What is a soft magnetic steel ?
Soft magnetic steels are low alloyed steels with about 3.0-3.4 weight % Si
that can be magnetized in a magnetic field at low energy loss.
The magnetic polarization associated with the magnetization can be produced, for example, by an electric current in a current-carrying coil around a magnetic core.
In all soft magnetic steels, the polarization leads to a magnetic flux density that is many times higher than that generated by the external magnetic field in air.
In plain terms a soft magnetic FeSi steel reinforces an externally imposed magnetic field.
Soft magnetic steels have a coercivity of less than 1000 A/m. If an external magnetic field exceeds the coercivity, the direction of the magnetic flux in the material is also reversed.
The most important functional properties of soft magnetic steels that matter for the field of electromobility, more specific, for the core of the electrical motors are low low eddy current losses, very high magnetic permeability, minimal hysteresis losses, and low anomalous losses.
Why are soft magnetic steels important for electrical car motors?
The magnetic properties of soft magnetic FeSi steels described above are essential to build electrical car engines that generate a large electromagnetic force at lowest possible electric power. Specifically the magnetic flux density is a key parameter that determines the torque of an electrical car motor.
Of high relevance for the efficiency of an electric motor are also the core losses.
They occur due to the constantly changing direction of the magnetic field.
This hysteresis loss costs valueable energy and thus battrery life time and produces heat which leads to unwanted thermal fatigue and thermal expansion of engine parts. The core losses depend strongly on the frequency with which the direction of the magnetic field switches during operation and of the type the soft magnetic material used.
Conventional electric engines operate at 50 Hz. In contrast high-performance car engines for electric and hybrid cars operate at service frequencies beyond 400 hertz. Electric cars thus require soft magnetic steels with high very flux density and minimal core losses at high service frequencies.
How are electrical FeSi steels for electromobility produced?
The production of electrical steel is very complex and among the most demanding metallurgical processing methods.
It can be divided into several process steps such as steel making and alloying, hot-rolling and cold strip production, heat treatment and strip coating, and finishing in the finishing lines.
The chemical composition of the steel melt, i.e. particularly the adjustment of the carbon, nitrogen, silicon and aluminium content and the contents of other alloying elements and the
prevention of undesired components such as chrome, and titanium, are done in the secondary laddle metallurgy.
An essential role in the development of electrical steel plays lies therefore in the quality of the primary and of secondary metallurgy.
Through appropriate measures the purity of the melts increased and their carbon content reaches values as low as 20 ppm - 30 ppm.