In the iron and steel making shops, some processes are so complex that it is difficult to formulate precise control models. In such cases, process control is made on the basis of the skilled operator's heuristic knowledge. However, utilization of skilled operator's heuristic knowledge is one of the major problems to control the process precisely. In the fuzzy set theory, skilled operator's knowledge is expressed as membership functions to control the process as well as skilled operators.
The fuzzy set theory was applied to the on-line control of return fine hopper level at the sintering plant in Fukuyama Works, Nippon Kokan K.K. This was the first application of fuzzy control to the practical operation. The commercial operation has started in August 1986. As the result, the deviation of return fine hopper level decreased from 12 to 4%/h and the amount of return fine decreased by about 2kg/t-sinter.

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A heat-resistant robot has been developed to repair cracks or opened joints in the chamber wall of coke ovens which are in hot operation. The robot is placed on the ovens and inserts its arm into the hot chamber through a charging hole. This repair robot has the following capabilities and characteristics:
1) Repair of the central portion of ovens under high temperatures up to 1000°C.
2) Capability to seal narrow cracks or opened joints with a plasma spray gun.
3) Compact and fully automated operation with a high level man-machine interface.
4) No disturbance of oven operation and no large firebrick temperature drop.
Repair tests on high temperature coke ovens have confirmed the capabilities and usefulness of this system. The control accuracy of the robot is about 3mm, which is accurate enough to repair and seal a narrow crack. The wall surface after repair has a good cross profile without protrusion. The time required for repair is short enough for utilization during the period between chargings. The repair material has been found to remain on the wall for more than 3 years without spalling.

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Formation of CO macroblowholes during solidification of Fe-C alloys containing below 0.3%C (mass%) cast into a mold has been studied. The critical compositions for blowhole formation at the carbon concentration of the melt, [%C]_L≤0.3 have been determined by examining 4 types of macroblowhole formation. The critical oxygen concentration for macroblowhole formation is constant at 0.01% in the range of [%C]_L=0.005-0.03, while it decreases as the carbon content increases at [%C]_L =0.05-0.15. It is constant at [%C]_L>0.15. No macroblowholes form at [%C]_L<0.0025. These changes of critical oxygen concentration with carbon concentration have been explained by the changes of microstructures, which correspond to the morphologies of solidification interface. The mechanism of macroblowhole formation has been explained by the fact that microporosities formed in the interdendritic region grows into macroblowholes. Especially the critical composition for blowhole formation at 0.1%C has been explained quantitatively by the calculation of the model. It is found that there is agreement between the calculated and the experimental results of the critical composition.

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A plasma-torch and refractory-lined furnace with a 20kg capacity were used to study the kinetics of decarburization of iron melts. Molten iron with an initial carbon content of either 1.1 or 2.6wt% was decarburized using an argon-hydrogen plasma jet. The rate of decarburization was found to be limited by mass transfer in the melt; a mass transfer coefficient of 2.76×10^-3cm/s was obtained. Additionally, it was determined that the decarburization reaction occurred only in the region of plasma jet impingement.

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Experiments on hot metal dephosphorization by using the mixtures of Fe-oxide and Mn-oxide as oxidant were achieved by the use of Tammann furnace. The results were analyzed by the coupled reaction model which consisted of mass transfer equations of each component. Among many unknown parameters in the model, the effective equilibrium constants of P, Mn and S and the mass transfer coefficient in slag phase were changed by the experimental condition and determined by fitting to the experimental result. The observed distribution ratio of P, S and Mn was consistent with the calculated value for each experiment and the applicability of the coupled reaction model was proved.
(1) When a mixture of Fe-oxide and Mn-oxide was used as oxidant, as the ratio of FeO to the total content of Fe-oxide and Mn-oxide in flux decreased, the effective equilibrium constants of P and S and the mass transfer coefficient in slag decreased and the effective equilibrium constant of Mn increased. This result indicated that the degree of dephosphorization, desulfurization, Mn recovery and the reaction rate of these reactions increased as the increase of Fe-oxide ratio in flux.
(2) The addition of CaF₂ and CaCl₂ did not significantly change the effective equilibrium constants but modified the mass transfer coefficient in slag. Together with the result of temperature dependence, the necessary amount of CaF₂ to obtain the equivalent reaction rate for each temperature was clarified.
(3) The activity coefficients of FeO, P₂O₅ and MnO converted from the effective equilibrium constants were compared with those values calculated by the thermodynamic models. However satisfactory agreements were not obtained especially for γ_MnO.

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The effect of gas bottom-blowing condition on the refining characteristics of a top- and bottom-blowing converter in comparison with that of top-blowing condition has been studied, on the basis of the amount of accumulated oxygen in the converter (Os), which represents the change in oxidation/reduction reactions between hot metal and slag.
It is possible to express quantitatively the relationships between hard blow/soft blow by the manipulation of top-blowing jets and strong agitation/weak agitation by that of bottom-blown gas. The manipulation of the flow rate of bottom-blown gas permits to control the blowing reaction more effectively than that of the top-blowing jet.
In connection with the above fact, dynamic control of blowing reaction in the top- and bottom-blowing converter has been done, with the flow rate of bottom-blown gas as a manipulated variable and the variation of Os as a controlled variable. By the application of the multistep optimum control theory to the blowing reaction model, the stability of blowing operation and metallurgical characteristics, i.e., the ranges of changes in (T.Fe) of slag, phosphorus and manganese concentration of molten steel at blow end, were narrowed appreciably.

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Experiments were done on the prediction of slopping through direct observation of in furnace condition with an image fiberscope and by image processing and on the suppression of slopping by coke breeze injection. As a result, the following findings were obtained.
(1) Slag foaming is detected by direct observation of the condition in converter with an image fiberscope.
(2) The slag expansion rate at the occurrence of slopping is 1.2-1.5 m³/s. Slopping is predicted about 30-40s before its occurrence by providing an observation hole at a location of 2.5m below the converter top.
(3) Slopping is automatically predicted by the identification of portions corresponding to the slag and the flame through processing of the image signals obtained by the camera, by the calculation of the ratio of area corresponding to the slag, and by the combination of change in ratio with the lapse of time with the process information.
(4) Slopping is suppressed nearly 100% by direct injection of coke breeze into the foaming slag at a rate of 200kg/min for 1-2min.
(5) It is considered that the suppression of slopping by direct injection of coke breeze into the foaming slag is attributable to the loss of foam stability by local reduction of slag (for example, decrease in local concentration of P₂O₅).
(6) The occurrence ratio of slopping is been reduced from 22 to 2.7% by the application of this slopping prediction and suppression system to the commercial process, resulting in the elimination of large disturbance for blowing control.

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During multipass submerged arc welding of thick alloy steel plates for special applications such as cryogenic and pressure vessel the consideration of HAZ morphology governing its toughness is of utmost importance. Besides the current voltage and welding speed the positioning and the angle of attack of the electrode are having a considerable influence on the thermal cycle of HAZ. In this work the effects of angle of attack of the electrode to workpiece, θ, (5 to 45°) and the distance between the electrode and the weld groove edge, φ, (0.4 to 3.0mm) on the morphology and the width of the HAZ have been studied during multipass submerged arc welding of a 108mm thick ASME-SA-203 Gr.B-582 steel plate. At a given φ the increase in θ enhances the width of HAZ and the grain size in HAZ adjacent to the fusion line. At a given θ the increase in φ reduces the width of HAZ and the grain size in HAZ adjacent to the fusion line. The hardness of HAZ measured in the region near to the fusion line has been found to increase with the increase in φ under a given θ. However, at a given φ the increase in θ has been found to decrease the hardness of HAZ. The variation in hardness of HAZ has been found to be influenced primarily by the presence of some small second phase colonies having a comparatively high hardness in the matrix. It has also been observed that the standard deviation of hardness in HAZ in general is comparatively lower when the welding is carried out at a relatively low θ (20°) by keeping φ not less than 1.0mm.

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Cast duplex stainless steel (CF3M) which contained 15% ferrite was aged at 500°C for 1000h, and fracture toughness tests were conducted at various loading rates and temperatures. Fracture resistance of the aged material reduced largely compared with that of unaged material. The fracture resistance of the aged material had strain rate and temperature dipendence. Fracture morphology in the aged material also changed by loading rate and test temperature. The role of embrittled frerrite phase on toughness reduction of duplex stainless steel have described qualitatively based on scanning electron microscope observations.

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Effects of carbon on the low-cycle fatigue behavior of 20Cr-15Ni and 15Cr-15Ni stainless steels have been investigated at room temperature. Thin foils sliced from the fatigue tested specimens were observed with electron microscopy. Single crystals produced by remelting some of the steels were deformed by compression, and sliced in the predetermined directions for electron microscopy of dislocation structure. The cooperative effects of carbon and silicon addition on proof strength, cyclic deformation behavior and dislocation configurations were also studied. It was shown that carbon induced fatigue softening as reported previously in a 0.18C-25Cr-20Ni steel. The softening was observed even at the small number of cycles, and was enhanced with an increase in carbon content and a decrease in strain amplitude. Carbon also inhibited the dislocations to form cellular configurations. It was revealed that such effects of carbon were enhanced by silicon addition and that silicon increased the solid solution hardenability of carbon. All these results were qualitatively consistent with the concept that solid solution hardening by some complexes between carbon and chromium atoms existed in the as-solution treated condition and broke down during cyclic deformation resulting in fatigue softening.

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