Characterization of a Continuously Cooled Dual-Phase Steel Microstructure

Continuous-cooling transformation behavior of a DP steel was analyzed from dilation curves with cooling rates that range between 10 °C/s and 98 °C/s and data taken in 10 °C/s increments. For a precise understanding of the problem, several metallographic techniques were used in order to determine which phases and types of transformation are present, the grain structure and crystal defects generated for each cooling rate, among other characteristics. The local distribution of the main alloying elements was analyzed by wave dispersive spectroscopy. From the dilation curves, the relative amount of transformed phase was estimated, as well as the first derivatives as a function of both temperature and time to analyze the characteristics of the transformation and correlate these with a characteristic microstructure. To further understand these results, the mobility of suitable alloying elements such as Cr, Mn, Al, and P was evaluated. The analysis showed that at lower cooling rates, 10 °C/s to 20 °C/s, the transformation occurs at temperatures above 700 °C (at which the majority of alloying atoms have good mobility) in a relatively slow process producing polygonal ferrite. At cooling rates greater than 40 °C/s, the transformation occurs below 700 °C in a relatively short time, where massive transformation takes place. Finally, a cooling rate of 30 °C/s gives a mixed transformation, producing an appreciably smaller grain structure with a high density of crystal defects.