Effect of Air Temperature on the Thermal Behavior and Mechanical Properties of Wire Rod Steel during Stelmor Cooling

Abstract

The effect of air temperature (T_a) on the thermal behavior and mechanical properties of steel wire rods is investigated during the Stelmor air cooling process using a numerical model and an offline cooling simulator. During the Stelmor cooling process, the temperature of the wire rod measured in the summer (28°C) is higher than that in the winter (4°C). The average temperature difference of the wire rod between the seasons is approximately 5°C. In addition, the tensile strength (TS) in the summer is lower than that in the winter: the average TS difference between the seasons is approximately 19 MPa. The different cooling rates of the wire rod depending on T_a are associated with the simple temperature difference between seasons instead of variations in the thermophysical properties of air with temperature. The variation in the cooling rate of the wire rod with T_a is affected significantly by forced convection because the absolute value of the forced convection is approximately 10 times higher than that of natural convection, and the heat flux by thermal radiation is almost unchanged by T_a. The forced convective heat transfer coefficient decreases with T_a because the Reynolds number decreases owing to the decrease in density and increase in kinematic viscosity of air as T_a increases. The deviation in temperature of the wire rod between the summer and winter seasons increases in a wire rod with a small diameter that is fabricated using high forced air because the amount of forced convection increases as the wire diameter decreases and the applied air velocity increases. It is concluded that different working conditions are necessary depending on the T_a, particularly when the wire diameter is small, the blower power is high, and the laying head temperature is high during the Stelmor cooling process.