Revealing the precipitation kinetics and strengthening mechanisms of a 450 MPa grade Nb-bearing HSLA steel

Abstract

This study investigates the precipitation kinetics and strengthening mechanisms of a 450 MPa grade Nb-bearing high strength low alloyed steel. The results reveal that Nb(C, N) and NbC particles with a mean size of 8.2 nm, are distributed in grain interior and grain boundaries (GBs). There are two predominant precipitation morphologies in the grain interior, namely disc-like and spherical shapes. The former precipitated in ferrite and the latter precipitated in high-temperature austenite, relying on the Baker-Nutting and Cube-Cube orientation relationships (ORs), respectively. The particle ratio of disc-like NbC precipitates varied from 1.75 to 2.36, which was consistent with the theoretical particle ratio of about 2.43. Besides, NbC or Nb(C, N) particles with a nearly disc-like morphology were found along GBs, also exhibiting the Baker-Nutting OR with ferrite. Thermodynamic calculations indicated that 39 ppm nitrogen increased the precipitation temperature by about 44 °C. Moreover, under heavy hot-rolling reduction, a significant fraction of Nb(C, N) and NbC precipitated in austenite. Precipitation kinetics analysis reveals that both the resultant heightened deformation stored energy (DSE) and trace nitrogen effectively accelerated the nucleation and growth transformation. The high DSE facilitated the strain induced precipitation in austenite, which presented a linear precipitation distribution. With the increased DSE, both the precipitation-temperature-time curve and nucleation rate time curve shifted towards higher temperature and shorter time. Eventually, NbC and Nb(C, N) particles induced strengthening contribution was estimated to be approximately 84 MPa. The main strengthening contributions in the material include grain boundary and solution strengthening.