Effect of plastic strain and forming temperature on magnetic properties of low-carbon steel

Claw poles are a key component of automobile generators. The output power performance of the generator is very dependent on the magnetic properties of its claw poles. Plastic deformation is known to significantly change the magnetic behavior of ferromagnetic materials in claw poles. In this paper, changes in the magnetic properties of low-carbon steel, used for claw pole components due to their plastic deformation, were investigated for different strains and temperatures. Ring-shaped material samples were prepared by machining and their magnetic properties were measured. The surface roughness was first evaluated and a machining process with an arithmetic average of roughness Ra 1.6 µm was selected as enabling the lowest measurement error. Hysteresis loops at different applied magnetic fields of the material were obtained for different plastic strains and forming temperatures. The magnetic parameters of magnetic flux density, coercivity, and remanence were obtained and compared with magnetic flux density as the primary focus. Results showed that machining, cold forming, and hot forming all led to lower magnetic flux density, larger coercivity, and smaller remanence. Magnetic flux density showed a sharp decrease at the start of plastic deformation, but as the strain increased, the decreasing trend gradually reached a constant value. The decrease was much larger for cold forming than for hot forming. For example, at 500 A/m, the degradation of magnetic flux density with a reduction percentage of 5% at room temperature was about 50%, while that of hot forming at 1200°C was about 10%. Results of this research may provide a reference for the future process design of hot-forged claw poles.