Role of Grain Boundary Strength on Yielding Behavior and Uniaxial Tensile Properties in Ferritic Steels

The yield-point phenomenon in recrystallized ferritic steels is often associated with the dislocation multiplication mechanism, wherein the yield drop can be attributed to the lack of mobile dislocations in materials. However, the yield-point phenomenon is not consistently observed in all recrystallized ferritic steels, implying that the dislocation multiplication mechanism has constraints in delineating the yielding behavior of these materials. Therefore, in this study, we introduced grain boundary strength as a critical parameter for elucidating the yielding behavior of recrystallized ferritic steels. Three types of steels-interstitial-free (IF) steel, precipitation-hardened (PH) steel, and Mn-added interstitial-free (IF-2Mn) steel-were analyzed for grain boundary strength using nanoindentation, and the reliability of this methodology was verified by Hall-Petch analysis. The IF steel, which lacked the yield-point phenomenon, demonstrated a much lower grain boundary strength than the PH and IF-2Mn steels, where the phenomenon occurred. Microstructural analysis confirmed that the enhanced grain boundary strengths of the PH and IF-2Mn steels were due to carbon and manganese segregation at the grain boundaries, respectively. Further, the grain boundary strength significantly influenced the tensile properties and yielding behavior. In PH steels, the enhanced grain boundary strength increased the yield strength owing to Hall-Petch hardening; however, it also increased the resistance to plastic deformation propagation, resulting in reduced ductility. In the IF-2Mn steels, the two specimens with different grain sizes exhibited similar yield strengths, which could be attributed to differences in the grain boundary strength. Our findings have significant implications for the design and optimization of ferritic steels.