We investigated the change in the characteristics of metal line castable for the blast furnace main trough by two methods. One was the analysis of samples which were used for a long term in actual furnaces, and the other was the analysis of samples after long heating times or multicycle heatings in the laboratory in order to understand how metal line castable deteriorate in long term use and which factors had an influence on the deterioration of those castable. The results showed that the SiC content in used castable decreased as the period of use became longer and heating-cooling cycle became more frequent. This tendency was especially common in castable containing spinel. The change in chemical composition observed in used castable was also observed in a castable sample after 15 cycles of heating at 1500℃ for 3 hours and cooling in a laboratory test. In this test, the modulus of rupture decreased and the apparent porosity increased, but the corrosion resistance did not change.
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