通过Gleeble-2000 试验机研究了Q345C钢连铸坯的高温热塑性。利用扫描电镜、金相显微镜、透射电镜观察了第Ⅰ、Ⅲ脆性温度区内拉伸试样断口部位的显微组织及形貌,分析了动态再结晶、相变、析出物等对微合金化钢高温延塑性的影响。结果表明：在1×10-3/s应变速率下, Q345C钢存在两个脆性温度区,即第Ⅰ脆性区（1200～1300℃）和第Ⅲ脆性区（600～875℃），无第Ⅱ脆性区出现；最高塑性出现在1050℃左右，断面收缩率（Z）达到85.8%；在第Ⅲ脆性区，沿奥氏体晶界析出膜状铁素体抗拉能力较低，晶界处存在夹杂物以及微合金元素的析出物，是钢的热塑性降低的主要原因。

Hot ductility of continuous casting Q345C slabs was tested by Gleeble-2000 thermal /strain simulation machine. Using scanning electron microscopy, metalloscopy and transmission electron microscopy, morphology characteristics and microstructure of fracture surface of tensile specimen in brittle temperature zone Ⅰ and Ⅲ were observed. Influences of dynamic recrystallization, phase change and precipitate on hot ductility of the niobium- microalloyed steel were analyzed. The results showed that: at the 1 × 10-3/s strain rate, there are two low ductility zones, namely brittle zone Ⅰ (1200-1300℃) and brittle zone Ⅲ (600-875℃)，the brittle zone Ⅱdoes not exist; the maximum ductility is around 1050℃ and area reduction （Z）reached 85.8%; in brittle zone Ⅲ, membranous lower tensile strength of ferrite along austenite grain boundaries, and the existence of inclusions and precipitates of micro-alloying elements in the grain boundaries is the main reasons for lower ductility of Q345C steel.