Abstract: The experimental data of 400 heats of steel produced by 110 t EAF + LF/VD refining + continuous casting + hoi rolling in a special steel plant are statistically analyzed. The hardenability [at (920 ± 10) °C for 50 min and then air cooling to room temperature,followed by holding at (880 ±5) °C for 30 min water quenching] and chemical composition of 20CrMnTiH steel are obtained by means of Jominy test and spectral analysis to get a linear relationship between the hardenability and the content of residual boron. By each steelmaking practice analysis it is found that the main source of B in steel is EAF tapping slag. The content of B in steel can be effectively controlled by controlling the amount of tapping slag in EAF tapping. In addition, when the amount of Al₂O₃ in the refining slag decreases from 30 % to 15% ~20% ,the content of B in steel can be controlled below 0. 000 2%.