Since the 1920s, steel has been the material of choice for automakers worldwide. Today, steel makes up around 65 % of an average automobile’s weight and is the backbone of the entire vehicle. On average, that is 900 kg of steel used per vehicle.
In order to enhance passenger safety and vehicle performance, reducing the weight of vehicles has become the top priority for the automotive industry today. Advanced High Strength Steel (AHSS) is the fastest growing material in today’s automotive industry and the key material when it comes to vehicle mass reduction .
In general, AHSS are steels with yield strengths higher than 550 MPa. They offer uniquely low weight, high strength and optimised formability, which allows automakers to use less material, greatly reducing a vehicle’s weight.
The properties of steel have significantly improved over the last century, from mild steel in the early 1900s to high strength low alloys (HSLA) in late 1970s and the introduction of the first generations of Advanced High Strength Steel (AHSS) in the 1990s .
In the last two decades, the steel industry has developed different alloying and processing combinations to produce steel microstructures providing higher strength for reduced steel section size and weight .
The impressive combination of high tensile strength and ductility, carefully selected chemical compositions and the multiphase microstructures of AHSS are designed to help the automotive industry meet low weight requirements . AHSS are not significantly lighter than traditional steels, but their strength allows automakers to manufacture very thin gauges, thus reducing the weight of the vehicles.
Classification of steel for automotive use
There are approximately 30 steel grades that are used today in the automotive industry and can be classified into three different designations :
- Metallurgical designation
- Strength designation
- Formability designation
The metallurgical designation provides information about composition, processing and microstructure of the steel. Steel for the automotive industry can be classified as traditional mild steel, conventional high strength steel (HSS) and advanced high strength steel (AHSS).
A second important classification method for the automotive industry is the strength of steel. The terms HSS and AHSS are generally used to designate all higher strength steels.
AHSS are sometimes called extra high-strength steels or ultra-high-strength steels for tensile strengths exceeding 780 MPa and 1000 MPa, respectively. However, the terminology used to classify high strength steels varies considerably throughout the world due to the constant development of new generations of AHSS.
The formability of steel is defined as its ability to be formed into simple and complex shapes by different manufacturing processes [1, 2]. The important parameters that characterise the formability are high work-hardening exponent and total elongation. While high a work-hardening exponent accounts for the ability of sheet metal to stretch and more uniformly distribute strain in the presence of the applied load, the total elongation determines the volume by which a steel can be stretched before failure.
Traditional mild steel
Mild or low carbon steels are steels with a tensile strength of 400 MPa and carbon content of 0.05%-0.25%. The microstructure of mild steel causes it to be relatively ductile and easy to form, being comprised of one phase, normally ferrite . Mild steels are commonly used in the body structure and trunk closures of vehicles, as can be seen in Fig.1 .
High strength steel
High strength low alloy (HSLA) steels were the first commonly used high strength steels in the automotive industry . These steels have higher tensile strengths of up to 800 MPa. They are not made to meet a specific chemical composition but rather specific mechanical properties . They have a low alloying and carbon content to retain formability and weldability, with copper, titanium, vanadium, and niobium added for strengthening purposes . HSS steels have been used in the areas of vehicles where energy absorption is important, as shown in fig 1.
The principal difference between conventional HSS and AHSS is in their microstructure. AHSS are multiphase steels with complex microstructures that contain phases such as ferrite, martensite, bainite and austenite .
The first generation of AHSS
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 [1, 3].
- DP steels have a tensile strength from 590 to 1400 MPa and are used in the crash zones of the vehicles.
- CP steels have a microstructure consisting of bainite in addition to martensite and ferrite, which makes them more formable than DP steels. Their tensile strength ranges from 800 to 1180 MPa and are generally used in car frames.
- TRIP steels have tensile strengths ranging from 590 to 1180 MPa. The microstructure of these steels, together with ferrite and martensite, contains retained austenite, which, when steel is deformed, transforms into strong martensite phases and so it can absorb a larger amount of energy. They are normally used for energy absorption in frontal and rear zone structures in vehicles.
- MS steels are the hardest steel class in the AHSS family. Their strength ranges from 900 MPa to 1700 MPa. These steels maybe have the highest strength but due to the higher amount of martensite formation in the microstructure, they have the lowest formability. They are used in vehicle bodies where deformation must be limited.
The second generation of AHSS
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 
Figure 2 shows that the first generation of AHSS 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 % 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 broad range of AHSS grades can be seen in fig 2.
The significance of AHSS
AHSS are now used for nearly every new vehicle design. They are predicted to replace approximately 60 % 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 % to 80 % more than steel .
The future of AHSS
New grades of AHSS are making vehicle body structures lighter by 25-39% 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 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.
A number of recent studies, compared and predicted the consumption of AHSS in automotive, construction and other applications [1, 5]. Not surprisingly, the demand for AHSS will increase in the coming years, with the automotive industry being the main driver.
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 S. K. Sarna, “Steels for Automotive Applications,” Ispat Guru, Nov. 21 2015, [Online]. [Accessed Apr. 4, 2019].
 D. A. Porter, K. E. Easterling and M. Y. Sherif, Phase Transformations in Metals and Alloys. New York: Taylor and Francis Group, 2009.
 K. Bachman, “Lightweighting still dominates Great Designs in Steel,” FMA The Fabricator, 23 Mar. 2018, [Online]. [Accessed Apr. 4, 2019].
 T. Team, “Trends in Steel Usage in the Automotive Industry,” Forbes, 20 May 2015, [Online]. [Accessed Apr. 4, 2019].
 C. M. Tamarelli, “AHSS 101 The Evolving Use of Advanced High Strength Steels for Automotive Applications,” Internship report, Dept. Materials Science and Engineering, Univ. of Michigan, MI, 2011.
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