304 stainless steel magnetic is caused by cold processing. The microstructure of 304 stainless steel is austenitic, which shows paramagnetism (weak response to the magnetic field). That is, without magnetism as the saying goes). In the cold working process of stainless steel, part of the austenite structure will be transformed into martensite or ferrite structure, and the steel with martensite or ferrite structure is magnetically philic. In general processing, the greater the deformation of the material, the lower the temperature, and the stronger the magnetic processing. The magnetism of stainless steel can be eliminated by heat treatment etc. Some stainless steels are less magnetophilic even when cold worked (e.g. 316).  Some stainless steels are inherently magnetomphilic (e.g. 400 series).