Effect Of Process Parameters Of Medium Manganese TRIP Steel On The Microstructure And Properties

High strength steels and ultra-high strength steels which can realize the auto lightweight,reduce fuel consumption and emissions on the premise of auto safety are in urgent need in order to meet the development trend of energy saving,consumption reduction,environmental protection and safety.As the typical steel of the third generation advanced high strength steel,medium manganese TRIP steel has drawn more and more attention and became a hot research topic for a long time.In this study,medium manganese TRIP steels with four different compositions based on the medium frequency induction melting furnace smelting were used as experimental materials,four major experiments like the continuous cooling transformation,hot deformation simulation,hot rolling and intermediate annealing were designed to analyze the effect of hot rolling and intermediate annealing on the microstructure and properties of medium manganese TRIP steel.Firstly,the continuous transformation curves corresponding to the rule of continuous cooling transformation were characterized by Gleeble3500 thermal simulation testing machine,so was the effect of the process of hot deformation simulation on the microstructure and mechanical properties.The results show that martensite transformation occurs under all cooling speeds and the content of martensite increases with the increase of cooling rate,there is a certain difference in the morphology of martensite between different components of martensite;banite transformation only occurs under a relatively lower cooling speed;the change of deformation temperature and deflection have little influence on the microstructure morphology of tested steels.The hot rolling test was carried out to analyze the effect of different finish rolling temperature on the microstructure and mechanical properties in accordance with the relationship between microstruture and deformation process parameters.The results show that under the precondition of same composition,the grain size of original austenite becomes finer and the martensitic lath bundle becomes larger with the reduction of finish rolling temperature;besides,with the reduction of finish rolling temperature,the tensile strength increases while the elongation decreases;the tested steel of sample No.1-3finally gets the optimum mechanical properties.Intermediate annealing treatments in different temperature were carried out between hot rolling and cold rolling in favor of improving the microstruture and obtaining the better matching of strength and plasticity.The results show that the quantity of martensite and ferrite in different components of tested steels decreases and the microstructure distributes more uniformly after annealing 10 hours in different annealing temperature,it can be also find that a certain number of retained austenite occurs from the XRD diffraction patterns;the tensile strength increases while the elongation decreases with the increase of annealing temperature.When the annealing temperature is 620°C,the tested steels achieve the optimum mechanical properties and the product of strength and elongation is up to 31.5 GPa·%.