The Science of Forming

You have previously formed parts with 50-ksi-yield-strength HSLA steel and feel confident about your capabilities to form most any high-strength steel. That is until a new RFQ lands on your desk. The part has a very complex shape with areas of large increases in length-of-line. Tight specifications limit dimensional variations. So far you are okay—until you look at the mechanical-property specifications for the part. The final part must have a martensitic microstructure with minimum yield strength of 150 ksi.

What is known about the specified martensitic steel? The as-received properties are 150 ksi min. yield strength, 220 ksi min. tensile strength, and a typical total elongation (2-in. gauge length) of 5 percent. The workhardening exponent (n-value) is so low that it is immeasurable by standard test procedures. The forming-limit diagram has an FLC0 of 15 percent. Springback is a major source of dimensional variation. Since the elastic stresses that cause springback are proportional to yield strength, the springback is about three times that of 50-ksi-yield steel. A usual mode of deformation for martensitic steel is rollforming because die forming is not possible or limited to very shallow, simple shapes.

To quote your part, you may need to break the part down into four separate pieces and then weld them together. Considering the extreme amount of springback compensation required and the introduction of dimensional variation in welding these four pieces, what is the probability of keeping dimensional tolerances?

Some industries, especially the automotive industry, have created many final parts with 150 ksi and higher yield strengths by taking a radically different approach. Nothing in the RFQ says the as-received steel must start with those high-strength properties; only the final part needs to meet those specifications. These companies successfully make the parts by hot forming the steel. The premise is simple. Process low-strength steel through the following steps in the schematic: 1) External to the die, cut and preheat the blank to high temperatures to lower the yield strength for maximum stretchability, 2) Quickly transfer the hot blank to the die and form the part without elastic springback, and 3) In-die quench the part to generate the required very high-strength final martensitic microstructure. Process information is amplified below.

• While several steels can be used in the hot-forming process, the most common is a boron-manganese (22MnB5) steel. The as-received properties are 50 ksi min. yield strength, 70 ksi min. tensile strength, and a typical total elongation of 23 to 27 percent. The blank-cutting tools must be designed to withstand these properties.

• The blank is heated to a target temperature of 1650 F. In less than 10 min., the microstructure transforms to austenite. At this temperature, austenite has a relatively constant yield strength of 6 ksi and allowable elongations greater than 50 percent. Because of the high temperature, the steel needs a protective coating to prevent formation of a surface oxide.

Schematic showing properties of hot-forming steel from 1) as-received to 2) hot forming to 3) final quenched part.

• The blank is transferred to a water-cooled die for immediate forming. At the end of the forming stroke, the punch is held at bottom dead center to allow the punch and die to quickly quench the formed part to a martensitic micro-structure. This locks in the geometrical shape of the part. The part properties now are 150 ksi min. yield strength, 220 ksi min. tensile strength and a typical total elongation of 5 percent. This is the strength specified for the final part.

• The very high strength and low elongations of the final part restrict final operations to special cutting, trimming and piercing equipment designed to withstand the high loads generated by these operations. No additional forming should be attempted.

The above steps describe the “direct” hot-forming process during which all forming is done on the hot blank in the water-cooled die. A second process, called an “indirect” process, also is used. Here the part is partially formed (to 90 percent complete) in traditional dies at room temperature. Instead of the blank being heated, the partially formed part is heated to 1650 F. The hot part is transferred to the die for forming the severe design features that tear during room-temperature forming. Quenching to the martensitic-steel microstructure completes the process.

One can argue that the process is too slow to be profitable. The total time for transfer to the die, forming and quenching is about 20-30 sec. If the part is small, multiple parts can be formed and quenched at the same time, reducing the process time allocated to each part. However, the real comparison must be against forming the part completely at room temperature with 150-ksi steel having only 5 to 6 percent total elongation. If making the part requires four subparts, four forming dies, a number of line dies to correct for springback, welding costs and uncontrollable dimensional variations, this usual forming mode could well exceed the cost of hot forming. If the part absolutely cannot be made at room temperature, is the cost now infinite?

Hot forming has been around for a number of years. The recent automotive weight-reduction programs (increased strength and reduced sheetmetal thickness) have allowed hot forming to prove itself as a competitive process. However, if you are set in your ways and always have processed parts the same way for decades, hot forming will never even come up on your radar. MF