Introduction to the Field of Invention:
This invention relates to an improved process for manufacturing higher (YS/TS) yield
ratio micro-alloyed wear resistant rails of railway.
Prior Art and Drawbacks:
Conventional 880 Mpa pearlitic rails have limitation of achievable (YS/TS) yield ratio
(0.50-0.55). The demand for rails processing higher stress ratio is increasing worldwide
due to the stringent service conditions being practiced under heavy haul railways sectors
having higher axle loads, higher speeds and higher traffic density.
Objects:
It is an object of the invention to propose an improved method for the manufacture of
micro-alloyed wear resistant rails having higher YS/TS ratio than so far produced.
It is another object to propose such a method, which will use a starting steel alloy of a
different composition.
It is another object of this invention to propose such a method, which can be
meticulously followed and commercially implemented.
It is yet another object to propose such a process which when commercially used will
yield consistent results so that the products obtained at different times will have uniform
property.
These and other objects will be clear from the following paragraphs.
Brief Details of the Invention:
Thus, according to this invention, there is provided an improved method for
manufacturing higher (YS/TS) yield ratio micro-alloyed wear resistant rails which
comprises the following steps:
a. Preparing a starting steel by BOF steel making steps having the following
composition:
Composition of proposed micro-alloyed steel:
Carbon : 0.50-0.75 wt %; Mn: 0.90-1.20 wt %; Si : 0.20-0.40 wt %; S : 0.025 wt %
max; P : 0.025 wt % max; Nb: 0.02-0.06 wt %; Al: 0.01 wt % max.
b. Continuously casting same to obtain blooms of required size allowing minimum pick
up of oxygen in the casting ladle.
c. Subjecting the cast blooms to a step of rolling to obtain the rails of required
dimensions and
d. Allowing the rolled rail to a slow cooling step.
In a preferred method, the starting steel preferably has the following composition.
Composition of experimental micro-alloyed rail steels:
C: 0.62-0.68 wt.%; Mn: 0.98-1.10 wt.%; Si:0.20-0.29 wt.%; S: 0.013-0.023 wt. %;
P: 0.019-0.024 wt.%; Nb: 0.025-0.052 wt.%; Al: O.Olwt. % max;
In this method, when the BOF step carried out, the sulphur content in the hot metal is
kept below 0.04% and before starting tapping of molten steel, gunny bags are used in
tap hole and the converter is tilted backwards to restrict entry of converter slag into the
ladle.
It is ensured that all the alloying elements are added before the ladle is 2/3 full to ensure
ease of operation and uniform results.
The rolled rails are re-heated at temperatures in the range of 1290±20° C and the finish
rolling is carried out at temperatures in the range of 900±30° C.
After the rolling, the rolled steel rails are subjected to slow cooling by stacking the rails
into a pit/s.
It is found that the finished pit cooled rails have higher stress ratio (0.585-0.590) and
the finished pit cooled rails exhibit the following mechanical properties:
Mechanical properties of as rolled pit cooled rails
0.2% P.S., MPa : 533-547; U.T.S., MPa: 908-934; YS/UTS Ratio: 0.585-0.590;
Elongation %: 12-15; Reduction in Area %:23-33; Hardness, BHN: 269-285;
CVN Energy Joules: 8-13;
Further Details of this Invention:
In the first stage, laboratory scale research was carried out to develop the chemistry of
higher yield strength rail steel by micro-alloyed with niobium.
A large varieties of chemistry of steel were tried in the Laboratory which were useful.
The range of the chemistries of experimental heats in shown in Table 1.
Table 1: Chemistry of proposed micro-alloyed steel

Out of a large variety of chemistries tried in laboratory as shown in Table 1, the rail
steel chemistry shown in Table 2 was found most suitable for achieving property
requirements of micro-alloyed yield ratio 880 MP a rail.
Table 2 : Chemistry of experimental micro-alloyed rail steels

Results obtained on laboratory scale were encouraging enough to justify their validation
by industrial scale experiments involving production of actual rails and investigation of
their properties. Five industrial heats of 120 tonne charge wt. were made in BOF furnace
at one of the manufacturing plant of the applicants and -
heats were processed through LF-RH-CC route. 25 blooms of 300x335 mm size were cast
from each heat. The blooms were processed into R-60/R-52 rails in rail and structural
mills of one of the manufacturing plant of the applicants.
Various steps of the process that were followed in the large scale experiment are as
follows :
Details of Process Parameters Followed:
BOF Steel making:
i. Sulphur in hot metal was kept < 0.40%
ii. Gunny bags were used in tap hole before tapping started and converter
was titled back to restrict entry of converter slag in ladle.
iii. All additions were completed before the ladle was 2/3 full.
iv. Tapping duration was kept between 4-6 mts.
v. Fe-Nb added in ladle furnace/VAD to attain 0.03% Nb in final cast analysis.
Continuous casting:
i. Blooms were cast in section size of 300x340 mm and marked properly
ii. Proper shrouding of ladle to tundish was ensured to maximize oxygen pick
up.
Rail Rolling:
i. Re-heating temperature : 1290±20° C
ii. Finish rolling temperature : 900±30° C
iii. After rolling rails stacked into pit for slow cooling.
Properties of slow cooled rails were evaluated at various test sites wherein the
mechanical properties, micro-structural features, fracture toughness, fatigue strength
and Charpy V-notch energy were determined.
The disperse precipitation of Nb carbo-nitrides and carbides resulting the pearlite colony
size finer has contributed significantly for the increase of yield strength with no
significant increase in tensile strength and hardness (Table 3). This has contributed
higher stress ratio (0.585-0.590) in comparison of conventional 880 Mpa grade rail steel
(0.50-0.55).
Table 3: Mechanical properties of as rolled pit cooled rails

It is evident from results that improved yield ratio and charpy absorbed energy has been
achieved. The prior austenitic grain size number of without micro-alloyed steel was 25-65 urn
(mixed grains) and that with micro-alloyed steel was 20-30 urn (uniform grains) (figure 1). The
fracture toughness of micro-alloyed rails are well within the acceptable range of 42 MPa Vm
minimum as per the IRS-T-12-96 Specification. The charpy absorbed energy of micro-alloyed rail
is quite superior than that reported for plain carbon conventional 880 MPa rails (6-8 Joules).
Special Advantages of the Invention:
Of the many advantages obtained, the following are very significant.
• Higher fracture toughness value (KlC at RT): > 45 MPa vm
• UTS: 908-934 MPa; Elng%: 12-15; YS: 533-547 MPa
• Yield ratio (YS/TS): 0.585-0.59
• Charpy notch toughness: 8-13 J
• Surface hardness: 269-285 BHN
• Uniform prior austenitic grain size (20-30 micron) resulting quite finer interiamellar spacing
(0.13 micron) and finer pearlite colony (3-6 micron).
We Claim:
1. An improved method for manufacturing higher (YS/TS) yield ratio micro-alloyed wear
resistant rails which comprises the following steps:
a. Preparing a starting steel by BOF steel making steps having the following
composition:
Carbon : 0.50-0.75 wt %;
Mn: 0.90-1.20 wt %;
Si : 0.20-0.40 wt %;
S : 0.025 wt % max;
P : 0.025 wt % max;
Nb: 0.02-0.06 wt %;
Al: 0.01 wt % max.
b. Continuously casting same to obtain blooms of required size allowing minimum
pick up of oxygen in the casting ladle.
c. Subjecting the cast blooms to a step of rolling to obtain the rails of required
dimensions and
d. allowing the rolled rail to a slow cooling step.
2. A method as claimed in claim 1, wherein, the starting steel preferably has the
following composition.
C: 0.62-0.68 wt.%;
Mn: 0.98-1.10 wt.%;
Si%:0.20-0.29 wt.%;
S: 0.013-0.023 wt. %;
P: 0.019-0.024 wt.%;
Nb: 0.025-0.052 wt.%;
Al: O.Olwt. % max;
3. A method as claimed in claim 1, wherein, the BOF step carried out, the sulphur content
in the hot metal is kept below 0.04%.
4. A method as claimed in claim 3, wherein, before starting tapping of molten steel,
gunny bags are used in tap hole and the converter is tilted backwards to restrict entry
of converter slag into the ladle.
5. A method as claimed in claims 1 to 4, wherein, all the alloying elements are added
before the ladle is 2/3 full to ensure ease of operation and uniform results.
6. A method as claimed in claims 1 to 5, wherein, the rolled rails are re-heated at
temperatures in the range of 1290±20° C.
7. A method as claimed in claim 6, wherein, the finish rolling is carried out at
temperatures in the range of 900±30° C.
8. A method as claimed in claims 1 to 7, wherein, the rolled steel rails are subjected to
slow cooling by stacking the rails into a pit/s.
9. A method as claimed in claims 1 to 8, wherein, the finished pit cooled rails have higher
stress ratio (YS/TS) in the range of 0.585-0.590.
10. A method as claimed in claim 9, wherein, the finished pit cooled rails exhibit the
following mechanical properties:
0.2% P.S., MPa : 533-547;
U.T.S., MPa: 908-934;
YS/UTS Ratio: 0.585-0.590;
Elongation %: 12-15;
Reduction in Area %:23-33;
Hardness, BHN: 269-285;
CVN Energy Joules: 8-13;
11. An improved method for manufacturing higher (YS/TS) yield ratio micro-alloyed wear
resistant rails substantially as herein described with reference to the examples.

The present invention is directed to a method for manufacturing micro-alloyed wear
resistant rails having high (YS/TS) yield ratio in the range of 0.585-0.590 which
comprises the following steps:
(i) Preparing a starting steels by BOF steel making steps having the following
composition: Carbon : 0.50-0.75 wt %; Mn: 0.90-1.20 wt %; Si : 0.20-0.40 wt%;
S : 0.025 wt % max; P : 0.025 wt % max; Nb: 0.02-0.06 wt %; Al: 0.01 wt % max;
(ii) Continuously casting same to obtain blooms of required size allowing minimum
pick up of oxygen in the casting ladle; (iii) Subjecting the cast blooms to a step of
rolling to obtain the rails of desired sections; and (iv) Allowing the rolled rail to a
slow cooling step.