22 February 2021

High strength steels offer the potential to lower CO2 emissions in the automotive industry

Researchers from the Department of Materials Science and Engineering have developed a completely new way of making high strength steel that is easy to adapt to mass manufacture.

Microstructure of a fine-grained steel
The left hand image shows the utra-fine grain size (each colour is an individual grain). The right hand image shows the atomic structure (each bright sphere is a atom).

The demand for cars and other road vehicles shows no signs of slowing down, yet legislation applies increasingly stringent control of CO2 emissions. Manufacturers are therefore seeking cost effective ways to reduce the impact that their products are having on the environment.

According to the World Steel Association (www.worldsteel.org), the average car contains 900kg of steel, so any way of reducing this will improve the performance of the vehicle in terms of its environmental impact.

As a result, manufacturers are on the lookout for lightweight components to place into their cars, and this is driving the steady growth of the global automotive advanced high strength steel (AHSS) market. According to the Allied Market Research, in 2018, the AHSS market generated $12.80 billion, and this is estimated to grow to $33.85 billion by 2026, rising at a compound annual growth rate of 13.1% from 2019 to 2026. 

The benefit of using high strength steels is that less material can be used in the vehicle, and so the total weight of components is reduced for the same level of performance.

Advanced high strength steels have been in development for more than four decades, but recent years have seen significant advances, with their strength increasing by 10% every year - a feat that no other material comes anywhere near to matching. Remarkably, this has to be achieved while the cost remains the same. However, this increase in strength has been increasingly difficult to achieve. 

Researchers in the Department of Materials Science and Engineering at the University of Sheffield have delivered a breakthrough that is published this month’s Nature (Gao et al. ‘Facile route to bulk ultrafine-grain steels for high strength and ductility’, Nature, Volume 590 Issue 7845, 11th February 2021 https://www-nature-com.sheffield.idm.oclc.org/articles/s41586-021-03246-3).

The paper describes a completely new way of making high strength steel that is easy to adapt to mass manufacture.

The work, led by Prof Mark Rainforth, with Dr Junheng Gao as the lead researcher, has shown how ultra-fine grained steel can be made to deliver world leading mechanical properties (strength of nearly 2GPa with an elongation of 45% - for comparison, a similar unmodified steel alloy had a strength of ~710MPa).

Chart showing the mechanical properties of Advanced High Strength Steel
Plot of the product of total strength and uniform elongation versus yield strength for high strength steels. Ideally, both properties should be as high as possible. The new Sheffield steels exceed the properties of other high strength steels.

The secret behind this success is the inclusion of an element which is usually avoided in steel production because of detrimental effects it can have on the properties of particular steel compositions.

Copper is normally considered a tramp element that must be removed from the steel making process. But copper is increasingly present with recycled steel and unavoidable by steelmakers wanting to include more recycled materials in their production process. It is therefore important to turn this ‘evil’ into a positive, which is exactly what the research team in Sheffield, alongside international collaborators, have done, paving the way for further light weighting in transport, leading to reduced CO2 emissions. 

The presence of copper results in a rapid precipitation of a copper phase when the steel is heat treated during processing. This phase restricts the growth of grains in the material’s microstructure, leaving an ultrafine-grained microstructure which imparts the high strength and superior ductility, but also enhances the thermal stability of the steel.

The combination of these properties makes these alloys particularly interesting to automotive manufacturers looking to increase their use of lightweight alloys in their vehicles.

Professor Raniforth and Dr Gao have been collaborating with the University of Science and Technology at Bejing, China, (Dr Suihe Jiang and Prof Zhaoping Lu) and the National Institute of Science and Technology, USA, (Dr Huairuo Zhang, Leo Bendersky and Prof Albert Davydov).

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