本研究设计制备一种中碳合金钢40Cr3Mn3Ni3Si2Mo，通过对实验钢热轧态直接回火、冷处理后回火及淬火态冷处理后回火试样进行组织表征和性能测试，探索组织演变规律，建立工艺-组织-性能的关系并分析强韧化机理，为高强塑性淬火-配分（Q&P）型超高强度钢的成分设计与工艺优化提供了依据。结果表明：实验钢热轧态回火可以获得回火马氏体+富碳奥氏体的复相组织，但由于残余奥氏体含量较高，钢的强度未达到超高强度钢的要求。将热轧态实验钢冷处理后低温回火，改善了组成相的比例和分布状态，使强塑性明显提高。实验钢进行紧密衔接的油淬、深冷及回火处理可以降低奥氏体稳定化的不利影响，获得更加优异的综合性能，此时钢的屈服强度为1506 MPa，抗拉强度为1895 MPa，伸长率为16.7%。同时提出一种提高1800~1900 MPa级超高强度钢强塑性的有效途径：通过成分和工艺优化控制马氏体相变动力学，在马氏体组织中保留20%左右的残余奥氏体，利用回火辅助配分处理提高其稳定性，此时钢的屈服强度小幅下降，降至1400~1600 MPa，但伸长率可以达到15%~18%。

A medium carbon alloy steel 40Cr3Mn3Ni3Si2Mo is designed and prepared， and tempering is carried out after hot rolling， hot rolling plus refrigerating treatment， as well as quenching plus refrigerating treatment respectively. Microstructural characterization and properties testing are conducted on the heat-treated samples to investigate microstructural evolution. The relationship between processing， microstructures and mechanical properties is established， and the mechanism of strengthening and toughening is illuminated. Consequently， the principles for composition design and process optimization of quenching and partitioning （Q&P） typed ultra-high strength steels with high strength and plasticity are figured out. The results show that multiphase microstructures of tempered martensite and carbon-enriched austenite are achieved through tempering on the hot-rolled testing steel， while its strength fails to reach the level of ultra-high strength steels due to high volume fraction of retained austenite. The match of strength and ductility of the hot-rolled testing steel increases significantly through low temperature tempering after refrigerating treatment because of the improved phase proportion and distribution. When the testing steel is refrigerating treated and tempered instantly after oil quenching to weaken the effect of austenite stabilization， more excellent comprehensive properties are achieved with 1506 MPa in yield strength， 1895 MPa in ultimate tensile strength and 16.7% in elongation. Moreover， a feasible approach to increase the strength and plasticity of 1800-1900 MPa graded ultra-high strength steels is proposed： through controlling the martensitic phase transformation kinetics by alloying design and process optimization， austenite of about 20% in volume fraction is retained within martensite after incomplete quenching， its stability is reinforced by tempering assistant partitioning. Thus， the elongation increases to 15%-18%， with the yield strength lowered slightly to 1400-1600 MPa.