Effect of Annealing Before Cold Forging on the Behavior of Abnormal Grain Growth during Carburizing

摘要

In order to clarify the mechanism of abnormal grain growth in steel for cold forging and carburizing, the effect of spheroidizing annealing on the behavior of abnormal grain growth during carburizing was investigated. Abnormal grain growth was observed in annealed steel, whereas it was suppressed in normalized steel. However, since both the normalized steel and the annealed steel have almost the same size distribution of Nb(C,N) nano-precipitates, the effect of annealing on abnormal grain growth was not explained by the conventional theory based on the pinning force by nano-precipitates. Spheroidized cementite was remained in the annealed steel which was immediately quenched from the quasi-carburizing temperature of 1 203 K. Dissolution of spheroidized cementite and abnormal grain growth took place simultaneously. Spheroidized cementite was thermally stabilized by Cr concentration. A DICTRA simulation was carried out to discuss the kinetics of cementite dissolution. Cr concentration affects the dissolution rate of spheroidized cementite through the relationship between the Cr content and the cementite size. The Cr content in cementite has a wide distribution, and the dissolution time is different in each cementite. This dissolution time lag among cementites causes a local and non-uniform decrease of the pinning force by cementite. As a consequence, abnormal grain growth is likely to occur in annealed steel.