Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

Abstract

Carbon and niobium (Nb) play an important role in influencing the ultimate microstructure and mechanical properties. In this regard, we elucidate here the impact of carbon and Nb on the microstructural evolution and precipitation behavior during continuous cooling of industrially processed microalloyed steels with varying carbon and Nb-content. The microstructure and precipitation evolution was studied via electron microscopy and related to the outcome of thermodynamic simulation. The increase of carbon content in steel increased the precipitation temperature of (Nb, Ti)(C, N), which led to relatively larger size (Nb, Ti)(C, N) precipitates. Furthermore, high carbon content contributed to stabilization of austenite and delayed the transformation of ferrite and bainite, such that martensite/austenite content (M/A) was obtained. The M/A islands in high carbon-containing steel contributed to highest strength and intermediate elongation. The high degree of (Nb, Ti)(C, N) precipitation in steel contributed to refinement of prior austenite grain size and strain accumulation, which increased ferrite and bainite start transformation temperature, resulting in higher volume fraction of polygonal ferrite. Polygonal ferrite in steel with high Nb-content was responsible for relatively low strength in comparison with steels with higher carbon or intermediate carbon-Nb contents. Granular bainite and lath bainite in steel with intermediate C and Nb-contents was characterized by best combination of strength and elongation. The outcomes of the thermodynamic simulations were consistent with the experimentally observed microstructure.