Effect Of Different Austenitizing Pretreatment Processes On Microstructure And Mechanical Properties Of TRIP780 Steel

With the improvement of people’s living standards,the demand for automobiles is also increasing.However,the emergence of energy crisis and the increasing pollution hinder the development of automobile industry,which forces automobiles to develop in the direction of energy saving and environmental protection,and at the same time to take into account their safety performance,the most direct way is to lighten the car.Transformation induced plasticity steel(TRIP steel)has both high strength and high plasticity due to its unique TRIP effect.It has good application prospects in the automotive industry.The effects of austenitizing pretreatment and traditional TRIP treatment on the structure and properties of TRIP 780 steel were studied in this experiment,and the optimum heat treatment parameters were also explored.Firstly,the optimum process parameters of traditional TRIP treatment were explored.The cold rolled TRIP 780 steel sheet was treated at different temperatures and times in the two-phase zone,and then at different temperatures and times in the bainite transformation zone.On the basis of the best traditional TRIP heat treatment process,the experiments of different austenitizing pretreatment heat preservation temperature and time distribution were carried out.The effects of different heat treatment processes on the microstructure of experimental steel were studied by means of optical metallographic microscope(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD)and transmission electron microscopy(TEM).The mechanical properties of experimental steel were tested by an electronic universal material testing machine.The main conclusions were as follows:Under the traditional TRIP treatment process,the mechanical properties of the experimental steel reach the best when the holding temperature of the two-phase zone is 800℃ and the holding time is 120s,and then the holding temperature of the bainite transformation zone is 420℃ and the holding time is 240s.The yield strength,ultimate tensile strength,elongation and product of strength and plastic of the steel are 433MPa,1108MPa,17.6%and 19500MPa·%.When the holding temperature in the two-phase zone is 800℃ and the holding time is 120s,the initial austenite content increases and the grain size is moderate,and more retained austenite can be obtained.The appropriate amount of bainite transformation is obtained when the holding temperature is 420℃ and holding time is 240s in the bainite transformation zone,which enables the initial austenite to better carry out the secondary enrichment process of carbon elements,increases the stability of austenite and obtains more retained austenite.The mechanical properties of the experimental steel after austenitizing pretreatment have been significantly improved,and its comprehensive mechanical properties have increased by 58.9%on average.When the austenitizing holding temperature is 925℃ and holding time is 90s,the mechanical properties of the experimental steel reach the best.The yield strength of the experimental steel is 510MPa,the ultimate tensile strength is 1123MPa,the elongation is 28%,and the strength-plastic product is 3144MPa·%.Compared with the traditional TRIP treatment process,the comprehensive mechanical properties of the experimental steel are improved by 61.2%.The holding temperature and holding time of austenitizing pretreatment of experimental steel mainly affect the grain size and matrix structure of the initial structure after TRIP treatment.When the pretreatment temperature is too low or the holding time is too short,the homogenization degree of austenite will be too low.When the pretreatment temperature is too high or the holding time is too long,the grain size of the matrix structure will be too large,which will affect the final mechanical properties.By analyzing the roll reversal pole diagrams of experimental steels with and without austenitizing pretreatment,it is found that the orientation of FCC phase along the rolling direction is<001>and<113>respectively,and the strength is 6.28 and 1.87,respectively.The Schmidt factors are 0.41 and 0.45,respectively.That is to say,there are more stable residual austenite in the experimental steels pretreated by austenitizing,which can make greater contribution to TRIP effect.