Microalloyed Steels

Microalloyed Steels Microalloyed steels are a type of alloy steels that contains small amounts of alloying elements (usually 0.05 % to 0.15 %). These steels are also sometimes called high strength low alloy (HSLA) steels. Though the work of strengthening steels through addition of small percentage of alloying elements started as early as 1916 in USA, the term ‘Microalloying’ (believed to be of Russian origin) was adopted by Prof. T. M. Noren-Brandel in 1962 and became pervasive as a result of the landmark conference ‘Microalloying 75’.  Strengthening by microalloying elements permits a dramatic reduction in carbon content which greatly improves weldability and notch toughness. Microalloyed steels have been developed originally for large diameter oil and gas pipelines. The technology of microalloying involves the addition of a fraction of a percent of the microalloying elements to simple low carbon mild steel. The use of ‘micro’ alloy concentrations, which produce remarkable changes in mechanical properties, distinguishes the technology from ‘alloying’ in the conventional sense (low alloy steels family) where concentration of the alloying elements may range from 0.25 % to one or two or possibly several percent. Microalloyed steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition. Microalloyed steels are a family of steels that contain 0.07 % to 0.12 % carbon, upto 2 % manganese and are strengthened by the elements niobium, vanadium and titanium added either singly or in combination. These elements are sometimes used in conjunction with other strengtheners such as boron, molybdenum and chromium, nickel, copper and rare earth metals. The microalloying elements are used to refine...

Nitrogen in Steels May23

Nitrogen in Steels

Nitrogen in Steels All steels contain some nitrogen which can enter the steel as an impurity or as an intentional alloying addition. The quantity of nitrogen in steels normally depends on the residual level arising from the steelmaking processes or the amount aimed in case of deliberate addition. There are significant differences in residual levels of nitrogen in steels produced from the two main steelmaking processes. Basic oxygen furnace (BOF) process generally results into lower residual nitrogen in steels, typically in the range of 30 to 70 ppm while electric arc furnace (EAF) process results into higher residual nitrogen, typically in the range of 70 to 110 ppm. Nitrogen is added to some steels (e.g. steels containing vanadium) to provide sufficient nitrogen for formation of nitride to achieve higher strength. In such steels nitrogen levels can increase to 200 ppm or higher. Nitrogen in the liquid steel is present in the form of solution. During the solidification of steel in continuous casting, three nitrogen related phenomena can happen. These are Formation of blow holes Precipitation of one or more nitride compounds Solidification of nitrogen in interstitial solid solution. The maximum solubility of nitrogen in liquid iron is around 450 ppm, and less than 10 ppm at ambient temperature (Fig 1). The presence of significant quantities of other elements in liquid iron affects the solubility of nitrogen. Mainly the presence of dissolved sulfur and oxygen limit the absorption of nitrogen because they are surface active elements. Fig 1 Solubility of nitrogen in iron Nitrogen is generally considered as undesirable impurity which causes embrittlement in steels and affects strain aging. However nitrogen produces a marked (intersititial solid solution) strengthening when diffused into the surface of the steel, similar to the strengthening observed during case hardening (Nitriding)....