dr. jitendra patel, cbmm consultant - harnessing the economic and performance benefits of nb

7/27/2019 Dr. Jitendra Patel, CBMM Consultant - Harnessing the Economic and Performance Benefits of Nb

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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.

dr. jitendra patel, cbmm consultant - harnessing the economic and performance benefits of nb

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