Effect of Deformation Temperatures on Flow Stress and Dislocation Structures of Low Carbon Steels

The relation between dislocation structures and tensile properties have been investigated in the temperature range from room temperature to 400°C using four kind of a-irons with different carbon concentration. The grain size dependence of the yield and flow stress have also been studied in the same temperature range.The specimens showed serrated flow curves with high flow stress and high work-hardening rate. The higher the solute carbon content was the higher the flow stress and work-hardening rate became, and the temperatures of serrated flow being observable moved to a slightly higher temperature by decreasing the carbon content of the specimens.The flow stresses of the specimens were analyzed in term of the Petch parameters k and σi. It was found that kf at the temperatures of blue brittleness region was identical with ky, but σ showed higher value than the value at room temperature. These high values of frictional stress well corresponded to high workhardening rates. The change of flow stress component, therefore, was almost related to the frictional stress (σi). In the temperature range of serrated flow, deformed structures showed high dislocation densities and very small cell structures. Rapid increase in flow stress and high work-hardening rates were closely related with high rate of dislocation-multiplication.