Development of Micro Alloyed Structural Steels - Steelworld

Technology
Development of Micro Alloyed
Structural Steels in
Secondary Steel Sector
- R. K. Bagchi, Director, NISST
Introduction
Plain carbon mild steels with varying
carbon contents were used for any
common constructional job. With
Thermo Mechanical Treatment (TMT)
process, it became possible to attain
higher strength values. Yield strength
values of common mild steels are 37-47
kgmm-2, whereas that for TMT steels is
55-65 kgmm-2. This improvement in
YS values shall be responsible in
tonnage saving of constructional
materials. But in turn may be offset by
high cost of TMT plant and
accessories. Moreover, TMT process
is applicable to round sections only. It
is also difficult to obtain desired
microstructure of ribbed bars to
designate those as proper TMT bars. A
reasonable percentage of tonnage TMT
steel bars are available in the market
which fall into this category.
Micro alloying was adopted to
produce structural steels at lower cost
and of consistent quality in secondary
steel sector comprising of induction
furnaces and re-rolling units.
Strength Specific Cost (SSC)
Strength specific cost can be
considered to be an important factor for
selection or development of a material.
It can be expressed in terms of total
cost incurred per tonne of finished
product to raise per unit strength (UTS)
of a material from a base value. Total
cost incurred may include all direct and
indirect cost components.
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Technology
The base UTS value for 12mm dia mild
steel structural ribbed bar is around 41
kgmm-2. 12mm dia TMT bars having
acceptable metallurgical quality
presents a value of 65kgmm-2. If an
amount of Rs 'X' is to be incurred per
ton to produce the TMT bars, then SSC
would be X/(65-41) or X/24. A lower
value of SSC is desirable when two
materials are to be compared.
Experimental-Methodology
Micro alloyed steels have been
developed through induction melting
furnace and controlled rolling route to
r e p l a c e T M T b a r s u s e d f o r
construction purposes.
An induction furnace was selected
for making steel ingots and a re-rolling
mill was chosen for rolling the ingots to
1 2 m m d i a r i b b e d b a r s . F o r
investigation, a base chemistry was
selected to have:
%C %Si %Mn %S %P %Cr %Ni %Mo %Cu
Min. .20 .25 .80 – -- ---------- traces ------------
Max. .25 .35 1.00 .04 .04
%Vanadium and %Niobium were
both varied. Nitrogen content was
aimed at 150ppm in some heats. Total
12 heats were cast. Melting was done
in a 500 kg (converted to 250 kg)
induction melting furnace, 3½” x 4½”
sized pencil ingots were cast and rolled
to 12mm dia ribbed bars. 3 different
sets of cooling parameters of rolled
products were adopted.
One set was made with full water
quenching, as applied in TMT
production (WQ). One large set was
made with out water cooling. It was air
cooled on cooling bed. One bar from
each heat of this set was twisted after
cooling [CTD(AC)] and rest was kept
as it is (AC). The third set was again
water cooled, but with much less water
as compared to the first set (WC), as
shown in Table 1:
Set
A
B
C
D
Heat No
1-12
1-12
1-12
1-12
TABLE-1 : FOUR DIFFERENT SETS OF TRIAL
Designate
Post Rolling Treatment
WQ
AC
CTD(AC)
WC
Results and Discussion
Samples were taken from each stage of
production. One test piece was cast
from each heat, it was normalized and
tested for chemical analysis through
d i r e c t r e a d i n g s p e c t r o m e t e r ,
metallographic studies like micro
structure, grain size and inclusion
rating, hardness in BHN, mechanical
tests like UTS, YS, %El and bendrebend
tests were carried out. Tests
pieces were cut from each heat of 4
sets of rolled products and each set
was tested for micro structure,
hardness. UTS, YS and %El. The
results have been presented in tabular
form in Table-2 for inclusion rating and
Table-3 through Table-8 for
Mechanical tests.
Inclusion Rating
In all the heats, levels of inclusion
types A, B & C were very good. Mixed
oxides were high in almost all the heats.
It is possible that some of these
particles could be seen in SEM analysis
and might have affected strength and
ductility values.
During steel melting, although
measures were taken to reduce
i n c l u s i o n l e v e l s , s o m e m o r e
precautions are needed to reduce the
level of mixed oxides in steels. It is
important to control both indigenous
and exogenous inclusions to improve
mechanical properties. These are
possible with little care in steel melting
and casting.
Mechanical Properties
Results of mechanical tests have
been presented below, from Table 3-
Table-8.
In unalloyed steel, AC resulted
Water quenching as in TMT
Normal air cooling in cooling bed
Normal air cooling and CTD (cold twisted deformed)
Water cooling with 50% lesser water as compared to set A
(The treatment designations would be used later in the report e.g. Heat No.1, set A with
water quench as 1AWQ and so on)
almost equal or better strength and
ductility as compared to WQ but were
lower than WC. In the cases of V
combinations, though strength values
under AC were lower (2-13) than those
under WQ, ductility values were better
in AC. Here again, UTS values were
higher under WC than those obtained
under WQ and AC. Though %
Elongation values were lower under
WC than under WQ and AC, % RA values
were more.
Strength went down with increasing
vanadium percentages, under almost all
cooling conditions. But %E values were
the lowest with WC and were higher
under AC conditions, with the highest
value obtained with V4. Vanadium
produces strengthening effect through
precipitation hardening and only AC
enhances strength without sacrificing
the ductility.
When V+N combinations were
used, similar trend as with was
followed. All the WC samples
developed LC. No appreciable
improvement observed with N
additions. %E values decreased with
more %V under AC and WQ. Under WQ
and WC, values of %E and %RA were
lower than obtained under AC. Faster
cooling enhanced strength values but
reduced the ductility. Nb combinations
also presented high strength and
ductility values almost comparable to
vanadium combinations. Different
cooling conditions did not appreciably
changed the UTS values but faster
cooling rates resulted in marked
reduction in ductility.
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Technology
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30VN
30VNbN
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
TABLE-2 : INCLUSION RATING OF TRIAL HEATS
Inclusion Rating (Thin/Thick)
Sulphide (a) Alumina (b) Silicate (c) Mixed oxides (d)
0.5/0.5
1/0.5
1/0.5
0.5/0.5
Test Piece
(NOR)
180
200
153
200
195
190
195
172
200
210
216
190
Water
Quench (WQ)
200
234
253
234
226
234
226
253
264
226
258
264
TABLE-3 : HARDNESS, (BHN) VALUES OF TRIAL HEATS
12 mm Dia Rolled Products
Air
CTD (AC)
Cooled (AC)
210
226
237
210
222
237
216
258
222
253
258
253
286
271
286
279
253
247
237
253
286
247
294
286
TABLE - 4 : UTS, KGMM-2 VALUES OF TRIAL HEATS
Test Piece Water
Air
CTD (AC)
(NOR) Quench (WQ) Cooled (AC)
63
67
54
51
65
59
61
62
58
54
50
68
70
81
83
74
78
83
77
72
86
75
83
87
69
79
70
61
73
75
65
75
78
65
81
76
80
86
82
82
84
84
94
78
87
68
88
90
2/1
2/1.5
2.5/1.5
1/0.5
2/0.5
1/-
1/-
0.5/-
1/-
1/-
1.5/-
0.5/-
0.5/-
0.5/-
-/-
1/-
-/-
1/-
-/-
-/-
1/-
0.5/-
-/-
-/-
-/-
-/-
-/-
0.5/-
0.5/-
-/-
0.5/-
0.5/-
-/-
-/-
-/-
0.5/-
-/-
2/-
2/-
2.5/-
1.5/-
2/-
1.5/-
1/-
Water
Cooled (WC)
237
286
271
237
231
301
231
247
258
258
264
258
Water
Cooled (WC)
79
89
83
72
85
83
85
75
84
73
82
88
There was no appreciable change
with V+Nb+N combination having
0.28%C. All the samples (other than
with V4) under CTD(AC) developed LC
and resulted mostly low %El and %RA
values, though the UTS values were
above 80 kgmm-2. CTD resulted in
raising dislocation density level of
already stresses rolled products.
Best combination of strength and
ductility values were obtained with V2
(UTS: 79, YS: 59, %El : 23 & % RA :
59) and Nb2 (UTS : 75, YS : 61, %El : 23
& %RA : 66) Typical values obtained
with 12mm2 TMT of SAIL/RINL, were
UTS : 65, YS : 56, %El : 25 & %RA : 45.
Summary of all the results have
been presented in Table-9.
Strength Specific Cost (SSC)
Commonly, a typical 12 mm dia
TMT bar of SAIL/RINL has a UTS value
of 60 kgmm-2 and that of an ordinary
mild steel ribbed bar is around 40
kgmm-2. Assuming the similar quality
of TMT bar is produced in the
secondary steel sector w i t h a n
annual production of 12,000 MT, total
cost of the TMT plant and accessories
as Rs 1.5 crores, IRR of 18% and an
expected pay back period of 5 yrs, the
SSC may be calculated as Rs 17.50.
In case of micro alloyed steels
with low %V/Nb, expected UTS level
shall be around 75 kgmm-2. As far as
the cost factor is concerned,
requirement of vanadium shall be less
than niobium, because in steel scrap,
usually little vanadium comes as
residual, but not niobium. Additional
cost of micro alloyed steels with low
vanadium(V2) shall be around Rs
200.00. In this case, the SSC may be
calculated as Rs 5.72 only, which is
about 2.5 times less.
Moreover, if due considerations are
given and applied in the design
calculations for constructional
activities, then there will be more
savings of constructional materials for
the users of micro alloyed steels than of
TMT bars.
Savings can be even more in case of
other sections of constructional
materials, because in those cases, the
difference in UTS shall be more (from
45 kgmm-2 to 79 kgmm-2) as the TMT
process can not be applied to the
51
November 2012

Technology
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
-2
TABLE-5: YS, KGMM VALUES OF TRIAL HEATS
Test Piece
(NOR)
40
43
44
35
43
46
60
40
39
36
47
47
Water
Quench (WQ)
51
63
73
64
65
68
68
49
76
64
70
74
12 mm Dia Rolled Products
Air
CTD (AC)
Cooled (AC)
49
59
55
48
57
59
51
61
61
50
67
61
64
83
62
64
-LC
-LC
-LC
60 LC
-LC
-LC
TABLE - 6 : %ELONGATION VALUES OF TRIAL HEATS
12 mm Dia Rolled Products
Test Piece
(NOR)
30
20
27
27
19
11
9
25
24
21
7
18
Water
Quench (WQ)
23
24
16
20
24
20
15
21
15
25
20
18
Air
Cooled (AC)
25
23
28
24
24
25
20
23
20
22
17
24
66
68
CTD (AC)
20
20
17
19
19
18
17
10
17
20
18
20
Water
Cooled (WC)
63
80
67
60
69 LC
72 LC
74 LC
63
74 LC
63
65 LC
71 LC
Water
Cooled (WC)
25
18
18
20
19
20
18
19
20
19
20
16
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
TABLE - 7 : %REDUCTION IN AREA VALUES OF TRIAL HEATS
12 mm Dia Rolled Products
Test Piece
(NOR)
52
53
63
30
50
17
23
48
34
29
5
13
Water
Quench (WQ)
57
46
51
54
56
64
55
33
51
54
54
39
Air
Cooled (AC)
61
59
57
59
54
61
65
66
63
60
56
54
CTD (AC)
26
38
44
58
58
51
18
37
32
60
47
45
Water
Cooled (WC)
66
55
64
70
49
51
58
67
58
62
65
53
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November 2012

Technology
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30VN
30VNbN
Code
P
V2
V4
V6
V2N
V4N
V6N
Nb2
Nb4
Nb6
30V4N
30VNb4N
TABLE - 8 : BEND-RE-BEND TEST RESULTS OF TRIAL HEATS
12 mm Dia Rolled Products
Test Piece
(NOR)
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
Water
Quench (WQ)
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
Air
Cooled (AC)
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
NC
CTD (AC)
LC
LC
LC
NC
NC
LC
LC
LC
NC
NC
LC
LC
Water
Cooled (WC)
TABLE - 9 : SUMMARY OF RESULTS
Test Piece
(Normalised)
Micro Structure Rolled Product, Mech. Props
VF(P) GS WQ AC WC WQ AC CTD WC
25 5-6 B FP FP Nor Nor
High
40
20
25
25
55
40
40
40
30
58
50
5
6
5-6
5-6
4-5
4-5
7
5-6
6-7
6-7
6
B
B
B
B
B
B
B
B
B
B
B
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
FP
High
High
Low
Low
High
Poor
Low
V High
Poor
High
Low
High
Nor
Poor
Nor
Nor
Low
Nor
High
Poor
V High
Nor
HIGH
&
BRITTLE
LC
NC
NC
LC
NC
NC
NC
LC
NC
LC
LC
NC
LC
Nor
VHigh
Nor
High
High
Low
Nor
Low
Low
Poor
Nor
sections other than rounds and
squares.
References
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Evolution of HSLA Steels”. Journal of
Metals, April 1982 pp 14.
E. E. Flecher, “A Review of the
Status, Selection and Physical
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B, Motgi, A. K. Chakraborty & R. L.
Kumar, “Micro alloyed Steels:
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405-408.
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1959, p 53.
W. B. Morrison, “Effect of Grain
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November 2012