Railway track and method of manufacturing a railway track

Abstract

Railway steel comprising the following elements: 0.25% ≤ C ≤ 0.8%; 1.0% ≤ Mn ≤ 2.0%; 1.40% ≤ Si ≤ 2%; 0.01% ≤ Al ≤ 1%; 0.8% ≤ Cr ≤ 2%; 0 ≤ P ≤ 0.09%; 0 ≤ S ≤ 0.09%; 0% ≤ N ≤ 0.09%; 0% ≤ Ni ≤ 1%; 0% ≤ Mo ≤ 0.5%; 0% ≤ V ≤ 0.2%; 0% ≤ Nb ≤ 0.1%; 0% ≤ Ti ≤ 0.1%; 0% ≤ Cu ≤ 0.5%; 0% ≤ B ≤ 0.008%; 0% ≤ Sn ≤ 0.1%; 0% ≤ Ce ≤ 0.1%; 0% ≤ Mg ≤ 0.10%; 0% ≤ Zr ≤ 0.10%; the remainder of the composition consisting of iron and unavoidable impurities resulting from the processing, the microstructure of said steel comprising, in area percentage, 2% to 10% of proeutectoid ferrite, the balance consisting of pearlite, wherein the interlamellar distance of the pearlite is between 100 nm and 250 nm.

Description

[0001] The present invention relates to a steel suitable for manufacturing a rail for railways, and in particular for trains operating in magnetic levitation or magnetic guidance based on repulsion and attraction principles.

[0002] The rail steel is developed for high speed railways or for dual purpose for freight and passenger railways. The load carrying capacity of the railway is increased irrespective of the purpose and is expected to increase in future. Therefore, it is necessary to develop a rail steel which is good in mechanical, electrical and magnetic properties such as electrical resistivity, magnetic permeability and tensile strength even in harsh working environment of the railway.

[0003] Therefore, a lot of research and development efforts have been put in to develop a material which is good in electrical resistivity and magnetic permeability while having high tensile strength at room temperature as well as at a temperature of 180°C and sufficient hardness.

[0004] The early research and development in the field of rail steel for railways has resulted in several methods for producing high strength and wear resistant rail steel, some of which are enumerated herein for clear understanding of the present invention:

[0005] US4350525 magnetic levitation railway magnetically active parts are made of a steel having a composition of 0% to 0.15% carbon, 0% to 0.045% phosphorous, 0% to 0.008% nitrogen, 0.75% to 2.0% silicon, 0.15% to 1.00% manganese, 0.02% to 0.07% soluble aluminium, 0.25% to 0.55% copper, 0.65% to 1.00% chromium, the remainder iron and unavoidable impurities, but the steel of US4350525 does not show a tensile strength of 900 MPa at 180°C.

[0006] WO2016019730 is a F-shaped rail for an induction core made of a soft magnetic steel and the chemical composition of the soft magnetic steel is: C: 0.005% to 0.15% by weight, Mn: 0.25% to 0.60%, Si: 0.30% to 1.0%, Re: 0.003% to 0.006%, both P and S less than 0.025%, the balance being Fe and trace impurities, but this steel also does not achieve a strength of 900 MPa at a temperature of 180°C.

[0007] Therefore, the object of the present invention is to solve these problems by making available a steel suitable for manufacturing a rail for railways which is good in mechanical operation while having:

[0008] - a tensile strength at 180°C greater than or equal to 900 MPa and preferably higher than 920 MPa,

[0009] - a hardness of at least 310 HV or more and preferably more than 315 HV or more,

[0010] - 40 Ωmm 2 / m or more and preferably 41 Ωmm 2 / m or more,

[0011] - a maximum magnetic permeability measured at 4000 A / m of 165 or more.

[0012] In a preferred embodiment, the steel according to the application can also have a tensile strength at room temperature greater than or equal to 950 MPa and preferably higher than 1000 MPa.

[0013] In a preferred embodiment, the steel according to the application can also have a polarisation measured at 40000 A / m greater than 1.5 T.

[0014] In a preferred embodiment, the steel according to the application can also have a magnetic flux density measured at 40000 A / m greater than 1.5 T.

[0015] Preferably, such a steel is suitable for manufacturing a guide rail and the steel is also suitable for other structural parts of a guide rail such as chassis members of a guide rail transport vehicle.

[0016] Another object of the present application is also to make available a method for manufacturing these mechanical parts which is compatible with conventional industrial applications while being robust to variations in manufacturing parameters.

[0017] Carbon is present in the steel of the application between 0.25% and 0.8%, carbon being an element required for increasing the strength of the steel of the application by generating pearlite. Carbon also ensures the electrical resistivity by contributing to the formation of cementite in the lamellar pearlite. But a carbon content less than 0.25% would not confer the tensile strength and the electrical resistivity due to the excessive formation of proeutectoid ferrite. On the other hand, at a carbon content exceeding 0.7%, the tensile strength is adversely affected due to the excessive formation of proeutectoid cementite during cooling after hot rolling. The further excessive formation of proeutectoid cementite during the operating life cycle of the guide rail is also detrimental to the guide rail. The carbon content is advantageously in the range of 0.27% to 0.75%, and more particularly 0.28% to 0.7%.

[0018] Manganese is added in the steel of the present application in the range of 1.0% to 2.0%. Manganese provides solid solution strengthening by helping to form cementite in the pearlite, increases hardenability, thereby increasing the electrical resistivity. Further, it also suppresses the ferrite transformation temperature and reduces the rate of ferrite transformation to control the formation of pro-eutectoid ferrite, thus helping to form pearlite. A quantity of at least 1.0% is required to impart strength as well as to help in the formation of pearlite. But when the manganese content is greater than 2.0%, it produces adverse effects such as it accelerates the transformation of austenite to martensite or bainite during cooling after hot rolling, which is detrimental to the steel of the present application as these microstructures adversely affect the electrical resistivity and permeability of the steel of the present application. Manganese content higher than 2.0% can also produce excessive segregation in the steel during solidification, as well as impair the homogeneity within the material, which can lead to surface cracks during the hot working process. The preferred limit of the presence of manganese is 1.0% to 1.8% and more preferably 1.0% to 1.5%.

[0019] Silicon is an essential element present in the steel of the present application in the range of 1.40% to 2%. Silicon imparts strength to the steel of the present application by solid solution strengthening and also acts as a deoxidizer. But since silicon is a ferrite forming element and also increases the Ac3 transformation point, which will push the austenite temperature to a higher temperature range, this is the reason for keeping the content of silicon to a maximum of 2%. Silicon content higher than 2% can also lead to temper embrittlement. The preferred limit of the presence of silicon is 1.45% to 1.8% and more preferably 1.45% to 1.6%.

[0020] The content of aluminum is 0.01% to 1%. Aluminum removes oxygen present in the molten steel to prevent the formation of gaseous phases during solidification. Aluminum also fixes nitrogen in the steel to form aluminum nitride, thereby reducing the size of the grains. Aluminum controls the size of the interlamellar distance of the pearlite and thereby increases the electrical resistivity while retaining sufficient permeability in the steel of the present application. Higher content of aluminum higher than 1% leads to the presence of coarse rich aluminum oxides which deteriorate the fatigue limit and brittle fracture of the steel rail. The preferred limit of the presence of aluminum is 0.02% to 0.9% and more preferably 0.02% to 0.5%.

[0021] Chromium is present in the steel of the present application in the range of 0.8% to 2%. Chromium is an essential element which provides strength to the steel by solid solution strengthening and a minimum of 0.2% is required to impart strength, but when used higher than 2%, the hardenability is increased beyond acceptable limits as undesirable phases such as bainite are formed after cooling, thereby impairing the ductility of the steel. Chromium addition higher than 2% also reduces the diffusion coefficient of carbon in austenite, thus delaying the formation of pearlite during cooling after hot rolling. The preferred limit of the presence of chromium is 0.9% to 1.9% and more preferably 0.9% to 1.6%.

[0022] The phosphorus content of the steel of the present invention is 0% to 0.09%. Phosphorus tends to segregate at the grain boundaries or co-segregate with manganese. For these reasons, it is recommended to use phosphorus as little as possible. In particular, a content higher than 0.09% can cause cracking due to intergranular interface disgregation, which can be detrimental to the tensile strength and wear resistance. The preferred limit of the phosphorus content is 0% to 0.05%.

[0023] Sulfur is included in the range of 0% to 0.09%. Sulfur forms MnS precipitates that can become elongated. If such elongated MnS inclusions are not aligned with the loading direction, the inclusions can have a considerable adverse effect on mechanical properties such as hardness and tensile strength. Therefore, the sulfur content is limited to 0.09%. The preferred range of the sulfur content is 0% to 0.05%, and more preferably 0% to 0.02%.

[0024] Nitrogen is in the steel of the present invention in an amount of 0% to 0.09%. Nitrogen is limited to 0.09% to avoid aging of the material and to prevent the precipitation of coarse aluminum nitride during solidification, which is detrimental to the mechanical properties of the steel. Nitrogen also forms nitrides and carbonitrides with vanadium, titanium, and niobium to impart strength to the steel of the present invention.

[0025] Nickel is an optional element and is added to the present invention in the range of 0% to 1% to increase the strength of the steel of the present invention. Nickel is beneficial in improving its pitting corrosion resistance. Nickel is added to the steel composition to reduce the diffusion coefficient of carbon in austenite, thereby promoting the formation of ferrite in pearlite. However, the presence of nickel content higher than 1% can cause the stabilization of retained austenite, which has a detrimental effect on the tensile strength. It is preferred to have nickel in the range of 0% to 0.9% in the steel of the present invention.

[0026] Molybdenum is an optional element and can be present in the present invention in the range of 0% to 0.5%. Molybdenum is added to impart hardenability and hardness to the steel by forming molybdenum-based carbides. However, the excessive addition of molybdenum increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit of the molybdenum content is 0% to 0.4%, and more preferably 0% to 0.2%.

[0027] Vanadium is an optional element of the present invention and has a content of 0% to 0.2%. Vanadium is effective in strengthening the strength of the steel through precipitation strengthening, particularly by forming carbides or carbonitrides. The upper limit is kept at 0.2% for economic reasons.

[0028] Niobium is present in the steel of the present invention from 0% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium will also influence the size of the microstructure constituents by its precipitation as carbonitrides and by retarding recrystallization during the heat process and thus refine the grain size. However, niobium contents higher than 0.1% are not economically interesting, as well as forming coarser precipitates which are detrimental to the tensile strength of the steel, furthermore, when the content of niobium is 0.1% or more, niobium is also detrimental to the hot ductility of the steel, creating difficulties during the steel casting and rolling.

[0029] Titanium is an optional element and is present from 0% to 0.1%. Titanium forms titanium nitrides which impart strength to the steel and refines the grains. The preferred limit of titanium is from 0% to 0.05%.

[0030] Copper is a residual element and can be present up to 0.5% due to the processing of the steel. Copper up to 0.5% does not affect any of the properties of the steel, but above 0.5% the hot workability is significantly reduced. And other elements such as tin, cerium, magnesium, boron or zirconium can be added individually or in combination in the following weight proportions: tin =< 0.1%, cerium =< 0.1%, magnesium =< 0.10%, 0% =< boron =< 0.008% and zirconium =< 0.10%. Up to the maximum content levels shown, these elements make it possible to refine the grains during solidification. The remainder of the composition of the steel is composed of iron and unavoidable impurities resulting from the processing.

[0031] The microstructure of the steel comprises:

[0032] The pearlite is the matrix microstructure constituent of the steel of the present invention and the area percentage must be at least 90% or more, and it is preferably from 90% to 99%, and more preferably from 93% to 98%. The pearlite is formed during the second cooling after hot rolling. The pearlite of the steel of the present invention is a lamellar structure. The lamellar structure of the pearlite of the present invention is an aggregate of ferrite and cementite, and the interlamellar distance of the pearlite of the present invention is from 100 nanometers to 250 nanometers. This interlamellar distance improves the use properties of the steel of the present invention such as tensile strength and electrical resistivity. When the interlamellar distance is greater than 250 nanometers, the steel will be soft and cannot reach the tensile strength, especially at 180°C, while whenever the interlamellar distance of the pearlite is less than 100 nanometers, the magnetic permeability of the steel is adversely affected. The preferred limit of the interlamellar distance is from 110 nanometers to 230 nanometers, and more preferably from 120 nanometers to 220 nanometers. The pearlite of the present invention also imparts use properties to the steel like magnetic permeability and hardness.

[0033] Pro-eutectoid ferrite is present in the steel of the present invention in an amount of 2 to 10 %. The pro-eutectoid ferrite forms on the grain boundaries of the prior austenite grains during the first cooling step after hot rolling and the pro-eutectoid ferrite is dispersed within the pearlite. The pro-eutectoid ferrite provides the steel of the present invention with ductility and magnetic permeability. If the content of the pro-eutectoid ferrite is greater than 10 %, the steel of the present invention will not be able to achieve the hardness. The preferred limit of the presence of the pro-eutectoid ferrite is 3 to 9 %, and more preferably 3 to 8 %.

[0034] In addition to the above described microstructure, the microstructure of the guideway does not contain microstructural constituents such as bainite, martensite and retained austenite.

[0035] The guideway according to the present invention can be produced by any suitable manufacturing process with the process parameters specified hereinafter.

[0036] Preferred exemplary methods are described herein, but the examples do not limit the scope of the disclosure and the aspects on which the examples are based. Additionally, none of the examples set forth in this specification are intended to be limiting, but rather merely to illustrate some of the many possible ways in which aspects of the disclosure can be practiced.

[0037] The preferred method comprises providing a semi-finished casting of a steel having a chemical composition according to the present invention. The casting can be finished in any form that can be manufactured or machined into a guideway for a railway, and in particular for magnetic levitation, for example an ingot or a bloom or a billet.

[0038] For example, a steel having the above described chemical composition is cast into a billet, which is then rolled in the form of a profiled bar that can serve as a semi-finished product for further rolling. Multiple rolling steps can be performed to obtain the desired semi-finished product.

[0039] To prepare the steel to be manufactured as a guideway, the semi-finished product can be used directly after rolling at high temperature, or it can first be cooled to room temperature and then reheated for manufacturing the guideway.

[0040] The semi-finished product is reheated at a temperature of Ac3 to Ac3+ 500°C, preferably Ac3+ 30°C to Ac3+ 450°C, and more preferably 1100°C to 1300°C, at which it is maintained for a time of 5 seconds to 1200 seconds to ensure a uniform temperature across the section of the semi-finished product and to ensure the formation of 100% austenite. The Ac3 is calculated according to KASATKIN, O.G. et al. Calculation Models for Determining the Critical Points of Steel in Metal Science and Heat Treatment, 26: 1-2, January-February 1984, 27-31.

[0041] If the reheating temperature of the semi-finished product is lower than Ac3, an excessive load is applied during rolling, in addition, the temperature of the steel can also decrease below the ferrite transformation start temperature, which will lead to the formation of ferrite during hot rolling. Additionally, for a given cooling rate or for a given chemical composition, the metallurgical transformation under strain can cause a significant change in the microstructure obtained. Thus, the microstructure obtained will be completely different from the target microstructure and therefore the mechanical properties as well as the electrical properties will be completely different. Therefore, the temperature of the semi-finished product is preferably high enough so that all the mechanical operations are carried out and completed in the 100% austenite temperature range. It is necessary to avoid reheating at a temperature higher than Ac3+ 500°C, because it is industrially expensive and can lead to the appearance of liquid regions that will affect the rolling of the steel.

[0042] The semi-finished product is then subjected to at least one hot rolling pass of Ac3 to Ac3+ 300°C, preferably with a reduction of 35% to 90%. The hot rolling can be done in several passes as required to obtain the hot lead from the semi-finished product. The preferred temperature for all the hot rolling is Ac3+ 30°C to Ac3+ 300°C, and the more preferred temperature is Ac3+ 50°C to Ac3+ 250°C.

[0043] It is necessary to maintain the finishing temperature above Ac3 and this is preferably in favor of recrystallization and the mechanical production of the structure. It is preferable that all the rolling passes, especially the finishing temperature, are carried out at a temperature greater than 1000°C, because below this temperature, the steel exhibits a significant decrease in rollability. In the case where the finishing temperature is lower than Ac3, it can lead to problems of deterioration of the final dimensions of the lead as well as the surface appearance. It can even cause cracking or complete failure of the lead.

[0044] The hot rail is then cooled in a two-step cooling process, wherein the first step cooling, starting from the exit of the final hot rolling, the hot rail is cooled at a cooling rate CR1 of 0.1 °C / s to 5 °C / s to a temperature T1 in the range of 480 °C to 550 °C. In a preferred embodiment, the cooling rate CR1 for such first step cooling is 0.1 °C / s to 3 °C / s, and more preferably 0.1 °C / s to 2 °C / s. The preferred T1 temperature for such first step is 490 °C to 530 °C, and more preferably 490 °C to 510 °C.

[0045] In the second step cooling, the hot rail is cooled from T1 to room temperature at a cooling rate CR2 of less than 5 °C / s. In a preferred embodiment, the cooling rate CR2 for the second step cooling is less than 3 °C / s, and more preferably less than 1 °C / s.

[0046] In a preferred embodiment, CR1 is higher than CR2.

[0047] When the hot rail reaches room temperature, the rail is obtained from the steel of the present application. Example

[0048] The following tests, examples, graphical examples and tables presented herein are essentially non-limiting and must be considered only for illustrative purposes, and will show the advantageous features of the present application.

[0049] The rails made from steels with different compositions are summarized in Table 1, wherein the rails were produced according to the process parameters as noted in Table 2, respectively. Thereafter, Table 3 summarizes the microstructure of the rails obtained during the trial, and Table 4 summarizes the evaluation results of the properties obtained.

[0050] Table 1

[0051]

[0052] Table 2

[0053] Table 2 summarizes the process parameters carried out on the semi-finished products made from the steels of Table 1. Trials I1 to I3 were used to manufacture rails according to the present application. Table 2 is as follows:

[0054]

[0055] The Ac3 value is determined by KASA TKIN, O.G. et al. Calculation Models for Determining the Critical Points of Steel in Metal Science and Heat Treatment, 26: 1-2, January-February 1984, 27-31.

[0056] Table 3

[0057] Table 3 illustrates the results of tests according to standards performed on different microscopes such as scanning electron microscope for determining the microstructure of both the inventive steel and the reference steel in area fraction. The results are mentioned here:

[0058] Steel sample % pearlite % proeutectoid ferrite Interlamellar distance of pearlite (nm) I1 95 5 125 I2 95 5 170 I3 97 3 211

[0059] Table 4

[0060] Table 4 illustrates the mechanical and magnetic properties of both the inventive steel and the reference steel. For determining the tensile strength, tests were performed according to the NF EN ISO 6892-1 / 2017 standard. The tests for measuring the resistivity and permeability of both the inventive steel and the reference steel were performed according to IEC-60404-13 and IEC-60404-4 respectively. The tests for measuring the hardness of both the inventive steel and the reference steel were performed according to EN-13674. The results of various mechanical tests performed according to standards are summarized.

[0061] Table 4:

[0062]

Claims

1. A steel for guide rails, comprising the following elements expressed in weight percentage: 0.25％≤C≤0.8％； 1.0% < Mn < 2.0%; 1.40% < Si < 2%; 0.01％≤Al≤1％； 0.8％≤Cr≤2％； 0≤P≤0.09％； 0≤S≤0.09％； 0％≤N≤0.09％； and possibly one or more of the following optional elements: 0% < Ni < 1%; 0% < Mo < 0.5%; 0％≤V≤0.2％； 0% < Nb < 0.1%; 0% < Ti < 0.1%; 0% < Cu < 0.5%; 0％≤B≤0.008％； 0% < Sn < 0.1%; 0% < Ce < 0.1%; 0% < Mg < 0.10%; 0％≤Zr≤0.10％； the remainder consisting of iron and unavoidable impurities resulting from the processing, the microstructure of said steel comprising, in area percentage, 2% to 10% of pro-eutectoid ferrite, the balance consisting of pearlite, wherein the interlamellar distance of said pearlite is comprised between 100 nm and 250 nm.

2. The steel for guide rails according to claim 1, wherein the composition comprises 0.27% to 0.75% of carbon.

3. The steel for guide rails according to claim 1 or 2, wherein the composition comprises 0.02% to 0.9% of aluminum.

4. The steel for guide rails according to claim 1 or 2, wherein the composition comprises 0.9% to 1.9% of chromium.

5. The rail steel of claim 1 or 2, wherein, said pearlite is 93% to 99%.

6. The rail steel of claim 1 or 2, wherein, said interlamellar distance of the pearlite is comprised between 110 nm and 230 nm.

7. The rail steel of claim 1 or 2, wherein, the tensile strength at 180°C is greater than 900 MPa.

8. The steel for guide rails according to claim 1 or 2, wherein the hardness of said steel is 310 HV or greater.

9. The rail steel of claim 1 or 2, wherein the steel has an electrical resistivity greater than 40 Ωmm 2 / m.

10. The steel for guide rails according to claim 1 or 2, wherein the maximum magnetic permeability of said steel measured at 4000 A / m is equal to greater than 165 or greater.

11. A method for producing a guide rail of steel, comprising the following sequential steps: - providing a steel composition according to any one of claims 1 to 4 in the form of a semi-finished product; - re-heating said semi-finished product to a temperature comprised between Ac3 and Ac3+500°C and maintaining said semi-finished product at said temperature comprised between Ac3 and Ac3+500°C for 5 seconds to 1200 seconds; - subjecting said semi-finished product to one or more hot rolling passes in the austenitic range, wherein the hot rolling temperature is comprised between Ac3 and Ac3+300°C to obtain a hot guide rail; - cooling the hot guide rail in two steps, wherein in step one, said hot guide rail is cooled from a temperature comprised between Ac3 and Ac3+300°C to a temperature T1 comprised between 480°C and 550°C at a cooling rate comprised between 0.1°C / s and 5°C / s, - thereafter, in step two, said hot guide rail is cooled from T1 to room temperature at a cooling rate lower than 5°C / s to obtain a guide rail.

12. The method according to claim 11, wherein the re-heating temperature of said semi-finished product is comprised between Ac3+30°C and Ac3+450°C.

13. The method according to any one of claims 11 or 12, wherein said temperature T1 is comprised between 490°C and 530°C.

14. The method according to claim 11 or 12, wherein the CR1 cooling rate is higher than CR2.

15. Use of a steel according to any one of claims 1 to 10 or of a guide rail produced according to the method of any one of claims 11 to 14 for manufacturing structural or safety parts of a guide rail transport car.

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