JP2004263271A

JP2004263271A - Method for manufacturing high-tensile galvanized steel plate

Info

Publication number: JP2004263271A
Application number: JP2003056901A
Authority: JP
Inventors: Yoshitsugu Suzuki; 善継鈴木; Kazuaki Kyono; 一章京野
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-03-04
Filing date: 2003-03-04
Publication date: 2004-09-24
Anticipated expiration: 2023-03-04
Also published as: JP3997931B2

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently manufacture a high-tensile galvanized steel plate of excellent surface property and excellent plating adhesiveness when performing galvanization on a high-tensile steel plate containing Si, Mn and Al. <P>SOLUTION: A high-tensile steel plate containing, by mass, one or more kinds of 0.1-2.0% Si, 1.0-3.5% Mn, and 0.1-2.5% Al is annealed in a non-oxidizing furnace type or direct-fire furnace type continuous annealing furnace and cooled, ≥ 70% of a surface concentrated layer of Si, Mn and Al formed on the surface of the high-tensile steel plate is removed by pickling, and the high-tensile steel plate is galvanized thereafter. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車，建材，家電等の分野で使用するのに好適な、めっき密着性に優れた高張力溶融亜鉛めっき鋼板の製造方法に関する。
【０００２】
【従来の技術】
近年、自動車，建材，家電等の分野において、高張力鋼板の用途が広がっている。特に自動車業界においては、車体を軽量化して燃費を向上するとともに、衝突安全性を高めるという観点から、高張力鋼板の使用量が急増している。このような高張力鋼板は、たとえば特公平３−５１７７８号公報に開示されているように、強度延性バランス等の機械的性質を改善するために、Ｓｉ，Ｍｎ，Ｔｉ，Ａｌ，Ｐ等の元素を添加したものが多い。
【０００３】
しかしＳｉ含有量を増加すると、焼鈍によって鋼板表面にＳｉ酸化物が形成され、化成処理に支障をきたすのみならず、電気亜鉛めっきの密着性，溶融亜鉛めっきの密着性が劣化することが知られている。特にＳｉを含有する高張力鋼の溶融亜鉛めっきでは、部分的に溶融亜鉛が付着せず不均一なめっき層が形成（いわゆる不めっき）されたり、あるいはプレス加工でめっき層が剥離する等が大きな問題となっている。
【０００４】
Ｓｉ含有量が０．１質量％を超えると、オールラジアントチューブ型（以下、ＲａｄｉａｎｔＴｕｂｅＦｕｒｎａｃｅを略してＲＴＦ型という）の焼鈍炉あるいは無酸化型（以下、ＮｏｎＯｘｉｄｉｚｅｄＦｕｒｎａｃｅを略してＮＯＦ型という）の焼鈍炉を有する連続式溶融亜鉛めっきラインで操業が著しく不安定になる。また素材となるスラブの熱間圧延工程で、Ｓｉ−Ｆｅ系スケールに由来する凹凸が発生し、最終製品である高張力溶融亜鉛めっき鋼板の品質が損なわれる。
【０００５】
ＳｉやＭｎを多量に含有する鋼板のめっき密着性を向上させる方法として、連続式溶融亜鉛めっきライン（以下、ＣｏｎｔｉｎｕｏｕｓＧａｌｖａｎｉｚｉｎｇＬｉｎｅを略してＣＧＬという）に鋼板を導入する前に電気めっきを行なう方法（たとえば特開平２−１９４１５６号公報）や、クラッド法でＳｉ，Ｍｎ含有量の少ない鋼板を表層に配置することによってめっき濡れ性を改善する方法（たとえば特開平３−１９９３６３号公報）が知られている。
【０００６】
一方、めっき層を十分に合金化させるためには、合金化処理の時間を延長したり、あるいは合金化温度を上昇させる必要がある。合金化時間を延長するためには、ＣＧＬの搬送速度を低下させなければならないので、生産性が低下し、製造コストの上昇を招く。一方、合金化温度を上昇させると、めっき層中のＦｅ含有量の制御が困難となり、合金化が過剰に進行しやすい。しかも、めっき密着性が劣化し、製品歩留りが低下する。
【０００７】
そこで、このような問題を回避しながら合金化を促進する技術として、特開昭５８−１２０７７１号公報には、還元焼鈍の前にＮｉ，Ｃｕ等をめっきすることによって、Ｆｅ−Ｚｎの合金化を促進する技術が開示されている。しかし、この技術では、Ｎｉ，Ｃｕ等をめっきする新たな設備を設置する必要がある。
また特公昭６４−１１１１１号公報には、Ｚｎ浴のＡｌ濃度および温度を変更して合金化処理の条件を一定にする技術が開示されている。しかし、Ｚｎ浴のＡｌ濃度や温度を変化させるのに長時間を要する。その間、ＣＧＬの操業を停止しなければならないので、生産性の低下を招く。しかも、Ｓｉ，Ｍｎ，Ａｌを多量に含有する鋼板にこの技術を適用すると、めっき層の剥離や不めっき等の問題が生じる。
【０００８】
特開平３−２４３７５１号公報には、Ｐ添加鋼を焼鈍した後、Ｐ濃化層を除去して合金化を促進する技術が開示されている。しかし、この技術を適用するＰ添加鋼は３５ｋｇｆ／ｍｍ^２級と呼ばれている強度の低い鋼種であり、高張力鋼板に適用するのは困難である。つまり特開平３−２４３７５１号公報に開示された技術を、４５ｋｇｆ／ｍｍ^２級と呼ばれる高張力鋼板や、さらに強度を高めた高張力鋼板に適用すると不めっき等の問題が生じる。しかも特開平３−２４３７５１号公報では連続焼鈍ライン（以下、ＣｏｎｔｉｎｕｏｕｓＡｎｎｅａｌｉｎｇＬｉｎｅを略してＣＡＬという）の連続焼鈍炉の型式には言及しておらず、無酸化炉型，直火炉型あるいはＲＴＦ型の連続焼鈍炉の特性に応じた操業条件を設定できない。
【０００９】
すなわち、これらの技術では高張力溶融亜鉛めっき鋼板のめっき密着性を改善するのは困難であった。高張力鋼板には、通常、Ｓｉ，Ｍｎ，Ａｌが複合添加されており、これらの元素を含有する高張力鋼板のめっき密着性を向上する技術が種々検討されている。
たとえば、特許第３１１０２３８号公報や特許第３１６２９０１号公報には、高張力鋼板を再結晶焼鈍し、次いで酸洗し、さらに再度焼鈍することによって、高張力鋼板のめっき密着性を改善する技術が開示されている。しかし、これらの技術では内部酸化層が生成されないので、自動車用鋼板等で要求される厳格なめっき密着性の仕様を満足するには不十分である。
【００１０】
特許第３１３０４７０号公報には、内部酸化層を形成することによって、ＳｉやＭｎの表面濃化を抑制する技術が開示されている。しかし、この技術では巻取り温度を上げることが重要とされているが、巻取り温度の上昇によって高張力鋼板の延性の劣化を招くという問題がある。
特開２０００−２９０７３０号公報には、連続焼鈍炉の露点を高くすることによって、熱間圧延の条件に関わらず、連続焼鈍炉で内部酸化層を生成する技術が開示されている。しかし、この技術では炉内に多量の水分を導入して、８００〜１０００℃の高温で加熱する際の雰囲気を加湿する必要がある。その結果、炉内に設置されるバーナー類，計測機器あるいは耐火物の耐用性が劣化するという問題がある。
【００１１】
以上に説明したように、Ｓｉ，Ｍｎ，Ａｌを添加した高張力鋼板に溶融亜鉛めっきを施すにあたって、従来の技術では、表面凹凸や不めっき等の表面欠陥が発生したり、十分なめっき密着性が得られないという問題が生じるばかりでなく、生産性の低下や歩留りの低下を招くという問題がある。
【００１２】
【特許文献１】
特公平３−５１７７８号公報
【特許文献２】
特開平２−１９４１５６号公報
【特許文献３】
特開平３−１９９３６３号公報
【特許文献４】
特開昭５８−１２０７７１号公報
【特許文献５】
特公昭６４−１１１１１号公報
【特許文献６】
特開平３−２４３７５１号公報
【特許文献７】
特許第３１１０２３８号公報
【特許文献８】
特許第３１６２９０１号公報
【特許文献９】
特許第３１３０４７０号公報
【特許文献１０】
特開２０００−２９０７３０号公報
【００１３】
【発明が解決しようとする課題】
本発明は上記のような問題を解消し、高強度化元素としてＳｉ，Ｍｎ，Ａｌのうちの１種または２種以上を含有する高張力鋼板であっても、溶融亜鉛めっきを施すにあたって、優れた表面性状を有し、かつ優れためっき密着性を有する高張力溶融亜鉛めっき鋼板を効率良く製造する方法を提供することを目的とする。
【００１４】
【課題を解決するための手段】
本発明者らは、高強度化元素としてＳｉ，Ｍｎ，Ａｌを含有する高張力鋼板に溶融亜鉛めっきを施す前に、直火炉型あるいは無酸化炉型の加熱帯を有する連続焼鈍炉を用いて再結晶焼鈍を施すことによって、高張力鋼板に高強度化元素の鋼板表面での濃化を防止することができ、その結果、高張力溶融亜鉛めっき鋼板のめっき密着性や表面性状を大幅に改善できることを見出した。
【００１５】
すなわち本発明は、Ｓｉ：０．１〜２．０質量％，Ｍｎ：１．０〜３．５質量％およびＡｌ：０．１〜２．５質量％のうちの１種または２種以上を含有する高張力鋼板を、無酸化炉型または直火炉型の加熱帯を有する連続焼鈍炉で焼鈍した後、Ｓｉ，ＭｎおよびＡｌのうち高張力鋼板に含有される元素の表面濃化層の７０％以上を酸洗によって除去し、しかる後に再加熱し、その後、高張力鋼板に溶融亜鉛めっきを施す高張力溶融亜鉛めっき鋼板の製造方法である。
【００１６】
また本発明は、Ｓｉ：０．１〜２．０質量％，Ｍｎ：１．０〜３．５質量％およびＡｌ：０．１〜２．５質量％のうちの１種または２種以上を含有する高張力鋼板を、加熱帯温度を４００〜８００ ℃、加熱帯雰囲気をＨ_２Ｏ：２〜３０体積％、Ｏ_２：５体積％以下、ＣＯ＋ＣＯ_２：２０体積％以下、残部をＮ_２および不可避的不純物とし、さらに均熱帯温度を７５０〜９５０ ℃、均熱帯雰囲気をＨ_２：１〜１０体積％、残部をＮ_２および不可避的不純物とした連続焼鈍炉を用いて焼鈍を施した後、Ｓｉ，ＭｎおよびＡｌのうち高張力鋼板に含有される元素の表面濃化層の７０％以上を酸洗によって除去し、しかる後に再加熱し、その後、高張力鋼板に溶融亜鉛めっきを施す高張力溶融亜鉛めっき鋼板の製造方法である。
【００１７】
前記した高張力溶融亜鉛めっき鋼板の製造方法の発明においては、好適態様として、焼鈍によって高張力鋼板の表層部にＳｉ，ＭｎおよびＡｌのうち高張力鋼板に含有される元素を内部酸化させることが好ましい。
また、内部酸化によって高張力鋼板の表層部に生じる酸素増加量が片面あたり０．０１ｇ／ｍ^２以上であることが好ましい。さらに、高張力鋼板に溶融亜鉛めっきを施した後、めっき層の合金化処理を施すことが好ましい。
【００１８】
【発明の実施の形態】
本発明を適用する高張力鋼板は、Ｓｉ：０．１〜２．０質量％，Ｍｎ：１．０〜３．５質量％およびＡｌ：０．１〜２．５質量％のうちの１種または２種以上を含有する。
これらの３種の元素は、高張力鋼板としての強度を確保することを主な目的として添加するものであるから、ここでは高強度化元素と記す。なお高強度化元素には、高張力鋼板の強度を確保する他に、下記のような作用がある。本発明では高強度化元素のこれらの作用を考慮して、それぞれの含有量を設定した。
【００１９】
Ｓｉ含有量が０．１質量％未満では、後述する本発明の連続焼鈍条件を適用しなくとも十分なめっき密着性が得られるから、本発明を適用する意義がない。一方、２．０質量％を超えると、後述する本発明の焼鈍条件を適用してもＳｉの表面濃化層が過剰に生成され、めっき密着性が十分に改善されない。しかも合金化処理の進行が抑えられるので、合金化時間の延長あるいは合金化温度の上昇が必要となる。その結果、生産性の低下あるいはめっき密着性の劣化を招く。したがって、Ｓｉは０．１〜２．０質量％の範囲内とした。
【００２０】
Ｍｎ含有量が１．０質量％未満では、後述する本発明の連続焼鈍条件を適用しなくとも十分なめっき密着性が得られるから、本発明を適用する意義がない。一方、３．５質量％を超えると、高張力溶融亜鉛めっき鋼板の溶接性が劣化する。したがって、Ｍｎは１．０〜３．５質量％の範囲内とした。
Ａｌ含有量が０．１質量％未満では、後述する本発明の連続焼鈍条件を適用しなくとも十分なめっき密着性が得られるから、本発明を適用する意義がない。またＡｌは、鋼板組織を残留オーステナイト相を有する組織とし、高張力溶融亜鉛めっき鋼板の強度延性バランスを改善するために有用であり、この意味でも０．１質量％以上とすることが好ましい。一方、２．５質量％を超えると、高張力溶融亜鉛めっき鋼板の溶接性が劣化する。したがって、Ａｌは０．１〜２．５質量％の範囲内とした。
【００２１】
本発明を適用する高張力鋼板に添加する元素は、必ずしも上記した３種の高強度化元素に限定するものではない。高強度化元素に加えてＣを添加し、さらにＰ，Ｓ，Ｔｉ，Ｎｂ，Ｃｒ，Ｍｏ，Ｂ，Ｎ，Ｖ等を必要に応じて添加しても良い。高強度化元素以外のこれらの元素の添加量は特定の範囲に限定せず、所定の強度が得られるように適宜添加する。これら元素の含有量は、Ｃ：０．２５質量％以下，Ｐ：０．２０質量％以下，Ｓ：０．０１質量％以下，Ｔｉ：０．３質量％以下，Ｎｂ：０．３質量％以下，Ｃｒ：１質量％以下，Ｍｏ：１質量％以下，Ｂ：０．００５質量％以下，Ｎ：０．０１質量％以下，Ｖ：０．３質量％以下であれば特に問題はない。
【００２２】
以上説明した元素以外の残部は、Ｆｅおよび不可避的不純物とすることが好ましい。また高張力鋼板の製造方法は、熱間圧延あるいは冷間圧延等、従来から知られている技術を使用する。
次に、本発明の最も重要な条件である連続焼鈍条件について説明する。
こうして所定の量の高強度化元素を含有する高張力鋼板を、無酸化炉型または直火炉型の加熱帯を有する連続焼鈍炉に装入して、焼鈍を施す。無酸化炉型または直火炉型の加熱帯は、炉内雰囲気の酸素分圧が比較的高いので、焼鈍の際の昇温過程で高強度化元素の内部酸化が促進される。こうして内部酸化を促進することによって、高強度化元素の酸化物からなる表面濃化層が表面に形成されるのを抑制する効果も得られる。このような表面濃化層の生成を抑制することによって、不めっき等の表面欠陥が防止でき、高張力溶融亜鉛めっき鋼板の表面性状を改善できる。
【００２３】
ところが加熱帯がＲＴＦ型の連続焼鈍炉では、加熱帯における炉内雰囲気の酸素分圧が低いので、内部酸化が進行しにくい。したがって本発明では、無酸化炉型または直火炉型の加熱帯を有する連続焼鈍炉を用いて焼鈍を施す。そして本発明では、雰囲気温度が８００℃以下と低い加熱帯において、Ｓｉ等の易酸化性元素の内部酸化を促進できるので、温度が高い均熱帯は必ずしも加湿する必要はなく、連続焼鈍炉の耐久性の観点からも有利である。
【００２４】
連続焼鈍炉の加熱帯の雰囲気は、酸素供給源としてＨ_２Ｏ：２〜３０体積％，Ｏ_２：５体積％以下，ＣＯ＋ＣＯ_２：２０体積％以下を含有し、残部はＮ_２および不可避的不純物とすることが好ましい。なお加熱帯として、この雰囲気を維持できるのであれば、加熱帯は無酸化炉型または直火炉型に限定されないが、加熱帯が無酸化炉型または直火炉型であれば、この雰囲気を満足する。
【００２５】
加熱帯雰囲気のＨ_２Ｏが２体積％未満では、内部酸化が進行しない。一方、３０体積％を超えると、高張力溶融亜鉛めっき鋼板の品質に悪影響は及ぼさないものの、炉内に設置されるバーナー類，計測機器あるいは耐火物の耐用性が劣化する。したがって、Ｈ_２Ｏは２〜３０体積％の範囲内とする。
加熱帯雰囲気のＯ_２が５体積％を超えると、高張力鋼板の表面が酸化され、高強度化元素の表面濃化層が形成され、高張力溶融亜鉛めっき鋼板の表面性状の劣化を招く。しかも連続焼鈍炉内の搬送ローラー等にピックアップが生じて、操業に支障をきたす。したがって、Ｏ_２は５体積％以下とする。ただし、Ｏ_２は大気から不可避的に混入する。そのため、０．０１体積％未満まで削減するためには多大な設備負荷や労力を要するので、Ｏ_２は０．０１体積％以上とするのが好ましい。
【００２６】
さらに加熱帯雰囲気にはＣＯおよび／またはＣＯ_２が含有されても良い。無酸化炉型あるいは直火炉型の連続焼鈍炉では、炉内のバーナーで燃料を燃焼させることによってＣＯやＣＯ_２が発生する。ただし、これらのガスの含有量はＣＯ＋ＣＯ_２が２０体積％を超えると、連続焼鈍後、高張力鋼板の表面にＳｉ，Ｍｎ，Ａｌの表面濃化層が形成され、高張力溶融亜鉛めっき鋼板の表面性状の劣化を招く。しかも連続焼鈍炉内の搬送ローラー等にピックアップが生じて、操業に支障をきたす。したがって、ＣＯ＋ＣＯ_２を２０体積％以下とする。つまりＣＯおよびＣＯ_２を含有する場合は合計２０体積％以下とし、ＣＯまたはＣＯ_２を含有する場合はそれぞれ２０体積％以下とする。好ましくは１５体積％以下である。
【００２７】
さらに加熱帯雰囲気はＨ_２を含んでも良い。その場合のＨ_２濃度は、後述する均熱帯と同等のＨ_２濃度とするのが好ましい。
加熱帯雰囲気の残部はＮ_２と不可避的不純物である。燃焼ガスによる空燃比は０．７５〜１．２が好ましいが、加熱帯雰囲気中のＨ_２Ｏが内部酸化に最も影響を及ぼすので、本発明では特定の範囲に限定しない。
【００２８】
連続焼鈍炉の加熱帯の温度は４００〜８００ ℃の範囲内とする。加熱帯温度が４００℃未満では、内部酸化が十分に進行しないので、めっき密着性が改善されない。一方、８００℃を超えると、炉体の温度が上昇して耐用性が劣化する。
連続焼鈍炉に装入された高張力鋼板は、こうして加熱帯で加熱された後、均熱帯へ搬送される。
【００２９】
連続焼鈍炉の均熱帯の雰囲気は、Ｈ_２：１〜１０体積％，を含有し、残部はＮ_２および不可避的不純物とすることが好ましい。
均熱帯雰囲気のＨ_２が１体積％未満では、高張力鋼板が加熱帯を通過する際に高張力鋼板の表面に生成するＦｅ酸化物が還元されず残存する。その結果、高張力溶融亜鉛めっき鋼板の表面欠陥（不めっき，めっきの剥離等）が発生しやすくなる。一方、１０体積％を超えると、加熱帯で生成した内部酸化層が還元され、めっき密着性改善の効果が得られない。したがって、Ｈ_２Ｏは２〜５体積％の範囲内とする。
【００３０】
均熱帯雰囲気の残部はＮ_２と不可避的不純物である。なお、均熱帯の雰囲気を上記した条件とするには、ＲＴＦ型の均熱帯を用いることが好ましい。
連続焼鈍炉の均熱帯の温度は７００〜９５０ ℃の範囲内とする。均熱帯温度が７００℃未満では、内部酸化が十分に進行しないので、めっき密着性が改善されない。一方、９５０℃を超えると、炉体の温度が上昇して耐用性が劣化する。
【００３１】
このような条件を満たすように設定した連続焼鈍炉を用いて焼鈍を行なうことによって、高張力鋼板に含有される易酸化性元素であるＳｉ，Ｍｎ，Ａｌを鋼板の表層部において内部酸化させることができる。
ここで内部酸化は、酸化量が鋼板の片面あたり０．０１〜１ｇ／ｍ^２となるように、上記した加熱帯および均熱帯の雰囲気，温度範囲内で操業条件を設定することが好ましい。酸化量は、上記の加熱帯および均熱帯の条件の連続焼鈍炉への通板前後での鋼板の単位面積あたりの酸素の増加量を指す。連続焼鈍における酸化量を片面あたりで０．０１ｇ／ｍ^２以上とすることにより、めっき密着性の向上効果をより一層安定して得ることができる。
【００３２】
酸化量が１ｇ／ｍ^２を超えると、鋼板表面に割れが生じ、めっき後の外観が悪くなることがあるので好ましくない。鋼種，連続焼鈍炉通板時のライン速度，加熱帯や均熱帯の詳細な温度設定および雰囲気設定と、酸化量との関係を予め実験的に求めておき、酸化量が上記の範囲となる操業条件を設定することが好ましい。なお、酸化性雰囲気である加熱帯での鋼板の滞在時間を長く、また加熱帯の温度を高くすることで、酸化量は増加することは言うまでもない。
【００３３】
なお内部酸化量は、高張力鋼板の内部酸化層中の酸素量を、たとえば「インパルス炉溶融−赤外線級手法」と呼ばれる方法等で測定することによって得られる。ただし、高張力鋼板の内部酸化量を求めるためには、素材（すなわち焼鈍を施す前の高張力鋼板）に含まれる酸素量を差引く必要がある。そこで本発明では、連続焼鈍後の高張力鋼板の両面の表層部を１００μｍ以上研磨して鋼中酸素濃度を測定し、その測定値を素材に含まれる酸素量Ｏ_Ｈとし、また連続焼鈍後の高張力鋼板の板厚方向全体での鋼中酸素濃度を測定して、その測定値を内部酸化後の酸素量Ｏ_Ｉとする。
【００３４】
こうして得られた高張力鋼板の内部酸化後の酸素量Ｏ_Ｉと、素材に含まれる酸素量Ｏ_Ｈとを用いて、Ｏ_ＩとＯ_Ｈの差（＝Ｏ_Ｉ−Ｏ_Ｈ）を算出し、さらに単位面積（すなわち１ｍ^２）あたりの量に換算した値（ｇ／ｍ^２）が内部酸化量である。
このようにして連続焼鈍炉で再結晶焼鈍を施した高張力鋼板の表層部には、高強度化元素の内部酸化層が形成され、かつその表面には、高強度化元素の酸化物からなる表面濃化層が形成される。この表面濃化層には、高張力鋼板に添加される他の元素の酸化物も含まれる。内部酸化層はめっき密着性を改善する効果を有する。
【００３５】
次に、連続焼鈍を施した鋼板に対して酸洗を施す。本発明では、上記した連続焼鈍の後には、鋼板表層のＳｉ，Ｍｎ，Ａｌといった易酸化性元素の多くは、鋼板表層の地鉄内部に酸化物として存在する。しかしながら、これら易酸化性元素の表面濃化を皆無にすることはできないので、これを酸洗により除去する必要がある。そこで、不めっきの防止に有効なこれらの元素の表面濃化層の除去率を定量化するために、上記した条件で連続焼鈍した後の鋼板、およびさらに酸洗した鋼板について、Ｓｉ，Ｍｎ，Ａｌをグロー放電発光分光分析（いわゆるＧＤＳ）で低スパッタリング測定（たとえばＦｅ換算で約０．０１μｍ／ｓｅｃ程度）した濃度プロファイルを基に定量化した。
【００３６】
その結果、Ｓｉ，Ｍｎ，Ａｌのうち鋼中に含有されている元素の表面濃化層を、酸洗により７０％の除去率で除去しておけば、溶融亜鉛めっきを施した後のめっき密着性は良好となることが分かった。図１は、ＧＤＳにより深さ方向の濃度プロファイルを測定した場合のＦｅおよびＳｉの分布の例を示すものであり、（ａ）は連続焼鈍後の高張力鋼板についての測定結果を示し、（ｂ）は酸洗後の高張力鋼板についての測定結果を模式的に示す。
【００３７】
本発明では、たとえばＳｉについて言えば、連続焼鈍後と酸洗後の鋼板について、図１に示すようなＳｉの濃度プロファイルを測定し、これら濃度プロファイルにおいて、表面近傍に存在するＳｉのピークについての測定強度の積算値（図１中の斜線部の面積に相当）をＳｉの表面濃化量として求め、下記の（１）式により表面濃化層の除去率を求めるものとする。
【００３８】

Ｓｉ，Ｍｎ，Ａｌのうち、上記した含有量範囲で鋼中に含有される元素全てについて、このようにして求められる除去率が７０％以上となるように酸洗を行なうことで、不めっきを防止することができる。なお、濃度プロファイルの測定にあたっては、測定時間は、表面濃化層を貫通し、鋼板内部（地鉄部分）が含まれるまでスパッタリングを行なうものとし、表面濃化量を求めるにあたっては、測定対象の元素の含有量の違いによってデプスプロファイルのベースラインが異なることを考慮する。
【００３９】
この除去率が９０％以上となるように表面濃化層を除去すると、高張力溶融亜鉛めっき鋼板の表面性状が著しく改善される。したがって、除去率を９０％以上とするのが一層好ましく、除去率を１００％とする（すなわち表面濃化層を完全に除去する）のが最も好ましい。
実際の酸洗条件については、上記の除去率が７０％以上となる条件を、鋼板の成分組成，連続焼鈍条件を勘案して、酸洗液の温度，濃度や浸漬時間を実験的に求めておけば良い。
【００４０】
好適な酸洗条件は、酸濃度０．１〜２０％，温度４０〜９０℃の酸洗液に１〜１００ｓｅｃ浸漬すれば良い。酸洗液の温度が４０℃未満もしくは浸漬時間が１ｓｅｃ未満では、表面濃化層が十分に除去されないので、高張力溶融亜鉛めっき鋼板の表面性状が劣化する。一方、酸洗液の温度が９０℃を超えるもしくは浸漬時間が１００ｓｅｃを超えると、高張力鋼板の表面が粗くなり、高張力溶融亜鉛めっき鋼板の表面性状が劣化する。しかも高張力鋼板の内部酸化層も除去されるので、めっき密着性改善の効果が得られない。
【００４１】
酸洗液の酸濃度が０．１％未満では、表面濃化層を除去するのに長時間を要するので、生産性が低下する。一方、２０％を超えると、高張力鋼板の表面が粗くなり、高張力溶融亜鉛めっき鋼板の表面性状が劣化する。しかも高張力鋼板の内部酸化層も除去されるので、めっき密着性改善の効果が得られない。また酸洗減量は、Ｆｅ換算で片面あたり０．０５〜５ｇ／ｍ^２が好ましい。
【００４２】
酸洗液として使用する酸の種類は、塩酸，硫酸，硝酸，リン酸等の一般に使用される酸を使用する。ただし安価に入手できる塩酸や硫酸を使用するのが好ましい。
酸洗が終了した後、高張力鋼板を通常溶融亜鉛めっき処理に使用されるＣＧＬに送給して溶融亜鉛めっきを施し、高張力溶融亜鉛めっき鋼板を製造する。ＣＧＬでは通常焼鈍炉にて再加熱をした後に溶融亜鉛めっきが施される。
【００４３】
ＣＧＬ焼鈍炉の加熱帯は、無酸化炉型，直火炉型，ＲＴＦ型のいずれを使用しても良い。ＣＧＬ焼鈍炉の均熱帯は、通常ＲＴＦ型であり、本発明においてもこれが好適である。ＣＧＬ焼鈍炉の焼鈍雰囲気（均熱帯の雰囲気）は、Ｆｅにとって還元性であれば良く、Ｈ_２を数体積％〜２５体積％程度含むＮ_２雰囲気が好ましい。
【００４４】
ＣＧＬ焼鈍炉の焼鈍温度は、高張力鋼板の成分や要求される強度に応じて適宜設定する。ただしＣＧＬ焼鈍炉の焼鈍温度が６５０℃未満では、酸洗後も残存する表面濃化層が十分に還元されず、高張力溶融亜鉛めっき鋼板の表面性状が劣化する。一方、ＣＧＬ焼鈍炉の焼鈍温度が、連続焼鈍炉の均熱帯温度を上回ると、内部酸化層が過剰に生成され、めっき密着性が十分に改善されない。したがって、ＣＧＬ焼鈍炉の焼鈍温度は６５０℃以上〜連続焼鈍炉の均熱帯温度以下の範囲内とするのが好ましい。
【００４５】
このようにしてＣＧＬ焼鈍炉で焼鈍を施した高張力鋼板は、従来の溶融亜鉛めっき処理と同様に、５００℃程度に降温した後、４４０〜５００ ℃の溶融亜鉛めっき浴にて溶融亜鉛めっきを施される。溶融亜鉛めっき浴のＡｌ濃度は、ＧＡの場合は０．１２〜０．１８質量％，ＧＩの場合は０．１４〜０．２４質量％が好ましい。こうして高張力鋼板は溶融亜鉛めっきを施され、高張力溶融亜鉛めっき鋼板となる。
【００４６】
溶融亜鉛めっき浴から高張力溶融亜鉛めっき鋼板が搬出されると、ガスワイピングでめっきの付着量（いわゆる目付量）を調整し、その後、必要に応じて合金化処理を施す。目付量は、製品の仕様として要求される特性に応じて設定されるので、本発明では特定の範囲に限定しない。ただし、耐食性を確保するためには２０〜９０ｇ／ｍ^２程度で十分である。
【００４７】
合金化処理を施す場合には、合金化処理の温度が４６０℃未満では、合金化処理に長時間を要するので生産性が低下する。一方、５４０℃を超えると、めっき密着性が劣化する。したがって、合金化処理の温度は４６０〜５４０ ℃の範囲内が好ましい。ただし、内部酸化層が十分に生成されている場合は５００〜５２０ ℃で合金化処理を施しても、めっき密着性改善の効化は損なわれない。
【００４８】
また合金化処理を施す場合は、溶融亜鉛めっき層中のＦｅ含有量は８〜１３質量％の範囲内が好ましい。溶融亜鉛めっき層中のＦｅ含有量は８質量％未満では、摺動性が劣るζ相が多量に生成するので、高張力溶融亜鉛めっき鋼板の耐フレーキング性が劣化する。一方、１３質量％を超えると、硬くて脆いΓ相が多量に生成するので、内部酸化層が形成されていてもプレス成形におけるめっきの剥離を防止できなくなる。
【００４９】
【実施例】
高張力鋼板の素材として、表１に示す組成のスラブを加熱炉で加熱（１２５０℃，４０分）し、引き続き熱間圧延で２．８ｍｍの熱延鋼板として５００℃で巻取った。次いで、酸洗で表面のスケール（いわゆる黒皮）を除去し、冷間圧延で１．６ｍｍの冷延鋼板（すなわち高張力鋼板）とした。
【００５０】
【表１】

【００５１】
これらの高張力鋼板の表面を清浄化した後、無酸化炉型，直火炉型の加熱帯およびＲＴＦ型の均熱帯を有する連続焼鈍炉で焼鈍を施した。これを発明例とする。
一方、比較例として加熱帯および均熱帯がＲＴＦ型の連続焼鈍炉で焼鈍を施した。
【００５２】
なお発明例および比較例は、いずれもＣＡＬに設置されている連続焼鈍炉を使用した。焼鈍後の鋼板について、内部酸化量を求めた。すなわち内部酸化量は、前記した「インパルス炉溶融−赤外線級手法」を用いてＯ_ＩとＯ_Ｈを測定し、その測定値からＯ_Ｉ−Ｏ_Ｈを算出して１ｍ^２あたりの量に換算して求めた。連続焼鈍炉の設定と、内部酸化量は表２に示す通りである。
【００５３】
【表２】

【００５４】
こうして焼鈍が終了した後、発明例と比較例の高張力鋼板を一旦冷却して酸洗を行ない、高張力鋼板の表面濃化層を除去した。酸洗液は５％塩酸を使用し、その温度は６０℃，浸漬時間は６ｓｅｃとした。
表面濃化層の除去率は、グロー放電発光分光分析装置（島津製作所製ＧＤＬＳ５０１７型）を使用し、試料電流２０ｍＡ，Ａｒガス流量８．３ｃｍ^３／ｓｅｃの条件で、高張力鋼板の表面から深さ方向に３０ｓｅｃスパッタリングしたプロファイルを基に、上記した方法で定量化した。スパッタリング速度は、Ｆｅ換算で約０．０１μｍ／ｓｅｃであった。
【００５５】
ただし各元素の含有量の違いによってデプスプロファイルのベースラインが異なることを考慮し、表面近傍の各元素の測定強度の積算値（図１中の斜線部の面積に相当）を求めるにあたっては、Ｓｉ，Ｍｎ，Ａｌのピークがいずれもスパッタリング時間０〜１５ｓｅｃまでに存在していることから、スパッタリング時間が１５〜３０ｓｅｃまでの積分値を、スパッタリング時間０〜１５ｓｅｃまでの積分値から差し引くことで、焼鈍後および酸洗後の表面濃化量を求めた。酸洗前後の試料について、その積算値を相対比較することによって、表面濃化層の除去率を求めた。
【００５６】
次に、ＣＧＬに設置されている焼鈍炉で焼鈍を施し、さらに溶融亜鉛めっき浴に浸漬して、高張力溶融亜鉛めっき鋼板を製造した。なお、ＣＧＬの焼鈍炉の加熱帯は、ＲＴＦ型，直火炉型，無酸化炉型のいずれかであり、均熱帯は全てＲＴＦ型である。また、溶融亜鉛めっき浴の出側でガスワイピングを行ない、目付量を片面あたり５０ｇ／ｍ^２に調整した。溶融亜鉛めっき浴のＡｌ濃度は、ＧＡ用として０．１３質量％，ＧＩ用として０．１６質量％とした。溶融亜鉛めっき浴の温度は４６０℃とし、高張力鋼板の温度も４６０℃に調整して浸漬した。
【００５７】
めっき層に合金化処理を施す場合は、４６０〜５８０ ℃の温度に１５ｓｅｃ保持して処理した。また、めっき層中のＦｅ含有量を原子吸光法で測定した。すなわち、２０質量％ＮａＯＨ−１０質量％トリエタノールアミン水溶液１９５ｃｍ^３＋３５質量％Ｈ_２Ｏ_２水溶液７ｃｍ^３を用いてめっき層を除去して、原子吸光法でめっき層中のＦｅ含有量を定量した。
【００５８】
酸洗による表面濃化層の除去率，ＣＧＬ焼鈍炉の加熱帯種類と均熱帯雰囲気は表３に示す通りである。
【００５９】
【表３】

【００６０】
こうして得られた高張力溶融亜鉛めっき鋼板を目視で観察し、その外観および不めっきの有無を評価した。
さらにＧＩについては、ボールインパクト試験を行ない、加工部を粘着テープで剥離し、めっきの剥離の有無を観察し、剥離が認められないものを良好，剥離が生じたものを不良として評価した。
【００６１】
ＧＡについては、高張力溶融亜鉛めっき鋼板に粘着テープを貼り、次いで粘着面を９０°曲げ−戻しを行ない、そのときの剥離量を蛍光Ｘ線によってＺｎカウント数を測定し、めっき密着性を耐パウダリング性として評価した。
これらの評価の結果を表４に示す。
【００６２】
【表４】

【００６３】
表４から明らかなように、発明例１〜１５は、いずれも外観が良好で、しかもめっき密着性も優れている。しかも既存の設備を利用できるので、新たな設備投資は必要ない。
【００６４】
【発明の効果】
本発明によれば、高強度化元素としてＳｉ，Ｍｎ，Ａｌを含有する高張力鋼板に溶融亜鉛めっきを施すにあたって、優れた表面性状を有し、かつ優れためっき密着性を有する高張力溶融亜鉛めっき鋼板を効率良く製造することができる。
【図面の簡単な説明】
【図１】ＧＤＳにより深さ方向の濃度プロファイルを測定した場合のＦｅおよびＳｉの分布の例を示すグラフであり、（ａ）は連続焼鈍後の高張力鋼板についての測定結果を示し、（ｂ）は酸洗後の高張力鋼板についての測定結果を模式的に示す。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet having excellent plating adhesion and suitable for use in the fields of automobiles, building materials, home appliances, and the like.
[0002]
[Prior art]
2. Description of the Related Art In recent years, applications of high-tensile steel sheets have been expanding in the fields of automobiles, building materials, home appliances, and the like. Particularly in the automotive industry, the use of high-tensile steel sheets has been rapidly increasing from the viewpoint of improving fuel efficiency by reducing the weight of a vehicle body and improving collision safety. Such a high-strength steel sheet is, for example, disclosed in Japanese Patent Publication No. 3-51778 in order to improve mechanical properties such as strength-ductility balance and the like, such as elements such as Si, Mn, Ti, Al, and P. In many cases.
[0003]
However, it is known that when the Si content is increased, Si oxides are formed on the steel sheet surface by annealing, which not only hinders the chemical conversion treatment, but also deteriorates the adhesion of electrogalvanized and the adhesion of hot-dip galvanized. ing. In particular, in hot-dip galvanizing of high-tensile steel containing Si, there is a large possibility that a non-uniform plated layer is formed without partially adhering molten zinc (so-called non-plated) or the plated layer is peeled off by pressing. Has become a problem.
[0004]
When the Si content exceeds 0.1% by mass, an all-radiant tube type (hereinafter, abbreviated as RTF type for Radiant Tube Furnace) or a non-oxidized type (hereinafter, abbreviated as NOF for Non Oxidized Furnace) is used. Operation becomes extremely unstable in a continuous hot-dip galvanizing line having an annealing furnace. Further, in the hot rolling step of the slab as a raw material, irregularities derived from the Si—Fe-based scale are generated, and the quality of the high-strength hot-dip galvanized steel sheet as a final product is impaired.
[0005]
As a method of improving the plating adhesion of a steel sheet containing a large amount of Si or Mn, a method of performing electroplating before introducing a steel sheet into a continuous galvanizing line (hereinafter, abbreviated as CGL) ( For example, Japanese Patent Application Laid-Open No. 2-194156) and a method of improving plating wettability by disposing a steel sheet having a small content of Si and Mn on a surface layer by a cladding method (for example, Japanese Patent Application Laid-Open No. 3-199363) are known. I have.
[0006]
On the other hand, in order to sufficiently alloy the plating layer, it is necessary to extend the time of the alloying treatment or to increase the alloying temperature. In order to extend the alloying time, the transport speed of the CGL must be reduced, so that the productivity is reduced and the production cost is increased. On the other hand, when the alloying temperature is increased, it becomes difficult to control the Fe content in the plating layer, and alloying tends to proceed excessively. In addition, the plating adhesion deteriorates, and the product yield decreases.
[0007]
Therefore, as a technique for promoting alloying while avoiding such a problem, Japanese Patent Application Laid-Open No. 58-120771 discloses a technique for forming an alloy of Fe-Zn by plating Ni, Cu or the like before reduction annealing. A technology that promotes is disclosed. However, in this technique, it is necessary to install new equipment for plating Ni, Cu, or the like.
Japanese Patent Publication No. 64-11111 discloses a technique for changing the Al concentration and the temperature of a Zn bath to keep the conditions of the alloying treatment constant. However, it takes a long time to change the Al concentration and the temperature of the Zn bath. In the meantime, the operation of the CGL must be stopped, which causes a decrease in productivity. Moreover, if this technique is applied to a steel sheet containing a large amount of Si, Mn, and Al, problems such as peeling of a plating layer and non-plating occur.
[0008]
Japanese Patent Application Laid-Open No. Hei 3-243751 discloses a technique for promoting alloying by annealing a P-added steel and then removing a P-enriched layer. However, the P-added steel to which this technology is applied is 35 kgf / mm. ² It is a low-grade steel grade called a grade, and it is difficult to apply it to high-tensile steel sheets. In other words, the technique disclosed in Japanese Patent Application Laid-Open No. H3-243751 is applied to 45 kgf / mm ² When applied to a high-strength steel sheet called a high-grade steel sheet or a high-strength steel sheet with further increased strength, problems such as non-plating occur. In addition, Japanese Patent Application Laid-Open No. Hei 3-243751 does not refer to the type of continuous annealing furnace of a continuous annealing line (hereinafter abbreviated as CAL for continuous annealing line). Operating conditions cannot be set according to the characteristics of the continuous annealing furnace.
[0009]
That is, it was difficult to improve the plating adhesion of a high-strength hot-dip galvanized steel sheet by these techniques. Generally, Si, Mn, and Al are added to a high-strength steel sheet in a complex manner, and various techniques for improving the plating adhesion of a high-strength steel sheet containing these elements have been studied.
For example, Japanese Patent No. 3110238 and Japanese Patent No. 3162901 disclose techniques for improving the plating adhesion of a high-strength steel sheet by recrystallizing and annealing a high-strength steel sheet, followed by pickling and then annealing again. Have been. However, since these techniques do not produce an internal oxide layer, they are not sufficient to satisfy the strict plating adhesion specifications required for steel sheets for automobiles and the like.
[0010]
Japanese Patent No. 3130470 discloses a technique for suppressing the surface concentration of Si and Mn by forming an internal oxide layer. However, in this technique, it is important to increase the winding temperature, but there is a problem that the increase in the winding temperature causes deterioration of the ductility of the high-tensile steel sheet.
Japanese Patent Application Laid-Open No. 2000-290730 discloses a technique in which the dew point of a continuous annealing furnace is increased to generate an internal oxide layer in the continuous annealing furnace regardless of hot rolling conditions. However, in this technique, it is necessary to introduce a large amount of water into the furnace and humidify the atmosphere when heating at a high temperature of 800 to 1000 ° C. As a result, there is a problem that the durability of burners, measuring instruments or refractories installed in the furnace is deteriorated.
[0011]
As described above, when hot-dip galvanizing is applied to a high-tensile steel sheet to which Si, Mn, and Al are added, in the conventional technology, surface defects such as surface irregularities and non-plating occur, and sufficient plating adhesion is obtained. Not only cannot be obtained, but also causes a decrease in productivity and a decrease in yield.
[0012]
[Patent Document 1]
Japanese Patent Publication No. 3-51778
[Patent Document 2]
JP-A-2-194156
[Patent Document 3]
JP-A-3-199363
[Patent Document 4]
JP-A-58-120772
[Patent Document 5]
Japanese Patent Publication No. 64-11111
[Patent Document 6]
JP-A-3-243751
[Patent Document 7]
Japanese Patent No. 3110238
[Patent Document 8]
Japanese Patent No. 3162901
[Patent Document 9]
Japanese Patent No. 3130470
[Patent Document 10]
JP 2000-290730 A
[0013]
[Problems to be solved by the invention]
The present invention solves the above-described problems, and is excellent in hot-dip galvanizing even a high-strength steel sheet containing one or more of Si, Mn, and Al as high-strength elements. It is an object of the present invention to provide a method for efficiently producing a high-strength hot-dip galvanized steel sheet having excellent surface properties and excellent plating adhesion.
[0014]
[Means for Solving the Problems]
The present inventors use a continuous annealing furnace having a direct-fired furnace type or a non-oxidation furnace type heating zone before hot-dip galvanizing a high-tensile steel sheet containing Si, Mn, and Al as a strengthening element. By performing recrystallization annealing, it is possible to prevent the high-strength steel sheet from concentrating on the steel sheet surface of high-strength elements, and as a result, significantly improve the plating adhesion and surface properties of the high-strength hot-dip galvanized steel sheet I found what I can do.
[0015]
That is, in the present invention, one or more of Si: 0.1 to 2.0% by mass, Mn: 1.0 to 3.5% by mass, and Al: 0.1 to 2.5% by mass are used. After the high-tensile steel sheet contained therein was annealed in a continuous annealing furnace having a heating zone of a non-oxidizing furnace type or a direct-fired furnace type, 70% of the surface-enriched layer of the element contained in the high-tensile steel sheet among Si, Mn and Al was removed. % Or more is removed by pickling, then reheated, and thereafter hot-dip galvanized on the high-strength steel sheet.
[0016]
In the present invention, one or more of Si: 0.1 to 2.0% by mass, Mn: 1.0 to 3.5% by mass, and Al: 0.1 to 2.5% by mass may be used. The high-strength steel sheet is heated at a heating zone temperature of 400 to 800 ° C. ₂ O: 2 to 30% by volume, O ₂ : 5% by volume or less, CO + CO ₂ : 20% by volume or less, the balance being N ₂ And unavoidable impurities, and the soaking temperature is 750-950 ° C. and the soaking atmosphere is H ₂ : 1 to 10% by volume, balance N ₂ And after performing annealing using a continuous annealing furnace as an unavoidable impurity, 70% or more of the surface-concentrated layer of the element contained in the high-strength steel sheet among Si, Mn and Al is removed by pickling. This is a method for producing a high-strength hot-dip galvanized steel sheet in which reheating is performed later, and thereafter hot-dip galvanizing is performed on the high-strength steel sheet.
[0017]
In the invention of the method for manufacturing a high-strength hot-dip galvanized steel sheet, as a preferred embodiment, an element contained in the high-strength steel sheet among Si, Mn, and Al is internally oxidized by annealing in a surface layer portion of the high-strength steel sheet. preferable.
Further, the amount of increase in oxygen generated in the surface layer of the high-tensile steel sheet due to internal oxidation is 0.01 g / m / side. ² It is preferable that it is above. Further, it is preferable to subject the high-strength steel sheet to hot-dip galvanizing, and then to perform alloying treatment of the plating layer.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
The high-strength steel sheet to which the present invention is applied is one of Si: 0.1 to 2.0% by mass, Mn: 1.0 to 3.5% by mass, and Al: 0.1 to 2.5% by mass. Or two or more are contained.
These three elements are added for the main purpose of ensuring the strength as a high-tensile steel sheet, and are therefore referred to as high-strength elements herein. The high-strength elements have the following effects in addition to ensuring the strength of the high-tensile steel sheet. In the present invention, the respective contents are set in consideration of these effects of the strengthening elements.
[0019]
If the Si content is less than 0.1% by mass, sufficient plating adhesion can be obtained without applying the continuous annealing conditions of the present invention described later, and thus there is no significance in applying the present invention. On the other hand, when the content exceeds 2.0% by mass, even if the annealing conditions of the present invention described later are applied, an excessively concentrated surface layer of Si is generated, and the plating adhesion is not sufficiently improved. Moreover, since the progress of the alloying treatment is suppressed, it is necessary to extend the alloying time or raise the alloying temperature. As a result, a decrease in productivity or a deterioration in plating adhesion is caused. Therefore, Si is set in the range of 0.1 to 2.0% by mass.
[0020]
If the Mn content is less than 1.0% by mass, sufficient plating adhesion can be obtained without applying the continuous annealing conditions of the present invention described later, and thus there is no significance in applying the present invention. On the other hand, when the content exceeds 3.5% by mass, the weldability of the high-strength galvanized steel sheet deteriorates. Therefore, Mn was set in the range of 1.0 to 3.5% by mass.
When the Al content is less than 0.1% by mass, sufficient plating adhesion can be obtained without applying the continuous annealing conditions of the present invention described later, and thus there is no significance in applying the present invention. Al is also useful for improving the strength-ductility balance of a high-strength hot-dip galvanized steel sheet by changing the steel sheet structure to a structure having a retained austenite phase. In this sense, it is preferably 0.1% by mass or more. On the other hand, if it exceeds 2.5% by mass, the weldability of the high-strength galvanized steel sheet deteriorates. Therefore, Al was set in the range of 0.1 to 2.5% by mass.
[0021]
The elements added to the high-strength steel sheet to which the present invention is applied are not necessarily limited to the above-mentioned three kinds of high-strength elements. C may be added in addition to the strengthening element, and P, S, Ti, Nb, Cr, Mo, B, N, V, etc. may be added as necessary. The addition amount of these elements other than the strengthening element is not limited to a specific range, and is appropriately added so as to obtain a predetermined strength. The contents of these elements are as follows: C: 0.25% by mass or less, P: 0.20% by mass or less, S: 0.01% by mass or less, Ti: 0.3% by mass or less, Nb: 0.3% by mass Hereinafter, there is no problem if Cr: 1% by mass or less, Mo: 1% by mass or less, B: 0.005% by mass or less, N: 0.01% by mass or less, and V: 0.3% by mass or less.
[0022]
The balance other than the elements described above is preferably Fe and unavoidable impurities. In addition, a conventionally known technique such as hot rolling or cold rolling is used for a method of manufacturing a high-tensile steel sheet.
Next, the continuous annealing condition, which is the most important condition of the present invention, will be described.
Thus, a high-tensile steel sheet containing a predetermined amount of a high-strength element is charged into a continuous annealing furnace having a non-oxidizing furnace type or direct-fired furnace type heating zone and subjected to annealing. In a heating zone of a non-oxidizing furnace type or a direct-fired furnace type, since the oxygen partial pressure of the atmosphere in the furnace is relatively high, the internal oxidation of the high-strength elements is promoted during the temperature rise process during annealing. By promoting internal oxidation in this manner, an effect of suppressing formation of a surface-concentrated layer made of an oxide of a high-strength element on the surface can be obtained. By suppressing the formation of such a surface-concentrated layer, surface defects such as non-plating can be prevented, and the surface properties of the high tensile galvanized steel sheet can be improved.
[0023]
However, in a continuous annealing furnace in which the heating zone is of the RTF type, the internal oxidation hardly proceeds because the oxygen partial pressure of the furnace atmosphere in the heating zone is low. Therefore, in the present invention, annealing is performed using a continuous annealing furnace having a heating zone of a non-oxidizing furnace type or a direct-fired furnace type. In the present invention, the internal oxidation of easily oxidizable elements such as Si can be promoted in a heating zone in which the ambient temperature is as low as 800 ° C. or less. It is also advantageous from the viewpoint of sex.
[0024]
The atmosphere in the heating zone of the continuous annealing furnace is H as an oxygen supply source. ₂ O: 2 to 30% by volume, O ₂ : 5% by volume or less, CO + CO ₂ : 20% by volume or less, the balance being N ₂ And it is preferable to use unavoidable impurities. The heating zone is not limited to the non-oxidizing furnace type or the direct-fired furnace type as long as this atmosphere can be maintained as the heating zone. However, if the heating zone is the non-oxidizing furnace type or the direct-fired furnace type, the atmosphere is satisfied. .
[0025]
H in heating zone atmosphere ₂ If O is less than 2% by volume, internal oxidation does not proceed. On the other hand, if it exceeds 30% by volume, the quality of the high-strength galvanized steel sheet is not adversely affected, but the durability of burners, measuring instruments or refractories installed in the furnace is deteriorated. Therefore, H ₂ O is in the range of 2 to 30% by volume.
O in heating zone atmosphere ₂ If it exceeds 5% by volume, the surface of the high-strength steel sheet is oxidized, a surface-concentrated layer of a high-strength element is formed, and the surface properties of the high-strength galvanized steel sheet are deteriorated. In addition, pick-up occurs on the transport rollers and the like in the continuous annealing furnace, which hinders the operation. Therefore, O ₂ Is 5% by volume or less. Where O ₂ Is inevitably mixed from the atmosphere. Therefore, a large facility load and labor are required to reduce the volume to less than 0.01% by volume. ₂ Is preferably 0.01% by volume or more.
[0026]
In addition, CO and / or CO ₂ May be contained. In a continuous annealing furnace of a non-oxidizing furnace type or a direct-fired furnace type, CO or CO is reduced by burning fuel with a burner in the furnace. ₂ Occurs. However, the content of these gases is CO + CO ₂ Exceeds 20% by volume, after continuous annealing, a surface-concentrated layer of Si, Mn, and Al is formed on the surface of the high-strength steel sheet, which causes deterioration of the surface properties of the high-strength galvanized steel sheet. In addition, pick-up occurs on the transport rollers and the like in the continuous annealing furnace, which hinders the operation. Therefore, CO + CO ₂ To 20% by volume or less. That is, CO and CO ₂ , When the total content is 20% by volume or less, CO or CO ₂ , Respectively, the content is 20% by volume or less. Preferably it is 15% by volume or less.
[0027]
Furthermore, the heating zone atmosphere is H ₂ May be included. H in that case ₂ The concentration is H, which is equivalent to ₂ The concentration is preferably set.
The rest of the heating zone atmosphere is N ₂ And unavoidable impurities. The air-fuel ratio of the combustion gas is preferably 0.75 to 1.2. ₂ The present invention is not limited to a specific range because O has the greatest effect on internal oxidation.
[0028]
The temperature of the heating zone of the continuous annealing furnace is in the range of 400 to 800 ° C. If the temperature of the heating zone is lower than 400 ° C., the internal oxidation does not sufficiently proceed, so that the plating adhesion is not improved. On the other hand, when the temperature exceeds 800 ° C., the temperature of the furnace body rises, and the durability deteriorates.
The high-tensile steel sheet charged into the continuous annealing furnace is heated in the heating zone in this way, and then transported to the soaking zone.
[0029]
The atmosphere of the tropics in the continuous annealing furnace is H ₂ : 1 to 10% by volume, the balance being N ₂ And it is preferable to use unavoidable impurities.
H with a uniform tropical atmosphere ₂ Is less than 1% by volume, the Fe oxide generated on the surface of the high-tensile steel sheet when the high-tensile steel sheet passes through the heating zone is not reduced and remains. As a result, surface defects (non-plating, peeling of plating, etc.) of the high-tensile hot-dip galvanized steel sheet are likely to occur. On the other hand, if it exceeds 10% by volume, the internal oxide layer generated in the heating zone is reduced, and the effect of improving the plating adhesion cannot be obtained. Therefore, H ₂ O is in the range of 2 to 5% by volume.
[0030]
The rest of the tropic atmosphere is N ₂ And unavoidable impurities. In addition, in order to set the above-mentioned conditions in a sotropic atmosphere, it is preferable to use an RTF-type sophomore.
The soaking zone temperature of the continuous annealing furnace is in the range of 700 to 950 ° C. If the soaking temperature is lower than 700 ° C., the internal oxidation does not proceed sufficiently, so that the plating adhesion is not improved. On the other hand, when the temperature exceeds 950 ° C., the temperature of the furnace body rises, and the durability deteriorates.
[0031]
By performing annealing using a continuous annealing furnace set to satisfy such conditions, Si, Mn, and Al, which are easily oxidizable elements contained in the high-strength steel sheet, are internally oxidized in the surface layer portion of the steel sheet. Can be.
Here, the amount of internal oxidation is 0.01 to 1 g / m per one side of the steel sheet. ² It is preferable to set the operating conditions within the above-mentioned heating zone, somitic atmosphere, and temperature range so that The oxidation amount indicates the amount of increase in oxygen per unit area of the steel sheet before and after passing through the continuous annealing furnace under the above-mentioned heating zone and soaking conditions. Oxidation amount in continuous annealing is 0.01 g / m per side ² By doing so, the effect of improving the plating adhesion can be obtained even more stably.
[0032]
Oxidation amount is 1 g / m ² If it exceeds 300, cracks occur on the surface of the steel sheet and the appearance after plating may deteriorate, which is not preferable. The relationship between the steel type, the line speed at the time of passing through the continuous annealing furnace, the detailed temperature setting and atmosphere setting of the heating zone and soaking zone, and the oxidation amount is experimentally obtained in advance, and the operation in which the oxidation amount is within the above range It is preferable to set conditions. It is needless to say that the longer the residence time of the steel sheet in the heating zone, which is an oxidizing atmosphere, and the higher the temperature of the heating zone, the greater the oxidation amount.
[0033]
The amount of internal oxidation can be obtained by measuring the amount of oxygen in the internal oxide layer of the high-strength steel sheet by, for example, a method called “impulse furnace melting-infrared ray method”. However, in order to determine the internal oxidation amount of the high-tensile steel sheet, it is necessary to subtract the amount of oxygen contained in the raw material (that is, the high-tensile steel sheet before annealing). Therefore, in the present invention, the surface layers on both sides of the high-strength steel sheet after continuous annealing are polished by 100 μm or more to measure the oxygen concentration in the steel, and the measured value is used as the oxygen content O in the material. _H And the oxygen concentration in the steel in the entire thickness direction of the high-tensile steel sheet after continuous annealing is measured, and the measured value is used as the oxygen content O after internal oxidation. _I And
[0034]
Oxygen content O after internal oxidation of the high strength steel sheet thus obtained _I And the amount of oxygen O contained in the material _H And O _I And O _H Difference (= O _I -O _H ) And further calculate the unit area (that is, 1 m ² ) (G / m ² ) Is the internal oxidation amount.
An internal oxide layer of a high-strength element is formed on the surface layer of a high-strength steel sheet that has been subjected to recrystallization annealing in a continuous annealing furnace in this manner, and the surface is made of an oxide of the high-strength element. A surface thickened layer is formed. This surface-concentrated layer also contains oxides of other elements added to the high-strength steel sheet. The internal oxide layer has the effect of improving plating adhesion.
[0035]
Next, pickling is performed on the steel sheet that has been subjected to continuous annealing. In the present invention, after the above-described continuous annealing, many easily oxidizable elements such as Si, Mn, and Al in the surface layer of the steel sheet exist as oxides in the ground iron in the surface layer of the steel sheet. However, since the surface concentration of these easily oxidizable elements cannot be completely eliminated, it is necessary to remove them by pickling. Therefore, in order to quantify the removal rate of the surface-enriched layer of these elements, which is effective in preventing non-plating, the steel sheet after continuous annealing under the above-mentioned conditions, and the steel sheet further pickled, were made of Si, Mn, Al was quantified based on a concentration profile obtained by low sputtering measurement (for example, about 0.01 μm / sec in terms of Fe) by glow discharge emission spectroscopy (so-called GDS).
[0036]
As a result, if the surface-enriched layer of the elements contained in the steel among Si, Mn, and Al is removed at a removal rate of 70% by pickling, the plating adhesion after hot-dip galvanizing is performed. The properties were found to be good. FIG. 1 shows an example of the distribution of Fe and Si when the concentration profile in the depth direction is measured by GDS. FIG. 1A shows the measurement results of a high-tensile steel sheet after continuous annealing, and FIG. ) Schematically shows the measurement results for the high-tensile steel sheet after pickling.
[0037]
In the present invention, for example, regarding Si, for a steel sheet after continuous annealing and after pickling, the concentration profiles of Si as shown in FIG. 1 are measured, and in these concentration profiles, the peaks of Si existing near the surface are measured. The integrated value of the measured intensity (corresponding to the area of the hatched portion in FIG. 1) is determined as the amount of surface concentration of Si, and the removal rate of the surface concentration layer is determined by the following equation (1).
[0038]

Of the elements contained in the steel in the above-mentioned content range among Si, Mn, and Al, pickling is performed so that the removal rate thus obtained is 70% or more, so that non-plating is performed. Can be prevented. In measuring the concentration profile, it is assumed that sputtering is performed until the inside of the steel sheet (ground iron portion) is included through the surface thickening layer for the measurement time. It is considered that the baseline of the depth profile differs depending on the content of the element.
[0039]
When the surface-concentrated layer is removed so that the removal rate becomes 90% or more, the surface properties of the high-strength galvanized steel sheet are significantly improved. Therefore, the removal rate is more preferably 90% or more, and most preferably the removal rate is 100% (that is, the surface-concentrated layer is completely removed).
Regarding the actual pickling conditions, the temperature, concentration, and immersion time of the pickling solution were experimentally determined in consideration of the component composition of the steel sheet and the continuous annealing conditions so that the above-mentioned removal rate was 70% or more. It is good.
[0040]
Suitable pickling conditions may be immersion in a pickling solution having an acid concentration of 0.1 to 20% and a temperature of 40 to 90 ° C. for 1 to 100 seconds. If the temperature of the pickling solution is less than 40 ° C. or the immersion time is less than 1 sec, the surface condensed layer is not sufficiently removed, and the surface properties of the high-strength galvanized steel sheet deteriorate. On the other hand, if the temperature of the pickling liquid exceeds 90 ° C. or the immersion time exceeds 100 seconds, the surface of the high-strength steel sheet becomes rough, and the surface properties of the high-strength hot-dip galvanized steel sheet deteriorate. In addition, since the internal oxide layer of the high-tensile steel sheet is also removed, the effect of improving the plating adhesion cannot be obtained.
[0041]
When the acid concentration of the pickling solution is less than 0.1%, it takes a long time to remove the surface-concentrated layer, and the productivity is reduced. On the other hand, if it exceeds 20%, the surface of the high-tensile steel sheet becomes rough, and the surface properties of the high-strength galvanized steel sheet deteriorate. In addition, since the internal oxide layer of the high-tensile steel sheet is also removed, the effect of improving the plating adhesion cannot be obtained. Further, the pickling loss is 0.05 to 5 g / m per side in terms of Fe. ² Is preferred.
[0042]
As the kind of acid used as the pickling solution, a commonly used acid such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid is used. However, it is preferable to use hydrochloric acid or sulfuric acid which can be obtained at low cost.
After the completion of the pickling, the high-tensile steel sheet is fed to a CGL used for a normal hot-dip galvanizing process and subjected to hot-dip galvanizing to produce a high-strength hot-dip galvanized steel sheet. In CGL, hot-dip galvanizing is usually performed after reheating in an annealing furnace.
[0043]
As a heating zone of the CGL annealing furnace, any of a non-oxidizing furnace type, a direct-fired furnace type, and an RTF type may be used. The soaking zone of the CGL annealing furnace is usually of the RTF type, which is also preferred in the present invention. The annealing atmosphere (sotropic atmosphere) of the CGL annealing furnace may be any reducing effect for Fe. ₂ Containing about several to 25% by volume of N ₂ An atmosphere is preferred.
[0044]
The annealing temperature of the CGL annealing furnace is appropriately set according to the components of the high-tensile steel sheet and the required strength. However, if the annealing temperature of the CGL annealing furnace is lower than 650 ° C., the surface concentrated layer remaining even after pickling is not sufficiently reduced, and the surface properties of the high tensile galvanized steel sheet deteriorate. On the other hand, when the annealing temperature of the CGL annealing furnace is higher than the soaking temperature of the continuous annealing furnace, the internal oxide layer is excessively generated, and the plating adhesion is not sufficiently improved. Therefore, it is preferable that the annealing temperature of the CGL annealing furnace be in the range of 650 ° C. or higher to the soaking zone temperature of the continuous annealing furnace.
[0045]
The high-strength steel sheet annealed in the CGL annealing furnace in this way is cooled to about 500 ° C in the same manner as in the conventional hot-dip galvanizing treatment, and then hot-dip galvanized in a hot-dip galvanizing bath at 440 to 500 ° C. Will be applied. The Al concentration of the hot-dip galvanizing bath is preferably 0.12 to 0.18% by mass for GA and 0.14 to 0.24% by mass for GI. Thus, the high-strength steel sheet is hot-dip galvanized, and becomes a high-strength hot-dip galvanized steel sheet.
[0046]
When the high-strength hot-dip galvanized steel sheet is carried out of the hot-dip galvanizing bath, the amount of plating (so-called basis weight) is adjusted by gas wiping, and thereafter, an alloying treatment is performed as necessary. The weight per unit area is set in accordance with the characteristics required as product specifications, and is not limited to a specific range in the present invention. However, in order to ensure corrosion resistance, 20 to 90 g / m ² A degree is enough.
[0047]
When the alloying treatment is performed, if the temperature of the alloying treatment is lower than 460 ° C., the productivity is reduced because the alloying treatment requires a long time. On the other hand, when the temperature exceeds 540 ° C., the plating adhesion deteriorates. Therefore, the temperature of the alloying treatment is preferably in the range of 460 to 540 ° C. However, when the internal oxide layer is sufficiently generated, the effect of improving the plating adhesion is not impaired even if the alloying treatment is performed at 500 to 520 ° C.
[0048]
In the case where an alloying treatment is performed, the Fe content in the hot-dip galvanized layer is preferably in the range of 8 to 13% by mass. If the Fe content in the hot-dip galvanized layer is less than 8% by mass, a large amount of ζ phase having poor slidability is generated, and the flaking resistance of the high-strength hot-dip galvanized steel sheet is deteriorated. On the other hand, when the content exceeds 13% by mass, a large amount of hard and brittle Γ phase is generated, so that even if an internal oxide layer is formed, it is not possible to prevent peeling of plating during press molding.
[0049]
【Example】
As a material of the high-tensile steel sheet, a slab having the composition shown in Table 1 was heated in a heating furnace (1250 ° C., 40 minutes), and subsequently hot-rolled to form a 2.8 mm hot-rolled steel sheet at 500 ° C. Next, the scale (so-called black scale) on the surface was removed by pickling, and a cold-rolled steel sheet of 1.6 mm (that is, a high-tensile steel sheet) was formed by cold rolling.
[0050]
[Table 1]

[0051]
After the surfaces of these high-tensile steel sheets were cleaned, they were annealed in a non-oxidizing furnace type, direct-fired furnace type heating zone, and RTF type continuous annealing furnace having a soaking zone. This is an invention example.
On the other hand, as a comparative example, the heating zone and the soaking zone were annealed in an RTF-type continuous annealing furnace.
[0052]
In each of the invention examples and the comparative examples, a continuous annealing furnace installed in CAL was used. For the steel sheet after annealing, the amount of internal oxidation was determined. That is, the amount of internal oxidation is determined using the above-mentioned “impulse furnace melting-infrared ray class method”. _I And O _H Is measured, and from the measured value, O _I -O _H 1m ² It was calculated by converting the amount per unit. The settings of the continuous annealing furnace and the amount of internal oxidation are as shown in Table 2.
[0053]
[Table 2]

[0054]
After the annealing was completed, the high-strength steel sheets of the invention example and the comparative example were once cooled and pickled to remove a surface-concentrated layer of the high-strength steel sheet. The pickling liquid used was 5% hydrochloric acid, the temperature was 60 ° C., and the immersion time was 6 seconds.
The removal rate of the surface condensed layer was determined using a glow discharge optical emission spectrometer (GDLS5017 manufactured by Shimadzu Corporation) at a sample current of 20 mA and an Ar gas flow rate of 8.3 cm. ³ / Sec was quantified by the above-described method based on a profile sputtered from the surface of the high-strength steel sheet in the depth direction for 30 sec. The sputtering rate was about 0.01 μm / sec in terms of Fe.
[0055]
However, taking into account that the base line of the depth profile varies depending on the content of each element, the integrated value of the measured intensity of each element near the surface (corresponding to the area of the hatched portion in FIG. 1) is determined by Si , Mn, and Al peaks are present before the sputtering time is 0 to 15 sec., So that the integrated value for the sputtering time 15 to 30 sec is subtracted from the integrated value for the sputtering time 0 to 15 sec. The amount of surface concentration after and after pickling was determined. The removal rate of the surface-concentrated layer was determined by relative comparison of the integrated values of the samples before and after pickling.
[0056]
Next, the steel sheet was annealed in an annealing furnace installed in CGL and further immersed in a hot-dip galvanizing bath to produce a high-strength hot-dip galvanized steel sheet. The heating zone of the CGL annealing furnace is any of the RTF type, the open flame type, and the non-oxidizing furnace type, and the soaking zone is all the RTF type. Further, gas wiping was performed on the outlet side of the hot-dip galvanizing bath, and the basis weight was 50 g / m per side. ² Was adjusted to The Al concentration in the hot-dip galvanizing bath was 0.13% by mass for GA and 0.16% by mass for GI. The temperature of the hot-dip galvanizing bath was 460 ° C, and the temperature of the high-strength steel sheet was also adjusted to 460 ° C and immersed.
[0057]
When performing an alloying treatment on the plating layer, the plating layer was kept at a temperature of 460 to 580 ° C. for 15 seconds. Further, the Fe content in the plating layer was measured by an atomic absorption method. That is, a 20 mass% NaOH-10 mass% triethanolamine aqueous solution 195 cm ³ + 35% by mass H ₂ O ₂ Aqueous solution 7cm ³ Was used to remove the plating layer, and the Fe content in the plating layer was quantified by an atomic absorption method.
[0058]
Table 3 shows the removal rate of the surface concentrated layer by pickling, the type of heating zone in the CGL annealing furnace, and the soaking atmosphere.
[0059]
[Table 3]

[0060]
The high tensile galvanized steel sheet thus obtained was visually observed, and its appearance and the presence or absence of no plating were evaluated.
Further, as for GI, a ball impact test was performed, the processed portion was peeled off with an adhesive tape, and the presence or absence of peeling of the plating was observed. Those with no peeling were evaluated as good, and those with peeling were evaluated as poor.
[0061]
As for GA, an adhesive tape was applied to a high-strength hot-dip galvanized steel sheet, and then the adhesive surface was bent and returned at 90 °. The powdering property was evaluated.
Table 4 shows the results of these evaluations.
[0062]
[Table 4]

[0063]
As is clear from Table 4, all of Invention Examples 1 to 15 have good appearance and excellent plating adhesion. Moreover, since existing equipment can be used, no new equipment investment is required.
[0064]
【The invention's effect】
According to the present invention, when hot-dip galvanizing is applied to a high-strength steel sheet containing Si, Mn, and Al as high-strength elements, high-strength hot-dip zinc having excellent surface properties and excellent plating adhesion is provided. A plated steel sheet can be manufactured efficiently.
[Brief description of the drawings]
FIG. 1 is a graph showing an example of the distribution of Fe and Si when the concentration profile in the depth direction is measured by GDS. FIG. 1 (a) shows the measurement results of a high-tensile steel sheet after continuous annealing, and FIG. ) Schematically shows the measurement results for the high-tensile steel sheet after pickling.

Claims

High strength steel sheet containing one or more of Si: 0.1 to 2.0% by mass, Mn: 1.0 to 3.5% by mass, and Al: 0.1 to 2.5% by mass. After annealing in a continuous annealing furnace having a non-oxidizing furnace type or direct-fired furnace type heating zone, 70% or more of the surface-enriched layer of the element contained in the high-strength steel sheet among the Si, Mn and Al is removed. A method for producing a high-strength hot-dip galvanized steel sheet, comprising removing by pickling, then reheating, and thereafter subjecting the high-strength steel sheet to hot-dip galvanizing.

High strength steel sheet containing one or more of Si: 0.1 to 2.0% by mass, Mn: 1.0 to 3.5% by mass, and Al: 0.1 to 2.5% by mass. The heating zone temperature is 400 to 800 ° C., the heating zone atmosphere is H ₂ O: 2 to 30% by volume, O ₂ : 5% by volume or less, CO + CO ₂ : 20% by volume or less, and the balance is N ₂ and inevitable impurities. After annealing using a continuous annealing furnace in which the soaking temperature is 750 to 950 ° C., the soaking atmosphere is H ₂ : 1 to 10% by volume, the balance is N ₂ and unavoidable impurities, the Si and Mn are obtained. And 70% or more of the surface-concentrated layer of the elements contained in the high-strength steel sheet among Al and Al is removed by pickling, followed by reheating, and thereafter, the high-strength steel sheet is hot-dip galvanized. For producing high-strength hot-dip galvanized steel sheet

The high-strength hot-dip galvanizing according to claim 1 or 2, wherein an element contained in the high-strength steel sheet among the Si, Mn, and Al is internally oxidized on a surface layer of the high-strength steel sheet by the annealing. Steel plate manufacturing method.

Method for producing a high-tensile galvanized steel sheet according to claim 3, wherein the oxygen increase occurring in the surface portion of said high-tensile steel plate by the internal oxidation is per side 0.01 g / m ² or more.

The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, 2, 3, or 4, wherein after the hot-dip galvanizing is applied to the high-strength steel sheet, an alloying treatment of a plating layer is performed.