Niobium Microalloyed Structural Steels

Niobium is a chemical element with symbol Nb (formerly known as Columbium, Cb). It is a soft, grey ductile transition metal. The main commercial source for niobium is the mineral pyrochlore. Brazil is the leading producer of niobium and ferroniobium (a niobium/iron alloy). Niobium was first used commercially in the early 20th Century to replace tungsten in tool steel production; since then, it has become a key element in the production of modern engineering materials. Its use has increased steadily as advances are made in the understanding of its beneficial properties as an alloying element.

Ferroniobium

Figure 1: Ferroniobium, obtained after industrial processing
(Photo: CBMM)

How is Niobium used in Steel?

Around 80% of niobium produced is used as a microalloying element in steels for the automotive industry, for oil and gas pipelines and in construction. Another significant end use of niobium is in the production of stainless steels in vehicle exhaust systems. Other applications include the use of niobium-containing superalloys in jet and rocket engines and in various superconducting materials. Niobium is also used in functional ceramics and catalysts for the optical and electronics industries.

Niobium Microalloyed Steels

A microalloyed steel contains small amounts of alloying elements (0.02-0.1 wt%), such as niobium, vanadium, titanium, molybdenum, zirconium and boron. These small percentages of alloying elements have a significant impact on many important properties for engineering applications. Adding niobium to steel causes the formation of niobium carbide and niobium nitride within the structure of the steel. These compounds improve the grain refining, retardation of recrystallization, and precipitation hardening of the steel which increase the toughness, strength, formability, and weldability of the microalloyed steel.

Steel properties like strength and toughness depend both on the chemical composition and processing procedures; steel producers use a wide range of different concepts to achieve the required balance of properties. Although the easiest way to improve the strength of steel is to increase its carbon content, this reduces other important properties like weldability, toughness and formability. Microalloying with elements like niobium, vanadium or titanium in amounts below 0.1wt % (1000 grams/tonne) is a cost-effective method of achieving a balanced combination of properties.

Most structural steels with strengths of 355 MPa and above contain niobium additions. Depending on the particular steel production concept adopted by the mill, as the strength increases beyond 355 MPa, the content of niobium is likely to increase to ensure the steel possesses the right balance of strength, toughness and weldability.

Niobium microalloyed high strength steel plates are used in a variety of applications, such as large diameter line pipe for the transmission of gas and oil, shipbuilding, offshore platforms, bridges and  energy generation structures such as wind turbines. Plates in excess of 50 mm thickness are common.

The Use of High Strength Steels in Construction

As structures become increasingly ambitious and complex, greater demands are placed on the performance of building materials and structural systems to ensure elegant, cost-effective and sustainable solutions. This has led to a greater reliance on microalloying with niobium to produce steels able to meet higher specifications. In 2000, an average approximate value of 40 grams of ferroniobium were added per tonne of steel, and this increased to 63 grams per tonne in 2008. Although Niobium represents less than 0.5% of the total cost of producing steel, it adds significant value by improving strength, toughness, weldability etc.

About half of all steel produced worldwide is used in the construction industry. An increase in infrastructure projects driven by urbanization, population growth, and replacement of aging infrastructure will ensure that strong demand for steel will continue. Indeed, the world steel consumption is expected to rise from 1500 million tonnes in 2000 to approximately 2800 million tonnes by the year 2050.

The use of a higher strength steel can enable substantial savings in structural weight and material costs; although high strength steels are more expensive than conventional structural steels, the price increase is less than the rate at which the strength increases. The reduction in weight leads to a cost saving in the foundations, welding, fabrication, transportation and erection. The weight savings made possible by HSS depend on the type of member and mode of loading, but in many practical situations might range from 10–40%.

The CO2 emissions resulting from the production of higher strength steels are only slightly higher than those for conventional structural steels – there is about a 7% increase in CO2 emissions for the production of steel plate (cradle to gate) when the yield strength doubles from 350 to 690 MPa. Hence significant reductions in CO2 emissions are possible.

The use of high strength steel designs can also result in more clearance, greater design freedom and less congestion, which can be very important in certain types of structures, including offshore oil and gas platforms.

Challenges of Designing with High Strength Steels

Even though plates and sections made from higher strength steels have been available for more than 20 years, only about 5% of structural steel used worldwide in construction is grade S420 and above.

One reason for the slow uptake in specification of high strength steels is that for most structures, the governing performance criterion is not strength, but stiffness (the critical failure mode being buckling instability or the need to limit deformation). The stiffness of a steel structure is heavily governed by the magnitude of the modulus of elasticity (E), which is the same value for all strength grades of structural steel. Therefore, the economy of high strength steel usage depends on the degree to which the enhanced strength can be exploited. Almost all applications of high strength steels in structures to date have involved steels with yield strengths of S420 or S460 and/or structural systems that postpone the onset of buckling or limit deflection.

Additional obstacles to the greater use of high strength steels have been associated with inexperience in fabrication and limited sources of supply, along with a general reluctance to change within the notoriously conservative construction industry.

Which Structural Applications are most suitable for High Strength Steels?

Three structural applications are described below where high strength steels can lead to significant weight savings:

1) Gravity columns in high-rise buildings

Providing horizontal movement (drift) or vibration do not govern design, the use of high strength steel for gravity columns leads to weight savings, for example a weight reduction of 15-25% can be realised by replacing an S355 steel with an S460 steel.

2) Long span trusses

High strength steels are most suitable for tension members such as the bottom chord in a truss, and for compression members with short buckling lengths, such as the top chords in a truss. The use of S460 steel generally allows a weight reduction exceeding 15 % when compared to a solution using S355 steel. This reduction in weight is a function of the truss span and the relative magnitude of the permanent (dead) loads compared to the variable (imposed) loads; generally, the structure deadweight is a considerable proportion of the design load, and so a reduction in deadweight is of great value.  Additionally, less stringent deflections limits apply for long span trusses because the overall height is large, and stiffness can be increased by increasing truss depth.

3) Bridges

Bridges often carry heavy, dynamic and cyclic loads over long spans and are designed for long service lives, typically 120 years. Designing structural bridge elements in high strength steel enables the cross section and the material thickness to be reduced. For a typical small span composite bridge, the weight reduction from the use of S460 instead of S355 can be as high as 25%. This reduction results in lower permanent loads and hence lower foundation bearing capacity, which is very important for the replacement of bridge decks on existing piers and abutments. The weight of a moveable bridge (which allows passage for boats) governs the design of the mechanical parts and for these types of bridges, high strength steel allows cost saving which far exceed the savings in material cost. Additionally, reduced material thickness is a great advantage in welded splices in bridges.

Case Study: High Strength Steels in The Friends Arena

High strength steels were used in the construction of the roof trusses supporting the retractable roof in The Friends Arena in Stockholm, the second largest indoor stadium in Europe (Figure 2). The four 17 m deep space trusses span the 162 m width of the stadium. The design was optimised by using different grades of steel in the various structural elements within the trusses. The key elements of the trusses, offering the greatest potential to exploit the advantages of high strength steel were the top and bottom chords and the outer diagonal member closest to the support points. S460 steel tubes were used for the top chords with a diameter of just over one metre. Higher grades of steel were used in the bottom chords and the outer diagonals (S690 and S900 respectively) as these are predominantly subject to tensile loading. The bottom chord was a U profile to simplify welded connections and the outer diagonal members were flat plate (Figure 3).

Friends Arena

Figure 2 Roof trusses at the Friends Arena (height 17 m, span 162 m)
(Photo: Sweco)

Diagonals

Figure 3: Diagonals connecting the truss to the end supports are made of double plates (steel grade S900, dimensions 40×220 mm, length 16m)

High strength steel made up 32% of the weight of the arena roof structure and led to a 584 tonnes (corresponding to 17%) reduction in the total roof weight compared to a roof in conventional S355 steel. The reduced tonnage and subsequent saving in the cost of fabrication (mainly due to less welding) led to a 15% cost saving compared to a traditional design using conventional strength steel. A life cycle assessment demonstrated that the use of high strength steels led to a reduction in greenhouse gas emissions of 17% (895,675 kg of CO2 equivalent), when analysed over the entire life cycle of the steel used.

This article was contributed by Nancy Baddoo, the Associate Director for The Steel Construction Institute.

 

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