Many studies have been conducted over the last 50 years or more to clarify the dissolution behavior of lime in slag. In this paper, the previous studies on enhancing the dissolution of lime are reviewed. The research subject is divided into industrial tests to verify the new technology, and laboratory tests to clarify the dissolution mechanism. To enhance the dissolution, the only feasible measure is to increase the interfacial area. For this purpose, various methods to increase surface area have been tried, including the decrease in lime size; powder blowing or injection; controlling the calcination conditions; and application of a pre-melted flux. Among them, powder blowing and the reuse of the refining slag are the most effective measures. For fundamental experiments, the immersion of a rotation rod in the slag, immersion of lime cube with a circular rotating rod, and the addition of lime to the slag under gas bubbling have been conducted. The mechanism and rate-controlling step of lime dissolution have become clear: the destruction of the dicalcium silicate layer is important to promote the dissolution of lime. Stirring-promoted fluid flow and the formation of CO2 inside of the quick lime are proposed as mechanisms of the destruction.
In steelmaking processes, quicklime is generally used to produce CaO-based slags, and its dissolution rate is important for steel refining. The dissolution rate of quicklime is conventionally measured by the rotating cylinder method using dense and hard lime samples which gives rates that are slower than estimated rates from the actual operation. The authors established a new method to measure the dissolution rate of quicklime by measuring the variation of slag composition and reported that the quicklime used in the actual operation had a much higher dissolving rate than that of completely calcined quicklime. The significant increase of the dissolution rate was caused by gas formation from the quicklime due to the thermal decomposition of residual limestone existing in quicklime. In this study, the dissolution rate of quicklime with the accompanying gas formation is quantitatively investigated by using quicklimes with different CO2 contents produced by a rotary kiln process through the direct observation of the dissolution behavior of quicklime particles and the change of the CaO content in the slag. The results revealed that quicklime emits the gas in two steps, and the second occurrence of gas formation effectively enhances the quicklime dissolution. The weight of the CaCO3 core differed among particles from the same grade of quicklime, and the corresponding dissolution rates were different as well. The dissolution rates of quicklime during the second foaming, however, were 5–10 times higher than without foaming and were similar regardless of the CO2 content in a quicklime particle.
Dissolution behavior of a 4%-CO2 quicklime particle in molten slag. (Online version in color.)
The dissolution rate of lime in the molten slag is important for the efficient of steelmaking reactions. The dissolution rates of quicklime were conventionally measured by a rotating cylinder method, and they were quite lower compared with the estimated rates from actual steelmaking operations. Previously, the authors reported that the quicklime used in the actual operation had a much faster dissolution rate than completely calcined lime. During the dissolution of quicklime used in the actual operation, quicklime emits CO2 gas twice, and the second gas formation effectively enhances the dissolution rate. Though the dissolution rates of quicklime with a CO2 content of 0, 2, 4, and 9 mass% had been analyzed, the dissolution rates were scattered. The reason for this scattering of the data was that the CO2 content of individual quicklime samples varied significantly within the same grade of quicklime, because the samples used in the previous study were produced by a rotary kiln process. Consequently, the dissolution rates were inconclusive, and the effect of the CO2 content in quicklime on the dissolution rate of quicklime could not be fully clarified. In this study, the CO2 content was controlled through the laboratory-based preparation of spherical quicklime samples and thus, the effect of the CO2 content on the dissolution rate of quicklime in the molten slag could be precisely analyzed. Eventually, this approach allowed to propose the dissolution rate of quicklime with gas formation due to the thermal decomposition of the CaCO3 core existing in the center of quicklime samples.
Relation between the CO2 content in spherical quicklime samples and mass transfer coefficients of CaO.
In the metal manufacturing process, it is considered that ultrasound technology can be applied in the promotion of solid dissolution such as lime dissolution to molten slag and solid metal addition to molten alloy. Generally, in the high-temperature reaction process, mass transfer has a large effect on the rate of reaction in isothermal conditions. In order to facilitate mass transfer, it is necessary to strengthen the agitation of the bath. However, it is impossible to avoid the erosion of the stirrer and container. Enhancement of stirring promotes mass transfer. This corresponds to thinning of the boundary layer thickness in the boundary layer theory. To further accelerate mass transfer, a new method of thinning the boundary layer of the solid surface without using mechanical stirring is needed.
In this study, the effect of ultrasound on the dissolution phenomena of solid in liquid was investigated by using a sucrose-water system. Applying ultrasound to a liquid induces ultrasonic radiation pressure, acoustic streaming and various nonlinear phenomena of cavitation. Especially, when the cavitation bubble collapses, a micro jet is generated and a strong flow occurs locally. It was thought that this micro jet collided with the solid sample and the surface became uneven. If the cavitation phenomena occur at the solid surface, it is possible to disturb the boundary layer and increase the mass transfer rate by thinning the boundary layer. It was found that the cavitation in the vicinity of the solid surface at the low ultrasound frequency caused a disturbance of the boundary layer of the solid surface and promoted dissolution rate of the solid.
Change in shape due to dissolution of the sucrose samples by using non-degassed distilled water. (a) without ultrasound (b) with ultrasound (28 kHz 1.07 W).
In the steel refining process, dissolution of quick-lime would accompany the formation of 2CaO·SiO2 on its surface. For proper understanding of heat supply to the lime phase from the molten slag through this 2CaO·SiO2 layer, thermal conductivity of this phase should be well known. The present study investigates the thermal conductivity of 2CaO·SiO2 bearing solid solution by hot-wire and hot-strip methods. Thermal conductivity of the solid solution was obtained to have the value from 0.28 to 1.18 W/m·K at temperatures from 298 to 1623 K and found much smaller than that of CaO. Its temperature dependence was positive as is often characterized for complex oxide system. Addition of FeO as solid solution component to 2CaO·SiO2 phase raised thermal conductivity and effect of change in concentration of P2O5 was also elucidated to have a minimum peak of thermal conductivity appearing with this change. On the basis of the thermal conductivity of 2CaO·SiO2 obtained and that of CaO, thermal behavior of practical process has been evaluated. The complete thermal decomposition time of CaCO3 contained by 4 mass% in the sphere of quick lime having the radius of 1 cm is estimated to be about 30 s. The appropriate control of residual ratio of CaCO3 and size of quick-lime would be necessary for the promotion of lime dissolution into the steelmaking slag.
Rate of 2CaO·SiO2 dissolution into molten CaO–FeO–SiO2 slag saturated with solid iron has been investigated considering the promotion of lime fluxing. A new diffusion couple method has been applied where 2CaO·SiO2 pellet was immersed into molten CaO–FeO–SiO2 slag in an iron crucible, slag convection being suppressed. Concentration profile of components was observed and the diffusion behavior was well understood. Considering the pseudo-binary inter-diffusion between components such as CaO and SiO2 and slag phase matrix, diffusivity of CaO and SiO2 in the present slag has been determined to have the values from 4.9×10−7 to 4.7×10−5 cm2/s at temperatures from 1573 to 1673 K. Assuming the dissolution mechanism as continuous formation and dissolution of 2CaO·SiO2 on the surface of CaO, complete dissolution time into the flowing slag has been estimated on the basis of the Ranz-Marshall empirical equation, and temperature is found to be controlling factor of the dissolution rate in the normal operation condition.
Dissolution of lime into molten slag is an important phenomenon in hot-metal dephosphorization treatment and should be suitably promoted in order to obtain an effective refining reaction and to recycle slag as some environmental resources. A lot of research has been conducted on the phenomenon, but the influence of P2O5 on the dissolution behavior of lime has never been studied despite the presence of P2O5 in the slag obtained during actual operation.
In this study, the dissolution behavior of lime in CaO–SiO2–FeO or CaO-SiO2-FeO-5.2 mass% P2O5 molten slag was investigated via a high-temperature laser microscope, an optical microscope, and a scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) in order to clarify the influence of P2O5 on the dissolution behavior of lime. We conclude that the addition of P2O5 to the slag accelerates the dissolution of lime in the molten slag mainly by increasing the CaO equilibrium content in the liquid slag saturated with 2CaO·SiO2.
P2O5 is a product of the dephosphorization reaction, and basically, its content is preferred to maintain low content for dephosphorization based on equilibrium theory. However, the experimental results obtained in this study clarify that the presence of P2O5 in molten slag is effective for the promotion of the dephosphorization reaction when the reaction is limited by the dissolution of lime.
Towards better understanding of the phosphorus removal from hot metal with low-basicity slags, electrochemical technique incorporating MgO-stabilized zirconia was conducted to measure simultaneously the activities of FeO and P2O5 within heterogeneous CaO–SiO2–P2O5–FeO slags. The FeO activity was fairly insensitive to the variation of Ca3P2O8 content in solid solutions between Ca2SiO4–Ca3P2O8, while the P2O5 activity increased with an increase in Ca3P2O8 content. By using the present values for activities, phosphorus distribution ratios were estimated between molten slag and carbon-saturated iron. The relationship between phosphorus distribution ratio and FeO content in molten slag was consistent with the phase diagram of the pseudo-ternary CaO–(SiO2+P2O5)–FeO system.
In steelmaking processes, there are incentives to reduce slag volume and CaO consumption. The key to meet these requirements is the better understanding of CaO dissolution mechanism into molten slag, which relies on the knowledge of the thermochemical properties of slags and fluxes used for dephosphorization. In this study, the liquidus compositions coexisted with solid CaO and Ca2SiO4–Ca3P2O8 solid solution simultaneously were determined in the quaternary system CaO–SiO2–P2O5–FexO at 1573 K. Measurements were also conducted on the FexO activities at temperatures between 1542 K and 1604 K by virtue of an electrochemical technique. By using the present experimental results, phosphorus distribution ratios were estimated.
The molten CaO-based slags used in modern hot metal pretreatment and steelmaking convertors often contain a dispersed gas phase. This is called foaming slag and is created by refining reactions that utilize metallurgical lime. The rheological behavior of foaming slag, which significantly affects the fluid flow of processes associated with the dissolution of lime, has been found to be controlled by the dispersed fraction of the gas phase. In the present study, a simulated slag foam was produced by dispersing inert gas bubbles in various liquids. The effect of varying the volume fraction of the dispersed gas phase on the impedance was then systematically investigated with cylindrical configurations of electrodes. The following equivalent circuit analyses on the semicircular Nyquist plots indicated that electrodes with foaming slag consisted of a series circuit of the solution resistance, a parallel junction of the double layer capacitance, and the resistance of the electric charge transfer. The Nyquist plots displayed semicircular shapes, and their diameters increased with the gas phase fraction despite the various viscosities of the liquid phases, which corresponded to the increase in the charge transfer resistance. The charge transfer resistances were calibrated on the basis of the increase in the electrode surface area and revealed a good linear relationship against the two-thirds power of the gas phase fraction in an ultrapure water and glycerol solution. This suggests a possible approach to quantitatively evaluating the gas phase fraction of a foaming slag by measuring the impedance.
Changes in the Nyquist diagrams of the glycerol aqueous solution with the gas phase fraction (vol%) characterized by using the steel apparatus: (a) 1410, (b) 520, (c) 220, and (d) 60 mPa∙s.
We conducted a three-dimensional fluid dynamics analysis of the solid-gas mixed-phase flow from a top blowing lance by considering the application of powder top blowing to hot metal dephosphorization in the steelmaking process. When a mixed-phase flow of gas and powder passed through a single-nozzle lance, the powder particles passed unevenly near the central axis of the nozzle and the gas flowed around the powder particles in a circular pattern. Thus, the maximum gas velocity shifted in the radial direction from the central axis of the nozzle. After the outlet of the single-nozzle lance, the gas velocity on the central axis increased and the maximum gas velocity shifted to the central axis. The velocity of the powder immediately after being discharged from the nozzle outlet was lower than the gas velocity but then gradually increased, while the gas velocity decreased. The position where the velocity of the powder, which has a greater inertial force, started to decrease was farther from the nozzle outlet than that for the gas velocity. When a multiple-nozzle lance with an angle of inclination is used, the powder particles may impinge on the nozzle inner wall. Thus, it is necessary to consider the wear of the nozzle inner wall and the dispersion region of the powder particles.
Phosphorus in steel is detrimental element for mechanical properties and dephosphorization treatment of hot iron is necessary to produce high-grade steel products. However, it is difficult to perform dephosphorization treatment to attain lower phosphorus content using CaO–SiO2–FeO flux without fluorite of which use is limited owing to stringent environmental regulations. In the present work, addition of ladle slag and lime powder top blowing method were applied in 2 t test converter to obtain the higher slag basicity. Obtained results are as follows:
(1) By simultaneously adopting lime powder top blowing and ladle slag addition, phosphorus content of hot iron decreased to 0.01 mass% under the condition of (CaO)/(SiO2)=2.
(2) By comparison of the results of the heats with and without ladle slag, the ratio of dephosphorization at hot spot by lime top blowing was estimated to be 35%.
(3) (FeO) content of CaO–FeO melt at hot spot was estimated 37% on a basis of oxygen flow rate except to be consumed for decarburization and lime feeding rate. CaO–FeO phase diagram shows that the melt is liquid at hot spot but solid in hot iron. It is suggested that low ratio of dephosphorization at hot spot is attributed to this result.
Phosphorus in steel is detrimental element for mechanical properties and dephosphorization treatment of hot iron is necessary to produce high-grade steel products. However, it is difficult to perform dephosphorization treatment to attain lower phosphorus content using CaO–SiO2–FeO flux without fluorite of which use is limited owing to stringent environmental regulations. It is because increasing slag basicity is difficult without slagging promotion agent. In the present work, addition of ladle slag and lime powder top blowing method were applied in 2 t test converter to obtain the higher slag basicity. Obtained results are as follows: (1) phosphorus content of hot iron decreased to 0.001 mass% under the condition of (CaO)/(SiO2)=3. (2) Rate constant, K, of dephosphorization reaction below [P]=0.02 mass% increased with increasing bottom gas flow rate, but K decreased under the condition of too much bottom gas flow rate. (3) Phosphorus distribution in the final stage of blowing was much higher than predicted values on a basis of thermodynamic relation between top slag and hot iron. (4) It is suggested that the FeO–CaO melt formed at hot spot is capable of decreasing the phosphorus content of hot iron to the ultralow range even at high temperature at hot spot.
In order to study the formation mechanism of Al2TiO5, the solid-state inter-reaction between Al2O3 and TiO2 under Ar atmosphere was investigated in the temperatures range from 1723 to 1873 K by the diffusion couple method. The phase of the inter-compound and the change of the concentration of Ti across the diffusion layers were confirmed by the electron probe microanalysis (EPMA). Based on the concentration profile of Ti element, the thickness of diffusion layers was obtained and the interdiffusion coefficients which were affected by temperature and concentration of Ti were calculated by the Wagner method. The experimental results indicate that the thickness of diffusion layers increases with the diffusion time. The magnitudes of interdiffusion coefficients were in the range of 10−10~10−12 cm2/s and the range of diffusion activity energy ED was from 266.77 kJ/mol to 309.96 kJ/mol.
A new process of selective enrichment and separation of Ti-bearing minerals from the Ti-bearing electric furnace slag (TEFS) with metallic iron as carrier was proposed in this paper. The thermodynamic analysis, melting process and selective reduction were carried out, with an emphasis on the effects of Fe2O3/slag ratio and H2 partial pressure on the processes. The results indicated that the major Ti-bearing mineral in TEFS was pseudobrookite after adding Fe2O3 into the molten slag. In the selective reduction process, the pseudobrookite particle in the modified slag was selectively reduced to the magnetic Ti–Fe enriched mineral particle embedded with metallic iron. Then the magnetic Ti–Fe enriched mineral particle was separated from the nonmagnetic silicate particle in magnetic separation process. Finally, the Ti–Fe enriched mineral with 61.1 wt% TiO2 and 18.31 wt% Total Fe was obtained. Furthermore, the recovery ratio of TiO2 and Total Fe were 85% and 78.6%, respectively. The contents of CaO and SiO2 of the Ti–Fe enriched mineral were 1.4 wt% and 1.9 wt%, which were much less than that of TEFS. The Ti-bearing mineral in TEFS was effectively concentrated as using the new selective enrichment and separation method.
The partitionless solidification and melting in Al–Cu alloy system are investigated by means of molecular dynamics simulations with an embedded atom method (EAM) potential. The solid-liquid interfacial velocity for solid-liquid biphasic systems of Al-rich alloys is examined with respect to temperature and Cu composition. The kinetic coefficient is then derived from the slope of the interfacial velocity with respect to temperature. Our results show that the kinetic coefficient is largely dependent on the Cu composition. It sharply decreases with addition of small amount of Cu. There is almost no partition at the solid-liquid interface within the time scale of the simulation since the solid-liquid interfacial velocity is very fast at temperatures away from the equilibrium temperature. Since it is not straightforward to measure the kinetic coefficient directly from experiments, it is significant in this study to derive the composition dependence of the kinetic coefficient for binary alloys directly from the MD simulation without any phenomenological parameters.
Reaction mechanism study of ferrosilicon synthesis was carried out by using reagent grade material, graphite and waste plastic, Bakelite as reducing agent over a temperature range of 1623 K to 1823 K (1350°C to 1550°C) under inert atmosphere. Reaction rate was determined by using off-gases evolving from reduction reactions. Results showed that reduction mechanism was predominantly controlled by chemical reactions with both the reducing agent. Initially Bakelite bearing pellet showed faster reaction rate compared to graphite due to volatiles generation and less crystalline nature of Bakelite derived carbon. Extent of reduction can be improved by increasing temperature; however Bakelite bearing pellet showed lower dependency on temperature compared to graphite. Activation energy for graphite and Bakelite pellet is 238.07 kJ/mol and 140.29 kJ/mol respectively. This comparative study will create new opportunities to use waste Bakelite as a reductant even at moderately lower temperature to synthesise ferrosilicon alloy.
Growing concerns over fossil CO2 emissions has created a considerable interest in an efficient utilization of renewable biomass in steel industry. Biomass lignin can be used as binder and reducing agent in the blast furnace briquettes. The traditional briquettes consist of various iron oxide-containing residues and cement is used as binder to give the proper mechanical strength. In the present study, cement (C) has been partially and totally substituted with lignin (L) to produce briquettes containing 0–12 wt.% lignin (L/C: 0, 10, 25, 50 and 100%). The mechanical strength has been evaluated based on drop test and tumbler index measurement. The partial replacement of cement with lignin up to 25% (3.0 wt.% lignin in briquettes) was exhibited adequate briquettes strength for blast furnace application. At higher substitution rate (L/C: 50 and 100%), the briquettes strength was sharply decreased. The briquettes with proper mechanical strength (L/C: 0, 10 and 25%) were subjected to self-reduction under inert atmosphere using thermogravimetric technique (TGA). The reduction rate of briquettes increased when increasing the cement substitution with lignin. The reduction took place in two main steps at 500–800°C and 800–940°C. Combined effect of gas diffusion and interfacial reaction were the rate determining step at the first stage while carbon gasification was controlling the second step of reduction. Interrupted reduction tests have been conducted to evaluate the compression strength after reduction. For all briquettes, the increased reduction temperature and lignin content deteriorated the briquette’s mechanical strength due to the effect of dehydration and lignin gasification.
A new estimation method of reduction rate constant in Ishida-Wen’s model, which be considered forming of reaction layer, for oxide aggregate such as sinter ore is proposed. Briquettes of FeO–Al2O3 mixture were prepared as a model of sinter ore and reduction test and analysis were taken place. Al2O3 cannot be reduced under this experimental condition so that it acts as a neutral material which obstruct reduction of FeO. Therefore, the influence of existence of Al2O3 in FeO–Al2O3 briquette on the reduction rate of FeO becomes clear. FeO–Al2O3 briquettes were reduced by H2 at 1173 K, and reduction rate constant kv were defined by Murayama’s method. As a result, kv of FeO–Al2O3 briquettes can be described as kv=ϕFeOkv°, where ϕFeO is volume ratio of FeO in FeO–Al2O3 briquette and kv° is reduction rate constant of pure FeO briquette.
To achieve stable operation of high ratio coke mixed charging, it is important to control coke segregation behavior in mixed layer at blast furnace top. In this study, the effect of charging conditions and burden particle diameter of mixed coke ratio at blast furnace top were analyzed by numerical model based on discrete element method (DEM). Rolling friction coefficient was calibrated by experiment and calculation. Then, segregation behavior of coke particle was numerically investigated by using above calculation parameters. As a result, following findings were obtained.
1) At reverse tilting and broad range charging pattern of rotating chute, granular flow to the furnace center was interrupted and mixed coke particle was distributed uniformly for the direction of furnace radius.
2) Coke segregation in mixed layer was decreased by lowering particle diameter ratio between coke and sinter. Therefore, for the improvement of coke mixed ratio in mixed layer, control of particle diameter of coke and sinter is effective.
During the top side-pouring twin-roll casting (TSTRC) of steel, the thermal field of the roll and the effects of parameters on the cooling power of roll surface were investigated using a two-dimensional model. The temperature and heat flux curves of roll surface appeared double-peak, especially when the rolling speed was high. The theories for the first and second peak of the curves and the relationship between the curve shape and the rolling speed were both discussed. Temperature variation curves in a revolution for different locations under the bottom roll surface were analyzed to reveal the temperature distribution of the roll. Besides, the cooling power of roll surface, measured by average temperature of roll surface, was of great significance in the TSTRC process. The influence of the rolling speed and the cooling water flux on the cooling power had been discussed and they were believed having positive and negative relativity with average temperature of roll surface, respectively. By comparison of average temperature range between two groups of simulation results, the cooling water flux was believed to affect the cooling power more than the rolling speed did. Furthermore, the increase of the cooling water flux was less effective in reducing the average temperature of roll surface especially when the water flux of the bottom roll was more than 6 m3/h. And to balance average surface temperature of two rolls, the suitable cooling water flux would be 4 m3/h and 6 m3/h for the upper and bottom roll respectively.
Thermodynamics of nitrogen and AlN formation in multi-component liquid iron alloys containing Mn, Al, Si and C was investigated by the metal/gas and the metal/nitride/gas equilibration techniques under reduced nitrogen partial pressures in Ar–N2 gas mixture. The N solubility was measured in binary Fe–Mn and Fe–Si alloy melts, and ternary Fe–Mn–Si, Fe–Al–Si, Fe–Mn–C and Fe–Si–C alloy melts over wide composition range at 1773–1873 K. Using Wagner’s formalism, the effect of alloying elements on the N solubility was described as the first- and second-order interaction parameters as well as the second-order cross-product parameters. The validity of the interaction parameters determined in the present study was checked by measuring the N solubility and AlN solubility product for a typical TWIP steel composition melt of Fe-25% Mn-0.3% Si-0.6% C-Al at 1723–1873 K.
For characterizing nanostructures embedded in a metallic matrix, a newly designed intermediate-angle neutron scattering (iANS: “irons”) instrument has been developed that shortens the distance between the sample and detector and is combined with a time-of-flight (TOF) technique. Since the momentum transfer (Q) resolution can be relaxed to provide an optimum Q-range when we focus on characterizing nanoscale heterogeneity, a much higher neutron flux can be utilized for the measurements than those available in a conventional small-angle neutron scattering (SANS) instrument. Consequently, iANS gives sufficiently high quality profiles for quantitative analysis on an absolute unit scale even using a compact accelerator driven neutron source (CANS). The results obtained at the Hokkaido University Neutron Source (HUNS) are compared to those obtained in large facilities. Some results obtained by iANS, are compared to those obtained by small angle X-ray scattering (SAXS) with respect to SAXS/SANS contrast variation.
Aiming at the heating process of copper alloy billets, thermal behavior in a walking beam reheat furnace is investigated. A simplified thermal model is presented to predict billet temperature distribution considering specific heat transfer characteristics. Hot gas blackness is corrected basing on composition of chamber combustion atmosphere during the computation of heat flux boundary condition which improves the model accuracy. A fully implicit scheme is employed to solve the discretization of conduction equation in finite volume method. As an inverse application, a practical method to optimize the chamber gas setting temperature is introduced to make a billet temperature track the technical curve well. Design effects are calculated and simulated with Matlab software. Application results for the hot rolled coil production illustrate the effectiveness of the proposed approach.
Niobium is a critical alloying element in modern steels, which is usually employed at microalloy concentrations in the hundredths of mass percent. Since its incorporation into industrial steels in 1958, Nb has enabled the development of the important class of high strength low alloy steels through its ability to limit prior austenite grain size and recrystallization. One of the mechanisms by which Nb is believed to limit grain growth is through solute drag on austenite grain boundaries. This study presents the first direct calculations of the binding energy for solute Nb at austenite grain boundaries, along with binding energies for additional important alloying elements in steel for which experimental data are available. The binding energies are then compared to select data sets for austenite recrystallization from the literature. The strong correlation between the calculated energies and the experimentally measured effects of the alloying elements confirms that the origin of the significant solute drag effect of Nb is the strong binding energy between solute Nb and austenite grain boundaries.
In this article, effects of carbide on the cooperative growth of pearlite have been explored based on a multi-phase field method. Optical microscopy observations of pearlite structure in eutectoid steels have been taken to validate the accuracy of the developed multi-phase field method. Besides, the simulation results are compared with the experimental results of Ridley and Frye, as well as the phase field simulation results of Steinbach, a good consistency has been found. Comparison results demonstrate that the growth morphology of pearlite is more accurate when the effects of carbide are considered. Influences of the carbide location and size have been considered to further analyze the cooperative growth of pearlite. Results demonstrate that the presence of carbide above the lamellar pearlite changes the growth situation by promoting the growth of cementite lamellar pearlite when it is just above lamellar cementite. Lamellar cementite in the vicinity of carbide easily melts off with the growth of lamellar pearlite, and the lamellar cementite evolves to form islands. Moreover, effects of different carbide diameter on the growth of lamellar pearlite have been explored. The final growth rate and growth morphology are basically the same despite variations in diameter. Relationship, which can be used to control the morphology and performance of lamellar pearlite, between the size of the carbide and the horizontal maximum bending distance have been obtained through statistical analysis of the transverse maximum bending distance of the lamellae.
Grey cast iron is traditionally thought to not deform plastically, because of the presence of a large number of graphite flakes, which act as crack initiation sites. Therefore, researchers had given up efforts to study its formability. However, our research suggests that, when the thickness of grey cast iron samples is decreased to 400 µm, they can be elongated by cold rolling, with the extension being as high as 156%, which is well beyond the forming limit of grey cast iron. This phenomenon confirms that the plasticity size effect occurs in thin grey cast iron samples, breaking the ceiling of their intrinsically low plasticity. We use classical plasticity theory and the surface-layer ratio theory to investigate the mechanism responsible for the deformation that occurs during micro-rolling. We also propose a method for improving the plasticity of extra-brittle materials.
Bainite in steel is an industrially useful structure. However, the controlling factor of its transformation start point is not known clearly. In this study, to clarify the effect of carbon content on the bainite transformation start temperature (Bs), we evaluated the dilatation curve and the microstructure in low carbon Fe–9Ni alloys. As a result, Bs decreased with increasing of carbon content. Furthermore, the driving force of partitionless transformation from fcc to bcc at Bs, which was calculated considering nickel segregation, was approximately constant at 400 J/mol in all alloys. This value is consistent with the driving force required for partitionless growth of ferrite, as reported in a previous study. This consistency suggests that Bs depends on the martensitic growth behavior of lath-shaped ferrite, which is determined by the supercooling starting from the T0 line.
In this investigation, a long-standing grain growth question is addressed in a Fe-3.5% Si steel. This material undergoes secondary growth of Goss oriented grains, but the underlying mechanisms of this growth remain unresolved despite extensive investigations for almost a century. In this investigation, a key question concerning this subject is resolved i.e. whether the nature of primary recrystallization grain boundaries solely determines the growth of Goss grains during secondary recrystallization. To address this issue undeformed and lightly deformed samples were subjected to recrystallization annealing under identical conditions. Although the extent of the deformation is light, such that it creates only limited modifications in the grain boundary structures, the recrystallization mechanism changes from abnormal secondary growth to conventional grain growth. In the former case, recrystallization commences at higher temperatures when the MnS inhibiting particles become dissolved, and in the latter mechanism, recrystallization occurs at lower temperatures due to the differences in stored energy between grains sharing a common boundary. This occurs through strain-induced grain boundary migration.
A method was developed for the measurement of BH curves for steels from room temperature to the Curie point (Tc). A closed magnetic circuit measurement was applied to obtain accurate temperature-dependent magnetic characteristics. BH curves were successfully measured at temperatures over 473 K (200°C), which was not possible before the development of this method.
In addition, at room temperature BH curves measured by the proposed method and a conventional method correspond well, which indicates the validity of the proposed method.
In the measurement for a medium-carbon steel and a electromagnetic soft iron, the difference in the magnetic flux density at 8000 A·m−1 (B8k) came from the low saturation magnetic flux density of cementite in the medium-carbon steel. B8k for the medium-carbon steel clearly decreased as the temperature increased from room temperature to 473 K (200°C), which corresponds to Tc for cementite.
In addition, B8k for the electromagnetic soft iron decreased almost linearly from 973 K (700°C) to 1048 K (775°C), whereas B8k for the medium-carbon steel decreased significantly from 998 K (725°C) to 1023 K (750°C). This difference indicates that magnetic transformation occurs only in the electromagnetic soft iron at around Tc, while magnetic and phase transformations, from ferrite to austenite, occur simultaneously in the medium-carbon steel. The differences in the temperature-dependent coercive force for electromagnetic soft iron, and for the low and medium-carbon steels were observed in detail.
The proposed method provided exact BH curves at high temperature, which were reflected by the complex and detailed magnetic behavior.