The goal of this work is to investigate the reduction of chromium from a quaternary slag by carbon dissolved in liquid steel. Laboratory scale experiments were conducted to study the reduction of chromium oxides in the slag by carbon dissolved in the melt. These experiments were made under different conditions of slag basicity and amount of added carbon. Thermodynamic calculations based on Double Sublattice model were applied using the commercial software Thermo-Calc, with the IRSID database. The results obtained showed good correlation with practical and calculated results, making it possible to predict equilibrium conditions of the system and to determine the activities of chromium oxides in the slag.
As an initial stage of the creation of a new solid material for carbon dioxide capture and storage (CCS), the behaviors of CO2 absorption and desorption were investigated in terms of Li2O–TiO2 compounds. The Li4TiO4 specimen began to absorb CO2 at 250°C with the formation of Li2TiO3 and Li2CO3 phases. The absorption rate was significantly enhanced at 710°C by the formation of the Li2CO3-based liquid phase. Decomposition of Li2CO3 was observed above 960°C in CO2 atmosphere and above 900°C in Ar-10%CO2 atmosphere. In the formation of solid Li2CO3 after Li4TiO4/CO2 reaction below 700°C, the reaction rate of CO2 absorption increased with the decreasing diameter of Li4TiO4 particles. The area ratio of the Li4TiO4 phase decreased and that of the Li2TiO3 phase increased in the Li4TiO4 sample after repeating the reaction path of CO2 absorption and desorption.
A tensile strength of a coke picked from different distance away from coke oven wall was experimentally measured to investigate the effect of carbonization process by using the diametral-compression test. Changes of the microscopic structure of the coke picked from different distance away from coke oven wall were also observed to evaluate the effect of those on coke strength. The tensile strength of the coke vertically picked from the distance of 0–60 mm away from coke oven wall was higher than one picked from the distance of 60–120 mm away from coke oven wall. The microscopic structure of the coke vertically picked from the distance of 0–60 mm from coke oven wall was more homogeneous than the one vertically picked from the distance of 60–120 mm away from coke oven wall. Furthermore, a fracture analysis using RBSM (Rigid Bodies-Spring Model) assuming diametral-compression test was carried out in order to investigate fracture behavior of the coke. Numerical results showed that the fracture started at the near center of analytical object and crack vertically propagates by shear stress and tensile stress. Strain-load curve of analytical result was in good agreement with one of experimental result. It is supposed that fracture analysis using RBSM assuming diametral-compression test can reproduce the fracture behavior of coke.
This paper proposes a novel process for the pre-treatment of high phosphorus iron ore by pre-reduction, air jet milling and screening methods. The dephosphorization behavior was evaluated as a function of such parameters as the milling gas kinetic energy, Ek and specific energy consumption, Esp. At a fixed milling gas kinetic energy, the dephosphorization treatment behavior was highly dependent on the solids feed rate and the degree of sintering of the pre-reduced phases. The proposed pre-treatment process involves a pre-reduction process by carbonaceous materials at 1200°C, and air jet milling using Ek of 325.24 kJ and Esp of 3.24 kJkg–1h followed by fine screen separation using a 25 μm screen size.
In order to analyze and explore the cost minimum operation in iron ore sintering, a differential equation of cost change with respect to various operational factors was derived from a constitutive formula of energy cost for a unit sinter production; and then the equation was modified to a few equations describing the cost-minimum state and determinants guiding directions for four different operational actions. As an example for the former equations, the operation under the minimum consumption ensures the relation between yield (η) and coke content (%C) like δη/δ(%C)=η/%C. The operation-guide determinants were derived for coke, bed height, bulk density and burnt lime action; then, the factors affecting the sign of each determinant were discussed. In any action case the ratio of partial costs was a critical factor to decide the direction of action. Moreover in case of bed height and burnt lime actions, the coefficient of yield with respect to bed height (βη-H) and that of bed permeation with respect to burnt lime content (βκ-BL) were critical, respectively.
Consistently high level of cleanliness needs to be ensured, particularly in grades of steel used for critical applications. In addition, preventing the formation of alumina rich non-deformable inclusions is important in high carbon steels for the deep drawing applications. In the present work, influence of billet caster up-gradation on the overall cleanliness of high carbon steel has been investigated, mechanical characteristics of oxide inclusions evaluated and assessment of the existing deoxidation practice have been done. In general, there has been improvement in the overall level of steel cleanliness due to increasing tundish capacity, submerge pouring of liquid steel and application of in-mold EMS. These measures have effectively controlled the inclusions originating from reoxidation of liquid steel. However, the characteristics of the oxide inclusions generated during the deoxidation process in ladle remained same even after the caster up-gradation. The conventional deoxidation practice was found to generate mostly inclusions of variable compositions and alumina rich non-deformable inclusions, which makes the steel less suitable for very fine wire drawing applications. Appropriate measures have been proposed for further improvement in the inclusion characteristics while processing those grades of steels during the secondary steelmaking.
The Ti deoxidation equilibrium of Fe–Ni, Fe–Cr and Fe–Cr–Ni alloys saturated with ‘Ti3O5’ and Ti2O3 phase was clarified in previous researches. Solubility of Fe, Cr and/or Ni oxides in ‘Ti3O5’ phase equilibrated with liquid Fe–Ni, Fe–Cr and Fe–Cr–Ni alloys at 1823 K to 1923 K were measured by SEM–EDS. It was confirmed that the solubility of Fe, Cr and/or Ni oxide in ‘Ti3O5’ phase at low Ti content increase with decrease of Ti content in alloys. Titanium content that dissolution of Fe, Cr and/or Ni oxide into ‘Ti3O5’ phase becomes apparent increases with increase of Cr and/or Ni content of the alloys. The activity of Ti deoxidation product, ‘Ti3O5’ phase, equilibrated with Fe–Ni, Fe–Cr and Fe–Cr–Ni alloys was also evaluated by Redlich-Kister type polynomial. Equilibrium between Ti and O at low Ti content of those alloys was analyzed using the activity of ‘Ti3O5’ phase.
Inert gas shrouding practices in water model tundishes were mathematically simulated using the finite volume based program ANSYS 12. The fluid within the tundish and the ladle shroud was assumed to be Newtonian and incompressible (water), and the flow within the shroud was assumed to be predominantly bubbly i.e. discrete bubbles are formed, and these move down the shroud along with the down-flowing water. Thus, the numerical model was developed using the Discrete Phase Modeling (DPM) approach, along with the standard k-ε turbulence model with two way turbulence coupling. Predicted flow fields, slag behaviour and bubble trajectories were investigated and then compared with experiments in the full scale and one third scale water model tundishes. Various experimental measurements compared well with predictions from the mathematical model, which was shown to slightly over-estimate depths of penetration of the bubble column by 5–15%.
The fluid flow in the SEN and its outflow characteristic at nozzle port, after installation of slide gate system to control molten steel flow rate, have been investigated by mathematical simulation using standard k-ε model, RSM and experimental measurement by Ultrasonic Doppler Velocimeter (UDV) respectively, and the adaptability of turbulence models has been discussed. The research results indicate that, due to the throttle action of slide gate, a secondary flow and a separated flow are generated under the slide gate, and a swirl flow appears at nozzle port, with the swirl direction from the clogging side via nozzle bottom to the opening side of the slide gate. The swirl direction of outflow at nozzle port calculated by standard k-ε model is contrary to the measurement result by UDV and, the calculated result obtained by RSM is close to the UDV data. Based on kinetic theory of molecular, theoretical deficiencies of the Boussinesq hypothesis has been discussed.
With the data of silicon content in the hot metal collected from the three blast furnaces as a sample space, the multiscale trend decomposition was employed to analyze the local trend of time series. The singularity of fluctuation at different scales was quantified by the Hurst index. The Horton-Strahler topological classification of ramified pattern was constructed to identify the difference between Hurst index computed on different time series. Simulation results demonstrate that time series of silicon content in the hot metal from the three different blast furnaces exhibits significant local singularity. The duration of increasing and decreasing trends as well as the fluctuation of silicon content series are the main cause of differences among the Hurst index computed.
The maximum size of single inclusion particles and clusters in an Fe–10 mass% Ni alloy deoxidized with Al or Ti/Al were examined using extreme value analysis. The results obtained from conventional two-dimensional observations of inclusions on a polished cross section of metal sample (the CS-method) were compared to those from three-dimensional investigations of inclusions on a film filter after electrolytic extraction (the EE-method). It was found that the EE-method can successfully be used as a reference method for estimation of the probable maximum size of single inclusion particles and clusters by using an extreme value distribution (EVD). The EVD results for single inclusion particles obtained from the EE-method agreed satisfactorily well with those from a conventional CS-method. However, this required identification as well as neglect of pores on an investigated cross section of a metal sample. The predicted maximum size of single inclusion particles in a 1 mm3 volume was confirmed by results from the EE-method.
Key parameters for controlled rolling and accelerated cooling process have been determined to industrially produce 45–50 mm heavy plates of microalloyed low carbon-equivalent SiMnCrNiCu steel for offshore structures and shipbuilding. The plates were hot rolled from continuously cast slabs with a four-high 5000 mm width mill and cooled with an accelerate-cooling system. The process was characterized by heavy finish rolling reduction ratio over 63% in austenite non-recrystallization region and interrupted accelerated cooling at 460°C to form quasipolygonal ferrite and acicular ferrite as majorities of microstructure. Yield-strength greater than 460 MPa was achieved at room temperature, and Charpy V notch impact energies of 150 to ~300 J were secured even at –80°C. Coarse granular bainite and/or degenerate upper bainite were identified harmful, causing cleavage fracture. The correlation between the fraction of high-angle grain boundaries (≥15 deg) and ductile-brittle transition temperature was derived quantitatively for the advanced heavy plates. The effect of heat input during heavy plate welding on the microstructure and mechanical properties in the coarse-grained heat affect zone was discussed.
The dynamic transformation of austenite was studied in a 0.06%C low carbon steel by deforming to strains of 0.25–5.0 at strain rates of 0.04 and 0.4 s–1 over the temperature range 877–917°C. All these temperatures are at or above the conventional Ae3. Two critical strains were detected, the first (about ε=0.2) was for the formation of strain-induced ferrite; the second (about ε=1.5) was the critical strain for the reverse transformation, which was gradual and only observed in specimens deformed at the lower strain rate. After deformation, the strain-induced ferrite was stable for about 60 s of isothermal holding. However, after this time, the reverse transformation began to take place, approaching saturation in about 240 s. The influence of strain, strain rate and temperature on the dynamic transformation is described. The results indicate that reverse transformation is unlikely to take place in the finishing stands of strip mills due to the high strain rates and short interpass times involved. It is also suggested that conventional phase diagrams do not apply to austenite undergoing deformation in rolling mills.
This study presents the development of refined three dimensional (3D) finite element (FE) models with the ability to reliably simulate the mechanical behavior of full-scale bolted T-stub connections. FE models incorporated material nonlinearity, geometric nonlinear behavior, several contact interactions between faying surfaces, and prescribed displacements for generating initial bolt pretension. These FE models were used to compare experimental test results, which verify that advanced FE modeling methods make a notable contribution to reproducing the overall behavior of connections and components accurately, including the moment-rotation curves. In addition, the FE models provide some useful information which is difficult to obtain during physical testing, i.e., the distribution of stress and strain, friction forces between shear faying surfaces, and bolt reaction forces. The validated FE models are also used for additional parametric studies so as to comprehensively understand their response mechanisms. Moreover, the observation of FE analysis results supports the statement that connection models presented herein were designed to reach the yielding of connection components when the structural beam produces its full plastic moment at the plastic hinge.
The surface and sub-surface of an intercritically annealed Si-bearing CMnSi TRIP steel were investigated by high resolution transmission electron microscopy after pre-oxidation in a N2+0.3 vol-% O2 atmosphere and subsequent reduction in a N2+10 vol-% H2 atmosphere. After the initial pre-oxidation the surface consisted of areas covered with a thick Fe3O4 oxide with an open porous structure and areas covered with a thin compact Fe3O4 film connected to the steel by xMnO.SiO2 oxide bridges connecting the oxide film to the matrix. Both porous and compact oxide contained embedded grains of MnO. A grain boundary network of amorphous SiO2 was formed in the steel subsurface under the porous Fe3O4 after the pre-oxidation treatment. No internal oxidation was observed below the compact Fe3O4 layer. Whereas, the Fe3O4 was fully reduced to metallic Fe after reduction annealing, the selective oxides MnO, c-xMnO.SiO2 (x>1) and a-xMnO.SiO2 (x<0.9) were unaffected by the reduction annealing. As no amorphous a-SiO2 or a-xMnO.SiO2 (x<0.9) film forming oxides were present at the surface of the Si-bearing TRIP steel after the pre-oxidation and reduction annealing, the treatment is believed to lead to an improved wettability of the surface by liquid Zn during hot dip galvanizing and Zn-coatings which are free of bare spot defects. Zn-coating adhesion problems may however arise from the formation of large pores at the Fe3O4/steel interface, which are still present after the oxide reduction.
The effect of dissolution of hydrogen on the defect structure of Cr2O3 was studied. The hydrogen flux and permeability through Cr2O3 scale were measured at 1273 K in the oxygen activities of 1.6 × 10–18 – 1.0 × 10–16. The oxygen activities on the both sides of the sample were controlled by fixing the H2/CO2 and CO/CO2 ratios to measure the hydrogen permeability without oxygen activity gradient through the sample. The measured hydrogen flux and permeability was proportional to aH21/2 and aO2–1/8 respectively. The activity dependences indicate that dissolved hydrogen is the neutral hydrogen at oxygen site as a minor defect and does not affect the oxidation behavior of chromia-forming alloys.
Over the last few years two new types of Ductile Iron (DI) have been developed based on Austempered DI (ADI), called Dual Phase ADI and Carbidic ADI (CADI), respectively. In the light of the experience acquired when studying these DI variants, the authors found motivating the possibility of obtaining a new type of multi-phase DI (MPDI), combining their main microstructural characteristics. A DI melt alloyed with 2%Cr, exhibiting free carbides and a pearlitic matrix in its as-cast condition, was used. The upper (UCT) and lower (LCT) critical temperatures of the Fe–C–Si equilibrium diagram region, where ferrite (α), austenite (γ) and graphite (Gr) coexist (called “intercritical interval”), were determined by heat treatment and microstructural analysis on samples previously annealed. Carbides stability was studied, and its dissolution was found to be negligible. It was possible to obtain free or allotriomorphic ferrite and austenite by isothermal austenitizing at temperatures within the intercritical interval, and then transform the remaining austenite into ausferrite after an austempering step. Therefore, multi- phase microstructures composed of graphite nodules, free carbides, free ferrite and ausferrite were obtained. The possibility of obtaining MPDI directly from the as-cast structure was also analyzed, and it was found that very similar microstructures to those previously annealed can be obtained by austenitizing samples at the same temperature range. These results proved that the pearlite → austenite transformation kinetics is rapid, as it took only one hour for the transformation to occur.
The microstructural evolution of Fe–0.2C–5Mn steel during intercritical annealing with holding time for up to 144 hours was examined by TEM and STEM. It was demonstrated by TEM that the martensite lath structure gradually transformed into a lamellar ferrite and austenite duplex structure. The partitioning of manganese from ferrite to austenite was found by STEM. Typical Kurdjumov-Sachs orientation relationship between austenite lath and ferrite lath was observed by electron back scattered diffraction (EBSD). Based on the analysis of the austenite lath thckening behavior, it was proposed that the Mn-partitioning in austenite dominated the microstructure evolution of the ultrafine lamellar ferrite and austenite duplex structure during annealing process.
Formations of Cu-rich precipitate in 18%Cr-1.5%Cu stainless steels without and with 0.5%Nb have been examined mainly using transmission electron microscopy. The specimens solution-treated at 1250°C were isothermally transformed at various temperatures between 500 and 850°C. Fine spherical Cu-rich solute zones were nucleated in the steels in the beginning of aging. These Cu-rich zones exhibited bcc structure and then transformed into 9R structure with twins during aging. The spherical 9R-Cu particles changed into rod-shaped fcc-Cu particles after prolonged aging. Laves phase of Fe2Nb type started to form in the Nb-added steel almost after finishing nucleation and growth of Cu particles. Addition of Nb in the steel delayed the nucleation of the Cu-rich zones due to slow diffusion of Nb atoms in ferrite.
The additional aging at 1273 K was done for the single crystal CMSX-4 pre-crept at interrupted the creep test at the strain of 0.01 at 1273 K-250 MPa. Changes in the γ' morphology and in the dislocation substructure at the γ/γ' interfaces with aging were investigated. The shape of γ' precipitates in CMSX-4 pre-crept specimen at the creep at the strain of 0.01, remained still cuboidal. But by employing the additional aging to this interrupted specimen the shape of γ' precipitates turned its shape to the rafted one perpendicular to the stress axis. A large number of the dislocations tangled were located at the γ/γ' interfaces. By the additional aging, these dislocations were rearranged regularly with increasing aging time. However, the decrease in the dislocation density at the γ/γ' interfaces with increasing aging time was not recognized. All dislocations at the γ/γ' interfaces were defined by the six-kinds of the <110> Burgers vectors, irrespective to aging time. Consequently, dislocations at the γ/γ' interfaces were regarded as not the misfit dislocations, but the mobile dislocations in the γ channels, and the formation of the rafted γ/γ' structure might be correlated with the rearrangement of the dislocations.
Industrial grade ferrotitanium (Fe–Ti) was used for the first time as the raw material to synthesize TiC. Mixtures of Fe–Ti and graphite were milled in a planetary ball mill for varying durations between 10 h and 40 h. The milled mixtures were subsequently heat treated at 1000°C for 15 min. The powder mixtures were characterized by using XRD and SEM-EDAX. In an attempt to purify the TiC, the milled and heat treated powder products were leached with dilute HCl. The XRD patterns of as milled heat treated powders indicated formation of crystalline TiC. SEM images also indicated grain refinement even though agglomeration to some extent was noticed. The mechanochemical route coupled with leaching thus offers a successful way to synthesize TiC from a cheap source that has not been attempted before.
Intergranular surface cracking occurring only in the inner decarburization zone of plain carbon steel tubes during cooling after the decarburization is understood in the light of the grain boundary segregation of phosphorus, the thermal tensile stress developed in the decarburization zone and the dependence of yield and grain boundary strengths on temperature.
The evolution of microstructure and texture during wire drawing of pearlitic steel was studied using scanning electron microscopy and X-ray diffraction. Wire drawing induces elongation of ferrite grains with the development of <110> fibre texture. The cementite lamellae also have a tendency to reorient themselves along the wire axis with increasing strain. Viscoplastic self-consistent simulations were employed to simulate the texture evolution in the ferrite phase.