JP2010018873A - Method for producing hot-dip galvannealed steel sheet - Google Patents
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本発明は、自動車、家電製品、建築材料等の用途に使用される表面外観に優れた合金化溶融亜鉛めっき鋼板の製造方法に関するものである。 The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having an excellent surface appearance used for applications such as automobiles, home appliances, and building materials.
溶融亜鉛めっき鋼板は、自動車、家電製品、建築材料等の広範な用途に用いられており、特に合金化溶融亜鉛めっき鋼板は、耐食性、スポット溶接性に優れることから、自動車用鋼板として広く使用されている。近年、自動車においては、車体の軽量化による燃費の向上、衝突安全性を高めるといったニーズから、この合金化溶融亜鉛めっき鋼板にも高強度化、薄物化のニーズが高まっている。 Hot-dip galvanized steel sheets are used in a wide range of applications such as automobiles, home appliances, and building materials. In particular, alloyed hot-dip galvanized steel sheets are widely used as automotive steel sheets because of their excellent corrosion resistance and spot weldability. ing. In recent years, in automobiles, there is a growing need for high strength and thinning of this alloyed hot-dip galvanized steel sheet because of the need to improve fuel efficiency by reducing the weight of the vehicle body and to improve collision safety.
これらの現状を踏まえ、更には強度延性バランスの確保という観点もあり、現在使用されている合金化溶融亜鉛めっき鋼板は、ＳｉやＭｎといった易酸化元素を添加したものが多くなっている。しかしながら、これら易酸化元素は、鋼板にめっきを行う前の焼鈍時に選択酸化されて、めっき濡れ性や合金化処理性を著しく阻害することが知られており、その制御を行うのは非常に難しい。以上の実情もあって、合金化溶融亜鉛めっき鋼板を安定的に製造するのは非常に難しいのが現状である。 In view of these present conditions, there is also a viewpoint of securing a balance between strength and ductility, and many of the galvannealed steel sheets currently used have added oxidizable elements such as Si and Mn. However, these easily oxidizable elements are known to be selectively oxidized during annealing before plating on a steel sheet, and significantly impair plating wettability and alloying processability, and it is very difficult to control the elements. . In view of the above circumstances, it is very difficult to stably manufacture an alloyed hot-dip galvanized steel sheet.
このような実情が勘案され、近年、合金化溶融亜鉛めっき鋼板について、種々の提案がなされている。 In consideration of such a situation, in recent years, various proposals have been made for galvannealed steel sheets.
特許文献１には、焼鈍工程で、鋼板表層に鋼板添加元素と焼鈍雰囲気の成分との反応物を形成させる合金化溶融亜鉛めっき鋼板の製造方法が開示されている。また、特許文献２には、Ｍｎを含む高張力鋼板の表面に、Ｓを含有するアンモニウム塩を付着させたのち、熱処理を施し、ついでめっき処理を施す溶融亜鉛めっき鋼板の製造方法が開示されている。これらの方法は、生成反応物として、Ｓｉ−Ｍｎ−ＯやＭｎＳを形成させようとする方法である。 Patent Document 1 discloses a method for producing an alloyed hot-dip galvanized steel sheet in which a reaction product of a steel sheet additive element and a component in an annealing atmosphere is formed on a steel sheet surface layer in an annealing process. Patent Document 2 discloses a method for producing a hot-dip galvanized steel sheet in which an ammonium salt containing S is attached to the surface of a high-tensile steel sheet containing Mn, and then heat treatment is performed, followed by plating. Yes. These methods are methods in which Si—Mn—O or MnS is formed as a product reactant.
特許文献３には、めっき溶に鋼板を浸漬させる前に、鋼板の表層をドライエッチングするという合金化溶融亜鉛めっき鋼板の製造方法が開示されている。また、特許文献４には、焼鈍後の鋼板を冷却制御することにより粒界偏析を減らそうという合金化溶融亜鉛めっき鋼板のめっき性改善方法が開示されている。 Patent Document 3 discloses a method for producing an alloyed hot-dip galvanized steel sheet in which the surface layer of the steel sheet is dry-etched before the steel sheet is immersed in the plating solution. Further, Patent Document 4 discloses a method for improving the plating property of an alloyed hot-dip galvanized steel sheet in which grain boundary segregation is reduced by controlling the cooling of the steel sheet after annealing.
更には、特許文献５には、焼鈍後に、Ｓｉ、Ｍｎ、Ａｌを含有する表面濃化層の７０％以上を酸洗により除去し、その後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が開示されている。また、特許文献６には、固溶Ｍｎ量を低減することにより、加熱炉内でのＭｎＯの発生を抑制しようとする合金化溶融亜鉛めっき用鋼板の製造方法が開示されている。 Furthermore, in Patent Document 5, after annealing, 70% or more of the surface concentrated layer containing Si, Mn, and Al is removed by pickling, and then a high-tensile hot-dip galvanized steel sheet that is hot-dip galvanized is manufactured. A method is disclosed. Patent Document 6 discloses a method for producing a steel sheet for alloying hot dip galvanization which attempts to suppress the generation of MnO in a heating furnace by reducing the amount of dissolved Mn.
しかしながら、これらの合金化溶融亜鉛めっき鋼板の製造方法は、その何れもが工程が複雑であり、容易に合金化溶融亜鉛めっき鋼板を製造することはできない。また、特に高いＭｎ量を含む鋼板において、表面外観に優れた合金化溶融亜鉛めっき鋼板を製造する方法として提案されたものでもなかった。 However, all of these methods for producing an alloyed hot-dip galvanized steel sheet have complicated processes, and an alloyed hot-dip galvanized steel sheet cannot be easily produced. Further, it has not been proposed as a method for producing an alloyed hot-dip galvanized steel sheet having an excellent surface appearance in a steel sheet containing a particularly high amount of Mn.
本発明は、上記従来の問題を解決せんとしてなされたもので、高いＭｎ量を含む鋼板であっても、合金化むらの原因となるＭｎＯが表面に生成される影響を低減することで、めっき皮膜の均一性および耐パウダリング性に優れ、表面外観に優れた合金化溶融亜鉛めっき鋼板を製造することができる合金化溶融亜鉛めっき鋼板の製造方法を提供することを課題とするものである。 The present invention has been made as a solution to the above-described conventional problems, and even by a steel sheet containing a high amount of Mn, by reducing the influence of the generation of MnO that causes uneven alloying on the surface, plating is performed. It is an object of the present invention to provide a method for producing an alloyed hot-dip galvanized steel sheet that can produce an alloyed hot-dip galvanized steel sheet that is excellent in film uniformity and powdering resistance and has an excellent surface appearance.
請求項１記載の発明は、質量％で、Ｃ：０．０２〜０．２％、Ｍｎ：２．０〜３．５％、Ｃｒ：０．０３〜０．５％、Ａｌ：０．０１〜０．１５％、Ｓｉ：０．０４％以下（０％を含む）、Ｐ：０．０３％以下（０％を含む）、Ｓ：０．０３％以下（０％を含む）を含有する鋼板から、焼鈍工程、めっき工程、合金化処理工程を経て合金化溶融亜鉛めっき鋼板を製造する合金化溶融亜鉛めっき鋼板の製造方法であって、焼鈍工程は、雰囲気中の酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で行うと共に、めっき工程は、５１０℃以上、６００℃未満の板温の鋼板を、亜鉛めっき浴に浸漬させることにより行うことを特徴とする合金化溶融亜鉛めっき鋼板の製造方法である。 Invention of Claim 1 is the mass%, C: 0.02-0.2%, Mn: 2.0-3.5%, Cr: 0.03-0.5%, Al: 0.01 ~ 0.15%, Si: 0.04% or less (including 0%), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%) A method for producing an alloyed hot-dip galvanized steel sheet, which is manufactured from a steel sheet through an annealing step, a plating step, and an alloying treatment step, wherein the annealing step comprises oxygen partial pressure PO 2 ( (Unit: atm) is performed under the condition that −log (PO 2 ) ≦ 23, and the plating step is performed by immersing a steel plate having a plate temperature of 510 ° C. or higher and lower than 600 ° C. in a galvanizing bath. It is the manufacturing method of the galvannealed steel plate characterized by the above.
請求項２記載の発明は、前記鋼板は、更に、質量％で、Ｃｕ：０．００３〜０．５％、Ｎｉ：０．００３〜１．０％、Ｔｉ：０．００３〜１．０％からなる群から選ばれた１種または２種以上を、合計で０．００３〜１．０％含有することを特徴とする請求項１記載の合金化溶融亜鉛めっき鋼板の製造方法である。 In the invention according to claim 2, the steel sheet is further in mass%, Cu: 0.003-0.5%, Ni: 0.003-1.0%, Ti: 0.003-1.0%. The method for producing a galvannealed steel sheet according to claim 1, wherein 0.003 to 1.0% in total of one or more selected from the group consisting of:
請求項３記載の発明は、前記鋼板は、更に、質量％で、Ｖ：０．００３〜１．０％、Ｎｂ：０．００３〜１．０％、Ｂ：０．０００２〜０．１％、Ｍｏ：０．００３〜１．０％からなる群から選ばれた１種または２種以上を含有することを特徴とする請求項１または２記載の合金化溶融亜鉛めっき鋼板の製造方法である。 In the invention according to claim 3, the steel sheet is further in mass%, V: 0.003-1.0%, Nb: 0.003-1.0%, B: 0.0002-0.1%. Mo: One or more selected from the group consisting of 0.003 to 1.0% are contained. The method for producing an galvannealed steel sheet according to claim 1 or 2, .
請求項４記載の発明は、前記鋼板は、更に、質量％で、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００１％からなる群から選ばれた１種以上を含有することを特徴とする請求項１乃至３のいずれかに記載の合金化溶融亜鉛めっき鋼板の製造方法である。 The invention according to claim 4 is characterized in that the steel sheet further comprises at least one selected from the group consisting of Ca: 0.0005 to 0.005% and Mg: 0.0005 to 0.001% by mass%. It contains, It is a manufacturing method of the galvannealed steel plate in any one of the Claims 1 thru | or 3 characterized by the above-mentioned.
本発明の合金化溶融亜鉛めっき鋼板の製造方法によると、Ｍｎの含有量が２．０〜３．５質量％と高い鋼板であっても、合金化むらの原因となるＭｎＯが表面に生成される影響を低減することで、めっき皮膜の均一性および耐パウダリング性に優れ、表面外観に優れた合金化溶融亜鉛めっき鋼板を製造することができる。 According to the manufacturing method of the galvannealed steel sheet of the present invention, even if the Mn content is as high as 2.0 to 3.5% by mass, MnO causing uneven alloying is generated on the surface. Therefore, it is possible to produce an alloyed hot-dip galvanized steel sheet having excellent plating film uniformity and powdering resistance and excellent surface appearance.
通常の合金化溶融亜鉛めっき鋼板の製造工程において実施される焼鈍においては、鋼板の主成分であるＦｅが酸化されることはないが、ＳｉやＭｎといった易酸化元素が添加されている場合、これらの易酸化元素が選択的に酸化されて鋼板表面への拡散が発生する。そのため、鋼板の表面には、これら易酸化元素単独の酸化物や複合酸化物が生成される。 In the annealing performed in the manufacturing process of a normal alloyed hot-dip galvanized steel sheet, Fe, which is the main component of the steel sheet, is not oxidized, but when oxidizable elements such as Si and Mn are added, The easily oxidizable elements are selectively oxidized and diffuse to the steel sheet surface. Therefore, oxides or complex oxides of these easily oxidizable elements are generated on the surface of the steel sheet.
易酸化元素のうちでも、Ｓｉは表面に濃化すると、鋼板最表面に薄い酸化層や粒界酸化を形成し、めっき性や合金化処理性を著しく劣化させるという問題を生じる。そのため、本発明では、易酸化元素のうちＭｎは添加するが、Ｓｉについては、不可避的不純物として混入することは容認するものの積極的には添加することはしない。 Among the easily oxidizable elements, when Si is concentrated on the surface, a thin oxide layer or grain boundary oxidation is formed on the outermost surface of the steel sheet, which causes a problem that the plating property and the alloying processability are significantly deteriorated. Therefore, in the present invention, Mn is added as an easily oxidizable element, but Si is allowed to be mixed as an inevitable impurity, but it is not positively added.
一方、Ｍｎも鋼板の表層に濃化するが、Ｓｉのように酸化層や粒界酸化を形成するのではなく、粒状の酸化物（ＭｎＯ）として成長するため、合金化処理時のＦｅの外方拡散の障害になることは少なくバリア効果はＳｉより小さい。また、添加量が少量であれば、合金化速度が速くなる傾向さえある。しかしながら、Ｍｎは強化能力が低いことから、大量に添加する必要がある。大量に添加すると、ＭｎＯが鋼板の表面に発生しやすくなるので、合金化挙動を複雑化し、制御を困難にしている。 On the other hand, Mn also concentrates on the surface layer of the steel sheet, but does not form an oxide layer or grain boundary oxidation like Si, but grows as a granular oxide (MnO). The barrier effect is less than that of Si. Further, if the addition amount is small, the alloying rate tends to increase. However, since Mn has a low reinforcing ability, it needs to be added in a large amount. When added in a large amount, MnO tends to be generated on the surface of the steel sheet, complicating the alloying behavior and making it difficult to control.
以上のような前提条件を勘案し、本発明者らは、ＭｎＯの生成形態と合金化の関係に着目し、検討した結果、合金化むらの詳細な発生メカニズムを突き止めることに成功した。 Considering the preconditions as described above, the present inventors paid attention to the relationship between MnO formation and alloying, and as a result, have succeeded in finding out the detailed generation mechanism of alloying unevenness.
その詳細メカニズムを、図１に基づき説明する。まず、図１（ａ）に示すように、大量のＭｎが添加された鋼板を高い酸素分圧下で焼鈍すると、鋼板の最表面に粒状酸化物であるＭｎＯが大量に生成する。その状態で、鋼板を溶融亜鉛めっきの亜鉛めっき浴に浸漬すると、図１（ｂ）に示すように、亜鉛めっき浴中に含まれるＡｌが、鋼板表面に生成したＭｎＯの酸素、および鋼板内部から拡散するＦｅと瞬時に反応し、鋼板と亜鉛めっき層の界面にＦｅ−Ａｌ−Ｏ合金層が形成される。図１（ｃ）に示すように、このＦｅ−Ａｌ−Ｏ合金層が合金化処理時の鋼板からのＦｅの拡散障壁となり、鋼板の合金化が阻害されることで、合金化むらを引き起こし、合金化溶融亜鉛めっき鋼板の表面外観を悪化させていることが分かった。 The detailed mechanism will be described with reference to FIG. First, as shown in FIG. 1A, when a steel plate to which a large amount of Mn is added is annealed under a high oxygen partial pressure, a large amount of MnO, which is a granular oxide, is generated on the outermost surface of the steel plate. In that state, when the steel sheet is immersed in a galvanizing bath for hot dip galvanization, as shown in FIG. 1 (b), Al contained in the galvanizing bath is generated from oxygen of MnO produced on the steel sheet surface and from the inside of the steel sheet. It reacts instantaneously with the diffusing Fe, and an Fe—Al—O alloy layer is formed at the interface between the steel sheet and the galvanized layer. As shown in FIG. 1 (c), this Fe—Al—O alloy layer becomes a diffusion barrier for Fe from the steel sheet during the alloying treatment, and the alloying of the steel sheet is inhibited, thereby causing uneven alloying. It was found that the surface appearance of the galvannealed steel sheet was deteriorated.
本発明者らは、この実情を解消するべく鋭意検討を行った結果、鋼板を亜鉛めっき浴へ浸漬する際の板温を適切に制御することで、Ｆｅ−Ａｌ−Ｏ合金層の生成を抑制することができ、その結果、合金化むらのない、合金化溶融亜鉛めっき鋼板を得ることができることを見出した。 As a result of intensive studies to eliminate this situation, the inventors of the present invention suppress the formation of the Fe—Al—O alloy layer by appropriately controlling the plate temperature when the steel plate is immersed in the galvanizing bath. As a result, it has been found that an alloyed hot-dip galvanized steel sheet without uneven alloying can be obtained.
通常、亜鉛めっきを行う際に、亜鉛めっき浴へ浸漬する際の鋼板の板温は４００〜５００℃程度であるが、表面にＭｎＯが発生した鋼板を、この板温条件で浸漬すると合金化むらが発生する。ここで、鋼板を亜鉛めっき浴へ浸漬する際の板温を、通常温度（４００〜５００℃）より高くすると、図２（ｂ）に示すように、Ｆｅの溶出と共にＭｎＯが亜鉛めっき浴内に排出されることとなり、鋼板表面におけるＦｅ−Ａｌ−Ｏ合金層の生成を防止することが可能となる。その結果、図２（ｃ）に示すように、合金化は均一に進行し、合金化むらの発生を低減することができる。 Usually, when performing galvanization, the plate temperature of the steel plate when immersed in the galvanizing bath is about 400 to 500 ° C., but when a steel plate with MnO generated on the surface is immersed under this plate temperature condition, uneven alloying occurs. Will occur. Here, when the plate temperature when the steel plate is immersed in the galvanizing bath is made higher than the normal temperature (400 to 500 ° C.), as shown in FIG. It will be discharged | emitted and it will become possible to prevent the production | generation of the Fe-Al-O alloy layer in the steel plate surface. As a result, as shown in FIG. 2 (c), alloying proceeds uniformly and the occurrence of uneven alloying can be reduced.
また、鋼材の焼鈍条件を、ＭｎＯが十分に生成されない条件である低酸素分圧とした上で、板温を通常温度（４００〜５００℃）より高くした場合、合金化溶融亜鉛めっき鋼板の表面に形成された亜鉛めっき層が剥離してしまう。従って、亜鉛めっき浴へ浸漬する際の鋼板の板温を単に制御するだけではなく、同時に、焼鈍条件も適切に調節しなければ、合金化の挙動を適性に制御することはできないということも確認した。 In addition, when the steel is annealed under a low oxygen partial pressure, which is a condition in which MnO is not sufficiently generated, and the plate temperature is higher than the normal temperature (400 to 500 ° C.), the surface of the alloyed hot-dip galvanized steel sheet The galvanized layer formed on is peeled off. Therefore, it is also confirmed that not only the temperature of the steel sheet when immersed in the galvanizing bath is controlled, but also the behavior of alloying cannot be controlled properly unless the annealing conditions are also adjusted appropriately. did.
本発明者らは、以上説明したように、亜鉛めっき浴へ浸漬する際の鋼板の板温を制御することと、焼鈍工程の雰囲気中の酸素分圧を調節することを、併せて実施することで、合金化むらのない、合金化溶融亜鉛めっき鋼板を確実に製造することができることを見出し、本発明を完成するに至った。 As described above, the inventors of the present invention are to carry out controlling the sheet temperature of the steel sheet when immersed in the galvanizing bath and adjusting the oxygen partial pressure in the annealing process atmosphere. Thus, the present inventors have found that an alloyed hot-dip galvanized steel sheet having no unevenness in alloying can be produced reliably, and have completed the present invention.
次に、本発明に用いる鋼材の成分限定理由について説明する。以下、各元素の含有量を％と記載するが、断りのない限り全て質量％を示す。 Next, the reasons for limiting the components of the steel material used in the present invention will be described. Hereinafter, although content of each element is described as%, unless otherwise indicated, all indicate mass%.
Ｃ：０．０２〜０．２％
Ｃは鋼の強度に大きく作用し、低温変態生成物の量や形態を変えることで、伸びや伸びフランジ性にも影響を与える。含有量が０．０２％未満では自動車用の高強度の鋼板とすることができず、一方、０．２％を超えて添加すると溶接性の低下を招く。従って、Ｃの含有量は、その下限を０．０２％、好ましくは０．０４％とし、その上限を０．２％、好ましくは０．１５％とする。
C: 0.02-0.2%
C greatly affects the strength of the steel, and affects the elongation and stretch flangeability by changing the amount and form of the low-temperature transformation product. If the content is less than 0.02%, a high-strength steel sheet for automobiles cannot be obtained. On the other hand, if the content exceeds 0.2%, weldability is deteriorated. Therefore, the lower limit of the C content is 0.02%, preferably 0.04%, and the upper limit is 0.2%, preferably 0.15%.
Ｍｎ：２．０〜３．５％
Ｍｎは強化元素であり、高強度を得るためと、加工性が非常に優れた高強度鋼板としての特性を得るためには、少なくとも２．０％以上添加することが必要である。一方、その含有量が多すぎると、伸びの低下、或いは炭素当量の増大があり、溶接性に悪影響を及ぼすため、３．５％以下とする必要がある。従って、Ｍｎの含有量は２．０〜３．５％とする。
Mn: 2.0 to 3.5%
Mn is a strengthening element, and it is necessary to add at least 2.0% or more in order to obtain high strength and in order to obtain characteristics as a high-strength steel plate having excellent workability. On the other hand, if the content is too large, there is a decrease in elongation or an increase in carbon equivalent, which adversely affects weldability. Therefore, the Mn content is set to 2.0 to 3.5%.
Ｃｒ：０．０３〜０．５％
Ｃｒは焼き入れ性を高め、組織強化を図る上で有効な元素である。また、Ｃｒはオーステナイト中にＣを濃化させ、その安定度を高め、マルテンサイトを生成させやすくするだけでなく、酸化物を鋼板表面に形成することによって、めっき性にも影響を与える。その含有量が０．０３％未満では、焼き入れ性の向上効果が期待できないので、その下限を０．０３％とする。一方、０．５％を超えて添加しても焼き入れ性の向上効果が飽和し、コスト面では不利になるので、その上限を０．５％とする。また、０．３％を超えて添加した場合、めっき性を損ねるので、その上限は０．３％とすることが好ましい。
Cr: 0.03-0.5%
Cr is an element effective in enhancing hardenability and strengthening the structure. Moreover, Cr not only concentrates C in austenite, increases its stability and facilitates the formation of martensite, but also affects the plating properties by forming an oxide on the steel sheet surface. If the content is less than 0.03%, the effect of improving hardenability cannot be expected, so the lower limit is made 0.03%. On the other hand, even if added over 0.5%, the effect of improving hardenability is saturated and disadvantageous in terms of cost, so the upper limit is made 0.5%. Further, if added over 0.3%, the plating property is impaired, so the upper limit is preferably made 0.3%.
Ａｌ：０．０１〜０．１５％
Ａｌは製鋼段階での脱酸剤として有効な元素であるので、０．０１％以上は添加する必要がある。しかしながら、その含有量が０．１５％を超えると、表面性状を悪化させるばかりか、製造コストの上昇を招く。従って、Ａｌの含有量は０．０１〜０．１５％とする。
Al: 0.01 to 0.15%
Since Al is an effective element as a deoxidizer in the steelmaking stage, it is necessary to add 0.01% or more. However, if its content exceeds 0.15%, not only the surface properties are deteriorated, but also the production cost is increased. Therefore, the Al content is set to 0.01 to 0.15%.
Ｓｉ：０．０４％以下（０％を含む）
Ｓｉはα層中の固溶Ｃ量を減少させることにより、伸びなどの加工性を向上させる元素である。但し、Ｓｉは鋼板表面に酸化皮膜を形成し、めっきの濡れ性を極端に劣化させる元素であるため、基本的には添加しない。しかしながら、不可避的に不純物として混入することがある元素であるため、その上限を、悪影響を及ぼす最低限の０．０４％とする。好ましくは、その上限を０．０３％に止める必要がある。
Si: 0.04% or less (including 0%)
Si is an element that improves workability such as elongation by reducing the amount of dissolved C in the α layer. However, since Si is an element that forms an oxide film on the surface of the steel sheet and extremely deteriorates the wettability of plating, it is basically not added. However, since it is an element that may inevitably be mixed as an impurity, the upper limit is set to 0.04%, which is the minimum that has an adverse effect. Preferably, the upper limit needs to be stopped at 0.03%.
Ｐ：０．０３％以下（０％を含む）
Ｐは高強度鋼板を得るために有効な元素であるが、０．０３％を超えるとめっきむらが生じやすくなり、また、合金化処理が困難になるので、基本的には添加しない。しかしながら、不可避的不純物として混入することがある元素であるため、その上限を０．０３％に止める必要がある。
P: 0.03% or less (including 0%)
P is an element effective for obtaining a high-strength steel sheet. However, if it exceeds 0.03%, uneven plating tends to occur, and alloying treatment becomes difficult, so basically it is not added. However, since it is an element that may be mixed as an unavoidable impurity, it is necessary to limit the upper limit to 0.03%.
Ｓ：０．０３％以下（０％を含む）
Ｓは熱間圧延時の熱間割れの原因になるほか、スポット割れ性を著しく損なう元素である。鋼中で析出物として固定されるが、その含有量が増加すると、伸びや伸びフランジ性の劣化を招くので、基本的には添加しない。しかしながら、不可避的不純物として混入することがある元素であるため、その上限を０．０３％に止める必要がある。
S: 0.03% or less (including 0%)
S is an element that causes hot cracking during hot rolling and significantly impairs spot cracking. Although it is fixed as a precipitate in steel, if its content is increased, elongation and stretch flangeability are deteriorated, so basically it is not added. However, since it is an element that may be mixed as an unavoidable impurity, it is necessary to limit the upper limit to 0.03%.
また、本発明に用いる鋼材は、以上の元素のほかはＦｅと不可避的不純物で構成されるが、必要に応じて更に以下の元素を含有しても良い。 In addition to the above elements, the steel material used in the present invention is composed of Fe and unavoidable impurities, but may further contain the following elements as necessary.
Ｃｕ：０．００３〜０．５％、Ｎｉ：０．００３〜１．０％
ＣｕとＮｉは鋼材自体の強度を向上させたり、めっき性を向上させたりすることができる有効な元素である。ＣｕやＮｉは鋼材の主成分であるＦｅより酸化しにくいため、ＣｕやＮｉが鋼材表面に濃化することにより、ＳｉやＭｎの酸化物形態を変化させてめっき性の低下を防止することが可能になる。そのような効果を得ることを考慮すると０．００３％以上の添加は必要ではあるが、過度の添加は、加工性の低下、コスト高をもたらすため、Ｃｕの場合、上限は０．５％、Ｎｉの場合、上限は１．０％とする。
Cu: 0.003-0.5%, Ni: 0.003-1.0%
Cu and Ni are effective elements that can improve the strength of the steel material itself or improve the plating property. Since Cu and Ni are less susceptible to oxidation than Fe, which is the main component of steel materials, Cu and Ni can be concentrated on the steel material surface to change the oxide form of Si and Mn and prevent deterioration of plating properties. It becomes possible. In consideration of obtaining such an effect, addition of 0.003% or more is necessary, but excessive addition causes a decrease in workability and high cost. Therefore, in the case of Cu, the upper limit is 0.5%. In the case of Ni, the upper limit is 1.0%.
Ｔｉ：０．００３〜１．０％
Ｔｉは炭化物を形成し、鋼を高強度化するために有効な元素である。また、ＣやＮを固定し、鋼板のｒ値を上昇させる効果もある。その効果を奏するためには、０．００３％以上の添加は必要であるが、過度の添加は、加工性の低下、コスト上昇をもたらすため、その上限を１．０％とする。
Ti: 0.003-1.0%
Ti is an effective element for forming carbides and increasing the strength of steel. In addition, C and N are fixed and the r value of the steel plate is increased. In order to exhibit the effect, addition of 0.003% or more is necessary, but excessive addition causes a decrease in workability and an increase in cost, so the upper limit is made 1.0%.
また、Ｃｕ、Ｎｉ、Ｔｉは複合添加することで、鋼板表面の清浄度を向上させることができ、Ｆｅの溶解時に鉄の複合酸化物を形成して、めっき性を向上させる作用もある。従って、これらの元素を複合添加する場合は、単独で含有する場合の上下限も考慮して、合計で０．００３〜１．０％とする。 In addition, Cu, Ni, and Ti can be added together to improve the cleanliness of the steel sheet surface, and have the effect of improving the plating properties by forming a complex oxide of iron when Fe is dissolved. Therefore, when these elements are added in combination, the total content is set to 0.003 to 1.0% in consideration of the upper and lower limits when contained alone.
Ｖ：０．００３〜１．０％、Ｎｂ：０．００３〜１．０％
ＶとＮｂは共に炭化物を形成し、鋼を高強度化するために有効な元素である。その効果を奏するためには、夫々０．００３％以上の添加は必要であるが、過度の添加は、加工性の低下、コスト上昇をもたらすため、夫々その上限を１．０％とする。
V: 0.003-1.0%, Nb: 0.003-1.0%
V and Nb together form carbides and are effective elements for increasing the strength of steel. In order to achieve the effect, 0.003% or more of each addition is necessary. However, excessive addition causes a decrease in workability and an increase in cost, so the upper limit is made 1.0%.
Ｂ：０．０００２〜０．１％
Ｂは溶接性を向上させると共に、焼入性を高める作用がある。その作用を効果的に発現させるには、０．０００２％以上添加することが好ましい。しかし、過度に添加すると、これらの作用が飽和するだけではなく、延性が劣化し、加工性が低下するようになるので、その上限を０．１％とする。
B: 0.0002 to 0.1%
B has the effect of improving weldability and improving hardenability. In order to effectively exhibit the action, it is preferable to add 0.0002% or more. However, if added excessively, these actions are not only saturated, but ductility is deteriorated and workability is lowered, so the upper limit is made 0.1%.
Ｍｏ：０．００３〜１．０％
Ｍｏはめっき性を損なわずに、固溶強化を図る上で有効な元素である。その効果を奏するためには、０．００３％以上の添加は必要であるが、過度の添加は、製造コストの上昇をもたらすため、その上限を１．０％とする。
Mo: 0.003-1.0%
Mo is an element effective in strengthening solid solution without impairing the plating property. In order to achieve the effect, addition of 0.003% or more is necessary, but excessive addition causes an increase in manufacturing cost, so the upper limit is made 1.0%.
Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００１％
Ｃａは介在物の形態を制御して、延性を高め、加工性を向上させる作用がある。その作用を効果的に発現させるには、０．０００５％以上添加する必要がある。しかし、過度に添加すると、鋼中の介在物量が増加して延性が劣化し、加工性が低下するようになるので、その上限を０．００５％とする。Ｍｇも鋼中でＣａと同様の働きをするが、その含有量は、Ｃａと同様の理由で０．０００５〜０．００１％とする。
Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.001%
Ca has the effect | action which controls the form of an inclusion, raises ductility, and improves workability. In order to effectively exhibit the action, it is necessary to add 0.0005% or more. However, if added excessively, the amount of inclusions in the steel increases, ductility deteriorates and workability decreases, so the upper limit is made 0.005%. Mg also has the same function as Ca in steel, but its content is set to 0.0005 to 0.001% for the same reason as Ca.
以下、本発明の合金化溶融亜鉛めっき鋼板の製造方法の一例を、その製造条件と共に詳細に説明する。 Hereinafter, an example of the manufacturing method of the galvannealed steel plate of this invention is demonstrated in detail with the manufacturing conditions.
まず、上記の成分を含有する鋼のスラブを熱間圧延した後、巻き取り、必要に応じて表面の酸洗を行った後、冷間圧延して下地鋼板を作製する。 First, a steel slab containing the above components is hot-rolled, wound up, and optionally pickled on the surface, and then cold-rolled to produce a base steel plate.
次に、連続式溶融亜鉛めっきラインにて下地鋼板の焼鈍を行う。例えば、この工程での焼鈍温度は７５０〜９００℃とし、焼鈍時間は２００秒以内とする。また、この焼鈍工程は、雰囲気中の酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で行う。酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）＞２３という低い条件で下地鋼板の焼鈍を行った場合、ＭｎＯの発生が極端に少なくなる。この条件で焼鈍した鋼板の板温を通常温度（４００〜５００℃）より高くして５１０℃以上とし、次のめっき工程で亜鉛めっき浴に浸漬すると、Ｆｅの拡散が過剰に促進されて合金化が過度に進行する。その結果、亜鉛めっきが剥離しやすくなり、耐パウダリング性が悪くなる。よって、焼鈍工程での雰囲気中の酸素分圧ＰＯ２（単位はａｔｍ）は、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で行う。 Next, the base steel sheet is annealed in a continuous hot dip galvanizing line. For example, the annealing temperature in this step is 750 to 900 ° C., and the annealing time is within 200 seconds. In addition, this annealing step is performed under the condition that the oxygen partial pressure PO 2 in the atmosphere (the unit is atm) satisfies −log (PO 2 ) ≦ 23. When the base steel sheet is annealed under a low oxygen partial pressure PO 2 (unit: atm) of −log (PO 2 )> 23, generation of MnO is extremely reduced. When the steel plate annealed under these conditions is made higher than the normal temperature (400 to 500 ° C.) to 510 ° C. or higher and immersed in a galvanizing bath in the next plating step, the diffusion of Fe is excessively promoted and alloyed. Progresses excessively. As a result, the galvanizing is easily peeled off and the powdering resistance is deteriorated. Therefore, the oxygen partial pressure PO 2 (unit: atm) in the atmosphere in the annealing process is performed under a condition satisfying −log (PO 2 ) ≦ 23.
焼鈍工程を終えた後、めっき工程での亜鉛めっき処理を行う。めっき浴としては、Ａｌを０．０５〜０．２０質量％含有する溶融亜鉛めっき浴を用いる。本発明では、亜鉛めっき浴に浸漬する際の鋼板の板温を、５１０℃以上、６００℃未満とする。 After finishing the annealing process, a galvanizing process in the plating process is performed. As the plating bath, a hot dip galvanizing bath containing 0.05 to 0.20% by mass of Al is used. In the present invention, the plate temperature of the steel sheet when immersed in the galvanizing bath is set to 510 ° C. or more and less than 600 ° C.
鋼板の板温を、亜鉛めっきを行う際の従来までの通常温度（４００〜５００℃）として、亜鉛めっき浴に浸漬すると、鋼板表面に発生したＭｎＯ粒子が、亜鉛めっき浴に浸漬後もそのまま鋼板と亜鉛めっき層の界面に残存することになり、そのＭｎＯの酸素（Ｏ）が、鋼板内部から拡散するＦｅと共に、亜鉛めっき浴中に排出されることになる。このＯとＦｅが、亜鉛めっき浴中のＡｌと瞬時に反応することで、鋼板と亜鉛めっきの界面にＦｅ−Ａｌ−Ｏ合金層が形成される。その結果、合金化むらを発生することになる。 これに対して、鋼板の板温を、５１０℃以上、６００℃未満として、亜鉛めっき浴に浸漬すると、ＭｎＯがＦｅ共に亜鉛めっき浴中に排出されることになり、Ｆｅ−Ａｌ−Ｏ合金層の形成が抑制されることになり、その結果、合金化むらの発生を抑制することができる。 When the plate temperature of the steel plate is immersed in a galvanizing bath at a normal temperature (400 to 500 ° C.) up to now when performing galvanizing, the MnO particles generated on the surface of the steel plate are still intact after being immersed in the galvanizing bath. And oxygen (O) of the MnO are discharged into the galvanizing bath together with Fe diffusing from the inside of the steel sheet. This O and Fe react instantaneously with Al in the galvanizing bath, whereby a Fe—Al—O alloy layer is formed at the interface between the steel plate and the galvanizing. As a result, alloying unevenness occurs. On the other hand, when the plate temperature of the steel sheet is set to 510 ° C. or more and less than 600 ° C. and immersed in the galvanizing bath, both MnO and Fe are discharged into the galvanizing bath, and the Fe—Al—O alloy layer As a result, the occurrence of uneven alloying can be suppressed.
この鋼板の亜鉛めっき浴への浸漬時間は、例えば５秒以内である。この浸漬後に鋼板を亜鉛めっき浴から引き出し、その鋼板の表面に付着した亜鉛めっきの付着量を調整する。その調整は、例えばガスワイパーによって６０±５ｇ／ｍ２の適正量に調整する。 The immersion time of the steel sheet in the galvanizing bath is, for example, within 5 seconds. After this immersion, the steel plate is pulled out from the galvanizing bath, and the amount of galvanizing attached to the surface of the steel plate is adjusted. The adjustment is made to an appropriate amount of 60 ± 5 g / m 2 by, for example, a gas wiper.
このめっき工程終了後に、続いて、合金化処理工程での合金化処理を行って、溶融亜鉛めっき鋼板を合金化溶融亜鉛めっき鋼板とする。例えば、この合金化処理の処理温度は４５０〜６００℃で、処理時間は６０秒以内である。以上の工程を経ることにより、めっき皮膜の均一性および耐パウダリング性に優れ、合金化むらが抑制された表面外観に優れた合金化溶融亜鉛めっき鋼板を製造することができる。 After the completion of this plating process, subsequently, an alloying process in the alloying process is performed to make the hot dip galvanized steel sheet an alloyed hot dip galvanized steel sheet. For example, the alloying treatment temperature is 450 to 600 ° C., and the treatment time is within 60 seconds. By passing through the above process, the alloyed hot-dip galvanized steel sheet excellent in the uniformity of the plating film and the powdering resistance and excellent in the surface appearance in which unevenness in alloying is suppressed can be produced.
以上説明したように、Ｍｎの含有量が２．０〜３．５質量％と高い鋼板を下地鋼板として、合金化溶融亜鉛めっき鋼板を製造するにあたっては、焼鈍時の酸素分圧ＰＯ２（単位はａｔｍ））を、−ｌｏｇ（ＰＯ２）≦２３を満たす適正な条件とした上で、めっき工程での鋼板の浸漬温度を、５１０℃以上、６００℃未満とすることで、耐パウダリング性、表面外観に優れた合金化溶融亜鉛めっき鋼板を製造することができる。 As described above, when producing an alloyed hot-dip galvanized steel sheet using a steel sheet having a high Mn content of 2.0 to 3.5% by mass as a base steel sheet, the oxygen partial pressure PO 2 (unit) during annealing is used. Is atm)) under appropriate conditions satisfying −log (PO 2 ) ≦ 23, and the immersion temperature of the steel sheet in the plating step is 510 ° C. or more and less than 600 ° C. An alloyed hot-dip galvanized steel sheet having an excellent surface appearance can be produced.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、本発明の趣旨に適合し得る範囲で適宜変更を加えて実施することも可能であり、それらは何れも本発明の技術的範囲に含まれる。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.
試験では、表１に示す成分組成の各種冷延鋼板を、１００×２５０ｍｍのサイズに加工し、溶融亜鉛めっきシミュレータを用いて、焼鈍、めっき、合金化処理という順を経ることで、合金化溶融亜鉛めっき鋼板の試験片を得た。 In the test, various cold-rolled steel sheets having the component compositions shown in Table 1 were processed into a size of 100 × 250 mm, and were subjected to annealing, plating, and alloying treatment using a hot-dip galvanizing simulator, and then alloyed and melted. A specimen of a galvanized steel sheet was obtained.
まず、表１に示す成分組成の各種冷延鋼板の表面を酸洗することで清浄化した後、Ｎ２−３％Ｈ２の雰囲気で焼鈍を行った。焼鈍条件については表２に示す。この焼鈍での焼鈍温度は７５０〜９００℃の範囲とし、焼鈍時間は１２０秒とした。−ｌｏｇ（ＰＯ２）は、表２に示すように、焼鈍温度を７５０〜９００℃の範囲とし、露点を−７５〜０℃の範囲で変化させることで調整した。 First, the surfaces of various cold-rolled steel sheets having the component compositions shown in Table 1 were cleaned by pickling, and then annealed in an atmosphere of N 2 -3% H 2 . The annealing conditions are shown in Table 2. The annealing temperature in this annealing was in the range of 750 to 900 ° C., and the annealing time was 120 seconds. -Log (PO 2), as shown in Table 2, the annealing temperature in the range of 750 to 900 ° C., was adjusted by changing the dew point in the range of -75~0 ℃.
この焼鈍後の鋼板の板温を、表３に示すように調整し、Ａｌを０．１３質量％含有する溶融亜鉛めっき浴に浸漬することで、鋼板の表面に亜鉛めっき層を形成した。尚、浸漬時間は２秒間とした。亜鉛めっき層形成後、ガスワイパーにより、その亜鉛付着量を６０ｇ／ｍ２に調整して、溶融亜鉛めっき鋼板を作製した。また、溶融亜鉛めっき浴の温度は、鋼板の板温と同一温度となるよう調整した。 The plate temperature of the steel plate after annealing was adjusted as shown in Table 3 and immersed in a hot dip galvanizing bath containing 0.13% by mass of Al to form a galvanized layer on the surface of the steel plate. The immersion time was 2 seconds. After forming the galvanized layer, the zinc adhesion amount was adjusted to 60 g / m 2 with a gas wiper to prepare a hot dip galvanized steel sheet. The temperature of the hot dip galvanizing bath was adjusted to be the same temperature as the steel plate temperature.
合金化処理は、めっき処理の直後、めっきシミュレータ内で赤外線加熱炉を使用することで行った。合金化温度は５５０℃、合金化時間は１０秒間とした。この試験では、得られた合金化溶融亜鉛めっき鋼板の試験片を用いてめっき特性の評価を行った。評価結果を表３に示す。 The alloying treatment was performed by using an infrared heating furnace in the plating simulator immediately after the plating treatment. The alloying temperature was 550 ° C. and the alloying time was 10 seconds. In this test, the plating characteristics were evaluated using a specimen of the obtained galvannealed steel sheet. The evaluation results are shown in Table 3.
まず、合金化処理後の各試験片の中央より１０ｍｍ角のサンプルを切り出して断面試料を作製し、合金化の有無を確認した。亜鉛めっき層のＦｅ含有量（質量％）は、ＳＥＭ−ＥＤＸにより分析した。このＦｅ含有量は、合金化の進行度合いを示し、含有量の多少で合金化むらを推測できる。Ｆｅ含有量が少ない場合は、合金化不足を生じ、Ｆｅ含有量が過剰な場合は、合金化過剰によるめっき剥離が発生する。 First, a 10 mm square sample was cut out from the center of each test piece after alloying treatment to produce a cross-sectional sample, and the presence or absence of alloying was confirmed. The Fe content (mass%) of the galvanized layer was analyzed by SEM-EDX. This Fe content indicates the degree of progress of alloying, and unevenness in alloying can be estimated depending on the content. When the Fe content is low, insufficient alloying occurs, and when the Fe content is excessive, plating peeling due to excessive alloying occurs.
合金化むらの発生状況の評価基準は、◎：合金化むらなし、○：合金化むらが面積率で１０％未満発生、△：合金化むらが面積率で１０％以上３０％未満発生、×：合金化むらが面積率で３０％以上発生とした。◎と○を合格とする。 The evaluation criteria for the state of occurrence of alloying unevenness are as follows: ◎: no alloying unevenness, ○: alloying unevenness occurs in an area ratio of less than 10%, Δ: alloying unevenness occurs in an area ratio of 10% or more and less than 30%, × : Alloying unevenness occurred at 30% or more by area ratio. ◎ and ○ are acceptable.
耐パウダリング性については、９０°曲げ戻しテストを実施した後、圧縮側表面の亜鉛めっき層をテープ剥離させ、剥離面積／全面積＝面積率（％）を求めた。耐パウダリング性の評価基準は、◎：めっき剥離なし、○：めっき剥離が面積率で５％以下発生、×：めっき剥離が面積率で１０％以上発生、××：不めっきが発生し、めっき部の略全面が剥離とした。◎と○を合格とする。 Regarding the powdering resistance, after performing a 90 ° bend-back test, the galvanized layer on the compression side surface was peeled off with tape, and peeled area / total area = area ratio (%) was determined. The evaluation criteria for powdering resistance are: ◎: no plating peeling, ○: plating peeling occurs 5% or less in area ratio, x: plating peeling occurs 10% or more in area ratio, xx: non-plating occurs, The substantially entire surface of the plating part was peeled off. ◎ and ○ are acceptable.
試験Ｎｏ．１、２、６、７、８、１１、１５〜２９は、本発明の実施例である。酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で焼鈍を行うと共に、亜鉛めっき浴に浸漬させる際の鋼板の板温を５１０℃以上且つ６００℃未満とした実施例では、合金化むらの発生は全て１０％未満の合格範囲で、表面外観に優れた合金化溶融亜鉛めっき鋼板を得ることができた。また、耐パウダリング性についても、評価基準のめっき剥離は多くても面積率で５％であって、全て合格範囲であった。 Test No. 1, 2, 6, 7, 8, 11, 15 to 29 are examples of the present invention. Annealing is performed under the condition that oxygen partial pressure PO 2 (unit: atm) satisfies −log (PO 2 ) ≦ 23, and the plate temperature of the steel sheet when immersed in the galvanizing bath is 510 ° C. or more and less than 600 ° C. In the examples, the occurrence of alloying unevenness was all within the acceptable range of less than 10%, and an alloyed hot-dip galvanized steel sheet excellent in surface appearance could be obtained. Moreover, regarding the powdering resistance, the plating peeling of the evaluation standard was 5% in area ratio at most, and all were within the acceptable range.
一方、酸素分圧ＰＯ２（単位はａｔｍ））が、−ｌｏｇ（ＰＯ２）＞２３の条件で焼鈍を行った比較例である試験Ｎｏ．４、１０、１４では、合金化むらが面積率で３０％以上発生し、耐パウダリング性についても、めっき剥離が面積率で１０％以上発生し、表面外観が悪くなった。特に、亜鉛めっき浴に浸漬させる際の鋼板の板温を５１０℃未満とした試験Ｎｏ．１４では、合金化が極度に進行したと思われ、亜鉛めっき層の剥離は顕著であり、表面外観は際立って悪くなった。 On the other hand, the oxygen partial pressure PO 2 (unit is atm)) is a test No. 1 which is a comparative example in which annealing was performed under the condition of −log (PO 2 )> 23. In 4, 10, and 14, alloying unevenness occurred in an area ratio of 30% or more, and regarding powdering resistance, plating peeling occurred in an area ratio of 10% or more, and the surface appearance deteriorated. In particular, in Test No. 1 in which the plate temperature of the steel plate when immersed in the galvanizing bath was less than 510 ° C. No. 14, the alloying seems to have progressed extremely, the galvanized layer was markedly peeled off, and the surface appearance was markedly worse.
試験Ｎｏ．３、５、９、１２、１３は、酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で焼鈍を行ったが、亜鉛めっき浴に浸漬させる際の鋼板の板温を５１０℃未満とした比較例である。これら比較例（試験Ｎｏ．１２を除く）では、耐パウダリング性については良好であったが、合金化むらが面積率で３０％以上発生し、表面外観が悪くなった。また、亜鉛めっき浴に浸漬させる際の鋼板の板温を４２０℃と極端に低くした試験Ｎｏ．１２では、ＭｎＯが大量に発生し、不めっきが発生してしまった。 Test No. 3,5,9,12,13, the oxygen partial pressure PO 2 (unit atm) is has been subjected to annealing under the conditions satisfying the -log (PO 2) ≦ 23, the steel sheet at the time of immersing in the galvanizing bath This is a comparative example in which the plate temperature was less than 510 ° C. In these comparative examples (excluding Test No. 12), the powdering resistance was good, but alloying irregularities occurred in an area ratio of 30% or more, and the surface appearance deteriorated. Further, test No. 1 in which the plate temperature of the steel plate when immersed in the galvanizing bath was extremely low as 420 ° C. In No. 12, a large amount of MnO was generated and non-plating occurred.
Claims (4)
焼鈍工程は、雰囲気中の酸素分圧ＰＯ２（単位はａｔｍ）が、−ｌｏｇ（ＰＯ２）≦２３を満たす条件で行うと共に、
めっき工程は、５１０℃以上、６００℃未満の板温の鋼板を、亜鉛めっき浴に浸漬させることにより行うことを特徴とする合金化溶融亜鉛めっき鋼板の製造方法。 In mass%, C: 0.02-0.2%, Mn: 2.0-3.5%, Cr: 0.03-0.5%, Al: 0.01-0.15%, Si: From steel sheet containing 0.04% or less (including 0%), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%), annealing process, plating process A method for producing an alloyed hot-dip galvanized steel sheet, which is produced through an alloying treatment step,
The annealing step is performed under the condition that the oxygen partial pressure PO 2 (unit: atm) in the atmosphere satisfies −log (PO 2 ) ≦ 23,
A plating process is performed by immersing a steel plate having a plate temperature of 510 ° C. or higher and lower than 600 ° C. in a galvanizing bath.
The steel sheet further contains one or more selected from the group consisting of Ca: 0.0005 to 0.005% and Mg: 0.0005 to 0.001% by mass%. Item 4. A method for producing an galvannealed steel sheet according to any one of Items 1 to 3.
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