Today many product designs tend to combine geometry complexity and part consolidation with the highest possible final strength steel required for in-service applications. Maximum part complexity usually requires superior stretchability as evidenced by high work hardening capability and defined by the n-value. Part consolidation might take three non-severe stampings and make one very severe large part. Imagine three separate stampings formed with extensive metal flow from the binder to provide maximum part depth. Lay the three stampings side-by-side in a straight line and successfully connect them with welds to make the final part. Now attempt to make all three attached stampings from a single blank in one die. There is no binder area to feed the middle stamping, which now must form almost completely by excessive stretch forming. Making the problem worse, increasing the strength of the as-received steel reduces the stretching capacity of the steel because the work hardening exponent (n-value) decreases with increasing strength for each type of steel. Finally, springback problems increase as the yield strength increases.
The hot-forming process can minimize all the above problems. Hot forming, also called hot stamping or press hardening, is a process used to form very thin, but also very strong metals into automotive parts to reduce weight while also increasing strength and safety. By performing the stamping while the steel is nearly molten, the process eliminates springback and allows for the forming of complex geometries while eliminating springback often associated with stamping advanced high strength steels.
Hot forming once was a novice technology; first used in the 1984 Saab 9000, hot-forming has steadily increased in popularity. For instance, the number of production stampings from hot-forming went up from 8 million per year in 1997 to 107 million per year in 2007 and continues to rise in all regions of the globe. For some recent vehicles, the percentage of these steels is close to 25% of the body in white (BIW) weight. Today, hot-forming is used to stamp product geometries with relatively complex shapes such as A-pillars, B-pillars, front/rear side members, bumpers, door rings, and more.
At the Steel Market Development Institute’s (SMDI) 2018 Great Designs in Steel conference in Livonia, Mich., several automakers and Tier 1 suppliers revealed 2019 vehicles where they used hot-stamped components to achieve lightweighting and strength for crash requirements (Figure 1). (Click the vehicle-name links to review the event presentations on the SMDI website.) For example, the new Chevy Silverado pick-up truck uses 5.5 percent hot stampings in the cab and truck box, and the 2019 Dodge Ram truck uses a 6-piece laser-welding door ring that is hot stamped. The new Acura RDX uses 15 percent press-hardened steels in its body structure.
Figures 2 and 3 show the process flow for direct vs. indirect hot forming. Our next blog will go into more detail about these two processes.
Benefits of Hot Forming
- Hot-forming has the highest potential for weight reduction of crash components.
- Tailored welded blanks with different combinations of thickness, properties, and surface coatings can be hot-formed as a single stamping.
- Controlling the temperature in various locations of the forming die can create zones with different strength levels in the final stamping.
- Springback issues eliminated, which is remarkable considering the extremely high final part strength.
- Manufactured stampings have low distortion.
- Stamping consolidation has high feasibility for success.
- Stampings have low directionality of properties measured by r-value anisotropy.
- A 10% increase in yield strength (about 100 MPa) bake hardening effect can further increase in-service strength.
- A-pillar upper
- B-pillar outer and reinforcement
- Front/rear side members
- BumperRoof rail
- Door ring
- Cross member
Some of these parts are often made with tailored welded blanks, mixing different thicknesses and different grades (500 MPa for the areas which must allow buckling during crash and 1500 MPa for areas where no deformation under crash is accepted).
1 2019 Chevy Silverado photo courtesy of General Motors Company
2019 Dodge Ram photo courtesy of FCA USA
2019 Acura RDX photo courtesy of Honda
2 Hot forming Process photos courtesy of M. Peruzzi, voestalpine Stahl GmbH