| Title of Invention | A METHOD OF HEAT TREATMENT OF A HOT ROLLED LOW CARBON STEEL SHEET PRIOR TO GALVANIZING AND PRODUCT PRODUCED THEREOF |
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| Abstract | This invention relates to a method of heat treatment of a hot rolled low carbon steel sheet prior to galvanizing and product produced thereof to achieve optimized coatability of zine on the steel sheet surface comprising keeping low carbon steel sheet in a reducing atmosphere electrical furnace being heated firstly to 550°C ± 5°C from room temperature at heating rate of 30 - 40°C/min; holding the same furnace temperature for a minute; then cooling the furnace to 460°C ± 5°C at the same cooling rate as during heating at 30 - 40°C/min;thereafter cooling the furnace to room temperature at cooling rate of 15°C/min;followed by galvanizing the withdrawn steel sheet to obtain an optimizing zinc coated sheet with carbide free grain interior and segregated carbide at the grain boundary. |
| Full Text | 2 FIELD OF THE INVENTION: The present invention relates to a method of heat treatment of hot rolled low carbon steel sheet to achieve optimized galvanizing operation. More specifically the invention relates to heat treatment of low carbon low alloy steel to reduce carbide segregation at the surface level of hot rolled steel sheets to achieve optimized galvanizing and to produce galvanized sheet. In HR steel sheets, the carbon from the matrix gets segregated at the surface/subsurface level during processing. The carbides form a thin film at the surface level. During galvanizing of HR steel sheets improper coating is observed due to presence of carbides at the surface. The carbide layer actually hinders the formation of inhibition layer of Fe2 Al5 which is the primary phase that forms during galvanizing. Therefore, poor coating is observed in hot rolled galvanized steel sheets. The present invention has yielded a heat treatment process with a particular furnace atmosphere, which can shift the carbide segregation to the grain boundaries and making comparatively clean steel surface. Sub-surface properties of HR steel coils are controlled by the segregation of alloying elements and this has implications on the Zinc coatability of the steel. Depending on the finish rolling temperature (FRT) and coiling temperature (CT) different alloying elements segregate at the sub surface level during the hot rolling process. It is observed that for low carbon steels in comparison to other alloying elements, carbon gets easily segregated at the surface as carbide. The presence of carbides at the surface hampers the formation of zinc layer during galvanizing operation. In order to achieve proper galvanizing the removal of carbide from the surface is necessary. The present invention is related to a method to reduce the carbide segregation at the surface level to achieve better galvanizing operation. 3 DESCRIPTION OF THE INVENTION: The main objective of the invention is to develop a heat treatment method on maintenance of specific furnace atmosphere so as to shift the carbide segregation to the grain boundaries from the grain interior and thus producing comparatively clean steel sheet surface. Another objective of the invention is to develop hot rolled (HR) Low carbon Aluminium killed (LACK) steel sheets having composition in weight % C-0.03, Mn-0.2-0.3, S-0.01-015, P-0.015-0.020, Si-.015-0.025, AI-0.05-0.08, N2-0.005- 0.006. Yet another objective of the invention is to galvanize the said heat treated steel sheet to achieve optimized Zn coating on the heat treated steel sheet surface. The said objectives are carried out on first employment of sub-surface transmission electron microscopy (TEM) before the heat treatment of steel sheet showing metallographic structure of segregated carbides at the surface level and also on detection presence of carbide detected by X-ray micro-analysis. Before the heat treatment the HR steel sheets are pickled in acid to remove oxides formed on the surface of the sheets during HR operations. Thereafter the steel sheets are given heat treatment in a furnace maintained at reducing atmosphere of 5% hydrogen filled with Nitrogen with dew point of the reducing atmosphere maintained at-40°C. Such a low dewpoint inside the furnace ensures that the furnace is free from oxygen and possibilities of oxidation at surface level is minimal. Changing of dew point may lead to the oxidation of the surface which will lead to improper Zn coating. The sheets before galvanizing operation is kept in the furnace and then heated gradually upto 550°C with reducing gas supply in 4 the furnace during heating and subsequent cooling of the heat treatment cycle. The steel sheets are thereafter galvanized. The pickled sheet after the heat treatment is evaluated of finding the morphology by TEM and by X-ray microanalysis (EDX) for detection of any carbide. It was found that all the carbides are found segregated at grain boundaries and the interior of grain was found free from carbides and the EDX microanalysis revealed no formation of oxygen in the steel sheet surface, thereby concluding that no oxidation has occurred during heat treatment of the sheets. The invention will be better understood from the following descriptions in which Fig 1(a) represents Sub-Surface Transmission Electron Microscopy (TEM) bright field image showing morphology of segregated carbides at the sub-surface level of test sample of H R sheets before heat treatment. Fig 1(b) represents Energy Dispersive X-ray Microanalysis (EDX) spectrum of test sample of HR steel sheet before heat treatment showing presence of Carbides with characteristic X-ray energy peak for carbon, oxygen and iron at sub-surface level of a sample measured in nm. Figure 2 represents schematically the heat treatment operation of a HR steel sheet sample prior to galvanizing. Figure 3 represents a photograph of the heat treated sample showing no external or internal oxidation. Figure 4 (a) represents TEM bright field of image of grain boundary segregation of carbide and grain interior free from carbides. 5 Figure 4 (b) represents EDX microanalysis of a heat treated sample showing X- ray characteristic peaks for carbon and iron and not any oxygen peak indicating no formation of oxide in the sample after heat treatment. The present invention is aimed to enhance the Zn coatability property on heat treated low carbon steel sheet by achieving a heat treatment cycle in a controlled reducing atmosphere maintained furnace before galvanizing the steel sheet, which heat treatment was aimed to shift the precipitation of carbides from inner grain surface to grain boundary, as the carbides so formed at the inner grain surface and sub surface of the inner grain encourage oxidation of the carbides thus resulting spots after galvanizing throughout the grain structure of the galvanized sheet and making the galvanized sheet unpleasant looking to the customer with irregular spotted galvanized sheet due to oxidation. Before developing heat-treating cycle, the HR steel sheet samples were first evaluated of the manner of carbide formation through TEM to find out the morphology of segregated carbides, which showed segregated carbides formation throughout the grain structure at the surface level as shown in Fig la. The presence of carbide was detected by EDX spectrum of X-ray from the carbide network deposited at the grain structure of sheet steel sample of low carbon steel as shown in Fig 1(b). Before doing any heat treatment operation the samples were pickled with acid to remove the oxides present at the surface of steel sheet. This pickling operation removes all the oxides formed during the hot rolling operations but can not remove the segregated carbides. In order to remove / reduce the carbides from the surface; one heat treatment cycle prior to the galvanizing operation is employed. 6 Heat treatment of the low carbon steel sheet was thought to disintegrate carbides so deposited by heating a steel sheet sample at a maximum temperature much below the eutectoid transformation temperature of 723°C, not to be interfered with any phase transformation. A heat treatment cycle was achieved for the purpose on heating HR steel sheet sample to 550°C ± 5°C on heating the sample kept in a furnace maintained with reduced atmosphere on heating at the rate of 30-40°C/min from room temperature, then maintaining the said temperature at 550° ± 5°C for a minute, thereafter cooling the furnace at the same rate of 30-40°C/min as like during heating, to cool the furnace to 460°C ± 5°C, followed by cooling to room temperature by maintaining cooling rate of 15°C/min. The said heat treatment cycle can be best understood from the schematic diagram as shown in Fig -2. The conditions maintained in the furnace was as follows:- • Reducing atmosphere - IV H2 5% • Dew point maintained inside the furnace: -40°C • The reducing gas supply was maintained during heating as well as during cooling operation. The furnace being heated electrically in an induction furnace or other known resistance-heating coil furnace and controlling heating rate and cooling rate accordingly. During the above heat treatment the iron carbides structure recrystalize when the carbides breaks down to form carbon, which move towards the grain boundary surfaces from all sides of grains and precipitated as carbides at the grain boundary while leaving behind some portion of iron (Fe) in the inner grain 7 and reduced concentration of carbides at the grain boundary, which feature of the invention satisfy the customer with much reduced spot due to oxidation of the carbides but in a pleasant look of regular spots at the grain boundary only on galvanization after the heat treatment of HR low carbon steel sheet. The overall surface of the heated sheets has retained the luster after the above mentioned heat treatment as it had after the pickling operations. For each set of experiments one sample was mirror polished prior to the heating. The surface of the sample remains unaltered after the reheating operation as shown in Fig 3. This indicates that no external oxidation took place. In order to know the morphology of the segregated carbides already present in the HR steel, sub-surface TEM was carried out. All the carbides are segregated at the grain boundaries only as shown in Fig 4 a. When the grain interior is looked into, no carbide is observed inside the grains. So, it can be concluded that all the carbides get segregated to the grain boundaries making the grain interior free from carbides. The EDX microanalysis as shown in Fig 4 b has not shown any oxygen. Therefore from the above results it can be concluded that under the above mentioned conditions external as well as internal oxidation has not occurred. The heat treated steel sheet free from any external or internal oxidation is finally galvanized to produce galvanized sheet with delayed and less external oxidation only at the grain boundary and not any oxidation at the inner grain surface hereto unknown in the prior state of art. The invention as herein narrated should not be read in a restrictive manner to construe the invention as many alterations of heating temperature and heating and cooling rates during heat treatment and reducing atmosphere conditions of 8 the furnace, adaptations of the heat treatment arrangement and modifications applied to various grades of low carbon low alloy steels are possible within the ambit and scope of the invention as described above with its technical features to find a solution of prior art difficulties as defined in the appended claims. 9 WE CLAIM: 1. A method of heat treatment of a hot rolled low carbon steel sheet prior to galvanizing to achieve optimized coatability of zine on the steel sheet surface comprising keeping low carbon steel sheet in a reducing atmosphere electrical furnace being heated firstly to 550°C ± 5°C from room temperature at heating rate of 30 - 40°C/min; holding the same furnace temperature for a minute; then cooling the furnace to 460°C ± 5°C at the same cooling rate as during heating at 30 - 40°C/min; thereafter cooling the furnace to room temperature at cooling rate of 15°C/min; followed by galvanizing the withdrawn steel sheet to obtain an optimizing zinc coated sheet with carbide free grain interior and segregated carbide at the grain boundary. 2. The method of heat treatment as claimed in claim 1 wherein the heat treated low carbon steel sheet is a low carbon alluminium killed (LACK) steel having composition in weight % GO.03-0.04, Mn-0.2-0.3, S-0.01- 0.015, P-0.015-0.020, Si-0.015-0.025, AI-0.05-0.08 and N-0.005-0.006. 3. The method of heat treatment as claimed in claim 1 wherein the reducing furnace is maintained with an inert atmosphere of Nitrogen fed with 5% hydrogen gas to maintain a furnace atmosphere of N2 - 5% H2 (reducing) with a dew point of said atmosphere maintained at - 40°C. 10 4. The method of heat treatment as claimed in claim 3 wherein the said atmosphere of N2 - 5% H2 is maintained by gas supply during heating and cooling of heat treatment cycle. 5. The method of heat treatment as claimed in the preceding claims wherein the iron - carbide grain structure in the interior grains of the steel during heating is recrystallised to disintegrate into carbon which propagate to the grain boundaries and segregate as carbides during cooling with less amount of carbide formation as found before the heat treatment. 6. The method of heat treatment as claimed in the preceding claims wherein the HR low carbon steel sheet sample is pre-evaluated prior to heat treatment by TEM bright field image and EDX spectrum of the carbide network showing morphology of segregated carbides at the surface level test sample and presence of carbides from characteristic X-ray energy peak for carbon, oxygen and iron at sub surface level of the sample. 7. The method of heat treatment claimed in claim 1 wherein before the heat treatment the HR low carbon steel sheet is acid pickled to remove all the oxides formed during hot rolling operation. 8. The method of heat treatment as claimed in the proceeding claims wherein the heat treated steel sheet sample is post evaluated through TEM bright field image and EDX spectrum showing segregated carbides formation at grain boundaries and oxygen free grain structure both at inner grain and grain boundary thus revealing that no external or internal oxidation has occurred during heat treatment cycle. 11 9. A galvanized low carbon steel sheet with optimized zine ratability obtained by the heat treatment steps and conditions maintained during heat treatment as claimed in the preceeding claims. Dated this 23rd day of November 2007. This invention relates to a method of heat treatment of a hot rolled low carbon steel sheet prior to galvanizing and product produced thereof to achieve optimized coatability of zine on the steel sheet surface comprising keeping low carbon steel sheet in a reducing atmosphere electrical furnace being heated firstly to 550°C ± 5°C from room temperature at heating rate of 30 - 40°C/min; holding the same furnace temperature for a minute; then cooling the furnace to 460°C ± 5°C at the same cooling rate as during heating at 30 - 40°C/min;thereafter cooling the furnace to room temperature at cooling rate of 15°C/min;followed by galvanizing the withdrawn steel sheet to obtain an optimizing zinc coated sheet with carbide free grain interior and segregated carbide at the grain boundary. |
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01591-kol-2007-correspondence others.pdf
01591-kol-2007-description complete.pdf
1591-KOL-2007-(22-02-2012)-ABSTRACT.pdf
1591-KOL-2007-(22-02-2012)-CLAIMS.pdf
1591-KOL-2007-(22-02-2012)-CORRESPONDENCE.pdf
1591-KOL-2007-(22-02-2012)-DESCRIPTION (COMPLETE).pdf
1591-KOL-2007-(22-02-2012)-DRAWINGS.pdf
1591-KOL-2007-(22-02-2012)-FORM 1.pdf
1591-KOL-2007-(22-02-2012)-FORM 2.pdf
1591-KOL-2007-ABSTRACT-1.1.pdf
1591-KOL-2007-AMANDED CLAIMS.pdf
1591-KOL-2007-CORRESPONDENCE 1.1.pdf
1591-KOL-2007-CORRESPONDENCE 1.2.pdf
1591-kol-2007-CORRESPONDENCE OTHERS 1.1.pdf
1591-KOL-2007-DESCRIPTION (COMPLETE)-1.1.pdf
1591-KOL-2007-DRAWINGS-1.1.pdf
1591-KOL-2007-EXAMINATION REPORT REPLY RECIEVED.pdf
1591-KOL-2007-EXAMINATION REPORT.pdf
1591-KOL-2007-GRANTED-ABSTRACT.pdf
1591-KOL-2007-GRANTED-CLAIMS.pdf
1591-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf
1591-KOL-2007-GRANTED-DRAWINGS.pdf
1591-KOL-2007-GRANTED-FORM 1.pdf
1591-KOL-2007-GRANTED-FORM 2.pdf
1591-KOL-2007-GRANTED-LETTER PATENT.pdf
1591-KOL-2007-GRANTED-SPECIFICATION.pdf
1591-KOL-2007-REPLY TO EXAMINATION REPORT.pdf
| Patent Number | 251529 | |||||||||||||||
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| Indian Patent Application Number | 1591/KOL/2007 | |||||||||||||||
| PG Journal Number | 12/2012 | |||||||||||||||
| Publication Date | 23-Mar-2012 | |||||||||||||||
| Grant Date | 21-Mar-2012 | |||||||||||||||
| Date of Filing | 23-Nov-2007 | |||||||||||||||
| Name of Patentee | TATA STEEL LIMITED | |||||||||||||||
| Applicant Address | JAMSHEDPUR | |||||||||||||||
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| PCT International Classification Number | C21D | |||||||||||||||
| PCT International Application Number | N/A | |||||||||||||||
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