FIELD OF THE INVENTION:
The present invention relates to production of Carbon-Manganese strap steel with high strength
and ductility. In particular, the invention is directed to an improved quality strap steel through
development of suitable microstructure. Advantageously, the invention is directed to achieving
much desired maximum strength-ductility combination, which would further enable reducing the
width of the strap without compromising on strength. This would further favour saving of
material and cost of strap steel. In particular the invention would favour obtaining a grade of
steel in the form of narrow strap, usually 1.27 mm thick and 31.75 mm wide having high
strength (UTS) in the range from 1040 MPa to 1100 MPa coupled with a ductility value (%
elongation) in the range of 8-10%(150mm gauge length), such as to withstand the nature of
load they are expected to experience during transportation being used as a packing material for
steel coils or bundle of sheets.
BACKGROUND ART:
Strap steel of 1.27 mm thickness is produced in some steel plants for packaging of steel coils
and sheets. Such steel straps are produced for use as a packing material for binding the fully
processed coils and packets of sheets after they are covered with HDPE cloth and galvanized
sheet. Additionally, a number of coils are utilized together with the help of 1.27mm strap in the
railway wagons for the supply of the coils to the customers at distant locations from the steel
plants. Hence, such straps need to have high strength as binding/packing of material for steel
coils/sheets of heavy weight, to withstand the load including sudden jerks, during transportation
and also should be reasonably ductile so that it is easily bent without failure during its
use/application as packing material.
US 6814817 states about a heat treated steel strap usable in a strapping machine, having
a tensile strength of at least about 170 KSI and an elongation of at least about 6.5%. the
steel strap is fabricated from a coiled steel reduced by cold rolling, the steel strap
composition comprising 0.3-0.36% Carbon, 0.90-1.25% Manganese, and 0.75-1.10%
Silicon; the strap being heated to a temperature of about 850°C to about 900°C; and
quenched to a temperature of about 370°C to about 510°C, the strap having a seal joint
break strength of about 4350 pounds when the strap has a width of about one inch and a
thickness of 0.03 inch.Preferably, the strap is preheated to a temperature of about 370°C to
about 540°C prior to heating, heated and subsequently quenched. This prior art further
states that that the addition of silicon in an amount of about 0.75 to about 1.10 percent by
weight of the material tends to reduce the embrittlement that the material would otherwise
exhibit. Thus this US prior art makes use of selective weight percent of Silicon to reduce
embrittlement and have attained a ductility of 6.5%.
In our co-pending patent application no. 708 CAL/2001 dated 24-12-2001, there is disclosed a
process for producing steel strap involving austempering after casting and rolling to achieve
improved strength properties. However, there has been a continuous need in the art to provide
for possible further improvement in quality of strap steel by achieving suitable microstructure,
which would favour increase in strength without loss of ductility (% elongation). The present
invention is thus specifically directed to providing the C-Mn steel strap with enhanced strength-
ductility properties achieved through selective microstructure of the grade of steel obtained
through selective use of changed process parameters.

OBJECTS OF THE INVENTION:
Thus the basic object of the invention is directed to providing the C-Mn steel strap with
enhanced properties achieved through selective microstructure of the grade of steel which
would on one hand achieve increased strength characteristics and on the other hand also
favours maintaining desired ductility (% elongation).
Another object of the present invention is directed to obtaining a grade of carbon-
manganese steel strap having high strength and reasonable ductility/percent elongation
such as to ensure the use of this steel strap as reliable packaging materials without failure
for steel coils and bundles of steel sheets for transportation to distant places from the
manufacturing plants, by rail/road transport.
A further object of the present invention is to obtain said high strength and ductility in steel
strap for packaging, usually in the size of 1.27mm thick and 31.75mm wide.
A further object of the present invention is to obtain said high strength and ductility in steel
strap for packaging wherein the improved strength and ductility properties are achieved
through development of selective all bainite microstructure with non-lamellar carbides in
said grade of steel by controlled parameters for selective alloy chemistry in the furnace, slab
casting and austempering processes.
A further object of the present invention is to obtain said high strength and ductility in steel
strap for packaging wherein provision for further reduction in the width of the steel strap
having higher strength (UTS) in the range of UTS: 1040-1100 MPa and ductility in the range
of 8 to 10% (150 mm gauge length).
SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided a Carbon-
manganese strap steel with high strength-ductility combination comprising:
a hundred % bainite microstructure with non-lamellar carbides providing for a high
level strength UTS 1040-1100 MPa and ductility in the range of 8-10%.

A-further aspect of the present invention directed to said Carbon-manganese strap steel
with high strength-ductility combination, comprising the weight % of:
C in the range of 0.40 to 0.45 ;
Mn in the range of 1.6 to 1.65;
P in the range of upto 0.025 (max.);
S in the range of upto 0.015 (max.);
Si in the range of 0.45 to 0.50; and
AI in the range of 0.025 to 0.040.
A still further aspect of the present invention directed to said Carbon-manganese strap
steel with high strength-ductility combination wherein the steel strap is of 1.27mm
thickness.
A still further aspect of the present invention directed to said Carbon-manganese strap
steel with high strength-ductility combination wherein the optical and SEM micrographs
are as illustrated in Figures 2a and 2b respectively.
Another important aspect of the present invention directed to said process for the
manufacture of carbon-manganese strap steel with high strength-ductility combination
comprising:
(i) preparing the steel in basic oxygen furnace (BOF) having the composition(wt. %)
C in the range of 0.40 to 0.45 ;
Mn in the range of 1.6 to 1.65;
P in the range of upto 0.025 (max.);

S in the range of upto 0.015 (max.);
Si in the range of 0.45 to 0.50; and
Al in the range of 0.025 to 0.040.
ii) processing for sulfur control before continuous casting;
iii) subjecting the slabs to continuous casting maintaining a superheat of upto
30°C max;
iv) soaking and hot rolling the trips maintaining finishing and coiling temperatures of
880 ± 10°C and 690 ± 10°C respectively, further processing and cold rolling to obtain
cold rolled coils which are slit into straps of suitable width;
v) austempering the straps thus obtained in a furnace maintained at 930°C-950°C to
get it austenitised;
passing the austenitised straps through lead bath maintained at 430 to 460°C to
thereby achieve selective bainitic transformation for desired strength and ductility
combination.
A still further aspect of the present invention directed to said process for the
manufacture of carbon-manganese strap stee%with high strength-ductility wherein
the casting speed is about 0.9 to 1.1 meter/min and preferably 1 meter/min;
slabs are soaked at a temperature range of 1240°C to 1260°C preferably at 1250°C
and hot rolled to 2.75 mm to 2.85 mm, preferably 2.8mm strips.
A further aspect of the present invention directed to said process for the manufacture of
carbon-manganese strap steel with high strength-ductility combination wherein the hot
rolled strips were pickled in HCL before cold rolling to coils of 1.25 to 1.3 mm preferably
1.27 mm thickness and the cold rolled coils are slit into straps of 31.5 mm to 32 mm
width straps preferably 31.75 mm width straps and subsequently austempered.

According to yet another aspect of the present invention directed to said process for the
manufacture of carbon-manganese strap steel with high strength-ductility combination
as claimed in anyone of claims 5 to 7 wherein the lead bath is maintained at a distance
of 1.1 to 1.3 meter, preferably 1 meter and after completion of bainitic transformation in
lead bath , the strap are passed through cooling tower and finally coiled.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:
Figure 1: is the illustration of the schematic diagram of austempering unit showing the
movement of cold rolled slitted strip through different sections of the unit during processing.
Figure 2(a): is the optical micrograph showing bainitic microstructure according to the
present invention ;
Figure 2(b): is the SEM micrograph showing non-lamellar carbide in bainite, according to
the present invention;
Figure 3 : is the illustration of the flow chart for the process used for production of C-Mn
strapping steel with high strength-ductility combination, according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING
FIGURES:
To obtain the combination of improved high strength and ductility properties in the
particular grade of steel used for producing the strap for packaging according to the present
invention, a number of laboratory trials were conducted to study, observe and record the
influence of alloying elements like C, Mn and Si on the hardenability of the resulting steel.
Different corresponding microstrutures of steel grades were obtained. The 100% bainitic
micro-structure with non-lamellar carbides resulted in the maximum strength-ductility
combination. Such microstructure was achieved during carrying out experimental trial
production of this product after austempering stage.
According to the present invention the alloy chemistry on Wt. % basis of the steel obtained
after melting in the basic oxygen furnaceand subsequent treatment in secondary refining
unit(SRU) for Sulphur control, as starting material was as follows:

C in the range of 0.40 to 0.45, Mn in the range of 1.6 to 1.65, P in the range of 0.025(max),
S in the range of 0.015(max), Si in the range of 0.45 to 0.50, Al in the range of 0.025 to
0.040.
This grade of steel was further processed through continuous casting. The slabs were cast
maintaining the superheat of 30°C max. and at a casting speed of about 1m/min. The slabs
were soaked at 1250°C and hot rolled to 2.8mm strips, maintaining finishing and coiling
temperature 880±10°C and 690±10°C respectively. Hot rolled strips were pickled in
Hydrochloric acid (HCL) media before cold rolling to further reduction in thickness to
1.27mm thick sheet wound in coils. Cold rolled coils were slit in 31.75mm width straps and
subsequently austempered.
Attention is first invited to Figure 1 that shows the schematic diagram of the austempering
unit, according to the present invention. The steel strap is passed with the help of an
uncoiler through a austenitising furnace maintained at 930-950°C at its different zones
where it gets austenitised over duration of about 2 min. Subsequently, it passes through a
lead bath at 1.2 meter distance and the temperature of said bath is maintained at 430-
460°C. After completion of bainitic transformation while passed through the lead bath, with
residence time for about 1/2 min, the strap passes through the cooling tower and finally
coiled as finished product.
The optical and SEM micrograph of the corresponding processed grade of strap material are
shown in the accompanying figures 2(a) and 2(b) respectively. The micrograph of the
steel strap according to the present invention shows that a 100% bainitic structure and the
carbide in the bainite is non-lamellar which is the characteristic of bainitic
transformation.The resulting steel garde was further tested for tensile strength and percent
elongation carried out in the shop itself using WNC Tensile Testing Machine. The test results
achieved are UTS: 1040-1100 MPa and %
elongation 8-10 (150 mm gauge length) which are substantially higher than
those specified in IRS: M-41 specification. Such microstructure is responsible for a high
level of strength such as UTS in the range of 1040 MPa to 1100 MPa, without any sacrifice in
the ductility, which is maintained in the range of 8-10% to favour the use of the steel strap
for bending during packaging without failure. The flow chart for the entire process and its
sequences to produce the C-Mn steel strap having high strength-ductility combination
properties, according to the present invention is illustrated in the accompanying Figure 3.
Because of very high strength of the product, the width of the strap are capable to be
reduced further from the existing 31.75mm in order to effect material saving.

We Claim:
1. Carbon-manganese strap steel with high strength -ductility combination comprising:
C in the range of 0.40 to 0.45 ;
Mn in the range of 1.6 to 1.65;
P in the range of upto 0.025 (max.);
S in the range of upto 0.015 (max.);
Si in the range of 0.45 to 0.50; and
Al in the range of 0.025 to 0.040.
having a hundred % bainite microstructure with non-lamellar carbides providing for a
high level strength (UTS) 1040-1100 MPa and ductility in the range of 8-10%.
2. Carbon-manganese strap steel with high strength -ductility combination as claimed
in claim 1 wherein the steel strap is of 1.27mm thickness.
3. Carbon-manganese strap steel with high strength -ductility combination as claimed
in anyone of claims 1 or 2wherein the optical and SEM micrographs are as illustrated
in Figures 2a and 2b respectively.
4. A process for the manufacture of carbon-manganese strap steel with high strength-
ductility combination as claimed in anyone of claims 1 to 3 comprising:
(i) preparing the steel in basic oxygen furnace (BOF) having the ladle chemistry (wt.
%) after SRU treatment:
C in the range of 0.40 to 0.45 ;

Mn in the range of 1.6 to 1.65;
P in the range of upto 0.025 (max.);
S in the range of upto 0.015 (max.);
Si in the range of 0.45 to 0.50; and
Al in the range of 0.025 to 0.040.
ii) processing for sulfur control in SRU before continuous casting;
iii) subjecting the slabs to continuous casting maintaining a superheat of upto
30°C max;
iv) soaking and hot rolling the strips maintaining finishing and coiling
temperatures of 880 ± 10°C and 690 ± 10°C respectively, further
processing and cold rolling to obtain cold rolled coils which are slit into
straps of suitable width;
v) austempering the straps thus obtained in a furnace maintained at 930-
950°C to get it austenitised;
passing the austenitised straps through a lead bath maintained at 430 to 460°C to
thereby achieve selective bainitic transformation for desired strength and ductility
combination.

5. A process for the manufacture of carbon-manganese strap steel with high strength-
ductility combination as claimed in claim 4 wherein
the casting speed is about 0.9 to 1,1 meter/min preferably 1 meter/min;
slabs are soaked at a temperature range of 1240°C to 1260°C preferably at 1250°C
and hot rolled to 2.75 mm to 2.85 mm preferably 2.8mm strips.
6. A process for the manufacture of carbon-manganese strap steel with high strength-
ductility combination as claimed in anyone of claims 4 or 5 wherein the hot rolled strips
were pickled in HCL before cold rolling to coils of 1.25 mm to 1.30 mm preferably 1.27
mm thickness and the cold rolled coils are slit into straps of 31.5 mm to 32 mm width
straps preferably 31.75 mm width straps and subsequently austempered.
7. A process for the manufacture of carbon-manganese strap steel with high strength-
ductility combination as claimed in anyone of claims 4 to 6 wherein the lead bath is
maintained at a distance of 1.1 meter to 1.3 meter preferably 1.2 meter and after
completion of bainitic transformation in lead bath , the straps are passed through cooling
tower and finally coiled.

ABSTRACT

TITLE: CARBON-MANGANESE STRAP STEEL WITH HIGH STRENGTH -DUCTILITY
COMBINATION.
Carbon-Manganese strap steel with high strength and ductility is for intended use in
packaging of coils and sheet piles. A selective alloy chemistry of the steel in BOF after
treatment for Sulphur control before continuous casting into slab with specified degree of
superheat, help achieve desired properties after hot and cold rolling with controlled
parameters and subjected to a austempering process at subsequent stage. The steel strap
obtained having microstructure showing 100% bainite with non-lamellar carbide in it,
demonstrating high strength-ductility properties is suitable for the application for packaging.
The grade of steel obtained in the form of narrow strap, usually 1.27 mm thick and 31.75
mm wide having high strength (UTS) in the range from 1040 MPa to 1100 MPa coupled with
a ductility value (% elongation) in the range of 8-10% is capable to withstand the load
during transportation. The present invention is directed to producing a grade of steel strap
having far superior strength without any loss of ductility.
Figure 2 a & b.