【作者】 丁毅；

【导师】 单爱党； 蒋建华；

【作者基本信息】 上海交通大学 ， 材料学， 2010， 博士

【摘要】 钢铁是主要的结构材料之一,在未来的很长时间里仍然是重要的结构材料。近年来,通过晶粒细化提高钢铁材料的性能成为材料学界和钢铁研究者的关注热点。本文以纯铁为研究对象,通过优化改进的异步轧制设备制备超细晶纯铁,研究了异步轧制纯铁产生的剪切应变的大小和分布,系统地研究了剪切应变对纯铁的组织、织构和性能的影响,试图为钢铁材料的超细晶化研究探索一种新的技术方法。本文通过有限元法模拟纯铁在异步轧制中的变形过程,研究了轧下量、摩擦系数和轧制方式对纯铁塑性流变的影响。有限元模拟结果表明,纯铁在同步轧制时产生单一的压应变,而在异步轧制时还产生额外的强烈剪切应变。随着轧下量的增加,异步轧制轧材的剪切应变逐步增加。在相同轧下量的时候,增大轧辊与轧材间的摩擦系数,能提高异步轧制轧材的剪切应变。使用在纯铁中植入异质金属铜片的方法对纯铁异步轧制后的变形行为进行直接观测。本文根据弹塑性基本理论推导了分析计算剪切应变和等效应变的计算公式。根据直接观测的数据计算了纯铁经过不同轧下量异步轧制后的等效应变,随着轧下量的增加,轧材的等效应变逐渐增大。纯铁经过异步轧制后产生了强烈的剪切应变,剪切应变沿厚度的分布不均匀,在上轧面具有最大值约为13.17,随着厚度的增加,在中心部位具有最小值为1.3左右。本文使用异步轧制方法成功制备了平均晶粒尺寸为0.9μm、大角度晶界达60%的等轴状超细晶纯铁,其CSL晶界达到35%左右,旋转轴都为[111]晶带轴。异步轧制超细晶纯铁的组织在轧制厚度方向上呈现一定的不均匀性,但是这种不均匀性不显著。这是由异步轧制过程中纯铁产生的剪切应变、等效应变分布不均匀导致的。异步轧制超细晶纯铁在400℃退火1小时后晶粒基本没有明显长大,没有再结晶现象发生。再结晶开始温度在400℃和500℃之间,再结晶过程在600℃时全部完成。在体心立方结构的异步轧制纯铁中发现了{100}面织构,该织构很难通过冷轧方式直接获得,该织构的形成是异步轧制过程中纯铁产生的剪切应变和压应变共同作用的结果。该织构高度依赖于轧制方式,纯铁经过同步轧制可以得到高斯取向G{110}<001>,但是通过异步轧制可以获得{100}面织构。异步轧制在纯铁中制备的{100}面织构在轧材厚度方向上分布均匀,没有明显的分层现象。异步轧制纯铁的{100}面织构在继续进行70%同步轧制后仍然是{100}面织构,但是在400℃退火后其{100}面织构就完全消失而形成其它织构。随着异步轧制轧下量的增加,纯铁的显微硬度和屈服强度逐渐增大,它们与轧下量基本呈线性关系。这种强度的提高既是晶粒细化的效果,也是加工硬化强化的结果。经过计算,当轧下量为90%时,纯铁的位错密度达到1.75×1015 m-2。异步轧制超细晶纯铁的断裂状态为韧性断裂,由大量等轴韧窝组成,随着轧下量的增加,韧窝的尺寸逐渐减小。异步轧制超细晶纯铁经过不同温度退火后获得不同晶粒尺寸的纯铁,它们的屈服强度服从Hall-Petch关系,Hall-Petch系数为0.583MN/m2/3。随着晶粒尺寸减小,纯铁的应变速率敏感因子逐渐减小。异步轧制超细晶纯铁虽然有很高的强度,但是延伸率很小,缺乏足够的加工硬化能力,具有严重的塑性不稳定性,这严重妨碍了其在工业上的实际应用。异步轧制超细晶纯铁极小的应变速率敏感因子使材料具有很小的空间保持稳定的塑性变形,从而使材料很快进入塑性不稳定阶段直至断裂。更多还原

【Abstract】 Improving the properties of steel through grain refinement has attracted much attention of the material researchers since steel plays an important role as a structural material. In this paper, pure iron is studied as a model material to produce ultrafine-grained (UFG) iron by asymmetrical rolling (ASR). The shear strain and its distribution in ASR processed iron have been studied. The effects of shear strain on microstructure, texture and mechanical property have been studied systemically. This research is the pioneer and the base for the grain refinement in steel.The deformation behavior of pure iron during ASR has been simulated through finite element module (FEM). Roles of reduction, friction coefficient and rolling processing on the deformation behavior of iron have been studied. The result of the FEM simulation shows that only the plane compressive strain exists in the rolled iron processed by symmetrical rolling. However, there is additional severe shear strain in ASR processed iron. The shear strain in the ASR processed iron is increasing with the increase of the rolling reduction. The shear strain also increases when the friction coefficient increases.The deformation in pure iron after ASR has been directly observed through the enbedded Cu plate. The equation to calculate the shear strain and equivalent strain has been proposed based on the elasticity-plasticity theory. The equivalent strain in iron with different rolling reductions has been estimated according to the data from the direct observation. With increasing rolling reduction, the equivalent strain of the rolled materials increases. Severe shear strain is induced in the ASR processed iron. Its distribution along the thickness is inhomogeneous. There are a maximum of 13.17 at upper surface and a minimum of 1.3 at the center.The equiaxed UFG pure iron with the average grain size of 0.9μm and 60% high angle boundary has been obtained through ASR. In the UFG pure iron by ASR, the fraction of the coincidence site lattice (CSL) grain boundary rotating about才axis is approximately 35%.The microstructure of the UFG pure iron from ASR is not uniform along the whole thickness. It is resulted from the distribution of the shear strain and equivalent strain. After annealing at 400℃for 1 h, the grain size of the UFG pure iron from ASR almost does not increase without recrystallization. The recrystallization occurs from 400℃to 500℃, and finishes at 600℃.The {100} texture has been realized in body-centered cubic pure iron by ASR without any post annealing. The rolling reduction is not the main reason for its formation. Its formation is strongly affected by the rolling process. Gauss orientation G {110} <001> has been obtained in the same iron processed by SR instead of the {100} texture by ASR. It is attributed to the different strain mode during different rolling process. The {100} texture in pure iron is uniform along the whole thickness. The {100} texture in iron subjected to ASR still remains after post SR with the reduction of 70% and disintegrates completely after annealing at 400℃.With increasing rolling reduction, the microhardness and the yield strength of the ASR processed pure iron increase, resulted from not only the grain refinement but also the work hardening. The density of the dislocation is up to 1.75×1015 m-2 in UFG pure iron after ASR with the 90% reduction. The fracture of the UFG pure iron is ductile with number of dimples, of which the size is decreases with increasing the reduction.Different grain sizes have been obtained from the UFG pure iron after annealled at different temperatures. The yield strength obeys the Hall-Petch relationship. The Hall-Petch constant is about 0.583 MN/m2/3. With decreasing grain size, the strain rate sensitivity (SRS) decreases. The UFG pure iron with high strength shows a little work hardening capability but severe plastic instability. The negligible SRS allows a small space to keep the deformation behavior stable. The deformation behavior localizes in narrow regions shortly and comes into a plastic instable stage. After that the tensile curve falls down and the sample plunges into fracture.更多还原