目前，世界范围内在高寒地区铺设的管道越来越多，低温管线钢受到了广泛关注。然而，我国对于低温管线钢的研发尚缺乏经验，正在兴建的中俄东线管道是我国第一条低温管道。严寒的气候条件对管线钢提出了更高的性能要求，尤其是低温韧性。因此，急需开发适用于低温环境下服役的高钢级管线钢。本文以组织调控和有效晶粒尺寸(取向差大于15°界面所包围的晶粒大小)细化为核心，结合热模拟和实验室轧制实验，对低温管线钢多元细晶组织调控技术进行了系统研究，并考察了该种组织的力学性能，尤其是低温韧性。 设计了两道次热模拟实验，通过优化再结晶区变形工艺结合临界区变形得到了由超细晶铁索体和粒状贝氏体组成的多元细晶组织，平均有效晶粒尺寸小于2 μm，其中包括：超细晶(EGS≤1μm，40-50％)铁素体+细晶(1≤EGS<3μm，40-50％)粒状贝氏体+纳米亚晶(≤500nm，20-30％)。与再结晶区低温段变形相比，临界区变形可以使组织的有效晶粒尺寸进一步细化。晶粒细化的最佳变形工艺区间为临界区靠近A〓的温度范围内，在临界区变形30％的变形量即可使组织的有效晶粒尺寸发生显著细化，并且随着变形量从30％增加到50％，组织细化的程度也更显著。 临界区变形细化有效晶粒尺寸的微观机制是：增强了的变形诱导铁素体相变和先共析铁素体连续动态再结晶两种过程共同作用的结果，其中变形诱导铁素体相变起主要作用。先共析铁素体发生连续动态再结晶可以通过亚晶不断吸收位错和锯齿状晶界直接截断扁平化的晶粒来实现。 在实验室轧制条件下，通过粗轧低温段大变形结合临界区精轧，也得到了由细晶铁素体和粒状贝氏体组成的多元细晶组织。实验钢板的平均有效晶粒尺寸细化到2.51 μm，使得-80℃条件下的夏比冲击功仍然大于400 J，在-20℃～-60℃的低温条件下DWTT断口剪切面积均达到100％，表现出优异的低温韧性。明确了大角度晶界的组织细化是控制低温DWTT的关键因素，而组织中粒贝的纳米亚晶对低温裂纹扩展没有明显的阻碍作用。 关键词：低温管线钢；临界区变形；多元细晶组织；粒状贝氏体；亚晶细化

 Currently, there are more and more pipelines lay in the alpine regions around the world. Therefore, the low-temperature pipeline steels are received a number of attentions. However, China has insufficient experience in the research and development of the low-temperature pipeline steel. The pipeline, Russia-China gas pipeline east line, being built is the first low-temperature pipeline in China. The poor climatic conditions require high performance for pipeline steels, especially the low-temperature toughness. Therefore, there is an urgent need to develop the high-grade pipeline steels which suit for servicing in the low-temperature environments. The key of the paper is the microstructure control and the refinement of the effective grain size (the size of grain enclosed by grain boundary with misorientation higher than 15°). The control technology of multiscale fine-grained microstructure for the low-temperature pipeline steel was systematically investigated by combining the thermal simulation and rolling test. The mechanical properties of this microstructure were investigated, especially the low-temperature toughness. Two-pass thermal simulation experiment was designed. The multiscale fine-grained microstructure composed of ultrafine ferrite and granular bainite was prepared by optimizing the recrystallization deformation process combined with the intercritical deformation. This microstructure had average effective grain size of less than 2 μm, which included ultrafine ferrite (EGS≤1 μm, 40-50%) + fine-grained granular bainite (1≤EGS≤3 μm, 40-50%) + nanoscale subgrain (≤500 nm, 20-30%). Compared with the deformation in low-temperature section of the non-recrystallization region, the intercritical deformation could further refine the effective grain size. When deforming in the intercritical region, the optimal region of deformation was the temperature range close to the A〓. The 30% deformation in the intercritical region could significantly refine the grains. As the deformation increases from 30% to 50%, the degree of the grain refinement was significant increase. The refinement mechanism during the intercritical deformation was the combine action of the enhanced DIFT and continuous dynamic recrystallization of pro-eutectoid ferrite. The cDRX process was confirmed by GAM analysis in conjunction with TEM observations, which were achieved by both the subgrain boundaries continuously trapping dislocations and the serrated boundary pinching off the elongated grains. Under the condition of library rolling, the multiscale fine-grained microstructure consisted of the fine-grained ferrite and granular bainite was also prepared by large roughing in low-temperature region combined with intercritical finishing. The average effective grain size of the steel plate reached to 2.51 μm. The impact energy was still higher than 400 J at -80℃, and the DWTT fracture shearing area was 100% at -20℃ ～ -60℃, which showed excellent low-temperature toughness. It was clear that the microstructure refinement of large angle grain boundaries was the key factor to control the low-temperature DWTT, and the nanoscale subgrains of granular bainite had no obvious hindrance to the low-temperature crack propagation. Key words: Low-temperature pipeline steel; intercritical deformation; multicale fine-grained microstructure; granular bainite; subgrain refining