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tribond® – composite for hot forming

To make tomorrow’s cars even lighter thyssenkrupp is taking a new approach to hot forming with the composite material tribond®. Unlike monolithic materials, tribond® makes it possible to combine conflicting properties in one product: maximum ductility and high strength.

The steel composite tribond® 1200 offers very high strength with high energy absorption through controlled deformation of the steel under axial stresses, while tribond® 1400 allows up to two and a half times higher bending angles compared with the standard material MBW® 1500.

tribond® – tailored property profiles in auto construction

  • Composite for hot forming combines high strength and high ductility, i.e. good formability
  • Possible uses in auto bodies:
    • tribond® 1200 for parts subjected to axial loads such as longitudinal members and crash boxes that need to display high energy absorption
    • tribond® 1400 for structural parts such as B-pillars subjected to high bending loads
  • Adaptable in line with application: Tailored material with an extremely high-strength steel core and two outer layers of ductile steel
  • High weight-reduction potential: Especially in crash-relevant areas, parts made from tribond® can weigh up to ten percent less than conventional cold-formed solutions – with the same performance
  • Easy to process: tribond® can be manufactured and processed on standard hot-forming lines without the need for modification
Coating
UC ZE/EG Z/GI ZF/GA ZM AS
Steel grade Comparison grade
tribond® 1200 - #25cf
tribond® 1400 - #25cf
Composite material for hot forming based on VDA 239-100.

Notes on applications and processing

Material characteristics

The steels used for the layer structure of tribond® have carefully selected and matched chemical compositions to deliver the required properties after hot forming. In the core layer the initial ferritic-pearlitic microstructure is transformed fully into martensite, while the outer layers of the composite retain a ferritic-pearlitic-bainitic microstructure. This combination of microstructures results in extremely high strengths (martensite) combined with very high ductility under bending load.

TTT diagram: Transformation behavior of tribond® core material

TTT diagram: Transformation behavior of tribond® face material

Examples of microstructure

tribond ® 1200 in as-delivered condition.

tribond ® 1200 in as-delivered condition.

tribond ® 1200 after press hardening.

tribond ® 1200 after press hardening.

tribond ® 1400 in as-delivered condition.

tribond ® 1400 in as-delivered condition.

tribond ® 1400 after press hardening.

tribond ® 1400 after press hardening.

In the hot roll cladding process, the high temperatures and pressures join the three layers inseparably by diffusion. This diffusion zone can be seen clearly in the micrographs of the material in as-delivered condition. Hot forming causes further diffusion, but if the process window is adhered to this does not have any significant impact on the properties of the part.

Forming

Like the production material MBW® 1500+AS, tribond®+AS displays outstanding hot forming behavior. In the austenitizing temperature range, the formability of these grades is comparable with that of mild deep-drawing steels at room temperature. This allows complex-shaped parts to be formed with low press forces in a single operation. By contrast with traditional cold forming, the part properties are mainly produced by cooling in the die and less by the forming process. Targeted control of cooling in the die also allows parts to be produced with graded functional properties (Tailored Tempering).

The one-stage process – direct hot forming – is used most commonly and is ideal for processing tribond®+AS. The AS coating offers good protection against scaling – a typical occurrence in hot forming – and thus improves die life. tribond® can be processed on standard production equipment when adhering to the process window for MBW® 1500+AS.

Joining

In both the as-delivered and the hot-formed (hardened) condition, the two tribond® variants for hot forming are suitable for welding with steels of the same or different grade on the condition that the welding parameters are matched to the material. The welding parameters are comparable with those for the production material MBW® 1500+AS. Resistance spot welding, shield gas welding and laser beam welding are particularly suitable.

Resistance spot welding

Resistance spot welding is the process of choice and the most commonly used welding process worldwide. In comparison with lower-strength steels, resistance spot welding of tribond®, like MBW® 1500, calls for higher electrode forces and longer welding times – optionally as multiple-impulse (pulsation) welding based on DIN EN ISO 18278-2. The welding ranges for both similar and dissimilar metal joints are wide for this strength class. The spotwelded joints are relatively ductile. Despite the high material strength and hardness of the weld joint, they generally suffer mixed mode failure in the chisel test with a relatively high amount of nugget pullout. Joint strengths follow the strengths of the base materials joined; in dissimilar joints, the joint strength is naturally influenced by the softer of the two materials.

Arc brazing and welding MIG/MAG

In electric arc joining processes, the material softens in the heat-affected zone. Alongside the filler, this must be taken into account in the part design. Especially when joining with zinc-coated steel sheet, the AS coating – which is thermally impacted by the hot-forming process – may restrict arc stability.

Laser beam welding

tribond® steels can be readily welded with both CO2 and solid state lasers. Welding with CO2 lasers is carried out using standard shield gases. In the non-heat-treated condition, the hot-dip aluminum coating should be removed locally prior to laser beam welding as otherwise AlSi inclusions may occur, which impact material strength. In the heat-treated condition, the coating is fully alloyed and does not need to be removed. In general, it should be noted that the strength of the base material decreases in the heat-affected zone of the weld.

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