Surface Characteristics of Continuously Annealed Steel Sheet in Overaged Condition

The overaging treatment of a continuous annealing line was simulated in the laboratory. The chemical state of the surface of low carbon steel sheet was determined by using ESCA and SEM. The surfaces of a conventional, low alloyed deep drawing quality (DDQ) steel and a dual phase (DP) steel alloyed with Mn and Si were characterised in simulated and production line conditions. The influence of the surface state of the steels on the formation of surface phosphates was investigated. During overaging in the temperature range 200–400° C in a protective H2+ N2atmosphere iron oxide is reduced to metallic iron. The alloying elements manganese and silicon are oxidised. Silicon especially is markedly enriched on the surface, in the form of pure Si02, due to its high diffusibility. The manganese and silicon oxides form particles on the surface during the overaging treatment. The results indicate that iron oxide is dissolved in the particles. Sodium and calcium are also present in their oxide states on the surfaces of the production line sheets; they originate in the rinsing waters used. Temper rolling reduces the amount of surface oxide particles to about half the prerolling value. It is suggested that this is due to the stripping of particles from the surface and the pressing of particles into the steel matrix. The phosphating experiments show that the steel sheets treated in the simulation furnace and taken from the production line form surface phosphates readily. The phosphate crystals are small, uniform, and cover the entire surface.