【作者】 沈晓辉；

【导师】 高霖；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 近年来，高速车轮需求量随着高速铁路交通的快速发展而不断增加。截止目前，我国高速车轮全部依赖于国外进口的局面仍未打破。高速车轮要求较高的强韧性能，对车轮的冶金质量、成形及热处理工艺等都有较高要求。我国高速车轮制造仍存在诸多理论、技术上的关键问题亟待解决，尤其是车轮轧制成形变形规律、多工步成形过程组织演变、热处理对提高车轮断裂韧性这一关键指标等方面的研究还相对肤浅，有些甚至是空白。鉴于此，本文基于“863”重点项目“高速动车组用车轮的研究与开发”，主要围绕高速车轮热成形，系统研究车轮压轧成形过程金属变形及组织演变规律，为提高成形精度、优化工艺及控制缺陷提供理论依据；并研究开发出提高断裂韧性的车轮热处理新技术。本文的主要研究内容和结果如下：（1）基于上限法开发出车轮终锻压力快速预测模型，并分析了工艺参数对锻压力的影响。探讨了轮辋顶端圆角半径对模具压靠时金属流动及锻压力的影响，从理论上说明了在终锻结束时轮辋外端角部难以完全充满的原因。（2）基于理论解析给出了车轮立式轧制时轧辊接触区面积的计算方法，分析得到主辊侧与辐板辊侧压入量的关系及其在轧制中的变化规律。研究发现，辐板辊侧的压入量稍大于主辊侧，且随着轮辋直径扩大，辐板辊侧压入量所占比例逐渐增大。得到车轮立式轧制的咬入条件及轮辋轧透条件，为车轮轧制工艺优化提供了理论指导。（3）通过车轮多工步热成形有限元分析，尤其是车轮轧制的三维模拟，系统分析了高速车轮成形过程的金属变形规律，得到以下主要结果：1）发现车轮轧制过程中轮辋断面存在两个难变形区，即轮辋内侧中部近表面区域和轮辋心部，难变形区金属在周向和径向受到拉伸作用；2）轧制时轮辋内外径的扩大主要发生在主辊变形区前后的两个影响区中，并阐明其发生机理；3）揭示了轧制中车轮的应力分布特点，轧制时辐板径向和周向受明显拉应力作用，变形区以外的轮辋及轮缘则周向受压；4）发现并解释了车轮轧制时辐板减薄现象及机理；5）得到主辊每圈进给量对车轮轧制的影响规律；6）解释了预成形坯轮缘局部欠充满是轧后轮辋发生椭圆现象主要原因之一，阐明了轮缘严重欠充满处轮辋内侧面产生折叠缺陷的根本原因是，主辊变形区中轮辋内侧靠近轮缘部位在径向和周向受强烈拉伸作用。这些结果对进一步厘清车轮轧制变形认识具有重要意义，并为车轮轧制工艺优化、轧制缺陷控制提供了理论指导。（4）基于Gleeble实验分析得出高速车轮钢热成形过程奥氏体组织演化模型，并通过二次开发将组织演化模型与车轮多工步热成形有限元模型相集成，结合实验验证，对车轮成形过程组织演变的进行了系统分析。得出以下主要规律：1）初锻中，坯料金属动态再结晶充分；终锻中，动态再结晶主要发生在轮毂中部和下部、辐板及轮辋区域；2）开锻温度每升高30℃，终锻后平均晶粒尺寸增加20~30m；随锻压速度的降低，轮辋中部金属的晶粒尺寸增大，而近表面金属晶粒尺寸有所细化；3）轧制中，轮辋仅外端部分金属发生动态再结晶，轮辋的大部分及辐板变形部位金属只能在轧制变形区间隙时间内发生部分静态再结晶；4）轧制中轮辋近表层金属晶粒细化明显，其中轮辋外端细晶区分布区域深度明显大于轮辋内端。车轮成形过程的组织演变研究为车轮成形工艺和后继热处理工艺的优化提供了新的视角。（5）开发出“预处理+终处理”车轮热处理新工艺，通过改善轮辋组织状态以提高轮辋断裂韧性。主要研究结果有：1）预处理中晶粒尺寸及其分布主要受温度影响，而初始组织状态的影响较小，通过一次预处理可以显著细化并改善晶粒尺寸分布均匀性；2）预处理加热温度合理控制范围为840~880℃，终处理选择840℃左右为宜；3）以870℃×2.5h预处理+840℃×2.5h终处理工艺进行实物车轮试制，结果表明，车轮的断裂韧性相对于传统工艺得到显著提高，各项力学性能也都能满足高速车轮的技术要求。用该工艺生产高速车轮的技术条件已通过评审，试制车轮正准备装车试验。更多还原

【Abstract】 The demand of high-speed railway (HSR) wheels has increased dramatically with the rapiddevelopment of HSR transportation in recent years. So far, the situation that HSR wheels are alldependent on foreign imports has not been broken in China. The HSR wheels require high strengthand toughness, which lead to higher requirements in metallurgy, hot forming and heat treatmentprocesses. There are some key issuers in the theories, technologies of HSR wheels manufacturing thatmust be resolved. This thesis has studied in depth metal deformation and microstructure evolutionduring HSR wheel multi-stage forming processes supported by the “863” key project titled of“Research and development of high speed EMU wheels”. At last, a new heat treatment process wasdeveloped to improve the wheel fracture toughness. The main contributions of the present thesis indetails are as follows:(1) A quick prediction model for railway wheel final forging load was developed based on the upperbound method, by which the effect of process parameters on forging load was analyzed. The effects offillet radius of the rim external side on the load and metal flow in the rim were investigated. Thephenomenon of underfill in the conner of external end of the rim during final forging was explained.(2) Based on analytical method, the methods for determining the contact area of rolls and the ratioof unilateral reductions of web roll side and back roll side were obtained. It was found that theunilateral reduction of the web roll side is slightly larger than that of the back roll side and with theincreasing of the rim radius the proportion of the reduction in web rolls side increases during rolling.The bite condition and plastic penetration condition of the rim were also obtained, which providetheoretical guidance for rolling optimization.(3) The metal deformation during HSR wheel multi-forming processes, especially for the rollingprocess, were analyzed systematically by FEM. The main results are as follows:1) It was found thatthere are two difficult-deformation regions in the rim, one of which is the area near the middle of riminternal side and the other is the center region of the rim. The metal in the these regions subjects totensile stresses in circumferential and radial directions.2) The expanding of the radius of rim mainlytakes place in the two affected zones before and after the main deformation zone.3) During rolling,the web subject to radial and circumferential tensile stresses, the metal of the rim outside thedeformation zones subject to compressive stress in circumferential direction.4) The phenomenon ofthe web thinning during rolling process was identified and explained.5) The effects of feedings perround on rolling deformation were obtained.6) Research indicated that the local underfill in the wheel flange is one of the reasons for the wheel ellipsing. The main cause of the folding defect occurs in theinternal side of the rim corresponding to the positioin of severely underfilling in the flange subject tointensive stretching action of the metal near the defect during rolling. These results further deepen theunderstanding of metal deformation characteristics during wheel rolling and provide guidance tooptimizing rolling process and reducing rolling defects.(4) Based on Gleeble experiments, the mathematical models for the recrystallization and graingrowth of a HSR wheel steel were derived. The evolution of the austenite grain size during the HSRwheel multi-stage forming processes was simulated by integrating the microstructure evolutionmodels with the finite element model based on programming the user subroutines. The main resultsare as follows:1) In the primary forging, almost all the metal subjects to full dynamic recrystallization;In the final forging, dynamic recrystallization only take place mainly in center and down part of thehub, the web and the rim areas.2) The average grain size increased by20~30m with the initialtemperature elevated every30℃; The grain size increases for the rim center and decreases for themetal near surface when forging speed reduces.3) During wheel rolling process, only part of externalend of the rim subject to partial dynamic recrystallization, while static recrystallization occurs in otherdeformation regions.4) The grains for the metal near to surface were refined evidently, thedistribution depth of the fine grains in the external end of the rim is larger than that of the internal end.(5) A new heat treatment of railway wheel, which involving a pre-treatment and a final treatment,was developed to improve the microstructure morphologies and increase the fracture toughness of theHSR wheel rim. The main results are as follows:1) It was found that the grain size and its distributionafter pre-treatment are mainly dependant on the temperature, while the initial microstructure has littleeffect on it.2) It is appropriate that the temperature of the pre-treatment be in the range of840~880℃,while the temperature of the final treatment be about840℃.3) The pilot production of HSR wheelwas conducted by the process of870℃×2.5h pre-treatment+840℃×2.5h final treatment.Contracted to the conventional technology, the new heat treatment process improves the fracturetoughness significantly and makes all the mechanical properties meet the specifications of the HSRwheel. The technical conditions for production HRS wheel had passed the technical assessment andthe pilot products are preparing to service test.更多还原