New Applications of Improved TM Plates
The optimization of the process parameters based on systematic development work has allowed us to extend the production and application range of TM steels. Increasing plate thickness and width, improved homogeneity and flatness, lower carbon equivalents, higher steel cleanliness and well-balanced combinations of strength and toughness properties could be achieved through well-equipped production facilities and optimized processing conditions. The experience gained from recent TM plate production gives evidence for the important role of process design plus control on the one hand, and the use of the powerful four-high rolling stands plus sophisticated cooling devices of DH-GTS on the other. Investigations on the material behavior of plates and fabricated components have been performed to convince construction engineers of the superior property combination of new TM plates in comparison to conventional materials. As a result, challenging projects in construction of bridges, buildings, and offshore platforms are presented in which TM plates were used due to their improved performance, especially their excellent weldability.
Creation of Superior TM Steel at Dillinger Hütte GTS Plate Mill
Dillinger Hütte GTS operates two plate mills, one at Dillingen (Germany), the other at Dunkerque (France). Both are rather comparable, but the following description mainly addresses the Dillinger site, abbreviated DH. DH operates a plate mill with two powerful four-high stands using slabs from their own steel shop. Together with all the other equipment that has been installed during the past two decades in the plate mill, the application of a great variety of processing routes has been enabled. As an important supplier to several large-diameter pipe mills all over the world, DH and GTS entered into the production of TM materials around 20 years ago, in conjunction with the production of heavy plates for a wide range of applications made by various rolling and heat treatment variants. Today, for the application of plate material in sophisticated structures, it is not sufficient simply to comply with basic material requirements in terms of strength and toughness. There is a demand for materials that provide an attractive set of fabrication qualities that offer the potential to save time and fabrication costs. With the new generation of TM steels, Dillinger Hütte GTS offers a program of materials that is accurately tailored to fabrication needs. The problem with conventional high-strength steels is that the required strength properties can only be achieved through the production of a certain alloy content and subsequent heat treatment. This has detrimental effects on suitability for welding, especially in the case of thick wall structures. Therefore, TM steel materials with a lean chemistry have been created based on efficient exploitation of strengthening mechanisms through the manufacturing process. Manufacturing of TM steels requires a technology in which each individual step is coordinated with the others very precisely.
Detailed TM Process
From a metallurgical point of view, the TM-process is as follows:
- Designed to produce a defined microstructure with low effective grain size and a superior level of mechanical properties
- Adapted to the chemical steel composition (low carbon equivalent, microalloying, high cleanliness due to sophisticated steelmaking and casting technology)
- A time and temperature controlled process (described by a specific TM rolling and cooling schedule in the plate mill)
The TM rolling process includes a variety of realization variants, which differ from normal hot rolling (as shown in Figure 2). The desired properties of the TM-rolled plates are obtained without a heat treatment cycle through a special time and temperature sequence of:
- Slab reheating to defined temperature
- A defined number of rolling stages at prescribed temperature ranges, interrupted by cooling periods
- Cooling after finishing rolling, either on air or in a water cooling line
The metallurgical result of TM-rolling is a reduction of the austenitic grain size and the final ferritic grain size. A modified type of ferrite with a high dislocation substructure can be produced for further strengthening of the steel through finishing in the x+y region. A mixed microstructure of very fine ferrite and bainite is obtained through accelerated cooling. The result is an increased strength and toughness level and an improved weldability due to the lean chemical composition. Figure 3 shows the schematic layout of the plate mill at DH.
The principle steps to be mentioned are the reheating of slabs in pusher type or bogie hearth furnaces, multi-stage TM-rolling at the two-high stands (with 5.5 m and 4.8 m barrel length of the work rolls) according to a well-defined schedule of rolling passes and the use of accelerated cooling in 30-m-long MULPIC equipment. One important contribution to the resulting material properties is the precise setting of the chemical composition in the steel plant. The steel is characterized by a very low carbon content and use of microalloying, typically with C ~ 0.1 % and Nb ~ .02% respectively. All grades are of a high purity, exemplified by an extremely low phosphorus and sulfur content. This is achieved through a specific route of steelmaking, especially the ladle metallurgy steps. More than 30% of the total plate production of DH is accomplished through the thermomechanical control process (with plate thicknesses up to 120 mm). These TM plates are used for line pipes, pipes for bends and risers, shipbuilding, offshore platforms and other constructions (e.g., bridges).
TM Plate Requirements for Extended Application
As listed in Table I, the thickness range of today’s TM plates for extended application has increased significantly compared to the early years of TM plate use.
The requirement profile can become rather complex when taking into account both the physical properties of base material and the aspects of fabrication of structures. The fulfillment of such a requirement profile will be illustrated, presenting typical results. Concerning structural offshore plates, several principal development trends should be noted. One is the substitution of normalized grades 355 (YS-level) with TM products with minimized levels of carbon equivalent. This already covers a thickness range up to 120 mm. The other development targets are the higher yield strength grades, mainly type 460 (and for some cases also 420 or 500 yield level). In this case, TM is exploited to produce the desired yield strength level without an exaggerated addition of alloying to avoid costs and impairment of weldability. The decisive elements of the approach are well-defined usage of microalloying elements, reproducibility of a high level of steel cleanliness and sufficient reduction of segregation phenomena through tight control of the casting technique. These steps (combined and balanced throughout the steel-making process) are, together with the TM process, the prerequisites to guarantee the required toughness properties for use even in the North Sea cold water region, as well as high ductility for testing in through thickness direction.
Plates with extreme thickness and/or width. One prerequisite for the production of plates with extreme weight and size is the layout of the rolling equipment, as described above and elsewhere. The second requirement for such products is that the available material be put into the reheating and rolling process, either as a CC-slab or ingot. The advantages of continuous casting have been exploited for almost three decades to produce heavy plates with an optimized level of homogeneity, both in regard to thickness and length of product. A defined total reduction ratio (i.e., slab to plate thickness) is necessary in order to meet the property requirements of a plate. The required total reduction ratio depends on the level of properties that has to be met, the features of the starting material (slab) and the rolling process itself. To achieve satisfactory internal quality, it is necessary to apply a rolling process that ensures nominal local deformation in the core of the material. The essential parameter of the rolling pass describing its efficiency in penetration of deformation into the core is its shape factor . The shape factor depends on the reduction per pass and the work roll radius, and expresses the size of the compressive stress zone in the material between the rolls. The higher the shape factor, the higher the efficiency in reducing porosity. In order to improve properties in mid-thickness, the rolling has to be performed with a number of passes (as low as possible and as high as possible reduction per pass), resulting in high shape factor respectively. To perform rolling with high shape factor (abbreviated: HS-rolling), a powerful rolling stand is necessary. For example, at the 5.5 m – four-high stand of Dillinger Hütte, which enables rolling up to a maximum force of 108.000 kN and torque up to 4,500 kNm, a reduction per pass of up to 60 mm can be performed. The efficiency of HS-rolling can be increased through low speed rolling, which is only possible with suitable roll bearing systems. It is clear that by using HS-rolling instead of low shape factor roiling (LS), the reduction of area can be considerably improved for a given total reduction ratio, and for a defined steel type and cleanliness level. Another aspect of the application can be seen in the first rolling stage of TM schedules, where HS-rolling assures a complete recrystallization of austenite in the core of the material. This is an important contribution to grain refinement, and thereby to a high level of strength and roughness. ACC-treated TM plates. TM rolling of plates with subsequent ACC treatment has become a standard production route for high grades of line pipe steels, as well as for offshore, construction and ship building steels with low carbon equivalent and good weldability. Since 1993, more than 500,000 tons have been cooled with the MULPIC-ACC-equipment at DH. In respect to the customer’s plate requirements and other important aspects, the following MULPIC process variants can be applied:
- ACC treatment with predefined cooling rate or with ideal cooling rate aiming for a cooling of the core as quickly as possible, but with a cooling stop temperatures for surface just above martensite start temperature
- DQ with high cooling rates and cooling stop temperatures for surface and core below Ms
- QST with high cooling rate to cool down the plate temperature only in the surface region below the martensite start temperature, and a subsequent self-tempering by the heat coming from the core of the plate
These process variants are precalculated by an off-line cooling model and stored as cooling schedules. The MULPIC process control (Figure 4) guarantees a sufficient accuracy and reproducibility of the cooling process, good homogeneity of properties and satisfying flatness of the final product.
Temperature homogeneity and control of flatness. According to Table 2, there are a lot of process parameters that have to be controlled and well-balanced to assure the flatness of plates and parts (e.g., lamella cut out of them). Important features of the right approach include sufficient understanding of the involved mechanisms and the aimed use of the equipment. This should be illustrated for the case of ACC based on the process rules that are derived from modeling work.
Cooling must be sufficiently homogenous in the longitudinal and transverse directions to achieve a satisfactory standard of flatness and consistency of mechanical and technological properties. The heat-transfer coefficient, which depends on the temperature, exerts the strongest influence on the temperature distribution after cooling. Temperature differences in a plate prior to cooling are increased after cooling by using the same cooling conditions. Even if the plate is flat after hot leveling, a local internal stress state will be produced because, during the natural air-cooling to ambient temperature, the colder areas are unable to follow the thermal contractions. In extreme cases the internal stresses can reach the yield stress of the material and produce a plastic deformation. This phenomenon is very important in regard to the edges and extremities of the plates. It is essential to reduce the temperature differences in the cooled plates in order to produce plates without high internal stresses and deformation. Therefore, the following methods are applied:
- There are two features that affect the temperature profile in the transverse direction during cooling in the MULPIC equipment of the Dillinger plate mill (Figure 5):
a) With crown repartition the amount of water on the sides of the cooling banks can be varied between 0 and 100% of the nominal value of the cooling schedule. In Figure 8a the temperature profile in the transverse direction is shown as being dependent on the crown repartition factor. Without crown repartition (W = 100) the edges are too cold. An undesired inversion of the temperature conditions can be produced by reducing the water in the outer parts too much (W = 0). An efficient adjustment of the crown repartition (W = opt) produces a well-balanced temperature profile in the transverse direction over almost the whole width close to the edges of the plates, which suffer a definite drop in temperature. b) The side-masks on the upper banks protect the edges of the plates from direct impingement of the water jets (Figure 5b). Depending on the extent of masking, the temperature profile is strongly influenced in the local region of the edges. A combination of both methods – an adjusted crown repartition plus an adapted side-masking – guarantees an optimized temperature profile in the transverse direction.
- The temperature differences at the head and tail ends of the plate are reduced by end masking. This is performed by following the material flow and coupling the opening and closing of the high speed valves with the position of the plate on the roller table. So the head and the tail receive less water on a defined area than the center part of the plate. This is to compensate for the loss of temperature produced by the phenomenon known as cold ends.
The application of these three methods for optimizing the homogeneity of temperature distribution in a cooled plate is appropriate for two reasons:
- A flat cooled plate without high temperature differences remains flat after cooling to the ambient temperature. This prevents internal stresses and avoids difficulties during processing of the plate by the customer.
- The homogeneity of the mechanical properties also demands a high level of temperature homogeneity.
TM-rolling was introduced at Dillinger Hütte in order to manufacture plates for large welded line pipes. Linepipe is expected to have high strength in order to withstand internal operating pressures, to have excellent toughness in arctic temperatures and, at the same time, to avoid welding problems during laying. This classical TM-rolling is used for many grades up to approximately 30 mm thick. At the time of publication, Dillinger Hütte has supplied around 5 million tons of these line pipe plates.
TM-Steel for Constructional Steelwork
An example of an outstanding project, the Maeslant Kering (the massive swinging gate in front of Rotterdam Harbor) is an important construction project in the Netherlands’ coastal protection system (Figure 6).
13,000 tons of TM-rolled plate of grade 355 have been supplied by Dillinger Hütte for this storm surge barrier being built by the BMK Group. Figure 7 presents the mechanical test results from tensile and ChV tests for two thickness classes. These distributions illustrate the balanced design, emphasizing toughness aspects.
For some specific parts, TM plates of thickness 120 mm have been used, based on a modified chemistry but still with a rather low carbon equivalent. For other projects like buildings and bridges, TM plates of grades 355 and 460 have been used in different countries across Europe and Asia. As an example of a recent bridge project, Figure 8 presents a view of the Erasmus Bridge with its impressive pylon construction.
TM Steel Use in Offshore Structures
For the purpose of safe living and working conditions on drilling platforms, the engineering of these artificial islands is subjected to extreme requirements. Not only must the plates themselves meet the highest demands, but also the welds that are laid under aggravated conditions on the construction site. The plates produced with the ACC installation are now especially suitable for this because they combine high strength and toughness with excellent weldability. The Italian oil company AGIP was convinced of this when they placed an order with Dillinger Hütte for the supply of more than 20,000 tons of plates Gr.355 for their North Sea platform TIFFANY in 1990. These plates have been TM-rolled and accelerated cooled up to 90 mm. In addition to weldability and internal cleanliness, the plates were systematically tested for mechanical properties before they were dispatched to the fabricator. Certain projects are currently running grades 450 (Caister Murdoch, Captain Field, Britannia,etc.) or grade 420 (Ekofisk or Njord). As an illustration, the properties of the 420 grade used for Ekofisk are plotted in Figure 9. They are the result of a thickness dependent combination of alloying and process design.
TM Steel Use in Shipbuilding
Containers are the future means of transport for most types of goods, and container ships are the fastest, most economical and environmentally compatible means of long-range transport (e.g., between continents). Shipyards in Germany and Korea build such ships. In 1991, the first Dillinger Hütte customers received TM-rolled and accelerated cooled high-strength shipbuilding plates for the cranes and box girder parts of these ships. The supplied plates (up to 65 mm thick) were welded without preheating or high energy welding processes (like electro gas welding), bringing substantial cost savings. In the meantime, GL 36 TM plate or similar steel types ex Dillingen are distributed throughout a lot of countries in Europe and Asia, covering thicknesses up to I00 mm and widths of up to approximately 4 m.
Fabrication and Weldability
The success of TMCP steels is mainly due the to their improved weldability. The job of the fabricators becomes easier and more economic as less preheating to prevent hydrogen induced cold cracking is required, and improved HAZ toughness allows for increased welding efficiency.
A low hardenability of steel in the HAZ is an indicator of good weldability. Excessive hardness increases the risk of hydrogen induced cold cracking and shows the presence of microstructures with unfavorable toughness properties. A suitable way to characterize the HAZ hardenability of a steel grade is to perform bead on plate (BOP) tests. A detailed description of the procedure for such a test is given in the Appendix of BS 7191. From the test weld beads, polished surfaces are prepared and the micro hardness in the coarse grained zone is measured. An important parameter for austenite decomposition and hence hardness is the cooling time between 800 and 500°C (t8/5), which depends on the heat flow and welding parameters. The weld cooling time can either be measured with thermocouples harpooned into the molten weld metal, or it can be calculated using appropriate formulae (German Stahl Eisen Werkstoffblatt 088). In order to provide general information about hardenability, weld beads were produced using different heat inputs resulting in different cooling times.
An example of HAZ hardness as a function of cooling time is given in Figure 10 for a 70 mm thick 450 N/mm2 yield strength TMCP steel. It was observed that with normal welding conditions the HAZ hardness of the steel was always below 300 Vickers hardness units. Post-weld heat treatment (PWHT) carried out at 580°C and 620°C applied for two hours reduced the hardness for weld cooling times t8/5 below 15 s. For t8/5 exceeding 15 s., since the welded structure was mainly ferritic, no tempering effect was observed. An increase of HAZ hardness as it has been experienced on normalized 450 N/mm2 yield strength steels due to important vanadium precipitation can be excluded for the steel composition of TM plates.
Resistance Against Delayed Cold Cracking in Tbi RAZ
In the past, steels of elevated yield strength and heavy plate thickness had to be preheated and minimum interpass temperatures respected to prevent hydrogen induced cracks in the HAZ. For a 355 N/mm2 yield strength steel, German standards recommended the use of preheating for plate thickness higher than 30 mm, or for 450 N/mm2 steel higher than 12 mm.
TMCP steels, on the other hand, can be welded up to important plate thicknesses without preheating. Safe welding conditions were assessed in Y-groove and CTS tests and the results were confirmed in shop and on site production welds. How the plate thickness and weld metal hydrogen level raises the required preheat temperature is shown in Figures 12 a and b. As for the older steel types, increasing constraint and hydrogen input require more precautions for TMCP steels, but the critical thickness is shifted to much higher values. For example, tubular constructions of 355 TMCP plates with wall thicknesses of up to 120 mm were successfully welded on site without preheating in the Netherlands (during construction of the “Maeslant Kering” storm barrier). From these figures it can be derived that the highest profit can be obtained if consumables with very low hydrogen input are used. Maximum levels of hydrogen in the deposit weld metal (as indicated in Table 3) are recommended, as they are commercially available.
Omitting preheating means that a lot of time and energy can be saved. The extent of savings depends largely on labor costs, mean material thickness and the potential for onsite heating and energy supply. Thus, it has to be calculated for each specific project. For jackets of offshore platforms steel, where high wall thicknesses are used, the costs for preheating become very important and may reach the price of the steel plate.
Toughness of the HAZ
High toughness requirements for the HAZ have often been a problem for normalized steel grades, particularly if high heat inputs have been applied by the fabricator. TMCP plates offer the potential to extend the heat input without impairing the toughness in the HAZ. As opposed to steels of higher carbon content, low carbon steels are less sensitive to higher cooling times, as plotted in Figure 12 for Grade 450. Consequently, a wide range of welding parameters can be tolerated.
The reduced sensitivity to variations in heat input increases the safety of a structure, as poorly controlled welding (e.g., due to poor workmanship) will not lead to elevated levels of embrittlement. The fabricator can also use the savings to fund high efficiency welding processes. Even in the case of HAZ with extremely high heat input (some 20 kJ/mm) used in single pass vertical up electrogas welding or single pass panel welding, acceptable toughness was obtained down to -20°C.
Material Requirements of Future Applications
Based on the collection of established requirements (Table 1), it can be recognized that today’s ranges are further extended to
- Higher plate thickness (e.g., for line pipe at extreme depth and for constructional steelwork with higher loads)
- Easier and quicker fabrication (e.g., through increased welding heat input [high energy welding])
- More severe high flatness tolerances (restricted flame-straightening after welding)
The essential steps in plate production and application are a) Steelmaking and casting b) TM rolling and cooling c) Fabrication The actual development work is directed to assure inherent weldability through further modification of the alloy design. The intention is to avoid brittle microstructural constituents even after extreme welding cycles. The installation of a high pressure water system in the cooling equipment of OH should be mentioned as one possible contribution towards extension of the process route facilities.
The further development of TM steel plates is based on increased potential in several fields, the most essential of which are steelmaking, plate rolling and cooling process, and study of fabrication behavior. To fill customer demands for safer and more cost-effective fabrication of structures and components from steel plates, plate suppliers have to assure a wide range of plate sizes, a well-balanced combination of strength and toughness, alloy approaches with excellent weldability, sufficient flatness of the plates and resistance against deflection during fabrication, a high level of reproducibility and homogeneity. This complex profile can only be achieved through the use of well-equipped production facilities and research and modeling work. The approaches that have been taken by Dillinger Hütte GTS are illustrated through the explanation of fundamental process design aspects, as well as by presenting experiences from various projects.
This article was written by A. Streisselberger, W. Schütz, J. Bauer, R. Hubo, F. E Hanus and was originally published by Dillinger Hütte.