JP3757027B2

JP3757027B2 - High strength hot rolled steel with excellent weldability, high strength steel wire and high strength steel bar using the same

Info

Publication number: JP3757027B2
Application number: JP14820797A
Authority: JP
Inventors: 憲二落合; 信彦茨木; 高明南田; 寛百▲崎▼
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1996-06-05
Filing date: 1997-06-05
Publication date: 2006-03-22
Anticipated expiration: 2017-06-05
Also published as: JPH1053814A

Description

【０００１】
【発明の属する技術分野】
本発明は、溶接性および靭延性に優れた高強度熱延鋼材および該熱延鋼材を伸線加工してなる高強度鋼線、並びに該熱延鋼材からなる高強度棒鋼に関するものであり、この熱延鋼材は、コンクリート補強用等として使用される鉄筋やＨ鋼、Ｌ鋼の如く異形断面形状のものを含む各種の棒鋼等として使用され、またこれを伸線加工することによって得られる鋼線は、溶接金網、自動車用シートの枠線、インサートワイヤ、各種ばね材等の素材として有用である。
【０００２】
【従来の技術】
熱間圧延後そのまま使用されるコンクリート補強用の鉄筋は、ＪＩＳＧ３１１２に規定されている。また、溶接金網、自動車用シート枠線、インサートワイヤ用鋼線の様に、優れた溶接性と曲げ加工性が要求される鋼線は、たとえば低炭素鋼の熱延鋼材に伸線加工を施すことによって製造されている。また高強度鋼線を製造する際には、パテンティング処理を施した高炭素鋼材に伸線加工を施す方法が一般的に採用されている。前者の溶接性と曲げ加工性を必要とする鋼線の製造に用いられる代表的なものは、ＪＩＳＧ３５０５に規定される軟鋼線材であり、また後者の高強度鋼線としてはＪＩＳＧ３５０６に規定される高炭素鋼線材が代表的に使用されている。
【０００３】
【発明が解決しようとする課題】
熱延のままで使用される鉄筋には、たとえば上記ＪＩＳＧ３１１２に規定される鉄筋コンクリート棒鋼等があり、化学成分や機械的特性が定められている。このうち最も高強度を有しているのはＳＤ４９０であり、その０．２％耐力は４９０〜６２５Ｎ／ｍｍ² であるが、’９５年に発生した阪神大震災以降、建造物を構成する鉄筋や基礎等の高強度化が望まれている。こうした要望に応えるため素材の炭素量を多くすることが考えられるが、そうすると、溶接時に過冷却組織が生成し易くなって溶接強度が劣悪になるといった問題が生じてくる。
【０００４】
また、ＰＣ鋼材をコンクリートパイルに縦筋として埋め込み、パイルの剪断破壊耐力を増大させるプレストレスコンクリート杭があり、最近の研究によると、震災時のプレストレスコンクリートパイル中に降伏強度が８０ｋｇｆ／ｍｍ² 程度の高強度の横拘束筋を配置してパイルに組成変形能を持たせ、地震により地盤に大きな曲げモーメントが生じた場合でも、これに順応して曲がり破壊しない高曲げ靭性のプレストコンクリートパイルが注目を集めている。
【０００５】
ところが、ＰＣ鋼材としてはＣ量が０．６％以上の高炭素鋼材が用いられており、これを溶接で固定すると溶接後の冷却時に過冷却組織が生じて材料の脆化が起こるため、ＰＣ鋼材を直接溶接することはできない。すなわち鋼材の強度と溶接性には相反する傾向があり、Ｃ量が０．２％を超えると溶接性が急激に悪くなると考えられている。
【０００６】
他方、熱延後に伸線加工を行うことによって製造される溶接金網などの場合、低炭素鋼を使用すると優れた溶接性は得られるが、低炭素鋼であるため高強度が得られ難い。即ち低炭素鋼では、素材そのものの強度が低いばかりでなく、伸線加工による加工硬化もあまり期待できないので、高強度化の目的を果たすことができない。これに対し高炭素鋼では、伸線加工前の素線自体の強度が高くしかも伸線時の加工硬化率も高いため、高強度の鋼線が得られ易い。反面高炭素鋼は、上記の様に炭素含有量が多いため、これを溶接用途に適用した場合には溶接後の急冷時に過冷却組織が生じ易く、脆化による靭延性の低下が大きな問題になってくる。
【０００７】
即ち従来の鋼材では、高強度を得ようとすると溶接性や溶接後の靭延性が悪くなり、一方優れた溶接性と曲げ加工性を確保しようとすると強度不足になり、優れた溶接性、曲げ加工性、靭延性および高強度を同時に満足させることはできない。
【０００８】
本発明は上記の様な事情に着目してなされたものであって、その目的は、高強度で加工性に優れると共に溶接性も良好で高い溶接強度を示し、コンクリート補強用鉄筋等として熱延のままで優れた性能を示し、或は熱延の後に伸線加工を施して溶接金網、自動車用シート枠線、インサートワイヤ、その他の各種機械用ばね等として有用な鋼線材を与える様な熱延鋼材を提供し、併せて該熱延鋼材を用いた溶接性に優れた高強度棒鋼および、該熱延鋼材を伸線加工してなる溶接性に優れた高強度鋼線を提供しようとするものである。
【０００９】
【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かかる高強度熱延鋼材は、
Ｃ：０．０５〜０．２２％
Ｓｉ：０．５〜２．０％
Ｍｎ：０．５〜２．０％
の要件を満たし、あるいは他の成分として、
Ｃｒ：１．５％以下（０％を含まない）
Ｍｏ：１．０％以下（０％を含まない）
Ｔｉ：０．５％以下（０％を含まない）
Ｖ：０．５％以下（０％を含まない）
Ｂ：０．０１％以下（０％を含まない）
Ｎｂ：０．１％以下（０％を含まない）
よりなる群から選択される少なくとも１種の元素を含み、好ましくはＡｌ，Ｐ，Ｓ，Ｎの各含有率が
Ａｌ：０．０８％以下（０％を含む）
Ｐ：０．０４％以下（０％を含む）
Ｓ：０．０４％以下（０％を含む）
Ｎ：１００ｐｐｍ以下（０ｐｐｍを含む）
である鋼を熱間圧延後調整冷却してなり、横断面組織中に占める（ベイナイト＋パーライト）面積率が１０〜９５％、残部がフェライト、マルテンサイトおよび残留オーステナイトの１種以上であり、好ましくは０．２％耐力が８００Ｎ／ｍｍ² 以上の強度を示すものであるところにその特徴を有している。
【００１０】
上記高強度熱延鋼材のうち、横断面組織中に占める（ベイナイト＋パーライト）面積率が４０〜９５％であるものは、特に高強度で伸線加工性がやや不足気味であるので、これは実質的に伸線加工することなくコンクリート補強用鉄筋などの棒鋼として有効に活用することができる。
【００１１】
また、（ベイナイト＋パーライト）面積率が４０〜９５％で伸線加工性の不足する熱延鋼材は、調整冷却の後４５０〜６５０℃で熱処理し加工性を高めてから伸線加工を施すことにより、また（ベイナイト＋パーライト）面積率が１０〜６０％で伸線加工性の良好な熱延鋼材はそのまま伸線加工を施し、伸線加工による加工硬化によって高強度化を図ることにより、溶接金網、自動車用シートの枠線、インサートワイヤ、各種ばね材等の素材となる鋼線として有効に活用することができる。
【００１２】
従って本発明では、上記熱延鋼材の特性に鑑み、（ベイナイト＋パーライト）面積率が４０〜９５％であるものは、その優れた強度特性を生かし実質的に伸線加工されていない状態の高強度棒鋼、あるいは該（ベイナイト＋パーライト）面積率が４０〜９５％である熱延鋼材を一旦軟化焼鈍し加工性を高めてから伸線加工した鋼線、若しくは（ベイナイト＋パーライト）面積率が１０〜６０％である加工性に優れた熱延鋼材については、これをそのまま伸線加工し加工硬化させることによって強度を高めた高強度鋼線についても権利範囲に包含している。
【００１３】
尚、上記伸線加工時に金属組織は若干引き伸ばされるが、上記（ベイナイト＋パーライト）面積率は本質的に変わらず、また、場合により調整冷却後に熱処理を行なうことがあっても、上記４５０〜６５０℃の熱処理温度条件ではその前後で（ベイナイト＋パーライト）面積率が本質的に変わることはなく、伸線加工によって得られる鋼線材の横断面組織は、伸線加工前の金属組織、即ち（ベイナイト＋パーライト）面積率が１０〜９５％で、残部がフェライト、マルテンサイトおよび残留オーステナイトの１種以上である組織がほぼそのまま維持されたものとなる。
【００１４】
また、（ベイナイト＋パーライト）面積率が４０〜９５％であり、実質的に伸線加工を行なうことなく棒鋼等として用いられる熱延鋼材の場合は、その表面に付着しているスケールによって溶接性が著しく損なわれるので、該熱延鋼材や棒鋼をコンクリート補強用鉄筋などとして実用化する際には、その表面に付着しているスケール量を０．４％以下に抑えることが望ましい。
【００１５】
【発明の実施の形態】
まず、本発明において鋼の化学成分を定めた理由について詳述する。
Ｃ：０．０５〜０．２２％
Ｃは強化元素および焼入れ性向上元素として作用する元素で、熱延後の調整冷却のままで棒鋼としたとき、あるいはその後必要により熱処理を施してから伸線加工を施して線材とした状態で、高強度を得るのに欠くことのできない元素であり、０．０５％未満では強化元素としての量が不足すると共に、焼入れ性も不十分となって満足のいく強度が得られなくなる。一方、Ｃ量が０．２２％を超えると、棒鋼あるいは鋼線としての靭延性が低下して加工時に断線等を起こし易くなるばかりでなく、溶接性も著しく劣化し、溶接後の急冷時に過冷却組織が生じて脆化による靭延性の低下が大きな問題となってくる。強度、加工性、溶接性を全て満足させるうえではＣ量を０．１０％以上、０．１７％以下にすることがより好ましい。
【００１６】
Ｓｉ：０．５〜２．０％
Ｓｉは溶製時の脱酸性元素として作用し、また溶接強度を高める上で重要な元素である。即ち、本発明は溶接性の改善に１つの重要な目的を有するものであり、溶接工程では鋼材が部分的に溶融するため、溶接金属中に大気中の酸素が取り込まれて気泡や酸化物の等介在物となり、溶接金属の物性を悪化させる。従って、こうした溶接時に混入する酸素に起因する問題を回避するには、鋼材中に脱酸元素としてＳｉを含有させる必要がある。またＳｉは、伸線加工を施して鋼線やばねとした時の耐へたり特性やレラクセーション特性を向上させる作用を有しており、更には固溶強化元素として作用しフェライトを強化すると共に焼入れ性を高めて高強度化を増進する作用も発揮する。こうしたＳｉの作用を有効に発揮させるには、Ｓｉを０．５％以上含有させることが必要であるが、Ｓｉ量が多過ぎると、延性が低下して加工性や伸線性等に悪影響が表われるばかりでなく、曲げ性能等にも悪影響が表われてくるので、２．０％以下に抑えるべきである。Ｓｉのより好ましい含有率の下限は０．７％、より好ましい上限は１．７％である。
【００１７】
Ｍｎ：０．５〜２．０％
Ｍｎは、上記Ｓｉと同様溶製時の脱酸に有効に作用する他、鋼の焼入れ性を高めて高強度化を図るうえで欠くことのできない元素である。殊に本発明では、後で詳述する如く金属組織中の（ベイナイト＋パーライト）分率を特定範囲に収めることによって高強度化を達成するものであり、そのためにはＭｎが必須の元素となる。Ｍｎの好適含有量は、Ｃその他の強化元素の含有率によっても変わってくるが、十分な焼入れ性を確保するには少なくとも０．５％以上含有させなければならない。しかし、Ｍｎによる強度向上効果は２．０％でほぼ飽和し、過度にＭｎ量を多くしてもそれ以上の改善効果は得られず、むしろＭｎが正偏析を起こして粒界強度の劣化を招くので、２．０％を上限として定めた。Ｍｎのより好ましい含有率の下限は０．７％、より好ましい上限は１．７％である。
【００１８】
本発明で用いられる鋼の必須構成元素は上記の３元素であり、残部は実質的にＦｅからなるものであるが、該鋼中には、焼入れ性向上による高強度化等を狙って下記の元素を含有させることも有効である。
【００１９】
Ｃｒ：１．５％以下、Ｍｏ：１．０％以下、Ｔｉ：０．５％以下、
Ｖ：０．５％以下、Ｂ：０．０１％以下、Ｎｂ：０．１％以下
これらの元素は、いずれも焼入れ性を高めて高強度化に寄与する成分である点で同効元素である。これらの元素のうち特にＣｒ、Ｍｏは、焼入れ性を高めて高強度化に寄与する。また本発明の鋼材は、前述の如く使用時に溶接熱の影響を受けるが、ＣｒやＭｏは、前述したＳｉと共に熱による軟化を抑える作用を有しており、溶接熱による強度低下を抑えて溶接強度を一層高める意味からも有効な元素である。またＶ、Ｔｉ、Ｎｂは、焼入れ性の向上に寄与する他、オーステナイト結晶粒の粗大化を防止して靭性を高める効果も発揮する。またＢは、粒界に集散することにより鋼の焼入れ性を向上させると共に、粒界へのＭｎやＰの偏析・析出を抑制する効果がある。そうした効果は、これらの元素の１種以上を含有させることによって発揮されるが、各元素の添加効果をより確実に発揮させる意味から好ましい下限値は、Ｃｒ：０．０５％程度以上、Ｍｏ：０．０５％程度以上、Ｔｉ：０．０１％程度以上、Ｖ：０．０５％程度以上、Ｂ：０．０００５％程度以上、Ｎｂ：０．０１％程度以上である。しかし、上限値を超えて過度に含有させてもそれ以上の改質効果は発揮されないので経済的に無駄である。
【００２０】
また、鋼中に不可避不純物として混入することの多いＡｌ，Ｐ，Ｓ，Ｎについては、以下に示す様な理由から、夫々上限値を下記の様に制限することが好ましい。
【００２１】
Ａｌ：０．０８％以下（０％を含む）
Ａｌは、精錬時に脱酸剤として作用し鋼中の酸素と結合しＡｌ₂ Ｏ₃ となって酸素を捕捉する他、窒素と結合してＡｌＮを生成して金属組織を微細化する作用を有しているが、多過ぎると却って結晶粒を粗大化し、また鋼中の非金属系介在物量の増大により靭性を劣化させるので、０．０８％以下に抑えるのが良い。なお、該Ａｌによってもたらされる上記作用効果を有効に発揮させる上では、０．００５％程度以上のＡｌを含有させることが望ましい。
【００２２】
Ｐ，Ｓ：夫々０．０４％以下（０％を含む）
ＰおよびＳは、鋼材の靭延性に悪影響を及ぼすばかりでなく、偏析を起こして焼入れ性にばらつきを生じさせる有害な元素であり、これらの欠点を生じさせないためには、ＰおよびＳをいずれも０．０４％以下、より好ましくは０．０２％以下に抑えるべきである。
【００２３】
Ｎ：１００ｐｐｍ以下（０ｐｐｍを含む）
Ｎは、熱延後の伸線加工を行なう際に時効によって鋼線の延性を悪化させる要因となり、その障害は１００ｐｐｍを超えると顕著に現われてくる。尚Ｎ量は少ない方が好ましく、勿論全く含まなくても構わないが、通常は不可避的に混入してくる元素であり、２０〜１００ｐｐｍの含有は許容される。
【００２４】
本発明の高強度熱延鋼材は、上記成分組成の要件を満たすという条件の下で、その断面組織が極めて重要であり、横断面組織中に占める（ベイナイト＋パーライト）分率（以下、Ｂ・Ｐ分率ということがある）が１０〜９５％で、残部がフェライト、マルテンサイトおよび残留オーステナイトの１種以上からなるものでなければならない。しかして断面組織中に占めるＢ・Ｐ分率は、熱延鋼材の強度と靭延性や加工性等に顕著な影響を及ぼし、上記の様にＣ量などを低めに抑えた成分系で本発明で意図する様な高強度を確保すると共に適度の加工性を保証するには、Ｂ・Ｐ分率を１０〜９５％の範囲とすることが極めて重要となる。
【００２５】
即ちＢ・Ｐ分率が１０％未満では、金属組織面からの強度向上効果が有効に発揮されなくなり、本発明で意図するレベルの高強度が得られなくなる。しかしＢ・Ｐ分率が９５％を超えて過度に高くなり過ぎると、曲げ加工性が著しく悪くなる他、組織中のマルテンサイト分率が高くなり、伸線加工におけるダイス寿命の低下や曲げ加工時に切断し易くなるといった問題が生じてくるので、Ｂ・Ｐ分率の上限は９５％と定めた。また本発明の熱延鋼材は、その目的の一つである高強度特性を有効に発揮させる意味から、０．２％耐力が８００Ｎ／ｍｍ² 以上であることが望ましく、こうした強度特性と優れた溶接性を兼備せしめることによって、従来材に対し卓越した優位性を誇ることができる。
【００２６】
尚上記のＢ・Ｐ分率は、前述の如く鋼材の強度と加工性に大きな影響を及ぼし、Ｂ・Ｐ分率の低いものは相対的に強度が低めで優れた加工性を有しており、一方Ｂ・Ｐ分率の高いものは相対的に高強度で加工性が不足気味となる。従って、本発明で規定する範囲のＢ・Ｐ分率を有するものであっても、該Ｂ・Ｐ分率が低めである熱延鋼材、例えばＢ・Ｐ分率が１０〜６０％程度である熱延鋼材は、そのままで曲げ加工性等が重視される用途に適用し、あるいは更に曲げ加工や伸線加工を施すことにより加工硬化を生じせしめ、所要の線径と強度にしてから使用される。
【００２７】
またＢ・Ｐ分率が高めである熱延鋼材、例えばＢ・Ｐ分率が４０〜９５％程度である熱延鋼材は、そのままで高強度が求められる鉄筋等の棒鋼などとして実用化され、あるいは、必要によっては一旦軟化焼鈍に付すことにより加工性を高めてから曲げ加工や伸線加工して実用化することも可能である。
【００２８】
なお、本発明の熱延鋼材を伸線加工してから鋼線として活用する場合、熱延工程で鋼材表面に生じたスケールは伸線加工工程で殆んど除去されるため、該表面スケールによって溶接性が阻害されることは殆んどないが、実質的に伸線加工を行なわず調整冷却の後実質的にそのままで棒鋼等として使用する場合、熱延工程で表面に生じたスケールは溶接性に顕著な悪影響を及ぼしてくる。従って、上記の様にＢ・Ｐ分率が４０〜９５％で熱延後実質的に伸線加工しないで棒鋼などとして用いられる熱延鋼材の場合は、こうしたスケールによる溶接性の劣化を抑えるため、スケール付着量を０．４％以下に抑えることが重要となる。
【００２９】
熱延ままの鋼材あるいは棒鋼としてのスケール付着量を少なく抑える為の具体的手段としては、たとえば熱延終了後の冷却開始温度を９２５℃程度以下の低めに抑え、酸化を抑える方法等を採用すればよい。
【００３０】
熱延鋼材に伸線加工を施して使用する場合、熱延鋼材の金属組織は伸線加工によって形状は若干変わるが、Ｂ・Ｐ分率自体は殆んど変わらず、また伸線加工に先立って加工性向上の為の熱処理を行なった場合でも、本発明で採用される４５０〜６５０℃の範囲の熱処理温度域では、僅かに組織の球状化傾向が認められるだけであってＢ・Ｐ分率自体は殆んど変わらない。従って、伸線加工によって得られる鋼線材の金属組織も、伸線加工前の組織であるＢ・Ｐ分率（面積率）で１０〜９５％の範囲内のものとなる。
【００３１】
尚上記において、Ｂ・Ｐ分率が同じであっても、（ベイナイト＋パーライト）組織の中のベイナイトとパーライトの面積比率［ベイナイト／パーライト×１００（％）：以下、Ｂ／Ｐ比率という］によっても強度や加工性は変わり、ベイナイトが多くなると強度は高くなる一方で加工性は低下傾向を示し、ベイナイトが少なくなると強度は低くなる一方で加工性は向上してくるが、本発明で定める上記Ｂ・Ｐ分率の範囲で高強度と優れた加工性を安定して確保するには、Ｂ／Ｐ比率で６０％程度以上、より好ましくは８０％程度以上が好ましい。
【００３２】
本発明で規定する上記Ｂ・Ｐ分率範囲を満たし、あるいは更に上記の好ましいＢ／Ｐ比率を有する熱延鋼材は、前記成分組成の要件を満足する鋼を鋳造した後、７５０〜１２５０℃、より好ましくは８００〜１１５０℃の範囲で熱間圧延を施し、その後の冷却過程で、２相域温度以上からの冷却を０．１〜３０℃／ｓｅｃ、より好ましくは０．５〜１５℃／ｓｅｃの速度で調整冷却することによって得ることができる。
【００３３】
かくして得られる本発明の熱延鋼材は、上記の様に適度の伸びを有すると共に優れた溶接性と高強度を有しており、そのままでコンクリート補強用鉄筋等の棒鋼として有効に実用化することができ、或はＢ・Ｐ分率が低めで加工性の優れたものについては、その後伸線加工等を施し、また伸線工程で断線を起こす恐れがある高Ｂ・Ｐ分率の熱延鋼材の場合は４５０〜６５０℃の温度で焼鈍を行い、上記Ｂ・Ｐ分率を実質的に変えることなく塑性変形能を高めてから伸線加工等を行ない、適度の断面寸法・形状に整えて実用化される。
【００３４】
【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。
【００３５】
実施例
図１は、用いる鋼中のＣ含有量が溶接強度に及ぼす影響を確認するために行なった実験結果を示したものであり、Ｓｉ：０．７５〜０．９０％、Ｍｎ：１．４０〜１．５５％のベース組成を有し、Ｃ量のみを０．０７〜０．４２の範囲で変えた熱延鋼線材（５．５ｍｍφ）を使用し、オーステナイト化した後水焼入れした鋼線について、絞りおよび引張強さを調べた結果を示したものである。水焼入れしてマルテンサイト組織とした理由は、溶接時の熱影響で溶接部の金属組織がマルテンサイト組織となり、この組織の強度や延性が溶接強度に大きな影響を及ぼすからである。即ち、溶接部が硬い鋼組織であっても延性がなければ溶接強度は低下し、又たとえ延性に優れたものであっても柔らかい鋼組織であれば十分な溶接強度が得られなくなるからである。
【００３６】
この図からも明らかである様に、鋼中のＣ量が０．２５％までは高レベルの引張強さを保っているが、Ｃ量が０．２２％を超えると延性の指標となる絞り値が急激に低下しており、溶接後の急冷で延性が確保できなくなることを確認できる。即ちこの結果より、本発明で意図するレベルの優れた溶接強度を確保するには、鋼中のＣ量を０．２２％以下に抑えなければならないことが分かる。
【００３７】
次に、表１に示す如く化学組成を種々変化させた鋼材を溶製・鋳造し、８００〜１１５０℃で熱間圧延を行なった後の冷却速度を種々変えることによって金属組織の異なる鋼材（直径１１．５ｍｍまたは７．５ｍｍの熱延鋼材）を作製し、夫々について０．２％耐力、伸び、スケール付着量を求めると共に十字溶接引張試験を行ない、表２に示す結果を得た。尚十字溶接引張試験は、２本の同一径の線材を十字状に電気抵抗溶接した後、交点を母材から引張り破壊し、このときの破断荷重を測定することによって溶接性を評価した。溶接時の据え込み量は１．５ｍｍ一定とした。
【００３８】
【表１】

【００３９】
【表２】

【００４０】
表１，２より次の様に考察することができる。
供試鋼Ａは、従来の低炭素鋼（軟鋼）を使用し、熱間圧延の後通常の調整冷却を行なったものであり、スケール付着量が多いため溶接性が悪く、しかもＳｉ量およびＭｎ量が不足する他、Ｂ・Ｐ分率も規定範囲に満たないため耐力が乏しく高強度化の目的が果たせない。また供試材Ｃは、Ｃ量のみが不足する比較例であり、硬さ不足の為０．２％耐力が低く、またスケ−ル付着量がやや多くて溶接強度も不足気味である。供試材Ｄは従来の高炭素鋼（硬鋼）を用いた例であり、０．２％耐力は良好であるが、スケール付着量が多く溶接時の破断荷重が低くて溶接性が乏しい。
【００４１】
供試材ＥはＳｉ量が多過ぎる比較例であり、伸びが低くて加工性が乏しく且つ０．２％耐力も不足気味であり、しかも溶接熱により焼入れ硬化を起こして溶接破断荷重が十分上がらなくなる。
【００４２】
これらに対し供試材Ｂ，Ｆ〜Ｌは本発明に規定要件を全て満足する実施例であり、熱延材としての０．２％耐力、伸びおよび溶接性のいずれにおいてもバランスのとれた優れた値が得られている。
【００４３】
尚図２（Ａ），（Ｂ）は、本発明に係る熱延鋼材の代表例として、表２に示した（Ｉ−１）および（Ｉ−２）の横断面組織を示す図面代用顕微鏡写真を示している。
【００４４】
次に上記表１，２に示した熱延鋼材の一部について、Ｂ・Ｐ分率の低いものについてはそのまま伸線加工を行ない、またＢ・Ｐ分率が高く且つ伸線加工率の高い場合は５５０℃で５時間の熱処理を施してから伸線加工を施し、得られた各伸線材について０．２％耐力、伸び、捻回値および溶接性を調べた。結果を表３に示す。
【００４５】
【表３】

【００４６】
表３からも明らかである様に、供試材Ａは、従来の低炭素鋼でＢ・Ｐ分率が低過ぎる比較例であり、捻回値が高く加工性は良好であるが、引張強さおよび溶接強度が低く、本発明の目的に合致しない。また供試材Ｄは、従来の高炭素鋼であり、伸線材としての引張強さは良好であるが捻回値が低く、またＣ量が多過ぎるため溶接後に過冷却組織が出現して強度劣化を起こし、溶接破断荷重が極端に低くなっている。
【００４７】
これらに対し、供試材Ｂ−１，２，３，Ｆ−１，２はいずれも本発明の規定要件を全て満足する実施例であり、Ｂ・Ｐ分率が低めであるものはそのまま伸線加工を施し、Ｂ・Ｐ分率が高めであるものは適度の熱処理を施すことによって、引張強さ、捻回値および溶接性のいずれにおいても優れた性能を示す伸線材が得られている。尚表３の熱延材と伸線材のＢ・Ｐ分率を見れば分かる様に、熱処理の有無に拘らず伸線加工の前後でＢ・Ｐ分率は殆んど変わっていない。尚図３（Ａ），（Ｂ）は、他の実験で得た伸線材の横断面組織を例示する図面代用顕微鏡写真［図３（Ａ）は、伸線加工歪みを０．３１とした伸線材でＢ・Ｐ分率６０％であるもの、図３（Ｂ）は、伸線加工歪みを１．８８とした伸線材でＢ・Ｐ分率５４％であるもの］であり、加工歪みが大きくなるにつれて横断面組織の形状はかなり変わってくるが、Ｂ・Ｐ分率の絶対値そのものは殆んど変化しない。
【００４８】
また図４は、上記表３に示した供試材Ａ（比較材）、Ｂ−１（本発明材）および供試材Ｄ（比較材）の十字溶接引張試験において、据え込み量と破断荷重の関係を調べた結果を示したグラフであり、従来の低炭素鋼および高炭素鋼よりなる伸線材（供試材Ａ，Ｄ）に比べて、本発明材（供試材Ｂ−１）は格段に優れた据え込み強度特性を有していることが分かる。
【００４９】
【発明の効果】
本発明は以上の様に構成されており、Ｃ量を低めに抑えた低炭素鋼をベース組成とすることによって優れた溶接性を確保し、且つＳｉ量やＭｎ量等を規定すると共に、横断面組織中に占めるＢ・Ｐ分率が１０〜９５％、残部がフェライト、マルテンサイトおよび残留オーステナイトのうち少なくとも１種および２種以上からなる混合組織とすることにより、優れた靭延性、加工性、溶接性を兼ね備えた高強度熱延鋼材を提供することができる。従ってこの熱延鋼材は、熱延ままで加工性の優れたものについては更に伸線加工等を施し、また伸線工程で断線を起こす恐れがある高Ｂ・Ｐ分率の熱延鋼材の場合は適当な温度で焼鈍を行い、マルテンサイトやベイナイト組織の塑性変形能を高めてから伸線加工等を行ない、適度の断面寸法・形状に整えることにより、溶接金網、自動車用シートの枠線、インサートワイヤ、各種ばね材の如き各種鋼線として有効に活用できる。また、Ｂ・Ｐ分率が４０〜９５％である高強度の熱延鋼材については、スケール付着量を抑えることによって溶接性の劣化を抑え、例えばコンクリート補強用鉄筋等に用いる棒鋼等として、熱延間まで有効に活用することができる。
【図面の簡単な説明】
【図１】鋼材中のＣ量と引張強さおよび絞りの関係を示すグラフである。
【図２】実施例で得た高強度熱延鋼材の代表的な横断面金属組織を示す図面代用顕微鏡写真である。
【図３】実施例で得た高強度伸線材の代表的な横断面金属組織を示す図面代用顕微鏡写真である。
【図４】実施例および比較例で用いた熱延鋼材の破断荷重と据込み量の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high strength hot rolled steel material excellent in weldability and tough ductility, a high strength steel wire formed by drawing the hot rolled steel material, and a high strength steel bar made of the hot rolled steel material. Hot-rolled steel is used as various types of steel bars, such as steel bars, H steel, L steel, etc., which are used for concrete reinforcement, etc., and is obtained by wire drawing. Is useful as a material such as a welded wire mesh, a frame of an automobile seat, an insert wire, and various spring materials.
[0002]
[Prior art]
Reinforcing bars for concrete reinforcement that are used as they are after hot rolling are defined in JIS G 3112. In addition, steel wires that require excellent weldability and bending workability, such as welded wire mesh, automobile seat frame wires, and insert wire steel wires, are drawn, for example, on hot rolled steel materials of low carbon steel. It is manufactured by. Moreover, when manufacturing a high-strength steel wire, the method of performing a wire drawing process to the high carbon steel material which performed the patenting process is generally employ | adopted. The typical one used for the production of steel wire that requires the former weldability and bending workability is a mild steel wire specified in JIS G 3505, and the latter high-strength steel wire conforms to JIS G 3506. The specified high carbon steel wire is typically used.
[0003]
[Problems to be solved by the invention]
Reinforcing bars that are used in hot rolling include, for example, reinforced concrete steel bars defined in the above JIS G 3112, and the chemical components and mechanical properties are determined. Among them, SD490 has the highest strength, and its 0.2% proof stress is 490 to 625 N / mm. ² However, since the Great Hanshin Earthquake that occurred in 1995, it is desired to increase the strength of the reinforcing bars and foundations that make up the building. In order to meet these demands, it is conceivable to increase the amount of carbon in the material. However, in this case, a problem arises that a supercooled structure is easily generated during welding and the welding strength is deteriorated.
[0004]
In addition, there is a prestressed concrete pile that embeds PC steel as a vertical streak in the concrete pile and increases the shear fracture strength of the pile. ² A high-strength laterally constrained reinforcing bar is placed to give the pile a compositional deformability. Even if a large bending moment is generated in the ground due to an earthquake, a high-bending toughness pressed concrete pile that does not flex and break It attracts attention.
[0005]
However, a high carbon steel material having a C content of 0.6% or more is used as the PC steel material, and if this is fixed by welding, a supercooled structure is formed during cooling after welding, and the material becomes brittle. Steel cannot be welded directly. That is, the strength and weldability of steel materials tend to conflict, and it is considered that the weldability deteriorates rapidly when the C content exceeds 0.2%.
[0006]
On the other hand, in the case of a welded wire mesh manufactured by performing wire drawing after hot rolling, excellent weldability is obtained when low carbon steel is used, but high strength is difficult to obtain because of low carbon steel. That is, in the low carbon steel, not only the strength of the raw material itself is low, but also work hardening by wire drawing cannot be expected so much, so the purpose of increasing the strength cannot be achieved. On the other hand, in high carbon steel, since the strength of the wire itself before drawing is high and the work hardening rate at the time of drawing is also high, it is easy to obtain a high strength steel wire. On the other hand, because high carbon steel has a high carbon content as described above, when it is applied to welding applications, a supercooled structure is likely to occur during rapid cooling after welding, and the deterioration of toughness due to embrittlement is a major problem. It becomes.
[0007]
In other words, with conventional steel materials, weldability and post-weld toughness deteriorate when attempting to obtain high strength, while when attempting to ensure excellent weldability and bending workability, strength is insufficient, and excellent weldability and bending are achieved. Processability, toughness and high strength cannot be satisfied simultaneously.
[0008]
The present invention has been made by paying attention to the above-described circumstances, and its purpose is to provide high strength and excellent workability, good weldability and high weld strength. The heat that shows excellent performance as it is, or that gives a steel wire useful as a welded wire mesh, car seat frame wire, insert wire, and other various mechanical springs by drawing after hot rolling. Providing a rolled steel material, and also to provide a high-strength steel bar excellent in weldability using the hot-rolled steel material, and a high-strength steel wire excellent in weldability formed by drawing the hot-rolled steel material Is.
[0009]
[Means for Solving the Problems]
The high-strength hot-rolled steel material according to the present invention that has solved the above problems is
C: 0.05 to 0.22%
Si: 0.5 to 2.0%
Mn: 0.5 to 2.0%
Meet other requirements, or as other ingredients,
Cr: 1.5% or less (excluding 0%)
Mo: 1.0% or less (excluding 0%)
Ti: 0.5% or less (excluding 0%)
V: 0.5% or less (excluding 0%)
B: 0.01% or less (excluding 0%)
Nb: 0.1% or less (excluding 0%)
At least one element selected from the group consisting of, preferably each content of Al, P, S, N
Al: 0.08% or less (including 0%)
P: 0.04% or less (including 0%)
S: 0.04% or less (including 0%)
N: 100 ppm or less (including 0 ppm)
The steel is hot-rolled and adjusted and cooled, and the area ratio (bainite + pearlite) in the cross-sectional structure is 10 to 95%, and the balance is one or more of ferrite, martensite and retained austenite, 0.2% proof stress is 800 N / mm ² The characteristic is that it exhibits the above strength.
[0010]
Among the high-strength hot-rolled steel materials, those having an area ratio of (bainite + pearlite) in the cross-sectional structure of 40 to 95% are particularly high-strength and slightly short of wire drawing workability. It can be effectively used as a steel bar such as a reinforcing steel bar for concrete reinforcement without substantially drawing.
[0011]
In addition, (bainite + pearlite) hot rolled steel with an area ratio of 40 to 95% and insufficient wire drawing workability should be heat treated at 450 to 650 ° C. after adjustment cooling to improve the workability before drawing. In addition, (bainite + pearlite) area ratio of 10-60% and hot-drawn steel material with good wire drawing workability is subjected to wire drawing as it is, and welding is achieved by increasing the strength by work hardening by wire drawing. It can be effectively used as a steel wire as a raw material for wire mesh, automobile seat frame wire, insert wire, various spring materials and the like.
[0012]
Therefore, in the present invention, in view of the characteristics of the hot-rolled steel material, those having an area ratio of (bainite + pearlite) of 40 to 95% take advantage of the excellent strength characteristics and are not in a state of being substantially drawn. Strengthened steel bar, or a steel wire that has been heat-treated and annealed once after hot-rolled steel material having a (bainite + pearlite) area ratio of 40 to 95% to improve workability, or (bainite + pearlite) area ratio of 10 About the hot-rolled steel material excellent in workability which is -60%, the high-strength steel wire which increased the intensity | strength by drawing this as it is and carrying out work hardening is also included in the right range.
[0013]
Although the metal structure is slightly stretched during the wire drawing process, the area ratio of (bainite + pearlite) is essentially unchanged, and even if heat treatment is performed after adjustment cooling in some cases, the above 450-650. Under the heat treatment temperature condition of 0 ° C., the area ratio does not change substantially before and after (bainite + pearlite), and the cross-sectional structure of the steel wire obtained by wire drawing is the metal structure before wire drawing, ie (bainite) + Perlite) The area ratio is 10 to 95%, and the structure in which the balance is one or more of ferrite, martensite and retained austenite is maintained almost as it is.
[0014]
In the case of a hot rolled steel material having a (bainite + pearlite) area ratio of 40 to 95% and being used as a bar steel or the like without substantially drawing, the weldability depends on the scale attached to the surface. Therefore, when the hot-rolled steel material or steel bar is put into practical use as a reinforcing steel bar for concrete reinforcement, it is desirable to suppress the amount of scale attached to the surface to 0.4% or less.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
First, the reason why the chemical components of steel are determined in the present invention will be described in detail.
C: 0.05 to 0.22%
C is an element that acts as a strengthening element and a hardenability improving element, and when it is a steel bar with adjusted cooling after hot rolling, or after being subjected to a heat treatment if necessary after that, after being drawn into a wire, It is an element indispensable for obtaining high strength. If it is less than 0.05%, the amount as a strengthening element is insufficient, and the hardenability is insufficient, so that satisfactory strength cannot be obtained. On the other hand, if the C content exceeds 0.22%, not only does the toughness as a steel bar or steel wire decrease, it becomes easy to cause disconnection or the like during processing, but the weldability also deteriorates remarkably, and excessively during rapid cooling after welding. A cooling structure is generated, and a decrease in toughness due to embrittlement becomes a serious problem. In order to satisfy all of the strength, workability, and weldability, the C content is more preferably 0.10% or more and 0.17% or less.
[0016]
Si: 0.5 to 2.0%
Si acts as a deacidifying element during melting and is an important element for increasing the welding strength. In other words, the present invention has one important purpose for improving weldability. In the welding process, the steel material is partially melted, so that oxygen in the atmosphere is taken into the weld metal and bubbles and oxides are formed. It becomes an inclusion, and deteriorates the physical properties of the weld metal. Therefore, in order to avoid such problems due to oxygen mixed during welding, it is necessary to contain Si as a deoxidizing element in the steel material. In addition, Si has the effect of improving the sag resistance and relaxation characteristics when it is drawn into a steel wire or spring, and further acts as a solid solution strengthening element to strengthen ferrite. At the same time, it also enhances hardenability and enhances strength. In order to effectively demonstrate the effect of Si, it is necessary to contain 0.5% or more of Si. However, if the amount of Si is too large, ductility is lowered and workability and wire drawing are adversely affected. In addition to being adversely affected, bending performance and the like are also adversely affected, and should be suppressed to 2.0% or less. The minimum of the more preferable content rate of Si is 0.7%, and a more preferable upper limit is 1.7%.
[0017]
Mn: 0.5 to 2.0%
Mn is an element indispensable for increasing the strength by increasing the hardenability of steel as well as effectively acting on deoxidation during melting as in the case of Si. In particular, in the present invention, as will be described in detail later, high strength is achieved by keeping the (bainite + pearlite) fraction in the metal structure within a specific range. For this purpose, Mn is an essential element. . The preferred content of Mn varies depending on the content of C and other reinforcing elements, but it must be contained at least 0.5% in order to ensure sufficient hardenability. However, the strength improvement effect by Mn is almost saturated at 2.0%, and even if the amount of Mn is excessively increased, no further improvement effect can be obtained. Rather, Mn causes positive segregation and deteriorates the grain boundary strength. Therefore, the upper limit is set to 2.0%. The minimum of the more preferable content rate of Mn is 0.7%, and a more preferable upper limit is 1.7%.
[0018]
The essential constituent elements of the steel used in the present invention are the above three elements, and the balance is substantially composed of Fe. In the steel, the following are aimed at increasing the strength by improving the hardenability. It is also effective to contain elements.
[0019]
Cr: 1.5% or less, Mo: 1.0% or less, Ti: 0.5% or less,
V: 0.5% or less, B: 0.01% or less, Nb: 0.1% or less
All of these elements are synergistic elements in that they are components that increase hardenability and contribute to high strength. Among these elements, especially Cr and Mo contribute to high strength by increasing hardenability. In addition, the steel material of the present invention is affected by welding heat during use as described above. However, Cr and Mo have the effect of suppressing softening due to heat together with the above-described Si, so that the strength reduction due to welding heat is suppressed and welding is performed. It is also an effective element from the viewpoint of further increasing the strength. V, Ti, and Nb contribute to the improvement of hardenability and also exhibit the effect of preventing the austenite crystal grains from coarsening and increasing the toughness. Further, B has the effect of improving the hardenability of the steel by concentrating at the grain boundaries and suppressing the segregation / precipitation of Mn and P at the grain boundaries. Such an effect is exhibited by containing one or more of these elements. However, from the viewpoint of more surely exhibiting the effect of addition of each element, preferred lower limit values are about Cr: 0.05% or more, Mo: About 0.05% or more, Ti: about 0.01% or more, V: about 0.05% or more, B: about 0.0005% or more, and Nb: about 0.01% or more. However, even if it is excessively contained exceeding the upper limit value, no further reforming effect is exhibited, so that it is economically wasteful.
[0020]
For Al, P, S, and N that are often mixed as inevitable impurities in steel, it is preferable to limit the upper limit values as follows for the following reasons.
[0021]
Al: 0.08% or less (including 0%)
Al acts as a deoxidizer during refining and combines with oxygen in the steel to form Al. ₂ O _Three In addition to scavenging oxygen, it has the effect of refining the metal structure by combining with nitrogen to produce AlN, but if it is too much, it will coarsen the crystal grains, and it will be non-metallic in steel Since the toughness is deteriorated by the increase in the amount of inclusions, it is preferable to keep it to 0.08% or less. In order to effectively exhibit the above-mentioned effects brought about by the Al, it is desirable to contain about 0.005% or more of Al.
[0022]
P, S: 0.04% or less each (including 0%)
P and S are harmful elements that not only adversely affect the toughness of steel materials but also cause segregation and cause variations in hardenability. To prevent these defects, both P and S are used. It should be suppressed to 0.04% or less, more preferably 0.02% or less.
[0023]
N: 100 ppm or less (including 0 ppm)
N becomes a factor that deteriorates the ductility of the steel wire due to aging when the wire drawing after hot rolling is performed, and the obstacle appears remarkably when it exceeds 100 ppm. In addition, it is preferable that the amount of N is small. Of course, it may not be contained at all, but usually it is an element that is inevitably mixed, and the content of 20 to 100 ppm is allowed.
[0024]
The high-strength hot-rolled steel material of the present invention has an extremely important cross-sectional structure under the condition that the above-mentioned component composition is satisfied, and the (bainite + pearlite) fraction (hereinafter referred to as B · P fraction) may be 10 to 95%, and the balance should be composed of one or more of ferrite, martensite and retained austenite. Thus, the B / P fraction in the cross-sectional structure has a significant effect on the strength, toughness, workability, etc. of the hot-rolled steel material, and the present invention is a component system in which the amount of C is kept low as described above. In order to secure the high strength as intended and to guarantee an appropriate workability, it is extremely important that the B / P fraction is in the range of 10 to 95%.
[0025]
That is, when the B / P fraction is less than 10%, the strength improvement effect from the metal structure surface cannot be effectively exhibited, and the high strength at the level intended by the present invention cannot be obtained. However, if the B / P fraction exceeds 95% and becomes excessively high, the bending workability will be remarkably deteriorated, the martensite fraction in the structure will be high, and the die life in wire drawing will be reduced and bending work will be performed. Since the problem of easy cutting sometimes occurs, the upper limit of the B / P fraction is set to 95%. In addition, the hot-rolled steel material of the present invention has a 0.2% proof stress of 800 N / mm in order to effectively exhibit the high strength characteristics which is one of its purposes. ² The above is desirable, and by combining such strength characteristics and excellent weldability, it is possible to boast of superiority over conventional materials.
[0026]
The B / P fraction mentioned above has a great influence on the strength and workability of the steel as described above, and those having a low B / P fraction have relatively low strength and excellent workability. On the other hand, those with a high B / P fraction have relatively high strength and lack of workability. Therefore, even if it has a B / P fraction within the range specified in the present invention, the hot-rolled steel material having a lower B / P fraction, for example, the B / P fraction is about 10 to 60%. Hot-rolled steel is used for applications where bending workability is important as it is, or by further bending and drawing to cause work hardening, and after using the required wire diameter and strength. .
[0027]
Further, hot rolled steel materials having a high B / P fraction, for example, hot rolled steel materials having a B / P fraction of about 40 to 95%, have been put into practical use as steel bars such as reinforcing bars that require high strength as they are, Alternatively, if necessary, it can be put to practical use by bending and wire drawing after improving workability by subjecting it to soft annealing once.
[0028]
In addition, when the hot-rolled steel material of the present invention is used as a steel wire after being drawn, the scale generated on the surface of the steel material in the hot-rolling process is almost removed in the drawing process. The weldability is hardly hindered, but the scale formed on the surface in the hot-rolling process is welded when it is used as a steel bar or the like after adjusting and cooling without substantially drawing. Has a significant negative effect on sex. Therefore, in the case of a hot-rolled steel material used as a bar steel, etc., with a B / P fraction of 40 to 95% as described above and substantially not drawn after hot rolling, in order to suppress the deterioration of weldability due to such scale. In addition, it is important to suppress the amount of scale adhesion to 0.4% or less.
[0029]
As a specific means for suppressing the amount of scale adhesion as a hot-rolled steel or steel bar, for example, a method of suppressing the oxidation by suppressing the cooling start temperature after the hot-rolling to about 925 ° C. or lower is employed. That's fine.
[0030]
When hot-rolled steel is used after drawing, the metal structure of the hot-rolled steel changes slightly due to drawing, but the B / P fraction itself remains almost unchanged, and prior to drawing. Even when heat treatment for improving workability is performed, in the heat treatment temperature range of 450 to 650 ° C. employed in the present invention, only a slight tendency to spheroidize the structure is observed, and B · P content The rate itself is almost unchanged. Therefore, the metal structure of the steel wire obtained by wire drawing is also within the range of 10 to 95% in terms of the B / P fraction (area ratio) that is the structure before wire drawing.
[0031]
In the above, even if the B / P fraction is the same, the area ratio of bainite to pearlite in the (bainite + pearlite) structure [bainite / perlite × 100 (%): hereinafter referred to as B / P ratio] However, the strength and workability change, and when the bainite increases, the strength increases while the workability tends to decrease, and when the bainite decreases, the strength decreases while the workability improves. In order to stably secure high strength and excellent workability in the range of B / P fraction, the B / P ratio is preferably about 60% or more, more preferably about 80% or more.
[0032]
The hot rolled steel material satisfying the above-mentioned B / P fraction range defined in the present invention, or further having the above-mentioned preferable B / P ratio, is 750 to 1250 ° C. after casting a steel that satisfies the requirements of the above component composition, More preferably, hot rolling is performed in the range of 800 to 1150 ° C., and cooling from the two-phase region temperature is 0.1 to 30 ° C./sec, more preferably 0.5 to 15 ° C./sec in the subsequent cooling process. It can be obtained by adjusting cooling at a rate of sec.
[0033]
The hot-rolled steel material of the present invention thus obtained has moderate elongation as described above and has excellent weldability and high strength, and can be effectively put into practical use as a steel bar for reinforcing steel for concrete reinforcement as it is. If the B / P fraction is low and the workability is excellent, then the wire is subjected to wire drawing, etc., and high B / P fraction hot rolling may cause disconnection in the wire drawing process. In the case of steel, annealing is performed at a temperature of 450 to 650 ° C., and the plastic deformation ability is increased without substantially changing the B / P fraction, and then wire drawing is performed to prepare an appropriate cross-sectional size and shape. And put to practical use.
[0034]
【Example】
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 as a matter of course, and is implemented with modifications within a range that can be adapted to the purpose described above and below. Of course, any of these is also included in the technical scope of the present invention.
[0035]
Example
FIG. 1 shows the results of experiments conducted to confirm the influence of the C content in the steel used on the welding strength. Si: 0.75 to 0.90%, Mn: 1.40 About a steel wire having a base composition of 1.55% and using a hot-rolled steel wire (5.5 mmφ) in which only the C content is changed in the range of 0.07 to 0.42, austenitized, and then water-quenched The results of examining the drawing and tensile strength are shown. The reason why the martensite structure is obtained by water quenching is that the metal structure of the welded portion becomes a martensite structure due to the heat effect during welding, and the strength and ductility of this structure greatly affect the weld strength. That is, even if the welded portion is a hard steel structure, if the ductility is not ductile, the weld strength is lowered, and even if it is excellent in ductility, if the soft steel structure is used, sufficient weld strength cannot be obtained. .
[0036]
As is clear from this figure, a high level of tensile strength is maintained up to 0.25% of the C content in the steel. However, if the C content exceeds 0.22%, it becomes a ductility index. It can be confirmed that the value is abruptly lowered and ductility cannot be ensured by rapid cooling after welding. That is, from this result, it is understood that the C content in the steel must be suppressed to 0.22% or less in order to ensure the excellent welding strength at the level intended by the present invention.
[0037]
Next, as shown in Table 1, steel materials having various metal structures (diameters) were prepared by melting and casting steel materials having various chemical compositions and changing the cooling rate after hot rolling at 800 to 1150 ° C. 11.5 mm or 7.5 mm hot-rolled steel material) was prepared, and 0.2% proof stress, elongation, and amount of scale adhesion were determined for each, and a cross-weld tensile test was performed, and the results shown in Table 2 were obtained. In the cross welding tensile test, two wires having the same diameter were subjected to electrical resistance welding in a cross shape, and then the intersection was pulled and broken from the base material. The weldability was evaluated by measuring the breaking load at this time. The amount of upsetting during welding was fixed at 1.5 mm.
[0038]
[Table 1]

[0039]
[Table 2]

[0040]
From Tables 1 and 2, it can be considered as follows.
Test steel A is a conventional low carbon steel (soft steel), which has been subjected to normal adjustment cooling after hot rolling, and has poor weldability due to a large amount of scale adhesion. In addition to the shortage of the amount, the B / P fraction is less than the specified range, so the proof strength is poor and the purpose of increasing the strength cannot be achieved. Further, the sample material C is a comparative example in which only the amount of C is insufficient, the 0.2% proof stress is low due to insufficient hardness, the amount of scale adhesion is slightly large, and the welding strength is insufficient. Specimen D is an example using conventional high carbon steel (hard steel), and 0.2% proof stress is good, but the amount of scale adhesion is large, the fracture load during welding is low, and weldability is poor.
[0041]
Specimen E is a comparative example in which the amount of Si is too large, the elongation is low, the workability is poor, the 0.2% proof stress is insufficient, and quenching hardening is caused by welding heat to sufficiently increase the welding fracture load. Disappear.
[0042]
On the other hand, specimens B and F to L are examples that satisfy all the requirements of the present invention, and are excellent in balance in any of 0.2% proof stress, elongation and weldability as hot rolled materials. Values are obtained.
[0043]
2A and 2B are drawing-substituting micrographs showing the cross-sectional structures of (I-1) and (I-2) shown in Table 2 as representative examples of the hot-rolled steel material according to the present invention. Is shown.
[0044]
Next, for some of the hot-rolled steel materials shown in Tables 1 and 2 above, those with a low B / P fraction are drawn as they are, and the B / P fraction is high and the drawing ratio is high. In this case, the wire drawing was performed after heat treatment at 550 ° C. for 5 hours, and 0.2% proof stress, elongation, twist value and weldability of each drawn wire were examined. The results are shown in Table 3.
[0045]
[Table 3]

[0046]
As is clear from Table 3, the test material A is a comparative example in which the B / P fraction is too low in the conventional low carbon steel, and the twisting value is high and the workability is good. And the weld strength is low and does not meet the object of the present invention. In addition, specimen D is a conventional high carbon steel, which has good tensile strength as a wire drawing material, but has a low twist value, and because of too much C content, a supercooled structure appears after welding and the strength is high. Deterioration has occurred, and the welding fracture load is extremely low.
[0047]
On the other hand, all of the test materials B-1, 2, 3, F-1, and 2 are examples that satisfy all of the prescribed requirements of the present invention, and those having a low B / P fraction are stretched as they are. A wire drawing material that has a high B / P fraction has been obtained by performing an appropriate heat treatment, thereby exhibiting excellent performance in any of tensile strength, twist value, and weldability. . As can be seen from the B / P fractions of the hot-rolled wire and the wire drawing material in Table 3, the B / P fraction is almost unchanged before and after the wire drawing regardless of the heat treatment. 3A and 3B are drawing-substituting micrographs illustrating the cross-sectional structure of the wire drawing material obtained in other experiments [FIG. 3A is a drawing with a wire drawing strain of 0.31. The wire has a B / P fraction of 60%, and FIG. 3B shows the wire drawn with a drawing strain of 1.88 and a B / P fraction of 54%]. As the size increases, the shape of the cross-sectional structure changes considerably, but the absolute value of the B · P fraction itself hardly changes.
[0048]
FIG. 4 shows the upsetting amount and the breaking load in the cross welding tensile test of the specimens A (comparative material), B-1 (present invention material) and the specimen D (comparative material) shown in Table 3 above. Is a graph showing the results of investigating the relationship of the present invention material (test material B-1) compared to the conventional wire drawing material (test materials A, D) made of low carbon steel and high carbon steel, It turns out that it has remarkably excellent upsetting strength characteristics.
[0049]
【The invention's effect】
The present invention is configured as described above, and by using a low carbon steel with a low C content as a base composition, excellent weldability is ensured, and the Si content, Mn content, etc. are defined, Excellent toughness and workability with a mixed structure consisting of at least one or more of ferrite, martensite and retained austenite with a B / P fraction in the surface structure of 10 to 95% and the balance being ferrite, martensite and retained austenite. Further, it is possible to provide a high-strength hot-rolled steel material having weldability. Therefore, this hot-rolled steel material is a hot-rolled steel material with a high B / P fraction that may be subjected to wire drawing or the like for those that are hot-rolled and have excellent workability, and may break in the wire drawing process. Is annealed at an appropriate temperature, and the wire deforming process is performed after increasing the plastic deformability of the martensite and bainite structures. It can be effectively used as various steel wires such as insert wires and various spring materials. For high-strength hot-rolled steel materials having a B / P fraction of 40 to 95%, the deterioration of weldability is suppressed by suppressing the amount of scale adhesion. For example, as steel bars used for concrete reinforcing bars, It can be used effectively up to the promenade.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the amount of C in steel, tensile strength, and drawing.
FIG. 2 is a drawing-substituting micrograph showing a representative cross-sectional metallographic structure of a high-strength hot-rolled steel material obtained in an example.
FIG. 3 is a drawing-substituting micrograph showing a representative cross-sectional metallographic structure of the high-strength wire drawing material obtained in the examples.
FIG. 4 is a graph showing the relationship between the breaking load and the upsetting amount of hot-rolled steel used in Examples and Comparative Examples.

Claims

C: 0.10 to 0.17 % (hereinafter, means mass% unless otherwise specified)
Si: 0.5 to 2.0%
Mn: 0.5-2.0% , and
Balance being steel having a steel hot rolled after adjustment cooled to name Ru bainite and pearlite structure consisting of Fe and unavoidable impurities,
0.2% proof stress is 800 N / mm ² or more,
Occupying the cross-sectional structure (bainite + pearlite) area ratio of 10-60%, the balance being ferrite state, and are 1 or more martensite and residual austenite, Ru der those used by applying it wiredrawing High strength hot rolled steel with excellent weldability.

C: 0.10 to 0.17 %
Si: 0.5 to 2.0%
Mn: 0.5-2.0% , and
Balance being steel having a steel hot rolled after adjustment cooled to name Ru bainite and pearlite structure consisting of Fe and unavoidable impurities,
0.2% proof stress is 800 N / mm ² or more,
Occupying the cross-sectional structure (bainite + pearlite) area ratio from 40 to 95%, the balance being ferrite, der one or more martensite and residual austenite is, heat-treated at 450 to 650 ° C. After adjusting cooling, drawing high strength hot rolled steel with excellent weldability, characterized in der Rukoto those used giving the process.

Steel is another component,
Cr: 1.5% or less (excluding 0%)
Mo: 1.0% or less (excluding 0%)
Ti: 0.5% or less (excluding 0%)
V: 0.5% or less (excluding 0%)
B: 0.01% or less (excluding 0%)
Nb: 0.1% or less (excluding 0%)
The hot-rolled steel material according to claim 1 or 2 , comprising at least one element selected from the group consisting of:

Each content of Al, P, S, N in steel is Al: 0.08% or less (including 0%)
P: 0.04% or less (including 0%)
S: 0.04% or less (including 0%)
N: 100 ppm or less (including 0 ppm)
The hot-rolled steel material according to any one of claims 1 to 3 .

Claim 1 high-strength steel wire and hot-rolled steel product as claimed excellent weldability, characterized by comprising wire drawing to one of the 4.

C: 0.10 to 0.17 %
Si: 0.5 to 2.0%
Mn: 0.5-2.0% , and
Balance being steel having a steel hot rolled after adjustment cooled to name Ru bainite and pearlite structure consisting of Fe and unavoidable impurities,
0.2% proof stress is 800 N / mm ² or more,
The area ratio (bainite + pearlite) in the cross-sectional structure is 40 to 95%, the balance is one or more of ferrite, martensite and retained austenite , and the surface scale adhesion is 0.4% or less. A high-strength hot-rolled steel material with excellent weldability, characterized in that it is used without any wire drawing .

Steel is another component,
Cr: 1.5% or less (excluding 0%)
Mo: 1.0% or less (excluding 0%)
Ti: 0.5% or less (excluding 0%)
V: 0.5% or less (excluding 0%)
B: 0.01% or less (excluding 0%)
Nb: 0.1% or less (excluding 0%)
The hot-rolled steel material according to claim 6 , comprising at least one element selected from the group consisting of:

Each content of Al, P, S, N in steel is Al: 0.08% or less (including 0%)
P: 0.04% or less (including 0%)
S: 0.04% or less (including 0%)
N: 100 ppm or less (including 0 ppm)
The hot-rolled steel material according to claim 6 or 7 .

A high-strength steel bar excellent in weldability comprising the hot-rolled steel material according to any one of claims 6 to 8 .