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.

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 elongation. CP steels with the bainitic matrix have superior formability because the difference between the hardness of bainite and martensite is relatively small. In bainitic CP steels polygonal ferrite is replaced by bainitic ferrite. The bainitic ferrite is strengthened by a high density of dislocations, i.e., > 1012/cm2, and by a fine dispersion of micro alloying second phase and carbonitrides or carbides.

CP steels are characterized by their tensile strength at a level of around 800 MPa, and quite often even more. The high strength of steel is achieved due to the contents of ?ne grained ferrite and interstitial bainite in the microstructure and the dispersion hardening by precipitates of carbides and nitrides. To obtain ?ne grained precipitates, additions of Nb, Ti or V are used.

CP steels have a high yield strength tensile strength ratio. Generally CP steels are easy to cold form and excellent for stretch forming and roll forming.

CP cold rolled steels have higher minimum yield strength in comparison with dual phase (DP) steels of identical tensile strength. Although the ultimate elongation of CP steels is lower than that of DP and TRIP (transformation induced plasticity) steels, CP steels have good formability for their high strength level. Forming limit curves can be used to define maximum strains without necking for different deformation paths.

CP steels are characterized by good work hardening even to minor deformation forces. Additionally, these steels have good bake hardening potential. Heat treatment of hot rolled CP steels at 500 up to 700 deg C can be used to increase the yield point upto 100 MPa. In addition, without compromising component properties, forming in the temperature range between 550 up to 650 deg C can be used to produce complex parts.

CP steels exhibit good roll forming, bending and hole expansion behaviour. Their key characteristics are high residual deformation and high energy absorption capacity.

CP steels have high wear resistance and hence have good durability. Hence these steels are used for parts with heavy wear.

CP steels display high fatigue strength. Based on stress-strain curves they display higher fatigue strength than dual-phase and retained austenite steels, but they are more sensitive to severe strain peaks, i.e. abusive loads. The good fatigue properties of CP steels can be used in suspension system parts such as suspension arms.

 Fig 2 below gives typical Wohler curves for a variety of CP steels. They are expressed as stress amplitude versus cycles to failure and are obtained with a stress ratio of R = 0.1 and repeated tensile loading.

Wohler curve for CP steels
Fig 2 Typical Wohler or SN curves for CP steels (R=0.1)

 The graph below (Fig 3) shows typical low cycle or EN curves for CP steels. These are expressed as strain amplitude versus number of reversals (one cycle equals two reversals.

EN curves for CP steels
Fig 3 Typical low cycle or EN curves for CP steels (R=-1)

 CP steels can be welded to other CP steels and to other common steel grades. The welding parameters must be matched to the material. To spot weld CP steels the same equipment can generally be used as for unalloyed deep-drawing steels.

Compared with lower strength steel grades, CP steels tend to have a lower electrical conductivity, which is why they require lower welding current with the same electrode force in spot welding. When resistance spot welding galvanized sheets, the welding currents must be raised due to the higher conductivity of the coating compared with the base metal. In addition, increasing the electrode force and welding time has a favorable effect on the welding current range.

As well as sheet grade, surface and thickness ratio, other factors such as the type of electrode used play an important role in determining the optimum joining parameters.

Applications of CP steels

CP steels are particularly suitable for weight saving manufacturing of cold formed, crash relevant parts of automobiles and have several automotive applications, particularly in body structure, suspension, and chassis components. Some car manufacturers have used CP steels in several components to improve rear crash safety.

CP steels are distinguished by good deformability and high capability to absorb energy during a collision. Because of these properties, CP steels ?nd application as a material for production of construction elements absorbing the energy of collisions, especially side crashes.

Given their high energy absorption capacity and fatigue strength, CP steel grades are particularly well suited for automotive safety components requiring good impact strength and for suspension system components. Examples are fender beams, cross members, side impact intrusion beams, door impact beams, bumpers, and reinforcements for B- pillar. Replacing conventional microalloyed steels with CP steels in B-pillar reinforcement can double its strength.