JP3620183B2 - Steel sheet for automobiles with little deterioration over time in impact resistance - Google Patents

Description

【０００８】
【発明の実施の形態】
以下、本発明の詳細と限定理由について説明する。
鋼板表層の清浄度（ｄｔ）と耐衝撃特性の経時的劣化との関係を調べるため、以下のような試験を行った。供試鋼板として化学成分、金属相および強度が同一で、表層の清浄度（ｄｔ）が種々異なる薄鋼板（板厚１．６ｍｍ）を作製した。これら薄鋼板の表層の清浄度（ｄｔ）は、鋼の精錬及び鋳造条件を制御することで調整し、また、圧延以降の製造条件を同じにすることで金属相および強度がほぼ同一になるようにした。各鋼板表面のスキューネス（Ｒｓｋ）は、製造工程の最終での調質圧延により−１．５〜＋１．０の範囲となるよう調整した。
【０００９】
これらの鋼板をプレス加工し、次いで電着塗装を施した後、複合サイクル腐食試験（「０．５％ＮａＣｌ水溶液（３５℃）によるＳＳＴ試験を３時間→６５℃、湿度１０〜１５％雰囲気にて乾燥を６時間→５５℃、湿度９０％以上の雰囲気にて３時間」を１サイクルとし、計５０サイクル実施）を実施し、試験後の各鋼板について高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。また、同様の条件で上記プレス加工ままの鋼板についても平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いの指標となるΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求め、このΔＦａｖｅ（このΔＦａｖｅ値のマイナス値が大きい程、耐衝撃特性の経時的劣化が大きい）と鋼板表層の清浄度（ｄｔ）との関係を調べた。その結果を図１に示す。なお、鋼板表層の清浄度（ｄｔ）はＪＩＳ Ｇ ０５５５の方法により測定された、鋼板表面から板厚方向でｔ／８（但し、ｔ：板厚）の深さまでの部分の測定値である。
【００１０】
図１によれば、鋼板表層の清浄度（ｄｔ）が０．０５％以下ではΔＦａｖｅのマイナス値は低いレベルを示し、耐衝撃特性の劣化はほとんど見られないが、清浄度（ｄｔ）が０．０５％を超えるとΔＦａｖｅのマイナス値が急激に増大し、耐衝撃特性の劣化が生じている。このような表層の清浄度（ｄｔ）が０．０５％を超える鋼板の複合サイクル腐食試験後の表面を観察した結果、多数の点錆が認められ、断面検鏡では錆発生部からのクラック発生も観察された。
以上の結果から本発明では、耐衝撃特性の経時的劣化を生じさせない条件として、鋼板表層の清浄度（ｄｔ）を０．０５％以下と定めた。なお、このように清浄度を規制する“鋼板表層”とは、鋼板表面から板厚方向で最大でもｔ／８（但し、ｔ：鋼板板厚）を超えない範囲の鋼板表層を対象とすることが好ましい。
【００１１】
このような鋼板表層の清浄度（ｄｔ）：０．０５％以下は、鋼の溶製後の二次精錬および鋳造時の介在物の巻込み対策を適切に行なうことにより達成できる。なお、本発明では鋼板の板厚中央部（板厚方向において上記“鋼板表層”以外の部分）の清浄度（ｄｔ）は特に規定しないが、特に厳しいプレス成形が行われる用途の鋼板については、上述した点錆の発生を抑制するという観点から、板厚中央部の清浄度（ｄｔ）についても０．０５％以下とすることが好ましい。
【００１２】
次に、鋼板のプレス加工後の錆発生とそれに伴う耐衝撃特性の経時的劣化を抑制するという観点から、鋼板表面の粗さパターンと上記耐衝撃特性の経時的劣化との関係を調べるため、以下のような試験を行った。供試鋼板として化学成分、金属相および強度が同一で、表面の粗さパターンが種々異なる薄鋼板（板厚１．６ｍｍ）を作製した。これら薄鋼板表面の粗さパターンは最終での調質圧延により調整し、また、圧延以降の他の製造条件を同じにすることで金属相および強度がほぼ同一になるようにした。また、各鋼板表層の清浄度（ｄｔ）は、鋼の精錬および鋳造条件を制御することで０．０５％以下に調整した。
【００１３】
これらの鋼板をプレス加工し、次いで電着塗装を施した後、図１の試験と同様に複合サイクル腐食試験（試験条件も図１の試験と同様）を実施した鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いの指標となるΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求め、このΔＦａｖｅと鋼板表面のスキューネス（Ｒｓｋ）との関係を調べた。その結果を図２に示す。
【００１４】
図２によれば、鋼板表面のスキューネス（Ｒｓｋ）が−１．５未満および＋１．０超になるとΔＦａｖｅが急激に増加し、耐衝撃特性の著しい経時的劣化を生じている。特に、鋼板表面のスキューネス（Ｒｓｋ）が−１．５未満の場合にはプレス加工時に局部的な型のかじりが発生し、その部位で発錆が多く認められたことから、Ｒｓｋ＜−１．５において耐衝撃特性が劣化した原因は、スキューネス（Ｒｓｋ）の過度の低下によってプレス加工時に局部的な加工の集中が発生したためであると考えられる。すなわち、スキューネス（Ｒｓｋ）が低くなりすぎるとプレス加工時の潤滑油の油だまりが少なくなって摺動抵抗が増し、この結果、プレス加工時の面圧が高くなって局部的な加工の集中が生じることにより型のかじりが発生し、この部位の発錆が著しくなったものと考えられる。一方、スキューネス（Ｒｓｋ）が＋１．０超の場合にはＲｓｋ＜−１．５の場合のような型のかじりは見られないが、発錆がランダムに認められたことから、Ｒｓｋ＞＋１．０において耐衝撃特性が劣化する原因は、スキューネス（Ｒｓｋ）が高くなりすぎると粗さ曲線のパターンが凸型で局部的にとがった状態になるため、プレス加工時に上記凸型の先端部が接触する金型との面圧が高くなり、その部分で局所的な加工の集中を生じる結果、発錆しやすくなるためであると考えられる。
【００１５】
以上の結果から本発明では、耐衝撃特性の経時的劣化を生じさせない条件として、鋼板表面の粗さ曲線の高さ方向における片寄り指標であるスキューネス（Ｒｓｋ）を−１．５〜＋１．０と定めた。
このようなスキューネス（Ｒｓｋ）で規定される表面粗度は、熱間圧延、冷間圧延、調質圧延等において適切にダル加工されたロールを用いて鋼板を圧延することによって得ることができる。
また、上記した鋼板表層の清浄度（ｄｔ）を規制することによる作用効果を適切に得るためには、鋼板の金属組織が主にフェライト相、フェライト相＋パーライト相、フェライト相＋ベイナイト相、フェライト相＋マルテンサイト相、ベイナイト相、フェライト相＋オーステナイト相＋ベイナイト相、フェライト相＋オーステナイト相＋ベイナイト相＋マルテンサイト相のいずれかであることが好ましい。なお、これらの金属相中には、一般に鋼中に不可避的に存在する金属間化合物や非金属介在物等の相が含まれていることを妨げない。
【００１６】
また、本発明は様々な強度を有する鋼板に適用することができる。すなわち、自動車用鋼板としては上記した様々な金属組織を有する鋼板が使用されるが、所望の強度に応じて、例えば以下のような使い分がなされる。
フェライト相→２７０〜５００Ｎ／ｍｍ^２
フェライト相＋パーライト相→４００〜８００Ｎ／ｍｍ^２
フェライト相＋ベイナイト相→４５０〜７５０Ｎ／ｍｍ^２
フェライト相＋マルテンサイト相→５００〜９００Ｎ／ｍｍ^２
ベイナイト相→５００〜９００Ｎ／ｍｍ^２
フェライト相＋オーステナイト相＋ベイナイト相→７００〜９００Ｎ／ｍｍ^２
フェライト相＋オーステナイト相＋ベイナイト相＋マルテンサイト相→７００〜９００Ｎ／ｍｍ^２

【００１７】
また、本発明の鋼板は、同じ強度レベルであっても要求される加工性の種類に応じて金属組織が適宜選択されることが好ましい。すなわち、鋼板に高い伸びフランジ性（バーリング性）が要求される場合には、フェライト相よりもフェライト相＋マルテンサイト相、フェライト相＋オーステナイト相＋ベイナイト相またはフェライト相＋オーステナイト相＋ベイナイト相＋マルテンサイト相の方が好ましく、また上記フェライト相＋マルテンサイト相よりもフェライト相＋ベイナイト相の方がより好ましく、さらにはこのフェライト相＋ベイナイト相よりもベイナイト相の方がより好ましい。また、鋼板に高い延性（全伸び）が要求される場合には、ベイナイト相よりもフェライト相の方が好ましく、またこのフェライト相よりもフェライト相＋マルテンサイト相またはフェライト相＋ベイナイト相の方が好ましく、さらにはこのフェライト相＋ベイナイト相よりもフェライト相＋オーステナイト相＋ベイナイト相またはフェライト相＋オーステナイト相＋ベイナイト相＋マルテンサイト相の方が好ましい。
【００１８】
また、本発明の作用効果は通常の冷延鋼板だけでなく、本発明が規定する鋼板の表面粗さが損われない限度で化成処理を施した鋼板においても得られるものであり、したがって本発明が対象とする鋼板にはこのような化成処理を施した鋼板も含まれる。
【００１９】
【実施例】
［実施例１］
化学成分が０．０９％Ｃ−０．０１％Ｓｉ−０．５０％Ｍｎ−０．１００％Ｐの４４０ＭＰａ級のフェライト＋パーライト鋼からなり、表１に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚１．６ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表１に示す。
【００２０】
表１によれば、本発明例であるＮｏ．１〜Ｎｏ．３では複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．４〜Ｎｏ．６は、本発明の規定する鋼板表層の清浄度（ｄｔ）、鋼板表面のスキューネス（Ｒｓｋ）のいずれかを満足していないため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００２１】
【表１】

【００２２】
［実施例２］
化学成分が０．００３％Ｃ−０．２５％Ｓｉ−２．００％Ｍｎ−０．０７５％Ｐ−０．０７％Ｔｉの４４０ＭＰａ級のフェライト鋼からなり、表２に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚１．６ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表２に示す。
【００２３】
表２によれば、本発明例であるＮｏ．７では複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．８は、鋼板表面のスキューネス（Ｒｓｋ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００２４】
【表２】

【００２５】
［実施例３］
化学成分が０．１０％Ｃ−０．５０％Ｓｉ−１．４０％Ｍｎ−０．０１５％Ｐの５９０ＭＰａ級のフェライト＋パーライト鋼からなり、表３に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚１．６ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表３に示す。
【００２６】
表３によれば、本発明例であるＮｏ．９は複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．１０は、鋼板表層の清浄度（ｄｔ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００２７】
【表３】

【００２８】
［実施例４］
化学成分が０．０９％Ｃ−０．０８％Ｓｉ−１．５５％Ｍｎ−０．０１０％Ｐの５９０ＭＰａ級のフェライト＋ベイナイト鋼からなり、表４に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚３．０ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表４に示す。
【００２９】
表４によれば、本発明例であるＮｏ．１１は複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．１２は、鋼板表面のスキューネス（Ｒｓｋ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００３０】
【表４】

【００３１】
［実施例５］
化学成分が０．０９％Ｃ−０．２０％Ｓｉ−０．７５％Ｍｎ−０．０７０％Ｐからなる５９０ＭＰａ級のフェライト＋マルテンサイト鋼からなり、表５に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚１．６ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表５に示す。
【００３２】
表５によれば、本発明例であるＮｏ．１３は複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．１４は、鋼板表層の清浄度（ｄｔ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００３３】
【表５】

【００３４】
［実施例６］
化学成分が０．１０％Ｃ−１．０８％Ｓｉ−１．６４％Ｍｎ−０．０１０％Ｐの５９０ＭＰａ級のフェライト＋オーステナイト＋ベイナイト鋼からなり、表６に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚１．６ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表６に示す。
【００３５】
表６によれば、本発明例であるＮｏ．１５は複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．１６は、鋼板表面のスキューネス（Ｒｓｋ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００３６】
【表６】

【００３７】
［実施例７］
化学成分が０．１５％Ｃ−０．１５％Ｓｉ−２．３０％Ｍｎ−０．１００％Ｐからなる７８０ＭＰａ級のフェライト＋パーライト鋼からなり、表７に示すような表層の清浄度（ｄｔ）を有する鋼板（板厚３．０ｍｍ）を調質圧延して、同表に示す表面粗度（Ｒｓｋ）に調整した。これらの鋼板をプレス加工し、次いで電着塗装を行なった後、複合サイクル腐食試験（試験条件は図１の試験と同様）を実施し、錆の発生状況を観察した。また、図１の試験と同様に、複合サイクル腐食試験を実施した上記鋼板とプレス加工ままの鋼板について、高速変形（変形速度：１１ｍ／ｓｅｃ）により圧壊する際の平均崩壊荷重を測定した。これら平均崩壊荷重の測定値に基づき、耐衝撃特性の経時的劣化の度合いを示すΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）を求めた。その結果を表７に示す。
【００３８】
表７によれば、本発明例であるＮｏ．１７は複合サイクル腐食試験後でも発錆はみられず、したがってΔＦａｖｅも小さい値となっており、耐衝撃特性の経時的劣化が効果的に抑制されていることが判る。これに対して比較例であるＮｏ．１８は、鋼板表層の清浄度（ｄｔ）が本発明の規定する上限を超えているため、複合サイクル腐食試験後に発錆し、このためΔＦａｖｅも大きくなっており、耐衝撃特性の経時的劣化が生じていることが判る。
【００３９】
【表７】

【００４０】
【発明の効果】
以上述べたように本発明によれば、鋼板表層の清浄度（ｄｔ）と鋼板の表面粗度（Ｒｓｋ）を適正化することによって、耐衝撃特性の経時的劣化の小さい薄鋼板を提供することができ、これを自動車用鋼板として利用することにより、製造から長期間経過した車体についても衝突安全性の向上を図ることができる。
【図面の簡単な説明】
【図１】鋼板表層の清浄度（ｄｔ）と鋼板の耐衝撃特性の経時的劣化の指標となるΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）との関係を示すグラフ
【図２】鋼板表面のスキューネス（Ｒｓｋ）と鋼板の耐衝撃特性の経時的劣化の指標となるΔＦａｖｅ（＝［複合サイクル腐食試験後の鋼板の平均崩壊荷重］−［プレス加工ままの鋼板の平均崩壊荷重］）との関係を示すグラフ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel plate having excellent impact resistance, and more particularly to a steel plate for automobiles having a small deterioration with time of impact resistance after press working.
[0002]
[Prior art]
In recent years, demand for automobile crash safety has been increasing against the backdrop of strengthening laws and regulations relating to automobile safety, and development of automobile steel sheets having excellent impact resistance has been actively conducted.
As one of such techniques, Japanese Patent Application Laid-Open No. 8-3678 defines the volume ratio of each metal phase of a structure composed of a ferrite phase and a martensite phase and the dislocation density in the ferrite phase under a specific component composition. An automotive steel plate has been proposed.
[0003]
[Problems to be solved by the invention]
This technology is intended to improve the impact resistance characteristics of the steel sheet by controlling the composition and structure of the steel sheet. However, according to the present inventors' investigation and examination, the impact resistance of the material formed by press-molding the thin steel sheet is investigated. The characteristics deteriorate over time, and even if the components and the structure control are performed as in the above-mentioned conventional technology, the substantial impact resistance characteristics that contribute to the collision safety of the automobile (that is, the impact resistance after a long time has passed since the vehicle body was manufactured) (Characteristics) was not sufficiently obtained. That is, the present inventors conducted a corrosion acceleration test on a sample obtained by press-molding a steel plate for automobiles as a model, and as a result of investigating the high-speed deformation characteristics (average collapse load when crushing) described later for this sample, It was found that the average collapse load of the sample rusted by the corrosion acceleration test was significantly inferior to that of the sample before the corrosion acceleration test, that is, the impact resistance was deteriorated with time.
[0004]
Conventionally, a technique for improving the impact resistance of a steel sheet has not been known from the viewpoint of suppressing such deterioration over time of the impact resistance.
Accordingly, an object of the present invention is to provide a steel sheet for automobiles in which deterioration over time of impact resistance characteristics is effectively suppressed, and collision stability can be appropriately ensured even after a long period of time has passed since the manufacture of the vehicle body. is there.
[0005]
[Means for Solving the Problems]
In view of the above-mentioned problems, the present inventors have conducted research to obtain a steel sheet in which deterioration over time of impact resistance characteristics is suppressed. As a method for evaluating the presence or absence of deterioration over time of impact resistance characteristics, It has been found that the method of measuring the average collapse load when the molded member is crushed during high-speed deformation is most suitable. For this reason, after changing the internal properties and surface properties of steel sheets having various metal structures (metal phases), performing corrosion acceleration tests on each steel sheet after pressing, and measuring the average collapse load, the impact resistance The presence or absence of deterioration of characteristics over time was examined. As a result, the temporal deterioration of the impact resistance is caused by the occurrence of rust on the surface layer of the steel sheet, and in order to suppress the temporal deterioration of the impact resistance due to such rusting, the following (1) It has been found that it is necessary to satisfy the condition of (3).
[0006]
(1) It is necessary to reduce the starting point of rusting as much as possible by setting the cleanliness (dt) in at least the steel sheet surface layer to a predetermined level or less.
(2) By restricting the surface roughness (Rsk) of the surface of the steel sheet to a predetermined range, the lubricity at the time of press working is improved, and the local concentration of processing is suppressed, so that It is necessary to prevent corrosion promotion.
(3) In addition, in order to appropriately obtain the effect by regulating the cleanliness (dt) of the steel sheet surface layer, the metal phase of the steel sheet is mainly composed of ferrite phase, ferrite phase + pearlite phase, ferrite phase + bainite phase, ferrite phase. + Martensite phase, bainite phase, ferrite phase + austenite phase + bainite phase, ferrite phase + austenite phase + bainite phase + martensite phase are preferable.
[0007]
The present invention has been made on the basis of such knowledge, and the constitution thereof is ferrite phase, ferrite phase + pearlite phase, ferrite phase + bainite phase, ferrite phase + martensite phase, bainite phase, ferrite phase + austenite phase + bainite. Phase, ferrite phase + austenite phase + bainite phase + martensite phase, and at least the cleanliness (dt) of the steel sheet surface layer is 0.05% or less, and the surface of the steel sheet is rough. It is a steel sheet for automobiles having a small deterioration with time of impact resistance, characterized in that skewness (Rsk), which is a deviation index in the height direction of the height curve, is −1.5 to +1.0.
Here, skewness (Rsk) indicates the symmetry of the roughness curve with respect to the center line of the amplitude distribution curve distribution, and is given by the following equation.
[Expression 1]

[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the details of the present invention and the reasons for limitation will be described.
In order to investigate the relationship between the cleanliness (dt) of the steel sheet surface layer and the temporal deterioration of the impact resistance, the following test was conducted. As the test steel sheet, thin steel sheets (thickness 1.6 mm) having the same chemical composition, metal phase and strength and different surface cleanliness (dt) were prepared. The cleanliness (dt) of the surface layer of these thin steel plates is adjusted by controlling the steel refining and casting conditions, and by making the production conditions after rolling the same, the metal phase and the strength are almost the same. I made it. The skewness (Rsk) of each steel sheet surface was adjusted to be in the range of −1.5 to +1.0 by temper rolling at the end of the manufacturing process.
[0009]
These steel sheets were pressed and then electrodeposited, and then combined cycle corrosion test (“SST test with 0.5% NaCl aqueous solution (35 ° C.) for 3 hours → 65 ° C., humidity 10-15% atmosphere” Drying for 6 hours → 55 ° C. and 3 hours in an atmosphere of 90% or higher ”as one cycle, 50 cycles in total), and each steel plate after the test was deformed at high speed (deformation speed: 11 m / sec) The average collapse load at the time of crushing was measured. Moreover, the average collapse load was measured also about the steel plate as the said press work on the same conditions. Based on these measured values of average collapse load, ΔFave (= [average collapse load of steel sheet after combined cycle corrosion test] − [average collapse load of as-pressed steel sheet, which becomes an index of the degree of deterioration of impact resistance over time] ]), And the relationship between the ΔFave (the greater the negative value of the ΔFave value, the greater the deterioration in impact resistance with time) and the cleanliness (dt) of the steel sheet surface layer was investigated. The result is shown in FIG. The cleanliness (dt) of the steel sheet surface layer is a measured value of a portion from the steel sheet surface to a depth of t / 8 (where t is the sheet thickness) in the sheet thickness direction, measured by the method of JIS G 0555.
[0010]
According to FIG. 1, when the cleanness (dt) of the steel sheet surface layer is 0.05% or less, the minus value of ΔFave shows a low level, and the deterioration of the impact resistance property is hardly seen, but the cleanliness (dt) is 0. If it exceeds 0.05%, the negative value of ΔFave increases rapidly, and the impact resistance characteristics deteriorate. As a result of observing the surface of the steel sheet with a surface layer cleanliness (dt) exceeding 0.05% after the combined cycle corrosion test, many spot rusts were observed, and in the cross-sectional speculum, cracks were generated from the rust generation part. Was also observed.
From the above results, in the present invention, the cleanliness (dt) of the steel sheet surface layer is set to 0.05% or less as a condition that does not cause deterioration of impact resistance with time. The “steel sheet surface layer” that regulates the cleanliness in this way is intended to cover the steel sheet surface layer in a range that does not exceed t / 8 at the maximum in the sheet thickness direction from the sheet surface. Is preferred.
[0011]
Such a cleanliness (dt) of the steel sheet surface layer of 0.05% or less can be achieved by appropriately performing secondary refining after melting of steel and measures for inclusion inclusion during casting. In the present invention, the cleanliness (dt) of the central portion of the steel plate (the portion other than the “steel plate surface layer” in the plate thickness direction) is not particularly defined. From the viewpoint of suppressing the occurrence of spot rust as described above, it is preferable that the cleanness (dt) of the central portion of the plate thickness is also 0.05% or less.
[0012]
Next, from the viewpoint of suppressing the aging after the press working of the steel sheet and the deterioration over time of the impact resistance characteristics associated therewith, in order to investigate the relationship between the roughness pattern of the steel sheet surface and the deterioration over time of the above impact resistance characteristics, The following tests were conducted. Thin steel plates (thickness 1.6 mm) having the same chemical composition, metal phase and strength and different surface roughness patterns were prepared as test steel plates. The roughness patterns on the surface of these thin steel plates were adjusted by temper rolling at the end, and the metal phase and strength were made substantially the same by making the other production conditions after rolling the same. The cleanliness (dt) of each steel sheet surface layer was adjusted to 0.05% or less by controlling the steel refining and casting conditions.
[0013]
After pressing these steel plates, and then applying electrodeposition coating, steel plates subjected to a combined cycle corrosion test (test conditions are the same as those in FIG. 1) as well as those in FIG. The average collapse load at the time of crushing by high-speed deformation (deformation speed: 11 m / sec) was measured. Based on these measured values of average collapse load, ΔFave (= [average collapse load of steel sheet after combined cycle corrosion test] − [average collapse load of as-pressed steel sheet, which becomes an index of the degree of deterioration of impact resistance over time] ]) And the relationship between ΔFave and the skewness (Rsk) of the steel sheet surface was examined. The result is shown in FIG.
[0014]
According to FIG. 2, when the skewness (Rsk) of the steel sheet surface is less than −1.5 and more than +1.0, ΔFave increases sharply, and the impact resistance characteristics deteriorate significantly over time. In particular, when the skewness (Rsk) of the steel sheet surface is less than −1.5, local mold galling occurred during press working, and a lot of rusting was observed at that site, so that Rsk <−1. The reason why the impact resistance characteristics deteriorated in No. 5 is considered to be that local concentration of processing occurred during press processing due to excessive decrease in skewness (Rsk). In other words, if the skewness (Rsk) is too low, the accumulation of lubricating oil during press working is reduced and the sliding resistance is increased. As a result, the surface pressure during press working is increased and the concentration of local processing is concentrated. As a result, the galling of the mold occurs, and it is considered that the rusting of this part became remarkable. On the other hand, when the skewness (Rsk) is greater than +1.0, no type galling is observed as in the case of Rsk <−1.5. However, since rusting was observed at random, Rsk> +1. The cause of the deterioration of the impact resistance at 0 is that if the skewness (Rsk) is too high, the pattern of the roughness curve becomes convex and locally sharp, so that the tip of the convex shape comes into contact during pressing. This is thought to be because the surface pressure with the mold to be used increases, and as a result of local concentration of processing at that portion, rusting tends to occur.
[0015]
From the above results, in the present invention, as a condition that does not cause deterioration of impact resistance with time, the skewness (Rsk), which is a deviation index in the height direction of the roughness curve of the steel sheet surface, is −1.5 to +1.0. It was determined.
Such surface roughness defined by skewness (Rsk) can be obtained by rolling a steel sheet using a roll that has been appropriately dulled in hot rolling, cold rolling, temper rolling, or the like.
In addition, in order to appropriately obtain the effect by regulating the cleanliness (dt) of the above steel sheet surface layer, the metal structure of the steel sheet is mainly composed of ferrite phase, ferrite phase + pearlite phase, ferrite phase + bainite phase, ferrite It is preferably any one of phase + martensite phase, bainite phase, ferrite phase + austenite phase + bainite phase, ferrite phase + austenite phase + bainite phase + martensite phase. In addition, it does not prevent that these metal phases contain phases, such as an intermetallic compound and a nonmetallic inclusion which generally exist inevitably in steel.
[0016]
In addition, the present invention can be applied to steel plates having various strengths. In other words, steel plates having various metal structures described above are used as the steel plate for automobiles, but for example, the following usage is made according to the desired strength.
Ferrite phase → 270-500 N / mm ²
Ferrite phase + pearlite phase → 400 to 800 N / mm ²
Ferrite phase + bainite phase → 450-750 N / mm ²
Ferrite phase + martensite phase → 500 to 900 N / mm ²
Bainite phase → 500 to 900 N / mm ²
Ferrite phase + austenite phase + bainite phase → 700 to 900 N / mm ²
Ferrite phase + austenite phase + bainite phase + martensite phase → 700 to 900 N / mm ²

[0017]
In the steel sheet of the present invention, it is preferable that the metal structure is appropriately selected according to the type of workability required even at the same strength level. That is, when high stretch flangeability (burring property) is required for the steel sheet, the ferrite phase + martensite phase, ferrite phase + austenite phase + bainite phase or ferrite phase + austenite phase + bainite phase + martens rather than the ferrite phase. The site phase is preferred, the ferrite phase + bainite phase is more preferred than the ferrite phase + martensite phase, and the bainite phase is more preferred than the ferrite phase + bainite phase. Further, when high ductility (total elongation) is required for the steel sheet, the ferrite phase is preferable to the bainite phase, and the ferrite phase + martensite phase or the ferrite phase + bainite phase is more preferable than the ferrite phase. More preferably, the ferrite phase + austenite phase + bainite phase or the ferrite phase + austenite phase + bainite phase + martensite phase is more preferable than the ferrite phase + bainite phase.
[0018]
The effects of the present invention can be obtained not only in a normal cold-rolled steel sheet, but also in a steel sheet that has been subjected to chemical conversion treatment as long as the surface roughness of the steel sheet specified by the present invention is not impaired. The steel plate to which No. 2 is intended includes steel plates subjected to such chemical conversion treatment.
[0019]
【Example】
[Example 1]
Cleanliness of surface layer (dt) as shown in Table 1 consisting of 440 MPa class ferrite + pearlite steel with a chemical component of 0.09% C-0.01% Si-0.50% Mn-0.100% P A steel plate having a thickness of 1.6 mm was temper-rolled to adjust the surface roughness (Rsk) shown in the same table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 1.
[0020]
According to Table 1, No. 1 is an example of the present invention. 1-No. In No. 3, no rusting was observed even after the combined cycle corrosion test, and therefore ΔFave was also a small value, indicating that the deterioration over time of the impact resistance characteristics was effectively suppressed. On the other hand, No. which is a comparative example. 4-No. No. 6 does not satisfy either the cleanliness (dt) of the steel sheet surface layer specified by the present invention or the skewness (Rsk) of the steel sheet surface, and therefore rusts after the combined cycle corrosion test, and ΔFave also increases. Thus, it can be seen that the impact resistance is deteriorated with time.
[0021]
[Table 1]

[0022]
[Example 2]
It consists of a 440 MPa grade ferritic steel with a chemical composition of 0.003% C-0.25% Si-2.00% Mn-0.075% P-0.07% Ti. A steel sheet (thickness 1.6 mm) having a degree (dt) was temper-rolled to adjust the surface roughness (Rsk) shown in the table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 2.
[0023]
According to Table 2, no. In No. 7, no rusting was observed even after the combined cycle corrosion test, and therefore ΔFave was also a small value, indicating that the deterioration over time of the impact resistance characteristics was effectively suppressed. On the other hand, No. which is a comparative example. No. 8, because the skewness (Rsk) of the steel sheet surface exceeds the upper limit defined by the present invention, rusting occurs after the combined cycle corrosion test, and therefore ΔFave also increases, resulting in deterioration of impact resistance with time. You can see that
[0024]
[Table 2]

[0025]
[Example 3]
Cleanliness (dt) of the surface layer as shown in Table 3 consisting of 590 MPa grade ferrite + pearlite steel with a chemical composition of 0.10% C-0.50% Si-1.40% Mn-0.015% P A steel plate having a thickness of 1.6 mm was temper-rolled to adjust the surface roughness (Rsk) shown in the same table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 3.
[0026]
According to Table 3, No. which is an example of the present invention. 9 shows no rusting even after the combined cycle corrosion test, and therefore ΔFave is also a small value, indicating that the temporal deterioration of the impact resistance is effectively suppressed. On the other hand, No. which is a comparative example. No. 10, the cleanliness (dt) of the steel sheet surface layer exceeds the upper limit defined by the present invention, and thus rusts after the combined cycle corrosion test. Therefore, ΔFave is also increased, and the impact resistance is deteriorated over time. You can see that it has occurred.
[0027]
[Table 3]

[0028]
[Example 4]
Cleanness of surface layer (dt) as shown in Table 4 consisting of 590 MPa grade ferrite + bainite steel with chemical components of 0.09% C-0.08% Si-1.55% Mn-0.010% P A steel plate having a thickness of 3.0 mm was temper-rolled to adjust the surface roughness (Rsk) shown in the same table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 4.
[0029]
According to Table 4, no. 11 shows no rusting even after the combined cycle corrosion test, and therefore ΔFave is also a small value, indicating that the deterioration over time of the impact resistance is effectively suppressed. On the other hand, No. which is a comparative example. No. 12, since the skewness (Rsk) of the steel sheet surface exceeds the upper limit prescribed by the present invention, rusting occurs after the combined cycle corrosion test, and therefore ΔFave also increases, resulting in deterioration of impact resistance with time. You can see that
[0030]
[Table 4]

[0031]
[Example 5]
Cleanliness of the surface layer as shown in Table 5 (Table 5) consisting of 590 MPa grade ferrite + martensitic steel consisting of 0.09% C-0.20% Si-0.75% Mn-0.070% P A steel plate having a thickness of dt) (plate thickness of 1.6 mm) was temper-rolled to adjust the surface roughness (Rsk) shown in the table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 5.
[0032]
According to Table 5, no. No rusting was observed even after the combined cycle corrosion test No. 13, and therefore ΔFave is also a small value, indicating that the deterioration over time of the impact resistance characteristics is effectively suppressed. On the other hand, No. which is a comparative example. No. 14, because the cleanliness (dt) of the steel sheet surface layer exceeds the upper limit prescribed by the present invention, rusting occurs after the combined cycle corrosion test. For this reason, ΔFave also increases, and the impact resistance characteristics deteriorate over time. You can see that it has occurred.
[0033]
[Table 5]

[0034]
[Example 6]
The chemical composition is composed of 590 MPa grade ferrite + austenite + bainite steel with 0.10% C-1.08% Si-1.64% Mn-0.010% P, and the cleanliness of the surface layer as shown in Table 6 ( A steel plate having a thickness of dt) (plate thickness of 1.6 mm) was temper-rolled to adjust the surface roughness (Rsk) shown in the table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 6.
[0035]
According to Table 6, No. which is an example of the present invention. 15 shows no rusting even after the combined cycle corrosion test, and therefore ΔFave is also a small value, indicating that the deterioration over time of the impact resistance is effectively suppressed. On the other hand, No. which is a comparative example. No. 16, since the skewness (Rsk) of the steel sheet surface exceeds the upper limit defined by the present invention, rusting occurs after the combined cycle corrosion test, and therefore ΔFave also increases, resulting in deterioration of impact resistance with time. You can see that
[0036]
[Table 6]

[0037]
[Example 7]
It consists of 780 MPa grade ferrite + pearlite steel consisting of 0.15% C-0.15% Si-2.30% Mn-0.100% P, and the cleanliness of the surface layer as shown in Table 7 (dt ) Was temper rolled to adjust the surface roughness (Rsk) shown in the same table. After pressing these steel sheets and then performing electrodeposition coating, a combined cycle corrosion test (the test conditions are the same as those in the test of FIG. 1) was performed, and the occurrence of rust was observed. Moreover, the average collapse load at the time of crushing by the high-speed deformation (deformation speed: 11 m / sec) was measured about the said steel plate which implemented the combined cycle corrosion test, and the steel plate as-pressed like the test of FIG. ΔFave (= [average collapse load of steel plate after combined cycle corrosion test] − [average collapse load of as-pressed steel plate]) indicating the degree of deterioration of impact resistance with time based on the measured values of average collapse load Asked. The results are shown in Table 7.
[0038]
According to Table 7, No. which is an example of the present invention. No rusting was observed even after the combined cycle corrosion test No. 17, and therefore ΔFave was also a small value, indicating that the deterioration over time of the impact resistance characteristics was effectively suppressed. On the other hand, No. which is a comparative example. No. 18, the cleanliness (dt) of the steel sheet surface layer exceeds the upper limit defined by the present invention, and thus rusts after the combined cycle corrosion test. Therefore, ΔFave is also increased, and the impact resistance is deteriorated over time. You can see that it has occurred.
[0039]
[Table 7]

[0040]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a thin steel sheet having a small deterioration in impact resistance over time by optimizing the cleanliness (dt) of the steel sheet surface layer and the surface roughness (Rsk) of the steel sheet. By using this as a steel plate for automobiles, it is possible to improve collision safety even for a vehicle body that has been in production for a long time.
[Brief description of the drawings]
FIG. 1 shows ΔFave (= [average collapse load of a steel plate after a combined cycle corrosion test] − [pressed steel plate as an index of deterioration over time of steel sheet surface layer cleanliness (dt) and impact resistance of a steel plate] Fig. 2 is a graph showing the relationship between the average collapse load]) and Fig. 2 ΔFave (= [average collapse of steel plate after combined cycle corrosion test], which is an index of the time-dependent deterioration of steel plate surface skewness (Rsk) and impact resistance of the steel plate Load]-[Average collapse load of pressed steel sheet]))

Claims (1)

Choose from ferrite phase, ferrite phase + pearlite phase, ferrite phase + bainite phase, ferrite phase + martensite phase, bainite phase, ferrite phase + austenite phase + bainite phase, ferrite phase + austenite phase + bainite phase + martensite phase And the degree of cleanliness (dt) at least in the steel sheet surface layer is 0.05% or less, and the skewness (Rsk) that is a deviation index in the height direction of the roughness curve of the steel sheet surface is − An automotive steel sheet having a small deterioration with time of impact resistance, which is 1.5 to +1.0.

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