Mechanism of Non-metallic Inclusion Formation and Modification and Their Deformation during Compact Strip Production (CSP) Process for Aluminum-Killed Steel

Abstract

The formation and modification of non-metallic inclusions for Al-killed steel during Compact Strip Production Process (CSP Process) was studied by industrial experiments. The thermodynamics for the formation of Al2O3, MgO·Al2O3 spinel, various calcium aluminates and CaS bearing inclusion were analyzed with the help of calculation software FactSage. It is found that the oxygen activity in liquid steel during LF (Ladle Furnace) refining is confirmed to be determined by the equilibrium between dissolved Al and Al2O3 in inclusion. There are two manners for Al2O3 inclusion modification during LF refining: the first route is followed by Al2O3-low modified calcium aluminates-liquid calcium aluminates; and the other is as Al2O3-MgO·Al2O3 spinel-CaO-MgO-Al2O3 multi-component inclusion. In addition, the liquid calcium aluminates have the tendency to be multi-component inclusion. Two types of CaS bearing inclusion could precipitate in a slab during solidification. The favorable modified inclusions deform very well along with steel matrix during rolling process even if they are associated with CaS bearing layer, while the no or low modified Al2O3 based inclusion would be rolled into pieces and the micro-cracks might be generated around the inclusions.

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

“…As for Ref. 17), it was presumed that Ca content might be similar to ours by considering the slag compositions despite no descriptions regarding Ca content in the middle stage of LF refining.…”

“…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.…”

“…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.…”