Title of Invention

A METHOD OF PRODUCING HIGH STRENGHT NON-MICROALLOYED GALVANNEALED STEEL STRIP AND STEEL STRIP PRODUCED THEREOF

Abstract This invention relates to a method of producing a high strength non-microalloyed galvannealed steel strip/sheet comprising the steps of preparing low carbon steel cast slabs of compositions in wt% Carbon 0.07 to .11, Manganese 1.45 to 1.62, Sulphur 0.001 to 0.007, Phosphorus 0.019 to 0.08, Silicon 0.017 to 0.032, Aluminium 0.020 to 0.045 and Nitrogen 20 to 40 ppm, hot rolling the cast slabs at 600 - 650°C to strip/sheet, cold rolling the strip/sheet and processing the strip/sheet through annealing at 700-750°C in a continuous oven in cycles to result fully recrystallised microstructure of the strip/sheet without any grain growth resulted from microalloying.
Full Text - 2 -
Field of Invention
This invention relates to production of high strength (440 MPa) galvannealed steel strip resulted from steel slab without microalloying addition and steel strip resulted thereof.
Background of the Invention
In the industrial production of steel strip, steel strips or sheets are galvanized to generate coated strips/sheets with Zinc in order to protect the steel surface from oxidisation
To maintain compatible strength characteristics of the steel strips/sheets the steel composition is maintained with microalloy additions cromium, nickel, copper, molybdenum etc which make the steel castely. But during the course of continuious annealing due to presence of higher level of carbon and Manganese, Manganese sulphides precipitated at the grain boundaries of the resulted steel microstructure and carbides are segregated in the resulted microstructure of the steel causing the steel strip to be oxidised resulting uncoated spots of the galvanized steel strips. Such prior steel strips are unsuitable to maintain strength characteristics in practical use or on forming of such steel strips.

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To remove the aforesaid difficulties of prior art the present invention proposes to develop steel strips/sheets of high strength by producing steel slab avoiding microalloying and treating the said slab to result strips/sheets of fully recrystallised microstructure.
Another objective of the invention is to prepare a steel composition avoiding microalloy additions thus maintaining cost characteristics.
According to a further objective of the invention it is proposed to produce giavannealed steel strips/sheets without any uncoated spots going through annealing of the galvanized steel strips/sheets.
A still further objective of the invention is to produce weldable strips/sheets by maintaining wider weldable lobe through weldibility test characteristics studies.
According to yet another objective of the invention it is proposed to develop steel strips/sheets maintaining compatible stretch flangibility characteristics maintained through hole expansion ratio test studies.
According to still yet another objective of the invention the microstructure of steel strips/sheets are maintained with delta phase.
The steel strips/sheets produced according to the invention can be used in all types of structural members which do not require extensive forming. The steel strips/sheets can be used successfully in auto industry.

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According to the invention there is provided a method of producing a high strength non-microalloyed galvannealed steel strip/sheet comprising the steps of preparing low carbon steel cast slabs of compositions in wt% carbon 0.07 to .11, Manganese 1.45 to 1.62, sulphur 0.001 to 0.007, phosphorus 0.019 to 0.08, Silicon 0.017 to 0.032, Aluminium 0.020 to 0.045 and Nitrogen 20 to 40 ppm,hot rolling the cast slabs at 600 - 650°C to strip/sheet, cold rolling the strip/sheet and processing the strip/sheet through annealing at 700-750°C in a continuous oven in cycles to result fully recrystallised microstructure of the strip/sheet without any grain growth resulted from microalloying.
Description of the Invention
The present invention will be best understood from the following description with reference to the accompanying drawings in which
Figure 1 represents microstructures of gleeble simulated sample at 740°C annealing temperature of line speed (a) 75 meter per minute (b) 85 meter per minute.
Figure 2 represents generated weldibility lobe with welding parameters as stated in Table 6.
Figure 3 represents the image of the sample after conducting the stretch flange test.

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Figure 4 represents Micrographs of steel strips/sheets of (a) 1.75 mm and (b) 1 mm thickness resulted through stretch flange test.
Figure 5 represents SEM micrographs for 1.75 mm and 1 mm thick steel strips/sheets.
Laboratory heat of 1 kg was taken in vacuum induction melting furnace. The chemical composition of the laboratory heat made is shown in the following Table 1.
Table 1: Chemical composition of the Laboratory Sample
c Mn S p Si Al N
0.07 1.62 0.011 0.008 0.017 0.05 Max 40 ppm Max
The solidified steel masses were subsequently hot rolled to 3mm thickness and finally cold rolled to lmm thickness. The cold rolled samples were subjected to annealing cycles at 740°C for the line speeds of 75 meter/minute and 85 meter/minute maintained through Gleeble-1500 thermo-mechanical simulator. The results are shown in the Table 2.

- 6 -Table 2: Mechanical properties obtained for the lab heat
Sample No. Annealing Temp (°C) Processed Speed (meter per minute) YS (MPa) UTS (MPa) EL% (50 GL)
1 740 75 341.00 453.00 -
2 740 85 350.00 467.00 -
The microstructure of Gleeble simulated samples have been shown in Figure 1. The microstructure showed full recrystallisation but no grain growth in all the cases.
Based on the laboratory experiments and previous plant trials one heat was made and processed. The chemical composition of plant heat made is shown in the Table 3. The hot rolling details of two slab are shown in Table 4.
Table 3: Chemical composition for the plant trial
c Mn S P Si Al N
0.11 1.45 0.007 0.019 0.032 0.044 39ppm

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Table 4: Details of hot rolling characteristics from plant trials
Slab Samples Thickness (mm) Width (mm) SDOT (°C) FRT CT YS (MPa) UTS (MPa) EL %
1 3.84 1190 1206 877 621 355 520 38
2 3.03 1160 1170 870 607 355 520 38
Where SDOT = Slab Drop Out Temperature FRT = Finish Rolling Temperature CT = Coiling Temperature
The hot rolled coils of slab samples 1 and 2 casted at 621 and 607°C were cold rolled to 1 and 1.75 mm thickness respectively and processed through annealing cycles at the laboratory. The details of the cold rolled coil processing conditions are detailed in the table 5.
Table 5: Details of cold rolling
Coil No. Thickness mm Width mm Annealing Temp°C Processed Speed (meter per minute) YS (Mpa) UTS (Mpa) B.% (50 GL)
1 1 1160 751 76 358.00 454.00 34
2 1 1160 743 76 358.00 454.00 34
3 1.73 1100 711 50 335.00 444.00 37
4 1.75 1160 736 62 335.00 444.00 37

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The above coil samples of 1 mm and 1.75 mm thickness and width 1160mm are cold roiled and subjected through annealing cycles at temperatures as defined in the above table and elongation % is evaluated through simulator at simulation value of 50 Gfeeble (GL).
Although the strength criteria of the product development was met but other properties weldability, stretch flangeability and microstructures were also evaluated through the following procedures.
Wefdabifity Studies
Weldibility lobe was determined for lmm and 1.75 mm thick steeJ samples. The nugget size used for weldability lobe was 3.5Vt to 5Vt. The sample and their welding details are given in the Table 6. The weldability lobe is shown in the Figure 2. It is observed that the lobe is quite wider and therefore it is evaluated that the steel has weldable characteristics.
Table 6: Welding parameters used during weldability studies
Thickness mm Coil Specimen Welding force (Kilo Newton) Electrode diameter
1 1 3.1 6mm
1.75 2 3.92 7mm

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Stretch Flange Test
Stretch Flangeability or hole expansion ratio is one of the important criteria for automotive grade steels. In the test 50mmx50mm square samples were taken and 5mm diameter hole was drilled. The hole was then subsequently expanded by pressing it with a conical punch having 60° angle. The hole expansion value, n, was calculated using the following equation.
df-do
n. = x 100
do
Where df is the final diameter and d0 is the initial diameter of the hole.
Image of the sample after the test is shown in the Figure 3. The result of the hole expansion test for 1mm and 1.75mm thick sheet is shown in the Table 7. It is observed that the hole expansion ratio is more than 150% which is considered as a very good stretch flange steel.
Table 7: Result of the stretch flange test
Thickness Mm Coil ID Initial Hole Dia (mm) Hole dia after Stretching (mm) Stretch flangeability (Final dia-Initial dia)xlOO/Initial dia
1 5A17001 5 13.76 178.6%
1.75 5A17331 5 13.93 175.2%

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The steel micrographs of test samples taken after stretch flangeability test have been shown in Figure 4. The micrographs are found to be identical to laboratory experimentation as shown in Fig. 1. Full recrystallisation and no grain growth is observed for both the thickness of test samples of Table 7. The EDS (Energy Dispersive Spectroscopy) analysis using scaning electron microscope (SEM) revealed that the galvannealing phototransformation is satisfactory as the iron content at the edge of the coating were found to be more than 9% for both the coils. The SEM micrograph is shown in the Figure 5 and the corresponding iron zinc profiles are represented in the Table 8. In figure 5 the number (1,2,3.) represents the various points on the coating starting from the edge of the substrate.
Table 8: Iron-zinc profile for lmm and 1.75mm gaivannealed samples from plant
lmm Lmm 1.75mm 1.75mm
Features Fe% Zn% Fe% Zn%
Feature 1 4.89 77.87 9.61 80.61
Feature 2 9.47 86.74 9.80 86.49
Feature 3 10.06 86.96 9.58 87.52
Feature4 9.18 86.56 10.63 87.01
Feature 5 9.49 86.59 10.19 87.54
Feature 6 13.13 85.08 11.37 85.86
Feature 7 46.16 50.45 27.01 68.58
Feature 8 89.71 1.96 92.26 0.83
The results obtained on the plant third trial indicate that one lab trial indicates that the YS and the UTS have been achieved to the targeted value. The stretch Mange test and weldabillty studies have proved that the steel is having very good hole expansion ratio and the steel is amenable for welding applications. The EDS analysis has shown that the steel strips/sheets according to the present invention can be gaivannealed, without any risk of surface oxidisation or decoated

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spots of the strips/sheets during continuous annealing operations of the galvanized coiled strips/sheets through developing fully recrystallised microstructure of the steel strips/sheets - thus avoiding any non microalloying in the steel, and by avoiding any risk of carbide or sulphide segregation.
The invention as herein described and illustrated through figures and tables should not be read in a restrictive manner as various adaptations, modifications and changes are possible as encampused within the scope of the appended claims.

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WE CLAIM
1. A method of producing a high strength non-microalloyed galvannealed
steel strip/sheet comprising the steps of preparing low carbon steel cast slabs of
compositions in wt% Carbon 0.07 to .11, Manganese 1.45 to 1.62, Sulphur 0.001
to 0.007, Phosphorus 0.019 to 0.08, Silicon 0.017 to 0.032, Aluminium 0.020 to
0.045 and Nitrogen 20 to 40 ppm, hot rolling the cast slabs at 600 - 650°C to
strip/sheet, cold rolling the strip/sheet and processing the strip/sheet through
annealing at 700-750°C in a continuous oven in cycles to result fully recrystailised
microstructure of the strip/sheet without any grain growth resulted from
microalloying.
2. A method as claimed in claim 1 wherein the steel strip is produced from
steel slab by annealing cycles at 740°C for the line speed of 75 meter/minute and
85 meter/minute.
3. A method as claimed in claims 1 and 2 wherein the hot rolled coils of strip
are cold rolled to lmm to 1.75mm thickness.
4. A method as claimed in the preceeding claim wherein the strip is hot
rolled to 3mm thickness before cold working.
5. A method as claimed in the preceeding claims wherein the strip/sheet
produced after annealing cycles has high mechanical strength of UTS 440 to 455
MPa,YS335TO350MPa.

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6. A method as claimed in the preceeding claims wherein the strip/sheet
produced is characterized by good weldibility of wider lobe as evaluated through
weldibility test using Ac welder.
7. A method as claimed in the preceeding claims wherein the strip/sheet
produced is characterized by hole expansion ratio of > 150% as evaluated
through Stretch Flange Test and the hole expansion value is determined by the
following equation

in which n is the expansion hole value,
df is the final diameter of the drilled hole of the strip and expanded by pressing with a conical punch and
d0 is the initial diameter of the drilled hole.
8. A method of producing a high strength non-microalloyed galvannealed steel strip/sheet as herein described and illustrated.

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9. A steel strip/sheet produced by the method as claimed in claim 1 having
composition in wt% C - 0.07 to .11, Mn - 1.45 to 1.62, S - 0.001 - 0.007, P -
0.019 to 0.08, Si - 0.017 to 0.032, Al - 0.020 to 0.045 and Nitrogen 20 to 44
ppm.
10. A steel strip/sheet as claimed in claim 9 wherein the strip/sheet is used in
automobile industry.

Dated this 13th day of September, 2006.
A METHOD OF PRODUCING HIGH STRENGTH NON-MICRQALLOYED GALVANNEALED STEEL STRIP AND STEEL STRIP PRODUCED
THEREOF
This invention relates to a method of producing a high strength non-microalloyed gaivannealed steel strip/sheet comprising the steps of preparing low carbon steel cast slabs of compositions in wt% Carbon 0.07 to .11, Manganese 1.45 to 1.62, Sulphur 0.001 to 0.007, Phosphorus 0.019 to 0.08, Silicon 0.017 to 0.032, Aluminium 0.020 to 0.045 and Nitrogen 20 to 40 ppm, hot rolling the cast slabs at 600 - 650°C to strip/sheet, cold rolling the strip/sheet and processing the strip/sheet through annealing at 700-750°C in a continuous oven in cycles to result fully recrystallised microstructure of the strip/sheet without any grain growth resulted from microalloying.

Documents:

00926-kol-2006 abstract.pdf

00926-kol-2006 claims.pdf

00926-kol-2006 correspondence others.pdf

00926-kol-2006 description[complete].pdf

00926-kol-2006 drawings.pdf

00926-kol-2006 form-1.pdf

00926-kol-2006 form-2.pdf

00926-kol-2006 form-3.pdf

00926-kol-2006 g.p.a.pdf

00926-kol-2006-correspondence-1.1.pdf

00926-kol-2006-correspondence-1.2.pdf

00926-kol-2006-form-1-1.1.pdf

00926-kol-2006-form-9.pdf

926-KOL-2006-ABSTRACT 1.1.pdf

926-KOL-2006-FORM 1 1.1.pdf

926-KOL-2006-GPA.pdf

926-kol-2006-granted-abstract.pdf

926-kol-2006-granted-claims.pdf

926-kol-2006-granted-correspondence.pdf

926-kol-2006-granted-description (complete).pdf

926-kol-2006-granted-drawings.pdf

926-kol-2006-granted-examination report.pdf

926-kol-2006-granted-form 1.pdf

926-kol-2006-granted-form 18.pdf

926-kol-2006-granted-form 2.pdf

926-kol-2006-granted-form 3.pdf

926-kol-2006-granted-gpa.pdf

926-kol-2006-granted-reply to examination report.pdf

926-kol-2006-granted-specification.pdf

926-KOL-2006-PETETION UNDER RULE 137.pdf

926-KOL-2006-REPLY TO EXAMINATION REPORT.pdf

abstract-00926-kol-2006.jpg


Patent Number 235089
Indian Patent Application Number 926/KOL/2006
PG Journal Number 26/2009
Publication Date 26-Jun-2009
Grant Date 24-Jun-2009
Date of Filing 13-Sep-2006
Name of Patentee TATA STEEL LIMITED
Applicant Address JAMSHEDPUR
Inventors:
# Inventor's Name Inventor's Address
1 MR. ANIL KUMAR VERMA TATA STEEL LTD.,JAMSHEDPUR-831 001
2 MR. RAJIV MISHRA TATA STEEL LTD.,JAMSHEDPUR-831 001
3 MR. NIKHILES BANDYOPADHYAY TATA STEEL LTD.,JAMSHEDPUR-831 001
4 MR. M.C. SADHU TATA STEEL LTD.,JAMSHEDPUR-831 001
5 MR. AVTAR SINGH SAINI TATA STEEL LTD.,JAMSHEDPUR-831 001
6 MR. VENUGOPALAN TATA STEEL LTD.,JAMSHEDPUR-831 001
PCT International Classification Number C 21 C 5/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA