Dual Phase Steel

Steels are classified as dual phase steels when they display a ferritic-martensitic microstructure. Dual phase steels are known for their high ultimate tensile strength. In large parts, DP steels are used in the automobile industry. Dual phase steel grades are standardized in the European standard EN 10346.

Properties of Dual Phase Steel

The microstructure of DP steels is made of a ferritic matrix and secondary phase martensites, which are positioned elliptically in the grain boundaries. DP steels are made of about 10 to 40 % of martensites. The higher that amount, the higher the ultimate tensile strength. Dual phase steels also display lower fracture strain and yield strength.
Furthermore, dual phase steels have a strong bake hardening effect. Bake hardening is an effect in which yield strength of materials increase after a controlled aging process at low temperatures (170°C). This means that DP steels can be strengthened after deformation in a relatively low heat.

As a result, dual phase steels can be deformed with little force (low yield point) without breaking (high ultimate strength) and then hardened to increase yield strength after deformation (strong bake hardening effect).

Composition of dual phase steel

Dual phase steels are composed of a mixture of ferrites and martensites. The hard phase is made of martensites, which is formed in steels with high carbon content. By quenching gamma-phase iron (austenite) rapidly, the carbon atoms cannot diffuse out quickly enough, meaning that they will form a martensitic crystal structure.

Steel alloys such as manganese, molybdenum, nickel and chromium can increase hardenability. The balance of alloys determines other material properties, such as weldability and ductility.

Production of Dual Phase Steel

Steels that are eligible to transforming into DP steels are low and medium carbon steels that contain carbon rich austenite and low carbon ferrites. The material is subjected to annealing at lower transformation temperatures (A1). Most steel types are held at around 800°C, depending on the composition present.

Next, the material is quenched to temperatures below the upper critical temperature (A3) at which ferrites and martensites start to form as a result of the formation of austenites.

Applications of Dual Phase Steel

Since deformation works well on DP steels, they are often used for deep-drawn parts. The automobile industry benefits from dual phase steel parts in particular. Examples are car doors and body panels.