Effect of cooling rate on microstructure evolution of rapidly cooled high-impurity steels

The effect of cooling rate on secondary dendrite arm spacing (SDAS) and prior austenite grain size (PAGS) has been investigated in high-phosphorous steels (0.1% P) subjected to a rapid cooling process of electron-beam (E-beam) surface remelting. A heat flow calculation was carried out to estimate the cooling rates of surface remelting experiments. The measured SDAS values depended on the cooling rate in good accordance with the Suzuki equation [A. Suzuki, T. Suzuki, Y. Nagaoka, Y. Iwata, J. Jpn. Inst. Met. 32 (1968) 1301-1304] up to the cooling rate of 105 K s−1. The classical grain growth modeling (CGM) [N. Yoshida, O. Umezawa, K. Nagai, ISIJ Int. 44 (2004) 547-555] was employed to clarify the effect of the cooling rates on the PAGS. The growth during the continuous cooling process was calculated for each cooling rate by CGM. With increasing the cooling rate, the growth of the prior austenite grains decreased and the PAGS converged to primary dendrite arm spacing (PDAS) when the cooling rates exceeded 103 K s−1. An analytic model that can be applied to predict the PAGS for cooling rates from 100 to 2 × 104 K s−1 was presented based on the present results.