JP3718586B2

JP3718586B2 - High cleanliness rolled steel

Info

Publication number: JP3718586B2
Application number: JP00267998A
Authority: JP
Inventors: 武司黒田; 信彦茨木
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1998-01-08
Filing date: 1998-01-08
Publication date: 2005-11-24
Anticipated expiration: 2018-01-08
Also published as: JPH11199982A

Description

【０００１】
【発明の属する技術分野】
本発明は、冷間加工性や疲労特性に優れた高清浄度圧延鋼材に関し、特に自動車用エンジンや懸架装置に用いられる高強度ばね、あるいはスチールコードやワイヤソー等として用いられる極細鋼線用として優れた性能を発揮する清浄度の高い圧延鋼材に関するものである。
【０００２】
【従来の技術】
近年自動車などの輸送機械においては、排ガス低減や燃費低減を期して車体の軽量化や高出力化の要請が強く、それに伴ってエンジン、サスペンション等に用いられる弁ばねや懸架ばねなどについても高応力設計が指向されている。こうした傾向の下で、ばね用鋼についても負荷応力の増加に対応するため一段と高疲労特性が要求される様になっている。またスチールコードやワイヤソー等の極細鋼線においても、軽量化や歩留向上の要請から細径化が指向され、それに伴って優れた伸線性の要求が高まっている。
【０００３】
ところが、これらばねや細径鋼線用の素材として用いられる従来の鋼材では、大型介在物を起点とする折損がしばしば発生し、優れた伸線性や疲労特性が得られないことがあり、こうした介在物に起因する伸線性や疲労特性の劣化を改善するため、鋼材の高清浄度化が求められている。
こうした状況の下で、高清浄度鋼やその製法について多くの研究が進められている。
【０００４】
例えば高清浄度鋼の製法に関し特公平６−１０２２４９号には、Ａｌ含有量が０．００２％以下のＳｉ脱酸鋼を用いて溶鋼処理および連続鋳造を行なう際に、溶鋼と接触する耐火物のＡｌ₂ Ｏ₃ 含有量を１０％以下に低減することによってアルミナ系介在物の混入量を可及的に低減し、高清浄化することにより疲労特性を高める技術を開示している。また特開昭６２−１０７０４４号には、組成がＡｌ₂ Ｏ₃ ：２０％以下、ＭｎＯ：１０〜８０％、ＳｉＯ₂ ：２０〜６０％で、且つＭｇＯ：１５％以下またはＣａＯ：５０％以下からなる非金属系介在物の総量をＪＩＳ規格で２０％以下に抑えることにより、疲労特性が改善される旨記載され、更に特公平６−７４４８５号には、圧延鋼材のＬ断面におけるサイズ1/d ≦５の非金属介在物の組成をＳｉＯ₂ ：３５〜７５％、Ａｌ₂ Ｏ₃ ：３０％以下、ＣａＯ：５０％以下、ＭｇＯ：２５％以下に制御することにより、疲労特性が高められる旨記述されている。
【０００５】
更に特開平６−１５８２２６号や同５−３２０８２７号には、疲労特性を高めるには上記酸化物系介在物の制御のみでは不十分であり、チタンを主成分とする炭化物や窒化物系介在物（以下、Ｔｉ系介在物という）の大きさを規定することで疲労特性が一層高められることを明らかにしている。
【０００６】
【発明が解決しようとする課題】
ところが、上記従来技術の殆んどは酸化物系介在物に注目しているだけで、それ以外の有害不純物の影響については明らかにされておらず、疲労特性について一層の性能向上を求める最近の需要者の要望には応じ切れなくなっている。
【０００７】
そこで最近では、酸化物系介在物以外の有害な介在物としてＴｉ系介在物（炭化物や窒化物など）についても検討されているが、それらにしても酸化物系介在物やＴｉ系介在物の最大サイズを規定するに止まっており、それら介在物の粒径分布状況まで追求して疲労特性や加工性等への影響を調べた公知技術は見られない。
【０００８】
即ちこれまでの介在物調査では、酸化物系介在物については、その介在物組成を制御して融点を低下させることによって熱間圧延による圧下に伴う介在物の微細化を進める方向の研究が殆んどであり、具体的には酸化物系介在物の融点が可及的に低くなる様な組成制御を行なうと共に、その粒径を１５μｍ程度以下に抑え、熱間圧延工程で該酸化物系介在物が微細化し得る様な組成制御を行ない、また熱間圧延工程で微細化し難いＴｉ系介在物については、その粒径を５μｍ以下に抑えることによって、介在物に起因する疲労破壊の改善を図っているのである。
【０００９】
ところが本発明者らが更なる疲労特性の改善を期して研究を進めるうち、それら従来の介在物制御では、近年における高強度化や高負荷応力化の要請に十分に対応することができず、それら不純介在物が伸線性や疲労特性に及ぼす影響をより厳密且つ定量的に把握しなければ、一段と高性能化を志向する需要者の要望には到底応えきれないと考えた。
【００１０】
本発明はこの様な状況の下で鋭意研究の結果完成されたものであって、その目的は、最近のばね用鋼や極細鋼線に要望される高レベルの伸線加工性や疲労特性にも安定して確実に応え得る様な高清浄度圧延鋼材を提供しようとするものである。
【００１１】
【課題を解決するための手段】
上記課題を解決することのできた本発明に係る高清浄度圧延鋼材とは、圧延鋼材の軸心を含む縦断面に観察される非金属系介在物の個数とサイズが、下記の要件を満たすものであるところに特徴を有している。
(1) 酸化物系介在物のうち、幅方向サイズが５μｍ以上のものが、２０個／１０００ｍｍ² 以下、
(2) 上記(1) の要件を満たす酸化物系介在物には、１５μｍ以上のものが実質的に存在しない、
(3) 上記(1) の要件を満たす酸化物系介在物のうち、１０μｍ以上１５μｍ未満である介在物の存在率が３０％以下であり、且つそれらの硬さがＨｖ：６５０未満、
(4) 上記(1) の要件を満たす酸化物系介在物のうち、５μｍ以上１０μｍ未満である介在物の存在率が７０％以上であり、且つそのうち８０％以上がＨｖ：６５０未満の硬さを有する介在物である、
(5) 酸化物以外のＴｉ系介在物の幅方向サイズが５μｍ以下である。
【００１２】
上記要件を満たす高清浄度圧延鋼材のうち、特にばね用として好ましいのは、下記化学成分、
Ｃ：０．４５〜０．７５質量％、
Ｓｉ：１．０〜２．５質量％、
Ｍｎ：０．２〜１．５質量％、
Ｃｒ：０．５〜１．５質量％、
を含み、あるいはこれらに加えて
Ｎｉ：０．５質量％以下（０質量％を含まない）、
Ｍｏ：０．４質量％以下（０質量％を含まない）、
Ｖ：０．２５質量％以下（０質量％を含まない）、
Ｎｂ：０．２５質量％以下（０質量％を含まない）、
よりなる群から選択される少なくとも１種の元素を含む鋼材であり、また極細線用として特に好ましいのは、
Ｃ：０．６０〜１．０５質量％、
Ｓｉ：０．１５〜１．２質量％、
Ｍｎ：０．１５〜１．２質量％、
を含む鋼材である。
【００１３】
【発明の実施の形態】
前述した如く介在物については、圧延工程で介在物が十分に微細化される様、その組成制御により融点を低下させる方法が主体であり、介在物サイズについては、圧延工程での該微細化の程度を考慮してその最大サイズを規定することによって十分に目的が達成できると考えられていた。
【００１４】
ところが本発明者らが検討したところでは、介在物組成や介在物サイズの上限を厳密に規定したとしても必ずしも確実な改質効果が得られるとは限らず、たとえ前述した様な従来技術の規定要件を満たすものであっても、最近一段と高度化している要求性能を満足できないことがあり、品質安定性に問題がある。
【００１５】
本発明では、こうした品質の不安定を解消し、伸線性や疲労特性についての高度の要求を安定して確実に達成し得る様な技術を確立するものであり、上記の様に非金属系介在物のサイズと個数および硬さが伸線性や疲労特性に及ぼす影響を定量的に把握し、高度の要求性能を確実に満たすための要件として、前述の如く非金属系介在物のサイズと個数および硬さを厳密に規定している。以下、それらの限定理由を明らかにする。
【００１６】
まず本発明では、圧延鋼材の軸心を含む縦断面に観察される非金属系介在物の幅方向サイズによって清浄度を評価する。しかして線材の折損や疲労破壊に影響を及ぼすのは、伸線方向に対して幅方向の介在物サイズであり、線材の長手方向サイズは殆んど影響を及ぼさないからである。
【００１７】
そして本発明では、非金属系介在物のうち酸化物系介在物について、まず
(1) 酸化物系介在物のうち、幅方向サイズが５μｍ以上のものが
２０個／１０００ｍｍ² 以下
であることが必須となる。即ちばね用鋼や極細鋼線用の圧延鋼材として優れた疲労特性や伸線性を確保するには、５μｍ以上の粗大な酸化物系介在物の個数を２０個／１０００ｍｍ² 以下に抑えることが前提となる。ここで幅方向サイズの下限を５μｍと定めたのは、５μｍ未満の酸化物系介在物の個数と折損の間には何らの相関性も認められず、該微細な酸化物系介在物は伸線加工時の折損や疲労破壊に殆んど悪影響を及ぼさない、という確認結果に基づいている。しかし、５μｍ以上の酸化物系介在物になると折損に明らかな影響が表われ、その個数が２０個／１０００ｍｍ² を超えると、以下に詳述する他の要件を如何にコントロールしても、本発明で意図する優れた特性を確実に得ることができなくなる。
【００１８】
本発明では、酸化物系介在物が上記(1) の要件を満足するという条件の下で、更に前記(2) 〜(4) の要件を満たすことが必須となる。(2) の要件として「上記酸化物系介在物には１５μｍ以上のものが実質的に存在しない」ことを規定したのは、１５μｍを超える粗大な酸化物系介在物が例え僅かでも存在するときは、該粗大介在物が起点となって折損を起こし、満足のいく伸線性や疲労特性が得られなくなるからである。
【００１９】
また(3) の要件として「上記酸化物系介在物のうち、１０μｍ以上１５μｍ未満である介在物の存在率が３０％以下であり、且つそれらの硬さがＨｖ：６５０未満」と定めたのは、１０μｍ以上１５μｍ未満の範囲の酸化物系介在物は単独では折損の原因になることはないが、その数が多くなるにつれて折損の発生に顕著な影響を及ぼす様になり、前記(1) の要件を満たす酸化物系介在物のうち３０％以上が１０μｍ以上１５μｍ未満の比較的粗大なものであるときは、折損に少なからず悪影響を及ぼす様になるからである。又折損の起点になるか否かはサイズのみならずその硬さも大きな影響を及ぼし、該サイズ範囲の酸化物系介在物に起因する折損を無くすには、ビッカース硬さでＨｖ：６５０以上のものを含まないことが必須となる。即ち、たとえ１０μｍ以上１５μｍ未満の幅方向サイズの介在物の存在率が３０％以下であったとしても、その中の一部がＨｖ：６５０を超える硬質の介在物であるときは、該硬質の酸化物系介在物を起点とする折損の可能性が生じてくるからである。
【００２０】
次に(4) の要件として「上記酸化物系介在物のうち、５μｍ以上１０μｍ未満である介在物の存在率が７０％以上であり、且つそのうち８０％以上がＨｖ：６５０未満の硬さを有する介在物」と定めたのは、該サイズ範囲の酸化物系介在物は折損に与える悪影響があまり認められず、その個数が全体の７０％以上である限り、伸線性や疲労特性に実質的な障害とはならない。ただしこのサイズ範囲の酸化物系介在物のうち硬質のものが多くなり過ぎると、たとえ相対的に微細なものと雖も折損原因になる恐れがでてくるので、該サイズ範囲の酸化物系介在物のうち少なくとも８０％以上は、ビッカース硬さでＨｖ：６５０未満の軟質なものでなければならない。
【００２１】
なお、酸化物系介在物の硬さの測定法は特に制限されないが、本発明では微細な介在物の個々の硬さを正確に測定できる様、ＡＫＡＳＨＩ製作所社製の微小硬さ測定計を用い、ＪＩＳＺ２２４４に準じるため圧子は２〜１０ｇのものを使用した。
【００２２】
なお上記(3),(4) では酸化物系介在物の硬さを規定したが、該硬さをＨｖ：６５０未満に抑えるには酸化物系介在物の組成制御が有効であり、好ましくは溶製時のスラグコントロールによって酸化物系介在物の組成がＳｉＯ₂ ：３５〜７５質量％、Ａｌ₂ Ｏ₃ ：３５質量％以下、ＣａＯ：５０質量％以下、ＭｇＯ：３０質量％以下となる様に制御することが望ましい。しかして、酸化物系介在物中のＳｉＯ₂ 量が３５質量％未満になると、介在物がＡｌ₂ Ｏ₃ 、ＣａＯまたはＭｇＯ主体の硬質介在物になり易く、逆に７５質量％を超えると、ＳｉＯ₂ 量が過度にリッチとなってやはり硬質介在物になるからである。またＡｌ₂ Ｏ₃ が３５質量％を超えると、コランダムやスピネル等の硬質介在物が生成し易くなり、またＣａＯが５０質量％を超え、あるいはＭｇＯが３０質量％を超えても、やはり酸化物系介在物が硬質化し易くなる。尚ＭｎＯについては特に規定していないが、ＭｎＯは酸化物系介在物を軟質化させると考えられており、多少のＭｎＯが含まれていても介在物起因の折損原因になることはない。
【００２３】
次に前記(5) の要件として「酸化物以外のＴｉ系介在物の幅方向サイズが５μｍ以下」と定めたのは、次の様な理由による。すなわち該Ｔｉ系介在物は、その形状が長方形に近く且つ角が非常にシャープで応力集中を起こし易い形状特性を有しており、相対的に微細なものであっても伸線性や疲労特性に顕著な悪影響を及ぼす。しかも該Ｔｉ系介在物は硬質で圧延による微細化も期待し難いので、スラグコントロールの段階で十分に微細化しておくことが必要であり、こうしたＴｉ系介在物の悪影響を確実に抑えるには、そのサイズを５μｍ以下にすることが必須となる。
【００２４】
この様に本発明では、非金属系介在物として含まれる酸化物系介在物のサイズと個数およびビッカース硬さを規定した点に特徴を有しているが、酸化物系介在物のサイズや個数は、圧延鋼材を溶製する際の清浄化処理条件、例えばランスからの吹込みガスや底吹きガスの種類や流量などにより溶鋼−スラグ反応を適正に制御すれば良く、またＴｉ系介在物については、鋼中のＴｉ含有量を１５ｐｐｍ以下に抑えると共に、Ｔｉ系介在物の主たる形成成分となるＮの含有量を５０ｐｐｍ以下に抑えれば、Ｔｉ系介在物のサイズと個数を十分に小さく且つ少なく抑えることができる。
【００２５】
また、被検面となる圧延鋼材の軸心を含む縦断面の面積は、試料毎の測定値のばらつきを少なくするため広い方が望ましく、少なくとも３０００ｍｍ² 以上の面積を測定し、望ましくは試料の長手方向から５〜１０箇所以上の縦断面をアトランダムに採取し、各縦断面からの測定値の平均値として求めれば、当該製造ロット全体の清浄度をほぼ正確に評価することができる。
【００２６】
又縦断面における被検面の位置も特に制限されないが、本発明者らの経験によると、ばね鋼の如く疲労特性が重要視される鋼材では、圧延線材の表面から深さ１／４・Ｄ（Ｄは線材の直径）までの位置を被検面とすることにより、介在物による疲労特性の良否をより正確に評価できることを確認している。これは、ばね鋼の場合、負荷応力の勾配が表面で最大となり軸心部では０となり、軸心部に存在する介在物等に由来する折損は殆んど問題にならないからである。
【００２７】
一方、極細鋼線の如く高度の冷間加工性が求められる鋼材の場合は、伸線負荷が鋼材全体に加わるので、被検面を縦断面の全域から万遍なく選択することが望ましい。
【００２８】
本発明では、上記の様に圧延鋼材の軸心を含む縦断面に観察される酸化物系介在物とＴｉ系介在物のサイズと個数、硬さを規定したところに特徴を有しており、こうした要件を規定することによってそれらの介在物を起点とする折損を確実に阻止し、より高レベルの伸線加工性や疲労特性が確実に発揮される様にしたところに特徴を有しているが、こうした特徴をばね用あるいは極細線材用としてより効果的に発揮させるには、それぞれの用途に応じた化学成分の鋼材を使用することが望ましいので、以下、ばね用と極細鋼線用に分けて夫々の好ましい化学成分について説明する。
【００２９】
まずばね用に用いられる鋼材としては、
Ｃ：０．４５〜０．７５質量％（以下、質量％を示す場合は単に％と記す）
Ｓｉ：１．０〜２．５％、
Ｍｎ：０．２〜１．５％、
Ｃｒ：０．５〜１．５％、
を含み、あるいは更に他の元素として
Ｎｉ：０．５％以下（０％を含まない）、
Ｍｏ：０．４％以下（０％を含まない）、
Ｖ：０．２５％以下（０％を含まない）、
Ｎｂ：０．２５％以下（０％を含まない）、
よりなる群から選ばれる少なくとも１種の元素を含む鋼材が好ましく、上記各元素の作用は下記の通りである。
【００３０】
Ｃ：０．４５〜０．７５％
Ｃは焼入れ焼戻し後の強度を確保するために必要な元素であり、Ｃ含有量が０．４５％未満では強度不足となり、一方０．７５％を超えると靭性が低下してばね成形時に折損を生じ易くなる。ばね用鋼としてのＣのより好ましい下限値は０．５０％、より好ましい上限値は０．７０％である。
【００３１】
Ｓｉ：１．０〜２．５％
Ｓｉは脱酸と介在物組成の制御に有効に作用する他、フェライト中に固溶して鋼素地の強度を高めるのに有効な元素であり、それらの効果を有効に発揮させるには１．０％以上含有させなければならない。しかし、多過ぎると靭性や疲労特性に悪影響を及ぼす様になるので、２．５％以下に抑えなければならない。ばね用鋼としてのより好ましいＳｉ量の下限値は１．２％、上限値は２．１％である。
【００３２】
Ｍｎ：０．２〜１．５％
Ｍｎは焼入れ性向上元素として有効に作用し、その効果は０．２％以上含有させることによって有効に発揮される。しかし、多過ぎると靭性や疲労特性に悪影響を及ぼす様になるので、１．５％以下に抑えなければならない。ばね用鋼としてのより好ましいＭｎ量の下限値は０．４％、上限値は１．２％である。
【００３３】
Ｃｒ：０．５〜１．５％
Ｃｒも鋼素材の強化に有効に作用し、またＣ活量を低下させて圧延や熱処理工程で脱炭を抑えると共に炭化物の黒鉛化を抑制して疲労強度を高める作用を有しており、こうした効果は０．５％以上含有させることによって有効に発揮される。しかしながら、多過ぎると靭性や延性に悪影響を及ぼす傾向が生じてくるので１．５％以下に抑えなければならない。Ｃｒ量のより好ましい下限値は０．６％、より好ましい上限値は１．２％である。
【００３４】
ばね用として使用する場合の必須元素は上記の４元素であるが、更に高強度のばねを得たい場合は、Ｎｉ，Ｍｏ，Ｖ，Ｎｂから選択される少なくとも１種の元素を下記含有量となる様に添加することが望ましい。
【００３５】
Ｎｉ：０．５％以下
Ｎｉは焼入れ性を高めて靭延性を高めると共にばね成形性を高める作用を有しており、それらの効果は、好ましくは０．１％以上含有させることによって有効に発揮される。しかし、それらの効果は０．５％で飽和するのでそれ以上の添加は経済的に無駄である。
【００３６】
Ｍｏ：０．４％以下
Ｍｏは焼入れ性を高めてばね用鋼の高強度化に寄与する元素であり、その効果は、好ましくは０．１％以上含有させることによって有効に発揮されるが、多過ぎると靭延性を極端に悪化させるので、０．４％以下に抑えなければならない。
【００３７】
Ｖ：０．２５％以下，Ｎｂ：０．２５％以下
これらの元素は焼入れ焼戻し等の熱処理時に結晶粒を微細化し、靭延性の向上に寄与し、それらの効果は夫々好ましくは０．０５％以上含有させることによって有効に発揮される。しかし、多過ぎると焼入れ加熱時に溶解しない粗大な炭化物が生成し、靭延性を却って低下させ疲労特性にも悪影響を及ぼす様になるので、夫々０．２５％以下に抑えなければならない。
【００３８】
次に、極細鋼線に用いられる鋼材としては、
Ｃ：０．６０〜１．０５％、
Ｓｉ：０．１５〜１．２％、
Ｍｎ：０．１５〜１．２％、
を含む鋼材が好ましく、これら各元素の作用は下記の通りである。
【００３９】
Ｃ：０．６０〜１．０５％
Ｃは強化元素として作用し、極細鋼線として十分な強度を確保するには０．６０％以上含有させなければならない。しかしＣは偏析し易い元素であり、多過ぎると中心偏析を起こして伸線加工時の断線を起こす原因になるので、１．０５％以下に抑えなければならない。極細鋼線用としてのＣのより好ましい下限値は０．６５％、より好ましい上限値は１．００％である。
【００４０】
Ｓｉ：０．１５〜１．２％
Ｓｉは脱酸と介在物組成の制御のため０．１５％以上含有させることが必要であるが、反面ＳｉはＡ₃ 変態点を高め、パテンティング処理中にオーステナイト組織の生成を阻害し、最終湿式伸線加工時に断線を起こす原因になることがあるので１．２％以下に抑えなければならない。極細鋼線用としてのより好ましいＳｉ量の下限値は０．２０％、上限値は１．０％である。
【００４１】
Ｍｎ：０．１５〜１．２％
Ｍｎは介在物組成の制御と焼入れ性の向上に有効に作用し、その効果は０．１５％以上含有させることによって有効に発揮される。しかし、多過ぎると偏析を起こして偏析部にマルテンサイトが生成し、伸線加工時に断線を起こし易くなるので１．２％以下に抑えなければならない。極細鋼線用としてのより好ましいＭｎ量の下限値は０．２０％、上限値は１．０％である。
【００４２】
本発明に係る高清浄度圧延鋼材をばね用、あるいは極細鋼線用として実用化する際の好ましい含有元素は以上の通りであり、残部は実質的にＦｅであるが、その他の元素については、上記各用途に応じた鋼材特性に悪影響を及ぼさない範囲で不可避不純物量の含有が許容される。
【００４３】
いずれにしても本発明によれば、前述の如く鋼材の圧延方向縦断面に現われる酸化物系介在物とＴｉ系介在物のサイズと個数、更には硬さを規定することによって、ばね用として卓越した疲労特性を有し、あるいは極細鋼線用として非常に優れた伸線加工性を有する鋼材を安定して提供し得ることになった。
【００４４】
【実施例】
次に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勿論可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００４５】
実施例
表１に示す符号Ａ１〜Ａ７，Ｂ１〜Ｂ１１の鋼材を溶製した。この溶製段階でＳｉ，Ｃｒ，Ｍｎなどの合金を添加して成分調整を行ない、また合金中に含まれるＴｉ量を調整することによって各溶鋼中のＴｉ系介在物量をコントロールした。また酸化物系介在物については、取鍋処理段階で溶鋼表面のＳｉＯ₂ −Ａｌ₂ Ｏ₃ −ＣａＯ−ＭｇＯ系スラグの組成割合をコントロールすると共に、ランスあるいは底吹きノズルから吹き込む不活性ガス（Ａｒなど）の流量を調整して溶鋼−スラグ反応を制御することによって、その組成とサイズ、個数を調整した。
【００４６】
得られた各溶鋼を用いて鋳造、鍛造、圧延を行ない、直径５．５ｍｍの線材を作製し、各圧延線材の軸心を含む縦断面を研磨して各断面に現われる介在物のサイズと個数を調べた。結果を表２に示す。
【００４７】
【表１】

【００４８】
【表２】

【００４９】
次いで、符号Ａ１〜Ａ４及びＢ１〜Ｂ６については、直径５．５ｍｍまで伸線加工した後オイルテンパー処理を行ない、引張強度がＡ１〜Ａ３，Ｂ１〜Ｂ４は２０５ｋｇｆ／ｍｍ² 、Ａ４，Ｂ５，Ｂ６は２１５ｋｇｆ／ｍｍ² のオイルテンパー鋼線を作製した。
【００５０】
その後ショットピーニング処理および２００℃焼鈍処理を行なった後、中村式回転曲げ疲労試験機を用いて疲労試験を行なった。このとき、試験応力はＡ１〜Ａ３，Ｂ１〜Ｂ４は８５ｋｇｆ／ｍｍ² 、Ａ４，Ｂ５，Ｂ６は８６．５ｋｇｆ／ｍｍ² とした。
【００５１】
結果は下記表３に示す通りであり、本発明の規定要件を満たすＡ１〜Ａ４の高清浄度鋼は、全サンプルについて回転曲げ回数３×１０⁷ の試験で折損が認められず優れた疲労特性を有しており、ばね用として卓越した性能を発揮し得るものであるのに対し、本発明で定める何れかの要件を欠くＢ１〜Ｂ６では、酸化物系介在物あるいはＴｉ系介在物を起点とする折損が高い確率で発生しており、ばね用としての性能に劣るものであることが分かる。
【００５２】
【表３】

【００５３】
また、符号Ａ５〜Ａ７およびＢ７〜Ｂ１１については、前記直径５．５ｍｍの線材を、伸線途中で鉛パテンティング処理を行ないながら直径１．５ｍｍの伸線材とした後、更に鉛パテンティング処理を行なって直径０．０５ｍｍの極細線材にまで湿式伸線加工を行ない、夫々について断線回数を調べた。
【００５４】
結果は下記表４に示す通りであり、本発明の規定要件を満たすＡ５〜Ａ７は、本発明で定めるいずれかの要件を欠くＢ７〜１１に比べて断線回数が格段に少ない。またＢ７〜Ｂ１１では、酸化物系介在物またはＴｉ系介在物を起点とする疲労破壊によって断線を起こしているが、Ａ５〜Ａ７で生じた断線の疲労起点に酸化物系介在物やＴｉ系介在物は確認されなかった。即ちＡ５〜Ａ７で生じた断線は、不純介在物以外の要因（具体的な要因は明確でない）によるものと思われるが、Ｂ７〜Ｂ１１では不純介在物を起点とする断線が優先的に起こり、それ以外の要因による断線は実質的に生じなかったものと思われる。いずれにしても本発明によれば、介在物を起点とする断線回数を大幅に減少でき、極細鋼線を得る際の伸線加工性を著しく高め得ることが分かる。
【００５５】
【表４】

【００５６】
【発明の効果】
本発明は以上の様に構成されており、圧延鋼材の軸心を含む縦断面に観察される特定サイズの酸化物系介在物の個数と硬さを規定すると共に、Ｔｉ系介在物のサイズを制限することによって、疲労特性や伸線加工性の優れた高清浄度圧延鋼材を提供し得ることになった。またこうした清浄度の要件に加えて、鋼材の化学成分を適正に制御すれば、ばね用として卓越した疲労特性を有し、或は極細鋼線用として卓越した伸線加工性を有する鋼材を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high cleanliness rolled steel material excellent in cold workability and fatigue characteristics, and particularly excellent for high-strength springs used in automobile engines and suspension systems, or for ultrafine steel wires used as steel cords and wire saws. The present invention relates to a rolled steel material with high cleanliness that exhibits excellent performance.
[0002]
[Prior art]
In recent years, in transport equipment such as automobiles, there has been a strong demand for lighter bodies and higher output for reducing exhaust gas and fuel consumption. Along with this, high stress is also applied to valve springs and suspension springs used for engines, suspensions, etc. Design is oriented. Under these trends, spring steel is also required to have higher fatigue characteristics in order to cope with an increase in load stress. Further, in ultra-thin steel wires such as steel cords and wire saws, diameter reduction is directed from the demand for weight reduction and yield improvement, and accordingly, the demand for excellent wire drawing is increasing.
[0003]
However, conventional steel materials used as materials for springs and small-diameter steel wires often cause breakage starting from large inclusions, and may not provide excellent wire drawing and fatigue characteristics. In order to improve the deterioration of the wire drawing property and fatigue characteristics caused by the object, it is required to increase the cleanliness of the steel material.
Under such circumstances, a lot of research is being conducted on high cleanliness steel and its manufacturing method.
[0004]
For example, Japanese Patent Publication No. 6-102249 relating to a manufacturing method of high cleanliness steel has a refractory which comes into contact with molten steel when performing molten steel treatment and continuous casting using Si deoxidized steel having an Al content of 0.002% or less. A technique is disclosed in which the amount of inclusions of alumina inclusions is reduced as much as possible by reducing the Al ₂ O ₃ content to 10% or less, and the fatigue characteristics are improved by high cleaning. Japanese Patent Application Laid-Open No. 62-107044 discloses that the composition is Al ₂ O ₃ : 20% or less, MnO: 10 to 80%, SiO ₂ : 20 to 60%, and MgO: 15% or less or CaO: 50% or less. It is described that the fatigue characteristics are improved by suppressing the total amount of non-metallic inclusions comprising 20% or less in accordance with JIS standards, and Japanese Patent Publication No. 6-74485 describes the size 1 / By controlling the composition of non-metallic inclusions with d ≦ 5 to SiO ₂ : 35 to 75%, Al ₂ O ₃ : 30% or less, CaO: 50% or less, MgO: 25% or less, fatigue characteristics can be improved. Is described.
[0005]
Furthermore, in JP-A-6-158226 and JP-A-5-320827, it is not sufficient to control the oxide inclusions alone to enhance the fatigue characteristics, and carbides and nitride inclusions mainly composed of titanium. It is clarified that the fatigue characteristics can be further improved by defining the size of Ti inclusions (hereinafter referred to as Ti-based inclusions).
[0006]
[Problems to be solved by the invention]
However, most of the above prior arts only pay attention to oxide inclusions, and the influence of other harmful impurities has not been clarified. It can no longer meet the demands of consumers.
[0007]
Recently, Ti-based inclusions (such as carbides and nitrides) have been studied as harmful inclusions other than oxide-based inclusions. Only the maximum size is defined, and there is no known technique that investigates the influence on the fatigue characteristics and workability by pursuing the particle size distribution of these inclusions.
[0008]
In other words, in the investigation of inclusions so far, with regard to oxide inclusions, there has been almost no research in the direction of increasing the refinement of inclusions due to reduction by hot rolling by controlling the inclusion composition and lowering the melting point. Specifically, the composition control is performed so that the melting point of the oxide inclusions is as low as possible, and the particle size is suppressed to about 15 μm or less, and the oxide type inclusions are subjected to a hot rolling process. The composition is controlled so that inclusions can be refined, and for Ti-based inclusions that are difficult to refine in the hot rolling process, the grain size is suppressed to 5 μm or less to improve fatigue fracture caused by inclusions. It is intended.
[0009]
However, as the inventors proceeded with research aimed at further improvement of fatigue characteristics, those conventional inclusion controls could not sufficiently meet the demand for higher strength and higher load stress in recent years, Unless the effects of these impure inclusions on wire drawability and fatigue properties were grasped more strictly and quantitatively, we thought that it would be impossible to meet the demands of consumers who are aiming for higher performance.
[0010]
The present invention was completed as a result of diligent research under such circumstances, and its purpose is to achieve a high level of wire drawing workability and fatigue characteristics required for recent spring steel and ultrafine steel wire. However, the present invention intends to provide a high cleanliness rolled steel material that can respond reliably and reliably.
[0011]
[Means for Solving the Problems]
The high cleanliness rolled steel according to the present invention that has solved the above-mentioned problems means that the number and size of non-metallic inclusions observed in the longitudinal section including the axis of the rolled steel satisfy the following requirements: It has the characteristic in that.
(1) Among oxide inclusions, those having a width direction size of 5 μm or more are 20 pieces / 1000 mm ² or less,
(2) In the oxide inclusions satisfying the requirement (1) above, there are substantially no inclusions of 15 μm or more.
(3) Of the oxide inclusions satisfying the above requirement (1), the presence of inclusions that are 10 μm or more and less than 15 μm is 30% or less, and their hardness is Hv: less than 650,
(4) Among oxide inclusions satisfying the above requirement (1), the presence of inclusions of 5 μm or more and less than 10 μm is 70% or more, and 80% or more of them has a hardness of Hv: less than 650 Is an inclusion having
(5) The width direction size of the Ti inclusions other than the oxide is 5 μm or less.
[0012]
Among the high cleanliness rolled steel materials that satisfy the above requirements, the following chemical components are particularly preferred for springs:
C: 0.45-0.75 mass%,
Si: 1.0 to 2.5% by mass,
Mn: 0.2 to 1.5% by mass,
Cr: 0.5 to 1.5% by mass,
Or in addition to these, Ni: 0.5% by mass or less (excluding 0% by mass),
Mo: 0.4 mass% or less (excluding 0 mass%),
V: 0.25 mass% or less (excluding 0 mass%),
Nb: 0.25 mass% or less (excluding 0 mass%),
It is a steel material containing at least one element selected from the group consisting of, and particularly preferable for ultrafine wires,
C: 0.60 to 1.05% by mass,
Si: 0.15 to 1.2% by mass,
Mn: 0.15 to 1.2% by mass,
It is a steel material containing.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the inclusion mainly includes a method of lowering the melting point by controlling the composition so that the inclusion is sufficiently refined in the rolling process, and the inclusion size is reduced in the rolling process. It was thought that the purpose could be sufficiently achieved by defining the maximum size in consideration of the degree.
[0014]
However, the present inventors have examined that, even if the upper limit of inclusion composition and inclusion size is strictly defined, a reliable reforming effect is not always obtained. Even if it satisfies the requirements, it may not be able to satisfy the recently demanded performance, and there is a problem in quality stability.
[0015]
In the present invention, such a quality instability is solved, and a technology capable of stably and surely achieving high demands on wire drawing and fatigue properties is established. As described above, the size and number of non-metallic inclusions and the number of non-metallic inclusions and Hardness is strictly specified. The reasons for these limitations will be clarified below.
[0016]
First, in the present invention, the cleanliness is evaluated by the width direction size of the nonmetallic inclusions observed in the longitudinal section including the axis of the rolled steel material. Thus, the breakage and fatigue fracture of the wire are affected by the inclusion size in the width direction with respect to the drawing direction, and the longitudinal size of the wire has little influence.
[0017]
And in this invention, about an oxide type inclusion among nonmetallic type inclusions,
(1) Among oxide inclusions, it is essential that the number in the width direction is 5 μm or more is 20 pieces / 1000 mm ² or less. That is, in order to ensure excellent fatigue characteristics and wire drawability as rolled steel for spring steel and ultrafine steel wire, the number of coarse oxide inclusions of 5 μm or more is assumed to be 20/1000 mm ² or less. It becomes. Here, the lower limit of the size in the width direction was set to 5 μm, and no correlation was observed between the number of oxide inclusions less than 5 μm and breakage, and the fine oxide inclusions were not stretched. This is based on the confirmation result that there is almost no adverse effect on breakage and fatigue fracture during wire processing. However, when the oxide inclusions are 5 μm or more, there is a clear influence on breakage. When the number exceeds 20/1000 mm ² , this requirement can be controlled no matter how other requirements described in detail below are controlled. The excellent characteristics intended by the invention cannot be obtained with certainty.
[0018]
In the present invention, it is essential that the oxide inclusions further satisfy the requirements (2) to (4) under the condition that the requirement (1) is satisfied. The requirement of (2) stipulates that “the inclusion of oxide inclusions of 15 μm or more is not substantially present” when there are even a few coarse oxide inclusions exceeding 15 μm. This is because the coarse inclusions cause breakage, and satisfactory drawability and fatigue characteristics cannot be obtained.
[0019]
In addition, the requirement (3) is that “the inclusion ratio of inclusions of 10 μm or more and less than 15 μm among the oxide inclusions is 30% or less and their hardness is Hv: less than 650”. The oxide inclusions in the range of 10 μm or more and less than 15 μm do not cause breakage alone, but as the number increases, the occurrence of breakage has a significant effect. This is because when 30% or more of oxide inclusions satisfying the above requirement are relatively coarse particles having a size of 10 μm or more and less than 15 μm, the breakage is adversely affected. In addition, whether or not it becomes the starting point of breakage has a great influence not only on the size but also its hardness. To eliminate breakage caused by oxide inclusions in the size range, the Vickers hardness is Hv: 650 or more. It is essential not to include. That is, even if the abundance of inclusions in the width direction of 10 μm or more and less than 15 μm is 30% or less, if some of them are hard inclusions exceeding Hv: 650, This is because there is a possibility of breakage starting from oxide inclusions.
[0020]
Next, as a requirement of (4), “of the oxide inclusions, the presence of inclusions of 5 μm or more and less than 10 μm is 70% or more, and 80% or more of them has a hardness of Hv: 650 or less. The inclusions that have been defined as “inclusions having the size range” have substantially no adverse effects on breakage in the oxide inclusions in the size range, and as long as the number of inclusions is 70% or more of the total, the drawability and fatigue characteristics are substantially reduced. It will not be a serious obstacle. However, if too much of the oxide inclusions in this size range are too hard, relatively fine ones and soot may cause breakage. At least 80% or more of the product must be soft with a Vickers hardness of less than Hv: 650.
[0021]
The method for measuring the hardness of oxide inclusions is not particularly limited, but in the present invention, a microhardness meter manufactured by AKASHI Corporation is used so that the individual hardness of fine inclusions can be accurately measured. In order to comply with JIS Z 2244, an indenter of 2 to 10 g was used.
[0022]
In the above (3) and (4), the hardness of the oxide inclusions is specified, but the composition control of the oxide inclusions is effective in order to keep the hardness below Hv: 650, preferably By controlling the slag during melting, the composition of oxide inclusions is SiO ₂ : 35 to 75% by mass, Al ₂ O ₃ : 35% by mass or less, CaO: 50% by mass or less, and MgO: 30% by mass or less. It is desirable to control. Thus, when the amount of SiO ₂ in the oxide inclusions is less than 35% by mass, the inclusions tend to be hard inclusions mainly composed of Al ₂ O ₃ , CaO or MgO. This is because the SiO ₂ amount becomes excessively rich and becomes hard inclusions. Further, if Al ₂ O ₃ exceeds 35% by mass, hard inclusions such as corundum and spinel are likely to be formed, and even if CaO exceeds 50% by mass or MgO exceeds 30% by mass, it is still an oxide. System inclusions are easily hardened. MnO is not particularly defined, but MnO is considered to soften oxide inclusions, and even if some MnO is contained, it does not cause breakage due to inclusions.
[0023]
Next, the reason (5) is that “the size in the width direction of the Ti-based inclusions other than oxide is 5 μm or less” is as follows. In other words, the Ti-based inclusion has a shape characteristic that is close to a rectangle and has very sharp corners and is likely to cause stress concentration. Has a significant adverse effect. Moreover, since the Ti-based inclusions are hard and it is difficult to expect miniaturization by rolling, it is necessary to sufficiently refine them at the stage of slag control. In order to reliably suppress the adverse effects of such Ti-based inclusions, It is essential to make the size 5 μm or less.
[0024]
As described above, the present invention is characterized in that the size and number of oxide inclusions included as nonmetallic inclusions and the Vickers hardness are defined, but the size and number of oxide inclusions. Can be used to properly control the molten steel-slag reaction according to the cleaning treatment conditions when melting the rolled steel material, for example, the type and flow rate of the blowing gas and bottom blowing gas from the lance. If the Ti content in the steel is suppressed to 15 ppm or less and the content of N, which is the main forming component of the Ti-based inclusions, is suppressed to 50 ppm or less, the size and number of Ti-based inclusions are sufficiently small and It can be kept low.
[0025]
In addition, the area of the longitudinal section including the axis of the rolled steel material to be the test surface is preferably wide in order to reduce the variation in the measured value for each sample, and the area of at least 3000 mm ² is preferably measured. If at least 5 to 10 vertical sections from the longitudinal direction are collected at random and obtained as an average value of the measured values from each longitudinal section, the cleanliness of the entire production lot can be evaluated almost accurately.
[0026]
Further, the position of the test surface in the longitudinal section is not particularly limited. However, according to the experience of the present inventors, in steel materials in which fatigue characteristics are regarded as important, such as spring steel, the depth is 1/4 · D from the surface of the rolled wire. It has been confirmed that the fatigue quality due to inclusions can be more accurately evaluated by setting the position up to (D is the diameter of the wire) as the test surface. This is because in the case of spring steel, the gradient of the load stress is maximized on the surface and becomes 0 in the shaft center portion, and breakage due to inclusions and the like existing in the shaft center portion hardly poses a problem.
[0027]
On the other hand, in the case of a steel material that requires a high degree of cold workability, such as an ultrafine steel wire, since the wire drawing load is applied to the entire steel material, it is desirable to select the test surface uniformly from the entire longitudinal section.
[0028]
In the present invention, as described above, the size and number of oxide inclusions and Ti inclusions observed in the longitudinal cross-section including the axis of the rolled steel material, characterized by having defined the hardness, By defining these requirements, it is possible to reliably prevent breakage originating from these inclusions, and to ensure that a higher level of wire drawing workability and fatigue properties are exhibited. However, in order to exhibit these characteristics more effectively for springs or ultrafine wire materials, it is desirable to use steel materials with chemical components suitable for each application. Each preferred chemical component will be described.
[0029]
First, as steel materials used for springs,
C: 0.45-0.75 mass% (Hereinafter, when showing mass%, it only describes as%)
Si: 1.0-2.5%,
Mn: 0.2 to 1.5%
Cr: 0.5 to 1.5%
Or as another element, Ni: 0.5% or less (not including 0%),
Mo: 0.4% or less (excluding 0%),
V: 0.25% or less (excluding 0%),
Nb: 0.25% or less (excluding 0%),
A steel material containing at least one element selected from the group consisting of the above elements is preferred, and the action of each element is as follows.
[0030]
C: 0.45-0.75%
C is an element necessary for ensuring the strength after quenching and tempering. If the C content is less than 0.45%, the strength is insufficient. On the other hand, if it exceeds 0.75%, the toughness is reduced and breakage occurs during spring forming. It tends to occur. A more preferable lower limit value of C as spring steel is 0.50%, and a more preferable upper limit value is 0.70%.
[0031]
Si: 1.0-2.5%
Si is an effective element for controlling deoxidation and inclusion composition, and is an element effective for increasing the strength of the steel substrate by solid solution in ferrite. It must be contained at 0% or more. However, if it is too much, it will adversely affect toughness and fatigue properties, so it must be kept to 2.5% or less. A more preferable lower limit value of Si amount for spring steel is 1.2%, and an upper limit value is 2.1%.
[0032]
Mn: 0.2 to 1.5%
Mn effectively acts as a hardenability improving element, and the effect is effectively exhibited by containing 0.2% or more. However, if it is too much, it will adversely affect toughness and fatigue characteristics, so it must be suppressed to 1.5% or less. The lower limit value of the more preferable amount of Mn as spring steel is 0.4%, and the upper limit value is 1.2%.
[0033]
Cr: 0.5 to 1.5%
Cr also effectively works to strengthen the steel material, and also has the effect of reducing the C activity to suppress decarburization in rolling and heat treatment processes and to suppress the graphitization of carbides to increase the fatigue strength. The effect is effectively exhibited by containing 0.5% or more. However, if too much, there is a tendency to adversely affect toughness and ductility, so it must be suppressed to 1.5% or less. A more preferable lower limit value of the Cr content is 0.6%, and a more preferable upper limit value is 1.2%.
[0034]
The essential elements when used for springs are the above four elements. However, in order to obtain a spring with higher strength, at least one element selected from Ni, Mo, V, and Nb is contained in the following content. It is desirable to add as follows.
[0035]
Ni: 0.5% or less Ni has the effect of enhancing hardenability and enhancing toughness and spring formability, and these effects are preferably exerted by containing preferably 0.1% or more. Is done. However, since their effects are saturated at 0.5%, further addition is economically wasteful.
[0036]
Mo: 0.4% or less Mo is an element that enhances hardenability and contributes to increasing the strength of spring steel, and the effect is preferably exhibited by containing 0.1% or more, If it is too much, the toughness will be extremely deteriorated, so it must be suppressed to 0.4% or less.
[0037]
V: 0.25% or less, Nb: 0.25% or less These elements refine crystal grains during heat treatment such as quenching and tempering and contribute to improvement of toughness, and their effects are preferably 0.05%, respectively. It is exhibited effectively by containing above. However, if the amount is too large, coarse carbides that do not dissolve during quenching and heating are generated, and the toughness is deteriorated and the fatigue properties are adversely affected. Therefore, the amount must be suppressed to 0.25% or less.
[0038]
Next, as steel materials used for extra fine steel wires,
C: 0.60 to 1.05%,
Si: 0.15 to 1.2%,
Mn: 0.15 to 1.2%,
A steel material containing is preferable, and the action of each of these elements is as follows.
[0039]
C: 0.60 to 1.05%
C acts as a strengthening element and must be contained in an amount of 0.60% or more to ensure a sufficient strength as an ultrafine steel wire. However, C is an element that easily segregates, and if it is too much, it causes center segregation and causes disconnection during wire drawing, so it must be suppressed to 1.05% or less. The more preferable lower limit value of C for extra fine steel wire is 0.65%, and the more preferable upper limit value is 1.00%.
[0040]
Si: 0.15-1.2%
Si needs to be contained in an amount of 0.15% or more in order to control the deoxidation and inclusion composition. On the other hand, Si increases the A ₃ transformation point and inhibits the formation of austenite structure during the patenting process. Since it may cause disconnection during wet wire drawing, it must be suppressed to 1.2% or less. The more preferable lower limit value of the Si content for the ultrafine steel wire is 0.20%, and the upper limit value is 1.0%.
[0041]
Mn: 0.15 to 1.2%
Mn effectively acts to control the inclusion composition and improve the hardenability, and the effect is effectively exhibited by containing 0.15% or more. However, if the amount is too large, segregation occurs and martensite is generated in the segregated portion, and breakage is likely to occur during wire drawing, so it must be suppressed to 1.2% or less. The more preferable lower limit of the amount of Mn for the ultra fine steel wire is 0.20%, and the upper limit is 1.0%.
[0042]
The preferred contained elements when the high cleanliness rolled steel material according to the present invention is put into practical use for springs or ultrafine steel wires are as described above, and the balance is substantially Fe, but for other elements, Inclusion of inevitable impurities is allowed within a range that does not adversely affect the properties of the steel material according to each application.
[0043]
In any case, according to the present invention, as described above, by defining the size and number of oxide inclusions and Ti inclusions appearing in the longitudinal section of the steel material in the rolling direction and the hardness, it is excellent for springs. Therefore, it has been possible to stably provide a steel material having excellent fatigue characteristics, or having excellent wire drawing workability for ultra fine steel wires.
[0044]
【Example】
EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. Of course, any of these is also included in the technical scope of the present invention.
[0045]
The steel materials of codes A1 to A7 and B1 to B11 shown in Example Table 1 were melted. In this melting stage, an alloy such as Si, Cr, Mn was added to adjust the components, and the amount of Ti inclusions in each molten steel was controlled by adjusting the amount of Ti contained in the alloy. For oxide inclusions, the composition ratio of SiO ₂ —Al ₂ O ₃ —CaO—MgO slag on the surface of the molten steel is controlled at the ladle treatment stage, and an inert gas (Ar) blown from a lance or bottom blowing nozzle is used. The composition, size, and number were adjusted by controlling the molten steel-slag reaction.
[0046]
Casting, forging, and rolling are performed using each of the obtained molten steels to produce a wire rod having a diameter of 5.5 mm, and the size and number of inclusions appearing in each cross-section by polishing the vertical cross-section including the axis of each rolled wire rod I investigated. The results are shown in Table 2.
[0047]
[Table 1]

[0048]
[Table 2]

[0049]
Next, for the codes A1 to A4 and B1 to B6, after drawing to a diameter of 5.5 mm, oil tempering is performed, and the tensile strengths of A1 to A3 and B1 to B4 are 205 kgf / mm ² and A4, B5 and B6. Produced an oil tempered steel wire of 215 kgf / mm ² .
[0050]
Thereafter, after shot peening treatment and 200 ° C. annealing treatment, a fatigue test was conducted using a Nakamura type rotary bending fatigue tester. In this case, test stress A1 to A3, B1 to B4 is ^{85kgf / mm 2, A4, B5} , B6 was 86.5kgf / mm ^2.
[0051]
The results are as shown in Table 3 below. The high cleanliness steels of A1 to A4 that satisfy the specified requirements of the present invention have excellent fatigue properties with no breakage observed in the test of 3 × 10 ⁷ rotation bending times for all samples. In the case of B1 to B6 that lack any of the requirements defined in the present invention, the oxide inclusions or Ti inclusions are the starting points. It can be seen that the breakage occurs at a high probability and is inferior in performance for a spring.
[0052]
[Table 3]

[0053]
Moreover, about the code | symbol A5-A7 and B7-B11, after making the said wire material of diameter 5.5mm into the wire drawing material of diameter 1.5mm, performing lead patenting processing in the middle of wire drawing, it is further lead patenting processing. Then, wet drawing was performed up to an ultrafine wire having a diameter of 0.05 mm, and the number of wire breaks was examined for each.
[0054]
A result is as showing in following Table 4, A5-A7 which satisfy | fills the prescription | regulation requirement of this invention has remarkably few frequency | counts of a disconnection compared with B7-11 which lacks any requirements defined by this invention. In B7 to B11, breakage is caused by fatigue failure starting from oxide inclusions or Ti inclusions, but oxide inclusions or Ti inclusions are present at the fatigue start points of breakage caused by A5 to A7. The thing was not confirmed. That is, the disconnection that occurred in A5 to A7 seems to be due to factors other than impure inclusions (specific factors are not clear), but in B7 to B11, disconnection starting from the impure inclusion occurs preferentially, It seems that disconnection due to other factors did not occur substantially. In any case, according to the present invention, it can be seen that the number of wire breaks starting from inclusions can be greatly reduced, and the wire drawing workability when obtaining an ultrafine steel wire can be remarkably improved.
[0055]
[Table 4]

[0056]
【The invention's effect】
The present invention is configured as described above, and defines the number and hardness of oxide inclusions of a specific size observed in the longitudinal section including the axis of the rolled steel material, and the size of the Ti inclusions. By limiting, it was possible to provide a high cleanliness rolled steel material having excellent fatigue characteristics and wire drawing workability. In addition to these cleanliness requirements, by properly controlling the chemical composition of the steel material, it is possible to obtain a steel material that has excellent fatigue characteristics for springs or excellent wire drawing workability for ultrafine steel wires. be able to.

Claims

C: 0.45-0.75 mass%,
Si: 1.0 to 2.5% by mass,
Mn: 0.2 to 1.5% by mass,
Cr: 0.5 to 1.5% by mass,
Wherein the balance being a rolled steel consisting essentially of Fe, characterized in that the number and size of non-metallic inclusions observed in a longitudinal section through the axis of the rolled steel material, meet the following requirements High cleanliness rolled steel.
(1) Among oxide inclusions, those having a width direction size of 5 μm or more are 20 pieces / 1000 mm ² or less,
(2) Oxide inclusions that satisfy the above requirement (1) are substantially free of those having a thickness of 15 μm or more.
(3) Of the oxide inclusions that satisfy the above requirement (1), the abundance of inclusions that are 10 μm or more and less than 15 μm is 30% or less, and their hardness is Hv: less than 650,
(4) Among oxide inclusions satisfying the above requirement (1), the abundance of inclusions of 5 μm or more and less than 10 μm is 70% or more, and 80% or more of them has a hardness of Hv: less than 650 Is an inclusion having
(5) The width direction size of the Ti inclusions other than the oxide is 5 μm or less.

C: 0.45-0.75 mass%,
  Si: 1.0 to 2.5% by mass,
  Mn: 0.2 to 1.5% by mass,
  Cr: 0.5 to 1.5% by mass,
And containing
  Ni: 0.5% by mass or less (excluding 0% by mass),
  Mo: 0.4 mass% or less (excluding 0 mass%),
    V: 0.25 mass% or less (excluding 0 mass%),
  Nb: 0.25 mass% or less (excluding 0 mass%),
The number of non-metallic inclusions observed in a longitudinal section including a rolled steel material containing at least one element selected from the group consisting of Fe and the balance substantially consisting of Fe. And high cleanliness rolled steel characterized by the following requirements:
(1) Among oxide inclusions, 20/1000 mm having a width direction size of 5 μm or more. ²² Less than,
(2) the above (1) The oxide inclusions satisfying the requirement of
(3) the above (1) Among the oxide inclusions satisfying the requirements of the above, the abundance of inclusions that are 10 μm or more and less than 15 μm is 30% or less, and their hardness is less than Hv: 650,
(Four) the above (1) Among the oxide-based inclusions that satisfy the above requirements, the presence rate of inclusions that are 5 μm or more and less than 10 μm is 70% or more, and 80% or more of them are inclusions having a hardness of Hv: 650,
(Five) The width direction size of Ti inclusions other than oxides is 5 μm or less.

The high cleanliness rolled steel material according to claim 1 or 2 , which is used for a spring.

C: 0.60 to 1.05% by mass,
Si: 0.15 to 1.2% by mass,
Mn: 0.15 to 1.2% by mass,
The balance is a rolled steel material substantially consisting of Fe, and the number and size of non-metallic inclusions observed in the longitudinal section including the axis of the rolled steel material satisfy the following requirements: High cleanliness rolled steel.
(1) Among oxide inclusions, 20/1000 mm having a width direction size of 5 μm or more. ²² Less than,
(2) the above (1) The oxide inclusions satisfying the requirement of
(3) the above (1) Among the oxide inclusions satisfying the requirements of the above, the abundance of inclusions that are 10 μm or more and less than 15 μm is 30% or less, and their hardness is less than Hv: 650,
(Four) the above (1) Among the oxide-based inclusions that satisfy the above requirements, the presence rate of inclusions that are 5 μm or more and less than 10 μm is 70% or more, and 80% or more of them are inclusions having a hardness of Hv: 650,
(Five) The width direction size of Ti inclusions other than oxides is 5 μm or less.

The high cleanliness rolled steel according to claim 4 , which is used for extra fine wires.