JP2004076823A - Rolling device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ball screw device capable of having an excellent wear resistance and being suitably used even when a high load is applied momentarily and in a short stroke service condition. <P>SOLUTION: At least a screw shaft 1 of the ball screw device is formed of an alloy steel containing C of 0.15 to 0.6 wt%, Si of 0.1 to 0.9 wt%, Mn of 0.1 to 1.5 wt%, and Cr of 0.5 to 4.0 wt%. In an exterior thread groove 2 of the screw shaft 1 complete after carbonizing or carbonitrizing, a total area ratio of carbide, nitride and carbonitride (hereinafter referred to as carbide, etc.) in the range from the surface to 50μm is 10% to 30%, and an area ratio of carbide, etc. with grain diameter of 1μm or less is the total area of 60% or more. Vickers hardness in the range of the rolling element diameter of 2% from the surface is Hv 700 or more, and the maximum grain diameter of the carbide, etc. is 5μm or less in a circle equivalent diameter. <P>COPYRIGHT: (C)2004,JPO

Description

【００３８】
表２の実施例および比較例の合金鋼に以下に述べるような熱処理を施した。なお、焼入れ・　焼戻しの熱処理条件を下記の条件範囲内で変えることで、　析出炭化物の表面積率や表面硬さ等の調整を行った。なお、表２の鋼種の欄は表１の合金鋼Ｎｏに該当する。
熱処理条件Ａ
浸炭窒化：８６０〜９２０°Ｃ保持後常温まで空冷または徐冷
雰囲気：Ｒｘガス＋エンリッチガス＋アンモニアガス
焼入れ：８２０〜８８０°Ｃ保持後油冷
焼戻し：　１６　０〜２４０°Ｃ２　〜３　時間保持
熱処理条件Ｂ
浸炭窒化：９２０〜９８０°Ｃ保持後常温まで空冷または徐冷
雰囲気：Ｒｘガス＋エンリッチガス＋アンモニアガス
焼入れ：８２０〜８８０°Ｃ保持後油冷
焼戻し：　１６　０〜２４０°Ｃ２　〜３　時間保持
熱処理条件Ｃ
浸炭窒化：９２０〜９８０°Ｃ保持後常温まで放冷
雰囲気：Ｒｘガス＋エンリッチガス
焼入れ：８２０〜８８０°Ｃ保持後油冷
焼戻し：　１６　０〜２４０°Ｃ２　〜３　時間保持
（寿命試験）
高負荷・短ストロークの使用条件下での疲労寿命を評価するために、　ボールねじ装置を作製した。作製したボールねじ装置の仕様は次の通りである。
【００３９】
ねじ軸径：８０ｍｍ
リード：２０ｍｍ
ねじ軸材：表１に示す合金鋼を上記熱処理条件Ａ〜Ｃで熱処理
ナット材：ＳＣＭ４２０浸炭
ボール材：　ＳＵＪ２ずぶ焼き
セパレータ：合成樹脂製
熱処理が完了したねじ軸やナットおよびボールには研削仕上げを施し、さらにボ−ルにはラップ仕上げを行った。以上のようにして作製したボールねじ装置を供試体として、　射出成形機を想定した試験条件で試験を行った。試験条件の詳細を以下に示す。
【００４０】
荷重：最大２　００ｋＮ
ストローク：２０ｍｍ
サイクルタイム：３秒
潤滑材：鉱油系グリース
試験温度：８０°Ｃ（ナット外形をヒータで加熱）
表２に実施例Ａ−１〜Ａ−２８及び比較例Ｂ−１〜Ｂ−７の試験片の表面から５　０μｍの位置（最表面層）における炭化物、窒化物及び炭窒化物の合計面積率とこれらの粒のうち粒径が１μｍ以下となるものの面積率を、また、表面からボール径（Ｄａ）の２％の位置（表面層）におけるビッカース硬さ、　円相当径で５μｍを超える炭化物、窒化物及び炭窒化物の存在有無、Ｃ濃度、　Ｎ濃度、残留オーステナイト量を、さらに摩耗試験と寿命試験の結果について示した。
【００４１】
なお、表２中には炭化物、窒化物及び炭窒化物を炭化物等と表記した。
【００４２】
【表２】

【００４３】
炭化物等の面積率測定は、表面から５０μｍまでの断面のうち任意の位置について、５０００倍の倍率でＳＥＭ観察し、２０視野分を画像解析して円相当径で０．１μｍ以上のものを抽出して算出した。
また、粗大炭化物の確認については、表面から２％Ｄａ位置までの断面のうち任意の位置について、１０００倍の倍率でＳＥＭ観察し、４０視野分について円相当径で５μｍを超える炭化物の有無について確認を行った。
【００４４】
Ｃ濃度及びＮ濃度の測定にはＥＰＭＡ（Ｅｌｅｃｔｏｒｏｎ　Ｐｒｏｂｅ　Ｍｉｃｒｏａｎａｌｙｓｅｒ）を、残留オーステナイト量はＸ線分析法を、また表面硬さの測定にはマイクロビッカース硬さ試験機を用い、それぞれ表面から２％Ｄａの位置における測定値を示した。
寿命試験は、ねじ軸の回転トルクもしくは振動が急激に増大するまでのショット数で評価し、比較例Ｂ−７のショット数を１とした比で示した。また、摩耗量に関しては１０万ショット時における軸の軌道溝の形状を測定し、最大摩耗深さを求め、　比較例Ｂ−７の摩耗深さを１として示した。なお、比較例Ｂ−７はＳＣＭ４４５に高周波焼入れを施し、ねじ軸の表面硬さをＨｖ６５１としたものである。
【００４５】
寿命試験の結果から、本発明による実施例Ａ−１〜Ａ−２８は各比較例と比較して優れた耐摩耗性と疲労寿命を有していることがわかる、また、　各実施例の中でも、　浸炭を行った実施例Ａ−１２〜Ａ−１４と、　それぞれ同じ鋼種を浸炭窒化した実施例Ａ−９、　Ａ−２３、　Ａ−２５では、浸炭窒化を行った方が耐摩耗性および疲労寿命が向上しており、　浸炭窒化を行う方がより好ましいといえる。
【００４６】
さらに、Ｖを０．６重量％以上２．０重量％以下含有し、９２０°Ｃ以上で浸炭窒化を行い、雄ねじ溝の完成品表面層のＮ濃度を０．１５重量％以上０．４重量％以下とした実施例Ａ−１７〜Ａ−２８では、　炭化物や窒化物および炭窒化物が微細に析出し、粒径１μｍ以下となる炭化物等の合計面積率が７０％を越えるため、特に優れた耐摩耗性が得られる。その中でも表面層におけるビッカース硬さがＨｖ７５０以上となる実施例Ａ−１７及びＡ−２０〜Ａ−２８においては、疲労寿命が比較例Ｂ−７の６倍以上と非常に優れていることがわかる。
【００４７】
比較例Ｂ−１およびＢ−２は、　最表面層において粒径１μｍ以下である炭化物等の面積率が６　０％を越え、　表面層のビッカース硬さがＨｖ７００以上となるため、摩耗量は比較的少ないが、　粒径５μｍを超える粗大な炭化物が存在したため、転動寿命は比較例Ｂ−７と比較して低下した。
比較例Ｂ−３〜５は表面層のビッカース硬さがＨｖ７００を下回ったため、　転動疲労寿命は向上せず、特に表面層において粒径が５μｍ以上の炭化物が見られた比較例Ｂ−５　は非常に短寿命となった。
【００４８】
また、比較例Ｂ−６は実施例Ａ−２１にセパレータを使用しなかった例であるが、　セパレータを使用しないとボール同士が競り合い、摩耗してしまうため、比較的早期に使用停止に至ることが確認された。
次に、ねじ軸の強化に加えてボールを強化したボールねじ装置を作製した。特に電動射出成形機や電動プレス機に使用されるボールねじ装置は負荷荷重が大きく、ボールと軌道溝との接触楕円が大きくなるため、　接触楕円がねじ溝端部からはみ出る、所謂ボールの乗り上げが問題となることがある。ボールの乗り上げが起こると、ねじ溝の端部によってボールが傷付いたり摩耗したりするため、　ボールを強化しておくことも必要である。
【００４９】
ボールには表１に示した材料に浸炭または浸炭窒化を行ったもの以外に、表３に示した材料に窒化処理を施したものも使用した。
【００５０】
【表３】

【００５１】
窒化処理に用いる素材については、Ｃ＋Ｎ：　０．　４５重量％以上１．３重量％以下含有し、Ｃｒ，Ｍｏ，Ｗ，Ｖ，Ｎｂ，Ｔｉ，Ａｌのうち少なくとも一種類以上添加し、必要に応じて添加する元素としてＮｉ，Ｃｏ，Ｎを含み、　その他Ｆｅ及び製鋼上必要な元素と不可避不純物を含む鋼を母材とするとよい、
以下に数値限定の臨界的意義と考慮すべき事項について述ぺる。
（Ｃ＋Ｎ：０．４５重量％以上１．３重量％以下）
ＣやＮは、基地をマルテンサイト化することにより、　焼入れ・　焼戻し後の硬さを向上させるために必要な元素である。また、　ＣｒやＶやＭｏ等の炭化物や窒化物を形成し、硬さや耐摩耗性の向上に寄与する。転動体として必要な芯部硬さを確保するためには、　Ｃ＋Ｎの合計含有量で０．４５重量％以上添加することが好ましい。特にＣの代わりにＮを添加するとＣ添加量を低く抑えられるため、共晶炭化物サイズを抑えることができるという効果がある。
【００５２】
Ｃを、１．３重量％を超えて添加すると、鋼中に巨大な共晶炭化物が生成しやすくなり、　転動疲労寿命を低下させる不具合がある。さらにはＭｓ点の低下により熱処理後の残留オーステナイトが過剰に生成し、転動体の芯部硬さが十分に得られない場合がある。また、Ｎを製鋼時の段階で０．２重量％を超えて添加すると、凝固の際に気泡が発生して鋼塊に多数の気孔が導入されることがあるため、母材のＮ添加量を好ましくは０．２重量％以下、さらに好ましくは０．１５重量％以下とする。以上の理由から、　Ｃ＋Ｎは０．４５重量％以上１．３重量％以下とすることが好ましく、さらには０．６重量％以上０．９重量％以下とすることが好ましい。
（Ｃｒ，Ｍｏ，Ｗ，Ｖ，Ｎｂ，Ｔｉ，Ａｌのうち、少なくとも一種類以上添加する）
Ｃｒ，Ｍｏ，Ｗ，Ｖ，Ｎｂ，Ｔｉ，Ａｌはいずれも窒化処理によって硬度が高い窒化物や炭窒化物を形成し、耐摩耗性を向上させるために有用な元素であるため、少なくとも一種類以上添加されることが好ましい。また、窒化処理は例えば５００°Ｃ以上で行われることが多く、窒化処理が行われた部材の芯部硬さは低下する。転動体は高い面圧を支承するために、　高い表面硬度を有することが必要であると同時に、芯部の硬度が低いと、高い接触圧力を受けた場合に窒化層の破損を招くことがあるため、芯部硬さも一定以上とすることが必要である。芯部硬さは少なくともＨｖ５５０以上、好ましくはＨｖ６５０以上とすることが好ましい。
【００５３】
鋼中に添加されるＣｒ，Ｍｏ，Ｗ，Ｖ等は窒化処理前の熱処理で炭化物を形成し、　硬さの向上に寄与するだけでなく、窒化処理中に二次硬化を起こして、芯部硬さの低下を妨げる効果もある。
以上のように、窒化後の表面硬度を高めて耐摩耗性を与えると共に、窒化処理後の転動体の芯部硬さを一定以上に保つために、これらの元素を一定量以上添加する必要がある。本発明者らは、以下に示すＣｒ，Ｍｏ，Ｗ，Ｖ，Ｎｂ，Ｔｉ，Ａｌの含有量の関係式Ｅｑ１が、２．０≦Ｅｑ１を満たせば、　十分な添加効果が得られることを見出した（表３参照）。また、下記のＥｑ１値が４．０重量％以上であれば転動体表面にさらに安定な窒化層が得られ、良好な耐摩耗性が得られる。
【００５４】
Ｅｑ１＝Ｃｒ重量％＋０．８５Ｍｏ重量％＋０．４８Ｗ重量％＋Ｖ重量％＋０．４５Ｎｂ重量％＋０．８９Ｔｉ重量％＋１．２Ａｌ重量％
さらに、Ｃｒ，Ｍｏ，Ｗ，Ｖ，Ｎｂ，Ｔｉ，Ａｌの添加量は以下の条件を満たすことが好ましい。
Ｃｒは焼入れ性、焼戻し軟化抵抗性を高め、さらに炭化物を形成して熱処理時の結晶粒粗大化を抑制し、耐摩耗性を付与する等の作用がある。また、窒化処理を行うとＣｒの窒化物や炭窒化物が析出し、耐摩耗性が向上する。しかし、１８．０重量％を超えて添加すると、母材の熱伝導度の低下による耐焼付き性の悪化や、δフェライト生成による靭性の低下が引き起こされる懸念がある。以上のことからＣｒ含有量は１８．０重量％以下、さらには１６．０重量％以下とすることが好ましい。また、転動体の安定した表面硬さを得るために、好ましくは２．０重量％以上、さらに好ましくは４．０重量％以上添加することが望ましい。
【００５５】
Ｍｏは焼入れ性及び焼戻し軟化抵抗性を著しく増大させる元素であり、耐摩耗性向上に寄与する炭化物を析出する。また、窒化処理により窒化物や炭窒化物を形成して硬度や耐摩耗性を高める作用がある。必要に応じて添加してよいが、過剰に添加してもその効果が飽和するだけでなく、靭性や加工性が低下するため、Ｍｏの添加量は７．０重量％以下、　より好ましくは５．０重量％以下とする。
【００５６】
ＷはＭｏと同様に焼入れ性を向上させ、さらに、　硬度の高い炭化物を形成することにより耐摩耗性を向上させる。また、窒化処理により、上記元素と同様に窒化物や炭窒化物を形成するため、必要に応じて添加してもよいが、加工性や靭性が低下するため、Ｗの添加量は１　０．　０重量％以下、さらに好ましくは５．　０重量％以下とする。
【００５７】
Ｖは強力な窒化物生成元素であり、硬度と耐摩耗性を高める作用がある。また、二次硬化に対する効果が大きく、窒化処理後の芯部硬さを一定以上に保つために非常に効果的な元素である。そのため、必要に応じて添加してよいが、多量に添加すると靭性や加工性が低下するため、添加量の上限を５．０重量％以下、　さらに好ましくは３．０重量％以下とする。
【００５８】
Ｎｂも強力な窒化物生成元素であり、窒化処理を行って耐摩耗性を向上させるのに有用な元素である。しかしながら、多量に添加すると靭性や加工性が低下するだけでなく、転動寿命を低下させる粗大なＮｂ介在物が生成するため、その上限は１．０重量％以下とする。
Ｔｉも上記添加元素と同様に、　窒化処理によって窒化物や炭窒化物を形成する元素であり、　必要に応じて添加してもよいが、添加量が０．５重量％を超えると転動寿命低下を引き起こす粗大なＴｉ介在物が生成しやすくなるため、　その含有量は０．５重量％以下とする。
【００５９】
ＡｌはＶ同様、強力な窒化物生成元素であり、　非常に硬度が高く摺動性に優れるＡｌ窒化物を析出する。しかし、その添加量が２．０重量％を超えると添加効果は飽和するだけでなく、Ａｌは製鋼時の脱酸剤として使用される元素でもあり、　鋼中に多量に含有させると粗大なＡｌ酸化物が多数残留し、転動寿命を低下させる虞れがある。以上のことから、　Ａｌ添加量は２．０重量％以下とする。
［必要に応じて添加する元素について］
Ｎｉは強力なオーステナイト安定化元素であり、δフェライトの生成を抑制し、さらに基地に固溶して靭性を向上させ高温特性を高める作用がある。しかし、必要以上に添加すると多量の残留オーステナイトが生成して十分な焼入れ硬さが得られなくなることがあるので、上限を４．５重量％以下とする。
【００６０】
ＣｏもＮｉと同様にオ―ステナイト安定化元素であり、δフェライトの生成を抑え、さらに基地中に固溶して炭化物の凝集を抑制し、高温硬さを向上させる作用がある。しかし、多量に添加すると加工性が低下するだけでなく、　添加コストが高くなるので上限を１　０．　０重量％以下とする。
［製鋼上必要な元素］
Ｓｉは、製鋼時の脱酸剤として必要な元素であり、０．１重量％以上添加されることが好ましい。また、焼戻し軟化抵抗性を高め、さらに表面に窒化処理を施すと窒化物を形成し、耐摩耗性向上に寄与する。しかし、多量に添加すると靭性や加工性を低下させるため上限を１．５重量％とする。
【００６１】
Ｍｎは脱酸剤として０．１重量％以上必要であるが、多量に添加すると鍛造性、被削性が低下するだけでなく、Ｓ，Ｐなどの不純物と共存して耐食性を低下させるので上限を１．５重量％とする。
［不可避不純物について］
上記に述べた合金元素の他に、不可避の不純物として、Ｐ≦０．　０２　重量％、Ｓ≦０．　０５重量％、　Ｃｕ≦０．　１０重量％、Ｏ≦１５ｐｐｍ等を含むことができるが、転動疲労寿命特性に有害な非金属介在物を極力少なくするためには、Ｏ≦１０ｐｐｍとすることが好ましい。
（共晶炭化物について）
鋼中に粗大な共晶炭化物が含まれると靭性が低下するだけでなく、窒化層中に粗大な共晶炭化物が存在した場合は、仕上げ加工中に窒化層が剥離することがある。そのため、鋼材の最大共晶炭化物径は円相当径で５μｍ以下、さらに３μｍ以下とすることが好ましい。
（表面硬さについて）
転動体完成品の表面硬さについては、表面硬さがＨｖ９　００に到達しない場合は、窒化処理による十分な効果は得られないため、表面の硬さは少なくともＨｖ９　００以上とする。また、　好ましくはＨｖ１０００以上とする。なお、本発明でいう表面硬さとは、完成品ボール表面をマイクロビッカース硬さ試験機を用い２００ｇの荷重で測定した値を指す。
（芯部硬さについて）
転動体完成品の芯部硬さはボール全体の剛性のため、一定以上の値としておくことが必要である。少なくともＨｖ５５０以上であることが好ましく、　より好ましくはＨｖ６００以上とする。
（窒化層について）
窒化処理を行うと、ボ−ル表面には二層構造を有する窒化層が形成される。最表面にはζ相−Ｆｅ_２Ｎ、　ε相−Ｆｅ_２−３、γ′相−Ｆｅ_４Ｎ、ＣｒＮ、Ｃｒ_２Ｎ等の化合物からなる化合物層が形成され、それよりも深い部分には、　窒素が固溶したマルテンサイト相に上記の化合物層を形成する窒化物が分散した、拡散層が形成されるようになる。しかし、最表面に形成される化合物層は摺動性に優れる反面、ポーラスで脆い層が形成されることがあり、　特に高荷重が負荷される電動射出成形機や電動プレス機等に使用されるボールねじ装置の場合では、　このポーラスな化合物層の剥離が懸念される。よって、このようなポーラスな化合物層が形成された場合は窒化処理後の仕上げ加工で除去することが望ましい。窒化処理による確実な効果を得るためには、　完成品表面の窒素濃度を０．５重量％以上とすることが好ましい。また、窒素濃度が６重量％を超えた場合は窒化物のみの化合物層となることから、好ましくは表面窒素濃度を０．５重量％以上６重量％以下とする。
【００６２】
また、高荷重を受けるような場合では、　応力体積に占める窒化層の割合が大きいほどボール表面の剛性が増し、結果として耐荷重性が向上する。そのため、できるだけ窒化層の厚さを大きくとることが好ましいが、必要以上の厚い窒化層を設けることは窒化処理時間の延長につながり、処理コストおよび窒化処理後の真球度低下による加工コストが増大する。さらには表面組織が粗くなって要求精度を満足できない場合もあるため、必要以上に窒化層を厚くすることは好ましくない。よって完成品の窒化層厚さはボール直径Ｄａの２　％以下とすることが好ましい。
【００６３】
また、窒化層厚さが少ないと耐摩耗性や耐焼付き性が向上しない。窒化処理前の真円度や粗さ、そして窒化層の付き回り性を考慮すると、少なくとも窒化層厚さは１０μｍ以上とすることが好ましく、さらに好ましくは２０μｍ以上とする。なお、ここでいう窒化層厚さとは窒素濃度が０．２重量％を超える部分の表面からの厚さを指す。
（ボールの作製方法について）
以下に窒化処理を施したボールの作製方法について述べる。
【００６４】
ボ−ル５の母材には本発明の合金鋼を使用する。合金鋼に冷間引き抜きなど施した線材に、　ヘッダー等で冷間加工を施し、球体成形を行う（　以下素球という）　。また、線材を転造などの方法で素球を形成してもよい、上記の方法で作製された素球にバリ等が見られる場合は、フラッシング等でバリ取りを行う。
次いで、焼入れ・　焼戻し等の熱処理を行う。なお、焼戻しの前に、　サブゼロやクライオ処理を行うようにしてもよい。また、　取り扱い上の表面キズ発生を防止するため、熱処理後にバレル、　ショットピーニング或いはボールピーニング等の機械的硬化加工によって、更に硬度を高めても良い。
【００６５】
一般に、熱処理後の真球度あるいは直径相互差等は非常に大きい。この焼入れされたままの状態のものを使用して窒化処理を行うと、窒化層は被処理物の形状に倣った形で生成するため、仕上げ加工後の窒化層厚さが不均一となる。また、窒化処理によって生じた応力バランスが崩れて目標精度が達成できないという不具合が発生する場合もある。そのため、焼入れ硬化後には研削加工（粗研削加工）などを行うことが好ましい（以下、半加工球と呼ぶ）　。半加工球の真球度は３．０μｍ以下、好ましくは１．０μｍ以下とすることが望ましい。なお、半加工球は完成品寸法に設定取り代を加算した目標寸法まで加工を行う。設定取り代は、目標とする精度まで仕上げ加工を行う際の必要な取り代を示すが、勿論窒化処理による膨縮量も含めたものを示している。
【００６６】
次いで、　半加工球に窒化処理を施す。この窒化処理は、例えば、ガス窒化、　塩浴窒化、イオン窒化等のいずれの方法を用いてもよい。
なお、　好ましい窒化処理の一形態として、　比較的低温で処理が可能なＮｖ窒化プロセス（エア・ウォータ株式会社の商標）　がある。このＮｖ窒化プロセスは、まず、例えばＮＦ_３などのフッ素系ガスを用いて、２００〜４００°Ｃでフッ化処理を施した後、５　０％Ｎ_２−５　０％ＮＨ_３の混合ガス雰囲気中で、４００〜４８０°Ｃに加熱することで、窒化層を形成する。
【００６７】
ここでフッ化処理を行うことで、窒化処理を阻害する被処理材表面のＣｒ酸化物等を除去し、　被処理材表面を活性化するフッ化層を形成するため、その後の窒化処理が４　００°Ｃの低温で行われても、　比較的均一な窒化層を形成することが可能となる。また、このフッ化処理が比較的低温で行われるため、窒化処理による精度劣化が抑制され、仕上げ加工を容易に行うことが可能となる。また、窒化処理による芯部硬さの低下が極力抑えられるという効果もある。
【００６８】
その結果、表面には窒化層が形成され、この窒化層にはζ相−Ｆｅ_２、ε相−Ｆｅ_２−３、γ′相−Ｆｅ_４Ｎ、ＣｒＮ、Ｃｒ_２Ｎ等の微細な析出物からなる化合物層が形成されて、耐摩耗性が飛躍的に向上する。
引き続いて、窒化処理が施されたのちに仕上げ加工を行い、ボールの表面粗さを０．１μｍＲａ以下、具体的にはＪＩＳＢ１５０１記載の等級６０以上の精度とすることが好ましい。転動部材の転動面に形成される油膜厚さは、接触面の２乗平均粗さをσ_１およびσ_２とすると、（σ_１ ^２＋σ_２ ^２）^１／２に反比例する。つまり、表面粗さが小さいほど良好に油膜形成が行われることになり、特に、電動射出成形機や電動プレス機などの用途で使用されるボールねじ装置では油膜形成が非常に困難なため、　表面粗さを高精度に仕上げておくことは非常に重要となる。よって、ボールの表面粗さを０．１μｍＲａ以下、更に好ましくは０．０３μｍＲａ以下とする。
【００６９】
なお、ボール完成品の窒化層厚さを１０μｍ以上２％Ｄａ以下とするために、　窒化処理後の窒化層厚さは仕上げ加工の取り代分を加味した厚さとすることが望ましい。
以上のようにして加工されたボールは表面硬さがＨｖ９００以上とされ、　窒素濃度は０．　５重量％以上６重量％以下とされる。そのため、耐焼付き性と耐摩耗性に優れ、ボールねじ装置の長寿命化に貢献する。
（その他の構成部材について）
ボール５以外のナット３、チューブ６等は従来の材料を使用してもよい。例えばナット３であればＳＣＭ４２０を浸炭または浸炭窒化したものや、ＳＣＭ４４５を高周波焼入したものなどがあけられる。ただし、少なくとも雌ねじ溝の転動面の表面硬さがＨｖ６５０以上であり、さらにその母材の鋼中酸素濃度は１５ｐｐｍ以下、好ましくは１０ｐｐｍ以下とし、共晶炭化物径は１０μｍ以下、好ましくは５μｍ以下とする。
【００７０】
上記の条件を満たすことができるなら、その母材の成分や熱処理方法は問わない。例えば、ＳＣＭ４２０等の従来から使用されている合金鋼に、ＭｏやＶなどを添加し浸炭窒化処理を行って耐摩耗性を向上させてもよい。もちろん、本発明による合金鋼および熱処理を適用することも差し支えない。また、ねじ溝転動面の表面粗さは上記した理由により、好ましくは０．　５μｍＲａ以下、　更に好ましくは０．１μｍＲａ以下とする。チューブ６は冷間圧延鋼ＳＰＣＣなどを使用すればよい。
（第２実施例）
本発明の合金鋼でボール５を形成し、ボール５間にセパレ−タ　を介装させたボールねじ装置の寿命ついて評価を行った。表３　に、　試験に用いた各種の合金鋼の主な化学成分を示す。
【００７１】
摩耗試験には、図３に示すボールオンディスク式の摩耗試験機を用いた。このボールオンディスク式の摩耗試験機は、円盤状の試験片にボ―ルを載せ、上から荷重Ｐを負荷しながら互いに接触状態で回転させるものである。試験条件の詳細を以下に示す。
面圧：３ＧＰａ
回転数：５０ｍｉｎ^−１
潤滑油：鉱油（粘度はＶＧ１０程度の低粘度油）
給油量：１ｃｃ／ｍｉｎ　軌道面上に給油
試験温度：室温
ボール材：表３に示す材料に下記の窒化処理条件Ａ〜Ｄで窒化したもの
ディスク材：ＳＵＪ２ずぷ焼入れ（表面粗さ０．１μｍ以下）
ボールには表３に示す材料を使用し、先に示したボールの作製方法に従って作製した。窒化処理は次の何れかの方法を用いた。
【００７２】
窒化処理Ａ
５００〜５７０°Ｃで１〜１０時間のガス窒化処理（５０％Ｎ_２−５０％ＮＨ_３混合ガス雰囲気中）
窒化処理Ｂ
５　００〜５７０°Ｃで１〜１０時間の塩浴軟窒化処理（ＫＣＮＯまたはＮａＣＮＯなどのシアン酸塩を主成分とする塩浴窒化）
窒化処理Ｃ
５００〜５７０°Ｃで１〜１０時間のガス軟窒化処理（５０％ＮＨ_３−５０％吸熱型変成ガス（ＣＯ、Ｎ_２、Ｈ_２を主成分とする））
窒化処理Ｄ　　　　　　・
３　００〜４００°Ｃでフッ化処理後（９０％Ｎ_２−１０％ＮＦ_３混合ガス雰囲気中）　、４　００〜５　００°Ｃで５〜５０時間窒化処理（５０％Ｎ_２−５　０％ＮＨ_３混合ガス雰囲気中）
（寿命試験）
高負荷・　短ストロークの使用条件下での疲労寿命を評価するために、本発明によるボールねじ装置を作製した。作製したボールねじ装置の仕様は以下に示す通りである。
ねじ軸径：８０ｍｍ
リード：２０ｍｍ
ねじ軸材：表１に示す合金鋼を上記熱処理条件Ａ〜Ｃで熱処理したもの
ナット材：ＳＣＭ４２０浸炭
ボール材：　表３に示す材料に上に示す窒化処理条件Ａ〜Ｄで窒化したもの
セパレータ：　合成樹脂製
熱処理が完了したねじ軸やナットおよびボールには研削仕上げを施し、さらにボールにはラップ仕上げを行った。以上のようにして作製したボールねじ装置を供試体として、射出成形機を想定した試験条件で試験を行った。試験条件の詳細を以下に示す、また、この条件は接触楕円が溝肩に乗り上げやすいように、表２で行った寿命試験よりも負荷荷重を大きく設定した
荷重：最大３００ｋＮ
ストローク：２０ｍｍ
サイクルタイム：２秒
潤滑材：鉱油系グリース
試験温度：８０°Ｃ（ナット外形をヒータで加熱）
試験結果を表４に示す。
【００７３】
【表４】

【００７４】
表面硬さおよび芯部硬さの測定にはボールオンディスク摩耗試験に使用したボールを使用した。測定にはマイクロビッカ―ス硬さ試験機を用い、測定荷重を２００ｇとした。表面硬さはボ−ル表面を、　芯部硬さはボールの中心部をそれぞれ測定した。
試験片の摩耗試験の結果は従来例Ｅ−１の摩耗量を１として比で示した。ボールねじ耐久試験は、　ねじ軸，ナツト，ボールのいずれかが表面損傷を起こすか、ねじ軸の回転トルクが急激に増大するまでのショット数で評価し、従来例Ｅ−１のショット数を１とした比で示した。なお、従来例Ｅ−１はボールをＳＵＪ２ずぶ焼とし、ねじ軸には表２に示すＡ−１２と同様なものを使用し、ナットにＳＣＭ４２０に浸炭処理したものを使用した。
【００７５】
なお、比較例のＤ−１〜Ｄ−４はいずれも、Ｅｑ１値が２．０を下回ったものである。比較例Ｄ−５およびＤ−６はＣ含有量が１．３重量％を超えるもので、粗大な共晶炭化物が見られ、窒化処理後の仕上げ加工時に窒化層の剥離が発生したため、摩耗試験およびボールねじ耐久試験は行わなかった。比較例Ｄ−７およびＤ−８は実施例Ｃ−９およびＣ−１０にセパレータを介装しなかったものである。
【００７６】
摩耗試験の結果からは、Ｅｑ１値が２．０以上である実施例Ｃ−１〜Ｃ−３０では表面硬さがＨｖ９００以上となるため、従来例Ｅ−１であるＳＵＪ２　ずぶ焼と比較して良好な耐摩耗性が得られることがわかる。特にＥｑ値が４．０以上であるものでは表面硬さがＨｖ１０００以上となり、さらに良好な耐摩耗性が得られる。Ｅｑ１値が２．０を下回る比較例Ｄ−１〜Ｄ−４では、窒化処理に伴って窒化物が析出するための合金元素が十分でないため、表面硬さが十分に向上せず、耐摩耗性も各実施例ほどには及ばない。
【００７７】
次にボールねじ耐久試験の結果からは、　ボールに本発明の合金鋼に窒化処理を施し、セパレータを介装させた各実施例は、ボールにＳＵＪ２ずぷ焼を使用し、同じくセパレータを介装させた従来例と比較して転動疲労寿命が２倍以上と向上していることがわかる。また、ねじ軸にＶを添加した浸炭窒化材を使用した実施例Ｃ−９は、ねじ軸にＳＣＭ４２０に浸炭を行った実施例Ｃ−１０と比較して、転動疲労寿命が向上する。これはＶを添加した浸炭窒化材はＳＣＭ４２０の浸炭材よりも耐摩耗性に優れるためである。
【００７８】
また、セパレータを介装しない比較例Ｄ−７およびＤ−８は、セパレータを介装した実施例Ｃ−９およびＣ−１０をはじめとする各実施例と比較して、十分な疲労寿命が得られていないことがわかる。このことから、ボールに耐摩耗性に優れる材料を使用するだけでは、転疲労寿命向上には不十分であり、セパレータと組み合わせることで、より好ましい転動疲労寿命が得られることがわかる。
【００７９】
【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、高荷重が瞬間的に加わる使用条件や、高荷重が瞬間的に加わり、且つ短ストロークの使用条件においても優れた耐摩耗性を有して好適に用いることができる転動装置を提供することができる。この場合、転動体に窒化処理を施せば、寿命向上により効果的である。
【図面の簡単な説明】
【図１】本発明の転動装置の実施の形態の一例であるボールねじ装置の説明図である。
【図２】図１のＡ−Ａ線断面図である。
【図３】ボールオンディスク摩耗試験機の説明図である。
【符号の説明】
１…ねじ軸（内方部材）
２…雄ねじ溝（軌道面）
３…ナット（外方部材）
４…雌ねじ溝（軌道面）
５…ボール（転動体）
６…チューブ
７…フランジ
８…平面（　切欠面）
９…チューブ押え
１０…シール
１１…セパレータ
１２…供試体
Ｐ…荷重[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ball screw used in a short stroke with a high load applied instantaneously, such as an electric injection molding machine or an electric press machine, in addition to a rolling bearing or a linear guide device to which a high load is applied instantaneously. The present invention relates to a rolling device such as a device.
[0002]
[Prior art]
A ball screw device, which is an example of a rolling device, includes a screw shaft (inner member) having a male screw groove (track surface) formed on the outer periphery thereof, and a female screw groove (track surface) opposed to the male screw groove formed on the inner periphery thereof. A formed nut (outer member), a number of balls (rolling elements) interposed between a male screw groove and a female screw groove, and a ball provided on the nut from one end of the female screw groove to the other end. And a circulating member such as a tube or a top having a circulating passage for circulating.
[0003]
During relative rotation between the screw shaft and the nut, the ball rolls with respect to both the male screw groove and the female screw groove, and circulates from one end of the female screw groove to the other end through the circulation passage.
As described above, many general ball screw devices have only a large number of balls loaded between a male screw groove and a female screw groove (hereinafter referred to as a total ball specification). However, in the ball screw device of the total ball specification, the balls that rotate in opposite directions contact each other at the contact points of the adjacent balls, causing slip at the contact points of the balls. Such a slip not only deteriorates the operability of the ball and causes a torque fluctuation, but also causes the ball to be worn or the ball, the male screw groove and the female screw groove to be seized due to frictional heat.
[0004]
In addition, ball scratches and indentations caused by contact between balls and competition between the balls easily cause scratches and indentations on the rolling surface of the thread groove, and the life is likely to be shortened due to early damage of the surface starting type.
In the case of ball competition, a spacer ball smaller than the diameter of the ball supporting the load by several tens {0 μm} (hereinafter referred to as a spacer ball specification) has been used. In this ball screw device of the spacer ball specification, the ball that supports the load and the spacer ball are in rolling contact with each other in the opposite directions, so that the sliding in the direction that prevents the rolling of the balls due to the pressing of the balls with each other is performed. No contact occurs. For this reason, the resistance that hinders the rolling of the ball becomes very small, so that a ball screw device with little fluctuation of the driving torque can be obtained, and early wear and seizure of the ball can be suppressed.
[0005]
However, the spacer ball type has a smaller number of balls for supporting a load than the total ball type, and the rigidity and the load capacity are reduced to about half. Therefore, the spacer ball type is not suitable for a ball screw device requiring a high load capacity.
Furthermore, in recent years, a ball screw device has been used as a substitute for a hydraulic cylinder in order to save energy and shorten the cycle of an injection molding machine and a press machine. Since the ball screw device used for (1) is required to have a high load capacity, the required load capacity cannot be satisfied with the spacer ball specification, or the size of the device cannot be avoided. There is.
[0006]
In addition, in the above-mentioned applications, it is often used under load conditions in which the ball temporarily stops and rotates reversely with the maximum load applied, and before and after the stop, lubricant is drawn into the contact surface between the ball and the male or female thread groove. Oil film formation is difficult because it is difficult.In addition, the stroke is short, and not only is the load applied at the same place on the thread groove, but the rolling surface of the male thread groove is structurally Since high surface pressure is generated, (1) especially for a screw shaft having a male thread groove, (2) load resistance and wear resistance are required.
[0007]
To cope with such a problem, Japanese Utility Model Laid-Open Publication No. Sho 63-178659 discloses a method in which separators having the same thickness and having a spherical concave surface on which a part of a spherical surface of a ball is slidably fitted are formed on both end surfaces in the axial direction. A ball screw device arranged between balls has been proposed (hereinafter, referred to as a separator specification).
The ball screw device of the separator type has the following features: (1) The point contact between balls as in the ball screw device of the total ball type or the spacer ball type described above is changed to the surface contact between the ball and the concave surface of the separator to reduce the unit surface pressure. Can be reduced.
[0008]
Further, since the distance between the balls supporting the load can be narrowed compared to the ball screw device of the spacer ball specification, there is an advantage that the load capacity can be made larger than that of the spacer ball specification.
JP-A-11-315835, JP-A-2001-21018, and JP-A-2000-199556 disclose techniques for improving the shape of a separator for the purpose of improving operability and oil film retention. Have been.
[0009]
[Problems to be solved by the invention]
However, in the above-described conventional ball screw device of the separator type, such as Japanese Utility Model Application Laid-Open No. 63-178659, special consideration is not sufficiently given to the material of each member such as the screw shaft, so that a high load is instantaneously applied. However, it is not sufficiently considered for applications such as electric injection molding machines and electric presses used with short strokes.
[0010]
In Japanese Patent Application Laid-Open No. 2000-212721, a steel containing ΔV is subjected to carbonitriding, and a material considering the nitrogen concentration and the carbon concentration on the surface of the finished product is taken out of one of the members of the ball screw device. However, since this technique does not take into account the prevention of competition between balls, there is still room for improvement for use in applications such as electric injection molding machines and electric presses. There is.
[0011]
The present invention has been made in order to solve such inconvenience, and has excellent wear resistance even under a use condition where a high load is momentarily applied or under a use condition where a high load is momentarily applied and a short stroke. It is an object of the present invention to provide a rolling device that can be suitably used by having a rolling device.
[0012]
[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. An outer member disposed on the outside, and a rolling device including a rolling element rotatably disposed between the two raceway surfaces,
At least the inner member of the inner member, the outer member and the rolling elements is C: 0.15% by weight or more. 6% by weight or less, Si: 0.1% by weight or more and 0.9% by weight or less, Mn: 0.1% by weight or more and 1.5% by weight or less, Cr: 0.5% or more and 4.0% or less. The carbide, nitride and carbonitride in the outermost surface layer of at least the raceway surface of the finished inner member formed of an alloy steel after carburizing or carbonitriding, with the range from the surface to 50 μm as the outermost surface layer Is not less than 10% and not more than 30%, the area ratio of carbide, nitride and carbonitride having a particle size of not more than 1 μm is not less than 60% of the total area. The Vickers hardness of the surface layer is set to Hv 700 or more, and the maximum particle size of carbide, nitride and carbonitride is set to 5 μm or less in circle equivalent diameter. .
[0013]
The invention according to claim 2 is characterized in that, in claim 1, the alloy steel contains V: 2.0% by weight or less, Mo: 3.0% by weight or less, and Ni: 4.5% by weight or less. I do.
The invention according to claim 3 is characterized in that, in claim 1 or 2, at least the concentration of C + N in the surface layer is set to 0.7% by weight or more and 1.7% by weight or less.
[0014]
According to a fourth aspect of the present invention, in any one of the first to third aspects, at least the retained austenite of the surface layer is set to 10% by volume or more and 45% by volume or less.
According to a fifth aspect of the present invention, in any one of the first to fourth aspects, a separator is interposed between the rolling elements.
[0015]
According to a sixth aspect of the present invention, in any one of the first to fifth aspects, the compressive residual stress on the surface of the inner member is set to 150 MPa or more.
The invention according to claim 7 is the invention according to any one of claims 1 to 6, wherein the finished product of the rolling element has a surface hardness of Hv900 or more and a range of 10 µm or more and 2% or less of the rolling element diameter from the surface. And a nitride layer having a nitrogen concentration of 0.5% or more and 6% or less.
[0016]
The invention according to claim 8 is characterized in that, in any one of claims 1 to 7, the inner member is a screw shaft of a ball screw device, and the outer member is a nut of a ball screw device.
In addition, as a rolling device of this invention, a rolling bearing, a linear guide device, a linear motion bearing, etc. other than a ball screw device are mentioned.
[0017]
The inner member means an inner ring when the rolling device is a rolling bearing, a guide rail when the linear guide device is the same, and a shaft when the linear bearing is the same.
The outer member means an outer ring when the rolling device is a rolling bearing, a slider when the linear guide device is the same, and an outer cylinder when the linear bearing is the same.
[Numerical limitations for each material]
(C: 0.15% by weight or more and 0.6% by weight or less)
C is an element necessary for improving the hardness after quenching and tempering by converting the matrix to martensite. In order to secure the necessary strength as a member of the rolling device, its content is set to 0.15% by weight or more. Also, 0. If added in excess of 6% by weight, a large amount of carbides will be formed at the stage of the raw material, so that not only the workability in plastic working and turning for forming the product before heat treatment will deteriorate, but also coarse eutectic carbides will be formed. It forms and adversely affects rolling fatigue life. Therefore, the carbon content of the material is set to 0.15% by weight or more and 0.6% by weight or less.
(Si: 0.1% by weight or more and 0.9% by weight or less)
Si is an element necessary as a deoxidizing agent in steel making. 1% by weight or more is required. Further, although the tempering softening resistance is increased, the addition of a large amount not only lowers the toughness and workability but also promotes decarburization. Therefore, the upper limit is set to 0.9% by weight.
(Mn: 0.1% by weight or more and 1.5% by weight or less)
Mn is used as a deoxidizing agent at 0 °. 1% by weight or more is necessary, but if added in a large amount, not only does the forgeability and machinability deteriorate, but also the corrosion resistance is reduced by coexistence with impurities such as S and P, so the upper limit is 1%. 5% by weight.
(Cr: 0.5% by weight or more and 4.0% by weight or less)
Cr enhances hardenability, strengthens the solid solution of the matrix, and precipitates carbide, nitride, and carbonitride on the surface layer of the rolling device by carburizing or carbonitriding, and provides wear resistance and rolling fatigue life. Has the effect of improving. However, when the content is 0. If the amount is less than 5% by weight, the effect of the addition is small. On the other hand, if the amount is added in a large amount, Cr oxide is formed on the surface, which prevents carbon and nitrogen from penetrating from the surface during carburizing or carbonitriding, thereby reducing heat treatment productivity. The upper limit is 4 to lower. 0% by weight.
(V: 2.0% by weight or less)
V forms an extremely fine and high-hardness carbide, nitride, and carbonitride, and is therefore an element effective for improving wear resistance and rolling life, and has an effect of increasing temper softening resistance. However, since the addition of a large amount decreases the toughness and workability, the content of 2.と す る 0% by weight or less. Particularly when surface hardness or wear resistance is required, it is preferable to add 0.6% by weight or more. In this case, the treatment temperature is more preferably 920 ° C. or more in order to precipitate fine V nitrides, carbides, and carbonitrides by carbonitriding.
(Mo: 3.0% by weight or less)
Mo is an element that remarkably increases the hardenability and the tempering softening resistance. Carbide or carbonitriding causes carbides, nitrides and carbonitrides to be precipitated on the surface layer of the rolling device, resulting in wear resistance and rolling fatigue life. It is an element effective for improving. However, if added in excess, the toughness decreases, so the upper limit is set to 3.0% by weight or less.
(Ni: 4.5% by weight or less)
Ni is an element effective for improving the toughness by forming a solid solution in the matrix. However, if it is added excessively, the amount of retained austenite in the surface layer becomes excessive and the hardness decreases, so the upper limit was made 4.5% by weight.
(About inevitable impurities)
In addition to the alloy elements described above, P ≦ 0. {02}% by weight, S ≦ 0.05% by weight, Cu ≦ 0. Although it may contain 10% by weight and O ≦ 15 ppm, it is preferable to set O ≦ 10 ppm in order to minimize nonmetallic inclusions harmful to the rolling fatigue life characteristics.
[Numerical limit in the range of 5 0 μm from the raceway surface of the finished inner member]
Abrasion resistance and seizure resistance are greatly affected not only by lubrication conditions and surface roughness, but also by the state of the surface microstructure. Particularly, in a ball screw device used for an injection molding machine, a press machine or the like, since lubrication conditions are particularly severe, some countermeasures by material and heat treatment are required.
[0018]
Further, in the case of the screw shaft (inner member) of the ball screw device, the polishing allowance after heat treatment is larger than that of a rolling element, and it is difficult to make the state of the outermost surface layer of the finished product coincide with all the rolling surfaces. is there. Therefore, if the outermost surface layer from the surface of any finished product raceway surface to 50 μm is defined as the outermost surface layer and the uppermost surface layer is within the numerical limitation described below, a certain level of wear resistance can be obtained on all the rolling surfaces. It becomes possible.
(Total area ratio of carbide, nitride and carbonitride is not less than 100% and not more than 30%)
Carbides, nitrides, and carbonitrides have an effect of improving wear resistance, but preferably have an area ratio of 1% or more. However, if the area ratio is too high, the rolling fatigue life is reduced. Therefore, the upper limit is 25% or less in area ratio, and more preferably 2 0% or less.
(The area ratio of carbides, nitrides, and carbonitrides having a particle size of 1 μm or less is 6% or more of the total area.)
Further, carbides, nitrides, and carbonitrides are fine, and the smaller the spacing between grains, the higher the effect of improving wear resistance. If carbides, nitrides and carbonitrides having a particle size of 1 μm or less are present in an amount of 60% or more of their total area, it is effective in improving wear resistance, and more preferably 70% or more. .
[Numerical limit in the range from the raceway surface of the finished inner member to 2% of the rolling element diameter]
In the range (surface layer) from the raceway surface of the finished inner member to 2% of the rolling element diameter (surface layer), consideration must be given not only to (1) wear but also to rolling fatigue life. Especially in ball screw devices used in injection molding machines and press machines, since the applied load is large and the load position is always constant, it is necessary to ensure sufficient rolling fatigue life even under such severe operating conditions. Careful consideration is required.
(Vickers hardness Hv700 or more)
In a ball screw device used for an injection molding machine or a press machine having a large load, a higher shear stress is generated than in a normal use, and the position where the shear stress is generated is deeper. Therefore, in order to obtain a sufficient rolling life, (1) the Vickers hardness of the surface layer is set to Hv700 or more, and (2) more preferably Hv750 or more.
(Carbide, nitride and carbonitride particle size is 5μm or less in circle equivalent diameter)
Carbides, nitrides, and carbonitrides increase the hardness by precipitating them in fine and large amounts, and have the effect of improving the rolling fatigue life.However, if coarse grains are present, they become a source of stress concentration, so rolling fatigue life May be reduced. If the maximum particle size of the carbide, nitride and carbonitride contained in the surface layer exceeds 5 μm in equivalent circle diameter, the rolling fatigue life is remarkably reduced, so that the maximum particle size is 5 μm or less.
[About separator]
Since the separator prevents contact between the balls, early wear and seizure of the balls can be prevented. In addition, the distance between the balls for supporting the load can be narrowed as compared with the case where the spacer balls are used, and the load capacity becomes almost the same as the total ball specification.
[0019]
Generally, the separator is made of a synthetic resin such as nylon.However, the material is not limited to resin, and can reduce the rolling resistance of the balls, and can use any material as long as it can prevent competition between the balls. Good. However, if a gap is formed between the ball and the separator, the separator may fall down, causing a malfunction of the ball screw device. Therefore, the separator must be elastically deformed to some extent so that the ball can always be given a pressurizing force in the linear motion direction.
[0020]
Further, in order to suppress the increase in the gap due to the abrasion of the separator, it is preferable that the separator is also made of a material having a certain degree of wear resistance. For example, a sintered body mainly containing a solid lubricant such as molybdenum disulfide, a porous sintered body, high-strength brass and the like can be mentioned.
[Other considerations]
(Retained austenite in the surface layer is 10% by volume or more and 45% by volume or less)
Retained austenite is softer than martensite, has higher toughness, has higher work hardening characteristics, and suppresses the occurrence and propagation of cracks. On the other hand, when the amount of retained austenite is increased, the dimensional stability decreases. However, according to surface treatment such as carburizing or carbonitriding, the amount of retained austenite in the entire member is smaller than that of SUJ2, which is a general bearing steel, and the amount of retained austenite can be increased only on the surface. It is also advantageous for stability.
[0021]
In the case of a general ball screw device, scratches and dents formed on the ball due to ball competition and wear may cause dents and scratches on the male screw groove, resulting in peeling and cracking. Furthermore, high loads are momentarily applied to ball screw devices used in electric injection molding machines, electric presses, etc., resulting in indentations on the rolling surface, coupled with deformation of the machine base and misalignment during installation. Therefore, it is preferable to contain residual austenite that can suppress the generation and propagation of cracks, and the amount is 10% or more, more preferably 15% or more by volume. .
[0022]
However, when the residual austenite exceeds 45% by volume, not only the rolling life is reduced due to the decrease in hardness, but also the residual austenite amount of the entire member including the core and the surface cannot be suppressed low, As a result, the dimensional stability decreases, so the amount of retained austenite is set to 45% by volume or less. In particular, when wear resistance is required, and when high surface hardness is required, it is preferable that the retained austenite is 35% by volume or less.
(C + N concentration of the surface layer is 0.7% by weight or more and 1.7% by weight or less)
In order to obtain the required surface hardness as a member of the rolling device, the C + N concentration of the surface layer needs to be 0.7% by weight or more. However, when the content is excessive, residual austenite is excessively generated on the surface, whereby the hardness may be reduced, or coarse carbides may be precipitated to lower the rolling fatigue life. 7% by weight. In particular, the upper limit of the carbon concentration is preferably 1.3% by weight or less in order to prevent coarse carbide precipitation.
[0023]
Further, nitrogen N is a very effective element for improving wear resistance, and is added to the surface layer by carbonitriding. However, if an attempt is made to obtain a high nitrogen concentration deeply in the production of large products, the heat treatment productivity decreases and the cost increases, so the upper limit was set to 0.4% by weight. In order to obtain particularly high surface hardness and excellent wear resistance, the nitrogen content is more preferably 0.15% by weight or more.
(Residual compressive stress is 150MPa or more)
Carburizing or carbonitriding causes residual compressive stress on the surface. Since the residual compressive stress has an effect of reducing the shear stress generated by rolling of the rolling element, it is preferable that the residual compressive stress is set to a value equal to or more than a certain value, preferably 150 MPa or more.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory view of a ball screw device which is an example of a rolling device according to an embodiment of the present invention, and FIG. 2 is a sectional view taken along line AA of FIG.
As shown in FIGS. 1 and 2, the ball screw device includes a screw shaft (inner member) 1 having a male screw groove (track surface) 2 formed on an outer periphery thereof, and a female screw groove (track surface) facing the male screw groove 2. ) 4 has a cylindrical nut (outer member) 3 formed on the inner periphery thereof, and a number of balls (rolling elements) 5 interposed between the male screw groove 2 and the female screw groove 4. A separator 11 is interposed between the balls 5 to avoid contact between the balls 5.
[0025]
The nut 3 has a flange 7 formed at one end thereof for fixing to a table or the like (not shown), and a flat surface (a cutout surface) 8 formed by cutting a part of the outer peripheral surface (upper part in FIG. 2). A tube (circulation member) 6 having a circulation passage is fixed to the nut 3, and the ball 5 rolling between the two screw grooves 2, 4 circulates through the tube 6. . In the drawing, reference numeral 9 denotes a tube holder for fixing the tube 6 on the flat surface 8 of the nut 3, and reference numeral 10 denotes a dust-proof plastic seal attached to both ends of the nut 3.
[0026]
Here, in the present embodiment, at least the screw shaft 1 ° is provided with C: 0.15% by weight or more. 6 wt% or less, Si: 0.1 wt% to 0.9 wt%, Mn: 0.1 wt% to 1.5 wt%, Cr: 0.5% to 4.0%, necessary Accordingly, the screw shaft 1 is formed from an alloy steel containing V: 2.0% by weight or less, Mo: 3.0% by weight or less, Ni: 4.5% by weight or less, and after carburizing or carbonitriding, the screw shaft 1 is completed. In at least the external thread groove 2 of the product, the range from the surface to 5.0 μm is defined as the outermost surface layer, and the total area ratio of carbides, nitrides and carbonitrides in the outermost surface layer is set to 10% or more and 30% or less. The area ratio of carbide, nitride and carbonitride having a diameter of 1 μm or less is set to 60% or more of the total area, and the Vickers hardness of the surface layer is set to 2% of the ball diameter from the surface as a surface layer. Hv 700 or more, and carbides, nitrides and The maximum particle size of carbonitride is set to 5 μm or less in circle equivalent diameter, so that a high load is instantaneously applied like an electric injection molding machine or an electric press machine, and it is excellent even under short stroke use conditions. The ball screw device has wear resistance and can be suitably used.
[0027]
Further, in this embodiment, the compression residual stress on the surface of the screw shaft 1 is 150 MPa or more, and the finished product of the ball 5 has a surface hardness of Hv900 or more, and is 10 μm or more from the surface and 2% of the ball diameter from the surface. A nitride layer having a nitrogen concentration of 0.5% or more and 6% or less is provided in the following range.
Next, a method for manufacturing the screw shaft 1 will be described.
[0028]
The screw shaft 1 is formed of an alloy steel satisfying the above numerical limitations, and is turned or rolled into a bar or a tube to be processed into a desired shape. Next, the screw shaft 1 is subjected to carburizing or carbonitriding, followed by quenching and tempering. After carburizing or carbonitriding, slow cooling or rapid cooling may be performed, followed by quenching and tempering by heating and holding again.
[0029]
Carburizing and carbonitriding are carried out at, for example, 820 to 980 ° C. In particular, when V is added in an amount of 0.6% by weight or more and 2.0% by weight or less in order to obtain extremely excellent wear resistance, In order to disperse a large amount and finely of carbides, nitrides and carbonitrides, it is preferable to perform carbonitriding at 920 ° C. or more. Note that a carbon-proof material is applied to a portion that does not particularly require mechanical strength, such as a shaft end of a screw weight.
[0030]
Further, quenching is performed, for example, at 780 to 880 ° C, and tempering is performed, for example, at 160 to 240 ° C. However, any of the above-mentioned conditions are examples, and allowance for finish grinding after heat treatment is made so that the outermost surface layer and the surface layer of the external thread groove 2 of the finished product of the screw shaft 1 fall within the numerical limits of the present invention. The conditions for carburizing, carbonitriding, quenching and tempering are also determined in consideration of the above.
[0031]
Further, the retained austenite of the surface layer is set to 10% to 45% by volume, the C concentration of the surface layer is set to 0.7% to 1.3% by weight, and the N concentration is set to 0.4% by weight or less. To do. Further, when the additive element of the alloy steel and the C concentration and the N concentration of the surface layer after carburizing and carbonitriding are high, and the tempering is performed at a high temperature, the retained austenite of the surface layer exceeds 45% by volume. Sub-zero processing or the like may be performed.
[0032]
The screw shaft 1 after the heat treatment is finished to a desired shape by grinding and polishing the shaft end, outer diameter, external thread groove 2 溝, and the like. Further, when heat treatment deformation is large such as a change in screw pitch, cutting may be performed on a necessary portion.
The nut 3, the ball 5, the tube 6, etc. other than the screw shaft 1 may use a conventional material. For example, the nut 3 is obtained by carburizing the SCM420 or the SCM445 is induction hardened. The ball 5 is obtained by soaking SUJ2. The tube 6 is formed by cold-rolled steel SPCC. Of course, the alloy steel and the heat treatment according to the present invention may be applied. The working method when the alloy steel and the heat treatment according to the present invention are applied may be in accordance with the respective conventional methods.For example, when there is a problem in workability before the heat treatment, spheroidizing annealing is performed before the working. Measures may be taken.
[0033]
Note that the present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the gist of the present invention.
For example, in the above-described embodiment, the alloy steel according to the present invention is used only for the screw shaft. However, the alloy steel according to the present invention may be applied to nuts and balls. In this case, if the ball is subjected to a nitriding treatment, it is still effective to improve the life.
[0034]
In the above-described embodiment, a ball screw device to which a high load is instantaneously applied and which is used under a short stroke use condition, such as an electric injection molding machine or an electric press machine, has been described. It can also be used as a device, and it goes without saying that the use of the alloy steel according to the present invention for balls and nuts as described above is still more effective.
[0035]
Further, in the above embodiment, the case where the present invention is applied to the screw shaft of the ball screw device is taken as an example. However, instead of this, at least the guide rail of the linear guide device, the inner ring and / or the outer ring of the rolling bearing are used. Even if the same material and processing as those of the above-described ball screw device are used, or the same material and processing is used for the rolling elements and the separator, the life can be effectively improved.
[0036]
【Example】
(First embodiment)
The wear resistance of the alloy steels of Examples and Comparative Examples according to the present invention and the fatigue life of a ball screw device in which a separator is interposed between balls 5 using the alloy steels of Examples and Comparative Examples for the screw shaft 1. An evaluation was performed. Table 1 shows various alloy steels No. used in the test. 1 to No. The 24 main chemical components are shown.
[0037]
[Table 1]

[0038]
The alloy steels of Examples and Comparative Examples in Table 2 were subjected to the following heat treatment. By changing the heat treatment conditions of quenching and tempering within the following conditions, (1) the surface area ratio and the surface hardness of the precipitated carbide were adjusted. The column of steel type in Table 2 corresponds to the alloy steel No. in Table 1.
Heat treatment condition A
Carbonitriding: Air cooling or slow cooling to room temperature after holding at 860-920 ° C
Atmosphere: Rx gas + enriched gas + ammonia gas
Quenching: oil cooling after holding at 820-880 ° C
Tempering: {16 0 to 240 ° C2 to 3 hours
Heat treatment condition B
Carbonitriding: Air-cooled or slowly cooled to room temperature after holding at 920-980 ° C
Atmosphere: Rx gas + enriched gas + ammonia gas
Quenching: oil cooling after holding at 820-880 ° C
Tempering: {16 0 to 240 ° C2 to 3 hours
Heat treatment condition C
Carbonitriding: Cool to room temperature after holding at 920-980 ° C
Atmosphere: Rx gas + enriched gas
Quenching: oil cooling after holding at 820-880 ° C
Tempering: {16 0 to 240 ° C2 to 3 hours
(Life test)
In order to evaluate the fatigue life under the conditions of high load and short stroke, a ball screw device was manufactured. The specifications of the manufactured ball screw device are as follows.
[0039]
Screw shaft diameter: 80mm
Lead: 20mm
Screw shaft material: heat treatment of alloy steel shown in Table 1 under the above heat treatment conditions A to C
Nut material: SCM420 carburized
Ball material: SUJ2 Suzuyaki
Separator: Synthetic resin
The heat-treated screw shaft, nut and ball were finished by grinding, and the ball was finished by lapping. Using the ball screw device manufactured as described above as a specimen, a test was performed under the test conditions assuming an injection molding machine. Details of the test conditions are shown below.
[0040]
Load: Maximum 200kN
Stroke: 20mm
Cycle time: 3 seconds
Lubricant: mineral oil grease
Test temperature: 80 ° C (Nut outer shape is heated by heater)
Table 2 shows the total area ratio of carbides, nitrides, and carbonitrides at a position of 5.0 μm (the outermost surface layer) from the surfaces of the test pieces of Examples A-1 to A-28 and Comparative Examples B-1 to B-7. And Vickers hardness at a position (surface layer) at 2% of the ball diameter (Da) from the surface, carbide having a diameter equivalent to a circle of more than 5 μm, The presence or absence of nitrides and carbonitrides, the C concentration, the ΔN concentration, the amount of retained austenite, and the results of the wear test and the life test were also shown.
[0041]
In Table 2, carbides, nitrides, and carbonitrides are described as carbides and the like.
[0042]
[Table 2]

[0043]
For the measurement of the area ratio of carbide, etc., SEM observation was performed at a magnification of 5000 times at an arbitrary position in the cross section from the surface to 50 μm, image analysis was performed for 20 visual fields, and those with a circle equivalent diameter of 0.1 μm or more were extracted. Was calculated.
Regarding the confirmation of coarse carbides, SEM observation was performed at an arbitrary magnification in a cross section from the surface to the 2% Da position at a magnification of 1000 times, and the presence or absence of carbides exceeding 5 μm in a circle equivalent diameter for 40 visual fields was confirmed. Was done.
[0044]
EPMA (Electron Probe Microanalyser) was used for the measurement of C concentration and N concentration, X-ray analysis was used for the amount of retained austenite, and a micro Vickers hardness tester was used for the measurement of surface hardness. The measured value at the position is shown.
The life test was evaluated by the number of shots until the rotational torque or the vibration of the screw shaft increased sharply, and the ratio was set to 1 with the number of shots of Comparative Example B-7 being one. Regarding the amount of wear, the shape of the raceway groove of the shaft at the time of 100,000 shots was measured, and the maximum wear depth was determined. The wear depth of Comparative Example B-7 was shown as 1. In Comparative Example B-7, SCM445 was subjected to induction hardening, and the surface hardness of the screw shaft was set to Hv651.
[0045]
From the results of the life test, it can be seen that Examples A-1 to A-28 according to the present invention have excellent wear resistance and fatigue life as compared with each Comparative Example. In Examples A-12 to A-14 in which carburizing was performed, and in Examples A-9, A-23, and A-25 in which the same steel type was carbonitrided, respectively, carbonitriding was better in wear resistance and fatigue. The life is improved, and it can be said that (1) carbonitriding is more preferable.
[0046]
Furthermore, V is contained in an amount of 0.6% by weight or more and 2.0% by weight or less, carbonitriding is performed at 920 ° C. or more, and the N concentration of the finished product surface layer of the external thread groove is 0.15% by weight or more and 0.4% by weight. % Or less in Examples A-17 to A-28, since carbides, nitrides, and carbonitrides are finely precipitated and the total area ratio of carbides having a particle size of 1 μm or less exceeds 70%, is particularly excellent. Abrasion resistance is obtained. Among them, in Examples A-17 and A-20 to A-28 in which the Vickers hardness in the surface layer is Hv750 or more, it is understood that the fatigue life is as excellent as 6 times or more of Comparative Example B-7. .
[0047]
Comparative Examples B-1 and B-2 have the following characteristics: (1) the area ratio of carbide or the like having a particle size of 1 μm or less in the outermost surface layer exceeds 6 0%, and (2) the Vickers hardness of the surface layer is Hv700 or more. The rolling life was reduced as compared with Comparative Example B-7 because coarse carbides exceeding 5 μm in particle diameter were present.
In Comparative Examples B-3 to B-5, since the Vickers hardness of the surface layer was lower than Hv700, the {rolling fatigue life was not improved, and in particular, Comparative Example B-5 in which a carbide having a particle size of 5 μm or more was observed in the surface layer} Very short life.
[0048]
Comparative Example B-6 is an example in which no separator was used in Example A-21. However, if the separator was not used, the balls would compete with each other and wear out, so that the use was stopped relatively early. Was confirmed.
Next, a ball screw device in which the ball was strengthened in addition to the strengthening of the screw shaft was manufactured. In particular, ball screw devices used in electric injection molding machines and electric presses have a large load, and the contact ellipse between the ball and the raceway groove is large. It may be. When the ball runs, the end of the thread groove will damage or wear the ball, so it is necessary to strengthen the ball.
[0049]
In addition to balls obtained by carburizing or carbonitriding the materials shown in Table 1, balls obtained by subjecting the materials shown in Table 3 to nitriding treatment were used.
[0050]
[Table 3]

[0051]
For the material used for the nitriding treatment, C + N: {0. 45 wt% or more and 1.3 wt% or less, at least one of Cr, Mo, W, V, Nb, Ti, and Al is added, and Ni, Co, and N are added as necessary elements. , It is good to use Fe and steel containing necessary elements and unavoidable impurities in steelmaking as a base material,
The critical significance of numerical limitation and matters to be considered are described below.
(C + N: 0.45% by weight or more and 1.3% by weight or less)
C and N are elements necessary for improving the hardness after quenching and tempering by turning the matrix into martensite. In addition, carbides and nitrides such as ΔCr, V, and Mo are formed, which contributes to improvement in hardness and wear resistance. In order to secure the core hardness required as a rolling element, it is preferable to add 0.45% by weight or more in terms of the total content of ΔC + N. In particular, when N is added instead of C, the amount of C added can be kept low, and thus there is an effect that the eutectic carbide size can be suppressed.
[0052]
If C is added in excess of 1.3% by weight, huge eutectic carbides are likely to be formed in the steel, and there is a problem that (1) the rolling fatigue life is reduced. Furthermore, the residual austenite after the heat treatment is excessively generated due to the decrease in the Ms point, and the core hardness of the rolling element may not be sufficiently obtained. Further, if N is added in excess of 0.2% by weight at the stage of steel making, bubbles may be generated during solidification and many pores may be introduced into the steel ingot. Is preferably 0.2% by weight or less, more preferably 0.15% by weight or less. For the above reasons, ΔC + N is preferably set to 0.45% by weight or more and 1.3% by weight or less, and more preferably 0.6% by weight or more and 0.9% by weight or less.
(Add at least one of Cr, Mo, W, V, Nb, Ti and Al)
Since Cr, Mo, W, V, Nb, Ti, and Al are all useful elements for forming nitride or carbonitride having high hardness by nitriding treatment and improving wear resistance, at least one of them is used. It is preferable to add above. Further, the nitriding treatment is often performed at, for example, 500 ° C. or higher, and the core hardness of the member subjected to the nitriding treatment is reduced. The rolling element must have a high surface hardness to support a high surface pressure, and at the same time, if the core has a low hardness, the nitride layer may be damaged when subjected to a high contact pressure Therefore, it is necessary that the hardness of the core is also not less than a certain value. The core hardness is at least Hv550 or more, preferably Hv650 or more.
[0053]
Cr, Mo, W, V, etc. added to the steel form carbides by heat treatment before nitriding, not only contributing to the improvement of hardness, but also secondary hardening during nitriding, resulting in a core portion. There is also an effect of preventing a decrease in hardness.
As described above, in order to increase the surface hardness after nitriding to provide abrasion resistance, and to maintain the core hardness of the rolling element after nitriding at a certain level or more, it is necessary to add a certain amount or more of these elements. is there. The present inventors have found that if the following relational expression Eq1 of the contents of Cr, Mo, W, V, Nb, Ti, and Al satisfies 2.0 ≦ Eq1, a sufficient addition effect can be obtained. (See Table 3). When the following Eq1 value is 4.0% by weight or more, a more stable nitrided layer can be obtained on the rolling element surface, and good wear resistance can be obtained.
[0054]
Eq1 = Cr wt% + 0.85Mo wt% + 0.48W wt% + V wt% + 0.45Nb wt% + 0.89Ti wt% + 1.2Al wt%
Further, it is preferable that the addition amounts of Cr, Mo, W, V, Nb, Ti, and Al satisfy the following conditions.
Cr has an effect of enhancing quenching properties and tempering softening resistance, and further, forms carbides to suppress coarsening of crystal grains during heat treatment and impart abrasion resistance. Further, when the nitriding treatment is performed, nitrides and carbonitrides of Cr are precipitated, and wear resistance is improved. However, if it is added in excess of 18.0% by weight, there is a concern that the seizure resistance may be deteriorated due to a decrease in the thermal conductivity of the base material, and the toughness may be reduced due to the formation of δ ferrite. From the above, the Cr content is preferably 18.0% by weight or less, more preferably 16.0% by weight or less. In order to obtain a stable surface hardness of the rolling elements, it is preferable to add 2.0% by weight or more, more preferably 4.0% by weight or more.
[0055]
Mo is an element which remarkably increases the hardenability and the tempering softening resistance, and precipitates carbide which contributes to the improvement of the wear resistance. Further, it has an effect of forming a nitride or a carbonitride by the nitriding treatment to increase hardness and wear resistance. Mo may be added as needed, but if added excessively, the effect is not only saturated, but also the toughness and workability are reduced. Therefore, the amount of Mo added is 7.0% by weight or less, preferably 5%. 0.0% by weight or less.
[0056]
W improves the hardenability similarly to Mo, and further improves the wear resistance by forming a carbide having a high hardness. In addition, nitrides and carbonitrides may be added as necessary to form nitrides and carbonitrides by the nitriding treatment in the same manner as the above-mentioned elements. However, since the workability and toughness are reduced, the amount of W added is about 0.1 to 0.1.以下 0% by weight or less, more preferably 5.と す る 0% by weight or less.
[0057]
V is a powerful nitride-forming element and has an effect of increasing hardness and wear resistance. In addition, the element has a large effect on secondary hardening, and is a very effective element for keeping the core hardness after nitriding at a certain level or more. Therefore, they may be added as needed, but if added in large amounts, the toughness and workability are reduced, so the upper limit of the amount added is 5.0% by weight or less, more preferably 3.0% by weight or less.
[0058]
Nb is also a powerful nitride-forming element, and is a useful element for improving the wear resistance by performing a nitriding treatment. However, when a large amount is added, not only the toughness and the workability are reduced, but also coarse Nb inclusions that reduce the rolling life are generated, so the upper limit is set to 1.0% by weight or less.
Ti is also an element which forms a nitride or a carbonitride by nitriding similarly to the above-mentioned additional elements, and may be added if necessary. However, if the addition amount exceeds 0.5% by weight, the rolling life Since coarse Ti inclusions that cause a decrease tend to be generated, the content thereof is set to 0.5% by weight or less.
[0059]
Al is a strong nitride-forming element, like V, and precipitates Al nitride having extremely high hardness and excellent slidability. However, when the addition amount exceeds 2.0% by weight, the effect of addition is not only saturated, but also Al is an element used as a deoxidizing agent in steel making. There is a possibility that a large number of oxides will remain and reduce the rolling life. From the above, the amount of Al added is set to 2.0% by weight or less.
[Elements to be added as necessary]
Ni is a strong austenite stabilizing element, and has an effect of suppressing the formation of δ ferrite, and further forming a solid solution in a matrix to improve toughness and enhance high-temperature characteristics. However, if it is added more than necessary, a large amount of retained austenite may be formed and sufficient quenching hardness may not be obtained. Therefore, the upper limit is set to 4.5% by weight or less.
[0060]
Co is an austenite stabilizing element like Ni, and has an effect of suppressing the formation of δ-ferrite, and also forming a solid solution in a matrix to suppress carbide aggregation and improve high-temperature hardness. However, when a large amount is added, not only does the processability deteriorate, but also {addition cost increases}, so the upper limit is 1 0.と す る 0% by weight or less.
[Elements required for steelmaking]
Si is an element necessary as a deoxidizing agent at the time of steel making, and is preferably added in an amount of 0.1% by weight or more. Further, when the tempering softening resistance is enhanced, and the surface is further subjected to nitriding treatment, a nitride is formed, which contributes to the improvement of wear resistance. However, if added in a large amount, the toughness and workability are reduced, so the upper limit is made 1.5% by weight.
[0061]
Mn is required to be 0.1% by weight or more as a deoxidizing agent. However, if added in a large amount, not only does the forgeability and machinability deteriorate, but it also coexists with impurities such as S and P to lower the corrosion resistance. To 1.5% by weight.
[About inevitable impurities]
In addition to the alloy elements described above, P ≦ 0. {02}% by weight, S ≦ 0. 05% by weight, Cu ≦ 0. 10 wt%, O ≦ 15 ppm, etc., but it is preferable to set O ≦ 10 ppm in order to minimize nonmetallic inclusions harmful to rolling contact fatigue life characteristics.
(About eutectic carbide)
When coarse eutectic carbides are contained in steel, not only is toughness reduced, but if coarse eutectic carbides are present in the nitrided layer, the nitrided layer may peel off during finishing. Therefore, the maximum eutectic carbide diameter of the steel material is preferably 5 μm or less, more preferably 3 μm or less, in terms of the equivalent circle diameter.
(About surface hardness)
Regarding the surface hardness of the finished rolling element, if the surface hardness does not reach Hv900, a sufficient effect of the nitriding treatment cannot be obtained, so that the surface hardness is at least Hv900. Also, preferably, Hv is 1000 or more. The surface hardness referred to in the present invention refers to a value obtained by measuring the surface of a finished product ball with a load of 200 g using a micro Vickers hardness tester.
(About core hardness)
The core hardness of the finished rolling element must be set to a certain value or more because of the rigidity of the entire ball. It is preferably at least Hv550 or more, and more preferably Hv600 or more.
(About nitrided layer)
By performing the nitriding treatment, a nitride layer having a two-layer structure is formed on the ball surface. Ζ phase-Fe on the outermost surface₂N, Δε phase-Fe_2-3, Γ 'phase -Fe₄N, CrN, Cr₂A compound layer composed of a compound such as N is formed, and in a deeper portion, a diffusion layer is formed in which the nitride forming the compound layer is dispersed in a martensite phase in which nitrogen is dissolved. Become. However, the compound layer formed on the outermost surface is excellent in slidability, but may form a porous and brittle layer, and is particularly used for an electric injection molding machine or an electric press machine subjected to a high load. In the case of the ball screw device, there is a concern that the porous compound layer may be peeled off. Therefore, when such a porous compound layer is formed, it is desirable to remove it by finishing after nitriding. In order to obtain a certain effect by the nitriding treatment, it is preferable that (1) the nitrogen concentration on the surface of the finished product is 0.5% by weight or more. When the nitrogen concentration exceeds 6% by weight, a compound layer containing only nitride is formed. Therefore, the surface nitrogen concentration is preferably 0.5% by weight or more and 6% by weight or less.
[0062]
In a case where a high load is applied, the rigidity of the ball surface increases as the ratio of the nitride layer to the stress volume increases, and as a result, the load resistance improves. Therefore, it is preferable to increase the thickness of the nitrided layer as much as possible. I do. Furthermore, since the required precision may not be satisfied due to a rough surface structure, it is not preferable to make the nitride layer thicker than necessary. Therefore, the thickness of the nitrided layer of the finished product is preferably 2% or less of the ball diameter Da.
[0063]
If the thickness of the nitrided layer is small, the wear resistance and seizure resistance are not improved. In consideration of the roundness and roughness before the nitriding treatment and the throwing power of the nitrided layer, the thickness of the nitrided layer is preferably at least 10 μm, more preferably at least 20 μm. Here, the nitride layer thickness refers to the thickness from the surface of the portion where the nitrogen concentration exceeds 0.2% by weight.
(About the method of making the ball)
Hereinafter, a method of manufacturing a ball subjected to a nitriding treatment will be described.
[0064]
The alloy steel of the present invention is used as a base material of the ball 5. Cold-drawing the alloy steel wire by cold-drawing with a header or the like to form a sphere (hereinafter referred to as elementary sphere). Further, the elementary sphere may be formed by rolling or the like of the wire rod. When burrs or the like are found on the elementary sphere produced by the above method, deburring is performed by flashing or the like.
Next, heat treatment such as quenching and tempering is performed. Note that, before tempering, a sub-zero or cryo treatment may be performed. Further, in order to prevent the occurrence of surface scratches during handling, the hardness may be further increased by mechanical hardening such as barreling, shot peening or ball peening after the heat treatment.
[0065]
Generally, the sphericity or diameter difference after heat treatment is very large. When nitriding is performed using the as-quenched state, the nitrided layer is generated in a shape following the shape of the object to be treated, so that the thickness of the nitrided layer after finishing becomes uneven. In addition, there may be a case where a stress balance generated by the nitriding treatment is broken and a target accuracy cannot be achieved. Therefore, it is preferable to perform a grinding process (rough grinding process) after quenching and hardening (hereinafter, referred to as a semi-processed ball). The sphericity of the semi-finished sphere is desirably 3.0 μm or less, preferably 1.0 μm or less. The semi-processed sphere is processed to a target size obtained by adding a set allowance to the finished product size. The set allowance indicates the necessary allowance for performing the finishing processing to the target accuracy, but of course includes the amount of expansion and contraction due to the nitriding treatment.
[0066]
Next, the semi-finished sphere is subjected to a nitriding treatment. For this nitriding treatment, for example, any method such as gas nitriding, salt bath nitriding, or ion nitriding may be used.
It should be noted that {a preferred form of nitriding treatment is {Nv nitriding process which can be carried out at a relatively low temperature (trademark of Air Water Corporation)}. This Nv nitridation process first involves, for example, NF₃Fluorinated at 200 to 400 ° C. using a fluorine-based gas such as₂-5 0% NH₃In a mixed gas atmosphere of 400 to 480 ° C. to form a nitrided layer.
[0067]
Here, by performing the fluoridation treatment, a Cr oxide or the like on the surface of the material to be treated that inhibits the nitriding treatment is removed. Even at a low temperature of 00 ° C., it is possible to form a relatively uniform nitrided layer. In addition, since the fluoridation is performed at a relatively low temperature, deterioration in accuracy due to the nitriding is suppressed, and the finishing can be easily performed. In addition, there is also an effect that a decrease in core hardness due to nitriding is suppressed as much as possible.
[0068]
As a result, a nitride layer is formed on the surface.₂, Ε phase-Fe_2-3, Γ 'phase -Fe₄N, CrN, Cr₂A compound layer comprising fine precipitates such as N is formed, and the wear resistance is dramatically improved.
Subsequently, it is preferable to perform a finishing process after the nitriding treatment, so that the ball has a surface roughness of 0.1 μmRa or less, specifically, a grade of 60 or more described in JISB1501. The oil film thickness formed on the rolling surface of the rolling member is obtained by calculating the root mean square roughness of the contact surface by σ.₁And σ₂Then (σ₁ ²+ Σ₂ ²)^1/2Is inversely proportional to In other words, the smaller the surface roughness, the better the oil film is formed. In particular, it is very difficult to form an oil film with a ball screw device used in applications such as electric injection molding machines and electric presses. It is very important to finish the roughness with high precision. Therefore, the surface roughness of the ball is set to 0.1 μmRa or less, more preferably 0.03 μmRa or less.
[0069]
In order to set the thickness of the nitrided layer of the finished ball to 10 μm or more and 2% Da or less, it is desirable that the thickness of the nitrided layer after the nitriding treatment be a thickness in consideration of the allowance for finishing.
The ball processed as described above has a surface hardness of Hv900 or more and a nitrogen concentration of 0.1. $ 5% by weight or more and 6% by weight or less. Therefore, it has excellent seizure resistance and wear resistance, and contributes to prolonging the life of the ball screw device.
(About other components)
The nut 3, the tube 6, etc. other than the ball 5 may use a conventional material. For example, if the nut 3 is used, a carburized or carbonitrided SCM420, a hardened SCM445, or the like may be used. However, at least the surface hardness of the rolling surface of the internal thread groove is Hv650 or more, and the oxygen concentration in steel of the base material is 15 ppm or less, preferably 10 ppm or less, and the eutectic carbide diameter is 10 μm or less, preferably 5 μm or less. And
[0070]
As long as the above conditions can be satisfied, the components of the base material and the heat treatment method are not limited. For example, Mo or V or the like may be added to a conventionally used alloy steel such as SCM420 to perform carbonitriding to improve wear resistance. Of course, the alloy steel and heat treatment according to the present invention may be applied. Further, the surface roughness of the thread groove rolling surface is preferably 0.1 mm for the above-mentioned reason. {5 μmRa or less, more preferably 0.1 μmRa or less. The tube 6 may use cold-rolled steel SPCC or the like.
(Second embodiment)
The life of the ball screw device in which the balls 5 were formed from the alloy steel of the present invention and the separator 5 was interposed between the balls 5 was evaluated. Table 3 shows the main chemical components of the various alloy steels used in the test.
[0071]
In the wear test, a ball-on-disk wear tester shown in FIG. 3 was used. In this ball-on-disk wear tester, a ball is placed on a disk-shaped test piece, and the test pieces are rotated in contact with each other while applying a load P from above. Details of the test conditions are shown below.
Surface pressure: 3GPa
Rotation speed: 50min^-1
Lubricating oil: mineral oil (viscosity is low viscosity oil of about VG10)
Refueling amount: 1 cc / min @ Refuel on track surface
Testing temperature: room temperature
Ball material: The material shown in Table 3 was nitrided under the following nitriding conditions A to D
Disk material: SUJ2 hardened (surface roughness 0.1 μm or less)
The materials shown in Table 3 were used for the balls, and the balls were manufactured according to the method for manufacturing the balls described above. For the nitriding treatment, any one of the following methods was used.
[0072]
Nitriding treatment A
Gas nitriding at 500-570 ° C for 1-10 hours (50% N₂-50% NH₃In a mixed gas atmosphere)
Nitriding treatment B
Salt bath nitrocarburizing treatment at 500 to 570 ° C for 1 to 10 hours (salt bath nitriding mainly composed of cyanate such as KCNO or NaCNO)
Nitriding treatment C
Gas nitrocarburizing treatment (50% NH) at 500 to 570 ° C for 1 to 10 hours₃-50% endothermic metamorphic gas (CO, N₂, H₂))
Nitriding D
After fluorination at 3 00 to 400 ° C (90% N₂-10% NF₃(In a mixed gas atmosphere), nitriding at 50-500 ° C. for 5-50 hours (50% N₂-5 0% NH₃In a mixed gas atmosphere)
(Life test)
A ball screw device according to the present invention was manufactured in order to evaluate the fatigue life under the conditions of high load and short stroke. The specifications of the manufactured ball screw device are as shown below.
Screw shaft diameter: 80mm
Lead: 20mm
Screw shaft material: heat-treated alloy steel shown in Table 1 under the above heat treatment conditions A to C
Nut material: SCM420 carburized
Ball material: The material shown in Table 3 was nitrided under nitriding conditions A to D shown above
Separator: Made of synthetic resin
The heat-treated screw shaft, nut, and ball were finished by grinding, and the ball was wrapped. Using the ball screw device manufactured as described above as a specimen, a test was performed under test conditions assuming an injection molding machine. The details of the test conditions are shown below. In these conditions, the applied load was set to be larger than that in the life test performed in Table 2 so that the contact ellipse could easily ride on the groove shoulder.
Load: up to 300kN
Stroke: 20mm
Cycle time: 2 seconds
Lubricant: mineral oil grease
Test temperature: 80 ° C (Nut outer shape is heated by heater)
Table 4 shows the test results.
[0073]
[Table 4]

[0074]
For the measurement of the surface hardness and the core hardness, the balls used in the ball-on-disk wear test were used. The measurement was performed using a micro Vickers hardness tester, and the measurement load was set to 200 g. The surface hardness was measured at the ball surface, and the core hardness was measured at the center of the ball.
The results of the abrasion test on the test pieces are shown as a ratio with the amount of abrasion of Conventional Example E-1 set to 1. In the ball screw durability test, (1) Evaluate the number of shots until any one of the screw shaft, nut, and ball causes surface damage or the rotational torque of the screw shaft sharply increases. The ratio was shown as follows. In Conventional Example E-1, the ball was SUJ2 soak-fired, the screw shaft used was the same as A-12 shown in Table 2, and the nut was obtained by carburizing SCM420.
[0075]
In addition, all of D-1 to D-4 of the comparative examples have Eq1 values lower than 2.0. In Comparative Examples D-5 and D-6, the C content exceeded 1.3% by weight, coarse eutectic carbides were observed, and the nitrided layer was peeled off at the time of finishing after the nitriding treatment. No ball screw durability test was performed. Comparative Examples D-7 and D-8 are Examples C-9 and C-10 without a separator.
[0076]
From the results of the abrasion test, in Examples C-1 to C-30 in which the Eq1 value was 2.0 or more, the surface hardness was Hv900 or more. It can be seen that good wear resistance can be obtained. In particular, when the Eq value is 4.0 or more, the surface hardness becomes Hv1000 or more, and further excellent wear resistance is obtained. In Comparative Examples D-1 to D-4 in which the Eq1 value is less than 2.0, the surface hardness is not sufficiently improved due to insufficient alloying elements for precipitation of nitrides due to the nitriding treatment, and wear resistance is low. The performance is not as good as in each embodiment.
[0077]
Next, from the results of the ball screw durability test, it was found that in each of the examples in which the alloy steel of the present invention was subjected to nitriding treatment on the ball and a separator was interposed, SUJ2 was used for the ball, and the separator was also interposed. It can be seen that the rolling fatigue life is improved to twice or more as compared with the conventional example. In addition, in Example C-9 using a carbonitrided material to which V was added to the screw shaft, rolling fatigue life was improved as compared with Example C-10 in which SCM420 was carburized in the screw shaft. This is because the carbonitrided material to which V is added has better wear resistance than the carburized material of SCM420.
[0078]
In Comparative Examples D-7 and D-8 without the separator, a sufficient fatigue life was obtained as compared with each of the examples including Examples C-9 and C-10 with the separator interposed. You can see that it has not been done. This indicates that simply using a material having excellent wear resistance for the ball is not sufficient for improving the rolling fatigue life, and that a more favorable rolling fatigue life can be obtained by combining the ball with a separator.
[0079]
【The invention's effect】
As is clear from the above description, according to the present invention, the use condition under which a high load is momentarily applied, and the high load is momentarily applied, and has excellent wear resistance even under a short stroke use condition. And a rolling device that can be suitably used. In this case, if the rolling element is subjected to a nitriding treatment, it is more effective to improve the life.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a ball screw device that is an example of a rolling device according to an embodiment of the present invention.
FIG. 2 is a sectional view taken along line AA of FIG.
FIG. 3 is an explanatory view of a ball-on-disk wear tester.
[Explanation of symbols]
1: Screw shaft (inner member)
2: Male thread groove (track surface)
3 ... Nut (outer member)
4: Female thread groove (track surface)
5 Ball (rolling element)
6 ... Tube
7 ... Flange
8 ... plane (notched surface)
9 ... Tube presser
10 ... Seal
11 ... separator
12 ... Specimen
P… Load

Claims (8)

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. A rolling device comprising a rolling element disposed so as to freely roll,
At least the inner member of the inner member, the outer member, and the rolling element is C: 0.15% by weight or more. 6 wt% or less, Si: 0.1 wt% to 0.9 wt%, Mn: 0.1 wt% to 1.5 wt%, Cr: 0.5 wt% to 4.0 wt% The carbide, nitride and carbonitride in the outermost surface layer are formed at least on the raceway surface of the inner member after carburizing or carbonitriding, at least on the raceway surface up to 50 μm from the surface. Is not less than 10% and not more than 30%, the area ratio of carbide, nitride and carbonitride having a particle size of not more than 1 μm is not less than 60% of the total area. The Vickers hardness of the surface layer is set to Hv700 or more, and the maximum particle size of carbide, nitride and carbonitride is set to 5 μm or less in circle equivalent diameter, with a range of 2% of the diameter as a surface layer. Rolling device. The rolling device according to claim 1, wherein the alloy steel contains V: 2.0% by weight or less, Mo: 3.0% by weight or less, and Ni: 4.5% by weight or less. 3. The rolling device according to claim 1, wherein at least the concentration of C + N in the surface layer is 0.7% by weight or more and 1.7% by weight or less. The rolling device according to any one of claims 1 to 3, wherein at least the retained austenite of the surface layer is set to 10% by volume or more and 45% by volume or less. The rolling device according to any one of claims 1 to 4, wherein a separator is interposed between the rolling elements. The rolling device according to any one of claims 1 to 5, wherein a compressive residual stress on a surface of the inner member is set to 150 MPa or more. The finished product of the rolling element has a surface hardness of Hv900 or more, and has a nitrided layer having a nitrogen concentration of 0.5% or more and 6% or less in a range of 10 μm or more and 2% or less of the rolling element diameter from the surface. The rolling device according to any one of claims 1 to 6. The rolling device according to any one of claims 1 to 7, wherein the inner member is a screw shaft of a ball screw device, and the outer member is a nut of a ball screw device.

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