"Microstructures matter now." So says James R. Fekete, technical fellow, Materials, Standards & Technology, Global Die & Press Center, Body Manufacturing Engineering, General Motors. He's talking about steel. Specifically, about the category of advanced high-strength steels (AHSS). He makes a related point, one that may change the way you think about what are comparatively new materials. (Comparatively? Well, consider this: the Iron Age started about 12 BC. These AHSS materials were categorized about 10 years ago to, Fekete says, "differentiate the materials from the HSLAs"-that's high-strength, low-alloy-"and bake hardenables." So in the grand scheme of things, AHSS is comparatively brand new.)
Fekete: "We all know about composites. There are hard things embedded in soft things. The hard things are about strength; the soft things are about manufacturability. The same is true for steel. It is really a composite. The principles behind the design of the materials is the same."
The design of the materials? Steel is a composite? Yes, the materials are designed. And the microstructures that he refers to make all the difference in terms of the strength of the materials. Specifically, according to Advanced High Strength Steel Applications Guidelines, version 3, by the International Iron and Steel Institute (www.worldautosteel.org
), "The principal difference between conventional HSS and AHSS is their microstructure. Conventional HSS are single phase ferritic steels. AHSS are primarily multi-phase steels, which contain ferrite, martensite, bainite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behavior." Which is to say they're designed, and note how there are the inclusions that add the mechanical properties, just like the carbon fiber in its polymer matrix.
According to Raj Mohan Iyengar, platform manager, Automotive Product Applications, Advanced Engineering, SeverStal North America, "Microstructures are nothing new. We've known of their importance for more than 50 years. What is new is how we've translated that basic knowledge into the practical manufacturing of steel." After all, this is about making end products-like structural elements for cars-which means making materials reliably at volume.
Achieving these advanced characteristics is essentially about the heating and cooling of the material. Ron Krupitzer, vice president, Automotive Applications, American Iron and Steel Institute, says that historically (we are talking about contemporary times here, not the aforementioned Iron Age), there was more attention paid to the heat treating of things like long bars and rods, not sheet. "Now thermal processing counts. The thermal history of the steel has become more important now." The importance is predicated on this: while HSS has yield strengths from 210 to 550 MPa and tensile strengths of 270 to 700 MPa, the ultra-high strength steels (UHSS), which is the extreme end of the AHSS category, have a yield strength of 550 MPa or more and a tensile strength of 700 MPa or more. (Yield strength, incidentally, relates to the amount of force applied at which permanent deformation occurs; tensile strength has to do with the tension load before the material fractures.)
As Fekete puts it simply, "The UHSS have a lot more of the hard stuff." And whereas the hard components in the materials more ordinarily known as "advanced composites" tend to make the materials more brittle, that's not the case for steel. He observes, "There's remarkable ductility for the strength."
And strength is a key attribute of these materials-but there must be sufficient ductility because the materials need to be formed. By and large, these AHSS materials are used for structural stampings, such as roof rails, carriers, B-pillar reinforcements, and other places where mass can be reduced and the inherent strength of the materials can provide advantage. This is particularly the case when there are issues related to safety, as the materials generally have an intrinsically high yield strength.
While it might seem that an AHSS would be more difficult to form than an HSS, that's not the case. In fact, it has more formability than HSS and consequently, there's more springback. So it is a matter of developing new design guidelines for stamping the material to accommodate that.
However, there is another issue. The strength of the material. Fekete says, "The issue you run into is that you may have a stamping machine that can make a part with HSS and now you put in an AHSS that has the same amount of formability, so you can make the part, but the press loads might go up significantly because the material itself has higher strength. If you're close to the limit of the stamping machine with the original part, you might go past it and might need a new press."
He adds, "Die material is another aspect. We used to make dies out of cast iron and then induction harden them. Now you're moving from cast iron to tool steels, and from just hardened to sophisticated tool coatings, inserts, and they're still wearing."
And there is even another kicker: "The steels are more expensive. The tools are more expensive."
Which might lead one to think that maybe AHSS isn't such a good idea. After all, it is more expensive. However, Fekete makes an important point. On the one hand, there are the vehicle designers who are trying to minimize overhangs, create sleek shapes, reduce the size of A- and B-pillars, and the like. On the other hand, there are the engineers, who have to deal with such things as rollover and crush-regulations. "You need to have some really high-strength materials that are reasonably manufacturable," Fekete says, adding, "Our engineers are finding that the box is getting smaller and smaller, and they don't have many alternatives." And AHSS gets them out of that box.