喷嘴射流特征对连铸二冷区蒸汽膜穿透行为的影响

 连铸二冷区铸坯表面温度通常高于900 ℃,此时喷淋液滴接触高温铸坯时不会湿润铸坯表面,仅在其上形成一层蒸汽膜,阻碍了液滴与铸坯表面接触传热。针对以上问题,以国内某钢厂连铸二冷区的扁平型水喷嘴为原型,建立了喷嘴射流仿真计算模型,并对所建模型进行了理论和实验室验证;采用数值模拟的方法研究了喷嘴自由射流区的流场分布,运用连铸喷嘴冷却检测系统测量获得了射流液滴粒径演变规律;结合数值模拟和实验室测定结果,定量分析了喷嘴在不同水流量下射流液滴冲击铸坯表面蒸汽膜深度的变化规律。结果表明,该喷嘴的最大射流速度在喷嘴出口处,射流在喷嘴出口附近出能维持较大的射流速度,且随着水量的增加,射流保持高射流速度的距离也增长;整体射流的轴向速度占比均在80%以上。当喷淋水量越大时,射流液滴粒径变得越小;随着距喷嘴出口距离的增加,射流中心处的液滴粒径逐渐增大并达到最大值;当水流量为9和12 L/min时,液滴粒径基本相同,这表明当水流量增加到一定值时,冷却水量的增加不影响液滴粒径分布。在不同水流量下,随着喷淋距离的增加,液滴穿透铸坯表面蒸汽膜深度呈先增大后略微减小的变化规律,在喷射距离为100~200 mm范围内时,液滴穿透深度最大,这表明喷射高度在该范围时,喷淋冷却效果最好。     

Metallurgical Aspects of Ultra Fast Cooling in front of the Down‐Coiler

The Ultra Fast Cooling technology developed by CRM has been implemented on the hot strip mill of Carlam (Arcelor group), just in front of the down‐coilers (late UFC). The results have demonstrated the controllability of the system and the great efficiency (≥ 3.5 MWm^?2, 300°C/s on 4 mm thick strip) of the cooling unit. Increasing the cooling rate in a controllable way makes the production of high strength steels easier and cheaper. The combination of laminar and UFC technologies opens the way for new cooling schedules. Multiphase microstructures are therefore easily generated by controlling the intermediate temperature ‐ between laminar cooling and UFC ‐ and the coiling temperature.     

Size and Velocity Characteristics of Droplets Generated by Thin Steel Slab Continuous Casting Secondary Cooling Air-Mist Nozzles

Direct spray impingement of high temperature surfaces, 1473 K to 973 K (1200 °C to 700 °C), plays a critical role in the secondary cooling of continuously cast thin steel slabs. It is known that the spray parameters affecting the local heat flux are the water impact flux w as well as the droplet velocity and size. However, few works have been done to characterize the last two parameters in the case of dense mists (i.e., mists with w in the range of 2 to 90 L/m²s). This makes it difficult to rationalize how the nozzle type and its operating conditions must be selected to control the cooling process. In the present study, particle/droplet image analysis was used to determine the droplet size and velocity distributions simultaneously at various locations along the major axis of the mist cross section at a distance where the steel strand would stand. The measurements were carried out at room temperature for two standard commercial air-assisted nozzles of fan-discharge type operating over a broad range of conditions of practical interest. To achieve statistically meaningful samples, at least 6000 drops were analyzed at each location. Measuring the droplet size revealed that the number and volume frequency distributions were fitted satisfactorily by the respective log-normal and Nukiyama–Tanasawa distributions. The correlation of the parameters of the distribution functions with the water- and air-nozzle pressures allowed for reasonable estimation of the mean values of the size of the droplets generated. The ensemble of measurements across the mist axis showed that the relationship between the droplet velocity and the diameter exhibited a weak positive correlation. Additionally, increasing the water flow rate at constant air pressure caused a decrease in the proportion of the water volume made of finer droplets, whereas the volume proportion of faster droplets augmented until the water flow reached a certain value, after which it decreased. Diminishing the air-to-water flow rates ratio, particularly below 10, resulted in mists of bigger and slower droplets with low impinging Weber numbers. However, increasing the air pressure maintaining a constant water flow rate caused a greater proportion of finer and faster drops with Weber numbers greater than 80, which suggests an increased probability of wet drop contact with a hot surface that would intensify heat extraction.     

Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surface Cracking of Heavy Forgings

18Mn18Cr0.6N steel was tension tested at 0.001 s^?1 to fracture from 1473 K to 1363 K (1200 °C to 1090 °C, fracture temperature) at a cooling rate of 0.4 Ks^?1. For comparison, specimens were tension tested at temperatures of 1473 K and 1363 K (1200 °C and 1090 °C). The microstructure near the fracture surface was examined using electron backscatter diffraction analysis. The lowest hot ductility was observed under continuous cooling and was attributed to the suppression of dynamic recrystallization nucleation.     

雾化冲击射流冷却热轧钢板的研究

利用有限元耦合场数值模拟计算方法对热轧钢板雾化冲击射流冷却进行模拟计算,结果表明,在其它条件相同的情况下,单喷嘴喷射时,在上喷和下喷的喷嘴附近冷却能力相同;多喷嘴喷射时,横向存在二次冷却,钢板表面横向温度分布总体上呈现逐渐降低的趋势。     

热装高炉高温煤气喷雾降温塔CFD数值分析

高炉炼铁采用原燃料热装工艺能显著提高铁前工序显热资源利用效率。为保证高温高炉煤气的净化回收系统安全,须在干法布袋除尘设施前设置1座喷雾蒸发降温塔。采用Fluent软件中的离散相模型对喷雾降温塔内的流场进行了数值模拟,分析得到塔内气相流场、温度场分布以及雾滴运动规律等信息,对比分析了不同喷嘴角度和不同喷雾方向对塔内喷雾降温效果的影响。分析结果表明：采用喷雾蒸发冷却工艺可迅速降低煤气温度,采用雾滴与气体逆流型式且喷嘴角度为30°时,效果最优,雾滴充满塔体整个截面且可完全蒸发。分析结果可为喷雾降温塔的结构设计和喷雾降温过程的优化提供参考。     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

Water spray cooling of stainless and C-Mn steel

This paper describes laboratory cooling experiments of C-Mn and stainless steel with a water/air nozzle. Experiments included use of both the steady state and the transient experimental method. The heat transfer coefficient was calculated both analytically and by a numerical method. The heat transfer coefficient was evaluated as a function of steel surface temperature, water flux, material and water/air flow. It was found that by increasing the water flux the heat transfer coefficient also increased. The Leidenfrost point was shifted to higher temperatures. Increasing air flow was also found to increase the heat transfer coefficient. Maximum heat transfer coefficient was twice as large for the C-Mn steel as for the stainless steel. Comparison of the steady state and transient experimental method showed good agreement in the temperature range 200 – 500°C, and with a water flux of 2I/m²s. Above 500°C it was difficult to obtain stable conditions for the steady state experiments.     

Experimental-calculation study of ion-plasma thermal barrier coatings for turbine blades made of intermetallic nickel superalloys

The ion-plasma thermal barrier coatings deposited onto samples and blades made of intermetallic VKNA-1V and VKNA-25 alloys are tested in a laboratory. The external ceramic layer of the thermal barrier coatings (TBC) is formed by magnetron sputtering of zirconium alloy targets and has a columnar structure. The influence of NiCrAlY(Re, Ta, Hf) + AlNiY(Hf) + ZrYGdO TBC on the long-term strength at a test temperature of 1200°C and on the high-cycle fatigue at a temperature of 900°C is studied. Blades with TBC are subjected to thermal cycling tests in the temperature range 950 ? 400°C and 1050 ? 400°C during air cooling and in the range 950 ? 200°C during water cooling at 500 cycles. The temperature fields in the cross section of a blade airfoil during thermal cycling are calculated. The laws of formation of fracture zones and the development of thermal fatigue cracks under the conditions that are close to the operating conditions of nozzle TBC-containing blades are investigated.     

Hot Ductility and Compression Deformation Behavior of TRIP980 at Elevated Temperatures

The hot ductility tests of a kind of 980 MPa class Fe-0.31C (wt pct) TRIP steel (TRIP980) with the addition of Ti/V/Nb were conducted on a Gleeble-3500 thermomechanical simulator in the temperatures ranging from 873 K to 1573 K (600 °C to 1300 °C) at a constant strain rate of 0.001 s^?1. It is found that the hot ductility trough ranges from 873 K to 1123 K (600 °C to 850 °C). The recommended straightening temperatures are from 1173 K to 1523 K (900 °C to 1250 °C). The isothermal hot compression deformation behavior was also studied by means of Gleeble-3500 in the temperatures ranging from 1173 K to 1373 K (900 °C to 1100 °C) at strain rates ranging from 0.01 s^?1 to 10 s^?1. The results show that the peak stress decreases with the increasing temperature and the decreasing strain rate. The deformation activation energy of the test steel is 436.7 kJ/mol. The hot deformation equation of the steel has been established, and the processing maps have been developed on the basis of experimental data and the principle of dynamic materials model (DMM). By analyzing the processing maps of strains of 0.5, 0.7, and 0.9, it is found that dynamic recrystallization occurs in the peak power dissipation efficiency domain, which is the optimal area of hot working. Finally, the factors influencing hot ductility and thermal activation energy of the test steel were investigated by means of microscopic analysis. It indicates that the additional microalloying elements play important roles both in the loss of hot ductility and in the enormous increase of deformation activation energy for the TRIP980 steel.     

Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

18Mn18Cr0.6N steel specimens were tensile tested between 1173 K and 1473 K (900 °C and 1200 °C) at 9 strain rates ranging from 0.001 to 10 s^?1. The tensile strained microstructures were analyzed through electron backscatter diffraction analysis. The strain rate was found to affect hot ductility by influencing the strain distribution, the extent of dynamic recrystallization and the resulting grain size, and dynamic recovery. The crack nucleation sites were primarily located at grain boundaries and were not influenced by the strain rate. At 1473 K (1200 °C), a higher strain rate was beneficial for grain refinement and preventing hot cracking; however, dynamic recovery appreciably occurred at 0.001 s^?1 and induced transgranular crack propagation. At 1373 K (1100 °C), a high extent of dynamic recrystallization and fine new grains at medium strain rates led to good hot ductility. The strain gradient from the interior of the grain to the grain boundary increased with decreasing strain rate at 1173 K and 1273 K (900 °C and 1000 °C), which promoted hot cracking. Grain boundary sliding accompanied grain rotation and did not contribute to hot cracking.     

Modeling of Mechanical Characteristics in Hot Deformation of 4130 Steel

In this investigation, hot compression tests were performed at 900 °C ? 1100 °C and strain rate of 0.001 ? 0.1 s^?1 to study hot deformation behavior and flow stress model of 4130 steel. Based on the classical stress–dislocation relations and the kinematics of the dynamic recrystallization, the flow stress constitutive equations of the work hardening‐dynamical recovery period and dynamical recrystallization period were established for 4130 steel, respectively. The validity of the model was demonstrated by comparing the experimental data with the numerical results. The agreement of this comparison is quite reasonable.     

A Study of Microstructure and Performance of Quenching Fe‐V‐W‐Mo Alloy

The critical points and time temperature transformation (TTT) curves of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co (Fe‐V‐W‐Mo) were measured, and the effects of quenching temperature and cooling modes on the microstructure and performance of Fe‐V‐W‐Mo alloy were investigated. The results showed that the hardness of Fe‐V‐W‐Mo alloy increased until the quenching temperature reached 1025°C and dropped down as the quenching temperature exceeded 1050°C in oil cooling. The hardness obtained in air cooling and spray cooling exhibited a similar tendency as that in oil cooling, but the temperature at which the highest hardness was obtained in these slower cooling processes changed to a higher range. The hot hardness and toughness of Fe‐V‐W‐Mo alloy increased with rising quenching temperature until it reached 1150°C, and from then on the toughness began to drop. The main reasons why the structures and properties of Fe‐V‐W‐Mo alloy obviously change under different quenching conditions are particularly analysed at last.     

Modelling the microstructural evolution during hot compression of low carbon steel

Compression tests have been performed on low carbon cylindrical specimens in the temperature range of 900–1100°C in a thermomechanical simulator at a strain rate of 10 s^?1. True stress/true strain and load-displacement curves have been characterised over a strain of 0 to 0.8 at above temperatures. The specimens were helium quenched after an incremental true strain of 0.2 for microstructural study. From the experimental data, flow stress of the material at high temperatures has been determined as a function of Zener-Hollomon parameter. The flow stress equation was employed in a coupled finite element flow formulation model to compute the load for various incremental displacements. The predicted results of load-displacement and change in specimen geometry during compression showed good agreement with the measured values. The predicted rise in temperature due to deformation was of the order of 52 to 34°C in the temperature range of 900 to 1100°C at a strain rate of 10 s^?1. The prior austenite grain size has been measured in the specimen compressed up to a strain of 0.6 at 1100°C and compared with the predicted austenite grain size employing the microstructural model. Metallographic study showed an equiaxed recrystallized grains network in most of the region at the center of the specimen with average grain size of 43 μm. A coarse deformed grain structure with few recrystallized grains at the intersection boundary of austenite grains was observed at the top surface and bulge surface with an average grain size of 74 and 84 μm, respectively. The model predicted the evidence of fully dynamically recrystallised grains at the center of the specimen with a grain size of 42 μm. The predicted grain size at the top and bulge surface has been calculated as 90 and 106 μm, respectively.     

Phase Evolution and Mechanical Behavior of 0.36 wt% C High Strength TRIP‐Assisted Steel

Phase evolution in a 0.36 wt% C steel has been studied by thermodynamic calculation and dilatometric analysis with an aim to achieve high strength TRIP‐assisted steel with bainitic microstructure. The equilibrium phase fraction calculated as the function of temperature indicated the formation of δ‐ferrite (≈98%) at 1417°C. In contrast, similar calculation under para‐equilibrium condition exhibited transformation of δ‐ferrite to austenite at the temperature below 1300°C. During further cooling two‐phase (α+γ) microstructure has been found to be stable at the intercritical temperature range. The experimentally determined CCT diagram has revealed that adequate hardenability is achievable in the steel under continuous cooling condition at cooling rate >5°C s^?1. In view of the aforesaid results, the steel has been hot rolled and subjected to different process schedule conducive to the evolution of bainitic microstructure. The hot rolled steel has exhibited reasonably good tensile properties. However, cold deformation of the hot rolled sample followed by intercritical annealing and subsequent isothermal bainitic transformation has resulted in high strength (>1000 MPa) with attractive elongation due to the favorable work hardening condition during plastic deformation offered by the multiphase microstructure.     

Tensile and fatigue properties of 17-4 PH stainless steel at high temperatures

The tensile and high-cycle fatigue properties for 17-4 PH* stainless steels in three different conditions were investigated at temperatures ranging from room temperature to 400 °C. Results indicated that the yield strength and fatigue strength for the three conditions at a given temperature took the following order: condition H900 > condition A> condition H1150. The yield strength of each condition decreased with increasing temperature except for condition A, which was tested at 400 °C with longer hold times, where a precipitation-hardening effect took place. The S-N curves showed that the fatigue strengths of each condition in the short-life regime were decreased with an increase in temperature. In the long-life regime, the fatigue strengths of condition A at 400 °C were greater than those at lower temperatures as a result of an in-situ precipitation-hardening effect. The fatigue strengths of condition H900 in the long life regime at 300 °C were superior to those at lower temperatures, due to the mechanisms of surface oxidation and thermal activation of dislocations. Fractography observations indicated that a shift of fatigue fracture from surface to internal crack initiation occurred at higher temperatures (300 °C and 400 °C) with long fatigue lives.     

Influence of on-line tempering parameters on microstructure of medium-carbon steel

A new process involving ultra-fast cooling(UFC)and on-line tempering(OLT)was proposed to displace austempering process,which usually implements in a salt/lead bath and brings out serious pollution in the industrial application.The optimization of the new process,involving the evolution of the microstructure of medium-carbon steel during various cooling paths,was studied.The results show that the cooling path affected the final microstructure in terms of the fraction of pearlite,grain size and distribution of cementite in pearlite.Increasing the cooling rate or decreasing the OLT temperature contributes to restraining the transformation from austenite to ferrite,and simultaneously retains more austenite for the transformation of pearlite.It is also noted that bainite was observed in the microstructure at the cooling rate of 45°C/s and the OLT temperature of 500°C.Through either increasing the cooling rate or decreasing the OLT temperature,the distribution of cementite in pearlite is more dispersed and grain is refined.Taking the possibility of industrial applications into account,the optimal process of cooling at 45°C/s followed by OLT at 600°C after hot rolling was determined,which achieves a microstructure containing nearly full pearlite with an average grain size of approximately 7μm and a homogeneously dispersed distribution of cementite in pearlite.     

Microstructural Evolution and Flow Analysis during Hot Working of a Fe-Ni-Cr Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 1173 K to 1323 K (900 °C to 1050 °C) and strain rates of 0.001 seconds⁻¹ to 1 second⁻¹ to investigate the hot deformation behavior of the austenitic stainless steel type 1.4563. The results showed that hot deformation at low temperatures, i.e., 1173 K to 1223 K (900 °C to 950°C), and at low and medium strain rates, i.e., 0.001 seconds⁻¹ to 0.1 seconds⁻¹, results in the dynamic formation of worm-like precipitates on existing grain boundaries. This in turn led to the restriction or even inhibition of dynamic recrystallization. However, at higher temperatures and strain rates when either the time frame for dynamic precipitation was too short or the driving force was low, dynamic recrystallization occurred readily. Furthermore, at low strain rates and high temperatures, there was no sign of particles, but the interactions between solute atoms and mobile dislocations made the flow curves serrated. The strain rate sensitivity was determined and found to change from 0.1 to 0.16 for a temperature increase from 1173 K to 1323 K (900 °C to 1050 °C). The variations of mean flow stress with strain rate and temperature were analyzed. The calculated apparent activation energy for the material was approximately 406 kJ/mol. The hyperbolic sine function correlated the Zener-Hollomon parameter and flow stress successfully at intermediate stress levels. However, at low levels of flow stress a power-law equation and at high stresses an exponential equation well fitted the experimental data.     

Dislocation creep controlled superplasticity in the high carbon steel 140NiCr16-6

Superplasticity in the alloyed high carbon-steel 140NiCr16-6 with phosphorus additions and a fine grained microdupiex structure – containing cementite in volume fractions of 22 % (Fe,Cr,Ni)₃C, particle size of about 1 μm and with a medium ferrite grain size of about 2 μm – has been investigated in the temperature regime of 550 to 675°C and in the strain rate range of 10^?5 to 5 · 10^?2 s^?1. Maximum strain rate exponents of m = 0,45 at 675°C with strain rates of the order of 10^?4 s^?1 have been determined. Maximum superplastic elongations of about 700 % were detected. At higher strain rates of 10^?3 s^?1 superplastic elongations of about 570 % were achieved. At relatively low test temperatures of 550°C elongations up to 230 % were recorded. The activation analysis in the temperature regime of 550 to 650°C show an activation energy for superplastic flow of 250 ± 20 kJ/mol. This is in agreement with the activation energy for lattice self diffusion of iron in α-iron. Above 650°C the activation energy decreases to 70 kJ/mol. This is due to a stress induced decrease in the eutectoid α-γ-transformation temperature from 685°C to somewhat lower temperatures during superplastic deformation. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range at relatively low temperatures above 550°C allows near net shape forming of complex parts applying low flow stresses.