Recent progress in grain refinement of steels was overviewed. After energetic research works since middle 1990's including big national projects in Japan, ultrafine grained steels whose mean grain size is smaller than 1μm have been realized. There are principally two ways to obtain ultrafine grained structure in steels: the ultimate thermomechanical processing using various kinds of phase transformation in steels and the intense straining using a kind of recovery and recrystallization phenomena. Large amount of plastic strain is the key factor in both case. Microstructural characteristics and mechanical properties of the ultrafine grained steels were summarized. Future advance and prospect in the ultrafine grained steels was discussed.
Raw materials of the iron ore sinter contain certain amounts of carbon and chlorine sources, which are indispensable elements to form dioxins. It has been also pointed out that the properties and contents of the solid fuels and chlorides in the raw mixture significantly affect the dioxin emissions during the iron ore sintering process. However, their control is not always practically possible because coke is the main fuel of the process and chlorides are unavoidable impurities contained in the raw materials. It has been reported that some substances act as catalysts to promote the formation of dioxins or as inhibitors to suppress it, although the detailed mechanisms have not been comprehensively understood yet. In the present study, the effect of the various additives to the raw mixture was examined by some series of sinter pot tests. Especially, copper compounds, e.g., metallic copper, copper oxide and copper chloride, were employed as catalysts and ammonia and its relating compounds to release the active nitrogen components in elevating temperatures, e.g., ammonium sulfate, urea, ethylene-di-amine and monoethanolamine, were acted as inhibitors. Further, the effects of the kinds of lime resources and iron ores on the dioxins emissions were studied.
The various factors have been pointed out which affect the dioxin emissions from the iron ore sintering process. One of the significant factors is solid fuel. It has been reported that metallurgical coke, which is the major fuel, gives the higher emissions than other carbonaceous materials, e.g., anthracite, coal, activated coke and coal. In this study, the effect of solid fuel on the dioxin emissions was experimentally studied in detail by a sinter pot test. The various coke samples corrected from different steel works were examined together with other carbonaceous materials.The effects of different pre-treatments to the coke samples, e.g., crushing, water or solvent rinsing, composting with a chloride and pre-heating in nitrogen, were further examined. The results of the present study suggest that although the other substances complicatedly affect also to the formation/decomposition and transportation of dioxins in the sintering bed, the properties of solid fuel directly and indirectly affect to the emissions. For instance, increase of the mixing ratio of coke, especially finer size of particles, increases the emission; and contrary; the water rinsing and preheating of coke decrease the emissions. The effect of nitrogen contained in the solid fuel is also discussed.
The method of the atomized water injection into the free space of coke oven chamber was studied to decrease the carbon deposits by controlling the atmospheric temperature. After the preliminary examinations, three injection lances were installed among four charging holes of an actual coke oven chamber. When the 1.7 kmol/h of water per lance was injected into the free space, the temperature decreased from 1210 to 1160 K and the carbon formation rate was decreased by 70 % (average in an oven length direction, respectively). A long-term (about two months) injection test showed that the remarkable decreases of the frequency of the manual de-carbonization operation held on the oven top and the incidence of the blockage of the standpipe. It was estimated that the decrease of the carbon deposits was brought not only by the depression of the pyrolysis reaction, but also by the dilution of the carbonization gas and the reduction of the carry-over of fines.
In order to improve the cleanliness of steel slabs cast at high speeds, the application of an intense static magnetic field in the continuous caster mold using superconducting magnets was examined for the first time in the world. Pilot scale experiments using a 5 ton steel melt were carried out. The maximum magnetic field intensity was 1.0 T. To clarify the possibility of high speed casting with a superconducting magnetic brake, casts were made at a maximum speed of 3.0 m/min. Surface and internal inclusions were remarkably reduced by application of the magnetic field. The results showed that the slab quality at a casting speed of 3.0 m/min with a static magnetic field of 1.0 T is better than that in conventional casting at 1.2 m/min without magnetic field control. Thus, these experiments confirmed the possibility of high speed casting of steel using superconducting magnets. A numerical analysis clarified the effect of the intensity of the magnetic field on the downward velocity of the molten steel in the mold. The calculated results were in good agreement with experimental data. The reduction in internal inclusions by the application of a strong magnetic field can be convincingly explained by the reduction in the downward velocity obtained in this simulation.
Effects of nature of matrix and retained austenite such as morphology, volume fraction and carbon content on strength and ductility in (0.10.3)%C-1.5%Si-1.5%Mn TRIP-aided steels were investigated. In order to change the nature of retained austenite, before heat treatment to obtain TRIP-aided steels, initial microstructure were arranged to three kinds of states, as-cold rolled, martensite and ferrite-pearlite. When initial microstructure was controlled to martensite, the highest elongation was obtained and ferrite-pearlite showed the lowest value. This tendency was obvious in high carbon containing steels. The reason why TRIP-aided steels whose initial microstructure was martensite showed the highest elongation was considered to be brought about by high stability of retained austenite during deformation. This stability comes from the fact that austenite was hemmed by ferrite lath having same orientation, in addition to the small size of it.
In recent years, a more sophisticated methodology to predict the remnant creep life is demanded as the operating duration of existing high temperature components have exceeded their design lives in various industries. In that sense, the strain rate based life assessment could lead to a better solution than conventionally applied methods relying upon hardness measurements, observation of microstructures and so on because strain rate is the one of the most sensitive indicator of damage accumulation by creep. In the present work, authors have examined the applicability of the omega method to creep curve description and life assessment using a service exposed 1.25Cr-0.5Mo steel operated for twenty three years in a refinery. The materials examined were two kinds of parent materials (a normalized and tempered plate and a forged flange) and a weld metal fabricated by a Shield Metal Arc Welding technique. Despite relatively large cast to cast variations in magnitudes of the parameters termed ε0 and Ω in the Omega method, consistent correlation between these parameters and the creep properties such as rupture life, a magnitude of strain and ductility were found. As reported in the previous works, ε0 and Ω can be expressed by the equation which is similar to that for Norton's law. Using these relationships, the creep behavior in terms of time-strain correlation from a low strain regime to rupture and time to rupture was predicted with satisfactory accuracy in the testing conditions in the present work.
For the purpose of creating super-low phosphorus stainless steel with high corrosion resistance, dephosphorization behavior of SUS316L stainless steel was experimentally studied by using Ca-CaF2 flux in a cold crucible levitation melting process. Dephosphorization, which proceeded immediately after flux addition, was more effective in lower temperature and depended upon the amount of Ca in Ca-CaF2 flux. And also rephosphorization phenomena were observed when the Ca concentration in molten metal decreased by vaporization of Ca in both molten metal and flux. In this reductional dephosphorization process, the relation between P and Ca concentrations in molten stainless steel just after dephosphorizing treatment by Ca-CaF2 flux addition, is described as following equation, [%P]2[%Ca]3 = 6.6× 10-11. The mechanism of dephosphorization and rephosphorization were discussed based on the analyzed P and Ca contents in molten metals. As a result, by the present dephosphorization process for a commercial stainless steel with 0.025% P, the super-low phosphorus stainless steel with phosphorus concentration below 0.0002% P was attained.