Evolution of nonmetallic inclusions in pipeline steel during LF and VD refining process

Abstract

The formation and evolution of nonmetallic inclusions in pipeline steel were investigated by SEM, EDS and INCA Feature Analysis System, with the industrial process of electric arc furnace → ladle furnace (LF) refining → vacuum degassing → continuous casting. The composition, size and amount of inclusions during refining process were discussed systematically. The results show that inclusions at each refining step are mainly small-particle inclusions (below 5 µm), and the total number of inclusions has been reduced significantly due to the refining effect of slag during LF refining. The calcium (Ca) treatment increases the amount of small inclusions. The types of inclusion are mainly Al2O3 and MnO–SiO2–Al2O3 before LF, and they are transformed into CaO–Al2O3, MgO–Al2O3 and CaO–MgO–Al2O3 during LF process. After Ca treatment, inclusions are changed to CaO–Al2O3–(CaS) and CaO–MgO–Al2O3–(CaS). Typical inclusions are still mainly CaO–Al2O3 and CaO–MgO–Al2O3 in tundish, but the composition of those inclusions has been changed and located to the low melting point region in ternary phase diagram. Such inclusions will further be removed as continuous casting approaches.

Characterization of Nonmetallic Inclusions of Al‐Killed Ca‐Treated Steels by Automated SEM/EDS and Its Application to Industrial Case Studies

“…[ 50–56 ] Furthermore, with an operation tightly controlled and metallurgical practices properly defined, it is also possible to improve steel cleanliness by reducing the amount of harmful inclusions during the vacuum treatment process. [ 51,56–64 ]…”

“…This improvement in the steel cleanliness can be attributed to the vigorous stirring that exists during the vacuum treatment, which helps in the coalescence and removal of inclusions. [ 58–60,62,64 ]…”

Ex Situ Examination of Matrix and Inclusions of API-X100 before and after Exposure to Bitumen at Elevated Temperature

“…Further, neither MgO nor SiO 2 were observed in Figure 2 d. As shown in Figure 2 c, the Al and Mg are bound with O, whereas the Ca and Mn are bound with S, indicating a separation within the same particle, i.e., into (Ca, Mn)S and (Mg, Al)O rich portions. During refining, the improvement of steel cleanliness and modification of Al 2 O 3 and MnS are achieved by the addition of Ca [ 24 ]. The sulfur in inclusions exists as CaS with the CaS formed by the direct reaction of S and Ca as well as the reaction of CaO with dissolved Al and S in liquid steel [ 24 ].…”

“…During refining, the improvement of steel cleanliness and modification of Al 2 O 3 and MnS are achieved by the addition of Ca [ 24 ]. The sulfur in inclusions exists as CaS with the CaS formed by the direct reaction of S and Ca as well as the reaction of CaO with dissolved Al and S in liquid steel [ 24 ].…”

“…It is inferred that the inclusion in Figure 2a [24]. The sulfur in inclusions exists as CaS with the CaS formed by the direct reaction of S and Ca as well as the reaction of CaO with dissolved Al and S in liquid steel [24].…”