Title of Invention	" A METHOD OF MANUFACTURING A HIGH STRENGTH AND HIGH NORMAL ANISOTROPY COLD ROLLED DUAL PHASE STEEL "
Abstract	This invention relates to a method of manufacturing a high strength and high plastic anisotropy dual phase steel comprising the steps of forming a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities; hot rolling the slab through finish rolling at ≥ 870° retaining the steel in the temperature range of 800° C to 700° C; cooling the steel sheet at a cooling rate of ≥ 30° C / sec and coiling at ≤ 520° C to produce a three phase steel sheet consisting of ferrite, bainite and martensite; the steel sheet being further austenised by heating at a temperature above 900° C for some time and furnace cooled to result in microstructure of ferrite, pearlite and bainite; the steel was then cold rolled in the range of 30% to 60%, the stock being finally heated in the temperature range of 700 - 800° C for 30 seconds to 4 minutes and quenched in water resulting a ferrite-martensite dual phase steel.

Title of Invention

" A METHOD OF MANUFACTURING A HIGH STRENGTH AND HIGH NORMAL ANISOTROPY COLD ROLLED DUAL PHASE STEEL "

Abstract

This invention relates to a method of manufacturing a high strength and high plastic anisotropy dual phase steel comprising the steps of forming a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities; hot rolling the slab through finish rolling at ≥ 870° retaining the steel in the temperature range of 800° C to 700° C; cooling the steel sheet at a cooling rate of ≥ 30° C / sec and coiling at ≤ 520° C to produce a three phase steel sheet consisting of ferrite, bainite and martensite; the steel sheet being further austenised by heating at a temperature above 900° C for some time and furnace cooled to result in microstructure of ferrite, pearlite and bainite; the steel was then cold rolled in the range of 30% to 60%, the stock being finally heated in the temperature range of 700 - 800° C for 30 seconds to 4 minutes and quenched in water resulting a ferrite-martensite dual phase steel.

Full Text	FIELD OF THE INVENTION The present invention is related to the production of high strength steel in the range of 700 MPa to 850 MPa with high formability in cold rolled and annealed condition. More specifically the proposed invention has derived steel sheets through characteristic process steps development on maintaining plastic anisotropy of 1.3 -1.5. BACKGROUND OF THE INVENTION The outer body parts of automotives (roof, bonnet, fender, pillars etc.) and white goods require high strength along with good formability. There have been a number of steels developed for this purpose. However, the dual phase steels available in the market have poor formability and cannot be formed easily. In order to have good formability, the yield strength has to be low; at the same time tensile strength of these steels must be high enough to improve crash worthiness. High strength ferrite-martensite dual phase steels have been developed keeping the above requirements in mind. Automotive steel sheets are required to have excellent press formability because many of them are press formed into automotive parts. The formability is represented by plastic anisotropy. So far the dual phase steels produced have low values of plastic anisotropy, therefore these cannot be used as automotive parts requiring high formability. This invention proposes a novel method to improve the plastic anisotropy of dual phase steels besides maintaining high strength. DESCRIPTION OF THE INVENTION According to one objective of the invention it is proposed to develop a high strength ferrite martensite dual phase steel having plastic anisotropy greater than 1 produced by cold rolling and annealing. Another objective of the invention is to produce high strength steel of 700 to 850 MPa and high plastic anisotropy of 1.3 - 1.5 having final compositions in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance substantially Fe and incidental impurities. A still another objective of the invention is to provide a novel sequential heat treatment procedure of hot rolling a steel slab of the above mentioned composition at austenising temperature range, cooling, coiling at ≤ 520° C, further heating to austenising temperature, annealing, cold rolling, heating to 700 - 800° C and water quenching to obtain dual phase ferrite-martensite with plastic anisotropy greater than 1. The property of a material is called the normal anisotropy and is commonly evaluated in terms of rm or r. This is defined by the relationship, rm = (r0 + r45 + r90) / 4, where the subscripts 0, 45 and 90 refer to inclinations of the longitudinal axes of tensile test pieces to the rolling direction of the sheet and each individual r-value is the ratio of width / thickness strain, as measured in a simple tensile test. After thorough studies and experiments it has been found that cold rolled steels with high stren gths and high plastic anisotropy can be produced by obtaining a microstructure consisting of ferrite and martensite phases. This is quite different from conventional dual phase steels where plastic anisotropy of more than one lhas not yet been reported. The present invention overcomes the limitation for application of dual phase steel in outer body of automotive steels. The present invention is directed to the production of high strength and high plastic anisotropy cold rolled steels having compositions of about 0.08 to 0.2 weight % of C, about 0.2 to 1 weight % Si, about 1.7 weight % or less of Mn, about 0.02 weight % or less P, about 0.03 weight % or less S, about 0.04 weight % of Al, about 0.2 weight % or less Cr, the balance being substantially Fe and incidental impurities. According to the invention there is provided a method of manufacturing a high strength and high plastic anisotropy dual phase steel comprising the steps of forming a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities; hot rolling the slab through finish rolling at ≥ 870° retaining the steel in the temperature range of 800° C to 700° C; cooling the steel sheet at a cooling rate of ≥: 30° C / sec and coiling at ≤ 520° C to produce a three phase steel sheet consisting of ferrite, bainite and martensite; the steel sheet being further austenised by heating at a temperature above 900° C for some time and furnace cooled to result in microstructure of ferrite, pearlite and bainite; the steel was then cold rolled in the range of 30% to 60%, the stock being finally heated in the temperature range of 700 - 800° C for 30 seconds to 4 minutes and quenched in water resulting a ferrite-martensite dual phase steel. The invention will be better understood from the following description with reference to microstructure of resulted steel and example. Figure 1 represents a microstructure consisting of elongated martensite in matrix of ferrite derived according to the present invention. Steel slab compositions were prepared to conduct the different experiments for the present invention. The compositions consisting essentially of 0.08 to 0.2 weight % of C, 0.2 to 1 weight % Si, ≤ 1.7 weight % of Mn, ≤ 0.02 weight % P, ≤ 0.03 weight % S, ≤ 0.04 weight % of Al, ≤ 0.2 weight % Cr, the balance being substantially Fe and incident impurities. All the steel slabs were hot-rolled under various conditions to manufacture steel sheets each having a thickness of 3.00 mm. Mechanical properties and microstructural characteristics were determined for the hot rolled steels from the test samples as illustrated in Table 1. The steels in the present invention have a tensile strength in the range of 700 MPa to 850 MPa and good formability. Presence of Cr suppresses the formation of pearlite and hence promotes bainitic transformation. Of all the common alloying elements the effect of chromium in promoting hardenability is maximum. It makes the steel apt for oil or air hardening. It reduces the critical cooling rate required for martensite formation, increases hardenability and thus improves the aptitude for heat treatment. Figure 1 shows a typical microstructure of one of the invented steels. Presence of elongated martensite in ferrite matrix is the special feature of this microstructure. The final ferrite-martensite structure is sufficient to produce a tensile strength of 700 to 850 MPa and the banded morphology causes high plastic anisotropy. The invention as narrated and illustrated herein 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 encompassed appended claims. WE CLAIM: 1. A method of manufacturing a high strength and high plastic anisotropy dual phase steel characterized in that a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities is subjected to the step of hot rolling and finish rolling at ≥ 870°, retaining the steel in the temperature range of 800°C to 700°C; cooling the steel sheet at a cooling rate of ≥ 30°C / sec and coiling at ≤ 520° C to produce a three phase steel sheet consisting of ferrite, bainite and martensite; the steel sheet being further austenised by heating at a temperature above 900°C for some time and furnace cooling to result in microstructure of ferrite, pearlite and bainite, followed by rolling cold in the range of 30% to 60%, the stock being finally heated in the temperature range of 700 - 800°C for 30 seconds to 4 minutes and quenched in water resulting a ferrite-martensite dual phase steel. 2. A method of manufacturing steel as claimed in claim 1, wherein the plastic anisotropy achieved in the resultant steel is more than 1 and in the range of 1.3 to 1.5 with high strength in the range of 700 - 850 MPa. 3. A high strength and high plastic anisotropy dual phase steel produced according to method of claim 1 having composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being substantially Fe and incidental impurities. 4. A method of manufacturing a high strength and high plastic anisotropy dual phase steel as herein described and illustrated. ABSTRACT A METHOD OF MANUFACTURING A HIGH STRENGTH AND HIGH PLASTIC ANISOTROPY DUAL PHASE STEEL This invention relates to a method of manufacturing a high strength and high plastic anisotropy dual phase steel comprising the steps of forming a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities; hot rolling the slab through finish rolling at ≥ 870° retaining the steel in the temperature range of 800° C to 700° C; cooling the steel sheet at a cooling rate of ≥ 30° C / sec and coiling at ≤ 520° C to produce a three phase steel sheet consisting of ferrite, bainite and martensite; the steel sheet being further austenised by heating at a temperature above 900° C for some time and furnace cooled to result in microstructure of ferrite, pearlite and bainite; the steel was then cold rolled in the range of 30% to 60%, the stock being finally heated in the temperature range of 700 - 800° C for 30 seconds to 4 minutes and quenched in water resulting a ferrite-martensite dual phase steel.

Full Text

FIELD OF THE INVENTION
The present invention is related to the production of high strength steel in the
range of 700 MPa to 850 MPa with high formability in cold rolled and annealed
condition. More specifically the proposed invention has derived steel sheets
through characteristic process steps development on maintaining plastic
anisotropy of 1.3 -1.5.
BACKGROUND OF THE INVENTION
The outer body parts of automotives (roof, bonnet, fender, pillars etc.) and white
goods require high strength along with good formability. There have been a
number of steels developed for this purpose. However, the dual phase steels
available in the market have poor formability and cannot be formed easily. In
order to have good formability, the yield strength has to be low; at the same
time tensile strength of these steels must be high enough to improve crash
worthiness.

High strength ferrite-martensite dual phase steels have been developed keeping
the above requirements in mind. Automotive steel sheets are required to have
excellent press formability because many of them are press formed into
automotive parts. The formability is represented by plastic anisotropy. So far
the dual phase steels produced have low values of plastic anisotropy, therefore
these cannot be used as automotive parts requiring high formability. This
invention proposes a novel method to improve the plastic anisotropy of dual
phase steels besides maintaining high strength.
DESCRIPTION OF THE INVENTION
According to one objective of the invention it is proposed to develop a high
strength ferrite martensite dual phase steel having plastic anisotropy greater
than 1 produced by cold rolling and annealing.

Another objective of the invention is to produce high strength steel of 700 to 850
MPa and high plastic anisotropy of 1.3 - 1.5 having final compositions in weight
% of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2
Cr, the balance substantially Fe and incidental impurities.
A still another objective of the invention is to provide a novel sequential heat
treatment procedure of hot rolling a steel slab of the above mentioned
composition at austenising temperature range, cooling, coiling at ≤ 520° C,
further heating to austenising temperature, annealing, cold rolling, heating to
700 - 800° C and water quenching to obtain dual phase ferrite-martensite with
plastic anisotropy greater than 1.
The property of a material is called the normal anisotropy and is commonly
evaluated in terms of rm or r. This is defined by the relationship, rm = (r0 + r45 +
r90) / 4, where the subscripts 0, 45 and 90 refer to inclinations of the longitudinal
axes of tensile test pieces to the rolling direction of the sheet and each individual
r-value is the ratio of width / thickness strain, as measured in a simple tensile
test.

After thorough studies and experiments it has been found that cold rolled steels
with high stren gths and high plastic anisotropy can be produced by obtaining a
microstructure consisting of ferrite and martensite phases. This is quite different
from conventional dual phase steels where plastic anisotropy of more than one
lhas not yet been reported. The present invention overcomes the limitation for
application of dual phase steel in outer body of automotive steels.
The present invention is directed to the production of high strength and high
plastic anisotropy cold rolled steels having compositions of about 0.08 to 0.2
weight % of C, about 0.2 to 1 weight % Si, about 1.7 weight % or less of Mn,
about 0.02 weight % or less P, about 0.03 weight % or less S, about 0.04 weight
% of Al, about 0.2 weight % or less Cr, the balance being substantially Fe and
incidental impurities.

According to the invention there is provided a method of manufacturing a high
strength and high plastic anisotropy dual phase steel comprising the steps of
forming a steel slab of composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤
1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and
incidental impurities; hot rolling the slab through finish rolling at ≥ 870° retaining
the steel in the temperature range of 800° C to 700° C; cooling the steel sheet at
a cooling rate of ≥: 30° C / sec and coiling at ≤ 520° C to produce a three phase
steel sheet consisting of ferrite, bainite and martensite; the steel sheet being
further austenised by heating at a temperature above 900° C for some time and
furnace cooled to result in microstructure of ferrite, pearlite and bainite; the steel
was then cold rolled in the range of 30% to 60%, the stock being finally heated
in the temperature range of 700 - 800° C for 30 seconds to 4 minutes and
quenched in water resulting a ferrite-martensite dual phase steel.
The invention will be better understood from the following description with
reference to microstructure of resulted steel and example.
Figure 1 represents a microstructure consisting of elongated martensite in
matrix of ferrite derived according to the present invention.

Steel slab compositions were prepared to conduct the different experiments for
the present invention. The compositions consisting essentially of 0.08 to 0.2
weight % of C, 0.2 to 1 weight % Si, ≤ 1.7 weight % of Mn, ≤ 0.02 weight % P,
≤ 0.03 weight % S, ≤ 0.04 weight % of Al, ≤ 0.2 weight % Cr, the balance
being substantially Fe and incident impurities. All the steel slabs were hot-rolled
under various conditions to manufacture steel sheets each having a thickness of
3.00 mm. Mechanical properties and microstructural characteristics were
determined for the hot rolled steels from the test samples as illustrated in Table
1.

The steels in the present invention have a tensile strength in the range of 700
MPa to 850 MPa and good formability. Presence of Cr suppresses the formation
of pearlite and hence promotes bainitic transformation. Of all the common
alloying elements the effect of chromium in promoting hardenability is maximum.
It makes the steel apt for oil or air hardening. It reduces the critical cooling rate
required for martensite formation, increases hardenability and thus improves the
aptitude for heat treatment.
Figure 1 shows a typical microstructure of one of the invented steels. Presence
of elongated martensite in ferrite matrix is the special feature of this
microstructure. The final ferrite-martensite structure is sufficient to produce a
tensile strength of 700 to 850 MPa and the banded morphology causes high
plastic anisotropy.
The invention as narrated and illustrated herein 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 encompassed appended claims.

WE CLAIM:
1. A method of manufacturing a high strength and high plastic
anisotropy dual phase steel characterized in that a steel slab of
composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤1.7 Mn, ≤ 0.02 P, ≤
0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental
impurities is subjected to the step of hot rolling and finish rolling at ≥
870°, retaining the steel in the temperature range of 800°C to 700°C;
cooling the steel sheet at a cooling rate of ≥ 30°C / sec and coiling at ≤
520° C to produce a three phase steel sheet consisting of ferrite, bainite
and martensite; the steel sheet being further austenised by heating at a
temperature above 900°C for some time and furnace cooling to result in
microstructure of ferrite, pearlite and bainite, followed by rolling cold in
the range of 30% to 60%, the stock being finally heated in the
temperature range of 700 - 800°C for 30 seconds to 4 minutes and
quenched in water resulting a ferrite-martensite dual phase steel.
2. A method of manufacturing steel as claimed in claim 1, wherein the
plastic anisotropy achieved in the resultant steel is more than 1 and in
the range of 1.3 to 1.5 with high strength in the range of 700 - 850 MPa.

3. A high strength and high plastic anisotropy dual phase steel produced
according to method of claim 1 having composition in weight % of 0.08 to
0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤ 0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the
balance being substantially Fe and incidental impurities.
4. A method of manufacturing a high strength and high plastic
anisotropy dual phase steel as herein described and illustrated.

ABSTRACT

A METHOD OF MANUFACTURING A HIGH STRENGTH AND
HIGH PLASTIC ANISOTROPY DUAL PHASE STEEL
This invention relates to a method of manufacturing a high strength and high
plastic anisotropy dual phase steel comprising the steps of forming a steel slab of
composition in weight % of 0.08 to 0.2 C, 0.2 to 1 Si, ≤ 1.7 Mn, ≤ 0.02 P, ≤
0.03 S, ≤ 0.04 Al, ≤ 0.2 Cr, the balance being Fe and incidental impurities; hot
rolling the slab through finish rolling at ≥ 870° retaining the steel in the
temperature range of 800° C to 700° C; cooling the steel sheet at a cooling rate
of ≥ 30° C / sec and coiling at ≤ 520° C to produce a three phase steel sheet
consisting of ferrite, bainite and martensite; the steel sheet being further
austenised by heating at a temperature above 900° C for some time and furnace
cooled to result in microstructure of ferrite, pearlite and bainite; the steel was
then cold rolled in the range of 30% to 60%, the stock being finally heated in
the temperature range of 700 - 800° C for 30 seconds to 4 minutes and
quenched in water resulting a ferrite-martensite dual phase steel.

Documents:

00835-kol-2007-abstract.pdf

00835-kol-2007-claims.pdf

00835-kol-2007-correspondence others 1.1.pdf

00835-kol-2007-correspondence others 1.2.pdf

00835-kol-2007-correspondence others.pdf

00835-kol-2007-description complete.pdf

00835-kol-2007-drawings.pdf

00835-kol-2007-form 1 1.1.pdf

00835-kol-2007-form 1.pdf

00835-kol-2007-form 18.pdf

00835-kol-2007-form 2.pdf

00835-kol-2007-form 3.pdf

00835-kol-2007-gpa.pdf

835-KOL-2007-(15-12-2011)-ABSTRACT.pdf

835-KOL-2007-(15-12-2011)-AMANDED CLAIMS.pdf

835-KOL-2007-(15-12-2011)-DESCRIPTION (COMPLETE).pdf

835-KOL-2007-(15-12-2011)-DRAWINGS.pdf

835-KOL-2007-(15-12-2011)-EXAMINATION REPORT REPLY RECEIVED.pdf

835-KOL-2007-(15-12-2011)-FORM-1.pdf

835-KOL-2007-(15-12-2011)-FORM-2.pdf

835-KOL-2007-(15-12-2011)-FORM-5.pdf

835-KOL-2007-(15-12-2011)-OTHERS.pdf

835-KOL-2007-(19-04-2012)-CORRESPONDENCE.pdf

835-KOL-2007-CORRESPONDENCE 1.1.pdf

835-kol-2007-CORRESPONDENCE.pdf

835-KOL-2007-EXAMINATION REPORT.pdf

835-KOL-2007-FORM 18.pdf

835-KOL-2007-FORM 3.pdf

835-KOL-2007-GPA.pdf

835-KOL-2007-GRANTED-ABSTRACT.pdf

835-KOL-2007-GRANTED-CLAIMS.pdf

835-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

835-KOL-2007-GRANTED-DRAWINGS.pdf

835-KOL-2007-GRANTED-FORM 1.pdf

835-KOL-2007-GRANTED-FORM 2.pdf

835-KOL-2007-GRANTED-SPECIFICATION.pdf

835-KOL-2007-OTHERS.pdf

835-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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Patent Number

253205

Indian Patent Application Number

835/KOL/2007

PG Journal Number

27/2012

Publication Date

06-Jul-2012

Grant Date

04-Jul-2012

Date of Filing

30-May-2007

Name of Patentee

TATA STEEL LIMITED.

Applicant Address

TATA STEEL LIMITED, JAMSHEDPUR

Inventors:

#	Inventor's Name	Inventor's Address
1	SAHA PODDAR, ARIJIT	TATA STEEL LIMITED. JAMSHEDPUR - 831 001
2	GOPE, N.	TATA STEEL LIMITED. JAMSHEDPUR - 831 001
3	BHATTACHARJEE, DEBASHISH	TATA STEEL LIMITED. JAMSHEDPUR - 831 001

PCT International Classification Number

C22C38/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA