Mechanical Properties And Microstructure Evolution Of A BQ&P Steel Under High Strain Rates

Q&P(queuing and partitioning)steel is a typical advanced high strength steel(AHSS)for the third generation of automobile.It has a combination of high strength and superior plasticity,and has a wide application prospect in the field of automobile.While BQ&P steel is a kind of Q&P steel with Bainite,the introduction of bainite can coordinate the deformation ability and improve the plasticity of the material.It is very important to study the mechanical behavior and microstructure evolution of BQ&P steel under high speed impact for the safety design and material selection of automobile.In this thesis,based on the mechanical behavior of a BQ&P(bainite-based quenching&partitioning)steel under the condition of quasi-static compression at room temperature(20?),the mechanical behavior of the steel under high strain rate and different temperatures was studied using Hopkinson dynamic loading device with pulse-shaping technique.SEM was used to observe the microstructure and impact fracture morphology after high-speed impact at different testing conditions.EBSD and TEM were used to characterize the evolution of the as-received microstructure,after quasi-static compression and dynamic impact at room temperature microstructure.The J-C constitutive equation was constructed using the experimental results obtained under different temperatures and strain rates,and the J-C parameters were fitted using experimental data.The results show that under the condition of high strain rates(2200/s?3600/s),the stress-strain curve fluctuates obviously,the yield strength of BQ&P steel increases with increasing strain rate,and the flow stress increases slowly with increasing strain.There are two kinds of bainite structure,martensite structure and M/A island structure in BQ&P steel.After quasi-static and high strain rate compression deformation,the microstructure was refined obviously,and no M/A island structure was nor visible.After high strain rate deformation,,the original tissue parallel to the impact direction is compressed into a flat"S"shape,while the tissue perpendicular to the impact direction is compressed together,or at a certain angle with the horizontal direction.The original grain size decreased from 2.78?m to 1.09?m and 2.14?m after deformation,and the volume fraction of retained austenite was significantly less.After dynamic impact,?-martensite with higher volume fraction also appeared.The dislocation densities of the three samples are 9.6×10¹⁴m^-2,6.6×10¹⁴m^-2and 1.5×10¹⁵m^-2,respectively.There are(100)and(110)textures in the as–received specimen,and(110),(112)and(358)textures in the quasi-static compression specimen,while(100),(122)and(103)textures in the dynamic impact specimen.There are massive retained austenite and film retained austenite in the as-received BQ&P steel.No film like retained austenite was observed after quasi-static compression,but no massive austenite was found after dynamic impact.The yield strength of BQ&P steel at low temperature is higher than that at room temperature,and the maximum strain is reduced.Most of the specimens fracture after impact,and the maximum strain is reduced.The yield strength at high temperature is lower than that at room temperature,and the maximum strain is relatively higher than that at room temperature and low temperature.All dynamic impact fracture specimens show shear fracture along 45°direction,and the fracture mode is the mixture of ductility-brittleness.The fracture surface at low temperature show a bright corlor,mainly brittle fracture.The high temperature impact fracture surface is very uneven,undulating seriously,and the fracture surface color is darker,most of which are ductile fracture.