JP3598087B2

JP3598087B2 - High-strength galvannealed steel sheet with excellent workability and method for producing the same

Info

Publication number: JP3598087B2
Application number: JP2001305116A
Authority: JP
Inventors: 和彦本田; 正春亀田; 康治佐久間; 秋男齋藤; 鉄生西山; 淳伊丹
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-10-01
Filing date: 2001-10-01
Publication date: 2004-12-08
Anticipated expiration: 2021-10-01
Also published as: JP2003105514A

Description

【０００１】
【発明の属する技術分野】
本発明は、高強度合金化溶融亜鉛めっき鋼板およびその製造方法に係わり、更に詳しくは優れた加工性を有し、種々の用途、例えば建材用や自動車用鋼板として適用できるめっき鋼板に関するものである。
【０００２】
【従来の技術】
耐食性の良好なめっき鋼板として合金化溶融亜鉛めっき鋼板がある。この合金化溶融亜鉛めっき鋼板は、通常、鋼板を脱脂後、無酸化炉にて予熱し、表面の清浄化および材質確保のために還元炉にて還元焼鈍を行い、溶融亜鉛浴に浸漬し、付着量制御した後合金化を行うことによって製造される。その特徴として、耐食性およびめっき密着性等に優れることから、自動車、建材用途等を中心として広く使用されている。
【０００３】
特に近年、自動車分野においては衝突時に乗員を保護するような機能の確保とともに燃費向上を目的とした軽量化を両立させるために、めっき鋼板の高強度化が必要とされてきている。
【０００４】
加工性を悪化させずに鋼板を高強度化するためには、ＳｉやＭｎ、Ｐといった元素を添加することが有効であるが、これらの元素の添加は合金化を遅延させるため、軟鋼に比べて高温長時間の合金化を必要とする。この高温長時間の合金化は、鋼板中に残存していたオーステナイトをパーライトに変態させ、加工性を低下させるため、結果として添加元素の効果を相殺することになる。
【０００５】
Ｓｉ添加高強度鋼板の合金化に関しては、特開平５−２７９８２９号公報において、連続溶融亜鉛めっきラインでも実現可能な製造方法が開示されているが、その製造条件の範囲が極めて広く記述されており、実際の生産における有用性に乏しい。また、特開平１１−１３１１４５号公報に開示されている製造方法は、残留オーステナイトを生成させるためにめっき後低温保持を行っているが、これは設備の増大を招くため生産性を悪化させる。
【０００６】
【発明が解決しようとする課題】
そこで、本発明は上記問題点を解決し、加工性の優れた高強度合金化溶融亜鉛めっき鋼板と、新たな設備を設置することなく、加工性の優れた高強度合金化溶融亜鉛めっき鋼板を製造する方法を提案するものである。
【０００７】
【課題を解決するための手段】
本発明者らは、高強度鋼板のめっき処理について鋭意研究を重ねた結果、Ｃ、Ｓｉ、Ｍｎが一定量以上添加された鋼にＮｉ、Ｃｕの一種または二種をめっきし、熱処理条件およびめっき条件を最適化した連続溶融亜鉛めっき設備でめっき処理することにより、加工性の優れた高強度合金化溶融亜鉛めっき鋼板を製造できることを見いだして本発明をなした。
【０００８】
すなわち、本発明の要旨とするところは、以下の通りである。
【０００９】
（１）質量％で、
Ｃ：０．０５〜０．１５％、
Ｓｉ：０．３〜２．０％、
Ｍｎ：１．０〜２．８％、
Ｐ：０．０３％以下、
Ｓ：０．０２％以下、
Ａｌ：０．００５〜０．５％、
Ｎ：０．００６％以下を含有し、
残部Ｆｅおよび不可避的不純物からなり、更に％Ｃ、％Ｓｉ、％ＭｎをそれぞれＣ、Ｓｉ、Ｍｎ含有量とした時に（％Ｍｎ）／（％Ｃ）≧１２かつ（％Ｓｉ）／（％Ｃ）≧４が満たされる高強度鋼板の上に、Ａｌ：０．０５〜０．５質量％、Ｆｅ：７〜１５質量％を含有し、残部がＺｎおよび不可避的不純物からなる合金化溶融亜鉛めっき層を有する鋼板であって、引張強さＦ（ＭＰａ）と伸びＬ（％）の関係が
Ｌ≧５１−０．０３５×Ｆ
を満足することを特徴とする加工性の優れた高強度合金化溶融亜鉛めっき鋼板。
【００１０】
（２）めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比が０．２以下であることを特徴とする（１）記載の加工性の優れた高強度合金化溶融亜鉛めっき鋼板。
【００１１】
（３）前記（１）記載の化学成分からなる組成のスラブをＡｒ_３点以上の温度で仕上圧延を行い、５０〜８５％の冷間圧延を施した後、Ｎｉ、Ｃｕの一種または二種をめっきし、その後連続溶融亜鉛めっき設備で７００℃以上８５０℃以下のフェライト、オーステナイトの二相共存温度域で焼鈍し、その最高到達温度から６５０℃までを平均冷却速度０．５〜１０℃／秒で、引き続いて６５０℃からめっき浴までを平均冷却速度１〜２０℃／秒で冷却して溶融亜鉛めっき処理を行うことによって、前記冷延鋼板の表面上に溶融亜鉛めっき層を形成し、次いで、前記溶融亜鉛めっき層が形成された前記鋼板に対し合金化処理を施すことによって、前記鋼板の表面上に合金化溶融亜鉛めっき層を形成する合金化溶融亜鉛めっき鋼板の製造方法であって、前記溶融亜鉛めっき処理を、浴中有効Ａｌ濃度：０．０７〜０．１０５ｍａｓｓ％、残部がＺｎおよび不可避的不純物からなる成分組成の溶融亜鉛めっき浴中で行い、そして、前記合金化処理を、
３００＋２０００×〔Ａｌ％〕−５０×√（０．５×Ｗ）≦Ｔ≦３５０＋２０００×〔Ａｌ％〕−２５×√（０．５×Ｗ）
但し、〔Ａｌ％〕：亜鉛めっき浴中の浴中有効Ａｌ濃度（ｍａｓｓ％）
Ｗ：Ｎｉめっき付着量（ｇ／ｍ^２）
を満足する温度Ｔ（℃）において行うことを特徴とする、加工性の優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１２】
（４）前記（３）に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法において、
Ｎｉめっきの付着量が０．１〜５ｇ／ｍ^２、Ｃｕめっきの付着量が０．１〜２ｇ／ｍ^２であることを特徴とする、加工性の優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１３】
（５）前記（３）または（４）に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法において、浴中有効Ａｌ濃度を、
〔Ａｌ％〕≦０．１０３−０．００８×〔Ｓｉ％〕
但し、〔Ｓｉ％〕：鋼板中のＳｉ含有量（ｍａｓｓ％）
を満足する浴中有効Ａｌ濃度（ｍａｓｓ％）において行うことを特徴とする、加工性の優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１４】
（６）前記（３）乃至（５）のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法において、溶融めっき後４００℃以下の温度に冷却されるまでの時間を１０秒以上１００秒以下とすることを特徴とする、加工性の優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１５】
（７）前記（３）乃至（６）のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法において、溶融亜鉛めっき浴の温度を４６０℃未満とすることを特徴とする、加工性の優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１６】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【００１７】
まず、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｎの数値限定理由について述べる。
【００１８】
Ｃはマルテンサイトや残留オーステナイトによる組織強化で鋼板を高強度化しようとする場合に必須の元素である。Ｃの含有量を０．０５％以上とする理由は、Ｃが０．０５％未満ではミストや噴流水を冷却媒体として焼鈍温度から急速冷却することが困難な溶融亜鉛めっきラインにおいてセメンタイトやパーライトが生成しやすく、必要とする引張強さの確保が困難であるためである。一方、Ｃの含有量を０．１５％以下とする理由は、Ｃが０．１５％を超えると、スポット溶接で健全な溶接部を形成することが困難となると同時にＣの偏析が顕著となり加工性が劣化するためである。
【００１９】
Ｓｉは鋼板の加工性、特に伸びを大きく損なうことなく強度を増す元素として０．３〜２．０％添加しかつＣ含有量の４倍以上の質量％とする。Ｓｉの含有量を０．３％以上とする理由は、Ｓｉが０．３％未満では必要とする引張強さの確保が困難であるためであり、Ｓｉの含有量を２．０％以下とする理由は、Ｓｉが２．０％を超えると強度を増す効果が飽和するとともに延性の低下が起こるためである。またＣ含有量の４倍以上の質量％とすることで、めっき直後に行う合金化処理のための再加熱でパーライトおよびベイナイト変態の進行を著しく遅滞させ、室温まで冷却後にも体積率で３〜２０％のマルテンサイトおよび残留オーステナイトがフェライト中に混在する金属組織とできる。
【００２０】
ＭｎはＣとともにオーステナイトの自由エネルギーを下げるため、めっき浴に鋼帯を浸漬するまでの間にオーステナイトを安定化する目的で１．０％以上添加する。またＣ含有量の１２倍以上の質量％を添加することにより、めっき直後に行う合金化処理のための再加熱でパーライトおよびベイナイト変態の進行を著しく遅滞させ、室温まで冷却後にも体積率で３〜２０％のマルテンサイトおよび残留オーステナイトがフェライト中に混在する金属組織とできる。しかし添加量が過大になるとスラブに割れが生じやすく、またスポット溶接性も劣化するため、２．８％を上限とする。
【００２１】
Ｐは一般に不可避的不純物として鋼に含まれるが、その量が０．０３％を超えるとスポット溶接性の劣化が著しいうえ、本発明におけるような引張強さが４９０ＭＰａを超すような高強度鋼板では靭性とともに冷間圧延性も著しく劣化するため、その含有量は０．０３％以下とする。Ｓも一般に不可避的不純物として鋼に含まれるが、その量が０．０２％を超えると、圧延方向に伸張したＭｎＳの存在が顕著となり、鋼板の曲げ性に悪影響をおよぼすため、その含有量は０．０２％以下とする。
【００２２】
Ａｌは鋼の脱酸元素として、またＡｌＮによる熱延素材の細粒化、および一連の熱処理工程における結晶粒の粗大化を抑制し材質を改善するために０．００５％以上添加する必要がある。但し、０．５％を超えるとコスト高となるばかりか、表面性状を劣化させるため、その含有量は０．５％以下とする。Ｎもまた一般に不可避的不純物として鋼に含まれるが、その量が０．００６％を超えると、伸びとともに脆性も劣化するため、その含有量は０．００６％以下とする。
【００２３】
また、これらを主成分とする鋼にＮｂ、Ｔｉ、Ｂ、Ｍｏ、Ｃｕ、Ｓｎ、Ｚｎ、Ｚｒ、Ｗ、Ｃｒ、Ｎｉ、Ｃｏ、Ｃａ、希土類元素（Ｙを含む）、Ｖ、Ｔａ、Ｈｆ、Ｐｂ、Ｍｇ、Ａｓ、Ｓｂ、Ｂｉを合計で１％以下含有しても本発明の効果を損なわず、その量によっては耐食性や加工性が改善される等好ましい場合もある。
【００２４】
次に、合金化溶融亜鉛めっき層について述べる。
【００２５】
本発明において合金化溶融亜鉛めっき層のＡｌ組成を０．０５〜０．５質量％に限定した理由は、０．０５質量％未満では合金化処理時においてＺｎ―Ｆｅ合金化が進み過ぎ、地鉄界面に脆い合金層が発達し過ぎてめっき密着性が劣化するためであり、０．５質量％を超えるとＦｅ−Ａｌ−Ｚｎ系バリア層が厚く形成され過ぎ合金化処理時において合金化が進まないため目的とする鉄含有量のめっきが得られないためである。
【００２６】
また、Ｆｅ組成を５〜１５質量％に限定した理由は、５質量％未満だとめっき表面に柔らかいＺｎ−Ｆｅ合金が形成されプレス成形性を劣化させるためであり、１５質量％を超えると地鉄界面に脆い合金層が発達し過ぎてめっき密着性が劣化するためである。好ましくは７〜１３質量％である。
【００２７】
更に本発明においてはめっき中にＮｉ：０．１〜１０質量％、Ｃｕ：０．０５〜３％の質量一種または二種以上を添加する。これらの元素を添加する理由は、合金化を促進するためである。合金化を促進する効果は、Ｎｉ、Ｃｕにおいて各々０．１、０．０５質量％以上でその効果が顕著になり始める。ただし、Ｎｉ、Ｃｕにおいて各々１０、３質量％を超えるとコスト高となるばかりか、外観を劣化させるため、その含有量は各々１０、３質量％以下とする。
【００２８】
本発明鋼板は、溶融亜鉛めっき浴中あるいは亜鉛めっき中にＰｂ、Ｓｂ、Ｓｉ、Ｓｎ、Ｍｇ、Ｍｎ、Ｃｒ、Ｃｏ、Ｃａ、Ｌｉ、Ｔｉ、Ｂｅ、Ｂｉ、希土類元素の一種または二種以上を含有、あるいは混入してあっても本発明の効果を損なわず、その量によっては耐食性や加工性が改善される等好ましい場合もある。合金化溶融亜鉛めっきの付着量については特に制約は設けないが、耐食性の観点から２０ｇ／ｍ^２以上、経済性の観点から１５０ｇ／ｍ^２以下であることが望ましい。
【００２９】
本発明において加工性の優れた高強度合金化溶融亜鉛めっき鋼板とは、引張強さＴＳが４９０ＭＰａ以上で、引張強さＦ（ＭＰａ）と伸びＬ（％）の関係が、Ｌ≧５１−０．０３５×Ｆ
を満足する性能を持つ鋼板である。
【００３０】
伸びＬを［５１−０．０３５×Ｆ］％以上と限定した理由は、Ｌが［５１−０．０３５×Ｆ］より低い場合、深絞り等の厳しい加工の時に破断する等加工性が不十分であるためである。
【００３１】
本発明において、更に加工性を向上させるためには、めっき層にζ相を発生させず、δ_１相主体のめっき層とする。ζ相の多い被膜は摩擦係数が高いため、プレス成形性を向上させるためには、鋼板の伸びを向上させるだけでなく、摩擦係数の低いδ_１相主体のめっき層とすることが必要となる。具体的には、めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比が０．２以下であることが有効である。めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比を０．２以下に限定した理由は、これらのＸ線回折強度比が０．２以下であればめき表面にほとんどζ相は存在せず、摩擦係数が低くなるためである。
【００３２】
次に、製造条件の限定理由について述べる。その目的はマルテンサイトおよび残留オーステナイトを３〜２０％含む金属組織とし、高強度とプレス加工性が良いことが両立させることにある。マルテンサイトおよび残留オーステナイトの体積率が３％未満の場合には高強度とならない。一方、マルテンサイトおよび残留オーステナイトの体積率が２０％を超えると、高強度ではあるものの鋼板の加工性が劣化し、本発明の目的が達成されない。
【００３３】
熱間圧延に供するスラブは特に限定するものではなく、連続鋳造スラブや薄スラブキャスター等で製造したものであれば良い。また鋳造後直ちに熱間圧延を行う連続鋳造−直送圧延（ＣＣ−ＤＲ）のようなプロセスにも適合する。
【００３４】
熱間圧延の仕上温度は鋼板のプレス成形性を確保するという観点からＡｒ_３点以上とする必要がある。熱延後の冷却条件や巻取温度は特に限定しないが、巻取温度はコイル両端部での材質ばらつきが大ききなることを避け、またスケール厚の増加による酸洗性の劣化を避けるためには７５０℃以下とし、また部分的にベイナイトやマルテンサイトが生成すると冷間圧延時に耳割れを生じやすく、極端な場合には板破断することもあるため５５０℃以上とすることが望ましい。冷間圧延は通常の条件でよく、フェライトが加工硬化しやすいようにマルテンサイトおよび残留オーステナイトを微細に分散させ、加工性の向上を最大限に得る目的からその圧延率は５０％以上とする。一方、８５％を超す圧延率で冷間圧延を行うことは多大の冷延負荷が必要となるため現実的ではない。
【００３５】
Ｎｉめっきの付着量は０．１〜５ｇ／ｍ^２、Ｃｕめっきの付着量は０．１〜２ｇ／ｍ^２とする。本発明においてＮｉめっきの付着量を０．１〜５ｇ／ｍ^２に限定した理由は、０．１ｇ／ｍ^２未満では合金化を促進する効果がみられないためであり、５ｇ／ｍ^２を超えると合金化を促進する効果が飽和しコスト高となるためである。Ｃｕめっきの付着量を０．１〜２ｇ／ｍ^２に限定した理由は、０．１ｇ／ｍ^２未満では合金化を促進する効果がみられないためであり、２ｇ／ｍ^２を超えると合金化を促進する効果が飽和しコスト高となるためである。
【００３６】
Ｎｉ、Ｃｕをめっきする方法は特に限定することなく、所定の付着量を得られればいかなる方法でも構わないが、電気めっきや置換めっきなどの方法が簡便で制御しやすい。
【００３７】
Ｎｉめっきは合金化温度を低下させることによって合金化を促進する効果が強く、Ｃｕめっきは鋼板表面の合金化のバラツキを抑えることによって合金化を促進する効果が強いため、これらを同時にめっきするとその相乗効果により、より少量のめっき付着量で合金化を促進させることができる。
【００３８】
ライン内焼鈍方式の連続溶融亜鉛めっき設備で焼鈍する際、その焼鈍温度は７００℃以上８５０℃以下のフェライト、オーステナイト二相共存域とする。焼鈍温度が７００℃未満では再結晶が不十分であり、鋼板に必要なプレス加工性を具備できない。８５０℃を超すような温度で焼鈍することは鋼帯表面にＳｉやＭｎの酸化物層の成長が著しく、めっき不良が起こりやすくなるため好ましくない。また引き続きめっき浴へ浸漬し、冷却する過程で、６５０℃までを緩冷却しても十分な体積率のフェライトが成長せず、６５０℃からめっき浴までの冷却途上でオーステナイトがマルテンサイトに変態し、その後合金化処理のための再加熱でマルテンサイトが焼き戻されてセメンタイトが析出するため高強度とプレス加工性の良いことの両立が困難となる。
【００３９】
鋼帯は焼鈍後、引き続きめっき浴へ浸漬する過程で冷却されるが、この場合の冷却速度はその最高到達温度から６５０℃までを平均０．５〜１０℃／秒で、引き続いて６５０℃からめっき浴までを平均１〜２０℃／秒とする。６５０℃までを平均０．５〜１０℃／秒とするのは加工性を改善するためにフェライトの体積率を増すと同時に、オーステナイトのＣ濃度を増すことにより、その生成自由エネルギーを下げ、マルテンサイト変態の開始する温度をめっき浴温度以下とすることを目的とする。６５０℃までの平均冷却速度を０．５℃／秒未満とするためには連続溶融亜鉛めっき設備のライン長を長くする必要がありコスト高となるため、６５０℃までの平均冷却速度は０．５℃／秒以上とする。
【００４０】
６５０℃までの平均冷却速度を０．５℃／秒未満とするためには、最高到達温度を下げ、オーステナイトの体積率が小さい温度で焼鈍することも考えられるが、その場合には実際の操業で許容すべき温度範囲に比べて適切な温度範囲が狭く、僅かでも焼鈍温度が低いとオーステナイトが形成されず目的を達しない。
【００４１】
一方、６５０℃までの平均冷却速度を１０℃／秒を超えるようにすると、フェライトの体積率の増加が十分でないばかりか、オーステナイト中Ｃ濃度の増加も少ないため、鋼帯がめっき浴に浸漬される前にその一部がマルテンサイト変態し、その後合金化処理のための加熱でマルテンサイトが焼き戻されてセメンタイトとして析出するため高強度と加工性の良いことの両立が困難となる。
【００４２】
６５０℃からめっき浴までの平均冷却速度を１〜２０℃／秒とするのは、その冷却途上でオーステナイトがパーライトに変態するのを避けるためであり、その冷却速度が１℃／秒未満では本発明で規定する温度で焼鈍し、また６５０℃まで冷却したとしてもパーライトの生成を避けられない。一方、６５０℃からめっき浴までを平均冷却速度２０℃／秒を超えるように鋼帯を冷却することはドライな雰囲気では困難である。
【００４３】
本発明の合金化溶融亜鉛めっき鋼板の製造において、用いる溶融亜鉛めっき浴はＡｌ濃度が浴中有効Ａｌ濃度Ｃで０．０７〜０．１０５ｍａｓｓ％に調整する。ここでめっき浴中の有効Ａｌ濃度とは、浴中Ａｌ濃度から浴中Ｆｅ濃度を差し引いた値である。
【００４４】
有効Ａｌ濃度を０．０７〜０．１０５ｍａｓｓ％に限定する理由は、有効Ａｌ濃度が０．０７％よりも低い場合には、めっき初期の合金化バリアとなるＦｅ−Ａｌ−Ｚｎ相の形成が不十分であってめっき処理時にめっき鋼板界面に脆いΓ相が厚くできるため、加工時のめっき皮膜密着力が劣る合金化溶融亜鉛めっき鋼板しか得られないためである。一方、有効Ａｌ濃度が０．１０５％よりも高い場合には、高温長時間の合金化が必要となり、鋼中に残存していたオーステナイトがパーライトに変態するため、高強度と加工性の良いことの両立が困難となる。
【００４５】
更に、本発明において合金化処理時の合金化温度を
３００＋２０００×〔Ａｌ％〕−５０×√（０．５×Ｗ）≦Ｔ≦３５０＋２０００×〔Ａｌ％〕−２５×√（０．５×Ｗ）
但し、〔Ａｌ％〕：亜鉛めっき浴中の浴中有効Ａｌ濃度（ｍａｓｓ％）
Ｗ：Ｎｉめっき付着量（ｇ／ｍ^２）
を満足する温度Ｔ（℃）において行う。
【００４６】
合金化温度Ｔを〔３００＋２０００×〔Ａｌ％〕−５０×√（０．５×Ｗ）〕℃以上、〔３５０＋２０００×〔Ａｌ％〕−２５×√（０．５×Ｗ）〕℃以下に限定した理由は、合金化温度Ｔが〔３００＋２０００×〔Ａｌ％〕−５０×√（０．５×Ｗ）〕℃よりも低いと合金化が進行しないか、あるいは合金化の進行が不十分で合金化未処理となりめっき表層が加工性の劣るη相やζ相に厚く覆われるためである。また、Ｔが〔３５０＋２０００×〔Ａｌ％〕−２５×√（０．５×Ｗ）〕℃よりも高いと、合金化が進み過ぎて本発明のめっき中Ｆｅ％を超え、加工時にめっき密着力が低下することが増えるためである。
【００４７】
本発明において合金化温度が高すぎると鋼中に残存していたオーステナイトがパーライトに変態し、目的の高強度と加工性を両立した鋼板を得ることができない。従って、Ｓｉの添加量が大きくなり難合金化するほど、加工性を向上させるためには、浴中有効Ａｌ濃度を低下させ合金化温度を下げることが有効となる。具体的には、
〔Ａｌ％〕≦０．１０３−０．００８×〔Ｓｉ％〕
但し、〔Ｓｉ％〕：鋼板中のＳｉ含有量（ｍａｓｓ％）
を満足する浴中有効Ａｌ濃度（ｍａｓｓ％）においてめっきを行う。
【００４８】
有効Ａｌ濃度を〔０．１０３−０．００８×〔Ｓｉ％〕〕％以下に限定する理由は、有効Ａｌ濃度が〔０．１０３−０．００８×〔Ｓｉ％〕〕％より高い場合には、高温長時間の合金化が必要となり、鋼中に残存していたオーステナイトがパーライトに変態し、加工性が劣化するためである。
【００４９】
溶融めっき後４００℃以下の温度に冷却されるまでの時間を１０秒以上１００秒以下に限定する理由は、１０秒未満ではオーステナイト中へのＣの濃化が不十分となり、オーステナイト中のＣ濃度が、室温でのオーステナイトの残留を可能とする水準まで到達しないためであり、１００秒を超えると、ベイナイト変態が進行し過ぎて、オーステナイト量が少なくなり、十分な量の残留オーステナイトを生成できないためである。好ましくは１０秒以上８０秒以下である。
【００５０】
本発明において合金化炉加熱方式については特に限定するものではなく、本発明の温度が確保できれば、通常のガス炉による輻射加熱でも、高周波誘導加熱でも構わない。また、合金化加熱後の最高到達板温度から冷却する方法も、問うものではなく、合金化後、エアーシール等により、熱を遮断すれば、開放放置でも十分であり、より急速に冷却するガスクーリング等でも問題ない。
【００５１】
溶融亜鉛めっき浴の温度を４６０℃未満に限定する理由は、４６０℃以上ではめっき初期の合金化バリアとなるＦｅ−Ａｌ−Ｚｎ相の形成が進み過ぎ合金化温度を上昇させるため、特にＳｉ添加量の高い鋼種で加工性を低下させる原因となりやすいためである。浴温の下限は特に限定しないが、亜鉛の融点が４１９．４７℃であることから、物理的にそれ以上の浴温でしか溶融めっきできない。
【００５２】
【実施例】
以下、実施例により本発明を具体的に説明する。
【００５３】
（実施例１）
表１に示す組成からなるスラブを１１５０℃に加熱し、仕上温度９１０〜９３０℃で４．５ｍｍの熱間圧延鋼帯とし、５８０〜６８０℃で巻き取った。酸洗後、冷間圧延を施して１．６ｍｍの冷間圧延鋼帯とした後、表２に示す付着量の電気Ｎｉめっき、置換Ｃｕめっきを行い、ライン内焼鈍方式の連続溶融亜鉛めっき設備を用いて表２に示すような条件の熱処理とめっきを行い、合金化溶融亜鉛めっき鋼板を製造した。
【００５４】
各鋼板からＪＩＳ５号試験片を切り出し、常温での引張試験を行うことにより、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。引張強さは４９０ＭＰａ以上を合格とし、伸びは〔５１−０．０３５×引張強さ〕％以上を合格とした。めっき被膜の付着量およびＦｅ、Ａｌ、Ｎｉ、Ｃｕ濃度は、被膜をインヒビター入りの酸で溶解し、ＩＣＰにより測定した。めっき中のＦｅ濃度は７〜１３％を合格とした。
【００５５】
めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比は、ｄ＝１．９００のピ−ク強度Ｉζ（４２１）とｄ＝１．９９０のピ−ク強度Ｉδ_１（２４９）をそれぞれ取り、バックグラウンドを除いた後のピ−ク強度比を表２に示した。
【００５６】
評価結果は表２に示す通りである。番号１は鋼中のＣ含有量が本発明の範囲外であるため引張り強さが不足した。番号２は鋼中のＳｉ含有量が本発明の範囲外であるため引張り強さ、伸びともに不合格であった。番号３は鋼中のＰ含有量が本発明の範囲外であるため伸びが不合格であった。番号７、８、１７は焼鈍時の最高到達温度が本発明の範囲外であるため伸びが不合格であった。番号９は鋼中のＭｎ含有量が本発明の範囲外であるため引張り強さ、伸びともに不合格であった。番号１２、２７は合金化温度が本発明の範囲外であるため伸びが不合格であった。番号１５は合金化温度が本発明の範囲外であるためめっき中のＦｅ％が不合格であった。番号２０、２８は最高到達温度から６５０℃までの平均冷速が本発明の範囲外であるため伸びが不合格であった。番号２４は鋼中のＭｎ含有量／Ｃ含有量が本発明の範囲外であるため伸びが不合格であった。番号２５は鋼中のＳｉ含有量／Ｃ含有量が本発明の範囲外であるため伸びが不合格であった。番号２９は６５０℃からめっき浴までの平均冷速が本発明の範囲外であるため伸びが不合格であった。番号３０は鋼中のＭｎ含有量が本発明の範囲外であるため伸びが不合格であった。番号３１は鋼中のＣ含有量が本発明の範囲外であるため伸びが不合格であった。これら以外の本発明品は、高強度で加工性が良好な合金化溶融亜鉛めっき鋼板であった。
【００５７】
また、めっき浴温４６０℃未満では、鋼中Ｓｉ含有量に関係なく高強度で加工性が良好な合金化溶融亜鉛めっき鋼板の製造が可能であった。一方、４７０℃では、番号５の低Ｓｉ含有量の場合や、番号３３の高Ｓｉ含有量で低Ｆｅ％の場合は製造可能であるが、番号３４の高Ｓｉ含有量でＦｅ％を上げようとすると、合金化温度を上げる必要があり、結果として伸びが不合格になる。
【００５８】
【表１】

【００５９】
【表２】

【００６０】
（実施例２）
表１のＨに示す組成からなるスラブを１１５０℃に加熱し、仕上温度９１０〜９３０℃で４．５ｍｍの熱間圧延鋼帯とし、５８０〜６８０℃で巻き取った。酸洗後、冷間圧延を施して１．６ｍｍの冷間圧延鋼帯とした後、表３に示す付着量の電気Ｎｉめっき、置換Ｃｕめっきを行い、ライン内焼鈍方式の連続溶融亜鉛めっき設備を用いて表３に示すような条件の熱処理とめっきを行い、合金化溶融亜鉛めっき鋼板を製造した。
【００６１】
各鋼板からＪＩＳ５号試験片を切り出し、常温での引張試験を行うことにより、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。引張強さは４９０ＭＰａ以上を合格とし、伸びは〔５１−０．０３５×引張強さ〕％以上を合格とした。めっき被膜の付着量およびＦｅ、Ａｌ、Ｎｉ、Ｃｕ濃度は、被膜をインヒビター入りの酸で溶解し、ＩＣＰにより測定した。めっき中のＦｅ濃度は７〜１３％を合格とした。
【００６２】
めっき密着性は、あらかじめ圧縮側に密着テープ（セロハンテープ）を貼った試験片を曲げ角度が６０゜となるようにＶ字状に試験片を曲げ、曲げ戻し後に密着テープをはがして、めっきの剥離の程度を目視で観察して、以下の分類で評価し、△以上を合格とした。
◎：めっき層の剥離幅が１ｍｍ未満のもの
○：めっき層の剥離幅が１ｍｍ以上６ｍｍ未満のもの
△：めっき層の剥離幅が６ｍｍ以上１２ｍｍ未満のもの
×：めっき層の剥離幅が１２ｍ以上のもの
【００６３】
評価結果は表３に示す通りである。番号４はめっき浴中の有効Ａｌ濃度が本発明の範囲外であるためめっき中のＦｅ％、めっき密着性が不合格であった。番号７はめっき浴中の有効Ａｌ濃度が本発明の範囲外であるため伸びが不合格になった．番号８はめっき浴中の有効Ａｌ濃度が本発明の範囲外である。ためめっき中のＦｅ％が不合格であった。これら以外の本発明品は、密着性が良好で、且つ、高強度で加工性が良好な合金化溶融亜鉛めっき鋼板であった。
【００６４】
【表３】

【００６５】
（実施例３）
表１のＨに示す組成からなるスラブを１１５０℃に加熱し、仕上温度９１０〜９３０℃で４．５ｍｍの熱間圧延鋼帯とし、５８０〜６８０℃で巻き取った。酸洗後、冷間圧延を施して１．６ｍｍの冷間圧延鋼帯とした後、表４に示す付着量の電気Ｎｉめっき、置換Ｃｕめっきを行い、ライン内焼鈍方式の連続溶融亜鉛めっき設備を用いて表４に示すような条件の熱処理とめっきを行い、合金化溶融亜鉛めっき鋼板を製造した。
【００６６】
各鋼板からＪＩＳ５号試験片を切り出し、常温での引張試験を行うことにより、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。引張強さは４９０ＭＰａ以上を合格とし、伸びは〔５１−０．０３５×引張強さ〕％以上を合格とした。めっき被膜の付着量およびＦｅ、Ａｌ、Ｎｉ、Ｃｕ濃度は、被膜をインヒビター入りの酸で溶解し、ＩＣＰにより測定した。めっき中のＦｅ濃度は７〜１３％を合格とした。
【００６７】
めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比は、ｄ＝１．９００のピ−ク強度Ｉζ（４２１）とｄ＝１．９９０のピ−ク強度Ｉδ_１（２４９）をそれぞれ取り、バックグラウンドを除いた後のピ−ク強度比を表４に示した。
【００６８】
評価結果は表４に示す通りである。番号６はＮｉめっき付着量が本発明の範囲外であるため、めっき中のＮｉ濃度が不合格になった。また、めっき中のＦｅ濃度が番号１〜５に比べ低くなり、めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比も０．２より大きくなっていた。番号７はＣｕめっき付着量が本発明の範囲外であるため、めっき中のＣｕ濃度が不合格になった。また、めっき中のＦｅ濃度が番号１〜５に比べ低くなり、めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比も０．２より大きくなっていた。これら以外の本発明品は、Ｎｉ、Ｃｕめっきの効果で合金化が促進され、高強度で加工性が良好な合金化溶融亜鉛めっき鋼板が得られた。
【００６９】
【表４】

【００７０】
（実施例４）
表１のＨに示す組成からなるスラブを１１５０℃に加熱し、仕上温度９１０〜９３０℃で４．５ｍｍの熱間圧延鋼帯とし、５８０〜６８０℃で巻き取った。酸洗後、冷間圧延を施して１．６ｍｍの冷間圧延鋼帯とした後、表５に示す付着量の電気Ｎｉめっき、置換Ｃｕめっきを行い、ライン内焼鈍方式の連続溶融亜鉛めっき設備を用いて表５に示すような条件の熱処理とめっきを行い、合金化溶融亜鉛めっき鋼板を製造した。
【００７１】
各鋼板からＪＩＳ５号試験片を切り出し、常温での引張試験を行うことにより、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。引張強さは４９０ＭＰａ以上を合格とし、伸びは〔５１−０．０３５×引張強さ〕％以上を合格とした。めっき被膜の付着量およびＦｅ、Ａｌ、Ｎｉ、Ｃｕ濃度は、被膜をインヒビター入りの酸で溶解し、ＩＣＰにより測定した。めっき中のＦｅ濃度は７〜１３％を合格とした。
【００７２】
めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比は、ｄ＝１．９００のピ−ク強度Ｉζ（４２１）とｄ＝１．９９０のピ−ク強度Ｉδ_１（２４９）をそれぞれ取り、バックグラウンドを除いた後のピ−ク強度比を表５に示した。摩擦係数は、平板の金型で防錆油を塗油した鋼板を挟み、荷重を掛けつつ一定速度で引き抜き、引き抜き荷重を押し付け荷重で除した値の１／２を使用した。
【００７３】
評価結果は表５に示す通りである。めっき層のζ相のミラー指数（４２１）面とδ_１相のミラー指数（２４９）面のＸ線回折強度比が０．２より大きいものに比べ、これらのＸ線回折強度比が０．２以下のものは摩擦係数が小さく良好な加工性を示した。
【００７４】
【表５】

【００７５】
【発明の効果】
以上述べたように、本発明は加工性に優れる高強度合金化溶融亜鉛めっき鋼板とその製造方法を提供することを可能としたものであり、産業の発展に貢献するところが極めて大である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same, and more particularly to a galvanized steel sheet having excellent workability and applicable to various uses, for example, steel sheets for building materials and automobiles. .
[0002]
[Prior art]
A galvannealed steel sheet is a plated steel sheet having good corrosion resistance. This alloyed hot-dip galvanized steel sheet is usually preheated in a non-oxidizing furnace after degreasing the steel sheet, subjected to reduction annealing in a reduction furnace to clean the surface and secure the material, and immersed in a hot-dip zinc bath, It is manufactured by alloying after controlling the amount of adhesion. It is widely used mainly for automobiles, building materials and the like because of its excellent corrosion resistance and plating adhesion.
[0003]
Particularly in recent years, in the field of automobiles, it has been required to increase the strength of plated steel sheets in order to ensure a function of protecting an occupant in the event of a collision and at the same time reduce the weight for the purpose of improving fuel efficiency.
[0004]
In order to increase the strength of the steel sheet without deteriorating the workability, it is effective to add elements such as Si, Mn, and P. However, since the addition of these elements delays alloying, compared to mild steel. Requires high temperature and long time alloying. The alloying at a high temperature for a long time transforms the austenite remaining in the steel sheet into pearlite and lowers the workability, thereby offsetting the effect of the added element.
[0005]
Regarding alloying of a Si-added high-strength steel sheet, Japanese Patent Application Laid-Open No. Hei 5-279829 discloses a production method that can be realized even with a continuous hot-dip galvanizing line, but the range of production conditions is described extremely widely. , Lacks usefulness in actual production. Further, in the manufacturing method disclosed in Japanese Patent Application Laid-Open No. 11-131145, low temperature is maintained after plating in order to generate retained austenite. However, this causes an increase in equipment and deteriorates productivity.
[0006]
[Problems to be solved by the invention]
Therefore, the present invention solves the above problems, and provides a high-strength galvannealed steel sheet with excellent workability and a high-strength galvannealed steel sheet with excellent workability without installing new equipment. It proposes a manufacturing method.
[0007]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on the plating treatment of a high-strength steel sheet. As a result, one or two types of Ni and Cu were plated on steel to which C, Si, and Mn were added in a certain amount or more, and heat treatment conditions and plating were performed. The present invention has been found that a high-strength galvannealed steel sheet having excellent workability can be manufactured by performing a plating treatment in a continuous hot-dip galvanizing facility with optimized conditions.
[0008]
That is, the gist of the present invention is as follows.
[0009]
(1) In mass%,
C: 0.05-0.15%,
Si: 0.3 to 2.0%,
Mn: 1.0 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: contains 0.006% or less,
The balance consists of the balance Fe and unavoidable impurities, and when% C,% Si and% Mn are C, Si and Mn contents, respectively, (% Mn) / (% C) ≧ 12 and (% Si) / (% C ) On a high-strength steel sheet satisfying ≧ 4, alloyed hot-dip galvanizing containing 0.05 to 0.5% by mass of Al and 7 to 15% by mass of Fe, with the balance being Zn and unavoidable impurities A steel sheet having a layer, wherein the relationship between tensile strength F (MPa) and elongation L (%) is L ≧ 51−0.035 × F
A high-strength galvannealed steel sheet with excellent workability, characterized by satisfying the following.
[0010]
(2) X-ray diffraction intensity ratio of Miller indices (249) plane of the Miller index (421) face of the ζ phase of the plating layer [delta] _1-phase is equal to or more than 0.2 (1) working according High strength galvannealed steel sheet with excellent heat resistance.
[0011]
(3) After finish rolling the slab having the chemical composition described in the above (1) at a temperature of _three or more points of Ar and performing cold rolling of 50 to 85%, one or two kinds of Ni and Cu are performed. And then annealed in a continuous hot-dip galvanizing facility at a temperature between 700 ° C. and 850 ° C. in the two-phase coexisting temperature range of ferrite and austenite. From the highest temperature to 650 ° C., the average cooling rate is 0.5-10 ° C. / In seconds, the hot-dip galvanizing process is performed by subsequently cooling from 650 ° C. to the plating bath at an average cooling rate of 1 to 20 ° C./sec to form a hot-dip galvanized layer on the surface of the cold-rolled steel sheet; Next, by performing an alloying process on the steel sheet on which the hot-dip galvanized layer is formed, a method for manufacturing an alloyed hot-dip galvanized steel sheet that forms an alloyed hot-dip galvanized layer on the surface of the steel sheet. Thus, the hot-dip galvanizing treatment is performed in a hot-dip galvanizing bath having an effective Al concentration in the bath: 0.07 to 0.105 mass% and a balance of Zn and unavoidable impurities. Processing,
300 + 2000 × [Al%]-50 × √ (0.5 × W) ≦ T ≦ 350 + 2000 × [Al%]-25 × √ (0.5 × W)
However, [Al%]: effective Al concentration in the bath of zinc plating bath (mass%)
W: Ni plating adhesion amount (g / m ² )
A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability, which is performed at a temperature T (° C.) satisfying the following.
[0012]
(4) The method for producing a high-strength galvannealed steel sheet according to (3),
A high-strength galvannealed steel sheet having excellent workability, characterized in that the amount of Ni plating is 0.1 to 5 g / m ² and the amount of Cu plating is 0.1 to ² g / m ^2. Manufacturing method.
[0013]
(5) The method for producing a high-strength galvannealed steel sheet according to (3) or (4), wherein the effective Al concentration in the bath is
[Al%] ≦ 0.103-0.008 × [Si%]
However, [Si%]: Si content in steel sheet (mass%)
A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability, wherein the method is performed at an effective Al concentration (mass%) in a bath that satisfies the following conditions.
[0014]
(6) In the method for producing a high-strength galvannealed steel sheet according to any one of (3) to (5), the time from hot-dip coating to cooling to a temperature of 400 ° C. or less may be 10 seconds or more and 100 or more. A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability, wherein the time is not more than seconds.
[0015]
(7) The process for producing a high-strength galvannealed steel sheet according to any one of (3) to (6), wherein the temperature of the hot-dip galvanizing bath is less than 460 ° C. Method of manufacturing high strength galvannealed steel sheet with excellent quality.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0017]
First, the reasons for limiting the numerical values of C, Si, Mn, P, S, Al, and N will be described.
[0018]
C is an essential element in the case where the steel sheet is to be strengthened by strengthening the structure by martensite or retained austenite. The reason why the content of C is set to 0.05% or more is that if C is less than 0.05%, cementite or pearlite is difficult to be rapidly cooled from the annealing temperature by using mist or jet water as a cooling medium in a galvanizing line. This is because it is easy to produce and it is difficult to secure the required tensile strength. On the other hand, the reason for setting the content of C to 0.15% or less is that if C exceeds 0.15%, it becomes difficult to form a sound welded portion by spot welding, and at the same time, segregation of C becomes remarkable and processing is performed. This is because the property deteriorates.
[0019]
Si is added in an amount of 0.3 to 2.0% as an element for increasing the strength without significantly impairing the workability of the steel sheet, particularly, elongation, and is set to be at least 4% by mass of the C content. The reason why the content of Si is set to 0.3% or more is that if the content of Si is less than 0.3%, it is difficult to secure the required tensile strength, and the content of Si is set to 2.0% or less. The reason for this is that if the content of Si exceeds 2.0%, the effect of increasing the strength is saturated and the ductility is reduced. Further, by setting the content by mass to 4 times or more of the C content, the progress of pearlite and bainite transformation is remarkably slowed by reheating for the alloying treatment performed immediately after plating, and the volume ratio is 3 to 3 even after cooling to room temperature. A metal structure in which 20% of martensite and retained austenite are mixed in ferrite can be obtained.
[0020]
Mn is added in an amount of 1.0% or more to stabilize austenite before immersing the steel strip in the plating bath in order to lower the free energy of austenite together with C. Further, by adding 12% by mass or more of the C content, the progress of the pearlite and bainite transformation is remarkably slowed by reheating for the alloying treatment performed immediately after the plating, and the volume ratio becomes 3% even after cooling to room temperature. A metal structure in which about 20% of martensite and retained austenite are mixed in ferrite can be obtained. However, if the amount is too large, the slab is liable to crack and the spot weldability deteriorates, so the upper limit is 2.8%.
[0021]
P is generally contained in steel as an unavoidable impurity. However, if its amount exceeds 0.03%, spot weldability is significantly deteriorated, and in a high-strength steel sheet having a tensile strength exceeding 490 MPa as in the present invention. Since the cold rolling property is significantly deteriorated as well as the toughness, its content is set to 0.03% or less. S is also generally contained in steel as an unavoidable impurity. However, if the amount exceeds 0.02%, the presence of MnS elongated in the rolling direction becomes remarkable, which adversely affects the bendability of the steel sheet. 0.02% or less.
[0022]
Al must be added as a deoxidizing element of steel and 0.005% or more in order to reduce the grain size of the hot-rolled material by AlN and suppress the coarsening of crystal grains in a series of heat treatment steps to improve the material quality. . However, if it exceeds 0.5%, not only does the cost rise, but also the surface properties deteriorate, so its content is made 0.5% or less. N is also generally contained in steel as an unavoidable impurity, but if the amount exceeds 0.006%, the brittleness is deteriorated along with the elongation, so the content is made 0.006% or less.
[0023]
In addition, Nb, Ti, B, Mo, Cu, Sn, Zn, Zr, W, Cr, Ni, Co, Ca, rare earth elements (including Y), V, Ta, Hf, Even if Pb, Mg, As, Sb, and Bi are contained in a total amount of 1% or less, the effect of the present invention is not impaired, and depending on the amount, corrosion resistance and workability are sometimes improved, which is preferable.
[0024]
Next, the alloyed hot-dip galvanized layer will be described.
[0025]
In the present invention, the reason why the Al composition of the alloyed hot-dip galvanized layer is limited to 0.05 to 0.5% by mass is that if less than 0.05% by mass, Zn—Fe alloying proceeds too much during the alloying treatment, and This is because the brittle alloy layer develops too much at the iron interface and the plating adhesion deteriorates. If it exceeds 0.5% by mass, the Fe-Al-Zn-based barrier layer is formed too thick and alloying occurs during the alloying treatment. This is because the plating does not proceed and a plating having the desired iron content cannot be obtained.
[0026]
The reason why the Fe composition is limited to 5 to 15% by mass is that if the content is less than 5% by mass, a soft Zn—Fe alloy is formed on the plating surface to deteriorate the press formability. This is because the brittle alloy layer develops too much at the iron interface and the plating adhesion deteriorates. Preferably it is 7-13 mass%.
[0027]
Further, in the present invention, one or two or more masses of Ni: 0.1 to 10% by mass and Cu: 0.05 to 3% are added during plating. The reason for adding these elements is to promote alloying. The effect of accelerating alloying starts to become significant when Ni and Cu are respectively 0.1% and 0.05% by mass or more. However, if the content of Ni and Cu exceeds 10 and 3% by mass, respectively, the cost is increased and the appearance is deteriorated.
[0028]
The steel sheet of the present invention contains one or more of Pb, Sb, Si, Sn, Mg, Mn, Cr, Co, Ca, Li, Ti, Be, Bi, and rare earth elements in a hot dip galvanizing bath or galvanizing. Even if it is contained or mixed, the effect of the present invention is not impaired, and depending on the amount thereof, it may be preferable that the corrosion resistance and workability are improved. There is no particular limitation on the amount of the galvannealed galvanized coating, but it is preferably 20 g / m ² or more from the viewpoint of corrosion resistance and 150 g / m ² or less from the viewpoint of economy.
[0029]
In the present invention, a high-strength galvannealed steel sheet having excellent workability has a tensile strength TS of 490 MPa or more, and a relation between the tensile strength F (MPa) and the elongation L (%) is L ≧ 51-0. 0.035 × F
It is a steel sheet with performance that satisfies.
[0030]
The reason for limiting the elongation L to [51-0.035 × F]% or more is that if L is lower than [51-0.035 × F], the workability such as breaking during severe processing such as deep drawing is not good. That is enough.
[0031]
In the present invention, in order to further improve the workability, a 層 phase is not generated in the plating layer, and a δ ₁ phase-based plating layer is used.被膜 Since the coating with many phases has a high friction coefficient, in order to improve the press formability, it is necessary not only to improve the elongation of the steel sheet but also to use a δ ₁ phase-based plating layer having a low friction coefficient . Specifically, it is effective X-ray diffraction intensity ratio of Miller indices (249) plane of the Miller index (421) face of the ζ phase of the plating layer [delta] ₁ phase is 0.2 or less. The reason why the X-ray diffraction intensity ratio between the 指数 phase Miller index (421) plane and the δ ₁ phase Miller index (249) plane of the plating layer is limited to 0.2 or less is that these X-ray diffraction intensity ratios are not more than 0.2. This is because if it is 2 or less, there is almost no ζ phase on the plating surface, and the friction coefficient is low.
[0032]
Next, reasons for limiting the manufacturing conditions will be described. The purpose is to provide a metal structure containing 3 to 20% of martensite and retained austenite, and to achieve both high strength and good press workability. If the volume fraction of martensite and retained austenite is less than 3%, high strength is not obtained. On the other hand, when the volume ratio of martensite and retained austenite exceeds 20%, the workability of the steel sheet is deteriorated although the strength is high, and the object of the present invention is not achieved.
[0033]
The slab to be subjected to hot rolling is not particularly limited, and any slab manufactured by a continuous cast slab or a thin slab caster may be used. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
[0034]
The finishing temperature of the hot rolling needs to be ₃ points or more of Ar from the viewpoint of securing the press formability of the steel sheet. The cooling conditions and coiling temperature after hot rolling are not particularly limited, but the coiling temperature is to avoid large variations in material at both ends of the coil and to avoid deterioration in pickling properties due to an increase in scale thickness. Is set to 750 ° C. or less, and when bainite or martensite is partially formed, ear cracks are apt to occur during cold rolling, and in extreme cases, the plate may be broken. Cold rolling may be performed under ordinary conditions, and the rolling ratio is set to 50% or more for the purpose of maximizing workability by dispersing martensite and retained austenite finely so that ferrite is easily work-hardened. On the other hand, performing cold rolling at a rolling reduction exceeding 85% is not practical because a large amount of cold rolling load is required.
[0035]
The amount of Ni plating is 0.1 to 5 g / m ² , and the amount of Cu plating is 0.1 to ² g / m ² . Reason for the deposition of Ni-plated in the present invention is limited to 0.1-5 g / ^{m 2,} in less than 0.1 g / ^{m 2} is because the effect of promoting the alloying absent, a 5 g / ^{m 2} If it exceeds, the effect of promoting the alloying is saturated and the cost is increased. The reason for limiting the amount of adhered Cu plating 0.1-2 g / ^{m 2,} in less than 0.1 g / ^{m 2} is because the effect of promoting the alloying not seen, exceeds 2 g / ^{m 2} Alloy This is because the effect of promoting the conversion is saturated and the cost increases.
[0036]
The method of plating Ni and Cu is not particularly limited, and any method may be used as long as a predetermined amount of coating can be obtained. However, methods such as electroplating and displacement plating are simple and easy to control.
[0037]
Ni plating has a strong effect of promoting alloying by lowering the alloying temperature, and Cu plating has a strong effect of promoting alloying by suppressing the variation in alloying on the surface of the steel sheet. Due to the synergistic effect, alloying can be promoted with a smaller plating adhesion amount.
[0038]
When annealing in a continuous hot-dip galvanizing equipment of an in-line annealing method, the annealing temperature is set to a ferrite-austenite two-phase coexisting region of 700 ° C. or more and 850 ° C. or less. If the annealing temperature is lower than 700 ° C., recrystallization is insufficient, and the steel sheet cannot have the required press workability. Annealing at a temperature exceeding 850 ° C. is not preferable because the growth of an oxide layer of Si or Mn on the surface of the steel strip is remarkable and plating failure is likely to occur. Further, in the process of immersion in the plating bath and cooling, ferrite with a sufficient volume ratio does not grow even if cooled slowly to 650 ° C., and austenite transforms to martensite during cooling from 650 ° C. to the plating bath. After that, martensite is tempered by reheating for alloying treatment and cementite precipitates, so that it is difficult to achieve both high strength and good press workability.
[0039]
After annealing, the steel strip is cooled during the subsequent immersion in the plating bath. In this case, the cooling rate is 0.5 to 10 ° C./sec on average from the highest temperature to 650 ° C., and subsequently from 650 ° C. The average up to the plating bath is 1 to 20 ° C./sec. An average of 0.5 to 10 ° C./sec up to 650 ° C. is obtained by increasing the volume fraction of ferrite in order to improve the workability and at the same time, increasing the C concentration of austenite, thereby lowering the free energy of formation of the ferrite. The purpose is to set the temperature at which site transformation starts to be equal to or lower than the plating bath temperature. In order to make the average cooling rate to 650 ° C. less than 0.5 ° C./sec, it is necessary to increase the line length of the continuous hot-dip galvanizing equipment, which increases the cost. Therefore, the average cooling rate to 650 ° C. is 0.1 mm. 5 ° C./sec or more.
[0040]
In order to make the average cooling rate to 650 ° C. less than 0.5 ° C./sec, it is conceivable to lower the maximum temperature and perform annealing at a temperature at which the volume fraction of austenite is small. The appropriate temperature range is narrower than the allowable temperature range, and if the annealing temperature is slightly low, austenite is not formed and the purpose is not achieved.
[0041]
On the other hand, when the average cooling rate up to 650 ° C. exceeds 10 ° C./sec, not only does the volume fraction of ferrite not sufficiently increase, but the C concentration in austenite is also small, so that the steel strip is immersed in the plating bath. Before transformation, a part of the martensite transforms, and thereafter martensite is tempered by heating for alloying treatment and precipitates as cementite, which makes it difficult to achieve both high strength and good workability.
[0042]
The reason why the average cooling rate from 650 ° C. to the plating bath is 1 to 20 ° C./sec is to prevent austenite from transforming to pearlite during the cooling, and when the cooling rate is less than 1 ° C./sec. Even if the steel is annealed at the temperature specified in the invention and cooled to 650 ° C., formation of pearlite cannot be avoided. On the other hand, it is difficult in a dry atmosphere to cool the steel strip so that the average cooling rate from 650 ° C. to the plating bath exceeds 20 ° C./sec.
[0043]
In the production of the alloyed hot-dip galvanized steel sheet of the present invention, the hot-dip galvanizing bath used is adjusted to have an Al concentration of 0.07 to 0.105 mass% in terms of the effective Al concentration C in the bath. Here, the effective Al concentration in the plating bath is a value obtained by subtracting the Fe concentration in the bath from the Al concentration in the bath.
[0044]
The reason that the effective Al concentration is limited to 0.07 to 0.105 mass% is that when the effective Al concentration is lower than 0.07%, the formation of an Fe—Al—Zn phase serving as an alloying barrier at the beginning of plating is difficult. This is because a brittle Γ phase can be thickened at the interface of the coated steel sheet at the time of the plating treatment and is insufficient, so that only an alloyed hot-dip galvanized steel sheet having poor adhesion of the plating film during processing can be obtained. On the other hand, when the effective Al concentration is higher than 0.105%, alloying at a high temperature for a long time is required, and austenite remaining in the steel is transformed into pearlite, so that high strength and good workability are required. Is difficult to achieve.
[0045]
Further, in the present invention, the alloying temperature during the alloying treatment is set to 300 + 2000 × [Al%] − 50 × − (0.5 × W) ≦ T ≦ 350 + 2000 × [Al%] − 25 × √ (0.5 × W )
However, [Al%]: effective Al concentration in the bath of zinc plating bath (mass%)
W: Ni plating adhesion amount (g / m ² )
At a temperature T (° C.) that satisfies
[0046]
The alloying temperature T is limited to [300 + 2000 × [Al%]-50 × √ (0.5 × W)] ° C. or higher and [350 + 2000 × [Al%]-25 × √ (0.5 × W)] ° C. or lower. The reason is that if the alloying temperature T is lower than [300 + 2000 × [Al%] − 50 × √ (0.5 × W)] ° C., the alloying does not proceed, or the progress of the alloying is insufficient and This is because the coating surface is not treated and the plating surface layer is thickly covered with an η phase or a ζ phase having poor workability. On the other hand, if T is higher than [350 + 2000 × [Al%] − 25 × √ (0.5 × W)] ° C., alloying proceeds excessively and exceeds Fe% in the plating of the present invention. This is because the decrease in the value increases.
[0047]
In the present invention, if the alloying temperature is too high, the austenite remaining in the steel is transformed into pearlite, and a steel sheet having the desired high strength and workability cannot be obtained. Therefore, in order to improve the workability as the addition amount of Si increases and the alloy becomes more difficult, it is effective to lower the effective Al concentration in the bath and lower the alloying temperature. In particular,
[Al%] ≦ 0.103-0.008 × [Si%]
However, [Si%]: Si content in steel sheet (mass%)
Plating is performed at an effective Al concentration (mass%) in a bath that satisfies the following conditions.
[0048]
The reason that the effective Al concentration is limited to [0.103-0.008 × [Si%]]% or less is that the effective Al concentration is higher than [0.103-0.008 × [Si%]]%. This is because alloying at a high temperature for a long time is required, and austenite remaining in the steel is transformed into pearlite, thereby deteriorating workability.
[0049]
The reason why the time required for cooling to a temperature of 400 ° C. or less after hot-dip plating is limited to 10 seconds or more and 100 seconds or less is that if it is less than 10 seconds, the concentration of C in austenite becomes insufficient, and the C concentration in austenite is low. However, because it does not reach the level at which austenite can be retained at room temperature. If it exceeds 100 seconds, bainite transformation proceeds too much, the amount of austenite decreases, and a sufficient amount of retained austenite cannot be generated. It is. Preferably it is 10 seconds or more and 80 seconds or less.
[0050]
In the present invention, the method of heating the alloying furnace is not particularly limited. As long as the temperature of the present invention can be secured, radiant heating using a normal gas furnace or high-frequency induction heating may be used. Also, the method of cooling from the highest plate temperature after alloying heating is not questionable. If the heat is shut off by air seal etc. after alloying, it is enough to leave it open and the gas that cools more rapidly There is no problem with cooling.
[0051]
The reason for limiting the temperature of the hot-dip galvanizing bath to less than 460 ° C. is that if the temperature is 460 ° C. or more, the formation of the Fe—Al—Zn phase, which becomes an alloying barrier in the initial stage of plating, progresses too much and raises the alloying temperature. This is because a high-amount steel type tends to lower workability. Although the lower limit of the bath temperature is not particularly limited, since the melting point of zinc is 419.47 ° C., hot-dip plating can be performed only at a physically higher bath temperature.
[0052]
【Example】
Hereinafter, the present invention will be specifically described with reference to examples.
[0053]
(Example 1)
A slab having the composition shown in Table 1 was heated to 1150 ° C to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C, and wound at 580 to 680 ° C. After pickling, cold rolling was performed to form a 1.6 mm cold-rolled steel strip, and then electric Ni plating and displacement Cu plating with the adhesion amounts shown in Table 2 were performed, followed by in-line annealing continuous galvanizing equipment. Was used to perform heat treatment and plating under the conditions shown in Table 2 to produce an alloyed hot-dip galvanized steel sheet.
[0054]
A JIS No. 5 test piece was cut out from each steel sheet and subjected to a tensile test at room temperature to determine a tensile strength (TS) and an elongation (El). The tensile strength was 490 MPa or more, and the elongation was [51-0.035 × tensile strength]% or more. The adhesion amount of the plating film and the concentrations of Fe, Al, Ni, and Cu were measured by dissolving the film with an acid containing an inhibitor and by ICP. The Fe concentration in the plating was determined to be 7 to 13%.
[0055]
X-ray diffraction intensity ratio of Miller indices (249) plane of the Miller index (421) face of the ζ phase of the plating layer [delta] ₁ phase, peak of d = 1.900 - click intensity Aizeta (421) and d = 1. The peak intensity Iδ ₁ (249) of 990 was taken, and the peak intensity ratio after removing the background is shown in Table 2.
[0056]
The evaluation results are as shown in Table 2. No. 1 was insufficient in tensile strength because the C content in steel was outside the range of the present invention. In No. 2, both the tensile strength and the elongation were rejected because the Si content in the steel was outside the range of the present invention. No. 3 failed in elongation because the P content in steel was outside the range of the present invention. Nos. 7, 8, and 17 failed the elongation because the maximum temperature during annealing was outside the range of the present invention. In No. 9, both the tensile strength and the elongation were unacceptable because the Mn content in the steel was outside the range of the present invention. Nos. 12 and 27 failed in elongation because the alloying temperature was outside the range of the present invention. In the case of No. 15, Fe% during plating was rejected because the alloying temperature was outside the range of the present invention. In Nos. 20 and 28, elongation was rejected because the average cooling rate from the highest temperature to 650 ° C. was out of the range of the present invention. No. 24 failed in elongation because the Mn content / C content in the steel was outside the range of the present invention. No. 25 failed in elongation because the Si content / C content in steel was outside the range of the present invention. No. 29 failed in elongation because the average cooling rate from 650 ° C. to the plating bath was out of the range of the present invention. No. 30 failed in elongation because the Mn content in the steel was outside the range of the present invention. No. 31 failed in elongation because the C content in steel was outside the range of the present invention. The products of the present invention other than these were alloyed hot-dip galvanized steel sheets having high strength and good workability.
[0057]
When the plating bath temperature was lower than 460 ° C., it was possible to produce a galvannealed steel sheet having high strength and good workability regardless of the Si content in the steel. On the other hand, at 470 ° C., a low Si content of No. 5 or a high Si content of No. 33 and a low Fe% can be manufactured, but the Fe% is increased at a high Si content of No. 34. Then, it is necessary to raise the alloying temperature, and as a result, elongation is rejected.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
(Example 2)
A slab having the composition shown in H of Table 1 was heated to 1150 ° C to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C, and wound at 580 to 680 ° C. After pickling, cold rolling was performed to form a 1.6 mm cold-rolled steel strip, and then electric Ni plating and displacement Cu plating with the adhesion amounts shown in Table 3 were performed, followed by in-line annealing continuous galvanizing equipment. Was used to perform heat treatment and plating under the conditions shown in Table 3 to produce an alloyed hot-dip galvanized steel sheet.
[0061]
A JIS No. 5 test piece was cut out from each steel sheet and subjected to a tensile test at room temperature to determine a tensile strength (TS) and an elongation (El). The tensile strength was 490 MPa or more, and the elongation was [51-0.035 × tensile strength]% or more. The adhesion amount of the plating film and the concentrations of Fe, Al, Ni, and Cu were measured by dissolving the film with an acid containing an inhibitor and by ICP. The Fe concentration in the plating was determined to be 7 to 13%.
[0062]
The plating adhesion was determined by bending the test piece in which the adhesive tape (cellophane tape) was previously applied to the compression side in a V-shape so that the bending angle was 60 °, peeling the adhesive tape after bending back, and removing the plating tape. The degree of peeling was visually observed, evaluated according to the following classification, and rated as △ or more.
◎: Peeling width of plating layer is less than 1 mm ：: Peeling width of plating layer is 1 mm or more and less than 6 mm △: Peeling width of plating layer is 6 mm or more and less than 12 mm X: Peeling width of plating layer is 12 m or more The thing of [0063]
The evaluation results are as shown in Table 3. In No. 4, since the effective Al concentration in the plating bath was outside the range of the present invention, Fe% in plating and plating adhesion were rejected. In the case of No. 7, elongation was rejected because the effective Al concentration in the plating bath was outside the range of the present invention. No. 8 indicates that the effective Al concentration in the plating bath is outside the range of the present invention. Therefore, Fe% during plating was rejected. The products of the present invention other than these were alloyed hot-dip galvanized steel sheets having good adhesion, high strength and good workability.
[0064]
[Table 3]

[0065]
(Example 3)
A slab having the composition shown in H of Table 1 was heated to 1150 ° C to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C, and wound at 580 to 680 ° C. After pickling, cold rolling was performed to obtain a 1.6 mm cold-rolled steel strip, and then electric Ni plating and displacement Cu plating with the adhesion amounts shown in Table 4 were performed. Was used to perform heat treatment and plating under the conditions shown in Table 4 to produce an alloyed hot-dip galvanized steel sheet.
[0066]
A JIS No. 5 test piece was cut out from each steel sheet and subjected to a tensile test at room temperature to determine a tensile strength (TS) and an elongation (El). The tensile strength was 490 MPa or more, and the elongation was [51-0.035 × tensile strength]% or more. The adhesion amount of the plating film and the concentrations of Fe, Al, Ni, and Cu were measured by dissolving the film with an acid containing an inhibitor and by ICP. The Fe concentration in the plating was determined to be 7 to 13%.
[0067]
X-ray diffraction intensity ratio of Miller indices (249) plane of the Miller index (421) face of the ζ phase of the plating layer [delta] ₁ phase, peak of d = 1.900 - click intensity Aizeta (421) and d = 1. Table 4 shows the peak intensity ratio after removing the background by taking the peak intensity Iδ ₁ (249) of 990 respectively.
[0068]
The evaluation results are as shown in Table 4. In the case of No. 6, the Ni concentration during plating was rejected because the amount of Ni plating was out of the range of the present invention. Further, the Fe concentration during plating was lower than that of Nos. 1 to 5, and the X-ray diffraction intensity ratio between the ζ phase Miller index (421) plane and the δ ₁ phase Miller index (249) plane of the plating layer was also 0.2. Was larger. In the case of No. 7, the Cu concentration during plating was rejected because the Cu plating adhesion amount was outside the range of the present invention. Further, the Fe concentration during plating was lower than that of Nos. 1 to 5, and the X-ray diffraction intensity ratio between the ζ phase Miller index (421) plane and the δ ₁ phase Miller index (249) plane of the plating layer was also 0.2. Was larger. With respect to the present invention products other than these, alloying was promoted by the effect of Ni and Cu plating, and an alloyed hot-dip galvanized steel sheet having high strength and good workability was obtained.
[0069]
[Table 4]

[0070]
(Example 4)
A slab having the composition shown in H of Table 1 was heated to 1150 ° C to form a 4.5 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C, and wound at 580 to 680 ° C. After pickling, cold rolling was performed to form a 1.6 mm cold-rolled steel strip, and then electric Ni plating and displacement Cu plating with the adhesion amounts shown in Table 5 were performed, followed by in-line annealing continuous galvanizing equipment. Was used to perform heat treatment and plating under the conditions shown in Table 5 to produce an alloyed hot-dip galvanized steel sheet.
[0071]
A JIS No. 5 test piece was cut out from each steel sheet and subjected to a tensile test at room temperature to determine a tensile strength (TS) and an elongation (El). The tensile strength was 490 MPa or more, and the elongation was [51-0.035 × tensile strength]% or more. The adhesion amount of the plating film and the concentrations of Fe, Al, Ni, and Cu were measured by dissolving the film with an acid containing an inhibitor and using ICP. The Fe concentration in the plating was determined to be 7 to 13%.
[0072]
X-ray diffraction intensity ratio of Miller indices (249) plane of the Miller index (421) face of the ζ phase of the plating layer [delta] ₁ phase, peak of d = 1.900 - click intensity Aizeta (421) and d = 1. The peak intensity Iδ ₁ (249) of 990 was taken, and the peak intensity ratio after removing the background is shown in Table 5. The coefficient of friction used was 1/2 of a value obtained by sandwiching a steel plate coated with rust-preventive oil in a flat mold, applying a load, extracting at a constant speed, and dividing the extraction load by the pressing load.
[0073]
The evaluation results are as shown in Table 5. The X-ray diffraction intensity ratio of the ζ phase Miller index (421) plane and the δ ₁ phase Miller index (249) plane of the plating layer is 0.2 The following had a small coefficient of friction and exhibited good workability.
[0074]
[Table 5]

[0075]
【The invention's effect】
As described above, the present invention makes it possible to provide a high-strength galvannealed steel sheet having excellent workability and a method for producing the same, and greatly contributes to industrial development.

Claims

In mass%,
C: 0.05-0.15%,
Si: 0.3 to 2.0%,
Mn: 1.0 to 2.8%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.005 to 0.5%,
N: contains 0.006% or less,
The balance consists of Fe and unavoidable impurities, and when% C,% Si and% Mn are C, Si and Mn contents, respectively, (% Mn) / (% C) ≧ 12 and (% Si) / (% C ) On a high-strength steel plate satisfying ≧ 4, Al: 0.05 to 0.5% by mass, Fe: 7 to 15% by mass, Ni: 0.1 to 10% by mass, Cu: 0 A steel sheet containing an alloyed hot-dip galvanized layer containing 0.05 to 3% by mass of one or two kinds, the balance being Zn and unavoidable impurities, and having a tensile strength F (MPa) and an elongation L (%) Is L ≧ 51−0.035 × F
A high-strength galvannealed steel sheet with excellent workability, characterized by satisfying the following.

The workability according to claim 1, wherein the X-ray diffraction intensity ratio between the ζ phase Miller index (421) plane and the δ ₁ phase Miller index (249) plane of the plating layer is 0.2 or less. Excellent high strength galvannealed steel sheet.

The slab having the composition of the chemical component according to claim 1 is subjected to finish rolling at a temperature of _three or more points of Ar, cold-rolled at 50 to 85%, and then plated with one or two of Ni and Cu. Then, in a continuous hot-dip galvanizing equipment, annealed in a dual phase coexisting temperature range of 700 ° C to 850 ° C ferrite and austenite, and from its highest temperature to 650 ° C at an average cooling rate of 0.5 to 10 ° C / sec. Subsequently, a hot-dip galvanizing treatment is performed by cooling the plating bath from 650 ° C. to an average cooling rate of 1 to 20 ° C./sec to form a hot-dip galvanized layer on the surface of the cold-rolled steel sheet. By performing an alloying treatment on the steel sheet having the hot-dip galvanized layer formed thereon, a method for producing an alloyed hot-dip galvanized steel sheet that forms an alloyed hot-dip galvanized layer on the surface of the steel sheet, The hot-dip galvanizing treatment is performed in a hot-dip galvanizing bath having an effective Al concentration in the bath of 0.07 to 0.105 mass% and a balance of Zn and unavoidable impurities, and the alloying treatment is performed.
300 + 2000 × [Al%]-50 × √ (0.5 × W) ≦ T ≦ 350 + 2000 × [Al%]-25 × √ (0.5 × W)
However, [Al%]: effective Al concentration in the bath of zinc plating bath (mass%)
W: Ni plating adhesion amount (g / m ² )
A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability, which is performed at a temperature T (° C.) satisfying the following.

The method for producing a high-strength galvannealed steel sheet according to claim 3,
A high-strength galvannealed steel sheet having excellent workability, characterized in that the amount of Ni plating is 0.1 to 5 g / m ² and the amount of Cu plating is 0.1 to ² g / m ^2. Manufacturing method.

The method for producing a high-strength galvannealed steel sheet according to claim 3 or 4, wherein the effective Al concentration in the bath is:
[Al%] ≦ 0.103-0.008 × [Si%]
However, [Si%]: Si content in steel sheet (mass%)
A method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability, wherein the method is performed at an effective Al concentration (mass%) in a bath that satisfies the following conditions.

The method for producing a high-strength galvannealed steel sheet according to any one of claims 3 to 5, wherein the time from hot-dip coating to cooling to a temperature of 400 ° C or less is 10 seconds or more and 100 seconds or less. A method for producing a high-strength galvannealed steel sheet having excellent workability, characterized in that:

The method for producing a high-strength galvannealed steel sheet according to any one of claims 3 to 6, wherein the temperature of the hot-dip galvanizing bath is set to less than 460 ° C. A method for producing a high-strength galvannealed steel sheet.