Determination of the bonding behaviour of carbon and nitrogen in micro-alloyed structural steels

An important presupposition in the purposeful development of micro-alloyed structural steels is the knowledge of the size, frequency distribution and chemical composition of the carbonitrides. For this reason, the precipitates of the carbonitrides were investigated on three specimens of a continuously cast steel, which was micro-alloyed with niobium and titanium. One of the specimens was examined in the as-cast condition, whereas the other two were thermomechanically rolled to plate. The methods employed comprised residue analysis, electron microscope examination including electron diffraction, and microbeam EDX analysis. The results show that even in the as-cast condition this steel exhibits thermostable precipitates which inhibit austenitic grain growth during reheating. With a sufficiently high reheating temperature, particularly favourable precipitation conditions are obtained. Stable titanium carbonitrides are present throughout the rolling phase. A substantial niobium proportion is initially dissolved and then, during the final rolling phase, available for precipitation hardening. The combination of fine grain hardening and precipitation hardening explains the excellent mechanical properties of steels micro-alloyed with niobium and titanium. (Author)