Investigation On Composition Design,Microstructure And Properties Of High Strength Rail Steels

In this dissertation,the thery of compositional design and processing of pearlitic rail steels with high properties were systematically investigated.The 80CrSiV pearlitic rail steel was developed and the effects of hot deformation and subsequent cooling processes on the microstructure and mechanical properties were also studied.The pilot tests were carried on the the steel and then the microstructure and mechanical properties were analyzed.In addition,the bainitic rail steels with high strength and toughness were also explored.The composition design theory of heavy-duty high-strength pearlitic rail steel has been obtained.In pearlitic rail steels containing 0.75 wt.%?[C]?0.8 wt.%,[V]?0.09wt.%,the Si,Mn,and Cr contents of the alloying elements should satisfy[Mn]+2.00[Si]+1.80[Cr]?3.6,[Mn]+0.67[Si]+1.53[Cr]?2.3 and[Mn]+0.91[Si]+2.50[Cr]?2.8.During the cooling from 850~550°C after deformation,by controlling the cooling rate of pearlitic rail steel between 60~120°C/min followed by coolling to room temperature at a cooling rate of 7°C/min,high-strength pearlitic rail steel with a full pearlite structure can be obtained.Besides,the interlamellar spacing of the 80CrSiV pearlitic rail steel has the lowest sensitivity to the final deformation temperatures and deformation amounts,while greatest sensitivity to the cooling rate,secondly to the final cooling temperature.Specially,with the cooling rate increases,the interlamellar spacing and mechanical properties of 80CrSiV pearlitic rail steel are more sensitive to the final cooling temperature.The pilot tests have been conducted on 80CrSiV pearlitic rail steel and the results show that by shortening the air cooling time,accelerating the cooling rate,lowering the final cooling temperature and implementing two accelerated cooling processes,the interlamellar spacing of 80CrSiV pearlitic rail steel can be effectively refined and then the full pearlitic rail steel with tensile strength of 1444 to 1537 MPa can be obtained.The work hardening ability of the steel is strongly affected by the pearlite interlamellar spacing and the instantaneous work hardening index will decrease with the dearease in pearlite interlamellar spacing.During the in-situ tensile tests,dislocations are more likely to pile up in the lamellar microstructures which are parallel or at acute angles to the tensile axis and then induce the formation of shear bands.Besides,the shear bands propagate along the directions of 45°with the tensile direction and eventually lead to the fracture of the steel.The longitudinal sections of the 80CrSiV pearlitic rail steel undergo different degrees of deformation during the Block-on-Ring wear conditions and the typical lamellae microstructures disappear.When the wear condition is 400 r/min×90 min,the wear rate is relieved because of that the formation of spheroidized carbide with a certain thickness on the contact surfaces.The microstructure and properties of bainitic rail steels with different compositions have been investigated and the high-strength,high-toughness bainitic steels with tensile strength ranging from 1425 to 1548 MPa and impact toughness ranging from 90 to 150J/cm~2 have been obtained.In addition,the results of low-temperature impact toughness of bainitic rail steels show that the effects of temperatures on work of crack formation for the first three steels with bainite microstructure are greater than the effects on work of crack propagation.However,the effects of low temperatures on the work of crack formation of the latter two steels with block ferrite and bainite is less than or close to that on work of crack propagation.The main reason is that the retained block ferrite with lower hardness not only has higher toughness and can change the direction of crack propagation,but also can increases the carbon content in the retained austenite and then increases the mechanical stability and effectively suppress the propagation of cracks.Although the toughness of 80CrSiV pearlitic steel rail steel is low,but its strength can reach 1537 MPa,the preparation process is simple and the production cost is low.It is one of the alternative materials for a new generation of high-strength pearlitic steel rail steel.The high-strength and tough bainitic rail steels can obtain high strength and toughness of1500 MPa and toughness of 150 J/cm~2 and deserve be further studied as a substitute material for pearlitic steel rail steel.