Molybdenum in Steels

Molybdenum in Steels Molybdenum (Mo) (atomic number 42 and atomic weight 95.95) has a density of 10.22 gm/cc. Melting point of Mo is 2610 deg C and boiling point is 5560 deg C. The phase diagram of the iron molybdenum (Fe-Mo) binary system is at Fig 1. Fig 1 Iron molybdenum binary system Mo is normally referred in short as ‘moly’.  It has many important uses in alloy steels, stainless steels, alloy cast irons and super alloys. It is a powerful hardenability agent and is a constituent of many heat treatable alloy steels. Mo retards softening at higher temperatures. Hence it is used in boiler and pressure vessel steels, as well as several grades of high speed and other tool steels. Mo improves the corrosion resistance of stainless steels. In HSLA (high speed low alloy) steels, it produces acicular ferrite structures. Mo is the basis for many of the as-rolled DP (dual phase) steels used in automotive applications. While Mo may often be used interchangeably with chromium (Cr) and vanadium (V), in many cases the properties it imparts are unique. Due to it, the use of Mo has increased considerably over the past several decades. Available forms Mo is supplied as ferro-molybdenum (Fe-Mo) and as molybdic oxide (MoO3). Fe – Mo contains a minimum of 60 % Mo. Silicon (Si) and copper (Cu) may be present in quantities up to 1 % each. It is relatively expensive and is sparingly used for addition. Technical MoO3 has a minimum of 57 % Mo. SiO2 is the main impurity, but it may also contain small amounts of Cu, sulfur (S), and phosphorus (P). MoO3 is supplied either in cans or as briquettes. MoO3 briquettes may also contain some amount of carbon (C). Considerable quantity of Mo is recovered...

Ferritic Bainitic Steels...

Ferritic Bainitic Steels  Ferritic bainitic steels are also known as FB steels. These steels are one of the types of advanced high strength steels which have been developed for automotive application. Since these steels have two phases, hence these steels are also a type of dual phase (DP) steel. FB steels are mostly available as hot rolled products. These steels are normally cold-drawn. Ferritic bainitic range of hot rolled high strength steels has been developed to meet weight reduction requirements of the automobiles. . They are fully killed steels and are usually available in four strength levels namely FB 450, FB 540, FB 560 and FB 590. FB family of steels extends the HSLA range of micro alloyed steels to include products combining high ultimate tensile strength (UTS) with excellent formability. Typical additions for grain refinement in these steels are Al (aluminum), B (boron), Nb (niobium), and Ti (titanium). These elements are added individually or in combination. Nitrogen (N) binding is also used sometimes. FB steels are with soft ferrite and hard bainite. They have a microstructure of fine ferrite and bainite. Their micro structure is finer than the typical DP steel. Strengthening is obtained by both grain refinement and second phase hardening with bainite. The micro structure of FB steels gives these steels a marked improved ductility. Fig 1 shows a typical microstructure for the FB steel. Fig 1 Typical micro structure of FB steel  FB steels are utilized to meet specific customer application requirements that require stretch flangeable (SF) or high hole expansion (HHE) capabilities for improved edge stretch capability. SF capabilities of FB steels are based on their ferrite bainite micro structure. The micro structure is usually even more finely tuned to be SF. This characteristic can be measured by the...

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...

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...