RECRYSTALLIZATION BEHAVIOR OF A C-MN STEEL, A TITANIUM-INTERSTITIAL-FREE STEEL AND A NIOBIUM-MICROALLOYED STEEL DURING STRIP ROLLING

In order to characterize the Lake Erie strip mill of Stelco, Canada, three grades of steel (C-Mn, interstitial free (IF) and niobium microalloyed) were subjected to torsional simulation. The flow stresses and microstructural behaviours have been compared. As anticipated, for long interpass times (typical of plate-rolling schedules) the temperature T(nr) of no recrystallization can be clearly established only for the niobium steel, the other grades undergoing nearly full recrystallization at all temperatures. Short interpass times, typical of strip-rolling schedules, lead to little pass-to-pass strain accumulation in the three steels. In the temperature range 1000-900-degrees-C, static recrystallization is largely responsible for the high degree of interpass softening in the C-Mn steel. In the case of the IF steel, during the latter stages of finishing, there is some strain accumulation, and both static and dynamic recrystallization occur to a significant degree. For the niobium steel, because of solute effects, the short interpass times (about 1 s) do not permit much static recrystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(C, N) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the niobium steel followed by the initiation of dynamic recrystallization would lead to lower rolling loads in the strip mill for this grade than in the plate mill at the same rolling temperature.