MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Effect of Cooling Rate on Phase Transformation and Microstructure of Nb–Ti Microalloyed Steel
Su ZhaoDonglai WeiRongbin LiLi Zhang
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2014 Volume 55 Issue 8 Pages 1274-1279

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Abstract

The cooling rate is a key factor of controlling the slab surface microstructures during continuous casting of steel. The effect of cooling rate on phase transformation and microstructure of Nb–Ti microalloyed steel was investigated by a confocal laser scanning microscopy and a Gleeble-3800 thermal simulation machine. The process of phase transformation can be analyzed through in situ observation. A critical cooling rate of 5 K·s−1 was revealed, below which the proeutectoid ferrite along austenite grain boundaries and widmanstatten structures were observed, and carbonitrides precipitated were also observed in the proeutectoid ferrite. With the increase of cooling rate, the quantity of the precipitates decreases while the width of the proeutectoid ferrite becomes smaller. The carbonitrides precipitated along the austenite grain boundary result in the decrease of the carbon concentration near the grain boundary, which is more favorable to form the proeutectoid ferrite as well as to change its width. When the cooling rate was greater than or equal to 5 K·s−1, the precipitates were dispersed uniformly in the grain, and the bainite was observed mainly.

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© 2014 The Japan Institute of Metals and Materials
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