Microstructural Characterization and Softening Mechanism of Ultra-Low Carbon Steel and the Control Strategy in Compact Strip Production Process

In this paper, the microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact strip production (CSP) processes were investigated. The softening mechanism was also discussed and the control strategy was proposed in order to obtain optimum properties. Result showed that the average ferrite grain sizes of austenite rolling sheet and multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higher yield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocation in the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanism of the sheet after multiphase rolling was the coarsening of ferrite grain. The combined role of {001} and {111} orientation resulted in a slight increase of the r and $\overline{\text{r}}$ value in the sheet after multiphase rolling. It was a wise choice to conduct rolling at the Ac₁ temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength of the sheets could be further reduced and the elongation could also be improved.