Smart Steel: Steel with new sustainable DNA

In addition to being CO2-free, metallurgical research is now increasingly concerned with meeting the requirements of the circular economy while at the same time not having to compromise on quality. The focus of research is now increasingly on metallic eternity properties such as endless recyclability and infinite service life. The first and at the same time most important finding of the research efforts: Decarbonization and the primacy of circularity will permanently change the DNA of steel.

The green transformation and the accompanying decarbonization of the steel industry herald nothing less than a technological paradigm shift: Traditional coke-coal smelting is being replaced by hydrogen-based direct reduction using renewable electricity. In the wake of this industrial-historical caesura, the way is now also being paved for the Circular Economy. In addition to direct-reduced iron, the electric arc furnaces primarily use scrap as a raw material. To ensure the high steel qualities to which we have been accustomed up to now, smart steels with completely new sustainable and circular properties are now required.

"Steel is a raw material with infinite recyclability," emphasizes Prof. Ulrich Krupp, head of the Institute of Ferrous Metallurgy at RWTH Aachen University and one of the masterminds behind the new circular thinking. Steel is a sustainable product per se, but it is only really smart if every point in the process chain is designed to be sustainable.

A brief look at the design scenario in this respect: currently, around 127 million tons of steel, of the roughly 1.9 billion tons produced annually worldwide (2022), are already being manufactured on the direct reduction route. While this is a pleasingly green development, it also brings with it completely new challenges: direct-reduced steel must meet the proven quality standards of blast furnace steel.

The main quality problem here is slag metallurgy. In the absence of this, phosphorus and nitrogen contents must be kept low in the electric arc furnace by other means. One possibility would be to achieve the desired pig iron quality by adding carbon. However, decarbonization means no carbon. Improving the properties in the second step, for example with heat treatment and mechanical forming processes, therefore seems to be much more effective.

Many steels are subjected to multiple heat treatments, e.g., to draw them apart again and roll them. At RWTH Aachen, for example, research is currently being carried out on air-hardening ductile steels (Lufthärtend duktile Stähle, LHD-steels). This means cooling the steel directly from the forging heat. The air cooling causes martensite to form and small carbides to precipitate. The end product is a high-strength steel with significantly lower emission values. There is always a great demand for high-strength steels; the automotive industry needs them, for example, to make passenger compartments and, more recently, battery surrounds for electric cars as crash-resistant as possible.

Many auto manufacturers have therefore in the meantime concluded agreements with their steel producers for the return of their scrap. Steel scrap is increasingly being melted down in the plants, both by the conventional route and in the electric arc furnace. The CO2 footprint per ton of steel can thus be reduced significantly, in some cases by 80-90 percent.

Good and bad scrap

The main problem with scrap is its separation, because not all scrap is optimally recyclable due to its composition. But here, too, sustainable improvement is already in sight: New systems equipped with artificial intelligence and high-performance sensor technology are helping with sorting, for example with regard to alloys.

Alloys of the most diverse kinds, which come together in scrap, are another hurdle in this context on the way to 100 percent circularity of steel. Substances such as copper and tin are difficult or impossible to remove from the molten metal.

When recycling a car, for example, the lack of sortability means that small engine elements as well as copper cables and hybrid material compounds come together in one melting pot. The problem is that copper makes steel less ductile. Its share in the melt must not exceed 1%. If steel scrap containing copper impurities is repeatedly melted, the copper content will continue to increase in the long term. Although steel is in principle infinitely recyclable, it would then have to be diluted either in the melt with low-contaminant scrap or even primary iron - which would inevitably reduce the sustainability factor again.

In addition, if tin from tinplate is also melted down, it is unknown what interaction this has with the copper on the steel. "Here we are currently still on terra incognita," says Prof. Ulrich Krupp, illustrating the current state of research.

Krupp therefore suggests: "In the future, we should think more leanly here, i.e. leave it as far as possible with iron, carbon, manganese and chromium and then try to achieve the necessary differentiation in the qualities via thermomechanical treatment strategies."

This would significantly simplify the circularity of steel and considerably reduce the need for costly sorting.

Long live steel

In terms of service life, research is also currently looking at new steel compositions and, as already mentioned, new processes in the hardening shop, which should, for example, bring greater corrosion resistance.

In additive manufacturing in particular, research into the optimum formulation of metal powders is currently running hot in order to make steel harder and more durable and to be able to process it permanently. With the help of welding robots, tired parts can already be renewed virtually endlessly.

In addition, intensive research is being conducted into the damage tolerance of steel. The microscopic cracks that develop after a certain time could be contained by the material itself. The phase transformation at the crack tip would locally lead to an increase in the specific volume. As a result, the crack is compressed and does not grow further.

Another method of giving steel new, and ultimately even everlasting, life is cold hardening. This is because cold forming increases strength. At fatigued points, e.g. a microstructural defect in an inclusion, cold forming can be used to make the steel stronger at this point. It can then be used again for further stresses.

Research is also being carried out at RWTH on the idea of a "steel memory". Using resonance and ultrasonic pulsators to test steel samples at frequencies of up to 20,000 Hz, the Aachen scientists are simulating a stress similar to that of 20 years in just one day. The 20,000 Hz here roughly correspond to the vibration of a wind turbine. The procedure shows how damage occurs, whether self-repair works, and thus provides information about the memory capacity of the steel in question.

By steel memory is meant that the phase transformation, which could be detected by an electrical or micromagnetic signal, reflects the damage state, e.g. that the steel would have reached 80% of its service life.

Before reaching the end of the product life cycle, material fatigue can then be reversed in this way by further rolling or surface treatment; in the best case, a steel product can even regain 100% of its original service life.

Three properties thus characterize the new DNA of steel: First, it is low in CO2 emissions; second, it has a long service life; and third, it is easy to recycle. AI is now increasingly helping to maintain consistent quality. The so-called digital shadow monitors the thermomechanical processes such as rolling, cooling and heat treatment and notes how the required temperatures and forces deviate from one another. This produces an image of the material's microstructure at hand, according to Prof. Krupp, "a digital shadow of the material's properties, so to speak."

After a while, the AI has then collected a vast amount of data that it can relate to the material properties of the steel. The links are made using machine learning and PINN, "physically-informed neural networks." The great hope: In the next step, this will allow processes in secondary metallurgy to be adapted in real time to the values determined and ensure consistent best possible quality in the future.