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In this paper, the influence of tempering temperatures on microstructures and tensile properties of a Cr-N alloyed medium Mn martensitic steel was studied. The microstructures formed after the tempering below 400 ℃ are composed of recovered martensite as the matrix, ultrafine retained austenite (RA) and carbonitrides. The tempering at 100 ℃ led to the best combination of 2080 MPa ultrahigh ultimate tensile strength (UTS) and 15% total elongation (TE), which is attributed to the prominent strain hardening capacity caused by both the gradually release of internal stress and the pronounced austenite-to-martensite transformation. The tempering at 400 ℃ resulted in the rapid increase of yield strength (YS) by ~500 MPa due to the relief of internal tensile stress and annihilation of dislocations and the best ductility because it produced the most stable RA grains with the highest C concentration for a sustainable austenite-to-martensite transformation over the large plastic straining. The further increase of temperature to 650 ℃ caused ferrite formed, which decrease both YS and strain hardening rate, leading to the lowest UTS. Moreover, it was found that higher N content increased YS but had little influence on both UTS and TE because it mainly contributed to enhanced precipitation of carbonitrides. It is then concluded that the strength and ductility of medium Mn martensitic steel could be increased by increasing the strain hardening capacity through tailoring both the internal stress in martensite and the mechanical stability of RA via a proper tempering treatment.

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