Title of Invention

AN IMPROVED PROCESS FOR PRODUCING FERRITIC STAINLESS STEEL STRIPS HAVING REDUCED YIELD POINT ELONGATION

Abstract An improved process for producing ferritic stainless steel strips having reduced yield point elongation, comprising the following steps in sequence : (i) selecting conventionally produced 4 mm thick hot rolled ferritic stainless steel strips of composition (by weight %): C-0.05, Mn-0.45, Si-0.20, P-0.02, S-0.001, Cr-16.80, Ni-0.25, Mo-0.01, N-0.03, Al-0.015 and Fe-the balance; (ii) annealing the strips in a continuous annealing furnace at a temperature of 900°C for 4 minutes; (iii) pickling the annealed strips in the standard manner; (iv) cold rolling the strips for reducing thickness thereof to 0.5 mm in two stages, the reduction in thickness in the first stage being 55 to 65% and that in the second stage being 65 to 75% with final annealing of the strips for 1 minute in a continuous annealing furnaces; characterised in that the strips are finally annealed at a temperature of 880°C and subjected to minimum one skin-passing.
Full Text The present invention relates to an improved process for producing ferritic stainless steel strips having reduced yield point elongation.
The ferritic stainless steel strips of thickness less than 1.0 mm containing 1796 by weight of chromium produced in the conventional process by applying the final annealing treatment at 850°C has the drawback of having an excessive yield point elongation, called Luders strain, of as high as 5%. The Luders strain in steel is caused by its inhomogeneous deformed and undeformed regions, popularly known as Luders bands. In the pressed components the Luder strain may lead to stretcher strain markings. In order to remove the Luders strain, the strips are subjected to a number of skin-passing in the skin pass mill of a steel plant at an increase in the production cost thereof. The mechanical properties of the strips such as yield strength (Y.S.), ultimate tensile strength (U.T.S.) and total elongation are also adversely affected by the Luders strain and the skin-passing applied to remove the same.
The objective of the invention is to provide a process for producing cold rolled ferritic stainless steel strips having reduced Luders strain and hence improved surface glaziness.
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Another objective is to provide a process for producing cold rolled ferritic stainless steel strips, the Luders strain in which can be removed by a reduced number of skin-passing and the mechanical properties of which are improved without being affected appreciably by skin-passings thereof.
A further objective is to lower the cost of production of the strips by reducing the number of skin-passing required to improve the surface appearance thereof.
A process for producing 1796 chromius containing stainless steel strips of thickness 0.5 mm by batch annealing has been disclosed in the Indian Patent No. 187375.
An improved process for producing ferritic stainless steel hot bands through high temperature continuous annealing has been disclosed in the Indian Patent No. 192027.
The contents of the specifications of the said two Indian Patents are included in the specification of the present invention by reference.
In developing the process of the present invention, the temperature for final annealing of the strips of cold rolled ferritic stainless steel has been optimised by carrying out detailed investigations on the effect of the variation in the final annealing temperature on the microstructure as well as on the mechanical properties thereof at an increasing number of skin-passing applied thereon. The mechanical properties studied are: yield strength (Y.S.), ultimate tensile strength (U.T.S.), plastic anisotropy (?), strain hardening exponent (n),
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total elongation (T.E.) and yield point elongation (Y.P.E)/ Luders strain (L.S.).
The steps followed in the process are : (i) to select stainless steel strips of 4.0 mm thickness and ferritic AISI-430 grade having nominal composition (by weight %) of C-0.05, Mn-0.45, Si-0.20, P-0.02, S-0.001, Cr-16.80, Ni-0.25, Mo-0.01, N-0.03, Al-0.015, and Fe-the balance; (ii) to anneal the strips in a continuous annealing furnace in the two-phase (a +? ) region thereof at a temperature of 895-910°C and more preferably of 900°C, for 4 minutes; (iii) to pickle the strips; (iv) to cold roll the strips for reducing the thickness thereof to 0.5 mm in two stages with reduction in thickness by 55 to 6596 in the first stage and by 65 to 7596 in the second stage with final annealing in a continuous annealing furnace at 880°C for 1 minute in the single phase (a) region.
Thus the invention provides an improved process for producing ferritic stainless steel strips having reduced yield point elongation, comprising the following steps in sequence : (i) selecting conventionally produced 4 mm thick hot rolled ferritic stainless steel strips of composition (by weight %) : C-0.05, Mn-0.45, Si-0.20, P-0.02, S-0.001, Cr-16.80, Ni-0.25, Mo-0.01, N-0.03, Al-0.015 and Fe-the balance; (ii) annealing the strips in a continuous annealing furnace at a temperature of 900°C for 4 minutes; (iii) pickling the annealed strips in the standard manner; (iv) cold rolling the strips for reducing thickness thereof to 0.5 mm in two stages, the reduction in thicness in the first stage being 55 to 6596 and that in the second stage being
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65 to 7556 with final annealing of the strips for 1 minute in a continuous annealing furnaces; characterised in that the strips are finally annealed at a temperature of 880°C and subjected to minimum one skin-passing.
The invention is described fully and particularly in an unrestricted manner with reference to the accompanying drawings in which
Figure 1 is an optical micrograph at 800X magnification of cold rolled ferritic stainless steel strips produced at a final annealing temperature of 880°C;
Figure 2 is an optical micrograph at 800X magnification of cold rolled ferritic stainless steel strips produced at a final annealing temperature of 910°C;
Figure 3 is an optical micrograph at 800X magnification of cold rolled ferritic stainless steel strips produced at a final annealing temperature of 930°C;
Figure 4 illustrates typical stress-strain curves of :
(A) cold rolled ferritic stainless steel strips produced at a final annealing temperature of 850°C; and
(B) cold rolled ferritic stainless steel strips produced at a final annealing temperature of 880°C.
Figure 5 is a flow chart showing the steps followed
in the process.
Referring to Figs, 1 to 3, it is noted that the microstructure of the strips finally annealed at 880°C consists of equiaxed ferrite grains containing coarse carbide particles (M23C6) in which M is a metal i.e. chromium and C is carbon, that of the strips finally annealed at 910°C has a mixed
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grain structure containing carbide particles and sheets of the particles, and that of the strips finally annealed at 930°C is composed of euiaxed ferrite grains and a small ( It is further noted that the grains in the strip
finally annealed at 880°C (Fig. 1) are more uniform in size,
and of coarser polygonal shape compared with the strips finally
annealed at 910°C and 930°C.
The mechanical properties of the strips finally annealed at the temperatures of 850, 880, 910 and 930°C have been studied both before skin-passing and after subjecting the same upto five skin-passings in sequence.
The test results obtained are presented in Table I from which it is noted that :
(a) the strips finally annealed at 880°C do not show any appreciable variation in the yield strength (Y.S.), ultimate tensile strength (U.T.S.) plastic anisotropy (? ), strain hardening exponent (? ), total elongation (T.E.) with increase in the number of skin-passings from 1 to 5;
(b) the yield point elongation (Luders strain) in the strips finally annealed at 880°C is completely removed by only one skin-passing, unlike the strips finally at 850 and 910°C, requiring
at least five skin-passings for removal of the same; and
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(c) the yield point elongation/Luders strain is 2.5 in the strips finally annealed at 880°C against 4.5 in the strips finally annealed at 850°C.
Referring to Fig. 4, it is noted that the stress-strain curve (B) of the strips finally annealed at 880°C does not show any dip at the start of the yield point elongation unlike the curve (A) of the strips finally annealed at 850°C, showing the said dip marked D.
Based on the test results obtained, the final annealing temperature of the strips has been optimised at 880°C.
Although only a single skin-passing is sufficient to remove the yield point elongation (luders strain) in the strips finally annealed at 880°C,the number of skin-passings for such strips may be increased upto 3 for attaining satisfactory surface glaziness thereof.
The advantages attained by the invented process are :-
1. The Luders strain in the strips produced in the process is reduced.
2. The Luders strain in the strips produced in the process is completely removed by a single skin-passing only.
3. The mechanical properties of the strips produced in the process remain practically unaffected by increasing the number
of skin-passing from 1 to 5.
4. The cost of production of the strips in the process is lowered because of the reduced number of skin-passings required for improving the surface appearance of the strips to the desired level.
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Table I : Variation of mechanical properties - yield strength (Y.S.), ultimate tensile strength (U.T.S.), plastic anisotropy (? ), strain hardening exponent (?), total elongation (T.E.) and yield point elongation (Y.P.E.)/Luders strain of cold rolled stainless steel strip with annealing temperature and number of skin-pass.

Number
of
skin-
passing
Mechanical Properties

Annealing Temperature 850 880 910
(°c)
930

(i) Y.S.
(MPa)
337
347
340
310

(ii) U.T.S.
(MPa)
468
482
525
550
0
(iii) ?

0.94
0.77
0.72
0.73

(iv) ?

0.18
0.25
0.26
0.22

(v) T. E.
%
30
33
33
33

(vi) Y.P.E. % (luders Strain)
4.5
2.5
1.8
0

(i) -

-
347
350
325

(ii) -

-
482
520
520
1
(iii) -

-
0.78
0.70
0.73

(iv) -

-
0.19
0.25
0.20

(v) -

-
28
33
32

(vi) -

-
0
2.5
0

(i) -

-
345
355
350

(ii) -

-
483
515
510
2
(iii) -

-
0.76
0.74
0.73

(iv) -

-
0.19
0.24
0.18

(v) -

-
28
33
30

(vi) -

-
0
1.7
0
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Table I (Contd.)

Number
Mechanical Properties
Annealing Temperature (°C)
skin;-
passing


850
880
910
930

-
344
350
370

(ii)
-
484
510
510
3
(iii)
-
0.75
0.73
0.73

(iv)
-
0.19
0.23
0.18

(v)
-
28
33
29

(vi)
-
0
1.6
0

(i)
-
342
345
370

(ii)
-
485
505
510
4
(iii)
-
0.75
0.75
0.70

(iv)
-
0.19
0.23
0.17

(v)
-
28
29
30

(vi)
-
0
1.3
0

(i)
-
340
340
375

(ii)
-
485
505
510
5
(ill)
-
0.75
0.82
0.65

(iv)
-
0.19
0.23
0.16

(v)
-
28
26
29

(vi)
0
0
0.2
0
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We Claim :-
1. An improved process for producing ferritic stainless
steel strips having reduced yield point elongation, comprising
the following steps in sequence : (i) selecting conventionally
produced 4 mm thick hot rolled ferritic stainless steel strips
of composition (by weight %): C-0.05, Mn-0.45, Si-0.20, P-0.02,
S-0.001, Cr-16.80, Ni-0.25, Mo-0.01, N-0.03, Al-0.015 and Fe-the balance; (ii) annealing the strips in a continuous annealing furnace at a temperature of 900°C for 4 minutes; (iii) pickling the annealed strips in the standard manner; (iv) cold rolling the strips for reducing thickness thereof to 0.5 mm in two stages, the reduction in thickness in the first stage being 55 to 65% and that in the second stage being 65 to 75% with final annealing of the strips for 1 minute in a continuous annealing furnace; characterised in that the strips are finally annealed at a temperature of 880°C and subjected to minimum one skin-passing.
2. The process as claimed in claim 1, wherein the strips are subjected to three skin-passings.
3. An improved process for producing ferritic stainless steel strips having reduced yield point elongation, substantially as herein described and illustrated in the accompanying drawings.
Dated this 26th day of October 1998.

(B. L. BANERJEE) of DePENNING & DePENNING Agent for the Applicants.
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An improved process for producing ferritic stainless steel strips having reduced yield point elongation, comprising the following steps in sequence : (i) selecting conventionally produced 4 mm thick hot rolled ferritic stainless steel strips of composition (by weight %): C-0.05, Mn-0.45, Si-0.20, P-0.02, S-0.001, Cr-16.80, Ni-0.25, Mo-0.01, N-0.03, Al-0.015 and Fe-the balance; (ii) annealing the strips in a continuous annealing furnace at a temperature of 900°C for 4 minutes; (iii) pickling the annealed strips in the standard manner; (iv) cold rolling the strips for reducing thickness thereof to 0.5 mm in two stages, the reduction in thickness in the first stage being 55 to 65% and that in the second stage being 65 to 75% with final annealing of the strips for 1 minute in a continuous annealing furnaces; characterised in that the strips are finally annealed at a temperature of 880°C and subjected to minimum one skin-passing.

Documents:

01900-cal-1998-abstract.pdf

01900-cal-1998-assignment.pdf

01900-cal-1998-claims.pdf

01900-cal-1998-correspondence.pdf

01900-cal-1998-description(complete).pdf

01900-cal-1998-drawings.pdf

01900-cal-1998-form-1.pdf

01900-cal-1998-form-2.pdf

01900-cal-1998-form-3.pdf

01900-cal-1998-form-5.pdf

01900-cal-1998-form-pa.pdf

1900-cal-1998-granted-abstract.pdf

1900-cal-1998-granted-acceptance publication.pdf

1900-cal-1998-granted-assignment.pdf

1900-cal-1998-granted-claims.pdf

1900-cal-1998-granted-correspondence.pdf

1900-cal-1998-granted-description (complete).pdf

1900-cal-1998-granted-drawings.pdf

1900-cal-1998-granted-form 1.pdf

1900-cal-1998-granted-form 2.pdf

1900-cal-1998-granted-form 3.pdf

1900-cal-1998-granted-form 6.pdf

1900-cal-1998-granted-letter patent.pdf

1900-cal-1998-granted-pa.pdf

1900-cal-1998-granted-reply to examination report.pdf

1900-cal-1998-granted-specification.pdf


Patent Number 194752
Indian Patent Application Number 1900/CAL/1998
PG Journal Number 30/2009
Publication Date 24-Jul-2009
Grant Date 09-Sep-2005
Date of Filing 26-Oct-1998
Name of Patentee STEEL AUTHORITY OF INDIA LTD.
Applicant Address ISPAT BHAWAN, LODHI ROAD, NEW DELHI
Inventors:
# Inventor's Name Inventor's Address
1 BIMAL KUMAR JHA RDCIS/SAIL, DORANDA, RANCHI-834002
2 PRITI JHA RDCIS/SAIL, DORANDA, RANCHI-834002
3 SUDHAKAR JHA RDCIS/SAIL, DORANDA, RANCHI-834002
4 CHANDI DUTTA SINGH RDCIS/SAIL, DORANDA, RANCHI-834002
PCT International Classification Number C21D 9/46
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA