Nam Hoe Heo

After final annealing, magnetic properties of the electrical steel including the lowest bulk sulfur content are best, while those of the steel including the additional aluminum is worst. Such excellent magnetic properties in the former steel is due to the strong cube + Goss texture. The sulfur highly segregated at grain boundaries and the surface Al₂O₃ layer is detrimental to the selective growth of the cube and Goss grains, resulting in the smaller grain size and the final poor magnetic properties. The bulk content of sulfur and aluminum should, therefore, be decreased to obtain the strong cube + Goss texture.

This research has been performed to evaluate the reheat cracking susceptibility of 9Cr T/P92 heat-resistant steels and find the difference in susceptibility between 2.25Cr T/P23 and 9Cr T/P92 heat-resistant steels. After the welding simulation at 1200°C and 1300°C, the prior austenite grain size is much smaller in the T/P92 steel than the T/P23 steel. This is attributed to the undissolved Nb-rich carbo-nitrides in the T/P92 during the welding simulation which inhibit the growth of prior austenite grains. After tensile test at 750°C for the evaluation of cracking susceptibility, the fracture mode is typically intergranular in the T/P23 steel, while that of the T/P92 is ductile. Therefore, the insensitivity to intergranular cracking of the T/P92 steel is mainly attributed to the much lower phosphorus segregation concentration at grain boundaries which is caused by the smaller prior austenite grain size. Calculation results finally confirm that the grain boundary segregation concentration of phosphorus decreases with decreasing grain size.