Forged-Rolled Steel Bar Offers Best of Both Worlds
Ovako AB is completing a multimillion-dollar investment at two mills (Hofors and Hällefors) in Sweden to produce forged-rolled black bar. The company’s production and R&D teams have implemented a new forged-rolled process. In a multi-pass forging step they consolidate the porosity out of cast ingots down to an intermediate dimension. The resulting billet is then hot-rolled down to the final required dimension. This produces black bar with an accurate shape and good surface finish, thereby reducing the need for final machining by the customer.
In recent years, Sweden’s Ovako AB has seen a substantial increase in customer demand for steel black bar in diameters between 170 mm (6.7 inches) and 230 mm (9.1 inches). It is used mainly in the bearing and mining industries for critical components. Black bar is formed by a hot-rolling process at a temperature of around 1200°C (2192°F). This is above the steel’s recrystallization temperature (typically 400-700°C), which makes it malleable and easy to form.
One challenge Ovako faces is that the porosity resulting from the casting process makes it difficult to achieve the internal soundness required for a high-quality rolled product at diameters greater than 160 mm (6.3 inches). In simple terms, rolling to a smaller final dimension produces a denser material, free of the porosities that can have a negative impact on the performance of the final product.
Hot forging is, of course, an ideal approach to providing a consolidated and much sounder internal structure. The downside of forging is that it is relatively expensive to produce bar in dimensions up to 230 mm, especially as time-consuming additional forging and reheating operations can be necessary. In addition, a forged bar is generally inferior to a rolled bar in terms of surface quality and dimensional tolerances.
Investment to Create a New Product Range
To meet demand for black bar at higher diameters up to 230 mm (9.1 inches), Ovako’s production and R&D teams established a new forged-rolled production flow. The process starts with the multi-pass forging of cast steel ingots down to an intermediate dimension. This consolidates the porosity. The resulting billet is then hot-rolled down to the final required dimension. This produces black bar with an accurate shape and good surface finish, thereby reducing the need for final machining by the customer.
A particular advantage of using the company’s Hofors and Hällefors plants is that they both already had forging and rolling processes in operation. The main focus of Ovako’s investment has been in the handling equipment to bring them together. New equipment at Hofors includes a manipulator to lift the ingots from the forging press to the pit-furnace hall and a modified ingot trolley and overhead crane. The main investments at Hällefors are a new saw and upgrading the ultrasonic testing equipment to suit heavy bar.