GRAF精錬による実用鋼の品質改善

概要

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Gas Refining Arc Furnace (GRAF) is an arc heating converter which is exclusively applied for reduction refining. It makes possible to realize an effective simultaneous refining (i.e. degassing, slag refining and powder injection etc. in a same period) by a strong Ar injection through the bottom tuyere.This paper provides some examples of quality improvement as shown below, which were obtained from operation results of the 3 ton furnace.(1) Clean carbon and low alloy steels of 10-15ppm [O] and 5-10ppm [S] can be produced by simultaneous reduction of [O], [N], [H] and [S] with the strong gas stirring. CaO-CaF2 slag decreases oxygen and sulfur in molten steel to lower level than CaO-Al2O3 slag. MgO content in CaO-Al2O3 slag must be kept low in order to promote desulfurization reaction.(2) Ultra low (C+N) stainless steels, such as 26% Cr-1% Mo steels of 180ppm (C+N), can be produced by high flow rate of argon gas injection which causes decarburization and nitrogen removal.(3) In making clean molten steel by GRAF treatment, it is possible to reduce casting defects and to increase Charpy impact energy in stainless steel castings. This results in reduction of repair time, compared with electric arc furnace melting.
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