THEREOF

The present invention relates to forged and hot rolled steel suitable for rails and manufacturing of rail components and particularly suitable for manufacturing of crossings, switch points, expansion parts or any other places where train wheels transfer from one track to another.

In most cases, austenitic manganese steel castings and fabricated components made out of existing rail steels are used today to manufacture various railways components such as crossings, switch points, expansion parts or any other places where train wheels transfer from one track to another.

Inspection, repair or replacement of these critical components is generally expensive and disruptive to the rail schedule. Yet, fabricated components or cast crossings suffer from several limitations. As an example, fabricated components is a cost effective option but it requires welding of multiple rail segments to form the desired geometry, which cause poor dimensional tolerances and heterogeneous properties of the assembly in the numerous zones affected by the welding heat. Cast austenitic manganese steel crossings, another common solution used today by most railways networks for higher tonnage and speed configurations, have rather low mechanical strength and dimensional stability of the transfer point is a concern. Periodic and manual built up repair a/o grinding during track maintenance inspection is required to maintain the accurate geometry required to control the load transfer. Furthermore, flash butt welding of cast 12%Mn steel to carbon steel rails requires the interposition of a Cr Ni insert and considerable precautions need to be taken during welding so as to prevent carbide precipitation in the HAZ that reduces the fracture toughness of the weld joint.

Within this objective, new high strength steels are continuously developed by the steel manufacturers, to offer steels for railway applications with improved hardness, strength, in-service wear and fatigue resistance as well as good machinability and weldability during manufacturing, track installation or repair. Intense research and development endeavors are put in to develop a material that is good in machinability while having high tensile strength that is above 1400 MPa and a hardness of 400 HBWwith adequate impact toughness. Earlier research and developments in the field of steels have resulted in several methods for producing high strength and good formability some of which are enumerated herein for conclusive appreciation of the present invention:

EP3894217 The invention relates to a steel product having a high energy absorption capacity under sudden stress, said steel product having the following alloy composition in wt%: 0.10 to less than 0.75 C, 0.10-4.00 Si, 0.50-4.00 Mn, 0.05-2.00 Cr, max. 0.025 N, max. 0.15 P, max. 0.05 S, remainder iron with impurities resulting from steel production, and the steel product having a microstructure which is lamellar. The minimum energy absorption capacity to be achieved of the steel product fulfils the following condition (equation 1): Emin = 450x D-2000 (1), where E is the energy absorption capacity in kilojoules (kJ), the numerical value 450 is in kilojoules per mm (kJ/mm), the numerical value 2000 is in kilojoules (kJ) and D is the thickness of the steel product in millimetres (mm). However the steel of EP3894217 is npt able to reache the tensile strength of 1400MPa or more.

To obtain a significant increase in wear resistance, hardness should be higher than 400 HB. This level is beyond the limit of conventional pearlitic steels. The new proposed steel fulfills the dimensional requirement of most common railways, typically 160 to 200 mm height. The use of monolithic steel plates offers enhanced design flexibility for the part manufacturer: they can be cut-to-size by flame cutting, are fully machinable to achieve a large variety of dimensions and designs with critical tolerances and fast manufacturing. Costs associated with individual patterns and castings are thus eliminated.

Therefore, the purpose of the present invention is to provide a hot rolled steel that has reduced, the internal hardness and a higher surface hardness so that the steel of the present invention thereby has improved wear resistance and rolling contact fatigue resistance as well as a long service life while having good elongation simultaneously. The surface properties within the scope of the present invention means properties measured in a portion located from a top surface layer of the hot rolled steel to a depth from 25 mm to 35 mm. Further the through thickness properties within the scope of the present invention means properties measured in any portion of the steel except in a portion located from a top surface layer of the hot rolled steel to a depth from 25 mm to 35 mm. Hence the purpose of the present invention is to solve these problems by making available a forged and hot rolled steel for rails and railway equipment that simultaneously combines different properties across its thickness.

The forged and hot rolled steel simultaneously has the following surface properties as well as the trough thickness properties :

The surface properties are as follows :

- an ultimate tensile strength greater than or equal to 1400 MPa and preferably above 1450 MPa,

- a yield strength greater than or equal to 1050 MPa,

- an impact toughness of 25 J/cm2 or more when measured at -40°C or more for a CVN type of sample,

- a hardness of 400 HBW or more,

The forged and hot rolled steel simultaneously have the following through thickness properties:

- a hardness of less than or equal to 400 HBW and preferable below 380 HBW

- a through-thickness elongation greater than or equal to 15% and preferably above 25%

Such a combination of properties brings the following advantages:

At the running surface of the track, the high strength improves dimensional stability of the point that controls the load transfer and increases the resistance of the part against rolling contact fatigue. The high hardness provides lower wear rate of the running surface and thus longer lifetime of a single component. In addition, the corresponding multiphase microstructure has the ability to rapidly work harden so that the surface hardness of the track in service can reach 550 HBW or above.

The bulk of the plate offers a moderate hardness for easier machining and maintain good ductility, particularly in the through-thickness direction which was found critical to ensure direct flash butt welding with rails according to applicable European standard EN 14583-3 for zero crack tolerance. Internal and external tests have shown that crossings made of this new steel exhibit lower wear rate than a crossing made of 360HB-class grade and explosion depth hardened manganese cast components currently used for rail turnouts.

Additionally, the material characteristics allow a wider range of applications such as tramways, conventional, high speed, heavy haul transportation. A final benefit of the proposed steel is its suitability for online UT inspection control contrary to cast austenitic components, which allows easier control during maintenance with enhanced safety for the personnel.

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.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

Carbon is present in the steel of present invention is from 0.2% to 0.3%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low-temperature transformation phases such as Bainite, But Carbon content less than 0.2% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.3%, the toughness is adversely impacted due to the excessive precipitation of cementite that accompanies bainite formation during the cooling after hot rolling or reheat quenching treatment. Further excessive formation of cementite is also detrimental for damage resistance of the rolling surface of rails. The carbon content is advantageously in the range 0.22% to 0.30% and more especially 0.25% to 0.3%.

Manganese is added in the present steel from 1 % to 2%. This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of bainite. An amount of at least 1 % is required to impart strength as well as to assist the formation of Bainite. But when Manganese content is present more than 2% it cause segregation which results in banded microstructure after annealing and this banded microstructure is deferential to the mechanical properties of the steel of present invention, process. The preferred limit for the presence of Manganese is from 1.1 % to 1.9% and more preferably from 1.2% to 1.7%.

Silicon is present in the steel of present invention from 0.5% to 1.2%. Silicon imparts the steel of present invention with strength through solid solution strengthening and also acts as a deoxidizer. Silicon is a constituent that can retard the precipitation of carbides during cooling after annealing or during the quenching, therefore, Silicon promotes formation of Bainite. But Silicon is also 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 1.2%. Further Silicon higher than 1.2% also enhances segregation. The preferred limit for the presence of Silicon is from 0.6% to 0.9%.

The content of the Aluminum is from 0.001% to 0.1 %. 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. But the deoxidizing effect saturates for aluminum content more than 0.1%. Aluminum also controls the grain size of the present steel by forming AIN. Higher content of Aluminum above 0.1% lead to the occurrence of coarse aluminum- rich oxides that deteriorate machinability and hot forging on steel. The preferred limit for the presence of Aluminium is from 0.01% to 0.09% and more preferably from 0.01 to 0.08%

Molybdenum is an essential element and may be present from 0.01 % to 0.5% in the present invention. Molybdenum is added to impart hardenability and hardness to steel by forming Molybdenum based carbides and also promote the formation of Martensite during the quenching and also retard the formation of coarse Niobium carbides or Niobium Carbonitrides. 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 0.03% to 0.4% and more preferably from 0.05% to 0.4%. Chromium is present from 0.5% to 1.5% 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 0.5% is required to impart the strength but when used above 1.5% increase the hardenability is beyond an acceptable limit due the formation of coarse cementite after cooling thereby impairing the forgeability as well as the ductility of the steel. Chromium addition also decreases the diffusion coefficient of carbon in the austenite same as nickel hence promote the formation of martensite during quenching The preferred limit for the presence of Chromium is from 0.6% to 1.4 % and more preferably from 0.6% to 1 %.

Phosphorus is content of the steel of present invention is from 0 % to 0.02%. 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 0.012% can cause rupture by intergranular interface decohesion which may be detrimental for the fatigue limit. The preferred limit for Phosphorus content is from 0% to 0.012%.

Sulphur is contained from 0 % to 0.02%. Sulphur forms MnS precipitates which improve the machinability and assists in obtaining a sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions become elongated. Such elongated MnS inclusions can have considerable adverse effects on mechanical properties such as striction and impact toughness if the inclusions are not aligned with the loading direction further higher sulphur content is also detrimental for the forgeability of the steel. Therefore, sulfur content is limited to 0.02%. A preferable range the content of Sulphur is 0 % from 0.015% and more preferably from 0% to 0.01 %to obtain the best balance between machinability and fatigue limit.

Nitrogen is limited to 0.09% in order to avoid the formation of a high amount of Aluminum or Titanium Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.08%.

Nickel can be added to the present invention from 0% to 1% to increase the strength of the steel present invention and to improve toughness. Nickel is also beneficial, in combination with low amounts of others elements such as Cu, Cr, Mo and Si alloying additions, in improving its pitting corrosion resistance. A minimum of 0.1 % is preferred to get such effects. Nickel is added into the steel composition to decreases the diffusion coefficient of carbon in the austenite thereby promoting the formation of martensite during the Carburization process as well as low temperature phases such as bainite. But the presence of nickel content above 1 % lowers the martensite start temperature hence leading to the excessive stabilization of residual austenite thereby having a detrimental impact on tensile strength and yield strength. Further Nickel is also restricted to 1% due to the economic reasons. It is preferred to have nickel from 0.1 % to 0.9% in the steel of present invention.

Vanadium is an optional element for the present invention and its 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 an optional element for the steel of present invention from 0% to 0.06% and suitable for forming carbo-nitrides to impart strength of the steel of present invention by precipitation hardening. Niobium will also toughness the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, microstructure formed at the end of the holding temperature and as a consequence after the complete austenitization lead to the hardening of the product. However, Niobium content above 0.06% is not economically interesting as well as forms coarser precipitates which are detrimental for the impact toughness of the steel and also when the content of niobium is 0.06% or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling. The preferred limit for niobium content is from 0% to 0.05%.

Titanium forms coarse TiN hence is an optional element and present from 0% to 0.1 %. Titanium forms titanium nitrides which impart steel with strength, but these nitrides may form during solidification process, therefore have a detrimental effect on impact toughness and elongation. Hence the preferred limit for titanium is from 0% to 0.05%. Copper usually is a residual element and may be present up to 1% 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. Copper is also beneficial, in combination with low amounts of others elements such as Cr, Mo, Si and Ni alloying additions, in improving its pitting corrosion resistance.

Boron is present in the steel from 0.0005% to 0.005% to improve the hardenability of the steel.

Other elements such as Tin, Cerium, Calcium, Bismuth, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Tin ^0.1%, Cerium ^0.1%, Magnesium 0.10%, Calcium 0.005%, Bismuth 0.05%, and Zirconium 0.10%. 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 rest of the composition is iron and unavoidable impurities, in particular resulting from the elaboration. More particularly, the composition of the forged and hot rolled steel part for rails and railways component consists of the above-mentioned elements.

The forged and hot rolled steel part for rails and railways component has a core microstructure comprising, in surface fractions or area%, of at least 80% bainite, an optional cumulative presence of Residual Austenite, Pearlite, ferrite from 0% to 10% and martensite from 1% to 20%.

Bainite is present in the steel according to the invention as a matrix phase and imparts strength to such steel. Bainite is present in the steel at least 80% by area fraction and preferably from 80% to 99% by area fraction and more preferably from 85% to 99%. Bainite is formed during cooling after annealing. Such bainite may include Cementite- Free Lath-Like Bainite, granular bainite, Upper bainite and Lower bainite or any other bainite.

Martensite is present in the steel from 1% to 20%. The martensite of the present invention can comprise both fresh and tempered martensite. However, fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of between 0% and 4%, preferably between 0 and 2% and even better equal to 0%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and preferably after below Ms temperature and more preferably between Ms-10°C and 20°C. Such martensite is then tempered during the holding at a tempering temperature Temper between 150°C and 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is between 2% and 19% and more preferably between 3% and 18%. martensite

The cumulative presence of Residual Austenite, Pearlite, ferrite does not affect adversely to the present invention till 10% but above 10% the mechanical properties may get impacted adversely. Residual Austenite may impart toughness and ductility to the steel of present invention. Hence the preferred limit for the cumulative presence ferrite, pearlite and residual austenite is kept from 0% to 8% and more preferably from 0% to 4%.

In addition to this microstructure in the core of the steel, it also includes a martensitic- enriched layer on all the surfaces of the steel of forged and hot rolled steel for rails and railways component up to a depth of 35mm or less and preferably up to a depth of 30mm or less and more preferably 28mm or less and showing a martensite percentage from 40% to 80% in area fraction, preferably from 42% to 78% more preferably from 45% to 75%. The martensite enriched layer is formed on the surfaces by the natural gradient of cooling rates that is established within the heavy section of the product during the quenching treatment. This hard layer preferably comprises any or all possible martensite kinds and notably fresh martensite, tempered martensite etc. This martensite layer imparts the steel of the invention with a surface hardness of 400 Hv or more which provides the final steel part good resistance against the wear and also impart the precision during the fitting of part with each other during operation of railways component.

In addition, all the surfaces of the steel of forged and hot rolled steel for rails and railways component up to a depth of 35mm or less and preferably up to a depth of 30mm or less and more preferably 28mm or less contain Bainite in an amount of 15% to 30%. In the frame of the present invention the bainite contained can comprise carbide-free bainite and/or lath bainite. Bainite provides an improved elongation. The preferred presence for bainite is from 18% to 30% and more preferably from 19% to 29%.

The remaining part of this surface layer comprises at least one phase among residual austenite, ferrite and cementite. In a preferred embodiment, the cumulated amounts of residual austenite, ferrite and cementite are limited to a maximum value of 10% or even better to 8 %.

A forged and hot rolled steel part for rails and railways component 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.

In this preferred embodiment the rail component considered for demonstrating preferred process according to the present invention is a rail crossing.

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 railways component that can have a cross section up to 250mm*1250mm.

For example, the steel having the above-described chemical composition is casted into a billet or ingot and this ingot or billet can act as a semi-finished product for further process steps of manufacturing. A preferred Semi-finished product has a cross section be from 0200mm to 01000mm

The semi-finished product after the casting can be used directly at a high temperature or may be first cooled to room temperature and then reheated for hot forging at a temperature ranging from 1000°C to 1300°C. The temperature of the semi-finished, which is subjected to hot forging, is preferably at least 1150°C and must be below 1300°C because when the temperature of the semifinished product is lower than 1150°C, excessive load is imposed on forging dies Therefore, the temperature of the semi-finished product is preferably sufficiently high so that hot forging can be completed in the austenitic temperature range. Reheating at temperatures above 1300°C must be avoided because they are industrially expensive.

A final finishing forging temperature, herein after referred as Tforging, must be kept above 900°C to have a structure that is favorable to recrystallization and forging. It is preferred to have final forging to be performed at a temperature greater than 950°C and preferably above 980°C because below this temperature the semi-finished product exhibits a significant drop in forging. The hot forging reduction ratio is from 25% to 90%. The hot forged thick steel plate obtained has preferably a thickness from 300mm to 500mm.

This hot forged thick steel plate can optionally be used directly at a high temperature or cooled to room temperature then reheated for hot rolling at a temperature ranging from 1100°C to 1300°C.

Then the hot forged thick steel plate is subjected to hot rolling, the preferable hot rolling start temperature is at least 1100°C and must be below 1300°C. In case the hot rolling start temperature is lower than 1100°C, excessive load is imposed on a rolling mill. Therefore, the hot rolling start temperature is preferably sufficiently high so that hot rolling can be completed in the in 100% austenitic range. Therefore, the preferred hot rolling start temperature is between 1150°C and 1275°C.

Hot rolling finishing temperature for the present invention is above 800°C and preferably from 800°C and 975°C and preferably between 825°C and 925°C. The forged and hot rolled thick plate after hot rolling finishing has thickness from 100mm to 250mm and preferably between 140mm to 220mm and more preferably between 160mm and 205mm.

The forged and hot rolled thick plate obtained in this manner is then cooled at a cooling rate above 0.1°C/s to temperature which is below or equal to 100°C to 20°C. Preferably, the cooling rate will be above 0.3°C/s. Thereafter the forged and hot rolled thick plate is thus obtained in this manner is subjected to a heat treatment, to ensure the target properties for forged and hot rolled steel part for rails and railways component and for further processing in to rail components.

In the heat treatment, the forged and hot rolled thick plate is subjected to heating to reach the soaking temperature TA from 850°C to 990°C, the preferred TA temperature is from 890°C to 960°, more preferably from 900°C to 950°C.

In the heating step, forged and hot rolled thick plate is heated from room temperature to the soaking temperature TA at a heating rate HR1 from 150°C/h to 10°C/s. It is preferred to have HR1 rate from 150°C/h to 5°C/s and more preferably from 0.01°C/s to 0.2°C/s

Then the forged and hot rolled thick plate is held at the annealing soaking temperature TA during 100 to 1000 seconds to ensure adequate transformation to Austenite microstructure of the strongly work-hardened initial structure thereby reducing the hardness of forged and hot rolled thick plate. Then the forged and hot rolled thick plate is cooled at a cooling rate CR1 which is more than 0.1 °C/s and preferably more than 0.2°C/s and more preferably more than 0.5°C/s to a cooling stop temperature range CS1 from 325°C to 15°C and preferably from 300°C to 20°C and more preferably from 275°C to 25°C. The forged and hot rolled thick plate can be optionally held at CS1 temperature during 5 to 500 seconds.

Then the forged and hot rolled thick plate is cooled to room temperature to obtain forged and hot rolled steel for rails and railways component.

Thereafter the forged and hot rolled steel for rails and railways component is subjected to at least one mechanical manufacturing operation. Mechanical operation may comprise machining, grinding, milling, levelling or any other suitable mechanical operation or manufacturing procedure. The mechanical operations can be performed at room temperature or a higher temperature as desired by condition of specific mechanical operation.

After the mechanical operation is performed on forged and hot rolled steel for rails and railways component it results in a rail component or rails. Further the obtained rail component or rails of forged and hot rolled steel may optionally be reheated to a tempering temperature Ttemper from 150°C to 250°C with a heating rate of at least 1 °C/s and preferably of at least 2°C/s and more of at least 10°C/s during 100 s to 600s. The preferred temperature range for tempering is from 180°C to 240°C and the preferred duration for holding at Ttemper is from 200 s to 500s.

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.

Forged and hot rolled steel for rails and railways component with different compositions is gathered in Table 1 , where the forged and hot rolled steel for rails and railways component is produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the forged and hot rolled steel for rails and railways component 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 steels 1 and 2 serve for the manufacture of forged mechanical part according to the invention. This table also specifies the steels for reference forged mechanical parts which are steels 3 and 4.

The table 2 is as follows:

All the steels were reheated to a temperature for hot rolling is 1250°C for all inventive

5 and reference examples. Further all the inventive and reference examples underwent mechanical manufacturing operations. All the steels were cooled to a CS1 temperature of 250°C.

I = according to the invention; R = reference; underlined values: not according to the io invention.

Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as optical or Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials and Xray 15 measurements for the determination of the residual austenite fraction (RA).

Table 3: microstructures of the Steel samples and the presence of Martensite in Martensite on surface layer is as follows : I = according to the invention; R = reference; underlined values: not according to the invention.

Table 4 exemplifies the mechanical properties of both the inventive steel parts and reference steel parts. In order to determine the tensile strength, tests are conducted in accordance of NF EN ISO 6892-1 standards. Tests to measure the toughness and fatigue are conducted in accordance of EN ISO 148-1 standard CVN specimen with V- notch at a temperature of -40°C.

Table 4 I = according to the invention; R = reference; underlined values: not according to the invention.