Precipitation Strengthening by Nanometer-sized Carbides in Hot-rolled Ferritic Steels

The mechanical properties of the hot-rolled plates of Ti steel and Ti-Mo steel after isothermal transformation in a temperature range of 600–700 °C for 60 min have been tested, and the microstructures of the matrix and the characteristics of precipitated nanometer-sized carbides have also been examined by scanning electron microscopy and transmission electron microscopy. The precipitation regularity of nanometer-sized carbides has been studied by thermodynamic method and the contributions of corresponding strengthening mechanisms to the total yield strength have been calculated. The tensile strength of hot-rolled Ti-Mo ferritic steel can achieve 780 MPa with an elongation of 20.0% after being isothermally treated at 600 °C for 60 min, and the tensile strength of Ti steel is 605 MPa with an elongation of 22.7%, according to the results of tensile tests. The critical nucleation size of (Ti, Mo)C is smaller than that of TiC at a given isothermal temperature, but the nucleation rate of (Ti, Mo)C is larger than that of TiC. The grain-refinement strengthening and precipitation strengthening contribute the main amount of the total yield strength. The major increase in yield strength with the decrease of isothermal temperature results from the contribution of precipitation strengthening. The contribution of precipitation strengthening to the yield strength of the steels has been estimated. The ferrite phase can be strengthened by about 400 MPa through precipitation strengthening in Ti-Mo steel isothermally treated at 600 °C for 60 min, which is about 200 MPa higher than that of Ti steel under the same conditions.