Study On Microstructure And Properties Of Titanium And Niobium Multi-micoralloycd Steel

Technology optimization is one of ways to improve the strength of microalloyed steel. Thispaper focuses on the influence of heating process and cooling modes after rolling on themicrostructure and properties of Titanium and Niobium microalloyed steels.First of all, experiments to study influence of both heating temperature and time ondissolving of microalloyed elements were conducted in laboratory. The microstructure andprecipitation of the samples were observed. The experimental results show that:(1) Experimentalsteel’s austenitic coarsening temperature is1250℃and the austenite grains have no significantchange with heating time after30min;(2) Before austenization, microalloyed elements exist inthe matrix in the form of (Ti，Nb)(C，N) compound particles. When the heating temperature is1200℃, only a few precipitation particles dissolve. But if the temperature increases to1250℃,most of the precipitations dissolve in the matrix.(3) Considering all the factors, the optimumheating temperature and time are1250℃and60min, respectively, for the experimental steel.Secondly, thermal simulation experiments of cooling modes were carried out in laboratoryand observe the microstructure and precipitation of the samples were analyzed. The resultsindicate that the later-fast-cooling has a more positive effects for improving functions ofmicroalloyed elements Ti and Nb on refining grain size and increasing the amount of fineprecipitates than the former-fast-cooling.Finally, two cooling modes were applied in actual production based on the laboratory testresults. The samples on different location of strip coil were taken and then tensile test,microstructure observation and precipitation analysis were performed. Industrial test resultsshow that the samples of later-fast-cooling has a higher strength. Compared with theformer-fast-cooling, later-fast-cooling has a more positive effects on precipitation strengtheningand fine grain strengthening to improve the strength of Ti-Nb microalloyed steels, which are thetwo main factors causing the strength difference of the samples with two cooling modes andprecipitation strengthening contributes the most important role. In addition, later-fast-cooling iseasily realized in production and no extra cost is needed. Therefore, the later-fast-cooling is aeffective method to improve the strength of microalloyed steels.