Processing of niobium-containing steels by Steckel mill rolling

The Steckel mill is a reversing rolling mill in which steel strip is coiled after each rolling pass in a heated coilbox positioned on either side of the mill. In addition to the lower capital costs associated with a single stand mill in comparison to multi-stand finishing mills, the Steckel mill offers a high degree of flexibility in the steel rolling process. IPSCO has pioneered the use of the Steckel mill for the production of flat-rolled high-strength steel for linepipe and structural applications. Much of this development has been based on an Nb-microalloying strategy. Niobium plays a key role in development of a fine-grained high strength steel in a number of ways: a) Nb(C,N) precipitation in austenite retards recrystallization effectively refining grain size; b) Solute Nb acts to enhance the hardenability of the steel, promoting formation of an acicular ferrite microstructure; and c) Nb(C,N) precipitation in ferrite subsequent to transformation provides substantial precipitation strengthening. This paper will review the use of niobium in microstructure development during Steckel mill rolling. (AU)