Advanced High-Strength Steels Add Strength and Ductility to Vehicle Design

What material is seeing the most rapid growth in automobiles? If you guessed aluminum or composites, you’d be wrong. It’s advanced high-strength steel (AHSS). The material comprised just a small fraction of cars and light trucks a few years ago, but it could grow to over 30% of vehicle weight within 10 years.

Although higher-priced, lower-volume vehicles like the Audi A8 have converted many of their parts to aluminum in response to fuel-economy pressures, the trend has been for moderately priced vehicles to stick with steel. These mainstream vehicles are using better manufacturing techniques — like laser-welded blanks, hydroformed components, and better joining techniques — in addition to containing up to 30% AHSS.

When the Honda Insight was first launched, it had among the highest percentages of alternate materials of any vehicle on the road. However, the latest version of the Insight is one of the most AHSS-intensive vehicles. Likewise, BMW came out with an aluminum front end on its 5-Series a few years ago but recently switched back to steel.

So what is AHSS, and what makes it so attractive automakers? Grades of AHSS have strengths to 1,500 MPa but retain the formability of lower-strength steels. In general, elongation, the property that equates to formability, degrades as strength increases. AHSS is formulated for more elongation at equivalent strengths.

Microstructure magic
The high-strength, high-ductility characteristics of AHSS come from the metals’ unique microstructures. Where most steels have primarily one microstructural phase, like ferrite, AHSS typically has a combination of martensite, bainite, and ferrite phases.

Each microstructural phase has a different crystalline or molecular structure. For instance, ferrite has a body-centered-cubic (BCC) structure, and martensite has a body-centered-tetragonal structure. Each structure has its own set of physical properties due to the forces within the crystals and the densities with which atoms are packed in a crystal cell.

Phases form during annealing, a process in which steel is heated to 850°C so that it becomes pure austenite. From there, the steel is cooled in a controlled manner that determines the final phase mixture (see “Transformative cooling”).

For instance, if the steel is cooled at a very slow rate, the austenitic phase will transform into ferrite without transitioning into any other phase. Conversely, very rapid cooling or quenching produces 100% martensitic steel (MART). A slow cool followed by a rapid quench produces dual-phase (DP), complex-phase (CP), or transformation-induced plasticity (TRIP) steels, all of which have AHSS properties.

Alloying elements like silicon, manganese, chromium, and molybdenum shift the phase lobes on the transformation diagram, making it possible to form the desired phase mix with feasible cooling cycles.

Forming and springback
The combination of phases keeps AHSS formable while they retain the high strength of martensite. Formability is especially important in automotive manufacturing where large presses and multipiece dies form parts.

AHSS parts are formable by traditional methods, but they typically need stronger die materials or coatings that boost die life. Manufacturers may also need higher tonnage presses to form these steels.

A key difference designers need to take into account when using AHSS is a greater degree of springback, the tendency of a metal to partially return to a previous shape. While all steel parts have some degree of springback when removed from the press, steels that undergo more work hardening or strengthening during forming have more springback.

There are several strategies for reducing springback. One is overforming the metal so it springs back to the desired dimensions. For example, to stamp a 90° angle into a sheet of steel, overstamp it to about 87° and it will springback to the desired angle.

Designers can also optimize radii in the edges and corners of the stamped piece. Other strategies involve changing the design of the finished part to minimize springback.

Structural strategies
Despite AHSS’ better strength, steel still has to compete with aluminum’s lower density. But aluminum may not be a panacea for automakers. For one thing, 30 times as much steel is produced than aluminum, so it can be hard to sustain production or keep prices down on an all-aluminum car.

Various research studies (see “High-strength history” sidebar) have shown that proper application of AHSS can cut a vehicle’s weight between 10 and 25%. When fuel economy is paramount, the 5% fuel-economy boost a 10% reduction in weight provides is a nice carrot. But designers should be aware of certain approaches that can take full advantage of AHSS’ capabilities.

Automotive bodies can be considered to be a combination of plates, beams, and joints — the intersections of two or more beams. If all the steel in a car was replaced with aluminum and no design changes were made other than making the aluminum parts thick enough to achieve the same overall stiffness and strength of the steel vehicle, the distribution of strength and stiffness would still be somewhat different on the aluminum car versus the steel one.

For mechanical responses that are proportional to the specific modulus — the modulus of elasticity divided by the density — there’s no advantage of switching from one metal to another from a weight perspective. For instance, the torsional stiffness of circular thin-walled tubes made from aluminum and steel of the same weight is the same. Looked at another way, two thin-walled tubes having the same torsional stiffness, one made out of steel and the other made out of aluminum, weigh the same.

However, the torsional stiffness of a plate is not directly proportional to specific modulus. A steel plate weighs twice as much as an aluminum plate of the same area (but not thickness) and the same torsional stiffness.

So, designers can improve structural efficiency and minimize the benefit of converting to aluminum in two ways. First, use higher strength steels to minimize the weight of plates in a vehicle. Second, get the vehicle’s internal beams to behave more like closed-section, circular tubes.

Advanced architectural elements like laser-welded blanks or hydroformed beams can help a steel body or frame compete with aluminum in terms of weight. Laser welding two sheets of dissimilar-thickness or dissimilar-grade steel together puts strength where it is needed in a single steel stamping.

Hydroforming a continuous hollow tube to the desired contour in a die using water pressure eliminates spot-welded sheet-metal beams. Because the tubes are continuously fastened and of a thicker gage than the sheet-metal beams, they have higher strength and stiffness.

Joints are a third area designers should consider. The joints of an all-steel vehicle will usually have lower strength and stiffness than those of its all-aluminum counterpart because the joints have typically higher stresses and steel is thinner gage than in the aluminum vehicle. So, strengthening and stiffening the steel vehicle’s joints will reduce the weight benefit of aluminum.

Designers can add internal stiffeners to boost joint strength. More manufacturers are also converting spot welds into continuous welds or adding continuous adhesives in the seams of the joint.

These strategies let automotive designers maintain or reduce vehicle weight in the face of increasing safety and crashworthiness requirements. One example is an upcoming change to the National Highway Transportation Safety Administration’s roof-crush requirements. Since 1994, vehicle roofs have had to withstand a load of 1.5 times the gross vehicle weight (GVW). In 2011, this will grow to 2.5 to 3 times GVW, necessitating thicker or stronger roofs.