The solidification behavior of slags is a complex subject due to the varied chemical compositions of steelmaking slags. However, recent experimental developments, using the fact that slags that do not contain significant quantities of transition metal oxides are transparent, has allowed the development of optical techniques to observe slag solidification behavior. While “curiosity based research” was the initiator of such studies, it has quickly become apparent that observation of the phenomenon of slag solidification has lead to a much deeper understanding of the role of slag solidification behavior in the process of the continuous casting of steel. In this paper findings from observation of slag solidification will be discussed with reference to developments in continuous casting.
As a special material with better mechanical properties than that of traditional glasses and ceramics, glass-ceramics which can be made from the bulk of industrial solid wastes from the perspective of protecting the environment, could be applied into the fields of construction and industry. For BOF slag, its residual iron element contained inside also can be recovered by a reduction process when it was melted to prepare parent-glass. This is an effective way to resolve the problems of how to utilize such metallurgical slags in large scale and recover the remaining metal component. In this study, the parents glass with different reduction degree were made from BOF slag, fly ash and varied amounts of coal powders, and glass-ceramics were obtained by respectively heating the various parents glass in air or in nitrogen. Results have shown that crystallization is occurred in all parents glasses heated in air, but is suppressed in samples heated in N2 except for the parent glass with TFe content of about 4.5%. Since the TFe content of parents glass is above 7.5%, the shape of crystalline exothermic peak is high and sharp, and the main crystal phases are diopside(-ferrian) and augit. A maximum bending strength of glass-ceramics heated in different atmospheres is obtained for samples with 8% coal powders mixed. The N2 atmosphere could promote the optimal bending strength and decrease its heating temperature.
The diffusion behavior of Mn, O, and S at the interface between the oxide and solid steel must be clarified to understand the reaction between solid steel and inclusions containing sulfur at heat-treatment temperatures. In a previous paper, we investigated the reaction between the MnO–SiO2–FeO oxide and an Fe–Mn–Si solid alloy by heat treatment using diffusion couple. In the diffusion couple experiments, fine particles were formed near the interface between the alloy and the bulk oxide after heat treatment owing to diffusion of oxygen from the bulk oxide to the alloy. In this study, by using the same diffusion couple method, we investigated the effect of sulfur on diffusion behavior. The width of the particle-precipitated zone (PPZ) and Mn-depleted zone (MDZ) decreased with increasing sulfur content of the bulk oxide and with decreasing sulfur content of the alloy. This result indicates that the diffusion of oxygen was suppressed by the diffusion of sulfur. The influence of sulfur on the formation of the MDZ and PPZ is discussed in terms of the difference in the driving force of sulfur diffusion and the direction of oxygen and sulfur diffusion.
Self-reduction experiments of mill scale with four potential carbonaceous reductants (charcoal, coal char, blast furnace coke and petroleum coke) were carried out by thermogravimetry (TG) in order to choose one allowing the highest reduction rate and overall conversion. The fastest rate of self-reduction was measured for the charcoal and the slowest for the petroleum coke. The Friedman method of kinetics data analysis was used to calculate apparent activation energy at different conversion rates. Based on these values it can be concluded that the Boudouard reaction controls the rate of self-reduction with charcoal up to about 60% of conversion and even more for others reductants. It has also been demonstrated that the morphology of the iron produced strongly depends on reactivity of carbonaceous reductant.
Influences of preparation technologies on properties of straw char (corn) and its use as a supplementary fuel for coke breeze in iron ore sintering were investigated. By means of one-stage carbonisation, produced straw char achieved similar volatile and fixed carbon contents to coke breeze, where recommended carbonisation temperature was 700°C, heating rate was 10–25°C/min, and carbonisation time was 30 min. Two-stage carbonisation, consisting of one low temperature stage and the other high temperature stage, facilitated the further removal of volatile matters, and making straw char increased in yield and fixed carbon content. Recommended one-stage carbonisation benefited reducing combustibility difference between straw char and coke breeze since densified strucuture was achieved. Compared with one-stage carbonisation, two-stage carbonisation made straw char more densified in structure and less difference in combustibility to coke breeze. For straw char prepared from one-stage and two-stage carbonisation, recommended replacement ratio to coke breeze were 20% and 40% respectively. The emission reduction of SOx for replacement ratio 20% and 40% were 10.98% and 38.15% respectively, and that of NOx were 12.67% and 22.53% respectively.
Co-injection of two or more reducing agents via tuyères may ensure technological, economic and environmental benefits and enhance the flexibility of use of various reductants. The blast furnace technology with top gas recycling, recently tested in the scope of the ULCOS program aiming at significant mitigation of CO2 emissions, is also based on co-injection of reducing gas, pulverised coal and pure oxygen. This paper presents results of a study on conversion behaviour of pulverised coal while co-injecting further substances with focus on tuyère assembly design. Factors affecting the coal particle conversion in the raceway derived from the combustion mechanism and constructional features of tuyère stock arrangement are firstly discussed. An experimental work was performed using the Multifunctional Injection Rig for Ironmaking (MIRI) at the Department of Ferrous Metallurgy, RWTH Aachen University. Tests simulating different scenarios for injection of coal, carbon monoxide and oxygen were conducted. Conversion degree determined by three methods and microstructural analysis were used to examine the effect of pre-mixing of coal with above mentioned components on its combustion behaviour. Results obtained give hints for optimising the lance arrangement.
This study aims to utilize the high Al2O3 pisolitic ore in sintering process by designing the quasi-particle where the pisolitic ore is used as nuclei and ultra-fine hematite and magnetite ores are employed as adhering fines. The assimilation behavior between nuclei and adhering fines was investigated through microstructure analysis and it was correlated to sinter quality. When ultra-fine hematite ore was used as adhering fines, the low viscous melt of CaO·Fe2O3 was formed in the assimilation. Since this results in the low extent of Al2O3 localization and the porous structure, the detrimental effect of Al2O3 on the strength was not fully controlled. On the other hand, for the ultra-fine magnetite ore, 3CaO·Fe2O3·3SiO2 melt with high viscosity was predominantly participated in the assimilation. The assimilation was suppressed by the formation of ‘interfacial layer’. Due to the dense structure and high extent of Al2O3 localization, the detrimental effect of Al2O3 on strength was reasonably controlled. The quasi-particle comprising high Al2O3 pisolitic ore and ultra-fine magnetite ore showed the equivalent sinter quality to the quasi-particle sample consisting of nuclei of dense hematite and adhering fines of ultra-fine hematite resulting in the high sinter quality.
Together with the conventional granulation, a proposed pre-wetting granulation for two commercial sinter mixtures consisted of six different iron ores and other auxiliary materials has been investigated under controlled laboratory conditions. A method to describe the size variation of the granules between different granulation trials was proposed and studied in the granulation of this study. It is a method by drawing a cumulative undersized curve (CUS curve) and a differential cumulative undersized curve (Δ‑CUS curve) into one figure. The results indicated that (1) with more water available for granulation, more particles in a sinter raw mixture would behave as the layering fines in granules and the upper size for these layering fines would increase from 1 mm to 4.5 mm in conventional granulation; (2) compared to the results from conventional granulation, the granulation with pre-wetting treatment on iron ore fines resulted in a tighter particle size distribution and a higher permeability before ignition. At similar moisture content, more particles can be shifted into the layering fines in pre-wetting granulation than in conventional granulation; (3) Due to the preferential granule growth and the deformation of the granules, a further increase in pre-wetting degree (PWD) from 65% to 80% had little influence on the optimal permeability before ignition. Furthermore, a sinter mixture with more materials that are high in moisture absorption capacity would lead to higher optimum moisture content in granulation.
The softening and melting dripping properties of increasing mixed rate of metalized pellets were investigated. The experimental results showed that the softening and melting properties of the lump ores or oxidized pellets were dramatically improved by interaction between lump ores (or oxidized pellets) and metalized pellets, while there was no obvious interaction between sinters and metalized pellets. On the basis of the constant basicity of mixed burden, the softening and melting start temperatures reduced and softening and melting interval became wide as the mixing rate of metalized pellets was increasing from 0% to 56%. The influence of the mixed rate of metalized pellets, less than 25%, on the properties of mixed burden is little. When the rate was more than 25%, the properties of mixed burden would rapidly deteriorate. With a large number of metalized pellets, especially 56% sinters replaced, the properties of mixed burden were worst. According to the element migration of burden interface observed, it was found that the diffusion of Ca and Si between oxidized pellets and sinters (or metalized pellets) was obvious. However, the phenomenon between metalized pellets and sinters was weaker. Thus, it was confirmed that the strong interaction must happen between burdens, the nature differences of which were larger.
The generator gas of melter gasifier inevitably carries a lot of dust, which would be transported to the freeboard for recycling, as a result of the existence of the fluidized bed. The recycling dust, with an attractive concentration of C and Fe, is considered as a useful secondary material, thus it is important to investigate the inherent characteristics and behavior of recycling dust in the freeboard. In the present work, the recycling dust characteristics are investigated through the granulometry, chemical, mineralogical and micro analysis. The recycling dust has an irregular shape and an uneven granulometric distribution with the following primary minerals: Fe, FeO, SiO2, 2CaO·SiO2 and 2CaO·Al2O3·SiO2. Furthermore, a three-dimensional mathematical model at steady state is developed to describe the recycling dust combustion and the freeboard temperature distribution, which significantly influences the recycling dust behavior. The region of high temperature above 3000 K can be observed in front of DB due to the combustion of C in the recycling dust, with a combustion ratio of almost 100%. Based on the above analysis, the recycling dust behavior is complex. Fe and FeO in the recycling dust may be oxidized and finally smelts into liquid iron, while other minerals smelt into molten slag. It is worth noting that the phenomena of circulation and accumulation of alkali metals, ZnO and S could occur, despite that most of them are discharged out of the melter gasifier through the generator gas and molten slag.
The effects of molar ratio of C/O and the particle size of char on carbothermic reduction of titanomagnetite (TTM) were investigated from a kinetic viewpoint at 1100°C employing thermogravimetric analysis (TGA) and quadruple mass spectrometry (QMS). An increase in molar ratio of C/O results in a higher rate of carbon gasification, leading to an increase in final fraction reduction of TTM since excessive amount of char in TTM and char composite shifted the equilibrium in carbothermic reduction of TTM from the wustite/Fe equilibrium to that by the carbon gasification. With decreasing the particle size of char, the carbothermic reduction of TTM was improved by the activation of carbon gasification.
In order to recycle the phosphorus in P-bearing steelmaking slag and make it used as slag phosphate fertilizer, the citric acid solubility of P-bearing steelmaking slag was researched. The research results show that the citric acid solubility of P-bearing steelmaking slag is decreased with the increasing of P2O5 and Fe2O3 content for the CaO–SiO2–Fe2O3–P2O5 slag system. Added CaF2 into slag can easily form the fluorapatite (Ca5(PO4)2F) that can’t be dissolved in 2% citric acid solution, which makes citric acid solubility of the slag decreased obviously. And added MgO, MnO or Na2O into slag can prevent the precipitation of β-Ca3(PO4)2 phase with low citric acid solubility, which makes the citric acid solubility of slag increased. While for the CaO–SiO2–Fe2O3–P2O5–X (MgO, MnO or Na2O) slag system, MgO and MnO in slag mainly enters into RO phase, which has no effect on the phosphorus existence form in slag and has little influence on citric acid solubility of the slag. The Na2O in slag changes the phosphorus existence form in slag, but the generated Na2Ca4(PO4)2SiO4 and Na3PO4 also have good citric acid solubility, which also has little effect on citric acid solubility. Al2O3 and TiO2 modification only increases the phosphorus content in phosphorus-rich phase, does not change the phosphorus existence form in slag, which makes the citric acid solubility increased slightly. While SiO2 modification prevents the precipitation of β-Ca3(PO4)2 phase with low citric acid solubility, which makes citric acid solubility of the slag increased significantly. The modified slag without fluoride makes the citric acid solubility increased to 95% and meets the requirement of producing slag phosphate fertilizer.
Cold model experiment on ion-exchange reaction between pearlite particles and HCl aq. was carried out in order to understand the effect of particles dispersion and operating factors on solid/liquid mass transfer rate in a mechanically-stirred vessel. Inner diameter of vessel was varied in conjunction with both bath depth as 400 (base) and 300 mm. Rotation speed and volume ratio of particles to liquid were changed between 0–240 rpm, 0.02–0.24 (–), respectively. When rotating speed increased, solid/liquid mass transfer rate increased moderately both in the regions I and III, whereas it increased in the region II. When impeller depth decreased, it was kept almost constant in the region I, increased in the region II and increased moderately in the region III. Solid/liquid mass transfer rate changed less than liquid/liquid one in the region II when rotation speed and impeller height were changed, whereas both of solid/liquid and liquid/liquid mass transfer rates were kept almost constant in the region I. The dimensionless equation on solid/liquid mass transfer rate of each region was given as a function of Sherwood number, Reynolds number, volume ratio of particles to liquid and bath depth normalized by vessel diameter. Dispersion ratio in the region II was ranged by solid/liquid mass transfer coefficient and rotation speed or impeller height of the transitions I–II and II–III. Solid/liquid mass transfer rate of mechanical stirring was larger than that of gas injection practice for the same supplied rate of energy into bath.
The capillary breakup of multiple molten oxide jets was investigated. A graphite nozzle head allowed producing an array of laminar slag jets which disintegrate into droplets due to the growth of instabilities. A high temperature furnace with optical access and a pressurized crucible was used to generate the coherent circular jets which were recorded using a high-speed camera. External perturbations by means of a pneumatic vibrator were applied to investigate the impact of the vibration frequency on the length of each jet strand, the drop formation rate, the spacing between consecutive droplets and the drop size distribution. Results show excellent consistency in droplet formation when external vibration is applied and very good agreement with theoretical predictions. The findings may be used to facilitate the design of a droplet heat exchanger in which the latent heat of a molten slag can be transferred to a counter-current gas in a predictable manner.
A new unpolluted deoxidation method for liquid steel was given in this paper. Four deoxidation experiments were done, under the conditions of applied voltage U=0, 2 V, 4 V and 6 V between the liquid steel and CaO(45 wt%)–Al2O3(55 wt%) molten slag. The results showed that the deoxidation rate increased as increasing of applied voltage. In addition, based on the oxygen diffusion behavior under the gradient of electrochemical potential, the kinetics formulae for deoxidation of steel with molten slag electrochemistry method are derived. The formulae can well describe the change of dissolved oxygen content in the steel. From the formulae it can be concluded that deoxidation rate could be enhanced by strengthening the stirring of steel, increasing the applied voltage, and selecting slag system with a high basicity and a low viscosity, etc.
The laboratory experiments of Al2O3 inclusions modified by calcium treatment in linepipe steels with the S content of 30 ppm and 310 ppm were performed at 1873 K. Particularly, samples were taken before calcium treatment ( 1 minute before the calcium addition) and at various times (typically 1 minute, 10 minutes and 30 minutes) after calcium treatment to study the transient inclusions evolution during calcium modification. Traditional modification mechanism of Al2O3 inclusions is that Al2O3 inclusion is directly modified by dissolved calcium or indirectly modified via CaO formed from calcium and oxygen. In the current study, after Ca–Si alloy power addition, CaS outer layer were promptly formed on the angular Al2O3 inclusion. With the reaction of the CaS outer layer and the Al2O3 core or [O], the Al2O3 inclusion was modified to a spherical liquid calcium aluminate. Moreover, the equilibrium curve of Ca–O and Ca–S, the stability diagram of inclusions and equilibrium precipitation of inclusions during solidification in linepipe steels were calculated to study the formation of inclusions.
As a fundamental study to clarify the agglomeration and coalescence of alumina inclusions in molten steel from the viewpoint of interfacial chemical interactions, it has been experimentally verified for the first time that significant agglomeration force is exerted between alumina particles in aluminum deoxidized molten steel by using a newly established experimental method. In this method, the agglomeration force exerted between alumina particles in molten steel is directly measured separately from the effect of molten steel flow. In addition, it has been quantitatively demonstrated that the contact angles measured between aluminum deoxidized molten steel and an alumina plate are larger than those between the molten iron-oxygen alloy and the alumina plate, which have already been measured by other researchers. Moreover, it has also been indicated by analyzing the actual measurement values of agglomeration force with an interaction model taking contact angles and interfacial properties into consideration that the agglomeration force between the alumina particles in aluminum deoxidized molten steel derives not from the van der Waals force but from the cavity bridge force occurring due to molten steel, which is unlikely to wet the alumina particles. Meanwhile, it has been assumed that the agglomeration force on spherical alumina inclusions in aluminum deoxidized molten steel calculated on the basis of the interaction model according to the cavity bridge force is greater than the buoyant force and drag force, and the alumina inclusions once coming into contact are therefore not prone to be simply dissociated even under molten steel flow. Thus, they maintain the agglomeration state and are subsequently sintered and form comparatively solidly bonded alumina clusters.
The duration of supercooling for erythritol, a promising phase change material with a melting point of 118°C, was investigated in a glass tube using three small specimen volumes of 0.025, 0.40, and 16.0 cm3 in glass tube diameters of 1.02, 10.0, and 27.3 mm, respectively. The supercooling duration was measured by the temperature increase of the specimen due to the release of latent heat under a constant degree of supercooling from 38 to 98°C. The supercooling duration was largely dependent on the supercooling degree and specimen sizes, and increased from approximately 0.1 to 20000 min with a decrease in the supercooling degree and specimen size. The effects of the supercooling degree and specimen size on the supercooling duration are discussed in terms of the Johnson-Mehl-Avrami equation and three nucleation theories based on the observed solidification position; homogeneous nucleation, heterogeneous nucleation from the wall, and heterogeneous nucleation from insoluble particles.
A generalized three dimensional mathematical model, which adopted the Euler-Lagrange approach, has been developed for the motion of inclusions in a continuous casting tundish with channel type induction heating. The inclusion trajectories were obtained by numerical solution of the motion equation including gravity, buoyancy, drag, lift, added mass, Brownian, electromagnetic pressure and thermophoretic forces. Besides, the collision and coalescence of inclusions and adhesion to the lining solid surfaces were also taken into account. The Brownian, Stokes and turbulent collision were clarified, separately. Then the effect of induction heating power on the inclusion behavior was demonstrated. A reasonable agreement between the experimental observation and numerical results was obtained. The results indicate that the electromagnetic pressure force significantly promotes the removal of inclusions, especially for the bigger inclusion. Although the thermophoretic force goes against the removal of inclusions, its influence is negligibly small. The removal ratio of inclusions in the tundish with induction heating increases from 67.45% to 96.43%, while the power varies from 800 kW to 1200 kW. The collision and coalescence should be included when model the inclusion motion, because it can promote the removal of inclusion. The turbulent and Brownian collision becomes more active with the increasing power, while the Stokes collision is just opposite.
The uniformity of strand solidification and lubrication in continuous casting mold has a great effect on the slab quality and production stability. In this work, an inverse problem model, based on online measured mold temperature, was developed to simulate mold heat transfer and strand solidification. Through quantifying the local heat flux and its distribution between the mold and strand shell, the temperature field and strand solidified behavior inside mold were obtained. Following Newton law of viscosity, Coulomb law of friction and mixed lubrication theory, the mathematical model of mold flux lubricated and frictional behavior was proposed in view of various contact states between mold flux and strand surface. The non-uniform distributions of slag film thickness, lubrication status and frictional stress were further investigated. The inverse problem model could reflect the non-uniformity of solidification and lubrication, and therefore would provide a worthwhile calculation method for exploring the complex lubricated and frictional behavior inside mold.
In the present study, inclusions in calcium-treated steel after RH treatment, in the tundish and in bloom were studied. Only two types of inclusions were detected in all steel samples, namely liquid calcium aluminate inclusions and inclusions of two phases with spinel in the center surrounded by the liquid calcium aluminate. The attachment of the inclusions on the inner surface of SEN was investigated for two types of refractory materials. The results indicated that liquid calcium aluminate inclusions could attach on the wall of SEN, when the refractory had big grain size and big cavities on the surface. On the other hand, tiny grain size and smooth surface of the SEN showed no attachment of the inclusions. The different behaviors of the two types of SEN were well explained using the results of flow calculation in the nozzle. The mechanism of the attachment was also discussed based on the experimental results and the CFD calculation. The tiny nodules formed on the surface of the inner nozzle due to inclusion attachment could be a source of macro inclusions.
A transient three-dimensional (3D) coupled mathematical model has been developed to explore the effect of the rotating electrode on the electromagnetic, flow, temperature fields as well as solidification in the electroslag remelting (ESR) process. Maxwell’s equations are solved by the finite volume formulation. The Joule heating and electromagnetic force (EMF), which are the source terms in the energy and momentum equations, are recalculated at each iteration as a function of the phase distribution. The movement of metal droplets is described by the volume of fluid (VOF) approach. Additionally, the solidification of the metal is modeled by an enthalpy-based technique, in which the mushy zone is treated as a porous medium with porosity equal to the liquid fraction. The rotation speed of the electrode is imposed on the inlet, and the melt rate changes with time proportionally to the Joule heating. The present work is the first attempt to investigate the innovative technology of ESR process with rotating electrode by a transient 3D comprehensive mathematical model. The metal droplets are thrown out by the centrifugal force without enough time to grow up in the process with rotating electrode. The melting rate increases because of the enhanced heat transfer in the vicinity of the electrode tip. With a 800A current, the melt rate increases from 0.0078 kg/s to 0.0114 kg/s, while the rotation speed ranges from 120 rpm to 160 rpm. Besides, the metal pool not only keeps the same depth, but also becomes wider in spite of a higher melting rate.
This paper proposed a new near-field imaging algorithm for Blast Furnace (BF) burden surface imaging. The algorithm was applied in a novel T-shaped MIMO radar. In the process of beam synthesis focusing, the proposed algorithm does precise phase compensation to realize the equiphase surface. It avoids interpolation, reduces the computational complexity, and is convenient for data batch processing concurrently, which greatly improves the efficiency of operations and enhances the real-time performance. Simulation results and the on-line test result demonstrate the effectiveness and high operational efficiency of the proposed MIMO radar imaging method which has great potential in BF burden surface imaging.
This paper is to determine the slab schedule on parallel hot rolling lines with consideration of chain precedence constraints, which restricts the slabs belonging to a chain to be successively processed without interruption in one production line. Moreover, energy loss increases as its waiting time before being processed increases, since the slabs dynamically arrive with high temperature. The objective of the problem is to minimize the total energy loss costs. Two kinds of energy loss cost that are linearly and nonlinearly dependent of its waiting time are concerned. The strongly NP-hard feature of the problem stimulates us to develop branch-and-pricing algorithm to solve it. Firstly, the model is formulated as a set partitioning model. For the strongly NP-hard sub-problem, dynamic programming based on state-space relaxation method is derived to tackle it. A branch-and-bound procedure is applied for the case that the solution obtained at the root node is not integral. The whole algorithm is implemented by C language on a Pentium IV 3.0 GHz PC. Results on 25 different problem scenarios demonstrate that the proposed algorithm can solve the instances up to 120 slabs optimally within a reasonable computation time. The gaps between optimal solution and lower bound are no more than 0.0395% for the linear case and 0.0315% for the nonlinear case, respectively.
Water jet impingement onto a moving steel strip surface was numerically simulated using a three-dimensional computational model. A continuous supply of cooling water causes residual water to accumulate on the moving strip surface, which reaches a certain water level according to the flow rate. Because this residual water significantly affects the water jet impingement and strip cooling, it is essential to accurately reflect its level in a simulation of strip cooling. In this study, we developed the concept of artificial sidewalls to apply a more realistic residual water level to a cooling simulation. For various water jet flow rates and longitudinal domain sizes, the residual water levels and impinging pressures were compared in cases with and without these artificial sidewalls.
The evolution of microstructure and texture during isothermal annealing of a heavily warm-rolled duplex steel (DSS) was studied. For this purpose a DSS steel was hot rolled, homogenized at 1448 K and subsequently warm-rolled to 90% reduction in thickness at 698 K and 898 K. This was followed by isothermal annealing at 1448 K for different time intervals up to 7200 seconds. Lamellar microstructure with alternate arrangement of deformed ferrite and austenite bands was observed in the as warm-rolled condition. The ferrite in DSS warm-rolled at 698 K showed much stronger α-fiber (Rolling direction (RD)//<110>) than γ-fiber (Normal direction (ND)//<111>) as compared to the ferrite in DSS warm-rolled at 898 K. The austenite in warm-rolled DSS showed a predominantly pure metal (or copper type) texture. Upon annealing the lamellar morphology of the as warm-rolled structure transformed into a bamboo type morphology for short annealing time but finally broke down with increasing isothermal annealing time due to mutual interpenetration of the two phases. Retention of deformation texture components and presence of annealing twins in austenite indicated discontinuous recrystallization. Despite differences of texture in the as warm-rolled condition the ferrite in annealed DSS showed much stronger α-fiber as compared to γ-fiber due to strong recovery behavior of ferrite during annealing. The texture evolution in the two phases was not affected by the presence of the other phase while grain growth was significantly restricted due to the presence of the other phase.
The surfaces of dies used for flat sliding tests of galvannealed steel sheet (GA) have been investigated using several electron microscopic techniques in order to clarify the adhesion mechanism. Two kinds of adhesive materials were identified on the die surface. One consists of an Al oxide – Fe–Zn alloy composite and tightly bonds to ridges of the die surface. This resembles the built-up edges formed on tool surfaces during metal-cutting operations. Another adhesive material is composed of Fe–Zn alloys located in the hollows of the die surface which do not bond or only loosely bond to the die surface. A new adhesion model consisting three steps is proposed; (1) the Al oxide – Fe–Zn alloy composite layers are formed on the ridges of the die surface. (2) The Fe–Zn intermetallics on the GA surfaces are cut by this layer and (3) accumulate in the hollow of the die surface.
This work studies the sliding wear behavior of PVD coated austempered ductile iron samples. The effects of the substrate surface finishing method (grinding and polishing) and coating material (CrN and TiN) on the wear behavior are evaluated. Coatings were applied in an industrial reactor. Deposition times were adjusted to obtain similar film thicknesses in both coating materials. Wear tests under dry sliding conditions were carried out with a pin–on–disc tribometer (ASTM G99). The steady–state friction coefficient and wear rate were calculated for each sample variant. The wear track of the discs was examined by using optical microscopy and stylus profilometry. The results obtained indicate that the uncoated and TiN coated samples show steady–state friction coefficients close to 0.8, while the CrN coated samples show steady–state values close to 0.4. The sliding wear tests do not produce the fracture and/or delamination of the films in any case. The specific wear rate of the CrN and TiN coated samples is close to zero, while that of the uncoated samples is higher. The wear rate of the uncoated samples is slightly higher for the ground ones. The specific wear rate of the pins (AISI 52100 bearing balls) is higher than that of the discs in all the cases. The wear rate of the pins tested against uncoated samples is higher for the ground ones. The wear rate of the pins tested against coated samples is higher for the polished and TiN coated ones.
Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve product performance and to reduce manufacturing cost. Pre-metered methods such as slot, slide and curtain coating are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, the coating gap and consequently the final film thickness deposited on the web will also be non-uniform. In this study we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane covered roll was visualized in order to find out how the uniformity of two layer coating is affected by the speed ratio between two rolls, layers wet thickness and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in two layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.
Formation of the reversed austenite by intercritical annealing has been studied in the 0.2C–5Mn steel by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the volume fraction of reversed austenite increases solely with increasing annealing temperature. While the volume fraction of retained austenite begins to go up around 575°C and exhibits a maximum value of 33.0% at 650°C; and when above 650°C, the retained austenite volume fraction decreases with increasing annealing temperature. At the same time, the concentrations of C and Mn in austenite which contribute a lot to the thermal stabilization of austenite are investigated to explain the variation of retained austenite volume fraction. And the microstructure evolution of different annealing temperatures is also observed.
The effect of the addition of 0.042 wt.% of titanium on the relation between the evolution of the microstructure and the softening kinetics of quenched martensite in high-purity Fe–C–Mn steel has been studied during tempering at 300 and 550°C. The evolution of the microstructure is characterized by measuring the cementite particle size, the martensite block size, the area fraction of martensite regions which contain a high dislocation density, the macroscopic hardness, the nano-hardness of martensite blocks boundaries, the nano-hardness of the matrix and the TiC-precipitate size during tempering. Nucleation of TiC-precipitates take place during annealing at 550°C and starts earlier in regions close to the block boundaries, after 5–10 minutes, and thereafter in the matrix, after 10–30 minutes, due to the higher dislocation density in the regions close to the block boundaries. The TiC-precipitates slow down the recovery in regions of high dislocation density compared to the alloy without TiC-precipitates. The TiC-precipitates increase the macroscopic hardness of the steel after 30 minutes annealing at 550°C. The growth of TiC-precipitates in martensite is simulated in good agreement with experimental observations by a model that takes into account: 1) capillarity effects, 2) the overlap of the titanium diffusion fields between TiC-precipitates, and 3) the effect of pipe diffusion of titanium atoms via multiple dislocations. The average, experimentally observed, TiC-precipitate size is 69 ± 48 Ti atoms.
Quenched and Partitioned (Q&P) steels have been shown in previous literature to exhibit attractive mechanical properties due to austenite and martensite present in the microstructure. However, questions have remained regarding the mechanisms at work during the partition step. In this work, x-ray diffraction and atom probe tomography were utilized to gain insights into phenomena that occur (such as carbon partitioning from martensite to austenite, carbon atom clustering/dislocation trapping, and carbide formation). Evidence of all of these mechanisms was observed.
The effect of the sulfur content on the inclusion and microstructure characteristics of steels where Ti2O3 and TiO2 have been added was studied. Based on the microscopic examinations, it is found in the steel samples with Ti2O3 additions that the area fraction of intragranular ferrite decreases from 52.68% to 39.09% as the sulfur content increases from 0.009 mass.% to 0.030 mass.%. In the steel samples with TiO2 additions, this value also decreases from 49.05% to 36.26% as the sulfur content increases. The nucleant inclusion was identified as a TiOx+MnS phase based on SEM-EDS measurements as well as on equilibrium calculations with thermodynamic calculation software, Thermo-Calc. Also, TiOx was found to be the nucleation site for an intragranular ferrite formation. Moreover, the nucleation probability increases with an increased inclusion size. It is also noted that the nucleation probability decreases slightly with an increased sulfur content. The minimum size of TiOx+MnS inclusions for an IGF nucleation is about 0.85 μm in the present samples. Furthermore, this minimum size of TiOx inclusions is shifted to a size of about 0.5 μm by excluding the depth of a MnS layer. In addition, the effective nucleation size range of TiOx inclusions in the steels, where Ti2O3 and TiO2 had been added, is smaller than that of TiN+Mn–Al–Si–Ti–O inclusions in steel samples where TiN had been added.
Deformation-induced ferrite transformation (DIFT) is one of the most effective ways of refining ferrite grains in steel. In this study, we employed a multi-phase-field (MPF) model to simulate both variations in macroscopic flow stress and microstructural evolution during DIFT. Using the MPF model, two-dimensional simulations of DIFT in a Fe–C alloy were performed to investigate the effects of strain rate, austenite grain size, and dynamic recrystallization (DRX) of the ferrite phase on flow stress curve and ferrite grain size. The results demonstrated that increasing the rate of ferrite nucleation by increasing the strain rate and reducing the austenite grain size is essential to obtaining fine-grained ferrite. The results of the simulations also indicated that it is important to reduce the interfacial mobility and increase the nucleation rate of the ferrite grains subjected to DRX in order to obtain ultrafine-grained ferrite by DIFT when it is accompanied by DRX of the ferrite phase. Thus, the MPF model is an effective tool for elucidating the correlation between the variation in the flow stress and the evolution of the ferrite grains during DIFT.
Effects of temperature, strain, strain rate and cooling rate on austenite grain size were investigated at first. The results show that by increasing the strain from 0.0 to 0.5 can significantly refine austenite grains for different deformation temperatures studied. However, by increasing the strain from 0.5 to 0.8 can not continue to refine austenite grains for higher deformation temperatures of 1100 and 1150°C, while austenite grains can be further refined for lower deformation temperature of 1000°C. The austenite grain size is proportional to ( is strain rate and p is the strain rate exponent) and v–q (v is cooling rate and q is cooling rate exponent). Moreover, using ultra fast cooling after hot rolling can significantly refine austenite grains. Then a low-carbon bainite steel with yield strength of 811 MPa and ductile-brittle transition temperature (DBTT) of –49°C produced by optimum recrystallization controlled rolling designed based on thermosimulation results and ultra fast cooling. The microstructural characteristics, mechanical properties and the mechanism of toughening were investigated in details. On the one hand, fine austenite grains with equivalent diameter of 16.5 μm is obtained by recrystallization repeated and suppressing recrystallized austenite grain growth using ultra fast cooling. On the other hand, recrystallized austenite grains are divided by fine bainite blocks with higher misorientation between each other. So the fine bainite microstructure is obtained. In addition, Prior austenite grain boundaries and bainite packet and block boundaries with higher misorientation can effectively arrest cracks propagation, resulting in better low-temperature toughness.