Effect of the Simulated and Pilot-scaled Thermomechanical Processes on the Microstructure, Precipitates and Mechanical Properties of V–N Alloyed Steel

The effect of the two-stage continuous cooling process with different finish cooling temperatures (FCTs) after 950°C high temperature finish deformation on microstructural evolution, precipitation and mechanical properties of V–N alloyed steel has been investigated under the simulated and pilot-scaled thermomechanical condition. Microstructural observation of dilatometry specimens indicates that FCT should be above 600°C to obtain microstructure composed of polygonal ferrite (PF), pearlite (P), and less than 5 vol. pct bainite (B) in steel. Based on the simulation result, three pilot-scaled cooling paths with FCTs of 950, 750 and 600°C are designed and all microstructure consists of PF + P. The decreased FCT contributes to a refinement in the effective ferrite grain size and interlaminar spacing of pearlite, coupled with a decrease in PF content. Conversely, the amount of high misorientation angle boundary (above 15°) is increased. Moreover, the lower FCT promotes the precipitation of refined nano-scaled V(C,N) particles including interphase and random precipitates. Furthermore, an optimal combination of strength, ductility and toughness has been acquired at 600°C, of which the yield strength, tensile strength, total elongation, uniform elongation, and impact energy at room temperature are 769 MPa, 935 MPa, 23 pct, 11 pct and 30 J, respectively. The negligible variation of mechanical properties after artificial ageing exhibits that the addition of 0.024 mass% N in vanadium alloyed steel is available.