Technical points of alloyed hot-dip galvanized steel sheet

Alloyed hot-dip galvanized steel sheet (GA sheet) has excellent performance in terms of stamping formability, corrosion resistance, paintability and weldability. It meets the development requirements of automobile lightweight and strong safety, which has increasingly caused the automobile industry Attention.
The alloyed hot-dip galvanized steel sheet was developed by the United States in the early 1960s. After its development has experienced a low period, in recent years, with the increase of anti-corrosion requirements, it has attracted people’s attention again. This is because in order to meet the anti-corrosion requirements of the car body, the quality of the coating must be controlled above 40g/m2, and the production cost of electro-galvanized sheets increases sharply with the increase of the thickness of the coating. Therefore, how to ensure the corrosion resistance of the steel surface without excessively increasing production costs has become an important issue. Research work has shown that if the steel sheet is immediately put into an alloying annealing furnace for alloying annealing after hot-dip galvanizing in a continuous galvanizing line, an iron-zinc alloy layer with an iron content of 10% can be obtained. The electrode potential of the iron-zinc alloy layer is still lower than the iron of the substrate, but higher than that of the pure zinc layer, which reduces the corrosion rate of the coating, increases the life span, and improves the corrosion resistance. In addition, the welding performance of alloyed galvanized steel sheet is also better than that of general galvanized steel sheet.

At present, the main technical points of concern in the development of GA steel plates are:

1. Since the composition of the Zn-Fe alloy in the coating has a great influence on the final surface quality, mastering and controlling the composition of the Zn-Fe alloy in the coating is the key technology for alloying hot-dip galvanized steel. After the surface layer of the steel plate is alloyed, the alloy layer on the surface is likely to become hard and brittle, and powdery flaking occurs during processing, that is, powdering. For this reason, studies have shown that adding an appropriate amount of aluminum (above 0.10%) to the zinc solution to control the iron content in the coating between 7% and 12% will help eliminate the hard and brittle phases in the coating and improve the coating’s performance Anti-chalking ability.

2. Adopt multi-layer GA steel plate or film composite coating steel plate. In the former, in order to improve cationic electro-coating characteristics and stamping formability, a two-layer GA steel plate with an Fe-rich layer on the upper layer is used. To ensure deep drawability, IF steel is often used for the substrate. The latter manages to form a Mg-containing phosphate film on the coating layer of 30g/m2, and its corrosion resistance perforation is better than that of ordinary thin-film organic composite coating boards. Since the late 1990s, the newly developed inorganic lubricating film GA steel sheet with Mn-P composite oxide film has been put into practical use due to its low cost and similar performance.

3. Research and develop high-strength GA steel plate. In order to adapt to the lightweight of the car body, the application of high-strength steel plates is expanding. However, the GA steel plate, which is the mainstream of the car body anti-rust plate, is restricted by the composition of the steel plate. That is, the added Si, Mn, etc. are easy to oxidize due to the need for high strength. The element will be firstly oxidized externally in the heat treatment process of the continuous hot-dip galvanizing production line, thus affecting the compactness of zinc. At the same time, oxides such as SiO2 and Mn2SiO4 will be formed on the surface of the steel sheet, causing the problem of not being plated. For this reason, GA high-strength steel sheets suitable for a variety of purposes, such as coating bake hardening (BH), precipitation strengthening, and structure control (DP, TRIP), have been developed and put into practical use.