AHSS of the 3rd generation in sight – highly ductile dual-phase steels
High strength combined with high elongation: Two conflicting goals that can already largely be resolved by modern multi-phase steels (AHSS). A new class of dual-phase steels made by thyssenkrupp Steel offers around 30 percent higher elongation – compared with conventional grades of this steel family. They bear the designation DH – Ductility High. These materials in the strength classes 600 and 800 MPa are currently being used successfully for series production. AHSS steels of the 3rd generation with strength values of 1,000 and 1,200 MPa are in an advanced testing stage und will open up completely new possibilities of cost-effective lightweight construction both in combustion-engine vehicles and in hybrid and electric vehicles.
Dual-phase steels are the classic universal grades when it comes to high and ultra-high strengths combined with good forming and joining properties. Their share in multi-phase lightweight construction steels for cold forming has been rising for years and accounts currently for about 90 percent. However, the use of increasingly higher strengths, especially for weight optimization, is limited by complex component geometries. Forming operations that can still be carried out easily at 600 MPa may no longer be possible if a higher strength is used.
If the strength of the materials used for a component is increased in order to reduce weight and if this leads to problems in the press shop, the new DH grades can be the solution to these problems. Their elongation reserves enable higher forming degrees, and the proven component geometry does not have to be elaborately modified. Thus, significant weight reductions of 10 to 11% could be determined for various design options of a front longitudinal member by increasing the strength from 600 to 800 MPa. Depending on the component in question, the use of the new DH grades is also attractive in terms of costs, which are reduced by 2% to more than 3%. Concepts for vehicles with conventional and electric drives were examined.
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The substitution of a conventional dual-phase grade by a DH variant with the same strength may make sense: If the geometric design borders on technical limits or if mispressing occurs or an increased residual elongation at fracture is used to enhance the crash performance of structural components.
Sights on the 3rd generation AHSS with strength values of 1,000 and 1,200 MPa
The advantage of steels with strength values of over 1,000 MPa is their improved ductility. Their high lightweight construction potential is often limited by the geometric complexity of modern structural components. At thyssenkrupp Steel, the development of modern multi-phase steels of the 3rd generation with up to 1,200 MPa is well advanced. They combine good local and global forming properties. High elongation at fracture as well as good hole expansion capability and high resistance to edge compression failure allow for the first time more complex component geometries with high stretching and deep-drawing proprtions in the forming process, thus tapping new cost-reduction and lightweight construction potential. Like the conventional dual-phase steels of this strength class, the new grades are protected against corrosion by the proven hot-dip galvanizing process.
Electromobility involves high crash behavior requirements
Lightweight construction still plays an important role also in vehicles with electrified powertrains. Although more kilos in electric cars are not necessarily tantamount to more emissions, and thanks to recuperation, the range is not affected to any significant extent either, crash scenarios of heavy battery vehicles pose new challenges for developers in the automotive industry and materials researchers alike. Both head-on collisions and rollovers are an enormous mechanical load on the car body structure. Strength alone is of limited use in the design of components such as side members and A-pillars, only in combination with a high energy absorption can optimum crash performance of these structures be achieved. Given their significant higher ductility, AHSS grades of the 3rd generation will in future play an important role in the development of electric vehicles with maximum car occupant safety. That’s why steel is the “Material of Mobility“.
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.
|Steel grade|| Reference grade
DIN EN 10152,
10338, 10346 /
|DP-W® 330Y580T||HDT580X / HR330Y580T-DP|
| DP-K® 290Y490T
||HCT490X / HR290Y490T-DP|
| DP-K® 330Y590T
||HCT590X / CR330Y590T-DP|
|DP-K® 330Y590T DH||–|
| DP-K® 420Y590T
|DP-K® 440Y780T||HCT780X / CR440Y780T-DP|
|DP-K® 420Y780T DH||– / CR440Y780T-DH|
|DP-K® 590Y980T||HCT980X / CR590Y980T-DP|
|DP-K® 700Y980T||HCT980XG / CR700Y980T-DP|
1. With different mechanical properties on request.
available; suitable for external parts
ZE/EG: Electrolytically galvanized
Z/GI: Hot-dip galvanized
ZM: ZM Ecoprotect®
Notes on applications and processing
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 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
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. In such cases, it is recommended to choose the yield-optimized variants. 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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