FIELD OF THE INVENTION
This invention relates to thin ultra low carbon steel sheets with high r bar value
and process for the production.
This invention further relates to thin, ultra low carbon steel sheets with high
average r bar value, which will help to reduce the weight of the product and
improve formability for packaging applications. The sheets are produced by
different combinations of cold rolling and annealing techniques within the
specified composition range, to have a thickness in the range of 180 to 300 urn
and a high average r bar value.
BACKGROUND OF THE INVENTION
Low carbon steel sheets are largely used for the production of Tin plate Tin plate
is the material used more widely in cans for preserving foods and for other
sophisticated applications. Conventionally, the term Tin plate refers to low
carbon mild steel sheet with a thin layer of tin coating on each surface of the
material. However, currently no suitable method is known, which leads to very
thin steel sheets with high r bar value.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 Flow chart of the process according to the invention
Fig. 2 Φ2 = 45° sections of the ODFs (Bunge) of (a) hot rolled (b)
primary cold roiled (C) continuous annealed (primary annealed) (d)
batch annealed (secondary annealed) steel.

OBJECTS OF THE INVENTIOM
It is therefore an object of this invention to propose thin ultra low carbon steel
sheets with high r bar value.
It is a further object of this invention to propose thin ultra low carbon steel
sheets with high r bar value, which are easy to produce.
Another object of this invention is to propose thin ultra low carbon steel sheets
with high r bar value which is cost - effective.
Yet another object of this invention is to propose thin ultra low carbon steel
sheets with high r bar value which are light in weight and have wide applications
in the packaging industry.
These and other objects and advantages of this invention will be apparent from
the ensuing description.
BRIEF DESCRIPTION OF THE INVENTION
According to this invention there is provided thin ultra low carbon steel sheets
with high r bar value, and a process for the production thereof.
In accordance with this invention, the steel selected for carrying out the process
has a composition (in wt%) of:


The steel is subjected to controlled hot rolling. The amount of hot rolling applied
is in the range of 50-90%. The Finish Rolling Temperature (FRT) is kept within
875°C - 975°C. The coiling temperature of the hot rolled material is in the range
650°C-775°C. The hot rolled plate is subjected to primary cold rolling (PCR)
followed by continuous annealing (CA) for 2 to 5000 seconds in the temperature
range 775°C-865°C in H2 atmosphere. The continuous annealed sheet is further
given secondary cold rolling (SCR) reduction, which brings down the thickness of
the sheet in the range 180 μm to 300 μm. The thin sheet produced after
secondary cold rolling is subjected to batch annealing in the temperature range
590 - 715°C for 900 to 10000 seconds in H2 atmosphere to obtain the thin ultra
low carbon steel sheets. The process has been outlined in the schematic flow
chart (Fig. 1). The steel sheets thus prepared were subjected to various tests to
ascertain the properties thereof.

The samples for X-Ray diffraction (XRD) are prepared by the manual polishing
technique. Crystallographic textures are determined from the mid thickness
regions of the sheets using a Philips X-Ray diffractometer. ODFs (Orientiation
Distribution Functions) are measured using MTM=FHM software and Φ2 = 45°
sections (Bunge notation) are determined therefrom. The average 4 bar value is
also calculated using the same solftware.
Experimental Results
Formability of steel sheets is greatly enhanced by the presence of strong and
homogeneous y fibre texture Fig. 2 (a-d) show the Ψ2 = 45° sections of the
ODFs of the sheet during different stages of processing. Fig. 2 (a) shows the
texture of the hot roiled sheet. The texture has been found very weak with
maximum ODF intensity 2 times random. The texture intensity increases to 11
times random as a result of primary cold rolling [Fig.2b]. The texture here can be
described as a combination of Y and a fibres. Continuous annealing (here
termed as primary annealing) of the cold rolled sheet produces a strong Y fibre
at the expense of a fibre [Fig.2c] . The ODF intensity found here is 8 times
random. Batch annealing of the secondary cold rolled sheet leads to the
development of very strong and uniform Y fibre with improved texture intensity
of around 16 times random [ Fig. 2d]. The r bar value of the steel sheet after
primary annealing (here continuous annealing) has been found to be in the
range 2.15 to 2.40. The r bar value of the sheet after secondary annealing (here
batch annealing) has been improved to the range 2.6 to 2.72.

Therefore, a combination of two stage cold rolling coupled with continuous
annealing and batch annealing technique, leads to the development of superior y
fibre texture, which results in the improvement of the r bar value to the range
2.6 to 2.72 in very thin steel sheets. Achieving such high r bar value makes the
steel sheets very light and highly formable and useful for light weight packaging
applications. The sheets will find use in major steel industries which are
interested to produce light weight thin steel sheets for packaging purpose, such
as beverage cans, food cans, toys, sophisticated cans, shielding sheet for
cathode ray tube in colored picture tube in Television.

WE CLAIM;
1. The low carbon steel sheets having a r bar value In the range of 2.6 to 2.72
and a thickness in the range of 180μm to 300μm.
2. The steel sheets as claimed in claim 1 having a composition in terms of
percent by weight as follows:
C : 0.001 - 0.005%
Mn : 0.05 - 0.2%
S : 0.004 - 0.012%
P : 0.005 - 0.015%
Si : 0.002 - 0.010%
Al .0.020 - 0.45%
N : 0.0025- 0.008%
Nb : 0.004 - 0.030%
Ti : 0.01 -0.05%
Balance : Fe
3. A process for producing thin ultra low carbon, steel sheets having high r
bar value, comprising the steps of subjecting steel to controlled hot rolling,
followed by coiling and primary cold rolling, to obtain the cold rolled steel,
subjecting the cold rolled steel to continuous annealing followed by a secondary
cold rolling to obtain thin steel sheets, subjecting the thin steel sheets to batch
annealing to obtain the thin ultra low carbon steel sheets.

4. The process as claimed in claim 3, wherein the steei used has a composition
in terms of percent by weight as follows:

5. The process as claimed in claim 3, wherein the hot rolling is carried out in
the temperature 875°C - 975°C.
6. The process as claimed in claim 3, wherein the amount of hot rolling applied
is in the range of 50-90%.
7. The process as claimed in claim 3, wherein the coiling temperature of the
sheet after hot rolling is in the range 650°C - 775°C.
8. The process as claimed in claim 3, wherein the continuous annealing
temperature is in the range of 775°C- 865°C.

9. The process as claimed in claim 3, wherein the time used for continuous
annealing is in the range 2 to 5000 seconds.
10. The process as claimed in claim 3 wherein second annealing temperature
is in the range 590 - 715°C.
11. The process as claimed in claim 3, wherein second annealing time involved is
in the range of 900 - 10000 seconds.
12. The process as claimed in claim 3 wherein the thin steel sheets are
subjected to batch annealing under hydrogen atmosphere.

The low carbon steel sheets having a r bar value in the range of 2.6 to 2.72 and a thickness in the range of 180 μm to 300 μm.