Effect of composition and prior austenite deformation on the transformation characteristics of ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of ultralow-carbon microalloyed steels containing additions of B and/or Mo to enhance hardenability. Each alloy was subjected to simulated recrystallisation and non-recrystallisation rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100°C/s. The resulting transformation products were analysed using optical microscopy in conjunction with hardness measurements which, together with the dilatometry data, facilitated the construction of CCT diagrams. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element content, through to lath bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. The combined addition of B and Mo was found to be most effective in increasing steel hardenability, while B was significantly more effective than Mo as a single addition. Large plastic deformation of the prior austenite markedly enhanced PF formation, both in the base and Mo-containing steels, thus decreasing their hardenability significantly. In contrast, the steels containing B displayed only very small decreases in hardenability, resulting from the lack of sensitivity to strain in the austenite of the non-equilibrium microstructure constituents forming in the absence of PF.