the fatigue strength of the non-heat treated portion is improved by refining the prior austenite crystal grains, finely dispersing the pro-eutectoid ferrite, and strengthening the ferrite.
[0108]
The metal structure of the raceway portion having a hardened layer formed by induction hardening mainly becomes a martensite structure, and a rolling fatigue life is required. Generally, when the C content is reduced, the amount of carbides is reduced and the strength of the base martensite is reduced, so that the rolling fatigue life is reduced. However, since V is added to the steel used in the present invention, fine carbides or nitrides of V are dispersed in martensite. The carbide or nitride of V has a very high hardness, and when it is finely dispersed, the wear resistance and the hardness are improved, so that the rolling fatigue life is improved.
[0109]
Further, Si forms a solid solution in martensite and strengthens the base material of martensite, and thus has an effect of improving the rolling fatigue life. Furthermore, even if tempering is performed after quenching steel with a reduced C content in order to significantly improve tempering resistance, it is possible to maintain good hardness and maintain a good rolling fatigue life because of a small decrease in hardness. .
By the above effects, even if the C amount is reduced, the rolling fatigue life can be favorably maintained by adding a predetermined amount of V and Si.
[0110]
Below, the alloy components of steel used in the present invention and the reasons for limiting the hardness are shown.
C is an element that significantly affects the hardness after hot forging and the hardness after quenching and tempering. If the content is less than 0.45% by weight, the hardness at the time of quenching is insufficient and the rolling fatigue of the raceway portion is insufficient. The life is shortened. Furthermore, since the hardness after hot forging is insufficient, the bending fatigue strength of the non-heat-treated portion also decreases. However, when C exceeds 0.5% by weight, the hardness after hot forging increases, the machinability and drilling workability decrease, and a working time is required to improve the working accuracy. Therefore, when Si is 0.3 to 1.5% by weight and V is 0.03 to 0.3% by weight, the C content of the present invention is 0. 45% by weight or more and 0.50% by weight or less.
[0111]
As described above, Si is an element that strengthens martensite and further enhances tempering resistance, thereby improving the rolling fatigue life. Further, by dissolving in the ferrite in the non-heat-treated part and improving the strength of the ferrite structure, the fatigue strength of the non-heat-treated part is also improved. When C is 0.45 to 0.50% by weight, the above effect is insufficient when Si is less than 0.3% by weight. However, when Si exceeds 1.5% by weight, hot forgeability is reduced. Therefore, the amount of Si of the present invention is set to 0.3% by weight or more and 1.5% by weight or less. Preferably, the Si content is set to 0.65% by weight or more and 1.0% by weight or less in consideration of the stable improvement of the rolling fatigue life and the fatigue strength of the non-heat treated portion and the productivity at the time of hot forging. .
[0112]
V is an important element that improves the fatigue strength of the non-heat treated part and the rolling fatigue life of the induction hardened part, as described above. V reduces the prior austenite grains and further contributes to the fine dispersion of proeutectoid ferrite, thereby improving the fatigue strength of the non-heat treated part. Further, since the carbide or nitride of V has a very high hardness, if it is finely dispersed in the martensitic structure of the induction hardened raceway, the wear resistance is improved and the rolling fatigue life is improved. . If V is less than 0.03% by weight, the above effects cannot be exhibited. When V exceeds 0.3% by weight, hot forgeability, machinability, and grindability decrease. Therefore, the V content of the present invention is set to 0.03% by weight or more and 0.3% by weight or less. Preferably, in consideration of the above effects and costs, the V content is 0.03% by weight or more and 0.1% by weight or less, more preferably 0.05% by weight or more and 0.1% by weight or less.
[0113]
Mn is an element that improves the hardenability of steel. However, if it exceeds 1.5% by weight, the machinability and drilling workability decrease. Therefore, the Mn content of the present invention is 1.5% by weight or less. Preferably, the Mn content is 0.5% by weight or more and 1.0% by weight or less in consideration of productivity during hardening, machinability, and drilling workability.
Cr is an element that improves the hardenability of steel, further strengthens the martensitic structure after quenching, and improves the rolling fatigue life. However, if it exceeds 1.0% by weight, the hot forgeability and machinability become poor. descend. Therefore, the Cr content of the present invention is set to 1.0% by weight or less. Preferably, the Cr content is 0.1% by weight or more and 0.5% by weight or less in consideration of improvement in productivity and rolling fatigue life during quenching and workability.
[0114]
S forms nonmetallic inclusions such as MnS in steel. MnS existing in the induction hardened raceway serves as a starting point of peeling due to rolling fatigue and reduces rolling fatigue life. Further, in a flanged bearing device in which the inner ring 3 is fixed to the hub wheel 2 by crimping (see reference numeral 9 in FIG. 1), MnS in the non-heat-treated portion may be a starting point of cracking of the crimping portion 9. .
[0115]
For the above two reasons, it is preferable that the amount of S is small. If the amount of S exceeds 0.035% by weight, the rolling fatigue life may be reduced or cracks of the caulked portion 9 may be increased. Is 0.035% by weight or less. Preferably, the amount of S is set to 0.020% by weight or less in consideration of securing a stable rolling fatigue life and preventing a crimped portion from cracking.
[0116]
O is an element that greatly affects the rolling fatigue of the induction hardened raceway portion. O is Al in steel₂O₃And other non-metallic inclusions, which serve as starting points for peeling due to rolling fatigue and reduce the rolling fatigue life. Therefore, in order to improve the rolling fatigue life, it is preferable that the O content is small. If the O content exceeds 15 ppm, the rolling fatigue life may be reduced. Therefore, the O content of the present invention is 15 ppm or less.
[0117]
The value of C + 0.2Si + 0.5V indicates the contribution of Si and V to rolling fatigue life when C is reduced. When C is reduced, the rolling fatigue life is reduced, but by adding Si and V, it is possible to suppress a reduction in the rolling fatigue life. However, when the value of C + 0.2Si + 0.5V is less than 0.55, the rolling fatigue life decreases. On the other hand, when it exceeds 0.75, the machinability and the drilling workability are reduced. Therefore, the value of C + 0.2Si + 0.5V in the present invention is 0.55 or more and 0.75 or less. Preferably, the value of C + 0.2Si + 0.5V is 0.60 or more and 0.70 or less in consideration of stable improvement of rolling life and productivity.
[0118]
Further, since the raceway portion of the inner member or the outer member receives a high surface pressure from the rolling elements, a high hardness that can withstand the high surface pressure is required to improve the rolling fatigue life. Therefore, the surface hardness of the raceway portion of the hardened layer formed by induction hardening is preferably Hv630 to Hv750. If the hardness of the raceway surface is less than Hv630, the rolling fatigue life is reduced due to insufficient hardness. On the other hand, if the alloy component specified in the present invention exceeds Hv750, the toughness is reduced and the impact resistance is reduced. Therefore, the surface hardness of the raceway portion of the hardened layer formed by the induction hardening of the present invention is Hv630 or more and Hv750 or less. More preferably, it is Hv700 or more for improving the rolling life.
[0119]
The non-heat-treated part around the flange requires rotational bending fatigue strength. Since the ferrite in the metal structure is finely dispersed and precipitated in the member used for the flanged bearing device of the present invention, the fatigue strength is improved. However, when it is less than Hv220, the fatigue strength of the non-heat-treated part decreases. On the other hand, if it exceeds Hv300, the machinability and drilling workability decrease. Therefore, the hardness of the non-refined part which is not subjected to the hardening treatment by the induction hardening of the present invention is Hv220 or more and Hv300 or less. More preferably, Hv 240 or more and Hv 280 or less in consideration of stable improvement in fatigue strength of the non-heat-treated portion and productivity during cutting and drilling.
[0120]
As the rolling elements used in the present invention, it is preferable to use high carbon chromium bearing steel such as SUJ2, or a carbonitrided high carbon chromium bearing steel. Further, as the shape of the rolling element used in the present invention, a ball or a roller can be used according to the application.
Furthermore, in a flanged bearing device of a type in which the inner ring 3 is fixed to the hub wheel 2 by caulking (see reference numeral 9 in FIG. 1), the inner ring 3 is preferably made of a high carbon chromium bearing steel such as SUJ2.
[0121]
[Example C]
The aforementioned bearing device with flange shown in FIG. 6 was manufactured.
The hub wheel 2 was formed into a predetermined shape by using steel having the alloy components shown in Table 8 and then performing hot forging at 1000 to 1150 ° C., followed by cutting and drilling. Thereafter, induction hardening and tempering were performed from the peripheral portion of the inner raceway surface 7b to the peripheral portion of the small-diameter step portion 8, thereby forming a hardened layer on the surface, and finishing by grinding. Table 8 also shows the surface hardness of the raceway portion subjected to induction hardening and the hardness of the non-heat treated portion not subjected to induction hardening.
[0122]
[Table 8]

[0123]
The outer ring 4 is manufactured using S53C, and the periphery of the outer ring raceway surface 10a and the periphery of the outer ring raceway surface 10b are subjected to induction hardening and have a surface hardened layer. The inner ring 3 and the rolling elements 5 are made of SUJ2, and are hardened from the surface to the core by a normal quenching process.
[Rotation test]
The bearing type of the manufactured bearing device with a flange is a double-row ball bearing having a rolling element pitch diameter of 49 mm, and has 12 balls in each row. Using this bearing device with a flange as a test bearing device, a rotation test was performed under the following conditions.
[0124]
Radial load Fr: 9800N
Axial load Fa: 4900N
Rotation speed: 300min^-1
The service life was determined at the time when the track portion was peeled off or at the time when cracks around the flange were confirmed. Table 8 also shows the life test results obtained from the rotation test. Note that the test life described in Table 8 is expressed as a ratio, with the test life of the conventional product shown in Comparative Example 3-1 being 1.0. FIG. 9 shows the relationship between C + 0.2Si + 0.5V and the test life.
[Processing test]
A drilling test was performed using the hub wheel 2 of the manufactured bearing device with flange. Under the following conditions, the flange 6 was drilled with a hole of φ8 mm and a depth of 13 mm.
[0125]
Drill material: SKH51
Cutting speed: 18mm / min
Feed speed: 0.15mm / rev
After the test, the flank wear of the drill bit was measured. Table 8 also shows the results. The wear amount in Table 8 is expressed as a ratio with the life of the conventional product shown in Comparative Example 3-1 as 1.0.
[0126]
The ferrite area ratio (%) also shown in Table 8 is such that the metal microstructure is revealed by mirror-polishing and etching the cross-section of the non-heat treated portion of the outer base 14 of the flange 6 provided in the hub wheel 2. , Observation photograph of metal microscope 0.1-0.3mm²Was measured by image analysis.
Similarly, the non-tempered part hardness (Hv) also shown in Table 8 is obtained by measuring the cross section of the non-tempered part of the outer base 14 of the flange 6 provided in the hub wheel 2 with a Vickers hardness tester. The track portion hardness (Hv) described in 8 is obtained by cutting out a cross section of the track portion provided in the hub wheel 2 and measuring a portion having a depth of 0.2 mm from the track groove surface with a Vickers hardness measuring machine.
[0127]
In Examples 3-1 to 3-10 shown in Table 8, since the alloy components were within the range specified in the present invention, the rotation test results were equal to or higher than those of the conventional product, and the workability of the drilling test was carried out. Has improved. In particular, in Examples 3-2, 3-3, 3-8, 3-9, and 3-10, the rotation test results were superior to the conventional products, and the workability was particularly improved.
[0128]
On the other hand, in Comparative Examples 3-2 and 3-3, the amount of C or Si contained in the steel was smaller than the range specified in the present invention, and C + 0.2Si + 0.5V was also smaller than the range specified in the present invention. The result is significantly poor.
In Comparative Example 3-4, the addition amount of V was small, so that the result of the rotation test was slightly inferior to that of the conventional product.
[0129]
In Comparative Examples 3-5 and 3-6, since the amount of Si or V contained in the steel was larger than the range specified in the present invention, the result of the rotation test was good, but the workability was significantly reduced. I have.
From the above, the alloy component in the range specified in the present invention, by defining the ferrite area ratio, the hardness of the non-heat treated part, and the hardness of the induction hardened raceway part in a predetermined range, the non-heat treated part A flanged bearing device with excellent workability can be obtained while maintaining good fatigue strength and high rolling life of the induction hardened raceway.
[0130]
Note that the present invention is not limited to the above embodiments, and can be appropriately changed without departing from the gist of the present invention.
For example, in each of the above embodiments, the case where the inner member is the hub wheel 2 is taken as an example, but the present invention may be applied to a flange of the hub wheel using the outer member as a hub wheel.
In each of the above embodiments, the case where the present invention is applied to only the hub wheel 2 is taken as an example. However, the present invention is not limited to this, and the present invention is applied to the inner ring 3 and the outer ring 4 in addition to the hub wheel 2. May be applied.
[0131]
【The invention's effect】
As is evident from the above description, according to the present invention, at least the microstructure of the flange is refined, so that the shape and dimensions of the flange are not changed, and the cost is increased due to an increase in the induction hardened portion. In addition, it is possible to increase the strength of the flange which is the weakest part of the rotating bending fatigue. As a result, the thickness of the flange can be reduced, and the weight of the flanged bearing device can be reduced.
[0132]
In addition, by optimizing the steel composition and hot forging conditions, it is possible to obtain a flanged bearing device with excellent rotational bending fatigue strength of the non-heat treated part around the flange and rolling fatigue strength of the induction hardened raceway part. As a result, the weight of the flanged bearing device can be reduced.
Furthermore, by defining the alloy components in a predetermined range, the ferrite area ratio, the hardness of the non-heat treated part, and the hardness of the orbital part subjected to induction hardening in a predetermined range, the fatigue strength of the non-heat treated part, A flanged bearing device with excellent workability can be obtained while maintaining the rolling life of the induction hardened raceway portion well.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an example of a bearing device with a flange.
FIG. 2 is an explanatory diagram of intersections of crystal grain boundaries.
FIG. 3 is a graph showing a relationship between S amount (% by weight) × 1000 + α area ratio (%) and tool life.
FIG. 4 is a graph showing a relationship between S content (% by weight) / α area ratio (%) and fatigue limit strength.
FIG. 5 is a photomicrograph of the binarized microstructure in the field of view, extracting only proeutectoid ferrite grains.
FIG. 6 is a cross-sectional view of a bearing device with a flange as a testing machine used in Examples.
FIG. 7 is a graph showing a relationship between a material heating temperature during forging (heating holding temperature) and a durable load in a rotation test.
FIG. 8 is a graph showing a relationship between an average cooling rate from the end of forging and 600 ° C. to a durable load in a rotation test.
FIG. 9 is a graph showing the relationship between C + 0.2Si + 0.5V and test life.
[Explanation of symbols]
1: Bearing device with flange
2. Hub ring (inner member)
3. Inner ring
4: Outer ring (outer member)
5 ... rolling element
6 ... Wheel mounting flange
7a, 7b ... inner ring raceway surface
8 ... Small diameter step
10a, 10b ... outer ring raceway surface
11 ... suspension mounting flange
14 ... outer base

Claims (12)

An inner member having a raceway surface on an outer surface, an outer member having a raceway surface facing the raceway surface of the inner member on the inner surface and disposed outside the inner member, and between the two raceway surfaces. In a bearing device with a flange provided with a rolling element rotatably arranged and a flange provided on at least one of the inner member and the outer member,
At least the member provided with the flange is made of carbon steel having a carbon content of 0.45% by weight or more and 0.65% by weight or less. A bearing device with a flange, characterized in that: The S content of the carbon steel is 0.003% by weight or more and 0.020% by weight or less, and the relationship between the S content and the area ratio of the proeutectoid ferrite structure is 10 ≦ (S content (weight %) × 1000 + area ratio (%) of pro-eutectoid ferrite structure ≦ 30. The area ratio of the proeutectoid ferrite structure with respect to the S content has a relationship of 1 ≦ (S content (% by weight) × 1000) / area ratio (%) of the proeutectoid ferrite structure ≦ 2. The bearing device with a flange according to claim 1 or 2, wherein: The area ratio of pro-eutectoid ferrite per unit area (10 mm ² ) of the root portion in the thickness direction outside of the flange is 3 to 15%, the maximum length of the pro-eutectoid ferrite is 200 μm or less, and further 180 μm. The flanged bearing device according to any one of claims 1 to 3, wherein the number of proeutectoid ferrites having the above length is five or less. The flanged bearing device according to any one of claims 1 to 4, wherein the member provided with the flange contains 0.05 to 0.3% by weight of V (vanadium). A plurality of rolling elements are arranged between the inner member and the outer member, and at least one of the inner member and the outer member has a flange for attaching to a fixed side or a rotating side, and at least In a method for manufacturing a flanged bearing device having a hardened layer by induction hardening around a raceway portion,
At least the member provided with the flange is made of C: 0.45 to 0.65% by weight, Mn: 0.5 to 1.5% by weight, Si: 0.1 to 1.0% by weight, Cr: 0. 01 to 0.5% by weight, S ≦ 0.025% by weight, O ≦ 15 ppm, balance is formed by hot forging using alloy steel composed of Fe and unavoidable impurities, and the material heating temperature during the hot forging A method for manufacturing a bearing device with a flange, wherein the temperature at the end of forging is 800 ° C. or more. The method for manufacturing a bearing device with a flange according to claim 6, wherein the average cooling rate from the temperature at the end of forging to 600 ° C is 0.5 to 5 ° C / sec. V: 0.01 to 0.2% by weight, Nb: 0.01 to 0.15% by weight, and Ti: 0.01 to 0.15% by weight. The method for manufacturing a bearing device with a flange according to claim 6 or 7, wherein: A plurality of rolling elements are arranged between the inner member and the outer member, and at least one of the inner member and the outer member has a flange for attaching to a fixed side or a rotating side, and at least In a flanged bearing device having a hardened layer by induction hardening around the raceway,
It is manufactured using the manufacturing method according to any one of claims 6 to 8, wherein the metal structure at the root of the flange includes a ferrite-pearlite structure, and the prior-austenite crystal grain size of the ferrite-pearlite structure is: A bearing device with a flange, wherein the number is four or more. A plurality of rolling elements are disposed between the inner member and the outer member, and at least one of the inner member and the outer member has a flange for attaching to a vehicle body side or a wheel side, and at least In a flanged bearing device for wheel support having a hardened layer by induction hardening around the raceway portion,
At least the member provided with the flange is made up of C: 0.45 to 0.50% by weight, Si: 0.3 to 1.5% by weight, V: 0.03 to 0.3% by weight, Mn ≦ 1. 5% by weight, Cr ≦ 1.0% by weight, S ≦ 0.035% by weight, O ≦ 15ppm, the balance being made of alloy steel comprising Fe and unavoidable impurities, and the value of C + 0.2Si + 0.5V being 0 0.55 to 0.75, the surface hardness of the raceway portion of the hardened layer by the induction hardening is Hv630 to Hv750, and the hardness of the portion not hardened by the induction hardening is Hv220 to Hv300. Bearing device with flange. The flanged bearing device according to claim 10, wherein an area ratio of proeutectoid ferrite in a non-heat-treated portion at least at a root portion of the flange attached to the wheel side is 5% or more and 15% or less. The inner member is a hub wheel, a wheel mounting flange as the flange is provided at one end of the hub wheel, and the inner ring is fitted into a small diameter step formed at the other end of the hub wheel. A track surface is formed on the outer peripheral surface of the inner ring and the outer peripheral surface of the intermediate portion in the axial direction of the hub wheel to form a double-row inner ring raceway surface, and the outer member is used as an outer ring, and Forming a double-row outer raceway surface corresponding to the double-row inner raceway surface, and forming a suspension mounting flange at an end of the outer race that is separated from the wheel mounting flange; The flange according to any one of claims 1 to 5, wherein the plurality of rolling elements are rotatably disposed between a raceway surface and the double-row outer raceway surface. With bearing device.

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