Effects of Nb addition and austenitizing temperature on the mechanical properties and microstructure of a low-carbon steel after martensite process

The effects of niobium addition (about 0.15 wt.%) and austenitizing temperature on the mechanical properties and microstructure of a low carbon steel (containing less than 0.2 wt.% carbon) after martensite process were investigated in this research. After casting and hot rolling at 1050 C, the samples were austenitized at 900, 1000, 1100 and 1200 C for one hour followed by quenching in water. The as-quenched microstructures consisted of martensite along with a small amount of widmanstatten ferrite. The maximum hardness and strength were obtained from the Nb-containing sample austenitized at 1200 °C. The hardness and tensile strength were 238 Vickers and 859 MPa, respectively. However, this sample showed an elongation of less than 1%. After 50% reduction in the thickness of the as-quenched samples by cold rolling, annealing was carried out at 600 °C for 90 min. Optical and scanning electron microscopy (SEM) showed that the microstructure of the niobium-free sample was completely annealed and the martensite process effects were eliminated. The tensile strength of this sample was approximately the same as that of the hot rolled specimen. On the other hand, the Nb-containing sample exhibited a fine-grained microstructure. In comparison with the as-quenched condition, the hardness of the microalloyed specimen was essentially preserved after martensite process. The results of the tensile test also revealed that the martensite process led to a good combination of strength and elongation in the microalloyed specimen because its tensile strength and elongation were higher than 700 MPa and 15%, respectively.