珠江钢厂CSP热连轧层流冷却热过程模拟研究

分析了带钢层流冷却的传热过程，将相变潜热引入到层流冷却热过程数学模型，用过程模拟的方法研究了珠江钢厂CSP热连轧层流冷却系统。讨论了喷水方式、喷嘴位置、板速和板厚对带钢温度场分布的影响。结果表明建立的模型对于珠钢CSP热连轧层流冷却系统有较好的适应性，能够对实际生产起到较好的指导性作用。     

带钢超快速冷却条件下的换热过程

 分析了轧后加速冷却过程中带钢表面的局部换热机理，认为冷却系统实现超快速冷却的关键在于扩大带钢表面射流冲击换热区的面积。确定了薄带钢实现超快速冷却所需的对流换热系数，并采用有限元分析工具ANSYS模拟得到了超快速冷却条件下不同厚度带钢的温度场。温度场的分布表明薄带钢在超快速冷却过程中具有较好的温度一致性。同时还表明随着带钢厚度增加，超快速冷却条件下厚度方向的温度梯度显著增大，对于带钢内部组织的均匀性将产生不利的影响。带钢厚度范围应是超快速冷却技术实际应用过程中的重要考虑因素。     

Modelling of heat transfer in hot strip mill runout table cooling

The need for accurate prediction and control of cooling profiles of steel strips on runout tables has led to the development of a mathematical model that is able to predict coiler temperatures under any given condition with an accuracy of ± 14 °C as well as calculating the entire temperature profile of a steel strip with sufficient accuracy. Comparisons with online strip temperature data at various locations of the runout table, which were obtained by a new experimental procedure, show that the effect of single cooling headers on the thermal response of a steel strip can be predicted. The model takes into account all relevant thermodynamic effects by means of a statistical approach. Heat transfer to the environment, steel thermophysical properties and phase transformation are modelled using B‐splines. Model adaptation is realised by fitting calculated and measured coiler temperatures of approximately 40000 strips with a least square method in order to gain optimal base values for the B‐spline functions. During model development special attention was paid to the model's capability of being re‐adjustable to a large variety of conditions as well as its local behaviour. Therefore, concepts like temperature‐dependent heat transfer coefficients, which are applicable only to one specific plant, have been avoided in favour of a more generalised formulation of the model that helps to gain insight into the physics of the processes involved, i.e. heat transfer of subcooled jet impingement boiling and film boiling. It was found that both cooling water and steel surface temperature have a large influence on heat transfer whereas the influence of strip speed can be neglected.     

Numerical determination of heat transfer coefficient for boiling phenomenon at runout table of hot strip mill

Abstract

The heat transfer coefficient during film boiling at the runout table of the hot strip mill is usually determined by experimental methods. Described in the present paper is a finite difference based model for analysis of the thermal behaviour of the strip during cooling at the runout table of the hot strip mill at Tata Steel, India. The model, developed for the prediction of strip temperature, is used to determine the heat transfer coefficient at the water/strip interface while water cooling occurs. A simple form of polynomial as a function of the strip surface temperature is proposed to describe the heat transfer coefficient at the water/strip interface. Good correlation has been found between model predicted temperatures considering the polynomial type heat transfer coefficient and the actual coiling temperature.     

改善中厚板轧后超快冷均匀性的措施及其应用

 以改善中厚板轧后冷却均匀性为目的,从高压水射流冲击换热原理出发,研究在超快速冷却条件下的改善钢板冷却均匀性方法。通过合理设计集管及其布置形式,采用上集管位置设定、水比设定以及钢板头尾速度遮蔽等措施,实现了在超快速冷却过程中钢板各个方向上的冷却均匀性。将上述措施用于在线生产,结果表明,钢板各向均匀性控制良好,长度方向95%以上的温度被控制在距离目标返红温度±25℃的范围之内。     

A parametric study of the accelerated cooling of steel strip

To gain insight into the effects of various parameters controlling the thermal behaviour of a quenched steel strip during the process of accelerated cooling by an array of planar water jets, a parametric study has been performed using a previously developed and validated mathematical model. The behaviour of the strip was characterized by its coiling temperature, top and bottom surface temperature variations and heat extraction in the jet impingement region, top surface heat extraction in the film boiling region, and top and bottom surface thermal penetration depths. Parametric variations included cooling system design conditions, such as the top and bottom nozzle widths, and operating conditions such as the top nozzle discharge velocity, the cooling water temperature, and the ratio of volumetric water flow rates applied at the top and bottom surfaces. The effects of steel strip parameters such as strip thickness and strip velocity were also considered.     

Experimental study of boiling heat transfer during subcooled water jet impingement on flat steel surface

Abstract

The growth in demand for high quality metal alloys has placed considerable emphasis on the type of cooling methods used in manufacturing processes, in particular, the production of highly tailored steel through controlled cooling on the runout table. The present study focuses on the heat transfer (cooling of hot rolled steel strips) on a runout table. The purpose of the study was to develop an efficient experimental method and collect temperature data under conditions similar to those that occur during industrial runout table conditions in a steelmill. Surface and internal temperatures were measured during transient cooling of a flat, upward facing fixed steel plate cooled by a highly subcooled single, circular, free surface jet of water. Measurements were made at stagnation and several streamwise distances from the stagnation point. A numerical, finite difference model was applied to calculate the surface heat flux using measured temperatures. The effect of water flowrate and subcooling on the overall heat transfer with emphasis on the maximum heat flux is discussed.     

多排气雾冲击射流换热过程的试验研究

气雾冷却是带钢连续镀锌后的一种强化冷却方式。气雾冷却装置的设计和运行的关键是掌握气雾换热系数。采用试验方法研究了多排气雾射流冷却高温钢板的换热系数,考察了喷气流量和喷水流量对换热系数的影响。试验结果表明：喷气流量对气雾换热系数影响可以忽略;喷水流量对换热系数影响显著,在喷水流量为0.96～1.59 m^3/h时,换热系数随喷水流量的增加而明显上升,最大可达5 650 W/（m^2.K）;喷雾冷却的换热系数远大于常规喷气冷却,能有效地强化镀后冷却。     

钢板轧后冷却过程的数值模拟及变形分析

 针对钢板冷却发生翘曲变形的问题，利用热弹塑性有限元法对中厚板冷却过程的温度场及应力应变场进行数值模拟。采用分步循环加载的方法精确模拟不同换热边界条件，温度场模拟结果与实测值吻合较好。将热分析得到的温度场作为载荷进行钢板冷却过程应力应变的模拟，模拟结果表明塑性应变差出现在水冷的开始阶段。因此为了获得平直的板形，必须调整上下集管水量比，尽量减小水冷开始时上下表面的应力和塑性应变差。     

An experimental study on heat transfer process of multiple mist impinging jets

Mist jet impingement cooling is an enhanced heat transfer method widely used after the continuous galvanizing process.The key of a successful design and operation of the mist jet impingement cooling system lies in mastering heat transfer coefficients.The heat transfer coefficients of high temperature steel plates cooled with multiple mist impinging jets were experimentally investigated,and the effects of gas and water flow rates on heat transfer coefficients were studied.The test results illustrate that the gas flow rate has little effect on the mist heat transfer rate.It is also found that the water flow rate has a great impact on the heat transfer coefficient.When the water flow rate ranges from 0.96m3/h to 1.59 m3/h,an increase in the rate will produce a higher heat transfer coefficient with a maximum of 5650 W/(m2·K).Compared with the conventional gas jet cooling,the heat transfer coefficient of the mist jet cooling will be much higher,which can effectively strengthen the after-pot cooling.     

带钢连退线风管式冷却过程的数值模拟

 喷射气体冷却是带钢连续退火和镀后冷却过程中最重要的冷却方法,不同的冷却器结构对冷却性能影响很大。针对带钢连续退火及镀后冷却风管式冷却器工艺过程,将计算区域划分为2个子区域,经过网格独立性检验,在一定网格数目基础上利用数值模拟软件进行数值模拟研究。通过对风箱风管区域研究获取冷却器的压力流量公式,对冲击射流冷却区域研究获取带钢表面对流换热系数及带钢冷却速度变化。典型工况下带钢表面平均对流换热系数为117.29W/(m2·K),带钢冷却速度14.0℃/s。     

Spray Cooling Heat Transfer and Calculation of Water Impact Density for Cooling of Steel Sheet Materials by Inverse Process Modelling

Modern steel grades require accurate temperature control during processing. The cooling section technology has to deliver prescribed cooling rates while fulfilling specific constraints, e.g. on the minimum surface temperature. For all material thicknesses, numerical cooling system set value prediction is advantageous and above 10 mm, the possible cooling rates and the experimental parameter determination are limited by physical constraints. Laboratory measurements provide quantitative experimental data on the heat transfer coefficient (HTC) depending on the cooling system parameter water impact density and the temperature difference. The desired final material properties determine temperature control and cooling rate. This information is used to predict the optimum cooling section set values for a specific cooling task. The inverse modelling calculations use a simple cooling section process model. Illustrative examples for optimum cooling of strip or sheet material using water spray cooling demonstrate the approach. Additionally, the physical limitations due to the finite heat conductivity of the strip are calculated and discussed.     

钢管冷却喷淋水量对换热系数的影响

 采用一种钢管外喷淋装置,通过在钢管中预埋热电偶测量钢管冷却期间的温降曲线,研究了喷淋水量对钢管冷却强度的影响。利用有限元模拟软件对测量结果进行分析,建立了钢管表面对流换热系数和水流密度的数学关系模型,将模型嵌入了钢管温度场计算软件当中,软件的计算精度进行了工程验证。结果表明,融合了该换热数学模型的钢管温度场模拟软件计算精度达到90％以上。     

热轧带钢层流冷却过程温度场研究

 建立了热轧带钢层流冷却过程中温度场的三维有限元模型,对3 mm厚带钢轧后冷却过程带钢温度场进行模拟计算,得出卷取温度比现场测量值低9.5 ℃,相对误差为1.42%,验证了模型和假设的合理性。研究了上喷嘴直径对带钢温度的影响,带钢上表面宽度方向上存在2种不同的冷却区域：位于喷嘴正下方层流冷却过程中交替经过冲击区和平流区的区域和位于两喷嘴之间层流冷却过程中只经过平流区的区域,这造成带钢宽度方向上温度分布不均匀。计算结果表明,喷水量保持不变的情况下,存在一个最佳喷嘴直径,使带钢宽度方向上温度分布更均匀。喷水速度保持不变,增加喷嘴的直径有利于带钢宽度上方温度均匀,但增加了喷水量,降低了带钢的卷取温度。     

Self-Learning and Its Application to Laminar Cooling Model of Hot Rolled Strip

The mathematical model for online controlling hot rolled steel cooling on run-out table （ROT for abbreviation） was analyzed, and water cooling is found to be the main cooling mode for hot rolled steel. The calculation of the drop in strip temperature by both water cooling and air cooling is summed up to obtain the change of heat transfer coefficient. It is found that the learning coefficient of heat transfer coefficient is the kernel coefficient of coiler temperature control （CTC） model tuning. To decrease the deviation between the calculated steel temperature and the measured one at coiler entrance, a laminar cooling control self-learning strategy is used. Using the data acquired in the field, the results of the self-learning model used in the field were analyzed. The analyzed results show that the self-learning function is effective.     

立式连续退火炉中带钢气体射流冷却数值模拟及温度场计算

气体射流冷却是立式连退炉内带钢快速冷却的主要方式,计算冷却过程中带钢温度场分布,对改善带钢连续退火工艺有着重要的指导意义。利用CFD模拟软件对冷却过程进行数值模拟,获得带钢表面换热系数分布规律。运用有限差分法,对带钢温度场分布进行数值仿真计算。研究了初始带钢温度对冷却过程中带钢温度场分布规律的影响。     

Heat Transfer during Multiple Jet Impingement on the Top Surface of Hot Rolled Steel Strip

Using a unique pilot facility a series of tests were conducted using three top jets to simulate the heat transfer that occurs during run‐out table (ROT) cooling. Steel samples instrumented with internal thermocouples were tested on this facility and the effect of top jet configuration (nozzle spacing of 40 to 115mm), and water flow rate (15 and 30 1/min) were quantified using moving plate samples. The multiple top jet work indicated that heat transfer across the plate width varies significantly and is high directly under the nozzle but decreases rapidly away from the nozzle in the interaction region. As cooling progresses a much larger wetted region occurs and more uniform cooling is experienced across the plate. Multiple jet cooling experiments have also confirmed that nozzle spacing does have an effect on heat transfer. This effect is predominate in the interaction region where closer nozzle spacing leads to enhanced and more uniform heat transfer in the lateral direction across the plate width away from the nozzle. As expected higher water flow rates led to higher heat transfer both under the nozzle and in the interaction region.     

Re-reddening on Strip Surface After Water Cooling

After water cooling,there is a big temperature difference between the center and the surface of strip,which leads to the heat transfer from the center to the surface,and the surface temperature can rise in a short time.The finite element method was used to simulate the phenomena of re-reddening on the surface of strip and to analyze the temperature field of hot rolled strip during laminar cooling,and the periodical variation curve of the cooling rate was obtained during water cooling and subsequent re-reddening.The results show that the critical line of the cooling rate is at 1/3 of the half-thickness from the strip surface.The regression model of the relation of re-reddening temperature,time,and distance from the surface was obtained in the re-reddening region.Re-reddening regularity on the surface of strip under the condition of different thickness and cooling rate was also studied.     

Modelling of microstructural evolution during accelerated cooling of hot strip on the runout table

Microstructural evolution in the hot strip after finishing and subsequent accelerated cooling on the runout table has been modelled in order to assess their suitability for further processing. Transient heat transfer and kinetics of phase change comprising austenite to ferrite plus pearlite have been coupled to ascertain temperature profile, taking into accout the heat generated during phase change. Johnson-Mehl-Avrami relation together with Scheil's rule of additivity have been invoked. Several process parameters such as, coefficient of heat transfer, temperature at the exit of finishing stand, thickness and the speed of strip have been varied to determine their influence on the extent of phases engendered on the runout table. It has been demonstrated that greater spreadout in cooling arrangement with relatively lower heat transfer coefficient ensures homogeneity in microstructure. Cooling from comparatively higher finishing temperatures may result in greater microstructural uniformity. Two grades of steel – namely 0.05C-0.23Mn-0.015Si and 0.08C-0.37Mn-0.06Si – were chosen to carry out plant trials to validate the model. Special features of the microstructure have been brought out and the mechanical properties have been correlated.     

Cooling of a moving steel strip by an array of round jets

Starting from slabs of known dimensions and chemical composition in a hot strip mill, homogeneous strips of predetermined geometry and mechanical properties may be produced. While the geometry and the surface quality are influenced by the deformation process, mechanical properties depend on the cooling process applied immediately after the last stand. Accelerated cooling of steel strips is one of the best ways to achieve both high cooling efficiency and desirable product qualities. A mathematical model is developed to predict the thermal behaviour of steel strips cooled by an array of round jets. Parameters such as the arrangement of the cooling line, nozzle diameter, jet velocity and temperature, and the strip chemical composition (thermophysical properties), thickness and velocity are considered. The governing equation was solved numerically, and the boundary conditions were imposed in different cooling regimes along the cooling line in the form of experimentally and analytically obtained heat transfer coefficients. The mathematical model was validated by comparing predictions for an industrial cooling line with measured starting and coiling temperatures.