Introduction to advanced high-strength steels - Part II

Part I of this two-part series presented an overview of advanced high-strength steels (AHSS). Part II addresses issues encountered when processing these grades.

Using AHSS in appropriate applications offers opportunities for reduced product weight, enhanced crash performance, manufacturing process consolidation, and cost reduction. However, because these grades have different microstructures, chemistries, and properties, die processing must be optimized to take advantage of the differences. Proactively addressing the associated challenges can help minimize costly tryout.

Springback

Figure 1 Dimensional Stability as a Function of Strength. Note: HSLA grade circled in red; all others are DP grades Source: M.F. Shi, G.H. Thomas, X.M. Chen, and J.R. Fekete, "Formability Performance Comparison Between Dual Phase and HSLA Steels," Proceedings-Process Symposium 43rd Mechanical Working and Steel Processing Conference, Iron & Steel Society, 2001.	Figure 2 Dimensional Stability as a Function of Tensile Strength Source: M. Kamura, Y. Omiya, J. Shaw, and M. Chen, "Formability and Spring Back Characterization of Advanced High Strength Steel," Society of Automotive Engineers (SAE), 2003-01-0522.

As with all high-strength steels, springback is a concern with AHSS. Without appropriate compensation, the higher strength of the AHSS grades usually leads to more springback compared to conventional HSS. The degree of springback correlates with the yield strength level after forming (rather than the yield strength in the flat sheet) and the tensile strength. As shown in Figure 1 and Figure 2, this correlation is independent of the microstructure or strengthening approach —high-strength, low-alloy (HSLA), dual-phase (DP), or transformation-induced plasticity (TRIP).

Comparing a DP steel with an HSLA steel, each with 350-MPa (50-KSI) yield strength and formed under the same conditions, the DP steel has much greater springback than the HSLA steel. This difference is because the parts made from DP steel have greater yield strength after forming (greater work hardening) compared with the HSLA steel parts. Also, the DP steel's tensile strength is about one-third greater than the HSLA steel's. Adjusting tooling parameters, such as the die radii and bead placement and severity, can control springback to manageable levels (Figure 3).

Figure 3 Controlling Springback By Changing Die Parameter Source: A.A. Konieczny, M.F. Shi, and C. Du, "An Experimental Study of Springback for Dual Phase Steel and Conventional High Strength Steel," Society of Automotive Engineers (SAE), 2001-01-3106.

Die adjustments can be made to achieve even greater stretch forming with TRIP steels. As shown in Figure 4, TRIP steel with a tensile strength of about 800 MPa performs similarly to 600- and 440-MPa grades with punch shoulder radii of 2 or 6 mm. This advantage is lost at the largest tested punch shoulder radius of 25 mm, presumably because the sheet metal does not stretch as much, which reduces the work-hardening effect. A tight die shoulder radius helps with the dimensional stability of TRIP (and non-TRIP) steels ( Figure 2). Although a tight radius reduces springback, it also increases forming challenges and press load requirements.

Figure 4 Stretch Forming Height of Different Microstructures. See Figure 2 for Legend and Property Information. Source: M. Kamura, Y. Omiya, J. Shaw, and M. Chen, "Formability and Spring Back Characterization of Advanced High Strength Steel," Society of Automotive Engineers (SAE), 2003-01-0522.

Figure 5 compares the performance of several steels in various simulative tests. The TRIP grade's draw depth is comparable to that of deep-drawing steel (DDS) and superior to that of other high-strength grades, except in hole flanging. In this stretched edge test, the hard second phases act as stress risers and inhibit the degree of edge elongation.

Figure 5 Performance of Several Steels in Various Simulative Tests Source: D. Cornette, T. Hourman, O. Hudin, J.P. Laurent, and A. Reynaert, "High Strength Steels for Automotive Safety Parts," Society of Automotive Engineers (SAE), 2001-01-0078.

Springback, side wall curl, and panel twist have their origins in unbalanced stresses in the formed part. These can be caused by part and die processing design (nonsymmetrical parts or cutouts, rapid changes in cross section, or unequal flange lengths) or forming process differences (uneven lubrication, die polishing, blank holder forces, or broken or worn draw beads).¹

Minimizing springback requires forming process consideration. Steel will work-harden as it is drawn over a radius, and this higher strength will cause both increased springback and side wall curl and make restrike operations difficult. As such, metal movement across these radii should be limited. Furthermore, a conventional closed-end draw process might have a reduced draw-depth capability compared with an open channel approach. To achieve the desired channel height while limiting movement across radii, a hat section can be formed using a different approach, like a two-stage gull-wing process. Although the general guidance is to minimize the number of forming operations, the gull-wing process eliminates having to rework the same areas (Figure 6).

Figure 6 Multiple-Stage Forming to Minimize Springback Source: "AHSS Guidelines" at www.WorldAutoSteel.org.

Another method for improving dimensional accuracy is to stretch the side walls at the bottom of the press stroke.²Also, by employing variable binder force control, it is possible to reduce springback and obtain a more uniform strain distribution.³Controlling metal flow of these higher-strength steels can increase the binder tonnage requirements by 20 percent compared with conventional HSLA steel grades,⁴and double that needed for mild steels.⁵In the right press, these are not overwhelming issues. However, a single-acting press with a nitrogen cushion is probably insufficient for processing AHSS grades.

Edge Cracking and Trimming

Martensite and other hard second-phases of the AHSS grades reduce stretch flangeability of these steels, with the issue becoming more severe as the strength increases. The part and process should be designed to minimize features that lead to the reduced stretch flangeability. For example, stress risers can be induced by abrupt changes in flange length. A gradual transition, free of sharp notches, will minimize edge cracking potential. In addition, length of line increases occurring during stretch flanging can be compensated for by using metal gainers.

A clean edge cut is especially important for blanked or sheared edges and punched holes on AHSS material. A consistent and appropriate clearance is vital to minimize burr height. Cutting tools also need to be sufficiently sharp. Compared to sharp tools, worn tools result in a 20 percent reduction in hole expansion (stretch flangeability) in mild steels, but a reduction of 50 percent or more in DP and TRIP grades.⁶

During trimming, it is important to support the panel and trim stock. This reduces the tendency for a bad burr, which in turn minimizes the tendency for edge cracking. Trim steels also need to be engineered with a higher strength in mind—the tensile strength of the AHSS grades can be substantially higher than that of conventional HSS.

Furthermore, during tool development, the blank die sometimes is the last one to be completed, necessitating the use of laser-cut blanks during prototype and even at the beginning of hard tool tryout. Laser-cut edges have much higher stretch flangeability than the sheared edge obtained from a conventional blank die, and as a result, tryout performance may be different.

Press and Tooling

One benefit of the AHSS grades is the ability to reduce the sheet steel thickness. However, the decreased thickness has the potential to increase wrinkling if die clearances are not adjusted to reflect the reduced gauge. Controlling the wrinkling requires higher press forces, which may lead to the need for higher-tonnage presses. The wrinkling, combined with the sheet steel's overall strength, increases the potential for die wear. Upgraded die materials, premium wear-resistant coatings, and strategically placed inserts all may be needed to address die wear. Also, the high press forces cause higher temperatures, which can cause lubricants to break down or burn, resulting in extreme wear conditions. Appropriate lubricants designed for these grades should be considered.

Important to Remember

Compared to conventional HSS, the AHSS grades typically have higher tensile strength and higher formability for a given yield strength. As such, the forming system needs to change accordingly. Higher-tonnage presses are needed, appropriate die materials with inserts should be used, and optimized lubricants and tool coatings should be considered. Springback control may necessitate using variable binder force control with the ability to achieve at least 25 percent higher total binder force. The great work-hardening benefits useful for kicking up formed panel strength (discussed in Part I of this series) make restrike operations something to avoid. Minimizing stretch flanging requires attention to the trim dies, trim steels, and clearance (burrs). Metal gainers should be used to balance length-of-line. All of these issues are magnified when a process tuned for conventional HSS is re-engineered for AHSS, rather than starting with an optimized AHSS design.

With these challenges in mind, you may be overwhelmed by the requirements to process AHSS. However, with a systematic and thorough understanding of metal movement and interaction with the forming and processing system, it is possible to realize the performance, weight, and cost benefits that these grades offer.

References

1. "AHSS Guidelines," www.WorldAutoSteel.org

2. C.D. Horvath and J.R. Fekete, "Opportunities and Challenges for Increased Usage of Advanced High Strength Steels in Automotive Applications," International Conference on Advanced High Strength Sheet Steels for Automotive Applications Proceedings, June 2004, Association for Iron & Steel Technology.

3. M. Milititsky, M. Garnett, C. Du, J. Wu, L. Zhang, P.J. Belanger, J.M. Prencipe, and E.D. Bishop, "Variable Binder Force for Springback Management," International Conference on Advanced High Strength Sheet Steels for Automotive Applications Proceedings, June 2004, Association for Iron & Steel Technology.

4. M.J. Lee, "Advanced High Strength Steel Technology in the Ford 500 and Freestyle," Great Designs in Steel Seminar, March 2005, American Iron & Steel Institute.

5. "AHSS Guidelines."

6. B. Carlsson, P. Bustard, and D. Eriksson, "Formability of High Strength Dual Phase Steels," Paper F2004F454, SSAB Tunnplt AB, Borlnge, Sweden (2004).