| In this paper,SWRS82 B wire rod steel and 70# steel were used as research materials.The isothermal transition TTT curve of SWRS82 B wire rod steel was measured by expansion method.After ultra-fast cooling austenite to the low-temperature bainite transformation region,and then performing isothermal transformation phase transition conditions,the separation rate and mechanics of the pearlite phase are studied by separating the subcooling degree and isothermal temperature and other related phase transition conditions.The effect of deep undercooling and re isothermal strip on isothermal transformation kinetics of SWRS82 B steel and 70# steel,and its influence on Pearlite characteristic size and mechanical properties were compared and studied,and the reasons for the difference were discussed.In the experiment,the DIL805 deformation dilatometer was used for heat treatment,and the TTT curves of SWRS82 B wire rod steel and 70# wire rod steel were measured for direct isothermal and subcooling and then isothermal transformation.The structure and morphology were observed by OM,SEM,EBSD and TEM,and the INSTRON8501 stretching machine carry out the stretching experiment,use the ZEISS SUPRA 55 scanning electron microscope to carry out the in-situ stretching experiment.The main conclusions are as follows:(1)The isothermal transformation TTT curve of SWRS82 B steel and 70# was measured by expansion method.After subcooling,the pearlite transformation kinetic curve shifted to the left,and the incubation period and transformation time of pearlite transformation were shortened from 4.69 s and 8.69 s to 3.78 s,5.7s,the temperature of "nose tip" increased from 560? to 590?,the pearlite transformation rate increased during the isothermal transformation stage.By comparing 70# wire rod steel and 82 B wire rod steel's pearlite incubation period and time-consuming transformation,it isfound that with the increase of carbon content,the rate of pearlite isothermal transformation becomes faster.(2)In this study,when the pearlite transformation is not completed,the state of the incomplete transformation is instantaneously "frozen" to explore the influence of the subcooling and isothermal process on the nucleation rate of pearlite.Compared with the direct isothermal process,the subcooling and isothermal process can significantly increase the nucleation rate of the pearlite structure.The nucleation rate of the structure is increased from 3.03% to 28.03%.The intragranular pearlite nucleation rate is also increased,and the higher the supercooling temperature is low,the greater the nucleation rate of pearlite tissue.By comparing the pearlite structure nucleation rate of 70# and 82 B wire rod steel,it is found that the nucleation rate of82 B wire rod steel with greater C content is greater.(3)The statistical results of pearlite colony size show that the colony size of the82 B large-grain supercooled sample is refined by 39.7% at the highest,and the colony size of the small-grain supercooled sample is refined by 30% at the highest.According to TEM statistics,the cementite thickness of the 82 B large-grain supercooled sample is refined by up to 56.2%,and the cementite thickness of the small-grain supercooled sample is refined by 49.3%.The sub-cooling and isothermal process refines the substructure of pearlite,and with the decrease of the sub-cooling temperature,the refinement effect becomes more obvious.The EBSD results show that the undercooling treatment has played a certain role in improving the uniformity of the structure.The 82 B wire rod steel sample has a higher distribution frequency in the small-angle grain boundary range,and the 70# sample has a higher distribution frequency in a large angle range.(4)In the case of a slight decrease in tensile strength and yield strength(<10%),the reduction of area of the undercooled sample is increased by 36.5% at the maximum,and the undercooling and isothermal process is substantially increased while the strength index is basically unchanged.The plastic properties.And the lower the supercooling temperature,the more obvious the improvement of plasticity index.Fracture analysis show that the fracture of the direct isothermal specimen show acleavage brittle fracture morphology,and the specimen had poor plastic toughness.The undercooled sample is ductile fracture,with better ductility.By comparison,the tensile strength and yield strength of the 70# steel sample are slightly lower than that of the 82 B sample,but the 70# sample after the supercooling treatment has doubled the reduction of area.(5)In-situ tensile test observations show that the direct isothermal sample has a larger pearlite colony size,poor coordination deformation between pearlite colonies,a large number of microcracks in the colony,and it is easy to initiate in the pearlite colony and along the Fe3C/ ?-Fe flakes expand.The coarse lamella structure of the direct isothermal sample is more involved in the uneven deformation behavior between the structures,resulting in the characteristics of high strength and poor plastic toughness.The pearlite colonies of the supercooled samples are small in size and uniformly distributed,and the coordination and deformation ability between the colonies is better.The cracks mainly initiate and expand between the pearlite colonies.The strength improvement caused by the thinning of the sheet spacing is weakened.The strength index is lower than that of the direct isothermal sample,but its plastic toughness is better.(6)Through SEM,TEM,etc.,the evolution of the pearlite microstructure during the cold deformation process was studied,and it was found that with the increase of the strain,the coarse lamella area with shear bands in the direct isothermal sample increased,and the cementite lamella was broken.The dislocation density in the ferrite lamella increases significantly.The supercooled sample has more fine lamella areas,the cementite fragmentation phenomenon is suppressed,and it has good plastic deformation ability.The microhardness test results show that as the amount of deformation increases,a significant work hardening effect occurs.However,under the same amount of deformation,the microhardness values of the supercooled samples are lower than those of the direct isothermal samples. |
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