Insight into austenite reversion and mechanical behavior of quenching and partitioning steel inherited hierarchical structure of martensite

In this paper, the start microstructure of full martensite and dual phases comprising ferrite and martensite were introduced into quenching and partitioning (Q&P) steel to tailor the microstructural characteristics and me-chanical behaviors, which was achieved by means of a pre-quenching treatment with various temperatures. It is revealed that the conventional Q&P sample possessed bimodal microstructure with predominantly blocky feature, while gradually changed from combination of blocky and lath structures into individually lath feature in pre-quenched Q&P samples. The reversed austenite presented different variants, which predominantly repro-duced the orientation of parent austenite and occasionally possessed twin relationship, low angle misorientation or extraneous with parent austenite. The twin-related austenite included annealing twin and reversed twin, whilst the latter showed twin relationship with both neighbored annealing twin and parent austenite. The variant type of reversed austenite significantly influenced the characteristics of its orientation relationship with neigh-bored pre-quenched martensite. The pre-quenching treatment was favorable to improve the carbon content of meta-stable austenite before Q&P treatment, while the final carbon content in retained austenite was mainly controlled by the synergistic carbon partitioning of martensite and bainite on basis of thermodynamic calcula-tion, accompanied with the partitioning of Mn and Si elements. Moreover, the pre-quenched martensite deformed coordinately in a unit of martensite packet and was beneficial to retard necking to improve ductility. The optimal mechanical properties were obtained with pre-quenching temperature near Ac3, possessing yielding strength of 623 MPa, ultimate tensile strength of 861 MPa and product of strength and elongation exceeding 28 GPa%, attributing to the positive TRIP effect originating from lath-like retained austenite with high carbon content and multi-level hierarchical microstructure inherited from pre-quenched martensite.