Ce addition enabling superior strength and ductility combination of a low-carbon low-manganese transformation-induced plasticity steel

Abstract

Transformation-induced plasticity (TRIP) steels doped with rare earth elements have attracted great attraction in the engineering field. Here we study microstructures and strengthening mechanisms of a low-carbon low-manganese TRIP steel containing Ce by applying electron back-scattered diffraction, energy dispersive spectroscopy and spherical aberration corrected transmission electron microscopy. The effect of Ce is three-fold: First, the Ce-rich inclusion forms as Ce is segregated at grain boundaries to absorb the impurities such as Ca and S, which reduces the interface energy for driving grain growth. Thus, the average grain size is refined from 2.1 μm to 1.4 μm, contributing to the elevated yield strength. Second, the addition of Ce leads to an increased density of geometrically necessary dislocations and strengthens the steel. Besides, Ce promotes the precipitation of cementite and refines the cementite, leading to the improved precipitation strengthening. As a result, the Ce-containing low-carbon low-manganese steel shows an increase of ∼100 MPa in yield strength compared with the Ce-free TRIP steel. In particular, the Ce addition improves the stability of the retained austenite, leading to the ductility increased from 38 ± 1% to 46 ± 1%. For the first time, the Ce inclusion was characterized at the atomic scale, revealing the multilayered structure composed of five atomic layers of Ca–S–Ce–Ce–S.