Spring Steel Strip (65Mn / 60Si2Mn / C75S / SUP9 / 51CrV4)

Spring steel strip and spring steel plate are both flat spring steel products but differ fundamentally in dimensions, delivery form, manufacturing process, dimensional precision, and typical application. Spring steel strip is a narrow flat product (width typically 5–600mm) produced by cold rolling and precision slitting, supplied in coiled form (continuous length wound into a coil), with tight thickness tolerances (±0.01–0.05mm) and width tolerance (±0.05mm achievable by precision slitting), bright cold-rolled surface finish, and often supplied in hardened and tempered condition (HRC 40–55) ready for direct spring stamping or winding without further heat treatment by the customer. The coiled form of strip enables continuous automated progressive die feeding at high press speeds (50–500 strokes per minute) for economical mass production of small stamped spring components. Spring steel plate is a wider, thicker flat product (width typically 100–2000mm, thickness 1–30mm) produced by hot rolling, supplied in cut lengths on pallets, with wider dimensional tolerances acceptable for the larger spring components it serves, and typically requiring heat treatment (quench and temper) by the spring manufacturer after forming — making it suitable for large components like automotive leaf springs, truck suspension springs, and industrial machinery leaf springs where individual spring dimensions are large enough to justify handling cut pieces rather than a continuous coil. The core material chemistry and spring performance requirements are similar between strip and plate, but the strip's tighter dimensional tolerances, better surface finish, coiled form factor, and pre-hardened supply condition make it essential for small precision spring components where plate would be impractical.

C75S, 65Mn, SK5, and SAE 1075 are the four most widely used spring steel strip grades globally, each designated under a different national standard but all representing nominally 0.70–0.80% carbon spring steel strip with similar target mechanical properties. C75S (EN 10132-4) is the European standard designation for narrow cold-rolled strip for springs with carbon 0.72–0.78%, manganese 0.50–0.80%, specified per EN 10132-4 which also defines dimensional tolerances, surface quality, decarburisation limits, and delivery conditions — the dominant specification for European automotive and electronics OEM spring supplier quality systems. 65Mn (GB/T 1222) is the Chinese standard spring steel with carbon 0.62–0.70% (slightly lower than C75S) and higher manganese 0.90–1.20% — the additional manganese compensates for the lower carbon in strengthening the steel and improving hardenability, and 65Mn is the most cost-competitive globally available spring strip grade produced in the largest volumes by Chinese cold rolling mills. SK5 (JIS G3311) is the Japanese standard spring steel strip with carbon 0.70–0.80%, manganese 0.25–0.60% — specified for Japanese automotive and electronics OEM suppliers following JIS quality systems, with equivalent performance to C75S in hardened and tempered condition. SAE 1075 (SAE J403) is the American specification with carbon 0.70–0.80%, manganese 0.40–0.70% — specified for North American OEM applications and is commercially interchangeable with C75S and SK5 in mechanical performance. For most spring stamping applications, these four grades are functionally interchangeable in the hardened and tempered supply condition at the same specified hardness (HRC 46–50), and the choice is typically determined by the applicable OEM specification or customer quality system requirements rather than technical performance differences.

Spring steel strip is supplied in five principal delivery conditions, each suited to different spring manufacturing processes and facility capabilities. Annealed (Soft) condition (≤210 HBW per EN 10132-4 / GB/T 1222) is the softest form, produced by spheroidised annealing at 690–720°C to convert lamellar pearlite to spheroidised carbides in a soft ferritic matrix, enabling maximum formability for complex stamping, deep drawing, bending around very small radii, and cold coiling without cracking. Annealed strip requires complete hardening and tempering heat treatment by the customer after spring forming to achieve final spring properties — this adds process steps but enables the most severe forming operations. Normalised condition (approximately 250–320 HBW) is produced by heating above Ac3 and air cooling, giving a uniform fine pearlite structure with intermediate hardness and ductility — suitable for moderate cold bending operations where annealed softness is unnecessary but full hardness is not required before forming. Cold Rolled Full Hard condition (work-hardened from cold rolling without subsequent annealing, tensile strength 1,200–1,600 MPa depending on grade and thickness reduction ratio) provides high strength through cold-work strengthening without heat treatment — economical for applications where the as-rolled strength is adequate (battery contacts, light spring clips) though the directional anisotropy and lack of tempered toughness limits suitability for high-fatigue applications. Hardened and Tempered (H+T) condition is the most commercially important supply condition for spring strip — strip is hardened (austenitized and quenched) then tempered at the steel mill to the specified hardness (HRC 40–55 per grade and customer specification), producing a uniform tempered martensitic microstructure with optimised combination of strength, hardness, and toughness for spring service. H+T strip enables direct spring stamping and forming without customer heat treatment, eliminating scale formation, distortion, and decarburisation risk in the customer's manufacturing process. Pre-Tempered (also termed Pre-Hardened or Spring Temper) is sometimes used synonymously with H+T for strip supplied in the final tempered spring condition. For most high-volume spring stamping operations in automotive and electronics manufacturing, Hardened and Tempered is the preferred specification as it enables direct progressive die production with consistent spring properties without additional heat treatment investment.

Edge condition is a critical quality parameter for spring steel strip because strip spring fatigue failures — particularly in high-cycle applications such as automotive relay contact springs, electronic connector springs, and constant-force clock springs — most frequently initiate at strip edge imperfections including microcracks, edge burrs, tool marks, and decarburised edge zones generated during slitting. The strip edge is simultaneously the zone of maximum bending stress in a transversely flexed flat spring and the region most susceptible to surface defects from the slitting process — making edge quality directly critical to spring service life. Four principal edge conditions are available for spring steel strip: Slit Edge (also called mill edge or as-slit edge) is produced by circular slitter blades cutting the strip to width from a wider master coil, resulting in a square-cut edge with a small burr (typically 0.01–0.05mm height depending on strip thickness and slitter condition). Slit edge is acceptable for applications where edge stresses are low (wide strips bent about their long axis) or where the edge will be tumbled, deburred, or surface-treated after stamping. Deburred Slit Edge applies a deburring operation after slitting (wire brush, tumbling, or abrasive belt) to remove the slit burr without modifying edge geometry — suitable for moderate fatigue duty applications. Roller Burnished Edge (or rolled edge) passes the slit edge through hardened roller burnishing tools that plastically compress and smooth the edge surface, introducing beneficial compressive residual stress in the edge zone (similar to shot peening effect on the strip face) and eliminating micro-cracks from slitting — this is the preferred edge condition for high-fatigue spring applications including automotive contact springs and precision electronic springs. Ground Edge is produced by edge grinding on an abrasive wheel or belt to remove the slit edge zone completely, achieving a smooth, square, burr-free edge with controlled edge radius and eliminating decarburised edge material — the highest quality edge condition for the most demanding spring fatigue applications in aerospace and medical device contact springs. When ordering spring strip for fatigue-critical applications, always specify roller burnished or ground edge and verify decarburisation depth at strip edge as well as strip face.

Specifying coil dimensions correctly is important for ensuring compatibility with your press feeding equipment, coil handling systems, and production efficiency when ordering spring steel strip. The key coil parameters to specify are inner diameter (ID), maximum outer diameter (OD), maximum coil weight, and winding direction. Inner Diameter (ID) must match your coil reel or decoiler mandrel diameter — common standard IDs are 300mm (for small strip widths up to approximately 50mm width processed on small precision presses), 400mm (medium strip, common for Japanese and Korean market equipment), 500mm (standard for most European and Chinese production press lines), 600mm (large coil handling systems, heavy press lines), and 760mm / 30-inch (North American market standard). Specifying an ID outside your equipment range requires an adaptor or cannot be used at all — always confirm ID before ordering. Maximum Outer Diameter (OD) is governed by your coil car or coil reel capacity — common maximum ODs are 1,000mm, 1,200mm, 1,500mm, and 1,800mm. Larger OD means more strip length per coil and fewer coil changes during production, improving efficiency, but requires adequate coil handling equipment capacity. Maximum Coil Weight must be within the rated capacity of your coil reel or decoiler — typical ratings are 500 kg (small precision decoilers), 1,000–2,000 kg (standard production press coil reels), 3,000–5,000 kg (heavy press line decoilers). For narrow strip widths (5–50mm) at thin gauges (0.1–1.0mm), coil weight per reel is naturally limited by the total strip length achievable within the OD constraint — specify maximum weight and let the mill optimise coil length. For wide thick strip, maximum weight is the binding constraint. Winding Direction specifies whether the free end of the strip exits from the top or bottom of the coil — specify 'eye to sky' (coil standing upright with axis vertical, strip exits from OD) or 'eye to wall' (coil lying on side with axis horizontal) to match your decoiler configuration. Custom coil splicing to produce extra-long coils (strip-to-strip welding) and traverse-wound (oscillate-wound) coils for very narrow strip widths are available for specialist applications requiring very long continuous strip runs between coil changes.