Precipitation behaviour and strengthening mechanisms of Nb-Ti microalloyed HSLA steel produced by CSP process

Compact strip production promotes the uniform and sufficient precipitation of the second phase particles in the steel by means of rapid solidification rate and large depressions, and improves the utilisation of microalloying elements. In this article, the Nb content in compact strip production hot-rolled strip was slightly increased, resulting in a significant improvement in mechanical properties. The results showed that two types of (Ti, Nb)(C, N) were observed in the investigated steels, in particular, coarse (Ti, Nb)(C, N) precipitated in austenite, relying on the K-S orientation relationships and fine (Ti, Nb)C precipitated in ferrite, relying on the B-N orientation relationships. Combined with the thermodynamic and kinetic analyses of Ti-Nb composite precipitation, the increase of Nb content could improve Gibbs free energy, increase the precipitated nucleation rate, and promote precipitation in austenite. Thereby, more precipitation precipitated in the process of rolling, which pinned grain boundaries, and then refined the ferrite grain size from 4.9 ± 0.37 μm to 3.9 ± 0.25 μm, resulting in stronger grain strengthening. As the temperature range decreases from austenite to ferrite, the main influence on nucleation shifts from Gibbs free energy to diffusion of solute elements, and therefore, the precipitation-temperature-time curve of ferrite is C-shaped. The increase of Nb not only raises the temperature at the nose point from 630 °C to 660 °C, but also improves the relative nucleation rate by 4 times within the coiling temperature. Thereby, more and finer precipitation precipitated in the process of coiling, which improved the precipitation strengthening from 45.92 to 86.29 MPa. The combined improvement of the two strengthening mechanisms increased the yield and tensile strengths of the investigated steels from 458 and 518 MPa to 517 and 592 MPa, respectively.