Effect of thermomechanical treatment and coiling temperature on the strengthening mechanisms of low carbon steels microalloyed with Nb

One of the options available for coping with current property requirements is to increase microalloying contents. As well as affecting austenite microstructural evolution, this can lead to a larger contribution of precipitation and dislocation density hardening to the mechanical strength. However, it also results in more complex interactions between the strengthening mechanisms, making it difficult to optimize operation parameters. In this work, plane strain compression tests were carried out to produce different austenite microstructures with two low carbon steels microalloyed with different Nb levels (0.04% and 0.11%), followed by coiling simulations in the 500 to 700 °C temperature range. The mechanical properties of the transformed microstructures were determined via tensile testing and the specimens were characterized using optical microscopy, EBSD and TEM. Similar austenite grain sizes and strain accumulation levels were obtained for both steels, and as a result, it was observed that increasing Nb content had a small effect on the grain size of the transformed microstructures. On the other hand, the coiling temperature significantly affected the mechanical strength. The lowest strength values are obtained for 700 °C; these increase significantly at 600 °C (≅90 and 140 MPa increase in the Yield Strength (YS) for the low and high Nb steels, respectively) and decrease slightly at 500 °C. At the conditions investigated, similar dislocation density levels were determined for both steels, which indicates that the larger YS increase observed for the high Nb steel must be mainly due to the effect of precipitates formed during or after phase transformation.