基于渗碳体调控低合金钢中块状逆变奥氏体与奥氏体晶粒尺寸

逆变奥氏体微观组织显著影响钢铁材料的最终组织性能，阐明块状奥氏体的形成规律对于精准掌握逆相变至关重要。本文以Fe–2.5Mn–1.5Si–0.35C合金为研究对象，通过OM、SEM和EBSD等手段研究了不同预回火条件下晶内块状奥氏体与最终奥氏体晶粒尺寸的演变规律。研究结果表明，随预回火温度自350 ℃升高至650 ℃，晶内块状奥氏体体积分数呈现出先增加后迅速降低的趋势；400 ℃预回火条件下，随预回火时间的延长，晶内块状奥氏体体积分数先增加后趋于稳定；预回火促使晶内块状奥氏体形成，导致最终奥氏体晶粒显著细化。随着预回火温度的升高，逆相变前渗碳体发生粗化，增加了晶内块状奥氏体的有效形核位点，此促进了晶内块状奥氏体的形成。此外，晶内块状奥氏体具有多重取向，晶内块状奥氏体的增加，使得逆相变后奥氏体晶粒显著细化。本研究提供了一种在不改变钢化学成分的条件下，通过控制渗碳体实现对逆相变晶内块状奥氏体形成和最终奥氏体晶粒尺寸调控的新方法。 

Controlling the formation of reverted globular austenite and the as-transformed austenite grain size in low-alloy steel based on cementite

Austenite reversion has been widely used in the traditional heat treatment of steels, and recently, it has been used in the fabrication of advanced high-strength steels. The microstructure of reverted austenite significantly influences the final microstructure and properties of steel; thus, it is crucial to understand the formation of globular austenite to accurately grasp its reversion behavior. In this paper, an Fe–2.5Mn–1.5Si–0.35C alloy was chosen as the research object, and the evolution of intragranular globular austenite and finally transformed austenite grain size were studied under different pre-tempering conditions using a metallographic optical microscope, scanning electron microscope, and electron backscatter diffraction. It was found that as the pre-tempering temperature was increased from 350 ℃ to 650 ℃, the volume fraction of intragranular globular austenite first increased and then rapidly decreased. At the pre-tempering temperature of 400 ℃, the volume fraction of intragranular globular austenite initially increased and remained stable thereafter, when the pre-tempering duration was increased from 1 to 10 h. Fine cementite particles were primarily formed immediately before the reversion in the non-tempered or low-temperature pre-tempered initial structures. This provided less effective nucleation sites for the formation of intragranular globular austenite. Therefore, lesser intragranular globular austenite grains were formed, thereby resulting in relatively coarse finally transformed austenite grains after reversion. The cementite particles were gradually coarsened as the pre-tempering temperature was increased to 550 ℃, thereby increasing the number of effective nucleation sites for the formation of intragranular globular austenite. Conversely, when the martensite samples were pre-tempered at a high temperature of 650 ℃, Mn is seriously enriched into the cementite particles before the reversion, largely reducing the driving force for reversion. This resulted in the growth of intragranular globular austenite under the partitioning local equilibrium mode, with a slow growth rate, resulting in a low volume fraction. Therefore pre-tempering can effectively promote the formation of intragranular globular austenite. Owing to its multiple orientations, increased intragranular globular austenite formation resulted in significantly refined austenite grains after reversion. This study provided a new strategy to regulate the formation of intragranular globular austenite and finally transformed austenite grain size by controlling the size and composition of cementite particles through pre-tempering without changing the chemical composition of the steel.