Rolling Mill
The Rail Rolling Process
The blooms are stored in an intermediate storage facility and introduced into a walking beam furnace (WBF) at ambient temperature, where they are heated to rolling temperature (1250°C). Depending on the production cycle of the steel mill, it is also possible to use the blooms directly in the WBF by hot-charging with an entrance temperature of approx. 200°C. This saves energy and is less taxing on the environment.
The blooms are discharged from the furnace with a very constant temperature across the crossection. The bloom is pre-rolled in a breakdown-mill to a leading pass section depending on the rail section. The remaining conversion to rails happens on the finishing line through a reduction sequence of several steps. Three identical rolling stands and power supplies are aligned in the finishing train. Depending on the section, a combination of universal- and duo stands are used. High-pressure water descaling plants guarantee a high surface quality of the rail.
The pass lubrication used in the previous rolling mill is no longer required, which is beneficial for the environment. Together with automated hydraulic gauge control we achieve such a high dimension precision, so that we can guarantee to achieve half-width tolerances. The roll change is also highly automated, allowing to change section within 20 minutes. This is the reason why we call our mill the Ultra-Flexible Rail Mill (UFR).
There is an automated hot stamping machine prior to the cooling bed roller table after a length of approx. 10 meters. This embosses the mill mark, i.e. the name of the rail, the bar number and the heat number into the web of each rail, at defined intervals.
A facility for the heat treatment of the rails is located between the rolling table and the walking beam cooling bed. The process of in-line hardening of the rail head was developed in Donawitz and is now known throughout the world as HSH®(Head Special Hardened) process.
Besides flat bottom rails, switch rails or grooved rails can also be hardened. The linear ruler that straightens and aligns the rail on the roller bed also transports rails from the HSH® plant. The rails are placed on a walking beam cooling bed that is 125 m long and 18 m wide. The rails cool there on air for approx. 3 hours to a temperature less than 60°C.
At the end of the walking beam cooling bed the rails are transported to a roller table and from there to the roller straightener. This consists of two parts, an eight spindle horizontal roller straightener followed by a seven spindle vertical roller straightener. Using a manipulator, the cooled rail is introduced vertically at its rolled length into the roller straightener and straightened over its entire length in one step.
Our straightening geometry chosen - i.e. the diameter of the rolls and the distance between the leveling rolls - was determined using the experience of decades and assures that the leveled rails are straight, even and that they have low residual stresses.
After the roller straightener the finishing area begins, where the rail is prepared for delivery.
The HSH® Process
The special process developed by voestalpine Schienen GmbH in Donawitz is fully integrated into the production line of rails. The entire rolled length of the rail is heat treated during this hardening process. An annual production capacity of more than 250,000 metric tons is available. The globally patented process therefore has a capacity unmatched by any other rail producer.
Within the hardening facility, the rail is held head down by manipulators. When a certain temperature is reached, the rail is lowered into the quenching bath in such a way, that only the rail head is suspended into the hardening solution. It remains there for a specific, pre-selected time in the same position, while a hard fine-pearlitic structure forms over the entire crossection of the rail head. Afterwards, the rail is lifted from the solution and positioned on a walking beam cooling bed, where it is processed in the same way as non-hardened rails that are produced in the traditional way.
The characteristics of HSH® rails are a result of the special head hardening technology.
The hardness is constant throughout almost the entire rail head, due to the homogeneous fine-pearlitic microstructure. This hardness distribution offers clear advantages, since contact with a rail material of high hardness is ensured during the whole life-span of the rail, even under excessive wear conditions. It is essential that the best possible resistance to cold deformation and abrasive wear be combined with sufficient toughness in the web and the base.
This heat treatment is also applied to tongue rails and grooved rails.
Head hardened tongue rails make it possible for switch manufacturers to offer a continuous, wear resistant track using highly wear resistant rail steels.
Head hardened grooved rails are a special product that makes savings over the useful life-span in excess of 15% possible for light rail public transport companies.
voestalpine Schienen GmbH has a correspondingly extensive list of references to show that our HSH® rails are used throughout the world. A direct comparison with the products of our competitors, for instance in Pueblo, USA, has proven HSH®-rails to be among the best of the world.
The combination of the characteristics "ultra-hard", "ultra-clean", "ultra-flat" and "ultra-long" results in a globally unique high performance product that enables the customer to be "One Step Ahead".
