To meet the unprecedented demand of environmental issues and tightened production cost, steel industry must develop the disruptively innovative process. In the present study, totally new steelmaking process of 'Solid State Steelmaking' (or S 3 process) without BOF process or liquid state oxidation process is proposed. The overview of the new process is as follows: (1) High carbon liquid iron from the ironmaking processes is directly solidified by using a strip casting process to produce high carbon thin sheets. (2) Then, the produced cast iron sheet is decarburized by introducing oxidizing gas of H2O or CO2 in a continuous annealing line to produce low carbon steel sheets. The most beneficial aspect of the S 3 process is the elimination of several steps such as BOF, and secondary refinement processes and no formation of inclusions. To investigate the feasibility of S 3 process, the cast iron strips with various high carbon content produced by a centrifugal slip casting method are decarburized at 1 248 K and 1 373 K by using H2O-H2 gas mixture and its kinetics of the decarburization is investigated. In the decarburization process, the carbon diffusion through the decarburized austenite phase but not the decomposition of cementite is the rate controlling step of the decarburizing process. It is found that 0.5 mass% C sheets can be produced from 3.89 mass% C sheets with the thickness of 1.0 mm within 30 min at 1 373 K. Based on these results, S 3 process is confirmed to be feasible as an alternative low cost steelmaking process although the further improvement of the process will be necessary.

In-situ observations of metal-slag separation behaviors between CaO-Al2O3 based slag, iron and graphite powder mixed pellets by a confocal laser-scanning microscope (CLSM) have been carried out to find the effective way to use high Al2O3 iron ore in CCA reduction processes. Since a particular type of high Al2O3 iron ore such as laterite contains small amount of NiO and Cr2O3, the effect of NiO and Cr2O3 on the metal-slag separation behaviors were also studied. The observed metal-slag separation behaviors were analyzed based on the equilibrium phase fractions calculated by FactSage and carbon diffusion simulation in an iron sphere particle with a spot carbon source condition. Based on these in-situ observations and carbon diffusion results, starting temperature of metal-slag separation was found to correspond to the eutectic temperature of CaO-Al2O3 based slag. The iron carburization was initiated by slag melting and most of iron particles were melted within about 20 s after slag melting. The rapid carburization after slag melting was introduced by the good wettability between solid iron and molten slag. NiO and Cr2O3 additions did not change the fundamental behaviors of metal-slag separation at least up to about 3 mass%. Based on these results, it was confirmed that high Al2O3 content iron ore can be used in CCA reduction process by using CaO-Al2O3 based slag.KEY WORDS: metal-slag separation; high Al2O3 iron ore; iron carburization; spot source diffusion; CaOAl2O3 based slag.

緒言To meet the unprecedented demand of environmental issues and tightened production cost, steel industry must develop the disruptively innovative process. In the present study, totally new steelmaking process of 'Solid State Steelmaking' (or S 3 process) without BOF process, or liquid state oxidation process is proposed. The overview of the new process is as follows: (1) High carbon liquid iron is directly solidified by strip casting process to produce high carbon thin sheets. (2) Then, the produced cast iron sheet is decarburized by introducing oxidizing gas of H 2 O or CO 2 in a continuous annealing line to produce low carbon steel sheets. The most beneficial aspect of the S 3 process is the elimination of several steps such as BOF, and secondary refinement processes and no formation of inclusions. To investigate the feasibility of S 3 process, the cast iron strips with various high carbon content produced by a centrifugal slip casting method are decarburized at 1248K and 1373K by using H 2 O-H 2 gas mixture and its kinetics of the decarburization is investigated. In the decarburization process, the carbon diffusion through the decarburized austenite phase but not the decomposition of cementite is the rate controlling step of the decarburizing process. It is found that 0.5 mass% C sheets can be produced from 3.89 mass% C sheets with the thickness of 1.0 mm within 30 min at 1373K. Based on these results, S 3 process is confirmed to be feasible as an alternative low cost steelmaking process although the further improvement of the process will be necessary.