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Steel makers are developing new grades and coatings that can help the automotive industry reduce weight without compromising safety
There is a significant need within the automotive industry to find ways of increasing safety in a crash situation, using lightweight designs,” explains Anke Meyer, marketing manager at SSAB. One of the solutions is electro-galvanised steels such as SSAB’s Docol 1400 MZE.
Electro-galvanising is really another term for zinc electroplating. This means the steel is immersed in an aqueous bath and electricity is used to move electrons from the anode to the cathode, thus inducing the zinc anodes to be oxidised and dissolve as zinc ions in the aqueous solution. They are transported as ions through the solution, and reduced as metal onto the work (the cathode).
In hot-dip galvanising the work is immersed in molten zinc. As it is withdrawn, the zinc cools and forms a coating of zinc on the work. Hot-dip galvanised coatings are almost always several times thicker and therefore several times as corrosion resistant. There are also some other practical differences in the two coatings for different applications.
“This is an extremely cost-effective alternative to other lightweight materials, such as aluminium, magnesium or carbon-fibre-reinforced plastics,” Meyer adds. “It can be used in a wide variety of applications from side impact beams to bumpers, sills and tunnel reinforcements, as well as cross and floor beams. It’s also suitable as battery protection for hybrids or electric cars.”
As with all new grades, safety is vital and Meyer confirms that Docal 1400 MZE has passed these tests with flying colours. “3D laser scanning of the safety parts made with the material presents perfect shape accuracy compared with the original CAD data,” she says. “Crash tests of side impact beams made with the steel show that the ultra-high-strength steel (UHSS) grade has a very large energy absorption capacity.
To manufacture complex body parts capable of withstanding impacts, automotive manufacturers and their suppliers are increasingly using hot press forming and this has required the development of new grades of steel. The steels used to make these parts need to be ultra-high-strength and as light as possible in order to achieve optimum safety and environmental performance.
Such is the growth in their use that hot-formed parts make up more than 20% by weight of some recently launched car models. The use of steels whose strength has been enhanced through the inclusion of boron is growing particularly rapidly. Tata Steel is developing zinc-coated boron steels, which offer superior corrosion resistance compared to other boron steel grades.
“The major progress of recent years has been the outstanding success of aluminium-silicon-coated grades for hot stamping,” says Arcelor Mittal’s Jean-Luc Thirion. “Hot-stamping grades may amount to more than 20% of grades used in the manufacturing of the body-in-white for some vehicles. These grades can have huge ultimate tensile strength after hot stamping, but remain very formable thanks to this specific forming process. Moreover, the springback issue is fully solved.
“Beside hot-stamping grades, numerous grades beyond 1000MPa are now on the market. Some of them can be stamped, other are mainly for roll-forming. For these applications, ArcelorMittal has developed a martensitic grade at 1700MPa.”
During the 15 last years, the steel industry has developed very high strength steel, up to 980MPa. Most widespread metallurgies are Dual Phase, combining ferrite phase for the formability and martensite for the strength. TRIP steels are also successful thanks to their outstanding forming properties due to residual austenite in the ferrite.
For the highest strengths, most grades are fully martensitic. Typical applications include bumper reinforcement beams, door intrusion beams, belt line reinforcements, springs, brackets, clips and other metal stampings
The future for AHSS consists in substituting more and more the conventional steel grades at 600 or 780MPa with grades at 980, 1180MPa and more. “In this way we will keep reducing the gauges,” Thirion explains. “In this objective, ArcelorMittal develops products with outstanding compromises between formability and strength. These grades that will open new, surprising possibilities are sometimes known as Third Generation AHSS. This new generation will be especially ready to meet the very demanding challenges of the automotive market in terms of reduction of CO2 emissions or fuel consumption that are planned after 2020. Thanks to them and their combination with the highest performing current solutions, I am convinced that steel will remain the material of choice for automotive industry in the future.”
Hot-dip galvanisation and electro-galvanisation remain the preferred coating solutions for automotive steels. For hotdip galvanisation, beside pure zinc coating and galvanealed (ZnFe), aluminium–silicon has proven very efficient for hotstamping thanks to its behaviour at high temperature.
ZnMg coatings are also growing on the automotive market thanks to their outstanding corrosion performances. “We are working a lot on these developments, both with regard to the development of innovative metallurgies and on adaptation of the process,” Thirion reveals. “Our development programme is very ambitious and will regularly bring new grades on the market in the next five years.
Some recent work by Tata Steel demonstrates that current steel grades can go a long way to satisfying future needs. Tata’s R&D department studied crash tests conducted on the WorldAutoSteel’s lightweight small car concept, the Future Steel Vehicle (FSV) and used analysis runs to produce an overview explaining the impact performance of the vehicle structure.
Tata re-engineered components for body-in-white, subframe and seating designs using its currently available steel grades and gauges. During these design studies, stiffness, crash performance and durability were optimised and steel grade specifications were confirmed.
Additional forming simulation techniques ensured the development of designs suitable for volume production, while cost studies resulted in designs that were cost-effective, taking into account every step of the manufacturing process, such as stamping and assembly.
“We can be more than satisfied with the results,” says Sander Heinhuis, marketing manager, automotive at Tata Steel in Europe. “Tata Steel delivers lightweight body-inwhite solutions weighing only a few kilos more than the FSV model, which had used steels not available for commercial use before 2020.
The Tata Steel solutions provide improved performance in terms of higher load capacity and crash safety without compromising front, rear and side impact performance. Further production cost savings over FSV can be achieved and the overall life-cycle cost is reduced due to the weight reductions. For manufacturers, the good news is that through our automotive engineering expertise, ease of manufacture can also be guaranteed.”
When upgrading to higher-strength materials there is always a concern about compromising on formability in relation to stamping and on weldability in relation to assembly processes – in short, about the ease of manufacture.
“Through its experience in supplying all major car manufacturers in Europe, Tata Steel knows what the automotive industry requires – the use of thinner-gauge material to produce improved strength for higher load capacities and better crash safety performance,” says Heinhuis. “The need to further reduce weight, emissions and costs, while at the same time increasing safety, does not only apply to body-in-white, but also to other areas like chassis and seating.”
As welding is still the main joining method, it is also important that these new steel grades allow proper forming and welding while still achieving the best possible corrosion protection. “Tata Steel’s advanced coating products address and efficiently solve this issue,” Heinhuis adds.
