Microstructure And Mechanical Properties Of Lamellar Structured Medium Carbon TRIP-assisted Steel

Transformation-induced plasticity steels with excellent comprehensive mechanical properties are a hotspot in the research of the third generation steel materials.The excellent properties of TRIP steels are attributed to the interaction of the various phases in the structure,especially the retained austenite.Obtaining more retained austenite by changing the heat treatment process is the key for better-performing TRIP steels.In this paper,the medium carbon steel with the martensite structure obtained by quenched was used as the initial structure.By means of the optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction techniques,DIL805A/D thermal dilatometer,tensile tester and oscillating impact tester,the effect of austempering temperature on microstructure and properties after complete austenitization and intercritical annealing was investigated to reveal the influence of the lamellar structure ferrite on microstructure and properties;the microstructure and mechanical properties of oil quenched experimental steel in different intercritical temperature and holding time were studied to find the heat treatment parameters that can fully exert the potential of the steel.The effects of austempering temperature on the microstructure and mechanical properties of complete austenitizing and intercritical annealing were investigated.The differences in microstructure and mechanical properties of the steel were compared.Compared with the microstructure and mechanical properties of complete austenitization,the austempered sample of the intercritical annealing structure is divided into finer austenite grains by the lamellar ferrite,The size of retained austenite was smaller and the volume fraction was higher;the strength decreases slightly,but the elongation increases significantly.After intercritical annealing,the austempered sample has higher product of strength and plasticity and impact toughness,showing better Comprehensive mechanical properties.With the decreases of austempering temperature,the size of retained austenite in the structure of both the complete austenitization and intercritical annealing increases gradually,and the volume fraction of the retained austenite shows a trend of increasing first and then decreasing;the finer bainite lath makes the yield strength higher,and the tensile strength decreases first and then increases,while the elongation shows the law of increasing first and then decreasing.The higher carbon content and the smaller size of the retained austenite increase the stability and inhibit the initiation and propagation of crack,cause the impact toughness lower.The effects of process parameters of intercritical annealing on the microstructure and mechanical properties of the lamellar structure TRIP steel were investigated.The results show that with the increase of intercritical temperature and holding time,the size of retained austenite austempered at 320°C increases gradually and the volume fraction rises first and then decreases after different annealing temperature kept for 10 min and the intercritical annealing temperature of 840°C for different time.obtained the highest volume fraction of retained austenit(24.1%)when kept at 840°C for 10 min.Because of the increase of hard bainite content in the structur and the decrease of the amount of soft ferrite,the yield strength and tensile strength showed a gradually increasing law,and the elongation showed a trend of increasing first and then decreasing.The sample was kept at 840°C for 10 min to obtain the highest Product of strength and plasticity(32.1GPa?%).When the holding time at 840°C was more than 30 min,the content of each phase in the austempered sample at 320°C tends to be stable,and the tensile properties remain substantially unchanged.