Carriage bolt (DIN603) also called round head square neck bolt. The square neck under round head locks into pre-drilled hole to prevent bolt rotation while tightening nut. Matched with hex nut and flat washer. Carbon steel grade 4.8/8.8 or stainless steel A2/A4. Surface: zinc plated, HDG, black oxide. Sizes M6~M20. Widely used for guardrail, timber construction, fencing and agricultural machinery.