DUAL-TEN^® (dual phase) Steels

DUAL-TEN^® (dual phase) steels are quickly becoming one of the most popular and versatile materials in today's automotive industry. Currently, these steels are most commonly used in structural applications where they have replaced more conventional HSLA steels. They offer a great opportunity for part weight reduction. The improved formability, capacity to absorb crash energy, and ability to resist fatigue have driven this substitution. Today's applications include front and rear rails, crush cans, rocker reinforcements, b/c pillar reinforcements, cowl inner/outer, back panels, cross members, bumpers and door intrusion beams. Recently, DUAL-TEN^® steels have gained in popularity in automotive closures.

DUAL-TEN^® steels are made in the following grades. You can find more specific material property information about these grades in our online brochure or by following the links provided.

	DUAL-TEN® 590/600 steel
	DUAL-TEN® 780/800 steel

In general, DUAL-TEN^® steel is a mixture of ferrite matrix and martensite islands decorating grain boundaries with possible addition of bainite. Formable DUAL-TEN^® steels contain approximately 5% - 15% martensite. Refer to Figure - 1.

     - Ferrite - soft phase, offers ductility
     - Martensite - hard phase, offers strength

Figure - 1: Example of DUAL-TEN^® 600 steel structure, 1000x

Characteristics of DUAL-TEN^® steels:

• Work hardening - DUAL-TEN^® steel displays a high and rapid initial work hardening rate. Even at low forming strain levels (2% - 3%), yield strength increases approximately 21-31 ksi (145-214 MPa). Refer to Figure - 2.
  

• Yield point elongation - Tested DUAL-TEN^® 590/600 and DUAL-TEN^® 780 steels show no yield point elongation. Refer to Figure - 3.
  

• Formability - Due to a higher work-hardening rate and absence of YPE, DUAL-TEN^® steels behave predictably in stamping processes (i.e. resistance to necking, plastic instability and kinking).
  

• FLD curves - For the needs of forming feasibility analysis, FLD can be approximated with sufficient accuracy by the conventional ASM-FLD calculated from n-value and thickness. Refer to Table - 1, Figure - 4 and Figure - 5.
  

• Springback - As compared to conventional HSLA steels, springback is easier to control due to consistent behavior during stamping. Refer to Figure - 6.
  

• Bendability - DUAL-TEN^® steel has very good bendability.  Refer to Figure - 7.
  

• Bake hardening - DUAL-TEN^® steels have an excellent bake hardening capacity. The increase in the yield strength resulting from typical paint baking cycle is approximately 5-10 ksi (35-70 MPa). Refer to Figure - 8 and Figure - 9.
  

• In-part strength - DUAL-TEN^® (dual phase) steels have a high ultimate tensile strength (UTS). UTS ranges from 72-175 ksi (500-1200 MPa) for available grades.
  

• Shelf life - DUAL-TEN^® steel displays no room temperature aging.
  

• Mass reduction capability - These steels have high potential for part downgaging and weight reduction (up to 25% as compared to equivalent conventional HSLA steels).
  

• Crash energy management - DUAL-TEN^® steels have a higher yield to tensile ratio as compared to conventional HSLA steels (0.5-0.6). This results in a higher capacity to manage vehicle crash energy.
  

• Fatigue performance - DUAL-TEN^® steels have a higher fatigue strength than equivalent conventional HSLA steels.
  

• Weldability - DUAL-TEN^® steel meets automotive application weldability needs.

Figure - 2: Example of work hardening rate for DUAL-TEN^® 590/600 steel: Instantaneous n-value as a function of engineering strain. Note the characteristic high n-value in the low plastic deformation range.

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Figure - 3: Examples of stress strain curves for DUAL-TEN^® steels as compared with various grades.

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Table - 1: Comparison of mechanical properties for DUAL-TEN^® 590 and HSLA 340 steels

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Figure - 4: Experimental forming limit curve for material thickness of 1.8mm. A conventional ASM FLC calculated from n-value and thickness can be used for DUAL-TEN^® steel with sufficient accuracy.

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Figure - 5: Experimental forming limit curve for material thickness of 1.2mm. A conventional ASM FLC calculated from n-value and thickness can be used for DUAL-TEN^® steel with sufficient accuracy.

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Figure - 6: Springback. Results of bending under tension test. Various back tension forces at constant ratio of bending radius to material thickness,
 R/t = 2.14. For the top samples, back tension is 85% YS, the middle is 75% YS, and the bottom is 50% YS. Two samples are shown in the picture for each case. Note consistent geometry for DUAL-TEN^® 590. Dimensional accuracy of parts made of DUAL-TEN^® steel can be controlled easier due to consistent material behavior in plastic deformation.

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Figure - 7: Bendability in a free-bending test. Results of the bendability test for DUAL-TEN^® 590 steel: a) inner radius close to zero, b) inner radius close to the half of the material thickness. Note material folding tendencies in the inner radius area. Bending with inner radius close to zero can be performed successfully without fracture on the outer bend line.

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Figure - 8: Bake hardening in paint baking cycle. Increase in yield strength due to bake hardening for DUAL-TEN^® 590 steel at various pre-strain levels.

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Figure - 9: Combined effect of work hardening (WH) and bake hardening (BH). Yield strength of DUAL-TEN^® 600 steel increases more than 260 MPa after deformation to 5% and baking in a typical paint baking cycle.

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