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

University of British Columbia

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. This control is, in turn, based on a better understanding of the flow stress behavior, restoration behavior, microstructure evolution, and precipitation behavior of steels during the processing. Very few studies have dealt with the deformation and microstructure evolution occurring during the hot and/or warm rolling of IF steels. 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. This was accomplished with the aid of axisymmetric compression tests, torsional rolling simulation tests, TEM observation on precipitation and substructures, and Kikuchi pattern analysis. Experimental tests were mainly carried out in the ferrite temperature range with the aim of providing guidance to the application of warm rolling of IF steels on a CSP line. The important results and conclusions of this research are as follows: (1) 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 deformation activation energies were measured to be 302kJ/mole and ~240kJ/mole for deformation in the austenite and in the ferrite regions, respectively. 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 Zener-Hollomon value, Z, 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, even though dynamic recovery is the dominant softening mechanism. (2) In the ferrite region, static recovery played a very important role in the softening process of IF steel; ~40% of the softening was attributed to static recovery. 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. This effect was strengthened by the reduced solute Nb content in one of the steels studied. Static recrystallization progressed slowly in the ferrite temperature range, especially for the Nb-rich Ti-Nb IF steel, and at lower temperatures. In constrast, the Nb-lean Ti-Nb IF steel recrystallized fully in 100 seconds at 800°C. (3) Precipitates found in the two IF steels studied were TiN, TiS, Ti₄C₂S₂, Ti(CN), and Nb(CN). Among them, The Ti₄C₂S₂ particles (~50nm in size) were randomly distributed. Ti(CN), and Nb(CN) particles(<20nm in size) were found in specimens prior to the 'first hit' deformation and their sizes increased to ~10-35nm after the 'first hit' deformation and after different holding times. It is thought that Ti₄C₂S₂ was formed both by in situ transformation from TiS and by separate formation. 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. The 5% recrystallization time increased ~30 times either by precipitate pinning or by a solute drag effect in the Nb-rich Ti-Nb IF steel, and increased 2.4 times either by precipitate pinning effect or by solute drag effects in the Nb-lean Ti-Nb IF steel. (4) Static and dynamic recovery dominated the softening process for early passes of the CSP rolling. 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". This apparent recrystallization took place, aided by dynamic recovery. The IF steel with less solute Nb exhibited a lower flow stress and easier occurrence of apparent dynamic recrystallization. (5) 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.

Thesis/Dissertation

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2009-06-30

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http://hdl.handle.net/2429/9883

Materials Engineering

University of British Columbia

UBCV

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