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Questions Answered Regarding Advanced High-Strength Steels

By: Stuart Keeler

Monday, February 1, 2016
 

Fifty years ago, high-strength low-alloy (HSLA) steels became available. And with yield strengths ranging from 30 to 80 ksi, they still find plenty of use today. Meanwhile, metallurgists and mechanical engineers around the world have remained hard at work creating another class of steels known as advanced high-strength steels (AHSS). These alloys are more complex than HSLA steels in composition, processing and application.

Here, I’m pleased to answer some of the questions being asked about AHSS grades by designers, buyers, press and repair shops, and others affected by these new steels.

• What factors drive AHSS research around the world?

The main driver is the North American automotive industry, working to increase fuel efficiency as required by new legislation. The primary technique is lightweighting, which seeks to reduce the thickness of stamped parts. Consequently, the strength of the steel must be increased to balance the thickness reduction. Unfortunately, reduced thickness and increased steel strength cause a reduction in stretchability.

• Are the AHSS grades simply an extension of HSLA steels?

No. HSLA and AHSS exhibit major differences in their microstructures and different microstructures create different properties. HSLA steels start with a specific ladle of molten metal that solidifies into a microstructure called ferrite phase. Meanwhile, AHSS grades comprise a mixed microstructure that includes one or more phases other than ferrite.

As steel solidifies, and depending on a number of variables, the grain size within the microstructure can become large or small. A relatively small grain size makes for stronger steel, because smaller grains have a larger ratio of strong grain boundaries to weaker inner grain volumes. To increase the strength of HSLA steels, manufacturers change the atomic cell structure by replacing iron atoms with atoms of other elements such as potassium, titanium and silicon.

The simplest AHSS grade, dual-phase (DP) steel, exhibits a microstructure of approximately 10 percent high-strength martensite, present as islands as the second phase within the ferrite microstructure. The martensitic phase helps to increase the general strength of the steel. Also, during deformation of DP steel, the boundaries between ferrite and martensite phases undergo a mixing that creates an increase in workhardening and instantaneous n-values.

• How does DP 350/600 differ from HSLA 350/450?

Both have the same 350-MPa (50-ksi) yield strength but a different tensile strength. During deformation, the DP material exhibits a strong interaction between the ferrite phase and the islands of martensite. This generates an increase in the instantaneous n-value curve that peaks at 3-percent engineering strain with an n-value of 0.20, compared to an n-value of 0.13 for HSLA 350/450. This higher initial n-value increases the stress-strain curve for the DP steel, generating a higher tensile strength. At a strain of 8 percent, the DP steel has exhausted its n-value advantage and the two steels exhibit the same properties.

• What benefit is gained with the higher n-values at such low strains?

Many parts have areas of localized high deformation called gradients, which contain zones of high stress that create narrow peaks of large deformation. Examples include long character lines, raised channels and localized tight bends. To comply with constancy-of-volume rules, high surface strains also create a corresponding narrow zone of local thinning. Tearing or other failure modes occur early from this forming process. High n-values accelerate workhardening in these high-strain zones that shut down the gradients as forming begins. DP steels are designed to deal with this gradient problem.

• What are the current primary applications for DP steels?

We find DP steels incorporated into the design of the passenger compartment, to absorb crash energy.

• The traditional n-values and forming-limit curves (FLCs) with initial high n-values for DP 350/600 are similar to those for HSLA 350/450. Why?

The higher instantaneous n-value for DP steel is exhausted by a strain of 8 percent. The traditional measurement range for n-values is from 10 to 20-percent strain. Therefore, the two steels will have the same values. Likewise, the FLC depends highly on the terminal n-value just prior to local necking and failure. Again, the two steels will exhibit the same terminal n-values.

• Does the DP steel have a series of steel grades with increased yield- and tensile-strength stress-strain curves similar to HSLA steels?

Yes. Nine DP grades have been produced, as described in “Advanced High-Strength Steels Application Guidelines, Version 5.0.” It’s available as a free download from www.worldautosteel.org. In addition to the 250-page guidelines, website visitors also can download another document containing details for 12 case studies describing use of AHSS. MF

 

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