Title of Invention

"A METHOD OF MAKING BATCH ANNEALED TI- STABILISED INTERSTITIAL FREE (IF) STEEL SHEETS WITH IMPROVED DRAWABILITY"

Abstract This invention relates to a method of making batch annealed Ti-stabilised interstitial free (IF) steel sheets / strips with improved drawability comprising the steps of developing a Ti-stabilized interstitial free (IF) steel sheets / strips through varied cold deformation and batch annealing at temperature ranges of 660-750° C of steel bar at orientation distribution function (ODF) &#966;2 = 45°, or finish hot rolling the hot band material produced at hot strip mill at hot spot temperature by maintaining hot rolling line below or above upper critical temperature (Ar3); securing uniform &#947;-fibre [111] <uvw> texture with negligible orientation density of rotated cube orientation {001} <110> and achieving high plastic strain ratio (rm) in the order of 2.3.
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FIELD OF THE INVENTION
The present invention relates to a method of making batch annealed Ti-stabilised Interstitial Free (IF) steel sheets or strips with improved drawability.
BACKGROUND OF THE INVENTION
Interstitial Free (IF) steels are widely used in auto industry for their excellent deep drawability character with high plastic strain ratio (rm), high elongation, high hardening index and low yield to tensile strength ratio.
With the tremendous technological development in auto industry to meet the continuous higher demand of auto cars, the auto industries are bound to develop low weight auto bodies and auto parts to enable engines of lower H.P. to run of reduced weight body parts and thus reducing consumption of fuel and cost.

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Industrial house is therefore engaged in building up research and development for industrial production of deep drawn Interstitial Free (IF) steels to reduce cost Of production by material savings through reduction of weight and with fuel efficiency but at the same time maintaining compatible strength characteristics of the auto bodies and auto parts.
The improvement in productivity in this respect is carried through by the prior knowledge and understanding of the recrystallisation texture and texture memory effect during sheet forming in the automotive industry.
In prior art practice of making deep drawable sheets investigation were made on sheet texture and through texture control, texture was formed in the directions in which the properties are optimum or in the direction of the intended use for any engineering material or alloy by selection of manufacturing process depending on several variables.

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Combining several types of deformation processing, means were developed for various deformation textures. For example a narrow strip was produced by conventional rolling followed by slitting or by wire drawing followed by flattening. Intermediate annealing operation, normally performed to soften a work piece for further deformation were also used for additional texture control similar to that obtained through recovery or recrystallization.
In the existing art of preparation of deep drawn steel sheet coils it has been observed that the steel sheets or strips during use under temperature, state of stress and impact on the auto body are possessing lower ductility, which may be attributable to sub-optimal process conditions.
The present invention is aimed to remove the drawbacks of prior art by developing a process to achieve improved drawability on exploiting the optimization effect of percent cold reduction and cold spot temperature.

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DESCRIPTION OF THE INVENTION
The present invention has been developed on sudden discovery while investigating the processing parameters of annealing cycle that the cold spot temperature (temperature attained by the spot in the coil which is located at 1/3 rd of the total thickness of the coil from the inner diameter and 2/3 rd from the outer diameter) of the existing cycle was in the range of 640-665° C, which is pretty low to fully recrystallize the material within a short period of ~1/21 hour.
One of the objective of the invention is to develop a Ti stabilized Interstitial Free (IF) steel which is recrystalized in the range of ~ 660° C - 710° C, whose recrystalization rate is pretty sluggish after 90% of recrystalization, though 660° C is observed to be insufficient to fully recrystallize the material within a short period of 1 hour.

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The main objective of the invention is to have a very strong homogenous gamma-fibre ({111)} texture) and have negligible orientation density of rotated cube orientation, {001} to achieve higher deep drawability, by optimizing processing parameters.
Another objective of the invention is to achieve optimum combination of percent cold reduction of 60, 65, 70, 75, 80, 85 and 90% and annealing temperatures in the range of 660-750° C on simulation of cold spot temperature to obtain higher deep drawability of steel sheet in terms of high plastic strain ratio (rm) in the order of 2.3.
According to the invention there is provided a method of making batch annealed Ti-stabilised interstitial free (IF) steel sheets with improved drawability comprising the steps of developing a Ti-stabilized interstitial free (IF) steel sheets/strips through varied cold deformation and batch annealing at temperature ranges of 660-750° C of steel bar at orientation distribution function (ODF) f2 = 45°, or finish hot rolling the hot band material produced at hot

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strip mill at hot spot temperature by maintaining hot rolling line below or above upper critical temperature (Ar3); securing uniform a-fibre [111] texture with negligible orientation density of rotated cube orientation {001} and achieving high plastic strain ratio (rm) in the order of 2.3.
DETAILED DESCRIPTION OF THE INVENTION
The proposed invention will be better understood from an embodiment as narrated hereinbelow with reference to the accompanying drawings in which
Figure la shows schematically a-fibres formed for 80% cold rolled
and batch annealed steel sheet, with orientations distribution function (ODF) f2 = 45° of rm = 1.91.
Figure lb shows schematically g-fibre formed for 80% cold rolled and batch annealed steel sheet at 680° C with ODF f2 = 45° and of rm = 2.3.
Figure 1C shows schematically y-fibre formed for 80% cold rolled and batch annealed steel sheet at 710° C with ODF f2 = 45° and rm = 2.3.

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Table 1 below shows the normal anisotropy (rm) values as in the existing art along with representative orientation distribution function (ODF) in Fig la, where ODF f2 = 45° of 80% cold reduced and batch annealed material in Figure l(a) shows the presence of appreciable amount of a-fiber, which is detrimental to the deep drawability.
Table 1: Normal anisotropy of the cold rolled and batch annealed material collected from the plant.
Material Normal Anisotropy (rm)
IF-0.8 mm (80%) 1.91
IF-1.0 mm (78%) 1.90
IF-1.2 mm (73%) 1.89
Hence in the present work an attempt has been ventured to exploit the optimization effect of percent cold reduction and cold spot temperature with a view to achieve improved deep drawability. It is well known that cold reduction

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plays a major role in recrystallization and thus developing the right kind of texture for desired deep drawability. Therefore, various combinations of percent cold reduction of 60, 65, 70, 75, 80, 85 and 90% and annealing temperatures in the range 660-750° C to simulate the cold spot temperature annealing were exploited to establish optimum combination of percent cold reduction and annealing temperature to obtain higher deep drawability. The ideal combination of A/ 80% of cold reduction and 680° C [Fig. l(b)] as cold spot temperature along with 80% cold reduction and 710° C [Fig. l(c)] as hot spot temperature, have been found to have resulted in higher rm value With uniform {111} texture. Whereas Fig l(a) shows quite a bit of spread in a-fibre, Figs. l(b) and (c) show beneficial g-fiber intensity with 16 X RI (RI -> random intensity) and 8 X RI respectively with minimum a-fiber intensity as compared to l(a), 6.4 X RI. Hence in test working process in Figures l(b) and l(c) Conditions produce higher deep drawability in terms of rm, which is ~ 2.3. Besides, as the processing history influences the final texture, i.e., hot band and subsequent cold rolled textures are inherited by the annealing (recrystallization) texture (texture memory effect), a detailed study of hot band and cold rolled materials have been

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carried out. It has been found that a strong starting texture consisting of intense rotated cube component has also been instrumental in resulting a strong rotated cube component in the cold-rolled-batch-annealed materials. This strong texture in hot band has been accounted for due to the finish rolling of the material below Ar3 temperature (893° C). This was rectified by using a coil box instead of bypass rolling in the hot rolling line to maintain the temperature just above the upper critical temperature and thus taking care of the desired texture.
The rm value obtained, by taking the starting material in the study as the hot rolled materials produced in the hot strip mill is ~ 2.3. However, if the starting hot band material with the use of coil box in the hot rolling line is taken, it is observed that the rm value would exceed ~2.3, provided modified annealing parameters are used.
The invention as herein narrated herein with an embodiment should not be read and construed in a restrictive manner as various modifications, alterations and changes are possible within the scope and limit of the invention as encampused in the appended claims.

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WE CLAIM
1. A method of making batch annealed Ti-stabilised interstitial free (IF) steel sheets / strips with improved drawability comprising the steps of:
- developing a Ti-stabilized interstitial free (IF) steel sheets / strips
through varied cold deformation and batch annealing at
temperature ranges of 660-750° C of steel bar at orientation
distribution function (ODF) f2 = 45°, or finish hot rolling the hot
band material produced at hot strip mill at hot spot temperature by
maintaining hot rolling line below or above upper critical
temperature (Ar3);
- securing uniform g-fibre [111] texture with negligible
orientation density of rotated cube orientation {001} and
achieving high plastic strain ratio (rm) in the order of 2.3.

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2. A method of making steel sheets / strips as claimed in claim 1, wherein
during investigating the processing parameters it was distinctly observed
that the cold spot temperature (temperature attained by the spot in the
coil which is located at 1/3 rd of the total thickness of the coil of sheet
from the inner diameter and 2/3rd from the outer diameter) of the existing
cold working and annealing cycle was in the range of 640-665° C, which
was found to be pretty low temperature to fully recrystallize the steel
sheet / strip within a short period of 1/2 to 1 hour.
3. A method as claimed in the preceeding claims wherein optimum
combination of percent cold working deformation of 60, 65, 70, 75, 80,
85 and 90% and batch annealing temperatures in the range of 660-750°
were simulated with cold spot temperature to achieve higher drawability
of steel sheet / strip in terms of high rm value of 2.3.
4. A method as claimed in the preceeding claims wherein the combination of
cold working deformation cold spot temperature was maintained as 80%
and 680° C respectively, to achieve the best drawability of the steel.

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5. A method as claimed in the preceeding claims wherein the combination of
cold making deformation and hot spot temperature was maintained at
80% and 710° C respectively to achieve best drawability of the steel.
6. A method as claimed in the preceeding claims wherein beneficial y-fibre
intensity within 8 X RI to 16 X RI- random intensity with minimum a-fibre
intensity was formed during cold working and batch annealing processing
parameter sequencing.
7. A method as claimed in the preceeding claims wherein a strong starting
texture consisting of intense rotated cube component can be employed in
the cold rolled batch annealed parameters sequencing resulting in a
strong rotated cube component sheet material.
8. A method as claimed in the preceeding claims wherein the strong texture
in hot band has been accounted for due to the finish rolling of the
material below Ar3 temperature (893° C).

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9. A method as claimed in the preceeding claims wherein hot rolling line is
maintained above the upper critical temperature (Ar3) by using a coil box
in the hot rolling line and achieving a high drawability exceeding rm value
of 2.3 by using starting hot band material to be rolled in the hot rolling
line provided with necessary modified annealing parameters.
10. A method of making batch annealed Ti-stabilised interstitial free (IF) steel
sheets / strips with improved drawability as herein described and
illustrated.

This invention relates to a method of making batch annealed Ti-stabilised interstitial free (IF) steel sheets / strips with improved drawability comprising the steps of developing a Ti-stabilized interstitial free (IF) steel sheets / strips through varied cold deformation and batch annealing at temperature ranges of 660-750° C of steel bar at orientation distribution function (ODF) φ2 = 45°, or finish hot rolling the hot band material produced at hot strip mill at hot spot temperature by maintaining hot rolling line below or above upper critical temperature (Ar3); securing uniform γ-fibre [111] texture with negligible orientation density of rotated cube orientation {001} and achieving high plastic strain ratio (rm) in the order of 2.3.

Documents:

01306-kol-2006 abstract.pdf

01306-kol-2006 assignment.pdf

01306-kol-2006 claims.pdf

01306-kol-2006 correspondence others.pdf

01306-kol-2006 description (complete).pdf

01306-kol-2006 drawings.pdf

01306-kol-2006 form-1.pdf

01306-kol-2006 form-2.pdf

01306-kol-2006 form-3.pdf

01306-kol-2006-correspondence_1.1.pdf

01306-kol-2006-correspondence_1.2.pdf

01306-kol-2006-form-1-1.1.pdf

01306-kol-2006-form-9.pdf

1306-KOL-2006-ABSTRACT.pdf

1306-KOL-2006-CANCELLED PAGES.pdf

1306-KOL-2006-CLAIMS.pdf

1306-KOL-2006-DESCRIPTION (COMPLETE).pdf

1306-KOL-2006-DRAWINGS.pdf

1306-KOL-2006-FORM 1.pdf

1306-KOL-2006-FORM 2.pdf

1306-KOL-2006-FORM 5.pdf

1306-KOL-2006-REPLY TO EXAMINATION REPORT.pdf

abstract-01306-kol-2006.jpg


Patent Number 242358
Indian Patent Application Number 1306/KOL/2006
PG Journal Number 35/2010
Publication Date 27-Aug-2010
Grant Date 24-Aug-2010
Date of Filing 05-Dec-2006
Name of Patentee TATA STEEL LIMITED
Applicant Address RESEARCH AND DEVELOPMENT AND SCIENTIFIC SERVICES DIVISION, JAMSHEDPUR-831 001
Inventors:
# Inventor's Name Inventor's Address
1 Banerjee,Kumkum of R&D Division TATA STEEL LIMITED. RESEARCH AND DEVELOPMENT AND SCIENTIFIC SERVICES DIVISION, JAMSHEDPUR-8310001
2 Gope, N.of R&D Division TATA STEEL LIMITED. RESEARCH AND DEVELOPMENT AND SCIENTIFIC SERVICES DIVISION, JAMSHEDPUR-831 001
3 Venugopalan, T. of office of DMD (s) TATA STEEL LIMITED. RESEARCH AND DEVELOPMENT AND SCIENTIFIC SERVICES DIVISION, JAMSHEDPUR-831 001
PCT International Classification Number C21D9/46
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA