Characterization of reduced-iron/substrate interface of hot-dip galvanized TRIP steel sheet
PENG Jun1, JIN Xin-yan2,3
1. Cold Rolling Plant, Baoshan Iron and Steel Co., Ltd., Shanghai 200941, China; 2. Research Institute, Baoshan Iron and Steel Co., Ltd., Shanghai 201999, China; 3. State Key Laboratory of Development and Application Technology of Automotive Steels(Baosteel), Shanghai 201999, China
Abstract：Oxidation-reduction process is an effective way to improve the galvanizability of advanced high strength steel (AHSS). When it is used to produce hot-dip galvanized AHSS,poor coating adhesion occurs occasionally. To figure out the influence of reduced-iron/substrate interface on coating adhesion of hot-dip galvanized AHSS,two TRIP steel sheets with different coating adhesion,which were industrially produced by oxidation-reduction process,were characterized by means of GDOES,SEM,FIB and TEM. It was found that a 150-300 nm thick reduced iron layer was peeled off together with the zinc coating from the steel substrate,indicating that the coating adhesion failure occurred at the reduced-iron/substrate interface instead of the coating/reduced-iron interface. It was totally different from the common coating adhesion failure caused by the absence of Fe2Al5 inhibition layer at the coating/substrate interface. Different degree of Si and Mn enrichment was found at the reduced-iron/substrate interface for the samples with different coating adhesion,in which the samples with poor coating adhesion showed higher Si and Mn enrichment. The bonding strength of the reduced-iron/substrate interface was significantly affected by the morphology of the interface oxides. When a continuous oxidation film was formed between the reduced-iron and substrate,poor interface bonding strength might cause coating adhesion failure easily. When a small amount of fine and discontinuous oxide particles was formed between the reduced-iron and substrate,the interface bonding strength was improved. To achieve good reduced iron/substrate interface bonding strength when oxidation-reduction process is performed on hot-dip galvanized AHSS,it is still necessary to promote the formation of internal oxidation by increasing the dew point of the annealing atmosphere. Therefore,the amount and the size of the oxides at the reduced-iron/substrate interface can be reduced,and the formation of continuous oxidation film can be prevented.
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2022, Vol. 57 
