[Tuna-Fish]

Iron exists in many different crystal structures in different kinds of steels. Some of these forms are magnetic, others are not. When you work the metal, such as by bending it, you can cause it to change from one structure to another.

304 is an austenitic stainless steel, and austenite is non-magnetic. But when it's cold worked, it turns into martensite, which is harder and magnetic. If you anneal it by heating it and letting it cool, it turns back into austenite.

[riiii]

Interesting. Do you know if it affects it's anti-rust properties after you work it / make it martensite?

[Tuna-Fish]

Martensite is less corrosion-resistant. But note that slight working like bending it a bit will only turn a small portion of it into martensite, most of it will remain austenitic.