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...

TRIP Steels

TRIP Steels  TRIP steels are high strength steels. TRIP stands for ‘transformation induced plasticity’.  They are new generation of low alloy steels. These steels offer outstanding combination of strength and ductility as a result of their micro structure. TRIP steels rely on the transformation of austenite grains into the harder phase of martensite during deformation for achieving their mechanical properties. The locations of these grains in the microstructure are of major importance because they influence the impact of the TRIP effect, the microstructural localization and therefore the macroscopical deformability of the material. Microstructure and composition  The microstructure of these steels is composed of islands of hard residual austenite and carbide free bainite dispersed in a soft ferritic matrix.  The retained austenite is embedded in a primary matrix of ferrite. In addition to a minimum of 5 % to 15 % of retained austenite, hard phases such as martensite and bainite are present in varying amounts. Austenite is transformed into martensite during plastic deformation (TRIP effect), making it possible to achieve greater elongations and lending these steels their excellent combination of strength and ductility. Fig 1 shows the typical microstructure of TRIP steel. Fig 1 Typical micro structure of TRIP steel  TRIP steels typically require the use of an isothermal hold at an intermediate temperature, which produces some bainite. The higher silicon and carbon content of TRIP steels also result in significant volume fractions of retained austenite in the final microstructure. TRIP steels use higher quantities of carbon than dual phase steels to obtain sufficient carbon content for stabilizing the retained austenite phase to below ambient temperature. Higher contents of silicon and/or aluminum accelerate the ferrite/bainite formation. They are also added to avoid formation of carbide in the bainite region. Silicon though a key element for the formation of retained austenite, is undesirable...

Advanced high strength steels for automotive application...

       Advanced high strength steels for automotive application Advanced high strength steels (AHSS) are being developed for automotive applications. These automotive grades are different when compared with the conventional low and high strength steels. AHSS have superior mechanical properties which are developed in it due to the steel’s structure and due to its distinct processing. AHSS are manufactured by adopting control cooling from austenite or austenite plus ferrite phases on the run out roller table in a hot rolling mill or in the cooling section of a continuous annealing furnace in cold rolled product. A comparison of AHSS with other types of high strength steel is given in Fig 1. Some types of these steels are described below. Fig 1 Comparison of high strength steel with AHSS Dual Phase (DP) steel This steel has two phases namely ferrite and martensite. The hard second phase of martensite is present in the form of islands in a matrix of ferrite. Higher is the volume fraction of second phase higher is the strength. During the production of this steel, a portion of the austenite phase is first converted into ferrite before rapid cooling to transform the remaining austenite to martensite. Some hot rolled steels can have a microstructure which contains considerable amount of bainite. The property of good ductility to this steel is imparted by soft ferrite phase which is generally continuous. During the working of this steel, lower strength ferrite phase gets strained giving the steel a distinctive high work hardening rate. The work hardening rate along with good elongation provides DP steel better ultimate tensile strength (UTS) values then conventional steel of similar yield strength (YS). Accordingly DP steel has low YS/TS ratios. DP steel also has a bake hardening effect which is...