No abstract

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 + N2 atmosphere 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.

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.