A thread forming screw with a double-lead, consisting of a high and low thread. The lower thread varies in height from 1/3 to 1/2 that of the higher thread, which is sharper and flatter than a standard thread.
18-8 stainless steel (SUS304) screws have good chemical resistance and may be mildly magnetic.
410 stainless steel  screws are stronger and more wear resistant than 18-8 stainless steel screws, these are magnetic and mildly chemical resistant. 
Steel screws are zinc plated to protect the steel from corroding in wet environments. Black zinc-plated screws blend in with dark surfaces and reduce glare. 

Special types available on request.