Influence of Composition and Thermal History on the Dynamic Recrystallisation and Subsequent Hot Ductility of Mild Steels

抄録

Torsion tests at strain rates 1×10^–3, 1.6×10^–1 and 1 s^–1 and at temperatures between 850 and 1100°C were carried out (1) on Al and Al-Ti mild steels after heating directly to the testing temperature, (2) on Al steel soaking for 30 min at 1200°C, then cooling the specimen to the testing temperature and (3) after soaking, cooling to 700°C and then heating to the testing temperature. Both steels showed a clear trough in ductility between 850 and 1050°C, the ductility being lowest for the directly heated Al treated steel and the soaked, cooled below A₁ and heated again, highest for the 1200°C soaked steel and intermediate for the directly heated Al-Ti steel. The lowest ductilities are associated with the inhibiting effect of AlN and TiN exerted on grain boundary migration and preventing the completion of dynamic recrystallisation, as shown also by their effect of increasing the peak strain and making the reaching of equilibrium dynamic recrystallised grain size difficult, which would markedly improve ductility. Soaking at 1200°C, by dissolving and/or coarsening the AlN precipitates has the marked effect of increasing ductility. The present results can be explained in terms of the drag exerted by the dispersed particles making dynamic recrystallisation and grain growth difficult.