Effect of microalloying with molybdenum and boron on the microstructure and mechanical properties of ultra-low-C Ti bearing steel

Abstract

In the present study, we have carried out controlled rolling followed by accelerated cooling to explore the microstructure and mechanical properties of Ti, Ti–Mo, and Ti–B microalloyed steels. The objective was to enhance the yield strength of Ti-bearing steel and simultaneously obtain good ductility and toughness. The microstructure of Ti and Ti–Mo steels consisted of polygonal ferrite and the effective grain size was reduced from 5.6μm in Ti-bearing steel to 4.3μm in Ti–Mo microalloyed steel, accompanied by increase in dislocation density. The microstructure of Ti–B steel was acicular ferrite with lath width in the range of ~0.2–0.4μm. The density of precipitates of 3–5nm size was high in all the three steels. Both strength and low temperature toughness were increased on microalloying with 0.09wt% Mo. In steel, containing 0.002wt% B, the yield strength was increased by ~105MPa, and high impact energy of 53.2J at −40°C was obtained. The impact energy was decreased to 14.3J at −60°C because free-B segregated to prior austenite grain boundaries and significantly deteriorated the low temperature toughness. The evolution of fracture surface with temperature was consistent with impact energy.