Flow stress, restoration and precipation behavior and modeling for two Ti-Nb stabilized IF steels in the ferrite region

During the thermomechanical processing of steels in a conventional hot-strip mill or in a compact strip production line, the control of shape and gage, and the concomitant changes in microstructure, are essential for producing quality as-rolled steel strip or for providing good base materials for manufacturing cold-rolled and annealed steel sheet with excellent formability. The flow stress behavior, static and dynamic restoration characteristics, precipitation behavior, and Compact Strip Production (CSP) rolling simulation behavior have been investigated on two Ti-Nb stabilized IF steels in this study.; Both dynamic recrystallization and dynamic recovery contributed to the softening exhibited by the flow stress curves obtained in the austenite region. Dynamic recovery was the dominant softening mechanism in the ferrite region. The measured values of the deformation activation energies are similar to the self-diffusion energies and confirm that there is a close relationship between these two processes. The temperature compensated strain rate for the transition from dynamic recovery to dynamic recrystallization was determined as 8.23 · 1011s-1 in the austenite region for the Nb-rich Ti-Nb IF steel. The constitutive equation derived from dislocation theory fit the measured curves well. The comparison also suggests that other softening mechanisms, besides dynamic recovery, contributed to the flow stress behavior for deformation in the ferrite region.; In the ferrite region, static recovery played a very important role in the softening process of IF steel ( ∼ 40%). Dynamic recovery during deformation reduced the effect of deformation strain on the recrystallization kinetics and intensified the effect of strain rate on the recrystallization kinetics. Static recrystallization progressed slowly in the ferrite temperature range, especially for the Nb-rich Ti-Nb IF steel, and at lower temperatures. In contrast, the Nb-lean Ti-Nb IF steel recrystallized fully in 100 seconds at 800°C.; Precipitates found in the two IF steels studied were TiN, TiS, Ti 4C2S2. Ti(CN), and Nb(CN). Among them, The Ti 4C2S2 particles were randomly distributed. Ti(CN), and Nb(CN) particles were found in specimens prior to the 'first hit' deformation and their sizes increased after the 'first hit' deformation and after different holding times. Ti(CN), and Nb(CN) were found to co-precipitate with Ti4C2S2 particles and to form as separate particles. Ti4C2S2, Ti(CN), and Nb(CN) precipitates and solute Nb effectively retarded recrystallization in the ferrite region of the two IF steels studied.; An "apparent dynamic recrystallization" contributed to the flow stress reductions in the later passes, when the appropriate temperature and composition were obtained. Very fine quasi-equiaxed ferrite grains were obtained after the CSP rolling simulation and could be explained by "apparent dynamic recrystallization". The IF steel with less solute Nb exhibited a lower flow stress and easier occurrence of apparent dynamic recrystallization.; The research suggests that warm-rolled IF steel bands with adequate microstructure and mechanical properties for cold-rolling and thinner hot strip products could be manufactured by CSP technology by optimizing the processing parameters to control precipitation distribution and microstructure. (Abstract shortened by UMI.)...