This article summarizes the current state of the art in the production and application of TMCP high strength steel plates for steel structures. Using the thermomechanical rolling process heavy steel plates with a yield strength of up to 500 MPa can be produced. These are also characterized by best fabrication properties, therefore these products can be efficiently used in big steel structures. For special elements high strength steel grades with a yield strength up to 690 MPa (and now even to 1100 MPa) are sometimes used which are produced by the quenching and tempering process. Typical application examples will be given.
Introduction to TMCP High Strength Steel
Steel — in particular heavy plates – is the most important input material for heavy steel structures. Therefore it is obvious that the improvement of the efficiency of steel products in design, fabrication and service life of a steel structure is a key element to develop also the efficiency of the steel structure.
One way of gaining a higher productivity is offered by the use of high strength steel, which can be defined as a product with a yield strength higher than 355 MPa. Under special constructional circumstances such high strength steel enables the designer to reduce cross-sections saving also considerable fabrication time and costs by smaller welds. Furthermore, also high strength steel grades with a good structural safety in particular against brittle fracture and excellent fabrication properties (welding) already exist. However, the weak points of designing with high strength steel such as fatigue or displacement restrictions will also be addressed.
High strength steel plates can be produced by various production processes which also influence the final using and fabrication properties of the steel. Here, mostly the thermomechanical rolling process is applied for the production of high strength steel plates as thereby also good fabrication and utilisation properties can be guaranteed. Such plates are today produced up to a yield strength of 500 MPa and have gained special attentiveness in large span landmark bridges. But also other fields of applications, such as industrial buildings or medium span bridges, can profit from these materials, Furthermore, some extra high strength steel with a yield strength up to 690 MPa is sometimes used for special structural elements in bridges and buildings. These steels are produced by a quenching and tempering process.
It can be seen that the production process of heavy plates has fundamental impact on the fabrication properties of a steel product. Therefore, the various production techniques which exist today for heavy plates will be described first. Secondly, it will be explained how this influences the fabrication properties, in particular welding of these high strength steel grades.
However, some peculiarities concerning design have to be taken into account when constructing with high strength steel, Finally, it will be shown how these steels can successfully be applied for steel construction.
Overview of Production Process of High Strength Steel
Weldable structural steels are delivered in the conditions: normalised, quenched and tempered, and thermomechanical controlled rolled, schematically shown in Figure 1. Figure 2 allows to compare typical microstructures for the above mentioned supply conditions.
For steel grades of moderate strength and toughness requirements a classical hot rolling and normalising of the steel is sufficient to obtain the necessary mechanical values. By this process route weldable structural steels up to S460N are produced.
Hot rolling is generally carried out at high temperatures above 950°C (process A in Figure 1). By re-heating the hot rolled plates to some 900°C followed by free cooling in air a refined microstructure of ferrite and pearlite (process B in Figure 1) is obtained. However with this process a higher strength of steel plates is mostly related to higher alloying contents influencing weldability in a negative way.
By quenching and tempering high strength structural steels can reach a yield strength of up to 1,100 MPa. This heat treatment (process C in Figure 1) applied subsequent to hot rolling, consists of an austenitisation, followed by quenching and finally tempering,
The aim of thermomechanical rolling (TM or TMCP) is to create an extremely fine grained microstructure by a skilled combination of rolling steps at particular temperatures and a close temperature control. The gain in strength obtained by the grain refinement allows to reduce effectively the carbon and alloy content of the TM-steel compared to normalised steel of the same grade. The improved weldability that results from the leaner steel composition is a major advantage of high strength steel TM-plates.
Depending on the chemical composition, the required strength and toughness properties and the plate thickness the “rolling schedule” is individually designed. Some typical TM-processes are shown in Figure 1. Especially for thick plates an accelerated cooling after the final rolling pass is beneficial for the achievement of the most suitable microstructure as it forces the transformation of the elongated austenite grains before recrystallisation can happen. For very thick plates and high strength steel grades a tempering process can be used after the accelerated cooling.
TM-rolled plates with minimum yield strength values of 500 MPa were supplied up to 100 mm for hydro-power, offshore platforms and special ships (Schutz & Schroter, 2005). Even higher yield strength classes up to 690 MPa are feasible by the TM-process, however, in a more limited thickness.
Figure 3 summarizes the historical development of high strength steel grades during the last decades
Available Grades and Construction Rules for High Strength Steel
Available High Strength Steel Grades
According to the various production processes described above, high strength steel plates for construction purposes with yield strength higher than 355 MPa are defined in three European standards:
EN 10025-3: Normalized fine grain steel: S420N, S420NL, S460N, and S460NL
EN 10025-4: Thermomechanical rolled fine grain steel: S420M, S420ML, S460M, and S460ML
EN 10025-6: Quenched and tempered fine grain steel: S460Q, S460QL, S460QL1, S500Q, S500QL, S500QL1, S550Q, S550QL, S550QL1, S620Q, S620QL, S620QL1, S690Q, S690QL,S690QL1, S890Q, S890QL, S890QL1, S960Q, and S960QL
In the above steel grade designation, the figure indicates the minimum yield strength of the product at the lowest available thickness. The following code is an abbreviation for the production process and the Charpy-V impact energy tested. Thus, the abbreviation N or M corresponds to a minimum toughness of 40 J at -20°C (longitudinal), whereas NL- or ML-grades are characterized as low-temperature grades with more than 27 J at -50°C. For quenched and tempered grades the following is valid:
- Q: 30J at -20°C
- QL: 30J at -40°C
- QL1: 30J at -60°C
For instance, Tables 1 and 2 summarize the mechanical properties of high strength TMCP rolled material according to EN 10025-4,
Application of High Strength Steel according to the Eurocode
According to the basic Eurocode EN 1993-1-1 steel grades up to a yield strength of S460 can be used for steel structures either in the normalized, the thermomechanically rolled or the quenched and tempered condition.
Furthermore, a new part of the Eurocode EN 1993-1-12 is in preparation, which will define rules for the application of steels up to a yield strength of 5690. Here, also some requirements on steel products have been adapted. Thus, for instance a tensile to yield strength ration fify of 1,05 is sufficient.
The most significant advantage of TM-plates compared with conventional steel grades of the same thickness is their outstanding suitability for welding characterised by two main features: on the one hand, preheating of thicker TM-plates can be significantly reduced or omitted completely, which allows significant savings in fabrication time and costs, On the other hand, TM-plates exhibit high toughness values and low hardening values in the heat affected zone (HAZ) after welding (Schroter, 2004).
These effects are due to the very low alloying contents (in particular carbon contents) which can be reached by this special rolling process. Thus, Table 3 compares the typical alloying content of a conventional S355J2G3 with that of a TM-steel of the same yield strength S355ML.
The table also indicates the values for the mostly used carbon equivalents, formulas which are used to judge the influence of the alloying elements on weldability. It can be seen that TMCP rolled steel shows much smaller carbon equivalents than normalized steel grades of the same yield strength.
Furthermore, TMCP rolled steel has an excellent toughness behavior. Figure 4 illustrates that the transition temperature between brittle and tough fracture behavior, which is often defined by the temperature, where a Charpy-V impact energy of 27 J is attained, can be significantly reduced by TMCP rolling in comparison to a conventional steel grade S355J2G3.
Thus, a reserve is given in order to ensure also outstanding toughness properties in the heat affected zone of the weld. As an example Figure 5 shows the Charpy-V-temperature transition curve of an S355ML measured in the heat affected zone in distance of 2 mm to the fusion line. It can be seen that even at —50°C a good toughness level can be reached resulting in a high safety of the welded joint against brittle fracture. Furthermore, good toughness levels in the heat affected zone are a prerequisite for the application of welding processes with high heat input. Thus, also the efficiency of the welding process can be increased by using TMCP rolled material.
Due to the higher carbon content and the risk of hydrogen-induced cracking a conventional S355J2G3 in thickness’ above 30 mm is normally preheated prior to welding. Due to its low alloying content, TM-steel S355M is normally not preheated if EN 1011-2 is applied for the calculation of preheating temperatures.
The economic benefit of avoiding preheating is clear: A time and money consuming step in the fabrication process of steel structures can be avoided and the production efficiency of the workshop be increased (Hever & Schröter 2003).
One big advantage of TMCP rolling technique is that even higher strength grades can be produced by an appropriate heat treatment without considerably increasing the alloying content. Thus Figure 6 shows the mechanical properties of two steel plates of the same chemical compositions but with different treatments after rolling. By applying an especially harsh cooling — the so-called heavy accelerated cooling — the yield and ultimate strength of the steel plates can be increased without any change of the chemical composition.
Thus, even a steel with a minimum yield strength of 460 MPa can be produced with acceptable carbon equivalents for best weldability. For instance, an S460M steel shows a carbon equivalent of 0.40 —0.42 % which may be lower than that for a conventional S355J2G3.
Thus, S460M enables the reduction of preheating temperatures in comparison to a conventional S460N. Furthermore, in most cases even for this higher-strength steel grade preheating can be omitted completely if special conditions on the welding process are fulfilled, as in particular the usage of low-hydrogen consumables (hydrogen content: HD) such as thoroughly dried basic electrodes. Figure 7 compares the necessary preheating temperatures for a S460M and S460N steel.
In order to reduce the danger of embrittlement in the heat-affected zone, steels unsusceptible to ageing are needed. The insusceptibility for ageing is shown on the material by notch impact tests on cold formed and artificially aged material. Figure 8 shows that the notch impact-temperature-transition curve moves towards higher temperatures when the steel is being aged, but relatively low transition temperatures can still be found even under these hard test conditions.
Further fabrication properties of TMCP-rolled steel can be found in Hanus & Hubo (1999) and Schroter (2004).
Quenched and Tempered High Strength Plates
Steels of 690 MPa yield became commercial about three decades ago. They were — like today – essentially produced by water quenching and tempering (QT). Nowadays QT-plates with a yield strength over 1,100 MPa have become commercial.
The aim of quenching and tempering is to produce a microstructure consisting mainly of tempered martensite. Some amounts of lower bainite are also acceptable, Quenching of high strength steels is performed after austenising at temperatures of 900 to 960°C. In order to suppress the formation of softer microstructure, essentially ferrite, during cooling an accelerated cooling is necessary. The fastest cooling is obtained by exposing the plate surfaces to a rapid water stream; by such an operation the very surface is cooled to a temperature below 300 °C within a few seconds. In the core of the plate cooling is considerably slower and the cooling rate decreases with increasing plate thickness. Therefore the alloying concept of thicker quenched and tempered plates has to be adapted to ensure sufficient hardening in the plate core.
If we consider the mechanical properties in the as quenched condition, the strength is considerably higher than required but the material is too brittle for most structural applications. A suitable tempering of the martensitic microstructure is necessary in order to get a satisfactory combination of tensile strength and toughness properties. By tempering the martensite, the supersaturation of carbon in the matrix is reduced by the precipitation of carbides leading to a relaxation in the atomistic scale. At the same heat treatment the high dislocation density associated with martensite formation is reduced. Both effects improve the toughness of the material, A 60 mm thick S890QL (EN 10025-6) is chosen for example that shows the influence of tempering on the properties. Figure 9 illustrates how the tensile properties are lowered with increasing temper parameter, Figure 10 the improvement of impact toughness, respectively.
The tempering conditions must be adapted to the particular chemical composition of the steel. The higher the toughness and strength requirements the closer gets the permitted range for the tempering conditions.
It has already been mentioned that the alloying compositions of quenched and tempered steel increases with growing plate thickness in order to ensure a sufficient hardening of the plate in the core region. Therefore, it is obvious that the carbon equivalent of a quenched and tempered steel increases with the plate thickness. An example is given by Table 4.
Due to higher strength and carbon equivalents quenched and tempered steel grades of a yield strength of 690 MPa and more show a more sophisticated fabrication behavior than thermomechanically rolled steel grades. Here especially the weldability should be examined in detail.
The temperature-time cycles during welding have a significant effect on the mechanical properties of a welded joint. Generally the cooling time from 800°C to 500°C (t815) is chosen to characterize the cooling conditions of an individual weld pass for the weld metal and the corresponding heat affected zone for higher strength steel. Increasing heat input and inter-pass temperature leads to slower cooling and, hence, longer cooling times t515, Knowing the welding parameters (like heat input and preheating temperature) and geometry t815 can be calculated according to standard EN 1011-2.
To achieve satisfactory weld metal properties the welding parameters must be limited with increasing yield strength. Acceptable properties for an S690 steel are normally obtained with cooling times between 6 s and 20 s. For lower cooling times the hardness of the heat affected zone may exceed limiting values with the risk of introducing cracks, see Figure 11. On the other hand, long cooling times result in poor strength and toughness values.
The window between upper and lower admissible cooling times gets the smaller the higher the basic yield strength of the steel is. This is related to a narrowing of the window of welding parameters, from which heat input and preheating temperature can be chosen. Figure 12 demonstrates that this parameter window also depends on other factors. The hydrogen content of the consumables H2 [measured in m1/100 g] has a significant influence on the preheating temperature. In order to reduce the risk of hydrogen-induced cracking the preheating temperature increases with growing hydrogen content. According to EN 1011 the necessary preheating temperature Tp [in °C] under the condition of normal constraint conditions also depends on the heat input Q [in kEmm], the carbon equivalent CET and the plate thickness d [in min]:
The fatigue resistance of welded structures limits the use of higher strength steel in dynamically loaded constructions. It is understood that the fatigue resistance of welded details performed under normal conditions is more or less the same for normal mild steel and for higher strength grades. Thus it can happen that the designer of steel structures cannot profit from the higher static strength of high strength grades if the construction is dynamically loaded.
On the other hand it is also known that the fatigue resistance of high strength steel can be improved by reducing the notch effect of the weld details. Apart form special precautions to perform high weld quality, this can be reached by special post-treatments of the weld.
In general these post-weld treatments are processes which reduce the notch effect of the weld details, such as the TIG dressing or a grinding of the weld. For instance the TIG dressing smoothes the weld notch by a further remelting of the transition zone between weld and base material.
But post treatments, such as shoot peening and hammering, exist which introduce compression stresses in the transition zone between weld and base material. By these processes the transition zone is plastically deformed inducing compression stresses. Also smaller defects in the weld are removed. However, these processes are rarely used in steel construction due to high vibrations, noise and also poor reproducibility.
The ultrasonic impact treatment (UIT) represents a relatively new post-treatment which does not show these disadvantages (Statnikov, 1975), Here, an ultrasonic transformer transforms ultrasonic waves into mechanical impulses. Thereby, the weld is hammered by hardened bolts with a frequency of 200 Hz and at the same time ultrasonic energy of 27 —55 kHz is introduced.
This process is now under close investigation in a German research project (Kuhlmann, Durr & Schroter). Figures 13 and 14 show some selected results of the fatigue test performed in this study for various post treatments of the weld. Here the fatigue resistance dot defined at a level where 50 % of the specimens are assumed to have failed under a constant cyclic load after 2 million load cycles is given. The R-value (ratio between lowest and highest) load is fixed at 0,1. The tests have been performed on small scale specimens simulating a transverse stiffener,
Although the number of tests given in these charts is very limited, it can be clearly seen, that the fatigue resistance can be improved by the different post treatment. The best results are obtained by the UIT treatment.
These results give the hope that the efficiency of dynamically loaded steel structures can be improved. It should be taken into account that the number of details for which the fatigue resistance is the determining factor in the design process, can be very small. Therefore, the improvement of the fatigue behavior in such local areas can increase significantly the efficiency of higher strength steel in the entire structure.
Examples of TMCP High Strength Steel in Use
The benefits of using high strength steel in steel structures are clear: In comparison to normal strength steel the size of the cross section can be reduced resulting in:
- a decrease of the dead weight of the structure, from which the substructure and the erection profit,
- reduced cross section of welded joints by which fabrication and inspection costs can be reduced and higher clearance heights under overpasses can be ensured.
For this reason higher strength steel are not only used for bigger landmark bridges but also for more convenient medium span bridges.
Figure 15 shows such a standard bridge, a bridge across a canal in Zuid Beveland, the southern part of the Netherlands. Here a girder construction of S460 was chosen in order to reduce the girder depth and to allow maximising the clearance height for the canal under the bridge.
As Another example Figure 16 shows the bridge of Remoulins in the South of France. For this continuous twin-girder construction with span lengths of 47, 66 and 51 m a combination of TM-steels S355ML and S460ML was used. The high strength S460ML was especially applied in the highly stressed pier region of the girders to reduce the maximum thickness. So only a maximum thickness of 80 mm instead of 120 mm (Figure 17) for the solution purely in S355ML was necessary resulting in weight reduction and an easier fabrication and erection procedure. Moreover, thanks to the choice of TM-material, fabrication costs could be additionally reduced by the avoiding of preheating. In total a weight reduction of more than 8 % could be obtained by using this special combination of materials
A typical example for the application of S690Q steel in medium span bridges in Germany is displayed in Figure 18. Here a composite bridge across the freight railroad centre in Ingolstadt with span lengths of 24 + 3*30 + 24 in is shown. The cross section consists of two 1.2 m-high plated girders in a distance of 7 m, cross beams in a distance of 7.50 in and a cantilevered concrete deck cast in-situ in a rhythm of 15 m. Here S690Q was applied for the connection between the girder and the piers formed by concrete filled steel tubes of 600 mm diameter. The 70 mm-thick lamella of S690 was welded to the girder to form a bending-stiff connection. Thus, a very efficient alternative for bearings was created.
However, the real domain of high strength steel grades such as S460M is still the construction of bigger bridges such as the new Rhine-bridge in the north of Dlisseldorf (Germany), which was opened for traffic middle of 2002 (Sedlacek, Eisel, Paschen & Feldmann, 2002). For this cable-stayed bridge with a central span length of 275 m the pylons had to be restricted to a height of 34 m due to their situation in the landing zone of the near airport, see Figure 19. Therefore, the high forces arising in the pylon heads could only be solved by selecting the high strength steel S460ML for these structural elements. Even plate thicknesses up to 100 mm have been used for the central parts of the pylon heads.
The most impressive example for the use of higher strength grades in bridge building is shown by Figure 20. The Millau Viaduct in the South of France, which was opened end of 2004, is the highest bridge in the world by a total height of 343 m, a deck height of up to 270 m and a length of 2,460 m. The total weight of steel plates used for this extraordinary bridge is 43,000 t, among this 18,000 t of 5460M. Apart from the pylons this steel grade was used for wider parts of the box girder in particular to reduce the weight during the incremental launching process and to optimize the thickness of welded elements for efficient and quick construction.
An example for the application of higher strength steel grades in building structures is shown in Figure 21. The structure shown there forms a part of the Sony-Center in Berlin. This truss structure is fixed on three columns and holds the lower apartment building which hangs above the facade of an ancient hotel. The truss girders consist of welded box sections made of S460 in thickness’ up to 110 mm. Due to the high stresses the nods between flanges and diagonals are built of lamella packages made of S690.
High Strength Steel – Conclusion
This article highlights the recent developments made by the steel industry in order to supply high strength steel products for more efficient steel buildings.
However, other obstacles may exist in the single countries which may hinder the utilisation of these efficient high strength steel products, for instance ancient building regulations penalising or even forbidding the use of these products. However, countries with such an attitude can strongly benefit from experiences made in other countries, where these “new” materials are already introduced widely in the market. Thus, for instance in Norway, more than half of all steel bridges use high strength TM-rolled steel, whereas these steels are still used only in special constructions in Germany. This simple example shows that steel structure design in a particular country cannot only profit form new material offered – it can also profit from the experiences made with these materials in other countries. Let’s use it to built more efficient, safe and also beautiful steel structures.
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This article was written by Falko Schroter – General Marketing Manager at Dillinger Hutte Copyright Dillinger Hutte