Reasearch On Microstructure And Mechanical Properties Of Cold-rolled TRIP Steel By Rapid Heating Continuous Annealing

Transformation-induced plasticity(TRIP)steel is one of the most widely used advanced high strength steel(AHSS)sheet due to its good combination of strength and ductility.Currently,as energy conservation and environmental protection are receiving more attention,the development of continuous annealing equipment technology in the direction of rapid response and short annealing processes induced an urgent demand to study rapid heating annealing process.Based on Baosteel rapid heated simulation equipment,the evolution of microstructure,mechanical properties and bake hardenability of cold-rolled TRIP780 steels under the rapid heating continuous annealing process were investigated by means of optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),electron back scattering diffraction(EBSD),electron microprobe(EPMA)and mechanical tensile testing.The main contents and findings of the dissertation are as follows:Firstly,the influence of heating rate on microstructure evolution of TRIP steel was studied.The results showed that the recrystallization of deformed ferrite was substantially complete before austenitization at a slow heating rate.Then austenite nucleated at recrystallized ferrite grain boundaries and grew into ferrite.As the heating rate increased,the overlap of ferrite recrystallization and austenite formation process appeared.In addition,the nucleation site and growth process of austenite also changed.At fast heating rates,austenite nucleated at cementite/deformation ferrite interface and the growth of austenite into the deformed ferrite was controlled by diffusion of carbon.In addition,the austenitizing dynamics during the process of continuous heating was analyzed based on the JMAK equation.Secondly,the microstructure evolution mechanism and mechanical properties of TRIP steel were studied under different rapid heating continuous annealing process parameters.The experimental results showed that the rapid heating could inhibit ferrite and pearlite recrystallization.The spacing and length of the pearlite lamella were extremely beneficial to austenite mucleation and growth of rapid heating specimen because the pearlite were still crushed before austenite formation.When the heating rate was 300?/s and intercritical annealing temperature was 800?,the tested steel showed a superior combination of strength and plasticity;the rapid heating rate of 300 ?/s refined the grain size to 1.5±1?m.When temperature was maintained at the bainite isothermal temperature of 420 ? after 200s,the tensile strength and elongation were 980MPa and 21%at room temperature,respectively.The matrix phase of blocky-bainite changed to big lath bainite ferrite.The tensile strength increased but elongation decreased when the annealing temperature increased above the complete austenitizing temperature(Ac3).Thirdly,the quasi-static deformation behavior and fracture characteristics in the uniaxial tension and the stretch-flangeability test were studied.The experimental results showed that the curves of the instantaneous work-hardening exponent n value versus true strain ? had three stages during the uniform plastic deformation:during the first stage,the n value was high,but it decreased slowly with ?,and at last decreased quickly with ?.In addition,the retained austenite stability of rapid heating is higher than slow heating conditions in the whole process of plastic deformation.The stability of retained austenite for 300?/s is lower than 5?/s during early stages,but the volume fraction of more stable retained austenite was still plentiful which could increase elongation subsequently.The crack formation and propagation behavior during the hole-expansion and uniaxial tension were both microvoid coalescence.When the heating rate was 30?/s and intercritical annealing temperature was 800 ?,the hole-expansion ratio of test steel was 37.6%.Fourthly,experiments were performed at four different dynamic uni-axialtensile strain rates(0.15m/s,1.5m/s,4.5m/s,and 7.5m/s)and the stress-strain curves were determined.The microstructure of the fractures were investigated under high strain rates.In addition,a strain rate dependent material constitutive model was developed to describe the rate-dependent deformation behavior based on the Johnson-Cook model.Finally,the bake hardening mechanism of TRIP and bake hardening(BH)steel were comparatively studied.The experimental results showed that Cottrell atmosphere formation and carbide precipitation were the key factors affecting the BH ability of ferrite BH steel.The bake hardening ability of the ULC bake hardening steels can be increased considerably by both the finer grain size rapid heating and by the lage temper rolling reduction.For TRIP steel,the increase in prestrain was accompanied by a gradual reduction in the volume fraction of retained austenite,which caused incremental changes of work hardening value.The BH value decreased with the prestrain increasing at the heating rate of 5 ?/s,while it remained unchanged for 300 ?/s.The bake hardening mechanism of TRIP steel was a complexity process encompassing bainite transformation,C enrichment of retained austenite,formation of cottrell atmosphere,carbide precipitation and bainite tempering.The BH value caused by the formation of Cottrell atmosphere and carbide precipitation at rapid heating rate was greater than at slow heating rate,which disguised the reduction of yield strength caused by significant reduction of austenite volume fraction during early stages substantially.