Total iron contents in iron ores have been accurately determined by JIS M 8212, in which iron ions in digested solutions of iron ores are reduced to divalent prior to redox titration. It is necessary for the iron reduction process that no reducing chemicals other than iron(II) in the decomposition solutions must not remain after the reduction with titanium(III). However, the redox reactions concerning the chemical species present in the decomposition solution has not been completely elucidated at the present time. In this paper, the redox reactions that occurred in the decomposition solution during the iron reduction in JIS M 8212 were studied by potentiometry and spectrophotometry under nitrogen atmosphere. The redox reaction of tin(II)/(IV) was very slow, causing significant effects on identifying the end point of the indicator for the iron reduction. The copper chloro-complexes were reduced with titanium(III) at a potential higher than that of indigo carmine used as a redox indicator, so that the reduced copper(I) gave a positive error to the potassium dichromate titration. The pen-tavalent vanadium was reduced with titanium (III) to form a complex with titanium, which also interfered with the potassium dichromate titration positively. To avoid these interferences, titanium(III) chloride was stoichiometrically added to the reaction mixture after addition of tin(II) chloride under nitrogen atmosphere so as to reduce only iron to divalent prior to the following redox titration. Combination of the proposed protocol with the potassium dichromate titration could successfully determine the iron content of certified reference materials of iron ores.
It has been realized that a massive-like transformation, in which δ phase (ferrite) transformed to γ phase (austenite) in the solid state during and after solidification, was selected in Fe-C steels. X-ray radiography confirmed that the massive-like transformation also occurred in Fe-18 mass%Cr-Ni alloys with Ni contents of 8, 11, 14 and 20 mass%Ni. According to the equilibrium phase diagram, δ phase is the primary phase in 8 and 11 mass%Ni alloys while γ phase in 14 and 20 mass%Ni alloys. Solidification was always initiated by nucleation of δ phase and consequently fine γ grains were formed by the massive-like transformation in 8 and 11 mass%Ni. On the other hand, nucleation of δ phase as a metastable phase was preferably selected at lower undercoolings (<50 K) in 14 and 20 mass%Ni and consequently the massive-like transformation occurred even in 14 and 20 mass%Ni alloys. Solidification of γ phase can be triggered by nucleation of δ phase followed by the massive-like transformation in the Fe-Cr-Ni with lower Cr/Ni values (the primary γ alloys). Moreover, the present study demonstrates that the massive-like transformation will be commonly observed in Fe-based alloys, in which δ and γ phases are competitive each other from a thermodynamic perspective.
Pitting corrosion behavior of SBHS500 steel in boric-borate buffer solutions containing chloride ions was investigated by macroscale and microscale polarization, immersion tests, optical microscopy, and scanning electron microscopy. Calcium sulfide inclusions (CaS) existed in the SBHS500 steel. When the specimen was immersed in a boric-borate buffer solution (pH 8.0) containing 10 mM NaCl for 24 h at 25°C, the steel matrix was not corroded. However, partial dissolution of the CaS inclusions was observed. Pitting occurred after the wet-dry corrosion test, and calcium and sulfur were detected near the center of the pit. From the results of the microscale polarization measurements, the pitting initiation sites for the SBHS500 steel were determined to be the CaS inclusions. No pitting was observed at the microscale electrode area without inclusions. In a boric-borate buffer solution containing 10 mM NaCl, the depassivation pH at the microscale electrode area without inclusions was 6.0. The depassivation at the microscale electrode area with the CaS inclusions occurred at approximately pH 6.6. The CaS inclusions in the SBHS500 steel were found to be a trigger of the depassivation of the steel matrix surrounding the inclusions.
The effect of blasting on hydrogen analysis was investigated with the aim of establishing a hydrogen analysis method for precisely measuring hydrogen that entered steel in a corrosive environment. The hydrogen existing states of the specimens blasted under various conditions were analyzed using thermal desorption analysis and the hydrogen visualization method by secondary ion mass spectrometry. The phenomenon of hydrogen entry into steel by blasting was demonstrated for the first time. It should be noted that the effect is remarkable in the case of a specimen with a large specific surface area, and the blasting becomes an inhibitory agent in the measurement of the hydrogen content in steel. The hydrogen source for increasing the hydrogen content due to blasting is mainly the water contained in the abrasive. The mechanism of increasing the hydrogen content in steel by blasting is that the fresh surface of the steel exposed by blasting reacts with the water in the abrasive, which results in the hydrogen generation and entry into steel. Additionally, the water in the abrasive remaining on the steel surface reacts with steel during the thermal desorption analysis to release hydrogen. To suppress the increase of hydrogen content by blasting, it is effective to use abrasive with low water content and to remove rust by repeating a short blasting time in order to suppress the temperature rise of the specimen.
Metallic Fe (hereinafter abbreviated as M.Fe) is suspended in steelmaking slags due to the stirring action during blowing and is mainly recovered via pulverization, classification, and magnetic separation. However, steelmaking slags are hard, and it is difficult to transform irregular-shaped and fine M.Fe in slags into free particles through the conventional pulverization method, which requires a large energy consumption. In this study, pulverization and separation experiments of steelmaking slags were performed using electrical pulse disintegration, which is completely different from the conventional pulverization method and capable of causing preferential fracture at the heterophase interface. As a result, several free particles of M.Fe with almost no slag attached were obtained from the coarse and fine pulverized particles. In addition, the electric field analysis results of a system where spherical M.Fe exists in a slag show that electric field concentration occurs in the front and back directions of the external magnetic field. The findings also show that a fracture can occur at the interface between the M.Fe and slag due to the combination of increased discharge probability, concentration of thermal energy, and generation of the Maxwell stress. Furthermore, the larger the pulverized mass, the higher the pulverization efficiency. In sum, electrical pulse disintegration may be advantageous for actual operations, where large quantities of oxides employed in the steel industry, such as steelmaking slag, spent refractories, and raw materials, should be treated in a short time with low energy consumption.
The application of high-strength steel sheets in automobiles has been increased to achieve low bodyweight and simultaneously enhance crashworthiness. High-strength steel sheets are susceptible to hydrogen embrittlement and it is essential to evaluate their delayed fracture resistance for appropriate use. Delayed fracture resistance is typically evaluated using the relationship between the amount of diffusible hydrogen and fracture strength obtained from a constant load test and the slow strain rate technique (SSRT). It is difficult to monitor the amount of diffusible hydrogen invading from the environment; the thermal desorption analysis is not a non-destructive analysis to obtain the amount of diffusible hydrogen and diffusible hydrogen is easily desorbed from the specimens. The hydrogen permeation test easily monitors the invasion of diffusible hydrogen. In this study, we evaluated the delayed fracture resistance of high-strength steel sheets using the hydrogen permeability obtained from the hydrogen permeation test. As a result, relationships between hydrogen permeability, mechanical properties obtained from SSRT, and the brittle fracture surface ratio were found to be consistent among various hydrogen invasion conditions, such as under hydrogen charging and corrosive environments. Furthermore, little diffusible hydrogen was detected using the hydrogen permeation test. Thus, delayed fracture resistance obtained from the relationship between hydrogen permeability and its mechanical properties proves the effectiveness of this method.
The formation of acicular silico-ferrite of calcium and aluminum (SFCA) with fine pores has been investigated from the perspectives of sintering temperature, cooling rate and Al2O3 concentration. Two types of hematite iron ore, lime stone, burnt lime and reagent grade Al2O3 powders were mixed so that the Al2O3 concentration was 0.8 mass% and 2 mass%. The mixed powders were uniaxially pressed into a shape of tablet, which was subjected to sintering. Both samples with low and high Al2O3 concentrations (LA and HA samples, respectively) were sintered for 10 min at constant temperatures in the range between 1215ºC and 1325ºC, and then cooled down in air. For LA samples sintered at 1255ºC, the effect of cooling rate (5ºC/min, 50ºC/min and 350ºC/min) was also examined. The samples after sintering were characterized by XRD and EPMA. XRD profiles indicated that only hematite and acicular SFCA existed above 1240ºC for LA sample and above 1230ºC for HA sample. For LA sample, in addition, EMPA indicated:
(i) the acicular SFCA melted above 1305ºC, and probably recrystallized from slag melt during the cooling cycle. The melting temperature increased with additions of Al2O3.
(ii) the fraction of fine pores except macro pores in the sinter increased with decreasing sintering temperature and decreasing cooling rate.
Hence, it is concluded that sintering at lower temperatures, more moderate cooling and higher Al2O3 concentration provide conditions suitable for formation of acicular SFCA with fine pores.
It is necessary to develop an innovative iron-making technology for low-grade iron ore containing a high concentration of P. In this study, we propose a new process for enriching phosphorus in the C2S phase by mixing high-P iron ore, CaO, and graphite in appropriate proportions and partially reducing it. In this study, high-P iron ore adjusted to various basicities and reducing agent ratios was heated at 1573 K in Ar atmosphere, and the obtained sample was analyzed by EPMA. The results showed that in the reduced sample obtained under the conditions of C/S = 2.0 and Target FetO = 60%, more than 95% of P was distributed to the C2S phase, and the P content in metallic iron was sufficiently low.
Reduction behavior of various multi-component calcium ferrites at 900°C were investigated by using in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy. Intermediate components were determined by XRD, and change in X-ray absorption spectra of Fe and Ca K-edges were analyzed to determine reaction rate constants. SFCA-I (Ca3(Ca,Fe)(Fe,Al)16O28) and SFCA (Ca2(Fe,Ca)6(Fe,Al,Si)6O20) consist of layered structure of spinel and pyroxene. Early stage of reduction reaction, diffraction peaks of spinel structure were observed which indicating SFCA-I and SFCA decomposed into these units at the first step of the reduction reactions. The spinel was reduced sequentially into FeOx then Fe. Intermediate component, Ca2(Fe,Al)2O5 originated in pyroxene module was hard to reduce and reaction was controlled by decomposition of this phase. Reduction of SFCA-I started later than SFCA (with 5.7 mol% Al2O3) under hydrogen gas reduction condition at 900°C. SFCA with a high aluminum content indicated lower reducibility than that with a low one.
There is a great demand for understanding the layering granulation of coarse and fine iron ore particles. For the understanding, a numerical simulation can be powerful approach. We here proposed a numerical simulation method, by which the deposition of fine particles with water on the surface of a coarse particle can be simulated. In the proposed model, surface of a coarse particle was modeled as a flat surface. The fine particles and water droplets were then deposited on the flat surface with minimizing the surface energy of the liquid and potential energy of the particles, resulting in a bed of the deposited particles with water under the equilibrium state with considering the influence of the physicochemical properties of the particles and liquid. First, an experiment of the deposition of spherical polymer beads with water droplet on the flat polymer sheet was performed. The simulation results showed an agreement with the experimental result, demonstrating the validity of the proposed simulation method. Second, influences on the liquid amount and contact angle (i.e., wettability) of the particles were analyzed. The simulation results suggest that the smaller contact angle (good wettability) can result in more rigid bed with less ungranulated particles.
Depletion of high-grade iron ore resources leads to increasing use of ore concentrates as raw materials for sinter. One of the methods to effectively utilize such concentrates is Mosaic EmBedding Iron Ore Sintering (MEBIOS), which pre-granulated green pellets are charged into sintering bed with the mixture of other raw materials. In this study, effects of the ore type and gangue mineral components on the strength of sintered pellet prepared of fine concentrates were examined. Green pellets were prepared using hematite and magnetite ores, burnt lime and alumina and mullite reagents, and then sintered at 1300°C. The strength of sintered pellet increases with increasing basicity (CaO/SiO2, C/S) at lower basicity region. The pellet using hematite ore with C/S above 1.5 showed higher strength than 980 N. It can be attributed to the melt formation during sintering. On the other hand, when using magnetite ore, higher strength than 980 N was obtained above C/S = 1.0. The reason is an acceleration of solid-state sintering by the volume expansion due to oxidation of magnetite to hematite. Increasing Al2O3 content leads to decreasing the strength of pellet because oxidation of magnetite is prevented by the increasing amount of formed melt.
For achieving high sinter yield and quality, various technologies are being implemented and developed to control the heat pattern during the sintering reaction. Further improvements in these technologies necessitate detailed time-course profiles of temperature at all sinter-bed heights; however, no technique has yet been reported for determining the temperature distribution in the top layers of the sinter bed at high spatial and time resolutions. Herein, detailed heat patterns in these layers were visualized by a newly developed pot test apparatus having ~300-mm sinter-bed height. The developed apparatus demonstrated the effect of ignition time on heat patterns during combustion and immediately after ignition. Ignition times of 30, 60, and 90 s demonstrated that the high-temperature holding time increased with an increase in ignition time, and this effect is more evident in the top layer. All parameters, including high-temperature holding time, flue gas composition, and sinter yield, suggest that a longer ignition time intensified coke combustion in the top half layer. The developed technique to measure the temperature in the top layer will quantitatively clarify the effect of segregation or ignition condition on the heat pattern in the top layer.