Why are stainless steel fasteners magnetic?

Our company uses 300 series austenitic stainless steel. The material properties of austenitic stainless steel are that after cold working, part of the austenite structure will become martensite and ferrite, while the martensite material properties reflect slightly magnetic properties. Generally, the magnetism of martensite increases with the increase of deformation, and also decreases with the increase of deformation temperature. Therefore, the hot formed austenitic stainless steel is almost non-magnetic, such as the nut formed by our company. However, the magnetic properties of products such as screws will be more obvious after cold forming.