【作者】 陈文；

【导师】 杨金堂； 李公法；

【作者基本信息】 武汉科技大学 ， 机械电子工程， 2011， 硕士

【摘要】 我国经济的高速增长对铁路运输的发展起到了强大的推动作用,要求重型铁轨可以承载运货量日益增大的火车,因此,钢轨轨端的热处理技术得到越来越多的重视。淬火冷却方式是重轨热处理最关键的工艺,同时也是重轨淬火质量的主要影响因素之一。目前国内外在钢轨的冷却工艺方面绝大部分采用喷雾冷却和喷风冷却。由于喷雾冷却淬火工艺控制要求严格,对钢轨表面状态敏感,因此质量问题时有发生。喷风冷却速度波动范围较小,对重轨表面状态不敏感,人为影响因素少,可以保证淬火的质量。目前国际上已普遍采用风冷淬火技术,国内的大部分淬火生产线已改建为风冷淬火线。因此,有必要研究喷风头流域内流场、风冷淬火下温度场的分布规律,以便为实际生产做出指导。本文以60kg/m重轨和喷风头作为研究对象,通过有限元软件FLUENT,对重轨轨端的风冷过程进行数值模拟。模拟结果与相关试验结果吻合良好。首先,对于风冷过程中的喷风头的流场进行模拟。对于入风口压力为0.4MPa的喷风头的内部、外部流场进行仿真,分析重轨表面的空气流速。然后,分析了喷风头与重轨踏面距离为10mm、15mm、20mm时的重轨表面气流速度,结果说明10mm的距离是三种情况中的最佳选择。其次,通过设定风冷的表面换热系数,对重轨风冷过程中的温度场进行数值模拟。在此过程中,充分考虑了材料的物性参数随温度变化情况的影响。模拟过程中得到重轨的瞬时温度分布,相关试验结果表明模拟是正确的。最后,根据U71Mn钢的CCT曲线得到其相变温度,由关键节点的降温曲线计算了重轨轨端在相变点的冷却速度,然后通过对比各组织的临界冷却速度,对重轨轨端最终的淬冷组织进行预测。相关试验结果表明预测结果是正确的。本文模拟了喷风头的内外部流场、重轨表面的流场和重轨在风冷过程中的温度场,并通过经验公式和CCT曲线图计算了重轨的最终淬冷组织。整个模拟结果是准确的,生产中可以通过此方法来对喷风头的尺寸参数进行研究、预测重轨的淬火组织,更好的指导工艺改进和现场生产。更多还原

【Abstract】 As economy of our country is developing at a high speed, the railway transport is required to develope deeply. The use of heavy rail can meet the demand of large amount of load, therefore heat-treatment of rail end is becoming more and more emphasized. Quench cooling type is one of the most important treatments in heavy rail heat-treatments, and it is also a major factor influencing quenching quality. The type of atomize-cool and air-cool are widely used in the world now. Because atomize-cool quench treatment is strictly required in controlling and sensitive to rail surface state, there is sometimes something wrong with the quality. While the velocity of air-cool treatment waves little, what’s more, air-cool treatment is not sensitive to rail surface state, influenced little by artificial factor that ensure the quality of quench.Air-cool treatment is recently used in the world, while most quench production lines are changed to air-cool quenching line. As a result, it is necessary to research the distribution regularity of the air-jet flow field and temperature field under air-cooling to support the production. As 60kg/m heavy rail and air-jet were researched, air-cooling process of heavy rail was simulated through finite element software FLUENT. The simulation result is consistent with the theoretic result.Firstly, air-jet flow field of air-cooling process was simulated. While the pressure of air inlet was 0.4MPa, inner and outer flow fields of air-jet were simulated, and air flow velocity of rail surface was analysed. Then, the air flow velocities were analysed when the distances between air-jet and rail tread were 10mm, 15mm, and 20mm. the results showed that the choice of 10mm among the three conditions was the best choice.Secondly, through setting heat transfer coefficient of rail surface in air-cooling process, the temperature field of air-cooling quench of heavy rail was numerical simulated. In calculating process, various physical parameters that changed by temperatures were fully considered. The temperature instant distribution was calculated in simulation. Relative tests showed that the numerical simulation was reasonable.Finally, the phase changing temperature of steel U71Mn was got based on its CCT curves. Through cooling curves of several key points, the cooling rate at phase transition point was calculated. By comparing with every microstructure’s critical cooling rate, the final cooling microstructure was predicted. Relative tests showed that the prediction was reasonable.This paper simulated the inner and outer flow field of air-jet, flow field of heavy rail surfaces, temperature field of heavy rail under cooling process, and calculated final cooling microstructure of heavy rail based on the empirical formulation and the CCT. The results of numerical simulation were correct. In actual production, the method can be applied to research the parameters of air-jet, and predict the quenching microstructure, in order to guide the improve of technology and actual production.更多还原