304 stainless is one of the commonly called 18/8 stainless steels (see your kitchen cutlery). This means they contain roughly 18% chromium and 8% nickel and are of an austenitic structure. It is mainly the chromium that gives the corrosion resistance.
When there is no nickel, the stainless grade family is the "400 series". 400 series are not as corrosion resistant as the 300 series and are magnetic (ferritic structure) , where the 300 series are non-magnetic. It's to do with the crystal orientations or lattice within the grains of the metal.
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However, under serious coldworking 300 series may become magnetic!!
Regards
2scoops