Study Of Retained Austenite Stability And Deformation Mechanism Of A Ferrite-based Low Density Steel

The automotive industry has been exploring approaches to achieving structural lightweighting of steel-based car body-in-white in order to cope with the increasingly serious issues of energy shortage and environmental pollution.One approach is to utilize advanced high strength steels(AHSS)and ultrahigh strength steels(UHSS)such as dual phase(DP)steels,transformation-induced plasticity(TRIP)steels,quenched & partition(QP)steels and martensite(MS)steels to replace low-strength mild steels,thereby thinning of the gauge of auto parts and weight savings realized.Another potential way is to reduce the density but maintain high strengths and toughness of C-Mn steels by alloying with abundant Al,thus increasing specific strengths of steels.The newly-designed steels with enhanced specific strengths are named Al-enriched lightweight steels.In view of the microstructural difference,lightweight steels can be generally categorized into three types: ferrite-based lightweight steels,ferrite-austenite duplex steels and austenite-based lightweight steels.In recent years,ferrite-based lightweight steels containing 3~6 wt% Al have attracted increasing attention because of their lower densities,excellent combinations of strength and ductility,and appreciable producibility.Since the microstructure consists of a major characteristic phase of ?-ferrite embedded with appreciable volume fraction of metastable retained austenite,ferrite-based lightweight steels are often called ?-TRIP steels.In the dissertation,a ?-TRIP steel with chemical compositions of Fe-1.1Mn-4.1Al-0.35C-0.38Si(wt.%)was studied in terms of microstructure,phase constitution as well as macroscopic and microscopic mechanical properties,using microstructural characterization techniques including optical microscopy(OM),scanning electron microscopy(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscopy(TEM),phase analysis techniques including X-ray diffraction(XRD)and magnetic probing,and mechanical characterization techniques including uniaxial tensile test and nanoindentation test.Investigative emphasis was placed on:(i)the effect of coiling temperature on the microstructure and mechanical properties of the hot rolled?-TRIP steel as well as the cold-rolled and continuously-annealed steel;(ii)the stability of retained austenite in the cold-rolled and continuously-annealed steel sheet and the strain-induced transformation behavior of retained austenite during tensile deformation at different temperatures;(iii)the feasibility of strengthening the ?-TRIP steel by forming dispersed precipitates inside the phase of ?-ferrite.Main results were obtained as follows:(1)Coiling temperature ranging between 400 °C and 700 °C significantly affects the microstructure and mechanical properties of the ?-TRIP steel.Basically the hot-rolled microstructure consists of ferrite bands and secondary phase bands,both bands aligned with the rolling direction.Ferrite bands are aggregates of ?-ferrite grains,while secondary phases bands are originated from the decomposition of antecedent austenite.The secondary phase band could show different variants and morphologies at different coiling temperatures.The secondary phase band is a bainite band at coiling temperatures lower than 450 °C.More specifically,the bainite band mainly consists of lower bainite together with blocky retained austenite at the coiling temperature of 400 °C,while it primarily contains carbide-free bainite being an aggregate of lath-shaped ferrite and austenite at the coiling temperature of 450 °C.The secondary phase band is a carbide band which mainly contains a pearlite structure at coiling temperatures higher than 500 °C.There are three types of carbides in the microstructure of hot-rolled plate: transitional ?-carbide present inside lower bainite,cementite present within carbide bands as well as at the boundaries between carbide bands and ?-ferrite bands,and ?-carbide present at ?-ferrite grain boundaries.The volume fraction of retained austenite reaches the peak value of 9.6 pct at the coiling temperature of 450 °C,and abruptly drops to zero when the coiling temperatures are higher than 500 °C.Metastable lath-shaped retained austenite with a higher volume fraction contributes to significant enhancement of elongation through the TRIP effect,leading to a uniform elongation of 25% and an elongation-to-failure of 32% at the coiling temperature of 450 °C.(2)At high coiling temperatures,carbides in the hot-rolled steel readily coarsen,promoting cracking initiation in the steel plate during subsequent cold rolling deformation.In the meantime,an increase in coiling temperature facilitates recovery of the ?-ferrite matrix and then reduces the resistance to cold deformation of the steel plate,which further effectively suppresses the crack propagation in the steel matrix and the occurrence of noticeable edge cracking.Taking manufacturability into consideration,the coiling temperature of the ?-TRIP steel plate after hot rolling was selected to be 700 °C for the present study.(3)Coiling temperature does not significantly affect mechanical properties of the cold-rolled and continuously-annealed steel sheet.Intercritical annealing at 830 °C gives rise to an optimal combination of strength and ductility for the annealed sheet.The yield strength,tensile strength and elongation are 500 MPa,805 MPa and 33%,respectively.The Ms temperature of the retained austenite in the annealed sheet is below-150 °C and thesMstemperature is about-10 °C.(4)During room temperature tensile deformation,retained austenite in the annealed steel sheet is subjected to strain-induced martensite transformation,with 68 percent of retain austenite transformed into twin martensite(martensite plates being 100~200 nm thick).Deformation twins are formed in many untransformed retained austenite grains.The corresponding tensile strength of the annealed sheet is 805 MPa.When tensile deformation is performed in the range of 25 to 150 °C,the stability of retained austinite increases with deformation temperature,and the martensitic transformation is gradually suppressed.Correspondingly,the strain hardening rate and tensile strength of the annealed sheet decrease.At the deformation temperature of 150 °C,the strain induced martensite transformation of retained austenite is completely inhibited and starts to be replaced by the strain induced bainite transformation.However,a small quantity of evolved bainite results in a low strain hardening rate and a local minimum tensile strength of 700 MPa.With deformation temperature further increasing,retained austenite increasingly transforms into bainite and tensile strength concurrently increases until the deformation temperature of 300 ° C reaches.At this temperature,90 percent of retained austenite transform into bainite in which the thickness of bainitic ferrite plates is similar to the thickness of martensite plates formed during room temperature deformation,and retained austenite films are situated in between bainitic ferrite plates(the formation of cementite inhibited due to the presence of Al and Si).In addition,the bainite ferrite is supersaturated with carbon at this relatively low deformation temperature because of restricted carbon diffusivity.As such,the strain induced bainite transformation induces the highest strain hardening rate up to high strains and leads to the peak tensile strength of 890 MPa at 300 °C for the present annealed steel sheet.As deformation temperature continuously increases,the driving force for bainitic transformation becomes remarkably reduced.On the other hand,due to its significantly increased diffusivity,carbon repelled from bainitic ferrite readily partitions into adjacent austenite and then contributes to stabilizing the austenite region.These two factors cause the progressive suppression of the austenite-to-bainite transformation with deformation temperature increasing,and a deformed microstructure is visible in retained austenite.Accordingly,the tensile strength of the annealed sheet decreases constantly.The above findings indicate that the strengthening mechanism of the ?-TRIP steel changes from strain-induced martensitic transformation(SIMT)and twin induced plasticity(TWIP)to strain-induced bainitic transformation(SIBT)to dislocation glide(DG)in sequence as deformation temperature increases.(5)Under appropriate conditions of thermomechanical processing and heat treatment,by alloying with Nb/Ti as well as Cu,fine precipitates form in the matrix of the ?-TRIP steel,thus providing a potential approach to developing high-strength ?-TRIP steel.For the Nb/Ti microalloyed ?-TRIP steel,the addition of Nb/Ti could slightly refine ferrite grains of the cold-rolled and annealed steel sheet,but can not eliminate the inhomogeneous grain size distribution and the banded microstructure.Precipitates formed inside ferrite grains have various sizes and appear to play different roles.Ti(C,N)fine particles in the sizes of 50~200 nm and(Nb,Ti)C fine particles in the sizes less than 20 nm inhibit recrystallization of ferrite grains by pinning dislocations and hindering grain boundary movement,resulting in the formation of a large number of sub-grain boundaries within the ?-ferrite grains.Coarse(Nb,Ti)(C,N)precipitates in the sizes of 5~10 ?m are observed to easily initiate cracking in the deformed steel sheet.The precipitation of carbide and carbonitride reduces the volume fraction and stability of retained austenite in the annealed steel sheet.Mechanically,the above microstructural features due to the Nb/Ti microalloying lead to an increase in the yield strength of the cold-rolled and annealed sheet by 65 MPa,while they cause insignificant variation of the tensile strength and a decrease of the elongation.For the hot-rolled Cu alloyed ?-TRIP steel,the addition of 1.0 wt.% copper could not refine ferrite and retained austenite grains,but it delays bainitic transformation during the coiling procedure subsequent to the hot rolling operation,leading to 17.1% volume fraction of austenite retained in the microstructure(In comparison,under the same hot-rolling process condition,the volume fraction of retained austenite in the hot-rolled Cu-free ?-TRIP steel is 9.6%).The homogeneous precipitation of ?-Cu particles in the sizes of about 2 to 4 nm strengthens ferrite and thus increases the yield strength of the hot-rolled Cu-alloyed steel by 83 MPa(i.e.,17% increment).As compared to the Cu-free steel,the composite effects of precipitation hardening and the transformation-induced-plasticity mechanism result in the higher work hardening rate,higher tensile strength of 805 MPa(increased by 130 MPa,namely 19% increment),and remarkable elongation-to-failure(~30%)for the copper-alloyed steel.The present study provided a theoretical basis and guidance for further development and application of low-density ?-TRIP steels.