| Microalloyed high strength steel(HSS)is one of the important research fields of high performance materials.A large number of works have been carried out to develop Nb,V and Ti microalloyed high strength steels.Compared with other microalloying elements,Ti has received more and more attention for its commercial advantage.However,it is hard to precisely control the property of the production due to the high activity of Ti,resulting in very low efficiency when using Ti-microalloyed.Moreover,there are few reports on the development of Ti-Nb composite microalloyed steel which Ti acts as the main strengthening element.Thus microstructure evolution,strengthening and toughening mechanisms in Ti-riched microalloyed steels need to be further studied.In the present work,Ti-Nb microalloyed high strength steels were designed to systematically study the precipitationbehavior,microstructure evolution and toughening mechanism,which can provide theoretical basis for the manufacture of Ti-Nb microalloyed high strength steel.The main content and conclusions are as follow:(1)Ti-Nb microalloyed steels with different compositions were smelted and rolled.The effects of Ti and Nb on the microstructure and properties of microalloyed HSS were systematically studied.Finally,the optimal amount of Ti and Nb in HSS was determined.(2)After studying the heating process of Ti-Nb microalloyed steel,the effects of austenitizing temperature and time on the austenite grain size and the solubility of microalloying elements were clarified,which provides theoretical guidance for the formulation of heating temperature in industrial production.(3)Both the critical strain of dynamic recrystallization and the true strain for full recrystallization in the Ti-Nb microalloyed steel were determined by single-pass and double-pass compression tests.To ensure complete recrystallization of austenite during high temperature rolling process,the cumulative pressure reduction of the upper and downstream frames in actual production must be greater than the critical strain(true strain)in order to effectively avoid the occurrence of mixed grain.It provides theoretical basis for the formulation of rolling technology in actual production.(4)Transition behavior during continuous cooling has been systematically studied.The effects of cooling modes on the microstructure and precipitation behavior in a Ti-Nb microalloyed steel were studied,which provides theoretical basis for the formulation of laminar cooling technology in the actual production process.(5)Based on the above experimental research,industrial trial production was carried out.Ti-Nb microalloyed high strength steel with good combination of strength and ductility was successfully trial-produced.Moreover,the industrial trial production results were consistent with the laboratory research results. |
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