Salzgitter Flachstahl expanding its portfolio to include further dual-phase steels with enhanced formability

10.09.2021 | Salzgitter Flachstahl GmbH

Picture of seat backrest side panel from Faurecia
Seat backrest side panel from Faurecia (by courtesy of Faurecia Autositze GmbH, Stadthagen, Germany).

Introducing the two new products CR330Y590T-DH-GI and CR440Y780T-DH-GI, Salzgitter Flachstahl is now covering an additional area of the automotive standard VDA239-100.

The first generation of dual-phase steels had already represented an attractive combination of low yield strength, high tensile strength, good formability, and strong hardening. The two new products that are now available, identifiable in the VDA239-100 standard by the abbreviation DH in the designation, exhibit increased elongation at break, thereby enabling the production of complex geometries in connection with relatively low forming forces. The high strain hardening combined with the high tensile strength makes it possible to produce components which, in the event of a vehicle crash, absorb significantly more energy than components made from conventional microalloyed steels - and even compared with conventional dual-phase steels of the same strength. These advantages are additionally enhanced by further strengthening during paint baking (bake-hardening effect). This property profile opens up a multitude of potential applications in the structure of vehicle bodies, e.g. in longitudinal members with increased degrees of freedom now available in terms of geometry.

 

Picture of the typical microstructure of the HCT780Xxtend® (CR440Y780T-DH-GI)
Fig. 2: Typical microstructure of a dual-phase steel with enhanced formability HCT780Xxtend® (CR440Y780T-DH-GI)

This property profile is achieved by the targeted generation of hard, strength-increasing martensite islands in the ferritic matrix of these steels, combined with low proportions of bainite and retained austenite (Fig. 2). 

 

Yield strength Rp0.2 Tensile strength Rm Total Elongation A80 Hardening exponent n BH2
MPa MPa % MPa
CR250Y440T-DP* 250 - 330 440 - 560 ≥ 27 ≥ 0.16 ≥ 30
CR290Y490T-DP 290 - 380 490 - 600 ≥ 24 ≥ 0.15 ≥ 30
CR330Y590T-DP 330 - 430 590 - 700 ≥ 20 ≥ 0.14 ≥ 30
CR440Y780T-DH 440 - 550 780 - 900 ≥ 18 ≥ 0.13 ≥ 30
CR440Y780T-DP 440 - 550 780 - 900 ≥ 14 ≥ 0.11 ≥ 30
CR590Y980T-DP 590 - 740 980 - 1130 ≥ 10 - ≥ 30
CR660Y980T-DP* 590 - 740 ≥ 980 ≥ 10 - -
CR700Y980T-DP 700 - 850 980 - 1130 ≥ 8 - ≥ 30
CR700Y980T-DH 700 - 850 980 - 1180 ≥ 13 - ≥ 30

* following VDA 239-100

Figure 3: Comparison of stress-strain curves in longitudinal direction of HCT600X (CR330Y590T-DP) vs. HCT780X (CR440Y780T-DP) vs. HCT780Xxtend® (CR440Y780T-DH)

Salzgitter Flachstahl is offering the dual-phase steels with increased formability CR330Y590T-DH and CR440Y780T-DH under the brand names HCT590Xxtend® and HCT780Xxtend® in hot-dip galvanized design. The brand name xtend® is based on the Latin extendere (= to stretch) and stands for the improved elongation of these steels. Similar to the dual-phase steels offered to date, the xtend® steels feature outstanding processing properties, including their weldability.

The comparison of the stress-strain curves of the three dual-phase steels HCT600X (CR330Y590T-DP), HCT 780XD (CR440Y780T-DP) and HCT780Xxtend® (CR440Y780T-DH) shown in Fig. 3 illustrates the performance of this new group of dual-phase steels with increased formability. In connection with the same strength class, component complexity can be increased by opting for xtend® steels. In addition, this material can also serve as a problem solver for raising production stability when increased demands are made on forming behavior. A comparison of the stress-strain curves of HCT600X and HCT780Xxtend® shows up another starting point for its use. The considerable difference in strength between the two steel grades is reflected to a minor extent in the difference in forming behavior, as determined by the elongation at break in this case. This makes it possible to produce components with increased strength, which can be used, for example, in car body construction to increase safety for the vehicle occupants without having to increase the vehicle weight. The increased formability also increases energy absorption in the event of a crash.

 

In order to illustrate the possible applications just described, a potential study (Fig. 3) was carried out. The manufacturability assessment based on a concrete component (seat-back side part) was carried out with the LS-Dyna simulation program, taking into account the input data (sheet thickness, flow curve, r-values and FLC) of the two materials (HCT600X+Z (CR330Y590T-DP-GI) in 1.4 mm; HCT780Xxtend®+Z (CR440Y780T-DH-GI) in 1.2 mm), as well as the tool geometry. 

 

Figure 4: Seat-back side panel from Faurecia as a component example of a potential study of the HCT780Xxtend®+Z (CR440Y780T-DH-GI)

Result: Thanks to the increase in strength, the sheet thickness can be reduced from 1.4 mm to 1.2 mm, resulting in weight savings of 14 percent. Due to the fact that less material is required as a result, component manufacturing entails lower CO2 emissions. As a result, the CO2 equivalent of this seat-back side panel under consideration is reduced by around 0.9 kg (excluding processing at the supplier).

Should you have any further questions please do not hesitate to contact your sales representative and your technical customer service representative.