RESEARCH ON TRANSFORMATION OF GRANULAR STRUCTURE IN LOW CARBON Mn-Si STEEL

Previous researches indicated that granular bainite (GB) and granular structure (GS) mainly differ in the shape and distribution of islands and the ferrite matrix: firstly, the GB islands distribute regularly and longitudinally in stripe shape, whereas the GS islands distribute disorderedly with irregular morphology; secondly, the matrix of the former is bainitic ferrite, however that of the latter is proeutectoid ferrite matrix. In addition, the properties of GB and GS differ significantly. Under specific conditions the strength and toughness of GB are superior to those of GS whose toughness is poor hence GS should be avoided in steels. It's important for phase transformation theory study to understand the morphology of GS and its corresponding transformation condition completely, furthermore, its properties can be improved by microstructure optimization. The granular structures of low carbon Mn-Si steels, especially the distribution of GS islands were studied. It was found that the microstructure of steels without Cr after air-cooling is CS consisting of proeutectoid ferrite and islands which distribute in the two forms: irregular and directional respectively. Their forming mechanisms, including forming process of islands and type of proeutectoid ferrite were discussed. Results show that the morphology and distribution of the islands reflect the morphology of proeutectoid ferrite, and the strip islands formation may be controlled by the ledgewise growth of proeutectoid ferrite. There are three formation types for the strip GS islands: (1) long islands formed among Widmannstatten ferritic plates; (2) short and paralleled islands formed in massive proeutectoid ferrites which are formed by ledgewise mechanism; (3) broad and paralleled islands formed in massive ferrites growing along certain low-energy crystal faces. The GS islands may distribute directionally under specific conditions, and not all microstructures of ferrite matrix and paralleled islands are GB. In existence of proper content Cr no granular structure appears in the studied low carbon Mn-Si steel after continuous cooling.