A thermodynamic model for calculating sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal has been developed by using a thermodynamic model for calculating mass action concentrations of structural units or ion couples of ironmaking slags based on the ion and molecule coexistence theory.The calculated mass action concentrations of structural units or ion couples in CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags equilibrated with carbon saturated hot metal at 1 773 K can be applied to represent reaction ability, like classic concept of activity. The calculated total sulphur distribution ratio shows an acceptable agreement with the tested sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal from desulphurization experiments at 1 773 K. Meanwhile, the developed thermodynamic model for calculating sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal can quantitatively determine the respective contribution of free CaO and MgO in CaO-SiO 2 -MgO-Al 2 O 3 slags. A very significant difference of desulphurization ability between free CaO and MgO has been found with free CaO accounting for 97 % desulphurization potential comparing with free MgO as about 3 % in CaO-SiO 2 -MgO-Al 2 O 3 slags equilibrated with carbon saturated hot metal at 1 773 K.

A sulphide capacity prediction model of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags has been developed based on the ion and molecule coexistence theory (IMCT) and verified by two groups of sulphide capacity data of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags by different researchers. A hot metal pretreatment slags of CaO-SiO 2 -MgO-Al 2 O 3 with high binary basicity is also applied to verify the feasibility of the developed IMCT model. The predicted sulphide capacity of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags at 1 773 K as well as high alumina CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags in a temperature range of 1 773-1 873 K by the developed IMCT model has higher accuracy than the measured as well as the predicted by other sulphide capacity prediction models. The calculated equilibrium mole numbers, mass action concentrations of structural units or ion couples and optical basicity are recommended to represent slag composition for correlating with sulphide capacity of the slags compared with mass percentage of components or binary slag basicity. The developed IMCT model can calculate not only the total sulphide capacity of the slags but also the respective sulphide capacity of free CaO and MgO in the slags. Largely increasing Al 2 O 3 content from 15 to 25 % and decreasing CaO content from 40 to 34 %, MgO content from 9 to 4 % can improve contribution of free CaO from 97 to 99 % while decreasing contribution of free MgO from 3 to about 1 % to the total sulphide capacity of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags.KEY WORDS: sulphide capacity; CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags; sulphur distribution ratio; sulphide capacity model; the ion and molecule coexistence theory; blast furnace ironmaking; desulphurization potential; mass action concentration; structural units; ion couples.plied to some limited slags without widespread acceptance from viewpoint of metallurgical physicochemistry. It is necessary and interesting to develop a sulphide capacity prediction model according to intrinsic relation of sulphide capacity and sulphur distribution ratio from new viewpoint.A sulphide capacity prediction model of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags has been developed according to the ion and molecule coexistence theory [22][23][24][25][26] (2) However, the desulphurization reaction between slag and metal can be traditionally written by ion exchange reaction as Therefore, the relationship between L S and C S 2Ϫ of a slag can be obtained from Eq. (6) as 14,19) .... (7) Obviously, the uneasily measured C S 2Ϫ of slags equilibrated with gas can be calculated from the easily measured L S of slags equilibrated with metal by Eq. (7). Sulphide Capacity Model Based on IMCTAccording to the reported L S prediction model 22) between CaO-SiO 2 -MgO-Al 2 O 3 slags and hot metal based on IMCT, [22][23][24][25][26] . .

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Deoxidation during electroslag remelting of S136 die steel was experimentally studied. The characteristics of inclusions in the electrode and ESR ingots were determined by image analyzer and SEM-EDS. The results show that the oxygen content can be reduced from 89 ppm in the electrode to the lowest (12 ppm) in the ingot only when protective Ar gas remelting in combination with specially designed slag deoxidation treatment were employed simultaneously. The proportion of the oxygen combined as oxide inclusions increases with decreasing the total oxygen content in ESR ingot. The original inclusions in the electrode are mainly large (Mn,Cr)S and the large inclusions in the form of Al 2 O 3 core surrounded by an outer sulfide layer, besides a few pure Al 2 O 3 inclusions. After ESR process, while only pure Al 2 O 3 inclusions with the size of about 1 mm were observed in ESR ingots. The large inclusions in the electrode were removed during ESR process. With higher oxygen content in the ingot, the contents of inclusions and large inclusions would be relatively higher. The results from industrial experiments have confirmed the availability of the present oxygen control technique. The mechanisms of oxygen behavior and control as well as inclusion evolution during ESR process were proposed based on experimental results along with thermodynamic analysis.

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