Overview
SMn420 is a sulfurized free-cutting steel grade primarily used for mass-produced precision components. It belongs to the JIS G 4804 standard (Japan) and is chemically optimized for automated machining processes. The steel's sulfur content forms manganese sulfide inclusions that improve chip breakability during turning, drilling, or milling operations. Compared to standard carbon steels, SMn420 reduces tool wear by 30-50% while maintaining adequate tensile strength (420 MPa minimum). Its balanced composition (0.07-0.13% sulfur, 0.9-1.3% manganese) makes it particularly suitable for high-speed CNC machining of small fasteners.
Physical and Chemical Properties
SMn420 exhibits typical density (7.85 g/cm³) and thermal conductivity (45 W/m·K) of low-alloy steels. The sulfur content (0.07-0.12%) creates discontinuous chip formation during machining, while manganese (0.9-1.3%) ensures proper sulfide distribution. Carbon content (0.12-0.18%) provides base strength without compromising machinability. The steel's microstructure consists of ferrite-pearlite with elongated manganese sulfide stringers. These inclusions reduce cutting forces by 15-20% compared to non-free-cutting grades. SMn420 maintains stable mechanical properties up to 250°C, making it suitable for components in moderate temperature environments.
Main Applications
Approximately 65% of SMn420 production is used for fasteners, including ISO metric screws (M3-M12), self-tapping screws, and specialized bolts for automotive assemblies. The automotive sector consumes nearly 40% of global SMn420 supply for engine components, transmission parts, and chassis fasteners. Other applications include hydraulic fittings, electrical connector pins, and watch components. The material's consistent machinability allows production of complex geometries like internal threads and undercuts without secondary operations. Some manufacturers use SMn420 for bushings and spacers where moderate corrosion resistance (uncoated) is acceptable.
Safety and Storage
SMn420 requires standard steel handling precautions. Machining generates sulfur-containing dust that may irritate respiratory systems—use local exhaust ventilation and PPE (N95 masks). The material is non-flammable but produces sparks during grinding operations. Store SMn420 in dry conditions (relative humidity below 60%) to prevent surface oxidation. Coiled wire forms should be kept on pallets with moisture barriers. For long-term storage (6+ months), apply light oil coating or vapor corrosion inhibitors. Avoid contact with chlorides or acidic substances that could initiate pitting corrosion.
B2B Procurement Guide
When procuring SMn420, specify: 1) Form (round bars, wire coils, or precision ground stock), 2) Diameter tolerance (±0.05mm for precision applications), 3) Sulfur content verification (certificate of analysis), and 4) Surface finish (hot-rolled, cold-drawn, or peeled). Major producers include Japanese steel mills (JFE Steel, Sanyo Special Steel) and Chinese manufacturers (Baosteel, Angang). Lead times typically range 4-8 weeks for customized dimensions. Consider ordering with anti-rust paper wrapping for international shipments. For high-volume contracts (100+ tons annually), negotiate alloy surcharges separately from base prices.
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