Research On Deformation-induced Pearlite Transformation In Vanadium-microalloyed High Carbon Steel

Ultrafine duplex microstructure consisted of ferrite and cementite particle could be obtained by deformation induced pearlite transformation(DIPT)in high carbon steel.The steel has good plasticity and toughness,high strength and hardness.Thus multifarious process consisted of hot rolling,cooling rolling and spheroidizing annealing could be replaced.In recent years,effect of deformation parameters on DIPT have been widely studied,however,research about effect of vanadium microalloying on DIPT,how does deformation impact on DIPT thermodynamically and dynamically and the application of DIPT has been rarely reported.To further understand and perfect the theory of DIPT,DIPT of vanadium-microalloyed eutectoid steel was studied deeply and systematically.Firstly,the possibility of transformation happening in the deformation process was verified in the vanadium-free steel.Secondly,microstructure evolution of DIPT and precipitation of vanadium carbides or carbonitrides in vanadium-containing steels was studied under deformation under different deformation parameters.Thirdly,effect of vanadium microalloying on DIPT was investigated by comparing fraction,microstructure and heat stability between vanadium-free steel and vanadium–containing steels.Fourthly,mechanism of DIPT and impact of vanadium microalloying on DIPT was analyzed by calculation of thermodynamic and dynamic rule.Finally,industrial application method was raised combined with available technique and equipment at manufacture field.The results showed that deformation would induce austenite to pearlite transformation when deformed from 640?to 720?,duplex microstructure consisted of submicron ferrite and nanoscale cementite particles with spheroidization rate above 95%after undergoing pearlite transformation,breaking of cementites,recrystallization of ferrite,precipitation of intragranular cementites and growing up of ferrite and cementite particles.The formed cementite particles were finer and stress in the deformation was lower compared with warm rolling of pearlite.The evolution of microstructure in vanadium microalloyed steel under different deformation conditions revealed that,with the decrease of deformation temperature,the decrease of deformation rate and the increase of deformation amount,the fraction of deformation-induced pearlite increased and the spheroidization of cementite was improved.When deformed at 680?with a deformation rate of 0.01s~-11 to a deformation amount of 75%,the vanadium microalloyed steel could realize complete transformation and spheroidization,and a uniform granular pearlite could be obtained.In addition,the recrystallization ratio of ferrite decreased with the decrease of deformation temperature.The experiment revealed that vanadium microalloying with addition of nitrogen promoted DIPT,the formed microstructure had the highest ratio of ferrite recrystallization,the most uniform grain size of ferrite compared with vanadium-free steel and vanadium-containing steel without addition of nitrogen.However,vanadium microalloying without addition of nitrogen would restrain DIPT.The difference of precipitation of vanadium carbides or vanadium carbonitrides was main reason for the difference of transformation.In addition,the formed microstructure of vanadium-microalloyed steel with the addition of nitrogen had better stability.The results revealed that the deformation storage energy would increased the equilibrium temperature of austenite to pearlite transformation,decreased the critial nucleation radius and shorten the incubation period.That's the reason that pearlite could be induced in the deformation process.Vanadium microalloying with the addition of nitrogen could make the deformation storage energy further increased,thus further promote the DIPT.The increase of dislocation nucleation rate is one of the main mechanism.The experiment results revealed that uniform granular pearlite could be obtained by multi-pass reduction or multi-pass reduction coupled with short time isothermal treatment.The problem of high critial deformation amount needed in the single-pass reduction.The online spheodization of high carbon steel could be utilized industrially with the improvement of rolling equipments.