Influence of running water on the heat transfer in continuous casting

The influence of running water on spray water cooling is experimentally looked at with the aid of a stationary working experimental plant. The running water was fed separately to the sample from above as a defined water film by using a slot nozzle. In addition to the spray nozzle pre-pressure, the velocity of the running water at the slot nozzle exit and the temperature of the running water, the angle of inclination of the sample plane was also varied. By employing three different full cone spray nozzles and setting different nozzle distances, a range of 450 to 2000 l/(m²·min) of the water impingement density was covered. To estimate the marginal influences, samples of 20 × 20 mm as well as 30 × 30 mm were used. Studies of initially pure spray water cooling without the additional running water film revealed a dependency of the heat transfer on the spray water impingement density, the spray water velocity and also the angle of inclination of the sample plane. For nozzles with a low water impingement density, a superimposing of a running water film on the spray water revealed a sharp increase in the heat transfer coefficients compared to pure spray water cooling. For nozzles with a high water impingement density and, therefore, with a high heat transfer already even for pure spray water cooling, the increase of the heat transfer coefficient is minor.     

连铸喷淋结晶器的传热数学模型

针对铸坯在结晶器内的凝固特性，建立了喷淋冷却结晶顺的传热数学模型。并数值计算了普通循环水冷结晶器与喷淋冷却结晶器的冶金参数，讨论分析，比较了两种冷却方式的冶金效果。     

Modelling water cooling of steel strip during hot rolling

Several tests of water cooling of steel samples are presented in the paper. The water spray and quenching conditions are considered. The temperature variations during the tests are monitored using the thermocouples embedded in the side of the samples. The experimental time – temperature profiles are compared with the results of calculations based on the finite element model. Comparison of the theoretical and experimental results allowed to suggest heat transfer coefficients of 10000 W/m²K for water in quenching, 6000 W/m²K for the top surface during water spray cooling and 4000 W/m²K for the bottom surface during water spray cooling.     

Ultra Fast Cooling of a Hot Steel Plate by Using High Mass Flux Air Atomized Spray

In the current research, the ultra fast cooling (UFC) of a hot stationary AISI‐304 steel plate has been investigated by using air atomized spray at different air and water flow rates. The initial temperature of the plate, before the cooling starts, is kept at 900°C or above. The spray was produced from a full cone internal mixing air atomized spray nozzle at a fixed nozzle to plate distance; and the average spray mass flux was varied from 130 to 370 kg m^?2 s by selecting different combinations of air and water flow rates. The surface heat flux and surface temperature calculations have been performed by using INTEMP software and the calculated results have been validated by comparing with the measured thermocouple data. The heat transfer analysis indicates that the cooling occurs in the transition boiling regime up to surface temperature of 500°C and thereafter it changes to nucleate boiling regime. The superposed flow of air on the hot plate enhances the cooling in the temperature range of 900–500°C by sweeping the partially evaporated droplets from the hot surface. However, due to the high percentage of fine water droplets in the resultant spray produced at higher air flow rates, the maximum cooling rate is achieved at the medium air flow rate of 30 N m³ h^?1. The cooling rate (182°C s^?1) produced by an air atomized spray is found to be in the UFC regime of a 6 mm thick steel plate. The findings of this research can be considered as the basis for the fabrication of cooling system in the run‐out table of a hot strip mill.     

Two-dimensional heat transfer model for secondary cooling of continuously cast beam blanks

Abstract

A two-dimensional heat transfer model was developed for the secondary cooling system during beam blank continuous casting. The finite element method was used to calculate the heat transfer. Accurate cooling boundary conditions in the secondary cooling zone are involved, including spray water cooling, water evaporation cooling, radiation cooling and roll contact cooling in the casting direction and non-uniform distribution of spray water flow density in the cross-section. The causes of longitudinal crack at the fillet during Q235 steel continuous casting were analysed on the basis of the simulation of the developed model, and then the spray water flow and the transverse nozzle layout were optimised. Practical results show that the surface quality of the beam blank improved after optimisations. Numerical results from the present model were validated using previous experimental measurements, which show good agreement.     

High-speed temperature measurements during rapid solidification of iron-silicon ribbons,produced by planar flow casting

In order to investigate the melt undercooling and the non-equilibrium solidification of crystalline Fe 5 wt.% Si melt spun ribbons, produced by planar flow casting (PFC), high speed temperature measurements and appropriate process simulations have been performed. Using a rotating fibre optical system with a fast response double pyrometer, the temperature radiation of the solidifying ribbon during the casting process has been recorded with a measuring frequency of 50 kHz. The obtained cooling curves have been interpreted by computer simulations. It is shown that with increasing wheel temperature the overall cooling becomes more efficient. This is caused by an improved wetting behaviour of the melt-wheel system and an increase in the heat transfer coefficient at the interface of the solidifying ribbon and the wheel from 6 · 10⁴ to about 2 · 10⁵ W/(m²K). The solidification of 100 to 200 μm thick ribbons takes place in a time interval of 2 to 5 ms. The average growth rate varies between 10 and 60 mm/s. The high cooling rate results in a fine dendritic solidification morphology with diminishing microsegregations.     

Compensation Control Model of Superheat and Cooling Water Temperature for Secondary Cooling of Continuous Casting

In this paper, a compensation control model of secondary cooling process of billet continuous casting for quality steel has been presented. The effects on the spray control of the various parameters such as steel superheat, casting speed, cooling water temperature and chemical component of steel were considered. The parameters of control model were determined to associate with the two‐dimensional heat transfer equation and solved by finite‐difference method. Effects of steel superheat and cooling water temperature on surface temperature, solidification structure and solidifying end point were discussed. Results indicate that steel superheat significantly affects solidification structure and solidifying end point but has a little effect on slab surface temperature. Moreover, secondary cooling water temperature affects surface temperature and solidifying end point but has a little effect on solidification structure. The surface temperature and solidifying end point can be maintain stabilized through applying the compensation control model when steel superheat and cooling water temperature vary. The models have been validated by industrial measurements. The results show that the simulations are in very good agreement with the real casting situation.     

Analysis of Mould,Spray and Radiation Zones of Continuous Billet Caster by Three‐dimensional Mathematical Model based on a Turbulent Fluid Flow

An analysis of mould, spray and radiation zones of a continuous billet caster has been done by a three‐dimensional turbulent fluid flow and heat transfer mathematical model. The aim was to reduce crack susceptibility of the billets and enhance productivity of the billet caster. Enthalpy‐porosity technique is used for the solidification. Turbulence is modelled by a realizable k‐ε model. The three‐dimensional mesh of the billet is generated by Gambit software, and Fluent software is used for the solution of equations. In various zones, different standard boundary conditions are applied. Enhanced wall treatment is used for the turbulence near the wall. In the mould region, Savage and Prichard expression for heat flux is applied. In the spray cooling zone, the heat transfer coefficient for surface cooling of the billet is calculated by knowing the water flow rate and the nozzle configuration of the plant. The model predicts the velocities in the molten pool of a billet, the temperature in the entire volume of billet, the heat transfer coefficient in the mould region, the heat flux in the cooling zone and radiation cooling zone, and the shell thickness at various zones. The model forecasts that the billet surface temperature up to the cutting region is above the austenite‐ferrite transformation temperature (which is accompanied by large volume change). The model predicts a temperature difference of maximum 700 K between the centre and surface of the billet. The entire solidification takes place at 11.0 m length at 3.0 m/min. For the same casting arrangement, increasing the casting speed up to 4.0 m/min has been explored. Based on the simulation results, recommendations to alter the spray water flow rate and spray nozzle diameter are presented to avoid a sudden change of temperature.     

The solidification of pure metals (and eutectics) under uni-directional heat flow conditions: II. Solidification in the presence of superheat

The generalized integral-profile method previously developed² to predict the solidification rates of pure metals and eutectic alloys under a range of cooling conditions has been extended to treat solidification rates in the presence of super heat. Heat transfer within the liquid metal has been characterized in terms of a single parameter for which a heat balance on the liquid yields an ordinary nonlinear equation. This equation is additional to the two equations derived previously for the growth of the solid metal, and the three equations have been solved simultaneously using a Runge-Kutta technique. Solutions have been obtained for heat transfer conditions that can be reproduced accurately in laboratory experiments. The results of a series of such experiments are also presented and shown to agree very well with the theoretical predictions. In carrying out the analysis, it has been shown that a number of different cooling and solidification modes can occur during the solidification of superheated liquid metal, the solidification process following one of two possible routes through these modes. This presents a useful approach to the analysis of this solidification problem and, indeed, of more complicated problems since the logic of the computer program used in the analysis is closely related to the logic of the solidification process. The work described in this paper was carried out while the authors were with the John Percy Research Group in Process Metallurgy at Imperial College, London.     

Optimal control of secondary cooling for medium thickness slab continuous casting

Abstract

A software to simulate the solidification, heat transfer and water flowrate distribution in slab continuous casting was developed by establishing a mathematical model for the heat transfer and solidification in medium thickness slab casting. This model was validated by pin shooting and surface temperature measurement experiments. A reasonable target surface control temperature was found by testing the high temperature mechanical properties of Nb bearing ship plate steel, and then the water flowrate of each loop of the secondary cooling zone was determined by the software. The influence of uneven secondary cooling in the slab width direction on the quality of the slab was also investigated, which provided data for the optimisation of the secondary cooling of the slab caster. On the basis of the above research, an optimisation scheme for a secondary cooling system was proposed. Experimental results showed that the quality of the slab was significantly improved after optimisation. The centreline macrosegregation was reduced, and the ratio of equiaxed grains was increased by 3·18%. In addition, the transverse cracking of the slab was almost eliminated.     

Analysis of heat flow and microstructure evolution during spray deposition of Fe-3C-1.5Mn alloy

A theoretical model for the concomitant solidification of droplets and preform during spray deposition has been proposed, based on heat-flow analysis. It has been unambiguously demonstrated that cooling rates approaching those in the rapid solidification (RS) regime can only be achieved when the droplets are still in free flight during the deposition process. The cooling rates in the droplets range from 10⁴–10⁶ Ks^?1 depending upon their size for the experimental conditions employed in the present studies. In contrast, the model predicted cooling rates for the deposits in the region of 10³–10⁴ Ks^?1. A hypoeutectic Fe-3C-1.5Mn-0.3Si has been chosen as an experimental alloy for studies relating to microstructural characterization. The microstructure of powder developed fully during solidification of droplets in free flight revealed dendritic morphology of the metastable austenitic phase, whereas the spray-deposited alloy exhibited characteristic homogeneous and refined substructure. The evolution of microstructure during spray deposition as also during atomization has been compared and discussed by invoking the proposed model.     

喷嘴喷淋距离对连铸小方坯二冷均匀性的影响

研究了不同喷淋距离下连铸小方坯二冷喷嘴的水量分布,建立了凝固传热模型分析了82B钢连铸坯的热行为。该模型特别考虑了二冷区铸坯表面宽度方向的水流密度分布,并根据铸坯表面测温结果进行了模型校正。采用凝固传热模型研究了喷嘴喷淋距离对连铸二冷均匀性的影响。结果表明:喷嘴喷淋距离的增加有助于提高二冷水横向分布的均匀性,导致铸坯表面温度横向均匀性降低、纵向均匀性提高。这些效果有助于改善铸坯内部裂纹,但是会对角部裂纹产生不利影响。在二冷区前段喷嘴采用低喷淋距离,二冷区末段采用高喷淋距离,既可以提高铸坯角部温度,又能降低表面最大回温速率,有助于同时改善连铸坯角部和内部裂纹。在此基础上,提出了一种连铸小方坯二冷喷嘴布置方式,即二冷区每段喷嘴喷淋距离沿拉坯方向逐渐增加,该方法有助于提高连铸坯“纵?横”冷却均匀性。      

Solidification mechanism of martensitic stainless steel

Abstract

In an unidirectional solidification experiment, an 8 kg stainless steel ingot with the composition 0·25%C, 17%Cr, and 1%Mn was solidified under continuous casting conditions. The dwell time of primary cooling was varied, followed by secondary spray cooling. Metallographic investigation, heat transfer, and segregation were carried out to study the solidification mechanism. The partition ratio of the elements present in ferrite and in austenite (martensite) was determined. It was indicated that the solidification follows: L → L + δ → L + δ + γ → δ + γ + carbides. Under high cooling rates γ austenite solidifies as a leading phase. The beginning of spray cooling has the main effect in controlling the obtained microstructures. Carbide thickening is observed in the rapidly cooled zone between the ferrite and the martensitic matrix. Tempered martensite increases by lowering the cooling rate, which gives more time for carbide dissolution and for carbon to diffuse into the ferrite, eventually increasing the austenite (martensite) fraction in the final matrix at the expense of ferrite.     

基于非调质钢凝固特性的二次冷却控制

提出了基于非调质钢凝固特性的二次冷却控制方法。在凝固特性研究方面,运用高温共聚焦显微镜、场发射扫描电镜研究了冷速对第二相粒子析出规律的影响,并阐释了先共析铁素体的相变机制。结果表明,第二相粒子在1086 ℃开始析出,并在912 ℃达到峰值。当冷速在0.1～5 ℃·s?1时,随着冷速增大,第二相粒子尺寸和数量均减小,且第二相粒子由晶界处的链状分布向晶体内的弥散分布过渡,提高冷速有助于削弱第二相粒子的钉扎作用,强化铸坯表层微观组织;在二冷配水优化方面,建立了考虑铸坯横向水量分布的凝固传热数学模型,提出了基于非调质钢凝固特性的二冷配水优化方案,即对出结晶器后的铸坯实施强冷,以满足控制第二相粒子析出的合理冷速和温度区间的要求。工业试验证实了技术方案的可行性。此外,研究表明,降低喷淋距离有助于改善连铸坯横向冷却不均匀性。本研究统筹考虑二冷水量与喷淋距离对非调质钢裂纹敏感性的影响,通过开展“纵?横”凝固冷却控制研究对连铸二次冷却进行系统优化,提出的二冷优化方案有助于改善连铸坯的表面及皮下裂纹。      

Mathematical Simulation of Process Parameters in the case of Rapid Solidification of Steel

Rapid solidification of steel was investigated experimentally under laboratory conditions by immersion of cold copper rods in steel baths. For a better understanding of the process parameters during rapid solidification, an explicit finite difference model was employed. In the calculation, a coefficient of heat transfer between a frozen steel shell and solid copper of α = 40 [KW/m²·K] is assumed in good agreement with experimental data derived from temperature measurements. The solidification parameters such as local time of solidification (LST), local time of the superheat reduction (LShRT), local solidification and cooling rates (LSR, LCR) and local heat flux density of solidification and superheat reduction (LSHFD, LShRHFD) can be calculated using the developed model, in dependence on the processing conditions. This influence of processing parameters, such as steel bath superheat, steel bath material, immersed body material, initial temperature of immersed body and immersed body geometry, were the subject of intensive investigations.     

Droplet solidification and gas-droplet thermal coupling in the atomization of a Cu-6Sn alloy

The accurate prediction of solidification cooling rates of droplets in gas atomization is critical to the production of quality powders and spray deposits. Despite many sophisticated on-line measuring instruments and numerous mathematical models, an experimentally verified analysis of droplet cooling in gas atomization is still lacking. Impulse atomization offers a unique single-fluid atomization technique with controlled gas-droplet heat transfer. This technique was, therefore, used to generate a reliable cell-spacing vs cooling-rate relationship for Cu-6Sn. Subsequently, the cell spacing was measured for gas-atomized (GA) particles that were collected with quench probes in known parts of the melt spray. Using the calibration work done on impulse atomization and the sampling positions in gas atomization, the effect of particle size, spray parameters, and location in the spray are analyzed. This analysis incorporates other on-line diagnostics in gas atomization to relate atomized droplets to GA processing conditions. A comparison between impulse atomization and gas atomization is also discussed. This work clearly demonstrates that the droplet cooling rate is more strongly affected by the gas temperature than by the droplet-gas relative velocity. In addition, the thermal Stokes number is a useful parameter to assess the amount of two-way thermal coupling in a given spray. Hence, sprays with low thermal Stokes numbers having droplets of the same size will experience lower cooling rates and have correspondingly higher gas temperatures.     

喷淋结晶器传热特性的实验研究

喷淋结晶器是一种新型结晶器，它是用特殊的喷嘴将水喷淋到结晶器的铜板表面上进行铸坯冷却的，这种冷却方式具有较高的传热速度，便于调节结晶器的热流密度，可以提高拉坯速度和铸坯质量，喷淋结晶器的传热现象是很复杂的，本文通过建立传热实验台，对喷淋冷却过程进行较细致的实验研究，找出喷水量和换热系数的关系等，为设计喷淋冷却结晶器提供必要的参数。     

水平连铸结晶器内真实换热边界计算及传递模型

连铸结晶器内冷却水的流动、传热和金属液的传热、凝固全耦合计算时,模型计算效率低、收敛性差,而不考虑真实冷却水流场只进行金属液传热-凝固耦合时,模型计算精度较低,可将冷却水流动-传热和金属液传热-凝固过程分别建模,并基于二次开发的温度场量原位传递程序,将模型串联耦合。结果表明,通过耦合流场计算得到的真实冷却边界替代传统均匀界面换热系数的边界条件施加方式,在保证模型计算效率和计算精度的前提下,可准确模拟铸坯晶粒的尺寸、取向和数量,经试验验证,模拟结果与试验结果高度吻合。     

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.     

基于目标温度的方坯连铸二冷配水方案优化

通过建立铸坯传热数学模型,对连铸凝固过程进行了解析。制定了二冷配水制度优化原则,基于目标温度控制原则开发出二冷配水控制模型。对不同钢种、拉速等工艺条件下的连铸工艺设计出二冷总水量、水量分配和水量与拉速的关系式,并对配水方案进行了凝固与传热的预测和验证。