Influence of operational parameters on silicon in hot metal from Corex

Abstract

Quality steel production at low cost demands superior quality of hot metal (HM) with low silicon (Si) and sulphur. High silicon is undesirable for its subsequent treatment during steelmaking, making the process costlier and less productive, hence there have been constant efforts to lower the silicon content in HM. Corex is the only commercially established and proven smelting reduction ironmaking technology alterative to conventional blast furnace (BF) ironmaking. While the mechanism of silicon transfer is largely understood in the BF, the Corex process still requires further operational guidance for its control. A statistical analysis of more than one year plant data has been carried out to understand the parameters influencing the silicon content and to provide directions for its prediction and control. Based on the understanding of the parameters affecting the silicon in HM, regression analysis was carried out and equations have been developed to predict the silicon in Corex HM. The present paper discusses the various parameters involved, the regression output for silicon prediction, its validation and its interrelation with operating parameters. The analysis indicates that high fuel rates due to high moisture, poor metallisation, high volatile matter, high slag volume, high coal fines and low production, along with high basicity and alumina in slag lead to high silicon in HM.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

Abstract

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.     

Variation in hot metal and slag composition during tapping of blast furnace

Abstract

To determine the quality of the hot metal and the thermal conditions inside the blast furnace, the composition of the hot metal and slag must be known. Obtaining representative metal and slag samples during tapping is thus highly important to blast furnace operation. The study covered in the present report focused on hot metal and slag composition variation during tapping from a commercial blast furnace. From the results, optimal sampling time points for obtaining elemental concentrations that can be taken as representative for the whole tapping sequence were identified. It was furthermore concluded that the reliability of hot metal composition data is significantly improved by averaging elemental concentrations determined from two samples, each taken at a particular time point. One sampling, however, was found to be adequate for slag. Results from the study also showed a fairly strong correlation between amounts of silicon and carbon, sulphur and carbon, and silicon and sulphur in the hot metal, while a weaker correlation between hot metal temperature and each of these elements was observed.     

Simplified approach for prediction of spread,roll load and torque in grooveless rolling of billets

Abstract

A simplified analytical approach has been developed to predict the spread, roll load and torque in the grooveless rolling of billets under various conditions of billet aspect ratio, height reduction ratio, friction coefficient and roll radius. The results obtained from the simplified analytical model are shown to be in fair agreement with those obtained from three-dimensional finite element simulation under the same conditions. The main merit of the proposed simplified analytical approach, as compared with three-dimensional finite element simulation, lies in the drastic reduction of computational time required in finite element simulation, which favours the effective use of such a simplified approach to design the rolling schedule and apply online control of grooveless rolling.     

Submerged liquid slag injection in steel melt: scaleup study using cold model and dimensional analysis

Abstract

Submerged injection of solid flux powder is used in the steel industry to eliminate impurities in an economical way. The efficiency of such an injection process is limited by the fact that only a fraction of the injected particles penetrate into the liquid melt, while the majority remain as bubble encapsulated solids, causing poor heat and mass transfer. Therefore, liquid slag injection can be considered a potential alternative technique in the refining of steel to improve the efficacy of mass transfer in such a process. In the present work, liquid slag injection in a steel melt has been simulated by means of laboratory scale cold model experiments in which, water, paraffin oil and benzoic acid have been used as low temperature analogues for liquid steel, slag and impurities, respectively. Through dimensional analysis it is observed that the modified Froude number can be considered as a criterion for scaling up such a process from a bench scale to a full scale system. A regression analysis has also been carried out to correlate the dimensionless mass transfer rate constant with the relevant dimensionless numbers, namely, dimensionless gas velocity, Froude number, aspect ratio and non-dimensional lance depth.     

Analytical model for continuous caster profile optimisation

Abstract

As a partial solution of the differential equation of the continuous caster pass line, the analytical relationship between the basic radius and the lengths of the radial and unbending zones was derived. Choosing rationally the length of the unbending zone at a given unbending strain rate, it is possible to reduce the height of a high productivity caster and the ferrostatic pressure within the strand shape to eliminate internal cracking. The software developed allows the temperature, stress, and strain distribution in the strand shell to be determined and, for assumed allowable strains due to bulging and unbending, the caster profile and the positioning of support and unbending rollers to be optimised. For the case of a continuous caster with a vertical mould, the same technique for its profile optimisation was proposed.     

Liquid Al2O3-CaO-MgO inclusion entrapment at delta ferrite intercellular boundaries during solidification

Abstract

The behaviour of liquid Al₂O₃-CaO-MgO inclusions at the δ ferrite/melt interface in aluminium killed and calcium treated steel has been observed in situ using a confocal scanning laser microscope equipped with a gold image furnace. Movement of inclusions parallel to the solid/melt front was observed (a) during solidification rates below 1 μm s^-1 or with nearly static fronts, (b) during remelting of the front or (c) when inclusions at the solid/melt interface were pushed by a newly arriving liquid inclusion. Upon sliding, the inclusions tended to become entrapped in the intercellular regions of the front. After entrapment, a critical solidification velocity for pushing/engulfment was measured and compared with corresponding experimental results for pushing/engulfment at planar fronts. Engulfment at the intercellular boundaries occurred at solidification velocities larger than a critical velocity V_cr=9·91×10^-9 R^-1/2 in the present study. The result follows the same trend with respect to inclusion size as that found for planar interfaces, but indicates that the critical velocity is ~20%. lower. This is attributed to the observation that engulfment at intercellular regions involves replacing a section of the intercellular boundary with the inclusion, which decreases the surface energy change owing to engulfment compared with engulfment at planar fronts.     

Separation of copper from steel

Abstract

Copper has been separated from iron and steel by chlorine–air mixtures at 800°C. It was found that, contrary to predictions based upon thermodynamics, cupric chloride was the favoured copper product rather than cuprous chloride. This was due to the high vapour pressure of cupric chloride. It was found that in order to prevent the reaction between iron and cupric chloride it was necessary to preoxidise the iron to form an impervious oxide film. Copper contents lower than 0·05 wt-% were readily obtained after 10 min exposure to the gas, even when starting with several per cent of copper mixed with the iron or steel.     

Effect of welding consumables on hydrogen induced cracking of armour grade quenched and tempered steel welds

Abstract

Armour grade quenched and tempered (Q&T) steels are susceptible to hydrogen induced cracking (HIC) in the heat affected zone after welding. Austenitic stainless steel (ASS) consumables are selected for welding Q&T steels as they have higher solubility for hydrogen in the austenitic phase and it is the most beneficial method for controlling HIC in Q&T steel welds. Recent studies reveal that high nickel steel and low hydrogen ferritic steel consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. In this investigation, an attempt has been made to study the effect of welding consumables on hydrogen induced cracking of Q&T steel welds by implant testing. Shielded metal arc (SMAW) welding process has been used for making welds using three different consumables, namely austenitic stainless steel, low hydrogen ferritic steel (LHF) and high nickel steel (HNS) to assess HIC by implant testing. The high nickel steel consumables exhibited a higher value of lower critical stress (LCS) and thus they offered a greater resistance to hydrogen induced cracking of armour grade Q&T steel welds than other consumables. The diffusible hydrogen content and the value of the LCS meets the specified limit for armour grade Q&T steel welds and hence, the LHF consumables can be accepted as an alternative to the to the traditionally used ASS consumables and the proposed HNS consumables.     

Surface inclusions and their evolution on strip rolled on CSP line and in following cold rolling process

Abstract

Hot and cold rolled strip samples with surface defects such as black pockmark, sliver and central cracking were collected from a compact strip production line followed by cold rolling. Microstructures and chemical compositions of the strip with defects were studied by scanning electronic microscopy and energy dispersive X-ray spectroscopy. Four typical types of defects on strip including surface spherical mould powder entrapment, centre spherical mould powder entrapment, surface linear mould powder entrapment and centre linear mould powder entrapment were studied by hot and cold rolling experiment in laboratory. It was observed that spherical mould powder entrapment was transformed into the surface black pockmark after the hot rolling and cold rolling process. Centre spherical mould powder entrapment led to central fracture in the hot and cold rolling process. Surface linear mould powder entrapment resulted in sliver and centre linear mould powder entrapment led to severe sliver.