EP4247987A1

EP4247987A1 - Steel for rails and a method of manufacturing of a rail thereof

Info

Publication number: EP4247987A1
Application number: EP20815939.2A
Authority: EP
Inventors: Diego CARRASCAL; David ALVAREZ DIEZ; José ARANCON ALVAREZ; Wilberth SOLANO ALVAREZ
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2020-11-17
Filing date: 2020-11-17
Publication date: 2023-09-27
Also published as: JP2025148451A; US20240002966A1; CN116472358B; ZA202304564B; MX2023005664A; JP2023551416A; CA3199316A1; KR20230078799A; CN116472358A; WO2022106864A1

Abstract

A steel for rail comprising of 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 composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising, by area percentage, 2% to 10% of Proeutectoid Ferrite, the balance being made of Pearlite wherein the pearlite having interlamellar spacing from 100nm to 250nm.

Description

STEEL FOR RAILS AND A METHOD OF MANUFACTURING OF A RAIL THEREOF

The present invention relates to a steel suitable for manufacturing of rails for railways and particularly for trains running on magnetic levitation or magnetic guiding based on repulsion and attraction principles.

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

Therefore, intense research and development endeavors are put in to develop a material that is good in resistivity and permeability while having high tensile strength at room temperature as well as at a temperature of 180°C that is above 900 MPa with adequate hardness.

Earlier research and developments in the field of steels for rails for railways have resulted in several methods for producing high strength and wear resistant steel for rails some of which are enumerated herein for conclusive appreciation of the present invention:

US4350525 magnetic suspension railroad magnetically active part is made of steel with the 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-% aluminum, soluble, 0.25 to 0.55-% copper, 0.65 to 1 .00-% chromium, remainder-iron with unavoidable impurities but the steel of US4350525 does not demonstrate to reach the tensile strength of 900 MPa at 180°C.

WO201 6019730 is a F-shaped rail for the induction core made of 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%, P and S are both less than 0.025%, the rest is Fe and trace impurities but this steel is also not able to reach the strength of 900MPa at a temperature of 180°C Hence the purpose of the present invention is to solve these problems by making available a steel suitable for mechanical operations for manufacturing rails for railways that simultaneously have:

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

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

- Resistivity of 40 Ωmm/m² or more and preferably 41 Ωmm/m² or more

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

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

In a preferred embodiment, the steel according to the invention may also have a polarization of more than 1 .5 T measures at 40000A/m.

In a preferred embodiment, the steel according to the invention may also have a Flux Density of more than 1 .5 T measures at 40000A/m.

Preferably, such steel is suitable for manufacturing of rails and the steel is also suitable for other structural parts of rails such as chassis members of the rail wagon.

Another object of the present invention is also to make available a method for the manufacturing of these mechanical parts that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.

Carbon is present in the steel of present invention is from 0.25% to 0.8%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing pearlite. Carbon also ensure the resistivity by assisting in the formation of cementite in lamellar pearlite. But Carbon content less than 0.25% will not be able to impart the tensile strength as well as resistivity due to the excessive formation of Proeutectoid ferrite. On the other hand, at a Carbon content exceeding 0.7%, the tensile strength is are adversely impacted due to the excessive formation of proeutectoid cementite during the cooling after hot rolling. Further excessive formation of proeutectoid cementite is also detrimental for the rail during its operational life cycle. The carbon content is advantageously in the range 0.27% to 0.75% and more especially 0.28% to 0.7%.

Manganese is added in the present steel from 1 .0% to 2.0%. Manganese provides solid solution strengthening , increases the hardenability by assisting in the formation of cementite in pearlite thereby increasing the resistivity. Further also suppresses the ferritic transformation temperature and reduces ferritic transformation rate to control the formation of Proeutectoid ferrite hence assisting in the formation of pearlite. An amount of at least 1 .0% is required to impart strength as well as to assist the formation of Pearlite. But when Manganese content is more than 2.0% it produces adverse effects such as it speed-up the transformation of Austenite to Martensite or bainite during cooling after hot rolling which are detrimental for the steel of present invention as these microstructure adversely affects the resistivity and permeability of the steel of present invention. Manganese content of above 2.0% can also get excessively segregated in the steel during solidification and homogeneity inside the material is impaired which can cause surface cracks during a hot working process. The preferred limit for the presence of Manganese is from 1 .0% to 1 .8% and more preferably from 1 .0% to 1 .5%.

Silicon is an essential element that is present in the steel of present invention from 1 .40% to 2%. Silicon impart the steel of present invention with strength through solid solution strengthening and also acts as a deoxidizer. But as Silicon is a ferrite former and also increases the Ac3 transformation point which will push the austenitic temperature to higher temperature ranges that is why the content of Silicon is kept at a maximum of 2%. Silicon content above 2% can also cause temper embrittlement.The preferred limit for the presence of Silicon is from 1 .45% to 1 .8% and more preferably from 1 .45% to 1 .6%.

The content of the Aluminum is from 0.01 % to 1 %. Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains. Aluminum allows the steel of present invention to have control over the size of the pearlite lamellar spacing and thereby increase the resistivity while retaining adequate permeability . Higher content of Aluminum above 1% lead to the occurrence of coarse aluminum-rich oxides that deteriorate fatigue limit and brittle fracture of steel rail. The preferred limit for the presence of Aluminium is from 0.02% to 0.9% and more preferably from 0.02 to 0.5%

Chromium is present from 0.8% to 2% in the steel of present invention. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 0.2% is required to impart the strength but when used above 2% increase the hardenability beyond an acceptable limit due the formation of undesired phases such as bainite after cooling thereby impairing the ductility of the steel. Chromium addition above 2% also decreases the diffusion coefficient of carbon in the austenite hence retards the formation pearlite during the cooling after hot rolling. The preferred limit for the presence of Chromium is from 0.9% to 1 .9 % and more preferably from 0.9% to 1 .6%.

Phosphorus is content of the steel of present invention is from 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 less as possible. Specifically, content over 0.09% can cause rupture by intergranular interface decohesion which may be detrimental for the tensile strength and wear resistance. The preferred limit for Phosphorus content is from 0% to 0.05%.

Sulphur is contained from 0 % to 0.09%. Sulphur forms MnS precipitates which can become elongated. Such elongated MnS inclusions can have considerable adverse effects on mechanical properties such as hardness and tensile strength if the inclusions are not aligned with the loading direction. Therefore sulfur content is limited to 0.09%. A preferable range the content of Sulphur is 0 % to 0.05% and more preferably 0% to 0.02%.

Nitrogen is in an amount from 0% to 0.09% in steel of present invention. Nitrogen is limited to 0.09% to avoid ageing of material and prevent the precipitation of coarse Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel. Nitrogen also forms nitrides and carbonitrides with vanadium titanium and niobium to impart strength to the steel of present invention. Nickel is an optional element and added to the present invention from 0% to 1 % to increase the strength of the steel present invention. Nickel is beneficial in improving its pitting corrosion resistance. Nickel is added into the steel composition to decreases the diffusion coefficient of carbon in the austenite thereby promoting the formation of Ferrite in pearlite. But the presence of nickel content above 1 % may lead to the stabilization of residual austenite thereby having a detrimental impact on tensile strength. It is preferred to have nickel from 0 % to 0.9% in the steel of present invention

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

Vanadium is an optional element for the present invention and is content is from 0% to 0.2%. Vanadium is effective in enhancing the strength of steel by precipitation strengthening especially by forming carbides or carbo-nitrides. Upper limit is kept at 0.2% due to the economic reasons.

Niobium is present in the Steel of present invention from 0% to 0.1 % and suitable for forming carbo-nitrides to impart strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process and thus refining the grain size. However, Niobium content above 0.1 % is not economically interesting as well as forms coarser precipitates which are detrimental for the tensile strength of the steel and also when the content of niobium is 0.1 % or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling.

Titanium is an optional element and present from 0% to 0.1 %. Titanium forms titanium nitrides which impart steel with strength and refine the grain size. The preferred limit for titanium is from 0% to 0.05%. Copper is a residual element and may be present up to 0.5% due to processing of steel. Till 0.5% copper does not impact any of the properties of steel but over 0.5% the hot workability decreases significantly. And

Other elements such as Tin, Cerium, Magnesium, boron or Zirconium can be added individually or in combination in the following proportions by weight: Tin Cerium Magnesium Boron and Zirconium Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

The microstructure of the Steel comprises:

Pearlite is the matrix microstructural constituent of the present steel and the area percentage presence must be at least 90% or more and it is preferred from 90% to 99% and more preferably from 93% to 98%. Pearlite is formed during the second step of cooling after hot rolling. Pearlite of the present steel is of lamellar structure. The lamellar structure of the pearlite of present invention is an aggregate of ferrite and cementite and the inter-lamellar spacing of the pearlite of present invention is from 100 nanometers to 250 nanometers. This inter-lamellar spacing improve the in-use properties of the steel of present invention such as tensile strength, and resistivity. When the inter-lamellar spacing is more than 250nanometer the steel will be soft and is not able to reach the tensile strength an especially the tensile strength at 180°C and whenever the inter-lamellar spacing of the pearlite is less than 100 nanometers the permability of the steel is adversely affected. The preferred limit for the inter-lamellar spacing is from 110 nanometers to 230 nanometers and more preferably from 120 nanometers to 220 nanometers. Pearlite of the present invention also impart the steel with in-use properties like Permeability and hardness.

Proeutectoid ferrite is present from 2% to 10% in the steel of present invention. Proeutectoid ferrite is formed during the first step of cooling after hot rolling on the grain boundaries of the prior austenite grains and Proeutectoid ferrite interspersed within the pearlite. Proeutectoid ferrite provides the present steel with ductility as well as the permeability. If the content of the Proeutectoid ferrite is more than 10% then the steel of present invention will not be able to achieve hardness. Preferred limit for the presence of Proeutectoid ferrite is from 3% to 9% and more preferably from 3% to 8%. In addition to the above-mentioned microstructure, the microstructure of the rail is free from microstructural components such as bainite, martensite and residual austenite.

A rail according to the invention can be produced by any suitable manufacturing process, with the stipulated process parameters explained hereinafter.

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

A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done in any form such as ingots or blooms or billets which is capable of being manufactured or processed into a rail for railways and particularly for magnetic levitation rails .

For example, the steel having the above-described chemical composition is casted in to a billet and then rolled in form of a bar. This bar can act as a semi-finished product for further rolling. Multiple rolling steps may be performed to obtain the desired semi- finished product.

In order to prepare for the steel to be manufactured as a rail, the semi-finished product can be used directly at a high temperature after rolling or may be first cooled to room temperature and then reheated for manufacturing the rail.

The semi-finished product is reheated from temperature Ac3 to Ac3 + 500° C, preferably from Ac3+30°C to Ac3 +450°C and more preferably from 1 100°C to 1300°C where it is held during 5 seconds to 1200 seconds to ensure homogenous temperature across the cross section of the semi-finished product as well as to ensure 100% austenite is formed. Thw Ac3 is claulated according to KASATKIN, O.G. et alii. Calculation Models for Determining the Critical Points of Steel in Metal Science and Heat Treatment, 26:1 -2, January-February 1984, 27-31.

If the reheating temperature of the semi-finished product is lower than Ac3, excessive load is imposed during the rolling further, the temperature of the steel may also decrease below the Ferrite transformation start temperature that will lead to the ferrite formation in during the hot rolling. Additionally the metallurgical transformation under strain can lead to significant change in the obtained microstructure for a given cooling rate or a given chemical composition. As a result, the obtained microstructure will be completely different from the targeted one and so the mechanical properties as well as the electrical properties . Therefore, the temperature of the semi-finished product is preferably sufficiently high so that all the mechanical operations are performed and completed in the 100% austenitic temperature range. Reheating at temperatures above Ac3 +500°C must be avoided because they are industrially expensive and can lead to the occurrence of liquid areas that will affect the rolling of the steel.

Then the semi-finished is subjected to at least one pass of hot rolling from Ac3 to Ac3 +300° C, preferably with a reduction from 35 to 90%. Hot rolling may be done in multiple passes that are required to have a hot rail from semi finished product. The preferred temperature for all the hot rolling is from Ac3 +30° C to Ac3 +300° C and more preferable temperature is from Ac3 +50° C to Ac3 +250° C.

A final rolling temperature must be kept above Ac3 and this is preferred a structure that is favorable to recrystallization and mechanical manufacturing. It is preferable to have all the rolling pass especially the final rolling temperature to be performed at a temperature greater than 1000°C, because below this temperature the steel exhibits a significant drop in the rollability. In case the final rolling temperature is less than Ac3 it can lead to issues regarding the final dimension of the rail as well as a deterioration of the surface aspect. It can even provoke cracks or a full failure of the rail.

The hot rail is then cooled in a two steps cooling process wherein the first step of cooling starts from the exit of final hot rolling, the hot rail being cooled down, at a cooling rate CR1 from 0.1 °C/s to 5°C/s, to a temperature T 1 which is in a range from 480°C to 550°C. In a preferred embodiment, the cooling rate CR1 for such first step of cooling is from 0.1 °C/s to 3°C/s and more preferably from 0.1 °C/s to 2°C/s. The preferred T1 temperature for such first step is from 490°C to 530°C and more preferably from 490°C to 510°C.

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

In a preferred embodiment, CR1 is higher than CR2.

When the hot rail reaches room temperature the rail is obtained from the steel of present invention.

EXAMPLES

The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention. Rails made of steels with different compositions is gathered in Table 1 , where the rail is produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the rail obtained during the trials and table 4 gathers the result of evaluations of obtained properties.

Table 1

Table 2

Table 2 gathers the process parameters implemented on semi-finished product made of steels of Table 1 . The trials 11 to I3 serve for the manufacture of rail according to the invention. The table 2 is as follows: Ac3 values were determined through KASATKIN, O.G. et alii. Calculation Models for Determining the Critical Points of Steel in Metal Science and Heat Treatment, 26:1 -2, January-February 1984, 27-31.

Table 3 Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels in terms of area fraction. The results are stipulated herein:

Table 4

Table 4 exemplifies the mechanical properties and magnetic properties of both the inventive steel and reference steels. In order to determine the tensile strength, tests are conducted in accordance of NF EN ISO 6892-1/2017 standards. Tests to measure the resistivity and permeability for both inventive steel and reference steel are conducted in accordance of IEC-60404-13 and IEC-60404-4 respectively . Tests to measure the hardness for both inventive steel and reference steel are conducted in accordance of EN-13674. The results of the various mechanical tests conducted in accordance to the standards are gathered.

Table 4:

Claims

1 . A steel for rail comprising of the following elements, expressed in percentage by weight: and can contain one or more of the following optional elements the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising, by area percentage, 2% to 10% of Proeutectoid Ferrite, the balance being made of Pearlite wherein the pearlite having interlamellar spacing from 100nm to 250nm.

2. Steel for rail according to claims 1 or 2, wherein the composition includes 0.27% to 0.75% of Carbon.

3. Steel for rail according to anyone of claim 1 to claim 2, wherein the composition includes 0.02 % to 0.9% of Aluminum.

4. Steel for rail according to anyone of claim 1 to 3, wherein the composition includes 0.9% to 1 .9% of Chromium.

5. Steel for rail according to anyone of claim 1 to 4, wherein, the Pearlite is between 93% and 99%.

6. Steel for rail according to anyone of claim 1 to 5, wherein, the interlamellar spacing of pearlite is from 1 10nm to 230nm.

7. Steel for rail according to anyone of claim 1 to 8, wherein, the tensile strength at 180°C is greater than 900 MPa.

8. Steel for rail according to anyone of claim 1 to 7, wherein said steel has hardness of 310Hv or more.

9. Steel for rail according to anyone of claim 1 to 8, wherein said steel has resistivity of more than 40Ωmm/m².

10. Steel for rail according to anyone of claim 1 to 9, wherein said steel has maximum permeability equal to more than 165 or more measure at 4000A/m.

11. A method of production a rail of steel comprising the following successive steps:

- providing a steel composition according to anyone of claims 1 to 4 in form of semi-finished product;

- reheating said semi-finished product to a temperature from Ac3 to Ac3 +500°C and where it is held from 5 seconds to 1200 seconds ;

- performing one or more hot rolling pass on the said semi-finished product in the austenitic range wherein hot rolling temperature shall be from Ac3 to Ac3 +300°C to obtain a hot rail;

- cooling hot rail in two-step cooling, wherein in step one the hot rail is cooled at an cooling rate from 0.1 °C/s to 5°C/s from Ac3 and Ac3 +300°C temperature to a temperature T1 ranging from 480 to 550°C,

- thereafter in step two the hot rail is cooled at a cooling rate less 5°C/s from T1 to room temperature to obtain a rail.

12. A method according to claim 1 1 , wherein the reheating temperature of the semi- finished product to is from Ac3+30°C to Ac3 +450°C.

13. A method according to anyone of claims 11 or 12, wherein the temperature T1 is from 490°C to 530°C. A method according to anyone of claims 11 to 13 , wherein the CR1 cooling rate is higher than CR2. Use of a steel according to anyone of claims 1 to 10 or of a rail produced according to the method of claims 11 to 14, for the manufacture of structural or safety parts of a rail wagon .