Effect of alumina on slag–metal separation during iron nugget formation from high alumina Indian iron ore fines

Abstract

Iron nuggets can be obtained from ore–coal composite pellets by high temperature reduction. Alumina in the ore plays a vital role in slag–metal separation during nugget formation, as it increases the liquidus temperature of the slag. In this study, the effect of carbon content, reduction temperature and lime addition on slag–metal separation and nugget formation of varying alumina iron ore fines were studied by means of thermodynamic modelling. The results were validated by conducting experiments using iron ore fines with alumina levels ranging from 1·85 to 6·15%. Results showed that increase in reduction temperature enhances slag metal separation, whereas increasing alumina and carbon content beyond the optimum level adversely affects separation. Carbon below the required amount decreases the metal recovery, and carbon above the required amount reduces the silica and alters the slag chemistry. Optimum conditions were established to produce iron nuggets with complete slag–metal separation using iron ore–coal composite pellets made from high alumina iron ore fines. These were reduction temperature of 1400°C, reduction time minimum of 15 min, carbon input of 80% of theoretical requirement and CaO input of 2·3, 3·0 and 4·2 wt-% for 1·85, 4·0 and 6·15 wt-% alumina ores respectively.     

Study on control technology of slag composition for low-sulfur ＆ low-oxygen steel

为实现高品质低硫、低氧钢的生产,有效降低钢中T．0、S含量,在考虑转炉下渣、炉渣氧化性以及钢水氧活度的条件下,研究确定了用于低硫、低氧钢冶炼的CaO-Si02-A1：03精炼渣系,并结合转炉下渣改质技术以及LF精炼钢包渣成分控制技术等工艺措施,制定了低硫、低氧钢的钢包渣改质制度。采用该技术生产的27CrMoNbV等圆管坯钢达到了成品w（T．0）≤15×10,w（S）≤0．005％的水平,实现了低硫、低氧钢的生产。     

Impact of slag–refractory lining reactions on the formation of inclusions in steel

Abstract

The present study was carried out to investigate the impact of slag– refractory lining reactions on the formation of inclusions during ladle treatment of steel. The experiments were conducted on an industrial scale in the ladle at Uddeholm Tooling AB in Hagfors, Sweden. The inclusion chemistry and population during ladle treatment were studied along with the composition of the ladle glaze, taken from the ladle lining. The inclusions in the steel were classified into four groups according to the Swedish standard SS 111116. SEM/EDS analyses were carried out to identify the phases present in both the inclusions and the ladle glaze. The number of inclusions in the steel before deoxidation was found to increase with the ladle age, i.e. the number of times the ladle had been in use. A similar increase was also found after vacuum degassing and before casting. A great portion of inclusions before casting was found to be supplied by ladle glaze. This observation was further confirmed and explained by thermodynamic analysis. The present results show that ladle glaze is a major source of inclusions in the ladle at Uddeholm Tooling.     

Research on slag modifying agents for CaO–Mg based hot metal desulphurisation

Abstract

To improve the slag/iron separation problems in CaO–Mg based hot metal desulphurisation, experiments were carried out to generate a more fluid slag and to reduce the amount of hot iron entrapped within the desulphurising slag. The optimised slag modifying agent effectively decreased the melting temperature and viscosity of CaO–Mg based desulphurising slag. The optimised modifying agent has been successfully applied to industrial production. Industrial tests show that the average desulphurisation exceeded 82% when using the 80CaO–15Mg–5CaF₂ desulphurising agent at 3·5–4·0 kg t^–1, and with an average final S of 0·005%. The skimming time decreased from nearly 12 min to ～6·5 min. The heat loss during desulphurisation decreased to ～19°C which is favourable to the subsequent steelmaking process. Average total Fe content in post-desulphurisation slag is 34% with 0·7–0·8 kg t^–1 modifying agent, a decrease of ～30% in iron loss compared with current status.     

Effect of TiO2 on the crystallisation behaviour of CaF2–CaO–Al2O3–MgO slag for electroslag remelting of Ti-containing tool steel

The crystallisation characteristics of CaF₂–CaO–Al₂O₃–MgO slags with various TiO₂ contents from 0 to 9.73 mass% were studied using a single hot thermocouple technique, SEM-EDS and X-ray diffraction. The crystallisation mechanism of TiO₂-bearing slag was identified based on kinetic analysis. It was found that increasing TiO₂ from 0 to 6.43 mass% inhibited the crystallisation ability of electroslag remelting-type CaF₂–CaO–Al₂O₃–MgO slag, whereas further increasing TiO₂ content up to 9.73 mass% enhanced the slag crystallisation signally. When increasing TiO₂ content to 6.43 mass%, the crystalline phase shift from CaO to Ca₁₂Al₁₄O₃₂F₂ and CaTiO₃ at high temperatures. At lower temperatures, the crystalline phase change from polygonal Ca₁₂Al₁₄O₃₂F₂ to need-like CaTiO₃. Further increasing TiO₂ content to 9.73 mass%, the crystalline phase are Ca₁₂Al₁₄O₃₂F₂ and CaTiO₃ in the range of 1473–1613?K. The crystallisation of crystalline phase in the isothermal crystallisation is surface nucleation and controlled by interface reaction when TiO₂ content is lower than 6.43 mass% in the slag. It is bulk nucleation and diffusion-controlled one-dimensional growth in slag with 9.73 mass% TiO₂.     

Control of sulphur for spring steel killed by Si–Mn and with low basicity slag

Experiments are carried out in the case of low basicity slag for spring steel killed by Si and Mn, then the changes of the sulphur content, the sulphur distribution ratio L_S and inclusions are investigated. Finally, the effect on desulphurisation of oxygen content in molten steel, the calculated and measured lgL_S and the deep desulphurisation of the vacuum degassing station are discussed. It is found that the most sulphur in liquid steel is mainly removed during the early stage of LF refining. The average sulphur content in steel and the sulphur distribution ratio after the soft bottom-blown are 0.0047% and 115, respectively. It is very accurate and credible to use the L_S model to predict L_S. During the early stage of LF refining, to reduce the oxygen content in steel quickly is very crucial for the rapid desulphurisation of the Si and Mn killed spring steel with low basicity slag. The desulphurisation of molten steel can be further carried out during the VD refining station and it is beneficial to reduce the sulphur content for the control of sulphide in spring steel killed by Si–Mn and with low basicity slag.     

Effect of SiO2 substitution with Al2O3 during high-Al TRIP steel casting on crystallization and structure of low-basicity CaO–SiO2-based mold flux

The crystallization and structure of non-conventional lime-silica-based mold fluxes after undergoing slag-steel interaction in casting high-AI transformation induced plasticity(TRIP)steel were studied.The results showed that the crystallization temperatures of the mold fluxes decreased with decreasing the SiO2/Al2O3 ratio,and CaO/MnO2 ratio had an opposite effect on the crystallization temperatures.The crystalline phases precipitated in the mold flux were Ca4Si2O7F2 and NaAlSiO4.Decreasing SiO2/Al2O3 ratio and increasing CaO/MnO2 ratio in the mold fluxes have no influence on the types of crystalline phases.The dominant crystalline phase precipitated in each mold flux was Ca4Si2O7F2 with dendritic morphology,except for part of that with globular morphology in the mold flux without MnO2 addition.NaAlSiO4 crystals are distributed in the space among Ca4Si2O7F2 crystals.The size of Ca4Si2O7F2 crystals in the slag with higher S1O2/AI2O3 ratio is smaller,which is attributed to the polymerization degree of the mold flux with increasing SiO2/Al2O3 ratio.[SiO4]-tetrahedral,[AlO4]-tetrahedral and T-O-T bending(T denotes Si or Al)depolymerized gradually with decreasing SiO2/Al2O3 ratio,and an opposite trend was observed for the case with increasing CaO/MnO2 ratio.The polymerization degree of the mold fluxes decreased,which would result in the decrease in the viscosity of the mold fluxes.     

Effect of scandium on the phase composition and hardening of casting aluminum alloys of the Al–Ca–Si system

The phase composition of the Al–Ca–Si–Sc system is investigated in aluminum corner uisng computational (Thermo-Calc) and experimental (optical microscopy, scanning electron microscopy, and electron probe microanalysis) methods. The influence of annealing on the structure and hardness of alloys containing 0.3 wt % Sc is investigated in the region up to 550°C. It is shown that the maximum in the hardening curve caused by the isolation of nanoparticles of the Al₃Sc (L1₂) is attained after annealing at temperatures of 300–350°C in alloys belonging to the phase region (Al) + Al₄Ca + Al₂Si₂Ca ((Al) is the aluminum-based solid solution). Scandium completely enters the (Al) composition in alloys of this region, while the silicon concentration is minimal in it. On the other hand, hardening is almost absent in alloys from the (Al) + (Si) + Al₂Si₂Ca phase region. The possibility in principle to form the casting alloys based on the (Al) + Al₄Ca + Al₂Si₂Ca eutectic hardened without quenching is substantiated.     

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.     

Development of a microfluidic-chip system based on parallel flow for intensified Gd(Ⅲ) extraction from nitrate media using cationic extractant

Microfluidic solvent extraction(micro SX) of gadolinium was conducted using a mono-and di-ester mixture(MDEHPA) as the cationic extractant. A microfluidic Y-Y channel was fabricated using CO_2-laser technique in a glass microchip used as the extraction system. Compared with batch extraction,extraction kinetic is found fast, and extraction equilibrium is attained within 15 s. Stoichiometry of the extracted complex is found to be Gd(NO_3)_3·3MDEHPA using log—log plot method. Additionally,the operating parameters and overall volumetric mass transfer coefficient(k_Lα) were investigated to determine the mass transfer performance. Optimal condition of microextraction for gadolinium using response surface methodology was determined(feed solutions 31 mg/L adjusted to pH value 2.5,extractant concentration 2.9 vol% and extraction time 13.5 s). In optimal condition, gadolinium extraction yield is obtained 95.5%. Findings of this study approve simplicity, portability, effectiveness, swiftness, and environmental friendliness microfluidic solvent extraction process and reveal that micro SX is useful in the field of extraction strategic metals present at low concentrations, which are otherwise not technically amenable or economically feasible to extract using current traditional methods.     

Effects of Al content on non-metallic inclusion evolution in Fe–16Mn–xAl–0.6C high Mn TWIP steel

In the present study, the effects of Al content on the evolution of inclusions formed in Fe–16Mn–xAl–0.6C high Mn TWIP steels were investigated experimentally and discussed based on the thermodynamic calculation with FactSage. The results showed that with the increase of Al content from 0.002 to 2.1?wt-% in the steels, the evolution route of the main oxide inclusions is MnO?→?Al₂O₃?→?MgAl₂O₄?→?MgO, the main sulphide inclusion is changed from MnS to MgS, and the dominant stable inclusion changes along the route of MnO?→?Al₂O₃/MnS?→?MnS?→?AlN. The main inclusion types in thermodynamic calculation results with FactSage are consistent with the observed results in the present experimental steels.     

Evaluation of B2O3 as replacement for CaF2 in CaO–Al2O3 based mould flux

In order to develop low fluoride or fluoride free CaO–Al₂O₃ based mould flux for casting high aluminium steel, an investigation was carried out to study the effect of substituting CaF₂ with B₂O₃ on heat transfer and crystallisation behaviour of CaO–Al₂O₃ based mould flux by employing a heat transfer simulator of mould flux and a single hot thermocouple technique. The results showed that addition of CaF₂ promoted heat transfer of CaO–Al₂O₃ based mould flux, which was opposite to the effect of CaF₂ on heat transfer in conventional CaO–SiO₂ based mould flux. Addition of CaF₂ inhibited crystallisation of CaO–Al₂O₃ based mould flux by lowering the start crystallisation temperature and prolonging the incubation time of crystallisation. B₂O₃ showed similar effects to CaF₂ on heat transfer and crystallisation of CaO–Al₂O₃ based mould flux, but its ability to promote heat transfer and suppress crystallisation was stronger than CaF₂. Ca₃B₂O₆ (melting temperature 1480°C) was found as the primary crystalline phase in fluoride free CaO–Al₂O₃ based mould flux compared with the primary crystalline phase Ca₂Al₃O₆F (melting temperature 1507°C) in fluoride bearing (20% CaF₂) CaO–Al₂O₃ based mould flux.     

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.     

Influence of hot isostatic pressing on the structure and properties of an innovative low-alloy high-strength aluminum cast alloy based on the Al–Zn–Mg–Cu–Ni–Fe system

Hot isostatic pressing (HIP) is applied for treatment of castings of innovative low-ally high-strength aluminum alloy, nikalin ATs6N0.5Zh based on the Al–Zn–Mg–Cu–Ni–Fe system. The influence of HIP on the structure and properties of castings is studied by means of three regimes of barometric treatment with different temperatures of isometric holding: t ₁ = 505 ± 2°C, p ₁ = 100 MPa, τ₁ = 3 h (HIP1); t ₂ = 525 ± 2°C, p ₂ = 100 MPa, τ₂ = 3 h (HIP2); and t ₃ = 545 ± 2°C, p ₃ = 100 MPa, τ₃ = 3 h (HIP3). It is established that high-temperature HIP leads to actually complete elimination of porosity and additional improvement of the morphology of second phases. Improved structure after HIP provides improvement properties, especially of plasticity. In particular, after heat treatment according of regime HIP2 + T4 (T4 is natural aging), the alloy plasticity is improved by about two times in comparison with the initial state (from ~6 to 12%). While applying regime HIP3 + T6 (T6 is artificial aging for reaching the maximum strength), the plasticity has improved by more than three times in comparison with the initial state, as after treatment according to regimes HIP1 + T6 and HIP2 + T6 (from ~1.2 to ~5.0%), which are characterized by a lower HIP temperature.     

Effect of volume–surface quenching on the impact toughness of 20GL steel intended for cast freight bogie solebars

The effect of volume–surface quenching (VSQ) by a rapidly moving water flow on the impact toughness at–60°C (KCV^–60) of cast low-carbon 20GL steel, which is used to produce freight bogie solebars, is studied. VSQ is shown to increase KCV^–60 of 20GL steel by a factor of 1.7–2.0 at a higher static and cyclic strength as compared to the normalized state.     

A study on precipitation kinetics of sigma phase in a hot-rolled super duplex stainless steel during isothermal aging based on the Johnson–Mehl–Avrami model

Isothermal aging treatment of 2507 super duplex stainless steel (SDSS2507) was conducted at 850°C after solution treatment at 1150°C. The characteristics of sigma (σ) precipitation kinetics in SDSS2507 were discussed and an improved JMA (Johnson–Mehl–Avrami) model had been developed. The results show that, the precipitation mechanism of σ phase in hot-rolled SDSS2507 was quite different to other ordinary DSSs. It can be found that σ phase precipitated via two mechanisms in this experiment. (1) The eutectoid decomposition δ?→?σ?+?γ₂. It dominated the precipitation kinetics of σ phase in aging 0–25?min, and at this time the volume fraction of σ phase increased rapidly. (2) After 25?min, the precipitation of σ phase absolutely relied on the transformation of γ?→?σ which is controlled by the diffusion of Mo and Ni in γ. In this period, the precipitation rate of σ phase was significantly decreased. In this case, the separate modeling on the basis of each transformation mechanism was adopted and then an improved JMA model was developed for the precipitation kinetics of σ phase in the whole aging process. As a result, a good agreement between the experimental data and this developed JMA model can be obtained.