Advanced High
Strength Steels (AHSS) are designed to provide an efficient answer to the actual
demands from the automotive industry, with more and more strict safety
regulations, constant weight reduction demands, CO2 emission
restrictions and good forming properties, all of them within a reasonable cost.
They are complex and sophisticated materials with a carefully selected chemical
composition and multiphase structures as a result of precise and controlled heating
and cooling processes. The hardening mechanisms involved for reaching the range
of properties such as strength, ductility, fracture toughness and fatigue are
extraordinarily diverse.
Dual phase steels
(DP), with a ferritic-martensitic structure, Tranformation-Induced Plasticity
steels (TRIP) and the Martensitic Steels (MS), belong to the second phase from
the 1st Generation of AHSS steels. Twinning Induced Plasticity steels
have been developed more recently and they are part of the 2nd
Generation of AHSS. They are materials with a more complex chemistry, with high
manganese and high silicon, along with exceptional mechanical properties that
are above the TRIP steels (Figure 1).
Figure 1. Tensile strength versus strain
for different stamping steels.
TWIP steels
have an austenitic structure at room temperature and therefore they require a
high manganese content (17-30%) within their chemical composition. This implies
certain specific challenges in the melting and secondary metallurgy processes.
During the forming
operations of these materials the microstructure suffers a change in the crystalline
orientation inside the grain, which is known as twinning (Figure 2).
Essentially,
twin boundaries must be considered true grain boundaries and the final result
is a steel with extraordinary forming properties and very high strength values
(Figure 3) along with a significant increase of the strain hardening exponent
(n value) (Figure 4).
Figure 2. Twins in
austenitic grains of a TWIP steel.
Figure 3. Stress-strain curves of
different AHSS.
Figure 4. Work hardening
versus true strain of different AHSS
According to
the information provided by these steel manufacturers it is possible to reach
up to 35% deformation with a tensile strength of 1400 MPa, as well as an
exceptional capacity to absorb impact energy. An example of its deformation limits
can be observed in the Nakajima forming test that compares this material with
an Interstitial Free steel (IF) (Figure 5).
Figure 5. Nakajima forming test.
On the other hand TWIP steels have a low yield strength in annealed
conditions (@250 MPa) and the high strength values are only reached after a
remarkable deformation of the material. An open working line to overcome this challenge
is the cold rolling of the material and/or some micro-alloying elements to
achieve precipitation hardening. In this regard, vanadium, niobium and titanium
are elements which are being analyzed.
One aspect to
consider in the TWIP steels is related to the precipitation of cementite in the
grain boundaries during annealing, that might result in a steel with hydrogen
embrittlement and delayed cracking. In this particular case, recent
developments propose the addition of aluminum between 1.5% and 2%, that avoids
the precipitation of cementite during cooling after the hot rolling and
annealing, due to a decrease of the reactivity and the diffusivity of the carbon
in the austenite.
These advanced
and innovative materials have a set of properties such as high strength,
excellent ductility values, good impact resistance capabilities, along with a
reasonable cost that makes them very interesting, enabling more energy-efficient
designs and allowing considerable weight reductions with the corresponding compliance
of the safety requirements.
One of the
main challenges in the use of these materials relies in the metallurgical
knowledge of the relationship between their mechanical properties and micro-structural
characteristics. Without any doubt this understanding will push the
introduction of TWIP steels in the automotive industry and in other sectors
such as railway, shipbuilding, piping and any other additional specific
applications that do not require magnetic materials.
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