Effects of Microstructural Evolution and Dislocation Properties on the Strengthening Behavior of Cold-Drawn Ultralow-Carbon Steel

The microstructural evolution and variations in the dislocation properties and the dislocation density of drawn ultralow-carbon (ULC) steel wires, which exhibited an ultimate tensile strength of over 1500 MPa at ε = 5.3, were investigated through scanning electron microscopy/electron backscatter diffraction (SEM/EBSD) analysis and synchrotron radiation X-ray diffraction (SR-XRD), respectively. The wire drawing did not cause systematic changes in the dislocation properties of the drawn ULC steel wire. Dislocation strengthening and the grain size effect, such as the Bailey–Hirsch relationship and the Langford–Cohen relationship, were found to be dominant in the early and subsequent stages of drawing, respectively. Moreover, tensile strength of the drawn ultralow-carbon steel wires at ε > ~ 4 showed a positive deviation from the extrapolated Langford–Cohen relationship in the early stages of drawing, which was found to be caused by changes in the character of grain boundaries during drawing, that is, an increase in the fraction of grains surrounded by boundaries with misorientation angles of 15 deg or greater and a relative decrease in the fraction of low-angle boundaries with misorientation angles of less than 5 deg contributed to the excessive strengthening in the drawn ULC steel wires.