| The rapid growth of China's car ownership has brought enormous pressure on oil energy consumption and environmental pollution,and has also made traffic safety problems increasingly prominent.As an important development trend in the automotive industry,the lightweight technology of automobiles has obvious advantages in terms of energy saving and emission reduction,handling,safety and battery life of fuel vehicles and new energy vehicles.At present,owing to the high maturity of structural design,the development and application of lightweight materials have become the main direction of current automotive lightweight technology.As an important raw material for the automotive industry,steel generally accounts for more than 65%of the dead weight of a car.It has the advantages of high strength,low production costs and ease of recycling,which will have an irreplaceable role for quite some time in the future.At this stage,the development of automotive steel materials is still focused on the development and large-scale application of advanced high strength steel(AHSS).As one of the representatives of the third generation of automotive advanced high-strength steel,medium manganese steel(MMS)has become the focus of research by experts,scholars,and automotive manufacturers because of its excellent mechanical properties that are comparable to those of high-manganese TWIP steel and its relatively low alloy cost.In terms of the current industrialized production of medium manganese steel,the production cycle of hood annealing is long,and the long holding time is likely to cause coarse grain size to reduce mechanical properties and increase brittleness.However,the heat treatment time of continuous annealing generally does not exceed 15 min,which cannot make MMS obtain sufficient element diffusion enrichment and reverse phase transformation austenite content.In this regard,the combination of short-time intercritical annealing-quenching and partitioning(IA-Q?P)process proved to be a good method to obtain retained austenite with proper mechanical stability in MMS.In this thesis,the IA-Q?P process is used to treat MMS,and research is carried out in terms of further improvement of retained austenite stability,investigation of annealing and partitioning process parameters,evolution of microstructure properties,elemental diffusion,toughening mechanism,etc.The following three aspects of the research results were obtained.(1)The effects of different passes of short-time cyclic heating and quenching pretreatment on the microstructure and properties of MMS were studied.(?)After pretreatment,the average size of each phase was drastically reduced,and the retained austenite morphology was changed from martensite/austenite(MA)island or lath-like to flake-like(film-like).Among them,the average size of retained austenite reached?0.14?m after three cycles,and a retained austenite conversion rate of?78.30%before and after the strain was obtained owing to its fine grain size and morphological stability.(?)After pretreatment,a wide range of rolling weaves evolved into irregular orientations,which not only contributed to the transformation-induced plasticity(TRIP)effect of differently oriented retained austenite but also increased the internal organization uniformity;As the Fe3C at the recrystallized grains and grain boundaries of each phase became finer and denser,the accumulated large amount of distortion energy and dislocations provided greater hardening resistance at the initial strain stage.(?)Combined with line scan and 3D-atom probe tomography(3D-APT)techniques,it was found that the reduction in the average size and the close encirclement of the surrounding"carbon donor"matrix resulted in a higher enrichment of C and Mn in the pretreated thin-film retained austenite than in the initial martensite or large slate-like retained austenite.(?)The dual enhancement of mechanical structure,elemental enrichment,higher dislocation density,and lattice distortion of the retained austenite of MMS by pretreatment resulted in a maximum?53.75%increase in total elongation(16.00%?24.60%),and it has obvious"three-stage"work hardening behavior and higher n-value.(2)The effects of different annealing temperatures on the matrix structure were studied.(?)As the annealing temperature decreases,more ferrite causes penetration cutting of austenite,resulting in the transformation of large lath retained austenite in M/A islands into fine lath(film-like).Among them,when annealed at 670?,the average lateral size of the retained austenite reaches?50-150 nm,and the transformation rate of retained austenite before and after strain reached the maximum.(?)As the annealing temperature increases,more carbon diffuses from the supersaturated martensite to the retained austenite,resulting in a continuous increase in the carbon content of the retained austenite.When the annealing temperature exceeds 700°C,a sufficient amount of carbon leads to over-stable retained austenite,while a large amount of hard-phase martensite matrix increases the resistance of austenite to martensitic phase transformation,resulting in a continuous decrease in total elongation.(?)Excessive dislocations and lattice distortions in the martensitic or ferrite matrix led to high hardening rates in the initial low-strain state.Proper retained austenite stability and resistance of the surrounding matrix to martensitic phase transformation are important for the work-hardening rates to remain stable at high strains;thus,MMS annealed at 670°C obtain the best combination of ultimate tensile strength of 1130MPa and total elongation 26.80%for the best combination.(3)The effects of different partitioning temperatures on the primary and secondary martensite contents,morphology,size,and retained austenite distribution were studied.(?)With an increase in the partitioning temperature,the bulk secondary martensite increases,the average size is relatively large,and the primary slat martensite is less and coarser.The retained austenite is mainly distributed in fine laths between the primary lath martensite phases,at the boundaries of massive secondary martensite or the original austenite grain boundaries.As the partitioning temperature decrease,more retained austenite was stored in the primary lath martensite phase.(?)At different partitioning temperatures,the residual austenite content and its carbon content show an opposite up-and-down trend,with more or less primary martensite not allowing a large amount of untransformed austenite to be effectively retained,owing to less supersaturated primary martensite or less untransformed austenite base to provide carbon.(?)As the partitioning temperature increases,the increase in hard-phase secondary martensite leads to poor coordination of deformation during strain,generating stress concentrations and a gradual increase in yield strength,which also leads to a high work hardening rate at the initial strain stage.The changes in the ultimate tensile strength and total elongation are mainly influenced by the retained austenite content and stability,which in turn control the persistence of the work-hardening rate and n value,where the"proper"content of hard-phase secondary martensite can provide just the right resistance to martensitic phase transformation of retained austenite and retard the occurrence of fracture.In addition,the hard-phase secondary martensite makes the largest contribution to hardness,but it is also important to note that the introduction of other substrates such as bainite reduces the overall hardness and limits the resistance of retained austenite to martensitic phase transformation. |
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