Theoretical understanding of the “quenching and partitioning” (Q&P) process allowed developing microstructures consisting of carbon-depleted martensite and retained austenite that deliver superior mechanical properties. Most of the models describing the Q&P process are limited to systems in which carbide precipitation in martensite and decomposition of austenite to bainite are totally suppressed. However, these reactions are often unavoidable, even in low-carbon steels containing a relatively high concentration of Si and Mn. This work investigates interactions between carbon partitioning, carbide precipitation and carbide-free bainite formation during the Q&P process of a 0.3C–1.6Si–3.5Mn (wt.%) steel with non-homogenous distribution of the alloying elements. It was found that prior to the partitioning step ɛ-carbide forms in martensite. The decomposition of this carbide is required for a full completion of the carbon partitioning from martensite to austenite. Slow kinetics of decomposition of ɛ-carbide retards the carbon partitioning process. Results show that a fraction of austenite becomes stable by carbon partitioning and does not decompose to bainite. In the specimens quenched to lower temperature, a higher fraction of austenite becomes stable and consequently a lower fraction of bainite is formed.

A 0.3C-1.6Si-3.5Mn (wt%) steel was subjected to different Q&P treatments, leading to different combinations of initial martensite, bainite, secondary martensite, and retained austenite. In this study, initial martensite refers to the martensite formed during the initial quenching step and then subjected to an isothermal treatment at 400 °C; secondary martensite refers to martensite formed during quenching from 400 °C to room temperature. The yield strength of each constituent phase was determined by applying physical models to the data obtained from detailed microstructural characterization. The yield strength (uncertainty of 5%) of the Q&P microstructures was calculated by using a composite law to account for the contribution of each constituent phase. The dependence of the yield strength on the microstructural features of the Q&P microstructures was revealed by using the approach developed in this work. For example, initial martensite (which has a high yield strength and is the dominant phase in the microstructures) had the greatest effect on the yield strength of the Q&P microstructures. Furthermore, the phase fraction and dislocation density of this phase increased with decreasing quenching temperature, leading to an increase in the yield strength of the material.