| Ultra-fine bainite steel has become one of the research hot spots of advanced high-strength steel in recent years due to the combination of high strength,good plasticity and toughness.The acceleration of ultra-fine bainite transformation and the improvement of mechanical properties have been achieved though optimizing alloy composition design,ausforming or prior transformation.However,the ultrafine bainite microstructure is in metastable state from the perspective of thermodynamics,and tends to coarse spontaneously to reduce the interface energy during reheating.The excellent mechanical properies will disappear when the bainite microstructure coarsening.Therefore,it is of great theoretical significance and practical value for the large-scale production and application of ultra-fine bainite steel to explore the rapid manufacter technology of ultra-fine bainite steel for industrial production and to clarify the facters affecting the formation and the thermal stability of the ultra-fine bainite microstructure at the same time.In this paper,the transformation kinetics and microstructure ecvolution of ultra-fine bainite during tempering process are studied systematically from Nb microalloying design,prior ferrite and prior martensite.The acceleration mechanism and the thermal in stability mechanism of ultra-fine bainite steel manufacterd by prior ferrite and prior martensite are analyzed,and the critical factors that affecting the thermal stability of bainite microstructure are identified,so as to explore the methods to improve the thermal stability of ultra-fine bainite steel.The specific research work and results are as follows:The effect of Nb elemet on transformation kinetics and thermal stability of bainite microstructure from medium-carbon ultra-fine bainite steel is compared and analyzed.The incubation time is shortened significantly,and the completion time of bainite transformation during austempering at 340? is shortented from 784.8 s to 687.0 s due to the addition of 0.02 wt.%Nb element.The bainitic ferrite plates are refined,and the carbon contents of the blocky and film-like retained austenite are also increased,thus improving the strength and toughness of the ultra-fine bainite steel.The precipitation strengthening caused by carbide after tempering at 500 ? compensates for the decrease in strength caused by the the coarsening of bainitic ferrite plates and the decrease in dislocation density within the paltes.The stable MC-type carbides,formed by Nb atoms and C atoms,can inhibit the recovery of bainitic ferrite plates and the growth of ferrite grains during tempering process.The effects of austenitizing temperature and austempering temperature on the transformation kinetics,microstructure and mechanical properties of direct isothermal bainite transformation are studied by means of thermal dilatometer and salt bath furnace.At the same austempering temperature,the optimal austenitizing temperature of 0.28C medium-carbon ultra-fine bainite steel is 1000?,when the completion time of bainite transformation is the shortest and the mechanical properties are the best.When the austempering temperature is lower than the Ms,the bainite nucleation sites and the selection of bainite variation are increases because of the introduction of prior martensite,resulting in the refinment of bainitic ferrite plates after the transformation.However,the diffusion of carbon atoms is slow at the low austempering temperature,and the completion time of bainite transformation is longer.The completion time of bainite transformation,affected by the combined effect of austenite supercooling degree and carbon atom diffusion,reaches the shortest time during austempering at 340?,when the austempering temperatures are higher than the Ms.According to the results of transformation kinetics and mechanical properties,the optimal process parameters of direct isothermal transformation of 0.28C medium-carbon ultra-fine bainite steel are 1000? austenitization+340? austempering.The effects of prior ferrite on the bainite transformation kinetics,microstructure morphology and mechanical properties are studied.Ultra-fine bainite steels with different volume fraction of prior ferrite are manufactured by controlling different forming temperature of prior ferrite.The ?/? interface introduced by the formation of prior ferrite provides more nucleation sites for the subsequent bainite transformation,thereby refining the bainite ferrite plates and accelerating the bainite transformation.With the increase of volume fraction of prior ferrite,the completion time of bainite transformation increases and the strength of ultra-fine bainite steel decreases.The carbon-rich of the untransformation austentie,caused by the formation of prior ferrite,reduces the carbon content of the film-like retained austenite in the ultra-fine bainite steel,which reduces the dynamic precipitation of carbide,and delaies the decomposition of retained austenite,and inhibits the coarsening of bainitic ferrite plates during tempering process.The yield strength of the tested steel tempering at 600 ? is comparabale to that of the untempered tested steel.Different volume fractions of prior martensite are formed by controlling different quenching endpoint temperatures before isothermal bainite transformation,to studied the effect of prior martensite on bainite transformation kinetics,micro structure.The introduction of prior martensite increases the bainite nucleation sites by changing the strain field of the adjacent prior austenite matrix.The prior martensite and the adjacent bainite bundle belong to the same Bain group,and the thickness of bainitic ferrite paltes is less than 100 nm after the bainite transformation is completed.As the volume fraction of prior martensite increases,the prior martensite splits the austentie matrix,which inhibiting the growth of baintic ferrite plates and prolonging the completion time of bainite transformation.The carbides within the prior martensite decomposes during the tempering process,so that the adjacent film-like retained austenite still maintains the original morphology when tempered at 600?,which can inhibiting the coarsening the bainitic ferrite plates.The yield strength of the ultra-fine bainite steel after tempering at 600? is 1014 MPa,which is signigicantly higher than that of the bainite steel manufactured by direct isothermal transformation at the same tempering temperature.The high-carbon retained austenite in the medium-carbon ultra-fine bainite steel transforms into martensite though plastic deformation,and the changed of the bainite microstructure and mechanical properties of the ultra-fine bainite steel during tempering process before and after deformation are compared and analyzed.The strength of the medium-carbon ultra-fine bainite steel is obviously increased,while the plasticity and toughness are decreased.The ultra-fine bainite steel manufactured by direct isothermal bainite transformation at 340?,the prior ferrite process at 810? and the prior martensite process at 320? do not show the obvious precipitation strengthen during tempering process,indicating that the thermal stability of high-carbon retained austentie is the critical factor affecting the thermal stability of ultra-fine bainite microstructure. |
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