Effect of boron on the hot ductility of the Nb-microalloyed steel in austenite region
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Fig. 5—Schematic illustration of the formation of the pelletlike corrosion products in a cavity.
process of the pelletlike corrosion products in a cavity. A corollary of the proposed mechanism should be that some pellets are hollow due to the gas bubbles trapped in them. This has been experimentally proved in this study. A pellet in a cavity was trenched by FIB and its cross section is shown in Figure 6. A hole (perhaps two connected holes) can be clearly seen inside the pellet. It should be stressed that, at the current stage, the proposed mechanism is still a postulated explanation for the formation of the pelletlike corrosion products in corrosion cavities. Further investigation is required to verify this mechanism. Nevertheless, in this study, the observation of the pellets in corrosion cavities at least signifies that the deposited corrosion products inside corrosion cavities could be much less protective than the normal corrosion products formed outside the cavities, and hence the corrosion inside a cavity should be much faster than outside. Furthermore, the hydrogen bubbles trapped in the pellets also signal a higher risk of ingress of hydrogen into the magnesium alloy from corrosion cavities than from outside the cavities, which might have an unexpected influence on the corrosion inside a cavity. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 50499330). 2316—VOLUME 37A, JULY 2006
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Effect of Boron on the Hot Ductility of the Nb-Microalloyed Steel in Austenite Region FARAMARZ ZARANDI and STEVE YUE Two grades of the Nb-microalloyed steel, one modified with B, were subjected to in situ melting and the thermal schedules experienced by the billet surface in the continuous casting process. The hot ductility was evaluated at various temperatures at the straightening stage of the process. FARAMARZ ZARANDI, Research Associate, and STEVE YUE, Professor, are with the Department of Metals and Materials Engineering, McGill University, M.H. Wong Building, 3610 University, Montreal, QC H3A 2B2, Canada
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