Strengthening And Toughening Mechanism Of Rare Earth Lanthanum In High Strength Bainite Rail Steel

Based on the powerful micro-alloying effects of rare earth lanthanum(La)in steel and the significant demand for bainite rails with high strength and toughness due to high-speed railway construction in China,the strengthening and toughening mechanism of La in bainite rail steel is explored.Bainite rail steels containing La are prepared by adding La and micro-alloying elements such as Mn,Cr and Mo.Three existing forms of La are identified.Moreover,the microstructure controlling technology of La strengthening and toughening is designed on the basis of La effects on the hot deformation behavior and phase transformation behavior,which are researched by means of Gleeble thermal simulation technology,thermal expansion method and in-situ observation method,combined with laser scanning confocal microscope(LSCM),transmission electron microscopy(TEM),Auger electron spectrometer(AES),electron probe microanalysis(EPMA),X-ray diffractometer(XRD)and other micro-characterization and micro-area composition analysis techniques.Finally,the mechanism of La achieving nano refinement of bainite ferrite(BF)plates and synergistic enhancement of strength and toughness are revealed.The thesis provides theoretical basis for enriching the microalloying theory of rare earth in steel and preparing new bainite steel with high strength and toughness due to adding rare earth.Three types of micro-alloyed La in bainite rail steel are found by TEM,EPMA and AES micro-characterization and micro-area composition analysis:La segregated on boundaries between grains and bainite ferrite(BF)plates,strain induced precipitation(M,La)3C and cementite θ-(Fe,La)3C formed during the process of phase transformation.The best La content in bainite rail steel is 0.015%which can achieve the most significant effect of nano refinement and synergistic enhancement of strength and toughness,making the tensile strength up to 1260 MPa and impact toughness at room temperature up to 72.8 J/cm2 by refining the BF packets,purifying and strengtheneng grain boundaries and deteriorating Al2O3 inclusions.In the rail blanking temperature zone of 1100～1200 ℃,the best hot workability can be obtained by La refining dynamic recrystallization grains due to La segregating on grain boundaries.In the rail finishing temperature range of 900～1000℃,the strengthening effect of La at high temperature and the "Zener effect" of(M,La)3C pinning grain boundaries are enhanced.As a result,the activation energy of hot deformation increases by 68 kJ/mol due to dynamic recrystallization suppressed by the competitive behavior between the "Zener effect" of(M,La)3C and dynamic recrystallization.So the significant deformation strengthening in the non-recrystallized region can be achieved which is also the reason why La affects the hot deformation behavior of bainite rail steel.Lanthanum enhances the hardenability of bainite rail steel,making the continuous cooling transformation curve shift to the lower right,delaying the bainite transformation and increasing the phase transformation activation energy by 31 kJ/mol.Thus,the high proportion of fine BF plates with fine film-like retained austenite(RA)between plates can be obtained at the slow air cooling rate range of 0.8～2.0℃/s.The in-situ observation results show that the excited nucleation-inhibited growth of La on BF plates is the reason why La affects the phase transformation behavior of bainite rail steel,and this effect is the most obvious at the phase transformation cooling rate of 0.8℃/s which helps achieve fine BF plates with high strength and toughness,as well as the 15～20 nm steps of BF plates.In this thesis,the phase transition controlled by diffusion mechanism in La-containing bainite rail steel is verified by the growth mechanism of such steps and the average growth rate of BF plates being 5.6×10-3 mm/s.The simulation results of the Gibbs Energy Balance(GEB)model also verifies the effect of La on shortening the growth time of BF plates and refining the width of BF plates.Compared with the bainite rail steel containing La,the average width of BF plates of steel containing 0.015%La can be refined by 35%because the nano ultrafine subunits can be achieved in BF plates due to the deformation strengthening effects of La and the excited nucleation-inhibited growth of La on BF plates.The boundaries of such subunits play a role as sub-grain boundaries due to the uneven dislocation density which is the main reason why La realizes the nano-refinement of BF plates.The reason of La realizing the synergistic enhancement of strength and toughness in bainite rail steel is that La enhances the strengthening and toughening effects of BF plates,substructures inside BF plates,precipitations and retained austenite between plates.Further,the corresponding mechanism are as follows:La enhances the interaction between BF plates and dislocations,strengthens the entanglement of C,θ-(Fe,La)3C and dislocations inside BF plates,and promotes the pinning effects of θ-(Fe,La)3C on the nano ultrafine subunits.The nano refinement of BF plates with an average width of 108 nm can be achieved at an air cooling rate of 0.8℃/s in bainite rail steel containing 0.015%La through the micro structure control process of strengthening deformation in the non-recrystallized zone and controlling cooling by three stages.The average size of nano ultrafine subunits is 20 nm × 32 nm,and there are a large number of fine twins with the thickness of 2～20 nm and dislocations with high density of 7.38×1014 cm-2 at the boundaries of such subunits.Meanwhile,some nano θ-(Fe,La)3C with the size of 55 nm inside BF plates are precipitated and the film-like or granular retained austenite with a ratio of about 5.4%between BF plates is formed.This unique fine structure with multi-level feature makes the bainite rail steel containing La have good strength,toughness and fatigue resistance.