The Significance of Mn Prepartitioning on Bainitic Transformation and Mechanical Properties of a Low-Carbon Bainitic Steel

The significance of manganese prepartitioning treatment (Mn-PrPT) in the intercritical region on bainitic transformation behavior, microstructure evolution, and mechanical properties of a low-carbon bainitic steel is elucidated, herein. The results indicate that the Mn-depleted and Mn-enriched regions existed in the austenite after austenitizing as a result of Mn-PrPT, which are evaluated by thermodynamic calculation, and validated by dilatometric and wavelength dispersive X-ray spectrometry (WDS) measurement. The introduction of Mn-PrPT before austenitizing has little effect on the prior austenite grain size, whereas it affects martensite start (M (S)) temperature remarkably, which increased by 30 degrees C. Moreover, the dilatation rate with Mn-PrPT shows a two-peak feature during the bainitic transformation, which is believed to be attributed to the varied bainitic transformation domain for Mn-depleted and Mn-enriched regions. In addition, the Mn-PrPT contributes to an increase in both fraction of retained austenite and its carbon concentration, and consequently, the elongation is significantly improved without compromising the tensile strength, leading to an enhancement of the product of strength and elongation. However, it has a slightly negative effect on impact toughness due to an increment in the fraction of blocky retained austenite with low mechanical stability.