Gregory Navarre details how steel bar with special properties boosts productivity
Designers and manufacturers can achieve improved functionality and greater efficiency by ordering steel bars with special properties in dimensional tolerances, mechanical performance and surface finish.
Producing steel bars by rolling can cause variations in these properties as well as the response to heat treatment. Precision control in the mill makes it possible to produce bars with defined and stable properties, which Ovako calls SP-Bar. For OEMs, this means less wastage, less downtime and fewer interruptions to production. It can also improve mechanical performance during a product’s lifetime.
In the case of tighter tolerances, consistency provides the advantages of improved yield and material savings, stable performance in forming operations and less waste. Furthermore, consistent mechanical properties such as toughness and hardenability will increase fatigue life and impact strength. Finally, consistently good surface quality means less scale and less need for finishing. This results in process savings from having cleaner quenching tanks and an improved production environment.
Precision Rolling Mill
The key to SP-Bar is the fine-section rolling mill at Ovako’s mill in Sweden, which carries out finishing rolling on steel bar at closely controlled temperatures, while it is in the austenitic phase. This ensures the bar will keep its dimensions. However, it also enables finer grain sizes and therefore tougher steel without the build-up of residual stress experienced during cold-rolling. Finally, it also reduces decarburisation, meaning less scale.
The fully automated facility has dedicated rolling stands for roughing, as well as shaping and finishing that provide multiple routes to deliver bar of 14-52mm diameter. Inter-stand water cooling creates consistent end-to-end temperatures for each bar and billet in a process that precisely controls the temperature of the steel without quenching it. Cutting shears and in-line measurement gauges provide dimensional control.
The SP-Bar technique was first developed in 2003 for a customer that wanted bar with a very tight tolerance for roundness. This was not possible at the time as higher rolling temperatures made the bar too soft to maintain perfect roundness. This was overcome by installing cooling stations that use water tubes for better control of rolling temperatures. Since then, the process has been further refined to reduce decarburisation and surface scale. This reduces waste during cold forming and provides a smoother surface finish. The SP-Bar process can also create bar with a small grain size, giving it good toughness and a consistent response to heat treatment, which helps to achieve the required hardness.
The process produces similar results to Kocks rolling, which applies rolling pressure from three axes to control tolerances and roundness. However, order sizes for Kocks blocks rolling are typically at least 200 tonnes. In comparison, SP-Bar can be produced in batches as small as 12 tonnes. This enables customers to try it out without committing to a large production run.
High And Low Alloyed Grades
The technique can be used on low-alloyed steel to improve efficiency for high-volume production, such as automotive components. It can also enhance the mechanical performance of high-alloyed grades, giving superior impact strength, toughness and hardenability. This makes the bar suitable for high-performance, safety-critical products such as fasteners
for wind turbines.
Around one third of SP-Bar is based on microalloyed grades. In this case, the advantage of carrying out finishing rolling in the austenitic phase is that it gives better control over the precipitation of alloying elements such as vanadium, titanium, niobium, nitrides and carbides. This makes it possible to produce steels with a very fine grain size – down to 12, which is the finest possible as measured by the ASTM specification – and excellent mechanical properties.
BSA Groupe is a French manufacturer that processes steel products and other alloys for manufacturers in the automotive, pharmaceutical, construction, aerospace, nuclear and defence industries. It has boosted its efficiency by switching to SP-Bar as a raw material for peeled bar.
Peeling removes imperfections and oxides from the surface of the steel bar. BSA has three peeling lines at its workshop, where it processes around 18,000 tonnes of bar per year in diameters from 8 to 100mm. This provides a flawless surface so that machine builders and OEMs can build high precision into their equipment.
BSA started using SP-Bar in 2019 because of its unusually tight dimensional tolerances and near-perfect roundness. Because there is minimal variation in the bar’s diameter the company has been able to reduce waste and improve its production yield.
Other customers in the automotive industry are also finding value from the special properties. One manufacturer is making automotive steering racks based on SP-Bar made from a microalloyed steel to achieve repeatable mechanical properties, with a secondary benefit being good yield.
SP-Bar provides a bespoke balance between the three characteristics of dimensional tolerances, surface finish and mechanical properties. That means its benefits are specific to individual customers and applications and can vary considerably according to their priorities. Many customers have found that it is an opportunity worth exploring.
Gregory Navarre is with Ovako