An investigation on microstructure evolution and mechanical properties of cryogenic steel rebars under different cooling conditions

In this study, industrial trial production of the microstructure evolutions and resultant mechanical properties and finite element model of the temperature variation in the cryogenic temperature rebar subjected to the TempCore processes were presented. The microstructure of tested steel rebar is composed of ferrite, pearlite and bainite when the cooling rate is less than or equal to 5 °C s−1. The pearlite disappears when the cooling rate is 10 °C s−1, and the microstructure is composed of granular bainite and a small amount of ferrite. The microstructure consists of martensite and bainite at cooling rate of 20°. Compared with the tested steel rebar with water flow rate of 450 m3 h−1, the tested steel rebar with water flow rate of 100 m3 h−1 has an optimum mechanical strength and a higher uniform elongation at room temperature, and has a higher NSR(notch sensitivity ratio) at −165 °C. The uniform elongation is 7.3% at room temperature and NSR is 1.14 at −165 °C. The ductile-brittle transition temperature of the tested steel rebar with water flow of 100 m3 h−1 is about −30 °C, while which of 450 m3 h−1 is 0 °C. The impact energy of the tested steel bar with water flow of 100 m3 h−1 is 2 ∼ 3 times that of 450 m3 h−1 at or below 0 °C, which is mainly due to its ultrafine acicular ferrite and bainite containing high dislocation density and sub-grains.