PROCESS FOR MANUFACTURING OF CORROSION RESISTANT RAIL STEEL
Field of the invention
The present invention relates to corrosion resistant rail steel composition and
process for manufacturing the same.
The steel compositions in accordance with the invention is effective in applications
where high strength as well as corrosion resistance is required and are particularly
useful for manufacture of rails for marine/coastal environment wherein steel rails
are continually subjected to the aggressiveness of saline atmosphere.
Background of the invention
Conventionally different alloying elements are added to steel in order to enhance
its corrosion resistance and mechanical properties. The need for a steel
composition which has enhanced corrosion resistance and has appreciable
mechanical properties is significant in the manufacture of rails especially for
coastal/marine areas. Such rails are continually subjected to the saline
atmosphere of the coastal/marine areas and require frequent rail replacements
due to severe corrosion in the foot regions of the rails. Instances of such severe
corrosion in foot locations of rails lead to frequent fractures on account of reduced
section due to localized corrosion.
Alloying elements like copper (Cu), chromium (Cr), molybdenum (Mo) and
phosphorus (P) are added in weathering steels for improved corrosion resistance
for structural applications. These alloying elements promote the formation of a
compact, crack-free, cohesive amorphous layer of iron oxyhydroxides (FeOOH) in
steel rust, thereby limiting the ingress of oxygen, water and aggressive ionic
species to the steel surface. However, use of such alloying elements for
enhancing corrosion resistance of rail steels has not yet been reported.
US 5830286 teaches a steel rail having wear resistance and internal breakage
resistance required for a heavy load railway, containing, in terms of percent by
weight, more than 0.85 to 1.20% of C, 0.10 to 1.00% of Si, 0.40 to 1.50% of Mn,
0.0005 to 0.0040% of B, at least one of 0.05 to 1.00% of Cr, 0.01 to 0.50% of Mo,
0.02 to 0.30% of V, 0.002 to 0.05% of Nb and 0.10 to 2.00% of Co, whenever
necessary, being acceleratedly cooled at a cooling rate of 5°C to 15°C/sec from
an austenite zone temperature to 650°C to 500°C, exhibiting a pearlite structure
having a hardness of at least Hv 370 within the range from the surface of the rail
head portion to a position having a depth of 20 mm from the head surface with this
head surface being the start point, and the difference of the hardness within this
range being not more than Hv 30.
US 5762723 teaches a steel composition having improved wear and damage
resistance required for a rail of a sharply curved zone of a heavy load railway,
comprising more than 0.85 to 1.20% of C, 0.10 to 1.00% of Si, 0.40 to 1.50% of
Mn and if necessary, at least one member selected from the group consisting of
Cr, Mo, V, Nb, Co and B, and retaining high temperature of hot rolling or a steel
rail heated to a high temperature for the purpose of heat-treatment, the present
invention provides a pearlitic steel rail having a good wear resistance and a good
damage resistance, and a method of producing the same, wherein a head portion
of the steel rail is acceleratedly cooled at a rate of 1°C to 10°C/sec from an
austenite zone temperature to a cooling stop temperature of 700°C to 500°C so
that the hardness of the head portion is at least Hv 320 within the range of a 20
mm depth.
US 5676772 teaches a high-strength bainitic steel rail having an excellent damage
resistance property, essentially consisting of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of
Si, 0.3 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% of S, and 0.3 to 4.0
wt% of Cr, a balance being Fe, and having a micro structure made of a bainitic
structure. This rail includes corner and head side portions having a Vickers
hardness of Hv 420 or higher, and a head top portion having a hardness of Hv420
or higher at a site 20-mm distant from a center of the head top portion in a width
direction, wherein the center of the head top portion has such a hardness
distribution that a hardness of the center of the head top portion is 10 to 70 lower
in Vickers hardness than that of the site 20-mm distant from the center of the head
top portion, a hardness of a section between the center of the head top portion
and the site 20-mm away from the center in the width direction increases
gradually from the center towards an outer side of the width direction, and a
difference between an actual hardness of the section, and a hardness obtained by
interpolating the hardness of the center of the head top portion and the hardness
of the site 20-mm away from the center in the width direction by straight line, is 10
or less in Vickers hardness.
JP 06248347 teaches a process for the production of rail steel. The process
improves the using service life of a rail by subjecting the top part and bottom part
of a rail of steel of a high temperature having a specified composition to
accelerate cooling under specified conditions and subsequently executing cooling
to an ordinary temperature at a specified rate. The steel comprises 0.15 to 0.45%
C, 0.15 to 2.00% Si, 0.30 to 2.00% Mn, 0.50 to 3.00% Cr and 0.10 to 0.60% Mo,
and the balance iron with inevitable impurities. The top part and bottom part of a
rail holding heat of high temperature obtained by subjecting the steel to hot rolling
or a rail heated to a high temperature for the purpose of heat treatment are
subjected to accelerated cooling from the temperature of austenitic region to 500
to 350°C at 1 to 10°C/sec. Successively, cooling is executed to room temperature
at 1 to 40°C/min. The steel is moreover incorporated with one or more kinds
among 0.05 to 4.00% Ni, 0.01 to 0.05% Nb and 0.05 to 0.50% Cu. In this way, the
rail excellent in surface damaging resistance can be obtained.
The above-mentioned prior arts describe the alloying of steel rails with Cr, Mo, Cu,
etc. and application of accelerated cooling techniques for the sole purpose of
achieving microstructures with higher hardness and wear resistance in rails. The
aspect of enhanced corrosion resistance is not investigated/ mentioned.
Objects of the invention
Thus the object of the invention is to provide a steel composition which is
corrosion resistant and can withstand the saline atmosphere of coastal/marine
environment.
A further object of the invention is to provide a steel composition which has
excellent mechanical properties.
The applicants after extensive research have found that if conventional rail steel is
alloyed with Copper (Cu) and Molybdenum (Mo) in specific ranges of amounts the
resulting steel composition has excellent corrosion resistance as well as
mechanical properties. The alloying process is preferably carried out following a
process scheme to achieve the desired end results.
Summary of the invention
Thus according to the present invention there is provided a corrosion resistant rail
steel composition comprising:
0.55 to 0.85 wt% of Carbon, 0.10 to 0.40 wt% of Silicon, 0.80 to 1.20 wt% of
Manganese, 0.001 to 0.04 wt% of Sulphur, 0.001 to 0.04 wt% Phosphorus, 0.10 to
0.50 wt% Copper and 0.001 to 0.35 wt% of Molybdenum, the rest being Iron with
Hydrogen content below 2 ppm.
According to another aspect of the present invention there is provided a process
for manufacture of corrosion resistant rail steel comprising:
(i) preparation of steel heats of predetermined composition in a basic oxygen
furnace (BOF);
(ii) secondary refining of said steel heats in a vacuum arc degasser (VAD);
(iii) soaking said refined steel in reheating furnace;
(iv) rolling of said soaked steel followed by gradual cooling.
Detailed description
In the present invention, the steel composition preferably comprises 0.65 to 0.75
wt% of Carbon, 0.20 to 0.30 wt% of Silicon, 0.90 to 1.10 wt% of Manganese, 0.00
to 0.03 wt% of Sulphur, 0.00 to 0.03 wt% Phosphorus, 0.20 to 0.40 wt% Copper
and 0.00 to 0.25 wt% of Molybdenum, the rest being Iron with Hydrogen content
less than 2 ppm.
According to a preferred aspect the steel composition comprises 0.66 to 0.71 wt%
of Carbon, 0.23 wt% of Silicon, 1.06 to 1.08 wt% of Manganese, 0.016 to 0.017
wt% of Sulphur, 0.023 to 0.03 wt% Phosphorus, 0.29 to 0.33 wt% Copper and
0.00 to 0.15 wt% of Molybdenum, the rest being Iron. The Hydrogen content is
preferably restricted to below 2 ppm.
According to a further preferred aspect of the invention, the steel composition
comprises 0.71 wt% of Carbon, 0.23 wt% of Silicon, 1.08 wt% of Manganese,
0.017 wt% of Sulphur, 0.023 wt% Phosphorus, 0.29 wt% Copper and 0.005 wt%
of Aluminum, the rest being Iron. The Hydrogen content is preferably restricted to
1.41 ppm.
According to another further preferred aspect of the invention, the steel
composition comprises 0.66 wt% of Carbon, 0.23 wt% of Silicon, 1.06 wt% of
Manganese, 0.016 wt% of Sulphur, 0.03 wt% Phosphorus, 0.33 wt% Copper, 0.15
wt% of Molybdenum and 0.004 wt% of Aluminum, the rest being Iron. The
Hydrogen content is preferably restricted to 1.81 ppm.
In the process for manufacture of the steel composition, steel heats having the
composition as stated above are first prepared in a basic oxygen furnace. The
liquid steel is then secondary-refined in a vacuum arc degasser. The primary
purpose of the secondary refining is to restrict the hydrogen content below 2 ppm.
This low hydrogen level in steel helps in controlling delayed micro-crack initiation.
This eliminates hydrogen-induced catastrophic failures in service. After secondary
refining the liquid steel is continuously cast into blooms. The blooms are soaked in
reheating furnace at a temperature of between 1250 to 1300°C for temperature
and compositional homogenization. Thereafter the blooms are rolled in 14 to16
passes with a finishing rolling temperature of 870 to 930°C. For manufacture of
rails the blooms are rolled in a rail mill and rails are air cooled to about 550°C
followed by slow cooling in pit to facilitate hydrogen removal. Finally, the rails are
straightened and inspected.
The steel rails have 1.8 to 2.2 times higher corrosion resistance than conventional
rail steels in aqueous sodium chloride solutions. Thus, the service life of these
rails in actual operating conditions is much higher than that of the conventional
rails. As a consequence, the rail replacements on account of corrosion damage
are reduced considerably. The maintenance cost for the present steel rails is also
significantly lower.
The invention is illustrated with reference to accompanying graphical
representation/flow diagram in which:
Figure 1 shows process flow diagram for manufacture of rail steel.
Figure 2 shows CVN impact toughness test results.
Figure 3 shows conditional fracture toughness test results.
Figure 4 shows fatigue crack growth rate test result.
Figure 5 shows corrosion resistance indices of rail steels in aqueous sodium
chloride
The following examples describe the best method of performing the process in
accordance with the present invention and tests conducted on the product
obtained.
Examples
Example 1: Process for manufacture of Copper bearing rail steel
Figure 1 shows the process flow diagram for the manufacture of steel rails. Liquid
steel having 0.71 wt% of Carbon, 0.23 wt% of Silicon, 1.08 wt% of Manganese,
0.017 wt% of Sulphur, 0.023 wt% Phosphorus, 0.29 wt% Copper and 0.005 wt%
of Aluminum, the rest being Iron was prepared in a basic oxygen furnace. The
liquid steel was then secondary-refined in a vacuum arc degasser. During the
secondary refining the Hydrogen content is restricted to 1.44 ppm. The secondary
refined liquid steel was cast continuously to blooms. The blooms were soaked in
reheating furnace at a temperature of 1280°C for temperature and compositional
homogenization. Thereafter the blooms were rolled in a rail mill in15 passes with a
finishing rolling temperature of 900°C to form rails. The rails were air cooled to
550°C followed by slow cooling in pit to facilitate hydrogen removal. Finally, the
rails were straightened and inspected.
Example 2: Process for manufacture of Copper-Molybdenum bearing rail steel
Figure 1 shows the process flow diagram for the manufacture of steel rails. Liquid
steel having 0.66 wt% of Carbon, 0.23 wt% of Silicon, 1.06 wt% of Manganese,
0.016 wt% of Sulphur, 0.03 wt% Phosphorus, 0.33 wt% Copper, 0.15 wt% of
Molybdenum and 0.004 wt% of Aluminum, the rest being Iron was prepared in a
basic oxygen furnace. The liquid steel was then secondary-refined in a vacuum
arc degasser. During the secondary refining the Hydrogen content was restricted
to 1.81 ppm. The secondary refined liquid steel was cast continuously to blooms.
The blooms were soaked in reheating furnace at a temperature of 1280°C for
temperature and compositional homogenization. Thereafter the blooms were
rolled in a rail mill in 15 passes with a finishing rolling temperature of 900°C to
form rails. The rails were air cooled to 550°C followed by slow cooling in pit to
facilitate hydrogen removal. Finally, the rails were straightened and inspected.
The following examples provide comparative testing and analysis of rail steel
manufactured in accordance with the present invention. The comparison was
done with conventional rail steel composition comprising 0.69 wt% Carbon, 0.29
wt% Silicon, 1.09 wt% Manganese, 0.017 wt% Sulphur, 0.033 wt% Phosphorus,
the rest being Iron.
Example 3: Experiment for testing tensile strength
For tensile tests, round tensile test specimens were obtained from head region of
the manufactured rails longitudinally in the rolling direction. The specimens were
machined to dimensions as per ASTM standard designation E 8M-96. A gauge
length of 25 mm and a gauge diameter of 6.25 mm were maintained for all tensile
specimens. Tensile tests were conducted in a 10T Universal Static Tensile
Testing Machine using 25 mm gauge length extensometer and a cross head
speed of 2 mm/min. The alloyed rail steels exhibited ultimate tensile strengths of
nearly 100 kg/mm2 as compared to 93 kg/mm2 for the conventional rail steel.
Example 4: Evaluation of interlamellar pearlite spacing
The average interlamellar spacing of pearlite was evaluated for rail steels in
accordance with the present invention and conventional rail steel. The
interlamellar pearlite spacing was found to be 0.19µm for the Copper-
Molybdenum bearing rail steel and 0.29 µm for Copper rail steel. For the
conventional rail steel the spacing was found to be 0.33 µm. Thus the alloyed rail
steels of the present invention exhibited smaller pearlite colonies with finer pearlite
and this reduced interlamellar pearlite spacing leads to higher strength and
hardness.
Example 5: Evaluation of Charpy V-notch (CVN) impact toughness
Charpy V-notch (CVN) impact test specimens of 10 X 10 X 55 mm size were
prepared longitudinally in the rolling direction from the head section of the rails. A
2 mm V-notch was cut in each specimen in the transverse direction. Impact
loading at room temperature was carried out in a Instrumented Impact Tester.
Figure 2 shows the CVN impact toughness test results. The Copper rail exhibited
CVN Impact Energy of 0.78 kg.m and the Copper Molybdenum 1.34 kg.m while
the conventional Carbon Manganese rail exhibited CVN Impact Energy of 0.64
kg.m.
Example 6: Experiment for testing fracture toughness
Fracture toughness tests were conducted at 25°C in a computer controlled 10 ton
capacity servo hydraulic Dynamic Testing Machine. 25 mm thick straight through
notched compact tension (CT) specimens were prepared from head of the rails as
per ASTM standard designation E 399-90 with the orientation of the notch
transverse to the rolling direction. The comparative static fracture toughness of the
rail steels is shown in Figure 3. While the conventional rail steel exhibited
conditional fracture toughness of 36.29 MPam1/2, the Copper rail steel exhibited
41.28 MPam1/2 and the Copper Molybdenum rail steel 50.02 MPam1/2.
Example 7: Experiment for evaluating fatigue crack growth rate
25 mm thick straight through notched compact tension (CT) specimens were
prepared from head portion of the rails as per ASTM standard designation E 399-
90 with the notch oriented transverse to the rolling direction. Fatigue crack growth
rate measurements were made in a computer controlled 10 ton capacity servo
hydraulic Dynamic Testing Machine in accordance with ASTM designation E 647-
95a. The comparative fatigue crack growth rates of the rail steels are shown in
Figure 4. The rail steels of the present invention exhibited lower fatigue crack
growth rates.
Example 8: Experiment for determining corrosion rates
For determining corrosion rates, electrochemical TAFEL polarization experiments
were carried out in a EG&G PARC Potentiostat with 15 mm diameter and 3 mm
thick polished disc specimens taken from foot region of the rails. A 3.5% NaCI test
solution, a saturated calomel reference electrode and a scan rate of 1mV/sec
were employed during the experimentation. Weight loss was also recorded for 70
X 50 X 5 mm size machined steel coupons, prepared from the foot portion of each
grade of rail, for 720 hours of static immersion in 3.5% NaCI solution at ambient
temperature. Machined and weighed steel coupons of 70 X 50 X 5 mm, taken
from foot section of the section of the rails of each grade, were also exposed to
salt fog in a salt spray chamber for 720 hours at a chamber temperature of 30°C.
A test solution of 5% NaCI was used to generate the salt mist. The test was
preformed in accordance with ASTM standard designation B 117. The
comparative corrosion resistance indices of the rail steels, as determined by
above three testing procedures, is demonstrated in Figure 5. The test revealed 1.8
to 2.2 times higher corrosion resistance for Copper and Copper Molybdenum rail
steels of the present invention over conventional carbon-manganese rail steel.
We claim:
1. Process for manufacture of corrosion resistant rail steel comprising:
(i) preparation of steel heats in a basic oxygen furnace (BOF), said steel heats
having composition comprising 0.55 to 0.85 wt% of Carbon, 0.10 to 0.40 wt% of
Silicon, 0.80 to 1.20 wt% of Manganese, 0.001 to 0.04 wt% of Sulphur, 0.001 to
0.04 wt% Phosphorus, 0.10 to 0.50 wt% Copper and 0.001 to 0.35 wt% of
Molybdenum, optionally aluminium the rest being Iron with Hydrogen content less
than 2 ppm;
(ii) secondary refining of said steel heats in a vacuum arc degasser (VAD) to
reduce the hydrogen content below 2 ppm;
(iii) soaking said refined steel in reheating furnace at temperature between 1250 to
1300°C.
(iii) rolling of said soaked steel followed by gradual cooling to facilitate hydrogen
removal.
2. Process as claimed in claim 1 wherein said steel heats has composition comprising:
0.65 to 0.75 wt% of Carbon, 0.20 to 0.30 wt% of Silicon, 0.90 to 1.10 wt% of
Manganese, 0.00 to 0.03 wt% of Sulphur, 0.00 to 0.03 wt% Phosphorus, 0.20 to 0.40
wt% Copper and 0.00 to 0.25 wt% of Molybdenum, the rest being Iron with Hydrogen
content less than 2 ppm.
3. Process as claimed in claim 2 wherein said steel heats has composition comprising:
0.71 wt% of Carbon, 0.23 wt% of Silicon, 1.08 wt% of Manganese, 0.017 wt% of
Sulphur, 0.023 wt% Phosphorus, 0.29 wt% Copper and 0.005 wt% of Aluminum, the
rest being Iron with Hydrogen content less than 1.41 ppm.
4. Process as claimed in claim 2 wherein said steel heats has composition comprising:
0.66 wt% of Carbon, 0.23 wt% of Silicon, 1.06 wt% of Manganese, 0.016 wt% of
Sulphur, 0.03 wt% Phosphorus, 0.33 wt% Copper, 0.15 wt% of Molybdenum and
0.004 wt% of Aluminum, the rest being Iron with Hydrogen content less than 1.81
ppm.
6. Process as claimed in claim 1, wherein said rolling in step (iv) is carried out in 14 to
16 passes.
7. Process as claimed in claim 6, wherein said rolling in step (iv) is preferably carried out
in 15 passes.
8. Process as claimed in claim 1, wherein the finishing rolling temperature during said
rolling in step (iv) is maintained between 870 to 930°C.
9. Process as claimed in claim 8, wherein the said finishing rolling temperature is
preferably maintained at 900°C.
10. Process as claimed in claim 1, wherein said slow cooling step (iv) is carried out till
temperature of about 550°C is reached followed by slow cooling in pit to facilitate
hydrogen removal.

A corrosion resistant rail steel composition comprises 0.55 to 0.85 wt% of Carbon,
0.10 to 0.40 wt% of Silicon, 0.80 to 1.20 wt% of Manganese, 0.001 to 0.04 wt% of
Sulphur, 0.001 to 0.04 wt% Phosphorus, 0.10 to 0.50 wt% Copper and 0.001 to
0.35 wt% of Molybdenum, the rest being Iron with Hydrogen content less than 2
ppm. The steel composition is preferably manufactured by a specific process
comprising steps of preparation of steel heats of predetermined composition in a
basic oxygen furnace (BOF), secondary refining of the steel heats in a vacuum arc
degasser (VAD), soaking the refined steel in reheating furnace and rolling of the
soaked steel followed by gradual cooling. The rail steels are particularly useful for
use in marine/coastal environment wherein steel rails are continually subjected to
the aggressiveness of saline atmosphere.