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 with excellent bendability and corrosion resistance of the bent portion, and a method for manufacturing the same. [Solution] In order to achieve the above objective, the present invention comprises a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, and an interfacial alloy layer, wherein Mg2Si is present on the outer surface of the plating film, and when line analysis is performed on the cross-section of the plating film at five or more locations with a pitch of 2-5 μm along the thickness direction of the plating film, from 1 μm below the outer surface to 1 μm before the interfacial alloy layer, and with a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is 1.5 or less, and the thickness of the interfacial alloy layer is 1 μm or less.

Claims

1. A hot-dip Al-Zn plated steel sheet comprising a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, 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, 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. On the outer surface of the aforementioned plating film, Mg 2 Si exists, In the cross-section of the plating film, when line analysis is performed at five or more locations along the thickness direction of the plating film at a pitch of 2 to 5 μm, from 1 μm below the outer surface to 1 μm before the interface alloy layer, and with a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is within 1.

5. 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 hot-dip Al-Zn plated steel sheet according to claim 1, 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.

4. A method for manufacturing a hot-dip Al-Zn plated steel sheet according to claims 1 to 3, A step of forming a plating film on a base steel sheet having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder consisting of Zn, Fe, and unavoidable impurities. 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 to the steel sheet, the heating time from room temperature to the maximum temperature reached is controlled to be 10 minutes or less, the cooling time from the maximum temperature reached to 150°C is less than 2 hours, and the cooling time from 150°C to room temperature is 3 hours or more.

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