Title of Invention

A METHOD OF HEAT TREATMENT OF A HOT ROLLED LOW CARBON STEEL SHEET PRIOR TO GALVANIZING AND PRODUCT PRODUCED THEREOF

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- 40°C/min;thereafter cooling the furnace to room temperature at cooling rate of 15°C/min;followed by galvanizing the withdrawn steel sheet to obtain an optimizing zinc coated sheet with carbide free grain interior and segregated carbide at the grain boundary.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.

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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

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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.

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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.

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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

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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

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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.

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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.

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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.

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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.

Documents:

01591-kol-2007-abstract.pdf

01591-kol-2007-claims.pdf

01591-kol-2007-correspondence others.pdf

01591-kol-2007-description complete.pdf

01591-kol-2007-drawings.pdf

01591-kol-2007-form 1.pdf

01591-kol-2007-form 2.pdf

01591-kol-2007-form 3.pdf

01591-kol-2007-gpa.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-FORM 1-1.1.pdf

1591-kol-2007-FORM 18.pdf

1591-KOL-2007-FORM 2-1.1.pdf

1591-KOL-2007-FORM 3.pdf

1591-KOL-2007-GPA.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-OTHERS 1.1.pdf

1591-KOL-2007-OTHERS-1.1.pdf

1591-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

abstract-01591-kol-2007.jpg


Patent Number 251529
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
Inventors:
# Inventor's Name Inventor's Address
1 DR. V SUBRAMANYA SARMA NOT AVAILABLE
2 PROF. B S MURTY NOT AVAILABLE
3 MR. A CHATTOPADHYAY NOT AVAILABLE
4 DR. D BHATTACHARJEE NOT AVAILABLE
PCT International Classification Number C21D
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA