Higher strength steels for magnetic applications in motors

Abstract

Motors are essential components in electric vehicles and there are at present worldwide efforts to improve their efficiency and reduce their weight. One of these areas concerns improving the quality of steel laminations that are used for their magnetic properties in rotors and stators. These are ‘soft’ magnetically, meaning that they can be easily magnetised and demagnetised. Improvements in magnetic quality, i.e. reductions in power loss during magnetisation, have been pursued for many years but the high-speed motors in vehicles are also subject to greater mechanical loading, in particular in the rotors of IPM-machines (Internal Permanent Magnet rotor machines). These are the dominating type of electric motor for electrified vehicles due to their high efficiency and high power/torque density. As a result, there are increasing requirements for new steel products that combine low power losses with high strength. This is difficult because most ways of raising the strength result in the magnetic characteristics becoming also undesirably harder. The present paper starts with a consideration of how magnetic power losses are defined and the factors that influence these. Then, different strengthening methods are considered together with their effects on magnetisation. Strengthening can be achieved by (i) solid solution alloying, (ii) grain refinement, (iii) work hardening, (iv) precipitation and (v) texture control. Most commonly, the requirements are in conflict. Only solid solution strengthening can confer benefits to both strength and power loss although extremely fine nanoprecipitation can raise the strength with little or no magnetic detriment. Based on this analysis, results from research publications and patents are summarised and reviewed.