Fabrication And Its Microstructure And Mechanical Properties Of Ultra-fine Grain High Carbon Steel

Grain refinement is an important method to improve the mechanical performanceof steel materials, because the materials have good combination of strength, hardness,toughness and ductility. It can not only prolong the service life of mechanicalequipment, but also reduce resources and alleviate environmental pressures on earthdue to the premature failure of mechanical equipment and engineering structure. It isof great significance to the sustainable development of human being. High carbon steelas an advanced material, possess excellent properties and has caused wide concern andresearch interest of the materials industry. How to obtain the excellent comprehensiveproperties of materials during the structure ultra-fining treatment is an important taskwhich has the important academic value and the project application value. Thus, it isimportant to investigate the fabrication, microstructure and mechanical properties ofultra-fine grained high carbon steel.The UFG materials (sub-micron grain size) was fabricated in the granularpearlitic steel after split hopkinson pressure bar (SHPB) and equal channel angularpressing (ECAP), respectively. SHPB method has a high strain rate and smalldeformation, while, ECAP has a small strain rate but large deformation. Themicrostructure, before and after processing, was analyzed by microscopic analysistools. The mechanical properties were investigated using mini-tensile tests and Vickersmicrohardness tester.Based on the stress-strain curves of SHPB at high strain rates, the flow stressincreases with the increase of the strain rate. The material appears positive strain ratesensitivity. The granular pearlitic steel was obviously refined and the ultra-microduplex structure (α+θ) was fabricated successfully. At the strain rate of3078.2s-1, the size of ferrite grain was about500~700nm and the size of the cementiteis about100~250nm. Microhardness increases from HV195to HV270. The high carbon steel with granular pearlitic structure was studied by ECAP viaroute BCat room temperature and650℃, respectively. The granular pearlitic structurewas obviously refined. After four passes by cold deformation, the size of ferrite grainwas about400nm and the size of the cementite is about100nm. After four passes bywarm deformation, the ferrite matrix is nearly homogeneous with an average grain sizeof450nm and the size of the cementite is about200nm.After four passes by cold deformation, the ultimate tensile strength and hardnessincrease from598MPa, HV195to1077MPa and HV309, respectively. After warmdeformation, the ultimate tensile strength and hardness first increase, then decreasewith the number of ECAP passes. After two passes, the maximum values of ultimatetensile strength and hardness are about870MPa and HV275, respectively. After fourpasses, the ultimate tensile strength and hardness are782MPa and HV258. After fourpasses, the elongation rate decreases from20.1%(before ECAP) to10.2%(colddeformation) and16.8%(warm deformation). Compared with original granularpearlitic structure, the mechanical performance of the material is significantlyimproved.The tensile fracture of ultra-fine grain high carbon steel indicates that the size andnumber of small and shallow dimple are increased with the number of ECAP. Afterfour passes by cold deformation, the fracture appears a little brittle cleavage fracture.The average size of few large and deep dimples about4μm, while a large number ofsmall and shallow dimples are distributed uniformly, and the average size is about1μm. After four passes by warm deformation, a large number of small and shallowdimples are about1μm.