Effect of deformation ratio and cooling rate on mechanical properties and microstructure of 0.08wt% C HSLA steel microalloyed with Nb and Mo

In this work, a low carbon steel microalloyed with Nb and Mo is thermomechanically processed. The effect of two important parameters; deformation percent and post deformation cooling rate; is studied. It was found that increasing amount of hot deformation decreases strength and increases elongation in C-Mn-Nb-Mo steel till a definite value then increases strength appreciably and decreases elongation with a less amount. This is due to the refinement of grain size. However, yield strength to ultimate strength ratio increases. Moreover, water quenched specimen had higher strength and lower ductility than air cooled ones. This is due to the smaller grain size and higher amount of hard phases like acicular ferrite and bainite. However, the effect of cooling rate weakens with increasing amount of deformation. This can be attributed to the following two reasons: (1) high amount of deformation increases nucleation sites and inhibit the chance for grains to grow even if long time is available. Thus, there is only a small difference in size between water quenched and air cooled specimens. (2) Deformation induced ferrite transformation which reduces the amount of austenite available for low temperature transformation during rapid cooling.