Formation and evolution of inclusions in the refining process of X80 pipeline steel

To further meet the requirements for using pipeline steel in extreme environments and to improve its safety in service, the inclusion control level in pipeline steel urgently needs improvement. In this paper, the variation laws of inclusion type, size, and composition in the refining process of X80 pipeline steel were studied through industrial trial sampling, and the evolution mechanism of inclusions during calcium treatment and steel cooling and solidification was analyzed using thermodynamic calculations with FactSage 8.1 software. The trial results showed mainly MgO–Al₂O₃ and MgO–Al₂O₃–CaO inclusions after LF refining in proportions of 25% and 75%, respectively, with sizes mainly distributed between 1–5 μm, and the proportion of inclusions of 1–2 μm and 2–5 μm were 56.0% and 37.3%, respectively. The contents of TO and N were reduced from 0.0022% and 0.0059% after LF refining to 0.0010% and 0.0035% after RH refining, respectively, and the number density of inclusions was reduced from approximately 23.07 mm⁻² after LF to 7.44 mm⁻², with an inclusions removal rate of approximately 67.8%. The inclusions were mainly MgO−Al₂O₃–CaO and CaS–Al₂O₃–CaO systems during calcium treatment, the average CaS content in the inclusions increased from 8% after RH refining to 36%, and the average CaO content decreased from 24% to 12%. After soft blowing, the SiO₂ content ranged from 0 to 2.5% in the inclusions smaller than 40 μm and from 6.0% to 8.0% in the inclusions larger than 40 μm, and the inclusions larger than 40 μm were mainly CaO–Al₂O₃–MgO–SiO₂, whose chemical composition is essentially identical to that of the refining slag, whose source is the refining slag involved; thermodynamic calculations show that when the Ca content is between 10.5×10^–6–15.8×10^–6, all spinel inclusions are modified, and all the inclusions are liquid calcium aluminates; when the steel is at casting temperature, the inclusions are mainly liquid calcium aluminates, and when the temperature is lowered to 1428 ℃, the liquid inclusions completely transform into solid. As the temperature drops below 1309 ℃, the type of inclusions essentially remains constant. During the entire temperature drop, the CaO content in the inclusions decreased, and the CaS content increased.