Influences of Vanadium and Thermo-mechanical Treatment on Microstructures of Medium Carbon Microalloyed Steels

Abstract

The austenite recrystallization behaviors during the hot-deformation and the characteristics of the ferrite-pearlite structures transformed during continuous cooling from the hot-deformed austenite were investigated for the medium carbon steels containing 0.4% carbon with 0 to 0.3% vanadium. The results are as follows: (1)The vanadium additions inhibit the recrystallization of hot-deformed austenite and elevate the austenite recrystallization temperatures in the medium carbon steels, more effectively than those in the low carbon steels. The austenite recrystallization-stop temperature of 0.4% carbon steel added 0.3% vanadium is over 200K higher than that of 0.4% carbon steel. This is considered to be due to the increase of the strain induced precipitation of VC in deformed austenite, through the increase of the super saturation of VC with the increase in the carbon content. (2)The ferrite-pearlite structures of 0.4%C-0.3%V steels hot-deformed at non-recrystallization temperature (below 1223K) are refined remarkably, through the increase of the ferrite nucleation site consisted of the flattend austenite grain boundary and the annealing twin formed in the austenite grain. (3)0.4%C-0.3%V steels hot-rolled at non-recrystallization temperature on the practical mill have superior toughness in spite of high tensile strength over 900N/mm2 because of the microstructure refinement through the non-recrystallization austenite deformation.