Title of Invention

A PROCESS FOR MINIZING OXIDE SCALE FORMATION DURING STELMOR COOLING IN A WIRE AND ROD MILL

Abstract

A process for minimizing oxide scale formation during Stelmor cooling in a wire and rod mill, comprising the steps of: - determining γ-α Fe transformation temperature of the grade of steel using a thermo-mechanical simulator; - and forming FeO type scale by using a modified cooling rate of ~ 1.4 C s-1 at 900°C LHT till 750°C and subsequently increasing the cooling rate to 8° C s-1 till reformer tub.

Full Text

FIELD OF INVENTION The present invention relates to a process for minimizing oxide scale formation during Stelmor cooling in a wire and rod mill. BACKGROUND OF THE INVENTION The oxide scale formed under continuous cooling for electrode wire non-rimming (EWNR) quality grade steel wire rods is a major concern among the electrode manufacturing industry who draw the hot rolled rods to lesser diameters of up to 1.6 mm starting from either 5.5 or 7mm wire rods. The oxide scale formed during Stelmor cooling at a wire rod mill for low carbon grades accounts for nearly 1 percent of the coil weight. There is therefore, a need to minimize the formation of oxide scale when the wire rods are cooled after hot rolling. SUMMARY OF THE INVENTION The main object of the present invention is to provide a process for minimizing the oxide scale formed after cooling of the wire rods after hot rolling. Another object of the present invention is to provide an improved yield to the user of the products from the wire and rod mill. Yet another object of the present invention is to reduce the wear of machinery used for mechanical descaling. The object of minimizing the formation oxide scale and other objects can be achieved by introducing modified cooling of electrode wire non-rimming wire rods. An accelerated cooling rate cannot however, be introduced just after the laying head as it can affect the tensile properties. To overcome this problem accelerated cooling rate is introduced after the phase transformation is completed. Conventional rolling with a cooling rate of 1.4° C s-1 at 900° C till 750° C can be carried out subsequently the cooling rate can be raised to 8° C s-1 till reformer tub. Thus the present invention provides a process for minimizing oxide scale formation during Stelmor cooling in a wire and rod mill, comprising the steps of: determining γα Fe transformation temperature of the grade of steel using a thermo-mechanical simulator; and forming FeO type scale by using a modified cooling rate of ~ 1.4 Cs -1 at 900° C LHT till 750° C and subsequently increasing the cooling rate to 8° Cs1 till reformer tub. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 shows the scale structure of oxide formation after normal cooling. Figure 2 shows the scale structure of oxide formation after accelerated cooling of the present invention. DETAILED DESCRIPTION In the present invention the oxide scale formed on low carbon steel during conventional wire rod rolling (cooling without any blower opened) after laying head temperature LHT, at 900°C is investigated. The γ-αFe transformation temperature of the grade is determined using a thermo-mechanical simulator, Gleeble-1500. Subsequently the formation of FeO type scale is achieved through modification in cooling rate (~1.4pCs-1 at 900°C LHT till 750°C and subsequently 8° Cs-1 till reformer tub). The oxides scale formed after hot rolling is characterized with the help of Raman Spectroscopy, EBSD technique, X-Ray diffraction and optical metallography. It is observed that the scale on wire rods produced through conventional cooling practice contain magnetite (Fe3O4) and hematite (Fe2O3) predominantly between the steel substrate and wustite layer whereas uniform wustite (FeO) layer with very less amount of transformed magnetite and proeutectoid magnetite is found on the wire rods with modified cooling practice. The amount of scale formed is evaluated by mechanical descaling and chemical pickling. In both descaling practices, modified cooling strategy at Stelmor conveyor showed a reduction in scale by 15% over conventional cooled wire rods. In conclusion, the enhanced cooling rate after y- aFe transformation on Stelmor conveyor has led to reduced amount of scale formation as well as higher percentage of FeO. The tensile properties of the scale showed minor increase in strength (~4 MPa). Trials were carried out as per following cooling schedule. (i) Conventional rolling: cooling rate, 1.4°Cs-1 after 900°C laying head temperature. The usual practice for producing cost effective wire rods of electrode wire non-rimming grade is to keep laying head temperature at 900°C, conveyor speed of 0.30m/s, no forced cooling at conveyor to achieve slowest cooling rate on air cooling for achieving coarse grained structure. (ii) Modified rolling practice: cooling rate, 1.4° Cs-1 at 900°C laying head temperature till 750°C (y-aFe transformation finish temperature was achieved) and subsequently the cooling rate was raised to 8°Cs-1 till reformer tub. Tranformation finish temperature was approximately 765°Cs-1 cooling rate by Gleeble simulation. Temperature at reformer wire and rod mill is~600°C for normal cooling. As the coil is compacted at this point, the cooling rate is further slower, hence the probablility of wustite to magnetite transformation is maximum. This results in residual magnetite on rod surface even after pickling or after mechanical descaling. The accelerated cooling is conducted after the y- aFe transformation finish temperature. This is achieved at around 750°C by opening two blowers (blowing capacities of 80000m3 / hr) on maximum capacity. The reform temperature after blower opening came down to ~410°C as against the normal temperature ~600°C. The figures of the type of oxides formed after normal and accelerated cooling are given in Figures 1 and 2 which shows lots of unwanted oxides Fe3O4 and Fe2O3 after normal cooling which is not desirable for pickling as well as mechanical descaling. Figure 2 shows scale structure after accelerated cooling after phase transformation which shows uniform layer of wustite as well as low wear of drawing dies during drawing. The type of oxide scale (FeO) most amenable for pickling/mechanical descaling at the customers end was engineered through modification of the temperature at the reformer end of the wire rod mill. The oxide scale formed was approximately 0.60% weight after the process modification. Pickling tests and mechanical descaling trials at customers end confirmed the modified nature of the oxide scale, which agreed well with the process design. The details of the trial data are given in Table 1 and Table 2. Thus by the method of the present invention the amount of oxide scale formed can be reduced and at the same time the oxide scale formed in this manner is desirable both for pickling as well as mechanical descaling. The electrode wire non-rimming (EWNR) grade after modified cooling can be used by electrode manufacturing industry with better yield and good drawing die life with consistent product quality. WE CLAIM 1. A Process for minimizing oxide scale formation during stelmor cooling in a wire and rod mill comprising the steps of: determining γ- α Fe Transformation temperature of the grade of steel using a thermo mechanical simulator; and forming FeO type scale by using a modified cooling rate of 1.4 CS-1 at 900°C 2 LHT till 750°C and subsequently increasing the cooling rate to 8°CS-1 till reformer tub, Characterized in that the said process hinder the formation of magnetite (Fe3O3) and hematite (Fe2CO3) predominantly between the steel substrate and wustite layer and enhance the formation of uniform wustite (FeO) layer. 2. The Process as claimed in claim 1 wherein, said thermo-mechanical simulator is glessble 1500 simulator. 3. The process as claimed in claim 1 wherein, the scale formed is evaluated by mechanical descaling and chemical pickling. 4. The process as claimed in claim 1 wherein, said modified cooling rate results in a reduction of scale formation by 15 % over conventional cooling. 5. The process as claimed in claim 4 wherein, said modified cooling rate results in a higher percentage of FeO scale formation. 6. A process for minimizing oxide scale formation during stelmor cooling in a wire and rod mill as substantially described and illustrated herein with respect to accompanying figures of metallurgical micro-structure. Dated this 15th day of March 2006 A process for minimizing oxide scale formation during Stelmor cooling in a wire and rod mill, comprising the steps of: - determining γ-α Fe transformation temperature of the grade of steel using a thermo-mechanical simulator; - and forming FeO type scale by using a modified cooling rate of ~ 1.4 C s-1 at 900°C LHT till 750°C and subsequently increasing the cooling rate to 8° C s-1 till reformer tub.

Documents:

222-KOL-2005-(12-12-2011)-FORM-27.pdf

222-kol-2005-granted-abstract.pdf

222-kol-2005-granted-claims.pdf

222-kol-2005-granted-correspondence.pdf

222-kol-2005-granted-description (complete).pdf

222-kol-2005-granted-drawings.pdf

222-kol-2005-granted-examination report.pdf

222-kol-2005-granted-form 1.pdf

222-kol-2005-granted-form 13.pdf

222-kol-2005-granted-form 18.pdf

222-kol-2005-granted-form 2.pdf

222-kol-2005-granted-form 3.pdf

222-kol-2005-granted-form 5.pdf

222-kol-2005-granted-gpa.pdf

222-kol-2005-granted-reply to examination report.pdf

222-kol-2005-granted-specification.pdf