Abstract: The mechanical properties, chemical composition control and cleanness control of HSLC steel were introduced and the strengthening mechanism was discussed. It is pointed out that the mechanical properties level of HSLC steel produced by EAF-CSP technology in Zhujiang Iron Steel Co, Ltd is located at the same position with that of microalloyed HSLA steel produced in foreign CSP plants. The main points of chemical composition and cleanness control were listed and the terminal point control as well as low nitrogen control technology of EAF was developed. It was found that there existed nano-scale sulfides, oxides, nitrides and carbides in HSLC steel. Similar to HSLA steel, the precipitation strengthening mechanism is available to HSLC steel. The nano-scale iron carbide would be the main strengthening phase.