Study On Dynamic Mechanical Behavior And Microstructure Evolution Of An Ultra-Fine-Grained High Carbon Steel Under The High Strain Rates

The ultrafine grained (UFG) steel has caused wide concern and research interestof the materials industry, due to its advanced mechanical performance, such as highstrength, high plastic mechanical performance, as well as superplastic processing athigh strain rates under the low temperature. To date, much work has been done onmicrostructure characteristics and on the static mechanical properties of UFG materials.However, there are a few information available on the dynamic mechanical behaviorand the microstructure evolution of UFG materials.The ultrafine micro-duplex structure (sub-micron grain size), with400nmequiaxed ferrite grains and150nm cementite particles, can be obtained after equalchannel angular pressing (ECAP). The dynamic mechanical behavior of UFG steel atdifferent high strain rates was investigated by SHPB. The microstructure evolutionwas observed by SEM and TEM. The results show that ASB can be formed in both ofthe lamellar pearlitic steel and the UFG steel, but the microstructure evolution in ASBis different. In the lamellar pearlitic steel, the pile-up of high density dislocationscontributes to the subgrains, then dynamic continuous recrystallization occurrs,leading to the formation of the equiaxed ferrite grains. Also, the fine cementiteparticles are precipitated during dynamic recovery process, which results in theultrafine micro-duplex structure of ASB. However, at the high strain rates, due to thecrystal distortion, the equiaxed ferrite grain of UFG steel is elongated, leading to theformation of deformation bands, then the dislocations in the elongated ferritegrainboundary arrange themselves into dislocation cells, which become sharper,forming subgrains. Dynamic recrystallization occurrs and more finer equiaxed ferritegrain is formed. Meanwhile, the cementite particles with a smaller grain size areprecipitated, which finally leads to the formation of ASB. The average strain rate of the lamellar pearlitic steel and the UFG steel increaseswith the projectile velocity. The strain rate of UFG steel is more sensitive than that ofthe lamellar pearlitic steel. Hardness increases from matrix to ASB and the hardness inASB is the highest. The value is HV226in the lamellar pearlitic steel. Micro hardnessin ASB increases with the projectile velocity and the value increased by29.2%fromHV266to HV292. Microhardness is HV246in UFG steel. The value in ASB fristdecreases, then increases with the projectile velocity increasing. When the projectilevelocity is33m/s, microhardness reaches to the maximum and the value is HV314which is a24.6%increase. This is consist with the results of nanoindentation hardness.After the dynamic compression test, macroscopic fracture was not observed in thelamellar pearlitic steel, two passes of ECAPed samples and UFG steel. From the cross-section of the lamellar pearlitic steel, microcrack with a width of100nm can beobserved, meanwhile, the cavity defect and some small microcracks can be found bythe longitudinal observation. The cavity defect is observed in two passes of ECAPedsamples and in local area the cavity defects run through the samples. However, thisdefects can be found in UFG steel. It indicates that microstructure is one of the mainfactors effected on fault germinating during the dynamic loading process. The morehomogeneous the microstructure is and the smaller the grain size is, the defects areless during the dynamic loading process.