*5.2.1 Metallurgy of Q&P steels*

Q&P steels are a series of C-Si-Mn, C-Si-Mn-Al, or other similar compositions that are processed by the quenching and partitioning (Q&P) heat treatment. Q&P steels possess an excellent combination of strength and ductility with a final microstructure of ferrite (in the case of partial austenitization), martensite and retained austenite. This microstructure makes them suitable to use in the automotive industry as new-generation AHSS. They are suitable for cold stamping of various structures and safety parts having complicated shape to improve fuel economy and promoting passenger safety.

It is possible to change the amount of retained austenite at room temperature and its stability with alloying elements as carbon, manganese, nickel, etc. based on the knowledge gained by duplex stainless steels. However, it affects the cost and may be detrimental concerning the welding properties. The third generation of AHSS grades were developed to overcome these disadvantages; few of the good examples are those third-generation AHSS that are based partly on the quenching and partitioning process (Q&P steels) and on the properties of medium manganese steels. In this case, the composition of steel is not adequate for keeping the retained austenite at room temperature, but annealing, cooling, and thermal processes are optimized to change the austenite's composition and decrease its Ms temperature. For medium-Mn steels, where a relatively larger manganese amount (typically between 5 and 8 wt. %) is characteristic, the thermal treatment is slightly simplified. The intercritical annealing provides a chance to form austenite and to increase its carbon and manganese content; then the steel is cooled down to

room temperature. The complex multiphase fine-grained microstructure together with the TRIP effect arising from the progressive transformation of the retained austenite during deformation provides the excellent mechanical behavior. By these processes, the UTS above 1200 MPa and uniform elongation larger than 12% can be achieved.
