JP4564189B2

JP4564189B2 - High toughness non-tempered steel for hot forging

Info

Publication number: JP4564189B2
Application number: JP2001043788A
Authority: JP
Inventors: 浩一磯部
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-02-20
Filing date: 2001-02-20
Publication date: 2010-10-20
Anticipated expiration: 2021-02-20
Also published as: JP2002241890A

Description

【０００１】
【発明の属する技術分野】
本発明は高靭性非調質鋼、特に質量％でＣ：０．１〜０．６％の炭素鋼及び低合金鋼からなる熱間鍛造用の非調質鋼に関するものである。
【０００２】
【従来の技術】
従来、自動車部品等の機械部品の多くは熱間鍛造により成型された後、焼入れ、焼戻し処理、いわゆる調質処理を施された後、切断、研削等の機械加工を経て製造されてきた。
【０００３】
しかしながら、省工程による生産性の向上や省エネルギー化の要求から、上述の調質処理省略が志向され、省エネルギーやコスト削減メリットから非調質鋼の適用が拡大している。
【０００４】
一般に調質処理を省略した非調質鋼では調質鋼に比べ靭性が低いために高い靭性が要求されない機械部品に使用されるなど、用途が狭い範囲に限定されるという問題があった。
【０００５】
従来、この非調質鋼として機械構造用炭素鋼にＶ、Ｎｂ等の元素を少量添加するマイクロアロイ型非調質鋼が提案されたが、熱間鍛造ままの組織は著しく粗大化したフェライト・パ−ライト組織のため靭性は極めて低く適用範囲は極めて限定されていた。
【０００６】
このような欠点を解消するため少量のＴｉを添加して結晶粒の粗大化を防止し靭性の改善を図る方法が提案されているが（例えば特開昭５６−３８４４８号公報）、その効果は必ずしも安定していない。
【０００７】
また、Ｚｒ、Ｔｉのうち１種又は２種以上を少量添加すると共に、鋳造工程での冷却速度を確保することでＭｎＳやＴｉＮ及びＺｒＮの内少なくとも１種を微細均一に分散させる熱間鍛造用非調質鋼の製造方法が提案されている（特公平６−７５７４７号公報）。
【０００８】
更に、Ａｌ、Ｖ、Ｎｂ、Ｔｉ、Ｂ等の炭、窒化物形成元素を添加すると共に、Ｃａ、Ｔｅ、Ｃｅ及びその他の稀土類金属、ミッシュメタル及びその混合物を０．００１〜０．０４ｗｔ％添加する高靭性非調質鋼及びその製造方法が提案されている（特開平６−３４０９４６号公報）。
【０００９】
また、特公平４−７０３８５号公報にはＺｒとＢ添加により熱間鍛造材の強度、靭性の改善を図る方法が提案され、更に稀土類元素の添加により靭性の改善を図る方法が提案されている。
【００１０】
【発明が解決しようとする課題】
本発明は上述の機械部品製造工程において、熱間鍛造後の熱処理を省略しても、常に安定して従来の非調質鋼で達成できないレベルの高靭性を有する非調質鋼を提供するものである。
【００１１】
【課題を解決する手段】
本発明者らは上述の機械部品製造工程において、熱間鍛造後の調質処理を省略しても、常に安定して従来の非調質鋼で達成できないレベルの高靭性を有する非調質鋼を実現する手段について種々の研究を重ね、鋼材の組成を適正に制御すると共に、硫化物や酸化物及び酸化物と硫化物が複合化した介在物サイズの減少を図ると共にそれら介在物のアスペクト比（長さ／幅）をある程度以下に制御することにより、熱間鍛造後に調質処理を省略しても高い靭性を安定して達成できることを見出すと共に、それらを実現する具体的な手段を見出し、本発明を完成した。
【００１２】
本発明の要旨は以下の通りである。
【００１３】
（１）質量％で、
Ｃ：０．１〜０．６％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．２〜２．０％、
Ｓ：０．００５〜０．５％、
Ｃｒ：０．１〜２．０％、
Ｔｉ：０．００５〜０．１％、
Ｎ：０．００３〜０．０２％、
Ａｌ：０．０７％以下（０％含む）、
Ｓｎ：０．００８〜０．１％、
更に、
Ｒｅｍ（稀土類金属）：０．００５〜０．１％、
及びＣａ：０．０００５〜０．００５％
を含み、且つ、
（Ｒｅｍ％＋Ｃａ％）／（Ｓ％）≧０．０５
の関係を満たし、残部Ｆｅ及び不可避的不純物からなり、介在物の平均円相当径が２０μｍ以下であり、介在物の平均アスペクト比（長さ／幅）が１０以下であることを特徴とする熱間鍛造用高靭性非調質鋼。
【００１４】
（２）更に、質量％で、
Ｚｒ：０．００７〜０．１％
を含み、且つ、
（Ｒｅｍ％＋Ｃａ％＋２×Ｚｒ％）／（Ｓ％）≧０．０５
の関係を満たすことを特徴とする上記（１）記載の熱間鍛造用高靭性非調質鋼。
【００１５】
（３）更に、質量％で、
Ｖ：０．０１〜０．５％
を含有することを特徴とする上記（１）又は（２）記載の熱間鍛造用高靭性非調質鋼。
（４）更に、質量％で、
Ｖ：０．０１〜０．５％、
Ｍｇ：０．０００５〜０．０１％
を含み、且つ、
（Ｒｅｍ％＋Ｃａ％＋Ｍｇ％）／（Ｓ％）≧０．０５
の関係を満たすことを特徴とする上記（１）記載の熱間鍛造用高靭性非調質鋼。
（５）更に、質量％で、
Ｎｂ：０．０１〜０．３％、及びＢ：０．０００３〜０．００５％
のうちの１種又は２種、
Ｐｂ：０．０１〜０．３％、及びＢｉ：０．０１〜０．３％
のうちの１種又は２種、
を含有することを特徴とする上記（１）乃至（４）のいずれかに記載の熱間鍛造用高靭性非調質鋼。
【００１６】
（６）更に、質量％で、
Ｎｉ：０．０１〜２．０％、
Ｍｏ：０．０１〜１．０％
の２種のうちの１種又は２種を含有することを特徴とする上記（１）乃至（５）のいずれかに記載の熱間鍛造用高靭性非調質鋼。
【００１７】
（７）更に、質量％で、
Ｐｂ：０．０１〜０．３％、
Ｂｉ：０．０１〜０．３％
のうちの少なくとも１種以上を含有することを特徴とする上記（１）乃至（４）及び上記（６）のいずれかに記載の熱間鍛造用高靭性非調質鋼。
【００１８】
【発明の実施の形態】
以下本発明の各請求項の要件について説明する。
【００１９】
まず、本発明の請求項１で鋼材成分を規定した理由は次の通りである。
【００２０】
Ｃは鋼材の強度を得るための必須成分であり、０．１％未満とすると機械構造用部品としての特性が得られなくなるし、０．６％超とすると熱間鍛造品の機械的特性のバラツキが大きくなり、安定した品質が得られにくい。
【００２１】
Ｓｉも強度を確保する上で有用な元素であるが、強度アップには少なくとも０．０１％以上必要であり、また、２．０％超えて含有するとフェライト地の脆化が激しくなるため、その含有量は２．０％以下に抑えるべきである。
【００２２】
Ｍｎは強靭化作用が大きくＳｉ同様有用な元素であり、且つ、０．２％未満では熱間延性が低く、鋼材の熱間圧延や熱間加工が困難である。また、２．０％を超えると切削性や溶接性等が低下するので制限した。
【００２３】
Ｓは被削性を向上させる元素であり、０．００５％未満では被削性が低く切削加工が困難となり、また、非調質鋼では熱間鍛造後の冷却過程でフェライトの生成を促進する等で靭性の向上をもたらす一方、ＭｎＳの増加により機械的特性の劣化や異方性の増大をもたらす。０．５％超では機械的特性の劣化や異方性の増大に加え、熱間延性も極端に低くなるため０．５％以下に制限した。
【００２４】
Ｃｒも強度上昇に有効な元素であるが、強度上昇のためには少なくとも０．１％以上必要であり、２．０％を超えると靭性が顕著に低下するため上限は２．０％とした。
【００２５】
Ｔｉはオーステナイト組織を微細化にして靭性の向上をもたらす元素であり、そのためには、少なくとも０．００５％以上必要であり、一方、０．１％を超えると粗大なＴｉＮ等が生成して靭性の劣化をもたらすため、上限を０．１％に設定した。
【００２６】
ＮはＡｌ、Ｔｉ等の窒化物形成元素と窒化物を形成してオーステナイト結晶粒を微細化するのに有用な元素である。上記効果を発揮させるには少なくとの０．００３％以上のＮが必要であり、０．０２％を超えて含有しても上記効果は飽和とするため、Ｎの上限は０．０２％とした。
【００２７】
Ａｌは一般には脱酸や結晶粒の制御にとって有用な元素であるが、Ｓｉ、Ｍｎ、Ｔｉを含有するケースではＡｌは必ずしも添加する必要がない。Ａｌを添加した分は脱酸や結晶粒制御に寄与し、Ａｌ量が０．０７％超では熱間加工性を害するため、Ａｌの範囲を０．０７％以下（０％含む）とした。
【００２８】
Ｓｎはフェライトに固溶し、強度の向上に寄与すると共に、本発明鋼では凝固時の初晶フェライトの増加やそれにともなうＳのデンドライト樹間への排出の抑制効果やＳｎ自身のデンドライト樹間に存在する液相への濃化により、最終凝固部に晶出する硫化物の小型化に寄与すると共に、後述するＲｅｍ、Ｃａ等の添加による硫化物の組成制御で熱間圧延や熱間鍛造での延伸を防止して硫化物のアスペクト比低減を図る際、その効果を増大させる作用を有することが判明した。そのような効果が得られるのは実施例に記載のように少なくとも０．００８％以上のＳｎを含有させる。また、Ｓｎの含有量が０．１％を超えると熱間延性が著しく低下するため、その上限を０．１％以下とした。上記Ｓｎ含有量の範囲でもＳｎによる熱間延性の低下が問題となる場合には、Ｎｉを添加し、熱間延性を確保するのが好ましい。
【００２９】
本発明者らが種々検討した結果、熱間圧延等で延伸したＭｎＳ等の硫化物が従来から言われているような機械的異方性の増大をもたらすばかりでなく、靭性の低下にも大きな影響を及ぼしていることが判明した。
【００３０】
そこで、延伸した硫化物による靭性低下を防止する方法について検討し、Ｒｅｍ（稀土類金属）：０．００５〜０．１％とＣａ：０．０００５〜０．００５％を含有させ、且つ、（Ｒｅｍ％＋Ｃａ％）／（Ｓ％）≧０．０５になるよう含有させると、ＭｎＳがＲｅｍとＣａが固溶する硫化物や硫化物と酸化物が複合化した介在物に変化することによって熱間での可塑性がＭｎＳに比べ大幅に減少し、熱間圧延や熱間鍛造によって介在物がＭｎＳのように延伸するのを極めて効果的に防止でき、その結果、延伸したＭｎＳ等による靭性の低下を大幅に改善できることが判明した。
【００３１】
また、上記条件を満足するようにＲｅｍとＣａを添加した場合、Ｒｅｍ又はＣａを含有するほぼ球状の酸化物が生成し、そのような形態変化も低温靭性の改善に寄与していることが判明した。
【００３２】
ＲｅｍとＣａの添加によって、硫化物の熱間加工時の延伸を効果的に抑制するには、Ｒｅｍが０．００５％以上必要であり、Ｃａも０．０００５％以上添加する必要があり、一方、Ｒｅｍを０．１％以上、Ｃａを０．００５％以上添加するとこれらの元素を含む硫化物のクラスーターが出現し易くなり、それらの生成によって靭性は却って低下する。更にＳ量に比べＲｅｍやＣａの添加量が不足しても硫化物の熱間加工時の延伸を十分防止できないため、（Ｒｅｍ％＋Ｃａ％）／（Ｓ％）≧０．０５になるよう含有させる必要がある。
【００３３】
更に、熱間鍛造用の非調質鋼において、介在物の円相当径の平均値が２０μｍ以下にすると共に、それらの介在物アスペクト比（長さ／幅）の平均値が１０以下になるよう制御すると、衝撃が加わった場合の伸びた硫化物系介在物が存在する時の様な介在物によるノッチ効果は大幅に緩和され、また、亀裂の伝播抵抗を増大させる等の効果で、硫化物による靭性低下を極めて効果的に改善できる。
【００３４】
ここで言う介在物とは酸化物、硫化物及び酸化物と硫化物が複合化した介在物であり、そのなかにはＲｅｍやＣａを含有する酸化物や硫化物及びそれらの複合した介在物も含まれる。
【００３５】
上記の介在物サイズの小型化を実現するには、成分を請求項１に規定した範囲に制御すると共に、鋼塊もしくは鋳片を製造する鋳造工程において、凝固中に硫化物等の介在物が晶出する場合、１５００℃〜１３００℃の温度区間の冷却速度をある程度確保することにより、凝固中に晶出する介在物の成長を抑えることも有効である。
【００３６】
鋳片や鋼塊の断面サイズが小さいほど上記凝固中の冷却速度を確保し易く、ＭｎＳ等凝固中に晶出する介在物の成長を抑制して、介在物の小型化を達成し易い。
【００３７】
また、硫化物等高温での可塑性を有する介在物はサイズが大きいほど熱間加工で延伸するため、上述のような手段で鋳造段階においても介在物サイズを小さくコントロールすることも介在物の熱間圧延や熱間鍛造等での延伸を抑制する上で有効である。
【００３８】
また、何らかの理由でＲｅｍやＣａの添加量が規制され、熱間加工による硫化物等の延伸を十分に防止できない場合において、請求項２に記載したＺｒ：０．００７〜０．１％を含有させ、且つ、（Ｒｅｍ％＋Ｃａ％＋２×Ｚｒ％）／（Ｓ％）≧０．０５になるよう含有させ、あるいは、請求項４に記載したＭｇ：０．０００５〜０．０１％を含有させ、且つ、（Ｒｅｍ％＋Ｃａ％＋Ｍｇ％）／（Ｓ％）≧０．０５になるように含有させ、硫化物の可塑性を低減することも熱間加工での延伸をより確実に防止する上で有効である。
【００３９】
Ｚｒ及びＭｇの添加により熱間加工時における硫化物の延伸防止を図るにはＺｒは０．００７％以上、Ｍｇは０．０００５％以上添加する必要がある。Ｚｒが０．１％超添加すると硬質なＺｒＯ₂等の酸化物が増加し、それが原因で靭性が低下する場合があり、Ｚｒの含有量の上限は０．１％とした。Ｍｇについては０．０１％超添加しても溶鋼中でのＭｇＳの生成に消費され、凝固中に晶出する介在物の形態制御には有効に作用しなくなるため、Ｍｇ含有量の上限については０．０１％とした。
【００４０】
一層強度や靭性の向上が必要な時は、請求項３又は４のようにＶの添加や請求項５のようにＮｂ、Ｂの添加が有効である。
【００４１】
ＶやＮｂは共に炭、窒化物を形成して強度や靭性を向上させる元素であり、Ｎｂについては熱間加工中オーステナイトの再結晶を抑制して結晶粒の微細化を促進するためその点でも、強度靭性を向上させる。Ｖ、Ｎｂとも０．０１％以上の添加で上記改善効果が現れ、また、Ｖの添加量が０．５％を超えると、Ｎｂも添加量が０．３％を超えると熱間延性の低下が激しいため、それぞれの添加量の上限を０．５％と０．３％とした。
【００４２】
一方、Ｂは非調質鋼でフェライトの生成を促進したり、硬度を向上させる効果を有するが効果が出現するのは０．０００３％以上であり、０．００５％を超えると熱間延性や靭性も低下するためＢの添加量は０．００５％に制限した。
【００４３】
焼入れ性や靭性の向上が必要な時は請求項６のようにＮｉ、Ｍｏの添加が有効である。
【００４４】
Ｎｉ、Ｍｏは焼入れ性を確保すると共に靭性を向上させるのに有効な元素であり、それらの効果を得るには少なくとも０．０１％以上添加する必要があり、Ｎｉについては２．０％を超えて添加しても、Ｍｏについては１．０％を超えて添加しても効果が飽和するのでＮｉの上限を２．０％、Ｍｏの上限を１．０％とした。
【００４５】
被削性向上のニーズがある場合には、請求項７のようにＰｂやＢｉの添加が被削性の向上に有効である。
【００４６】
被削性改善効果を得るには、ＰｂとＢｉでは０．０１％以上添加する必要があり、一方、どの元素も０．３％を超えて添加すると著しく熱間延性を劣化させるためＰｂ、Ｂｉの２元素共に添加量の上限を０．３％とした。
【００４７】
【実施例】
（実施例１）
本実施例では、請求項１、請求項５に該当する発明鋼とそれらに対応する各比較鋼の引張り強度や低温靭性を調査、対比するため、表１に示した化学組成を有する１５０ｋｇの鋼塊を真空溶解炉で溶製すると共に鋼塊が凝固する際の冷却速度を大幅に変化させた。本発明鋼と一部の比較鋼の鋼塊を鋳型内で放冷したのに対し、比較鋼Ｎｏ．９、１０の鋼塊の溶製では、カオール等の断熱材を用いた断熱ボックスに鋳型を設置すると共にガス加熱で断熱ボックス内に設置された鋳型の周囲を加熱して上記冷却速度大幅に低下させ硫化物等の凝固中に晶出する介在物の粗大化を図った。
【００４８】
製造した鋼塊は１２５０℃に加熱後熱間鍛造で９０φの丸棒に成型後一旦室温まで冷却して介在物調査用サンプルを採取した。その後９０φの丸棒を１２５０℃に再度加熱して熱間鍛造することにより５０φの丸棒に成型した。鍛造終了後の１１００℃から室温までは空冷にて冷却した。そのようにして得られた５０φの丸棒より切削加工にてＪＩＳ４号引張り試験片やＪＩＳ３号衝撃試験片を採取し、室温で引張り試験やシャルピー試験を実施、強度と低温靭性を評価した。各試験の結果を表１に示す。尚、介在物径や介在物のアスペクト比（長さ／幅）の測定では、光学顕微鏡を用いて９０φの丸棒縦断面に存在する介在物を４００倍の倍率で２０視野ほど写真観察し、その写真中の介在物全てを対象に画像解析装置で解析して、介在物の円相当径とアスペクト比（長さ／幅）の平均値を求めた。
【００４９】
【表１】

【００５０】
表１からも明らかなように、本発明鋼を用いて熱間鍛造後空冷したものは、比較鋼を用いて熱間鍛造し、その後空冷したものに比べ、強度、低温靭性共に高い傾向を示した。逆にＳｎやＲｅｍ、Ｃａの含有量、（Ｒｅｍ＋Ｃａ）／Ｓの比が本発明の請求項１で規定した範囲を逸脱したり、成分系が規定範囲を満足していても介在物の平均円相当径や平均アスペクト比が規定範囲を外れると強度や靭性が本発明鋼に比べ劣化するのが分かる。
【００５１】
また、凝固時の冷却速度を大幅な低減を図った比較鋼Ｎｏ．９と１０と本発明鋼のＮｏ．１〜３、６を比較すると、比較鋼Ｎｏ．９では介在物の平均円相当径と平均アスペクト比が、Ｎｏ．１０では介在物の平均円相当径が本発明鋼より増大しており、そのことに起因して強度や衝撃値が低下していることが分かる。比較鋼Ｎｏ．１２では（Ｒｅｍ＋Ｃａ）／Ｓの比が請求項１の範囲を満足しておらず、そのため介在物の平均アスペクト比が大きく、強度や衝撃値も発明鋼に比べかなり低下している。
【００５２】
（実施例２）
表２に化学組成を示した鋼を高炉、転炉法で溶製し、２２０ｍｍ角のブルーム鋳片に鋳造後、分塊圧延、棒鋼圧延で９０φの丸棒に成型した。
【００５３】
上記の方法で得られた丸棒を用いて１２５０℃に加熱後熱間鍛造し、５０φの丸棒に成型した後、鍛造終了後１１００℃から室温まで空冷した。そのようにして得られた材料より、実施例１と同様、ＪＩＳ４号引張り試験片やＪＩＳ３号衝撃試験片を切削加工により採取して室温にて引張り試験やシャルピー試験を実施した。各試験の結果を表２に併記した。
【００５４】
【表２】

【００５５】
本試験でも介在物の円相当径や介在物のアスペクト比の平均値は実施例１と同様の画像解析装置を用いる方法で９０φの丸棒の縦断面で計測した。
【００５６】
請求項１に該当する発明鋼はＮｏ．１、４、１０、１１の発明鋼であり、Ｎｏ．１２、１３、１５、２１、２２、２３はその比較鋼である。また、請求項２に該当する発明鋼はＮｏ．８、９の発明鋼であり、Ｎｏ．１９、２０はその比較鋼である。Ｎｏ．３、５、６、９の鋼材は請求項３に該当する発明鋼であり、その比較鋼がＮｏ．１４、１６、１７、２０の鋼材である。更に、請求項４及び請求項５に該当する発明鋼はそれぞれＮｏ．７とＮｏ．５、６であり、それらに対応する比較鋼はそれぞれＮｏ．１８とＮｏ．１６、１７である。
【００５７】
本実施例においても、各請求項に該当する発明鋼を用いて熱間鍛造した場合は、各発明鋼に対応する比較鋼を用いた場合に比べ、室温での引張り強度や靭性は高い値が得られており、本発明鋼が優れた熱間鍛造用非調質鋼であることが本実施例でも確認された。
【００５８】
また、表２より分かるように本実施例においても成分系が各請求鋼の範囲を満足していなかったり、成分系は規定範囲を満足しても、それ以外の介在物の平均円相当径や平均アスペクト比が規定範囲を外れると本発明鋼に比較して強度や靭性がかなり低下しているのが分かる。
【００５９】
【発明の効果】
以上説明したように、本発明の鋼では調質処理を施さなくても高い強度や靭性値が得られると共に、調質処理の省略によって大幅なエネルギーや製造コストの削減が可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high toughness non-heat treated steel, in particular, a non-heat treated steel for hot forging made of carbon steel and low alloy steel having C: 0.1 to 0.6% by mass.
[0002]
[Prior art]
Conventionally, many machine parts such as automobile parts have been manufactured by hot forging, quenching, tempering, so-called tempering, and then machining such as cutting and grinding.
[0003]
However, due to demands for improved productivity and energy savings due to process savings, the above-mentioned refining treatment is omitted, and the application of non-heat treated steel is expanding due to energy saving and cost reduction benefits.
[0004]
In general, non-tempered steel that has not been subjected to tempering treatment has a problem that its use is limited to a narrow range, such as being used for machine parts that do not require high toughness because it has lower toughness than tempered steel.
[0005]
Conventionally, a microalloy type non-heat treated steel has been proposed in which a small amount of elements such as V and Nb are added to the carbon steel for machine structure as a non-heat treated steel. Due to the pearlite structure, the toughness was extremely low and the application range was extremely limited.
[0006]
In order to eliminate such drawbacks, a method has been proposed in which a small amount of Ti is added to prevent coarsening of crystal grains and to improve toughness (for example, JP-A-56-38448). Not necessarily stable.
[0007]
For hot forging, finely and uniformly disperse at least one of MnS, TiN, and ZrN by adding a small amount of one or more of Zr and Ti and securing a cooling rate in the casting process. A method for producing non-heat treated steel has been proposed (Japanese Patent Publication No. 6-75747).
[0008]
Furthermore, while adding carbon, nitride forming elements such as Al, V, Nb, Ti, B, etc., 0.001 to 0.04 wt% of Ca, Te, Ce and other rare earth metals, misch metals and mixtures thereof A high toughness non-heat treated steel and a method for producing the same have been proposed (Japanese Patent Laid-Open No. 6-340946).
[0009]
Japanese Patent Publication No. 4-70385 proposes a method for improving the strength and toughness of hot forged materials by adding Zr and B, and a method for improving toughness by adding rare earth elements. Yes.
[0010]
[Problems to be solved by the invention]
The present invention provides a non-heat treated steel having a high toughness level that cannot always be achieved with a conventional non-heat treated steel even if the heat treatment after hot forging is omitted in the above-mentioned machine part manufacturing process. It is.
[0011]
[Means for solving the problems]
In the machine part manufacturing process described above, the present inventors have obtained a non-tempered steel having a level of high toughness that cannot always be achieved with conventional non-tempered steel even if the tempering treatment after hot forging is omitted. Through various researches on the means to achieve this, the steel composition is controlled appropriately, and sulfides, oxides, and oxide / sulfide composite inclusions are reduced in size, and the aspect ratio of these inclusions is reduced. By controlling (length / width) to a certain extent or less, it is found that high toughness can be stably achieved even if the tempering treatment is omitted after hot forging, and specific means for realizing them are found, The present invention has been completed.
[0012]
The gist of the present invention is as follows.
[0013]
(1) In mass%,
C: 0.1-0.6%
Si: 0.01 to 2.0%,
Mn: 0.2 to 2.0%,
S: 0.005 to 0.5%,
Cr: 0.1 to 2.0%,
Ti: 0.005 to 0.1%,
N: 0.003 to 0.02%,
Al: 0.07% or less (including 0%),
Sn: 0.008 to 0.1%,
Furthermore,
Rem (rare earth metal): 0.005 to 0.1%,
And Ca: 0.0005 to 0.005%
And including
(Rem% + Ca%) / (S%) ≧ 0.05
And the balance is composed of the remaining Fe and inevitable impurities, the average equivalent circle diameter of inclusions is 20 μm or less, and the average aspect ratio (length / width) of inclusions is 10 or less. High toughness non-tempered steel for hot forging.
[0014]
(2) Furthermore, in mass%,
Zr: 0.007 to 0.1 %
And including
(Rem% + Ca% + 2 × Zr %) / ( S %) ≧ 0.05
The high toughness non-tempered steel for hot forging according to the above (1), characterized by satisfying the relationship:
[0015]
(3) Furthermore, in mass%,
V: 0.01 to 0.5 %
The high toughness non-tempered steel for hot forging according to the above (1) or (2), which contains
(4) Furthermore, in mass%,
V: 0.01-0.5%
Mg: 0.0005 to 0.01%
And including
(Rem% + Ca% + Mg%) / (S%) ≧ 0.05
The high toughness non-tempered steel for hot forging according to the above (1), characterized by satisfying the relationship:
(5) Furthermore, in mass%,
Nb: 0.01-0.3% and B: 0.0003-0.005%
One or two of these,
Pb: 0.01 to 0.3% and Bi: 0.01 to 0.3%
One or two of these,
The high toughness non-heat treated steel for hot forging according to any one of the above (1) to (4), comprising:
[0016]
( 6 ) Furthermore, in mass%,
Ni: 0.01 to 2.0%,
Mo: 0.01 to 1.0%
The high toughness non-tempered steel for hot forging according to any one of the above (1) to ( 5) , comprising one or two of the two types.
[0017]
( 7 ) Furthermore, in mass%,
Pb: 0.01-0.3%
Bi: 0.01-0.3 %
The high toughness non-heat treated steel for hot forging according to any one of (1) to (4) and (6) above , comprising at least one of the above .
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The requirements of each claim of the present invention will be described below.
[0019]
First, the reason why the steel material components are defined in claim 1 of the present invention is as follows.
[0020]
C is an essential component for obtaining the strength of the steel material. If it is less than 0.1%, the characteristics as a machine structural part cannot be obtained, and if it exceeds 0.6%, the mechanical characteristics of the hot forged product are not obtained. The variation becomes large and it is difficult to obtain stable quality.
[0021]
Si is also an element useful for securing the strength, but at least 0.01% or more is necessary for increasing the strength, and if it exceeds 2.0%, the embrittlement of the ferrite ground becomes severe. The content should be kept below 2.0%.
[0022]
Mn has a strong toughening effect and is a useful element like Si, and if it is less than 0.2%, the hot ductility is low, and it is difficult to hot-roll and hot-work steel materials. Further, if it exceeds 2.0%, the machinability, weldability, and the like are lowered, so the limit is imposed.
[0023]
S is an element that improves machinability, and if it is less than 0.005%, the machinability is low and cutting is difficult, and non-heat treated steel promotes the formation of ferrite in the cooling process after hot forging. While the toughness is improved by, for example, the increase in MnS causes the deterioration of mechanical properties and the increase in anisotropy. If it exceeds 0.5%, the mechanical properties deteriorate and the anisotropy increases, and the hot ductility becomes extremely low.
[0024]
Cr is also an element effective in increasing the strength, but at least 0.1% or more is necessary for increasing the strength, and if it exceeds 2.0%, the toughness is remarkably reduced, so the upper limit was made 2.0%. .
[0025]
Ti is an element that refines the austenite structure to improve toughness, and for that purpose, at least 0.005% or more is necessary. On the other hand, if it exceeds 0.1%, coarse TiN or the like is generated and toughness is increased. Therefore, the upper limit was set to 0.1%.
[0026]
N is an element useful for forming a nitride with a nitride-forming element such as Al or Ti to refine the austenite crystal grains. In order to exhibit the above effect, at least 0.003% or more of N is necessary, and even if the content exceeds 0.02%, the above effect is saturated, so the upper limit of N is 0.02%. did.
[0027]
Al is generally an element useful for deoxidation and control of crystal grains, but Al is not necessarily added in the case of containing Si, Mn and Ti. The amount of Al added contributes to deoxidation and crystal grain control, and when the Al content exceeds 0.07%, hot workability is impaired. Therefore, the Al range is set to 0.07% or less (including 0%).
[0028]
Sn dissolves in ferrite and contributes to the improvement of strength. In the steel of the present invention, the increase in primary crystal ferrite during solidification and the effect of suppressing the discharge of S into the dendrite trees, and between the dendrite trees of Sn itself. Concentration to the existing liquid phase contributes to miniaturization of the sulfide crystallized in the final solidified part, and also by hot rolling and hot forging by controlling the composition of sulfide by adding Rem, Ca, etc. described later. It has been found that when the aspect ratio of the sulfide is reduced by preventing the stretching of the film, the effect is increased. Such an effect is obtained by containing at least 0.008% or more of Sn as described in Examples. Moreover, since hot ductility will fall remarkably when Sn content exceeds 0.1%, the upper limit was made into 0.1% or less. Even in the range of the Sn content, when a decrease in hot ductility due to Sn becomes a problem, it is preferable to add Ni to ensure hot ductility.
[0029]
As a result of various investigations by the present inventors, sulfides such as MnS stretched by hot rolling or the like not only cause an increase in mechanical anisotropy as conventionally known, but also greatly reduce the toughness. It turns out that it has an influence.
[0030]
Therefore, we investigated how to prevent the decrease in toughness due stretched sulfide, Rem (rare Ruikin Genus): 0.005% to 0.1% and Ca: 0.0005 to 0.005% is contained, and, When (Rem% + Ca%) / is contained so that the (S%) ≧ 0.05, MnS oxide and sulphide or sulphide R em and C a forms a solid solution is changed to inclusions complexed As a result, the hot plasticity is greatly reduced compared to MnS, and the inclusions can be extremely effectively prevented from being stretched like MnS by hot rolling or hot forging. As a result, due to the stretched MnS, etc. It has been found that the reduction in toughness can be greatly improved.
[0031]
Also, in the case of adding Rem and Ca so as to satisfy the above condition, that the oxide of approximately spherical shape having containing an R em or Ca is generated, which contributes to such morphological change improvement in low temperature toughness Turned out .
[0032]
The addition of Re m and C a, In order to effectively suppress the stretching during hot working of sulfides, Rem is required than 0.005%, it is necessary to add Ca also less than 0.0005% On the other hand, if Rem is added at 0.1% or more and Ca is added at 0.005% or more, sulfide clusterers containing these elements are likely to appear, and the toughness is lowered by their formation. Furthermore, even if the amount of Rem or Ca added is insufficient compared to the amount of S, it is not possible to sufficiently prevent stretching during hot working of sulfide, so that (Rem% + Ca%) / (S%) ≧ 0.05 is contained. It is necessary to let
[0033]
Further, in the non-heat treated steel for hot forging, the average value of the equivalent circle diameter of inclusions is set to 20 μm or less, and the average value of the inclusion aspect ratio (length / width) is set to 10 or less. When controlled, the notch effect due to inclusions such as the presence of elongated sulfide inclusions when an impact is applied is greatly mitigated, and the effect of increasing the propagation resistance of cracks, etc. Can reduce the toughness due to the above.
[0034]
The inclusions mentioned here are oxides, sulfides, and inclusions in which oxides and sulfides are compounded, including oxides and sulfides containing Rem and Ca, and complex inclusions thereof. .
[0035]
In order to reduce the size of the inclusions, the components are controlled within the range defined in claim 1, and in the casting process for producing a steel ingot or slab, inclusions such as sulfides are solidified during solidification. In the case of crystallization, it is also effective to suppress the growth of inclusions that crystallize during solidification by securing a cooling rate in a temperature range of 1500 ° C. to 1300 ° C. to some extent.
[0036]
The smaller the cross-sectional size of the slab or steel ingot, the easier it is to secure the cooling rate during solidification, and the growth of inclusions that crystallize during solidification, such as MnS, is suppressed, and the inclusions can be made smaller.
[0037]
In addition, since inclusions having high temperature plasticity such as sulfides are stretched by hot working as the size increases, it is also possible to control the inclusion size to be small in the casting stage by the above-described means. This is effective in suppressing stretching during rolling or hot forging.
[0038]
Further, when the amount of addition of Rem or Ca is restricted for some reason and stretching of sulfides and the like due to hot working cannot be sufficiently prevented, Zr described in claim 2: 0.007 to 0.1 % is contained And (Rem% + Ca% + 2 × Zr %) / ( S %) ≧ 0.05 or Mg according to claim 4: 0.0005 to 0.01%. In addition, (Rem% + Ca% + Mg%) / (S%) ≧ 0.05 is added to reduce the plasticity of the sulfide in order to more reliably prevent stretching during hot working. It is valid.
[0039]
In order to prevent the stretching of sulfides during hot working by adding Zr and Mg, it is necessary to add 0.007% or more of Zr and 0.0005% or more of Mg. If Zr exceeds 0.1%, hard oxides such as ZrO ₂ increase, which may cause toughness to decrease. The upper limit of the Zr content is set to 0.1%. Even if Mg is added in excess of 0.01%, it is consumed in the formation of MgS in the molten steel, and it will not work effectively for the form control of inclusions that crystallize during solidification. 0.01%.
[0040]
When further improvement of strength and toughness is required , addition of V as in claim 3 or 4 and addition of Nb and B as in claim 5 are effective.
[0041]
V and Nb are elements that improve the strength and toughness by forming charcoal and nitride, and Nb also suppresses the recrystallization of austenite during hot working and promotes the refinement of crystal grains. , Improve strength toughness. When V and Nb are added in an amount of 0.01% or more, the above improvement effect appears. When the amount of V exceeds 0.5%, the hot ductility decreases when the amount of Nb also exceeds 0.3%. Therefore, the upper limit of each addition amount was set to 0.5% and 0.3%.
[0042]
On the other hand, B is a non-tempered steel and has the effect of promoting the formation of ferrite and improving the hardness, but the effect appears at 0.0003% or more, and when it exceeds 0.005%, hot ductility and Since the toughness also decreases, the amount of B added is limited to 0.005%.
[0043]
When it is necessary to improve hardenability and toughness, addition of Ni and Mo is effective as described in claim 6 .
[0044]
Ni and Mo are effective elements for ensuring hardenability and improving toughness, and in order to obtain these effects, it is necessary to add at least 0.01% or more, and Ni exceeds 2.0%. Even if the addition of Mo exceeds 1.0%, the effect is saturated, so the upper limit of Ni is 2.0% and the upper limit of Mo is 1.0%.
[0045]
When there is a need for improvement of machinability, the addition of Pb and Bi as in claim 7 is effective for improving machinability.
[0046]
To obtain the machinability improving effect, it is necessary to more than the added pressure of 0.01% in the Pb and Bi, on the other hand, in order to degrade significantly the hot ductility which element is also added in excess of 0.3% Pb and the upper limit of the addition amount of 2 elements both of Bi is 0.3%.
[0047]
【Example】
Example 1
In this example, in order to investigate and contrast the tensile strength and low-temperature toughness of the invention steels corresponding to claims 1 and 5 and the corresponding comparative steels, 150 kg steel having the chemical composition shown in Table 1 was used. The ingot was melted in a vacuum melting furnace and the cooling rate when the steel ingot solidified was greatly changed. The ingots of the steel of the present invention and some comparative steels were allowed to cool in the mold. In the melting of 9, 10 steel ingots, the mold is placed in a heat insulation box using a heat insulating material such as cahors, and the surroundings of the mold placed in the heat insulation box are heated by gas heating, and the cooling rate is greatly reduced. The inclusions crystallized during the solidification of sulfides and the like were coarsened.
[0048]
The manufactured steel ingot was heated to 1250 ° C., then formed into a 90φ round bar by hot forging, and then cooled to room temperature, and an inclusion investigation sample was collected. Thereafter, the 90φ round bar was heated again to 1250 ° C. and hot forged to form a 50φ round bar. After forging was completed, air cooling was performed from 1100 ° C. to room temperature. JIS No. 4 tensile test pieces and JIS No. 3 impact test pieces were collected by cutting from the 50φ round bar thus obtained, and subjected to tensile tests and Charpy tests at room temperature to evaluate strength and low-temperature toughness. The results of each test are shown in Table 1. In addition, in the measurement of the inclusion diameter and the aspect ratio (length / width) of the inclusions, the optical microscope is used to observe the inclusions existing in the vertical section of the 90φ round bar about 20 fields of view at a magnification of 400 times. All inclusions in the photograph were analyzed with an image analyzer, and the average value of the equivalent circle diameter and aspect ratio (length / width) of the inclusions was determined.
[0049]
[Table 1]

[0050]
As is clear from Table 1, the steel that had been air-cooled after hot forging using the steel of the present invention showed higher tendencies in both strength and low-temperature toughness than those that were hot-forged using comparative steel and then air-cooled. It was. Conversely, even if the Sn, Rem, Ca content and the ratio of (Rem + Ca) / S deviate from the range defined in claim 1 of the present invention or the component system satisfies the specified range, the average circle of inclusions It can be seen that when the equivalent diameter and the average aspect ratio are out of the specified range, the strength and toughness deteriorate compared to the steel of the present invention.
[0051]
In addition, comparative steel No. 1 which greatly reduced the cooling rate during solidification. Nos. 9 and 10 and steel Nos. 1 to 3 and 6 are compared, comparative steel No. In No. 9, the average equivalent circle diameter and average aspect ratio of inclusions were 10 shows that the average equivalent circle diameter of inclusions is larger than that of the steel according to the present invention, and the strength and impact value are reduced due to this. Comparative steel No. No. 12, the ratio of (Rem + Ca) / S does not satisfy the range of claim 1, so that the average aspect ratio of inclusions is large, and the strength and impact value are considerably lower than those of the inventive steel.
[0052]
(Example 2)
Steels having chemical compositions shown in Table 2 were melted by a blast furnace and a converter method, cast into a 220 mm square bloom slab, and then formed into 90φ round bars by split rolling and bar rolling.
[0053]
The round bar obtained by the above method was heated to 1250 ° C. and then hot forged, formed into a 50φ round bar, and then air-cooled from 1100 ° C. to room temperature after forging was completed. From the material thus obtained, as in Example 1, JIS No. 4 tensile test piece and JIS No. 3 impact test piece were collected by cutting and subjected to tensile test and Charpy test at room temperature. The results of each test are also shown in Table 2.
[0054]
[Table 2]

[0055]
In this test as well, the average equivalent circle diameter of inclusions and the average value of the aspect ratio of inclusions were measured on a vertical section of a 90φ round bar by the same method as that used in Example 1.
[0056]
The invention steel corresponding to claim 1 is No. 1. No. 1, 4, 10, and 11 of the invention steel. 12, 13, 15, 21, 22, 23 are comparative steels. The invention steel corresponding to claim 2 is No. No. 8 and 9 invention steel. 19 and 20 are comparative steels. No. The steel materials 3, 5, 6, and 9 are invention steels corresponding to claim 3, and the comparative steel is No. 3. 14, 16, 17, and 20 steel materials. Further, invention steels corresponding to claims 4 and 5 are No. 7 and no. No. 5 and No. 6, and the corresponding comparative steels are No. 18 and No. 16 and 17.
[0057]
Also in this example, when hot forging using the invention steel corresponding to each claim, the tensile strength and toughness at room temperature are higher than when using the comparative steel corresponding to each invention steel. In this example, it was confirmed that the steel of the present invention is an excellent non-heat treated steel for hot forging.
[0058]
Further, as can be seen from Table 2, even in this example, the component system does not satisfy the range of each claimed steel, or even if the component system satisfies the specified range, the average equivalent circle diameter of other inclusions or It can be seen that when the average aspect ratio is outside the specified range, the strength and toughness are considerably reduced as compared with the steel of the present invention.
[0059]
【The invention's effect】
As described above, with the steel of the present invention, high strength and toughness values can be obtained without performing a tempering treatment, and it is possible to significantly reduce energy and manufacturing cost by omitting the tempering treatment.

Claims

% By mass
C: 0.1-0.6%
Si: 0.01 to 2.0%,
Mn: 0.2 to 2.0%,
S: 0.005 to 0.5%,
Cr: 0.1 to 2.0%,
Ti: 0.005 to 0.1%,
N: 0.003 to 0.02%,
Al: 0.07% or less (including 0%),
Sn: 0.008 to 0.1%,
Furthermore,
Rem (rare earth metal): 0.005 to 0.1%,
And Ca: 0.0005 to 0.005%
And including
(Rem% + Ca%) / (S%) ≧ 0.05
And the balance is composed of the remaining Fe and inevitable impurities, the average equivalent circle diameter of inclusions is 20 μm or less, and the average aspect ratio (length / width) of inclusions is 10 or less. High toughness non-tempered steel for hot forging.

Furthermore, in mass%,
Zr: 0.007 to 0.1%
And including
(Rem% + Ca% + 2 × Zr%) / (S%) ≧ 0.05
The high toughness non-tempered steel for hot forging according to claim 1, wherein:

Furthermore, in mass%,
V: 0.01 to 0.5%
The high toughness non-tempered steel for hot forging according to claim 1 or 2, characterized by comprising:

Furthermore, in mass%,
V: 0.01-0.5%
Mg: 0.0005 to 0.01%
And including
(Rem% + Ca% + Mg%) / (S%) ≧ 0.05
The high toughness non-tempered steel for hot forging according to claim 1, wherein:

Furthermore, in mass%,
Nb: 0.01-0.3% and B: 0.0003-0.005%
One or two of these,
Pb: 0.01 to 0.3% and Bi: 0.01 to 0.3%
One or two of these,
The high toughness non-tempered steel for hot forging according to claim 1, comprising:

Furthermore, in mass%,
Ni: 0.01 to 2.0%,
Mo: 0.01 to 1.0%
The high toughness non-tempered steel for hot forging according to any one of claims 1 to 5, comprising one or two of the two types.

Furthermore, in mass%,
Pb: 0.01-0.3%
Bi: 0.01 to 0.3%
The high toughness non-heat-treated steel for hot forging according to any one of claims 1 to 4 and 6, characterized by containing at least one of the above.