Grain boundary character and superplasticity of fine-grained ultra-high carbon steel : Superplasticity and its applications

The characteristics and superplasticity of the (a + 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 (a + θ) microduplex structure with equi-axed a grains of 0.4 pm in diameter and spheroidized θ particles of 0.2 μm in diameter is obtained. The a matrix exhibits a recovered structure in which most of a grain boundaries are low-angle boundaries, resulting in rather smaller elongation at 973 K. Heavy cold rolling and annealing of pearlite produces an (a + θ) microduplex structure which consists of the coarse-grain region (d α ∼ 0.4 μm) with high-angle a boundaries and the fine-grain region (d α ∼ 0.2 μm) with low-angle a boundaries. Superplasticity in this specimen is slightly better than the warm-rolled specimen. When pearlite was austenitized in the (y + θ) region, quenched and tempered at the temperature below A 1 , an (a + θ) microduplex structure in which a and θ grain sizes are nearly the same as in the warm-rolled specimen and most of a boundaries are of high-angle one is formed. Such ultra-fine a grains are formed through the recovery of the fine (a' 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 y grain size is more uniform. When cold-rolled pearlite was austenitized and air-cooled, an (a + θ) microduplex structure with high-angle a boundary is formed. However, since the a 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.