Fabrication of ultrafine grains in low carbon steel by cold rolling and annealing

An ultrafine-grained (UFG) low carbon steel was fabricated by cold rolling and annealing of acicular ferrite (AF) derived from twin-roll strip casting. The steel strip with 80 % AF and 20 % grain boundary ferrite was produced by twin-roll strip casting. When the strip was cold rolled by reduction of 80 % and annealed at 550 °C for 60 min, an UFG steel with an average ferrite grain size of 0.38 μm was obtained. A quasi in-situ observation upon cold-rolled samples demonstrates that the AF lath is subdivided into some ultrafine ferrite grains with mean grain size of 0.29 μm. The ultrafine grains with the same/similar orientation tend to form a banded cluster. Extending annealing time from 60 to 120 min, the steel exhibits a bimodal microstructure composed by ultrafine grains with an average size of 0.58 μm and coarse grains with an average size of 1.97 μm. With the extension of annealing time, the yield strength (YS) of the samples decreases, while the correspond total elongation (TE) increases. Increased annealing time leads to a larger grain size, reducing the contribution of grain boundary strengthening to YS. All of samples with ultrafine grain shows the ductile fracture after tensile tests.