PROCESS TECHNOLOGY FOR PRODUCTION OF ALUMINIUM KILLED
BLOOMS FOR ANGLES WITH IMPROVED GALVANISABILITY FOR
TRANSMISSION LINE-TOWER APPLICATION
FIELD OF INVENTION
The present invention relates to process technology for production of aluminium
killed blooms with innovative alloy chemistry of steel composition (by weight %)
containing: C- 0.16 to 0.20%, Mn-1.20 to 1.40%, S- = 0.035%, P- = 0.035%, Si-
0.15 to 0.25%, Al- =0.02%, V- =0.03% and CE- 0.36 to 0.42 suitable for making
angles with improved galvanisability for transmission line-tower application with
yield strength = 350 MPa through BOF - LF - CC.The present invention achieves
weldability of CE = 0.42, tensile properties of Yield strength = 350 MPa, UTS : =
490 MPa, % Elongation : = 22 and improved galvanisability in terms of minimum
thickness ( = 70 micron) as per IS 4759 in TLT member angles made from
blooms of 230 X 160 mm / 350 x 150 mm size produced through BOF-LF-LTS-
CC route.
PRIOR ART AND DRAWBACKS
The general practice with reference to the quality of steel is to specify the use of
steel for tower members, although some authorities have instead specified the
use of steel manufactured by either the open hearth or electric furnace process
for tower members, although some authorities have instead specified the use of
steel manufactured by either the open hearth or electric furnace process. The
usual standards specified are ASTM A-7, BSS 15, and German Steel Standard St
37. IS: 226-1975, Specification for structural Steel (Revised), is currently adopted
in India.
Depending upon the requirements of the transmission system, various line
configurations have to be considered - ranging from single circuit horizontal to
double circuit vertical structures and with single or V strings in all phases, as well
as any combination of these.
Towers are classified according to their use independent of the number of
conductors they support. Angle towers, sometimes called semi-anchor towers,
are used where the line makes a horizontal angle greater than two degrees. As
they must resist a transverse load from the components of the line tension
induced by this angle, in addition to the usual wind, ice and broken conductor
loads, they are necessarily heavier than suspension towers. Unless restricted by
site conditions, or influenced by conductor tensions, angle towers should be
located so that the axis of the cross-arms bisects, the angle formed by the
conductors.
The US 7921005 document discloses methods for selecting a conductor
configuration of an overhead power transmission line tension section between
two dead-end towers when the conductor configuration includes at least two
different types of conductors. Geographical profile information and overhead
power transmission line preferences are used along with predetermined selection
rules when selecting the appropriate conductor configuration.
The CN101935815 document relates to a transmission line fastening piece alloy
coating and a preparation process thereof. The alloy coating is characterized by
comprising the following components in percentage by weight: 0.045 to 3 percent
of aluminium, 0.03 to 3 percent of magnesium, 0.03 to 3 percent of rare earth,
0.01 to 2 percent of copper and the balance of zinc. The preparation process
comprises the following steps of: 1) washing a coating piece with an acid and
water, namely performing acid washing on the coating piece for 5 to 30 minutes
before a hot dip process by adopting 10 to 20 percent hydrochloric acid; and
washing the coating piece with the water after the acid washing; 2) fluxing
treatment, namely performing fluxing on the coating piece obtained from the step
1) in a fluxing agent for 20 to 120 seconds at the temperature of between 50 and
120 DEG C; and 3) performing the hot dip process. The alloy coating can
effectively prolong the anti-corrosion lifetime of a hot dip coating of a fastening
piece in a transmission tower in power industry, make the fastening piece of a
stressed part of the transmission tower have prolonged anti-corrosion lifetime
under complex corrosive environment conditions, ensure that the thickness of a
coating is not increased, and simultaneously improve the anti-corrosion lifetime
for over 3 times compared with hot-dip plated pure zinc in industry.
The CN101552385 document relates to a graphite aluminium-magnesium alloy
grounding material which is obtained through the following steps : placing the
graphite into a mixed solution of hydrochloric acid and nitric acid for immersing
and acid washing twice for changing the inner structure, after rinsing until the pH
value gets to 7.0, dewatering for high-temperature swelling, expanding the
graphite worm to 50-70 times of normal volume; adding magnesium powder,
aluminium powder, manganese powder and zinc powder, mixing to uniform;
executing extrusion forming to the graphite worm under the pressure of 5-8 MPa
for forming a graphite worm bland; and executing high temperature sintering.
According to the invention, the graphite is added with metallic ions of magnesium,
aluminium, manganese and zinc. The graphite aluminium-magnesium alloy
grounding material of the invention has the advantages of stable resistance value
of product, good corrosion resistance; cathodic protection provided for the main
grounding system, effectively reduced grounding resistance and delayed
corrosion efficiency of main grounding network. The graphite aluminium-
magnesium alloy grounding material of the invention is suitable for the grounding
of high soil resistivity areas (mountain area, sand land, etc.) including the tower of
transmission line, meteorological station and ecological monitoring station.
The US5493766 document relates to a process for hot working blooms produced
by continuous casting and steel ingots produced with moulds. The process is
intended for effective use of the latent heat in cast blooms or steel ingots, and in
particular, for preventing surface cracks that occur frequently in cast blooms and
ingots of aluminium killed steel as they are worked by hot rolling.
The CN101186998 document relates to a long-acting anti-corrosion alloy coating
of a transmission line tower and process technology, belonging to the field of the
hot dip of the metallic material of the transmission line tower in electric industry,
and the invention is applicable to the corrosion preservation of the hot dip of the
surface of the transmission line tower. The anti-corrosion alloy coating is
characterized in that the invention comprises the following alloy elements with
certain quality content, including that zinc is 32% to 35%, aluminium is 43% to
65%, magnesium is 0.5% to 8%, rare earth nickel is 0.5% to 3%, and titanium is
0.01% to 0.2%. The technique of preparing the coating is characterized in that
the hot dip means that coating members are placed in the coating liquid
according to the alloy dispensation of the long-acting anti-corrosion alloy coating
of a transmission line tower, the temperature of the coating liquid is 430 DEG C
to 450 DEG C, and the dip period is three to ten minutes. The alloy coating is
simple in hot dip technique. The existing hot dipping device can completely
satisfy the manufacture need, the temperature of the hot dip technique is 20 to 40
DEG C lower than that of the existing zinc plating, the process technique of the
existing hot dipping can be employed in pre-process and post-process, the
brightness of the surface after the dipping is high, the thickness of the alloy layer
is even, the thickness of the coating is more than 86 micrometers, and the hot dip
lasts for 3 to 10 minutes.
Transmission line towers (TLT) are assembled using steel angles of different
sizes which in turn are produced by hot rolling of steel blooms. The TLT member
angles must conform to specifications laid by the tower manufacturer. High
strength TLT members must have yield strength of 350 MPa min. while having
uniform galvenisability. Primarily a suitable alloy design is essential in obtaining
the required combination of strength and galvenisability. Concept of solution
strengthening, grain is refining and precipitation hardening along with
strengthening component on account of dislocation density and texture play
significant role in development of strength in steel which in turn depends on both
alloy design and the selection of hot rolling parameters. On the other hand certain
elements e.g. Si and P influence the galvenisability properties. There is no
reference to process technology for production of Al-killed steel blooms suitable
for making angles with improved galvanisability for transmission line-tower
application with yield strength = 350 MPa.
OBJECT OF INVENTION
The main objective of the invention is to produce innovative alloy chemistry of
steel composition (by weight %) containing: C- 0.16 to 0.20%, Mn-1.20 to
1.40%, S- = 0.035%, P- = 0.035%, Si- 0.15 to 0.25%, Al- =0.02%, V- =0.03% and
CE- 0.36 to 0.42 for production of aluminium killed blooms suitable for making
angles with improved galvanisability for transmission line-tower application.
It is therefore an object of the invention to produce aluminium killed blooms
suitable for making angles with improved galvanisability for transmission line-
tower application with yield strength = 350 MPa.
It is therefore an object of the invention to produce aluminium killed blooms
suitable for making angles with improved galvanisability for transmission line-
tower application with weldability (i.e. CE = 0.42), tensile properties (Yield
strength = 350 MPa, UTS = 490 MPa, % Elongation = 22).
It is further object of the invention to produce aluminium killed blooms suitable for
making angles with improved galvanisability for transmission line-tower
application with improved galvanisability in terms of minimum thickness ( =70
micron) as per IS 4759 in TLT member angles made from blooms of 230 X 160
mm / 350 x 150 mm size produced through BOF-LF-LTS-CC route yield strength
= 350 MPa.
These and other objects of the invention will be clear from the following
paragraphs.
BRIEF DETAILS OF THE INVENTION
In present innovation, specially designed innovative alloy chemistry of steel
composition (by weight %) containing: C- 0.16 to 0.20%, Mn-1.20 to 1.40%, S- =
0.035%, P- = 0.035%, Si- 0.15 to 0.25%, Al- =0.02%, V- =0.03% and CE- 0.36 to
0.42 for production of aluminium killed blooms suitable for making angles with
improved galvanisability for transmission line-tower application has been used.
Transmission line towers (TLT) are used by power grid to transmit electric power
from generating units to far places of customer-end.
These towers are made by assembling the respective angles made of high
strength steel which are produced as per IS 2062 E 350 grade. Weldability (CE =
0.42), tensile properties (Yield strength = 350 MPa, UTS = 490 MPa, %
Elongation = 22), and galvanisability in terms of minimum thickness ( =70 micron)
are the quality requirements to be met by each member angles used in
assembling the TLT. Processing technology developed at DSP to make
aluminium killed blooms of 230 X 160 mm / 350 x 150 mm size suitable for
making angles with improved galvanisability for transmission line-tower
application with yield strength = 350 MPa through CC route include achieving the
specified chemistry through BOF-LF-LTS, casting into bloom-caster blooms
having facilities of shroud and SEN under suitably selected process parameters,
rolling into angles and hot dip galvanizing ensured uniform thickness of
galvanized coating besides achieving the targeted tensile properties including
yield strength = 350 MPa.
Achieving weldability (i.e. CE = 0.42), tensile properties (Yield strength = 350
MPa, UTS = 490 MPa, % Elongation = 22), and improved galvanisability in
terms of minimum thickness ( = 70 micron) as per IS 4759 in TLT member angles
made from blooms of 230 X 160 mm / 350 x 150 mm size produced through
BOF-LF-LTS-CC route was the object of innovation.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1: shows a typical macro of the cast bloom.
Fig. 2: shows typical macro after rolling of angle.
Fig. 3: shows a typical view after galvanizing; numbers on the coated surface
shows the thickness of Zn coating at that location.
Fig. 4: All the tensile properties are in the targeted range primarily because of
suitable alloy design, and the evolving microstructure.
DETAILED DESCRIPTION OF THE INVENTION
Galvanized steel first became an important material for pole manufacturers in the
1950's. As relatively new industry, the pole industry is just now becoming aware
of possible problems that can be associated with corrosive soils, mechanical
damage, storage and stock rotation, ground level and underground / water table
corrosion, coatings requirements and interference induced from other structures.
Specifications from paint and coating suppliers do not always take these issues
into consideration. As a result, the service life of a utility pole can be diminished if
the pole specifier doesn't take into consideration the different environments that
utility poles can be exposed to.
Galvanized steel poles are protected from corrosion attack due to both barrier
effect and also due to galvanic (sacrificial) action of zinc. It provides long term
protection both above ground and underground portion of poles. The key point in
long service life is that the soil in that location provided the protective layer on
galvanized surface.
The production of high quality galvanized steel poles depends on the
metallurgical reaction between steel and molten zinc. The micro-structural
characteristics, grain structure (spangle formation), surface segregation and
corrosion of galvanized coatings depend on both steel and bath compositions,
coating processes and post-coating processing. Factors often associated with
corrosion failure of galvanized steel poles are improper thickness, excessive
brittle inter-metallic alloy layer, galvanizing, substrate surface preparation if
coated, storage conditions, soil service conditions, or unsuitable coating selection
for the soil exposure in service.
The present invention relates to processing technology for production of
aluminium killed blooms of 230 X 160 mm / 350 x 150 mm size suitable for
making angles with improved galvanisability for transmission line-tower
application with yield strength = 350 MPa through BOF-LF-LTS-Continuous
Bloom Caster (having facilities of shroud and SEN) route aimed at selection of
suitable alloy design, heat making and casting parameters.
Innovative alloy design included the consideration of metallurgical requirements
of final galvanized angle on one hand and the process requirements / constraints
of steel making, concast and hot rolling mill on the other. BOF-LF-LTS-CC
defines the broad process route. Process technology has been developed at
DSP, which ensures the quality requirements of galvanized angles as given
below:
YS: 350 MPa min.
UTS: 490 MPa min.
Elongation: 22% min.
CE: 0.42 max.
Zn weight on Coating: 610 gm / M2 min.
Zn coating thickness: 86 micron (av) & 70 micron min.
Following important innovative technological intervention resulted in successful
development of aluminium killed blooms of 230 X 160 mm / 350 x 150 mm size
suitable for making angles with improved galvanisability for transmission line-
tower application with yield strength = 350 MPa:
• Innovative Alloy Design as shown in Table 1 based on consideration of end
property of product and the BOF-LF-LTS-Continuous Bloom Caster (having
facilities of shroud and SEN) route.
• Mn was controlled in conjunction with bath C to restrict CE = 0.42
• Si was restricted in the range 0.15 - 0.25 wt.%.
• Si was controlled in conjunction with P (Si + 2.5 P = 0.25%).
• Mn / S ratio was kept at 30 minimum.
• V was kept at 0.03 wt. % minimum.
• Al was kept at 0.02 wt. % minimum.
In view of weldability requirement entailing CE = 0.42, tensile properties viz. Yield
Strength: 350 MPa min., Ultimate Tensile Strength: 490 MPa min., elongation:
22% min., and galvanisability requirements viz. Zn weight on Coating: 610 gm /
M2 min. and Zn coating thickness: 86 micron (av) & 70 micron min., role of C, Mn
Si and P towards solid solution strengthening, and adverse impairment of
galvanisability through Sandelin Effect were considered. Accordingly, these
elements were adjusted in the range shown in Table 1.
C was controlled at ~ 0.18 wt. % whereas Mn was controlled in conjunction with
bath C to restrict CE = 0.42. In view of Sandelin effect which leads to uneven
coating due to differential distribution of Si on steel surface under certain
concentration range, Si was restricted in the range 0.15 - 0.25 wt.%. Actual aim
value of Si in steel was controlled in conjunction with P (Si + 2.5 P = 0.25%). Mn /
S ratio was kept above 30 for ensuring defect free cast structure in blooms. For
precipitation strengthening, vanadium was kept at 0.03 wt. % minimum. To
ensure complete killing of steel, alloy design envisaged 0.02 wt. % minimum
Aluminium in the steel.
Table 2 shows the salient processing parameters of the developed technology.
All the tensile properties are in the targeted range primarily because of suitable
alloy design, and the evolving microstructure (Fig. 4). Thickness of Zn coating in
the range (74 -125 micron) is well within the specified values indicating good
galvanizability.
While this invention has been described with an emphasis upon preferred
embodiments, it will be obvious to those of ordinary skill in the art that variations
in the preferred methods may be used and that it is intended that the invention
may be practiced otherwise than as specifically described herein. Accordingly,
this invention includes all modifications encompassed within the spirit and scope
of the invention as defined by the claims that follow.
WE CLAIM:
1. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application
having innovative alloy chemistry of steel composition (by weight %) containing:
C- 0.16 to 0.20wt.%, Mn- 1.20 to 1.40wt.%, S- = 0.035wt.%, P- = 0.035wt.%, Si-
0.15 to 0.25wt.%, Al- =0.02wt.%, V- =0.03wt.% and CE- 0.36 to 0.42.
2. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the Mn in conjunction with bath C is restricted to
maintain CE=0.42.
3. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the Si is restricted in the range 0.15-0.25wt.% for
even casting.
4. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the Si in steel was controlled conjunction with P.
5. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the Mn : S ratio is kept above 30 to get defect free
cast structure in blooms.
6. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the V is maintained at 0.03 wt.% for strengthening
precipitation.
7. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the Aluminium is maintained at 0.02 wt.% to ensure
complete killing of steel.
8. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein the specification of the aluminium killed blooms are
of 230 X 160 mm / 350 x 150 mm.
9. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein for achieving the improved galvanizability along with
tensile properties as follows:
YS: 350 MPa min,
UTS: 490 MPa min,
Elongation: 22% min,
CE: 0.42 max,
Zn weight on Coating: 610 gm / M2 min., and
Zn coating thickness: 86 micron (av.) & 70 micron min.
10. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application as
claimed in claim 1, wherein process enroutes through BOF-LF-LTS-Continuous
Bloom Caster.
11. A process technology for production of aluminium killed blooms for making
angles with improved galvanisability for transmission line-tower application
substantially as herein described.

ABSTRACT

The present invention relates to process technology for production of aluminum
killed blooms with innovative alloy chemistry of steel composition (by weight %)containing: C- 0.16 to 0.20%, Mn- 1.20 to 1.40%, S- ≤ 0.035%, P- ≤ 0.035%, Si-0.15 to 0.25%, Al- ≥0.02%, V- ≥0.03% and CE- 0.36 to 0.42 suitable for making
angles with improved galvanisability for transmission line-tower application with
yield strength ≥ 350 MPa through BOF - LF - CC. The present invention
achieves weldability of CE ≤ 0.42, tensile properties of Yield strength ≥ 350 MPa,
UTS ≥ 490 MPa, % Elongation ≥ 22 and improved galvanisability in terms of
minimum thickness ( ≥ 70 micron) as per IS 4759 in TLT member angles made
from blooms of 230 X 160 mm / 350 x 150 mm size produced through BOF-LF-
LTS-CC route.