JP2011127216A - Plated steel sheet and method of producing the same - Google Patents
Plated steel sheet and method of producing the same Download PDFInfo
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Abstract
Description
本発明は、めっき密着性に優れるＳｉ及びＭｎ含有溶融亜鉛めっき鋼板（ＧＩ）および合金化溶融亜鉛めっき鋼板（ＧＡ）に関し、詳細には、自動車、家電、建材等の分野で、年々複雑化する加工に対しても良好なめっき密着性を確保し得るＳｉ及びＭｎ含有溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板に関するものである。以下では、説明の便宜上、これらの鋼板をまとめて「めっき鋼板」で代表させる場合がある。 The present invention relates to hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA) containing Si and Mn, which are excellent in plating adhesion, and in particular, in the fields of automobiles, home appliances, building materials, etc. The present invention relates to a hot-dip galvanized steel sheet containing Si and Mn and an alloyed hot-dip galvanized steel sheet that can ensure good plating adhesion to processing. Hereinafter, for convenience of explanation, these steel plates may be collectively represented by “plated steel plates”.
近年、自動車や家電等の軽量化の目的で、強度、延性、加工性に優れた鋼鈑の需要が急増している。鋼鈑にＳｉやＭｎを添加すると、強度を損なうことなく延性や加工性を向上できることから、このような特性を満たす鋼鈑としてＳｉ及びＭｎ含有鋼が使用されている。また、上記鋼板には耐食性も要求されることから、Ｓｉ及びＭｎ含有鋼に耐食性を付与した溶融亜鉛めっき鋼板（ＧＩ鋼板）や合金化溶融亜鉛めっき鋼板（ＧＡ鋼板）のニーズが年々高まりつつある。 In recent years, the demand for steel plates excellent in strength, ductility, and workability has been increasing rapidly for the purpose of reducing the weight of automobiles and home appliances. When Si or Mn is added to the steel sheet, ductility and workability can be improved without impairing the strength. Therefore, steel containing Si and Mn is used as a steel sheet satisfying such properties. Moreover, since corrosion resistance is also required for the steel sheet, the needs for hot dip galvanized steel sheets (GI steel sheets) and alloyed hot dip galvanized steel sheets (GA steel sheets) that give corrosion resistance to Si and Mn-containing steels are increasing year by year. .
溶融亜鉛めっき鋼板や合金化溶融亜鉛めっき鋼板のめっき鋼板は、一般に以下の方法で製造される。まず、スラブを熱延、冷延の後、必要に応じて熱処理を行なった薄鋼板（母材鋼板）を用意する。母材鋼板の表面は、前処理工程にて脱脂および／または酸洗して洗浄しても良い。次に、予熱炉内で母材鋼板表面の油分を燃焼除去した後、非酸化性雰囲気または還元性雰囲気の焼鈍炉内で加熱して再結晶焼鈍を行う。その後、非酸化性雰囲気中または還元性雰囲気中で鋼板をめっきに適した温度まで冷却し、大気に触れることなく微量のＡｌ（約０．１〜０．２質量％程度）を添加した溶融亜鉛浴中に浸漬することによって溶融亜鉛めっき鋼板が得られる。 In general, a galvanized steel sheet or a galvannealed steel sheet is produced by the following method. First, after the slab is hot-rolled and cold-rolled, a thin steel plate (base material steel plate) that is heat-treated as necessary is prepared. The surface of the base steel plate may be cleaned by degreasing and / or pickling in the pretreatment step. Next, after the oil on the surface of the base steel plate is burned and removed in the preheating furnace, recrystallization annealing is performed by heating in an annealing furnace in a non-oxidizing atmosphere or a reducing atmosphere. Thereafter, the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere, and a small amount of Al (about 0.1 to 0.2% by mass) is added without being exposed to the air. A hot-dip galvanized steel sheet is obtained by dipping in a bath.
また、合金化溶融亜鉛めっき鋼板は、溶融亜鉛めっき後、合金化炉内で熱処理することによって得られる。 The alloyed hot-dip galvanized steel sheet is obtained by heat treatment in an alloying furnace after hot-dip galvanizing.
しかしながら、ＳｉやＭｎは易酸化性元素であり、鋼板表面に濃化し易い。すなわち、易酸化性元素を含有する鋼板を加熱処理すると、これらの元素が選択的に酸化され、鋼鈑表面（鋼板とめっき層との界面側）に濃化して酸化物（Ｓｉ−Ｍｎ−Ｏなど）を形成する。これらの酸化物は、めっき処理時の溶融亜鉛との濡れ性を著しく低下させるため、不めっきや合金化不良を招く。これらの易酸化性元素は、非酸化性雰囲気中または還元雰囲気中でも濃化を抑制することが困難なため、Ｓｉ及びＭｎ含有鋼板では、上記酸化物による問題の改善が求められている。 However, Si and Mn are easily oxidizable elements and are easily concentrated on the steel sheet surface. That is, when a steel sheet containing an easily oxidizable element is heat-treated, these elements are selectively oxidized and concentrated on the steel sheet surface (the interface side between the steel sheet and the plating layer) to form an oxide (Si-Mn-O). Etc.). Since these oxides remarkably reduce wettability with molten zinc during the plating process, non-plating and poor alloying are caused. Since these easily oxidizable elements are difficult to suppress concentration even in a non-oxidizing atmosphere or a reducing atmosphere, the Si and Mn-containing steel sheets are required to improve the problems caused by the oxides.
そこで、ＳｉやＭｎの易酸化性元素含有鋼板を用いて溶融亜鉛めっき、更には合金化を行なうにあたり、所謂「酸化−還元法」が用いられている。この「酸化−還元法」は、熱延および冷延を行なった鋼板に対し、酸化性雰囲気下での酸化および還元性雰囲気下での還元を行なってから、所定の溶融亜鉛めっき処理、更には合金化処理を行なう方法である。詳細には、まず、焼鈍炉を酸化性雰囲気として加熱（酸化）することにより、鋼板の表面に、鋼板側から順に、Ｆｅと、Ｓｉ、Ｍｎなどの易酸化性元素の酸化物（Ｆｅ−Ｓｉ−Ｍｎ−Ｏ）から主に構成される内方酸化層と；鉄酸化物（Ｆｅ−Ｏ）から主に構成される外方酸化層が形成される。次に、還元炉を還元性雰囲気として加熱（還元）することにより、上記の鉄酸化物が還元されてめっき濡れ性に優れた還元鉄（Ｆｅ）の層が鋼板表層（外側）に形成される。次いで、溶融亜鉛めっき、更には合金化処理が行なわれる。 Therefore, a so-called “oxidation-reduction method” is used for hot dip galvanization and further alloying using a steel plate containing Si and Mn easily oxidizable elements. This “oxidation-reduction method” is a method in which a hot-rolled and cold-rolled steel sheet is subjected to oxidation in an oxidizing atmosphere and reduction in a reducing atmosphere, followed by a predetermined hot dip galvanizing treatment, This is a method of alloying treatment. Specifically, first, by heating (oxidizing) an annealing furnace as an oxidizing atmosphere, Fe, Si, Mn, and other oxides of easily oxidizable elements (Fe—Si) are sequentially formed on the surface of the steel plate from the steel plate side. An inner oxide layer mainly composed of -Mn-O) and an outer oxide layer mainly composed of iron oxide (Fe-O) are formed. Next, by heating (reducing) the reducing furnace as a reducing atmosphere, the iron oxide is reduced, and a reduced iron (Fe) layer having excellent plating wettability is formed on the surface layer (outside) of the steel sheet. . Next, hot dip galvanizing and further alloying treatment are performed.
しかしながら、この「酸化−還元法」によれば、めっき層と鋼鈑との界面にＳｉやＭｎ
などの易酸化性元素の酸化物（Ｓｉ−Ｍｎ−Ｏ）が不適切に濃化する場合があり、その結果、めっき鋼板の加工時にめっきが剥離するなどの問題がある。
However, according to this "oxidation-reduction method", Si or Mn is formed at the interface between the plating layer and the steel sheet.
The oxide (Si-Mn-O) of an easily oxidizable element such as the above may be inappropriately concentrated, and as a result, there is a problem that the plating is peeled off when the plated steel sheet is processed.
このようなめっき剥離の問題に対し、例えば特許文献１〜３に記載の方法が提案されている。このうち特許文献１および特許文献２には、鋼板とめっき層との界面から５μｍ以下の鋼板側の結晶粒界と結晶粒内にＳｉを含む酸化物が存在するように制御された溶融亜鉛めっき鋼板が開示されている。特許文献３には、鋼板の表面に所定量のＳｉＯ２の内部
酸化物を含むように制御された合金化溶融亜鉛めっき鋼板が開示されている。
For example, methods described in Patent Documents 1 to 3 have been proposed for such a problem of plating peeling. Among them, in Patent Document 1 and Patent Document 2, hot dip galvanization controlled so that an oxide containing Si exists in the crystal grain boundary and the crystal grain on the steel sheet side of 5 μm or less from the interface between the steel sheet and the plating layer. A steel sheet is disclosed. Patent Document 3 discloses an alloyed hot-dip galvanized steel sheet that is controlled so as to include a predetermined amount of an internal oxide of SiO 2 on the surface of the steel sheet.
しかしながら、上記の方法はいずれも、ＳｉやＭｎの鋼板表面への濃化（表面酸化）を抑制するため、鋼板とめっき層の界面付近に粒界酸化や内部酸化層を積極的に発生させることに主眼を置いたものであり、めっき密着性が不充分な場合がある。 However, all of the above methods positively generate grain boundary oxidation and internal oxide layers near the interface between the steel plate and the plating layer in order to suppress the concentration (surface oxidation) of Si and Mn on the steel plate surface. In some cases, the plating adhesion is insufficient.
本発明は上記事情に鑑みてなされたものであり、その目的は、めっき密着性に優れたＳｉ及びＭｎ含有溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板、並びにその製造方法を提供することにある。 This invention is made | formed in view of the said situation, The objective is to provide Si and Mn containing hot-dip galvanized steel plate and alloyed hot-dip galvanized steel plate excellent in plating adhesiveness, and its manufacturing method. .
上記課題を解決することのできた本発明の溶融亜鉛めっき鋼鈑は、素地鋼板に合金化されたまたは合金化されていない溶融亜鉛めっき層が形成されためっき鋼板であって、前記素地鋼板と前記溶融亜鉛めっき層との界面に、Ｓｉ−Ｍｎ−Ｏおよび鉄亜鉛合金を含む酸化物含有層を有し、前記酸化物含有層の素地鋼板側の表面は、網目状の凸部と、該凸部によって分割された複数の凹部とを有し、インターセプト法で算出した前記凹部の平均直径が３．０μｍ以上１０．０μｍ以下であり、前記凸部の平均幅が０．２０μｍ以上３．０μｍ以下であるところに要旨を有している。 The hot-dip galvanized steel sheet of the present invention that has solved the above-mentioned problems is a plated steel sheet in which a hot-dip galvanized layer that is alloyed or not alloyed is formed on a base steel sheet, and the base steel sheet and the above-mentioned It has an oxide-containing layer containing Si-Mn-O and an iron-zinc alloy at the interface with the hot-dip galvanized layer, and the surface of the oxide-containing layer on the base steel plate side has a mesh-like convex part and the convex part. An average diameter of the concave portion calculated by the intercept method is 3.0 μm to 10.0 μm, and an average width of the convex portion is 0.20 μm to 3.0 μm. It has the gist.
好ましい実施形態において、前記凹部の平均直径は４μｍ以上９μｍ以下であり、前記凸部の平均幅は０．８μｍ以上２．２μｍ以下である。 In a preferred embodiment, the concave portion has an average diameter of 4 μm or more and 9 μm or less, and the convex portion has an average width of 0.8 μm or more and 2.2 μm or less.
好ましい実施形態において、上記素地鋼板は、質量％で（以下、鋼中成分について、全て同じ。）、Ｃ：０．０４％以上０．２％以下、Ｓｉ：０．１％以上３％以下、Ｍｎ：０．１％以上３％以下、Ａｌ：０．０６％以下（０％は含まない）を含有する。 In preferable embodiment, the said base steel plate is the mass% (Hereinafter, all are the same about the component in steel.), C: 0.04% or more and 0.2% or less, Si: 0.1% or more and 3% or less, Mn: 0.1% or more and 3% or less, Al: 0.06% or less (0% is not included).
好ましい実施形態において、上記素地鋼板は、更にＣｒを０．３％以下（０％を含まない）含有している。 In a preferred embodiment, the base steel sheet further contains 0.3% or less (not including 0%) of Cr.
好ましい実施形態において、上記素地鋼板は、更にＴｉを０．０５％以下（０％を含まない）含有している。 In a preferred embodiment, the base steel sheet further contains 0.05% or less (not including 0%) of Ti.
好ましい実施形態において、上記素地鋼板は、更にＮｉを２％以下（０％を含まない）、Ｃｕを２％以下（０％を含まない）、Ｍｏを２％以下（０％を含まない）、およびＢを０．０１％以下（０％を含まない）よりなる群から選択される少なくとも１種を含有している。 In a preferred embodiment, the base steel sheet further includes Ni 2% or less (excluding 0%), Cu 2% or less (not including 0%), Mo 2% or less (not including 0%), And B contains at least one selected from the group consisting of 0.01% or less (excluding 0%).
好ましい実施形態において、上記素地鋼板は、更にＮｂを１％以下（０％を含まない）、Ｖを１％以下（０％を含まない）、およびＷを０．３％以下（０％を含まない）よりなる群から選択される少なくとも１種を含有している。 In a preferred embodiment, the base steel sheet further includes Nb of 1% or less (excluding 0%), V of 1% or less (not including 0%), and W of 0.3% or less (including 0%). And at least one selected from the group consisting of:
好ましい実施形態において、上記素地鋼板は、更にＣａ、Ｍｇ、およびＲＥＭよりなる群から選ばれるすくなくとも１種の元素を０．０３％以下（０％を含まない）含有している。 In a preferred embodiment, the base steel sheet further contains 0.03% or less (not including 0%) of at least one element selected from the group consisting of Ca, Mg, and REM.
好ましい実施形態において、上記素地鋼板は、残部：鉄および不可避不純物である。 In a preferred embodiment, the base steel sheet is the balance: iron and inevitable impurities.
上記課題を解決し得た本発明に係る溶融亜鉛めっき鋼板の製造方法は、素地鋼板を酸化
雰囲気下で熱処理し、素地鋼板表面に酸化層を形成する酸化工程と、前記酸化工程の後、還元雰囲気下で熱処理して前記素地鋼板表面の酸化層を還元する還元工程と、前記還元工程の後、溶融亜鉛めっき処理するめっき工程と、を含み、前記酸化工程において、酸素分圧を０．０００１０体積％以上０．１体積％以下に制御すると共に、素地鋼板が６５０℃以上７５０℃以下の温度範囲になる時間を２０秒以上７０秒以下に制御するところに要旨を有するものである。
The method for producing a hot-dip galvanized steel sheet according to the present invention that has solved the above-described problems includes an oxidation process in which a base steel sheet is heat-treated in an oxidizing atmosphere to form an oxide layer on the base steel sheet surface, and after the oxidation process, reduction A reduction step of reducing the oxide layer on the surface of the base steel sheet by heat treatment under an atmosphere, and a plating step of hot dip galvanization after the reduction step, wherein the oxygen partial pressure is 0.00010 in the oxidation step The gist is that the time when the base steel sheet is in the temperature range of 650 ° C. or more and 750 ° C. or less is controlled to 20 seconds or more and 70 seconds or less while being controlled to be not less than 0.1% by volume.
本発明によれば、めっき層と素地鋼板との界面に形成される酸化物含有層（主にＦｅ−Ｚｎ合金＋Ｓｉ−Ｍｎ−Ｏから構成される）の素地鋼板側に形成される凹凸形態が適切に制御されているため、素地鋼板とのめっき密着性に優れた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る。 According to this invention, the uneven | corrugated form formed in the base steel plate side of the oxide content layer (it is mainly comprised from Fe-Zn alloy + Si-Mn-O) formed in the interface of a plating layer and a base steel plate. Since it is appropriately controlled, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet excellent in plating adhesion with the base steel sheet can be obtained.
本発明者らは、めっき密着性に優れた溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板（以下、これらをまとめて「めっき鋼板」で代表させる場合がある。）を提供するため、検討を重ねてきた。その結果、めっき処理後のめっき鋼板において、めっき層と素地鋼板との界面に形成される酸化物含有層（主にＦｅ−Ｚｎ合金＋Ｓｉ−Ｍｎ−Ｏから構成される）の素地鋼板側に形成される凹凸形態［後記する図１（ｃ）の模式図を参照］が、最終製品であるめっき鋼板のめっき密着性に大きく寄与することを突き止めた。 In order to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet (hereinafter sometimes collectively referred to as “plated steel sheet”), the present inventors have made extensive studies. I came. As a result, in the plated steel sheet after the plating treatment, the oxide-containing layer (mainly composed of Fe—Zn alloy + Si—Mn—O) formed at the interface between the plating layer and the base steel sheet is formed on the base steel sheet side. It was found that the uneven form [see the schematic diagram of FIG. 1 (c) described later] greatly contributes to the plating adhesion of the plated steel sheet as the final product.
本明細書では、めっき鋼板を構成するめっき層と鋼板のうち、当該鋼板を特に「素地鋼板」と呼ぶ。上記「素地鋼板」は、めっき鋼板の製造過程では、熱延および冷延を行なった後であって、めっき前の鋼板に対応している。以下では、説明の便宜上、「素地鋼板」を、単に「鋼板」と略記する場合がある。 In the present specification, among the plating layers and steel plates constituting the plated steel plate, the steel plate is particularly referred to as a “base steel plate”. The “base steel plate” corresponds to the steel plate before plating after hot rolling and cold rolling in the manufacturing process of the plated steel plate. Hereinafter, for convenience of explanation, “base steel plate” may be simply abbreviated as “steel plate”.
以下、図１を参照しながら、上記の凹凸形態について詳しく説明する。 Hereinafter, the above-described uneven form will be described in detail with reference to FIG.
図１は、素地鋼板に対し、前述した「酸化−還元法」を施したときの、酸化処理後、還元処理後、めっき処理後の各段階における表面状態（めっき前の鋼板内部または鋼板表層の状態、及びめっき後の鋼板とめっき層界面近傍の状態）を模式的に示した図である。 FIG. 1 shows the surface state in each stage after the oxidation treatment, after the reduction treatment, and after the plating treatment when the above-described “oxidation-reduction method” is applied to the base steel plate (inside of the steel plate before plating or on the surface layer of the steel plate). It is the figure which showed typically a state and the state of the steel plate after plating, and the state of the plating layer interface vicinity.
まず、焼鈍炉での酸化処理により、図１（ａ）に示すように、鋼板の表面には、鋼板側から順に、内方酸化層（Ｆｅ−Ｓｉ−Ｍｎ−Ｏ）と外方酸化層（Ｆｅ−Ｏ）が形成されると共に、鋼板の内部に、粒界酸化物（Ｓｉ−Ｍｎ−Ｏ）が形成される。 First, as shown in FIG. 1 (a), an inner oxide layer (Fe—Si—Mn—O) and an outer oxide layer (Fe—Si—Mn—O) and an outer oxide layer ( Fe—O) is formed, and grain boundary oxide (Si—Mn—O) is formed inside the steel sheet.
次に、還元炉での還元処理により、図１（ｂ）に示すように、外方酸化層（Ｆｅ−Ｏ）は還元されて純鉄層（Ｆｅ）となるが、内方酸化層（Ｆｅ−Ｓｉ−Ｍｎ−Ｏ）や粒界酸化物（Ｓｉ−Ｍｎ−Ｏ）中の酸化物（Ｓｉ−Ｍｎ−ＯまたはＳｉ−Ｏ）は還元されず、そのまま残留する。 Next, as shown in FIG. 1B, the outer oxide layer (Fe—O) is reduced to a pure iron layer (Fe) by the reduction treatment in the reduction furnace, but the inner oxide layer (Fe -Si-Mn-O) and the oxide (Si-Mn-O or Si-O) in the grain boundary oxide (Si-Mn-O) remain without being reduced.
この状態でめっき処理を行なうと、図１（ｃ）に示すように、最表面にめっき層（Ｆｅ−Ｚｎ合金）が形成されると共に、ＳｉおよびＭｎが鋼板表面側に拡散し、Ｓｉ−Ｍｎ−Ｏの存在領域が広域化する。このとき、粒界酸化（結晶粒界に限定されず、結晶粒内も含む。）で優先的に合金化が起こり、鋼板とめっき層との界面の鋼板側に凹凸が生じる（図１（ｃ）を参照）。この凹凸が所謂くさび効果を奏し、この凹凸形態を適切に制御することにより、めっき層と鋼板の密着性が飛躍的に向上することが判明し、本発明を完成した。 When plating is performed in this state, as shown in FIG. 1 (c), a plating layer (Fe—Zn alloy) is formed on the outermost surface, and Si and Mn diffuse to the steel sheet surface side. -O's existence area becomes wider. At this time, alloying occurs preferentially by grain boundary oxidation (not limited to crystal grain boundaries, but also within crystal grains), and irregularities are formed on the steel sheet side at the interface between the steel sheet and the plating layer (FIG. 1 (c). )). This unevenness has a so-called wedge effect, and it has been found that the adhesion between the plating layer and the steel sheet is drastically improved by appropriately controlling the form of the unevenness, thus completing the present invention.
すなわち、本発明のめっき鋼板（溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板の両方を含む）は、素地鋼板と溶融亜鉛めっき層との界面に、Ｓｉ−Ｍｎ−Ｏおよび鉄亜鉛合金を含む酸化物含有層を有し、上記酸化物含有層の表面を素地鋼板側から観察したとき、後記する図３に示すように、網目状の凸部によって分割された複数の凹部を有しており、隣接する凹部と凹部の幅を凸部の幅としたとき、インターセプト法で求めた凹部の平均直径：３．０μｍ以上１０．０μｍ以下、且つ、凸部の平均幅：０．２０μｍ以上３．０μｍ上であるところに特徴がある。好ましくは、上記凹部の平均直径は４μｍ以上９μｍ以下であり、上記凸部の平均幅値は０．８μｍ以上２．２μｍ以下である。 That is, the plated steel sheet of the present invention (including both hot-dip galvanized steel sheet and galvannealed steel sheet) is an oxide containing Si—Mn—O and an iron-zinc alloy at the interface between the base steel sheet and the hot-dip galvanized layer. When the surface of the oxide-containing layer is observed from the base steel plate side, the product-containing layer has a plurality of concave portions divided by mesh-shaped convex portions as shown in FIG. When the width of the adjacent concave portion and the concave portion is the width of the convex portion, the average diameter of the concave portion determined by the intercept method: 3.0 μm or more and 10.0 μm or less, and the average width of the convex portion: 0.20 μm or more and 3.0 μm There is a feature in the above. Preferably, the average diameter of the concave portion is 4 μm or more and 9 μm or less, and the average width value of the convex portion is 0.8 μm or more and 2.2 μm or less.
参考のため、後記する表３のＮｏ．５（本発明のめっき鋼板）における、凹部の平均直径と凸部の平均幅を後記する手順に従って観察したＳＥＭ写真を図３に；従来のめっき鋼板における凹部の平均直径と凸部の平均幅を後記する手順に従って観察したＳＥＭ写真を図４に、それぞれ示す。これらの図を対比すると明らかなように、本発明のめっき鋼板は従来のめっき鋼板に比べて、凹部の平均直径が小さく、且つ、凸部の平均幅が小さくなっている。すなわち、本発明によれば、従来に比べて幅の狭い凸部が多く形成されているため、前述した楔効果によるめっき密着性向上作用が大きく発揮されると思料される。詳細は、後記する図５および図６を参照して説明する。 For reference, No. in Table 3 to be described later. 5 (the plated steel sheet of the present invention), the SEM photograph observed in accordance with the procedure described below for the average diameter of the concave portion and the average width of the convex portion is shown in FIG. The SEM photograph observed according to the procedure mentioned later is shown in FIG. As is clear from comparison of these figures, the plated steel sheet of the present invention has a smaller average diameter of the recesses and a smaller average width of the projections than the conventional plated steel sheet. That is, according to the present invention, it is considered that the plating adhesion improving effect by the wedge effect described above is greatly exhibited because many convex portions having a narrower width than the conventional one are formed. Details will be described with reference to FIGS. 5 and 6 to be described later.
本発明では、上記酸化物含有層の鋼板側の表面状態（凹凸状態）を観察するに当たり、独自の方法を採用した。この方法は、アンモニア水溶液を用いてめっき層のみを溶解除去した後、電解により鋼板を溶解し、残存する酸化物含有層を鋼板側から観察する方法であり、凹凸状態を精度良く評価するために本発明者らが考案したものである。上記の表面状態を観察する代表的な方法としては、めっき層と鋼板との界面（断面）を観察する方法があるが、断面の切り方によって凹凸状態が大きく変化し、ばらつきが大きくなるため、本発明では、以下の方法を採用した。 In the present invention, a unique method was adopted in observing the surface state (unevenness state) of the oxide-containing layer on the steel plate side. In this method, only the plating layer is dissolved and removed using an aqueous ammonia solution, then the steel plate is dissolved by electrolysis, and the remaining oxide-containing layer is observed from the steel plate side. The present inventors have devised. As a representative method of observing the surface state, there is a method of observing the interface (cross section) between the plating layer and the steel sheet, but the uneven state greatly changes depending on how to cut the cross section, and the variation becomes large. In the present invention, the following method is adopted.
以下、図２を参照しながら、上記方法を詳しく説明する。 Hereinafter, the above method will be described in detail with reference to FIG.
まず、図２（ａ）に示すように、めっき処理後のめっき鋼板を用意する。図２（ａ）は前述した図１（ｃ）と同じである。 First, as shown in FIG. 2A, a plated steel sheet after plating is prepared. FIG. 2A is the same as FIG. 1C described above.
次に、上記めっき鋼板をアンモニア水溶液に浸漬する。これにより、めっき層（Ｆｅ−Ｚｎ合金）のみが溶解除去される。本発明において「酸化物含有層」とは、めっき鋼板をアンモニア水溶液に浸漬後、溶解せずに残存した層を意味する。めっき鋼板をアンモニア水溶液に浸漬すると、めっき層を構成する亜鉛とアンモニアが錯体を形成してめっき層は溶解するが、鋼板や、鋼板とめっき層との間に形成される酸化物を含む層（Ｆｅ＋Ｓｉ−Ｍｎ−Ｏ）は溶解されない。 Next, the plated steel sheet is immersed in an aqueous ammonia solution. Thereby, only the plating layer (Fe—Zn alloy) is dissolved and removed. In the present invention, the “oxide-containing layer” means a layer that remains without being dissolved after the plated steel sheet is immersed in an aqueous ammonia solution. When the plated steel sheet is immersed in an aqueous ammonia solution, the zinc and ammonia constituting the plating layer form a complex and the plating layer dissolves, but the steel sheet or a layer containing an oxide formed between the steel sheet and the plating layer ( (Fe + Si—Mn—O) is not dissolved.
ここで、めっき層の溶解に用いられるアンモニア水溶液の濃度は特に限定されず、めっき層を溶解できる濃度に制御されていれば良い。具体的にはめっき層の組成や厚さなどによっても相違するが、例えば、約２５〜３０質量％のアンモニア水溶液を用いることが好ましい。 Here, the concentration of the aqueous ammonia solution used for dissolving the plating layer is not particularly limited as long as it is controlled to a concentration at which the plating layer can be dissolved. Specifically, although it varies depending on the composition and thickness of the plating layer, for example, it is preferable to use an aqueous ammonia solution of about 25 to 30% by mass.
次に、電解により鋼板を溶解する。電解条件は、以下のとおりである。
電解液：１０％アセチルアセトン−１％テトラメチルアンモニウムクロリド−メタノール溶液（１０％ＡＡ系電解液）
電解装置：北斗電工製のＨＡ１５１
電流密度＜２０ｍＡ／ｃｍ2
Next, the steel sheet is melted by electrolysis. The electrolysis conditions are as follows.
Electrolytic solution: 10% acetylacetone-1% tetramethylammonium chloride-methanol solution (10% AA-based electrolytic solution)
Electrolyzer: HA151 manufactured by Hokuto Denko
Current density <20 mA / cm 2
電解後に残存した酸化物含有層の凹凸状態を、図２（ｃ）に示すように鋼板側からＳＥＭ観察（倍率１０００倍）する。 The uneven state of the oxide-containing layer remaining after the electrolysis is observed with a SEM (magnification 1000 times) from the steel plate side as shown in FIG.
上記のようにして酸化物含有層の凹凸状態を鋼板側から観察したときのＳＥＭ観察写真を、図３（本発明例）および図４（比較例）に示す。 SEM observation photographs when the uneven state of the oxide-containing layer is observed from the steel sheet side as described above are shown in FIG. 3 (invention example) and FIG. 4 (comparative example).
これらの図に示すように、粒界酸化物（Ｓｉ−Ｍｎ−Ｏ）に相当する部分と、そうでない部分との間で凹凸の高低差が見られた。詳細には上記の酸化物含有層は、粒界酸化物に対応する、網目状の凸部によって分割された複数の孔を有しており、本明細書では、上記複数の孔を「凹部」と呼び、隣接する凹部と凹部の幅を「凸部の幅」と呼ぶ。 As shown in these figures, a difference in level of unevenness was observed between the portion corresponding to the grain boundary oxide (Si—Mn—O) and the portion not. Specifically, the oxide-containing layer has a plurality of holes divided by network-like convex portions corresponding to the grain boundary oxide, and in the present specification, the plurality of holes are referred to as “concave portions”. The widths of the adjacent concave portions and the concave portions are referred to as “the width of the convex portion”.
本発明では、上記「凹部」の平均直径と上記「凸部の平均幅」を、めっき密着性の指標として用いている。凹部の平均直径は、インターセプト法で算出した。これらの測定方法の詳細は、以下のとおりである。 In the present invention, the average diameter of the “concave portion” and the “average width of the convex portion” are used as indicators of plating adhesion. The average diameter of the recess was calculated by the intercept method. Details of these measuring methods are as follows.
まず、凹部の平均直径の測定方法を説明する。上記のようにして得られた電解後の酸化物含有層を、鋼板側からＳＥＭ（走査型電子顕微鏡）で観察し、ＳＥＭ写真（１００ｍｍ×１２０ｍｍ）を得る。倍率は１０００倍とし、合計３視野（１視野は１００μｍ×１２０μｍ）を観察する。凹凸形状を観察するという観点からすると、ＳＥＭによる２次電子像を用いることが好ましく、後記する実施例では、２次電子像を用いた。ＳＥＭ写真に観察される複数の孔（凹部）のそれぞれについて、各凹部を横切るように、左右の水平線及び上下の垂直線を複数本引く。後記する実施例では、左右に２本、上下に３本、合計５本の線を引いた。各写真上に引かれた合計５本の線分（凹部を横切る線と線の長さ）の和を、各線分が通る凹部の数で除した長さを算出し、その平均値を、凹部の平均直径とした。 First, a method for measuring the average diameter of the recesses will be described. The oxide-containing layer after electrolysis obtained as described above is observed with a SEM (scanning electron microscope) from the steel plate side, and an SEM photograph (100 mm × 120 mm) is obtained. The magnification is 1000 times, and a total of three visual fields (one visual field is 100 μm × 120 μm) are observed. From the viewpoint of observing the concavo-convex shape, it is preferable to use a secondary electron image by SEM, and in the examples described later, a secondary electron image was used. For each of a plurality of holes (recesses) observed in the SEM photograph, a plurality of horizontal lines on the left and right and a plurality of vertical lines on the upper and lower sides are drawn so as to cross each recess. In the examples described later, a total of five lines were drawn, two on the left and right and three on the top and bottom. Calculate the length by dividing the sum of the total of five line segments drawn on each photo (the line and the length of the line crossing the recess) by the number of recesses through which each line segment passes, and calculate the average value as the recess The average diameter.
凸部の平均幅の測定方法は、以下のとおりである。凹部の平均直径を測定する場合と同様の方法で電解後の酸化物含有層を鋼板側からＳＥＭで観察し、ＳＥＭ写真（１００ｍｍ×１２０ｍｍ）を得る。ただし、凸倍の幅を精度良く測定するため、倍率は２０００倍と高めた。また、凸部の幅については、視野間のバラツキが少ないため、１視野（１００μｍ×１２０μｍ）のみ観察を行なった。各ＳＥＭ写真において、隣接する２つの凸部を任意にとり、凸部と凸部の間隔の最小値を測定する。合計５個の間隔を測定し、その平均を「凸部の平均幅」とした。 The method for measuring the average width of the convex portions is as follows. The oxide-containing layer after electrolysis is observed with a SEM from the steel sheet side in the same manner as in the case of measuring the average diameter of the recesses, and an SEM photograph (100 mm × 120 mm) is obtained. However, in order to accurately measure the width of the convex double, the magnification was increased to 2000 times. Further, regarding the width of the convex portion, since there was little variation between the visual fields, only one visual field (100 μm × 120 μm) was observed. In each SEM photograph, two adjacent convex portions are arbitrarily taken, and the minimum value of the interval between the convex portions is measured. A total of five intervals were measured, and the average was defined as “average width of convex portions”.
このようにして測定される「凹部の平均直径」及び「凸部の平均幅」と、めっき密着性との関係を調べたところ、以下の知見が得られた。めっき密着性は、後記する実施例に記載のめっき密着性試験（テープ剥離試験）で評価したものであり、テープ剥離幅によりめっき密着性を評価している。後記する実施例では、テープ剥離幅が５．０ｍｍ以下のものを合格（めっき密着性に優れる）と評価している。 When the relationship between the “average diameter of the concave portion” and “average width of the convex portion” thus measured and the plating adhesion was examined, the following knowledge was obtained. The plating adhesion is evaluated by a plating adhesion test (tape peeling test) described in Examples described later, and the plating adhesion is evaluated by the tape peeling width. In the examples described later, a tape peeling width of 5.0 mm or less is evaluated as acceptable (excellent plating adhesion).
図５は、凹部の平均直径と、テープ剥離幅との関係を示すグラフである。ここでは、凸部の平均幅を１〜２μｍ（一定）とした。 FIG. 5 is a graph showing the relationship between the average diameter of the recesses and the tape peeling width. Here, the average width of the protrusions was set to 1 to 2 μm (constant).
図５に示すように、凹部の平均直径が３．０μｍ付近と１０．０μｍ付近で変曲点が見られた。よって、凹部の平均直径を３．０〜１０．０μｍに制御すると、めっき密着性が飛躍的に改善されることが分かった。凹部の平均直径が１０．０μｍを超えると、めっき層と鋼板との界面の凹凸が減少し、くさび作用による密着性向上作用が有効に発揮されないと考えられる。また、凹部の平均直径が３．０μｍを下回ると、逆に上記界面が平坦となり、密着性改善効果が得られないと考えられる。凹部の好ましい平均直径は、４μｍ以上９μｍ以下である。 As shown in FIG. 5, inflection points were observed when the average diameter of the recesses was around 3.0 μm and 10.0 μm. Therefore, it was found that when the average diameter of the recesses is controlled to 3.0 to 10.0 μm, the plating adhesion is drastically improved. If the average diameter of the recesses exceeds 10.0 μm, the unevenness at the interface between the plating layer and the steel sheet is reduced, and it is considered that the adhesion improving effect due to the wedge effect is not effectively exhibited. On the other hand, when the average diameter of the recesses is less than 3.0 μm, the interface is conversely flattened, and it is considered that the effect of improving the adhesion cannot be obtained. A preferable average diameter of the recesses is 4 μm or more and 9 μm or less.
次に、図６を参照する。図６は、凸部の平均幅と、テープ剥離幅との関係を示すグラフである。ここでは、凹部の平均直径を５〜８μｍ（一定）とした。 Reference is now made to FIG. FIG. 6 is a graph showing the relationship between the average width of the protrusions and the tape peeling width. Here, the average diameter of the recesses was set to 5 to 8 μm (constant).
図６に示すように、凸部の平均幅を０．２０〜３．０μｍの範囲に制御すると、めっき密着性が向上することが分かる。凸部の平均幅が０．２０μｍを下回ると、楔効果が弱くなって剥離が生じ易くなり、一方、３．０μｍを超えると界面が平坦になるため、密着性は低下する。凸部の平均幅の好ましい平均は、０．８μｍ以上２．２μｍ以下である。 As shown in FIG. 6, it can be seen that the plating adhesion is improved by controlling the average width of the protrusions in the range of 0.20 to 3.0 μm. When the average width of the protrusions is less than 0.20 μm, the wedge effect is weakened and peeling easily occurs. On the other hand, when the average width is more than 3.0 μm, the interface becomes flat, and the adhesion is lowered. A preferable average of the average width of the convex portions is 0.8 μm or more and 2.2 μm or less.
以上、本発明を最も特徴付ける酸化物含有層の凹凸状態について説明した。 In the above, the uneven | corrugated state of the oxide containing layer which characterizes this invention most was demonstrated.
次に、本発明に用いられる素地鋼板の組成について説明する。 Next, the composition of the base steel sheet used in the present invention will be described.
上記素地鋼板は、Ｃ：０．０４％以上０．２％以下、Ｓｉ：０．１％以上３％以下、Ｍｎ：０．１％以上３％以下、Ａｌ：０．０６％以下（０は含まない）を含有している。 The base steel plate has C: 0.04% to 0.2%, Si: 0.1% to 3%, Mn: 0.1% to 3%, Al: 0.06% (0 is Does not contain).
Ｃ：０．０４〜０．２％
Ｃは、鋼板の強度向上に必要な元素であり、そのため、Ｃ量を０．０４％以上とする。好ましいＣ量は０．０５％以上であり、より好ましくは０．１０％以上である。しかし、Ｃを過剰に添加すると冷間加工性が低下するため、Ｃ量の上限を０．２％とする。Ｃ量の好ましい上限は０．１５％であり、より好ましくは０．１３％である。
C: 0.04 to 0.2%
C is an element necessary for improving the strength of the steel sheet. Therefore, the C content is 0.04% or more. A preferable amount of C is 0.05% or more, and more preferably 0.10% or more. However, if C is added excessively, the cold workability deteriorates, so the upper limit of C content is 0.2%. The upper limit with the preferable amount of C is 0.15%, More preferably, it is 0.13%.
Ｓｉ：０．１〜３％
Ｓｉは、延性や加工性を劣化させることなく強度を高めるのに有用な元素であり、このような作用を有効に発揮させるため、Ｓｉ量を０．１％以上とする。Ｓｉは易酸化性元素であるため、従来では、Ｓｉを０．１％以上含有させると合金化溶融亜鉛めっき層の外観性状とめっき密着性が劣化するという問題があった。これに対し、本発明では、焼鈍炉内の雰囲気および加熱条件を適切に制御して酸化層を形成しているため、素地鋼板と合金化溶融亜鉛めっき層の間に形成される酸化物含有層にＳｉを濃化させることができ、素地鋼板にＳｉを０．１％以上含有させても良好な外観性状とめっき密着性を確保できる。好ましいＳｉ量は、０．３％以上であり、より好ましくは０．５％以上、更に好ましくは１．０％以上である。しかしＳｉ量が３％を超えると延性が劣化するため、上限を３％とする。Ｓｉ量の好ましい上限は２．５％であり、より好ましくは２％である。
Si: 0.1 to 3%
Si is an element useful for increasing the strength without deteriorating ductility and workability, and the Si amount is set to 0.1% or more in order to effectively exhibit such action. Since Si is an easily oxidizable element, conventionally, when Si is contained in an amount of 0.1% or more, there is a problem that the appearance property and plating adhesion of the alloyed hot-dip galvanized layer deteriorate. On the other hand, in the present invention, since the oxide layer is formed by appropriately controlling the atmosphere and heating conditions in the annealing furnace, the oxide-containing layer formed between the base steel plate and the galvannealed layer Si can be concentrated, and good appearance properties and plating adhesion can be ensured even if the base steel sheet contains 0.1% or more of Si. A preferable Si amount is 0.3% or more, more preferably 0.5% or more, and further preferably 1.0% or more. However, if the Si content exceeds 3%, ductility deteriorates, so the upper limit is made 3%. The upper limit with preferable Si amount is 2.5%, More preferably, it is 2%.
Ｍｎ：０．１〜３％
Ｍｎは、強度と靭性を確保するために必要な元素であり、こうした作用を有効に発揮させるため、Ｍｎ量を０．１％以上とする。本発明によれば、後述するように焼鈍炉での酸化加熱条件を適切に制御しているため、Ｍｎを０．１％以上添加してもめっき密着性の低下などを回避することができる。好ましいＭｎ量は０．３％以上である。しかし、Ｍｎの
過剰添加は延性低下を招くため、上限を３％とする。Ｍｎ量の好ましい上限は２．８％であり、より好ましくは２．５％以下である。
Mn: 0.1 to 3%
Mn is an element necessary for ensuring strength and toughness, and the Mn content is set to 0.1% or more in order to effectively exert such effects. According to the present invention, as will be described later, the oxidation heating conditions in the annealing furnace are appropriately controlled. Therefore, even if 0.1% or more of Mn is added, it is possible to avoid a decrease in plating adhesion. A preferable amount of Mn is 0.3% or more. However, excessive addition of Mn causes a drop in ductility, so the upper limit is made 3%. The upper limit with the preferable amount of Mn is 2.8%, More preferably, it is 2.5% or less.
Ａｌ：０．０６％以下（０％は含まない）
Ａｌは、脱酸剤として作用するほか、焼鈍時におけるオーステナイト結晶粒粗大化の防止に有用な元素である。しかし、Ａｌを過剰に添加しても上記作用は飽和するほか、結晶粒が不安定になって材質にムラが生じ易くなるため、Ａｌ量の上限を０．０６％とする。Ａｌ量の好ましい上限は０．０５％であり、より好ましくは０．０４％である。
Al: 0.06% or less (excluding 0%)
In addition to acting as a deoxidizer, Al is an element useful for preventing austenite grain coarsening during annealing. However, even if Al is added excessively, the above action is saturated, and the crystal grains become unstable and unevenness of the material tends to occur. Therefore, the upper limit of the Al amount is set to 0.06%. The upper limit with preferable Al amount is 0.05%, More preferably, it is 0.04%.
本発明に用いられる素地鋼板は、上記元素を基本元素として含有し、残部は鉄および不可避不純物である。不可避不純物のうち、例えばＰは０．０２％以下（０％は含まない）、Ｓは０．００４％以下（０％は含まない）、Ｎは０．０１％以下（０％は含まない）とすることが好ましい。 The base steel sheet used in the present invention contains the above elements as basic elements, and the balance is iron and inevitable impurities. Among inevitable impurities, for example, P is 0.02% or less (0% is not included), S is 0.004% or less (0% is not included), and N is 0.01% or less (0% is not included). It is preferable that
Ｐ：０．０２％以下（０％は含まない）
Ｐは、セメンタイトの析出を遅延して変態を抑制する作用があるが、過剰添加は、延性やめっき密着性の低下を招くため、０．０２％以下に制御することが好ましい。より好ましいＰ量は０．０１％以下であり、更に好ましくは０．００５％以下である。
P: 0.02% or less (excluding 0%)
P has an action of suppressing the transformation by delaying the precipitation of cementite. However, excessive addition causes a decrease in ductility and plating adhesion, so it is preferably controlled to 0.02% or less. A more preferable amount of P is 0.01% or less, and further preferably 0.005% or less.
Ｓ：０．００４％以下（０％は含まない）
Ｓは、ＭｎＳなどの硫化物系介在物を形成し、熱間圧延時に偏析して鋼板の脆化を招くため、０．００４％以下に制御することが好ましい。より好ましいＳ量は０．００３％以下である。
S: 0.004% or less (excluding 0%)
S forms sulfide inclusions such as MnS and segregates during hot rolling to cause embrittlement of the steel sheet. Therefore, S is preferably controlled to 0.004% or less. A more preferable amount of S is 0.003% or less.
Ｎ：０．０１％以下（０％は含まない）
Ｎは、粗大な窒化物を形成し、曲げ性や穴拡げ性を劣化させ、また溶接時のブローホールの原因となるため、０．０１％以下に制御することが好ましい。より好ましいＮ量は０．００５％以下である。
N: 0.01% or less (excluding 0%)
N forms coarse nitrides, degrades bendability and hole expandability, and causes blowholes during welding. Therefore, N is preferably controlled to 0.01% or less. A more preferable N amount is 0.005% or less.
本発明に用いられる素地鋼板は、更に他の選択元素を含有しても良く、例えば、以下のようにＣｒやＴｉ、Ｎｉなどを含有することができる。 The base steel sheet used in the present invention may further contain other selective elements, and can contain, for example, Cr, Ti, Ni and the like as follows.
Ｃｒ：０．３％以下（０％を含まない）
Ｃｒは、鋼板の強度向上に有効な元素である。このような作用を有効に発揮させるため、Ｃｒ量を好ましくは０．０１％以上、より好ましくは０．０４％以上、更に好ましくは０．０８％以上とする。しかし、Ｃｒの過剰添加は延性の低下を招くため、Ｃｒ量の上限は０．３％とすることが好ましい。Ｃｒ量のより好ましい上限は０．２５％であり、更に好ましくは０．２％である。
Cr: 0.3% or less (excluding 0%)
Cr is an element effective for improving the strength of the steel sheet. In order to effectively exhibit such an action, the Cr amount is preferably 0.01% or more, more preferably 0.04% or more, and further preferably 0.08% or more. However, since excessive addition of Cr causes a drop in ductility, the upper limit of Cr content is preferably 0.3%. The upper limit with more preferable Cr amount is 0.25%, More preferably, it is 0.2%.
Ｔｉ：０．０５％以下（０％を含まない）
Ｔｉは、脱酸剤として有用な元素である。このような作用を有効に発揮させるため、Ｔｉ量を好ましくは０．０１％以上、より好ましくは０．０２％以上とする。しかし、Ｔｉの過剰添加は靱性の低下を招くため、Ｔｉ量の上限は０．０５％とすることが好ましい。Ｔｉ量のより好ましい上限は０．０４％である。
Ti: 0.05% or less (excluding 0%)
Ti is an element useful as a deoxidizer. In order to effectively exhibit such an action, the Ti content is preferably 0.01% or more, more preferably 0.02% or more. However, excessive addition of Ti causes a decrease in toughness, so the upper limit of Ti content is preferably 0.05%. A more preferable upper limit of the Ti amount is 0.04%.
Ｎｉ、Ｃｕ、Ｍｏ、およびＢは焼き入れ性向上に有用な元素であり、これらの元素を単独又は併用することができる。具体的には以下の通りである。 Ni, Cu, Mo, and B are elements useful for improving the hardenability, and these elements can be used alone or in combination. Specifically, it is as follows.
Ｎｉ：２％以下（０％を含まない）
Ｎｉは、焼入れ性向上に有用な元素である。Ｎｉを適量を添加するとＣＡＬ焼鈍、冷却時にマルテンサイト比率が増大し、またマルテンサイトのラス構造が微細化され、次工程のＣＧＬ焼鈍時における２相域再加熱・冷却処理時の焼入れ性が良好となる。また冷却後の最終的な複合組織が良好なものとなるため、各種成形加工性を向上させることができる。このような作用を有効に発揮させるため、Ｎｉ量を好ましくは０．１％以上、より好ましくは０．２％以上とする。しかし、Ｎｉは高価な元素であり、製造コストの上昇を招くため、Ｎｉ量の上限は２％とすることが好ましい。Ｎｉ量のより好ましい上限は１．５％、更に好ましくは１．０％である。
Ni: 2% or less (excluding 0%)
Ni is an element useful for improving hardenability. When an appropriate amount of Ni is added, the martensite ratio increases during CAL annealing and cooling, the lath structure of martensite is refined, and the hardenability during the two-phase reheating / cooling process during CGL annealing in the next process is good. It becomes. Moreover, since the final composite structure after cooling becomes favorable, various molding processability can be improved. In order to effectively exhibit such an action, the Ni content is preferably 0.1% or more, more preferably 0.2% or more. However, since Ni is an expensive element and causes an increase in manufacturing cost, the upper limit of the Ni amount is preferably 2%. A more preferable upper limit of the amount of Ni is 1.5%, more preferably 1.0%.
Ｃｕ：２％以下（０％を含まない）
Ｃｕは、Ｎｉと同様、焼入れ性向上に有用な元素である。ＣｕもＮｉと同様の作用により各種成形加工性を向上させることができる。このような作用を有効に発揮させるため、Ｃｕ量を好ましくは０．１％以上、より好ましくは０．２％以上とする。しかし、Ｃｕは高価な元素であり、製造コストの上昇を招くため、Ｃｕ量の上限は２％とすることが好ましい。Ｃｕ量のより好ましい上限は１．５％、更に好ましくは１．０％である。
Cu: 2% or less (excluding 0%)
Cu, like Ni, is an element useful for improving hardenability. Cu can improve various processability by the same action as Ni. In order to effectively exhibit such an action, the amount of Cu is preferably 0.1% or more, more preferably 0.2% or more. However, since Cu is an expensive element and causes an increase in manufacturing cost, the upper limit of the amount of Cu is preferably 2%. A more preferable upper limit of the amount of Cu is 1.5%, more preferably 1.0%.
Ｍｏ：２％以下（０％を含まない）
Ｍｏは、めっき性を損ねることなく、固溶強化を図る上で重要な元素である。またＮｉやＣｕと同様、焼入れ性向上に有用な元素である。ＭｏもＣｕやＮｉと同様の作用により各種成形加工性を向上させることができる。このような作用を有効に発揮させるため、Ｍｏ量を好ましくは０．１％以上、より好ましくは０．２％以上とする。しかし、Ｍｏは高価な元素であり、製造コストの上昇を招くため、Ｍｏ量の上限は２％とすることが好ましい。Ｍｏ量のより好ましい上限は１．５％、更に好ましくは１．０％である。
Mo: 2% or less (excluding 0%)
Mo is an important element in strengthening the solid solution without impairing the plating property. Moreover, like Ni and Cu, it is an element useful for improving hardenability. Mo can improve various moldability by the same action as Cu and Ni. In order to effectively exhibit such an action, the Mo amount is preferably 0.1% or more, more preferably 0.2% or more. However, since Mo is an expensive element and causes an increase in manufacturing cost, the upper limit of the amount of Mo is preferably 2%. A more preferable upper limit of the Mo amount is 1.5%, and more preferably 1.0%.
Ｂ：０．０１％以下（０％を含まない）
Ｂは、焼入れ性向上に有用な元素である。このような作用を有効に発揮させるため、Ｂ量を好ましくは０．０００１％以上、より好ましくは０．０００２％以上とする。しかし、Ｂを過剰に添加すると、めっき性が低下するため、Ｂ量の上限は０．０１％とすることが好ましい。Ｂ量のより好ましい上限は０．００５％、更に好ましくは０．００１％である。
B: 0.01% or less (excluding 0%)
B is an element useful for improving hardenability. In order to effectively exhibit such an action, the B content is preferably 0.0001% or more, more preferably 0.0002% or more. However, if B is added excessively, the plating property is lowered, so the upper limit of the amount of B is preferably 0.01%. A more preferable upper limit of the amount of B is 0.005%, more preferably 0.001%.
Ｎｂ、Ｖ、およびＷは強度向上に有用な元素であり、これらの元素を単独又は併用することができる。具体的には以下の通りである。 Nb, V, and W are elements useful for improving the strength, and these elements can be used alone or in combination. Specifically, it is as follows.
Ｎｂ：１％以下（０％を含まない）
Ｎｂは、微量の添加で微細組織を得ることができ、靱性を劣化させることなく強度を高めるのに有用な元素である。このような作用を有効に発揮させるため、Ｎｂ量を好ましくは０．００１％以上、より好ましくは０．００５％以上とする。しかし、Ｎｂを過剰に添加すると炭化物が過剰に生成し、マルテンサイトの体積率減少やその析出強化によって強度と加工性のバランスを失わせるため、Ｎｂ量の上限は１％とすることが好ましい。Ｎｂ量のより好ましい上限は０．５％、更に好ましくは０．１％である。
Nb: 1% or less (excluding 0%)
Nb is an element useful for increasing the strength without degrading the toughness because a fine structure can be obtained with a small amount of addition. In order to effectively exhibit such an action, the Nb amount is preferably 0.001% or more, more preferably 0.005% or more. However, when Nb is added excessively, carbides are generated excessively, and the balance between strength and workability is lost due to a decrease in the martensite volume fraction and its precipitation strengthening. Therefore, the upper limit of the Nb content is preferably 1%. A more preferable upper limit of the amount of Nb is 0.5%, more preferably 0.1%.
Ｖ：１％以下（０％を含まない）
Ｖは、Ｎｂと同様、強度を高めるのに有用な元素である。このような作用を有効に発揮させるため、Ｖ量を好ましくは０．００１％以上、より好ましくは０．００５％以上とする。しかし、Ｖを過剰に添加すると、製造コストの上昇を招くだけでなく、降伏点（降伏比）が上昇して加工性が低下するため、Ｖ量の上限は１％とすることが好ましい。Ｖ量のより好ましい上限は０．５％、更に好ましくは０．１％である。
V: 1% or less (excluding 0%)
V, like Nb, is an element useful for increasing the strength. In order to effectively exhibit such an action, the V amount is preferably 0.001% or more, more preferably 0.005% or more. However, excessive addition of V not only increases the manufacturing cost but also increases the yield point (yield ratio) and decreases the workability, so the upper limit of the V amount is preferably 1%. The upper limit with more preferable V amount is 0.5%, More preferably, it is 0.1%.
Ｗ：０．３％以下（０％を含まない）
Ｗは、析出物強化や、フェライト結晶粒の成長を抑制して細粒強化、および再結晶の抑制による転移強化によって、強度を高めるのに有用な元素である。このような作用を有効に発揮させるため、Ｗ量を好ましくは０．００１％以上、より好ましくは０．００５％以上とする。しかし、Ｗを過剰に添加すると、炭窒化物の析出が過剰となって成形性が低下するため、Ｗ量の上限は０．３％とすることが好ましい。Ｗ量のより好ましい上限は０．２％、更に好ましくは０．１％である。
W: 0.3% or less (excluding 0%)
W is an element useful for increasing the strength by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and transition strengthening by suppressing recrystallization. In order to effectively exhibit such an action, the W amount is preferably 0.001% or more, more preferably 0.005% or more. However, if W is added excessively, precipitation of carbonitrides becomes excessive and moldability is lowered, so the upper limit of W content is preferably 0.3%. A more preferable upper limit of the amount of W is 0.2%, more preferably 0.1%.
Ｃａ、Ｍｇ、およびＲＥＭよりなる群から選ばれる少なくとも１種の元素：０．０３％以下（０％を含まない）
Ｃａ、Ｍｇ、ＲＥＭは、脱酸剤として作用する元素である。このような作用を有効に発揮させるため、Ｃａ、Ｍｇ、ＲＥＭよりなる群から選ばれる１種以上の元素を合計量で好ましくは０．００２％以上、より好ましくは０．００３％以上とする。しかし、これら元素を過剰に添加すると、成形性が低下するため、Ｃａ、Ｍｇ、およびＲＥＭよりなる群から選ばれる１種以上の元素の合計量の上限は０．０３％とすることが好ましく、より好ましくは０．０２％、更に好ましくは０．０１％である。
At least one element selected from the group consisting of Ca, Mg, and REM: 0.03% or less (excluding 0%)
Ca, Mg, and REM are elements that act as a deoxidizer. In order to effectively exhibit such an action, the total amount of one or more elements selected from the group consisting of Ca, Mg, and REM is preferably 0.002% or more, more preferably 0.003% or more. However, if these elements are added excessively, the moldability decreases, so the upper limit of the total amount of one or more elements selected from the group consisting of Ca, Mg, and REM is preferably 0.03%, More preferably, it is 0.02%, More preferably, it is 0.01%.
なお、本発明において、ＲＥＭとは、ランタノイド元素（ＬａからＬｕまでの１５元素）およびＳｃ（スカンジウム）とＹ（イットリウム）を含む意味である。これらの元素のなかでも、Ｌａ、ＣｅおよびＹよりなる群から選ばれる少なくとも１種の元素を含有することが好ましく、より好ましくはＬａおよび／またはＣｅを含有するのがよい。 In the present invention, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.
以上、本発明のめっき鋼板（溶融亜鉛めっき鋼板または合金化溶融亜鉛めっき鋼板）について説明した。 The plated steel sheet (hot dip galvanized steel sheet or galvannealed steel sheet) of the present invention has been described above.
次に、本発明のめっき鋼板を製造する方法について説明する。前述したように、本発明のめっき鋼板は、めっき層と素地鋼板との界面に形成される酸化物含有層（主にＦｅ−Ｚｎ合金＋Ｓｉ−Ｍｎ−Ｏから構成される）のうち素地鋼板側に形成される凹凸形態が適切に制御されている点に最大の特徴があり、具体的には、上記凹部の平均直径が３．０μｍ以上１０．０μｍ以下であり、且つ、凸部の平均幅が０．２０μｍ以上３．０μｍ以下を満足していることが必要である。このようなめっき鋼板を製造するには、めっき前の素地鋼板に対し、焼鈍炉での酸化加熱処理（酸化工程）、次いで還元炉での還元処理（還元工程）を行なう「酸化−還元」法を適用するに当たり、特に上記の酸化工程において、酸素分圧を０．０００１０体積％以上０．１体積％以下（１.０ｐｐｍ以上１０００ｐｐｍ以下）に制御すると共に、素地鋼板が６５０℃以上７５０℃以下の温度範囲になる時間を２０秒以上７０秒以下に制御することが必要である。 Next, a method for producing the plated steel sheet of the present invention will be described. As described above, the plated steel sheet of the present invention is the base steel sheet side of the oxide-containing layer (mainly composed of Fe—Zn alloy + Si—Mn—O) formed at the interface between the plating layer and the base steel sheet. The feature is that the concavo-convex shape formed on the surface is appropriately controlled, specifically, the average diameter of the concave portion is 3.0 μm or more and 10.0 μm or less, and the average width of the convex portion Needs to satisfy 0.20 μm or more and 3.0 μm or less. In order to manufacture such a plated steel sheet, an “oxidation-reduction” method is performed in which the base steel sheet before plating is subjected to an oxidation heat treatment (oxidation process) in an annealing furnace and then a reduction treatment (reduction process) in a reduction furnace. In particular, in the above oxidation step, the oxygen partial pressure is controlled to 0.00010 volume% or more and 0.1 volume% or less (1.0 ppm or more and 1000 ppm or less), and the base steel sheet is 650 ° C. or more and 750 ° C. or less. It is necessary to control the time within the temperature range of 20 seconds to 70 seconds.
すなわち、本発明に係るめっき鋼板の製造方法は、素地鋼板を酸化雰囲気下で熱処理し、素地鋼板表面に酸化層を形成する酸化工程と、前記酸化工程の後、還元雰囲気下で熱処理して前記素地鋼板表面の酸化層を還元する還元工程と、前記還元工程の後、溶融亜鉛めっき処理するめっき工程と、を含み、前記酸化工程において、酸素分圧を１．０ｐｐｍ以上１０００ｐｐｍ以下に制御すると共に、素地鋼板が６５０℃以上７５０℃以下の温度範囲になる時間（６５０〜７５０℃の素地鋼板の保持時間）を２０秒以上７０秒以下に制御するところに特徴がある。 That is, in the method for producing a plated steel sheet according to the present invention, the base steel sheet is heat-treated in an oxidizing atmosphere, and an oxidation process for forming an oxide layer on the base steel sheet surface is performed. A reduction step of reducing the oxide layer on the surface of the base steel sheet, and a plating step of galvanizing after the reduction step, and in the oxidation step, the oxygen partial pressure is controlled to 1.0 ppm or more and 1000 ppm or less The base steel sheet is characterized in that the time during which the base steel sheet is in the temperature range of 650 ° C. or higher and 750 ° C. or lower (the holding time of the base steel sheet of 650 to 750 ° C.) is controlled to 20 seconds or longer and 70 seconds or shorter.
まず、本発明の方法を最も特徴付ける酸化工程を説明する。 First, the oxidation process that best characterizes the method of the present invention will be described.
（焼鈍炉での酸化工程）
上記酸化工程では、まず、酸素分圧を０．０００１０体積％以上０．１体積％以下に制御する。本発明者らの基礎実験によれば、凹部の平均直径は、焼鈍炉での酸素分圧と密接に関連しており、酸素分圧を上記範囲に制御すれば、凹部の平均直径を、めっき密着性に有用な「３．０μｍ以上１０．０μｍ以下」の範囲内に制御することができる。
(Oxidation process in annealing furnace)
In the oxidation step, first, the oxygen partial pressure is controlled to 0.00010 volume% or more and 0.1 volume% or less. According to the inventors' basic experiment, the average diameter of the recess is closely related to the oxygen partial pressure in the annealing furnace, and if the oxygen partial pressure is controlled within the above range, the average diameter of the recess is plated. It can be controlled within the range of “3.0 μm to 10.0 μm” useful for adhesion.
この点について、もう少し詳しく説明すると、本発明者らの基礎実験によれば、粒界酸化は焼鈍雰囲気中の酸素分圧が少なくなる程発生し易くなり、めっき後の凹部の平均直径が増加する傾向にあることが分かった。また、本発明者らの基礎実験によれば、粒界酸化の発生頻度（凹部の微細化に繋がる）と鋼板表面の結晶粒径との間に、高い相関関係はなく、表面の結晶粒径が微細な鋼板について、当該鋼板の粒界酸化も微細に発生するとは必ずしも言えないことが分かった。すなわち、酸化のし易さの指標である酸素分圧が、粒界酸化の発生頻度（微細の程度）に大きく影響していることが判明した。 This point will be explained in more detail. According to the basic experiments of the present inventors, grain boundary oxidation is more likely to occur as the oxygen partial pressure in the annealing atmosphere decreases, and the average diameter of the recesses after plating increases. It turned out that there was a tendency. Further, according to the basic experiments of the present inventors, there is no high correlation between the occurrence frequency of grain boundary oxidation (leading to the miniaturization of the recesses) and the crystal grain size of the steel sheet surface, and the crystal grain size of the surface However, it has been found that the grain boundary oxidation of the steel sheet is not necessarily generated finely. That is, it has been found that the oxygen partial pressure, which is an index of the ease of oxidation, greatly affects the frequency of occurrence of grain boundary oxidation (the degree of fineness).
そこで、酸素分圧を種々変化させた雰囲気下で焼鈍を行なったときの、酸素分圧と、めっき鋼板における凹部の平均直径との関係を詳細に調べたところ、焼鈍炉の酸素分圧が０．０００１０体積％（１ｐｐｍ）未満では、凹部の平均直径が３．０μｍを下回ることが分かった。これは、酸素分圧が小さいと、Ｓｉが粒界に偏析し易くなって粒界酸化の発生頻度が高くなり、後に形成される網目状の凸部が微細化されるためである。一方、焼鈍炉の酸素分圧が０．１体積％（１０００ｐｐｍ）を超えると粒界酸化の発生頻度が非常に少なくなり、凹部の平均直径が１０．０μｍを超えることが分かった。焼鈍炉の好ましい酸素分圧は、０．００５体積％以上０．０９体積％以下（５０ｐｐｍ以上９００ｐｐｍ以下）である。 Thus, when the relationship between the oxygen partial pressure and the average diameter of the recesses in the plated steel sheet was investigated in detail when annealing was performed in an atmosphere with various oxygen partial pressures, the oxygen partial pressure in the annealing furnace was 0. It was found that the average diameter of the recesses is less than 3.0 μm at less than 0.0010 volume% (1 ppm). This is because, when the oxygen partial pressure is small, Si is easily segregated at the grain boundaries, the frequency of occurrence of grain boundary oxidation is increased, and the network-shaped convex portions to be formed later are miniaturized. On the other hand, it was found that when the oxygen partial pressure in the annealing furnace exceeds 0.1 volume% (1000 ppm), the frequency of occurrence of grain boundary oxidation becomes very small, and the average diameter of the recesses exceeds 10.0 μm. The preferable oxygen partial pressure of the annealing furnace is 0.005 vol% or more and 0.09 vol% or less (50 ppm or more and 900 ppm or less).
更に上記酸化工程では、素地鋼板が６５０℃以上７５０℃以下の温度範囲になる時間を２０秒以上７０秒以下に制御する。本発明者らの基礎実験によれば、凸部の平均幅は、焼鈍炉における特定温度領域（６５０〜７５０℃）の保持時間（在炉時間）と密接に関連しており、この保持時間を上記範囲に制御すれば、凸部の平均幅を、めっき密着性に有用な「０．２０μｍ以上３．０μｍ以下」の範囲内に制御することができる。上記の在炉時間は、例えば、焼鈍炉内の温度や通板速度を調整して制御することができる。 Further, in the oxidation step, the time for the base steel sheet to be in the temperature range of 650 ° C. or higher and 750 ° C. or lower is controlled to 20 seconds or longer and 70 seconds or shorter. According to the basic experiments of the present inventors, the average width of the convex portion is closely related to the holding time (in-furnace time) of the specific temperature region (650 to 750 ° C.) in the annealing furnace. By controlling to the above range, the average width of the convex portion can be controlled within the range of “0.20 μm or more and 3.0 μm or less” useful for plating adhesion. The in-furnace time can be controlled, for example, by adjusting the temperature in the annealing furnace and the plate passing speed.
この点について、もう少し説明すると、本発明者らの基礎実験によれば、焼鈍炉で生成する粒界酸化の幅を増加する（太らせる）と、凸部の平均幅も増加することが分かった。ここで、粒界酸化の幅増加のためには、鋼板温度を高める方法が有効であるが、温度の温度上昇に伴い、めっき層と鋼板の界面に濃化するＳｉ−Ｍｎ−Ｏも増加してしまい、界面が脆化してめっき剥離を招くため、この方法は採用し難い。そこで、本発明者らは、特に
鋼板温度が６５０℃から７５０℃の比較的低温度域に着目し、当該温度域での保持時間（在炉時間）を適切に制御することで、凸部の平均幅を調節できないか更に検討を行なった。その結果、当該温度域での保持時間を２０秒以上７０秒以下に制御すると、所望の凸部の平均幅が得られることが判明した。
To explain this point a little further, according to the basic experiments of the present inventors, it was found that when the width of grain boundary oxidation generated in the annealing furnace is increased (thickened), the average width of the convex portion also increases. . Here, in order to increase the width of grain boundary oxidation, a method of increasing the steel plate temperature is effective, but as the temperature rises, Si-Mn-O concentrated at the interface between the plating layer and the steel plate also increases. As a result, the interface becomes brittle and plating peeling occurs, so this method is difficult to adopt. Therefore, the present inventors pay particular attention to a relatively low temperature range where the steel sheet temperature is 650 ° C. to 750 ° C., and appropriately controlling the holding time (in-furnace time) in the temperature range, thereby Further investigation was made as to whether the average width could be adjusted. As a result, it was found that when the holding time in the temperature range is controlled to 20 seconds or more and 70 seconds or less, a desired average width of the convex portions can be obtained.
上記温度域での保持時間が２０秒未満では、粒界酸化がやせているため、凸部の平均幅が０．２０μｍを下回る。一方、上記温度域での保持時間が７０秒を超えると、粒界酸化が太り過ぎて凸部の平均幅が３．０μｍ超えてしまう。上記温度域での好ましい保持時間は、３０秒以上６０秒以下である。 When the holding time in the above temperature range is less than 20 seconds, the grain boundary oxidation has faded, so the average width of the protrusions is less than 0.20 μm. On the other hand, if the holding time in the above temperature range exceeds 70 seconds, the grain boundary oxidation becomes too thick, and the average width of the protrusions exceeds 3.0 μm. A preferable holding time in the above temperature range is 30 seconds or more and 60 seconds or less.
このように本発明では、焼鈍炉での上記温度域での保持時間を適切に制御しているが、焼鈍炉の抽出温度（出口温度と同義）は、当該温度域の上限（７５０℃）と一致しても良いし、超えていても良い。ただし、焼鈍炉の抽出温度が７５０℃を超えると、上記保持時間による効果（粒界酸化の幅増加）よりも、酸化層と鋼板との界面に形成される内方酸化層（Ｆｅ−Ｓｉ−Ｍｎ−Ｏ）が増加するようになり、めっき剥離の原因を招くため、７５０℃を超える温度域の保持時間は、できるだけ短く制御することが推奨される。また、このようなめっき剥離の問題をできるだけ回避するためには、焼鈍炉の抽出温度は、低い方が良く、７５０℃以下であることが好ましい。 Thus, in this invention, although the holding time in the said temperature range in an annealing furnace is controlled appropriately, the extraction temperature (synonymous with outlet temperature) of an annealing furnace is the upper limit (750 degreeC) of the said temperature range. They may match or exceed. However, when the extraction temperature of the annealing furnace exceeds 750 ° C., the inner oxide layer (Fe—Si—) formed at the interface between the oxide layer and the steel sheet is more than the effect by the holding time (increase in the width of grain boundary oxidation). It is recommended that the holding time in the temperature range exceeding 750 ° C. be controlled as short as possible. Moreover, in order to avoid such a problem of plating peeling as much as possible, the extraction temperature of the annealing furnace should be low, and is preferably 750 ° C. or lower.
以上、本発明の製造方法を最も特徴付ける焼鈍炉での酸化雰囲気および所定温度での保持時間について説明した。本発明では、上記工程を特に制御することによって凹部の平均直径および凸部の平均幅を適切に制御したところに最大の特徴があり、その他の工程は特に限定されず、上記要件を制御できるように、通常用いられる方法を適切に選択して採用すれば良いが、以下、推奨される方法を、工程順に説明する。 As described above, the oxidizing atmosphere and the holding time at a predetermined temperature in the annealing furnace that characterize the manufacturing method of the present invention most have been described. The present invention has the greatest feature in that the average diameter of the concave portions and the average width of the convex portions are appropriately controlled by particularly controlling the above steps, and the other steps are not particularly limited so that the above requirements can be controlled. In addition, a method that is usually used may be appropriately selected and adopted, but the recommended method will be described below in the order of steps.
繰返し述べるように、本発明ではめっき前の素地鋼板に対し、焼鈍炉での酸化処理、還元炉での還元処理を行なった後、常法に従い、溶融亜鉛めっき処理、更には必要に応じて、合金化処理を行い、所望のめっき鋼板（溶融亜鉛めっき鋼板または合金化溶融亜鉛めっき鋼板）を製造するが、焼鈍炉での酸化処理の前に、予備加熱炉での加熱処理（前処理）を行っても良い。この予備加熱処理は、所望の酸化層形成のため、通常行なわれるものである。 As described repeatedly, in the present invention, the base steel plate before plating is subjected to an oxidation treatment in an annealing furnace, a reduction treatment in a reduction furnace, and according to a conventional method, a hot dip galvanizing treatment, and further, if necessary, Alloying treatment is performed to produce the desired plated steel sheet (hot dip galvanized steel sheet or galvannealed steel sheet), but before the oxidation treatment in the annealing furnace, the heat treatment (pretreatment) in the preheating furnace is performed. You can go. This preheating treatment is usually performed for forming a desired oxide layer.
上記予備加熱炉の好ましい加熱条件は、室温から、おおむね４００〜５５０℃程度までとし、加熱時間（予備加熱炉でのトータル時間）は、おおむね３０〜７０秒程度とする。また、上記予備加熱炉での好ましい加熱雰囲気は、酸素量：おおむね１ｐｐｍ（０．０００１０体積％）〜１体積％程度、水蒸気量：おおむね１５〜２５体積％程度である。 Preferred heating conditions for the preheating furnace are from room temperature to approximately 400 to 550 ° C., and the heating time (total time in the preheating furnace) is approximately 30 to 70 seconds. Moreover, the preferable heating atmosphere in the said preheating furnace is oxygen amount: about 1 ppm (0.00010 volume%)-about 1 volume%, and water vapor amount: about 15-25 volume%.
予備加熱炉で予備加熱された鋼板は、次いで焼鈍炉へ供給され、酸化加熱処理を行なう。 The steel plate preheated in the preheating furnace is then supplied to the annealing furnace and subjected to oxidation heat treatment.
焼鈍炉での雰囲気について、酸素分圧は上記のとおりであるが、水蒸気量はおおむね、１５〜２５体積％程度に制御することが好ましい。これにより、酸化層の厚さが適切に制御されるので、還元後に充分な厚さの純鉄層を確保でき、合金化不良を抑制することができる。 Regarding the atmosphere in the annealing furnace, the oxygen partial pressure is as described above, but the amount of water vapor is preferably controlled to about 15 to 25% by volume. Thereby, since the thickness of an oxide layer is controlled appropriately, a pure iron layer having a sufficient thickness can be secured after reduction, and poor alloying can be suppressed.
焼鈍炉内の雰囲気に含まれる酸素量や水蒸気量は、焼鈍炉内の加熱に用いられるバーナーに供給する燃焼ガスの流量や、燃焼ガスと空気の流量比（空燃比）を調整することによって制御することができる。焼鈍炉内の酸素量は、例えば、磁気式濃度計を用いて測定でき、水蒸気量は、例えば露点計を用いて測定できる。 The amount of oxygen and water vapor contained in the atmosphere in the annealing furnace is controlled by adjusting the flow rate of the combustion gas supplied to the burner used for heating in the annealing furnace and the flow rate ratio (air-fuel ratio) of the combustion gas and air. can do. The amount of oxygen in the annealing furnace can be measured using, for example, a magnetic densitometer, and the amount of water vapor can be measured using, for example, a dew point meter.
焼鈍炉では、予備加熱炉の抽出温度から約７００〜９００℃までの温度範囲を、おおむね３０〜７０秒（焼鈍炉でのトータル時間）かけて加熱することが好ましい。焼鈍炉での６５０〜７５０℃での在炉時間（保持時間）は、前述したとおりである。 In the annealing furnace, it is preferable to heat the temperature range from the extraction temperature of the preheating furnace to about 700 to 900 ° C. over approximately 30 to 70 seconds (total time in the annealing furnace). The in-furnace time (holding time) at 650 to 750 ° C. in the annealing furnace is as described above.
次に還元炉に供給し、還元雰囲気下で酸化層を還元する。還元炉内の雰囲気は、所望となるＦｅ層（還元層）が形成されるよう、還元性ガス雰囲気に制御すればよい。還元性ガス雰囲気としては、窒素雰囲気が挙げられ、水素ガスを含むＨ2ガス含有Ｎ2ガス雰囲気下で行なっても良い。還元炉内の温度は、おおむね８００〜９５０℃程度、還元時間（還元炉でのトータル時間）は、おおむね３０秒〜３分程度に制御することが好ましい。 Next, it is supplied to a reduction furnace, and the oxide layer is reduced in a reducing atmosphere. The atmosphere in the reduction furnace may be controlled to a reducing gas atmosphere so that a desired Fe layer (reduction layer) is formed. Examples of the reducing gas atmosphere include a nitrogen atmosphere, which may be performed in an N 2 gas atmosphere containing hydrogen gas and containing H 2 gas. It is preferable to control the temperature in the reduction furnace to about 800 to 950 ° C. and the reduction time (total time in the reduction furnace) to about 30 seconds to 3 minutes.
次に、常法に従い、溶融亜鉛めっきを行なう。必要に応じて合金化処理を行なっても良い。これらの溶融亜鉛めっき条件や合金化条件は特に限定されず、公知の条件を採用できる。例えば溶融亜鉛めっき浴の温度は、おおむね４００〜６００℃程度に制御することが好ましい。また、合金化温度は、約５００〜６００℃程度に制御することが好ましい。合金化溶融亜鉛めっき層の好ましい付着量は、３０〜７０ｇ／ｍ2程度である。 Next, hot dip galvanization is performed according to a conventional method. An alloying treatment may be performed as necessary. These hot dip galvanizing conditions and alloying conditions are not particularly limited, and known conditions can be adopted. For example, the temperature of the hot dip galvanizing bath is preferably controlled to about 400 to 600 ° C. The alloying temperature is preferably controlled to about 500 to 600 ° C. The preferable adhesion amount of the alloyed hot-dip galvanized layer is about 30 to 70 g / m 2 .
以下、実施例を挙げて本発明をより具体的に説明するが、本発明は下記実施例によって制限されず、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に包含される。 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 may be implemented with appropriate modifications within a range that can meet the purpose described above and below. All of these are possible within the scope of the present invention.
まず、表１、表２に示す化学成分を含有する鋼Ａ〜Ｗ（残部は鉄および不可避不純物）を溶製し、スラブを製造した。スラブを製造するにあたって、ＲＥＭはＬａを約２５％とＣｅを約５０％含有するミッシュメタルを用いて成分調整した。得られたスラブを１２００℃に加熱し、熱間圧延して厚さ２．５ｍｍの熱延鋼板を作製した。熱間圧延の巻取り温度は５００℃とした。これを酸洗してスケールを除去し、冷間圧延を行なって厚さ２．０ｍｍの薄鋼板を作製した。 First, steels A to W containing the chemical components shown in Tables 1 and 2 (the balance is iron and inevitable impurities) were melted to produce slabs. In manufacturing the slab, REM was prepared by using misch metal containing about 25% La and about 50% Ce. The obtained slab was heated to 1200 ° C. and hot-rolled to produce a hot-rolled steel sheet having a thickness of 2.5 mm. The coiling temperature for hot rolling was 500 ° C. This was pickled, the scale was removed, and cold rolling was performed to produce a thin steel plate having a thickness of 2.0 mm.
次に、予備加熱炉で室温から４５０℃まで加熱した後、焼鈍炉で加熱して酸化層を形成し、この酸化層を還元炉で還元した後、溶融亜鉛めっきを行ない、溶融亜鉛めっき鋼板を得た。一部については、更に合金化を行ない、合金化溶融亜鉛めっき鋼板を得た。 Next, after heating from room temperature to 450 ° C. in a preheating furnace, an oxidation layer is formed by heating in an annealing furnace, and after this oxidation layer is reduced in a reduction furnace, hot dip galvanization is performed to obtain a hot dip galvanized steel sheet. Obtained. About some, it alloyed further and obtained the galvannealed steel plate.
予備加熱炉、焼鈍炉、および還元炉における具体的な条件は次の通りである。なお、薄鋼板の表面温度は放射温度計を、酸素量は磁気式濃度計を、水蒸気量は露点計を、それぞれ用いて測定した。 Specific conditions in the preheating furnace, annealing furnace, and reduction furnace are as follows. The surface temperature of the thin steel plate was measured using a radiation thermometer, the oxygen content was measured using a magnetic densitometer, and the water vapor content was measured using a dew point meter.
まず、予備加熱炉内は、燃焼ガスの排ガス雰囲気とし、酸素を０．５体積％と水蒸気を２０体積％含有するように調整した。燃焼ガスとしてはＣＯＧガスを用い、このＣＯＧガスは、５５体積％のＨ2ガスと６体積％のＮ2ガスを含み、残部は炭化水素ガスで構成されている。なお、予備加熱炉での全在炉時間（トータル時間）は３０〜５０秒である。 First, the inside of the preheating furnace was adjusted to contain an exhaust gas atmosphere of combustion gas and contain 0.5% by volume of oxygen and 20% by volume of water vapor. As the combustion gas, COG gas is used. This COG gas contains 55% by volume of H 2 gas and 6% by volume of N 2 gas, and the remainder is composed of hydrocarbon gas. The total in-furnace time (total time) in the preheating furnace is 30 to 50 seconds.
次に、焼鈍炉では、ＣＯＧガスと空気の混合ガスをバーナーで燃焼させて上記薄鋼板を４５０℃から加熱した。ここでは、ＣＯＧガスの流量、およびＣＯＧガスと空気の流量比（空燃比）を制御することにより、焼鈍炉内の雰囲気ガスに含まれる酸素分圧を表２に示す範囲に調整した。更に、鋼板の通板速度を制御することにより、焼鈍炉内における、薄鋼板の温度が６５０〜７５０℃までの時間（保持時間）を表３、表４に示す範囲に調整した。焼鈍炉での全在炉時間（トータル時間）は３０〜５０秒であり、焼鈍炉出口における薄鋼板の温度は、７００〜８５０℃であった。 Next, in the annealing furnace, a mixed gas of COG gas and air was burned with a burner, and the thin steel sheet was heated from 450 ° C. Here, the partial pressure of oxygen contained in the atmospheric gas in the annealing furnace was adjusted to the range shown in Table 2 by controlling the flow rate of COG gas and the flow rate ratio (air-fuel ratio) of COG gas and air. Furthermore, the time (holding time) until the temperature of the thin steel plate in the annealing furnace was 650 to 750 ° C. was adjusted to the ranges shown in Tables 3 and 4 by controlling the plate passing speed of the steel plate. The total furnace time (total time) in the annealing furnace was 30 to 50 seconds, and the temperature of the thin steel sheet at the outlet of the annealing furnace was 700 to 850 ° C.
次に、上記薄鋼板を還元炉に供給し、酸化層を還元した。ここでは、ラジアントチューブを備えた還元炉を用い、上記薄鋼板の温度を間接的に上げる方式で、鋼板温度を９５０℃まで加熱した。還元炉内の雰囲気は、Ｈ２を２０体積％含有するＮ２ガス雰囲気とした。 Next, the said thin steel plate was supplied to the reduction furnace, and the oxide layer was reduced. Here, the steel sheet temperature was heated to 950 ° C. by a method of indirectly raising the temperature of the thin steel sheet using a reduction furnace equipped with a radiant tube. The atmosphere in the reduction furnace was an N 2 gas atmosphere containing 20% by volume of H 2 .
次に、上記の還元性雰囲気を維持したまま冷却し、大気と接触させることなく上記薄鋼板を溶融亜鉛浴に浸漬し、溶融亜鉛めっきを行った。溶融亜鉛浴の温度は４５０℃とした。 Next, it cooled, maintaining said reducing atmosphere, the said steel plate was immersed in the hot dip galvanizing bath without making it contact with air | atmosphere, and hot dip galvanization was performed. The temperature of the molten zinc bath was 450 ° C.
更に合金化処理を行なう場合は、合金化炉で５００℃に加熱した。 Furthermore, when performing alloying process, it heated at 500 degreeC with the alloying furnace.
このようにして得られた溶融亜鉛めっき鋼板または合金化溶融亜鉛めっき鋼板を用い、前述した方法に基づき、凹部の平均直径と凸部の平均幅を算出した。 Using the hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet thus obtained, the average diameter of the recesses and the average width of the protrusions were calculated based on the method described above.
また、上記めっき鋼板の密着性を、以下のようにして評価した。 Moreover, the adhesiveness of the said plated steel plate was evaluated as follows.
（めっき密着性の評価）
上記めっき鋼板を縦１００ｍｍ×横２００ｍｍ×厚２ｍｍの板形状試験片に加工し、Ｖ曲げ曲げ戻し試験を行ってめっき密着性を評価した。このＶ曲げ曲げ戻し試験は、実際のプレス成形よりも厳しい条件を模擬したものである。詳細には、Ｖ曲げ試験用の金型（曲げ角度６０°）を用いて上記試験片をＶ曲げ加工した後、更にプレスで上記試験片を平坦に戻す曲げ戻し加工を行った。曲げ戻し加工を行ったときの内側の面（変形部）にセロハンテープ（ニチバン社製「セロテープ（登録商標）ＣＴ４０５ＡＰ−２４」）を貼り付け、手で剥がした後、テープに付着しためっき層の剥離幅を測定した。本実例では、剥離幅が５ｍｍ以下のものを合格（めっき密着性に優れる）と評価した。
(Evaluation of plating adhesion)
The plated steel sheet was processed into a plate-shaped specimen having a length of 100 mm, a width of 200 mm, and a thickness of 2 mm, and a V-bending / returning test was performed to evaluate the plating adhesion. This V-bend / bend-back test simulates conditions more severe than actual press molding. Specifically, the V-bending test was performed on the test piece using a V-bending test die (bending angle 60 °), and then bending back was performed to return the test piece to a flat shape using a press. The cellophane tape ("Cello Tape (registered trademark) CT405AP-24" manufactured by Nichiban Co., Ltd.) is applied to the inner surface (deformed part) when the bending back process is performed. The peel width was measured. In this example, a strip width of 5 mm or less was evaluated as acceptable (excellent plating adhesion).
これらの結果を表３、表４に併記する。表３中、Ｎｏ．１〜１５は、溶融亜鉛めっき鋼板の例であり、このうちＮｏ．１〜８は、焼鈍炉での６５０〜７５０℃の保持時間を５０秒と一定にし、焼鈍炉での酸素分圧を変化させた例；Ｎｏ．９〜１５は、焼鈍炉での酸素分圧を５００ｐｐｍ（０．０５体積％）と一定にし、６５０〜７５０℃の保持時間を変化させた例である。一方、Ｎｏ．１６〜２３は、合金化溶融亜鉛めっき鋼板の例であり、このうちＮｏ．１６〜１９は、焼鈍炉での６５０〜７５０℃の保持時間を３５秒と一定にし、焼鈍炉での酸素分圧を変化させた例；Ｎｏ．２０〜２３は、焼鈍炉での酸素分圧を３００ｐｐｍ（０．０３体積％）と一定にし、６５０〜７５０℃の保持時間を変化させた例である。 These results are also shown in Tables 3 and 4. In Table 3, No. 1 to 15 are examples of hot-dip galvanized steel sheets. Nos. 1 to 8 are examples in which the holding time at 650 to 750 ° C. in the annealing furnace was made constant at 50 seconds and the oxygen partial pressure in the annealing furnace was changed; Nos. 9 to 15 are examples in which the oxygen partial pressure in the annealing furnace is kept constant at 500 ppm (0.05% by volume) and the holding time at 650 to 750 ° C. is changed. On the other hand, no. Nos. 16 to 23 are examples of galvannealed steel sheets. Nos. 16 to 19 are examples in which the holding time at 650 to 750 ° C. in the annealing furnace was kept constant at 35 seconds and the oxygen partial pressure in the annealing furnace was changed; 20-23 are examples in which the oxygen partial pressure in the annealing furnace was kept constant at 300 ppm (0.03% by volume) and the holding time at 650-750 ° C. was changed.
表４中、Ｎｏ．２８、２９、３３〜３６、３９〜４１は、溶融亜鉛めっき鋼板の例である。一方、Ｎｏ．２４〜２７、３０〜３２、３７、３８は、合金化溶融亜鉛めっき鋼板の例である。 In Table 4, No. 28, 29, 33-36, 39-41 are examples of hot-dip galvanized steel sheets. On the other hand, no. 24-27, 30-32, 37, 38 are examples of galvannealed steel sheets.
表３、表４より、以下のように考察できる。 From Tables 3 and 4, it can be considered as follows.
まず、表３のＮｏ．２〜７、１０〜１４（以上、溶融亜鉛めっき鋼板）、１７、１８、２１、２２（以上、合金化溶融亜鉛めっき鋼板）はいずれも、鋼中成分、凹部の平均直径および凸部の平均幅が、すべて本発明の要件を満足する例であり、密着性試験後のテープ剥離幅が５．０ｍｍ以下と小さく、めっき密着性に優れている。このうち、凹部の平均直径および凸部の平均幅が、本発明の好ましい要件を満足するＮｏ．４〜６、１１〜１３、１８、２１は、テープ剥離幅が３．０ｍｍ以下と更に小さくなっており、めっき密着性に一層優れている。 First, in Table 3, No. 2-7, 10-14 (above, hot dip galvanized steel sheet), 17, 18, 21, 22 (more, alloyed hot dip galvanized steel sheet) are all components in steel, average diameter of recesses and average of protrusions All the widths are examples satisfying the requirements of the present invention, the tape peeling width after the adhesion test is as small as 5.0 mm or less, and the plating adhesion is excellent. Among these, the average diameter of the concave portions and the average width of the convex portions satisfy the preferable requirements of the present invention. Nos. 4-6, 11-13, 18, and 21 have a tape peeling width of 3.0 mm or less, and are further excellent in plating adhesion.
これに対し、本発明の要件のいずれかを満足しない下記の例は、めっき密着性が低下した。 On the other hand, in the following examples that did not satisfy any of the requirements of the present invention, the plating adhesion decreased.
まず、Ｎｏ．１は、焼鈍炉での酸素分圧が低い例であり、凹部の平均直径が小さくなってめっき密着性が低下した。 First, no. No. 1 is an example in which the oxygen partial pressure in the annealing furnace is low, and the average diameter of the recesses is reduced and the plating adhesion is reduced.
Ｎｏ．８は、焼鈍炉での酸素分圧が高い例であり、凹部の平均直径が大きくなってめっき密着性が低下した。 No. No. 8 is an example in which the oxygen partial pressure in the annealing furnace is high, and the average diameter of the recesses is increased and the plating adhesion is lowered.
Ｎｏ．９は、上記温度域の保持時間が短い例であり、凸部の平均幅が短くなってめっき密着性が低下した。 No. No. 9 is an example in which the holding time in the above temperature range is short, and the average width of the convex portions is shortened, and the plating adhesion is lowered.
Ｎｏ．１５は、上記温度域の保持時間が長い例であり、凸部の平均幅が長くなってめっき密着性が低下した。 No. No. 15 is an example in which the holding time in the temperature range is long, and the average width of the protrusions is increased, resulting in a decrease in plating adhesion.
Ｎｏ．１６は、焼鈍炉での酸素分圧が低い例であり、凹部の平均直径が小さくなってめっき密着性が低下した。 No. No. 16 is an example in which the oxygen partial pressure in the annealing furnace is low, and the average diameter of the recesses is reduced and the plating adhesion is reduced.
Ｎｏ．１９は、焼鈍炉での酸素分圧が高い例であり、凹部の平均直径が大きくなってめっき密着性が低下した。 No. No. 19 is an example in which the oxygen partial pressure in the annealing furnace is high, and the average diameter of the recesses is increased and the plating adhesion is lowered.
Ｎｏ．２０は、上記温度域の保持時間が短い例であり、凸部の平均幅が短くなってめっき密着性が低下した。 No. No. 20 is an example in which the holding time in the above temperature range is short, and the average width of the convex portions is shortened, and the plating adhesion is lowered.
Ｎｏ．２３は、上記温度域の保持時間が長い例であり、凸部の平均幅が長くなってめっき密着性が低下した。 No. No. 23 is an example in which the holding time in the above temperature range is long, and the average width of the convex portion is increased, and the plating adhesion is lowered.
表４のＮｏ．２８、２９、３３〜３６、３９〜４１（以上、溶融亜鉛めっき鋼板）、Ｎｏ．２４〜２７、３０〜３２、３７、３８（以上、合金化溶融亜鉛めっき鋼板）はいずれも、鋼中成分、凹部の平均直径および凸部の平均幅が、すべて本発明の要件を満足する例であり、密着性試験後のテープ剥離幅が５．０ｍｍ以下と小さく、めっき密着性に優れている。このうち、凹部の平均直径および凸部の平均幅が、本発明の好ましい要件を満足するＮｏ．２４、２６、２７、３０、３１、３３、３５、３８、４０、４１は、テープ剥離幅が３．０ｍｍ以下と更に小さくなっており、めっき密着性に一層優れている。
No. in Table 4 28, 29, 33-36, 39-41 (above, hot dip galvanized steel sheet), no. Examples 24 to 27, 30 to 32, 37 and 38 (all above, alloyed hot dip galvanized steel sheet) are examples in which the components in the steel, the average diameter of the recesses, and the average width of the protrusions all satisfy the requirements of the present invention. The tape peeling width after the adhesion test is as small as 5.0 mm or less, and the plating adhesion is excellent. Among these, the average diameter of the concave portions and the average width of the convex portions satisfy the preferable requirements of the present invention. 24, 26, 27, 30, 31, 33, 35, 38, 40, and 41 have a tape peeling width of 3.0 mm or less and are further excellent in plating adhesion.
Claims (10)
前記素地鋼板と前記溶融亜鉛めっき層との界面に、Ｓｉ−Ｍｎ−Ｏおよび鉄亜鉛合金を含む酸化物含有層を有し、
前記酸化物含有層の素地鋼板側の表面は、網目状の凸部と、該凸部によって分割された複数の凹部とを有し、
インターセプト法で算出した前記凹部の平均直径が３．０μｍ以上１０．０μｍ以下であり、前記凸部の平均幅が０．２０μｍ以上３．０μｍ以下であることを特徴とするめっき鋼板。 A plated steel sheet in which a hot-dip galvanized layer that is alloyed or not alloyed is formed on a base steel sheet,
At the interface between the base steel sheet and the hot-dip galvanized layer, an oxide-containing layer containing Si-Mn-O and an iron-zinc alloy is provided.
The surface of the oxide-containing layer on the base steel plate side has a mesh-like convex part and a plurality of concave parts divided by the convex part,
An average diameter of the recesses calculated by an intercept method is 3.0 μm or more and 10.0 μm or less, and an average width of the protrusions is 0.20 μm or more and 3.0 μm or less.
素地鋼板を酸化雰囲気下で熱処理し、素地鋼板表面に酸化層を形成する酸化工程と、
前記酸化工程の後、還元雰囲気下で熱処理して前記素地鋼板表面の酸化層を還元する還元工程と、
前記還元工程の後、溶融亜鉛めっき処理するめっき工程と、を含み、
前記酸化工程において、酸素分圧を０．０００１０体積％以上０．１体積％以下に制御すると共に、素地鋼板が６５０℃以上７５０℃以下の温度範囲になる時間を２０秒以上７０秒以下に制御することを特徴とするめっき鋼板の製造方法。 A method for producing the plated steel sheet according to claim 1,
An oxidation step of heat-treating the base steel sheet in an oxidizing atmosphere to form an oxide layer on the base steel sheet surface;
After the oxidation step, a reduction step of reducing the oxide layer on the base steel sheet surface by heat treatment in a reducing atmosphere;
After the reduction step, including a plating step of galvanizing treatment,
In the oxidation step, the oxygen partial pressure is controlled to 0.00010 volume% or more and 0.1 volume% or less, and the time for the base steel sheet to be in the temperature range of 650 ° C. or more and 750 ° C. or less is controlled to 20 seconds or more and 70 seconds or less. A method for producing a plated steel sheet, comprising:
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