Effect of low-temperature isothermal holding on microstructure and mechanical properties of hot rolled high carbon Nb microalloyed steel plate

Two high carbon steels, one plain carbon (HC0) and the other, Nb microalloyed (HC1) are selected in the present research. Both experimental steels were initially hot rolled after complete homogenisation and then air cooled to room temperature. These hot rolled plates were then subjected to low temperature isothermal holding followed by air-cooling to room temperature. Isothermal holding above the martensite start temperature (Ms) results in the formation of bainite with some amount of high carbon martensite whereas, isothermal holding below Ms leads to the formation of predominantly martensite. The increase in isothermal holding temperature from 200°C to 300°C results in lowering the hardness, estimated yield strength and ultimate tensile strength values marginally for HC0 whereas, it decreases the hardness, estimated yield strength and ultimate tensile strength values significantly for HC1 steel which may be attributed to the expected formation of some amount of coarse niobium carbide (NbC).