TRACK PART MADE OF A HYPEREUTECTOID STEEL

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Austrian Application Serial No. A 201/2018, filed Jul. 10, 2018, the contents of which are hereby incorporated by reference in its entirety.

The invention relates to a track part, in particular a rail for rail vehicles, made of a hypereutectoid steel comprising a rail foot, a rail web and a rail head portion.

The invention further relates to a method for producing such a track part.

Recently, the weight of transported loads in rail traffic and the traveling speed have been steadily increased to increase the efficiency of rail transportation. Rail tracks are therefore subject to difficult operating conditions and must have higher quality and strength to withstand the higher loads. The specific problems that are occurring are the sharp increase in wear, especially for rails mounted in arches, and the occurrence of material fatigue damage, which mainly develops at the running edge, which is the main contact point of the rail with the wheels in the arch. This leads to rolling contact fatigue damage (RCF). Examples of RCF surface damage are e.g. head checks (rolling fatigue), spalling (flaking), squats (plastic surface deformations), slip waves and scratches. These damage to the surface results in shortened rail life, increased noise emissions and operational disabilities. The increased occurrence these problems is also accelerated by the ever-increasing traffic loads. The immediate consequence of this development is an increased need for maintenance of the rails. However, the growing need for maintenance is in conflict with the ever-decreasing maintenance windows. Higher train densities are increasingly reducing the periods when rails can be changed or machined.

Although the mentioned RCF damage can be eliminated at an early stage by grinding, the rail, however, is to be replaced in case of severe damage. Therefore, attempts have been made in the past to improve both the wear resistance and the resistance to RCF damage in order to increase the life cycle of the rails.

With respect to pearlitic rail steels, increasing strengths have been found to have a very favorable effect on wear behavior and RCF resistance, leading to the development of hypereutectoid rail steels in the past.

Hypereutectoid steels are known for producing rails, for example from EP 2388352 A1. The iron-carbon diagram shows an eutectoid at a carbon content (C content) of 0.77% by weight of carbon and at a temperature of 723° C., at which point a fixed direct phase transition from the austenite phase to the perlite phase occurs on cooling. Perlite is preferred over other steel modifications for rails in terms of wear resistance and elongation at break, as it is best for wear due to the lamellar structure.

By definition, the pearlitic structure comprises a ferrite phase, wherein the ferrite content in the perlite phase can be regarded as a tough and ductile phase and as a fixed variable, because the C-content of the rail steel varies only in very narrow limits, and the pearlitic structure further comprises a cementite phase, wherein the ferrite and the cemetite phases are arranged in lamellar relationship to each other.

Higher carbon contents than 0.77 wt % are preferable for greater hardness and hence wear resistance, because higher carbon content in the steel results in strengthening the thickness of cementite lamellae in the perlite phase and hence to an increase of the wear resistant cementite phase portion. Too high carbon contents are to be avoided also in the perlite phase, since the cementite can occur not only in the pearlite lamellae, but also as a separate phase in the microstructure.

These microstructure portions, called secondary cementite precipitates, are significantly influenced by a combination of alloying and heat treatment technology. Here, the knowledge of the precipitation temperature of the secondary cementite is of crucial importance and one should not fall below the precipitation temperature before the subsequent heat treatment. In addition, the cooling rate during the heat treatment process should be as high as possible to suppress the precipitation of secondary cementite. Excessive levels of secondary cementite can adversely affect the breaking properties of steels, as the breaking behavior becomes increasingly intercrystalline.

Essential for the quality of a rail is thus a high wear resistance, for which essentially the hardness (for example indicated as Brinell hardness) of the rail is characteristic. However, the desirable hardness inevitably goes hand in hand with a reduction in toughness, which is detrimental to the durability of the track part, especially in heavy load, where the rail is subjected to particularly high bending stresses when driving over a rail vehicle.

The invention therefore aims to improve a track part, in particular a rail, which is to consist of a low-alloy steel for reasons of cost and ease of welding, to the effect that due to a high hardness of the material even at elevated wheel loads, the wear resistance in the rail head is increased so far that a use time of longer than 30 years can be ensured. Finally, the track part should be well weldable and have similar other material properties as proven steels used in the rail construction, such as e.g. a similar electrical conductivity and a similar coefficient of thermal expansion.

Furthermore, the invention aims to provide a simple manufacturing method for a track part according to the invention, which is characterized by a short process time (avoidance of incandescent phases), high reproducibility and high cost-effectiveness. The method shall be suitable for producing long rails of e.g. over 100 m in length, with specification-compliant material properties over the entire rail length to be ensured.

To achieve this object, the invention according to a first aspect provides a track part, in particular a rail of the type mentioned above, which is characterized in that a hypereutectoid steel with the following directional analysis is used:

- 0.98-1.17 wt.-% of C
- 0.90-1.35% wt.-% of Mn
- 0.70-1.10% wt.-% of Si
- 0.15-0.70 wt.-% of Cr

and the steel, at least in the head portion of the rail, has a pearlitic structure that is substantially free of secondary cementite networks.

In the development of the rail according to the invention, this steel composition has proven to be extremely suitable, since a hardness in the range of 460 HB and higher could be achieved and the rail according to the invention at the same time has a sufficient elongation at break for the heavy load range. In the context of the present invention, the feature that at least in the head portion of the rail a pearlitic structure is substantially free of secondary cementite networks, means that at most 5% of the existing secondary cementite precipitates are in the form of secondary cementite networks. A secondary cementite network means a microstructure in which the former austenite grains are completely surrounded by a closed network of secondary cementite.

Secondary cementite precipitates reduce the carbon supply for the formation of pearlitic cementite lamellae needed for wear resistance. For these reasons, the suppression of secondary cementite in hypereutectoid rail steels is considered to be advantageous.

The concentration of manganese in the present steel is selected to shift the formation of embrittling secondary cementite to lower temperatures due to its austenite stabilizing effect, thereby enabling fine grain rolling at low rolling temperatures of hypereutectoid rail steels while suppressing secondary cementite precipitation.

The lower limit of 0.9 wt.-% manganese was chosen because the precipitation temperature of embrittling secondary cementite shifts to higher temperatures if one falls below this lower limit, which would result in that the suppression of the same can not be guaranteed in the subsequent heat treatment. The upper limit of 1.35 wt.-% manganese was chosen to ensure castability of the steel.

The alloy composition of the present invention provides a lower limit of the carbon content of 0.98 wt % and an upper limit of 1.17 wt %. The lower limit was chosen here in view of a sufficient strength, since carbon brings the required strength. The upper limit was chosen in order to avoid the precipitation of secondary cementite networks, especially from a depth of approx. 5-10 mm below the rail head surface, even with a lower heat dissipation during forced cooling after rolling. If secondary cementite networks were formed in the interior of the rail head due to a too low heat dissipation or a too low cooling rate, the material properties would deteriorate significantly, so that a sufficient elongation at break, such as, for example, an elongation at break of at least 8%, could no longer be achieved.

In the context of the invention, particular preference has been given to steel compositions which are distinguished by the fact that C is present in amounts of 1.05-1.17 wt.-%, preferably 1.06-1.15 wt.-%, and particularly preferably 1.08 wt.-%. These carbon contents ensure the best possible balance between wear resistance and elongation at break.

The alloy composition of the invention provides for silicon a lower limit of 0.70 wt.-% and an upper limit of 1.10 wt.-%. The lower limit was chosen to ensure the effectiveness of silicon to suppress embrittling secondary cementite precipitates. This extends the process window of the heat treatment so that a secondary cementite network can be avoided. The upper limit is based on the background that, if this limit is exceeded, the required electrical conductivity of the rails would decrease in such a way that in some cases impairments in the signaling technology would be possible.

The alloy composition according to the invention provides for chromium a lower limit of 0.15 wt.-% and an upper limit of 0.70 wt.-%. It has been found that chromium, starting at a content of 0.15 wt.-%, has a marked effect on the cross-section hardening of the rail and suppresses the formation of secondary cementite, which in turn widens the process window of the heat treatment, so that the secondary cementite structure cannot be formed as a network. The upper limit of 0.70 wt.-% was chosen because the weldability of the rail is made difficult or impossible with increasing chromium content.

In an overall consideration of the alloy composition according to the invention, the invention is based on a particular selection of the quantitative ranges of the individual alloy constituents, the individual alloying constituents having partially opposite effects. For example, a higher carbon content is desirable for achieving high strengths, but as the carbon content increases, the disadvantage of the increasing austenite-to-perlite-transition temperature has to be considered. Although high silicon contents are responsible for the suppression of embrittling secondary cementite precipitates, they also increases the austenite-to-perlite-transition temperature. Overall, secondary cementite precipitates at relatively high temperatures due to the relatively high carbon and silicon content, so that secondary cementite precipitates can occur in an enhanced form also prior to the heat treatment process. Thus, the use of high carbon content in combination with silicon is actually counterproductive. However, high silicon concentrations also have a diffusion-inhibiting effect on carbon and thus minimize secondary cementite. To take advantage of this positive effect, the negative effect of the shift in the transformation temperature must be compensated. This is achieved according to the invention by the addition of the appropriate amounts of the austenite-stabilizing element manganese.

In order to increase the elongation at break, according to a preferred embodiment of the present invention, the directional analysis of the rail steel may be formed such that Al (aluminum) is additionally used in amounts of 0.01-0.06 wt.-%. This leads to a minimization of the pearlite grain size, which is beneficial to the elongation at break. From the same point of view as the addition of Al, V (vanadium) may additionally be used in amounts of from 0.07 to 0.20 wt.-%, in particular from 0.10 to 0.20 wt.-%, as in a preferred embodiment of the present invention. It has been found that already starting from a vanadium content of 0.07 wt.-%, an increase in strength and a grain refining effect may be achieved. The strength decreases again from the upper limit of 0.2 wt.-%, since too much C from the matrix is bound.

Likewise, according to a preferred embodiment of the present invention, Nb (niobium) may additionally be used in amounts of 0.010-0 0.030 wt.-% in order to minimize the pearlite size and thereby positively influence the breaking elongation of the rail according to the invention.

From the same point of view as the addition of Al, Ti (titanium) may additionally be used in amounts of 0.015-0.05 wt.-%.

It has been found to be particularly preferred that in addition V is used in amounts of 0.07 to 0.20 wt.-%, in particular 0.10 to 0.2 wt.-%, together with Nb in amounts of 0.010 to 0.030 wt.-%. This simultaneous use of V and Nb resulted in a particularly high elongation at break, as will become apparent from the embodiments of the present invention set forth below.

From the same point of view it has been found to be particularly preferred that in addition Al is used in amounts of 0.01-0.06 wt.-% together with Nb in amounts of 0.01-0.03 wt.-%.

Further, by adding the above-mentioned alloying elements (Al, Ti, V, Nb), the grain refining effect can be increased by a nitrogen content set in the steel in the range of 40 to 120 ppm, which corresponds to a preferred embodiment of the present invention.

In particular, with the composition according to the present invention, a steel quality is achieved which enables the production of a track part in which the steel, at least in the head portion of the rail, has a tensile strength greater than 1500 MPa, an elongation at break of greater than 8% and a Brinell hardness (according to EN ISO 6506-1) of greater than 460 HB, as corresponds to a preferred embodiment of the present invention.

The inventive method for producing a track part according to the invention is characterized in that a hypereutectoid steel having a composition according to any one of claims 1 to 9 is taken from a furnace at a temperature of 1000-1300° C., then rolled at a final rolling temperature of 850-950° C. and is then subjected to forced cooling to a temperature of 450° C. to 600° C. The furnace is preferably a walking beam furnace. The steel with the composition according to the invention is removed from the furnace and rolled to the desired shape of the track part, in particular rail. Here, in order to avoid secondary cementite precipitates in the non-eutectoid transition from the austenite phase to the perlite phase, according to the present invention, one does not fall below a final rolling temperature, i.e. a temperature of the steel at the end of the rolling mill, of 850° C., since secondary cementite precipitates at the perlite grain boundaries, especially in the formation of secondary cementite networks, can lead to unacceptable embrittlement of the rail. The conditions in the rolling mill are selected by means of the accumulated degree of transformation within continuous rolling passes on the finishing scale such that a recrystallization-controlled rolling process is achieved by means of deformation induced precipitations and precipitates in solution taking into account the forming speed of the reduction per pass, the temperature and the alloy composition, which allows to realize a small former austenite grain size of 8 μm to 35 μm, at least in the head portion of the rail. According to the invention this is followed by a rapid cooling to below 600° C., in which temperature range no secondary cementite is precipitated any more, creating a very wear-resistant fine-pearlitic microstructure with sufficient elongation at break for the heavy-duty application.

According to a preferred embodiment of the present invention, the forced cooling takes place at least in the head portion of the rail, in order to ensure at least there the pearlitic structure.

Here, the cooling rate is chosen so high that a substantial suppression of secondary cementite precipitation takes place, but no formation of undesirable secondary phases such as wear-promoting bainite or martensite occurs.

In order to precipitate as little as possible secondary cementite in the pearlite structure, the process according to the invention is preferably further developed in that the forced cooling takes place in a bath of cooling medium not being pure water. With non-pure water cooling media evaporation effects on the surface to be cooled of the rail can be avoided, which leads on the one hand to improved heat transfer from the hot steel to the cooling medium and, consequently, in the interest of avoiding secondary cementite precipitates to a more rapid cooling, and on the other hand, suppresses the emergence of soft staining the surface of the rail.

A particularly effective cooling succeeds in the interest of avoiding secondary cementite precipitates, when the forced cooling takes place in a polymer bath with a temperature of 10-70° C., as provided according to a preferred embodiment of the present invention. Particularly preferably, the method is carried out such that, to avoid secondary cementite precipitates, the forced cooling is performed at a rate of at least 4° C./sec, preferably at least 8° C./sec, more preferably at least 12° C./sec. In this way, the area of the formation of secondary cementite precipitates is quickly passed through in the iron-carbon diagram, so that the embrittlement of the rail steel can be effectively avoided.

The invention will be explained in more detail with reference to embodiments of the invention.

EXAMPLE 1

A rail for railway vehicles was made of a hypereutectoid steel with the following directional analysis according to the method of the invention:

1.13 wt.-%
C

1.28 wt.-%
Mn

0.87 wt.-%
Si

0.39 wt.-%
Cr

0.15 wt.-%
V

0.03 wt.-%
Nb