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Fig. 2—Graph showing the Elongation-TensileStrength relationships of current and futurehigher strength steels. Courtesy of 

WorldAutoSteel.

(DP) steel and the transformation induced plasticity (TRIP) steel

mentioned in the editorial by Brad Kuvin. DP steel has the longest

production history and is the AHSS most commonly used. Some TRIP

steel is used in production but most is still under evaluation.

The second target is to improve the strengths without changing the

stretchability or bendability. These are the complex phase (CP) steel

and the martensitic (MS) steel. As one might imagine, the extremely

high yield and tensile strengths of the strongest MS grade creates

stretchability and bendability problems. These problems can be solved

by changing the processing to hot forming. Instead of starting with a

very strong MS steel, a common boron-based steel is heated to 850 C.

 At this temperature the strength of the steel is extremely low and thesteel is highly formable. The preheated blank is inserted into a chilled

die, quickly formed, and then quenched as it remains in the die until the microstructure changes to martensite. The

benefits are the ability to form complex shapes; an extremely high strength martensitic final structure; and no

problems with springback. The disadvantage is a longer cycle time in the die.

The current AHSS are shown in Fig. 2 as Generation 1. In the upper right corner of the graphic is a Generation 2

steel called twinning induced plasticity (TWIP) already undergoing production studies. This steel has a very high n-

value for excellent stretchability. This steel has a very high austenite component.

 Another steel with a very high austenite component is the 300 series of stainless steel, which historically has been

called the “formable” high strength steel. When plotted in Fig. 2, the 301 and 304 stainless-steel alloys would be

within the band of Generation 2. The current problems with Generation 2 steels are the high alloying cost and the

resulting poorer weldability.

The question also has been raised about the real need for such a dramatic improvement in formability parameters.

Therefore, for the past several years researchers at various universities and steel companies have been attemptingto develop higher strength steels that fall into a Generation 3 band. These steels would have less cost and easier 

 joining procedures than Generation 2, but still have the ability to make more complex parts than the current AHSS.

Even now some of these new steels are being called designer steels tailored to meet very specific and tough

production and in-service requirements. The recent increases in fuel efficiency being proposed for trucks, busses

and other heavier vehicles are sure to further challenge metallurgists and physicists to make steels perform even

better. MF

 

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