OA20006A

OA20006A - Track Part Made of a Hypereutectoid Steel.

Info

Publication number: OA20006A
Application number: OA1202100006
Authority: OA
Inventors: Jürgen GORIUPP; Mario Kuss
Original assignee: Voestalpine Rail Technology Gmbh
Priority date: 2018-07-10
Filing date: 2019-07-03
Publication date: 2021-08-31

Abstract

In a track part, in particular rail for railway vehicles, made of a hypereutectoid steel, comprising a rail foot, a rail web and a rail head portion, a hypereutectoid steel with the following directional analysis is used: 0.98- 1.17 wt.-%of C 0.90 - 1.35% wt.-%ofMn 0.70- 1.10% wt.-% of Si 0.15-0.70 wt.-% of Cr and wherein the steel, at least in the head portion of the rail, has a pearl itic structure that is substantially free of secondary cementite networks

Description

Track part made of a hypereutectoid Steel

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 hâve been steadily increased to increase the efficiency of rail transportation. Rail tracks are therefore subject to difficult operating conditions and must hâve higher quality and strength to withstand the higher loads. The spécifie 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 émissions and operational disabilities. The increased occurrence these problems is also accelerated by the ever-increasing traffic loads. The immédiate conséquence of this development is an

0 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 2 5 rail, however, is to be replaced in case of severe damage. Therefore, attempts hâve been made in the past to improve both the wear résistance and the résistance to RCF damage in order to increase the life cycle of the rails.

With respect to pearlitic rail steels, increasing strengths hâve been found to hâve a very

0 favorable effect on wear behavior and RCF résistance, leading to the development of hypereutectoid rail steels in the past.

Hypereutectoid steels are known for producing rails, for example from EP 2388352 Al.

The iron-carbon diagram shows an eutectoid at a carbon content (C content) of 0.77% by weight of carbon and at a température 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 résistance and élongation at break, as it is best for wear due to the lamellar structure.

By définition, 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, 10 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 préférable for greater hardness and hence wear 15 résistance, 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 résistant 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 précipitâtes, are significantly influenced by a combination of alloying and heat treatment technology. Here, the knowledge of the précipitation température of the secondary cementite is of crucial importance and one should not fall below the précipitation température before the

5 subséquent heat treatment. In addition, the cooling rate during the heat treatment process should be as high as possible to suppress the précipitation of secondary cementite.

Excessive levels of secondary cementite can adversely affect the breaking properties of steels, as the breaking behavior becomes increasingly intercrystalline.

0 Essential for the quality of a rail is thus a high wear résistance, for which essentially the hardness (for example indicated as Brinell hardness) of the rail is characteristic.

However, the désirable hardness inevitably goes hand in hand with a réduction 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 résistance 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 hâve 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 specificationcompliant 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- 1.17wt.-%ofC

0.90 - 1.35% wt.-% of Mn

0.70 -1.10% wt.-% of Si

0.15-0.70 wt.-%ofCr 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 élongation at break for the heavy load range. In the context of the présent 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 précipitâtes 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 précipitâtes reduce the carbon supply for the formation of pearlitic cementite lamellae needed for wear résistance. For these reasons, the suppression of secondary cementite in hypereutectoid rail steels is considered to be advantageous.

The concentration of manganèse in the présent Steel is selected to shift the formation of embrittling secondary cementite to lower températures due to its austenite stabilizing effect, thereby enabling fine grain rolling at low rolling températures of hypereutectoid rail steels while suppressing secondary cementite précipitation.

The lower limit of 0.9 wt.-% manganèse was chosen because the précipitation température of embrittling secondary cementite shifts to higher températures if one falls below this lower limit, which would resuit in that the suppression of the same can not be guaranteed in the subséquent heat treatment. The upper limit of 1.35 wt.-% manganèse 2 0 was chosen to ensure castability of the Steel.

The alloy composition of the présent 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 2 5 was chosen in order to avoid the précipitation 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 30 sufficient élongation at break, such as, for example, an élongation 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 présent 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 résistance and élongation at 5 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 précipitâtes. This extends the 10 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 2 0 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 considération of the alloy composition according to the invention, the invention is based on a particular sélection of the quantitative ranges of the individual 2 5 alloy constituents, the individual alloying constituents having partially opposite effects.

For example, a higher carbon content is désirable for achieving high strengths, but as the carbon content increases, the disadvantage of the increasing austenite-to-perlite-transition température has to be considered. Although high Silicon contents are responsible for the suppression of embrittling secondary cementite précipitâtes, they also increases the

0 austenite-to-perlite-transition température. Overall, secondary cementite précipitâtes at relatively high températures due to the relatively high carbon and Silicon content, so that secondary cementite précipitâtes 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 hâve a diffusioninhibiting effect on carbon and thus minimize secondary cementite. To take advantage of this positive effect, the négative effect of the shift in the transformation température must be compensated. This is achieved according to the invention by the addition of the appropriate amounts of the austenite-stabilizing element manganèse.

In order to increase the élongation at break, according to a preferred embodiment of the présent 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 bénéficiai to the élongation 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 présent invention. It has been found that already starting from a vanadium content of 0.07 wt-%, an increase in strength and a grain refming 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 présent invention, Nb (niobium)

0 may additionally be used in amounts of 0.010- 0 .030 wt.-% in order to minimize the pearlite size and thereby positively influence the breaking élongation of the rail according to the invention.

From the same point of view as the addition of Al, Ti (titanium) may additionally be used 2 5 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

0 élongation at break, as will become apparent from the embodiments of the présent 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 éléments (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 présent invention.

In particular, with the composition according to the présent invention, a Steel quality is 10 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 élongation 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 présent 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 fumace at a température of 1000 - 1300°C, then rolled at a final rolling température of 850 ·· 950°C and is then subjected to forced cooling to a température of 450°C to 600°C. The furnace is preferably a walking beam furnace. The

0 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 précipitâtes in the non-eutectoid transition from the austenite phase to the perlite phase, according to the présent invention, one does not fall below a final rolling température, i.e. a température of the Steel at the end of the rolling mill, of 850°C, 2 5 since secondary cementite précipitâtes 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

0 induced précipitations and précipitâtes in solution taking into account the forming speed of the réduction per pass, the température and the alloy composition, which allows to realize a small former austenite grain size of 8 pm to 35 pm, at least in the head portion ofthe rail. According to the invention this is followed by a rapid cooling to below 600°C, in which température range no secondary cementite is precipitated any more, creating a very wear-resistant fine-pearlitic microstructure with sufficient élongation 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 précipitation 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 évaporation 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 précipitâtes 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 précipitâtes, when the forced cooling takes place in a polymer bath with a température 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 précipitâtes, 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 précipitâtes 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

0.15wt.-% V

0.03 wt.-% Nb

Companion and trace éléments, remainder Fe.

A rail with a tensile strength of 1580 MPa/mm², an élongation at break (As) of 8.5% and a hardness (RS) of 475 HB (Brinell hardness) was obtained.

Example 2:

1.12 wt.-% C

l.I0 wt.-% Mn

0.85 wt.-% Si

0.45 wt.-% Cr

0.15 wt.-% V

0.015 wt.-% Nb

Companion and trace éléments, remainder Fe.

A rail with a tensile strength of 1550 MPa/mm², an élongation at break (As) of 9.2% and a hardness (RS) of 470 HB (Brinell hardness) was obtained.

Example 3:

0.98 wt.-%C

1.15 wt.-%Mn

0.95 wt.-%Si

0.55 wt.-%Cr

Companion and trace éléments, remainder Fe.

A rail with a tensile strength of 1515 MPa/mm², an élongation at break (As) of 9.7% and a hardness (RS) of 463 HB (Brinell hardness) was obtained.

Example 4:

.01 wt.-%C

0.90 wt.-%Mn

0.90 wt.-%Si

0.53 wt.-%Cr

Companion and trace éléments, remainder Fe.

A rail with a tensile strength of 1500 MPa/mm², an élongation at break (As) of 9.6% and a hardness (RS) of 460 HB (Brinell hardness) was obtained.

Example 5:

.06 wt.- % C

1.20 wt.-%	Mn
0.95 wt.-%	Si
0.56 wt.-%	Cr
0.15 wt.-%	V

0.015 wt.-% Nb

Companion and trace éléments, remainder Fe.

A rail with a tensile strength of 1570 MPa/mm², an élongation at break (Ai) of 9.2% and a hardness (RS) of 478 HB (Brinell hardness) was obtained.

The upper limit values for the accompanying and trace éléments are listed in the context of the présent invention as follows:

0.017 wt.-%	P
0.017 wt.-%	S
0.10 wt.-%	Ni
0.15 wt.-%	Cu
0.05 wt.-%	Ti
0.02 wt.-%	Mo
0.03 wt.-%	Sn

0.003 wt.-% O

1.4 ppm 0.012 wt.-%	H As
0.01 wt.-%	Pb
0.01 wt.-%	Co
0.01 wt.-%	Sb
0.012 wt.-%	N

Rest: Fe

The rails produced according to Examples 1 to 5 hâve a purely pearlitic microstructure essentially free of secondary cementite networks according to Fig. 1

The rail material microstructure, at least in the standard tensile test position of the rail (10 mm below the running edge), has a pearlitic structure below 3%-nital-etching substantially free of secondary cementite networks corresponding to the classification chart in Fig. 3.

The cementite lamella thickness is significantly increased in the case of the rail according to the invention compared with a rail from the prier art (rail R400HT according to EN 13674-1), as can be seen from Fig. 2.

The degree of secondary cementite itself can be assessed with the aid of a classification chart for assessing the secondary cementite précipitâtes on the microstructure, as shown in Fig. 3.

... free of secondary cementite

... very few traces of secondary cementite

... isolated contiguous secondary cementite structures

... closed secondary cementite network

The wear résistance of rails corresponding to the examples was measured by means of a test device according to AT 409766 B (wheel-rail test bench) and compared with that of conventional rail steels according to EN 13674-1 (Fig. 4).

The results obtained show that the wear résistance of the invention examples could be significantly increased compared to the commercially available railroad tracks, whereby the increased demands on the product properties can be significantly better fulfilled with the aid of the invention.

Claims:

Claims

Claims:

1. Track part, in particular rail for railway vehicles, made of a hypereutectoid Steel, comprising a rail foot, a rail web and a rail head portion, wherein a hypereutectoid Steel with the following directional analysis is used:

0.98- 1.17wt.-%ofC

0.90 - 1.35% wt.-% of Mn

0.70 - 1.10% wt.-% of Si

0.15-0.70 wt.-%ofCr and the Steel, at least in the head portion of the rail, has a pearlitic structure that is substantially free of secondary cementite networks with at most 5% of the existing secondary cementite précipitâtes being in the form of secondary cementite networks, wherein the hypereutectoid Steel additionally contains optionally Al in an amount of 0.01 - 0.06 wt.-% and optionally V in an amount of 0.07 to 0.20 wt.-%, in particular 0.10 to 0.20 wt.-%, and optionally Nb in an amount of 0.01 - 0.03 wt.-% and optionally Ti in an amount of 0.015 - 0.05 wt.-%, wherein the hypereutectoid Steel, at least in the head portion of the rail, has a tensile strength greater than 1500 MPa, an élongation at break of greater than 8% and a Brinell hardness greater than 460 HB.
2. Track part according to claim 1, characterized in that C is présent in amounts of 1.05 to 1.17 wt.-%, preferably 1.06 to 1.15 wt.-%, and particularly preferably 1.08 wt.-%.
3. Track part according to claim 1 or 2, characterized in that the hypereutectoid Steel additionally contains N in an amount ranging from 40 to 120 ppm.
4. A method for producing a track part according to any one of claims 1 to 3, 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 température of 1000 - 1300°C, then rolled at a final rolling température of 850 - 950°C and is then subjected to forced cooling to a température of 450°C to 600°C, wherein 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..
5. The method according to claim 4, characterized in that the deformation in the

5 température range 1000-850°C at least in the head portion of the rail has an accumulated comparative degree of deformation of min 1.4.
6. The method according to claim 4 or 5, characterized in that the forced cooling takes place at least in the head portion of the rail.
7. The method according to any one of claims 4 to 6, characterized in that the forced cooling takes place in a bath of cooling medium not being pure water.
8. The method according to any one of claims 4 to 7, characterized in that the forced 15 cooling takes place in a polymer bath having a température of 15 - 50°C.