Research On Quenching And Partitioning Process Of TRIP800Steel And Hot Stamping Steel

Research on strengthening and toughening process of metallic material has long beenan important subject for material workers. With the rapid development of automotiveindustry, automobiles are facing higher requirements in terms of energy-saving,environmental protection and security, the future trend of automotive industry isdecreasing the weight of automobile body to reduce fuel consumption and emissions onthe premise that security is guaranteed. Under such circumstances, the demand for thethird generation of automobile steel with low alloy and high product of strength andplasticity is more urgent.In this thesis, TRIP800steel and two kinds of hot stamping steel for automobileswere treated by Q&P process according to the quenching and partitioning processproposed by Speer et al, the Q&P steel with significantly enhanced mechanical propertieswas obtained, relation between microstructure and mechanical properties of specimenstreated by Q&P process was experimentally investigated by means of metallographicmicroscope, scanning electron microscope, X-ray diffraction and universal material testingmachine, effects of quenching temperature, partitioning temperature and partitioning timeon microstructure, mechanical properties and retained austenite content of the tested steelwere analyzed in detail, the main research contents include the following points:TRIP800steel which contains plenty of Al was treated by Q&P process through saltbath method, the results indicate that the microstructure of the tested steel treated by Q&Pprocess is a mixed structure composed of ferrite matrix and martensite ormartensite-austenite distributed over it in blocky type, the blocky retained austenite in thesize of2~3μm is mainly distributed at the boundary of ferrite or the boundary betweenferrite and martensite, with the decrease of quenching temperature or increase ofpartitioning time, the microstructure of martensite or martensite-austenite changes; themechanical properties of TRIP800steel are notably improved after treated by Q&P process, the optimal mechanical properties are obtained at the quenching temperature of250℃, the partitioning temperature of400℃, and the partitioning time of60s, the productof strength and plasticity reaches22238MPa·%; the tensile fracture surfaces of allspecimens show fine features of plastic fracture; the retained austenite content of thetested steel reaches11.5%after treated by Q&P process, which increases substantiallycompared with that of the original TRIP steel.Hot stamping steel22MnB5was treated by1-step and2-step Q&P process, theresults indicate that the microstructure of22MnB5steel treated by Q&P process is mainlymartensite and retained austenite, with a small amount of carbide precipitation, with theincrease of quenching temperature and partitioning temperature or the extension ofpartitioning time, the microstructure of martensite and carbide changes; the mechanicalproperties of22MnB5steel treated by Q&P process are greatly improved compared withthose gained though traditional direct quenching process, the product of strength andplasticity reaches maximum(20435MPa·%) at the quenching temperature and partitioningtemperature of325℃and the partitioning time of60s; the retained austenite content ofthe specimens treated by Q&P process can reach5.5%.Research on hot stamping steel developed by Wuhan Iron and Steel Group Corp. andtreated by1-step Q&P process indicates that the microstructure of the tested steel is amixed structure mainly composed of martensite and austenite, with a small amount ofprecipitated carbide, with the increase of quenching temperature and partitioningtemperature or the extension of partitioning time, the microstructure shows a feature oftempering which is more and more obvious, and this results in lower strength of thespecimens; the mechanical properties of hot stamping steel from WISCO are significantlyimproved after treated by Q&P process, the optimum mechanical properties are obtainedat the quenching temperature and partitioning temperature of230℃and the partitioningtime of90s, the product of strength and plasticity reaches24017MPa·%; the retainedaustenite content of the tested steel can reach6.6%after treated by Q&P process.