Effect of reheating temperature on the microstructure and mechanical properties of hot-deformed Nb and Nb/V microalloyed high-carbon pearlite steels

Niobium is a promising alloying element for improving the mechanical properties of high-carbon steels, yet optimized processing routes are required to fully exploit its beneficial effects. Reheating temperature during hot rolling is a key parameter, and the present work investigates its effect on the microstructure and mechanical properties of Nb and Nb/V microalloyed steels. It was found that pearlite transformation shifts to a lower temperature region during continuous cooling as the reheating temperature rises from 1180 °C to 1230 °C due to the increased prior austenite grain size and decreased pearlite growth rate. This leads to an increase in pearlite volume fraction and a refinement of pearlite interlamellar spacing, with a maximum reduction of 16%, but also causes coarsening of pearlite colonies and nodules. The coarsening of colonies and nodules is weak in steels with single Nb addition. By contrast, the colony and nodule sizes increase by about 100% in the Nb/V combined-addition steel as the reheating temperature rises. In addition, the strength of single Nb-addition steels increases by about 25–40 MPa without a significant decrease in toughness as the reheating temperature rises, whereas a simultaneous reduction in strength (by about 42 MPa) and in toughness is expected in the steel with Nb/V combined addition. The relationship between microstructure, strength and toughness was analyzed to explain these results.