Hot utility of aluminium and titanium containing steels with and without cyclic temperature oscillations

摘要

The hot ductility of alow and high Al containing steel having 0.1 percent C and 0.4 percent Mn has been examined. The steels were heated to 1350 deg C and then cooled to test temperatures in the range 1000-700 deg C at 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 determined. Cooling directly to the test temperature gave a hot ductility trough for the low Al containing steel which spanned the temperature range from the Ae_3to the Ar_3 (undf formed). Failure in the trough was intergranular ductile due to microvoid coalescence at MnS inclusions situated in the softer deformation induced ferrite phase surrounding the y grains. Ductility was good at temperatures above the Ae_3 because of dynamic recrystallisation and good below the Ar_3as large amounts of ferrite formed before deformation. Increasing the Al content 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 remained on reheating to the test temperature. The undercooling in the y range also encouraged precipitation of AIN which increased the temperature for the onset of dynamic recrystallisation. The troughs were consequently raised by 50-100 K. The single cycle caused additional AIN 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 ductility had already been seriously impaired by marked AIN precipitation. A Ti containing steel with a high Al content and a higher Mn steel with low Al content were also studied, but without undercooling or cycling. The Ti containing steel gave a very narrow trough due to (i) removal of N so that AIN could not form and (ii) a finer grain size owing to the presence of TiN particles. The finer grain size reduced the depth of the trough but also encouraged deformation induced ferrite to form so that ductility recovered at temperatures close to the Ae_3. Increasing the Mn content shifted the trough to lower temperatures in accordance with the steels' lower transformation temperatures and gave a shallower trough due to reduced precipitation of the finer MnS inclusions at the gamma grain boundaries.