Cr5锻钢支承辊材料的接触疲劳损伤定量表征及强度退化机制研究

The Cr5 type backup roll is widely applied in rolling due to its better performance than the Cr2 and Cr3 type. However, the contact fatigue failure of Cr5 backup roll is inevitable and the mechanism is unclear until now. The rolling contact fatigue tests will be implemented on a Cr5 backup roll steel to determine its stress-life relationship. Based on the test results, the fatigue damage accumulation model will be established by modifying the linear fatigue damage accumulation model. The fatigue damage degree computation model of backup roll’s section will be established by combining the fatigue damage accumulation model and the section’s damage distribution model established by our previous research. Furthermore, the characterization mathematic model of damage degree and failure criterion based on hardness will be established by statically analyzing the relationship between the hardness vibration and damage degree of test samples’ section. Meanwhile, the transmission electron microscopy(TEM) will be used to analyze the morphology of microstructure and the electron back scattering diffraction(EBSD) be employed to measure the grain sizes in the damage influenced regions. The analyze results will be combined with the stress distribution and fracture mechanics theory to establish the relationship between the size of grains, formed by the fragment of original grains due to accumulation of fatigue damage,and the fatigue strength of material. Further works will be done based on the model established and morphology observed to illustrate the breakage mechanism of microstructure. The TEM, EBSD and atom probe tomography (APT) will be employed to analyze the interaction between the matrix phase, carbides, dislocation density and elements distribution for further investigating the strength degradation mechanism of Cr5 backup roll material under cyclic contact load.

Cr5系支承辊作为目前带钢轧制广泛采用的轧辊，其性能较Cr2和Cr3系有较大提高，但使用中的接触疲劳失效仍不可避免，且失效机理尚不明确。本项目拟设计并开展Cr5支承辊材料的接触疲劳试验，确定其应力-寿命关系；基于试验结果对线性损伤累积模型进行修正并建立支承辊材料非线性损伤累积模型的基础上，联合已开展研究中建立的截面损伤分布模型，建立截面的损伤度分布计算模型。然后，统计分析测试得到的截面不同深度处硬度变化值与损伤度理论计算值间的关系，建立基于硬度的损伤度表征数学模型和失效判断准则；同时利用透射电镜对组织形貌的观察和EBSD对晶粒尺度的分析结果，结合应力分析和断裂力学理论，建立组织破碎后晶粒尺度与接触疲劳强度间的定量关系，探讨损伤累积引起的组织破碎机理；最后，结合微观组织的定性和定量分析，探讨接触载荷作用下微观组织的变化规律及相互间的影响，揭示Cr5支承辊材料的接触疲劳强度退化机理。

项目以目前带钢轧机支承辊广泛采用的Cr5合金钢材料为研究对象，针对其服役过程中接触疲劳失效机制不明确，疲劳损伤后组织变化从宏观层面无法有效定量检测的问题，开展了针对此种材料的接触疲劳损伤定量表征及强度退化机制研究。首先通过接触疲劳试验测试获得了Cr5支承辊材料的应力-寿命关系，并确定了5级应力水平下的特征寿命；然后开展了5级应力水平下的中断接触疲劳试验；基于试验确定的应力寿命关系及线性疲劳损伤累积理论建立了支承辊截面的接触疲劳损伤分布模型；同时利用扫描电镜、电子背散射衍射和透射电镜对接触疲劳试验后试样的微观组织进行了分析，结合科氏原子扩散理论和位错累积理论建立的位错胞及C原子迁移模型，研究了Cr5锻钢支承辊材料在接触疲劳损伤累积过程中的微观组织破碎机理及强度退化机理；结果表明：接触滚压引起的压应力作用于支承辊表面时，位于次表面的板条与板条之间的相互挤压使得与板条走向呈45度方向的切应力引起的位错逐步累积并侵蚀板条，当累积到一定程度时导致板条切断是支承辊材料随着损伤累积内部板条组织逐渐破碎并纳米化的根本原因；伴随损伤累积过程，材料中微观元素尤其是C元素的迁移及位错变化是引起支承辊材料在接触疲劳过程中强度退化内在原因，并基于此建立了支承辊材料接触疲劳损伤程度与硬度变化之间的定量关系模型，确定了支承辊接触疲劳失效的硬度变化门槛准则。最后基于项目的部分研究成果在企业生产现场进行了推广应用，其中基于建立的接触疲劳损伤模型和硬化关系模型对宝山钢铁股份1420轧机支承辊和迁钢20辊森吉米尔轧机中间辊硬化评估中开展了现场应用，理论计算结果与现场测试基本一致。