WO2022085287A1 - めっき鋼板 - Google Patents

めっき鋼板 Download PDF

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Publication number
WO2022085287A1
WO2022085287A1 PCT/JP2021/030398 JP2021030398W WO2022085287A1 WO 2022085287 A1 WO2022085287 A1 WO 2022085287A1 JP 2021030398 W JP2021030398 W JP 2021030398W WO 2022085287 A1 WO2022085287 A1 WO 2022085287A1
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Prior art keywords
steel sheet
phase
plating layer
plating
plated steel
Prior art date
Application number
PCT/JP2021/030398
Other languages
English (en)
French (fr)
Inventor
卓哉 光延
武寛 高橋
浩史 竹林
Original Assignee
日本製鉄株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to CN202180051908.3A priority Critical patent/CN116113719A/zh
Priority to KR1020237006212A priority patent/KR20230043162A/ko
Priority to MX2023002315A priority patent/MX2023002315A/es
Priority to EP21882428.2A priority patent/EP4234736A4/en
Priority to JP2022556435A priority patent/JP7360082B2/ja
Priority to US18/040,903 priority patent/US20230295775A1/en
Publication of WO2022085287A1 publication Critical patent/WO2022085287A1/ja

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B15/043—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material of metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
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    • C—CHEMISTRY; METALLURGY
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C—CHEMISTRY; METALLURGY
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • Y10T428/263—Coating layer not in excess of 5 mils thick or equivalent
    • Y10T428/264—Up to 3 mils
    • Y10T428/265—1 mil or less

Definitions

  • the present invention relates to a plated steel sheet.
  • This application claims priority based on Japanese Patent Application No. 2020-175786 filed in Japan on October 20, 2020, the contents of which are incorporated herein by reference.
  • hot-dip galvanized steel sheets such as alloyed hot-dip galvanized steel sheets have been applied mainly in the Japanese domestic market.
  • the alloyed hot-dip galvanized steel sheet is plated with improved weldability and post-painting corrosion resistance by subjecting the steel sheet to hot-dip galvanizing and then alloying heat treatment to diffuse Fe from the steel sheet (base steel sheet) into the plated layer. It is a steel plate.
  • hot-dip galvanized steel sheets are required to be further improved in corrosion resistance such as corrosion resistance after painting and red rust resistance.
  • molten Al—Zn-based plated steel sheets have been widely put into practical use as highly corrosion-resistant plated steel sheets.
  • the plating layer of such molten Al—Zn-based plating is a dendrite-like ⁇ - (Zn, Al) phase (Al primary crystal part: Al—Zn-based binary state diagram, etc., which first crystallizes from the molten state.
  • the ⁇ - (Zn, Al) phase that crystallizes as.
  • the Zn phase and Al phase formed in the gaps between the dendrite-like Al primary crystals. It is formed from a structure consisting of (Zn / Al mixed phase structure). Since the Al primary crystal portion is passivated and the Zn / Al mixed phase structure has a higher Zn concentration than the Al primary crystal portion, the corrosion is concentrated in the Zn / Al mixed phase structure. As a result, the corrosion progresses in a worm-eaten manner in the Zn / Al mixed phase structure, and the corrosion progress path becomes complicated, so that the corrosion does not easily reach the base steel sheet. As a result, the hot-dip Al—Zn-based plated steel sheet has excellent corrosion resistance as compared with the hot-dip galvanized steel sheet having the same thickness of the plated layer.
  • the plated steel sheet When such a hot-dip Al—Zn-based plated steel sheet is used as an automobile outer panel, the plated steel sheet is provided to an automobile manufacturer or the like in a state of being plated by a continuous hot-dip plating facility, and is processed into a panel component shape there. After that, chemical conversion treatment, electrodeposition coating, intermediate coating, and top coating are generally applied for automobiles.
  • the outer panel using the molten Al—Zn-based plated steel sheet is caused by the unique plating phase structure consisting of the above-mentioned two phases of Al primary crystal part and Zn / Al mixed phase structure when the coating film is damaged.
  • Patent Document 1 discloses a hot-dip Zn-based plated steel sheet having excellent corrosion resistance after painting.
  • the plating layer contains Zn, Al, Mg and Si, and the plating layer contains 5% or more of the lamella structure in which the layered Zn phase and the layered Al phase are alternately arranged. If so, it is disclosed that the swelling of the coating film in the coated state is suppressed.
  • Patent Document 2 Al: 25 to 90% and Sn: 0.01 to 10% are contained in mass%, and more than one selected from the group consisting of Mg, Ca and Sr is contained in a total of 0.
  • a fused Al—Zn-based plated steel sheet is disclosed, which comprises a plating layer containing 01 to 10%.
  • Sn destroys the Al oxide film formed around the ⁇ -Al phase described above and increases the solubility of the ⁇ -Al phase, so that both the ⁇ -Al phase and the Zn-rich phase are present. It is disclosed that the uniform corrosion of the plating layer to be dissolved can suppress the selective corrosion of the Zn-rich phase and improve the corrosion resistance after coating.
  • Patent Document 3 a molten Zn—Al—Mg alloy plated steel sheet in which the ratio of [Al / Zn / Zn 2 Mg ternary eutectic structure] to the outermost surface of the plating layer is 60 area% or more is used as a base material.
  • a chemical conversion-treated steel sheet in which the surface of the plating layer is covered with a chemical conversion film is disclosed.
  • Patent Document 4 contains Al: 0.18 to 5%, and further, Mg: 0.01 to 0.5%, La: 0.001 to 0.5%, and Ce: 0.
  • a zinc-based alloy-plated steel sheet containing one or more of 001 to 0.5% and having a Zn—Al-based alloy plating in which the balance is Zn is disclosed.
  • Patent Document 4 shows that zinc-based alloy plating such as Zn-Al-Mg-based and Zn-Al-Mg-Si-based has higher corrosion resistance than conventional Zn-based plating.
  • Automotive parts may be used in an environment where water collects, and it is required to have sufficient corrosion resistance even in such a harsh environment.
  • the Zn-Al-Mg-based plating containing a certain amount or more of Al has excellent sacrificial anticorrosion properties, but in an environment where water collects, the coating film is easily peeled off, and the corrosion resistance after coating is improved. It turned out that there was a problem.
  • Patent Documents 1 to 4 do not consider the corrosion resistance after coating in such a harsh environment.
  • Patent Document 1 it is necessary to perform complicated heat history processing for tissue control, and there is also a problem that the manufacturing cost increases. Further, the plated steel sheet of Patent Document 2 is inferior in adhesion to the electrodeposition coating film for automobiles. Further, in Patent Document 2, since Sn addition is indispensable, there is a problem that the alloy cost increases and it becomes difficult to manage the plating bath. Further, in Patent Document 3, the corrosion resistance is improved by controlling the composition of the chemical conversion film. Further, in order to enhance the reactivity with the chemical conversion film, the main phase of the plating layer is a ternary eutectic structure of Al / Zn / Zn 2 Mg. Therefore, when a normal chemical conversion treatment is performed, it is considered that the chemical conversion treatment property is improved, but the corrosion resistance after coating is not sufficiently obtained.
  • Japanese Patent No. 6350780 Japanese Patent Application Laid-Open No. 2015-214747 Japanese Patent No. 4579715 Japanese Patent Application Laid-Open No. 2006-249521
  • An object of the present invention is to provide a plated steel sheet having excellent corrosion resistance after coating, on the premise of a hot-dip galvanized steel sheet.
  • the present inventors have studied a steel sheet having a plating layer containing Al, Mg and Zn (Zn—Al—Mg alloy plated steel sheet) for improving the corrosion resistance after coating.
  • the corrosion resistance after coating is improved by incorporating La and / or Ce in the plating layer and controlling the plating structure on the surface of the plating layer.
  • the area ratio of the lamella structure of the (Al—Zn) phase and the MgZn 2 phase is increased, and the area ratio of the (Al—Zn) dendrite is decreased, so that after coating. It was found that the corrosion resistance is improved.
  • the present invention has been made based on the above findings, and the gist thereof is as follows.
  • the plated steel sheet according to one aspect of the present invention has a steel sheet and a plating layer formed on at least a part of the surface of the steel sheet, and the chemical composition of the plating layer is Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: 0.0001 to 0.5000% in total, Si: 0 to 2.00%, Ca: 0 to 2.00% , Fe: 0 to 2.00%, Sb: 0 to 0.50%, Sr: 0 to 0.50%, Pb: 0 to 0.50%, Sn: 0 to 1.00%, Cu: 0 to
  • the plating layer contains 1.00%, Ti: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%, and the balance is made of Zn and impurities.
  • the area ratio of the lamella structure in which the (Al—Zn) phase and the MgZn 2 phase are arranged in layers is 10 to 95%
  • the lamella spacing of the lamella structure is 2.5 ⁇ m or less
  • (Al— Zn) A plated steel sheet having a dendrite area ratio of 10% or less.
  • the chemical composition of the plated layer is Al: 11.00 to 30.00%
  • La + Ce in mass%. It may contain one or more of 0.0010 to 0.1000% in total.
  • the plated layer may have an area ratio of the lamella structure of 60 to 95% on the surface.
  • the plated layer may have an area ratio of the lamella structure of 80 to 95% on the surface.
  • the plated steel sheet according to the embodiment of the present invention (the plated steel sheet according to the present embodiment) has a steel sheet and a plating layer formed on at least a part of the surface of the steel sheet. Further, in the plated steel sheet according to the present embodiment, the chemical composition of the plated layer is mass%, Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: 0 in total.
  • the area ratio of the lamellar structure in which the (Al—Zn) phase and the MgZn 2 phase are arranged in layers on the surface is 10 to 95%, and the area ratio of the lamellar structure is 10 to 95%.
  • the lamellar spacing is 2.5 ⁇ m or less, and the area ratio of (Al—Zn) dendrite is 10% or less.
  • the plated layer is important for the plated steel sheet according to the present embodiment, and the type of the steel sheet is not particularly limited. It may be determined according to the applicable product, the required strength, the plate thickness, and the like. For example, a hot-rolled steel sheet described in JIS G3193: 2008 or a cold-rolled steel sheet described in JIS G3141: 2017 can be used.
  • the plated steel sheet according to the present embodiment includes a plated layer on at least a part of the surface of the steel sheet.
  • the plating layer may be formed on one side of the steel sheet or may be formed on both sides.
  • the amount of adhesion of the plating layer is preferably 15 to 250 g / m 2 .
  • Al 6.00 to 35.00%
  • Al is an element effective for ensuring corrosion resistance after coating in a plating layer containing aluminum (Al), zinc (Zn), and magnesium (Mg). Further, it is an element necessary for forming a lamella structure in the plating layer of the plated steel sheet according to the present embodiment.
  • Al contributes to the formation of an alloy layer (Al—Fe alloy layer) and is also an effective element for ensuring plating adhesion.
  • the Al content is set to 6.00% or more.
  • the Al content is preferably 11.00% or more.
  • the Al content is set to 35.00% or less. It is preferably 30.00% or less.
  • Mg 3.00 to 15.00%
  • Mg is an element having an effect of enhancing the corrosion resistance of the plating layer after coating. Further, it is an element necessary for forming a lamella structure in the plating layer of the plated steel sheet according to the present embodiment.
  • the Mg content is set to 3.00% or more.
  • the Mg content is preferably 5.00% or more.
  • the Mg content is set to 15.00% or less.
  • the Mg content is preferably 11.00% or less.
  • La + Ce 0.0001 to 0.5000% in total La and Ce are effective elements for stabilizing the lamella structure in the plating layer. Even if La and Ce are not contained in the plating layer, a lamella structure may be formed in a region other than the surface layer portion inside the plating layer. However, if the total content of La and Ce is less than 0.0001%, no lamella structure is formed on the surface layer. Therefore, it is not possible to sufficiently secure the area ratio of the lamella structure on the surface of the plating layer.
  • the total content of La and Ce exceeds 0.5000%, the viscosity of the plating bath increases, which often makes it difficult to build the plating bath itself, and it is not possible to produce a plated steel material having good plating properties. .. Therefore, it is preferable that the total content of La and Ce is 0.5000% or less.
  • Si 0 to 2.00%
  • Si is an element that forms a compound together with Mg and contributes to the improvement of corrosion resistance after painting. Further, Si suppresses the formation of an excessively thick alloy layer formed between the steel sheet and the plating layer when forming the plating layer on the steel sheet, thereby improving the adhesion between the steel sheet and the plating layer. It is also an element that has the effect of enhancing. Therefore, it may be contained.
  • the Si content is preferably 0.10% or more. More preferably, it is 0.20% or more.
  • the Si content is more than 2.00%, excess Si is crystallized in the plating layer, the lamella structure is not sufficiently formed, and the corrosion resistance after coating is lowered. In addition, the workability of the plating layer is also reduced. Therefore, the Si content is set to 2.00% or less. The Si content is more preferably 1.50% or less. Si does not necessarily have to be contained, and the lower limit of the content is 0%.
  • Ca 0 to 2.00%
  • the amount of dross that is likely to be formed during the plating operation decreases as the Mg content increases, and the plating manufacturability is improved. Therefore, Ca may be contained. It is not always necessary to contain Ca, and the lower limit is 0%. However, in order to obtain the above effect, the Ca content is preferably 0.03% or more, and more preferably 0.10% or more.
  • the Ca content is high, the lamella structure is not sufficiently formed, and further, Ca-based intermetallic compounds such as CaZn 11 phase are generated as other intermetallic compound phases in an area ratio of 10% or more, and after coating. Corrosion resistance is reduced.
  • the Ca content is set to 2.00% or less.
  • the Ca content is preferably 1.00% or less.
  • Fe 0 to 2.00% Fe is mixed into the plating layer as an impurity when the plating layer is manufactured. Fe may be contained up to about 2.00%, but if it is within this range, the adverse effect on the characteristics of the plated steel sheet according to the present embodiment is small. Therefore, the Fe content is preferably 2.00% or less. It is more preferably 1.50% or less, still more preferably 1.00% or less. On the other hand, as described above, Fe is mixed in the plating layer as an impurity. Since it is extremely costly to completely prevent the contamination of Fe, the Fe content may be 0.10% or more.
  • the chemical composition of the plating layer of the plated steel sheet according to the present embodiment basically has the above-mentioned chemical composition, and the balance is Zn and impurities.
  • the content of impurities is preferably 5.0% or less, more preferably 3.0% or less.
  • the plating layer of the plated steel sheet according to the present embodiment may further contain, for example, Sb, Pb, Cu, Sn, Ti, Sr, Ni, Mn in the following range instead of a part of Zn (intention). It does not matter whether it is added as a target or contained as an impurity). Since these elements do not necessarily have to be contained, the lower limit of the content is 0%.
  • Sb 0 to 0.50% Sr: 0 to 0.50%
  • Pb 0 to 0.50%
  • Sr, Sb, and Pb are contained in the plating layer, the appearance of the plating layer is changed, spangles are formed, and the improvement of metallic luster is confirmed. Therefore, it may be contained.
  • the content of these elements exceeds 0.50%, various intermetallic compound phases are formed, and processability and corrosion resistance are deteriorated.
  • the Sr content is 0.50% or less
  • the Sb content is 0.50% or less
  • the Pb content is 0.50% or less.
  • Sn 0 to 1.00%
  • Sn is an element that increases the Mg elution rate in the plating layer containing Zn, Al, and Mg. As the elution rate of Mg increases, the corrosion resistance of the flat surface portion deteriorates. Therefore, the Sn content is preferably 1.00% or less.
  • Cu 0 to 1.00%
  • Ni 0 to 1.00%
  • Mn 0 to 1.00%
  • the content of one or more of Cu, Ni, Ti, and Mn is preferably 0.01% or more.
  • the content of these elements becomes excessive, the viscosity of the plating bath increases, which often makes it difficult to build the plating bath itself, and it is not possible to manufacture a plated steel sheet having good plating properties. Therefore, it is preferable that the content of each element is 1.00% or less.
  • the chemical composition of the plating layer is measured by the following method. First, an acid solution obtained by exfoliating and dissolving the plating layer with an acid containing an inhibitor that suppresses corrosion of the base iron (steel material) is obtained. Next, the chemical composition of the plating layer can be obtained by measuring the obtained acid solution by ICP analysis.
  • the acid type is not particularly limited as long as it is an acid that can dissolve the plating layer.
  • the chemical composition is measured as an average chemical composition.
  • the plated layer of the plated steel sheet according to the present embodiment contains a lamella structure in which the (Al—Zn) phase and the MgZn 2 phase are arranged in a layer on the surface, and the area ratio thereof is 10 to 10 or more. It is 95%. Further, in the plated layer of the plated steel sheet according to the present embodiment, the area ratio of (Al—Zn) dendrite is 10% or less on the surface. (For comparison, FIG.
  • FIG. 1 shows a microstructure photograph of the surface of the plating layer of a conventional galvanized steel sheet.
  • the lamellar structure in which the (Al—Zn) phase and the MgZn 2 phase are arranged in layers has high corrosion resistance, and the (Al—Zn) phase and the MgZn 2 phase are arranged in layers on the surface. It was found that the corrosion resistance is improved by forming the lamellar structure at a certain area ratio or more.
  • the area ratio of the lamella structure in which the (Al—Zn) phase and the MgZn 2 phase, which contribute to the improvement of the corrosion resistance after coating, are arranged in a layer on the surface of the plating layer is determined. 10% or more.
  • the area ratio of the lamella structure on the surface is preferably 60% or more, more preferably 80% or more.
  • the lamella structure has the effect of improving not only corrosion resistance after painting but also LME resistance.
  • the area ratio of the lamella tissue is set to 95% or less.
  • the area ratio of the (Al—Zn) dendrite that reduces the corrosion resistance after painting is set to 10% or less on the surface.
  • the area ratio of the (Al—Zn) dendrite is preferably small, and may be 0%.
  • the lamellar structure is a structure in which the (Al—Zn) phase and the MgZn 2 phase are arranged in layers, and the finer the lamellar spacing is, the better the corrosion resistance and LME resistance after coating are. The effect will be greater.
  • the lamella spacing at which sufficient performance can be obtained is 2.5 ⁇ m or less (2500 nm or less), preferably 500 nm or less.
  • Such lamellar tissue is also called feather-like tissue.
  • the ternary eutectic structure of Zn / Al / MgZn 2 is composed of Zn phase: 45 to 60%, MgZn 2 phase: 35 to 45%, and Al phase: 3 to 10% in area%, whereas it has a lamellar structure. Is a structure in which the fraction of each phase is% in area, Zn phase: 0 to 10%, MgZn 2 phase: 40 to 65%, and Al phase: 30 to 45%.
  • a massive MgZn 2 phase and a ternary eutectic structure of Zn / Al / MgZn 2 are used as phases other than the above-mentioned lamella structure and (Al—Zn) dendrite.
  • Other intermetallic compounds may be included.
  • the total of the balance is preferably 50% or less, more preferably 30% or less.
  • the massive MgZn 2 phase contributes to the improvement of corrosion resistance after painting.
  • the area ratio of the massive MgZn 2 phase is 5% or more.
  • the area ratio of the MgZn 2 -phase is preferably 40% or less.
  • the area ratio of the ternary eutectic structure of Zn / Al / MgZn 2 is preferably 45% or less. If the area ratio of the ternary eutectic structure of Zn / Al / MgZn 2 exceeds 45%, there is a concern that the corrosion resistance after painting may decrease. Further, since the MgSi 2 -phase and other intermetallic compounds reduce the corrosion resistance after coating, the area ratio is preferably 10.0% or less. More preferably, the total area ratio is 10.0% or less. Examples of other intermetallic compound phases include CaZn 11 phase, Al 2 CaSi 2 phase, and Al 2 CaZn 2 phase.
  • the structure of the plating layer (area ratio of each phase, lamella spacing) is measured by the following method. From the plated steel sheet according to the present embodiment, a sample having a size of 25 mm in the direction perpendicular to the rolling direction ⁇ 15 mm in the rolling direction was taken, embedded in a resin so that the surface of the plating layer of this sample became an observation surface, and polished. After that, an SEM image and an element distribution image by EDS are obtained.
  • the area ratio of the lamella structure, massive MgZn 2 -phase, Zn / Al / MgZn 2 ternary eutectic structure, (Al—Zn) dendrite, and other intermetallic compounds in the plating layer differs from the cross-sectional EDS mapping image of the plating layer.
  • a total of 5 fields (magnification 1500 times: 60 ⁇ m ⁇ 50 ⁇ m / 1 field) are photographed from 5 samples in each field, and calculated from the images.
  • the (Al—Zn) phase composed of Al and Zn and the MgZn 2 phase are arranged in layers and the lamella spacing is 4 ⁇ m or less, it is judged to have a lamella structure, and the (Al—Zn) phase and the MgZn 2 phase are combined. If the minor axis of the (Al—Zn) phase or MgZn 2 phase is more than 4 ⁇ m, it is judged that they are (Al—Zn) dendrite and massive MgZn 2 phase, respectively, and the lamella spacing is 4 ⁇ m or less.
  • the lamellar spacing of the lamellar tissue is determined by measuring the spacing from the SEM observation to the adjacent phase for the phase having the lowest area ratio among the phases forming the lamellar tissue, and calculating the average of the 10 points.
  • the area ratios of the Zn phase, the MgZn 2 -phase, and the Al phase constituting the lamellar structure and the Zn / Al / MgZn 2 ternary eutectic structure are determined by using image processing software or the like for the region where the structure exists on the cross-sectional SEM image. It can be obtained by enclosing with a line using and calculating the area of the area surrounded by the line.
  • the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (IV).
  • (I) Annealing step of reducing and annealing the steel plate (II) Plating step of immersing the steel plate in a plating bath containing Al, Mg, Zn, and one or two of La and Ce to obtain a plating original plate (III).
  • the plated original plate is subjected to an average cooling rate of 15 ° C./sec or higher at (Al-Zn / MgZn 2 binary eutectic temperature -30) ° C to (Al-Zn / MgZn 2 binary eutectic temperature -10) ° C.
  • Controlled cooling step of cooling to the cooling stop temperature (IV) After the controlled cooling step, a slow cooling step of cooling to 335 ° C or lower so that the average cooling rate to 335 ° C is 5 ° C / sec or less.
  • the steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by a known method is annealed (reduction annealing).
  • the annealing conditions may be known conditions, for example, heating to 750 to 900 ° C. in a 5% H2 - N2 gas atmosphere with a dew point of ⁇ 10 ° C. or higher and holding for 30 to 240 seconds.
  • the plating step in the temperature lowering process after annealing, the steel sheet is immersed in a plating bath to form a plating layer, and the plating base plate is obtained.
  • the plating bath is mass%, Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: 0.0001 to 0.5000%, Si: 0 to 2.00%, Ca: It is preferable that it contains 0 to 2.00% and the balance is Zn and impurities. Further, Fe, Sb, Sr, Pb, Sn, Cu, Ti, Ni and Mn may be contained as required. Since the composition of the plating bath is substantially the same as the composition of the plating layer to be formed, it may be adjusted according to the chemical composition of the plating layer to be obtained.
  • Control cooling process In the controlled cooling step, the plating original plate (pulled up from the plating bath) after the plating step is cooled after adjusting the amount of plating adhesion with a wiping gas such as N2 .
  • a wiping gas such as N2 .
  • Al-Zn / MgZn 2 binary eutectic temperature -30 ° C to (Al-Zn / MgZn 2 binary eutectic temperature -10) so that the average cooling rate is 15 ° C./sec or more. Cool to a cooling stop temperature of ° C.
  • the temperature is higher than (Al—Zn / MgZn binary eutectic temperature ⁇ 10) ° C.
  • (Al—Mg) the condition that the ⁇ phase and the MgZn 2 phase are eutectic solidified cannot be satisfied, and as a result, (Al—Mg). It causes a lot of dendrites.
  • the upper limit of the average cooling rate does not have to be limited, but the average cooling rate may be 40 ° C./sec or less due to restrictions such as equipment.
  • the Al—Zn / MgZn binary eutectic temperature can be obtained from, for example, a liquid phase projection drawing of a Zn—Al—Mg ternary system.
  • slow cooling process In the slow cooling step, the plating original plate after the control cooling is stopped is cooled to 335 ° C or lower so that the average cooling rate up to 335 ° C is 5 ° C / sec or less.
  • the nucleation of the lamella structure formed in the controlled cooling step grows, and the area ratio of the predetermined lamella structure is obtained on the surface.
  • the average cooling rate up to 335 ° C is more than 5 ° C / sec, the nuclear growth becomes insufficient and the area ratio of the lamella tissue becomes insufficient.
  • the plated steel sheet according to the present embodiment can be obtained.
  • a cold-rolled steel sheet (0.2% C-2.0% Si-2.3% Mn) having a plate thickness of 1.6 mm was prepared. After cutting this steel sheet into 100 mm ⁇ 200 mm, annealing and hot-dip plating were continuously performed using a batch-type hot-dip plating test device. Annealing was performed at 860 ° C. for 120 seconds in an atmosphere containing 5% H 2 gas and the balance consisting of N 2 gas in a furnace having an oxygen concentration of 20 ppm or less and having a dew point of 0 ° C. rice field.
  • the steel sheet After annealing, the steel sheet was air-cooled with N2 gas, and when the steel sheet temperature reached + 20 ° C., it was immersed in a plating bath having a bath temperature shown in Table 1 for about 3 seconds. After adjusting the adhesion amount of plating to 40 to 80 g / m 2 with N2 gas on the plating original plate on which the plating layer was formed, controlled cooling and slow cooling were performed under the conditions shown in Table 2 to cool to room temperature.
  • the controlled cooling shutdown temperature was in the range of (Al—Zn / MgZn 2 binary eutectic temperature ⁇ 30) ° C. to (Al—Zn / MgZn 2 binary eutectic temperature ⁇ 10) ° C.
  • the temperature of the steel sheet was measured using a thermocouple spot-welded to the center of the original plating plate.
  • the composition of the formed plating layer was as shown in Table 1.
  • the rest of Table 1 was Zn and 5.0% or less impurities.
  • the area ratio of each phase contained in the plated layer and the lamella spacing of the lamella structure were measured by the following methods. A sample having a size of 25 mm in the direction perpendicular to the rolling direction ⁇ 15 mm in the rolling direction was taken from the obtained plated steel sheet, embedded in a resin so that the surface of the plating layer of this sample became an observation surface, and polished. An SEM image and an element distribution image by EDS were obtained.
  • the area ratio of the lamella structure, massive MgZn 2 -phase, Zn / Al / MgZn 2 ternary eutectic structure, (Al—Zn) dendrite, and other intermetallic compounds in the plating layer differs from the cross-sectional EDS mapping image of the plating layer.
  • a total of 5 fields (magnification 1500 times: 60 ⁇ m ⁇ 50 ⁇ m / 1 field) were photographed from 5 samples in each field, and calculated from the images.
  • the lamellar spacing of the lamellar tissue was determined by measuring the spacing from the SEM observation to the adjacent phase for the phase having the lowest area ratio among the phases forming the lamellar tissue, and calculating the average of the 10 points. ..
  • the area was%, and the structure was composed of Zn phase: 0 to 10%, MgZn 2 phase: 40 to 65%, and Al phase: 30 to 45%.
  • the corrosion resistance of the obtained plated steel sheet after painting was evaluated. Specifically, a sample of 50 ⁇ 100 mm is taken from a plated steel sheet, treated with Zn phosphoric acid (SD5350 system: a standard manufactured by Nippon Paint Industrial Coding Co., Ltd.), and then electrodeposition coating (PN110 Powernicks (registered trademark)). ) Gray: A standard manufactured by Nippon Paint Industrial Coding Co., Ltd.) was carried out so that the thickness was 20 ⁇ m, and the baking was performed at a baking temperature of 150 ° C. for 20 minutes.
  • Zn phosphoric acid SD5350 system: a standard manufactured by Nippon Paint Industrial Coding Co., Ltd.
  • electrodeposition coating PN110 Powernicks (registered trademark)
  • Gray A standard manufactured by Nippon Paint Industrial Coding Co., Ltd.
  • the steel sheet was subjected to V-bending using a mold having a radius of curvature of 10 mm at 60 °, and after bending back, it was further immersed in a 5% NaAl aqueous solution at 50 ° C. for 500 hours only on the surface subjected to V-bending.
  • a tape peeling test was conducted in which the adhesive tape was attached and instantly peeled off, and the ratio of the area where the coating was peeled off to the area where the adhesive tape was attached was determined and evaluated as follows. (evaluation) B: Peeling area ratio 25% or more A: Peeling area ratio less than 15 to 25% AA: Peeling area ratio less than 10 to 15% AAA: Peeling area ratio less than 10%
  • the obtained steel sheet was spot-welded under the following conditions, the cross section of the welded portion was observed, and the length of the crack (LME crack) was evaluated. That is, No. 1 described in the table.
  • Two plated steel plates 1 to 30 are stacked, and the current-carrying electrodes are pressed so that the striking angle is 7 ° and the load is 400 kgf, and the current pattern is set to a nugget diameter of 3.5 x ⁇ t to 5.5 x.
  • Spot welding was performed with the setting set to ⁇ t (t: plate thickness).
  • a DR6 ⁇ type Cu—Cr electrode according to the JIS standard was used as the current-carrying electrode.
  • No. 1 which is a comparative example. 1, 4, 6, 11, 18, 23, 25, and 30 were inferior in corrosion resistance after coating because the chemical composition of the plating layer and the composition of the structure on the surface of the plating layer were outside the scope of the present invention.
  • the present invention it is possible to provide a plated steel material having superior corrosion resistance after painting as compared with the conventional galvanized steel sheet for automobiles, and it is possible to contribute to the development of the industry by extending the life of the galvanized steel sheet for automobiles.

Abstract

このめっき鋼板は、鋼板と、前記鋼板の表面の少なくとも一部に形成されためっき層とを有し、前記めっき層の化学組成が、質量％で、Ａｌ：６．００～３５．００％、Ｍｇ：３．００～１５．００％、Ｌａ＋Ｃｅ：合計で０．０００１～０．５０００％、及びＺｎを含有し、前記めっき層は、表面において、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織の面積率が１０～９５％であり、前記ラメラ組織のラメラ間隔が、２．５μｍ以下であり、(Ａｌ－Ｚｎ)デンドライトの面積率が１０％以下である。

Description

めっき鋼板
　本発明はめっき鋼板に関する。
　本願は、２０２０年１０月２０日に、日本に出願された特願２０２０－１７５７８６号に基づき優先権を主張し、その内容をここに援用する。
　近年、自動車構造部材には、防錆の観点からめっき鋼板が使用され、主に日本国内市場では合金化溶融亜鉛めっき鋼板等の溶融亜鉛めっき鋼板が適用されている。合金化溶融亜鉛めっき鋼板は、鋼板に溶融亜鉛めっきを施した後に合金化熱処理し、めっき層内に鋼板（下地鋼板）からＦｅを拡散させることによって、溶接性や塗装後耐食性を向上させためっき鋼板である。しかしながら、溶融亜鉛めっき鋼板に対しては、塗装後耐食性や耐赤錆性などの更なる耐食性の向上が求められている。
　溶融亜鉛めっき鋼板の耐食性を向上させる方法として、Ｚｎ系めっきへのＡｌの添加が挙げられる。例えば建材分野では高耐食性めっき鋼板として溶融Ａｌ－Ｚｎ系めっき鋼板が広く実用化されている。こうした溶融Ａｌ－Ｚｎ系めっきのめっき層は、溶融状態から最初に晶出したデンドライト状のα－(Ｚｎ，Ａｌ)相（Ａｌ初晶部：Ａｌ－Ｚｎ系二元状態図等において、初晶として晶出するα－（Ｚｎ，Ａｌ）相。必ずしもＡｌリッチな相ではなく、ＺｎとＡｌの固溶体として晶出。）と、デンドライト状のＡｌ初晶部の隙間に形成したＺｎ相とＡｌ相とからなる組織（Ｚｎ／Ａｌ混相組織）から形成される。Ａｌ初晶部は不動態化しており、かつ、Ｚｎ／Ａｌ混相組織はＡｌ初晶部に比べＺｎ濃度が高いので、腐食はＺｎ／Ａｌ混相組織に集中する。結果として、腐食はＺｎ／Ａｌ混相組織を虫食い状に進行し、腐食進行経路が複雑になるので、腐食が容易に下地鋼板に到達しにくくなる。これにより、溶融Ａｌ－Ｚｎ系めっき鋼板は、めっき層の厚みが同一の溶融亜鉛めっき鋼板に比べ優れた耐食性を有する。
　このような溶融Ａｌ－Ｚｎ系めっき鋼板を自動車外板パネルとして使用する場合、めっき鋼板は連続式溶融めっき設備でめっきが施された状態で自動車メーカー等に供され、そこでパネル部品形状に加工された後に化成処理、さらに電着塗装、中塗り塗装、上塗り塗装の自動車用総合塗装が施されることが一般的である。しかしながら、溶融Ａｌ－Ｚｎ系めっき鋼板を用いた外板パネルは、塗膜に損傷が生じた際、上述したＡｌ初晶部とＺｎ／Ａｌ混相組織の二相から成る独特なめっき相構造に起因して、傷部を起点にＺｎの優先溶解（Ｚｎ／Ａｌ混相組織の選択腐食）が塗膜／めっき界面で発生する。これが塗装健全部の奥深くに向けて進行して大きな塗膜膨れを起こす結果、十分な耐食性（塗装後耐食性）を確保できないという課題がある。
　耐食性向上を目的に、Ａｌ－Ｚｎ系めっきへさらにＭｇ等の元素を添加することも検討されている。しかしながら、Ｍｇを添加しても溶融Ａｌ－Ｚｎ系めっき鋼板には依然として不動態皮膜を有するＡｌ初晶部が形成されると推測されるので、塗装を施した後、塗膜に損傷が生じたときの耐食性（塗装後耐食性）の課題は解消されていない。
　このような課題に対し、特許文献１には、塗装後耐食性に優れた溶融Ｚｎ系めっき鋼板が開示されている。特許文献１では、めっき層がＺｎ、Ａｌ、Ｍｇ及びＳｉを含有し、めっき層中に層状Ｚｎ相と層状Ａｌ相とが交互に整列したラメラ組織が面積率の合計値で５％以上含有される場合、塗装した状態での塗膜膨れが抑制されると開示されている。
　特許文献２には、質量％で、Ａｌ：２５～９０％及びＳｎ：０．０１～１０％を含有し、さらに、Ｍｇ、Ｃａ及びＳｒからなる群より選択される一種以上を合計で０．０１～１０％含有しためっき層を有することを特徴とする、溶融Ａｌ－Ｚｎ系めっき鋼板が開示されている。特許文献２では、Ｓｎによって、上述したα－Ａｌ相の周囲に形成されるＡｌ酸化膜が破壊され、α－Ａｌ相の溶解性が上がるので、α－Ａｌ相とＺｎリッチ相との両方が溶解するめっき層の均一腐食が起こることで、Ｚｎリッチ相の選択腐食を抑制でき、塗装後耐食性が向上すると開示されている。
　また、特許文献３には、めっき層最表面に占める〔Ａｌ／Ｚｎ／Ｚｎ２Ｍｇの三元共晶組織〕の割合が６０面積％以上である溶融Ｚｎ－Ａｌ－Ｍｇ合金めっき鋼板を基材とし、めっき層表面が化成皮膜で覆われた化成処理鋼板が開示されている。
　また、特許文献４には、Ａｌ：０．１８～５％を含有し、さらに、Ｍｇ：０．０１～０．５％、Ｌａ：０．００１～０．５％、および、Ｃｅ：０．００１～０．５％のうちのいずれか１種または２種以上を含有し、残部がＺｎからなるＺｎ－Ａｌ系合金めっきを有する亜鉛系合金めっき鋼板が開示されている。特許文献４では、Ｚｎ－Ａｌ－Ｍｇ系、Ｚｎ－Ａｌ－Ｍｇ－Ｓｉ系などの亜鉛系合金めっきは、従来のＺｎ系めっきよりも耐食性が高いことが示されている。
　自動車用部品は、水が溜まる環境で使用される場合があり、そのような厳しい環境でも十分な耐食性を有することが求められる。本発明者らが検討した結果、Ａｌを一定量以上含むＺｎ－Ａｌ－Ｍｇ系めっきは、優れた犠牲防食性を有する一方、水が溜まる環境では、塗膜剥離が生じ易く、塗装後耐食性に課題があることが分かった。しかしながら、特許文献１～４では、このような厳しい環境での塗装後耐食性については考慮されていなかった。
　また、特許文献１の技術では、組織制御のために複雑な熱履歴の処理を施す必要があり、製造コストが増加するという課題もある。
　また、特許文献２のめっき鋼板では、自動車用電着塗膜との密着性が劣る。また、特許文献２ではＳｎ添加を必須とするので、合金コストが増加し、さらにはめっき浴の管理が難しくなるという課題もある。
　また、特許文献３では、化成皮膜の構成を制御することで耐食性の向上を図っている。また、化成皮膜との反応性を高めるため、めっき層についてはＡｌ／Ｚｎ／Ｚｎ２Ｍｇの三元共晶組織を主相としている。そのため、通常の化成処理を行う場合には、化成処理性は向上するものの、塗装後耐食性が十分に得られないと考えられる。
　従って、従来、近年の自動車構造部材に求められる、十分な塗装後耐食性を確保できる溶融亜鉛系めっき鋼板は提案されていなかった。
日本国特許第６３５０７８０号公報 日本国特開２０１５－２１４７４７号公報 日本国特許第４５７９７１５号公報 日本国特開２００６－２４９５２１号公報
　本発明は上記の課題に鑑みてなされた。本発明は、溶融亜鉛系めっき鋼板を前提として、塗装後耐食性に優れるめっき鋼板を提供することを目的とする。
　本発明者らは、Ａｌ、Ｍｇ及びＺｎを含むめっき層を有する鋼板（Ｚｎ－Ａｌ－Ｍｇ合金めっき鋼板）において、塗装後耐食性を向上させるための検討を行った。
　その結果、めっき層にＬａ及び／またはＣｅを含有させ、めっき層の表面において、めっき組織を制御することで、塗装後耐食性が向上することを新たに見出した。具体的には、めっき層の表面において、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とのラメラ組織の面積率を大きくするとともに、(Ａｌ－Ｚｎ)デンドライトの面積率を小さくすることで、塗装後耐食性が向上することを見出した。
　本発明は上記の知見に基づいてなされたものであり、その要旨は以下の通りである。
［１］本発明の一態様に係るめっき鋼板は、鋼板と、前記鋼板の表面の少なくとも一部に形成されためっき層とを有し、前記めっき層の化学組成が、質量％で、Ａｌ：６．００～３５．００％、Ｍｇ：３．００～１５．００％、Ｌａ＋Ｃｅ：合計で０．０００１～０．５０００％、Ｓｉ：０～２．００％、Ｃａ：０～２．００％、Ｆｅ：０～２．００％、Ｓｂ：０～０．５０％、Ｓｒ：０～０．５０％、Ｐｂ：０～０．５０％、Ｓｎ：０～１．００％、Ｃｕ：０～１．００％、Ｔｉ：０～１．００％、Ｎｉ：０～１．００％、及びＭｎ：０～１．００％、を含有し、残部がＺｎ及び不純物からなり、前記めっき層は、表面において、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織の面積率が１０～９５％であり、前記ラメラ組織のラメラ間隔が、２．５μｍ以下であり、(Ａｌ－Ｚｎ)デンドライトの面積率が１０％以下である、めっき鋼板。
［２］上記［１］に記載のめっき鋼板では、前記めっき層の前記化学組成が、質量％で、Ａｌ：１１．００～３０．００％、Ｍｇ：５．００～１１．００％、Ｌａ＋Ｃｅ：合計で０．００１０～０．１０００％、の１種以上を含有してもよい。
［３］上記［１］または［２］に記載のめっき鋼板では、前記めっき層は、前記表面において、前記ラメラ組織の面積率が６０～９５％であってもよい。
［４］上記［３］に記載のめっき鋼板では、前記めっき層は、前記表面において、前記ラメラ組織の面積率が８０～９５％であってもよい。
　本発明の上記態様によれば、塗装後耐食性に優れるめっき鋼板を提供することができる。
比較例であるＮｏ．２３のめっき層の表面の組織写真である。 発明例であるＮｏ．１９のめっき層の表面の組織写真である。 発明例であるＮｏ．１９のめっき層の表面の組織写真である。
　本発明の一実施形態に係るめっき鋼板（本実施形態に係るめっき鋼板）は、鋼板と、鋼板の表面の少なくとも一部に形成されためっき層とを有する。また、本実施形態に係るめっき鋼板は、めっき層の化学組成が、質量％で、Ａｌ：６．００～３５．００％、Ｍｇ：３．００～１５．００％、Ｌａ＋Ｃｅ：合計で０．０００１～０．５０００％、Ｓｉ：０～２．００％、Ｃａ：０～２．００％、Ｆｅ：０～２．００％、Ｓｂ：０～０．５０％、Ｓｒ：０～０．５０％、Ｐｂ：０～０．５０％、Ｓｎ：０～１．００％、Ｃｕ：０～１．００％、Ｔｉ：０～１．００％、Ｎｉ：０～１．００％、及びＭｎ：０～１．００％を含有し、残部がＺｎ及び不純物からなる。また、本実施形態に係るめっき鋼板のめっき層は、表面において、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織の面積率が１０～９５％であり、前記ラメラ組織のラメラ間隔が、２．５μｍ以下であり、(Ａｌ－Ｚｎ)デンドライトの面積率が１０％以下である。
＜鋼板＞
　本実施形態に係るめっき鋼板はめっき層が重要であり、鋼板の種類については特に限定されない。適用される製品や要求される強度や板厚等によって決定すればよい。例えば、ＪＩＳ　Ｇ３１９３：２００８に記載された熱延鋼板やＪＩＳ　Ｇ３１４１：２０１７に記載された冷延鋼板を用いることができる。
＜めっき層＞
　本実施形態に係るめっき鋼板では、鋼板の表面の少なくとも一部にめっき層を備える。めっき層は鋼板の片面に形成されていてもよく、両面に形成されていてもよい。
　めっき層の付着量は、１５～２５０ｇ／ｍ２が好ましい。
［化学組成］
　本実施形態に係るめっき鋼板のめっき層の化学組成について説明する。各元素の含有量の％は、質量％を意味する。また、「～」を挟んで示される数値範囲は、その両端の値を、上下限として含む。
Ａｌ：６．００～３５．００％
　Ａｌは、アルミニウム（Ａｌ）、亜鉛（Ｚｎ）、マグネシウム（Ｍｇ）を含むめっき層において、塗装後耐食性を確保するために有効な元素である。また、本実施形態に係るめっき鋼板のめっき層において、ラメラ組織を形成するために必要な元素である。また、Ａｌは、合金層（Ａｌ－Ｆｅ合金層）の形成に寄与し、めっき密着性を確保するために有効な元素でもある。上記効果を十分に得るため、Ａｌ含有量を６．００％以上とする。Ａｌ含有量は、好ましくは１１．００％以上である。
　一方、Ａｌ含有量が３５．００％超であると、(Ａｌ－Ｚｎ)デンドライトの面積率が高くなり、塗装後耐食性やめっき層の切断端面の耐食性が低下する。そのため、Ａｌ含有量は３５．００％以下とする。好ましくは３０．００％以下である。
Ｍｇ：３．００～１５．００％
　Ｍｇは、めっき層の塗装後耐食性を高める効果を有する元素である。また、本実施形態に係るめっき鋼板のめっき層において、ラメラ組織を形成するために必要な元素である。上記効果を十分に得るため、Ｍｇ含有量を３．００％以上とする。Ｍｇ含有量は、好ましくは５．００％以上である。
　一方、Ｍｇ含有量が１５．００％超であると、ラメラ組織が十分に形成されず、塗装後耐食性が低下する上、めっき層の加工性が低下する。また、めっき浴のドロス発生量が増大する等、製造上の問題が生じる。そのため、Ｍｇ含有量を１５．００％以下とする。Ｍｇ含有量は、好ましくは１１．００％以下である。
Ｌａ＋Ｃｅ：合計で０．０００１～０．５０００％
　Ｌａ及びＣｅは、めっき層中でラメラ組織を安定化するために有効な元素である。めっき層にＬａ、Ｃｅを含有させなくても、めっき層の内部において表層部以外の領域では、ラメラ組織を形成することができる場合がある。しかしながら、ＬａとＣｅの合計含有量が０．０００１％未満では表層部にラメラ組織が形成されない。このため、めっき層の表面におけるラメラ組織の面積率を十分に確保できない。
　一方、ＬａとＣｅの合計含有量が０．５０００％を超えると、めっき浴の粘性が上昇し、めっき浴の建浴そのものが困難となることが多く、めっき性状が良好なめっき鋼材を製造できない。そのため、ＬａとＣｅの合計含有量を０．５０００％以下とすることが好ましい。
Ｓｉ：０～２．００％
　Ｓｉは、Ｍｇとともに化合物を形成して、塗装後耐食性の向上に寄与する元素である。また、Ｓｉは、鋼板上にめっき層を形成するにあたり、鋼板とめっき層との間に形成される合金層が過剰に厚く形成されることを抑制して、鋼板とめっき層との密着性を高める効果を有する元素でもある。そのため含有させてもよい。上記効果を得る場合、Ｓｉ含有量を０．１０％以上とすることが好ましい。より好ましくは０．２０％以上である。
　一方、Ｓｉ含有量を２．００％超にすると、めっき層中に過剰なＳｉが晶出し、また、ラメラ組織が十分に形成されず、塗装後耐食性が低下する。また、めっき層の加工性も低下する。従って、Ｓｉ含有量を２．００％以下とする。Ｓｉ含有量は、より好ましくは１．５０％以下である。Ｓｉは必ずしも含有させる必要はなく、含有量の下限は０％である。
Ｃａ：０～２．００％
　Ｃａがめっき層中に含有されると、Ｍｇ含有量の増加に伴ってめっき操業時に形成されやすいドロスの形成量が減少し、めっき製造性が向上する。そのため、Ｃａを含有させてもよい。Ｃａは必ずしも含有させる必要はなく、下限は０％であるが、上記効果を得る場合、Ｃａ含有量を０．０３％以上とすることが好ましく、０．１０％以上とすることがより好ましい。
　一方、Ｃａ含有量が多いとラメラ組織が十分に形成されず、さらにはＣａＺｎ１１相をはじめとしたＣａ系金属間化合物がその他の金属間化合物相として面積率で１０％以上生成し、塗装後耐食性が低下する。また、めっき層の平面部の塗装後耐食性そのものが劣化する傾向にあり、溶接部周囲の耐食性も劣化することがある。そのため、Ｃａ含有量は２．００％以下とする。Ｃａ含有量は、好ましくは１．００％以下である。
Ｆｅ：０～２．００％
　Ｆｅはめっき層を製造する際に、不純物としてめっき層に混入する。Ｆｅは、２．００％程度まで含有されることがあるが、この範囲であれば本実施形態に係るめっき鋼板の特性への悪影響は小さい。そのため、Ｆｅ含有量を２．００％以下とすることが好ましい。より好ましくは１．５０％以下、さらに好ましくは１．００％以下である。
　一方、上述の通り、Ｆｅは不純物としてめっき層に混入する。Ｆｅの混入を完全に防ぐには著しくコストがかかるので、Ｆｅ含有量を０．１０％以上としてもよい。
　本実施形態に係るめっき鋼板のめっき層の化学組成は、上記の化学組成を有し、残部がＺｎ及び不純物であることを基本とする。不純物の含有量は、５．０％以下であることが好ましく、３．０％以下であることがより好ましい。
　しかしながら、本実施形態に係るめっき鋼板のめっき層は、更にＺｎの一部に代えて、例えば、Ｓｂ、Ｐｂ、Ｃｕ、Ｓｎ、Ｔｉ、Ｓｒ、Ｎｉ、Ｍｎを以下の範囲で含んでもよい（意図的な添加であるか、不純物としての含有であるかは問わない）。これらの元素は必ずしも含まなくてもよいので含有量の下限は０％である。
Ｓｂ：０～０．５０％
Ｓｒ：０～０．５０％
Ｐｂ：０～０．５０％
　Ｓｒ、Ｓｂ、Ｐｂがめっき層中に含有されると、めっき層の外観が変化し、スパングルが形成されて、金属光沢の向上が確認される。そのため含有させてもよい。上記効果を得る場合、Ｓｂ、Ｓｒ、Ｐｂの１種以上を０．０１％以上含有させることが好ましい。一方、これらの元素の含有量が０．５０％超になると、様々な金属間化合物相が形成され、加工性および耐食性が悪化する。また、これらの元素の含有量が過剰になるとめっき浴の粘性が上昇し、めっき浴の建浴そのものが困難となることが多く、めっき性状が良好なめっき鋼板を製造できない。そのため、Ｓｒ含有量を０．５０％以下、Ｓｂ含有量を０．５０％以下、Ｐｂ含有量を０．５０％以下とすることが好ましい。
Ｓｎ：０～１．００％
　Ｓｎは、Ｚｎ、Ａｌ、Ｍｇを含むめっき層において、Ｍｇ溶出速度を上昇させる元素である。Ｍｇの溶出速度が上昇すると、平面部耐食性が悪化する。そのため、Ｓｎ含有量を１．００％以下とすることが好ましい。
Ｃｕ：０～１．００％
Ｔｉ：０～１．００％
Ｎｉ：０～１．００％
Ｍｎ：０～１．００％
　これらの元素は、耐食性の向上に寄与する元素である。そのため、含有させてもよい。上記効果を得る場合、Ｃｕ、Ｎｉ、Ｔｉ、Ｍｎの１種以上の含有量を、０．０１％以上とすることが好ましい。一方、これらの元素の含有量が過剰になるとめっき浴の粘性が上昇し、めっき浴の建浴そのものが困難となることが多く、めっき性状が良好なめっき鋼板を製造できない。そのため、各元素の含有量をそれぞれ１．００％以下とすることが好ましい。
　めっき層の化学組成は、次の方法により測定する。
　まず、地鉄（鋼材）の腐食を抑制するインヒビターを含有した酸でめっき層を剥離溶解した酸液を得る。次に、得られた酸液をＩＣＰ分析で測定することで、めっき層の化学組成を得ることができる。酸種は、めっき層を溶解できる酸であれば、特に制限はない。化学組成は、平均化学組成として測定される。 
［めっき層に含まれる組織（相）］
　本実施形態に係るめっき鋼板のめっき層は、例えば図２に示すように、表面において、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織を含み、その面積率が１０～９５％である。また、本実施形態に係るめっき鋼板のめっき層では、表面において、(Ａｌ－Ｚｎ)デンドライトの面積率が１０％以下である。（比較のため、図１に従来のめっき鋼板のめっき層の表面の組織写真を示す。）
　一般に、Ｚｎ、Ｍｇ、Ａｌを含むめっき浴に浸漬した鋼板を冷却すると、めっき層中には、初晶である（Ａｌ－Ｚｎ）デンドライトと、Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織とが生成される。この（Ａｌ－Ｚｎ）デンドライトは、耐食性が低いので、鋼板の塗装がなされた場合であっても塗膜に疵が入った場合等には、めっき層の内部で腐食が進行し、塗膜膨れが生じる。これに対し、めっき層の表面の耐食性が高ければ、塗膜に疵が入った場合であっても、めっき層の表面において腐食の進行を抑制することができる。本発明者らの検討の結果、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織は耐食性が高く、表面に（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織を一定面積率以上で形成することで、耐食性が向上することが分かった。
　そのため、本実施形態に係るめっき鋼板のめっき層では、めっき層の表面において、塗装後耐食性の向上に寄与する（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織の面積率を１０％以上とする。表面におけるラメラ組織の面積率は、６０％以上が好ましく、８０％以上がより好ましい。ラメラ組織は塗装後耐食性だけでなく、耐ＬＭＥ性も向上させる効果を有する。
　一方、上述した化学組成を前提とした場合、ラメラ組織の面積率を９５％超とするのは、工業上容易ではない。そのため、ラメラ組織の面積率を９５％以下とする。
　また、本実施形態に係るめっき鋼板のめっき層では、表面において、塗装後耐食性を低下させる(Ａｌ－Ｚｎ)デンドライトの面積率を１０％以下とする。(Ａｌ－Ｚｎ)デンドライトの面積率は少ない方が好ましく、０％であってもよい。
　ラメラ組織は、例えば図３に示すように、（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだ組織であるが、そのラメラ間隔は微細であるほど塗装後耐食性及び耐ＬＭＥ性の向上効果が大きくなる。十分な性能が得られるラメラ間隔は２．５μｍ以下（２５００ｎｍ以下）であり、好ましくは５００ｎｍ以下である。このようなラメラ組織は、羽毛状組織とも呼ばれる。
　Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織が、面積％で、Ｚｎ相：４５～６０％、ＭｇＺｎ２相：３５～４５％、Ａｌ相：３～１０％からなるのに対し、ラメラ組織は、各相の分率が、面積％で、Ｚｎ相：０～１０％、ＭｇＺｎ２相：４０～６５％、Ａｌ相：３０～４５％となる組織である。
　本実施形態に係るめっき鋼板のめっき層には、表面において、上述のラメラ組織及び(Ａｌ－Ｚｎ)デンドライト以外の相として、塊状ＭｇＺｎ２相、Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織、その他の金属間化合物を含んでもよい。残部の合計は、５０％以下であることが好ましく、３０％以下であることがより好ましい。
　塊状ＭｇＺｎ２相は塗装後耐食性の向上に寄与する。十分な効果を得る場合、塊状ＭｇＺｎ２相の面積率を５％以上とすることが好ましい。一方、加工性の点からは、ＭｇＺｎ２相の面積率は、４０％以下であることが好ましい。
　Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織の面積率は４５％以下とすることが好ましい。Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織の面積率が４５％超であると塗装後耐食性の低下が懸念される。
　また、ＭｇＳｉ２相およびその他の金属間化合物は、塗装後耐食性を低下させるので、それぞれ面積率で１０．０％以下とすることが好ましい。より好ましくは合計の面積率で１０．０％以下である。その他の金属間化合物相としては、例えばＣａＺｎ１１相、Ａｌ２ＣａＳｉ２相、Ａｌ２ＣａＺｎ２相などが挙げられる。
　めっき層の組織（各相の面積率、ラメラ間隔）については、以下の方法で測定する。
　本実施形態に係るめっき鋼板から、圧延方向に直角方向に２５ｍｍ×圧延方向に１５ｍｍのサイズのサンプルを採取し、このサンプルのめっき層の表面が観察面となるように、樹脂に埋め込み、研磨した後、ＳＥＭ像ならびにＥＤＳによる元素分布像を得る。めっき層の、ラメラ組織、塊状ＭｇＺｎ２相、Ｚｎ／Ａｌ／ＭｇＺｎ２三元共晶組織、（Ａｌ－Ｚｎ）デンドライト、その他の金属間化合物の面積率は、めっき層の断面ＥＤＳマッピング像を異なる５サンプルから、各１視野で合計５視野(倍率１５００倍：６０μｍ×５０μｍ／１視野)を撮影し、画像から算出する。
　その際、ＡｌとＺｎとから成る（Ａｌ－Ｚｎ）相とＭｇＺｎ２相とが層状に並び、ラメラ間隔が４μｍ以下であればラメラ組織と判断し、（Ａｌ－Ｚｎ）相とＭｇＺｎ２相とが隣り合っていても、（Ａｌ－Ｚｎ）相またはＭｇＺｎ２相の短径が４μｍ超であれば、それぞれを、（Ａｌ－Ｚｎ）デンドライト、塊状ＭｇＺｎ２相と判断し、ラメラ間隔４μｍ以下のＺｎ相とα相とＭｇＺｎ２とのラメラ組織であればＺｎ／Ａｌ／ＭｇＺｎ２三元共晶組織と判断し、（Ｚｎ、Ａｌ、Ｍｇ、Ｓｉ）以外の金属が相中に１０％以上含有される場合は、その他の金属間化合物であると判断する。
　また、ラメラ組織のラメラ間隔は、ＳＥＭ観察からラメラ組織を形成する相の内、最も面積率が低い相について隣接する相までの間隔を測定し、その１０か所平均を算出することで求める。
　また、ラメラ組織、Ｚｎ／Ａｌ／ＭｇＺｎ２三元共晶組織を構成するＺｎ相、ＭｇＺｎ２相、Ａｌ相の面積率は、断面ＳＥＭ像上の当該組織の存在する領域を、画像処理ソフトなどを用いて線で囲い、線で囲んだ領域の面積を算出する方法で求めることができる。
＜製造方法＞
　次に、本実施形態に係るめっき鋼板の好ましい製造方法について説明する。本実施形態に係るめっき鋼板は、製造方法によらず上記の特徴を有していればその効果は得られる。しかしながら、以下の方法によれば安定して製造できるので好ましい。
　具体的には、本実施形態に係る鋼板は、以下の工程（Ｉ）～（ＩＶ）を含む製造方法によって製造可能である。
（Ｉ）鋼板を還元焼鈍する焼鈍工程
（ＩＩ）鋼板をＡｌ、Ｍｇ、Ｚｎ、並びに、Ｌａ及びＣｅのうち１種または２種を含むめっき浴に浸漬してめっき原板とするめっき工程
（ＩＩＩ）前記めっき原板を、１５℃／秒以上の平均冷却速度で、(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－３０)℃～(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－１０)℃の冷却停止温度まで、冷却する制御冷却工程
（ＩＶ）前記制御冷却工程後、３３５℃までの平均冷却速度が５℃／秒以下となるように、３３５℃以下まで冷却する緩冷却工程
［焼鈍工程］
　焼鈍工程では、めっき工程に先立って、公知の方法で得られた鋼板（熱延鋼板または冷延鋼板）に対し、焼鈍（還元焼鈍）を行う。焼鈍条件については公知の条件でよく、例えば露点が－１０℃以上の５％Ｈ２－Ｎ２ガス雰囲気下で７５０～９００℃に加熱して、３０～２４０秒保持する。
［めっき工程］
　めっき工程では、焼鈍後の降温過程で、鋼板をめっき浴に浸漬させてめっき層を形成させて、めっき原板とする。
　めっき浴は、質量％で、Ａｌ：６．００～３５．００％、Ｍｇ：３．００～１５．００％、Ｌａ＋Ｃｅ：０．０００１～０．５０００％、Ｓｉ：０～２．００％、Ｃａ：０～２．００％を含み、残部がＺｎ及び不純物からなることが好ましい。さらに、Ｆｅ、Ｓｂ、Ｓｒ、Ｐｂ、Ｓｎ、Ｃｕ、Ｔｉ、Ｎｉ、Ｍｎを必要に応じて含んでもよい。めっき浴の組成は形成されるめっき層の組成と略同一となるので、得たいめっき層の化学組成に応じて調整すればよい。
［制御冷却工程］
　制御冷却工程では、めっき工程後の（めっき浴から引き上げた）めっき原板を、Ｎ２などのワイピングガスでめっき付着量を調整した後、冷却する。冷却に際しては、平均冷却速度が１５℃／秒以上となるように、(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－３０)℃～(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－１０)℃の冷却停止温度まで冷却する。
　上記の条件で冷却を行うことで、(Ａｌ－Ｚｎ)デンドライトの生成が抑制されるとともに、ラメラ組織の生成核が形成され、続く緩冷却工程においてラメラ組織が生成する。
　平均冷却速度が１５℃／秒未満であると、（Ａｌ－Ｚｎ）相とＭｇＺｎ２相とがラメラ組織を形成することなく、(Ａｌ－Ｚｎ)デンドライトが多量に生成し、塗装後耐食性が低下する。
　また、冷却停止温度が(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－３０)℃よりも低いと、続く緩冷却工程でラメラ組織を十分な量生成させることが困難となる。また、(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－１０)℃よりも高いと、α相とＭｇＺｎ２相とが共晶凝固する条件を満たすことができなくなり、結果として（Ａｌ－Ｍｇ）デンドライトが多く生成する原因となる。
　平均冷却速度の上限は限定する必要はないが、設備等の制約から平均冷却速度を４０℃／秒以下としてもよい。
　Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度は、例えばＺｎ－Ａｌ－Ｍｇ三元系の液相面投影図から求めることが可能である。
［緩冷却工程］
　緩冷却工程では、制御冷却停止後のめっき原板を、３３５℃までの平均冷却速度が５℃／秒以下となるように、３３５℃以下まで冷却する。
　この緩冷却工程によって、制御冷却工程で形成されたラメラ組織の生成核が成長し、表面において、所定のラメラ組織の面積率が得られる。
　３３５℃までの平均冷却速度が５℃／秒超となると、核成長が不十分となり、ラメラ組織の面積率が不十分となる。
　上記の製造方法によれば、本実施形態に係るめっき鋼板が得られる。
　焼鈍、めっきに供する鋼板として、板厚１．６ｍｍの冷延鋼板(０．２％Ｃ－２．０％Ｓｉ－２．３％Ｍｎ)を準備した。
　この鋼板を１００ｍｍ×２００ｍｍに切断した後、バッチ式の溶融めっき試験装置を用いて、焼鈍及び溶融めっきを続けて行った。
　焼鈍に際しては、酸素濃度２０ｐｐｍ以下の炉内において、Ｈ２ガスを５％含有し、残部がＮ２からなるガスからなり、露点０℃である雰囲気の下で、８６０℃で１２０秒間焼鈍を行った。
　焼鈍後、鋼板をＮ２ガスで空冷して、鋼板温度が浴温＋２０℃に到達したところで、表１に示す浴温のめっき浴に約３秒間浸漬させた。
　めっき層が形成されためっき原板に対し、Ｎ２ガスでめっきの付着量を４０～８０ｇ／ｍ２に調整した後、表２に示す条件で制御冷却及び緩冷却を行って室温まで冷却した。制御冷却停止温度は、いずれも、(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－３０)℃～(Ａｌ－Ｚｎ／ＭｇＺｎ２二元共晶温度－１０)℃の範囲内であった。
　鋼板の温度はめっき原板中心部にスポット溶接した熱電対を用いて測定した。
　形成されためっき層の組成は、表１に示す通りであった。表１の残部は、Ｚｎ及び５．０％以下の不純物であった。
　得られためっき鋼板に対し、めっき層に含まれる各相の面積率、及びラメラ組織のラメラ間隔を以下の方法で測定した。
　得られためっき鋼板をから圧延方向に直角方向に２５ｍｍ×圧延方向に１５ｍｍのサイズのサンプルを採取し、このサンプルのめっき層の表面が観察面となるように、樹脂に埋め込み、研磨した後、ＳＥＭ像ならびにＥＤＳによる元素分布像を得た。めっき層の、ラメラ組織、塊状ＭｇＺｎ２相、Ｚｎ／Ａｌ／ＭｇＺｎ２三元共晶組織、（Ａｌ－Ｚｎ）デンドライト、その他の金属間化合物の面積率は、めっき層の断面ＥＤＳマッピング像を異なる５サンプルから、各１視野で合計５視野(倍率１５００倍：６０μｍ×５０μｍ／１視野)を撮影し、画像から算出した。
　また、ラメラ組織のラメラ間隔は、ＳＥＭ観察からラメラ組織を形成する相の内、最も面積率が低い相について隣接する相までの間隔を測定し、その１０か所平均を算出することで求めた。
　ラメラ組織については、いずれも面積％で、Ｚｎ相：０～１０％、ＭｇＺｎ２相：４０～６５％、Ａｌ相：３０～４５％で構成される組織であった。
　また、得られためっき鋼板に対し、塗装後耐食性を評価した。
　具体的には、めっき鋼板から５０×１００ｍｍのサンプルを採取し、Ｚｎりん酸処理(ＳＤ５３５０システム：日本ペイント・インダストリアルコーディング社製規格)を実施し、その後、電着塗装(ＰＮ１１０パワーニックス（登録商標）グレー：日本ペイント・インダストリアルコーディング社製規格)を厚みが２０μｍになるように実施して、焼き付け温度１５０℃、２０分で焼き付けを行った。その後、鋼板を６０°、曲率半径が１０ｍｍの金型を用いたＶ曲げに供し、曲げ戻した後、さらに５０℃、５％ＮａＡｌ水溶液に５００ｈ浸漬し、Ｖ曲げ加工が施された面のみに接着テープを貼り付け瞬時に引き剥がすテープ剥離試験し、接着テープを貼り付けた面積に対し、塗装が剥離した面積の割合を求め、これによって以下のように評価した。
（評価）
Ｂ　　:剥離面積割合２５％以上
Ａ　　：剥離面積割合１５～２５％未満
ＡＡ　：剥離面積割合１０～１５％未満
ＡＡＡ：剥離面積割合１０％未満
　また、得られた鋼板に対し、以下の条件でスポット溶接を行い、溶接部の断面を観察し、亀裂（ＬＭＥ割れ）の長さで評価した。
　すなわち、表に記載のＮｏ．１～３０のめっき鋼板を２枚重ね合わせ、打角が７°、荷重が４００ｋｇｆとなるように、通電電極を押し当て、電流パターンを、ナゲット径が３．５×√t～５．５×√t（ｔ：板厚）となるよう設定してスポット溶接を行った。通電電極にはＪＩＳ規格におけるＤＲ６φ型のＣｕ－Ｃｒ電極を用いた。
　スポット溶接後、打角を設けた方向と並行に鋼板の板厚方向に切断した。切断後、機械研磨と化学研磨とにより鏡面研磨に仕上げた溶接部の断面を光学顕微鏡で観察し、コロナボンド直外部のＬＭＥ亀裂長さを測定した。
　亀裂の有無によって以下のように判断した。
（評価）
Ａ　：０．３ｍｍ以下の亀裂あり
ＡＡ：亀裂無し
　結果を表２に示す。
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
　表１～２から分かるように、発明例であるＮｏ．２、３、５、７～１０、１２～１７、１９～２２、２４、２６～２９については優れた塗装後耐食性が得られている。
　一方、比較例である、Ｎｏ．１、４、６、１１、１８、２３、２５、３０はめっき層の化学組成、めっき層の表面における組織の構成が本発明の範囲外だったため、塗装後耐食性に劣っていた。
　本発明によれば、従来の自動車用めっき鋼板よりも塗装後耐食性に優れるめっき鋼材を提供することができ、自動車用めっき鋼板の長寿命化を通して、産業の発展に貢献することができる。
　１　　（Ａｌ－Ｚｎ）相とＭｇＺｎ２相とのラメラ組織
　２　　(Ａｌ－Ｚｎ)デンドライト
　３　　Ｚｎ／Ａｌ／ＭｇＺｎ２の三元共晶組織

Claims (4)

  1. 　鋼板と、
    　前記鋼板の表面の少なくとも一部に形成されためっき層と
    を有し、
    　前記めっき層の化学組成が、質量％で、
    　　Ａｌ：６．００～３５．００％、
    　　Ｍｇ：３．００～１５．００％、
    　　Ｌａ＋Ｃｅ：合計で０．０００１～０．５０００％、
    　　Ｓｉ：０～２．００％、
    　　Ｃａ：０～２．００％、
    　　Ｆｅ：０～２．００％、
    　　Ｓｂ：０～０．５０％、
    　　Ｓｒ：０～０．５０％、
    　　Ｐｂ：０～０．５０％、
    　　Ｓｎ：０～１．００％、
    　　Ｃｕ：０～１．００％、
    　　Ｔｉ：０～１．００％、
    　　Ｎｉ：０～１．００％、及び
    　　Ｍｎ：０～１．００％、
    を含有し、残部がＺｎ及び不純物からなり、
    　前記めっき層は、表面において、
    　　（Ａｌ－Ｚｎ)相とＭｇＺｎ２相とが層状に並んだラメラ組織の面積率が１０～９５％であり、
    　　前記ラメラ組織のラメラ間隔が、２．５μｍ以下であり、
    　　(Ａｌ－Ｚｎ)デンドライトの面積率が１０％以下である
    ことを特徴とする、めっき鋼板。
  2. 　前記めっき層の前記化学組成が、質量％で、
    　　Ａｌ：１１．００～３０．００％、
    　　Ｍｇ：５．００～１１．００％、
    　　Ｌａ＋Ｃｅ：合計で０．００１０～０．１０００％、
    の１種以上を含有することを特徴とする、請求項１に記載のめっき鋼板。
  3. 　前記めっき層は、前記表面において、前記ラメラ組織の面積率が６０～９５％である
    ことを特徴とする、請求項１または２に記載のめっき鋼板。
  4. 　前記めっき層は、前記表面において、前記ラメラ組織の面積率が８０～９５％である
    ことを特徴とする、請求項３に記載のめっき鋼板。
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