Surface Characteristics of Continuously Annealed Steel Sheet in Heat Treated and Pickled Conditions

Abstract

The aim of the present work was to simulate the chemical state of the surface of cold rolled steel sheet which results from some of the treatments used on a continuous annealing line (CAL, developed by Nippon Kokan). By using ESCA, GDOS, and SEM the surfaces of two different steels were characterised. One steel was a low alloyed, conventional deep drawing quality (DDQ) steel, the other a dual phase (DP) steel alloyed with manganese and silicon. The surface was studied after four different subprocesses: as received after cold rolling; after annealing and gas cooling; after water quenching; and after pickling. On annealing at high temperature in a protective atmosphere consisting of nitrogen and hydrogen the iron oxides are completely reduced. However, the elements manganese and silicon, which have high affinities to oxygen, will form oxides on the surface, distributed as discrete particles. The DP steel, which has a relatively high bulk concentration of manganese and silicon, exhibits a larger surface coverage of these oxides than the low alloyed DDQ steel. On water quenching the metallic part of the surface, consisting of iron, is reoxidised, forming a rather thick iron oxide film that almost covers the oxide islands. This thick film is unacceptable and must be removed by pickling. Simulation of the pickling process shows that the iron oxide formed during water quenching is completely removed. The surface after pickling consists of a thin iron oxide film formed during pickling and rinsing. The manganese and silicon oxide islands are not completely removed by pickling, as can be seen from the ESCA analyses and SEM. However, pickling reduces the difference in surface coverage of these oxides between the two types of steel.