The evolution of precipitates in a Nb-Ti microalloyed HSLA steel during reheating

摘要

Precipitation in Nb-Ti microalloyed HSLA steel was investigated in both the hot-rolled andthe reheated conditions by means of analytical transmission electron microscopy, includingenergy dispersive X-ray (EDX) spectrometry and Gatan Imaging Filter (GIF) elementalmapping. Complex precipitates (Nb,Ti)(C,N), rather than the stoichiometric niobium carbides(NbC), were found in the hot-rolled steel, in spite of the low level of titanium and nitrogen.Regarding composition and morphology two kinds of precipitates were observed. The mainpart consists of precipitates with an irregular spherical shape ranging in size between 40 and80 nm. A smaller fraction of larger precipitates (>100 nm) was found with a cuboidmorphology. In general, both of them were niobium rich carbonitrides, although a clearlyhigher Ti/Nb ratio was found in the cuboid precipitates. However, the average ratio of Ti toNb was higher than the nominal ratio in the steel as a whole. This indicated that someniobium was still in solution in the hot-rolled HSLA steel. The solute niobium re-precipitatedto small niobium carbides (10～20 nm), at the lowest reheating temperature, 900°C, and theoriginal precipitates also grew to a larger size with a lower Ti/Nb ratio. However, theseprecipitates tended to become smaller when increasing the reheating temperature. The EDXanalysis also indicated an increase in the ratio of titanium to niobium when compared withthe treatment at lower temperature. However, a large number of precipitates containingniobium in association with titanium were still found in samples reheated at 1200°C, whichdemonstrated a greater thermodynamic stability of the complex precipitates compared to NbC.A thermodynamic model has been successfully applied to understand this dissolutionbehavior of the complex Nb-Ti precipitation that occurred during the reheating process.