材料期刊网

对低碳Mn--Si钢中的粒状组织进行了实验研究, 考察了粒状组织中小岛的形状及其分布. 结果表明, 不含Cr时, 空冷后形成了由先共析铁素体基体+岛状物所组成的粒状组织,其中的岛状物形态及分布可以是无规则的, 也可以是长条状的小岛沿一定方向分布于基体上. 对于这2种形态的粒状组织, 尤其是对后者形态的形成和铁素体基体的分析表明, 小岛的形状和分布是先共析铁素体形态的反映, 条形规则排列的小岛可能受先共析铁素体基体台阶长大机制控制, 粒状组织小岛在一定条件下也可能具有方向性. 加适量Cr后, 连续冷却后的组织中不出现粒状组织.

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 GS 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.