Effect of low-temperature annealing on two-different severely cold-rolled steels

Abstract

The present work is on the effect of low-temperature annealing (or tempering) treatment of two severely cold rolled ultra-high-strength steels. Cold rolling of hot rolled steels significantly enhanced the dislocation density in martensitic and bainitic steels, leading to increase in the YS and UTS of both the steels. Further, these hot rolled high strength steels were cold rolled to about 70% reduction. It is proposed that the cold rolling process might have improved the stability of retained austenite within a range of 15 – 20 mJm–2 through a dynamic interaction process between retained austenite and dislocations during each rolling pass. While the initial passes were associated with the mechanical stability of retained austenite in the steels, the final stage was influenced by the rolling strain and heat generated during cold rolling process. The carbon partitioning at the dislocation sites too had a similar effect on increasing YS and UTS during relatively low-temperature annealing in both the steels. There was a difference in the YS values of two highly deformed steels in post annealing condition, which was due to the intrinsic hardness of the phases present in those steels, nevertheless, both the martensitic and bainitic steels had similar work hardening rates at each annealing temperature. It is also proposed that activities like carbon-dislocation pinning, recovery, and precipitation during annealing in any highly deformed steel may be governed through a similar mechanism, which can work independently irrespective of the phases present in the microstructure.