Effect of coiling conditions on the strengthening mechanisms of Nb microalloyed steels with high Ti addition levels

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.