Effect of residual elements from scraps on steel processing and service properties of typical steel grades

Lead Research Organisation: University of Warwick
Department Name: WMG

Abstract

A significantly increased use of scrap in steel manufacturing process has been widely recognized as a low CO2 production pathway, which is in particular attractive for the UK steel industry considering the over-supply of steel scrap in the UK and its projected growth in quantity in the 2020's. However, the quality (e.g. residual elements) and applicability of the scraps to the specific use of steel is not clear, while out-of-spec impurities in scraps are found having detrimental effects on the steel processing and the quality of the steel produced.
This project aims to study the influence of residual elements (Cu, Co, and their combination with other impurities Ni, Cr, As, Sn, Sb, Mo, etc) on the processability and properties (e.g. hot shortness, descaling, strength, ductility, surface quality) of typical steel grades identified with industry partner Tata Steel. The work will be carried out by using the world leading research facilities at the Advanced Steel Research Centre funded by EPSRC and HEFCE at WMG, the University of Warwick.
This is an EPSRC/Tata Steel iCASE studentship, and the project align well with the EPSRC research areas of "Materials Engineering - Metals & Alloys" and "Manufacturing Technologies".

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/T517641/1 30/09/2019 31/01/2026
2267821 Studentship EP/T517641/1 30/09/2019 29/09/2023 Prince Svipira
 
Description [1] Knowledge GAP Objective - MSWI Scrap Steel Characterisation Conclusion

Incinerator scrap surface oxidation is composed of iron oxide hydroxyl compounds, and the bulk texture is acidic due to the presence of moisture/water vapor during the corrosion process. Three main hydroxyl oxides are formed at 900 degrees of incineration (abundant moisture and hydrochloric acid presence) namely Goethite, Maghemite, and Lepidocrocite. 6 total oxide phases are identified as Goethite, Lepidocrocite, Maghemite, Wustite, Magnetite, and Hematite. The latter 3 are high-temperature oxides formed in limited or dry moisture conditions. Trace elements also aid in producing oxides in local sites. Detection of other hydroxyl-based oxides from elements such as aluminum to form Gibbsite and Sillimanite is also observed. Current research proposes 3 main iron oxide phases for clean steals over the last 4 to 5 decades, this research would like to emphasize a contradiction in relation to only 3 oxides existing especially for the iron ores containing acidic red oxides that for chloride or hydroxyl compounds suggesting most oxidation-corrosion mechanisms observed involve a hybrid mixture of iron oxides and iron oxyhydroxides. Oxide formulae for municipal solid waste incinerator scrap are as follows oxyhydroxides (a-FeOOH) Goethite, , lepidocrocite (?-FeOOH), and maghemite ?-Fe2O3. The bulk iron oxide formed is , wüstite FeO, magnetite Fe3O4 and hematite Fe2O3. In conclusion of chapter 3.2.1.2, MSWI surface texture is of red appearance meaning Goethite is a common outer oxide formed in moisture-rich environments as opposed to hematite-formed in oxidising dry environments with oxygen only. The hierarchy of the inner layers of the surface oxides is fundamentally dependent on the stoichiometry ratios of molecules. Goethite (1st layer) , Lepidocrocite (2nd layer) and Maghemite (3RD layer), Hematite (4th layer), Magnetite (5th layer) and Wustite (6th layer). XRD is not yet able to list all oxides in hierarchical order until a solid research database is produced in relation to displacement reactions of surface metallic oxides and metallic oxyhydroxides.

[2] Knowledge gap objective - BOS Furnace Melting Behavior Conclusion

Copper % increase results in high oxidation activity at the surface this is observed by the sudden increase in thickness of hematite, wustite, and Magnetite. Average oxide scale growth is observed to increase to nearly double the value 2.36x the value of 0.005% copper. Copper increase in steel lifecycle increases the oxidation of low-carbon steel. It is also worth noting the copper trace element is normally detected at the outer surface region using SEM-EDS due to a larger electronegative force controlled by electron density. The thermodynamic interactions induce segregation at the upper surface of low-carbon steel. As the copper % increases more oxygen is ionised to form (wustite, hematite, and magnetite) layers, as a result, the oxide scale thickness and weight gain increase respectively to each other. The same is also observed for the thickness interface layers of Wustite and Magnetite. Copper and Zinc have many implications on the primary-secondary metallurgical processes they both affect melting temperature and decrease the solubility of other residual elements during heat treatment as such the hot shortness is more prone to increase. Both the nucleation rate and growth rate of ferrite are decreased by the addition of copper, and this is considered to be caused by the reduction in austenite grain boundary energy due to the segregation of copper and the solute drag effect by copper. Zinc is present in municipal solid waste incinerator steel scraps as a waste product from galvanized steel products. The removal of Zinc in low-carbon steel is limited by the chlorine availability suggesting this impurity is already present in large concentrations as zinc ferrite (Izan.Jaafar.et.al). Equilibrium displacement reactions lead to a similar conclusion: as long as HCl or Cl2 is available. Its evident that strong reducing or oxidising agents controlling the amount of zinc dust escaping into the Basic Oxygen furnace, and catalyst radicals such as chlorine increase the rate of chemisorption. The concentration of Zinc in Municipal Solid Waste fly ash is almost 10-fold compared to factory scraps, given Zinc is semi-volatile, chemical differences in the physical properties of the fly ash are observed from XRF, WDXRF, SEM-EDS, and XRD results where 14-21 residual elements are detected.

[3] Knowledge gap - BOS Furnace Melting Behavior Conclusion

A simulation using a computational fluid dynamics module in COMSOL is conducted for an ingot of diameter 30mm and length 170mm which is charged into the BOS furnace to measure the temperature of the ingot over a period of 20 minutes at a temperature of 1800 Kelvin. Results highlight high exothermic heat release shift due to an increase in chlorine and Iron oxyhydroxide impurities on the low carbon steels. The heat signatures obtained from both graphs demonstrate increased exothermic heat release due to the conduction and convection of heat. For Iron-Oxide 60.28 (J/mol*K) and Iron-Oxyhydroxide (heat capacity), 100.671 (J/mol*K) an increase in temperature by a magnitude of greater than 400-degree fold between non-oxidized and oxidized samples is observed. A knowledge gap in Multiphase modeling of scraps (Municipal solid waste low carbon steel scraps) of various compositions melting in an open furnace leaves the industry wondering if current blast furnaces and basic oxygen furnaces can handle the temperature increase (<2000 degrees celsius). Incinerator scrap surfaces are oxidised with unique oxide phases namely oxyhydroxides Goethite, Maghemite, Lepidocrocite, and non-oxyhydroxides Wustite, Hematite, and Magnetite. The bulk oxides are Magnetite and Maghemite. Iron oxide is the bulk composition measured across all samples and XRD results are in assessment to determine if the oxides formed are Magnetite (Fe3O4) , Maghemite (?-Fe2O3), Goethite (a-FeOOH), Hematite (a-Fe2O3) and Wustite (FeO). WDXRF-XRF indicates the presence of both acidic and basic oxides in MSWI scrap , the residual elements are Fe (98.940 to 99.435 ), C (0.02) ,Al (0.04) , Mn (0.5) and Cu (0.005). Thermochemistry properties of chlorine create acidic conditions of iron - oxyhydroxides (hydrochloric acid < 37.5%) to form surface red oxides. The furnace temperature increases from 1100 degrees ? to 1500 ? and is recorded as the baseline value meaning for a basic iron oxide thrown into the furnace for a duration of 60 seconds, the temperature increases by 340 ? due to the exothermic nature of the oxides.
Exploitation Route 1.1 Residual Elements Impact on BOS Slopping and crude steel end Properties

Municipal solid waste steel scrap consists of battery chemicals (cobalt), electrical coating powders (zinc), carbon (from organics), chlorine (plastic HCl compounds), and silicates adsorbed from the incineration process. On the left-hand side surface oxide scale thickness growth (<1mm) for the combined iron oxide - oxyhydroxide is observed from the conventional high-temperature process. The EDS micrographs show <20 elements (on the surface and in the substrate) detected; no carbon is quantified due to being a lighter element >6 (atomic mass spectrometry). Energy Dispersive Spectroscopy is not able to detect elements lighter than 11. A potential problem that could arise in the use of MSWI BOS furnaces is the increase in carbon concentration because a lot more carbon could be escaping into the life cycle adsorbed onto alloy steel scraps. Carbon concentration increase in BOS furnace is associated with slopping, currently, there is a knowledge GAP in the BOS primary-secondary steel-making process.

1.2 Pyrolysis to produce Crude Steel Product

A knowledge gap exists in relation to why the steel industry can't utilise <50% scrap and hot melt per tonne perhaps this could be due to the limitations in energy produced from thermochemistry. An alternative route is pyrolysis which vaporises impurities leaving pure metals. With the increase in supply and demand across all industries due to urbanization aided by advances in engineering/manufacturing sectors incinerator, scrap becomes a cheap product by nature in comparison to iron ore as it can be harnessed in millions of tonnes through heat treatment methods i.e., pyrolysis, etc. Research indicates incinerator scrap harnessing techniques currently implemented are efficient to support global demand with the right melting and purification methods such as pyrolysis in electric furnaces using i.e., renewable energy microgrids.

1.3 Effect of residual Elements on Sigma Embrittlement

A small proportion of residual elements left in the molten steel mixture mainly include metallic impurities and non-metallics like silicon, carbon, phosphorus, or Sulphur which can have a detrimental effect on surface properties resulting in embrittlement, cracking, or poor mechanical properties. Whilst these processes are not needed if using fundamental physics i.e., pyrolysis to obtain pure elements it is quite evident why secondary metallurgy is not needed due to the fact residual or trace elements increase via compounding effect. This increases exponentially over the given years as we recycle and incinerate scrap steel bringing sustainability and health-related problems to society. Physics -chemistry-related properties are crystal structure controlled from a molecular perspective, thus metal matrix formations and physio-chemical properties, or characteristics can be gated this way for pure elements. For alloys made with residual elements the product of metallic combinations brings a lot of other non-homogenous characteristics detrimental to not only manufacturing industries but also the quality of life (i.e., increased radiation exposure from Terbium, Erbium, or Chlorides).
Sectors Aerospace

Defence and Marine

Agriculture

Food and Drink

Chemicals

Creative Economy

Education

Electronics

Energy

Environment

Government

Democracy and Justice

Manufacturing

including Industrial Biotechology

Transport

Other
URL http://Potential 3 new projects are proposed for future studies
 
Description As the world shifts towards sustainability and clean forms of energy; recycling and reuse of raw materials from end-of-life steel products has been gaining positive momentum. In the steel industry, this is not only seen as profitable but also necessary in order to cope with the demand for steel production. Incinerator scrap is categorised as a form of obsolete scrap. A transition can be observed in the early 2000s where 70% of obsolete scrap is utilised at double the rate 30 years back. Home and process scrap are identified by a positive trend relative to crude steel manufacturing, however, for incinerator scrap, the rise is driven by previously manufactured steel products that have now reached the end of life. According to Janke. et.al 70% of obsolete products return to the steel life cycle at 20 years in most cases. The remaining 30% are landfilled for a long duration and experience harsh environmental conditions such as oxidation and corrosion. Global carbon dioxide emissions reached 2.6 billion tonnes in 2020. According to IEA, the steel industry is not on track to meet its carbon reduction emissions target by 2050 unless new steel-making methods are proposed. Scrap steel reuse in the steel-making process has been a topic of interest over the last few decades with some significant studies being conducted to improve scrap utilisation from 1960-1990. The benefits include the use of scrap as a source of iron raw material and also leverage on the lifecycle of the steel-making process by avoiding other carbon dioxide emitting factors such as mining, smelting, and logistics of iron ore raw material. Steelmakers will need to use scrap as the main source of raw material and this requires a clear understanding of the scrap composition and its impact on the steel-making process. The global steel industry has been fined with stringent legislation and ambitious targets which all have a part to play in greenhouse gas emissions reduction in accordance with the 2050 Sustainable Development Goal (SDG) target. With global warming on the rise the Paris agreement SDG goal requires greenhouse gas emitting industries to play a part in reducing the concentration of particles due to their nature of trapping heat in the atmosphere for decades thus resulting in high-temperature increase. One great solution to mitigate this risk is to reuse scrap steel from end-of-life products that have been dumped in preparation for incineration or scrap yard storage. In addition, the scrap can act as a supplementary raw material of iron ore in the steel-making process. Currently, there is no legal requirement for steel manufacturers to rely on this process however given the scarcity of iron ore and the exhaustive mechanisms required to mine it, one can begin to understand why scrap is becoming a sustainable and potential alternative to iron ore. With nearly 66 million tons of waste being produced in the UK every year most of this is in the form of steel scrap from end-of-life vehicles and related mechanical products. This scrap can be categorised into groups before incinerating it in a Municipal solid waste incinerator which operates at 900-1200 degrees Celsius. Grating mechanisms within the furnace allow for light elements to be quickly converted to gas and extracted in form of fumes. The remaining heavy elements require higher heat to melt and due to their dense nature will settle to the bottom of the furnace leaving all impurities and light elements at the top. This comes with a challenge since waste products incinerated include organic and non-organic species meaning we may potentially end up with a non-homogenous mixture of scrap steel and other toxic impurities co-existing. Currently, the separated methods involve the use of mechanical and magnetic sieving methods to extract steel from non-iron-containing products this improves the overall quality of the scrap batch extracted.
First Year Of Impact 2020
Sector Education,Environment,Manufacturing, including Industrial Biotechology,Transport,Other
Impact Types Societal

Economic

Policy & public services