| Two kinds of P added interstitial free high strength (IFHS) steels with different Ticontent are studied in this thesis. Characterization of precipitates in experimental steelsis carried out by means of Transmission electron microscopy (TEM) and EnergyDispersive X-ray Spectrometer (EDS) to study the precipitation behavior duringproduction process. Special attention is paid to investigate the mechanism of FeTiPprecipitation. Distribution of fine precipitates and orientation information of the grainsaround them are obtained via Scanning electron microscopy (SEM) and Electronbackscatter diffraction (EBSD) technique. Effect of precipitates on grain orientation isdiscussed. Auger electron spectroscopy (AES) is used to study the segregation behaviorof P at grain boundary during the recrystallization annealing process. Three principalconclusions can be summarized from the results. There are totally six kinds ofprecipitates obtained in the experimental steels which precipitate in the order of TiN,TiS, Ti4C2S2, MnS, TiC and FeTiP. FeTiP forms in the recrystallization annealingprocess by the way of slowly diffusing P atoms into the earlier formed FeTi precursorswhich observed only in steel with higher Ti content. Precipitation time of FeTiP, whichis between90~120s when annealed at810oC and60~90s at840oC, decreases with theincrease of annealing temperature. When annealing at higher temperature, grainboundary breaks away from the precipitates reducing the content of {111} and{111}<112> texture component while the total {111} texture is more uniform.Non-equilibrium segregation of P at the boundary occurs during the recrystallizationprocess, which is caused by the solute atom atmosphere around dislocations formed atthe recovery stage and then enhanced by the solute atom-vacancy complexes left bygrain boundary migration during the grain growth. The critical time show its appearanceuntil the grains reach the limit size. |
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