Microstructure evolution of medium carbon steel during manufacture process for non-quenched and tempered oil well tubes

The microstructure evolution of medium carbon microalloyed steel during the manufacture sub-processes of N80 grade hot rolling seamless oil well tubes was investigated by compression simulation on Gleblee 1500 thermal simulator and industrial experiments of seamless tubes production. The results show that the tested steel has good anti-coarsening ability at equalization sub-procedure. The quantitative analysis of austenite grain size has illustrated that the average austenite grain size in the steel would almost always continuously increase in the tube manufacture process till just before the stretch-reduction-diameter deformation. The most effective microstructure refinement in industrial experiment did not occur in austenite state, but mainly in austenite decomposition following the stretch-reduction-diameter process. The intra-granular ferrite formation should be very useful for effective refinement of the whole microstructure. The variation tendencies of austenite grain size in simulated and industrial experiments are nearly the same, while the optical microstructures and precipitation characterization are quite different.