FIELD OF THE INVENTION
The invention relates to the development of a method to determine
the shelf life of bake hardening steel in industrial practice.
BACKGROUND OF THE INVENTION
The steel for automobile body panel applications requires higher
strength for better dent resistance and good formability for defect
free formation of the component of automobile. In view of the above,
bake hardening steel was developed having a good combination of
yield strength and formability and it gives rise to an appreciable
increase in yield strength of 30-50 MPa during commercial baking
operation (around 17°C/20min) of the formed component. This extra
increase in strength leads to an improvement in dent resistance. A
thinner gauge of this material can also be used replacing the
conventional thicker material. In advanced automobile, use of bake
hardening steels is well established due to the additional strength
achieved during the commercial baking operation. This bake
hardening is primarily achieved by the presence of controlled amount
of interstitial carbon in solid solution after cold rolling and annealing.
Ultra low carbon (ULC) steel is the prime material for bake hardening
(BH) grade, since it possesses excellent formability and it is easier to
control the solute carbon to a desired level at the

steel making stage. In addition, these steels can easily be processed
through various routes such as batch annealing, hot dip galvanizing
galvannealing route and continuous annealing route. However,
automakers are concerned about the room temperature aging
problem in bake hardening steels due to inappropriate amount of
solute elements, which may cause stretcher strain during forming at
automobile manufacturer's place. This leads to a rejection of
material adding to an extra cost for manufacturing. The requirement
is that the material should have sufficient shelf life so that it should
not degrade during the transportation and storage before the forming
operation. The knowledge of the shelf life is necessary for both steel
makers and automakers. For steel makers, it is important to know
the shelf life in order to control the inventory and decide a period for
its storage, transportation and its delivery, whereas for automobile
component manufacturers it is useful in deciding its storage period
and controlling inventory to produce the automobile components
without any rejection. There is no standard available to evaluate the
shelf life of bake hardening steel and hence different automakers use
their own technique for its evaluation. Some of the them are using
the conventional method i.e. strain aging, which is mainly based on
the rise in the flow stress after accelerated aging test at 100 °C after
7.5-10% tensile deformation. The method does not suit for
predicting the room temperature aging behaviour of ultra low carbon
steel in skin-passed condition, specially to be used for manufacturing
auto-components, where stretcher strain caused by yield point

elongation (YP-EL) of the input material is the main concern. Hundy has
presented an expression for simulating the aging behaviour at different
temperature based on Cottrell-Bilby model and the prediction made by it
deviates from the actual room temperature test data in case of skin-passed
or temper rolled material. Carrying out actual room temperature test for
shelf life evaluation is not possible in view of long time and large number
of samples required for this. Thus there is a need to formulate a simple
method for the prediction of shelf life in an industrial practice. In view of
the limited work in this area, the present invention focuses on evaluating
the strain aging behaviour of ultra low carbon bake hardening steel and
then formulating a simple method to determine the shelf life applicable in
an industrial practice.
A prior Indian Patent Application No. 763/KOL/2006 entitled "Development
of cold rolled batch annealed bake hardening steel article and a process of
preparing the same for automobile application" which discloses an
invention relates to a process of producing cold rolled batch annealed bake
hardening steel article and a process of preparing the same by comprising
an ultra low carbon steel in a LD converter of composition in weight % C -
< 0.0035, Mn 0.2-0.6, Al -<0.03, S -<0.02, P - 0.03 - 0.06, N - <0.0050
and Nb and/or Ti <0.02, by controlling through RH degasser to maintain

required level of carbon and other impurities, continuously casting the steel
into slabs, hot rolling the slabs and finishing the resulted strip at required
temperature ranges and coiling according to the specification, cold rolling
the hot rolled coils with > 65% deformation, the resulted cold rolled coils
being annealed in batch annealing furnace with optimum annealing cycle
and the annealed coils being given skin pass rolling with deformation >
equal 0.8%, the resulted steel article being characterization evaluated for
Bake Hardening through the course of giving a test specimen of the
resulted cold rolled steel an elongation of 2%, heating at 170°C for 20
minutes in a furnace and then air cooled and finally calculating for
difference between yield stress after baking and flow stress after 2%
elongation.
This invention has not focused any information about the life-span of Bake
hardening Steel.
The present invention emphasizes on the life-span of Bake hardening Steel.
OBJECTS OF THE INVENTION:
- the object of the invention is to determine the shelf life of bake
hardening steel in an industrial practice.

- another object of the invention is to reduce the rejection loss of bake
hardening steel for steel manufacturers and ancillary users.
- further object of the invention is to optimize production/inventory
control of steel manufacturers and ancillary users.
- yet another object of the invention is to formulate a simple method
for ascertainment of shelf life of bake hardening steel.
SUMMARY OF THE INVENTION:
The shelf life is defined as the maximum period of storage of finally
processed cold rolled annealed sheets at room temperature prior to use,
without showing any stretcher strain in the component after forming.
Aging kinetics of cold rolled-annealed ultra low carbon bake hardening
steel has been studied in the temperature range of 50-100°C and based on
these results, the activation energy for the aging process was determined
as 21-24 kCal/mole, which is higher than that of diffusion of carbon or
nitrogen in iron. Using this activation energy, Hundy equation was

modified. The shelf life predicted using Hundy equation is significantly
lower than that of actual test data, whereas the predictions made using
modified Hundy equation (from higher activation energy) is close to the
actual room temperature aging data. In the present invention a simple
method has been developed to determine the shelf life of ultra low carbon
steel in an industrial practice.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
1. Fig - 1 shows an experimental evaluation of shelf life and its
validation with the actual room temperature test (assumed as 30°C).
2. Fig - 2 shows relationship between the shelf life and yield point
elongation (YP-EL).
3. Fig - 3 shows relationship between aging time and yield point
elongation (YP-EL).
4. Fig - 4 shows slopes of various steel.
5. Table - III shows a simulated relationship data of yield point
elongation (YP-EL) at 100°C/lh (%) and shelf life.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENT OF
THE INVENTION:
Cold rolled ultra low carbon (ULC) steels alloyed with Mn and P were taken
for present investigations. The bake hardening strength in these steels
were found to vary from 31 to 43 MPa. For assessing the kinetics of aging,
the tensile specimens were aged for various periods at different aging
temperatures (Ta) i.e. 50, 70, 85 and 100°C. The yield point elongation
(YP-EL) was then measured by conducting tensile test. To study the room
temperature aging behaviour, the samples were aged for various periods in
oil bath furnace maintained at 30°C and then yield point elongation (YP-EL)
were similarly measured.
Kinetics of aging process was assessed using following equation, which is
derived form of Johnson - Mehl - Avrami (J - M - A) equation:

Where ty = time for a given fraction of transformation, and Q = Activation
energy for the governing process. In the present analysis, the time for
yield point appearance was used as ty. The In (ty) was plotted for different
steels as a function of 1/T and the activation energy calculated from the

slope for the different steels was found to be in the range of 21 - 24
kCal/mole.
Hundy derived the following correlation using Cottrell-Bilby equation log

Where, K = constant = Q/R (log10e), Q = Activation energy, R = gas
constant, (for carbon, K = 4400; for nitrogen, K = 4000), Tr = room
temperature (K), T = artificial aging temperature (K), tr = aging time at
room temperature, Tr t = aging time at artificial aging temperature T. It
enables the aging time at any temperature to be converted to an
equivalent aging time at room temperatures. The constant K was derived
from the activation energy for the diffusion of C or N in iron. The above
equation is reported to deviate from the actual room temperature test data
in temper rolling condition. This could be due to the higher activation
energy in the skin passed or temper rolled material. The average activation
energy determined in the present results was used to re-calculate the exact
K-value of the Hundy equation and thus modified Hundy equation can be
represented as follows:

The shelf life was predicted by both Hundy equation and modified Hundy

equation. The predictions made by Hundy equations are much lower than
that of actual room temperature test data, whereas the prediction made
using modified equation matches well with them. In the present work, the
shelf life was defined as the time of room temperature aging when the
yield point elongation (YP-EL) becomes 0.2%.
This level of point elongation may be acceptable by the automobile
manufacturers. To determine the exact shelf life time for yield point
elongation (YP-EL) - 0.2%, it is necessary to conduct a large number of
tests at a high temperature for simulation. In the present invention a
simple method has been designed for evaluating the shelf life in an
industrial practice, yield point elongation (YP-EL) value at the aging
temperature of 100°C for 1h was selected as the aging parameter for
accelerated aging test. A relationship between the shelf life as a function of
yield point elongation (YP-EL) for 100°C/lh has been established as shown
in fig. 2. The laid down criteria is shown in table-Ill. Based on this
relationship, the expected shelf life can be assessed by a single simple test
and can be utilized in industrial condition as a routine basis. The selecting
"100°C" as the test temperature is important since this temperature can be
achieved accurately in a boiling water bath and it can be used in place of
using furnace.

A typical graph for the experimental evaluation of shelf life for such sample
steel is shown in Fig 1.
The stretcher strain in the formed component is related to the yield point
elongation of the material before forming. It has been observed that there
is no occurrence of stretcher strain up to a certain yield point elongation
(YP-EL) value. A limit of 0.2% yield point elongation (YP-EL) for no
stretcher strain has been considered.
One can decide any test temperature or aging time for simulation. In the
present method, aging parameter as 100°C for 1h has been selected for
the convenience of the test in industrial condition. Test temperature as
100°C has been selected since this is the boiling point of water. The
samples can be heat-treated in boiling water easily without any variation in
temperature. An appropriate time of lh has been decided since too long
time period may not be possible for conducting this test on routine basis in
an industrial condition. Shorter time period may result in a very low yield
point elongation (YP-EL) causing an error in measurement.
Similar aging parameter (100°C/lh) has also been suggested in a paper
reported by Hundy for simulation for aging process.

The relationship has been established and shown in Fig 2. And shelf life
can be ascertained from the chart as shown in Table - III from above
experiments.


WE CLAIM:
1. A method to predict the shelf life of bake hardening steel in an
industrial practice, which comprises:
an accelerated aging treatment of cold rolled - annealed bake
hardening steel sheet at a temperature of 100°C (preferably in
boiling water) for 1 hour;
measurement of yield point elongation (YP-EL) by conducting
tensile test;
characterized in that prediction of shelf life of bake hardening
steel can be ascertained with respect to yield point elongation
(YP-EL) from the following relation:
0.3 > YP-EL means < 1 month shelf life
0.1 < YP-EL < 0.3 means > 1 month but < 2 months shelf life
YP-EL < 0.1 means > 3 months shelf life
YP-EL = 0 means > 6 months shelf life
2. The method to predict the shelf life of bake hardening steel as
claimed in claim 1 wherein ultra low carbon (< 0.004%) steel is
generally used.

ABSTRACT

A METHOD TO DETERMINE THE SHELF LIFE OF BAKE HARDENING
STEELS IN AN INDUSTRIAL PRACTICE
The present is provided with a method to predict the shelf life of bake
hardening steel in an industrial practice, which comprises an accelerated
aging treatment of cold rolled - annealed bake hardening steel sheet at a
temperature of 100°C (preferably in boiling water) for 1 hour;
measurement of yield point elongation (YP-EL) by conducting tensile test;
characterized in that prediction of shelf life of bake hardening steel can be
ascertained with respect to yield point elongation (YP-EL) from the
relations 0.3 > YP-EL means < 1 month shelf life, 0.1 < YP-EL < 0.3
means > 1 month but < 2 months shelf life, YP-EL < 0.1 means > 3
months shelf life, YP-EL = 0 means > 6 months shelf life.