This paper deals with the presence of High Density Inclusions (HDI) in VAR melted titanium ingots. For performance and economical reasons, the elimination of these inclusions is of utmost importance for the titanium industry. However, very few studies have considered dissolution aspects of HDIs and accurate data on their dissolution rates still lack in the literature. In the present study, we investigate the mass transport driven dissolution of some HDIs (tungsten and molybdenum) in CPTi, Ti64 and Ti17 baths. This has been done by allowing the partial dissolution of cylindrical rods in molten titanium for various controlled periods of time. Dissolution rates have been determined by measuring the dimensions of these samples before and after the experiments. In some cases, the chemical composition of the solidified bath near the sample has also been measured by Scanning Electron Microscope. It has been evidenced that the dissolution kinetics depends highly on the liquid metal agitation and temperature. The results also revealed that the dissolution of both tungsten and molybdenum is higher in pure titanium than in the investigated alloys. A numerical model describing the mass transport driven dissolution was used to determine dissolution rates numerically and to compare them to experimental results.
Mechanical stirring is widely used for hot metal desulfurization in Japan. In this process, solid lime is added as flux and emulsified into molten iron using a vortex formed by the stirrer. However, in addition to the added solid lime, the liquid top slag on the ladle is emulsified and forms granules. To clarify the roles of the solid lime and liquid slag in hot metal desulfurization, the reaction rates of slag, solid lime, and slag with solid lime are determined and the interfacial layers are observed. The results are summarized as follows: (1) The desulfurization rate is very slow when a solid lime rod is immersed into hot metal without slag. The reason for the slow desulfurization is the formation of an interfacial layer, which inhibits the mass transfer of sulfur. (2) Because sulfur is not detected inside the solid CaO, the mass transfer of sulfur from the liquid slag to the solid CaO does not occur. Therefore, it is believed that solid CaO does not play a direct role in the desulfurization reaction, and thus, the reaction is solely due to the liquid slag. A good relationship between the desulfurization rates and the sulfide capacity of the liquid slag is found. (3) When tricalcium aluminate forms at the interface, the desulfurization rate is increased by the immersion of a CaO rod. This is due to the supply of CaO to the slag because this interfacial layer does not inhibit the mass transfer.
The oxidative removal of Cu from carbon-saturated iron via the Ag phase into B2O3 flux was attempted at 1523 K. The Cu content was reduced from 4 to below 0.2 mass%, and Cu in the molten iron could be removed into the B2O3 flux by the method proposed in this study. Furthermore, the Cu distribution ratio between the B2O3 flux and Ag, LCu(flux-Ag) (= [mass% Cu](in flux)/[mass% Cu](in Ag)) and the activity coefficients of Cu2O in the B2O3 flux, γCu2O(in flux), owing to the oxygen partial pressure were measured. The greatest value of LCu(flux-Ag) was found to be 17 at 0.6 atm of oxygen partial pressure. Using this value, the distribution ratio of Cu between the B2O3 flux and carbon-saturated iron is calculated to be 120 at 1523 K. To predict the behavior of solute elements in the Ag phase, a numerical calculation of mass transfer based on Fick's second law is performed and the optimum execution method of oxidative removal of Cu is discussed.
To meet the unprecedented demand of environmental issues and tightened production cost, steel industry must develop the disruptively innovative process. In the present study, totally new steelmaking process of ‘Solid State Steelmaking’ (or S3 process) without BOF process or liquid state oxidation process is proposed. The overview of the new process is as follows: (1) High carbon liquid iron from the ironmaking processes is directly solidified by using a strip casting process to produce high carbon thin sheets. (2) Then, the produced cast iron sheet is decarburized by introducing oxidizing gas of H2O or CO2 in a continuous annealing line to produce low carbon steel sheets. The most beneficial aspect of the S3 process is the elimination of several steps such as BOF, and secondary refinement processes and no formation of inclusions. To investigate the feasibility of S3 process, the cast iron strips with various high carbon content produced by a centrifugal slip casting method are decarburized at 1248 K and 1373 K by using H2O–H2 gas mixture and its kinetics of the decarburization is investigated. In the decarburization process, the carbon diffusion through the decarburized austenite phase but not the decomposition of cementite is the rate controlling step of the decarburizing process. It is found that 0.5 mass% C sheets can be produced from 3.89 mass% C sheets with the thickness of 1.0 mm within 30 min at 1373 K. Based on these results, S3 process is confirmed to be feasible as an alternative low cost steelmaking process although the further improvement of the process will be necessary.
An integrated steel plant with two blast furnaces with the option to use biomass to partially substitute fossil reductants was simulated. A thermodynamic blast furnace model was used, combined with simpler models of the other unit processes (sintermaking, cokemaking, basic oxygen furnace, hot stoves and power plant) and a nonlinear model of the biomass conversion with respect to the processing temperature. Given an aim steel production rate for the plant, the economics of the plant was optimized by minimizing the specific costs of liquid steel, considering costs of raw materials, energy and CO2 emissions. Limited supply of sinter and coke was optimally allocated to the two blast furnaces and the effects of restrictions in the biomass, oxygen and oil supply on the operating states were studied. An analysis was also undertaken to study how the production rate of the plant would affect the optimal state. The results demonstrate that a non-uniform distribution of the resources can be economically justified, in particular for cases where the blast furnaces operate under different constraints.
Ludwigite is the most abundant boron resource in China. In order to fully utilize the complex ore and meet the boron and iron consumption of the country, a new process was proposed in this research. Fine ludwigite and pulverized coal were pelletized and then reduced isothermally at high temperatures. The influence of reduction conditions, such as reduction temperature and C/O (mole ratio), on the metallization degree and melting morphology of the samples, were studied in detail. The pellet could not melt well at higher or lower temperature than 1673 K. The increase in C/O could improve the reduction and melting rate of the pellet. The appropriate temperature and C/O were 1673 K and 1.2 in the work. When the pellet was reduced at 1673 K for 12 min, the iron and slag separated in a clean manner. The apparent activation energy of reduction stage was 180.17 kJ/mol. The boron content of nugget and boron oxide (B2O3) content of the slag were 0.065% and 20.01%, respectively. In addition, the microstructure, phase transformation and phases composition of the samples were characterized by means of XRD, SEM, EDS and optical microscope. The efficiency of extraction of boron (EEB) of the slag was 86.46% when it was cooled in a slow method. The boron-bearing nugget and boron-rich slag were good raw materials for steelmaking and borax production.
This paper presents a new approach towards the evolution of non-metallic inclusion (NMI) populations in Ti-alloyed Al-killed steels, based on an extensive inclusion analysis campaign at Tata Steel Europe, IJmuiden Works. Automated SEM techniques were used to characterize the inclusion populations in 120 steel samples taken from nine heats out of two casting series of this steel grade. As NMI in Ti-alloyed Al-killed steels are overwhelmingly dominated by chemically simple Al2O3, most of the process relevant information lies in the analysis of particle size distribution during the secondary steelmaking process. The population density function (PDF) concept was applied, for the first time, to the characterization of inclusion size distributions sampled from secondary steelmaking practice. Two size distribution forms predominate in the entire dataset: i) Lognormal size distributions associated with active nucleation and growth of alumina (deoxidation and reoxidation), indicating net transfer of matter between NMI and solutes in liquid steel and ii) Power-law size distributions, associated with an inclusion population in chemical equilibrium with the melt and subject to collision/breakup processes controlling the distributions. Based on inclusion PDF observations, it is found that the size distribution of alumina inclusions suspended in steel melt, after equilibration and effective float out of large inclusions, tends to approach a Reference Distribution of power-law type function (f(r) = a ⋅ r–3.5) that appears to be a fundamental feature of the alumina-steel system. This Reference Distribution can guide efforts to improve and engineer inclusion populations for a better controlled steel product.
The corrosion behavior of MgO–C refractory used in oxygen blowing process in the manufacture of ferromanganese was investigated for different slag compositions. Finger Rotating Test (FRT) was introduced to observe the dependence of the corrosion behavior on various conditions: rotation speed, immersion time, slag, basicity, temperature and slag composition. As a result, the corrosion rate of MgO–C refractory increases with increasing time and rotation speed. Also, the strong corrosion of MgO–C refractory occurred when MnO is included in the slag composition. A kinetic study has been carried out with different slag contents.
The viscous behavior of the CaO–Al2O3–12 mass%Na2O–12 mass%CaF2 based slag system with various concentrations of Li2O has been studied using the rotating spindle method to understand the effects on the viscosity with these additives. Li2O additions up to 1 mass% significantly lowered the viscosity by breaking the [AlO4]-tetrahedral network structure of molten fluxes, but Li2O concentrations above this level was comparatively less effective in lowering the viscosity. The viscosity was lower at higher temperatures and from an Arrhenius relationship, the activation energy was calculated to be between approximately 180 to 190 kJ/mol. Fourier Transform Infra-Red (FTIR) analysis of as-quenched slag samples showed the characteristic Al–O stretching vibration in the wavenumber of 800 cm–1 and 660 cm–1. Slags with higher viscosity showed a slightly wider trough near the 800 cm–1 and 660 cm–1 Al–O stretching bands. XPS analysis of slags with various concentrations of Li2O indicated the fraction of bridged oxygen (Oo) decreased and the non-bridged oxygen (O–) increased with higher concentrations of Li2O. Li2O additions up to 1 mass% significantly decreased the bridged oxygen fraction, which seem to correlate well with the viscosity measurements.
The dissolution of δ-ferrite in continuously cast slabs of SUS304 has been studied during heat treatment in the temperature range of 1373 to 1473 K. The dissolution behavior can be expressed by Kolmogorov–Johnson–Mehl–Avrami equation. The dissolution rate is affected by annealing temperature and secondary dendrite arm spacing. Moreover, the numerical methodology for multi-phase field method has been performed in Fe–Cr–Ni alloy. It is thought that dissolution behavior of δ-ferrite during heat treatment can be also predicted by this method.
An integrated Case Based Reasoning model is proposed to predict the endpoint temperature of molten steel in AOD. Case's problem part is represented by a set of feature attributes and a set of state attributes. The boundary value of state attributes can be obtained by M5P. Case similarity is computed based on Grey Relational Degree with different weights of attributes in order to solve the problem of obtaining the accurate results with incomplete information. Entropy Weight Method is adopted to determine the weights of attributes. A two-step case retrieval, composed of rough search and delicate search, is provided to decrease the search time greatly. Seven methods, Multiple Linear Regression, M5P, Artificial Neural Network, Cased Based Reasoning with case similarity based on Euclidean Distance and Equal Weights of attributes (CBR_ED_EW), Cased Based Reasoning with case similarity based on Euclidean Distance and Different Weights of attributes (CBR_ED_DW), Cased Based Reasoning with case similarity based on Grey Relational Degree and Equal Weights of attributes (CBR_GRD_EW) and Cased Based Reasoning with case similarity based on Grey Relational Degree and Different Weights of attributes (CBR_GRD_DW), are employed for a comparison. The results show that CBR_GRD_DW outperforms the other methods, and the integrated Case Based Reasoning model is effective in predicting the endpoint temperature of molten steel in AOD.
For the precise control of tension and thickness in a tandem cold mill during acceleration and deceleration, we have developed a nonlinear model including the rolling speed as a time-varying parameter. It is demonstrated from simulation results that nonlinear receding horizon control by real-time optimization results in a satisfactory performance for the nonlinear model.
Blast Furnace (BF) Gas Top pressure Recovery Turbine (TRT) unit is the most popular energy-saving equipment in iron making process. It is desired to control top pressure of BF precisely and reliably after installing TRT. The dynamic mathematical model of top pressure with TRT is the key point of controller design. However, it is difficult to establish the dynamic mathematical model of TRT from first principles. In this paper, the dynamic mathematical model of No. 1 TRT in Hangzhou Iron & Steel Group Company is first estimated by subspace identification method. For safety reasons, we adopt closed-loop identification method. Then a model predictive controller is proposed based on the identified model. Results show that the identified model can capture the dynamic properties of top pressure process accurately. Moreover, the model predictive controller gives excellent performance.
Traces of Si(IV), P(V), and As(V) in an iron sample solution were converted into heteropoly molybdic acids at 0.5 M acidity. They were extracted into a tiny amount of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]), leaving the iron(III) matrix in the solution. The ionic liquid was diluted with methanol and analyzed by high-performance liquid chromatography with spectrophotometric detection at 310 nm. The distinct, well-separated peak was obtained for Si(IV), while the peaks of P(V) and As(V) were not separated from each other. To discriminate between P(V) and As(V), the extracted molybdoarsenic acid was decomposed with tetramethylammonium hydroxide before the HPLC analysis. The proposed method allowed the determination of Si, P, and As at low- to sub-μg g–1 levels in high-purity iron metals.
This study clarifies that the initial scale structure affects the transformation behavior of wüstite. The initial scale structure is controlled by limiting the temperature of nitrogen gas before the transformation of wüstite. The bilayer scale of magnetite and wüstite transforms from the magnetite/wüstite interface, while the monolayer scale of wüstite alone generates magnetite precipitates at the scale/steel interface preferentially. Furthermore, the monolayer scale takes longer to transform compared with the bilayer scale. These results indicate that the transformation behavior of wüstite can be controlled by the initial scale structure.
A novel swing arc system was developed to realize high quality narrow gap GMA welding at low cost. This system uses a motor of hollow axis to turn directly the micro-bent conductive rod and then to weave circularly the arc around itself axis of torch, and can automatically search for the midpoint of symmetrical swing trail and detect precisely swing frequency in real time. Three mathematical models are also presented to calculate precisely key swing variables from torch structure and process parameters. Experimental results show that weld surface curvature and the penetration into groove sidewalls increase and weld sectional thickness decreases with increasing swing frequency and at-sidewall staying time of arc, while bottom shape of bead varies from single to twin peaks. This swing arc process thus improved obviously narrow gap weld formation.
Iron coatings were prepared by electrodeposition in a bath containing citrate to improve the galvanizability of silicon-bearing steel. The iron coating electrodeposited from the bath containing citrate contains more than 4 mass% oxygen. This oxygen originates from the incorporation of the hydroxide of Fe3+ during electroplating. The oxygen-containing coating improves the galvanizability of the silicon-bearing steel by suppressing the surface segregation of silicon during annealing. This is because silicon in the steel reacts with oxygen in the oxygen-containing coating to form internal oxides during annealing.
This work studies the influence of the PVD processing parameters on the characteristics of TiN and CrN coatings deposited on ADI substrates, austempered at 360°C, with different nodule counts and surface roughnesses. Coatings were applied by arc ion plating using an industrial reactor and different sets of parameters, with BIAS voltages, arc currents, chamber pressures and substrate temperatures varying from –100 to –250 V, 60 to 65 A, 0.7 to 2.8 Pa and 280 to 450°C, respectively. The effect of the different depositions conditions on the substrates microstructure was also analyzed. The existing phases, preferred orientation, surface topography, film thickness, hardness and adhesion of each coating were determined. The retained austenite content and hardness of each substrate were computed before and after coating deposition. The results obtained indicate that the different deposition conditions and coating materials evaluated do not generate significant changes neither in the resulting topography nor in the coating adhesion, which can be related to indices between HF1 and HF2. Coating adhesion was not affected by different substrate roughnesses. The combined reduction of BIAS voltage, arc current and chamber pressure leads to a decrease of TiN growth rate and hardness, while high substrate temperatures promotes an increase in TiN and CrN growth rates. Substrate temperatures around 300°C with deposition times of up to 240 min do not promote noticeable changes on the ausferritic microstructure, while temperatures of 400°C and above translate into a clear microstructural deterioration, even for short deposition times.
Two stage pack cementation processes are developed to form dual Fe–Al–Cr layers on surfaces of SNCM439 steels. The first 550°C treatment assists to modulate adequate aluminum activity for the formation of iron-rich intermetallics. In the second 750°C treatment stage, simultaneous chromizing and aluminizing treatments are achieved by first forming a FeAl ferritic layer and then a surface layer with higher Cr content at later time. The current study examines the effects of second stage 750°C holding time, activator concentration, and Cr:Al ratio on coating structures. Fe–Al coatings consisting of Fe3Al and FeAl intermetallic phases are observed to form initially. This Fe–Al layer accounts for 25 to 32 μm thickness of the coatings and show good adherence with the substrates. The coating thickness increases parabolically with 750°C holding time. With prolonged treatment at 750°C, surface concentration of aluminum in powder packs drops with treatment time and increasing concentration of activator. A peak concentration exists at a depth below substrate surface. Aluminum is back diffused from the steel surface into the powder packs. The growth of Fe–Al intermetallics slows down. Surface layer then forms a thickness of 6 μm coating with 2–5 wt.% of chromium. Samples treated for longer than 6 h with over 12 wt.% of NH4Cl activator concentration or with Cr:Al ratio higher than 90:10 induce earlier chromium infusion and lead to porous coating structure due to Kirkendall effect. Eventually chromium carbide forms to cease further growth of the dual coating structures.
In the vacuum carburization of steels, short-time carburization is usually followed by a diffusion period to eliminate the filmy cementite (θ) grown on austenite (γ) grain boundaries. The surface C content in γ was found to increase past the solubility of θ due to radical reaction with decomposed hydro-carbon gas. The thermodynamic condition was recognized as the metastable equilibrium of supersaturated γ with graphite. Based on these results, a calculation model for not only C concentration profiles but also quantitative evaluation of grain-boundary θ was developed. The existing model estimates the amount of θ with the equilibrium fractions for local C contents in a framework of the finite difference method (FDM). However, with the corresponding surface condition of equilibrium γ plus θ with graphite, it overrates the amount of θ observed after several minutes of carburization. In the developed model, a parabolic law was assumed for the thickening of θ and a rate controlling process was considered for partitioning of Si. The local Si concentrations at the γ/θ interface were determined from the C isoactivity condition with supersaturated γ. The change in the rate coefficient (αSi) was also validated using multicomponent diffusion simulations. Coupled with the CALPHAD software, a one-dimensional FDM program calculates the increment of θ using the updated local equilibrium. Predictions of the C profile and θ volume fraction corresponded much better with the experimental results than existing models.
The stress exponent of the minimum creep rate, n, and the activation energy of creep, Qc, were obtained for the oxide dispersion-strengthened nickel-based superalloy MA754 by conducting creep tests at 1223–1273 K in the stress range of 130–190 MPa. The values of n and Qc in MA754 were determined to be 26 and 962 kJ/mol, respectively. The causes of these high values were determined by measuring the internal stress, σi, using the strain dip test. The ratio of σi to applied stress, σa, was very high at lower stresses, while at higher stresses, σi reached its saturated value with increasing stress. In this stress range, the ratio of σi to σa decreased drastically with increasing stress, which reflected the large increase in creep rate. Such a large increase in creep rate with increasing applied stress led to an unpredictably higher value of n. With increasing temperature, the saturated σi decreased, which resulted in a relatively large creep rate. The larger creep rate at high temperatures led to a large value of Qc. In addition, the dislocation density, ρ, of the interrupted creep specimens at the time of the minimum creep rate increased with increasing stress and reached the saturated value. This change in ρ with stress must be reflected in a large change in n and Qc through a change in σi.
Maraging steels are a group of materials with high strength and toughness. Titanium is the most widely used element in the above group, which helps to improve the mechanical properties by the formation of coherent Ni3Ti precipitates during the aging process. However, due to the high affinity of Ti to elements such as C, N, S and O, harmful inclusions of these elements may form in the steels if due care is not exercised during the processing. The present research explores the effects of titanium, within the range of 1–2 weight percent, on the mechanical properties of C300-grade maraging steels produced by vacuum arc remelting (VAR) technique. Then there is a discussion of the formation of deleterious titanium-rich inclusions and their effect on the mechanical properties of steel.
To satisfy the demand for high-strength steel sheets with excellent strength–ductility–hole expansion ratio balance, high-strength steels consisting of a ferrite or bainitic ferrite matrix strengthened by a large amount of finely dispersed precipitates have been developed. In this study, the influence of the precipitation-forming elements Ti and Nb on the strength–ductility–hole expansion ratio balance of these steels has been investigated. The strength–ductility–hole expansion ratio balance of steel with Ti added was superior to that with Nb added. The reason for the superiority of steel with added Ti was discussed from various viewpoints such as microstructure, texture, the condition of the pierced surface, sulphide formation, the nucleation and propagation behaviour of cracks during hole expansion, etc. We concluded that the inferior strength–ductility–hole expansion ratio balance of the steel with added Nb was mainly caused by the formation of large textural colonies and a detrimental influence of MnS. From these inferences, we proposed a combined addition of Ti and Nb, and succeeded in improving the strength–ductility–hole expansion ratio balance of the advanced high-strength steel.