JP3773604B2

JP3773604B2 - High-strength cold-rolled steel sheet or hot-dip galvanized steel slab excellent in deep drawability and method for producing the same

Info

Publication number: JP3773604B2
Application number: JP27149296A
Authority: JP
Inventors: 孝松元; 征一浜中; 裕一肥後; 照夫田中
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1996-09-20
Filing date: 1996-09-20
Publication date: 2006-05-10
Anticipated expiration: 2016-09-20
Also published as: JPH1096051A

Description

【０００１】
【産業上の利用分野】
本発明は、高度の加工性が要求される自動車の車体部品等に適した高強度冷延鋼板又は溶融めっき鋼板用のスラブ及びその製造方法に関する。
【０００２】
【従来の技術】
地球環境保護の運動が高まる中で、自動車に関して排ガス対策，省エネルギー等を狙って燃費の低減が従来以上に強く求められている。燃費低減の有力な方策の一つに車体の軽量化があり、各種の高強度鋼板が提案されている。高強度鋼板は、車体の安全性を向上させる上でも有効な材料である。
なかでも、フロントサイドメンバー等の自動車車体部品に使用される冷延鋼板は、深絞り成形を主体とする過酷な成形が施され、しかも製品そのものは重要な保安部品としての高い部品強度が必要とされる。そのため、自動車車体部品においては、従来よりも格段に優れた加工性をもち、しかも従来以上の高強度を呈する冷延鋼板や溶融めっき鋼板が望まれている。
【０００３】
高加工性及び高強度の要求に応えるものとして、たとえば特開昭６１−２７６９２７号公報，特開昭６１−２７６９３０号公報等にみられるように、極低炭素鋼にＴｉ，Ｎｂ等の炭窒化物形成元素を含有させたＩＦ鋼をベースとし、Ｓｉ，Ｍｎ，Ｐ等の固溶強化元素を多量に含有させたスラブを使用し、各種の加工性に優れた自動車用高強度冷延鋼板がこれまで製造されてきた。しかも、自動車車体の構造部品は、重要保安部品としての価値が高いことから、年々その需要が増大し、少量生産から通常材同様の大量生産といえる状況にまで発展してきている。
【０００４】
【発明が解決しようとする課題】
高強度化のため従来以上にＳｉ，Ｍｎ，Ｐ等の固溶強化元素を多量に添加し、しかも生産性向上のために連続鋳造時に冷却速度を上げて製造したスラブでは、従来みられなかった硬質の擬ベイナイト組織が生成するようになる。硬質の擬ベイナイト組織は、表面割れ等の欠陥を多数発生させる原因であり、結果として製品歩留りを低下させる。そのため、大量生産を狙ったものの、逆に大幅な製造コストの上昇を招くことになる。
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、Ｔｉ又はＮｂを添加した成分系においてスラブの組成及び鋳造組織を特定することにより、生産性の阻害や製造コストの上昇を招くことなく、加工性に優れた高強度冷延鋼板及び溶融めっき鋼板を提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明の高強度冷延鋼板又は溶融めっき鋼板用スラブは、その目的を達成するため、Ｃ：０．０００５〜０．０１重量％，Ｓｉ：１．０重量％以下，Ｍｎ：０．５〜３．０重量％，Ｐ：０．０５〜０．２重量％，Ｓ：０．０００５〜０．０２重量％，Ａｌ：０．００５〜０．１０重量％，Ｎ：０．００７重量％以下，Ｔｉ：０．０１〜０．１重量％及び／又はＮｂ：０．０１〜０．１重量％を含み、残部がＦｅ及び不可避的不純物からなる組成をもち、擬ベイナイト組織−ポリゴナルフェライト組織の混合比率で擬ベイナイト組織が４０％以下である鋳造組織をもつことを特徴とする。
【０００６】
このスラブは、更にＣｕ：０．０３〜０．５重量％及びＮｉ：０．０３〜０．５重量％を含むことができる。また、更にＢ：０．０００２〜０．００１重量％，Ｚｒ：０．０１〜０．１重量％，Ｖ：０．０１〜０．１重量％の１種又は２種以上を含んだ組成のスラブも使用される。
製造に際しては、凝固後のスラブが９５０〜７５０℃の温度域にあるとき、５〜５０℃／秒の冷却速度でスラブを冷却することが必要である。
【０００７】
【実施の形態】
本発明者等は、冷延鋼板及び溶融めっき鋼板の強度や耐食性に応じて設定される成分・組成をもつスラブにおいて、連続鋳造されたスラブの鋳造組織が冷延鋼板及び溶融めっき鋼板の特性に及ぼす影響を種々調査・研究した。その結果、Ｔｉ及び／又はＮｂを含む成分系をもつスラブのミクロ組織における擬ベイナイト組織を４０％以下に規制するとき、生産性の阻害及び製造コストの上昇を招くことなく、加工性に優れた高強度冷延鋼板及び溶融めっき鋼板を製造できることを見い出した。
強度向上のためにＳｉ，Ｍｎ，Ｐ等の固溶強化元素を多量に添加した鋼材を連続鋳造すると、擬ベイナイトが生成し易い。擬ベイナイトは、凝固後のスラブの冷却速度が遅くなるほど生成量が少なくなるが、過度に遅い冷却速度では生産性が阻害される。本発明者等の研究によるとき、９５０〜７５０℃の温度域における冷却速度を５〜５０℃／秒にすると、擬ベイナイト組織の混合比率が４０％以下に規制され、擬ベイナイトに起因する表面割れが抑制され、生産性を低下させることなく加工性に優れた高強度冷延鋼板及び溶融めっき鋼板用のスラブが得られることが判った。
【０００８】
以下、本発明スラブの合金成分，含有量，製造条件等を説明する。
Ｃ：０．０００５〜０．０１重量％
ＴｉＣ，ＮｂＣ等の炭化物として固定される成分であるが、Ｃ含有量が低いほどランクフォード値や伸びが改善され、Ｃの固定化に必要なＴｉ，Ｎｂ等の添加量を低減できる。そのため、本発明においては、Ｃ含有量の上限を０．０１重量％に規定した。しかし、極端にＣ含有量を下げるためには製鋼工程で過度の脱炭精錬を必要とし、製造コストの上昇を招くことから、Ｃ含有量の下限を０．０００５重量％に規定した。
Ｓｉ：１．０重量％以下
鋼材を強化する作用を呈し、必要とする強度に応じて添加量が決定される。しかし、Ｓｉ含有量が１．０重量％を超えると、化成処理性，めっき性等が低下する傾向がみられる。
【０００９】
Ｍｎ：０．５〜３．０重量％
Ｓｉと同様に鋼材を強化する作用を呈し、必要とする強度に応じて添加量が決定される。鋼材を強化する作用は、０．５重量％以上のＭｎ含有量で顕著になる。しかし、３．０重量％を超える多量のＭｎが含まれると、Ａｒ₃ 変態点が急激に低下し、再結晶焼鈍がγ域になり、ランクフォード値や伸びが低下しやすい。
Ｐ：０．０５〜０．２重量％
Ｓｉと同様に鋼材を強化する作用を呈し、目標強度に応じて必要量が添加される。しかし、０．０５重量％未満のＰ添加では、鋼材を強化する作用が不十分である。逆に０．２重量％を超える多量のＰが添加されると、加工性，耐二次加工性，めっき性等が劣化する。
【００１０】
Ｓ：０．０００５〜０．０２重量％
Ｍｎ，Ｔｉ等と硫化物を形成し、炭化物系析出物の生成に影響を及ぼし、ランクフォード値を向上させる作用を呈する。しかし、熱間加工時の割れを誘発させる成分であるため、上限を０．０２重量％に規制した。また、Ｓ含有量を過度に低減することは、製鋼工程で脱硫精錬に多大なコストがかかることから、Ｓ含有量の下限を０．０００５重量％に設定した。
Ａｌ：０．００５〜０．１０重量％
脱酸剤として添加されると共に、Ｎを固定する作用を呈する。このような作用は、０．００５重量％以上のＡｌ含有量で顕著になる。しかし、０．１０重量％を超える多量のＡｌが含まれると、酸化物系介在物が増加し、加工性，表面性状等が劣化する。
【００１１】
Ｎ：０．００７重量％以下
不可避的に含まれる成分であり、Ｔｉ等で固定される。しかし、０．００７重量％を超える多量のＮが含まれると、Ｎの固定に必要なＴｉ，Ｎｂ等の添加量を多くすることが要求され、析出物の増加に起因して加工性が劣化する。
Ｔｉ：０．０１〜０．１重量％
鋼中に侵入型として固溶するＣ及びＮを炭窒化物として固定すると共に、Ｓと結合して硫化物を形成する合金成分である。硫化物を形成し、固溶状態のＣ及びＮを十分に減少させることにより加工性の向上を図るためには、０．０１重量％以上のＴｉが必要とされる。しかし、０．１重量％を超える多量の添加は、Ｔｉによる加工性改善効果が飽和し、却って製造コストの上昇を招く。
【００１２】
Ｎｂ：０．０１〜０．１重量％
Ｔｉと同様の炭窒化物形成元素であり、鋼中のＣ及びＮを固定し、加工性を向上させる作用を呈する。また、Ｔｉとの複合添加では、複合析出物を形成し、比較的粗大な析出物とすることによっても加工性が改善される。このような効果は、Ｎｂ含有量が０．０１重量％以上で顕著になり、０．１重量％で飽和する。
Ｃｕ：０．０３〜０．５重量％
必要に応じて添加される合金成分であり、耐食性を改善する作用を呈する。Ｃｕの添加効果は、０．０３重量％以上の含有量で顕著になり、０．５重量％で飽和する。
【００１３】
Ｎｉ：０．０３〜０．５重量％
Ｃｕに起因した熱間脆性を防止する作用を呈することから、耐食性を改善するためにＣｕを添加した系で有効な合金成分である。このような作用を得るためには、Ｃｕ含有量とほぼ同量のＮｉ含有量にすることが好ましく、そのためＮｉを添加する場合にはその範囲を０．０３〜０．５重量％に設定する。
Ｂ：０．０００２〜０．００１重量％
必要に応じて添加される合金成分であり、優先的に結晶粒界に偏析し、Ｐに起因する粒界脆化を抑制する作用を呈する。また、プレス成形時に二次加工割れを防止する作用もある。このような作用は、０．０００２重量％以上のＢで顕著になる。しかし、０．００１重量％を超える多量のＢが含まれると、結晶粒の成長が阻害され、却って加工性が劣化する。
Ｚｒ及びＶ：０．０１〜０．１重量％
必要に応じて添加される合金成分であり、炭窒化物を形成してＣやＮを固定する作用を呈する。また、Ｔｉ，Ｎｂ等と複合添加するとき、加工性を向上させる作用も呈する。これらの作用は、０．０１重量％以上のＺｒ又はＶで顕著になるが、０．１重量％で飽和する。
【００１４】
鋳造組織：擬ベイナイト組織が４０％以下の擬ベイナイト
−ポリゴナルフェライトの混合組織
以上のように成分・組成が調整された溶鋼を連続鋳造し、スラブとする。得られるスラブは、冷却速度に応じて低温変態生成物の一つである擬ベイナイト組織やポリゴナルフェライト組織等に変化するミクロ組織を持っている。擬ベイナイト組織は、特に転移密度が高く硬質であり、スラブに表面割れ等の欠陥を発生させる原因となる。他方、ポリゴナルフェライト組織は、転移密度が小さく軟質であり、表面割れに対する抵抗力が大きい。
表面割れに及ぼす擬ベイナイト組織及びポリゴナルフェライト組織の影響を調査・研究した結果、擬ベイナイト組織−ポリゴナルフェライト組織の混合比率で擬ベイナイト組織を４０％以下に規制すると、擬ベイナイト組織の影響が抑制され、スラブに表面割れ等の欠陥が全く発生しないことを解明した。この理由については必ずしも明確ではないが、軟質のポリゴナルフェライト組織が支配的になっていることに原因があるものと推察される。
【００１５】
スラブの冷却条件：９５０〜７５０℃の温度域を５〜５０℃／秒の冷却速度
擬ベイナイト組織が４０％以下のミクロ組織を得るためには、凝固後のスラブが９５０〜７５０℃の温度域にあるとき、５０℃／秒以下の冷却速度でスラブを冷却することが必要である。この温度域は、フェライトとオーステナイトの２相域にあり、γ相からポリゴナルフェライト組織又は擬ベイナイト組織を生成させる上で有効である。温度域が特定条件を外れると、擬ベイナイト相４０％以下の組織が得られない。また、５０℃／秒を超える冷却速度では、何れの鋼成分においても擬ベイナイト相４０％以下の組織が得られない。
ただし、過度に遅い冷却速度は生産性を低下させる要因となるので、冷却速度の下限を５℃／秒に設定する。
本発明に従って組成及び組織が調整されたスラブは、熱間圧延，酸洗，冷間圧延工程を経て、再結晶焼鈍された高強度冷延鋼板、或いは再結晶焼鈍後に溶融めっきされた溶融めっき鋼板として使用される。
【００１６】
熱間圧延では、加熱温度１０００〜１２００℃の低温加熱を使用し、仕上げ温度Ａｒ₃ 変態点以上，６００℃以上の高温巻取りを採用することが好ましい。低温加熱及び高温巻取りは、鋼中に侵入型として固溶しているＣ及びＮをＦｅ₃ Ｃ，ＡｌＮ等の炭化物，窒化物，炭窒化物として固定し、加工性を改善する上で有効である。また、Ａｒ₃ 変態点に達しない仕上げ温度で熱延すると、加工性劣化の原因となる熱延集合組織が形成される。
熱延コイルは、脱スケール酸洗後、冷間圧延される。冷間圧延では、十分な深絞り性を得るために、圧下率を６０％以上に設定することが好ましい。冷間圧延後の再結晶焼鈍は、連続焼鈍工程、又はＺｎ，Ａｌ等の溶融めっきを施す場合には溶融めっき設備内の還元焼鈍炉で施される。なお、めっき条件は特に規制されるものではなく、工業的に通常採用されている条件が選定される。また、焼鈍後の冷延鋼板及び溶融めっき後の溶融めっき鋼板に対しては、５％以下の調質圧延を施すこともできる。
【００１７】
【実施例】
溶解炉で溶鋼を表１の組成に調整した後、連鋳鋳型に注入し、凝固後の冷却過程で９５０〜７５０℃の温度域における冷却速度を１０〜１２０℃／秒の範囲で変化させて鋳造し、５０ｋｇの鋼塊を製造した。
【００１８】

【００１９】
得られた鋼塊の表面を肉眼で観察し、表面割れの有無を調査した。引き続き、磁粉探傷試験で表面割れの個数をカウントした。また、光学顕微鏡を用いて鋼塊のミクロ組織を観察した。調査結果を表２に示すように、本発明に従って鋼塊の冷却速度を５０℃／秒以下に規制するとき、擬ベイナイトの組織比率が４０％以下に抑えられ、表面割れが発生ないことが判った。これに対し、鋼塊の冷却速度を５０℃／秒を超える試験番号５，７の比較例では、擬ベイナイトの組織比率が４０％を大きく超え、表面割れが多発した。
また、表面割れの発生及びミクロ組織と鋳造時の冷却速度との関係を調査したところ、両者の間に図１に示す関係が成立していた。すなわち、ミクロ組織において擬ベイナイト組織−ポリゴナルフェライト組織の混合比率で擬ベイナイト組織が４０％以下の場合及び凝固後９５０〜７５０℃の温度領域を５０℃／秒以下の冷却速度で冷却して鋳造する場合には、何れも表面割れの発生が全く観察されず、良好なスラブが得られることが判る。
【００２０】

【００２１】
表面割れのない鋼塊を厚さ３５ｍｍの鋼片に熱間鍛造した後、１１３０℃に加熱し、熱間圧延した。熱間圧延の仕上げ温度は、８９０〜９３０℃で且つＡｒ₃ 変態点以上に設定した。仕上げ板厚は、後続する冷間圧延での圧延率を考慮し、５ｍｍに設定した。熱延仕上げ後、６５０〜７２０℃に加熱したソルトバス中に鋼帯を装入して所定温度に加熱し、約２時間保持することにより、熱延鋼帯の巻取りに相当する処理を施した。
引き続き、脱スケール酸洗し、圧延率８０％の冷間圧延により板厚１．０ｍｍの冷延鋼板とした。この冷延鋼板を、再結晶温度以上の温度８５０℃で連続焼鈍した。
【００２２】
得られた冷延鋼板の機械的性質を、ＪＩＳ５号引張試験片を使用して測定した。ランクフォード値は、１５％の引張予歪みを与えた後、３点法で測定し、Ｌ方向（圧延方向），Ｄ方向（圧延方向に４５度方向）及びＣ方向（圧延方向に直交する方向）の平均値［（ｒ_L ＋２ｒ_D ＋ｒ_C ）／４］として求めた。また、直径９０ｍｍに打ち抜いたブランクを用いて絞り比２．７の三段階絞りで直径３３ｍｍの平底円筒カップを成形した後、液体窒素及び有機溶剤からなる各種温度の冷媒に浸漬しながら、先端角６０度のポンチを円筒上部から押し込み、脆性割れが発生しない最低温度を測定した。この測定温度を、二次加工割れ発生温度とした。
【００２３】
調査結果を表３に示すように、本発明に従って冷却速度が規制された鋼塊から製造された冷延鋼帯では、０．２％耐力，引張強さ，伸び共に優れ、ランクフォード値が１．２以上，二次加工割れ脆化温度が−１００℃以下と加工性に優れていることが判る。これに対し、スラブの冷却速度が本発明で規定した５０℃／秒以下であっても、組成に関する条件を満足しない試験番号１５では、ランクフォード値が低く、二次加工割れ脆化温度も高いことから、二次加工割れが発生し易い材料であった。
【００２４】

【００２５】
実施例２：
表４に示す組成をもつ溶鋼を転炉及び脱ガス炉で精錬し、連続鋳造時の引抜き速度，冷却水量等を調節してスラブの冷却速度を変化させながら厚み２５０ｍｍ，単重１３トンのスラブを製造した。
【００２６】

【００２７】
得られたスラブについて、表面割れ，ミクロ組織等を実施例１と同様に調査した。表５の調査結果にみられるように、本発明に従って鋼塊の冷却速度を５０℃／秒以下に規制するとき、擬ベイナイトの組織比率が４０％以下に抑えられ、表面割れが発生しないことが判った。これに対し、鋼塊の冷却速度を５０℃／秒を超える試験番号２０，２５の比較例では、擬ベイナイトの組織比率が４０％を大きく超え、表面割れが多発した。
【００２８】

【００２９】
スラブ段階で表面割れの発生した比較例では、熱間圧延以降の工程通板ができなかったので、表面割れのないスラブのみを加熱炉で１１３０℃に加熱し、熱間圧延した。熱間圧延の仕上げ温度を８９０〜９１０℃で且つＡｒ₃ 変態点以上に、仕上げ板厚を５ｍｍに、巻取り温度を６５０〜７２０℃に設定した。
熱延鋼帯を塩酸系酸洗液槽を備えた連続酸洗ラインに通板して脱スケール酸洗した後、冷間圧延機に送り、冷延率８０％で冷間圧延した。得られた板厚１．０ｍｍの冷延鋼板を焼鈍工程に送り、再結晶温度以上で焼鈍し、冷延鋼板の製品とした。一部の鋼帯については、再結晶温度以上で還元焼鈍を施し、めっき浴温を４５０℃とした連続溶融めっきラインに通板し、溶融亜鉛めっき鋼板の製品とした。
得られた各製品の機械的性質を実施例１と同様に調査した。表６の調査結果にみられるように、何れの製品鋼帯も０．２％耐力，引張強さ等の機械的特性に優れ、高い伸び，ランクフォード値及び低い二次加工割れ脆化温度を示すことから良好な加工性をもつことが判る。
【００３０】

【００３１】
【発明の効果】
以上に説明したように、本発明においては、組成が特定されたＴｉ，Ｎｂを含む成分系の鋼材を使用し、鋳造スラブのミクロ組織及び連続鋳造時の冷却速度を制御することにより、生産性の阻害及び製造コストの上昇を招くことなく、高品質のスラブが製造される。得られたスラブは、深絞り性に優れた高強度冷延鋼板及び溶融めっき鋼板用に使用される。
【図面の簡単な説明】
【図１】ミクロ組織及び表面割れの発生に及ぼす鋳造時の冷却速度の影響[0001]
[Industrial application fields]
The present invention relates to a slab for a high-strength cold-rolled steel sheet or hot-dip plated steel sheet suitable for automobile body parts and the like that require a high degree of workability, and a method for manufacturing the slab.
[0002]
[Prior art]
As the movement to protect the global environment grows, automobiles are strongly demanded to reduce fuel consumption with the aim of exhaust gas countermeasures and energy saving. One of the most effective measures for reducing fuel consumption is to reduce the weight of the vehicle body, and various high-strength steel sheets have been proposed. A high-strength steel plate is an effective material for improving the safety of the vehicle body.
Above all, the cold-rolled steel sheets used for automobile body parts such as front side members are subjected to severe forming mainly by deep drawing, and the product itself requires high part strength as an important safety part. Is done. Therefore, in automotive body parts, cold-rolled steel sheets and hot-dip plated steel sheets that have remarkably superior workability than conventional ones and that exhibit higher strength than conventional ones are desired.
[0003]
In order to meet the demands for high workability and high strength, carbonitriding such as Ti, Nb or the like is used for ultra-low carbon steel as seen in, for example, JP-A-61-276927 and JP-A-61-276930. A high-strength cold-rolled steel sheet for automobiles that uses a slab containing a large amount of solid solution strengthening elements such as Si, Mn, P, etc. It has been manufactured so far. Moreover, the structural parts of automobile bodies are highly valuable as important safety parts, so that their demand is increasing year by year, and the situation has been developed from a small volume production to a mass production similar to ordinary materials.
[0004]
[Problems to be solved by the invention]
In slabs manufactured by adding a large amount of solid solution strengthening elements such as Si, Mn, P, etc. to increase the strength and increasing the cooling rate during continuous casting in order to improve productivity, it was not seen in the past A hard pseudo-bainite structure is generated. The hard pseudo-bainite structure causes many defects such as surface cracks, resulting in a decrease in product yield. Therefore, while aiming for mass production, conversely, a significant increase in manufacturing cost will be caused.
The present invention has been devised to solve such problems. By specifying the composition and cast structure of the slab in the component system to which Ti or Nb is added, productivity is hindered and manufacturing cost is increased. An object of the present invention is to provide a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet that are excellent in workability.
[0005]
[Means for Solving the Problems]
The slab for high-strength cold-rolled steel sheet or hot-dip galvanized steel sheet according to the present invention is C: 0.0005 to 0.01% by weight, Si: 1.0% by weight or less, Mn: 0.5 to 3.0 wt%, P: 0.05 to 0.2 wt%, S: 0.0005 to 0.02 wt%, Al: 0.005 to 0.10 wt%, N: 0.007 wt% or less , Ti: 0.01 to 0.1% by weight and / or Nb: 0.01 to 0.1% by weight, with the balance being composed of Fe and inevitable impurities , pseudo-bainite structure-polygonal ferrite structure It is characterized by having a cast structure whose pseudo-bainite structure is 40% or less at a mixing ratio of
[0006]
The slab may further contain Cu: 0.03-0.5 wt% and Ni: 0.03-0.5 wt%. Further, the composition further includes one or more of B: 0.0002 to 0.001 wt%, Zr: 0.01 to 0.1 wt%, and V: 0.01 to 0.1 wt%. Slabs are also used.
In production, when the slab after solidification is in a temperature range of 950 to 750 ° C., it is necessary to cool the slab at a cooling rate of 5 to 50 ° C./second.
[0007]
Embodiment
The present inventors, in the slab having a component and composition set according to the strength and corrosion resistance of the cold-rolled steel sheet and hot-dip steel sheet, the cast structure of the continuously cast slab is the characteristics of the cold-rolled steel sheet and hot-dip plated steel sheet Various influences were investigated and studied. As a result, when the pseudo-bainite structure in the microstructure of a slab having a component system containing Ti and / or Nb is regulated to 40% or less, it is excellent in workability without causing an inhibition of productivity and an increase in manufacturing cost. It has been found that high-strength cold-rolled steel sheets and hot-dip plated steel sheets can be produced.
When a steel material to which a large amount of a solid solution strengthening element such as Si, Mn, P or the like is added for strength improvement is continuously cast, pseudobainite is likely to be generated. The amount of pseudo bainite produced decreases as the cooling rate of the slab after solidification decreases, but productivity is hindered at an excessively low cooling rate. According to the study by the present inventors, when the cooling rate in the temperature range of 950 to 750 ° C. is 5 to 50 ° C./second, the mixing ratio of the pseudo bainite structure is regulated to 40% or less, and surface cracks caused by pseudo bainite. It was found that slabs for high-strength cold-rolled steel sheets and hot-dip galvanized steel sheets having excellent workability without lowering productivity were obtained.
[0008]
Hereinafter, the alloy components, content, production conditions and the like of the slab of the present invention will be described.
C: 0.0005 to 0.01% by weight
Although it is a component fixed as a carbide such as TiC and NbC, the lower the C content, the more the Rankford value and the elongation are improved, and the addition amount of Ti, Nb and the like necessary for C fixation can be reduced. Therefore, in this invention, the upper limit of C content was prescribed | regulated to 0.01 weight%. However, in order to extremely reduce the C content, excessive decarburization and refining is required in the steel making process, leading to an increase in production cost. Therefore, the lower limit of the C content is defined as 0.0005% by weight.
Si: 1.0% by weight or less The effect of strengthening the steel material is exhibited, and the addition amount is determined according to the required strength. However, when the Si content exceeds 1.0% by weight, the chemical conversion property, the plating property and the like tend to decrease.
[0009]
Mn: 0.5 to 3.0% by weight
The effect of strengthening the steel material is exhibited in the same manner as Si, and the addition amount is determined according to the required strength. The effect | action which strengthens steel materials becomes remarkable with Mn content of 0.5 weight% or more. However, when a large amount of Mn exceeding 3.0% by weight is contained, the Ar ₃ transformation point is rapidly lowered, recrystallization annealing is in the γ region, and the Lankford value and elongation are likely to be lowered.
P: 0.05 to 0.2% by weight
Similar to Si, the steel material is strengthened, and a necessary amount is added according to the target strength. However, when P is added in an amount of less than 0.05% by weight, the effect of strengthening the steel material is insufficient. On the other hand, when a large amount of P exceeding 0.2% by weight is added, workability, secondary work resistance, plating property and the like deteriorate.
[0010]
S: 0.0005 to 0.02% by weight
It forms sulfides with Mn, Ti, etc., affects the formation of carbide-based precipitates, and exhibits the effect of improving the Rankford value. However, since it is a component that induces cracking during hot working, the upper limit was regulated to 0.02% by weight. Further, excessively reducing the S content requires a great deal of desulfurization refining in the steelmaking process, so the lower limit of the S content was set to 0.0005% by weight.
Al: 0.005 to 0.10% by weight
In addition to being added as a deoxidizer, it exhibits an action of fixing N. Such an effect becomes remarkable when the Al content is 0.005% by weight or more. However, if a large amount of Al exceeding 0.10% by weight is contained, oxide inclusions increase and workability, surface properties, etc. deteriorate.
[0011]
N: 0.007% by weight or less is an inevitable component, and is fixed with Ti or the like. However, when a large amount of N exceeding 0.007% by weight is contained, it is required to increase the amount of Ti, Nb, etc. required for fixing N, and the workability deteriorates due to an increase in precipitates. To do.
Ti: 0.01 to 0.1% by weight
It is an alloy component that fixes C and N dissolved as interstitial steel in steel as carbonitride and combines with S to form sulfide. In order to improve the workability by forming sulfides and sufficiently reducing C and N in a solid solution state, 0.01% by weight or more of Ti is required. However, the addition of a large amount exceeding 0.1% by weight saturates the workability improving effect by Ti, and causes an increase in manufacturing cost.
[0012]
Nb: 0.01 to 0.1% by weight
It is a carbonitride-forming element similar to Ti, and has the effect of fixing C and N in steel and improving workability. In addition, in the composite addition with Ti, the workability is also improved by forming a composite precipitate and forming a relatively coarse precipitate. Such an effect becomes remarkable when the Nb content is 0.01% by weight or more, and is saturated at 0.1% by weight.
Cu: 0.03 to 0.5% by weight
It is an alloy component added as necessary, and exhibits the effect of improving the corrosion resistance. The effect of addition of Cu becomes significant at a content of 0.03% by weight or more, and saturates at 0.5% by weight.
[0013]
Ni: 0.03 to 0.5% by weight
Since it exhibits the effect of preventing hot brittleness caused by Cu, it is an effective alloy component in a system to which Cu is added in order to improve corrosion resistance. In order to obtain such an effect, it is preferable to make the Ni content approximately the same as the Cu content. Therefore, when Ni is added, the range is set to 0.03 to 0.5% by weight. .
B: 0.0002 to 0.001% by weight
It is an alloy component that is added as necessary, preferentially segregates at the grain boundaries, and exhibits the effect of suppressing grain boundary embrittlement due to P. It also has the effect of preventing secondary processing cracks during press molding. Such an effect becomes remarkable at B of 0.0002% by weight or more. However, if a large amount of B exceeding 0.001% by weight is contained, the growth of crystal grains is inhibited, and the workability is deteriorated.
Zr and V: 0.01 to 0.1% by weight
It is an alloy component added as necessary, and exhibits an action of forming carbonitride and fixing C and N. Moreover, when adding together with Ti, Nb, etc., the effect | action which improves workability is also exhibited. These effects become significant with 0.01% or more by weight of Zr or V, but saturate with 0.1% by weight.
[0014]
Casting structure: Molten steel with components and compositions adjusted as described above is mixed with a pseudo-bainite-polygonal ferrite mixed structure having a pseudo-bainite structure of 40% or less, and a slab is obtained. The obtained slab has a microstructure that changes to a pseudo-bainite structure or a polygonal ferrite structure, which is one of low-temperature transformation products, according to the cooling rate. The pseudo-bainite structure has a particularly high transition density and is hard, and causes a defect such as a surface crack in the slab. On the other hand, the polygonal ferrite structure has a small transition density and is soft, and has a high resistance to surface cracks.
As a result of investigating and studying the influence of pseudo-bainite structure and polygonal ferrite structure on surface cracking, if the pseudo-bainite structure is regulated to 40% or less by the mixing ratio of pseudo-bainite structure-polygonal ferrite structure, the effect of pseudo-bainite structure is It was clarified that defects such as surface cracks did not occur at all in the slab. The reason for this is not necessarily clear, but it is assumed that the cause is that the soft polygonal ferrite structure is dominant.
[0015]
Slab cooling conditions: 950 to 750 ° C temperature range 5 to 50 ° C / sec cooling rate In order to obtain a microstructure with a pseudo-bainite structure of 40% or less, the solidified slab has a temperature range of 950 to 750 ° C. It is necessary to cool the slab at a cooling rate of 50 ° C./second or less. This temperature range is in the two-phase region of ferrite and austenite, and is effective in generating a polygonal ferrite structure or a pseudobainite structure from the γ phase. If the temperature range is outside the specific conditions, a structure with a pseudo-bainite phase of 40% or less cannot be obtained. Further, at a cooling rate exceeding 50 ° C./second, a structure having a pseudo-bainite phase of 40% or less cannot be obtained in any steel component.
However, an excessively slow cooling rate causes a decrease in productivity, so the lower limit of the cooling rate is set to 5 ° C./second.
A slab whose composition and structure are adjusted in accordance with the present invention is a hot-rolled, pickled, and cold-rolled process, and is a high-strength cold-rolled steel sheet that has been recrystallized annealing, or a hot-dip plated steel sheet that has been hot-dip plated after recrystallization annealing Used as.
[0016]
In hot rolling, it is preferable to use low-temperature heating at a heating temperature of 1000 to 1200 ° C. and adopt high-temperature winding at a finishing temperature Ar ₃ transformation point or higher and 600 ° C. or higher. Low temperature heating and high temperature winding are effective in improving the workability by fixing C and N, which are solid solution in steel, as carbides, nitrides, carbonitrides such as Fe ₃ C and AlN. It is. In addition, when hot rolling is performed at a finishing temperature that does not reach the Ar ₃ transformation point, a hot rolled texture that causes workability deterioration is formed.
The hot rolled coil is cold-rolled after descaling and pickling. In cold rolling, it is preferable to set the rolling reduction to 60% or more in order to obtain sufficient deep drawability. The recrystallization annealing after the cold rolling is performed in a continuous annealing process, or in a reduction annealing furnace in a hot dipping facility when hot dipping such as Zn or Al is performed. In addition, the plating conditions are not particularly limited, and conditions that are usually employed industrially are selected. Moreover, temper rolling of 5% or less can be applied to the cold-rolled steel sheet after annealing and the hot-dip plated steel sheet after hot dipping.
[0017]
【Example】
After the molten steel was adjusted to the composition shown in Table 1 in the melting furnace, it was poured into a continuous casting mold, and the cooling rate in the temperature range of 950 to 750 ° C. was changed in the range of 10 to 120 ° C./second in the cooling process after solidification. A 50 kg steel ingot was produced by casting.
[0018]

[0019]
The surface of the obtained steel ingot was observed with the naked eye, and the presence or absence of surface cracks was investigated. Subsequently, the number of surface cracks was counted in the magnetic particle testing. Moreover, the microstructure of the steel ingot was observed using an optical microscope. As shown in Table 2, when the cooling rate of the steel ingot is regulated to 50 ° C./second or less according to the present invention, the structure ratio of pseudo-bainite is suppressed to 40% or less and surface cracks do not occur. It was. On the other hand, in the comparative examples of test numbers 5 and 7 in which the cooling rate of the steel ingot exceeded 50 ° C./second, the structure ratio of pseudobainite greatly exceeded 40%, and surface cracks occurred frequently.
Further, when the occurrence of surface cracks and the relationship between the microstructure and the cooling rate during casting were investigated, the relationship shown in FIG. 1 was established between them. That is, when the pseudo-bainite structure is 40% or less in the microstructure of the pseudo-bainite structure-polygonal ferrite structure, and after cooling, the temperature range of 950 to 750 ° C. is cooled at a cooling rate of 50 ° C./second or less. In either case, no cracks are observed at all, and it can be seen that a good slab can be obtained.
[0020]

[0021]
A steel ingot having no surface crack was hot forged into a steel piece having a thickness of 35 mm, and then heated to 1130 ° C. and hot rolled. The finishing temperature of hot rolling was set to 890 to 930 ° C. and above the Ar ₃ transformation point. The finishing plate thickness was set to 5 mm in consideration of the rolling ratio in the subsequent cold rolling. After the hot rolling finish, the steel strip is inserted into a salt bath heated to 650 to 720 ° C., heated to a predetermined temperature, and held for about 2 hours to perform a process equivalent to winding the hot rolled steel strip. did.
Subsequently, descaling and pickling were performed, and a cold rolled steel sheet having a thickness of 1.0 mm was obtained by cold rolling at a rolling rate of 80%. This cold-rolled steel sheet was continuously annealed at a temperature of 850 ° C. above the recrystallization temperature.
[0022]
The mechanical properties of the obtained cold-rolled steel sheet were measured using a JIS No. 5 tensile test piece. The Rankford value is measured by a three-point method after giving a tensile pre-strain of 15%, and is measured in the L direction (rolling direction), D direction (45 ° direction in the rolling direction), and C direction (direction perpendicular to the rolling direction). ) Average value [(r _L + 2r _D + r _C ) / 4]. In addition, after forming a flat bottom cylindrical cup with a diameter of 33 mm with a three-stage drawing with a drawing ratio of 2.7 using a blank punched out to a diameter of 90 mm, the tip angle is immersed in various temperature refrigerants composed of liquid nitrogen and organic solvents. A 60 degree punch was pushed from the upper part of the cylinder, and the lowest temperature at which no brittle cracking occurred was measured. This measurement temperature was defined as a secondary processing crack occurrence temperature.
[0023]
As shown in Table 3, the cold rolled steel strip manufactured from the steel ingot with the cooling rate regulated according to the present invention is excellent in 0.2% proof stress, tensile strength and elongation, and the Rankford value is 1 as shown in Table 3. .2 or more, it can be seen that the secondary work crack embrittlement temperature is −100 ° C. or less and excellent workability. On the other hand, even if the cooling rate of the slab is 50 ° C./second or less as defined in the present invention, the test number 15 that does not satisfy the conditions regarding the composition has a low Rankford value and a high secondary work crack embrittlement temperature. Therefore, it was a material in which secondary processing cracks are likely to occur.
[0024]

[0025]
Example 2:
Smelting steel with the composition shown in Table 4 is refined in a converter and degassing furnace, and the slab has a thickness of 250 mm and a single weight of 13 tons while changing the cooling speed of the slab by adjusting the drawing speed and cooling water amount during continuous casting. Manufactured.
[0026]

[0027]
About the obtained slab, surface cracks, microstructures, etc. were investigated in the same manner as in Example 1. As seen in the investigation results in Table 5, when the steel ingot cooling rate is regulated to 50 ° C./second or less according to the present invention, the structure ratio of pseudo-bainite is suppressed to 40% or less, and surface cracks do not occur. understood. On the other hand, in the comparative examples of test numbers 20 and 25 in which the cooling rate of the steel ingot exceeded 50 ° C./second, the structure ratio of pseudobainite greatly exceeded 40%, and surface cracks occurred frequently.
[0028]

[0029]
In the comparative example in which the surface crack occurred in the slab stage, the process plate after the hot rolling could not be performed. Therefore, only the slab without the surface crack was heated to 1130 ° C. in the heating furnace and hot rolled. The hot rolling finishing temperature was set to 890 to 910 ° C. and above the Ar ₃ transformation point, the finishing plate thickness was set to 5 mm, and the winding temperature was set to 650 to 720 ° C.
The hot-rolled steel strip was passed through a continuous pickling line equipped with a hydrochloric acid-based pickling bath and descaled and pickled, then sent to a cold rolling mill and cold rolled at a cold rolling rate of 80%. The obtained cold-rolled steel sheet having a thickness of 1.0 mm was sent to the annealing step and annealed at a recrystallization temperature or higher to obtain a cold-rolled steel sheet product. Some steel strips were subjected to reduction annealing at a temperature higher than the recrystallization temperature and passed through a continuous hot dipping line with a plating bath temperature of 450 ° C. to obtain hot dip galvanized steel sheet products.
The mechanical properties of each product obtained were investigated in the same manner as in Example 1. As can be seen from the survey results in Table 6, all the steel strips are excellent in mechanical properties such as 0.2% proof stress and tensile strength, and have high elongation, Lankford value and low secondary work crack embrittlement temperature. As can be seen, it has good workability.
[0030]

[0031]
【The invention's effect】
As described above, in the present invention, by using a component steel material containing Ti and Nb whose composition is specified, productivity is controlled by controlling the microstructure of the cast slab and the cooling rate during continuous casting. High quality slabs can be manufactured without hindering the production and increasing the manufacturing cost. The obtained slab is used for high-strength cold-rolled steel sheets and hot-dip plated steel sheets having excellent deep drawability.
[Brief description of the drawings]
Fig. 1 Effect of cooling rate during casting on the occurrence of microstructure and surface cracks

Claims

C: 0.0005 to 0.01 wt%, Si: 1.0 wt% or less, Mn: 0.5 to 3.0 wt%, P: 0.05 to 0.2 wt%, S: 0.0005 -0.02 wt%, Al: 0.005-0.10 wt%, N: 0.007 wt% or less, Ti: 0.01-0.1 wt% and / or Nb: 0.01-0. 1% by weight, the balance is composed of Fe and inevitable impurities , and has a high workability with a cast structure having a pseudo bainite structure-polygonal ferrite structure with a pseudo bainite structure of 40% or less. Slab for high-strength cold-rolled steel sheet or hot-dip plated steel sheet.

The slab for high-strength cold-rolled steel sheets or hot-dip plated steel sheets according to claim 1, further comprising a composition containing Cu: 0.03-0.5 wt% and Ni: 0.03-0.5 wt%.

The composition further comprises one or more of B: 0.0002 to 0.001 wt%, Zr: 0.01 to 0.1 wt%, V: 0.01 to 0.1 wt%. The slab for high-strength cold-rolled steel sheets or hot-dip plated steel sheets according to 1 or 2.

The high strength cold rolling excellent in workability according to any one of claims 1 to 3, wherein the slab is cooled at a cooling rate of 5 to 50 ° C / sec when the solidified slab is in a temperature range of 950 to 750 ° C. Manufacturing method of slab for steel plate or hot dipped galvanized steel plate.