Study On Microstructure And Properties Of Ultrahigh Strength Steel Under High Speed ​​Impact

This thesis mainly researches on the ultra high strength steel, Launching a series of studies on the mechanical properties and microstructure under high temperature and high speed impact environment. The first is ultra high strength steel in high temperature, The high temperature tensile test is mainly performed using MTS880electronic universal testing machine under a series of different temperature, and with the help of Metallurgical microscope, scanning electron microscope to analysis of the microstructure and fracture morphology of the high temperature tensile test samples, studying the high temperature softening resistance; The second is ultra high strength steel in high speed, it is mainly used the quasi static device and Split Hopkinson Pressure Bar (SHPB) device to experiment the quasi static and dynamic compression test on the cylindrical specimens, Studying the three kinds of ultra high strength steel of high strain rate sensitivity and adiabatic shear sensitivity; and with the help of Metallurgical microscope, scanning electron microscope and transmission electron microscopy to study the deformation and failure mechanism under the high speed impact loading conditions.The main findings are listed as following:Three kinds of ultra high strength steel in the temperature range of400～700℃, High temperature softening resistance of G54steel is the strongest, followed by A100steel, the high temperature softening resistance of G31steel is the worst. when the test temperature is600"C, G54secondary hardening steel has poor plasticity, it may be due to the impurity element segregation at grain boundary, so that the grain boundary strength weakened, the formation of the source of stress concentration, under the effect of stress on grain boundaries to crack initiation, crack propagation along grain boundaries, grow up, resulting in intergranular fracture; when the test temperature is700℃, G54secondary hardening steel has high plasticity, it may be due to the formation of austenite films in the grain boundary;In the range of10-3～5000s-1strain rate, with the increase of strain rate, the flow stress of material is also gradually improving in the three kinds of ultra high strength steel, they are all have the strain rate hardening effect or strain rate sensitivity. In the10-3～1000s-1range of strain rate, A100and G54steel relatively high strain rate sensitivity, low strain rate sensitivity in G31steel; In strain rate range of1000～5000S-1, A100and G31steel has higher strain rate sensitivity, lower strain rate sensitivity of G54steel. under high strain rate loading conditions, with the increase of the strain, the flow stress of the material has not increased but decreased, it is mainly related to the deformation strengthening, the strain rate hardening and the thermal softening of the material, indicating the specimen in the process of deformation due to adiabatic temperature rise leads to thermal softening effect gradually dominated by.Three kinds of ultra high strength steels at high strain rate impact fracture morphology, from a macro perspective is45degrees of shear failure. From the micro perspective fracture morphology is elongated parabolic dimple. It belongs to the ductile fracture and the parabolic shaped elongated dimple is formed of the shear stress. In the three kinds of ultra high strength steels, the adiabatic shear sensitivity and lowest for G31steel, adiabatic shear sensitivity of G54, A100steel is relatively close, but were higher than that of G31steel. The adiabatic shear sensitivity of ultra high strength steel is related to the composition of the steel, the thermal conductivity and the strength of steel. The higher the material yield ratio or Strong guide ratio, the more easily of the formation of adiabatic shear band.