Influence of Si and Process Parameters on the Microstructure and Properties of Continuously Annealed Low C-Nb-Ti Strip Steel

The effects of silicon (Si) addition and continuous annealing (CA) parameters on the microstructure and mechanical properties of low carbon Nb-Ti steels were investigated. Steels with and without Si were subjected to CA simulations, varying annealing temperature, line speed (LS), and cold work (CW) levels. Low-temperature thermomechanical controlled processing (TMCP) during hot rolling produced a fine polygonal ferrite matrix with uniformly distributed, spherical cementite - finer and more homogeneously dispersed in the Si-containing steel. Surface oxides in the as-rolled Si steel consisted mainly of wüstite and magnetite, with no deleterious hematite or fayalite observed due to high temperature descaling. Recrystallization during CA began near 650°C and completed above 780°C but was delayed by Si addition, higher line speeds, and moderate cold work. The final ferrite grain size remained fine, averaging 4–5 μm, across a broad annealing temperature range, aided by effective grain boundary pinning from carbonitrides. In the 690–760°C annealing range, the Si-containing steel exhibited increased strength due to solid solution strengthening, carbonitride precipitation hardening and restricted recrystallisation. Despite this, elongation was preserved through the formation of fine, soft, ductile, uniformly dispersed spherical cementite (Fe 3 C) in the Si steel. Higher levels of cold work reduced strength slightly after annealing above 780°C but improved elongation due to full recrystallisation and coarsening of NbTi (C,N) particles.