JP3806653B2 - Steel for electrical parts excellent in cold forgeability and electrical conductivity, electrical parts excellent in electrical conductivity, and manufacturing method thereof - Google Patents
Steel for electrical parts excellent in cold forgeability and electrical conductivity, electrical parts excellent in electrical conductivity, and manufacturing method thereof Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、自動車や電車、船舶用などを対象とする各種電装部品に使用される電気部品、およびその原材料である電気部品用鋼材、並びに電気部品の製造方法に関するものであり、特に成形加工時において高歩留まりで寸法精度に優れた部品を得ることができ(以下、この特性を単に「冷間鍛造性」ということがある)、かつ良好な電気伝導性を確保することのできる電気部品用鋼材、および該鋼材を用いて得られる電気伝導性に優れた電気部品、並びにこれら電気部品用鋼材および電気部品の製造に有用な方法に関するものである。
【0002】
【従来の技術】
電気部品用材料には、その特性として、製品の省電力化・小型化を図るべく、通電時のエネルギー損失が小さく、かつジュール発熱の小さいことが要求される。従って直流用電気部品等に用いられる材料には、電気抵抗が低く電気伝導性に優れていることが求められる。
【0003】
電気伝導性を有する材料として、銅、ニッケルまたはC量が0.05%以下の極低炭素鋼等が挙げられ、特に電気伝導性を重視する場合は銅が使用され、耐食性を重視する場合はニッケル材が使用されるなど用途に応じて材料が選択される。
【0004】
ところで近年は、製造コストの低減を背景とした部品構造の簡素化に伴い、電装部品に対し、特性として電気伝導性とともに好適な剛性も兼ね備えていることが要求されつつある。
【0005】
この様に、電装部品としてはある程度高い強度を確保して剛性を高めることが必要であるが、一方で強度が高すぎると、製造コスト低減の一手段として切削加工にかわり冷間鍛造により部品成形を行うときに、変形抵抗が大きすぎて良好に冷間鍛造を行えないといった問題がある。
【0006】
電気伝導性および電装部品の強度を確保できるほどの剛性の両特性を実現するには、銅を用いた場合、他の構造材料との複合が必要となるので、製造工程が複雑となり、製造コストの増加や製品の十分な小型化を図ることができないといった問題が生じる。またニッケル材を用いた場合は、適度の剛性は確保できるもののコストが高くなるといったことが懸念される。
【0007】
また電装部品には、ジュール熱を速やかに部品の外部に伝達し、温度上昇を抑制するための、優れた熱伝導特性も兼備していることが要求される。図1は、電気抵抗率と熱伝導度の関係を調べたものであるが、この図1に示されるように熱伝導度と電気抵抗率との間には相関関係があり、電気抵抗率の低い材料ほど熱伝導度が高く、熱伝導性に優れる傾向を示すが、電気抵抗率が約11μΩcmを超えるとその改善効果が低くなるため、電気伝導性と熱伝導性を実用レベルで両立させるには、電気抵抗率を11μΩcm以下とする必要がある。即ち、熱伝導性を高めるためにも、電気伝導性をより向上させる必要がある。
【0008】
ところで電気伝導性に優れた鋼材は、表面処理における被膜−基材の密着性向上や、該被膜の基材熱伸縮への追従性向上といった技術の向上に伴い、鋼材に被覆された被膜のクラック・ボイド等を抑制してさびの発生を防止できることから、耐食性の要求される分野においても適用されるなど要望はますます高まっている。
【0009】
この様な電装部品に用いる鋼材に関する技術として、例えば特許第2910288号には、ひずみ時効硬化の原因となるCやNの低減をはじめ、合金成分や圧延条件を調整することによって、変形抵抗の増加を抑え、且つ優れた磁気特性を得る技術が提案されている。しかしこの技術は、冷間鍛造時の工具寿命と磁気特性に主眼を置いてなされたものであって、電気部品として必要な良好な電気伝導性と好適な剛性の両特性を確保するには更なる改善を要するものである。
【0010】
【発明が解決しようとする課題】
本発明は、この様な事情に鑑みてなされたものであって、その目的は、電気伝導性に優れ、かつ冷間鍛造において精度よく高歩留まりで加工することのできる電気部品用鋼材と、この様な鋼材を用いて得られる電気伝導性に優れた電気部品、更にはこれらの製造方法を提供することにある。
【0011】
【課題を解決するための手段】
本発明に係る冷間鍛造性と電気伝導性に優れた電気部品用鋼材とは、質量%で、C:0.02%以下(0%を含む)、Si:0.1%以下(0%を含まない)、Mn:0.1〜0.5%、P:0.02%以下(0%を含む)、S:0.02%以下(0%を含む)、Al:0.01%以下(0%を含む)、N:0.005%以下(0%を含む)、O:0.02%以下(0%を含む)を満たし、残部Feおよび不可避的不純物からなり、金属組織がフェライト単相組織であるところに要旨を有する。
【0012】
この様な鋼材を得るにあたっては、熱間圧延に際して1000〜1200℃に加熱し、仕上げ圧延を850℃以上で行った後、800〜500℃間の冷却を平均冷却速度0.5〜10℃/secで行うのがよい。
【0013】
更に本発明は、この様な鋼材を用いて得られる電気伝導性に優れた電気部品も規定するものであって、該電気部品は、C:0.02%以下(0%を含む)、Si:0.1%以下(0%を含まない)、Mn:0.1〜0.5%、P:0.02%以下(0%を含む)、S:0.02%以下(0%を含む)、Al:0.01%以下(0%を含む)、N:0.005%以下(0%を含む)、O:0.02%以下(0%を含む)を満たし、残部Feおよび不可避的不純物からなり、金属組織が平均結晶粒径100μm以上のフェライト単相組織であるところに要旨を有する。
【0014】
またその製造方法は、前記鋼材を用いて所定の部品形状に成形加工後、850〜950℃で2時間以上焼鈍するところに特徴を有し、該焼鈍前に加工ひずみが0.15以上の冷間加工または熱間加工を行うことを好ましい実施形態とする。
【0015】
尚、前記「0%を含む」とは、0%の場合を排除しないことを示し、前記「フェライトの平均結晶粒径」とは、フェライト結晶粒の短径と長径の平均値をいうものとする。
【0016】
【発明の実施の形態】
本発明者らは、前述した様な状況の下で、成形加工時には優れた冷間鍛造性を発揮し、かつ良好な電気伝導性を確保することのできる電気部品用鋼材、およびこの様な鋼材を用いて得られる電気伝導性に優れた電気部品の実現を目指し、化学成分組成や金属組織の影響など様々な角度から検討を行った。その結果、本発明で規定する成分組成とし、かつ金属組織を本発明で規定するフェライト単相組織とすれば、電気伝導度を高めることができ、更に成形加工時には変形抵抗を大幅に低減して寸法精度の良好な冷間鍛造を行うことができることを見出し、上記本発明に想到した。以下、本発明で金属組織および化学成分組成を規定した理由について詳述する。
【0017】
金属材料の電気伝導度は自由電子(伝導電子)の散乱状態と相関があり、材料固有の要因(電子−電子相互作用、電子−フォノン相互作用)に加え、結晶粒界、析出物および格子欠陥といった製造条件により決定される構造要因に大きく影響を受け、析出物は粒界に偏析し易く、また粒界では結晶方位が乱れることから自由電子が散乱され易くなる。このため、金属組織における結晶粒界が多く存在すると、電気抵抗が増加し易いのである。
【0018】
本発明者らは、電気伝導性を向上すべく粒界を減少させるにあたっては、フェライトの平均結晶粒径を100μm以上とする必要があることを見出した。平均結晶粒径が100μm未満の場合には、上述の通り、電気抵抗が増加し易いことから、良好な電気伝導性を発揮させることができないのである。前記フェライトの平均結晶粒径は、好ましくは130μm以上である。尚、熱処理時間(製造コスト)を費やして前記フェライトの平均結晶粒径が大きくしすぎても電気伝導性向上効果は飽和するだけであるので、約250μm以下に留めるようにする。
【0019】
尚、フェライト単相組織とするにあたっては、パーライトの生成を抑制するため、鋼材中の炭素量を極めて少なくするのが有効である。
【0020】
次に化学成分組成が電気伝導性および機械的特性に及ぼす影響について調べた。その結果、化学成分については、電気抵抗を増加させるC、Si、Mn、P等の化学成分を必要最小限に抑制することが有効であることを見出した。
【0021】
C:0.02%以下(0%を含む)
C(炭素)は鋼材の強度と延性のバランスを支配する基本元素であり、添加量を低減するほど強度は低下し、延性は向上する。
【0022】
Cは、鋼中に固溶してひずみ時効硬化を生じさせるので、変形抵抗の増大を抑制して冷間鍛造性を向上させるには、C量が極力少ないほうが望ましい。図2は、鋼中の炭素量が電気伝導度と変形抵抗値に及ぼす影響を調べたものであり、実験は、鋼中炭素量の異なる5試料につき、直流400mAの電流を流したときの抵抗値から電気伝導度を測定し、円柱試料の端面拘束圧縮試験から変形抵抗値を測定したものである。この図2から分かるように、良好な電気特性を確保する観点からC量は少ない方がよく、工業用Ni材と同等以上の電気特性を満足し、かつ上述した様に良好な熱伝導性を確保するにあたっては、電気抵抗率を11μΩcm以下にして電気伝導性を高めるのがよいことから、本発明では炭素含有量を0.02%以下に抑えるようにした。好ましくは0.01%以下である。
【0023】
Si:0.1%以下(0%を含まない)
Siは鋼の溶製時に脱酸剤として作用し、酸素による電気抵抗率の上昇を抑制するが、含有量が多過ぎると冷間鍛造性を阻害する。従って本発明では、部品成形時の冷間鍛造性を確保する観点から、Si含有量の上限を0.1%とした。より好ましくは、0.05%以下である。
【0024】
Mn:0.1〜0.5%
Mnは脱酸剤として有効に作用するとともに、鋼中のSと結合してMnSを形成することにより、Sによる脆化を抑制する。この様な効果を有効に発揮させるにはMnを0.1%以上、好ましくは0.2%以上含有させるのがよい。しかしMn量が多過ぎると、析出するMnSの粒径が大きくなって電気抵抗率が上昇するため、0.5%を上限とする。好ましくは0.3%以下である。
【0025】
P:0.02%以下(0%を含む)
P(リン)は、鋼中で粒界偏析を起こして冷間鍛造性や電気特性に悪影響を及ぼす有害元素である。従って本発明では、Pの含有量を0.02%以下、好ましくは0.01%以下とする必要があり、この様にP量を制限することで、優れた冷間鍛造性および電気特性を保証することができる。
【0026】
S:0.02%以下(0%を含む)
S(硫黄)は、上記の様に鋼中でMnSを形成し、S量が多くなり過ぎると多量にMnSが析出して冷間鍛造性と電気伝導性を著しく劣化させるので、0.02%以下、好ましくは0.01%以下に抑える。
【0027】
Al:0.01%以下(0%を含む)
Alは、固溶Nを捕捉しAlNとなって結晶粒の微細化を促進させる。その結果、結晶粒界を増加させることとなり電気抵抗の増大を招く。従って、本発明ではAl量を0.01%以下、好ましくは0.005%以下に抑えるようにする。
【0028】
N:0.005%以下(0%を含む)
上記の様にN(窒素)は、Alと結合しAlNを形成して電気特性を害するが、それに加え、Alなどにより固定されなかったNは固溶Nとして鋼中に残存し、これも電気抵抗の増大を招く。また固溶Nは、ひずみ時効による変形抵抗の増加を引き起こす原因にもなる。よって、何れにしてもN量は極力少なく抑えるべきであるが、鋼材製造の実操業面も考慮し、且つ前記弊害を実質的に無視し得る程度に抑えることのできる0.005%を上限値として定めた。
【0029】
O:0.02%以下(0%を含む)
O(酸素)は常温では鋼に殆ど固溶せず、AlやSiなどの元素と結合して硬質の酸化物系介在物となり、電気抵抗を大幅に増加させることとなる。ゆえにO含有量は極力低減すべきものであり、0.02%以下に抑える必要がある。O含有量は0.01%以下に低減するのが好ましく、より好ましくは0.005%以下にするのがよい。
【0030】
またCu、Ni、Cr、Tiについては、鋼中にこれらの元素の析出物が生じると電気抵抗の大幅な増加を招くため、それぞれCuを0.02%以下、Niを0.02以下、Crを0.05%以下、Tiを0.01%以下とすることが望ましい。
【0031】
本発明で規定する元素は上記の通りであり、残部成分は実質的にFeであるが、該鋼材中に、上記説明したものの他、原料、資材、製造設備等の状況によって持ち込まれる不可避的不純物、更には、本発明の課題達成に悪影響を与えないAs等の許容元素が含まれる場合も、本発明で用いる鋼材または鋼部品に包含される。
【0032】
本発明に係る電気部品用鋼材の製造に際しては、上記化学成分の要件を満たす鋼材を常法により溶融してから鋳造すればよいが、冷間鍛造性に優れ、且つ焼鈍後の状態で良好な電気伝導性を得るには、1000〜1200℃に加熱して熱間圧延し、850℃以上の仕上げ温度で圧延を終了した後、800〜500℃の温度域を0.5℃/sec以上、10℃/sec以下の平均冷却速度で冷却することが極めて有効となる。以下、これらの条件を定めた理由を説明する。
【0033】
<熱間圧延に際しての加熱温度>
合金成分を母相に完全に固溶させるため、加熱温度はできるだけ高温である方が望ましい。図3は、本発明の成分組成を満たす鋼材を用い、加熱温度:850〜1100℃の範囲内で変化させて加熱した後、熱間圧延を行ったときの割れ発生率を測定したものである。この図3から明らかなように、加熱温度が低過ぎると異相が局所的に生成して圧延時の割れ発生を招く危険性があり、しかも、低温側では圧延時のロール負荷が上昇して生産性の低下を招くことにもなる。従って加熱温度は1000℃以上、好ましくは1100℃以上に設定するのがよい。一方、該加熱温度が高すぎるとフェライト結晶粒の粗大化が顕著となり、部品成形時の冷間鍛造性の低下を招くので、1200℃以下、好ましくは約1150℃以下に抑えるのがよい。
【0034】
<仕上げ圧延温度>
仕上げ圧延温度が低過ぎると、微細に析出するMnSの粒径および密度に偏りが生じ易くなる。従って、MnSを微細かつ均一に析出させて電気特性への影響を極力抑制するには、仕上げ圧延を850℃以上、好ましくは900℃以上で行うのがよい。
【0035】
<熱間圧延後の800〜500℃温度域の冷却速度>
熱間圧延後の冷却速度が速すぎると原子空孔の増加をもたらし、焼鈍後において所望の電気特性が得られ難くなる。よって800〜500℃の温度域の冷却速度を10℃/sec以下、好ましくは5℃/sec以下とするのがよい。一方、該温度域の冷却速度が遅過ぎると生産性が低下する他、析出物であるMnSが粗大化するので、0.5℃/sec以上、好ましくは1.0℃/sec以上とする。
【0036】
なお、冷却速度を制御する温度域を800〜500℃の範囲と定めたのは、800℃を超える温度域では、フェライト相への変態が進まないため、金属組織への影響がほとんどなく、また500℃未満の温度では、フェライト相への変態がほぼ完了するからである。
【0037】
<焼鈍条件>
かくして得られる電気部品用鋼材を用いて電気部品を製造するに当たっては、該鋼材を冷間鍛造したのち焼鈍に付されるが、上記電気部品用鋼材の特長を活かして優れた電気伝導性を発揮する部品とするには、焼鈍を850℃以上950℃以下の温度域で2時間以上行うのがよい。
【0038】
図4は、焼鈍時間と焼鈍温度がフェライト平均結晶粒径に及ぼす影響を調べたものであり、実験は、化学成分組成がC:0.004%、Si:0.005%、Mn:0.22%、P:0.007%、S:0.007%、Al:0.003%、Ti:0.002%を満たす鋼材を用い、焼鈍温度:800〜950℃、焼鈍時間:30分〜4時間の範囲内で変化させて焼鈍を行っている。
【0039】
この図4から明らかなように、850℃未満では所望のフェライト結晶粒径とするのに長時間を要し、実用的でない。一方、過度に焼鈍温度を高めても、所望のフェライト結晶粒径とする効果は殆ど変わらないので、焼鈍温度範囲を850℃以上で950℃以下とした。
【0040】
また焼鈍時間が短すぎると、焼鈍温度を高めに設定したとしても焼鈍時間不足でフェライト結晶粒を十分に粗大化させることができないので、少なくとも2時間、好ましくは3時間以上焼鈍するのがよく、長すぎても所望のフェライト粒径とする効果はほとんど変わらないことから、6時間以下に抑えるのがよい。
【0041】
前記焼鈍に際しては、加工ひずみが0.15以上の冷間加工または温間加工を施すことによって、焼鈍を行ったときに結晶粒成長が顕著となり、電気伝導性を一層向上させることができるので望ましい。
【0042】
図5は、焼鈍前の冷間加工ひずみ量が電気抵抗率に及ぼす影響を調べたものであり、加工ひずみ量を0〜2.2の間で変化させた試料を作成し、該試料の変形抵抗率を円柱試料(高さ/外径=1.5)の端面拘束圧縮試験(圧縮率:80%)により測定したものである。この図5に示される通り、加工ひずみが0.15以上の加工を焼鈍前に行うことによって、より電気抵抗率を低減、即ち、電気伝導度をより高めることができるのである。より好ましくは0.2以上の加工ひずみを加えるようにする。尚、図5は、冷間加工を行って加工ひずみを加えた場合の結果であるが、焼鈍前に温間加工を行った場合にも同様の効果が見られた。
【0043】
【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が、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【0044】
表1に示す化学成分の供試鋼材を溶製し、鋳造した後、表2に示す条件で熱間圧延を行い、直径10mmの線材を得た。得られた線材から採取した試料に850℃で3時間の焼鈍を施した後、該試料の金属組織を観察した。また、前記線材を伸線加工して得た鋼線について、冷間鍛造性と焼鈍後の電気特性を調べた。
【0045】
金属組織の観察は次の方法で行った。即ち、線材の横断面を露出させた状態で支持基材内に埋め込み、研磨後、5%のピクリン酸アルコール液に15〜30秒間浸漬して腐食させた後、光学顕微鏡によりD/4(D:線材の直径)部位の組織を100〜400倍で10視野を写真撮影し、該写真からフェライト平均結晶粒径を求めた。
【0046】
一方、冷間鍛造後の部品の寸法精度は、変形抵抗と相関が強いことから、冷間鍛造性は、端面拘束圧縮時の変形抵抗値をもって評価した。変形抵抗の測定には、直径10mm×高さ15mmの前記鋼線試料を用い、常温での端面拘束圧縮における80%圧縮時(ひずみ速度10/s)の荷重から求めた。本発明ではこの変形抵抗が500N/mm2以下の場合を○、500N/mm2を超える場合を×とした。
【0047】
電気特性として電気抵抗率を、通電法にて通電電流と発生電圧を測定した後、通電電流と発生電圧の比から算出した。尚、通電試験に際しては、電極の接続長さを線径の10倍、電圧端子間距離を線径の20倍とし、通電方向を正逆2通り実施することによって、接触抵抗や偏流、熱起電力などの影響を除去した。本発明では、電気抵抗率が11μΩcm以下の場合を電気伝導性が良好であるとして○、電気抵抗率が11μΩcmを超える場合を電気伝導性が好ましくないとして×と評価した。
【0048】
表2に各試料の金属組織、変形抵抗および電気抵抗の測定結果を併せて示す。
【0049】
【表1】
【0050】
【表2】
【0051】
表2から次のように考察することができる。尚、以下のNo.は表2における実験No.を示す。
【0052】
No.1〜5は、本発明で規定する化学成分組成および金属組織を満足するものであり、いずれも工業用Ni材と同等以上の電気伝導性を有し、且つ優れた冷間鍛造性も兼備していることがわかる。
【0053】
これに対し、No.6〜17は、本発明で規定する化学成分を外れるか、または本発明で規定する方法で製造を行わなかったものであり、伸線時に割れが生じたり、所望の電気伝導性が得られなかったり、あるいは変形抵抗の低減効果が十分でない等の不具合が生じた。
【0054】
No.6〜9は、鋼材の化学成分組成が要件を満足するものであるが、本発明で規定する条件で製造を行わなかったため、上記不具合が生じたものと考えられる。即ち、No.6では焼鈍温度が低すぎたため、焼鈍での再結晶が十分に進まず電気伝導性を低下させる結果となった。No.7では、焼鈍時間が短いため、焼鈍にて再結晶が十分に進まず、粒界面積の多い組織となり、電気伝導性を低下させる結果となった。またNo.8では、圧延時の冷却過程におけるフェライトの粒成長が十分進まず、焼鈍後においても粒界面積が多くなり、電気伝導性が低下する結果となった。No.9からは、焼鈍における再結晶を促進するには、焼鈍前に加工ひずみが0.15以上の加工を行うのがよいことがわかる。
【0055】
またNo.10〜17は、本発明で規定する条件で製造を行ったものであるが、鋼材の化学成分組成が本発明で規定する範囲を外れているため、上記不具合が生じたものと考えられる。No.10は、C量が本発明で規定する上限を超えて含有したものであるので、電気伝導性が大幅に劣化する結果となった。No.11は、Si量が上限を超えていることから、冷間鍛造性と電気伝導性のどちらもが劣化する結果となった。No.12は、Mnが本発明で規定する条件を超えて添加されており、生成したMnSがフェライト結晶粒の成長を抑制し、また析出したMnSが伝導電子を散乱させるため、電気伝導性が低下したものと考えられる。No.13は、Pが過剰に含有され、粒界にPが偏析して結晶粒の成長を抑制しているため、電気伝導性が劣化する結果となった。No.14は、Sが過剰に含有されていることから、析出するMnSが粗大化し、電気伝導性が劣化することとなった。
【0056】
No.15は、Alが本発明で規定する上限を超えて含有しているため、多量のAlNが形成され、該AlNにより結晶粒の成長が抑制されて電気伝導性が著しく低下することとなった。
【0057】
No.16は、窒素が本発明で規定する上限を超えて存在するため、ひずみ時効による変形抵抗の増大に加え、電気伝導性も劣化することとなった。No.17は、酸素が本発明で規定する上限を超えて過剰に存在するため、電気伝導性が劣化する結果となった。
【0058】
【発明の効果】
本発明は上記のように構成されており、寸法精度の良好な冷間鍛造を行うことができるとともに、優れた電気伝導性を確保することのできる電気部品用鋼材、およびこの様な鋼材を用いて優れた電気伝導性を発揮する電気部品が得られることとなり、Ni等コストのかかる材料を用いるものでないことから、自動車や電車、船舶用などを対象とする各種電装部品を安価で提供できることとなった。
【図面の簡単な説明】
【図1】電気抵抗率と熱電導度の関係を調べたグラフである。
【図2】鋼中の炭素量が電気伝導度と変形抵抗値に及ぼす影響を調べたグラフである。
【図3】熱間圧延に際しての加熱温度と熱間圧延時の割れ発生率の関係を示すグラフである。
【図4】焼鈍時間と焼鈍温度がフェライト平均結晶粒径に及ぼす影響を調べたグラフである。
【図5】加工ひずみ量が電気抵抗率に及ぼす影響を調べたグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrical component used for various electrical components intended for automobiles, trains, ships, and the like, a steel material for electrical components that is a raw material thereof, and a method for manufacturing the electrical component, particularly during molding processing. Can obtain parts with high yield and excellent dimensional accuracy (hereinafter, this characteristic may be simply referred to as “cold forgeability”), and can secure good electrical conductivity. In addition, the present invention relates to an electrical component having excellent electrical conductivity obtained by using the steel material, and a method useful for manufacturing the steel material for electrical component and the electrical component.
[0002]
[Prior art]
Electrical component materials are required to have low energy loss during energization and low Joule heat generation in order to save power and reduce the size of products. Accordingly, materials used for DC electrical components and the like are required to have low electrical resistance and excellent electrical conductivity.
[0003]
Examples of materials having electrical conductivity include copper, nickel, or ultra-low carbon steel having a C content of 0.05% or less, and copper is used particularly when electrical conductivity is important, and when corrosion resistance is important. The material is selected according to the application, such as using a nickel material.
[0004]
In recent years, along with the simplification of the component structure against the background of the reduction in manufacturing cost, it has been demanded that the electrical component also has a suitable rigidity as well as electrical conductivity as a characteristic.
[0005]
In this way, it is necessary to increase the rigidity by securing a certain degree of strength as an electrical component, but if the strength is too high, the part is formed by cold forging instead of cutting as a means of reducing the manufacturing cost. There is a problem that the cold forging cannot be performed satisfactorily because the deformation resistance is too large.
[0006]
In order to achieve both the characteristics of electrical conductivity and rigidity sufficient to ensure the strength of electrical components, the use of copper requires a combination with other structural materials, which complicates the manufacturing process and increases the manufacturing cost. The problem arises that the increase in the number of products and the size of the product cannot be reduced sufficiently. Further, when nickel material is used, there is a concern that although moderate rigidity can be ensured, the cost increases.
[0007]
In addition, the electrical component is required to have excellent heat conduction characteristics for quickly transmitting Joule heat to the outside of the component and suppressing temperature rise. FIG. 1 shows the relationship between electrical resistivity and thermal conductivity. As shown in FIG. 1, there is a correlation between thermal conductivity and electrical resistivity. The lower the material, the higher the thermal conductivity and the better the thermal conductivity. However, when the electrical resistivity exceeds about 11 μΩcm, the improvement effect is reduced, so that both electrical conductivity and thermal conductivity can be achieved at a practical level. Needs to have an electrical resistivity of 11 μΩcm or less. That is, it is necessary to further improve electrical conductivity in order to increase thermal conductivity.
[0008]
By the way, the steel material having excellent electrical conductivity has been improved by improving the coating-base adhesion in the surface treatment and improving the follow-up ability of the coating to the thermal expansion and contraction of the coating.・ Since voids can be suppressed to prevent the occurrence of rust, there is an increasing demand for applications in areas where corrosion resistance is required.
[0009]
As a technique related to steel materials used for such electrical components, for example, Patent No. 2910288 includes an increase in deformation resistance by adjusting the alloy components and rolling conditions, including the reduction of C and N that cause strain age hardening. There has been proposed a technique for suppressing the magnetic field and obtaining excellent magnetic characteristics. However, this technology has been made with a focus on tool life and magnetic properties during cold forging, and has been further improved to ensure both good electrical conductivity and suitable rigidity required for electrical components. It is necessary to improve.
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and its purpose is to provide a steel material for electrical parts that has excellent electrical conductivity and can be processed with high yield in cold forging with high accuracy. Another object of the present invention is to provide an electrical component having excellent electrical conductivity obtained by using such a steel material, and further a method for producing them.
[0011]
[Means for Solving the Problems]
The steel material for electrical parts excellent in cold forgeability and electrical conductivity according to the present invention is mass%, C: 0.02% or less (including 0%), Si: 0.1% or less (0%) Mn: 0.1 to 0.5%, P: 0.02% or less (including 0%), S: 0.02% or less (including 0%), Al: 0.01% Satisfying the following (including 0%), N: 0.005% or less (including 0%), O: 0.02% or less (including 0%), consisting of the balance Fe and unavoidable impurities, The main point is that it is a ferrite single phase structure.
[0012]
In obtaining such a steel material, it is heated to 1000 to 1200 ° C. during hot rolling, finish rolling is performed at 850 ° C. or higher, and cooling between 800 to 500 ° C. is performed at an average cooling rate of 0.5 to 10 ° C. / This should be done in seconds.
[0013]
Furthermore, the present invention also defines an electrical component having excellent electrical conductivity obtained by using such a steel material, and the electrical component is C: 0.02% or less (including 0%), Si : 0.1% or less (excluding 0%), Mn: 0.1 to 0.5%, P: 0.02% or less (including 0%), S: 0.02% or less (0% Al): 0.01% or less (including 0%), N: 0.005% or less (including 0%), O: 0.02% or less (including 0%), and the balance Fe and It is composed of inevitable impurities and has a gist in that the metal structure is a ferrite single phase structure having an average crystal grain size of 100 μm or more.
[0014]
In addition, the manufacturing method is characterized in that after forming into a predetermined part shape using the steel material, annealing is performed at 850 to 950 ° C. for 2 hours or more, and a cooling strain of 0.15 or more is applied before the annealing. It is a preferred embodiment to perform hot working or hot working.
[0015]
The term “including 0%” means that the case of 0% is not excluded, and the term “average ferrite grain size” means the average value of the minor axis and major axis of the ferrite crystal grains. To do.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Under the circumstances as described above, the present inventors have demonstrated excellent cold forgeability at the time of molding, and can secure good electrical conductivity, and such steel materials. Aiming at the realization of electrical parts with excellent electrical conductivity obtained by using, we studied from various angles such as chemical composition and influence of metal structure. As a result, if the component composition specified in the present invention is used and the metal structure is a ferrite single-phase structure specified in the present invention, the electrical conductivity can be increased, and the deformation resistance can be greatly reduced during the molding process. The inventors have found that cold forging with good dimensional accuracy can be performed and have arrived at the present invention. Hereinafter, the reason why the metal structure and the chemical component composition are defined in the present invention will be described in detail.
[0017]
The electrical conductivity of metallic materials correlates with the scattering state of free electrons (conducting electrons), and in addition to material-specific factors (electron-electron interaction, electron-phonon interaction), grain boundaries, precipitates, and lattice defects The precipitate is easily segregated at the grain boundary, and the crystal orientation is disturbed at the grain boundary, so that free electrons are easily scattered. For this reason, if there are many crystal grain boundaries in the metal structure, the electrical resistance tends to increase.
[0018]
The inventors have found that in order to reduce the grain boundary in order to improve electrical conductivity, the average crystal grain size of ferrite needs to be 100 μm or more. When the average crystal grain size is less than 100 μm, as described above, the electrical resistance is likely to increase, so that it is not possible to exhibit good electrical conductivity. The average crystal grain size of the ferrite is preferably 130 μm or more. Note that, even if the heat treatment time (manufacturing cost) is spent and the average crystal grain size of the ferrite is too large, the effect of improving electrical conductivity is only saturated, so it is limited to about 250 μm or less.
[0019]
In forming a ferrite single phase structure, it is effective to extremely reduce the amount of carbon in the steel material in order to suppress the formation of pearlite.
[0020]
Next, the effect of chemical composition on electrical conductivity and mechanical properties was investigated. As a result, it has been found that it is effective to suppress chemical components such as C, Si, Mn, and P that increase electrical resistance to the minimum necessary.
[0021]
C: 0.02% or less (including 0%)
C (carbon) is a basic element that governs the balance between the strength and ductility of the steel material, and the strength decreases as the amount added decreases, and the ductility improves.
[0022]
Since C dissolves in steel and causes strain age hardening, it is desirable that the amount of C is as small as possible in order to suppress an increase in deformation resistance and improve cold forgeability. Fig. 2 shows the effect of carbon content in steel on the electrical conductivity and deformation resistance. The experiment shows the resistance when a DC current of 400 mA is applied to 5 samples with different carbon content in steel. The electrical conductivity was measured from the value, and the deformation resistance value was measured from the end face constrained compression test of the cylindrical sample. As can be seen from FIG. 2, it is better that the amount of C is small from the viewpoint of securing good electrical characteristics, the electrical characteristics are equivalent to or better than those of industrial Ni materials, and good thermal conductivity is achieved as described above. In securing, it is preferable to increase the electrical conductivity by setting the electric resistivity to 11 μΩcm or less. Therefore, in the present invention, the carbon content is suppressed to 0.02% or less. Preferably it is 0.01% or less.
[0023]
Si: 0.1% or less (excluding 0%)
Si acts as a deoxidizer during the melting of steel and suppresses an increase in electrical resistivity due to oxygen, but if the content is too large, it inhibits cold forgeability. Therefore, in the present invention, the upper limit of the Si content is set to 0.1% from the viewpoint of ensuring the cold forgeability during component forming. More preferably, it is 0.05% or less.
[0024]
Mn: 0.1 to 0.5%
Mn effectively acts as a deoxidizing agent and combines with S in steel to form MnS, thereby suppressing embrittlement due to S. In order to exhibit such an effect effectively, it is preferable to contain 0.1% or more, preferably 0.2% or more of Mn. However, if the amount of Mn is too large, the particle size of precipitated MnS increases and the electrical resistivity increases, so 0.5% is made the upper limit. Preferably it is 0.3% or less.
[0025]
P: 0.02% or less (including 0%)
P (phosphorus) is a harmful element that causes grain boundary segregation in steel and adversely affects cold forgeability and electrical characteristics. Therefore, in the present invention, the P content must be 0.02% or less, preferably 0.01% or less. By limiting the P content in this way, excellent cold forgeability and electrical characteristics can be obtained. Can be guaranteed.
[0026]
S: 0.02% or less (including 0%)
S (sulfur) forms MnS in the steel as described above, and if the amount of S is excessively large, MnS is precipitated in a large amount and remarkably deteriorates cold forgeability and electrical conductivity. Hereinafter, it is preferably suppressed to 0.01% or less.
[0027]
Al: 0.01% or less (including 0%)
Al captures solute N and becomes AlN to promote refinement of crystal grains. As a result, the crystal grain boundary is increased and the electrical resistance is increased. Therefore, in the present invention, the Al content is suppressed to 0.01% or less, preferably 0.005% or less.
[0028]
N: 0.005% or less (including 0%)
As described above, N (nitrogen) binds to Al to form AlN and harms the electrical characteristics. In addition, N that is not fixed by Al or the like remains in the steel as solute N, which is also electrically Increases resistance. Solid solution N also causes an increase in deformation resistance due to strain aging. Therefore, in any case, the N amount should be suppressed as much as possible, but the upper limit is set to 0.005% which can be suppressed to such an extent that the above-mentioned adverse effects can be substantially ignored in consideration of the actual operation of steel production. As determined.
[0029]
O: 0.02% or less (including 0%)
O (oxygen) hardly dissolves in steel at room temperature, and is combined with elements such as Al and Si to form hard oxide inclusions, which greatly increases electrical resistance. Therefore, the O content should be reduced as much as possible, and should be suppressed to 0.02% or less. The O content is preferably reduced to 0.01% or less, and more preferably 0.005% or less.
[0030]
For Cu, Ni, Cr, and Ti, if precipitates of these elements occur in the steel, the electrical resistance is greatly increased. Therefore, Cu is 0.02% or less, Ni is 0.02 or less, Cr Is preferably 0.05% or less and Ti is 0.01% or less.
[0031]
The elements defined in the present invention are as described above, and the remaining component is substantially Fe, but inevitable impurities brought into the steel material depending on the situation of raw materials, materials, manufacturing equipment, etc. in addition to those described above Furthermore, the case where an allowable element such as As that does not adversely affect the achievement of the object of the present invention is also included in the steel material or steel part used in the present invention.
[0032]
In the production of the steel for electrical parts according to the present invention, a steel material that satisfies the above-mentioned chemical component requirements may be cast after being melted by a conventional method, but it is excellent in cold forgeability and good in a state after annealing. In order to obtain electrical conductivity, it is heated to 1000 to 1200 ° C. and hot rolled, and after rolling at a finishing temperature of 850 ° C. or higher, a temperature range of 800 to 500 ° C. is 0.5 ° C./sec or higher. Cooling at an average cooling rate of 10 ° C./sec or less is extremely effective. Hereinafter, the reason for setting these conditions will be described.
[0033]
<Heating temperature during hot rolling>
In order to completely dissolve the alloy components in the matrix phase, it is desirable that the heating temperature be as high as possible. FIG. 3 shows the crack occurrence rate when hot rolling is performed after changing the heating temperature within the range of 850 to 1100 ° C. and using the steel material satisfying the component composition of the present invention. . As is apparent from FIG. 3, if the heating temperature is too low, a foreign phase is locally generated and there is a risk of causing cracks during rolling, and on the low temperature side, the roll load during rolling is increased and produced. It will also cause a decline in sex. Therefore, the heating temperature is set to 1000 ° C. or higher, preferably 1100 ° C. or higher. On the other hand, if the heating temperature is too high, the ferrite crystal grains become prominent and the cold forgeability at the time of component molding is lowered, so it is preferable to keep it at 1200 ° C. or less, preferably about 1150 ° C. or less.
[0034]
<Finishing rolling temperature>
If the finish rolling temperature is too low, the particle size and density of the finely precipitated MnS tend to be uneven. Therefore, in order to deposit MnS finely and uniformly and suppress the influence on the electric characteristics as much as possible, the finish rolling is preferably performed at 850 ° C. or higher, preferably 900 ° C. or higher.
[0035]
<Cooling rate of 800 to 500 ° C. after hot rolling>
If the cooling rate after hot rolling is too high, the number of atomic vacancies increases, and it becomes difficult to obtain desired electrical characteristics after annealing. Therefore, the cooling rate in the temperature range of 800 to 500 ° C. is 10 ° C./sec or less, preferably 5 ° C./sec or less. On the other hand, if the cooling rate in the temperature range is too slow, the productivity is reduced and the precipitate MnS is coarsened, so the temperature is 0.5 ° C./sec or more, preferably 1.0 ° C./sec or more.
[0036]
Note that the temperature range for controlling the cooling rate was determined to be in the range of 800 to 500 ° C., because in the temperature range exceeding 800 ° C., the transformation to the ferrite phase does not proceed, so there is almost no influence on the metal structure. This is because at temperatures below 500 ° C., the transformation to the ferrite phase is almost complete.
[0037]
<Annealing conditions>
When manufacturing electrical parts using the steel materials for electrical parts obtained in this way, the steel materials are cold forged and then subjected to annealing, but they exhibit excellent electrical conductivity by taking advantage of the characteristics of the above steel materials for electrical parts. In order to obtain a part to be annealed, it is preferable to perform annealing at a temperature range of 850 ° C. to 950 ° C. for 2 hours or more.
[0038]
FIG. 4 shows the effect of the annealing time and annealing temperature on the ferrite average crystal grain size. In the experiment, the chemical composition was C: 0.004%, Si: 0.005%, Mn: 0.00. Steel material satisfying 22%, P: 0.007%, S: 0.007%, Al: 0.003%, Ti: 0.002% is used, annealing temperature: 800-950 ° C., annealing time: 30 minutes- Annealing is performed within a range of 4 hours.
[0039]
As apparent from FIG. 4, when the temperature is lower than 850 ° C., it takes a long time to obtain a desired ferrite crystal grain size, which is not practical. On the other hand, even if the annealing temperature is excessively increased, the effect of obtaining the desired ferrite crystal grain size is hardly changed. Therefore, the annealing temperature range is set to 850 ° C. or more and 950 ° C. or less.
[0040]
Also, if the annealing time is too short, even if the annealing temperature is set high, the ferrite crystal grains cannot be sufficiently coarsened due to insufficient annealing time, so it is preferable to anneal at least 2 hours, preferably 3 hours or more, Even if it is too long, the effect of obtaining a desired ferrite particle diameter is hardly changed, so it is preferable to suppress it to 6 hours or less.
[0041]
In the annealing, it is desirable to perform cold working or warm working with a working strain of 0.15 or more, so that crystal grain growth becomes remarkable when annealing is performed, and electrical conductivity can be further improved. .
[0042]
FIG. 5 shows the effect of the cold work strain amount before annealing on the electrical resistivity. A sample was prepared by changing the work strain amount between 0 and 2.2. The resistivity was measured by an end face constrained compression test (compression rate: 80%) of a cylindrical sample (height / outer diameter = 1.5). As shown in FIG. 5, the electrical resistivity can be further reduced, that is, the electrical conductivity can be further increased, by performing processing with a processing strain of 0.15 or more before annealing. More preferably, a processing strain of 0.2 or more is applied. Note that FIG. 5 shows the results when cold working is performed and machining strain is applied, but similar effects were also seen when warm working was performed before annealing.
[0043]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.
[0044]
Sample steels having chemical components shown in Table 1 were melted and cast, and then hot-rolled under the conditions shown in Table 2 to obtain a wire having a diameter of 10 mm. The sample taken from the obtained wire was annealed at 850 ° C. for 3 hours, and then the metal structure of the sample was observed. The steel wire obtained by drawing the wire was examined for cold forgeability and electrical characteristics after annealing.
[0045]
The metal structure was observed by the following method. That is, it was embedded in a supporting substrate in a state where the cross section of the wire was exposed, polished, immersed in a 5% picric acid alcohol solution for 15 to 30 seconds to be corroded, and then subjected to D / 4 (D : Diameter of wire) 10 fields of view were photographed at 100 to 400 times the structure of the region, and the average ferrite grain size was determined from the photograph.
[0046]
On the other hand, since the dimensional accuracy of the parts after cold forging has a strong correlation with the deformation resistance, the cold forgeability was evaluated by the deformation resistance value at the time of end face constrained compression. The deformation resistance was measured by using the steel wire sample having a diameter of 10 mm and a height of 15 mm from a load at 80% compression (
[0047]
As electrical characteristics, the electrical resistivity was calculated from the ratio of the energized current and the generated voltage after measuring the energized current and the generated voltage by the energization method. In the energization test, the connection length of the electrode is 10 times the wire diameter, the distance between the voltage terminals is 20 times the wire diameter, and the energization direction is carried out in two forward and reverse directions. Eliminated power and other effects. In the present invention, the case where the electrical resistivity was 11 μΩcm or less was evaluated as “good”, and the case where the electrical resistivity exceeded 11 μΩcm was evaluated as “poor” because the electrical conductivity was not preferable.
[0048]
Table 2 also shows the measurement results of the metal structure, deformation resistance, and electrical resistance of each sample.
[0049]
[Table 1]
[0050]
[Table 2]
[0051]
From Table 2, it can be considered as follows. The following No. Is the experiment No. in Table 2. Indicates.
[0052]
No. 1 to 5 satisfy the chemical component composition and the metal structure defined in the present invention, both have electrical conductivity equal to or higher than that of industrial Ni materials, and also have excellent cold forgeability. You can see that
[0053]
In contrast, no. Nos. 6 to 17 are the chemical components defined in the present invention or are not manufactured by the method defined in the present invention, and cracking occurs at the time of wire drawing, and desired electrical conductivity cannot be obtained. Or problems such as insufficient deformation resistance reduction occurred.
[0054]
No. In Nos. 6 to 9, although the chemical composition of the steel material satisfies the requirements, it was considered that the above problem occurred because the production was not performed under the conditions specified in the present invention. That is, no. In No. 6, since the annealing temperature was too low, recrystallization during annealing did not proceed sufficiently, resulting in a decrease in electrical conductivity. No. In No. 7, since the annealing time was short, recrystallization did not proceed sufficiently by annealing, resulting in a structure with a large grain interface area, resulting in a decrease in electrical conductivity. No. In No. 8, the ferrite grain growth did not sufficiently progress in the cooling process during rolling, and the grain boundary area increased even after annealing, resulting in a decrease in electrical conductivity. No. From FIG. 9, it can be seen that, in order to promote recrystallization during annealing, it is preferable to perform processing with a processing strain of 0.15 or more before annealing.
[0055]
No. Nos. 10 to 17 were manufactured under the conditions specified in the present invention. However, the chemical component composition of the steel material is out of the range specified in the present invention, so it is considered that the above-described problems occurred. No. No. 10 was contained in such a manner that the C content exceeded the upper limit defined in the present invention, which resulted in a significant deterioration in electrical conductivity. No. No. 11 had a result that both the cold forgeability and the electrical conductivity deteriorated because the Si amount exceeded the upper limit. No. In No. 12, Mn was added in excess of the conditions specified in the present invention, the generated MnS suppressed the growth of ferrite crystal grains, and the deposited MnS scattered conduction electrons, so the electrical conductivity decreased. It is considered a thing. No. No. 13 contained P in excess, and P segregated at the grain boundaries to suppress the growth of crystal grains, resulting in deterioration of electrical conductivity. No. In No. 14, since S was excessively contained, the deposited MnS was coarsened and the electrical conductivity was deteriorated.
[0056]
No. No. 15 contains Al in excess of the upper limit defined in the present invention, so a large amount of AlN is formed, and the growth of crystal grains is suppressed by the AlN, resulting in a marked decrease in electrical conductivity.
[0057]
No. In No. 16, since nitrogen exceeds the upper limit specified in the present invention, in addition to an increase in deformation resistance due to strain aging, electrical conductivity is also deteriorated. No. No. 17 resulted in the deterioration of electrical conductivity because oxygen was present in excess beyond the upper limit defined in the present invention.
[0058]
【The invention's effect】
The present invention is configured as described above, and it is possible to perform cold forging with good dimensional accuracy and to ensure excellent electrical conductivity, and to use such a steel material. Therefore, it is possible to provide various electric parts for automobiles, trains, ships, etc. at low cost, because it is possible to obtain electric parts exhibiting excellent electric conductivity and not using costly materials such as Ni. became.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between electrical resistivity and thermal conductivity.
FIG. 2 is a graph showing the effect of carbon content in steel on electrical conductivity and deformation resistance.
FIG. 3 is a graph showing the relationship between the heating temperature during hot rolling and the crack generation rate during hot rolling.
FIG. 4 is a graph showing the effect of annealing time and annealing temperature on the average ferrite grain size.
FIG. 5 is a graph showing the effect of machining strain on electrical resistivity.
Claims (5)
C :0.02%以下(0%を含む)、
Si:0.1%以下(0%を含まない)、
Mn:0.1〜0.5%、
P :0.02%以下(0%を含む)、
S :0.02%以下(0%を含む)、
Al:0.01%以下(0%を含む)、
N :0.005%以下(0%を含む)、
O :0.02%以下(0%を含む)を満たし、残部Feおよび不可避的不純物からなり、金属組織がフェライト単相組織であることを特徴とする冷間鍛造性と電気伝導性に優れた電気部品用鋼材。% By mass (the same applies below)
C: 0.02% or less (including 0%),
Si: 0.1% or less (excluding 0%),
Mn: 0.1 to 0.5%
P: 0.02% or less (including 0%),
S: 0.02% or less (including 0%),
Al: 0.01% or less (including 0%),
N: 0.005% or less (including 0%),
O: satisfying 0.02% or less (including 0%), remaining Fe and unavoidable impurities , excellent in cold forgeability and electrical conductivity, characterized in that the metal structure is a ferrite single phase structure Steel for electrical parts.
Si:0.1%以下(0%を含まない)、
Mn:0.1〜0.5%、
P :0.02%以下(0%を含む)、
S :0.02%以下(0%を含む)、
Al:0.01%以下(0%を含む)、
N :0.005%以下(0%を含む)、
O :0.02%以下(0%を含む)を満たし、残部Feおよび不可避的不純物からなり、金属組織が平均結晶粒径100μm以上のフェライト単相組織であることを特徴とする電気伝導性に優れた電気部品。C: 0.02% or less (including 0%),
Si: 0.1% or less (excluding 0%),
Mn: 0.1 to 0.5%
P: 0.02% or less (including 0%),
S: 0.02% or less (including 0%),
Al: 0.01% or less (including 0%),
N: 0.005% or less (including 0%),
O 2: satisfying 0.02% or less (including 0%), remaining Fe and inevitable impurities, the metal structure being a ferrite single phase structure having an average crystal grain size of 100 μm or more. Excellent electrical component.
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JP2002030081A JP3806653B2 (en) | 2002-02-06 | 2002-02-06 | Steel for electrical parts excellent in cold forgeability and electrical conductivity, electrical parts excellent in electrical conductivity, and manufacturing method thereof |
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JP5671869B2 (en) * | 2010-08-09 | 2015-02-18 | 新日鐵住金株式会社 | Non-oriented electrical steel sheet and manufacturing method thereof |
BRPI1004950A2 (en) * | 2010-09-03 | 2012-07-03 | Incotep Ind E Com De Tubos Especiais De Precisao Ltda | low carbon steel composition for electric conduction purposes in electrolytically reduced reduction tanks |
DE102013207738A1 (en) * | 2013-04-26 | 2014-10-30 | Sgl Carbon Se | Cathode block with a groove of varying depth and filled gap |
CN106460119B (en) | 2014-06-02 | 2019-01-11 | 新日铁住金株式会社 | Steel wire rod |
CA3039303A1 (en) | 2016-10-11 | 2018-04-19 | Nippon Steel & Sumitomo Metal Corporation | Steel wire rod and manufacturing method of steel wire rod |
US20190316238A1 (en) | 2016-10-11 | 2019-10-17 | Nippon Steel Corporation | Steel wire and coated steel wire |
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