Tribological behavior of coatings for continuous casting of steel

There are a large number of steel making processes in which great demands are made on the surface behavior of several components that come into direct contact with steel under various conditions. Continuous casting is mainly a heat-extraction process. The mold must rapidly transfer heat from the steel to the cooling water. In continuous casting, steel solidification starts when it comes in contact with the mold linerʹs interior surface. The key job of every mold consists in cooling the molten steel in a controlled way. The mold is a major element in the overall economics of a continuous casting plant which explains the number of innovative approaches to increase the working life (length of time during which the mold shows acceptable dimensional stability to meet the quality standards) or to satisfy the new demands to be met by the mold liners. Coating the mold inner is a firmly established practice, in particular with electrolytic surface modification treatments, to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. Several pin-on-disk tests were carried out to determine the friction and wear behavior of different coatings. The friction partner for all coatings was a K30 (WC–Co 9%) chip, the sliding speed was 10 cm/s at a temperature of 250°C and a load of 5 N. The sliding time was 200 h (720 000 revolutions for a radius of friction 16 mm). Additional tests for shorter times allowed verification of the morphological evolution of the wear track. All coatings were also evaluated using a scratch test. This test introduces stresses at the interface between the coating and the substrate as the sample is displaced at constant speed. The critical load (Lc) recorded from the scratch test translates the complex intrinsic properties of a specific coating into a very reproducible figure of great practical significance. This paper presents a tribological characterization of conventional electrolytic coatings, bare copper alloys and some new surface solutions for continuous steel casting molds.