HCT780Xxtend® dual phase steel with increased formability

24.05.2017 | Salzgitter Flachstahl GmbH

Dual phase steels have a favorable combination of relatively low yield strength, high tensile strength and strong hardening. This combination of properties makes them excellent for use in car bodies. The low yield strength allows the manufacture of complex geometries with relatively low forming forces. The strong hardening in conjunction with the high tensile strength makes it possible to create elements that absorb significantly more energy in the event of a vehicle crash than elements made of conventional, microalloyed steels. These advantages are further improved by extra hardening when the paint is baked in (bake hardening effect).

Thanks to these favorable properties, dual phase steels have been used in a significant portion of structural elements in vehicle bodies. The possibility of also using these steels in corrosion-protected, hot-dipped zinc coated versions has also contributed to this success.
These properties are achieved by the selective creation of hard, strengthening martensite islands in the ferritic matrix of these steels. The selective cooling during the production and the corresponding chemical analysis make it possible to adjust the strength by altering the martensite content.

Steel Grade Yield Strength
Tensile Strength
Elongation 		Hardening exponent
n 		BH2
according to VDA Rp0,2
MPa 		Rm
MPa
A80 mm
% 		n4-6 n10-20/Ag MPa
HCT780X CR440Y780T-DP 440 - 550 780 - 900 ≥ 14 ≥ 0,15 ≥ 0,11 ≥ 30
HCT780Xxtend® CR440Y780T-DH 440 - 550 780 - 900 ≥ 18 ≥ 0,18 ≥ 0,13 ≥ 30

Table 1: Mechanical characteristics of cold-rolled dual phase steels in accordance with VDA 239


The formability of these steels has been further improved as a consistent extension of their successful use (Table 1). This is done by analytical and process changes that set in a low content of bainite and residual austenite in the microstructure (Figure 1). These changes allow the manufacture of solider, more complex components and consequently a further increase in passenger safety without necessitating an increase in the vehicle weight. On the other hand, the improved formability helps simplify the manufacturing of such elements (Figure 2), which makes it possible to avoid procedures that are more complex and more cost-intensive. The increased formability can also be used for greater energy absorption in the event of a crash (Figure 3) and consequently it contributes to improved passenger safety.

Photo of a typical microstructure of a dual phase steel
Figure 1: Typical microstructure of a dual phase steel with increased formability HCT780Xxtend®+Z
Graphic of the forming limit curves (FLC) for HCT780X+Z and HCT780Xxtend
Figure 2: Forming limit curves (FLC) for HCT780X+Z and HCT780Xxtend®+Z
Stress/strain diagram for HCT780X+Z and HCT780Xxtend®+Z
Figure 3: Stress/strain diagram for HCT780X+Z and HCT780Xxtend®+Z

Salzgitter Flachstahl offers CR440Y780T-DH dual phase steel with increased formability under the brand name HCT780Xxtend® with a hot-dipped zinc coating. The brand name xtend® stands for the improved expansion provided by these steels. Like the previously offered dual phase steels, the xtend® steels also offer excellent processing characteristics, including weldability.
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