Forming AHSS Not a Radical Change from Conventional HSS
Forming of AHSS is not a radical change from forming conventional HSS. The major acquisition of new knowledge and experience needed for forming higher strength steels in general has been gained gradually over the years with ever-increasing strengths available in the HSLA grades. Now new demands for improved crash performance, while reducing mass and cost, have spawned a new group of steels that improve on the current conventional base of HSS.
The AHSS solve two distinct automotive needs by two different groups of steels. The first group as a class has higher strength levels with improved formability and crash-energy absorption compared to the current HSLA grades. This requirement is fulfilled by the DP and TRIP grades of steel, which have increased values of the work hardening exponent. The second is to extend the availability of steel in strength ranges above the HSLA grades. This area is covered by the CP and MS grades. Originally targeted only for chassis, suspension, and body-in-white components, AHSS are now being applied to doors and other body panels. Additional steels highlighted previously in Figure 1-1B are designed to meet specific process requirements. These include increased edge stretch flangeability, strengthening after forming, or increased springback tolerances.
The improved capabilities the AHSS bring to the automotive industry do not bring new forming problems but certainly accentuate problems already existing with the application of any higher strength steel. These concerns include higher loads on presses and tools, greater energy requirements, and increased need for springback compensation and control. In addition, AHSS have greater tendency to wrinkle due to lack of adequate hold-down and often a reduction in sheet thickness.
Matching exact mechanical properties of the intended steel grade against the critical forming mode in the stamping not only requires an added level of knowledge by steel suppliers and steel users, but mandates an increased level of communication between them. A specific example is total elongation versus local elongation. Total elongation has been the traditional measure of the steel’s general stretchability over wide areas of the stamping – required length of line deformation. Now, local elongation over very small gauge lengths found in stretch flanging, hole expansion, and blanked edge extension is as important as total elongation. The modification of microstructure to create DP and TRIP steels for increased work hardening exponent, greater stretchability and crash energy absorption, and higher total elongations reduces local elongation and edge stretchability – and vice versa.
New emphasis is being placed on determining specific needs of the stamping, highlighting critical forming modes, and identifying essential mechanical properties. The interaction of all inputs to the forming system means the higher loads and energy needs of AHSS also place new requirements on press capacity, tool construction/protection, lubricant capabilities, process design, and maintenance.
The Advanced High-Strength Steel Application Guidelines (AHSS Guidelines) Guidelines addresses the forming issues, drawn from the experience of WorldAutoSteel's global members and partners. To learn more about forming AHSS, click the link.
