Abstract: Self-shielded flux-cored wires with 1wt%, 3wt%, 5wt% and 7wt% CaCO₃ were prepared, and then used to weld ultra-low carbon bainitic steels. The effects of CaCO₃ content on the micromorphology, phase composition and slag detachability of slag after welding were studied. The results show that with the increase of CaCO₃ content, the micromorphology of internal surface of slag changed from massive to rod-like, then to dendritic, and finally to granular. When CaCO₃ content was 1wt% and 3wt%, the slag was composed of composite oxide of BaO and TiO₂ (BaTiO₃), spinel compound (MgMn₂O₄ or MnAl₂O₄) and BaMg₃Al₂Si₂O₁₀F₂ composite oxyfluoride. When CaCO₃ content was improved to 5% and 7%, respectively, the BaMg₃Al₂Si₂O₁₀F₂ phase disappeared and Ca₂SiO₄ phase formed. With the increase of CaCO₃ content in the flux-cored wire, the slag detachability decreased.