Effect of recovery and recrystallization on microstructure and magnetic properties of Fe-0.4P rolled sheets

Powder metallurgical processing of the soft magnetic Fe-P alloys is well known. In recent times, the wrought metallurgical processing of these alloys is gaining importance due to the possibility of achieving a good combination of soft magnetic properties by optimising the microstructure through carefully designed heat treatment. The Fe-P alloys are mainly focussed towards realizing an alternative and cost effective alloy to conventional electrical steels. Since not much work on bulk Fe-P alloy system has been reported, we have put efforts on processing Fe-P alloy by wrought metallurgy route to bring out comprehensively the dependence of the microstructure on magnetic properties. Fe-0.4 wt.% P alloy sheets (~0.5 mm) were prepared by adopting the conventional melting, forging and rolling, and heat treated at various temperatures (500 – 1000 °C). Recovery and recrystallization behaviour was investigated using electron back scattered diffraction technique and correlated with the microstructural parameters and mechanical/magnetic properties. Transmission electron microscopy studies revealed the formation of Fe3P nanoprecipitates (2-4 nm) leading to a reduction in lattice strain as confirmed from X-Ray diffraction data analysis. Heat treatment at 1000 °C/1h resulted in the formation of fully recrystallized coarse grain structure with coercivity of 80 A/m. Upon further aging at 300 - 500 °C/0.5 h, the coercivity reduced to 43 A/m with a core loss of 347 W/kg. The attractive combination of soft magnetic properties was attributed to the coarse grain and nanoprecipitate microstructure with reduction in lattice strain. The results were also compared with commercial non-oriented Si-steels (M700-50A and M400-50A).