Effect Of Q&P Processes On Microstructure And Mechanical Properties Of Cu-TRIP Steel

With greenhouse effect and energy shortage getting worse,the concept of energy saving and environmental protection becomes an urgent target.The automotive industry,therefore,has to reduce car weight under the premise of safety.Transformation induced plasticity(TRIP)steel is widely used in automobiles due to its good comprehensive mechanical properties as a kind of advanced high strength steel(AHSS).Generally,Cu is considered to be an inclusion element in TRIP steel;however,it can also be used for increasing strength and corrosion resistance in other steels.At the present stage,researches on the effect of Cu in TRIP steel are scarcely reported.Quenching and partitioning(Q&P)process,as a new type of heat treatment,has greatly improved the content and stability of retained austenite,which provides a new method for obtaining high strength and plasticity simultaneously.In this thesis,Cu was added to TRIP steel as an additive element creatively,and the role of Cu in microstructure and properties was studied after heat treating with one-step and two-step of Q&P processes,by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),transmission electron microscope(TEM),hardness tests,and tensile tests.Finally,preparation process of product of strength and elongation which is as high as 55900MPa%in Cu-TRIP steel was designed.The main research results are as follows:(1)The phase transformation points of the investigated steel were measured as a basis for process parameters.The results show that Acl,Ac3,Ms,and Mf are 702?,864?,233?,and 122?,respectively.The CCT curves were protracted according to the expansion curves at different cooling rates.The bainite region is 350??420?.The microstructure consists of ferrite,bainite,martensite,and perlite at the cooling rates of 0.1?/s and 0.2?/s,but only martensite exists with the acceleration of cooling rate.(2)One-step of Q&P processes,i.e.,partitioning at the range of quenching temperatures,were applied to figure out the effects of intercritical annealing temperature,time,partitioning temperature,and time on microstructure,austenite,and mechanical properties of the investigated steel.The results show that the microstructure is made up of ferrite,martensite-austenite,and austenite.Properties under different parameters vary from each other showing low elongation but high tensile strength over 2000MPa.After intercritical annealing at 820? for 180s and partitioning at 180? for 300s,tensile strength and elongation are 2201MPa and 15%,respectively,and product of strength and elongation is 33105MPa·%.(3)The investigated steel was heat treated by using two-step of Q&P processes,i.e.,partitioning at a point above the quenching temperature.The relationships between microstructure and properties under different partitioning temperature and time were explored.The results show that the microstructure consists of ferrite,martensite-austenite,bainite,and austenite.With increasing temperature and time,tensile strength decreases,but elongation rises.After intercritical annealing at 820? for 180s,quenching at 180? for 10s,and partitioning at 400? for 300s,tensile strength is 1300MPa,elongation is 43%,and product of strength and elongation is as high as 55900MPa·%.(4)Differences between one-step and two-step of Q&P processes were analyzed,and the optimum heat treatment parameters after Q&P processes were discussed.The results show that grains become finer,and more bainite and austenite are obtained after two-step processing.Tensile strength decreases but elongation increases to?40%.Although tensile strength is as high as 2200MPa after one-step processing,however,a product of strength and elongation of 33105MPa·%is still significantly lower than the results obtained after two-step processing.Therefore,the optimum heat treatment parameters after Q&P processes are intercritical annealing at 820? for 180s,quenching at 180? for 10s,and partitioning at 400? for 300s,after which the investigated steel reveals excellent combination of tensile strength(1300MPa)and elongation(43%)with a product of strength and elongation of 55900MPa·%.(5)The effects of Cu on microstructure and mechanical properties of the investigated steel were studied.The results reveal that Cu precipitates occurred after one-step and two-step processing,nevertheless,these precipitates are rod-like after one-step processing with average length and width of 30nm and 5nm,respectively,while they turn to be spherical after two-step processing with an average diameter of 8nm.Two kinds of Cu precipitates can improve the strength of TRIP steel,but the rod-like ones are more effective.