Why is 304 stainless steel pipe magnetic when used

In life, we often use whether the tube is magnetic to judge the quality of stainless steel, but in fact, this method is very unscientific. For example, our common zinc tube alloy, tube copper alloy, etc. can generally imitate the appearance and color of stainless steel. And it is not magnetic, so it is easy to be mistaken for stainless steel. Even 304 steel, which is the most commonly used pipe at present, will have different degrees of magnetic properties after cold processing, so we cannot rely on whether it is There is a magnetic tube to judge the quality of stainless steel, so why is the 304 stainless steel tube magnetic?
         Under normal circumstances, tube stainless steel can be divided into two types according to its organizational structure at room temperature. One is tube austenitic stainless steel, such as 304, 321 tube, 316 tube, 310 tube, etc., and the other is tube martensitic or ferritic stainless steel. Stainless steel, pipes are like 430, 420, 410, etc., while austenitic stainless steel is non-magnetic or weakly magnetic, martensite or ferrite is magnetic, so 304 stainless steel pipes are non-magnetic Or if it is magnetic.
         And we often say that 304 stainless steel is non-magnetic stainless steel, which means that its magnetic index is lower than a certain value. That is to say, general stainless steel has a certain degree of magnetic properties. The obtained austenitic stainless steel is non-magnetic or weakly magnetic, while ferritic and martensitic stainless steel is magnetic. This is due to composition segregation or improper heat treatment during smelting, resulting in austenitic 304 stainless steel A small amount of martensite or ferrite structure appears in the 304 stainless steel tube, so that weak magnetism will appear in the 304 stainless steel tube, and the tube is that after the 304 tube stainless steel is cold-worked, the structure will also transform into martensite, and with the cold The greater the tube processing deformation, the more martensitic transformation tubes, and the stronger the magnetic tube will be.
         Therefore, the magnetism of a material is determined by whether the molecular arrangement is regular and the electron spin isotropic. It can be considered as the physical property of the material, while the corrosion resistance of the material is determined by the chemical composition of the material. Yes, it is the chemical properties of the material and has nothing to do with whether the material is magnetic or not.