Transformation Behavior,Microstructure And Mechanical Properties Of Ausformed Nanostructured Bainite

Large-strain ausforming and low-temperature bainite transformation were carried out in a medium C-Si rich alloy steel on the Gleeble-3800 thermomechanical simulator.The effects of ausforming on transformation kinetics,microstructure and hardness of samples were studied.The effect of ausrolling on microstructures and mechanical properties of nanobainite steel in medium C-Si rich steel was investigated by warm rolling and hot rolling.In addition,the effect of tempering on microstructures and mechanical properties of warm rolling nanostructured bainite was investigated.The M_s was reduced by ausforming at 400-600? with a strain of 75%in a medium C-Si rich alloy steel on the Gleeble-3800 thermomechanical simulator.The bainite transformation can proceed under a lower temperature and the microstructure was refined,so nanostructured bainite was obtained in medium carbon steel.The volume fraction of retained austenite was increased by ausforming.Besides,the incubation duration of the isothermal transformation was reduced but the growth of bainite was hindered by ausforming.Nevertheless,the entire process of bainite transformation was accelerated.The effect of ausforming on bainite transformation kinetics depended not only on the ausforming temperature and strain but also on the isothermal transformation temperature.The reduction in transformation time was more significant for the lower isothermal transformation temperature than for the higher values.The transformation rate at the initial stage increased with decreasing ausforming temperature and the transformation rates of the ausformed samples were lower than those of the non-ausformed samples at the final stage.Compared with the non-ausformed transformation,the maximum rate of ausformed transformation was faster and occurred earlier,and the transformed fraction at the maximum transformation rate was lower.In addition,the maximum rate of the ausformed bainite transformation was increased and the time of reaching the maximum rate was shortened with decreasing the ausforming temperature.The hardness increased because of the plastic deformation of austenite.Among the samples isothermally transformed at identical temperatures,hardness was greatest for the samples ausformed at 600?.The M_s was reduced by warm rolling and hot rolling in a medium C-Si rich alloy steel.The bainite transformation can proceed under a lower temperature and the microstructure was refined,so nanostructured bainite was obtained in medium carbon steel.The variants of bainite were reduced by ausforming,especially for large strain,but were not obvious for hot rolling.The hardness was increased by warm rolling and improved a little by hot rolling.The impact energy was decreased by warming rolling and improved little by hot rolling.The yield strength decreased after warm rolling but slightly increased after hot rolling.However,the total elongation was increased only by warming at 580? and isothermally transformed at 240?.The highest product of tensile strength and ductility of 28587 MPa%was obtained in the sample warm rolled at 580 ~oC and isothermally transformed at 240 ~oC.The highest UTS of 2581 MPa was obtained in the sample warm rolled at 500 ~oC and isothermally transformed at 200 ~oC and higher than that of high-C nanostructured bainite.The highest hardness was 682 HV and all the hardness was above 600 HV,which was comparable to the nanostructured bainite obtained by isothermal transformation at low temperature in high C high Si steel.The effect of tempering on the microstructure and mechanical properties of samples warm rolled and isothermally transformed was investigated.The volume fraction of retained austenite slightly decreased after tempered at 200-400?,and decreased with the increasing of tempering temperature.The bainite ferrite plates coarsened significantly after tempering.The hardness decreased after tempering and reduced with the increasing of tempering temperature.The tempering resistance was high when the bainite was tempered at 200-300?.The impact energy slightly increased after tempering but not monotonically and firstly increased and then decreased.The ultimate tensile strength decreased significantly after tempering at 200-350? but appeared little difference between 350? and 400?.The yield strength increased after tempering and firstly increased after tempered at 200-350? and then decreased at 400?.The elongation increased except samples tempered at 200?.The elongation firstly increased after tempered at 200-350? and then decreased at 400?.