Mechanical Properties Of Low-temperature Bainitic Steel And Phase Transformation Under High Magnetic Field

Low-temperature bainitic steel has the characteristics of ultra-high strength,high ductility and good toughness.It is an important scientific problem in the field of steel materials to obtain low-temperature bainitic steel with excellent mechanical properties in a short time by controlling the microstructure.The tensile properties of two kinds of low-temperature bainitic steels with different sulfur content under the same heat treatment and the different austenitising temperatures were investigated.It was found that sulfur content and austenitising temperature lead to significant difference in tensile properties.Except for traditional heat treatment,we studied the low-temperature bainite transformation under high magnetic field.The results showed that the high magnetic field accelerated low temperature bainite transformation significantly,affected the carbon partitioning between bainitic ferrite and retained austenite,and induced the precipitiation of nano-cementite.Meanwhile,these experimental phenomena were analyzed and explained by establishing physical model and calculating the thermodynamic free energy.The main results are as follows:?1?The elongation of high sulphur steel?1.0±0.5%?is significantly lower than that of low sulphur steel?11.4±1.5%?.The reason is that the effect of sulphur on austenite grain refinement in high sulphur steel is more obvious than that in low sulphur steel,and finer austenite grain accelerates bainite transformation.This leads to an increase in the volume fraction of bainite and more carbon atoms are partitioned to retained austenite.In high sulfur steel,the volume fraction of retained austenite is relatively low,and TRIP effect cannot occur due to its high stability.The contribution of retained austenite to elongation is not significant.We also characterized and analyzed the stability of retained austenite by the carbon concentration of retained austenite,the variation of retained austenite during tensile test,the local tensile elongation and Ms temperature.?2?With the increasing of austenitising temperature,the elongation of high sulfur steel increases gradually.The TRIP effect,which contributes to the improve elongation,almost did not occur at 850?due to the relatively low volume fraction of retained austenite and its high carbon concentration,and a large number of MnS inclusions.The contribution of TRIP effect to elongation is greatly enhanced by the higher volume fraction of retained austenite and its moderate stability at 950?and 1050?,respectively.Besides,the more MnS was dissolved in austenite during higher temperature austenitizeing.All the above factors promoted the elongation.?3?High magnetic field accelerates the low temperature bainite transformation,refines the thickness of bainite lath and makes the microstructure more uniform.Thermodynamic free energy calculation shows that the magnetic free energy produced by ferromagnetic bainite ferrite is-133.90 J/mol with a 12 Tesla magnetic field.The driving force of transformation increases from-1159.00 J/mol without magnetic field to-1292.90 J/mol with magnetic field.The increase of phase transformation driving force promotes the nucleation of bainite ferrite.?4?High magnetic field promotes the carbon partition from bainitic ferrite to retained austenite.Retained austenite is more carbon-rich under high magnetic field.It is found that the T₀-T₀'curve shifts to the right,and the carbon concentration in retained austenite is increased when the bainitic ferrite and the retained austenite have same free energy.In addition,during the carbon partition,a large number of carbon atoms are segregated at the bainite ferrite/retained austenite interface,leaning toward the side of the retained austenite.As a result,the carbon concentration of retained austenite near the interface is significantly higher than that of other parts of retained austenite.?5?High magnetic field promotes the precipitation of nano-cementite in low temperature bainite.The nucleation barrier of cementite was decreased and the nucleation rate was increased under high magnetic field.