MODERN HIGH STRENGTH NIOBIUM MICROALLYED STRUCTURAL STEELS

MODERN HIGH STRENGTH NIOBIUM MICROALLYED STRUCTURAL STEELS

Dr Jitendra Patel

CBMM Technology Suisse S.A.

KEY WORDS: NIOBIUM, TMCP, HIGH STRENGTH, STEEL, PLATE, BEAMS, CONSTRUCTION Abstract Niobium (Nb) is widely applied as a key micro-alloy in the production of modern high strength structural steels and in particular allows a combination of thermal and mechanical treatment during plate production via the TMCP route. A consequential metallurgical benefit is the development of very fine microstructures resulting in high strengths as well as high toughness in spite of a lean steel chemical composition. 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. This paper presents and discusses some of the recently developed niobium bearing high strength structural steels that have been employed in recent projects which have offered substantial savings in terms of material fabrication costs for a wide range of applications such as bridges and buildings. Background and Introduction The application of steel in the construction of civil building structures dates back well over 225 years. Indeed, one of the well-known structures using cast iron is the Ironbridge (Shropshire, UK) built in 1779 using approximately 338t of cast iron. However, although this was based on a carbon content of 2-4%, the structure put into place the beginnings of all modern steel bridge construction practices. The application of steels in such civil construction projects allowed the following advantages: (i) reduced construction time; (ii) reduced costs; (iii) reduced maintenance; (iv) greater load bearing capability, and; (v) better strength to weight ratios. Interestingly, these advantages still hold strong in today’s environmentally conscious and economically sensitive markets, the construction sector is utilising ever greater tonnages 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. 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-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 -20oC (designated M) or -50oC (designated ML). Before discussing the requirements in modern steels a steel construction example is given to show another advantage of this approach, i.e. reduced weight and therefore transportation savings. Figure 1 shows the administration centre of a bank in Frankfurt/Main, Germany, completed in 1997 with 56 stories. It has the shape of a triangle with one side being 60m long. With a height of 260m (300m with spire) it is one the tallest skyscrapers in Europe. Design and cost reasons resulted in the decision to rely on a steel construction saving 60,000t of material if the alternative solution of reinforced concrete was used (the total cost was US$350m). In total the building applied 19,500t of steel of which 40% consisted of thermo-mechanically rolled S355M steel was used for plate thicknesses exceeding 30mm (up to 80mm in some areas), whereas in highly loaded girders and columns S460M was applied (Hulka and Bauer, 1999). Modern Niobium High Strength Steels (HSS) for Construction The application of steels with higher yield strengths affords a significant reduction in material thickness and consequently, weight. Naturally, the optimal reduction is highest when only uniaxial stresses occur, i.e. by doubling the yield strength only half of the wall thickness is required to carry the equivalent load (Lessels, 1987). Nevertheless, even with consideration to other stresses that derive from bending or torsional loads, the weight reduction is still noteworthy (Younger, 1975). This effect can be demonstrated by the example of beams with different strength levels, Figure 2 (TradeArbed, 1989), indicating that a higher strength of the steel allows the usage of lighter profiles. Although the manufacture of such HSS may in some cases correspond to a premium product, the ultimate savings made in employing such steels has been proven to offset a higher material cost. Furthermore, the lower weight reduces fabrication costs as well as such important costs such as transportation and handling. The vast majority of steel construction requires welding, and even in this regard, the application of modern high strength steels permit savings also to be made here. With a reduction in wall thickness, there is also a reduction in the weld metal volume which amounts to an exponent of two, and importantly it is the weld metal volume that determines the production time needed for fabrication of the component (an ever increasing economic driver is time for completion of the project). This obvious economic benefit can only be used if the construction is safe. Fundamental studies have shown that firstly, the steel must not be brittle at the intended working temperature and secondly, it should exhibit sufficient ductility to withstand any crack

propagation. The results presented in Figure 3 (Hubo, 1990) from fracture mechanics tests show that larger flaws and higher strength steels ask for higher toughness in order to have a safer construction. As seen from the figure, the necessary fracture mechanics value increases exponentially with yield strength and therefore it is the challenge of steel metallurgists to develop steels which possess both higher strengths as well as better toughness. As discussed later, 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. Indeed, the European design codes for steel construction (EN 1993 - Eurocode 3) describes a safety analysis based on fracture mechanics and also a practical approximation by using the more widely (and easily undertaken) Charpy V-notch impact test. For modern structural steels, there is also a requirement to produce steels which have high levels of cleanliness, providing better toughness characteristics, and also improvements in ductility (see Figure 4). Such steels typically exhibit low levels of sulphide and oxide inclusions as well as reducing the volume fraction of the second phase microconstiuent, pearlite by utilising lower carbon contents (Gladman, Holmes and McIvor, 1971). This practise is widely used in the manufacture of modern high strength low alloy (HSLA) steels together with application of niobium (Nb) to invoke grain refinement as the major strengthening mechanism (as well as improved toughness). It should be noted that other strengthening mechanisms, i.e. precipitation, dislocation, or solid solution hardening and especially the traditional means of steel strengthening i.e. applying a higher carbon content, all have a toughness impairing effect. Therefore for steel development, the focus has been on refinement of grain size.

Furthermore, a low carbon and highly clean steel also has a positive effect on weldability. Studies have shown that first, the steel must not be brittle at the intended working temperature and second, it should exhibit sufficient ductility to withstand any crack propagation. For example, for offshore plates the trend in crack tip opening displacement (CTOD) test requirements demonstrates the increasing performance requirements of modern structural plate in which recent projects (e.g. Sahkalin) have required testing at -40oC and Charpy toughness CVN requirements down to -60oC for steels with 460MPa yield strength. Fracture mechanics tests show that larger flaws and higher strength steels ask for higher toughness in order to have a safer construction. The challenge for steel metallurgists is to better understand 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;

• Ca-treatment to modify any sulphide inclusions making them more globular - today the typical sulphur content in an aluminium deoxidised steel has <0.003 %S;

• Soft reduction to shrink cavities and minimise macro-segregation during continuous casting, and;

• Production of a maximum slab thickness – to allow greater core conditioning during rolling.

Modern HSLA steels always rely on a carbon content which avoids the peritectic reaction during solidification, i.e. below 0.09%, most typically being around 0.07% or less. This low carbon content not only helps to avoid surface cracks during continuous casting, but also improves the weldability with regard to the toughness in the heat affected zone (HAZ), see Figure 5 (Heisterkamp, Hulka and Batte, 1990). During the peritectic reaction an additional shrinkage as result of the transformation of primary !-ferrite into austenite occurs causing inter-dendritic inclusion of liquid steel. This is naturally enriched in alloying elements, as the major alloying element in HSLA steels is manganese it exhibits a segregation ratio typically as high as twice the bulk steel composition and this segregation is the origin of local brittle zones in the HAZ resulting in poor toughness. Today’s modern generation of high strength structural steel plate are increasingly utilising a low carbon TMCP route consequently generating the higher strengths, toughness and better weldability. Furthermore, some of the recent developments and improvements in plate mill technology has permitted some additional heat treatment procedures (normalising, quenching and tempering) to be eliminated (Porter, 2006) resulting in better product margins. Table 1 shows typical chemical compositions for 50mm plate, either heat-treated or thermomechanically rolled, with the carbon equivalents indicating the necessary conditions for welding. As can be seen in the table, besides TMCP another possible solution for economical production is the application of the ‘HTP concept’ (Hulka and Gray, 2001), which is based on a higher amount of niobium in solid solution, thus allowing austenite processing at higher temperatures and increasing plate mill output. Figure 6 shows mechanical properties of a 0.03%C–1.80%Mn–0.10%Nb steel and plates of various thicknesses rolled under relatively relaxed mill processing conditions. The strength and toughness properties are excellent, even at relatively thick plate dimensions.

The Thermo-mechanical Rolling Process Most modern HSLA steels are pearlite reduced or even pearlite free and their processing is dominated by thermo-mechanical rolling (TM), often applied together with enhanced cooling after rolling (TMCP). The definition of TM processing is given in the standards for structural steels, such as EN 10113(3) – 1993, but in general it can be defined as a process which aims to achieve a structure with a fine effective grain size that permits a favorable combination of service properties. Importantly, the rolling schedule is tailored to the steel composition, and is composed of the following steps governed in terms of time and temperature (Streisselberger, 2001):

• Slab reheating - with a defined drop out temperature typically at 1200-1250oC; • Rolling - on the basis of a tailored pass sequence with finish rolling in the

nonrecrystallized austenite or austenite-ferrite two-phase zone; • Cooling - either in air or in the stack, or in accelerated form in the cooling line, down to a

defined final cooling temperature, and; • Optional - additional heat treatment (tempering) stage. The essential benefits of TM/TMCP are based on the effects of microalloying, of which the key element is niobium (Nb) as it: • Retards/suppresses recrystallization of austenite between the individual rolling passes

allowing strain accumulation during rolling, subsequently leading to the formation of very fine grains during transformation;

• Forms very fine carbonitride precipitates which block dislocation movement in the atomic lattice and thus results in increases in yield strength and tensile strength (precipitation hardening), and;

• Retards, if in solid solution, the austenite to ferrite transformation. It is these effects of niobium which are exploited during processing 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 strength and tensile strength values. As with all elements, the alloying content of niobium must be chosen with respect to the process route and property requirement of applied plate, see Figure 7 (Hulka, 1991). Over the last few years TMCP of heavy plates at 500MPa and above have been developed for the constructional sector (Streisselberger, 2001). These grades are characterised by excellent toughness levels after welding even with the high strength properties. In cases in which high loads or large spans have to be designed, columns, piles and girders are normally welded and made from heavy plates (e.g. column bases for the Taipei 101). This design and construction method shows an economical advantage at sizes greater than a girder height of about 600mm because the cross sections of the supporting structure can be adapted individually to the constructional task by using only a minimum of steel. The steel grade S355 is predominantly applied for these applications, although even heavier plates at S460 are increasingly being used (Streisselberger, 2001). However, thick plates ask for another necessary consideration, the total available deformation is rather limited for the pre-rolling and the characteristic thermo-mechanical rolling. It has been found that also the failure by lamellar tearing is not only dependent on the cleanliness of the steel, but also asks for a compact core of the plate. In order to bring the deformation to the core of the rolled plate a higher deformation per pass is helpful. Figure 8 (Hulka and Bauer, 1999), shows that besides a high total deformation, a high deformation per pass helps to prepare a sound core of the plate.

Although all modern TMCP structural steels make use of Nb as the primary microalloying element, the current EN 10025-4 standard only accepts 0.05%Nb maximum (see Table 2 for the structural steel grades according to Europe and ASTM standards). When compared to the recently harmonised linepipe API 5L / ISO 3183 standards, this allows higher levels of Nb to be used as an advantage when C is less than 0.10 - 0.12% and only limiting the total level of microalloying through Nb+V+Ti < 0.15, thereby allowing Nb to be alloyed up to 0.15% in the absence of other microalloying elements. Therefore, with the known application of lower C (0.03-0.045%) and higher Nb (0.075-0.10%) steels for API X70 (483MPa minimum yield strength) and X80 (552MPa minimum yield strength) linepipe that exhibit excellent low temperature toughness and HAZ CTOD properties a revision of the EN 10025-4 specification to permit Nb levels towards 0.10% with a corresponding decrease in carbon content should be called for. Suitable amendment of EN 10025-4 would enable an overall leaner alloy design to be used and also permit much of the experience gained from processing of modern API 5L / ISO 3183 linepipe steels to be transferred across to construction steels. This would also enable steelmakers to quickly adapt the same steel chemistries for two different end applications by suitably adjusting the processing parameters only and thereby enable additional savings. Furthermore, such modern linepipe grades have also been designed to strain capacity (rather than conventional stress based design) to account for seismic / unstable ground condition and therefore possess sufficient plastic strain capacity as well as being supplied to within an upper yield ratio. Niobium High Strength H-Beams In the construction of tall buildings, bridges and industrial halls requiring large load bearing capabilities, the use of H-bearing piles are widely used. These piles are driven deep into the subsoil by hydraulic impact hammers and quite often have to penetrate highly resistant constituent layers of rock, marlstone, dense sand and gravel layers. Therefore, such steels are required to have high axial load bearing capabilities, as well as good impact properties. Today several producers around the world are capable of making Nb bearing high strength H-beams with yield strengths of 460 and 500MPa. Typically these beams are made via the quenching and self-tempering route (QST) whereby after the last pass of thermo-mechanical rolling (at around 850oC), intense water-cooling is applied via directed spray nozzles. However, the cooling is interrupted before the core is cooled and thus the retained heat from within the core tempers the outer-cooled layer. The self-tempering is achieved at approximately 600oC and the principles and temperature flow of the process are shown in Figure 9 (Weber et al., 2008). However, for the majority of mills where such technology advantages do not exist, such H-beams and wide flange beams are conventionally rolled. Here semis are generally reheated at a temperature of around 1250°C and are hot rolled in 13 to 20 passes. The flange reductions can be anything from 4 to 22% per pass and the rolling end temperature is often at 1,000°C or

above. As the material is asymmetric the temperature profile will also be different along the cross-section. For H-beams and wide flange beams the hot points are found in the web-flange junction and the coldest location is the middle height of the web (i.e. the neutral point of the beam cros-section). Depending upon the profile of the beam the temperature difference between these two points can reach 100oC, which will influence the developing microstructure and residual strains. As the finish rolling temperatures are generally nearer 1,000oC, small additions of titanium and niobium microalloying can be used to control the as-reheated (soaked) austenite grain size which helps in producing a refined recrystallised microstructure. Studies have demonstrated that a deformation of 15% per pass is sufficient to obtain the desired microstructure and mechanical properties for a Nb-alloyed S355 grade steel (equivalent to Grade 50), of which a typical commercial beam composition is given in Table 3. Naturally, as the flange thickness increases the steel chemical composition must be adjusted to account for the reduced through thickness reduction and slower cooling rates. This normally means an increase in the carbon equivalent and niobium content to obtain the required tensile properties of S355 as a function of the flange thickness as shown in Figure 10 (Donnay and Grober, 2001). In this case as the finishing rolling temperature is high, a significant portion of the niobium will remain in solid solution. The niobium will aid to the strength increase by retarding the austenite transformation and this will lead to a finer ferrite grain size and a certain amount of bainite in the microstructure as the material is air-cooled. Figure 11 highlights the increasing amount of bainite formed with greater solute niobium. For the S355 steel, the microstructure will consists of about 80% ferrite and nearly 20% bainite. Some small fraction of pearlite will also be present. As the beam air-cools additional strengthening is provided by the available solute niobium forming fine precipitates of NbC in the ferrite during and/or after transformation from austenite. The degree of this contribution will be determined by the size and volume fraction of the precipitate, as defined by the Ashby Orowan relationship (see Figure 12). Value Addition of Niobium Alloying The advantage of using high strength steel for columns in structures compared to a lower strength S235 beam is shown in Figure 13 (Donnay and Grober, 2001). Using an identical design load but with three different geometric cross sections in three different steel types, S235, S355 and S460 the advantages of high strength steels are evident. Although S355 will save weight and costs compared to S235, it can be seen that use of S460 will lead to a further 14% weight saving. In terms of material costs the savings for S460 are about 10% compared to S355 and 25% compared to S235. All of this is made possible by small additions of niobium in both plate and beams allowing the development of stronger, tougher and easily weldable steels even for the thickest sections.

Some Recent Applications of HSS Plate There are a big variety of constructions projects that use HSS such as hangars, offshore platforms, sport stadiums, etc. the most impressive are bridges and skyscrapers due to their visibility. To underline the merits of modern Nb-bearing plate steels some examples are given as shown in Table 4. Included in the table are two recent examples from India. The first is the new corporate headquarters for ICICI Bank in the financial district of Hyderabad. Here, the latest steel composite floor decking system along with 10,000 tonnes of BS4-1:2005 universal heavy sections were used. This combination produced significant cost savings in the overall construction costs and also offered more scope to the architects. The construct now marks it as India’s largest steel framed building and one of the world top 20 buildings with the largest floor area in a single building. The 20 storey building measures 342m in length, 88m wide and 70m tall and 18,500 tonnes of steel were used in its construction. The second example has been marked as “India’s first signature bridge”, the Yamuna Bridge costing nearly $250 million will have a main span of 251m and a total length of 675m. It is a cable-stayed bridge with a 154m tall pylon. All of the structural steels that will be used are the equivalent of S355 and for the pylon S460 has also been used. In total approximately 8,000t of steel will be used at thicknesses up to 150mm. The bridge is schedule for completion in 2013. Concluding Remarks In recent years the usage of Nb based high strength TMCP structural steels has increased, however, this has been coupled with greater material performance requirements such as low temperature toughness, low yield ratios and limiting the need for preheating prior to welding. Modern plate and beam rolling mills are capable of producing heavy plates and H-beams with yield strengths of 500MPa. To achieve this along with other performance challenges, cleaner steels are required with lower carbon contents, microalloying with Nb, and good management of the hot rolling process with respect to the developing microstructure. There is an opportunity to accelerate the use of Nb-bearing high strength structural steels based on the experience gained from processing of modern API 5L / ISO 3183 linepipe steels (e.g. X80 552MPa minimum yield) which can be designed with strain capacity (rather than conventional stress based design) to account for seismic / unstable ground condition and therefore possess sufficient plastic strain capacity as well as being supplied to within an upper yield ratio. Such steels exhibit excellent low temperature toughness and HAZ CTOD properties but currently they cannot be applied to structural steels (EN 10025-4) due to limitations on the chemical composition (at 0.05%Nb). With a revision of the EN 10025-4 specification to permit Nb levels towards 0.10% with a corresponding decrease in carbon content would enable a greater degree of technology transfer and further would also enable steelmakers to quickly adapt the same steel chemistries for two different end applications by suitably adjusting the processing parameters only and thereby enable additional cost savings.

Acknowledgements The author would like to thank Dr Marcos Stuart, Director, CBMM Technology Suisse for permission to publish this paper. References B. Donnay and H. Grober, International Conference Proceedings Niobium 2001, Florida, December 2001, TMS, pp.777-800 T. Gladman, B. Holmes and I.D. McIvor, The Iron and Steel Institute, London (UK), 1971, pp.68-71. F. Heisterkamp, K. Hulka and A.D. Batte, The metallurgy, welding and qualification of microalloyed (HSLA) steel weldments, AWS, Miami (Fl), 1990, pp.659-681 R. Hubo, VDI-progress report, series 18/book 80,VDI, Düsseldorf (Germany), 1990 K. Hulka and J.M. Gray, Niobium Science & Technology, Niobium 2001 Ltd., TMS, 2001, pp.587-612 K. Hulka, Microstructure and Properties of Microalloyed and Other Modern HSLA Steels, Pittsburgh, 1991 K. Hulka and J. Bauer, Steel for Fabricated Structures, ASM Int., Metals Park (OH), 1999, pp.11-19 J. Lessels, HSLA Steels, IISI, Brussels (Belgium), 1987, Chapter 7 D. Porter, Nordic Welding Conference ‘06, 8-9 November 2006, Tampere, Finland A. Streisselberger, V. Schwinn and R. Hubo, Niobium Science & Technology, Niobium 2001 Ltd., TMS, 2001, pp.625-646 TradeARBED S.A., HISTAR, Luxembourg (Lux.), 1989 L. Weber, L. Cajot and J Gerardy, International Conference Proceedings on New Development on Metallurgy and Applications of High Strength Steels, Buenos Aires 2008, pp.787-798 D.G. Younger, Metal Progress 107, 1975, No.5, pp.43-47

Fig. 1 Commerzbank Tower (Source: CityForum ProFrankfurt)

Fig. 2 Weight and costs reduction by using HSLA steels

Fig. 3 Ductility requirements for steel construction

Fig. 4 Reduction of area in thickness direction of S355 plate

Fig. 5 Heat affected zone toughness of Grade S355 plates

Fig. 6 Mechanical properties of plates produced with ‘HTP steel’

Fig. 7 Mechanical properties of a 0.08%C-1.50% Mn steel as a function of Nb content and rolling conditions

Fig. 8 Reduction of area in thickness direction as result of total deformation and deformation per pass

Fig. 9 The principles of the Quenching and Self-Tempering (QST) process (Weber and Cajot, 2008)

Fig. 10 Steel alloying contents for S355 from controlled rolling (Donnay and Grober, 2001)

Fig. 11 Correlation between solute niobium and bainite

Fig. 12 Precipitation hardening by NbC influence of volume fraction and particle size

Figure. 13 Weight savings using high strength grades compared to conventional steel qualities (Weber and Cajot, 2008)

200 %

132 %116 %100 %

0 %

91 %86 %

Relative weight Relative material costs

Buc

klin

g le

ngth

: 3,5

m

100 %

Steel grade S 235 S 355 S 460SectionUltimate load kN

HE 300 B2976

HE 300 A3129

HE 280 A3102

Table 1. Typical chemical composition, wt% of 50mm plate high strength structural steels

Steel grade S355 S355 S460 S460 S460 API X80

Processing N TM N QT TM + ACC 0.10%Nb

TM + ACC C 0.15 0.07 0.15 0.10 0.07 0.03

Si 0.40 0,30 0.40 0.35 0.25 0.30

Mn 1.50 1.50 1.50 1.45 1.55 1.80

P 0.012 0.012 0.012 0.012 0.012 0.012

S 0.004 0.004 0.004 0.004 0.004 0.004

Al 0.03 0.03 0.03 0.03 0.03 0.03

N 0.005 0.005 0.005 0.005 0.005 0.005

Ti 0.015 0.015 0.015 0.015 0.015 0.015

V none none 0.12 none 0.04 none

Nb 0.04 0.04 0.04 0.04 0.04 0.10

Cu none none 0.60 0.30 none 0.20

Ni none none 0.60 0.60 0.25 0.10

Mo none none none 0.25 none None

CE 0.40 0.31 0.50 0.45 0.36 0.36

pcm 0.23 0.15 0.28 0.22 0.17 0.14

(N = normalised, TM = thermomechanically rolled, QT = quenched plus tempered, TM + ACC =

thermomechanically rolled plus accelerated cooled)

Table 2. Structural steel grades according to European and ASTM standards (sections of 30mm thickness and unit weight lower than 634kg/m) (Weber and Cajot, 2008)

Standard Grades Ladle analysis

C max

Mn max

P max

S max

Si max

Al min

Nb max

V max

CE max

S355J0 0.20 1.6 0.04 0.04 0.55 - - - 0.45 S355M 0.16 1.6 0.035 0.03 0.5 0.02 0.05 0.1 0.39

S355ML 0.16 1.6 0.03 0.025 0.5 0.02 0.05 0.1 0.39 EN 10113-3 (1993)

S420M 0.18 1.7 0.035 0.03 0.5 0.02 0.05 0.12 0.45 S420ML 0.18 1.7 0.03 0.025 0.5 0.02 0.05 0.12 0.45

S460M 0.18 1.7 0.035 0.03 0.6 0.02 0.05 0.12 0.46 S460ML 0.18 1.7 0.03 0.025 0.6 0.02 0.05 0.12 0.46

S355G4 0.16 1.6 0.035 0.03 0.5 0.02 0.05 0.1 S355G11 0.14 1.65 0.025 0.015 0.55 0.015 0.04 0.06

EN 10225

(2000)

S355G12 0.14 1.65 0.02 0.007 0.55 0.015 0.04 0.06 S420G3 0.14 1.65 0.025 0.015 0.55 0.015 0.05 0.08 S420G4 0.14 1.65 0.02 0.007 0.55 0.015 0.05 0.08

S460G3 0.14 1.7 0.025 0.015 0.55 0.015 0.05 0.08 S460G4 0.14 1.7 0.02 0.007 0.55 0.015 0.05 0.08 A36 (1997) 0.26 0.8 0.04 0.05 0.4 - - -

A572 (1997) Gr50 0.23 1.35 0.04 0.05 0.4 - 0.05 0.15 Gr65 0.23 1.65 0.04 0.05 0.4 - 0.05 0.15

A992 (1998) Gr50 0.23 1.5 0.035 0.045 0.4 - 0.05 0.11

A913 (1997) Gr50 0.12 1.6 0.03 0.03 0.4 - 0.05 0.06 0.38 Gr65 0.16 1.6 0.03 0.03 0.4 - 0.05 0.06 0.43

Standard Grades Tensile test Impact test

Re MPa

Rm MPa

A+ min %

Temp. °C

Energy J

EN 10113-3 (1993)

S355M 345 450-610 22 -20 40 S355ML -50 27 S420M 400 500-660 19 -20 40 S420ML -50 27 S460M 440 530-720 17 -20 40 S460ML -50 27

EN 10225

(2000)

S355G4 450-610

460-620

-20 50 S355G11 345 22 -40 50 S355G12 -40 50 S420G3 410 500-690 19 -40 60 S420G4 -40 60 S460G3 440 530-720 17 -40 60 S460G4 -40 60

A36 (1997) 250 400-550 18

A572 (1997) Gr50 345 450 18 Gr65 450 550 15

A992 (1998) Gr50 345-450 450 18

A913 (1997) Gr50 345 450 18 20 54 Gr65 450 550 15 20 54

+EN standards: A5d, ASTM standards: A200

Table 3. Chemical composition of 20mm beams in S355 (finish rolling temperature at 1050°C)

C Si Mn P S Ti Nb (Cu+Cr+Ni) N

0.08 0.2 1.5 <0.02 <0.02 0.015 0.022 0.4 0.0085

Table 4. Recent examples of structures utilising high strength steels

Structure Description High Strength Steels Used

Taipei 101 Tower Taiwan

509m tall sky scrapper, 101 floors above ground – opened in late 2003.

95,000t of HSS including TMCP Grade SM570M up to 80mm thick (pcm <0.28).

Oresund Bridge Sweden

7,845m long bridge linking Denmark and Sweden – opened in 2000.

82,000t of HSS including Grade 65 (S460M) 42-78mm thick. Resultant cost savings over $25M.

Viaduc de Millau France

Long bridge 270m above ground – opened in 2004.

43,000t of HSS including Grade 65 (S460MC) up to 80mm thick and also 120mm thick.

Two International Finance Centre Hong Kong - China

415m tall sky scrapper, 88 floors above ground – opened in late 2003.

90mm thick plate used for 6 steel-concrete columns from foundation to 6th floor (5,000t) as well as TMCP HSS <40mm.

ICICI Bank India (Hydrebad)

20-storey, 75m tall, 342m wide, 88m deep. World top 20 building with floor space in single building – opened 2010.

18,500 tonnes of steel used, of which 10,000 tonnes of BS4 Sections. A unique composite steel floor system was also used.

Yamuna Bridge India (Delhi)

Cable-stayed bridge spanning 251m and total length of 675m. It is 35m wide (4 lanes). Main Pylon is 154m high – currently constructing.

S355 and S460 grade steels used, with plate thickness up to 150mm. 8,000 tonnes of steel used for the pylons.