Influence of Annealing Process on Microstructures, Mechanical and Magnetic Properties of Nb-Containing High-Strength Non-Oriented Silicon Steel

As the core material of transaction motor for electrical/hybrid vehicles, the non-oriented silicon steel (NOSS) sheets require not only the good magnetic properties, i.e. high permeability and low iron loss, but also high yield strength to resist the centrifugal force during the high speed rotation. In this work, Nb element was added into the conventional NOSS to improve the strength without sacrificing the good magnetic properties too much. The effects of annealing process on the microstructures, magnetic and mechanical properties of Nb-containing high-strength non-oriented cold-rolled silicon steel were studied. The increases of annealing temperature and time both lead to the reduced segreation of Nb at grain boundaries and the solution and ripening of precipitates, which means the decreased suppression on the migration of grain boundaries; thus, the recrystallized grains start to grow; particularly, the density of {111}<112> texture component may increase to deteriorate the magnetic flux density, B-50. The best mechanical and magnetic properties cannot be achieved at the same time. The annealing process at 940 degrees C. for 270 s could lead to the best combination of mechanical and magnetic properties, which include B-50 of 1.69 T, the iron loss P-1.5/50 of 4.86 W/kg and P-1.0/400 of 30.47 W/kg, resulting from both the segregation of solute Nb at grain boundaries and the extensive precipitation which refrains the grain growth and development of harmful gamma texture. Therefore, the yield strength is increased due to both grain refinement and precipitation strengthening without greatly sacrificing the permeability and iron loss.