【作者】 喻海良；

【导师】 刘相华；

【作者基本信息】 东北大学 ， 材料加工工程， 2008， 博士

【摘要】 近年来,随着钢铁工业的发展,对钢铁产品质量的要求越来越高,而轧制钢材上裂纹和夹杂物等是影响产品质量的主要因素。因此,本文针对轧制过程中轧件裂纹萌生、扩展与闭合和夹杂物等的演变行为开展模拟与实验研究,论文的主要研究内容和主要进展如下：1.提出多道次轧制过程中有限元模拟的几何更新方法。并对多道次立—平轧制过程轧件变形行为进行有限元模拟,分析了立辊孔型等因素对轧制过程中轧件头尾不均匀形状、轧件翻边行为、轧件应变场等的影响,通过轧制实验验证了数值模拟结果,两者吻合很好。2.采用几何模型更新方法对多道次立—平轧制过程中轧件角部横向裂纹演变行为进行了有限元模拟。首次采用裂纹表面接触压力分析轧制条件对裂纹闭合程度的影响。获得了轧制过程中裂纹形状、裂纹表面接触压力和裂纹尖端应力等计算结果,分析了立辊孔型、摩擦系数、初始裂纹尺寸等因素对裂纹演变行为的影响,通过轧制实验验证了数值模拟结果的可靠性。同时,应用该方法对多道次立—平轧制过程中轧件边部表面纵向裂纹演变行为进行了模拟研究,分析了裂纹在不同截面的投影形状。在数值模拟结果的基础上,分析了轧件表面裂纹消失的演变行为。3.通过对多道次轧制过程中轧件内部纵向裂纹演变行为的三维模拟研究,获得了轧制过程中裂纹形状和表面接触压力等计算结果。分析了不同轧制阶段裂纹形状的演变行为,并分析了立辊孔型、裂纹位置等因素对裂纹扩展与闭合的影响。4.通过对热轧过程中轧件内部裂纹愈合行为的轧制实验,分析了不同压下率下裂纹形状、裂纹断口形貌的演变和裂纹愈合区域剪切强度的变化规律。采用热力模拟实验的方法对轧件内部裂纹愈合行为进行模拟,采用扫描电镜分析了不同加热速度、加热温度、保温时间、应变速度、压下率、压下道次等条件下裂纹的形貌,并分析了各因素对裂纹愈合行为的影响。在实验结果的基础上,提出了轧制变形条件下轧件内部裂纹的愈合机理。5.利用扫描电镜分析了轧件内部常见的夹杂物形状,通过能谱分析检测了夹杂物的主要化学成份。在此基础上,首次对多道次轧制过程轧件内部微米尺寸的夹杂物演变行为进行了三维数值模拟,获得了不同夹杂物尺寸、位置、工作辊直径等条件下夹杂物形状、夹杂物及其附近区域应变场、夹杂物在轧件中的相对位置,分析了夹杂物变形行为对轧件内部裂纹产生的影响。6.轧制过程中,轧件基体围绕夹杂物存在着较大的应变梯度,夹杂物直径增加,对于难变形夹杂物,夹杂物与基体之间应变梯度增加。对夹杂物与轧件基体脱离时夹杂物变形行为进行了二维数值模拟,分析了夹杂物尺寸、形状和位置对轧制过程中夹杂物周围裂纹萌生的影响。通过对轧制过程轧件裂纹、夹杂物的演变行为研究,丰富了轧制条件下裂纹和夹杂物的演变行为的理论,同时,研究结果对企业提高轧件质量具有指导作用。更多还原

【Abstract】 A further requirement is needed to improve the quality of rolled products with the rapid development of steel industry in recently years. And the cracks and the inclusions in rolled steel are the main factors that affect the quality of rolled products. In this thesis, the simulation researches on the initiation, the propagation, and the closure of cracks and the evolution behavior of inclusions in slabs during rolling have been carried out by using the finite element method (FEM) and the experiments. The research contents and results are as follows:1. The geometric updating method was proposed for FE simulation of the slab deformation behavior during multi-pass rolling. With this method, the deformation behavior of slabs during multi-pass vertical-horizontal rolling was carried out. The non-uniform shape of the slab head and tail, the metal flow at the slab corner and the strain distribution in slab after every pass during rolling were analyzed under various vertical roll shapes, etc. The numerical results were in good agreement with the physical simulation ones.2. The evolution behavior of transversal cracks on slab corner during multi-pass vertical-horizontal rolling was simulated using the geometric updating method. The contact pressure on crack surfaces was firstly used for analyzing the crack closure under various rolling conditions. The crack shape, the contact pressure on crack surfaces, and the crack tip stress during rolling were investigated which were adopted to analyze the influence of the vertical roll shape, the friction coefficient, and the initial crack size on the growth and closure of cracks. The calculated crack shapes after every pass during rolling are in good agreement with that obtained by physical simulation. At the same time, the evolution behavior of longitudinal cracks on the slab surface during multi-pass vertical-horizontal rolling was also simulated with the same method, and the projection of crack shapes in different sections were analyzed. And the evolution mechanism of surface cracks on slabs during rolling was analyzed.3. The evolution behavior of internal longitudinal cracks in slabs during vertical-horizontal rolling was simulated with the three-dimensional FEM. The crack shapes at different stages in rolling deformation zone were obtained, and the contact pressure on crack surfaces during rolling were adopted to analyze the effect of the vertical roll shape, and the crack location in slab on the growth and closure of cracks.4. The physical simulation was carried out on the healing of internal cracks in slab during hot rolling. The SEM micrographs of the crack shape, the SEM micrographs of crack fracture surface and the shear strength on the crack healing surface zone were analyzed under various rolling reductions. Meanwhile, a series of experiments were carried out for the internal crack healing by using the thermo-mechanical simulator. The SEM micrographs of crack shapes were obtained under different the heating rate, the heating temperature, the holding time, the strain rate, the reduction, and the deformation passes, and the influences of the above-mentioned factors on the crack healing were discussed. The mechanisms of internal crack healing during rolling were proposed based on the experimental results.5. The inclusion shape in strips was analyzed by SEM, and the chemical compositions of inclusions were analyzed by the energy spectrum analysis. Based on the experimental results, the evolution behavior of inclusions in strips during multi-pass rolling was carried out with the three-dimensional numerical simulation when the inclusions are combined with the matrix. The inclusion shape, the strain distribution around inclusion and the relative position of inclusions in strips after rolling were obtained considering a variety of inclusion sizes, positions, work roll diameters, etc. The influence of deformation behavior of inclusions on the crack initiation was discussed.6. The strain gradient around inclusions gradually increases with increasing the hard inclusions sizes during rolling. The evolution of inclusions and the initiation of cracks during rolling were simulated by using a two-dimensional numerical model when inclusions are separated with the matrix. The influence of inclusion size, shape, and position on the crack initiation around inclusions after rolling was analyzed.Analysis of the evolution behavior of cracks and inclusions in slab during rolling is important for understanding the mechanism of defects, and the results are useful for guiding the production.更多还原