Method for Manufacturing a Rail and Corresponding Rail

This is a Continuation of U.S. Ser. No. 16/767,105, filed on May 26, 2020, which is a national phase of PCT/IB2018/059349 filed Nov. 27, 2018 which claims priority to International Patent Application PCT/IB2017/057424, filed on Nov. 27, 2017. All of the above are hereby incorporated by reference herein.

The present invention concerns a method for producing a steel rail having excellent mechanical properties and wear and rolling contact fatigue resistances, as well as a corresponding steel rail.

In recent years, train speed and load have been increased to improve railroad transportation and contact stresses can exceed 2000 MPa. These more severe service conditions require new rails with higher wear and rolling contact fatigue resistance, especially for heavy industrial railway traffic.

Wear and rolling contact fatigue (RCF) are two important factors that may cause a delayed failure in the railway track. Whereas the mechanisms for wear have been fully studied and are well understood, and wear is nowadays managed in the railway system, RCF is still not sufficiently understood to have efficient solutions to prevent the formation of RCF defects, which can cause progressive deterioration and a premature maintenance of the rail.

The traditional approach for the development of new rail steels to address wear and RCF has been to increase steel hardness and strength. In the case of conventional pearlitic grades for railways, this increase has been achieved during the last 40 years by decreasing the interlamellar spacing, by adding costly alloying elements or through head hardening. Nevertheless, this increase in resistance to wear is generally accompanied by a decrease in toughness. The aforementioned challenges are showing that despite all the research that has been taken place to develop new microstructures with enhanced mechanical properties, pearlitic steel grades have already reached their limits in terms of wear and rolling contact fatigue performance, which means that the existing railway grades cannot cope with the most demanding in-service conditions.

Bainitic steels, comprising for example lower bainite microstructure, have been considered as the next generation of advanced high strength steels and candidate materials for heavy-duty rails and railway-crossings due to a good combination of hardness, strength and toughness.

Bainitic steels comprising lower bainite microstructure provide good wear resistance but do not achieve a sufficient RCF resistance.

Especially, WO1996022396A1 discloses a method for producing a high strength wear and rolling contact fatigue resistant rail. The rail is produced from a steel having a composition comprising 0.05% to 0.5% C, 1.00% to 3.00% Si and/or Al, 0.50% to 2.50% Mn and 0.25% to 2.50% Cr. The rail is produced by air cooling the steel from the finish hot rolling temperature.

EP 1 873 262 discloses a method for manufacturing high-strength guide rails, from a steel comprising 0.3% to 0.4% C, 0.7% to 0.9% Si, 0.6% to 0.8% Mn and 2.2% to 3.0% Cr. The manufacturing method comprises air cooling the steel after formation of a bainitic structure. However, EP 1 873 262 does not teach any specific cooling rate.

EP 0 612 852, US2015218759 and US201514702188 disclose methods for producing bainitic rails by accelerated cooling. However, these rails do not show a sufficient Rolling Contact Fatigue resistance.

Therefore, it remains desirable to produce steel rails.

An object of this present disclosure is to provide a method of manufacturing high performance rail having excellent rolling-contact fatigue resistance and wear resistance.

Especially, it is desirable to produce a steel rail wherein the rail head has a tensile strength of at least 1300 MPa, a yield strength of at least 1000 MPa, a total elongation of at least 13% and a hardness of at least 420 HB and preferably of at least 430 HB together with excellent rolling-contact fatigue resistance and wear resistance.

A method is provided for manufacturing a rail comprising a head, the method comprising the following successive steps:

The method for manufacturing a rail may further comprise one or more of the following features, taken along or according to any technically possible combination,

A hot rolled steel part is also provided having a chemical composition comprising, by weight percent:

The hot rolled steel part may further comprise one or more of the following features, taken along or according to any technically possible combination:

An embodiment of a railaccording to the invention is depicted in.

The railcomprises a headand a foot, the footand the headbeing connected to each other through a support.

As depicted in, the supporthas a maximal width strictly inferior to the maximal width of the head, notably at least inferior to 50% to the maximal width of the head.

Likewise, the support has a maximal width strictly inferior to the maximal width of the foot, notably at least inferior to 50% to the maximal width of the foot.

The head, the footand the supportare made integral.

The rail, in particular the headof the rail, is manufactured from a steel having a chemical composition comprising, by weight percent:

In this alloy, carbon is the alloying element having the main effect to control and adjust the desired microstructure and properties of the steel. Carbon stabilizes the austenite and thus leads to its retention even at room temperature. Besides, carbon allows achieving a good mechanical resistance and the desired hardness, combined with a good ductility and impact resistance.

A carbon content below 0.20% by weight leads to the formation of a non-sufficiently stable retained austenite, insufficient hardness and tensile strength, and insufficient rolling-contact fatigue and wear resistances. At carbon contents above 0.60%, the ductility and impact resistance of the steel are deteriorated by the appearance of center-segregation. Therefore, the carbon content is comprised between 0.20% and 0.60% by weight.

The carbon content is preferably comprised between 0.30% and 0.60% by weight percent.

The silicon content is comprised between 1.0% and 2.0% by weight. Si, which is an element which is not soluble in the cementite, prevents or at least delays carbide precipitation, in particular during bainite formation, and allows the diffusion of carbon into the retained austenite, thus favoring the stabilization of the retained austenite. Si further increases the strength of the steel by solid solution hardening. Below 1.0% by weight of silicon, these effects are not sufficiently marked. At a silicon content above 2.0% by weight, the impact resistance might be negatively impacted by the formation of large size oxides. Moreover, an Si content higher than 2.0% by weight might lead to a poor surface quality of the steel.

Preferably, the Si content is comprised between 1.25% and 1.6% by weight.

The manganese content is comprised between 0.60% and 1.60% by weight, and preferably between 1.09% and 1.5%. Mn has an important role to control the microstructure and to stabilize the austenite. As a gammagenic element, Mn lowers the transformation temperature of the austenite, enhances the possibility of carbon enrichment by increasing carbon solubility in austenite and extends the applicable range of cooling rates as it delays perlite formation. Mn further increases the strength of the material by solid solution hardening, and refines the structure. Below 0.6% by weight, these effects are not sufficiently marked. At contents above 1.6%, Mn favors the formation of too large a fraction of martensite, which is detrimental for the ductility of the product.

The chromium content is comprised between 0.5% and 2.2% by weight. Cr is effective in stabilizing the retained austenite, ensuring a predetermined amount thereof. It is also useful for strengthening the steel. However, Cr is mainly added for its hardening effect. Cr promotes the growth of the low-temperature-transformed phases and allows obtaining the targeted microstructure in a large range of cooling rates. At contents below 0.5%, these effects are not sufficiently marked. At contents above 2.2%, Cr favors the formation of too large a fraction of martensite, which is detrimental for the ductility of the product. Moreover, at contents above 2.2%, the Cr addition becomes unnecessarily expensive.

When present, the molybdenum content is comprised between 0.01% and 0.3% by weight. In the steel of the present disclosure, Mo may be present as an impurity, in a content which is generally of at least 0.01%, or added as a voluntary addition. When added, the Mo content is preferably of at least 0.10%. When added, Mo improves the hardenability of the steel and further facilitates the formation of lower bainite by decreasing the temperature at which this structure appears, the lower bainite resulting in a good impact resistance of the steel. At contents greater than 0.3% by weight, Mo can have however a negative effect on this same impact resistance. Moreover, above 0.3%, the Mo addition becomes unnecessarily expensive.

When present, the vanadium content is comprised between 0.01% and 0.30%. Vanadium is optionally added as a strengthening and refining element. When added, the V content is preferably of at least 0.10%. Below 0.10%, no significant effect on the mechanical properties is noted. Above 0.30%, under the manufacturing conditions according to the present disclosure, a saturation of the effect on the mechanical properties is noted. When V is not added, V is generally present as an impurity in a content of at least 0.01%.

The remainder of the composition is iron and unavoidable impurities. In this respect, nickel, phosphorus, sulfur, nitrogen, oxygen and hydrogen are considered as residual elements which are unavoidable impurities. Therefore, their contents are at most 0.05% Ni, at most 0.025% P, at most 0.020% S, at most 0.009% N, at most 0.003% O and at most 0.0003% H.

The rail, in particular the headof the rail, has a microstructure consisting of, in surface fractions:

The bainite can include granular bainite and lath-like carbide free bainite. In the frame of the present disclosure, carbide free bainite will designate bainite containing less than 100 carbides per surface unit of 100 square micrometer.

Preferably, the surface fraction of bainite in the microstructure of the headis higher than or equal to 56%.

The retained austenite and the tempered martensite are generally present as M/A constituents, located between the laths or plates of bainite.

The austenite is also contained in the bainite between the laths or plates of bainite.

The retained austenite has an average carbon content comprised between 0.83% and 1.44%, preferably higher than 1.3%.

Preferably, the surface fraction of retained austenite in the microstructure of the headis comprised between 18% and 23%.

The tempered martensite is contained in the bainite between the laths or plates of bainite, and in the M/A components.

The martensite is tempered martensite and preferably self-tempered martensite. Generally, the tempered martensite has a low carbon content, i.e. an average C content strictly lower than the average C content in the steel.

Preferably, the surface fraction of tempered martensite in the microstructure of the headis comprised between 14.5% and 22.5%.

The headof the railhas a hardness of at least 420 HB, generally comprised between 430 HB and 470 HB, a tensile strength of at least 1300 MPa, generally comprised between 1300 MPa and 1450 MPa, a yield strength of at least 1000 MPa, generally comprised between 1000 MPa and 1150 MPa, and a total elongation of at least 13%, generally comprised between 13% and 18%.

The manufacturing of the railaccording to the present disclosure can be done by any suitable method.