【作者】 刘云飞；

【导师】 刘宏民； 彭艳；

【作者基本信息】 燕山大学 ， 机械设计及理论， 2011， 博士

【摘要】 作为热轧带钢产品的重要品种之一，管线钢具有高强度、高韧性、高止裂性、低的包申格效应和良好的焊接性能，广泛应用于石油、天然气运输行业。管线钢轧后冷却过程中微观组织演变直接决定着管线钢最终的组织状态，进而决定最终的力学性能，因此对管线钢轧后冷却过程中微观组织演变的预测和控制已经成为当前微合金钢研究热点之一。本文以X80级别管线钢轧后冷却过程中微观组织演变为研究对象，通过考虑相变潜热、相变和温度三者之间相互影响关系，建立了包括奥氏体相变热力学、相变动力学、相变潜热以及温度场等模型在内的耦合组织预测模型，深入研究了控轧控冷工艺条件下X80管线钢奥氏体相变规律，并通过热模拟试验验证了该模型的准确性，为管线钢冷却过程中微观组织演变的预测和控制提供了重要的理论依据和实际参考。采用KRC模型计算了X80管线钢的相界面浓度、相变驱动力以及相平衡开始温度等热力学参数，并分析了变形量、变形速率和变形温度对其热力学参数的影响规律。通过热力学模型的建立，为轧后冷却阶段组织演变过程的预测提供必要的计算前提。建立相变动力学模型，包括：相变孕育期模型、相变实际温度模型、相变体积分数计算模型以及铁素体晶粒尺寸预测模型，计算分析了变形和冷却速度对相变动力学的影响。其中在计算铁素体相变温度时考虑了合金元素Nb的固溶现象对铁素体相变抑制作用，完善了含Nb微合金钢铁素体相变开始温度模型。从相变机理出发，建立了基于相变过程焓变量的相变潜热模型，并与DSC实验结果进行对比，显示了模型具有较好的准确性，并分析了碳含量和冷却速度对相变潜热的影响规律。提出了连续冷却过程中各个相变温度下相变潜热的计算方法，并与实验数据进行对比，吻合较好。利用有限差分法建立了层流过程中温度场模型，模拟分析了相变潜热、带钢厚度、辊道速度以及水流密度对其温度分布的影响。根据已建立的温度场模型，结合现场实际冷却情况，针对钢板温度不均匀现象从冷却方式和设备参数两个方面提出了改进措施，取得了较好的效果。并基于传热学原理计算了上下水量比，利用模拟数据回归了边部遮蔽量与板宽、板厚、水流密度的关系式，且计算结果与现场实际数据吻合良好，证明了边部遮蔽量计算模型的合理性。在已有的连续冷却相变模型的基础上，通过含内热源的二维温度场模型，将相变潜热与相变模型联系起来，进行迭代计算，建立了奥氏体相变、相变潜热和温度场耦合模型。将计算结果与实验结果及普通奥氏体相变模型计算结果进行比较可以发现，耦合模型计算结果与普通奥氏体相变模型计算结果相比，更贴近实验结果，具有较高的计算精度。对两种不同Nb含量的X80管线钢进行了Gleeble热模拟实验，结合金相观察分析了不同的控轧控冷工艺条件和合金元素Nb对管线钢中奥氏体相变规律的影响，并验证了耦合模型的准确性。总体而言，本文建立的管线钢轧后冷却过程微观组织演变模型，考虑了相变潜热和合金元素Nb对相变的影响，提高了组织预测精度，对科学的制定控轧控冷工艺，特别是对更准确的控制冷却过程具有一定的理论价值和实际参考意义。更多还原

【Abstract】 As one important product of hot rolled strip, the pipeline steel is widely used inpetroleum and natural gas transportation industry because of high strength, hightoughness, high crack, low bauschinger effect and good welding performance. Themicrostructure evolution of pipeline steel during the cooling process not only decide thefinally phase transformation organization but also decide to the finally mechanicalproperty of pipeline steel, which become the hotspot study of alloy steel to predict andcontrol the microstructure evolution during the cooling process.In the present paper, a new integrated mathematical model for prediction ofmicrostructure evolution during rolling and cooling was developed for X80 pipeline steelby considering the relationship between the transformation, latent heat and temperature,which consists of transformation thermodynamic, transformation kinetics, latent heat andtemperature field models. The influence of controlled rolling and controlled cooling onmicrostructure evolution was thoroughly investigated, which provides importanttheoretical evidence for predicting and controlling the microstructure evolution ofpipeline steel during cooling process. The innovate works and corresponding results areas follows.The phase interface concentration, transformation force and phase equilibriumtemperature of X80 pipeline steel was calculated by KRC model, and the effect of hotdeformation on the thermodynamic parameters was analyzed, which provides thenecessary calculation hypothesis for predicting the microstructure evolution during thecooling process.The transformation kinetics model of X80 pipeline steel was established, whichincluded phase incubation period model, phase transformation temperature model, phasetransformation volume fraction model and ferrite grain size model, the effect of hotdeformation and cooling rate on the transformation kinetics was analyzed. Whencalculating the ferrite phase starting temperature, the effect of the solute drag of Nbshould be considered, which improved the ferrite phase starting temperature model for the Nb micro-alloyed steel.Based on the phase transformation mechanism, a mathematical model of latent heatfor X80 pipeline steel was established, which considered the enthalpy change during thephase transition. The results given by the model were in good agreements with the DSCexperimental results. According to the variation of latent heat, combined with theory andexperiment, a new method to calculate the latent heat at any temperature was proposed,the results given by the method were in accord with the measured value.The temperature field model of X80 pipeline steel during laminar cooling wasestablished by using finite difference method, the effect of latent heat, plate thickness,conveyor speed and water discharge density on the temperature distribution. According tothe temperature field model of X80 pipeline steel, and combined with actual coolingconditions, some improvements in connection with uneven temperature strip wereproposed from the cooling method and device parameters, which achieved good results. Itcalculated the optimum water flow based on the heat transfer theory, a formula tocalculate edge masking amount was regressed through the relationship between edgemasking amount and plate width, plate thickness, water discharge density. Throughcomparing the results with field data, it proved the edge masking model reasonableness.A new model for austenite transformation volume fraction and temperature in thetwo-dim unstable temperature fields considering latent heat was established. Bycomparing volume fraction of the three phase obtained by kinetic model, temperaturecoupling model and experiment, it is indicated that the coupling model has superiorityand the calculation accuracy of the three values.The continuous cooling transformation behaviors of two different Nb content of X80pipeline steel are simulated on Gleeble-3500 thermomechanical simulator, it ananlysesthe influences of cooling rate, deformation and addition of Nb element on transformationof X80 pipeline steel with metallographic observation and verifies the veracity ofcoupling model. Overall, the coupling model for prediction of microstructure evolutionduring rolling and cooling is developed for X80 pipeline steel in the present paper, whichimproves the predicted precision of microstructure evolution during the cooling process.The effect of latent heat and alloy element Nb on the austenite transformation is studied. It has a certain theory value and practical reference sense to formulate the scientificcontrolled rolling and controlled cooling and more accurate control the cooling process.更多还原