Why does stainless steel have magnetism?

People often use magnets to check the quality of stainless steel. A magnet without magnetism is considered genuine and genuine, while the opposite is considered a counterfeit. In fact, this is an extremely one-sided and unrealistic method of identifying errors
Austenitic stainless steel is non-magnetic or weakly magnetic; Martensitic or ferritic stainless steel is magnetic. But after cold processing, the microstructure of the processed area of austenitic stainless steel will also transform into martensite. The greater the processing deformation, the more martensitic transformation and the greater the magnetism. The material of the product will not change. The material of stainless steel should be tested using more professional methods. (Spectral detection, stainless steel identification liquid detection)