A laboratory study has been made on the mechanical properties of hot-direct-rolled steels containing a small amount of titanium.
In the steels processed by hot-direct-rolling, even a small amount of titanium content (0.005-0.02 wt%) which has little effect on the strengthening of the steels processed by conventional reheat-rolling was found to be very effective both in the strengthening and in the low temperature toughening. Chemical analyses and electron microscopy confirmed that in the hot-direct-rolling process, most of titanium is in super-saturated solid-solution during cooling after solidification down to the temperature at the start of hot rolling. The titanium in solution precipitates as very fine particles of titanium nitride during the hot rolling.
From a further detailed study of the change in mechanical properties with nitrogen content for the direct-rolled low-titanium steels, it was concluded that the higher nitrogen content steels (more than 40 ppm) are strengthened mainly by TiN particles of non-embrittling type with precipitate during hot rolling. For the lower nitrogen content steels (less than 20 ppm), the main strengthening is caused by TiC particles of embrittling type which precipitate after hot rolling.