This paper summarized the status of comprehensive strengthening mechanism of steel. By using chemical phase analysis, X-ray small-angle scattering, RTO and high-resolution TEM observation, the properties of nano-scale cementite precipitates in Ti micro-alloyed high strength weathering steels produced by thin slab continuous casting and rolling process were analyzed. Except nano-scale TiC, cementites precipitates less than 36 nm and high volume fraction were also found in Ti micro-alloyed high strength weathering steels.The volume fraction of cementite less than 36 nm is 4.4 times as much as that of TiC in the same size. Cementite with high volume fraction has stronger precipitation strengthening effect than that of nano-scale TiC, which can not be ignored.The precipitation strengthening contributions of nano-scale precipitates of different types and sizes should be calculated respectively according to the mechanisms of dislocation intersection and dislocation bypass, and then be added up with the contributions of solid solution strengthening and grain refinement strengthening.A formula for calculate the yield strength of low carbon steel is proposed, and the calculated yield strength considering the precipitation strengthening contributions of nano-scale precipitates and the comprehensive strengthening mechanism of steels matches the experimental results well. The calculated σ_s=630~676 MPa, while the examined σ_s=630~680 MPa. The reason for grain refinement strengthening can not directly plus dislocation strengthening and the performance form of phase transformation's influence on steel strength were debated.The applications for comprehensive strengthening theory were summarized, and several scientific questions for further study were put forward.