Since early twenty-first century, railway transportation has seen a global revival. Among all the modes of transportation, railway is the most environment-friendly and energy efficient mode of transportation on the land. At present, in order to sufficiently release the potential of railway transportation, railway transportation is being developed toward a direction of high speed and heavy load. Steel rail is an important part of the track structure, and the quality of the steel rail is directly related to the safety and efficiency of railway operations. In order to meet the requirements of the construction and development of the railway, especially heavy-haul railway, there is an urgent need to improve the strength, wear resistance and fatigue resistance of the steel for railway. Generally, the steel for a traditional steel rail, for example, with China code of U75Mn, has a carbon content of 0.6-0.8 wt %, and belongs to eutectoid steels, which has a tensile strength of ≥880 MPa and a hardness ranging from 260 to 300 HB. In order to improve the strength and wear resistance, elements such as Nb, V, rare earth, and so on are added to original composition of the pearlitic rail, and the processes such as controlled rolling are used to improve the mechanical properties thereof, as described in Chinese Patents CN 10447230 and CN 11077350, U.S. Pat. Nos. 5,658,400 and 4,767,475, etc., which disclose rails corresponding to China codes of U75V, U76NbRe, U77MnCr, etc. The microstructures of these types of rails are all pearlite structures. Among them, the most widely used rails are U75Mn and U75V rails. Comparing with U71Mn rail, U75V rail has a higher carbon content, and further comprises silicon and vanadium elements. Accordingly, U75V rail has an improved strength and significantly superior wear resistance, but a poorer plasticity and toughness than U71Mn rail. Meanwhile, U71Mn rail has superior resistance fatigue crack growth, fracture toughness and weldability performances over U75V rail.
In order to further improve the service life of the rail, researchers have conducted numerous studies and explorations in recent years on the design of the alloy composition of the rail, and have invented and developed many novel ultra-fine pearlite rail with high overall properties by adding elements such as Cr, Mo, V, Ti, Nb, Co, Cu, Ni, B, N, Al, Zr, etc. to ordinary rails having eutectoid carbon contents, and adopting new rolling processes and heat treatment techniques, such as those described in U.S. Pat. RE42,668, U.S. Pat. Nos. 7,972,451 and 8,361,246, United States Patent Applications US 2004035507-A1 and US 2003192625-A1; Chinese Patent Applications CN 101818312A, CN 1884606A, and CN 1522311A; Japanese Patent Application Nos. JP 2005256022-A, JP 2002363696-A and JP 7126741-A; etc. These patented technologies improved the strength and wear resistance and fatigue resistance of the rail to different degrees. Some specific thermal treatments were also performed after rolling to improve the strength and wear resistance of the rail, such as those described in Chinese Patent CN1155013, etc. Furthermore, a bainite rail is manufactured by introducing a bainite microstructure into the rail to improve its obdurability, such as those described in Chinese Patent Applications CN102899471A, CN103160736A, and CN102936700A; U.S. Pat. No. 5,676,772; etc. The microstructures of these rails are in a macro- or micro-scale, but not a nano-scale, and then the performance potential of these rails were not thoroughly released.
It has been showed that a pearlitic structure has excellent overall mechanical properties when it is refined to a nanoscale (see Scripta Materialia, 2012, 67(1): 53-56). Accordingly, it is believed that a nano-pearlite rail would have a high strength, as well as an excellent wear resistance and fatigue resistance. However, the average interlamellar spacing of pearlite in conventional pearlite rails is in a submicron or even micron scale, while that in the so-called ultra-fine pearlite rail is also more than 150 nm. It is very difficult to reach a nanoscale (>100 nm) in a manufacturing process.
Several inventions relating to pearlite rails addressed the ultra-fine pearlite. For example, Chinese Patent Application CN1884606A discloses a pearlite structure manufactured mainly by a heat deformation process, which has an insufficient hardness, tensile strength, and elongation. Chinese Patent Applications CN1522311A and CN1140473A filed by Japanese applicants describe rails having carbon contents up to 1.2 wt. % and 1.4 wt %, respectively, and high contents of Mn, but being free of Al. It is impossible for these rails to have a nanoscale microstructure. In order to obtain a rail having a fine pearlite structure, it is required by the technology patented by the Japanese to rapidly cool the rail immediately after rolling to avoid growth of the pearlite structure. This increases the complexity of the process for manufacturing rails, and makes it impractical.
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