【作者】 周鹏；

【导师】 马庆贤；

【作者基本信息】 清华大学 ， 机械工程， 2017， 博士

【摘要】 锻造过程中金属微观组织的转变机制对工件成形的精确性以及工件机械性能的控制具有重要的影响。由于大型锻件锻造是一个多工步、多道次的过程,变形温度、应变速率、应变量、初始奥氏体晶粒尺寸、道次间的保温时间等因素都会影响锻件最终的组织。本文将取自600吨级钢锭的30Cr2Ni4MoV钢和取自特大型水轮发电机镜板坯件的25CrMo4钢进行了高温压缩实验,基于热变形流变数据,结合变形后的微观组织,研究了典型轴类和饼类锻件材料的晶粒细化机制,为大型锻件的制造工艺提供理论依据。对30Cr2Ni4MoV钢的室温组织进行了分析,室温组织中存在粗大树枝晶和严重的“混晶”现象。在Gleeble-1500热压缩模拟试验机上进行了单道次和双道次压缩实验。根据单道次压缩实验所得流变曲线,研究了不同变形温度和应变速率条件下30Cr2Ni4MoV钢和25CrMo4钢的高温变形力学行为,建立了动态再结晶动力学和晶粒尺寸模型。基于动态再结晶动力学模型预测了流变应力与变形温度、应变速率、应变的关系。通过双道次压缩实验,研究了变形温度、应变速率、应变、初始晶粒尺寸对25CrMo4钢静态再结晶和亚动态再结晶速率和微观组织的影响,并分别建立了静态再结晶和亚动态再结晶的动力学模型。模型预测值与实验值相吻合,可以为实际生产提供理论依据。基于单道次压缩流变曲线,得到了30Cr2Ni4MoV钢和25CrMo4钢的应变速率敏感指数并研究了变形温度和应变速率对其影响;结合动态再结晶动力学和微观组织研究了变形温度、应变速率、应变对功率耗散率的影响;建立了基于Prasad失稳准则的热加工图,根据功率耗散率的大小和Prasad失稳准则将热加工图进行了分区,结合不同区域的微观组织研究了热加工图不同区域的变形机制。动态再结晶是锻造过程晶粒细化最优机制,当应变为0.7时,30Cr2Ni4MoV钢的锻造工艺参数设定区间为950?1200?C/0.03?0.5s^-1,最佳锻造工艺参数为:1100?C/0.25s^-1;25CrMo4钢的最佳开坯工艺参数区间为:1100?1200?C/0.03?0.5s^-1,终锻的最佳工艺参数区间为:1000?1100?C/0.03?0.5s^-1。更多还原

【Abstract】 The deformation mechanism of the microstructure during forging has an important influence on the forming precision and the control of mechanical properties.Heavy foging is a process consisting of multi-pass and multi-step,so the deformation mechanism of grain refinement is affected by the deformation temperature,stain rate,stain,initial austenite grain size and interpass time.It is highly necessary to research the effect of the deformation condition and the initial microstructure on the mechanism of the grain refinement.In this research,the 30Cr2Ni4MoV steel sampled from 600t ingot and the 25CrMo4 steel taken from the mirror plate billet are compressed at high temperature.Combining with the microstructure observation,the grain refinement mechanism can supply the theory basis for the processing control of the heavy forging.Following is the main research content:The coarse grain structure of the 30Cr2Ni4MoV steel at room termperature,which is composed of dendrite and mixed grain,is investigated.The single compression deformation is carried out in the Gleeble-1500 thermo-mechanical simulator and the flow curves under different defomation temperatures and strain rates are obtained.The dynamic recrystallization（DRX）behavior of the 30Cr2Ni4MoV steel and 25CrMo4steel is investigated.Based on the experimental flow curves and the microstructure,the DRX kinetics model and DRX grain model are established.With the help of the DRX kinetics model,the flow stress is expressed as a function of deformation temperature,strain rate and stain.The double hit compression deformation of 25CrMo4 steel is carried out in the Gleeble-1500 thermo-mechanical simulator and the flow curves under different deformation conditions and interpass times are obtained.Based on the flow curves,the volume fraction of the static recrystallization（SRX）and metadynamic recrystallization（MDRX）is calculated.The effects of the deformation temperature,strain rate,strain and initial grain size on the recrystallization rate and microstructure of the SRX and MDRX are discussed.Furthermore,the kinetics models of SRX and MDRX are constructed and the predicted results agree well with the experimental results.Based on the obtained flow curves,the power dissipation maps of the30Cr2Ni4MoV steel and 25CrMo4 steel at different strains are developed and the effect of the strain on the efficiency of power dissipation is discussed in detail.The processing maps at different strains are obtained by superimposing the instability maps on the power dissipation maps.When the strain is 0.7,according to the processing map and the metallographic observation,the optimum domain of hot deformation for 30Cr2Ni4MoV steel is in the temperature range of 950¹200?C and strain rate range of 0.03⁰.5s^-1,with a peak efficiency of 0.41 at 1100?C and 0.25s^-1 which are the optimum hot working parameters.For the 25CrMo4 steel,the domain occurs at the temperature in the range1100¹200?C and strain rate in the range 0.03⁰.5s^-1 is recommended for the blooming process.In view of the control of the grain size for the final product,the optimum parameters for the finish forging process should be located at the temperature in the range 1000¹100?C and strain rate in the range 0.03⁰.5s^-1.更多还原