Thanks to the constant flow of technology in the automotive industry, today's vehicles house many new features that have developed in a nonlinear fashion through periods of stability as well as through periods of rapid change.

In the field of materials, for example, steel has been a very stable product from its first significant use in vehicle body structures, around 1915, until about the year 2000. Major improvements during that span focused on uniformity and low-strength formability, such as the development of interstitial-free steels. Improvements in uniformity of chemistry and physical properties have resulted from process technologies such as ladle metallurgy and continuous casting, and in computer-controlled rolling and annealing. High-strength low-alloy (HSLA) steels, more than 30 years old, illustrate how materials have improved in uniformity.

The enabling technologies from the manufacturing viewpoint, such as forming and joining, that support the use of these steels have, until recently, also been well-defined and readily understandable by OEMs and their supplier teams.

Rapid Changes in the New Century

However, since 2000, the global automotive industry has seen rapid improvements in automotive steels, especially with the application of advanced high-strength steels (AHSS) including dual-phase (DP) alloys and transformation-induced-plasticity (TRIP) steels. TRIP steels contain martensite and retained austenite and have higher formability than DP steels, which contain primarily ferrite and martensite.

These improved steel products represent a major change in how steels are designed and manufactured into automotive components and vehicle structures. In fact, predictions by major OEMs throughout the world suggest that AHSS products will replace, in large percentages, the conventional mild and HSLA steels applied throughout body and chassis structures.

Many reasons explain this rapid shift in steel technology for vehicle structures. Major drivers include the continuing need for better mass efficiency for fuel economy; requirements for increased crash performance in frontal, side and rollover tests; the marketing need to maintain affordability; and the environmental requirements of recyclability, sustainability and reduced CO2 emissions.

In 2002, a major global project referred to as ULSAB-AVC (Ultra-Light Steel Autobody Advanced Vehicle Concept) was released to the automotive industry. It demonstrated that a vehicle using AHSS, with a cleverly designed steel body structure, could be as efficient in mass and performance as a vehicle made from alternative light-weighting materials (Fig. 1). The concept also proved out at a projected price of less than half that of vehicles designed with similar objectives through the government-sponsored PNGV (Partnership for a New Generation Vehicle) program.

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Fig. 1-The ULSAB-AVC project illustrated dozens of applications for advanced high-strength steels.

The strength of the ULSAB-AVC project was its extensive use of high-strength steels throughout the entire vehicle structure-body, chassis, seats etc.-with AHSS as the vast majority of these steels. Prior to that project, the most extensive uses of AHSS, with martensite- or bainite-containing microstructures, were in bumpers, door beams and other components that could be integrated easily into a vehicle on an individual-part basis.

What Makes AHSS so Great?

So, what exactly are AHSS grades, and why do they perform better than the more traditional grades of high-strength steels?

First some definitions have to be established to classify the various families of high-strength steels. The ULSAB-AVC project set definitions for many of these grades; the broadest categories are listed below.

• High-strength steels (HSS)-steel products with yield strengths from 210 to 550 MPa
• Ultra-high strength steels (UHSS) -steels with yield strengths greater than 550 MPa
• Advanced high-strength steels (AHSS)-steels that overlap the range of strengths defined above by HSS and UHSS, as shown in Fig. 2.

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Fig. 2-Strength-formability relationships for mild, conventional high-strength steels (HSS) and advanced high-strength steels (AHSS).

The principal differences between conventional HSS, UHSS and AHSS lie in their microstructures. AHSS are primarily ferrite-phase (soft) steels with varying percentages of martensite, bainite and retained austenite sufficient to produce favorable combinations of elongation and strength. This enhanced elongation or formability at elevated strength arises primarily from their high strain-hardening capacity, exemplified by their low ratio of yield strength (YS) to ultimate tensile strength (UTS).

Enhanced Formability

When working with conventional steels, reduced formability is one consequence of selecting higher strength levels. This became a major issue during the early introduction of HSLA steels in the 1970s. Considerable use of HSLA was made possible by the steel-processing improvements mentioned above, which enhanced uniformity. However, enhanced uniformity only enabled a limited use of HSLA because many parts simply could not be stamped in those grades.

The advent of AHSS enables the stamping, as well as hydroforming, of many more parts, and enables the manufacture of vehicles with the very high percentages of AHSS as required by the ULSAB-AVC concept.