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Advanced HSS are a class of steel used primarily in sheet form for automotive structures. The microstructures of these types of steels ae basically of multiphase with ferrite as the dominant phase; however, grades introduced recently have been fully martensitic based or austenite based.
The discussion introduces the requirement of an automotive body structure, then the different classes of AHSS hat have been used in the automotive industry and their typical characteristic tensile and formability properties. The specific properties that are required for automotive body structure that AHSS should serve are formability and good resistance to crash damage.
Being the successor of HSS, the AHSS has higher strength and formability balance. AHSS is preferred over other steels by automotive sectors for its higher strength, lighter weight and formability. Lighter weight increases the efficiency whereas formability helps in manufacturing and shock absorption during a crash.
Steel with tensile strength greater than 780 MPa are referred as Ultra High Strength steel. Steel with tensile strength greater than 1000 MPa are referred as Gigapascal Steel.
3 generations of AHSS are known till date.
1st generation of AHSS consists of ferritic base and the steels under this category are Dual Phase steel, Complex Phase steel, Transformation induced plasticity steel, Martensite Steel, etc. These steels had higher tensile strength but were not formable to the requirement.
This resulted in development of 2nd generation of AHSS which had the austenitic base. It consists of Twinning induced plasticity steel and Lighter weight induced plasticity steel. It shows the required properties of high tensile strength and formability but has limited use due to its higher cost.
The cost is high due to use of stabilizers which stabilizes the austenite phase.
So as to overcome the problem of having the optimum strength to elongation balance along with lower cost, 3rd generation AHSS was developed and is still under research. It has a higher strength phase with a ductile austenitic phase. The desired property for 3rd generation AHSS is to have a tensile strength of around 1200 MPa with an elongation of around 30%.
Steel being an alloy of carbon and iron serves many purposes such as for construction, automation, industries, tools, weaponry and daily use products. Most of the structures and equipment are made of steels as it provides better strength and machinability.
But among all the steels that are present, the steel which was used by the automotive sector was the high strength steel (HSS). This steel was considered for production of automotive due its strength and ability to survive crash efficiently. Still formability was not enough for proper manufacturing of the vehicles.
Hence a new steel called Advanced High Strength Steel (AHSS) was developed which led to an era of lighter automotive. This material provides a balance between strength to elongation to weight which was way better the characteristics of HSS. Development and Research is still going on the AHSS as it is believed as the future of the automotive sector.
What made us to search for a new material like AHSS?
Several factors drive material selection for automotive applications, including safety, fuel
efficiency, environmentalism, manufacturability, durability, and quality. Manufacturability which includes body structure which in turn requires balanced relationship between tensile strength and formability, which was not found in earlier alloys of iron. Thus, in order to increase performance and fuel efficiency weight of the body has to be reduced. Thus, the AHSS material serves us a better than any other.
1st generation AHSS:
These newer steels have enhanced strength and formability achieved through the development of more complex microstructures through controlled cooling processes. Basically 1st generation AHSS are martensite – ferrite-based materials.
Dual phase (DP):
The microstructure of DP steel consists of a soft ferrite matrix and discreet hard martensitic particles. The combination of hard and soft phases results in an excellent strength-ductility
balance along with increase in strength with increasing amount of martensite.
If the combination i.e. martensite – ferrite is hot-rolled and cooling is done carefully in a controlled manner to produce the ferritic-martensitic structure from austenite. If continuously annealed or hot-dipped, the final structure is produced from a dual phase ferritic-austenitic structure that is rapidly cooled to transform some of the austenite to martensite.
The soft ferrite in the final DP material is exceptionally ductile and absorbs strain around the martensitic particles, enabling it to have uniform elongation.
DP steels can absorb a lot of strain energy.
FB (Ferrite – Bainite) Steel:
FB steel is also DP, with soft ferrite and hard bainite. FB steel performs well under dynamic loading conditions, making it well suited to carry vibration loads. Often
cold-drawn FB is used for profiles, mechanical parts, cross beams and reinforcements, and wheels. FB is also recommended for suspension and chassis. Because FB
has good fatigue properties in dynamic load conditions, it is an outstanding candidate for shock towers and control arms.
In MS steels, nearly all austenite is converted to martensite. The resulting martensitic matrix contains a small amount of very fine ferrite or bainite phases. This structure is typically formed during a slight quench following hot-rolling, annealing. Increasing the carbon content increases strength and hardness. MS has relatively low elongation, but post-quench tempering can improve ductility, allowing for adequate formability considering its
extreme strength Because MS steel has such high strength to weight ratio, it is weight and cost effective. It is often selected for body structures, ancillary parts, and tubular
structures. MS grades are recommended for bumper reinforcement and door
intrusion beams, rocker panel inners and reinforcements, side sill and belt line reinforcements, springs, and clips
TRIP (Transformation-Induced Plasticity) Steel:
As the name itself suggests a material having certain combination transforms itself due to the induced force and remains plastic afterwards.
As the material have multi-phase microstructure with a soft ferrite matrix embedded with hard phases. TRIP has a high carbon content to stabilize the meta-stable
austenite below ambient temperatures. Silicon or aluminium are often included to
accelerate the ferrite/bainite formation.
Behavior of trip steel:
TRIP steel received its name for its unique behaviour during plastic strain: in addition to the dispersal of hard phases, the austenite transforms to martensite. This transformation allows the high hardening rate to endure at very high strain levels, hence “Transformation-Induced Plasticity.”
With less stability, the transformation begins almost as soon as deformation transpires. With more stability, the austenitic transformation to martensite is delayed until higher levels of strain are reached, typically beyond those of the forming process. In highly stabilized TRIP steel automotive parts, this delay can allow austenite to remain until a crash event transforms it to martensite.
HF steel is typically boron-based, containing 0.002-0.005 percent boron, and may even be called “boron steel.” The processes used to produce HF bestow a unique combination of properties. “Direct hot-forming” may be used to deform the blank in the austenitic state (at high temperatures).
In direct hot-forming, the boron-based steel is blanked at room temperature and then heated to high enough temperature for austenisation. The steel is then formed while hot and quenched in the forming tool, developing the martensitic microstructure.
Parts made from HF steel benefit from several material advantages, including high strength and improved (reduced) spring back. The part remains in the die through the cooling phase, and so spring back is virtually not observed Repairability is limited, however, because HF steel becomes brittle through the work hardening of a crash event; the heat required to straighten the damage degrades the strength of the part.
2nd generation AHSS:
TWIP (Twinning induced plasticity) Steel:
TWIP Steel is an austenitic based steel which is completely different from the conventional and 1st generation HSS in case of strength and formability. It contains manganese (around 17-24%) which enables it to exist at room temperature. It also shows good weldability.
TWIP steel received its name for its particular mode of deformation i.e., Twinning. In this deformation slip causes the formation of symmetric twin boundaries. These boundaries act more like grain boundaries as it restricts the movement of dislocation through the material. They also strengthen TWIP steel and increases the work hardening rate. Though it has a good strength to elongation balance still its use is limited due to its higher cost and affinity o corrosion.
3rd generation AHSS:
TBF (bainite +ferrite + retained austenite)
Q&P (Tempered martensite + lower bainite + retained austenite)
This process exploits novel martensitic steel containing retained austenite, based on the fact that carbon can diffuse from superheated martensite into neighbouring untransformed austenite and stabilize it at random temperature.
The steel is first treated by an initial partial or full austenization and then followed by an interrupted quench to a temperature between the martensitic start (Ms) and martensitic finish temperature (Mf) resulting in untransformed retained austenite.
Following to this annealing is done or so called partitioning treatment either at / above the initial quench temp with enhanced silicon alloying suppressing the cementite precipitation, it is anticipated that retained austenite will be enriched with carbon expected to escape from supersaturated martensite phase in which it has very low solid solubility.
The treatment should then produce fine circular aggregate structure of carbon depleted and potentially carbide free martensite lathe interwoven with retained austenite stabilized by carbon enrichment.
As a result with a composition of 0.2%C, 1-1.5%Al and 1-1.5% Mn, Q&P steel shows an ultra-high strength of 1000 – 1400 MPA with adequate ductility of 10 – 20%.
This uses the TRIP effect for the formation of TBF steel.
TBF steels can be produced b heat treatment and multistep thermal processing austenising (or annealing in the gamma region) and following to this isothermal transformation (or austempering) at temp between bainite start (Bs) and the martensite start (Ms) temperature of steels.
During isothermal transformation, the transformation of austenite to bainite ferrite without any carbide and the carbon enriched austenite is retained with a small amount of martensite.
As the isothermal transformation is employed at temperatures between the Ms and Mf after austenization some of austenite first transforms to martensite and then most of the remaining austenite transforms to bainite ferrite because of the lowered Ms temp due to carbon partitioning in austenite.
The retained austenite fraction increases with increasing isothermal transformation temp is 0.2 %C – 1.5% Si – 1.5%Mn TBF steel.
AHSS being an ideal material for automotive sector requires more research to get better crash performance & increased vehicle efficiency. Current research aims to continue to expand the broad spectrum of HSS & AHSS.
One area of particular interest is the “third generation steels” which can help to cover the gap between strength – elongation of the material.
Another research area that draws the attention of researchers the most is the thermal process that takes place inside the material while quenching or annealing of the steel.
Along with the microstructural study of the materials, study of thermodynamics of materials can also serve of better advancements towards development of newer and better microstructures.
Study of AHSS continues o grow and evolve. Many grads and types of AHSS has been studied and developed to meet the unique and varied performance requirements of the vehicle. Each area of AHSS has been studied and these steels are improving the strength and safety of cars on the roads today.
It also offers flexibility to automotive engineers to design new components along with light weight solutions. Continuous study of 1st,2nd and 3rd generations tell us why Research and Development is still required in this area.
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