| The development of advanced high strength steel with low cost,high performance to help ensure the safety and comfort,improve the degree of light weight of the car,thereby reducing the emissions and environmental pollution.The quenching and partitioning(Q&P)processes can produce a third generation advanced high strength(AHSS)steel with martensite and retained austenite as the main structure(Q&P steel),and the Q&P steel has the transformation induced plasticity(TRIP)effect,Q&P process through carbon partitioning improves the stability of austenite,which make the austenite is retained at room temperature(retained austenite);the retained austenite can be transformed into martensite to absorb a large amount of impact energy under the action of stress,which can significantly improve the safety of the vehicle.At the same time,martensite provides ultra-high strength for Q&P steel,when use the Q&P steel to replace the low strength automotive materials,the thickness of the automobile parts can be properly reduced,effectively improve the degree of automobile lightweight.In order to further investigates Q&P process and the mechanical stability of retained austenite,the different conditions processes was designed in this paper,the microstructure of the test steels was analyzed by optical microscope(OM)and scanning electron microscope(SEM),the contents and distribution of C-Mn elements of test retained austenite were tested by X ray energy dispersive spectrometer(EDS),the mechanical properties of test steelss were gotten by universal tensile test,the content of retained austenite was measured by X ray diffractometer(XRD);This paper mainly studies the influence of comprehensive partitioning behavior of the C-Mn elements on microstructure and properties of test steels,established the carbon partitioning model,defined the starting point and end point of carbon partitioning,the migration rate of martensite and austenite(α’/γ)interface at different carbon partitioning temperatures was calculated,and studied the effect of tensile stress on the mechanical stability of retained austenite in TRIP steel,the main research results are as follows:(1)In the salt bath quenching processes,when the martensitic transformation is completed,the carbon atoms has spread to the α’/γ interface,it leads to the unequal of the chemical potential of the Fe atoms on the side of the martensite and the side of the austenite in α’/γ interface,which provides the driving force for the migration of the α’/γ interface,therefore,the α’/γ interface migration and carbon partition start simultaneously.At this time,the chemical potential of Fe atoms on the side of martensite is lower than that of austenite,in the subsequent partitioning,the α’/γ interface will migrate to the austenite.(2)The carbon partition temperature plays a decisive role in the migration rate of the α’/γ interface increases.The migration rate of the α’/γ interface is not obvious when the temperature is below 300℃;However,when the temperature increased from 300℃ to 400℃,the migration rate of the α’/γ interface increases rapidly.(3)C and Mn elements content in retained austenite of test steels is far more than the original content of C and Mn elements,the experiment proved Mn partitioning is effective.At the same time,through the analysis of the results of EDS scanning,this paper also designed a kind of composition ratio of AHSS steel,which has the characteristic of low carbon,medium manganese and contain copper,the aim is to adapt to the Mn partitioning in Q&P process.(4)The changes trend of the product of strength and ductility is related to the tensile strength and elongation increase or decrease the proportion;the change of elongation is more likely to affect the product of strength and ductility.(5)The fracture position of the samples appeared the ferromagnetic phenomenon.The test steels contains α-Fe(ferrite and martensite),α-Fe will appeared the characterized of ferromagnetism when it is below the Curie temperature.Therefore,in the tensile process,the magnetic domains in the ferrite and martensite,in order not to be destroyed,arranged in the same direction to resist the tensile stress.Thus,the two groups with different polarity are formed in the fractures,the test steels to resist the destroyed of tensile stress through the mutual attraction of the two groups with different polarity.(6)The research results show that changed regulation of the mechanical stability and transition rate of retained austenite was determiated by the stress induced the dislocation multiplication,pile-up and DARA effect.The changed regulation of retained austenite mechanical stability and transition rate obviously divided into three stages with the increase of strain,the stress causes the mechanical stability of retained austenite to play a different role in the different stages. |
PDF Full Text Request