Preparation Of Nano-Grained Q235 Steel Plates And Their Microstucture And Mechanical Properties

In this paper, the ultra fine grained and the nano-grained steel sheets were prepared by a new process of severe multi-pass cold rolling (accumulated reduction of 93.6%) and low temperature annealing of Q235 steel plates(6^thickness×60^width×260^lengthmm³) with starting structure of lath martensite Andthe effects of the annealing temperature and annealing time on the microstructure and the mechanical property of the severe cold-rolled samples of lath martensite were studied using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Scanning electron microscop (SEM), microhardness test and tensile tests.The microstructure observation indicates that, the results of XRD scheer method are in good agreement with that of TEM dark field technique, the difference between them is 1.3-1.6 nm. And, by adjusting annealing temperature and time, the ultra fine grained and the nano-grained steel sheets with average grain size from 4.7μm to 18 nm were obtained.SEM observation of the tensile fracture indicated that, when annealed below 500℃, tensile fracture showed delamination, when annealed at 600℃, the delamination character disappeared. Then recrystalization dynamics of cold-rolled lath martensite was studied by the method of remained work hardening and the recrystallization apparent energy was about 151 kJmol^-1. In addition, the mechanisms of the refining and strengthening, and the reasons for the ultra high strength and high brittleness resulted from the new method for refining grain of low carbon steel were also discussed.