Abstract
This study investigated the effects of surface conditions on phosphatability for Si- and Mn-containing high-strength cold-rolled steel sheets. Si-, Mn-, and Fe-containing oxides of approximately 3 μm form on the steel surface after annealing. During 3 s pickling using nitric acid, these oxides are partially removed, exposing the steel surface. Additional pickling (6 s pickling time) mostly removes Si-, Mn-, and Fe-containing oxides, but Fe-based oxides of 50 nm maximum thickness are formed on the steel surface. These observations suggest that Si-, Mn- and Fe-containing oxides (about 3 μm thickness) and Fe-based oxides (about 50 nm thickness) interfere with the steel dissolution in the phosphate solution. Results demonstrate the importance of controlling the Fe-based oxide formation as well as that of Si- and Mn-containing oxides to obtain good phosphatability.
