| The warm-forming process of medium-Mn steel is an effective technology to manufacture the 1500 MPa-grade automotive part.It holds the advantages of low austenizing temperature,low stamping temperature,low quenching rate,as well as high and well-balanced mechanical properties.Therefore,it has been an important way to realize automotive lightweight and improve crash safety.In this thesis,aiming at the key problems during the warm forming process of medium-Mn steel,such as unclear phase transformation behavior,incomplete process optimization,lack of comprehensive evaluation of mechanical properties and formability,the martensitic transformation behavior and properties of warm forming of medium-Mn steel are studied based on micro and macro analysis by combining experiment with simulation.Specific research contents are as follows.(1)The martensitic transformation behavior under the coupling condition has been studied.The influencing rules of loading mode,load,fonning temperature,strain and strain rate on the Ms temperature are analyzed by the radial expansion method and high-temperature radial tensile/compressive test.The effects of temperature-load conditions on martensitic transformation behavior under the coupling condition of thermo-mechanical-martensitic transformation are revealed.The martensitic transformation process is mastered.The research results can provide theoretical support for improving the flexibility of automotive part design and warm forming process design.(2)The mechanical properties of warm-formed medium-Mn steel have been studied,and the mathematical model between process parameters and mechanical properties has been established.Considering the mechanical properties of strength,plasticity and toughness,the mathematical relationship models between process parameters and mechanical properties are established and validated by the extended orthogonal test combined with the progressively-optimized linear regression method.The influence of warm forming process parameters and their interaction on mechanical properties is analyzed.The micro-morphologies of medium-Mn steel under high/low scoring processes are compared.The optimized range of process parameters for mechanical properties is obtained.The results can provide sufficient experimental and theoretical basis for achieving the design of target mechanical property of medium-Mn steel parts.(3)The formability of warm forming the medium-Mn steel has been studied,and the formability analysis model and the minimum thickness calculation model of the real medium-Mn steel automotive B-pillar have been established.The thickness distribution model of automotive B-pillar is established by finite element software LS-DYNA to show the formability of warm-fonned medium-Mn steel.The validity of model is examined by benchmarking analysis with the experiments.Further,the minimum thickness of automotive B-pillar is selected as the evaluation index of formability.The calculation model of minimum thickness is established by Latin hypercube sampling method and Kriging method,and the effect of process parameters on thinning is analyzed.The optimized range of process parameters of formability is obtained.The results can lay the foundation of simulation and experiment for the practical application of medium-Mn steel.(4)A comprehensively comparative study on micro and macro properties of warm-formed medium-Mn steel and hot-formed 22MnB5 steel has been carried out.Microscopically,by comparing the surface and core morphologies before and after warm/hot forming,the influence mechanism of warm/hot forming process on microstructures is analyzed,and the high mechanical properties of warn-formed medium-Mn steel are scientifically explained;macroscopically,the tensile strength,total elongation,tear strength and unit propagation energy are compared to quantitatively analyze the advantage of warm-formed medium-Mn steel on strength,plasticity and toughness.The results can lay the foundation for large-scale industrial application of in edium-Mn steel. |
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