JP4947565B2

JP4947565B2 - A method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability.

Info

Publication number: JP4947565B2
Application number: JP2001039425A
Authority: JP
Inventors: 良久高田; 正芳末廣; 高志荒牧
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-02-16
Filing date: 2001-02-16
Publication date: 2012-06-06
Anticipated expiration: 2021-02-16
Also published as: JP2002241896A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車、建築、電気等の部材として有用な高強度鋼板の製法に関し、低コストでプレス成形時の張出し成形性およびめっき密着性に優れる高強度溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板の製法に関するものである。
【０００２】
【従来の技術】
自動車等のクロスメンバーやサイドメンバー等の部材は、近年の燃費節減の動向に対応すべく軽量化が検討されており、材料面では、薄肉化しても強度が確保されるという観点から高強度化が進められている。ところが、一般に材料のプレス成形性は強度が上昇するに従って劣化するので、上記部材の軽量化を達成するためには、プレス成形性と高強度性の両特性を満足する鋼板の開発が求められている。成形性の指標値には引張試験における伸びをはじめとしてｎ値やｒ値があるが、一体成形によるプレス工程の簡略化が課題となっている昨今では均一伸びに相当するｎ値の大きいことがなかでも重要になってきている。
【０００３】
このため、鋼中に含有する残留オーステナイトの変態誘起塑性を活用した熱延鋼板および冷延鋼板が開発されている。これは高価な合金元素を含まずに０．０７〜０．４％程度のＣと０．３〜２．０％程度のＳｉおよび０．２〜２．５％程度のＭｎのみを基本的な合金元素とし、二相域で焼鈍後３００〜４５０℃内外の温度でベイナイト変態を行うことが特徴の熱処理により残留オーステナイトを金属組織中に含む鋼板であり、例えば、特開平１−２３０７１５号公報や特開平２−２１７４２５号公報等で開示されている。この種の鋼板は連続焼鈍で製造された冷延鋼板ばかりでなく、例えば、特開平１−７９３４５号公報のようにランアウトテーブルでの冷却と巻取温度を制御することにより熱延鋼板でも得られることが開示されている。
【０００４】
自動車の高級化を反映して耐食性および外観を向上させることを目的として、自動車部材のめっき化が進んでおり、現在では、車内に装着される特定の部材を除いた多くの部材に、亜鉛めっき鋼板が用いられている。従って、これらの鋼板には、耐食性の観点から溶融Ｚｎめっきを施すかあるいは溶融Ｚｎめっき後合金化処理した合金化溶融Ｚｎめっきを施して使用することが有効であるが、これらの高張力鋼板のうち、Ｓｉ含有量が高い鋼板の場合には鋼板表面が酸化膜を有しやすいため、溶融Ｚｎめっきの際に微小不めっき部が生じたり、合金化後の加工部のめっき密着性が劣るなどの問題があり、優れた加工部めっき密着性を有し、かつ耐食性の優れた高Ｓｉ系の高張力高延性合金化溶融Ｚｎめっき鋼板は実用化されていないのが現状である。
【０００５】
しかしながら、例えば特開平１−２３０７１５号公報や特開平２−２１７４２５号公報等で開示されている鋼板は０．３〜２．０％のＳｉを添加し、その特異なベイナイト変態を活用し残留オーステナイトを確保しているため、二相共存温度域で焼鈍後の冷却や３００〜４５０℃内外の温度域での保持をかなり厳格に制御しないと意図する金属組織が得られず、強度や伸びが目標の範囲をはずれる。この熱履歴は工業的には連続焼鈍設備や熱間圧延後のランアウトテーブルと巻取工程において実現されはするが、４５０〜６００℃ではオーステナイトの変態がすみやかに完了するので４５０〜６００℃に滞留する時間を特に短くするような制御が要求され、３５０〜４５０℃でも保持する時間によって金属組織が著しく変化するので所期の条件からはずれると陳腐な強度と伸びしか得られない。さらに、４５０〜６００℃に滞留する時間が長いことやめっき性を悪くするＳｉを合金元素として含むことから溶融めっき設備を通板させてめっき鋼板とはできず、表面耐食性が劣るため広範な工業的利用が妨げられているという問題点がある。
【０００６】
上記問題を解決するために、例えば、特開平５−２４７５８６号公報や特開平６−１４５７８８号公報等では、Ｓｉ濃度を規制することでめっき性を改善した鋼板が開示されている。この方法ではＳｉの変わりにＡｌを添加することで残留オーステナイトを生成されている。しかしながら、ＡｌもＳｉと同じようにＦｅよりも酸化しやすいので、鋼板表面にＡｌやＳｉが濃化し酸化膜を有しやすく、十分なめっき密着性を有することができないという問題点がある。また、特開平５−７０８８６号公報にＮｉを添加することでめっき塗れ性を改善するという方法が開示されている。しかしながら、この方法ではめっき塗れ性を阻害するＳｉやＡｌとＮｉの関係が開示されてはいない。
【０００７】
また、例えば、特開平４−３３３５５２号公報や特開平４−３４６６４４号公報等において高Ｓｉ系高強度鋼板の合金化溶融めっき方法としてプレＮｉめっき後急速低温加熱して溶融Ｚｎめっき後合金化処理する方法が開示されている。しかしながら、この方法ではＮｉプレめっきが必要になるので新たな設備が必要になるという問題点がある。また、この方法では最終組織に残留オーステナイトを残存させることができないし、その方法についても言及されていない。
そこで、本発明はかかる問題点を解決し、表面耐食性を向上するため溶融めっき設備でも製造可能でかつ、プレス成形性の良好な高強度鋼板の組成と金属組織の特徴を見いだしたものである。
【０００８】
【発明が解決しようとする課題】
本発明は、上記の様な問題点を解決し、プレス成形性およびめっき密着性の良好な高強度溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板を効率よく製造する方法を提供しようとするものである。
【０００９】
【課題を解決するための手段】
本発明者らは、上記目的を達成できる高強度溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板の製造方法を提供するべく、めっき性と鋼板成分との関係について鋭意検討を行い、鋼板表層に着目することで本発明を完成させたものであり、その趣旨とするところは、
（１）質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜２．５％、Ａｌ：０．０１〜１．５％、Ｎｉ：２％未満、Ｃｕ：２％未満、Ｓｎ：１％未満を含有し、かつ、ＳｉとＡｌの関係が、０．４（％）≦Ｓｉ＋０．８Ａｌ（％）を満足し、残部不可避的不純物を含むＦｅからなる鋳片（Ｂ層）に、表層（Ａ層）の組成が、質量％で、ＮｉとＣｕとＳｉとＡｌの関係が、Ｎｉ＋Ｃｕ（％）≧１／４Ｓｉ＋１／３Ａｌ（％）となるように鋳造凝固時にＮｉやＣｕを含有したパウダーを使った後、熱間圧延、冷間圧延し、６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍し、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、溶融亜鉛めっきを施し、その後に５℃／ｓ以上の冷却速度で２５０℃以下に冷却することを特徴とする、めっき密着性およびプレス成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１３】
（２）質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜２．５％、Ａｌ：０．０１〜１．５％、Ｎｉ：２％未満、Ｃｕ：２％未満、Ｓｎ：１％未満を含有し、かつ、ＳｉとＡｌの関係が、０．４（％）≦Ｓｉ＋０．８Ａｌ（％）を満足し、残部不可避的不純物を含むＦｅからなる鋳片（Ｂ層）に、表層（Ａ層）の組成が、質量％で、ＮｉとＣｕとＳｉとＡｌの関係が、Ｎｉ＋Ｃｕ（％）≧１／４Ｓｉ＋１／３Ａｌ（％）となるように鋳造凝固時にＮｉやＣｕを含有したパウダーを使った後、熱間圧延、冷間圧延した後、６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍し、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、Ｆｅ：８〜１５％、Ａｌ：１％以下と残部不可避的不純物を含むＺｎからなる合金化溶融亜鉛めっきを施し、４７０〜６００℃の範囲の温度域で５秒〜２分保持し、その後５℃／ｓ以上の冷却速度で２５０℃以下に冷却することを特徴とする、めっき密着性およびプレス成形性に優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１４】
（３）質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜２．５％、Ａｌ：０．０１〜１．５％、Ｎｉ：２％未満、Ｃｕ：２％未満、Ｓｎ：１％未満を含有し、かつ、ＳｉとＡｌの関係が、０．４（％）≦Ｓｉ＋０．８Ａｌ（％）を満足し、残部不可避的不純物を含むＦｅからなる鋳片の鋳造凝固時後、熱間圧延する際に、表層（Ａ層）の組成が、質量％で、ＮｉとＣｕとＳｉとＡｌの関係が、Ｎｉ＋Ｃｕ（％）≧１／４Ｓｉ＋１／３Ａｌ（％）となるようにＮｉやＣｕを含有した化合物あるいは合金鉄を前記鋳造スラブ表面に付加した後、加熱、圧延し、冷間圧延した後、６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍した後、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、溶融亜鉛めっきを施し、その後に５℃／ｓ以上の冷却速度で２５０℃以下に冷却することを特徴とする、めっき密着性およびプレス成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１５】
（４）質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．２〜２．０％、Ｍｎ：０．２〜２．５％、Ａｌ：０．０１〜１．５％、Ｎｉ：２％未満、Ｃｕ：２％未満、Ｓｎ：１％未満を含有し、かつ、ＳｉとＡｌの関係が、０．４（％）≦Ｓｉ＋０．８Ａｌ（％）を満足し、残部不可避的不純物を含むＦｅからなる鋳片の鋳造凝固時後、熱間圧延する際に、表層（Ａ層）の組成が、質量％で、ＮｉとＣｕとＳｉとＡｌの関係が、Ｎｉ＋Ｃｕ（％）≧１／４Ｓｉ＋１／３Ａｌ（％）となるようにＮｉやＣｕを含有した化合物あるいは合金鉄を前記鋳造スラブ表面に付加した後、加熱、圧延し、冷間圧延した後、６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍した後、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、Ｆｅ：８〜１５％、Ａｌ：１％以下と残部不可避的不純物を含むＺｎからなる合金化溶融亜鉛めっきを施し、その後に５℃／ｓ以上の冷却速度で２５０℃以下に冷却することを特徴とする、めっき密着性およびプレス成形性に優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１８】
（５）質量％で、前記（１）〜（４）のいずれか１項に記載の質量％で、さらに、Ｍｏ：０．２％未満、Ｃｒ：１％未満、Ｖ：０．３％未満、Ｂ：０．０１％未満、Ｎｂ：０．１％未満、Ｔｉ：０．１％未満の内少なくとも１種以上を含有し、かつ、ＳｉとＡｌの関係が、０．４（％）≦Ｓｉ＋０．８Ａｌ（％）を満足し、残部不可避的不純物を含むＦｅからなる鋳片（Ｂ層）であることを特徴とする、めっき密着性およびプレス成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
（６）前記冷間圧延後、６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍した後、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、さらにその温度域で２０分以下保持することを特徴とする前記（１）〜（４）のいずれか１項に記載のめっき密着性およびプレス成形性に優れた高強度溶融亜鉛系めっき鋼板の製造方法にある。
【００１９】
【発明の実施の形態】
本発明者らは、Ｎｉ，Ｃｕを含有させることによりめっき性を改善させることを見出し、かつ、低コストで製造するために、複層構造にすることでめっき性改善に寄与するＮｉ，Ｃｕを表層側にのみ含有することに着目し本発明を完成させた。本発明における成分および製造方法の限定理由は、低コストでプレス成形性およびめっき密着性の良好な高強度溶融亜鉛めっき鋼板または高強度合金化溶融亜鉛めっき鋼板を提供するためであり、以下に詳細に説明する。
本発明の最も重要な点は、鋼板を複層にさせ表層側はめっき性を満たすために、内層側は成形性を満たすことで低コストでプレス成形性およびめっき密着性の良好な高強度溶融亜鉛めっき鋼板または高強度合金化溶融亜鉛めっき鋼板を得ることにある。そのために、表層側のＡ層と内層側のＢ層のそれぞれの成分やＡ層とＢ層の比率が決定する。
【００２０】
Ａ層側は、めっき性の観点から求まる。本発明では良好なプレス成形性を得るために成形性を担うＢ層の中に残留オーステナイトを含有させる。オーステナイトを安定化させるためには、鋼中にセメンタイトに溶解しないＳｉやＡｌを含有させる必要がある。このＳｉやＡｌはＦｅよりも易酸化性元素でありめっき前の焼鈍時に鋼板表層に濃化し酸化物を形成しめっき性を阻害する。そのため、本発明ではＦｅよりも難酸化性元素のＮｉ，Ｃｕを活用することでＳｉ，Ａｌを含んでいるのにもかかわらずめっき性を確保させることを特徴にしている。
【００２１】
本発明者らによる実験の結果、Ｎｉ，Ｃｕ濃度がＳｉ，Ａｌと共に「Ｎｉ＋Ｃｕ（％）≧１／４Ｓｉ＋１／３Ａｌ（％）」以上にすることで良好なめっき密着性が得られることを見出した。この際にＮｉ，Ｃｕの下限は特に規定する必要がなく、また、上限についても規制されるものではなく、Ｓｉ．Ａｌとの上記式を満足すれば良好なめっき性が得られる。従って、純Ｎｉめっき、純Ｃｕめっきであってもよい。また、めっき性を確保させるためには、Ａ層は片面０．５μｍ以上必要なことを見出した。
【００２２】
Ｂ層側は成形性の観点から求まる。以下にそれぞれの元素の限定理由を示す。Ｃはオーステナイト安定化元素であり、二相共存温度域およびベイナイト変態温度域でフェライト中から移動しオーステナイト中に濃化する。その結果、化学的に安定化されたオーステナイトが室温まで冷却後も２〜２０％残留し、変態誘起塑性により成形性を良好とする。Ｃが０．０５％未満だと２％以上の残留オーステナイトを確保するのが困難であり、目的を達せられない。また、Ｃ濃度が０．２％を超すことは溶接性を悪化させるので避けなければならない。
【００２３】
Ｓｉはセメンタイトに固溶せず、その析出を抑制することにより３５０〜６００℃におけるオーステナイトからの変態を遅らせる。この間にオーステナイト中へのＣ濃化が促進されるためオーステナイトの化学的安定性が高まり、変態誘起塑性を起こし、成形性を良好とするのに貢献する残留オーステナイトの確保を可能とする。Ｓｉの量が０．２％未満だとその効果が見いだせない。一方、Ｓｉ濃度を高くするとめっき性を確保させるためのＡ層側に必要なＮｉ，Ｃｕ量が増加しコストが高くなるので２．０％を上限とした。
【００２４】
Ｍｎはオーステナイト形成元素であり、また二相共存温度域での焼鈍後３５０〜６００℃に冷却する途上でオーステナイトがパーライトへ分解するのを防ぐので、室温まで冷却した後の金属組織に残留オーステナイトが含まれるようにする。０．２％未満の添加ではパーライトへの分解を抑えるのに工業的な制御ができないほどに冷却速度を大きくする必要があり、適当ではない。一方、２．５％を超すとバンド組織が顕著になり特性を劣化させるし、スポット溶接部がナゲット内で破断しやすくなり好ましくない。
【００２５】
Ａｌは脱酸材としても用いられると同時に、Ｓｉと同じようにセメンタイトに固溶せず、３５０〜６００℃での保持に際してセメンタイトの析出を抑制し、変態の進行を遅らせる。しかしＳｉよりもフェライト形成能が強いため変態開始は早く、ごく短時間の保持でも二相共存温度域での焼鈍時よりオーステナイト中にＣが濃化され、化学的安定性が高まっているので、室温まで冷却後の金属組織に成形性を悪化させるマルテンサイトは僅かしか存在しない。このためＳｉと共存すると３５０〜６００℃での保持条件による強度や伸びの変化が小さく、高強度で良好なプレス成形性を得やすくなる。そのため、Ａｌは０．０１％以上の添加が必要である。また、Ｓｉと共に「Ｓｉ＋０．８Ａｌ」が０．４％以上になるようにしなければならない。一方、Ａｌ濃度が１．５％を超すとＡｌもＳｉと同様にめっき性を確保させるためのＡ層側に必要なＮｉ，Ｃｕ量が増加しコストが高くなるので１．５％を上限とした。
【００２６】
最終製品としての本発明鋼板の延性はＢ層側に含まれる残留オーステナイトの体積率に左右される。金属組織に含まれる残留オーステナイトは変形を受けていない時は安定に存在するものの、変形が加えられるとマルテンサイトに変態し、変態誘起塑性を呈するので良好な成形性が高強度で得られる。残留オーステナイトの体積率が２％未満でははっきりとした効果が認められない。一方、残留オーステナイトの体積率が２０％を超すと極度に厳しい成形を施した場合、プレス成形した状態で多量のマルテンサイトが存在する可能性があり二次加工性や衝撃性において問題を生じることがあるので、本発明では残留オーステナイトの体積率を２０％以下とした。組織はその他、フェライト、ベイナイト、マルテンサイトおよび炭化物を含むものである。
【００２７】
また、Ａ層とＢ層の比率は成形性、めっき性およびコストの観点から求まる。Ａ層の厚さを低くするとめっき性が劣化する。そのため、Ａ層は片面０．５μｍ以上とした。Ａ層の厚さの上限は特に規定する必要はないが、Ａ層の比率を高くすると必要なＮｉ，Ｃｕ量が増大し高コストになるので、Ａ層の全板厚に対する比率を１／４以下とすることが望ましい。この際に、Ａ層とＢ層のそれぞれの成分は平均の組成であり板厚方向に組成が分布を有していても問題はない。図１に、Ｎｉ，Ｓｉ，Ａｌについて本発明での板厚方向の成分の分布の一例を示す。
本発明は以上のＡ層及びＢ層を基本成分とするが、Ｂ層側に以下の不可避的に混入する成分を１種以上含むことにより特性がさらに向上する。
【００２８】
Ｎｉは、Ｍｎと同じようにオーステナイト生成元素であると同時に強度およびめっき密着性を向上させる。さらにＮｉにはＳｉやＡｌと同じようにセメンタイトに固溶せず、３５０〜６００℃での保持に際してセメンタイトの析出を抑制し、変態の進行を遅らせる。また、ＮｉはＦｅよりも難酸化性元素であり鋼板表層に濃化しめっき性をも改善させる。ただ、Ｎｉは高コストになるので２％未満とした。
ＣｕもＮｉやＭｎと同じようにオーステナイト生成元素であると同時に強度およびめっき密着性を向上させる。Ｃｕを２．０％以上に高くするとＣｕ析出物が生成するため材質が悪化するのでＣｕ添加量は２％未満とした。
【００２９】
Ｓｎは強度およびめっき密着性を向上させる。しかし、Ｓｎ添加量を増大させると熱間圧延時に割れが発生する懸念が多くなるのでＳｎ添加量は１％未満とした。
Ｍｏ，Ｃｒ，Ｖ，Ｂ，Ｎｂ，Ｔｉは強度を上げる元素であり、Ｍｏ：０．２％未満，Ｃｒ：１％未満、Ｖ：０．３％未満、Ｂ：０．０１％未満、Ｔｉ：０．１％未満、Ｎｂ：０．１％未満のうちの少なくとも１種以上を必要に応じて添加することは本発明の趣旨を損なうことはない。これら元素の効果は上記の上限で飽和するのでそれ以上の添加はコストが高くなる。
【００３０】
本発明のＢ層は以上の成分とするが、これらの元素およびＦｅ以外にＣｏ，Ｚｎ，Ｍｇ，Ｔａ，Ｔｅ，Ｂｅ，Ｒｕ，Ｏｓ，Ｒｈ，Ｉｒ，Ｐｄ，Ｐｔ，Ａｇ，Ａｕ，Ｃｄ，Ｈｇ，Ｇｅ，Ｐｂ，Ｓｂ，Ｂｉ，Ｓｅ，Ｔｅなどその他の一般鋼に対して不可避的に混入する元素を含むものであり、これら元素を例えば全体で０．０１％以下含んでいても本発明の趣旨を何ら損なうものではない。また、ＰおよびＳは溶接性の観点からそれぞれ、０．０５％未満、０．０３％未満とすることが望ましい。これらの元素は不可避的に混入する元素としたが、本発明においては、特に付随的成分として扱い、これらの条件を満たす場合は本発明の範囲として包含するものである。
【００３１】
本発明では上記鋼板の上にＺｎめっき層またはＺｎ合金めっき層を有しているが、以下に詳細に説明する。
Ｚｎめっき層としてはＡｌ：１％以下と残部Ｚｎおよび不可避的不純物を含むものである。めっき中のＡｌ含有率を１％以下にしたのは、Ａｌ含有率が１％を超えるとめっき中に偏析したＡｌが局部電池を構成し、耐食性が劣化するからである。また、Ｚｎ合金めっき層としてはＦｅ：８〜１５％、Ａｌ：１％以下と残部Ｚｎおよび不可避的不純物からなるものである。めっき層中のＦｅ含有率を８％以上としたのは、８％未満では、化成処理性（リン酸塩処理）塗膜密着性が良好となるためである。また、Ｆｅ含有率を１５％以下としたのは１５％超では、過合金となり加工部のめっき密着性が劣化するためである。また、めっき中のＡｌ含有率を１％以下にしたのは、Ａｌ含有率が１％を超えるとめっき中に偏析したＡｌが局部電池を構成し、耐食性が劣化するからである。
【００３２】
本発明でのＺｎめっき層およびＺｎ合金めっき層は以上であるが、その他Ｍｎ，Ｐｂ，Ｆｅ，Ｓｂ，Ｎｉ，Ｃｕ，Ｓｎ，Ｃｏ，Ｃｄ，Ｃｒなど不可避に混入するものを、本発明においては不可避的成分と定めたが、これらの元素は、特に付随的成分元素と同等に含んでもよい。また、Ｍｇ，ＣａはＺｎめっき時に鋼板表層の酸化物を還元し、めっき密着性を改善させる効果を有するので、付随的成分としてそれぞれ８％未満、１％未満含んでもよい。これら元素はいずれも本発明の範囲として包含するものである。また、Ｚｎ合金めっき層厚みについては特に制約は設けないが、耐食性の観点から０．１μｍ以上、加工性の観点からすると１５μｍ以下であることが望ましい。
【００３３】
次に、本発明の溶融亜鉛めっき鋼板および本発明の合金化溶融亜鉛めっき鋼板の製造方法について説明する。
本発明の溶融亜鉛めっき鋼板は、上記の様な成分組成を得るために、鋳造凝固時、熱間圧延する際、冷間圧延後Ｚｎめっきを行う焼鈍の前に、鋼板表層にＮｉ，Ｃｕを含有させ、その後に６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍した後、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、場合によってはさらにその範囲の温度域で２０分以下に保持した後に、溶融亜鉛めっきを施し、その後に５℃／ｓ以上の冷却速度で２５０℃以下に冷却することにより得られる。
【００３４】
また、本発明の合金化溶融亜鉛めっき鋼板は、上記の様な成分組成を得るために、鋳造凝固時、熱間圧延する際、冷間圧延後Ｚｎめっきを行う焼鈍の前に、鋼板表層にＮｉ，Ｃｕを含有させ、その後に６５０〜９００℃の二相共存温度域で１０秒〜１０分焼鈍した後、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却し、場合によってはさらにその範囲の温度域で２０分以下に保持した後に、溶融亜鉛めっきを施し、その後に４５０〜６００℃の範囲の温度域で５秒〜２分保持してから５℃／ｓ以上の冷却速度で２５０℃以下に冷却することにより得られる。
【００３５】
以下に、それぞれの製造条件の理由について記す。
本発明者らは、Ｎｉ，Ｃｕを含むＡ層を製造するためにラボ試験を行った。試験は、０．１％Ｃ，１．２％Ｓｉ，１．５％Ｍｎを含む炭素鋼の鋳造時にＮｉまたはＣｕを含有させたパウダーを使った後、熱間圧延、冷間圧延、焼鈍、めっき試験を行い鋼板断面観察を行った。その結果、パウダー中にＮｉ，Ｃｕを含有させることで鋼板表層にＮｉ，Ｃｕを多く含む層を得ることが出来た。
【００３６】
調査した結果、この表層側のＡ層のＮｉ，Ｃｕ，Ｓｉ，Ａｌ量がＮｉ＋Ｃｕ＞１／４Ｓｉ＋１／３Ａｌの関係を満たし、Ａ層が片面０．５μｍ以上である場合に、めっき性およびプレス成形性に優れた高強度めっき鋼板が得られた。鋳造時により、鋼板表層にＮｉ，Ｃｕを含ませることは電磁攪拌や電磁ブレーキを使うことでより促進できるので、これらを使うことが望ましい。また、パウダーは金属パウダーだけでなく、合金や酸化物でも同様の効果が得られる。この際に、Ｎｉ，Ｃｕのパウダー中への含有量を調整することでＡ層の厚みを調整することができる。コストとの関係からＮｉまたはＣｕのパウダー中の比率は重量％で５０％未満とすることが望ましい。
【００３７】
また、０．１％Ｃ，１．２％Ｓｉ，１．５％Ｍｎを含む炭素鋼の鋳造後のスラブ表層にＮｉ粉、Ｃｕ粉を散布した後、熱間圧延、冷間圧延、焼鈍、めっき試験を行い鋼板断面観察を行った。その結果、鋼板表層にＮｉ，Ｃｕを多く含む層を得ることが出来、さらに表層側のＡ層のＮｉ，Ｃｕ，Ｓｉ，Ａｌ量がＮｉ＋Ｃｕ＞１／４Ｓｉ＋１／３Ａｌの関係を満たし、Ａ層が片面０．５μｍ以上である場合に、めっき性およびプレス成形性に優れた高強度めっき鋼板が得られた。ここでＮｉ，Ｃｕは金属粉、Ｎｉ，Ｃｕを含む合金でもよいし、酸化物でもかまわない。これらの散布量を調整することで、Ａ層の厚みを調整することができる。
【００３８】
さらに、０．１％Ｃ，１．２％Ｓｉ，１．５％Ｍｎを含む炭素鋼を鋳造、熱間圧延、冷間圧延後、電気めっきでＮｉやＣｕを表層にめっきした後、焼鈍、めっき実験を行い、鋼板断面観察を行った。その結果、鋼板表層にＮｉ，Ｃｕを多く含む層を得ることが出来、さらに表層側のＡ層のＮｉ，Ｃｕ，Ｓｉ，Ａｌ量がＮｉ＋Ｃｕ＞１／４Ｓｉ＋１／３Ａｌの関係を満たし、Ａ層が片面０．５μｍ以上である場合に、めっき性およびプレス成形性に優れた高強度めっき鋼板が得られた。Ｎｉ，Ｃｕのめっきは電気めっきである必要は必ずしもなく、置換めっきでも問題はない。また、めっき種としてＮｉ，Ｃｕだけではなく、これらの酸化物あるいは合金でもなんら変わりはない。これらのめっき量を調整することで、Ａ層の厚みを調整することができる。
【００３９】
本発明では、上記の手段によりＡ層とＢ層を形成させた後に焼鈍および溶融亜鉛めっきを施すことによりめっき密着性及びプレス成形性に優れた高強度溶融亜鉛めっき鋼板および高強度合金化溶融亜鉛めっき鋼板を製造する。焼鈍条件及びめっき条件は、特に、プレス成形性の観点から求まる。
冷間圧延後の冷延鋼板の連続焼鈍では、まず〔フェライト＋オーステナイト〕の二相組織とするためにＡｃ₁変態点以上Ａｃ₃変態点以下の温度域に加熱が行われる。このときに加熱温度が６５０℃未満であると、セメンタイトが再固溶するのに時間がかかり過ぎオーステナイトの存在量もわずかになるので、加熱温度の下限は６５０℃とした。また、加熱温度が高すぎるとオーステナイトの体積率が大きくなり過ぎてオーステナイト中のＣ濃度が低下することから、加熱温度の上限は９００℃とした。均熱時間としては、短すぎると未溶解炭化物が存在する可能性が高く、オーステナイトの存在量が少なくなる。また、均熱時間を長くすると結晶粒が粗大になる可能性が高くなり強度延性バランスが悪くなる。よって、本発明では保持時間を１０秒〜１０分の間とした。
【００４０】
均熱後は、２〜２００℃／ｓの冷却速度で３５０〜５００℃まで冷却する。これは、二相域に加熱して生成させたオーステナイトをパーライトに変態させることなくベイナイト変態域に持ち越し、引き続く処理により室温では残留オーステナイトとベイナイトとして所定の特性を得ることを目的とする。この時の冷却速度が２℃／ｓ未満では冷却中にオーステナイトの大部分がパーライト変態をしてしまうために残留オーステナイトが確保されない。また、冷却速度が２００℃／ｓを超えると冷却終点温度が幅方向、長手方向でずれが大きくなり均一な鋼板を製造することができなくなる。
【００４１】
この後、場合によっては３５０〜５００℃の範囲内で２０分以下に保持してもよい。このＺｎめっき前に温度保持をすることでベイナイト変態を進行させＣの濃縮した残留オーステナイトを安定化させることができ、より安定して強度、伸びの両立した鋼板を製造できる。二相域からの冷却終点温度が５００℃を超える温度になると、その後の温度保持を行うとオーステナイトの炭化物への分解が起こりオーステナイトを残存できなくなる。また、冷却終点温度が３５０℃未満になるとオーステナイトの大半がマルテンサイトに変態するので、高強度にはなるもののプレス成形性が悪化することと、Ｚｎめっき時に鋼板温度を上げる必要があり、熱エネルギーを与える必要があるため非効率になる。保持時間が２０分を超えるとＺｎめっき後の加熱で炭化物析出と未変態オーステナイトの消失による強度とプレス成形性両方の劣化になるので保持時間を２０分以下とした。
【００４２】
溶融亜鉛めっき鋼板を製造する場合は溶融亜鉛めっき後、５℃／ｓ以上の冷却速度で２５０℃以下に冷却する。ここでＺｎめっき時にベイナイト変態を進行させ炭化物をほとんど含まないベイナイトとその部分から掃き出されたＣが濃化しＭ_S点が室温以下に低下した残留オーステナイト、および二相域加熱中に清浄化が進んだフェライトの混在した組織を現出させ、高強度と成形性を両立させている。そのため、保持後の冷却速度を５℃以下としたり、冷却終点温度が２５０℃以上とすると冷却中にＣの濃化したオーステナイトも炭化物を析出してベイナイトに分解するため、変態誘起塑性により加工性を改善する残留オーステナイト量が減少してしまうので目的を達し得ない。残留オーステナイトをより残存させるために、溶融亜鉛めっき後３５０〜４００℃の温度範囲に５分以内保持した方が望ましい。
【００４３】
また、合金化溶融亜鉛めっき鋼板を製造する際には溶融亜鉛めっき後、４５０〜６００℃の温度域で５秒〜２分保持し、その後５℃／ｓ以上の冷却速度で２５０℃以下に冷却する。ここでは、ＦｅとＺｎの合金化反応と、組織的な観点からもとまる。本発明鋼ではＳｉやＡｌが含まれるためにオーステナイトからベイナイトへの変態が二段階に分離することを活用し、炭化物をほとんど含まないベイナイトとその部分から掃き出されたＣが濃化しＭｎ点が室温以下に低下した残留オーステナイト、および二相域加熱中に清浄化が進んだフェライトの混在した組織を現出させ、高強度と成形性を両立させている。
【００４４】
保持温度が６００℃を超えるとパーライトが生成するために残留オーステナイトが含まれなくなり、また、合金化反応が進みすぎめっき中のＦｅ濃度が１５％を越えてしまう。一方、加熱温度が４５０℃以下になるとめっきの合金化反応速度が遅くなり、めっき中のＦｅ濃度が低くなる。また、保持時間が５秒以下ではベイナイトが十分に生成せず、未変態のオーステナイト中へのＣ濃化も不充分なため冷却中にマルテンサイトが生成し成形性か劣化すると同時に、めっきの合金化反応が不充分になる。
【００４５】
また、保持時間が２分以上になるとめっきの過合金化が生じ成型時にめっき剥離などが生じやすくなる。さらに、保持後の冷却速度を５℃以下としたり、冷却終点温度が２５０℃以上とするとベイナイト変態がさらに進み、前段の反応でＣの濃化したオーステナイトも炭化物を析出してベイナイトに分解するため、変態誘起塑性により加工性を改善する残留オーステナイトの量が減少してしまうので目的を達し得ない。
溶融亜鉛めっき温度はめっき浴の融点以上５００℃以下が望ましい。５００℃以上になるとめっき浴からの蒸気が多大になり操業性が悪化するためである。また、めっき後の保持温度までの加熱速度については特に規定する必要はないが、めっき組織や金属組織の観点から３℃／ｓ以上が望ましい。
【００４６】
なお、以上説明した工程における各温度、冷却温度は規定の範囲内であれば一定である必要はなく、その範囲内で変動したとしても最終製品の特性はなんら劣化しないし向上する場合もある。
また、めっき密着性をさらに向上させるために、鋼板焼鈍時の雰囲気を調節し、始め鋼板表面を酸化させ、その後還元することによりめっき前の鋼板表面の清浄化を行ってもよい。さらに、めっき密着性を改善するために焼鈍前に鋼板を酸洗あるいは研削することで鋼板表面の酸化物を取り除いても問題はない。これら処理をすることでめっき密着性がさらに向上する。
【００４７】
【実施例】
表１に成分を示した鋼（Ｂ層）を表２に記載した条件でＮｉ，Ｃｕを含むＡ層形成および、焼鈍、めっきを行い、その後０．５％で調質圧延することで板厚１ｍｍの鋼板を製造した。
Ｎｉ，Ｃｕを含むＡ層を形成するために、下記の方法を用いた。ＮｉあるいはＣｕパウダーを用いる場合には、Ｎｉ，Ｃｕを質量％で１０％を含むパウダーを用いた。また、Ｎｉ，Ｃｕ混合パウダーはそれぞれ質量％で１０％含有させたパウダーを用いた。熱延前のＮｉ，Ｃｕ吹きつけは、鋳造後で熱間圧延前のスラブ表層を室温でＮｉ，Ｃｕ金属粉をスプレーで付着させた。
【００４８】
製造した鋼板は、下記に示す「引っ張り試験」「残留オーステナイト測定試験」「溶接試験」「Ａ層濃度およびＡ層の幅およびＡ層とＢ層の比率測定」「めっき性」「めっき密着性」「めっき層中濃度測定」の試験を行った。また、めっき付着量は片面５０ｇ／ｍ² になるようにした。
「引っ張り試験」はＪＩＳ５号引張試験片を採取し、ゲージ厚さ５０ｍｍ、引張速度１０ｍｍ／ｍｉｎで常温引っ張り試験を行った。
「残留オーステナイト測定試験」は、表層より板厚の１／４内層を化学研磨後、Ｍｏ管球を用いたＸ線回折でα−Ｆｅとγ−Ｆｅの強度から求める５ピーク法と呼ばれる方法で測定した。
【００４９】
「溶接試験」は、溶接電流：１０ｋＡ、加圧力：２２０ｋｇ、溶接時間：１２サイクル、電極径：６ｍｍ、電極形状：ドーム型，先端６φ−４０Ｒの溶接条件でスポット溶接を行い、ナゲット径が４√ｔ（ｔ：板厚）を切った時点までの連続打点数を評価した。評価基準は○：連続打点１０００点超、△：連続打点５００〜１０００点、×：連続打点５００点未満とした。ここでは、○を合格とし、△・×は不合格とした。
「Ａ層濃度」は、めっき鋼板の断面部深さ方向のＥＰＭＡ分析による測定で行った。「Ａ層の幅」はめっき鋼板の鋼板／めっき界面部をＦＩＢ法で切り出した試料をＴＥＭでのＥＤＳ分析で求めた。測定の際は標準試料を用い検量線を作成した。
【００５０】
「めっき密着性」は、めっき鋼板の６０度Ｖ曲げ試験を実施後テープテストを行い、以下の基準に従い評価した。
テープテスト黒化度（％）
評価：◎ … ０〜１０
評価：○ … １０〜２０未満
評価：△ … ２０〜３０未満
評価：× … ３０以上
（◎と○が合格、△・×は不合格）
「めっき層中濃度測定」は、アミン系インヒビターを入れた５％塩酸でめっき層を溶かした後、ＩＣＰ発光分析法で測定した。
【００５１】
性能評価試験結果を表３に示す。本発明である試料１〜８は残留オーステナイトが２〜２０％で５５０ＭＰａ以上でありながら全伸びも３０％以上であり、高強度とプレス成形性の良好さを両立していると同時に、めっき性や溶接性も満足した溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板である。それに対し、試料９はＣ濃度が低いために、試料１０はＣ濃度が高いために、試料１１は鋼中ＳｉとＡｌの関係を満たしていないために、試料１２はＭｎ濃度が高いために、試料２３はＡ層のＮｉ，Ｃｕ，Ｓｉ，Ａｌの関係を満たしていないために、残留オーステナイト量、高強度とプレス成形性の両立、めっき性、溶接性を全ては満足しておらず、本発明の目的を達し得ない。
また、本発明鋼であっても処理条件の一つに問題があると、試料１３〜２３のように残留オーステナイト量、高強度とプレス成形性の両立、めっき性、溶接性を全ては満足しておらず、本発明の目的を達し得ない。
【００５２】
【表１】

【００５３】
【表２】

【００５４】
【表３】

【００５５】
【発明の効果】
以上説明したように、本発明によれば、プレス成形性およびめっき密着性の良好な高強度合金化溶融亜鉛めっき鋼板および該鋼板を効率よく製造することができる。
【図面の簡単な説明】
【図１】本発明におけるＮｉ，Ｃｕ，Ｓｉ，Ａｌの板厚方向の濃度分布を例示した図である。[0001]
BACKGROUND OF THE INVENTION
  The present invention is a high-strength steel plate useful as a member for automobiles, architecture, electricity, etc.Made ofHigh-strength hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet that are excellent in stretch formability and plating adhesion during press forming at low costMade ofIt is about the law.
[0002]
[Prior art]
Lightweight parts for automobiles such as cross members and side members are being studied to meet the recent trend of fuel efficiency savings, and in terms of materials, the strength is increased from the viewpoint that strength is ensured even if it is made thinner. Is underway. However, since the press formability of the material generally deteriorates as the strength increases, the development of a steel sheet that satisfies both the press formability and the high strength properties is required in order to achieve weight reduction of the above members. Yes. There are n values and r values, including elongation in tensile tests, as index values of formability. However, in recent years when simplification of the pressing process by integral molding has become a problem, the n value corresponding to uniform elongation is large. Especially important.
[0003]
For this reason, hot-rolled steel sheets and cold-rolled steel sheets utilizing transformation-induced plasticity of retained austenite contained in steel have been developed. This is basically composed of about 0.07 to 0.4% of C, about 0.3 to 2.0% of Si, and about 0.2 to 2.5% of Mn without containing expensive alloy elements. It is a steel sheet containing residual austenite in the metal structure by heat treatment characterized by being an alloying element and performing bainite transformation at temperatures in and out of 300 to 450 ° C. after annealing in a two-phase region. For example, JP-A-1-230715 and It is disclosed in JP-A-2-217425. This type of steel sheet can be obtained not only with cold-rolled steel sheets manufactured by continuous annealing, but also with hot-rolled steel sheets by controlling the cooling at the run-out table and the coiling temperature as disclosed in, for example, JP-A-1-79345. It is disclosed.
[0004]
In order to improve the corrosion resistance and appearance reflecting the upgrading of automobiles, the plating of automobile parts is progressing, and at present, galvanization is applied to many parts except for specific parts installed in the car. A steel plate is used. Therefore, it is effective to use these steel sheets by applying hot-dip Zn plating from the viewpoint of corrosion resistance or by applying alloyed hot-dip Zn plating that has been alloyed after hot-dip Zn plating. Among them, in the case of a steel sheet with a high Si content, the surface of the steel sheet tends to have an oxide film, so that a fine non-plated part occurs during hot-dip Zn plating, or the plating adhesion of the processed part after alloying is inferior. However, the present situation is that a high-Si high-tensile and highly ductile alloyed hot-dip Zn-plated steel sheet having excellent work-part plating adhesion and excellent corrosion resistance has not been put into practical use.
[0005]
However, for example, steel sheets disclosed in JP-A-1-230715, JP-A-2-217425 and the like add 0.3 to 2.0% Si, and utilize their unique bainite transformation to retain retained austenite. Therefore, if the cooling after annealing in the two-phase coexisting temperature range and the holding in the temperature range of 300 to 450 ° C are not controlled strictly, the intended metal structure cannot be obtained, and the strength and elongation are the targets. Out of range. Although this heat history is industrially realized in continuous annealing equipment, a run-out table after hot rolling and a winding process, the austenite transformation is completed promptly at 450 to 600 ° C, so it stays at 450 to 600 ° C. Control is required to particularly shorten the time to be performed, and the metal structure changes significantly depending on the holding time even at 350 to 450 ° C. Therefore, if it deviates from the intended condition, only stale strength and elongation can be obtained. Furthermore, since it has a long residence time at 450 to 600 ° C. and contains Si as an alloying element that deteriorates plating properties, it cannot be made into a plated steel sheet by passing through a hot dipping facility, and has a wide range of industries due to poor surface corrosion resistance. There is a problem that general use is hindered.
[0006]
In order to solve the above problem, for example, JP-A-5-247586, JP-A-6-145788, and the like disclose a steel sheet that has improved plating properties by regulating the Si concentration. In this method, residual austenite is generated by adding Al instead of Si. However, Al, like Si, is more easily oxidized than Fe. Therefore, there is a problem that Al or Si is concentrated on the surface of the steel sheet and an oxide film is easily formed, and sufficient plating adhesion cannot be obtained. Japanese Patent Application Laid-Open No. 5-70886 discloses a method of improving plating coatability by adding Ni. However, this method does not disclose the relationship between Si, Al, and Ni, which hinders the coatability.
[0007]
Also, for example, as a method for alloying and hot-plating high Si-based high-strength steel sheets in JP-A-4-333552 and JP-A-4-346644, etc., pre-Ni plating and rapid low-temperature heating and alloying treatment after hot-dip Zn plating A method is disclosed. However, since this method requires Ni pre-plating, there is a problem that new equipment is required. Further, this method cannot leave residual austenite in the final structure, and the method is not mentioned.
Therefore, the present invention solves such problems and finds the composition of the high-strength steel sheet and the characteristics of the metal structure that can be manufactured even by hot dipping equipment and improve the surface corrosion resistance.
[0008]
[Problems to be solved by the invention]
  The present invention solves the above-mentioned problems and provides high strength with good press formability and plating adhesion.Hot dip galvanized steel sheet andAlloyed galvanized steelBoardAn object of the present invention is to provide a method for producing efficiently.
[0009]
[Means for Solving the Problems]
  In order to provide a method for producing a high-strength hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that can achieve the above-mentioned object, the present inventors have conducted intensive studies on the relationship between plating properties and steel sheet components, and focused on the steel sheet surface layer. The present invention has been completed by doing so,
(1) By mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.5%, Al: 0.01 to 1.5%, Ni: less than 2%, Cu: less than 2%, Sn: less than 1%And the relation between Si and Al satisfies 0.4 (%) ≦ Si + 0.8 Al (%), and the surface layer (A The composition of the layer) is mass%, and a powder containing Ni or Cu is used at the time of casting solidification so that the relationship between Ni, Cu, Si and Al is Ni + Cu (%) ≧ 1 / 4Si + 1 / 3Al (%) After that, it is hot-rolled and cold-rolled, annealed for 10 seconds to 10 minutes in a two-phase coexisting temperature range of 650 to 900 ° C., cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, and melted. A method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability, characterized by performing galvanization and then cooling to 250 ° C. or lower at a cooling rate of 5 ° C./s or higher.
[0013]
  (2) By mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.5%, Al: 0.01 to 1.5%, Ni: less than 2%, Cu: less than 2%, Sn: less than 1%And the relation between Si and Al satisfies 0.4 (%) ≦ Si + 0.8 Al (%), and the surface layer (A The composition of the layer) is mass%, and a powder containing Ni or Cu is used at the time of casting solidification so that the relationship between Ni, Cu, Si and Al is Ni + Cu (%) ≧ 1 / 4Si + 1 / 3Al (%) Then, after hot rolling and cold rolling, annealing for 10 seconds to 10 minutes in a two-phase coexisting temperature range of 650 to 900 ° C., cooling to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, Fe: 8 to 15%, Al: not more than 1%, and alloying hot dip galvanizing composed of Zn containing inevitable impurities, and holding for 5 seconds to 2 minutes in a temperature range of 470 to 600 ° C. It is cooled to 250 ° C or lower at a cooling rate of ° C / s or higher. Manufacturing method of adhesion and a high strength galvannealed steel sheet excellent in press formability can.
[0014]
  (3) By mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.5%, Al: 0.01 to 1.5%, Ni: less than 2%, Cu: less than 2%, Sn: less than 1%And the relationship between Si and Al satisfies 0.4 (%) ≦ Si + 0.8 Al (%), and the cast slab made of Fe containing the remaining inevitable impurities is subjected to hot rolling after casting solidification. In this case, the composition of the surface layer (A layer) is mass%, and Ni and Cu are contained so that the relationship between Ni, Cu, Si and Al is Ni + Cu (%) ≧ 1 / 4Si + 1 / 3Al (%) After adding the above-mentioned compound or alloyed iron to the cast slab surface, heating, rolling, cold rolling, annealing in a two-phase coexistence temperature range of 650 to 900 ° C. for 10 seconds to 10 minutes, and then 2 to 200 ° C. Plating adhesion and press formability, characterized by cooling to 350-500 ° C. at a cooling rate of / s, applying hot dip galvanizing, and then cooling to 250 ° C. or less at a cooling rate of 5 ° C./s or more For producing high-strength hot-dip galvanized steel sheets with excellent resistance.
[0015]
  (4) By mass%, C: 0.05 to 0.2%, Si: 0.2 to 2.0%, Mn: 0.2 to 2.5%, Al: 0.01 to 1.5%, Ni: less than 2%, Cu: less than 2%, Sn: less than 1%And the relationship between Si and Al satisfies 0.4 (%) ≦ Si + 0.8 Al (%), and the cast slab made of Fe containing the remaining inevitable impurities is subjected to hot rolling after casting solidification. In this case, the composition of the surface layer (A layer) is mass%, and Ni and Cu are contained so that the relationship between Ni, Cu, Si and Al is Ni + Cu (%) ≧ 1 / 4Si + 1 / 3Al (%) After adding the above-mentioned compound or alloyed iron to the cast slab surface, heating, rolling, cold rolling, annealing in a two-phase coexistence temperature range of 650 to 900 ° C. for 10 seconds to 10 minutes, and then 2 to 200 ° C. Is cooled to 350 to 500 ° C. at a cooling rate of / sec, Fe: 8 to 15%, Al: 1% or less, and alloyed hot dip galvanizing composed of Zn containing the inevitable impurities is applied, and then 5 ° C./s Cooling to 250 ° C. or lower at the above cooling rate Manufacturing method of plating adhesion and high strength galvannealed steel sheet excellent in press formability.
[0018]
  (5) By mass%, and by mass% of any one of (1) to (4),further,At least one of Mo: less than 0.2%, Cr: less than 1%, V: less than 0.3%, B: less than 0.01%, Nb: less than 0.1%, Ti: less than 0.1% In addition, the relationship between Si and Al satisfies 0.4 (%) ≦ Si + 0.8 Al (%), and is a cast slab (B layer) made of Fe containing the remaining inevitable impurities. A method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability.
(6) After the cold rolling, after annealing for 10 seconds to 10 minutes in a two-phase coexisting temperature range of 650 to 900 ° C., cooling to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, and further the temperature It is in the method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability according to any one of (1) to (4), characterized by holding for 20 minutes or less in the region. .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have found that the plating property is improved by containing Ni and Cu, and in order to manufacture at a low cost, Ni and Cu contributing to the improvement of the plating property by forming a multilayer structure. The present invention was completed by paying attention to the inclusion only on the surface layer side. The reason for the limitation of the components and the production method in the present invention is to provide a high-strength hot-dip galvanized steel sheet or high-strength galvannealed steel sheet with good press formability and plating adhesion at a low cost. Explained.
The most important point of the present invention is that high-strength melting with good press formability and good plating adhesion is achieved at low cost by making the steel sheet into multiple layers and satisfying the plateability on the surface layer side, and satisfying the formability on the inner layer side. The object is to obtain a galvanized steel sheet or a high-strength galvannealed steel sheet. For this purpose, the respective components of the A layer on the surface layer side and the B layer on the inner layer side and the ratio of the A layer and the B layer are determined.
[0020]
The A layer side is determined from the viewpoint of plating properties. In the present invention, retained austenite is contained in the B layer responsible for formability in order to obtain good press formability. In order to stabilize austenite, it is necessary to contain Si or Al that does not dissolve in cementite in the steel. Si and Al are more easily oxidizable elements than Fe, and concentrate on the surface layer of the steel sheet during annealing before plating to form an oxide and inhibit the plating property. For this reason, the present invention is characterized in that, by using Ni, Cu, which is a less oxidizable element than Fe, the plating property is ensured despite containing Si and Al.
[0021]
As a result of experiments by the present inventors, it was found that good plating adhesion can be obtained by setting the Ni and Cu concentrations to “Ni + Cu (%) ≧ 1 / 4Si + 1 / 3Al (%)” or more together with Si and Al. . At this time, the lower limits of Ni and Cu do not need to be specified, and the upper limit is not restricted. If the above formula with Al is satisfied, good plating properties can be obtained. Therefore, pure Ni plating or pure Cu plating may be used. Moreover, in order to ensure plating property, it discovered that A layer needed 0.5 micrometer or more of single side | surfaces.
[0022]
The B layer side is determined from the viewpoint of moldability. The reason for limitation of each element is shown below. C is an austenite stabilizing element, which moves from the ferrite in the two-phase coexistence temperature range and the bainite transformation temperature range and concentrates in the austenite. As a result, chemically stabilized austenite remains 2 to 20% even after cooling to room temperature, and the formability is improved by transformation-induced plasticity. If C is less than 0.05%, it is difficult to secure 2% or more retained austenite, and the purpose cannot be achieved. Further, if the C concentration exceeds 0.2%, weldability is deteriorated, so it must be avoided.
[0023]
Si does not dissolve in cementite, but delays transformation from austenite at 350 to 600 ° C. by suppressing its precipitation. During this time, C concentration in the austenite is promoted, so that the chemical stability of the austenite is enhanced, transformation-induced plasticity is caused, and retained austenite contributing to good formability can be secured. If the amount of Si is less than 0.2%, the effect cannot be found. On the other hand, if the Si concentration is increased, the amount of Ni and Cu necessary for the A layer side to ensure the plating property is increased and the cost is increased, so 2.0% was made the upper limit.
[0024]
Mn is an austenite forming element and prevents austenite from decomposing into pearlite in the course of cooling to 350 to 600 ° C. after annealing in the two-phase coexisting temperature range. Therefore, residual austenite is formed in the metal structure after cooling to room temperature. To be included. If the addition is less than 0.2%, it is necessary to increase the cooling rate to such an extent that industrial control cannot be performed in order to suppress decomposition into pearlite. On the other hand, if it exceeds 2.5%, the band structure becomes prominent and the characteristics are deteriorated, and the spot welded portion tends to break in the nugget, which is not preferable.
[0025]
Al is also used as a deoxidizing material, and at the same time, it does not dissolve in cementite like Si, suppresses the precipitation of cementite during the holding at 350 to 600 ° C., and delays the progress of transformation. However, since the ferrite forming ability is stronger than Si, the transformation starts quickly, and even in the case of holding for a very short time, C is concentrated in the austenite from the annealing in the two-phase coexisting temperature range, and the chemical stability is increased. There is little martensite that deteriorates the formability in the metal structure after cooling to room temperature. For this reason, when coexisting with Si, changes in strength and elongation under holding conditions at 350 to 600 ° C. are small, and high strength and good press formability are easily obtained. Therefore, Al needs to be added in an amount of 0.01% or more. Further, “Si + 0.8Al” must be 0.4% or more together with Si. On the other hand, if the Al concentration exceeds 1.5%, the amount of Ni and Cu required for the A layer side to ensure the plating property as well as Si increases and the cost increases, so 1.5% is the upper limit. did.
[0026]
The ductility of the steel sheet of the present invention as the final product depends on the volume ratio of retained austenite contained on the B layer side. Residual austenite contained in the metal structure exists stably when it is not deformed, but when deformed, it transforms into martensite and exhibits transformation-induced plasticity, so that good formability is obtained with high strength. If the volume fraction of retained austenite is less than 2%, no clear effect is observed. On the other hand, if the volume fraction of retained austenite exceeds 20%, extremely severe molding may occur, and a large amount of martensite may exist in the press-formed state, causing problems in secondary workability and impact properties. Therefore, in the present invention, the volume ratio of retained austenite is set to 20% or less. In addition, the structure contains ferrite, bainite, martensite and carbide.
[0027]
Further, the ratio of the A layer and the B layer is determined from the viewpoints of formability, plating property, and cost. When the thickness of the A layer is lowered, the plating property is deteriorated. Therefore, the A layer is 0.5 μm or more on one side. The upper limit of the thickness of the A layer need not be specified, but if the ratio of the A layer is increased, the amount of Ni and Cu required increases, resulting in higher costs. The following is desirable. At this time, each component of the A layer and the B layer has an average composition, and there is no problem even if the composition has a distribution in the thickness direction. FIG. 1 shows an example of the distribution of components in the thickness direction in the present invention for Ni, Si, and Al.
The present invention uses the above A layer and B layer as basic components, but the characteristics are further improved by including one or more of the following components inevitably mixed on the B layer side.
[0028]
Ni, like Mn, is an austenite-forming element and at the same time improves strength and plating adhesion. Further, Ni does not dissolve in cementite like Si and Al, but suppresses precipitation of cementite during holding at 350 to 600 ° C., and delays the progress of transformation. Ni is a less oxidizable element than Fe and concentrates on the surface layer of the steel sheet to improve the plating property. However, since Ni becomes expensive, it was made less than 2%.
Cu, as well as Ni and Mn, is an austenite-forming element and at the same time improves strength and plating adhesion. If Cu is increased to 2.0% or more, Cu precipitates are generated and the material quality deteriorates. Therefore, the amount of Cu added is less than 2%.
[0029]
Sn improves strength and plating adhesion. However, if the Sn addition amount is increased, there is a concern that cracks occur during hot rolling, so the Sn addition amount is set to less than 1%.
Mo, Cr, V, B, Nb, and Ti are elements that increase the strength. Mo: less than 0.2%, Cr: less than 1%, V: less than 0.3%, B: less than 0.01%, Ti : Addition of at least one of Nb: less than 0.1% and Nb: less than 0.1% as necessary does not impair the spirit of the present invention. Since the effects of these elements are saturated at the above upper limit, addition of more elements increases the cost.
[0030]
The B layer of the present invention has the above components. In addition to these elements and Fe, Co, Zn, Mg, Ta, Te, Be, Ru, Os, Rh, Ir, Pd, Pt, Ag, Au, Cd, It contains elements inevitably mixed in other general steels such as Hg, Ge, Pb, Sb, Bi, Se, Te, and the present invention even if these elements contain, for example, 0.01% or less in total. It doesn't detract from the purpose. P and S are preferably less than 0.05% and less than 0.03%, respectively, from the viewpoint of weldability. These elements are inevitably mixed elements, but in the present invention, they are treated as incidental components, and if these conditions are satisfied, they are included in the scope of the present invention.
[0031]
In the present invention, a Zn plating layer or a Zn alloy plating layer is provided on the steel plate, which will be described in detail below.
The Zn plating layer contains Al: 1% or less, the balance Zn and unavoidable impurities. The reason why the Al content during plating is 1% or less is that when the Al content exceeds 1%, Al segregated during plating constitutes a local battery and the corrosion resistance deteriorates. The Zn alloy plating layer is composed of Fe: 8 to 15%, Al: 1% or less, the balance Zn and inevitable impurities. The reason why the Fe content in the plating layer is 8% or more is that if it is less than 8%, the chemical conversion treatment (phosphate treatment) coating film adhesion becomes good. Further, the reason why the Fe content is set to 15% or less is that if it exceeds 15%, it becomes an overalloy and the plating adhesion of the processed part deteriorates. The reason why the Al content during plating is 1% or less is that when the Al content exceeds 1%, the Al segregated during plating constitutes a local battery and the corrosion resistance deteriorates.
[0032]
In the present invention, the Zn plating layer and the Zn alloy plating layer in the present invention are as described above. In addition, in the present invention, other components such as Mn, Pb, Fe, Sb, Ni, Cu, Sn, Co, Cd, and Cr are inevitably mixed. Although defined as inevitable components, these elements may be included in the same manner as the incidental component elements. Moreover, since Mg and Ca have the effect of reducing the oxide on the surface layer of the steel sheet during Zn plating and improving the plating adhesion, they may be included as less than 8% and 1%, respectively, as incidental components. All of these elements are included within the scope of the present invention. The thickness of the Zn alloy plating layer is not particularly limited, but is preferably 0.1 μm or more from the viewpoint of corrosion resistance and 15 μm or less from the viewpoint of workability.
[0033]
Next, the manufacturing method of the hot dip galvanized steel sheet of the present invention and the galvannealed steel sheet of the present invention will be described.
In order to obtain the component composition as described above, the hot dip galvanized steel sheet of the present invention has Ni, Cu on the steel sheet surface layer before casting and solidifying at the time of casting solidification and before performing the Zn plating after cold rolling. And then annealed for 10 seconds to 10 minutes in a two-phase coexistence temperature range of 650 to 900 ° C., then cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, and in some cases, further within that range. After maintaining at a temperature range for 20 minutes or less, hot dip galvanization is performed, and then it is obtained by cooling to 250 ° C. or less at a cooling rate of 5 ° C./s or more.
[0034]
In addition, the alloyed hot-dip galvanized steel sheet of the present invention is formed on the surface layer of the steel sheet prior to annealing to perform Zn plating after cold rolling during casting solidification and hot rolling in order to obtain the above component composition. After containing Ni and Cu, and then annealing for 10 seconds to 10 minutes in a two-phase coexistence temperature range of 650 to 900 ° C., cooling to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s. Furthermore, after maintaining in the temperature range of the range for 20 minutes or less, hot dip galvanization is performed, and then the temperature range of 450 to 600 ° C. is maintained for 5 seconds to 2 minutes, and then a cooling rate of 5 ° C./s or more. At 250 ° C. or lower.
[0035]
The reason for each manufacturing condition will be described below.
The present inventors conducted a laboratory test to produce an A layer containing Ni and Cu. In the test, after using a powder containing Ni or Cu at the time of casting of carbon steel containing 0.1% C, 1.2% Si, 1.5% Mn, hot rolling, cold rolling, annealing, A plating test was performed and the cross section of the steel sheet was observed. As a result, it was possible to obtain a layer containing a large amount of Ni and Cu on the steel sheet surface layer by incorporating Ni and Cu in the powder.
[0036]
As a result of the investigation, when the Ni, Cu, Si, Al amount of the A layer on the surface layer side satisfies the relationship of Ni + Cu> 1 / 4Si + 1 / 3Al, and the A layer is 0.5 μm or more on one side, plating properties and press molding A high-strength plated steel sheet having excellent properties was obtained. Inclusion of Ni and Cu in the surface layer of the steel sheet at the time of casting can be further promoted by using electromagnetic stirring and electromagnetic braking, so it is desirable to use these. Further, the same effect can be obtained by using not only metal powder but also an alloy or oxide as the powder. At this time, the thickness of the A layer can be adjusted by adjusting the content of Ni and Cu in the powder. In view of the cost, the ratio of Ni or Cu in the powder is preferably less than 50% by weight.
[0037]
Moreover, after sprinkling Ni powder and Cu powder on the slab surface layer after casting of carbon steel containing 0.1% C, 1.2% Si, 1.5% Mn, hot rolling, cold rolling, annealing, A plating test was performed and the cross section of the steel sheet was observed. As a result, it is possible to obtain a layer containing a large amount of Ni and Cu on the surface layer of the steel sheet. Further, the amount of Ni, Cu, Si and Al of the A layer on the surface layer side satisfies the relationship of Ni + Cu> 1 / 4Si + 1 / 3Al. When the thickness was 0.5 μm or more on one side, a high-strength plated steel sheet having excellent plating properties and press formability was obtained. Here, Ni and Cu may be metal powder, an alloy containing Ni and Cu, or an oxide. The thickness of the A layer can be adjusted by adjusting these application amounts.
[0038]
Furthermore, after casting, hot rolling, cold rolling carbon steel containing 0.1% C, 1.2% Si, 1.5% Mn, after plating Ni or Cu on the surface layer by electroplating, annealing, A plating experiment was conducted, and the cross section of the steel sheet was observed. As a result, it is possible to obtain a layer containing a large amount of Ni and Cu on the surface layer of the steel sheet. Further, the amount of Ni, Cu, Si and Al of the A layer on the surface layer side satisfies the relationship of Ni + Cu> 1 / 4Si + 1 / 3Al. When the thickness was 0.5 μm or more on one side, a high-strength plated steel sheet having excellent plating properties and press formability was obtained. The plating of Ni and Cu does not necessarily need to be electroplating, and there is no problem even with displacement plating. Moreover, not only Ni and Cu but also these oxides or alloys are not changed as plating types. By adjusting the amount of plating, the thickness of the A layer can be adjusted.
[0039]
In the present invention, a high-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip zinc which are excellent in plating adhesion and press formability by performing annealing and hot-dip galvanizing after forming the A layer and the B layer by the above means Manufacture plated steel sheets. Annealing conditions and plating conditions are determined particularly from the viewpoint of press formability.
In the continuous annealing of the cold-rolled steel sheet after cold rolling, first, in order to obtain a two-phase structure of [ferrite + austenite], Ac₁Above the transformation point Ac_ThreeHeating is performed in a temperature range below the transformation point. At this time, if the heating temperature is less than 650 ° C., it takes too much time for the cementite to re-dissolve, and the amount of austenite is small, so the lower limit of the heating temperature is set to 650 ° C. Moreover, since the volume ratio of austenite will become large too much and the C density | concentration in austenite will fall when heating temperature is too high, the upper limit of heating temperature was 900 degreeC. If the soaking time is too short, there is a high possibility that undissolved carbide is present, and the austenite content is reduced. Further, if the soaking time is lengthened, there is a high possibility that the crystal grains become coarse and the balance of strength and ductility is deteriorated. Therefore, in the present invention, the holding time is set between 10 seconds and 10 minutes.
[0040]
After soaking, it is cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s. The purpose of this is to carry over the austenite produced by heating in the two-phase region to the bainite transformation region without transforming it to pearlite, and to obtain predetermined characteristics as retained austenite and bainite at room temperature by subsequent treatment. If the cooling rate at this time is less than 2 ° C./s, most of the austenite undergoes pearlite transformation during cooling, so that retained austenite cannot be secured. On the other hand, when the cooling rate exceeds 200 ° C./s, the end point temperature of cooling is greatly shifted in the width direction and the longitudinal direction, and a uniform steel sheet cannot be produced.
[0041]
Thereafter, in some cases, it may be kept within a range of 350 to 500 ° C. for 20 minutes or less. By maintaining the temperature before the Zn plating, the bainite transformation can be advanced to stabilize the C-concentrated retained austenite, and a steel sheet having both strength and elongation can be manufactured more stably. When the end point temperature of cooling from the two-phase region exceeds 500 ° C., if the temperature is maintained thereafter, austenite is decomposed into carbides and austenite cannot remain. Also, when the cooling end point temperature is less than 350 ° C., most of the austenite is transformed into martensite. However, although the strength becomes high, the press formability deteriorates, and the steel plate temperature needs to be raised during Zn plating. It becomes inefficient because it is necessary to give If the holding time exceeds 20 minutes, heating after Zn plating results in deterioration of both strength and press formability due to carbide precipitation and disappearance of untransformed austenite, so the holding time was set to 20 minutes or less.
[0042]
When manufacturing a hot dip galvanized steel sheet, after hot dip galvanization, it cools to 250 degrees C or less with the cooling rate of 5 degrees C / s or more. Here, bainite transformation proceeds during Zn plating, and bainite containing almost no carbide and C swept out from the portion are concentrated._SA structure in which retained austenite whose point has dropped to room temperature or lower and ferrite that has been cleaned during two-phase heating has been mixed is revealed to achieve both high strength and formability. Therefore, if the cooling rate after holding is 5 ° C. or lower, or if the end temperature of cooling is 250 ° C. or higher, C-concentrated austenite also precipitates carbides and decomposes into bainite. The amount of retained austenite that improves the content is reduced, so the objective cannot be achieved. In order to make residual austenite remain more, it is desirable to keep it within a temperature range of 350 to 400 ° C. within 5 minutes after hot dip galvanization.
[0043]
Moreover, when manufacturing an alloyed hot-dip galvanized steel sheet, after hot-dip galvanizing, hold at a temperature range of 450 to 600 ° C. for 5 seconds to 2 minutes, and then cool to 250 ° C. or less at a cooling rate of 5 ° C./s or more. To do. Here, it originates from the alloying reaction of Fe and Zn and a systematic viewpoint. In the steel of the present invention, since Si and Al are contained, the transformation from austenite to bainite is separated in two stages, bainite containing almost no carbide and C swept from the part are concentrated, and the Mn point is increased. A structure in which residual austenite lowered to room temperature or lower and ferrite that has been cleaned during two-phase heating has been mixed is revealed to achieve both high strength and formability.
[0044]
When the holding temperature exceeds 600 ° C., pearlite is generated, so that retained austenite is not included, and the alloying reaction proceeds too much, and the Fe concentration in the plating exceeds 15%. On the other hand, when the heating temperature is 450 ° C. or lower, the alloying reaction rate of the plating becomes slow, and the Fe concentration in the plating becomes low. In addition, when the holding time is 5 seconds or less, bainite is not sufficiently formed, and C concentration in untransformed austenite is insufficient, so that martensite is generated during cooling and formability deteriorates. The chemical reaction becomes insufficient.
[0045]
Further, if the holding time is 2 minutes or longer, plating is over-alloyed and plating peeling is likely to occur during molding. Furthermore, if the cooling rate after holding is set to 5 ° C. or lower, or the cooling end point temperature is set to 250 ° C. or higher, the bainite transformation further proceeds, and the austenite enriched with C in the preceding reaction also precipitates carbides and decomposes into bainite. The objective cannot be achieved because the amount of retained austenite that improves workability is reduced by transformation-induced plasticity.
The hot dip galvanizing temperature is preferably not lower than the melting point of the plating bath and not higher than 500 ° C. This is because when the temperature is 500 ° C. or higher, the vapor from the plating bath increases and the operability deteriorates. Moreover, it is not necessary to prescribe | regulate especially the heating rate to the holding temperature after plating, but 3 degree-C / s or more is desirable from a viewpoint of a plating structure or a metal structure.
[0046]
It should be noted that the temperatures and cooling temperatures in the processes described above do not have to be constant as long as they are within a specified range, and even if they fluctuate within the ranges, the characteristics of the final product may not be deteriorated and may be improved.
Further, in order to further improve the plating adhesion, the steel plate surface before plating may be cleaned by adjusting the atmosphere during annealing of the steel plate, first oxidizing the steel plate surface, and then reducing it. Furthermore, there is no problem even if the steel plate is pickled or ground before annealing to remove the oxide on the steel plate surface in order to improve the plating adhesion. By performing these treatments, the plating adhesion is further improved.
[0047]
【Example】
  Thickness of steel (B layer) whose components are shown in Table 1 by forming A layer containing Ni and Cu, annealing and plating under the conditions described in Table 2, and then temper rolling at 0.5% A 1 mm steel plate was produced.
  In order to form the A layer containing Ni and Cu, the following method was used. When Ni or Cu powder was used, powder containing 10% by mass of Ni and Cu was used. Moreover, Ni and Cu mixed powder used the powder which contained 10% by mass%, respectively. Ni and Cu spraying before hot rolling was performed by spraying Ni and Cu metal powder at room temperature on the slab surface layer after casting and before hot rolling..
[0048]
The manufactured steel sheets are shown in the following “tensile test”, “residual austenite measurement test”, “welding test”, “A layer concentration, A layer width and A layer to B layer ratio measurement”, “plating property”, “plating adhesion” The test of “concentration measurement in plating layer” was performed. Moreover, the amount of plating adhesion is 50g / m on one side.² I tried to become.
In the “tensile test”, a JIS No. 5 tensile test piece was collected, and a normal temperature tensile test was performed at a gauge thickness of 50 mm and a tensile speed of 10 mm / min.
“Residual austenite measurement test” is a method called a 5-peak method in which a 1/4 inner layer of the plate thickness is chemically polished from the surface layer, and is obtained from the strength of α-Fe and γ-Fe by X-ray diffraction using a Mo tube. It was measured.
[0049]
In the “welding test”, spot welding was performed under the welding conditions of welding current: 10 kA, pressing force: 220 kg, welding time: 12 cycles, electrode diameter: 6 mm, electrode shape: dome shape, tip 6φ-40R, and a nugget diameter of 4 The number of continuous hit points up to the time when √t (t: thickness) was cut was evaluated. The evaluation criteria were as follows: ◯: Over 1000 continuous hit points, Δ: Continuous hit points of 500 to 1000 points, and X: Continuous hit points of less than 500 points. Here, ○ was accepted and Δ · x was rejected.
"A layer density | concentration" was performed by the measurement by the EPMA analysis of the cross-section part depth direction of a plated steel plate. “Width of layer A” was obtained by EDS analysis with a TEM of a sample obtained by cutting the steel plate / plating interface portion of the plated steel plate by the FIB method. At the time of measurement, a standard curve was prepared using a standard sample.
[0050]
“Plating adhesion” was evaluated according to the following criteria by performing a tape test after a 60-degree V-bending test of the plated steel sheet.
Tape test blackness (%)
Evaluation: ◎ ... 0-10
Evaluation: ○ ... less than 10-20
Evaluation: △ ... Less than 20-30
Evaluation: ×… 30 or more
(◎ and ○ pass, △ ・ × fail)
“Measurement of concentration in plating layer” was measured by ICP emission spectrometry after dissolving the plating layer with 5% hydrochloric acid containing an amine-based inhibitor.
[0051]
  The performance evaluation test results are shown in Table 3. Samples 1 to 1 according to the present invention8Has a retained austenite of 2 to 20% and a total elongation of 30% or more while being 550 MPa or more, and has both high strength and good press formability, and at the same time satisfying plating properties and weldability. It is a plated steel plate and an galvannealed steel plate. In contrast, the sample9Has a low C concentration, so the sample10Has a high C concentration.11Does not satisfy the relationship between Si and Al in steel.12Has a high Mn concentration.23Does not satisfy the relationship of Ni, Cu, Si, and Al in the A layer, and therefore does not satisfy all of the retained austenite amount, high strength and press formability, plating properties, and weldability. The purpose cannot be achieved.
  In addition, even if the steel of the present invention has a problem in one of the processing conditions,13~23As described above, the amount of retained austenite, compatibility between high strength and press formability, plating properties and weldability are not all satisfied, and the object of the present invention cannot be achieved.
[0052]
[Table 1]

[0053]
[Table 2]

[0054]
[Table 3]

[0055]
【The invention's effect】
As described above, according to the present invention, a high-strength galvannealed steel sheet having good press formability and plating adhesion and the steel sheet can be efficiently produced.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a concentration distribution in the thickness direction of Ni, Cu, Si, and Al in the present invention.

Claims

% By mass
C: 0.05 to 0.2%
Si: 0.2-2.0%,
Mn: 0.2 to 2.5%
Al: 0.01~1.5%,
Ni: less than 2%,
Cu: less than 2%,
Sn: less than 1% , and the relationship between Si and Al is
0.4 (%) ≤ Si + 0.8 Al (%)
And the balance of Ni, Cu, Si, and Al is Ni + Cu (%) ≧% of the composition of the surface layer (A layer) in the cast slab (B layer) containing Fe inevitable impurities remaining. After using Ni or Cu-containing powder at the time of casting solidification so as to be 1 / 4Si + 1 / 3Al (%), hot rolling and cold rolling are performed for 10 seconds in a two-phase coexistence temperature range of 650 to 900 ° C. Annealing for 10 minutes, cooling to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, applying hot dip galvanizing, and then cooling to 250 ° C. or less at a cooling rate of 5 ° C./s or more A method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability.

% By mass
C: 0.05 to 0.2%
Si: 0.2-2.0%,
Mn: 0.2 to 2.5%
Al: 0.01~1.5%,
Ni: less than 2%,
Cu: less than 2%,
Sn: less than 1% , and the relationship between Si and Al is
0.4 (%) ≤ Si + 0.8 Al (%)
And the balance of Ni, Cu, Si, and Al is Ni + Cu (%) ≧% of the composition of the surface layer (A layer) in the cast slab (B layer) containing Fe inevitable impurities remaining. After using powder containing Ni or Cu at the time of casting solidification so as to be 1 / 4Si + 1 / 3Al (%), after hot rolling and cold rolling, 10 seconds in a two-phase coexistence temperature range of 650 to 900 ° C. Annealed for 10 minutes, cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, Fe: 8 to 15%, Al: 1% or less, and alloyed molten zinc composed of Zn containing the inevitable impurities remaining Plating adhesion, press molding, characterized in that plating is performed and held in a temperature range of 470 to 600 ° C. for 5 seconds to 2 minutes, and then cooled to 250 ° C. or less at a cooling rate of 5 ° C./s or more. production of high strength galvannealed steel sheet excellent in sex Law.

% By mass
C: 0.05 to 0.2%
Si: 0.2-2.0%,
Mn: 0.2 to 2.5%
Al: 0.01~1.5%,
Ni: less than 2%,
Cu: less than 2%,
Sn: less than 1% , and the relationship between Si and Al is
0.4 (%) ≤ Si + 0.8 Al (%)
And the balance of Ni, Cu, Si, and Al when the composition of the surface layer (A layer) is mass% when hot rolled after casting solidification of a slab made of Fe containing the balance inevitable impurities. However, after adding Ni or Cu containing compound or alloy iron to the casting slab surface so that Ni + Cu (%) ≥ 1/4 Si + 1/3 Al (%), after heating, rolling, cold rolling, After annealing for 10 seconds to 10 minutes in a two-phase coexistence temperature range of 650 to 900 ° C., the sample is cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, and hot dip galvanized, and then 5 ° C./s. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in plating adhesiveness and press formability characterized by cooling to 250 degrees C or less with the above cooling rate.

% By mass
C: 0.05 to 0.2%
Si: 0.2-2.0%,
Mn: 0.2 to 2.5%
Al: 0.01~1.5%,
Ni: less than 2%,
Cu: less than 2%,
Sn: less than 1% , and the relationship between Si and Al is
0.4 (%) ≤ Si + 0.8 Al (%)
And the balance of Ni, Cu, Si, and Al when the composition of the surface layer (A layer) is mass% when hot rolled after casting solidification of a slab made of Fe containing the balance inevitable impurities. However, after adding Ni or Cu containing compound or alloy iron to the casting slab surface so that Ni + Cu (%) ≥ 1/4 Si + 1/3 Al (%), after heating, rolling, cold rolling, After annealing for 10 seconds to 10 minutes in a two-phase coexisting temperature range of 650 to 900 ° C., the sample is cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s, Fe: 8 to 15%, Al: 1% or less And the alloyed hot dip galvanizing composed of Zn containing the inevitable impurities of the balance, and holding for 5 seconds to 2 minutes in a temperature range of 470 to 600 ° C., then to 250 ° C. or less at a cooling rate of 5 ° C./s or more. Plating adhesion and press, characterized by cooling Method of producing a high strength galvannealed steel sheet excellent in shape retention.

% By mass
In mass% of any one of Claims 1-4,
further,
Mo: less than 0.2%,
Cr: less than 1%,
V: less than 0.3%,
B: less than 0.01%,
Nb: less than 0.1%,
Ti: containing at least one of less than 0.1%, and the relationship between Si and Al
0.4 (%) ≤ Si + 0.8 Al (%)
A method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability, characterized in that it is a cast slab (B layer) made of Fe containing the remaining inevitable impurities.

After the cold rolling, annealing is performed for 10 seconds to 10 minutes in a two-phase coexisting temperature range of 650 to 900 ° C., and then cooled to 350 to 500 ° C. at a cooling rate of 2 to 200 ° C./s. The method for producing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion and press formability according to any one of claims 1 to 4, wherein the content is maintained for a minute or less.