Influence of Microalloying and Isothermal Treatment on Microstructure and Mechanical Properties of High Carbon Steel

The influence of isothermal holding (IH) near the pearlite start temperature (P_s≈610 °C) and above the martensite start temperature (M_s≈245 °C) were studied in two hot-rolled (FRT≈1000 °C) high carbon steels, without (HC0) and with Nb microalloying (HC1). Optical microscopy (OM), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were utilised for observing the microstructural constituents whereas, nanoindentation and microindentation were performed to estimate the average hardness and elastic modulus values of various phase constituents. It has been observed that hot deformation in the austenite region results in the formation of a dual phase-type microstructure consisting of fine pearlite lamella and ferrite grains with some amount of degenerated pearlite. IH at 570 °C leads to a higher volume percentage of ferrite and coarsening of pearlite whereas, IH at 300 °C results in mixed phases comprising bainite, a very low amount of martensite/retained austenite (M/RA), pearlite and some amount of ferrite. Hot rolled microalloyed steel when isothermally held near the P_s results in a significant enhancement of ductility than hot rolled and air-cooled condition while sacrificing tensile strength marginally, which may be attributed to a higher percentage of ferrite and refinement of pearlite due to Nb addition. The estimated yield strength values correlate well with those experimental yield strength values, but a little discrepancy has been noticed for the ultimate tensile strength values. Possible interpretations are suggested in this paper.