Tesla Inc (TSLA)

[boston745]

Even though wrought austenitic stainless steels are non-magnetic in the annealed condition they may develop magnetic response when cold worked. Cold work can transform some austenite to martensite.

This has a dramatic effect on tensile strength and even more so on yield strength; a heavily cold drawn grade 304 wire can achieve a tensile strength of up to around 2000MPa. Such a highly worked 304 will also be very strongly attracted to a magnet.

http://www.kimballphysics.com/media/images/MCF/magnetic_permeability.jpg

Any process which can change the crystal structure of stainless steel can cause austenite to be converted to the ferromagnetic martensite or ferrite forms of iron. These processes include cold working and welding. It is also possible for austenite to spontaneously convert to martensite at low temperatures. To complicate matters further, the magnetic properties of these alloys depend on the alloy composition. Within the allowed ranges of variation of Ni and Cr, significant differences in magnetic properties may be observed for a given alloy.

https://www.greenwoodmagnetics.com/resource/what-is-the-difference-between-304-and-316-stainless-steel/

“We’re rapidly changing alloy constituents and forming methods, so traditional names like 304L will become more of an approximation.”

If I had to guess, Musks team are trying to increase the steals magnetic properties which would be why 304L would be an approximation.