Title of Invention	A METHOD OF PRODUCING PHOSPHORUS FREE INTERSTITIAL FREE HIGH STRENGTH (IFHS) FORMABLE AND WELDABLE STEEL SHEET / STRIP WITH IMPROVED DRAWABILITY
Abstract	This invention relates to a method of producing phosphorus free interstitial free high stength (IFHS), improved drawable and weldable steel sheet / strip comprising the steps of: preparing a steel slab of IFHS grade without phosphorous addition during melting having a composition in weight % of C - < 0.0030, Mn - 0.5 - 0.8, S - < 0.01, P - < 0.012, Si - < 0.015, Al - 0.04 - 0.05, Ti - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - < 30; reheating the slab at 1150 - 120° C; finish rolling the steel strip / sheet at temperature within 900 - 910° C above Ar3 temperature of the steel; coiling the strip / sheet at 700° C; cold rolling the coiled strip / sheet with 73 - 77 % reduction; batch annealing the cold rolled sheet / strip maintaining hot spot temperature 720° C and cold spot temperature 690° C and skin pass rolling the annealed sheet / strip with 0.5 % temper elongation.

Title of Invention

A METHOD OF PRODUCING PHOSPHORUS FREE INTERSTITIAL FREE HIGH STRENGTH (IFHS) FORMABLE AND WELDABLE STEEL SHEET / STRIP WITH IMPROVED DRAWABILITY

Abstract

This invention relates to a method of producing phosphorus free interstitial free high stength (IFHS), improved drawable and weldable steel sheet / strip comprising the steps of: preparing a steel slab of IFHS grade without phosphorous addition during melting having a composition in weight % of C - < 0.0030, Mn - 0.5 - 0.8, S - < 0.01, P - < 0.012, Si - < 0.015, Al - 0.04 - 0.05, Ti - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - < 30; reheating the slab at 1150 - 120° C; finish rolling the steel strip / sheet at temperature within 900 - 910° C above Ar3 temperature of the steel; coiling the strip / sheet at 700° C; cold rolling the coiled strip / sheet with 73 - 77 % reduction; batch annealing the cold rolled sheet / strip maintaining hot spot temperature 720° C and cold spot temperature 690° C and skin pass rolling the annealed sheet / strip with 0.5 % temper elongation.

Full Text	-2- FIELD OF THE INVENTION The present invention relates to the development of batch annealed high strength P free IF steel sheet. More particularly the invention relates to development of a high strength of minimum UTS level of 340 MPA and improved formable and weldable steel grade with improved drawability adaptable mainly for manufacturing automotive body panels. BACKGROUND OF THE INVENTION The modern automobile industry is driven by some important factors, such as (a) Energy saving, or less fuel consumption (b) Less emission (c) Improved passenger safety -3- In order to satisfy all these basic conditions, the modern car manufacturing involves in development of more high strength steel grades, which should also be combined with certain degree of formability. Interstitial free (IF) steel grades (both mild and high strength grades) are usually processed through continuous annealing route. However, due to the lack of continuous annealing facilities in India, the steel manufacturers are compelled to process interstitial free steels through batch annealing route. While soft interstitial free steel grades can be manufactured conveniently through batch annealing route, it is difficult to process the high strength versions of the same family, because of the complex physical metallurgy involved in processing of these grades. It is now possible to manufacture IFHS (interstitial free high strength) grade through batch annealing, with appreciable drawability. However, the drawability of IFHS (interstitial free high strength) grades is always less than that of soft IF steel, even if it is manufactured through continuous annealing route. -4- The present invention is related with development of a new chemical composition IFHS (interstitial free high strength) grade with minimum UTS of 340 MPa. The drawability of this batch annealed steel is quite high, and in fact very close to that of soft IF steel. Chemical composition and processing route have been designed in a way as to develop excellent drawability, combined with strength and adequate resistance against secondary work embrittlement. Prior art drawbacks The crux of the present invention is the development of a new chemical composition. Usually, the IFHS (interstitial free high strength) steels are strengthened by the addition of P, which is considered as a very effective solid solution strengthener. Mn is added to IFHS (interstitial free high strength) steels, as a supporting strengthening element. -5- Drawbacks connected with hither to known processes and advantage of the present invention Addition of P leads to a few problems, some of which are listed below: • Segregation of P at grain boundaries leads to secondary work embrittlement effect. • B is added to control this effect, but B itself affects drawability in a deleterious manner. • P has a strong affinity towards the stabilizing elements like Ti, thereby forming FeTiP type precipitates, this process drastically affects the stabilization of steel, as well as the drawability. • The above effects can be controlled considerably through CAL processing, but the processing of IFHS is extremely difficult through BAF. -6- Objective of the invention It is therefore an object of the present invention to propose a new concept of chemical composition, in which P will not be added at all as a strengthening element in IFHS steel. Another object of the invention is to obtain steel without any secondary work embrittlement due to non addition of P and therefore avoiding any need to B addition; however, to be on safe side, a few steel makers add a small quantity of B (2-3 ppm) even in the soft IF grades, in order to make these grades suitable for using in the extreme cold service conditions, this practice may be included in this case also, under mutual agreement between the supplier and customer. Yet another object of the invention is to secure any chance of precipitate formation like FeTiP. -7- A still another object of the invention is to conveniently process through batch annealing route. A further object of the invention is to obtain the steel very close to that of high quality soft IF steel. A still further object of the invention is to result poor dent resistance of soft IF steel and also at the same time developing high strength steel grades. The present invention has a potentiality to replace the soft IF (interstial free) grade in many applications, due to the high Lankford value of the processed material. SUMMARY OF THE INVENTION The invented product is a steel grade for automotive stamping applications. The grade has ultra low carbon based steel composition, strengthened by suitable addition of manganese, in order to achieve a minimum strength level of 340 MPa (UTS). -8- The processing schedules, including steel making, hot rolling, cold rolling and annealing, have been determined appropriately so as to achieve desired drawability and strength. The steel grades are also adequately weldable. Illustration of the invention Chemical composition of the proposed steel is stated as below: c% Mn% S% P% Si% Al% Ti% Nb% N (ppm) Physical processing (a) Hot charging in steel making furnace is not required in this grade, since there is no added P. (b) Slab drop out around - 1200° C. (c) Finish rolling higher than Ar3 temperature - 900 - 910° C. (d) Coiling at 700° C. 9 (e) cold rolling thee hot rolled strip at 73-77 % reduction (f) Batch annealing is carried out with hot spot temperature 720° C and cold spot temperature 690° C. Test results (two results are given below): Steel Grades Width (mm) Thickness (mm) YS (MPa) UTS (MPa) %EI r-bar IFHS-340 Without P 1180 0.8 184 341 45 2.11 1160 0.9 183 345 44 2.01 Desirable properties 145-245 340 min 40 min 1.4 min Secondary work embrittlement property: OK upto -30° C . Variables (a) Chemical parameters: Mn - Increases strength of the steel; Nb also adds to strength to some extent. -10- (b) Physical parameters: Slab drop out temperature, finish rolling temperature, coiling temperature, cold reduction and batch annealing temperature, all have significant effects on final mechanical properties of all the steel grades. The invention as herein narrated with an exemplary embodiment should not be read and construed in a restrictive manner as various modifications, alterations and adaptations are possible within the scope and ambit of the invention as defined in the appended claims. -11- WE CLAIM 1. A method of producing phosphorus free interstitial free high stength (IFHS), formable and weldable steel sheet / strip with improved drawability comprising the steps of: - preparing a steel slab of IFHS grade without phosphorous addition during melting, having a composition in weight % of C - Mn - 0.5 - 0.8, S - 0.05, Ti - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - - reheating the slab at 1150-1200°C. - finish rolling the steel strip / sheet at temperature within 900 - 910° C, above Ar3 temperature of the steel; - coiling the strip / sheet at 700° C; - cold rolling the coiled strip / sheet with 73 - 77 % reduction; -12- - batch annealing the cold rolled sheet / strip maintaining hot spot temperature 720° C and cold spot temperature 690° C and - skin pass rolling the annealed sheet / strip with 0.5 % temper elongation. 2. A method of producing IFHS steel sheet / strip as claimed in claim 1, wherein steel sheet resulted has YS - 145 - 245 MPa and UTS - 340 - 360 MPa. 3. A method of producing IFHS steel sheet / strip for automotive body panel as claimed in claim 1, wherein secondary embrittlement of the steel is avoided due to non-addition of P during steel making and also avoiding any Boron addition to control secondary work embrittlement. 4. A method as claimed in claims 1 and 3, wherein Boron is added in the steel in case the steel sheet / strip is used in extreme cold service condition. 5. A method as claimed in claims 1, wherein the steel has high drawability having % elongation of minimum 40 and minimum r-bar of 1.4. -13- 6. A method of producing IFHS steel strip / sheet as claimed in claim 1, wherein the Manganese content is usually maintained at 0.5 - 0.6 %, but for increased UTS Manganese content is kept at 0.7 - 0.8 %. 7. A steel strip / sheet having composition in weight % C - 0.8, S - 0.04 - 0.05, N (ppm) - 8. A steel sheet / strip produced according to method of claims 1. Dated this 2nd day of January 2003. This invention relates to a method of producing phosphorus free interstitial free high stength (IFHS), improved drawable and weldable steel sheet / strip comprising the steps of: preparing a steel slab of IFHS grade without phosphorous addition during melting having a composition in weight % of C - 0.0030, Mn - 0.5 - 0.8, S - - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - 120° C; finish rolling the steel strip / sheet at temperature within 900 - 910° C above Ar3 temperature of the steel; coiling the strip / sheet at 700° C; cold rolling the coiled strip / sheet with 73 - 77 % reduction; batch annealing the cold rolled sheet / strip maintaining hot spot temperature 720° C and cold spot temperature 690° C and skin pass rolling the annealed sheet / strip with 0.5 % temper elongation.

Full Text

-2-
FIELD OF THE INVENTION
The present invention relates to the development of batch annealed high
strength P free IF steel sheet. More particularly the invention relates to
development of a high strength of minimum UTS level of 340 MPA and improved
formable and weldable steel grade with improved drawability adaptable mainly
for manufacturing automotive body panels.
BACKGROUND OF THE INVENTION
The modern automobile industry is driven by some important factors, such as
(a) Energy saving, or less fuel consumption
(b) Less emission
(c) Improved passenger safety

-3-
In order to satisfy all these basic conditions, the modern car manufacturing
involves in development of more high strength steel grades, which should also
be combined with certain degree of formability.
Interstitial free (IF) steel grades (both mild and high strength grades) are usually
processed through continuous annealing route. However, due to the lack of
continuous annealing facilities in India, the steel manufacturers are compelled to
process interstitial free steels through batch annealing route.
While soft interstitial free steel grades can be manufactured conveniently
through batch annealing route, it is difficult to process the high strength versions
of the same family, because of the complex physical metallurgy involved in
processing of these grades. It is now possible to manufacture IFHS (interstitial
free high strength) grade through batch annealing, with appreciable drawability.
However, the drawability of IFHS (interstitial free high strength) grades is always
less than that of soft IF steel, even if it is manufactured through continuous
annealing route.

-4-
The present invention is related with development of a new chemical
composition IFHS (interstitial free high strength) grade with minimum UTS of
340 MPa. The drawability of this batch annealed steel is quite high, and in fact
very close to that of soft IF steel. Chemical composition and processing route
have been designed in a way as to develop excellent drawability, combined with
strength and adequate resistance against secondary work embrittlement.
Prior art drawbacks
The crux of the present invention is the development of a new chemical
composition. Usually, the IFHS (interstitial free high strength) steels are
strengthened by the addition of P, which is considered as a very effective solid
solution strengthener. Mn is added to IFHS (interstitial free high strength)
steels, as a supporting strengthening element.

-5-
Drawbacks connected with hither to known
processes and advantage of the present invention
Addition of P leads to a few problems, some of which are listed below:
• Segregation of P at grain boundaries leads to secondary work
embrittlement effect.
• B is added to control this effect, but B itself affects drawability in a
deleterious manner.
• P has a strong affinity towards the stabilizing elements like Ti, thereby
forming FeTiP type precipitates, this process drastically affects the
stabilization of steel, as well as the drawability.
• The above effects can be controlled considerably through CAL processing,
but the processing of IFHS is extremely difficult through BAF.

-6-
Objective of the invention
It is therefore an object of the present invention to propose a new concept of
chemical composition, in which P will not be added at all as a strengthening
element in IFHS steel.
Another object of the invention is to obtain steel without any secondary work
embrittlement due to non addition of P and therefore avoiding any need to B
addition; however, to be on safe side, a few steel makers add a small quantity of
B (2-3 ppm) even in the soft IF grades, in order to make these grades suitable
for using in the extreme cold service conditions, this practice may be included in
this case also, under mutual agreement between the supplier and customer.
Yet another object of the invention is to secure any chance of precipitate
formation like FeTiP.

-7-
A still another object of the invention is to conveniently process through batch
annealing route.
A further object of the invention is to obtain the steel very close to that of high
quality soft IF steel.
A still further object of the invention is to result poor dent resistance of soft IF
steel and also at the same time developing high strength steel grades.
The present invention has a potentiality to replace the soft IF (interstial free)
grade in many applications, due to the high Lankford value of the processed
material.
SUMMARY OF THE INVENTION
The invented product is a steel grade for automotive stamping applications. The
grade has ultra low carbon based steel composition, strengthened by suitable
addition of manganese, in order to achieve a minimum strength level of 340 MPa
(UTS).

-8-
The processing schedules, including steel making, hot rolling, cold rolling and
annealing, have been determined appropriately so as to achieve desired
drawability and strength. The steel grades are also adequately weldable.
Illustration of the invention
Chemical composition of the proposed steel is stated as below:

c% Mn% S% P% Si% Al% Ti% Nb% N (ppm)
Physical processing
(a) Hot charging in steel making furnace is not required in this grade, since
there is no added P.
(b) Slab drop out around - 1200° C.
(c) Finish rolling higher than Ar3 temperature - 900 - 910° C.
(d) Coiling at 700° C.

9
(e) cold rolling thee hot rolled strip at 73-77 % reduction
(f) Batch annealing is carried out with hot spot temperature 720° C and cold
spot temperature 690° C.
Test results (two results are given below):

Steel
Grades Width
(mm) Thickness
(mm) YS
(MPa) UTS
(MPa) %EI r-bar
IFHS-340
Without P 1180 0.8 184 341 45 2.11

1160 0.9 183 345 44 2.01

Desirable properties 145-245 340 min 40 min 1.4 min
Secondary work embrittlement property: OK upto -30° C .
Variables
(a) Chemical parameters: Mn - Increases strength of the steel; Nb also adds
to strength to some extent.

-10-
(b) Physical parameters: Slab drop out temperature, finish rolling
temperature, coiling temperature, cold reduction and batch annealing
temperature, all have significant effects on final mechanical properties of
all the steel grades.
The invention as herein narrated with an exemplary embodiment should not be
read and construed in a restrictive manner as various modifications, alterations
and adaptations are possible within the scope and ambit of the invention as
defined in the appended claims.

-11-
WE CLAIM
1. A method of producing phosphorus free interstitial free high stength
(IFHS), formable and weldable steel sheet / strip with improved
drawability comprising the steps of:
- preparing a steel slab of IFHS grade without phosphorous addition
during melting, having a composition in weight % of C - Mn - 0.5 - 0.8, S - 0.05, Ti - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - - reheating the slab at 1150-1200°C.
- finish rolling the steel strip / sheet at temperature within 900 -
910° C, above Ar3 temperature of the steel;
- coiling the strip / sheet at 700° C;
- cold rolling the coiled strip / sheet with 73 - 77 % reduction;

-12-
- batch annealing the cold rolled sheet / strip maintaining hot spot
temperature 720° C and cold spot temperature 690° C and
- skin pass rolling the annealed sheet / strip with 0.5 % temper
elongation.

2. A method of producing IFHS steel sheet / strip as claimed in claim 1,
wherein steel sheet resulted has YS - 145 - 245 MPa and UTS - 340 -
360 MPa.
3. A method of producing IFHS steel sheet / strip for automotive body panel
as claimed in claim 1, wherein secondary embrittlement of the steel is
avoided due to non-addition of P during steel making and also avoiding
any Boron addition to control secondary work embrittlement.
4. A method as claimed in claims 1 and 3, wherein Boron is added in the
steel in case the steel sheet / strip is used in extreme cold service
condition.
5. A method as claimed in claims 1, wherein the steel has high drawability
having % elongation of minimum 40 and minimum r-bar of 1.4.

-13-
6. A method of producing IFHS steel strip / sheet as claimed in claim 1,
wherein the Manganese content is usually maintained at 0.5 - 0.6 %, but
for increased UTS Manganese content is kept at 0.7 - 0.8 %.
7. A steel strip / sheet having composition in weight % C - 0.8, S - 0.04 - 0.05, N (ppm) - 8. A steel sheet / strip produced according to method of claims 1.
Dated this 2nd day of January 2003.

This invention relates to a method of producing phosphorus free interstitial free
high stength (IFHS), improved drawable and weldable steel sheet / strip
comprising the steps of: preparing a steel slab of IFHS grade without
phosphorous addition during melting having a composition in weight % of C - 0.0030, Mn - 0.5 - 0.8, S - - 0.05 - 0.06, Nb - 0.04 - 0.05, N - (ppm) - 120° C; finish rolling the steel strip / sheet at temperature within 900 - 910° C
above Ar3 temperature of the steel; coiling the strip / sheet at 700° C; cold
rolling the coiled strip / sheet with 73 - 77 % reduction; batch annealing the cold
rolled sheet / strip maintaining hot spot temperature 720° C and cold spot
temperature 690° C and skin pass rolling the annealed sheet / strip with 0.5 %
temper elongation.

Documents:

00021-kol-2008-abstract.pdf

00021-kol-2008-claims.pdf

00021-kol-2008-description complete.pdf

00021-kol-2008-form 1.pdf

00021-kol-2008-form 2.pdf

00021-kol-2008-form 3.pdf

00021-kol-2008-gpa.pdf

21-KOL-2008-(10-12-2013)-ABSTRACT.pdf

21-KOL-2008-(10-12-2013)-CLAIMS.pdf

21-KOL-2008-(10-12-2013)-CORRESPONDENCE.pdf

21-KOL-2008-(10-12-2013)-DESCRIPTION (COMPLETE).pdf

21-KOL-2008-(10-12-2013)-FORM-1.pdf

21-KOL-2008-(10-12-2013)-FORM-2.pdf

21-KOL-2008-(10-12-2013)-OTHERS.pdf

21-KOL-2008-(20-03-2014)-CLAIMS.pdf

21-KOL-2008-(20-03-2014)-CORRESPONDENCE.pdf

21-KOL-2008-CORRESPONDENCE OTHERS.pdf

21-kol-2008-form 18.pdf

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Patent Number

263079

Indian Patent Application Number

21/KOL/2008

PG Journal Number

41/2014

Publication Date

10-Oct-2014

Grant Date

30-Sep-2014

Date of Filing

02-Jan-2008

Name of Patentee

TATA STEEL LIMITED

Applicant Address

JAMSHEDPUR

Inventors:

#	Inventor's Name	Inventor's Address
1	BASUDEV BHATTACHARYA	NOT APPLICABLE

PCT International Classification Number

B05D7/14, C21D1/19

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA