Effect of microalloying elements Nb and V on phase transformation,microstructure,and strengthening mechanisms in high-strength ferrite-pearlite steels: A comparative analysis

Abstract

The phase transformation, microstructure, mechanical properties and valence electron structure of 20MnSi steels with different Nb and V contents were investigated by using thermal expansion, micro-tensile, Empirical Electronic Theory (EET) of solids and molecules, among others. The results indicate that the Ar₃ (the starting temperature of the transformation from undercooled austenite to ferrite) and the content of proeutectoid ferrite both significantly decrease with the increase of Nb content, while the change with V content is small, which is because the number of covalent electron pairs in C-Nb bonds is higher than that in C-V bonds. The carbonitrides precipitate increase the Ar₃ of Nb-V composite steels with the increase of Nb content. In contrast to the decrease in tensile strength/yield strength (Rm/Rel) ratio with increasing V content in V-microalloyed steels, the Rm/Rel ratio in Nb-V composite microalloyed steels increases with increasing Nb content, which is attributed to the precipitation of carbonitrides and the microhardness of pearlite in Nb-V composite microalloyed steels are increased with the increasing Nb content, leading to a greater increment in tensile strength than in yield strength.