Smart Steel & Steel Innovation

Plan of the DR plant at thyssenkrupp

Smart Home and Smart Grid - what would they be without Smart Steel?

The term Smart Steel describes steel products with improved or new properties. These properties are achieved by integrating sensors and actuators, using new methods such as nanotechnology or coating techniques.

In the context of green or environmentally friendly steel, smart steel refers to the development and production of sustainable steel products that are characterised by three main properties: firstly, they are low in CO2, secondly, they are durable and of high quality, and thirdly, they are easy to recycle.

Process stages for steel production in Germany and Europe (source: VDEh)

However, hydrogen-based direct reduction as a low- CO2 technology poses metallurgical challenges that make it more difficult to fulfil the criteria of "high quality, long service life" and "easy to recycle". One challenge with regard to the quality of the end product lies in the fact that direct reduction takes place at temperatures below the melting point of iron.

The end of the slag

In traditional blast furnace metallurgy, the slag serves as an important medium for removing impurities and unwanted elements from the pig iron. The slag binds these impurities, allowing them to be separated from the iron.

In the direct reduction process, however, iron ore is not melted but reduced to solid sponge iron in its solid state with the aid of reducing gas. There is no slag formation, which makes it more difficult to control the chemical composition of the iron produced. This can have a negative impact on the quality of the steel and lead to poorer weldability, mouldability and corrosion resistance. Secondly, the gangue components (non-metallic minerals in the iron ore) remain in the product. In the form of undesirable inclusions, these can impair the quality of the steel, reduce its toughness and make it susceptible to cracking.

"With direct reduction, we are looking at a completely new metallurgy. There is de facto no more slag metallurgy. This form of metallurgy must be carried out downstream with the help of an electric arc furnace or a melting unit," explains Prof Dr Ulrich Krupp, Head of the Institute of Ferrous Metallurgy at RWTH Aachen University.

As Dr Hans Bodo Lüngen and Prof Dr Peter Schmöle from the VDEh Steel Institute explain, "limited" slag metallurgy does take place in the downstream steel production stage in the electric arc furnace. However, the amount of slag is restricted by the process. For this reason, special pellets are used in direct reduction plants, which have significantly lower gangue components and therefore higher iron contents compared to blast furnace pellets. However, as such iron-rich ores are only available in limited quantities, a global transformation from the blast furnace-converter to the direct reduction-electric furnace route is not feasible.

New solutions must therefore be found for secondary metallurgy.

Submerged arc furnace (SAF)

Smart solutions for high-quality electrical steels

One approach being pursued by thyssenkrupp and Tata Steel, for example, is the use of an additional melting unit, a submerged arc furnace (SAF). By melting down the DRI into liquid pig iron with appropriate slag metallurgy, the known pig iron quality could be achieved. On this basis, the well-known oxygen blowing process of the converter route could then be carried out and the secondary metallurgy adapted accordingly, explains Krupp.

The existing oxygen steelworks with its metallurgical advantages could continue to operate and the steel qualities produced to date could also be produced in the future.

Testing the fatigue behaviour of steels in the "Gigaforte". Testing metallic materials at up to 1000 Hz makes it possible to make statements about the cyclic load capacity and therefore the service life in a very short time.

As the proportion of scrap in total steel production increases, the accumulation of by-elements also becomes more important. Copper in particular plays a role, as it is a noble element that cannot be transferred to the slag by oxygen treatment. If there is a localised accumulation of elemental copper, cracks occur at these points during forming. These lead to a serious deterioration in the mechanical properties of the material.

"The 'red problem' is exacerbated by the increasing proportion of copper in the material cycle, for example due to electromobility. More and more copper - including complete electric motors - ends up in scrap and later in electric arc furnaces, where it either has to be diluted with copper-free scrap from production or low-quality products are created," says a report by Krupp and his colleague Alexander Gramlich, a postdoctoral researcher at RWTH Aachen University.

Clever idea: copper-resistant steels

Another approach to compensate for the lack of slag metallurgy is therefore research into contamination-tolerant steels, for example with regard to the enrichment of copper in recycled steel, as well as methods to improve the fault tolerance and fatigue resistance of steel. Krupp reports interesting results. In a study, he, Alexander Gramlich and Hauke Springer present the effects of adding 0.5 % and 1.0 % copper to the reference alloy 42CrMo4 on its mechanical properties. The results show that copper contamination, which is unavoidable in steel recycling, has the potential to increase the hardness and strength of high-strength steels if it can be controlled during processing.

Benteler's Steel/Tube products also utilise the modified metallurgy of direct reduction. The company has succeeded in manufacturing free-cutting steel in an electric arc furnace and producing seamless tubes from it. A special feature of Benteler Smartcut® free-cutting steel is its particularly high sulphur content. This results in faster, more productive machining with better chip formation and longer tool life.

"With various sulphur contents from at least 0.10 % sulphur - around three to five times the usual 0.015 to 0.035 % for machining tubes - Benteler Smartcut® tubes have excellent machinability. This means that the tubes can be processed better and more cost-effectively. In addition, material savings of more than 50% compared to bar steel are not uncommon," reports Helwig Brabander, Head of Engineering Hydraulics / Precision Technology.

Smart influence: artificial intelligence and 3D printing in the green transformation

Ensuring and monitoring steel quality is increasingly being carried out with the support of AI. The so-called "digital shadow" comprises the passively generated data that is created through interaction with digital systems. These data sets can be used to monitor processes such as rolling, cooling and heat treatment, for example to determine whether the target and actual state of temperatures and forces differ. This results in an image of the expected material properties.

The data is linked using machine learning and physics-informed neural networks (PINNs), which learn from a combination of data and physical laws. The predictions made by the PINNs can then be sent back to the process to optimise its efficiency. In this way, AI helps to improve plant efficiency, which in turn leads to a reduction in CO2 emissions.

Above all, however, processes in secondary metallurgy can be adapted in real time to the values determined and ensure the best possible quality on a consistent basis.

In addition to AI, 3D printing plays a key role in the green transformation of metallurgy. It offers great potential for the environmentally friendly design of production processes. In combination with topology optimisation, it enables the production of complex steel components with minimal material waste and the manufacture of lighter and more efficient steel structures, which can lead to energy savings in various applications. This approach is relevant for specialised applications, such as in the automotive and aerospace sectors, where customised and high-performance components are required.

Deutsche Edelstahlwerke develops smart metal powders for 3D printing

Deutsche Edelstahlwerke is developing metal powders that are specially designed for additive manufacturing in 3D printing. These enable the production of steels with new property profiles.

One example is the material Printdur HSA (High Strength Austenite), a corrosion-resistant austenite with double strength that is produced without cracks, pores or defects. Another new development is Printdur HCT, which has high hardness, corrosion and tempering resistance as well as improved wear resistance and is free from nickel and cobalt, making it particularly advantageous in terms of sustainability.

A major advantage of 3D printing is the high degree of customisation, which makes it ideal for the production of special tools. For example, cooling channels can be designed exactly as required using the injection moulding process, which would not be possible using conventional drilling methods. In practice, Printdur HCT has been used, for example, to produce the feet of closure moulds, significantly increasing their service life.

Smart future: nanotechnology

The self-repair of a crack in steel is a modern concept in materials science that is based on the idea that certain materials are able to "heal" damage in their microstructure independently under specific conditions. These concepts of self-repair in steels are still largely at the research stage and not necessarily widespread in commercial applications. However, they have the potential to significantly improve the service life and reliability of steel products.

"Self-repairs" in steels could be realised through phase transformations, for example. In certain steels, a crack can cause a phase transformation to occur at the crack tip. This transformation leads to a change in volume in the material, which exerts pressure on the crack so that it closes or at least inhibits its growth.

In addition, certain metals or compounds could be embedded in steel alloys as "healing" agents that are activated during crack development. This could be achieved in the steel matrix by means of microcapsules that release corresponding substances when damaged. These agents could then diffuse into the crack and fill it or inhibit crack formation through localised reactions.

Another method is iron-based shape memory alloys (SMA), which return to their original shape even after severe deformation caused by heat, i.e. they "remember" their original state. The shape memory alloy memory®-steel was developed for the construction industry. This utilises the effect that a preload is created when deformation is prevented. For example, SMA rods set in concrete can be "activated" by heat. They "want" to contract into their original shape. However, as they are encased in concrete, this is not possible. This creates a preload. The systems are used to reinforce reinforced concrete and steel structures as well as in new buildings.

Nanotechnology and additive manufacturing will continue to change the steel industry. The processes offer potential breakthroughs in material properties and production processes. By manipulating materials at the atomic level, it is possible to produce steels with unprecedented strength, durability and flexibility. These advances could lead to lighter, more efficient designs and vehicles, which would have a significant impact on various industries.

Outlook

"Smart steel" is an expression of a new steel industry characterised by sustainability, technological innovation and an increased focus on the circular economy. The integration of material science, digital monitoring and intelligent manufacturing techniques opens up ways to increase efficiency, conserve resources and minimise environmental impact. While the challenges are significant, particularly in terms of quality assurance and adapting to new metallurgical processes, current developments offer an optimistic outlook for the future. Research into impurity-tolerant steels and the application of AI and 3D printing in production are just a few examples of how the steel industry is evolving to meet the demands of a more sustainable world.