For a long time now, steel has been the preferred material for automakers worldwide. Using steel has let automobile manufacturers accomplish sought after standards of strength and safety for their vehicles at relatively low costs vis-a-vis other materials. According to the International Organization of Motor Vehicle Manufacturers, 97.3 million vehicles were produced in 2017, a 2.36% increase compared to 2016. And on average, 900 kg of steel is used per vehicle.
Steel is the backbone of the entire vehicle. In cars, these days, steel contributes to 65% of the weight and plays a huge role in the vehicles we see today. It protects the occupants of a car, reacts to road loads, provides comfort, and makes attachment points available to other components of the vehicle. Automobile manufacturers utilise steel for automotives because it is the toughest and most reasonable material accessible today for the application and can be engineered in a lot of diverse ways to fulfil the requirements of crash safety and the performance of the vehicle.
Steel distribution in vehicles
The steel in a vehicle is distributed as follows:
34% is used in the body structure, panels, doors and trunk closures for high-strength and energy absorption in case of a crash
23% is in the drive train, consisting of cast iron for the engine block and machinable carbon steel for the wear resistant gears
12% is in the suspension, using rolled high-strength steel strip
The remainder is found in the wheels, tyres, fuel tank, steering and breaking systems
The role of AHSS
Advanced high-strength steels (AHSS) are now used for nearly every new vehicle design. AHSS make up as much as 60% of today’s vehicle body structures making lighter, optimised vehicle designs that enhance safety and improve fuel efficiency.
With escalating concerns about human-induced greenhouse gases, global legislators have passed more stringent vehicle emission regulations through 2020, while considering further, aggressive targets for the next ten years. Automakers are searching for new materials and engineering capabilities to meet requirements that often conflict. As an example, structural applications require materials characterised by high strength and stiffness, often achieved with greater thickness. But fuel economy and emissions are positively impacted when component thickness is reduced.
New vehicle designs with complex geometries are aesthetically pleasing, but difficult to form and join, compromised further by thickness reduction to achieve mass reduction targets. The global steel industry continues to develop new grades of steel, defined by ever-increasing strength and formability capabilities, continually reinventing this diverse material to address these opposing demands. These Advanced High-Strength Steels are characterised by unique microstructures and metallurgical properties that allow OEMs to meet the diverse functional requirements of today’s vehicles.
The AHSS family
Besides, AHSS are complex, sophisticated materials, with carefully selected chemical compositions and multiphase microstructures, resulting from precisely controlled heating and cooling processes. Various strengthening mechanisms are employed to achieve a range of strength, ductility, toughness, and fatigue properties. The AHSS family includes Dual Phase (DP), Complex-Phase (CP), Ferritic-Bainitic (FB), Martensitic (MS or MART), Transformation-Induced Plasticity (TRIP), Hot-Formed (HF), and Twinning-Induced Plasticity (TWIP). These first and second generation AHSS grades are uniquely qualified to meet the functional performance demands of certain parts.
For example, DP and TRIP steels are excellent in the crash zones of the car for their high energy absorption. For structural elements of the passenger compartment, extremely highstrength steels, such as, Martensitic and boron-based Press Hardened Steels (PHS) result in improved safety performance. Recently, there has been an increased funding and research for the development of the “3rd Generation” of AHSS. These are steels with special alloying and thermo-mechanical processing to achieve improved strength-ductility combinations compared to present grades, with potential for more efficient joining capabilities at lower costs. The broad range of properties is best illustrated by the famous Steel Strength Ductility Diagram, captured in the figure.
AHSS grades contain significant alloying and two or more phases. The multiple phases provide increased strength and ductility not attainable with single phase steels, such as, high strength, low alloy (HSLA) grades. HSLA materials achieve their strength through alloying and solid solution hardening, whereas AHSS are produced by using specific alloys and precise thermomechanical processing.
Reducing vehicle weight
In the past, steels with tensile strength (UTS) levels in excess of 550 MPa were generally categorised as AHSS, and the name “ultra high-strength steels” was reserved for tensile strengths exceeding 780 MPa. However, today, there are multiple phase AHSS with tensile strengths as low as 440 MPa. So, using strength as the threshold for whether a steel qualifies as AHSS is no longer suitable. AHSS, with tensile strengths of at least 1000 MPa, is often called ‘GigaPascal steel’ (1000 MPa = 1GPa). Third Generation AHSS seeks to offer comparable or improved capabilities at a significantly lower cost.
New grades of Advanced High-Strength Steels enable car-makers to reduce vehicle weight by 25-39% compared to conventional steel. When applied to a typical five-passenger family car, the overall weight of the vehicle is reduced by 170 to 270 kg, which corresponds to a lifetime saving of 3 to 4.5 tonnes of greenhouse gases over the vehicle’s total life cycle. This saving in emissions represents more than the total amount of CO2 emitted during the production of all the steel in the vehicle.
WorldAutoSteel, worldsteel’s automotive group, completed a three-year programme in 2013 that delivers fully engineered, steel intensive designs for electric vehicles. Known as the FutureSteelVehicle (FSV), the project features steel body structure designs that reduce the mass of the bodyin - weight to 188 kg and reduce total life cycle greenhouse gas (GHG) emissions by almost 70%. The FSV study commenced in 2007 and concentrates on solutions for cars that will be produced in 2015-2020. Today, we see the material portfolio developed through the FSV programme progressively being introduced into new products.
The global transportation industry is a significant contributor to greenhouse gas emissions and accounts for about 24% of all man-made CO2 emissions (International Energy Agency, CO2 Emissions from Fuel Combustion Highlights, 2018 Edition, p 13). Regulators are addressing this challenge by setting progressive limits on automotive emissions, fuel economy standards or a combination of both. Many of the existing regulations began as metrics to reduce oil consumption and focused on extending the number of kilometres/litres (miles/gallon) a vehicle could travel. This approach has been extended into the regulations, which now limit GHG emissions from vehicles.
Extending the fuel economy metric to meet objectives to reduce emission has unintended consequences. Low - density alternative materials are being used to reduce vehicle mass. These materials may achieve lighter overall vehicle weights, with corresponding reductions in fuel consumption and use phase emissions. However, the production of these low-density materials is typically more energy and GHG intensive, and emissions during vehicle production are likely to increase significantly. These materials are often not able to be recycled and need to be sent to landfill. Numerous life cycle assessment (LCA) studies show how this can lead to higher emissions over the entire life cycle of the vehicle as well as increased production costs.
A key factor in understanding the real environmental impact of a material is its LCA. An LCA of a product looks at resources, energy and emissions from the raw material extraction phase to its end-of-life phase, including use, recycling and disposal.
the best out of the power of steels used for parts, while augmenting safety and bringing down weight. The importance of steel stands by the fact that automakers continue to develop materials that will live up to forthcoming requirements.