Nanoscale spheroidized cementite induced ultrahigh strength-ductility combination in innovatively processed ultrafine-grained low alloy medium-carbon steel

We describe here innovative processing of low alloy medium-carbon steel with a duplex microstructure composed of nanoscale spheroidized cementite (Fe3C) in an ultrafine-grained (UFG) ferritic steel. After multi-pass rolling and intermittent annealing at 550 degrees C for 300 s, the obtained UFG-1 steel showed an average ferrite grain size of similar to 430 nm, containing nanoscale spheroidized cementite (Fe3C) particles with an average size of similar to 70 nm. On annealing at 600 degrees C for 300 s, the average size of ferritic grains was increased to similar to 680 nm and the average size of spheroidized Fe3C particles increased to similar to 90 nm, referred as UFG-2 steel. Tensile tests indicated that UFG-1 steel had high yield strength (sigma(y)) of 1260 MPa, and ultimate tensile strength (sigma(UTS)) of 1400 MPa. These values are higher than that of UFG-2 steel (sigma(y) = 1080 MPa and sigma(UTS) = 1200 MPa), suggesting that the strengthening contribution is a cumulative effect of decrease in ferrite grain size and nanoscale cementite. The incoherent interfaces between nanosized particles and the matrix acted as a strong barrier to dislocation motion. The study underscores that nanosized precipitates not only provide strength but also contribute to ductility, which is very encouraging for improving the ductility of medium-carbon steels.