Effect of phosphorus on Hall-Petch coefficient in ferritic steel

The effect of phosphorus on the Hall-Petch coefficient (k_y) of polycrystalline ferritic steel was investigated by using interstitial free steel (IF steel) and an ultralow carbon (60 ppm C) steel containing different amount of phosphorus. The k_y of IF steel was around 100 MPa·μ m^1/2 regardless of the amount of phosphorus. On the other hand, the k_y of ultralow carbon steel containing phosphorus was higher than that of the IF steels, but significantly lower than that of the ultralow carbon steel without phosphorus. The analysis by Auger electron spectroscopy revealed that carbon segregates at ferrite grain boundary, and the amount of segregated carbon tended to be reduced by the addition of phosphorus. From these results, it can be concluded that phosphorus does not directly influence the ky of polycrystalline ferritic iron, but indirectly decreases the k_y by reducing the amount of carbon segregated at grain boundary