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Dual-phase steels:
A tailored portfolio for the next generation of lighter automobiles

We at thyssenkrupp have widened our portfolio of cold-rolled dual-phase steels well beyond the conventional high-strength steels. We have added new grades which as well as offering high energy absorption and deformation resistance also display enhanced cold-forming properties – and allow substantial weight savings in the car body.

For the vehicles of the future: eco-friendly, resource-efficient, safe, comfortable, and with low production and running costs.

Product range, DIN EN and VDA

thyssenkrupp supplies the following steel grades as per the product information or the reference steel grades in accordance with the respective standards.

Dual-phase steel

SURFACE FINISHING
Steel grade Reference grade
DIN EN 10152,
10338, 10346 /
VDA 239-100
-/UC ZE/EG Z/GI ZF/GA ZM AS ZA AZ
DP-W® 330Y530T #25cf #25cf
DP-W® 330Y580T HDT580X /­ HR330Y580T-DP #25cf #25cf
DP-W® 3300Y580T #25cf
DP-W® 700 #25cf #25cf
DP-K® 290Y490T
HCT490X / HR290Y490T-DP #25 #25 #25 #25 #25
DP-K® 330Y590T
HCT590X / CR330Y590T-DP #25cf #25cf #25cf #25cf #25cf
DP-K® 330Y590T DH #25cf
DP-K® 34/60 HF

#25cf #25cf #25cf
DP-K® 420Y590T

#25cf
DP-K® 440Y780T HCT780X / CR440Y780T-DP #25cf
DP-K® 420Y780T DH – / CR440Y780T-DH #25cf
DP-K® 590Y980T HCT980X / CR590Y980T-DP #25cf #25cf
DP-K® 60/98 #25cf #25cf
DP-K® 700Y980T HCT980XG / CR700Y980T-DP #25cf #25cf #25cf #25cf
DP-K® 780Y1180T #25cf
DP-K® 900Y1180T #25cf

available
available; suitable for external parts

-/UC: Uncoated
ZE/EG: Electrolytically galvanized
Z/GI: Hot-dip galvanized
ZF/GA: Galvannealed
ZM: ZM Ecoprotect®
ZA: galfan®
AZ: galvalume®
AS: Aluminium Silicium Coated

Notes on applications and processing

Material characteristics

Hot and cold-rolled dual-phase steels offer a particularly attrative combination of high strength, low yield point, good cold formability and weldability due to their concerted microstructure of ferrite and martensite components.

This high strain-hardening capacity reduces the risk of local constriction of the material during the forming process and induces a strong increase in the component’s yield point in the worked areas even at low degrees of deformation. The final strength of the component is also determined by the high bakehardening potential.

The microstructure consists predominantly of a soft ferrite matrix, in which a second, hard, mainly martensitic phase is embedded in pockets. The ferrite content is up to 90%. In addition to martensite, austenite and bainite components can also exist, thus improving formability. In nital etching, the grain boundaries are well contrasted. Color etching according to Klemm contrasts the grain surfaces. The grain surfaces of the ferrite appear in brown or blue hues; martensite is brown; structurally weak martensite and austenite are shown in white.

Structural example of cold-rolled DP steels

Microstructural contrasting through etching with nital

Microstructural contrasting through etching with nital.

Microstructural contrasting with color etching according to Klemm

Microstructural contrasting with color etching according to Klemm.

Forming

Hot-rolled dual-phase steels DP-W® are particularly suitable for weight-saving production of wheels, chassis parts, profiles, body reinforcements, etc. Cold-rolled dual-phase steels, steels DP-K® are suitable for both complex structural parts, e. g., side members and cross members, as well as stretch-formed exterior parts with special requirements in terms of buckling strength (doors, roofs, trunk lids). The choice of the right type for a given strength level must also be made with a special focus on the actual anticipated forming stresses. This allows optimum leveraging of specific benefits so that the steels can also be used for difficult drawn parts.

Due to the good strain-hardening behavior, expressed by a relatively high n-value, dual-phase steels exhibit high resistance to local constriction, as a larger area of material is involved in the deformation zone due to greater strain hardening. The microstructure of dual-phase steels, composed of hard martensite and soft ferrite, which promotes strain hardening, as well as the distinct cutting-edge hardening in mechanical cutting substantially impact the good forming potential in the trim cutting-edge area. For engineering design, e.g., in case of through-hole extensions or the height of drawn flanges in corner areas, this must be taken into consideration. Small bending and drawing radii relative to the respective thicknesses should thus be avoided. Under high shear stress, dual-phase steels react much more sensitively than high-strength micro-alloyed steel grades. Plasticization must be designed to be as homogeneous as possible in order to improve the dimensional accuracy of the worked components. The presses should have high pressing and hold-down force potentials. As a guideline, the tensile strength level should be considered here, compared with known materials. Upstream straightening equipment must also be designed accordingly. Special attention must be paid, as the strength level of dual-phase steels increases, to the design of the forming and cutting tools. Tool requirements are exacting, especially in cutting. In addition to a sufficient hardness of> 60 HRC, it is important to select suitable tool materials to simultaneously ensure high ductility, thus preventing premature breaking of the cutting-edges. By means of specific rounding of the cutting edge in the order of about 50 μm, the edge strength of the tools can be optimized. The cutting gap must be designed to take the material thickness into account and should be (as a guideline) ≥ 10% of the sheet thickness.

A sufficient supporting hardness must be achieved for the forming tools. A segmented structure of the forming tools is common today. In highly stressed areas, the use of high speed steels may be necessary. These include 1.3343 or equivalent materials produced by powder sintering. In addition, tool coatings such as CVD (TiC-TiN coating) can minimize tool wear.

Processing instructions for joining

DP steels principally support welding well in same-grade joints or in hybrid joints with other common steel grades. The precondition is welding parameters matched to the material.

Resistance spot welding

For spot welding of dual-phase steels, the same equipment can be used as for welding unalloyed deep drawing steels. Compared to same-thickness steel grades of lower strength, the welding zone tends to shift toward lower currents. At the same time, the setting range narrows slightly, but this can be largely compensated for by increasing the electrode force and welding currents. An extension of the current flow times, or for example the use of multi-pulse welding in line with SEP 1220-2, can also have a favorable effect on the width of the welding zone.

In resistance spot welding of galvanized sheets, the welding currents must be increased due to the higher conductivity of the coating compared with the base material (substrate). In addition to this, increasing the electrode force and welding time has a favorable effect on the welding zone. In addition to the sheet type, surface and thickness combination, other factors, e.g., the type of electrode used, play an important rolein determining optimum joining parameters.

MIG arc brazing

Information sheet DVS 0938-2 “Arc brazing” describes brazingof steels up to a tensile strength of approximately 500 MPa. As the material described is above this tensile strength, it is advisable to check the component-specific suitability of brazing.

Fatigue strength and crash performance

As already described in the product information, dual-phase steels have a high strain-hardening capacity combined with high yield point values. The high yield point and high tensile strength are evidenced by high fatigue limits. Fine and dispersed distribution of martensite and ferrite avoids any adverse effect on the fatigue limit on account of the strength difference between the structural components of ferrite and martensite. Higher strength values due to work hardening as an effect of deformation, including the bake-hardening effect, contributeto the advantageous material behavior. The high level of strength and the high strain-hardening capacity make dual-phase steels ideal for crash energy absorbing components.

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