| Nanostructured bainitic steel has been widespread concern due to its excellent comprehensive properties.However,the ultra-long bainitic isothermal transformation time severely limits its application.Therefore,shortening bainitic transformation time has important significance for the development and application of nanostructured bainitic steel.In this paper,high-carbon bainitic steel was taken as the research object.From the design of V microalloying and the regulation of two-step austempering process,the effects of the introduction of second phase of VC and two-step austempering treatment parameters,especially the second-step temperature and the first-step bainitic fraction on the bainitic transformation kinetics,microstructure and properties of high-carbon nanostructured bainitic steel were systematically studied by means of by using the transformation kinetics analysis methods such as dilatometer and in-situ TEM,microstructure characterization methods,such as SEM,EBSD and XRD,and the performance test methods,such as tensile,impact,wear,fatigue.At the same time,the mechanism of accelerating bainitic transformation of high-carbon nanostructured bainitic steel was revealed.The high-carbon steel was microalloyed by adding a small amount of V element.The second phase of VC possessing a low mismatch bainitic ferrite can precipitate by mediumtemperature holding at the bay region,so that the activation energy of bainitic nucleation at the VC/austenite interfaces reduces,which provides the preferential nucleation position for bainitic nucleation and significantly shortens the incubation period by about 91%.Because the intragranular nucleation hinders the growth of bainitic ferrite,the total transformation time of bainite is only shortened by 12%.VC precipitation reduces the size of blocky retained austenite and makes more geometrical orientation of bainitic,thereby improving the strength and impact toughness of high-carbon nanostructured bainitic steel.After first-step austempering treatments of high-carbon steel at different temperatures,with the increase in austempering temperature,the incubation period and total transformation time of bainitic transformation significantly shortened,which can be shortened by up to 1.4 h.In-situ TEM observation results show that the broadening rate of bainitic subunits restricts the growth of bainite.The reduction of stress and strain at the phase interfaces can accelerate the transformation of bainitic subunits.With the increase in austempering temperature,especially the austempering temperature from 290 ℃ to 330 ℃,the bainitic structure is greatly coarsened.With the increase in autemperign temperature,the strength and hardness decrease,the plasticity increases,and the impact toughness decreases first and then increases greatly.Under the determined first-step bainitic transformation process(austempering at210 °C for 50 min),the effects of introducing the second-step medium-temperature bainite on the two-step bainitic transformation,microstructure and properties of ultrafine bainitic steel were studied.The results show that the increase in the second-step austempering temperature promotes the diffusion of carbon,reduces the carbon enrichment at the bainitic ferrite/austenite interfaces,and reduces the activation energy of bainite nucleation,thus shortening the bainitic transformation time.The higher the second-step austempering temperature,the shorter the total transformation time.With the increase in second-step temperature,the thickness of bainitic ferrite lath,the size of blocky retained austenite and the content of retained austenite increase gradually.At lower second-step temperatures of250 and 290 °C,the bainitic microstructure significantly refines compared to the 210 °C austempered sample.When the second-step temperature is 290 and 330 °C,the impact toughness is higher than that of the 210 °C sample.The optimal second-step austempering temperature of the two-step austempering processes under the set first-step process for the test steel is 290 ℃.The effects of the first-step bainitic fraction on the whole two-step bainitic transformation and microstructure of ultrafine bainitic steel were further studied.The results show that with the increase in the first-step bainitic fraction,the shortening ratio of the bainitic transformation time gradually decreases.When the first-step fraction is 0.25%,the bainitic transformation time is reduced from 24 h to 1.4 h,and the shortening ratio is as high as 94%.Compared with the 210 °C austempering process,the microstructure of the two-step austempering process with different first-step bainitic fractions at the secondorder temperature of 290 °C is finer than that of the 210 °C austempered process,and the retained austenite content is higher.With the increase in the first-step bainitic fraction,the thickness of bainitic ferrite plate and the size of retained austenite decrease,and the length of bainitic sheaves increases slightly.The hardnesses of the two-step austempered samples with first-step bainitic fraction greater than 22% are higher than 58 HRC,which meets the performance requirements of bearings.Meanwhile,when the first-step bainitic fraction is 50% and the second-step temperature is 290 °C,the two-step austempered sample shows higher wear resistance and better fatigue performance than the 210 °C sample.Therefore,considering the shortening amplitude of bainitic transformation time and the final comprehensive performance,the first-step bainitic transformation amount is controlled to 50% at 210 °C,and the bainitic transformation is further carried out at the second-step temperature of 290 °C,which is the preferred heat treatment process path for the test steel in this study.Rapid phase transformation process suitable for high-carbon nanostructured bainitic steel was successfully developed in the paper.The process can significantly shorten the heat treatment cycle,and make the test steel have excellent mechanical properties,which provides important basic theoretical support for the development and application of highcarbon nanostructured bainitic steel. |
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