Dual phase steels

Dual Phase steels offer a good combination of strength and stampability as a result of their microstructure, in which a hard martensitic or bainitic phase is dispersed in a ductile ferritic matrix. These steels have high strain hardenability. This gives them not only good strain redistribution capability and thus stampability, but also finished part mechanical properties, including yield strength, that are far superior to those of the flat metal. The yield strength of Dual Phase steels is further increased by the paint baking (also called Bake Hardening, BH) process.

This high level of mechanical strength obtained in parts lends these steels excellent fatigue strength and good energy-absorption capacity, making them suitable for use in structural parts and reinforcements. Their strong strain hardening combined with a pronounced bake hardening effect gives them excellent potential for reducing the weight of structural parts and even skin parts, especially in the case of CR290Y490T-GI-E Extragal^® or Ultragal^®.

Given their high energy absorption capacity and fatigue strength, cold-rolled Dual Phase Steels are particularly well suited for the manufacture of automotive structural and safety parts such as longitudinal beams, crossmembers and reinforcements.

CR290Y490T-GI-E Extragal^® or Ultragal^® steel can be used to make visible parts with 20% higher dent resistance than conventional high strength steels, resulting in a potential weight saving of some 15%.

As a result of its mechanical strength, hot-rolled Dual Phase 600 can be used to reduce the weight of structural parts by decreasing their thickness. Relevant automotive applications include:

• Wheel webs
• Lightweight sections
• Strut mounts
• Fasteners

ArcelorMittal has an extensive database on the forming and service properties of the entire range of Dual Phase steels. To incorporate these steels at the design stage, a team of experts is available to carry out specific studies based on either modelling or characterisation testing. 

LCE: Low Carbon Equivalent grade used to optimise properties in service.

The above table, for information only, summarises the cross-references between the ArcelorMittal product range, European standards and VDA standard 239-100.

These cross-references are given for information only. ArcelorMittal steel grades generally offer tighter mechanical properties (see table below).

A_80mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 80 mm (ISO 6892-1 type 2 (EN20x80))
A_50mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 50 mm (ISO 6892-1 type 1 (ASTM12.5x50) or type 3 (JIS25x50)
A%: Percentage elongation after fracture using a proportional specimen with L₀ = 5.65 (So)^1/2
Ag %: Percentage plastic extension at maximum force
BH₂: Increase in yield strength between a reference condition after a 2% plastic pre-strain and the condition obtained after heat treatment (170°C-20minutes)

LCE: Low Carbon Equivalent grade, used to optimise properties in service.

The service properties of Dual Phase steels are guaranteed by manufacturing process control. The controlled (temperature, cooling rate) annealing cycle in particular ensures that the Dual Phase microstructure is achieved and mechanical properties are reproducible.

Please enquire about the availability of products shown as being under development or left blank in the table.

LCE: Low Carbon Equivalent grade, used to optimise properties in service.

Dual Phase steels offer a good combination of strength and stampability as a result of their good ductility and strain hardening capacity from the onset of deformation, which ensure uniform strain distribution and reduced local thinning. For example, in the case of a Dual Phase 500 steel, the yield strength increases by about 120 MPa after 2% of plastic strain in uniaxial tension (a phenomenon known as work hardening or WH2). The yield strength can be further increased through bake hardening (BH2) after paint curing.

CR290Y490T-GI-E Extragal^® or Ultragal^® steel can also be used to manufacture skin parts, as a result of its excellent behaviour in expansion and in tension broadly.

Dual Phase steels can be drawn on conventional tools, subject to settings being optimised. For example, stamping pressure should be increased by approximately 20% for a CR330Y590T-DP steel, compared to a micro-alloyed (HSLA) type steel of the same thickness. It should be noted that these steels, especially the highest grades, are sensitive to the so-called springback phenomenon. Component geometry must be carefully studied during design (small die radius, reinforcement perpendicular to bends to stiffen open parts, etc.) and drawing sequence definition (overbending, calibration tool, punch stroke, increased blank-clamping force, etc.).

ArcelorMittal has developed expertise in controlling springback by means of part and drawing tool design (OUTIFO method).

For more information about the forming of Dual Phase steels in special thicknesses and with special coatings, please contact us.

Although Dual Phase steels are more highly alloyed than HSLA steels, they can be readily welded using conventional resistance spot welding processes, provided the parameters used in industrial conditions are adjusted. The table below gives  examples, for information only, of some spot weld properties for Dual Phase steel similar joints complying with the ISO 18278-2 standard.

For coated (galvanised and alloy galvanised) products, electrode service life tests show values characteristic of the type of coating considered. The Dual Phase substrate does not affect the service lifespans obtained.

In butt or lap MAG (Metal Active Gas) arc welding, maximum hardness in the fusion zone does not exceed 300 HV for a HR330Y580T-DP steel, whatever the parameters. The weld runs meet ISO 25817 Class B requirements. Recommended welding consumables are:

• Filler: G3Si1 NF EN 440;
• Protective gas: Ar + 8% CO2.

Dual Phase steels have excellent mechanical strength in laser lap welding. Based on long shop-floor experience in characterising its products, ArcelorMittal can provide technical assistance in adjusting the welding parameters for Dual Phase steels.