Mechanism of increased N content in controlling the M-A constituent of low-carbon Mo-V-N steel

The effect of nitrogen content on the martensite-austenite (M-A) constituent in low-carbon Mo-V-N steels produced by an isothermal transformation, γ→α+γ′ (metastable austenite), at 550 °C and air-cooling was investigated via optical microscope, transmission electron microscopy, electron probe microanalysis, and DICTRA software. An increased nitrogen content from 0.0030wt% to 0.0165 wt% decreased (increased) the carbon (nitrogen) diffusion during α→γ′; meanwhile, the amount of M-A constituent and the initial martensite transformation temperature (Ms) increased. Furthermore, the inner structure of the M-A constituent varied from twin-type M-A to lath-type M-A.