C :0.13-0 .18 Si :0.15-0 .35 Mn :0.60-0 .85 P: 0.03 following S: 0.03 following Cr :0.9-1 .2 Mo :0.15-0 .3 Mechanical properties: tensile strength σb (MPa): ≥ 440 (45) yield strength σs (MPa): ≥ 295 (30) elongation δ5 (%): ≥ 22, the section shrinkage rate ψ (%): ≥ 60 impact energy Akv (J): ≥ 94 impact toughness αkv (J / cm 2) : ≥ 118 (12) hardness: ≤ 179HB sample size: sample blank size of 30mm heat treatment specifications: quenching 900 ° C, air cooling; tempering 650 ° C, air-cooled.