低合金高强钢微观组织转变机制

摘要：通过高温激光共聚焦显微镜原位观察低合金高强钢（Q345R）在热循环过程中微观组织演变规律，并结合扫描电镜和纳米压痕，研究不同类型铁素体形核、长大机制及硬度。结果表明，铁素体可在晶界、亚晶界及夹杂物上形核，多边形铁素体及链状晶界铁素体主要在晶界上形核，而侧板条铁素体可在晶界及亚晶界上形核，而针状铁素体则主要在夹杂物及已形成的铁素体上形核，且奥氏体晶界及晶内亚结构尺寸控制了铁素体尺寸；讨论了夹杂物特征参数对针状铁素体形核的影响规律，Al、Mg、Ca、S等元素的含量达到一定比值且尺寸在5μm以下的复合夹杂物更容易成为针状铁素体的诱导核心；硬度实验结果表明，不同类型铁素体组织硬度存在差别，针状铁素体硬度最大可达到4GPa。

Microstructure evolution mechanism of HSLA steel

The microstructure evolution of low-alloy high-strength steel (Q345R) during thermal cycling was observed by high-temperature laser confocal microscopy. The nucleation, growth mechanism and hardness of different types of ferrite were studied by scanning electron microscopy and nanoindentation. Experiment results proved that ferrite can nucleate at grain boundary, sub grain boundary and inclusions. Polygonal ferrite and chain like grain boundary ferrite mainly nucleate on the prior austenite grain boundary. The ferrite side plates (FSP) can nucleate on the prior austenite grain boundary and sub grain boundary. Furthermore, the acicular ferrite (AF) mainly nucleates on the inclusions and ferrite. In addition, the size of the prior austenite grain boundaries and sub microstructures confine the size of AF and FSP. Meanwhile, the effect of characteristic parameter of inclusions on AF nucleation was discussed. Only when the Al, Mg, Ca, S content in inclusion reach a value and the size is smaller than 5μm, the inclusions can usually be the nucleation cores for AF. The hardness test results show that there are differences in the hardness of different types of ferrite, and the hardness of acicular ferrite can reach 4GPa.