Aldone,
as you know , Ferritic structure of Fe-C diagram are important in terms of magnetic properties. Then,the steel grades with very low Carbon ( left side of Fe- C ) are usually applied for magnetic purpose. Better magnetic properties will be obtained is Ferrite structure is "added" of Silicon. ( Silicon- Iron alloys for relais, solenoide core, ecc ecc). In addition , Ferritic stainless steel grades have same good behaviour. Even alloys steels ( as Sae 8640 ) could find some application but magnetic properties could be low. Same situation are shown by soft Martensitic stainless steel or Precipitation hardening SS. These last grades have highest coercive force, poor magnetic permeabilty . However, they could be some usefulness if higher Rm or Rp0,2 were required.
At the end of story , the different strucutures of Fe-C diagram give following "performances":
1) Ferrite = highest Permeability and lowest coercitivity ( Hc), low retentivity ( Br)
2) Ferrite + Perlite = lower permeability and higher Hc than 1)
3) Perlite and Perlite + Fe3C lowest permeabilty and highest Hc than 2)
4) Fresh Martensite or low temperature tempered Martensite have lowest permeability and highest Hc. I point up Martensite is not a structure of Fe-C but a trasformation of Austenite when is fast cooled starting from temperature above A3.
5) Austenite doesn'exist at room temperature in Fe-C diagram but Austenite of Austenitic Stainless Steels ( as 302 , 304 , 316 and so on ... ) has permeabilty close to 1,00 ( one).
Obviously, Hc , Br and permeability of above structures strongly depend on heat treatment ( i.e magnetic annealing ), strain hardening and stresses ( as straightening for instance ).
If you want to have more data and some survival kit about magnetic properties of different grades , you can verify the sites of Carpenter ( USA ) , Valbruna Stainless ( USA and Italy) , Ugine ( France), Daido ( Japan )just to say the most important Steelmakers of Magnetic Grades. Sorry for sintetic and general information usefull only to have an idea about this subject.
