Nanoscale precipitation and its influence on strengthening mechanisms in an ultra-high strength low-carbon steel

Abstract

Nanoscale precipitation and its influence on the strengthening mechanisms in low-carbon ultra-high-strength steel, hot-rolled at different temperatures and subjected to various isothermal aging conditions, are studied. The steel has a yield strength of ∼1730 MPa and elongation-to-failure of ∼13% under the peak aging condition, of which ∼740 MPa is contributed by precipitation strengthening. The precipitation strengthening mechanisms, including both the shearing mechanism and the Orowan mechanism, are quantitatively analyzed based on the mechanical properties and precipitate properties determined by small-angle neutron scattering, and atom probe tomography (APT). The APT results show that the average Guinier radius of the nanoscale precipitates under the peak aging condition is 1.4 nm at a number density of 6.19 × 1023m−3. These nanoscale precipitates consist of a Cu-enriched core, in which the depletion of Fe and enrichment of Cu change monotonically towards the center of the precipitates, whereas the concentrations of Ni, Al, and Mn exhibit diffused enrichment near the precipitate-matrix interfaces. Until the peak aging, precipitation strengthening mainly arises from shearing mechanism, among which the order and modulus strengthening mechanisms play the most significant role. Beyond peak aging, the shearing mechanism is not valid and the Orowan mechanism is the dominant contributor to the increase in yield strength due to the coarsening of the precipitates. The effect of the matrix microstructure on strength is also addressed and discussed.