Steel for rail and method for manufacturing the rail

Abstract

To provide steel for rails, excellent in mechanical, electrical, magnetic characteristics such as resistivity, permeability, tensile strength and the like, even under severe operation environment.SOLUTION: Steel for rails is provided, containing 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%, with the balance consisting of iron and inevitable impurities resulting from working. The fine structure of this steel includes 2 to 10% pro-eutectoid ferrite by area percentage, with the remainder being pearlite, and the pearlite has an interlamellar spacing of 100 nm to 250 nm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to steel suitable for the manufacture of rails for railways, in particular for trains that run on magnetic levitation or magnetic guidance, based on the principles of repulsion and attraction. [Background technology]

[0002] Rail steels are being developed for high-speed rail or dual-use rail for both freight and passenger rail. The cargo-carrying capacity of rail has been increasing regardless of the application and is expected to continue to increase in the future. Therefore, it is necessary to develop rail steels that have good mechanical, electrical, and magnetic properties, such as resistivity, magnetic permeability, and tensile strength, even in the harsh working environment of rails.

[0003] Therefore, vigorous research and development efforts are being made to develop materials that have sufficient hardness, high tensile strength at room temperature, and a tensile strength exceeding 900 MPa at 180°C, while also having good resistivity and magnetic permeability.

[0004] Early research and development in the field of steel for railway rails resulted in several methods for producing high strength wear resistant steel for rails, some of which are listed here for a final understanding of the present invention.

[0005] The magnetically active parts for magnetic suspension railways in US4350525 are manufactured from steel having the following composition: 0-0.15% carbon, 0-0.045% phosphorus, 0-0.008% nitrogen, 0.75-2.0% silicon, 0.15-1.00% manganese, 0.02-0.07% aluminum, soluble, 0.25-0.55% copper, 0.65-1.00% chromium, the remainder being iron along with unavoidable impurities; however, the steel in US4350525 has not been demonstrated to reach a tensile strength of 900 MPa at 180°C.

[0006] WO2016019730 discloses an F-type rail for induction cores made of soft magnetic steel, which has a chemical composition of 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%, with P and S each being less than 0.025%, and the remainder being Fe and trace impurities. However, this steel also fails to achieve a strength of 900 MPa at a temperature of 180°C. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 4,350,525 [Patent Document 2] International Publication No. 2016 / 019730 Summary of the Invention

[0008] The object of the present invention is therefore to solve these problems by making available a steel suitable for mechanical operations to manufacture railway rails, which simultaneously has: - tensile strength at 180 ° C of more than 900 MPa, preferably more than 920 MPa - Hardness of at least 310Hv and preferably 315Hv or more - 40Ωcm / m 2 or more, preferably 41 Ωmm / m 2 Resistance above - Maximum magnetic permeability of 165 or greater measured at 4000A / m.

[0009] In a preferred embodiment, the steel according to the invention may also have a tensile strength at room temperature of at least 950 MPa, preferably more than 1000 MPa.

[0010] In a preferred embodiment, the steel according to the invention may also have a polarization of more than 1.5 T at 40000 A / m.

[0011] In a preferred embodiment, the steel according to the invention may also have a flux density of more than 1.5 T measured at 40000 A / m.

[0012] Preferably, such steel is suitable for the manufacture of rails, and also other structural parts of rails, such as chassis members of rail wagons.

[0013] Another object of the present invention is to make available a method for manufacturing these mechanical parts that is stable towards manufacturing parameter shifts, yet compatible with conventional industrial applications. DETAILED DESCRIPTION OF THE INVENTION

[0014] The carbon content of the steel of the present invention is between 0.25% and 0.8%. Carbon is an element necessary for increasing the strength of the steel by forming pearlite. Carbon also ensures the resistivity by aiding in the formation of cementite in lamellar pearlite. However, a carbon content below 0.25% does not provide resistivity or tensile strength due to excessive formation of proeutectoid ferrite. On the other hand, a carbon content above 0.7% adversely affects tensile strength due to excessive formation of proeutectoid cementite during cooling after hot rolling. Furthermore, excessive formation of proeutectoid cementite is detrimental to the rail during its operating life cycle. The carbon content is advantageously in the range of 0.27% to 0.75%, particularly in the range of 0.28% to 0.7%.

[0015] Manganese is added to the present steel in an amount of 1.0% to 2.0%. Manganese provides solid solution strengthening, aids in the formation of cementite in pearlite, and increases hardenability by increasing resistivity. Furthermore, it suppresses the ferrite transformation temperature and slows the ferrite transformation rate, controlling the formation of proeutectoid ferrite and thus aiding in the formation of pearlite. A content of at least 1.0% is necessary not only to aid in the formation of pearlite but also to impart strength. However, manganese content above 2.0% adversely affects the resistivity and permeability of the present steel, as these microstructures adversely affect the steel's resistivity and magnetic permeability. Manganese content above 2.0% can also have adverse effects on the present steel, such as increasing the rate of austenite transformation to martensite or bainite during cooling after hot rolling. Manganese content above 2.0% can excessively segregate in the steel during solidification, impairing the internal homogeneity of the material and potentially causing surface cracking during the hot working process. The preferred limit for manganese presence is 1.0% to 1.8%, more preferably 1.0% to 1.5%.

[0016] Silicon is an essential element present in the steel of the present invention at 1.40% to 2%. Silicon imparts strength to the steel of the present invention through solid solution strengthening and also acts as a deoxidizer. However, since silicon is a ferrite former and also raises the Ac3 transformation point, which pushes the austenite temperature into a higher temperature range, the silicon content is kept to a maximum of 2%. Silicon contents above 2% can also cause temper embrittlement. The preferred limit for silicon presence is 1.45% to 1.8%, more preferably 1.45% to 1.6%.

[0017] The aluminum content is 0.01% to 1%. Aluminum removes oxygen present in molten steel and prevents it from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing the grain size. Aluminum allows the steel of the present invention to control the size of the pearlite lamellar spacing, thereby increasing resistivity while maintaining adequate permeability. High aluminum contents above 1% lead to the formation of coarse aluminum-rich oxides, which worsen the fatigue limit and brittle fracture of steel rails. The preferred limit for the presence of aluminum is 0.02% to 0.9%, more preferably 0.02% to 0.5%.

[0018] Chromium is present in the steel of the present invention at 0.8% to 2%. Chromium is an essential element that imparts strength to steel through solid solution strengthening, and a minimum of 0.2% is required to impart strength. However, use of more than 2% increases hardenability beyond acceptable limits due to the formation of undesirable phases such as bainite after cooling, thereby impairing the ductility of the steel. Chromium additions greater than 2% also reduce the diffusion coefficient of carbon in austenite, thus delaying the formation of pearlite during cooling after hot rolling. The preferred limit for chromium presence is 0.9% to 1.9%, more preferably 0.9% to 1.6%.

[0019] The phosphorus content of the steel of the present invention is 0% to 0.09%. Phosphorus tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, it is recommended to use as little phosphorus as possible. Specifically, a content exceeding 0.09% can cause fracture due to intergranular interface delamination, which can have a negative effect on tensile strength and wear resistance. The preferred limit for the phosphorus content is 0% to 0.05%.

[0020] Sulfur is contained in a range of 0% to 0.09%. Sulfur forms MnS precipitates that can be elongated. These elongated MnS inclusions can have a significant adverse effect on mechanical properties such as hardness and tensile strength if the inclusions are not aligned in the load direction. Therefore, the sulfur content is limited to 0.09%. The preferred range of sulfur content is 0% to 0.05%, and more preferably 0% to 0.02%.

[0021] Nitrogen is present in the steel of the present invention in an amount of 0% to 0.09%. Nitrogen is limited to 0.09% to avoid material aging, which has a negative effect on the mechanical properties of the steel, and to prevent the precipitation of coarse aluminum nitrides during solidification. Nitrogen also forms nitrides and carbonitrides with vanadium, titanium, and niobium, imparting strength to the steel of the present invention.

[0022] Nickel is an optional element and is added to the present invention at 0% to 1% to increase the strength of the steel of the present invention. Nickel is beneficial for improving its pitting corrosion resistance. The addition of nickel to the steel composition reduces the diffusion coefficient of carbon in austenite, thereby promoting the formation of ferrite in pearlite. However, nickel contents greater than 1% can lead to the stabilization of retained austenite, thereby adversely affecting tensile strength. It is preferred that the steel of the present invention have 0% to 0.9% nickel.

[0023] Molybdenum is an optional element and may be present in the present invention at 0% to 0.5%. Molybdenum is added to impart hardenability and hardness to steel by forming molybdenum-based carbides. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit for the molybdenum content is 0% to 0.4%, more preferably 0% to 0.2%.

[0024] Vanadium is an optional element in the present invention, and its content is 0% to 0.2%. Vanadium has the effect of increasing the strength of steel, particularly by forming carbides or carbonitrides, which cause precipitation strengthening. For economic reasons, the upper limit is kept at 0.2%.

[0025] Niobium is present in the steel of the present invention at 0% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also influences the size of microstructural constituents through its precipitation as carbonitrides and by retarding recrystallization during the heating process, thus refining the grain size. However, niobium contents greater than 0.1% form coarse precipitates that are not economically attractive and have a negative effect on the tensile strength of the steel. Niobium contents greater than 0.1% also have a negative effect on the hot ductility of the steel, leading to difficulties during casting and rolling of the steel.

[0026] Titanium is an optional element present at 0% to 0.1%. Titanium forms titanium nitrides which impart strength to the steel and refine the grain size. The preferred limit for titanium is 0% to 0.05%.

[0027] Copper is a residual element and may be present up to 0.5% depending on the processing of steel. Up to 0.5% copper does not affect any of the steel's properties, but above 0.5%, hot workability is significantly reduced.

[0028] 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%. These elements, up to the maximum content levels indicated, allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of the steel includes:

[0030] Pearlite is a matrix microstructural component of the present steel, and its presence by area percentage must be at least 90%, preferably 90% to 99%, and more preferably 93% to 98%. Pearlite is formed during the second cooling step after hot rolling. The pearlite of the present steel has a lamellar structure. The lamellar structure of the present pearlite is an aggregate of ferrite and cementite, and the interlayer spacing of the present pearlite is 100 to 250 nanometers. This interlayer spacing improves the in-service properties of the present steel, such as tensile strength and resistivity. If the interlayer spacing exceeds 250 nanometers, the steel becomes soft and cannot reach the tensile strength, especially at 180°C. When the pearlite interlayer spacing is less than 100 nanometers, the magnetic permeability of the steel is adversely affected. The preferred limit for the interlayer spacing is 110 to 230 nanometers, more preferably 120 to 220 nanometers. The pearlite of the present invention also imparts in-service properties to the steel such as magnetic permeability and hardness.

[0031] The steel of the present invention contains 2% to 10% pro-eutectoid ferrite. Pro-eutectoid ferrite is formed at the grain boundaries of prior austenite grains in the first cooling step after hot rolling, and is dispersed within pearlite. Pro-eutectoid ferrite imparts ductility as well as magnetic permeability to the steel. If the pro-eutectoid ferrite content exceeds 10%, the steel of the present invention will not be able to achieve the required hardness. The preferred limit for the presence of pro-eutectoid ferrite is 3% to 9%, more preferably 3% to 8%.

[0032] In addition to the above microstructure, microstructural constituents such as bainite, martensite and retained austenite are absent from the rail microstructure.

[0033] Rails according to the present invention can be manufactured by any suitable manufacturing process, using the prescribed process parameters described below.

[0034] Although preferred exemplary methods are demonstrated herein, the examples do not limit the scope of the disclosure and the underlying aspects of the embodiments. Furthermore, any examples described herein are not intended to be limiting, but merely to describe some of the many possible ways in which various aspects of the present disclosure may be implemented.

[0035] A preferred method comprises providing a semi-finished casting of steel having a chemical composition according to the invention, in any form such as an ingot or bloom or billet, which can be manufactured or processed into rails for railways, in particular for magnetic levitation rails.

[0036] For example, steel having the above chemical composition may be cast into a billet and then rolled into a bar, which may serve as a semi-finished product for further rolling. Multiple rolling steps may be performed to obtain the desired semi-finished product.

[0037] To prepare the steel for production as rails, the semi-finished product may be used hot directly after rolling, or it may be first cooled to room temperature and then reheated to produce rails.

[0038] The semi-finished product is reheated to a temperature of Ac3 to Ac3 + 500°C, preferably Ac3 + 30°C to Ac3 + 450°C, more preferably 1100°C to 1300°C, and held for 5 to 1200 seconds to ensure 100% austenite formation and uniform temperature across the cross section of the semi-finished product. Ac3 is calculated (calculated) according to KASATKIN, OG, 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.

[0039] If the reheating temperature of the semifinished product is lower than Ac3, excessive loads may be applied during rolling, causing the steel temperature to drop below the ferrite transformation start temperature, which would lead to ferrite formation during hot rolling. Furthermore, metallurgical transformations under strain can significantly change the resulting microstructure for a given cooling rate or chemical composition. As a result, the resulting microstructure may be quite different from the target, resulting in different mechanical and electrical properties. Therefore, the temperature of the semifinished product should be high enough so that all mechanical operations are performed and completed in the 100% austenite temperature range. Reheating at temperatures above Ac3 + 500°C should be avoided, as it is industrially expensive and may lead to the development of liquid regions that affect the rolling of the steel.

[0040] The semi-finished product is then subjected to at least one pass of hot rolling at Ac3 to Ac3 + 300°C, preferably with a reduction of 35 to 90%. Hot rolling can be done in multiple passes as required to have a hot rail from the semi-finished product. The preferred temperature for all hot rolling passes is Ac3 + 30°C to Ac3 + 300°C, with the more preferred temperature being Ac3 + 50°C to Ac3 + 250°C.

[0041] The final rolling temperature must be kept above Ac3, which is favorable for recrystallization and a structure favorable for mechanical fabrication. It is preferable to carry out all rolling passes, especially the final rolling temperature, at temperatures above 1000°C, because below this temperature the steel exhibits a significant decrease in rollability. A final rolling temperature below Ac3 can lead to problems with the final rail dimensions and a deterioration in the surface morphology. This can even cause cracks or complete failure of the rail.

[0042] The hot rail is then cooled in a two-stage cooling process, the first stage of cooling starting at the exit of the final hot rolling mill, in which 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 of such first stage of cooling is 0.1°C / s to 3°C / s, more preferably 0.1°C / s to 2°C / s. The preferred T1 temperature of such first stage is 490°C to 530°C, more preferably 490°C to 510°C.

[0043] In the second cooling step, 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 in the second cooling step is less than 3°C / s, more preferably less than 1°C / s.

[0044] In a preferred embodiment, CR1 is higher than CR2.

[0045] Once the hot rail has reached room temperature, the rail is obtained from the steel of the invention. [Example]

[0046] The following tests, examples, illustrative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0047] Table 1 summarizes the rails made with steels of different compositions, where rails are manufactured according to the process parameters specified in Table 2, respectively. Table 3 then summarizes the microstructures of the rails obtained during the trials, and Table 4 summarizes the evaluation results of the obtained properties.

[0048] [Table 1]

[0049] Table 2 summarizes the process parameters carried out on semi-finished products made from the steels of Table 1. Trials I1 to I3 serve to manufacture rails according to the invention. Table 2 is as follows:

[0050] [Table 2]

[0051] Ac3 values ​​were determined according to KASATKIN, OG, 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.

[0052] Table 3 illustrates the results of tests carried out according to standards on different microscopes, such as scanning electron microscopes, for determining the microstructure in area fractions of both the steel of the invention and the reference steel. The results are specified herein.

[0053] [Table 3]

[0054] Table 4 illustrates the mechanical and magnetic properties of both the steel of the present invention and the reference steel. To determine the tensile strength, tests are performed according to the NF EN ISO 6892-1 / 2017 standard. Tests to measure the resistivity and magnetic permeability of both the steel of the present invention and the reference steel are performed according to IEC-60404-13 and IEC-60404-4, respectively. Tests to measure the hardness of both the steel of the present invention and the reference steel are performed according to EN-13674. The results of the various mechanical tests performed according to the standards are summarized below.

[0055] [Table 4]

Claims

1. Rail steel containing the following elements expressed in weight percent: 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 optionally the following 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 of the composition being composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel contains, in area percentage, 2 to 10% proeutectoid ferrite and the remainder being made up of pearlite, the pearlite having an interlamellar spacing of 100 nm to 250 nm.

2. 3. Steel for rails according to claim 1 or 2, wherein said composition comprises 0.27% to 0.75% carbon.

3. 3. Steel for rails according to claim 1, wherein the composition comprises 0.02% to 0.9% aluminum.

4. Steel for rails according to any one of claims 1 to 3, wherein said composition comprises 0.9% to 1.9% chromium.

5. 5. Steel for rails according to claim 1, wherein the pearlite is between 93% and 99%.

6. 6. Steel for rails according to claim 1, wherein the interlayer spacing of pearlite is 110 nm to 230 nm.

7. Steel for rails according to any one of claims 1 to 8, having a tensile strength at 180 ° C. of more than 900 MPa.

8. The steel for rails according to any one of claims 1 to 7, having a hardness of 310 Hv or more.

9. 40 Ωmm / m ２ 9. Steel for rails according to claim 1, having a resistivity of more than

10. Steel for rails according to any one of claims 1 to 9, having a maximum magnetic permeability of 165 or more measured at 4000 A / m.

11. A method for manufacturing steel rails comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 4 in the form of a semi-finished product, - reheating the semi-finished product to a temperature of Ac3 to Ac3+500°C and holding it there for 5 to 1200 seconds; - subjecting said semi-finished product to one or more hot rolling passes in the austenitic range of hot rolling temperatures between Ac3 and Ac3+300°C to obtain a hot rail; - cooling the hot rail in two stages, in which in stage 1, said hot rail is cooled at a cooling rate of between 0.1°C / s and 5°C / s from a temperature of Ac3 and Ac3+300°C to a temperature T1 in the range of 480-550°C; then, in step 2, cooling said hot rail from T1 to room temperature at a cooling rate of less than 5°C / s to obtain a rail.

12. 12. The method of claim 11, wherein the reheat temperature of the semi-finished product is from Ac3+30°C to Ac3+450°C.

13. 13. The method according to claim 11, wherein the temperature T1 is between 490°C and 530°C.

14. The method according to any one of claims 11 to 13, wherein the CR1 cooling rate is higher than CR2.

15. Use of a steel according to any one of claims 1 to 10 or a rail manufactured by the method according to claims 11 to 14 for the manufacture of structural or safety parts of a rail wagon.

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