Influence of heat transfer on the development of residual stresses in quenched steel cylinders

In this paper results of systematic FE-calculations about the influence of characteristic points of the temperature dependent heat transfer coefficient, especially the Leidenfrost point and the point of maximum heat transfer coefficient on the development of residual stresses are discussed. The numerical investigations were carried out for SAE 1045 and 4140 steel cylinders with 10 and 20 mm 0 quenched in water and oil, respectively. In this work experimentally determined h, T-curves are linearly approximated in the successive stages of heat transfer. Changes of the Leidenfrost-temperature do not influence the middle plane residual stresses of the cylinders investigated. Increasing maximum heat transfer coefficients and low temperatures of maximum heat transfer coefficient, respectively, cause higher magnitudes of residual stress. The development of residual stresses is determined by the temperature dependent gradient of the heat flux density δq/δT in the temperature range of martensitic transformation. Increasing Leidenfrost-temperatures cause more homogeneous stress and residual stress states at the surface of quenched cylinders due to the symmetrical cooling of the sample in axial as well as in radial direction. In particular, it was shown that during immersion cooling of cylindrical parts the heat transfer is locally dependent. Simulating immersion cooling this dependence has to be considered using effective local heat transfer coefficients.     

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.     

Determination of the interfacial heat transfer coefficient in a metal-metal system solving the inverse heat conduction problem

Simulation of several industrial processes involving solidification of metals requires characterization of heat transfer coefficient at the solidifying metal/metal-substrate interface. In the present investigation an attempt has been made to estimate this heat transfer coefficient, h_c, using simulated experiments in which the heat transfer from a heated stainless steel block (simulating solidifying metal) to a water cooled copper block (simulating metal-substrate) is monitored by continuously recording temperatures at a few internal locations both within the metal block and the substrate block. The problem of determining the interfacial heat transfer coefficient is recognized to be an inverse heat conduction problem (IHCP). A numerical method is employed to solve IHCP and to determine the h_c from the transient history of temperatures at a few locations. The effect of the physical nature of the interface, as well as the cooling conditions prevailing at the outer surface of the substrate on h_c is examined and discussed. While the physical nature of the interface, i.e. roughness on the metal as well as the substrate surfaces, has a significant effect on h_c, the cooling conditions have only a marginal effect. The h_c in the present investigation remains more or less time invariant.     

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.     

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.     

小方坯结晶器水温水速对传热过程影响

通过建立结晶器内钢液和水的二维对流-传热耦合模型过程,研究了小方坯结晶器冷却水入口温度和流速对铜管温度和结晶器内平均热流的影响.该模型使用Fluent进行求解,模拟了钢液和冷却水的流动和传热,凝固坯壳的生长,以及热量以辐射和导热两种通过保护渣和气隙.通过将坯壳厚度和铜管温度与其他研究的结果进行对比来验证模型准确性.研究结果表明,结晶器冷却水的温度显著影响铜管的冷面温度,水温超过313 K会导致铜管冷面最高温度超过水的沸点.水流速升高0.49 m·s-1能够消除水温升高4 K带来的不利影响.      

A new method for evaluating cooling capacity of blast furnace cooling stave

The detailed process of the heat transfer of the cooling stave in blast furnace (BF) has been systematically analyzed and the simplified mathematical model was constructed based on heat transfer theory. Precise definitions of the cooling capacity, stable working slag thickness and safe working slag thickness were put forward so as to evaluate the cooling capacity of cooling stave systematically. The results show that 95% of heat is carried off by cooling water through convection and the heat taken away through convective heat transfer between furnace shell and atmosphere only account for 5%. The entire heat transfer process can be divided into four modules and the cooling system is divided into three parts. The cooling capacity φ is defined and function curve of temperature of cooling stave hot surface T_b with changes of brick thickness is drawn and the safe working area and stable working area are put forward.     

Growth Rate,Microstructure and Phase Composition of Oxide Scales for Three Typical Steels in Simulated Continuous Casting Process

Growth rate,microstructure and phase composition of scale layer formed during oxidation in 56%H2O-9%O2-N2 and following continuous cooling in ambient air were experimentally investigated by means of optical microscopy,scanning electron microscopy(SEM),energy-dispersive spectroscopy(EDS)and X-ray diffraction(XRD)for45,20 CrMnTi and TTS443 Msteels respectively,to examine the effects of strand surface temperature and steel composition on the scale formation in simulated continuous casting process.The growth rates were found to be approximately parabolic and the calculated activation energy of TTS443 Msteel is much higher than those of the two other steels.For 45 and 20CrMnTi steels,the scale layers were looser and a distinct gap formed at the scale-substrate interface at higher strand surface temperature.The dominant phases within the scale were iron oxides except for FeO·Cr2O3phase simultaneously existing in the oxide scale of 20 CrMnTi steel.On the other hand,the scale layer formed on TTS443 Msteel was compact and tightly attached to the steel surface.At both lower and higher strand surface temperature,iron oxide was main phase in external layer of the scale,while chromia was dominant in inner layer with an appreciable Cr enrichment.     

炉缸冷却壁对流换热系数计算及烘炉传热特性

 炉缸冷却壁冷却性能主要体现在冷却水与水管间的对流传热。因为工程上常用计算对流换热系数的经验公式不能满足不同的水流状态从而导致炉缸热应力分析误差较大，所以以某高炉炉缸结构为例，首先利用传热学准数方程推导出冷却水处于不同流动状态时对应的综合对流换热系数表达式，同时探讨了对流换热系数经验公式的适用范围；然后通过迭代计算推导出了冷却水处于层流状态下考虑衰减热阻时的综合对流换热系数表达式；最后对烘炉状态下炉缸侧壁传热模型进行瞬态传热与冷却分析，得到了微水烘炉甚至闭水烘炉的热工依据，可为初步制定高炉烘炉制度进行评估和完善。     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 2: determination of material properties of oxide scales on steel under spray-water cooling conditions

In order to model heat transfer from the slab to the cooling agent in the continuous casting process the thermophysical properties of the surface layer must be taken into account. For this purpose thermal conductivities and thermal diffusivities of wustite as well as of two carbon steels were measured. The oxide scale growth for six steels was measured as a function of time and at temperatures between 900 and 1200°C in air and in steam. Complex heterogeneous oxide scales were examined by optical and scanning electron microscopy, by SIMS, as well as by Mössbauer spectroscopy. Segregation of alloying elements in the metal-oxide contact zone was found. Cracks due to thermal stresses result in spalling of the scale. Therefore, a straightforward use of the data for process modelling can principally not be recommended without restrictions.     

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m² K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m² K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m² K). The heat transfer coefficient of graphite into air is 12 W/(m² K) at a mold surface temperature up to 600°C.     

分形微通道换热过程强化研究进展

随着微纳制造技术的快速发展,微电子芯片、微反应器和微燃料电池等微型器件受到了研究者越来越多的关注。微型器件的应用不仅对加工工艺和材料具有较高的要求,而且需要高效的热管理来维持其性能。特别是对于高集成度和高频化的高性能微电子芯片而言,超高的热流密度不仅会严重制约芯片的性能,而且会显著影响芯片的寿命和可靠性。鉴于传统的风冷和液体单相对流换热冷却方式无法满足散热需求,具有高换热系数的微通道换热技术成为解决微型器件散热问题的重要途径。然而,常规的微通道换热技术普遍存在着高流动阻力和非均温性的难题,限制了该技术的实际规模化应用。近年来,研究者开发出一系列新型的分形微通道技术用于换热过程强化。本文系统总结了不同类型的分形换热微通道（包括Y、H、T、Ψ、康托、科赫等分形结构）,并对各分形微通道的原理和性能进行了着重介绍,最后对分形微通道换热的现存挑战和未来发展方向分别进行了分析和展望,以期为换热过程强化的发展提供新的研究思路。      

冷却工艺对高速连铸结晶器传热的影响

 为了分析冷却水的供水工艺对结晶器铜壁和冷却水温度场的影响,基于结晶器铜壁热电偶实测温度,构建铸坯/铜壁传热反问题和铜壁/冷却水正问题数学模型,采用ANSYS建立铸坯/铜壁/冷却水数值分析模型,对薄板坯结晶器温度场进行耦合传热分析,解析不同冷却工艺对高速薄板坯连铸结晶器内传热行为的影响。结果表明,水缝内冷却水流动方向对铜壁温度场具有显著影响,采用自上而下“反向供水”,比常规冷却工艺时铜壁热面温度峰值降低117 ℃,铜壁侧冷却水最高温度降低24 ℃,有效改善铜板工作状态,抑制冷却水局部沸腾趋势。提高冷却水速度可以进一步降低铜壁和冷却水温度,冷却水温度对铜壁温度场影响较小。     

Study of heat transfer through layers of casting flux: experiments with a laboratory set-up simulating the conditions in continuous casting

The heat transfer through layers of casting flux has been studied with a laboratory set-up which consists of a water-cooled heat flux probe made of copper and an electrically heated steel plate with a trough to hold the flux sample. The measured heat flux density values were converted to system conductivities k_sys which contain the radiation/conduction conductivity in the flux layer and the contact resistance at the probe side. It was found that k_sys is almost independent from the layer thickness. Ten commercial casting fluxes were investigated and the data for k_sys are given as functions of composition and strand surface temperature.     

水流量对热轧钢板层流冷却过程对流换热系数的影响

提高带钢层流冷却控制模型的精度,关键是建立精确的对流换热系数与冷却工艺之间的关系.采用有限差分法和反向热传导法,获得了实验条件下钢板表面的对流换热系数及表面温度.研究了不同水流量(0.9~2.1 m3·h-1)对换热系数与表面温度变化规律的影响.在层流冷却过程中,对流换热系数与表面温度呈非线性关系;在距离驻点70 mm内,水流量对换热系数随表面温度变化规律没影响;远离驻点70 mm外,对流换热系数比随远离冲击区驻点距离的增加而减小.采用所确定的换热系数计算得到的温降曲线与实测曲线吻合较好.      

Influence of gases dissolved in cooling water on heat transfer during stable film boiling

Results of investigations concerning the determination of the influence of the water quality on the heat transfer of quenched hot metal surfaces are presented. With an immersion quenching device, tests have been carried out for various cooling water qualities. Apparently there was no correlation between the water quality and the cooling process (Leidenfrost temperature). High concentrations of dissolved gases yield stable vapour films (low Leidenfrost temperatures are measured). Salt displaces the dissolved gases from the water; the Leidenfrost temperature rises: the higher the salt concentration, the lower the gas concentration and, therefore, the higher the Leidenfrost temperature. If salt concentrations are high or the cooling water is degassed, the vapour film falls apart just after Immersion. An extended model considering the Influence of the gases allows the correlation of the water quality/salt concentration and the heat transfer/Leidenfrost temperature.     

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.     

Calculation of temperature and heat flux in quenched cylinders for different wetting processes

The cooling of pieces being quenched in evapourable fluids is mainly determined by the process of wetting, which is defined as the sequence of the three known cooling stages film boiling, nucleate boiling and convective heat transfer. All these determine the properties of the finished product. The effect of different wetting processes and heat transfer coefficients on the time-dependent distribution of temperature and heat flux in quenched cylinders was examined using numerical methods. Only transformation-free quenching was considered. When the three cooling stages simultaneouly take place on the sample's surface the radial heat fluxes are superimposed by high axial heat fluxes, which could not be pointed out experimentally before. The consequences may be great axial differences of structure and hardness, especially for steels with low hardenability, as well as residual stresses. The axial temperature gradients can be reduced by producing high wetting velocities. An optimum wetting process involves a very fast breakdown of the vapour envelope on the whole surface of the quenched piece.     

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.     

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.