FIELD OF APPLICATION
This invention relates to a process for producing formable cold rolled steel sheets
with low planar anisotropy.
BACKGROUND OF THE INVENTION
When producing axially symmetrical parts by deep-drawing operation from a
sheet blank, earing as well as variable wall thickness may be observed despite a
symmetrical operating stress. If a material exhibits uniform magnitude of
mechanical properties in all directions, the material is termed as Isotropic, and
the earing would be low. 'Earing' is a result of non-uniform plastic flow due to
heterogeneous strain distribution over the sheet plane. If the plastic flow
properties vary with angle, the flow of the metal will be uneven in different
directions, and this will give rise to peaks and troughs around the periphery of
the drawn cup, and earing will occur. And the phenomenon is known as plastic
anisotropy. Conventionally produced low carbon steels in cold rolled annealed
condition using batch annealing process possess good drawability along with
high planar anisotropy (∆r>0.3). High planar anisotropy (∆r) is undesirable since
it leads to ear formation during the forming operation of components causing
higher rejections. (Ears occur most frequently at 0° and 90° or at 45° to the
original rolling direction. Anisotropy is measured in terms plastic strain ratio (r-
value), and planar anisotropy (∆r).
where, are true width and true thickness strain respectively.
where, the suffixes indicate the angle of tensile axis with
respect to rolling direction of the sheet.

With ∆ r> 0, ears are formed at 0° and 90° to the rolling direction and with ∆ r<
0, earing appears at 45°. Since planar anisotropy is caused by a few undesirable
texture components in the sheet material, only a stringent control can solve this
problem and the answer lies in controlling the material chemistry, hot rolling,
cold rolling and annealing parameters. The isotropic grade steels, which are
also, called non-scalloping grades, are ideal choice for many deep drawn parts.
These include bearing cages, drawn cylinders, eyelets and ferrules etc. The strip
will draw into a cup free of scalloping, or earing. It has been reported that, in
case of low carbon steels, ears appear at 0° and 90° to the rolling direction,
whereas, steel grades of higher strength show ears at 45°. [1] '
Lower planar anisotropy is difficult to achieve in steel sheets processed with the
conventional chemistry and process parameters.
SUMMARY OF THE INVENTION
The main object of the present invention is to produce formable cold rolled steel
sheets with low planar anisotropy in cold rolled and batch annealed condition
which can be achieved with a suitable chemistry.
The chemistry selected in the present invention is different from the conventional
deep drawing quality steel. In deep drawing quality steel, the use of stabilizing
element is not in practice since it is detrimental to the development of texture
beneficial for the formability properties. In the present invention use of micro
alloying element i.e. 71 has been found to be beneficial for achieving low planar
anisotropy. The process parameters are different from conventional deep
drawing steel and suitable for the production of low planar anisotropy steels.

The process parameters include hot rolling parameters: finish rolling and coiling
temperatures, cold rolling parameters: optimum deformation and annealing
parameters: annealing temperature and time.
In conventional route, higher finish rolling temperature and lower coiling
temperature are in practice whereas as in the present invention, these
parameters are different from the above mentioned. Cold rolling reduction
above 70% is desirable for deep drawing steel whereas for low planar anisotropy
steel, either deformation in the lower range (< 40 %) or in higher range
(> 90 %) has been found suitable. Similarly, the annealing parameters for these
steels are different than those for conventional annealing practice for deep
drawing applications.
The cold rolled strips produced with the optimized parameters show a low value
of planar anisotropy with a good combination of yield strength and tensile
strength. These combinations of properties are developed in the present
invention and are expected to be used for many applications such as bearing
cages, ferrules, lipstick cases and automotive panel applications. The result of
low planar anisotropy were confirmed by the texture measurements by X-ray
diffraction analysis.
Thus the present invention provides a process for producing formable cold rolled,
steel sheets with low planar anisotropy, comprising the steps of: selecting and
controlling material chemistry and producing low carbon liquid steel in LD
converter; continuously casting steel into slabs; hot rolling the cast slabs into
strips with appropriate finish rolling and coiling temperatures; cold rolling said
strips into coils with suitable degree of cold reduction; annealing the coils;
maintaining appropriate soaking temperature; and giving a skin pass deformation
in a skin pass mill.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The invention will now be described with the help of the accompanying drawing
where
Figure 1 shows a flow chart for the process of the present invention.
DETAILED DESCRIPTION
In the present invention detailed laboratory scale studies have been conducted
for achieving low planar anisotropic properties in low carbon steels with and
without small titanium addition. Through optimizing laboratory scale processing
and subsequently, commercial processing has resulted into steel sheets with r-
bar 1.0, ∆ r 0.1, yield strength (MPa)=220 min., ultimate tensile strength (MPa)=
340 min.,% El= 40 min. and n = 0.2 min.
In the following, the effects of chemical composition, hot rolling, cold rolling and
batch annealing parameters in case of two low carbon grade steels on influence
of planar anisotropy has been elucidated.
Three types of low carbon steels were used for the present investigation, one of
them being Ti-bearing, with less than 350 ppm Ti content. The thickness of all
the as received hot rolled strips varied between 1.5 to 3.0 mm. The finish rolling
temperatures (FRT) and coiling temperatures (CT) of the above-mentioned steel
grades were near Ae3 temperature and below 600° C respectively. The hot-
bands were then pickled, cold rolled and annealed in laboratory. During this
investigation, in the experimental cold rolling mill, the degree of cold reduction
was varied. During subsequent batch annealing, the annealing temperature and
soaking time were also varied either during laboratory simulation or in Basic
Oxygen process in order to ascertain their effect on planar anisotropy of the steel
sheets.

A microstructural examination carried out on all hot bands using optical
microscopy on a polished and etched cross section of thickness reveals the grain
size. Subsequently, hardness and uniaxial static tensile properties were
determined. Smaller coupons from the as received hot rolled strips were
then pickled in specially HCI prepared HCI solution and subsequently given
various cold deformations in experimental cold rolling mill less than 80%. Steels
A and B were given lower cold deformation whereas Steel C was given varied
degrees of cold deformation. After cold rolling the coupons were annealed in a
lab scale muffle furnace. In batch annealing simulation in lab, for steels A and B,
the soaking temperature was varied in a range, and less than or equal to 750° C,
with a suitable heating rate and soaking time simulating commercial batch-
annealing cycle. While in case of steel C, the soaking temperature and time
were kept constant. It is worth mentioning here that, in case of steel C, in order
to do similar kind of study in commercially rolled and batch annealed material,
the cold deformation was varied, whereas the annealing cycles were also varied
accordingly.
Detailed optical microstructural studies were carried out with help of image
analyzer. Uniaxial static tensile test were done on all the cold rolled, annealed
samples. Tensile properties have been determined in transverse direction only,
for laboratory cold rolied annealed samples, whereas, the variation of tensile
properties was studied in 0, 22.5, 45, 67.5 and 90° for the commercially rolled
sheets of steel C. ericsson cupping values (ECV) were determined for steel sheet
C as an index of stretchability.

In order to explore the degree of randomization in the samples of steels A and B,
intensity ratio of two planes [I(211)/ I(200)] were found out with the help of X-ray
diffraction analysis. Microtextures on the surfaces of commercially hot rolled,
cold rolled, annealed and skin passed sheets C were studied in detail with help of
electron black scatter diffraction (EBSD).
Out of the few influential factors to reduce planar anisotropy of batch-annealed
cold rolled sheets, the degree of cold reduction proceeding recrystallization
annealing is of decisive importance. Only with either very low or sufficiently high
cold reductions can a low planar anisotropy be achieved, and thus earing is
avoided [1].
However, from the viewpoint of commercial manufacturing, the lower cold
reduction range is inappropriate. Adjusting the finish rolling temperature (FRT),
coiling temperature (CT) to the final cold rolling deformation can also help to
obtain a batch-annealed strip with low level of ears, provided the recrystallization
kinetics can be slowed down precipitation of nitrides and / or carbides of Ti/Nb.
Steel A, which was given low cold deformation, and subsequently annealed at
various temperatures less than or equal to 750° C for less than 4 hours and also
normalized at a temperature less than 700° C, gave an indication that with low
deformation and without any microalloying addition, a low carbon steel can give
low planar anisotropy. The microstructural studies in case of both annealed and

normalized strip samples, showed equiaxed, polygonal ferrite grains, ideal for
giving isotropic (low planar anisotropy) properties. From Table 1, it can be seen
with an increase in recrystallization annealing temperature, the hardness of the
strip has decreased and also grain growth has taken place. It is to be noted
here, that no firm conclusion could be made from planar anisotropy behaviour of
the steel, as the values of ∆r and rm had no clear pattern of variance with the
annealing temperature. In order to explore degree of randomization in the strips
by X-ray technique, samples from one of the coupons, was taken for analysis to
find out the intensity ratio between two planes (i.e I(211) / I(200)) of interest
I(211) / I(200) =2.26 [2] for a randomized structure).

In case of steel B, which is of richer chemistry, when given a further lower cold
reduction as compared to Steel A, and subsequently similarly annealed or
normalized, gave equiaxed, polygonal ferrite grains and hence isotropic
properties. The randomization the microstructure was verified by X-ray analysis,
as in the earlier case. The various mechanical as well as planar anisotropic
properties as achieved in these lab cold rolled heat-treated strip samples have
been shown in Table 2.


Steel C, which was a Ti bearing steel, was given various cold reductions varying
in between 20 % to 80 %, followed by suitable annealing cycle. As can be seen
from Table 3, low planar anisotropic properties were obtained when the cold
deformation was restricted medium range of deformation, whereas with higher
cold deformation, anisotropy increased, even with a low deformation, the strip
showed higher planar anisotropy. Hence for industrial processing i.e. cold rolling
and annealing, it was decided to go for a medium range of cold reduction. And
following which again two different annealing cycles were given, in order to
study the effect of change in annealing, time and temperature on planar
anisotropy of sheets. Table 4 shows various properties achieved in case of
industrially processed strips. Microstructural studies clearly indicated equiaxed
polygonal ferritic grains in case of both laboratory treated material as well as in
industrially treated material.

The texture studies of the samples from the industrially processed sheets
showed that addition of 71 does not cause a change in the orientation density of
the a-fiber and y-fiber. From the orientation distribution functions (ODF) studies,
the hot strip starting texture was almost random. Ti is believed to slow down
the recrystallization kinetics by precipitation of TIN and subsequently, TIC. EBSD
studies showed that in case of lower deformation {112} <110> orientation on
the a-fiber is distinctly emphasized, whereas {001} <110> appears moderately.
On the contrary, with cold deformation increasing i.e. the {111} <112>
component on y-fiber is on the higher side, increasing the planar anisotropic
properties in the sheet.



With the present chemistry and subsequent hot rolling and cold rolling
parameters, no significant change in mechanical properties were found with
change in annealing time and temperature. Although, it can be predicted that
with higher annealing temperature above Ae1, there might be a decrease in yield
strength but due to the chances of strip windings getting stuck together in case
of present case of batch annealing furnace, maximum annealing temperature
was kept around Ae1.
The flow chart for the method for producing formable cold rolled steel sheets
with low planar anisotropy of the present invention is shown in Figure 1 of the
accompanying drawing. The steps comprise controlling material chemistry and
producing liquid steel in LD converter. Continuously casting steel into slabs of
210 mm in thickness. Hot rolling the slabs with appropriate finish rolling and
rolling temperature into strips. Cold rolling the strips in suitable sizes having less
than 80 % deformation. The coils are then annealed at a soaking temperature
less than 750° C and then given a skin pass deformation at the skin pass mill.

WE CLAIM:
1. A process for producing formable cold rolled steel sheets with low planar
anisotropy, comprising the steps of:
- selecting and controlling material chemistry and producing low
carbon liquid steel in LD converter;
- continuously casting steel into slabs;
- hot rolling the cast slabs into strips with appropriate finish rolling
and coiling temperatures;
- cold rolling said strips into coils with suitable degree of cold
reduction;
- annealing the coils;
- maintaining appropriate soaking temperature; and
- giving a skin pass deformation in a skin pass mill.

2. The method as claimed in claim 1, wherein the material chemistry
selected is carbon between 0.01 and 0.10 %, manganese between 0.1
and 1.0 %, silicon between 0.01 and 0.10 %, sulfur less than 0.035 %,
phosphorous less than 0.035 %, aluminum between 0.02 and 0.10 %,
titanium between 0.01 and 0.10 %, nitrogen 0.002 and 0.05 %.
3. The method as claimed in claims 1 or 2, wherein said continuously cast
steel slabs are of 210 mm in thickness.
4. The method as claimed in the preceding claims, wherein the finish rolling
temperature is near Ae3 temperature.

5. The method as claimed in the preceding claims, wherein the coiling
temperature is less than 600° C.
6. The method as claimed in the preceding claims wherein the hot rolled
strip is pickled in specially prepared HCI solution before giving cold
deformation.
7. The method as claimed in the preceding claims, wherein the cold rolling is
carried out with cold deformation between 20-95 %.
8. The method as claimed in the preceding claims, wherein the annealing is
carried out at a soaking temperature of less than 750° C for less than 4
hours.
9. Formable cold rolled steel sheets with low planar anisotropy with the
following chemistry carbon betweep 0.01 and 0.10%, manganese
between 0.1 and 1.0%, silicon between 0.01 and 0.10%, suffer less than
0.035%, phosphorous less than 0.035%, aluminum between 0.02 and
0.10%, titanium between 0.01 and 0.10%, nitrogen 0.002 and 0.05%.
10. The method for producing formable cold rolled steel sheets with low
planar anisotropy substantially as herein described.

This invention relates to a process for producing
formable cold rolled steel sheets with low planar anisotrophy. The process comprising the steps of selecting and controlling
material chemistry and producing low carbon liquid steel in LD converter; continuously casting steel into slabs; hot rolling the
cast slabs into strips with appropriate finish rolling and coiling temperatures, cold rolling the strips into coils with suitable degree of cold reduction; annealing the coils;
maintaining appropriate soaking temperature; and giving a skin pass deformation in a skin pass mill.