| With the requirement of reducing automobile weight for energy saving, the application of high strength steel sheets on automobile has been popularized increasingly, due to not only achieving weight reduction of automobile, but also improving the passive safety of automobile. Due to its weight reduction, passive safety and good formability, hot stamping steel has been applied more and more widely in the automobile industry. The coating of hot stamping steel prevented the surface oxidation and decarbonization of steel plate during heating. In this paper, the microstructure change of coating and effect on microstructure and properties were studied, it has important guiding significance to the development of coated hot stamping steel.The effect of annealing process and protective atmosphere on galvanizability of hot stamping steel was studied, the results showed that:the thermit reaction between effective aluminum and oxides affected the reaction of Fe-Al interface. Dew point also affected the galvanizability because it resulted in the change of partial pressure of oxygen. Annealed at 780℃ and-50℃ dew point, galvanized steel sheets had excellent quality.After Heated at temperature higher than 860℃, the quenched organization was fully martensite. In order to obtain the ideal tensile strength and hardness, the best heating process was heated at 860 to 900℃, and the holding time was 3 to 5min.The variation of GI coating microstructure in heating process was studied. After heated at 900℃, the coating was almost a-Fe(Zn), only a small amount of gamma phase presented in the surface. According to the law of diffusion, the relationship between diffusion coefficient and temperature was calculated, and the phase transition model of coating in heating process was given. Control the heating process of temperature at 870-900℃ and holding time for 2～5min ensured Al2O3 layer on coating surface, it avoided the zinc evaporation.The microstructure and properties of Al-Si coating were studied. Heated at higher temperature, the microstructure of Al-Si coating became to Fe3Al phase. The Fe3Al phase showed the plastic. According to the diffusion kinetics of Fe-Al-Si alloys and isothermal diagram, microstructure of coating after heated at different temperature was determined.Cracks formed in GI coating during the hot stamping, and the liquid zinc clearly propagated from the coating into the substrate steel with cracks. The mechanism of crack propagation was studied. Hot stamped below 782℃, the number of cracks was reduced. After hot stamping, coating also provided cathodic protection and played a good corrosion protection on the substrate. |
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