Title of Invention

ACID FREE PROCESS AND SYSTEM FOR REMOVAL OF OXIDE SCALE FROM STEEL STRIPS

Abstract A process and a system for producing steel sheet/strips free of oxide scale or mill scale comprising electrolytic pickling to effectuate a mechanical scrubbing of the scales by generation of gas bubbles followed by subjecting the loosened scales to high frequency ultrasonic agitation to peel off the loose scales and provide said metal free of said scales. The process and system used from the purpose of the invention are environment friendly, simple and safe to carry out.
Full Text The present invention relates to a process for producing steel sheet/strips free of oxide scale or mill scales and in particular to a process involving electrolytic descaling of oxide/mill scales from plain carbon steels and an apparatus for carrying out such electrochemical descaling of plain carbon steel.
It is known that oxide scales are usually formed on the steel sheet or strip surface during hot rolling which have to be removed before further processing involving cold rolling, coating, etc. to obtain the required finished product appearance and properties. Various processes have been in use for providing refined molten metals and also finished metal products with good surface finish.
Patent No. 5443699 discloses a method for electrochemical refining of an impurity-containing low carbon steel melt using a solid electrolyte ionic conductor to remove the impurity from the melt. Also disclosed is an apparatus for performing the method, and a continuous method for electrochemical refining of a low carbon steel melt.
Patent No. 5445685 teaches ultrahigh carbon steels with superplastic properties produced by heating a steel containing ferrite and carbide phases to a soaking temperature for a sufficient time that the major portion of the carbides dissolve into the austenite matrix, and then cooling tha steel to obtain a steel having substantially spheroidal carbides. New alloy compositions contain aluminum and solute additions which promote the formation of a fine grain size and improve the resistance of the carbides to coarsening at the forming temperature.
Patent No. 5536338 discloses a method of annealing of siderurgical products comprised of carbon steel having a chromium content in the range 0.15-1.5 wt% and a manganese content also in the range 0.15 - 1.5 wt%, whereby said products are held in an annealing furnace under nitrogen atmosphere to bring about globalization of the pearlite ; characterized in that oxygen is added to this atmosphere, in an amount greater than or equal to 0.5 vol. %.
Pickling is also known to achieve chemical removal of oxide scale or mill scale from the metal surface by mineral acids.

Patent No. 5202002 discloses a process of pickling surfaces of a steel-based metallic material wherein the surfaces of the metallic material are pickled at a high speed by dipping and then pickling the surfaces of the metallic material in an aqueous solution of hydrochloric acid or an aqueous solution of hydrochloric acid-nitric acid mixture while at least one kind of ion selected from a group comprising a platinum ion, a palladium ion and a rhodium ion is contained in a hydrochloric acid or pickling treatment in the aforementioned aqueous solution by feeding a direct current between two electrodes, one of the electrodes being an anode composed of the metallic material and the other one being a cathode electrode disposed opposite to the anode, or an anode and a cathode comprising one pair of electrode plates disposed at both sides of a surface of the metallic in the aqueous solution, and feeding a direct curent between the anode and the cathode to thereby dissolve off a scale formed on the surface of the metallic material in the aqueous solution by an indirect feeding of the direct current.
Patent No. 5292374 teaches a process and plant for pickling steel strip. The plant comprises at least one elongated, preferably shallow pickling vat having a supporting structure of steel, glass fibre reinforced synthetic resin or polypropylene and a lining of acid and abrasion resistance material-including PVC, carbon bricks, graphite filled and/or aluminium oxide filled polyester resin.
Patent No. 4713154 discloses a method of continuously annealing and pickling steel strips pickling is effected at least by nitric acid electrolytic treatment. The pickling apparatus comprises at least a nitric acid electrolytic cell.
Patent No. 5837061 discloses a process for post-treatment of pickled steel products by rinsing with water in continuous rinsing plants, inert gas or rare gas is blown into at least one of the rinsing sections of the rinsing plant. This prevents the formation of hydrolysis stains on the surface of the steel products in case of the strip standing still.
Thus white pickling steel products is well known, different types of mineral acids and organic acids are used in the presently followed pickling processes for the

removal of oxide/mill scales from steel strips/sheets and for the purpose, commercial sulphuric and hydrochloric acid are widely used. However, such conventional acid pickling are associated with drawbacks like environmental pollution, reduced plant equipment life, sludge disposal, hazardous working conditions, etc.
It is thus the basic object of the present invention to provide a process of producing steel sheet/strips free of oxide/mill scales following pickling procedure which would be environment friendly.
Another object of the present invention is to provide a process of producing steel sheet/strips free of oxide/mill scales involving electrolytic descaling and ultrasonic cleaning.
Yet further object of the present invention is to provide a process of acid pickling of steel sheet or strip surface to remove oxide/mill scaling which would be simple and safe to carry out.
Yet further object is directed to provide an apparatus for electrochemical pickling of steel sheet/strips to free it from oxide/mill scaling.
Yet further object is directed to provide a system useful for providing steel strips/sheets free of oxide/mill scales involving electrochemical pickling unit and an ultrasonic cleaning unit.
Thus according to one aspect of the present invention there is provided in a process for producing steel sheet/strips free of oxide/mill scales comprising :
subjecting the steel sheet/strip to be treated to electrolytic pickling comprising mechanical scrubbing of the scales by gas bubbles generated by electrolytic process to thereby looses said scales on said metal surface; and
subjecting the metal with said loose scale to high frequency ultrasonic agitation to peel off the loose scale and provide said metal free of said scales.

Preferably, a neutral salt such as namely, sodium sulphate having favourable ionic conductivity and mobility, solubility, availability and cost is used for said electrochemical treatment. Also advantageously the periodic reverse electrocleaning method is used for utilising the advantages of both oxygen and hydrogen gas evolution on steel for descaling.
According to another aspect of the present invention there is provided a system for descaling of metals comprising :
an electrolytic means having an electrolytic cell with a pair of graphite electrodes
and said metal to be descaled as an intermediate electrode and a neutral salt
electrolyte solution;
means ror periodic reversing of the polarity of said electrodes;
means for heating said electrolyte; and
an ultrasonic clearing tank.
Preferably, a bipolar electrochemical cleaning equipment with programmable periodic reverse system is used. An ultrasonic cleaning tank is attached to the cleaning set up. The operating parameters for the process like current density, cell voltage, bath composition, temperature and time of descaling Were optimised for best descaling depending upon the nature of the scales.
Electrolytic cleaning tank can vary, but it essentially constitutes two counter electrodes and one working electrode (work piece), electrolyte, heating element and temperature probe. Electrolytic cell is conventional but part modification or alteration has been done to make it suitable for our invention.
In the ultrasonic cleaning tank, micron size bubbles form and grow due to alternating positive and negative pressure waves in a solution (by a process termed cavitation). The bubbles subjected to these alternating pressure waves continue to grow until they reach resonant size. Just prior to implosion there is tremendous amount of energy stored in the bubble itself which travels through the solution like a shock wave and causes vigourous agitation in

the bath. After electrolytic cleaning, the plain carbon steel sample is immediately transferred to ultrasonic cleaning tank for removal of loosened scale.
The objects/advantages and details of the invention are explained hereunder in greater detail with reference to non-limiting exemplary embodiment discussed hereunder with relation to the accompanying figures wherein Fig. 1 is a schematic illustration of the system for descaling in accordance with the present invention.
As shown in Figure 1 the electrochemical cleaning equipment consists of an
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electrolytic cell/tank (EC) for holding the neutral salt solution (NS). The cell contains one working electrode, the plain carbon steel (CS) sample to be pickled and two graphite counter electrodes (E1, E2) on either side of the working electrode. The electrodes are fixed to the sample holders and are placed in the electrolytic tank containing the working solution or the electrolyte. The electrodes are connected to a stabilised DC source equipped (DCE) with a programmable periodic reverse (PR) system. The PR system is so configured that the polarities of the electrodes are reversed after a fixed time interval. This time interval can be varied from 6 seconds to 99 minutes. Suitable mechanism is also provided for lifting and lowering the electrodes and for adjusting the inter electrode distance. An immersion heater (IH) and a thermocouple are provided in the electrolytic tank for heating the electrolyte. The heating is controlled by a programmbale digital temperature indicator cum controller (DIC). An ultrasonic cleaning tank (UCT) equipped with a transducer of frequency in the range 30-40 kHz and heating arrangement (30°-70° Centigrade) is also provided.
Method of carrying out the process of pickling using the system of the invention.
The electrolytic pickling involves a two fold operation. First, the oxide scale is loosened by the mechanical scrubbing action of gas bubbles generated during the electrolytic process. Next the loose scale is peeled off under high frequency ultrasonic agitation. The scrubbing action is due to the hydrogen evolution when the working steel electrode is cathode (negative polarity) and oxygen evolution when the working steel electrode is anode (positive polarity). The periodic

reverse system is programmed in such a way that the final cycle of operation is anodic to avoid any smut formation.
The optimum operating parameters for electrochemical descaling are as follows :
a) Sodium sulphate bath concentration 120-150 g/l
b) Bath temperature 60-80°C
c) Current density 20-30 A/dm2
d) Time of electrolysis 3-9 minutes
e) Frequency of periodic reverse Cathode - 5 to 20 seconds
Anode = 5 to 20 seconds
0.5-2.0 g/l of additives like Sodium lauryl sulphate and Ammonium sulphate are added to the electrodescaling bath for improving wettability and for pH stabilisation.
The process of descaling of sheet/strip metal and the system of the invention thus achieves pickling using acid free neutral salt solution which is essentially environment friendly and cost effective.
The invention would provide for-substantially total removal of scale from plain carbon steel in shortest possible time and with lowest possible temperature.
Electrolytic pickling in accordance with the present invention in neutral salt solutions eliminates many drawbacks of conventional acid pickling like environmental pollution due to acid fume generation, reduced plant equipment life, sludge disposal, hazardous working conditions, etc. Neutral electrolytic process has advantages like less hazardous working conditions in the shop floor, reduced emission of polluting substances and corrosion initiating streams, better surface condition of treated steel strip, increased plant equipment life, easy handling of waste. Electrolytic pickling of the invention remove scale twice as fast as conventional acid pickling and would be more economical where time is limited.



WE CLAIM:
1. An acid free process for producing steel sheet/strips free of oxide/mill scales
comprising:
subjecting the steel sheet/strips to be treated to electrolytic pickling using acid free neutral salt solution and in the process achieve mechanical scrubbing of the scales by gas bubbles generated by electrolytic process to thereby loosen such scales on said metal surface, said treatment to electrolytic pickling being carried out in presence of Sodium sulphate bath having a concentration of between 120-150 g/l, wherein the Bath temperature is between 60-80°C, the current density applied is between 20-30 A/dm2, such that electrolysis is carried out for 3-9 minutes and the frequency of periodic reverse applied is cathode = 5 to 20 seconds and anode = 5 to 20 seconds, further 0.5 -2.0 g/l of additives selected from Sodium lauryl sulphate and Ammonium sulphate being added to the electrodescaling bath for improving wettability and for pH stabilization; and
subjecting the metal with said loose scales to high frequency ultrasonic agitation to peel off the loose scales from said metal surface and thereby provide the metal steel sheet/strips free of oxide/mill scales.
2. A process as claimed in claim 1 wherein said neutral salt is selected to have ionic
conductivity, mobility and solubility preferably, sodium sulphate.
3. A process as claimed in anyone of claims 1 to 2 wherein said electrolytic process
comprise periodic reverse electro cleaning method to favour gas evolution for
descaling purposes.
4. A process as claimed in anyone of claims 1 to 3 wherein a bipolar
electrochemical cleaning equipment with programmable periodic reverse system
is used.
5. A process as claimed in anyone of claims 1 to 4 wherein an ultrasonic cleaning
tank is attached to the cleaning set up.
6. A process as claimed in anyone of claims 1 to 5 wherein the operating
parameters for the process like current density, cell voltage, bath composition,
temperature and time of descaling are determined based upon the nature of the
scales.
7. A process as claimed in anyone of claims 1 to 6 wherein said electrolytic
cleaning tank used comprise conventional electrolytic cleaning tank comprising
two counter electrodes and one working electrode (work piece), electrolyte,
heating element and temperature probe.
8. A process as claimed in anyone of claims 1 to 7 wherein said ultrasonic cleaning
tank is adapted and operated to generate micron sized bubbles form and growth
by alternating positive and negative pressure waves in a solution preferably by
cavitation process.
9. A process as claimed in claim 8 wherein said bubbles subjected to these
alternating pressure waves are continued to grow until they reach resonant size
and implode, releasing tremendous amount of stored energy which travels
through the solution like a shock wave and causes vigorous agitation in the bath.
10.A process as claimed in anyone of claims 1 to 9 wherein after electrolytic cleaning, the plain carbon steel sample is immediately transferred to ultrasonic cleaning tank for removal of loosened scale.
11. A system for use in removal of oxide/mill scales from steel sheet/strips using the acid free process as claimed in anyone of claims 1 to 10 comprising: an electrolytic means having an electrolytic cell with a pair of electrodes and said metal to be descaled as an intermediate electrode and a neutral salt electrolyte solution;
means for periodic reversing of the polarity of said electrodes ; means for heating said electrolyte ; and an ultrasonic clearing tank.
12. A system as claimed in claim 11 wherein said electrodes comprise pair of
graphite electrodes.
13. A system as claimed in anyone of claims 11 or 12 wherein said electrochemical
cleaning equipment consists of an electrolytic cell/tank for holding the neutral salt
solution, said cell comprising one working electrode, the plain carbon steel
sample to be pickled and two graphite counter electrodes on either side of the
working electrode, said electrodes being fixed to the sample holders and are
placed in the electrolytic tank containing the working solution of the electrolyte.
14. A system as claimed in claim 14 wherein said electrodes are connected to a
stabilised DC source equipped with a programmable periodic reverse system.
15. A system as claimed in claim 14 wherein said periodic reverse system is
adapted such that the polarities of the electrodes are reversed after a fixed time
interval preferably, varying between 6 seconds to 99 minutes.
16. A system as claimed in anyone of claims 12 to 15 comprising means for lifting
and lowering the electrodes and for adjusting the inter electrode distance.
17. A system as claimed in anyone of claims 12 to 16 comprising an immersion
heater and a thermocouple provided in the electrolytic tank for heating the
electrolyte.
18. A system as claimed in claim 17 comprising means for controlling the heating
comprising a programmbale digital temperature indicator cum controller.
19. A system as claimed in anyone of claims 12 to 18 wherein said ultrasonic
cleaning tank comprises a transducer of frequency in the range 30-40 kHz and
heating arrangement (30°-70° centigrade).
21. A system as claimed in anyone of claims 12 to 20 wherein said periodic reverse
system is adapted in such a way that the final cycle of operation is anodic to avoid any
smut formation.
22. A process for producing steel sheet/strips free of oxide/mill scales and a system for
carrying out the same substantially as hereindescribed and illustrated with reference to
the accompanying figures.

Documents:

1469-del1999-abstract.pdf

1469-del1999-claims.pdf

1469-del1999-correspondence-others.pdf

1469-del1999-correspondence-po.pdf

1469-del1999-description (complete).pdf

1469-del1999-drawings.pdf

1469-del1999-form-1.pdf

1469-del1999-form-19.pdf

1469-del1999-form-2.pdf

1469-del1999-form-3.pdf

1469-del1999-gpa.pdf


Patent Number 195204
Indian Patent Application Number 1469/DEL/1999
PG Journal Number 43/2007
Publication Date 26-Oct-2007
Grant Date 10-Oct-2007
Date of Filing 09-Nov-1999
Name of Patentee STEEL AUTHORITY OF INDIA LIMITED,
Applicant Address ISPAT BHAWAN, LODI ROAD, NEW DELHI-110 003, INDIA,
Inventors:
# Inventor's Name Inventor's Address
1 BHATTACHARYYA AMITABH, RESEARCH AND DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834 002, STATE OF BIHAR, INDIA,
2 KILARU RAVI, B) RESEARCH AND DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834 002, STATE OF BIHAR, INDIA,C)
3 SRIKANTI SRIKANTH, B) RESEARCH AND DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834 002, STATE OF BIHAR, INDIA,C)
4 MISHRA KUNJ BIHARI, B) RESEARCH AND DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834 002, STATE OF BIHAR, INDIA,C)
5 JHA SUDHAKER, B) RESEARCH AND DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834 002, STATE OF BIHAR, INDIA,C)
PCT International Classification Number B32B-15/04
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA