JP2004256831A - Steel material for nitriding excellent in magnetic property after nitriding, and its formed body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for nitriding excellent in magnetic properties after nitriding to which an inexpensive forming method such as plastic working can be applied and in which sufficient increase in hardness after nitriding and sufficient hardening depth can be obtained and superior magnetic properties after nitriding can be obtained and also to provide its formed body. <P>SOLUTION: The steel material for nitriding excellent in magnetic properties after nitriding has a composition containing, by mass ratio, 0.01 to 0.10% C, 0.005 to 1.0% Si, 0.010 to 3.00% Mn, 0.001 to 0.150% P, 0.0002 to 0.0100% N, 0.15 to 3.00% Cr, <0.010% Ti and <0.010% Nb, further containing any one of, by mass, (1) 0.01 to 0.10% V, (2) 0.05 to 2.00% Al and (3) 0.01 to 0.10% V and 0.005 to 2.00% Al, and having the balance iron with inevitable impurities. The formed body using this steel material has, in at least a part thereof, a surface nitrided layer in which precipitates with a size of ≤40 nm are uniformly dispersed. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、窒化後の磁気特性に優れた窒化用鋼材およびその成形体に関し、詳しくは、成形性にも優れる前記窒化用鋼材、およびこの鋼材からなる成形体、すなわち、例えば電磁力により駆動する機械部品、工具など、耐摩耗性、耐疲労強度が必要とされる部品に用いられる、加工性および磁気特性に優れかつ耐摩耗性に優れた成形体に関する。
【０００２】
【従来の技術】
窒化用鋼材およびその成形体に関する従来技術として、プレス加工や曲げ加工等の安価な成形法で形を成形でき、かつ窒化性、すなわち窒化による硬度上昇にすぐれる鋼板、および該鋼板を用いた経済性、生産性に優れ、かつ加工性、耐摩耗性に優れたプレス成形体の提供を目的とした、（１）重量比でＣ：０．０１〜０．０８％未満、Ｓｉ：０．００５ 〜１．００％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５超〜５．００％、Ａｌ：０．０６０ 超〜２．００％を含有し、さらに、Ｔｉ：０．０１０ ％以上でかつ４Ｃ〔％〕未満、Ｖ：０．０１０ 〜１．００％の１種または２種を含有し、残部が鉄および不可避的不純物からなる成形性に優れた窒化用鋼板、ならびに（２） （１） に記載の鋼板からなるプレス成形体において該成形体の少なくとも片面に硬質窒化層を有するプレス成形体が知られている（特許文献１参照）。
【０００３】
【特許文献１】
特開平９−２５５４３号公報
【０００４】
【発明が解決しようとする課題】
特許文献１記載の技術では、成形性に優れた窒化用鋼板、及び該鋼板からなる工具、機械構造用部品、自動車の部品など、耐摩耗性、耐疲労強度、耐焼付性を必要とされる部品に用いられる加工性と耐摩耗性に優れたプレス成形体が対象となっている。
【０００５】
一方、例えば電磁ブレーキなど、磁力を介して駆動する機械部品および工具では、部品間の接触による摩耗およびチッピングによる疲労強度低下を防止する目的で表面硬化処理が行われる。この場合、機械加工後の寸法精度を維持するために、例えば５５０ 〜６００ ℃程度の低温処理を基調とする窒化処理が施される。このような機械部品では、磁力を介して駆動する部位と接触駆動する部位とを別々に作製し、組み立てる工程が採用されており、したがって、耐摩耗性、硬度といった機械的性質と磁気特性との両方に優れたものが要望されている。
【０００６】
しかしながら、特許文献１記載の技術のように、Ｔｉ添加を基調とするものは、粒子径が５０ｎｍを超えるＴｉ炭窒化物が多数析出しやすく、これら析出物は磁壁移動を抑制するため、磁気特性が悪化しやすい問題がある。また、Ｔｉ以外の炭窒化物形成元素としてＣｒ、Ｖ、Ｎｂなどを用いた場合でも、同様の問題が発生する場合があって、磁気特性と機械的性質とを安定して両立させることは困難であった。
【０００７】
このように従来技術では困難であった磁気特性と機械的性質の両立が課題であり、特に合金鋼で問題になった高強度、難加工性による研削加工などの機械加工費の上昇に対し、安易かつ低コストな塑性加工により形成することも課題である。すなわち、本発明の目的は、塑性加工などの安価な成形法を適用でき、かつ窒化後の硬度上昇、硬化深さが十分であり、さらに窒化後の磁気特性が良好な、窒化後の磁気特性に優れた窒化用鋼材およびその成形体を提供することにある。
【０００８】
【課題を解決するための手段】
前記目的を達成した本発明の特徴とするところは以下のとおりである。
（１）質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１．０ ％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０． ０１０％未満、Ｎｂ：０．０１０ ％未満、Ｖ：０．０１〜０．１０％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。
【０００９】
（２）質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１．０ ％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０．０１０ ％未満、Ｎｂ：０．０１０ ％未満、Ａｌ：０．０５〜２．００％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。
（３）質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１． ０％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０． ０１０％未満、Ｎｂ：０．０１０ ％未満、Ｖ：０．０１〜０．１０％、Ａｌ：０．００５ 〜２．００％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。
【００１０】
（４）（１）〜（３）のいずれかに記載された鋼材を用いた成形体であって、該成形体の少なくとも一部に、サイズ４０ｎｍ以下の析出物が均一に分散した表面硬化層を有する成形体。
【００１１】
【発明の実施の形態】
以下に、鋼の化学成分組成を限定した理由について述べる。
Ｃは、鋼の成形性に影響を及ぼす元素であり、０．１０％超では、加工性が低下し、また、他の元素を加えたときの成形性の悪化が助長されるため、０．１０％以下とした。一方、０．０１％未満では機械構造用としての強度が不足しあるいは結晶粒界強度が低下する懸念があるため、０．０１％以上とした。窒化処理前の成形における加工性を確保する観点および窒化処理後の化合物層（表面硬化層）直下の硬度を適性化する観点から、好ましくは０．０３〜０．０８％である。
【００１２】
Ｃｒは、窒化硬化に非常に重要な元素であり、特に表層の硬度を確保するのに適した元素であり、かつ磁化過程で重要な役割を果たす磁壁移動の妨げにならない微細な析出粒子を形成するのに有利な元素である。添加されたＣｒの含有量が０．１５％未満では十分な硬度上昇が得られないため、０．１５％以上を含有するものとし、一方、３．００％を超えると微細析出粒子が凝集し磁壁移動の抵抗が増加するため、３．００％以下とした。析出形態の適正化および必要硬度を達成する観点から、０．５０〜２．５０％が好ましい。
【００１３】
Ａｌは、脱酸成分として必要に応じて添加される。脱酸成分として添加する場合は、ブローホール等の欠陥の発生を防止するため、０． ００５％以上含有させる。一般に、アルミキルド鋼では、０．０３０ 〜０．０５０ ％含有するように添加されている。また、Ａｌは、窒素との親和力が強く、かつ形成される窒化アルミの硬度が高いことから、表層を非常に硬くする元素であるため、本発明では表面硬度を高める目的でＡｌを添加することができる。この目的で添加する場合は、０．０５０ ％以上含有させることが必要となる。好ましくは０．０８０ ％以上である。一方、２．００％を超えると成形性が劣化するので、２．００％を上限とする。
【００１４】
Ｖは、適量のＣｒあるいはさらにＡｌとの複合添加により表面硬度を高める働きがあるので、本発明ではＡｌに代えて、またはＡｌと共に添加される。しかし、ＣｒおよびＡｌに比較して窒化物形成の自由エネルギー変化が小さいため、窒素の内方拡散が進行しやすく、硬化深さが著しく深くなる。したがって、所望の硬化分布に合わせてＶ添加量を調整する必要がある。０．０１％未満の含有では窒化による硬度上昇が小さいので０．０１％を下限とする。一方、Ｖを過剰に添加すると、鋼中の炭素を析出物として、結晶粒間の接着力を弱め、Ｔｉほどではないが、スラブ割れを起こしやすくする危険性があり、とくに、０．１０％を超えて含有すると延性が低下し、かつ透磁率も低下するので、０．１０％を上限とする。好ましくは０．０３〜０．０８％である。
【００１５】
以上が本発明に係る鋼材の窒化処理性向上のための重要成分元素であるが、本発明では、さらに、鋼板としての成形性を確保するために、以下の範囲の元素を含有成分とする。
Ｓｉは、０．００５ ％未満では製鋼コストが大幅に高くなり経済的ではないので、０．００５ ％以上としたが、コスト的に成り立つのであればこれ未満であってもよい。一方、１．００％を超えると熱延時に低融点スケールが生成しやすいので外観不良を招き、また延性も低下してくるので、１．００％以下とした。窒化処理時に表面酸化層形成によるバラツキをなくす意味で、好ましくは０．０７％以下である。
【００１６】
Ｍｎは、０．０１０ ％未満では製鋼コストが大幅に高くなり経済的ではないので、０． ０１０％を下限とし、一方、３．００％を超えると高い成形性が得られなくなるので、３．００％を上限とする。強度、成形性の面から０．１０〜２．５０％が好ましい範囲である。
Ｐは、成形性を損なわずに強度を上昇させうる元素であり、強度レベルに応じて添加するが、０．００１ ％未満にするには製鋼コストが大幅に高くなり経済的ではないので０．００１ ％を下限とし、一方、０．１５０ ％を超えると結晶粒界への偏析が顕在化し脆化する問題が発生するため０．１５０ ％を上限とする。窒化処理時の結晶粒界およびその近傍での析出粒子の析出量を適正化する観点から、Ｐは０．０８％以上の含有が好ましい。
【００１７】
Ｎは、０．０００２％未満では製鋼コストが大幅に高くなり経済的ではないので０．０００２％を下限とし、一方、０．０１００％を超えると加工性が劣化するので０．０１００％を上限とする。好ましくは０．００５０％以下である。
ＴｉおよびＮｂは、窒化処理においてＮ拡散を抑制する働きがあるため、本発明では含有させないのがベストである。しかしながら過度の低減化は製鋼コストの上昇をまねくので０．０１０ ％未満を許容範囲とした。また、いずれも０．０１０ ％以上含有すると、熱間圧延過程および冷延‐焼鈍過程において、磁壁移動のピンニングとなる炭化物、窒化物および炭窒化物を形成させるため、この意味でも０．０１０ ％未満とする。
【００１８】
本発明に係る鋼材を製造する方法は、製鋼段階で上記の化学組成になるように鋼成分調整すること以外は、特に限定されない。例えば鋼材に鋼板を充てる場合、鋳造後の加熱、圧延条件などは通常採用される各種条件のうちのいずれを用いてもよい。熱間圧延を行う場合、熱間圧延以前および熱間圧延については特に規定しないが、熱延鋼板から成形体を製造する場合は、加工性向上のために ５００℃以上で巻取りを行うことが好ましい。
【００１９】
該熱延鋼板を冷間圧延し、引続き焼鈍することにより鋼板の板厚精度および加工性が向上する。この場合、熱間圧延以前および熱間圧延については特に規定しない。
冷間圧延では５０〜８０％の圧下率で圧延するのが好ましい。５０％以上の冷間圧延により高い成形性がもたらされる。最も望ましくは６０％以上である。一方、８０％を超える冷間圧延を行うと、再結晶焼鈍の集合組織が加工性を損なうため、８０％以下が好適である。冷間圧延に引き続く再結晶焼鈍は、箱型焼鈍、連続焼鈍のいずれであってもよい。焼鈍条件については特に規定しないが、再結晶温度以上でかつ粗大粒が生成しない ９００℃以下で行うことが好ましい。またその後、 加工性の向上や外観のために、本発明に係る鋼板に調質圧延や、塗油、固体潤滑剤の塗布等を施すことは、何ら差支えない。
【００２０】
上記化学成分組成になる鋼板を用いて、塑性加工により成形体を得た後に、窒化処理を施すことで、微細なＣｒ窒化物が均一に析出することにより、表面硬度および成形体内部の硬度を上昇させることができる。この微細なＣｒ窒化物は４０ｎｍ以下の析出物サイズを有し、磁壁移動の抵抗にならないため、窒化処理を施しても磁気特性が悪化せず、優れた透磁率を得ることができる。ここで、析出物サイズは、透過型電子顕微鏡により析出物を観察し、画像処理により円相当径を求めて評価した。なお、平均円相当径が３０ｎｍ以下となることが好ましい。ここでの表面硬度とは、表面より深さ５０μｍ の位置での断面硬度を指す。好ましい表面硬度はＨｖ ５００以上である。塑性加工による転位密度の大きさに関らず、微細なＣｒ窒化物析出による硬化が起こるので、成形体の各部位での塑性加工歪量の変化に起因した硬度差は発生しにくい。
【００２１】
また、最表層には主成分として鉄窒化物が形成されるが、好ましくは炭素を含有する鉄窒化物が形成するように、窒化処理ガス中に炭素源（例えばＲＸガスまたはＣＯ_２ ガス等）を供給することが好ましい。これにより、耐摩耗性に優れた表面窒化層が形成される。表面窒化層は、表面亀裂の発生を抑制し、耐疲労強度、耐焼付性を向上させる。
【００２２】
本発明でいう加工性に係る加工とは、成形体の形状によって選択されるあらゆる成形加工を包含し、例えば、絞り加工、曲げ加工、しごき加工、打ち抜き加工等々のいずれであっても何ら差支えない。
成形体を所定の形状に成形後、引き続き窒化処理を施すことにより成形体表面に表面硬化層（窒素化合物層）と拡散層とからなる表面窒化層を形成する。ここにいう拡散層とは、表面硬化層から鋼材内部に向かう微細なＣｒ窒化物主体の窒素化合物析出層を指す。また、窒化処理したままでの前記表面硬化層（窒素化合物層）およびその下層の拡散層の合計厚みを窒化深さと称する。
【００２３】
窒化処理としては、ガス窒化処理、ガス軟窒化処理、塩浴窒化処理、イオン窒化処理、酸窒化処理、浸硫窒化処理など各種の処理方法があり、表面硬化層を形成する処理方法ならいずれでも構わないが、磁壁移動を妨げる４０ｎｍを超える析出物が形成されないことが必要である。好ましくは、表面硬化層の緻密性および靭性を高めるため、浸炭性ガスを含有する雰囲気で窒化処理を行うことである。また、必要な拡散層を得るために適宜に処理時間や処理温度を代えることができる。また、表面硬化層が過大であれ、研削等の手段により、表面硬化層の厚みを窒化処理ままでのそれより減じて、厚みを調節したり、表面の粗度を調整しても何ら差支えない。また、表面硬化層は厚みが１μｍ 以上あればよいが、十分な耐摩耗性を得るには１０μｍ 以上の厚みであることが好ましい。一方、数百μｍ の厚みにするのは、表面硬化層の耐摩耗性向上効果が飽和することに加え、脆くなるのでむしろ好ましくない。
【００２４】
【実施例】
以下、実施例を挙げて本発明をより具体的に説明する。
〔実施例１〕
表１に示す化学成分組成になる鋼を溶製し、常法に従い連続鋳造によりスラブとした。このスラブを加熱炉中で１２００℃まで加熱し、 ９１０℃以上の仕上げ温度で熱間圧延し、その後６００ ℃で巻取り、さらに酸洗を行って、得られた熱延鋼板から試験片を採取し脱脂した後、ＮＨ_３ ガスとＲＸガスの混合雰囲気ガス中で５７０ ℃×３時間の条件で窒化処理し、油冷した。この試験片について、マイクロビッカース硬度計を用い表面から深さ５０μｍ の位置で測定した硬度（Ｈｖ）をもって窒化性を評価した。直流磁化特性は２５ｃｍエプスタイン測定によりμｍａｘ で評価した。窒化処理後の成形体の電磁駆動性能を評価するため、エプスタイン試験片（板厚×３０ｍｍ幅×２８０ｍｍ 長さ）を圧延方向と圧延方向から９０度方向の二方向から、合わせて５００ｇを超える最少枚数を用意し、上記と同様に窒化処理を施し、その後、直流磁化測定を行い最大透磁率μｍａｘ を求めた。高い電磁駆動性能を得るためには、μｍａｘ が６００ｅｍｕ以上であることが望ましい。析出物サイズ（円相当径）は透過型電子顕微鏡写真の画像処理から求めた。これらの結果を表１に併記する。
【００２５】
【表１】

【００２６】
表１より、本発明例は、表面硬度およびμｍａｘ ともに良好な値を示すことがわかる。比較例８，１０，１１，１２，１３では所望の表面硬度は得られているが、磁壁移動の妨げとなるサイズ４０ｎｍ超の析出物（炭化物、窒化物および炭窒化物）が窒化処理前から存在し、あるいは窒化処理により形成されて存在するため、窒化処理後の磁気特性が悪い。
【００２７】
〔実施例２〕
表２に示す化学成分組成になる鋼を溶製し、常法に従い連続鋳造によりスラブとした。このスラブを加熱炉中で１２００℃まで加熱し、 ９１０℃以上の仕上げ温度で熱間圧延し、その後 ６００℃で巻取り、ついで酸洗し、７０％の圧下率で冷間圧延した後、 ８００℃×６０秒の条件で再結晶焼鈍を行い、得られた冷延鋼板に、処理時間および処理温度を種々変えた窒化処理を施して該鋼板の表面硬度を変化させ、析出物サイズ（円相当径）と直流磁化特性のμｍａｘ および硬度（Ｈｖ）との対応関係を調べた結果を表２に示す。なお、窒化処理では、ＮＨ_３ ガスとＲＸガスの混合雰囲気ガス中で処理し、処理後は油冷により冷却した。また、直流磁化特性のμｍａｘ ，硬度（Ｈｖ），析出物サイズ（円相当径）は、上述の〔実施例１〕と同一の方法で測定した。
【００２８】
【表２】

【００２９】
本発明例１〜７はいずれも、Ｈｖ５００ 以上の表面硬度と６００ｅｍｕ以上の最大透磁率を示し、かつ析出物サイズは４０ｎｍ以下である。なお、析出物サイズ（円相当径）と、直流磁化特性のμｍａｘ および硬度（Ｈｖ）との対応関係を整理したグラフを図１に示す。
【００３０】
【発明の効果】
本発明によれば、高い窒化性をもち、かつ窒化後の磁気特性に優れた成形体を製造でき、これにより経済性および生産性にすぐれた耐摩耗性、耐疲労強度、高透磁率を兼ね備えた工具、機械構造用部品などに用いられる成形体が得られるという効果を奏する。
【図面の簡単な説明】
【図１】析出物サイズと表面硬度Ｈｖおよび最大透磁率μｍａｘ との対応関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitriding steel material excellent in magnetic properties after nitriding and a molded body thereof, and more specifically, the nitriding steel material excellent in formability and a molded body made of the steel material, that is, driven by, for example, electromagnetic force. The present invention relates to a molded article having excellent workability and magnetic properties and excellent wear resistance, which is used for parts that require wear resistance and fatigue strength, such as machine parts and tools.
[0002]
[Prior art]
As a conventional technique related to nitriding steel and its molded body, a steel sheet that can be shaped by an inexpensive molding method such as press working or bending, and has excellent nitriding properties, that is, increased hardness due to nitriding, and economy using the steel sheet (1) C: 0.01 to less than 0.08% by weight, Si: 0.005 for the purpose of providing a press-molded body excellent in workability and productivity, and excellent in workability and wear resistance ~ 1.00%, Mn: 0.010 ~ 3.00%, P: 0.001 ~ 0.150%, N: 0.0002 ~ 0.0100%, Cr: over 0.15 ~ 5.00% , Al: more than 0.060 to 2.00%, and further, Ti: 0.010% or more and less than 4C [%], V: 0.010 to 1.00% 1 type or 2 types A steel sheet for nitriding that contains and the balance is iron and inevitable impurities and has excellent formability, In addition, a press-formed body comprising a hard nitride layer on at least one side of the formed body of the steel sheet described in (2) (1) is known (see Patent Document 1).
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 9-25543
[Problems to be solved by the invention]
The technology described in Patent Document 1 requires wear resistance, fatigue strength, and seizure resistance, such as a steel sheet for nitriding excellent in formability, a tool made of the steel sheet, a machine structural part, and an automobile part. A press-molded body excellent in workability and wear resistance used for parts is targeted.
[0005]
On the other hand, for machine parts and tools that are driven via magnetic force, such as electromagnetic brakes, surface hardening is performed for the purpose of preventing wear due to contact between parts and fatigue strength reduction due to chipping. In this case, in order to maintain the dimensional accuracy after machining, a nitriding treatment based on a low-temperature treatment of, for example, about 550 to 600 ° C. is performed. In such mechanical parts, a process of separately manufacturing and assembling a part driven by magnetic force and a part driven by contact is adopted, and therefore, mechanical properties such as wear resistance and hardness and magnetic characteristics are used. What is excellent in both is desired.
[0006]
However, as in the technique described in Patent Document 1, those based on the addition of Ti are likely to precipitate a large number of Ti carbonitrides having a particle diameter exceeding 50 nm. There is a problem that is easy to get worse. Further, even when Cr, V, Nb, or the like is used as a carbonitride-forming element other than Ti, the same problem may occur, and it is difficult to stably achieve both magnetic properties and mechanical properties. Met.
[0007]
In this way, it is a challenge to achieve both magnetic properties and mechanical properties that were difficult with the prior art, especially against the increase in machining costs such as grinding due to high strength and difficult workability, which was a problem with alloy steel, Forming by easy and low-cost plastic working is also an issue. That is, the object of the present invention is to apply an inexpensive molding method such as plastic working, and to increase the hardness after nitriding, the curing depth is sufficient, and the magnetic properties after nitriding are good, and the magnetic properties after nitriding An object of the present invention is to provide a nitriding steel material excellent in the above and a molded body thereof.
[0008]
[Means for Solving the Problems]
The features of the present invention that achieve the above object are as follows.
(1) By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1.0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150% , N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: 0.0. A steel for nitriding containing less than 010%, Nb: less than 0.010%, and V: 0.01 to 0.10%, the balance being iron and inevitable impurities and excellent magnetic properties after nitriding.
[0009]
(2) By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1.0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150% N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: less than 0.010%, Nb: less than 0.010%, Al: 0.05 to 2.00% A steel material for nitriding that is excellent in magnetic properties after nitriding, containing iron and the inevitable impurities.
(3) By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1. 0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: 0 . Magnetic after nitriding containing less than 010%, Nb: less than 0.010%, V: 0.01 to 0.10%, Al: 0.005 to 2.00%, the balance being iron and inevitable impurities Steel for nitriding with excellent characteristics.
[0010]
(4) A surface-hardened layer using a steel material according to any one of (1) to (3), wherein a precipitate having a size of 40 nm or less is uniformly dispersed in at least a part of the formed body. A molded body having
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the chemical composition of steel is limited will be described below.
C is an element that affects the formability of steel. If it exceeds 0.10%, the workability deteriorates, and the deterioration of formability when other elements are added is promoted. 10% or less. On the other hand, if it is less than 0.01%, the mechanical structure strength is insufficient, or there is a concern that the grain boundary strength is lowered, so the content was made 0.01% or more. From the viewpoint of ensuring workability in molding before nitriding treatment and from the viewpoint of optimizing the hardness just under the compound layer (surface hardened layer) after nitriding treatment, it is preferably 0.03 to 0.08%.
[0012]
Cr is a very important element for nitriding and hardening, especially suitable for securing the hardness of the surface layer, and forms fine precipitated particles that do not interfere with domain wall movement, which plays an important role in the magnetization process. It is an advantageous element to do. If the content of the added Cr is less than 0.15%, a sufficient increase in hardness cannot be obtained. Therefore, the content should be 0.15% or more. On the other hand, if the content exceeds 3.00%, fine precipitated particles aggregate. Since the resistance of domain wall movement increases, the content is set to 3.00% or less. From the viewpoint of achieving appropriate precipitation form and necessary hardness, 0.50 to 2.50% is preferable.
[0013]
Al is added as necessary as a deoxidizing component. When added as a deoxidizing component, in order to prevent the occurrence of defects such as blowholes, 0. 005% or more is contained. In general, aluminum killed steel is added so as to contain 0.030 to 0.050%. In addition, Al is an element that makes the surface layer extremely hard because it has a strong affinity with nitrogen and the hardness of the formed aluminum nitride, so in the present invention, Al is added for the purpose of increasing the surface hardness. Can do. When adding for this purpose, it is necessary to make it contain 0.050% or more. Preferably it is 0.080% or more. On the other hand, if it exceeds 2.00%, formability deteriorates, so 2.00% is made the upper limit.
[0014]
V has a function of increasing the surface hardness by adding a suitable amount of Cr or further combined with Al. Therefore, V is added in place of Al or together with Al in the present invention. However, since the change in the free energy of nitride formation is smaller than that of Cr and Al, the inward diffusion of nitrogen is likely to proceed, and the hardening depth becomes extremely deep. Therefore, it is necessary to adjust the V addition amount in accordance with a desired curing distribution. If the content is less than 0.01%, the increase in hardness due to nitriding is small, so 0.01% is made the lower limit. On the other hand, if V is added excessively, carbon in the steel becomes a precipitate, which weakens the adhesive strength between crystal grains and is not as high as Ti, but may cause slab cracking, especially 0.10%. If the content exceeds V, the ductility is lowered and the magnetic permeability is also lowered, so the upper limit is made 0.10%. Preferably it is 0.03 to 0.08%.
[0015]
The above is an important component element for improving the nitriding property of the steel material according to the present invention. In the present invention, in order to further secure the formability as a steel sheet, the following range of elements is included.
If Si is less than 0.005%, the steelmaking cost is significantly increased and it is not economical. Therefore, the Si content is set to 0.005% or more. On the other hand, if it exceeds 1.00%, a low melting point scale is likely to be formed during hot rolling, resulting in poor appearance and reduced ductility. In order to eliminate variation due to the formation of the surface oxide layer during nitriding, it is preferably 0.07% or less.
[0016]
If Mn is less than 0.010%, the steelmaking cost will be significantly increased, which is not economical. On the other hand, 0.10% is set as the lower limit, whereas if it exceeds 3.00%, high formability cannot be obtained, so 3.00% is set as the upper limit. 0.10 to 2.50% is a preferable range in terms of strength and formability.
P is an element that can increase the strength without impairing the formability, and is added depending on the strength level. However, if it is less than 0.001%, the steelmaking cost is significantly increased and it is not economical. On the other hand, the lower limit is 001%. On the other hand, if it exceeds 0.150%, segregation at the grain boundaries becomes obvious and embrittlement occurs. Therefore, the upper limit is 0.150%. From the viewpoint of optimizing the amount of precipitated particles precipitated at and near the crystal grain boundaries during nitriding, the P content is preferably 0.08% or more.
[0017]
If N is less than 0.0002%, the steelmaking cost is significantly increased and it is not economical, so 0.0002% is set as the lower limit. On the other hand, if it exceeds 0.0100%, the workability deteriorates, so 0.0100% is set as the upper limit. And Preferably it is 0.0050% or less.
Since Ti and Nb have a function of suppressing N diffusion in the nitriding treatment, it is best not to contain them in the present invention. However, excessive reduction leads to an increase in steelmaking costs, so the allowable range is less than 0.010%. Further, when 0.010% or more is contained in each case, carbides, nitrides, and carbonitrides that serve as pinning of domain wall motion are formed in the hot rolling process and the cold rolling-annealing process. Less than.
[0018]
The method for producing the steel material according to the present invention is not particularly limited except that the steel components are adjusted so as to have the above chemical composition in the steel making stage. For example, when a steel plate is used as a steel material, any of various conditions that are usually employed may be used for heating after casting, rolling conditions, and the like. When hot rolling is performed, pre-hot rolling and hot rolling are not particularly specified. However, when forming a formed body from a hot-rolled steel sheet, it may be wound at 500 ° C. or higher for improving workability. preferable.
[0019]
The hot-rolled steel sheet is cold-rolled and subsequently annealed to improve the thickness accuracy and workability of the steel sheet. In this case, there is no particular restriction on hot rolling and before hot rolling.
In cold rolling, it is preferable to roll at a rolling reduction of 50 to 80%. High formability is brought about by cold rolling of 50% or more. Most desirably, it is 60% or more. On the other hand, when cold rolling exceeding 80% is performed, the texture of recrystallization annealing deteriorates workability, so 80% or less is preferable. Recrystallization annealing subsequent to cold rolling may be either box-type annealing or continuous annealing. The annealing conditions are not particularly specified, but it is preferable to carry out the annealing at 900 ° C. or lower which is not lower than the recrystallization temperature and does not produce coarse grains. Further, for improving the workability and the appearance, there is no problem in subjecting the steel sheet according to the present invention to temper rolling, oil application, solid lubricant application, or the like.
[0020]
Using a steel plate having the above chemical composition composition, after obtaining a molded body by plastic working, by performing nitriding treatment, fine Cr nitride is uniformly precipitated, so that the surface hardness and the hardness inside the molded body are increased. Can be raised. Since this fine Cr nitride has a precipitate size of 40 nm or less and does not become a resistance to domain wall motion, even if nitriding is performed, the magnetic properties are not deteriorated, and excellent magnetic permeability can be obtained. Here, the precipitate size was evaluated by observing the precipitate with a transmission electron microscope and obtaining the equivalent circle diameter by image processing. The average equivalent circle diameter is preferably 30 nm or less. The surface hardness here refers to the cross-sectional hardness at a depth of 50 μm from the surface. A preferable surface hardness is Hv 500 or more. Regardless of the dislocation density due to plastic working, hardening due to fine Cr nitride precipitation occurs, so that a hardness difference due to changes in the amount of plastic working strain at each part of the compact is unlikely to occur.
[0021]
Further, iron nitride is formed as a main component in the outermost layer, but preferably a carbon source (for example, RX gas or CO ₂ gas) in the nitriding gas so that iron nitride containing carbon is formed. Is preferably supplied. Thereby, the surface nitride layer excellent in abrasion resistance is formed. The surface nitride layer suppresses the occurrence of surface cracks and improves fatigue resistance and seizure resistance.
[0022]
The processing related to workability in the present invention includes all molding processes selected depending on the shape of the molded body, and may be any of drawing, bending, ironing, punching, and the like. .
After forming the formed body into a predetermined shape, a nitriding treatment is subsequently performed to form a surface nitrided layer comprising a surface hardened layer (nitrogen compound layer) and a diffusion layer on the surface of the formed body. The diffusion layer referred to here refers to a fine Cr nitride-based nitrogen compound precipitation layer that goes from the surface hardened layer to the inside of the steel material. Further, the total thickness of the surface hardened layer (nitrogen compound layer) and the diffusion layer below the surface nitrided layer is referred to as nitriding depth.
[0023]
As nitriding treatment, there are various treatment methods such as gas nitriding treatment, gas soft nitriding treatment, salt bath nitriding treatment, ion nitriding treatment, oxynitriding treatment, sulfur nitriding treatment, and any treatment method for forming a surface hardened layer. Although it does not matter, it is necessary that precipitates exceeding 40 nm that prevent domain wall movement are not formed. Preferably, the nitriding treatment is performed in an atmosphere containing a carburizing gas in order to improve the denseness and toughness of the surface hardened layer. Moreover, in order to obtain a required diffused layer, processing time and processing temperature can be changed suitably. In addition, even if the surface hardened layer is excessive, there is no problem even if the thickness of the surface hardened layer is reduced by grinding or other means to adjust the thickness or the surface roughness. . The surface hardened layer may have a thickness of 1 μm or more, but is preferably 10 μm or more in order to obtain sufficient wear resistance. On the other hand, a thickness of several hundred μm is not preferable because the effect of improving the wear resistance of the surface hardened layer becomes saturated and becomes brittle.
[0024]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples.
[Example 1]
Steels having the chemical composition shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. This slab is heated to 1200 ° C in a heating furnace, hot-rolled at a finishing temperature of 910 ° C or higher, then wound at 600 ° C, pickled, and a specimen is taken from the obtained hot-rolled steel sheet. After degreasing, nitriding was performed in a mixed atmosphere gas of NH ₃ gas and RX gas under conditions of 570 ° C. × 3 hours, followed by oil cooling. About this test piece, nitriding property was evaluated by the hardness (Hv) measured at a position of a depth of 50 μm from the surface using a micro Vickers hardness tester. The direct current magnetization characteristics were evaluated at μmax by 25 cm Epstein measurement. In order to evaluate the electromagnetic drive performance of the molded body after nitriding, the Epstein test piece (plate thickness x 30 mm width x 280 mm length) is minimum over 500 g in total from the rolling direction and the 90 ° direction from the rolling direction. A number of sheets were prepared and subjected to nitriding treatment in the same manner as described above, and then direct current magnetization measurement was performed to obtain the maximum magnetic permeability μmax. In order to obtain high electromagnetic drive performance, it is desirable that μmax is 600 emu or more. The precipitate size (equivalent circle diameter) was determined from image processing of a transmission electron micrograph. These results are also shown in Table 1.
[0025]
[Table 1]

[0026]
From Table 1, it can be seen that the inventive examples show good values for both surface hardness and μmax. In Comparative Examples 8, 10, 11, 12, and 13, the desired surface hardness was obtained, but precipitates (carbides, nitrides, and carbonitrides) with a size exceeding 40 nm that hindered domain wall movement were observed before nitriding. Since it exists or is formed by nitriding, the magnetic properties after nitriding are poor.
[0027]
[Example 2]
Steels having the chemical composition shown in Table 2 were melted and slabs were obtained by continuous casting according to a conventional method. The slab was heated to 1200 ° C. in a heating furnace, hot-rolled at a finishing temperature of 910 ° C. or higher, then wound at 600 ° C., then pickled, and cold-rolled at a reduction rate of 70%. Recrystallization annealing was performed under the conditions of ° C x 60 seconds, and the resulting cold-rolled steel sheet was subjected to nitriding treatment with various treatment times and treatment temperatures to change the surface hardness of the steel sheet, and the precipitate size (equivalent to a circle) Table 2 shows the results of investigating the correspondence between the diameter) and the DC magnetization characteristics μmax and hardness (Hv). In the nitriding treatment, treatment was performed in a mixed atmosphere gas of NH ₃ gas and RX gas, and after the treatment, cooling was performed by oil cooling. Further, μmax, hardness (Hv), and precipitate size (equivalent circle diameter) of DC magnetization characteristics were measured by the same method as in the above-mentioned [Example 1].
[0028]
[Table 2]

[0029]
Inventive Examples 1 to 7 all show a surface hardness of Hv500 or more and a maximum magnetic permeability of 600 emu or more, and the precipitate size is 40 nm or less. FIG. 1 shows a graph in which the correspondence between the precipitate size (equivalent circle diameter), the direct current magnetization characteristic μmax and the hardness (Hv) is arranged.
[0030]
【The invention's effect】
According to the present invention, a molded body having high nitriding properties and excellent magnetic properties after nitriding can be produced, thereby combining wear resistance, fatigue strength strength, and high magnetic permeability excellent in economy and productivity. It is possible to obtain a molded body used for a tool, a machine structural part, or the like.
[Brief description of the drawings]
FIG. 1 is a diagram showing the correspondence between precipitate size, surface hardness Hv, and maximum magnetic permeability μmax.

Claims (4)

質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１．０ ％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０．０１０ ％未満、Ｎｂ：０．０１０ ％未満、Ｖ：０．０１〜０．１０％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1.0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: less than 0.010%, Nb: less than 0.010%, V: 0.01 to 0.10%, A steel for nitriding that is excellent in magnetic properties after nitriding, with the balance being iron and inevitable impurities. 質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１．０ ％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０．０１０ ％未満、Ｎｂ：０．０１０ ％未満、Ａｌ：０．０５〜２．００％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1.0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: less than 0.010%, Nb: less than 0.010%, Al: 0.05 to 2.00%, A steel for nitriding that is excellent in magnetic properties after nitriding, with the balance being iron and inevitable impurities. 質量比で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．００５ 〜１． ０％、Ｍｎ：０．０１０ 〜３．００％、Ｐ：０．００１ 〜０．１５０ ％、Ｎ：０．０００２〜０．０１００％、Ｃｒ：０．１５〜３．００％、Ｔｉ：０．０１０ ％未満、Ｎｂ：０．０１０ ％未満、Ｖ：０．０１〜０．１０％、Ａｌ：０．００５ 〜２．００％を含有し、残部が鉄および不可避的不純物からなる窒化後の磁気特性に優れる窒化用鋼材。By mass ratio, C: 0.01 to 0.10%, Si: 0.005 to 1. 0%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100%, Cr: 0.15 to 3.00%, Ti: 0 Less than 0.010%, Nb: less than 0.010%, V: 0.01 to 0.10%, Al: 0.005 to 2.00%, the balance being iron and inevitable impurities Steel for nitriding with excellent magnetic properties. 請求項１〜３のいずれかに記載された鋼材を用いた成形体であって、該成形体の少なくとも一部に、サイズ４０ｎｍ以下の析出物が均一に分散した表面窒化層を有する成形体。It is a molded object using the steel materials as described in any one of Claims 1-3, Comprising: The molded object which has the surface nitride layer in which the precipitate of size 40nm or less was disperse | distributed uniformly to at least one part of this molded object.

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