In order to establish the adequate method to reduce high segregation at the top center of rimmed steel ingot, the effect of the ingot dimension was investigated in the 1st report. In this paper, the effect of the capping method, the track time and sulphur content in the ladle which are connected with segregation during solidification are investigated; moreover, origins of the high segregation at the top center of the ingot and inverse U typed streak segregations are discussed.
The obtained results are as follows.
(1) It is scarcely possible to reduce the high segregation of a large ingot of low carbon rimmed steel (about 16t weight), by improving only the form of the ingot-cap (e.g.enlarging) and the capping method (e.g.put the cap earlier, water-cooling of the ingot top) as far as the normal open top ingot is concerned.
(2) By shortening the track time, the high segregating zone is enlarged, as the is tpeak at the top center of the ingot is removed to top side, while the 2nd peak is done to bottom side, at the sametime the latter is often raised abnormally.It about 2hr and 50min.of the track time is nccessary (though complete solidifying time-about 30min) to prevent the increase of the segregating zone, as the thickness of the ingot is 800mm (ingot weight is about 14t).
(3) It is effective for segregation to reduce sulphur content in the ladle, because the max.segregating ratio is increased linearly in accordance with the increase of sulphur content in the ladle, and its gradient is comparatively large, when sulphur content is 0.015 to 0.021% in the ladle.
(4) It is assumed that there are two kinds of segregation which are made up of the high segregating zone at the top center of the ingot;the 1st peak appears mainly by the rising and concentration of sulphur in molten steel during rimming action and the adiabatic effect of trapped CO bubbles in the segregating zone, while the 2nd peak occurs through the coagulation and precipitation of sulphide in the final solidifying zone.And it seems that the rising of sulphur in that case is connected with inverse U type streak segregations which are heavily distributed in the core zone.
