Microstructure development during multi-pass deformation in a low carbon steel with a leaner composition, finer grain size, and higher strength

Low-carbon strip steels for structural applications are produced by slab reheating, multi-pass hot rolling and controlled cooling. In this paper, a commercial structural strip grade steel has been compositionally modified, by reducing the Mn content to offer a different cost base with a proactive addition of Nb, to control grain size without altering processing parameters. The multi-pass hot rolling schedule and controlled cooling, representing industrial strip run out table cooling conditions, were simulated using a Gleeble HDS-V40 thermo-mechanical simulator. Interrupted tests were carried out to obtain the prior austenite grain size (PAGS) distributions after different hot deformation passes. The base composition steel showed substantial grain size refinement by recrystallization during the first three deformation passes, but no further refinement during the remaining four passes with a final PAGS of 34±0.87μm before controlled cooling. In the Nb-modified steel, recrystallization was delayed in the early passes, leading to undesired partial recrystallization; however, full rolling schedule refined the PAGS to 15±0.44 μm with narrow size distributions through recrystallization, which resulted in finer transformed ferrite grains after controlled cooling. Higher hardness in the Nb containing steel was attributed to finer ferrite grain size, NbC precipitation strengthening and Nb solute strengthening.