Study on the precipitation and coarsening of TiN inclusions in Ti-microalloyed steel by a modified coupling model

A modified coupling model, considering microsegregation, precipitation and growth of inclusion, and variable solute partition coefficient (k(i)), was established to investigate the precipitation and coarsening of TiN inclusion for the 0.053 wt% Ti-microalloyed steel. The influence of TiN inclusion on the steel impact toughness was revealed. Results show that the effect of phase composition on both the k(Ti) and k(N) is greater than the effect of temperature, and the k(Ti) and k(N) are in the range from 0.20 to 0.30 and from 0.28 to 0.45 during solidification, respectively. The change of the k(i) in different crystalline phases significantly affects the microsegregation and TiN precipitation. The TiN precipitation is promoted by segregation and the TiN starts to precipitate around 1754 K (corresponding to f(s) = 0.82) in the mushy zone. The predicted size of TiN inclusion is between 7.57 and 16.681im at a conventional cooling rate (1-5 K/s), which is consistent with the measured results in the slab. The square or triangle TiN inclusions with large size are harmful to the impact toughness. Many TiN inclusions can be found at the junction of the cleavage cracks. To control the TiN size and ensure the impact toughness, the cooling rate for the Ti-0.053 wt% steel solidification should be enhanced and greater than 10 K/s. Furthermore, reducing the Ti content to 0.03 wt% is a more effective way to control the TiN size. (C) 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).