Tensile properties of an acicular ferrite and martensite/austenite constituent steel with varying cooling rates

The microstructures of an acicular ferrite (AF) and martensite/austenite (M/A) constituent steel processed with cooling rate from 5 to 25 °C/s after hot deformation were characterized. The tensile properties were evaluated from the thermomechanical control process (TMCP) treated samples. The results indicate that as the cooling rate increased, AF grain is refined, M/A constituent becomes smaller and distributes more dispersively accompanied by the slight decrease of amount. The refinement of AF effective grain and the rising dislocation density could significantly induce the enhancement of yield strength (YS), and the relationship between the YS and the AF effective grain size defined with the low boundary tolerance angle (2–6°) follows the Hall–Petch approach. The increasing yield ratio (YR) shows that the strain hardening capacity reduces as a result of the microstructure evolution while increasing the cooling rate.