Development of mould fluxes based on lime–alumina slag system for casting high aluminium TRIP steel

Abstract

With excellent strength and ductility properties, transformation induced plasticity (TRIP) steels are superior to ordinary commercial steels and have great application potential in the automobile industry. However, the continuous casting of TRIP steels is associated with challenges owing to its high aluminium content, approximately 0?5–1?8%. The reaction of aluminium near the steel/slag interface continuously changes the basicity of conventional lime–silica based mould fluxes causing variation in the flux physical properties between the solidifying shell and mould wall. Therefore, the heat transfer process is unstable and the surface quality of the as-cast slab is not guaranteed.

To reduce the reactivity of aluminium in TRIP steel during casting, it is suggested to replace the lime–silica based fluxes with a lime–alumina based mould flux, based on laboratory measurements and industrial trials.

The trials showed great improvement in the slab surface quality and no depression defect was noticed so far.     

Optimisation of geometry of 185 × 185 mm square billet mould of Sidenor continuous casting machine

Abstract

Industrial trials conducted at Sidenor Basauri Works, and numerical modelling at Centro Sviluppo Materiali (CSM), have been combined in order to find a square 185 × 185 mm billet peripheral geometry that improves the quality of cast products by eliminating the occurrence of off corner cracks and depressions, and reducing the percentage of rejection. The idea is to change the square shape of the mould section to a new profile having a larger corner radius and outward bulging faces. Experimental trials using moulds with convex faces and larger corner radii resulted in an interesting decrease in the percentage of rejected bars. The presence of longitudinal defects has not been completely eliminated, but the adoption of moulds with larger corner radii has been demonstrated to be successful. Numerical modelling has been used to verify that the larger corner radius is the most important variable that guarantees the improved result, while it is less sensitive to different deflections of the faces. This encourages the use of moulds with flat faces, which produce billets that are much easier to handle with conventional tools for their transfer. It can be concluded that by using moulds with a corner radius of 40 mm, instead of conventional moulds with 10 mm corner radius, and flat faces, the risk of longitudinal off corner defects is significantly reduced.     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

Mechanism of explaining liquid friction and flux consumption during non‐sinusoidal oscillation in slab continuous casting mould

Abstract

The mechanism of explaining liquid friction and flux consumption in a non‐sinusoidal oscillation mould is expounded by proposing a series of indices for estimating interface phenomena between mould and metal in meniscus and calculating the effect of non‐sinusoidal oscillation parameters on these indices. The results show that the liquid flux is periodically infiltrated into the gap between oscillating mould and withdrawing slab by the negative pressure caused by widening flux channel from the last stage of positive strip time to the last stage of negative strip time. The surface cracks on the slab, even breakout, are formed during the positive strip time and existing surface cracks can be healed within the cumulative negative strip time. The casting performances were improved by decreasing oscillation frequency and increasing amplitude and non‐sinusoidal oscillation factor.

On explique la friction liquide et la consommation de flux dans une coquille à oscillation non‐sinusoïdale en proposant une série d’indices pour estimer les phénomènes d’interface entre la coquille et le métal dans le ménisque et pour calculer l’effet des paramètres d’oscillation non‐sinusoïdale sur ces indices. Les résultats montrent que le flux liquide s’infiltre périodiquement dans l’espace entre la coquille oscillante et la brame en retrait par la pression négative causée par l’élargissement du canal de flux à partir de la dernière étape de temps positif de démoulage jusqu’à la dernière étape de temps négatif de démoulage. Les fissures de surface de la brame, et même la percée, sont formées lors du temps positif de démoulage et les fissures de surface existantes peuvent être réparées lors du temps négatif cumulatif de démoulage. Le rendement du moulage s’améliorait en diminuant la fréquence d’oscillation et en augmentant l’amplitude et le facteur d’oscillation non‐sinusoïdale.     

Hot deformation behavior and finite element simulation of Mg–8.3Gd–4.4Y–1.5Zn–0.8Mn alloy

To study the hot deformation behavior of Mg–8.3Gd–4.4Y–1.5Zn–0.8Mn (wt%) alloy, hot compression tests were conducted using a Gleeble–3500 thermal simulator at temperatures ranging from 653 to 773 K, true strain rates of 0.001–1 s^?1, and a deformation degree of 60%. Results of hot compression experiments show that the flow stress of the alloy increases with the strain rate. The true stress–true strain curves are corrected by correcting the effect of temperature rise in the deformation process. Activation energy, Q, equal to 287380 J/mol and material constant, n, equal to 4.59 were calculated by fitting the true stress–true strain curves. Then, the constitutive equation was established and verified via finite element simulation. Results of the hot processing map show that the probability of material instability increases with the degree of deformation, which indicates that the material is not suitable for large deformation in a single pass. On the whole, the alloy is appropriate for multipass processing with small deformation and a suitable processing temperature and strain rate are 733 K and 0.01 s^?1, respectively.     

Effect of Mn/Ni ratio variation on microstructure of W–Ni–Mn alloy

Abstract

Tungsten heavy alloys (WHA) such as W–Ni–Cu and W–Ni–Fe are usually used as kinetic energy penetrators (KEP). However, the amount of penetration of these alloys is not sufficient due to their mushrooming effect that occurs as they impact their targets. On the other hand, KEP made of depleted uranium (DU) in spite of their excellent penetrating properties are not a very good substitute for WHA due to their environmental problems. Therefore, in order to increase the penetration depth of WHA penetrators, a new brand of WHA namely W–Ni–Mn alloys have been developed. The present paper deals with the microstructural improvement of such an alloy system, so that it can provide a potential candidate material to be used as KEP, having sufficient penetration depth. For developing this material, various ratios of Mn/Ni powder were mixed with 90 wt-% pure tungsten powder before compaction and sintering in order to investigate the amount of solubility of W in Ni–Mn matrix. In addition to study the effect of this ratio on the re precipitation and growth of W particles within the matrix after being subjected to sintering process, the results of the present study indicated that the grain refinement of W grains is possible by addition of Mn to W–Ni heavy alloys, so that the higher the amount of Mn/Ni ratio up to certain amount, the smaller will become the W grain size after sintering process. Worth mentioning that according to the results obtained by other researchers for WHA penetrators, the finer W grain size, the deeper the penetration of KEP would be. In addition, the results of the present study show that by selecting a suitable sintering cycle, one may obtain a dense microstructure having a density of ～100%, i.e. 99·6%.     

Microstructural evolution and hardness property of in situ Al–Mg2Si composites using one-step gravity casting method

In the present investigation, Al–X?wt-% Mg₂Si (X?=?0, 5, 10, 15 and 20) in situ composites are successfully synthesised by one-step gravity casting technique. Commercially pure Al, Mg and Si are used as raw materials. Microstructural evaluation and correlation of micro- and bulk hardness properties have been studied on developing composites. The composites consist of mainly three phases: matrix (α-Al), reinforcing (primary Mg₂Si) and binary eutectic (Al–Mg₂Si) phase. Primary Mg₂Si particles are formed by pseudo-eutectic transformation during solidification and surrounded by matrix and binary eutectic phase. It is found that Mg₂Si concentration has a significant impact on morphology and volume per cent of the above-mentioned phases. Primary Mg₂Si particles’ size and volume per cent increase with increasing wt-% of Mg₂Si. Volume per cent of individual phases and Mg₂Si concentration have great impact on hardness properties of composites. Bulk hardness increases with increasing wt-% of Mg₂Si concentration, but micro-hardness of primary Mg₂Si particle decreases slightly. Mg₂Si concentration also has significant impact on micro-hardness of individual phases.     

Hot-tearing susceptibility of Al–Zn alloys

The modern concepts of the causes of hot tearing are considered and the influence of the solid fraction growth rate of an alloy is studied. The hot-tearing susceptibility (HTS) of binary Al–(5–73) wt % Zn alloys is investigated using backbone tests. The HTS is found to be maximal at ~25 wt % Zn. This maximum cannot be explained by a change in the effective solidification range, since this range of the alloys decreases monotonically with increasing zinc content. The calculations of nonequilibrium solidification by the Scheil–Gulliver model and the solid fraction growth rate of the alloys under study demonstrate that the increase in the HTS induced by an increase in the zinc content from 5 to 25 wt % is related to the decrease in the solid fraction growth rate at the final stages of solidification. The decrease in the HTS at >25 wt % Zn is associated with an increase in the fraction of eutectic in the alloys (the solid fraction growth rate during the eutectic reaction tends toward infinity) and with a change in its morphology.     

Surface Oxidation and Wettability of Fe–Mn and Fe–Mn–Si-Alloyed Steel After Annealing

Metallurgical and Materials Transactions A - The surface oxidation and wettability of Mn and Si-alloyed steel after annealing at different conditions are studied with scanning electron microscope...     

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.     

Features of Structure Formation in an Al–Fe–Mn Alloy upon Crystallization with Various Cooling Rates

Russian Journal of Non-Ferrous Metals - Specific features of the microstructure formation of an Al–2.5% Fe–1.5% Mn alloy owing to the cooling rate during casting and during laser...     

Isothermal Solidification of an Al–Zn Alloy

In diffusion brazing, a brazing alloy layer thickness introduced into the gap between brazed parts is very small, which hinders the study of the isothermal solidification process. In this work, the isothermal solidification of an Al–Zn alloy is investigated at large volume and mass of the initial liquid that takes part in isothermal solidification. Zinc cylinders are caulked in the holes bored in aluminum cubes. The samples thus assembled are held at 500 ± 10°C for different times. The volume fraction of isothermal solidified crystals has been determined, and the growth rate of isothermal crystals is shown to decrease upon holding. The crystals formed during isothermal solidification width are up to 380 μm and length more than 1 mm. They are observed in the samples that are held more than 2 days. The zinc content in the isothermal solidified crystals corresponds to its mean content in the aluminum solid solution at 490–510°C, according to the Al–Zn phase diagram. The composition of the former liquid that surrounded the crystals during isothermal solidification coincides with the equilibrium composition of the liquid in the Al–Zn system at a temperature of 490–510°C.     

Nucleation in Zn–Al alloys grain refined with Ti

Abstract

In order to understand more fully the grain refinement of Zn–Al alloys by Ti, the α′ grains in chill cast 7 and 24% Al alloys have been studied by successive polishing and etching of microsections and by scanning electron microscopy of deeply etched specimens.

Idealized models have been deduced for these grains and it appears that lobed forms nucleate and grow preferentially in the 〈111〉 directions of the nucleus cubes. Increased supercooling causes more complex branching. When visible nuclei are absent the 24% Al alloy grains appear to have had a primitive plate-like morphology soon after nucleation.

The results are unique in the field of heterogeneous nucleation because the nuclei are relatively large and have a well-defined crystalline morphology so that their orientations relative to the early grain shapes can be easily studied.

Résumé

Afin de mieux comprendre l'action du Ti sur l'affmage des grains d'alliages Zn–Al, les grain α′ d'alliages à 7 et 24% d'Al, refroidis brusquement après coulée, ont été étuditiés par polissages et attaques successifs de microsections et par microscopie électronique à balayage d'échantil1ons attaques profondément.

Des modèles idéalisés ont été doouits pour ces grains. Il semble que les formes lobées germent et croissent d'une manière privilegiée dans les direction 〈111〉 des germes cubiques. Une surfusion plus prononcée engendre un embranchement plus complexe. Quand il n'y a pas de germes visibles, il semble que les grains de l'alliage à 24% d'Al aient eu une morphologie primitive en forme de plaques peu après la germination.

Les résultats sont uniques dans le domaine de la germination hétérogène parce que les germes sont relativement gros et possèdent une morphologie cristalline bien définie, de telle sorte que leurs orientations par rapport aux formes initiales des grains peuvent être étudiées facilement.