FIELD OF THE INVENTION
The present invention relates to low carbon, low alloy and low cost steel chemistry
formulated for improved corrosion resistance. The invention also relates to a process of
manufacturing a low carbon, low alloy steel for improved corrosion resistance. The invention
further relates to the manufacture of water pipelines with improved corrosion resistant
property.
BACKGROUND AND THE PRIOR ART
Conventionally, carbon steels have a two-phase microstructure with "ferrite" (a-Fe; an
allotrope of iron with bcc crystal structure) and "pearlite" as microstructural phases. The
"pearlite" phase itself is composed of two constituent phases, namely, "ferrite" and
"cementite" (a compound of chemical formula, Fe3C) which normally appear in the form of
alternating plates or lamellae in steel microstructure. This two-phase microstructure of
conventional carbon steel induces the formation of "micro-galvanic cells" in practical
environments such as water, in which the "ferrite" phase acts as anode and corrodes
preferentially, while the "cementite" (Fe3C) phase acts like a cathode during the corrosion
process.
CN1292429 teaches a low-alloy steel for making oil production casing pipe with C02 and
sea water corrosion resistance. The steel composition gives a very wide working range for
alloying additions and lacks specificity. The steel has higher C and Mn contents with
intentional alloying additions of Cr and Mo.
US3997372 discloses a costlier steel (due to micro-alloying with Cb, V) and employs
precipitation-strengthening mechanism for achieving high strength. US4826543 and
US4946516 teaches the manufacture of a quenched and tempered steel plate with
tempered martensitic steel microstructure. Also, the composition incorporates significantly
higher alloying additions of Si, Mn, Ni, Mo and Cr for stress corrosion cracking resistance.
The steel described is, thus, very costly.
2

US4397685 teaches the production of an ultra low carbon steel using basic oxygen
process. As such, it covers only the steel making aspects and excludes from its ambit, the
details of subsequent processing/ forming and finished steel aspects like product type,
chemical composition, microstructure and properties.
The present inventors have found that lowering the carbon content of steel reduces the
amount of two-phase lamellar aggregate, viz., pearlite and the resultant steel matrix tends to
acquire a near single-phase ferritic microstructure, whereby the "micro-galvanic cell"
formation is largely eliminated and the steel corrosion rate is lowered markedly. It has been
noted that while the lowering of carbon content in steel improves its corrosion resistance, its
tensile strength is impaired. Tensile strength is one of the important parameters which is of
concern to the steel manufacturers since the nature of steel alloy and the products made
from the same depends largely on the tensile strength. This also determines the durability
and longevity of the material and therefore extends the life of the material which proves to
be economic for both manufacturers and the end users.
The selection of piping materials for transportation and handling of industrial cooling and
scrubbing waters in integrated steel plants calls for the recognition of the fact that the water-
side environment is normally more aggressive than the external environment. Often, the
heat and mass transfer associated with a cooling water recirculation system adversely
affects the corrosion behaviour of containment surfaces, which can be predicted by normal
thermodynamic and corrosion kinetic considerations evaluated under static immersion
conditions in a laboratory. The corrosive damage to internal surfaces of pipelines
transporting cooling waters is a complex phenomenon. The damage is often a result of
synergistic effect of corrosion and erosion because of solid suspended particles moving in
water streams at high velocities.
In the past, the measures employed for protection against water corrosion in steel plants
have concentrated mostly on the use of protective paints and coatings. Use of corrosion
inhibitors has also been investigated with moderate success. However, it is only recently
that low-alloy corrosion resistant steels have been studied and applied for fabrication of
water pipelines on a large scale.
3

The present inventors have found that the spirally-welded (SW) steel pipes covered under
the Indian Standard specification IS:5504, stipulates the achievement of mechanical
(tensile) properties: (a) an yield strength of 240 MPa minimum, (b) an ultimate tensile
strength of 410 MPa minimum and (c) a uniform elongation of 20% minimum. The
realization of the as-specified tensile properties, therefore, necessitated further modification
of the steel chemistry, especially when the carbon content was lowered to 0.07-0.09 wt%.
The decrease in carbon content of steel was compensated by increasing the manganese
content to 0.85-0.95 wt%; manganese provides substitutional solid solution strengthening to
the steel and helps in achieving requisite tensile properties. Additionally, 0.20-0.25 wt%
copper is also incorporated in the LCLA steel chemistry to promote protective passive film
formation on pipeline steel under prolonged water service.
Thus there is need for a low carbon low alloy (LCLA) steel, for application in water pipelines.
The present invention has been specifically designed for application in pipelines used for
transporting and handling raw, industrial or potable waters. The carbon (0.07-0.09 wt%),
manganese (0.85-0.95 wt%) and copper (0.20-0.25 wt%) contents of this novel LCLA steel
are more specific and denotative. Besides, this steel has no intentional alloying of Cr, Mo
and rare earths and is definitely much cheaper than the steel in the prior art.
OBJECTS OF THE INVENTION
Thus one object of the present invention is to provide a steel alloy composition having
improved corrosion resistant property.
Another object of the present invention is to provide a process for manufacturing the steel
alloy composition.
Yet another object of the present invention is to manufacture water pipelines having
improved corrosion resistant property.
4

SUMMARY OF THE INVENTION
Accordingly, one aspect of the present invention is to provide a steel alloy composition for
manufacturing water pipelines, said composition comprising
0.20 to 0.25 wt% Copper;
0.75 to 0.95 wt% Manganese; and
0.07 to 0.09 wt% Carbon so as to reduce the amount of two-phase lamellar aggregate
(pearlite) of carbon steel and thus acquiring a near single-phase ferrite microstructure
whereby micro-galvanic cells are largely eliminated resulting in lower corrosion rate.
Another aspect of the present invention is to provide a process for manufacturing a steel
alloy composition comprising 0.20 to 0.25 wt% Copper; 0.75 to 0.95 wt% Manganese; and
0.07 to 0.09 wt% Carbon, so as to reduce amount of two-phase lamellar aggregate (pearlite)
and thus acquiring a near single-phase ferrite microstructure whereby micro-galvanic cells
are largely eliminated resulting in lower corrosion rate, said process comprising steps of:
forming liquid steel in basic oxygen furnace;
deoxidizing and alloying said liquid steel;
continuous casting of the liquid steel to form slabs;
soaking the slabs in a reheating furnace at an appropriate temperature; and
hot rolling the soaked slabs so as to form steel strips.
Yet another aspect of the present invention is to provide a process for manufacturing
pipelengths comprising steel alloy composition with 0.20 to 0.25 wt% Copper; 0.75 to 0.95
wt% Manganese; and 0.07to 0.09 wt% Carbon,so as to reduce the amount of two-phase
lamellar aggregate (pearlite) and thus acquiring a near single-phase ferrite microstructure
whereby micro-galvanic cells are largely eliminated resulting in lower corrosion rate, said
process comprising steps of:
forming liquid steel in basic oxygen furnace;
deoxidizing and alloying said liquid steel;
continuous casting of the liquid steel to form slabs;
soaking the slabs in a reheating furnace at an appropriate temperature;
hot rolling the soaked slabs so as to form steel strips; and
spiral welding of said steel strips to form pipelengths.
5

DETAILED DESCRIPTION OF THE INVENTION
Steel alloy composition of the present invention comprises 0.20 to 0.25 wt% Copper; 0.75 to
0.95 wt% Manganese; and 0.07 to 0.09 wt% Carbon.
The steel heat is prepared for making a relatively low carbon, low alloy steel, following
known practices for producing a clean steel, with good control of desired contents of small
percentages of alloying elements. Thus the basic melt is achieved in a usual manner, as in a
standard electric or basic oxygen furnace, appropriate attention being paid to the desired
low carbon content. The carbon levels as low as 0.07 to 0.09 wt% are effectively obtainable
without special treatment of the melt after tapping.
While the lowering of carbon content in steel improves its corrosion resistance, its tensile
strength is impaired. The spirally-welded (SW) steel pipes are covered under the Indian
Standard specification IS:5504, which stipulates the achievement of mechanical (tensile)
properties: (a) an yield strength of 240 MPa minimum, (b) an ultimate tensile strength of 410
MPa minimum and (c) an overall elongation of 20% minimum. The realization of the as-
specified tensile properties, therefore, necessitates further modification of the steel
chemistry, especially when the carbon content is lowered to 0.07 to 0.09 wt%. The decrease
in carbon content of steel was compensated by increasing the manganese content to 0.85 to
0.95 wt%; manganese provides substitutional solid solution strengthening to the steel and
helps in achieving requisite tensile properties. Additionally, 0.20-0.25 wt% copper was also
incorporated in the LCLA steel chemistry to promote protective passive film formation on
pipeline steel under prolonged water service.
The steel composition of this invention must be fully deoxidized. Deoxidation is preferably
achieved by addition of aluminum to molten steel, e.g. in the ladle. Although conceivably
other deoxidation practices may be followed and therefore manner of deoxidation as
described herein is not a restriction to scope of the invention.
The liquid steel is then fed into continuous casting to slabs.
Soaking of the steel slabs is done in Reheating furnace at 1250 - 1300°C.
These steel slabs are then hot rolled to 8-10 mm steel strip.
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The hot-rolled steel strip coils are then spirally welded to Low Carbon Low Alloy (LCLA)
Steel Pipes of 24" dia, 8 mm thickness and 33" dia, 10 mm thickness.
The carbon content is lowered to 0.07-0.09 wt%. The decrease in carbon content of steel is
compensated by increasing the manganese content to 0.85-0.95 wt%; manganese provides
substitutional solid solution strengthening to the steel and helps in achieving requisite tensile
properties. Additionally, 0.20-0.25 wt% copper is also incorporated in the LCLA steel
chemistry to promote protective passive film formation on pipeline steel under prolonged
water service.
This compositional adjustment is made for achievement of (a) near single-phase ferritic
microstructure (b) mechanical properties as per IS:5504 and (c) improved corrosion
resistance in steel water pipelines.
The serendipitous outcome of the present invention is the superior performance of LCLA
steel under dynamic corrosion testing in water (refer to Fig.6).
In the process of the present invention the carbon content is lowered so as to result in lower
volume fraction of the two-phase aggregate, namely, pearlite. The resultant steel matrix
tends to acquire a more single-phase ferrite microstructure and exhibits lowering of
corrosion rate through elimination of micro-galvanic cells. Copper (Cu) is added to promote
a protective passive film formation on the steel surface under prolonged water service.
Manganese (Mn) content in increased proportion helps in achieving the requisite tensile
properties (yield strength, tensile strength and elongation) and compensates for the
decrease in carbon content.
The composition/ chemistry of the steel alloy is shown in Table 1.
Table 2 shows the typical chemical composition of the LCLA steel achieved in an industrial
heat along with the typical composition of a regular IS:2062 pipeline steel in use in SAIL
steel plants.
The tensile properties achieved for the newly-developed LCLA steel and the specified
minimum values for pipeline steels as per IS.5504 (1969) and IS:2062 (1999) are given in
Table 3 for comparison. The tensile properties of welded LCLA steel were also determined
7

and the results are given in Table 3. The yield strength (YS), ultimate tensile strength (UTS)
and percentage elongation for LCLA steel were found to meet the stipulations of minimum
design-property requirement of Indian Standard designation for spirally-welded carbon steel
pipe, IS:5504(1969).
The waters for industrial use are transported and distributed through an extensive network
of large and small diameter pipelines and conduits running into several tens of kilometers.
These water pipelines are usually fabricated from 6-10 mm Indian Standard IS:2062 grade
hot-rolled mild steel strip coils and conform to the design-property requirements of Indian
Standard IS:5504 (1969) for a spirally-welded steel pipe.
In the integrated steel plants, the water pipelines have been observed to suffer damage
predominantly on the internal working surfaces due to corrosion (and erosion) in the
presence of complex environment of waters with varying pH, temperature, flow and
microbiological activity and containing suspended solid matter, dissolved gases and salts
such as dissolved oxygen (O2), chlorides (CI"), fluorides (F"), sulphates (S042"), etc.
The most commonly used constructional material for industrial water transportation pipelines
is IS:2062 grade mild steel with 0.20 weight% carbon (max) as principal alloying element.
This steel, in as-hot rolled condition, contains ferrite and pearlite as microstructural phases;
the two-phase microstructure promotes the formation of microgalvanic cells in corrosive
environment. Pearlite grains, which contain alternate layers of cementite and ferrite phases,
are especially the favoured sites for preferential corrosive attack.
The internal working surfaces of steel pipelines transporting industrial waters in SAIL steel
plants also suffer premature corrosion damage due to the presence of aggressive chemical
species such as dissolved oxygen (O2), dissolved carbon dioxide (CO2), chlorides (CI"),
fluorides (F'), sulphates (S042"), carbonates (CO32") etc in the waters.
Spirally-welded mild steel (MS) pipes conform to the design-property stipulations of Indian
Specifications, IS:2062 (1999) / IS:5504 (1969). These are mostly used for transportation of
various types of industrial waters. The overall service life of these pipes can be as low as 7
years depending on factors such as quality of water, operating parameters and other
conditions.
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The present inventors have developed a low-cost, lean-chemistry, corrosion resistant steel
for fabrication of pipes handling and transporting different types of waters in Steel Plants.
For augmenting the service life of water pipelines, the conventionally-used IS:2062 steel
chemistry is modified and redesigned in order to improve its corrosion resistance in water
service. The carbon content in the steel is lowered from 0.15-0.20 weight% to 0.07-0.09
weight% and minor alloying addition of 0.20-0.25 weight% copper (Cu) is incorporated in the
steel chemistry for achieving adequate corrosion resistance. The decrease in carbon
content is counterbalanced with a corresponding increase in the manganese (Mn) content to
0.75-0.95 weight% for achieving the stipulated tensile properties of the Indian Standard
IS:5504 specification for a spirally-welded pipe. Thus, a low-cost, lean-chemistry, low carbon
low alloy Cu-bearing steel with adequate corrosion resistance is designed to derive longer
service life than that realized with conventional mild steel pipelines transporting industrial
raw water.
Six commercial heats (total weight: 385 tonnes) of the newly-designed low-carbon low-alloy
(LCLA) steel were made through Basic Oxygen Furnace (BOF)- Continuous Casting (CC)
process route at Steel Plant. The slabs were subsequently rolled to 21 nos. of 8 and 10 mm
thick hot-rolled (HR) strip coils in the Hot Strip Mill (HSM). The HR coils are then fabricated
into 300 metres of 33" diameter, 10 mm thick and 762 metres of 24" dia, 8 mm thick spirally-
welded pipes. The overall length of the pipeline made at Spirally Welded Pipe Plant (SWPP)
was 1062 metres, which was installed and commissioned for transportation of industrial raw
water at several locations inside the steel plant and elsewhere.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Fig 1 shows the comparative hardness profiles across the weldments in IS:2062 and LCLA
pipeline steels.
Fig.2 shows the typical microstructures of LCLA and IS:2062 pipeline steels taken at 1000 X
magnification in an optical microscope.
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Figures 3, 4 and 5 show the comparative corrosion performance of LCLA and IS:2062
pipeline steels under laboratory test conditions involving, variously, static immersion in
industrial cooling water, tap water and distilled water respectively.
Figure 6 illustrates comparative steady-state corrosion rates of LCLA and IS:2062 pipeline
steels as a function of average flow velocity of distilled water.
Figure 7 is a Process Chart for Manufacture of Corrosion Resistant Low Carbon Low Alloy
(LCLA) Steel.
DETAILED DESCRIPTION OF THE ACCOMPANYING FIGURES
Fig. 1 shows the comparative hardness profiles across the weldments in IS:2062 and LCLA
pipeline steels. The LCLA steels were found to exhibit lower hardness values in the weld
and heat-affected zone (HAZ) regions than the IS:2062 pipeline steel in use at steel plants.
The achievement of hardness and tensile properties in welded LCLA steel clearly underlines
its good weldability.
Fig.2 shows the typical microstructures of LCLA and IS:2062 pipeline steels taken at 1000 X
magnification in an optical microscope. The LCLA steel is found to exhibit ferrite-pearlite
microstructure with very fine-grained ferrite and lower volume fraction of pearlite (~9%) in
comparison to the regular pipeline steel which shows relatively coarse grains of ferrite and a
higher volume fraction (-16%) of pearlite. The very low volume fraction of pearlite in LCLA
steel is attributable to its extremely low C content (0.07 weight%).
Figures 3, 4 and 5 show the comparative corrosion performance of LCLA and IS:2062
pipeline steels under laboratory test conditions involving, variously, static immersion in
industrial cooling water, tap water and distilled water respectively. The graphs clearly reveal
slower corrosion kinetics, lower weight losses and impeding of corrosion rates over the
entire 60-day test period for the low-carbon low-alloy Cu-bearing LCLA pipeline steel.
Figure 6 shows the comparative corrosion rates of LCLA and IS:2062 steels in distilled
water under dynamic, flowing conditions of the test medium. The graph shows the variation
10

in corrosion rates of the steels as a function of average flow velocity of distilled water and
reveals substantially slower corrosion kinetics for LCLA steel for the range of water
velocities studied.
TABLE 1
Designed chemical composition in weight% of corrosion-resistant low carbon low
alloy (LCLA) steel for water pipelines

c Mn Si S P Cu Al
0.07-0.09 0.75-0.95 0.25-0.40 0.02 max 0.035 max 0.20-0.25 0.02-0.04
TABLE 2
Chemical composition of pipeline steels for water service (weight%)

Pipeline Steel C Si Mn S P Cu Al
LCLA* 0.07 0.37 0.91 0.018 0.031 0.23 0.02
Existing SP pipe: IS:2062/ IS:5504 0.13 0.14 0.83 0.014 0.027 - -
TABLE 3
Tensile properties of LCLA steel vis-a-vis specified minimum values as per IS:5504
and IS:2062 standard specifications

Tensile
Property Specified (min) as
per iS:5504 (1969) Specified (min) as
per IS:2062 (1999) Achieved
(Range) Achieved for
welded
specimens
YS (MPa) 240 240 290-354 366
UTS (MPa) 410 410 420-456 450
%Elongation 20 23 30-34 31
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WE CLAIM
1. A steel alloy composition for manufacturing water pipelines, said composition
comprising
0.20 to 0.25 wt% Copper;
0.75 to 0.95 wt% Manganese; and
0.07 to 0.09 wt% Carbon so as to reduce the amount of two-phase lamellar
aggregate (pearlite) of carbon steel and thus acquiring a near single-phase ferrite
microstructure whereby micro-galvanic cells are largely eliminated resulting in lower
corrosion rate.
2. A steel alloy composition as claimed in claim 1 further comprising 0.25 to 0.40 wt%
Silicon, 0.02 max wt% Sulphur, 0.035 max wt% Phosphorous and 0.02 to 0.04 wt%
Aluminium.
3. A steel alloy composition as claimed in claims 1 and 2 having yield strength of 290
MPa to 354 MPa.
4. A steel alloy composition as claimed in claims 1 and 2 having ultimate tensile
strength of 420 MPa to 456 MPa.
5. A pipeline for water supply being made of steel alloy composition as claimed in
claims 1 and 2 comprising :
0.07 wt% carbon, 0.37 wt% silicon, 0.91 wt% Manganese, 0.018 wt% sulphur, 0.031
wt% phosphorous, 0.23 wt% copper and 0.02 wt% aluminium.
6. A process for manufacturing a steel alloy composition comprising 0.20 to 0.25 wt%
Copper; 0.75 to 0.95 wt% Manganese; and 0.07 to 0.09 wt% Carbon.so as to reduce
the amount of two-phase lamellar aggregate (pearlite) and thus, acquiring a near
single-phase ferrite microstructure whereby micro-galvanic cells are largely
eliminated resulting in lower corrosion rate, said process comprising steps of:
forming liquid steel in basic oxygen furnace;
deoxidizing and alloying said liquid steel;
continuous casting of the liquid steel to form slabs;
soaking the slabs in a reheating furnace at an appropriate temperature; and
hot rolling the soaked slabs so as to form steel strips.
12

7. A process for manufacturing pipelengths of steel alloy composition comprising 0.20 to
0.25 wt% Copper; 0.75 to 0.95 wt% Manganese; and 0.07 to 0.09 wt% Carbon,.so
as to reduce amount of two-phase lamellar aggregate (pearlite) and thus, acquiring a
near single-phase ferrite microstructure whereby micro-galvanic cells are largely
eliminated resulting in lower corrosion rate, said process comprising steps of:
forming liquid steel in basic oxygen furnace;
deoxidizing and alloying said liquid steel;
continuous casting of the liquid steel to form slabs;
soaking the slabs in a reheating furnace at an appropriate temperature;
hot rolling the soaked slabs so as to form steel strips; and
spiral welding of said steel strips to form pipelengths.
13
8. Process as claimed in claims 6 and 7, wherein the steel slab soaking temperature
ranges from 1250° to 1300°C.
9. A steel alloy composition and process for manufacturing the same as herein
substantially described and illustrated with reference to the accompanying figures.

A steel alloy composition and a process for manufacturing the steel alloy
composition and water pipelines with improved corrosion resistant property. The
composition comprises 0.20 to 0.25 wt% copper, 0.75 to 0.95 wt% manganese
and 0.07 to 0.09 wt% carbon such that the amount of two-phase lamellar
aggregate (pearlite) of carbon steel is reduced. Thus a near single-phase ferrite
microstructure is acquired whereby micro-galvanic cells are largely eliminated
resulting in lower corrosion rate.