Title of Invention | "A METHOD OF MAKING BATCH ANNEALED TI- STABILISED INTERSTITIAL FREE (IF) STEEL SHEETS WITH IMPROVED DRAWABILITY" |
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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) 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] <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. |
Full Text | -2- 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. -3- 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. -4- 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. -5- 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. -6- The main objective of the invention is to have a very strong homogenous gamma-fibre ({111)} 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 -7- strip mill at hot spot temperature by maintaining hot rolling line below or above upper critical temperature (Ar3); securing uniform a-fibre [111] 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. -8- 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 -9- 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} -10- 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. -11- 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] orientation density of rotated cube orientation {001} and achieving high plastic strain ratio (rm) in the order of 2.3. -12- 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. -13- 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). -14- 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] |
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01306-kol-2006 correspondence others.pdf
01306-kol-2006 description (complete).pdf
01306-kol-2006-correspondence_1.1.pdf
01306-kol-2006-correspondence_1.2.pdf
1306-KOL-2006-CANCELLED PAGES.pdf
1306-KOL-2006-DESCRIPTION (COMPLETE).pdf
1306-KOL-2006-REPLY TO EXAMINATION REPORT.pdf
Patent Number | 242358 | ||||||||||||
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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:
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PCT International Classification Number | C21D9/46 | ||||||||||||
PCT International Application Number | N/A | ||||||||||||
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