A model for calculating solubility of hydrogen in steel

Abstract

A model has been developed for calculating the hydrogen solubility in steel on the basis of composition (S, Ti, B) and grain size, with the objective of improving the control of hydrogen content in steel products. The effects of sulphur are in agreement with previous experience, but complex effects have been observed with variations in titanium and boron contents. The model defines trap sites in crystallographic terms for the various phases and features of interest. Segregation effects have been studied using laboratory casts and with composites using continuously cast slab from which the natural segregation has been removed. Segregated regions absorb large amounts of hydrogen, but the crack threshold is reduced such that the composites have much lower safe hydrogen levels than unsegregated material. The effects of assumptions in the model on number of traps and binding energies are considered.     

High speed continuous casting of steel billets: Part 1: General overview

Abstract

This paper is the first of a series of two, providing an overview of the results of mould heat transfer analyses conducted over years of industrial trials. The effects of casting speed, carbon content, superheat, water flowrate, electromagnetic stirring, and mould lubricant are discussed in terms of the midface mould heat transfer. Mould corner heat transfer is also discussed. Finally, an overview of mould design is given, as well as an analysis that reveals that both excessively steep and shallow meniscus tapers cause high heat transfer. Alternative designs to the conventional straight tapered moulds are discussed briefly. The focus is on the primary cooling zone, and therefore billet quality issues are not extensively covered.     

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.     

Modelling of steel shrinkage and optimisation of mould taper for high speed continuous casting

Abstract

In high speed continuous casting, optimisation of mould taper is key for intensifying heat transfer and for improving the quality of the cast products. Mathematical modelling has been carried out by combining heat transfer, steel shrinkage and parabolic continuous taper model in order to optimise the mould taper profile. These models have been assembled to a set of software, the inputs of which include the steel grade, casting speed, casting temperature, length and the cross-section of mould tube, while the outputs consist of surface temperature of the strand, thickness of solidified shell, thermal linear expansion coefficient, steel shrinkage, distortion of the mould tube wall, the actual air gap, total taper and the continuous taper profile. Optimum mould taper has a parabolic profile which is tapered inwards that changes continuously along the length of the strand in order to achieve reduction in air gap while avoiding distortion of the mould tube.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

Melting speed of mould powders: determination and application in casting practice

Abstract

Controlled melting of mould powder on top of the meniscus is essential for efficient mould flux performance. This paper presents two complementary methods to determine the melting speed of a mould powder. One method measures the displacement of a prepressed cylinder of mould powder at a fixed temperature. This method yields qualitative, but reproducible, results which can be related to flux composition. In the second method, a sample of mould powder is melted on top of a steel bath. The data are interpreted with the help of an improved theoretical melting model, resulting in quantitative values of melting speed and thickness of the molten flux layer. The results of both methods agree well with measured plant data.     

Break temperatures of mould fluxes and their relevance to continuous casting

Abstract

The break temperatures of mould fluxes are important since they help to control the horizontal heat transfer and lubrication between the steel shell and the mould, and consequently affect occurrences of longitudinal cracking and sticker breakout in continuous casting. Break temperatures T _br have been determined for both steady state and dynamic measurement of the viscosity, and equations relating T _br to chemical composition have been obtained for both cases. It has been found that T _br can be affected by (i) cooling rates and (ii) fluorine losses during the measurements.     

Effect of solidus temperature and crystalline phase of mould flux on heat transfer in continuous casting mould

Abstract

Pieces of mould flux film obtained from a commercial continuous casting machine and measurements of the mould temperature have been used in an investigation to explain the mechanism of heat transfer in the mould. Comparison with the results of numerical calculations, indicates that the main factor responsible for the reduction of heat transfer with a high basicity flux is the contact resistance at the interface of the mould wall and solid flux film. The results show that a relatively high contact resistance is introduced when using a crystalline, high solidus mould flux because of the stability of the contact resistance.     

On heat transfer and liquid mixing in the continuous casting of steel

Calculations are presented for the interaction of heat and fluid flow in the continuous casting of steel. In Part 1 it is shown that for typical superheat conditions the solidification process and the profile of the solidus line are relatively insensitive to the flow pattern in the molten phase. In Part 2 expressions are developed for predicting the dispersion of a tracer added to the molten pool, in the solidified slab. This dispersion is markedly affected by the flow pattern, thus tracer techniques may be used for characterizing the conditions in the molten pool. The effect of mixing on the floatability of inclusions is examined in Part 3. It is found that mixing may appreciably modify the relationship between particle size and floatability.     

Simulation of coagulation of non-metallic inclusions in tundish and their trapping into solidified shell in continuous casting mould

Abstract

A mathematical model simulating the coagulation and flotation of non-metallic inclusions in the tundish and continuous casting mould was developed based on turbulence coagulation and Stokes coagulation. From the simulation in the tundish, it was found that Stokes coagulation is dominant in the tundish. In the mould, the simulation result was that argon bubbles become the desirable sites where alumina inclusions are gathered and form large alumina clusters. The influence of the liquid steel flow in the mould applied by an in-mould electromagnetic stirring technique on the trapping rate of non-metallic inclusions into the solidified shell was examined. The horizontal flow in front of the shell was found to be effective for disturbing the trapping and decreasing the population of large non-metallic inclusions in the shallow surface layer of cast slabs. This effect was interpreted by the lift force applied to non-metallic inclusions in the velocity boundary layer formulated.     

连续铸钢结晶器钢水液位自动控制及其实践

介绍连续铸钢结晶器钢水液位自动控制技术的国内外状况,包括液位检测技术、控制算法和执行器等在实际生产过程中的应用情况及存在的问题。针对上述状况进行了评估并提出解决问题的建议。     

Simulation of heat transfer and strain behaviour in bloom casting mould

Abstract

An optimum casting model was developed to simulate the effect of mould flux on bloom heat transfer and strain behaviour based on a 3D MiLE method, and the influence of casting speed and superheat on bloom heat transfer and lubrication were also investigated. The simulation results showed that solidified shell thickness growth conforms to a Square Root Law, and that the model predicted results are basically in agreement with previous data in the literature, and provide confidence in model. The bloom temperature distribution range in the corner area is smaller than that in the mid-face, and the corner regions form a high cracking risk zone. The hot tearing indicator and effective stress in the corner area are significantly greater than that in the mid-face, so the corner area is the dangerous zone of cracking; The mould flux lubrication in the bloom mid-face is better than for the bloom corner region, due to a higher shell temperature and a fluid slag; The increasing of casting speed can delay air gap formation of the bloom corner area, improving the lubrication conditions, but when the casting speed is changed, it is also necessary for the mould flux viscosity and crystallization temperature be changed also. Increasing the superheat has little influence on the completely solidified distance of liquid flux in the bloom corner area.     

Three-dimensional analysis and design of petal-like mould for continuous casting of steels

Abstract

Understanding the shrinkage behaviour of a steel billet is very important for designing the continuous casting mould, and conversely, a well designed mould is beneficial when matching the shrinkage behaviour of the billet. The shrinkage behaviour of a billet is hard to measure in situ during continuous casting but can be calculated by numerical simulation. A three-dimensional finite element model has been built to simulate the thermal and stress fields of the billet in the mould. The dynamic thermal boundary condition, the effect of ferrostatic pressure and the temperature dependent thermophysical parameters have been considered in the model. The shrinkage of billet when considering ferrostatic pressure is on average 0·08 mm smaller than when not considering ferrostatic pressure. The temperature and stress distributions are analysed in the present paper, and based on this analysis, a novel petal-like mould was designed and its taper determined. The designed mould has been tested in industrial practice showing better lifetime and billet quality.     

Mathematical modelling of fluid flow,heat transfer and solidification phenomena in continuous casting of steel

A steady state, two-dimensional mathematical model for continuous billet casting operations has been developed. Towards this, governing equations of fluid flow and heat transfer together with their appropriate set of boundary conditions were derived and solved numerically via a control volume based implicit finite difference procedure (e.g., SIMPLE). The effect of various assumptions and procedures applied to modelling of turbulence phenomena, thermal buoyancy, flow through the mushy zone, free surface conditions etc., on the sensitivity of the computed results was investigated computationally. Of all these, modelling of heat and fluid flow phenomena in the mushy region was found to have relatively more effect on the predicted results. In addition to these, a set of three different billet casting operations reported in literature were simulated mathematically and direct comparisons were made between predicted and observed solidified shell profiles. Such comparisons demonstrated reasonable to excellent agreement between theory and experiments.     

Viscous properties of new mould flux based on aluminate system with CeO_2 for continuous casting of RE alloyed heat resistant steel

The conventional mould fluxes can not be applied to the continuous casting of RE alloyed heat resistant steel, because severe slag-metal interface reactions occur generally in the mold. To restrain the interface reaction and improve conditions for continuous casting, a new mould flux based on aluminate system was devised. The viscous properties were investigated. Scanning electron microscopy and X-ray diffraction were applied to detect and characterize the crystalline phases in the continuous cooling process. The results showed that appropriate addition of CeO_2 could avoid the precipitation of CaO and decrease the viscosity of the mould flux. Increasing the mass ratio of CaO /Al_2O_3, especially to a value exceeding 1, could worsen the stability of the mould flux. With a content of less than 14 wt.%, Li_2O could reduce the viscosity and breaking temperature, but its effect could be weakened for the promoted precipitation of LiAlO_2. To obtain a mould flux with stable viscous properties, such as viscosity and breaking temperature, appropriate contents of CeO_2 and Li_2O should be controlled to around 10 wt.% and 14 wt.%, while the mass ratio of CaO /Al_2O_3 should not be more than 1.     

高效连铸抛物线锥度结晶器的研究与设计

应用凝固理论对高效连铸条件下结晶器内初生坯壳生长规律、铸坯的凝固收缩特性进行了系统研究。利用结晶器锥度设计的最小气隙原则,建立结晶器设计计算模型,开发出高效连铸抛物线锥度结晶器设计系统。该系统可依据工艺条件设计出满足不同钢种和拉速条件的结晶器锥度、内腔曲线方程、上下口以及任意间位置的内腔尺寸,为结晶器的生产提供操作数据和理论依据。最后分别讨论了钢种、结晶器长度、拽坯速度、铸坯断面尺寸与结晶器锥度、凝固收缩系数的关系。     

Simulation and verification of solidification process of stainless steel 316 in the mould during continuous casting

采用三维有限元技术,研究了连铸过程中高温区6铁素体含量对钢水凝固的影响。通过对316不锈钢板坯浇铸过程的模拟,获得了凝固过程中6铁素体含量对坯壳出口温度、角部温度及坯壳厚度变化的影响规律。研究发现铁素体含量能够极大增加坯壳纵向温度,但对横向温度的梯度的变化影响较小。通过分析横向温度的变化,确定在距角部40mm左右处坯壳最薄。同时,也研究了出口坯壳厚度变化规律,随着8铁素体含量的增加,出口处坯壳厚度逐渐减小,厚度差逐渐变大。     

Analysis of variability and non-uniformity of mould heat extraction for round billet continuous casting in plant trials

Abstract

Maintaining a stable and uniform heat transfer from steel shell to mould is important to produce high quality casting billet. In the present paper, a large amount of measured data of heat flux and temperature for round billet continuous casting mould from a plant trial has been analysed to shed light on the variability and non-uniformity of mould heat transfer around the perimeter. The results show that the variability and non-uniformity of heat extraction from the steel through the mould is affected slightly by operational parameters, such as pouring temperature, casting speed, meniscus, electromagnetic stirring current, but strongly by the steel carbon content and mould powder type. The installation of the mould in caster machine determines the magnitude of non-uniformity of heat transfer to a great extent. The relative root mean square (rRMS) of mould heat flux, presenting the variability and non-uniformity of mould heat transfer around the perimeter in transverse section, has wider range of variation and higher mean value compared with that of temperature. When the abnormality of heat transfer happens, such as deposit, the non-uniformity of mould heat transfer is also studied.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 1: heat transfer through hot-oxidized steel surfaces cooled by spray-water

For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density ·_s only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.     

Microstructural engineering applied to the controlled cooling of steel wire rod: Part I. Experimental design and heat transfer

The goal of this study was to develop a mathematical model which incorporates heat flow, phase transformation kinetics, and property-structure-composition relationships to predict the mechanical properties of steel rod being control cooled under industrial conditions. Thus, the principles of microstructural engineering have been brought to bear on this interdisciplinary problem by combining computer modeling with laboratory measurements of heat flow, austenite decomposition kinetics, microstructure and mechanical properties, and industrial trials to determine heat transfer and obtain rod samples under known conditions. Owing to the length and diversity of the study, it is reported in three parts,_[8191]the first of which is concerned with the heat flow measurements. A relatively simple and reliable technique, involving a preheated steel rod instrumented with a thermocouple secured at its centerline, has been devised to determine the cooling rate in different regions of the moving bed of rod loops on an operating Stelmor line. The measured thermal response of the rod has been analyzed by two transient conduction models (lumped and distributed parameter, respectively) to yield overall heat-transfer coefficients for radiation and convection. The adequacy of the technique has been checked by cooling instrumented rods under well-defined, air crossflow conditions in the laboratory and comparing measured heat-transfer coefficients to values predicted from well-established equations. The industrial thermal measurements have permitted the characterization of a coefficient to account for radiative interaction among adjacent rod loops near the edge and at the center of the bed. Formerly Graduate Student, The University of British Columbia.