The effect of carbon content on solidification of steel in the continuous casting mold

This investigation examines the effect of steel carbon content on microsegregation and strand deformation during the first stage of solidification in the continuous casting mold. Calculation of microsegregation for phosphorus indicates a minimum at 0.10 wt pct C, and a maximum around 0.25 wt pct C of the decrease in solidus temperature. This leads to very different effective shell thicknesses and determines whether or not the strand shell can contract. As a result, mainly steels around 0.10 wt pct C can produce a finite gap in the early stages of strand formation, explaining the pronounced waviness of the surface of such steels. On the other hand, steels with more than 0.20 wt pct C are forced by ferrostatie pressure to remain in contact with the mold wall leading to uniform shell growth and smooth strand surfaces but also undergoing enhanced mold friction.     

Optimisation of cutting length of continuously cast strands

Abstract

Weight per metre and strand circumference were measured, using sensitive methods, on 177 mm round strands, with particular attention being paid to phase transformation and thermal shrinkage in the vicinity of the peritectic. The following results were achieved for a broad range of steel grades. With constant mould dimensions, the weight per metre of round continuously cast strand increases as casting speed rises. Soft steels, such as grade S35 containing 0·10%C, have a low weight per metre, whereas harder grades, such as C60 with 0·60%C, have the highest weight per metre. Low alloyed steels and oilfield tubular grades occupy rankings between these extremes. Martensitic and more highly resulphurised round billets have a conspicuously low weight per metre. The following definitive influencing factors on weight per metre became apparent: expansion of the mould tube under exposure to heat; shrinkage as a result of δ - γ transformation; creep processes under exposure to ferrostatic pressure; density of the compact steel; and porosity in the strand centre. These influences lessen in the order in which they are listed; they are, in some cases, contradictory, and balance one another out.     

包晶相变对连铸坯初生坯壳凝固收缩的影响

基于Ｆｅ－Ｃ合金的微观组织结构,建立了碳钢线性热膨胀系数计算模型,计算出不同碳含量的钢各在不同温度下的瞬时线性热膨胀系数,并将计算相应用于铸坯热－弹－塑性应力模型,研究了包晶相变对连铸坯初生坯壳凝固收缩的影响,模拟结果表明：浇铸碳含量在０．１％附近的包晶钢时,初生坯壳在靠近弯月面区域和角部区域的收缩很不规划,容易诱发表面缺陷。     

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.     

Design of corners of mould in square billet casting

Abstract

In continuous casting, heat flow optimisation in the mould is key to improving the quality of the product and production savings. The heat flow influences, and is influenced by, several phenomena of a mechanical or metallurgical nature, so its optimisation should include these. In particular, shrinkage of the strand and solid phase formation are among the most influencing factors affecting the cooling of the solidifying product. The present paper describes a model implemented by a software tool that can carry out simulation of shell formation of carbon steel within the mould, for rectangular shapes. The first aim of the software is to simulate formation of the solid shell in the strand and the deformations to which this solid is subjected. Deformations are a result of both thermal shrinkage, related to phase changes, and stresses caused by metallostatic pressure or the mould-shell interaction. The output of the model consists of temperature maps of the strand, maps of formation of the shell and the ideal mould contour.     

Analysis of thermomechanical behaviour in billet casting with different mould corner radii

Abstract

A finite element thermal stress model to compute the thermomechanical state of the solidifying shell during continuous casting of steel in a square billet casting mould has been applied to investigate longitudinal cracks. A two-dimensional thermoelastoviscoplastic analysis was carried out within a horizontal slice of the solidifying strand which moves vertically within and just below the mould. The model calculates the temperature distributions, the stresses, the strains in the solidifying shell, and the intermittent air gap between the casting mould and the solidifying strand. Model predictions were verified with both an analytical solution and a plant trial. The model was then applied to study the effect of mould corner radius on longitudinal crack formation for casting in a typical 0·75%/m tapered mould with both oil and mould powder lubrication. With this inadequate linear taper, a gap forms between the shell and the mould in the corner region. As the corner radius of the billet increases from 4 to 15 mm, this gap spreads further around the corner towards the centre of the strand and becomes larger. This leads to more temperature non-uniformity around the billet perimeter as solidification proceeds. Longitudinal corner surface cracks are predicted to form only in the large corner radius billet, owing to tension in the hotter and thinner shell along the corner during solidification in the mould. Off corner internal cracks form more readily in the small corner radius billet. They are caused by bulging below the mould, which bends the thin, weak shell around the corner, creating tensile strain on the solidification front where these longitudinal cracks are ultimately observed.     

包晶钢结晶器液面周期性波动原因及控制

针对鞍钢股份有限公司鲅鱼圈钢铁分公司板坯连铸在生产包晶系列钢种时．连铸结晶器钢液面周期性剧烈波动的原因进行了分析。包晶钢液在凝固过程的包晶相变造成初生坯壳的不均匀生长是导致结晶器出现液面周期性剧烈波动的主要原因。通过调整结晶器冷却水强度和控制保护渣传热性能,有效地控制了结晶器液面周期性波动,提高了铸坯质量。     

攀钢连铸坯角横裂缺陷产生的原因与对策

对攀钢Q235G钢连铸坯出现的角横裂缺陷进行了调查分析，在此基础上确定了连铸保护渣是引起角横裂的主要原因。并对保护渣的配方进行了改进，通过试验，研制出了适合攀钢普碳包晶钢用的连铸保护渣，有效地控制了Q235G钢连铸坯角的横裂缺陷。     

Initial development of thermal and stress fields in continuously cast steel billets

A mathematical model has been developed to compute the thermomechanical state of the shell of continuously cast steels in a round billet casting mold. The model determines the temperature distributions, the stresses in and the gap between the casting mold and the solidifying strand. The effect of variations in steel carbon content and mold taper on the thermal, displacement, and stress fields are examined. Comparisons with available experimental observations verify the predictions of the model. The model demonstrates that the thermal shrinkage associated with the phase change from delta-ferrite to austenite in 0.1 Pct C steel accounts for the decreased heat transfer observed in that alloy, as well as its susceptibility to cracking. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

Friction Forces between Mould and Strand Shell during Billet Casting

During continuous casting processes lubrication between mould and strand shell is very important for reaching and maintaining a good surface quality of both the as‐cast strand and the finished product. The lubrication is influenced by the properties of the mould powder but also by the friction forces caused by the periodic movements of mould and the descending strand shell. In the present research measurements were carried out to investigate the friction forces during 150 mm sq. billet casting of different steel grades using mould powder or granules and submerged entry nozzles. The friction forces reached values up to 6 kN. Since friction forces are depending on upward and downward movements within the mould‐strand‐system, the evaluation of a rheogram shows the typical change of pressure and tension. Maximum friction forces arise during the positive strip time of the rising mould, introducing tensions and cracks in the as‐cast strand. On the other hand lowest frictions forces are generated by the descending mould within the negative strip time leading to the healing of faults in the as‐cast strand. These industrial experiments indicate a correlation between friction forces and surface quality of as‐cast strands.     

连铸与模铸高铁车轴钢的高周疲劳破坏行为

连铸车轴钢能否达到模铸车轴钢的性能水平是其能否应用的一个关键。对此,采用旋转弯曲疲劳试验及疲劳裂纹扩展速率试验对比研究了连铸与模铸工艺生产的高铁车轴钢的高周疲劳破坏行为。结果表明,工业试制的连铸车轴钢的强度和疲劳极限均低于模铸车轴钢,且前者的疲劳裂纹扩展速率略高于后者。疲劳断口分析表明,疲劳断裂大部分起源于试样表面基体。微观组织分析表明,尽管两者的微观组织均为高温回火马氏体,但连铸车轴钢中原奥氏体晶粒尺寸及碳化物均略大于模铸车轴钢。金相评级法及夹杂物极值统计法的结果均表明,连铸车轴钢中的夹杂物尺寸明显大于模铸车轴钢。因此,为了以连铸工艺取代模铸工艺,还需要进一步优化连铸车轴钢的成分、冶金生产和热处理等工艺,以获得优良的冶金质量和组织性能。     

On the Formation of Centreline Segregation in Continuous Slab Casting of Steel due to Bulging and/or Feeding

Centreline macrosegregation is often observed in continuous slab casting of steel. Two of the main macrosegregation formation mechanisms are bulging and feeding. Both were studied and compared in the current work by using a two‐phase volume averaging model considering only columnar solidification. The casting of the strand itself is modelled by applying a predefined velocity following the casting speed and solid shell deformation (e.g. bulging). Three different cases are simulated and discussed. (i) The first case considers the influence of the feeding flow during solidification without taking bulging into account. Negative macrosegregation is observed in the centre of the casting in this case. (ii) The second case takes the flow caused by series of bulging along the solidifying strand shell into account, and is, therefore, representative for an ideal situation where bulging takes place without solidification shrinkage. In this case positive centreline segregation is found. (iii) The last case shows the results of a simulation which combines both shrinkage‐ and bulging‐induced flows. It is found that under the current casting conditions the bulging effect dominates over the shrinkage effect, and so positive centreline segregation is predicted.     

连铸保护渣性能与钢种、工艺参数关系的初探

从低、中、高碳钢等钢种本身性能及凝固收缩特点出发研究了不同钢种用保护渣性能特点,并分析了拉速、铸坯断面形状、尺寸及结晶器振动特性等连铸工艺参数对保护渣性能的要求,找出了不同钢种及工艺条件对保护渣性能设计的特点。     

Mould Powder Requirements for High‐speed Casting

Mould powders play an important role in the stability of the continuous casting process of steel. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer between the developing steel shell and the mould. Sufficient lubrication requires an undisturbed melting of mould powders and uniform infiltration of mould slag. Based on the casting practice in IJmuiden, it is found that these demands become even more important for the applied high casting speeds in thin slab casting at 5 to 6 m/min. At Corus RD&T, mould powders were characterised by X‐ray diffraction and subsequent fully quantitative Rietveld analysis. Additionally, the melting of mould powders has been studied in‐situ using high‐temperature X‐ray diffraction, to gain crucial knowledge about melting relations. Slag rims obtained from the thin slab caster mould were characterised using extended microscopic techniques in order to describe the mechanisms of rim formation and growth. Finally, slag films obtained after casting were characterised. As a result, not only the melting process of mould powder, but also the mechanism of formation and growth of slag rims is much better understood. This knowledge will be applied to define the demands on the composition and properties of mould powder for even higher casting speeds.     

不同角部形状特厚矩形坯凝固传热及应变数值模拟

 为了更加有效控制和减少连铸坯的角部横裂纹质量缺陷,根据其形成的机制,针对两种新型铸坯模型,即圆角和倒角模型进行研究。通过建立特厚连铸矩形坯在凝固过程的传热模型并进行数值模拟,得到铸坯在凝固过程沿拉速方向上温度场和坯壳厚度的分布规律,并在此基础上建立热力耦合模型,分析铸坯的应力变化,讨论了产生裂纹的可能性。研究结果表明,通过对比传统直角模型,得出圆角和倒角模型对铸坯角部温度场和应力场两个方面的分布状况都有改善,即新铸坯模型角部温度在连铸矫直段有效避开了钢的高温脆性区,同时降低了铸坯角部的应力值,减小了角部裂纹产生的可能性。     

Effect of Grade on Thermal–Mechanical Behavior of Steel During Initial Solidification

Thermal–mechanical analysis of solidification is important to understand crack formation, shape problems, and other aspects of casting processes. This work investigates the effect of grade on thermal–mechanical behavior during initial solidification of steels during continuous casting of a wide strand. The employed finite element model includes non-linear temperature-, phase-, and carbon content-dependent elastic–viscoplastic constitutive equations. The model is verified using an analytical solution, and a mesh convergence study is performed. Four steel grades are simulated for 30 seconds of casting without friction: ultra-low-carbon, low-carbon, peritectic, and high-carbon steel. All grades show the same general behavior. Initially, rapid cooling causes tensile stress and inelastic strain near the surface of the shell, with slight complementary compression beneath the surface, especially with lower carbon content. As the cooling rate decreases with time, the surface quickly reverses into compression, with a tensile region developing toward the solidification front. Higher stress and inelastic strain are generated in the high-carbon steel, because it contains more high-strength austenite. Stress in the δ-ferrite phase near the solidification front is always very small, owing to the low strength of this phase. This modeling methodology is a step toward designing better mold taper profiles for continuous casting of different steels.     

Effects of Mold Powder Chemistry on Shell Growth,Momentum and Heat Transfer in a Billet Mold

Effects of mold powder chemistry on shell growth and thinning have been studied using Computer Fluid Dynamic techniques under conditions of constant casting speed and steel superheat for a peritectic steel in a billet caster. Two mold powders were considered; a basic powder suitable for peritectic steels and an acid powder not recommendable for this steel in order to emphasize the importance of chemistry on shell stability. Numerical results indicate a strong interaction between powder composition and steel flow‐heat transfer phenomena. The acid powder creates recirculating flows at both sides of the entry jet that transport sensible heat to the shell inducing its remelting and thinning leading, eventually, to a strand breakout. Meanwhile, the basic powder induces a single recirculating flow in the internal radius side of the mold without severe shell thinning. A colder meniscus is predicted using the acid powder which is in agreement with the casting practice experience. Powder infiltration of the basic powder in between the mold hot wall and the strand provides a powder shell with a macroscopically smooth surface while the acid powder yields irregular infiltration. Buoyancy forces along the mold working height and mold curvature play a fundamental role on the generation of the recirculating flows. Interaction between powder chemistry and fluid flow‐heat transfer are two‐way coupled phenomena that must be considered for powder design purposes.     

重轨钢大方坯角部纵裂的形成与控制

针对重轨钢生产中出现的角部纵裂,从结晶器精度、保护渣性能、浇铸温度、结晶器液面波动和工艺操作等因素对角部纵裂的形成进行了分析.研究表明:结晶器精度、保护渣性能不佳和操作不规范是导致角部纵裂发生的主要原因.通过加强结晶器的检查、改进保护渣性能、优化水口结构和规范操作等措施,能有效地控制重轨钢角部纵裂的发生.     

Solidification and Particle Entrapment during Continuous Casting of Steel

Avoiding particle entrapment into the solidifying shell of a steel continuous caster is important to improve the quality of the continuous cast product. Therefore, the fluid flow dynamics in the steel melt and mushy zone, heat transfer and solidification of the steel shell, as well as the motion and entrapment of inclusion particles during the casting process were investigated using computational models. Solidification of the strand shell is modelled with an enthalpy‐formulation by assuming a columnar morphology in the mushy zone. The motion of particles is tracked with a Lagrangian approach. When the particles reach the solidification front, they can be entrapped/engulfed into the solid shell or pushed away from the solidification front, depending on the mushy zone morphology and the forces acting on them. The current paper focuses on the mould region at a steel continuous caster, including the submerged entry nozzle (SEN) and 1.2 m length of the strand. The results are validated with plant measurements and demonstrate the potential of the model to predict fluid flow, shell growth and the positions and the amount of entrapped/engulfed particles in the solidifying strand.     

Investigation of Friction Force between Mould and Strand Shell under Sinusoidal and Non‐sinusoidal Oscillation in Continuous Slab Casting

Friction force is an important parameter to evaluate powder lubrication and reflect the interaction between mould and strand shell. A non‐sinusoidal oscillation pattern emerges under high speed continuous casting, the characteristic of which is that the time of upward motion is longer than that of the downward motion within one cycle. In the present research experiments were performed on a slab continuous caster with a strand size of 1000 mm ×120 mm. The friction forces of sinusoidal and non‐sinusoidal oscillation under the same parameters were calculated and comparatively analysed. The results show that the friction forces of both oscillation patterns approximately change along with velocity, and present a trend to a trapezoid wave, but there is a phase difference between the friction force and relative velocity. Compared with sinusoidal oscillation, the non‐sinusoidal oscillation pattern exhibits some important advantages as follows; the maximal and average compressive forces and the compressive force work acting on the strand shell are all higher, the maximal tension is lower, and the actual amount of negative strip is higher. These advantages effect better healing of surface cracks and strand demoulds, and reduce the generation and expansion of cracks, consequently improve the surface quality.