You can either attempt to look up the value, OR you can calculate it based on the following equation;
thermal diffusivity = thermal conductivity/density * heat capacity
thermal conductivity at 1100 deg K = 25.9 W/m K
density at 1100 deg K = 7620 kg/m^3
heat capacity at 1100 deg K = 0.64 kJ/kg K
These values are for AISI Type 1026 CS and were from "Transport Phenomena in Materials Processing" by Poirier and Geiger