JP4166412B2

JP4166412B2 - Method for producing hot-dip galvanized steel sheet

Info

Publication number: JP4166412B2
Application number: JP2000120713A
Authority: JP
Inventors: 真樹子井尻; 一章京野
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2000-04-21
Filing date: 2000-04-21
Publication date: 2008-10-15
Anticipated expiration: 2020-04-21
Also published as: JP2001303225A

Description

【０００１】
【発明の属する技術分野】
本発明は、MnおよびＰを含有する高張力鋼板に溶融亜鉛めっきを施して、溶融亜鉛めっき鋼板を製造する方法に関するものである。
【０００２】
【従来の技術】
近年、自動車の軽量化および安全性の向上を目的として、高強度の自動車用鋼板のニーズが高まっている。鋼板の高強度化を図るためには、鋼板成分としてMnやＰを添加する方法が知られている。
ところがMnやＰを添加した高張力鋼板は、焼鈍時に、MnやＰが鋼板表面あるいは粒界に濃化するという現象が起こる。自動車用鋼板は腐食防止の目的で亜鉛めっきを施すが、MnやＰの濃化が生じると、溶融亜鉛めっきを施す場合に、不めっきやめっきムラ等の原因になる。またMnやＰが濃化した部分は、溶融亜鉛めっきを施した後の合金化処理には長い時間がかかる。
【０００３】
そのため、高張力鋼板の合金化処理を行なう場合は、合金化処理時間を長くしたり、合金化処理温度を高くして合金化処理を行なう必要がある。しかし合金化処理時間を延長すると生産性が低下し、合金化処理温度を高くすると合金化炉の耐火物が著しく損耗するという問題があった。
そこで溶融亜鉛めっきに先立って鋼板表面に硫黄化合物を塗布してＰの濃化を抑制する技術が提案されている。
【０００４】
たとえば、硫黄化合物として、硫酸ナトリウム、チオ硫酸ナトリウム、亜硫酸ナトリウム等の無機硫酸塩を用いる技術が開示されているが（特開平5-148603号公報、特開平11-50220号公報など）、形成される皮膜は摩擦などにより剥離しやすく、剥離が生じた部分は不めっきやめっきムラが生じてしまい、不めっきやめっきムラの問題は十分に解決されていない。
【０００５】
【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、皮膜の剥離を防止し、不めっきやめっきムラのない溶融亜鉛めっき鋼板の製造方法を提供することを目的とし、かつ溶融亜鉛めっき後の合金化処理の速度を向上することによって生産性を向上できる溶融亜鉛めっき鋼板の製造方法を提供することをも目的とする。
【０００６】
【課題を解決するための手段】
本発明者は、種々の実験を行なった結果、硫黄化合物の皮膜形成処理を行なう際に用いる溶液中にアルカリ金属やアルカリ土類金属が含有される場合は、めっき性が阻害されたり、合金化速度が低下する原因になることを見出した。たとえば二亜硫酸ナトリウムの加温水溶液（ｐＨ３〜６）に鋼板を浸漬した場合は、硫黄を含む皮膜が形成されるが、鋼板表面に細かい多孔質のさびが発生し、皮膜の剥離が生じやすい。また多孔質のさびの中に水溶液が染み込むと、洗浄してもNaを除去することが難しい。
【０００７】
Naが多量に付着すると、還元雰囲気下で焼鈍した後も鋼板表面にNaが残留するため、めっきのムラが生じたり、合金化速度が低下する。しかも焼鈍によって融点の低い金属酸化物が生成するので、搬送中にロールに付着し、後続の鋼板表面に再付着したり、疵の原因になる。
そこで、亜硫酸，硫化水素およびチオ硫酸アンモニウムの内から選ばれる１種または２種以上を含む水溶液を用いて皮膜形成処理を行なえば、短時間で硫黄化合物を含む皮膜が強固に形成され、剥離し難く、めっきのムラを防止し、かつ合金化速度を向上し、しかも後工程に及ぼす悪影響も抑えられるという知見を基に本発明をなすに至った。
【０００８】
本発明は、MnおよびＰを含有する高張力鋼板を、亜硫酸，硫化水素およびチオ硫酸アンモニウムの内から選ばれる１種または２種以上を含有する水溶液と接触させることによって、高張力鋼板の表面に硫黄化合物を含む皮膜を、Ｓ量に換算して５〜 200 mg ／ｍ ² の範囲で形成する皮膜形成処理を行なった後、還元雰囲気下にて焼鈍し、次いで溶融亜鉛めっきを施す溶融亜鉛めっき鋼板の製造方法である。
【０００９】
前記した発明においては、第１の好適態様として、溶融亜鉛めっきを施した後、合金化処理を行なうことが好ましい。
また第２の好適態様として、高張力鋼板が、Mnを0.05〜３mass％含有し、Ｐを 0.003〜0.1 mass％含有することが好ましい。
【００１０】
【発明の実施の形態】
MnおよびＰを含有する高張力鋼板を所定の厚さに冷間圧延した後、圧延油を洗浄するためにアルカリ脱脂またはアルカリ電解脱脂を行ない、必要に応じて酸洗を行なう。こうして洗浄された鋼板表面に、硫黄化合物を含む皮膜を形成するために皮膜形成処理を行なう。
【００１１】
なお高張力鋼板のMn含有量が0.05mass％未満の場合は、皮膜形成処理した後の不めっき等の問題は生じ難い。Mn含有量が３mass％を超える場合は、高張力鋼板の強度が飽和してしまう。したがってMn含有量は0.05〜３mass％の範囲とすることが好ましい。
またＰ含有量が 0.003mass％未満の場合は、高張力鋼板としての強度が得られない。Ｐ含有量が 0.1mass％を超える場合は、鋼板に割れが生じやすい。したがってＰ含有量は 0.003〜 0.1mass％の範囲とすることが好ましい。
【００１２】
上記した化学成分は、所望の特性に応じて適宜含有することができる。たとえば、Ｃ：0.0005〜0.5 mass％、Ｓ：0.05mass％以下、Nb： 0.001〜0.20mass％、Ｂ： 0.005mass％以下、Ti： 0.1mass％以下、Cr：0.05mass％以下が許容できる。
皮膜形成処理を行なう前に、鋼板を脱脂して、鋼板表面を親水性にしておくことが好ましい。脱脂が不十分である場合は、皮膜形成処理によって形成される皮膜が不均一となり、その結果、溶融亜鉛めっきや合金化処理の速度が不均一となるからである。
【００１３】
皮膜形成処理は、亜硫酸，硫化水素およびチオ硫酸アンモニウムの内から選ばれる１種または２種以上を含有する水溶液と接触させることによって行なう。たとえば、亜硫酸ガスを水に溶解させた溶液（以下、亜硫酸水という），硫化水素ガスを水に溶解させた溶液（以下、硫化水素水という）またはチオ硫酸アンモニウム水溶液を用いて、これらの溶液中に鋼板を浸漬させて皮膜形成処理を行なう。
【００１４】
これらの溶液中の硫黄化合物の濃度は 0.1〜30mass％の範囲が好ましい。硫黄化合物の濃度が 0.1mass％未満の場合は皮膜が形成されず、30mass％を超える場合はめっきのムラを生じるからである。さらに好ましくは、亜硫酸，硫化水素の場合は 0.1〜10mass％、チオ硫酸アンモニウムの場合は１〜30mass％の範囲である。水溶液のｐＨは３〜６が好ましい。ｐＨが低すぎると鋼板が溶解し、ｐＨが高すぎると皮膜が形成され難い。また、亜硫酸水あるいは硫化水素水を用いる場合は常温で皮膜形成処理を行なえるが、チオ硫酸アンモニウム水溶液を用いる場合は70〜90℃に加熱して皮膜形成処理を行なうのが望ましい。
【００１５】
なお、亜硫酸ガスまたは硫化水素ガスの雰囲気中で、皮膜形成処理を行なっても良い。
鋼板表面に付着する皮膜の量は、Ｓ量に換算して５〜200 mg／ｍ²の範囲とする。皮膜の付着量が、Ｓ量に換算して５mg／ｍ²未満の場合は、皮膜が少ないのでめっきのムラが発生し、しかも合金化速度は向上しない。200 mg／ｍ²を超える場合は、皮膜形成処理した後の焼鈍で硫黄化合物が十分還元されず、鋼板表面に残留して、めっきのムラが生じる。
【００１６】
なお鋼板表面の皮膜の量は、焼鈍炉内雰囲気の水素濃度や搬送速度等に応じて適正な範囲に維持する必要があり、前記したようにＳ量に換算して５〜200 mg／ｍ²の範囲が望ましいが、めっき性と生産性の観点から10〜120 mg／ｍ²の範囲が一層好ましく、さらに20〜80mg／ｍ²の範囲がより一層好ましい。
こうして生成した硫黄化合物を含む皮膜は、強固に鋼板表面と結び付いているため、ロール等に再付着することはない。したがって、皮膜形成処理を行なった鋼板を後工程に搬送する途中で汚染されたり、付着量のムラは生じない。
【００１７】
皮膜形成処理を行なった後、洗浄水で洗浄して、余剰の硫黄化合物を除去することが好ましい。
本発明においては、洗浄水で洗浄する方法は特定の構成に限定しないが、水槽に収容した洗浄水に鋼板を浸漬する方法、あるいはスプレーノズルを用いて洗浄水を鋼板に吹き付ける方法等が好ましい。
【００１８】
こうして皮膜形成処理を行なった鋼板を、水素を含む還元雰囲気の下で 700〜900 ℃の温度範囲で焼鈍し、次いで溶融亜鉛めっきを施す。この溶融亜鉛浴のAl濃度は、0.05〜0.2 mass％の範囲であることが好ましい。溶融亜鉛浴のAl濃度が0.05mass％未満の場合は粗大なFe−Zn粒子がまばらに生成してめっき層の均一性が損なわれ、Al濃度が 0.2mass％を超える場合はFe−Al合金が生成してZnのめっき層が形成されないからである。さらに必要に応じて 450〜600 ℃に加熱して合金化処理を行なっても良い。
【００１９】
【実施例】
Mnを 0.5mass％含有し、Ｐを0.04mass％含有する高張力鋼板を冷間圧延した後、アルカリ電解脱脂を行ない、さらに硫黄化合物を含む皮膜を形成するために皮膜形成処理を行なった。皮膜形成処理は、80℃に加熱した濃度10mass％のチオ硫酸アンモニウム水溶液に鋼板を10秒間浸漬して行なった。皮膜形成処理の後、ただちに流水で洗浄し、熱風乾燥機で乾燥した。鋼板表面に形成された硫黄化合物を含む皮膜中のＳ量（以下、Ｓ付着量という）を原子吸光法で測定した。
【００２０】
次に溶融めっきシミュレーターにて、水素濃度７mass％の還元雰囲気の下で焼鈍した。焼鈍温度は 810℃、焼鈍時間は55秒であった。その後、 470℃まで冷却し、 470℃に保持した溶融亜鉛浴に２秒間浸漬し、溶融亜鉛めっき鋼板を製造した。なお溶融亜鉛浴のAl濃度は0.14mass％，Fe濃度は0.05mass％であった。こうして得られためっき層中のZn量（以下、Zn付着量という）をアルカリ電解法によって測定した。
【００２１】
次に、この溶融亜鉛めっき鋼板に合金化処理を行なった。合金化処理は、溶融亜鉛めっき鋼板を 470℃に保持したＫＮＯ₃の溶融塩浴に浸漬して行なった。この浸漬時間（すなわち合金化処理時間）を変化させて、各々めっき層中へのFe拡散量をアルカリ電解法によって測定した。また、めっきのムラの有無を目視で調査した。その結果を発明例１〜６として表１に示す。なお発明例１〜６のサンプルの、硫黄皮膜形成後の表面を指で擦ったが、皮膜の剥離は生じなかった。
【００２２】
また硫黄化合物を含む皮膜を形成するための皮膜形成処理を行なわなかった例を、比較例１〜６として表１に示す。
【００２３】
【表１】

【００２４】
発明例１と比較例１は合金化処理を行なわなかった例であるが、発明例１の方が、めっき層中へのFe拡散量が大きいので合金化速度が速い。しかも発明例１はムラのない良好なめっきが得られた。
発明例２〜６は合金化処理を行なった例であり、いずれもめっき層中へのFe拡散量が発明例１より増加している。つまり発明例２〜６は、合金化速度が発明例１より一層増加し、しかもムラのない良好なめっきが得られた。
【００２５】
また発明例１〜６および比較例１〜６によると、合金化処理時間とめっき層中へのFe拡散量との関係は図１に示す通りであり、発明例の方が、めっき層中へのFe拡散量が大きい。つまり発明例１〜６は十分にめっきが施され、しかもそのめっき層の合金化速度が速く、しかもムラのない良好なめっきが得られた。
次に、硫黄化合物を含む皮膜を形成するための皮膜形成処理に用いる水溶液の種類と皮膜形成処理に要する時間の効果を調査した。
【００２６】
すなわち発明例７として、80℃に加熱した濃度10mass％のチオ硫酸アンモニウム水溶液に鋼板を18秒間浸漬して皮膜形成処理を行ない、さらに溶融亜鉛めっき後の合金化処理は38秒間行なった。その他の条件は前記の発明例１〜６と同じであるので、説明を省略する。
さらに発明例８として濃度３mass％の亜硫酸水に鋼板を５秒間浸漬して皮膜形成処理を行ない、発明例９として濃度４％の硫化水素水に鋼板を５秒間浸漬して皮膜形成処理を行なった。いずれも溶融亜鉛めっき後の合金化処理は38秒間行なった。
【００２７】
比較例としてアルカリ金属であるNaを含む水溶液を用いて皮膜形成処理を行なった。つまり80℃に加熱した濃度 9.6mass％の過硫酸ナトリウム水溶液に鋼板を20秒間浸漬して皮膜形成処理を行なった例を比較例７とし、80℃に加熱した濃度10.1mass％の硫酸ナトリウム水溶液に鋼板を10秒間浸漬して皮膜形成処理を行なった例を比較例８とした。いずれも溶融亜鉛めっき後の合金化処理は38秒間行なった。
【００２８】
この発明例７〜９と比較例７〜８について、Ｓ付着量，Zn付着量およびめっき層中へのFe拡散量を測定した。また、めっきのムラの有無を目視で調査した。その結果を表２に示す。なお硫黄皮膜形成後のサンプル表面を指で擦ったところ、発明例７〜９では剥離は生じなかったが、比較例７，８では一部剥離が発生した。
【００２９】
【表２】

【００３０】
発明例７〜９は皮膜の剥離がなく、ムラもない良好なめっきが得られたのに対して、比較例７〜８は皮膜の剥離が発生し、Ｓ付着量が少なくムラが発生した。また発明例７〜９の方が、めっき層中へのFe拡散量が大きいので、合金化速度が速いことが分かる。
【００３１】
【発明の効果】
本発明では、皮膜剥離がなく、しかもめっきムラのない良好な溶融亜鉛めっき鋼板を製造できる。さらに溶融亜鉛めっき後の合金化速度を向上することによって生産性を向上できる。
【図面の簡単な説明】
【図１】合金化処理時間とめっき層中へのFe拡散量との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-dip galvanized steel sheet by subjecting a high-tensile steel sheet containing Mn and P to hot-dip galvanizing.
[0002]
[Prior art]
In recent years, the need for high-strength steel sheets for automobiles has been increasing for the purpose of reducing the weight and safety of automobiles. In order to increase the strength of a steel plate, a method of adding Mn or P as a steel plate component is known.
However, in a high-strength steel sheet to which Mn and P are added, a phenomenon occurs in which Mn and P are concentrated on the steel sheet surface or grain boundaries during annealing. Automotive steel sheets are galvanized for the purpose of preventing corrosion. However, when Mn and P are concentrated, non-plating and uneven plating are caused when hot dip galvanization is performed. Further, the portion where Mn and P are concentrated takes a long time for the alloying treatment after the hot dip galvanizing.
[0003]
Therefore, when alloying the high-tensile steel plate, it is necessary to lengthen the alloying treatment time or increase the alloying treatment temperature to perform the alloying treatment. However, when the alloying treatment time is extended, productivity is lowered, and when the alloying treatment temperature is increased, there is a problem that the refractory in the alloying furnace is significantly worn.
Therefore, a technique has been proposed in which a sulfur compound is applied to the steel sheet surface prior to hot dip galvanizing to suppress P concentration.
[0004]
For example, a technique using an inorganic sulfate such as sodium sulfate, sodium thiosulfate, or sodium sulfite as a sulfur compound has been disclosed (JP-A-5-148603, JP-A-11-50220, etc.). The film to be peeled easily peels off due to friction or the like, and unplated or uneven plating occurs in the peeled portion, and the problems of unplating and uneven plating have not been sufficiently solved.
[0005]
[Problems to be solved by the invention]
The object of the present invention is to solve the above problems, to prevent peeling of the film, and to provide a method for producing a hot dip galvanized steel sheet without unplating or plating unevenness, and an alloying treatment after hot dip galvanizing Another object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet that can improve productivity by increasing the speed of the steel sheet.
[0006]
[Means for Solving the Problems]
As a result of various experiments, the present inventor has found that when an alkali metal or alkaline earth metal is contained in the solution used for the film formation treatment of the sulfur compound, the plating property is hindered or alloyed. It has been found that this causes a decrease in speed. For example, when a steel sheet is immersed in a heated aqueous solution of sodium disulfite (pH 3 to 6), a film containing sulfur is formed, but fine porous rust is generated on the surface of the steel sheet, and the film tends to peel off. In addition, when the aqueous solution soaks into the porous rust, it is difficult to remove Na even by washing.
[0007]
When a large amount of Na adheres, Na remains on the steel plate surface even after annealing in a reducing atmosphere, resulting in uneven plating or a decrease in alloying speed. Moreover, since a metal oxide having a low melting point is generated by annealing, it adheres to the roll during conveyance and reattaches to the surface of the subsequent steel sheet, or causes wrinkles.
Therefore, if a film formation treatment is performed using an aqueous solution containing one or more selected from sulfurous acid, hydrogen sulfide, and ammonium thiosulfate, a film containing a sulfur compound is formed firmly in a short time and is difficult to peel off. The present invention has been made on the basis of the knowledge that plating unevenness can be prevented, the alloying speed can be improved, and adverse effects on subsequent processes can be suppressed.
[0008]
In the present invention, a high-strength steel sheet containing Mn and P is brought into contact with an aqueous solution containing one or more selected from sulfurous acid, hydrogen sulfide, and ammonium thiosulfate, whereby sulfur on the surface of the high-strength steel sheet. A hot-dip galvanized steel sheet which is subjected to a hot-dip galvanizing after annealing in a reducing atmosphere after performing a film-forming treatment for forming a film containing a compound in the range of 5 to 200 mg / m ² in terms of S amount. It is a manufacturing method.
[0009]
In the above-described invention, as a first preferred embodiment, it is preferable to perform an alloying treatment after hot dip galvanizing.
As a second preferred embodiment, high-tensile steel sheet, the Mn containing 0.05～3Mass%, preferably contains P 0.003 to 0.1 mass%.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
After cold rolling a high-strength steel sheet containing Mn and P to a predetermined thickness, alkali degreasing or alkaline electrolytic degreasing is performed to wash the rolling oil, and pickling is performed as necessary. A film forming process is performed in order to form a film containing a sulfur compound on the surface of the steel plate thus cleaned.
[0011]
When the Mn content of the high-tensile steel plate is less than 0.05 mass%, problems such as non-plating after the film formation treatment are unlikely to occur. When the Mn content exceeds 3 mass%, the strength of the high-tensile steel plate is saturated. Accordingly, the Mn content is preferably in the range of 0.05 to 3 mass%.
Moreover, when P content is less than 0.003 mass%, the intensity | strength as a high-tensile steel plate cannot be obtained. If the P content exceeds 0.1 mass%, the steel sheet is likely to crack. Therefore, the P content is preferably in the range of 0.003 to 0.1 mass%.
[0012]
The above-described chemical components can be appropriately contained according to desired characteristics. For example, C: 0.0005 to 0.5 mass%, S: 0.05 mass% or less, Nb: 0.001 to 0.20 mass%, B: 0.005 mass% or less, Ti: 0.1 mass% or less, Cr: 0.05 mass% or less are acceptable.
It is preferable to degrease the steel plate to make the steel plate surface hydrophilic before performing the film formation treatment. This is because when the degreasing is insufficient, the film formed by the film forming process becomes non-uniform, and as a result, the speed of hot dip galvanizing or alloying process becomes non-uniform.
[0013]
The film formation treatment is performed by contacting with an aqueous solution containing one or more selected from sulfurous acid, hydrogen sulfide and ammonium thiosulfate. For example, a solution in which sulfurous acid gas is dissolved in water (hereinafter referred to as aqueous sulfurous acid), a solution in which hydrogen sulfide gas is dissolved in water (hereinafter referred to as aqueous hydrogen sulfide), or an aqueous solution of ammonium thiosulfate is used. The steel sheet is immersed and a film formation process is performed.
[0014]
The concentration of the sulfur compound in these solutions is preferably in the range of 0.1-30 mass%. This is because when the concentration of the sulfur compound is less than 0.1 mass%, no film is formed, and when it exceeds 30 mass%, uneven plating occurs. More preferably, in the case of sulfurous acid and hydrogen sulfide, the range is 0.1 to 10 mass%, and in the case of ammonium thiosulfate, the range is 1 to 30 mass%. The pH of the aqueous solution is preferably 3-6. If the pH is too low, the steel sheet is dissolved, and if the pH is too high, a film is hardly formed. In addition, when sulfite water or hydrogen sulfide water is used, the film formation treatment can be performed at room temperature. However, when an aqueous solution of ammonium thiosulfate is used, it is desirable to perform the film formation treatment by heating to 70 to 90 ° C.
[0015]
Note that the film formation treatment may be performed in an atmosphere of sulfurous acid gas or hydrogen sulfide gas.
The amount of the film adhering to the steel sheet surface is in the range of 5 to ²⁰⁰ mg / m ² in terms of S amount. When the coating amount is less than 5 mg / m ² in terms of S amount, the coating is small and uneven plating occurs, and the alloying speed is not improved. When it exceeds 200 mg / m ² , the sulfur compound is not sufficiently reduced by annealing after the film formation treatment, and remains on the surface of the steel sheet, resulting in uneven plating.
[0016]
The amount of the coating on the surface of the steel sheet needs to be maintained in an appropriate range according to the hydrogen concentration in the atmosphere in the annealing furnace, the conveyance speed, and the like, and as described above, 5 to ²⁰⁰ mg / m ² in terms of the S amount. Although the scope of the desirable, even more preferably in the range from the viewpoint of productivity of 10 to 120 mg / m ² and plating, yet the range of 20 to 80 mg / m ² is more preferable.
The film containing the sulfur compound thus produced is firmly bonded to the surface of the steel sheet and therefore does not reattach to the roll or the like. Therefore, the steel plate that has undergone the film formation treatment is not contaminated in the course of being transported to a subsequent process, and unevenness in the amount of adhesion does not occur.
[0017]
After performing the film formation treatment, it is preferable to remove excess sulfur compounds by washing with washing water.
In the present invention, the method of cleaning with cleaning water is not limited to a specific configuration, but a method of immersing a steel plate in cleaning water accommodated in a water tank, a method of spraying cleaning water on a steel plate using a spray nozzle, or the like is preferable.
[0018]
The steel sheet thus subjected to the film formation treatment is annealed in a temperature range of 700 to 900 ° C. in a reducing atmosphere containing hydrogen, and then hot dip galvanized. The Al concentration of the molten zinc bath is preferably in the range of 0.05 to 0.2 mass%. When the Al concentration in the molten zinc bath is less than 0.05 mass%, coarse Fe-Zn particles are generated sparsely and the uniformity of the plating layer is impaired. When the Al concentration exceeds 0.2 mass%, the Fe-Al alloy This is because the Zn plating layer is not formed. Further, if necessary, the alloying treatment may be performed by heating to 450 to 600 ° C.
[0019]
【Example】
After cold-rolling a high-tensile steel plate containing 0.5 mass% Mn and 0.04 mass% P, alkaline electrolytic degreasing was performed, and a film formation treatment was performed to form a film containing a sulfur compound. The film formation treatment was performed by immersing the steel sheet in an aqueous solution of ammonium thiosulfate having a concentration of 10 mass% heated to 80 ° C. for 10 seconds. After the film formation treatment, it was immediately washed with running water and dried with a hot air dryer. The amount of S in the film containing a sulfur compound formed on the surface of the steel sheet (hereinafter referred to as S adhesion amount) was measured by atomic absorption spectrometry.
[0020]
Next, annealing was performed in a reducing atmosphere with a hydrogen concentration of 7 mass% by a hot dipping simulator. The annealing temperature was 810 ° C and the annealing time was 55 seconds. Then, it cooled to 470 degreeC and immersed in the hot dip zinc bath hold | maintained at 470 degreeC for 2 second, and the hot dip galvanized steel plate was manufactured. The Al concentration in the molten zinc bath was 0.14 mass%, and the Fe concentration was 0.05 mass%. The amount of Zn in the plating layer thus obtained (hereinafter referred to as Zn deposition amount) was measured by an alkaline electrolysis method.
[0021]
Next, this galvanized steel sheet was alloyed. The alloying treatment was performed by immersing the hot dip galvanized steel sheet in a KNO ₃ molten salt bath maintained at 470 ° C. The amount of Fe diffusion into the plating layer was measured by an alkaline electrolysis method by changing this immersion time (that is, alloying treatment time). In addition, the presence or absence of uneven plating was visually examined. The results are shown in Table 1 as Invention Examples 1 to 6. In addition, although the surface of the samples of Invention Examples 1 to 6 after the sulfur film was formed was rubbed with a finger, the film did not peel off.
[0022]
Moreover, the example which did not perform the film formation process for forming the film | membrane containing a sulfur compound is shown in Table 1 as Comparative Examples 1-6.
[0023]
[Table 1]

[0024]
Inventive Example 1 and Comparative Example 1 are examples in which no alloying treatment was performed. Inventive Example 1 has a higher alloying speed because the amount of Fe diffusion into the plating layer is larger. In addition, Invention Example 1 obtained good plating with no unevenness.
Inventive Examples 2 to 6 are examples in which alloying treatment was performed, and in each case, the amount of Fe diffusion into the plating layer was increased from that of Inventive Example 1. That is, in Invention Examples 2 to 6, the alloying rate was further increased as compared with Invention Example 1, and good plating with no unevenness was obtained.
[0025]
Further, according to Invention Examples 1 to 6 and Comparative Examples 1 to 6, the relationship between the alloying treatment time and the amount of Fe diffusion into the plating layer is as shown in FIG. 1, and the invention example is into the plating layer. The amount of Fe diffusion is large. In other words, Invention Examples 1 to 6 were sufficiently plated, and the plating layer had a high alloying rate and good plating with no unevenness was obtained.
Next, the effect of the kind of aqueous solution used for the film formation process for forming the film | membrane containing a sulfur compound and the time required for a film formation process was investigated.
[0026]
That is, as Invention Example 7, the steel sheet was immersed in an aqueous solution of ammonium thiosulfate having a concentration of 10 mass% heated to 80 ° C. for 18 seconds to perform a film formation treatment, and the alloying treatment after hot dip galvanization was performed for 38 seconds. Since other conditions are the same as those of the first to sixth invention examples, a description thereof will be omitted.
Further, as Invention Example 8, the steel sheet was immersed in sulfite water with a concentration of 3 mass% for 5 seconds to perform film formation treatment, and as Invention Example 9, the steel sheet was immersed in hydrogen sulfide water with a concentration of 4% for 5 seconds to perform film formation treatment. . In any case, the alloying treatment after hot dip galvanization was performed for 38 seconds.
[0027]
As a comparative example, a film formation treatment was performed using an aqueous solution containing Na which is an alkali metal. In other words, the example in which the steel sheet was immersed in an aqueous solution of sodium persulfate having a concentration of 9.6 mass% heated to 80 ° C. for 20 seconds was used as Comparative Example 7, and the aqueous solution of sodium sulfate having a concentration of 10.1 mass% heated to 80 ° C. An example in which the steel sheet was immersed for 10 seconds to perform the film formation treatment was referred to as Comparative Example 8. In any case, the alloying treatment after hot dip galvanization was performed for 38 seconds.
[0028]
About this invention example 7-9 and comparative examples 7-8, S adhesion amount, Zn adhesion amount, and the amount of Fe diffusion in a plating layer were measured. In addition, the presence or absence of uneven plating was visually examined. The results are shown in Table 2. When the surface of the sample after the formation of the sulfur film was rubbed with a finger, no peeling occurred in Invention Examples 7 to 9, but partial peeling occurred in Comparative Examples 7 and 8.
[0029]
[Table 2]

[0030]
Inventive Examples 7 to 9 did not peel off the film, and good plating with no unevenness was obtained, whereas in Comparative Examples 7 to 8, peeling of the film occurred, and the S adhesion amount was small and unevenness occurred. It can also be seen that Invention Examples 7 to 9 have a higher alloying rate because the amount of Fe diffusion into the plating layer is larger.
[0031]
【The invention's effect】
In the present invention, it is possible to produce a good hot dip galvanized steel sheet free from coating peeling and without plating unevenness. Furthermore, productivity can be improved by improving the alloying speed after hot dip galvanization.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between alloying time and the amount of Fe diffusion into a plating layer.

Claims

By bringing a high-strength steel sheet containing Mn and P into contact with an aqueous solution containing one or more selected from sulfurous acid, hydrogen sulfide and ammonium thiosulfate, the surface of the high-strength steel sheet contains a sulfur compound. Hot-dip zinc characterized in that after the film is formed in a range of 5 to 200 mg / m ² in terms of S amount, the film is annealed in a reducing atmosphere and then hot-dip galvanized. Manufacturing method of plated steel sheet.

The method for producing a hot dip galvanized steel sheet according to claim 1, wherein an alloying treatment is performed after the hot dip galvanizing.

The high-strength steel sheet, the Mn containing 0.05～3Mass%, a manufacturing method of hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that it contains P 0.003 to 0.1 mass%.