Analysis of the cool bending fracture of HRB400

The tests were conducted on 400MPa hot-rolled ribbed bar(HRB400). The cool bending fracture appeared frequently in the production was analyzed. The research result showed that the cool bending fracture of rebar were caused by element segregation of C, Mn, inclusions, the existence of banded structure, widmannstatten structure and bainite structure in the center and edge of rebar. The inclusions are the root cause of internal crack and one of the reasons to fracture. Reasonable cooling rate can refine grain size, while the overquick cooling speed could result in the formation of widmannstatten structure in acicular or plume, increased the brittleness of steel and reduce its cold bending properties. On this basis, some improvement opinions were put forward: using the raw material with low Mn, S and P; reducing the S content with pretreatment of hot metal; controlling the content of C and Mn respectively in C: 0.18 ~ 0.23%, Mn:1.20~1.30%; extending argon blowing time at argon flushing station, using electromagnetic stirring technology; controlling the cooling rate at 15 ℃/s.