Effect of Nitrogen Content and Cooling Rate on Transformation Characteristics and Mechanical Properties for 600 MPa High Strength Rebar

Abstract

To obtain appropriate chemical composition and thermo-mechanical schedules for processing the V-N microalloyed 600 MPa grade high strength rebar, the microstructure analysis during dynamic continuous cooling and tensile tests of three experimental steels with different nitrogen contents were conducted. The results show that increasing nitrogen content promotes ferrite transformation and broadens the bainite transformation interval, when the nitrogen content is in the range of 0.019–0.034 mass%. Meanwhile, the martensite start temperatures decrease and the minimal cooling rate to form martensite increases. To achieve a good combination of strength and ductility, the cooling rates should be controlled in the range of 0.5–3°C/s, leading to the microstructure of ferrite, pearlite and less than 10% bainite (volume fraction). Furthermore, all the experimental steels satisfy the performance requirement of 600 MPa grade rebar and the rebar with nitrogen content of 0.034 mass% shows the highest strength through systematically comparative investigation.

Keywords

• 600 mpa high strength rebar
• cooling rate
• microstructure
• mechanical property
• precipitation