An asymptotic model of the mold region in a continuous steel caster

A model is developed to simulate the solidification of the steel shell in the mold region of the continuous casting process. Conduction-dominated temperature fields in the mold, mold flux, steel shell, and molten steel regions are determined through the development of an evolution equation for the solidifying front. This equation is derived in the limit of small aspect ratio, mold width to height, using asymptotic methods. These results are coupled with a lubrication-theory model for the mold flux region. This model assumes a temperature-dependent viscosity for the mold flux and allows for solidification of the flux at temperatures below a critical value. System response to changing casting speeds, superheat, mold wall temperatures, and mold flux properties is investigated.     

板坯连铸结晶器钢液行为的水模拟试验

以某钢厂250mm×2 100mm断面板坯连铸结晶器为研究对象,采用水力模拟的方法研究了拉速在0.88m/min条件下不同水口结构参数对钢液行为的影响。研究表明:1号水口(原方案)在浸入深度为57mm时,液面波峰谷差值达到3.67mm,卷渣严重,冲击深度为205mm;优化设计的5号水口在浸入深度为57mm时,液面波峰谷差值减小为2.74mm,卷渣现象基本消失,冲击深度为177mm。确定5号水口为最优水口结构参数,浸入深度以57~67mm为宜。     

Study of transient flow and particle transport in continuous steel caster molds: Part I. Fluid flow

Unsteady three-dimensional flow in the mold region of the liquid pool during continuous casting of steel slabs has been computed using realistic geometries starting from the submerged inlet nozzle. Three large-eddy simulations (LES) have been validated with measurements and used to compare results between full-pool and symmetric half-pool domains and between a full-scale water model and actual behavior in a thin-slab steel caster. First, time-dependent turbulent flow in the submerged nozzle is computed. The time-dependent velocities exiting the nozzle ports are then used as inlet conditions for the flow in the liquid pool. Complex time-varying flow structures are observed in the simulation results, in spite of the nominally steady casting conditions. Flow in the mold region is seen to switch between a “double-roll” recirculation zone and a complex flow pattern with multiple vortices. The computed time-averaged flow pattern agrees well with measurements obtained by hot-wire anemometry and dye injection in full-scale water models. Full-pool simulations show asymmetries between the left and right sides of the flow, especially in the lower recirculation zone. These asymmetries, caused by interactions between two halves of the liquid pool, are not present in the half-pool simulation. This work also quantifies differences between flow in the water model and the corresponding steel caster. The top-surface liquid profile and fluctuations are predicted in both systems and agree favorably with measurements. The flow field in the water model is predicted to differ from that in the steel caster in having higher upward velocities in the lower-mold region and a more uniform top-surface liquid profile. A spectral analysis of the computed velocities shows characteristics similar to previous measurements. The flow results presented here are later used (in Part II of this article) to investigate the transport of inclusion particles.     

连铸结晶器及设备冷却水系统优化

结合玉溪新兴钢铁有限公司的生产实践,通过对现场调研收集的大量实际运行数据进行分析,得到方坯、板坯设备及结晶器供水情况,提出该循环水系统的最佳设备改造方案。生产结果表明,改进后,结晶器水、设备冷却水水温下降10℃,软水使用量明显下降。     

A water model study of the flow asymmetry inside a continuous slab casting mold

The work studies the extent of asymmetric flow in water models of continuous casting molds of two different configurations. In the molds where fluid is discharged through multiple holes at the bottom, the flow pattern in the lower portion depends on the size of the lower two recirculating domains. If they reach the mold bottom, the flow pattern in the lower portion is symmetrical about the central plane; otherwise, it is asymmetrical. On the other hand, in the molds where the fluid is discharged through the entire mold cross section, the flow pattern is always asymmetrical if the aspect ratio is 1:6.25 or more. The fluid jet swirls while emerging through the nozzle. The interaction of the swirling jets with the wide sidewalls of the mold gives rise to asymmetrical flow inside the mold. In the molds with lower aspect ratios, where the jets do not touch the wide side walls, the flow pattern is symmetrical about the central plane.     

Study of transient flow and particle transport in continuous steel caster molds: Part II. Particle transport

Particle motion and capture in continuous steel casters were simulated using a Lagrangian trajectory-tracking approach, based on time-dependent flow fields obtained from large-eddy simulations (Part I of this article). A computation was first conducted on a water model of a full-scale standard slab caster, where measurements were available. It simulated the transport of 15,000 plastic particles and their removal by a screen positioned near the mold top surface. The computation shows the screenremoval fractions to be 27±5 pct for 0 to 10 seconds and 26±2 pct for 10 to 100 seconds, which agrees with previous measurements. The flow exiting the nozzle was relatively uniform, and turbulent motion in the domain was very chaotic, so particle removal did not depend on the initial location of particles introduced in the nozzle port. A computation of motion and capture of 40,000 small inclusions (10 and 40 μm) was then performed in an actual thin-slab steel caster. The particles moved through the mold region with an asymmetrical distribution, which was caused by transients in fluid turbulence in the lower recirculation region, rather than by inlet variations at the nozzle port. Only about 8 pct of these small particles were removed to the top surface. This removal fraction was independent of both particle size and density, likely because all the simulated particles were too small to deviate significantly from the surrounding fluid flow. Finally, the computational results were further processed to predict the ultimate distribution of impurity particles in the solid thin slab after a short burst of inclusions entered the mold. They were reprocessed to reveal the distribution of total oxygen content for a steady inclusion supply from the nozzle. The results of this work confirm the important role of flow transients in the transport and capture of particles during continuous casting and can serve as a benchmark for future simplified models.     

板坯结晶器内钢液三维流场数学模型的研究

重点研究了浸入式水口浸入深度、拉速等工艺参数对结晶器内钢液流动状态的影响.研究结果表明:改变这些影响因素在一定程度上可以起到改善钢液流动状态的目的,但通过改变这些影响因素并不能完全实现对结晶器内钢液流动状态的有效控制,尤其是对液面波动和对窄面冲击强度的控制.     

小扁坯钢水转炉区域控制情况

针对4#小扁坯铸机不配精炼炉的实际情况,制定相应的钢水脱氧制度、温度制度以及时序的控制和用包制度.保证了4#小扁坯铸机的生产顺行.     

Casting of steel from the tundish of a continuous caster with a sliding gate

The continuous casting of steel in a section continuous caster with a three-plate sliding gate placed in the tundish is considered. The main causes of casting-channel clogging in casting are shown to be related to the displacement of the central plate with respect to the two other plates and to high heat removal through the tundish nozzle walls. The results obtained are used to find the rational displacement of the central plate and to correct the casting channel diameter.     

Numerical study of steady turbulent flow through bifurcated nozzles in continuous casting

Bifurcated nozzles are used in continuous casting of molten steel, where they influence the quality of the cast steel slabs. The present study performs two-dimensional (2-D) and three-dimensional (3-D) simulations of steady turbulent(K- ε) flow in bifurcated nozzles, using a finite-element (FIDAP) model, which has been verified previously with water model experiments. The effects of nozzle design and casting process operating variables on the jet characteristics exiting the nozzle are investigated. The nozzle design parameters studied include the shape, angle, height, width, and thickness of the ports and the bottom geometry. The process operating practices include inlet velocity profile and angle as well as port curvature caused by erosion or inclusion buildup. Results show that the jet angle is controlled mainly by the port angle but is steeper with larger port area and thinner walls. The degree of swirl is increased by larger or rounder ports. The effective port area, where there is no recirculation, is increased by smaller or curved ports. Flow asymmetry is more severe with skewed or angled inlet conditions or unequal port sizes. Turbulence levels in the jet are higher with higher casting speed and smaller ports. Formerly Research Assistant, Department of Mechanical and Industrial Engineering. Formerly Research Assistant in the same department.     

Numerical investigation of the interface in a continuous steel casting mold water model

In this study, the steady-state Navier-Stokes equations are solved on a curvilinear nonorthogonal grid, following the finite volume approximation, with a pressure prediction-correction method, for the case of a flow in a model steel casting mold. The steel flow is simulated by water flow and the slag layer by an oil film, following conditions of previous experimental studies. The simulation aims at the understanding of the free wave and the interface surface wave behavior and the mechanism that leads to the breakup of the steel-slag interface, and thus induction of impurities inside the final steel product. Boundary conditions are set on the free and the interface surfaces, and an adaptive grid mechanism is used in order to update the grid’s shape so as to follow the wave formation. Several cases have been considered with the inlet velocity parameter, and results concerning the velocity field and the generated waves are reported. It is shown that a critical casting speed exists that leads to wave instability, which may be associated with emulsification phenomena.     

板坯连铸机浸入式水口结构优化的水模研究

通过水力模型实验,研究了水口结构对南钢板坯连铸机结晶器内钢水流动特性的影响.研究结果表明:使用现有水口结晶器内液面波动大且存在明显裸露并有严重的卷渣现象,适当增加浸入式水口出口面积,在不改变水口其它结构参数和插入深度条件下,能够获得液面波动小,保护渣覆盖良好且没有卷渣的钢水流动特性,不仅能够实现提高拉速的要求,还能提高铸坯质量.     

Aspects of a technology for obtaining semifinished products of low-carbon steel on a continuous section caster

The Chelyabinsk Metallurgical Combine has mastered a technology for obtaining continuous-cast semifinished products of low-carbon ([C] < 0.10%) steels SAE1006 and SAE1008. These steels are cast in an open stream on a continuous section caster with 100 × 100 mm molds. It was determined that good-quality semifinished products and favorable casting conditions are assured by deoxidizing the steel with secondary aluminum during casting. The amount of aluminum used here is 1.2 kg/ton. The aluminum is introduced into the ladle before the ferroalloys. The value of the ratio [Mn]:[S] in the finished steel should be no lower than 22, while the value of [Mn]:[Si] should be at least 2.5. Casting speed is kept at or below 5.2 m/min, and the maximum pressure exerted on the semifinished product by the withdrawing rollers is 35 ton-f. Scanning electron microscopy and x-ray phase analysis were used to determine the composition of the sulfide compounds in the steels when the ratio [Mn]:[S] has the values 14.8 and 17.7. __________ Translated from Metallurg, No. 7, pp. 59–62, July, 2007.     

A numerical and experimental study of the solidification rate in a twin-belt caster

A numerical and experimental study was carried out to investigate the solidification process in a twin-belt (Hazelett) caster. The numerical model considers a generalized energy equation that is valid for the solid, liquid, and mushy zones in the cast. Ak-ε turbulence model is used to calculate the turbulent viscosity in the melt pool. The process variables considered are the belt speed, strip thickness, nozzle width, and heat removal rates at the belt-cast interface. From the computed flow and temperature fields, the local cooling rates in the cast and trajectories of inclusions were computed. The cooling rate calculations were used to predict the dendrite arm spacing in the cast. The inclusion trajectories agree with earlier findings on the distribution of inclusion particles for near horizontally cast surfaces. This article also reports the results of an experimental study of the measurement of heat flux values at the belt-cast interface during the solidification of steel and aluminum on a water-cooled surface. High heat fluxes encountered during the solidification process warranted the use of a custom-made heat flux gage. The heat flux data for the belt surface were used as a boundary condition for the numerical model. Objectives of the measurements also included obtaining an estimate of the heat-transfer coefficient distribution at the water-cooled side of the caster belt. Y.G. KIM, formerly Graduate Student, Materials Engineering Department, Drexel University.     

A mathematical model of a single-roll continuous strip caster based on fluid-mechanics considerations

A mathematical model based on fluid mechanics considerations has been formulated to describe a single-roll continuous strip caster. The heat transfer related phenomena are incorporated in the model through an empirical over-all parameter. The model provides useful interrelationships between the final strip thickness and the operation and design parameters, e.g. the speed of rotation of the caster drum, the stand-off distance, the nozzle passage gap and the liquid metal head in the reservoir. Simulating the process, using the model, it has been possible to predict the effect of these parameters on the final strip thickness. The model allows, though indirectly, the prediction of the conditions which would lead to choking of the liquid metal pool due to premature solidification. Comparing the results of this model with those obtained using the model based on heat balance considerations only, it is noted that the former predicts higher values of final strip thickness.     

Numerical model for the minimization of intermixed round bars in a four line continuous caster

The grade transition process in a four line round billets continuous caster was analyzed. A numerical model was developed to determine, quickly and accurately, the length and location of intermixed steel in the bar. The model is based on “tank in series” or “volumes” mathematical models and was calibrated with physical water model measurements and full three-dimensional (3-D) turbulent numerical calculations. Comparison of the model with measurements performed on intermixed steel bars is also presented. The good agreement between these plant measurements and the numerical results validated the model. The model was implemented on PCs with a visual interface that allowed the easy input and output of data, and the program is operated in a steel shop. Finally, the code was employed to evaluate different strategies to fill the tundish during grade transitions. The “double dilution” process, which consists of filling the tundish in two steps, proved to be useful to reduce the amount of downgraded steel.