The effects of niobium content (0.011% ~0.055% ) and heating temperature (1100 - 1250 ℃)on mi¬crostructure and properties of HRB400 steel rebar 20 mm plat are studied by thermal simulator Gleeble

high temperature la¬ser con-focal microscope and transmission electron microscope. In 20 mm plate with heating at 1180 °C for 1h

with in¬creasing content of niobium addition

the bainite content in steel increases to inhibit ferrite transformation

leads to increasing yield strength. Addition of 0.024% niobium

the microstructure has 50. 8% ferrite

39.0% perlite and 10.2% bainite

wliich corresponds to yield strength of 426 MPa

tensile strength of 685 MPa

elongation of 24. 0% and reduction of area 61.3%. The size of niobium precipitates gradually refine with the increasing of heating temperature

the number is gradually increasing

and the disperse distributed niobium-containing precipitates with a size range of 7 ~ 88 nm can be produced at a heating temperature of 1180 °C.