Complex Phase Steels

Complex Phase Steels  The complex phase (CP) steels belong to the group of advanced high strength steels (AHSS) grade, which gain their strength through extremely fine grain size and a micro structure containing martensite in small amounts, and pearlite embedded in the ferrite/bainite matrix. A very high grain refinement is achieved by precipitation of micro alloying elements such as niobium (Nb), or titanium (Ti), or retarded recrystallization. The advantage of the CP steels is that cold forming, without subsequent quenching and tempering, is possible, thus implying a considerable cost saving potential. CP steels are currently being produced as hot rolled steel strips as well as cold rolled advanced high strength steels, which are hot dip galvanized for corrosion protection. The chemical composition of CP steels, and also their microstructure, is very similar to that of TRIP steels, but, additionally it contains some quantities of Nb, Ti and or V (vanadium) to cause the precipitation strengthening effect. Typically, CP steels have no retained austenite in the microstructure, but contain more hard phases like martensite and bainite. The microstructure of CP steels is composed of a very fine ferrite with the high volume fraction of hard phase, For cold shaped products, a triple phase steel containing ferrite, bainite and martensite can be designed which are obviously more difficult to produce. The bainitic complex phase microstructure exhibits better strain hardening and strain capacity than that for fully bainitic micro structure. It involves a strength graded microstructure where the martensite and bainitic ferrite phases are separated by a third phase of intermediate strength. Fig 1 shows typical micro structure of CP steels. Fig 1 Typical micro structure of CP steels  Properties of CP steels The mechanical properties of CP steels are characterized by continuous yielding and high uniform...