Advanced High Strength Steel: Nomenclature, Grades, and Applications
Advanced High Strength Steel (AHSS) refers to steel grades with an excellent combination of ductility, toughness and strength that allows them to be machined into thinner sheets delivering superior tensile strength to traditional steel.
Steel is a material that has been used globally and has been a major factor in advancing societies and economies. It fuelled the Industrial Revolution and has been a fundamental building block of housing, infrastructure, manufacturing, and transport systems. The automotive industry is the largest consumer and driver of steel advancements as the development of lightweight steel grades is essential to meet ever more stringent quality and efficiency standards. The increase in vehicle performance, weight reduction, cost, reliability and fuel efficiency demanded by the automotive industry can be met, at least in part, by the widespread adoption of AHSS [1].
Here, you will learn about:
- The nomenclature of advanced high strength steels
- The different AHSS grades
- AHSS’s involvement in the automotive industry
Nomenclature
The nomenclature of advanced high strength steels became available when the Ultra Light Steel Automotive Body - Advanced Vehicle Concept (ULSAB-AVC) consortium adopted an identification standard that specified the ultimate tensile strength (UTS) and yield strength (YS) [1].
The AHSS nomenclature follows this system:
XX aaa/bbb
where,
XX = steel type
aaa = minimum value of yield strength (YS)
bbb = minimum value of ultimate tensile strength (UTS)
|
AHSS Grade |
Classification |
|
Dual-Phase Steel |
DP |
|
Complex-Phase Steel |
CP |
|
Martensitic Steel |
MS |
|
Transformation-induced Plasticity |
TRIP |
|
Twinning-induced Plasticity |
TWIP |
|
AUST SS |
AHSS grading
Advanced High Strength Steel grades can be classified into first and second-generation variants.
First-generation AHSS family
Dual-phase steel
Dual-phase (DP) steel is versatile in the automotive industry due to its excellent strength and ductility combination. As the name implies, DP steels are made up of a dual microstructure of a soft ferrite matrix and 10 to 40% volume of martensite. DP steels with the duplex microstructure reach a tensile strength of 500 to 1200 MPa [1], [2].
Complex-phase steel
Complex-phase (CP) steels consist of a ferrite-bainite microstructure matrix with small parts of hard phased martensite, retained austenite and pearlite. CP steels have an ultimate strength exceeding 800 MPa and a tensile strength of 420 to 1030 MPa, with high formability and energy absorption, making it a suitable option to contend with automotive crash forces [1].
Transformation-induced plasticity
Transformation-induced plasticity (TRIP) steels are new AHSS grades developed for high strength, formability and ductility. Transformation-induced plasticity refers to the mechanisms in which soft austenite is transformed into hard martensite by plastic deformation. TRIP steels usually contain 0.1 to 0.4 wt% carbon and other alloying elements such as aluminium, titanium, silicon, nickel and vanadium. TRIP tensile strength ranges from 500 to 1050MPa, with total elongation rates at 12 to 32% [1], [2].
Martensitic steels
Martensitic (MS) steels are produced when the rapid cooling of the austenite traps carbon. Unable to diffuse, the carbon atom transforms face-centred cubic crystal structures to body-centred cubic crystal structures. This is the hardest transformation of austenite, therefore martensitic steels must be treated to soften and improve ductility. The tensile strength of this AHSS steel grade ranges from 720 to 1680 MPa [1].
Second-generation of AHSS family
Twinning-induced plasticity
Twinning-induced plasticity (TWIP) steels have a high manganese content of 22 to 30% and, in addition to iron, consist of other elements such as carbon, silicon and aluminium. These AHSS types have excellent strengths and ductility. The tensile strength ranges from 1100 to 1650 MPa and elongation from 60 to 90%. As the name implies, TWIPs are named after the mechanical twinning that is characterised by deformation and symmetrical discontinuities by two crystalline regions that structurally mirror each other [1].
Austenitic stainless steel
Austenitic stainless steel (AUST SS) consists of more than 50% Iron, 16 to 26% Cr and less than 35%Ni. AUST SS is characterised by its excellent corrosion resistance, high formability and good fatigue strength. The yield strength of this steel grade ranges from 200 to 1400 MPa and its tensile strength range spans between 900 and 1200 MPa [1].
AHSS in the automotive industry
AHSS’s primary market driver is the automotive industry. One key attribute of advanced high strength steel in the automotive manufacturing industry is its crashworthiness. This is the ability of a vehicle to deform while providing enough space for passenger survival in frontal and side collisions. A vehicle may be divided into safety and crushing zones. The safety zone is the compartment which the passengers and driver occupy and therefore must have high strength. The crushing zone is designed to receive the impact, deform, and absorb crash energy to protect passengers [3].
Sources
[1] M.Y. Demeri, Advanced High-Strength Steels: Science, Technology, and Applications, OH: ASM International, 2013.
[2] D.J. Schaeffler, “Introduction to advanced high-strength steels - Part I”, 2005 [Online]. Available: https://www.thefabricator.com/stampingjournal/article/metalsmaterials/introduction-to-advanced-high-strength-steels---part-i [Accessed on Nov. 11, 2019]
[3] H. Safari and H. Nahvi, “Improving automotive crashworthiness using advanced high strength steels”, International Journal of Crashworthiness, vol. 23 (6), pp. 645-659, 2017.
Sidebar content references:
[a] W. Bleck, "Advanced High Strength Steels for the automotive industry - from microstructures to nanostructures", Steel and Iron, 134, 7, pp. 25-34, 2014.
[b] C. Lesch, N. Kwiaton, and F.B. Klose, "Advanced High Strength Steels (AHSS) for Automotive Applications − Tailored Properties by Smart Microstructural Adjustments", Steel Research International, 88, 10, pp. 1-21, 2017.