Influence of chromium content on the microstructure and mechanical properties of thermomechanically hot-rolled low-carbon bainitic steels containing niobium

The effect of chromium content in the range of 1 wt.%-4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 wt.%) steel plates containing 0.06 wt.% Nb has been studied. In these microalloyed 700 MPa grade steels, the aim was to achieve a robust bainitic microstructure with a yield strength of 700 MPa combined with good tensile ductility and impact toughness. Continuous cooling transformation diagrams of deformed and non-deformed austenite were recorded to study the effect of Cr and hotdeformation on the transformation behavior of the investigated steels. Depending on the cooling rate, the microstructures consist of one or more of the following microstructural constituents: bainitic ferrite, granular bainite, polygonal ferrite, and pearlite. The fraction of bainitic ferrite decreases with decreasing cooling rate, giving an increasing fraction of granular bainite and polygonal ferrite and a reduction in the hardness of the transformation products. Polygonal ferrite formation depends mainlyon the Cr content and the cooling rate. In both deformed and non-deformed austenite, increasing the Cr content enhances the hardenability and refines the final microstructure, shifting the ferrite start curve to lower cooling rates. Preceding austenite deformation promotes the formation of polygonal ferrite at lower cooling rates, which leads to a decrease in hardness. In hot-rolled and direct-quenched plates, decreasing the Cr content promotes the formation of polygonal ferrite leading to an increase in the impact toughness and elongation but also a loss of yield strength.