| With the advancement of social and economic growth,the total number of automobiles in our country has steadily increased in recent years.At the same time,the traffic driving safety problem is becoming increasingly prominent,it has brought great pressure on the environment and energy.This has led to the growing need in the automobile industry for the protection of the environment,energy efficiency,and safety performance.Automobile lightweight is a viable strategy and methodology for addressing these problems.At the moment,the major techniques to lighten the vehicle are to use new materials,new processes,and structural optimization of the body frame.Steel is still the most often used material in automobile bodywork today,accounting for more than 65% of the vehicle’s weight.It will be an irreplaceable and significant material for the automotive industry for some time to come due to its benefits of high strength,low cost,and ease of recycling.At this stage,the focus of automotive steel research and development is still primarily on the development and application of advanced high-strength steel(AHSS).At present,advanced high-strength steel has developed into the third generation,with its excellent strong plasticity and low cost attracting the attention of experts or scholars at home and abroad.Among them,Medium manganese steel(MMS)is a typical representative of the third generation of advanced high-strength steel.Austenite in high-strength steels has become a hot topic of current research because of its excellent mechanical properties through the Transformation induced plasticity(TRIP)effect and Twin induced plasticity(TWIP).The addition of the alloying element Al to medium manganese steels can significantly reduce the steel’s density while improving its plastic toughness.As most modern vehicle components are complicated in design,they must be stretched and shaped numerous times to be completed.At the same time,the production cycle for manganese steel hood annealing is long,and the extended holding time tends to coarsen the microstructure grain,reducing the mechanical properties of manganese steel.However,due to the short heat treatment time,the use of continuous annealing production methods is not beneficial for sufficient manganese element diffusion in dielectric manganese steels to retain as much austenite as possible at room temperature.In conclusion,this thesis provides a method for combining pretreatment with intercritical annealing-quenching and partitioning(IA-Q&P).It has been investigated as a method for obtaining stable retained austenite in medium manganese steels.In this thesis,medium manganese steels were treated using a combination of pretreatment and quenching and partitioning processes to investigate the microstructure composition/content control,element diffusion partitioning mechanism,retained austenite mechanical stability,work-hardening behavior,toughening mechanism,obtaining the following three findings.(1)The effect of varying the numbers of stretching to 4% pre-strain and intercritical annealing-quenching and partitioning(IA-Q&P)processes on the microstructure and properties of Al-free hot-rolled Mn steels was investigated.It was discovered that subjecting hot-rolled Al-free Mn steels to varying numbers of stretches up to 4% pre-strain in combination with the IA-Q&P heat treatment process significantly improved the material’s plastic toughness compared to the conventional IA-Q&P heat treatment process,resulting in a significant increase in post-break elongation.With the increase in the number of stretches,a large number of deformation zones and dislocations within the original austenite grains can effectively refine the austenite grains.When subjected to the IA-Q&P heat treatment process,the martensite microstructure of the phase transformation also inherits a large number of deformation dislocations in the deformed austenite,enhancing the ability of the ferrite/remained austenite/martensite to accommodate the dislocations and thus improving the plastic toughness of the material.After several stretches,the microstructure generates a significant number of deformation bands and dislocations,and small carbides precipitate during the IA-Q&P heat treatment,giving more diffuse nucleation points for the austenite.At the same time,the grain microstructure is being refined,the distance of carbon atom diffusion dispersion is being reduced,and the stability of austenite is being improved.After pre-strain treatment,austenite with different phase anisotropy and different morphological dimensions has different gradients of stability in the microstructure.When subjected to external stress,a discontinuous TRIP effect is produced,resulting in the removal and redistribution of stresses in the microstructure,as well as the generation of back stresses in the microstructure.At the same time,the combined effect of retained austenite’s TRIP effect and ferrite’s synergistic deformation greatly increases the plasticity of the tested steel.(2)To investigate the effect of different annealing times in the IA-Q&P process on the microstructures and mechanical properties of lightweight Al-bearing Mn steels.Dimensions within the same region of microstructure were discovered to have a reciprocal connection.Because δ-ferrite divides the microstructure of medium manganese steels into multiple small areas.The size of the δ-ferrite reduces relative when the retained austenite/martensite/α-ferrite distributed in small areas divided by δ-ferrite increasingly grows.This might be the reason for the microstructure’s changing grain size fluctuations.The increase in annealing time was found to have a lower influence on the tested steel’s tensile strength and a higher effect on its plasticity.The microstructure retained austenite was found to nucleate and develop along the high-angle grain boundaries of the ferrite/martensite and to have a diffuse distribution.Because of the retained austenite’s polymorphism and multiscale structure,as well as its modest stability.When subjected to external stress,the tested steel tends to preferentially move in the {111}-<1-10> slid system.At the same time,the plasticity of the tested steel is strengthened by the fact that the tested steel’s recrystallized and substructured microstructure can,to some extent,compensate for microstructural defects.The work-hardening behavior and distribution of Mn element content in the microstructure after heat treatment was found to be closely related to the strong plasticity of Mn steels.The difference in the length of the work-hardening curve’s SIII stage manifests itself macroscopically as a difference in the value of the contribution to tensile strength.The longer the SIII stage plateau,the longer it takes for the TRIP effect in Mn steels to occur.This indicates that medium manganese steel has an excellent degree of plasticity.By comparing the IA-Q&P process with different annealing times,it was found that a suitably short annealing time also resulted in the best mechanical properties of the tested steel.(3)The influence of different tensile pre-strain-IA-Q&P processes on the microstructures and mechanical properties of light-weight Al-containing cold-rolled Mn steels was investigated.The cold-rolled medium manganese steel was found to significantly improve the plastic toughness of the tested steel after treatment with the pre-strain-IA-Q&P process.Compared to the Q&P process,this procedure may greatly enhance elongation after a fracture while maintaining tensile strength.The pre-strain-IA-Q&P process refined the microstructure grains(martensite,austenite,ferrite),and the capability of the microstructure to accept dislocations in medium manganese steels was enhanced.When the external force reaches a certain degree,the sudden opening of dislocations and their mutual cancellation causes stress relaxation and redistribution in the medium manganese steel microstructure.Thus,the TRIP effect and microstructure interactions work together to improve the plasticity of medium manganese steels.It was found that the new pretreatment-Q&P process encourages the enrichment and stability of C and Mn elements in austenite,allowing the existence of block and granular retained austenite in the microstructure to be stable at room temperature.The retained austenite in the medium manganese steel can maintain the TRIP effect at various strain stages when exposed to certain external stresses.This enhances the tested steel’s strong plasticity even further. |
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