METHOD FOR MANUFACTURING OF ANTI RIDGING FERRITIC STAINLESS STEEL SHEETS WITH SINGLE STAGE COLD ROLLING
INVENTORS:
• NAGARAJAN P
• LKSINGHAL
ASSIGNEE Jindal Stainless Limited
REFERENCE CITED:
4070208 2/2005 Hayami et al
5868875 2/1999 Yoshitake et al
6113710 11/2003 Takahashi et al.
TECHNICAL FIELD
The subject matter described herein generally relates to a method for manufacturing anti ridging sheets and particularly relates to a method for manufacturing anti ridging sheets with single stage cold rolling.
BACKGROUND
Generally, ferritic stainless steel sheets are known for use in various commercial applications. Processes like ridging and roping result in the spoiling of the surface finish, particularly during stretching or drawing of sheets, thereby marring the appearance of the product. Thus, anti ridging of ferritic stainless steel sheets is important for enhancing the surface finish and eventually the life cycle of the sheets.
Conventionally, it is known that the homogenization of stainless steel sheets at a temperature above or under the dual phase (Austenite + Ferrite) region, i.e. above Ac3 and below Acl, can prevent ridging of the sheets. Such a treatment can randomize the texture in the local region within the sheets.
Generally, anti ridging ferritic stainless steel sheets are produced by prolonged bell annealing of hot rolled coils at a temperature below Acl followed by two stage cold rolling with intermediate continuous annealing. However, this is a time consuming and expensive process step.

Over the years, research to develop antiridging stainless steel sheets by different means is going on. Hayami et al have produced anti ridging stainless steel sheets by warm rolling after hot rolling without cooling the hot rolled (HR) coil at a controlled temperature of about 700 C. They have also disclosed in their US patent 4070208 that continuous annealing for a shorter duration after warm rolling is sufficient to destroy the texture colonies as against the conventional bell annealing generally required. However, this requires an additional expensive unit for warm rolling.
Ota etal in their patent have developed antiridging ferritic stainless steel sheets by adding about 0.003 Boron. As per this patent, during hot rolled sheet annealing Boron combines with Nitrogen in steel to produce fine precipitates on dislocations introduced by pre-rolling, thereby suppressing recovery of the dislocations and promoting recrystallization.
Yoshitake et al, in their US patent 5868875, discloses the development of a ferritc stainless steel with antiridging characteristics by controlling the grain structure during casting. The method of US 5868875 eliminates the annealing of hot rolled coil by controlling the cast grain structure. For this purpose Electro magnetic stirring in the caster is required.
Further, US 6113710 advocates the generation of anti ridging characteristics in Ferritic stainless steel sheets by adding Mo, V in their component composition or by properly combining the component composition and the hot rolling conditions. Mo and V are very expensive elements.
Thus, there is a need for a method for manufacturing anti ridging stainless steel sheets, which overcomes the above mentioned drawbacks.
SUMMARY:
An object of the present subject matter is to provide a simple and efficient method for manufacturing anti-ridging stainless steel sheets.

Yet another object of the present subject matter is to provide an economical method for producing anti-ridging stainless steel sheets.
Accordingly, the subject matter described herein is directed to a method for manufacturing of anti ridging sheets with single stage cold rolling without any annealing of HR coil. Ferritic stainless steel melt with appropriate chromium content is continuously cast into slab or into a bloom. The cast steel contains chemical composition as per standard specified in ASTM 240 for AISI 430. Such slab/bloom is hot rolled into coil as per normal practice. As hot rolled coil or cut sheets are then descaled and cold rolled in the direction perpendicular to the hot rolling direction, with a cold reduction of about 55-80%. This sheets are then annealed for recrystallization at about 750-820 C, again descaled and skinpassed. Thus, annealing of hot rolled coil prior to cold rolling is not necessary to get anti ridging characteristics. Further, two-stage cold rolling with intermediate annealing is also not required as is the case in the conventional methods.
BRIEF DESCRIPTION OF DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Fig. 1 shows a pole figure of a stainless steel sheet manufactured by orientating the direction of cold rolling at 90 degrees from the direction of hot rolling.
Fig. 2 shows a pole figure of uni-directionally cold rolled stainless steel Isheet.
DETAILED DESCRIPTION:
The subject matter described herein relates to a method for manufacturing of anti ridging 430 sheets with single stage cold rolling without any annealing of HR coil. In the present subject

matter, cold rolling is carried out perpendicular to the direction of hot rolling, thereby disturbing the banded structure to suppress ridging in the ferritic stainless steel sheets.
In one embodiment of the present subject matter, about 200mm thick slabs of 430 are prepared by electric arc furnace (EAF), argon oxygen decarburization process (AOD) and continuous casting process simultaneously. The slabs are heated to around 1100 C and then subjected to hot rolling to form a hot rolled coil. In one preferred embodiment, the hot rolled coil thus formed has a thickness of about 3.5mm. The hot rolled coil is then cut into sheets suitable for cross rolling. These sheets are then descaled with acid and then cold rolled in the direction perpendicular to the direction of hot rolling. Thus, the annealing operation after cold rolling is eliminated in the present subject matter. After providing a reduction of around 55-80%, the cold rolled sheet is annealed for re-crystallization at about 750 - 820 C. This annealed sheet is then again descaled and skinpassed.
The final cold rolled annealed pickled sheet with a thickness of about 0.75mm, thus produced, is then subjected to various tests, such as ridging resistance test and texture evolution test. The results of these tests are as follows:
RIDGING RESISTANCE TEST:
The ridging resistance is evaluated by giving 20% tensile strain to JIS No 5 tensile test piece. The maximum roughness in the direction perpendicular to the tensile direction is measured by a surface roughness tester. The evolution of ridging resistance is based on the following standard:
Ridging grade 0.5; Rmax <5 micron meter
Ridging grade 1.0; Rmax 5-10 micron meter
Ridging grade 1.5; Rmax 10-15 micron meter
Ridging grade 2.0; Rmax 15-30 micron meter
Ridging grade 2.5; Rmax >30 micron meter

The cold rolled annealed and pickled ferrtic stainless steel sheets produced from the subject matter shows ridging values 2.97 micron meter, which falls under ridging grade 0.5 and considered as better resistance to ridging. The ridging and roping values are given in Table 1.
TABLE 1:
I ^^^''d""s I Thickness I Surface Roughness I g^^^^^^ Roughness after
Material ^
(VHN) (mm) (nm) 20% elongation(nm)
J430 cross cold ^^^^^ ^^^^
164 0.75
■■o'l'^g Rz-1.24 Rz-2.97
J430 Unidirection Ra-0.13 Ra-0.76
•^o'd ""O'l'^g Rz-1.62 Rz-14.62
TEXTURE EVALUATION TEST:
The preferred orientations i.e. the non-random orientation of single crystal lattices within a material, is usually obtained by Electron backscatter diffraction (EBSD). The crystal co-ordinate system is used to represent the orientation of the crystal.
It is known that weaker structure without any preferred orientated colonies of grains result in reduced ridging.
Fig. 1 illustrates a pole figure of the sheet manufactured by orientating the direction of cold rolling at 90 degrees from the direction of hot rolling. As shown herein, a very weak texture is produced without having any preferred orientation with intensity of 4.37.
Fig. 2 illustrates a pole figure of uni-directionally cold rolled sheet showing a strong texture with higher intensity of texture components up to 5.87 in the intensity scale.
A Pole figure shows the projected position of a particular set of crystallographic planes which have been projected on to a sphere and then on to a circle.

Thus, the stainless steel sheet processed in the above subject matter in which the direction of cold rolling is performed at 90 degrees from the direction of hot rolling gives a weak texture, thereby eliminating ridging of stainless steel sheets substantially.
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.

WE CLAIM:
1. A method for manufacturing of AISI 430 coils or sheets having anti-ridging characteristics &
comprising the steps of:
a. Manufacturing of slabs or blooms of AISI 430 through EAF-AOD-CCS route.
b. Hot rolling of slabs or blooms to coils.
c. De-scaling of coils or sheets.
d. Cold Rolling of the de-scaled coils or sheets.
e. Annealing of the cold rolled coils or sheets
f. De-scaling of the Annealed Cold Rolled coils or sheets.
g. Skin passing of the descaled Annealed Cold Rolled coils or sheets.
2. A method as described in claim 1 wherein said chemical composition of slabs or blooms of AISI 430 strictly conform to ASTM A240 standard.
3. A method as described in claim 1 wherein hot rolled coil is taken for cold rolling without any annealing.
4. A method as described In claim 1 wherein said cold rolling takes place in a direction perpendicular to the hot rolling direction.
5. A method as described in claim 1 wherein cold reduction varies in the range of 55 - 80%.
6. A method as described in claim 1 wherein said annealing of the cold rolled coil or sheet varies in the temperature range of 750 - 820 OC for recrystallization.
7. A method as described in claim 1 wherein two stage cold reduction with intermediate annealing is not required to get anti-ridging characteristics of AISI 430 coils or sheets.
Dated this 'S-U^ day of J*W*^ 20f 2-Signature