Relaxation of Deformed Austenite and Refinement of Bainite in a Nb-Containing Microalloyed Steel
摘要
By means of stress relaxation and strain-induced precipitati on in deformed austenite, bainite with micron sheaves size was obtaine d in Nb-containing steel. Microstructures of deformed samples isotherm ally relaxed for various time followed by cooling in water were examin ed. Stress relaxation test and transmission electron microscopy (TEM) were employed to detect precipitation of microalloy elements, such as Nb, Ti, during isothermal holding after deformation. All the samples w ere constituted by lath-like bainite along with acicular ferrite, but the size of bainitic sheaves and the amount of acicular ferrite were c hanged with relaxation time. To achieve optimum refinement, relaxation should be confined in the stage when the precipitates have sufficient ly grown and started to coarsen. The sample having not undergone relax ation does not exhibit obvious refinement despite of its higher disloc ation density. These results indicate that relaxation promotes bainite to refine, which is because deformed austenitic grains are divided by dislocation walls formed during relaxation and acicular ferrite forme d before bainitic transformation.