Doctor of Philosophy
Department of Metallurgy and Materials Engineering
Misra, Satish, Austenite recrystallisation and ferrite formation in hot rolled titanium microalloyed steels, Doctor of Philosophy thesis, Department of Metallurgy and Materials Engineering, University of Wollongong, 1986. http://ro.uow.edu.au/theses/1607
The effect of titanium on austenite recrystallisation and ferrite formation after single pass hot rolling has been investigated in low carbon 0.076 and 0.052 wt% Ti steels having Ti in excess of that for the stoichiometric TiN composition. Rolling reductions were given in the range 10-60% in temperature range 850-1100°C and the deformed samples were held at the rolling temperature up to 1800 sec in order to study the progress of recrystallisation. The deformed austenite was transformed isothermally to ferrite at 690°C for up to 1800 sec. The kinetics of both austenite recrystallisation and ferrite formation have been compared quantitatively with a C-Mn reference steel having a base composition similar to the 0.076% Ti steel.
An increase in the extent and temperature of reduction increased the rate of recrystallisation in all steels. For a starting austenite grain size (d0) of 175 μm, the 0.076% Ti steel showed a 'dual' recrystallisation behaviour, with kinetics similar to a C-Mn reference steel (d0 = 375 μm) for temperatures above 1030°C but significantly retarded kinetics for temperatures below 1030°C. The retardation in the titanium modified steel was associated with TiC precipitation and high activation energies for recrystallisation which were strain dependent. A similar retardation of recrystallisation was observed in the C-Mn steel for temperatures < 950°C and it was inferred that A1N precipitation delayed recrystallisation, in a similar way to TiC, by stabilising the deformed structure of the austenite. For similar starting grain sizes (175 μm and 186 μm), the two Ti steels showed little difference in recrystallisation kinetics.
This work is the first to show that the activation energy for recrystallisation under hot working conditions can be strain dependent. It is proposed that such strain dependence Is likely to be a characteristic of steels in which precipitation can strongly inhibit or modify nucleation of recrystallised grains.
The presence of Ti retarded both recrystallisation and grain growth, and these effects are considered to be primarily due to Ti in solution and presence of relatively coarse TiN particles at higher temperatures (>=1050°C); and at lower temperatures (< 1050°C) to the additional influence of TiC precipitation. However, the retarded recrystallisation was not observed at lower temperatures when the initial grain size decreased from 175 μm to 58 μm in the 0.076% Ti steel. It is inferred that the finer initial grain size results in recrystallisation being accelerated to the extent that it precedes precipitation of TiC.
Existing equations for predicting the recrystallisation times for the Ti steels were found to be inconsistent with the experimental data and empirical equations were developed which predicted the observed recrystallisation times within a factor of two. Equations were also developed for the C-Mn and Ti steels which related rolling strain and the Initial grain size to the recrystallised grain sizes.
The rate of ferrite formation was found to depend on the state of the austenite before transformation. Increase in deformation below the recrystallisation temperature significantly increased the ferrite nucleation rate per unit boundary area (of austenite boundaries and deformation bands); finer ferrite was obtained from partially- and non-recrystallised austenite in comparison with recrystallised austenite, as a result of the effect of higher retained strain on nucleation rate. The presence of Ti also retarded the rate of transformation and promoted the formation of polygonal ferrite.
The main contributions of the microalloying element Ti in ferrite grain refinement are considered to be (i) refinement of the starting austenite grain size, leading to a fine recrystallised grain size on hot working; and (ii) retardation of austenite recrystallisation during finish rolling at low temperatures which results in high ferrite nucleation rates on transformation from deformed austenite.