Ultrafine-Grained Pure Iron Prepared By Asymmetric Rolling And Its Microstructure Property

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.