One of the biggest focusses in the automotive industry today is the reduction of weight. Here, Nina Samodajev, process engineer at Siemens Gamesa, writing on behalf of Matmatch, examines the growing use of advanced high strength steel (AHSS) to reduce vehicle mass.
Since the 1920s, steel has been the material of choice for automakers worldwide. Today, steel makes up around 65 per cent of an average automobile’s weight and is the backbone of the entire vehicle. On average, that is 900 kg per vehicle.
In order to enhance passenger safety and vehicle performance, reducing the weight of vehicles has become the top priority for the automotive industry today. AHSS is the fastest growing material in today’s automotive industry and the key material when it comes to vehicle mass reduction.
In general, AHSS are steels with yield strengths higher than 550 megapascals (MPa). They offer uniquely low weight, high strength and optimised formability, which allows automakers to use less material, greatly reducing a vehicle’s weight.
The properties of steel have significantly improved over the last century, from mild steel in the early 1900s to high strength low alloys (HSLA) in late 1970s and the introduction of the first generations of AHSS in the 1990s.
In the last two decades, the steel industry has developed different alloying and processing combinations to produce steel microstructures providing higher strength for reduced steel section size and weight.
To create the combination of high tensile strength and ductility, carefully selected chemical compositions and the multiphase microstructures of AHSS are designed to help the automotive industry meet low weight requirements. AHSS are not significantly lighter than traditional steels, but their strength allows automakers to manufacture very thin gauges, thus reducing the weight of the vehicles.
Mild or low carbon steels are steels with a tensile strength of 400 MPa and carbon content of 0.05 to 0.25 per cent. The microstructure of mild steel causes it to be relatively ductile and easy to form, being comprised of one phase, normally ferrite. Mild steels are commonly used in the body structure of vehicles.
How strong is high strength steel?
HSLA steels were the first commonly used high strength steels in the automotive industry. These steels have higher tensile strengths of up to 800 MPa. They are not made to meet a specific chemical composition but rather specific mechanical properties. They have a low alloying and carbon content to retain formability and weldability, with copper, titanium, vanadium and niobium added for strengthening purposes. HSS steels have been used in the areas of vehicles where energy absorption is important, such as crumple zones.
The principal difference between conventional HSS and AHSS is in their microstructure. AHSS are multiphase steels with complex microstructures that contain phases such as ferrite, martensite, bainite and austenite.
Part 2 is here.