FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)


TITLE OF THE INVENTION An Improved Method Of Manufacturing Automobile Body Panel
APPLICANTS
TATA MOTORS LIMITED,
An Indian company having its registered office at
Bombay House, 24 Homi Mody Street,
Hutatma Chowk, Mumbai 400 001
Maharashtra, India
INVENTORS
SUBHRAJYOTI GHOSAL MUKULCHANDRA M OGALE
& BHIMSEN GALGALI R
An Indian nationals of TATA MOTORS LIMITED,
An Indian company having its registered office at
Bombay House, 24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in
which it is to be performed.


FIELD OF INVENTION
This invention relates to a method of manufacturing automobile body panel and more particularly relates to a method of bake hardening of automobile body panel in which the parameters are controlled to achieve higher bake hardening values.
BACKGROUND OF THE INVENTION
One of the most important objectives in the development of automotive steel sheet is the combination of strength and formability. Formability is required when the sheet is shaped into an automobile body panel, and high strength is required after assembly. Bake-hardenable steel sheet was developed by exploiting the fact that these two properties are not needed simultaneously.
Dislocations are introduced by press forming a steel sheet, and strength is increased by the action of work hardening in which accumulated dislocations prevent the movement of other dislocations. When an automobile body is being manufactured, painting and baking are carried out after assembly. These processes involve heating the steel body panels to about 443K (170.c). At this temperature, the carbon atoms dissolved in the steel diffuse by jumping between lattice points, which occurs 103 to 105 times a second, segregating in the regions around dislocations where the stresses are compressive. This results in locking of the dislocations which is called strain aging. This mechanism makes the steel panels harder after baking than after press forming, and is referred to as bake hardening.
Three parameters are responsible for the bake hardening effect. They are the pre-straining during forming, baking temperature and duration of baking.
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OBJECTS OF INVENTION
Main object of this invention is to improve the strength of automobile body panel by controlling bake hardening parameters such as pre-straining, baking temperature and baking time.
SUMMARY OF INVENTION
According to this invention an improved method of manufacturing automobile body panel comprising the steps of; preparing a press forming die to attain a pre-strain in the range of 2-3% during forming said body panel, forming said body panel using said die, baking said formed body panel at a temperature in the range of 180°C to 240°C, continuing said baking process for a time in the range of 10 to 25 minutes, air cooling said baked body panel.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows the graph for determining bake hardening index. Figure 2 shows the effect of pre-strain on bake hardening process Figure 3 shows the effect of temperature on bake hardening process Figure 4 shows the effect of baking time on bake hardening process
DETAILED DESCRIPTION OF INVENTION
The strength of automobile body panel is increased after bake hardening operation. The method of bake hardening involves the steps of straining the body panel during forming and baking for a specified time under predetermined temperature. The strength increases in the assembled auto components can be divided into two parts a) work hardening - the strength increases due to forming operation b) bake hardening -strength increase due to paint baking. The parameters which control bake hardening
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are i) pre-strain level in the component during forming ii) baking temperature iii) baking time.
The level of straining of a component can be controlled with the help of optimum die design. Areas which are very much important form passenger safety & dent or crash resistance can be designed in such a way that those areas become highly dent resistant so that energy absorption at those locations during crash will be maximum. This way optimum component design is possible. The die designed and prepared to attain a predetermined pre-strain in the component during forming. The steel sheet is pre -strained during forming the automobile component in the press forming die to a desired plastic strain level. The baking of the pre-strained formed components parts are carried out in a paint baking oven at predetermined time & temperature with temperature accuracy of +/- 1°C. After the baking the parts are air cooled.
The baking value is determined using the figure 1,
The bake hardening and strain hardening index of the material can be determined as
follows,
Bake Hardening Index (BHI) = C-B
Where B = Flow stress at given pre-strain value
C = Upper yield stress after baking in a given baking temperature and
baking time Strain Hardening Index (SHI) = B-A Where A = 0.2% proof stress of the unstrained and unbaked material
B = Flow stress at given pre-strain value
Referring to Figure 2 & Table 1 shows the effect of pre-straining on the bake hardening index values. The figure reveals that for all the baking temperature the baking value is the maximum at the pre-strain range between 2 to 3%. As the pre-strain value increases the baking value starts decreasing.
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Table 1

Actual Baking effect due to Temperature and Strain change (Time 20
minutes)
Strain Temperature in °C

140 150 160 170 180 190 200 210 220 230 240
2% 56 64 71 78 87 95 95 95 94 94 90
3% 49 55 62 69 77 86 85 85 85 86 81
5% 34 41 49 54 64 71 71 70 70 71 65
This phenomenon can be explained by the dislocation theory. With the increase of pre-strain the strain hardening of the material increases. This imparts more number of mobile dislocations in the material matrix. Because of this during the subsequent baking operation the possibility of locking of all mobile dislocations with carbon atoms starts reducing. This in turn increases density of unlocked dislocations in the matrix and causing the decrease in the BHI with the increase of pre-straining values. So when the pre-straining value increases from 3% to 5% the value of BHI starts decreasing.
Referring to Figure 3 & Table 1 shows the effect of temperature on the bake hardening index values. As the baking temperature increases from 140°C, the BHI value also starts increasing. It reaches the maximum value at 190°C. With the increase of baking temperature from 190°C the BHI value remains unchanged up to 240-250°C. If the baking temperature exceeds 250°C the BHI starts decreasing. Identical behavior of material can be observed in all the pre-strain values i.e. 2, 3 and 5%.
This phenomenon can be explained by precipitation kinetics and dislocation locking theory. As the baking temperature starts increasing from 140°C, the rate of diffusion of the carbon atoms towards the dislocations also increases. Hence the possibility of mobile dislocation locking by the carbon atoms also increases. This in turn causes the increase in the density of locked dislocations in the material matrix and increase in the strength. At 190°C the total process reaches saturation and hardly carbon atoms remain balance to diffuse and lock the dislocations. With baking temperature more
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than 190°C, the BHI values remain practically unchanged up to baking temperature 240°C. From 250°C baking temperature the BHI values show a reducing trend because of the coarsening effect. The maximum BHI can be achieved at baking temperature of 190°C.
Referring to figure 4 and table 2 shows the effect of baking time on the bake hardening index values. It shows that the BHI values increase with the increase of the baking time. It reaches the maximum value at 25 minutes of baking time and then starts decreasing. The same trend can be observed in all the baking temperature from 140°C to 190°C. The phenomenon is more prominent in the middle range of temperatures. At higher temperature like 190°C the phenomenon is not so prominent and practically it remains constant within the entire range (10 to 25 min) of time.
Table 2

Actual Baking effect due to Time change (Strain 2%)
Time (min) Temperaf ture in °C

140 150 160 170 180 190
10 44 52 56 63 70 94
15 48 57 62 70 78 95
20 56 64 70 78 87 95
25 62 70 75 83 90 95
30 51 65 66 78 80 85
This phenomenon can be explained with the help of the precipitation kinetics. At lower temperature as the duration of baking increases the number of carbon atom diffusing towards the dislocations to lock them increases, which in turns cause the increase in the BHI with the increase in the baking time. When the baking time is more than 25 minutes the BHI starts decreasing as almost all the available carbon atoms (depends on the activation energy required for diffusion and energy available to cause diffusion) complete their diffusion process within 25 minutes. Hence no more carbon atoms remain available to cause diffusion and coarsening also starts with long holding at particular temperature. At higher baking temperature because of the higher diffusion rate of carbon atoms, within a short period of baking most of the carbon
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atoms complete their diffusion process and hence the saturation of BHI takes place early. Also at higher temperature the decrease in the BHI value because of increased baking time is more prominent as compared to that at lower temperature. This is because of the higher coarsening effect at higher baking temperature.
The strength of automobile body panel can be increased by pre-straining in the range of 2-3% during forming the component and then heating the component at a temperature in the range of 180° to 240° C for a period in the range of 10 to 25 minutes. For maximum increase in strength of the component it is heated at a temperature of 190° for 25 minutes. The press forming die has been designed to attain a pre-strain in the range of 2-3% during forming the component.
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WE CLAIM
1. An improved method of manufacturing automobile body panel comprising the steps
of;
a) preparing a die to attain a pre-strain in the range of 2-3% during forming said body panel,
b) forming said body panel using said die,
c) baking said formed body panel at a temperature in the range of 180°Cto240°C
d) continuing said baking process for a time in the range of 10 to 25 minutes
e) air cooling said baked body panel.

2. The improved method of manufacturing automobile body panel as claimed in claim 1, wherein said body panel is baked at a temperature of 190° C.
3. The improved method of manufacturing automobile body panel as claimed in claim 1, wherein said body panel is baked for a period of 25 minutes
4. The improved method of manufacturing automobile body panel as claimed in claim 1, wherein said baking is paint baking of said body panel.
5. The improved method of manufacturing automobile body panel substantially as herein described with reference to accompanying drawings
Dated this 21st Day of May 2008
TATA MOTORS LIMITED By their Agent & Attorney
(Karuna Goleria) of DePENNING & DePENNING
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