U.S. patent application number 14/055319 was filed with the patent office on 2014-04-17 for method for heat-treating bainite steel rail.
The applicant listed for this patent is PANGANG GROUP PANZHIHUA IRON & STEEL RESEARCH INSTITUTE CO., LTD.. Invention is credited to Yong DENG, Hua GUO, Zhenyu HAN, Jihai JIA, Jianhua LIU, Chunjian WANG, Hui YAO, Jun YUAN, Ming ZOU.
Application Number | 20140102603 14/055319 |
Document ID | / |
Family ID | 47571956 |
Filed Date | 2014-04-17 |
United States Patent
Application |
20140102603 |
Kind Code |
A1 |
HAN; Zhenyu ; et
al. |
April 17, 2014 |
METHOD FOR HEAT-TREATING BAINITE STEEL RAIL
Abstract
A method for heat-treating a bainite steel rail is disclosed.
The method includes: cooling a rolled steel rail naturally to lower
a surface temperature of a rail head of the steel rail to
460.degree. C. -490.degree. C.; cooling the steel rail forcely at a
cooling rate of 2.0.degree. C./s-4.0.degree. C./s to lower the
surface temperature of the rail head to 250.degree. C.-290.degree.
C.; placing the steel rail in an ambient temperature until the
surface temperature of the rail head is more than 300.degree. C.;
performing a tempering on the steel rail in a heating furnace at
300.degree. C.-350.degree. C. for 2 h-6 h; and air cooling the
steel rail to the ambient temperature. Steel rails heat-treated by
present method may have a stable retained austenite structure and a
good mechanical performance.
Inventors: |
HAN; Zhenyu; (Panzhihua,
CN) ; ZOU; Ming; (Panzhihua, CN) ; JIA;
Jihai; (Panzhihua, CN) ; GUO; Hua; (Panzhihua,
CN) ; LIU; Jianhua; (Panzhihua, CN) ; DENG;
Yong; (Panzhihua, CN) ; WANG; Chunjian;
(Panzhihua, CN) ; YUAN; Jun; (Panzhihua, CN)
; YAO; Hui; (Panzhihua, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PANGANG GROUP PANZHIHUA IRON & STEEL RESEARCH INSTITUTE CO.,
LTD. |
Sichuan Province |
|
CN |
|
|
Family ID: |
47571956 |
Appl. No.: |
14/055319 |
Filed: |
October 16, 2013 |
Current U.S.
Class: |
148/581 |
Current CPC
Class: |
C21D 9/04 20130101 |
Class at
Publication: |
148/581 |
International
Class: |
C21D 9/04 20060101
C21D009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2012 |
CN |
201210394058.X |
Claims
1. A method for heat-treating a bainite steel rail, comprising:
cooling a rolled steel rail naturally to lower a surface
temperature of a rail head of the steel rail to 460.degree.
C.-490.degree. C.; cooling the steel rail forcely at a cooling rate
of 2.0.degree. C./s-4.0.degree. C./s to lower the surface
temperature of the rail head to 250.degree. C.-290.degree. C.;
placing the steel rail in an ambient temperature until the surface
temperature of the rail head is more than 300.degree. C.;
performing a tempering on the steel rail in a heating furnace at
300.degree. C.-350.degree. C. for 2 h-6 h; and air cooling the
steel rail to the ambient temperature.
2. The method for heat-treating the bainite steel rail of claim 1,
wherein, cooling the steel rail forcely is performed by applying a
cooling medium onto the rail head.
3. The method for heat-treating the bainite steel rail of claim 2,
wherein, the cooling medium includes an air-water gas or a
compressed air.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a method for heat-treating
a bainite steel rail, more particularly, to a method for
heat-treating a high-performance bainite steel rail containing
stable retained austenite.
BACKGROUND ART
[0002] A steel rail is a critical component for guiding trains and
transferring a wheel load to a track bed. The quality of the steel
rail directly influences operation efficiency and security of a
railway. In recent years, with continuing increase of axel load and
carrying gross weight of the railway, a steel rail of higher
performance is required.
[0003] Researches have proved that a carbide-free
bainite/martensite dual-phase steel composed of bainite type
ferrite, retained austenite and a trace of martensite has great
potential in the field of steel rail, and a lot of work has been
done in researching, manufacturing and applying a bainite steel
rail. At present, processes of manufacturing a bainite steel rail
mainly include:
[0004] (1) rolling a billet into a steel rail, and then directly
air cooling the same to an ambient temperature, so that a complex
phase steel rail of upper baisite (trace)+carbide free
baisite+retained austenite (a small amount)+martensite (trace)
being obtained, and the strength and toughness of the steel rail
being improved by stabilizing the retained austenite of the steel
rail and transforming the martensite into tempered martensite
through a subsequent tempering process compared to a air
cooling.
[0005] (2) increasing contents of metals such as Mo, Ni, V, Ti,
etc. in a steel rail, and directly air cooling a rolled steel rail
to an ambient temperature, so that a complex phase steel of carbide
free baisite+retained austenite+martensite having good strength and
toughness being obtained.
[0006] Since the method does not include an austenite stabilizing
step, the retained austenite in the steel rail tends to be
transformed into a brittle marstensite by external forces such as
impact of train wheels, etc.
[0007] (3) rolling a billet into a steel rail, beginning to apply a
cooling medium onto the steel rail at a phase region of austenite
so as to acceleratedly cool the steel rail to 300-500.degree. C. at
a cooling rate of 1-10.degree. C./s, and air cooling the steel rail
to an ambient temperature, so that a bainite steel rail having good
strength and toughness being obtained. The method was disclosed by
patent application of CN1095421A. Researches indicate that the
steel rail produced by this method tends to be bent and deformed
due to a non-uniform distribution of thermal capacity of cross
section of the steel rail in the accelerated cooling process, which
reduces the straightness of the steel rail, and a relatively large
residual stress will be formed in a subsequent straightening
process, thereby influencing safety of the steel rail.
SUMMARY
[0008] The purpose of the present disclosure is to overcome the
above defects of prior arts and provide a method for heat-treating
a bainite steel rail.
[0009] The method for heat-treating the bainite steel rail
according to the present disclosure comprises: cooling a rolled
steel rail naturally to lower a surface temperature of a rail head
of the steel rail to 460.degree. C.-490.degree. C.; cooling the
steel rail forcely at a cooling rate of 2.0.degree.
C./s-4.0.degree. C./s to lower the surface temperature of the rail
head to 250.degree. C.-290.degree. C.; placing the steel rail in an
ambient temperature until the surface temperature of the rail head
is more than 300.degree. C.; performing a tempering on the steel
rail in a heating furnace at 300.degree. C.-350.degree. C. for 2
h-6 h; and air cooling the steel rail to the ambient
temperature.
[0010] According to an exemplary embodiment of the present
disclosure, cooling the steel rail forcely may be performed by
applying a cooling medium onto the rail head.
[0011] According to an exemplary embodiment of the present
disclosure, the cooling medium may include an air-water gas or a
compressed air.
[0012] Steel rails heat-treated by present method may have a stable
retained austenite structure and a good mechanical performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and/or other aspects and advantages of the present
disclosure will become apparent and easily understood by reference
to the following description of exemplary embodiments in
conjunction with the accompanying drawings, wherein:
[0014] FIG. 1 is a schematic view of section hardness test
positions of a rail head according to China Railway Industry
Standard.
[0015] FIG. 2 is a microstructure photograph of a bainite steel
rail obtained by a heat-treating method according to an exemplary
embodiment of the present disclosure.
[0016] FIG. 3 is a microstructure photograph of a bainite steel
rail obtained by a conventional heat-treating method.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0017] In a rolling process of a bainite steel rail, a billet
containing components of a bainite steel rail is usually fed into a
heating furnace and heated at a soaking temperature of 1200.degree.
C.-1300.degree. C. for no less than 2 h, and a fast-to-slow heating
mode is usually adopted for the soaking. Then, the billet is rolled
into a steel rail of a required section in a rolling mill after
being kept at a certain temperature for a predetermined time.
Furthermore, the final rolling temperature is usually about
900.degree. C.-1000.degree. C.
[0018] Hereinafter, a method of heat-treating a steel rail (for
example, a steel rail obtained by the above rolling) will be
described in details by referring to exemplary embodiments of
present disclosure. However, the present disclosure is not limited
to heat-treating the above rolled steel rail.
[0019] First, a rolled steel rail is cooled down naturally to lower
a surface temperature of a rail head to about 460.degree.
C.-490.degree. C., and then, the steel rail is cooled down forcely
at a cooling rate of about 2.0.degree. C./s-4.0.degree. C./s to
lower the surface temperature of the rail head to about 250.degree.
C.-290.degree. C.
[0020] According to an exemplary embodiment of the present
disclosure, the rolled steel rail may be erected behind a roller
table and air cooled until the surface temperature of the rail head
reduces to about 460.degree. C.-490.degree. C., and then the steel
rail may be cooled down forcely by applying a cooling medium onto
the rail head, for example, by applying a cooling medium onto top
surface and two side surfaces of the rail head. Here, the cooling
rate of the steel rail may be controlled to be about 2.0.degree.
C./s-4.0.degree. C./s. However, those skilled in the art will
realize that the forced cooling of the steel rail is not limited
thereto. In addition, according to an exemplary embodiment of the
present disclosure, the cooling medium may include a mixed gas of
water and air or a compressed air.
[0021] In the case that a steel rail is air cooled after being
rolled, a phase transformation temperature of a bainite steel rail
is about 350-400.degree. C. In prior arts, an accelerated cooling
begins at a phase region of austenite, as there is a wide range
from the accelerated cooling temperature to the phase
transformation temperature of bainite steel rail, a relatively long
cooling time is required, and too much cooling medium will be
consumed. In addition, in the case that the accelerated cooling
begins at the phase region of austenite, when the surface of the
rail head is acceleratedly cooled by the cooling medium in the
process of accelerated cooling, heat from core of the rail head and
rail web diffuses to the surface of the rail head via heat
transfer, so that the rail head is difficult to complete phase
transformation at a larger degree of under cooling and the
toughness of the rail head decreases from the surface to the core,
thus a complete hardening can not be achieved. Therefore, there is
no benefit to significantly improving the performance of the rail
steel by performing the accelerated cooling at a temperature
between the phase region of austenite and 490.degree. C. However,
according to the exemplary embodiment of the present disclosure,
when the surface of the rail head is cooled naturally down to about
460.degree. C.-490.degree. C., the temperatures of the rail web and
rail bottom are less than 500.degree. C. When the accelerated
cooling is performed at this time, the surface temperature of the
rail head has been decreased prominently, and the heat from the
core of the rail head cannot effectively compensate for the surface
of the rail head. Meanwhile, as the temperature of 460.degree.
C.-490.degree. C. is relatively close to the phase transformation
temperature of bainite, the entire section, especially the core of
the rail head is allowed to complete the phase transformation at a
larger degree of under cooling. Therefore, the steel rail
heat-treated by the method according to the exemplary embodiment of
the present disclosure may have better performances, compared to
that heat-treated by a conventional method.
[0022] In addition, if the cooling rate is less than 2.degree. C./s
in the process of forced cooling, the surface temperature of the
rail head is difficult to decrease quickly, thus the temperature of
the core of the rail head can not be reduced effectively.
Meanwhile, the heat from the core of the rail head may be
transferred to the surface of the rail head, which is not favorable
to improve the overall performance of the steel rail. If the
cooling rate is more than 4.degree. C./s, too much martensite is
generated due to the high cooling rate of the surface of the rail
head. Although the martensite may be transformed into tempered
marstensite by a subsequent tempering process, the tranformation is
not complete, and the residual marstensite structure is not
beneficial to the safe use of the steel rail.
[0023] In addition, if the final temperature of the surface of the
rail head is more than 290.degree. C. after the forced cooling
process, although a fine bainite structure is formed in the surface
of the rail head, a coarse bainite structure will be formed at the
core of the rail head due to high temperature, which may finally
degrade performance of the steel rail at ambient temperature, and
is not beneficial to uniformity of performance in the entire
section of the rail head. If the final temperature of the surface
of the rail head is less than 250.degree. C., a large amount of
martensite is generated in bainite structure, which is difficult to
be removed by a subsequent tempering process, so that toughness and
plasticity of the steel rail decrease prominently, and even the
steel rail cannot be used.
[0024] Next, the steel rail is placed in an ambient temperature
until the surface temperature of the rail head is more than
300.degree. C. Then, a tempering is performed on the steel rail in
a heating furnace at 300.degree. C.-350.degree. C. for 2 h-6 h.
[0025] According to an exemplary embodiment of the present
disclosure, the surface temperature of the rail head may rise by
50-60.degree. C. due to the heat from the core of the rail head and
the rail web, when the steel rail is placed in air after the forced
cooling process. Therefore, the surface temperature of the rail
head may naturally rise to be more than 300.degree. C. after the
above forced cooling to 250.degree. C.-290.degree. C. When the
above forced cooling is completed, the heat from the rail web and
the core of the rail head may still be transferred to the surface
of the rail head, that is, the entire section of the steel rail is
in a soaking status. When the average temperature of the steel rail
is about 300-350.degree. C. after a period of soaking, and the
tempering is performed on the steel rail, so that the time for
tempering may be reduced prominently and a more uniform performance
may be obtained. In addition, a fine bainite structure has been
formed through the above forced cooling, retained austenite is
included between lamellas of lamellate bainite type ferrite, and
the retained austenite is not stable, which needs to be further
stabilized by tempering, so as to obtain stable steel rail having
good toughness and strength. Although the temperature rise of the
steel rail after the forced cooling process can play the role of
tempering to some extent, the function is limited. The reason is
that the heat from the rail web and rail bottom may only keep a
short period of temperature rise, and when the steel rail reaches a
soaking state, the temperature of the entire section decreases
simultaneously to an ambient temperature in a very short time.
Therefore, tempering relied on temperature rise of the steel rail
itself has limited influence on the structure, and a part of the
retained austenite in the steel rail is still in a metastable
state. Therefore, the tempering process by heating at about
300.degree. C.-350.degree. C. is adopted in the present
disclosure.
[0026] According to an exemplary embodiment of the present
invention, the temperature of the heating furnace is about
300.degree. C.-350.degree. C. When the tempering temperature is
less than 300.degree. C., the toughness and plasticity of the steel
rail, especially the impact toughness at -40.degree. C. are
obviously decreased, thus the high-toughness feature of the bainite
steel rail at low temperature does not take effect. When the
tempering temperature is more than 350.degree. C., although the
toughness and plasticity are improved, the strength and hardness
tend to decrease, which is not beneficial to obtain steel rails
having good overall performance. In addition, when the tempering
time is less than 2 h, a part of the retained austenite in the
steel rail is in a metastable state, and the purpose of stabilizing
the retained austenite can not be achieved. When the tempering time
is more than 6 h, the transformation of the retained austenite in
the steel rail is completed, i.e., the purpose of tempering is
achieved, thus there is no obvious benefit to prolonging the
tempering time.
[0027] At last, the tempered steel rail is air cooled to an ambient
temperature, so as to obtain a steel rail having a stable retained
austenite structure and a good mechanical performance.
[0028] Hereinafter, the method for heat-treating the
high-performance bainite steel rail having a stable retained
austenite structure is described in details in conjunction with
particular exemplary embodiments of present disclosure.
[0029] Table 1 shows chemical components of bainite steel rails
according to exemplary embodiments of the present disclosure and
comparative examples, however, the heat treatment method of the
present disclosure is not limited to be used for the steel rails
having the chemical components in table 1.
TABLE-US-00001 TABLE 1 Chemical Components (wt %) C Si Mn P S Cr Mo
Embodiment 1 and 0.23 1.58 1.97 0.010 0.006 0.80 0.30 Comparative
Example 1 Embodiment 2 and 0.25 1.20 2.10 0.011 0.005 1.22 --
Comparative Example 2 Embodiment 3 and 0.26 1.75 1.65 0.011 0.007
0.50 0.36 Comparative Example 3 Embodiment 4 and 0.21 0.80 1.95
0.014 0.009 1.05 0.32 Comparative Example 4 Embodiment 5 and 0.24
1.10 2.05 0.012 0.004 0.95 0.37 Comparative Example 5 Embodiment 6
and 0.26 1.30 1.87 0.013 0.006 1.53 0.25 Comparative Example 6
[0030] Billets having the above components were rolled into steel
rails of 60 kg/m, and heat-treating methods in table 2 were
respectively applied to the steel rails, wherein a subsequent
tempering was applied in the embodiments, while no tempering was
applied in the comparative examples.
TABLE-US-00002 TABLE 2 Initial Final Temperature Temperature
Temperature of Steel of of Rails when Temperature Accelerated
Accelerated Accelerated being of Heating Cooling Cooling Cooling
feeding into Furnace Tempering (.degree. C.) Rate (.degree. C./s)
(.degree. C.) furnace (.degree. C.) (.degree. C.) Time (h)
Embodiment 1 485 3.5 280 330 300 4.1 Embodiment 2 472 2.0 289 312
320 4.8 Embodiment 3 469 2.4 252 305 330 5.9 Embodiment 4 461 2.1
267 308 350 5.1 Embodiment 5 488 3.9 256 302 310 4.2 Embodiment 6
490 3.1 288 321 320 4.6 Comparative 760 1.8 350 -- -- -- Example 1
Comparative 780 2.4 381 -- -- -- Example 2 Comparative 820 2.2 364
-- -- -- Example 3 Comparative 880 3.1 425 -- -- -- Example 4
Comparative 690 2.9 346 -- -- -- Example 5 Comparative 570 1.9 315
-- -- -- Example 6
[0031] The steel rails were aired cooled to ambient temperature
after the above processes, and mechanical properties thereof were
shown in tables 3 and 4.
TABLE-US-00003 TABLE 3 Impact Property Tensile Property (Aku/J)
Proportion Rp0.2 Ambient of Retained (MPa) Rm (MPa) A/% Z/%
Temperature -40.degree. C. Austenite Embodiment 1 1130 1380 18.5 62
95 68 6.3% Embodiment 2 1180 1410 16.5 58 85 59 7.2% Embodiment 3
1150 1430 17.0 62 87 61 7.8% Embodiment 4 1210 1490 17.5 56 98 64
7.1% Embodiment 5 1200 1390 17.5 56 102 72 6.6% Embodiment 6 1160
1420 19.0 50 88 58 6.9% Comparative 1025 1340 16.0 52 85 52 11.8%
Example 1 Comparative 1040 1320 14.0 50 64 46 10.2% Example 2
Comparative 1080 1370 13.5 44 62 42 12.1% Example 3 Comparative
1105 1400 15.5 40 62 38 10.8% Example 4 Comparative 1060 1310 15.0
46 72 48 11.6% Example 5 Comparative 980 1300 16.0 42 60 40 10.0%
Example 6
TABLE-US-00004 TABLE 4 Surface Section Hardness (HRC) Hardness
A.sub.1 B.sub.1 C.sub.1 A.sub.4 B.sub.5 C.sub.5 (HBW) Embodiment 1
44.0 44.5 45.0 44.0 43.5 44.0 435 Embodiment 2 43.0 44.0 44.5 43.5
44.0 43.5 430 Embodiment 3 44.5 44.5 45.0 44.5 44.0 44.0 438
Embodiment 4 45.0 45.5 45.0 44.5 44.5 44.5 440 Embodiment 5 43.5
44.0 44.0 44.0 43.5 44.0 428 Embodiment 6 43.0 44.0 44.5 43.5 43.5
44.0 425 Comparative 41.5 42.0 42.0 40.0 40.5 41.0 415 Example 1
Comparative 41.0 42.0 41.5 41.5 42.0 41.5 418 Example 2 Comparative
42.0 41.5 41.5 41.0 41.0 40.5 420 Example 3 Comparative 42.5 42.0
42.5 41.5 42.0 42.0 425 Example 4 Comparative 40.5 40.0 40.5 40.0
40.0 40.5 406 Example 5 Comparative 39.5 40.0 40.0 39.5 40.0 39.5
398 Example 6
[0032] FIG. 1 is a schematic view of section hardness test
positions of a rail head according to China Railway Industry
Standard. The section hardness test points A.sub.1, B.sub.1,
C.sub.1, A.sub.4, B.sub.5, and C.sub.5 of the rail head in table 4
is shown in FIG. 1, wherein A.sub.1, B.sub.1, C.sub.1 respectively
represent three positions of the surface of the rail head, and
A.sub.4, B.sub.5, and C.sub.5 respectively represent three
positions in the core of the rail head.
[0033] FIG. 2 is a microstructure photograph of a bainite steel
rail obtained by a heat-treating method according to an exemplary
embodiment of the present disclosure. FIG. 3 is a microstructure
photograph of a bainite steel rail obtained by a conventional
heat-treating method.
[0034] Based on analyses of tables 1-4 and FIGS. 2 and 3, it can be
seen that in the condition of same chemical components, and
smelting and rolling processes, the method of heat-treating the
rolled steel rail may have obvious influence on the final
performance of the steel rail. Particularly, with the heat-treating
method according to the embodiments of the present disclosure, the
microstructure of the steel rail is a mixed structure composed of
lath-shaped bainite ferrite, discrete retained austenite film
alternatively distributed between the laths in a lamella shape, and
a small amount of twin marstensite. In same eutectic cell, the
bainite ferrite laths have uniform orientation, distance between
laths is small, such fine structure can obviously improve strength
and hardness of the steel rail and slightly improve toughness and
plasticity of the steel rail, thus the function of the grain
refinement strengthening to improve overall performance of the
steel rail is sufficiently achieved. In addition, the decrease of
the content of the retained austenite may obviously decrease the
tendency of the retained austenite to be transformed into brittle
marstensite by external force at ambient temperature, i.e., the
retained austenite have a better mechanical stability, which is
more beneficial to the reliability and safety in train operation.
In comparison, when the conventional heat-treating method is
adopted, although the structure is still composed of bainite
ferrite, retained austenite, and a small amount of marstensite,
since the final temperature of the accelerated cooling is
relatively high, the entire section of the rail head is difficult
to complete phase transformation at a larger degree of under
cooling, causing that the structure in the steel rail is coarse,
and a small amount of eutectoid ferrite is precipitated. Since the
function of accelerated cooling to improve performance of the
bainite steel rail cannot be sufficiently achieved, the steel rail
has low hardness and strength as well as unsatisfying toughness and
plasticity. Furthermore, in the conventional heat-treating method,
the retained austenite has a high proportion, coarse size, and
morphology of continuous and closed distribution, and tends to be
transformed into marstensite under the impact of the wheels of
trains, causing brittle failure of the steel rail, which may result
in train operation accident. However, the steel rail heat-treated
by the method of the embodiment of the present disclosure can not
only meet higher performance requirements for railway steel rail,
but also ensure safe operation of trains.
[0035] In conclusion, the method for heat-treating high-performance
bainite steel rail of the present disclosure is applied to rolled
steel rails having residual heat, in the condition of the same
chemical components and smelting and rolling processes, the steel
rails heat-treated by the method of the embodiments of the present
disclosure have better strength and toughness compared to that
heat-treated by conventional methods. In addition, the steel rails
heat-treated by methods of embodiments of the present disclosure
are suitable for heavy haul railways having relatively high
requirements for contact fatigue damage and wearability.
[0036] Although the method for heat-treating a bainite steel rail
has been described by referring to particular embodiments of the
present disclosure, those skilled in the art should realize that
within the spirit and scope of the present disclosure, various
amendments and changes may be made.
* * * * *