Study On Microstructure Evolution And Mechanical Prop-erties Of Ultra-Fine-Grained High Carbon Steel At Ultra-High Strain Rates

The ultrafine grained (UFG) steel has caused wide concern and research interestof the materials industry, due to its high strength, high plastic mechanical performance,as well as good superplastic processing at high strain rates under the low temperature.To date, much work has been done on microstructure evolution and on the mechanicalproperties and fatigue behavior of UFG materials, including ferrous metal, non-ferrousmetal and their alloys. However, there are a few information available on the dynamicmechanical behavior of UFG materials.The UFG materials (sub-micron grain size) can be obtained after equal channelangular pressing (ECAP) by using numerical simulations and experimental studies inthis paper. Subsequently the surface of the original lamellar pearlite and the ultra-microduplex structure (α+θ) were treated using the shock wave induced by the highpower density, short pulse laser beam, then the microstructure and mechanicalproperties of the original lamellar pearlite and the ultra-microduplex structure (α+θ)are investigated. The main results are as follows:ECAP process of multiple passes via route Bc in a fully pearlitic structured T8steel was numerically simulated with coupled thermo-mechanical finite elementmethod (FEM), which is based on the finite element software DEFORM-3D. The sizeand distribution of the equivalent strain field at any time and any place were gained.Then, the T8steel with a fully pearlitic structure was studied by ECAP via route Bc at650℃. The ultra-microduplex structure was formed in the high carbon steel after fourpasses ECAP, in which the sizes of ferrite grains and cementite particles were about0.4μm and0.15μm, respectively. After four passes, the tensile strength of the lamellarpearlite was decreased, but the yield strength, the elongation and the reduction of areawere obviously increased, respectively. Hardness of the ultra-microduplex structureand the lamellar pearlite are almost at the same level. The tensile fracture of the ultra- microduplex structure is composed of many small dimple structures, which is typicalductile fracture. However the fracture of original lamellar pearlite appears brittlecleavage fracture.ECAP process in a fully pearlitic structured GCr15steel was successfully carriedout at the room temperature and650℃via route Bc. After two passes by warmdeformation, the ferrite matrix is nearly homogeneous with an average grain size of0.4μm and the size of spherical cementite is about0.1μm. After one pass of ECAP,the hardness is from HRC42(original state in a fully pearlitic) to HRC38(colddeformation) and HRC27(warm deformation). After two passes by warm deformation,the hardness increases from about HRC27to HRC32.After laser shock processing (LSP), the cementite lamellae was bent, fracturedand spheroidized. The spheroidization of the cementite lamellae was aggravating withthe LSP passes and LSP pulse energy increasing. Ferrite grain after LSP was furtherrefinement with the grain size from0.4μm (before LSP) to0.15μm. The cementitedissolution in the ultra-microduplex structure was observed after LSP. With the LSPpasses and LSP pulse energy increasing, the lattice parameter of ferrite was increasedand the cementite dissolution was aggravating. After LSP, microhandness of thelamellar pearlite and the ultra-microduplex structure were increased by18%,30%, andthe maximum values of surface residual stresses were-280MPa and-239MPa,respectively.