Dual Phase Steel: Part One

The term dual phase steels, or DP steels, refers to a class of high strength steels which is composed of two phases, two distinct combinations of the same alloy. Normally DP steel contains a ferrite matrix with a dispersed second phase of martensite, retained austenite or bainite. DP steels were developed in the 1970s, driven by the need for high strength steels without sacrificing formability or increasing costs. In particular, the automotive industry has demanded steel grades with high tensile elongation to ensure formability, high tensile strength to ensure fatigue and crash resistance, and low alloy content to maintain weldability without influencing production cost. Decades on, the demand for DP steels is still strong.

Figure 1 is a diagram that relates the ductility to the strength of the most common grades of steel. Considering their strength, formability, weldability and cost, dual phase (DP) steel, as one kind of advanced high strength steel (AHSS), can meet the requirements of the automotive industry. Its special microstructural features, hard martensite embedded in a soft ferrite matrix, enables the steel to possess both good formability and high strength.

Figure 1: Strength-to-elongation relationships for different steel grades

As mentioned, dual phase steels are characterized by a microstructure consisting of about 75-85% ferrite (alpha phase iron) with the remainder being a mixture of martensite, lower bainite, and retained austenite (Figure 2). The name "dual phase" was coined in the mid-1970s to describe ferrite-martensite microstructures (I), but dual phase steels usually contain more than the two phases implied by their name. They are essentially low carbon steels that are thermomechanically processed to have better formability than ferrite-pearlite steels of similar tensile strength.

Figure 2: Scanning electron micrograph of a dual phase steel. The microstructure consists of a fine-grained ferrite matrix with a uniform distribution of about 20% transformation product, which consists of martensite, retained austenite and bainite.

Properties of DP steels

DP steels, with their hard phase islands (martensite) embedded in a soft phase (ferrite), have unique properties. They offer high strength, low yield-to-tensile strength ratio, high initial work hardening rate, continuous yielding behavior, bake hardenability, and no room temperature aging effects. These properties mainly depend on the grain size, amount, distribution and carbon content of the martensite phase. Compared to conventional high strength steels (Figure 3) and mild steel, the strength of DP steels is significantly greater without any loss of formability. Therefore, DP steels allow enhanced design flexibility and provide a significant thickness and weight reduction in structural components.

Figure 3: Comparison of different AHSS

Dual-phase steels can be produced as both hot rolled and cold rolled material. When cold rolled, the properties develop along continuous annealing lines where there is even greater control over thermal treatment.

DP Steels can also be produced as HD Galvanized, HD Galvannealed and Electro Galvanized.

Both hot and cold-rolled DP steels offer an incredibly advantageous combination of low yield, high-tensile strength, easy cold working, and weldability due to their ferrite-martensite imbued lattice microstructure.

The carbon content of dual-phase steels enables the formation of martensite at practical cooling rates, which increases the hardenability of the steel. Generally, higher carbon will promote a stronger steel and a higher fractional percentage of martensite.

In DP steels the soft Ferrite phase is generally continuous, giving these steels excellent formability. When DP deforms, the strain is concentrated in the lower strength Ferrite phase surrounding the hard islands of martensite, which creates the very high initial work hardening rate exhibited by these steels.

Due to high-strain hardenability, dual-phase steels also have a high strain redistribution capacity. This means improved drawability, as well as finished part mechanical properties (yield strengths) that are higher than the initial blank.

DP steels also have a bake hardening effect that is an important benefit over conventional HSLA-type materials. The bake hardening effect is the increase in yield strength resulting from elevated temperature aging created by the curing temperature of the paint bake cycle.

DP grades can be produced from 500 to 1200 MPa minimum tensile strength with 5-35% total elongation.

Formability

DP steels offer an excellent combination of strength and drawability as a result of their strain hardening capacity from the beginning of deformation. This capability ensures homogeneous strain redistribution and reduces local thinning.

Dual-phase steels can be drawn on conventional tools, provided the settings are properly adjusted. For example, drawing pressure may be increased by approximately 20% for a Dual-Phase 600, compared to a micro-alloyed (HSLA) type steel of the same thickness.

Applications in Automobiles

As one could expect from a material with a high-tensile strength, dual-phase steels are well suited for automobile parts that are meant to absorb a lot of energy during an impact.

Dual-phase steels are often used in the following automobile applications:

References

1. Y. Granbom: Structure and mechanical properties of dual phase steels-An experimental and theoretical analysis, Doctoral thesis, KTH Royal Institute of Technology, 2010, SE-100 44 Stockholm, Sweden, ISBN 978-91-7415-740-6;

2. M. S. Rashid: Dual Phase Steels, Ann. Rev. Mater. Sci. 1981. 11:245-66, Accessed JUNE 2020;

3. C. Chang: Correlation between the Microstructure of Dual Phase Steel and Industrial Tube Bending Performance, University of Windsor, Windsor, Ontario, Canada, 2010, Electronic Theses and Dissertations. 178;

4. Dual-Phase Steels: An Introduction