Intercritical treatments within the dual-phase (α+γ) region were applied on HSLA pipeline steels, for acquiring a low yield ratio (YR) well balanced with desirable strength. Intercritical cooling treatment (ICT), step cooling treatment (SCT) as well as direct cooling treatment (DCT) after full austenization were designed to obtain an optima multiphase microstructure. Effects of cooling rate in DCT routine and intercritical temperatures in ICT and SCT routines on microstructural evolution and corresponding mechanical properties were also investigated. Especially, the SCT treatment applied with an intercritical temperature of 750 °C produces a microstructure composited of “soft” coarse polygonal ferrite, and “hard” acicular bainite and lath martensite containing large amounts of dislocation tangles or networks generated by deformation. Such multiple phase constituents guarantee the high strength and remarkable ductility on deformation, meanwhile cleavages propagation is hindered by the high-angle boundaries of bainite and martensite sheaves, which leads to the lowest YR ∼0.61 combined with highest tensile strength among all. In addition, by using the Swift equation to elucidate the relationship between the phase component and yield ratio, it is found that simply increasing the fraction of low-temperature transformed phases, like high-strength acicular bainite and lathed martensite, or the percentage of soft polygonal ferrite for good ductility, can hardly solve the problem how to achieve ultralow-YR pipeline steels balanced with enhanced strength. The present result proves that, through utilizing the proposed SCT heat treatment on pipeline steels, an ultralow yield ratio ∼0.61 achieved is synchronized with a desirable strength, which efficiently overcomes the trade-off limit between the strength and yield ratio when applying conventional heat-treatment routines. The fact indicates that, rationally adjusting the content of multi-phase microstructure through optimizing the intercritical treatment conditions, can enable us of realizing the synchronous improvement of the YR and strength in HSLA pipeline steels for real engineering.

在HSLA管线钢上进行了双相（α+γ）区域内的临界处理，以获得低屈服比（YR），并具有理想的强度。设计了充分奥氏体化后的临界冷却处理（ICT），逐步冷却处理（SCT）以及直接冷却处理（DCT），以获得最佳的多相组织。还研究了DCT例行程序中的冷却速率以及ICT和SCT例行程序中的临界温度对微观组织演变和相应的力学性能的影响。尤其是，在750°C的临界温度下进行的SCT处理会产生由“软”粗多边形铁素体与“硬”针状贝氏体和板条马氏体复合而成的微观结构，其中包含大量因变形而产生的错位缠结或网状结构。这种多相成分可确保高强度和出色的延展性，YR约为0.61，具有最高的拉伸强度。此外，通过使用Swift方程阐明相成分与屈服比之间的关系，发现仅增加低温相变相的比例（如高强度针状贝氏体和板条马氏体）或软质百分比多边形铁素体具有良好的延展性，很难解决如何实现超低YR的问题管线钢平衡，强度提高。目前的结果证明，通过在管道钢上利用建议的SCT热处理，可以使〜0.61的超低屈服比与所需的强度同步，从而有效地克服了在应用常规热加工时强度与屈服比之间的取舍极限。治疗程序。事实表明，通过优化临界条件优化合理调整多相组织含量，可以使实际工程中HSLA管线钢的YR和强度同步提高。