Multi-phase steels are complex to cut and form, requiring specific tooling materials. The tooling alloys which have been used for decades, such as D2, A2 or S7, are reaching their load limits and often result in unacceptable tool life. The mechanical properties of the sheet steels achieve tensile strengths of up to 1800 MPa with elongations of up to 40%. Additionally, the tooling alloys are stressed by the work hardening of the material during processing.
The challenge to process AHSS quickly and economically makes it necessary for suppliers to manufacture tooling with an optimal tool steel selection. The following case study illustrates the tooling challenges caused by AHSS and the importance of proper tool steel selection.
A manufacturer of control arms changed production material from a conventional steel to an Advanced High-Strength Steel (AHSS), HR440Y580T-FB, a Ferrite-Bainite grade with a minimum yield strength of 440 MPa and a minimum tensile strength of 580 MPa. However, the tool steels were not also changed to address the increased demands of AHSS, resulting in unacceptable tool life and down time.
According to the certified metal properties, the 4 mm thick FB 600 material introduced into production had a 525 MPa yield strength, 605 MPa tensile strength, and a 20% total elongation. These mechanical properties did not appear to be a significant challenge for the tool steels specified in the existing die standards. But the problems encountered in production revealed serious tool life problems.
To form the FB 600 the manufacturer used D2 steel. D2 was successful for decades in forming applications. This cold work tool steel is used in a wide variety of applications due to its simple heat treatment and its easily adjustable hardness values. In this case, D2 was used at a hardness of RC 58/60.
While tools manufactured from D2 can withstand up to 50,000 load cycles when forming conventional steels, these particular D2 tools failed after only 5,000 – 7,000 cycles during the forming of FB 600. The first problems were detected on a curl station where mechanical overload caused the D2 tools to break catastrophically, as seen in Figure 1 below. Since the breakage was sudden and unforeseeable, each failure of the tools resulted in long changeover times and thus machine downtime.
Since the cause of failure was a mechanical breakage of the tools, a tougher alternative was consequently sought. These alternatives, which included A2 and DC53 were tested at RC 58-60 and unfortunately showed similar tool life and failures.
Metallurgical analysis indicated that the failure resulted from insufficient impact strength of the tool steel. This was caused by the increased cross-cut that the work-hardened AHSS exerted on the curl. As an alternative material, a cold work steel with a hardness of 58-60, a tensile strength of about 2200 – 2400 MPa and high toughness was sought. These properties could not be achieved with conventional tool steels. The toolmaker used a special particle metallurgy (PM) tool steel to obtain an optimum combination of impact strength, hardness and wear resistance.
Particle metallurgy (PM) tool steels, due to their unique manufacturing process, represent improvements in alloy composition beyond the capabilities of conventional tool steels. Materials with a high alloy content of carbide formers such as chromium, vanadium, molybdenum and tungsten are readily available. The PM melting process ensures that the carbides are especially fine in particle size and evenly distributed (reference Table 1). This process results in a far tougher tool steel compared to conventional melting practices.
The manufacturer selected Z-Tuff PM® to be used at a hardness of RC 58-60. Employing the identical hardness as the conventional cold work steel D2, a significant increase in impact strength (nearly 10X increase as measured by un-notched Charpy impact values) was realized due to the homogeneous microstructure and the more evenly distributed precipitates. This positive effect of the PM material led to a significant increase in tool life. By switching to the PM tool steel, the service life is again at the usual 40000 – 50000 load cycles. By using a steel with an optimal combination of properties, the manufacturer eliminated the tool breakage without introducing new problems such as deformation, galling, or premature wear.
AHSS creates tooling demands that challenge the mechanical properties of conventional tool steels. Existing die standards may not be sufficient to achieve consistent and reliable performance for forming, trimming and piercing AHSS. Proper tool steel grade selection is critical to ensuring consistent and reliable tooling performance in AHSS applications. Powder metallurgical tool steels offer a solution for the challenges of AHSS.