In order not to increase the oxygen content of the steel bath, extra-low carbon rimmed steel must be melted by slag of the basicity as low as possible, and moreover the sulphur must be removed effectively and opportunely by that slag in the basic open hearth process.
Consequently the rate and the period of desulphurization in the process were so important that test melts A (charged 6T lime in 113T), B (charged 4T lime in 113T) and C (charged 5T lime in 113T and poured sand before pouring Mn) were melted in a 100T basic open hearth, and change of -ds/dt, (s)/s, excess base, fluidity of slag and slag composition for time in the process were investigated and best method of melt was discussed.
1) In melt A, charged lime floated all at once about 10mn. before melt down and the rate of desulphurization was the greatest, and max. rate was attained to within 20mn. after melt-down. But in melt B and C the max. rate was attained to about 40mn. after melt-down and the rate of desulphurization was low.
2) In melt A, excess base increased rapidly from 10mn. before melt-down and its value was 0.2-0.3 at melt-down and did not increase remarkably in the refining period. On the other hand, in melt B, the value of excess base was <0.1 at melt-down and increased remarkably in the refining period, In melt C, it was half-way between A and B.
So it was recognized that in the basic process when charged lime was increased to 6T, the rate of desulphurization was the greatest and the max. rate was attained to within 20mn. after melt down. But when more lime was used in the refining period, the basicity of slag at before pouring Mn increased. So it is necessary to control the basicity as follows:
1. In refining period, lime must be used as little as possible.
2. Slag before melt down in which FeO content is much must be used effectively.
3. Basicity must be risen at the beginning of refining period.
4. C-O boil must be used effectively in desulphurization.
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