2019 Volume 83 Issue 10 Pages 353-362
The changes in the states of carbon (C) together with hardness and tensile properties of low C steel (0.045C-0.34Mn in mass％) quenched from 710℃ and aged at 50℃ were investigated as a function of aging time using TEM and atom probe tomography. Vickers hardness commences to increase at about 1.1 × 104 s, exhibits significant increase at 5.8 × 104 s (16 h) and keeps peak hardness till 8.6 × 105 s (10 d) followed by decrease after further aging time. In the beginning of peak aging, C clusters form having irregular shape close to a sphere with the diameter of about 10 nm. The number of C atoms is about 700 atoms, and the C content is in the range of 1 - 2 at％ at 1.0 × 105 s (28 h), where enrichment of elements except for C is not observed. Meanwhile in the end of peak aging, the precipitates with the plate shape (about 1 nm in width and 12 nm in length) having the C content of more than 10 at％ are distributed with the {100} habit plane. The transition from C clusters to fine carbides was confirmed. Lower yield strength (LYS) is the lowest for the specimen with solute C, and significantly increases for the specimen with C clusters and fine carbides in this order. LYS is inferred to be determined by the cutting mechanism for C cluster specimen and presumably Orowan mechanism for fine carbide specimen. The work hardening for solute C specimen and C cluster specimen is high, while carbide specimen shows less work hardening. C cluster is postulated to be decomposed into solute C by being sheared by dislocations, causing work hardening and relatively good uniform elongation. Post uniform elongation (l-El) was the lowest for C cluster specimen followed by fine carbide specimen with the same strength level. This is because dynamic strain aging caused by solute C promotes the strain localization leading to the deterioration in l-El.
