The variations of non-metallic inclusion was measured by microscopic method during the refining process of 50ton acid open-hearth furnace (lime process) in the case of chromium alloy steel, and the relation between the inclusions and the refining conditions was investigated. In this report are indicated the refining conditions to make refined steels.
The most profitable refining conditions for reduction of non-metallic inclusion were as follows:
(1) Fit silicon amount at melt-down is about 0.4 per cent.
(2) Chromium amount at melt-down must be as little as possible.
(3) Fit decarburizing amount at the oring period is about 0.6-0.7 per cent and at the liming period is about 0.5 percent.
(4) Fit use of ore is about 25-30kg per ton and lime-stone is about 7-10kg per ton.
(5) Average decarburizing velocity must be within about 0.0024-0.0028 C% per min.
(6) The oxygen amount in molten steel must be decreased as fast as possible after deoxidation.
(7) At the time of pigging, ferro-silicon and ferro-manganese are to be added simultaneously with the preliminary deoxidizer.
(8) Before ferro-chromium addition, large quantity of ferro-silicon must not be added.
(9) Fit silicon amount before tapping-off is about 0.33-0.40 per cent.
(10) The necessary ratio of Si/Mn ranges 1/1.5 to 1/2 to reduce the non-metallic inclusion.