The transition from vehicles powered by fossil fuels to ones running off electric batteries is having a profound effect on the car industry. Despite this major change, some priorities of automobile production are unchanged, especially high safety standards.
“The architectures of electric vehicles are definitely changing, but not so much the materials we use to produce them,” says Yuki Toji, a senior researcher at the Steel Research Laboratory at JFE Steel in Chiba, Japan. “Automobile companies have to protect batteries to avoid explosions in the case of a crash, and ultrahigh-strength steel is the best way to achieve this.”
A weight-loss plan
Changing engine technology is not the only way the automotive industry is working to go green. A major push is being made to reduce the mass of cars to both increase fuel efficiency and reduce emissions of carbon dioxide. Because steel components make up most of an automobile’s mass, there is an intense research focus to make it both more formable, and stronger (and thus lighter, since less is needed for the same structural function).
Nearly two decades ago after its establishment, JFE Steel’s answer to this challenge is a product range called ultrahigh-strength steel (UHSS) sheets. Using advanced processing technology that allows raw materials to be cooled, quenched or annealed on demand, JFE Steel developed a grade of steel with controlled distribution of carbon and other alloying elements, which dramatically improves tensile strength while maintaining formability.
While manufacturers eagerly adopted UHSS sheets for external parts such as bumpers, problems began to emerge when they turned their attention to components designed to protect passengers in the cabin. Interior parts including pillars and rockers are typically fabricated using a machine that presses steel sheets into a die, forming the metal to meet a desired shape. The high strength of UHSS made it difficult to deform.
Making the most of the microscale
Toji and his colleagues at JFE Steel realized that small fluctuations in carbon concentrations in UHSS could be affecting its ductility. “Steel has a microstructure where regions can have different hardness because of differing carbon contents,” he explains. “If we deform these microstructures, they elongate and produce microvoids — a weak point between hard and soft phases that can generate a big crack.”
As the team set out to modify their processing scheme to improve UHSS formability, another obstacle was revealed. They needed a non-destructive technique that could analyse the steel sheets and identify microstructures responsible for cracking. One prospective instrument, an electron probe microanalyser, seemed to fit their requirements. This approach, which involves scanning samples with electron beams to release characteristic X-rays of carbon atoms, is routinely used for characterizing steel. But getting X-ray measurements accurate enough to resolve microscale phases would require some modifications.
“Carbon is notoriously difficult to measure because it’s everywhere; there’s contamination even in our microscope, which is inside a vacuum chamber,” says Haruo Nakamichi, another senior researcher at JFE Steel who specializes in analytical microscopy. “It’s challenging to take these readings precisely.”
Nakamichi and his co-workers used a multi-part approach to suppress hydrocarbon contamination in electron probe microanalysis. They developed a heating stage and plasma cleaners to expel unwanted particles from the steel samples, where they can be sucked into a super-cold liquid-nitrogen trap.
With the new cleaning procedure in place, the JFE Steel researchers successfully produced two-dimensional maps showing carbon-atom distributions at a sub-micrometre resolution. These maps helped the team identify processing changes that can impart UHSS sheets with more formability.
“It’s better if the microstructures are uniform, since the strain concentration is smaller than in conventional UHSS,” says Toji. “This is the key to achieving good performance and high elongation.”
Helping customers build momentum
The unique properties of UHSS can present challenges for manufacturers accustomed to conventional mild steel. To help ease this transition, JFE Steel has created a dedicated facility termed the Customer Solutions Lab. Here, prospective partners can see existing advanced automotive manufacturing technologies and cooperate with a team of experts to develop customized designs of their own.
“We promote an approach known as early vendor involvement,” says Keiji Ueda, a senior researcher at JFE Steel. “We want to be involved from the earliest stage of automobile production, not only for material development but also to propose new forming technology.”
“Each customer has their own standards, and we produce materials to better satisfy their specifications,” adds Toji. “We perform tests with customers, such as crash simulations, and collaboratively we discuss their needs — more elongation, for instance, or how a slight change in shape can enhance the forming process.”
JFE Steel wants to expand the range of application of its products, and develop even greater high-performance, high-tensile strength steel sheets. “JFE Steel is pleased to contribute to the realization of a sustainable society by supporting the development of safe, environmentally friendly vehicles,” says Ueda.