Despite the development of new materials for use in body-in-white, steel remains steadfast as stronger, lighter incarnations of the metal continue to arise.
Engineers may lament the fact that vehicle users know little, and care less, about the materials that make up their vehicles. Yet two factors have come together in the past decade that have begun to change this scenario. Widespread public discussion of environmental issues and increasing legislative safety intervention have pushed the global motor industry to become much more open about its innovations and achievements. While computerisation and robotics may have been the headline innovations of the end of the 20th Century, perhaps the success story for the first decade of the 21st Century has been the successful repositioning of the image of steel in both designers’ and end-users’ minds.
It is almost as if this once boring and traditional material has acquired a new sexy image. Turn the pages of new car brochures and you’ll be sure to see multicoloured cutaways of the vehicle structure and its safety cell, with frequent reference to ‘new’, ‘ special’ and ‘lightweight’ high-strength steels.
Part of the fight back by steel producers against the threatened encroachment of other materials has been a consequence of their ability to co-operate. Business historians of the future may speculate whether the significant concentration in the industry assisted this process, or whether it was a product of robust industry leadership and a newly-awoken capacity for both technical and commercial innovation. Whatever the case, a key part of the repositioning of steel from an old, outdated material competing only on price with new high-tech, ecofriendly alternatives, has been the collaborative concept developments that have both captured headlines and shown in stark practical terms the possibilities of the material.
It is now 20 years since a consortium of 33 of the world’s major steel companies commissioned Porsche to complete the initial ULSAB (Ultra Light Steel Auto Body) project, encompassing conceptual designs for compact and midsize vehicles with the then latest high-strength steels and innovative design solutions. It spawned numerous more localised follow-up projects, often in association with universities, which kept the potential of the material in the headlines. Most recently, the ScaLight (Scalable Auto Body Lightweight Concept) project coordinated by vehicle design house Karmann (in association with steelmakers Salzgitter Mannesmann Forschung) illustrates areas of application for advanced steels. Importantly, it has moved away from the usual blue sky concept thinking toward a concern with the economic viability of the illustrated solution. It is a scalable basic architecture for the cost-efficient series production of a variety of different vehicles. Robert Schultz, Project Manager for Karmann, has emphasised that “ScaLight is aimed at maximum practical relevance” with materials and methods that are already available or about to be launched on a commercial basis.
Global steel industry co-operation has continued with the recent commissioning from EDAG Engineering & Design AG of a Future Steel Vehicle (FSV) project, focused specifically on developing models of steel body construction appropriate for the new generations of hybrid, electric and fuel-cell powered vehicles. Elsewhere, companies such as Stadco, conscious of the potential of these new market opportunities, have begun to develop lightweight BIW projects of their own which specifically address the requirements of vehicles using alternative power units. For the future, the role of these new materials will become more critical with the continued introduction of hybrid vehicles which, as far as one can forsee, will lead to significant increases in vehicle weights.
Steelmakers worldwide have been robust and effective in defence of their product. Despite the pundits affection for more exotic materials, technical innovation and cutting edge marketing, the material’s natural qualities have conspired to allow steel to remain centre stage when it comes to automotive mass production. High-strength steels, with carefully tailored chemistry and sophisticated heat treatments, have shown that significant weight reduction is entirely compatible with the continued use of steel-based structures. Its inherent attributes of superior formability and weldability, along with the massive existing knowledge base relating to the material, stacks things up in its favour.
More recently though, sophisticated life-cycle analysis (LCA) techniques and energy usage assessments have also gone a significant way to reinforcing advanced high strength steel’s claim as a material able to minimise the environmental impacts of vehicle production and use.
High-strength low-alloy (HSLA) steels are formed by the addition of small quantities of alloying elements and offer the opportunity for mass reduction. Advanced high strength steel (AHSS) includes dual-phase steels such as US Steel’s DUAL-TEN grades and Transformation-Induced Plasticity (TRIP) steels, with tensile strengths commonly in the range 500-800MPa. These categories of steel are gaining acceptance in the auto industry, as they are significantly easier to form than HSLA grades, with similar initial yield strengths, but then offer much-improved final part strength.
These characteristics are produced through the continuous heat treating process that creates martensite in the steel microstructure. The combined effect of work hardening and bake hardening induced in the typical paint baking cycles combines to produce increases of about 75MPa in the yield strength of TRIP steels such as 780/800T HDGA. Steels of this type can meet demands for high formability combined with a high crash energy management capacity. Additionally, TRIP steels exhibit better ductility at a given strength level. This is commonly attributed to the transformation of retained austenite to martensite during the plastic deformation phase, though metallurgists have pointed out the majority of the uniform elongation has its origins in the composite deformation behaviours of the major phases. Whatever its origins, this ductility frees designers from some of the constraints about shape and complexity that have limited the application of previous high-strength steels.
ArcelorMittal has been developing a range of high-stiffness sandwich materials that attempt to widen the scope and performance of steel. Usilight is the latest offering, a steel/ polymer composite. The material is a three-layer composite consisting of a rigid polymer core and two identical deep drawing quality steel skins, with the polymer core size being adapted to the functional specification of the part to be constructed. Early use in roof canopies emphasises the potential of such composite steel-based materials in large panels requiring stiffness. Tests suggest the material is comparable in recyclability and acceptance of traditional paint processes. Joining techniques, however, have to be specially adapted as most traditional welding techniques cannot be used. Bonding process and self-piercing rivets are preferred attachment methods.
Production methods also influence weight saving. The increased adoption of laser welding, for example, allows the reduction of flange overlaps compared with those used in spot welding. As laser marking, trimming and welding equipment and operating costs fall, their adoption has shown a significant increase. Corus, for example, points to other production developments such as tailored roll blanks, where the mill roll is controlled and adapted to produce a steeped blank profile in a batch roll process, which offers benefits for manufacturers when used in strength-critical areas.
Kobe Steel and voestalpine Krems have recently sought to promote ultra-high-strength steel (UHSS) and roll forming techniques to manufacturers in Japan and Asia.
Traditional press forming cannot cope readily with UHSS and, as a result, processing of large components has proved problematic. Roll forming provides an alternative. Complex components with length and cross-sectional variation can be produced in a cost-effective way, as roll forming equipment involves comparatively modest investment. At EuroBLECH 2008 in Hannover, voestalpine Automotive planned to present its phs-ultraform steel range, which provides complex formability in steels up to 1,500MPa, combined with hot-dip galvanised corrosion protection.
Elsewhere, observers have noted the changing attitudes of the major manufacturers to emerging markets. Increasingly, the industry has seen the emergence of global platforms designed for reconfiguration to meet the needs of different markets, but crucially retaining the crashworthiness and performance capabilities demanded in mature markets.
Not only do manufacturers now require platforms and bodyshells to be designed to meet myriad performance and deformation targets, but they also need to be able to be manufactured in different plants around the world, with all the tooling and supply-chain challenges this generates. BIW specialists Stadco, for example, has analysed future trends and identified the growing emphasis on the globalisation of platforms and the pressures this generates on the supply base.
Markets such as India and South America with high rates of growth and a developing supplier infrastructure will increasingly demand European and American levels of protection and performance, and it is these demands that will impact upon future growth for various grades of high-strength steels. Despite the recent economic downturn, steelmakers appear to remain confident of long-term growth potential and the need to invest to benefit from it.
At the moment, concentration in the steel industry is being accompanied by significant reorientation of capacity to supply UHSS and AHSS across the global market. In Europe and America, ArcelorMittal has committed investment to meet its customers’ needs in this respect, while in Asia, Baosteel and other suppliers regularly announce increases in capacity in automotive sheet production, with attention now focusing upon advanced high-strength grades.
A decade ago, low-carbon steels would be used for stiffness dominant upper structure parts with thickness minimised and apertures used to eliminate unstressed metal. Body side inners have tended to be made using low-carbon laser-welded blanks, while primary load paths would use high-strength steels with dual-phase steel applied to rocker sections to increase compressive strength for crash energy management. HSLA steels would be used for local stiffening and to address crucial areas of load transfer in rocker and crush box areas as well as in the roof sections.
Now, low-carbon steels are more often relegated to close out panels, while various grades of high-strength steels would form the rest of the structure. With TRIP steels offering both ductility and strength, they are increasingly chosen for crucial areas, but joinability and global availability remain concerns.
Probably more than 60 per cent of the steels used in cars being announced today were not in existence five years ago, such has been the pace of change and development. In the Jaguar XF, 25 grades of high-strength and ultrahigh- strength steels are used. UHSS is used extensively to provide upper-body crash protection for the passenger cell and is also found in the sill area. Nissan, for example, uses 22 per cent UHSS in the Altima, again primarily for crash protection, with special attention being paid to side intrusion. Similarly, the recent Smart cars have used high-strength steel extensively in screen pillar area and doorframe reinforcement, as has the Alfa 159. At Honda, the new Acura MDX is reported to use in excess of 50 per cent high-strength steel grades in its body construction.
The new Laguna 111 from Renault is reportedly lighter than its predecessor and uses steels of up to 1,200MPa in the B-pillars and other safety-critical parts of the structure.
Driven by the demands of Euro NCap ratings, ultra-highstrength steels are increasingly used as reinforcing elements in the door and sill areas to provide the necessary resilience, even in small, mass-market vehicles like the new Fiat Punto. Audi are perhaps typical of new model introductions, using UHSS as reinforcement in the centre tunnel and inner sills as well as in the B-posts and front firewall. UHSS makes up 11 per cent of the weight of the Avant BIW, with 22 per cent AHSS, 29 per cent HSLA and the remaining 38 per cent being conventional mild steel.
Ultra-high-strength steels with 550-1400MPa and hotstamped steels up to 1,200MPa are offering designers other options. Importantly, steel is process-friendly and has excellent manufacturability. As Roy Platz of Ispat Inland points out, controlling the uniformity of mechanical processes is extremely important in developing vehicles where performance conforms to design intent. This is particularly relevant to issues such as crashworthiness.
Through processes such as continuous annealing, the uniformity of steel can be closely controlled and improvements in process monitoring have gone hand in hand with the developments in the new high-strength steel grades.
The performance advantage of high-strength steels can be taken either as a weight reduction, an increase in strength, or a combination of both. While passenger car utilisation of HSS grades has been driven by the issue of passenger protection, the drivers in the commercial vehicle segment have been rather different. With fuel cost pressures, payload efficiency has become of increasing concern and the ability to use HSS grades in chassis and structural components to safely increase vehicle payload capacity has been welcomed by manufacturers and users alike. Manufacturers have noted also the benefit of the new-generation steels in offering enhanced corrosion protection, making the best use of zinc and improving e-coat paint flow.
Dent resistance is another key issue being addressed by the new range of steel grades for exterior panel work.
Rephosphorised steels are characterised by their acquisition of a significant strength increase resulting from work hardening while being formed and shaped. Solid solutions such as phosphorous or silicon are added to increase initial strength, while the levels of additional strength through formation can be controlled using the chemistry, as this is determined by the amount of carbon remaining in the solution. The result is a steel grade offering auto designers and users real benefits. With low yield strength prior to forming and high in part strength after manufacturing, they are ideal for BIW application. When used in deep draw applications, they offer high dent resistance when formed and can assist in down-gauging as part of weight reduction.
High-strength steels are frequently used as part of the impact energy management strategy. The challenge is to ensure robust and consistent energy absorption characteristics when progressive folding gets harder to achieve in axial crushing, as material strength increases.
As a consequence, designers have to consider the inclusion of soft regions at natural folding distances, the global geometry of the structure and impact zone and the use of inserts and foams. Multi-phase steels offer high strength, but high stresses on thin gauge material pose challenges from a design perspective. The requirement is for the sheets to be loaded in skin section rather than plate section.
Loadings need to be carefully assessed and appropriate design choices made. In particular, designs using highstrength steel put increased emphasis upon measures to avoid local bending, while thinner gauges, which may be theoretically acceptable from a structural point of view, can have adverse noise, vibration and harshness (NVH) outcomes. Designers find geometrical aspects play an important part in establishing appropriate NVH characteristics, with increased beam wall height being an option, along with continuous welding or adhesive solutions, closed sections and the addition of specific geometrical stiffeners and triangulation.
Use of high-strength steels also puts greater demands on machine suppliers and tool maintenance programmes in production. High-quality tool steels are required when punching or shaping high-strength materials and the use of worn equipment can quickly erode quality. In welding with high-strength steels, increased electrode force and longer weld times are usually employed. When using them in BIW applications, it is best to operate with a developed blank, as close to the final shape as possible; using finish trim operations is essential. Other design procedures reflect the fact that high-strength steels tend to resist compression due to the hardness of the material, thus darts and notches are commonly used to maintain part configuration.
The production of the BIW is central to the core of carmaking and brand identity, so outsourcing is naturally limited, yet there is no doubting the close collaboration between steel suppliers, machine makers and the major manufacturers. Corus, for example, speaks of working from the earliest concept stages of vehicle and component projects, through to prototyping and final production, advising on material optimisation, component design and formability. Of course, the identification of appropriate methods of handling, joining and shaping high-strength materials is a continuing challenge and is key to the cost effective utilisation of these materials. SSAB Swedish Steel has developed high-strength steel specifically adapted for roll forming, which has an obvious outlet in the automotive sector. Their product – branded Docol Roll – minimises the risk of bending or twisting of the profile. Roll forming offers high productivity and low production costs, while producing parts with a high degree of consistency.
The ScaLight body concept, while using UHSS. also uses a variety of forming processes, including roll forming and hydroforming, along with innovative jointing technologies.
Produced in-house by Karmann, it offers OEMs the opportunity to create a range of low-volume derivatives.
A combination of steel grades within a single component or system is increasingly required either for engineering reasons – to ensure the deformation pattern that is required, for example – or on more mundane cost grounds. In such cases, designers can call upon sophisticated production and joining techniques from their steel suppliers. Tailored welded blanks allow designers to specify precisely where high-strength materials are required, allowing a range of material thicknesses in a single panel section. Volume production of non-linear Nd:YAG laser-welded blanks is well established, with steel suppliers continually adapting and refining materials and processes in light of their work with carmakers.
It would be wrong to think that the impact of high-strength steels is just confined to major components within the BIW. Speak to Tier Two suppliers and the impacts of new material development increasingly affect their practice.
Pressavon, a Tier Two manufacturer of the pressed metal bracketry and fasteners that are required to secure pressure fluid lines to the BIW, has been working with its customers to generate weight savings. “In the quest for weight reduction, even the most humble of components have come under scrutiny,” says Peter Rush, Marketing Manager. “So we have actively developed the use of high-strength alloys to provide a bracket in 2.0mm-thick Docol, which will more than adequately replace a 3.0mm mild steel version.”
BMW researchers have reiterated the complexity of the process, including the selection of appropriate materials in the configuration of an optimised lightweight body structure. Their solution for recent models has been an adventurous, but carefully integrated mix of materials. In future years it is likely that this path will be followed by other manufacturers, but there is little doubt that the emerging family of high-strength steels, in various forms, will have a crucial role within the mix.