Effects Of Electropulsing Treatment On The Microstructure And Mechanical Properties Of Cu-enriched Nanoscale Precipitate-strengthened Steel

Cu-enriched nanoscale precipitate-strengthened steel was developed on high strengthand low alloy steel.Ultra-high strength can be obtained by controlling nano-phase precipitation and matrix phase in the optimized treatment process,and good plasticity can also be maintained.Electropulsing treatment(EPT)is a new material processing technology,which is efficient,energy saving,environmental and time saving,and other characteristics.When EPT applys on the metal material,it is different from the routine heat treatment(RHT),It has a big effect on the miscrostucture,and can improve the performance due to a variety of physical effects of coupling.The effects of EPT,s time,frequency,current density and rolling deformation on the microstructure and mechanical properties of cold-rolled Cu-rich nanostructured strengthened steel were studied,and the dislocation density and the initial stage of recrystallization were also studied,the mechanism of EPT was analyzed,and compared with RHT.Cold-rolled 40%of samples was selected for the following studies.When the fixed frequency was 300Hz and the current density was 55-56A/mm~2,different time of EPT was processed.When the electric pulse was applied for a short time,the nano-phase precipitated.At the same time,the matrix phase structure maintained the original rolled morphology and the sample was in the recovery phase.When the fixed time of treatment was 15min and the current density was 55-56A/mm~2,the EPT with different frequencies was processed.At300Hz and 350Hz,the microstructure of the sample maintains the original cold-rolled morphology,small grains born in the matrix grain boundaries at 400Hz.EPT reduces the recrystallization onset temperature compared to RHT.The macro texture of{001}<110>,{111}<110>and{111}<112>under different time and different electric pulses were observed,with the extension of time and the increase of frequency,the texture of{111}<110>turns to a more stable{223}<110>texture.The texture was similar to that of RHT.When the frequency fixed at 300Hz and the processing time was 15min,the EPT with different current density was processed for 80%cold rolled samples.With the increase of the current density,the microstructure of the sample gradually recrystallized,the impact on the organization was mainly Joule heating effect when the temperature was high.Different rolling deformation samples were processed by EPT,and the results indicated the larger the rolling deformation,the easier was the recrystallization.Compared with RHT,EPT obtained more recrystallized grains.Under the different electric pulse processing conditions,the phase in the sample was the same as RHT,all of which were ferrite structures.Under the different time of EPT,the mechanical properties can be improved,when the time reached 20min,the tensile strength reached 1297MPa,which was 393MPa higher than that of the raw materials.Compared with RHT,the tensile strength increased by 155MPa,which should be due to the higher volume fraction of nano-phase after EPT.With the increase of frequency,the strength first increased and then decreased.Under different electric current density,the better mechanical properties obtained under the lower current density.When the current density was 41A/mm~2,the tensile strength was 1401MPa,and it was a 524MPa increase compared with the raw material,while the tensile strength had an increase 171MPa compared with RHT.The main reason was that EPT could significantly reduce the temperature of the aging peak under isochronism,and the electric pulse promoted the precipitation and growth of the nano-phase.The process of deformation followed by EPT could obtain a lower yield ratio and enhance the reliability of the material,while having a higher work hardening rate,and RHT did not change the yield ratio.The cracks were found in the tensile fracture when the strength was high or the deformation was large.The analysis of dislocation density showed that the dislocation density in the sample after EPT was lower than that of RHT,mainly because of the presence of the electron wind force,which could promote the movement of the dislocation during the EPT.At different frequency of EPT,recrystallization began at 400 Hz and the recrystallization temperature was reduced by 65?compared with RHT.In the initial stage of recrystallization,the grains in the matrix were dominated by deformed grains,and the grain orientation in the specimen was mainly[111]and[001].The recrystallized grains were mainly located at the small-angle grain boundaries.CSL grain boundaries was mainly 3,low angle grain boundaries had a large proportion in the misorientation analysis.The microtexture was dominated by the?-direction texture,and its recrystallization behavior was similar to RHT.In conclusion,EPT can promote the recrystallization process.