HOT DUCTILITY OF ALUMINUM AND TITANIUM-CONTAINING STEELS WITH AND WITHOUT CYCLIC TEMPERATURE OSCILLATIONS

The hot ductility of a low and high Al containing steel having 0.1%C a nd 0.4%Mn has been examined, The steels were heated to 1350 degrees C and then cooled to test temperatures in the range 1000-700 degrees C a t 60 K min(-1) and strained to failure at a strain rate of 10(-3) s(-1 ) The effects on hot ductility of undercooling by 100 K before testing and giving a full single cycle with an amplitude of 100 K were also d etermined, Cooling directly to the test temperature gave a hot ductili ty trough for the low Al containing steel which spanned the temperatur e range from the Ae(3) to the Ar-3 (undeformed). Failure in the trough was intergranular ductile due to microvoid coalescence at MnS inclusi ons situated in the softer deformation induced ferrite phase surroundi ng the gamma grains. Ductility was good at temperatures above the Ae(3 ) because of dynamic recrystallisation and good below the Ar-3 as larg e amounts of ferrite formed before deformation. Increasing the Al cont ent extended the trough to higher temperatures due to precipitation of AIN which encouraged grain boundary sliding and delayed the onset of dynamic recrystallisation. Undercooling by 100 K caused ferrite to be introduced before testing for temperatures below the Ar-3, and this re mained on reheating to the test temperature. The undercooling in the g amma range also encouraged precipitation of AIN which increased the te mperature for the onset of dynamic recrystallisation, The troughs were consequently raised by 50-100 K. The single cycle caused additional A IN to be precipitated resulting in an extension of the trough for the low Al containing steel, but not in the high Al steel, since its ducti lity had already been seriously impaired by marked AIN precipitation. A Ti containing steel with a high Al content and a higher Mn steel wit h low Al content were also studied, but without undercooling or cyclin g. The Ti containing steel gave a very narrow trough due to (i) remova l of N so that AIN could not form and (ii) a finer grain size owing to the presence of Ti N particles. The finer grain size reduced the dept h of the trough but also encouraged deformation induced ferrite to for m so that ductility recovered at temperatures close to the Ae(3). Incr easing the Mn content shifted the trough to lower temperatures in acco rdance with the steels' lower transformation temperatures and gave a s hallower trough due to reduced precipitation of the finer MnS inclusio ns at the gamma grain boundaries.