Papers by Keyword: Mean Flow Stress

Abstract: When austenite is deformed within the austenite phase field, it partially transforms dynamically into ferrite. Here, plate rolling simulations were carried out on an X70 steel using rough rolling passes of 0.4 strain each. The influence of the number of roughing passes on the grain size and volume fraction of induced ferrite was determined. Up to three roughing passes applied at 1100 °C followed by 5 finishing passes at 900 °C were employed. The sample microstructures were analysed by means of metallographic techniques. Both the critical strain to the onset of dynamic transformation as well as the grain size decreased with pass number during the roughing simulations. For the finishing passes, the mean flow stresses (MFS`s) applicable to each schedule decreased when a higher number of roughing passes was applied. The volume fraction of dynamically formed ferrite retained after simulated rolling increased with the roughing pass number. This is ascribed to the increased amount of ferrite retransformed into austenite and the finer grain sizes produced during roughing. The forward transformation is considered to occur displacively while the retransformation into austenite during holding takes place by a diffusional mechanism. This indicates that both dynamic transformation (DT) and dynamic recrystallization were taking place during straining.

Abstract: Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, A_r3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium A_e3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.