FERRITE GRAIN REFINEMENT IN SEAMLESS PIPES THROUGH INTRAGRANULAR NUCLEATION ON VN

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.