Effect of Mn on strain accumulation in Nb microalloying steels

High Mn is added in line pipe steels to strengthen them. Also, Mn lowers the phase transformation temperature, which is an advantage because it lowers the rolling temperature for grain size control. However, Mn retards the kinetics of precipitation of NbC in thermo-mechanical processing in Nb microalloying steels. In high-grade line pipe steels, the control of strain accumulation behavior is the key to the control of grain size. Strain accumulation can only be obtained when static recrystallization between passes could be suppressed. The effect of NbC precipitates on suppressing recrystallization has been widely investigated, mostly focusing on the Zener pinning of NbC on grain boundaries. However, it is applicable in large strain and long inter-pass time. In short inter-pass time, such as strip mill rolling, recrystallization behavior is controlled by the complicated interaction of precipitation with recovery and recrystallization. In this paper, the effect of Mn on strain accumulation during multi-pass strip mill rolling based on the interaction of precipitates with the activation energy of grain boundary migration and recovery, and the thermodynamics and kinetics of recrystallization are discussed systematically.