High-strength steels: the bruisers among steel products

High-strength steels enable the thickness of the material to be reduced without sacrificing the structural strength characteristics, and with improved crash test results.

The strength of a material is often characterised by its yield strength, i.e. the minimum force that must be applied to the material in order to produce a permanent (plastic) deformation or by its drawing load, i.e. the minimum force that must be applied in order to have necking involved in the permanent deformation.

“High-strength steel” or HSS, is steel with a yield strength (YS) of 220 Mpa (1 Mpa or megapascal is equal to 1 million N/m˛). “Ultra-high-strength steels” or UHSS possess a tensile strength (TS) of 550 Mpa or higher.

A major metallurgical challenge is to produce steel grades with a high YS but that also remain sufficiently deformable (deep-drawable). If lighter car bodies, for example, are to be manufactured from steel, the steel strip must also exhibit perfect deforming behaviour during pressing, whilst at the same time possessing a sufficiently high YS value in order for a thinner (and therefore lighter) strip to have the necessary strength, for example to ensure the safety of passengers.

Lighter car bodies obviously result in lower fuel consumption, and therefore also in lower exhaust emissions. Each reduction by 100 kg in vehicle weight leads to a drop in fuel consumption by 0.7 l per 100 km.

Steel is in this respect an ideal material, since it can combine high strength with excellent deformability much better than aluminium cars, for example. Car manufacturers seek to achieve optimum safety by manufacturing cars with an undeformable passenger compartment and a deformable, energy-absorbing engine compartment.

The strength and deformability of steel are determined chiefly by its composition (addition of alloying elements, reduction of the sulphur and/or carbon content) and by thermal treatment of the steel during the rolling, annealing and coating processes, or even during the final processing of the strip (take for example the “bake hardening” principle. This means that steel gets stronger during the baking of the paint layer on the car body).

Typical examples of high-strength steels are:

  • Microalloyed steels (alloyed with Nb and/or Ti) for suspension components, longitudinal runners on heavy goods vehicles, car passenger cells and reinforcement elements on cars.
  • Ferritic-bainitic steels (with very low sulphur content) for wheel rims and wheel disks.
  • Dual-phase hot rolled steels (with added chromium and very low sulphur content) for wheel disks.
  • Dual-phase cold rolled and coated steels for exposed components and components for increasing the crash resistance of car bodies.
  • TRIP (TRansformation Induced Plasticity) steels for wheel rims and wheel disks.
  • Degassed high-strength steel for exposed components of the car bodywork (generally hot dip galvanised strip).
  • “Bake hardening” steel for exposed components of the car bodywork (controlled annealing cycle and skinpass rolling: both electrolytically galvanised and hot dip galvanised strip).

Powered by aware