Carbon footprint on the road: the true colours of stainless steel

Posted on 17 May 2024 by The Manufacturer

When automotive designers want to minimise the carbon footprint of their vehicle, it makes sense to choose the lightest, and therefore most fuel-efficient, material or metal that meets their safety, durability and performance targets. But will it still have the lowest carbon emissions when you factor in the energy used to make it? This was the question asked in a ground-breaking project to analyse three competing materials – aluminium, carbon steel and stainless steel – during both the manufacture and use phases.

The results proved counter-intuitive, with aluminium, the lightest material, having the highest carbon footprint by far. Stefan Lindner – Outokumpu Lead Technical Manager Mobility & Transport, BL Advanced Materials, explains.

Minimising global warming potential

Structural designers in the automotive industry constantly seek ways to lower the weight of their vehicles to improve fuel efficiency, with the aim of reducing carbon footprint and consequent global warming potential (GWP). This ‘lightweighting’ approach relies on a combination of innovative design and materials.

Exotic materials like carbon fibre are not appropriate for volume production vehicles. Therefore, the choice is generally regarded as standard carbon steel or aluminium. Stainless steel is, however, an important third player. Its high strength promotes lightweight design, while production based largely on recycled scrap results in an inherently low carbon footprint.

The challenge for automotive manufacturers is how to obtain through-life GWP data for their vehicles on a rigorous and repeatable basis. That is why Outokumpu and FKA, the Aachen-based research partner for the automotive industry, decided to collaborate on a first-of-its-kind project to evaluate the carbon footprint of a real-world structural component across both production and use phases.

Three scopes in a ‘cradle to gate’ carbon footprint

It can be difficult to compare the carbon footprint of materials from different suppliers due to variations in reporting. In fact, two products can appear identical in terms of composition and mechanical dimensions, yet their carbon footprints could be significantly different. One example is that steel mills often take differing approaches to the procurement of low carbon energy for their processes, as well as the sourcing of low carbon raw materials.

In addition, carbon reporting often varies between suppliers and countries. Not all reporting methods consider carbon dioxide (CO2) emissions from ‘cradle to gate’. That is why it is so important to ensure that like is compared with like.

This is where the ISO 14040 environmental management standard offers clarification. It provides a framework to assess the CO2 emissions of products and raw materials and breaks their carbon footprint down into three scopes:

  • Scope 1 covers direct emissions from a producer’s own operations. For stainless steel, that could include burning of fossil fuels to heat furnaces.
  • Scope 2 covers indirect emissions. These arise from the generation of electricity used on production sites.
  • Scope 3 covers indirect emissions from the supply chain such as the extraction and processing of raw materials and transport. This is where particular care is needed as not all material producers provide data under Scope 3.

Sustainable stainless steel

Stainless steel can be claimed to be sustainable because it is 100% recyclable, efficient and long-lasting. Furthermore, its production is based on the use of scrap – Outokumpu’s stainless steel produced in 2023 contained 95% scrap. This is one of the reasons why the company can offer stainless steel with the world’s lowest carbon footprint, as illustrated in Figure 1.

Figure 1: Outokumpu’s stainless steel carbon footprint

Driving for zero carbon with Circle Green

The bar on the far right of Figure 1 represents the world’s first towards-zero carbon stainless steel. Known as Circle Green, it has the lowest carbon footprint in the industry – a reduction of up to 92% when compared to the average. Green energy plays a significant role in the production of Circle Green, along with the use of special raw materials, such as organic coke for ferro-chrome production, and optimised scrap management.

Circle Green is not a new grade of stainless steel. Instead, it is a philosophy that Outokumpu has adopted towards minimising the carbon footprint of existing grades. That enables customers to adopt Circle Green with no concerns about existing approvals, because it is effectively the same material but produced by an even greener route.

Testing for the suitability of material substitution

The first stage in the project was to confirm that the alternative materials could deliver the required performance when used in the construction of a structural automotive component. The example used was the battery tray for a typical 2023 model-year, five-door passenger EV as shown in Figure 3.

A battery tray was modelled using three different materials

The wall thickness of the tray varied between 0.8 and 1mm. The higher strength of the stainless steel allowed a reduction in section, and therefore weight reduction, compared with the reference carbon steel. Even more weight was saved by using aluminium, along with a small increase in energy storage capacity.

The performance of the materials was modelled in a crush test and bollard test to ensure that in all cases the required air gap was maintained between the battery cells and the tray under impact.

Carbon footprint in the production phase

Once Outokumpu knew that the competing materials met the performance criteria, the project moved on to evaluate their carbon footprint in the production phase as shown in Figure 4. This considered a comprehensive range of aspects including:

  • Raw material production
  • Manufacturing processes – such as deep-drawing, stamping, extrusion, bending etc
  • Joining processes
  • Corrosion protection

Publicly available data was used where possible. The first of the three stainless steels listed is Forta H-Series. This is a new generation of nickel-free, fully austenitic manganese-chromium alloyed grade. It has been developed for safety-critical structural vehicle components. With a yield strength ≈ 800MPa and a tensile strength of around 1,000MPA in the temper-rolled condition, and in combination with high elongation to fracture, it opens new opportunities in lightweight engineering and design. The material also has very high energy absorption in the event of an impact.

The other two stainless steels were standard 1.4301, a widely used chromium-nickel steel with an attractive 2H finish and the same material produced via the Circle Green route. Both were temper-rolled to a yield strength ≈ 800MPa.

Moving on to the use phase

To complete the investigation, the company evaluated the carbon footprint of the three types of material in the use phase. Outokumpu used as a reference the vehicle that the battery tray was taken from, with this key data:

  • Energy storage of 58kWh
  • Lifetime mileage of 160,00 km
  • Energy consumption of 15.2kWh / 100km

The Fuel Reduction Value (FRV) was used to compare the possible energy savings. The FRV indicates how much energy (in kWh) is saved by each 100kg reduction in vehicle weight over a driving distance of 100km.

The analysis also considered the very different energy mixes in four regions:

  • EU: 275g CO2-eq / kWh
  • Norway: 30.0g CO2-eq / kWh
  • USA: 387.8g CO2-eq / kWh
  • China: 549.3g CO2-eq / kWh

Two scenarios were considered: the present day, and a future more circular economy with a higher level of scrap reuse.

Stainless steel currently has the lowest carbon footprint

In the present-day scenario, there is a relatively low level of circularity. Figure 5 shows the relative carbon footprint of the three materials under the EU energy mix. Stainless steel offers a carbon footprint saving of 29.9% against carbon steel. But there is even greater significance in that it also shows a reduction of 112.8% against aluminium.

These results will surprise many designers, as they would expect that aluminium, being the lightest material, will have the lowest carbon footprint. It is indeed true that the material enables the construction of the most fuel-efficient vehicle. However, because aluminium is produced using very energy-intensive processes it has a very high carbon footprint in production. This initial debt in terms of carbon emissions cannot be paid off by aluminium’s contribution to energy efficiency in the use phase, even over a 160,000km vehicle life.

Figure 5: Stainless steel shows a very significant reduction in carbon footprint across the production and use phases

Aluminium still loses in a future circular economy

The same analysis was carried out to factor in the impact of a future more circular economy, with a higher level of recycling as shown in Figure 6.

Outokumpu did see major improvements in the carbon footprint of three materials. Yet the balance still does not tip in favour of aluminium. It remains higher than stainless steel, although the margin is reduced to 13.6%.

Figure 6: The impact of the circular economy on the relative carbon footprints

The influence of regional energy mixes

The final stage of the investigation was to look at the influence that the regional energy mix had on both the production and use phases. This is shown in Figure 7.

The energy mix has a major impact on both phases. Because Norway has an energy mix based almost entirely on renewables, there is a remarkably low carbon footprint in the use phase. This contrasts with China, where most of the electricity is produced by coal, with the highest figure for the use case.

A clear overall pattern emerged showing that no matter how favourable the energy mix, aluminium always has a higher carbon footprint than stainless steel.

Figure 7: Regional energy mix has a significant influence on carbon footprint

Fully-informed material selection relies on a holistic picture

The key conclusion from this study is that it is essential to include both the production and use phases to understand the full carbon footprint implications of an automotive material. The advantages of a lighter material that delivers excellent fuel efficiency in use can be outweighed by the carbon footprint of the processes used in its production.

Furthermore, when reviewing different scenarios such as a future more circular economy or a favourable energy mix, stainless steel always delivers a lower carbon footprint than aluminium – when using current figures for aluminium production. This gap could be increased further by manufacturers adopting Circle Green, the towards-zero carbon stainless steel.

Aluminium is less dense than stainless steel but with current production technology it has a much heavier global warming potential.

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