Progress of hot metal treatment technology in Japan and scientific researches which supported the technology are outlined. To meet the increasingly severe customer requirements for steel properties, integrated steelmakers developed technologies for purifying molten steel, centering on the divided refining process, consisting of hot metal treatment, BOF decarburization and secondary refining. The hot metal treatment processes were put into practice at almost all steelworks in the 1980 s. Since the 1990 s, all companies have improved and restructured the hot metal treatment facilities, aiming at not only improvement of refining efficiency so as to achieve a higher degree of purity steel with higher productivity at lower cost, but also reduction in slag volume with the environmental problems taken into consideration. The directions in which hot metal treatment technology is to be pursued in the future are also commented briefly.
The development and prospects of the combined blowing converter in Japan are discussed from the historical point of view. Most combined blowing technologies were developed in the 1980s and completed in the 1990s. Aspects relevant to future converter technologies are reviewed, including multiphase flow, the coupled-reaction model, bottom gas injection phenomena, and post combustion.
As steel industry of our country developed, upsizing of steel materials and the mass production of various steel grades such as the high alloyed steel and the high quality steel have been requested. Crude steel was mass-produced from the late 1950 s by the primary refining process of the BOF and the electric arc furnace. After the initial period, the secondary refining technology replaced a part of the primary refining process function, improved the efficiency of the primary refining process by adding the new function and supplied the molten steel to the casting process stably. Furthermore, the secondary refining process enabled the production of the high quality steel materials and progressed rapidly in the after 70 s. In our country, nearly 100% of the BOF steel and the electric arc furnace steel are treated by the secondary refining process now. Vacuum degassing is performed for approximately 80% of BOF steel. A variety of secondary refining processings are carried out in the electric furnace for the production of special steels, and the LF processing ratio reaches 95% in the electric arc furnace for common steel production. Secondary refining process comes to take charge of a reduction refining function. In this paper, the development of the secondary refining process and technology, and its history are looked back, and the future prospects are reviewed.
The continuous casting technology of steels in Japan has been well-developed over the past 60 years since the introduction of a continuous casting machine and its technology from Europe. The continuous casting ratio of steels has reached over 98%. A large number of researchers have been conducted on fundamental researches for the improvement of continuous casting technologies. Some of the development histories are summarized in the book, entitled ìHistory of Steel Continuous Casting Technology in Japanî in 1996. In recent years, a near-net-casting has been also developed for higher efficiencies and productivity in many countries. This paper reviews the histories of development in the continuous casting in Japan on technologies regarding (1) Pouring to mold, (2) mold lubrication, (3) cooling of a solidifying shell, (4) machine type, (5) reduction of casting defects, (6) tundish, (7) measurement and control processes and (8) different types continuous castings. The fundamental researches on the major casting defects including macrosegregation, cracking and inclusions that are formed in a continuous casting are also reviewed. Some brief comments on the further development of continuous casting in the future are described.
In recent years, system technologies have been developed remarkably and also indispensable for steelmaking processes, especially for guaranteeing higher quality of products as well as realizing higher efficiency and lower cost in manufacturing. In this paper, a state-of-the-art survey concerning the development and the utilization of system technologies is presented with some typical examples, and future directions are discussed based on the analysis of the current situations and issues.
The simulation model for hot metal dephosphorization process by multi-phase slag was developed by the ISIJ research group of “Process Simulation for Dephosphorization of Pig Iron by Multi-Phases”. The model employed a multi-scale model, which was an integration of macro- and meso-scale models. In the macro-scale model, the reactions between the liquid slag and the metal were described by a coupled reaction model. In the meso-scale model, the fraction of solid phase was calculated from the numerated phase diagram data, and the distribution of P2O5 between solid and liquid slag phases was evaluated by thermodynamic data. The dissolution of flux was also considered in the model. The program runs on a normal Windows personal computer (PC) with Microsoft C++ 2010. The pull-down menu is available for selecting jobs. The input and output data files are written in CSV style, which can be handled easily by a spreadsheet processor such as Microsoft Excel. One calculation requires less than a second when a typical PC is used. This program was applied the various experiments carried out by five steelmaking companies in Japan and the reasonable results were obtained. The use of the program is sufficiently easy and enables the user to apply it to processes control as well as process simulation.
Owing to the increasing demand of steel in East Asia, it will become necessary to use low-grade iron ore in blast furnaces. The use of low-grade iron ore will increase the phosphorus content in hot metal. In India, iron ore containing 0.1% or more phosphorus has already been mined and used, and the phosphorus content in hot metal has been 0.25% or more. In this paper, the conditions for decreasing the phosphorus content in steel from hot metal with high phosphorus content are discussed on the basis of a simulation model developed by the ISIJ research group– “Process Simulation for Dephosphorization of Pig Iron by Multi-Phases.” Using this simulation model, the optimum conditions for BOF decarburization treatment with or without a hot metal dephosphorization process were determined to decrease the phosphorus content in steel at the end point of BOF operation to 0.015% from 0.3% in the hot metal. The results showed that when the phosphorus content decreased to 0.015% by the hot metal dephosphorization treatment before BOF decarburization, it was possible to produce steel with only a 4 kg/t increase in lime consumption. On the other hand, steel production by the single slag operation of the BOF treatment was very difficult. In the case of the double slag operation, although steel production became possible, the unit consumption of lime increased to 20 kg/t or more. The obtained results established the high potential of the hot metal dephosphorization process for the treatment of hot metal with high phosphorus content.
To clarify the optimum dephosphorization condition with CaF2 less fluxes, laboratory experiments were performed. The effect of CaF2 addition, CaO diameter, temperature and slag basicity on the rate of hot metal dephosphorization were evaluated. Addition of small amount of CaF2 drastically increased the rate of dephosphorization. This effect was attributed to the decrease in solid fraction of slag. At the conditions of the target basicity of 2.0 and using powder CaO, optimum temperature range was observed. At low temperature condition of 1270 °C, lower rate of dephosphorization was obtained. These phenomena are attributed to the loss of slag fluidity.
It has been reported that aluminum in liquid iron increase the rate of desulfurization by lowering oxygen potential and producing liquid calcium aluminate. In this study, the variations of the oxygen partial pressures during sulfur removal from hot metal were determined by employing an electrochemical technique involving stabilized zirconia electrolyte. The present experimental results demonstrated that aluminum could decrease the oxygen potentials in carbon-saturated liquid iron and thereby could decrease the equilibrium sulfur contents. The addition of nepheline as a substitute for fluorspar improved the rate of desulfurization due to the formation of liquid phase.
To achieve high-efficiency intermediate deslagging in MURC (Multi-Refining Converter) process is an important issue. In order to obtain the basic knowledge concerning the fluid behavior during intermediate deslagging, simulation using water model was carried out and the effects of tilting pattern, properties of liquids and the shape of vessel mouth (installation of weir) were evaluated. As the results, the followings were observed. 1)The metal starts spilling before reaching the inclined angle that is geometrically calculated from the remaining volume inside the vessel due to the uplift of metal edge. 2)Reduction of deslagging time with the same deslagged amount becomes possible by using the slowdown pattern in tilting rate near the end of deslagging. 3) Metal is easy to spill when slag viscosity is high and the density difference between metal and slag is small. 4) Deslagging is improved by installation of weir to the vessel mouth. Based on the results, a simple model was built by focusing on the balance between shear force and gravitational force (buoyant force) and the metal-spill-point and deslagged amount were estimated. Furthermore, from the detailed analysis using computational fluid dynamics, the above-mentioned phenomena were reproduced.
In order to increase the heat efficiency of the chromium ore smelting reduction furnace, a heated ore addition technology using a burner in the converter was developed. 5 ton scale pilot converter tests were conducted. Based on the results of the pilot converter tests, this technology has been applied to the actual process. The results are summarized as follows: 1)Compared with the conventional process (without burner), the amount of effective heat transfer increased by 18% with addition of heated ore using the burner in 5 ton converter. 2)A decrease in the off-gas temperature and reduction of the thermal load on the refractory were also confirmed in 5 ton converter. 3)The heat transfer ratio of burner combustion with addition of heated ore increased up to about 90%, depending on the increase of the ore feeding rate in 5 ton converter. 4)From the results of a numerical simulation, it was revealed that the total sensible heat of all particles increases as the ore feeding rate increases. As a result, the ore particles heated by the flame function as a medium of heat transfer from the flame to the molten metal and slag. 5)This technology was applied to an actual smelting reduction furnace at JFE Steel East Japan Works (Chiba). As in the tests with 5 ton converter, the heat transfer ratio of burner combustion with addition of heated ore was about 90%. The supplied energy per unit of chromium ore added to the furnace decreased by 17%.
A study was carried out to understand the effect of non-metallic inclusion compositions on nozzle clogging of a Fe-Cr-Ni-Mo system stainless steel, aiming at developing the consecutive continuous casting technology of two ladles. At first, nozzle clogging took place leading to the reduction in casting speed. It was found that the major oxide phase in the accretion observed on the nozzle wall consisted of MgO・Al2O3 spinel oxides. The inclusions observed in the sample taken from the tundish were spinel which was consistent with the accretion composition. The phase stability diagram was calculated with the thermodynamic data to understand how to control the inclusion compositions to prevent nozzle clogging. It was suggested that the inclusion composition should be controlled to MgO with high melting point as a countermeasure. It was confirmed that this control brought the successful casting practices avoiding nozzle clogging. The consistency of the inclusion compositions with the phase stability diagram proved the validity to apply thermodynamic data based on the standard taking infinite dilute solution for the present alloy system.
Non-metallic inclusions in steel considerably affect various properties of products and thus it is important to understand the formation and evolution behaviors of inclusions in steel. Much attention is paid to steel grades containing Ti because of its excellent mechanical properties and it is well known that such functions are mainly created by Ti-based inclusions. In the present study, the effect of Ti addition and cooling patterns on the inclusion properties such as size and composition was observed to understand fundamental behaviors of inclusions in steel containing Ti and N. Ti addition had following effects: (1) decrease of average size of inclusions, (2) increase of the fraction of TiN and other Ti-containing inclusions, (3) promotion of the transition of MnS single phase inclusion into Mn-Ti based inclusions. Slower cooling rate promoted TiN formation and growth. Inclusions with several phases were observed and its formation mechanism was discussed by considering the morphology and composition of oxide phases at center of multi-phase inclusions.
In order to improve the surface qualities of continuously-cast slabs, the pulsative electromagnetic casting (EMC) technique was developed. However, when the plant test was carried out, bleeds occurred at the slab surface. In this work, the mechanism of formation of the bleeds was investigated experimentally, and the numerical simulation of the flow of molten steel and mold flux was also conducted. The thickness of molten mold flux above steel decreased due to the increase in flux consumption when applying the electromagnetic force and the interface between the molten steel and flux fluctuated locally due to the interference of molten steel flows. The bleeds were considered to be caused by insufficient lubrication with mold flux. As countermeasures, the local fluctuation of interface between the molten steel and flux was stabilized by applying a suitable coil current, and the thickness of the molten mold flux was increased by using a flux with a higher melting rate. Finally the bleeds were diminished and a smooth cast surface without defects was obtained at the plant test of the pulsative EMC technique by applying these countermeasures.
In order to clarify the mechanism of suppression of unbalanced flow in the mold under high throughput conditions in continuous casting, the relationship the distribution of defects entrapped in the solidified shell was measured by ultrasonic defect detection, and the correlation between the defects distribution and the unbalanced flow in the mold when using an electromagnetic brake was investigated. The main results are summarized as follows. (1) By using a new ultrasonic testing system, rapid and accurate measurement could be achieved for the depth of defects with diameters of more than 0.6 mm in the region 2-10 mm from the slab surface. (2) The measured value of the effect of the electromagnetic brake on decreasing molten steel momentum was consistent with the calculated value. Molten steel momentum could be reduced by more than 50% when the Stuart number was more than 3.5. (3) The position of entrapped defects in the solidified shell in the mold was influenced by the throughput condition and the magnetic flux density of the electromagnetic brake.
Optical properties were measured for synthesised mould flux containing iron oxides, and radiative heat transfer from the steel shell to the mould was estimated to investigate the effect of CaF2 additions to mould fluxes on heat transfer in continuous casting process from the perspective of absorption by iron oxides. The sample compositions were designed so that T.CaO/SiO2=1 and Fe2O3 concentration was 1 mass%. All the glassy samples show optical properties and radiative heat transfer similar to each other, suggesting that there is no effect of CaF2 on the basicity of flux. In addition, there was no change in the absorption wavelength and the absorptivity by divalent Fe ion, suggesting that the coordination of Fe ions and the electric charge strength do not change with additions of fluoride. On the other hand, crystallised samples show reflectivity increase and transmissivity decrease with increasing CaF2 concentration, resulting in the decrease of the radiative heat transfer. The fluoride facilitates the crystallization of the flux, and induces a decrease in radiative heat transfer. The Fe ions were mainly contained in the glassy portion rather than in crystals (cuspidine or CaF2) in the crystallised samples. To optimize the refractive index in the glassy portion is a possible way to reduce further radiative heat transfer of mould flux containing iron oxides.
Continuous casting of hypo-peritectic steel was conducted with a pilot slab caster. Such experimental data as local heat flux, thickness of solidified shell or mold flux film, and dendrite primary arm spacing were obtained. On the basis of these experimental results, influence of mold flux on initial solidification in the mold was discussed. With mild cooling by crystallization of mold flux, local heat flux and solidification rate decreased in the mold. Their changes quantitatively correspond to each other. Dendrite primary arm spacing increased with the mild cooling. Relationship between the arm spacing and cooling rate was established and cooling rate on quite initial stage of solidification was estimated. Cooling rate at 1 mm thickness of solidified shell was estimated as about 10,000-17,000 K/min and changed by mold flux. Unevenness of the solidified shell thickness becomes remarkable when the shell grows to be 1 mm thick. Relation between the unevenness and the cooling rate was discussed, and critical cooling rate against the uneven solidification was observed around 17,000 K/min. Thermal resistance of mold flux film was also evaluated and it was clarified that thermal resistance in the film is larger than that by air gap, and Crystallization in the film contributes to increase of both resistances.