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Next Generation of Automotive Steels helping Lightweighting

The automotive industry has been using steel as a de facto material since the very first commercialized car designed by Karl Benz in 1885. The car had tubings made of steel while the driver compartment was created with wood panels. Since then the usage of steels has increased exponentially in automotive. Currently the automotive industry accounts for more than 12% of the global steel production and is a major customer for the various grades of alloy steels produced specifically for automotive usage. The reason for the high usage of steels across automobiles lies in the mechanical properties of the material. Steel not only offers designers a very good option for material strength and torsion rigidity, as required for body strength and crash worthiness, but arguably also provides the cheapest material option. Thus, the safety and design requirements are met at lower costs vis-à-vis other material options with steels.

Steels though, have been traditionally heavier than their other material counterparts, like aluminum, magnesium, plastics and carbon fibers. This resulted in the total vehicle weight being on the higher side if built mainly with steel components as opposed to alternate materials. While this did put pressure on fuel economy, it has not been an urgent issue for automotive OEMs until the last decade or so. However, recently there has been stricter focus on controlling vehicular emissions and improving fuel economy because of the increasing global pollution levels. Governments across the globe have set stricter vehicular emission and fuel economy targets to be met by all OEMs. Reducing vehicle weight or light weighting of the component systems has thus been one of the major enablers for the automotive companies to meet these stringent goals. This is where steel has been losing to its lighter counterparts. Aluminum has been increasingly taking the place of steel over the last 15 years, and at this rate is expected to increase its penetration even further by 2025. Magnesium has started to find automotive application and has overcome the difficulties in its manufacturability. Carbon fiber is also finding applications in luxury and premium segment vehicles, owing to its exceptional light weight and material characteristics. All this results in decreasing share for steel in automotive business for steels, as it had been inherently branded as the “heavier” material option.

However, replacing steel with other alternative materials is not the inherently easier option for OEMs and vendors alike. While cost is probably the foremost advantage that steel offers, there are also other factors which can help steel retain its share, both now and in future, if it can sort out the weight dynamics. Changing the material from steel to any other alternative requires automotive players to not only spend additionally on their research and design but also affects their supply chain and sourcing strategy. In today’s times of design somewhere, source anywhere and manufacture elsewhere, any change of material introduces an unnecessary complexity in the entire supply chain systems which OEMs try to avoid. The global platform designs also need to be homogeneous, while maintaining materials that are available easily around the manufacturing locations to manage lower costs of production. Quite clearly, it is difficult to replace steel totally from cars but at the same time it needs to improve itself on the cost to weight advantages scale to stay competitive. While steel has been seemingly trailing behind for some years, recently the steel manufacturing companies and vendor companies have responded strongly with advanced steels that are lighter and stronger as well as process innovations that have helped cut weight from steel structures in a car.

Over the years, steels have improved their yield strength, while improving upon their elongation percent to allow for higher toughness as well as strength. This has led to the development of various grades of high strength steels by careful combination of alloying elements and controlling the microstructure. As a result, they are stronger and can provide the same strength and torsional rigidity in a lighter structure. This allows for significant weight savings. High strength steels (HSS) have their yield strength typically in the range of 210-550 MPa, while the Ultra High Strength Steels (UHSS) have yield strengths more than 550 MPa. While these two grades have been used in automobiles over the years, the development of Advanced High Strength Steels (AHSS) has enabled the automotive industry to reap the benefits of lower cost steel options with reduced weight and equivalent strength. The yield strength of AHSS typically overlaps those of HSS and UHSS and is a result of a carefully controlled multi-phase microstructure attained through innovative heat treatment processes that allow for strain hardening. Thus, the mechanical properties of these steels are equivalent or better than their normal steel counterparts, allowing for even lighter components that help knock down vehicle weight. Add to this the cost and availability advantage, and it is very easy to understand why most OEMs and vendors have started to move to increased usage of AHSS in their portfolio.

Steel mills and design groups across the globe have been working on various modes of elemental and impurity control to attain varied mechanical properties in AHSS. Some of these methods include adding micro-alloying elements like vanadium in order to control the high temperature hardness and fatigue, while other methods include introduction of nickel and aluminum in the steel to allow for dispersed clusters of aluminum, helping steel to be less dense and brittle while retaining its strength. The AHSS family of microstructure includes, Martensitic Phase, Dual Phase, Transformation-induced Plasticity(TRIP), Twinning Induced Plasticity (TWIP), Complex-Phase (CP), Hot-formed(HF) and Ferritic-Bainitic(FB) Phases. Each of them has unique functional characteristics for usage in car components. The DP and TRIP phase steels are found to have high energy absorption and are therefore suitable for crash resistant zones of the car, whereas the Martensitic Phase AHSS is suitable for structural elements of the passenger cell because of their higher strength.

Nano steels are another generation of steel alternative comprises that improve the mechanical characteristics of the material by effecting change in the grain matrix and crystal sizes at a nano structure level. The resultant material is claimed to be 30% lighter than conventional steel and offers enormous light weighting opportunity. The company NanoSteel, which makes these special grades of steel, is also providing metal powder options of the same suitable for additive manufacturing which is another top-up to reduce weight of the components. The high formability of all the advanced grades of steel as mentioned above allows for innovative product design which helps to cut down some of the weight, even at the design stage through integration or design change. Additionally, there are other types of steels like Press Hardened Steels (PHS) and Bake Hardened Steels (BHS) that enable a reduction in weight through process oriented changes. PHS or Hot Stamped Steels are obtained by heating boron steels to high temperature and pressing/stamping them while they are still hot, in order to get a 100% martensitic structure which has optimum hardenability. PHS provide equivalent strength in considerable thinner cross sections and allow for complex shapes to be made easily. PHS are therefore gaining penetration majorly in the Body-In-White (BIW) component areas helping reduce weight of the frame. BHS on the other hand are used primarily for car bodies as they have high formability, improved dent resistance and yield strength while offering substantial weight reduction through thinner and lighter panels. Steels in the bake hardening range are generally designed for moving and visible (doors, hood, tailgates, roof etc) and structural (underbody, reinforcement, cross member) parts in the automotive. It takes advantage of carbon strain aging through controlled annealing processes of the ultra-low carbon and vacuum degassed steel.

All these advancements and innovations around steels have led to the resurgence of material usage across OEMs. Automotive companies have started to move to a multi-material approach of light weighting, primarily for their BIW and chassis components, from the earlier uni-material approach with aluminum. Automotive majors like BMW, Audi and Ford have recently introduced multi-material BIW and Monocoque to take best advantage of the cost and weight reductions that the new generations of steel are offering. Mass OEMs on the other hand are now uniquely placed to reduce further weight from their mid and entry segment vehicle by utilizing these lighter options of their base steel, without the need to migrate to costlier options like magnesium, carbon fiber and others. In fact, the steel industry in its on-going research for Future Steel Vehicles (FSV) has recently claimed over 35% weight savings with such AHSS and other advance steel grades, as compared to baseline steels. The optimized FSV body structure, made from steel, is claimed to weigh a meagre 177 kgs, thus putting steel totally at par with aluminum body designs and arguably at a lower total cost. Techniques such as hydrogen quenching and three-dimensional roll forming stand poised to disrupt the current balance of cost per pound of lightweighting activities. Some steel providers are even claiming that 2000 MPa steel is around the corner.

Lifecycle assessments, which include the production carbon footprint, can heavily favor higher strength steel use instead of premium materials. There are easements to this rule such as production plants which use entirely renewable energy. High strength steel use in BIW structures of one of the mass OEMs has already reached a penetration rate of 80%. Also, some premium segment vehicles with aluminum intensive structures have begun reducing their aluminum content. The battle between advance steels and other lighter material alternatives is just warming up, and the steel industry as well as research firms look set to soon regain lost turf, while aiming to increase penetration of these lighter steels in automotive. But perhaps the biggest winner will be the final customer who may get stiffer and safer cars at affordable value.

 

Suman Saurabh