A ‘flat-head washer’ is not usually the name of a single component, but rather refers to a screw with a flat head and a flange (washer), or a combination of a flat-head screw and a washer; in fastener jargon, the term ‘washer’ refers to either a washer or a flange.
‘Huasi’ is actually a phonetic translation of the English word ‘washer’, but in many fastener products, it specifically refers to a flange structure. Therefore, ‘flat-head huasi’ often refers to hexagon socket flat-head flanged screws; these screws have a flat head but feature an integral flange at the base, which helps distribute pressure and prevent loosening. In the field of self-tapping screws, the term also refers to flat-head self-tapping screws with integrated washers, where the self-tapping section can drill a hole directly, whilst the head is flat and serves as a washer.‌‌‌

Carbon Steel Variant (Preferred for General Indoor Fittings)
Surface: Blue-white zinc or coloured zinc (yellow zinc) electroplating for rust protection
Advantages: Affordable, high hardness; easily drills through thin steel sheets and light-gauge steel studs
Applications: Indoor suspended ceilings, plasterboard, light-gauge steel studs, indoor colour-coated steel sheets
Stainless Steel Variant (For Outdoor / Damp Environments / Aluminium Applications)
410 Stainless Steel: High hardness and strong cutting power; specifically designed for aluminium alloy doors and windows, insect screens and aluminium profiles. The fine-threaded, small-cut design prevents slipping and does not damage the aluminium; commonly used for outdoor canopies and advertising hoardings.
304/316 Stainless Steel: Superior rust resistance; suitable for coastal and highly corrosive environments; drilling power is slightly weaker than that of 410.