Cause Analysis of Low Impact Energy for Hot-Rolled High Strength Steel at Low Temperature

The reasons for low-temperature impact energy fluctuating and impact energy slightly lower than normal of hot-rolled high strength sheet were analyzed by observation of microstructure and impact fractures using OM and SEM as well as element section distribution in lengthwise section using EPMA.The results show that the microstructure of the steel was mainly composed of acicular ferrites and polygonal ferrites.However the pearlite segregation was observed in the center along the thickness direction of the steel.The enrichment of Mn was also found in the segregation,which could influence the transformation of austenite to ferrite to form pearlite segregation and led to the impact energy lower than normal.And large granular inclusions such as TiC and NbC were found in the segregation band,and the occurrence of micro-cracks in the interface between the inclusions and matrix can decrease the impact energy of the hot-rolled high strength steel during rolling process.