This paper reviews multiphase flow models for continuous casting of steel from classical models to recent methods. These multiphase flow models are classified into six groups in this paper: quasi-multiphase models, multi-fluid models, moving grid methods, interface tracking methods, particle based models/methods and hybrid models. For each model, the governing equations are summarized and the inherent advantages and disadvantages are discussed. Example applications of each model are presented from previous literature, illustrating typical results and accuracy that can be obtained. The objective of this paper is to guide readers to choose an appropriate multiphase flow model for their application. Argon gas bubble effects on the flow pattern can be modeled with simple mixture models, Eulerian-Eulerian models, Multiple size group models which also track bubble size distributions, and Discrete-Phase Models (DPM). Gas pockets and slug flow, which can occur inside the nozzle are more complex, and hybrid models which combine different models together, such as Eulerian-Eulerian, level-set, Volume of Fluid (VOF), and DPM, appear promising. The shape of the slag/steel interfacial profile, level fluctuations, and slag entrainment can be modeled with free surface methods, such as moving grid, VOF, or other interface tracking methods. Particle transport and entrapment, including inclusions and gas bubbles can be added via DPM models and also require a capture criterion model. Solidification and meniscus phenomena require flow models coupled with heat transfer and solidification.
To improve the sedimentation efficiency during the smelting reduction process of converter slag for iron extraction, the interaction and sedimentation behavior of liquid iron droplets have been studied under the condition of experimental crucible scale by numerical simulation. The effects of slag viscosity and density difference between slag and iron droplet have also been investigated. The results show that sedimentation process in molten converter slag can be divided into early and latter settling stages, and increasing the coalescence of small droplets in the middle and lower region of slag pool and accelerating the droplet sedimentation at later settling stage are crucial for shortening the settling time of whole iron droplet cluster. During the sedimentation process, the coalescence phenomenon is accompanied, whereas rebound may not happen probably due to the relatively large metal-slag interfacial tension, small slag viscosity and small relative velocities of colliding droplets (≤8×10−3 m·s−1) caused by the creeping flow. The sedimentation velocities for a droplet is increased by about 13%–17% after coalescence. The sedimentation efficiency with interaction is significantly higher than that without interaction when iron droplets coalesce frequently. As the slag viscosity increases from 0.065 Pa·s to 0.195 Pa·s, the sedimentation efficiency decreases from 90% to 59%, while it increases from 89% to 92% with the density difference increasing from 3700 kg·m−3 to 4300 kg·m−3 at the settling time of 24 s. Therefore, molten slags with better fluidity and lower density are favored in the practical iron extraction process.
To understand the effect of MgO as additive on the reduction roasting of beach titanomagnetite concentrate, the phase transformations, metallization degree and Fe–Mg–O phase diagram were studied in this paper. Results indicated that with the increasing of MgO dosage, more Mg2+ diffused into the magnetite lattice, replaced some of the Fe2+, and formed a large of MgFe2O4. So adding MgO not only did not effectively improve iron recovery, but baffled the reduction of iron oxides. Besides, the effect of CaCO3 on the growth of iron particles was also studied by optical microscope and Qwin image analysis software. Tests revealed that an appropriate CaCO3 dosage can facilitate the growth of iron particles. Because CaCO3 promoted the reaction of iron oxides, FeO content in the slag decreased and more nuclei of iron crystal was formed, resulting in the growth of iron particles. However, excessive CaCO3 increased the slag melting point, which hindered the diffusion and growth of iron particles. At last, the best product indexes were obtained by magnetic separation when CaCO3 dosage was 4%.
The study discusses the oxidation induration and swelling behavior of chromium-bearing vanadium titanomagnetite pellets (CVTP) with V2O5 additions, and the reduction swelling index (RSI) and compressive strength (CS) of reduced CVTP were investigated with simulated coke oven gas (COG) injection into the blast furnace (BF). The results show that the CS of CVTP decreases and the porosity of CVTP increases with increasing V2O5 additions. The proportion of microsize pore size distribution of CVTP between 0 to 5 µm decreases notably while the pore size distribution between 5 to 30 µm increases with increasing V2O5 additions. The V2O5 mainly exists in the form of V2Ti3O9 and V1.93Cr0.07O3 in CVTP and V2TiO5 in reduced CVTP. The V-bearing spinels on the grain boundaries with fragmentized and prismatic structure restrain the CS of CVTP. The CS of reduced CVTP decreases and RSI increases with increasing V2O5 additions. The V2O5 addition facilitates the aggregation and diffusion of metallic iron particles, and the shape of the metallic iron whiskers transform round dot to prismatic. The pores and intervals enlarge, and thickness of lamellar crystals thickens gradually with increasing V2O5 additions. The study could supply the theoretical and technical basis for the utilization of CVTP and other V-bearing ores with COG recyclable technology.
SEM images of reduced CVTP with V2O5 additions: (a) 0 mass%; (b) 2 mass%; (c) 4 mass%; (d) 6 mass%.
A new accumulated material-type distributing chute on blast furnace (BF) is designed to prolong the service life of distributing chute on BF according to powder mechanics theory, and a Comparative simulations study between the accumulated material-type distributing chute and smooth-plate straight chute is conducted using discrete element method. Additionally, the influence of shape of transverse ribs in accumulated area and particle size on the impact of chute are discussed. Results show that this type of distributing chute on BF has formed a stable material accumulation under the impact zone of material flow and its material cushion forms a certain thickness, which greatly reduces the impact and abrasion of material flow. The impact force of the material flow on the chute is reduced to 10.1%–17.2%, and the sliding velocity of particles near the bottom plate is reduced to 29.1%–33.4% compared with the smooth-plate straight chute. Therefore, the new accumulated material-type distributing chute has good anti-impact ability and abrasion resistance. Shape of transverse ribs in accumulated area and particle size have a relatively small effect on the impact of chute after discussing.
The effects of Ti on the crystallisation during cooling of complex Ca-rich ferrite iron ore sinter bonding phases SFCA and Fe-rich SFCA was investigated using in situ synchrotron X-ray diffraction. Cooling of a high temperature (T = 1623 K) assemblage comprising magnetite and melt in synthetic iron ore sinter mixtures showed that increasing the Ti concentration to 1, 3 and 6 mass% TiO2 resulted in the formation of SFCA to be favoured over Fe-rich SFCA. Fe-rich SFCA was not observed to form at all during cooling in the 6 mass% TiO2 mixture. The absence of Fe-rich SFCA in the high-Ti experiment was rationalised on the basis that the SFCA structure likely accommodates more Ti4+ than the Fe-rich SFCA structure, thereby stabilising SFCA relative to Fe-rich SFCA. Observation of Fe-rich SFCA in sinter may be an indicator that the localised Ti concentration within a sinter blend is likely to be low.
Utilization of iron ore-carbon composite for blast furnace has a possibility of the method to decrease carbon dioxide emission from iron and steel industry. Major iron ores consist of Fe2O3, Fe3O4, and FeOOH, which require several reduction steps through FeO to form metallic iron. When graphite and pure FeO obtained by pre-reduction are used for the raw material of composite, the reduction of iron oxide proceeds at lower temperature than that using conventional iron ore. In the actual process, raw materials such as iron ores and coke contain impurities of silica, alumina and sulfur. Furthermore, limestone is used as fluxing materials to control basicity of slag. In this study, the effect of types of carbonaceous materials and CaO addition on the reduction behavior of the composite using FeO prepared by pre-reduction was examined. Reduction temperatures of FeO composite using coke and biomass char were higher than that using graphite. The reason can be explained by the effect of sulfur contained in the carbonaceous materials. The addition of CaO which reacted with sulfur led to lowering the reduction temperature.
Based on the structure of slag, the revised structure based viscosity model was improved for viscosity prediction of the fully liquid slag in the Al2O3–CaO–MgO–SiO2 quaternary system and its subsystems. Experimental procedures and available data in the literature have been critically reviewed. In this modified model, the oxygen ions bonded with non-compensated Al3+ ions were defined as excess bridge oxygens. The concentration of different types of oxygen ions are calculated and used to express the activation energy. The present model is capable of predicting the viscosities in the Al2O3–CaO–MgO–SiO2 quaternary system and its subsystems over the wide composition and temperature ranges above liquidus within experimental uncertainties; the average of relative errors for this model was found to be 17.97%. CaO has a greater ability to decrease the viscosity than that of MgO in the system without Al2O3 because of the weaker bond strength of CaO. A viscosity maximum occurs for MO–Al2O3–SiO2 (M=Ca or Mg) slag with a fixed SiO2 content. The estimated viscosities decrease with the increase of MgO content, decrease with increasing of the Al2O3/SiO2 ratio at 5 mass% of MgO, and keep almost constant or even slightly increase with the increase of the Al2O3/SiO2 ratio at 10 mass% MgO.
The temperature field distribution of the upper part of a blast furnace (BF) is the result of the combination of charge distribution and gas flow. It is of great significance for both researchers and operators to study the thermal state and the phenomenon of the BF. The graphite box method is used in this study to obtain the temperature field distribution in the upper part of a 125 m3 BF. Graphite boxes with a variety of different melting point metals were loaded into the BF with the charge. And the temperature field distribution was obtained after the boxes were taken out with position and temperature information during the dissection process. The results illustrate that the graphite boxes are unevenly distributed in the BF, which was related to the distribution of BF materials. Furthermore, the temperature field distribution is asymmetric, and the isotherms present an irregular “W” shape, which is caused mainly by the simultaneous development of the edge airflow and the central airflow. Moreover, the shape of the softening and melting zone observed in the dissection process has a good correspondence with the temperature field. We concluded that the deflection of the temperature field and the softening and melting zone is related to the strong gas flow in the direction of No. 3 tuyere, the existence of accretion in the hearth, and the filling of refractory materials in tuyeres before blowing out.
Using a furnace focused 8 microwave beams of 16 kW at 2.45 GHz to the center of applicator, 9149 g of molten pig iron was produced from 14049 g of the mixed powdery resources of Romeral iron ore and graphite during 780 min under N2 gas. The yield of pig iron was 99.3%. The resources was intermittently supplied with the rate of 35 g/min for 20 min. By each supply of resources of 700 g, temperature rapidly decreased and increased to near 1300°C and molten pig iron poured out from a reaction chamber with generating white light emission.
Growth mechanism and structure evolution during nucleation of Calcium borosilicate (Ca11Si4B2O22) crystal in CaO–SiO2–B2O3 based fluorine-free (F-free) mold flux have been investigated in this article. The results suggested that during nucleation, the existed free oxygen ions tend to depolymerize Si–O–B units and break down borosilicate structure. Then, the transformation between [BO4]-tetrahedral and [BO3]-trihedral would further depolymerize borosilicate structure. Next, the decomposed borosilicate will connect with the formed Q2(Si) and the existed dissociative Q1(Si), and finally form a long-range ordered borosilicate structure with a certain symmetry. Finally, Ca2+ ions would associate with the borosilicate structure to form Calcium borosilicate crystal nucleus. With the formation of the crystal nucleus, the crystals first precipitated at the boundary of the thermocouple and exhibited grain-shape particles orientated in a line dispersively. Then, the grain-shape crystals at the boundary became thicker and formed dendrite structure. Subsequently, the secondary dendrites were observed to form and grew on the primary dendrites axis. Ultimately, Calcium borosilicate crystals were well distributed. The precipitated crystal phase of the F-free mold flux was β-polymorphs Calcium borosilicate. The kinetics of the isothermal crystallization at 1373 K is constant number of nuclei, 3-dimensional growth by diffusion control.
We fabricated the Fe-18Cr-2Ni-1Mo-0.2C-0.2Mn-0.1Si heat-resistant steels containing 3 to 12% Co by casting in the presence of a rotary electromagnetic stirring (EMS). The effect of EMS on the Fe–Cr–Co steels was evaluated by the size and type of inclusions and the examination of mechanical properties. The morphology and composition of inclusions were observed, and the formation mechanism of the inclusions was analyzed through thermodynamic calculation using Thermo-Calc. The results showed that there were mainly two types of inclusions generated in the solidification process, including the Al2O3 corundum and the MnO. Cr2O3 spinel. The application of EMS during solidification has shown a significant effect on the exclusion of large inclusions. Both the number density and the size of inclusions decreased under the effect of EMS. The mechanical properties at elevated temperatures were investigated considering the effect of cobalt addition and EMS. The strength of the steel increased with increasing cobalt content. At 800°C, an increase in cobalt from 3 to 12% resulted in a substantial strength increase concomitant with an increase in elongation rate. EMS reduced the ultimate tensile strength but enhanced the ductility of the steels. These effects were discussed relating to an early suppression of the δ–ferrite formation and the exclusion of large inclusions.
Efforts have been made to develop fluorine-free mould fluxes for the continuous casting of steel process. In this work the crystallization behaviour of fluorine-free mould slags based on the CaO–Al2O3–B2O3 system was investigated by differential scanning calorimeter (DSC) and scanning electron microscopy equipped with energy dispersive spectroscopy(SEM-EDS). The crystallization kinetics for Ca3Al2O6 primary crystals was analysed by combining modified Avrami analysis with Friedman isoconversional method. Avrami parameter n is close to 4 for samples with the ratios w(CaO)/w(Al2O3)=1 and w(CaO)/w(Al2O3)=1.2, indicating a crystallization mechanism of continuous bulk nucleation and 3D crystal growth. The Avrami parameter n for samples with w(CaO)/w(Al2O3)=0.9 is close to 3, indicating instantaneous bulk nucleation and 3D crystal growth. The crystallization rate constant is the highest and half crystallization time is the lowest for the samples with w(CaO)/w(Al2O3)=0.9, indicating the fastest crystallization. In the initial stage, effective activation energies were mainly determined by the undercooling values. In the final stage, kinetic barrier for crystallization could have some influence on crystallization; for the investigated mould fluxes crystallization in the final stage is retarded by increasing w(CaO)/w(Al2O3) ratio. Thus crystallization mechanisms were elucidated and effective activation energy of crystallization for the first crystal which precipitates from melt was determined.
The hot ductility of steels containing 0–0.06 wt.%Nb has been evaluated through γ grain growth experiments and hot stage tensile tests of the α + γ two phase region in order to clarify the roles of NbC precipitation and γ grain size evolution resulting from Nb-initiated solute drag on hot ductility in this important material property.
The experimental results show that (1) a decrease in γ grain size as a result of Nb-initiated solute drag improves hot ductility, (2) for a given γ grain size, hot ductility decreases with increasing Nb content because the corresponding increase in NbC precipitation fraction increases strength, and (3) the variation in ductility with Nb content is smaller when the γ grain size is smaller. These competing effects of γ grain size and NbC precipitation affect the strain incompatibility between the α and γ phases, leading to the onset of surface cracking during continuous casting when the incompatibility is high. The underlying mechanisms controlling ductility in Nb-containing steels are demonstrated using a model that partitions strain between the α and γ phases.
Air-mist jet is increasing in number of facilities for a variety of applications in surface cooling. It can provide a good balance of high heat removal capability. The present study focused on the velocity characteristics of the air-mist jet produced by a fan-shaped nozzle under different operating conditions during secondary cooling of continuous casting. To this end, the authors conducted experimental research, employing optical techniques, i.e., particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) to measure a fanlike flow field in an air-mist jet. However, the researchers observed different results between the PIV and LDV measurements, mainly at the outlet of the nozzle region. This article identifies velocity characteristics of air-mist jet during secondary cooling of continuous casting, which is divided into two parts for velocity distribution structure of air-mist jet, and self-similar and well described by a Gaussian distribution for the whole flow field. The study provides a suggestion to optimise the flow field of a fanlike air-mist nozzle for different working conditions. Furthermore, the behavior of gas-liquid two-phase flow can be understood by examining the difference in velocity characteristics between the PIV and LDV measurements of the air-mist jet.
In double cold rolling process, rolling energy and oil consumption is normally controlled separately, thereby causing a high comprehensive cost. This study investigated a calculation model of plate-out oil film thickness on strip surface, oil film thickness in deformation zone, friction coefficient, bite angle, forward slip, rolling force, rolling power, rolling energy consumption, and rolling oil consumption. Subsequently, the effect of emulsion flow and concentration on rolling energy and oil consumption comprehensive cost was quantitatively analyzed. On this basis, an objective function of rolling energy and oil consumption comprehensive cost was proposed, and the corresponding comprehensive optimization control technology for rolling energy and oil consumption was developed. Through a field application of this technology, the reduction of rolling energy and oil comprehensive consumption cost was achieved by optimizing emulsion flow and concentration comprehensively. Thus, a significant economic benefit was created with further popularization and application values.
Defect detection on bar surface is a challenging task due to the complex and variable bar surface conditions. Traditional pattern recognition methods are widely used to detect defects in the industry, however most of existing methods are not very universal for all kinds of defects. Meanwhile because of the limited number of defective samples, traditional deep learning methods are not very effective in practice. This paper addresses these issues and proposes a novel few-shot learning method which combines with attention mechanism. Our method is built by a Convolutional Neural Network (CNN) which extracts image features, and a Relation Network (RN) which calculates the similarity score between a pair of images, predicts image categories through similarity scores. Firstly, in order to extract more effective and discriminative features, we introduced Squeeze-and-Excitation Networks (SENet) as an attention module into our method which can enhance effective features and weaken invalid features. Secondly, unlike traditional object detection techniques which mainly focus on foreground information, background information is also necessary in our method, because we need to utilize background information to distinguish pseudo and real defects. So in our method, we replaced Max-Pooling with Mean-Pooling. Finally, in order to solve the low efficiency of parameter update caused by sharp dropping of loss function values on our dataset, we use L1Loss and BCELoss to replace Mean square error loss function. Experiment results show that the proposed method can achieve an average accuracy rate of 97.25% on our data set, increased by 7.92% compared with state-of-the-art.
As an environmentally hazardous waste, silica fume was considered as a potential alternative for cement and SiO2 production. The structure of Si–O was highly relevant to the reactivity of Si conversion for efficient utilization. In this study, the characteristic and chemical structure of Si–O in silica fume were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). Deconvolution of XPS and FTIR spectra into elementary profiles was carried out to analyze the structural components. As a result, the valence state, bonding structure and elementary unit in the Si–O network of silica fume were determined. Then, the reactivity silica fume with alkali solution was studied involving the effects of NaOH concentration and temperature. The staged kinetics behavior was associated with the structure of Si–O bonds, and the activation energies were determined. The results thus provided fundamental information for the utilization of silica fume for SiO2 production and geopolymer.
The nucleation potency of different types of inclusions for acicular ferrite (AF) plates was quantitatively evaluated for Ti-containing welds. The objective of this investigation was to determine the relative efficiency of AF nucleation among the inclusions and to understand the effect of nucleation potency on the AF content. A series of bead-in-groove welds having Ti concentrations in the range of 0.002 to 0.091 wt.% were used in this study. The nucleation event of the inclusions was examined using scanning electron microscopy for a large number of inclusions of various sizes. Then, the nucleation probability curves were constructed as a function of inclusion size for each weld. The experimental results demonstrated that the nucleation probability increased with the inclusion size for all welds. However, this size effect varied with the Ti content of the welds substantially. By employing two different parameters to represent the difference of nucleation potency in a quantitative manner, it was found that the Ti effect on AF content resulted from the corresponding change in nucleation potency.
Zn–Mg coatings with Mg compositions ranging from 5 wt.% to 15 wt.% were synthesized on high-strength steels using the electro-magnetic heating deposition process, and the effects of a 1-µm thick Zn interlayer on the formability and adhesion of the Zn–Mg coatings were investigated. The Zn–Mg coatings with the Zn interlayer showed little difference in terms of microstructure, crystal structure and corrosion resistance compared to the Zn–Mg coatings without the Zn interlayer. In contrast, the Zn interlayer significantly contributed to the improvement of the formability and adhesion of the Zn–Mg coating. The delamination area of the Zn–Mg coating with the Zn interlayer after deformation was extensively reduced, and the results of the lap shear tests revealed that the maximum adhesion strength of the Zn–Mg coatings with the Zn interlayer was approximately twice as high as that of the Zn–Mg coatings without the Zn interlayer. The maximum adhesion strength of the Zn/Zn–Mg coatings with 15 wt.% Mg, which showed the best corrosion resistance was measured to be slightly over 10 MPa, pending more research to increase the adhesion strength of the Zn–Mg coatings with high content of Mg over approximately 20 MPa necessary.
The present work has studied the kinetics of partial reversion of martensite into austenite, which occurred during overaging between 520 and 600°C in a maraging 350 steel. The microstructural modifications were followed using optical microscopy, scanning electron microscopy, X-ray diffraction and ferritoscopy. The observations carried out using microscopy showed that the reverted austenite is formed at interface regions such as grain boundaries, boundaries of the packets and martensite laths boundaries of the martensitic structure starting at 520°C, while the reverted austenite observed inside the martensite lath was formed starting at 560°C. The maximum percentage volumetric fractions of retained austenite were 18%, 25% and 37%, respectively for 520, 560 and 600°C. The higher the aging temperature, the faster the volumetric fraction attained its plateau of maximum constant value. The activation energy for the kinetics of the austenite reversion process has been determined as being 332 kJ/mol. The Avrami constant for the kinetics of the reverted austenite at 520°C was close to 1, while at 560°C was close to 2. The values determined for the activation energy and of the Avrami constants have been discussed and correlated with the possible transformation mechanisms.
The precipitation of iron carbides is a crucial factor that determines the properties of tempered martensite. However, the effect of alloying elements on the carbon concentration of ε carbide has not yet been clarified. In this work, we studied the effect of alloying elements on the carbon concentration of ε carbide using first-principles calculations and a three-dimensional atom probe. The first-principles calculations showed that ε carbide with a lower carbon concentration tends to form by the inclusion of Si. The carbon concentration in ε carbide measured by the three-dimensional atom probe was consistent with the first-principles calculations.
The effect of grain boundary precipitates on hydrogen embrittlement was investigated with a stable austenitic stainless steel, Fe-20Cr-11Ni-5Mn-2Mo-0.5Si-0.3N-0.1C (wt.%). The steels were aged to form M23C6 along the grain boundaries and electrochemically charged with hydrogen. Based on the fractography and silver decoration results, it was found that hydrogen seemed to be concentrated at M23C6/austenite interfaces, and have promoted crack initiation and propagation. Accordingly, hydrogen-induced ductility loss was intensified in the aged alloys.
Indian iron ore mining industry disposes large amount of ultra fines containing high gangue minerals, thereby lead to loss of iron values and environmental pollution. Selective flocculation studies have been thought of for beneficiation of ultra fines. Response of different types of starches to iron ore ultra fines generated by the hydro-cyclone in Joda Iron Ore Washing Plants, Orissa, India has been studied. The starches used are: (i) maize starch (MS), (ii) potato starch (PS) and (iii) causticized potato starch (CPS). The order of selectivity as flocculant towards iron bearing minerals is observed as MS<PS<CPS. This can be attributed to the different C-H chain morphology of the starches. MS has more amylopectin than PS. Amylopectin adsorbed strongly onto all oxides minerals as carbonyl groups attached to C-2 and C-3 atoms of starch form surface complex with surface atom of all oxide minerals whereas amylose has a more adsorption tendency to hematite only. Amylose has less number of end groups than amylopectin, thus exhibiting lesser adsorption density than amylopectin. The selective adsorption characteristics of PS to hematite further improves by modification. Iron content and the iron recovery of the concentrate depend on flocculant dosage as well as settling time. A good concentrate is obtained suitable for pellet feed with Fe content in the concentrate increased from 57.8 mass% to 66.3 mass% and with an iron recovery of 66.5% by this process using CPS as a flocculating agent under optimized conditions. The tailing generated is suitable for building materials like tiles.