The effect of agitation on the crystallization behavior of molten 50CaO–50SiO2 and 45CaO–45SiO2–10R2O (R = Li, Na, or K, mol%) fluxes were systematically investigated by the measurement of their electrical capacitance over a wide temperature range from liquid region to well below the liquidus temperature. It is well known that the electrical capacitance of liquids is generally much higher than that of solids owing to the differences in their respective polarization mechanisms. These differences were exploited as a sensitive indicator of the crystallization of molten calcium silicates in an experimental furnace equipped with an electrical capacitance measuring system. The system comprised a Pt-based alloy crucible and a rotating rod that allowed evaluation of the effect of agitation generated by the rod, connected to a capacitance meter (LCR meter). As expected, at a particular temperature, the electrical capacitance of the molten calcium silicates underwent a precipitous decrease by roughly three orders of magnitude, which was dependent on the chemical composition. This indicated the presence of crystallization and this was confirmed by corresponding microstructural characterization. It was also found that, for the measurements acquired with rotating rod agitation, the temperatures at which the capacitance underwent the sharp decrease were higher than that identified without the agitation. This suggests that the agitation effect induced by the rotating rod accelerates the crystallization of molten calcium silicates.
The main focus of the present study was the influence of liquid metal properties on the particle size during water atomisation. Experiments for liquid iron showed that alloy additions of carbon and sulphur decreased the particle size. Moreover, it was indicated that the reduced d50 value at increased %C and %S contents may be related to a decreased viscosity and surface tension respectively. An alternative mechanism could be that raised superheats at increased carbon contents increased the total available time for atomisation. This may also have decreased the particle size. The influence of surface tension and viscosity on the d50 value was further analysed with a theoretical d50 model proposed in a previous work. A reduced viscosity from 6.8 to 4.3 mPa s decreased the d50 value with 33%. In addition, the particle size was estimated to decrease with 27% by decreasing the surface tension from 1850 to 900 mN m–1.
Sulfur dioxide is one of the precursors of acid rain and desulfurization of the exhaust gases in iron ore sintering plants is essential. For the purpose of reducing SO2 emissions of iron ore sintering, flue gas circulation sintering (FGCS) process was employed to enrich SO2 content in off-gas in this work. Effects of simulated flue gas components involving O2, CO, SO2 and H2O(g) content and temperature on iron ore sintering properties were investigated in a laboratory scale sinter pot. Experimental results show that, compared with the conventional sintering process, the sinter quality became obviously worse when the O2 content in inlet gas was less than 18%. The presence of CO in inlet gas improved the sintering properties, so did the sensible heat of the heated inlet gas. However, either an excessive SO2 content of more than 1000 ppm or H2O content of more than 5% in inlet gas would be detrimental for sintering and results in the sulfur retention in finished sinter. Thus, in the practice of FGCS, it is necessary to add O2 content up to no less than 18%, and keep H2O content less than 5% in the circulated flue gas. Beside, FGCS process is capable of enriching SO2 in off-gas with a peak value of 2006 ppm when the SO2 content of inlet gas was 500 ppm, while the maximum SO2 content was 1256 ppm in the conventional sintering process. SO2 concentrated in off-gas is more readily to be captured by the current flue gas desulfurization (FGD) technologies.
Density measurements of CaO–MnO–SiO2 slags were carried out using the Archimedean method. The density decreased with increasing temperature, and the negative temperature coefficient (–dρ/dT) was in the range of 1.46–3.10 × 10–1 kg/m3 K. At a fixed CaO content (25 wt%) and at a fixed SiO2 content (30 wt%), the density of CaO–MnO–SiO2 slag increased with increasing MnO content. The molar volumes of CaO, MnO and SiO2 at 1773 K were estimated 21.0, 17.2, and 28.0 × 10–6 m3/mol, respectively. The molar excess volume was expressed by VEx = ΩXSiO2 (1–XSiO2), where Ω = –8.97 × 10–6 m3/mol at 1773 K. The thermal expansion coefficient decreased with increasing XSiO2 because of the enhanced silicate network structure.
The furnace with maximum 20 kW was constructed using focused 8 microwave beams at 2.455 GHz into the center of applicator (electromagnetic load chamber) to make high density electromagnetic field. By heating with the power of 17.5 kW during over 35 min, pig iron has been effectively produced from 1 kg of the mixed powder of magnetite ore and 18 mass% graphite. When wustite was reduced to iron, luminescence was emitted from reactor. The rate constant of pig iron making per microwave power was 4×10–3%·s–1·kW–1. The energy efficiency was 60% for heating resources and the reduction of magnetite to wustite and 15% for the reduction of wustite to iron and molten pig iron. The concentrations of impurities of phosphorous and silicon in pig iron were much lower than those in pig iron of blast furnace because of vaporization during the reduction of iron ore under rapid heating and the low oxygen potential in existence with graphite powder. The sulfur content in pig iron was a little higher than blast furnace pig iron because of little slag.
During iron ore sintering, material coalescence in the bed determines the physical properties of the agglomerated product. Sinter density and structural quantification by 2D image analysis were used to elucidate the degree of coalescence and densification achieved during sintering. In the first program, samples - representing increasing temperatures - were obtained from three locations down a sintered bed. Sinter density, determined by liquid pycnometry, was found to be strongly dependent on green granulated bed bulk density. Image analysis results indicated a strong dependence between sinter density and porosity. Results also show that more coalescence occurs for blends containing porous ores at increasing sintering temperatures. When sinter basicity and/or coke rate is low the effect of temperature on coalescence is less pronounced for all blends. The second program compared coalescence using 19 to 21 mm sinter from eight different pot tests and blends containing significant porous ores. For the same ore blend sintered under different conditions, measured trends in sinter density and porosity were in line with expectations but the changes were not large because the material in sinter has a high specific gravity of over four and changes in porosity were less than 5%. It was concluded that small increases in coke level could decrease sinter porosity by 5%, resulting in increased sinter tumble strength. Both programs show that when melt volume is high, small increases in temperature have a significant effect on coalescence in the flame front.
A new numerical iron ore sintering model was developed recently. It takes into account most of the significant physico-chemical processes in sintering. In this study results from the model are compared with experimental results from twenty five sinter pot tests. Results indicate that the model can simulate the iron ore sintering process, as reasonable correlations between predicted and measured results were obtained in many areas. The good comparisons also indicate that the key sub-models, which have significant effects on results, viz., coke combustion, fluxes calcination, drying and condensation as well as heat and mass transfer, describe the sub-processes well. The phenomena of steady-state waste gas composition (SSWGC) and steady-state waste gas temperature (SSWGT) were simulated and analyzed by the model. A total of nine important input variables were identified and their influence on sintering time and three critical parameters which determine heat transfer during sintering were considered in the sensitivity studies. Results showed that bed bulk density, solid and gas thermal capacities, coke level and diameter and post-ignition airflow rate have the greatest influences on sintering time and the temperature profile parameters. This paper also gives suggestions on how the model can be improved.
In the blast furnace operation equipped with parallel bunker-type bell-less top, the uneven charging of materials in circumferential direction is an important issue to be avoided. This unexpected circumferential imbalance is caused by dynamics of burden materials in charging system. Full analysis is necessary for overcoming this problem. In this paper, on-site measurements such as charging rates of coke at different flow control gate openings, burden falling trajectories in the air, height distribution of burden stock surface at furnace peripheral and at orthogonal two diameter directions, were done during the filling-up stage. Dislocation of center point was found in this measurement. Mechanical equations of burden materials flow are derived for three successive regions such as bunker-to-chute colliding point, materials motion on the chute and free falling in the air. Influence of operational factors such as flow control gate opening, rotating speed and tilting angle of chute, and stock line on circumferential weight and falling point distribution was investigated. By making use of the mathematical model, H value and K value are introduced to quantitatively evaluate the circumferential height and ore/coke distribution in several charges. Measured results and simulation results matched well. Observed dislocation of center point was explained well by the mathematical simulation. It is concluded that change of rotation direction is thought to be much effective in correcting the unevenness of circumferential height distribution. On the other hand, application of bunker change in turn at coke and ore charging is effective to realize circumferential even distribution of ore/coke.
COREX is the first industrially proven smelting reduction process in the world. In this paper, a static model has been developed based on mass and heat balances. Further, a zoned model for the melter-gasifier was established on the basis of the static model. The model is capable of calculating the consumption of iron ores, coal, fluxes, the volume and composition of the slag and the volume and composition of the reducing gas from melter-gasifier. The model enables the examination of the changes of material and heat flows induced by the variations of operation parameters and raw materials chemical compositions. The model allows optimization of operation parameters of the COREX process, which can thus be used to improve the plant operation under different conditions.
Condition monitoring of the blast furnace system plays an important role in the safe and efficient production of high quality hot metal. This article proposes to use a state space model for monitoring the dynamic blast furnace system. The blast furnace data is assumed to be generated by some independent non-Gaussian source signals and a state space model is used to extract the source signals. An optimization problem with the objective function of minimum Kullback-Leibler divergence, i.e., maximum independence between the source signals is constructed. The system matrix and non-Gaussian signals are obtained by solving the optimization problem. Based on the extracted signals, the support vector data description (SVDD) is used for constructing monitoring statistics. Operational data collected from a real blast furnace containing both normal and faulty data are analyzed and used to test the proposed monitoring strategy. The proposed method is then compared with the dynamic independent component analysis (DICA) based monitoring strategy. It is shown that the state space model based monitoring strategy is more appropriate for monitoring of blast furnace faults.
With increasing competition in the global steel market, strip casting technology potentially offers an efficient, economical and environmentally-friendly approach to the production of hot-rolled, coiled steel. This review provides a summary of the basic theory and history in the developments of strip casting operations of steels, along with technical discussions regarding various strip casting initiatives that have been carried out in the past, as well as present. Two strip casting processes are discussed in detail; Twin-Roll Casting (TRC) and Horizontal Single-Belt Casting (HSBC). With its inevitable logic, the emergence of strip casting technology could have an enormous impact on the world's steel industry. This present paper reviews the progress of strip casting technology for steel from a historical perspective, and this will be followed by a sequel, reviewing recent technical developments in the field.
The chemical composition and morphology of M7C3 eutectic carbides in 19 mass% Cr–2.8 mass% C white iron with up to 4.7 mass% V additions have been studied. Eutectic colonies are mainly composed of a very fine rod-like carbides at the center and become coarser rod-like or blade-like with increased distance from the center. The volume fraction, size and distribution of rod-like and blade-like carbides in the eutectic colonies are changing with increasing vanadium content in the alloys. The formation of the eutectic colonies of different morphology is the consequence of the segregation of alloying elements in the alloy melt, which was confirmed by EDS analysis of the chemical composition of carbides. Three different compositions of M7C3 carbides were found in all tested alloys. The main difference between them is in the amount of chromium and iron and in the degree of their replacement by vanadium. Due to different melt composition in particular zones, the constitutional undercooling, and subsequently the growth rate, will be different, which will induce the formation of eutectic colonies of different morphologies.
The kinetics of a solid-liquid interface of iron with a triple point is investigated by performing a large-scale molecular dynamics (MD) simulation. A grain boundary groove evolves at the triple point of a solid-liquid interface with a bccΣ3(111) tilt grain boundary during solidification, whereas a solid-liquid interface with a twin boundary of small grain boundary energy is almost planar except in the vicinity of the triple point. The propagation velocity of the solid-liquid interface without a triple point is almost proportional to the degree of undercooling. On the other hand, that of the interface with the triple point deviates from linearity as the degree of undercooling decreases due to the balance between the driving force of the solid-liquid interface induced by undercooling and the pulling force excited by the grain boundary. Moreover, it is found that the equilibrium temperature at which the solid-liquid interface does not move is in close agreement with the curvature undercooling estimated by the Gibbs-Thomson effect. These insights are newly obtained by the large-scale MD simulation performed on a graphic processing unit (GPU), which achieves about 100 times the speed of our previous simulation performed on a single central processing unit (CPU).
The effect of cooling rate and Ti additions on the mechanical properties and carbides characteristics such as morphology, size distribution and composition was studied in high-chromium cast irons containing Fe–17 mass%Cr–4 mass%C. Based on the size distribution, composition and morphology, M7C3 type carbides were roughly classified into “primary M7C3 carbides” and “eutectic M7C3 carbides” with a 11.2 μm border size. Thereafter, the change of the solidification structure and especially the refinement of carbides size were determined. It was found that both the size and number values should be summarized systematically for primary M7C3 carbides and eutectic M7C3 carbides, respectively. Also, TiC carbides with a high formation temperature can not only act as a nuclei of M7C3 carbides, but they also contain a Ti(C, N) core. In the as-cast condition, the bulk hardness of the cast irons increases with an increased Ti content. In addition, the wear loss increases with an increased Ti content. Neither the bulk hardness nor the wear loss did change too much with an increased cooling rate.
The effect of Na2O on the crystallization kinetics of mold fluxes for casting medium carbon steel was studied in this article. The Continuous-Cooling-Transformation (CCT) diagrams and Time-Temperature-Transformation (TTT) diagrams of mold fluxes with different Na2O concentrations were constructed by using single and double hot thermocouple technique (SHTT an DHTT). The results indicated that the crystallization temperature was increased with increasing Na2O content, and the incubation time of isothermal crystallization was decreased correspondingly. X-ray diffraction analysis results suggested that cuspidine was the base crystal for all the mold fluxes, and other Na bearing crystals like Na4Al2Si2O9 (2Na2O·Al2O3·2SiO2) and Ca2Si2Na2O7 (2CaO·2SiO2·Na2O) were formed with the increase of Na2O content. It was concluded that the addition of Na2O would significantly change the crystallization behaviors of medium carbon mold fluxes.
Electroslag remelting process will be an important method for manufacturing heavy casing and forging due to its ability to improve the cleanness, macrostructure, microstructure, the distribution and size of non-metallic inclusions. In this paper, a finite element coupling model has been developed based on four electrodes configuration mode for manufacturing large ingot. Maxwell equation, Faraday's law, Navier-Stokes equation, heat transfer equation has been adopted in the model to analyze the current density, Joule-heat density, magnetic induction intensity and electromagnetic force and fluid flow distribution. The impact of slag depth, electrodes immersion depth and filling ratio on the molten pool depth has been analyzed. Results indicated that four electrodes with two sets of bifilar configuration are better than single phase with one electrode type. The molten pool is shallower for four electrodes than conventional single electrode mode, which is in favor of improving large ingot quality. The larger slag pool depth is the shallower the molten pool is. The depth of molten steel pool increases with the increasing of electrode immersion depth under the same slag depth condition. In addition, increasing filling ratio can reduce the depth of molten steel pool, but the filling ratio should be less than 0.6 according to area for general materials. Comparing to the single electrode mode four electrodes configuration mode is in favor of manufacturing large ingot.
Solid-liquid interfacial energy of steel during solidification was measured predicted from the both experimental techniques of unidirectional solidification and thermal analysis applying the dendrite growth model and heterogeneous nucleation model. Solid-liquid interfacial energy changed depending on primary phase during solidification, i.e., that of primary δ phase was larger than that of γ phase. When the primary phase was the same, solid-liquid interfacial energy increased with increasing C content. Primary dendrite arm spacing changed depending on solid-liquid interfacial energy. A trace amount of Bi which had the effect of a decrease in the solid-liquid interfacial energy of steel during solidification decreased primary and secondary dendrite arm spacing, significantly.
Chatter is the most important limitation in high speed rolling of thin strips in modern industrial cold rolling mills. To study chatter in rolling, it is necessary to set up models for the rolling process as well as the mill stand. Existing models of the rolling process are analytic and are based on many simplifying assumptions. In current work Arbitrary Lagrangian Eulerian (ALE) finite element method is utilized for the first time to model the chatter vibrations in rolling, which relaxes many of these assumptions. The model has the benefit of mass translation to computational region by using ALE technique, which tremendously reduces huge computational requirements of the common Lagrangian models. The presented model is updated for simulating the chatter mechanism by means of some online velocity sensors and signal filtering method in applying the ALE boundary conditions. Results of the finite element model were compared with the experimental measurements obtained from a full scale industrial mill. Two main chattering characteristics, i.e., the critical rolling speed and the chatter frequency, obtained from the simulation program were found to be in good agreement with that of the experimental measurements.
Blistering is a phenomenon in which oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects in steel products during hot-rolling processes. The present study investigated the effect of oxygen and the scale layer structure on blister initiation when steel is oxidized at high temperatures, and the mechanism of blister formation is discussed. The following conclusions are drawn. Blisters are not formed when the scale is a wustite mono-layer, but start to nucleate when the scale layer structure changes from the wustite mono-layer to a three-layered scale comprised of hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO). The compressive stress in the oxide scale that is applied due to the oxide scale growth is largely released when scale layer structure changes from the wustite mono-layer to the three-layered scale. This result suggests that the compressive stress is not the main factor for blister initiation. It is deduced that the pressure of the CO, CO2, and N2 gases generated at the scale/steel interface is the main factor for blister formation. The hematite layer on the top surface of scale presumably acts as a barrier of gas permeability.
Three dimensional numerical analysis model of thermal stress and deformation of slab in reheating furnace were constructed. Using this model, temperature distribution, thermal stress and deformation of 36%Ni–Fe alloy slab showing the unique thermal expansion property, called “Invar alloy”, were simulated. During slab reheating, the surface part of the slab is always a high temperature compared with the internal portion. The difference of temperature between surface and center parts of slab causes the difference of thermal expansion, and stress acts on each part of the slab. The magnitude and the direction of stress vary in accordance with plastic deformation through the reheating phase. Tensile stress can be generated on the surface part of the slab during heating. If the tensile stress acting on the surface part is large enough, surface defects may be easily caused. Therefore, it is suggested that attention should be paid to the heating pattern in order to avoid surface defects of 36%Ni–Fe alloy.
In this study, the electrochemical behaviour of different parts of welded AISI 304L stainless steel in the simulated Pressurized Water Reactor (PWR) water at room temperature (25 deg.C) was studied by the different electrochemical techniques such as the potentiodynamic polarization measurements and electrochemical impedance spectroscopy, and the compositions of the passive film formed on the surface were investigated by XPS. In addition, the surface images of samples after electrochemical tests were also carried out by SEM. The results indicated that the heat affected zone has the worst corrosion behaviour. The heat affected zone has more pits than the other parts, which may lead to reducing corrosion resistance. In addition, the passive film of different zones of the welded stainless steel has the different chemical compositions. The Cr element on the fusion zone and heat affected zone are mainly the Cr(met), Cr2O3, and Cr (OH)3.while on the base metal surface is the Cr(met), Cr2O3,Cr (OH)3 and CrO3.
Oxidation-resistant Fe–Al alloy coatings were formed on Fe substrate by a process using electrodeposition of Al and annealing. Electrodeposition in a dimethylsulfone-aluminum chloride bath containing an additive at 110°C yielded a dense Al layer adherent to the Fe substrate. Annealing of the Fe substrate with the electrodeposited Al layer generated an Fe2Al5 layer on the surface at 700°C and 800°C, and an FeAl layer at 900°C through interdiffusion. Oxidation tests at 600°C and 800°C confirmed that the FeAl layer formed through the process could act as an oxidation-resistant coating.
A new approach to predict the propagation of abnormal grain growth is presented. The growth rate of an existing large (abnormal) grain is compared to the growth rate of the surrounding (normal) grains. A very simple argument based on the reduction of grain boundary surface serves as driving force, whereas the pinning force is due to second phase precipitates. The model is successfully tested and validated on a low alloy steel presenting various state of precipitation depending on thermal history.
Hydrogen embrittlement of a Fe–18Mn–0.6C–1.5Al steel was observed in tensile deformation during cathodic hydrogen charging. The fracture mode was quasi-cleavage fracture. The relationship between diffusible hydrogen content and fracture stress was arranged by the power law like that for ferritic and Al-free TWIP steels. The Al addition did not affect the magnitude of the degradation of hydrogen embrittlement property at the same current density in TWIP steels. However, the Al-added steel showed a suppression of hydrogen entry and a larger total elongation in comparison to those of the Al-free TWIP steel in the same environment, although the Al addition decreased fracture stress. The larger elongation is one of the reasons for why the Al addition improves the hydrogen embrittlement property of cup specimens.
The effect of heat treatment on the microstructure and mechanical properties of Ti-alloyed hypereutectic High Chromium Cast Iron (HCCI) containing Fe–17 mass%Cr–4 mass%C–1.5 mass%Ti was investigated. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It was found that the number of fine secondary M7C3 carbides with a size below 1 μm increases with lower holding temperatures and shorter holding times during heat treatment. The number of coarse primary M7C3 carbides with a size above 11.2 μm increases with increasing holding temperatures and longer holding times. In addition, the number of TiC carbides increases with increasing holding times, and martensite units are more refined at longer holding times and lower holding temperatures, respectively. Moreover, the volume fraction of martensite increases with increased holding times. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI.
The demand of green and recycling technology has become, nowadays, a crucial issue for the scientific community. In this work, a preliminary approach for safe and reliable “black slag” use is discussed on chemical and microstructural characterization basis. Different EAF slag types, coming from several steelmaking plants, are analyzed. The characterization is aimed to explain the diffusion, dissolution and releasing mechanisms of the polluting elements that affect the black slags and avoid their recycle as alternative filler materials. Furthermore, the work focuses on the leaching inhibition of these dangerous species in the environment. Experiments involve SEM-EDS and SEM-EBS analysis and elution tests, according to the UNI EN 12457 standard, on high FeO content slag, on slag featured by chromium high content and on slag with low FeO content. The analysis allows to recognize the phases responsible for the polluting elements releasing and to identify the phases to be promoted in order to obtain a safe and inert by-product.