To evaluate the suitable sinter properties and desirable measures for blending flux in blast furnace burdens under the high-rate pulverized coal injecting operation, the off-line model experiments, actual sinter production and actual blast furnace operation have been carried out. Main results obtained are summarized as follows: 1. Sinter with low slag content was produced by reducing fluxes having SiO2-MgO and CaO components in sinter raw mixture, and lump fluxes were charged from the top or fine flux was injected through the tuyere in blast furnace to maintain the tapped slag compositions. By application of this measures, permeability in the lower part of blast furnace was improved. The test results showed that the fine flux injection is the most effective in comparison with other measures. 2. According to the hot model trial, the top charged or tuyere injected fluxes were almost slagged at the boundary of raceway. 3. Sinter with low SiO2-low MgO content produced at Muroran No. 6 sintering machine has a high reducibility in the lumpy zone due to the increase of hematite and has a good permeability in the cohesive zone due to the decrease of FeO contained melt in blast furnace.
Experimental and theoretical studies were carried out to understand the behavior of sulfur content in circulating type degasser. A sulfur content in molten steel decreases with time after flux addition or powder injection, and the final content is determined by amount of flux, composition of flux and amount of stirring gas. A kinetic model for desulfurization was developed to estimate the desulfurization rate and to predict the sulfur content. This model clarified the volumetric coefficient for desulfurization effects on the stirring energy. The approach takes into consideration the transient contribution due to the particles dispersed in the bulk metal in case of flux injection in molten steel at vacuum vessel.
Experimental and theoretical studies were carried out to understand the mechanism of deoxidation with soluble gas in molten steel in an RH degasser. A large quantity of soluble gas was dissolved in molten steel, after decreasing the pressure rapidly during degassing, inclusions were trapped by fine bubbles and are then floated and removed. Experiments demonstrated the deoxidation rate was larger than that of conventional argon gas bubbling. A kinetic model for deoxidation in the RH degasser was developed to estimate the deoxidation rates in a vacuum vessel and in a ladle, and to predict the behavior of oxygen content.
In order to produce the strip with high productivity in cold rolling of stainless steel, tandem mills have been used. Under the circumstance, tribological problems such as friction pick up and reduction of surface brightness occurred. The purpose of this study is to evaluate the antiseizure property in cold rolling of stainless steel. In order to evaluate the property, evaluating system by means of the developed simulation testing machine by the author is used. The coefficient of friction and the limitation line of friction pick up are measured and the interface temperatures which the friction pick up occurs are calculated from the experimental data. From these results, the maximum rolling speed in actual tandem mill of stainless steel is estimated. By means of the evaluating system, it is estimated that the maximum rolling speed in tandem mill with lubricant having high lubricity is about 200m/min.
Changes in microstructure of steels deformed near the Ar3 transformation temperature and the effect of reduction on the Lankford value of low carbon steels were studies. Conclusions obtained are as follows: 1. Equiaxed ferrite grains are produced by high-reduction rolling near the Ar3 temperature, and the deformation temperature range for the equiaxed ferrite expands with increasing reduction. 2. Ferrite grain size number after the high-reduction rolling is in proportion to logarithm of the Zener-Hollomon parameter. Numerical model for the prediction of steel microstructure indicates that austenite grains undergo dynamic recrystallization after the high-reduction rolling. 3. On a commercial basis, the high-reduction rolling was achieved in the final three stands with a reduction more than 50% near the Ar3 temperature at each rolling pass. Average r-value of the steel sheets produced is approximately 1.0 and fairly isotropic. Elongation of the sheets is relatively higher than that of commercial steel sheets produced through conventional reduction. Carbon was found to have little effect on the average r-value of the sheets produced with high reduction; the average of r-value is constantly 1.0 even though the carbon content is changed from 30 ppm to 0.05 mass%.
The warm bonding process for 21Ni-6Cr-Fe/36Ni-Fe bimetal has been studied and we have confirmed that bond strength was improved drastically. Furthermore, the cause of improved bond strength has been pursued and the effect of warm bonding has been examined. The bond strength of warm bonded bimetal at 200°C was about doubled than that of cold bonded bimetal at room temperature. As a result of observation of the bonded surface separated from the bonded part, as for the warm bonded bimetal, tight bond traces were magnified 2.5 times than those of the cold bonded bimetal. Adsorption water on the surface of the material is released by heating before bonding, so oxygen which remained on the bonded part declined sharply. A calculated result of the relationship between bonding reduction and bond contribution area agreed with very well a tendency of measured results of the relationship between bonding reduction and bond strength. Therefore, the reason why the bond strength of warm bonding is higher, can be explained by the expansion of the bond contribution area. The bonding reduction can decrease in warm bonding, because the bond strength of the warm bonding at 200°C at 50% reduction is equal to the bond strength of the cold bonding at room temperature at 65% reduction. The range of manufacturing will be expanded in the case of warm bonding because a bonding load was reduced 30%.
We investigated the adhesion between electrolytically chromium coated steel (ECCS) and poly(ethylene terephthalate)(PET) film by T-peel test. The ECCS and the PET film were heat sealed at a temperature between 80°C and 300°C, and then the peel strength was measured at a peel temperature of 20°C and a peel speed of 200 mm/min. The peel strength increased with increasing heat seal temperature up to 200°C and then it slightly decreased with increasing heat seal temperature above 200°C because the molecular weight of the PET film decreased with the degradation of PET in this temperature range. Secondly, the peel strength of the sample heat sealed at 300°C was measured at a peel temperature between-30°C and 100°C and a peel speed between 1 mm/min and 1000 mm/min. The peel strength depended on the peel temperature and speed and then it was brought into the time-temperature superposition principle. The master curve obtained from the principle showed that the peel strength was large under the conditions of low temperature and high speed and it was small under the conditions of high temperature and low speed.
The present study is related to a steel plate with remarkably improved fracture propagation arrest capability (crack arrestability) to minimize damage to a steel structures by preventing the propagation of cracks which may initiate due to unexpected accidents or the like. The steel plate is a hybrid-type steel plate (with multi-layer structure in its thickness direction) whose surface layer has remarkably high resistance to brittle fracture. The surface layer consists of ultra fine grains of 1 to 3 μm. The level of grain size has realized on an industrial basis for the first time in the world, and crystal orientation which is resistant to brittle fracture. Even if a brittle crack propagates in the plate, the surface layers may not fracture or fracture in a ductile manner absorbing crack propagation energy (forming so-called "shear-lip") so that the brittle crack arrestability is remarkably improved. As a new technology for producing this steel plate economically on an industrial basis, a "metallurgical phenomenon in which the microstructure of steel is refined to ultra fine grains by rolling during temperature rise" was found: When an accelerated cooling is applied interrupting rolling the plate, the surface layer of the plate is once cooled, but subsequently re-heated by internal heat in the mid-thickness of the plate. If rolling is carried out during the re-heated process, the metallurgical phenomenon described above can be realized in the commercial manufacturing process.
To improve the deep drawing of high-strength TRIP-aided dual-phase (TDP) sheet steel, Swift cup test of a 0.19C-1.54Si-1.52Mn (mass%) TDP steel was investigated at temperatures between 20°C and 250°C. The steel exhibited a larger limiting drawing ratio (LDR) than those of the conventional ferrite-bainite and ferrite-martensite dual-phase steels without retained austenite. Furthermore, the LDR was enhanced by warm drawing at about 150°C, at which the strain-induced transformation of the retained austenite particles is suppressed most. Such excellent deep drawability was caused by large local necking resistance due to "the transformation hardening" and "the stress relaxation" resulting from the strain-induced martensite transformation at the cup wall just above the punch bottom, as well as low drawing resistance of the shrinking flange.
In order to conserve the energy resources and to reduce the dependence to oil, the large coal fired power plant which was improved heat efficiency has been currently developed. A very effective method of improving the heat efficiency of steam power plant is the raise of steam conditions. It is important to develop the high temperature turbine valve forging which is used as the major structural material of this steam power plant. Modified 9Cr-1Mo (Grade 91) steel has been incorporated to the parts of boiler in thermal plants due to its excellent properties at high temperatures. In recent years, this steel has also been applied to the parts of turbine in ultra super critical plants. Former experience with a process for manufacturing forged valves of low alloy steel in thermal power plants have enabled the application of F91 steel to forge as valves. A 25-ton ingot was made by the basic electric furnace and was molded to valve bodies by combination of forging and shaping with Gas. The manufacture of trial valve with F91 steel have successfully been performed. They exhibited high quality, good creep rupture properties and excellent toughness.
Recently, gas turbine inlet temperature has been raised in order to improve thermal efficiency of power plant. The turbine inlet temperature of 1350°C has already been attained in the newly developed gas turbine. In case of the combined cycle power plant, optimization of compressor pressure ratio corresponding to the increase of turbine inlet temperature is required to improve the thermal efficiency. Development of higher creep strength material for disk, therefore, is needed to actualize higher thermal efficiency gas turbine. Extensive studies, based on previous experience, by using several heats as laboratory scale have been made for the investigation of effects of chemistry and heat treatment on mechanical properties. These laboratory studies indicated that the high purity 2.25Cr-Mo-V-Nb-N steel is suitable to the disks required to the advanced type gas turbine. A trial disk has successfully been manufactured with this steel from 1000 mm dia. electroslag remelting (ESR) ingot. Several evaluation revealed good material properties which meet the requirements of advanced type gas turbine disk, especially in toughness and aging embrittlement redundant. The manufacture of actual disks have successfully been performed. They exhibited satisfactory quality in creep rupture and toughness.