Tramp Elements Research Articles

An investigation on effect of tramp elements and solidification processing on homogenization kinetics of AA 7xxx series aluminum alloys

AA 7068 Aluminum alloy is produced via conventional solidification (CS) and controlled diffusion solidification (CDS) processing and homogenized at 450 °C for 6 and 12 h. Optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectroscope (EDS) were used to examine the as-cast and homogenized samples. It was found that although the CS sample is more chemically homogenized in the as-solidified state, the CDS sample exhibits a higher homogenization rate during the treatment. The calculated phase diagram (CALPHAD) methodology was employed to study the effect of silicon and iron on the solidification path and homogenization of the alloy. Silicon consumes magnesium atoms on the surface of the α-Al phase to form the Mg2Si phase. Therefore, the magnesium concentration on the α-Al phase required for the formation of the σ phase must be increased. Calculations shows that all the Mg atoms at this stage are completely supplied from the bulk of the αAl phase. It can be hypothesized that during the last stage of solidification, a solid/liquid diffusion couple can be created between the primary αAl and the remaining liquid for several minutes (4 min at a cooling rate of 0.2 oC/s). As a result of the activation of this couple, the Mg atoms diffuse from the α-Al bulk to its surface, and vacancies diffuse from the surface to the bulk, forming a line of Kirkendall voids. With CDS processing, the surface of the primary phase is less Mg-concentrated than that in the CS process and this significantly reduces the formation of the Mg2Si phase. Conversely, the vacancy and Mg concentration of the αAl phase in CDS is higher than that of CS and this lets other alloying elements other than Mg diffuse to the primary phase, decreasing the homogenization time. The presence of Si in the alloy leads to the formation of the Mg2Si phase, which significantly reduces the final Mg concentration in the α-Al phase after full homogenization. In the pure alloy, the Mg concentration is 2.42 wt%, while in the FeSi-containing alloy, it is reduced to 1.58 wt% due to solid solution and dispersion strengthening mechanisms. Through CDS processing, the negative impact of Si can be reduced by disrupting the mechanism of Mg2Si formation.

On the Role of Tramp Elements for Surface Defect Formation in Continuous Casting of Steel

On the Role of Tramp Elements for Surface Defect Formation in Continuous Casting of Steel

The Melting Behavior of Hydrogen Direct Reduced Iron in Molten Steel and Slag: An Integrated Computational and Experimental Study

Direct reduced iron (DRI) and hot briquetted iron (HBI) are essential feedstocks for tramp element control in the electric arc furnace (EAF). Due to greenhouse gas (GHG) concerns related to CO2 emissions, hydrogen as a substitute for natural gas and a reductant in DRI production is being widely explored to reduce GHG emissions in ironmaking. This study examines the melting behavior of hydrogen DRI (H-DRI) pellets in the EAF containing low-carbon (0.1 wt.%) molten steel and molten slag. A computational heat transfer model was developed to predict the melting behavior of H-DRI pellets. To validate the model, a set of experimental laboratory simulations was conducted by immersing H-DRI in a molten steel bath and slag. The temperature history at the center of the pellet during melting and the shell thickness at different melting stages were utilized to validate the model. The simulation results agree with the experimental measurements of steel balls and H-DRI in different metallic molten steel and slag baths.

Influence of Tramp Elements on Surface Properties of Liquid Medium‐Carbon Steels

The transformation of the steel industry is inevitably linked to increased recycling rates of steel. However, end‐of‐life scrap is often contaminated with tramp elements like copper, molybdenum, and tin and while their influence on mechanical properties of steels is well described, their effect on nonmetallic inclusions remains largely unknown. Therefore, herein, two medium‐carbon steels are alloyed with up to 1 wt% of the listed tramp elements. The influence these elements have on the inclusion behavior in liquid steel is investigated using the sessile drop method in contact with alumina and zirconia as well as in steel/slag melting experiments to evaluate the formation and separation of new inclusions. Additionally, a semiempirical approach using CALPHAD to model the surface tension of steels with tramp elements is done. Copper and tin decrease the surface tension, while molybdenum increases the surface tension. It is demonstrated that most investigated alloys lead to a decrease of wetting angle as compared to the nonalloyed steels. The shown trends in the sessile drop experiments are lower contact angles with increased surface tension while a higher number of inclusions occurs with a decrease in surface tension. Thus, this study demonstrates that tramp elements affect the oxidic cleanness of steels.

Abatement of CO2 emissions by lowering hot metal ratio (HMR) in basic oxygen steelmaking (BOS) process

One of the most effective ways to reduce carbon dioxide (CO2) emissions in the oxygen steelmaking process is to lower the hot metal ratio (HMR) in the basic oxygen furnace (BOF). However, if the HMR is lowered, the amount of oxygen blowing during decarburisation in the BOF converter increases due to a lack of heat source. This reduces the cleanliness of the molten steel because the end point oxygen content in the molten steel increases. Hence, we sought to ensure the molten steel quality even at a decreased HMR of about 10%p compared to the current level to reduce carbon emissions in BOF steelmaking. An appropriate amount of silicon in hot metal was suggested to maintain the lowered HMR. In addition, a process optimisation for solving dephosphorisation as well as tramp elements problem was proposed even if the scrap charge increased. As a result, CO2 emissions from steel mills decreased from 2.20 t-CO2/t-steel to 2.06 t-CO2/t-steel.

Modeling solute drag during austenite–ferrite transformation with ab initio binding energies

The solute drag effect can have a considerable impact on the austenite to ferrite transformation. Most models describing the solute drag effect require knowledge of solute binding energies, which due to lack of accurate data typically are treated as fit parameters. In this study we use density functional theory (DFT) calculations to estimate the effective binding energies of the substitutional alloying elements in an ultra low carbon steel. For the first time these energies are used to calculate the solute drag effect considering site competition for the austenite to ferrite phase transformation. The depletion of elements in solid solution as a result from precipitation is computed and the solute drag contribution of each element is calculated. Comparison of the binding energies with the resulting solute drag effect to literature data shows reasonable agreement. The outlined approach points the way to future alloy development based on interface-controlled integrated computational materials engineering. Another field of application is given by the circular economy driven transition in the steel industry from the BF/BOF- to the EAF-based production route with increased utilization of scrap material introducing new tramp elements.

Electrorefining for copper tramp element removal from molten iron for green steelmaking

Steel recycling is crucial for lowering greenhouse gas emissions and reducing the pressure on natural resources within the manufacturing sector. Yet, the challenge of effectively dealing with impurities like carbon and tramp elements (copper), in scrap materials, persists as a major obstacle that current technologies fail to fully overcome. This study introduces an innovative oxysulfide electrolyte for electrorefining that significantly enhances steel recycling by effectively removing carbon and copper impurities from molten iron. This method not only achieves simultaneous extraction of these elements but also generates liquid iron and sulfur as by-products. By applying an electromotive force, this approach propels non-spontaneous electrochemical reactions, achieving refining efficiencies surpassing traditional methods. Results indicate a substantial reduction in carbon levels and copper to as low as 790 ppmw from 0.43 wt% Cu from molten iron can be achieved. The electrorefining process using this electrolyte offers a promising solution for improving scrap recycling in secondary steel production, highlighting its potential for reducing emissions and overcoming current technological limitations.

Influence of Surface Segregation of Tin on the Phosphatability of Cold‐Rolled Steel Sheets

The increasing use of iron scrap to reduce greenhouse gas emissions results in the enrichment of steel with the tramp element Sn. However, the effect of Sn on phosphatase activity remains unclear. Therefore, this study aims to investigate the effect of the Sn content on the phosphatase activity of cold‐rolled steel plates. The phosphatability of the cold‐rolled steel sheets is evaluated by adding several tens of ppm of Sn. The surface characteristics of the materials are analyzed using X‐Ray photoelectron spectroscopy and transmission electron microscopy. Additionally, the reactivities of the materials in the phosphate treatment solution are assessed using an electrochemical evaluation method. Notably, a significant decrease in phosphatase activity is observed, which is attributed to the inhibition of the phosphate reaction by the Sn oxide layers formed on the steel sheet surface, with a layer thickness of a few nanometers. These Sn surface oxides can be removed by pickling with HCl, and the initial phosphate reaction occurs rapidly in areas where the oxides are removed.

Methodology for qualifying the use of green steel in body components: A contribution to the life cycle engineering of steel

The use of materials with low environmental footprint will become more important for global car manufacturers in the future to comply the requirements and achieve the sustainability targets. The vehicle body of an electric vehicle is largely made of different steel grades and its production has a major impact on greenhouse gas emissions within the production stage. Future steel production will include a scrap-based electric arc furnace route. This route produces CO2-reduced steel products. However, these products are dependent on the scrap quality. Due to the high scrap content, impurities or tramp elements can be in the molten steel, reducing the steel end-quality or the properties of the final material and limiting reuse of the steel scrap. Therefore, a methodology is developed to perform an experimental analysis for data collection. Through this methodology, a recovery-process-structure-property correlation can be established and thus the influence of manufacturing-related impurities on the mechanical properties can be investigated. A case study shows initial research results using exemplary steel grades. The concept can examine the possibility of using steel with low carbon footprint to advance the life cycle engineering (LCE) idea in terms of recycling steel scrap and to reduce the environmental impact of body components by using CO2-reduced steel.

Influence of silicon and tramp elements on the high-temperature oxidation of steel in direct casting and rolling processes

Influence of silicon and tramp elements on the high-temperature oxidation of steel in direct casting and rolling processes

The Behavior of Phosphorus in the Hydrogen‐Based Direct Reduction—Smelter Ironmaking Route

Direct reduction (DR) nowadays relies on high‐grade iron ores, characterized by a high total iron content and low contents of tramp elements. Since their supply is limited and cost‐intensive, applying lower‐grade ores is a relevant topic for the future. Therefore, the behavior of phosphorus in unbeneficiated magnetitic ore during hydrogen‐based DR is studied. Phosphorus remains strongly bound as apatite whether raw or preoxidized fine ore is reduced in a fluidized bed or raw lump ore is reduced under shaft furnace conditions. That is an essential factor for the subsequent melting step, whose behavior is evaluated using thermodynamic calculations and published data from the literature. Although the smelter‐reducing conditions are not ideal for phosphorus withdrawal, one can expect it to be better than the blast furnace. Combining that with the outstanding dephosphorization capacity of the basic oxygen furnace (BOF), the route DR‐smelter‐BOF appears optimal for processing high‐phosphorus iron ores.

Sub-rapid solidified high copper-bearing steel with excellent resistance to hot shortness

The removal of accumulated impurity elements during scrap refining has been regarded as the bottleneck to limiting scrap usage, among which copper is the crucial tramp element in scrap steel recycling, resulting in the final product's hot shortness and surface cracking. This study proposes sub-rapid solidification-based strip casting technology to eliminate the copper impurity introduced problems. The results reveal that strip casting could improve copper behavior in the as-cast sample. More importantly, the sub-rapid solidified sample has excellent high-temperature tensile properties and resistance to hot shortness. It suggested that strip casting technology could produce copper-bearing steel, which can tackle the hot shortness problem and provide a unique idea for steelmaking with scrap as raw materials.

Development of Wet Chemical Analysis Technique for Tramp Elements (M = As, Pb, Sb, and Sn) in Silver-M Alloys

Wet chemical analysis techniques for four elements (M = As, Pb, Sb, and Sn) in Ag – M binary alloys were investigated with emphasis on the choice of solvent acid and the characteristic wavelength used in the ICP-AES (Inductively-Coupled Plasma Atomic Emission Spectrometer) analysis. The elements are representative tramp elements in ferrous scrap. The activity of these elements needs to be increased to remove them efficiently during the molten scrap refining process. The activity of these elements in the molten scrap (molten iron alloy) is usually measured by a chemical equilibration technique with molten Ag. Therefore, performing an accurate and reliable chemical analysis of these elements in the molten iron alloy and the molten Ag alloy is important. Preliminary tests using conventional acids (hydrochloric acid, nitric acid) resulted in unreliable results. In the present study, the proper choice of acids as solvents was investigated for each element M in the Ag-M alloys. Several synthesized Ag-M alloys of known compositions were analyzed using two ICP-AES systems independently, for cross-checking. As and Pb in Ag alloys could be successfully dissolved in the nitric acid-based solution. On the other hand, Sb and Sn in Ag alloys did not dissolve in the nitric acid-based solution completely, leaving some precipitates. It was found that the addition of hydrofluoric acid could resolve this problem. In addition to this, the effect of the mass of the Ag-M alloy and wavelength selection during ICP-AES analysis on the accuracy and the reproducibility were investigated. An optimized procedure for the wet chemical analysis of these elements in Ag-M alloys is reported.

Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV_{10}) of 320 J cm^{-2} compared to 66 J cm^{-2} in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

How much sorting is required for a circular low carbon aluminum economy?

Aluminum recycling follows a downcycling dynamic where wrought alloys are transformed into cast alloys, accumulating tramp elements at every cycle. With the saturation of stocks of aluminum and the reduction of the demand for cast alloy due to electrification of transport, improvement in the recycling system must be made to avoid a surplus of unused recycled aluminum, reduce the overall environmental impacts of the industry, and move toward a circular economy. We aim to evaluate the potential environmental benefits of improving sorting efforts by combining operations research, prospective material flow analysis, and life cycle assessment. An optimization defines the optimal sorting to minimize climate change impacts according to different sorting efforts, dismantling conditions, and collection rates. Results show how the improvement of sorting can reduce by around 30% the greenhouse gas emissions of the industry, notably by reducing unused scrap generation and increasing the recycled content of the flows that supply the demand of aluminum. The best performance is achievable with four different sorting pathways. Further improvements occur with a better dismantling and an increase of collection rates, but it requires more sorting pathways. Results point to different closed-loop recycling initiatives that should be promoted on priority in specific sectors, like the building and construction sector and the aluminum cans industry. To implement a better material circularity, the mobilization of different stakeholders is needed. From a wider perspective, the article shows how operations research can be used to project a circular future in a specific industry. This article met the requirements for a Gold–Gold JIE data openness badge described at http://jie.click/badges.

Evaporation of the tramp element antimony from molten steel under reduced pressure treatment

ABSTRACT The evaporation of antimony (Sb) in molten steel under reduced pressure is investigated with the gas phase pressure of 19–670 Pa, smelting temperature of 1823–1973 K and initial Sb content of 0.033–0.1 wt%. The findings indicate that Sb removal efficiency increases when smelting temperature rises or gas phase pressure decreases. However, the initial Sb content has little effect on the Sb removal. The Sb removal ratio can reach 94.1% within 40 min of treatment. The range of the apparent evaporation rate constant, k Sb, is 0.61 × 10−5 to 29.13 × 10−5 m s−1. The evaporation of Sb is limited by the gas phase mass transfer at 1903 K and 19–670 Pa. Additionally, the apparent activation energy of Sb evaporation, E Sb, is 128 kJ mol−1 at 226 Pa and 1823–1973 K, and the rate of the interfacial reaction severely restricts the evaporation of Sb.

Thermodynamic modeling of the Fe-Sn system including an experimental re-assessment of the liquid miscibility gap

The usage of low-grade ferrous scrap has increased over decades to decrease CO2 emissions and to produce steel products at a low cost. A serious problem in melting post-consumer scrap material is the accumulation of tramp elements, e.g., Cu and Sn, in the liquid steel. These tramp elements are difficult to remove during conventional steelmaking processes. Sn is considered as one of the most harmful tramp elements because, together with Cu, it sometimes induces the liquid metal embrittlement in high-temperature ferrous processing, e.g., continuous casting and hot rolling. Furthermore, the chemical interaction between Fe and Sn plays an important role in the Sn smelting process. The raw material used in the Sn smelting process is SnO2 (cassiterite), in which Fe3O4 is a gangue in the Sn ore. In the process, the reduction of Fe3O4 is unavoidable, which results in forming a Fe-Sn alloy (hardhead). The recirculation of the hardhead decreases the furnace capacity and increases the energy consumption in the smelting. The need to efficiently recover Sn from secondary resources is therefore inevitable. The CALculation of PHAse Diagrams (CALPHAD) approach helps to predict the equilibrium state of the multicomponent system. Previously reported studies of the Fe-Sn system show inconsistencies in the calculations and the experimental results. Mainly the miscibility gap in the liquid phase was under debate, as experimental data of the phase boundary are scattered. Experimental study and re-optimization of model parameters were carried out with emphasis on the correct shape of the miscibility gap. Three different experimental techniques were employed: differential scanning calorimetry, electromagnetic levitation, and contact angle measurement. The present thermodynamic model has higher accuracy in predicting the solubility of Sn in the body-centered cubic (bcc), compared to previous assessments. This is achieved by re-evaluating the Gibbs energies of the FeSn and FeSn2 compounds and the peritectic reaction related to Fe5Sn3. Also, the inconsistencies related to the miscibility gap around XSn = 0.31-0.81 were resolved. The database developed in the present study can contribute to the development of a large CALPHAD database containing tramp elements.

Effect of Bi and Ca on the Solidification Parameters of Sr-Modified Al-S-Cu (Mg) Alloys.

The effect of tramp elements, mainly Bi and Ca, on the thermal characteristics of Sr-modified Al–Si–Cu and Al–Si–Cu–Mg alloys has been investigated using thermal analysis, X-ray radiography, and field emission scanning electron microscopy (FESEM) techniques. The high affinity of Bi to interact with Sr results in an increase in the Al-Si eutectic temperature, and hence an increase in the size of eutectic silicon particles. In contrast, the Ca–Sr interaction seems to have no significant effect on the alloy thermal behavior. The effect of these interactions on porosity formation has been discussed. Hot zones may be formed in thin cavities, in particular, near the bottom of the mold, leading to formation of unexpected coarse porosity, mostly shrinkage type. The study also highlights the significance of other parameters on porosity formation, such as no melt degassing, SrO, Al2O3 (strings or bifilms), as well as the presence of iron-based intermetallics.

Analysis of grain boundary embrittlement by Cu and Sn in paramagnetic γ−Fe by first-principles computational tensile test

Ironmaking processes using steel scrap are becoming increasingly important in the effort to realize a decarbonized society, but their utilization is limited by the surface hot shortness related to cracking at paramagnetic \ensuremath{\gamma}-Fe grain boundaries (GBs) due to tramp elements such as Cu and Sn. To understand its electronic origin, GB embrittlement due to Cu and Sn was investigated by first-principles computational tensile tests (FPCTTs) on paramagnetic \ensuremath{\gamma}-Fe GBs. The paramagnetic \ensuremath{\gamma}-Fe GB was modeled as the $\mathrm{\ensuremath{\Sigma}}5(310)\phantom{\rule{0.28em}{0ex}}\mathrm{GB}$ in the antiferromagnetic double-layer (AFMD) configuration. Both Cu and Sn reduce the fracture stress, fracture energy, and fracture strain of the \ensuremath{\gamma}-Fe GB. The effect of Sn was more pronounced than that of Cu, which agrees well with experimental results on surface hot shortness. Crystal orbital Hamiltonian population (COHP) analysis of the electronic structure during the tensile processes revealed that the atomic orbitals of Cu are more localized than those of Fe in \ensuremath{\gamma}-Fe, resulting in the breaking of Cu--Fe bonds at a lower strain and stress, which leads to GB embrittlement. In addition to this effect, Sn increases the bond lengths between adjacent Fe atoms and weakens the Fe--Fe bonds, causing significant GB embrittlement. The obtained knowledge contributes to the elucidation of the mechanism of the surface hot shortness. Furthermore, the FPCTT using the \ensuremath{\gamma}-Fe GBs in the AFMD configuration is useful to investigate the effects of various solute elements on paramagnetic \ensuremath{\gamma}-Fe GBs, and is expected to be utilized to improve material properties governed by GB fracture.

Role of phosphorus as micro alloying element and its effect on corrosion characteristics of steel rebars in concrete environment

This communication reports the effect of phosphorus (P) added in micro concentration range in steel on kinetics, mechanism and growth of passive film in contact of chloride contaminated concrete. Electrochemical impedance spectroscopy, direct-current polarization, mass loss and Raman spectroscopic techniques were used to arrive at the findings. The results showed that an intentional addition of P in steel (0.064%) makes it more prone to uniform and localized corrosion (about 1.1 and 1.7 times) than the steel having low phosphorus (< 0.016%, present as tramp element) exposed under wet/dry conditions in simulated pore solution added with chloride and in the absence of this ion. A similar effect is also noted for the rebars embedded in mortars. Identification of corrosion products formed on steel rebars surface by Raman spectroscopy reveals thermodynamically stable maghemite and goethite phases on the surface of low P content steel. Unstable phase of lepidocrocite is recorded on the surface of higher phosphorus steel rebars. The findings are discussed with experimental evidence and taking clues from the published literature to arrive at plausible mechanism for this behaviour.