Effects of Mn content and annealing temperature on the microstructure of a G-phase strengthened Fe–20Cr–Ni–Si–Ti ferritic steel

Effect of Mn content and annealing temperature on the microstructure of a classic G-phase strengthened Fe–20Cr–3Ni–3Si–1Ti ferritic steel was extensively examined using advanced electron microscopy techniques (SEM, EBSD and TEM) and theoretical predictions (thermodynamic calculation). Firstly, a ternary precipitating phase named τ (Fe₇TiSi₂) which is isotypic to the structure of α-Mn (I-43m), existing at high-temperature (∼1050 °C), was discovered for the first time. Secondly, the ferrite matrix was observed to decompose into the mixture of the martensite, austenite and σ-phase at the mid-temperature region (700–850 °C). Thirdly, the nano-sized G-phase was discovered at both the intragranular sites and the grain boundaries after aging at 550 °C, behaving a strong over-aging resistance with the increased Mn content. Finally, phase stabilities of the detected τ, σ, G, ferrite, martensite and austenite phases with different Mn content were discussed under different annealing temperatures. These findings pave a new pathway to design the microstructure of G-phase steel via optimal annealing strategies.