The present study deals with the effect of constituent phase (austenite and martensite) characteristics on the microstructure and mechanical properties of an advanced high strength steel obtained through the quenching and partitioning (Q&P) process. The thermomechanical processing route is purposefully employed to modify the microstructures, to produce better mechanical properties. The final microstructures include first and second (fresh) martensite and retained austenite with film or blocky morphology in a wide range of size. The martensite laths mainly contribute in increasing the strength, and the retained austenite positively affects the ductility. Presence of a high fraction of high angle grain boundaries in the martensite laths indicate that the prior austenite grains are fully recrystallized in the thermomechanical process preceding the Q&P treatment. Although a lower carbon content is observed in the initial martensite of the specimen that is deformed during annealing treatment before the Q&P process, nevertheless, a higher work hardenability is found, because the lower carbon content is compensated by increasing the volume fraction of retained austenite and decreasing the martensite islands average sizes. The latter also the great film-like retained austenite fraction besides the low amount of fresh martensite is found to improve the mechanical behavior significantly.

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Quenching and partitioning (Q&P) process of a 0.3C-1.3Si-3.2Mn (wt%) steel with Mn segregation is studied experimentally and theoretically. During initial quenching a higher fraction of martensite forms in Mn-poor regions compared to Mn-rich regions. In the partitioning process, austenite in Mn-poor regions is surrounded with a higher fraction of martensite than austenite in Mn-rich regions and therefore receives a larger amount of carbon. When carbon partitioning is not sufficient to stabilize austenite, a higher fraction of martensite forms, during final quenching, in Mn-poor regions. Lowering the quenching temperature in the Q&P process reduces inhomogeneity in the distribution of phases.

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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.@en

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.@en

Microstructural development during the Q&P process of a 0.3C-1.6Si-3.5Mn (wt. %) steel with an inhomogeneous chemical composition is studied by applying local EPMA and optical microscopy techniques. During the initial Q&P quenching, the fraction of formed martensite is higher in the Mn/C-poor regions than Mn/C-rich regions. Due to the lower fraction of M1, austenite in the Mn/C/Si-rich regions has lower probability to receive sufficient carbon than austenite in the Mn/C/Si-poor regions. Consequently, during the final quench higher fraction of M2 is formed in the Mn/C/Si-rich than in the Mn/C/Si-poor regions.@en