Characteristics of Inclusions in Low Carbon Al-Killed Steel during Ladle Furnace Refining and Calcium Treatment

Abstract

A plant trial of the productions of LCAK steel was performed, and characteristics of inclusions during LF refining and calcium treatment were investigated. Besides, thermodynamic diagram among magnesium, aluminum, and oxygen as well as calcium, magnesium, aluminum, and oxygen in the steel melt were studied to understand the fundamentals of inclusion modification by calcium treatment in LCAK steel. Furthermore, the change mechanisms of oxide inclusions and the precipitation of calcium sulfide were discussed. The experimental results showed that oxide inclusions were partly changed along with the path of Al2O3→MgO·Al2O3→(MgO·)CaO·Al2O3, which was a little different from the thermodynamic calculation results due to the limited kinetic conditions. Calcium treatment somehow modified the inclusions, however, many large inclusions were generated, and the modification effect would be dramatically decreased by the formation of large amount of CaS inclusions, which appeared with three main distribution forms. The formation of inclusions after calcium treatment were discussed based on the thermodynamic analysis. In order to reach the target of modification, sulfur concertration in steel should be reduced to a small amount to decrease the formation of CaS.

Factors to Determine Inclusion Compositions in Molten Steel during the Secondary Refining Process of Case-Hardening Steel

“…16) In the following, the condition for the inclusion change of Al 2 O 3 →CaO-Al 2 O 3 is proposed by comparing the previous studies. 7,8,12,[15][16][17][18] The Ca contents were compared to estimate the condition because it was very difficult to compare the activities of Ca because their experimental situations were very different from each other. Some were for carbon steels and tool steels, while some were for stainless steels.…”

“…It was reported that both MgO·Al 2 O 3 and the CaO-Al 2 O 3 system inclusions were detected in the refining stages of Al-killed steels. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] According to the report, MgO·Al 2 O 3 inclusions have the potential to cause nozzle clogging due to its high sinterability because they are stable with a solid state at the steelmaking temperatures.…”

“…3,9) In addition to the above studies, a number of researchers have conducted work on the inclusion evolutions in detail including the variations from Al 2 O 3 to the CaO-Al 2 O 3 system, 6) from Al 2 O 3 to MgO·Al 2 O 3 or partially to MgO, [7][8][9][10] and from MgO·Al 2 O 3 to the CaO-Al 2 O 3 system or to the CaO-MgO-Al 2 O 3 system. [11][12][13][14][15][16][17][18] It should be pointed out that most of researches stated above were done in laboratory scales. Thus, information about inclusion formation in practice is still lacking to understand inclusion evolution occurring in the practical refining stages.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Discussion in the following sections focuses on how each phenomenon is related to the variation at each process based on the thermodynamic calculation results. 39) Another cause could be the slag entrapment caused by the gas stirring.…”

Thermodynamic Model for Prediction of Slag-Steel-Inclusion Reactions of 304 Stainless Steels

“…[12][13][14][15][16][17]23,24) A large number of thermodynamic models based on thermodynamic and phase equilibrium data for a given system has been developed, 25) which can be used to predict the relationship between compositions of slag, steel, and inclusions in relatively simple systems, such as Al-O, [26][27][28] AlMg-O, [29][30][31] Al-Ca-O, [32][33][34] Al-Ti-O, [35][36][37] Al-Si-Ca-O, 38,39) Al-Si-Mn-O, [40][41][42] etc. However, the thermodynamic models can hardly be applied to a multicomponent system due to complicated calculations and lack of thermodynamic data.…”