Abstract
Nb addition is known as an effective method in order to restrain recrystallization and improve strength and toughness of alloy steels because of grain fining. However, in manufacturing large ingots with low cooling rate, there is a possibility to lead to create large eutectic Nb carbide which harms to the mechanical properties. The purpose of the present study is to make clear the creation behavior of Nb carbide in such large ingots. Moreover, by solution treatment tests, the proper heat conditions to dissolve them are verified. The results obtained are as follows. 1. There are no remarkable macro segregation of Nb and carbon, which lead to create Nb carbides. 2. Nb carbides are mostly distributed around the center and the top of the ingots. 3. They are created due to solute condensation in the middle of dendrite arms. Therefore, amount of carbides is in proportion to the addition amount of Nb and the dendrite arm spacing. 4. As a result of solution treatment test, experimental formulae of solution rate are obtained to predict the disappearance behavior of the carbides.
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