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

摘要

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.OC). After heavy warm rolling of pearlite, an (α + θ) microduplex structure with equi-axed α grains of 0.4μm to diameter and spheroidized θ particles of 0.2 μm in diameter is obtained. The α matrix exhibits a recovered structure in which most α grain boundaries are low-angle boundaries, resulting in rather smaller elongation at 973K. Heavy cold rolling and annealing of pearlite producess an (α+θ) microduplex structure which consists of the coarse-grain region (d{sub}α ～0.4 μm) with high-angles boundaries and the fine-grain region ({sub}α～0.2 μm) with low-angles 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 A{sub}1, an (α+θ) microduplex structure in which α and θ grain sizes tire nearly the same as in the warm-rolled specimen and most of α 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. 2μm), 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.