This is a high strength, high toughness, low-carbon martensitic hardening steel. This steel has high hardenability, quenching in oil completely diameter of 60 ~ 80mm (> 95% martensite) (100mm diameter at the center receive 85% martensite), quenching in water completely diameter up to 110 ~ 120mm (> 95% martensite) (160mm diameter at the center receive 85% martensite). The good performance of steel heat treatment, quenching in oil deformation and cracking tendency of small, low tendency to decarburization, good performance of the forging process, welding is still good, but the complex shape parts should be preheated before welding to the above 300 ℃, slow cooling after welding . Its biggest drawback is poor cutting. This steel is usually in the quenching and tempering conditions used. Compared with 35CrMo quenched and tempered steel, the ductility, toughness much the same, but significantly increased strength, ultimate strength of which increased by 50 percent; and 40CrNiMo compared to quenching and tempering steel, although the intensity is slightly more, but the ductility toughness is significantly improve, especially the impact of much higher value; compared with quenched and tempered steel 40Cr, -60 ℃ low temperature impact values ​​at 5 times higher; the static notch sensitivity of quenched and tempered steel in general comparable, but not with such intensity quenching tempering temperature and isothermal quenching medium carbon steel compared to the gap effect was more low; and carbon alloy steel quenching and tempering or tempering temperature, high-temperature tempering compared to the fatigue notch sensitivity were also low; compared with quenched and tempered steel 35CrMo, overload sensitivity (ie, overload fatigue sensitivity) lower; with equal strength compared to other steel, high fracture toughness.