This research work introduces the concept of “useful arc power” and the thermal model, first introduced by Dittmer and Krüger, to establish the arc length at any stage of the heat in Alternate Current Electric Arc Furnaces (AC-EAF), based on the estimation of the fraction of the energy transferred to the metallic load by radiation. Radiation is the most effective way to transfer heat in an arc furnace in presence of metallic scrap (bore-in and early meltdown). On the other hand, if the arc is not adequately covered with slag, radiation is extremely dangerous to the furnace integrity. When the furnace is fully loaded, scrap protects the walls and cooling panels and then arc radiation must be maximized. To increase energy efficiency, and at the same time reduce circuit power losses, the arc length should be controlled. However, arc instability prevents to increase radiation, as desired, and a compromise must be reached between arc length and arc stability. In this work AC-EAF electric circuit is modeled and analyzed under different heat stages. Electrodes, anode and cathode, fall regions can be considered as energy losses and their associated power may be deducted for the estimation of the “useful arc power” and for the definition of the operational currents in the heat process, particularly during flat bath conditions (late meltdown and refining). As a result of the present study it is proved that current setpoints play an important role for energy saving at any stage of the heat. Finally, experimental results obtained from an industrial steel factory validate this approach to optimize the electrical energy consumption per ton of liquid steel in AC-EAF.
Phase Equilibria in the MnO–FeO–MnS–FeS–SiO2 liquid phase at SiO2 saturation under reducing condition were experimentally investigated in the temperature range from 1200°C to 1400°C in order to provide a fundamental knowledge on new FerroManganese (FeMn) alloy process. High temperature equilibration, quenching and Electron Probe Micro–Analysis (EPMA) were employed to obtain equilibrium compositions of liquid phase which was separated into oxide–rich liquid and sulfide–rich liquid. Concentration of Mn defined as in the sulfide–rich liquid was always lower than RMn in the oxide–rich liquid. In order to understand the liquid separation and the distribution of Mn in the two liquid phases, a thermo-dynamic modeling of this liquid oxysulfide was performed by taking into account strong chemical Short–Range Ordering (SRO) in the framework of the Modified Quasichemical Model in the Quadruplet Approximation. Contrary to the general understanding that Mn attracts S stronger than Fe does and it would have resulted higher Mn content in the sulfide–rich liquid, the present experimental results show that Fe is enriched in the sulfide–rich liquid. This implies that Fe attracts S stronger than Mn does in the liquid phase concerned in the present study. Such a behavior is attributed to the fact that Mn is bound by SiO2 through a formation of (Mn–O–Si) Second–Nearest–Neighbor (SNN) pair, thus oxide–rich liquid attracts more Mn while Fe is distributed more to sulfide–rich liquid. A number of points to be considered for the production of low phosphorus FeMn alloy through the two-phase liquid separation are discussed.
An empirical model has been developed based on optical basicity to calculate the sulfide capacity of metallurgical molten slag. The model estimated sulfide capacities agree well with the experimental data for CaO–MgO–FeO–MnO–TiO2–Al2O3– SiO2–CaF2 multicomponent slags. It is found that the abilities of increasing sulfide capacity in MO–SiO2 (M=Ca, Mg, Fe and Mn) melts follow the order: MnO>FeO>CaO>MgO. Also, substitution of CaO for CaF2 decreases the sulfide capacity and substitution of TiO2 for SiO2 increases the sulfide capacity.
The nitrogen solubility in liquid Mn–Si, Mn–Si–Fe, Mn–Si–C and Mn–Si–Fe–C alloys has been measured by the gas-liquid metal equilibration technique in the temperature range of 1673–1773 K. The additions of silicon, iron and carbon significantly decreased the nitrogen solubility in liquid manganese alloys. The experimental results were thermodynamically analyzed by the Wagner’s formalism to determine the first- and the second-order interaction parameters of silicon, iron and carbon on nitrogen in liquid manganese. The thermodynamic parameters can be used to predict the nitrogen solubility in ferromanganese and silicomanganese alloy melts as functions of the melt composition and temperature at given nitrogen partial pressures.
The formation mechanisms of the complex Ca-rich ferrite iron ore sinter bonding phases SFCA and SFCA-I, during heating of a synthetic sinter mixture in the range 298–1623 K and at pO2 = 0.21, 5 × 10–3 and 1 × 10–4 atm, were determined using in situ X-ray diffraction. SFCA and, in particular, SFCA-I are desirable bonding phases in iron ore sinter, and improved understanding of the effect of parameters such as pO2 on their formation may lead to improved ability to maximise their formation in industrial sintering processes. SFCA-I and SFCA were both observed to form at pO2 = 0.21 and 5 × 10–3 atm, with the formation of SFCA-I preceding SFCA formation in each case, but via distinctly different mechanisms at each pO2. No SFCA-I was observed at pO2 = 1 × 10–4 atm; instead, a Ca-rich phase designated CFAlSi, formed at 1420 K. By 1456 K, CFAlSi had decomposed to form melt and a small amount of SFCA. Such a low pO2 during heating of industrial sinter mixtures is, therefore, undesirable, since it would not result in the formation of an abundance of SFCA and SFCA-I bonding phases. In addition, CFA phase, which was determined by Webster et al. (Metall. Mater. Trans. B, 43(2012), 1344) to be a key precursor phase in the formation of SFCA at pO2 = 5 × 10–3 atm, was also observed to form at pO2 = 0.21 and 1 × 10–4 atm, with the amount decreasing with increasing pO2.
Slag entrainment during steel teeming-drain operations from a steel ladle impacts negatively steel cleanliness and quality. In the present work water modeling and mathematical simulations using a multiphase model for momentum and heat transfer were employed to understand the mechanisms of vortex funnel drain and sink drain flows. The critical bath height for vortex development increases with steel throughput and valve gate opening. Six stages during vortex development are identified, passing from a dimple formation on the bath surface until sink drain which begins when the bath level has the same magnitude as the nozzle diameter. This later drain pattern is independent from any other teeming-draining variable being only a function of the bath height and nozzle diameter; when they are about the same the metal-interface collapses. The temperature gradients originated by the heat losses of the ladle to the surroundings provide buoyancy forces that are large enough to influence liquid motion in the ladle. At large bath levels steel observes long-vertical recirculating flows and at low bath levels these flows change to horizontal-circular recirculating flows that become a seed for later vortex development. These buoyancy forces increase the critical height for vortex development and slag entrainment.
Steel flow phenomena and Ce2O3 inclusion behavior are presented in this paper. A three-dimensional model was developed to describe the steel flow phenomena and the inclusion behavior during a teeming process. The Kim-Chen modified k-ε turbulent model was used to simulate the turbulence properties and the Height-of-Liquid model was used to capture the interface between gas and steel. A Lagrangian method was then used to track the inclusions and to compare the behaviors of different-size inclusions in the steel flow. In addition, a statistical analysis was carried out by the use of a stochastic turbulence model to investigate the behaviors of different-size inclusions at different nozzle regions. The results show that the steel flow was the most turbulent at the connection region of the straight pipe part and the expanding part of the nozzle. All inclusions with a diameter smaller than 20 μm were found to have a similar trajectory and velocity distribution in the nozzle. However, inertia force and buoyancy force were found to play an important role for the behaviors of large-size inclusions/clusters. The statistical analysis results indicate that the regions close to the connection region between different angled nozzle parts seem to be very sensitive with respect to deposition of inclusions.
The present article is a sequel to the previous review on the history of near net shape strip casting facilities. The present review focuses on technical progress made in strip casting over the last three decades. Strip casting is a revolutionary technology that promises the hope for an efficient, economical and environmentally-friendly process to produce hot-rolled, steel sheets. This review provides a summary of the theory, recent research, and progress, in the developments of strip casting operations for steels, along with technical discussions regarding the characteristics and design features of steel strip casting machines. Two strip casting processes are discussed in detail; the Twin-Roll Casting (TRC) process and the Horizontal Single-Belt Casting (HSBC) process. Particular emphasis is placed on topics such as the commercial potential for strip casting technology in the steel industry, and the economic and environmental advantages of direct strip production, versus current continuous casting, fixed mold technologies.
The aim of the present work is to achieve a better understanding of the liquid steel flow patterns in a billet mould when is fed by a misaligned nozzle using two important tools analysis, physical and mathematical modelling. The numerical model includes the government Navier-stokes equations, the k-ε model, and the VOF model for the multiphase air/steel/flux system. These equations are solves through the segregated model embedded in FLUENT®. The physical model was built at 1:1 scale, where red ink and video recording are employed to visualize the fluidynamics. One nozzle deviation is applied towards the mould radius. The results indicate that a centred nozzle position does not guarantee symmetrical flow patterns inside the mould due to its curvature design. Because of curvature, at the normal nozzle alignment the jet trajectory is closer to the inner mould radius. Even when the nozzle deviations are small like 1° or 2°, the results show that the fluid flow consequences are significant and negatives in most of the cases. The worst cases induce an impact of the jet to one of the mould walls and a very unstable meniscus with strong vortexes formation. Under the present configuration, 1° deviation of the nozzle towards the inner mould radius is good enough to achieve symmetrical flow patterns and to obtain a better meniscus control.
The main aims of the present research is the study of steel flow under temperature gradient to understand the convective effects on the flow patterns inside the mold and its effects on the shell growth kinetics under more realistic conditions. In order to achieve this, a non-isothermal mathematical model is developed based on the Navier-Stokes equations together with the k-ε turbulence model, the volume of fluid model to solve the multiphase system air-slag-steel and a solidification model. Comparing isothermal and non-isothermal results, it is observed that the buoyancy forces are large enough to modify radically the lower recirculation flows inducing shorter and upwards streams; however, the upper recirculation flows do not show strong changes. Shell growth does not necessarily follow a steady parabolic growth and it is more dependent on the washing effects of convective steel streams. Therefore, shell thickness reports heterogeneous and irregular magnitudes through the four faces of the slab. In addition, mold curvature provides uneven shell growth in the inner side of the slab while in the outer side the shell thickness observes a more regular growth. Shell thickness is irregular and discontinuous all around the upper periphery of the slab; therefore, this region is very sensitive to cracking. Finally, the numerical results for liquid steel solidification are compared to published results of shell growth showing a very good qualitative agreement.
Fluorine-free mould flux has been required to reduce corrosion of continuous casters. In this paper, Na2Ca2Si3O9 has been applied to an alternative crystalline phase of Ca4Si2O7F2 (cuspidine) that crystallized in commercial flux films. Effects of basicity described as T.CaO/SiO2 and additions of Li2O, Na2O, MgO and/or MnO into Na2Ca2Si3O9 composition mould flux on viscosity and solidification temperature were studied and the solidified specimens were examined by X-ray diffraction to identified crystalline phases.The viscosity of the mould flux is reduced with increaing of the basicity or the basic components contents. The solidification temperature decreased slightly as the basicity increased from 0.47 to 0.60, whereas it increased steeply with increasing of the basicity from 0.60 to 0.70. The solidification temperature also steeply increased when large amounts of these basic components were added. With increasing of the basic components amounts, the first peak intensity of the target phase, Na2Ca2Si3O9, decreased while that of Na2Ca2Si2O7, which had higher melting point than the target phase, increased. It is indicated that the solidification temperature is related to Na2Ca2Si2O7 crystallization.Carbon steel was cast with one of the developed fluorin-free mould fluxes and slab without any surface cracking was obtained.
A generalized macroscopic two-dimensional mathematical model has been developed to simulate the transport phenomena occurring during the solidification of ternary alloy systems. The model is based on a fixed-grid enthalpy based control volume approach and is capable of capturing the effects of dendritic arm coarsening on the transport characteristics of the solidification process. Microscopic features pertaining to complex thermo-solutal transport mechanisms and dendritic arm coarsening are numerically modelled through a novel formulation of latent enthalpy evolution, consistent with the phase change morphology of general multi-component alloy systems. Numerical simulations are performed for ternary steel alloy and the resulting convection and macrosegregation patterns are analyzed. It is observed that dendritic arm coarsening leads to an increased effective permeability of the mushy region resulting in an enhanced macrosegregation. The numerical results are also tested against experimental results reported in the literature, and very good agreement is found in this regard.
In order to investigate bubble behavior before inclined solidified front, a numerical simulation model is developed, in which level set method and modified heat transfer equation is applied to simulate the dynamic evolution of gas-liquid interface and solidification process under a fixed grid frame. Meanwhile, an in-situ experiment of bubble behavior before inclined ice solidified front was exacted to validate the numerical model. The effects of bubble diameter, inclined angle of solidified front and cooling rate on its behavior are investigated both numerically and experimentally. The results show that as bubble diameter increasing, inclined angle increasing and cooling rate decreasing, the bubble entrapment possibility is decreasing. Each critical point for entrapment is also studied numerically.
A new model that can quantitatively evaluate the permeability for columnar dendritic structures was developed by modifying the Kozeny constant in Kozeny-Carman’s equation. The modified Kozeny constant consists of two terms: one accounting for the flow direction for primary arms of columnar dendrites and the other accounting for the tortuosity of channels in the dendritic structures. The permeability calculated by this new model was compared with that obtained in our previous simulations [Y. Natsume et al.: Tetsu-to-Hagané, 99 (2013), 117] and from experiments other researchers [K. Murakami et al.: Acta metall., 31 (1983), 1417, 32 (1984), 1423, Liu et al.: Mater. Sci. Tech., 5 (1989), 1148] and the values were found to be in fairly good agreement with the compared values. In addition, we investigated the obtained quantitative model to determine permeability for use in computational studies of macrosegregation. To evaluate the permeability quantitatively using Kozeny-Carman’s equation, the value of the specific surface area for dendrites is required. We introduced an assumption that the inverse of the specific surface area for columnar dendrites is proportional to the secondary arm spacing. By using this assumption in our modified model, the permeability can be determined using only the dendrite arm spacing and liquid volume fraction.
The integrated scheduling for the continuous casting (CC) and hot rolling (HR) process remains challenging in the iron and steel production. Taking into account the technological and practical constraints, we establish a mathematical model with the objective of maximizing the number of slabs processed in hot charge rolling (HCR) or direct hot charge rolling (DHCR) mode. Using the decomposition strategy, a hybrid algorithm is proposed based on the combination of the mathematical programming and constraint programming (CP) methods. Computational results demonstrate that the hybrid algorithm is efficient and effective for solving the integrated scheduling problem (ISP) of steel plants.
In this paper, we propose active vibration control of a strip in a continuous galvanizing line (CGL) using positive position feedback (PPF) control. The control system includes five pairs of electromagnetic actuators and controllers. First, the overall system was modeled using the three-dimensional finite element modeling (FEM) package ANSYS. The Krylov subspace technique was then used to reduce the order of the model. Finally, PPF control was applied to control the vibration. The stability condition was derived from the stiffness matrix concept, which shows the relationship between the DC gain of the controller and that of the system. Root locus analysis was performed to validate the stability condition derived. The results of the software simulations and the experiments demonstrate the effectiveness of the proposed controller under various tension conditions.
Nickel and silicon are attractive alloying elements for high-strength low-alloyed (HSLA) steel production. However, it is well known that the presence of Ni and Si in the steel can impair the surface quality, making it unsuitable for certain markets. The combined effects of Ni+Si on the oxide scale formation are still relatively unknown. Literature is dealing mostly with steels containing combinations of Ni and Si, with either traces of nickel (0.1%) or very high (8–16%) nickel levels. At Tata Steel we explored the effect of an optimum composition selected to achieve steel properties (0.15%Si and 1%Ni) on the formation of oxide in the reheating furnace and its descalability. Pilot hydraulic descaling trials were performed on blocks of three steel grades, applying reheating and hydraulic descaling in conditions closely resembling the industrial practice.The oxidation experiments show that synergistic effects occurring during the oxidation of alloys containing Ni (1.1%) are already obvious at relatively low levels of Si of 0.05%. This effect is even enhanced at higher Si levels of 0.15% and consists of increasing the adherence of oxide scale to the steel substrate by forming an entangled layer with oxidic pegs.In order to maximize descalability of (Ni,Si)-alloyed steels slabs, the metal/scale entanglement has to be minimised. In this respect, it was found that the slab surface temperature is the most important parameter. A gentle, smooth reheating process is required in which slab surface temperatures exceeding 1300°C should be avoided.
The thermal histories of steel strips in a multi-bending process after hot rolling were clarified by numerical analysis in order to investigate the appropriate conditions for grain refinement. The temperature oscillates cyclically at the strip surface, whereas the temperature amplitude is relatively small at the center of thickness. The temperature distribution in the strip thickness direction depends on the transfer speed and cooling conditions, that is, air or water cooling. In industrial production, water-cooling equipment would be necessary in order to cancel the accumulated heat generation caused by bending deformation and to maintain the strip temperature in the appropriate range for grain refinement.
The effects of titanium content on weld microstructure and inclusion characteristics have been studied using the two different bainitic welds fabricated to have similar oxygen content. Analytical transmission electron microscopy analysis with thin foil specimens was carried out to investigate inclusion characteristics focusing on the crystal structure and the chemical features of the constituent phases. Then, these results were related with the proportion of acicular ferrite measured under an optical microscope. For a weld containing 0.002 wt.% Ti, a Ti-rich oxide layer containing manganese is present on the surface of amorphous inclusions and appears to be responsible for acicular ferrite appreciably formed in the bainitic structure, ~50%. When the titanium content increases to 0.07 wt.% Ti2O3 inclusions were formed accompanying with manganese-depleted regions and eventually results in a significant increase in acicular ferrite content over 90%. Therefore, the manganese depletion developed along with the formation of Ti2O3 inclusions is concluded to be a possible mechanism for acicular ferrite nucleation in the high titanium welds.
The purpose of this study was to examine the effect of boronising heat treatment on the corrosion behaviour (in two different corrosive media) and wear properties (on two different counter sliding discs) of the DIN 1.4777 quality cast steel containing 1.7% C, 30% Cr and 1.1% Si. The steel supplied as cast was exposed to homogenisation heat treatment at 1150°C for 3,5 hours and then to boronising heat treatment at 900°C for 8 hours using the powder of Ekabor 2. An optical light microscope, SEM and XRD analyses were used to conduct microstructural characterisation of the steel investigated. Electrochemical potentiodynamic polarisation measurements were taken to evaluate corrosion behaviours of the examined steels. Wear tests were conducted in a pin-on-disc type wear device by using a load between 10 N and 60 N. While the corrosion resistance of the boronising heat treated steel deteriorated by the pitting damage mechanism within corrosive media, uniform corrosion damage enhanced the corrosion resistance of the examined boronised steel. Oxidative adhesion, cracking of oxide and/or boride layer, and severe plastic deformation mechanisms were dominant during the wear tests. Severe plastic deformation and cracking of the oxide and/or boride layer caused wear mechanism to transform from mild to severe.
Magnetite (Fe3O4) particles were synthesized by oxidation of a hydroxyl chloride green rust (GR(Cl–)) suspension at room temperature. The formation process of Fe3O4 particles was characterized by X-ray diffraction, magnetization and electrochemical measurements. The results showed that a small amount of fine Fe3O4 particles were nucleated when the supernatant solution of the as-synthesized GR(Cl–) suspension was replaced by deaerated water. By controlling the injection of oxygen gas at room temperature, Fe3O4 particles of about 70 nm in diameter formed from such GR(Cl–) suspension, while goethite (α-FeOOH) particles were mainly obtained from the as-synthesized GR(Cl–) suspension under the same oxidation conditions. Hence, the saturation magnetization of final oxidation products obtained from the GR(Cl–) suspension in which the supernatant solution was replaced was about 60 emu/g, which was six times larger than that obtained from the as-synthesized GR(Cl–) suspension. In the early stage of the oxidation process, the oxidation-reduction potential (ORP) in the GR(Cl–) suspension in which supernatant solution was replaced was lower than that in the as-synthesized GR(Cl–) suspension. In addition, the value of pH of the former suspension was higher than that of the latter suspension. It is concluded that the formation of Fe3O4 particles is enhanced in solution with relatively low ORP and high pH.
The dynamic transformation behavior of deformed austenite was studied in a 0.79%C high carbon steel over the temperature range 743–823°C. The experiments were carried out in torsion under an atmosphere of argon and 5% H2. All these temperatures are above the orthoequilibrium Ae3 temperature of the steel. Strains of 0.25–4 were applied at a strain rate of 4 s–1. The experimental parameters were varied in order to determine the effects of strain and temperature on the formation of strain-induced ferrite and cementite. The critical strain for dynamic transformation was about 0.20. The volume fractions of the transformed phases increased with strain and decreased with temperature. The observed ferrite structures are Widmanstätten in nature and appear to have formed displacively. The carbon diffusion times required for formation of the observed spheroidal cementite particles were calculated; these are consistent with the occurrence of interstitial diffusion during deformation. Similar calculations indicate that substitutional diffusion does not play a role during dynamic transformation.
The effect of carbon content and deformation temperature on ultra-grain refinement of two plain carbon steels was studied. The steels samples were severely deformed by means of warm torsion tests. For both steels, the final microstructure consisted of ultrafine ferrite grains and small dispersed cementite particles depend on the test temperature. Increase in carbon content led to a decrease in the average ferrite grain size due to the presence of a higher volume fraction of cementite particles. Increasing in deformation temperature led to an increase in ferrite grain size. In addition, a critical strain for ultra grain refinement was reached for both steels. This critical deformation represents a minimum accumulated equivalent strain beyond which further significant grain size refinement is not more achievable. The higher the C content, the smaller this critical deformation was.
A variant selection of ausformed lath martensite in an Fe–9Mn alloy (MS = 602 K) and ausformed lenticular martensite in an Fe–25Ni–0.5C alloy (MS = 220 K) are compared quantitatively using EBSD measurements combined with a method for the reconstruction of austenite orientation. Differences were found in the selected variants by ausforming between those two martensite morphologies. In ausformed lath martensite, variants with habit planes parallel to the active slip planes in austenite were preferentially selected. This variant selection in lath martensite may be attributed to the preferential nucleation on the microband structures in austenite and lack of growth inhibition from the microband boundaries. On the other hand, in ausformed lenticular martensite, variants with habit planes parallel to the compression plane were preferentially selected. The reason for this is that lenticular martensite whose habit plane is parallel to the compression plane can grow easily in austenite grains elongated along the compression plane. This means fewer disturbances for those variants to grow with a habit plane parallel to the compression plane than for other variants.