Scanning electron microscopy observations of fractured steel rod samples were made to clarify the formation and dissolution behaviour of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature. A large number of coarse size BN type inclusions of 2 to 5 mm were distributed at the bottom of the dimples. They did not dissolve during annealing at 1150uC. However, they had a tendency to begin dissolving and reducing its particle size with time at 1200uC and dissolved completely in a short time at 1250uC. From the chemical analyses of boron and nitrogen in many high Cr heat resistant steels and SEM observations of their fractured samples, critical boron and nitrogen concentrations for the formation of coarse size BN type inclusions of 0 . 001%B and 0 . 015%N were found, and the solubility limit of BN was represented as log [%B]522 . 45log [%N]26 . 81 at 1150uC.

Synopsis(I) Reduction of molten iron oxide and Fe0 bearing slag by H2 Ar plasma was studied using water cooled Cu crucible. The sample weights were 25 to 75g, the flow rate of mixture-gas was 20 1/mm n and DC electric power of plasma was 8.3 kW. Results obtained were as follows:(1) The reduction of molten iron oxides proceeds linearly with time and the reaction rate is proportional to the partial pressure of atomic hydrogen. Therefore, it is considered that the rate determining step is the chemical reaction between Fe0 and the atomic hydrogen formed by thermal dissociation in the plasma.(2) The rate of reduction of Fe0 bearing slag is lower than that of molten iron oxide and is proportional to the Fe0 concentration in slag. It is presumed that the reduction rate is controlled by both the chemical reaction rate of Fe0 with atomic hydrogen at the gas-solid interface and the mass transport rate of Fe0 across the boundary layer between the interface and the molten slag bulk.(3) The reduction of molten iron oxide and Fe0 bearing slag by H2 Ar plasma takes place only on the cavity formed at the surface of melt by the momentum of plasma jet gas.(II) Continuous melting of pre-reduced ore powder, obtained by a fluidized bed reduction was examined using MgO crucible and H2 Ar plasma.Following results were obtained:(1) Carry-over loss of the pre-reduced ore powder during the melting in plasma arc furnace was small, when the condition of powder feeding and plasma arc were properly chosen.(2) Reduction of Fe0 in slag, accompanied in fed material, by H2-Ar plasma, could be described by a simple model of continuous melting and reduction, based on experimental results of the reduction of Fe0 bearing slag as described (1-2).With this model, the rate of reduction during continuous melting was determined.

a cast structure or a forged and rolled structure, by assuming the same heat treatment condition and also by the appearance of the d-phase. As previously reported, 9) even the low C-9Cr-3Co-3W-V, Nb-N-B steel 18) showed the same formation behavior under the same heat treatment condition and the same contents of boron and nitrogen as the ones in this study. In other words, the formation of boron nitride inclusions is assumed to be affected mainly by the added contents of boron and nitrogen and the condition of heat treatments, and is hardly influenced by the steel structure. The Methods to Avoid the Formation of CoarseSize Boron Nitride Inclusions Boron and nitrogen that would help improve the creep strength of newly developed high Cr heat resistant steels would not perform efficiently if a large number of coarse boron nitride inclusions form. The growth of boron nitride inclusions of 20 to 30 mm are assumed to have undesirable influences on the mechanical properties of steel such as fatigue, thermal fatigue and impact. Therefore, the cooling rates after the hot working process should be controlled to avoid the formation of these inclusions. They can be avoided by adapting fast cooling rates or by controlling the heat treatments of normalizing or tempering to relatively thin and small size products such as heat exchange tubes.However, it will be difficult to employ fast cooling rates to thick and large size products such as main steam pipes that are produced from several ton steel ingots. The amount of boron and nitrogen concentrations which do not form boron nitride inclusions and the minimum adequate nitrogen contents that is needed to precipitate vanadium and niobium nitrides will have to be determined. Figure 12 shows the relation between boron and nitrogen in high Cr heat resistant steels that is necessary to form coarse size boron nitride inclusions 9) and the limits of boron and nitrogen concentrations in P122 and P92 steels by the ASME code.3) As shown in Fig. 12, boron and nitrogen concentrations are include in the area of boron nitride formation for both P122 and P92 steels. Therefore, we should try to obtain the boron and nitrogen concentration limit at the left side of the plot and the lowest point in the concentration limit area, to avoid the formation of coarse size boron nitride inclusions as much as possible. ConclusionsThe influences of remelting, forging, rolling and heat treatment on the boron nitride inclusion that were formed in P122 heat resistant steel containing 0.003 mass% boron and 0.06 mass% nitrogen were investigated by the SEM observation of the fractured surface. The following conclusions were drawn from this investigation.(1) The temperature where the boron nitride inclusion formation in P122 steel begins is between 1 150 to 1 200°C.(2) After forging and rolling, boron nitride inclusions form agglomerated groups of 20 to 30 mm during very slow cooling or of 10 to 20 mm during medium slow cooling.(3) Re-dissolution of boron nitride inclusions into the steel matrix begins above 1 200°...

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