Effect of normalizing temperature on microstructure and mechanical properties of a Nb-V microalloyed large forging steel

The microstructure of a microalloyed large forging steel with different normalizing temperatures ranging from 820 °C to 940 °C were characterized. The evolution of austenite formation was determined in a range of heating temperature from 730 °C to 940 °C. The mechanical properties were evaluated by tensile test and Charpy V-notch impact test. The relationship between the microstructure and the properties was discussed. The results indicated that the microstructure composed of fine-grained layers (FGL) and coarse-grained layers (CGL) was obtained at 820 °C. The finest and most homogeneous microstructure and optimal comprehensive mechanical properties were obtained at the normalizing temperature 880 °C. There was a Hall-Petch relationship between the yield strength and the average grain size, and a linear relationship between the impact energy (KV2) and the reciprocal of the square root of the grain size (D−1/2). Both the strength and toughness of the steel can be attributed to grain refinement.