Showing papers by "Lifeng Zhang published in 2017"

Transformation of Oxide Inclusions in Type 304 Stainless Steels during Heat Treatment

Abstract: Heat treatment of Type 304 stainless steel in the range of 1273 K (1000 °C) to 1473 K (1200 °C) can transform manganese silicate inclusions to manganese chromite (spinel) inclusions. During heat treatment, Cr reacts with manganese silicate to form spinel. The transformation rate of inclusions depends strongly on both temperature [in the range of 1273 K to 1473 K (1000 °C to 1200 °C)] and inclusion size. A kinetic model, developed using FactSage macros, showed that these effects agree quantitatively with diffusion-controlled transformation. A simplified analytical model, which can be used for rapid calculations, predicts similar transformation kinetics, in agreement with the experimental observations.

A kinetic model for Ca treatment of Al-killed steels using FactSage macro processing

Abstract: A kinetic model was developed to investigate the evolution of inclusions during the Ca treatment of Al-killed steels. Evolutions of steel chemistry and inclusion composition in Al-killed Ca-treated steels from previous experimental works were accurately predicted using the current kinetic model. The model was widely used to predict the evolution of inclusions during the Ca treatment of Al-killed steels with varying concentrations of Al, S, O and Ca. During the Ca injection of Al-killed steels, a large number of CaS and CaO inclusions are transiently generated in the Ca-rich zone due to the excessive Ca. Al2O3 inclusions are gradually modified to calcium aluminates in the bulk steel with the addition of Ca. The superfluous Ca or S may lead to the formation of solid CaS in the liquid steel.

Transformation of Inclusions in Pipeline Steels During Solidification and Cooling

Abstract: Experimental and thermodynamic considerations on the transformation of inclusions during cooling process of pipeline steel were carried out. In plant trials, CaO-Al2O3 type inclusions in molten steel were fully or partially transformed into MgO-CaO-Al2O3-CaS type in a slab depending on the size. The transformation details were revealed by thermodynamic calculations. The deviations between experimental and calculated results are discussed.

Evolution of Oxide Inclusions in Si-Mn Killed Steels During Hot-Rolling Process

Abstract: The evolution of oxide inclusions in Si-Mn killed steels refined by slags of different basicity during a four-pass industrial hot-rolling process was investigated using an automated microscopy system. High-basicity refining slag induced the formation of CaO- and Al2O3-containing inclusions, while refining slag with 0.8 basicity induced dominant inclusions of SiO2 and MnO-SiO2. CaO-SiO2-Al2O3 inclusions mainly formed endogenously during solidification and cooling of Ca-containing steels, where Ca originated from slag-steel reactions. However, the larger-sized higher-CaO inclusions originated from slag entrainment. Different inclusions presented different hot-rolling behaviors. The inclusion composition changed by deformation and new phase formation. The dominant oxide types were unchanged under refinement by low-basicity slag; however, they changed under refinement with high-basicity slag. The deformation index of inclusions decreased with increasing accumulated reduction (AR) of the steel. The difference in deformation index between different inclusion types was the largest in the first rolling stage and decreased in subsequent stages. SiO2-CaO and SiO2-MnO-CaO inclusions had larger deformation indices during hot rolling but smaller indices in the last two stages. High-basicity slag increased inclusion complexity; from the perspective of cold-drawing performance, low-basicity refining slag is better for the industrial production of tire-cord steels.

Characterization of the Three-Dimensional Morphology and Formation Mechanism of Inclusions in Linepipe Steels

Abstract: The characterization of the three-dimensional (3D) morphology and modification of inclusions in linepipe steel plates were investigated in the present work. The deoxidation products, Al2O3 inclusions, were modified into deformable calcium aluminate inclusions by calcium treatment in order to efficiently prevent nozzle clogging. The effect of sulfur content on the morphological characteristics and chemical composition of inclusions by calcium treatment were analyzed by extracting inclusions from steel matrix using non-aqueous electrolyte and automatic scanning electron microscopy connects with energy-dispersive spectrometer. The evaluation of the effect of calcium treatment was proposed using the 3D morphology of inclusions. Meanwhile, the modification and control of inclusions in the linepipe steel were studied by thermodynamic calculation on calcium treatment with the strategy of reducing the size of inclusions before calcium treatment, and controlling the contents of [Al], [Ca], and total oxygen in a reasonable narrow range during calcium treatment. The modification and transformation process of alumina inclusions was revealed as Al2O3 → MgO-Al2O3 → CaO-MgO-Al2O3-CaS (after calcium treatment) → CaO-Al2O3-CaS. A kind of high-melting MgO-Al2O3 core was wrapped by the low-melting component of CaO-Al2O3 indicating an incomplete reduction of MgO and Al2O3 by calcium.

Kinetic Modeling for the Dissolution of MgO Lining Refractory in Al-Killed Steels

Abstract: A kinetic model for the dissolution of the magnesia refractory in Al-killed steels was developed to predict the change of chemical compositions in the molten steel. Coupled reaction model and empirical equations were employed to calculate reactions between the refractory and the molten steel. The calculated result showed good agreement with the experimental value from the literature. The relationship between the mass transfer coefficient and the stirring energy dissipation in the molten steel was obtained as $$ k = \left( {5.6 - 4.7 \times e^{{ - 96.6\dot{\varepsilon }}} } \right) \times 10^{ - 4} $$ m/s. The formation of spinels is dependent on the chemical composition of the molten steel. The MgO refractory may react with Al and O in the liquid steel. Meanwhile, the decomposing of the MgO refractory also plays an important role in the dissolution of the magnesia refractory in Al-killed steels.

Characterization of MnS Particles in Heavy Rail Steels Using Different Methods

Abstract: Characterization of the MnS particles in continuous casting (CC) blooms and rolled rails of U75V steel using the methods of traditional two-dimensional (2D) microscope observation, three-dimensional (3D) micro-CT detection, and electrolytic extraction is performed. MnS particles from the surface to the interior, inside of columnar dendritic zone, and the equiaxed zone of the CC bloom are discussed. The morphology of MnS changes from elliptical and spherical near the surface of bloom to strip-like, petal-like, polyhedral, and irregular in the center of bloom gradually. The size of both pure MnS particles and complex inclusions which consist of a MnS outer layer and an oxide core increase from the edge to the center of bloom. However, MnS is elongated along the rolling direction during rolling process. The morphology of the cross section of the elongated MnS particles presents as globular and flaggy shape in the head and bottom of rail, and spindle shape in the waist of rail. The precipitation of MnS particles is studied on the basis of thermodynamics, the calculating results show that MnS particles are generated towards the end of solidification (solid fraction fS = 0.94), the amount of MnS particles are affected by the initial concentration of Mn and S in molten steel during solidification.

A Reaction Model for Prediction of Inclusion Evolution During Reoxidation of Ca-Treated Al-Killed Steels in Tundish

Abstract: A reaction model was developed to investigate evolutions of steel chemistry and inclusion compositions during reoxidation of Ca-treated Al-killed steels in a tundish. The evolutions of Al2O3, CaO, MgO, and CaO in inclusions were accurately predicted using the current reaction model. The reaction model can widely predict the evolution of inclusions during tundish reoxidation of Ca-treated Al-killed steels with varying concentrations of Al, Ca, and absorbed oxygen.

Effect of snorkel shape and number of nozzles on mixing phenomena in the RH process by physical modeling

Abstract: In the current study, the effect of snorkel shape and the effect of the number of nozzles on recirculation flow rate and the local mixing time in the RH process were investigated using physical modeling experiments and industrial trials. Meanwhile, the velocity fields were measured with a Particle Image Velocimetry technique. The results show that oval snorkels provided much better mixing conditions in comparison with round snorkels. The improvement in recirculation rate was found to be in the order of 25% and the increase in decarburization rate was about 50%. It was attributed to an increase in the cross-sectional area from round to oval snorkels. The best mixing conditions as a function of the number of nozzles was obtained with 12 nozzles for round snorkels and 18 nozzles for oval snorkels. For the same stirring conditions, oval snorkels are less sensitive to changes in mixing time by changing the number of nozzles. Finally, the treatment time using oval snorkels could be decreased by 50% in comparison with round snorkels according to the variation of the concentration of carbon.

Effect of slag basicity adjusting on inclusions in tire cord steels during ladle furnace refining process

Abstract: Three heats of industrial trials and thermodynamic calculations were performed to investigate the effect of slag basicity adjusting on inclusions in tire cord steels. It was found that the application of low basicity slag can effectively avoid the formation of Al2 O3 -rich inclusions, while a high basicity slag was beneficial to improve the cleanliness of tire cord steels. It should be noted that adjusting slag basicity from 1.5–2.0 to 0.8–1.0 during the LF refining process can hardly lower the Al2 O3 in inclusions. Thus, it was suggested to use the low slag basicity during the whole LF refining process to avoid the formation of Al2 O3 -rich inclusions. Moreover, thermodynamic calculation of slag-steel-inclusion reactions of tire cord steels was performed to better understand the observed experimental results.

Mathematical modeling on the fluid flow during RH degassing process

Abstract: A mathematical model combining VOF and DPM has been developed to investigate the effect of the lift force and the bubble size on the fluid flow in the RH process. The effect of the type of gas blowing was also discussed. The calculated results were validated against the experimental data from water modeling using a particle image velocimetry technique (PIV). In the current model, the interfacial behavior between the molten steel and the gas phase, and the motion of argon bubbles can be captured and tracked simultaneously. The results indicated that the speed of the molten steel and the recirculation rate increased with increasing the lift force. In order to accurately simulate the fluid flow in the RH process, the optimum lift coefficient was 0.1. However, an inverse tendency was obtained for the bubble size. With the bubble size increasing, the gas volume fraction in the up-leg snorkel changed from a double-peak distribution to a single-peak distribution, which weakened the interaction between the molten steel and gas bubbles and decreased the recirculate rate. To improve the recirculate rate and good stirring, small size gas bubbles should be adopted. In addition, the symmetrical flow of the molten steel in the down-leg snorkel was converted into a rotating flow pattern when increasing gas flow rates in one nozzle or two nozzles. Meanwhile, the recirculate rate decreased and the slag entrainment at the free surface might occur. Therefore, it was of great importance to ensure an equal gas flow rate in each nozzle in actual operations.

Mechanism and Control of Sulfide Inclusion Accumulation in CET Zone of 37Mn5 Round Billet

Abstract: In the current study, the R/2 accumulation was found from the results of sulfide inclusion distribution across the diameter of 37Mn5 round billet using ASPEX. In order to reveal the mechanism and furthermore control this phenomenon, the distribution of sulfur content and the macrostructure of billet after etching were analyzed. The results showed that there was obvious segregation in the same position as sulfide inclusion accumulation, where the columnar-to-equiaxed transition (CET) took place. Therefore, the mechanism of sulfide inclusion accumulation in the CET zone was discussed on the basis of the mechanism of R/2 sulfur segregation. The interlaced and coarse dendrites in the CET zone blocked solute concentrated from select crystallization, thus caused sulfur segregation near R/2. Higher sulfur content and larger secondary dendrite arm spacing (SDAS) in the CET zone benefited the precipitation and growth of sulfide inclusions. The effect factors of CET position were discussed on the purpose that by proper cooling conditions, the morphology of solidification structure can be controlled, by which the element segregation can be modified; thereafter, the control of sulfide inclusion distribution in the billet can be realized.

Effect of addition of Al-based slag deoxidizer on MgO.Al2O3 inclusions in 3Si-Fe steels

Abstract: In the current study, laboratory experiments between CaO-Al2 O3 -SiO2 -MgO-TiO2 - MnO-Fe2 O3 slags and 3Si-Fe melts with the addition of an Al-based slag deoxidizer with the main composition of 45% metallic Al, 25% CaO, 13% Al2 O3 , 4% SiO2 and 2.3% MgO were carried out at 1873 K. The addition of an Al-based slag deoxidizer increased not only Al but also Mg in the steel due to the reduction of MgO in slag and crucible by Al, which was responsible for the evolution of inclusions from MgO. Al2 O3 to pure MgO. log(a .Mg /a 2 Al . a 2 O ) in steel linearly increases by increasing log(a s MgO /a s Al2 O3 ) in slag with the slope close to unity. In addition, the relationship between log(X MgO /X Al2 O3 ) in the inclusions and log(a .Mg /a 2 Al . a 2 O ) in steel exhibits a linear correlation with a slope close to 1. Consequently, the prediction of compositions of slag, steel and inclusions can be achieved based on the relationships in the present study. The calculated stability diagram of inclusions in Mg-Al-O-0.035% C-0.04 % Mn-3-Si-Fe melts at 1873K can be used to predict the formation of Al2 O3 , MgO. Al2 O3 and MgO inclusions based on the steel composition.

Comparison of 2D and 3D morphology of non-metallic inclusions in steel using different methods

Abstract: In the current work, the two-dimensional (2D) morphology of MnS inclusions with different shapes, including spherical, rod-like, flower-like, polyhedron and aggregated cluster, as well as spherical oxides were observed. The three-dimensional (3D) morphology of inclusions extracted from steel using acid dissolution, but the MnS was reacted away with acid during the extraction of the inclusion from the steel matrix. It is shown that the 2D method is hardly accurate to detect the morphology and size of inclusions comparison 3D morphology of inclusions. Therefore, a method of separation, extraction and analysis of non-metallic inclusions in steel was presented. Electrolytic extraction using non-aqueous electrolytes were performed to dissolve the conductive steel matrix and remain nondestructive inclusions. Scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS) was used to obtain the integrated 3D morphology, size and chemical composition of inclusions. The morphology of MnS inclusions was reclassified on the basis of the integrated 3D morphology under the following categories: elliptical, spherical, rod-like, plate-like, irregular or polyhedron, dendrite and patch MnS precipitated on the surface of oxides. Non-aqueous electrolysis extraction with the advantage of nondestructive to inclusions, low cost, and high efficiency, and the subsequent analysis of morphology, size and chemical composition is more reliable and accurate than the traditional method. What is more, it allows a better understanding of the origin and formation of inclusions, which is helpful for the control and removed of inclusions in steels.

Modeling transient evolution of inclusion in Si-Mn-killed steels during the ladle mixing process

Abstract: A reaction model was developed to investigate the transient evolution of inclusions during the ladle mixing process The evolutions of Mn and Si in Si-Mn-killed steels from previous works were accurately predicted using the current reaction model During the mixing process of the added SiMn ferroalloy, it can be simulated that Si and Mn concentrations first fluctuated and then gradually trended to be stable in various reaction zones The fluctuation of steel chemistry led to the transient variation of the inclusion composition during the mixing process The reaction model was widely used to predict the transient evolution of inclusions during the ladle mixing process of Si-Mn-killed steels with varying sequences of ferroalloy addition and different concentrations of Si, Mn, and Al

Motion of Single Bubble and Interactions between Two Bubbles in Liquid Steel

Abstract: Gas injection is widely applied in secondary metallurgical processes and continuous casting.1–3) Bubbles injected into the liquid steel to homogenize the chemical composition and temperature distribution as well as remove non–metallic inclusions from liquid steel. The efficiency of inclusions removal by bubble flotation depends considerably on various factors such as bubble size, shape, rising trajectory, bubbles interaction or breakup and so on. Hence, the prediction of bubble formation, rising motion and the interactions between bubbles provides the theoretic guidance for the efficiency of inclusion removal. Oryall et al.4) reported the jet behavior into a mercury converter-shaped vessel and measured the distribution of gas volume fraction and bubble frequency. Andreini et al.5) achieved the bubble formation frequencies and bubble rising velocities in tin, lead, or copper melts through utilizing noises generated by bubbles. Using X-ray cinematography, Motion of Single Bubble and Interactions between Two Bubbles in Liquid Steel

Effect of different removal conditions on the growth and removal of inclusions in the molten steel in a two-strand tundish

Abstract: In the current study, mathematical models were developed to predict the concentration, size distribution of inclusions in a two-strand tundish. The effect of the floating velocity, the velocity of the molten steel, and the turbulent fluctuation velocity on the removal of inclusions at the interface between the molten steel and slag was also studied. Meanwhile, the composition, amount, and size of inclusions in the steel were detected and the content of the total oxygen was obtained from the industrial trial. The calculated results were well validated by the measured ones. The strong stirring energy was greatly beneficial to the collision and growth of inclusions in the molten steel. When only considering the floating velocity, the removal fraction of inclusions was 23.4%. Furthermore, the contribution of the velocity of the molten steel and the turbulent fluctuation velocity to the removal fraction of inclusions at the top surface in the tundish was 23.2% and 14.5%, respectively. Finally, the mathematical model only considering the floating velocity can accurately predict the growth and removal of inclusions in the tundish.

Bubble Motion and Gas-Liquid Mixing in Metallurgical Reactor with a Top Submerged Lance

Abstract: Abstract The multiphase fluid mixing and bubble motion in a scale-down Isa smelting furnace with a top submerged lance were investigated through cold water model experiments. The mixing time in the liquid bath was determined by a novel criterion, and the bubble formation frequency and gas-liquid contact area were also measured by high-speed photographing. The empirical equations of mixing time (Tm) relating to mixing energy (ε) were Tm,s=913ε−0.5 ${T_{{\\rm{m}},s}} = 913{\\varepsilon ^{ - 0.5}}$ for solid tracer and Tm,l=624ε−0.48 ${T_{{\\rm{m}},l}} = 624{\\varepsilon ^{ - 0.48}}$ for liquid tracer, respectively. The mixing time trended down with the increasing of bubble formation frequency, affected slightly by the bubbles with small size and decreased obviously when increasing the gas-liquid contact area. The optimal conditions of gas flow rate, bath height, lance submerged height and lance diameter to guarantee high-efficiency mixing and low-level surface fluctuation/splashing were obtained.

Quantifying Cathode Water Transport via Anode Relative Humidity Measurements in a Polymer Electrolyte Membrane Fuel Cell

Abstract: A relative humidity (RH) measurement based on pressure drop analysis is presented as a diagnostic tool to experimentally quantify the amount of excess water on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC). Ex-situ pressure drop calibration curves collected at fixed RH values, used with a set of well-defined equations for the anode pressure drop, allows for an estimate of in-situ relative humidity values. During the in-situ test, a dry anode inlet stream at increasing flow rates is used to create an evaporative gradient to drive water from the cathode to the anode. This combination of techniques thus quantitatively determines the changing net cell water flux. Knowing the cathodic water production rate, the net water flux to the anode can explain the influence of liquid and vapor transport as a function of GDL selection. Experimentally obtained quantified values for the water removal rate for a variety of cathode gas diffusion layer (GDL) setups are presented, which were chosen to experimentally vary a range of water management abilities, from high to low performance. The results show that more water is transported to the anode when a GDL with poor water management capabilities is used, due to the higher levels of initial saturation occurring on the cathode. At sufficiently high concentration gradients, the anode removes more water than is produced by the reaction, allowing for the quantification of excess water saturating the cathode. The protocol is broadly accessible and applicable as a quantitative diagnostic tool of water management in PEMFCs.

Experimental Study on Scale-Up of Solid–Liquid Stirred Tank with an Intermig Impeller

Abstract: The scale-up of a solid–liquid stirred tank with an Intermig impeller was characterized via experiments. Solid concentration, impeller just-off-bottom speed and power consumption were measured in stirred tanks of different scales. The scale-up criteria for achieving the same effect of solid suspension in small-scale and large-scale vessels were evaluated. The solids distribution improves if the operating conditions are held constant as the tank is scaled-up. The results of impeller just-off-bottom speed gave X = 0.868 in the scale-up relationship ND X = constant. Based on this criterion, the stirring power per unit volume obviously decreased at N = N js, and the power number (N P) was approximately equal to 0.3 when the solids are uniformly distributed in the vessels.

Influence of Hot Charge on Blast Furance Performance for Iron Making

Abstract: A two-dimensional gas-solid model has been developed for examination of the blast furnace (BF) performance under hot charge operation. Simulation conditions were obtained from the actual operation conditions of the BF with inner volume of 2500 m3 and molten iron output of 6250 ton hot metal (THM) per day. The simulation results indicate that hot charge operation could change the thermal state of the BF in the region above cohesive zone (CZ) and has little influence on the thermal state of CZ zone and the region below CZ; it could intensify both the indirect reduction of the ore and the gasification of the coke in the region above the CZ. The optimum temperature for hot charge operation is 750 K, under which, a coke rate reduction of 25 kg/THM could be obtained.

Effect of Various Aluminum Content on the Formation of Inclusion

Abstract: More attention is paid to high aluminum steel which attains hardness, abrasive resistance and corrosion resistance. However, the development of subsurface nitrides and oxides in a nitrogen-rich atmosphere during smelting process and solidification makes a significant effect on the properties of high aluminum steel. It is important to understand how theses nitrides and oxides form and their consequences for the quality of steel products. In the current study, the effect of various aluminum content on the formation of inclusions in a nitrogen-rich atmosphere was investigated using vacuum induction furnace and high-temperature contact angle device. The growth and evolution of the precipitation in a subsurface oxygen-depleted region were evaluated by means of optical microscope and scanning electron microscope. The depth of the internal oxide and nitride zones were different in the samples containing various aluminum content. Increasing aluminum content to a given value has an influence on modifying the surface oxide scale composition and increasing its continuity, which gradually decreased the internal precipitation zones with increasing aluminum content. For the sample with higher aluminum content, a thick continuous oxide layer formed and completely prevented nitrogen penetration into the bulk of the samples.