JP4410984B2 - High-strength steel wire rod with excellent cold drawability - Google Patents

Description

【００２２】
【表２】

【００２３】
表１、表２および図１から、断線破面に現れた介在物には、アルミナやスピネルといった硬質のアルミナ系介在物（以下、アルミナやスピネルを単に「アルミナ系介在物」と総称することがある）の他、ジルコニアやジルコン等のジルコニア系介在物（以下、ジルコニアやジルコンを「ジルコニア系介在物」と総称することがある）も含まれており、図１から、断線破面に現れる介在物はアルミナ系介在物とジルコニア系介在物とで９５個数％以上占めていることがわかる。
【００２４】
即ち、冷間伸線時の断線を極力低減するには、アルミナ系介在物のみを抑制したのでは不十分であり、断線頻度のより小さい冷間伸線性に優れた高強度鋼線材を得るには、アルミナ系介在物とともにジルコニア系介在物も抑制しなければならないのである。
【００２５】
また、表１および表２に示す介在物組成を詳細に解析すると、
（ａ）破断面に現れた介在物がジルコニア（ＺｒＯ₂）である場合は、該ＺｒＯ₂が７０質量％以上を占めており、
（ｂ）破断面に現れた介在物がジルコン（ＺｒＯ₂・ＳｉＯ₂）である場合には、ＺｒＯ₂：５０質量％以上かつＳｉＯ₂：２０質量％以上を占めており、
（ｃ）破断面に現れた介在物がアルミナ系介在物（Ａｌ₂Ｏ₃、スピネル）である場合には、Ａｌ₂Ｏ₃が７０質量％以上を占めていることがわかる。
【００２６】
従って、鋼線材の冷間伸線性を向上させる手段としては、例えば、長径２０μｍ以上の介在物組成をＺｒＯ₂：７０質量％未満としたりすること等が考えられる。しかし、これらの組成を有する介在物は、耐火物や取鍋に付着したスラグに起因するところが大きく、実用的規模で実施する際に精錬条件等を変更したとしても、介在物組成を変えることは極めて困難である。
【００２７】
そこで本発明者らは、鋼中に存在する長径２０μｍ以上の介在物の組成が上記（ａ）〜（ｃ）のいずれかに該当する場合であっても、冷間伸線性に優れた高強度鋼線材および該鋼線材の製造に用いる鋼を得るべく様々な方法について検討した。
【００２８】
その結果、追って詳述する本発明の方法を採用し、長径２０μｍ以上の上記組成の介在物の個数を下記の通り制御すればよいことを見出した。
【００２９】
Ｘ≦０．２１×［Ｃ］^-1.34 …（１）
｛式中、Ｘは、介在物組成が上記（ａ）〜（ｃ）のいずれかである長径２０μｍ以上の介在物の、鋼５０ｇ当たりの合計個数を示し、［Ｃ］は鋼中Ｃ量（質量％）を示す｝
この様に介在物個数を制御する理由について以下に説明する。
【００３０】
まず、Ｃ量が約０．７％、約０．８％、約１．０％の鋼線材に冷間伸線を施して、断線指数と長径２０μｍ以上のアルミナ、スピネル、ジルコニアおよびジルコンの合計個数との関係を調べた。図２に、上記断線指数と上記介在物合計個数との関係をＣ量別に示す。図２には、断線指数の許容上限値が２０（冷間伸線性を高レベルに維持できる状態）であることを併せて示している。
【００３１】
尚、上記「断線指数」とは、以下の加工を行ったときの鋼線材１０トン当たりの湿式伸線の断線回数をいう。
【００３２】
熱間圧延後の鋼線材（直径：５．５ｍｍ）を用い、１次伸線（直径２．５ｍｍまで伸線）→ 熱処理（空気パテンティング処理）→ 二次伸線（直径０．８ｍｍまで伸線）→ 熱処理（鉛パテンティング処理）およびブラスメッキ処理 → 湿式伸線（直径０．１５ｍｍまで伸線）。
【００３３】
図２より、断線指数を許容上限値以下とするには、Ｃ量に応じて介在物個数の許容量上限を把握する必要があり、例えば、Ｃ量が約０．７％の場合には、上記介在物を約０．３１個／（鋼５０ｇ）以下、Ｃ量が約０．８％の場合には、上記介在物を約０．２６個／（鋼５０ｇ）以下、Ｃ量が約１．０％の場合には、上記介在物を約０．２１個／（鋼５０ｇ）以下にすべきであることがわかる。
【００３４】
そこで本発明者らは、製造する高強度鋼線材のＣ量に応じて、２０μｍ以上の上記介在物個数の許容量上限を把握するため、前記図２の結果から式（１）を導き出した。
【００３５】
Ｘ≦０．２１×［Ｃ］^-1.34 …（１）
｛式中、Ｘは、介在物組成が上記（ａ）〜（ｃ）のいずれかである長径２０μｍ以上の介在物の鋼５０ｇ当たりの合計個数を示し、［Ｃ］は鋼中Ｃ量（質量％）を示す｝
冷間伸線性が一段と優れた鋼線材を得るには、前記図２に示す断線指数が１５以下であることが好ましく、この様に断線指数を１５以下とするには、上記式（１）のＸが（０．１５８×［Ｃ］^-1.34）以下となるよう、Ｃ含有量に応じて介在物個数を低減することが望ましい。
【００３６】
尚、この様に介在物の組成やサイズを制御するにあたり、本発明では下記の評価方法を採用する。
【００３７】
本発明では、鋼中のアルミナ系介在物およびジルコニア系介在物の個数を、鋼材を酸溶解することによって抽出される酸不溶性の介在物で評価する。上述した通り、アルミナ等の硬質介在物は、鋼中に僅かしか存在せず断面観察ではほとんど観察されないため、断面観察の結果では鋼全体の介在物組成等を正確に評価できないからである。
【００３８】
ところで鋼中介在物のうちアルミナ系介在物は、酸溶解法でほぼ確実に抽出できることが知られているが、ジルコニアやジルコン等のジルコニア系介在物は酸溶解で抽出できるか不明である。そこで、まず対象介在物であるジルコン、ジルコニア、アルミナおよびスピネルが、鋼材を酸溶解することで確実に抽出できるかどうかの確認実験を行った。
【００３９】
実験は次の様にして行った。即ち、粒度１０〜５０μｍに分級した酸化物粉末（ジルコン、ジルコニア、アルミナ、スピネル）を、各０．１ｇずつ計りとり、９０℃の温硝酸溶液に入れて５時間後の重量変化を調べた。その結果を表１に示す。
【００４０】
尚、前記ジルコン粉末、ジルコニア粉末およびアルミナ粉末としては、工業用耐火物原料粉末を用いた。またスピネル粉末としては、Ａｌ₂Ｏ₃、ＭｇＯおよびＭｎＯ試薬を用いて合成したものを粉砕して使用した。
【００４１】
【表３】

【００４２】
表３より、ジルコンとジルコニアも、アルミナやスピネルといったアルミナ系介在物と同様、酸にほとんど溶けないことが分かる。
【００４３】
従って、鋼中に存在するジルコニア系介在物は、鋼材を酸溶解したのちアルミナ系介在物とともに酸不溶物として溶液中に残存し、該溶液を濾過すれば残渣として取得できる。従って、この残渣（介在物）の組成分析と個数計測を行えば、アルミナ系介在物とともにジルコニア系介在物についても定量評価を行うことができる。
【００４４】
本発明は、上記酸溶解に供する鋼材量まで規定するものではないが、鋼材量は多い方が好ましい。その理由は、上述した通り、鋼中に含まれるアルミナ系介在物やジルコニア系介在物の量は僅かであるため、酸溶解に供する鋼材量が少なすぎると、分析結果が鋼材全体の介在物組成を示すものとは言い難くなるからである。
【００４５】
図３は、チャージＮｏ．（Ａ１〜Ａ５およびＢ１〜Ｂ３）別のアルミナ系介在物量の異なる鋼線材を用い、酸溶解に供する鋼材量を２００〜３０００ｇの範囲で変化させたときの、鋼線材１００ｇあたりの長径２０μｍ以上のアルミナ系介在物の個数を調べた結果を示している。
【００４６】
この図３より、鋼線１００ｇあたりに存在する長径２０μｍ以上のアルミナ系介在物の個数は、酸溶解に供した鋼材量（酸溶解量）が約１００〜１０００ｇでは介在物個数にかなりのバラツキが見られるが、約１５００ｇ以上になるとほぼ一定の値を示すことがわかる。従って、酸溶解には１５００ｇ以上の鋼材を用いることが好ましく、より好ましくは２０００ｇ以上用いるのがよい。
【００４７】
上記酸分解には、用いる酸溶液等について一般的な方法・条件を採用することができる。また本発明では、上記の通り鋼材試料を酸溶解して抽出される介在物の組成とサイズを電子線マイクロアナライザー（ＥＰＭＡ：electron probe microanalyzer）によって測定するのがよい。
【００４８】
次に本発明で成分組成を規定した理由を説明する。
【００４９】
Ｃ：０ . ７〜１ . ０％
Ｃは鋼の強度の向上に有効な元素であり、０．７％以上含有させるのがよい。しかしＣ含有量が増加すると、中心偏析が生じやすくなって冷間加工性が損なわれるので、１．０％以下に抑える。
【００５０】
Ｓｉ：０ . １〜０ . ４％
Ｓｉは脱酸作用を有する元素であり、この様な作用を有効に発揮させるには、０．１％以上含有させるのがよい。しかしＳｉ量が過剰になると、脱酸生成物としてＳｉＯ₂量が多量に生成し冷間加工性が低下するので０．４％以下にするのがよい。
【００５１】
Ｍｎ：０ . ２〜０ . ６％
Ｍｎは、Ｓｉと同様に脱酸作用を有するとともに介在物制御作用も有しており、これらの作用を有効に発揮させるには０．２％以上とするのがよい。但し、Ｍｎ量が過剰になると、鋼材が脆化して冷間加工性が低下するため０．６％以下にするのがよい。
【００５２】
Ａｌ：０．００３％以下（０％を含む）
Ａｌが過剰に存在すると、アルミナやスピネル等の非延性介在物が多量に生成して伸線時に断線が生じ易くなる。従って、Ａｌ量は０．００３％以下に抑える必要がある。
【００５３】
またＮｉやＣｕ、Ｃｒを添加して、以下の特性を付与することも有効である。
【００５４】
Ｎｉ：０ . ０１〜１．０％
Ｎｉは、鋼線の強度上昇への寄与は少ないが、鋼線の靭性を高めるのに有効な元素である。この様な効果を十分に発揮させるには、Ｎｉを０．０１％以上含有させることが好ましい。しかし過剰に含有させても、その効果は飽和し製造コストの増加につながるので、１．０％以下に抑えることが好ましい。
【００５５】
Ｃｕ：０．０１〜１．０％および／または
Ｃｒ：０．０１〜１．５％
Ｃｕは、析出硬化作用を発揮して鋼線の高強度化に寄与する元素である。この様な効果を有効に発揮させるには、０．０１％以上含有させることが好ましい。しかしながら過剰に添加すると、結晶粒界に偏析し、鋼材の熱間圧延工程で割れやキズを発生させる原因になるので、１．０％以下の範囲内で含有させるのがよい。
【００５６】
Ｃｒは、冷間伸線時の加工硬化を促進する作用があり、比較的低い加工率で冷間伸線した場合でも高強度鋼線が得られ易い。しかも、Ｃｒは鋼の耐食性を高める作用も有しているので、タイヤのゴム補強材等として用いる場合に鋼線の腐食を有効に抑制する。この様な効果を発揮させるには、０．０１％以上含有させることが好ましい。しかしながら過剰に多量させると、パーライト変態に対する焼き入れ性が高くなってパテンティング処理が困難となる他、二次スケールの緻密化が促進されて、メカニカルデスケーリングや酸洗で該スケールを除去し難くなるので、Ｃｒ量は１．５％以下に抑えるのが好ましい。
【００５７】
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、該鋼中に微量の不可避不純物の含有が許容されるのは勿論のこと、前記本発明の作用に悪影響を与えない範囲で、更に他の元素を積極的に含有させた鋼を使用することも可能である。
【００５８】
次に本発明で製造方法を規定した理由について述べる。前記式（１）に示す通り、Ｃ量に応じて制御すべき介在物個数は異なるので、高強度鋼線材用鋼の製造方法もＣ量別に定める必要がある。そこで本発明者らは、前記介在物の個数が、前記式（１）を満足する高強度鋼線材用鋼および高強度鋼線材を製造するための方法について検討を行った。その結果、下記（i）（ii）の要件を満たすようにすることが有用であることをつきとめた。
（i）二次精錬後に使用する取鍋の内壁材としてアルミナ系のもの（アルミナが約９０質量％以上のもの）を用いること。
（ii）鋼の炭素含有量：［Ｃ］（質量％）に応じ、以下の条件で操業すること。
【００５９】
▲１▼０．７％≦［Ｃ］＜０．８％の場合は、
二次精錬前に除滓を実施してスラグ量を７.５kg/ｔ以下とする。
【００６０】
▲２▼０．８％≦［Ｃ］＜０．９％の場合は、
前回受鋼鋼種がＡｌ含有量：０．０１５質量％以下である取鍋を使用し、
かつ、二次精錬前に除滓を実施してスラグ量を１.５kg/ｔ以下とする。
【００６１】
▲３▼０．９％≦［Ｃ］≦１．０％の場合は、
受鋼初回の取鍋または前回受鋼鋼種が同鋼種である取鍋を使用し、
二次精錬前に除滓を実施してスラグ量を１.５kg/ｔ以下とし、かつ、
二次精錬後のスラグ塩基度を１〜２とする。
【００６２】
以下、上記製造方法を規定した理由について述べる。
（i）まず、本発明の高強度鋼線材用鋼を製造するにあたっては、二次精錬に使用する取鍋の内張材としてアルミナ系のもの（アルミナが約９０質量％以上を占めるもの）を用いる必要がある。Ｚｒを含む取鍋を用いた場合、精錬等を行う際に、取鍋の内壁材を構成するジルコニアやジルコンといった化合物が溶鋼に混入しやすく、溶鋼にこれらが混入するとそのままジルコニア系介在物として存在するためである。
【００６３】
尚、本発明では、アルミナ系介在物も極力抑制すべきであり、アルミナ系の内壁材を取鍋に用いた場合には、アルミナ系介在物としての混入が懸念される。しかし、アルミナ製の取鍋内壁材を使用しても、後記する実施例で明らかにする通り、アルミナ系介在物の生成は十分に抑制できるので、取鍋内壁材としてアルミナ系のものを用いても差し支えない。
【００６４】
また本発明は、取鍋以外のタンディッシュやタンディッシュノズル等の設備の材質まで特定するものではないが、アルミナ系介在物の混入を防止するには、タンディッシュにシリカ系の材質のもの、タンティッシュノズルにジルコニア系の材質のものを用いることが望ましい。
（ii）本発明では、二次精錬に使用する取鍋の内壁材にアルミナ系のものを用いることに加え、鋼の炭素含有量：［Ｃ］（質量％）に応じて、以下の条件で操業する必要がある。
【００６５】
▲１▼０．７％≦［Ｃ］＜０．８％の場合
Ｃ量が上記範囲内にある場合、二次精錬前に除滓を実施してスラグ量を７.５kg/ｔ以下に減少させるのがよい。この様にスラグ量を低減するのは次の理由による。
【００６６】
即ち、Ｃ量が上記範囲内にある場合に、上記介在物個数Ｘを規定範囲内に抑えるには、アルミナ系介在物の個数を約０．２個／鋼５０ｇ以下に抑えておくことが有効であり、この様に、アルミナ系介在物個数を抑制するには、図４に示すとおり、二次精錬後のスラグ中のアルミナ濃度を約１２％以下に抑えるのがよい。
【００６７】
そしてこの様に二次精錬後のスラグ中のアルミナ濃度を約１２質量％以下に抑えるには、二次精錬後のスラグ中アルミナ濃度と二次精錬前のスラグ量との関係を示した図５から分かる通り、二次精錬前に除滓を実施してスラグ量を７.５kg/ｔ以下に減少させるのが大変有効だからである。
【００６８】
該スラグ中のアルミナ濃度が約１２％を超えると、スラグ中のアルミナが鋼中に巻き込まれ易くなり、アルミナ系介在物の量が増加しやすくなる。
【００６９】
アルミナ系介在物の発生量をより抑制して上記介在物個数Ｘを更に低減するには、上記スラグ中のアルミナ濃度を１０％以下に抑えることが好ましく、そのためには、図５から二次精錬前のスラグ量を４．５ｋｇ/ｔ以下とすることが好ましいことがわかる。
【００７０】
▲２▼０．８％≦［Ｃ］＜０．９％の場合
前記図２に示される通り、Ｃ量が増加するにつれて、優れた冷間伸線性を確保するため介在物個数をより低減しなければならない。従って、Ｃ量が上記範囲にある場合には、使用する取鍋の前回受鋼鋼種を規定し、介在物の混入をより一層抑制する必要がある。
【００７１】
Ｃ量が上記範囲にある場合、取鍋として、前回受鋼鋼種がＡｌ含有量：０．０１５％以下のものを使用する。図６は、取鍋の前回受講鋼種のＡｌ濃度が該取鍋を使用して同一鋼種を受鋼したときに生ずるアルミナ系介在物個数に及ぼす影響を調べたものであるが、この図６に示されるように、前回受鋼鋼種がＡｌ含有量の高いものであると、生成したアルミナが取鍋に残留したままとなり、次に受鋼した時にアルミナ系介在物として鋼中に混入し易くなる。好ましくは、前回受鋼鋼種がＡｌ含有量：０．０１％以下である取鍋を使用するのがよい。
【００７２】
またＣ量が上記範囲にある場合、二次精錬前に除滓を実施してスラグ量を１.５kg/ｔ以下とするのがよい。
【００７３】
即ち、Ｃ量が上記範囲内にある場合に上記介在物個数Ｘを規定範囲内に抑えるには、アルミナ系介在物個数を約０．１個／鋼５０ｇ以下に抑えておくことが有効であり、この様に、アルミナ系介在物個数を抑制するには、前記図４に示すとおり、二次精錬後のスラグ中のアルミナ濃度を約８％以下に抑えるのがよい。
【００７４】
そしてこの様に二次精錬後のスラグ中のアルミナ濃度を約８％以下に抑えるには、二次精錬後のスラグ中アルミナ濃度と発生するアルミナ系介在物の個数の関係を示した前記図５から分かる通り、二次精錬前に除滓を実施してスラグ量を１．５kg/ｔ以下に減少させるのが大変有効だからである。
【００７５】
上記スラグ中のアルミナ濃度は約７．４％以下に抑えることが好ましく、そのためには、二次精錬前のスラグ量を０．５ｋｇ/ｔ以下とすることが好ましい。
【００７６】
▲３▼０．９％≦［Ｃ］≦１．０％の場合
前記図２に示される通り、Ｃ量が上記範囲にある場合には、前記▲１▼、▲２▼の場合よりも更に介在物個数を低減させる必要がある。従って、製造は下記条件で行う必要がある。
【００７７】
まず、前記▲２▼の場合と同様に、取鍋の前回受鋼鋼種について規定するが、この場合は、受鋼初回の取鍋または前回受鋼鋼種が同鋼種である取鍋を使用する必要がある。尚「受鋼初回の取鍋」とは、使用後の取鍋を冷却し、オフラインで取鍋耐火物を更新あるいは補修して、付着スラグや地金等が内壁にほとんど付着していない状態の取鍋をいう。
【００７８】
即ち、上記Ｃ量の場合には、この様に受鋼初回である取鍋を用いるか、前回受鋼鋼種が同鋼種である取鍋を使用することで、アルミナ系介在物の発生を効果的に抑制することができる。
【００７９】
また、Ｃ量が上記範囲にある場合には、二次精錬前に除滓を実施してスラグ量を１.５kg/ｔ以下とする必要がある。この様にスラグ量を低減するのは、Ｃ量が０．８％≦［Ｃ］＜０．９％である場合と同様に、二次精錬後のスラグ中のアルミナ濃度を約８％以下にコントロールするためである。該スラグ中のアルミナ濃度が約８％を超えると、前述した如くスラグ中のアルミナが鋼中に巻き込まれ易く、アルミナ介在物が生成しやすくなる。尚、上記スラグ中のアルミナ濃度は約７．４％以下に抑えることが好ましく、そのためには、二次精錬前のスラグ量を０．５ｋｇ/ｔ以下に抑えることが好ましい。
【００８０】
更にＣ量が上記範囲内にある場合は、二次精錬後のスラグ塩基度を１〜２とする必要がある。
【００８１】
図７は、二次精錬後のスラグ塩基度（ＣａＯ／ＳｉＯ₂）と鋼材５０ｇ中の長径２０μｍ以上のアルミナ系介在物の関係を示したものである。この図７から、アルミナ系介在物を低減するには、該塩基度を１以上にすることが効果的であることがわかる。
【００８２】
一方、図８は、二次精錬後のスラグ塩基度（ＣａＯ／ＳｉＯ₂）とＣａＯ含有介在物個数の関係を示したものである。この図８から、スラグ系介在物であるＣａＯ含有介在物を低減するには、前記塩基度を２以下に抑えることが有効であることがわかる。
【００８３】
【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。
【００８４】
実施例
溶銑を２４０トン転炉で一次精錬し、これを取鍋に受鋼したのち、取鍋加熱精錬設備で二次精錬を行って成分調整をした。
【００８５】
後述する表４のＡ１〜Ａ５、Ｂ１〜Ｂ５、およびＣ１〜Ｃ５（本発明の製造方法で行った実施例）は、下記の方法で鋼を溶製した。また、表４のＮｏ．ＡＡ１〜ＡＡ５、Ｎｏ．ＢＢ１〜ＢＢ５およびＮｏ．ＣＣ１〜ＣＣ５は、後述する通り、本発明の規定を外れる条件で鋼を溶製した。
【００８６】
取鍋の内張り耐火物には、ジルコニア系介在物の不可避的混入を極力避けるためアルミナ系のものを用いた。取鍋スライドバルブ、タンディッシュ本体、タンディッシュスライドバルブ等には、該設備等からアルミナ等が混入すると、そのままアルミナやスピネルといった硬質のアルミナ系介在物となりうるので、ジルコニア系耐火物を使用した。
【００８７】
そして鋼中炭素量：［Ｃ］が０．７％≦［Ｃ］＜０．８％の場合には、前記二次精錬前に、スラグ厚を測定し、スラグ密度：3000ｋｇ/m³としてスラグ量を求め、スラグ量が７.５ｋｇ/ｔ以下となるようスラグを除去（除滓）した。
【００８８】
また鋼中炭素量：［Ｃ］が０．８％≦［Ｃ］＜０．９％の場合には、前記二次精錬前に、スラグ量が１．５ｋｇ/ｔ以下となるよう除滓し、かつ取鍋として前回受鋼鋼種がＡｌ含有量：０．０１５％以下のものを用いた。
【００８９】
鋼中炭素量：［Ｃ］が０．９％≦［Ｃ］≦１．０％の場合には、二次精錬前に、スラグ量が１．５ｋｇ/ｔ以下となるよう除滓してスラグ厚を薄くし、かつ受鋼鋼種が初回から同一である取鍋を用いた。
【００９０】
更に［Ｃ］が０．９％≦［Ｃ］≦１．０％の場合、取鍋として、前回の受鋼において二次精錬後のスラグ塩基度（ＣａＯ／ＳｉＯ₂）を１〜２に調製して取鍋精錬を行ったものを用いた。
【００９１】
この様にスラグ塩基度を調整することで、取鍋内張りのアルミナ耐火物とスラグ中のＣａＯおよびＳｉＯ₂が反応し、溶鋼と接触する耐火物表面に精錬スラグとの反応層が形成され、アルミナ製耐火物表面の純粋なＡｌ₂Ｏ₃が、断線の原因となり難いＡｌ₂Ｏ₃−ＣａＯ−ＳｉＯ₂系の複合酸化物にかわる。従って、不可避的に混入するアルミナやスピネルといった硬質のアルミナ系介在物を低減でき、また仮にＡｌ₂Ｏ₃が混入した場合でもその混入量はごく少量で、Ａｌ₂Ｏ₃の割合が７０質量％未満と断線の生じ難い介在物組成とすることができる。
【００９２】
この様な取鍋を用いて行う二次精錬も、二次精錬後のスラグ塩基度（ＣａＯ／ＳｉＯ₂）が１〜２となるよう調製して取鍋精錬を行い、アルミナ系内張耐火物からのアルミナやスピネルといった硬質のアルミナ系介在物の発生を抑制した。
【００９３】
こうして得られた溶鋼を鋳造、更には鍛造を行って、直径３〜１０ｍｍの鋼線材を得た。この鋼線材から介在物組成分析用に１５００ｇの試料を切り出し、該試料を酸溶解して介在物を抽出し、ＥＰＭＡで介在物の組成分析を行った。
【００９４】
酸溶解による介在物の抽出および介在物の組成分析は次のようにして行った。まず、純水、硝酸(濃度６０％）および硫酸(濃度９６％）を５５：２５：１（体積比）の割合で混合した酸溶液をビーカに調製し、これに鋼線材試料を入れた。次に試料を入れたビーカを加熱し、溶液温度を９０〜９５℃に保持しながら鋼線材のマトリックスを溶解した。そして、溶解後に篩目が１０μｍのフィルターで濾過を行った。
【００９５】
フィルター上に残った介在物のうち、長径２０μｍ以上である介在物の組成分析およびその個数計測を行った。これらの介在物測定には、ＥＰＭＡ装置（島津製作所製EPMA-8705）を用い、設定条件を、
加速電圧：２０ｋＶ、
試料電流：０．０１μＡ、
ビーム径：を直径３μｍ、
Ｘ線：Ｋα線
とし、特性Ｘ線の波長分散分光法で長径２０μｍ以上の介在物の中央部分を定量評価した。
【００９６】
定量対象元素は、Ａｌ，Ｍｎ，Ｓｉ，Ｍｇ，Ｃａ，Ｚｒ，Ｏ（酸素）とした。定量に際しては、これらの元素濃度が既知の物質を上記方法で測定し、Ｘ線強度と元素濃度との関係を検量線として予め作成し、該検量線を用いて試料測定時のＸ線強度から各元素濃度を求めた。
【００９７】
そして上記各元素がＡｌ₂Ｏ₃，ＭｎＯ，ＳｉＯ₂，ＭｇＯ，ＣａＯ，ＺｒＯ₂を構成すると仮定し、上記ＥＰＭＡで求めた各元素濃度から、各介在物のＡｌ₂Ｏ₃，ＭｎＯ，ＳｉＯ₂，ＭｇＯ，ＣａＯまたはＺｒＯ₂の組成（質量％）を算出した。
【００９８】
該組成から、ＺｒＯ₂：７０質量％以上のものをジルコニア（ＺｒＯ₂）、ＺｒＯ₂：５０質量％以上かつＳｉＯ₂：２０質量％以上のものをジルコン（ＺｒＯ₂・ＳｉＯ₂）であるとし、これらをジルコニア系介在物として個数を測定した。また、Ａｌ₂Ｏ₃が７０質量％以上のものをアルミナ系介在物（Ａｌ₂Ｏ₃またはスピネル）として個数を測定した。
【００９９】
尚、上記濾過時には、酸に不溶のＳｉＯ₂系介在物もフィルター上に多数存在するが、本発明では、ＳｉＯ₂系介在物単体は本発明の制御対象でなく、上記の通りＺｒＯ₂、またはＺｒＯ₂とともに検出されるＳｉＯ₂、またはＡｌ₂Ｏ₃を制御対象とする。
【０１００】
表４に、介在物の酸溶解抽出に供した鋼線材成分組成、鋼線材中のジルコニア系介在物（ジルコニア、ジルコン）の個数、アルミナ系介在物（アルミナ、スピネル）の個数、および、それぞれの鋼線材を冷間伸線したときの断線指数を示す。尚、前記断線指数とは、上述の通り、以下の加工を行ったときの鋼線材１０トン当たりの断線回数をいう。
【０１０１】
熱間圧延後の鋼線材（直径：５．５ｍｍ）を用い、１次伸線（直径２．５ｍｍまで伸線）→ 熱処理（空気パテンティング処理）→ 二次伸線（直径０．８ｍｍまで伸線）→ 熱処理（鉛パテンティング処理）およびブラスメッキ処理 → 湿式伸線（直径０．１５ｍｍまで伸線）。
【０１０２】
【表４】

【０１０３】
表４から次のように考察できる。Ｎｏ．Ａ１〜Ａ５は、炭素量が約０．７％の鋼材を用い、Ｎｏ．Ｂ１〜Ｂ５は、炭素量が約０．８％の鋼材を用い、Ｎｏ．Ｃ１〜Ｃ５は、炭素量が約１．０％の鋼材を用いて、いずれも本発明で規定する方法で製造したものである。
【０１０４】
これらの結果から、本発明の規定要件を満たす方法で製造すれば、硬質のアルミナ系介在物やジルコニア系介在物の個数が非常に少なく、断線指数が２０以下と冷間伸線性に優れた高強度鋼線材が得られることがわかる。
【０１０５】
これらに対し、Ｎｏ．ＡＡ１〜ＡＡ５は炭素量が約０．７％の鋼材を用い、Ｎｏ．ＢＢ１〜ＢＢ５は炭素量が約０．８％の鋼材を用い、またＮｏ．ＣＣ１〜ＣＣ５は炭素量が約１．０％の鋼材を用いたものであるが、いずれも本発明の規定要件を欠く例である。従って、アルミナ系介在物個数またはジルコニア系介在物の個数が本発明で規定する上限を超えており、いずれも断線指数が２０を超えて高く冷間伸線性に劣っていることが分かる。
【０１０６】
即ち、Ｎｏ．ＡＡ１〜ＡＡ５は、いずれも二次精錬前のスラグ量が７．５ｋｇ／ｔを超えたため、介在物個数が規定上限を超えている。また、Ｎｏ．ＢＢ１およびＮｏ．ＢＢ２は前回受鋼鋼種がＡｌ量：０．０１５％超である取鍋を用い、Ｎｏ．ＢＢ３およびＢＢ４は二次精錬前のスラグ量が１．５ｋｇ／ｔを超えたため、またＮｏ．ＢＢ５は前回受鋼鋼種がＡｌ量：０．０１５％超である取鍋を用い、かつ二次精錬前のスラグ量が１．５ｋｇ／ｔを超えたため、いずれについても、特にアルミナ系介在物が多量に生じて介在物個数が規定上限を超えている。
【０１０７】
Ｎｏ．ＣＣ１およびＮｏ．ＣＣ２は、二次精錬後のスラグ塩基度が１未満であるため、アルミナ介在物個数が過剰となり、介在物個数が規定範囲を超える結果となった。またＮｏ．ＣＣ３およびＣＣ４は、前回受鋼鋼種がＡｌ濃度の高い異鋼種である取鍋を用いたため、またＮｏ．ＣＣ５は、二次精錬前のスラグ量が１．５ｋｇ／ｔを超えたため、いずれについても、アルミナ系介在物やジルコニア系介在物が多量に生じて介在物個数が規定範囲を超える結果となった。
【０１０８】
【発明の効果】
本発明は以上のように構成されており、冷間伸線性に優れた高強度鋼線材および該鋼線材を得るための高強度鋼線材用鋼とこれらの製造方法を提供する。本発明の高強度鋼線材を冷間伸線に用いれば、冷間伸線時の断線発生が極めて少なくなるので、例えば自動車用タイヤに用いられるスチールコード等に適用できる鋼線を高い歩留まりで効率よく製造することができる。
【図面の簡単な説明】
【図１】冷間伸線時に生じた鋼線材の断線破面に見られる介在物の種類を示したグラフである。
【図２】介在物個数と断線指数の関係を鋼中Ｃ量別に示したグラフである。
【図３】酸溶解量と介在物の抽出個数の関係を調べたグラフである。
【図４】二次精錬後のスラグ中アルミナ濃度とアルミナ系介在物個数との関係を調べたグラフである。
【図５】二次精錬前のスラグ量と二次精錬後のスラグ中アルミナ濃度との関係を調べたグラフである。
【図６】前回受鋼鋼種のＡｌ濃度とアルミナ系介在物個数との関係を調べたグラフである。
【図７】二次精錬後のスラグ塩基度とアルミナ系介在物個数の関係を調べたグラフである。
【図８】二次精錬後のスラグ塩基度とＣａＯ含有介在物個数の関係を調べたグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel wire rod excellent in cold workability, a high-strength steel wire rod steel for obtaining the steel wire rod, and a production method thereof, and is particularly used for automobile tires and the like. High-strength ultrafine steel wires such as steel cords with excellent fatigue resistance (hereinafter, wire steel before cold drawing is called “steel wire”, and wire steel after cold drawing is called “steel wire”. ) To produce a high-strength steel wire rod that has good cold-drawability and is less likely to break, high strength steel wire rod steel for obtaining such a steel wire rod, and the like. It is related to a useful method.
[0002]
[Prior art]
For example, a high-strength ultrafine steel wire used as a tire cord for automobiles is obtained by finally drawing a steel wire to an ultrafine diameter of about 0.5 mm. Alumina is used in the steel wire used for cold drawing. (Al₂O_Three) And spinel (MgO · Al₂O_Three), Zirconia (ZrO₂), Zircon (ZrO₂・ SiO₂), Silica (SiO₂) And the like (hereinafter, sometimes simply referred to as “inclusions”), there is a problem of disconnection during cold drawing starting from these.
[0003]
In order to prevent disconnection during cold drawing starting from such inclusions, it has been proposed to define the composition of inclusions and the ratio of appearing in the steel material cross section (for example, Patent Document 1 and Patent Document 2). And Patent Document 3).
[0004]
In order to achieve such a regulation, hard inclusions that can be generated or mixed in the manufacturing process are eliminated as much as possible, or the inclusions are made harmless by softening the hard inclusions so that they do not start as cracks. It has been broken.
[0005]
Specific methods for softening inclusions include, for example, controlling the composition of slag during refining, or adding inclusions such as Ca or Mg to molten steel to change the inclusions generated during the deoxidation process to a low melting point. And a method of modifying the material so that it can be easily stretched or miniaturized during cold wire drawing has been proposed.
[0006]
In these methods, inclusions are evaluated by the composition and number of inclusions appearing on the steel cross section. When evaluating a large number of soft inclusions in steel, it can be regarded as average data of the whole steel if a certain amount of steel cross section is observed. However, since hard inclusions such as alumina and spinel that are inevitably mixed in from the refractory that constitutes the ladle inner wall are only slightly present in the steel and are hardly observed in the cross-sectional observation, the result of the cross-sectional observation is It cannot be regarded as data indicating the overall inclusion composition.
[0007]
Therefore, it has been proposed that a certain amount of steel material is dissolved in acid to extract insoluble alumina, and the cold workability is evaluated by the number of alumina in 50 g of the steel material (for example, Patent Document 4).
[0008]
However, because it is applied to the above steel cords, etc., when hard processing is performed to an extremely fine diameter of about 0.2 mm or less in the cold wire drawing process, only hard alumina-based inclusions such as alumina and spinel are reduced. Only by doing this, the frequency of disconnection is hardly reduced, and the cold-drawing property cannot be sufficiently improved.
[0009]
[Patent Document 1]
Japanese Patent Publication No. 6-74484
[Patent Document 2]
Japanese Patent Publication No. 6-74485
[Patent Document 3]
JP-A-62-99437
[Patent Document 4]
JP 9-125200 A
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and its purpose is to suppress the formation of inclusions that are likely to cause disconnection particularly during cold drawing, and to achieve high strength with improved cold drawing properties. An object of the present invention is to provide a steel wire, a steel for a high-strength steel wire to obtain such a steel wire, and a production method useful for obtaining the steel and the steel wire.
[0011]
[Means for Solving the Problems]
The manufacturing method of the steel for high-strength steel wire rods excellent in cold drawability according to the present invention is mass% (hereinafter the same for chemical components)
C: 0.7 to 1.0%,
Si: 0.1 to 0.4%,
Mn: 0.2 to 0.6%,
Al: 0.003% or less (including 0%)
When manufacturing high strength steel wire rod steel that satisfies
Depending on the carbon content of the steel: [C] (mass%), the alumina-based one (alumina is about 90% by mass or more) is used as the ladle lining material used for secondary refining. It is characterized by operating under the conditions of
[0012]
(1) If 0.7% ≦ [C] <0.8%,
Before secondary refining, stripping is performed to reduce the slag amount to 7.5 kg / t or less.
(2) If 0.8% ≦ [C] <0.9%,
Use the ladle whose previous steel type is Al content: 0.015% or less,
In addition, the slag amount is reduced to 1.5 kg / t or less by removing the slag before secondary refining.
(3) If 0.9% ≦ [C] ≦ 1.0%,
Use the ladle for the first time of receiving steel or the ladle whose previous steel type is the same steel type,
Before secondary refining, stripping is performed to reduce the slag amount to 1.5 kg / t or less, and
Slag basicity after secondary refining is set to 1-2.
[0013]
The secondary refining refers to a process in which molten steel that has been coarsely refined in advance is guided to another simple refining apparatus (for example, a ladle), and refining such as deoxidation is performed with the refining apparatus to further increase cleanliness. .
[0014]
The present invention also defines a method for producing a high-strength steel wire rod excellent in cold drawability characterized by being obtained by processing the steel produced in this way into a linear shape.
[0015]
Moreover, the steel for high-strength steel wire rod of the present invention is
C: 0.7 to 1.0%,
Si: 0.1 to 0.4%,
Mn: 0.2 to 0.6%,
Al: steel satisfying 0.003% or less (including 0%),
Among inclusions extracted by acid dissolution, inclusions detected by an electron probe microanalyzer (EPMA) whose inclusion composition is any of the following (a) to (c) and whose major axis is 20 μm or more The total number (X) of the product per 50 g of the steel satisfies the following formula (1).
(A) ZrO₂: 70% by mass or more,
(B) ZrO₂: 50% by mass or more and SiO₂: 20% by mass or more,
(C) Al₂O_Three: 70 mass% or more.
[0016]
X ≦ 0.21 × [C]^-1.34    ... (1)
{In the formula, X represents the total number of inclusions with a major axis of 20 μm or more whose inclusion composition is any of the above (a) to (c) per 50 g of steel, and [C] is the amount of C in steel ( Mass%)}
The steel may further contain Ni: 0.01 to 1.0% as another element, and Cu: 0.01 to 1.0% and / or Cr: 0.01 to 1 It may contain 5%.
[0017]
The present invention also includes a high-strength steel wire rod excellent in cold drawability obtained using the above steel for high-strength steel wire rod.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Under the circumstances as described above, the present inventors have studied from various angles in order to obtain a steel wire rod that is less likely to be broken even when a strong cold wire drawing process is performed.
[0019]
First, the present inventors collected a large number of steel wires broken at the time of cold drawing, and examined the composition and the like of individual nonmetallic inclusions that appeared on the broken surface. Tables 1 and 2 show the composition of inclusions appearing on the broken surface, the major axis of the inclusions, and the type (classification) of the inclusions. Moreover, the pie chart which classified the inclusion which appeared in the disconnection fracture surface shown in Table 1 and Table 2 is shown in FIG.
[0020]
As shown in Tables 1 and 2, since no cracks or the like starting from inclusions of less than 20 μm were found on the fracture surface caused by the fracture during cold drawing, the major axis was 20 μm in the present invention. The above inclusions were controlled.
[0021]
[Table 1]

[0022]
[Table 2]

[0023]
From Tables 1 and 2 and FIG. 1, inclusions appearing on the broken surface are hard alumina type inclusions such as alumina and spinel (hereinafter, alumina and spinel may be simply referred to as “alumina type inclusions”). In addition, zirconia inclusions such as zirconia and zircon (hereinafter, zirconia and zircon may be collectively referred to as “zirconia inclusions”) are included. It can be seen that 95% by number or more of the inclusions are composed of alumina inclusions and zirconia inclusions.
[0024]
That is, in order to reduce the disconnection at the time of cold drawing as much as possible, it is not sufficient to suppress only alumina inclusions, and to obtain a high-strength steel wire rod excellent in cold drawing with less disconnection frequency. Therefore, zirconia inclusions must be suppressed together with alumina inclusions.
[0025]
Moreover, when the inclusion composition shown in Table 1 and Table 2 is analyzed in detail,
(A) Inclusions appearing on the fracture surface are zirconia (ZrO₂), The ZrO₂Occupies 70% by mass or more,
(B) Inclusions appearing on the fracture surface are zircon (ZrO₂・ SiO₂) Is ZrO₂: 50% by mass or more and SiO₂: 20% by mass or more,
(C) The inclusions appearing on the fracture surface are alumina inclusions (Al₂O_Three, Spinel), Al₂O_ThreeOccupies 70% by mass or more.
[0026]
Therefore, as a means for improving the cold drawability of the steel wire rod, for example, an inclusion composition having a major axis of 20 μm or more is made of ZrO.₂: It can be considered to be less than 70 mass%. However, inclusions having these compositions are largely caused by slag adhering to refractories and ladles, and even if refining conditions are changed when implemented on a practical scale, it is not possible to change the inclusion composition. It is extremely difficult.
[0027]
Therefore, the inventors have high strength excellent in cold drawability even when the composition of inclusions having a major axis of 20 μm or more present in steel corresponds to any of the above (a) to (c). Various methods were studied to obtain a steel wire and a steel used for manufacturing the steel wire.
[0028]
As a result, it has been found that the method of the present invention, which will be described in detail later, is adopted, and the number of inclusions having the above composition having a major axis of 20 μm or more may be controlled as follows.
[0029]
X ≦ 0.21 × [C]^-1.34    ... (1)
{In the formula, X represents the total number of inclusions with a major axis of 20 μm or more whose inclusion composition is any of the above (a) to (c) per 50 g of steel, and [C] is the amount of C in steel ( Mass%)}
The reason for controlling the number of inclusions in this way will be described below.
[0030]
First, cold drawing is applied to a steel wire having a C content of about 0.7%, about 0.8%, and about 1.0%, and the total of the disconnection index and alumina, spinel, zirconia, and zircon having a major axis of 20 μm or more. The relationship with the number was examined. FIG. 2 shows the relationship between the disconnection index and the total number of inclusions for each C amount. FIG. 2 also shows that the allowable upper limit of the disconnection index is 20 (a state in which the cold drawability can be maintained at a high level).
[0031]
The “disconnection index” refers to the number of times of wet wire drawing per 10 tons of steel wire when the following processing is performed.
[0032]
Using hot-rolled steel wire (diameter: 5.5 mm), primary wire drawing (drawing to 2.5 mm diameter) → heat treatment (air patenting treatment) → secondary wire drawing (drawing to 0.8 mm diameter) Wire) → Heat treatment (lead patenting treatment) and brass plating treatment → Wet wire drawing (drawing to a diameter of 0.15 mm).
[0033]
From FIG. 2, in order to make the disconnection index below the allowable upper limit value, it is necessary to grasp the upper limit of the number of inclusions according to the C amount. For example, when the C amount is about 0.7%, When the inclusion is about 0.31 piece / (steel 50 g) or less and the C amount is about 0.8%, the inclusion is about 0.26 piece / (steel 50 g) or less and the C amount is about 1 In the case of 0.0%, it is understood that the inclusions should be about 0.21 pieces / (steel 50 g) or less.
[0034]
Therefore, the present inventors derived the formula (1) from the result of FIG. 2 in order to grasp the upper limit of the allowable number of inclusions of 20 μm or more according to the C amount of the high-strength steel wire to be manufactured.
[0035]
X ≦ 0.21 × [C]^-1.34    ... (1)
{In the formula, X represents the total number of inclusions with a major axis of 20 μm or more whose inclusion composition is any of the above (a) to (c) per 50 g of steel, and [C] is the amount of C in the steel (mass %)}
In order to obtain a steel wire material with further excellent cold drawability, the disconnection index shown in FIG. 2 is preferably 15 or less, and in order to make the disconnection index 15 or less in this way, the above formula (1) X is (0.158 × [C]^-1.34) It is desirable to reduce the number of inclusions according to the C content so as to be the following.
[0036]
In the present invention, the following evaluation method is employed in controlling the composition and size of inclusions.
[0037]
In the present invention, the number of alumina inclusions and zirconia inclusions in the steel is evaluated by acid-insoluble inclusions extracted by dissolving the steel with an acid. As described above, hard inclusions such as alumina are present in the steel in a small amount and are hardly observed in the cross-sectional observation. Therefore, the inclusion composition and the like of the entire steel cannot be accurately evaluated as a result of the cross-sectional observation.
[0038]
By the way, it is known that alumina inclusions among inclusions in steel can be extracted almost certainly by an acid dissolution method, but it is unclear whether zirconia inclusions such as zirconia and zircon can be extracted by acid dissolution. Therefore, first, a confirmation experiment was conducted to confirm whether the inclusions zircon, zirconia, alumina, and spinel could be reliably extracted by acid-dissolving the steel material.
[0039]
The experiment was performed as follows. That is, 0.1 g of each oxide powder (zircon, zirconia, alumina, spinel) classified to a particle size of 10 to 50 μm was weighed and placed in a 90 ° C. warm nitric acid solution, and the change in weight after 5 hours was examined. The results are shown in Table 1.
[0040]
As the zircon powder, zirconia powder and alumina powder, industrial refractory raw material powder was used. As spinel powder, Al₂O_ThreeThen, those synthesized using MgO and MnO reagents were crushed and used.
[0041]
[Table 3]

[0042]
From Table 3, it can be seen that zircon and zirconia are almost insoluble in acid as well as alumina inclusions such as alumina and spinel.
[0043]
Accordingly, the zirconia inclusions present in the steel remain in the solution as an acid insoluble matter together with the alumina inclusions after the steel material is acid-dissolved, and can be obtained as a residue by filtering the solution. Therefore, if composition analysis and number counting of this residue (inclusions) are performed, it is possible to quantitatively evaluate zirconia inclusions as well as alumina inclusions.
[0044]
Although this invention does not prescribe | regulate to the amount of steel materials with which the said acid dissolution is provided, the one where the amount of steel materials is large is preferable. The reason is that, as described above, since the amount of alumina inclusions and zirconia inclusions contained in the steel is very small, if the amount of the steel material used for acid dissolution is too small, the analysis result shows the inclusion composition of the entire steel material. This is because it is difficult to say.
[0045]
FIG. (A1 to A5 and B1 to B3) When different steel wire materials having different amounts of alumina inclusions are used and the amount of steel material used for acid dissolution is changed in the range of 200 to 3000 g, the major axis per 100 g of the steel wire material is 20 μm or more. The result of investigating the number of alumina inclusions is shown.
[0046]
According to FIG. 3, the number of alumina inclusions having a major axis of 20 μm or more present per 100 g of steel wire has a considerable variation in the number of inclusions when the amount of steel material used for acid dissolution (acid dissolution amount) is about 100 to 1000 g. As can be seen, it shows a substantially constant value when it is about 1500 g or more. Therefore, it is preferable to use 1500 g or more of steel for acid dissolution, and more preferably 2000 g or more.
[0047]
For the acid decomposition, general methods and conditions for the acid solution to be used can be employed. In the present invention, as described above, the composition and size of inclusions extracted by dissolving a steel sample with an acid may be measured by an electron probe microanalyzer (EPMA).
[0048]
Next, the reason for defining the component composition in the present invention will be described.
[0049]
C: 0 . 7-1 . 0%
C is an element effective for improving the strength of steel, and is preferably contained in an amount of 0.7% or more. However, if the C content is increased, center segregation is likely to occur and cold workability is impaired.
[0050]
Si: 0 . 1 to 0 . 4%
Si is an element having a deoxidizing action, and in order to exert such an action effectively, it is preferable to contain 0.1% or more. However, when the amount of Si is excessive, SiO is used as a deoxidation product.₂Since a large amount is generated and cold workability is deteriorated, the content is preferably 0.4% or less.
[0051]
Mn: 0 . 2-0 . 6%
Mn has a deoxidizing action as well as Si, and also has an inclusion controlling action. In order to effectively exhibit these actions, Mn is preferably 0.2% or more. However, if the amount of Mn becomes excessive, the steel material becomes brittle and cold workability is lowered, so it is preferable to make it 0.6% or less.
[0052]
Al: 0.003% or less (including 0%)
If Al is present in excess, a large amount of non-ductile inclusions such as alumina and spinel are generated, and breakage is likely to occur during wire drawing. Therefore, the amount of Al needs to be suppressed to 0.003% or less.
[0053]
It is also effective to add the following characteristics by adding Ni, Cu, or Cr.
[0054]
Ni: 0 . 01-1.0%
Ni contributes little to increasing the strength of the steel wire, but is an effective element for increasing the toughness of the steel wire. In order to sufficiently exhibit such an effect, it is preferable to contain 0.01% or more of Ni. However, even if contained excessively, the effect is saturated and leads to an increase in manufacturing cost.
[0055]
Cu: 0.01 to 1.0% and / or
Cr: 0.01 to 1.5%
Cu is an element that exerts a precipitation hardening action and contributes to increasing the strength of the steel wire. In order to exhibit such an effect effectively, it is preferable to make it contain 0.01% or more. However, if added excessively, it segregates at the grain boundaries and causes cracks and scratches in the hot rolling process of the steel material, so it is preferable to contain it within a range of 1.0% or less.
[0056]
Cr has an action of promoting work hardening during cold drawing, and a high-strength steel wire is easily obtained even when cold drawing is performed at a relatively low processing rate. And since Cr also has the effect | action which raises the corrosion resistance of steel, when using as a rubber reinforcement material etc. of a tire, corrosion of a steel wire is suppressed effectively. In order to exhibit such an effect, it is preferable to contain 0.01% or more. However, if the amount is excessively large, the hardenability to pearlite transformation becomes high and the patenting process becomes difficult, and the densification of the secondary scale is promoted, and it is difficult to remove the scale by mechanical descaling or pickling. Therefore, the Cr content is preferably suppressed to 1.5% or less.
[0057]
The elements defined in the present invention are as described above, and the remaining component is substantially Fe, but it is a matter of course that a small amount of inevitable impurities are allowed to be contained in the steel. It is also possible to use steel that further contains other elements as long as it does not adversely affect the steel.
[0058]
Next, the reason why the manufacturing method is specified in the present invention will be described. As shown in the above formula (1), the number of inclusions to be controlled differs depending on the C amount, and therefore, the manufacturing method of the steel for high strength steel wire rods needs to be determined for each C amount. Therefore, the present inventors have studied a method for producing a high strength steel wire rod and a high strength steel wire rod in which the number of inclusions satisfies the formula (1). As a result, it was found useful to satisfy the following requirements (i) and (ii).
(I) Use an alumina-based material (alumina content of about 90% by mass or more) as the inner wall material of the ladle used after secondary refining.
(Ii) Carbon content of steel: Operate under the following conditions according to [C] (% by mass).
[0059]
(1) If 0.7% ≦ [C] <0.8%,
Remove the slag to 7.5 kg / t or less after secondary refining.
[0060]
(2) If 0.8% ≦ [C] <0.9%,
Use a ladle whose previous steel type is Al content: 0.015 mass% or less,
In addition, stripping is performed before secondary refining to reduce the slag amount to 1.5 kg / t or less.
[0061]
(3) If 0.9% ≦ [C] ≦ 1.0%,
Use the ladle for the first time of receiving steel or the ladle whose previous steel type is the same steel type,
Before secondary refining, stripping is performed to reduce the slag amount to 1.5 kg / t or less, and
Slag basicity after secondary refining is set to 1-2.
[0062]
The reason why the above manufacturing method is specified will be described below.
(I) First, in producing the steel for high strength steel wire rod of the present invention, an alumina-based material (alumina occupies about 90% by mass or more) is used as a ladle lining material used for secondary refining. It is necessary to use it. When using a ladle containing Zr, when smelting, etc., compounds such as zirconia and zircon that make up the inner wall material of the ladle are likely to be mixed into the molten steel, and when these are mixed into the molten steel, they exist as zirconia inclusions as they are It is to do.
[0063]
In the present invention, alumina inclusions should also be suppressed as much as possible. When an alumina inner wall material is used for a ladle, there is a concern of mixing as alumina inclusions. However, even if an alumina ladle inner wall material is used, the production of alumina inclusions can be sufficiently suppressed, as will be clarified in the examples described later. There is no problem.
[0064]
In addition, the present invention is not limited to the material of the tundish and tundish nozzle other than the ladle, but in order to prevent the inclusion of alumina inclusions, the tundish is made of a silica-based material, It is desirable to use a zirconia-based material for the tan tissue nozzle.
(Ii) In the present invention, in addition to using an alumina-based material for the inner wall material of the ladle used for secondary refining, depending on the carbon content of steel: [C] (mass%), It is necessary to operate.
[0065]
(1) When 0.7% ≤ [C] <0.8%
When the amount of C is within the above range, it is preferable to reduce the slag amount to 7.5 kg / t or less by performing stripping before secondary refining. The reason for reducing the amount of slag in this way is as follows.
[0066]
That is, when the amount of C is within the above range, it is effective to keep the number of alumina inclusions to about 0.2 / 50 g of steel or less in order to keep the number of inclusions X within the specified range. Thus, in order to suppress the number of alumina inclusions, as shown in FIG. 4, the alumina concentration in the slag after secondary refining is preferably suppressed to about 12% or less.
[0067]
FIG. 5 shows the relationship between the alumina concentration in the slag after the secondary refining and the slag amount before the secondary refining in order to suppress the alumina concentration in the slag after the secondary refining to about 12% by mass or less. As can be seen from the above, it is very effective to reduce the slag amount to 7.5 kg / t or less by removing the slag before secondary refining.
[0068]
If the alumina concentration in the slag exceeds about 12%, the alumina in the slag is likely to be caught in the steel, and the amount of alumina inclusions tends to increase.
[0069]
In order to further reduce the number of inclusions X by further suppressing the generation amount of alumina inclusions, it is preferable to suppress the alumina concentration in the slag to 10% or less. For this purpose, secondary refining from FIG. It can be seen that the previous slag amount is preferably 4.5 kg / t or less.
[0070]
(2) When 0.8% ≦ [C] <0.9%
As shown in FIG. 2, as the amount of C increases, the number of inclusions must be further reduced in order to ensure excellent cold drawing. Therefore, when the amount of C is in the above range, it is necessary to specify the previous steel receiving steel type of the ladle to be used, and to further suppress inclusion inclusions.
[0071]
When the amount of C is in the above range, as the ladle, the last steel receiving steel type is Al content: 0.015% or less. Fig. 6 shows the effect of the Al concentration of the steel grade that the ladle took last time on the number of alumina inclusions generated when the same steel type was received using the ladle. As shown, if the previous steel receiving steel type has a high Al content, the generated alumina will remain in the ladle and will be easily mixed into the steel as an alumina inclusion when the steel is next received. . It is preferable to use a ladle whose previous steel receiving steel type is Al content: 0.01% or less.
[0072]
In addition, when the C amount is in the above range, it is preferable to remove the slag before the secondary refining so that the slag amount is 1.5 kg / t or less.
[0073]
That is, in order to keep the number X of inclusions within the specified range when the C content is in the above range, it is effective to keep the number of alumina inclusions to about 0.1 / 50 g of steel or less. Thus, in order to suppress the number of alumina inclusions, the alumina concentration in the slag after secondary refining is preferably suppressed to about 8% or less as shown in FIG.
[0074]
In order to suppress the alumina concentration in the slag after secondary refining to about 8% or less in this way, the relationship between the alumina concentration in the slag after secondary refining and the number of generated alumina inclusions is shown in FIG. As can be seen from the above, it is very effective to reduce the slag amount to 1.5 kg / t or less by removing the slag before secondary refining.
[0075]
The alumina concentration in the slag is preferably suppressed to about 7.4% or less, and for that purpose, the amount of slag before secondary refining is preferably 0.5 kg / t or less.
[0076]
(3) When 0.9% ≦ [C] ≦ 1.0%
As shown in FIG. 2, when the C amount is in the above range, the number of inclusions needs to be further reduced as compared with the cases (1) and (2). Therefore, the production needs to be performed under the following conditions.
[0077]
First, as in the case of the above (2), the last steel receiving steel type of the ladle is specified. In this case, it is necessary to use the first ladle of the receiving steel or the ladle whose previous steel receiving steel type is the same steel type. There is. In addition, “the first ladle for receiving steel” means that the ladle after use is cooled and the ladle refractory is renewed or repaired offline, so that the attached slag, metal, etc. are hardly attached to the inner wall. Say the ladle.
[0078]
That is, in the case of the above C amount, the use of a ladle that is the first steel receiving in this way, or the use of a ladle that is the same steel type as the previous steel receiving steel, can effectively generate alumina inclusions. Can be suppressed.
[0079]
In addition, when the C amount is in the above range, it is necessary to remove the iron before the secondary refining so that the slag amount is 1.5 kg / t or less. In this way, the amount of slag is reduced as in the case where the amount of C is 0.8% ≦ [C] <0.9%, and the alumina concentration in the slag after secondary refining is about 8% or less. This is for control. When the alumina concentration in the slag exceeds about 8%, as described above, the alumina in the slag is easily entangled in the steel, and alumina inclusions are easily generated. The alumina concentration in the slag is preferably suppressed to about 7.4% or less, and for that purpose, the amount of slag before secondary refining is preferably suppressed to 0.5 kg / t or less.
[0080]
Furthermore, when C amount exists in the said range, it is necessary to make the slag basicity after secondary refining 1-2.
[0081]
FIG. 7 shows the basicity of slag after secondary refining (CaO / SiO₂) And alumina inclusions having a major axis of 20 μm or more in 50 g of steel. FIG. 7 shows that it is effective to reduce the basicity to 1 or more in order to reduce alumina inclusions.
[0082]
On the other hand, FIG. 8 shows slag basicity (CaO / SiO2) after secondary refining.₂) And the number of CaO-containing inclusions. From FIG. 8, it can be seen that it is effective to reduce the basicity to 2 or less in order to reduce CaO-containing inclusions, which are slag inclusions.
[0083]
【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.
[0084]
Example
The hot metal was firstly refined in a 240-ton converter and the ladle was steel-received, and then secondary refining was performed in a ladle heating and refining equipment to adjust the components.
[0085]
A1 to A5, B1 to B5, and C1 to C5 (Examples carried out by the production method of the present invention) in Table 4 described later melted steel by the following method. In Table 4, No. AA1 to AA5, no. BB1 to BB5 and No. CC1-CC5 melted steel on the conditions which remove | deviate the prescription | regulation of this invention so that it may mention later.
[0086]
The ladle refractory was made of alumina to avoid the inevitable mixing of zirconia inclusions. Zirconia refractories were used for ladle slide valves, tundish main bodies, tundish slide valves, etc., when alumina or the like was mixed from the equipment or the like, so that they could be used as hard alumina inclusions such as alumina and spinel.
[0087]
When the carbon content in steel: [C] is 0.7% ≦ [C] <0.8%, the slag thickness is measured before the secondary refining, and the slag density is 3000 kg / m.^ThreeThen, the slag amount was determined, and the slag was removed (removed) so that the slag amount was 7.5 kg / t or less.
[0088]
Further, when the carbon content in steel: [C] is 0.8% ≦ [C] <0.9%, the steel is removed so that the slag amount is 1.5 kg / t or less before the secondary refining. And as a ladle, the last steel receiving steel type used Al content: 0.015% or less.
[0089]
Carbon content in steel: When [C] is 0.9% ≦ [C] ≦ 1.0%, slag is removed before secondary refining so that the slag amount is 1.5 kg / t or less. A ladle having a reduced thickness and the same steel grade from the first time was used.
[0090]
Furthermore, when [C] is 0.9% ≦ [C] ≦ 1.0%, the slag basicity (CaO / SiO2) after secondary refining in the previous steel receiving as a ladle₂) Was prepared to 1-2 and ladle refining was used.
[0091]
By adjusting the slag basicity in this way, the ladle lining alumina refractory and the slag CaO and SiO₂Reacts to form a reaction layer with smelting slag on the surface of the refractory that comes into contact with the molten steel, and pure Al on the surface of the refractory made of alumina₂O_ThreeIs difficult to cause disconnection₂O_Three-CaO-SiO₂Instead of complex oxides. Therefore, hard alumina inclusions such as alumina and spinel that are inevitably mixed can be reduced.₂O_ThreeEven if it is mixed, the mixed amount is very small, Al₂O_ThreeIf the ratio is less than 70% by mass, the inclusion composition is less likely to cause disconnection.
[0092]
Secondary refining using such a ladle is also applied to slag basicity (CaO / SiO2) after secondary refining.₂) Was adjusted to 1-2, and ladle refining was performed to suppress the generation of hard alumina inclusions such as alumina and spinel from the alumina-lined refractory.
[0093]
The molten steel thus obtained was cast and further forged to obtain a steel wire having a diameter of 3 to 10 mm. A sample of 1500 g was cut out from this steel wire for inclusion composition analysis, the sample was acid-dissolved to extract inclusions, and the composition analysis of inclusions was performed with EPMA.
[0094]
Extraction of inclusions by acid dissolution and composition analysis of inclusions were performed as follows. First, an acid solution in which pure water, nitric acid (concentration 60%) and sulfuric acid (concentration 96%) were mixed at a ratio of 55: 25: 1 (volume ratio) was prepared in a beaker, and a steel wire sample was put therein. Next, the beaker containing the sample was heated to dissolve the matrix of the steel wire rod while maintaining the solution temperature at 90 to 95 ° C. And it filtered with the filter with a 10 micrometers sieve after melt | dissolution.
[0095]
Among the inclusions remaining on the filter, the composition analysis of inclusions having a major axis of 20 μm or more and the number of the inclusions were measured. For these inclusion measurements, an EPMA device (EPMA-8705 manufactured by Shimadzu Corporation) was used, and the setting conditions were
Accelerating voltage: 20 kV
Sample current: 0.01 μA,
Beam diameter: a diameter of 3 μm,
X-ray: Kα ray
Then, the central part of inclusions having a major axis of 20 μm or more was quantitatively evaluated by wavelength dispersion spectroscopy of characteristic X-rays.
[0096]
The determination target elements were Al, Mn, Si, Mg, Ca, Zr, and O (oxygen). For quantification, substances having known element concentrations are measured by the above method, and the relationship between the X-ray intensity and the element concentration is prepared in advance as a calibration curve, and the calibration curve is used to determine the X-ray intensity at the time of sample measurement. The concentration of each element was determined.
[0097]
And each of the above elements is Al₂O_Three, MnO, SiO₂, MgO, CaO, ZrO₂From the concentration of each element obtained by the above EPMA, the Al content of each inclusion is assumed.₂O_Three, MnO, SiO₂, MgO, CaO or ZrO₂The composition (mass%) of was calculated.
[0098]
From this composition, ZrO₂: 70% by mass or more of zirconia (ZrO₂), ZrO₂: 50% by mass or more and SiO₂: 20% by mass or more of zircon (ZrO₂・ SiO₂), And these were used as zirconia inclusions to measure the number. Al₂O_ThreeWith an alumina inclusion (Al₂O_ThreeOr the number was measured as spinel).
[0099]
During the filtration, SiO insoluble in acid.₂Many system inclusions are also present on the filter.₂The system inclusion alone is not the control object of the present invention, and as described above, ZrO₂Or ZrO₂SiO detected with₂Or Al₂O_ThreeIs the control target.
[0100]
Table 4 shows the composition of steel wire materials subjected to acid dissolution extraction of inclusions, the number of zirconia inclusions (zirconia, zircon) in the steel wire, the number of alumina inclusions (alumina, spinel), and the respective The disconnection index when the steel wire is cold drawn. In addition, the said disconnection index | exponent means the frequency | count of disconnection per 10 tons of steel wire materials when the following processes are performed as above-mentioned.
[0101]
Using hot-rolled steel wire (diameter: 5.5 mm), primary wire drawing (drawing to 2.5 mm diameter) → heat treatment (air patenting treatment) → secondary wire drawing (drawing to 0.8 mm diameter) Wire) → Heat treatment (lead patenting treatment) and brass plating treatment → Wet wire drawing (drawing to a diameter of 0.15 mm).
[0102]
[Table 4]

[0103]
From Table 4, it can be considered as follows. No. A1 to A5 use steel materials having a carbon content of about 0.7%. B1 to B5 are steel materials having a carbon content of about 0.8%. C1 to C5 are produced by a method defined in the present invention, using a steel material having a carbon content of about 1.0%.
[0104]
From these results, if manufactured by a method that satisfies the prescribed requirements of the present invention, the number of hard alumina inclusions and zirconia inclusions is very small, and the disconnection index is 20 or less, which is excellent in cold drawability. It turns out that a strength steel wire is obtained.
[0105]
In contrast, no. AA1 to AA5 use steel materials having a carbon content of about 0.7%. BB1 to BB5 use steel materials having a carbon content of about 0.8%. CC1 to CC5 are steel materials having a carbon content of about 1.0%, but any of them is an example lacking the requirement of the present invention. Therefore, it can be seen that the number of alumina inclusions or the number of zirconia inclusions exceeds the upper limit defined in the present invention, and both are high in disconnection index exceeding 20 and inferior in cold drawability.
[0106]
That is, no. As for AA1-AA5, since the amount of slag before secondary refining exceeded 7.5 kg / t, the number of inclusions exceeds the specified upper limit. No. BB1 and No. BB2 uses a ladle in which the last steel receiving steel type has an Al content of more than 0.015%. In BB3 and BB4, the amount of slag before secondary refining exceeded 1.5 kg / t. Since BB5 used a ladle whose previous steel type was Al: more than 0.015%, and the amount of slag before secondary refining exceeded 1.5 kg / t, in both cases, especially alumina inclusions A large amount of inclusions exceeds the specified upper limit.
[0107]
No. CC1 and No. In CC2, since the slag basicity after the secondary refining is less than 1, the number of inclusions of alumina becomes excessive, and the number of inclusions exceeds the specified range. No. Since CC3 and CC4 used a ladle whose previous steel type was a different steel type with a high Al concentration, No. As for CC5, since the amount of slag before secondary refining exceeded 1.5 kg / t, in both cases, a large amount of alumina inclusions and zirconia inclusions occurred, resulting in the number of inclusions exceeding the specified range. .
[0108]
【The invention's effect】
The present invention is configured as described above, and provides a high-strength steel wire rod excellent in cold drawability, a high-strength steel wire rod steel for obtaining the steel wire rod, and a production method thereof. If the high-strength steel wire of the present invention is used for cold drawing, the occurrence of disconnection during cold drawing is extremely reduced, so that the steel wire applicable to, for example, steel cords used for automobile tires can be efficiently produced with high yield. Can be manufactured well.
[Brief description of the drawings]
FIG. 1 is a graph showing the types of inclusions found on a broken fracture surface of a steel wire rod produced during cold drawing.
FIG. 2 is a graph showing the relationship between the number of inclusions and the disconnection index according to the amount of C in steel.
FIG. 3 is a graph showing the relationship between the amount of dissolved acid and the number of inclusions extracted.
FIG. 4 is a graph showing the relationship between the alumina concentration in the slag after secondary refining and the number of alumina inclusions.
FIG. 5 is a graph showing the relationship between the amount of slag before secondary refining and the concentration of alumina in the slag after secondary refining.
FIG. 6 is a graph obtained by investigating the relationship between the Al concentration of the steel receiving steel type and the number of alumina inclusions.
FIG. 7 is a graph showing the relationship between the basicity of slag after secondary refining and the number of alumina inclusions.
FIG. 8 is a graph showing the relationship between the basicity of slag after secondary refining and the number of inclusions containing CaO.

Claims (3)

C: 0.7 to 1.0%
Si: 0.1 to 0.4%,
Mn: 0.2 to 0.6%,
Ni: 0.01 to 1.0%,
Al: 0.003% or less (including 0%)
With the balance being Fe and inevitable impurities,
Among inclusions extracted by acid dissolution , zirconia inclusions having a composition of the following (a) and having a major axis of 20 μm or more , detected by an electron probe microanalyzer (EPMA ), or a composition of the following ( b) and a zirconia inclusion that is a zircon inclusion having a major axis of 20 μm or more, and an alumina inclusion that has a composition of the following (c) and a major axis of 20 μm or more, and the zirconia inclusion and the total number (X) (the steel 50g per) is a high strength steel wire rod excellent diameter 3~10mm the cold drawing property, characterized in that satisfies the following formula (1) of the alumina-based inclusions.
(A) ZrO ₂ : 70% by mass or more,
(B) ZrO ₂ : 50% by mass or more and SiO ₂ : 20% by mass or more,
(C) Al ₂ O ₃ : 70% by mass or more.
X ≦ 0.21 × [C] ^−1.34 (1)
{Wherein [ C] represents the amount of C in steel (% by mass)} As other elements,
The high-strength steel wire rod excellent in cold drawability according to claim 1, containing Cu: 0.01 to 1.0% and / or Cr: 0.01 to 1.5%. A high-strength steel wire rod excellent in cold drawability drawn using the high-strength steel wire rod according to claim 1 .

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