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The influence of various solution annealing treatments on the microstructure and mechanical properties of a new Cr–Mn austenitic stainless steel with 1.15 wt% (C + N) and 0.51 C/N ratio has been investigated. A sequential solution annealing (SSA) treatment is designed to sufficiently dissolve and lower numerous precipitates including carbides and nitrides in the austenite matrix. In this process, the hot-forged steel is firstly solution heat treated at 1120 °C for 2 h, then continuously heated to 1180 °C for 0.5 h, and further heated to 1200 °C for 0.5 h, followed by water quenching. The experimental results show that the austenite grain size increases, and number of precipitates decreases, with increasing temperature and time of solution annealing treatments. The highest impact energy is obtained after this SSA treatment due to a more homogeneous grain distribution, a smaller amount of carbides remaining on the grain boundary, and solid solution strengthening of interstitial (C + N) atoms.