FIELD OF THE INVENTION
The present invention relates to a process of manufacturing steel strap involving
normalization heat treatment. More particularly, the present invention is directed to
developing strap steel usually of 1.27 mm thickness with a good combination of improved
strength and ductility achieved by development of a suitable microstructure through
normalization heat treatment route, instead of the conventional austempering process.
Normalization heat treatment involves austenitization followed by simply air cooling. The
strap steel produced according to the process of the present invention, favours obtaining a
microstructure consisting of mostly bainite having non-lamellar carbide distribution with a
small amount of ferrite and pearlite that resulted in the maximum strength-ductility
combination. The desired properties and the microstructure in the resulting steel strap could
be achieved by following producing said steel in BOF and subsequent processing in slab
casting, soaking, hot rolling as well as cold rolling and then slitting, with controlled process
parameters at each and every stage of said process. The steel strap having the desired
strength and ductility has prospects of wide industrial application in rail and road
transportation of coils as well as other heavy structural items in transit.
BACKGROUND ART
It is known in the art of steel Strap production and use, that the same is a vital packing
material for binding the fully processed coils/packets of sheets after they are covered with
HDPE cloth and galvanized sheet. Moreover, a number of such wrapped coils are tied
together with the help of 1.27 mm strap on the carriage platform of Railway wagons for the
supply of the coils to the customers. Therefore, the strap has to withstand high strength
during its use. It should also be reasonably ductile so that it is easily bent without failure
during application. Thus, a good combination of strength and ductility is the essential
requirement for the strap. It is also a common experience in the related art, that the
increase in strength of steel strap is achieved with simultaneous loss of ductility (%
elongation). Conventionally, the strap steel of 1.27mm thickness is produced using
austempering heat treatment process, starting with a plain C - Mn (nominal chemistry- C:
0.40%, Mn: 1.35%, and Si: 0.22%) steel and processing through BOF-SRU-Casting-HR-
Soaking-CR-HT route. In the conventional austempering process, steel strap is passed
through a lead bath after austenitization.

In our co-pending patent application no. 708/CAL/2001 dated 24-12-2001, there s
disclosed a process for producing steel strap involving austempering after casting and rolling
to achieve improved strength properties. Also, in our co-pending application no.
272/KOL/2007 dated 23-02-2007, a steel strap having 100% bainite microstructure with
still higher strength associated with moderately favorable elongation properties have been
claimed but through a relatively cost expensive process and also an environmentally
hazardous, because of using a Lead - bath treatment. However, there has been a
continuous need in the art to develop a quality of strap steel adopted for packaging of steel
sheet coils for transportation, achieving suitable microstructure and good strength-ductility
combination properties in resulting strap in a simple productive, cost effective and
environment friendly process, showing increased strength without loss of ductility (%
elongation) and avoiding hazardous Lead - bath treatment. The present invention is tnus
specifically directed to a process for developing the C-Mn steel strap with desired strength-
ductility properties achieved through selective microstructure of the grade of steel obtained
through selective use of preferred range of process parameters and specifically a
normalization heat treatment of strap by simple air cooling after austenittzation, eliminating
the need for conventional lead bath treatment and thus achieving economy and
environment benefiting process.
There has been therefore a continuous need to develop a process for manufacturing steel
strap for application in packaging and transportation of sheet coils, that would on one hand
have sufficient strength to wrap very heavy coils and on the other hand be ductile enough
such that during wrapping and fastening of heavy sheet coil packs, the steel does not buckie
or yield due to bending through sharp angles or subjected to sudden jerk loads during
transit. The microstructure and chemical composition of such steel strap as end product
would be capable to ensure said combination of superior strength and ductility properties for
suitability and reliability of service of said steel strap obtained through conventional
austenitizing rout, in packaging and allied applications. The present invention is an attempt
specifically directed to a process for developing the C-Mn strap steel with desired strength
ductility properties achieved through developing selective microstructure of the grade of
steel obtained through use of preferred range of process parameters and specifically a
normalization heat treatment of strap by simple air cooling after austenitization, eliminating
the need for conventional lead bath treatment and thus achieving cost effective and
environment benefiting process.

OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing of a steel strap
variety having a combination of desired strength and ductility properties adopted for
application in packaging of heavy large diameter steel sheet coils during rail/road
transportation and a process for its manufacture following a simple normalization heat
treatment process by air cooling after austenitization.
Another object of the present invention is directed to developing said process for
manufacturing steel straps with good combination of strength and improved elongation
properties which would not fail even if bent through acute/sharp angles during packaging
/tying for transportation of sheet coils.
A still further object of the present invention is directed to developing a process for
manufacturing steel strap having improved strength-ductility combination wherein said
strap obtained following a process of heat treatment involving austenitizing followed by air
cooling and control on other process parameters in different steps.
A still further object of the present invention is directed to developing a process for
manufacturing steel strap having desired strength-ductility combination wherein said
process ensures obtaining a fine grained microstructure comprising mostly bainite witn a
small amount of ferrite and pearlite contributing the much desired maximum strength-
ductility combination in resulting strap.
A still further object of the present invention is directed to developing a process for
manufacturing steel strap having desired strength-ductility combination wherein said
process ensures achieving a good strength-ductility combination of UTS: 900 MPa min. and
% El. 8 min. (150 mm GL) in the strap after normalization heat treatment.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a process for production of steel
strap through normalization heat treatment comprising
(i) Preparing the steel in basic oxygen furnace (BOF) having the composition (wt. %j

C in the range of 0.40 to 0.45 and preferably 0.43;
Mn in the range of 1.60 to 1.65 and preferably 1.63;
P in the range of 0.025 max.;
S in the range of 0.015 max.;
Si in the range of 0.50 to 0.55 and preferably 0.52; and
Al in the range of 0.025 to 0.040 and preferablyO.030.
ii) Subjecting the slabs to continuous casting;
iv) hot rolling and cold rolling to obtain cold rolled coils which are slit into straps of
desired width; and finally,
v) Subjecting to normalization heat treatment comprising austenitization followed by air-
cooling.
Another aspect of the present invention is directed to a process for production of steel strap
comprising,
i) Preparing the steel in basic oxygen furnace (BOF) having said composition ;
ii) Processing for sulfur control before continuous casting;
iii) Subjecting the slabs to continuous casting maintaining a superheat of up to
30°C max;
iv) Soaking and hot rolling to 2.75 to 2.85 mm preferably 2.80 mm strips, and
maintaining the finishing and coiling temperatures of 870 ± 10°C and 670 ± 10°C
respectively. Further processing and cold rolling to obtain cold rolled coils which are
slit into straps of suitable width;

v) Heating the straps thus obtained in a furnace maintained at 890°C-935°C to get ic
austenitized;
vi) subjecting the slit and austenitized straps to normalization heat treatment by passing
the austenitised straps through ambient air and then through a cooling tower to be
air cooled to a temperature of 25 to 30 and preferably 25°C, to thereby achieve
selective bainitic transformation in combination with small amounts of ferrite and
pearlite, directed to favour desired strength and ductility combination properties of
said strap steel.
A still further aspect of the present invention is directed to a process for production of steel
strap, wherein normalization heat treatment is carried out by simple air cooling and thus
making the process simple, productive and economic.
According to another aspect of the present invention, it is directed to a process for
production of steel strap, wherein the process is environment friendly as it is free of use of
lead bath.
A still further aspect of the present invention is directed to a steel strap with high strength-
ductility combination comprising, C:0.40-0.45% wt.; Mn: 1.60-1.65% wt.; P: 0.025
(max.)%wt; S: 0.015 (max.)% wt.; Si: 0.50-0.55% wt. and Al: 0.025-0.040 % wt and
having mostly bainite and wherein the carbide in the bainite is non-lamellar.
According to an important aspect of the present invention, it is directed to a steel strap with
high strength-ductility combination having UTS: 900 MPa min. and % El. 8 min (150 mm
GL).
said steel strap with high strength-ductility combination, wherein said strap steel is of size
preferably 1.27mm thick and about 3.75mm wide.
According to yet another aspect of the present invention, it is directed to a steel strap with
high strength-ductility combination, wherein the micro structure in said strap steel after
normalization comprises fine bainite with non lamellar distribution of carbide, in combination
with small amounts of fine pearlite and ferrite contributing to the desired high strength-
ductility combination.

The present invention and its objects and advantages are described in greater details with
reference to the accompanying non-limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the flow diagram for the conventional process route for manufacturing steel
strap.
Figure 2: is the illustration of the flow chart for the process route for making steel strap
having desired combination of strength and ductility involving normalization heat treatment
after austenitization step, according to the present invention.
Figure 3: is the schematic diagram of the normalization heat treatment process for steel
strap according to the present invention, providing a good combination of strength and
ductility and desired microstructure of the steel to favor such properties.
Figure 4(a) and 4(b): are the illustrations of the Optical and SEM micrographs,
respectively of the high strength-ductility steel strap product of the present invention,
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention is directed to developing of strap steel of size 1.27 mm thick and
31.75mm wide, with a good strength-ductility combination achieved through processing of
plain C, Mn steel having nominal chemistry ( C - 0.43 %, Mn - 1.63 %, P - 0.02 %, S -
0.012 %, Si - 0.52 %, and Al - 0.03 %), to obtain suitable microstructure in the resulting
steel strap through normalization heat treatment route, with selective control on process
parameters at different steps, instead of the conventional austempering process.
Normalization heat treatment involves austenitization followed by air-cooling while
conventional austempering involves austenitization followed by lead bath treatment.
Reference is invited to the accompanying Figure 1, which illustrates the flow chart for the
conventional austempering process wherein the strap is passed through a lead bath after
austenitization.
Laboratory experimentations were carried out to study the influence of C, Mn and Si on the
hardenability of the steel. A wide variety of microstructures were developed by suitably
altering the process parameters including heat making in the Induction Furnace, ingot

casting, the soaking temperature and holding time, finish rolling and coiling temperatures in
hot rolling, cold rolling and normalization heat treatment. The microstructure consisting of
mostly bainite with a small amount of ferrite and pearlite resulted in the maximum
strength-ductility combination. Same microstructure has also been achieved during scaling
up the process to commercial production of this product after normalization i. e.
austenitization of the strap followed by simply air cooling.
Reference is now invited to the accompanying Figure 2 that illustrates the flow chart
corresponding to the process steps of the present invention directed to the production of
steel straps involving normalization heat treatment after austenitization. It is apparent from
the said process flow chart that in the first step the plain C-Mn steel is produced in Basic
Oxygen Furnace (BOF) ensuring nominal chemistry, with the feed of BF hot metal, iron
scrap and ferroalloys comprising High Carbon Ferro-Manganese, Ferro-Silicon etc., in
desired proportion. It is then processed through Secondary Refining Unit (SRU) for sulphur
control before being cast through a continuous casting machine. The slabs are cast
maintaining a super heat of 30°C max. and a casting speed of 0.90 to 1.1 meters / minute
and preferably about 1 meter/minute. The slabs are then soaked at 1240 °C to 1260 °C and
preferably at 1250 °C. The slabs are then hot rolled to 2.8mm strips maintaining the
finishing and coiling temperatures in the range of 870 ± 10°C and 670 ± 10 °C,
respectively. Hot rolled strips are then pickled in HCL, and then cold rolled to 1.27 mm thick
coils. Cold rolled coils are slit into straps of desired width of 31.5 to 32.0 mm and preferably
about 31.75 mm. The steel straps so obtained are subsequently subjected to normalization
heat treatment.
Reference is now invited to the accompanying Figure 3 that illustrates the schematic
diagram of the normalization heat treatment process for steel strap according to the present
invention providing a good combination of strength and ductility and desired microstructure
of the steel to favor such properties. The strap is passed with the help of an uncoiler
through an austenitizing furnace maintained at 890°C-935°C where it gets austenitized.
Subsequently, it passes horizontally in air at room temperature over a stretch of about 6
meters and then passes through a cooling tower for air cooling and finally coiled in the coiler
unit.
Thus the present process for the manufacture of carbon-manganese strap steel with high
strength-ductility combination, involving normalization heat treatment after austenitization
comprises the following:

i) preparing the steel in basic oxygen furnace (BOF) having the ladle composition (wt.
%) after the ladle furnace treatment
C in the range of 0.40 to 0.45 and preferably 0.43;
Mn in the range of 1.60 to 1.65 and preferably 1.63;
P in the range of 0.025;
S in the range of 0.015;
Si in the range of 0.50 to 0.55 and preferably 0.52; and
Al in the range of 0.025 to 0.040, and preferably 0.030
ii) processing in the ladle furnace for sulfur control and final composition adjustment
through ferroalloy additions, before continuous casting;
iii) subjecting the slabs to continuous casting maintaining a superheat of up to 30 °C
max.;
iv) soaking and hot rolling the strips maintaining finishing and coiling temperatures of
870 ± 10 °C and 670 ± 10 °C respectively. Further processing and cold rolling to
obtain cold rolled coils which are slit into straps of suitable width;
v) austenitizing the straps thus obtained in a furnace maintained at 890 °C-935 °C
followed by air cooling in a horizontal condition up to 6 meters and then through a
cooling tower to a temperature of 20 to 30 °C, preferably 25 °C to achieve selective
bainitic transformation in combination with small amounts of ferrite and pearlite,
directed to favour desired strength and ductility combination properties of said strap
steel.
The chemical composition (Wt %) of the end product after normalization heat treatment,
obtained following the process of the invention is as given in the following Table 1:

Table 1: Al














C Mn P S Si

0.40-0.45 1 .60-1.65 0 .025 (max.) 0 .015 (max.) 0. 50-0. 55 0. 025-0 .040
The optical and SEM micrographs of the resulting strap product obtained foliowing the heat
treatment are shown in Figures 4(a) and 4(b) respectively. It consists of mostly bainite.
The carbide in the bainite is non-lamellar which is the characteristic of bainitic
transformation. It has also got a small amount: of ferrite and pearlite. Such microstructure is
responsible for achieving a good strength-ductility combination of UTS: 900 MPa min. and
% El. 8 min. (150 mm GL) in the strap according to the present invention following the
normalization by simple air cooling after austenitization, as compared to that of UTS: 800-
950 MPa and % El. 8-10 (150 mm GL) with the conventional chemistry and austempering in
the Lead bath.
It is thus possible by way of the present invention to develop a process for production of
steel strap of size, preferably 1.27mm thick and about 3.75mm wide, involving
normalization heat treatment after austenitization by simple air cooling and selective control
on finish roll and coiling temperatures in hot rolling as well as control on other process
parameters so as to achieve desired microstructure in the resulting heat treated strap steel
comprising bainite with non-lamellar carbide distribution and also containing ferrite and
pearlite in small amounts and thus , favoring good combination of strength of UTS:900 MPa
min. and ductility (percent elongation) in the order of 10-11%, conforming to IRS: P-41
specification. The present invention is thus directed to a simple, productive, economic and
environment friendly process for production of steel strap having excellent strength and
ductility combination capable of wide industrial application in transport and steel industry for
loading and packaging of heavy coils and other steel sections in rail/road carriage and
wagon.

We Claim:
1. A process for production of steel strap through normalization heat treatment
comprising of the following;
i) preparing the steel in basic oxygen furnace (BOF) having the ladle composition
(wt. %) after ladle furnace treatment;
C in the range of 0.40 to 0.45 and preferably 0.43;
Mn in the range of 1.60 to 1.65 and preferably 1.63;
P in the range of 0.025 max.;
S in the range of 0.015 max.;
Si in the range of 0.50 to 0.55 and preferably 0.52; and
Al in the range of 0.025 to 0.40 and preferably 0.030
ii) subjecting the slabs to continuous casting;
iii) hot rolling and cold rolling to obtain cold rolled coils which are slit into straps of
desired width; and finally,
iv) subjecting to normalization heat treatment comprising austenitization followed by
air cooling.
2 A process for production of steel strap as claimed in claim 1 comprising
i) preparing the steel in basic oxygen furnace (BOF) having said composition;
ii) processing for sulfur control before continuous casting;
iii) subjecting the slabs to continuous casting maintaining a superheat of up to
30°C max;
iv) soaking and hot rolling the slabs maintaining finishing and coiling
temperatures of 870 ± 10 °C and 670 ± 10 °C respectively, further
processing and cold rolling to obtain cold rolled coils which are slit into straps
of suitable width;
v) Heating the straps thus obtained in a furnace maintained at 890°C -935°C to
get it austenitized;
vi) subjecting the straps to normalization heat treatment by passing the
austenitised straps through ambient air and then through a cooling tower to

be air cooled to a temperature of 25 °C to 30 °C and preferably 25 °C , to
thereby achieve selective bainitic transformation in combination with small
amounts of ferrite and pearlite, directed to favour desired strength and
ductility combination properties of said strap steel.
3. A process for production of steel strap as claimed in anyone of claims 1 or 2, wherein
normalization heat treatment is carried out by simple air cooling and thus making the
process simple, productive and economic.
4. A process for production of steel strap as claimed in anyone of claims 1 to 3, wherein
the process is free of use of lead bath and is environment friendly.
5. A steel strap with high strength-ductility combination comprising, C:0.40-0.45% wt.;
Mn: 1.60-1.65% wt.; P: 0.025 (max.)%wt.; S: 0.015 (max.)% wt.; Si: 0.50-0.55%
wt. and Al: 0.025-0.040 % wt and having mostly bainite with a small amount of
ferrite and pearlite and wherein the carbide in the bainite is non-iamellar.
6. A steel strap with high strength-ductility combination as claimed in claim 5 having
UTS: 900 MPa min. and % El. 8 min (150 mm GL).
7. A steel strap with high strength-ductility combination as claimed in claims 5 or 6,
wherein said strap steel is of size preferably 1.27mm thick and about 31.75mm wide.
8. A steel strap with high strength-ductility combination as claimed in claims 5 to 7,
wherein the micro structure in said strap steel after normalization comprises fine
bainite with non lamellar distribution of carbide, in combination with small amounts
of fine pearlite and ferrite contributing to the desired high strength-ductilky
combination.
9. A steel strap with high strength-ductility combination and a process for its
manufacture involving normalization heat treatment by simple air cooling after
austenitization, as herein described with reference to the accompanying non limiting
drawings.

The present invention relates to a process of manufacturing steel strap through
normalization heat treatment. More particularly, the present invention is directed to
developing steel strap usually of 1.27 mm thickness with a good strength-ductility
combination property achieved through normalization heat treatment route that involves
austenitization followed by simply air cooling. The strap steel having microstructure
consisting of mostly bainite with non-lamellar carbide distribution and a small amount of
ferrite and pearlite. The higher strength-ductility combination comprises UTS:900 MPa mm.
and ductility (percent elongation) of 8 % min. (150 mm GL), conforming to IRS: P-41
specification. The desired properties and the microstructure in the resulting steel strap
developed from steel produced in BOF and subsequent processing with controlled process
parameters in slab casting, soaking, hot rolling, cold rolling, slitting and normalization. The steel strap having superior strength-ductility combination is capable of wide industrial application in rail and road transportation of coils as well as other heavy structural items in transit.