QUENCH & PARTITIONING RESPONSE OF A MO-ALLOYED CMNSI STEEL

Quenching & Partitioning (Q&P) has been recently proposed as a fundamentally new way of producing martensitic steels containing a considerable amount of retained austenite. It consists of a two step thermal treatment: after soaking, the steel is quenched to a predetermined temperature in the Ms-Mf range to produce a partially martensitic, partially austenitic microstructure. The second, so-called partitioning step, aims at carbon enrichment of the austenite by carbon depletion of the martensite and carbon transport to the austenite, in the absence of carbide formation. Thus, carbon stabilised austenite is retained in the microstructure after final quenching to room temperature. Remarkably high strength/ductility combinations and significant volume fractions of retained austenite are obtained via Q&P processing of a Mo-alloyed CMnSi steel. The instantaneous strain hardening behavior shows a pronounced dependency on the partitioning temperature (PT) applied. Continuously decreasing n-values (with strain) are obtained at low partitioning temperatures where increasing the PT results in flattening of the strain hardening curves. Partitioning at 450 degrees C results in positive n-value vs strain behavior similar to TRIP steels. In addition, oscillations in the strain hardening curves are observed which have been hypothesized to be linked to a different dislocation substructure of the partitioned martensite. Linear arrays of straight screw dislocations are observed after partitioning at 450 degrees C. The occurence of this dislocation substructure may be related to the retarding effect of Mo on dislocation mobility