Deformation induced ferrite and its influence on the elevated temperature tensile flow stress–elongation curves of plain C–Mn and Nb containing steels

Abstract

The flow stress–elongation curves for a strain rate of 3 × 10⁻³ s⁻¹ have been obtained for plain C–Mn and Nb containing steels over the temperature range 600–1000°C after solution treating at 1330°C and cooling at 60 K min⁻¹ to the test temperature. The type of curve depended on whether the part of the trough at the low temperature end of the hot ductility curve was wide or narrow. Of the two phases, austenite and ferrite, ferrite independent of whether it is deformation induced or not, has been shown to have very much the better ductility. For plain C–Mn steels giving narrow trough behaviour, deformation induced ferrite (DIF) forms in equilibrium or near equilibrium amounts from the austenite, so that large amounts of DIF can form just below the Ae₃. Ductility at the low temperature end of the trough is then dependent almost entirely on the amount of ferrite present. The improvement in ductility that occurs as the temperature is decreased, can therefore be related to the Ae₃ temperature, because a large reduction in flow stress occurs close to the Ae₃ in the curve of peak stress against temperature. This DIF is believed to form almost immediately on yielding in both steels with narrow and wide trough behaviour. However, in steels showing wide trough behaviour, although DIF forms readily at close to the Ae₃, it only forms as a thin film surrounding the austenite grains. This film grows very slowly in thickness as the temperature is reduced. In this case, it is necessary for the test temperature to be below the Ar₃, so there is sufficient ferrite present prior to deformation for ductility to improve and this temperature can be recognised by a discontinuity in the curve of strain to the peak stress against temperature as well as in the curve of peak stress against the temperature. It is therefore possible from analysis of the flow curves, by plotting the peak stress against temperature and the strain to the peak stress against temperature to obtain the Ae₃ for narrow troughs and the Ar₃ for wide troughs. In contrast, to the very low strain required to produce DIF, the strain for the onset of dynamic recrystallisation (DRX) in these coarse grained steels is in excess of 20%. The temperature at which the strain to reach the peak stress for ferrite changed to that needed for DRX could also be used to determine the temperature for the onset of DRX.