Patent Application
20130193223
The present invention relates to a steel rail material, more particularly, a steel rail adapted to be used in a high speed or quasi-high speed railway and a method of manufacturing the same.
There have been three main kinds of railways nowadays in the world, i.e., heavy haul railway, high speed railway, and mixed passenger and freight railway. As for steel rails for the heavy haul railway, because of generally 25 t-40 t of an axle load of a train, great contact stress between wheel and rail, and harsh forces, a carbon steel or alloy steel rail having more than 0.75% of C, a tensile strength of 1200 MPa or more, and a full pearlite structure is generally used to ensure that the steel rail has excellent resistance to wear. As for the high speed railway, since it is mainly used in passenger transport and the train has a light axle load, steel rails for the high speed railway are generally required to have an excellent antifatigue property. As for the mixed passenger and freight railway, since it is used not only for passenger transport, but also to ensure the particularity of cargo transport, the used steel rail is required to have both predetermined resistance to wear and predetermined antifatigue property to reach a balance therebetween. As for the steel rails used for the mixed passenger and freight railway, a hot-rolled or heat-treated steel rail having 0.70%-0.80% of C and a tensile strength of 900-1100 MPa is generally used, a steel rail having a tensile strength of 1200 MPa may be used for a railway having curve with a small radius, and the steel rails used for the mixed passenger and freight railway have a metallurgical structure with a dominant component of pearlite and, partially, a tiny amount of ferrite. Since the steel rails used for both high speed and quasi-high speed railways are required to have predetermined antifatigue properties, hot-rolled U71Mn steel rails having a tensile strength of 900 MPa and 0.65%-0.76% of C are widely used in the high speed and quasi-high speed railways.
However, practical application shows that a crack which has already been generated in an upper or side surface of a head portion of a steel rail is difficult to be worn away due to a relatively light axle load of generally 11-14 tons of a high speed train and little wear-out between wheel and rail in the practical operation, and under repeated wheel-rail contact force, propagation of the crack may be in turn aggravated, resulting in tendency of fracture of the steel rail, which seriously endangers running safety of the train. On the other hand, if a wear rate of the steel rail is improved by a method of only decreasing strength and hardness of the steel rail, a plastic flow may occur in a surface of the steel rail to cause deviation in cross-sectional dimension of the steel rail so that the train cannot run along the railway, and a service lifetime of the railway may be also shortened due to excessive wear-out of the steel rail. Accordingly, as for the high speed or quasi-high speed railways, a balance is difficult to be made between wear-out and rolling contact fatigue of the hot-rolled steel rail having a dominant component of pearlite.
In order to improve the rolling contact fatigue property of the steel rails for the high speed and quasi-high speed railways, there have been mainly two methods at present. A first method is to periodically grind an upper end of the steel rail by using a railway-grinding train, but this method has a problem in that the railway-grinding train is expensive, and meanwhile, there is a high traffic density on the high speed and quasi-high speed railways so that no sufficient grinding time can be spared. A second method is to improve the wear rate of the steel rail surface so that a fatigue layer is worn away through continuous wheel-rail wear-out before fatigue damage occurs. The wearing characteristic of the steel rail is affected by its hardness, and thus the hardness of the steel rail may be reduced so as to facilitate wear-out. However, simply reducing hardness may result in plastic deformation occurring on an upper surface of the steel rail after running a period of time, frequently accompanied by damages such as crack and peeling, which also negatively effect the lifetime and transportation safety of the steel rail.
In recent years, in order to improve contact fatigue damage property of a steel rail for a high speed railway, a steel rail having a dominant component of bainite, a small amount of martensite, and residual austenite has been developed. Chinese Patent No. CN1074058C discloses a bainite-based steel rail with excellent bonding characteristic in its welding portion and a method of manufacturing the same. The bainite-based steel rail includes 0.15%-0.40% of C, 0.1%-0.2% of Si, 0.15%-1.10% of Mn, less than or equal to 0.035% of P and S, as well as Cr, Nb, Mo, V, Ni and other elements.
However, in theory, a steel rail having a bainite structure, especially a lower bainite structure, has a significantly improved toughness and plasticity and an advantage in running safety as compared with a pearlite-based steel rail having the same strength level, but in terms of wear-out and rolling contact fatigue properties, its theoretical values are not consistent with its practical values. The structure and performance of bainite depend on morphologies, distribution and interaction of ferrite and carbide. For example, the carbide is solid-solved in the ferrite or distributed along grain boundaries of the ferrite, the steel rail may have significantly different hardness. The hardness directly determines the wear property, and thus extremely strict requirements for process control and production processes of steel rails are needed in order to obtain an ideal structural form. In addition, in the case of the bainite-based steel rail disclosed in Chinese Patent No. CN1074058C, in order to obtain an ideal bainite structure, a strict control process for the steel rail is required, a large amount of valuable elements need to be added, causing the manufacturing cost of the steel rail to be much higher than the existing pearlite-based series rail, and even if the performances of the steel rail manufactured are excellent, it will be difficult to be mass-manufactured and widely used.
Therefore, manufacture of the bainite steel rail and wide application thereof to the high speed or quasi-high speed railway are limited due to strict manufacturing process as well as addition of a quantity of valuable alloys, thus a high manufacturing cost equal to or more than twice of the existing pearlite steel rails. In addition, it still needs to be further verified whether the fatigue property of the bainite steel rail is superior to that of the existing pearlite steel rail or not.
Thus, there is an urgent need for a pearlite-based steel rail which has a low manufacturing cost while having excellent resistance to wear and fatigue damage to be suitable for high speed or quasi-high speed railway applications.
An object of the present invention is to solve the above described problems existing in the prior art, and to provide a steel rail suitable for a high speed or quasi-high speed railway having an excellent rolling contact fatigue property.
The present invention provides a steel rail for high speed and quasi-high speed railways including 0.40-0.64% by weight of C, 0.10-1.00% by weight of Si, 0.30-1.50% by weight of Mn, less than or equal to 0.025% by weight of P, less than or equal to 0.025% by weight of S, less than or equal to 0.005% by weight of Al, more than 0 and less than or equal to 0.05% by weight of a rare earth element, more than 0 and less than or equal to 0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities, wherein a head portion of the steel rail has a uniformly mixed microstructure of pearlite and 15-50% of ferrite at a room temperature.
According to one embodiment of the present invention, the steel rail includes 0.45-0.60% by weight of C, 0.15-0.50% by weight of Si, 0.50-1.20% by weight of Mn, less than or equal to 0.025% by weight of P, less than or equal to 0.025% by weight of S, less than or equal to 0.005% by weight of Al, more than 0 and less than or equal to 0.05% by weight of a rare earth element, more than 0 and less than or equal to 0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities. According to another embodiment of the present invention, the steel rail may include at least one of 0.01-0.15% of V, 0.02-0.20% of Cr, and 0.01-0.05% of Ti. According to yet another embodiment of the present invention, the steel rail may include at least one of 0.02-0.08% of V, 0.10-0.15% of Cr, and 0.01-0.05% of Ti.
According to one embodiment of the present invention, the head portion of the steel rail has a uniformly mixed microstructure of pearlite and 15-30% of ferrite at the room temperature.
The present invention provides a method of manufacturing the steel rail described above including smelting and casting molten steel, rolling steel rail, controlled cooling after rolling, and air-cooling, wherein the controlled cooling after rolling may include making the steel rail stand upright on a roll table, transferring the steel rail to a heat treatment unit through rotation of the roll table, and blowing cooling medium onto the steel rail by the heat treatment unit to uniformly cool the head portion of the steel rail at a cooling rate of 1-4° C./s until a temperature of a top side of the head portion decreases to 350-550° C.
According to the present invention, the method may further include after finishing rolling during the rolling steel rail, cooling the steel rail to a temperature lower than an austenitic phase zone, and then heating the steel rail to a temperature in the austenitic phase zone at a rate of 1-20° C./s, followed by the controlled cooling after rolling.
According to one embodiment of the present invention, the cooling medium may be at least one of compressed air, a mixture of water and air, and a mixture of oil and air.
According to the present invention, the smelting and casting molten steel may include smelting the molten steel by using a converter, an electric furnace or an open-hearth furnace, performing a vacuum treatment on the molten steel, casting the molten steel to a billet or a slab, and cooling the billet or the slab or directly transferring the billet or the slab to a heating furnace to increase a temperature thereof. The rolling steel rail may include feeding a billet or a continuously cast slab which has been heated to a certain temperature and kept for a certain period of time into a rolling machine to roll the billet or the continuously cast slab to a steel rail having a required cross-section. During the rolling steel rail, the temperature of the billet or the continuously cast slab may be increased to 1200-1300° C., and kept for 0.5-2 h.
According to the present invention, the method may further include after the controlled cooling after rolling, placing the cooled steel rail in the air to be naturally cooled to a room temperature.
In the present invention, by reducing the content of carbon element in a steel rail, with controlled cooling after rolling, toughness and plasticity and a yield strength of the steel rail can be improved while maintaining the levels of strength and hardness of the existing steel rail for the high speed railway, and an energy value required for initiating and expanding microcracks formed at the surface of the steel rail due to fatigue can be increased, and thus under the same conditions, the rolling contact fatigue property of the steel rail can be improved, thereby finally improving the service lifetime and the transportation safety of the steel rail.
The above and other objects and feature of the present invention will become more apparent by the following description in conjunction with the accompanying drawings, in which:
With the development of high speed and quasi-high speed railways, steel rails are required to have excellent comprehensive performances to ensure safety and longevity of high speed railways. A train runs along steel rails at a high speed, thus the steel rails are required to have excellent toughness and plasticity, and excellent rolling contact fatigue performance, in addition to an appearance with high flatness, high accuracy of geometric dimensions and defect-free. As for the current steel rails used for high speed and quasi-high speed railways, wear-outs of surfaces the steel rails due to wheel-rail contact friction need to be decreased as possible so as to ensure a long lifetime; meanwhile, in order to ensure that microcracks which have been generated in a surface of a steel rail can be timely worn away before expanding inwardly, a certain wear rate needs to be further ensured, which is in contradiction with increasing a service lifetime of the steel rail. However, both decreasing wear-out and improving rolling contact fatigue property seem to be not fundamentally resolved.
Therefore, in the present invention, by reducing the content of C element in a steel rail, with controlled cooling after rolling, toughness and plasticity and a yield strength of the steel rail can be improved while maintaining the levels of strength and hardness of the existing steel rail for the high speed railway, and an energy value required for initiating and expanding microcracks formed at the surface of the steel rail due to fatigue can be increased, and thus under the same conditions, the rolling contact fatigue property of the steel rail can be improved, thereby finally improving the service lifetime and the transportation safety of the steel rail.
In particular, the present invention provides a steel rail for high speed and quasi-high speed railways including 0.40-0.64% by weight of C, 0.10-1.00% by weight of Si, 0.30-1.50% by weight of Mn, less than or equal to 0.025% by weight of P, less than or equal to 0.025% by weight of S, less than or equal to 0.005% by weight of Al, more than 0 and less than or equal to 0.05% by weight of a rare earth element (RE), more than 0 and less than or equal to 0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities. Preferably, the steel rail for high speed and quasi-high speed railways according to the present invention includes 0.45-0.60% by weight of C, 0.15-0.50% by weight of Si, 0.50-1.20% by weight of Mn, less than or equal to 0.025% by weight of P, less than or equal to 0.025% by weight of S, less than or equal to 0.005% by weight of Al, more than 0 and less than or equal to 0.05% by weight of a rare earth element, more than 0 and less than or equal to 0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities. In the following description, the contents of the mentioned substances are based on weight percentages, unless stated otherwise.
The steel rail for high speed and quasi-high speed railways according to the present invention has a uniformly mixed metallurgical structure of pearlite and 15% to 50% of ferrite (preferably, pearlite and 15% to 30% of ferrite) at room temperature, an elongation after fracture of more than or equal to 15%, a yield strength (REl) of more than or equal to 550 MPa, and a fracture toughness KIC of more than or equal to 40 MPam1/2 at −20° C.
Hereinafter, the reasons for limiting the chemical components of the steel rail according to the present invention to the above-described ranges will be first described.
C is one of the most important and economical elements in the steel rail to endow it with an appropriate strength, hardness and resistance to wear. In the steel rail according to the present invention, when the content of C is less than 0.40% by weight, the wear property may be reduced because the amount of carbides in the metallurgical structure is too small to be concentrated below a head tread of the steel rail, resulting in reduced service lifetime of the steel rail due to being worn too fast; at the same time, due to reduction in hardness, a plastic flow zone is formed in the tread of the steel rail, and such defects as flash and the like are prone to be generated, endangering running safety of a high speed train. In the steel rail according to the present invention, when the content of C is more than 0.64 wt %, the strength and hardness of the steel rail will be excessively high by a subsequent heat treatment process. As for this, on the one hand, the cracks which have been generated can not be worn timely to expand, so that there is an increased tendency for the steel rail to be laterally fractured; on the other hand, the excessively high hardness of the steel rail accelerates the wear rate of a wheel, significantly reducing the service lifetime of the train. In addition, under the same conditions, the improvement in the strength of the steel rail is necessarily accompanied by reduced toughness and plasticity, which can not meet safety requirements as well. Therefore, the content of C is defined to be between 0.40% and 0.64% in the present invention so that a rigidity required for the steel rail can be better satisfied, while matching the hardness of the rail and the hardness of the wheel with each other and improving safety of the rail in use. Preferably, the content of C is defined to be between 0.45% and 0.60%.
Si, as a main added element in the steel rail, usually exists in ferrite and austenite in a form of solid solution to increase the strength of the metallurgical structure. In the steel rail according to the present invention, when the content of Si in the steel rail is less than 0.10% by weight, the amount of the solid solution will be too low, resulting in an unobvious strengthening effect, and when the content of Si is more than 1.00% by weight, the toughness and plasticity, and ductility of the steel rail will be reduced. In addition, when the content of Si in the steel rail is relatively high, a lateral performance of the steel rail may be significantly deteriorated, negatively affecting the safety of the steel rail in use. Therefore, in the present invention the content of Si is defined to be between 0.10% and 1.00%, especially when 0.15 wt %<Si %<0.50 wt %, the effect is remarkable.
Mn may form a solid solution together with Fe to improve the strength of ferrite and austenite. Meanwhile, Mn is an element for forming carbide, and may partially substitute for Fe atoms after entering into cementite to increase the hardness of the carbide, thereby finally increasing the hardness of the steel rail. In the steel rail according to the present invention, when the content of Mn in the steel rail is less than 0.50% by weight, a strengthening effect is not satisfactory, and the performances of the steel rail may be slightly improved only through the solid solution effect. When the content of Mn is more than 1.20% by weight, the hardness of the carbide in the steel rail is too high so that the steel rail may not obtain an ideal strength-toughness match, and more importantly, in a controlled cooling process during manufacturing the steel rail, carbon atoms in an austenite state may not be sufficiently diffused at a relatively rapid cooling rate due to an effect of Mn dragging solute atoms, thus a saturated or supersaturated state is formed, and abnormal structures such as bainite, martensite which are prohibited to occur in a pearlite-based steel rail, and the like are easily generated. Therefore, the content of Mn is defined to be between 0.30% and 1.50% in the present invention, especially when 0.50 wt %<Mn %<1.20 wt %, the effect is remarkable.
Al is prone to combine with oxygen in the steel to form Al2O3 or other complex oxides, which may remain in the steel if insufficiently floating, and which, as a heterogeneous phase, may damage continuity of the matrix when the steel rail is used. The inclusion forms a fatigue crack source under a repeated stress, and further expanding of the fatigue crack source may increase a tendency of laterally brittle fracture of the steel rail. Therefore, the content of Al should not exceed 0.005% so as to improve the purity of the steel rail and to ensure the safety.
RE (rare earth element) facilitates deformation of nonmetallic inclusions, while improving the purity of the steel. In addition, RE also decreases the damage of impurities such as S, As, etc. to properties of steel products, and improves the fatigue property of a rail steel. However, when the content of RE is more than 0.05%, it is easy to promote generation of coarse inclusions, thereby seriously deteriorating properties of steel products. As for the steel rail for a high speed or quasi-high speed railway, it is highly important to improve the steel purity and reduce the damage of nonmetallic inclusions to the steel matrix. Therefore, in the present invention, the content range of RE added is defined to less than or equal to 0.05%, especially when the content of RE is more than 0.010 wt % and less than 0.020 wt %, the effect is remarkable.
In the present invention, the total content of V, Cr and Ti is required to be not more than 0.20%. The reasons are as follows: the microstructure and properties of the steel rail are directly determined by the content of C as a main strengthening element of steel, and as the content of C decreases, the ratio of ferrite in the microstructure gradually increases and the ratio of pearlite decreases. Meanwhile, it is difficult for the ferrite as a soft phase in the steel to bear repeated wear of the wheel, and even through a heat treatment, the increment in strength of the ferrite matrix is also limited. Therefore, alloy elements such as V, Cr and/or Ti, etc. are required to be added to strengthen the ferrite matrix so that the wear property may be improved while improving toughness and plasticity of the rail. Hereinafter, the purpose and range of adding the above three alloy elements will be described in detail.
V in the steel has a very low solubility at the room temperature, and usually forms V(C, N) with C and N in the steel to refine grains and to improve toughness and plasticity while strengthening the matrix, and thus is one of the strengthening elements usually used in the carbon steel. In the steel rail according to the present invention, when the content of V is less than 0.15%, the above effects may be well achieved; when the content of V is further increased, the strength will be further improved, while toughness, especially impact performance, is significantly decreased, that is, the ability of the steel rail to resist impact is weakened, which is not suitable for high safety required by the steel rail for high speed railway. When the content of V is less than 0.01%, the strengthening effect is hardly to be exhibited due to a limited amount of the precipitated V. Thus, when V is added alone, the content of V is defined in a range of 0.01% to 0.15%, and especially when the content of V falls within a range of 0.02%≦V %≦0.08%, the effect is more remarkable.
Cr may form a continuous solid solution with Fe and form a variety of carbides with C, and is also one of primary strengthening elements in the steel. In addition, Cr may allow the distribution of the carbides in the steel to be uniform, and improve the wear property of the steel. Compared with V, Cr has a biggest advantage in economy. However, if the content of Cr is relatively high, welding performance may be adversely affected. In the present invention, the ratio of ferrite in the steel increases due to the decrease in the content of C, and thus solid-solution strengthening elements are required to be added to improve the strength of the ferrite so as to ensure the wear property of the rail in use. Meanwhile, since the high speed or quasi-high speed train has a light axle load, the wear is limited. Therefore, the content of Cr is defined in a range of 0.02% to 0.20%, and especially when the content of Cr falls within a range of 0.10%≦Cr %≦0.15%, the effect is more remarkable.
In the steel, Ti refines austenite grains during heating, rolling and cooling, and finally increases the toughness and plasticity of the metallurgical structure as well as rigidity. In the steel rail according to the present invention, when the content of Ti is more than 0.05%, TiC is excessively generated due to Ti being a strong element for forming carbonitride, causing excessively high hardness of the steel rail, and on the other hand, excessive TiC may be concentrated to form coarse carbides, not only reducing the toughness and plasticity, but also making a contact surface of the steel rail be prone to crack and resulting in fracture under an impact load. In the steel rail according to the present invention, when the content of Ti is less than 0.01%, the amount of the formed carbonitride is limited, causing its effect to be hardly exhibited. Therefore, in the present invention, the content of Ti is defined in a range of 0.01% to 0.05%.
The steel rail for the high speed or quasi-high speed railway has a low strength, required elements such as V, Cr, Ti and the like play limited effects of solid-solution strengthening and precipitation strengthening. Meanwhile, the toughness and plasticity has been significantly improved due to the reduction in the carbon content in the present invention, and the wear property of the steel rail may be improved only by the above alloy elements. Accordingly, the total amount of V, Cr and Ti in the steel rail is defined to be not more than 0.20% (0<V+Cr+Ti≦0.20%) in the present invention.
Hereinafter, a method for manufacturing a steel rail for high speed and quasi-high speed railways according to the present invention will be described in detail.
According to the present invention, a method for manufacturing a steel rail for high speed and quasi-high speed railways according to the present invention includes the following steps.
(1) Smelting and Casting Molten Steel
First, a molten steel having the following composition is smelted by using a converter, an electric furnace or an open-hearth furnace: 0.40-0.64% of C, 0.10-1.00% of Si, 0.30-1.50% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, less than or equal to 0.005% of Al, more than 0 and less than or equal to 0.05% of a rare earth element (RE), more than 0 and less than or equal to 0.20% of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities. Then, after LF (Ladle Furnace) refining (i.e., secondary refining) and a vacuum treatment, the molten steel is cast to a billet or a slab, and the billet or the slab is cooled or directly transferred to a heating furnace to increase a temperature thereof.
(2) Rolling Steel Rail
The temperature of a continuously cast billet or slab is increased to a certain temperature (preferably 1200° C.-1300° C.) and kept for 0.5-2 h, and then the continuously cast billet or slab is fed into a rolling machine to be rolled to a steel rail with a required cross-section.
(3) Controlled Cooling after Rolling
The steel rail is generally kept at a temperature of more than 800° C. after finishing rolling, and at this time, the steel rail may achieve various performances by controlling a cooling rate of a rail head portion of the steel rail. For the steel rail still having surplus heat after rolling, because of rolling characteristics of a rolling machine, the steel rail contacts a roll table at rail head side and rail base corner of a side thereof, while only the rail head portion is practically used. In the present invention, the controlled cooling is performed by firstly making the steel rail stand upright on the roll table, and transferring the steel rail to a heat treatment unit through rotation of the roll table. Before this, nozzles of the heat treatment unit for cooling a top side and both lateral sides of the rail head portion has started blowing cooling medium having appropriate pressure and flow rate, generally 2−15 kPa in an atmospheric environment. When the steel rail goes through the nozzles sequentially arranged by rotation of the roll table, the rail head portion is uniformly cooled at a cooling rate of 1-4° C./s. When an infrared temperature detecting device located above the heat treatment unit detects a temperature of the top side of the rail head portion drops to 350-550° C., the controlled cooling is stopped, thereby completing the controlled cooling of the head portion of the steel rail.
In the present invention, a medium for accelerated cooling may be at least one of compressed air, a mixture of water and air, and a mixture of oil and air. Under the teaching of the present invention, those skilled in the art can determine the medium for accelerated cooling to be used based on actual needs. Specifically, in the case of using the compressed air and the mixture of water and air as the medium for accelerated cooling, the ratio therebetween may be determined on the basis of common selections.
(4) Air-Cooling
After the temperature of the head portion of the steel rail reaches a temperature range at which the accelerated cooling is finished, the steel rail is placed in the air to be naturally cooled, and then is treated by subsequent processes.
In addition, an on-line heat treatment process is used in the above step (3). In the present invention, however, an off-line heat treatment process may also be used. The off-line heat treatment is a process in which the steel rail is firstly air-cooled to a room temperature after being rolled, and then heated by an induction heating device to a temperature in austenitic phase zone, typically 900-1100° C., and finally the rail head portion is subjected to accelerated cooling. In particular, after a steel billet or slab is rolled into a steel rail by the aforementioned steps, the steel rail is naturally cooled to a temperature lower than the austenitic phase zone, and then re-heated to a temperature falling in the austenitic phase zone or above 800° C., followed by being subjected to the process of the step (3), thereby obtaining the product of the present invention as well. In the present invention, when a billet or slab is rolled into a steel rail and cooled to a temperature below the austenitic phase zone, the steel rail is heated to a temperature range of 800-1000□ at a rate of 1-20° C./s, and then the process of step (3) is repeated, in which, uniformly cooling is performed on the rail head portion at a cooling rate of 1-4° C./s and is stopped when the temperature of the rail head portion drops to 350-550° C., and subsequently the steel rail is naturally cooled to the room temperature in the air. Here, it should be noted that when the steel rail naturally cooled is re-heated to a temperature in the austenitic phase zone, various heating rates may be applied based on factors such as specific equipment conditions, etc., for example, the steel rail can be either slowly heated to a temperature in the austenitic phase zone at a rate of 1V/s, or rapidly heated to a temperature in the austenitic phase zone at a rate of 20° C./s.
The method of manufacturing a steel rail according to the present invention is substantially the same as that of the prior art, except for the step of controlled cooling after rolling, and thus detailed description of identical contents will be omitted. In the present invention, after the finishing rolling, the rail head portion is uniformly cooled at a cooling rate of 1-4° C./s, and when the temperature of the rail head portion drops to 350-550° C., the cooling is stopped. Performances of a final product is determined by the selection on the cooling processes, and thus in the present invention, when the steel rail containing the above components is cooled at a rate of less than 1 □/s, a strength of the steel rail equivalent to that of an existing steel rail for a high speed or quasi-high speed railway cannot be achieved by refining ferrite and pearlite grains in the microstructure, and an insufficient ferrite matrix strength may cause the steel rail in use to hardly bear vertical loads of a train, so that a top side of a rail head portion has a size deviation due to plastic flow, while generating excessive wear, which not only reduces a service life of the steel rail, but also endangers running safety. On the other hand, when the cooling rate is more than 4° C./s, the diffusion rate of the carbides in the steel reduces to increase a possibility of generation of bainite and martensite structures which are expressly prohibited to occur in a pearlite-based steel rail. Moreover, if the cooling rate is too high, the strength of the steel rail will be significantly increased, and although energy required for crack initiation and propagation may be increased at the same time, cracks which have been generated can not be removed by wear between the wheel and the rail, adversely affecting the running safety.
In the present invention, the temperature at which the accelerated cooling is terminated is 350-550° C. for the reasons as follow. The steel rail containing the above components is accelerated cooled from the austenite phase zone, and phase transition has been completed at a rail surface to a depth of at least 15 mm below the surface at about 550° C.; at this time, heat existing inside the rail head portion will be transferred outwards, and if the accelerated cooling is terminated, the temperature of the surface of the rail may rise due to thermal conduction such that the refined microstructure which has formed is roughened, not facilitating transition of the internal microstructure of the rail head portion at a relatively great degree of supercooling, and thus the effect of heat treatment can not be fully achieved. If the temperature at which the accelerated cooling is terminated is lower than 350° C., the steel rail has entered into a bainite transformation zone, which is not conducive to obtain stable pearlite and ferrite microstructures, thereby increasing a tendency of generating abnormal microstructures.
In the present invention, the accelerated cooling is performed only on a rail head portion, while a rail waist and a rail base are subjected to natural air-cooling to reach a room temperature for reasons as follow. The rail waist of the steel rail, as a connector between the rail head portion and the rail base, indirectly receives a load from a train and needs to have a certain stiffness, while it also receives a normal force generated by steering the train. The rail base applies a force directly to railway sleepers to determine a running trajectory of the train, and finally transfers the load to a track bed. As for the high speed and quasi-high speed railways, a train has an axle load of 11 t-14 t lower than an axle load of 25 t-40 t of a train traveling on a mixed passenger and freight railway or a heavy haul railway, and has a large line curve radius of greater than typically 1000 m, and the rail waist and the rail base can bear limited vertical and normal forces. In addition, the accelerated cooling has a limited effect on toughness and plasticity indices and has no significant effect on the safety of the steel rail in use as compared with air-cooling.
The steel rail obtained by using the method of manufacturing a steel rail according to the present invention may have a mixed microstructure of fine pearlite and fine ferrite (15%-50%) in the rail head, have a strength reaching an equivalent level of strength of an existing steel rail for a high speed or quasi-high speed railway while significantly improving toughness and plasticity and yield strength thereof, improve the ability to resist impact loads while increasing the energy required for crack initiation and propagation of a surface layer of the steel rail, and ultimately improve the rolling contact fatigue properties to protect the transporting safety of the railway. Meanwhile, the method according to the present invention requires no modification in the existing equipments during the manufacturing processes, and thus the manufacturing processes are simple, convenient and flexible.
Hereinafter, the present invention will be described in more detail in conjunction with examples. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.
To obtain a steel rail having a composition as listed in Table 2 below, smelting by a converter, LF refining, vacuum degassing, continuous casting for billet, heating by a billet heating furnace, and rail rolling were sequentially performed, wherein the steel rail was rolled at a finishing rolling temperature of 903° C. and then was placed for 40 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 800° C., compressed air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 3.1° C./s; and when the temperature of the top surface of the rail head portion reached 520° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 1.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 910° C. and then was placed for 45 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 780° C., compressed air and a mixture of oil and air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.9° C./s; and when the temperature of the top surface of the rail head portion reached 514° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 2.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 900° C. and then was placed for 42 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 770° C., a mixture of oil and air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.7° C./s; and when the temperature of the top surface of the rail head portion reached to 530° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 3.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 890° C. and then was placed for 35 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 790° C., a mixture of water and air and a mixture of oil and gas began to be blown so as to uniformly cool the rail head portion at a cooling rate of 3.0° C./s; and when the temperature of the top surface of the rail head portion reached to 495, and temperatures of a rail waist and a rail base were respectively greater than 550° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 4.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 915° C. and then was placed for 50 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 780° C., compressed air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.8° C./s; and when the temperature of the top surface of the rail head portion reached to 528° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 5.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 922° C. and then was placed for 53 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 795° C., compressed air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.1° C./s; and when the temperature of the top surface of the rail head portion reached to 519° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 6.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 918° C. and then was placed for 49 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 800° C., compressed air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.2° C./s; and when the temperature of the top surface of the rail head portion reached to 531° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 7.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 907° C. and then is placed for 48 seconds; after that, when a temperature of a top surface of a rail head portion decreased to 785° C., compressed air and a mixture of water and air began to be blown so as to uniformly cool the rail head portion at a cooling rate of 2.3° C./s; and when the temperature of the top surface of the rail head portion reached to 526° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 8.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. Specifically, in this example, the steel rail was rolled at a finishing rolling temperature of 895° C., was firstly air-cooled to a room temperature, and then a rail head portion was re-heated to 900° C. by using a line-frequency induction heating device at a rate of 5° C./s; after that, when the rail head portion was naturally air-cooled to 760° C., a mixture of water and air and compressed air were blown so as to uniformly cool the rail head portion at a cooling rate of 2.2° C./s; and when the temperature of the top surface of the rail head portion reached 510° C., and temperatures of a rail waist and a rail base were respectively greater than 600° C. after blowing, the steel rail was placed in the air to be naturally cooled to a room temperature, thereby obtaining Sample 9.
Except for steps of controlled cooling after rolling, a steel rail was manufactured by using the same method as that in Example 1. After being rolled into a desired section, the steel rail was directly placed in air to be cooled to a room temperature, thereby obtaining an existing steel rail for a high speed or quasi-high speed railway of Comparative Example 1.
Mechanical properties of the steel rails according to the present invention and the prior art are shown in Table 3 below.
It can be seen from Table 3 above that the steel rails of Examples 1 and 3 according to the present invention have strengths at the same level with the steel rail of Comparative Example 1, but have elongations increased by about 50% than the steel rail of Comparative Example 1. The steel rails of Examples 2 and 8 according to the present invention have tensile strengths (Rm) slightly lower than the steel rail of Comparative Example 1, but have yield strengths (Rel) higher than the steel rail of Comparative Example 1, this will effectively prevent surface fatigue cracks from being generated in the steel rails in use under the same conditions; meanwhile, the steel rails of Examples 2 and 8 may satisfy wear requirements since the practical wear of a steel rail for a high speed railway is small due to a low contact stress between the rail and the wheels. Furthermore, the steel rail of Example 2 according to the present invention has an elongation after fracture increased by about 75% than that of the steel rail of Comparative Example 1, thereby improving the safety in use. Compared with Comparative Example 1, the steel rails of Example 4, Example 6, Example 7 and Example 8 in the present invention have improved strengths and hardnesses, while having plasticities significantly improved, so that the overall performances are improved. As for Example 9 using secondary heating, its performances may also meet the requirements of steel rails for a high speed or quasi-high speed railway because ferrite grains are refined.
Impact energies (Aku) at different temperatures of the steel rails according to the present invention and the prior art are shown in Table 4 below.
It can be seen from Table 4 above that, as compared with the steel rail manufactured according to the prior art, the steel rails manufactured by the method according to the present invention have significantly improved impact toughness at normal and low temperatures, and especially, the toughnesses of the steel rails in Example 2 and Example 8 have been increased to be nearly doubled due to the use of low carbon content and a micro-alloying process. As for the steel rails according to Examples 4 and 6 having relatively high carbon contents without alloying, the impact toughnesses are also improved by 25%. Thus, it can be seen that the reduction in the carbon content and the controlled cooling after rolling are advantageous to improve the toughness of the rail steel. Therefore, the steel rail manufactured by the method of the present invention can provide more effective protection for use safety of trains traveling on high speed railways in a cold area regardless of impact between the rail and the wheel resulting from irregular railway conditions or other reasons.
Wear properties of the steel rails according to the present invention and the prior art are shown in Table 5 below.
The steel rails according to the present invention were ground against the steel rail of the prior art as a comparative sample by means of rolling-sliding wear so that the wear properties of the steel rails are compared at the same conditions. The specific experimental conditions and parameters are as follow:
Type of a test device: Type MM-200;
Sizes of samples: a thickness of 10 mm, an inner diameter of 10 mm, and an outer diameter of 36 mm;
Testing load: 980N;
Sliding difference: 10%;
Testing environment: at a normal temperature and air cooling;
Rotating speed: 200 r/min;
Total rotating numbers of grinding: 200,000; and
Numbers of testing objects: three pairs (their arithmetic mean values were calculated as results).
The results for wear testing are shown in Table 5, and a schematic view showing the wearing is shown in
It can be seen from Table 5 above that the wear property of the steel rail of Example 8 in the present invention is slightly inferior to that of Comparative Example 1. Since a high speed train has a relatively lighter axle load and a steel rail for the high speed train has a relatively lower wear rate, a relatively lower wear property facilitates to remove fatigue cracks generated at a surface of a rail head portion of the steel rail by wearing, and thus greatly helps to improve the rolling contact fatigue property. Wear properties of the steel rails according to Examples 5 and 6 are equivalent to the wear property of the steel rail of Comparative Example 1, and thus the steel rails according to Examples 5 and 6 are also suitable for high speed or quasi-high speed railway applications.
Fatigue crack propagating rates of the steel rails according to the present invention and the prior art are shown in Table 6 below. A device for testing crack propagating rate, ISTRON 8801, was used to study a rule of a rate at which a length or depth of cracks propagates in a direction vertical to a stress direction. The slower the crack propagating rates are, the more beneficial to prevent the cracks from propagating under the same conditions.
It can be seen from Table 6 above that the steel rails manufactured by the method according to the present invention have a crack propagating rate lower than that of the steel rail in Comparative Example 1, and thus the present invention may help to prevent cracks from propagating under the same conditions.
Fracture toughnesses (KIC) at a low temperature (−20° C.) and a normal temperature (20° C.) of the steel rails according to the present invention and the prior art are shown in Table 7 below. A device for testing fracture toughness, ISTRON 8801, was used to measure the fracture toughnesses. The fracture toughness KIC is a mechanical property index exhibiting an ability of a material to resist crack propagation. The higher the value of KIC is, the stronger the ability of the steel rail to resist crack propagation and the safer the train runs.
It can be seen from Table 7 above that the fracture toughnesses of the steel rails manufactured according to the method of the present invention are higher than that of the steel rail of Comparative Example 1 under the same conditions, at both the normal temperature and the low temperature. By comparison, it can be found that the fracture toughness is significantly improved as the carbon content in the steel reduces. Therefore, the reduction in the carbon content of the steel rail helps to obtain higher fracture toughness.
Axial fatigue performances of the steel rails according to the present invention and the steel rail of Comparative Example 1 are shown in Table 8 below. Axial fatigue performances of the steel rails were measured by using a method of increasing and decreasing a stress amplitude by a PQ-6 bending fatigue testing machine under a testing condition that each group of samples has a fatigue lifetime greater than 5×106 when a total strain amplitude is 1350με.
It can be seen from Table 8 above that both the steel rails manufactured according to the method of the present invention and the steel rail manufactured according to the prior art meet standard requirements, and the fatigue limits of the steel rails according to the present invention are higher than the fatigue limit of the steel rail manufactured according to the prior art.
In the existing steel rail for high speed and quasi-high speed railways, the rail head portion has a microstructure of a great amount of pearlite and less than 5% of ferrite, whereas according to the steel rail for high speed and quasi-high speed railways according to the present invention, the rail head portion has a uniformly mixed microstructure of pearlite and 15% to 50% of ferrite at the room temperature by reducing the content of C in the steel rail in conjunction with the controlled cooling after rolling. The steel rail for high speed railways includes ferrite having a ratio increased to 15% to 50% in the microstructure. This is advantageous in that: (1) the existing steel rail for high speed railways has a microstructure containing a dominant component of pearlite and less than 5% of a ferrite structure, and it has been found that wear between the high speed trains and rails barely occurs during a certain period of running, resulting in that it is difficult for the pearlite structure with significantly good wear properties to play its role, and on the contrary, microcracks generated at a rail head surface contacting the wheels will be hardly removed because of no wear, but may expand toward the inside of the steel rail under repeated action from the wheels, and finally form contact fatigue damages such as cracks, drops, etc., which may cause a risk of broken rail. When the ratio of the ferritic structure increases, since ferrite belongs to a soft phase in the steel and has a wear property far inferior to pearlite, the steel rail may have a certain wear generated in use so as to ensure the cracks at the surface of the steel rail to be worn away timely. However, if a certain ratio of ferrite is obtained by simply decreasing the content of C in the steel, the service life of the steel rail may also be adversely affected due to excessive wear. Thus, the expected effect can be achieved only by strengthening the ferrite matrix, and in order to improve the strength of the matrix, there are three ways, i.e., solid solution strengthening of alloy elements, precipitation strengthening, and grain refining strengthening by a heat treatment. If a heat treatment process is performed alone, a strengthening effect from cementite may be enhanced while the strength of the ferrite matrix is increased, which may cause an excessively high strength. Thus, some micro-alloying elements are added to mostly strengthen the ferrite matrix, while slightly improving toughness and plasticity. In addition, if the ratio of ferrite exceeds 50%, the ratio of pearlite will be decreased, which cannot ensure a certain degree of the wear property, also causing the steel rail incapable of being applied to high speed railways. (2) The increase in the ratio of ferrite in the steel rail means a significant enhancement of the toughness and plasticity, and a relatively higher elongation as well as impact toughness will greatly reduce a possibility of broken rail under the same impact load, which is definitely beneficial to ensure the running safety.
In summary, by comparing the metallurgical microstructures, common mechanical properties and special mechanical properties of the steel rail according to the present invention under various conditions with those of the existing steel rail for high speed railways, it can be seen that, in the present invention, by reducing the content of C element in the steel rail in conjunction with the controlled cooling after rolling, the levels of strength and hardness of the existing steel rail for high speed railways are maintained, meanwhile, both the toughness and plasticity and the yield strength of the steel rail are remarkably improved, that is, the energy value required for initiating and expanding microcracks formed at the surface of the steel rail due to fatigue can be increased, and thus under the same conditions, the rolling contact fatigue property of the to steel rail can be improved, thereby finally improving the service lifetime and the transportation safety of the steel rail.
The present invention is not limited to the above embodiments, and various variation and modifications can be made therein without departing from the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201010270439.8 | Sep 2010 | CN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CN2011/079278 | 9/2/2011 | WO | 00 | 3/26/2013 |
