Abstract: The content,type,size and origin of large inclusions in two kinds of low cost carbon structural steel Q235 D in a steel factory in north China were studied systematically by inclusion tracing.The results shows: 120 tons BOF—CC process,the average content of T[O]in steady casting billets is 76×10-6,and the average content of [N]is 28×10-6,the content of large inclusions in the steady billets is 31.3 mg/10 kg; The average content of T[O]in mixed casting billet is 82×10-6,and the average content of [N]is 30×10-6,the content of large inclusions in the mixed casting billet is 38.8 mg/10 kg.Inclusions containing only slag tracer in ladle were about 5 %,containing only about 22 % of inclusions in tundish slag,and 27% of two tracers including ladle slag and tundish slag.120 tons BOF—ladle slag modification—CC process,The tapping temperature of this process is 15 ℃ higher than that of the original process,5 kg/t ladle slag modifier and 5 kg/t lime adding ladle with deoxidizer,and stir until the end of tapping,then blowing quietly 12 min and static 20 min after cover the ladle lip,and delivery to the continuous casting.Typical final slag composition of ladle: 43% CaO,36% Al₂O₃,9% SiO₂,6% Mg O,5%（ Mn O+Fe O）.After the modification,the content of T[O]and large inclusions in the steel decreased greatly.The addition of ladle top slag modifier increases the adsorption capacity of steel slag to large inclusions significantly.It can achieve that have no nozzle clogging in 30 times pouring cycle,and the effect of modification treatment is obvious.