Enhanced phosphatability by decorating ferrite layer on the surface of a multi-phase steel

Abstract

The poor phosphatability in the multi-phase steel is a critical issue to be solved to enhance the life of steel under the corrosion environment. The strategy of decorating the ferrite layer on the surface through decarburization annealing is applied to improve the phosphatability in the multi-phase steel. Different surface states are achieved by controlling dew points at −40, 30, and 60 °C during the annealing of cold-rolled sheets. Higher dew point results in thick external oxide on the top surface and deep decarburized ferrite layers around the sub-surface. The effect of the external oxidation and the decarburized matrix on the phosphatability is investigated by systematic analyses of the decarburized and phosphate-coated specimens. The external oxide layer on the steel surface degrades the phosphate coverage. On the other hand, more than 90% of phosphate coverage is realized with the decarburized matrix after pickling removes the external oxide layer. The enhanced phosphatability after the decarburization annealing and pickling can be understood by the improved dissolution rate of the ferrite layer. The study gives a way to overcome the intrinsic limitation of poor phosphatability in multi-phase steel.