CN116472358A - Guide rail steel and method for manufacturing guide rail thereof - Google Patents

Abstract

A steel for guide rail comprising the following elements: 0.25% C0.8%; 1.0% or less Mn or less than 2.0%;1.40% Si 2%;0.01% or less Al or less than 1%;0.8% or less Cr or less than 2%; 0.09% or less of P.ltoreq.0.09%; 0.ltoreq.S.ltoreq.0.09%; 0% +.ltoreq.N.ltoreq.0.09%; 0% or less Ni or less than 1%;0% +.o.mo.o.0.5%; 0% +.ltoreq.V.ltoreq.0.2%; 0% +.ltoreq.Nb +.ltoreq.0.1%; 0% +.ltoreq.Ti +.ltoreq.0.1%; 0% +.Cu +.0.5%; 0% +.ltoreq.B.ltoreq.0.008%; 0% +.ltoreq.Sn +.0.1%; 0% +.ltoreq.Ce +.ltoreq.0.1%; 0% +.ltoreq.Mg +.ltoreq.0.10%; 0% +.ltoreq.Zr +.0.10%; the remainder consists of iron and unavoidable impurities resulting from the working, the microstructure of the steel comprising, in percentage by area, from 2% to 10% of proeutectoid ferrite, the remainder consisting of pearlite, wherein the interlayer distance of pearlite is from 100nm to 250nm.

Description

Steel for guide rail and manufacturing method thereof

The invention relates to a steel suitable for the manufacture of guide rails for railways, and in particular for trains running in magnetic levitation or magnetic guidance based on the principle of repulsion and attraction.

Steel for guide rails was developed for high-speed railways or for dual use in both freight and passenger railways. Regardless of use, the load carrying capacity of railways has increased and is expected to increase in the future. Therefore, it is necessary to develop steels for guide rails that are good in mechanical properties, electrical properties, and magnetic properties such as electrical resistivity, magnetic permeability, and tensile strength even in the harsh working environment of railways.

Therefore, a great deal of research and development effort has been devoted to developing materials with high tensile strength above 900 MPa at room temperature and at a temperature of 180° C. and sufficient hardness while being good in terms of electrical resistivity and magnetic permeability.

Early research and development in the field of rail steels for railways has resulted in several methods for producing high strength and wear resistant rail steels, some of which are listed here for a clear understanding of the invention:

US4350525 Magnetically active parts of maglev railways are made of steel with the following composition: 0% to 0.15% carbon, 0% to 0.045% phosphorus, 0% to 0.008% nitrogen, 0.75% to 2.0% silicon, 0.15% to 1.00% manganese, 0.02% to 0.07% soluble aluminum, 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.

WO2016019730 is an F-shaped guide rail for an induction core made of soft magnetic steel, and the chemical composition of the soft magnetic steel is: by weight C: 0.005% to 0.15%, Mn: 0.25% to 0.60%, Si: 0.30% to 1.0%, Re: 0.003% to 0.006%, both P and S are less than 0.025%, the rest is Fe and trace impurities, but This steel also cannot reach the strength of 900MPa at a temperature of 180°C.

Therefore, the object of the present invention is to solve these problems by making available a steel suitable for mechanical operations for the manufacture of rails for railways at the same time:

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

- a hardness of at least 310 Hv or greater and preferably greater than 315 Hv or greater,

- a resistivity of 40 Ωmm/m ² or greater and preferably 41 Ωmm/m ² or greater,

-165 or greater maximum permeability measured at 4000A/m.

In a preferred embodiment, the steel according to the invention may also have a tensile strength at room temperature greater than or equal to 950 MPa and preferably higher than 1000 MPa.

In a preferred embodiment, the steel according to the invention may also have a degree of polarization measured at 40000 A/m of greater than 1.5T.

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

Preferably, such steel is suitable for the manufacture of the guide rail and also for other structural parts of the guide rail, such as chassis components of the guide rail trolley.

Another object of the invention is also to make available a method for the manufacture of these mechanical parts compatible with conventional industrial applications and at the same time robust to variations in manufacturing parameters.

Carbon exists in the steel of the present invention at 0.25% to 0.8%, and carbon is an element required for increasing the strength of the steel of the present invention by producing pearlite. Carbon also ensures electrical resistivity by contributing to the formation of cementite in lamellar pearlite. However, a carbon content of less than 0.25% will fail to impart tensile strength and electrical resistivity due to excessive formation of Proeutectoid ferrite. On the other hand, at a carbon content exceeding 0.7%, the tensile strength is adversely affected due to excessive formation of pro-eutectoid 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%.

Manganese is added in the steel of the present invention at 1.0% to 2.0%. Manganese provides solid solution strengthening by contributing to the formation of cementite in pearlite, increasing hardenability and thus electrical resistivity. In addition, the ferrite transformation temperature is suppressed and the ferrite transformation rate is reduced to control the formation of pro-eutectoid ferrite, thus facilitating the formation of pearlite. Amounts of at least 1.0% are required to impart strength and to aid in pearlite formation. However, when the manganese content is greater than 2.0%, it produces adverse effects such as accelerating the transformation of austenite to martensite or bainite during cooling after hot rolling, which is harmful to the steel of the present invention because these microstructures adversely affect the electrical resistivity and magnetic permeability of the steel of the present invention. Manganese contents above 2.0% may also produce excessive segregation in the steel during solidification, as well as impair homogeneity within the material, which may lead to surface cracks during the hot working process. The preferred limit for the presence of manganese is 1.0% to 1.8%, and more preferably 1.0% to 1.5%.

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

The content of aluminum is 0.01% to 1%. Aluminum removes the oxygen present in the molten steel to prevent the oxygen from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, which reduces the size of the grains. Aluminum enables the steel of the invention to control the size of the pearlite interlayer distance and thereby increase the resistivity while retaining sufficient magnetic permeability. Higher contents of aluminum above 1% lead to the appearance of coarse aluminum-rich oxides which deteriorate the fatigue limit and brittle fracture of the steel rail. The preferred limit for the presence of aluminum is 0.02% to 0.9%, and more preferably 0.02% to 0.5%.

Chromium is present in the steel of the invention at 0.8% to 2%. Chromium is an essential element that provides strength to steel through solid solution strengthening and requires a minimum of 0.2% to impart strength, but when used above 2%, increases hardenability beyond acceptable limits because of the formation of undesirable phases such as bainite after cooling, compromising the ductility of the steel. Chromium additions above 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 the presence of chromium is 0.9% to 1.9%, and more preferably 0.9% to 1.6%.

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 higher than 0.09% may cause cracks due to decohesion at the interface between grains, which may be detrimental to tensile strength and wear resistance. Preferred limits for phosphorus content are 0% to 0.05%.

Sulfur is included at 0% to 0.09%. Sulfur forms MnS precipitates which can become elongated. If such elongated MnS inclusions are misaligned with the loading direction, the inclusions may have considerable adverse effects on mechanical properties such as hardness and tensile strength. 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%.

Nitrogen is present in the steel of the 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 which is detrimental to the mechanical properties of the steel during solidification. Nitrogen also forms nitrides and carbonitrides with vanadium, titanium and niobium to impart strength to the steel of the present invention.

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 in improving its pitting 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 contents higher than 1% may lead to the stabilization of retained austenite and thus have a detrimental effect on the tensile strength. It is preferred to have 0% to 0.9% nickel in the steel of the invention.

Molybdenum is an optional element and may be present in the present invention from 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 addition cost of alloying elements, so its content is limited to 0.5% for economical reasons. The preferred limit of molybdenum content is 0% to 0.4%, and more preferably 0% to 0.2%.

Vanadium is an optional element of the present invention and is present in an amount of 0% to 0.2%. Vanadium is effective in increasing the strength of steel by precipitation strengthening, especially by forming carbides or carbonitrides. The cap was kept at 0.2% for economic reasons.

Niobium is present in the steel of the invention at 0% to 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the invention by precipitation hardening. Niobium will also affect the size of the microstructural components and thus refine the grain size by its precipitation as carbonitrides and by delaying recrystallization during the heating process. However, a niobium content higher than 0.1% is economically unimportant, and forms coarser precipitates that are detrimental to the tensile strength of the steel. In addition, when the niobium content is 0.1% or more, niobium is also detrimental to the hot ductility of the steel, thereby causing difficulties during casting and rolling of the steel.

Titanium is an optional element and is present from 0% to 0.1%. Titanium forms titanium nitrides that give strength to steel and refine grains. The preferred limit for titanium is 0% to 0.05%.

Copper is a residual element and may be present up to 0.5% due to processing of the steel. Up to 0.5% copper does not affect any properties of the steel, but above 0.5%, 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 ratios: tin≦0.1%, cerium≦0.1%, magnesium≦0.10%, 0%≦boron≦0.008% and zirconium≦0.10%. Up to the indicated maximum content levels, these elements make it possible to refine the grains during solidification. The remainder of the composition of steel consists of iron and unavoidable impurities resulting from processing.

The microstructure of steel consists of:

Pearlite is the matrix microstructural component of the steel of the present invention and must be present in an area percentage of at least 90% or more, and it is 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 steel of the present invention has a layered structure. The layered structure of the pearlite of the present invention is an aggregate of ferrite and cementite, and the interlayer distance of the pearlite of the present invention is 100 nm to 250 nm. This interlayer distance improves the service properties of the steel of the invention such as tensile strength and electrical resistivity. When the interlayer distance is greater than 250 nm, the steel will be soft and cannot achieve tensile strength, especially at 180°C, while whenever the interlayer distance of pearlite is less than 100 nm, the magnetic permeability of the steel will be adversely affected. A preferable limit of the interlayer distance is 110 nm to 230 nm, and more preferably 120 nm to 220 nm. The pearlite of the present invention also imparts service properties to steel like magnetic permeability and hardness.

Proeutectoid ferrite is present in the steel of the invention at 2% to 10%. Proeutectoid ferrite forms on the grain boundaries of prior austenite grains during the first cooling step after hot rolling and the proeutectoid ferrite is dispersed in the pearlite. Proeutectoid ferrite provides ductility and magnetic permeability to the steel of the present invention. If the content of pro-eutectoid ferrite is greater than 10%, the steel of the invention will not achieve hardness. The preferred limit for the presence of proeutectoid ferrite is 3% to 9%, and more preferably 3% to 8%.

In addition to the microstructure described above, the microstructure of the guide rail does not contain microstructural components such as bainite, martensite and retained austenite.

The guide rail according to the invention may be produced by any suitable manufacturing process with the noted process parameters described hereinafter.

A preferred exemplary method is described herein, but the example does not limit the scope of the disclosure and the aspects on which the example is based. Additionally, any examples set forth in this specification are not intended to be limiting, but merely set forth some of the many possible ways in which aspects of the present disclosure might be put into practice.

A preferred method consists in providing a semi-finished casting of steel having a chemical composition according to the invention. The casting may be done in any form, such as an ingot or a bloom or billet, that can be manufactured or processed into rails for railways, and in particular for magnetic levitation.

For example, steel with the chemical composition described above is cast into billets and then rolled in the form of steel shapes that can serve as semi-finished products for further rolling. Multiple rolling steps can be performed to obtain the desired semi-finished product.

In order to prepare the steel to be manufactured as guide rails, the semi-finished product can be used directly at high temperature after rolling, or it can first be cooled to room temperature and then reheated for the manufacture of guide rails.

The semi-finished product is reheated at a temperature from Ac3 to Ac3+500°C, preferably Ac3+30°C to Ac3+450°C, and more preferably from 1100°C to 1300°C, where it is held for a period of 5 seconds to 1200 seconds to ensure a uniform temperature across the cross-section of the semi-finished product and to ensure 100% austenite formation. Thw Ac3 is calculated according to KASATKIN, O.G. et al. Calculation Models for Determining the Critical Points of Steel in MetalScience and Heat Treatment, 26:1-2, Jan-Feb 1984, 27-31.

If the reheating temperature of the semi-finished product is lower than Ac3, an excessive load will be applied during rolling. In addition, the temperature of the steel may also drop below the ferrite transformation initiation temperature, which will cause ferrite formation during hot rolling. Additionally, for a given cooling rate or a given chemical composition, metallurgical transformations under strain can cause significant changes in the microstructure obtained. Consequently, the obtained microstructure will be completely different from the target microstructure, and thus the mechanical properties as well as electrical properties will be completely different. Therefore, the temperature of the semi-finished product is preferably high enough that all mechanical operations are performed and completed within the 100% austenitic temperature range. Reheating at temperatures above Ac3+500°C has to be avoided as it is industrially expensive and may lead to liquid zones which would affect the rolling of the steel.

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

The finishing temperature must be kept above Ac3 and this is preferably a structure that favors recrystallization and mechanical fabrication. It is preferred that all rolling passes, especially the final rolling temperature, be carried out at a temperature above 1000° C., since below this temperature the steel exhibits a significant drop in rollability. In the case where the finish rolling temperature is lower than Ac3, it may cause problems regarding the final dimensions of the guide rail and deterioration of the surface appearance. It may even cause cracks or complete failure of the guide rail.

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

In the second cooling step, the thermal 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 cooling step is less than 3°C/sec, and more preferably less than 1°C/sec.

In a preferred embodiment, CR1 is higher than CR2.

The rails are obtained from the steel of the invention when the hot rails have reached room temperature.

Example

The following tests, examples, graphical examples and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only and will show the advantageous features of the invention.

Table 1 summarizes guide rails made of steel with different compositions, the guide rails being produced in each case according to the process parameters as indicated in table 2. Thereafter, Table 3 summarizes the microstructures of the rails obtained during the tests, and Table 4 summarizes the evaluation results of the properties obtained.

Table 1

Table 2

Table 2 summarizes the process parameters carried out on the semi-finished products made of the steels of Table 1 . Trials I1 to I3 were used to manufacture the guide rail according to the invention. Table 2 is as follows:

Ac3 values were determined by KASATKIN, O.G. et al. Calculation Models for Determining the Critical Points of Steel in Metal Science and Heat Treatment, 26:1-2, Jan-Feb 1984, 27-31.

table 3

Table 3 exemplifies the results, in area fraction, of tests carried out according to the standard on different microscopes, such as scanning electron microscopes, for determining the microstructure of both the inventive steel and the reference steel. The result is noted here:

steel sample Pearlite% Proeutectoid ferrite % Interlayer distance of pearlite (nm) I1 95 5 125 I2 95 5 170 I3 97 3 211

Table 4

Table 4 illustrates the mechanical and magnetic properties of both the inventive and reference steels. To determine the tensile strength, tests are carried out according to the NFEN ISO 6892-1/2017 standard. The tests for measuring the electrical resistivity and magnetic 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 and reference steels were performed according to EN-13674. Summarizes the results of various mechanical tests carried out according to the standard.

Table 4:

Claims (15)

1. A steel for guide rails, expressed in weight percentage, comprising the following elements: 0.25%≤C≤0.8%; 1.0%≤Mn≤2.0%; 1.40%≤Si≤2%; 0.01%≤AI≤1%; 0.8%≤Cr≤2%; 0≤P≤0.09%; 0≤S≤0.09%; 0%≤N≤0.09%; and can contain 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 remaining composition is composed of iron and unavoidable impurities caused by processing, the microstructure of the steel contains 2% to 10% proeutectoid ferrite by area percentage, and the balance is composed of pearlite, wherein the interlayer distance of the pearlite is 100nm to 250nm. 2. The steel for guide rails according to claim 1 or 2, wherein the composition comprises 0.27% to 0.75% carbon. 3. The steel for a guide rail according to any one of claims 1 to 2, wherein the composition contains 0.02% to 0.9% of aluminum. 4. The steel for guide rails according to any one of claims 1 to 3, wherein the composition contains 0.9% to 1.9% of chromium. 5. The steel for a guide rail according to any one of claims 1 to 4, wherein the pearlite is 93% to 99%. 6. The steel for a guide rail according to any one of claims 1 to 5, wherein the interlayer distance of pearlite is 110 nm to 230 nm. 7. The steel for guide rails according to any one of claims 1 to 8, wherein the tensile strength at 180° C. is greater than 900 MPa. 8. The steel for a guide rail according to any one of claims 1 to 7, wherein the steel has a hardness of 310 Hv or more. 9. The steel for guide rails according to any one of claims 1 to 8, wherein the resistivity of the steel is greater than 40Ωmm/ ^m2 . 10. The steel for guide rails according to any one of claims 1 to 9, wherein the steel has a maximum magnetic permeability equal to greater than 165 or greater measured at 4000 A/m. 11. A method of producing a guide rail of steel comprising the sequential steps of: - 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 from Ac3 to Ac3+500°C and maintaining the semi-finished product at the temperature from Ac3 to 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, where the hot-rolling temperature should be from Ac3 to Ac3+300°C to obtain hot rails; - cooling the thermal rail in two-step cooling, wherein in step one the thermal rail is cooled from a temperature of Ac3 to Ac3+300°C to a temperature T1 in the range of 480°C to 550°C at a cooling rate of 0.1°C/sec to 5°C/sec, - Thereafter, in step two, the thermal rail is cooled from T1 to room temperature at a cooling rate of less than 5° C./second to obtain a rail. 12. The method according to claim 11, wherein the reheating temperature of the semi-finished product is from Ac3+30°C to Ac3+450°C. 13. The method according to any one of claims 11 or 12, wherein the temperature T1 is from 490°C to 530°C. 14. The method of any one of claims 11 to 13, wherein CR1 is cooled at a higher rate than CR2. 15. Use of a steel according to any one of claims 1 to 10 or a guide rail produced according to the method of claims 11 to 14 for the manufacture of structural or safety components of a rail transport vehicle.