Deformation Induced Ferrite Research Articles

Effect of Dissolved and Precipitated Niobium in Microalloyed Steel on Deformation Induced Ferrite Transformation (DIFT)

Hot deformation processing was designed to study the effects of niobium (Nb) on DIFT. A prestrain of 0.51 at 880 °C for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 °C for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite (DIF) obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.

Influence of Vanadium on Grain Growth of Deformation Induced Ferrite in Low Carbon Steel during Continuous Cooling Process

The effects of vanadium on the deformation induced ferrite transformation (DIFT) in low carbon steel during heavy deformation at the temperature slightly higher than Ar3 and on the coarsening of DIFT microstructure during the continuous cooling processes after deformation were investigated using a thermo‐simulator. The results show that vanadium has little effect on the volume fraction of DIFT microstructure under heavy deformation, whereas the deformation induced ferrite (DIF) grains are refined with increasing vanadium content. The steel containing a small amount of vanadium exhibits similar velocity of grain growth compared to vanadium free steel, while the vanadium remarkably inhibits grain growth in a steel containing a high amount of vanadium during the continuous cooling process. The diffusion activation energy of grain boundaries for all the tested steels is calculated and the influencing mechanism of vanadium on the grain growth during the continuous cooling process is discussed.

Thermal Stability of High Temperature Deformation Induced Ferrite in a Low Carbon Steel

A series of unidirectional compression tests for a low carbon steel were performed on a thermal simulating machine Gleeble 3 500, and the thermal stability of high temperature deformation induced ferrite (DIF) was studied systematically through the post-treatments such as isothermally hot holding time above the austenite–ferrite equilibrium transformation temperature Ae3, cooling rates and annealing temperatures after deformation. The results show that the DIF volume fraction decreases during isothermally hot holding and finally DIF disappears. The DIF grains showed rapid growth at low annealing temperatures (<200°C) and relatively stable at middle annealing temperatures (200–500°C), lastly rapid growing at high annealing temperatures (500–700°C). Compared with the microstructure of DIF with martensite in quenched specimens, the proeutectoid ferrite and pearlite appear in controlled cooled specimens at low cooling rates (i.e. 10°C/s and 1°C/s). The present experiments revealed that DIF is not stable at the high temperature and hardly conserved in hot mills.

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

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.

Influence of deformation induced ferrite transformation on grain refinement of dual phase steel

A controlled rolling process was simulated by thermomechanical simulator (Gleeble1500) to produce as-hot-rolled dual phase steel. The microstructure of as-hot-rolled dual phase steel using deformation induced ferrite (DIF) transformation could be refined to as fine as that of Intermediate Quenching (martensite as a starting microstructure). DIF transformation was influenced by austenite grain size, amount of strain and strain rate, and the grain of about 2 μm could be produced by heavy deformation of 80%. However, the ferrite growth occurred during the intercritical isothermal holding after deformation. Microalloying such as Nb, V was effective for suppressing the ferrite growth. Thus, microalloying elements that could restrain ferrite grain coarsening were required to produce fine grained dual phase steel.