Page 21 - MetalForming May 2012

Mild Steel
Bake Hard HSLA
DUAL-TEN® 590 DUAL-TEN® 590 IBFTM
DUAL-TEN® 780 DUAL-TEN® 780 IBFTM
DUAL-TEN® 980 DUAL-TEN® 890 IBFTM
MULTI-TENTM 1180
MART-TENTM 1500 PHS-TENTM 1500
(HSLA) grades with strength levels around 450 MPa. Use of grades with strength in excess of 450 MPa was extremely limited, since they lacked sufficient ductility to stampers to achieve the required complex shapes. However, automotive designers required higher-strength steels to keep pace with increasing crash-safety and fuel-economy mandates.
To meet this challenge, the steel industry developed the advanced high- strength steels. In particular, United States Steel created its USS Safety Steel family of steels, including Dual-Ten and Trip-Ten grades in strength levels of 590 to 980 MPa. These new steels pro- vide the added strength and ductility necessary to achieve improved crash safety, with thinner gauges that enable mass reduction. They also provide suf- ficient ductility to allow stamping for high-volume automotive production.
In 1999, United States Steel received the first North American production application of these grades when Dual- Ten 590 was applied to a front upper rail to improve front crash performance, while reducing the weight typically associated with such a part. Since 1999, demand for the high-strength USS Safe- ty Steel family of steels has increased.
“These new grades represent 10 per- cent of the flat-rolled steel in a typical vehicle,” says Mike Juddo, United States Steel’s director of product technology, “or 160 lb. per vehicle, with some vehi- cles reaching well in excess of 300 lb. Looking to the future, our customers expect to increase the use of these high- strength grades to 40 percent, or 590 lb. on a typical vehicle, with some vehicles including as much as 650 lb.”
Springback, Die Life and Other Challenges
The application of these high- strength grades initially created signifi- cant manufacturing challenges. Metal- formers were faced with the difficulty of forming components within the steels’ restricted forming envelopes while maintaining part dimensional tolerances challenged by springback. Other chal- lenges revolved around die life, since the strength of the steels being stamped
approached that of the die steels. This resulted in die wear and fatigue under the required forming load. Over time, die technology evolved to address these challenges, allowing the use of Dual- Ten 590 to mature to where it is now a regularly used material at many auto- motive-stamping facilities. Likewise, Dual-Ten 780 and 980 are rapidly pro- gressing down the same path.
Meanwhile, some of the challenges faced when forming these new grades were not anticipated and rarely expe- rienced with conventional mild and high-strength steels. As demand grew for applications of Dual-Ten 780 and 980, stampers began to experience some previously uncommon stamp- ing failures, such as cracking of sheared edges during stretching and shear frac- ture at tight bend radii. These failures— associated with areas in the stamping that undergo a significant amount of localized deformation—have been termed “local formability” failures. While traditional necking and splitting of stampings has been understood and predicted for several decades using analytical tools available for simula- tion and for use in the field, no such tools exist for these new failure modes.
“At United States Steel’s Automotive Center (in Troy, MI), we are working to develop technology to predict local formability failure such as shear frac- ture and edge cracking,” says Dr. Ming Shi, United States Steel’s manager of advanced applications technology, “and to identify more robust manufacturing techniques to prevent these failure modes. We are making progress in the lab, as well as through partnerships with universities and our customers.”
As an example, a common failure experienced in production has been at the sheared edge of a stamped blank. An extensive study conducted at the United States Steel Automotive Cen- ter found that significant improve- ments can be achieved with the shear die configuration by optimizing the shear die clearance, cutting angles and shearing speed. When implemented correctly, a blanking die that employs these techniques can provide a 25-per-
The USS Safety Steel family of steels, which includes the popular Dual-Ten 590 and 980 grades, “represent 10 percent of the flat-rolled steel in a typical vehicle,” says Mike Juddo, United States Steel’s director of product technology, “or 160 lb. per vehicle, with some vehicles reaching well in excess of 300 lb. Looking to the future, our customers expect to increase the use of these high-strength grades to 40 percent, or 590 lb. on a typical vehicle.”
cent improvement in edge-fracture strain compared to conventional blank- ing-die designs.
IBF Steels
United States Steel also is taking a product-development approach to addressing local formability issues, evi- denced by the recent development and commercialization of IBF (Improved Bending and Flanging) steel grades. Through microstructure optimization, these grades provide a 50- to 100-per- cent improvement in local formability properties compared to traditional Dual-Ten 780 and 980 grades. In a recent stamping study conducted by the Auto/Steel Partnership to evaluate several 980-MPa tensile-strength steels on a part prone to local formability failure, an IBF 980 grade provided supe- rior performance and was able to meet the forming requirements. IBF grades have been a game changer, enabling the use of 780- and 980-MPa steels in critical applications that would not otherwise be possible with traditional dual-phase grades.
“With the application of 980-MPa
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USS Safety Steel®