超快冷工艺下X70管线钢微观组织及M/A岛演变

Microstructural characteristic and martensite-austenite constituent of X70 pipeline steel under ultra-fast cooling process

采用超快冷与层流冷却相结合的冷却工艺对厚度为12.7 mm的X70管线钢进行轧制,分析讨论不同超快冷终冷温度下实验钢的微观组织及马氏体-奥氏体(M/A)岛演变规律,并进一步给出实验钢的最佳轧制工艺参数。研究结果表明:当超快冷终冷温度在570~360℃范围内时,实验钢组织可归类为:1)准多边形铁素体(QF)+贝氏体铁素体(BF)+针状铁素体(AF)+M/A岛;2)AF+BF+M/A岛;3)AF+BF+板条贝氏体(LB)+M/A岛。随着超快冷终冷温度由570℃降至440℃,M/A岛所占面积百分比变化不大,M/A岛长度减小;随着超快冷终冷温度进一步降至360℃,M/A岛长度变化不大但体积分数降低。当超快冷终冷温度为440℃时,实验钢拉伸性能及低温韧性最优。针对实验用X70管线钢,控制冷却最佳工艺制度为终轧830℃+超快冷却至410~470℃+层流冷却至320~370℃+卷取。

Ultra-fast cooling （UFC） combined with laminar cooling techniques were used to process X70 pipeline strip with a thickness of 12.7 mm. The evolutional disciplines of microstructure and martensite-austenite （M/A） constituent were analyzed and discussed under different UFC processes, and optimal processing parameters of UFC were further determined. The results show that when ultra-fast cooling interrupt temperature decreases from 570 to 360 ℃, the microstructure of the steel sample can be categorized as follows： 1） quasi-polygonal ferrite＋bainitic ferrite （BF）＋ acicular ferrite （AF） ＋M/A constituent; 2） AF＋BF＋ M/A constituent; 3） AF＋BF＋lath bainite （LB）＋M/A constituent. Specifically, when ultra-fast cooling interrupt temperature decreases from 570 to 440 ℃, the area fraction of M/A constituent does not change significantly and the length decreases obviously. When interrupt temperature further decreases to 360 ℃, the length of M/A constituent does not change obviously and the volume fraction decreases significantly. The tested steel exhibits the most excellent mechanical properties of tensile tests and Charpy impact tests under ultra-fast cooling interrupt temperature of 440 ℃. For the tested X70 pipeline steel, the optimum ultra-fast cooling processing parameters are concluded as follows： finishing rolling at 830 ℃＋ultra-fast cooling to 410？470 ℃＋laminar cooling to 320？370 ℃＋coiling.