Title of Invention	A PROCESS FOR PRODUCING HIGH STRENGHT FORMABLE STEEL FOR EXTERIOR PANEL OF AUTO BODY
Abstract	The invention provides a process for producing high strength formable steel stripfor exterior panel of auto body. The chemistry of steel is appropriately selected by adding substitutional elements like manganese, silicon, phosphorus, etc. for producing interstitial free high strength (IF-HS) grade steel. The transformation temperature of IF-HS is determined at different cooling rates by a dilatometer in a thermo-mechanical simulator. The parameters for hot rolling and cold rolling are optimized based on the results of the simulator. The final cold rolled strip is batch annealed for the desired grade of IS-HS steel strip.

Title of Invention

A PROCESS FOR PRODUCING HIGH STRENGHT FORMABLE STEEL FOR EXTERIOR PANEL OF AUTO BODY

Abstract

The invention provides a process for producing high strength formable steel stripfor exterior panel of auto body. The chemistry of steel is appropriately selected by adding substitutional elements like manganese, silicon, phosphorus, etc. for producing interstitial free high strength (IF-HS) grade steel. The transformation temperature of IF-HS is determined at different cooling rates by a dilatometer in a thermo-mechanical simulator. The parameters for hot rolling and cold rolling are optimized based on the results of the simulator. The final cold rolled strip is batch annealed for the desired grade of IS-HS steel strip.

Full Text	FIELD OF INVENTION The present invention relates to a process for producing high strength formable steel for exterior panel of auto body. BACKGROUND OF INVENTION Most of the steel sheets used in the auto body are in the cold rolled and annealed condition in order to induce strength and desired crystallographic texture for achieving good formability. Traditionally, Al-killed EDD grade has been used in the automobile industry. Subsequently interstitial free (IF) grades have found an increasing use in the car body due to their superior formability. Though the formability of IF steel is very good (r~2.0), the strength is quite low (YS 160 MPa) leading m to deterioration in dent resistance properties as dent resistance is directly proportional to yield strength. A general guideline for assessing dent resistance is : Load to produce 0.1 mm deep dent = constant which accounts for geometry and panel stiffeners effects = yield strength after 2% strain and pant bake = as formed panel thickness = 2-2.4 depending on over all panel stiffeners. Consequently, the material is not suitable for outer body panels of automobiles. Moreover, if the thickness of an auto body panel is reduced to achieve weight reduction for improving fuel economy, the strength of the sheet is to be increased suitably to maintain the dent resistance. These considerations have prompted the development of high strength grades with good formability. The strength of IF steel can be enhanced by resorting to solid solution strengthening by addition of variety of substitutional elements like Mn, Si, P, etc. The addition of these elements impairs the formability to some extent. These elements also affect hot rolling, cold rolling annealing parameters, surface properties and final properties significantly. The use of Phosphorus is the most effective strengthening element. An addition of 0.01 wt% of phosphorus raises the strength by about 12 MPa, while 0.10% of Si and 0.10% of manganese individually contribute to 5 MPa increase in yield strength. However there are constraints in using these elements. The efficacy of P depends on the stabilising element used and it has tendency to segregate at ferrite grain boundaries and consequently results in strain- induced or cold-work embrittlement (fracture during dynamic straining of a severly formed component.) Secondly the extent of segregation depends on the chemical composition and processing route followed (more in batch annealed material as compared with continuous annealed product). A small addition of B improved the resistance of the steel to cold work embrittlement, but combined use of Nb and 8 has synergistic effect for formation of low temperature transformation product affecting hot rolling parameters and cold rollability of strip. The use of silicon and manganese requires large ferroalloy addition leading to many operational problems. Silicon also leads to adherent scale formation causing picking problems. Use of ferromanganese contributes increase in carbon leading to stabilisation problem. SUMMARY OF THE INVENTION An object of this invention is to propose a process for producing high strength formable steel. Another object of this invention is to propose a process for producing high strength formable steel for dent resistant applications. Still another object of this invention is to propose a process for producing high strength formable steel which is efficient. Further objects and advantages of this invention will be more apparent form the ensuing description. These objects are achieved by judicious selection of chemistry based on the considerations described earlier. Before hot rolling to strips the transformation temperature of interstitial free-high strength (IF-HS) are to be determined at different cooling rates using a dialatometer in a thermomechanical simulator like Geeble-1500. Based on the result of the thermochemical simulator the hot rolling parameters and cold rolling parameters are optimized. A process for producing high strength formable steel strip for exterior panel of auto body, comprising the steps of: selecting appropriately the chemistry of steel by adding substitutional elements like manganese, silicon, phosphorus, etc., for producing interstitial free high strength (IF-HS) grade steel; determining transformation temperature of (IF-HS) at different cooling rates by a dialometer in a thermo-mechanical simulator; optimizing parameters for hot rolling and cold rolling based on the results of the simulator; and batch annealing the final cold rolled strip for the desired grade of IS- HS steel strip. At the outset of the description which follows, it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the invention is not intended to be taken restrictively. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is a continuous cooling transformation diagram of interstitial free high strength formable steel. Fig. 2 shows typical transmission electron micrographs of high strength formable steel strip in hot rolled condition. DETAILED DESCRIPTION OF INVENTION After judicious selection of chemistry based on the considerations as detailed earlier, commercial heat of interstitial high strength grade can be made. Before hot rolling to strips, the transformation temperatures of IF—HS can be determined at different cooling rates using a dilatometer in a thermomechanical simulator like Gleeble—1500. Based on that, a continuous cooling transformation diagram can be plotted as shown in Fig. 1. It is evident from Fig. 1 that the transformation temperatures IF—HS are lower. In accordance with this invention, it has now been found that at the slowest cooling rate (0.50 C s-1) the microstruture consists of polygonal ferrite whereas at the highest cooling rate of 50 0C s-1 both polygonal and acicular ferrite are observed. The hardness of the continuously cooled samples show an increase with coaling rate. Based on the results of the thermomechanical simulator, the hot rolling parameters (finish rolling temperature, cooling strategy at run out table and cooling temperature) are selected. Hot rolled strip is characterized. The hot rolled strip is sufficiently soft for cold rolling with minimum 75% deformation. Detailed transmission electron microscopy of hot rolled steel can be carried out for studying microstructural detail and nature of precipitates. Typical transmission electron micrographs (TEMs) of hot roplled strip are shown in Fig. 2. The cold rolled strip after batch annealing display the following properties : Yield strength : 200-260 MPa, UTS : > 350 MPa, Eln (80mm GL) = 32 %, r- bar value > 1.6, n—value 0.22 High strength interstitial free steel for dent resistant applications is produced by judicious selection of chemistry,optimising hot rolling and cold rolling parameters. The cold rolled strip and annealed (batch) displays the following properties: Yield strength : 200-260 MPa, UTS: > 350 MPa, Eln (80mm 6L) = 32 M, r- bar value > 1.6, n-value: >0.22. WE CLAIM 1. A process for producing high strength formable steel strip for exterior panel of auto body, comprising the steps of: - selecting appropriately the chemistry of steel by adding substitutional elements like manganese, silicon, phosphorus, etc., for producing interstitial free high strength (IF-HS) grade steel; - determining transformation temperature of (IF-HS) at different cooling rates by a dialometer in a thermo-mechanical simulator; - optimizing parameters for hot rolling and cold rolling based on the results of the simulator; and - batch annealing the final cold rolled strip for the desired grade of IS-HS steel strip. 2. The process as claimed in claim 1, wherein the cooling rate used is from 0.5° C per second to 50° C per second. 3. The process as claimed in claim 1 or 2, wherein the batch annealed cold rolled strip has the following properties: Yield strength 200 - 260 MPa UTS > 350 MPa Eln (80 mm GL) = 32 % r-bar value > 1.6 n-value > 0.22 4. A process for producing high strength formable steel for exterior panel of auto body substantially as herein described and i 1 lustrated. Dated this 24th day of September, 2003. The invention provides a process for producing high strength formable steel strip for exterior panel of auto body. The chemistry of steel is appropriately selected by adding substitutional elements like manganese, silicon, phosphorus, etc. for producing interstitial free high strength (IF-HS) grade steel. The transformation temperature of IF-HS is determined at different cooling rates by a dilatometer in a thermo-mechanical simulator. The parameters for hot rolling and cold rolling are optimized based on the results of the simulator. The final cold rolled strip is batch annealed for the desired grade of IS-HS steel strip.

Full Text

FIELD OF INVENTION
The present invention relates to a process for producing high
strength formable steel for exterior panel of auto body.
BACKGROUND OF INVENTION
Most of the steel sheets used in the auto body are in the cold
rolled and annealed condition in order to induce strength
and desired crystallographic texture for achieving good
formability. Traditionally, Al-killed EDD grade has been used in
the automobile industry. Subsequently interstitial free (IF)
grades have found an increasing use in the car body due to their
superior formability. Though the formability of IF steel is very
good (r~2.0), the strength is quite low (YS 160 MPa) leading
m
to deterioration in dent resistance properties as dent resistance
is directly proportional to yield strength. A general guideline
for assessing dent resistance is :
Load to produce 0.1 mm deep dent
= constant which accounts for geometry and panel
stiffeners effects
= yield strength after 2% strain and pant bake
= as formed panel thickness
= 2-2.4 depending on over all panel stiffeners.
Consequently, the material is not suitable for outer body panels
of automobiles. Moreover, if the thickness of an auto body panel
is reduced to achieve weight reduction for improving fuel
economy, the strength of the sheet is to be increased suitably to
maintain the dent resistance. These considerations have prompted
the development of high strength grades with good formability.
The strength of IF steel can be enhanced by resorting to solid
solution strengthening by addition of variety of substitutional
elements like Mn, Si, P, etc. The addition of these elements
impairs the formability to some extent. These elements also
affect hot rolling, cold rolling annealing parameters, surface
properties and final properties significantly. The use of
Phosphorus is the most effective strengthening element. An
addition of 0.01 wt% of phosphorus raises the strength by about
12 MPa, while 0.10% of Si and 0.10% of manganese individually
contribute to 5 MPa increase in yield strength. However there are
constraints in using these elements. The efficacy of P depends on
the stabilising element used and it has tendency to segregate at
ferrite grain boundaries and consequently results in strain-
induced or cold-work embrittlement (fracture during dynamic
straining of a severly formed component.) Secondly the extent of
segregation depends on the chemical composition and processing
route followed (more in batch annealed material as
compared with continuous annealed product). A small addition of B
improved the resistance of the steel to cold work embrittlement,
but combined use of Nb and 8 has synergistic effect for formation
of low temperature transformation product affecting hot rolling
parameters and cold rollability of strip.
The use of silicon and manganese requires large ferroalloy
addition leading to many operational problems. Silicon also leads
to adherent scale formation causing picking problems. Use of
ferromanganese contributes increase in carbon leading to
stabilisation problem.
SUMMARY OF THE INVENTION
An object of this invention is to propose a process for producing
high strength formable steel.
Another object of this invention is to propose a process for
producing high strength formable steel for dent resistant
applications.
Still another object of this invention is to propose a process
for producing high strength formable steel which is efficient.
Further objects and advantages of this invention will be more
apparent form the ensuing description.
These objects are achieved by judicious selection of chemistry based on the
considerations described earlier. Before hot rolling to strips the transformation
temperature of interstitial free-high strength (IF-HS) are to be determined at different
cooling rates using a dialatometer in a thermomechanical simulator like Geeble-1500.
Based on the result of the thermochemical simulator the hot rolling parameters and cold
rolling parameters are optimized.
A process for producing high strength formable steel strip for exterior panel of auto
body, comprising the steps of: selecting appropriately the chemistry of steel by adding
substitutional elements like manganese, silicon, phosphorus, etc., for producing
interstitial free high strength (IF-HS) grade steel; determining transformation
temperature of (IF-HS) at different cooling rates by a dialometer in a thermo-mechanical
simulator; optimizing parameters for hot rolling and cold rolling based on the results of
the simulator; and batch annealing the final cold rolled strip for the desired grade of IS-
HS steel strip.
At the outset of the description which follows, it is to be understood that the ensuing
description only illustrates a particular form of this invention. However, such a particular
form is only an exemplary embodiment and the teachings of the invention is not
intended to be taken restrictively.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 is a continuous cooling transformation diagram of
interstitial free high strength formable steel.
Fig. 2 shows typical transmission electron micrographs of high
strength formable steel strip in hot rolled condition.
DETAILED DESCRIPTION OF INVENTION
After judicious selection of chemistry based on the
considerations as detailed earlier, commercial heat of
interstitial high strength grade can be made. Before hot rolling
to strips, the transformation temperatures of IF—HS can be
determined at different cooling rates using a dilatometer in a
thermomechanical simulator like Gleeble—1500.
Based on that, a continuous cooling transformation diagram can be
plotted as shown in Fig. 1. It is evident from Fig. 1 that the
transformation temperatures IF—HS are lower.
In accordance with this invention, it has now been found that at

the slowest cooling rate (0.50 C s-1) the microstruture consists of
polygonal ferrite whereas at the highest cooling rate of 50 0C s-1
both polygonal and acicular ferrite are observed. The hardness
of the continuously cooled samples show an increase with coaling
rate.
Based on the results of the thermomechanical simulator, the hot
rolling parameters (finish rolling temperature, cooling strategy
at run out table and cooling temperature) are selected. Hot
rolled strip is characterized. The hot rolled strip is
sufficiently soft for cold rolling with minimum 75% deformation.
Detailed transmission electron microscopy of hot rolled steel can
be carried out for studying microstructural detail and nature of
precipitates. Typical transmission electron micrographs (TEMs)
of hot roplled strip are shown in Fig. 2.
The cold rolled strip after batch annealing display the following
properties :
Yield strength : 200-260 MPa, UTS : > 350 MPa, Eln (80mm GL) =
32 %, r- bar value > 1.6, n—value 0.22
High strength interstitial free steel for dent resistant
applications is produced by judicious selection of
chemistry,optimising hot rolling and cold rolling parameters. The
cold rolled strip and annealed (batch) displays the following
properties:
Yield strength : 200-260 MPa, UTS: > 350 MPa, Eln (80mm 6L) = 32
M, r- bar value > 1.6, n-value: >0.22.
WE CLAIM
1. A process for producing high strength formable steel strip for exterior panel of
auto body, comprising the steps of:
- selecting appropriately the chemistry of steel by adding substitutional
elements like manganese, silicon, phosphorus, etc., for producing
interstitial free high strength (IF-HS) grade steel;
- determining transformation temperature of (IF-HS) at different cooling
rates by a dialometer in a thermo-mechanical simulator;
- optimizing parameters for hot rolling and cold rolling based on the results
of the simulator; and
- batch annealing the final cold rolled strip for the desired grade of IS-HS
steel strip.
2. The process as claimed in claim 1, wherein the cooling rate used is from 0.5° C
per second to 50° C per second.
3. The process as claimed in claim 1 or 2, wherein the batch annealed cold rolled
strip has the following properties:
Yield strength 200 - 260 MPa
UTS > 350 MPa
Eln (80 mm GL) = 32 %
r-bar value > 1.6
n-value > 0.22
4. A process for producing high strength formable steel for
exterior panel of auto body substantially as herein described and
i 1 lustrated.
Dated this 24th day of September, 2003.
The invention provides a process for producing high strength formable steel strip
for exterior panel of auto body. The chemistry of steel is appropriately selected
by adding substitutional elements like manganese, silicon, phosphorus, etc. for
producing interstitial free high strength (IF-HS) grade steel. The transformation
temperature of IF-HS is determined at different cooling rates by a dilatometer in
a thermo-mechanical simulator. The parameters for hot rolling and cold rolling
are optimized based on the results of the simulator. The final cold rolled strip is
batch annealed for the desired grade of IS-HS steel strip.

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Patent Number

222923

Indian Patent Application Number

384/CAL/2002

PG Journal Number

35/2008

Publication Date

29-Aug-2008

Grant Date

27-Aug-2008

Date of Filing

24-Jun-2002

Name of Patentee

GOPE, N.

Applicant Address

TATA STEEL LIMITED, RESEARCH AND DEVELOPMENT DIVISION, JAMSHEDPUR

Inventors:

#	Inventor's Name	Inventor's Address
1	GOPE, N.	TATA STEEL LIMITED, RESEARCH AND DEVELOPMENT DIVISION, JAMSHEDPUR 831 001
2	TIWARY, S.K	TATA STEEL LIMITED, RESEARCH AND DEVELOPMENT DIVISION, JAMSHEDPUR 831 001
3	MOHANTY, O.N.	TATA STEEL LIMITED, RESEARCH AND DEVELOPMENT DIVISION, JAMSHEDPUR 831 001

PCT International Classification Number

C21C 5/54

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA