Analysis of work hardening mechanisms in Quenching and Partitioning steels combining experiments with a 3D micro-mechanical model

Quenching & Partitioning (Q & P) steels owe their good strength-ductility combinations to the martensite/ austenite (alpha'/gamma) mechanical interactions and to the formation of mechanically-induced martensite (alpha'(mech)) through the transformation-induced plasticity (TRIP) effect. An essential role is played by carbon, whose distribution among the phases can be modified through the Q & P route. This study presents a methodology to systematically and quantitatively examine the influence of the alpha'/gamma mechanical interactions on the overall work hardening of the steel with respect to the role of carbon in the martensite. The methodology rests on the generation of a 3D micro-mechanical model that allows to derive, by crystal plasticity simulations, the overall response of a mechanically-stable alpha'/gamma virtual microstructure. In combination with theoretical knowledge on hardening, the comparison between the experimental and simulated mechanical responses enables the quantification of the influence of the martensite carbon content and distribution on the overall TRIP strengthening contribution of the steel. The approach is applied to two low carbon Q & P-processed alpha'/gamma microstructures of similar initial volume fractions of austenite and alpha & PRIME;mech formation kinetics with strain, but one containing a Nbmicroaddition and displaying improved strength-ductility values. It is shown that the martensite strength and work hardening ability might additionally enhance or partially counteract the strengthening contribution from the austenite-to-alpha'(mech) transformation during uniaxial loading. The results of this study highlight that the processing-dependent properties of the carbon-depleted martensite should be considered in the optimization of Q & P processed steels.

Automated summary

This study systematically examines the influence of martensite carbon content and distribution on the overall TRIP strengthening contribution of Q & P processed steels through a combination of crystal plasticity simulations and experimental data analysis. The results show that the strength and work hardening ability of martensite may affect the strengthening contribution from the austenite-to-alpha'(mech) transformation during uniaxial loading.