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WorldAutoSteel Guidelines for AHSS shed light on lightweighting

New opportunities, new challenges around the curve

The international steel industry has seized the opportunity to create new families of alloys with higher strength and greater formability than those that existed just a few years ago to meet higher requirements for vehicular fuel efficiency.

Advanced high-strength steel (AHSS) grades are produced with chemistries and rolling practices that are optimized to achieve tightly engineered microstructures and properties.

Just as AHSS are made differently in the steel mill, stamping manufacturers need to process them differently. Forming as usual will not be sufficient to achieve robust production. The successful user will embrace best practices, even though they may be a change from existing practices. Forming, joining, and maintenance practices must evolve to produce optimized AHSS components.

WorldAutoSteel, an international consortium of leading steelmakers producing these cutting-edge AHSS grades, released its Advanced High-Strength Steels Application Guidelines, Version 5.0 (http://www.world

autosteel.org/download_files/index.php?path=AHSS+Guidelines+V5), intended to help manufacturers work through the differences encountered when forming and welding AHSS. Following are some highlights from the guides.

Section 1—Introduction to AHSS

The guidelines begin with a chapter identifying the nomenclature and family types of AHSS. They are:

  • Dual Phase (DP)
  • Transformation-induced Plasticity
  • (TRIP)
  • Complex Phase (CP)
  • Martensite (MS)
  • Hot Formed (HF)
  • Twinning-induced Plasticity (TWIP)

Because each of these families includes several grades with varying strength levels, the specific grade is referred to by its minimum yield strength and minimum tensile strength in megapascals (MPa, where 6.9 MPa = 1 KSI = 1,000 PSI). For example, DP350/600 is a dual-phase steel with 350 MPa (50.7 kilopounds per square inch, KSI) minimum yield strength and 600 MPa (87 KSI) minimum tensile strength.

A phase is a physically distinctive form of matter, such as a solid, liquid, gas, or plasma. A phase of matter is characterized by having relatively uniform chemical and physical properties, according to about.com.

More than 30 separate AHSS grades are projected to be commercially available by 2015 to 2020. At least half of them are already on the market. Some service centers stock the lower-strength DP steels. There does not appear to be a risk associated with aging AHSS grades—none is documented at least (see Figure 1).

Section 2—Metallurgy of AHSS

The steel mills combine new recipes with tight process control to create the engineered microstructures with the unique mechanical properties associated with these families of grades. The DP steels and TRIP steels are the two most widely used.

The WorldAutoSteel group published a book of application guidelines, Advanced High-Strength Steels Application Guidelines, Version 5.0, to help stampers and other manufacturers understand how to use AHSS. Image courtesy of WorldAutoSteel.

DP steels consist of islands of a hard, brittle metallurgical phase called martensite surrounded by a soft ductile matrix of a phase called ferrite—hence the term dual phase.

Because the matrix is continuous, forming strains initially concentrate in the ductile phase. This phenomenon leads to the high work-hardening values of DP steels. Higher-strength grades can be formed with greater concentrations of martensite.

TRIP steels contain a dispersion of hard phases in a ferrite matrix, so this family also has a fast work-hardening rate like the DP steels. What makes TRIP steels unique is that new martensite is produced as a metallurgical phase called retained austenite, which is transformed with increasing forming strains. This allows for the high work-hardening action to continue as increasing strains are put upon the part during the forming operation. The part will tend to fail at the weakest, thinnest location. Therefore, the greater work-hardening capacity of these grades means that more complex parts can be formed from them.

Even with the greater formability, tensile strength is 600 to 1,000 MPa, depending on the grade.

However, the use of exacting amounts of alloying elements is required to get these desired properties–most of which make welding more challenging (see Figure 2).

Section 3—Forming and Manufacturing

When it comes to sheet metal formability, n-value is the most important parameter. The n-value can be measured in a tensile test. Steels with lower n-value tend to concentrate strains faster, leading to early failure. A higher n-value means that the part will have a more uniform strain distribution.

The most widely available and widely used high-strength, low-alloy (HSLA) steel is 350/450 (350 MPa minimum yield strength/450-MPa minimum tensile strength). It is also known as HSLA 50, because the minimum yield strength converts to 50 KSI. The typical n-value for HSLA 350/450 is around 0.17. That is the same as DP350/600; however, it has the same n-value only when it is calculated between 10 percent strain and uniform elongation. While the n-value for the HSLA grade is essentially constant over the entire range of strains, the n-value for DP steels is significantly higher when calculated over a low strain range.

To ensure that they receive material with the correct properties, some stampers ask for AHSS with an n-value calculated between 4 and 6 percent strain, as well as the more conventional uniform elongation between 10 percent and 20 percent.

From the stamper’s perspective, this means that parts made from DP steels will be less negatively affected by the initial strain concentration that ultimately could lead to failure. After the binder closes and the beads are set, as the punch makes contact, the sheet metal blank starts to strain. Part and process features that reduce the increasing strains, such as larger radii and better lubrication, allow more complex parts to be formed. Similarly, sheet metals with higher n-values during the initial stages of the press stroke–like the DP steels–also allow for the stamping of more complex parts. In addition to the improved formability, DP steels form better and have a 30 percent greater tensile strength than HSLA steels of the same incoming yield strength (see Figure 3).

Martensite is the “strength element” in the microstructure of AHSS, but it is also a brittle phase. This brittleness increases the risk for edge cracking throughout the stamping process—blanking, trimming, or flanging operations. Cutting tooling must be sharp and aligned. Punching clearances must be optimized for the chosen work material. The ancient and disproven “rule of 10 percent on each side” does not necessarily apply to AHSS. Part design should be modified to reduce areas of sheared-edge stretch (see Figure 4).

Tool wear is always a concern, but it is more of a problem with AHSS parts. It is AHSS’s higher strength and additional structural benefits that allow for use of thinner gauges. However, thinner gauges are more vulnerable to wrinkling, and AHSS’s higher strength is more likely to keep the binder open, which leads to more wrinkling. Not only do the wrinkles restrict metal flow, which would lead to premature failure, they score (damage) the tool surface. Engineered tool materials, tool surface coatings, and lubricants are critical for consistent part quality (see Figure 5).

Figure 1
The most recently developed generations of AHSS are stronger than previous generations with similar elongation characteristics. Image courtesy of WorldAutoSteel.

The press used to stamp AHSS grades is critical. Traditionally, the only consideration was whether the press could withstand the maximum force required to form the stamping. However, the rapid work hardening of AHSS means that sufficient tonnage must be available at the appropriate part of the cycle—not just at bottom dead center. The work-hardening characteristics and higher tensile strength of the AHSS grades also mean that greater press energy is required to form it, compared with HSLA steels of the same yield strength.

Springback control is more challenging with AHSS grades. A tight punch radius will help maintain dimensions but limits metal flow and leads to premature thinning. It is easier to minimize springback and maintain dimensional accuracy via part and process design changes that reduce unbalanced stresses than use tighter radii. Using beads, darts, or mechanical stiffeners will help to lock in the elastic stresses and prevent them from contributing to part twist, side wall curl, and angular change.

Down the Road

AHSS entering the marketplace will become common in short order. The sooner stampers adapt to the new approaches needed to form them, the better off they will be.

Much like the HSLA grades developed in the 1970s, these AHSS grades were developed with the automotive industry in mind. Over time, however, it’s likely that they will find their way into any industry that puts a premium on lightweighting or complex, high-strength parts.

Finally, many other details provided in the WorldAutoSteel’s Advanced High-Strength Steels Application Guidelines 5.0 are not covered in this brief overview. More than 100 pages are devoted to joining alone! A 200-page supplement contains numerous case studies. A revision to the guidelines is planned for 2015. More details of the guidelines will be explored in a regular column on www.thefabricator.com in 2015.

About the Author
Engineering Quality Solutions Inc.

Daniel J. Schaeffler

President

P.O. Box 187

Southfield, Michigan 48037

248-539-0162

Engineering Quality Solutions Inc. is a provider of practical solutions for sheet metal forming challenges.