JP3698046B2 - High-strength hot-dip galvanized steel sheet excellent in workability and plating property and method for producing the same - Google Patents
High-strength hot-dip galvanized steel sheet excellent in workability and plating property and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、自動車の内板、外板などとしての使途に好適な高強度溶融亜鉛めっき鋼板（高強度合金化溶融亜鉛めっき鋼板を含む）の製造方法に関するものである。
【０００２】
【従来の技術】
近年、自動車の安全性、軽量化および低燃費化、ひいては地球環境の改善の観点から、自動車用の鋼板として、高強度の溶融亜鉛めっき鋼板を適用する傾向が増加している。その中で、とくに高強度溶融亜鉛めっき鋼板を製造するためには、めっき性がよく、かつ溶融亜鉛めっき浴を通過し、あるいはさらに合金化処理が施された後で、所望の強度と加工性が得られなければならない。
【０００３】
一般に、鋼板の強度を増加させるには、Mn、Si、Ｐなどの固溶強化元素やTi、Nb、Ｖなどの析出強化元素を添加している。かかる元素を添加した鋼板を連続溶融亜鉛めっきライン（ＣＧＬ）で処理すると、亜鉛めっき性を劣化させることが知られている。
このように合金元素の含有量は、強度とめっき性の面で相反する作用を及ぼすので、連続溶融亜鉛めっきラインにて、めっき性がよい高強度溶融亜鉛めっき鋼板を製造することは極めて困難なものであった。また、高強度溶融亜鉛めっき鋼板は、一般に、伸びなどの加工性に関わる特性が劣るため、加工性のよい溶融亜鉛めっき鋼板を製造することは、さらに困難なことであった。
【０００４】
ところで、従来の加工性を高めた高強度鋼板としては、フェライト素地にマルテンサイトを主相とする低温変態相（残留オーステナイトも含む）を含む複合組織鋼板が知られている。この複合組織鋼板は常温非時効で降伏比が低く、加工性および加工後の焼付硬化性が優れており、（α＋γ）域温度で加熱後、水冷やガス冷却などで急冷することにより製造される。
【０００５】
【発明が解決しようとする課題】
しかしながら、この複合組織鋼板を、500 ℃程度の温度で溶融亜鉛めっき、あるいはさらに合金化処理した場合に、フェライト素地中に分散しているマルテンサイトが焼もどしされて、引張強さ、伸びは低下し、上降伏点が現れて降伏比の上昇、さらには降伏伸びの発生が起こってしまう。
焼もどし軟化は、Mn、Siなどの合金元素が少ないほど生じやすく、一方、これら合金元素が多い場合には、溶融亜鉛めっき性が低下してしまう。結局、複合組織鋼板においても、めっき工程でマルテンサイトが焼き戻しされるので、その特徴である加工性と高強度を両立させ、かつ良好なめっき性を発揮させることは、従来技術の下では困難であった。
【０００６】
そこで、本発明は、従来技術が抱えている上記問題を解決するために、連続溶融亜鉛めっきラインなどの設備を用いて溶融亜鉛めっきしても、良好な加工性と高強度をともに満たし、しかも良好なめっきが得られる高強度溶融亜鉛めっき鋼板の製造方法を提案することを目的とする。
本発明の具体的目的は、加工性と高強度を表す指標として、ＴＳ：590 MPa 以上、ＴＳ×Ｅｌの値： 15000 MPa・％以上を満たしつつ、良好なめっきが得られることにある。
【０００７】
【課題を解決するための手段】
発明者らは、上記課題の解決に向けて鋭意研究した結果、Mo、Crを添加せずとも、また、残留オーステナイト相と焼もどしマルテンサイト相を含まなくても、加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板を製造しうることを知見し、本発明を完成するに至った。
【０００８】
すなわち、本発明は、以下の構成を要旨とするものである。
(1) 質量％で、
Ｃ：0.01〜0.20％、 Si：1.0 ％以下、
Mn：1.5 超〜3.0 ％、 Ｐ：0.10％以下、
Ｓ：0.05％以下、 Al：0.10％以下、
Ｎ：0.010 ％以下を含み、かつ
Ti、NbおよびＶから選ばれるいずれか１種または２種以上を合計で、0.010 〜1.0 ％含有し、残部はFeおよび不可避的不純物の組成からなるとともに、フェライト相の面積率が50％以上、かつフェライト相の平均結晶粒径が10μｍ以下であって、第２相からなるバンド状組織の厚みが、Ｔｂ／Ｔ≦0.005 （ただし、Ｔｂ：バンド状組織の板厚方向平均厚み、Ｔ：鋼板板厚）の関係を満たす金属組織を有することを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板。
【００１０】
(2) 上記(1)において、鋼組成がさらにCaおよびREMのうちの１種または２種を合計で0.001〜0.01％含有する組成からなることを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板。
【００１１】
(3) 上記(1) または (2)のいずれか１項に記載の鋼組成からなるスラブを、熱間圧延して、750〜450℃で巻き取り、次いで、そのまま或いはさらに冷間圧延を行い、得られた熱延板または冷延板を、750℃以上に加熱し、この温度からの冷却途中で溶融亜鉛めっきを行うことを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１２】
(4) 上記(1) または (2)のいずれか１項に記載の鋼組成からなるスラブを、熱間圧延して、750〜450℃で巻き取り、次いで、そのまま或いはさらに冷間圧延を行い、得られた熱延板または冷延板を、750℃以上に加熱し、この温度からの冷却途中で溶融亜鉛めっきを行い、次いで合金化処理を行うことを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１３】
(5) 上記(1) または (2)のいずれか１項に記載の鋼組成からなるスラブを、熱間圧延して、750〜450℃で巻き取り、次いで、そのまま或いはさらに冷間圧延を行い、得られた熱延板または冷延板を、一旦750℃以上に加熱し、冷却してから酸洗し、さらに700℃以上に加熱して、この温度からの冷却途中で溶融亜鉛めっきを行うことを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１４】
(6) 上記(1) または (2)のいずれか１項に記載の鋼組成からなるスラブを、熱間圧延して、750〜450℃で巻き取り、次いで、そのまま或いはさらに冷間圧延を行い、得られた熱延板または冷延板を、一旦750℃以上に加熱し、冷却してから酸洗し、さらに700℃以上に加熱して、この温度からの冷却途中で溶融亜鉛めっきを行い、次いで合金化処理を行うことを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
(7) 上記 (5) または (6) において、前記鋼組成が、さらに Cu および Ni のうちの１種または２種を合計で 3.0 ％以下含有する組成からなることを特徴とする加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１５】
【発明の実施の形態】
これらの発明では、以下の (a) (b)の効果が相乗的に発揮され、Mn、Siを大量に添加することなく、かつ強化元素として、Mo、Crを添加せずとも、また、組織的に残留オーステナイト相と焼もどしマルテンサイト相を含まなくても、冷却前のγ粒を微細にすることができ、このため、α相→γ相へのＣ、Mnの濃化が促進され、γ相を有効にマルテンサイト化し、加工性およびめっき性に優れた高強度溶融亜鉛めっき鋼板を製造可能とするものである。
(a) Ti、NbおよびＶから選ばれるいずれか１種または２種以上の添加で生成する、TiＣ、NbＣ、ＶＣなどの炭化物による結晶粒界移動のピン止め作用により、α（フェライト）結晶粒を１０μｍ以下に微細化できる。このため、これをさらにめっき前の工程で加熱するときに、α（フェライト）＋γ（オーステナイト）の２相域中で生成、成長するγ粒、あるいはγ（オーステナイト）単相域でのγ粒の粗大化が抑制される効果。
(b) 加熱前から存在し、Ｃ、Mnを多量に含有した第２相からなるバンド状組織の厚みが、Ｔｂ／Ｔ≦0.005 （ただし、Ｔｂ：バンド状組織の板厚方向平均厚み、Ｔ：鋼板板厚）の関係を満たすように分散させる加熱の効果。
本発明においては、こうした相乗効果によって、めっき性に有害なCrをほとんど含有させる必要がないのでめっき性が極めて良好であり、また、Moを添加していないので、加熱前に存在するバンド状組織の厚みが比較的薄く、めっき性の観点から不利な１回ＣＧＬ（溶融亜鉛めっき）法のときに、さほどの高温加熱を施さなくても加工性の良好な高強度溶融亜鉛めっき鋼板を製造できる。
【００１６】
次に、本発明の基になった実験結果について説明する。
（実験１）
化学組成が0.08％Ｃ−0.01％Si−1.9 ％Mn−0.011 ％Ｐ−0.002 ％Ｓ−0.04％Al−0.0022％Ｎ−0.02％Ti−0.05％Nbで厚み30ｍｍのシートバーを、1200℃に加熱し、５パスで厚さ2.8 ｍｍの熱延板とした。その後、巻取り温度（ＣＴ）相当処理として 400℃, 650 ℃で各１hrの熱処理を行った。次いで、酸洗後、冷間圧延して1.4 ｍｍの冷延板とし、 700℃〜850 ℃に１分間加熱保持し、10℃／ｓの速度で 500℃まで冷却して、溶融亜鉛めっき後、40ｓ間保持後、10℃／ｓの速度で 550℃まで加熱して合金化処理し、ただちに10℃／ｓの速度で室温まで冷却した。その後、圧下率1.0 ％の調質圧延を行った。
得られた溶融亜鉛めっき鋼板について、ＪＩＳ５号引張試験片により引張特性（ＴＳ、ＹＳ、Ｅｌ）を調査するとともに、めっき性を調べた。めっき性は次の基準で評価した。
○：不めっき欠陥なし（めっき性良好）
△：不めっき欠陥一部発生（めっき性やや良好）
×：不めっき欠陥多数発生（めっき性不良）
得られた結果を図１に示す。図１から、巻取り温度が650 ℃で、めっき前の加熱温度が750 ℃以上であるとき、ＴＳ：590 MPa 以上、Ｅｌ：25％以上を達成できることがわかる。
【００１７】
（実験２）
実験１と同じ成分でＣＴ相当処理を 400℃〜700 ℃に変化させた板厚1.6 mmの冷延板を用い、750 ℃に１分間保持（１回目加熱）を行い、10℃／ｓの速度で室温まで冷却して、酸洗後、750 ℃に１分間保持（２回目加熱）し、10℃／ｓの速度で 500℃まで冷却して、溶融亜鉛めっきし、40ｓ間保持後、10℃／ｓで 550℃まで加熱して合金化処理し、ただちに10℃／ｓの速度で室温まで冷却した。その後、圧下率1.0 ％の調質圧延を行った。
得られた溶融亜鉛めっき鋼板について、実験１と同様にして、引張特性とめっき性を調べた。その結果、２回の加熱（１回目加熱と２回目加熱）を行った場合（図２の○）には、図２に示すように、引張特性、めっき性ともに、１回の加熱だけの実験１と同様の実験（図２の●）の場合よりもさらに改善できることがわかる。
【００１８】
上記各実験から、高Mn含有量により高強度化した場合であっても、高温巻取り、めっき前の高温加熱あるいは２回の加熱処理により、めっき性や機械特性が改善されることがわかった。このような効果が得られる理由として、以下のようなことが考えられる。すなわち、高温巻取り、２回の加熱処理は、鋼板直下に内部酸化層を生成し、これがめっき性に有害なMnの鋼板表面への濃化を抑制すること、また、一度高温加熱して生成しためっき性に有害なMnの表面濃化層が、２回目加熱の前に酸洗で除去されること、めっき前の高温加熱は、Ｃ、Mn濃度の高いバンド組織を溶解・分散し、マルテンサイトなどの第２相の生成に有利に作用すること等が考えられる。
【００１９】
次に、本発明において成分組成および製造条件を上記範囲に限定した理由について説明する。 (成分組成は質量％で表す)
Ｃ：0.01〜0.20％
Ｃは鋼の重要な基本成分の一つであり、とくに本発明では、Ti、NbおよびＶの炭化物を析出して強度上昇に寄与するほか、低温で生成するベイナイト相、マルテンサイト相を通じて強度の向上に寄与する元素である。Ｃ量が、0.01％未満では、上記析出物はもちろん、ベイナイト相、マルテンサイト相も生成しにくく、一方、0.20％超ではスポット溶接性が劣化することから、その含有範囲を0.01〜0.20％とする。なお、好ましいＣ量は0.03〜0.15％である。
【００２０】
Si：1.0 ％以下
Siはα相中の固溶Ｃ量を減少させることにより、伸びなどの加工性を向上させる元素であるが、1.0 ％超のSi量の含有はスポット溶接性およびめっき性を損ねるので上限を1.0 ％とする。なお、好ましいSi量は0.5 ％以下である。また、Si量を0.005 ％未満まで低下させるにはコストの上昇を伴うので、下限は0.005 ％とするのが望ましい。
【００２１】
Mn：1.5 超〜3.0 ％
Mnは本発明における重要成分の一つであり、複合組織においては変態を抑制し、γ相を安定化させる元素である。しかし、1.5 ％以下の含有ではその効果がなく、一方、3.0 ％超えるとスポット溶接性およびめっき性を著しく損なう。よって、Mnは1.5 超〜3.0 ％、好ましくは1.6 〜2.5 ％の範囲で含有させる。
【００２２】
Ｐ：0.10％以下
Ｐは高強度化を安価に達成するうえで有効な元素であるが、0.1 ％を超えて含有するとスポット溶接性を著しく損なうので上限を0.10％とする。なお、Ｐ量は0.05％以下に抑えるのが望ましい。また、Ｐ量を0.001 ％未満まで低下させるにはコストの上昇を伴うので、下限は0.001 ％に止めるのが望ましい。
【００２３】
Ｓ：0.05％以下
Ｓは熱延時の熱間割れを引き起こす原因になるほか、スポット溶接部のナゲット内破断を誘発するので、極力低減するのが望ましい。よって、本発明では上限を0.05％以下とする。なお、0.010 ％以下に抑制するのがより好ましい。また、Ｓ量を0.0005％未満まで低下させるにはコストの上昇を伴うので、下限は0.0005％に止めるのが望ましい。
【００２４】
Al：0.10％以下
Alは製鋼段階での脱酸剤として、また時効劣化を引き起こすＮをAlＮとして固定する有効な元素である。しかし、0.10％超えて含有すると製造コストの上昇を招くので、Al量は0.10％以下に抑える必要がある。なお、好ましい含有量は0.050 ％以下である。また、Al量が0.005 ％未満では脱酸が不十分になりやすいので、下限は0.005 ％とするのが望ましい。
【００２５】
Ｎ：0.010 ％以下
Ｎは時効劣化をもたらすほか、降伏点（降伏比）の上昇、降伏伸びの発生を招くことから、0.010 ％以下に抑制する必要がある。なお、好ましいＮ量は0.0050％以下である。また、Ｎ量を0.0005％未満まで低下させるにはコストの上昇を伴うので、下限は0.0005％に止めるのが望ましい。
【００２６】
Ti、NbおよびＶ：合計で0.01〜1.0 ％
Ti、NbおよびＶは炭化物を形成し、鋼を高強度化するのに有効な元素であり、１種または２種以上を合計で0.01〜1.0 ％を含有させる。これらの元素は合計量で0.01％以上の含有で上記効果が得られるが、1.0 ％を超えて含有するとコスト上の不利を招くほか、微細析出物が多くなりすぎて、冷延後の回復・再結晶を抑制し、延性（伸び）を低下させる。よって、これらの元素は合計量で0.01〜1.0％、好ましくは0.010 〜0.20％の範囲で含有させる。
【００２７】
Cu、Ni：合計で3.0 ％以下
Cu、Niはマルテンサイトなどの第２相を形成し、鋼を高強度化するのに有用な元素であり、必要に応じて添加する。しかしながら、合計量で3.0 ％を超えて含有すると、コスト高となるだけでなく、降伏点を低下させるので、高降伏比が求められるときには不利となる。このため、Cu、Niの含有量は合計で3.0 ％以下の範囲で含有させる。なお、好ましい含有範囲は、合計量で0.010 〜3.0 ％の範囲である。
【００２８】
Ca、REM ：合計で0.001 〜0.01％
Ca、REM は、介在物および硫化物の形態を制御し、穴拡げ性を改善する効果を有するので、合計で0.001 ％以上含有させるのが好ましい。しかし、合計で0.01％を超えて含有しても効果が飽和しコスト高を招く。このため、CaおよびREM の含有量は合計で0.001 〜0.01％とする。なお、より好ましい含有範囲は、合計量で0.002 〜0.005 ％である。
【００２９】
フェライト相：面積率で50％以上
本発明は高度な加工性が要求される自動車用鋼板を対象としており、フェライト相が面積率で50％未満では必要な延性、伸びフランジ性を確保することが困難となる。なお、さらに良好な延性が要求される場合には、面積率で75％以上のフェライト分率とすることが望ましい。フェライトとしては、いわゆるフェライトのみでなく、炭化物の析出を含まないベイニティックフェライト、アシキュラーフェライトも含むものとする。
フェライト相の観察方法および評価方法は、鋼板の断面が観察面になるように樹脂に埋め込み「純水100 mlに対してピロ亜硫酸ナトリウム１ｇを添加した水溶液」と「エタノール100 mlに対してピクリン酸４ｇを添加した液」を１：１の割合で混合した液中に、室温で120 秒間浸漬してエッチングし、フェライト相（黒色部）と第２相（白色部）とに分離し、倍率1000倍の画像解析装置にて、フェライトの面積率を求めた。
【００３０】
フェライト相の平均結晶粒径：10μｍ(0.01mm)以下
焼鈍でα＋γの２相域に加熱した際に、フェライト粒径が10μｍ超の大きさでは、フェライト粒界から生成するオーステナイト粒は自ずと大きくなってしまう。当然、この大きなオーステナイト粒は冷却中に比較的大きなマルテンサイトやベーナイトなどの第２相に変態し、割れの起点となって穴拡げ性を低下させてしまう。よって、本発明では、第２相を微細化し、穴拡げ性の向上をはかるためにフェライト粒径を10μｍ以下とした。
ここで、平均結晶粒径は断面組織写真からＡＳＴＭに規定された求積法により算出した値と、同じく切断法により求めた公称粒径（例えば、梅本ら：熱処理24（1984）334 に解説あり）のより大きい方を採用する。また、本発明では第２相の種類（マルテンサイト、ベーナイト、パーライト、セメンタイトなど）については特に限定する必要がない。
【００３１】
バンド状組織：Ｔｂ／Ｔ≦0.005 の厚み
バンド状組織は、Ｃ、Mn量の多い鋼において、主にスラブの冷却段階で結晶粒界に沿って凝集したＣ、Mnの濃化層が、熱延時あるいはその後の冷却時に引き延ばされて、圧延方向、板幅方向に列状、層状をなして形成した第２相群からなるものである。これらバンド状組織の平均厚みＴｂと板厚Ｔの比Ｔｂ／Ｔを0.005以下とする理由は、本発明のようにMn含有量が多い場合、熱延板の組織中にＣ、Mnを主成分とするバンド状の第２相組織が厚くなり、フェライト素地中に硬質なマルテンサイトを均一に分散させた高強度鋼板を製造しにくくなるからである。よって、効率よく高強度鋼板を製造するためには、バンド状の第２相中に濃化しているＣ、Mnを分散しておく必要があり、その目安となるのがバンド状組織の平均厚みＴｂと板厚Ｔの比であり、Ｔｂ／Ｔ≦0.005 であれば良好な結果が得られるからである。
バンド状組織の平均厚み：Ｔｂは、鋼板の断面が観察面になるように樹脂に埋め込み、３％ナイタール液中に室温で１５秒間浸漬してエッチングし、倍率1000倍の画像解析装置にて、列状、層状の第２相組織２０点についてそれぞれの厚みを測定し、２０点の平均値から求めた。
【００３２】
次に、本発明における製造条件について述べる。
以上に述べた成分組成からなる鋼スラブを常法にしたがい熱間圧延し、750 〜450 ℃で巻き取る。巻取温度が450 ℃未満では、TiＣ, NbＣなどの炭化物が生成しにくく、強度不足になりやすい。また、鋼板の表面直下に内部酸化層を形成しにくく、鋼板表面でのMn濃化を抑制できなくなるからである。一方、750 ℃を超えて巻き取ると、スケール厚みが厚くなり酸洗効率が悪くなる他、コイル長手方向の先端部、中央部、後端部、およびコイル幅方向のエッジ部、中央部の間で材質変動が大きくなるからである。なお、好ましい巻取温度は700 〜550 ℃である。
【００３３】
この熱延板を必要により酸洗して脱スケールを行い、熱延のまま、或いはさらに冷間圧延した後、連続溶融亜鉛めっきラインにて750 ℃以上に加熱、冷却し、冷却途中で溶融亜鉛めっきを行う。
また、２回の加熱を行う場合には、先ずはじめに連続焼鈍設備等で750 ℃以上に加熱（１回目加熱）、冷却したのち、次に連続溶融亜鉛めっきラインにて700℃以上に加熱（２回目加熱）、冷却し、冷却途中、好ましくは600 〜 420℃で溶融亜鉛めっきを行う。
【００３４】
めっき前に、一旦、750 ℃以上の温度域（好ましくは、750 〜900 ℃）に加熱して冷却することによって、バンド状組織中に濃化しているMn等を分散させ、効率よくフェライト＋マルテンサイトの複合組織を形成させて、加工性の向上をはかることが可能になる。すなわち、本発明のようにMn含有量が多い場合、熱延板中にはバンド状をなした第２相組織が形成されやすく、γ相中のMn等の濃度が低下して複合組織形成に不利になる。そこで、このバンド状組織の厚みを薄くし、細かく分散させておけば、連続溶融亜鉛めっきラインのめっき過程、あるいはさらに合金化処理過程などで500 ℃近傍に保持された場合に、γ相中のMn等の濃化量が増すので、フェライト素地中にマルテンサイト相を好適に分散させることが可能になるのである。
【００３５】
また、２回の加熱を行う場合の２回目加熱は700 ℃以上で行う。２回目加熱は必然的に連続溶融亜鉛めっきラインで行うことになる。２回目加熱温度が700 ℃に満たないと、連続溶融亜鉛めっきラインにおいて鋼板表面が還元されず、めっき不良を生じやすくなる。この２回目加熱温度は、好ましくは750 〜800 ℃の範囲がよい。なお、２回の加熱を行う場合には、１回目加熱で生成したMn等の表面濃化層を除去し、その後のめっき性を高めるために酸洗するのが望ましい。
以上の加熱工程を経てから、溶融亜鉛めっきを行い、場合によっては、溶融亜鉛めっきを行った後、引き続き合金化処理を行ってもよい。
【００３６】
実施例１
表１に示す化学組成で、厚さ300 ｍｍの連続鋳造スラブを、1200℃に加熱し、３パスの粗圧延後、７スタンドの仕上げ圧延機で厚さ2.5 ｍｍの熱延板として巻き取った。酸洗後、熱延板のまま、または熱延板をさらに板厚1.2 ｍｍに冷延後、 (1)連続焼鈍ラインでの１回目加熱−酸洗−連続溶融亜鉛めっきラインでの２回目加熱−亜鉛めっき、または (2)連続溶融亜鉛めっきラインでの加熱−亜鉛めっき、の工程でめっきし、さらに一部分から採取したサンプルについては合金化処理した。これらの製造条件を表２および表３に示す。
【００３７】
【表１】
なお、加熱後のＣＧＬ条件としては、加熱〜めっきまでの鋼板の平均冷却速度を10℃／ｓとし、めっき浴（浴組成：0.15％Al−Zn、浴温： 470℃）に浸漬（浸漬時間：１秒間）したのち、ガスワイピングにより６０ｇ／ｍ２の目付量に調整した。その後、490 ℃まで加熱し、２０秒間保持したのち、平均冷却速度を20℃／ｓで200 ℃まで冷却した。
得られた鋼板を供試材として、機械的特性、めっき性、スポット溶接性などについて調査した。その結果を表２および表３に併せて示す。
【００３９】
ここで、機械的特性、めっき性、合金化処理性、スポット溶接性は以下の方法で評価した。
・機械的特性（引張試験、穴拡げ試験により調査）
鋼板より圧延直角方向に採取したＪＩＳ Ｚ ２２０４に規定の５号試験片を用い、ＪＩＳ Ｚ ２２４１に規定の方法で降伏強さ（ＹＳ）、引張強さ（ＴＳ）、破断伸び（Ｅｌ）、降伏伸び（ＹＥｌ）を測定した。
伸びフランジ性は、ＪＦＳ Ｔ １００１に規定の方法により、穴拡げ率（λ）を測定した。
・めっき性
良好：不めっき欠陥なし
やや良好：不めっき欠陥一部発生
不良：不めっき欠陥多数発生
・合金化処理性
良好：合金化ムラの全くないもの
やや良好：わずかに合金化ムラのあるもの
不良：合金化ムラの著しいもの
・スポット溶接性
スポット溶接は、溶接電極：ドーム型先端径６φ、電極加圧力：3.10 kN 、溶接電流：７kA、加圧時間：25 cyc、セットアップ時間：3 cyc 、溶接時間：13 cyc、保持時間：25 cycの溶接条件で行った。溶接後、ＪＩＳ Ｚ ３１３６の引張剪断試験による引張荷重（ＴＳＳ）と、ＪＩＳ Ｚ ３１３７の十字型引張試験による引張荷重（ＣＴＳ）を負荷し、板厚1.2 ｍｍの場合の基準引張剪断荷重である8787Ｎ以上で、かつ延性比（ＣＴＳ／ＴＳＳ）が0.25以上のものを「優」、これらに値を満たさないものを「劣」として評価した。
【００４０】
表１〜表３から、発明例は、ＴＳ：590 〜690MPaレベルで、Ｅｌ：25％以上の引張特性を有し、ＴＳ×Ｅｌの値： 15000 MPa・％以上でＴＳ×Ｅｌバランスも良好であり、めっき性、合金化処理性、スポット溶接性についてもとくに問題がないことがわかった。
【００４１】
【表２】
【表３】
実施例２
表４に示す化学組成で、厚さ300 ｍｍの連続鋳造スラブを、1200℃に加熱し、３パスの粗圧延後、７スタンドの仕上げ圧延機で厚さ3.0 ｍｍの熱延板として表５に示す温度で巻き取った。酸洗後、熱延板のまま、または熱延板をさらに板厚1.2 ｍｍに冷延後、 (1)連続焼鈍ラインでの１回目加熱−酸洗−連続溶融亜鉛めっきラインでの２回目加熱−亜鉛めっき、または (2)連続溶融亜鉛めっきラインでの加熱−亜鉛めっき、の工程でめっきし、さらに一部分から採取したサンプルを合金化処理した。これらの製造条件を表５に示す。
なお、加熱後のＣＧＬ条件としては、加熱〜めっきまでの鋼板の平均冷却速度を10℃／ｓとし、めっき浴（浴組成：0.15％Al−Zn、浴温： 470℃）に浸漬（浸漬時間：１秒間）したのち、ガスワイピングにより６０ｇ／ｍ２の目付量に調整した。その後、490 ℃まで加熱し、２０秒間保持したのち、平均冷却速度を20℃／ｓで200 ℃まで冷却した。
得られた鋼板を供試材として、機械的特性、めっき性、スポット溶接性などについて同様にして調査した。その結果を表５に併せて示す。
その結果、発明例は、ＴＳ×Ｅｌバランスが良好であり、高強度であるにもかかわらず、めっき性、合金化処理性、スポット溶接性について何ら問題がないことがわかった。
【００４４】
【表４】
【表５】
実施例３
表６に示す化学組成で、厚さ300 ｍｍの連続鋳造スラブを、1200℃に加熱し、３パスの粗圧延後、７スタンドの仕上げ圧延機で厚さ3.0 ｍｍの熱延板として表７に示す温度で巻き取った。酸洗ののち、板厚1.2 ｍｍに冷延し、連続焼鈍ラインでの１回目加熱−酸洗−連続溶融亜鉛めっきラインでの２回目加熱−亜鉛めっきの工程でめっきし、さらに合金化処理を行った。これらの製造条件を表７に示す。
なお、加熱後のＣＧＬ条件としては、加熱〜めっきまでの鋼板の平均冷却速度を10℃／ｓとし、めっき浴（浴組成：0.15％Al−Zn、浴温： 470℃）に浸漬（浸漬時間：１秒間）したのち、ガスワイピングにより６０ｇ／ｍ２の目付量に調整した。その後、490 ℃まで加熱し、２０秒間保持したのち、平均冷却速度を20℃／ｓで200 ℃まで冷却した。
得られた鋼板を供試材として、機械的特性、めっき性、スポット溶接性などについて同様にして調査した。その結果を表７に併せて示す。
その結果、発明例は、ＴＳ×Ｅｌバランスが良好であり、高強度であるにもかかわらず、めっき性、合金化処理性、スポット溶接性について何ら問題がないことがわかった。
【００４７】
【表６】
【表７】
【発明の効果】
以上説明したように本発明によれば、めっき性に何ら問題のない、降伏比が低く、ＴＳ×Ｅｌバランスが良好な高強度溶融亜鉛めっき鋼板（高強度合金化溶融亜鉛めっき鋼板を含む）を提供することが可能になる。したがって、この発明は、自動車の軽量化、低燃費化を可能とするので、地球環境の改善にも大きく貢献する。なお、本発明は、めっきなしの熱延鋼板や冷延鋼板、また電気亜鉛めっき鋼板にも適用することができ、同様な効果が期待できる。
【図面の簡単な説明】
【図１】引張強さ（ＴＳ）、降伏強さ（ＹＳ）、伸び（Ｅｌ）及びめっき性に及ぼす連続溶融亜鉛めっきラインにおける加熱温度の影響を示すグラフである。
【図２】引張強さ（ＴＳ）、降伏強さ（ＹＳ）、伸び（Ｅｌ）及びめっき性に及ぼす巻取温度および２回の加熱有無の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet (including a high-strength alloyed hot-dip galvanized steel sheet) suitable for use as an inner plate or an outer plate of an automobile.
[0002]
[Prior art]
In recent years, there has been an increasing tendency to apply high-strength hot-dip galvanized steel sheets as steel sheets for automobiles from the viewpoints of safety, weight reduction, fuel consumption reduction, and improvement of the global environment. In particular, in order to produce a high-strength hot-dip galvanized steel sheet, the desired strength and workability are obtained after having good plating properties and passing through a hot-dip galvanizing bath or further alloying treatment. Must be obtained.
[0003]
Generally, in order to increase the strength of a steel sheet, solid solution strengthening elements such as Mn, Si, and P and precipitation strengthening elements such as Ti, Nb, and V are added. It is known that when a steel sheet to which such an element is added is processed in a continuous hot dip galvanizing line (CGL), the galvanizing property is deteriorated.
As described above, the alloying element content has a conflicting effect in terms of strength and plating properties, so it is extremely difficult to produce a high-strength hot-dip galvanized steel sheet with good plating properties in a continuous hot-dip galvanizing line. It was a thing. Further, since high strength hot dip galvanized steel sheets generally have poor workability characteristics such as elongation, it is more difficult to manufacture hot dip galvanized steel sheets with good workability.
[0004]
By the way, as a conventional high-strength steel sheet with improved workability, a composite structure steel sheet containing a low-temperature transformation phase (including residual austenite) whose main phase is martensite in a ferrite base is known. This composite steel sheet is non-aging at room temperature, has a low yield ratio, has excellent workability and bake hardenability after processing, and is manufactured by heating at (α + γ) range temperature and then rapidly cooling with water cooling or gas cooling. .
[0005]
[Problems to be solved by the invention]
However, when this composite steel sheet is hot dip galvanized or further alloyed at a temperature of about 500 ° C, the martensite dispersed in the ferrite base is tempered, and the tensile strength and elongation decrease. However, the upper yield point appears, resulting in an increase in yield ratio and further yield elongation.
Temper softening tends to occur as the number of alloy elements such as Mn and Si decreases. On the other hand, when the number of these alloy elements is large, the hot dip galvanizing property is lowered. Eventually, martensite is also tempered in the steel plate in the composite structure, so it is difficult to achieve both good workability and high strength, which are the characteristics of the steel plate, and to exhibit good plating properties. Met.
[0006]
Therefore, the present invention satisfies both good workability and high strength even when hot dip galvanizing is performed using equipment such as a continuous hot dip galvanizing line in order to solve the above-described problems of the prior art. It aims at proposing the manufacturing method of the high intensity | strength hot-dip galvanized steel plate from which favorable plating is obtained.
A specific object of the present invention is to obtain satisfactory plating while satisfying TS: 590 MPa or more and TS × El value: 15000 MPa ·% or more as indices representing workability and high strength.
[0007]
[Means for Solving the Problems]
As a result of intensive research aimed at solving the above-mentioned problems, the inventors have excellent workability and plating properties without adding Mo and Cr, and without containing a retained austenite phase and a tempered martensite phase. The present inventors have found that a high-strength hot-dip galvanized steel sheet can be produced, and have completed the present invention.
[0008]
That is, the gist of the present invention is as follows.
(1) By mass%
C: 0.01 to 0.20%, Si: 1.0% or less,
Mn: more than 1.5 to 3.0%, P: 0.10% or less,
S: 0.05% or less, Al: 0.10% or less,
N: Including 0.010% or less, and
One or two or more selected from Ti, Nb and V are contained in a total amount of 0.010 to 1.0%, the balance is composed of Fe and inevitable impurities, and the ferrite phase area ratio is 50% or more. And the average crystal grain size of the ferrite phase is 10 μm or less, and the thickness of the band-like structure composed of the second phase is Tb / T ≦ 0.005 (where Tb: the thickness in the thickness direction of the band-like structure, T: steel plate A high-strength hot-dip galvanized steel sheet excellent in workability and plating properties, characterized by having a metal structure satisfying the relationship of (sheet thickness).
[0010]
(2) the above(1)The steel composition further comprises a composition containing 0.001 to 0.01% of one or two of Ca and REM in total, and is a high-strength hot-dip galvanized steel sheet excellent in workability and plating properties.
[0011]
(3) the above(1) Or (2)A hot-rolled sheet or a cold-rolled sheet obtained by hot-rolling a slab having the steel composition described in any one of the above items and winding it at 750 to 450 ° C. Is heated to 750 ° C. or higher, and hot dip galvanizing is performed during cooling from this temperature. A method for producing a high-strength hot dip galvanized steel sheet excellent in workability and plating properties.
[0012]
(Four) the above(1) Or (2)A hot-rolled sheet or a cold-rolled sheet obtained by hot-rolling a slab having the steel composition described in any one of the above items and winding it at 750 to 450 ° C. Is heated to 750 ° C. or higher, hot-dip galvanized during cooling from this temperature, and then alloyed, and a method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and plating properties .
[0013]
(Five) the above(1) Or (2)A hot-rolled sheet or a cold-rolled sheet obtained by hot-rolling a slab having the steel composition described in any one of the above items and winding it at 750 to 450 ° C. Once heated above 750 ° C and cooledPicklingA method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and plating properties, characterized by further heating to 700 ° C. or higher and performing hot dip galvanizing during cooling from this temperature.
[0014]
(6) the above(1) Or (2)A hot-rolled sheet or a cold-rolled sheet obtained by hot-rolling a slab having the steel composition described in any one of the above items and winding it at 750 to 450 ° C. Once heated above 750 ° C and cooledPicklingFurthermore, heating to 700 ° C or higher, hot-dip galvanizing in the middle of cooling from this temperature, followed by alloying treatment, production of high-strength hot-dip galvanized steel sheet with excellent workability and plating properties Method.
(7) the above (Five) Or (6) Wherein the steel composition is further Cu and
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In these inventions, the following effects (a) and (b) are synergistically exhibited, without adding a large amount of Mn and Si, and without adding Mo and Cr as strengthening elements. Even if the residual austenite phase and the tempered martensite phase are not included, the γ grains before cooling can be made finer. For this reason, the concentration of C and Mn from the α phase to the γ phase is promoted, It effectively makes the γ phase martensite and makes it possible to produce a high-strength hot-dip galvanized steel sheet excellent in workability and plating properties.
(a) α (ferrite) crystal grains due to the pinning action of grain boundary migration by carbides such as TiC, NbC, and VC, which are produced by the addition of one or more selected from Ti, Nb and V Can be miniaturized to 10 μm or less. For this reason, when this is further heated in the pre-plating step, γ grains formed and grown in a two-phase region of α (ferrite) + γ (austenite), or γ grains in a single phase region of γ (austenite) The effect of suppressing coarsening.
(b) The thickness of the band-like structure existing before heating and comprising the second phase containing a large amount of C and Mn is Tb / T ≦ 0.005 (where Tb: average thickness in the thickness direction of the band-like structure, T : Steel plate thickness) The effect of heating to satisfy the relationship.
In the present invention, such a synergistic effect makes it unnecessary to contain Cr that is harmful to the plating property, so that the plating property is very good, and since Mo is not added, the band-like structure existing before heating is present. Can be manufactured with a high strength hot-dip galvanized steel sheet with good workability even if it is not subjected to high-temperature heating at the time of the one-time CGL (hot dip galvanizing) method, which is relatively thin and disadvantageous from the viewpoint of plating properties .
[0016]
Next, experimental results on which the present invention is based will be described.
(Experiment 1)
Heat a sheet bar with a chemical composition of 0.08% C-0.01% Si-1.9% Mn-0.011% P-0.002% S-0.04% Al-0.0022% N-0.02% Ti-0.05% Nb to a thickness of 1200 ° C And it was set as the hot-rolled board of thickness 2.8mm by 5 passes. Thereafter, heat treatment was performed at 400 ° C. and 650 ° C. for 1 hour each as a coiling temperature (CT) equivalent treatment. Next, after pickling, cold rolled into a 1.4 mm cold rolled plate, heated and maintained at 700 ° C to 850 ° C for 1 minute, cooled to 500 ° C at a rate of 10 ° C / s, and after hot dip galvanization, After holding for 40 s, it was alloyed by heating to 550 ° C. at a rate of 10 ° C./s, and immediately cooled to room temperature at a rate of 10 ° C./s. Thereafter, temper rolling with a rolling reduction of 1.0% was performed.
About the obtained hot-dip galvanized steel sheet, the tensile properties (TS, YS, El) were investigated with a JIS No. 5 tensile test piece, and the plating property was examined. The plating property was evaluated according to the following criteria.
○: No plating defects (good plating properties)
Δ: Some non-plating defects occur (slight plating performance)
×: Many non-plating defects occur (plating failure)
The obtained results are shown in FIG. FIG. 1 shows that when the coiling temperature is 650 ° C. and the heating temperature before plating is 750 ° C. or higher, TS: 590 MPa or higher and El: 25% or higher can be achieved.
[0017]
(Experiment 2)
Using a cold-rolled sheet with a thickness of 1.6 mm with the same components as in
About the obtained hot dip galvanized steel sheet, it carried out similarly to
[0018]
From the above experiments, it was found that even when the strength was increased by the high Mn content, plating properties and mechanical properties were improved by high-temperature winding, high-temperature heating before plating, or two heat treatments. . The reason why such an effect can be obtained is as follows. That is, high-temperature winding and two heat treatments generate an internal oxide layer directly under the steel sheet, which suppresses the concentration of Mn, which is harmful to plating properties, on the steel sheet surface, and once heated at a high temperature The Mn surface enriched layer, which is harmful to the plating properties, is removed by pickling before the second heating, and the high temperature heating before plating dissolves and disperses the band structure with high C and Mn concentration, For example, it may be advantageous to generate a second phase such as a site.
[0019]
Next, the reason why the component composition and production conditions are limited to the above ranges in the present invention will be described. (Ingredient composition is expressed in mass%)
C: 0.01-0.20%
C is one of the important basic components of steel. In particular, in the present invention, Ti, Nb and V carbides are precipitated to contribute to the increase in strength, and the strength is increased through the bainite phase and martensite phase generated at low temperatures. It is an element that contributes to improvement. If the C content is less than 0.01%, not only the precipitates, but also the bainite phase and martensite phase are difficult to be formed. On the other hand, if it exceeds 0.20%, the spot weldability deteriorates, so the content range is 0.01 to 0.20%. To do. A preferable amount of C is 0.03 to 0.15%.
[0020]
Si: 1.0% or less
Si is an element that improves workability such as elongation by reducing the amount of solute C in the α phase, but the content of Si exceeding 1.0% impairs spot weldability and plating properties, so the upper limit is 1.0. %. A preferable Si amount is 0.5% or less. Moreover, since reducing the Si content to less than 0.005% involves an increase in cost, the lower limit is preferably set to 0.005%.
[0021]
Mn: more than 1.5 to 3.0%
Mn is one of the important components in the present invention, and is an element that suppresses transformation and stabilizes the γ phase in the composite structure. However, if the content is 1.5% or less, the effect is not obtained. On the other hand, if it exceeds 3.0%, the spot weldability and the plating property are remarkably impaired. Therefore, Mn is contained in the range of more than 1.5 to 3.0%, preferably 1.6 to 2.5%.
[0022]
P: 0.10% or less
P is an element effective for achieving high strength at a low cost, but if it exceeds 0.1%, spot weldability is significantly impaired, so the upper limit is made 0.10%. Note that the P content is desirably 0.05% or less. In addition, reducing the amount of P to less than 0.001% involves an increase in cost, so it is desirable to keep the lower limit at 0.001%.
[0023]
S: 0.05% or less
In addition to causing hot cracking during hot rolling, S induces breakage in the nugget of the spot weld, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the upper limit is made 0.05% or less. In addition, it is more preferable to suppress to 0.010% or less. In addition, since the cost is increased to reduce the amount of S to less than 0.0005%, the lower limit is desirably limited to 0.0005%.
[0024]
Al: 0.10% or less
Al is an effective element for fixing N as AlN as a deoxidizer in the steelmaking stage and causing aging deterioration. However, if the content exceeds 0.10%, the production cost increases, so the Al content must be suppressed to 0.10% or less. A preferable content is 0.050% or less. Further, if the Al amount is less than 0.005%, deoxidation tends to be insufficient, so the lower limit is preferably 0.005%.
[0025]
N: 0.010% or less
N not only causes aging deterioration, but also increases the yield point (yield ratio) and yield elongation, so it is necessary to suppress N to 0.010% or less. A preferable N amount is 0.0050% or less. Further, since reducing the N amount to less than 0.0005% involves an increase in cost, the lower limit is desirably limited to 0.0005%.
[0026]
Ti, Nb and V: 0.01 to 1.0% in total
Ti, Nb, and V form carbides and are effective elements for increasing the strength of steel, and one or more elements are contained in a total of 0.01 to 1.0%. The above effects can be obtained if the total content of these elements is 0.01% or more. However, if the content exceeds 1.0%, there will be a cost disadvantage, too much fine precipitates will increase, and recovery after cold rolling. Recrystallization is suppressed and ductility (elongation) is reduced. Therefore, these elements are contained in a total amount of 0.01 to 1.0%, preferably 0.010 to 0.20%.
[0027]
Cu, Ni: 3.0% or less in total
Cu and Ni are elements useful for forming a second phase such as martensite and increasing the strength of steel, and are added as necessary. However, if the total content exceeds 3.0%, not only the cost is increased, but also the yield point is lowered, which is disadvantageous when a high yield ratio is required. For this reason, the total content of Cu and Ni is contained within a range of 3.0% or less. In addition, a preferable content range is the range of 0.010 to 3.0% in total amount.
[0028]
Ca, REM: 0.001 to 0.01% in total
Ca and REM have the effect of controlling the form of inclusions and sulfides and improving the hole expandability. Therefore, it is preferable to contain Ca and REM in a total amount of 0.001% or more. However, even if the total content exceeds 0.01%, the effect is saturated and the cost is increased. Therefore, the total content of Ca and REM is set to 0.001 to 0.01%. A more preferable content range is 0.002 to 0.005% in total.
[0029]
Ferrite phase: 50% or more in area ratio
The present invention is intended for automotive steel sheets that require high workability. When the ferrite phase is less than 50% in area ratio, it is difficult to ensure the required ductility and stretch flangeability. In addition, when better ductility is required, the ferrite fraction is preferably 75% or more in area ratio. The ferrite includes not only so-called ferrite but also bainitic ferrite and acicular ferrite which do not include carbide precipitation.
Ferrite phase observation method and evaluation method are embedded in resin so that the cross section of the steel sheet becomes the observation surface, “aqueous solution with 1 g of sodium pyrosulfite added to 100 ml of pure water” and “picric acid in 100 ml of ethanol” 4 "added liquid" is mixed at a ratio of 1: 1, etched by immersion for 120 seconds at room temperature, separated into a ferrite phase (black part) and a second phase (white part), and a magnification of 1000 The area ratio of ferrite was determined with a double image analyzer.
[0030]
Average grain size of ferrite phase: 10μm (0.01mm) or less
When the ferrite is heated to the α + γ two-phase region by annealing, the austenite grains generated from the ferrite grain boundaries are naturally enlarged if the ferrite grain size exceeds 10 μm. Naturally, this large austenite grain transforms into a second phase such as relatively large martensite or bainite during cooling, and becomes a starting point of cracking, thereby reducing hole expansibility. Therefore, in the present invention, the ferrite particle diameter is set to 10 μm or less in order to refine the second phase and improve the hole expansibility.
Here, the average crystal grain size is explained by the value calculated by the quadrature method prescribed in ASTM from the cross-sectional structure photograph and the nominal grain size obtained by the same cutting method (for example, Umemoto et al .: Heat treatment 24 (1984) 334). The larger one is used. In the present invention, the type of the second phase (martensite, bainite, pearlite, cementite, etc.) need not be particularly limited.
[0031]
Band-like structure: Tb / T ≦ 0.005 thickness
In the steel with a large amount of C and Mn, the band-like structure is mainly formed by the concentrated layer of C and Mn that is agglomerated along the grain boundaries in the cooling stage of the slab during hot rolling or subsequent cooling. The second phase group is formed in the form of rows and layers in the rolling direction and the plate width direction. The reason why the ratio Tb / T between the average thickness Tb and the plate thickness T of these band-like structures is 0.005 or less is that when the Mn content is large as in the present invention, C and Mn are the main components in the hot-rolled plate structure. This is because the band-shaped second phase structure becomes thick and it becomes difficult to produce a high-strength steel sheet in which hard martensite is uniformly dispersed in the ferrite base. Therefore, in order to produce a high-strength steel plate efficiently, it is necessary to disperse C and Mn concentrated in the band-like second phase, and the average thickness of the band-like structure is the standard. This is because it is the ratio of Tb to the plate thickness T, and good results can be obtained if Tb / T ≦ 0.005.
The average thickness of the band-like structure: Tb is embedded in a resin so that the cross section of the steel sheet becomes the observation surface, etched by immersion in a 3% nital liquid at room temperature for 15 seconds, and with an image analysis device with a magnification of 1000 times. The thickness of each of the 20 points in the row and layered second phase structures was measured and determined from the average value of 20 points.
[0032]
Next, manufacturing conditions in the present invention will be described.
The steel slab having the component composition described above is hot-rolled according to a conventional method and wound at 750 to 450 ° C. When the coiling temperature is less than 450 ° C., carbides such as TiC and NbC are not easily generated, and the strength tends to be insufficient. Moreover, it is difficult to form an internal oxide layer directly under the surface of the steel sheet, and Mn concentration on the steel sheet surface cannot be suppressed. On the other hand, if the winding temperature exceeds 750 ° C., the thickness of the scale increases and the pickling efficiency deteriorates, and the coil longitudinal direction tip, center, rear end, and coil width direction edge, between the center This is because the material fluctuation increases. A preferable winding temperature is 700 to 550 ° C.
[0033]
This hot-rolled sheet is pickled and descaled as necessary, and hot-rolled or further cold-rolled, and then heated and cooled to 750 ° C or higher in a continuous hot-dip galvanizing line. Plating is performed.
When heating twice, first heat to 750 ° C or higher with a continuous annealing facility (first heating), cool, then heat to 700 ° C or higher with a continuous galvanizing line (2 (Second heating), cooling, and hot-dip galvanizing is performed during cooling, preferably at 600 to 420 ° C.
[0034]
Prior to plating, by heating and cooling to a temperature range of 750 ° C. or higher (preferably 750 to 900 ° C.), Mn and the like concentrated in the band-like structure are dispersed, and ferrite + martense is efficiently dispersed. It becomes possible to improve the workability by forming a complex structure of the site. That is, when the Mn content is large as in the present invention, a band-like second phase structure is easily formed in the hot-rolled sheet, and the concentration of Mn and the like in the γ phase is lowered to form a composite structure. It will be disadvantageous. Therefore, if the thickness of this band-like structure is made thin and finely dispersed, it will remain in the γ phase when it is kept near 500 ° C in the plating process of a continuous hot dip galvanizing line or further in the alloying process. Since the concentration of Mn and the like increases, the martensite phase can be suitably dispersed in the ferrite base.
[0035]
In addition, the second heating when performing the heating twice is performed at 700 ° C. or higher. The second heating is necessarily performed in a continuous hot dip galvanizing line. If the second heating temperature is less than 700 ° C., the surface of the steel sheet is not reduced in the continuous hot dip galvanizing line, and plating failure tends to occur. The second heating temperature is preferably in the range of 750 to 800 ° C. In addition, when performing twice heating, it is desirable to remove the surface thickening layer, such as Mn produced | generated by the first heating, and to pickle in order to improve subsequent plating property.
After the above heating step, hot dip galvanization is performed, and in some cases, hot dip galvanization may be performed, and then alloying treatment may be performed.
[0036]
Example 1
A continuous casting slab having a chemical composition shown in Table 1 and having a thickness of 300 mm was heated to 1200 ° C., roughly rolled for 3 passes, and then wound as a hot rolled sheet having a thickness of 2.5 mm by a 7-stand finish rolling mill. . After pickling, the hot-rolled sheet remains as it is, or the hot-rolled sheet is further cold-rolled to a thickness of 1.2 mm. (1) First heating in the continuous annealing line-Pickling-Second heating in the continuous hot-dip galvanizing line -Zinc plating, or (2) Plating in the process of heating in a continuous hot dip galvanizing line-Zinc plating, and samples taken from a part were alloyed. These production conditions are shown in Tables 2 and 3.
[0037]
[Table 1]
In addition, as CGL conditions after heating, the average cooling rate of the steel plate from heating to plating is 10 ° C./s, and it is immersed in a plating bath (bath composition: 0.15% Al—Zn, bath temperature: 470 ° C.) : 1 second), then gas wiping to 60 g / m2The basis weight was adjusted. Then, after heating to 490 degreeC and hold | maintaining for 20 second, it cooled to 200 degreeC with the average cooling rate of 20 degree-C / s.
The obtained steel plate was used as a test material to investigate mechanical properties, plating properties, spot weldability, and the like. The results are also shown in Table 2 and Table 3.
[0039]
Here, mechanical properties, plating properties, alloying properties, and spot weldability were evaluated by the following methods.
・ Mechanical properties (Investigated by tensile test and hole expansion test)
Using No. 5 test piece specified in JIS Z 2204 collected in the direction perpendicular to the rolling direction from the steel sheet, yield strength (YS), tensile strength (TS), elongation at break (El), yield by the methods specified in JIS Z 2241 Elongation (YEl) was measured.
For stretch flangeability, the hole expansion rate (λ) was measured by the method prescribed in JFS T 1001.
・ Plating properties
Good: No plating defects
Slightly good: Some non-plating defects occurred
Defect: Many non-plating defects occur
・ Alloyability
Good: No unevenness in alloying
Slightly good: Slightly uneven alloying
Defect: Significant unevenness in alloying
・ Spot weldability
Spot welding consists of welding electrode: dome shaped tip diameter 6φ, electrode pressure: 3.10 kN, welding current: 7 kA, pressurization time: 25 cyc, setup time: 3 cyc, welding time: 13 cyc, holding time: 25 cyc The welding conditions were used. After welding, the tensile load (TSS) by the tensile shear test of JIS Z 3136 and the tensile load (CTS) by the cross-type tensile test of JIS Z 3137 are loaded, and 8787N which is the standard tensile shear load when the plate thickness is 1.2 mm The above evaluation was made as “excellent” when the ductility ratio (CTS / TSS) was 0.25 or more, and “inferior” when not satisfying these values.
[0040]
From Table 1 to Table 3, the invention example has a tensile property of El: 25% or more at TS: 590 to 690 MPa level, TS x El value: 15000 MPa ·% or more, and TS x El balance is also good. In addition, it was found that there are no particular problems with respect to plating properties, alloying properties, and spot weldability.
[0041]
[Table 2]
[Table 3]
Example 2
A continuous cast slab having a chemical composition shown in Table 4 and having a thickness of 300 mm is heated to 1200 ° C., roughly rolled for 3 passes, and then hot rolled with a thickness of 3.0 mm on a 7-stand finish rolling mill. It was wound up at the indicated temperature. After pickling, the hot-rolled sheet remains as it is, or the hot-rolled sheet is further cold-rolled to a thickness of 1.2 mm. (1) First heating in the continuous annealing line-Pickling-Second heating in the continuous hot-dip galvanizing line -Zinc plating, or (2) Plating in the process of heating in a continuous hot dip galvanizing line-Zinc plating, and a sample taken from a part was alloyed. These manufacturing conditions are shown in Table 5.
In addition, as CGL conditions after heating, the average cooling rate of the steel sheet from heating to plating is 10 ° C / s, and it is immersed in a plating bath (bath composition: 0.15% Al-Zn, bath temperature: 470 ° C) (immersion time) : 1 second), then gas wiping to 60 g / m2The basis weight was adjusted. Then, it heated to 490 degreeC and hold | maintained for 20 second, Then, it cooled to 200 degreeC with the average cooling rate of 20 degree-C / s.
Using the obtained steel plate as a test material, the mechanical properties, plating properties, spot weldability and the like were investigated in the same manner. The results are also shown in Table 5.
As a result, it was found that the invention example had no problem with respect to plating properties, alloying properties, and spot weldability, despite having a good TS × El balance and high strength.
[0044]
[Table 4]
[Table 5]
Example 3
A continuous cast slab having a chemical composition shown in Table 6 and having a thickness of 300 mm is heated to 1200 ° C., roughly rolled for 3 passes, and then hot rolled with a thickness of 3.0 mm on a 7-stand finish rolling mill. It was wound up at the indicated temperature. After pickling, it is cold-rolled to a thickness of 1.2 mm, plated in the first heating-continuous annealing line-pickling-second hot-dip galvanizing line-zinc plating process, and further alloyed went. These production conditions are shown in Table 7.
In addition, as CGL conditions after heating, the average cooling rate of the steel plate from heating to plating is 10 ° C./s, and it is immersed in a plating bath (bath composition: 0.15% Al—Zn, bath temperature: 470 ° C.) : 1 second), then gas wiping to 60 g / m2The basis weight was adjusted. Then, after heating to 490 degreeC and hold | maintaining for 20 second, it cooled to 200 degreeC with the average cooling rate of 20 degree-C / s.
Using the obtained steel plate as a test material, the mechanical properties, plating properties, spot weldability and the like were investigated in the same manner. The results are also shown in Table 7.
As a result, it was found that the invention examples had no problem with respect to the plating property, alloying property, and spot weldability, although the TS × El balance was good and the strength was high.
[0047]
[Table 6]
[Table 7]
【The invention's effect】
As described above, according to the present invention, high-strength hot-dip galvanized steel sheets (including high-strength alloyed hot-dip galvanized steel sheets) that have no problem in plating properties, have a low yield ratio, and have a good TS × El balance. It becomes possible to provide. Therefore, the present invention makes it possible to reduce the weight and fuel consumption of automobiles, thus greatly contributing to the improvement of the global environment. In addition, this invention can be applied also to the hot-rolled steel plate and cold-rolled steel plate without plating, and an electrogalvanized steel plate, and can anticipate the same effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of heating temperature in a continuous hot dip galvanizing line on tensile strength (TS), yield strength (YS), elongation (El) and plating properties.
FIG. 2 is a graph showing the influence of winding temperature and presence / absence of heating twice on tensile strength (TS), yield strength (YS), elongation (El), and plating properties.
Claims (7)
を要旨構成とするものである。A slab comprising the steel composition according to any one of claims 1 or 2 is hot-rolled and wound at 750 to 450 ° C and then subjected to cold rolling as it is or further, and the obtained hot rolling is obtained. The plate or cold-rolled plate is once heated to 750 ° C or higher, cooled and pickled , further heated to 700 ° C or higher, hot dip galvanized during cooling from this temperature, and then alloyed. A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and plating properties, characterized in that it is performed. "
Is the gist configuration.
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| JP2000211028 | 2000-07-12 | ||
| JP2000318914A JP3698046B2 (en) | 1999-10-22 | 2000-10-19 | High-strength hot-dip galvanized steel sheet excellent in workability and plating property and method for producing the same |
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| JP4911123B2 (en) * | 2002-03-29 | 2012-04-04 | Ｊｆｅスチール株式会社 | Cold rolled steel sheet with ultrafine grain structure |
| KR100949694B1 (en) * | 2002-03-29 | 2010-03-29 | 제이에프이 스틸 가부시키가이샤 | Cold rolled steel sheet having ultrafine grain structure and method for producing the same |
| JP3901039B2 (en) * | 2002-06-28 | 2007-04-04 | Ｊｆｅスチール株式会社 | Ultra-high strength cold-rolled steel sheet having excellent formability and method for producing the same |
| JP3896075B2 (en) * | 2002-12-16 | 2007-03-22 | 新日本製鐵株式会社 | Manufacturing method of high-strength hot-rolled steel sheet with excellent stretch flangeability |
| JP4569071B2 (en) * | 2003-03-24 | 2010-10-27 | Ｊｆｅスチール株式会社 | Manufacturing method of hot-dip galvanized steel sheet with excellent stretch flangeability |
| JP4408386B2 (en) * | 2004-04-19 | 2010-02-03 | 新日本製鐵株式会社 | High-strength steel with fine grain structure |
| AU2006336816B2 (en) * | 2006-01-26 | 2011-09-15 | Giovanni Arvedi | Strip of hot rolled micro-alloyed steel for obtaining finished pieces by cold pressing and shearing |
| JP4736853B2 (en) * | 2006-02-28 | 2011-07-27 | Ｊｆｅスチール株式会社 | Precipitation strengthened high strength steel sheet and method for producing the same |
| WO2008110670A1 (en) * | 2007-03-14 | 2008-09-18 | Arcelormittal France | Steel for hot working or quenching with a tool having an improved ductility |
| JP4837604B2 (en) * | 2007-03-16 | 2011-12-14 | 新日本製鐵株式会社 | Alloy hot-dip galvanized steel sheet |
| JP5440370B2 (en) * | 2010-05-12 | 2014-03-12 | 新日鐵住金株式会社 | Alloyed hot-dip galvanized steel sheet and method for producing the same |
| JP2012082499A (en) * | 2010-10-14 | 2012-04-26 | Sumitomo Metal Ind Ltd | Hot-dipped steel sheet and method for producing the same |
| WO2016193268A1 (en) * | 2015-06-03 | 2016-12-08 | Salzgitter Flachstahl Gmbh | Deformation-hardened component made of galvanized steel, production method therefor and method for producing a steel strip suitable for the deformation-hardenening of components |
| WO2018030400A1 (en) | 2016-08-08 | 2018-02-15 | 新日鐵住金株式会社 | Steel sheet |
| MX2019012110A (en) | 2017-04-28 | 2019-11-28 | Nippon Steel Corp | High strength steel sheet and method for manufacturing same. |
| WO2019187031A1 (en) | 2018-03-30 | 2019-10-03 | 日本製鉄株式会社 | High-strength steel sheet having excellent ductility and hole expansion property |
| CN115074639A (en) * | 2021-03-15 | 2022-09-20 | 上海梅山钢铁股份有限公司 | Hot-rolled steel plate for automobile beam with tensile strength of 600MPa |
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