Fresh martensite (FM) is often present in medium-Mn steels, especially when containing lower Mn content, due to the insufficient thermal stability of reverted austenite; this FM is brittle, largely deteriorating the ductility. In this paper, large ductility and high yield strength are achieved in an Al/Si-added medium-Mn steel containing 2.7Mn via a two-step partitioning heat treatment, i.e. intercritical annealing (IA) followed by low-temperature partitioning (LTP). We show that, during the IA, C and Mn atoms partition from the pre-quenched martensite to reverted austenite; Al addition reduces the size of reverted austenite and promotes C and Mn enrichment in the reverted austenite by decelerating its growth kinetics. This enables the reverted austenite more thermally stabilized, thereby reducing the amount of FM and increasing the amount and mechanical stability of retained austenite (RA) at room temperature. During the LTP, accompanied with the recovery of dislocations and the suppression of carbide precipitation by Al and Si, C atoms further partition from FM to RA, which enables the RA more mechanically stabilized and thereby sustains the high strain hardening to larger strains. Simultaneously, the FM becomes less hard and less brittle due to C atoms depletion and dislocations recovery, alleviating the stress/strain localization and favoring the uniform plastic deformation. Furthermore, the decrease in mobile dislocation density that is accompanied with the recovery of dislocations is believed to be mainly responsible for the enhanced yield strength of the steel. The present results indicate that the synergetic effects of the primary element partitioning (promoted by Al) during IA, which increases the thermal stability of reverted austenite, and the secondary element partitioning (enhanced by Al and Si) and as well dislocation recovery during LTP, which increases the mechanical stability of RA and the uniformity of plastic deformation, significantly enhance both the ductility and yield strength of medium-Mn steel with low Mn content.

中文翻译：

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通过两步分配热处理提高 2.7Mn 钢的延展性和屈服强度


新鲜马氏体 （FM） 通常存在于中等锰钢中，尤其是当锰含量较低时，由于还原奥氏体的热稳定性不足;这种 FM 很脆，在很大程度上恶化了延展性。在本文中，通过两步分配热处理，即临界间退火 （IA） 和低温分配 （LTP），在添加 Al/Si 的 2.7Mn 中锰钢中实现了大延展性和高屈服强度。我们表明，在 IA 期间，C 和 Mn 原子从预淬火马氏体分裂到回归奥氏体;Al 添加减小了还原奥氏体的尺寸，并通过减慢还原奥氏体的生长动力学来促进还原奥氏体中 C 和 Mn 的富集。这使得还原的奥氏体具有更高的热稳定性，从而减少了 FM 的量，并增加了室温下残余奥氏体 （RA） 的量和机械稳定性。在 LTP 期间，伴随着位错的恢复以及 Al 和 Si 对碳化物沉淀的抑制，C 原子进一步从 FM 分裂到 RA，这使得 RA 在机械上更加稳定，从而维持对更大应变的高应变硬化。同时，由于 C 原子的耗尽和位错恢复，FM 变得不那么坚硬和更脆，从而减轻了应力/应变的局部化并有利于均匀的塑性变形。此外，随着位错的恢复，移动位错密度的降低被认为是钢屈服强度提高的主要原因。 目前的结果表明，IA 过程中一次元素分配（由 Al 促进）的协同效应，增加了还原奥氏体的热稳定性，以及 LTP 期间次元素分配（由 Al 和 Si 增强）以及位错恢复，这增加了 RA 的机械稳定性和塑性变形的均匀性， 显著提高低 Mn 含量的中等 Mn 钢的延展性和屈服强度。