Abstract: Pumping agents can improve the fluidity of paste slurries. However,the effecting mechanism of pumping agents on the coagulability of paste slurries is not made clear. To solve this problem,a pumping agent was added for compression strength tests and environmental scanning electron microscopy(ESEM) observations. Compression strength test results indicate that the pumping agent can improve the coagulability of paste slurries,and the more the pumping agent amount,the better the coagulability of paste slurries.ESEM observation results show that large size floccules and pores decrease after adding the pumping agent,and the paste's microstructure is changed. The fractal dimensions of the paste's microstructure images were calculated based on the image processing technology and fractal theory,and the structure was quantitatively analyzed. The mechanism of improving the coagulability of paste slurries was discussed. It is found that the coagulability of paste slurries is affected by changing the size and number of flocculants,the porosity and the hydration reaction.
Abstract: In order to investigate the characteristics of deformation and water loss of hardened paste of water-rich filling material under aging effect,the creep property of water-rich filling materials with a certain mass ratio of water to solid was studied in different stress conditions. The deformation characteristics,water loss and its relationship with external load on the filling body in the process of creep were analyzed by using scanning electron microscopy and differential thermal analysis-thermal gravimetry(DTA-TG). It is found that the instability and failure critical stress of water-rich filling materials with a 2.0 water-to-solid mass ratio is 1.96 MPa,which is 90% of their compressive strength. Creep cannot affect the content of combined water in water-rich filling materials. But unbound water in water-rich filling materials can run off when instability and failure happen. The relative loss value of unbound water content is of linear positive correlation with the external load level. The loss of unbound water can induce more interspaces in waterrich filling materials. These cracks are quickly pressed and the hardening body produces large deformation under the external load,which leads to the local instability of the filling body and thus affects the overall stability of the goaf.
Abstract: The feasibility and mechanisms of using the blast furnace dust as a reductant in direct reduction-low intensity magnetic separation of Indonesian seaside titanomagnetite were investigated. The results show that the blast furnace dust can be used as an alternative to coal as a reductant under the condition of fluorite as an additive. Under the synergistic effect of the blast furnace dust and fluorite,the recovery and grade of iron in the reduction iron powder can be increased and the content of TiO2 can be decreased at the same time during the direct reduction process. Simultaneously,iron in the blast furnace dust can also be recovered. A comparison of three different kinds of blast furnace dusts indicates that the composition of the blast furnace dust can influence the effect of reduction.Under the same dust dosage,the Jinxin blast furnace dust(referred to as JX) show the best reduction performances. The optimum conditions of roasting reduction are the blast furnace dust dosage of 30%,the fluorite dosage of 10% and reduction at 1250℃ for 60 min. The reduction iron powder,with 91.28% Fe grade,0.93% TiO2 content and 89.19% total iron recovery including iron in the run-of-mine and blast furnace dust,is obtained after the roasted product is processed by using a process of two-staged grinding and two-staged magnetic separation.
Abstract: The adsorption mechanism of HCl gas on the sinter surface was simulated and calculated based on the density functional theory,and the adsorption characteristics were experimentally investigated at different reaction temperatures,sinter particle sizes and HCl gas rates. It is found that the maximum adsorption energy of HCl on the α-Fe2O3(001) surface is -175.91 k J·mol-1,which is chemical adsorption. Cl atoms react with Fe atoms of the substrate surface and form Fe-Cl bonding. After adsorption,the bonding length of Fe-O becomes shorter,and the binding energy gets bigger,which makes the structure of Fe2O3 become more closely. The combination of Cl atoms and H atoms becomes weaker because Fe-Cl bonding forms. Temperature has a great influence on the chlorine adsorption content on the sinter surface. The chlorine adsorption content of sinter obviously increases with increasing temperature,gradually decreases with increasing sinter particle size,and quickly increases with increasing HCl gas rate.
Abstract: In order to deeply understand the carbothermic reduction mechanism of boron-bearing iron concentrate,the isothermal reduction of boron-bearing iron concentrate/carbon composite pellets was performed with high purity graphite as a reductant,and was kinetically analyzed by the integration method. The reduction temperature was set as 1000,1050,1100,1150,1200,1250 and 1300℃,and the C/O molar ratio was 1.0. When the reduction degree ranged from 0.1 to 0.8,the reduction temperature had important effect on the apparent activation energy and rate controlling step. If the reduction temperature was not greater than 1100℃,the average apparent activation energy was 202.6 k J·mol-1,and the rate controlling step was carbon gasification. But if the reduction temperature was higher than 1100℃,the average apparent activation energy was 116.7 k J·mol-1,and the reduction rate was the mixed control of carbon gasification and Fe O reduction reaction. When the reduction degree was not less than 0.8(reduction temperature 〉1100℃),the reduction might be controlled by carbon diffusion in metallic iron. Carbon gasification is the main rate controlling step for the reduction of the composite pellet due to the intense chemical inhibition effect of boron in the boron-bearing iron concentrate on carbon gasification.
Abstract: A water/carbon dioxide system was used to study bubble growth dynamics in a molten steel/(N2,H2) supersaturation system. Bubble nucleation and growth mechanism models in the aqueous solution and the molten steel were respectively established.Bubble growth dynamics in the water/carbon dioxide system and the molten steel/(N2,H2) system was studied based on three different kinds of bubble growth mathematical models. Water modeling experimental data were used to validate the mathematical models. The influences of preprocessing pressure,post-processing pressure and the depth of molten steel on bubble growth were analyzed in the molten steel/(N2,H2) system. It is found that when using the technology of inclusion removal by bubble flotation,preprocessing pressure has significant effect on the bubble growth. However,post-processing pressure blocks the bubble growth,and with increasing post-processing pressure,the influence gradually strengthens. The depth of molten steel has retarding effect on the bubble growth,and with the increasing of the depth of molten steel,the influence gradually weakens. Compared with nitrogen bubbles,hydrogen bubbles in molten steel grow faster.
Abstract: The slag entrapment process and the flow field of a specific ladle at critical flow rate are investigated based on particle image velocimetry(PIV) technology,and the change of fluid velocity at the region where slag is entrapped into steel during the whole slag entrapment process is studied by water modelling. The influence of the kinematic viscosity of slag on the critical slag entrapment velocity is analyzed quantitatively. The critical slag entrapment velocity collected from experiments is compared with the calculated one by the traditional theory. It is shown that the critical slag entrapment process can be divided into 8 stages from the occurrence to the end,and the fluid velocity at the point where slag is entrapped into steel increases first,then decreases and increases again during the 8 stages. The kinematic viscosity of slag has great influence on slag entrapment. When the kinematic viscosity of slag increases,the critical flow rate and critical slag entrapment velocity enlarge,and the linear correlation relationship between the critical slag entrapment velocity and the kinematic viscosity of slag becomes more obvious. A relational expression between the critical slag entrapment velocity and the kinematic viscosity of slag is derived from experimental data by liner fitting. At the end of this article,a correctional expression of the critical slag entrapment velocity is put forward based on the experimental data and the theoretically calculated values.
Abstract: The floating and collision behavior of inclusions in the floating process was numerically simulated by using the Lattice Boltzmann method. It is found that the floating velocity simulation results of different size inclusion particles are almost the same as the theoretical value,which demonstrates that the motion behavior of solid inclusion particles can be investigated concisely and effectively by using the numerical algorithm adopted in this paper. When an inclusion particle with a diameter of 80 μm locates below an inclusion particle with a diameter of 40 μm and floats up at the same time,the inclusion particle with a diameter of 80 μm can catch up with the inclusion particle with a diameter of 80 μm,collide with each other and grow up into a big inclusion cluster. When the inclusion particles with diameters of 80 μm and 40 μm floats up separately,the floating velocity of the inclusion cluster is bigger than them. For the inclusion particle with a diameter of 40 μm,the floating velocity after collision with the bigger size inclusion particle increases by 300% compared with that of floating separately. In the steelmaking process,it is necessary to take measures to enhance collision and coagulation in the floating process,which will improve the floating velocity of inclusions especially for small size inclusions and have a great importance on the cleanliness of steel.
Abstract: Low-carbon steel and medium-carbon steel were taken as the objects of research in this paper. In order to study the effects of process parameters on the initial solidification behavior of continuously cast billets,a CA-FE coupled method was used to simulate the initial solidification behavior of billets in the mold during practical continuous casting. The dependences of initial billet shell thickness at the exit of the mold upon superheat and casting speed were investigated and the microstructure morphologies of the two steels were compared at the same time. It is shown that the shell thickness decreases with increasing superheat and casting speed,while the casting speed has a more significant impact. The shell thickness of different steels decreases at different gradients under the same condition. The lower the superheat temperature,the more compact the columnar crystals will be,which is useful to improve the quality of continuous casting billets,while the casting speed has less influence. With the constraint of shell thickness at the exit of the mold,the superheat is taken to be 15℃,and the casting speeds of low-carbon steel and medium-carbon steel could not surpass 2.2m·min-1 and 2.5 m·min-1,respectively. According to this result,we can design the optimum drawing speeds for different steel grades to improve the efficiency of continuous casting. Moreover,the model results show that the columnar crystals of low-carbon steel is more developed than those of medium-carbon steel.
Abstract: The mineralogical composition,microstructure and TiO2 distribution rule of low titanium-containing blast furnace slags after water cooling and air cooling were investigated by X-ray diffraction,polarization microscopy and scanning electron microscopy.The results show that main mineralogical compositions in these slags are all vitreous,gehlenite,perovskite and merwinite,but the contents of mineral components in the two kinds of slags have much larger differences. The average contents of gehlenite and perovskite in the air-cooled slag are 62.5% and 12.5%,which are 2.27 and 1.92 times as large as those in the water-cooled slag,respectively.The content of vitreous in the air-cooled slag is less than a third of that in the water-cooled slag. The mineralogical microstructure of the water-cooled slag is pretty different from that of the air-cooled slag. Gehlenite in the water-cooled slag is dentate,but in the watercooled slag it is feathery and needle-shaped,and its grain size is smaller. Perovskite forms as star points and dendritic in the watercooled slag and the air-cooled slag,respectively. Merwinite is spindle-shaped both in the water-cooled slag and the air-cooled slag. It could be concluded that TiO2 in the water-cooled slag mainly distributes in perovskite,vitreous glassy and gehlenite,but TiO2 in theair-cooled slag mainly distributes in perovskite and gehlenite,and the distribution ratio of TiO2 in the air-cooled slag is 8.41% higher than that in the water-cooled slag. Air cooling is more beneficial to increase TiO2 content in perovskite.
Abstract: A comprehensive effect of Al2O3 content and binary basicity B on the enrichment behavior of phosphates in rapidly quenched CaO-SiO2-FeO-P2O5-Fe2O3-Al2O3 steelmaking slags was investigated through thermodynamic calculations and experimental determination. It is found that the determined mass percentage of P2O5 in the phosphate-enriched phase nC2S-C3P depends on the forming of free C2S in the steelmaking slags and increasing the SiO2 content can result in an obviously decreasing tendency of formed C2S. Adding Al2O3 as a dilute agent for decreasing the viscosity and melting point of the steelmaking slags can be easily bonded with free C2S to form a gehlenite(C2AS) solid solution. Thus,adding Al2O3 in the steelmaking slags can decrease the amount of free C2S in the steelmaking slags,which affects phosphate enrichment. In order to obtain a greater phosphate enrichment in the steelmaking slags,a coupling relationship between the binary basicity B and the mass percentage of Al2O3 in the initial steelmaking slags should be kept as(% Al2O3)=-27.70 + 21.62 B under the conditions of(Al2O3) 〈20.0% and the binary basicity B〉1.3.
Abstract: High density green compacts of annealed iron powder with a density of 7.70 g·cm-3 were prepared by high velocity compaction(HVC) combined with powder annealing. Iron soft magnetic materials with high performance and high density were obtained after sintering. The HVC behavior of the annealed powder was investigated,and the effects of sintering temperature and sintering time on the magnetic properties and grain size of the soft magnetic materials were studied. The results show that the density of the green compacts increases with increasing compaction velocity,and the maximum density is 7.70 g·cm-3(relative density of 98.10%). Specimens sintered at 1450℃ for 4 h have the optimum magnetic properties,with the sinter density of 7.85 g·cm-3(relative density 99.96%),the maximum permeability of 13.60 m H·m-1,the saturation induction of 1.87 T,and the coercive force of 56.50 A·m-1.
Abstract: Low density steels attract the attention of structural steel researchers due to their good comprehensive mechanical properties and low density characteristic. In this study,the thermodynamic equilibrium states of Fe-Mn-Al-C alloys with medium manganese and aluminum contents at different temperatures were calculated by Thermo-Calc software with TCFE 7 date base. The statistical data of phase ratio in the dual phase region were summarized and then processed by translation and correction methods to plot the Schaeffler diagram,which is applicable to design the composition and phase ratio of Fe-Mn-Al-C alloys with medium manganese and aluminum contents. Possible phases in these steels with different compositions were discussed according to the calculation of martensitic transformation temperatures. The accuracy and applicability of the Schaeffler diagram were verified by the experimental phase ratio and phase type of several typical Fe-Mn-Al-C alloys. The information of phase ratio and phase type in Fe-Mn-Al-C alloys can be obtained easily from the plotted Schaeffler phase diagram.
Abstract: In view of slow phase transformation rate in high-carbon,silicon-rich and low-alloy bainite steel(nanostructured bainite steel),the effects of alloy elements on the overall kinetics of low temperature bainite phase transformation were analyzed by using the thermodynamic theory of bainite phase transformation,and a composition of 0.83C-2.44Si-0.43Mn-0.73Al(mass fraction,%) was designed to develop a novel nanostructured bainitic steel. The overall phase transformation kinetics of this new designed nanobainitic steel at different temperatures were investigated by dilatometry. The effects of heat treatment parameters on the microstructure and mechanical properties were analyzed by scanning electron microscopy,X-ray diffraction and electron backscatter diffraction. It is found that the nanobainitic steel,which exhibited excellent mechanical properties with an ultimate tensile strength of 1401 MPa,a total elongation rate of 42.21% and a strength and ductility product of 59136 MPa·%,can be obtained by austempering at 350℃. In addition,transformation-induced plasticity(TRIP) effect appears during tensile testing at room temperature. When austempered at 230℃,the nanobainitic steel has a finer microstructure with the thickness of bainite ferrite laths less than 100 nm,and its tensile strength can reach up to 2169 MPa.
Abstract: The relationship of creep rupture life and microstructural evolution under different high temperature oxidation conditions for Cr35Ni45 Nb alloy was studied by using SEM and quantitative electronic probe analysis. It was found that the microstructure gradually varied both in the edge and inside of the tube with increasing treatment time. The oxidation sequence showed that discontinuous oxidation films firstly developed at different places from the surface of the tube specimen,then they became continuous,and the carbide depleted zone developed as well during the high temperature oxidation process. As time went by,the oxidation films ruptured with temperature fluctuation,the characteristic of a higher oxygen partial pressure during the aging process resulted in the untimely recovery of the broken oxide films,and internal oxidation occurred at grain boundaries. Furthermore,creep rupture tests under different pretreating conditions of oxidation in air and argon gas atmosphere showed that the newly-built thin and continuous oxidation films improved the creep properties,while with increasing aging time,carbide depleted zone formation,oxidation film rupture and internal oxidation could decrease the creep properties of this alloy.
Abstract: This article is focused on the corrosion behavior of Ni-base alloy C276 exposed to 550-650℃/25 MPa supercritical water. The oxidation kinetics,oxide morphology,element distribution and structure of the oxide film were analyzed by using weight gain measurement,scanning electron microscopy,energy spectrometry,and X-ray diffraction analysis. It is shown that C276 alloy exhibits general corrosion in the supercritical water,and the weight gain is subject to parabolic growth kinetics. The weight gain of C276 alloy at 600℃ is approximately three times that of C276 alloy at 550℃,while the weight gain shows a decline when the temperature increases to 650℃. The oxide film is not distinguishable and is mainly consisted of(Ni,Fe) Cr2O4. A large number of Ni O oxide particles are observed on the oxide film. The protective ability of the oxide film is dependent upon Cr content,and the higher the Cr content,the better the protective ability is.
Abstract: Ag@Pt core-shell nanoparticles(Ag@Pt NPs) composed of an Ag nanoparticle core and a Pt shell were prepared by the seed-mediated growth method and self-assembly technology. The electrocatalytic properties of Ag@Pt NPs to methanol before and after invalidation were measured and compared during the electrocatalytic process. The invalidation mechanism was studied by transmission electron microscopy,high-resolution electron microscopy,and X-ray photoelectron spectroscopy. The results show that the cavitation phenomena of Ag@Pt NPs occur during the cyclic voltammetry process with a threshold voltage of 0.5 V and become more obvious with the increase of time. Ag@Pt particles turn into Pt hollow shells coated by Ag after cavitation,which is the reason that the catalytic property of Ag@Pt NPs to methanol distinctly decreases during the electrocatalytic process.
Abstract: A reaction bonded SiC ceramic used in a suspension preheater inner cylinder was comparatively analyzed before and after use by X-ray diffractometer,scanning tunneling microscopy and energy dispersive spectroscopy,and its damage mechanism was studied. The results show that residual silicon metal in the Si C ceramic and silicon carbide in the surface are firstly oxidized into SiO2 under the high temperature atmosphere and the liquid viscosity of SiO2 decreases,resulting in the formation of an oxidation layer because of the existence of an alkaline steam of K2O(g),Na2O(g),KCl(g) and NaCl(g) and chlorides. Afterwards the oxidation layer is scoured and frayed by high-speed air flow,leading to a new interface. With the cycle repeating,the outside of the Si C ceramic becomes to be thinner little by little and appears to rupture till damage. As a result,the promotion of densification and the reduction of residual silicon metal are effective ways to improve the use ability of the reaction bonded Si C ceramic in a suspension preheater.
Abstract: In order to analyze the influence of asymmetric factors on the shape(such as wedge and single edge wave) of rolled pieces in 2250 mm hot tandem rolling,an asymmetric shape calculation model involving rolls and rolled pieces was established,which is integrated by a roll deformation calculation model based on the influence function method,a transverse distribution model of tension stress and a simplified transversal distribution model of rolling force. The results show that the wedge of incoming pieces has a greater effect on the wedge of rolled pieces than on the flatness. The stiffness asymmetry of up-stream stands and down-stream stands mainly affect the wedge and flatness of rolled pieces,respectively. A temperature difference of rolled pieces within 40℃ has a little effect on the flatness and has negligible effect on the wedge. The deviation of rolled pieces affects both the wedge and flatness dramatically. The admissible ranges of asymmetric factors were determined with a target of achieving a good strip shape.
Abstract: In a pulse generator circuit,the output and stability of the system are affected by stray parameters. In order to improve the quality of the output waveform,restrain the oscillation of the output and ensure components long-term reliable operation,it is necessary to analyze and compensate stray parameters in the pulse generator circuit. Through analyzing the actual waveform of the output,a stray parameter module for the pulse generator circuit and a new method to analyze and compensate stray parameters in the pulse generator circuit are introduced in this paper. Simulation results reflect the actual waveform accurately and no overshoot of the output is achieved after joining the compensation circuit.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
