ArcelorMittal is continuing to develop stronger, lighter steels for use in the automotive industry. A looks at the latest materials, and seeing what the future holds for steel

"The three most important factors in automotive steel production are safety, weight-saving and cost, and we need to offer the best compromise between the three.”  Jean-Luc Maurange, Vice-President of ArcelorMittal’s Worldwide Automotive division takes a pragmatic view when describing his business. Steel prices have been steadily rising, but it appears ArcelorMittal is well positioned to retain – even extend – its customer base, by continuing to offer products and services carmakers require.

“When ArcelorMittal was created, we decided to dedicate a specific branch of the organisation to automotive business, as it was such an important segment for the group. This was the origin of Worldwide Automotive. We function as a sales and information platform for our global automotive customers in Europe, North America, Brazil and South Africa. We have designed it to be a customer-orientated organisation, which means we have a team for each customer – for example Toyota, Ford, GM, Hyundai and Magna. Each team is lead by a global account manager, with members responsible for sales and marketing, technical assistance, logistics and research and development, for example. Internally, we are an extension of our production facilities, offering technical assistance and coordinating with R&D.

“We have tried to put the teams as close as possible to the decision making centres of our customers. For instance, the GM team is located in Detroit, the Daimler team in Stuttgart, the PSA team in Paris, while the remainder of each team is based near the facilities, as they are needed.” Despite the far-reaching size of the company, Maurange is still eager to expand into markets as yet untapped.

The search for new markets
”We are the closest to what you could call a global steel company, but we do not have a significant presence in Asia, at least for the flat product used in automotive,” explains Maurange. “We believe China is the country where the automotive market will develop most rapidly, so it is important for us to become a key supplier there.” To do this, the company will be required to invest in new processing plants, a decision that required extended planning.

“It is important to ensure the stability of the value chain. Investment in a steel-making facility represents a major outlay of capital, one that requires 10 to 15 years of future planning to gauge the value of the location in regards to material supply.

“With automotive steel, you need to be able to produce or process close to your customer facilities. We have the same demand from carmakers as other subcontractors to have JIT delivery and make supply-chain efficiencies, and you cannot do that by shipping material from Europe to the US, or from the US to Brazil. Perhaps it could be done on an exceptional basis, for a very specific product, but as a major supplier we need to have production and processing close to our customers.

It is where ArcelorMittal is unique, as our network of plants is well positioned. Our European competitors have more centric production facilities, shipping to where the material is needed. A central location may be more efficient in terms of cost, but that has to be balanced by the increased transport cost and logistics problems, and lack of flexibility.” ArcelorMittal has more than 40 specific lines producing a wide range of finished products for the automotive industry.

“We deliver hot- and cold-rolled steel in coils, we can deliver it in blanks, cut it into sheets of different thicknesses and produce shaped blanks. These are all monolithic steel. We also deliver laser-welded blanks, with different types of steel welded together to optimise the properties of the blank. We cut the steel and weld it. In a door, for example, we can weld different thicknesses across the panel, increase strength and improve weight performance and finish.” Within these individual product areas, ArcelorMittal is constantly looking to improve the products it makes available to customers.

“We have several resident engineers based in our automotive technical development centres,” says Maurange. “These engineers follow OEM development projects very closely, which allows us to provide the best steel solutions at the vehicle design stage. This way we can make carmakers aware of our recent developments and how they can be incorporated in future models. Developing these kinds of partnerships with our customers has proven very successful.”

Producing lighter, stronger steel
Through its research centres, the company has positioned itself as a leader of research into improving the steels and processing methods it offers the auto industry. But what is it about steel that allows it to be stronger, yet not as heavy? “In talking about our new steels, you have to go back to the process of producing them. To get a stronger steel, in very simplistic terms, you have to put more alloy element into it, adapting the thermal cycles during production. So instead of regular steel, you will have what we call TRIPsteel (transformationinduced plasticity), which features an alloy element like vanadium, to make the steel stronger, while still allowing it to be shaped into thinner, lighter parts. You want to make it stronger, because that is safer, but you also need to transform it.

“With this type of steel, we have been able to reduce the weight of some structural parts by 20 per cent. This steel is more expensive, but not by 20 per cent, so the economics is very favourable, even in comparison to other materials. You could achieve weight savings by using other materials, by incorporating more plastic or aluminium, but then your cost is going up and you potentially compromise the cost/ safety balance.” According to Maurange, the industry had reached the limit of what it could do with cold stamping. In response to this, ArcelorMittal recently introduced an improved forming technology. “To produce lighter steels, you have to work on the metallurgy and then on the application process. Changing the process allows you to alter the product. We have made tremendous progress this way, working on the metallurgy and process applications in parallel.

“For instance, we have developed a new kind of stamping – hot stamping – which is now being widely used in operations across Europe by companies such as Gestamp, Benteler and Magna. The technique involves heating the part and then stamping it when hot. This allows you to get the required shape more easily than with cold stamping. The hot stamping offsets the hardness of the steel, so you get the best of both worlds – a malleable, formable steel that returns a rigid, durable product. “As the steel cools, it doesn’t change the shape or size, it simply increases in strength. This concept was very well received by carmakers and subcontractors, who then invested in hot-stamping facilities. Volkswagen, for example, has made a huge investment in this technique.”

Explaining the process to customers
As each development is, by definition, new to the company’s customers, making sure that the latest process is fully understood is paramount to its success. As such, the company offers strong support in the form of data and simulations of how the new process will affect the steel. “When we began talking about hot stamping, we offered simulation data that showed the projected end result when the steel was transformed in this way. Due to the complexity of the material, we needed a lot of sampling to figure out how the steel would behave. This sampling has been replaced by simulation data, which we provide to carmakers.”

When it comes to new techniques like hot stamping, do you develop the necessary tooling, or do you work in conjunction with tooling manufacturers? “We don’t develop the tools, but we gather feedback from our users. This allows us to assist in tool development. In the case of hot stamping steel, we worked with stampers to optimise the thermal cycle. Initially, this kind of installation was quite large, with a very long furnace, and we succeeded in reducing the size of the furnace, raising the temperature faster. “We don’t consider ourselves as only steel suppliers, we are supplying steel solutions. Beyond providing a product, solutions need to be economically feasible, so we work on easing its implementation and improving its process performance. All this goes into creating and optimising the line. Moreover, we have specialists in regular stamping who can visit customers having difficulty processing a part. They assist by adjusting the tooling, which is part of the solution.”

Tailor-made steel solutions
Talking about what prompts the decision to develop a new material or process, Maurange offers insight as to ArcelorMittal’s close relationship with OEMs. “When discussing material in the vehicle development process, we either suggest using a particular type of steel, or propose a new innovation. What we have is not necessarily what they will think to use, even if they have a pretty good knowledge of what materials are available.”

To demonstrate what the company could offer carmakers, ArcelorMittal developed the ArcelorMittal Body Concept (ABC) project. Unveiled in 2004 and built in-house with subcontractor partners, the body-in-white featured what were then the best steel solutions for each module. “We built the structural part of the car using the latest steels,” explains Maurange. “This served to highlight the different steels used throughout the car, to show carmakers what we can do with our products. It was important for us to demonstrate that these steels are not conceptual products, they are existing products, available now. With ABC, we succeeded in decreasing the body weight by 21 per cent, and reducing CO2 emissions by 10 per cent. We’re now working on a second concept, aiming for the same increase in performance.”

The ABC features a series of different steels, colourcoded in the chassis to demonstrate optimal usage points. For example, researchers have used ArcelorMittal’s own Usibor for parts of the B-pillar assembly; a steel specifically developed for use in the hot-stamping process. “Usibor steel incorporates a boron alloy. The boron makes the steel stiff. The first use of boron in steel was for agricultural products, making ploughs. Fifteen years ago, if I’d suggested making car parts from this kind of steel, I would have been laughed at. But now there are many chassis parts made of Usibor. I believe that by next year there will not be a single car produced in Europe that does not include at least some steel of this type. America is also moving in this direction.

“In some accidents, an SUV can flip over, its roof collapsing. Until now, there was no efficient way to prevent this. You could add more material to the upper portion of the body, but that would compromise weight saving. Usibor is our solution for this, strengthening the support pillars.” In producing blanks and other products, there is always the issue of scrappage. ArcelorMittal has been working with companies to reduce or even eliminate this by-product. “The manufacturing industry generates scrap. For example, when shaping a door blank, the window cut out will be scrap – up to 40 per cent wastage. We are working with our customers to improve the current process. We now have a scrap buy-back programme, meaning there is little or no waste for our customers. Additionally, this is our steel, so it has its own quality guarantee.”

Asked if ArcelorMittal has considered providing completed blanks to OEM customers, Maurange points out that the idea has proven unsuccessful in the past. “For economic reasons, people in the industry have been concentrating on their core business. But as downstream process is part of the performance of steel, we have decided to strengthen our relationship with stampers by assisting them over the long term, with technical collaboration. This is evident in our financial participation with companies such as Gestamp and Magnetto.

“Stamping companies need to be run by experts in that business. It is a different business from that of creating automotive steel. But there are benefits from working in close cooperation. They help us by accelerating R&D of new products, and it’s good for them to make best use of the support we can provide. It doesn’t make sense to have full ownership in this area.”

The coating process – a challenge for the future
Another area where ArcelorMittal is making advancements is in steel coatings. Maurange points out that the company was in an advantageous position when the industry switched from uncoated to coated steels, already being involved in developing cost-effective coating solutions. “When the market switched from uncoated to coated material, we were able to offer a very efficient galvanizing solution, using Extragal coated steel for exposed parts. This steel returns an excellent surface finish. With a roof or side panels, paint does not hide the defects, so we focused on this area, a much bigger market. “Our next challenge is to improve the coating process so the coating can be differentiated depending on the part. “Until recently, there were two ways to coat a material. First, there is the electrolytic method, where you dip the metal in the zinc bath and the electric charge makes the zinc stick to the part. Then we started to use the zinc bath, which we use to produce Extragal, probably our most widely-known product. Now we have an even better product – Ultragal.

“What we’re investigating is depositing zinc on the material in a vacuum, which will allow us to vary the thickness of zinc applied to the steel. When it is introduced, it will be a breakthrough for the industry.” Of course, cost is the motivating factor behind the drive for this new technology.

“When zinc was $1,400 a tonne, people didn’t mind how much it cost to galvanize the steel. Now it’s $4,000 a tonne, that’s a huge cost increase, so we decided to improve the process. We are investigating vacuum physical vapour deposition, developed in-house, which allows us to provide a wide range of coating thicknesses across the same product, as well as incorporating new elements to improve corrosion resistance.”

When questioned about how far development of steel can advance, Maurange presents an optimistic view. “There are many things we can still do; we’re not at the limit. We don’t even know where the limit is. We have products for the next five years or so. It’s difficult to anticipate what will happen over the next 20 years, but we have ideas. There are still fields to be explored for the car of tomorrow.”

ArcelorMittal’s ongoing research

Montataire is one of the two research facilities of ArcelorMittal located in France (Maizières-lès-Metz is the second). The operation is dedicated to developing new steel solutions and technologies for use in the automotive sector. Jérôme Guth, Manager, Product Development Auto, oversees new projects through development, defining how they are brought to market.

AMS: What are you working on at Montataire?

JG: We have many new products and technologies in the pipeline. Our research and development is divided into three main areas: new products, applications (how our customers use our products, forming, jointing, painting, etc), and also process (new, steel-related industrial processes). We develop products with specific mechanical properties, improving the way these products are implemented.

ArcelorMittal has developed the widest range of steel products available globally in terms of mechanical properties, from the mildest to the strongest steels, and including surface properties. All our products have intrinsic mechanical, functional and surface properties. With surface, improving corrosion resistance is one area, but that is already mastered and controlled. We have other functionalities, like tribology (the properties related to friction). This bridges the gap with R&D, our special processes concerning physical vapour deposition. This process will allow us to develop products with new surface functionalities, improved corrosion protection and specific surface properties in terms of cleanliness, for the application of topcoats.

AMS: Laser welding is becoming more widespread. What makes the process so appealing to carmakers?

JG: The technology of tailor-welded blanks is primarily to adapt the thickness of a part. For example, a door is made of two parts, the outer and inner panels. The inner part needs to be reinforced at the hinges because it handles the most stress. In the classic design, the inner part and the reinforcement are stamped separately. The parts are then welded together, resulting in one that is reinforced at the hinges. You can avoid the separate stamping by producing a flat blank made of two rectangles, one that covers the location of the hinges with a thicker or stronger material, and the rest of the part with a lower-strength material. The two rectangles are laser-welded together, resulting in a thin and formable join, and then the single part is stamped. In doing that, you avoid producing the additional reinforcements and can reinforce only where it is needed. You save parts and weight, as well as the cost of developing the second die. In a single blank, you have several functions and resistances. It is very interesting to adapt the thickness and material type to the mechanical requirements of the parts, in terms of rigidity and stiffness.

AMS: As regards laser welding, when two grades of steel come together, does that create a third type of material?

JG: It’s a transition, we anticipate the change between the two parts. We have experimented with various lasers, with different power and wavelengths, to achieve the best results. The technology of laser welding is evolving. We have to anticipate and keep up with this, and laser technology will continue to evolve.

We use laser welding for a range of automotive applications, and our customers also use it for 3D welding. It can replace spot welding when you’re hemming, for example, a car door or hood. You can use short laser welds, completed by robots, that don’t have any contact with the material. Focus the laser beam on the joint area and make a dash or other shape to join the two parts, it’s almost instantaneous. You can guide a laser through fibre glass, like a laser pointer. The productivity of a laser is greater, plus it allows you to reduce the widths of the welding electrodes. Advantages are weight reductions, because the flanges are narrower, and speed, returning better mechanical properties and improved productivity.

All these technologies have been implemented at various carmakers, and we are continuing to work on this application research because there are still many avenues to explore in the field of automotive steel.

AMS: Do you see a problem and then work on a solution, or do you make an advance and then tailor the process to what carmakers need?

JG: We work on solutions. We have developed a range of steel solutions for generic parts and functions not linked to a specific car or specific customer, but a general function in the car – the front bumpers, side panels and rear rails. We develop the best combinations of materials and technology (joining, etc), while also accepting the development costs. We have been working on all the functions of the body – the front, the bodyside – a very important function for side impact. We have a catalogue of bodysides, with different sets of materials, showing that, depending on which are combined, it will result in a particular crash behaviour, weight and cost. This way we demonstrate that before we are asked to work on specific parts of specific models, we can show the engineers the added value we offer. This is working before we are involved with a project. The steel solution portfolio that we have developed is very effective in demonstrating the benefit of our product ranges.

AMS: What have you done to advance bodysides?

JG: When you configure the bodyside, the function is designed taking into account side impact. With today’s side-impact regulations – Euro NCAP tests – we have taken into account crash testing, crash barriers, speed, energy, etc. So the bodyside is compacting by design. The requirement of the carmaker is to limit the intrusion of the obstacle into the safety cell, for which the B-pillar is key. It must be very strong, but it must not collapse or break. For this application, we have developed boron steels like Usibor, a steel grade which is hot-stamped.

We deliver the material to carmakers and Tier One suppliers who heat the blank in furnaces and take it into the die while it is still hot, at about 800°C. The part is then simultaneously quenched and formed in the die. In doing this, you perform both forming and metallurgical operations, transforming the ferrite into martensite. This results in a fully martensitic structure with very high tensile strengths. These can reach up to 1,500 megapascals.

With all car bodies being coated, for corrosion prevention, this process has to be compatible with pre-coated steel. A standard zinc coating would not resist the process. So we have developed a specific coating for this process, which is aluminium silicium. The advantage is that this aluminium can resist the heat treatment and the quenching process, so at the end you have a formed panel that includes the required coating.

AMS: How does the coating product withstand 800°C?

JG: It is a mix of aluminium and silicium that diffuses into the steel matrix during the hot forming, becoming an integral part of the steel. Part of the process of hot stamping is to ensure you have this layer, to make sure that the coating has become part of the substrate. Usibor, plus the aluminium silicium coating, has become a commercial success because it fully matches the requirements of the carmakers. It allows weight reductions of up to 30 per cent because, when the part is so stiff, you can integrate reinforcements that carmakers would have produced separately. In the past, the B-pillar would have been made of separate parts, but with this technique, you can combine this into a single part of lower thickness that is very strong.

AMS: How does the cost of hot stamping compare to the previous method?

JG: It is a business case. Maybe you have some more initial cost in the process, but you save parts, weight and additional coating operations. There are other types of boron steels that are uncoated, where you have to coat the parts afterwards, but if you balance all this, then Usibor products become very competitive. We do a general analysis with carmakers to show how this technique can reduce their costs.

We also propose to combine hot stamping and tailorwelded blanks. That way, you can go even further in the way energy is absorbed. In the B-pillar particularly, you need different behaviour from the bottom to the top. In the bottom, you need a little bit of ductility and deformation. If you were to have a pure Usibor B-pillar, there might be a lack of deformability in the bottom parts, so what we do is propose a combination of Usibor 1500 in the upper part and Ductibor in the lower part. Ductibor is a more ductile Usibor. Combining the two grades into a laser-welded blank allows carmakers to optimise the crash behaviour of the part.

A steel solution is a combination of material and process. Hot forming, laser welding, boron steels and coating for specific parts with particular requirements. Steel is not plug and play, but it is adaptable to every situation.

AMS: What are you developing that’s not yet on the market?

JG:We will extend the range of our high-strength steels by increasing the properties of tensile strength and the elongation (the ability of the material to deform without breaking). On the one hand, you have mild steels, which can deform a lot, and ultra high-strength steels, including Usibor, with very little elongation. On the other hand, within this range, there are a lot of metallurgical families and we are working to enlarge this range over generations of products. Our goal is to produce very strong steels with high deformations.

AMS: How can something that is rigid by nature also be deformable?

JG: For that we have other types of steels – TRIPsteels, for example, where TRIP stands for transformation-induced plasticity. TRIPsteels combine strength and deformability. They are named that way because we produce a material that contains a base of residual austenite. Austenite is very ductile, and can be transformed into martensite just by forming, without any heating or quenching process. That means the mechanical properties of the final parts are much higher than on the blank.

AMS: So why go through the process of hot stamping to get your martensite product if you can achieve it without?

JG: It will not result in the same amount of martensite, it will not be a fully-martensitic steel. TRIPsteels are, after forming, partly martensitic. Because you still want to have deformation and forming ability, it’s a mixture of different bases.