Investigating precipitation kinetics of multi-variant ε-carbides in martensite steel manufactured by twin-roll casting process

The modified precipitation theory was employed to directly predict the multi-variant ε-carbide precipitation from thermodynamics and growing and ripening kinetics. Three distinct variants were identified: Variants 1 and 2 were the perpendicular plate-like ε-carbides, while the granular ε-carbides were Variant 3. The particle sizes of Variants 1 and 2 were usually larger than those of Variant 3. The mean aspect ratios of Variants 1 and 2 were 4.96, 4.62 and 4.35 larger than those (1.72, 1.63 and 1.56) for the granular ε-carbides when coiled at 140, 200 and 250 °C, respectively. Thermodynamic analysis indicated that Variants 1 and 2 are easier to nucleate than Variant 3. The growing kinetic analysis implied that the relative nucleation time and precipitation time for Variants 1 and 2 were about 8 and 5 orders of magnitude less than those for Variant 3, respectively. The ripening kinetics further displayed that the ripening rate was similar for Variants 1, 2 and 3. In addition, the dislocation density has weak influence on ε-carbide nucleation. These findings suggest that the precipitation thermodynamic and kinetic models could be extended to second phase precipitation in other materials systems. Besides, nano-scale ε-carbides, fine block size and nano-twins, as well as medium-density dislocations, jointly caused the optimal match between strength and total elongation when coiled at 140 °C.