The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling

In this paper, two low carbon microalloyed steels, named as steel A and steel B, were fabricated by ultra fast cooling (UFC). In both steels, the microstructures containing quasi polygonal ferrite (QF), acicular ferrite (AF) and granular bainite (GB) can be obtained by UFC process. The amount of AF in steel B is more than that in steel A. The size and distribution of precipitates (Nb/Ti carbonitrides) in steel B are finer and more dispersed than those of in steel A due to relatively low finish cooling temperature. The mechanical properties of both steels are effectively enhanced by UFC process. UFC process produces low-temperature transformation microstructures containing a significant amount of AF. The mechanical properties of steel B were more satisfactory than those of steel A due to the finer average grain size, the greater amount of the volume fractions and smaller size of secondary phases.

本研究通过超快速冷却（Ultra Fast Cooling, UFC）工艺制备了两种分别命名为钢A与钢B的低碳微合金钢。两种钢材经UFC工艺处理后，均可获得包含准多边形铁素体（Quasi Polygonal Ferrite, QF）、针状铁素体（Acicular Ferrite, AF）与粒状贝氏体（Granular Bainite, GB）的显微组织。其中钢B内的针状铁素体（AF）含量高于钢A。由于终冷温度相对更低，钢B内的析出相（铌/钛碳氮化物）尺寸更细小、分布更弥散，优于钢A。两种钢材的力学性能均通过UFC工艺得到有效提升。UFC工艺可获得含有大量针状铁素体（AF）的低温转变显微组织。钢B的力学性能优于钢A，这归因于其更细小的平均晶粒尺寸、更高的第二相体积分数以及更细小的第二相尺寸。