Some aspects of the development of the production program of Dillinger Hüttenwerke (DH) over the past 10 years are illustrated. The two main groups have been TM line pipe plates and customer plates for a wide range of sizes and applications.
The driving forces for entering into new markets—especially those with extreme specifications—have been on the one hand a systematic extension of equipment and facilities, and on the other hand the gaining of know-how in the fields of process control and metallurgical product development.
The presentation addresses the possibilities resulting from the installation of further reheating facilities, the powerful 5.5m-4-high stand with its process control devices and the MULPIC-accelerated cooling equipment, and the exploitation of this combined technology based on specific strategies to fulfill unique combinations of specification requirements or to assure a safe fabrication route or application service on the customer side.
The benefits of such progress can be seen in the more sophisticated products that are contributing to more satisfying steel structures.
As an example of such tailor-made plates, longitudinally tapered plates should be mentioned. It has to be emphasized that quality assurance systems and feedback from the fabricator or user side to the plate producer are key elements for the well-controlled development of process and product.
Dillinger Hüttenwerke (DH) is operating a plate mill with two powerful 4-high stands using slabs and ingots from their own steel shop. Together with all the other equipment that has been installed over 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 entered into the production of TMCP materials more than 15 years ago, in conjunction with the production of heavy plates for a wide range of applications made through numerous rolling and heat treatment variants.
Figure 1 gives an illustration of the development of production tonnage during this recent history. Whereas the importance of TM-rolling (especially together with ACC) has been highlighted in other papers, this presentation will focus on those processing variants that allow for special combinations of customers’ product requirements both in terms of geometry and physical properties.
These product groups are listed in Table 1:
Figure 2 gives an illustration of the various process stages that contribute more or less to the geometrical and physical product properties.
2. PLATE MILL LAYOUT AND EQUIPMENT
The layout of the plate mill (Figure 3) is determined by the following design features:
- Sufficient and flexible reheating facilities: three pusher-type furnaces (i.e. up to seven slab rows can be charged in parallel), three bogie hearth furnaces (esp. for ingots or small size slabs or non-ferrous materials)
- High-pressure descaling unit (180 bar) in front of the stands
- Two powerful 4-high reversing stands (Figure 4):
- First: 5.5 m barrel length, mainly for pre-rolling, including broadside rolling
- Second: 4.8 m barrel length, for finish rolling
- Both: Automatic gauge control for thickness control and roll bending devices for flatness control
- Process control: PLATE model for pass schedule calculation
- Space between stands: 105 m
- MULPIC-ACC-equipment: 30-m-long water pillow system for accelerated cooling or direct quenching (more details on the MULPIC process control are given in chapter 3.4)
- 5.2 m hot leveler
- Heat treatment facilities for normalizing, quenching and tempering
- Cooling beds and inspection bed (for 50 m rolled length)
- In-line automatic US-testing (100% body testing)
- Two shearing lines (up to 52 mm thick plates)
- Torch-cutting (for heavy thickness range)
- Cold levelers
- Shot-blasting (+ painting)
The principal material flow during plate production occurs along one hall of 1.2 km length.
The heart of the plate mill is the rolling part with its two 4-high reversing stands. Their general performance has been described elsewhere.
Attention should be drawn to those aspects having relevancy for the topic of this paper. In particular, during the design of the 5.5 m roughing stand the foundation for the future production and delivery program was laid. This high barrel length allows the use of longer and heavier concast slabs, especially during broadside rolling. Thus, the program was not only extended to heavier, thicker and wider plates, but also an increase in the mean value of slab weight has been achieved, which means a higher production efficiency.
When the 5.5 m stand is used as a finishing stand, plates of up to approximately 5.2 m width can be delivered. At large diameter pipe mills, the production of 64-inch pipes has been enabled with plates from Dillingen. For general structures or large vessels, such wide plates allow for spare welding work.
While specifying the technical features of the stand, a lot of very different application cases had to be taken into account. The most important of them are illustrated in Figure 5 in terms of rolling force and torque situation. This covers normal hot rolling of plain carbon manganese steel plates, which get their final properties through additional heat treatment. Thickness reduction per pass is in the range of 30 to 40 mm due to relatively high rolling temperatures. In extreme cases (e.g., low total reduction from slab to plate), the single reductions are increased up to 60 mm.
The purpose of such rolling a technique will be explained later in this paper (chapter 3.2). In the case of TM-rolling, the finishing passes are applied in the range of Ar3, or even lower temperatures (i.e., without softening in the interpass times by recrystallization due to microalloying of high strength steels). As a consequence, even with reductions as low as 10 mm, the rolling loads are considerably greater than those during normal rolling.
The torques become high even with moderate forces when they are combined with high thickness reduction. That is why the 4.8 m stand and its motor were prepared for maximum torques of 3200 kNm per roll, and the 5.5 m stand as high as 4500 kNm per roll, based on a synchronous type of drive motor.
Further user-orientated application examples of the equipment are presented in the subsequent chapters. As shown in Table 1, each special product group has been based on a process design exploiting the possibilities of the installed equipment.
3. SPECIAL PROCESS AND PRODUCT VARIANTS
3.1 Plates with extreme size
One prerequisite for the production of plates with extreme weight and size is the design of the rolling equipment as described above. The second requirement for such products is the available material to be put into the reheating and rolling process.
Table 2 presents a list of slab and ingot sizes coming from the steelmaking plant at Dillingen, which is located at a distance of less than 1 km from the plate mill. The gap between the two is used for slab finishing and storage (i.e., slab yard). These circumstances are also favorable for warm or hot charging of CC material. As each plate is rolled to ordered dimensions (which can vary widely), a large variety of slab and ingot sizes is necessary to assure in all processing stages a safe and reproducible handling.
Based to the ordered delivery condition (e.g., as rolled, normalized, quenched and tempered, and TM-processed), a range of available plate sizes can be defined and collected in the delivery program (Figure 6).
As a result, the utmost limits of the production program cover thicknesses ranging from 5 to 420 mm, widths ranging from narrow lamella to 5.2 m, and lengths ranging from rolled up to 48 m, and delivered up to 36 m and 28 m for heat-treated plates. This is all available in a large variety of steel compositions and grades that are not dealt with in detail here.
It should be mentioned that extreme plate sizes and weights require appropriate transport facilities, both in the mill (which include cranes, roller tables, trucks and railway systems) and also for the transport to the customer (which includes special trucks or wagons for inclined transport of large-width plates, and ship transport from Dillingen’s harbor).
As an additional service to the customer, plates can be delivered shot-blasted and painted, and even with edge preparation or pre-forming by the DH fabrication shop.
3.2 Plates with extreme thicknesses
For almost three decades the advantages of continuous casting have been exploited to produce heavy plates with an optimized level of homogeneity, both over the thickness and length of the product. By using CC-slabs and bottlenecks in steel and plate production, complications with ingots can be avoided. In this way, the yield of the whole process has been brought to a considerably higher level.
The use of CC-slabs is restricted to a certain maximum plate thickness. A defined total reduction ratio (i.e., slab to plate thickness) is necessary in order to meet the property requirements on 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.
A sophisticated and “safe” production route is required for production of thick plates (e.g., 125 mm starting from CC-slabs with thickness 300 mm), which corresponds to a total reduction of 2.4 times. “Safe” means plates with satisfactory internal quality both in terms of soundness (no harmful porosity) and properties (even in the core)—especially toughness and through-thickness properties.
To achieve this goal, it is necessary to apply a rolling process that extends to the core of the material a local deformation comparable to the nominal deformation. The essential parameter of the rolling pass describing its efficiency in penetration of deformation into the core is its shape factor m .
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 as low a number of passes possible respectively, and as high a reduction per pass as possible (i.e., high shape factor).
One prerequisite to perform rolling with high shape factor (abbreviated: HS-rolling) is a powerful rolling stand. For example, on the 5.5m-4-high stand of Dillinger Hütte, which enables rolling up to a maximum force of 108000 kN and torques up to 4500 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.
Figure 7 demonstrates the effect of HS-rolling on the reduction of area values in through-thickness direction. It is clear that by using HS-rolling instead of normal rolling (i.e., low shape factor rolling [LS]), the reduction of area can be considerably improved for a given total reduction ratio and for a defined steel type and cleanliness level.
As the HS-rolling is applied to use CC-slabs on place of ingots for a certain thickness range, it also allows for the extension of the possible plate thickness range for a given ingot thickness (e.g., 800 mm), as well as higher defined plate requirement values.
Another application example 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 the grain refinement, and thereby to a high level of strength and toughness.
3.3 Longitudinally tapered plates
One performance feature of a plate mill has to be seen in the accuracy and homogeneity of thickness over the entire length of each plate. To avoid thickness deviations along the plate, sophisticated control mechanisms and their technical realization have been implemented. They are widely known as AGC (automatic gauge control) systems with various modes of application (relative, absolute, feed forward, etc.). Based on this technique of control of defined constant thickness over plate length, another exploitation of the system was created (by using it for the production of variable plate thickness over length).
From the construction point of view, a lot of structural components (e.g., beams, box girders or similar parts, such as in bridges) have to bear load distributions that are changing over length.
The answer of the plate producer is LP-plates (i.e., longitudinally tapered plates). These allow for the reduction of costs by decreasing the weight of the entire construction or the number and extent of weld seams and machining of transition parts. These advantages, compared to the conventional construction case, are illustrated in Figure 8.
The maximum values for thickness change and gradient have recently been increased to 55 mm and 8 mm/m respectively.
From the process point of view, Figure 9 explains the principle of the method. The desired profile is fed into the process computer. The rolling is performed in a reversing mode by increasing the defined deviation from parallel faces pass by pass. The thickness deviation for each longitudinal position is applied via the AGC-positioning system (i.e., a hydraulic oil/grease system), the data of which are described in Table 3.
Figure 10 shows a variety of different profile forms that can be selected according to the load distribution of the structural component. Subsequent to the final rolling pass, the required flatness is assured by warm leveling. In this process stage, the upper rolls have to be able to follow the right profile.
The benefits of LP-plates have been recognized by design engineers. Therefore, several structures can be shown as successful application examples of this by-product idea of process development.
3.4 Heat-treated plates involving ACC-treatment
TM-rolling of plates with subsequent ACC-treatment—the TMCP-process—has become a standard production route for high grades of line-pipe steels, as well as for offshore, construction and ship blinding steels with low carbon equivalent and high weldability. For example, in 1993 more than 300,000 tons were cooled at the MULPIC-ACC equipment at DH, a large portion of them as line-pipe steels for sour gas service).
In 1988, a side-charging furnace called ELO (for “Einlegeofen”) was installed between the roughing and finishing stand in a by-pass (Figure 3). By its hearth area of 6 x 19 m2 and a theoretical capacity of 15 t/h, the ELO allows for the charging of plates with extreme size, thereby extending the range and flexibility of heat treatment of plates.
Further, its location not too far from the MULP1C-ACC equipment, which is situated behind the finishing stand, opens a new production route (ELO+MULPIC), which allows for the application of advanced process routes, or to develop alternative methods for heat treatment of steels including subsequent water cooling based on defined cooling schedules.
The most commonly performed applications for process route are:
- Austenizing plus water cooling with hot-leveling either before or subsequent to the cooling
- Reheating at defined temperature plus rolling plus cooling plus an additional heat treatment (e.g., tempering) in ELO
In respect to the customers’ plate requirements and other important aspects shown in Figure 11, 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 temperature for surface just above martensite start temperature.
- NQ, or in special cases DQ, with high cooling rates and cooling stop temperatures for surface and core below Ms 8.
- OST with high cooling rate for a short time to bring the plate temperature down only in the surface region, below the martensite start temperature and subsequently with switched-off cooling for sett-tempering of this microstructure by the heat coming from the core of the plate.
The courses of temperature for these process variants are pre-calculated by an off-line cooling model and stored as cooling schedules. The MULPIC process control guarantees a sufficient accuracy and reproducibility of the cooling process, good homogeneity of properties and satisfying flatness of the final product.
The following examples for application demonstrate the range of flexibility for each production route:
NOMQ: Using the ELO-MULPIC-process route, NO-process can be performed for plate thicknesses above 40 mm, width up to 4500 mm, plate length up to 18 m and a maximum specific weight per meter of 4 tons.
QST: An illustrative example is the application of the QST-process for thick plates with defined hardness profile from the surface to the core combined with severe flatness requirements. The dimensions of the treated plates are thickness: 170 mm, width: 3000 mm, length: 10 m. On the one hand, hardness on surface has to be adjusted above 245 HV10 because of high wear resistance requirements. On the other hand, a steep hardness gradient from surface to core is desired to produce a good machinability for the core.
Flatness deviation has to be adjusted to equal or less than 8 mm per total plate length (of 10 m). As a result of QST, a defined martensitici/ferritic perlitic microstructure distribution and a hardness profile over the plate thickness with a difference of 100 HV10 has been adjusted. Flatness is for 95% of plates within the required range, without additional flattening.
3.5 Normalizing rolled plates
In order to avoid normalizing treatment in the furnace, as well as to reduce production time, normalizing rolling can be applied. Normalizing rolling has been defined in standards such as SEW 082 or BS 4360. Following these and other definitions, N-rolling means a rolling procedure with the final deformation at well-defined temperatures above the Ar3 temperature (i.e., in the -seven, typically in the normalizing temperature range). As a result, the material condition is generally equivalent to that obtained by normalizing. Thus, normalizing rolled plates can be normalized (e.g., due to hot forming process) without risk of failing the specified mechanical properties of the steel grade.
Concerning the equivalency from N-rolling and N-treatment in the furnace, more detailed definitions have been given for supervised steel applications, such as for pressure vessels by the VdTÜV material sheets. To assure both the material properties in N-rolled and N-treated conditions are within the specified limits and their differences are within the range of equivalency, it is necessary to apply a sophisticated process design approach.
Figure 12 illustrates the interference of the effects of chemical composition and rolling process parameters to gain properties at target value c within a tolerance of ± d. Taking into account both the slab composition and the actual rolling parameters during the on-line process with the support of a metallurgical modelization, the properties can be predicted and compared to the specification. In terms of risk for non-conformity, a test of the individual plate is initiated before release.
The performance of the model has been checked and confirmed on a statistical base.
Figure 13 gives evidence and confidence for prediction quality and assurance of the equivalency criteria.
In addition to these aspects of property control, other advantages of N-rolling should be mentioned. As there is no need for a furnace treatment, plate sizes (as they can be rolled and transported) have been made available in condition N via rolling at the stand with barrel length 5.5 m, passing restrictions in furnace geometry.
Compared to plates normalized in the furnace, normalizing rolled plates have a better surface condition because there is no additional oxidation of the surface due to reheating after rolling.
3.6 Combination possibilities of the presented processes
After the presentation of the specific process routes having been developed to deliver products with a combination of properties superior to classical hot-rolled heat-treated plates, there remains the question of further development steps. Without expanding too much on the future potential, possible combinations of the above-presented process routes and ideas should be sought.
Such combination examples are:
a) Longitudinal profile + N-rolling
b) Special ACC route after austenizing in furnace (e.g., to reproduce TMCP properties)
c) HS-rolling interpass ACC (to cool the surface region, allowing for stronger penetration of the deformation into the core)
d) HS-rolling for clad products
Several of these combination possibilities are currently being exploited during production of a significant quantity (e.g., by HS-rolling applied to CC-slabs for extreme final plate thickness [which would require ingots if HS were not possible], N-rolling applied to plates with extreme width or length [which cannot pass through a furnace with restricted dimensions] or rolling and processing of non-ferrous materials [using the flexibility in working range of the equipment and of the on-line process control]).
The presentation has shown how DH has implemented various equipment and facilities to allow for the production of tailor-made plates to fill the needs of different users. It has to be mentioned that some of the process routes have only been developed after systematic comparison of real needs on the user side and possibilities on the producer side. As a consequence, we expect that both sides together will introduce or derive further production processes that are mutually beneficial.
This article originally appeared at Dillinger Hütte and is reproduced with permission.