Hot-dip Al-Zn plated steel sheet and method for manufacturing the same

Abstract

The objective is to provide a hot-dip Al-Zn plated steel sheet and a method for manufacturing the same, which are stable and have excellent bendability and corrosion resistance of the bent portion. [Solution] In order to achieve the above objective, the present invention provides a hot-dip Al-Zn plated steel sheet comprising: a plating film having a composition containing Al: 45-65 mass% and Si: 1.0-3.0 mass% with the remainder being Zn, Fe and unavoidable impurities; and an interface alloy layer formed on the interface side of the plating film with the underlying steel sheet, the interface alloy layer containing Fe, Al, Si, Zn and unavoidable impurities, wherein the plating film has dendrites mainly consisting of primary Al crystals and dendrite gaps containing Al-Zn eutectic crystals, the Si-containing phase consisting of elemental Si and / or Si compounds formed in the dendrite gaps does not extend continuously from the outermost surface of the plating film to the interface alloy layer, and the thickness of the interface alloy layer is 1 μm or less.

Claims

1. A hot-dip Al-Zn plated steel sheet comprising: a plating film having a composition consisting of Al: 45 to 65 mass%, Si: 1.0 to 3.0 mass%, and at least one selected from the group consisting of Mg, Cr, Mn, V, Mo, Ti, Ca, Ni, and Co: 10 mass or less, with the remainder being unavoidable impurities containing Zn and at least Fe; and an interface alloy layer formed on the interface side between the plating film and the underlying steel sheet, containing Fe, Al, Si, Zn, and unavoidable impurities, wherein The aforementioned plating film has dendrites mainly composed of primary Al crystals and interdendritic gaps containing Al-Zn eutectic crystals. The primary Al crystal comprises an α-Al phase matrix and Zn precipitates, wherein the Zn content in the matrix is ​​30% by mass or less. The amount of the aforementioned plating film attached is 45 g / m² per side of the steel plate. 2 More than 120g / m 2 The following: Si single crystal and Mg formed in the dendrite gap 2 Si, CrSi 2 , Mn 11 Si 9 , VSi 2 , MoSi 2 , TiSi 2 , CaSi 2 , NiSi 2 and CoSi 2 The Si-containing phase composed of at least one selected from the group consisting of does not continuously extend from the outermost surface of the plating film to the interfacial alloy layer. A hot-dip Al-Zn plated steel sheet characterized by having a thickness of 1 μm or less of the aforementioned interface alloy layer.

2. The hot-dip Al-Zn plated steel sheet according to claim 1, characterized in that the average of the maximum diameters of the Zn precipitates in the Al primary crystal (when observing the Al primary crystal with an extremely low-acceleration SEM (acceleration voltage of 3kV, magnification of 20,000 times or more) in three or more fields of view, the maximum diameters of 10 Zn-based precipitates present in each field of view are measured in descending order, and the average of these measured values) is 100 nm or more.

3. The Si compound constituting the Si-containing phase is Mg 2 A hot-dip Al-Zn plated steel sheet according to claim 1 or 2, characterized in that it is Si.

4. The hot-dip Al-Zn plated steel sheet according to claim 1 or 2, characterized in that the area ratio of the Si-containing phase observed in the cross-section in the thickness direction of the plating film is 10% or less of the entire plating film.

5. The hot-dip Al-Zn plated steel sheet according to claim 1 or 2, characterized in that the Al-Zn eutectic in the dendrite gaps does not have a stripe-like structure with a period of 2 μm or less.

6. A method for manufacturing a hot-dip Al-Zn plated steel sheet according to claim 1 or 2, A step of forming a plating film on a base steel sheet having a composition containing Al: 45 to 65 mass%, Si: 1.0 to 3.0 mass%, and at least one selected from the group consisting of Mg, Cr, Mn, V, Mo, Ti, Ca, Ni, and Co, with the remainder being unavoidable impurities containing Zn and at least Fe, The process includes, after the formation of the aforementioned plating film, a step of subjecting the steel sheet to a thermal history such that the maximum temperature reached is between 150°C and 277°C. In the process of forming the aforementioned plating film, the average cooling rate of the steel plate during the cooling period from the plating bath temperature to 300°C after immersing the base steel plate in the plating bath and removing it from the bath is controlled to 20°C / second or more. A method for manufacturing a hot-dip Al-Zn plated steel sheet, characterized in that, in the step of imparting the thermal history, the heating time of the steel sheet on which the plating film is formed is controlled to be 10 minutes or less from room temperature to the maximum temperature reached, and the cooling time from the maximum temperature reached to room temperature is 3 hours or more.

7. A method for manufacturing a hot-dip Al-Zn plated steel sheet according to claim 6, characterized in that the cooling time of the steel sheet from the highest temperature reached to room temperature is 5 hours or more.

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