Grain boundary character and superplasticity of fine-grained ultra-high carbon steel

The characteristics and superplasticity of the (α + θ) microduplex structures formed by various thermomechanical processings were studied in an ultra-high carbon steel (Fe-1.4Cr-1.0C). After heavy warm rolling of pearlite, an (α + θ) microduplex structure with equi-axed α grains of 0.4 μm in diameter and spheroidized θ particles of 0.2 μm in diameter is obtained. The α matrix exhibits a recovered structure in which most of α grain boundaries are low-angle boundaries, resulting in rather smaller elongation at 973 K. Heavy cold rolling and annealing of pearlite produces an (α + θ) microduplex structure which consists of the coarse-grain region (d_α ∼ 0.4 μm) with high-angle α boundaries and the fine-grain region (d_α ∼ 0.2 μm) with low-angle α boundaries. Superplasticity in this specimen is slightly better than the warm-rolled specimen. When pearlite was austenitized in the (γ + θ) region, quenched and tempered at the temperature below A₁, an (α + θ) microduplex structure in which α and θ grain sizes are nearly the same as in the warm-rolled specimen and most of ce boundaries are of high-angle one is formed. Such ultra-fine α grains are formed through the recovery of the fine (α′ lath martensite + θ) mixture during tempering. This microduplex structure exhibits superior superplasticity. Heavy warm rolling prior to the quenching and tempering improves total elongation further because the distribution of prior γ grain size is more uniform. When cold-rolled pearlite was austenitized and air-cooled, an (α + θ) microduplex structure with high-angle α boundary is formed. However, since the α grain size was relatively large (ca. 2 μm), its superplastic performance is poor. Finally, more simplification of processing for superplasticity was attempted. Further improvement of superplasticity was achieved by omitting the tempering in the quenching and tempering treatment.