JP3975689B2 - Slab, thin steel plate, and manufacturing method thereof - Google Patents

Slab, thin steel plate, and manufacturing method thereof Download PDF

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JP3975689B2
JP3975689B2 JP2001132265A JP2001132265A JP3975689B2 JP 3975689 B2 JP3975689 B2 JP 3975689B2 JP 2001132265 A JP2001132265 A JP 2001132265A JP 2001132265 A JP2001132265 A JP 2001132265A JP 3975689 B2 JP3975689 B2 JP 3975689B2
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slab
mass
hot rolling
steel
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JP2002020836A (en
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修二 中居
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Nippon Steel Corp
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Sumitomo Metal Industries Ltd
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Description

【0001】
【発明の属する技術分野】
本発明は、スラブ、薄鋼板、それらの製造方法に関する。より詳述すれば、本発明は、耐時効性に優れかつプレス成形性に優れたボロン添加薄鋼板とその製造方法およびそのためのスラブとその連続鋳造方法に関し、特に、スラブおよび薄鋼板の表面欠陥を抑制し、自動車や家電用途に適した良好な表面性状を有する薄鋼板を製造できるスラブとその連続鋳造方法ならびに薄鋼板とその製造方法に関する。
【0002】
【従来の技術】
ボロンは鋼中に含有されると変態点温度を低下させるため、例えば熱延鋼板においては仕上げ圧延温度を低下でき、薄物の製品製造が容易になる。また、自動車部品などにプレス成形して用いる加工用鋼板においては、ボロン添加により窒素時効が抑制されたり、極低炭鋼においては粒界結合力を上げて二次加工脆性が抑制されるなど、ボロン添加鋼の加工性は優れたものとなることが、広く知られている。
【0003】
一方、ボロンを添加すると連続鋳造時にスラブ表面に微細な割れが生じ、そのまま熱間圧延を行うと表面疵を生じ、製品表面欠陥により外観を損なうことがある。
【0004】
従来にあっても、このスラブの表面割れを防止するための手段がいくつか提案されている。
例えば、特開昭56-20119号公報の開示する方法は、表層のB量を低下して割れを抑制する方法である。
【0005】
特開昭56-80354号公報には、B量を逆に30ppm 以上とすること、およびそれ以下の時には窒素との関係で特定領域に制御すること、さらにはΔT( 断面収縮率が60%となる温度域の広さ) を50℃以下にすることにより割れを抑制する方法が開示されている。
【0006】
ところで、最近の連続鋳造技術の進歩によりスラブ厚が250mm 以上と厚肉化され、さらに、冷却技術が進歩し鋳込み速度も、例えば1.2m/min以上と高速化している。このため、実際には鋳込み速度が速い領域で表面疵が発生することが多く、上述のような従来技術ではかならずしも抑制しきれていない。
【0007】
特開昭57-60050号公報の開示する発明ではTiを過剰に添加することで、窒素による弊害を取り除こうとするものであるが、表面疵を抑制するためには過剰量のTiの添加が必須となりその分コスト高にならざるを得ない。また、Tiの過剰添加は製品の延性を劣化させる危険がある。
【0008】
さらに特開平6-297088号公報では鋳込み速度に応じてボロン、窒素、酸素を制御する方法が開示されているが、連続鋳造機前において鋼中成分を制御することは実際的には不可能に近く、実用的ではない。
【0009】
【発明が解決しようとする課題】
最近の連続鋳造技術の進歩により、スラブ厚が250mm以上となりさらには鋳込み速度が例えば1.2m/min 以上、特定的には1.4m/min以上と全体に高速化しているため、従来技術では抑制しきれない速度領域での表面疵発生抑制方法が求められている。
【0010】
ここに本発明の一般的な課題は、上述のような従来技術の問題点を解消し得る技術を開発することである。
より特定的には、本発明の課題は、ボロン添加鋼の高速鋳込を行う際に、スラブの表面割れの見られない連続鋳造方法を提供することである。
【0011】
さらに本発明の課題は耐時効性およびプレス成形性に優れた薄鋼板の製造方法を提供することである。
【0012】
【課題を解決するための手段】
発明者は、ボロン添加鋼による連続鋳造後のスラブ表面割れの原因について詳細に調査を実施したところ、次のような知見を得た。
【0013】
(1) 割れ原因は固溶ボロンそのものよりむしろ、スラブ表面のオーステナイト粒界に過剰に析出したボロンの析出物(以下、代表してBNと称する)が主な原因であり、これに鋳込み中のバルジングや曲げ応力が加わるとき発生する。
【0014】
(2) そこで、根本原因であるBNの析出量を抑制するため、スラブ厚250mm 以上の場合における連続鋳造条件を成分に応じて制御し、ボロンと窒素が固溶状態で鋳込みを完了する、すなわちBNを析出させずに鋳造を完了しスラブとすることを着想した。
【0015】
(3) しかし、そのままではボロン添加の本来の目的であるBNを析出させることができないため、プレス成形性が劣ることがある。そこで、スラブ加熱温度および熱間圧延仕上げ温度並びに巻き取り温度を特定することにより、BNの析出を促進させ所望の性能を発現させる。
【0016】
このように、鋳造条件および熱延条件を特定条件に制御することで製品表面ならびに製品性能の優れた薄鋼板を製造できることを知り、本発明を完成した。
すなわち、ボロン添加鋼による連続鋳造後のスラブ表面割れの原因はスラブ表面のオーステナイト粒界に過剰に析出したBNが主な原因であり、これに鋳込み中のバルジングや曲げ応力が加わるとき発生する。そこで、連続鋳造後の冷却速度を上げ、BNとして析出させることなく、すなわち、ボロンと窒素を固溶状態で凝固させるのである。
【0017】
しかし、鋳込み中の冷却速度、あるいは温度を測定することは実際的に困難である。そこで、冷却速度に代わる実際的指標として鋳込み速度をその成分に応じて制御する方法を検討した。結果、ボロン添加鋼において、その成分値に応じて下記(1)式になるよう、連続鋳造速度(Vc;m/min. )を調整することがBNの析出防止に有効であることが判明し、本発明に至った。ただし、Tiを添加した場合はBNが形成されにくく、Vcへの依存性は小さくなる。
【0018】
すなわち、本発明は、次の通りである。
(1)添加鋼であって、質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有し、凝固時にBNを析出させず、BおよびNが固溶状態で含有される連続鋳造スラブ。
【0019】
(2)前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする上記(1)記載の連続鋳造スラブ。
【0020】
(3) 前記鋼組成が、さらに、質量%で、Nb:0.002 〜0.060 %、およびV:0.002 〜0.050 %の一種または二種を含むことを特徴とする上記(1) または(2) 記載の連続鋳造スラブ。
【0021】
(4)質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有する溶鋼からのスラブの連続鋳造方法であって、連続鋳造速度(Vc)を下記(1)式にしたがって調整することを特徴とするスラブの連続鋳造方法。
【0022】
1.2<Vc≦-3×108[B×(N-14/48Ti)]2
-11500B×(N-14/48Ti)+2.03・・・(1)
ただし、Vc:m/min.B、N:質量%、Tiを含まないときはTi=0
(5)前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする上記(4)記載のスラブの連続鋳造方法。
【0023】
(6) 前記鋼組成が、さらに、質量%で、Nb:0.002 〜0.060 %、およびV:0.002 〜0.050 %の一種または二種を含むことを特徴とする上記(4) または(5) 記載のスラブの連続鋳造方法。
【0024】
(7) 請求項4ないし6のいずれかに記載の連続鋳造方法で得られたスラブを直接、保持後または加熱後、熱間圧延を行うに際し、下記条件で熱間圧延を行うことを特徴とする表面性状と成形性に優れる薄鋼板の製造方法。
【0025】
スラブ加熱温度≦1250℃、
仕上げ温度 ≧Ar3 点温度、そして
巻き取り温度 ≧500℃。
【0026】
(8) 前記熱間圧延において、該熱間圧延を粗熱間圧延と仕上げ熱間圧延とに分けて行い、該粗熱間圧延後の粗熱間圧延材を加熱または保熱してから仕上げ熱間圧延を行うことを特徴とする上記(7) 記載の薄鋼板の製造方法。
【0027】
(9)添加鋼であって、質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有し、BおよびNが固溶状態で存在する連続鋳造スラブから製造され、熱間圧延段階で析出したBNを含有することを特徴とする薄鋼板。
【0028】
(10)前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする上記(9)記載の薄鋼板。
【0029】
(11)前記鋼組成が、さらに、質量%で、Nb:0.002 〜0.060 %、およびV:0.002 〜0.050 %の一種または二種を含むことを特徴とする上記(9) または(10)記載の薄鋼板。
【0030】
【発明の実施の形態】
本発明は、ボロン添加鋼の連続鋳造に際して、B、NおよびTiの含有量を特定範囲に限定するが、これは鋳造速度との相関でBNを析出させずに連続鋳造が可能な条件を規定するものである。なお、以下において「%」は特にことわりがない限り「質量%」を意味する。
【0031】
図1は、鋼成分として質量%でB =0.0002〜0.0060、N ≦0.0065、Ti=0〜0.070 、を含有する溶鋼をスラブ厚250mm 以上に鋳込むにあたり、鋳込み速度Vcを種々変更し、成分と製品表面疵との関係を調査した結果を示すグラフである。
【0032】
図1に示すように、B、N、Ti量に応じて鋳込み速度を(1)式によって制御することにより、表面性状の良好なスラブが得られた。

Figure 0003975689
Vc: m/min 、N 、B 、Tiは質量%である。
【0033】
ここで、TiはBよりも窒化物を容易に作るため、その成分量に見合う分をあらかじめ控除して、回帰式を求めたものが(1)式である。
このように、B%およびN%により、さらに場合によりTiにより、Vcの上限値が影響される理由については明らかでないが次のように考えられる。
【0034】
すなわち、Vcが高くなるにつれ、スラブの冷却速度は相対的に低下して、BNが析出しやすくなること、また、同一Vcであれば、B%、N%の高い方がBNの析出量が増すためと考えられる。
【0035】
また、Vc≦1.2m/min. の領域ではB、Nが上限域でも表面不良が発生せず、本発明の対象から除外した。
このようにして製造されたスラブは、直接、高温のまま熱間圧延が行われるか、あるいは一旦保持後または加熱後、熱間圧延が行われる。いわゆる直送圧延である。
【0036】
しかし、そのままでは薄鋼板におけるボロン添加の本来の目的であるBNを析出させることができない。BNが析出しないと、窒素は固溶状態にあるため常温時効性に劣り、さらには製品性能のうちYS(降伏点)が高くEL(伸び)が低いため、プレス成形性の劣ることがある。
【0037】
ここに、BNの析出に影響する条件は、スラブ加熱温度、圧延仕上げ温度、そして巻取り温度である。
まず、スラブ加熱温度が高いとBNはBとNに分解、固溶状態になる。そこでスラブ加熱温度は極力低めとし、BNとして析出しやすい状況にする。このため加熱上限温度は1250℃とした。下限温度は仕上げ出口温度をAr3 点以上とするため、圧延装置特性を考慮して適宜決定できる。
【0038】
さらに、仕上げ温度は、Ar3 点以上が通常の熱間圧延方法だが、析出を促進するためにはAr3 点直上で終了し、オーステナイト粒に圧延歪みをなるべく多く導入することが望ましい。
【0039】
このための手段として粗熱間圧延材 (粗バー) を加熱あるいは保熱して、スラブ加熱温度を低温に抑制しながら仕上げ温度を所定以上に保つことは極めて有効である。
【0040】
加熱あるいは保熱の方法として例えば、粗バーを誘導加熱方法で加熱する粗バーヒータ、ガスバーナ加熱、直接粗バーに電流を流す通電加熱方式などの適用が望ましい。特に、粗バーヒータは好適である。
【0041】
熱延終了後はコイル状に巻き取るが、この時、BNの析出を十分行わせるため巻き取り温度は500 ℃以上、好ましくは530 ℃以上とする。これより低い温度ではBNの析出が不十分となり、固溶窒素の弊害で、製品の性能が十分発現されない。
【0042】
ここに、連続鋳造法により得られたスラブにおいて、「凝固時にBNを析出せず、B およびN が固溶状態で含有される」か否かは、例えば、BNの総量(total BN)を分析し、それぞれの原子量比から析出物としてのN%を算出し、一方、Nの総量(total N) を分析してこれとの差を固溶Nとして、その量で決定できる。例えば析出状態のN (BNとしてのN) が70%以上であるときをBNが析出していると考え、その量が70%未満のとき、BNが実質上析出していないと判断する。
【0043】
一方、薄鋼板製品の場合にも、「熱間圧延の段階で析出したBN」であるか否かは、BNの総量(Total BN)を分析し、それぞれの原子量比から析出物としてのN%を算出し、Nの総量(Total N) を分析してこれとの差を固溶Nとして、その量で決定できる。そのときに90%以上のNが析出状態である場合、本発明にかかる薄鋼板であるとすることができる。
【0044】
本発明においては、対象となるB添加鋼は、上述の範囲のB、N、およびさらに必要によりTiが含有されている限り特に制限ないが、本発明により得られるスラブに熱間圧延を施して得られる薄鋼板が自動車用部品に加工される用途を前提にするときは、鋼組成を次のように規定できる。
【0045】
すなわち、鋼成分として質量%で、C≦0.12%、Mn:0.05〜1.60%、P≦0.100 %、S≦0.020 %、Al≦0.100 %を含有する薄鋼板である。
さらに、最近のように自動車の軽量化が必要とされるとき、析出強化元素を添加して高強度化する事が望まれる。この場合、C%を0.01〜0.12%とし、さらにTi:0.070 %以下、Nb=0.002〜0.060 %、V=0.002〜0.050 %の1種または2種以上を配合すればよい。
【0046】
自動車の車体、特にボディ用には成形性の優れた極低炭素鋼が用いられるが、この場合、C含有量を0.010 %以下とし、さらに、Nb:0.002 〜0.060 %、V:0.002 〜0.050 %の1種または2種を含むことができる。またC含有量によらずCu、Ni、Cr、Snなどの不純物元素をそれぞれ0.05%以下含むことは何ら支障はない。
【0047】
また、薄鋼板の製品には、熱延鋼板、あるいはこれを用いて酸洗、冷間圧延後連続焼鈍あるいはバッチ焼鈍してから調質圧延を施した冷延鋼板、さらには熱延鋼板および冷延鋼板を母材とした亜鉛めっき鋼板、電気めっき鋼板が含まれる。
【0048】
ここに、本発明において鋼組成を上述のように規定した理由についてさらに説明すると次の通りである。
B:下限は実用的に分析可能な0.0002%とした。上限についても表面性状に対する制約はないが、N%に見合う量として0.0060%とする。好ましくは、0.0050%以下である。
【0049】
N:薄鋼板用には低いほど望ましい。製品性能の劣化を抑制するため上限は0.0065%とする。好ましくは、0.0050%以下である。
C:少ないほど軟質で、自動車用、家電用などにプレス成形するのに好適であるが保証強度などを考慮するとボロン添加鋼で実際的上限は0.12%である。
【0050】
Mn:Cと同様な理由により、下限を0.05%、上限を1.60%とする。
P:Cと同様な理由により、上限を0.100 %とする。
S:不純物として、低いほど望ましく、上限を0.020 %とする。好ましくは0.015 %以下である。
【0051】
Al:脱酸成分として添加するが、必要以上に添加しても効果がなく上限を0.100 %とする。
Ti:極低炭素鋼においてはN、S、Cなどと析出物を構成することにより、それぞれの弊害を除去するために添加されることがある。低炭素鋼においては炭化物の構成により、強度を上昇させる目的で添加されるが上限についてはその効果が減少するため0.070 %とする。好ましくは、0.0002〜0.0065%である。
【0052】
Nb:Tiと類似して、極低炭素鋼においてはN、Cなどと析出物を構成することにより、それぞれの弊害を除去するために添加される。
低炭素鋼においては炭化物の構成により、強度を上昇させる目的で添加されるが、分析の下限が0.002 %であることからこれを下限とした。上限についてはその効果が減少するため0.060 %とする。
【0053】
V:Nbと類似して、極低炭素鋼においてはN、Cなどと析出物を構成することにより、それぞれの弊害を除去するために添加される。
低炭素鋼においては炭化物の構成により、強度を上昇させる目的で添加されるが、分析の下限が0.002 %であることからこれを下限とした。上限についてはその効果が減少するため0.050 %とする。
【0054】
【実施例】
本例では、表1に示す鋼組成の溶鋼を、Vcを変化させて連続鋳造によりスラブ (厚さ 270mm) とし、そのときのスラブ表面の性状を観察した。次いで、このようにして得られたスラブを熱間圧延して2.0mm 、2.3mm 厚の熱間圧延製品を得た。一部の熱延鋼板はさらに圧下率65〜85%で冷間圧延を行い厚さ0.75〜1.0mm の冷延鋼板とし焼鈍工程を経て冷延製品とした。冷延鋼板の1部にはさらに電気Znめっきを施こした。また一部の冷延鋼板は圧延後、連続溶融亜鉛めっきを行って、各々めっき製品とした。
【0055】
表1に、溶鋼の成分、連続鋳造のときのVc、得られたスラブおよび薄鋼板のの表面の判定結果を示す。表面判定が不合格 (×印) のものは手入後さらに熱間圧延に供した。
【0056】
表1には、スラブにおけるtotal N%に対するBNとして析出したN%の比を示す。Ti無添加の場合、スラブの表面判定は、BNの析出量に依存していることが明らかである。
【0057】
ここでスラブ表面性状の判定は、その外観判定により、そのまま熱間圧延に供しうるものを合格として○印表示とした。また何らかの表面手入れを必要とするものを不合格として×印表示とした。
【0058】
【表1】
Figure 0003975689
【0059】
次に、得られた薄鋼板について機械特性の評価試験を行い、その結果を表2に示す。表2には熱間圧延条件も併せて示す。
表2には、析出BNはTi添加鋼を除き、スラブ加熱温度および巻取温度に依存しており、残量の機械特性はこれに依存していることが明らかである。
【0060】
本発明による製造方法によれば、YSが小さく、伸びが大きく、ΔYP値が小さく、薄鋼板の機械性能はいずれにおいても良好な結果が得られている。
【0061】
【表2】
Figure 0003975689
【0062】
【発明の効果】
本発明によれば、ボロンや窒素量の如何にかかわらず、表面性状の良好なスラブおよび薄鋼板が得られる。さらに製品の表面および性能はプレス加工後も含め不具合を生じることなく使用できる。特に本発明により得られたスラブを用いて製造した薄鋼板製品の表面は良好であり自動車部品として外観上も、成形性も何ら遜色のないものが得られた。
【0063】
このように、性能が良好な製品が効率よく生産できることは地球環境保護に寄与し、かつ品質に対する信頼性を高めることができる。
【図面の簡単な説明】
【図1】鋼組成と鋳込み速度との関係で製品表面疵の判定結果を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a slab, a thin steel plate, and a method for producing them. More specifically, the present invention relates to a boron-added thin steel sheet excellent in aging resistance and press formability, a manufacturing method thereof, and a slab and a continuous casting method therefor, in particular, surface defects of the slab and the thin steel sheet. It is related with the slab which can manufacture the thin steel plate which has the favorable surface property suitable for a motor vehicle or household appliance use, its continuous casting method, and a thin steel plate and its manufacturing method.
[0002]
[Prior art]
When boron is contained in the steel, the transformation point temperature is lowered. For example, in the case of a hot-rolled steel sheet, the finish rolling temperature can be lowered, and the production of a thin product is facilitated. In addition, in processing steel sheets used by press forming for automobile parts, etc., nitrogen aging is suppressed by adding boron, and in ultra-low carbon steel, secondary boundary brittleness is suppressed by increasing grain boundary bonding force, etc. It is widely known that the workability of boron-added steel is excellent.
[0003]
On the other hand, when boron is added, fine cracks are generated on the surface of the slab during continuous casting. If hot rolling is performed as it is, surface flaws may occur and the appearance may be impaired due to product surface defects.
[0004]
Even in the prior art, several means for preventing the surface crack of the slab have been proposed.
For example, the method disclosed in Japanese Patent Application Laid-Open No. 56-20119 is a method for suppressing cracks by reducing the amount of B in the surface layer.
[0005]
In Japanese Patent Laid-Open No. 56-80354, the amount of B is set to 30 ppm or more, and if it is less than that, it is controlled to a specific region in relation to nitrogen, and ΔT (cross-sectional shrinkage is 60%. A method of suppressing cracking by setting the temperature range to 50 ° C. or less is disclosed.
[0006]
By the way, with the recent progress of continuous casting technology, the slab thickness has been increased to 250 mm or more, and further, the cooling technology has advanced and the casting speed has been increased to, for example, 1.2 m / min or more. For this reason, surface flaws often occur in a region where the casting speed is high, and the conventional techniques as described above are not necessarily suppressed.
[0007]
In the invention disclosed in Japanese Patent Laid-Open No. Sho 57-60050, an excessive amount of Ti is used to remove the harmful effects of nitrogen. However, in order to suppress surface flaws, it is essential to add an excessive amount of Ti. Therefore, the cost must be increased accordingly. Moreover, excessive addition of Ti has a risk of deteriorating the ductility of the product.
[0008]
Furthermore, Japanese Patent Laid-Open No. 6-97088 discloses a method for controlling boron, nitrogen and oxygen in accordance with the casting speed, but it is actually impossible to control the components in the steel before the continuous casting machine. Close and not practical.
[0009]
[Problems to be solved by the invention]
Due to recent advances in continuous casting technology, the slab thickness has increased to 250 mm or more, and the casting speed has increased to, for example, 1.2 m / min or more, specifically 1.4 m / min or more. There is a need for a method for suppressing the occurrence of surface flaws in a speed range that cannot be achieved.
[0010]
Here, a general problem of the present invention is to develop a technique capable of solving the problems of the prior art as described above.
More specifically, an object of the present invention is to provide a continuous casting method in which surface cracks of slabs are not observed when high-speed casting of boron-added steel is performed.
[0011]
Furthermore, the subject of this invention is providing the manufacturing method of the thin steel plate excellent in aging resistance and press-formability.
[0012]
[Means for Solving the Problems]
The inventor conducted a detailed investigation on the cause of slab surface cracking after continuous casting with boron-added steel, and obtained the following knowledge.
[0013]
(1) The main cause of cracking is boron precipitates (hereinafter referred to as BN) which are excessively precipitated at the austenite grain boundaries on the slab surface, rather than solid solution boron itself. Occurs when bulging or bending stress is applied.
[0014]
(2) Therefore, in order to suppress the precipitation amount of BN, which is the root cause, the continuous casting conditions in the case of a slab thickness of 250 mm or more are controlled according to the components, and the casting is completed when boron and nitrogen are in a solid solution state. The idea was to complete the casting without depositing BN into a slab.
[0015]
(3) However, BN, which is the original purpose of boron addition, cannot be precipitated as it is, and press formability may be inferior. Therefore, by specifying the slab heating temperature, the hot rolling finishing temperature, and the winding temperature, the precipitation of BN is promoted and the desired performance is exhibited.
[0016]
Thus, the present invention was completed by knowing that a thin steel sheet having excellent product surface and product performance can be produced by controlling casting conditions and hot rolling conditions to specific conditions.
That is, the cause of slab surface cracking after continuous casting with boron-added steel is mainly BN precipitated excessively at the austenite grain boundary on the slab surface, and occurs when bulging or bending stress during casting is applied to this. Therefore, the cooling rate after continuous casting is increased so that boron and nitrogen are solidified in a solid solution state without being precipitated as BN.
[0017]
However, it is practically difficult to measure the cooling rate or temperature during casting. Therefore, a method for controlling the casting speed according to the component as a practical index instead of the cooling speed was examined. As a result, it was found that adjusting the continuous casting speed (Vc; m / min.) So as to satisfy the following equation (1) according to the component value of boron-added steel is effective in preventing precipitation of BN. The present invention has been reached. However, when Ti is added, BN is not easily formed, and the dependence on Vc is reduced.
[0018]
That is, the present invention is as follows.
(1) B- added steel in mass%, B: 0.0002 to 0.0060%, N: 0.0065% or less , C: 0.12% or less (however, C: 0.01% or less) Steel) containing Mn: 0.05 to 1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, the balance being Fe and impurities A continuous cast slab having a composition, containing no BN during solidification, and containing B and N in a solid solution state.
[0019]
(2 ) The continuous cast slab according to (1), wherein the steel composition further contains, by mass%, Ti: 0.070% or less.
[0020]
(3) The steel composition according to (1) or (2) above, wherein the steel composition further contains one or two of Nb: 0.002 to 0.060% and V: 0.002 to 0.050% in mass%. Continuous casting slab.
[0021]
(4) By mass%, B: 0.0002 to 0.0060%, N: 0.0065% or less, C: 0.12% or less (except for C: 0.01% or less), Mn: Slabs from molten steel containing 0.05 to 1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and having a steel composition comprising the balance Fe and impurities The continuous casting method of slab characterized by adjusting the continuous casting speed (Vc) according to the following formula (1).
[0022]
1.2 <Vc ≦ -3 × 10 8 [B × (N-14 / 48Ti)] 2
-11500B × (N-14 / 48Ti) +2.03 ... (1)
However, Vc: m / min.B, N: mass%, and Ti = 0 when not containing Ti
(5) the steel composition, by mass%, Ti: above, characterized in further comprising containing 0.070% or less (4) continuous casting method of a slab according.
[0023]
(6) The steel composition according to (4) or (5) above, wherein the steel composition further contains one or two of Nb: 0.002 to 0.060% and V: 0.002 to 0.050% in mass%. Slab continuous casting method.
[0024]
(7) When hot rolling is performed directly after holding or heating the slab obtained by the continuous casting method according to any one of claims 4 to 6, the hot rolling is performed under the following conditions: A method for producing a thin steel sheet having excellent surface properties and formability.
[0025]
Slab heating temperature ≦ 1250 ° C.
Finishing temperature ≧ Ar 3 point temperature and winding temperature ≧ 500 ° C.
[0026]
(8) In the hot rolling, the hot rolling is divided into rough hot rolling and finishing hot rolling, and the hot rolling material after the hot rolling is heated or heat-retained before finishing heat. The method for producing a thin steel sheet according to the above (7), characterized by performing hot rolling.
[0027]
(9) B- added steel in mass%, B: 0.0002 to 0.0060%, N: 0.0065% or less , C: 0.12% or less (however, C: 0.01% or less) Steel) containing Mn: 0.05 to 1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, the balance being Fe and impurities A thin steel sheet having a composition and containing BN produced from a continuously cast slab in which B and N are present in a solid solution state and precipitated in a hot rolling step.
[0028]
(10) The steel sheet according to (9), wherein the steel composition further contains, by mass%, Ti: 0.070% or less .
[0029]
(11) The steel composition according to (9) or (10) above, wherein the steel composition further contains one or two of Nb: 0.002 to 0.060% and V: 0.002 to 0.050% by mass%. Thin steel plate.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the content of B, N and Ti is limited to a specific range during continuous casting of boron-added steel, but this defines the conditions under which continuous casting can be performed without precipitating BN in correlation with the casting speed. To do. In the following, “%” means “% by mass” unless otherwise specified.
[0031]
Fig. 1 shows various changes in the casting speed Vc when casting molten steel containing B = 0.0002 to 0.0060, N ≤ 0.0065, Ti = 0 to 0.070 as a steel component to a slab thickness of 250 mm or more. It is a graph which shows the result of having investigated the relationship with a product surface flaw.
[0032]
As shown in FIG. 1, a slab with good surface properties was obtained by controlling the casting speed according to the formula (1) according to the amounts of B, N, and Ti.
Figure 0003975689
Vc: m / min, N 1, B 2 and Ti are mass%.
[0033]
Here, since Ti makes nitride more easily than B, the equation (1) is obtained by subtracting the amount corresponding to the component amount in advance and obtaining the regression equation.
As described above, the reason why the upper limit value of Vc is influenced by B% and N% and further by Ti in some cases is not clear, but is considered as follows.
[0034]
That is, as Vc increases, the cooling rate of the slab decreases relatively, and BN is likely to precipitate. If the same Vc, the higher the B% and N%, the more BN precipitates. It is thought to increase.
[0035]
Further, in the region of Vc ≦ 1.2 m / min., Surface defects did not occur even when B and N were in the upper limit region, and were excluded from the subject of the present invention.
The slab thus produced is directly hot-rolled at a high temperature, or once held or heated, it is hot-rolled. This is so-called direct rolling.
[0036]
However, as it is, BN which is the original purpose of boron addition in the thin steel sheet cannot be precipitated. If BN does not precipitate, nitrogen is in a solid solution state, so that it is inferior in normal temperature aging, and further, YS (yield point) is high and EL (elongation) is low in product performance, so press formability may be inferior.
[0037]
Here, the conditions affecting the precipitation of BN are the slab heating temperature, the rolling finishing temperature, and the winding temperature.
First, when the slab heating temperature is high, BN decomposes into B and N and enters a solid solution state. Therefore, the slab heating temperature is set as low as possible so that it is likely to precipitate as BN. For this reason, the heating upper limit temperature was set to 1250 ° C. The lower limit temperature can be appropriately determined in consideration of rolling mill characteristics since the finishing outlet temperature is set at Ar 3 or higher.
[0038]
Further, although the finishing temperature is a normal hot rolling method at an Ar 3 point or higher, in order to promote precipitation, it is desirable to finish the process immediately above the Ar 3 point and introduce as much rolling strain as possible to the austenite grains.
[0039]
As a means for this purpose, it is extremely effective to heat or heat the rough hot-rolled material (coarse bar) and keep the finishing temperature at a predetermined level or more while suppressing the slab heating temperature to a low temperature.
[0040]
As a heating or heat retaining method, for example, a rough bar heater that heats a rough bar by an induction heating method, a gas burner heating, or an energization heating method in which a current is directly supplied to the rough bar is desirable. In particular, the coarse bar heater is suitable.
[0041]
After the hot rolling is completed, the coil is wound into a coil shape. At this time, the winding temperature is set to 500 ° C. or higher, preferably 530 ° C. or higher, in order to sufficiently precipitate BN. If the temperature is lower than this, the precipitation of BN becomes insufficient, and the performance of the product is not sufficiently exhibited due to the harmful effect of solid solution nitrogen.
[0042]
Here, in the slab obtained by the continuous casting method, whether or not “BN is not precipitated during solidification and B and N are contained in a solid solution state” is analyzed, for example, by analyzing the total amount of BN (total BN) Then, N% as a precipitate is calculated from each atomic weight ratio, and on the other hand, the total amount of N (total N) is analyzed, and the difference from this is determined as solid solution N. For example, when the precipitated N (N as BN) is 70% or more, it is considered that BN is precipitated, and when the amount is less than 70%, it is determined that BN is not substantially precipitated.
[0043]
On the other hand, in the case of a thin steel plate product, whether or not it is “BN precipitated in the hot rolling stage” is determined by analyzing the total amount of BN (Total BN) and calculating the N% as a precipitate from each atomic weight ratio. Is calculated, the total amount of N (Total N) is analyzed, and the difference from this is determined as solid solution N. At that time, when 90% or more of N is in a precipitated state, the steel sheet according to the present invention can be considered.
[0044]
In the present invention, the target B-added steel is not particularly limited as long as it contains B and N in the above-mentioned range, and further Ti if necessary, but the slab obtained by the present invention is subjected to hot rolling. When it is assumed that the obtained thin steel sheet is processed into an automobile part, the steel composition can be defined as follows.
[0045]
That is, it is a thin steel plate containing C ≦ 0.12%, Mn: 0.05 to 1.60%, P ≦ 0.100%, S ≦ 0.020%, and Al ≦ 0.100% in terms of mass% as steel components.
Furthermore, when it is necessary to reduce the weight of an automobile as in recent years, it is desired to increase the strength by adding a precipitation strengthening element. In this case, C% is set to 0.01 to 0.12%, and further, Ti: 0.070% or less, Nb = 0.002 to 0.060%, V = 0.002 to 0.050%, or one or more may be blended.
[0046]
Extremely low carbon steel with excellent formability is used for automobile bodies, especially for bodies. In this case, the C content is 0.010% or less, and Nb: 0.002 to 0.060%, V: 0.002 to 0.050% 1 type or 2 types can be included. Further, regardless of the C content, the inclusion of 0.05% or less of each of impurity elements such as Cu, Ni, Cr and Sn has no problem.
[0047]
Thin steel products include hot-rolled steel sheets, cold-rolled steel sheets that have been subjected to temper rolling after pickling, cold rolling, continuous annealing or batch annealing, and hot-rolled steel sheets and cold-rolled steel sheets. This includes galvanized steel sheets and electroplated steel sheets that use rolled steel sheets as base materials.
[0048]
Here, the reason why the steel composition is defined as described above in the present invention will be further described as follows.
B: The lower limit was set to 0.0002% which can be practically analyzed. There is no restriction on the surface properties for the upper limit, but the amount commensurate with N% is 0.0060%. Preferably, it is 0.0050% or less.
[0049]
N: The lower the better for thin steel sheets. The upper limit is set to 0.0065% to suppress deterioration of product performance. Preferably, it is 0.0050% or less.
C: The smaller the amount, the softer it is and the more suitable it is for press forming for automobiles, home appliances, etc., but considering the guaranteed strength, the practical upper limit is 0.12% for boron-added steel.
[0050]
For the same reason as Mn: C, the lower limit is set to 0.05% and the upper limit is set to 1.60%.
P: For the same reason as C, the upper limit is made 0.100%.
S: The lower the impurity, the better. The upper limit is made 0.020%. Preferably it is 0.015% or less.
[0051]
Al: added as a deoxidizing component, but if added more than necessary, there is no effect and the upper limit is made 0.100%.
Ti: In ultra-low carbon steel, it may be added in order to remove each harmful effect by constituting precipitates with N, S, C and the like. In low carbon steel, it is added for the purpose of increasing strength due to the composition of carbides, but the upper limit is set to 0.070% because the effect is reduced. Preferably, it is 0.0002 to 0.0065%.
[0052]
Similar to Nb: Ti, in ultra-low carbon steel, it is added in order to remove each harmful effect by constituting precipitates with N, C and the like.
In low carbon steel, it is added for the purpose of increasing the strength due to the composition of carbides, but the lower limit of the analysis is 0.002%, so this was made the lower limit. The upper limit is set to 0.060% because the effect decreases.
[0053]
V: Similar to Nb, in ultra-low carbon steel, it forms a precipitate with N, C, etc., and is added to remove the respective harmful effects.
In low carbon steel, it is added for the purpose of increasing the strength due to the composition of carbides, but the lower limit of the analysis is 0.002%, so this was made the lower limit. The upper limit is 0.050% because the effect decreases.
[0054]
【Example】
In this example, the molten steel having the steel composition shown in Table 1 was made into a slab (thickness: 270 mm) by continuous casting while changing Vc, and the properties of the slab surface at that time were observed. Subsequently, the slab thus obtained was hot-rolled to obtain hot-rolled products having a thickness of 2.0 mm and 2.3 mm. Some of the hot-rolled steel sheets were further cold-rolled at a rolling reduction of 65 to 85% to obtain cold-rolled steel sheets having a thickness of 0.75 to 1.0 mm, and were subjected to an annealing process to obtain cold-rolled products. One part of the cold-rolled steel plate was further subjected to electric Zn plating. Some of the cold-rolled steel sheets were continuously hot-dip galvanized after rolling to obtain plated products.
[0055]
Table 1 shows the determination results of the components of the molten steel, the Vc at the time of continuous casting, the surface of the obtained slab and the thin steel plate. Those whose surface judgment was unacceptable (x mark) were subjected to hot rolling after purchase.
[0056]
Table 1 shows the ratio of N% deposited as BN to total N% in the slab. When Ti is not added, it is clear that the surface determination of the slab depends on the amount of BN deposited.
[0057]
Here, the determination of the slab surface property was determined by the appearance determination, and a mark that can be subjected to hot rolling as it was as a pass was indicated as a circle. Moreover, the thing which requires a certain surface care was made disqualified and was set as x mark display.
[0058]
[Table 1]
Figure 0003975689
[0059]
Next, the obtained thin steel sheet was subjected to a mechanical property evaluation test, and the results are shown in Table 2. Table 2 also shows hot rolling conditions.
In Table 2, it is clear that the precipitated BN depends on the slab heating temperature and the coiling temperature except for the Ti-added steel, and the remaining mechanical properties depend on this.
[0060]
According to the production method of the present invention, YS is small, elongation is large, ΔYP value is small, and good results are obtained in any mechanical performance of the thin steel sheet.
[0061]
[Table 2]
Figure 0003975689
[0062]
【The invention's effect】
According to the present invention, a slab and a thin steel plate having good surface properties can be obtained regardless of the amount of boron and nitrogen. Furthermore, the surface and performance of the product can be used without any problems including after pressing. In particular, the surface of the thin steel plate product produced using the slab obtained according to the present invention was good, and an automotive part having no inferior appearance and formability was obtained.
[0063]
Thus, the ability to efficiently produce a product with good performance contributes to the protection of the global environment and can improve the reliability of quality.
[Brief description of the drawings]
FIG. 1 is a graph showing the determination result of product surface flaws in relation to steel composition and casting speed.

Claims (11)

添加鋼であって、質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有し、凝固時にBNを析出させず、BおよびNが固溶状態で含有される連続鋳造スラブ。A B-added steel, by mass%, B: 0.0002~0.0060%, N : 0.0065% or less, C: 0.12% or less (except C: unless 0.01% or less ), Mn: 0.05 to 1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and having a steel composition comprising the balance Fe and impurities. A continuous cast slab containing B and N in a solid solution state without causing BN to precipitate during solidification. 前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする請求項1記載の連続鋳造スラブ。The steel composition, by mass%, Ti: 0.070% claim 1 Symbol placement of continuously cast slab characterized in that it further comprises the following. 前記鋼組成が、さらに、質量%で、Nb:0.002〜0.060%、およびV:0.002〜0.050%の一種または二種を含むことを特徴とする請求項1または2記載の連続鋳造スラブ。Said steel composition further contains, by mass%, Nb: from 0.002 to 0.060%, and V: claim 1 or 2, characterized in that it comprises 0.002 to 0.050% of one or two Continuous slab as described. 質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有する溶鋼からのスラブの連続鋳造方法であって、連続鋳造速度(Vc)を下記(1)式にしたがって調整することを特徴とするスラブの連続鋳造方法。
1.2<Vc≦-3×108[B×(N-14/48Ti)]2
-11500B×(N-14/48Ti)+2.03・・・(1)
ただし、Vc:m/min.B、N:質量%、Tiを含まないときはTi=0
% By mass: B: 0.0002 to 0.0060%, N: 0.0065% or less, C: 0.12% or less (except when C: 0.01% or less), Mn: 0.05 Continuous casting of slabs from molten steel containing steel composition consisting of ˜1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, the balance being Fe and impurities It is a method, Comprising: Continuous casting speed (Vc) is adjusted according to following (1) Formula, The continuous casting method of the slab characterized by the above-mentioned.
1.2 <Vc ≦ -3 × 10 8 [B × (N-14 / 48Ti)] 2
-11500B × (N-14 / 48Ti) +2.03 ... (1)
However, Vc: m / min.B, N: mass%, and Ti = 0 when not containing Ti
前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする請求項記載のスラブの連続鋳造方法。The steel composition, by mass%, Ti: 0.070% continuous casting process according to claim 4, wherein the slab, characterized in that it further comprises containing less. 前記鋼組成が、さらに、質量%で、Nb:0.002〜0.060%、およびV:0.002〜0.050%の一種または二種を含むことを特徴とする請求項または記載のスラブの連続鋳造方法。Said steel composition further contains, by mass%, Nb: from 0.002 to 0.060%, and V: Claim 4 or 5, characterized in that it comprises a from 0.002 to 0.050% of one or two The continuous casting method of the slab as described. 請求項ないしのいずれかに記載の連続鋳造方法で得られたスラブを直接、保持後または加熱後、熱間圧延を行うに際し、下記条件で熱間圧延を行うことを特徴とする表面性状と成形性に優れる薄鋼板の製造方法。
スラブ加熱温度≦1250℃、
仕上げ温度≧Ar点温度、そして
巻き取り温度≧500℃
A surface property characterized by performing hot rolling under the following conditions when performing hot rolling directly after holding or heating the slab obtained by the continuous casting method according to any one of claims 4 to 6. And manufacturing method of thin steel sheet with excellent formability.
Slab heating temperature ≦ 1250 ° C.
Finishing temperature ≧ Ar 3 point temperature and coiling temperature ≧ 500 ° C.
前記熱間圧延において、該熱間圧延を粗熱間圧延と仕上げ熱間圧延とに分けて行い、該粗熱間圧延後の粗熱間圧延材を加熱または保熱してから仕上げ熱間圧延を行うことを特徴とする請求項記載の薄鋼板の製造方法。In the hot rolling, the hot rolling is divided into rough hot rolling and finish hot rolling, and the hot hot rolled material after the rough hot rolling is heated or heat-retained before finishing hot rolling. The method for producing a thin steel sheet according to claim 7, wherein the method is performed. 添加鋼であって、質量%で、B:0.0002〜0.0060%、N:0.0065%以下、C:0.12%以下(ただしC:0.01%以下の場合を除く)、Mn:0.05〜1.60%、P:0.100%以下、S:0.020%以下、Al:0.100%以下を含有し、残部Feおよび不純物からなる鋼組成を有し、BおよびNが固溶状態で存在する連続鋳造スラブから製造され、熱間圧延段階で析出したBNを含有することを特徴とする薄鋼板。 B added steel, in mass%, B: 0.0002 to 0.0060%, N: 0.0065% or less , C: 0.12% or less (except for the case of C: 0.01% or less) ), Mn: 0.05 to 1.60%, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and having a steel composition comprising the balance Fe and impurities. A thin steel sheet comprising BN produced from a continuously cast slab in which B and N are present in a solid solution state and precipitated in a hot rolling stage. 前記鋼組成が、質量%で、Ti:0.070%以下をさらに含有することを特徴とする請求項記載の薄鋼板。The thin steel sheet according to claim 9 , wherein the steel composition further contains, by mass%, Ti: 0.070% or less. 前記鋼組成が、さらに、質量%で、Nb:0.002〜0.060%、およびV:0.002〜0.050%の一種または二種を含むことを特徴とする請求項9または10記載の薄鋼板。It said steel composition further contains, by mass%, Nb: from 0.002 to .060%, and V: claim 9 or 10, characterized in that it comprises a 0.002 to 0.050% of one or two The described thin steel sheet.
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