2014 年 54 巻 7 号 p. 1611-1620
The hot ductility of Nb–Ti bearing ship plate steel has been obtained using the Gleeble 1500 thermal-mechanical simulator, the results showed that the ductility trough of the reduction of area (R.A) curve of Nb–Ti bearing ship plate steel is wider than other steels without Nb. In order to investigate the reasons of the loss of the ductility, a series of studies have been done. It was found that Nb(C,N) particles with a mean size of 9 nm precipitate at 950°C and induce the formation of the nuclei of grain boundary ferrite, which could lead to strain concentration at grain boundaries as well as the poor uniformity of ferrite films thickness. This causes rapid decrease of the R.A values with the temperature cooling from 1000 to 950°C. When the temperature falls to 900°C, a large number of Nb(C,N) particles precipitate, which promote further formation of the grain boundary ferrite films. Therefore, the thickness of ferrite films formed at different time makes a difference and presents a poor uniformity. As the temperature down to 850°C, hot ductility further reduces because of the worse uniformity of ferrite films thickness which leads to strain concentration at the grain boundary. When temperature is close to Ae3 (816°C), the formation and growth of ferrite films become faster due to austenite-ferrite phase transformation, which leads to the uniformity of ferrite film thickness at worst, so that the R.A value exhibits a rapid decrease. After the temperature is lower than Ae3, ferrite films are markedly thickened, which compensate parts of adverse effects from the poor uniformity of ferrite films thickness and reduce the strain concentration at grain boundaries. In this way, ductility begins to recover.
