The first generation of the AHSS family includes dual phase (DP), complex-phase (CP), martensitic (MS) and regular transformation-induced plasticity (TRIP).
This first generation has more formability than HSLA at the same strength level. This is due to its multiphase microstructure, which contains ferritic and martensitic phases for a balance between formability and strength. The unique microstructure is created by special heat treatments.
The second generation of AHSS includes a new generation of transformation-induced plasticity (TRIP), hot-formed (HF) and twinning-induced plasticity (TWIP) steels. Both the first and second generation of AHSS are designed to meet the functional performance demands of certain parts in the automotive industry.
The first generation has very limited formability. The formability of the second generation is significantly higher than the first, however, because of the high cost of alloying elements, they are very expensive. Therefore, the third generation of AHSS is currently under development. These steels are aimed to have improved strength-ductility ratios and are expected to achieve over 35 per cent in structural weight reduction.
Since the metallurgy of AHSS grades is still a new field compared to conventional HSS, only two types of the third generation AHSS are currently in production.
The significance of AHSS
AHSS are now used for nearly every new vehicle design. They are predicted to replace approximately 60 per cent of currently used conventional HSS.
Since AHSS can be manufactured at very thin gauges and maintain the same strength as mild steels, designers can easily replace conventional steels with AHSS. This is not the case when replacing steel with other lightweight materials, such as aluminium or fibre-reinforced composite materials. These non-ferrous materials are expensive, incompatible with existing manufacturing processes and have higher production and manufacturing costs.
Cost/benefit analysis shows that steel parts are stronger and cheaper than other lightweight materials. The most popular lightweight material in competition with steel is aluminium. Although aluminium used in the automotive industry has been rising modestly, body structures fabricated with aluminium cost 60 per cent to 80 per cent more than steel.
What does the future hold for AHSS?
New grades of AHSS are making vehicle body structures lighter by 25 to 39 per cent compared to conventional steels. When applied to a typical five-passenger family vehicle, the total weight of the vehicle can be reduced by 170 to 270 kg. Vehicle weight reduction is also a crucial factor for fuel efficiency. Using a lower amount of steel per vehicle reduces both material and fuel costs, thus profiting the environment.
Simply put, if automakers were to use steel with a tensile strength of 1000 MPa instead of 500 MPa, steel consumption would be reduced by half.