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Steels alloyed with high Ti addition levels present an interesting combination of high strength and formability, principally due to the high levels of precipitation hardening that can be attained. However, their mechanical properties can be highly sensitive to variations in the processing route. In this work, dilatometry tests were performed to study the effect of coiling conditions on the microstructures and hardening mechanisms of a reference Nb microalloyed steel (0.03%Nb) and two high Ti-Nb steels (0.05%Ti-0.03%Nb, 0.1%Ti-0.03%Nb). Coiling temperatures from 550 °C to 675 °C and cooling rates of 0.01 °C/s and 0.03 °C/s were considered. A significant increase in hardness was observed for the high Ti-Nb steel samples. While the grain size and dislocation hardening were similar for all steels, much higher precipitation strengthening values in the range of 69–163 MPa and 100–307 MPa were calculated for the 0.05%Ti and 0.1%Ti steels, respectively. As a consequence, high yield strength values (over 700 MPa) were estimated for coiling temperatures greater than 625 °C for the Ti10Nb3 steel. However, it was also observed that the mechanical behavior of this steel greatly depended on coiling conditions: maximum mechanical strength was achieved at 625–650 °C, while it decreased significantly for temperatures between 550 °C and 600 °C and at 675 °C. The small size and density of the precipitates detected with TEM support the large precipitation hardening effect calculated in these conditions.

In recent years, steels microalloyed with high Ti levels have received increasing attention due to the interesting combination of high strength and formability because of the dispersion of nanometric sized titanium carbides that can be formed within the matrix. However, one of the problems related to these compositions is that their performance can be highly sensitive to variations in the processing route. To study this, in this work coiling simulations were performed by dilatometry tests with a reference Nb microalloyed steel (0.03%Nb) and a high-Ti steel (0.03%Nb-0.1%Ti) using temperatures from 550 to 675oC. The mechanical behaviour of the samples was characterized using Vickers hardness. A large hardness increase was observed in the high-Ti steel samples, resulting in estimated yield strength increases between 69 and 214 MPa. This improvement in the mechanical behaviour was very dependent on the coiling temperature; the maximum hardness was observed at 625oC-650oC, while this decreased drastically for temperatures from 550oC to 600oC and at 675oC. EBSD and TEM analysis has been performed to study the contribution of microstructural constituents and precipitation to the observed mechanical behaviour.