Effects of niobium and heat treatment on microstructure and mechanical properties of low carbon cast steels

• The yield and ultimate tensile strength of Nb-microalloyed cast steel can be improved by 60MPa and 35MPa, respectively.
• The impact toughness of Nb-microalloyed cast steel can be improved by 25.6 % with ductility remained almost the same.
• It will be most effective in grain refining by adding 0.044 wt % Nb and the grains can be refined by 34.6 %.
• The grains will be coarsened when normalizing and tempering temperatures are higher than 900 °C and 550 °C, respectively.

The present paper describes an attempt to reveal the effects of microalloying element niobium (Nb), normalizing and tempering temperatures on microstructure, strength and especially toughness of low carbon microalloyed cast steels. Four kinds of microalloyed cast steels with different contents of niobium have been examined by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD), tensile testing and room temperature Charpy V-notch impact toughness testing. As compared to Nb-free steel, the grains in the Nb-microalloyed cast steels are refined and the average grain sizes are about 20.8 ~ 34.6% lower. Moreover, fine spherical NbC precipitates with a diameter of about 1–15 nm are formed in the Nb-microalloyed cast steels. When normalized at 900 °C and tempered at 550 °C, the yield strength (YS) and ultimate tensile strength (UTS) of the microalloyed cast steel with 0.044 wt.% of Nb are increased to 350 MPa and 520 MPa from 290 MPa and 485 MP, respectively, as compared to Nb-free cast steel. Meanwhile, the impact energy is improved by 25.6% with ductility remained almost the same.

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