Materials Science Forum Vols. 500-501 (2005) pp 519-526
online at http://www.scientific.net
© (2005) Trans Tech Publications, Switzerland
Online available since 2005/Nov/15
Application of Niobium in Quenched and Tempered
High-Strength Steels
K. Hulka1a, A. Kern2b and U. Schriever2c
1
Niobium Products Company GmbH, Germany,
Thyssen Krupp Stahl, Profit Center Heavy Plate, Germany
a
kh@niobium.de,
b
andreas.kern@tks-cs.thyssenkrupp.com, c udo.schriever@tks-cs.thyssenkrupp.com
2
Keywords: High-strength steels, light-weight construction, heavy plate, niobium, mechanical properties, welding, wear resistance
Abstract. This article offers an overview on the ways and means of the application of Nb in
quenched and tempered high-strength steel plates. Thereby, the outstanding role of Nb to control the
austenite microstructure during rolling or heat treatment and to contribute to effective precipitation
hardening is discussed. It is shown, that Nb is very effective to retard the transformation processes
during quenching. For high-strength constructional steels with up to 1100 MPa yield strength and
for wear resistant steels, the improvement in strength and toughness properties in Nb microalloyed
steels as compared to Nb-free steels is pointed out. A remarkable effect is the improvement in
toughness and brittle fracture resistance due to a very fine microstructure and finely dispersed Nb
carbonitrides in a martensitic microstructure. Fields of application for the new Nb microalloyed
high-strength structural steels are presented too.
Introduction
The weight of the structure itself has a substantial effect on the working load and thus on the economy of heavy loaded steel constructions. A reduction of the weight of the construction itself but
maintaining its loading capacity, given by the strength and safety of the construction, is of prime
importance. Fig. 1 shows examples indicating that e.g. under tensile stresses the plate thickness can
be reduced by around 60% by using the steel grade S960 instead of S355 [1-3].
The steel industry fulfills the wishes for light-weight design by providing thermomechanically
rolled and water-quenched plus tempered structural steels up to a minimum yield strength of 1100
MPa. Despite their high strength, these structural steels exhibit excellent toughness, good cold
formability and good weldability. Fig. 2 gives an overview about the production process of waterquenched and tempered plates from the steelmaking to the final product.
Usually the quenching operation applies high-performance lines and starts with reheating to
temperatures above Ac3, which have to be rather homogeneous distributed from the surface to the
center. Then the plates are quenched by using pressurised water to obtain a transformation of the
microstructure into martensite or bainite. Besides the chemical composition, also the tempering
process, which follows the quenching operation, is a relevant factor controlling the mechanical
properties [2]. Apart from the conventional reheat-quench plus tempering, quenching can done also
directly from the rolling heat. This is known as direct quenching.
High-strength structural steels are applied in many fields of heavy machine building. Prominent
examples include the construction of mobile cranes, of off-highway vehicles and pressure vessels.
The chemical compositions of the steel grades get adapted to obtain the mechanical properties necessary for each specific application. High-strength quenched and tempered structural steels usually
comprise a carbon content of less than 0.2% and Mn contents of max. 2%. These steels may contain
also certain additions of elements, which retard the diffusion-controlled transformation process and
thus increase the hardenability of the steels, such as molybdenum, chromium or nickel [3,4].
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520
Microalloying for New Steel Processes and Applications
The high strength quenched and tempered steel grades often contain also certain quantities of
the microalloying elements V and B. Being in solid solution, the latter is especially effective in retarding the transformation, since it restricts the diffusion of iron and carbon along grain boundaries.
B can thereby partially replace the above-mentioned, more expensive alloying elements. Since B
has a strong affinity to nitrogen, the formation of B nitrides must be prevented to make use of this
effect [5]. Usually, Ti is added in quantities above the stochiometric value (typically > 0,04 %Ti) to
fully fix the nitrogen. In this case coarse Ti carbonitrides will be formed, resulting in related unfavourable effects on ductility and toughness. Besides a certain effect in retarding the transformation,
the addition of V mainly causes an increase in strength by precipitation hardening in the martensitic
microstructure during the tempering process. However, the precipitation of V carbonitrides can lead
to toughness impairment and has detrimental effects on the properties of the weldment. This is particularly observed after stress relieving, which is often employed in heavy-loaded structures [6].
Plate thickness reduction, %
80
70
Oxygensteelmaking
converter
Load-bearing capacity = const.
Vacuum
facility
TNfacility
Continous
casting facility
Rolling
60
50
40
S1100
30
S960
20
Direct quenching
S 690
10
Testing
Cutting
Shipping
S 355
0
350
450
550
650
750 850 950
Yield Strength, MPa
1050 1150 1250
Tempering
Quenching
Austenitizing
Fig. 1. Reduction of plate thickness in relation Fig. 2. Production route of high-strength steels
to grade S355 by using high-strength steels
Usage of Niobium in Quenched and Tempered Steels
Niobium is not a rare element; its content in the Earth’s crust is 24g/t and thus it is more widespread
than cobalt, lead or molybdenum and even ten times more common than tantalum. Even though it
has many interesting applications as a metal and oxide, its dominant role is as a microalloying element in the steel industry. While the traditional application of Nb as a stabilising element in
stainless steel remained almost constant, its usage as microalloying element in Europe in the last
two decades increased drastically and the technology of thermomechanical rolling, originally developed for large-diameter pipes, spread to other steel products, such as for steel structures and for
automobiles. When Nb is used in a microalloy, its typical proportion is below 0.10%.
The most prominent and the outstanding role of Nb in steel is during austenite processing,
where first the cubic carbonitrides of Nb are dissolved in the upper austenite region and then get reprecipitated again during rolling. Thereby, austenite grain coarsening as well as recrystallization
processes are effectively influenced, resulting in a fine-grained microstructure. Grain refinement
and precipitation hardening processes in the ferrite add to optimized property combinations, which
is consequently used in the fabrication of thermomechanically rolled steels for line pipes, shipbuilding, bridges, automobiles, etc., with a domain in the range of yield strength between 350 and 700
MPa at the most [7].
In the course of intensive studies on high-strength, quenched and tempered structural steels, indications were found that the advantages of microalloying with Nb could also be used for this group
of steels [5]. Nb has several positive effects on metallurgical mechanisms, which determine the
properties of quenched and tempered steels, such as grain refinement the retardation of transformation and precipitation hardening [8]. A schematic assessment about the effects of Nb in comparison
to other commonly used microalloying elements is given in Fig. 3.
During reheating to the quenching temperature the equilibrium condition for Nb(C,N) in austenite will be reached, promoted by the fact that the plate has already passed the transformation of α to
γ and back from γ to α. Other than for vanadium, where a relevant amount will exist in solid solu-
Materials Science Forum Vols. 500-501
521
tion in the lower austenite region of 900 to 950 °C, at these temperatures and the typical carbon
level of quenched and tempered steel practically all the Nb will exist in form of Nb(C,N), Fig. 4
Microalloying
Affinity to C, N
Fine precipitates
Retardation of
transformation
Grain refinement
Nb
++
+
+++
+++
V
+
++
o
o
Ti
+++
+/ -
+
+
+ : positive
- : negative
effect
1) depending
1)
on Ti - content
effect
o : no significant
effect
Fig. 3. Metallurgical effect of Nb in high-strength steels
Fig. 4. Solubility isotherms of Nb carbonitride and V nitride
This is a role of Nb known from normalized steel [9]. The equally distributed and fine Nb carbonitrides of typically less than 20 nm diameter control successfully the austenite grain size and
thus the microstructure after transformation. An additional effect is achieved in the boron containing quenched and tempered steels. In these steels the necessary fixing of nitrogen is usually made
by microalloying Ti, which has a higher affinity to nitrogen than boron. Modelling [10, 11] of the
thermodynamics of dissolution and precipitation of Nb(C,N), AlN and TiN in austenite is described
generally by Eq. 1:
B
log[Me][C]x [ N](1− x ) = A −
(1)
T
(A and B are constants, x specifies the C proportion in the carbonitride).Taking into account the
Cr,Mo,Ni,Mn,Si
Wagner interaction parameters ε Nb
, ε CCr,Mo,Ni,Mn,Si and ε NCr,Mo,Ni,Mn,Si Fig. 5 demonstrated that
microalloying with Nb (≤ 0.03%) in combination with increased Al levels up to 0.10% develops
both AlN and Nb(CN), but also results in a significantly higher content of free nitrogen at 900°C
then known for TiN formation. However, it is remarkable that also in the Nb plus Al steel the content of dissolved nitrogen is below the solubility limit for the undesired development of B nitrides.
Therefore this microalloy combination can be used to protect boron, which has the positive effect
that it avoids the formation of coarse TiN. In steelmaking a well-aimed sequence of adding the microalloying elements into the liquid steel is necessary during the secondary metallurgy operation.
In order to test the possibility of this approach, the transformation behaviour of boron-free steel
was compared with this of boron-containing steels, and in the latter the nitrogen was either fixed by
microalloying with Ti or with Nb. The results are shown in Fig. 6. The addition of B, together with
522
Microalloying for New Steel Processes and Applications
an adequate fixing of nitrogen, shifts the formation of ferrite to longer times and thus increase the
hardenability. It is important to note that the microalloying with Nb has practically the same effect
as the addition of Ti. Additional analytical investigations of the extracted phases showed that about
90% of the entire B content exists in solid solution in the steel, and this is true when either Ti or Nb
is used as microalloy. However, due to the lower formation temperatures of the AlN and Nb(C,N)
particles in the Nb microalloyed steel, a substantially finer particle dispersion exists compared to
the TiN steels, with correspondingly more favourable effects on the material properties [5].
Base: 0,16% C, 0,0060% N, 0,0025% B, additions of Mn, Mo, Cr
T= 900°C
1,0E-03
9,0E-04
(BN-formation)
8,0E-04
Fig. 5. N-fixation in boron
containing high strength steels
with Ti or Nb (results of thermodynamic calculations)
solute N-content, %
minimum N-Content for BN-formation [ ]
7,0E-04
(no BN-formation)
6,0E-04
5,0E-04
4,0E-04
3,0E-04
2,0E-04
1,0E-04
0,0E+00
Transformation temperature [°C]
Ti
1000
800
600
400
200
0
1000
800
600
400
200
0
1000
800
600
400
200
0
Nb+Al
Without Boron
C
F
B
Si
Basic analysis [%]
Mn Cr Mo
B
0,18 0,22 0,85 0,30 0,40 0,0022
M
Austenitizing temperature: 920°C
With Boron and Titanium (0,04%)
Fig. 6. Effect of Ti and
Nb on the transformation
of S690QL with boron
B
M
With Boron and Niobium (0,02%)
B
M
1
10
100
Cooling time t 8/5 [s]
1000
Corresponding plate thickness (10 – 50 mm) through water quenching
400
accelerated cooled
700
V
650
Nb
Ti
0,02
0,04
0,06
Soluble Atom [wt %]
300
200
Nb steels
V steels
100
Fig. 7. Effect of Nb on
transformation
behaviour and precipitation
hardening [after 12, 13]
0
600
0
Yield strength increase [MPa]
Transformation temperature [°C]
750
0,08
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14
Soluble Nb or V-content [wt %]
When direct quenching from rather high finish rolling temperatures is applied, a certain amount
of Nb will remain in solid solution, supported by the fact that under these conditions the equilibrium
Materials Science Forum Vols. 500-501
523
will not be reached. In this case the effect of Nb in retarding the transformation can be used effectively, which is much stronger than this of solute V or Ti, Fig. 7a [12]. Furthermore, Nb in solid
solution also offers a higher precipitation hardening potential than V during the annealing process
following the quenching operation. For example, a yield point increase of 100 MPa can be achieved
with either 0.06% V or just around 0.02% Nb (Fig. 7b) [13].
The studies on the effect of Nb in quenched and tempered steels demonstrated that optimized
results are already achieved with Nb levels of not more than 0.04%. This level corresponds to the
Nb content, which could be dissolved during reheating to the rolling temperature. Nb in these steels
is almost on the same order as in most of the thermomechanically rolled heavy plate steels.
Nb in High-Strength Structural Steel Plates with Minimum Yield Strength up to 1100 MPa
The high-strength, water-quenched and tempered structural steels N-A-XTRA and XABO® are suitable to fulfil severe requirements in strength and toughness, e.g. necessary for mobile crane construction. These steel grades range from a minimum yield strength of around 550 up to 1100 MPa.
With XABO® 1100, showing a minimum yield strength of 1100 MPa, actually a climax in the development of high-strength structural steels has been reached [1]. The most important alloying elements, the carbon equivalent CET and the specified mechanical properties of these high-strength
structural steels are shown in Fig. 8.
Furthermore, also water-quenched and tempered structural steels with a minimum yield strength
of 500 MPa and a low carbon equivalent CET are also available and get used e.g. in offshore engineering. The desired mechanical property combinations are adjusted by controlled tempering of the
quenched plates at temperatures up to 700 °C. The characteristics of these steels include excellent
combinations of high strength and good toughness. This is necessary because their use in heavyloaded and sometimes safety-relevant structures often demands brittle fracture resistance at low
subzero operating temperatures.
Special steel Quality acc. Alloying
T KS
Euro norm additions
N-A-XT RA 550
S550QL
N-A-XT RA 620
S620QL
N-A-XT RA 700
®
XABO 890
®
S690QL
S960QL
®
S1100QL
Condition
of
delivery
0,31
QT
S890QL
XABO 960
XABO 1100
CrMoB
T yp. CET
(%) *)
CrMoNiV
0,36
Rm
( MPa )
Min A 5
( MPa)
Min R e
550
640 - 820
16
620
15
700
700 - 890
770 - 940
890
940 - 1100
11
960
980 - 1150
10
27
27
1100
1200 - 1500
8
27
(%)
Min Av (J)
-20°C -40°C -60°C
27
27
14
27
*) for a plate thickness of 10 mm
CET = C +
Mn + Mo
Cr + Cu
Ni
+
+
10
20
40
Fig. 8. Mechanical properties of high-strength steels
Consequent optimisation of the analytical concepts including the usage of Nb < 0.04%, allowed
to further improve the property combinations of these steels. A substantial increase in toughness
could be achieved by refining the microstructure as a result of the fine distribution of carbonitrides.
Fig. 9 compares the Charpy-V-notch impact energy of steels with and without niobium microalloying. It is obvious that e.g. the impact energy in all steel grades up to the high strength of the B
alloyed steel N-A-XTRA 700 gets improved by around 70 J when replacing Ti by Nb microalloying,
while the strength properties remain unchanged. This applies to both conventional reheat-quenching
plus tempering as well as to direct quenching from the rolling heat, followed by tempering. A doubling of the impact energy is also observed in the steels with strength levels up to 1100 MPa.
Besides the Charpy-V-notch impact energy also the transition temperature T27, which is the temperature, where the impact toughness of 27 J is obtained and is a characteristic value to describe
brittle fracture, is shifted to more favourable values by Nb microalloying as shown by the example
in Fig. 10. According to the brittle fracture concepts of EUROCODE 3, a lower transition temperature reduces the growth of cracks and thereby improves the susceptibility of the construction to brittle fracture. Furthermore, this has a simultaneously positive effect on the fatigue properties of
welded structures, which are often exposed to cyclic loading. Experimental investigations and
mathematical approximation of the fatigue properties of the high-strength structural steels XABO®
524
Microalloying for New Steel Processes and Applications
1100 demonstrated [3] that an improvement in the impact toughness by Nb addition (see Fig. 10)
results in an improvement of the service life of the cyclic loaded structure of a welded crane component by about 10 to 20%.
300
Impact energy (Charpy-V, transverse) [J]
XABO 500
250
Temperature:
Plate thickness:
N-A-XTRA 700
XABO 960 / 1100
- 40°C
≤ 10 mm
- 60°C
20 - 40 mm
- 80°C
40 - 60 mm
200
Fig. 9. Toughness properties of high
strength steels after quenching and
tempering
150
100
50
Direct quenching
0
without Nb
with Nb
without Nb
without Nb
with Nb
with Nb
Toughness
properties of
high-strength
steels
Of particular
importance
for
the after
application
27
250
Transition temperature (Charpy-V) T
[°C]
Impact energy (Charpy-V, transverse) [J]
of high-strength structural steels is their behaviour
during fabrication. Structural components are usually manufactured by cold forming and/or welding. Due to the low carbon content, these steels can be easily welded using common welding processes. They also exhibit a high resistance towards cold cracking due to their low carbon equivalent,
when high-quality welding additives are used. More data about this are summarised in EN 1011. Of
course, the welding conditions for high-strength structural steels also influence the mechanical
properties of the welded joint. In this context one usually defines a suitable cooling time t8/5, since
this characteristic value combines various welding parameters including the heat input into one
number. This characteristic value has a direct influence on the toughness of the welded joint. Fig.
11 shows with the example of N-A-XTRA 700, that microalloying with Nb allows a significant improvement in the properties of welded joint, similar as the results in the base metal. This is particularly true for high t8/5 times, corresponding to high heat input welding. The achieved optimisation of
the microstructure shifts the characteristic transition temperature T27 for the characteristic cooling
times t8/5 from 10 to 25 s to values being approximately 20 K lower.
with Nb
200
without Nb
150
100
50
Av 27J
0
-150
∆ T27
-100
-50
0
Temperature [°C]
50
0
-20
b
out NNb
wwitithhout
-40
-60
wwitithhNNbb
-80
-100
-120
0
5
10
15
20
25
30
35
40
cooling time t8/5 [s]
welding conditions
on the
toughness on
in the
Fig. 10. Charpy-V toughness and transition Influence
Fig. 11.ofInfluence
of welding
conditions
the toughtemperature of N-A-XTRA 700
ness in the HAZ of N-A-XTRA 700
Niobium in Wear-Resistant Steel Plates
Machines and other equipment in industry, agriculture and for constructions often apply heavy
plate, which has to guarantee a high wear resistance. The trade name of this group of plates at
Thyssen Krupp Stahl AG (TKS) is XAR, available in various strength levels and thicknesses. Typical fields of application are machines to excavate raw materials such as coal, ores, stone etc. Fig. 12
provides an overview about the chemical composition of heavy plate made from wear-resistant special structural steels XAR. These steels comprise as characteristic alloy elements Mn, Cr, Mo and Ni
Materials Science Forum Vols. 500-501
525
and are based on a carbon content up to 0.40%. Consequently they attain high hardness values of
400 to 600 HB. Typical plate thicknesses range up to 100 mm [14].
In the most important fields of application for wear-resistant steels, the characteristic wear type is
ploughing leading to abrasive wear. Thereby usually the plate surface gets scratched when exposed
to an abrasive and hard material such as sand or minerals and is finally removed - this wear mechanism is called abrasion. A high hardness of the material is one important for good wear resistance.
Furthermore, also a higher toughness of the material improves the wear resistance and thus reduces
the material loss, because it changes the wear mechanism from micro-ploughing into micromachining [14, 15].
TKS
steel grade
Hardness
(H B)
Plate
thickn ess
(m m )
max.
C
max.
Si
m ax.
Mn
max.
Cr
m ax.
C he mical
com position
(% )
Ni
max.
typ.
CET , %
Mo
m ax.
8 mm
40 m m
XAR 400
360 - 440
100
0,20
0,80
1,50
1,00
0,50
0,26
0,37
XAR 450
410 - 490
100
0,22
0,80
1,50
1,30
0,50
0,38
0,38
XAR 500
450 - 530
100
0,28
0,80
1,50
1,00
(1,50)
0,50
0,41
XAR 600
550 - 630
40
0,40
0,80
1,50
1,50
1,50
0,50
0,55
0,41
0,55
Mn+Mo
Cr+Cu
Ni
Fig. 12. Chemical composition of wear-resistant special structural
steels
(CET=
C+(Mn+Mo)/10+(Cr+Cu)/20 + Ni/40)
Extensive and systematic studies at the Mannheim Technical College on the wear resistance of a
variety of steels [14] demonstrated that the addition of Nb is very favourable, since it increases substantially the toughness of the material by refining the microstructure and improving the precipitation state of the carbonitrides, similar as described for the water-quenched plus tempered structural
steels. One result of toughness data is shown in Fig. 13 by the example of the XAR 400 steel. The
studies on the abrasive wear properties showed that the increase in toughness connected with Nb
microalloying results in an improvement on the wear behaviour due to the explained changed in the
wear mechanism. In fact, the results with the XAR 450 steel indicate that the service life under the
abrasive wear by hard minerals can be increased by around 20% using Nb microalloying (Fig. 14).
thickness: 5 - 12 mm
80
60
with
Nb
ut
witho
40
Life time factor (S355 = 1)
Impact energy [J]
2,5
Nb
20
-50
-40
-30
-20
-10
Temperature [°C]
0
10
Fig. 13. Charpy-V toughness of XAR 400
2
1,5
without
Nb
with
Nb
1
0,5
0
S355
XAR 450
XAR 450
Fig. 14. Increase of life time in XAR 450 through
addition of Nb
As a result of these systematic studies on the wear properties of various steels, the model
PROWEAR, based on a computer-aided correlation and regression analysis, was developed to predict the wear resistance and the resulting service life [14]. This model defines the combined effects
of the chemical composition of the steel, the hardness and the environmental conditions on the wear
behaviour and allows an economical, time-saving and praxis-oriented support of steel selection exposed to abrasive wear. A comparison of the predicted wear properties with the results measured
under practical conditions demonstrates good correlation. This is shown in Fig. 15, where the mean
526
Microalloying for New Steel Processes and Applications
abrasion data determined for a skip made of the niobium micro-alloyed XAR 450 and XAR 500 steel
grades after use in the Chino Mines in New Mexico are compared with corresponding calculated
values. The computer program PROWEAR thereby permits a good, integral determination of the
wear properties in real abrasive systems.
2
measured
1,8
calculated
(PROWEAR)
Wear rate, µm/h
1,6
1,4
Fig. 15. Calculated and measured wear rates in a
truck dump body used in Chino Mines/New Mexico
1,2
1
0,8
0,6
0,4
0,2
Steel: XAR 450/XAR 500 Nb
Condition: dry resp. wet, pH=6
0
Summary and Future Trends
The use of Nb as a microalloying element in high-strength, water-quenched and tempered steels for
heavy machine construction and other application is worthwile. In general, the advantageous effects
of Nb applied in normalised and thermomechanically rolled steels can also be exploited in quenched
plus tempered steel grades. In particular, it refines the grain size and thus improves the toughness of
the material guaranteeing a high resistance against brittle fracture. Furthermore, in boron microalloyed steels it allows to substitute Ti, improving the cleanliness of the steel. Also the performance
after welding and the resistance against wear are improved. Generally said, one achieves a higher
quality level for high-strength quenched and tempered steels when microalloying with Nb.
Actual developments have to consider even higher requirements in toughness at increasingly
lower temperatures, e.g. for pressure vessel constructions. To comply with this demand, the use of
Nb in quenched and tempered steels will further increase in the future. A limiting factor for this
solution can be seen in specifications of the actually valid standards DIN EN 10137 and also the
ASTM/ASME regulations.
References
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[2] H.J. Tschersich, H.J. Kaiser, A. Kern, U. Schriever: Stahl (2002), p. 36
[3] W. Bleck, H. Hummel, A. Kern, U. Schriever: Stahlbau Vol. 73 (2004), p. 901
[4] B. Müsgen: Metal Construction Vol. 17 (1985), Nr. 8, p.495
[5] A. Kern, B. Müsgen, U. Schriever: Thyssen Technische Berichte Vol. 25 (1993), p. 67
[6] H. de Boer, U. Schriever: Thyssen Technische Berichte Vol. 21 (1989), p. 135
[7] M. Cohen and W.S. Owen: Microalloying 75, Union Carbide Corp., New York, 1977, p. 2
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[9] H. Baumgardt, H. de Boer and F. Heisterkamp: Niobium ’81, TMS, Warrendale, 1984, p. 883
[10] A. Kern, W. Reif: Steel Research Vol. 57 (1985), p. 321
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[12] S. Okaguchi, T. Hashimoto and H. Otani: Thermec 88, ISIJ, Tokyo (Japan), 1988, p. 114
[13] A. von den Steinen, S. Engineer, E. Horn and G. Preis: Stahl und Eisen Vol. 95 (1975), p.209
[14] A. Kern, P. Feinle, U. Schriever: Proc. EUROMAT'01, München (2001), p. 765
[15] K.H. Zum Gahr: DGM-Informationsgesellschaft, Oberursel (1987), p. 21
Materials Science Forum Vols. 500-501
Microalloying for New Steel Processes and Applications
doi:10.4028/0-87849-981-4
Application of Niobium in Quenched and Tempered High-Strength Steels
doi:10.4028/0-87849-981-4.519
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