超快冷工艺下X80管线钢的DWTT裂纹扩展行为

Crack Propagation Behavior during DWTT for X80 Pipeline Steel Processed via Ultra-fast Cooling Technique

用OM、SEM以及EBSD技术研究了超快冷工艺制备的18.4 mm X80管线钢的微观组织及晶体学取向,用SEM观察和分析了DWTT断口的表面形貌和裂纹扩展行为,并讨论了超快冷改善管线钢止裂性能的作用机制。结果表明,用超快冷工艺制备的X80管线钢的组织为AF+CB+M/A岛,其中AF组织大约占83%,CB大约占17%,组织的有效晶粒尺寸大约为3.5μm,大角晶界百分比大约为40.9%;在DWTT断裂过程中,实验钢中较大比例的AF和细小M/A岛减小了有效晶粒尺寸,提高了材料的止裂性能。超快冷提高管线钢止裂性能作用机制为:超快冷促进了AF形成,提高了细小弥散分布的M/A岛含量,弱化了AF相变过程中变体的选择,进而降低了有效晶粒尺寸,提高了大角晶界密度,改善了材料的止裂性能。

The microstructure and crystallographic features of X80 pipeline steel of 18.4 mm in thick- ness, which was prepared by ultra-fast cooling （UFC） technique, were characterized by means of optical microscope, electron scanning microscopy, and EBSD technique. The performance of the steel by drop weight tear test （DWTT） was investigated in terms of the crack propagation and the morphology of frac- tured surfaces, while the mechanism related with the crack arresting ability of the steel resulted from UFC treatment was revealed . The results show that the microstructure of the steel is primarily composed of AF, CB, and M/A island, and the area fractions of AF and CB are -83%, and ~17%, respectively. The ef- fective grain size is ~3.5 prn, and the fraction of high-angle boundary is -40.9%. The steel with higher fractions of AF and small-sized M/A island possesses smaller effective grain size, which is beneficial to the crack arresting ability of the steel. The mechanisms related to the enhancement of the crack arresting property may be ascribed to that the UFC promots the formation of AF, and increases the amount of small-sized M/A island by increasing cooling rate. Additionally, the variant selection during bainite transfor- mation is weakened by UFC Thus, the effective grain size is decreased, and the density of high-angle boundary is increased.