FERRITE GRAIN REFINEMENT IN SEAMLESS PIPES THROUGH INTRAGRANULAR NUCLEATION ON VN

Abstract

Mill rolling process of seamless pipe was simulated with the aim of applying grain refinement through the
formation of intragranular ferrite on VN precipitated inside austenitic grains, for production of as-hot rolled
microstructures, which are currently attained only by in-line normalising. Tests were performed on V+N and
V+N(Ti) steels using two types of schedules with long and short transfer/heating times prior to sizing between
930-830°C. It was found that transfer/heating time between high temperature rolling and low temperature
sizing could be used for precipitation of VN in austenite which in turn can nucleate intragranular ferrite
grains on cooling. To facilitate the precipitation process of VN a sub-micro-addition of Ti was used which also
helps to restrain austenite grain growth during piercing and pipe forming. Subsequent V(C,N) precipitation
of the remaining vanadium in the ferrite contributes precipitation strengthening. Hot rolling followed by
intragranular ferrite formation in 0.1%V-0.015%N-0.005%Ti steel is able to develop a fine ferrite-pearlite
microstructure with an average ferrite grain size of 7um.