Showing papers in "Isij International in 2006"

Predicting the Critical Stress for Initiation of Dynamic Recrystallization

Abstract: The critical stress for initiation of dynamic recrystallization (DRX) can be identified from the inflection point on the strain hardening rate (qds/de) versus flow stress (s) curve. This kind of curve can be described by an equation that fits the experimental q-s data from zero to the peak stress. Such a curve must have an in- flection point and the simplest relation that has such properties is a third order equation. Hot compression tests were carried out on a 304 H stainless steel over the temperature range 900-1 100°C and strain rate range 0.01-1 s � 1 to a strain of 1. An appropriate third order equation was fitted to the strain hardening data. The results show that the critical stress at initiation s c�� B/3A where A and B are coefficients of the third order equation. It is evident that this value depends on the deformation condi- tions. The stress-strain curve was then normalized with respect to the peak stress, leading to a normalized value of the critical stress (uc) equal to ucs c/s p�� B� /3A� . Here Aand Bare coefficients of the normal- ized third order equation. This value is constant and independent of the deformation conditions.

Crystallization of CaO–SiO2–TiO2 Slag as a Candidate for Fluorine Free Mold Flux

Abstract: The time-temperature-transformation diagram (TTT diagram) of CaO–SiO2–TiO2 slag has been determined by using differential thermal analysis (DTA) in order to investigate the crystallization of the slag as a candidate for fluorine free mold flux. The incubation time of CaOSiO2TiO2 in CaO–SiO2–TiO2 slag is as small as that of cuspidine (3CaO2SiO2CaF2) in commercial mold fluxes. This finding indicates that the CaOSiO2TiO2 crystallizes rapidly in the slag film between the mold and the steel in the continuous casting machine, similar to cuspidine in commercial mold fluxes. CaOSiO2TiO2 in CaO–SiO2–TiO2 slag is substituted for cuspidine in commercial mold fluxes. The thickness of the crystalline layer of CaOSiO2TiO2 is smaller than that of cuspidine. In order to improve the heat transfer control of CaO–SiO2–TiO2 slag, it is necessary to decrease the incubation time of CaOSiO2TiO2 at high temperature.

Tempering Kinetics of the Martensitic Phase in DP Steel

Abstract: The increase in the yield stress of dual phase (DP) steels, resulting from the static strain ageing phenomenon, commonly referred to as bake hardening (BH), gives an important contribution to the additional in-service strength of outer auto body parts, e.g. with respect to the dent resistance of the components made with DP steel. In order to understand this large BH effect, the role of the different constituents of the DP steel during this process needs to be considered. The various stages of tempering phenomena taking place in the martensite phase were investigated in detail by means of precision dilatometry and X-Ray Diffraction (XRD). The succession of the various tempering reactions that are characterised by typical volume changes was determined using both constant heating rate and isothermal dilatometric tests. The measurements made it possible to distinguish five distinct stages of structural changes during tempering: (I) the redistribution of carbon atoms, (II) the precipitation of η- or e-carbide, (III) the formation of Hag-carbide, (IV) the decomposition of retained austenite, and (V) the transformation of transition carbides to cementite.

The Ability of Slags to Absorb Solid Oxide Inclusions

Abstract: The capture rate of solid oxide-inclusion particles from molten steel by molten slag depends on the rate of steel film drainage (which occurs at certain particle velocities), interfacial separation, and dissolution into the slag. In this study the capture of common oxide inclusions of sizes 2.5–200 µm and with velocities ranging from their terminal velocities to 0.3 m·s−1 approaching the interface between molten iron and slags with chemistries corresponding to ladle, tundish and mold slags are investigated. Calculations, based on a model available in literature, show that film drainage (when applicable) is rapid enough to be ignored. A sensitivity analysis based on the slag properties show that the interfacial energy between slag and inclusion is the most pertinent property that could hinder interfacial separation. However, the interfacial tension needed to achieve this has to be a minimum of 0.41 N/m which is unreasonable for the case of common oxide inclusions such as Al2O3, MgO, ZrO2 and MgAl2O4. The final step of dissolution was found based on studies with Confocal Scanning Laser Microscope experiments, to be significantly slower than the other steps. For a 100 µm particle, in the slags/inclusions investigated a correlation between slag viscosity, η [Pa·s] and super saturation, ΔC [kg·m2−3] with inclusion dissolution time, τ [s] was shown to be, τ=2.04·10−2/(C/η).

Influence of Mineral Matter on Coke Reactivity with Carbon Dioxide

Abstract: A method to improve blast furnace efficiency and make it sustainable is to lower the temperature of the iron oxide reduction using a highly reactive coke. Understanding the role of coal rank, maceral composition and mineral matter in coke in the gasification reaction under these new conditions is of major importance. Four cokes prepared from Australian coals of varying rank, maceral composition and ash composition were gasified with carbon dioxide. The rank and maceral composition of the parent coals did not appear to be related to the reactivity of the cokes. However, coke reactivity increased with increasing total amount of catalytic minerals in crystalline phases such as metallic iron, iron sulfides. Calcium sulfide could be a potential catalyst for the gasification reaction. Iron, potassium and sodium present in the amorphous phase did not appear to have any effect on coke reactivity. Calcium was present only in the crystalline phases. Knowing the form and amount of the mineral phases that catalyse the gasification reaction in coke would improve the ability to predict coke reactivity. This knowledge would contribute to more efficiently matching cokes to blast furnace requirements.

Viscosities of CaO-SiO2-Al2O3-(R2O or RO) melts

Abstract: The effect of adding R2O (R=Li, Na and K) or RO (R=Ba, Mg) on the viscosities of CaO–SiO2–Al2O3 (CaO/SiO2=0.67, 1.00 or 1.22, Al2O3=20 mass%) melts has been measured by rotating crucible viscometer. In addition, structural characterizations of these quenched vitreous samples have been investigated by 27Al and 29Si MAS-NMR spectra.The viscosities of CaO–SiO2–Al2O3–R2O quaternary melts decreased with increasing the additive content of Li2O or Na2O. However, the viscosity of the melts increased with increasing the additive content of K2O. In the case of CaO–SiO2–Al2O3–RO quaternary melts, the viscosities of the melts with CaO/SiO2=0.67 decreased with increasing the additive content of BaO or MgO, however, the viscosities of the melts with CaO/SiO2=1.00 and 1.22 increased with increasing the additive content of BaO.In the case of CaO–SiO2–Al2O3–R2O, the analysis of 27Al MAS-NMR spectra and 29Si MAS-NMR spectra indicated that the degree of polymerization of silicate anions in the glasses decreased with the addition of Li2O or Na2O, and that the degree of polymerization of aluminosilicate anions in the glasses increased with the addition of K2O. It was estimated from the results that the increase in viscosities of CaO–SiO2–Al2O3–K2O melts was dependent on increase of aluminosilicate anions in the melts with the addition of K2O.In the case of CaO–SiO2–Al2O3–RO, the analysis of 27Al MAS-NMR spectra and 29Si MAS-NMR spectra indicated that the degree of polymerization of silicate and aluminosilicate anions in the glasses decreased with the addition of MgO. The degree of polymerization of aluminosilicate anions in the glasses increased with the addition BaO. It was estimated from the results that the increase in viscosities of CaO–SiO2–Al2O3–BaO melts was dependent on increase of aluminosilicate anions in the melts with the addition of BaO.

Effects of Hot Deformation and Subsequent Austempering on the Mechanical Properties of Si-Mn TRIP Steels

Abstract: In the present paper, effects of hot deformation and subsequent austempering on the mechanical properties of hot rolled Si–Mn TRIP steels were investigated. Thermomechanical controlled processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling reduction, temperatures and austemperings with various isothermal holding duration were applied. The results have shown that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. Ultimate tensile strength, yield strength and total elongation increase with increasing the amount of deformation and decreasing finish rolling temperature due to the stabilization of retained austenite. Tensile strength and total elongation can reach the maximum values (791 MPa and 36%, respectively), and isothermal holding 20 min at 400°C after hot deformation has been proved to be the optimum treatment.

Phosphorous Partition between 2CaO·SiO2 Particles and CaO–SiO2–FetO Slags

Abstract: The behavior of phosphorous transfer from P2O5-containing CaO–SiO2–FetO slags to 2CaO·SiO2 particles homogeneously dispersed in slag has been studied by using a microprobe analysis. The maximum phosphorus distribution ratio between 2CaO·SiO2 particle and slag is obtained at the nose composition of 2CaO·SiO2 primary phase region in CaO–SiO2–FetO phase diagram and the temperature dependence of this distribution ratio is small. The phosphorous transfer rate from slag to a 2CaO·SiO2 particle with 20 to 50 µm is considerably fast and a 2CaO·SiO2 particle changes to the particle with the composition of 2CaO·SiO2–3CaO·P2O5 solid solution within 5 s. In the case of particles present in cluster, only the rim part (5 µm) of a particle changes to the composition of 2CaO·SiO2–3CaO·P2O5 solid solution within 5 s, but small size particles with 3 to 8 µm completely change to the particles with the composition of 2CaO·SiO2–3CaO·P2O5 solid solution within 5 s.

Z-phase Formation during Creep and Aging in 9–12% Cr Heat Resistant Steels

Abstract: The precipitation behavior of the Z phase was investigated after long-term creep exposure in ASME-T91, T92, T122 without δ-ferrite, and T122 with δ-ferrite through elemental mapping using EF-TEM. The Z phase was identified by comparing the Cr map with the V map. Most of the Z phase was observed around prior austenite grain boundaries and/or packet boundaries in all of the steels examined. In T122 with δ-ferrite, the Z phase also precipitates around the δ-ferrite. In particular, the number of MX carbonitrides was very small in T122 with a large amount of the Z phase. The main metallic composition of the Z phase in T91 was the same as that in T92. In T122, the Z phase contained a lower Nb content. The main metallic composition of the Z phase around the δ-ferrite was the same as that in the other areas. There was no large difference in the size distribution of the Z phase among the steels. The mean diameter of the Z phases for T122 with δ-ferrite was relatively large in spite of a shorter creep exposure in contrast with T91 and T92. The number density of the Z phases increased with increasing creep exposure time except in the case of T91. The order of the number density was T122 with δ-ferrite, T91, T122 without δ-ferrite, and T92. In crept samples, the amount of Z phase in the gauge portion was higher than that in the grip portion, meaning that stress and/or strain promotes the formation of a Z phase during creep exposure.

Thermal Conductivity of the CaO–Al2O3–SiO2 System

Abstract: Thermal conductivity of the CaO–Al2O3–SiO2 system, which is one of the most important silicate melts in iron- and steelmaking processes, was measured using non-stationary hot wire method in the range from liquidus temperature to 1 873 K. Measurements were carried out at various compositions, and iso-thermal conductivity line of the CaO–Al2O3–SiO2 system was drawn in iso-thermal sections at 1 673 K, 1 773 K, and 1 873 K. Thermal conductivity decreased with basicity increase, when CaO/SiO2 ratio is smaller than unity, whereas it showed constant value when CaO/SiO2 ratio is larger. In case Al2O3 content was varied at constant CaO/SiO2 ratio of 0.39 and 0.90, thermal conductivity showed maximum at 15–20 mass% Al2O3, suggesting that Al2O3 behaves as an amphoteric oxide. In the temperature range of interest, the thermal conductivity of each composition decreased as temperature rises. Temperature dependence showed deviation from linearity with the reciprocal of absolute temperature, which was considered to be due to the thermally-induced depolymerisation of the silicate structure at higher temperature. Also, thermal conductivity was found to conform to an exponential function of 1/T during deploymerization with the apparent activation energy.

Microstructure Control of Steels through Dispersoid Metallurgy Using Novel Grain Refining Alloys

Abstract: This paper describes how small inclusions (designated dispersoids) can be used to control the microstructure of steels. The term "dispersoids" refers to oxides, sulphides, nitrides and carbides which are in the 1 μm size range and capable of promoting grain refinement during solidification by a process of epitaxial nucleation or in the solid state through intragranular nucleation of ferrite. Such particles are sufficiently small to be harmless from a toughness point of view, but at the same time large enough to act as potent nucleation sites during phase transformation. The dispersoids can either be created by balanced additions of strong oxide and sulphide forming elements to an impure steel melt or be added directly into the liquid steel through a specially designed master alloy containing the nucleating particles. In both cases it is possible to manipulate the steel microstructure in a positive direction, but the latter method, involving the use of a master alloy, has probably a wider industrial application. The direction of the research now being undertaken at NTNU/SINTEF to make the grain refining alloys commercially available is briefly described towards the end of the paper.

Design of Advanced Bainitic Steels by Optimisation of TTT Diagrams and T0 Curves

Abstract: Cementite is responsible of the limited application of conventional bainitic steels, however it has been proof that cementite precipitation during bainite formation can be suppressed by the judicious use of silicon in medium carbon steels In this work, thermodynamic and kinetic models were used to design steels with an optimum bainitic microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite Using these models, a set of seven carbide free bainitic steels with a 03 wt% carbon content were proposed for manufacturing The work presented here is concerned with the microstructural and mechanical characterisation of the steels manufactured Except for the steel with the highest content of alloying elements, all the grades present the same microstructure composed of carbide-free upper bainite and retained austenite after hot rolling and a two-steps cooling Theirs tensile strengths range from 1600 to 1950 MPa while keeping a uniform elongation equal to 4% and a total elongation over 10% Regarding toughness at room temperature, they match quenched and tempered martensitic steels

Optimization of Ironmaking Process for Reducing CO2 Emissions in the Integrated Steel Works

Abstract: Global warming is a common subject in steel industry in every country. International cooperation will be required using the Kyoto. Mechanism from global aspect. In the integrated steel works, there are various means to decrease reducing agent at blast furnace, however, preferable way to reduce CO 2 emissions must be chosen considering energy balance in whole steel works, and energy saving must be actively pursued. Injection of waste plastics and carbon neutral materials such as biomass is better alternative. In the near future, hydrogen will attract attention as a clean energy source even in the steel works. Regarding oxygen blast furnace and smelting reduction, the possibility of CO 2 reduction is dependent on optimum system design of total process including outside process. Charge of prereduced sinter and high reactivity coke to blast furnace leads to reduction of CO 2 , keeping current blast furnace facility and capability.

Characteristics of Particle Size Distribution of Deoxidation Products with Mg, Zr, Al, Ca, Si/Mn and Mg/Al in Fe–10mass%Ni Alloy

Abstract: In the experiments of an Fe–10mass%Ni alloy deoxidized with Mg, Zr, Al, Ca, Mn/Si and Al/Mg at 1873 K, the characteristics of particle size distributions have been studied as a function of holding time, amount of deoxidant and order of deoxidant addition. The peak height in particle size distribution decreases and the modal value increases with increasing holding time. The width of the distribution curve is larger for Al2O3 particles and smaller for liquid CaO–Al2O3 and MnO–SiO2 particles. The number of particles increases with an increase of deoxidant for a given initial oxygen content and holding time. In the case of Al followed by Mg deoxidation, the particle size distribution is almost the same as that obtained by only Al. The size distribution in case of Mg followed by Al deoxidation is almost the same as that by only Mg.

Dynamic Oxidation of the Ti-bearing Blast Furnace Slag

Abstract: The effects of the dynamic oxidation on the physical chemical characteristics of the Ti-bearing blast furnace slag, on the selective enrichment, precipitation and growth of the Ti component and the drop of metallic Fe from the slag have been investigated. Air was blown into the molten slag as oxygen resource through a lance during the dynamic oxidation process. It was found that the TiC, TiN, (Ti2O3), Fe and (FeO) in the slag were oxidized, and the temperature of slag increased and the viscosity of slag decreased. These were not only in favour of the selective enrichment of the Ti component into the perovskite phase, but also in favour of the precipitation and growth of the perovskite phase, meanwhile promoted the coalescence, growth and drop of the metallic Fe droplets in the molten slag under an air agitation condition.

Reaction Mechanism between Solid CaO and FeOx–CaO–SiO2–P2O5 Slag at 1 573 K

Abstract: Solid CaO and FeOx–CaO–SiO2–P2O5 slag were reacted for 2 to 2 400 s at 1 573 K. The interface of CaO and slag were observed and analyzed by SEM/EDS. The CaO–FeO layer was formed beside solid CaO. The thickness of the CaO–FeO layer increased with time. Next to the CaO–FeO layer, 2CaO·SiO2 phase was formed in the melt and high content of FeO was included in the liquid. The activities of FeO and CaO for each phase were evaluated and reaction mechanism between solid CaO and FeOx–CaO–SiO2–P2O5 slag was discussed. The activity of FeO for 2CaO·SiO2 saturated melt is larger than that for the CaO–FeO layer, therefore, Fe2+ diffuses from slag phase to solid CaO. Then the CaO–FeO layer is formed beside solid CaO. The pass of the slag composition change accompanied by CaO dissolution are represented in the phase diagram for the FeOx–CaO–(SiO2+P2O5) pseudo ternary system.

Calculation of Thermophysical Properties of Ni-based Superalloys

Abstract: Reliable thermophysical data of Ni-based superalloys are needed for the mathematical modelling of solidification in casting applications. This paper derives equations and procedures to calculate the following properties of these alloys from chemical composition of the alloy: the γ′ phase content, liquidus and solidus temperatures, density and thermal expansion coefficient, heat capacity and enthalpy, electrical resistivity, thermal diffusivity and conductivity, viscosity, surface tension and contact angle. Estimates are also for the diffusion coefficient and emissivity. This work has revealed an urgent need for reliable property measurements on liquid Ni-based alloys for the following: heat capacity, electrical resistivity, thermal diffusivity and conductivity, emissivity and diffusion coefficients.

Behavior of Phosphorous Transfer from CaO–FetO–P2O5(–SiO2) Slag to CaO Particles

Abstract: Lime particles (0.5 to 1 mm) were added by dropping to CaO–FetO–P2O5 slag at 1 400°C, followed by stirring and the reaction products near a CaO particle/slag interface were determined by a microprobe analysis (EPMA-EDX). The 3CaO·P2O5 and 4CaO·P2O5 phases were crystallized depending on the amount of added CaO particles which are rapidly dissolved after the contact with a molten slag. The phosphorous distribution ratios between calcium phosphates and CaO–FetO slag obtained at temperatures between 1 400 and 1 570°C are higher than those between 2CaO·SiO2–3CaO·P2O5 solid solution and CaO–SiO2–FetO slag. Lime particles (0.3 to 0.5 mm) are added to CaO–SiO2–FetO–P2O5 slag at 1 400°C and the reaction products near a CaO particle/slag interface are determined by EPMA-EDX. The 2CaO·SiO2–3CaO·P2O5 solid solution layer is formed on the surface of a CaO particle and thereafter the CaO–FetO slag layer is formed between a CaO particle and 2CaO·SiO2–3CaO·P2O5 solid solution layer which is surrounded by CaO–SiO2–FetO–P2O5 slag. It is also experimentally confirmed that the meso-phase slag refining is superior to conventional slag refining and the discrepancy between the theoretical and experimental results obtained by the meso-phase slag refining is discussed.

Production of Pig Iron from Magnetite Ore–Coal Composite Pellets by Microwave Heating

Abstract: Magnetite ore–coal composite pellets with about 10, 15 and 20 mm diameter were rapidly smelted to produce pig iron by microwave heating in N2 gas. A microwave generator with 5 kW maximum power at 2.45 GHz was employed. Carbon content in pig iron was about 2 mass% near the liquidus line in the Fe–C system. Slag was easily separated from pig iron. By XRD analysis it is realized that the reduction of magnetite started at about 800°C and was completed to be pig iron at about 1350°C. The heating rate of pellets was independent of their mass but dependent on applied power because of self heating. According to the increase of heating rate, the level of impurities in pig iron decreased less than blast furnace.

Refinement of Solidification Microstructure and Austenite Grain by Fine Inclusion Particles

Abstract: The effect of deoxidation products of Ce2O3, ZrO2 and MgO particles on solidification microstructure has been studied in Fe–10mass%Ni, Fe–0.20mass%C–0.02mass%P and Fe–0.50mass%C–1mass%Mn alloys. The degree of the equiaxed crystallization is explained by the lattice misfit parameter between γ (or δ)-Fe and oxide. The single-phase solidification microstructure of Fe–10mass%Ni and Fe–0.50mass%C–1mass%Mn alloys is well related to austenite grain boundaries under the inhibition of grain growth by pinning. The correspondence between solidification structure and initial austenite grain has been studied in two-phases solidification of Fe–0.15 (or 0.30)mass%C–1mass%Mn–1mass%Ni alloy. The γ-grain size decreases with decreasing the lattice misfit parameter between γ-Fe and oxide and increases with decreasing the Zener pinning force. The number of γ-grains to that of primary δ-grains per unit area in a cross section increases with decreasing the aforementioned lattice misfit parameter, indicating that more than one nucleation event per δ-grain occurs at δ-ferrite grain boundary during δ to γ transformation.

Principles of Billet Soft-reduction and Consequences for Continuous Casting

Abstract: The production of continuously cast products that show very low center segregation and are thus more homogeneous is an important objective, particularly for high-speed continuous casting of billets, in meeting quality requirements that are becoming increasingly stringent. In addition to the well-known methods of reducing macro-segregation in continuous casting, inline thickness reduction of the partially solidified strand, already used successfully in slab and bloom casting, offers an alternative for producing material with low segregation level. In the present report, which considers the feasibility and effectiveness of a mechanical soft-reduction for the continuous casting of billets, the influence of the soft-reduction position from the metallurgical point of view, the reduction parameter and the roll diameter in combination with the structural design of the soft-reduction unit was assessed comparatively. One strand of the billet caster S4 was modified for this purpose and soft-reduction trials were carried out on high carbon steel grade D72C for wire rod application and spring steel grade 54SiCr6 at casting speeds between 2.80 and 3.60m/min. The conclusions drawn from the test results with respect to required solid fraction in the liquid core, effective reduction rate and necessary roll diameter formed the basis for designing the soft-reduction unit of billet caster S0 and for predicting favourable process parameters. First results of billet soft-reduction trials on the caster S0 are presented.

Evaluation of Surface Tension of Molten Ionic Mixtures

Abstract: A thermodynamic model was derived to evaluate the surface tension of molten ionic mixtures by considering the ratio of the radius of cation to that of anion. The present model reproduces the following characteristics of the composition dependence of surface tension in molten alkaline–halide ionic mixtures precisely.• almost linear change with the composition in common cation systems:• large concave feature of the composition dependence in common anion systems.The present model can be applied to evaluate the surface tension molten ionic mixtures with complex anion such as sulfate as well as molten SiO2 based binary systems considering effective ionic radii for complex anions.

Lattice and Radiation Conductivities for Mould Fluxes from the Perspective of Degree of Crystallinity

Abstract: The lattice and radiation conductivities have been determined for commercial mould fluxes in glassy and partially crystalline states as functions of the degree of crystallinity to confirm whether or not more crystallisation of mould fluxes is always effective in slow cooling in continuous casting. Lattice conductivities, refractive indices and absorption/extinction coefficients were measured on glassy and partially crystallised samples from commercial mould fluxes. The lattice conductivities of mould fluxes increased with increasing the degree of crystallinity at temperatures around 773 K and more prominent increase was observed where the degree of crystallinity exceeded about 20%, which would be due to the contact between crystal grains precipitated. However, fluxes having higher degrees of crystallinity showed negative temperature coefficients in the lattice conductivities, particularly at higher temperatures, and thus there was a case where the lattice conductivities decreased with increasing the degree of crystallinity at higher temperatures. On the other hand, the radiation conductivities tended to decrease with increasing the degree of crystallinity and became almost constant where the degree of crystallinity exceeded about 15%. As a consequence of this, more crystallisation does not always lead to slow cooling in continuous casting: the degree of crystallinity should be controlled to be about 15% where the partially crystalline phase exists around 773 K.

Simulation of Microstructure and Behavior of Interfacial Mold Slag Layers in Continuous Casting of Steel

Abstract: A computational model of heat transfer, solidification and interface behavior during the continuous casting of steel is applied to interpret the crystallization behavior of slag layers in the interfacial gap between the mold and the steel shell. A mechanism for the formation of this crystalline layer is proposed that combines the effects of a shift in the viscosity curve, a decrease in the liquid slag conductivity due to partial crystallization, and an increase in the solid slag layer roughness corresponding to a decrease in solid layer surface temperature with distance down the mold. When the shear stress exceeds the slag shear strength before the axial stress accumulates to the fracture strength, the slag could shear longitudinally inside the layers. The predictions are consistent with measurements conducted in the real process and with the microstructure of analyzed slag samples.

Microstructure and Precipitation Behavior of Nb, Ti Complex Microalloyed Steel Produced by Compact Strip Processing

Abstract: A comprehensive microstructure analyses were conducted for CSP processed Nb, Ti microalloyed steel, especially focusing on the precipitation behavior of the microalloying elements Nb and Ti. After coiling, the steel exhibits mainly a ferrite microstructure. The average ferrite grain size is 5.3 μm. The ferrite has a transitional morphology from polygonal ferrite to non-polygonal ferrite and is characterized by a moderate dislocation density of 2.47E+10/cm2. A high density of Nb, Ti complex star-like or cruciform shaped precipitates exist in the steel. They are Nb-rich and the average size is around 150 nm. About 49% Nb of the total in the steel is tied up in star-like precipitates, thus remarkably reducing the amount of Nb available for austenite conditioning, transformation temperature control and precipitation as small strengthening particles in ferrite. The main strengthening mechanisms found in the steel are the grain refinement and dislocation strengthening. Of the total yield strength, they represent contributions of 44% and 24%, respectively. There is a very little precipitation strengthening in the steel. It is thought that Nb, Ti complex star-like precipitate is prone to form in Ti-containing niobium microalloyed steel produced by compact strip processing.

Characterization of Thermal Annealing Effects on the Evolution of Coke Carbon Structure Using Raman Spectroscopy and X-ray Diffraction

Abstract: An experimental study was conducted to monitor the evolution of coke carbon structure during thermal annealing in a temperature range from 1273 to 2473 K in a bench-scale reactor. Coke carbon structure was characterized by using Raman Spectroscopy and the X-ray Diffraction. The Raman spectra of most of the cokes displayed two broad peaks G* (1620 cm−1) and D* (1360 cm−1). Both Raman peaks were deconvoluted into five peaks namely G, D, D′, R1 and R2. On the basis of area under the respective band peaks, new structural parameters were obtained to quantify graphitic (G), graphitic defect (D) and random (R) carbon fractions of cokes.XRD analysis was used to show that stack height carbon crystallite, Lc, of coke increases with increasing annealing temperature while the impact of annealing duration was not significant particularly up to 1873 K. On the other hand, average carbon crystallite width, La, did not improve significantly up to 1873 K, and increased rapidly after subsequent rise in the annealing temperature. It appears that during annealing up to 1873 K, modification of coke carbon structure could occur due to loss of basal carbon as a consequence of in-situ gasification.The lateral expansion of carbon crystallite, La, was related to relative intensity or shape of Raman band peaks such that both parameters did not change significantly up to an annealing temperature of 1873 K. At higher annealing temperatures, La values of coke increased with decreasing D/G ratio. Lateral expansion of carbon crystallite was attributed to progressive reduction of defects of graphitic carbon of coke, which can be monitored by D fraction of Raman Analysis. Combined Raman and XRD analysis suggested that rapid graphitization of coke may not occur along all dimensions until the annealing temperature exceeds 1873 K.Combining XRD and Raman analysis would provide a comprehensive evaluation of the evolution of coke carbon structure at different temperatures and their subsequent implications on the efficiency of various ironmaking operations.

Large Inclusions in Plain-carbon Steel Ingots Cast by Bottom Teeming

Abstract: Inclusions in industrial-cast bottom-teemed ingots of plain carbon steel are investigated using ultrasonic detection, optical microscope observation, and SEM analysis. The composition, size distribution, entrapment locations, and sources of ingot inclusions were revealed by examining all the macro-inclusions (larger than 20 μm) that were observed in 35 000 mm 2 of sample surface area. Based on 78 non-sulfide inclusions observed, around 3.23×10 7 macro-inclusions per m 3 steel exist in the ingot, with a size distribution increasing with decreasing size. Inclusions are distributed uniformly within a given horizontal section through the ingot, but with more found towards the bottom. The largest inclusions exceed 7 mm and originate from mold flux in the ingot. The largest inclusion source appears to be reoxidation, as evidenced by 59% of the ingot inclusions composed of pure alumina clusters and lumps. Eroded refractories from the ladle well block and ladle inner nozzle bricks accounted for 31% of the ingot inclusions.

Effect of High-temperature Field on Supersonic Oxygen Jet Behavior

Abstract: In the steelmaking process, the behavior of the top blown oxygen jet is an important factor for controlling BOF or EAF operation. Because the temperature in the BOF is very high, jet behavior is still not fully understood. In this study, supersonic oxygen jet behavior in a high-temperature field was investigated by measuring the velocity and temperature of the oxygen jet in a heated furnace, and the results were compared with a jet model proposed by previous researchers. The results showed that velocity attenuation of the jet was restrained and the potential core length was extended in a high-temperature field. Under these experimental conditions, the results were in good agreement with the jet model using Pr=0.715 and Sc=0.708 proposed by Kleinstein. Supersonic jet behavior for SCOPE-JET nozzle, which can be applied in EAF operation to obtain high-energy efficiency, was also investigated. SCOPE-JET had a long potential core and attenuation of the jet in the axial direction was extremely restrained. It was shown to be possible to obtain the jet behavior of SCOPE-JET nozzle using the jet model with the adiabatic flame temperature as the ambient temperature and appropriate temperature attenuation behavior in the axial direction.

Numerical simulation of solidification structure formation during continuous casting in Fe-0.7mass%C alloy using cellular automaton method

Abstract: A numerical model was developed for the simulation of solidification structure formation during the continuous casting process of an Fe–0.7mass%C alloy. In this model, the cellular automaton method was combined with heat transfer calculation during the continuous casting process. The effect of electromagnetic stirring (EMS) during the continuous casting process was introduced as an increase in thermal conductivity in liquid. Furthermore, the effect of fragmentation of dendrites due to fluid flow during EMS was taken into account in the model as an increase in the formation of crystals. Simulations of grain structure formation in continuously cast billets with and without EMS were carried out, and the degree of fragmentation was evaluated from a comparison between experimentally observed and simulated macrostructures of the billets.

Modeling of Macrosegregation and Solidification Grain Structures with a Coupled Cellular Automaton—Finite Element Model

Abstract: A coupled Cellular Automaton (CA)-Finite Element (FE) model is presented for the prediction of solidification grain structures coupled with the calculation of solid and liquid flow induced macrosegregation. The model is applied to simulate the solidification of a Pb-48wt%Sn alloy in a rectangular cavity cooled down from only one of its vertical boundaries. The algorithm and the numerical implementation of the coupling between the CA and FE methods are first validated by considering a single grain developing with no undercooling. Such a CAFE simulation is shown to retrieve the solution of a purely FE method simulation for which the grain structure is not accounted for. Several applications of the model are then presented to quantify the effects of the grain structure on the final macrosegregation map. In particular, the effect of the undercooling of the columnar front, the presence of equiaxed grains nucleated in the undercooled liquid, as well as the transport and sedimentation of equiaxed grains are investigated. Although good validation is reached when comparing computed and measured segregation profiles available in the literature for the chosen configuration, it is concluded that refined experimental data are required to further validate the predictions of a coupled CAFE model.