We tried to confirm the conversion of Ca3P2 to calcium phosphate by oxygen injection into the molten Reducing Dephosphorization (RDP) slag as well as to find the stoichiometry of calcium phosphate compound by the treatment. It was experimentally confirmed that the Ca3P2 was converted to beta-3CaO·P2O5 and 4CaO·P2O5 phases by oxygen injection treatment of the molten RDP slag by XRD analysis and thermodynamic estimation.
In this paper, the pretreatment of specularite concentrates through roller press was present to improve the granulation of blends comprising some 10–30% specularite concentrates and enhance the sintering performance. It was shown that the content of +3 mm fractions of granulation mixture is elevated by 10% and the sinter mixture permeability is elevated by 35%. In the meantime, a sinter tumble index increasing from 62.00 to 64.13% was attainted with similar sinter productivity and at lower fuel rate. The mechanism of the pretreating concentrates process to improve sintering performance was demonstrated from mineralogy analysis that better permeability and improved reactivity which help to form more calcium ferrite and good microstructure of sinter, further proven by the better metallurgical performance of sinter.
Reduction of ore is the key process in its conversion to the metal form, and the reducibility of ore fragments is therefore a crucial parameter in smelting operations. At constant oxygen fugacity, reducibility is controlled by the texture of the ore fragments, which determines the transport length from reduction front to fragment interface, and the chemistry of the ore fragments, which impacts element mobility within the crystal lattice. Their relative contribution was studied here for iron-ore reduction by combining compositional analyses and thermo-gravitational reduction experiments on individual ore fragments. Results indicate that despite large, and ore-characteristic differences in chemistry, ore-fragment composition has a negligible impact on reducibility. The large variations among bulk ores; e.g. the start of hematite-to-magnetite reduction varies by over 300°C, is therefore attributable to ore-texture effects. Porous, goethite-dominated ores show the highest reducibility, followed by fractured and layered fragments and finally dense ore fragments.
The simulation of cohesive zone in the blast furnaces is conducted using 2-dimensional discrete element method. The simulation is divided into two parts; (1) the group simulation of the flow of coke and ore particles in the cohesive zone when the reduced ore particles start to shrink and disappear with melting, and (2) the biaxial compression test simulations of breakable single coke particle with receiving x and y directional stresses which is evaluated from the group simulation. In addition, a gasification model is developed and combined with the coke model for the biaxial compression test simulation. The gasification process is simulated by removing element particles for a fixed amount. In the group simulation, coke particles have been found to move actively not only the axial (horizontal) direction but also the radial (vertical) direction due to the elimination of ore particles in the melting zone. Due to these movements, the coke layers are severely distorted and the coke layer thickness is much decreased. The developed stresses on particles in the group simulation are recorded and this stress information is applied to a non-gasified and the gasified coke model to simulate the coke breakage behaviors. In the non-gasified case, both of the volume breakage and the surface abrasion are not observed. In the gasified case, the surface abrasion and fragmentation occur, but the volume breakage is not observed under the present simulation condition.
Steelmaking contributes by more than 5% to the world’s anthropogenic CO2 emissions, so new ways to reduce the emissions in this industrial sector must be found. During a transition to more sustainable production concepts, also economic factors must be considered. In this paper the potential of using direct reduced iron (DRI) from the FASTMET process with rotary hearth furnace (RHF) technology, as a partial substitute of pellets in a blast furnace (BF) was studied. Simplified mathematical models of the different operations in a steel plant, including RHF, are combined with a more detailed model of the BF and the entire system is optimized by non-linear programming with respect to costs. The objective of the presented study is to analyze the prerequisites for an economical operation of an integrated steel plant equipped with an RHF, under different raw material prices and varying costs of CO2 emission allowances. The blast furnace operation parameters are also analyzed for different amounts of DRI charged. The results illustrate the conditions under which it would be beneficiary in a steel plant to integrate the RHF and BF technologies.
In this study, the formation of calcium ferrites during heating and cooling was investigated by in situ and real-time observation using a newly developed system, i.e., “quick X-ray diffraction (Q-XRD),” and an in situ laser microscope. In the new Q-XRD, a specimen was heated up to 1773 K, and X-ray diffraction patterns were measured using a pixel-array area detector with an interval as short as a few seconds. In situ observation both of crystal structure and microstructure successfully revealed the effects of heating and cooling rates on the sintering reaction in the CaO–Fe2O3 system with special attention to overheating and overcooling phenomena. The first continuous cooling transformation (CCT) concept for iron ore sintering was proposed to understand overcooling phenomena when the molten oxide cooled down to room temperature and magnetite (Fe3O4), hematite (Fe2O3), and various types of calcium ferrite were formed. The CCT diagram for sintering provides crucial and fundamental information on the sintering accompanying solidification, precipitation, and formation of calcium ferrites from the molten oxide, and can be used as a guideline for controlling sintering processes.
Dephosphorization process of high phosphorus oolitic iron ore by acid leaching and leaching kinetics were investigated in the paper. The high phosphorus ore samples (51%T.Fe, 0.52%P) used in this work were analysed by SEM-EDS and XRD, which showed their oolitic structure and mineral phases. Among the three kinds of acids (H2SO4, HCl and HNO3), sulfuric acid was the most appropriate one for dephosphorization, and structure changes after leaching were also investigated through the SEM and EDS analysis. The effects of acidity, particle size, stirring speed and temperature were researched in detail. Through acid leaching, phosphorus could be removed effectively, and iron loss was negligible, which was also studied by thermodynamic calculation. The experimental data could be well described by the unreacted shrinking core model and rate controlling step was found to be chemical reaction between apatite and acid. The apparent activation energy was calculated as 45.02 kJ/mol.
COURSE50 (CO2ultimate reduction in steelmaking process by innovative technology for Cool Earth 50) carried out COG and reformed COG (RCOG) injection operation trials at LKAB’s experimental blast furnace in Luleå in cooperation with LKAB and Swerea MEFOS. Operation trials were successfully carried out. Input of C in both COG and RCOG injection periods decreased comparing the base period, because of increase in H2 reduction instead of C direct reduction that is a huge endothermic reaction. However poor penetration depth of injected gas from shaft tuyere made furnace efficiency worse. Hot top gas injection increased temperature of top gas and upper part of the furnace. Efficiency of hot top gas injection was not clear as sinter degradation did not occur in the base period.
In iron ore pellet production, an increased green pellet temperature has a positive influence on the straight grate induration furnace. Previous studies show that this can lead to both higher production capacity of the induration furnace and also reduced specific oil consumption. The aim of this study is to apply the process integration concept to analyze how to maintain a high green pellet temperature by using the available heat more effectively. The heat is today used for the indoor climate and is delivered by electricity boilers and a waste heat recovery boiler that is installed next to the induration furnace, using hot gases from the furnace to heat water. This work will however assume that one can use the heat from the waste heat recovery boiler to preheat the material stream. By optimizing the cold section of the pelletizing plant, using process integration and the mathematical modeling approach in combination with the newly developed Optimal Solution Space Method, the results show that it is possible to increase the green pellet temperature and also reduce the energy cost by using the available energy in a more efficient way. It is also showing that by retrofitting the system so that the waste heat from the waste heat recovery boiler is used to preheat the material stream, this also leads to reduced energy cost and also increased production of iron ore pellet since the temperature of green pellet will be higher.
Screws in COREX shaft furnace are used to discharge burdens to the latter processor - the melter gasifier. They play a very important role in the uniform drawdown pattern, which directly affects the uniform gas distribution and further the smooth operation in the COREX shaft furnace. Therefore, a three dimensional model is established based on the discrete element method (DEM) in the present work. The model is used to investigate the effect of the bottom diameter of COREX shaft furnace, the cylinder height and the screw flight diameter on particle descending velocity along the radius during discharging process. Results show that the descending velocity decreases along the radial direction. It is better to decrease the bottom diameter of COREX shaft furnace, or the screw flight diameter, or to increase the cylinder height in order to achieve a uniform descending velocity along the radius. An optimization model is also proposed for uniform drawdown pattern in the end.
In-situ observations of the liquid high carbon iron (HCI) and slag flows in coke bed was carried out by using high temperature X-ray fluoroscopy at 1773 K. Along with the observation, 2-dimensional multiphase computational simulations of the liquid HCI and slag flows in the coke bed was carried out to investigate the effect of the slag on the HCI flow in the coke bed. The liquid HCI cannot pass through the coke bed with the coke diameter of 3–5 mm, however, the HCI can pass through the same size coke bed if the HCI comes into contact with the slag in the coke bed. When the HCI and slag contact each other on the coke surface, the contact angle of the HCI with slag decreases and its wettability increases. On the other hand, the slag’s contact angle increases and it changes to non-wettable phase. Based on the experimental and simulation results, it is confirmed that the contact angle change due to the HCI-slag contact makes them pass though the narrow coke slit which is small enough to prevent both of the liquid phases from flowing down if they do not contact each other. Based on the capillary rise model, the driving force of the HCI penetration into the coke bed will be the energy reduction by extending area of the coke surface covered with the liquid HCI.
The inclusions and clusters in steel samples of two similar steel grades of high-silicon non-calcium treated (HSiNC) stainless steels were investigated and compared during ladle treatment and continuous casting. Samples of liquid steel and slag were taken at different stages of the ladle treatment and casting during two plant trials: Low Al steel (LAl) and High Al steel (HAl). After electrolytic extraction of the steel samples, characteristics of inclusions and clusters (such as morphology, composition, size and number) were investigated in three dimensions (3D) by SEM in combination with EDS. Moreover, the composition of typical inclusions and clusters was analyzed on a polished cross section of steel samples. Spherical (SP), irregular and regular (IR) inclusions and clusters (CL) were observed in the samples from both heats. It was found that the morphology and composition of inclusions and clusters in both heats were significantly changed during the ladle treatment and casting. Most of inclusions (44–98%) in a Low Al steel are MgO–CaO–SiO2–Al2O3 spherical inclusions. The compositions of IR inclusions and clusters in steel samples of a High Al steel were mostly MgO·Al2O3 spinel, but also the complex SP inclusions containing Al2O3–MgO–CaO–SiO2. In addition, phase stability diagram based on Darken’s quadratic formalism and Redlich-Kister type polynomial was estimated for both heats at a non-infinite solution.
A kinetic model to predict chemical composition changes in molten steel, slag, and inclusions in ladle refining was developed and used to elucidate the mechanism underlying the change in the chemical composition of the inclusions. The coupled reaction model was applied to estimate the reaction between molten steel/slag and molten steel/inclusion originating from the slag. The thermodynamic calculation software, FactSage6.3, was employed to obtain the activity of each component in the slag phase. Empirical equations were applied to the reaction between the slag and the refractory. The resulting model can calculate changes in (1) the composition of each element in the molten steel, slag, and the inclusion originating from the slag, (2) the amount of inclusion originating from the slag and the deoxidation products, and (3) the ratio of the inclusion originating from the slag and the deoxidation products to the total inclusion. The calculated results were found to agree with the operational results of a 165 t ladle refining process reported in the literature. The deoxidation products altered from alumina to a MgO·Al2O3 spinel-type inclusion due to an increase in the Mg content of steel. In the average composition changes of each element in the total inclusions, calculated results for the MgO and Al2O3 contents were also found to agree with the operational results.
A kinetic model to simulate the reactions in a ladle furnace was developed in the previous paper. The following parameters were considered in this model; (1) ratio of the entrapment of slag in the molten steel, (2) ratio of the floatation of the deoxidation products and inclusions originating from the slag, (3) ratio of the agglomeration of deoxidation products with inclusions originating from the slag and (4) ratio of the volume of the bulk zone to the total volume of molten steel and that of slag phase. These parameters were optimized using sensitivity calculation by comparison with operational results as the parameters affected the amount and composition of inclusions. Then, the method to suppress the formation of MgO·Al2O3 spinel-type inclusion was discussed using the optimized parameters. The calculated results showed that the formation of MgO·Al2O3 spinel-type inclusion could be suppressed by optimizing the additional amount of Al, initial content of MgO in the slag, and slag basicity in addition to the Ca treatment. The changes in the inclusions calculated using the kinetic model were in good agreement with those predicted by the phase stability diagram. The developed model was useful for optimizing the operation of a ladle furnace.
High temperature experiments were carried out to investigate the effect of aluminum and BN addition on the performance of Al2O3–SiO2–SiC–C ramming refractory against slag attack. The slag utilized was a typical ironmaking one composed of 45.4% SiO2, 36.6%CaO, 15.9% Al2O3 and balanced by MgO, P2O5 and S. The temperature was set at 1823 K. Infrared (IR), Scanning Electron Microscopy-Energy Dispersive Spectrometer (SEM-EDS) and X-ray Diffraction (XRD) results show that Al is very effective in inhibiting CO and CO2 gas formation resulting from the main in-situ reaction between silica and carbon in the mix and consequently improving the slag corrosion resistance of the material. On the other hand, the sample with BN addition becomes more porous and hence exhibits a deteriorated resistance against slag attack compared to the standard refractory sample due to the fact that the decomposition of BN releases gases. The post-mortem analysis of refractory samples from the Siphon Box of a Cupola Furnace after intense industry service was conducted using an X-ray and SEM-EDS to compare physical properties and the degradation mechanism between the samples after industry service and those used in high temperature laboratory experiments. The resonance of the results for both types evidences the validity of applying knowledge obtained in the current research to actual industrial processes.
In order to provide a prediction tool for sulfide/oxide/oxysulfide inclusion evolution in Mn–Al steel with a Ca addition/CaO-based flux, a comprehensive thermodynamic database for the inclusion system composed of CaO–MnO–Al2O3–CaS–MnS–Al2S3 was developed in the present study. Activity of MnS in a CaS–MnS sulfide solid solution was experimentally determined by employing a chemical equilibrium technique at 1400°C and 1500°C. The measured activity exhibits a positive deviation from an ideal behavior, which is in consistent with the known two-phase separation of the sulfide solid solution at lower temperature (Tcr = ~ 1200°C). Based on the activity and the phase diagram data available in literature, a thermodynamic modeling of the CaS–MnS system was carried out. The following excess Gibbs free energy of the CaS–MnS sulfide solid solution was obtained:
Furthermore, using available thermodynamic modeling results for other constituent sub-systems, a larger thermodynamic database of the CaO–MnO–Al2O3–CaS–MnS–Al2S3 system was developed. A Modified Quasichemical Model in the quadruplet approximation was used to model the Gibbs free energy of the oxysulfide liquid solution. Comparisons between the model calculation and available experimental data show good agreement. The developed thermodynamic model and the database were used to predict unexplored phase diagrams with various nMn/(nCa + nMn) ratio, and sulfide capacity of the CaO–MnO–Al2O3 oxide liquid phase. The database can be used along with software for Gibbs free energy minimization in order to calculate any phase diagram section or thermodynamic property.
The formation and modification of non-metallic inclusions for Al-killed steel during Compact Strip Production Process (CSP Process) was studied by industrial experiments. The thermodynamics for the formation of Al2O3, MgO·Al2O3 spinel, various calcium aluminates and CaS bearing inclusion were analyzed with the help of calculation software FactSage. It is found that the oxygen activity in liquid steel during LF (Ladle Furnace) refining is confirmed to be determined by the equilibrium between dissolved Al and Al2O3 in inclusion. There are two manners for Al2O3 inclusion modification during LF refining: the first route is followed by Al2O3-low modified calcium aluminates-liquid calcium aluminates; and the other is as Al2O3– MgO·Al2O3 spinel-CaO–MgO–Al2O3 multi-component inclusion. In addition, the liquid calcium aluminates have the tendency to be multi-component inclusion. Two types of CaS bearing inclusion could precipitate in a slab during solidification. The favorable modified inclusions deform very well along with steel matrix during rolling process even if they are associated with CaS bearing layer, while the no or low modified Al2O3 based inclusion would be rolled into pieces and the micro-cracks might be generated around the inclusions.
The castability and microstructures produced from strip casting simulations of three compositions in the 200 series stainless steels have been examined. The nucleation density was similar for all three compositions. The as-cast microstructure showed very fine austenite grains of 10–20 μm in width. Retained delta ferrite was observed in the inter-dendritic regions, and was likely to be stabilised by the segregation of Cr into these regions. An analysis of the crystallography expected of different solidification sequences is presented, but a strict adherence to the Kurdjumov-Sachs orientation relationship was not found in these samples.
The effect of niobium and vanadium additions (0.5 mass% and 2 mass%) on the as-cast microstructure and properties of hypoeutectic white cast iron containing 19 mass% Cr and 2.9 mass% C, has been examined. NbC carbides present in the structure of tested Fe–Cr–CNb alloys, due to their characteristic morphology, show higher wear resistance and toughness than M7C3 carbides. Increasing amount of this type of carbides, caused by the increase of niobium in the alloy, contributes to the improvement of wear resistance and dynamic fracture toughness. The alloy containing 2% Nb gives the best compromise between wear resistance and fracture toughness. This alloy shows about 23% greater dynamic fracture toughness and about 25% greater abrasion wear resistance than the basic Fe–Cr–C alloy. Besides, the secondary carbides which precipitate in the matrix regions of the tested Fe–Cr–C–V white irons also influence the abrasion behaviour and fracture toughness. The alloy containing 0.5% V has approximately the same fracture toughness but lower wear resistance than alloy with 2% Nb.
The effect of Mg and Ti addition on the solidification macrostructure of ferritic stainless steel was evaluated in order to obtain fine equiaxed grains. Equiaxed solidification was promoted by adding Mg and Ti. Ti addition affects the amount of TiN as a nucleation agent of δ-Fe and the increase in constitutional undercooling. Mg addition produces spinel oxide in the molten steel, which accelerates the TiN formation. [Al] content also have an influence on the equiaxed grain formation.
In this study, in order to obtain complete size distribution results, the maximum carbide size in a Fe-17 mass% Cr-4 mass% C hypereutectic High Chromium Cast Iron (HCCI) produced with different cooling conditions, titanium additions and heat treatment conditions was determined by using the statistics of extreme values (SEV) method. In addition, the shape factor, circularity, was estimated in order to classify the type of carbides (primary M7C3 carbides, TiC carbides and secondary M7C3 carbides). Compared to the smaller size carbides, such as TiC carbides and secondary M7C3 carbides, it was found that the slope of the extreme value distribution (EVD) regression lines is lower for the large sized carbides such as primary M7C3 carbides than for the smaller carbides. Moreover, it was found that the circularity value for the larger size carbides is higher than for the smaller carbides. Furthermore, the estimated and observed maximum carbide sizes were compared with each other for all carbide types. The characteristic of the different carbide types are summarized and classified based on the shape factor. Finally, the relationship between the carbide size distribution including the maximum carbides size and mechanical properties is discussed based on the combination of a size distribution analysis and a maximum size analysis.
A Sendzimir rolling mill (ZRM) uses a work roll with a small diameter to roll high strength steel. On the other hand, the work roll is often bent because its diameter is very small compared with its length. From roll bending, a complex wave shape appears in the rolled steel plates. In order to solve this problem, an AS-U roll is used to control the vertical rolling load on the plate. A neural-fuzzy control is applied to the shape control system in a ZRM because of the complexity, nonlinearity, and multi-input multi-output (MIMO) characteristics of rolling mills. The current shape control in a ZRM is not a fully automatic shape control. If the shape control were fully automatic, saturation can occur at the AS-U actuator. To solve this problem, the shape recognition performance should be improved and the fuzzy gain should be modified. In this study, to improve shape control performance, an echo state network (ESN) was applied instead of a multi-layer perceptron (MLP) at the neural network, and the fuzzy gain was set to change depending on error by adding P gain. Finally, the shape control system was evaluated through simulation.
We numerically analyze the cooling process of a steel plate in a run out table (ROT). As a preliminary attempt to obtain the best cooling capacity in a facility with limited space (i.e., the length of the ROT), the number of nozzles representing the quantity of supplied cooling water, and the spacing between the nozzles, are changed for several cases without changing other factors. Thus, the effects of the nozzle arrangement on cooling performance are investigated. Cooling histories of the plate, the heat flux on the plate’s surface, shapes of the residual water distribution, and cooling capacity are obtained for various cases.
X-ray fluorescence (XRF) yield x-ray absorption fine structures (XAFS) have been investigated in terms of quantitative analysis for steel sheets containing 31–2100 ppm of Nb. Nb concentrations in the samples were determined with high accuracy from the height of the K-edge jump. The detection limit was estimated down to the single digit ppm range. The combination of this technique with the conventional extended x-ray absorption fine structure (EXAFS) analysis for an identical XAFS spectrum provides both quantity and chemical state information of trace elements in the given volume of steel samples.
A quantitative analysis of copper in steel scraps was investigated by using laser induced plasma spectrometry. A Nd:YAG laser at a wavelength of 532 nm was employed to generate the plasma under reduced argon atmospheres. Several experimental parameters, such as an argon pressure, a gate width and a delay time of the spectrometer were optimized to obtain a calibration curve having higher sensitivity and better linearity. Several line pairs of copper and iron were also investigated as a candidate of the analytical line for measuring the intensity ratio converted into the atomic ratio of copper to iron in steel scraps. The Cu I 324.754-nm line was more suitable for the analytical line than the Cu I 327.396-nm line due to its higher intensity. Line pairs of Cu I 324.754 nm to several iron lines, Fe I 381.583 nm, Fe I 382.043 nm, and Fe II 276.750 nm, were measured to compare criteria of their calibration curves, such as a slope, an intercept and a regression coefficient. As a result, it was recommended that a calibration curve for the intensity ratio of Cu I 324.754 nm/Fe II 276.750 nm could be employed in the actual application because it had a regression coefficient of almost unity over a concentration range up to 1.0 mass% Cu in standard reference samples of iron-copper binary alloy. The limit of detection, based on three times the standard deviation of the emission intensity at 324.75 nm in a pure iron sample, was estimated to be the Cu/Fe atomic ratio of 0.00059.
Friction welding techniques are widely used in several industrial sectors. A non-destructive inspection is mandatory in post-processing for repair quality evaluation. The present study examines through conventional ultrasonic technique Friction Hydro Pillar Processing (FHPP) repairs on ASTM A36 low carbon steel plates. The repairs were made using four different axial forces, 200, 250, 300 and 350 kN with constant rotational speed of 1000 rpm. The features of the echogram signals obtained made possible to identify and distinguish microcracks, lack of bound, clustered inclusions and isolated inclusions, forming three critical regions within the weld. It was seen that high processing forces increases probability to generate microcracks from discontinuities. These ultrasonic results were related and validated with micrographic analysis and it was possible to make a relationship between echograms features and micrographs to differentiate the discontinuities founded within the repairs. This information could be used as a guideline for operating procedure to locate discontinuities.
The use of chrome-manganese stainless steels (Cr–Mn SSs) of 200-series grade has tremendously increased in past few years in various applications like construction, home accessories, office appliances, light poles etc. Their mechanical properties, weldability and corrosion/oxidation resistance provide the best all-round performance stainless steels at relatively low cost. Therefore, they serve as an appropriate alternative to 300-series stainless steels now days. Similar to 300-series steels, Cr–Mn SSs are also submitted to various fabrication practices like welding, hot rolling, cold working, solution annealing or stress relieving processes. This leads to change in their grain size due to higher operating temperature during service. The grain size has a major influence on the intergranular corrosion (IGC) developed due to sensitization phenomena. Several investigations have been made on the influence of grain size on IGC behaviour of 300-series stainless steels, but this information about Cr–Mn SS is scanty till now. Therefore, in this paper an attempt has been made to evaluate the effect of grain growth on degree of sensitization (DOS) of Cr–Mn SS and finally concluded that the DOS decreases with increase in grain size.
The aim of this study is to investigate the correlation between the hardenability curves and microstructure of a low carbon steel after the pack carburization process. Pack carburization experiments were conducted on 1.5920 steel. Sodium carbonate (Na2CO3) was used as an energizer material during the process. Samples were carburized with the various amounts of energizer material at 925°C for different times. The case depths of carburized samples were measured using microhardness test. The results indicated that the incorporation of energizer material led to the specific changes in the case depth of cementation steel. The microstructural characterizations of the samples were also carried out in order to determine the effect of energizer material on the microstructure and hardenability curves of the pack carburized steel. Recent scientists have believed the addition of energizer materials causes to increase the case depth according to carburizing phenomenon. This study proved the case depth of steel may decrease during the carburizing process due to the decarburizing phenomenon. Also, we showed any changes in the hardenability profile refer to the formation of specific phase after the process.
The effect of austenite grain size on martensitic transformation, particularly with regard to martensite structure, Ms/Mf temperatures, and mechanical properties was investigated in 0.1C–5Mn martensitic steel. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 μm. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. Grain boundary density, especially high-angle grain boundary density, increases with decreasing austenite grain size. Tensile strength slightly increases though austenite grain size decreases about one tenth. However, reduction in area significantly improves particularly at refined grain sizes of 30 μm. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off between strength and ductility. Low C–5Mn martensitic steel produced from fine austenite shows a great possibility having an excellent total balance of strength, ductility and toughness.
The deformation structure of low-carbon lath martensitic steels was analysed by transmission electron microscopy with Kikuchi pattern analysis. Retained austenite films on the martensite lath boundaries are transformed into high-carbon martensite films by light deformation. These high-carbon martensite films protect the martensite lath boundaries from reconstruction. Specific deformation structures of low-carbon lath martensite, such as kinked laths, irregularly bent lamellas and lamellar dislocation cells, were thus formed. Tempered lath martensite structures without retained austenite films easily disappeared by light deformation. The relationship between mechanical properties, such as the work-hardening ratio, and the development of the deformation structure was also clarified.
The formation of strain-induced ferrite was investigated in two low carbon steels and in a 0.036% Nb microalloyed low carbon steel at temperatures above the Ae3. Two distinct stages were observed, the first of which was characterized by the formation of Widmanstätten colonies with very fine plates of similar orientation. In the second stage, observed after further straining, the initial plates coalesced into polygonal ferrite grains. The transition from stage I to stage II was retarded by additions of carbon and niobium and slightly by an increase in strain rate. EBSD analysis revealed that the small misorientations (<1.5°) between Widmanstätten plates allow them to coalesce into grains. The 60° misorientations between some of the colonies suggest that they form from twinned regions of the parent austenite. A common characteristic of the polygonal grains is the presence of planar high angle boundaries, a feature inherited from the original Widmanstätten microstructure.
The interdiffusion of Ni, Cr, and Mo across the interface in the diffusion couples of type444/type316L stainless steels was simulated using the DICTRA software. The distance of interface migration and the Ni diffusion distance into the ferrite phase were calculated and compared with the experimental results previously reported by a present author. The calculated results agree very well with the experimental results. An analytical model based on the analytical solutions for the semi-infinite solute diffusion equations has been proposed. In the model, all solute elements are considered and diffusion paths are calculated. The supersaturations of the solutes defined both for the ferrite phase and the austenite phase are calculated from the calculated diffusion paths. According to the model it is shown that the temperature dependency of the interface migration distance and the Ni diffusion distance can be quantitatively evaluated by the supersaturation for Ni.
For understanding the transition from diffusive to displacive austenite reversion mechanism in steel, the effect of heating rate on austenite reversion behavior was investigated in 0.15%C–5%Mn steel. Austenite reversion temperature first increased gradually with the heating rate owing to the superheating effect and then remained at a constant temperature above a critical heating rate. In response, the austenite formed by rapid heating exhibited a coarse prior austenite grain structure, indicating the occurrence of displacive reversion even in low-alloy steel.
Failure criteria based on ductility associated with triaxiality are often used to define the strength of metal alloys as a function of the load modalities. However a discussion of the results with regards to the microstructural evolution and the behavior of the different phases is still not investigated, especially in multi-phase alloys. In this paper a ductile failure criterion was calibrated on α/β titanium alloy (Ti–6Al–4V). The applied approach is based on different experimental tests supported by numerical analyses carried out by detailed FE models. Specimens were tested on a multiaxial test system to investigate the failure induced by different states of stress triaxiality. Results have been discussed with regards to the textures induced in the lattice of the α-phase and β-phase, which are a direct consequence of the rotations associated with the dissipative plastic strains. Schmid factors at failure have been also calculated to determine which slip system have been activated by the different load modalities. The proposed approach could be also extended to steel.
Carbon dissolution investigations were carried out at 1550°C on chars from waste CDs as an alternative resource for ironmaking. Waste CDs/DVDs consist of polycarbonate material (thermoplastics polymer) and these produced a significant volume of residual char (~19 wt%) during heat treatment at 1550°C for 15 minutes. The carbon content of chars was determined to be 89% C. XRD and RAMAN analysis were used for the structural characterization of chars. Carbon dissolution investigations using the sessile drop method showed a very rapid carbon pickup reaching 4.12%C within 2 minutes (overall rate constant K: 19.2 × 10–3 s–1). SEM investigations confirmed the absence of oxides and other ash impurities in the interfacial region. The contact angle was seen to change from 79° to 100° after 3 minutes and 105° after 15 minutes of contact. These studies have shown that carbon from waste material could prove to be a valuable carbon resource and be used to partially replace coal and coke during iron making for carburization.