Dual Phase Steels

Dual Phase Steels  The term dual phase steels, or DP steels, refers to a class of high strength steels which is composed of two phases namely a purely ferrite matrix and a dispersed second phase of martensite (5 % to 30 %). In addition to martensite, small amounts of bainite and residual austenite may exist. DP steels were developed in the 1970s. The development was driven by the need for new high strength steels without reducing the formability or increasing costs. DP steels starts as a low or carbon steel and is quenched from a temperature above A1 but below A3 on a continuous cooling transformation diagram. This results in a microstructure consisting of a ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). The desire to produce high strength steels with formability greater than micro alloyed HSLA (high strength low alloy) steel led the development of DP steel. DP steels are low carbon micro alloyed steels. The steel microstructure consists of a very hard phase of martensite in a soft formable ferrite matrix (Fig 1). The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower strength ferrite phase surrounding the islands of martensite, creating the unique high work hardening rate exhibited by these steels. Fig 1 Micro structure of DP steel  The steel behave like composite materials where the ferrite matrix assures high cold formability, and the martensite is the strengthening element. The correct proportion between the two phases allows a continuous yield point, low yielding stress, and a high elongation value, a smooth flow stress curve with a high strain hardening coefficient, and better plasticity and formability. The microstructure of steel gives a good combination of high tensile strength, low yield-to-tensile...