تاثير كرنش بر ميزان پيشرفت استحاله ديناميكي آستنيت به فريت در يك فولاد ميكرو آلياژي

Effect of Strain on the Development of Strain-Induced Transformation of Austenite to Ferrite in a Microalloyed Steel

In the present research, laboratory thermomechanical processing using single pass hot compression was carried out on a low carbon Nb-Ti microalloyed steel to study the effect ofdeformation strain on the microstructural evolution. The objective is to understand how to attain a homogeneous, equiaxed and ultrafine ferrite (i.e. grain sizes < 3pm). Deformation schedules were conducted at two phase region (i.e. between Ad-ArJ). Hot compression experiments were executed at constant temperature with strains from 0.25 to J, and strain rate of 0.001s-1. The physical processes that occur during deformation were studied by analyzing the true stress- true strain hot flow curves. Examination of the microstructural evolution was carried out by optical microscopy. Results show that at the deformation temperature lower than Ar3, in two phase region (a+r), there is not any evidence of dynamic softening events, and both austenite andferrite are work hardened during the deformation. Very fine and equiaxed ferrite grains (EFG) ofabout 1.6 urn are obtained by deformation at 785 0 Cfor the strain of1. This has been attributed to the occurrence of strain-induced transformation (SIT) of austenite to ferrite. With increasing strain, EFG size reduces and its volume fraction increases. It was inferred that the amount of strain plays an important role in SIT. This is by controlling the amount ofactivation energy requiredfor SIT, when the specimen experiences the same strain rate.