TiN formation in steels depending of titanium and nitrogen

Abstract

In today’s steel automotive industry, press hardened steels meet requirements of a lower CO2 consumption and of a better safety of drivers. These steels contain titanium nitrides which can downgrade the in-use properties. From this laboratory study, with a wide range of N (20–120 ppm) combined with high Ti variations (20–110.10−3 wt.%), clear conclusions can be highlighted. Titanium content is the key parameter to drive inclusions content, more than N, as there is a high segregation of Ti in the last liquid, due to a low partition coefficient combined with a low back-diffusion coefficient. Due to this supersaturation in Ti, TiC can be formed. It does not mean that N has no effect. It has to be noticed that the density of inclusions coarser than two microns is highly dependent of the product [Ti] × [N]. Carbon is a very important factor too to take into account as the surpersaturation in Ti in presence of austenite is higher than in presence of delta-ferrite. This gives a few guidelines in the design of inclusions controlled new grades.