Abstract: In order to study the effect of Co content on microstructure and mechanical properties of the CB2 steel， the contents of Co in CB2 steel were adjusted to 0.5%， 1%， 1.5%， 2% and 3%， respectively. Using optical microscope (OM)， scanning electron microscopy (SEM)， and tensile test at room temperature， the effect of different Co content on the microstructure and mechanical properties was studied， associated with the Thermo-Calc and JMatpro-7.0 simulations. It is found that when the content of Co in the steel increases， the chromium equivalent value increases and the δ-ferrite content decreases. Especially， when the content of Co is 1.5%， the δ-ferrite is appears. Meanwhile， with the increase of Co content， the tensile strength of the steel increases， while the elongation increases firstly and then decreases. The results indicate that the steel with the Co content of 1.5%has a good comprehensive property， where the ultimate tensile strength reaches 805.13MPa， and the elongation is 20.4% and Brinell hardness is 260， respectively.

Key words: CB2 steel, Co content, microstructure, mechanical properties, comprehensive properties