Metallurgy of Highly Wear-Resistant Indefinite-Chill Work Roll Materials for Hot Rolling Mills

Indefinite-chill materials are used as shell materials for cast work rolls for surface-critical applications in hot rolling mills. Besides a smooth surface quality, a low sticking tendency and low sensitivity against incidents in the rolling mill, the work rolls need the highest wear resistance possible. The microstructure of the indefinite-chill material consists of various carbides (cementite up to 40 area-%) and up to 5 area-% of graphite embedded in tempered martensite. To increase the wear resistance of this material group, the comparably soft cementite has to be replaced by more wear resistant carbides such as MC, M2C or M6C. This can be achieved by increasing the amount of carbide forming elements such as Nb, V, Mo, W or Cr. Nevertheless it is important to maintain a certain amount of graphite in the microstructure to avoid sticking to the rolled material and to lower the sensitivity against mill incidents. It is well known that high amounts of carbide forming elements limit the graphite precipitation and therefore a sophisticated alloying concept is required for this material type. Not only the effects of matrix elements such as Si, Mn, Ni and Co but also the effects of Cr, Mo, W, Nb and V were studied in an intensive research project. This work gives an insight in the results of the project based on the example of the effects of Si and Cr on the phase amounts and the composition of the cementite phase.