Patent Application
20150218759
The invention relates to a track part, in particular a rail for rail vehicles, made from low-alloy steel.
The invention, moreover, relates to a method for producing a track part from a hot-rolled section, and a device for carrying out said method.
In recent times, the weights of the loads transported by railroad traffic and the travelling speeds have continuously increased in order to enhance the efficiency of rail transport. Railroad tracks are, therefore, subjected to aggravated operating conditions and, as a result, have to be of higher quality to withstand higher loads. Concrete problems involve strongly increased abrasion, in particular of rails mounted in curves, and the occurrence of damage due to material fatigue, which is primarily created on the running edge, which constitutes the main point of contact between the rail and the wheels in a curve. Rolling-contact fatigue (RCF) damage will result. Examples of RCF surface damage include headchecks (rolling fatigue), spalling (chipping off), squats (plastic surface deformations), slip waves, and corrugations. These types of surface damage will result in shortened rail service lives, elevated noise emissions, and operational impediments. The increased occurrence of such failures, in addition, will be accelerated by the constantly growing traffic loads. The direct consequence of such a development is an elevated rail maintenance demand. The growing maintenance demand is, however, in contradiction to the constantly decreasing maintenance windows. Higher train densities will increasingly reduce the periods to work on rails.
Although the above-cited types of damage can be eliminated in their early stages by grinding, the rail will have to be exchanged when heavy damage has occurred. Many attempts have, therefore, been made in the past to improve both the wear resistance and the resistance to RCF damage, in order to increase the life cycles of rails. Among others, this has been done by the introduction and use of bainitic rail steels.
Bainite is a structure that can form during the thermal treatment of carbon-containing steel by isothermal transformation or continuous cooling. Bainite is formed at temperatures and cooling rates ranging between those necessary for the formation of perlite and martensite, respectively. Unlike with the formation of martensite, flip-over processes in the crystal lattice and diffusion processes are coupled in this case, thus enabling different transformation mechanisms. Due to its dependence on the cooling rate, carbon content, alloying elements and the hence resulting formation temperature, bainite does not have a characteristic structure. Bainite, like perlite, consists of ferrite and cementite (Fe3C) phases, yet differs from perlite in terms of shape, size and distribution. Basically, bainite is distinguished in two main structural forms, i.e. upper bainite and lower bainite.
From AT-407057 B, a rail material has become known, in which a structural transformation of austenite is explicitly only formed in the range of lower bainite, thus imparting to the profiled rolling stock a hardness of at least 350 HB, in particular 450-600 HB.
A bainite base structure can also be obtained by higher portions of alloying components such as, e.g. a higher chromium content of 2.2 to 3.0% by weight, as described in the documents DE 1020060308915 A1 and DE 102006030816 A1. The high portion of alloying components will, however, involve undesiredly high costs and complex welding engineering tasks. DE 202005009259 U1 also describes a bainitic, high-strength track part of high-alloy steel comprising, in particular, high alloy portions of Mn, Si and Cr. With such a high-alloy steel, the bainite formation can be triggered in a simple manner by cooling in static air. By contrast, low-alloy steels enable bainite formation only at controlled cooling.
Accordingly, DE-1533982, for instance, describes a method for heat-treating rails, in which a rail still having rolling temperature is taken up by a lifting device upon leaving the roll stand and immersed, with the rail head down, in a fluidized bed kept at constant temperature, where it is cooled wherein a bainite crystalline structure is obtained in that the temperature of the fluidized bed is chosen between 380 and 450° C. and the rail is left in the fluidized bed between 300 and 900 seconds, depending on the temperature of the latter.
Another way of producing high-strength rails from low-alloy steels having bainite structures for achieving an enhanced resistance against fatigue damage by rolling contact has become known from EP-612852 B1. The head of the rail is subjected to an accelerated cooling from the austenitic range at a rate of 1-10° C./s until a cooling interruption temperature of 500-300° C. is reached. After that rapid cooling, the rail head is further cooled down to almost room temperature by applying natural cooling with heat recovery or forced cooling at a rate of 1-40° C./min.
Although the formation and propagation of cracks on the rail head could be slowed down by the above-cited measures, they could, however, not be prevented.
The invention, therefore, aims to improve a track part, in particular rail, which is to be made of low-alloy steel for cost reasons, and for reasons of welding engineering, to the effect that even with elevated wheel loads, no rolling contact fatigue damage and, in particular, no cracks will occur either on the running edge or on the running surface. Furthermore, the wear resistance is to be increased so as to ensure lifetimes of more than 30 years. Finally, the track part is to be perfectly weldable and exhibit also other material properties similar to those of steels that have so far proven successful in rail construction, e.g. a similar electrical conductivity and a similar thermal expansion coefficient.
The invention further aims to provide a simple production method that is characterized by a short process time (avoidance of annealing phases), a high reproducibility, and a high efficiency. The method is to be suitable for producing long rails having, for instance, lengths of more than 100 m, wherein constant material properties are to be ensured over the entire rail length.
To solve this object, the invention according to a first aspect provides a track part of the initially mentioned type, which is further developed such that the steel in the rail head of the track part comprises a ferrite portion of 5-15% by volume and a multiphase bainite structure consisting of upper and lower bainite portions. Due to the combination of a ferrite structure with a bainite structure, excellent toughness properties and a sufficiently high hardness will be achieved. The ferrite structure component serves as a plasticity carrier and prevents possibly formed cracks from extending into the material as headchecks. The ferrite portion imparts a continuous network with intercalated bainite to the overall structure. In this context, it is referred to a percolation threshold that has to be reached in order to achieve such a formation of contiguous areas (clusters). The ferrite is preferably an acicular ferrite. As opposed to a non-acicular structure, and as opposed to a perlite structure, the acicular structure is characterized by a higher tensile strength and wear resistance. The acicular ferrite has a microstructure that is characterized, by needle-shaped crystallites or grains, said crystallites being not uniformly oriented but present in a completely disoriented fashion, which will positively influence the toughness of the steel. The disoriented arrangement of the grains causes the individual grains to mutually interlock, which, in combination with the multiphase bainite effectively prevents the formation and propagation of cracks. In particular, it is thereby ensured that cracks possibly formed on the surface (headchecks) will not grow into the material depth as is, for instance, the case with a perlite structure. The track part will thus only be subjected to wear such that its period of use can be precisely determined and any further observation as to the formation of cracks can be obviated.
What is furthermore decisive is the presence of a multiphase bainite comprising upper and lower bainite portions. The upper bainite is formed in the upper temperature range of bainite formation and has a needle-shaped structure similar to that of martensite. In said upper temperature range of bainite formation, there are favorable diffusion conditions allowing the carbon to diffuse to the grain boundaries of the ferrite needles. Irregular and interrupted cementite crystals are consequently formed. Due to the irregular distribution, the structure frequently has a granular appearance such that the upper bainite is sometimes also referred to as granular bainite. The lower bainite forms under isothermal and continuous cooling in the lower temperature range of bainite formation. By the formation of ferrite the austenite is enriched with carbon, with further cooling the austenite ranges are converted into ferrite, cementite, acicular bainite and martensite. Bainitizing will reduce internal stresses and increase the toughness.
The mixing ratio between lower and upper bainites may basically be varied within wide limits as a function of the respective requirements. The choice of the mixing ratio will, in particular, determine the hardness of the steel. In the context of the invention it is preferably provided that the portion of upper bainite is 5-75% by volume, in particular 20-60% by volume, and the portion of lower bainite is 15-90% by volume, in particular 40-85% by volume.
The ferrite portion is preferably 8-13% by volume.
The formation of carbide from the austenite is the prerequisite for a complete bainite transformation. Since carbides take up large amounts of carbon, they constitute carbon sinks withdrawing carbon from the austenite. If the formation of carbide is prevented or retarded, for instance by silicon as alloying element, major amounts of austenite will not be transformed. They will consequently be partially or completely present as residual austenite after quenching to room temperature. The amount of residual austenite depends on how much the martensite start temperature has dropped in the remaining austenite. In the context of the invention it will be advantageous if portions of austenite and/or martensite as low as possible are left. In this respect, the invention therefore preferably provides that the steel in the rail head of the track part comprises a residual martensite/austenite portion of <2% by volume.
As already pointed out above, low-alloy steels are used according to the invention in order to minimize costs and improve weldability. In general, the low-alloy steel in the context of the invention preferably comprises silicon, manganese and chromium as well as, optionally, vanadium, molybdenum, phosphorus, sulfur and/or nickel as alloying components.
Steel in the context of the invention shall be referred to as low-alloy steel if none of the alloying components is present in a portion higher than 1.5% by weight.
Particularly good results could be achieved with a low-alloy steel having the following reference analysis:
A particularly good aptitude for highly loaded track sections will preferably be provided if the track part has a tensile strength Rm higher than 1150 N/mm2 in the head region. Furthermore, the track part has a hardness of above 340 HB in the head region.
According to a second aspect, the invention provides a method for producing the above-described track part, by which the track part is produced from a hot-rolled section, wherein the rail head of the rolled section is subjected to controlled cooling immediately upon leaving the roll stand at rolling heat, said controlled cooling comprising in a first step accelerated cooling until reaching a first temperature allowing the formation of ferrite, in a second step maintaining said first temperature to effect the formation of ferrite, in a third step further cooling within a temperature range allowing the formation of multiphase bainite until a second temperature, and in a fourth step maintaining said second temperature. Such controlled cooling is preferably performed by immersing at least the rail head into a liquid coolant, as is known per se.
The first step preferably starts at a temperature of 740-850° C., in particular about 790° C., and preferably ends at a temperature of 450-525° C. The cooling effected during the first step must be controlled in such a manner as to reach the ranges of ferrite, and subsequently bainite, formation in the time-temperature-transformation diagram, wherein no transformation is to take place in the perlite stage, in particular. To this end, the accelerated cooling in the first step is preferably performed at a cooling rate of 2-5° C./s. In order to achieve said cooling rate, it is preferably proceeded in such a manner that the track part is completely immersed in the coolant during the first step.
In the second step, the temperature of preferably 450-525° C. is maintained, while the portion of ferrite, in particular the portion of acicular ferrite, which is important for the use properties, is formed at a volume portion of 5-15%, in particular 8-13%, in particular about 10%. The maintenance of the temperature is preferably achieved in that the track part is held in a position removed from the coolant during the second step.
In a third step, further controlled cooling is performed for the required limitation of the ferrite portion, thus causing the formation of a mixture of upper and lower bainite structures (multiphase bainite). The temperature range in which bainite formation occurs preferably ranges between 450-525° C. and 280-350° C., i.e. that the rail head of the track part is cooled from 450-525° C. to 280-350° C. in the bainite formation phase. Said third step preferably extends for a period of 50-100 s, in particular about 70 s. In the bainite formation phase it will preferably do to immerse the track part into the coolant merely by the rail head.
When subsequently keeping the temperature of the track part to preferably range from 280-350° C. in the fourth step, the hardness of the track part is finally fixed as a function of the temperature position, wherein falling below the martensite start temperature (usually about 280° C.) is to be avoided, since in that temperature range too many martensitic, brittle structural components might form. The maintenance of the temperature during the fourth step is preferably ensured by cyclic head immersion, i.e. the track part is cyclically immersed into the coolant and removed from the coolant.
Since the bainite phase formation temperature range and the martensite start temperature depend on the alloying elements of the respective steel and their respective percentages, the value of the first temperature and the value of the second temperature have to be precisely determined beforehand for the respective steel. The temperature of the rail is continuously measured during the controlled cooling, wherein the cooling and maintenance steps are started and terminated, respectively, when the respective temperature thresholds are reached. Since the surface temperature of the rail can vary over the entire length of the track part, yet cooling is performed uniformly for the whole track part, it is preferably proceeded in such a manner that the temperatures are detected on a plurality of measuring points distributed over the length of the track part and a temperature mean value is formed, which is used for controlling said controlled cooling.
In the bainite formation phase, austenite is transformed to bainite as completely as possible. This occurs at temperatures below the formation of perlite as far as to the martensite start temperature both isothermally and during continuous cooling. By the austenite flipping over slowly, ferrite crystals strongly oversaturated with carbon and having cubic space-centered crystal lattices are formed, departing from the grain boundaries or crystal defects. Due to the higher diffusion rate in the cubic space-centered lattice, the carbon precipitates in the form of spherical or ellipsoid cementite crystals within the ferrite grain. The carbon can also diffuse into the austenite range and form carbide.
In the context of the invention, cooling and temperature-holding during the third and fourths steps are effected in such a manner as to form multiphase bainite. In a first sub-step, continuous cooling is effected at a lower cooling rate than in a second sub-step, in which the temperature is abruptly lowered until the second temperature is reached. During the first sub-step, primarily upper bainite is formed. After the abrupt cooling, the second temperature is maintained in the fourth step while lower bainite is formed. The maintenance time of the second temperature during the fourth step determines the extent of the formation of lower bainite.
Upper bainite is comprised of acicular ferrite arranged in packets. Between the individual ferrite needles, there are more or less continuous films of carbides extending in parallel with the needle axis. Lower bainite by contrast is comprised of ferrite plates within which carbides are formed at angles of 60° relative to the needle axis.
During the controlled cooling by means of liquid coolant, the coolant undergoes three phases of the quenching procedure. In the first phase, i.e. the vapor film phase, the temperature on the surface of the rail head is so high that the coolant will rapidly evaporate while forming a thin, insulating vapor film (Leidenfrost effect). This vapor film phase, among others, is strongly dependent on the vapor formation heat of the coolant, the nature of the surface of the track part, e.g. scale, or the chemical composition and configuration of the cooling tank. In the second phase, i.e. the boiling phase, the coolant comes directly into contact with the hot surface of the rail head and immediately starts to boil, which results in a high cooling rate. The third phase, i.e. the convection phase, starts as soon as the surface temperature of the track part has dropped to the boiling point of the coolant. In this range, the cooling rate is substantially influenced by the flow rate of the coolant.
During the controlled cooling provided by the invention, the coolant is preferably in the vapor film phase during the first step. It is preferably further proceeded in such a manner that cooling during the third step is controlled so as to cause the coolant to initially form a vapor film on the surface of the rail head and then boil on said surface. Thus, a transition from the vapor film phase into the boiling phase takes place. The vapor film phase extends over the length of the above-mentioned first sub-step, in which primarily upper bainite is formed. After having reached the boiling phase, the temperature abruptly drops to the second temperature, i.e. to preferably 280-350° C.
The transition from the vapor film phase to the boiling phase usually takes place in a relatively uncontrolled and spontaneous manner. Since the rail temperature is subject to certain production-related temperature deviations over the entire length of the track part, there is the problem that the transition from the vapor film phase to the boiling phase takes place at different times in different length regions of the track part. This would lead to a non-uniform crystalline structure, and hence non-uniform material properties, over the length of the track part. In order to harmonize the time of the transition from the vapor film phase to the boiling phase over the entire length of the rail, a preferred mode of operation provides that, during the third step, a film-breaking, gaseous pressure medium such as, e.g., nitrogen is supplied to the rail head along the entire length of the track part in order to break the vapor film along the entire length of the track part and trigger the boiling phase.
It can, in particular, be proceeded such that the state of the coolant during the third step is monitored along the entire length of the track part and the film-breaking, gaseous pressure medium is supplied to the rail head as soon as the first appearance of the boiling phase is noticed in a portion of the track part length.
The film-breaking, gaseous pressure medium is preferably supplied to the rail head about 20-100 s, in particular about 50 s, after the beginning of the third step.
According to a further aspect of the invention, a device for carrying out the above-described method is proposed, comprising a cooling tank corresponding to the length of the track part and capable of being filled with coolant, a lifting and lowering device for the track part to immerse the track part into the cooling tank and lift it out of the same, a temperature-measuring device for measuring the temperature of the track part, pressure medium generating means for injecting pressure medium into the coolant, means for controlling the temperature of the coolant, and a control device to which the measurements of the temperature measuring device are fed and which interacts with the lifting and lowering device for controlling the lifting and lowering operations, and with the means for controlling the temperature of the coolant as a function of the temperature measurements, and furthermore with the pressure-medium generating means.
In a preferred manner, sensors for detecting coolant boiling on the surface of the rail head are provided, whose sensor measurements are fed to the control device in order to activate the pressure medium generating means as a function of the sensor measurements. In particular, a plurality of sensors may be provided for detecting coolant boiling on the surface of the rail head, which sensors are distributed over the length of the cooling tank.
In a preferred manner, the sensor measurements of the plurality of sensors are fed to the control device, said control device activating the pressure medium generating means as soon as at least one sensor has detected coolant boiling on the surface of the rail head.
The control device is advantageously configured to perform controlled cooling which comprises in a first step accelerated cooling until reaching a first temperature allowing the formation of ferrite, in a second step maintaining said first temperature to effect the formation of ferrite, in a third step further cooling within a temperature range allowing the formation of multiphase bainite until a second temperature, and in a fourth step maintaining said second temperature.
The control device may, in particular, be configured to reduce the temperature of the rail head in the first step to a first temperature of 450-525° C. at a cooling rate of 2-5° C./s, to keep the temperature of the rail head in the second step at the first temperature, and to reduce the temperature of the rail head during the third step to a second temperature of 280-350° C., preferably for a period of 50-100 s, in particular about 70 s.
The control device is preferably configured to activate the pressure medium generating means during the third step.
In the following, the invention will be explained in more detail by way of exemplary embodiments.
A low-alloy steel having the following reference analysis was formed by hot-rolling to a running rail with a standard rail profile:
Immediately upon leaving the roll stand, the rail was subjected to controlled cooling at rolling heat. Said controlled cooling will be explained below with reference to the time-temperature-transformation diagram represented in
In a first exemplary embodiment, a low-alloy steel having the following reference analysis was formed by hot-rolling to a running rail with a standard rail profile:
The following fine structure was achieved in the rail head by the above-described controlled cooling:
The fine structure is illustrated in
Due to the higher portion of upper bainite, a lower hardness of the rail head than in the subsequent, second exemplary embodiment was achieved. The following material properties were measured.
Tensile strength: 1162 MPa
0.2% yield strength: 977 MPa
Breaking elongation: 14.4%
Notched impact test:
In the second exemplary embodiment, the same low-alloy steel as in Example 1 was taken and formed by hot-rolling to a running rail with a standard rail profile. Controlled cooling was performed similarly as in Example 1, yet the temperature in the fourth step was maintained for a longer period than in Example 1. The following fine structure was obtained in the rail head:
The fine structure is illustrated in
The following material properties were measured.
Tensile strength: 1387 MPa
0.2% yield strength: 1144 MPa
Breaking elongation: 12.6%
Notched impact test:
| Number | Date | Country | Kind |
|---|---|---|---|
| A 990/2012 | Sep 2012 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AT2013/000107 | 6/27/2013 | WO | 00 |
