Towards coal based continuous steelmaking Part 1 – Iron ore fines and scrap to low carbon steel via melt circulation

Abstract

Integrated iron- and steelmaking is effected by depositing a mixture of powdered coal and iron ore fines onto a moving melt surface without prior agglomeration. The resulting metallised solid raft is propelled out of the ironmaking loop onto the melt surface of the first of two steelmaking loops. By progressively adding oxygen to the gas first produced in ironmaking, decarburisation is conducted not with oxygen directly but rather by CO₂ and H₂O so that subsurface formation of CO is never permitted, via careful manipulation of the rates of gas phase mass transfer, interfacial chemical kinetics and liquid phase mass transfer. During ironmaking, infiltration of the melt via capillary rise greatly enhances the rate of metallisation. All the endothermic heat is supplied from beneath the circulating melt, which picks up its heat when post-combustion of CO and H₂ is completed after gases have first passed through the steelmaking loops.     

DRI production using cold bonded carbon bearing pellets Part 2 – Rotary kiln process modelling

Abstract

A new cold bonding technology for producing coal bearing composite pellet was developed. Laboratory tests showed that the cold bonded pellet has enhanced mechanical strength, from which good quality DRI can be produced. Based on the laboratory test results, a rotary kiln process was designed for producing DRI. Because of the high metallisation rate of the pellets and the corresponding novel operation conditions, the proposed rotary kiln process has significantly higher energy efficiency than the SL/RN process, hence emitting less CO₂.     

Scrap melting in continuous process rotary melting furnace Part 1 – Bench scale furnace trials

Abstract

A heat transfer model was developed for scrap melting in an oxyfuel fired continuous process rotary melting furnace (CPRMF), which was envisioned as a replacement for the electric arc furnace in minimill steelmaking. The results are presented in a two part series. This first paper describes the bench scale CPRMF, the experimental procedure, and the operational results which will be used to validate the model. Two groups of copper melting trials were performed to explore aspects of furnace operation such as oxygen enrichment and slag thickness. The experimental results indicated that furnace thermal efficiency was significantly improved by moving from 37 to 80% oxygen in the combustion air and, extrapolating the results, would be ～30% for oxy fuel firing. With the constant oxygen level of 53%, the introduction of a slag layer significantly lowered the efficiency from 22 to 18% for 1·3 cm slag and 12% for 2·5 cm slag.     

F-free mould powders for low carbon steel slab casting – technological parameters and industrial trials

The choice of the mould powder for slab casting is a difficult task because mould powders have many important functions during the continuous casting of steel. CaF₂ is a key ingredient in conventional mould slags since it reduces the viscosity, the liquidus temperature and the break temperature. Fluorine in mould powders is undesirable from the environmental and health points of view due to the following reasons: (i) evolves easily from slags, producing health-injurious gaseous substances, such as hydrofluoric acid; (ii) creates problems for storage and utilisation of solid waste and (iii) causes machinery corrosion. Aim of the present work is to describe the development of a new F-free mould powder for low carbon steel slab casting replacing CaF₂ with B₂O₃. Laboratory tests and industrial trials were performed considering the technological parameters viscosity, break temperature and crystallisation tendency. From laboratory tests it was concluded that important technological parameters are similar when comparing the F-bearing mould powder (reference) and the new F-free B-bearing mould powder for low carbon steel slab casting: viscosity at 1300°C, break temperature, and crystallisation tendency. It was observed during industrial trials that a significant decrease of the submerged entry nozzle erosion was observed. The results were similar when comparing the F-bearing and the F-free performance: slag pool thickness measurements, melting behaviour, Al₂O₃ absorption, mould powder consumption and slabs superficial quality.     

Inclusion characterisation – tool for measurement of steel cleanliness and process control: Part 1

Abstract

Owing to increasing demand for high surface and internal quality finished steel products, it is imperative to study the factors that limit the production of such steels. In particular, non-metallic inclusions affect steel cleanliness and cause defects leading to worsening of desired mechanical properties and service life of steel products. Other detrimental effects of inclusions are poor steel castability often resulting in slab downgrades and rejections, increased costs associated with recycling of liquid steel and refractories, and even shut down of the caster. Inclusion engineering is thus important to achieve process and quality control on a daily production basis. This article, in two parts, addresses inclusion characterisation as a tool for understanding and improving process conditions, minimising nozzle clogging and reducing sliver rejections in Ti stabilised ultra low carbon steels, Ca treated low carbon Al killed steels and advanced high strength steels. The paper begins with a survey of techniques followed by examples of use of techniques to resolve steelmaking and casting issues that affect quality of steel. Part I explains the use of an automated scanning electron microscope to correlate inclusion data with industrial process conditions. Examples include effect of different samplers, influence of ferroalloy quality and temperature control practices, ratio of elemental chemistries, ladle stirring etc. on quality of steel.     

Direct reduced iron production using cold bonded carbon bearing pellets Part 1 – Laboratory metallisation

Abstract

A new cold bonding technology for producing coal bearing composite pellets was developed. Alumina cement was used as binder, which gave high mechanical strength to the pellet even at elevated temperatures. Laboratory test results showed that the metallisation rate of the pellets was high owing to the intimate contact of the particulates of coal and the iron ore in the pellet. The developed cold bonding method can also be used to recycle electric arc furnace (EAF) dust, from which valuable zinc and lead can also be recovered.     

Microstructural evolution in ultra-low-carbon steel weldments—Part I: Controlled thermal cycling and continuous cooling transformation diagram of the weld metal

The transformation behavior and microstructural evolution of the as-deposited weld metal from an ultra-low-carbon (ULC) weldment were characterized by dilatometry, optical microscopy, transmission electron microscopy, and microhardness measurements. These results were used to construct a continuous cooling transformation (CCT) diagram for this weld metal. The major microconstituents observed in this ULC weldment were (in order of decreasing cooling rate) coarse autotempered martensite, fine lath martensite, lath ferrite, and degenerate lath ferrite. No polygonal ferrite was observed. These results were also used to develop criteria to differentiate between the two predominant microstructures in these ULC steels, lath martensite, and lath ferrite, which can look quite similar but have very different properties.     

Water–air online quenching process of 3Cr2Mo steel based on numerical simulation

A three-dimensional finite element model was established to investigate the water–air online quenching process of 3Cr2Mo steel with 130-mm thickness. The temperature, metallographic structure and stress–strain fields of the steel were calculated under single-pass continuous quenching, multi-pass continuous quenching and multi-pass interrupted quenching (MPIQ) processes. The results show that the three quenching processes can avoid the pearlite appearance, and MPIQ process could be more effective to decrease the brittleness of steel. Besides, MPIQ process is able to reduce stress–strain, minimise deformation and avoid cracking problem. The hardness and the metallographic structure were tested after MPIQ process and tempering. After tempering, the metallographic structures were all tempered sorbite, and the hardness difference of the whole steel was less than 3HRC with no cracks. It can draw the conclusion that the MPIQ process is a suitable quenching process for 3Cr2Mo steel.     

Theoretical concept on slag–oxygen sensors to measure oxide activities related to FeO,SiO2 , and CaO contents of steelmaking slags

Abstract

A theoretical concept is presented on the slag–oxygen sensors for in situ measurements of the FeO, SiO₂ , and CaO contents of steelmaking slags in the furnace and in the ladle. The purpose of this disclosure is to stimulate R & D interest in academia and technical institutes to further the development of new measuring devices applicable in steelmaking operations. The slag–oxygen sensor conceived* consists of two dissimilar electrodes such that when immersed in molten slag there will be a difference in oxygen potentials at the slag/ electrode interface between the two electrodes, registering an open circuit cell emf. Examples are given of different types of electrodes for different oxides in the slag; also, equations are derived for the theoretical relation between the oxide activity and sensor emf reading.     

Starch consolidation of M3/2 high speed steel powder – influence of microstructure on mechanical properties

Abstract

The influence of microstructure on the mechanical properties of starch consolidated super solidus liquid phase sintered AISI type M3/2 high speed steel powder has been evaluated. Hardness measurements, Rockwell C indentation and scratch testing were used to evaluate the mechanical properties and light optical microscopy and scanning electron microscopy were used for post-test characterisation. The results show that it is possible to starch consolidate and sinter large particle size high speed steel powder to obtain microstructures with high mechanical strength. However, the results show a strong correlation between the as sintered microstructure and the resulting mechanical properties and illuminate the importance of having a dense and isotropic microstructure in order to meet engineering requirements in demanding applications. Consequently, the failure mechanisms observed during indentation and scratch testing can be related to residual pores, present in the low temperature sintered samples, and a coarse microstructure with eutectic carbides, present in the high temperature sintered samples.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5–FeAl2 powders Part 2 – Densification mechanism analysis

Abstract

The use of Fe₂Al₅–FeAl₂ prealloyed powders and heating rates >150 K min^?1 overcomes the formation of density restricting Kirkendall porosity in the Fe–Al system. X-ray diffraction, electron probe micro analysis and differential thermal analysis suggest that the absence of a persistent liquid, experienced when liquid phase sintering with elemental powders, is overcome. Homogenisation is greater during heating at a rate of 20 K min^?1 than for 150, 250 or 400 K min^?1 and homogenous Fe₃Al forms across the compact at temperatures below the melting point of the liquid forming constituent, indicating that a liquid will not form under such processing conditions. The maximum density achieved under the processing conditions in the present study is 92% of theoretical density. The presence of large pores shortly after liquid formation suggests that the remaining porosity is largely due to powder agglomeration during mixing.     

Optimisation of process conditions in powder injection moulding of microsystem components using robust design method Part 2 – Secondary design parameters

Abstract

Powder injection moulding (PIM) is a proper fabrication method for microsystem technology components. This paper studies the process control of PIM to create thin walled, high aspect ratio geometries, which can be easily found in microtechnology based electro chemical, mechanical and biological systems (MECS). The powder used in this study is gas atomised 316L stainless steel with a median particle size of 10 μm. The effects of reducing the thickness of high aspect ratio geometries on the secondary design parameters including the maximum wall shear stress, cooling time and standard deviations of the melt front velocity and areas are studied. The study shows process parameters including fill time, feedstock injection temperature, mould wall temperature and switchover position can be optimised using the Taguchi robust design method.     

Properties investigation of CaO–Al2O3–SiO2–Li2O–B2O3–Ce2O3 mould flux with different w(CaO)/w(Al2O3) for heat-resistant steel continuous casting

The new CaO–Al₂O₃–SiO₂–Li₂O–B₂O₃–Ce₂O₃ mould flux was devised to realise smooth continuous casting of Ce-bearing heat-resistant steel. The new devised mould flux was based on calcium-aluminate system, so the w(CaO)/w(Al₂O₃) has great influence on the properties of the slag, which is similar to the basicity in the silicate system. The melting temperature, viscous properties, slag structure and crystalline phases with different w(CaO)/w(Al₂O₃) were investigated. The melting temperature of the mould flux could remain relatively steady with w(CaO)/w(Al₂O₃) in the range of 1.0–1.82. The main network former in the new slag was [AlO₄]-tetrahedron. The network formed by [AlO₄]-tetrahedron was destroyed by increasing w(CaO)/w(Al₂O₃), the viscosity decreased consequently. The mould flux show weaker crystallisation tendency with increasing w(CaO)/w(Al₂O₃). When the temperature decreased to 1100°C, the change of the fully crystallised phases with increasing w(CaO)/w(Al₂O₃) was as follows: Li₂O·Al₂O_3?+?2CaO·Al₂O₃·SiO_2?→?Li₂O·Al₂O_3?→?Li₂O·Al₂O_3?+?3CaO·Al₂O_3?+?CaCeAlO₄.     

Mixed burden softening and melting phenomena in blast furnace operation Part 2 – Mechanism of softening and melting and impact on cohesive zone

Abstract

The present work was designed to improve the current understanding of the softening and melting (SM) mechanism of ferrous materials and to identify their potential impact on the cohesive zone in the blast furnace (BF). The lump ore, direct reduced iron (DRI) and hot briquetted iron (HBI) were individually subjected to industrial trials and their results were compared with those obtained in the laboratory (SM) tests described in first part of the present paper. The difference between melting and 10% bed shrinkage temperature obtained from the results of laboratory and industrial tests was compared and it was found within 10% limits. The SM temperatures were also correlated to the basicity, gangue and flux contents of the mixed burden. The liquid slag mass fraction was computed using FactSage software and compared with the deformation of the burden bed. These results indicated presence of a small amount of liquid at temperatures close to the softening temperature of the burden suggesting that the deformation of solid phases is the primary cause for softening of the burden. The melting of the mixed burden is dependent on the melting point of DRI and melt exudation occurs close to the meltdown of the burden. The impact of ferrous burdens on the cohesive zone was predicted on the basis of experimental results which indicated DRI/HBI is an excellent blast furnace feed material to improve the productivity and decrease the coke rate in the BF.     

A thermodynamic model for calculating manganese distribution ratio between CaO–SiO2–MgO–FeO–MnO–Al2O3–TiO2–CaF2 ironmaking slags and carbon saturated hot metal based on the IMCT

A thermodynamic model (IMCT-L_Mn) for calculating manganese distribution ratio between CaO–SiO₂–MgO–FeO–MnO–Al₂O₃–TiO₂–CaF₂ slags and carbon saturated liquid iron have been developed based on the ion and molecule coexistence theory. The predicted manganese distribution ratio shows a reliable agreement with the measured ones. With the aid of the IMCT-L_Mn model, the respective manganese distribution ratio of (Mn²⁺?+?O^2?), MnO·SiO₂, 2MnO·SiO₂, MnO·Al₂O₃, MnO·TiO₂, and 2MnO·TiO₂ are investigated. The results indicate that the structural units SiO₂?+?FeO play a key role in CaO–SiO₂–MgO–FeO–MnO–Al₂O₃–TiO₂–CaF₂ slags in demanganisation process in the course of hot metal treatment at 1673?K. The manganese distribution ratio at a given binary basicity range increases with CaF₂ content, while that decreases with TiO₂ content at different binary basicity scopes, which demonstrate that high Mn in the metal is favoured by TiO₂ content. In the present study, various critical experiments are carried out in an effort to clarify the effect of temperature on demanganisation ability, indicating that the lower temperature of molten metal is, the faster the rate of demanganisation reaction is and the shorter the thermodynamic equilibrium time is and the lower end-point Mn content is. It can be deduced from the obtained experimental results that the greater oxygen potential of slags or iron-based melts, lower content of basic oxides in slags, and lower temperature at reaction region is benefit for demanganisation reaction.     

Evolution and coarsening of Al2O3 dispersoids at 500 °C to 600 °C in a mechanically alloyed Al-Ti-O based material

The formation and coarsening of Al₂O₃ dispersoids have been investigated at 500 °C, 550 °C, and 600 °C in a mechanically alloyed (MA) extrusion of composition Al-0.35wt pct Li-1wt pct Mg-0.25wt pct C-10vol pct TiO₂ for times up to 1500 hours. In the as-extruded condition, the dispersed phases included Al₃Ti, Al₄C₃, MgO, cubic TiO (C-TiO), monoclinic TiO (M-TiO), TiO₂, and a small amount of Al₂O₃. However, numerous Al₂O₃ dispersoids (various polymorphs: η, γ, α, and δ) with “block-shaped” morphology were formed after heat treatment due to reduction of C-TiO, M-TiO, and TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed conclusively the transformation of these phases to additional Al₂O₃ and Al₃Ti. High resolution TEM showed that the α-Al₂O₃ dispersoids exhibited some lattice matching with the α-Al matrix. Coalescence of the block-shaped Al₂O₃ dispersoids occurred after heat treatment, and Al₄C₃ also became attached to them. The length and width of the block-shaped Al₂O₃ dispersoids increased by a factor of ∼1.55 between 340 and 1500 hours at 600 °C.     

Phase Equilibria Studies in the System ZnO-“FeO”-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-SiO<Subscript>2</Subscript> Relevant to Imperial Smelting Furnace Slags: Part I

The phase equilibria and the liquidus temperatures in the system ZnO-“FeO”-Al₂O₃-CaO-SiO₂ have been determined experimentally in equilibrium with metallic iron. Specifically, the effects of Al₂O₃ concentrations in Imperial Smelting Furnace slags are identified, and the results are presented in the form of pseudo-ternary sections ZnO-“FeO”-(Al₂O₃ + CaO + SiO₂) in which CaO/SiO₂ = 0.93 and (CaO + SiO₂)/Al₂O₃ = 5.0 and 3.5, respectively. It was found that, in the presence of Al₂O₃, the spinel phase is formed, the spinel primary phase field expands, and the wustite and melilite primary phase fields are reduced in size with an increasing Al₂O₃ concentration. The implications of the findings to industrial practice are discussed.