About Advanced High Strenght-Steels (AHSS)

AHSS is a high-strength steel family that comprises a wide range of complex multiphase microstructures, formed by ferrite, bainite, martensite and retained austenite in different proportions. Such complex microstructures confer them an outstanding combination of mechanical properties and formability, compared to the conventional mild steels, which have a ferritic microstructure.


AHSS are characterised by showing very high strength, relatively good formability, and excellent crash performance, which make them especially suitable for structural and safety-related auto body components. The higher strength and enhanced impact performance compared to conventional steels allow for weight savings in structural parts through sheet downgauging and, at the same time, improving passenger safety.  

Strength ductility diagram for various types type of steels, including conventional steels and AHSS grades.


The great demands of the automotive industry have multiplied the research efforts towards the development of novel AHSS concepts in order to meet the increasingly exigent performance targets.

As a result, a large number of AHSS grades with complex and unique microstructures, carefully adjusted by controlling the chemical compositions and the thermomechanical processing routes, have been developed in the last two decades.

1st generation of AHSS

Dual-Phase (DP), Complex Phase (CP), Martensitic (MS), Press-hardened (PHS) or Hot formed (HF), and Transformation-Induced Plasticity (TRIP) steels. This generation is characterised by showing better formability than single-phase high strength low alloyed (HSLA) steels of similar strength.

2nd generation of AHSS

Includes Twinning-Induced Plasticity (TWIP) and austenitic stainless steels. These steels present very high strength and extremely high ductility when compared to the 1st generation of AHSS. However, their production complexity and elevated costs, together with other problems of delayed cracking and poor weldability, have limited their application

3rd generation

Developed to cover the gap between the 1st and the 2nd generation of AHSS, the 3rd generation presents superior strength and enhanced formability than 1st generation AHSS but at significantly lower production costs.

Such AHSS family comprises steels with ultrafine microstructural constituents, such as martensite or bainite, produced in non-equilibrium conditions, in combination with retained austenite (RA). Bainite and martensite contribute to increasing the strength, whereas the stress-induced transformation of RA (TRIP effect) contributes to further optimize ductility and strength. Some of the steels developed under this classification are TBF (TRIP-aided bainitic ferritic) and Q&P (quenching and partitioning) steels. Other 3rd generation TRIP-assisted steels, such as medium-Mn or δ-TRIP steels, and nanoprecipitation steels are currently under development.

Problems of AHSS sheet metal

However, new solutions bring new challenges, and not all are advantages. The fact is that working with this type of steel implies some changes in part design and in manufacturing processes. The high strength of AHSS, together with their complex damage and fracture mechanisms and higher cracking susceptibility, are the main source for most frequent manufacturing problems.


Premature tool wear and failure due to the high forces required for forming.

Difficult prediction of the part final dimensions, because of the high and unpredictable springback

Not homogeneous sheet thinning, which can lead to the fracture of the part in subsequent forming operations

Localised microcracking, as edge cracking, due to lack of local ductility for very high strength grades

The need for accurate characterisation

Overcoming such problems require a profound knowledge of the material properties and behaviour during forming. Unfortunately, conventional characterisation and modelling procedures do not allow a full description of AHSS behaviour and manufacturability.

Global vs local formability

For AHSS, the concept of formability is more complex than in conventional mild steels and it is divided into two categories: global and local formability. Global formability refers to the material’s resistance against the formation of localized necking in deformation modes where relatively large regions of material are deformed simultaneously (stretch forming, drawing, etc.). It is typically evaluated by means of Forming Limit Curves (FLCs) according to ISO 12004. On the other hand, local formability is more related to the fracture resistance of the material when the deformation is applied in a localized zone (tight-radius bending, stretch flanging, hole expansion, etc.).

Edge cracking and crack formation under impact loading are two of the main cracking problems related to the local formability of AHSS. This kind of fractures cannot be described by conventional fracture and ductility criteria based on FLCs or elongation values from uniaxial tensile tests. Therefore, aimed at improving material selection and optimising new material design, there is an increasing need for the identification of the material properties that best describe these crack-related failures.

Fracture toughness has shown to be a relevant material property to understand the cracking behaviour of AHSS sheets. It has been applied to select the most appropriate material in sheet metal forming and to rank their fracture and crash performance.

Thus, fracture toughness evaluation of AHSS sheets has become relevant for steelmakers, part producers and carmakers to enhance material selection and optimize the microstructure of future AHSS grades.

examples of crack-related fractures
Examples of crack-related fractures in high strength steel sheets