Recrystallization kinetics of microalloyed steels deformed in the intercritical region

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I.

INTRODUCTION

C O N T R O L L E D rolling involves deformation below the no-recrystallization temperature, Tn,, which leads to a microstructure of elongated (i.e., pancaked) austenite grains. I11 If the hot-rolled product is rapidly cooled, an extra degree of refinement is achieved because of the higher nucleation rates associated with higher undercooling. Rolling in the austenite range alone and without the use of accelerated cooling results in a ferritic microstructure with a yield strength of about 450 MPa (~65 ksi) in low-carbon (weldable) steels. Additional deformation in the austenite plus ferrite region can increase the yield strength by about 100 MPa (--~15 ksi), basically due to work hardening of the ferrite, I2] and it has been reported that for the best combination of strength and toughness, total strains of 30 to 50 pct should be used and a minimum of 50 pct of ferrite must be present in the microstructure.[31 Although the success of intercritical rolling relies on keeping the deformed ferrite in its work-hardened state, the kinetics of recrystallization after deformation in the austenite plus ferrite temperature range have not been investigated in detail. In the literature, there is a disproportional amount of information concerning austenite recrystallization and very little related to that of ferrite. Since the latter has a high stacking fault energy, recovery is expected to be the most important dynamic softening mechanism, although Glover and Sellars, I4'51 working with a vacuum-melted and a zone-refined iron, found that dynamic recrystallization can occur after heavy E.A. SIMIELLI, Professor, is with the Department of Metallurgy, lnstituto de Pesquisa Tecnol6gica do Estado de S~o Paulo (IPT), Cidade Universit~ia, S5o Paulo, SP 05508, Brazil. S. YUE and J.J. JONAS, Professors, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ H3A 2A7, Canada. Manuscript submitted November 9, 1990. METALLURGICAL TRANSACTIONS A

deformation at high temperatures. With regard to static softening, Talbot, t61 working with a zone-refined iron deformed at room temperature, reported that, during annealing, a very stable substructure developed and that recrystallization was completely suppressed.

II.

M A T E R I A L S AND M E T H O D S

Three steels with the chemical compositions given in Table I were used in this work. The first contains only Mn as an alloying element and was used as a reference for the other steels. The second contains Mo, which is expected to exert a solute drag effect. Niobium was added to the third steel to investigate the pinning exerted by carbonitride precipitates. The steels also contained A1 as a deoxidant and grain refiner; they were induction melted, and the ingots were rolled down to 12.7-ram-thick plates. Cylindrical samples 7.6 m m in diameter by 11.4 m m in height were machined from the plates in order to provide specimens for compression testing. The latter was carried out on an MTS machine coupled with a radiant furnace. The interrupted compression