Chemical composition:
Chemical Composition
C Si Mn P S Ni Cr Cu
≤
70 0.67 to 0.75 0.17 to 0.37 0.50 to 0.80 0.30 0.25 0.25 0.035 0.035
75 0.72 to 0.80 0.17 to 0.37 0.50 to 0.80 0.30 0.25 0.25 0.035 0.035

Mechanical properties:
Grades blank sample
Size
/ mm heat treatment process of the mechanical properties of the steel hardness delivery status
/ HBS
Normalizing quenching and tempering σb / MPa σs / MPa δ5 (%) ψ (%) Ak / J ≤
≥ No heat annealed steel
7,025,790,715,420,930,269 229
75 samples 820 4,801,080,880,730,285,241

Characteristics and Uses:
70: 65, strength and elasticity than steel pin high, similar to other properties. Hardenability low, spring wire diameter not exceeding 12-15mm hardenability, not only for the manufacturing section of the spring, and the wheel circle.
75: No. 65 steel with similar performance, its flexibility somewhat less than 65 steel, high strength, low hardenability, usually in the quenched and tempered state. Is not used in the manufacture of high strength, smaller cross-sectional dimensions of the coil spring, plate spring, the wear of mechanical parts.