Study on phosphating property and improvement measures of low carbon aluminum killed steel automobile plate
In order to solve the problem of coarse crystallization and insufficient coverage of the phosphating film on low-carbon aluminum killed steel automotive panels under the lower limit condi-tions of the phosphating process,the key influencing factors on the phosphating performance of low-carbon aluminum killed steel automotive panels were studied using scanning electron microscopy(SEM),surface profiler,glow spectrometer(GDS),and transmission electron microscopy(TEM),and improvement strategies were also proposed.The results show that when the phosphating process is at the lower limit,namely the concentration of accelerator and total acid is low,while the concentration of free acid is high,the phosphate at the interface between the steel plate and the phosphating solution is not easily saturated,and the difficulty of phosphating nucleation increases,leading to problems such as coarse phosphating crystallization and insufficient coverage.However,when there are a large number of small oxide particles evenly distributed on the surface of the steel plate,the oxide and the iron matrix can form a large number of primary batteries,which can improve the local phosphating acid etching and deposition reaction speed,and thus improve the phosphating performance.By controlling the carbon content of low carbon aluminum killed steel and the temperature of annealing rapid cooling section and over aging section,carbides with small size and evenly distributed are successfully precipi-tated on the steel plate surface.Due to the potential difference between carbide and iron matrix,the number of primary batteries on the steel plate surface is also increased,which promotes the nucleation of phosphating.The grain size of the obtained phosphating film is under 5 μm,the coverage rate is 100%.
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