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For the purpose oj illl'estigatillg the illtemal state oj a blast Jumace ill olleratioll, H igashida No.5 B .F. ha villg the illller volume oj 646 m" was water-quenched while ill a lIormal operatillg cOlldilioll alld dissected ill 1968. This was the first attempt to dissect a commercial blast Jumace in J apall. Hirohata No. I B.F. with the ;/11le, volume oj 1-107 m" which had beell operated with cOllsiderably high productivity, was lIext dissected ill 1970. Theil , Kukioka No. 4 B.F. havillg the illller volume oj I 279 "," was dissected ill 197 1 to illvestigate the characteristics of a low coke rat;o bLast Ji/rnace. D escriptiolls oj th e dissectioll Ilrocedu re. general observatiolls all the internal stale oj these blast furnaces, characteristics oj each furnace. alld feiatiollshijlS betweell the illiemal state alld operalillg cOllditiolls are givell in this re/lort.

Mathematical models are pro/Josed Jor anaiYzing the blast Jurnace /Ier-Jorman.ce. H igh to/I-gas-/lressure operation and oxygen-enriched o/leration are studied with the pro/losal models. The /lroduction rate and the jiMI ratio /lredicted Jrom the models agree very well uith the o/leralion data.Calculations show lhat the tojJ-gas /Jressure ,hould o/ltimum/y be 3 kglcm 2 -C alld lhe ratio oj oxygen-enrichment about 2 0 0 , Also, /'ossibility oj /Jredict-iJlg /Jroduction limit is discussed.

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