Dt300 Microstructures And Properties Of Steel And Hot Deformation Behavior Research

The effect of different heat treatment processes on the microstructure and mechanical properties of different components DT300tested steels is investigated in this thesis, and the action of alloying element; the grain growth law and the effect of carbide forming element Nb in grain refinement. Hot deformation behavior of tested steels also has been studied to show the law of microstructure variation in different hot deformation conditions. The main results obtained are as follows.(1) The best normalizing temperature range of DT300tested steels is900to920℃, tested steels quenching at850to900℃after normalizing, then tempering at300to350℃, obtain the best mechanical property and the tensile strength and impact achieve more than1700MPa and60J, the microstructure of tested steels is low carbon lath martensite and small dispersion precipitation ε-carbide and fair size retain austenite films between martensite lath. When the tempering temperature up to400℃, the ε-carbide gets coarsening; when the tempering temperature reaches500℃, ε-carbide changes into Fe3C, strength and toughness fall rapidly because of the one-step temper embrittlement.(2) For the different components of DT300tested steels in which content of Ni and Mo close to the upper boundary, tested strength exceeds our expectation, but tested toughness is just to meet the requirement; tested toughness exceeds our expectation, but tested strength is lower slightly when content of Ni and Mo close to the low boundary. Generally, we select middle and low as the level of content of Ni and Mo.(3) Soften microstructures that low carbon a-Fe and spheroidized cementite are obtained under the condition of annealing at660℃for20h after normalizing and air cool to room temperature. But, obtaining hardening martensite and can not improve processability when we choose hot delivery as the way to annealing at660℃after normalizing.(4) Compared the DT300steel without Nb element, the average austenite grain size of DT300steel with Nb element is more smaller, wihich is mainly because NbC precipitation in the austenite grain boundary, and result of restraining the growth of austenite grain. With the Transmission Electron Microscope (TEM) the NbC carbide precipitated in the tested steels was observed. Austenite grain activation energy of Nb microalloyed DT300experiment steel is206.2kJ/mol under the condition of heating at800to1200℃for60min, and we get the growth law of austenite grain. D2=43.56×108exp-206.2kj/mol——RT(5) Under the temperature800to1150℃and the strain rate of0.01to10s-1, the activation energy of the DT300tested steels is386.39kJ/mol and the hot deformation equation is built.