Introduction to advanced high-strength steels - Part I
Advanced high-strength steels (AHSS) offer enhanced formability. This article discusses the properties and performance of various grades.Part I of this two-part series presents an overview of advanced high-strength steel (AHSS) grades. Part II addresses issues encountered when processing these grades.
Several new commercialized and near-commercialized advanced high-strength steels (AHSS) that exhibit high strength and enhanced formability are being offered around the world. These steels have the potential to effect cost and weight savings while improving performance.
The increased formability allows for greater part complexity, which leads to fewer individual parts (cost savings) and more manufacturing flexibility. Fewer parts mean less welding (cost and cycle-time savings) and weld flanges (mass and weight savings). Depending on design, the higher strength can translate into better fatigue and crash performance, while maintaining or even reducing thickness.
YS and UTS are minimum values, other values are typical.
Source: ULSAB-AVC TTD#6 (Technology Transfer Dispatch #6)
available from www.ULSAB-AVC.org or www.autosteel.org.
The first of this two-part series examines the similarities and differences between conventional HSS and the various AHSS grades. Figure 1 lists some mechanical properties of the grades discussed in this article. The values shown in the table are for comparison only, with the specific properties and ranges likely varying somewhat among steel companies. In addition, n value is a function calculated over a specific strain range and is more prone to vary as a function of the chosen strain range in the AHSS grades compared with the conventional HSS grades. As a result, it is important to consider data from the appropriate strain range pertinent to the specific forming operation.
Conventional High-strength Steels
It is generally accepted that the transition from mild steel to HSS occurs at a yield strength of about 210 megapascals (MPa) [30 kilopounds per square inch (KSI)]. For yield strength levels below 280 to 350 MPa (40 to 50 KSI), a simple carbon-manganese (CMn) steel typically is used. The composition of these steels is similar to low-carbon mild steels, except they have more carbon and manganese to increase the strength to the desired level. This approach usually is not practical for yield strengths greater than 350 MPa (50 KSI) because of a drop-off in elongation and weldability.
One approach to achieving yield strengths between 280 and 550 MPa (40 and 80 KSI) is to use high-strength, low-alloy (HSLA) steels, also known as microalloyed (MA) steels. This family of steels usually has a microstructure of fine-grained ferrite that has been strengthened with carbon and/or nitrogen precipitates of titanium, vanadium, or niobium (columbium). Adding manganese, phosphorus, or silicon further increases the strength. These steels can be formed successfully when users know the limitations of the higher-strength, lower-formability trade-off.
Another approach to achieving these yield strength levels is to use AHSS grades. Dual-phase (DP), transformation-induced plasticity (TRIP), high hole expansion (HHE), complex-phase (CP), and martensitic steels are some of the grades collectively referred to as AHSS.
Dual-phase (DP) Steels
Formed Panel Strength
Source: J. R. Shaw, K. Watanabe, and M. Chen, "Metal Forming Characterization and Simulation of Advanced High Strength Steels," Society of Automotive Engineers (SAE), 2001-01-1139.
DP steels have a microstructure of mainly soft ferrite, with islands of hard martensite dispersed throughout. The strength level of these grades is related to the amount of martensite in the microstructure.
As the product arrives from the steel mill, its yield strength typically is much lower than its tensile strength, with a YS-to-TS ratio of about 0.6. (For comparison, the YS-to-TS ratio for HSLA steels is closer to 0.75.) The lower yield strength at a given tensile strength translates to higher elongation values and better formability.
In addition, the work-hardening response to deformation is different between DP and HSLA steels. HSLA steels begin to lose formability as soon as deformation starts. As a result of the soft ferrite matrix of DP steels, they can maintain their formability further into the press stroke and can better distribute the strains across the part.
DP steels usually are bake-hardenable (strengthening occurs after the steel goes through a paint-bake cycle), whereas HSLA steels do not exhibit this characteristic (Figure 2). Between this bake hardenability and the higher level of work hardenability, it is not unusual to see an increase in yield strength of about 140 MPa (20 KSI) after forming and baking. In comparison, HSLA steels may have an increase of about 20 MPa (3 KSI).
Enhanced energy absorption is another DP steel characteristic. For a given yield strength, the DP steel tensile strength is higher than that of HSLA steels, which enhances crash performance. If crash performance equivalent to that of an HSLA steel is desired, using a DP steel may allow for downgauging of about 10 percent. 1
DP steel weldability usually is similar to that of HSLA steels, although different parameters may be required. The welding current range is almost the same (about 3 kiloamps), but the actual currents may be somewhat shifted. 2
Stress-Strain Curves for HSLA, DP, and TRIP Steels
[350 MPa (50 KSI) Yield Strength]
Source: A. Konieczny, "Advanced High Strength Steels - Formability," Great Designs in Steel Seminar, February 2003, American Iron and Steel Institute, and AHSS Guidelines at www.WorldAutoSteel.org.
Transformation-induced Plasticity (TRIP) Steels
Like DP steels, the microstructure of TRIP steels is comprised of mainly soft ferrite. While DP steels have martensite as the only other phase, TRIP steels have a combination of martensite, bainite, and retained austenite. The various levels of these phases give TRIP steels their unique balance of properties. Figure 3 shows the stress-strain curves for a HSLA steel, a DP steel, and a TRIP steel, each with a yield strength of about 350 MPa (50 KSI).
As forming continues, the retained austenite in TRIP progressively transforms to martensite with increasing strain. This leads to a volume and shape change within the microstructure, which accommodates the strain and increases the ductility. In TRIP steels, the high work-hardening rate persists at higher strains, while that of DP begins to diminish. This work-hardening difference is one of the primary reasons for the enhanced formability of DP steels over HSLA steels, and what gives TRIP steels a further advantage over DP steels (Figure 4).
The strain level of retained austenite-to-martensite transformation can be engineered through carbon content adjustment. If lower carbon levels are used, transformation starts at the beginning of forming, leading to excellent formability and strain distribution at the strength levels produced. At higher carbon levels, retained austenite is more stable and persists into the final part. The transformation occurs at strain levels beyond those produced during stamping and forming. Transformation to martensite occurs during subsequent deformation, such as a crash event, and provides greater crash energy absorption.
Work-hardening of HSLA, DP, and TRIP Steels
Source: A. Konieczny, "Advanced High Strength Steels – Formability," Great Designs in Steel Seminar, February 2003, American Iron and Steel Institute; AHSS Guidelines at www.WorldAutoSteel.org.
The additional alloying required to get the TRIP effect makes spot welding more challenging compared with DP steels. This can be addressed with modified welding cycles.
High Hole Expansion (HHE) Steels also known as Stretch Flanging High-strength (SFHS) Steels
For applications that require a high degree of sheared edge elongation (hole flanging), HHE steels are increasingly being used. The microstructure is primarily ferrite and bainite, with some retained austenite. These steels exhibit high strength, high formability (although less than some other AHSS grades), and high sheared edge extension (hole flanging) capability. The ferrite-bainite microstructure is associated with high hole expansion values. Parts stamped from these grades are replacing cast and forged parts made from other materials.
Complex-phase (CP) Steels
CP steels are characterized by a very fine microstructure of ferrite and a higher volume fraction of hard phases (martensite and bainite), strengthened further by fine carbon or nitrogen precipitates of niobium, titanium, or vanadium.
These steel grades have been used for parts that require high energy-absorption capacity, such as bumpers and B-pillar reinforcements.
Martensitic steels have a microstructure that is 100 percent martensite. Minimum tensile strengths of this family of steels are typically between 900 and 1,500 MPa (130 and 220 KSI). These grades can be made directly at the steel mill (quenching after annealing) or via postforming heat treatment. Because of its limited elongation, mill-produced martensite typically is roll-formed. More complex shapes can be fabricated by hot forming and quenching a lower carbon grade.
Based on the targeted strength level, these grades can have carbon content typical of low-carbon steel or levels greater than 0.20 percent. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel also are used in various combinations to increase hardenability. As a result, adjustments in the welding procedure may be needed.
Typical applications for martensitic steels usually are those that require high strength and good fatigue resistance, with relatively simple cross sections (although profiles of hot stamped parts are becoming more complex). Good candidates for martensitic parts include door intrusion beams, bumper reinforcement beams, side sill reinforcements, and belt line reinforcements.
When combined with appropriate manufacturing techniques, advanced high-strength steels offer opportunities for reduced product weight, enhanced crash performance, manufacturing process consolidation, and cost reduction.
These engineered steels are being used in more applications throughout various manufacturing industries, and their use should continue to grow as die and process engineers become familiar with the different techniques that are required for manufacturability. The second article of this two-part series will highlight some of these techniques and issues that should be considered when processing these grades.
1. J.R. Fekete, A.M. Stibich, and M.F. Shi, "A Comparison of the Response of HSLA and Dual Phase Sheet Steel in Dynamic Crush," Society of Automotive Engineers (SAE), 2001-01-3101.
2. M. Kamura, Y. Utsumi, Y. Omiya, and Y. Kawamoto, "Crashworthiness and Spot Weldability of Galvannealed DP800 Steel Sheet," Society of Automotive Engineers (SAE), 2001-01-3094.
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