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Harnessing the Economic and Performance Benefits of
Nb-High Strength Structural Steels in India
Dr Jitendra Patel 1,2
1
Director, International Metallurgy Ltd England2 Consultant, CBMM Technology Suisse SA - Switzerland
Introduction
Like many growing nations, the skyline of several of Indias leading metropolitan cities is
rapidly changing. In cities like Mumbai availability of land is limited and at a premium,
therefore the financial and economic challenges of development on inner city sites are
pushing building designs taller and taller. This not only challenges architects to design taller
and greater, but this also gives the structural engineer significant challenges in deriving
designs which meet the competing functional requirements of tall buildings in a cost effective
manner.
Development of tall buildings involves a delicate balancing act of meeting the aspirations of
the client, architect, regulatory authorities and the myriad of other design professionals
working on the scheme. This balancing act often means that the main structural elements
providing stability need to be sized to minimize their impact on the building. It is in these
situations that the structural engineer starts to consider the use of high strength steel to help
achieve their goals.
This paper highlights some of the drivers in the structural design of tall buildings and larger
structures. It provides an example of savings in material that can be made if higher strength
steels are used, and in particular steels microalloyed with niobium (Nb) in conjunction with a
lower carbon content.
High strength steels
In todays environmentally conscious and economically sensitive markets the construction
sector is utilising ever-greater quantities of higher strength steels. This not only affords for
audacious projects but also permits significant savings to be made and enabling an earlier
return on investment. However, apart from the standard mechanical properties, such steels
have to meet increasing performance requirements taking into consideration on-site
weldability, cold temperature performance and other environmental conditions such as
seismic zones. One of the key ways in achieving this is through the use of niobium (Nb)
microalloying which affords both the development of higher strengths as well as enhanced
toughness. It is the metallurgical benefits of micro-alloying with niobium which are exploited
during the processing of the steel thereby making it possible to reduce alloying element
contents and carbon content while maintaining high toughness values and good weldability at
identical or higher yield and tensile strength values. As with all elements, the alloying content
of niobium must be chosen with respect to the process route and property requirements of the
applied plate. Traditionally higher strengths have been reached by increasing the amounts of
alloying elements. However, this results in higher hardenability and may lead to an increased
risk of brittle fracture and hydrogen induced cracking when welded if the correct welding
parameters are not applied.
Today, modern mills are capable of producing steels plates with yield strengths of 500MPa at
thicknesses approaching 100mm. These steels are classed as fine-grained weldable
microlloyed steels, which exhibit excellent toughness both within the base metal and the heat-
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affected zone (HAZ) of the welded joint. Typically these steels are made via the thermo-
mechanically controlled processed (TMCP) route coupled with accelerated cooling and
generally do not require any pre-heating prior to welding. The standard (EN 10025-4) covers
TMCP steel grades with a minimum specified yield strengths of 275, 355, 420 and 460MPa
and specified minimum impact toughness down to -20C (designated M) or -50C (designated
ML).
Thermo-mechanically controlled processed (TMCP)
Figure 1 illustrates the general evolution of structural steels. In particular it highlights the
different processing routes from TMCP (moderate yield strengths and higher toughness)
through to steels produced via the quenched and tempered (Q+T) route for the very high yield
strengths (1,100MPa). For TMCP steels, excellent properties can be achieved by a leaner
alloying content coupled with small amounts of microalloying with niobium.
1,000
800
600
400
200
0 1945 1955 1965 1975 1985 1995 2005
Year
YieldStrength(MPa)
S355J2 S355N
S690Q
S890QS960Q
S1110Q
S500M
TMCP
Q+T
N andN-rolled
S460N S460M
S355M
Figure 1. General evolution of structural steel
The principles of Nb-microalloying and its beneficial effects as a grain refiner (higher
strengths and improved toughness) and permitting reduction of carbon and other alloys are
well established. Figure 2 highlights the advantage of reduced carbon equivalent (CEq) for a
given strength of TM-rolled steels in comparison to traditional normalised routes.
Nevertheless, it is important to note that, dependant on the required strength and toughness,
for a given plate thickness, the correct processing schedule must be applied giving
consideration to the steel chemical composition. Such factors are important for structural
plates subjected to through-thickness loading, and both EN 1993-1-10 and EN 10164 are
normally specified for through-thickness ductility properties. The challenge for steel
metallurgists is therefore to develop steels that possess both higher strengths (i.e. above
350MPa) as well as better toughness. This can be achieved through a better understanding of
the alloying and entire process route. Typically this will involve:
Reducing the volume fraction of the pearlite by utilising lower carbon content - a low
carbon and highly clean steel also has a positive effect on weldability;
Vacuum degassing during secondary metallurgy to minimise sulphur, nitrogen, hydrogen
and the total oxygen content. Overall, this will result in reduced tramp elements and a
cleaner steel; Calcium-treatment to modify any sulphide inclusions making them more globular - today
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the typical sulphur content in an aluminium deoxidised steel has
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Mo none none none 0.25 none
CEq 0.40 0.31 0.50 0.45 0.36
(N = normalised, TM = thermomechanically rolled, QT = quenched plus tempered,
TM + ACC = thermomechanically rolled plus accelerated cooled)
To help maintain good low temperature toughness, any free nitrogen must be minimised. The
addition of the nitride forming elements of aluminium and/or titanium reduces the freenitrogen by the formation of nitrides and also contributes to grain refinement thereby, like Nb,
also having positive effect on toughness. Furthermore, the formation of TiN precipitates also
provides control of the grain size in the HAZ and accompanying intercritically reheated
zones. An important area for the fabricator of high strength steels is the prevention of
hydrogen assisted cold cracking in the weld. Preheating and post-weld-heating are standard
precautions against these defects from occurring but are timely and costly. Furthermore, they
are also practical challenges when undertaking such heat-treatments on site. The advantage of
using TMCP steels (i.e. lower carbon at
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in the beams making up the flooring system. For commercial offices, the main trade off is
column spacing (span) and construction depth to minimize floor-to-floor height (thus
generating more office floor area within the building). To keep section sizes at a minimum
and maintain stiffness within prescribed deflection limits, the optimum choice of steel grade
is S355.
It is important to note that for longer spans, as floor beams tend to have a serviceabilityrequirement or are stiffness driven, there is little advantage in using higher strength steels
greater than S355. However, where build construction zones are constricted, an increasingly
important processing route for beams are pre-fabricated sections with holes for service
integration. In such cases good weldability becomes an essential property for the high
strength steels used (where steels such as lower carbon Nb-microalloyed varieties provide
such performance properties).
As to be expected, the major area where high strength steel can make significant economic
and performance benefits is in high-rise construction for structural components where
strength is the key driver, and as building designs become taller this becomes even more of a
major factor. The key structural components where high strength steels could potentially be
used in high-rise designs are for example:
Columns and braces individual elements acting predominantly in compression that form
part of the vertical load carrying system or perimeter tube/diagrid system;
Shear walls individual elements that usually form part of the interior core system;
Link beams individual elements that link shear walls together to form cores;
Outrigger trusses/walls trusses or walls that link perimeter columns to interior cores;
Belt trusses trusses that link perimeter columns together;
Transfer systems structures that help transfer vertical load from individual elements to
other vertical load carrying elements, and;
Connections connections between members in any system or component.
Taller buildings tend to result in larger columns (and braces in diagrid structures) because
they must be able to pick up load down the structure (in particular highlighting the importance
of flange thickness) and the resulting size of the columns can become an aesthetic problem for
the architect, as often the space at the bottom of the building is often so important. Therefore,
in such cases higher strength beams with yield strengths up to 450MPa (65ksi) are allowed
(e.g. ASTM A992) for framing purposes. Typically such heavier (or jumbo) beams will have
a serial size of 356x406 (W14x16) up to 1086kg/m (730lb/ft) and have flange thickness of
125mm (4.91 inches). However, as the bottom of high-rise steel framed structures requires
columns and braces outside the normal section sizes, they are fabricated directly from plate
and thus require good weldability performance (i.e. ideally no pre- or post-weld heat
treatment). Therefore, high strength steel has advantages in terms of reducing member size,
improving constructability by reduced member weight whilst ensuring good weldability
(where the steel is via a low carbon route;
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on the architecture, space and cost. High strength steels can be used to reduce member sizes
to reduce impact on space and to reduce weight to ensure that such members can be more
easily placed within the structure during construction.
Finally, steel framed structures have to be connected together to make effective framing
systems. This is especially important for tubular perimeter and diagrid forms. The preferred
method of connecting members together is in the form of bolted connections to reduce theneed for on-site welding. The forces often transferred in the main lateral stability members
can be substantial meaning that the plates and bolts required to transfer the forces make the
connections very cumbersome. Perimeter forms have to be integrated with the faade system
and sometime connection sizes clash at pinch points with the faade. High strength steel can
be used to reduce plate sizes and ensure easier integration of the main structure with the
faade.
Overall, higher strength means a lower weight per component, it reduces fabrication costs as
well as important costs such as transportation and handling. With a reduction in wall
thickness, there is also a reduction in the weld metal volume (amounting to an exponent of
two). It is the weld metal volume that determines the production time needed for fabrication
of the component - an ever increasing economic driver as frame fabrication is often on the
critical path for project completion. As high strength steels microalloyed with niobium afford
fine grains, it is well suited where cold formability and weldability is a key aspect of the
fabrication. The benefits are especially greater when a lower carbon steel (
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Figure 3. Impressions of Imperial Tower (Source: Adrian Smith + Gordon Gill Architecture)
India Tower provides a glimpse of the future and introduces new structural forms (linked
cores and linked segmented structures) that will be required to meet the aspiration of building
taller and taller. Such forms will require new construction materials with higher strengths,
ductility, toughness and durability where microalloying elements such as niobium are
expected to feature in the steel makeup. Higher strength steels will have a significant role to
play both today and in the future in bringing those dreams to reality.
Closing remarks
The use of high strength steel in present day high rise construction is suited to certain areas of
the structural framing where its properties are beneficial in reducing member sizes thus
ensuring more effective integration of the framing system with the building envelope and
floor space. Adopting strengths such as S355 saves overall weight and hence costs compared
to S235 and selective use of even higher strength grades such as S460 can lead to a further
weight savings in columns, for example (up to 14% in comparison with S355). In terms of
material costs the savings for S460 are about 10% compared to S355 and 25% compared to
S235 (see Figure 4).
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Figure 4. Advantages of high strength steel S355 and S460 compared to S255
All of this is made possible by small microalloying additions of niobium in both plate and
beams and higher strength reinforcement bar allowing the development of stronger, tougher
and easily weldable steels, even for the thickest sections. It is important that structural
engineers are made more aware of the advantages of high strength steel in meeting the
aspirations of the client and other stakeholders in the project and in particular the value added
benefits of the properties of such steels when microalloyed with elements such as niobium.
Furthermore, with the established fact that such steels are in fact readily available in India
today.
Finally, in the future, as pressure on premium land continually increases, developers willdemand greater returns and buildings will need to become higher as a consequence. It is
certain that more opportunities to use high strength steel will present themselves as new
structural forms evolve.
Acknowledgements
The author would like to express sincere appreciation to Dr OConner (Technical Director,
WSP Cantor Seinuk, London) for providing examples and information for this paper and
CBMM Technology Suisse S.A. for granting permission for publication.
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