Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels
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I.
INTRODUCTION
I T is well known that the deformation of austenite in the temperature range where recrystallization is retarded increases the rate of ferrite formation.~-l~ Such deformation also improves the mechanical properties of microalloyed steels which have been finish rolled at temperatures below the Ar 3, i.e., in the austenite plus ferrite, two phase range. "-15 This improvement is primarily due to the refinement of the ferrite grain size attributable to austenite pancaking; additional contributions are made by work hardening and recovery of the deformed ferrite and by the development of the {001} texture, which lowers the impact transition temperature. II The present study was undertaken with the objective of determining the effect of deformation on the isothermal austenite-to-ferrite transformation in a plain C and two microalloyed steels. Experiments were carried out under dynamic conditions in order to study the effect of strain and strain rate on the formation of ferrite during deformation. A second set was tarried out under static conditions with different prestrains and prestraining strain rates. In this way, the effect of the alloying additions on the kinetics and morphology of ferrite formation could be determined in some detail. The influence of temperature and strain rate on the ferrite microstructure was also established, and in particular, the manner in which temperature increase refines the ferrite structure.
II. MATERIALS AND EXPERIMENTAL PROCEDURE The chemical compositions of the three steels studied are listed in Table I. Compression samples 11.43 mm in height and 7.62 mm in diameter were machined from the as-reE. ESSADIQI is Research Associate, National Research Council Canada, IGM, Boucherville, PQ J4B 6Y4. J.J. JONAS is CSIRANSERC Professor of Steel Processing, Department of Metallurgical Engineering, McGill University, 3450 University Street, Montreal, PQ, Canada H3A 2A7. Manuscript submitted January 26, 1987. METALLURGICALTRANSACTIONS A
ceived plates, with their longitudinal axes parallel to the rolling direction. Specimens were first normalized at 1000 ~ to eliminate the rolling texture. They were subsequently annealed for 30 minutes at 1030 (plain C), 1060 (Mo steel), and 1100 (Mo-Nb-V steel) ~ and then quenched. This treatment ensured the dissolution of the Nb(CN) and V(CN) particles present at room temperature and produced an austenite grain size of about 100/.J,m. 16'17 Each sample was then reheated into the austenite region for 5 minutes (930 ~ for the plain C and Mo steels, and 1030 ~ for the Mo-Nb-V steel) and cooled to the test temperature at about 1~ At the completion of the latter procedure, each of the three steels exhibited a grain size of around 25/zm. Isothermal compression tests were carried out on an Instron machine modified for constant strain rate testing.18 A first series of tests was conducted at a strain rate of 7.4 x 10 -4 s -1 in the temperature range from 790 to 840 ~ for the plain C and Mo steels and from 800 to 880 ~ for the Mo-Nb-V steel. A prestrain o
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