Strengthening mechanism of Nb and Ti complex microalloyed strip steel produced by thin slab casting and direct rolling process

The Nb and Ti complex microalloyed steels with two strengths were produced by thin slab casting and direct rolling (TSCR) process. The microstructural analyses show that both coils mainly consist of ferrites and have the same precipitation characters. The ferrites in the coil with higher yield strength present non-ploygonal morphology, finer grain size and higher dislocation density. A large number of complex star-like precipitates exist in both coils, which have the average sizes of 153142 nm and account for about 50% of the total Nb content. Ferrite grain refinement strengthening is the predominant mechanism and contributes (43-46)% to total yield strength. Precipitation strengthening effect is very weak and only contributes (4-6)% to total yield strength. Dislocation strengthening and grain refinement strengthening make coil possess higher yield strength.