JP3821036B2 - Hot rolled steel sheet, hot rolled steel sheet and cold rolled steel sheet - Google Patents

Description

【００２７】
得られた熱延鋼板のうち厚さ２．６ｍｍの鋼板を用いて、熱延鋼板としての表面性状、すなわち、表面疵の発生状況と表面凹凸状態、フェライトが組織に占める面積割合、フェライトの平均結晶粒径とアスペクト比、隣接フェライト粒間の結晶方位差、析出物が組織に占める面積割合と最小径、残部相の面積割合、及び第二相の平均結晶粒径を調査した。
【００２８】
また、JIS Z 2201に規定される５号引張試験片を圧延方向に対して０°、４５°、９０°の方向から採取して引張試験を行い、降伏強さ（ＹＳ）、引張強さ（ＴＳ）、破断伸び（ＥＬ）及びｒ値を測定した。穴拡げ率（λ）は、直径１０ｍｍの打ち抜き穴のバリをダイ側にして、頂角が６０°の円錐ポンチを打ち抜き穴に圧入し、クラックが板厚を貫通するまで押し拡げた時の穴径ｄを測定し、初期穴径ｄ_０ に対する比として下記式（ａ）で求めた。
【００２９】
λ＝｛（ｄ−ｄ₀）／ｄ₀｝×１００（％） ・・・ （ａ）
得られた熱延鋼板のうち厚さ４ｍｍの鋼板は、表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍ以下で、且つアスペクト比が２０以下の鋼塊を素材とするものについてだけ、上記の厚さ２．６ｍｍの熱延鋼板と同様の項目について調査した。更に、その後、通常の方法で酸洗し、圧下率５０〜８０％の冷間圧延を行い、その後８００℃で６０秒焼鈍して厚さ０．８〜２．０ｍｍの冷延鋼板にした。この冷延鋼板からＪＩＳ Ｚ ２２０１に規定される５号引張試験片を圧延方向に対して０°、４５°、９０°の方向から採取して引張特性を調査した。
【００３０】
熱延鋼板の表面性状は、「表面疵指数」及び引張試験後の鋼板の「表面荒さ指数」で評価した。ここで、「表面疵指数」は、表面疵発生面積比、すなわち、疵が発生した面積を鋼板表面の総面積で除して１００倍した％表示の値に基づき、表面疵発生面積比が５％未満の場合を表面疵指数１、表面疵発生面積比が５％以上で１５％未満の場合を表面疵指数２、表面疵発生面積比が１５％以上で３０％未満の場合を表面疵指数３、表面疵発生面積比が３０％以上の場合を表面疵指数４と定めた。
【００３１】
また、引張試験後の鋼板の「表面荒さ指数」は、JIS B 0601で規定された最大高さ（Ｒｙ）が２０μｍ未満の場合を表面荒さ指数１、Ｒｙが２０μｍ以上で４０μｍ未満の場合を表面荒さ指数２、Ｒｙが４０μｍ以上で６０μｍ未満の場合を表面荒さ指数３、Ｒｙが６０μｍ以上の場合を表面荒さ指数４と定めた。
【００３２】
熱延鋼板の成形性に関しては、ＴＳとＥＬの積（以下、「ＴＳ×ＥＬバランス」という）が１６０００ＭＰａ・％以上で、且つＴＳとλの積（以下、「ＴＳ×λバランス」という）が４５０００ＭＰａ・％以上であれば、一般的な成形方法であるプレス成形において問題なく成形できるので、優れた成形性を有すると評価される。
【００３３】
さらに、冷延鋼板の成形性に関しては、ＴＳ×ＥＬバランスが１５０００ＭＰａ・％以上で、且つＴＳとｒ値の積（以下、「ＴＳ×ｒバランス」という）が５００ＭＰａ以上であれば、優れた成形性を有すると評価される。
【００３４】
上述の調査において、フェライトの平均結晶粒径及びフェライトと析出物とを除く残部相のなかで面積割合が最大である第二相の平均結晶粒径は、光学顕微鏡又は走査電子顕微鏡（ＳＥＭ）によって組織を１０視野撮影し、この組織写真を用いて直線切断法により測定した平均切片長さを１．１３倍した値とした。
【００３５】
組織に占めるフェライトの面積割合、フェライトと析出物とを除く残部相が組織に占める面積割合は、画像解析装置を用いて上記の１０視野の組織写真を解析し、それらの平均値から求めた。隣接するフェライト結晶粒間の結晶方位差は、電子線後方散乱法（ＥＢＳＰ）によって測定した。
【００３６】
残留オーステナイト量は、板厚中心面についてＸ線回析により測定した。Ｍｏ−Ｋｄ線を入射Ｘ線として使用し、残留オーステナイトの｛２２０｝面、｛３１１｝面、｛２００｝面、｛１１１｝面の各面のＸ線強度比を測定し、平均値から残留オーステナイトの体積率を求めた。本発明では、この体積率を残留オーステナイトの面積割合とした。
【００３７】
析出物の最小径と面積割合は、透過電子顕微鏡（ＴＥＭ）で組織を１０視野撮影し、この組織写真を画像解析して求めた。なお、面積割合にはその平均値を用いた。さらに、引張試験と穴拡げ試験は前記の方法で行い、引張特性と穴拡げ率を測定した。
【００３８】
図１は、熱延鋼板の表面性状と成形性に及ぼす鋳片表層部の結晶粒の最大径とアスペクト比の影響を示す図である。用いられた熱延鋼板は、組織に占めるフェライトの面積割合が７０％以上で、フェライトと第二相の平均結晶粒径がいずれも５０μｍ以下である。
【００３９】
図１において黒く塗りつぶしたものが、良好な表面性状および優れた成形性を有する熱延鋼板を示している。すなわち、表面疵指数が２以下、且つ表面荒さ指数が２以下で、良好な表面性状であり、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％以上、且つＴＳ×λバランス値が４５０００ＭＰａ・％以上で、優れた成形性である。一方、白抜きのものは、表面疵指数が３以上、又は表面荒さ指数が３以上のいずれかに該当して表面性状が劣るか、又は、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満で成形性が劣る熱延鋼板であることを示す。
【００４０】
図１から、表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍ以下、且つ前記表層部の結晶粒のアスペクト比が２０以下の鋳片を熱間圧延した熱延鋼板であれば、表面性状及び成形性に優れることがわかる。
【００４１】
さらに、図１に用いられた鋼板のように、フェライトと第二相の平均結晶粒径がいずれも５０μｍ以下、且つ組織に占めるフェライトの面積割合が７０％以上であっても、フェライト粒界のうち大角粒界が７０％未満の場合、又はフェライト粒のアスペクト比が３を超える場合には、表面性状又は成形性が劣ることが確認できる。図２は、熱延鋼板の成形性に及ぼすフェライト粒径と第二相粒径の影響を示す図である。用いられた熱延鋼板は、表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍ以下、且つ前記表層部の結晶粒のアスペクト比が２０以下の鋳片を熱間圧延して、組織に占めるフェライトの面積割合が７０％以上のものである。
【００４２】
図２において黒く塗りつぶしたものが、ＴＳ×ＥＬバランス及びＴＳ×λバランスともに良好で、優れた成形性を有する熱延鋼板を示す。一方、白抜きのものは、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満のいずれかに該当し、成形性が劣る熱延鋼板を示す。
【００４３】
図２から、フェライトの平均結晶粒径が５０μｍ以下で、且つ第二相の平均結晶粒径が５０μｍ以下の場合にＴＳ×ＥＬバランスとＴＳ×λバランスが良好であることがわかる。
【００４４】
さらに、図２から、フェライトの平均結晶粒径が５０μｍ以下で、且つ第二相の平均結晶粒径が５０μｍ以下であっても、フェライト粒界のうち大角粒界が７０％未満の場合、フェライト粒のアスペクト比が３を超える場合、又は最小径が５ｎｍ未満、或いは最小径が５ｎｍの析出物の面積割合が２％を超える場合には、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満となり、成形性が劣ることがわかる。
【００４５】
図３は、熱延鋼板の成形性に及ぼすフェライト相の面積割合と第二相の面積割合の影響を示す図である。用いられた熱延鋼板は、表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍ以下、且つ前記表層部の結晶粒のアスペクト比が２０以下の鋼塊を熱間圧延して、フェライトの平均結晶粒径が０．５〜４５μｍ、第二相の平均結晶粒径が０．３〜４０μｍである。
【００４６】
図３において黒く塗りつぶしたものが、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％以上、且つＴＳ×λバランス値が４５０００ＭＰａ・％以上である、優れた成形性を有する熱延鋼板を示す。一方、白抜きのものは、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満となり、成形性が劣る熱延鋼板であることを示す。
【００４７】
図３から、フェライトが組織に占める面積割合が７０％以上の場合に、成形性に優れることがわかる。さらに、図３から、フェライトが組織に占める面積割合が７０％以上であっても、フェライト粒界のうち大角粒界が７０％未満の場合、フェライト粒のアスペクト比が３を超える場合、又は最小径が５ｎｍ未満、或いは最小径が５ｎｍ以上の析出物の面積割合が２％を超える場合には、ＴＳ×ＥＬバランス値は１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満となり、成形性が劣ることが確認できる。
【００４８】
次に、冷延鋼板での特性を確認するため、表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍ以下、且つアスペクト比が２０以下の鋼塊を素材にして、熱間圧延された厚さ４ｍｍの熱延鋼板を酸洗して冷間圧延の後、焼鈍して厚さ０．８ｍｍの冷延鋼板を作製した。得られた冷延鋼板を用いて、引張試験を実施した。
【００４９】
図４は、冷延鋼板の成形性に及ぼす熱延鋼板でのフェライト粒径と第二相粒径の影響を示す図である。用いた冷延鋼板は、組織に占めるフェライトの面積割合が７０％以上である熱延鋼板から作製したものである。前述の通り、冷延鋼板の成形性は、ＴＳ×ＥＬバランス値が１５０００ＭＰａ・％以上、且つＴＳ×ｒバランス値が５００ＭＰａ以上を満たすか否かで評価する。
【００５０】
図４において黒く塗りつぶしたものが、ＴＳ×ＥＬバランス及びＴＳ×ｒバランスともに良好で、優れた成形を有する冷延鋼板を示す。一方、白抜きのものは、ＴＳ×ＥＬバランス値が１５０００ＭＰａ・％未満、又はＴＳ×ｒバランス値が５００ＭＰａ未満であり、成形性が劣る冷延鋼板を示している。
【００５１】
図４から、熱延鋼板においてフェライトの平均結晶粒径が５０μｍ以下、且つ第二相の平均結晶粒径が５０μｍ以下であれば、冷延鋼板においてＴＳ×ＥＬバランス及びＴＳ×ｒバランスともに良好であることがわかる。
【００５２】
さらに、図４から、熱延鋼板においてフェライトの平均結晶粒径が５０μｍ以下、且つ第二相の平均結晶粒径が５０μｍ以下であっても、熱延鋼板においてフェライト粒界のうち大角粒界が７０％未満の場合、フェライト粒のアスペクト比が３を超える場合、又は最小径が５ｎｍ未満、或いは最小径が５ｎｍ以上の析出物の面積割合が２％を超える場合には、冷延鋼板のＴＳ×ＥＬバランス値が１５０００ＭＰａ・％未満、又はＴＳ×ｒバランス値が５００ＭＰａ未満で、冷延鋼板の成形性が劣ることがわかる。
【００５３】
図５は、冷延鋼板の成形性に及ぼす熱延鋼板でのフェライト相の面積割合と第二相の面積割合の影響を示す図である。用いられた冷延鋼板は、フェライトの平均結晶粒径が０．５〜４５μｍであり、第二相の平均結晶粒径が０．３〜４０μｍである熱延鋼板を素材として製造されたものである。
【００５４】
図５において黒く塗りつぶしたものが、成形性に優れた冷延鋼板である。一方、白抜きのものは、ＴＳ×ＥＬバランス値が１５０００ＭＰａ・％未満、又はＴＳ×ｒバランス値が５００ＭＰａ未満のいずれかに該当し、成形性に劣る冷延鋼板であることを示している。
【００５５】
図５から、熱延鋼板においてフェライトが組織に占める面積割合が７０％以上の場合に、ＴＳ×ＥＬバランス及びＴＳ×ｒバランスが良好であることがわかる。さらに、図５から、熱延鋼板においてフェライトが組織に占める面積割合が７０％以上であっても、熱延鋼板においてフェライト粒界のうち大角粒界が７０％未満の場合、フェライト粒のアスペクト比が３を超える場合、又は最小径が５ｎｍ未満、或いは最小径が５ｎｍ以上の析出物の面積割合が２％を超える場合には、冷延鋼板のＴＳ×ＥＬバランス値が１５０００ＭＰａ・％未満、又はＴＳ×ｒバランス値が５００ＭＰａ未満で、冷延鋼板の成形性が劣ることが明らかである。
【００５６】
上述の通り、鋳片の段階及び熱延鋼板の段階での金属ミクロ組織を適正化することにより、熱延鋼板の表面性状が良好になり、更に、ＴＳ×ＥＬバランス値も向上して成形性を高めることができる。更に、上述の適正組織を有する熱延鋼板を用いて冷間圧延及び焼鈍することにより、冷延焼鈍鋼板のＴＳ×ＥＬバランス値及びＴＳ×ｒバランス値ともに高めて、冷延鋼板は優れた成形性を具備することが可能になる。
【００５７】
鋼板の熱間圧延では、仕上げ圧延完了温度の変動幅、すなわち、仕上げ圧延完了の最高温度と最低温度の差である△ＦＴ（℃）は、熱間での粗圧延完了温度の変動幅△ＲＴ（℃）の影響を受ける。
【００５８】
図６は、粗圧延完了温度の変動（△ＲＴ）と仕上げ圧延完了温度の変動（△ＦＴ）が熱延鋼板の成形後の寸法精度等に及ぼす影響を示す図である。ここでは、仕上げ圧延完了温度がＡｅ_３点以上の場合を示している。また、熱延鋼板の成形後の寸法精度等は、ＴＳ×ＥＬバランスの変動（以下、△（ＴＳ×ＥＬ）で示す）とＴＳ×λバランスの変動（以下、△（ＴＳ×λ）で示す）で評価する。
【００５９】
通常、熱延鋼板の成形性の評価において、△（ＴＳ×ＥＬ）が２００ＭＰａ・％を超えるか、又は△（ＴＳ×λ）が１５００ＭＰａ・％を超えると、熱延鋼板（熱延コイル）内の特性変動が大きくなり、各種プレス成形法で成形した場合に問題が発生する。具体的には、プレス成形後に発生するスプリングバック量の変動が大きくなって、寸法精度良くプレス成形することが困難になる、または、プレス成形時に割れが発生することである。したがって、プレス成形後の寸法精度を良好にし、割れを防止するためには、△（ＴＳ×ＥＬ）を２００ＭＰａ・％以下、且つ△（ＴＳ×λ）を１５００ＭＰａ・％以下を満足する必要がある。
【００６０】
図６において黒く塗りつぶしたものが、成形後の寸法精度等が良好である熱延鋼板を示す。一方、白抜きのものは、△（ＴＳ×ＥＬ）が２００ＭＰａ・％を超えるか、又は△（ＴＳ×λ）が１５００ＭＰａ・％を超える熱延鋼板であることを示す。
【００６１】
図６に示す結果から、下記式（１）及び（２）を満足する場合に、△（ＴＳ×ＥＬ）を２００ＭＰａ・％以下、且つ△（ＴＳ×λ）が１５００ＭＰａ・％以下の条件を具備し、成形後の寸法精度等が良好であることがわかる。
△ＦＴ≦０．８×△ＲＴ ・・・ (１)
５０℃＜△ＦＴ≦１００℃ ・・・ (２)
同様に、図６に示す結果から、上記式（１）及び（２）を満足しない場合であっても、下記式（３）を満足する場合に、成形後の寸法精度等が良好であることがわかる。
△ＦＴ≦５０℃ ・・・ (３)
熱間での仕上げ圧延における低温オーステナイト領域での歪みは、仕上げ圧延完了後の冷却工程で生じる相変態を通じて、フェライト、第二相及び析出物の生成挙動に影響を及ぼす。すなわち、低温のオーステナイト領域での歪み、特にＡｅ_３点＋１００℃〜Ａｅ_３点の温度域での歪みが大きいほど、フェライト変態の駆動力と核生成速度が増加するため、フェライトの結晶粒径とアスペクト比の低減、組織に占めるフェライトの面積割合とフェライト粒界に占める大角粒界が存在し、残部層がフェライト粒間に均一に微細分布したフェライト主相の均一微細複相組織が得られる。
【００６２】
さらには、フェライト粒界に占める大角粒界の割合が増加し、隣接フェライト粒間の結晶方位差の大半が１５度以下で、大角粒界に囲まれたフェライト粒としては最大径で３０μmを超えるほどの粗大になることはない。これらの特徴を有するフェライト主相の均一微細複相組織は、高成形性を得るには最適である。
【００６３】
また、Ａｅ_３点直上での圧下率が大きいほど、仕上げ圧延完了温度がＡｅ_３点に近いほど、鋼板表面層の歪みも増加する。鋼板表面層は、鋼板のその他の部分に比べて歪みが大きいだけでなく、冷却開始が早く、同じ冷却速度で冷却しても冷却速度が大きくなる。このため、鋼板最表層の仕上完了温度をＡｅ_３点直上とすれば、鋼板最表層のフェライト相と第二相それぞれの平均結晶粒径は、鋼板のその他の部分に比べてより一層小さくすることができる。
【００６４】
上記の効果は、Ａｅ_３点直上の温度域での圧下率が大きいほど顕著に得られる。７０％以上の合計圧下率を得る温度域の上限温度は、Ａｅ_３点＋７０℃とすればより好ましい。さらに、Ａｅ_３点＋５０℃とすれば極めて好ましい。
【００６５】
なお、本発明の鋼板を製造する際に、Ａｅ_３点が非常に重要となる。従来技術では、Ａｒ_３点を仕上げ圧延完了温度の基準（例えば、Ａｒ_３点＋５０℃）としていたが、一般にＡｒ_３点は冷却速度によって異なるフェライト変態開始温度であるため、冷却速度による差が大きく、一義的に定めることができない。したがって、管理温度の指標としては適当ではない。
【００６６】
本発明者は、ラボ試験材と実機材を調査した結果、以下のことを明らかにした。
１）本発明の鋼板を製造する温度域で仕上げ圧延を完了する場合、仕上げ圧延完了前では累積歪みが非常に高く、Ａｅ_３点以下になれば直ちにフェライト相が生成し始める状態にあるため、管理温度としては、一義的に決まらないＡｒ_３点よりＡｅ_３点の方が、冶金的にも実用的にもより適切であること、
２）冷却速度が小さい場合のＡｒ_３点を仕上げ圧延完了温度の指標にとると、一般に冷却速度が大きい場合のＡｒ_３点より高いため、冷却速度が大きい鋼板最表層では仕上げ圧延完了前にフェライト相が生成し始め、圧延によって伸長・粗大化してしまうこと。
【００６７】
上記の知見に基づき、本発明では、従来技術と異なり、化学組成によって一義的に定まるＡｅ_３点を仕上げ圧延完了温度の基準とした。なお、本発明でいうＡｅ_３点は、前述の方法で測定される温度を指す。
【００６８】
更には、歪み誘起により、オーステナイト域で生成する析出物のサイズを大きくし、高温域からフェライト変態が開始してフェライト温度域で生成する析出物が一層高温から生成するようになって粗大化する。
【００６９】
熱間での仕上げ圧延完了温度の変動幅△ＦＴを低減することにより、上述の低温オーステナイト領域であるＡｅ_３点＋１００℃〜Ａｅ_３点の温度域での仕上げ圧延の合計圧下率を増加させ、この温度域での累積歪みを増加させることが容易になる。
図７は、仕上げ圧延完了温度の変動△ＦＴと仕上げ圧延時の累積歪みが熱延鋼板の成形性に及ぼす影響を示す図である。具体的には、同図では、スタンドの出側温度が被圧延鋼板のＡｅ_３点〜Ａｅ_３点＋１００℃にある仕上げ圧延スタンド数をｎとし、前記ｎ台のスタンドのうちｉ番目のスタンドにおける圧下時の真歪みをε_ｉ としたとき、熱間での仕上げ圧延完了温度の変動幅△ＦＴとε_ｉ とが、熱延鋼板のＴＳ×ＥＬバランス値とＴＳ×λバランス値に及ぼす影響を整理している。なお、仕上げ圧延完了温度がＡｅ_３点以上の場合の結果を示している。
【００７０】
図７において黒く塗りつぶしたものが、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％以上、且つＴＳ×λバランス値が４５０００ＭＰａ・％以上で、優れた成形性を有する熱延鋼板である。一方、白抜きのものは、ＴＳ×ＥＬバランス値が１６０００ＭＰａ・％未満、又はＴＳ×λバランス値が４５０００ＭＰａ・％未満であり、成形性が劣る熱延鋼板である。
【００７１】
図７に示す結果から、△ＦＴとε_i の関係が下記式（４）及び（５）を満たすようにすることで、熱延鋼板のＴＳ×ＥＬバランスとＴＳ×λバランスを高めることができる。これは、この条件を満たすように熱間圧延することにより、比較的容易にオーステナイト域で歪みを効果的に累積させることができるからである。
△ＦＴ｛Σ（０．８ ^n-i ×ε i ）｝≦３００ ・・・（４）
△ＦＴ≦１００℃ ・・・ (５)
さらに、この熱延鋼板を適正な条件で酸洗し冷間圧延の後、焼鈍することによって、ＴＳ×ＥＬバランス及びＴＳ×ｒバランスが良好な冷延鋼板が得られることを確認している。
【００７２】
【発明の実施の形態】
本発明が規定する要件について、鋼板の化学組成、鋼板の金属組織および鋼板の製造方法に区分して説明する。以下の説明において、各元素の含有量の％表示は、質量％を意味する。
（Ａ）鋼板の化学組成
Ｃ：０．０００２〜０．２５％、
Ｃ含有量が多いほどフェライトの面積割合が減少し、硬質な残部相の面積割合が増加して、延性や深絞り性に悪影響を及ぼすので、Ｃの含有量は少ない方がよく、０．２％以下とする必要がある。一方、Ｃ含有量が０．０００２％未満では、フェライト粒が極端に粗大化し、高い延性を安定して得られず、鋼板の成形時に表面肌荒れが生じ易くなる。更に、Ｃ含有量を０．０００２％未満に低下させるには、特殊な製鋼技術を必要とするのでコストも嵩む。したがって、Ｃの含有量を０．０００２〜０．２５％とする。なお、Ｃ含有量の上限は、０．１５％とすることが好ましく、０．１％とすれば一層好ましい。
【００７３】
Ｓｉ：０．００３〜３．０％
Ｓｉは、加工性を損なうことなく、鋼の強度を向上させる作用を有する。更に、フェライトの生成を促進して、フェライト量を増加させる作用もある。こうした効果を発揮させるためには、少なくとも０．００３％を含有させる必要がある。しかし、その含有量が３．０％を超えると、鋼の加工性が低下するし、鋼の表面性状も劣化する。したがって、Ｓｉの含有量を０．００３〜３．０％とする。なお、Ｓｉ含有量の上限は１．５％とすることが好ましく、１．０％とすれば一層好ましい。
【００７４】
Ｍｎ：０．００３〜３．０％
Ｍｎは、Ｓによる鋼の熱間脆性を防止する作用を有する。更に、鋼を固溶強化する作用もある。こうした効果を発揮させるためには、少なくとも０．００３％を含有させる必要がある。Ｍｎは０．０１％以上含有させるのが好ましく、０．０５％以上含有させるのが一層好ましい。
【００７５】
一方、Ｍｎを過剰に含有させると、成形性が劣化してしまうだけでなく、熱延後の冷却過程で十分なフェライトを生成させることが困難になり、延性と溶接性が損なわれることがあり、特に、Ｍｎ含有量が３．０％を超えるとその弊害が顕著になる。したがって、Ｍｎの含有量を０．００３〜３．０％とする。なお、Ｍｎ含有量の上限は２．５％とすることが好ましく、２．０％とするのが一層好ましい。
【００７６】
Ａｌ：０．００２〜２．０％
Ａｌは、鋼を脱酸するとともに、フェライトの生成を促進して、フェライト量を増加させるために０．００２％以上含有させる必要がある。Ａｌを含有させることで、後述のＴｉなど任意添加元素の歩留りを高めることもできる。一方、２．０％を超えて含有させても、前記の効果は飽和し、コストが嵩むばかりである。したがって、Ａｌの含有量を０．００２〜２．０％とした。なお、Ａｌ含有量の上限は１．２％とすることが好ましく、０．１％とすれば極めて好ましい。
【００７７】
本発明の鋼板は、上記の各成分元素に加えて更に、下記の第１群〜第４群のうちの１群以上を含んでもよい。
【００７８】
第１群：Ｂ：０．０００２〜０．０１％
Ｂには鋼の焼入れ性を高める作用があるので、冷却過程でフェライト相や残部相の結晶粒径や面積割合を制御する際に活用してもよい。又、Ａｅ_３ 点を低下させる作用があるので、オーステナイト温度域で仕上げ圧延を完了するのが困難な場合にＢを含有させることは有効で、特に厚さが２．０ｍｍ以下の薄物の熱延鋼板を製造する場合に極めて効果的である。
【００７９】
Ｂには、極低炭素鋼板を絞り加工する際に発生するおそれがある「二次加工割れ」を防止する作用もある。このため、前記した目的でＢを含有させてもよいが、Ｂ含有量が０．０００２％未満ではその効果が得難い。しかし、Ｂを０．０１％を超えて含有させるとフェライトの生成が著しく抑制されたり、二次加工割れを防止する作用が飽和するうえ、却って鋼板を脆くすることがある。したがって、Ｂを添加する場合には、その含有量を０．０００２〜０．０１％とする。なお、Ｂを添加する場合、Ｂ含有量の上限は０．００７％とすることが好ましく、０．００５％とすれば一層好ましい。
【００８０】
第２群：Ｔｉ、Ｎｂ、Ｖ及びＺｒ：１種以上を合計で０．００５〜１．０％
Ｔｉ、Ｎｂ、Ｖ及びＺｒには、鋼に含有される固溶Ｃ、固溶Ｎ、固溶Ｓを析出物として固定して無害化する作用があり、特に冷延焼鈍鋼板の深絞り性を向上するのに有効である。更に、延性や深絞り性をそれほど損なうことなく、鋼の強度を高める作用を有する。したがって、鋼の深絞り性や強度を効率よく高めるために、Ｔｉ、Ｎｂ、Ｖ、Ｚｒを１種以上含有させてもよいが、その含有量が合計で０．００５％未満ではその効果が得難い。
【００８１】
一方、合計で１．０％を超えると上記効果は飽和するので、逆に延性や深絞り性が低下し、降伏比が高くなり、プレス成形時の形状凍結性が劣化する。したがって、Ｔｉ、Ｎｂ、Ｖ及びＺｒを添加する場合には、１種以上を合計で０．００５〜１．０％含有させる。なお、合計含有量の下限は０．０１％とするのがよく、０．０２％とすれば一層よい。又、合計含有量の上限は０．５％とするのがよく、０．３％とすれば一層よい。
【００８２】
第３群：Ｃｒ、Ｍｏ、Ｃｕ及びＮｉ：１種以上を合計で０．００５〜３．０％
Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉには焼入れ性を向上させる作用があるので、冷却過程でのフェライトや残部相の結晶粒径や面積割合を制御するのが容易になる。上記焼入れ性を高めることに加えて、Ｃｕには耐食性を高める作用もある。このため、前記した目的でＣｒ、Ｍｏ、Ｃｕ、Ｎｉを１種以上含有させてもよいが、その含有量が合計で０．００５％未満ではその効果が得難く、一方、合計で３．０％を超えると上記効果は飽和するうえ、逆に延性が低下する。したがって、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉを添加する場合には、１種以上を合計で０．００５〜３．０％含有させるのがよい。なお、合計含有量の下限は０．０５％とするのがよく、０．１％とすれば一層よい。又、合計含有量の上限は２．０％とするのがよく、１．０％とすれば一層よい。
【００８３】
第４群：Ｃａ：０．０００１〜０．００５％及びＲＥＭ（希土類元素）：０．０００１〜０．２０％のうちの１種以上
Ｃａ及びＲＥＭには介在物の形状を調整して冷間加工性を改善する作用があるので、冷間加工性を高める目的で含有させてもよいが、Ｃａ、ＲＥＭともにその含有量が０．０００１％未満ではその効果が得難い。一方、Ｃａを０．００５％を超えて、ＲＥＭを０．２０％を超えて含有させてもその効果は飽和し、コストが嵩むばかりである。したがって、Ｃａ、ＲＥＭの１種以上を添加する場合には、Ｃａの含有量は０．０００１〜０．００５％、ＲＥＭの含有量は０．０００１〜０．２０％とするのがよい。
本発明においては、不純物元素としてのＰ、Ｓ、及びＮの含有量を次の通り規定する。
Ｐ：０．１５％以下
Ｐは、結晶粒界に偏析して鋼を脆化させ、特にその含有量が０．１５％を超えると鋼の脆化が著しくなる。したがって、不純物としてのＰの含有量を０．１５％以下とした。なお、Ｐ含有量は０．１２％以下とすることが好ましく、０．１０％以下とすれば一層好ましい。なお、例えばＣ含有量が０．０２％程度以下の極低Ｃ鋼の場合には、不純物としてのＰを０．０００２％以上含有しておれば、フェライト粒が極端に粗大化することを抑制できるので微量のＰを含有していてもよい。
【００８４】
Ｓ：０．０５％以下
Ｓは硫化物系介在物を形成して加工性の低下をきたし、特にその含有量が０．０５％を超えると加工性の低下が著しくなる。したがって、不純物としてのＳの含有量を０．０５％以下とした。なお、Ｓ含有量は０．０３％以下とすることが好ましく、０．０１％以下とすれば一層好ましい。なお、例えばＣ含有量が０．０２％程度以下の低Ｃ鋼や極低Ｃ鋼の場合には不純物としてのＳを０．０００２％以上含有しておれば、フェライト粒が極端に粗大化することを抑制できるので微量のＳを含有していてもよい。
【００８５】
Ｎ：０．０１％以下
Ｎの含有量は加工性を高めるために少ないほど良いが、０．０１％以下であれば本発明においては影響が小さい。したがって、不純物としてのＮの含有量を０．０１％以下とした。なお、Ｎ含有量は０．００７％以下とすることが好ましく、０．００５％以下とすれば一層好ましい。なお、例えばＣ含有量が０．０２％程度以下の極低Ｃ鋼の場合には、不純物としてのＮを０．０００５％以上含有しておれば、フェライト粒が極端に粗大化することを抑制できるので微量のＮを含有してもよい。
（Ｂ）鋼板の金属組織
熱延鋼板の組織に占めるフェライトの面積割合が７０％に満たない場合には、フェライトよりも強度が高い第二相が増えるため高強度が得られるが、熱延鋼板の延性や冷延焼鈍鋼板の延性、深絞り性などの成形性が大幅に劣化してしまう。したがって、熱延鋼板の組織に占めるフェライトの面積割合を７０％以上とする。なお、フェライトの面積割合は８０％以上とするのが好ましく、９０％以上とすれば一層好ましい。なお、熱延鋼板の組織に占めるフェライトの面積割合は１００％に近い値であってもよい。
【００８６】
熱延鋼板のフェライトの平均結晶粒径が５０μｍを超えると、たとえ組織に占めるフェライトの面積割合が７０％以上であっても、熱延鋼板の表面がプレス成形などの加工時に肌荒れを起こし、表面荒さが大きくなって表面性状が低下し、高延性を安定して得ることができない。
【００８７】
同時に、熱延鋼板のフェライトの平均結晶粒径が大きくなり、特に５０μｍを超えると、冷間圧延後の焼鈍時に旧熱延板粒界近傍から生成する｛１１１｝再結晶集合組織の発達が抑制され、高いｒ値が得られない。したがって、熱延鋼板のフェライトの平均結晶粒径を５０μｍ以下とした。なお、フェライトの平均結晶粒径は１０μｍ以下とすることが好ましく、５μｍ以下とすれば一層好ましい。このフェライトの平均結晶粒径は小さいほどよいが、フェライトの平均結晶粒径を０．５μｍ以下にするには極めて特殊な技術が必要となってコストが嵩むので、工業的規模での下限は０．５μｍ程度である。前記の図１〜５で示したように、熱延鋼板のフェライト粒界のうち大角粒界の割合が７０％未満の場合、フェライトのアスペクト比が３を超える場合には、熱延鋼板において所望の表面性状と成形性が得られず、更に、冷延鋼板においても優れた成形性が得られない。したがって、熱延鋼板のフェライト粒界のうち大角粒界の割合を７０％以上、フェライトのアスペクト比を３以下とした。
【００８８】
更に、フェライト粒界のうち大角粒界の割合は８０％以上とすることが好ましく、９０％以上とすれば一層好ましい。フェライト粒界のうち大角粒界の割合は１００％に近い値であっても構わない。
【００８９】
フェライトはそのアスペクト比が３以下の所謂「等軸フェライト」であることを必要とする。等軸フェライトのなかでも、アスペクト比が２以下の場合が一層好ましく、アスペクト比が１に近い値であれば極めて好ましい。前記の図２及び図４で示したように、熱延鋼板において、フェライトと析出物（セメンタイトを除く）を除く残部相のなかで面積割合が最大である第二相の平均結晶粒径が５０μｍを超えると、熱延鋼板において優れた成形性が得られず、冷延鋼板においても所望の成形性が得られない。
【００９０】
特に、前記第二相の平均結晶粒径が５０μｍを超えると、熱延鋼板においては、引張変形時や穴拡げ変形時にフェライトと第二相の界面から発生するクラックがフェライト粒界で伝播を阻止され難くなり、しかも変形が局在化しやすくなる。このため、延性や穴拡げ性が低下する。又、冷延焼鈍鋼板においては、熱延板の第二相が大きくなると、第二相近傍でのすべり系のランダム化により冷間圧延後の焼鈍時に｛１１１｝再結晶集合組織の発達が困難になり、ＴＳ×ＥＬバランス値とＴＳ×ｒ_mバランス値が低下する。
【００９１】
したがって、前記第二相の平均結晶粒径を５０μｍ以下とした。なお、前記第二相の平均結晶粒径は１０μｍ以下とすることが好ましく、５μｍ以下とすれば一層好ましい。この第二相の平均結晶粒径は小さいほどよいが、第二相の平均結晶粒径を０．１μｍ以下にするには、特殊な技術が必要でコストが嵩むので、工業的規模での下限は０．１μｍ程度である。
【００９２】
第二相を均一かつ微細に分散させることにより、熱延鋼板ではＴＳ×ＥＬバランス値とＴＳ×λバランス値をともに向上させ、冷延鋼板ではＴＳ×ＥＬバランスとＴＳ×ｒバランスをともに向上させることができる。特に、フェライト相と第二相の平均結晶粒径をともに５μｍ以下とし、さらに、フェライト相の面積割合を８０％以上、フェライト粒界の８０％以上を大角粒界とし、同時にアスペクト比を３以下とし、残部層として残留オーステナイトを含むベイナイトやマルテンサイトを均一微細に分散させることによって、上述のバランス値を一層向上させることができる。
前記の図２〜５で示したように、熱延鋼板において、析出物の最小径が５ｎｍを超えるか、或いは最小径が５ｎｍ以上の析出物の面積割合が２％を超える場合には、熱延鋼板において所望の成形性が得られず、冷延鋼板においても所望の成形性が得られない。ここで、面積割合を２％以下と限定しているのは、析出強化による強度上昇に起因する成形性の劣化を抑制するためである。
【００９３】
したがって、熱延鋼板において、最小径が５ｎｍ以上の析出物の面積割合が組織の２％以下と規定した。なお、析出物の最小径は１０ｎｍ以上とすることが好ましく、１００ｎｍ以上とすれば一層好ましい。析出物の最小径の上限は２μｍ程度であっても構わない。又、析出物の最大径は５μｍ程度とすることが好ましい。析出物が占める面積割合の下限は０．０００１％であることが好ましい。一方、析出物が占める面積割合の上限は１％であることが好ましく、０．５％であれば一層好ましい。
【００９４】
熱延鋼板が上記の条件を満足する場合、特に、フェライト粒界の７０％以上が大角粒界で、面積割合で組織の７０％以上が平均結晶粒径１μｍ以下の等軸フェライトであり、フェライト相と析出物を除く残部相のなかで面積割合が最大である第二相の平均結晶粒径が１μm以下であれば、熱延鋼板の延性や冷延焼鈍鋼板の延性や深絞り性が飛躍的に向上する。このため、極めて優れた成形性を確保したい場合には、熱延鋼板の組織をフェライト粒界の７０％以上が大角粒界で、面積割合で組織の７０％以上が平均結晶粒径１μｍ以下の等軸フェライトとし、面積割合が最大である第二相の平均結晶粒径が１μm以下とするのがよい。
【００９５】
さらに、本発明の熱延鋼板では、鋼板最表層のフェライト相と第二相の平均結晶粒径をともに５μｍ以下にするのが望ましい。ここで、鋼板最表層と規定するのは、鋼板表面から板厚中心に向かって２結晶粒入った板厚方向の位置までと定義し、最大で鋼板表面から４０μmまでとなる。
なお、粗圧延に供する鋼塊の表面から１０ｍｍ深さまでの表層部の結晶粒の最大径が１０ｍｍを超えると表面割れ感受性が高くなって、例えば、オーステナイト粒界割れである連続鋳造時の縦割れや横割れ、熱間圧延時の横割れといった表面割れが発生し易くなり、熱延鋼板において表面疵が発生して表面性状が劣化する場合がある。又、前記表層部の結晶粒のアスペクト比が２０を超えると、上記表面割れが発生しやすくなって、熱延鋼板において表面疵が発生して表面性状が劣化する場合がある。更に、表層部の結晶粒方位は主として｛１００｝集合組織であるため熱間圧延で再結晶が生じにくくなり、熱間圧延による微細化や結晶粒界の大角化が困難になる場合もある。
【００９６】
したがって、粗圧延に供する鋼塊は、その表面から１０ｍｍ深さまでの表層部において最大径が１０ｍｍ以下の結晶粒であり、しかも、前記表層部の結晶粒のアスペクト比が２０以下であることが望ましい。上記の表層部組織を有する鋼塊は、例えば、鋼の凝固時に液相線温度〜１３００℃の温度域における平均冷却速度を１０℃／秒以上とすることによって得ることができる。なお、上記結晶粒の最大径は５ｍｍ以下であれば更に好ましく、３ｍｍ以下であれば極めて好ましい。又、結晶粒のアスペクト比は１０以下であれば更に好ましく、５以下であれば極めて好ましい。
（Ｃ）鋼板の製造方法
本発明の製造方法を、粗圧延、熱間圧延及び冷間圧延に分けて説明する。
（Ｃ−１）鋳片の粗圧延
前記の化学組成を有する鋼を凝固させた鋳片は、その温度が１２８０℃を超える場合には、一旦１２８０〜９５０℃に冷却してから、直接に熱間での粗圧延を行う。又、粗圧延前に鋳片を再加熱する場合には、９５０〜１２８０℃に再加熱し、次いで粗圧延を行う必要がある。
【００９７】
鋳片を冷却する際の上限温度、又は再加熱する際の上限温度が１２８０℃を上回れば、鋼の鋳込み時に粗大析出しているＭｎＳ、ＡｌＮ、ＴｉＳ、Ｔｉ_４ Ｃ_２ Ｓ_２ などの析出物が再固溶し、熱間圧延時に微細析出して成形性が低下することがある。一方、鋳片を冷却する際の温度が９５０℃を下回ったり、再加熱する際の加熱温度が９５０℃を下回れば、所望のミクロ組織が得られずに成形性が低下する場合がある。
【００９８】
したがって、鋳片は１２８０〜９５０℃に冷却してから直接に熱間での粗圧延を行うか、粗圧延前に鋳片を再加熱する場合には、９５０〜１２８０℃に再加熱する必要がある。なお、鋳片を冷却する際の上限温度、又は再加熱する際の上限温度は１２５０℃とするのが好ましく、１１５０℃とすれば更に好ましい。再加熱する時間は、オーステナイト結晶粒が粗大にならない範囲で鋳片の寸法に応じて適宜選定すればよい。
【００９９】
粗圧延は、少なくともその最終圧延パスを、Ａｅ_３ 点〜１１５０℃の温度域で行うことが望ましい。更に、粗圧延の合計圧下率は４０％以上とするのが望ましい。これは、オーステナイト結晶粒が微細化して、オーステナイトからのフェライト変態後のフェライト結晶粒も微細化するからである。更に、粗圧延の合計圧下率は、５０％以上確保するのが好ましい。
【０１００】
粗圧延を終えた粗圧延材は、仕上げ圧延を開始するまでに再加熱、又は保熱処理を行い、仕上げ圧延を開始する直前の粗圧延材の圧延長手方向と幅方向における温度差を１４０℃以下とするのがよい。これによって、仕上げ圧延完了温度の変動幅（△ＦＴ（℃））とその後の冷却過程での温度変動に起因するミクロ組織の変動を抑制することができ、その結果鋼板の長手方向と幅方向の特性を均一化できる。
【０１０１】
更に、前記のように粗圧延材を仕上げ圧延の開始までに再加熱、又は保熱処理を行うことで、熱間での仕上げ圧延完了温度の変動幅△ＦＴ（℃）が小さくなって、熱延鋼板の圧延長手方向と幅方向のミクロ組織と特性が均一になるし、熱延鋼板における成形性も高まる。
なお、仕上げ圧延を開始する直前の粗圧延材の圧延長手方向と幅方向における温度差は１２０℃以下とすればより好ましく、１００℃以下とすれば極めて好ましい。
【０１０２】
前記のように粗圧延材を仕上げ圧延の開始までに再加熱、又は保熱処理を行うことにより、仕上げ圧延をオーステナイト域で完了する場合には△ＦＴを低減することができる。これにより、仕上げ圧延完了温度をＡｅ_３ 点の直上にしても、鋼板全体に亘ってＡｅ_３ 点以下に温度低下させることなく圧延することが容易になり、低温オーステナイト域での累積歪みを増加させることができる。
【０１０５】
更に、仕上げ圧延前に粗圧延材を再加熱、又は保熱処理することにより、粗圧延前の鋳片温度を低温にしても、仕上げ圧延完了温度を大幅に低下させることなく圧延できるので、仕上げ圧延時の熱間変形抵抗の増加も抑制でき、熱間圧延機に過負荷をかけることなく圧延することができる。すなわち、前記の処理により粗圧延前のスラブ温度を低温にすることができるため、熱間圧延時の微細析出や鋳片表層部の粒界酸化の抑制も可能になり、表面性状の良好な高成形性の熱延鋼板や冷延鋼板を得ることが容易になる。
【０１０６】
鋳片を粗圧延し、コイルボックスを用いてコイル状に巻き取った後で巻き戻して仕上げ圧延を行うプロセス、又は粗圧延材の先端部を先行する粗圧延材の後端部と接合した後で仕上げ圧延を行う連続仕上げ圧延プロセスは、コイルまたは鋼板内の特性を均一化するのに有効であるが、これらのプロセスと仕上げ圧延前に粗圧延材を再加熱、又は保熱する処理を組み合わせることにより、特性をより均一化することが可能になる。
【０１０７】
粗圧延材の表層部を冷却してその組織をオーステナイトからフェライトに変態させた状態から、仕上げ圧延前に粗圧延材を再加熱又は保熱する処理によってオーステナイトに逆変態させることで、この鋼板表層部の逆変態したオーステナイトから、再び変態生成するフェライトの粒径を効率的に微細化し粒界を大角化することができる。
【０１０８】
なお、仕上げ圧延開始までに粗圧延材を加熱又は保熱する手段は、粗圧延材を高周波誘導加熱で加熱する方式、ロールを通じて粗圧延材に直接電流を流して加熱する直接通電加熱方式、燃焼ガスを用いるガスバーナーで粗圧延材を加熱するガス加熱方式等どんな方法でも用いることができる。
（Ｃ−２）熱間仕上げ圧延
前記の金属組織を得るために、粗圧延後にその圧延完了温度がＡｅ₃点以上で、前記式（１）及び（２）又は前記式（３）を満足する条件で仕上げ圧延をし、若しくは前記式（４）及び（５）を満たす条件で熱間仕上げ圧延する必要がある。これは、前記の図６及び図７で示したように、これらの条件を満たさない場合には、良好な成形性を有する熱延鋼板が得られないからである。
【０１０９】
熱間仕上げ圧延の完了温度がＡｅ_３点を下回ると、仕上げ圧延後に所望の鋼板組織が得られない場合があり、変形抵抗が大きくなって圧延自体が困難となる場合がある。したがって、仕上げ圧延の完了温度をＡｅ_３点以上として、熱間仕上げ圧延する必要がある。
【０１１０】
仕上げ圧延では、（Ａｅ_３点＋１００℃）〜Ａｅ_３点の温度範囲で合計圧下率が７０％以上確保する必要がある。これにより、低温のオーステナイト領域での累積加工歪みを増加させ、フェライト変態の駆動力と核生成速度を促進して、フェライトの結晶粒径とアスペクト比を小さくし、組織に占めるフェライトの面積割合の増加と、それによる未変態オーステナイト相へのＣ濃化を図るとともに、フェライト粒界に占める大角粒界の割合を増加させることとしている。
【０１１１】
仕上げ圧延に際しては、熱間潤滑剤を用いて圧延ロールと被圧延材との間の摩擦係数が０．２以下となるようにして圧延してもよい。これにより板厚方向の加工変形が均一化されるので、熱延鋼板や冷延鋼板の板厚表層部のＥＬやｒ値がより向上する。この結果、板全体のＥＬやｒ値を一層向上させることができる。熱間潤滑剤は慣用されるものでよく、例えば摩擦係数の低減が可能な機械油などを用いればよい。
【０１１２】
なお、仕上げ圧延機の入側で鋼片の先端を先行する被圧材の後端と接合して粗圧延材を連続して圧延する、所謂「仕上げ連続圧延法」により圧延すれば、熱間潤滑圧延を行う際に発生する恐れがある被圧延材とロール間のスリップ現象や噛み込み不良を防止できるので、特性が均一化されるし、歩留りも向上する。
【０１１３】
仕上げ圧延終了後には、２秒以内に平均冷却速度３０℃／秒以上で６００〜８００℃の温度範囲内まで水冷する必要がある。すなわち、最終圧延パス終了後冷却開始までの時間を短くして、仕上げ圧延での累積歪みの開放を抑制して、フェライトの結晶粒径とアスペクト比を小さくし、組織に占めるフェライトの面積割合が増加するとともに、フェライト粒界に占める大角粒界の割合を増加させる。
【０１１４】
上記の作用を発揮するためには、平均冷却速度を３０℃／秒以上にする必要がある。更に、６００〜８００℃の温度範囲で冷却停止するのは、６００℃未満まで冷却すると、フェライトがベイニティックフェライト化して、延性が低下する。冷却が８００℃を超えた範囲で停止すると、フェライト粒が生成しても粗大化し、又中間空冷時に第二相へのＣ濃化が進まないため、延性も低下する。
【０１１５】
次いで３〜１５秒の間空冷した後、さらに平均冷却速度３０℃／秒以上で水冷して巻き取ることにしている。３〜１５秒の間空冷を必要としているのは、フェライトの生成を促進するとともに、第二相へのＣ濃化を進展させて、延性の向上を図るためである。その後、平均冷却速度３０℃／秒以上で水冷して巻き取ることにしているのは、平均冷却速度３０℃／秒未満では、第二相中に炭化物が析出して、微細なフェライトに対する最適な第二相としてのベイナイト、残留オーステナイト及びマルテンサイトが得られない。
【０１１６】
巻取温度は、第二相としてのベイナイト、残留オーステナイト及びマルテンサイトを用いる場合には、それぞれの組織を得るための適正な温度を選択する。ベイナイトを得る場合には、巻取温度は３００〜５５０℃、残留オーステナイトを得る場合には、巻取温度は３５０〜５００℃、マルテンサイトを得る場合には、巻取温度は２００℃以下をそれぞれ選択する。
【０１１７】
なお、仕上げ圧延後の冷却停止温度と巻取温度の熱延鋼板（熱延コイル）内での変動は、特性変動を抑制するため、望ましくは１００℃以下、より望ましくは６０℃以下にするのがよい。
（Ｃ−３）冷間圧延び再結晶焼鈍
熱延鋼板は、酸洗などの方法で表面の酸化物や汚れを除去した後冷間圧延され再結晶焼鈍される。冷間圧延後の焼鈍時に｛１１１｝再結晶集合組織を発達させるために、冷間圧延前に熱延鋼板を６５０〜９００℃に加熱して焼鈍を施し、熱延鋼板に｛１１１｝集合組織を発達させてもよい。
【０１１８】
冷間圧延は、圧延集合組織を発達させ、再結晶焼鈍工程でｒ値の向上と面内異方性の最小化に好ましい｛１１１｝集合組織を発達させるために、圧下率５０％以上として最終板厚に加工する。
【０１１９】
再結晶焼鈍は、冷間圧延により導入された圧延集合組織から、深絞り性に好ましい集合組織を発達させるために、６００〜９５０℃の温度範囲で行う必要がある。焼鈍温度が６００℃より低いと、長時間の焼鈍でも再結晶が十分に進行しない場合があり、一方、９５０℃を超えると、ｒ値が低下する場合がある。
【０１２０】
焼鈍方法は特に規定されるものではなく、箱焼鈍法や連続焼鈍法、又は、溶融亜鉛メッキ処理や合金化溶融亜鉛メッキ処理の際に通常おこなわれる連続焼鈍法など任意の方法で行えばよい。
【０１２１】
冷間圧延、再結晶焼鈍の後、通常の方法により、圧下率が１０％未満の調質圧延（スキンパス）を行ったり、溶融亜鉛めっき、合金化溶融亜鉛めっき、電気めっき、有機被覆コーティング等の表面処理を施してもよい。これらの処理を受けた鋼板は、プレス加工を施された後、例えば自動車、家電製品、鋼構造物などに使用される。
【０１２２】
【実施例】
本発明の鋼板の効果を確認するために、表２、３に示す化学組成の鋼種ＡＡ〜ＣＤを真空溶解炉を用いて鋳造し、熱間鍛造により７０ｍｍ厚の鋳片を作製した。これらの鋳片を用いて熱延鋼板を製造した。なお、表２、３には参考値として液相線温度（ＴＬ）、Ａｅ₃点の温度を示している。
【０１２３】
【表２】

【０１２４】
【表３】

【０１２５】
得られた鋳片を用いて、表４、５に示す条件に基づき、実験室規模で鋳片加熱、粗圧延及び仕上げ圧延を行い、厚さ２．６ｍｍ、幅２５０ｍｍの熱延鋼板を製造した。粗圧延材の加熱は、実験室規模の誘導加熱装置を用いて行なった。
【０１２６】
鋼ＡＢでは７００℃巻取、鋼ＡＤと鋼ＡＩでは６００℃巻取後８００℃で焼鈍するという条件で熱延鋼板を製造した。
【０１２７】
鋼ＡＣ、鋼ＡＧ、鋼ＡＭ、鋼ＡＺ、鋼ＢＩ、鋼ＣＡ、鋼ＡＫについては表４、５に示す条件で熱間圧延を行い、３．５〜５．３ｍｍの熱間圧延鋼板も製造した。この３．５〜５．３ｍｍ厚の熱延鋼板については更に酸洗し、後述する表８に示す条件で冷間圧延した後、さらに再結晶焼鈍及び圧下率０．６％の調質圧延を施して厚さ０．８〜１．３ｍｍ、幅２５０ｍｍの冷間圧延鋼板、溶融亜鉛めっき鋼板、及び合金化溶融亜鉛めっき鋼板を製造した。再結晶のための焼鈍は連続焼鈍法、及び溶融亜鉛めっきと合金化溶融亜鉛めっきの際に通常おこなわれる連続焼鈍法で行った。
【０１２８】
得られた熱延鋼板と冷延焼鈍鋼板について、トップ部、ミドル部及びボトム部の３箇所の両エッジ部、１／２幅の合計９箇所から、JIS Z 2201 の５号試験片（０°、４５°、９０°の３方向）及び穴拡げ試験片を採取して、降伏強さ、引張強さ、破断伸び、穴拡げ率、ｒ値、及びそれらの変動を調査した。各特性は鋼板内の最小値として、ＴＳ×ＥＬバランス値、ＴＳ×λバランス値及びＴＳ×ｒ値の変動は鋼板内の最大値から最小値を引いた値として求めた。調査結果を表４〜７と表８に示す。
【０１２９】
【表４】

【０１３０】
【表５】

【０１３１】
【表６】

【０１３２】
【表７】

【０１３３】
【表８】

【０１３４】
表４〜７と表８の結果から明らかなように、本発明で規定する化学組成、金属組織及び製造条件で製造した鋼板は、本発明で規定する条件を外れて製造した鋼板に比較して高い成形性を有し、特性値の鋼板内変動が小さく、表面性状に優れるという特徴を有している。
【０１３５】
【発明の効果】
本発明の熱延鋼板は、表面性状が良好であり、特性値の鋼板内変動が小さく、更に成形性に優れるので、自動車、家電製品、鋼構造物などの用途に好適である。本発明の製造方法によれば、表面性状が良好であり、且つ成形性に優れる熱延鋼板及び冷延鋼板を効率的に製造することができる。
【図面の簡単な説明】
【図１】熱延鋼板の表面性状と成形性に及ぼす鋳片表層部の結晶粒の最大径とアスペクト比の影響を示す図である。
【図２】熱延鋼板の成形性に及ぼすフェライト粒径と第二相粒径の影響を示す図である。
【図３】熱延鋼板の成形性に及ぼすフェライト相の面積割合と第二相の面積割合の影響を示す図である。
【図４】冷延鋼板の成形性に及ぼす熱延鋼板でのフェライト粒径と第二相粒径の影響を示す図である。
【図５】冷延鋼板の成形性に及ぼす熱延鋼板でのフェライト相の面積割合と第二相の面積割合の影響を示す図である。
【図６】粗圧延完了温度の変動（△ＲＴ）と仕上げ圧延完了温度の変動（△ＦＴ）が熱延鋼板の成形後の寸法精度等に及ぼす影響を示す図である。
【図７】仕上げ圧延完了温度の変動△ＦＴと仕上げ圧延時の累積歪みが熱延鋼板の成形性に及ぼす影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel sheet and a method for producing a hot-rolled steel sheet and a cold-rolled steel sheet, and more specifically, heat excellent in formability, particularly ductility and deep drawability used for automobiles, home appliances, steel structures and the like. The present invention relates to a rolled steel sheet and a method for producing the same, and a method for producing a cold rolled steel sheet having excellent formability using the hot rolled steel sheet as a material for cold rolling.
[0002]
[Prior art]
Hot-rolled steel sheets and cold-rolled steel sheets used for automobiles, home appliances, steel structures and the like are required to have excellent formability, in particular, good ductility and deep drawability. The ductility is represented by the elongation at break in the tensile test, and the deep drawability is a characteristic represented by the Rankford value (hereinafter referred to as “r value”) in the tensile test using a plate-shaped test piece. The r value tends to decrease as the tensile strength of the steel plate increases.
[0003]
It is known that reducing the impurity elements such as C, P, S and N contained in the steel will improve the ductility of the hot-rolled steel sheet, but the ferrite grains become extremely coarse due to the reduction of these impurities. To do. As a result, not only high ductility cannot be stably obtained, but surface roughness may easily occur when the steel sheet is formed. Further, when the impurity elements are reduced, the austenite grains become coarse at the time of casting of the steel.For example, during continuous casting of steel or when the steel ingot is roughly rolled after heating, it exhibits austenite grain boundary cracks and surface cracks. Is likely to occur. In addition, in order to reduce impurities in the steelmaking and steelmaking stages, it is necessary to lengthen the vacuum degassing treatment time and increase the amount of desulfurization treatment agent. Become.
[0004]
  Conventionally, a technique for obtaining a hot-rolled steel sheet having excellent surface properties and ductility has been proposed. For example, in JP-A-10-8139, a slab immediately after continuous casting is used.Ar _ThreeAfter rough rolling immediately without cooling below the point to form a rough bar, the rough bar is cooled to a specific temperature range, and the cooled rough bar is heated at a specific heating rate until a specific temperature increase occurs. Then, a manufacturing method has been proposed in which finishing is performed by hot rolling (hereinafter sometimes simply referred to as “hot rolling”). This production method is intended to prevent surface cracking during hot rolling and improve local elongation of the steel sheet by suppressing formation of molten FeS and fine precipitation of Mn-based sulfides.
[0005]
Generally, surface cracks during hot rolling depend not only on precipitates but also on the crystal grain size of the material to be rolled, in particular, the crystal grain size on the surface, and surface cracks are more likely to occur as the crystal grain size on the surface increases. . However, in the production method proposed in the above publication, the low carbon steel (C content is 0.01 to 0.07 mass%) which is easy to coarsen the crystal grains of the steel slab during continuous casting is used. No consideration is given to the prevention of coarsening of crystal grains, particularly the prevention of coarsening of surface crystal grains of continuous cast slabs. For this reason, even if it uses the manufacturing method proposed by the said gazette, the hot-rolled steel plate excellent in surface property cannot be obtained stably.
[0006]
The formability of a cold-rolled steel sheet depends greatly on the properties of the hot-rolled steel sheet that is the material. For this reason, in recent years, in order to obtain a cold-rolled steel sheet having excellent formability, studies on manufacturing conditions at the hot rolling stage have been conducted. As a result, in order to obtain a “cold rolled steel sheet excellent in formability” having a small in-plane anisotropy and a high r value, in the ferrite phase after recrystallization annealing (hereinafter simply referred to as “ferrite”) It has been reported that a {111} texture may be developed.
[0007]
Here, the small in-plane anisotropy means the r value in each direction of 0 °, 45 °, and 90 °.₀ , R₄₅, R₉₀The maximum value (r_max ) And minimum value (r_min ) Is small. Further, in the following description, the ferrite phase is simply referred to as “ferrite”, and in the following description, the “phase” in the structure is omitted, the austenite phase is referred to as “austenite”, the pearlite phase is referred to as “pearlite”, etc. .
[0008]
The {111} texture occurs from the vicinity of the ferrite grain boundary. For this reason, in order to develop the {111} texture, it is necessary to reduce the ferrite crystal grain size generated by transformation from austenite during hot rolling and increase the ferrite grain interface area.
[0009]
In CAMP-ISIJ Vol. 3 (1990) 785, CAMP-ISIJ Vol. 3 (1990) 786 and JP-A-1-177321, a hot-rolled steel sheet with a small ferrite grain size that leads to the realization of a high-formability cold-rolled steel sheet In order to manufacture the steel, a technique of rapidly cooling the steel after finish rolling in the austenite region has been reported. In this technique, ferrite grains generated by ferrite transformation from austenite are to be refined, thereby obtaining a hot rolled steel sheet having a relatively fine ferrite grain structure.
[0010]
In order to develop a {111} recrystallization texture during annealing after cold rolling, it is effective to make the ferrite grain boundary of the hot-rolled steel sheet a so-called “large-angle grain boundary”. Therefore, even if the ferrite grains are refined, the {111} recrystallization texture cannot be effectively developed if the number of sub-crystal grain boundaries having so-called “small-angle grain boundaries” increases. Therefore, in the techniques reported in the above-mentioned CAMP-ISIJ Vol. 3 (1990) 785, CAMP-ISIJ Vol. 3 (1990) 786 and JP-A-1-177321, grain boundaries such as large-angle boundaries and small-angle boundaries are used. No consideration has been given to the properties, and it is merely intended to refine the ferrite grains after transformation, so even if the hot-rolled steel sheet produced using these techniques is used as the raw material, A rolled steel sheet does not necessarily have a small in-plane anisotropy and is sufficiently high to ensure a uniform r value.
[0011]
As described above, the conventional hot-rolled steel sheet and cold-rolled steel sheet formability improving techniques have been problematic in that improvements in ductility, deep drawability, and surface properties cannot be obtained stably.
[0012]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described problems relating to hot-rolled steel sheets and cold-rolled steel sheets, and is excellent in formability, in particular, ductility and deep drawability, and has good surface properties, such as automobiles and home appliances. It aims at providing the manufacturing method of a hot-rolled steel plate suitable for uses, such as a steel structure, and a hot-rolled steel plate and a cold-rolled steel plate.
[0013]
[Means for Solving the Problems]
  The gist of the present invention is the hot rolled steel sheet (1), (2) below, the hot rolled steel sheet manufacturing method (3) to (7), and the cold rolled steel sheet manufacturing method (8).
(1) By mass%, C: 0.0002 to 0.25%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0% and Al: 0.002 to 2.0% The balance is Fe and impurities, P in the impurities is 0.15% or less, S is 0.05% or less, and N is 0.01% or less, and the area ratio is 70% or more of the metal structure. Is the ferrite phase and the average grain size is50μm or less, the aspect ratio is 3 or less, 70% or more of the ferrite grain boundaries are composed of large angle grain boundaries, the maximum diameter of the ferrite phase formed at the large angle grain boundaries is 30 μm or less, and the minimum diameter is 5 nm or more. The average grain size of the second phase having the largest area ratio among the remaining phases excluding the ferrite phase and the precipitate is less than 2% of the microstructure of the metal structure.50A hot-rolled steel sheet characterized by having a large-angle grain boundary of a ferrite phase between the nearest second phases, which is not more than μm.
[0014]
In this steel plate, the average crystal grain size of each of the ferrite phase and the second phase of the outermost layer of the steel plate is preferably 5 μm or less.
(2) Furthermore, the hot-rolled steel sheet of the above (1) may contain one or more elements from the following groups as necessary.
[0015]
  First group: B: 0.0002 to 0.01%
  Second group: Ti, Nb, V and Zr: 0.005 to 1.0% in total of one or more types,
  Third group: Cr, Mo, Cu and Ni: 0.005 to 3.0% in total of one or more kinds,
  Group 4: Ca: 0.0001 to 0.005% and REM (rare earth element): 0. One or more of 0001-0.20%
(3) When continuously casting the steel having the chemical composition described in (1) or (2) above, from the start of solidification of the molten steel until the solidified part at a position 10 mm from the solidified shell surface reaches 1300 ° C. The solidified layer within 10 mm from the surface of the solidified shell is cooled to a cooling rate of 10 ° C./second or more to form a slab, and then the slab is roughly rolled in a temperature range of 950 to 1280 ° C. (Ae_ThreePoint + 100 ° C) to Ae_ThreeThe total rolling reduction is 70% or more in the temperature range of points, and the finishing temperature is Ae._ThreeAnd finish rolling under conditions that satisfy the following formulas (1) and (2) or satisfy the following formula (3), and an average cooling rate of 30 ° C./sec or more within 2 seconds after finishing rolling. A method for producing a hot-rolled steel sheet, comprising: water cooling to a temperature range of 600 to 800 ° C., air cooling for 3 to 15 seconds, and further water cooling at an average cooling rate of 30 ° C./second or more.
              △ FT ≦ 0.8 × △ RT (1)
              50 ℃ <△ FT ≦ 100 ℃ (2)
              ΔFT ≦ 50 ° C (3)
  Here, ΔFT is the fluctuation range (° C.) of the hot finish rolling completion temperature, and ΔRT is the fluctuation range (° C.) of the hot rough rolling completion temperature.
(4) Above (3)alikeIn the method of producing a hot-rolled steel sheet by finish rolling after rough rolling, when continuously casting the steel having the chemical composition described in the above (1) or (2), Ae_ThreePoint + 100 ° C) to Ae_ThreeThe total rolling reduction is 70% or more in the temperature range of the point, the finishing temperature Ae_ThreeHot finish rolling is performed under conditions that satisfy the following formulas (4) and (5), and within a range of 2 to 2 seconds after the finish rolling, up to a temperature range of 600 to 800 ° C. at an average cooling rate of 30 ° C./second or more. A method for producing a hot-rolled steel sheet, comprising water-cooling and then air-cooling for 3 to 15 seconds, and further water-cooling at an average cooling rate of 30 ° C / second or more.
              △ FT {Σ (0.8 ⁿⁱ × ε i )} ≦ 300 (4)
              △ FT ≦ 100 ℃ (5)
  Here, ΔFT is the fluctuation range (° C.) of the hot finish rolling end temperature, n is an integer, and the exit temperature of the finish rolling stand of the material to be rolled is Ae_ThreePoint-Ae_ThreeThe number of finish rolling stands at the point + 100 ° C. is indicated, and ε i indicates the true strain at the time of reduction in the i-th stand from the upstream among the n rolling stands.
(5) The hot-rolled steel sheet wound by the production method according to the above (3) or (4) is cold-rolled at a reduction rate of 50% or more, and then annealed within a temperature range of 600 to 950 ° C. A method for producing a cold-rolled steel sheet.
[0016]
Among the technical terms defined in the present invention, the aspect ratio refers to a maximum value among the values of (maximum diameter) / (minimum diameter) of each crystal grain of the phase.
[0017]
The “maximum diameter” of a crystal grain refers to the longest diameter of the crystal grain, and the “minimum diameter” of the crystal grain refers to the shortest diameter of the crystal grain. For example, an optical microscope or a scanning electron microscope (SEM) The structure was photographed with several fields of view, and the “maximum diameter” and “minimum diameter” obtained by linear cutting using this structure photograph were multiplied by 1.13 to obtain the “maximum diameter” and “ "Minimum diameter".
Similarly, the average crystal grain size of the phase is, for example, an average section length of 1.13 obtained by taking several fields of the tissue with an optical microscope or a scanning electron microscope (SEM) and measuring the tissue by a linear cutting method. The doubled value was adopted.
[0018]
The large-angle grain boundary means that the orientation difference between adjacent ferrite crystal grains is 15 ° or more. The crystal orientation difference between adjacent ferrite crystal grains can be measured by, for example, an electron beam backscattering method (EBSP).
[0019]
The area ratio of the phase was determined from, for example, an average value obtained by taking several views of the tissue with an optical microscope or a scanning electron microscope (SEM), analyzing the above-described tissue photograph using an image analyzer.
Precipitate refers to carbides (excluding cementite), nitrides, sulfides, oxides, phosphides, borides, and composite products thereof, and the minimum diameter refers to the shortest diameter as described above. . The minimum diameter of the precipitate is obtained, for example, by taking a few fields of view of the structure with a transmission electron microscope (TEM) and obtained directly from the structure photograph. The area ratio of the precipitate is also the number of the structures with the transmission electron microscope (TEM). The field of view is photographed, the tissue photograph is subjected to image analysis, and the phase area ratio is obtained in the same manner as described above.
[0020]
The second phase refers to various phases such as cementite other than ferrite, pearlite, bainite, martensite, and austenite remaining without transformation (hereinafter referred to as “residual austenite”). Furthermore, each temperature prescribed | regulated by this invention points out the surface temperature of a to-be-measured material.
[0021]
Ae₃The point usually refers to a temperature at which a ferrite phase starts to form from an austenite phase in an equilibrium state. However, since a very long time is required for the measurement, in the present invention, a transformation point measuring device (for example, a FORD made by Fuji Electric Koki Co., Ltd.) is used. Master) and Ac at 1.0 ° C / s₃It was defined as the temperature at which the ferrite phase starts to form when heated to the point (temperature of completion of austenite) + 50 ° C. and then cooled at 0.1 ° C./s.
[0022]
For some steel types with different components, Ac₃As a result of comparing the temperature at which the ferrite phase starts to form when heated at a point + 50 ° C. and then cooled at 0.001 ° C./s, which is almost equivalent to an equilibrium state, and at 0.1 ° C./s, The difference between steel types is within 10 ° C, and the temperature at which the ferrite phase begins to form when cooling at 0.1 ° C / s is defined as Ae.₃It has been confirmed that the point is not a problem in practice.
[0023]
  In order to improve the formability of hot-rolled steel sheets and cold-rolled steel sheets, the present inventor has conducted various studies on the chemical composition, structure and hot rolling conditions of the steel. As a result, the following knowledge could be obtained.
(A) having a specific chemical composition and having an average crystal grain size of a ferrite phase as a metal structure50The maximum diameter of the ferrite phase formed at the large-angle grain boundary is 70 μm or less, the area ratio is 70% or more of the metal structure, the aspect ratio is 3 or less, and 70% or more of the ferrite grain boundaries are composed of large-angle grain boundaries. Of the second phase having an area ratio of 30 μm or less and a minimum diameter of 5 nm or more of the second phase having a maximum area ratio of 2% or less of the metal structure and the remaining phase excluding the ferrite phase and the precipitate. Average grain size is50A hot-rolled steel sheet having a grain size boundary of the ferrite phase between the nearest second phases, which is not more than μm, exhibits excellent formability.
[0024]
Furthermore, a hot-rolled steel sheet in which the average crystal grain size of each of the ferrite phase and the second phase in the outermost layer of the steel sheet is 5 μm or less is excellent in fatigue characteristics. Further, after hot galvanizing is applied to the hot-rolled steel sheet, the alloying reaction is promoted when the plated phase is alloyed by heat treatment. For this reason, although it has the outstanding formability, even high Si system steel which is a difficult-to-alloy material can be alloyed. Thereby, a high Si alloying hot-dip galvanized steel sheet having high formability can be obtained.
(B) A cold-rolled steel sheet and a plated steel sheet excellent in formability and fatigue characteristics can be obtained by cold-rolling and annealing the hot-rolled steel sheet under specific conditions.
(C) If the slab to be subjected to rough rolling has a maximum diameter of 10 mm or less in the surface layer part from the surface to a depth of 10 mm, and the aspect ratio of the crystal grain in the surface layer part is 20 or less, The surface properties of the rolled steel sheet are improved.
(D) The slab of (c) can be obtained by setting the average cooling rate in the temperature range of the liquidus temperature (TL) to 1300 ° C. to 10 ° C./second or more when the steel is solidified.
(E) If the relationship between the variation range of the hot finish rolling temperature (ΔFT (° C.)) and the variation range of the hot rough rolling completion temperature (ΔRT (° C.)) satisfies a specific condition, The microstructure and characteristics of the hot rolled steel sheet in the rolling longitudinal direction and the width direction are uniform.
(F) In the finish rolling stand group, the exit temperature of the stand is Ae of the steel sheet to be rolled.₃ Ε, which is the true strain during rolling at each stand when in the specific temperature range associated with the point_i If the relationship between the fluctuation range ΔFT (° C.) of the finish rolling temperature between the hot and the hot satisfies a certain condition, strain can be effectively accumulated in the austenite region, and as a result, forming in the hot rolled steel sheet Can increase the sex.
(G) Reheating or keeping the rough rolled material, and setting the temperature difference in the rolling longitudinal direction and the width direction of the rough rolled material immediately before starting the finish rolling to 140 ° C. or less, the finish rolling completion temperature The fluctuation of the microstructure caused by the fluctuation and the temperature fluctuation in the subsequent cooling process can be greatly suppressed, and as a result, the longitudinal and width characteristics of the steel sheet can be made uniform.
(H) When starting rough rolling directly hot without reheating the slab, by cooling the slab to 1280-950 ° C. and then rough rolling, or when reheating the slab In addition, by carrying out rough rolling after reheating to 950 to 1280 ° C., the precipitate present in the steel ingot can be re-dissolved and the amount of precipitation finely precipitated during hot rolling can be reduced, and at the time of reheating Since the austenite grain coarsening can be suppressed and the ferrite grain coarsening of the hot-rolled steel sheet can be suppressed, the formability of the hot-rolled steel sheet can be improved.
[0025]
Below, the experimental results that the present inventors have obtained the above knowledge will be described systematically. In order to grasp the characteristics of steel plates of each chemical composition, steels having the chemical composition shown in Table 1 are melted under various conditions to form steel ingots, which are hot-rolled and hot rolled to thicknesses of 4.0 mm and 2.6 mm. A steel plate was produced. In addition, about each said steel, the crystal grain diameter and aspect ratio of the surface layer part from the surface in a steel ingot stage to 15 mm depth were investigated.
[0026]
[Table 1]

[0027]
Of the obtained hot-rolled steel sheet, a 2.6 mm-thick steel sheet is used, and the surface properties of the hot-rolled steel sheet, that is, the occurrence of surface flaws and surface irregularities, the area ratio of ferrite in the structure, the average of ferrite The crystal grain size and aspect ratio, the crystal orientation difference between adjacent ferrite grains, the area ratio and minimum diameter occupied by precipitates, the area ratio of the remaining phase, and the average crystal grain diameter of the second phase were investigated.
[0028]
In addition, a No. 5 tensile test piece specified in JIS Z 2201 was sampled from 0 °, 45 °, and 90 ° directions with respect to the rolling direction and subjected to a tensile test, yield strength (YS), tensile strength ( TS), elongation at break (EL) and r value were measured. The hole expansion rate (λ) is the hole when the burr of a punched hole with a diameter of 10 mm is the die side, a conical punch with a vertex angle of 60 ° is press-fitted into the punched hole, and expanded until the crack penetrates the plate thickness. Diameter d is measured and initial hole diameter d₀ It calculated | required by following formula (a) as ratio with respect to.
[0029]
  λ = {(dd₀) / D₀} × 100 (%) (a)
  Of the obtained hot rolled steel sheetthicknessA 4 mm steel plate has a thickness of 2.6 mm as described above only for a steel ingot having a maximum surface grain diameter of 10 mm or less and an aspect ratio of 20 or less from the surface to a depth of 10 mm. The same items as the rolled steel sheet were investigated. Furthermore, it pickled by the normal method after that, cold-rolled with a reduction rate of 50 to 80%, and then annealed at 800 ° C. for 60 seconds to obtain a cold-rolled steel sheet having a thickness of 0.8 to 2.0 mm. From this cold-rolled steel sheet, No. 5 tensile test piece defined in JIS Z 2201 was sampled from directions of 0 °, 45 ° and 90 ° with respect to the rolling direction, and the tensile properties were investigated.
[0030]
The surface properties of the hot-rolled steel sheet were evaluated by the “surface roughness index” and the “surface roughness index” of the steel sheet after the tensile test. Here, the “surface wrinkle index” is based on a surface flaw occurrence area ratio, that is, a value expressed as a percentage expressed by dividing the area where wrinkles are generated by the total area of the steel sheet surface and multiplying by 100. If the ratio is less than%, the surface defect index is 1, the surface defect generation ratio is 5% or more and less than 15%, the surface defect index is 2, and the surface defect generation ratio is 15% or more and less than 30%. 3. The surface flaw index was 4 when the surface flaw generation area ratio was 30% or more.
[0031]
The “surface roughness index” of the steel sheet after the tensile test is the surface roughness index 1 when the maximum height (Ry) specified by JIS B 0601 is less than 20 μm, and the surface roughness when the Ry is 20 μm or more and less than 40 μm. The surface roughness index 3 was determined when the roughness index 2 and Ry were 40 μm or more and less than 60 μm, and the surface roughness index 4 was determined when the Ry was 60 μm or more.
[0032]
Regarding the formability of the hot-rolled steel sheet, the product of TS and EL (hereinafter referred to as “TS × EL balance”) is 16000 MPa ·% or more, and the product of TS and λ (hereinafter referred to as “TS × λ balance”) is If it is 45000 MPa ·% or more, it can be molded without any problem in press molding, which is a general molding method, and thus it is evaluated as having excellent moldability.
[0033]
Further, regarding the formability of the cold-rolled steel sheet, if the TS × EL balance is 15000 MPa ·% or more and the product of TS and r value (hereinafter referred to as “TS × r balance”) is 500 MPa or more, excellent forming is achieved. It is evaluated that it has sex.
[0034]
In the above-described investigation, the average crystal grain size of ferrite and the average crystal grain size of the second phase having the largest area ratio among the remaining phases excluding ferrite and precipitates are measured with an optical microscope or a scanning electron microscope (SEM). The tissue was photographed in 10 fields of view, and the average section length measured by the linear cutting method using this tissue photograph was taken as a value multiplied by 1.13.
[0035]
The area ratio of ferrite occupying the structure and the area ratio of the remaining phase excluding ferrite and precipitates occupying the structure were obtained by analyzing the above ten-view structure photographs using an image analyzer and calculating the average value thereof. The crystal orientation difference between adjacent ferrite crystal grains was measured by an electron beam backscattering method (EBSP).
[0036]
The amount of retained austenite was measured by X-ray diffraction on the plate thickness center plane. Using Mo-Kd rays as incident X-rays, the X-ray intensity ratios of the {220}, {311}, {200}, and {111} planes of residual austenite are measured, and the residual is determined from the average value. The volume fraction of austenite was determined. In the present invention, this volume ratio is defined as the area ratio of retained austenite.
[0037]
The minimum diameter and area ratio of the precipitates were obtained by taking 10 views of the structure with a transmission electron microscope (TEM) and analyzing the image of the structure. In addition, the average value was used for the area ratio. Further, the tensile test and the hole expansion test were performed by the above-described methods, and the tensile characteristics and the hole expansion rate were measured.
[0038]
  FIG. 1 is a graph showing the influence of the maximum diameter of crystal grains and the aspect ratio of the slab surface layer on the surface properties and formability of a hot-rolled steel sheet. The used hot-rolled steel sheet has an area ratio of ferrite of 70% or more in the structure, and the average grain size of ferrite and second phase is both50It is below μm.
[0039]
In FIG. 1, the black-painted one indicates a hot-rolled steel sheet having good surface properties and excellent formability. That is, the surface wrinkle index is 2 or less, the surface roughness index is 2 or less, and the surface properties are good, the TS × EL balance value is 16000 MPa ·% or more, and the TS × λ balance value is 45000 MPa ·% or more. The moldability. On the other hand, the white one has a surface wrinkle index of 3 or more, or a surface roughness index of 3 or more, and the surface properties are inferior, or the TS × EL balance value is less than 16000 MPa ·%, or TS × λ Balance value is less than 45000 MPa ·%, indicating that the steel sheet is inferior in formability.
[0040]
From FIG. 1, if the hot rolled steel sheet is obtained by hot rolling a slab having a maximum diameter of 10 mm or less of the surface layer crystal grains from the surface to a depth of 10 mm and an aspect ratio of the crystal grains of the surface layer of 20 or less, It turns out that it is excellent in surface properties and moldability.
[0041]
  Furthermore, as in the steel plate used in FIG. 1, both the average crystal grain sizes of ferrite and second phase are50Even when the area ratio of ferrite in the structure is 70% or more and the ferrite grain boundary has a large-angle grain boundary of less than 70%, or the ferrite grain aspect ratio exceeds 3, It can be confirmed that the property or moldability is inferior. FIG. 2 is a diagram showing the influence of the ferrite grain size and the second phase grain size on the formability of the hot-rolled steel sheet. The used hot-rolled steel sheet is obtained by hot rolling a slab having a maximum diameter of crystal grains of the surface layer portion of 10 mm or less from the surface to a depth of 10 mm and an aspect ratio of crystal grains of the surface layer portion of 20 or less. The area ratio of ferrite occupying 70% is 70% or more.
[0042]
In FIG. 2, the black-painted one shows a hot-rolled steel sheet having both excellent TS × EL balance and TS × λ balance and excellent formability. On the other hand, a white sheet corresponds to a TS × EL balance value of less than 16000 MPa ·%, or a TS × λ balance value of less than 45000 MPa ·%, and indicates a hot-rolled steel sheet having inferior formability.
[0043]
  From Fig. 2, the average crystal grain size of ferrite is50μm or less, and the average crystal grain size of the second phase is50It can be seen that the TS × EL balance and the TS × λ balance are good in the case of μm or less.
[0044]
  Further, from FIG. 2, the average crystal grain size of ferrite is50μm or less, and the average crystal grain size of the second phase is50Even if it is not more than μm, the area of precipitates where the large grain boundary is less than 70% of the ferrite grain boundary, the aspect ratio of the ferrite grain exceeds 3, the minimum diameter is less than 5 nm, or the minimum diameter is 5 nm When the ratio exceeds 2%, the TS × EL balance value is less than 16000 MPa ·%, or the TS × λ balance value is less than 45000 MPa ·%, indicating that the moldability is poor.
[0045]
FIG. 3 is a diagram showing the influence of the area ratio of the ferrite phase and the area ratio of the second phase on the formability of the hot-rolled steel sheet. The hot-rolled steel sheet used was hot-rolled with a steel ingot having a maximum diameter of the surface layer crystal grains of 10 mm or less from the surface to a depth of 10 mm and an aspect ratio of the crystal grains of the surface layer part of 20 or less. The average crystal grain size is 0.5 to 45 μm, and the average crystal grain size of the second phase is 0.3 to 40 μm.
[0046]
In FIG. 3, the black-painted steel sheet shows a hot-rolled steel sheet having excellent formability with a TS × EL balance value of 16000 MPa ·% or more and a TS × λ balance value of 45000 MPa ·% or more. On the other hand, the white ones indicate that the TS × EL balance value is less than 16000 MPa ·%, or the TS × λ balance value is less than 45000 MPa ·%, indicating that the steel sheet is inferior in formability.
[0047]
FIG. 3 shows that the formability is excellent when the area ratio of ferrite to the structure is 70% or more. Furthermore, it can be seen from FIG. 3 that even if the area ratio of ferrite in the structure is 70% or more, the ferrite grain boundary has a large-angle grain boundary of less than 70%, the ferrite grain aspect ratio exceeds 3, or When the area ratio of precipitates having a small diameter of less than 5 nm or a minimum diameter of 5 nm or more exceeds 2%, the TS × EL balance value is less than 16000 MPa ·%, or the TS × λ balance value is less than 45000 MPa ·%, It can be confirmed that the moldability is inferior.
[0048]
Next, in order to confirm the properties of the cold-rolled steel sheet, it is hot-rolled using a steel ingot having a maximum diameter of crystal grains of the surface layer part from the surface to a depth of 10 mm and an aspect ratio of 20 or less. The hot-rolled steel sheet having a thickness of 4 mm was pickled, cold-rolled and then annealed to produce a cold-rolled steel sheet having a thickness of 0.8 mm. A tensile test was carried out using the obtained cold-rolled steel sheet.
[0049]
FIG. 4 is a diagram showing the influence of the ferrite grain size and the second phase grain size in the hot-rolled steel sheet on the formability of the cold-rolled steel sheet. The cold-rolled steel sheet used was prepared from a hot-rolled steel sheet having a ferrite area ratio of 70% or more in the structure. As described above, the formability of the cold-rolled steel sheet is evaluated based on whether the TS × EL balance value satisfies 15000 MPa ·% or more and the TS × r balance value satisfies 500 MPa or more.
[0050]
In FIG. 4, the black-painted steel sheet has a good TS × EL balance and TS × r balance, and shows a cold-rolled steel sheet having excellent forming. On the other hand, the white one indicates a cold-rolled steel sheet having a TS × EL balance value of less than 15000 MPa ·% or a TS × r balance value of less than 500 MPa and inferior formability.
[0051]
  From FIG. 4, the average crystal grain size of ferrite in the hot-rolled steel sheet is50μm or less and the average crystal grain size of the second phase is50If it is below μm, it can be seen that both the TS × EL balance and the TS × r balance are good in the cold-rolled steel sheet.
[0052]
  Furthermore, from FIG. 4, in the hot rolled steel sheet, the average crystal grain size of ferrite is50μm or less and the average crystal grain size of the second phase is50Even if it is less than μm, if the large-angle grain boundary is less than 70% of the ferrite grain boundaries in the hot rolled steel sheet, the aspect ratio of the ferrite grains exceeds 3, or the minimum diameter is less than 5 nm, or the minimum diameter is 5 nm or more. When the area ratio of the precipitates exceeds 2%, the TS × EL balance value of the cold-rolled steel sheet is less than 15000 MPa ·%, or the TS × r balance value is less than 500 MPa, and the formability of the cold-rolled steel sheet is inferior. I understand.
[0053]
FIG. 5 is a diagram showing the influence of the area ratio of the ferrite phase and the area ratio of the second phase in the hot-rolled steel sheet on the formability of the cold-rolled steel sheet. The cold-rolled steel sheet used was manufactured from a hot-rolled steel sheet having a ferrite average crystal grain size of 0.5 to 45 μm and a second phase average crystal grain diameter of 0.3 to 40 μm. is there.
[0054]
In FIG. 5, the blackened steel sheet is a cold-rolled steel sheet having excellent formability. On the other hand, the white one corresponds to either a TS × EL balance value of less than 15000 MPa ·% or a TS × r balance value of less than 500 MPa, indicating a cold-rolled steel sheet having inferior formability.
[0055]
FIG. 5 shows that the TS × EL balance and the TS × r balance are good when the area ratio of ferrite in the structure of the hot-rolled steel sheet is 70% or more. Further, from FIG. 5, even when the area ratio of ferrite in the structure of the hot-rolled steel sheet is 70% or more, when the large-angle grain boundary is less than 70% among the ferrite grain boundaries in the hot-rolled steel sheet, the aspect ratio of the ferrite grain Is less than 5 nm, or when the area ratio of precipitates having a minimum diameter of 5 nm or more exceeds 2%, the TS × EL balance value of the cold-rolled steel sheet is less than 15000 MPa ·%, or It is clear that the TS × r balance value is less than 500 MPa, and the formability of the cold-rolled steel sheet is inferior.
[0056]
As described above, by optimizing the metal microstructure at the slab stage and the hot-rolled steel sheet stage, the surface properties of the hot-rolled steel sheet are improved, and the TS × EL balance value is also improved and formability is improved. Can be increased. Furthermore, by cold rolling and annealing using the hot-rolled steel sheet having the appropriate structure described above, both the TS × EL balance value and the TS × r balance value of the cold-rolled annealed steel sheet are increased, and the cold-rolled steel sheet is excellently formed. Can be provided.
[0057]
In hot rolling of a steel sheet, the fluctuation range of the finish rolling completion temperature, that is, the difference between the highest temperature and the lowest temperature of completion of the finish rolling ΔFT (° C.) is the fluctuation range of the hot rough rolling completion temperature ΔRT Influenced by (℃).
[0058]
FIG. 6 is a diagram showing the influence of fluctuation in the rough rolling completion temperature (ΔRT) and fluctuation in the finishing rolling completion temperature (ΔFT) on the dimensional accuracy after forming the hot-rolled steel sheet. Here, the finish rolling completion temperature is Ae₃The case of more than a point is shown. In addition, the dimensional accuracy and the like after forming the hot-rolled steel sheet are indicated by changes in TS × EL balance (hereinafter referred to as Δ (TS × EL)) and TS × λ balance (hereinafter referred to as Δ (TS × λ)). ).
[0059]
Usually, in the evaluation of formability of a hot-rolled steel sheet, when Δ (TS × EL) exceeds 200 MPa ·% or Δ (TS × λ) exceeds 1500 MPa ·%, the inside of the hot-rolled steel sheet (hot-rolled coil) The fluctuation of the characteristics becomes large, and problems occur when molding is performed by various press molding methods. Specifically, the variation in the amount of springback that occurs after press molding becomes large, making it difficult to press mold with high dimensional accuracy, or cracking during press molding. Therefore, in order to improve the dimensional accuracy after press molding and prevent cracking, it is necessary to satisfy Δ (TS × EL) of 200 MPa ·% or less and Δ (TS × λ) of 1500 MPa ·% or less. .
[0060]
In FIG. 6, the black-painted steel sheet indicates a hot-rolled steel sheet having good dimensional accuracy after forming. On the other hand, the white one indicates that the steel sheet is a hot-rolled steel plate having Δ (TS × EL) exceeding 200 MPa ·% or Δ (TS × λ) exceeding 1500 MPa ·%.
[0061]
From the results shown in FIG. 6, when the following formulas (1) and (2) are satisfied, Δ (TS × EL) is 200 MPa ·% or less and Δ (TS × λ) is 1500 MPa ·% or less. In addition, it can be seen that the dimensional accuracy after molding is good.
△ FT ≦ 0.8 × △ RT (1)
50 ℃ <△ FT ≦ 100 ℃ (2)
Similarly, from the results shown in FIG. 6, even if the above formulas (1) and (2) are not satisfied, the dimensional accuracy after molding is good when the following formula (3) is satisfied. I understand.
△ FT ≦ 50 ℃ (3)
Strain in the low-temperature austenite region in hot finish rolling affects the formation behavior of ferrite, second phase, and precipitates through phase transformation that occurs in the cooling process after completion of finish rolling. That is, the strain in the low temperature austenite region, particularly Ae₃Point + 100 ° C to Ae₃The greater the strain in the temperature range of the point, the higher the driving force and nucleation rate of the ferrite transformation, so the ferrite grain size and aspect ratio are reduced, the ferrite area ratio in the structure and the large angle in the ferrite grain boundary A uniform fine multiphase structure of a ferrite main phase in which grain boundaries exist and the remaining layer is uniformly finely distributed between ferrite grains can be obtained.
[0062]
Furthermore, the proportion of large-angle grain boundaries in the ferrite grain boundaries increases, and most of the crystal orientation difference between adjacent ferrite grains is 15 degrees or less, and the maximum diameter of ferrite grains surrounded by the large-angle grain boundaries exceeds 30 μm. It will not be as coarse. The uniform fine multiphase structure of the ferrite main phase having these characteristics is optimal for obtaining high formability.
[0063]
Ae₃The higher the rolling reduction just above the point, the higher the finish rolling completion temperature is Ae.₃The closer to the point, the greater the distortion of the steel sheet surface layer. The steel sheet surface layer not only has a larger strain than the other parts of the steel sheet, but also starts cooling quickly, and the cooling rate increases even when cooled at the same cooling rate. For this reason, the finish completion temperature of the steel sheet outermost layer is set to Ae.₃If it is just above the point, the average crystal grain size of the ferrite phase and the second phase of the outermost surface layer of the steel sheet can be made even smaller than the other parts of the steel sheet.
[0064]
The above effect is Ae₃The larger the rolling reduction in the temperature range just above the point, the more remarkable it is. The upper limit temperature of the temperature range for obtaining a total rolling reduction of 70% or more is Ae₃The point + 70 ° C. is more preferable. In addition, Ae₃The point + 50 ° C. is extremely preferable.
[0065]
When producing the steel sheet of the present invention, Ae₃The point is very important. In the prior art, Ar₃The reference point of the finish rolling completion temperature (for example, Ar₃Point + 50 ° C), but generally Ar₃Since the point is the ferrite transformation start temperature that varies depending on the cooling rate, the difference due to the cooling rate is large and cannot be determined uniquely. Therefore, it is not suitable as an index for the management temperature.
[0066]
As a result of investigating laboratory test materials and actual equipment, the present inventor has clarified the following.
1) When finishing rolling is completed in the temperature range for producing the steel sheet of the present invention, the cumulative strain is very high before finishing rolling, and Ae₃Since the ferrite phase immediately begins to form when the temperature falls below the point, Ar is not uniquely determined as the management temperature.₃From point Ae₃The point is more appropriate both metallurgically and practically,
2) Ar when cooling rate is low₃If the point is taken as an index of the finish rolling completion temperature, it is generally Ar when the cooling rate is high.₃Because it is higher than the point, the ferritic phase starts to form before the finish rolling is completed at the outermost surface layer of the steel plate with a high cooling rate, and it is elongated and coarsened by rolling.
[0067]
Based on the above findings, in the present invention, unlike the prior art, Ae is uniquely determined by the chemical composition.₃The point was used as a reference for the finish rolling completion temperature. In the present invention, Ae₃The point refers to the temperature measured by the method described above.
[0068]
Furthermore, due to strain induction, the size of precipitates generated in the austenite region is increased, and ferrite transformation starts from a high temperature region, and precipitates generated in a ferrite temperature region are generated from a higher temperature and become coarser. .
[0069]
Ae which is the above-mentioned low-temperature austenite region by reducing the variation width ΔFT of the finish rolling completion temperature in hot.₃Point + 100 ° C to Ae₃It becomes easy to increase the total rolling reduction of the finish rolling in the temperature range of the point and increase the cumulative strain in this temperature range.
FIG. 7 is a diagram showing the influence of the variation ΔFT of the finish rolling completion temperature ΔFT and the cumulative strain during finish rolling on the formability of the hot-rolled steel sheet. Specifically, in the same figure, the outlet side temperature of the stand is Ae of the steel sheet to be rolled.₃Point-Ae₃The number of finish rolling stands at the point + 100 ° C. is n, and the true strain at the time of reduction in the i-th stand among the n stands is ε_i , FT and ε of fluctuation range of hot finish rolling completion temperature_i Has arranged the influence which it has on the TS × EL balance value and the TS × λ balance value of the hot-rolled steel sheet. The finish rolling completion temperature is Ae.₃The result in the case of more than a point is shown.
[0070]
In FIG. 7, the black-painted steel sheet has a TS × EL balance value of 16000 MPa ·% or more and a TS × λ balance value of 45000 MPa ·% or more and has excellent formability. On the other hand, the white steel sheet has a TS × EL balance value of less than 16000 MPa ·% or a TS × λ balance value of less than 45000 MPa ·%, and is a hot-rolled steel sheet having inferior formability.
[0071]
  From the results shown in FIG. 7, ΔFT and ε_i By satisfying the following relationships (4) and (5), the TS × EL balance and the TS × λ balance of the hot-rolled steel sheet can be increased. This is because strain can be effectively accumulated in the austenite region relatively easily by hot rolling so as to satisfy this condition.
              △ FT {Σ (0.8 ⁿⁱ × ε i )} ≦ 300 (4)
              △ FT ≦ 100 ℃ (5)
  Furthermore, it has been confirmed that a cold-rolled steel sheet having good TS × EL balance and TS × r balance can be obtained by pickling the hot-rolled steel sheet under appropriate conditions, annealing after cold rolling, and annealing.
[0072]
DETAILED DESCRIPTION OF THE INVENTION
The requirements defined by the present invention will be described in terms of the chemical composition of the steel sheet, the metal structure of the steel sheet, and the manufacturing method of the steel sheet. In the following description,% display of the content of each element means mass%.
(A) Chemical composition of steel sheet
C: 0.0002 to 0.25%,
As the C content increases, the area ratio of ferrite decreases, and the area ratio of the hard residual phase increases, which adversely affects ductility and deep drawability. % Or less is required. On the other hand, if the C content is less than 0.0002%, the ferrite grains become extremely coarse, high ductility cannot be stably obtained, and surface roughness is likely to occur during the formation of the steel sheet. Furthermore, in order to reduce the C content to less than 0.0002%, a special steelmaking technique is required, so the cost is increased. Therefore, the content of C is set to 0.0002 to 0.25%. The upper limit of the C content is preferably 0.15%, and more preferably 0.1%.
[0073]
Si: 0.003-3.0%
Si has an action of improving the strength of steel without impairing workability. Further, it has the effect of promoting the generation of ferrite and increasing the amount of ferrite. In order to exhibit such an effect, it is necessary to contain at least 0.003%. However, if the content exceeds 3.0%, the workability of the steel is lowered, and the surface properties of the steel are also deteriorated. Therefore, the Si content is set to 0.003 to 3.0%. Note that the upper limit of the Si content is preferably 1.5%, and more preferably 1.0%.
[0074]
Mn: 0.003 to 3.0%
Mn has the effect of preventing hot brittleness of the steel due to S. Furthermore, it also has the effect of strengthening the solid solution. In order to exhibit such an effect, it is necessary to contain at least 0.003%. Mn is preferably contained in an amount of 0.01% or more, and more preferably 0.05% or more.
[0075]
On the other hand, if Mn is contained excessively, not only the formability will deteriorate, but it will be difficult to produce sufficient ferrite in the cooling process after hot rolling, and ductility and weldability may be impaired. Particularly, when the Mn content exceeds 3.0%, the adverse effect becomes remarkable. Therefore, the Mn content is set to 0.003 to 3.0%. Note that the upper limit of the Mn content is preferably 2.5%, and more preferably 2.0%.
[0076]
Al: 0.002 to 2.0%
Al needs to be contained in an amount of 0.002% or more in order to deoxidize steel and promote the formation of ferrite to increase the amount of ferrite. By including Al, the yield of an optional additive element such as Ti described later can be increased. On the other hand, if the content exceeds 2.0%, the above effect is saturated and the cost is increased. Therefore, the content of Al is set to 0.002 to 2.0%. Note that the upper limit of the Al content is preferably 1.2%, and extremely preferably 0.1%.
[0077]
The steel plate of the present invention may further include one or more of the following first to fourth groups in addition to the above-described component elements.
[0078]
First group: B: 0.0002 to 0.01%
Since B has an effect of enhancing the hardenability of the steel, it may be utilized when controlling the crystal grain size and area ratio of the ferrite phase and the remaining phase in the cooling process. Ae₃ Since it has the effect of lowering the point, it is effective to contain B when finishing rolling is difficult to complete in the austenite temperature range, and in particular, a thin hot-rolled steel sheet having a thickness of 2.0 mm or less is manufactured. It is extremely effective in some cases.
[0079]
B also has an effect of preventing “secondary processing cracks” that may occur when the ultra-low carbon steel sheet is drawn. For this reason, B may be contained for the above-mentioned purpose, but if the B content is less than 0.0002%, it is difficult to obtain the effect. However, if B is contained in excess of 0.01%, the formation of ferrite is remarkably suppressed, the action of preventing secondary work cracking is saturated, and the steel sheet may be made brittle. Therefore, when adding B, the content is made 0.0002 to 0.01%. When B is added, the upper limit of the B content is preferably 0.007%, and more preferably 0.005%.
[0080]
Second group: Ti, Nb, V, and Zr: 0.005 to 1.0% in total of at least one kind
Ti, Nb, V and Zr have the effect of fixing the solid solution C, solid solution N, and solid solution S contained in the steel as precipitates and making them harmless, especially the deep drawability of cold-rolled annealed steel sheets. It is effective to improve. Furthermore, it has the effect | action which raises the intensity | strength of steel, without impairing ductility and deep drawability so much. Therefore, in order to efficiently increase the deep drawability and strength of the steel, one or more of Ti, Nb, V, and Zr may be contained. However, if the total content is less than 0.005%, it is difficult to obtain the effect. .
[0081]
On the other hand, if the total exceeds 1.0%, the above effect is saturated, and conversely, ductility and deep drawability are lowered, the yield ratio is increased, and the shape freezing property at the time of press forming is deteriorated. Therefore, when adding Ti, Nb, V and Zr, 0.005 to 1.0% in total is included. The lower limit of the total content is preferably 0.01%, and more preferably 0.02%. Further, the upper limit of the total content is preferably 0.5%, and more preferably 0.3%.
[0082]
Third group: Cr, Mo, Cu and Ni: 0.005 to 3.0% in total of 1 or more types
Since Cr, Mo, Cu, and Ni have an effect of improving the hardenability, it becomes easy to control the crystal grain size and area ratio of ferrite and the remaining phase in the cooling process. In addition to enhancing the hardenability, Cu also has the effect of enhancing corrosion resistance. For this reason, one or more of Cr, Mo, Cu, and Ni may be contained for the above-described purpose. However, if the total content is less than 0.005%, it is difficult to obtain the effect. If it exceeds%, the above effect is saturated and, on the contrary, the ductility is lowered. Therefore, when adding Cr, Mo, Cu, Ni, it is good to contain 0.005 to 3.0% of 1 or more types in total. Note that the lower limit of the total content is preferably 0.05%, and more preferably 0.1%. The upper limit of the total content is preferably 2.0%, and more preferably 1.0%.
[0083]
Group 4: One or more of Ca: 0.0001 to 0.005% and REM (rare earth element): 0.0001 to 0.20%
Ca and REM have the effect of improving the cold workability by adjusting the shape of the inclusions. Therefore, Ca and REM may be contained for the purpose of improving the cold workability. If it is less than 0001%, it is difficult to obtain the effect. On the other hand, even if Ca exceeds 0.005% and REM exceeds 0.20%, the effect is saturated and the cost is increased. Therefore, when one or more of Ca and REM are added, the Ca content is preferably 0.0001 to 0.005% and the REM content is 0.0001 to 0.20%.
In the present invention, the contents of P, S, and N as impurity elements are defined as follows.
P: 0.15% or less
P segregates at the grain boundaries and embrittles the steel. Particularly when the content exceeds 0.15%, the embrittlement of the steel becomes remarkable. Therefore, the content of P as an impurity is set to 0.15% or less. The P content is preferably 0.12% or less, and more preferably 0.10% or less. For example, in the case of ultra-low C steel having a C content of about 0.02% or less, if the content of P as an impurity is 0.0002% or more, the ferrite grains can be prevented from becoming extremely coarse. Since it is possible, a trace amount of P may be contained.
[0084]
S: 0.05% or less
S forms sulfide inclusions, resulting in a decrease in workability. Particularly, when the content exceeds 0.05%, the workability decreases remarkably. Therefore, the content of S as an impurity is set to 0.05% or less. Note that the S content is preferably 0.03% or less, and more preferably 0.01% or less. For example, in the case of low C steel or extremely low C steel with a C content of about 0.02% or less, if S as an impurity is contained in an amount of 0.0002% or more, ferrite grains become extremely coarse. Since this can be suppressed, a trace amount of S may be contained.
[0085]
N: 0.01% or less
The content of N is preferably as small as possible in order to improve the workability, but if it is 0.01% or less, the influence is small in the present invention. Therefore, the content of N as an impurity is set to 0.01% or less. The N content is preferably 0.007% or less, and more preferably 0.005% or less. For example, in the case of extremely low C steel having a C content of about 0.02% or less, if N as an impurity is contained by 0.0005% or more, ferrite grains can be prevented from becoming extremely coarse. Since it is possible, a trace amount of N may be contained.
(B) Metal structure of steel sheet
When the area ratio of ferrite in the structure of the hot-rolled steel sheet is less than 70%, a high strength is obtained because the second phase having a higher strength than the ferrite increases, but the ductility of the hot-rolled steel sheet and the cold-rolled annealed steel sheet can be obtained. The formability such as ductility and deep drawability is greatly deteriorated. Therefore, the area ratio of ferrite in the structure of the hot-rolled steel sheet is set to 70% or more. The area ratio of ferrite is preferably 80% or more, and more preferably 90% or more. The area ratio of ferrite in the structure of the hot-rolled steel sheet may be a value close to 100%.
[0086]
  The average grain size of ferrite in hot-rolled steel sheet is50If it exceeds μm, even if the area ratio of ferrite in the structure is 70% or more, the surface of the hot-rolled steel sheet is roughened during processing such as press forming, the surface roughness is increased and the surface properties are lowered. High ductility cannot be obtained stably.
[0087]
  At the same time, the average grain size of ferrite in hot-rolled steel sheet is increased, especially50If it exceeds μm, the development of {111} recrystallized texture generated from the vicinity of the old hot-rolled plate grain boundary during annealing after cold rolling is suppressed, and a high r value cannot be obtained. Therefore, the average grain size of ferrite in hot-rolled steel sheet50μm or less. The average crystal grain size of ferrite is preferably 10 μm or less, and more preferably 5 μm or less. The smaller the average crystal grain size of this ferrite, the better. However, in order to make the average crystal grain size of ferrite 0.5 μm or less, a very special technique is required and the cost is increased, so the lower limit on the industrial scale is 0. About 5 μm. As shown in FIGS. 1 to 5 above, when the ratio of large-angle grain boundaries is less than 70% of the ferrite grain boundaries of the hot-rolled steel sheet, when the ferrite aspect ratio exceeds 3, it is desirable in the hot-rolled steel sheet. The surface properties and formability are not obtained, and excellent formability is not obtained even in a cold-rolled steel sheet. Therefore, the ratio of the large-angle grain boundaries in the ferrite grain boundaries of the hot-rolled steel sheet is set to 70% or more, and the ferrite aspect ratio is set to 3 or less.
[0088]
Furthermore, the ratio of the large-angle grain boundary in the ferrite grain boundary is preferably 80% or more, and more preferably 90% or more. The ratio of the large-angle grain boundary in the ferrite grain boundary may be a value close to 100%.
[0089]
  The ferrite needs to be a so-called “equal axis ferrite” having an aspect ratio of 3 or less. Among equiaxed ferrites, an aspect ratio of 2 or less is more preferable, and an aspect ratio close to 1 is very preferable. As shown in FIGS. 2 and 4, in the hot rolled steel sheet, the average grain size of the second phase having the largest area ratio among the remaining phases excluding ferrite and precipitates (excluding cementite) is as follows.50When it exceeds μm, excellent formability cannot be obtained in a hot-rolled steel sheet, and desired formability cannot be obtained even in a cold-rolled steel sheet.
[0090]
  In particular, the average crystal grain size of the second phase is50If it exceeds μm, cracks generated from the interface between the ferrite and the second phase at the time of tensile deformation or hole expansion deformation are hard to be prevented from propagating at the ferrite grain boundary, and deformation is likely to be localized. . For this reason, ductility and hole expansibility fall. In cold-rolled annealed steel sheets, when the second phase of the hot-rolled sheet becomes large, it is difficult to develop a {111} recrystallized texture during annealing after cold rolling due to the randomization of the slip system near the second phase. TS × EL balance value and TS × r_mThe balance value decreases.
[0091]
  Therefore, the average grain size of the second phase is50μm or less. The average crystal grain size of the second phase is preferably 10 μm or less, and more preferably 5 μm or less. The smaller the average crystal grain size of the second phase, the better. However, in order to reduce the average crystal grain size of the second phase to 0.1 μm or less, a special technique is required and the cost increases. Is about 0.1 μm.
[0092]
By uniformly and finely dispersing the second phase, both the TS × EL balance value and the TS × λ balance value are improved in the hot-rolled steel sheet, and both the TS × EL balance and the TS × r balance are improved in the cold-rolled steel sheet. be able to. In particular, the average crystal grain size of the ferrite phase and the second phase are both 5 μm or less, the area ratio of the ferrite phase is 80% or more, 80% or more of the ferrite grain boundary is a large angle grain boundary, and the aspect ratio is 3 or less at the same time. In addition, the above balance value can be further improved by uniformly and finely dispersing bainite and martensite containing retained austenite as the remaining layer.
As shown in FIGS. 2 to 5, in the hot-rolled steel sheet, when the minimum diameter of the precipitate exceeds 5 nm, or the area ratio of the precipitate having the minimum diameter of 5 nm or more exceeds 2%, A desired formability cannot be obtained in a rolled steel sheet, and a desired formability cannot be obtained even in a cold-rolled steel sheet. Here, the reason why the area ratio is limited to 2% or less is to suppress the deterioration of formability due to the strength increase due to precipitation strengthening.
[0093]
Therefore, in the hot-rolled steel sheet, the area ratio of precipitates having a minimum diameter of 5 nm or more is defined as 2% or less of the structure. The minimum diameter of the precipitate is preferably 10 nm or more, and more preferably 100 nm or more. The upper limit of the minimum diameter of the precipitate may be about 2 μm. The maximum diameter of the precipitate is preferably about 5 μm. The lower limit of the area ratio occupied by the precipitate is preferably 0.0001%. On the other hand, the upper limit of the area ratio occupied by the precipitate is preferably 1%, and more preferably 0.5%.
[0094]
When the hot-rolled steel sheet satisfies the above conditions, in particular, 70% or more of the ferrite grain boundaries are large-angle grain boundaries, and 70% or more of the structure in terms of area ratio is equiaxed ferrite having an average crystal grain size of 1 μm or less. If the average grain size of the second phase, which has the largest area ratio among the remaining phases excluding the phases and precipitates, is 1 μm or less, the ductility of the hot-rolled steel sheet, the ductility of the cold-rolled annealed steel sheet, and the deep drawability will jump dramatically. Improve. For this reason, in order to ensure extremely excellent formability, the structure of the hot-rolled steel sheet is such that 70% or more of the ferrite grain boundaries are large-angle grain boundaries, and 70% or more of the structure in terms of area ratio is an average crystal grain size of 1 μm or less. It is preferable to use equiaxed ferrite and the average crystal grain size of the second phase having the largest area ratio to be 1 μm or less.
[0095]
Furthermore, in the hot-rolled steel sheet of the present invention, it is desirable that both the average crystal grain size of the ferrite phase and the second phase of the outermost layer of the steel sheet be 5 μm or less. Here, the steel sheet outermost layer is defined as the position in the plate thickness direction including two crystal grains from the steel plate surface toward the plate thickness center, and the maximum is 40 μm from the steel plate surface.
In addition, when the maximum diameter of the crystal grains in the surface layer portion from the surface of the steel ingot to be subjected to rough rolling to a depth of 10 mm exceeds 10 mm, the surface cracking sensitivity becomes high, for example, longitudinal cracks during continuous casting that are austenite grain boundary cracks. Surface cracks such as cracks, transverse cracks, and transverse cracks during hot rolling are likely to occur, and surface defects may occur in hot-rolled steel sheets, resulting in deterioration of surface properties. Moreover, when the aspect ratio of the crystal grains of the surface layer portion exceeds 20, the surface cracks are likely to occur, and surface defects may occur in the hot-rolled steel sheet and the surface properties may deteriorate. Furthermore, since the crystal grain orientation of the surface layer portion is mainly a {100} texture, recrystallization hardly occurs during hot rolling, and it may be difficult to refine by hot rolling or increase the angle of crystal grain boundaries.
[0096]
Therefore, the steel ingot to be subjected to rough rolling is preferably a crystal grain having a maximum diameter of 10 mm or less in the surface layer part from the surface to a depth of 10 mm, and the aspect ratio of the crystal grain of the surface layer part is preferably 20 or less. . The steel ingot having the surface layer structure can be obtained, for example, by setting the average cooling rate in the temperature range from the liquidus temperature to 1300 ° C. to 10 ° C./second or more when the steel is solidified. The maximum diameter of the crystal grains is more preferably 5 mm or less, and most preferably 3 mm or less. Further, the aspect ratio of the crystal grains is more preferably 10 or less, and extremely preferably 5 or less.
(C) Steel plate manufacturing method
The production method of the present invention will be described separately for rough rolling, hot rolling and cold rolling.
(C-1) Rough rolling of slab
When the temperature exceeds 1280 ° C., the slab obtained by solidifying the steel having the chemical composition is once cooled to 1280-950 ° C. and then directly hot-rolled. Moreover, when re-heating a slab before rough rolling, it is necessary to reheat to 950-1280 degreeC, and to perform rough rolling then.
[0097]
If the upper limit temperature for cooling the slab or the upper limit temperature for reheating exceeds 1280 ° C, MnS, AlN, TiS, Ti, which are coarsely precipitated during casting of the steel₄ C₂ S₂ Such precipitates may be re-dissolved and finely precipitated during hot rolling, resulting in reduced formability. On the other hand, if the temperature at which the slab is cooled is lower than 950 ° C., or the heating temperature at the time of reheating is lower than 950 ° C., the desired microstructure may not be obtained and the formability may deteriorate.
[0098]
Therefore, when the slab is cooled to 1280-950 ° C. and then directly subjected to hot rolling, or when the slab is reheated before rough rolling, it is necessary to reheat to 950-1280 ° C. is there. The upper limit temperature for cooling the slab or the upper limit temperature for reheating is preferably 1250 ° C, more preferably 1150 ° C. What is necessary is just to select the time to reheat suitably according to the dimension of slab in the range in which an austenite crystal grain does not become coarse.
[0099]
In rough rolling, at least the final rolling pass is₃ It is desirable to carry out in a temperature range of point to 1150 ° C. Furthermore, the total rolling reduction of rough rolling is desirably 40% or more. This is because the austenite crystal grains are refined and the ferrite crystal grains after ferrite transformation from austenite are also refined. Furthermore, it is preferable to secure 50% or more of the total rolling reduction of rough rolling.
[0100]
After the rough rolling, the rough rolled material is reheated or heat-treated before the finish rolling is started, and the temperature difference between the rolling longitudinal direction and the width direction of the rough rolled material immediately before the finish rolling is started is 140 ° C. The following is recommended. Thereby, the fluctuation range of the finish rolling completion temperature (ΔFT (° C.)) and the fluctuation of the microstructure due to the temperature fluctuation in the subsequent cooling process can be suppressed, and as a result, the longitudinal direction and the width direction of the steel sheet can be suppressed. The characteristics can be made uniform.
[0101]
Further, by performing reheating of the rough rolled material before the start of finish rolling or heat treatment as described above, the fluctuation range ΔFT (° C.) of the finish rolling completion temperature in hot is reduced, and hot rolling is performed. The microstructure and properties in the rolling longitudinal direction and the width direction of the steel sheet become uniform, and the formability of the hot-rolled steel sheet increases.
In addition, it is more preferable if the temperature difference in the rolling longitudinal direction and the width direction of the rough rolled material immediately before the finish rolling is started is 120 ° C. or less, and it is extremely preferable if the temperature difference is 100 ° C. or less.
[0102]
As described above, when the rough rolling material is reheated or subjected to heat treatment before the start of finish rolling, ΔFT can be reduced when finish rolling is completed in the austenite region. As a result, the finish rolling completion temperature is set to Ae.₃ Even if it is just above the point, Ae over the entire steel plate₃ It becomes easy to roll without lowering the temperature below the point, and the cumulative strain in the low temperature austenite region can be increased.
[0105]
Furthermore, by reheating or holding heat treatment the rough rolled material before finish rolling, even if the slab temperature before rough rolling is lowered, rolling can be performed without significantly reducing the finish rolling completion temperature. The increase in hot deformation resistance at the time can also be suppressed, and rolling can be performed without overloading the hot rolling mill. That is, since the slab temperature before rough rolling can be lowered by the above-described treatment, it is possible to suppress fine precipitation during hot rolling and grain boundary oxidation of the slab surface layer portion, and to improve the surface property. It becomes easy to obtain formable hot-rolled steel sheets and cold-rolled steel sheets.
[0106]
A process in which a slab is roughly rolled, wound into a coil using a coil box, and then rolled back to finish rolling, or after joining the leading end of the rough rolled material with the trailing end of the preceding rough rolled material The continuous finish rolling process, in which finish rolling is performed, is effective for uniformizing the properties in the coil or the steel sheet, but combines these processes with the process of reheating or keeping the rough rolled material before finishing rolling. Thus, the characteristics can be made more uniform.
[0107]
By cooling the surface layer portion of the rough rolled material and transforming its structure from austenite to ferrite, the steel sheet surface layer is transformed back to austenite by reheating or heat retaining the rough rolled material before finish rolling. From the austenite that has undergone reverse transformation of the part, it is possible to efficiently refine the grain size of the ferrite that is again transformed and increase the grain boundary.
[0108]
  In addition, the means for heating or holding the rough rolled material before the start of finish rolling is a method of heating the rough rolled material by high frequency induction heating, a direct current heating method in which a current is directly supplied to the rough rolled material through a roll, and combustion. Any method such as gas heating method that heats rough rolled material with gas burner using gasButCan be used.
(C-2) Hot finish rolling
  In order to obtain the metal structure, the rolling completion temperature is Ae after rough rolling._ThreeIt is necessary to perform finish rolling under the conditions satisfying the formulas (1) and (2) or the formula (3) or hot finish rolling under the conditions satisfying the formulas (4) and (5). is there. This is because, as shown in FIG. 6 and FIG. 7, a hot-rolled steel sheet having good formability cannot be obtained unless these conditions are satisfied.
[0109]
Completion temperature of hot finish rolling is Ae₃Below the point, a desired steel sheet structure may not be obtained after finish rolling, and deformation resistance may increase and rolling itself may be difficult. Therefore, the finish rolling completion temperature is set to Ae.₃More than the point, it is necessary to hot finish rolling.
[0110]
In finish rolling, (Ae₃Point + 100 ° C) to Ae₃It is necessary to ensure a total rolling reduction of 70% or more in the temperature range of the point. This increases the cumulative working strain in the low temperature austenite region, promotes the ferrite transformation driving force and nucleation rate, reduces the ferrite grain size and aspect ratio, and reduces the ferrite area ratio in the structure. The increase and the C concentration to the untransformed austenite phase are attempted, and the ratio of the large-angle grain boundary in the ferrite grain boundary is increased.
[0111]
In finish rolling, hot rolling may be used so that the friction coefficient between the rolling roll and the material to be rolled is 0.2 or less. As a result, processing deformation in the plate thickness direction is made uniform, so that the EL and r value of the plate thickness surface layer portion of the hot-rolled steel plate and cold-rolled steel plate are further improved. As a result, the EL and r value of the entire plate can be further improved. The hot lubricant may be a commonly used one, and for example, a machine oil capable of reducing the friction coefficient may be used.
[0112]
If rolling is performed by the so-called `` finishing continuous rolling method '' in which the rough rolled material is continuously rolled by joining the leading end of the steel slab to the trailing end of the preceding pressure member on the entry side of the finish rolling mill. The slip phenomenon between the material to be rolled and the roll, which may occur when performing the lubrication rolling, and the biting failure can be prevented, so that the characteristics are made uniform and the yield is improved.
[0113]
  After finishing rolling, it is necessary to water-cool within 2 seconds to a temperature range of 600 to 800 ° C. at an average cooling rate of 30 ° C./second or more. In other words, the time from the end of the final rolling pass to the start of cooling is shortened, the release of accumulated strain in finish rolling is suppressed, and the ferrite grain size and aspect ratio are reduced.TheThe area ratio of ferrite occupying the structure is increased, and the ratio of large-angle grain boundaries occupying the ferrite grain boundaries is increased.
[0114]
In order to exhibit the above action, the average cooling rate needs to be 30 ° C./second or more. Furthermore, the reason for stopping the cooling in the temperature range of 600 to 800 ° C. is that when it is cooled to below 600 ° C., the ferrite becomes bainitic ferrite and the ductility is lowered. If the cooling is stopped in the range exceeding 800 ° C., the ferrite grains are coarsened even when they are formed, and the C concentration to the second phase does not proceed during the intermediate air cooling, so that the ductility is also lowered.
[0115]
Next, after air cooling for 3 to 15 seconds, the water is further cooled at an average cooling rate of 30 ° C./second or more and wound. The reason why air cooling is required for 3 to 15 seconds is to promote the formation of ferrite and to promote the concentration of C to the second phase to improve the ductility. Then, the water cooling at an average cooling rate of 30 ° C./second or more is to be taken up, and if the average cooling rate is less than 30 ° C./second, carbide precipitates in the second phase, which is optimal for fine ferrite. Bainite, retained austenite and martensite as the second phase cannot be obtained.
[0116]
As the coiling temperature, when bainite, retained austenite and martensite as the second phase are used, an appropriate temperature for obtaining each structure is selected. When obtaining bainite, the coiling temperature is 300 to 550 ° C., when obtaining retained austenite, the coiling temperature is 350 to 500 ° C., and when obtaining martensite, the coiling temperature is 200 ° C. or less. select.
[0117]
In addition, the fluctuation | variation in the hot-rolling steel plate (hot-rolling coil) of the cooling stop temperature and coiling temperature after finish rolling is desirably 100 ° C. or less, more desirably 60 ° C. or less in order to suppress characteristic variation. Is good.
(C-3) Cold drawing recrystallization annealing
The hot-rolled steel sheet is subjected to cold rolling and recrystallization annealing after removing surface oxides and dirt by a method such as pickling. In order to develop a {111} recrystallized texture during annealing after cold rolling, the hot-rolled steel sheet is heated to 650-900 ° C. and annealed before cold rolling, and the {111} texture is applied to the hot-rolled steel sheet. You may develop.
[0118]
In cold rolling, a rolling texture is developed, and in order to develop a {111} texture that is preferable for improving r value and minimizing in-plane anisotropy in the recrystallization annealing process, the rolling reduction is 50% or more. Process to plate thickness.
[0119]
Recrystallization annealing needs to be performed in a temperature range of 600 to 950 ° C. in order to develop a texture preferable for deep drawability from a rolled texture introduced by cold rolling. If the annealing temperature is lower than 600 ° C., recrystallization may not proceed sufficiently even if annealing is performed for a long time. On the other hand, if it exceeds 950 ° C., the r value may decrease.
[0120]
The annealing method is not particularly defined, and may be performed by an arbitrary method such as a box annealing method, a continuous annealing method, or a continuous annealing method that is usually performed during hot dip galvanizing treatment or alloying hot dip galvanizing treatment.
[0121]
After cold rolling and recrystallization annealing, temper rolling (skin pass) with a reduction rate of less than 10%, hot dip galvanization, alloyed hot dip galvanization, electroplating, organic coating, etc., is performed by ordinary methods. A surface treatment may be applied. The steel plates subjected to these treatments are used for automobiles, home appliances, steel structures and the like after being subjected to press working.
[0122]
【Example】
  In order to confirm the effect of the steel sheet of the present invention, steel types AA to CD having chemical compositions shown in Tables 2 and 3 were cast using a vacuum melting furnace, and a slab having a thickness of 70 mm was produced by hot forging. Hot rolled steel sheets were manufactured using these slabs. In addition,Tables 2 and 3For reference values, liquidus temperature (TL), Ae_ThreeThe temperature of the point is shown.
[0123]
[Table 2]

[0124]
[Table 3]

[0125]
Using the obtained slab, based on the conditions shown in Tables 4 and 5, slab heating, rough rolling and finish rolling were performed on a laboratory scale to produce a hot rolled steel sheet having a thickness of 2.6 mm and a width of 250 mm. . The rough rolled material was heated using a laboratory-scale induction heating apparatus.
[0126]
  Hot rolled steel sheets are manufactured under the conditions that steel AB is wound at 700 ° C., steel AD and steel AI are rolled at 600 ° C. and then annealed at 800 ° C.It was.
[0127]
For steel AC, steel AG, steel AM, steel AZ, steel BI, steel CA, and steel AK, hot rolling is performed under the conditions shown in Tables 4 and 5, and hot rolled steel sheets of 3.5 to 5.3 mm are also manufactured. did. The hot-rolled steel sheet having a thickness of 3.5 to 5.3 mm is further pickled, cold-rolled under the conditions shown in Table 8 to be described later, and further subjected to recrystallization annealing and temper rolling with a reduction rate of 0.6%. Thus, cold-rolled steel sheets, hot-dip galvanized steel sheets, and galvannealed steel sheets having a thickness of 0.8 to 1.3 mm and a width of 250 mm were manufactured. Annealing for recrystallization was performed by a continuous annealing method and a continuous annealing method usually performed during hot dip galvanizing and alloying hot dip galvanizing.
[0128]
About the obtained hot-rolled steel sheet and cold-rolled annealed steel sheet, from JIS Z 2201 No. 5 test piece (0 ° , 45 °, and 90 ° in three directions) and hole expansion test specimens were collected and examined for yield strength, tensile strength, elongation at break, hole expansion ratio, r value, and variations thereof. Each characteristic was determined as a minimum value in the steel plate, and fluctuations in the TS × EL balance value, TS × λ balance value, and TS × r value were determined by subtracting the minimum value from the maximum value in the steel plate. The survey results are shown in Tables 4-7 and Table 8.
[0129]
[Table 4]

[0130]
[Table 5]

[0131]
[Table 6]

[0132]
[Table 7]

[0133]
[Table 8]

[0134]
As is clear from the results in Tables 4 to 7 and Table 8, the steel sheets manufactured with the chemical composition, metallographic structure and manufacturing conditions specified in the present invention are compared with the steel sheets manufactured outside the conditions specified in the present invention. It has the characteristics that it has high formability, the fluctuation of the characteristic value in the steel sheet is small, and the surface property is excellent.
[0135]
【The invention's effect】
The hot-rolled steel sheet of the present invention has favorable surface properties, small fluctuations in the characteristic value of the steel sheet, and excellent formability, and is therefore suitable for applications such as automobiles, home appliances, and steel structures. According to the production method of the present invention, it is possible to efficiently produce a hot-rolled steel plate and a cold-rolled steel plate having good surface properties and excellent formability.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing the influence of the maximum diameter and aspect ratio of crystal grains of a slab surface layer on the surface properties and formability of a hot-rolled steel sheet.
FIG. 2 is a diagram showing the influence of ferrite grain size and second phase grain size on the formability of a hot-rolled steel sheet.
FIG. 3 is a diagram showing the influence of the area ratio of a ferrite phase and the area ratio of a second phase on the formability of a hot-rolled steel sheet.
FIG. 4 is a diagram showing the influence of ferrite grain size and second phase grain size in hot-rolled steel sheets on the formability of cold-rolled steel sheets.
FIG. 5 is a diagram showing the influence of the area ratio of the ferrite phase and the area ratio of the second phase in the hot-rolled steel sheet on the formability of the cold-rolled steel sheet.
FIG. 6 is a diagram showing the influence of fluctuation in rough rolling completion temperature (ΔRT) and finishing rolling completion temperature (ΔFT) on the dimensional accuracy and the like after forming a hot-rolled steel sheet.
FIG. 7 is a diagram showing the influence of variation ΔFT of finish rolling completion temperature ΔFT and cumulative strain during finish rolling on the formability of a hot-rolled steel sheet.

Claims (9)

In mass%, C: 0.0002 to 0.25%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0% and Al: 0.002 to 2.0% The balance consists of Fe and impurities, P in the impurities is 0.15% or less, S is 0.05% or less and N is 0.01% or less, and 70% or more of the metal structure in the area ratio is the ferrite phase. The average crystal grain size is 50 μm or less, the aspect ratio is 3 or less, 70% or more of the ferrite grain boundaries are composed of large angle grain boundaries, and the maximum diameter of the ferrite phase formed at the large angle grain boundaries is 30 μm or less. The average grain size of the second phase in which the area ratio of the precipitate having a minimum diameter of 5 nm or more is 2% or less of the metal structure and the area ratio is the maximum among the remaining phases excluding the ferrite phase and the precipitate diameter of the 50 [mu] m or less, a large ferrite phase between the closest second phase Hot-rolled steel sheet, wherein a grain boundary exists. The hot rolled steel sheet according to claim 1, wherein the average crystal grain size of each of the ferrite phase and the second phase of the outermost layer of the steel sheet is 5 µm or less. The hot-rolled steel sheet according to claim 1 or 2, containing 0.0002 to 0.01% of B in place of part of Fe. The hot-rolled steel sheet according to any one of claims 1 to 3 , which contains 0.005 to 1% in total of at least one of Ti, Nb, V and Zr instead of a part of Fe. The hot-rolled steel sheet according to any one of claims 1 to 4, which contains 0.005 to 3% in total of at least one of Cr, Mo, Cu and Ni instead of a part of Fe. 6. Instead of a part of Fe, one or two of Ca: 0.0001 to 0.005% and REM (rare earth element): 0.0001 to 0.2% are contained. The hot-rolled steel sheet according to any one of the above. When continuously casting the steel having the chemical composition according to any one of claims 1 to 6, the time between the start of solidification of the molten steel and the solidified part at a position 10 mm from the solidified shell surface reaches 1300 ° C. After cooling the solidified layer within 10 mm from the surface to a cooling rate of 10 ° C./second or more to form a slab, and then roughly rolling the slab in a temperature range of 950 to 1280 ° C., (Ae ₃ points + 100 ° C.) -Ae Conditions where the total rolling reduction is 70% or more in the temperature range of ₃ points, the finishing temperature is ₃ points or more of Ae, and the following expressions (1) and (2) are satisfied, or the following expression (3) is satisfied And finish cooling with water, and within 2 seconds after finish rolling, the water is cooled to a temperature range of 600 to 800 ° C. at an average cooling rate of 30 ° C./second or more, then air-cooled for 3 to 15 seconds, and then an average cooling rate of 30 ° C. Winding with water cooling at a speed of more than 1 second Manufacturing method of hot-rolled steel sheet and butterflies.
△ FT ≦ 0.8 × △ RT (1)
50 ℃ <△ FT ≦ 100 ℃ (2)
ΔFT ≦ 50 ° C (3)
here,
ΔFT: Fluctuation width of finish rolling temperature in hot (° C)
ΔRT: Fluctuation range of hot rolling completion temperature (° C) When continuously casting the steel having the chemical composition according to any one of claims 1 to 6, the time between the start of solidification of the molten steel and the solidified part at a position 10 mm from the solidified shell surface reaches 1300 ° C. A solidified shell within 10 mm from the surface is cooled to a cooling rate of 10 ° C./second or more to form a slab, and then the slab is roughly rolled in a temperature range of 950 to 1280 ° C., and then (Ae ₃ point + 100 ° C. ) To Ae ₃ points, the total rolling reduction is 70% or more, the finishing temperature Ae is ₃ points or more, and hot finish rolling is performed under the conditions satisfying the following formulas (4) and (5). Within 2 seconds, the water is cooled to a temperature range of 600 to 800 ° C. at an average cooling rate of 30 ° C./second or more, then air-cooled for 3 to 15 seconds, and further cooled with water at an average cooling rate of 30 ° C./second or more. A method for producing a hot-rolled steel sheet.
ΔFT {Σ (0.8 ⁿⁱ × ε i )} ≦ 300 (4)
△ FT ≦ 100 ℃ (5)
here,
ΔFT: Fluctuation width of hot finish rolling end temperature
n: integer, the number of finish rolling stands where the exit side temperature of the finish rolling stand of the material to be rolled is between Ae ₃ points to Ae ₃ points + 100 ° C.
εi: True strain at the time of rolling down at the i-th stand from the upstream among the n rolling stands A hot-rolled steel sheet wound by the production method according to claim 7 or 8 is cold-rolled at a reduction rate of 50% or more and then annealed within a temperature range of 600 to 950 ° C. Production method.

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