U.S. patent application number 09/814710 was filed with the patent office on 2001-10-04 for method for heat-treating profiled rolling stock.
This patent application is currently assigned to VOEST ALPINE SCHIENEN GmbH & Co.. Invention is credited to Moser, Alfred, Pointner, Peter, Prskawetz, Georg.
Application Number | 20010025674 09/814710 |
Document ID | / |
Family ID | 25595837 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010025674 |
Kind Code |
A1 |
Prskawetz, Georg ; et
al. |
October 4, 2001 |
Method for heat-treating profiled rolling stock
Abstract
A railroad rail of a steel alloy having a pearlitic
microstructure, which rail has a good long-term serviceability,
high ductility and high abrasion resistance of the working surface
at the rail head, the rail being formed of a steel alloy having a
chemical composition in weight % of 0.4 to 1.0 carbon, 0.1 to 1.2
silicon, 0.5 to 3.5 manganese, up to 1.5 chromium, optionally other
alloy elements below 1 weight %, the rest being iron and
impurities, and at least the rail head having a portion of fine
pearlitic microstructure and a hardness between 340 HB and 425 HB
down to a sufficient depth from the top surface, the arrangement
and the extension of the portion of fine pearlitic microstructure
in the rail cross section being even along the entire length of the
rail.
Inventors: |
Prskawetz, Georg; (Leoben,
AT) ; Pointner, Peter; (Leoben, AT) ; Moser,
Alfred; (Leoben, AT) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1941 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
VOEST ALPINE SCHIENEN GmbH &
Co.
Donawitz
AT
|
Family ID: |
25595837 |
Appl. No.: |
09/814710 |
Filed: |
March 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09814710 |
Mar 23, 2001 |
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09570455 |
May 12, 2000 |
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6224694 |
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09570455 |
May 12, 2000 |
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08320408 |
Oct 3, 1994 |
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Current U.S.
Class: |
148/333 ;
148/337; 148/581 |
Current CPC
Class: |
C21D 2211/009 20130101;
C21D 2221/02 20130101; C21D 1/63 20130101; C21D 8/00 20130101; C21D
9/04 20130101 |
Class at
Publication: |
148/333 ;
148/581; 148/337 |
International
Class: |
C22C 038/18; C21D
009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 1994 |
AT |
A 1431/94 |
Claims
What is claimed is:
1. A railroad rail of a steel alloy having a pearlitic
microstructure, which rail has a weight of 30 to 100 kg/m, good
long-term serviceability, high ductility and high abrasion
resistance of the working surface at the rail head, characterized
in that the rail is formed of a steel alloy having a chemical
composition in weight % of
1 carbon (C) 0.4 to 1.0 silicon (Si) 0.1 to 1.2 manganese (Mn) 0.5
to 3.5 chromium (Cr) up to 1.5,
optionally other alloy elements below 1 weight %, the rest being
iron (Fe) and impurities occurring in the manufacturing process,
and that at least the rail head has a portion of fine pearlitic
grain microstructure and a hardness between 340 HB and 425 HB down
to a sufficient depth from the top surface, whereas the remaining
rail portions have a hardness which is lower by more than 10 to 40
HB than that in the head portion, and that the arrangement and the
size and extension, respectively, of the portion of fine pearlitic
grain microstructure and increased hardness in the rail cross
section is even along the entire length of the rail and that the
rail shows a good weldability.
2. The rail of claim 1, characterized in that the central area at
the base of the rail, opposite the web, has a higher hardness than
the portions in the peripheral parts of the base and in the
web.
3. The rail of claim 1, characterized in that the hardness in the
transition from the upper head portion to the lower head portion
and to the web portion decreases continuously.
4. A process for the manufacture of a railroad rail of a steel
alloy having a pearlitic microstructure, which rail has a weight of
30 to 100 k/gm, good long-term serviceability, high ductility and
high abrasion resistance of the working surface at the rail head,
characterized in that the rail which has a chemical composition in
weight % of
2 carbon (C) 0.4 to 1.0 silicon (Si) 0.1 to 1.2 manganese (Mn) 0.5
to 3.5 chromium (Cr) up to 1.5,
optionally other alloy elements below 1 weight %, the rest being
iron (Fe) and impurities occurring in the manufacturing process, is
shaped, during the last pass of the multiple longitudinal rolling,
at a reduction rate of 1.8 to 8% and aligned straight in its
longitudinal direction at a temperature between 770.degree. C. and
1050.degree. C., whereafter the rail is mounted in vertically
suspended position with the head down and is allowed to cool slowly
in still air to a temperature of 5 to 120.degree. C. above the
Ar.sub.3 temperature at a rate of 3.degree. C./min, and upon
reaching this temperature at least the rail head is dipped, in its
entire longitudinal extension, into a cooling liquid and is cooled,
within a range between 800.degree. C. and 450.degree. C., with
increased cooling intensity and at a rate of 1.6 to 2.4.degree.
C./s, to the temperature of conversion of the austenitic grain
microstructure into a fine pearlitic grain microstructure, followed
by lifting the rail out of the cooling liquid, placing it onto a
cooling bed and allowing it to cool slowly in still air.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 09/570,455 filed May 12, 2000 which is a
continuation of U.S. patent application Ser. No. 0/320,408 filed
Oct. 3, 1994, now abandoned, which claims priority under 35 U.S.C.
.sctn. 119 of Austrian Patent Application No. A 1431/94 filed Jul.
19, 1994, the disclosures of all three documents are expressly
incorporated by reference herein in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a method for heat-treating profiled
rolling stock, in particular track or railroad rails, with an
increased heat removal from portions of the profile surface during
cooling in the gamma range of the basic iron material, wherein a
conversion into a fine pearlitic grain of increased strength, in
particular increased wear resistance and increased hardness takes
place in the desired cross-sectional area(s), particularly in the
head area of rails, and, if required, a deformation or bending by a
thermally caused warping of the rolling stock, in particular the
rail, perpendicularly to the longitudinal axis is decreased,
preferably essentially prevented, during cooling to room
temperature, particularly following a structural conversion in the
more heavily cooled cross-sectional area(s), and an increased
rigidity and fatigue strength under reversed bending stresses is
achieved.
[0004] The invention further relates to a device for the heat
treatment of profiled rolling stock, in particular track or
railroad rails, essentially including at least one stand-by area
for the rolling stock at the roller table, with a rolling stock
positioning device, a cooling treatment area, with devices for
partial high intensity heat removal from the surface of the rolling
stock and a final cooling area for cooling the rolling stock to
room temperature, as well as depositing, transverse transporting,
stopping and manipulating device.
[0005] Finally, the invention relates to profiled rolling stock, in
particular to a track or railroad rail, including of a rail head of
an at least partial pearlitic grain structure, a rail base and a
web between the rail head and the rail base.
[0006] 2. Discussion of Background Information
[0007] Profiled rolling stock, in particular track or railroad
rails, is mainly produced from basic iron alloys with weight-%
contents between 0.4 and 1.0 C, 0.1 and 1.2 Si, 0.5 and 3.5 Mn, if
required up to 1.5 Cr, as well as other alloy elements at
concentrations below 1%, the rest being iron and impurities
occurring in the manufacturing process. Based on the usual
dimensions, for example a weight between 30 to 100 kg/m, and the
ratio of cross section to circumference of rails resulting
therefrom, during cooling of the rolling stock from the conversion
heat in still air, for example on cooling beds and the like, a
conversion of the grain from an austenitic into a rough pearlitic
structure, possibly having portions of ferrite, because of slow
cooling, takes place. The previously mentioned materials having the
above structure have a hardness in the range between 250 HB to 350
HB.
[0008] An increase in traffic and larger axial loads, as well as
the desire to improve the durability of rails in practical use has
resulted in a multitude of suggestions for increasing the strength
and wear resistance of the material. In the course of this it is
possible to achieve more advantageous or improved material
properties with a hardness of 400 HB and above by measures in
respect to heat treatment and/or alloy techniques.
[0009] However, rails should be easy to weld in the field for
reasons, among others, of forming shock-free sections or multiple
lengths, so that measures in respect to alloy techniques for
increasing the hardness or strength and durability of the material
can mostly be applied on a small scale only due to the welding
problems and are aimed to a heat treatment matched to the
composition of the steel (German Patent Publication DE-C 34 46 794,
European Patent Publications EP-B-0 187 904, EP-B-0 186 373). For
economic reasons, such methods have also not proven themselves on a
large scale.
[0010] To increase the useful properties of rails and switch parts
made from the above mentioned materials it is possible and known to
one skilled in the art to provide a fine pearlitic material
structure by a thermal tempering treatment. In the process it is
important to set appropriate cooling conditions or cooling rates
for the cool-down from the austenitizing temperature. For example,
European Patent Publication EP-B-0 293 002 suggests for this
purpose to perform, after an initially high cooling intensity, a
practically isothermic structural conversion at approximately
530.degree. C. It is furthermore known from German Published,
Non-Examined Patent Application DE-OS 28 20 784 to perform
hardening of rails of a defined composition in boiling water and to
achieve a desired cooling intensity for setting a fine pearlitic
structural state by additives and movement steps.
[0011] In accordance with Austrian Patent AT-PS-323 224 it had also
been suggested to produce rails with a homogeneous fine pearlitic
structure from a selected alloy by the application of defined
cooling parameters, for example a cooling speed between 10 and
20.degree. C./s down to a temperature of no more than 550.degree.
C. However, the above steps have the common disadvantage that,
depending on the mass concentration of the rolling stock profile,
an even cooling intensity of the surface can cause different
cooling speeds and structural forms in the zones close to the
surface, and that it is often necessary to take elaborate
precautions to prevent undesired local structural form or material
properties, in particular excessive hardness and brittleness, in
parts of the rail which are primarily stressed by bending.
[0012] In many cases it was also proposed to provide in a directed
manner a heterogeneous microstructure in the cross section of a
rail in accordance with the respective stresses. For example, a
method is known from German Patent Publication DE-C-30 06 695, in
accordance with which a conversion over the entire cross section is
caused from the rolling heat by cooling the rail, after which the
head of the rail is re-austenitized by inductive heating and
subsequently hardened. In accordance with WO94/02652 it was further
proposed to cool the rail head to a surface temperature between 450
and 550.degree. C. in a cooling medium of a specially set cooling
intensity and in this way to create a fine pearlitic grain therein.
A device for the suspended hardening of rails in accordance with
German Patent Publication DE-C-40 03 363 is suitable for such
treatment.
[0013] However, the inhomogeneous cooling over the cross section of
profiled rolling stock can lead to curvatures or deviations from
the straightness at room temperature. To avoid this disadvantage it
has been proposed (German Patent Publication DE-A-42 37 991) to
transport or cool rails suspended, preferably with the head down,
on a cooling bed, however, a directed formation of a heterogeneous
grain structure over the cross section is hardly possible here.
[0014] All of the methods and devices known up to now have the
common disadvantage that although they disclose solutions in
limited areas or regarding individual method steps leading to the
desired goal in the manufacture of profiled rolling stock,
overcoming all the problems in a satisfactory way cannot be shown
in connection with an economical production of long rails of high
quality and with special finishing properties.
SUMMARY OF THE INVENTION
[0015] The invention is intended to provide relief in this area and
its object is, while removing the disadvantages of the known
production types, to recite a novel method by which profiled
rolling stock having particularly advantageous useful properties
can be produced. It is a further object of the invention to make
available a device especially for executing the method and to
design rolling stock, in particular a rail, for highest
stresses.
[0016] In a method in accordance with the species this object is
attained in that the rolling stock, in particular the rail, at an
average temperature of at most 1100.degree. C., preferably at most
900.degree. C., but at least 750.degree. C., and aligned straight
in its longitudinal direction during its plastic shaping, is in its
aligned state moved into a transverse direction and held there and,
in a first step of cooling the rolling stock or the rail, it is
allowed to cool evenly to a temperature below 860.degree. C.,
preferably approximately 820.degree. C., in particular to 5 to
120.degree. C. above the Ar.sub.3 temperature of the alloy with the
same local cooling intensity, preferably essentially by radiation
in still air. In a second step of cooling, heat is removed from the
rolling stock in the longitudinal direction with an intensity which
locally is essentially the same, but viewed in cross section is
circumferentially different, and the cooling intensity in at least
one zone at the circumference of the profiled rolling stock is
increased, wherein the larger cooling intensity(ies) are assigned
to the area(s) with a large ratio of the cross section to the
circumference or with a large portion of volume in respect to the
surface or with a high mass concentration and/or those with locally
high temperatures of the rolling stock, and the area(s) of a
cooling speed increased in this manner is (are) brought to the
conversion temperature, under which cooling condition a fine
pearlitic grain structure free of martensite is formed. Then, in a
subsequent step, cooling to room temperature at the same local
cooling intensity, for example in still air, is performed.
[0017] It is important that a straight alignment of the rolling
stock during plastic shaping takes place and this is performed
within a temperature range between 750.degree. C. and 1100.degree.
C. It has been found that lower temperatures than 750.degree. C.
can lead to partially resilient bending with deviations from the
straight alignment and as a result to inhomogeneous cooling
intensity in the longitudinal direction of the rail. In most cases
rolling stock temperatures above 1100.degree. C. cause a growth of
the austenite bodies or the formation of rough grains, by which the
material properties can be disadvantageously affected in the end.
Based on straight aligned rolling stock, it has been found to be
important for the formation of a fine pearlitic area of the cross
section which is evenly developed in the longitudinal direction
that the rolling stock is held and allowed to cool evenly in a
first cooling step to a temperature below 860.degree. C. at the
same local cooling intensity. In the process it is possible, on the
one hand, to compensate a local inhomogeneity of the temperature
distribution in the longitudinal direction which possibly might
have been caused by the partial resting on a transverse transport
device, on the other hand an axially symmetrical or
center-symmetrical temperature distribution is provided in the
cross section of the profiled rolling stock and in this way its
straightness is stabilized. It is particularly advantageous to
perform this compensating cooling to a temperature of 5 to
120.degree. C. above the Ar.sub.3 temperature of the alloy in order
to provide advantageous conditions for a partial conversion of the
grain into a fine pearlitic structural shape in portions of the
cross section. In this case the Ar.sub.3 temperature is the
temperature at which a conversion of the gamma grid into the alpha
grid of the alloy begins at a cooling velocity of 3.degree.
C./min.
[0018] Cooling of the rolling stock at an intensity of heat removal
which in the longitudinal direction is essentially the same but,
viewed in cross section is different circumferentially is known per
se. However, it is important to assign the areas of increased
cooling intensity of the surface to correspond with the mass
concentration of the rolling stock. In connection with a straight
alignment, compensating cooling and setting of a symmetrical
temperature distribution and an assignment of the cooling areas it
is possible to maintain a cooling speed, which is different over
the cross-sectional areas, but essentially the same in the
longitudinal direction of the rolling stock. In this connection it
is important to set the value of the cooling speed with which the
selected area of the rolling stock is brought to the conversion
temperature in a manner known per se. As can be seen in FIG. 3,
which is a time-temperature conversion diagram of an alloy of known
composition and is known to one skilled in the art, in the course
of higher rates of cooling from the Ar.sub.3 temperature, for
example the curves c and d, martensite parts are formed in the
grain, because of which the material achieves greater hardness, but
loses considerably in elasticity and has increased breaking
tendencies and the intended use is no longer possible. Low cooling
rates, for example those of the curve h, create a rough pearlitic
soft grain structure. Thus, it is important to set the local
cooling rates high enough that martensite formation during
conversion is prevented in every case, but that a fine pearlitic
grain is created in the area of increased cooling intensity.
Following the complete grain conversion, the rolling stock is
brought to room temperature at the same local cooling intensity in
order to reduce or to essentially prevent bending of the rolling
stock.
[0019] It is particularly advantageous if the heat treatment is
performed by the hot forming heat following the hot forming of the
rolling stock at a deforming degree of 1.8 to 8%, preferably 2 to
5%, during the last tapping at a temperature of at least
750.degree. C. and at most 1050.degree. C. A final deformation with
a deformation degree or a cross-sectional reduction of 1.8 to 8%
causes an advantageous austenite grain refining if conversion takes
place in a temperature range between 770.degree. C. to 1050.degree.
C. It has been shown that lesser conversion degrees than 1.8 cause
a particularly strong rough grain or grain growth in places, but
larger conversions than 8% cause a large temperature increase in
central or interior areas, apparently because of released
conversion energy, because of which inhomogeneities in the grain
can be caused locally and reductions in quality can occur.
[0020] In view of receiving essentially straight aligned or axially
aligned rolling stock after cooling to room temperature and
particularly in view of rails having increased rigidity and fatigue
strength under reversed bending stresses, it is of great advantage
if in the second step of cooling the cooling intensity is increased
in two or more zones at the circumference of the profiled rolling
stock. In this manner, it is possible to achieve increased hardness
and increased strength of the material in several areas of a
cross-sectional surface close to the surface because of a finer
pearlitic structure of the grain. In case of bending stresses of
the rolling stock, wherein the cross-sectional zones which are
farthest distant from the neutral grain or zero line show the
greatest stress, it is now possible to embody at least two of these
peripheral zones to have increased strength. It has been found that
with a rail it is also possible to increase the fracture toughness
of the material the base area.
[0021] In a preferred manner, the portion of the rolling stock
having the largest mass concentration, for example the head of the
rail, is cooled in a dipping process or by being dipped into a
cooling liquid, while simultaneously heat is removed by via lesser
cooling intensity, for example compressed air or air-water
spraying, from the rolling stock part(s) with a lesser mass
concentration, for example the base of the rail, which are intended
to be provided with increased cooling. Proceeding in this way it is
possible to counteract the formation of a high interior tension
state and thermal warping of the rolling stock.
[0022] In order to prevent a disadvantageous martensite formation
and to achieve a fine pearlitic structure of the grains in the
alloys on an iron base mentioned at the outset, it is advantageous
if the degree of cooling intensity, in particular the composition
of the cooling liquid for the dip cooling, is set in such a way
that, in the temperature range between 800.degree. C. to
450.degree. C., cooling of the zone close to the surface of the
dipped part in particular is achieved at 1.6 to 2.4.degree. C./s,
preferably at approximately 2.0.degree. C./s. This cooling speed is
preferred for economical reasons, because when a desired quality of
the rolled product has been achieved, a short cooling time in the
second step is required and in this way a large throughput is
achieved.
[0023] To minimize the curvature it has been shown to be
advantageous if with profiled rolling stock of T-shaped cross
section such as is present, for example, at the base of a rail, the
zone or surface opposite the web is cooled at higher intensity,
preferably by compressed air or an air-water mixture. In the
process it has been found in view of the improvement of the long
term properties to be particularly advantageous if the surface zone
located opposite the web of increased cooling intensity is embodied
to be essentially symmetrical in respect to the web axis and is
laterally limited.
[0024] Furthermore, if it is intended to prevent an increased
cooling intensity of the areas of the cross section of the profiled
rolling stock which are distal in respect to a mass concentration
or a web juncture and/or to protect these areas from an increased
heat removal or at least to heat them briefly, it is possible to
provide a grain of the same or decreased material strength in the
edges of the rolling stock. Surprisingly this lowers the danger of
breaking, particularly in case of sharp and/or changing continuous
stresses of the rolled material.
[0025] It is possible to achieve a special strength of the shape if
the cooling intensity at the surface of the profiled rolling stock,
in particular the rail, is set in such a way that the zones in
which the conversion of the gamma grain takes place during cooling
are essentially embodied to be parallel symmetrical and/or parallel
to the neutral plane, preferably concentric to the line of the
center of gravity or the center of gravity of the cross-sectional
surface.
[0026] In order to achieve an essentially completely even local
cooling intensity in the longitudinal direction and to maintain the
heat transfer into the cooling medium stable, it can be provided in
accordance with the invention that the rolling stock, a part of
which in respect to the cross section is dipped into a cooling
liquid in a dip tank, is moved in this longitudinal direction
relative to the cooling liquid container or dip tank during cooling
and/or that at least during the time in which a portion of the
rolling stock is dipped into the cooling liquid the latter is
charged with an oscillation or is made to oscillate. It has been
found that these measures decisively improve the homogeneity of the
achieved quality.
[0027] A device of the type mentioned at the outset for the
integral solution of the problems when producing profiled rolling
stock having special properties, is distinguished in accordance
with the invention in that the roller bed in the stand-by area has
a rolling stock positioning device, known per se, and a device for
the straight or axially aligned positioning of the profiled rolling
stock during its plastic shaping, has a transverse transport device
for a straight or axially aligned transfer of the rolling stock
essentially perpendicularly to its axis from the stand-by area into
the cooling treatment area, in which area a device, known per se,
for hardening rolling stock, in particular the head of rails, via
cooling liquid in a dip tank with holding and manipulation devices
and a controllable additional cooling device for more intense
cooling of at least one further area of the rolling stock, in
particular the base of a rail, is disposed and that the final
cooling area has a support for the rolling stock for its cooling to
room temperature.
[0028] It was found that the straight or axially aligned
positioning is important, particularly in connection with heat
treatments to be performed partially in respect to the cross
section or in partial areas of a profiled rolling stock. By
preventing a curvature over the entire length or partial areas
thereof it is possible to maintain the predetermined cooling
conditions or the cooling intensities of the rolling stock even,
viewed in the axial direction, so that differences in strength or
hardness along a generatrix of the profile are eliminated. Research
has shown that different distances from the wall of a coolant
reservoir and/or from the spray cooling axis can cause overly
proportional deviations of the hardness and strength values.
[0029] During positioning it is furthermore important that the
rolling stock is subjected to plastic shaping by appropriate
devices in order to prevent elastic returns to a possibly partially
curved shape. In order to avoid the necessity of later
straightening it is of great importance to bring the profiled
rolling stock in an axially aligned manner into a cooling area by a
straight-line transverse transport. In addition to this a
manipulation device is provided in the cooling area, by which the
transfer, holding, dipping into a cooling liquid tank or hardening
of partial areas of the rolling stock as well as the transfer into
a final cooling area are possible. In the process at least one
additional cooling device can be provided for the intensified
cooling of further cross-sectional areas.
[0030] In a further development of the invention it is of advantage
that the additional cooling device can be placed against the
rolling stock and its cooling intensity is controllable, so that a
further local heat removal corresponding to the method can be
set.
[0031] An embodiment is also advantageous, wherein the additional
cooling device has parts for forming a local cooling unit flow
which is essentially uninterrupted in the longitudinal or axial
direction of the rolling stock and limited in the transverse
direction and, if required, has a device for preventing an
increased heat removal from the surface(s) adjoining the cooled
surface. In this way, it is possible to form sharply limited
cooling zones and to exclude adjacent areas from an intensified
heat removal process or to create a lesser material hardness in
them, wherein in accordance with a further embodiment the
additional cooling device is designed as a moving pressure or spray
cooling device.
[0032] The homogeneity of the hardness and strength values in the
longitudinal direction of the profiled rolling stock can be further
increased if the rolling stock can be moved in the cooling liquid
in the longitudinal axial direction in respect to the dip tank
and/or in respect to the additional cooling device, and/or if
installations are disposed on the dip tank and/or in the cooling
liquid itself by which the cooling liquid can be turbulently moved
and/or set to oscillate. It was found that relative movements as
well as oscillation movements or pressure waves between the cooling
medium and the work piece even out the local cooling intensity and
create advantageous heat treating conditions.
[0033] A rail in accordance with the invention, particularly one
produced in accordance with one of the previously mentioned
methods, possibly produced in an above described device is
distinguished in that in its cross section the rail shows great
material strength and hardness values in the upper area of the
head, which values are reduced in the lower head area in the web
and the peripheral parts of the base, and that in the center area
in the bottom area of the base there are increased hardness values
of the material compared with the peripheral parts and the web,
wherein particularly even quality characteristics are achieved if
essentially equal material hardness values have been set
symmetrically with the main axis of the cross-sectional profile or
symmetrically to the perpendicular axis of the cross-section of the
rail. Such a rail displays improved use properties even under
increased demands such as high axial loads and/or high frequency of
use and/or small radii of curvature of the line.
[0034] The present invention is directed to a method for
heat-treating profiled rolling stock, including track and railroad
rails, having a profiled surface and increased heat removal from
portions of the profiled surface during cooling in the gamma range
of an iron based alloy material. A conversion into a fine pearlitic
grain of increased strength, increased wear resistance, and
increased hardness takes place in desired cross-sectional areas in
a head area of the rails, and, if required, a deformation or
bending, by a thermally caused warping of the profiled rolling
stock, perpendicularly to the longitudinal axis thereof, is one of
decreased and prevented, during cooling to room temperature.
Following a structural conversion in more heavily cooled
cross-sectional areas of the profiled rolling stock, an increased
rigidity and fatigue strength, under reversed bending stresses,
occurs. The method includes aligning the profiled rolling stock, at
an average temperature of between 750.degree. C. and 1100.degree.
C., straight in its longitudinal direction by plastic shaping, and
moving the aligned profiled rolling stock, in its aligned state, in
a transverse direction and holding same there. The method also
includes evenly cooling of the aligned profiled rolling stock, in a
first cooling, to a temperature below 860.degree. C., with the same
local cooling intensity, by radiation, in still air, removing heat
from the first cooled profiled rolling stock, in a second cooling,
in the longitudinal direction with an intensity which locally is
essentially the same, but, when viewed in cross section, is
circumferentially different, and increasing the cooling intensity
in at least one zone at the circumference of the first cooled
profiled rolling stock. Greater cooling intensities are assigned to
areas with one of a large cross sectional ratio relative to the
circumference and a large portion of volume with one of respect to
the surface and a high mass concentration. The method further
includes increasing a cooling speed of areas of the increased
intensely cooled profiled rolling stock having locally high
temperatures, and bringing these areas to a conversion temperature,
under which cooling conditions, a fine pearlitic grain structure,
free of martensite, is formed, and cooling the increased intensely
cooled profiled rolling stock from the conversion temperature to
room temperature, at the same local cooling intensity, in still
air.
[0035] According to a feature of the instant invention, the average
temperature of the profiled rolling stock is a maximum of
900.degree. C.
[0036] In accordance with another feature of the present invention,
the method may further include cooling a portion of the profiled
rolling stock having the largest mass concentration in one of a
dipping process and by dipping same into a cooling liquid, while,
simultaneously removing heat via lesser cooling intensity,
including one of compressed air and air-water spraying, from at
least one rolling stock part having a lesser mass concentration,
including the base of the rail, by providing increased cooling. The
portion of the increased intensely cooled profiled rolling stock
with respect to its cross section, which is dipped into a cooling
liquid in a dip tank, can be moved in a longitudinal direction
relative to one of the cooling liquid container and the dip tank,
during cooling. Further, at least during the time in which the
portion of the increased intensely cooled profiled rolling stock is
dipped into the cooling liquid, the method can further include
oscillating the cooling liquid. The largest mass concentration can
include the head of the rail. The method can further include
setting the degree of cooling intensity, including the composition
of the cooling liquid for the dipping, in such a way that, in the
temperature range between 800.degree. C. to 450.degree. C., cooling
of the zone close to a surface, particularly of the dipped part, is
achieved at a cooling rate of 1.6 to 2.4.degree. C./s, and
preferably about 2.0.degree. C./s.
[0037] According to another feature of the invention, in the first
cooling, the temperature can be about 820.degree. C.
[0038] In accordance with still another feature of the invention,
in the first cooling, the temperature can range from 5 to
120.degree. above the Ar.sub.3 temperature of the iron alloy
material.
[0039] The aligning can further include hot forming the aligned
profiled rolling stock, heat treating the aligned profiled rolling
stock with hot forming heat at a degree of deformation ranging
between about 1.8 to 8%, during a last tapping of the iron based
alloy material, at a temperature in the range of 750.degree. C. to
1050.degree. C. The degree of deformation ranges from about 2 to
5%.
[0040] Further, in the second cooling, the cooling intensity can be
increased in at least one of one and two zones at the circumference
of the profiled rolling stock.
[0041] Moreover, the profiled rolling stock can have a T-shaped
cross section, and the method can further include cooling, at the
base of the rail, the zone or surface opposite the web of the
T-shaped cross sectional rail, by one of compressed air and an
air-water mixture. A surface zone, located opposite the web of
increased cooling intensity can be essentially symmetrical with
respect to the web axis and is limited in lateral extent.
[0042] According to a further feature of the present invention, the
method can also include avoiding an increased cooling intensity in
areas of the cross section of the first step cooled profiled
rolling stock that are remote in distance from at least one of a
mass concentration and a web juncture. Further, the method can
include one of protecting the remote areas from increased heat
removal and by at least briefly heating said areas.
[0043] In accordance with yet another feature of the instant
invention, the method can include setting the cooling intensity, at
the surface of the increased intensely cooled profiled rolling
stock in such a way that the zones, in which the conversion of the
gamma grain takes place during cooling, are essentially one of
parallel symmetrical and parallel to a neutral plane thereof. The
zones can be concentric with one of the line of the center of
gravity and the center of gravity of the cross-sectional
surface.
[0044] The present invention is directed to a method for
heat-treating profiled rolling stock, adapted for track and
railroad rails, having a profiled surface comprising an alloy. The
method includes aligning the profiled rolling stock straight in its
longitudinal direction by plastic shaping at an average temperature
of between approximately 750.degree. C. and 1100.degree. C., and
transversely moving the aligned profiled rolling stock to a holding
area. The method also includes evenly cooling the profiled rolling
stock to a temperature above that at which a conversion of the
gamma grid into the alpha grid of the alloy begins at a cooling
velocity of 3.degree. C./min, whereby a partial conversion of the
grain into a fine pearlitic structural shape occurs in portions of
a cross-section of the profiled rolling stock, and unevenly
removing heat from the profiled rolling stock, whereby a structural
conversion occurs in more heavily cooled cross-sectional areas of
the profiled rolling stock, whereby rigidity and fatigue strength,
under reversed bending stresses, are increased. The method further
includes cooling the increased intensely cooled profiled rolling
stock from the conversion temperature to room temperature, at the
same local cooling intensity, in still air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] The present invention is further described in the detailed
description which follows, in reference to the noted plurality of
drawings by way of non-limiting examples of exemplary embodiments
of the present invention, in which like reference numerals
represent similar parts throughout the several views of the
drawings, and wherein:
[0046] FIG. 1 illustrates a course for the heat treatment of rails
in accordance with the features of the instant invention;
[0047] FIG. 2 illustrates a cross section of a rail; and
[0048] FIG. 3 graphically illustrates a time-temperature conversion
diagram of a rail material.
DETAILED DESCRIPTION OF THE INVENTION
[0049] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description taken with the drawings making apparent to those
skilled in the art how the several forms of the present invention
may be embodied in practice.
[0050] As schematically shown in FIG. 1, profiled rolling stock,
such as a rail, is positioned in a stand-by area A at a roller
table 21 by movable bumpers or the like, for example (not shown).
The rail 1 is then aligned straight by alignment device 22 and 23,
wherein a centering type of the alignment device 22 which also
corrects a vertical curvature is advantageous. Following the
alignment of the rolling stock 1, there is a transverse transport
to a support 2 in a cooling area B and placement into a
manipulation device with holding device 24, wherein holding during
the movement must be performed in such a way that there is no
bending transversely to the longitudinal axis. In a manner known
per se, the rolling stock or the rail 1 is partially immersed by
the holding device 24 into a cooling liquid 37 in a dip tank 38.
Herein it is important that the distance of the surface of the rail
1 from the wall of the dip tank is equally great on both sides over
its length. For intensifying and particularly for an equalization
of the cooling intensity of a rolling stock surface, in an
advantageous manner the rolling stock 1 can be movable in the dip
tank 38 or the cooling medium 37 in a longitudinal direction in an
amount of, for example 0.5 to 5 m. It is also possible to use
oscillation generators (not shown) in the cooling medium 37 or on
the dip tank, which cause the cooling medium to oscillate at a
frequency of, for example, 100 to 800 oscillations/min, which
advantageously affects the cooling intensity. A cooling medium
inlet is identified by numeral 39.
[0051] An additional cooling device 3 can be placed on or attached
to a flat part of the rolling stock, possibly on the base 13 of a
rail 1. Such an additional cooling device can have a water supply
32 and an air supply 33 and form a spray 31 directed to a surface
part of the rolling stock or the base of the rail. To provide a
decreased cooling intensity to the peripheral parts 132 (FIG. 2)
and to form a zone of increased material hardness only in a central
area 131 (FIG. 2) of a rolling stock or rail base area, it can be
advantageous to provide a cooling medium removal, for example, by
an aspirating device 34, which is connected to a source of vacuum
35 or similar type device.
[0052] After cooling of the rolling stock, in particular a rail 1,
immersed into a cooling medium 37 and in particular of a portion
thereof located opposite it and subjected to a spray 31, below the
conversion temperature of the material of an intensity causing a
fine pearlific grain, for example in accordance with FIG. 3 to
approximately 500.degree. C. at a cooling rate in accordance with
curve f, the rail can be placed on a support 25 in the final
cooling area C for cooling to room temperature.
[0053] As represented in FIG. 2, a rail 1, in accordance with the
invention, has three areas of different grain structure or
hardness, wherein the transition areas are embodied to be
continuous. A fine pearlitic zone 111 of hardness values between
340 and 390 HB, possibly up to 425 HB, is provided in the rail head
11 and makes a downward transition into a zone 112 of reduced
hardness, for example 300 to 340 HB. In the adjoining web 12, which
in actual use must have a large degree of toughness, hardness
values between 280 and 320 HB have accordingly been provided. A
pearlitic grain of a rougher structure or lamella formation and a
hardness between 280 to 320 HB, the same as in the web 12, is
provided in the peripheral areas 132 of the rail base 13.
Initiation of a rupture or break is prevented to a large extent by
this grain embodiment and the material properties of reduced
hardness values. However, an area 131 of increased material
strength and hardness values of 300 to 350 HB and more is formed in
the center of the bottom of the base 13. As has been determined,
such a distribution in accordance with the invention of the
mechanical properties of the material across the cross section of a
rail cause high stability and advantageous long term behavior,
particularly under difficult conditions.
[0054] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to an exemplary
embodiment, it is understood that the words which have been used
herein are words of description and illustration, rather than words
of limitation. Changes may be made, within the purview of the
appended claims, as presently stated and as amended, without
departing from the scope and spirit of the present invention in its
aspects. Although the present invention has been described herein
with reference to particular means, materials and embodiments, the
present invention is not intended to be limited to the particulars
disclosed herein; rather, the present invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims.
* * * * *