Effect of Sulfur in Steel on Transient Evolution of Inclusions During Calcium Treatment
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INTRODUCTION
THE major objectives of calcium treatment in the steel secondary refining process include (1) to diminish nozzle clogging and to improve castability by controlling the composition of inclusions[1-4] and (2) to improve the product property of the steel by controlling the shape of inclusions.[5-7] Excessive or insufficient addition of calcium can bring adverse effects. The excessive addition of calcium is detrimental to castability due to the formation of solid CaS inclusions[8-11] and the reaction between the redundant calcium in the molten steel and the alumina containing in the slag or in the refractory, which lowers the cleanliness of the steel.[12,13] If an insufficient amount of calcium is added, inclusions are hardly modified to target liquid ones so that the castability is hardly improved.[8-10] Hence, the composition of inclusions, such as CaO, CaS, and Al2O3, should be well controlled in order to obtain the desired properties of steel.[14] Thermodynamic calculation was YANG LIU, LIFENG ZHANG, YING ZHANG, HAOJIAN DUAN, YING REN, and WEN YANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted July 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
extensively performed to accurately predict the composition of inclusions considering the effect of the dissolved aluminum ([Al]s), the sulfur, the total calcium (T.Ca), and the total oxygen (T.O.) in steels.[8,12,15-17] Sulfur in the molten steel has a significant effect on inclusions during calcium treatment.[13,18-25] The formation of CaS easily occurred in steels with high sulfur and aluminum contents.[26] The generation of CaS prevented calcium from reacting with Al2O3 when the sulfur content was high in the molten steel.[4,24,27,28] Lu et al.[29] and Verma et al.[30] proposed that the extent of calcium capture depended on the sulfur content in the steel. Formation and evolution mechanisms of CaS can be summarized as follows: (1) inclusions of CaS are generated as transient phase[29-31] immediately upon calcium injection and then decompose due to the evaporation of calcium,[32] as shown in Reaction [1]; (2) inclusions of CaS form as a transient phase and then react with the alumina to yield modified inclusions by Reaction [2];[30,31,33,34] (3) inclusions of CaO-Al2O3-CaS form first and then convert to CaS-Al2O3 inclusions in steels with low T.O. and high sulfur contents since the reaction between CaO and dissolved S occurs, as shown in Reaction [3];[35] and (4) inclusions of CaS precipitate in the generated CaO-CaS-Al2O3 inclusions due to the reverse reaction of Eq. [2].[28] In the equations, symbol [ ] denotes an element dissolved in steel melt, () denotes a
component dissolved in liquid slag inclusions, and subscript ‘‘inc’’ denotes a solid (saturation) phase.[20] ðCaSÞinc ¼ ½Ca þ ½S
½1
3ðCaSÞinc þ ðAl2 O3 Þinc ¼ 3ðCaOÞinc þ 2½Al þ 3½S ½2
ðCaOÞinc þ ½S ¼ ðCaSÞinc þ ½O
½3
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