JP3874931B2

JP3874931B2 - Steel plate for processing with excellent weld strength and its manufacturing method

Info

Publication number: JP3874931B2
Application number: JP14372598A
Authority: JP
Inventors: 裕一肥後; 征一浜中; 進藤原; 広宣本田
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1998-05-26
Filing date: 1998-05-26
Publication date: 2007-01-31
Anticipated expiration: 2018-05-26
Also published as: JPH11335774A

Description

【０００１】
【産業上の利用分野】
本発明は、自動車用，建材用等に使用される溶接部強度に優れた加工用鋼板及びその製造方法に関する。
【０００２】
【従来の技術】
自動車の車体や建材等に使用される鋼板には、プレス成形性，深絞り性を始めとして厳しい加工性が要求される。加工性は、Ｃ含有量の低減やＴｉ，Ｎｂ等の添加によって改善されるが、Ｃ含有量を低減すると溶接熱影響部が粗粒化し易く、アーク溶接，スポット溶接等で組み立てられた構造体の強度が低下する。
溶接熱影響部の粗粒化による強度低下を防止するため、特開昭６３−２０６４５１号公報では極低炭素冷延鋼板に含まれるＴｉ量を規制している。しかし、強度低下の防止には、非常に多量のＴｉ添加が必要とされ、製造コストの上昇を招く。多量のＴｉ添加は、却って延性を劣化させる原因にもなる。
溶接熱影響部の粗粒化自体は、Ｍｇを単独添加し、或いはＭｇ，Ｔｉを複合添加してＭｎ−Ｓｉ−Ｍｇ系酸化物やＴｉ−Ｍｇ系酸化物を鋼中に分散させるとき防止されることが特開平８−２３２０４４号公報，特開平９−３１０１１９号公報，特開平９−３１０１４７号公報等に紹介されている。
【０００３】
【発明が解決しようとする課題】
特開平８−２３２０４４号公報の鋼材は、大型建造物等に使用される熱延鋼板を対象としたＣ含有量が比較的多い鋼種であり、微量のＭｇを含ませ、Ｍｎ−Ｓｉ−Ｍｇ系酸化物によって大入熱溶接時に熱影響部の粗粒化を抑制している。特開平９−３１０１４７号公報の鋼材も、同様に大型建造物等に使用される熱延鋼板を対象としたＣ含有量が比較的多い鋼種であり、Ｔｉ−Ｍｇ合金を溶鋼に添加してＴｉ−Ｍｇを主体とする粒径０．０５〜０．５μｍの複合酸化物を析出させることにより熱影響部の粗粒化を抑制している。
しかし、Ｔｉ−Ｍｇ系の酸化物は、単独での粒径が１μｍ以下であってもクラスターを生成し易い。クラスターが生成すると、結晶粒の粗大化を抑制する作用が弱くなる。この影響は、加工性改善のためにＣ含有量を低下した鋼種ほど顕著に現れ、スポット溶接やアーク溶接等の入熱量が少ない溶接による熱影響部を粗粒化し、溶接強度を低下させる原因となる。
【０００４】
【課題を解決するための手段】
本発明は、このような問題を解消すべく案出されたものであり、微細なＭｇＯを鋼中に分散・析出させることにより、Ｃ含有量が低い鋼種にあっても溶接熱影響部の結晶粒の粗大化を防止し、溶接部強度，加工性等に優れた鋼板を提供することを目的とする。
本発明の加工用鋼板は、その目的を達成するため、Ｃ：０．０００５〜０．０１０重量％，Ｓｉ：０．０２〜１．０重量％，Ｍｎ：０．１〜２．５重量％，Ｐ：０．００５〜０．１重量％，Ｓ：０．０２重量％以下，Ｍｇ：０．００１重量％以上０．０１重量％未満，Ａｌ：０．０２重量％以下を含み、残部がＦｅ及び不可避的不純物からなる組成で、１μｍ以下の微細なＭｇＯが均一に分散析出した組織をもつことを特徴とする。この加工用鋼板は、必要に応じＴｉ及び／又はＮｂを合計で０．０２〜０．１０重量％含むことができる。更には、Ｂ：０．０００５〜０．００５重量％を添加してもよい。
この加工用鋼板は、前掲した組成をもつスラブを仕上げ温度８５０℃以上，巻取り温度６００〜７５０℃で熱間圧延した熱延鋼帯として製造される。或いは、熱延鋼帯に酸洗，冷延率５０〜８５％の冷間圧延を施し、更に均熱温度６５０〜８８０℃で連続焼鈍することにより製造される。また、酸洗性を向上させるため、酸洗に先立って熱延鋼帯を高圧下冷延してもよい。この場合には、酸洗後の冷間圧延における冷延率と合わせた合計冷延率を５０〜８５％の範囲に調整する。
【０００５】
【作用】
本発明者等は、溶接時の熱影響による結晶粒の粗大化に及ぼす酸化物系介在物の影響を調査・研究した。その結果、前掲のＭｎ−Ｓｉ−Ｍｇ系やＴｉ−Ｍｇ系酸化物等は、比較的粗大な複合酸化物の形態をとり、Ｃ含有量を下げた鋼種では結晶粒の粗大化抑制に十分でないことが判明した。すなわち、本発明が対象としている薄鋼板をスポット溶接又はアーク溶接するとき、Ｃ含有量が０．０１重量％を超える鋼種にあっては固溶Ｃの影響や低いγ→α変態温度の影響を受け、酸化物が分散していなくても、溶接強度に悪影響を及ぼす粗粒化が発生しない。
ところが、結晶粒は一般にＣ含有量が低いものほど粗大化し易く、Ｃ含有量が０．０１重量％以下になると大入熱溶接だけでなく単純なスポット溶接やアーク溶接等によっても熱影響部の粗粒化が生じる。この粗粒化は、必然的に粗大な複合酸化物の形態をとるＭｎ−Ｓｉ−Ｍｇ系やＴｉ−Ｍｇ系酸化物等では十分に抑制できない。そこで、本発明者等は、鋼中に微細なＭｇＯを分散析出させることにより、入熱量が僅かな溶接法でみられる結晶粒の粗大化を防止することを検討した。その結果、各合金成分の含有量を特定し、更には製造条件を特定するとき、結晶粒の粗大化防止に有効に働く微細なＭｇＯが鋼中に均一分散することが判った。
【０００６】
以下、本発明の加工用鋼板に含まれる合金成分，含有量，製造条件等を説明する。
Ｃ：０．０００５〜０．０１０重量％
Ｃ含有量の低減に起因する溶接熱影響部の粗粒化は、０．０１０重量％を超えるＣを含む鋼種では発生しなくなる。しかし、本発明は、溶接熱影響部の粗粒化が問題となる低炭素鋼を対象としたものであることから、Ｃ含有量の条件を０．０１０重量％に設定した。Ｃ含有量の低減は、加工性を改善する上では有効である。しかし、Ｃ含有量を０．０００５重量％未満にすることは、大量生産ラインでは非常に困難であり、製造コストを上昇させる原因になる。
Ｓｉ：０．０２〜１．０重量％
高強度化に有効な合金成分であり、０．０２重量％以上の含有量でＳｉの作用が顕著になる。しかし、１．０重量％を超える多量のＳｉが含まれると、低温靭性を劣化させる傾向がみられる。
【０００７】
Ｍｎ：０．１〜２．５重量％
高強度化に有効な合金成分であり、０．１重量％以上の含有量でＭｎの作用が顕著になる。しかし、２．５重量％を超える多量のＭｎが含まれると、延性や低温靭性を劣化させる傾向がみられる。
Ｐ：０．００５〜０．１重量％
高強度化に有効な合金成分であり、０．００５重量％以上の含有量でＰの作用が顕著になる。しかし、０．１重量％を超える多量のＰが含まれると、低温靭性が大幅に低下する傾向がみられる。
Ｓ：０．０２重量％以下
熱間での延性を低下させる有害元素であり、低ければ低いほと好ましい。しかし、本発明の加工用鋼板にあっては、Ｓ含有量を０．０２重量％以下にするとき、Ｓ起因の悪影響が現れない。
【０００８】
Ｍｇ：０．００１〜０．０２重量％
１μｍ以下のＭｇＯとして鋼中に析出し、溶接熱影響部の粗粒化を防止する作用を呈する合金成分である。１μｍ以下の微細なＭｇＯを十分に析出させるためには、Ｍｇ添加量を０．００１重量％以上にする必要がある。０．００１重量％未満のＭｇ含有量では、ＭｇＯの析出量が不足し、ＭｇＯに比較して粗大なＭｎ−Ｓｉ−Ｍｇ系酸化物，Ａｌ₂ Ｏ₃ ，ＳｉＯ₂ 等の酸化物が析出するため、十分な粗粒化防止効果が得られない。Ｍｇの作用は含有量０．０２重量％で飽和し、それ以上添加しても却って鋼材コストを上昇させることになる。なお、本願では、０．０１重量％未満のＭｇを含有するものについてのみ、請求している。
Ｍｇの添加形態は特に拘束されるものではないが、金属ＭｇをＣａＯと混合して鉄皮に包んだＭｇ含有ワイヤを溶鋼中に連続投入するワイヤフィード法によるＭｇ脱酸で添加することができる。この場合、溶鋼中のフリー酸素が高い状態でＭｇが添加されるため、脱酸生成物としてＭｇＯが得られる。
或いは、Ａｌ，Ｍｎ，Ｓｉ等である程度脱酸した後、Ｆｅ−Ｓｉ−Ｍｇ等の合金としてＭｇを添加することもできる。このＭｇ添加方法は、ワイヤフィード法等による溶鋼の直接Ｍｇ脱酸に比較してＭｇの歩留が良いためコスト的に有利であり、Ａｌ，Ｍｎ，Ｓｉ系酸化物として鋼中に存在していた酸化物のＭｇ還元によりＭｇＯが生成する。
何れの添加方法によっても、Ｍｇ添加量が本発明で規定した条件を満足するとき、約１μｍ以下の微細なＭｇＯが鋼中に分散・析出したスラブが得られる。微細なＭｇＯ析出物は、結晶粒界の移動を阻止するピン止め作用を呈し、溶接熱影響部の粗粒化が抑制されるものと推察される。
【０００９】
Ａｌ：０．０２重量％以下
脱酸剤として添加される合金成分であり、Ｍｇ添加による脱酸量を調整するため０．０２重量％以下の範囲で補足的に添加される。なお、Ｍｇ添加による脱酸が十分な場合には、Ａｌ添加を省略することも可能である。
Ｔｉ及び／又はＮｂ：合計で０．０２〜０．１０重量％
必要に応じて添加される合金成分であり、鋼中の固溶Ｃを固定して鋼板の延性を改善する作用を呈する。延性改善効果は、Ｔｉ及び／又はＮｂの合計添加量が０．０２重量％以上で顕著になる。しかし、０．１０重量％を超える合計添加量でＴｉ及び／又はＮｂを添加すると、延性の低下や鋼材コストの上昇等の原因となる。
Ｂ：０．０００５〜０．００５重量％
ＴｉやＮｂを添加して加工性を改善した鋼板にあっては、固溶Ｃの減少に伴って粒界強度が低下し、過酷な条件下でプレス加工される用途等では加工後の耐二次加工割れ性が劣化する虞れがある。耐二次加工割れ性は、更に微量のＢを添加することにより改善される。Ｂの添加効果は、０．０００５重量％以上で顕著になり、０．００５重量％で飽和する。Ｂを０．００５重量％を超えて添加しても、却って鋼材コストを上昇させる。
【００１０】
熱間圧延：仕上げ温度８５０℃以上，巻取り温度６００〜７５０℃
以上のように成分設計された鋼材を溶製した後、連続鋳造で得られたスラブを熱間圧延する。熱間圧延では、仕上げ温度が８５０℃以上となる条件が採用される。仕上げ温度が８５０℃を下回ると、熱間変形抵抗が大きく変動し、ゲージハンチング等の形状不良や、場合によっては板切れ等の製造上のトラブルが発生し易くなる。熱間圧延された鋼帯は、ＡｌＮを十分に析出させるため６００℃以上の温度で巻き取られる。巻取り温度が６００℃未満では、ＡｌＮの析出が十分でなく、また微細なＴｉＣ等の析出によって冷延鋼板の延性及びランクフォード値が低下する。逆に、７５０℃を超える温度で巻き取ると、自重によりコイルが変形する虞れがあり、酸洗性も劣化する。
【００１１】
冷間圧延：冷延率５０〜８５％
熱延鋼帯は、連続酸洗ラインで酸洗された後、冷間圧延される。或いは、酸洗に先立って熱延鋼帯を高圧下冷延するとき、酸洗工程の負荷が軽減される。
冷間圧延工程では、後続する焼鈍時に再結晶を促進させるため、冷延率が５０％以上に調整される。５０％未満の冷延率では、再結晶温度が上昇すると共に再結晶後の結晶粒が粗大化し、加工後の鋼板表面に肌荒れが発生し易くなる。
しかし、８５％を超える高冷延率の冷間圧延は、タンデム式の圧延機においても１パス通板では困難であり、複数パスが必要になるため製造コストを上昇させる。
【００１２】
連続焼鈍：均熱温度６５０〜８８０℃
冷延鋼帯は、連続焼鈍ラインに通板され、均熱温度６５０〜８８０℃で再結晶焼鈍される。均熱温度が６５０℃に達しないと、再結晶が十分に進行せず、得られた鋼板の延性が低下する。逆に、８８０℃を超える高温焼鈍では、極低炭素鋼であっても一旦はγ変態が生じるため、鋼板の延性が低下する。
焼鈍された鋼帯は、そのままでも使用されるが、溶融Ｚｎめっき，溶融Ａｌめっき等による防錆処理を施しても良い。
【００１３】
【実施例】
実施例１：
表１に示した組成のスラブを連続鋳造法で製造した。なお、Ｍｇは、Ｍｇ＋ＣａＯ含有ワイヤを取鍋中の溶鋼に連続投入するＭｇ脱酸法で添加した。一部のスラブについては、Ｍｇ＋ＣａＯ含有ワイヤの投入に先立ってＡｌで溶鋼を予備脱酸した。
【００１４】

【００１５】
各スラブを１１８０℃まで加熱し、仕上げ温度９００℃，巻取り温度６５０℃の条件で熱間圧延し、板厚２．３ｍｍの熱延鋼帯を得た。
熱延鋼帯を酸洗した後、ＭＡＧ溶接による突合せ溶接試験に供した。試験片の溶接部を中央にしてＪＩＳ５号引張試験片を切り出し、引張試験に供した。試験結果を溶接部のない素材鋼板の試験結果と比較することにより、溶接による強度低下の有無を調査し、引張試験による破断位置を調査した。
また、熱延鋼帯から圧延方向と平行にＪＩＳ３号サブサイズ衝撃試験片を切り出し、−１０℃のシャルピー衝撃試験に供した。試験後の破面を観察し、延性破面が８０％以上の場合を○，８０％未満の場合を×として低温靭性を評価した。
更に、Ｂ添加が耐二次加工割れ性に及ぼす影響を調査するため、鋼種番号２０〜２３については、熱延鋼帯を総絞り比２．１でカップ成形した後、カップの耳を切断してバリを研磨，除去し、−３０℃に冷却した冷媒中でカップを横方向から押し潰した。そして、カップが破壊することなく変形したものを○，カップの壁部が脆性破壊したものを×として、耐二次加工割れ性を評価した。
調査結果を、本発明の請求項１〜３に対応させて表２〜４にそれぞれ示す。
【００１６】

【００１７】
各合金成分が本発明で規定した範囲にある鋼種番号１〜７の鋼板は、突合せ溶接部の引張試験でも強度低下がみられず、破断位置が何れも母材部になっていた。また、低温靭性にも優れていた。
これに対し、Ｍｇを添加していない鋼種番号８では、引張試験の結果、母材部に比較して突合せ溶接部の強度が低下し、伸びも素材鋼板に比較して低い値を示した。破断位置も溶接熱影響部になっており、溶接時の入熱で結晶粒が粗大化した影響が窺われる。本発明で規定した範囲を超えるＣを含む鋼種番号９では、Ｍｇ添加していなくても引張試験で強度低下が検出されず、破断位置も母材部になっていた。Ｓｉ，Ｍｎ，Ｐの強化元素を本発明で規定した範囲を超えて含有する鋼種番号１０〜１２では、延性及び低温靭性に劣っていた。
【００１８】

【００１９】
Ｔｉ，Ｎｂを添加した本発明の請求項２に対応する鋼種番号１３〜１６の鋼板は、Ｔｉ，Ｎｂ以外の合金成分をほぼ同量含む鋼種番号２と比較して延性に優れており、鋼中の固溶Ｃ，Ｎを固定するＴｉ，Ｎｂの添加効果が窺われる。しかも、引張試験では母材部で破断したことから突合せ溶接部の強度低下がみられず、ＭｇＯの微細分散が熱影響部の粗粒化を有効に抑制していることが窺われる。
これに対し、Ｍｇ無添加の鋼種番号１７では、Ｔｉ添加によって延性が良好となる傾向はみられるが、引張試験によって熱影響部が破断し、突合せ溶接部の強度が低下していた。Ｔｉ，Ｎｂを過剰に添加した鋼種番号１８，１９では、延性の更なる向上はみられず、却って延性が若干低下していた。
【００２０】

【００２１】
Ｂを添加した鋼種番号２０，２１では、優れた耐二次加工割れ性からＢ添加の効果が窺われる。また、突合せ溶接部も強度低下しておらず、低温靭性にも優れていた。
他方、Ｍｇ無添加の鋼種番号２２では、Ｂ添加によって耐二次加工割れ性は改善されているが、引張試験により熱影響部に破断が生じ、突合せ溶接部の強度が低下していた。また、Ｂ無添加の鋼種番号２３では、突合せ溶接部の強度低下はみられないが、耐二次加工割れ性に劣っていた。
【００２２】
実施例２：
表１に示した組成のスラブを連続鋳造法で製造した。なお、Ｍｇは、Ｍｇ＋ＣａＯ含有ワイヤを取鍋中の溶鋼に連続投入するＭｇ脱酸法で添加した。一部のスラブについては、Ｍｇ＋ＣａＯ含有ワイヤの投入に先立ってＡｌで溶鋼を予備脱酸した。
各スラブを１１８０℃まで加熱し、仕上げ温度９００℃，巻取り温度６５０℃の条件で熱間圧延し、板厚３．２ｍｍの熱延鋼帯を製造した。
熱延鋼帯を連続酸洗ラインに通板して酸洗した後、タンデム式の冷間圧延機を用いて圧下率７５％で冷間圧延し、板厚０．８ｍｍの冷延鋼帯にした。次いで、連続焼鈍ラインにおいて８３０℃×均熱６０秒で連続焼鈍した。焼鈍された鋼帯に、更に伸び率０．８％の調質圧延を施した。
【００２３】
調質圧延後の鋼帯から圧延方向と平行にＪＩＳ５号試験片を切り出し、引張試験により機械的性質を調査した。
また、同じ鋼帯から切り出された５０ｍｍ×２００ｍｍの短冊状試験片を２枚１組とし、電極チップ径５．５ｍｍ，加圧力２２０ｋｇ，電流７．６ｋＡ，溶接時間１０秒の条件下、４０ｍｍ間隔で５点スポット溶接した。そして、中間の３点について溶接部の断面を顕微鏡観察し、熱影響部の組織を調査した。調査の結果、熱影響部の粗粒化がみられなかったものを○，著しく粗粒化したものを×として粗粒化状況を評価した。
更に、冷延鋼板を総絞り比２．４でカップ成形し、カップの耳を切断してバリを研磨，除去した後、０℃に冷却した溶媒中で頂角６０℃の円錐ポンチでカップを押し広げ、素材鋼板の低温靭性を調査した。そして、カップが破壊することなく変形したものを○，カップの壁部が脆性破壊したものを×として低温靭性を評価した。
また、Ｂ添加が耐二次加工割れ性に及ぼす影響を調査するため、鋼種番号２０〜２３については、冷延鋼帯を総絞り比２．４でカップ成形した後、カップの耳を切断してバリを研磨，除去し、−３０℃に冷却した冷媒中でカップを横方向から押し潰した。そして、カップが破壊することなく変形したものを○，カップの壁部が脆性破壊したものを×として、耐二次加工割れ性を評価した。
調査結果を、本発明の請求項１〜３に対応させて表５〜７にそれぞれ示す。
【００２４】

【００２５】
各合金成分が本発明で規定した範囲にある鋼種番号１〜７の鋼板は、スポット溶接による熱影響部に粗粒化が観察されず、０℃の耐二次加工割れ性で評価した低温靭性にも優れていた。
これに対し、Ｍｇを添加していない鋼種番号８では、スポット溶接による熱影響部に著しい粗粒化が観察され、母材部に比較して突合せ溶接部の強度が低下していた。本発明で規定した範囲を超えるＣを含む鋼種番号９では、Ｍｇ添加していなくても熱影響部の粗粒化が観察されなかった。Ｓｉ，Ｍｎ，Ｐの強化元素を本発明で規定した範囲を超えて含有する鋼種番号１０〜１２では、素材が延性に乏しく、０℃の耐二次加工割れ性で評価した低温靭性も劣っていた。
【００２６】

【００２７】
Ｔｉ，Ｎｂを添加した本発明の請求項２に対応する鋼種番号１３〜１６の鋼板は、Ｔｉ，Ｎｂ以外の合金成分をほぼ同量含む鋼種番号２と比較して延性に優れており、鋼中の固溶Ｃ，Ｎを固定するＴｉ，Ｎｂの添加効果が窺われる。しかも、引張試験では母材部で破断したことから突合せ溶接部の強度低下がみられず、ＭｇＯの微細分散が熱影響部の粗粒化を有効に抑制していることが窺われる。
これに対し、Ｍｇ無添加の鋼種番号１７では、Ｔｉ添加によって延性が良好となる傾向はみられるが、スポット溶接による熱影響部が著しく粗粒化しており、突合せ溶接部の強度が低下していた。Ｔｉ，Ｎｂを過剰に添加した鋼種番号１８，１９では、延性の更なる向上はみられず、却って延性が若干低下していた。
【００２８】

【００２９】
Ｂを添加した鋼種番号２０，２１では、スポット溶接による熱影響部が粗粒化しておらず、０℃及び−３０℃の耐二次加工割れ性で評価した低温靭性にも優れていた。
他方、Ｍｇ無添加の鋼種番号２２では、Ｂ添加によって−３０℃の耐二次加工割れ性は改善されているが、スポット溶接による熱影響部が粗粒化しており、突合せ溶接部の強度が低下していた。また、Ｂ無添加の鋼種番号２３では、熱影響部の粗粒化は観察されなかったが、−３０℃の耐二次加工割れ性に劣っていた。
【００３０】
実施例３：
表１に掲げた鋼種番号１，１４，２０の鋼材を用い、熱延巻取り温度を５５０〜７００℃の範囲で４水準変化させる以外は実施例２と同じ条件下で冷延鋼帯を製造した。得られた各冷延鋼帯から切り出された試験片を引張試験に供し、０．２％耐力，引張強さ，伸び及びランクフォード値を求めた。
表８の調査結果にみられるように、本発明で規定した巻取り温度よりも低い５５０℃で巻き取った鋼帯では、延性及びランクフォード値の双方が劣っていた。これに対し、本発明で規定した巻取り温度の条件を満足する鋼帯では、延性，伸び共に良好であった。また、延性，伸びは、巻取り温度が高くなるほど向上する傾向を示した。
【００３１】

【００３２】
実施例４：
表１に掲げた鋼種番号１，１４，２０の鋼材を用い、連続焼鈍時に均熱温度を６００〜９００℃の範囲で４水準変化させる以外は実施例２と同じ条件下で冷延鋼帯を製造した。得られた各冷延鋼帯から切り出された試験片を引張試験に供し、０．２％耐力，引張強さ，伸び及びランクフォード値を求めた。
表９の調査結果にみられるように、本発明で規定した均熱温度よりも低い６００℃で焼鈍した鋼帯では、再結晶が不十分なため、延性及びランクフォード値の双方が劣っていた。また、本発明で規定した均熱温度より高い９００℃では、鋼帯が一旦γ域まで加熱されるため、延性及びランクフォード値の双方が劣っていた。これに対し、本発明で規定した均熱温度の条件を満足する鋼帯では、延性，伸び共に良好であった。
【００３３】

【００３４】
【発明の効果】
以上に説明したように、本発明の加工用鋼板は、微細なＭｇＯを鋼中に分散させているので、加工性改善のためにＣ含有量を低減した鋼種であるにも拘らず、溶接熱影響部の結晶粒が粗大化することがなく、良好な溶接強度を呈する。また、橋梁等の大型構造材用の厚鋼板を大入熱溶接する場合にあっても熱影響部の粗粒化が防止され、良好な溶接強度をもつ溶接構造体が得られる。[0001]
[Industrial application fields]
The present invention relates to a steel plate for processing excellent in weld strength used for automobiles, building materials, and the like, and a method for producing the same.
[0002]
[Prior art]
Steel sheets used for automobile bodies and building materials are required to have severe workability including press formability and deep drawability. Workability is improved by reducing the C content or adding Ti, Nb, etc., but if the C content is reduced, the weld heat affected zone tends to become coarser, and the structure is assembled by arc welding, spot welding, etc. The strength of is reduced.
In order to prevent a decrease in strength due to coarsening of the weld heat-affected zone, Japanese Patent Application Laid-Open No. 63-206451 regulates the amount of Ti contained in the ultra-low carbon cold rolled steel sheet. However, a very large amount of Ti is required to prevent the strength from being lowered, which leads to an increase in manufacturing cost. A large amount of Ti addition causes the ductility to deteriorate.
The coarsening of the weld heat affected zone itself is prevented when Mg is added alone or Mg and Ti are added together to disperse Mn-Si-Mg-based oxide and Ti-Mg-based oxide in steel. JP-A-8-232044, JP-A-9-310119, JP-A-9-310147 and the like are introduced.
[0003]
[Problems to be solved by the invention]
The steel material of JP-A-8-232044 is a steel type having a relatively large C content for hot-rolled steel sheets used for large buildings and the like, containing a small amount of Mg, and Mn-Si-Mg-based Oxide suppresses coarsening of the heat affected zone during high heat input welding. A steel material disclosed in JP-A-9-310147 is also a steel type having a relatively large C content for hot-rolled steel sheets used for large buildings and the like, and Ti-Mg alloy is added to molten steel to form Ti. -Precipitation of the heat-affected zone is suppressed by precipitating a composite oxide mainly composed of Mg and having a particle size of 0.05 to 0.5 µm.
However, Ti—Mg-based oxides easily form clusters even when the particle size by itself is 1 μm or less. When clusters are generated, the action of suppressing the coarsening of crystal grains becomes weak. This effect appears more prominently in steel grades with a reduced C content for workability improvement, and causes the heat-affected zone due to welding with low heat input, such as spot welding and arc welding, to be coarse and reduce weld strength. Become.
[0004]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem. By dispersing and precipitating fine MgO in steel, the crystal of the weld heat affected zone can be obtained even in a steel type having a low C content. An object of the present invention is to provide a steel sheet that prevents grain coarsening and has excellent weld strength, workability, and the like.
In order to achieve the object, the steel plate for processing of the present invention has C: 0.0005 to 0.010% by weight, Si: 0.02 to 1.0% by weight, Mn: 0.1 to 2.5% by weight. , P: 0.005 to 0.1 wt%, S: 0.02 wt% or less, Mg: 0.001 wt% or more and less than 0.01 wt% , Al: 0.02 wt% or less, the balance being It is a composition comprising Fe and inevitable impurities, and has a structure in which fine MgO of 1 μm or less is uniformly dispersed and precipitated. This processing steel plate can contain 0.02 to 0.10 wt% in total of Ti and / or Nb as required. Further, B: 0.0005 to 0.005% by weight may be added.
This working steel sheet is manufactured as a hot-rolled steel strip obtained by hot rolling a slab having the above-described composition at a finishing temperature of 850 ° C. or more and a winding temperature of 600 to 750 ° C. Alternatively, it is manufactured by subjecting a hot-rolled steel strip to pickling, cold rolling at a cold rolling rate of 50 to 85%, and further annealing at a soaking temperature of 650 to 880 ° C. Moreover, in order to improve pickling property, you may cold-roll a hot-rolled steel strip under high pressure prior to pickling. In this case, the total cold rolling rate combined with the cold rolling rate in the cold rolling after pickling is adjusted to a range of 50 to 85%.
[0005]
[Action]
The present inventors investigated and studied the influence of oxide inclusions on the coarsening of crystal grains due to the thermal effect during welding. As a result, the aforementioned Mn-Si-Mg-based and Ti-Mg-based oxides take the form of relatively coarse composite oxides, and steel types with a low C content are not sufficient for suppressing the coarsening of crystal grains. It has been found. That is, when spot welding or arc welding is performed on a thin steel sheet that is the subject of the present invention, the effect of solid solution C or the effect of a low γ → α transformation temperature is exerted on steel types having a C content exceeding 0.01% by weight. Even if the oxide is not dispersed, coarsening that adversely affects the welding strength does not occur.
However, crystal grains are generally more coarse as the C content is lower. When the C content is 0.01% by weight or less, not only large heat input welding but also simple spot welding, arc welding, etc. Coarse graining occurs. This coarsening cannot be sufficiently suppressed by a Mn—Si—Mg-based or Ti—Mg-based oxide that necessarily takes the form of a coarse composite oxide. Therefore, the present inventors studied to prevent coarsening of crystal grains observed in a welding method with a small amount of heat input by dispersing and precipitating fine MgO in steel. As a result, it was found that when the content of each alloy component is specified, and further the manufacturing conditions are specified, fine MgO that effectively works to prevent coarsening of crystal grains is uniformly dispersed in the steel.
[0006]
Hereinafter, alloy components, contents, production conditions, and the like included in the steel sheet for processing according to the present invention will be described.
C: 0.0005 to 0.010% by weight
The coarsening of the weld heat-affected zone due to the reduction of the C content does not occur in steel types containing C exceeding 0.010% by weight. However, since the present invention is intended for low-carbon steel in which coarsening of the weld heat-affected zone is a problem, the C content condition was set to 0.010% by weight. Reduction of the C content is effective in improving workability. However, it is very difficult to make the C content less than 0.0005% by weight in a mass production line, which causes an increase in manufacturing cost.
Si: 0.02 to 1.0% by weight
It is an alloy component effective for increasing the strength, and the effect of Si becomes remarkable when the content is 0.02% by weight or more. However, when a large amount of Si exceeding 1.0% by weight is contained, the low temperature toughness tends to be deteriorated.
[0007]
Mn: 0.1 to 2.5% by weight
It is an alloy component effective for increasing the strength, and the action of Mn becomes remarkable when the content is 0.1% by weight or more. However, when a large amount of Mn exceeding 2.5% by weight is contained, there is a tendency to deteriorate ductility and low temperature toughness.
P: 0.005 to 0.1% by weight
It is an alloy component effective for increasing the strength, and the action of P becomes remarkable when the content is 0.005% by weight or more. However, when a large amount of P exceeding 0.1% by weight is contained, the low temperature toughness tends to be greatly reduced.
S: 0.02% by weight or less S is a harmful element that lowers hot ductility. However, in the steel sheet for processing of the present invention, when the S content is 0.02% by weight or less, no adverse effect due to S appears.
[0008]
Mg: 0.001 to 0.02% by weight
It is an alloy component that precipitates in the steel as MgO of 1 μm or less and exhibits the action of preventing coarsening of the weld heat affected zone. In order to sufficiently precipitate fine MgO of 1 μm or less, the amount of Mg needs to be 0.001% by weight or more. If the Mg content is less than 0.001% by weight, the amount of MgO deposited is insufficient, and oxides such as coarser Mn—Si—Mg-based oxides, Al ₂ O ₃ , and SiO ₂ are precipitated as compared with MgO. Therefore, a sufficient effect of preventing coarsening cannot be obtained. The action of Mg is saturated at a content of 0.02% by weight, and even if it is added more than that, the cost of the steel material is increased. In this application, only those containing less than 0.01% by weight of Mg are claimed.
Although the addition form of Mg is not particularly restricted, it can be added by Mg deoxidation by a wire feed method in which a Mg-containing wire in which metallic Mg is mixed with CaO and wrapped in an iron skin is continuously charged into molten steel. . In this case, since Mg is added in a state where free oxygen in the molten steel is high, MgO is obtained as a deoxidation product.
Or after deoxidizing to some extent with Al, Mn, Si, etc., Mg can also be added as an alloy, such as Fe-Si-Mg. This Mg addition method is advantageous in terms of cost because the yield of Mg is good compared with direct Mg deoxidation of molten steel by wire feed method or the like, and is present in steel as an Al, Mn, Si-based oxide. MgO is generated by Mg reduction of the oxide.
In any addition method, when the added amount of Mg satisfies the conditions specified in the present invention, a slab in which fine MgO of about 1 μm or less is dispersed and precipitated in the steel is obtained. It is presumed that the fine MgO precipitate exhibits a pinning action that prevents the movement of the crystal grain boundary and suppresses the coarsening of the weld heat affected zone.
[0009]
Al: 0.02% by weight or less Al is an alloy component added as a deoxidizer, and is supplementarily added in a range of 0.02% by weight or less in order to adjust the amount of deoxidation by adding Mg. In addition, when deoxidation by Mg addition is sufficient, it is also possible to omit Al addition.
Ti and / or Nb: 0.02 to 0.10% by weight in total
It is an alloy component added as necessary, and exhibits the effect of fixing the solid solution C in the steel and improving the ductility of the steel plate. The effect of improving ductility becomes significant when the total amount of Ti and / or Nb added is 0.02% by weight or more. However, if Ti and / or Nb is added in a total addition amount exceeding 0.10% by weight, it causes a decrease in ductility, an increase in steel material cost, and the like.
B: 0.0005 to 0.005% by weight
In steel sheets with improved workability by adding Ti or Nb, the grain boundary strength decreases as the solid solution C decreases, and in applications such as press working under severe conditions, the resistance to post-processing There is a possibility that the next processing cracking property is deteriorated. The secondary work cracking resistance is further improved by adding a trace amount of B. The effect of addition of B becomes significant at 0.0005% by weight or more, and saturates at 0.005% by weight. Even if B is added in excess of 0.005% by weight, the steel material cost is increased.
[0010]
Hot rolling: Finishing temperature 850 ° C or higher, coiling temperature 600-750 ° C
After the steel material having the component design as described above is melted, the slab obtained by continuous casting is hot-rolled. In hot rolling, a condition where the finishing temperature is 850 ° C. or higher is employed. When the finishing temperature is lower than 850 ° C., the hot deformation resistance largely fluctuates, and it is easy to cause manufacturing problems such as shape defects such as gauge hunting and, in some cases, sheet breakage. The hot-rolled steel strip is wound up at a temperature of 600 ° C. or higher in order to sufficiently precipitate AlN. When the coiling temperature is less than 600 ° C., the precipitation of AlN is not sufficient, and the ductility and the Rankford value of the cold-rolled steel sheet decrease due to the precipitation of fine TiC or the like. On the contrary, if it winds up at the temperature exceeding 750 degreeC, there exists a possibility that a coil may deform | transform with dead weight and pickling property will also deteriorate.
[0011]
Cold rolling: cold rolling rate 50-85%
The hot-rolled steel strip is pickled in a continuous pickling line and then cold-rolled. Alternatively, when the hot-rolled steel strip is cold-rolled under high pressure prior to pickling, the load of the pickling process is reduced.
In the cold rolling process, the cold rolling rate is adjusted to 50% or more in order to promote recrystallization during the subsequent annealing. When the cold rolling rate is less than 50%, the recrystallization temperature rises and the crystal grains after recrystallization become coarse, and the surface of the steel sheet after processing tends to be rough.
However, cold rolling with a high cold rolling ratio exceeding 85% is difficult even with a tandem rolling mill with a single pass plate, and requires a plurality of passes, which increases the manufacturing cost.
[0012]
Continuous annealing: Soaking temperature 650-880 ° C
The cold-rolled steel strip is passed through a continuous annealing line and recrystallized and annealed at a soaking temperature of 650 to 880 ° C. If the soaking temperature does not reach 650 ° C., recrystallization does not proceed sufficiently, and the ductility of the obtained steel sheet decreases. On the contrary, in high temperature annealing exceeding 880 ° C., even if it is an extremely low carbon steel, γ transformation occurs once, so that the ductility of the steel sheet is lowered.
The annealed steel strip is used as it is, but may be subjected to a rust prevention treatment by hot dip Zn plating, hot dip Al plating or the like.
[0013]
【Example】
Example 1:
Slabs having the compositions shown in Table 1 were produced by a continuous casting method. Mg was added by the Mg deoxidation method in which an Mg + CaO-containing wire was continuously added to the molten steel in the ladle. For some slabs, the molten steel was pre-deoxidized with Al prior to the introduction of the Mg + CaO-containing wire.
[0014]

[0015]
Each slab was heated to 1180 ° C. and hot-rolled under conditions of a finishing temperature of 900 ° C. and a winding temperature of 650 ° C. to obtain a hot-rolled steel strip having a plate thickness of 2.3 mm.
After pickling the hot-rolled steel strip, it was subjected to a butt welding test by MAG welding. A JIS No. 5 tensile test piece was cut out with the welded portion of the test piece at the center and subjected to a tensile test. By comparing the test result with the test result of the material steel plate without the welded portion, the presence or absence of strength reduction due to welding was investigated, and the fracture position by the tensile test was investigated.
Further, a JIS No. 3 subsize impact test piece was cut out from the hot-rolled steel strip in parallel with the rolling direction, and subjected to a Charpy impact test at -10 ° C. The fracture surface after the test was observed, and the low-temperature toughness was evaluated with a case where the ductile fracture surface was 80% or more and a case where the ductile fracture surface was less than 80%.
Furthermore, in order to investigate the effect of B addition on the resistance to secondary work cracking, for steel types 20 to 23, after hot-rolled steel strip was cup-formed with a total drawing ratio of 2.1, the ears of the cup were cut. Then, the burrs were polished and removed, and the cup was crushed from the lateral direction in a refrigerant cooled to -30 ° C. Then, the resistance to secondary work cracking was evaluated by assuming that the cup was deformed without breaking, and the cup wall was brittlely broken.
The investigation results are shown in Tables 2 to 4, respectively, corresponding to claims 1 to 3 of the present invention.
[0016]

[0017]
In the steel plate Nos. 1 to 7 in which the respective alloy components are in the range defined by the present invention, no strength reduction was observed even in the tensile test of the butt welded portion, and all the fracture positions were the base material portion. Moreover, it was excellent in low temperature toughness.
On the other hand, in steel type number 8 to which no Mg was added, the strength of the butt welded portion was lower than that of the base metal portion as a result of the tensile test, and the elongation was also lower than that of the raw steel plate. The rupture position is also a welding heat affected zone, and the influence of coarsening of crystal grains due to heat input during welding is observed. In steel type No. 9 containing C exceeding the range defined in the present invention, no strength reduction was detected in the tensile test even when Mg was not added, and the fracture position was also the base material part. Steel types Nos. 10 to 12 containing Si, Mn, and P strengthening elements beyond the range defined in the present invention were inferior in ductility and low temperature toughness.
[0018]

[0019]
Steel plates of steel types 13 to 16 corresponding to claim 2 of the present invention to which Ti and Nb are added have excellent ductility compared to steel type No. 2 containing almost the same amount of alloy components other than Ti and Nb. The effect of adding Ti and Nb for fixing solid solution C and N therein is expected. Moreover, in the tensile test, the strength of the butt welded portion is not reduced because the base material portion is broken, and the fine dispersion of MgO effectively suppresses the coarsening of the heat affected zone.
On the other hand, in steel type No. 17 to which no Mg was added, there was a tendency that the ductility was improved by addition of Ti, but the heat-affected zone was broken by the tensile test, and the strength of the butt weld was reduced. In steel types Nos. 18 and 19 to which Ti and Nb were added excessively, no further improvement in ductility was observed, but the ductility was slightly lowered.
[0020]

[0021]
In the steel type numbers 20 and 21 to which B is added, the effect of B addition is seen from the excellent secondary work cracking resistance. Further, the strength of the butt weld was not lowered and the low temperature toughness was excellent.
On the other hand, in steel type No. 22 with no addition of Mg, the resistance to secondary work cracking was improved by addition of B, but the tensile effect was broken in the heat-affected zone and the strength of the butt weld was reduced. Moreover, in steel type number 23 without B addition, although the strength fall of the butt-welding part was not seen, it was inferior to secondary work cracking resistance.
[0022]
Example 2:
Slabs having the compositions shown in Table 1 were produced by a continuous casting method. Mg was added by the Mg deoxidation method in which an Mg + CaO-containing wire was continuously added to the molten steel in the ladle. For some slabs, the molten steel was pre-deoxidized with Al prior to the introduction of the Mg + CaO-containing wire.
Each slab was heated to 1180 ° C. and hot-rolled under conditions of a finishing temperature of 900 ° C. and a winding temperature of 650 ° C. to produce a hot-rolled steel strip having a thickness of 3.2 mm.
The hot-rolled steel strip is passed through a continuous pickling line and pickled, and then cold-rolled at a reduction rate of 75% using a tandem cold rolling mill to form a cold-rolled steel strip having a thickness of 0.8 mm. did. Next, continuous annealing was performed at 830 ° C. × soaking for 60 seconds in a continuous annealing line. The annealed steel strip was further subjected to temper rolling with an elongation of 0.8%.
[0023]
A JIS No. 5 specimen was cut out from the steel strip after temper rolling in parallel with the rolling direction, and the mechanical properties were examined by a tensile test.
In addition, a set of two 50 mm × 200 mm strip test pieces cut out from the same steel strip, with an electrode tip diameter of 5.5 mm, a pressure of 220 kg, a current of 7.6 kA, and a welding time of 10 seconds, a 40 mm interval. 5 point spot welding. And the cross section of the welding part was observed with the microscope about three middle points, and the structure | tissue of the heat affected zone was investigated. As a result of the investigation, the coarsening state was evaluated with ○ indicating that no coarsening was observed in the heat-affected zone, and x indicating extremely coarsening.
Furthermore, after cold-rolled steel sheet was cup-formed with a total drawing ratio of 2.4, the ears of the cup were cut and burrs were polished and removed. The low temperature toughness of the steel sheet was investigated. Then, the low temperature toughness was evaluated with ○ indicating that the cup was deformed without breaking, and x indicating that the cup wall was brittlely broken.
Moreover, in order to investigate the influence which B addition has on the secondary work cracking resistance, after forming a cold-rolled steel strip into a cup with a total drawing ratio of 2.4 for steel types 20 to 23, the ears of the cup were cut. Then, the burrs were polished and removed, and the cup was crushed from the lateral direction in a refrigerant cooled to -30 ° C. Then, the resistance to secondary work cracking was evaluated by assuming that the cup was deformed without breaking, and the cup wall was brittlely broken.
The investigation results are shown in Tables 5 to 7, respectively, corresponding to claims 1 to 3 of the present invention.
[0024]

[0025]
Steel sheets of steel types 1 to 7 in which the respective alloy components are in the range specified in the present invention are not observed for coarsening in the heat-affected zone by spot welding, and low temperature toughness evaluated by secondary work cracking resistance at 0 ° C. It was also excellent.
On the other hand, in Steel No. 8 to which no Mg was added, significant coarsening was observed in the heat-affected zone due to spot welding, and the strength of the butt weld was reduced compared to the base metal. In steel type No. 9 containing C exceeding the range defined in the present invention, no coarsening of the heat affected zone was observed even when Mg was not added. In steel types Nos. 10 to 12 containing Si, Mn, and P strengthening elements beyond the range specified in the present invention, the material is poor in ductility, and the low temperature toughness evaluated by secondary work cracking resistance at 0 ° C is also inferior. It was.
[0026]

[0027]
Steel plates of steel types 13 to 16 corresponding to claim 2 of the present invention to which Ti and Nb are added have excellent ductility compared to steel type No. 2 containing almost the same amount of alloy components other than Ti and Nb. The effect of adding Ti and Nb for fixing solid solution C and N therein is expected. Moreover, in the tensile test, the strength of the butt welded portion is not reduced because the base material portion is broken, and the fine dispersion of MgO effectively suppresses the coarsening of the heat affected zone.
On the other hand, in steel type No. 17 with no Mg added, there is a tendency that ductility is improved by addition of Ti, but the heat-affected zone due to spot welding is remarkably coarsened, and the strength of the butt weld is reduced. It was. In steel types Nos. 18 and 19 to which Ti and Nb were added excessively, no further improvement in ductility was observed, but the ductility was slightly lowered.
[0028]

[0029]
In steel type numbers 20 and 21 to which B was added, the heat-affected zone by spot welding was not coarsened, and the low-temperature toughness evaluated by secondary work cracking resistance at 0 ° C. and −30 ° C. was also excellent.
On the other hand, in steel type No. 22 with no Mg added, the resistance to secondary work cracking at −30 ° C. is improved by the addition of B, but the heat-affected zone by spot welding is coarsened, and the strength of the butt weld is increased. It was falling. Moreover, in the steel type number 23 without B addition, although the coarsening of the heat affected zone was not observed, it was inferior in secondary work crack resistance at -30 ° C.
[0030]
Example 3:
A cold-rolled steel strip is manufactured under the same conditions as in Example 2 except that steel types 1, 14, and 20 listed in Table 1 are used and the hot rolling coiling temperature is changed in four levels within the range of 550 to 700 ° C. did. Test pieces cut out from the obtained cold-rolled steel strips were subjected to a tensile test, and 0.2% proof stress, tensile strength, elongation, and Rankford value were determined.
As can be seen from the investigation results in Table 8, both the ductility and the Rankford value were inferior in the steel strip wound at 550 ° C., which is lower than the winding temperature defined in the present invention. On the other hand, in the steel strip satisfying the coiling temperature condition defined in the present invention, both ductility and elongation were good. In addition, the ductility and elongation tended to improve as the coiling temperature increased.
[0031]

[0032]
Example 4:
A steel strip of steel type Nos. 1, 14, and 20 listed in Table 1 was used, and a cold-rolled steel strip was formed under the same conditions as in Example 2 except that the soaking temperature was changed by 4 levels in the range of 600 to 900 ° C. during continuous annealing. Manufactured. Test pieces cut out from the obtained cold-rolled steel strips were subjected to a tensile test, and 0.2% proof stress, tensile strength, elongation, and Rankford value were determined.
As seen in the investigation results in Table 9, the steel strip annealed at 600 ° C., which is lower than the soaking temperature defined in the present invention, was inferior in both ductility and Rankford value due to insufficient recrystallization. . Further, at 900 ° C., which is higher than the soaking temperature defined in the present invention, the steel strip is once heated to the γ region, so both the ductility and the Rankford value were inferior. On the other hand, in the steel strip satisfying the soaking temperature condition defined in the present invention, both ductility and elongation were good.
[0033]

[0034]
【The invention's effect】
As described above, the steel plate for processing according to the present invention has fine MgO dispersed in the steel, so that the welding heat is reduced despite the fact that it is a steel type having a reduced C content for improving workability. The crystal grains in the affected part are not coarsened and exhibit good welding strength. Further, even when thick steel plates for large structural materials such as bridges are subjected to high heat input welding, coarsening of the heat affected zone is prevented, and a welded structure having good welding strength can be obtained.

Claims

C: 0.0005 to 0.010 wt%, Si: 0.02 to 1.0 wt%, Mn: 0.1 to 2.5 wt%, P: 0.005 to 0.1 wt%, S: 0.02 wt% or less, Mg: 0.001 wt% or more and less than 0.01 wt% , Al: 0.02 wt% or less, with the balance consisting of Fe and unavoidable impurities , with a fineness of 1 μm or less A steel plate for processing with a welded structure with a structure in which MgO is uniformly dispersed and precipitated.

Furthermore, the steel plate for work excellent in the welded part intensity | strength of Claim 1 which contains 0.02-0.10 weight% of Ti and / or Nb in total.

Furthermore, B: The steel plate for a process excellent in the weld part intensity | strength of Claim 1 or 2 containing 0.0005 to 0.005 weight%.

Manufacturing of a steel sheet for processing excellent in weld strength, characterized by hot rolling a slab having the composition according to any one of claims 1 to 3 at a finishing temperature of 850 ° C or higher and a winding temperature of 600 to 750 ° C. Method.

A slab having the composition according to any one of claims 1 to 3 is hot-rolled at a finishing temperature of 850 ° C or higher and a winding temperature of 600 to 750 ° C, and then pickled and cold-rolled at a cold rolling rate of 50 to 85%. And further annealing at a soaking temperature of 650 to 880 ° C., and a method for producing a working steel sheet having excellent weld strength.