JP3752109B2

JP3752109B2 - Method for producing high-tensile hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet

Info

Publication number: JP3752109B2
Application number: JP23144299A
Authority: JP
Inventors: 正文清水; 広司入江; 貴之山本
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1999-08-18
Filing date: 1999-08-18
Publication date: 2006-03-08
Anticipated expiration: 2019-08-18
Also published as: JP2001059133A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車用防錆鋼板に使用される高張力溶融亜鉛めっき鋼板、およびこの様な溶融亜鉛めっき鋼板のめっき層を合金化した合金化溶融亜鉛めっき鋼板等を製造する為の有用な方法に関するものであり、殊にＳｉ，Ｍｎ，Ｃｒを添加した鋼板のめっき性を向上させて不めっき部分のない良好な表面外観を呈する各種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、地球温暖化防止を目的として自動車の軽量化ニーズが高まっており、安全性および軽量化の観点から素材の高張力化が強く要求されている。一方、加工性の良好な高張力鋼板を得るという観点から、鋼板にＳｉ，Ｍｎが添加され、また必要によってＣｒが添加されるが、これら元素を添加した鋼板はめっき性が劣化するという問題がある。即ち、Ｓｉ，ＭｎおよびＣｒ等の元素は、易酸化性元素という理由から還元性雰囲気中で優先的に酸化されて鋼板表面に濃化し、これがめっき濡れ性を著しく劣化させ、いわゆる不めっき部分を発生させてめっき外観を損なうことになる。
【０００３】
上記の様な問題を解決する為に、これまでにも様々な技術が提案されており、例えば特開昭５７−７０２６８号、特開平２−１５６０５６号、同４−３３３５５２号等には、溶融亜鉛めっき処理前の鋼板に対して、Ｆｅ，Ｎｉ，Ｃｏ等の金属の１種または２種以上からなるめっき層をプレめっきし、還元性雰囲気で優先的に酸化されるＳｉ，Ｍｎ，Ｃｒ等の鋼板表面濃度を抑制する方法が開示されている。
【０００４】
また、無酸化炉（ＮＯＦ）タイプの焼鈍炉を有する溶融亜鉛めっき設備では、燃焼ガスの空気比を制御することによって直火炉内を酸化雰囲気とし、Ｆｅの酸化皮膜を優先的に生成させて還元炉内でのＳｉ，Ｍｎ，Ｃｒ等の鋼板表面濃度を抑制する方法（例えば、特開昭５５−１２２８６５号、特開平４−２７６０５７号、特開平６−８１０９６号等）も知られている。
【０００５】
【発明が解決しようとする課題】
ところで、間接加熱方式の焼鈍炉（竪型焼鈍炉）を備えている連続溶融亜鉛めっき設備においては、上記の様な従来技術をこの様に溶融めっきラインに適用する為には、プレめっき法では一般的に電気めっき法を用いるものであるので、設備投資費が高くなりまた製造コストも高くなるという問題がある。
【０００６】
一方、ＮＯＦタイプの焼鈍炉による酸化還元法では、基本的には、バーナの空気比を制御して適正な鉄の酸化皮膜を生成するものであるが、微量の酸素量を調整するものであるので、個々のバーナの調整が困難となり、酸素量が不安定になって、結果的に酸素皮膜量の生成も不安定になるという別の問題がある。
【０００７】
本発明はこうした状況の下でなされたものであって、その目的は、間接加熱方式の焼鈍炉を有する連続溶融亜鉛めっき設備を採用しても、不めっきが生じない様な溶融亜鉛めっき鋼板や合金化溶融亜鉛めっき鋼板を製造する為の有用な方法を提供することにある。
【０００８】
【課題を解決するための手段】
上記目的を達成し得た本発明の製造方法とは、Ｓｉ：０．１〜２％（質量％の意味、以下同じ）、Ｍｎ：０．５〜３％、Ｃｒ：１％以下（０％を含まない）を夫々含有し、且つＳｉ，ＭｎおよびＣｒの合計含有量が０．６〜５％である鋼板を用いて溶融亜鉛めっき鋼板を製造するに際して、鋼板を大気雰囲気中、２００〜６５０℃で５〜１００秒酸化処理し、その後Ｈ₂−Ｎ₂混合ガス雰囲気の焼鈍炉で、６００〜９００℃で６０〜６００秒還元することとし、この際酸化時間（Ｓ１）と還元時間（Ｓ２）（秒）について下記（１）式の関係を満足させ、引き続き溶融亜鉛めっきを施す点に要旨を有するものである。
４×（Ｓ１）−６０≦（Ｓ２）≦１０×（Ｓ１）＋６０ ……（１）
また、上記の様な方法によって得られた溶融亜鉛めっき鋼板のめっき層を合金化することによって、不めっき部分のない特性の良好な合金溶融亜鉛めっき鋼板を得ることができる。
【０００９】
【発明の実施の形態】
連続溶融亜鉛めっき設備に備えられている焼鈍炉（間接加熱タイプの焼鈍炉）で、Ｓｉ，ＭｎおよびＣｒが所定量含有する鋼板を直接的に還元すると、前述の如く鋼板表面にこれらの成分の表面濃化が生じて不めっき部分が発生することになる。そこで本発明者らは、鋼板表面に鉄の酸化皮膜を安定的に生成させた後に還元焼鈍を行なって、添加元素の表面濃化を防止することについて検討した。
【００１０】
その結果、還元焼鈍炉に導入する前に行なう酸化条件と還元焼鈍炉にける還元条件が、夫々の板温度と時間に影響されることが判明し、これら板温度と時間を適切に制御して酸化条件と還元条件を規定してやれば、添加元素の表面濃化が防止できて上記目的が見事に達成されることを見出し、本発明を完成した。以下、本発明の製造方法で規定する各要件について説明する。
【００１１】
本発明方法は、焼鈍炉に導入する前に鋼板を２００〜６５０℃で酸化処理するものであるが、このときの温度（板温度）が２００℃未満では酸化皮膜が十分に生成せず、また６５０℃を超えると酸化皮膜の生成量が多過ぎて、通常の還元焼鈍炉による還元処理ではその条件を如何に設定しても鉄の酸化皮膜の残存によって不めっき部分が発生することになる。
【００１２】
上記酸化温度による酸化処理は、その時間を５〜１００秒とする必要がある。この酸化時間が５秒未満では鉄の酸化皮膜の生成が不十分であり、１００秒を超えると酸化皮膜の生成量が多過ぎて、上記と同様の現象を生じることになる。尚、酸化処理の為の加熱方式については、特に限定するものではなく、間接加熱、誘導加熱、直火加熱、通電加熱のいずれも採用することができる。また、このときの雰囲気は大気中が最も一般的である。
【００１３】
一方、還元処理の際の温度（板温度）は、６００〜９５０℃とする必要がある。この温度が６００℃未満であると十分な還元が行なえず、鉄の酸化皮膜が残存して不めっき部分が発生することになる。また、還元温度が９５０℃を超えると、急速に還元が進んで鉄の酸化皮膜が高温状態で還元されて添加元素の濃化が発生することになる。こうしたことから、還元処理の際の温度は６００〜９５０℃と規定した。
【００１４】
上記還元温度による還元処理は、その時間を６０〜６００秒とする必要がある。この還元時間が６０秒未満では還元不足となり、６００秒を超えると還元過剰となって上記と同様な不都合が生じることになる。
【００１５】
本発明方法では、上記の様な酸化条件（板温度・時間）および還元条件（板温度・時間）で酸化処理および還元処理を行なうものであるが、本発明の目的を達成するためには、このときの酸化時間（Ｓ１）と還元時間（Ｓ２）の関係も適切に制御する必要がある。即ち、本発明方法では、上記酸化時間（Ｓ１）と還元時間（Ｓ２）が下記（１）式の関係を満足する必要がある。
４×（Ｓ１）−６０≦（Ｓ２）≦１０×（Ｓ１）＋６０ ……（１）
【００１６】
上記（１）式で規定される範囲を含めて、本発明で規定する酸化時間（Ｓ１）と還元時間（Ｓ２）の範囲を図１に示す。即ち、図１のハッチングで示した範囲内にある様にして酸化時間（Ｓ１）と還元時間（Ｓ２）を制御した後、溶融亜鉛めっきを行ない、必要によって合金化処理を行なうことによって、不めっきの発生を防止して良好な表面外観を有する溶融亜鉛めっき鋼板や合金化溶融亜鉛めっき鋼板を製造することができたのである。即ち、図１のハッチングで示した範囲を外れると、いずれも不めっきを発生することになる。
【００１７】
上記の様に本発明では、間接加熱タイプの焼鈍炉を備えた連続溶融亜鉛めっき設備において、還元焼鈍炉に導入する前に鋼板表面に鉄の酸化皮膜を形成させつつ、そのときの酸化条件とその後の還元条件を適切にコントロールすることによって、不めっきを発生させるＳｉ，ＭｎおよびＣｒ等の元素の表面濃化がめっき前に生成することを抑制し、不めっきのない良好な表面外観を得ることができたのである。
【００１８】
但し、上記の様に酸化条件および還元条件を規定しても、鋼板中のＳｉ，ＭｎおよびＣｒの各含有量が並びにこれらの合計含有量が所定の範囲を超えていれば、これらの元素の表面濃化を抑制することができない。また、鋼板中のＳｉが０．１％未満、Ｍｎが０．５％未満、Ｃｒが不純物レベルであって、且つこれらの元素の合計含有量が０．６％未満になると、通常の焼鈍炉による処理によっても不めっきの問題は発生しないが、高張力鋼として必要な引張り強さ（例えば、３４０ＭＰａ以上）を達成することが困難になる。こうしたことから、本発明で用いる素地鋼板における上記元素の含有量を、Ｓｉ：０．１〜２％、Ｍｎ：０．５〜３％、Ｃｒ：１％以下（０％を含なない）、且つＳｉ，ＭｎおよびＣｒの合計含有量を０．６〜５％と規定した。
【００１９】
尚、上記Ｓｉ，ＭｎおよびＣｒ以外の成分として、本発明で用いる鋼板にはＣ，Ａｌ，Ｐ，Ｓ等の基本成分の他、必要によってＴｉ，Ｎｂ，Ｍｏ，Ｖ，Ｚｒ，Ｎ，Ｂ等の各種元素が含まれるが、これらの含有量については特に限定するものではなく、素地鋼板として通常含有される程度であれば良い。また、これら以外にも本発明で用いる鋼板には、その特性に影響を与えない程度の微量成分も含み得るものであり、こうした鋼板も本発明で用いる素地鋼板に含まれるものである。
【００２０】
以下、本発明を実施例によって更に詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に徴して設計変更することはいずれも本発明の技術的範囲に含まれるものである。
【００２１】
【実施例】
Ｓｉ，ＭｎおよびＣｒを下記表１に示す様に含んだ各種鋼板を用い、大気中で酸化処理を施した後、間接加熱タイプの焼鈍炉（Ｈ₂−Ｎ₂混合ガス雰囲気）にて還元処理を施した。次いで、これらの処理を行なった鋼板について、通常の条件で溶融亜鉛めっきおよび合金化処理を行なった。得られた合金化溶融亜鉛めっきについて、めっき性を調査した。このときのめっき性については、目視にて観察し、不めっきの無い良好な外観のものを◎、不めっきが発生したものについては×として評価した。これらの結果を、酸化処理条件および還元処理条件と共に下記表１に示す。
【００２２】
【表１】

【００２３】
この結果から、次の様に考察できる。まず、Ｎｏ．２〜５、７〜１０のものは、本発明で規定する要件を満足する実施例であり、いずれの条件においても不めっきのない良好な外観が得られていることが分かる。
【００２４】
これに対して、Ｎｏ．１１〜２０のものは、本発明で規定する要件のいずれかを欠く比較例であり、いずれも不めっきが発生しており、良好な外観を得ることはできなかった。即ち、Ｎｏ．１１のものは、鋼板中のＳｉ含有量が２％を超えており、Ｎｏ．１２のものはＳｉ，ＭｎおよびＣｒの合計含有量が５％を超えており、本発明で規定する酸化還元条件を満足していても良好な外観を得ることはできなかった。
【００２５】
また、Ｎｏ．１３のものは酸化時間（Ｓ１）が短い、Ｎｏ．１４のものは還元時間（Ｓ２）が長い、Ｎｏ．１５のものは酸化板温度が低い、Ｎｏ．１６のものは還元加熱板温度が高い、Ｎｏ．１７は還元時間（Ｓ２）が短い、Ｎｏ．１８のものは酸化板温度が高い、Ｎｏ．１９のものは酸化時間（Ｓ１）が長い、Ｎｏ．２０のものは還元温度が低い、夫々の場合を示したものであり、いずれの条件においても良好な外観を得ることはできなかった。
【００２６】
【発明の効果】
本発明は以上の様に構成されており、酸化条件と還元条件の関係を適正なものとすることによって、鋼板に添加されるＳｉ，ＭｎおよびＣｒ等の元素の焼鈍時における表面濃化を抑制して不めっきの無い良好なめっき外観を有する溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板が得られた。
【図面の簡単な説明】
【図１】本発明で規定する酸化時間（Ｓ１）と還元時間（Ｓ２）の範囲を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention is a useful method for producing a high-tensile hot-dip galvanized steel sheet used for automobile anticorrosive steel sheet, and an alloyed hot-dip galvanized steel sheet obtained by alloying the plated layer of such hot-dip galvanized steel sheet. In particular, the present invention relates to a method for producing various plated steel sheets that improve the plating properties of steel sheets to which Si, Mn, and Cr are added and that have a good surface appearance with no unplated portions.
[0002]
[Prior art]
In recent years, there is an increasing need for weight reduction of automobiles for the purpose of preventing global warming, and high tension of materials is strongly demanded from the viewpoint of safety and weight reduction. On the other hand, from the viewpoint of obtaining a high-tensile steel plate with good workability, Si and Mn are added to the steel plate, and Cr is added if necessary, but the steel plate to which these elements are added has a problem that the plateability deteriorates. is there. That is, elements such as Si, Mn and Cr are preferentially oxidized in a reducing atmosphere due to the fact that they are easily oxidizable elements and concentrate on the steel sheet surface, which significantly deteriorates the plating wettability, so-called non-plated portions. If generated, the plating appearance will be impaired.
[0003]
In order to solve the above problems, various techniques have been proposed so far. For example, Japanese Patent Application Laid-Open Nos. 57-70268, 2-1556056, 4-333552, etc. Si, Mn, Cr, etc. that are pre-plated with a plating layer made of one or more metals such as Fe, Ni, Co, etc., and preferentially oxidized in a reducing atmosphere on the steel sheet before galvanizing treatment A method for suppressing the steel sheet surface concentration is disclosed.
[0004]
Moreover, in hot dip galvanizing equipment having a non-oxidizing furnace (NOF) type annealing furnace, the air ratio of the combustion gas is controlled to create an oxidizing atmosphere in the direct-fired furnace, and the oxide film of Fe is preferentially generated and reduced. Methods for suppressing the surface concentration of steel sheets such as Si, Mn, Cr, etc. in the furnace are also known (for example, JP-A Nos. 55-122865, 4-276057, 6-81096, etc.).
[0005]
[Problems to be solved by the invention]
By the way, in a continuous hot dip galvanizing facility equipped with an indirect heating type annealing furnace (vertical annealing furnace), in order to apply the above-described conventional technology to the hot dip plating line in this way, Since the electroplating method is generally used, there is a problem that the capital investment cost is high and the manufacturing cost is also high.
[0006]
On the other hand, in the oxidation-reduction method using a NOF type annealing furnace, basically, an appropriate iron oxide film is generated by controlling the air ratio of the burner, but a minute amount of oxygen is adjusted. Therefore, it is difficult to adjust individual burners, the oxygen amount becomes unstable, and as a result, the generation of the oxygen film amount becomes unstable.
[0007]
The present invention has been made under these circumstances, and its purpose is to provide a hot dip galvanized steel sheet that does not cause non-plating even when a continuous hot dip galvanizing facility having an indirect heating annealing furnace is employed. The object is to provide a useful method for producing galvannealed steel sheets.
[0008]
[Means for Solving the Problems]
The production method of the present invention that can achieve the above-mentioned object is: Si: 0.1 to 2% (meaning mass%, the same shall apply hereinafter), Mn: 0.5 to 3%, Cr: 1% or less (0%) When the hot dip galvanized steel sheet is produced using a steel sheet containing 0.6 to 5% of the total content of Si, Mn and Cr, the steel sheet is 200 to 650 in the atmosphere. Oxidation treatment at 5 ° C. for 5 to 100 seconds, followed by reduction at 600 to 900 ° C. for 60 to 600 seconds in an annealing furnace in an H ₂ —N ₂ mixed gas atmosphere. At this time, oxidation time (S1) and reduction time (S2 ) (Second) satisfies the relationship of the following formula (1), and subsequently has a gist in that hot dip galvanization is performed.
4 × (S1) −60 ≦ (S2) ≦ 10 × (S1) +60 (1)
Further, by alloying the plated layer of the hot dip galvanized steel sheet obtained by the method as described above, an alloy hot dip galvanized steel sheet having good properties with no unplated portion can be obtained.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
When a steel sheet containing a predetermined amount of Si, Mn and Cr is directly reduced in an annealing furnace (indirect heating type annealing furnace) provided in a continuous hot dip galvanizing facility, as described above, these components are added to the steel sheet surface. Surface thickening occurs and an unplated portion is generated. Therefore, the present inventors examined reducing the surface concentration of the additive elements by performing reduction annealing after stably forming an iron oxide film on the steel sheet surface.
[0010]
As a result, it was found that the oxidation conditions performed before introduction into the reduction annealing furnace and the reduction conditions in the reduction annealing furnace were influenced by the respective plate temperatures and times, and these plate temperatures and times were appropriately controlled. The inventors have found that if the oxidation conditions and the reduction conditions are defined, the surface concentration of the additive element can be prevented and the above object can be achieved brilliantly, and the present invention has been completed. Hereinafter, each requirement prescribed | regulated with the manufacturing method of this invention is demonstrated.
[0011]
In the method of the present invention, the steel sheet is oxidized at 200 to 650 ° C. before being introduced into the annealing furnace. However, if the temperature (plate temperature) at this time is less than 200 ° C., an oxide film is not sufficiently formed. When the temperature exceeds 650 ° C., the amount of oxide film produced is too large, and in the reduction treatment using a normal reduction annealing furnace, an unplated portion is generated due to the remaining iron oxide film regardless of the conditions.
[0012]
The time required for the oxidation treatment at the oxidation temperature is 5 to 100 seconds. If this oxidation time is less than 5 seconds, the formation of an iron oxide film is insufficient, and if it exceeds 100 seconds, the amount of oxide film produced is too large, resulting in the same phenomenon as described above. The heating method for the oxidation treatment is not particularly limited, and any of indirect heating, induction heating, direct fire heating, and electric heating can be employed. The atmosphere at this time is most commonly in the air.
[0013]
On the other hand, the temperature (plate temperature) during the reduction treatment needs to be 600 to 950 ° C. When this temperature is less than 600 ° C., sufficient reduction cannot be performed, and an iron oxide film remains and an unplated portion is generated. Further, when the reduction temperature exceeds 950 ° C., the reduction proceeds rapidly, and the iron oxide film is reduced at a high temperature, and the concentration of the additive element occurs. For these reasons, the temperature during the reduction treatment was defined as 600 to 950 ° C.
[0014]
The reduction treatment at the reduction temperature needs to be 60 to 600 seconds. If the reduction time is less than 60 seconds, the reduction is insufficient, and if it exceeds 600 seconds, the reduction is excessive and the same disadvantages as described above occur.
[0015]
In the method of the present invention, oxidation treatment and reduction treatment are performed under the oxidation conditions (plate temperature / time) and reduction conditions (plate temperature / time) as described above. To achieve the object of the present invention, It is necessary to appropriately control the relationship between the oxidation time (S1) and the reduction time (S2) at this time. That is, in the method of the present invention, the oxidation time (S1) and the reduction time (S2) need to satisfy the relationship of the following formula (1).
4 × (S1) −60 ≦ (S2) ≦ 10 × (S1) +60 (1)
[0016]
FIG. 1 shows the ranges of the oxidation time (S1) and the reduction time (S2) defined in the present invention, including the range defined by the above formula (1). That is, after controlling the oxidation time (S1) and the reduction time (S2) so as to be within the range indicated by hatching in FIG. 1, hot dip galvanization is performed, and if necessary, alloying treatment is performed, so that non-plating is performed. It was possible to produce hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets having a good surface appearance. In other words, any out of the range shown by hatching in FIG. 1 causes non-plating.
[0017]
As described above, in the present invention, in a continuous hot dip galvanizing facility equipped with an indirect heating type annealing furnace, an iron oxide film is formed on the steel sheet surface before being introduced into the reduction annealing furnace, and the oxidation conditions at that time By appropriately controlling the subsequent reduction conditions, surface enrichment of elements such as Si, Mn, and Cr that cause unplating is prevented from being generated before plating, and a good surface appearance without unplating is obtained. It was possible.
[0018]
However, even if the oxidation conditions and the reduction conditions are defined as described above, if each content of Si, Mn and Cr in the steel sheet and the total content thereof exceeds a predetermined range, these elements Surface thickening cannot be suppressed. Further, when Si in the steel sheet is less than 0.1%, Mn is less than 0.5%, Cr is at an impurity level, and the total content of these elements is less than 0.6%, a normal annealing furnace Although the problem of non-plating does not occur even by the treatment by, it becomes difficult to achieve the tensile strength (for example, 340 MPa or more) necessary for high-tensile steel. Therefore, the content of the above elements in the base steel sheet used in the present invention is as follows: Si: 0.1 to 2%, Mn: 0.5 to 3%, Cr: 1% or less (excluding 0%), And the total content of Si, Mn and Cr was defined as 0.6 to 5%.
[0019]
As components other than Si, Mn, and Cr, the steel plate used in the present invention includes basic components such as C, Al, P, and S as well as Ti, Nb, Mo, V, Zr, N, B, and the like as necessary. However, the content of these elements is not particularly limited as long as it is normally contained as a base steel sheet. In addition to these, the steel plate used in the present invention can also contain trace components that do not affect the properties thereof, and such a steel plate is also included in the base steel plate used in the present invention.
[0020]
Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.
[0021]
【Example】
Using various steel plates containing Si, Mn and Cr as shown in Table 1 below, after oxidation in the atmosphere, reduction treatment is performed in an indirect heating type annealing furnace (H ₂ -N ₂ mixed gas atmosphere). Was given. Next, the steel sheet subjected to these treatments was subjected to hot dip galvanizing and alloying treatment under normal conditions. The obtained alloyed hot dip galvanizing was examined for plating properties. The plating properties at this time were visually observed, and those having a good appearance without unplating were evaluated as ◎, and those having non-plating were evaluated as x. These results are shown in Table 1 below together with oxidation treatment conditions and reduction treatment conditions.
[0022]
[Table 1]

[0023]
From this result, it can be considered as follows. First, no. Those of 2 to 5 and 7 to 10 are examples that satisfy the requirements defined in the present invention, and it can be seen that a good appearance without unplating is obtained under any conditions.
[0024]
In contrast, no. Nos. 11 to 20 are comparative examples lacking any of the requirements defined in the present invention, and any of them exhibited non-plating, and a good appearance could not be obtained. That is, no. No. 11 has a Si content in the steel sheet exceeding 2%. In No. 12, the total content of Si, Mn and Cr exceeded 5%, and even if the oxidation-reduction conditions specified in the present invention were satisfied, a good appearance could not be obtained.
[0025]
No. No. 13 has a short oxidation time (S1), No. No. 14 has a long reduction time (S2). No. 15 has a low oxide plate temperature. No. 16 has a high reduction heating plate temperature. No. 17 has a short reduction time (S2). No. 18 has a high oxide plate temperature, No. No. 19 has a long oxidation time (S1). No. 20 shows the respective cases where the reduction temperature is low, and a good appearance could not be obtained under any conditions.
[0026]
【The invention's effect】
The present invention is configured as described above, and suppresses surface concentration during annealing of elements such as Si, Mn and Cr added to the steel sheet by making the relationship between the oxidation condition and the reduction condition appropriate. As a result, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a good plating appearance with no unplating were obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing a range of an oxidation time (S1) and a reduction time (S2) defined in the present invention.

Claims

Si: 0.1 to 2% (meaning of mass%, the same shall apply hereinafter), Mn: 0.5 to 3%, Cr: 1% or less (excluding 0%) , and Si, Mn and Cr When a hot dip galvanized steel sheet is produced using a steel sheet having a total content of 0.6 to 5%, the steel sheet is oxidized in an air atmosphere at 200 to 650 ° C. for 5 to 100 seconds, and then H ₂ —N. _In an annealing furnace in a mixed gas atmosphere, reduction is performed at 600 to 900 ° C. for 60 to 600 seconds. At this time, the relationship of the following formula (1) is satisfied with respect to oxidation time (S1) and reduction time (S2) (seconds). A method for producing a high-tensile hot-dip galvanized steel sheet, which is subsequently subjected to hot-dip galvanizing.
4 × (S1) −60 ≦ (S2) ≦ 10 × (S1) +60 (1)

A method for producing an alloyed hot- dip galvanized steel sheet obtained by alloying a plated layer of a hot-dip galvanized steel sheet obtained by the method of claim 1 to obtain an alloyed hot- dip galvanized steel sheet.