The distribution ratio of phosphorous between the CaO–CaF2 (–SiO2) flux and SiMn alloy melts at 1823 K was measured under strongly reducing atmosphere. Furthermore, thermodynamic and kinetics analyses were carried out for the environmental stability of reducing refining slags containing Ca3P2 under wet cooling conditions from the effect of slag composition on the evolution of PH3 (phosphine) gas. The distribution ratio of phosphorous between the CaO–CaF2 (–SiO2) flux and SiMn metal phases increased with increasing CaO concentration in the flux, followed by a constant value. The composition for the saturating distribution ratio of phosphorous was in good accordance to the saturation content of CaO in the CaO–CaF2 flux at 1823 K. When the Vee ratio (= CaO/SiO2) of the dephosphorization slag was greater than about 1.35, the lime and dicalcium silicate phases precipitated during solidification, resulting in an increase in the evolution rate of PH3 gas under wet and dry conditions due to an increase in the reaction area. However, when the Vee ratio of the slag was lower than about 1.35, fluorite, cuspidine, and wollastonite phases precipitated from the phase diagram, resulting in less amount of PH3 evolution during cooling because the reaction between Ca3P2 and H2O was restricted to the surface of bulk slag.
In this study the magnesium diffusion behaviour was studied in pellets with fine and course olivine, with and without additional fine quartzite (<20 μm) after isothermal reduction at 1000–1300°C. It was found that, by using a fine olivine (<38 μm) the whole magnesium content of the olivine was dissolved evenly in the wustite and in the slag, already at 1000°C, in agreement with the equilibrium tie-lines of the FeO–MgO–SiO2 phase diagram. This lead the liquid slag to precipitate into fayalitic olivine and the Al, Na, K, Ca, P-content to enrich in remaining inclusions in the olivine. This crystallization did not occur in the sample with only bentonite addition, or in the sample with unreactive olivine at these temperatures. However, with further addition of fine quartzite, the slag of the sample with coarse olivine also crystallized. In the samples reduced at 1000–1100°C, magnesium gradients could be detected in the slag phase around coarse olivine particles until entering the interaction volume of an interfering particle at around ~600 μm, or occasionally at distances of more than 1 mm. For the coarse olivine the main rise in magnesium occurs at 1200°C when the olivine particle cores begin to dissolve. The dissolution of all magnesium of the 2.5% olivine addition during oxidation lead to 6.5% Mg in the crystallized slag phase. The increase in melting point resulting from this compared to fayalite with no magnesium is ~50°C, according to thermodynamic calculations.
The characteristics of large-scale equipments, continuous process, close system and severe environment make grate-kiln process hard for parameter detection and difficult to control. The accuracy and normalization of grate-kiln process control need to be improved. Therefore, an expert system providing real time control guidance for grate-kiln pellet production is presented. The “roasting temperature of pellet in kiln & gas temperature in gas hood of preheating section” dominated control strategy is put forward and control rules for grate-kiln process are established based on expertise. A multistage reasoning strategy of “parameter status estimation – reason analysis – adjustment measure selection” is developed and forward reasoning is adopted. The application of software, which is programmed with Visual C++, is outlined. The results show that, with the assistance of expert system, stability rate of grate-kiln pellet production is increased from 91.0% to 94.2%, FeO content of finished pellet is 0.05% lower, compressive strength is 86 N.pellet–1 higher, first grade rate of pellet is increased by 2.54%.
The relationship between the strength and the microstructure of ferro-coke with hyper-coal (HPC) addition is investigated. In particular, we focused on the adhesiveness of coal particles. The strength of ferro-coke was evaluated by a tensile strength test and coke microstructure with increasing the amount of HPC addition was observed. In the observation of the microstructure, absolute maximum length and roundness of pores were measured. The result indicated that the strength of ferro-coke increases with an increase in the amount of HPC, because voids between coal particles decrease and adhesiveness of coal particles improve. A pore roundness of less than 0.2 intends coke strength, and the index decreases with HPC addition. We found that a pore roundness of less than 0.05 is included in voids between coal particles. It is suggested that mutual adhesiveness of coal particles is one of the factors affecting coke strength, and this factor can be evaluated by pores of roundness less than 0.05.
The Fe2O3 particles with the diameter between 74 and 150 μm were pre-reduced with CO at 923 K to precipitate the carbon on the surface. Then the particles were reduced by CO at 1173 K in the fluidized bed for examining the efficiency of the precipitated carbon for preventing the sticking. The result showed that the sticking of particles with high metallization ratio could be retarded or prevented by the precipitated carbon. For clarifying the prevention mechanism of the sticking, the carbon in different states was characterized by X-ray Photoelectron Spectroscopy (XPS) and its effects on sticking were discussed. The carbon on the surface was divided into two states, the graphitic carbon and the carbon in Fe3C. On account of the sticking of Fe3C at approximately 1054 K and the sticking of the sponge iron at the micron scale, the structural change of the precipitated iron by the carburization and the formation of Fe3C only retard the sticking. Therefore, the prevention of the sticking at 1173 K was attributed to the graphitic carbon. Due to the formation of metallic iron layer avoiding the consumption of the carbon and the formation of Fe3C restraining the dissolution of the graphitic carbon, the graphitic carbon accumulated on the surface of the particles, resulted in nC/nFe above 1.0, so that Fe3C and the metallic iron was coated by the graphitic carbon to block their contact among the particles.
Sulphur and alkalis in the blast furnace gas have been associated affecting the reduction swelling behaviour of iron ore pellets. A tube furnace was used in this study to examine the dynamic reduction swelling behaviour of olivine and acid pellets in CO–CO2–N2 atmosphere with sulphur and potassium in gaseous phases up to 1100°C simulating the conditions in the blast furnace shaft. No abnormal swelling was detected in sulphur or potassium containing CO–CO2–N2 atmospheres during dynamic reduction. Instead, sulphur in the reducing atmosphere was associated with pellet contraction and FeO–FeS melt formation which became more dominant with increasing sulphur partial pressures. In the extreme case, having a maximum of 1.0 vol-% S2 gas in the reducing atmosphere, the reduction reaction of wüstite to metallic iron was hindered. The formation of FeO–FeS liquid phase extends the cohesive zone towards the blast furnace top and lower temperatures and decreases the gas permeability. Furthermore, large amounts of potassium in the reducing atmosphere (max. 0.03 vol-%) led to swelling and cracking in the olivine pellets still remaining in the range of normal swelling.
A process with coal-based direct reduction followed by magnetic separation is presented for recovering metallic iron from high-phosphorus oolitic hematite in this study. Ca(OH)2 and Na2CO3 were used as additives in the reduction roasting. A Direct Reduction Iron (DRI) with 93.28 mass% Fe, and 0.07 mass% P can be obtained at a recovery percentage of 92.30 mass% under optimal conditions. The mechanisms of Ca(OH)2 and Na2CO3 were investigated by XRD and SEM with EDS. It showed that fluorapatite was reduced to P smelt into metallic iron without additives while the hematite was reduced. The addition of Ca(OH)2 can not only inhibit the reduction of fluorapatite but also promote the reduction of hematite. Na2CO3 can promote the separation of iron from slag, meanwhile it may also inhibit the reduction of fluorapatite at the presence of 15 mass% Ca(OH)2. Under optimal conditions, phosphorus remained as fluorapatite in the slag and can be removed by grinding and magnetic separation.
When the chemical potential of dissolved oxygen in a Fe–Al–O alloy is above the super-saturation level, Al2O3 inclusions will be precipitated; electrochemical reaction analysis can provide explanations of the kinetic and thermodynamic mechanisms in the formation and decomposition of Al2O3. In this study, an external potential was supplied to the electrochemical system consisting of electrodes and a MgO-stabilized ZrO2 electrolyte in order to control interfacial oxygen levels and oxide inclusions in Fe–Al–O alloys of 30, 70, 170, 560 and 1500 ppm Al at 1823 K. Critical levels of interfacial oxygen concentration were kinematically determined by concentration overvoltage using the Tafel relationship and the Nernst equation. From these experiments, the formation of Al2O3 requires interfacial and crystallization energy. Also energy for decomposition of Al2O3 should be needed the energy to break Al–O bonding and to dissociate into the Fe melt. The results of the formation and decomposition of Al2O3 at the interface between the Fe–Al–O alloy and solid electrolyte are indicated in scanning electron microscope (SEM) images and through electro-probe microscope analyzer (EPMA) analysis.
The argon-oxygen-decarburization (AOD) process is a common metallurgical treatment to decarburize high-chromium steel melts using oxygen and inert gas injection through sidewall tuyeres and a toplance. AOD converters are characterized by a fast and efficient decarburization, whereby the oxidation of chromium is reduced compared to treatments for regular steel grades like the LD process. However, low-frequency oscillations with large amplitudes can occur during the process and influence the converter's structural integrity. The aim of plant engineering is the development of an AOD converter using a vessel design that provides a fast decarburization rate and effective mixing, whereby the oscillation's amplitude and the chromium losses are as low as possible. The oscillation of the vessel is induced by the fluid flow. In this study a numerical model is presented, where the oscillation model is integrated in the CFD (computational fluid dynamics) solver by subroutines. The numerical models for both, fluid flow and vessel oscillation, are validated by experiments carried out with a 1:4 scale water model. In a further step, the numerical models are transferred to the actual AOD process. The results of the simulations are compared to experimental results obtained in plant trials. The numerical model developed in the present study can be used as a tool to design AOD vessels that fulfill the above mentioned criteria to satisfy an efficient, reliable and stable process.
The evolution mechanism of inclusions in Al-killed alloyed steel during secondary refining process was studied by industrial experiments and thermodynamic calculations. It is found that during the tapping process, Al–O deoxidization reaction is very close to equilibrium with the formation of many Al2O3 clusters. With the slag/steel reaction, inclusions vary with the route as Al2O3 inclusions→MgO–Al2O3 system inclusions→CaO–MgO–Al2O3 system inclusions, and finally change into globular inclusions surrounded by CaO–Al2O3 outer layer, of which the melting point is lower than liquid steel temperature. Since MgO is less stable than CaO and it is easier to be reduced by Al, dissolved Mg is generated earlier and faster than Ca before LF refining, thus the MgO–Al2O3 system inclusions form at first. The mapping photos of inclusions show that the evolution mechanism of MgO–Al2O3 system inclusions into CaO–MgO–Al2O3 system inclusions is Ca element substitution for Mg element in MgO–Al2O3 inclusions. The line scanning shows that there is also the reaction of Ca element substitution for Al element in the outer CaO–Al2O3 layer without MgO.
The effect of the slag and steel chemistry on the desulphurisation behaviour of stainless steel by using lime-alumina based slag has been studied on a laboratory scale. The sulphide capacity of the slag and sulphur distribution ratio between slag and steel were calculated with the aid of FactSage software. A kinetic model was developed to predict the evolution of sulphur content in molten steel during the treatment, and a good agreement between experimental data and modelling results was obtained. The sulphur removal rate in molten steel was found to be accelerated with increasing temperature and initial sulphur content in the steel. The desulphurisation capability of the optimized lime-alumina based slag was found to be similar as that of lime-fluorspar based slag.
A new slurry-making method consisting of two-stages of electro-magnetic(EM) stirring, was developed to manufacture high-quality slurries for rheo-diecasting of a near eutectic Al–Si based piston alloy. In the 1st stage of EM stirring, the optimized process conditions, such as the pouring temperature and the EM induction intensity were investigated to control the microstructural evolution of the primary Si particles. The 2nd stage of EM stirring was carried out to obtain fine and uniform globular α-Al particles and intermetallic compounds. High quality slurries with fine and uniform microstructures for rheo-diecasting can be obtained through the two-stages of EM stirring. Mechanical properties, such as hardness, high temperature tensile strength and relative wear resistance were evaluated on the specimens taken from the rheo-diecast products, and the results are compared with those of the conventional metal mold cast piston.
The pinning effects of different particles on grain growth were investigated in Fe-20 mass% Cr alloys deoxidised with Ti and Zr. More specifically, in-situ observations of the specimen surface were made during heat treatment at 1200 and 1400°C in a High Temperature - Confocal Scanning Laser Microscope (HT-CSLM). Initially, primary and secondary particles were investigated using thermodynamic equilibrium calculations and the SEM/EDX observations. Thereafter, the pinning effect of secondary nitride particles on grain boundary migration and the kinetics of the grain growth process were investigated. It was found that secondary nitride particles generally have a considerable effect on the pinning of grain boundary migration during heating treatment. This is especially true for heat treatment at 1400°C. Despite that the pinning effect of TiN particles decreases due to dissolution of these particles, the implicit pinning effects of ZrO2, ZrO2–ZrN and ZrO2–ZrN–TiN particles appear. Thus, despite that TiN individually is ineffective in causing grain-boundary pinning at high-temperature, TiN is effective as a compound with ZrO2 and ZrN in pinning grain-boundaries at high temperatures. The changing of the uniformity of grain size distributions during grain growth at different N contents and temperatures was discussed based on the consideration of the geometric standard deviation of the grain size distribution (σ g).
Euler-Euler Large Eddy Simulation (EELES) scheme has been developed to simulate the two-phase flow of argon gas and molten steel in slab continuous casting mold. The Euler-Euler approach is used to describe the equations of motion of the two-phase flow. The drag force, lift force and virtual mass force are incorporated in this model. Both turbulence of argon gas and molten steel are simulated using large eddy simulation (LES). Simulation results agree acceptably well with the water model experimental measurements of instantaneous flow structures. The flow pattern in the lower recirculation zone is expected to be asymmetrical between the left and right sides of the mold. The flow pattern is changeover; the direction of flow deviation is different from time to time. The time intervals for changeover appeared to vary randomly. The long-term asymmetry in the lower roll is due to the turbulent nature instead of the variation of other operating parameters. The turbulent flow in the mold includes multiple vortices. Those vortexes make the flow field to be more complex. Two typical transient flow structures, consisting of clockwise or counterclockwise rotational direction vortices, are found in the upper roll.
In Fe–C–V–W–Cr–Mo high speed steels, the nature of carbides during the solidification are discussed as a function of C, V and W content by the help of Partial Equilibrium (PE) approximation and thermodynamic calculations. The results show that the solidification path and carbide precipitation can be reasonably predicted by the Partial Equilibrium approximation for cooling rate lower than 10–13 K min–1. From the viewpoint of hardness control by carbides, it is found that among the main carbides MC, M6C and M7C3, the increase of C favours the formation of MC, M7C3 but decreases the hardness of M7C3 by increasing the Fe content in it. Meanwhile, the increase of V only increases the amount of MC and V content therein, and the increase of W largely increases the amount of M6C and W content in it. As a result, the addition of V and W improves the hardness of MC and M6C carbides.
A rheo-diecasting using a high-strength Al alloy has been investigated to estimate its applicability to the manufacturing of automobile suspension parts to replace the hot-forging process. The alloy used in the present study was based on the Al–Si–Mg system, which was developed recently for high mechanical performance in rheo-diecasting. Two important aspects were considered in this effort to use rheo-diecasting in practical applications: (i) optimization of the manufacturing conditions for semi-solid slurries and (ii) a proper mold design for rheo-diecasting. Electromagnetic stirring of the melt was carried out to obtain high-quality slurries. Two types of gating systems were considered: the vertical gate type and the horizontal gate type. A horizontal type of gating system with an injection velocity of 0.5–1.5 m/s, as designed using computer simulation of the fluid flow, was successfully applied to the rheo-diecasting of tension arms. X-ray inspection and microstructure observation were carried out to analyze the formation of casting defects. No defects, such as shrinkage porosity and eutectic segregation, were found in the rheo-diecast products. An evaluation of the rheo-diecast tension arms was carried out through a tensile test, a durability test and assembled module tests. The rheo-diecast products showed significant improvements in their tensile properties, such as the yield strength of 281 MPa, the tensile strength of 331 MPa, and the elongation of 11.8%, which are superior to those values of high pressure diecasting products. In the durability and assembled module tests, the rheo-diecast tension arms satisfied the specifications of hot-forged tension arms.
In hot rolling processes, the rough rolling process is an essential step for making the medium size bar for final strip products. After rough rolling, the bar usually has longitudinal bending called camber owing to several reasons. Camber should be reduced because it may cause various problems including clogging of finishing mill and strip edge folds. The difficulties to designing a camber-reducing controller are to have a good mathematical model of the rolling process and, consequently, lack of an appropriate validation scheme of the designed control algorithm before applying it to a real system. In this paper, a three-dimensional simulator for the rough rolling process using FEM (Finite Element Method) is constructed. An output feedback fuzzy controller that does not require a mathematical model is then designed to reduce camber including the lateral bar movement called side-slipping by tilting the roll. A FEM simulation combined with the proposed controller is carried out to show the effectiveness of this scheme. The results show that camber and side-slipping can be effectively reduced while achieving a reasonable wedge profile.
The aim of this work is to study residual stresses (RS) in PVD TiN and CrN coated ADI substrates with different nodule counts, austempering temperatures and surface finishing methods (grinding and polishing). Coatings were applied by arc ion plating using an industrial reactor and different sets of processing parameters. Residual stress measurements were performed by x-ray diffraction using the sin2ψ method along two principal axes on the samples surface (parallel and perpendicular to the substrate abrasion direction). The film thickness, hardness and adhesion of each coated sample were also evaluated. The results obtained indicate that RS in TiN and CrN coated samples are compressive irrespective of the different substrates, surface finishing methods and processing parameters utilized. The parallel and perpendicular RS do not vary significantly, indicating a rotationally symmetric biaxial stress state. The RS of the coated samples are not influenced by the different substrate characteristics regarding microstructure, hardness and surface roughness. The microhardness and RS of TiN and CrN coated samples increase with film thickness. The increase in substrate temperature together with the decrease in the values of BIAS voltage, arc current and chamber pressure lead to microhardness and RS reduction. Grinding produces surface hardening and reduction of the compressive RS in the substrates, but causes no variations in the RS of the TiN and CrN coated samples. The adhesion strength quality of TiN and CrN coatings to ADI substrates can be related to indices ranging from HF1 to HF2.
A cellular automaton model has been developed to simulate the austenite nucleation and growth of hypoeutectoid steel during heating process. In the model, the dissolution of pearlite, the transformation of ferrite into austenite and austenite grain coarsening were simulated. To validate the model, dilatometric and quenching experiments were carried out. The dilatometric experiment was conducted using a DIL805A dilatometer, and experimental data was employed to study phase transformation kinetics and validate the model. While the quenching experiment was conducted using a chamber electric furnace, and metallographic examination was carried out. The simulated results were compared with the experimental results and the capability of the model for quantitatively predicting the microstructure evolution of the steel in heating process was assessed.